Leading Edge
Review
Hallmarks of Cancer: The Next Generation
Douglas Hanahan1,2,* and Robert A. Weinberg3,*
1The Swiss Institute for Experimental Cancer Research (ISREC), School of Life Sciences, EPFL, Lausanne CH-1015, Switzerland
2The Department of Biochemistry & Biophysics, UCSF, San Francisco, CA 94158, USA
3Whitehead Institute for Biomedical Research, Ludwig/MIT Center for Molecular Oncology, and MIT Department of Biology, Cambridge,
MA 02142, USA
*Correspondence: dh@epfl.ch (D.H.), weinberg@wi.mit.edu (R.A.W.)
DOI 10.1016/j.cell.2011.02.013
The hallmarks of cancer comprise six biological capabilities acquired during the multistep development
of human tumors. The hallmarks constitute an organizing principle for rationalizing the
complexities of neoplastic disease. They include sustaining proliferative signaling, evading growth
suppressors, resisting cell death, enabling replicative immortality, inducing angiogenesis, and activating
invasion and metastasis. Underlying these hallmarks are genome instability, which generates
the genetic diversity that expedites their acquisition, and inflammation, which fosters multiple hallmark
functions. Conceptual progress in the last decade has added two emerging hallmarks of
potential generality to this list—reprogramming of energy metabolism and evading immune
destruction. In addition to cancer cells, tumors exhibit another dimension of complexity: they
contain a repertoire of recruited, ostensibly normal cells that contribute to the acquisition of hallmark
traits by creating the ‘‘tumor microenvironment.’’ Recognition of the widespread applicability
of these concepts will increasingly affect the development of new means to treat human cancer.
INTRODUCTION
We have proposed that six hallmarks of cancer together constitute
an organizing principle that provides a logical framework for
understanding the remarkable diversity of neoplastic diseases
(Hanahan and Weinberg, 2000). Implicit in our discussion was
the notion that as normal cells evolve progressively to
a neoplastic state, they acquire a succession of these hallmark
capabilities, and that the multistep process of human tumor
pathogenesis could be rationalized by the need of incipient
cancer cells to acquire the traits that enable them to become
tumorigenic and ultimately malignant.
We noted as an ancillary proposition that tumors are more than
insular masses of proliferating cancer cells. Instead, they are
complex tissues composed of multiple distinct cell types that
participate in heterotypic interactions with one another. We depicted
the recruited normal cells, which form tumor-associated
stroma, as active participants in tumorigenesis rather than
passive bystanders; as such, these stromal cells contribute to
the development and expression of certain hallmark capabilities.
During the ensuing decade this notion has been solidified and
extended, revealing that the biology of tumors can no longer
be understood simply by enumerating the traits of the cancer
cells but instead must encompass the contributions of the
‘‘tumor microenvironment’’ to tumorigenesis.
In the course of remarkable progress in cancer research
subsequent to this publication, new observations have served
both to clarify and to modify the original formulation of the hallmark
capabilities. In addition, yet other observations have raised
questions and highlighted mechanistic concepts that were not
integral to our original elaboration of the hallmark traits. Motivated
by these developments, we now revisit the original hallmarks,
consider new ones that might be included in this roster,
and expand upon the functional roles and contributions made
by recruited stromal cells to tumor biology.
HALLMARK CAPABILITIES—CONCEPTUAL PROGRESS
The six hallmarks of cancer—distinctive and complementary
capabilities that enable tumor growth and metastatic dissemination—continue
to provide a solid foundation for understanding
the biology of cancer (Figure 1; see the Supplemental Information
for downloadable versions of the figures for presentations).
In the first section of this Review, we summarize the essence
of each hallmark as described in the original presentation in
2000, followed by selected illustrations (demarcated by subheadings
in italics) of the conceptual progress made over the
past decade in understanding their mechanistic underpinnings.
In subsequent sections we address new developments that
broaden the scope of the conceptualization, describing in turn
two enabling characteristics crucial to the acquisition of the six
hallmark capabilities, two new emerging hallmark capabilities,
the constitution and signaling interactions of the tumor microenvironment
crucial to cancer phenotypes, and we finally discuss
the new frontier of therapeutic application of these concepts.
Sustaining Proliferative Signaling
Arguably the most fundamental trait of cancer cells involves their
ability to sustain chronic proliferation. Normal tissues carefully
control the production and release of growth-promoting signals
that instruct entry into and progression through the cell growthand-division
cycle, thereby ensuring a homeostasis of cell
646 Cell 144, March 4, 2011 ª2011 Elsevier Inc.
number and thus maintenance of normal tissue architecture and
function. Cancer cells, by deregulating these signals, become
masters of their own destinies. The enabling signals are
conveyed in large part by growth factors that bind cell-surface
receptors, typically containing intracellular tyrosine kinase
domains. The latter proceed to emit signals via branched intracellular
signaling pathways that regulate progression through
the cell cycle as well as cell growth (that is, increases in cell
size); often these signals influence yet other cell-biological properties,
such as cell survival and energy metabolism.
Remarkably, the precise identities and sources of the proliferative
signals operating within normal tissues were poorly understood
a decade ago and in general remain so. Moreover, we still
know relatively little about the mechanisms controlling the
release of these mitogenic signals. In part, the understanding
of these mechanisms is complicated by the fact that the growth
factor signals controlling cell number and position within tissues
are thought to be transmitted in a temporally and spatially regulated
fashion from one cell to its neighbors; such paracrine
signaling is difficult to access experimentally. In addition, the
bioavailability of growth factors is regulated by sequestration in
the pericellular space and extracellular matrix, and by the actions
of a complex network of proteases, sulfatases, and possibly
other enzymes that liberate and activate them, apparently in
a highly specific and localized fashion.
The mitogenic signaling in cancer cells is, in contrast, better
understood (Lemmon and Schlessinger, 2010; Witsch et al.,
2010; Hynes and MacDonald, 2009; Perona, 2006). Cancer cells
can acquire the capability to sustain proliferative signaling in
a number of alternative ways: They may produce growth factor
ligands themselves, to which they can respond via the expression
of cognate receptors, resulting in autocrine proliferative
stimulation. Alternatively, cancer cells may send signals to stimulate
normal cells within the supporting tumor-associated
stroma, which reciprocate by supplying the cancer cells with
various growth factors (Cheng et al., 2008; Bhowmick et al.,
2004). Receptor signaling can also be deregulated by elevating
the levels of receptor proteins displayed at the cancer cell
Figure 1. The Hallmarks of Cancer
This illustration encompasses the six hallmark
capabilities originally proposed in our 2000 perspective.
The past decade has witnessed
remarkable progress toward understanding the
mechanistic underpinnings of each hallmark.
surface, rendering such cells hyperresponsive
to otherwise-limiting amounts
of growth factor ligand; the same
outcome can result from structural alterations
in the receptor molecules that
facilitate ligand-independent firing.
Growth factor independence may also
derive from the constitutive activation of
components of signaling pathways operating
downstream of these receptors,
obviating the need to stimulate these
pathways by ligand-mediated receptor
activation. Given that a number of distinct downstream signaling
pathways radiate from a ligand-stimulated receptor, the activation
of one or another of these downstream pathways, for
example, the one responding to the Ras signal transducer,
may only recapitulate a subset of the regulatory instructions
transmitted by an activated receptor.
Somatic Mutations Activate Additional Downstream
Pathways
High-throughput DNA sequencing analyses of cancer cell
genomes have revealed somatic mutations in certain human
tumors that predict constitutive activation of signaling circuits
usually triggered by activated growth factor receptors. Thus,
we now know that 40% of human melanomas contain
activating mutations affecting the structure of the B-Raf protein,
resulting in constitutive signaling through the Raf to mitogenactivated
protein (MAP)-kinase pathway (Davies and Samuels
2010). Similarly, mutations in the catalytic subunit of phosphoinositide
3-kinase (PI3-kinase) isoforms are being detected in
an array of tumor types, which serve to hyperactivate the PI3kinase
signaling circuitry, including its key Akt/PKB signal
transducer (Jiang and Liu, 2009; Yuan and Cantley, 2008). The
advantages to tumor cells of activating upstream (receptor)
versus downstream (transducer) signaling remain obscure, as
does the functional impact of crosstalk between the multiple
pathways radiating from growth factor receptors.
Disruptions of Negative-Feedback Mechanisms that
Attenuate Proliferative Signaling
Recent results have highlighted the importance of negativefeedback
loops that normally operate to dampen various types
of signaling and thereby ensure homeostatic regulation of the
flux of signals coursing through the intracellular circuitry (Wertz
and Dixit, 2010; Cabrita and Christofori, 2008; Amit et al.,
2007; Mosesson et al., 2008). Defects in these feedback mechanisms
are capable of enhancing proliferative signaling. The
prototype of this type of regulation involves the Ras oncoprotein:
the oncogenic effects of Ras do not result from a hyperactivation
of its signaling powers; instead, the oncogenic mutations
affecting ras genes compromise Ras GTPase activity, which
Cell 144, March 4, 2011 ª2011 Elsevier Inc. 647
operates as an intrinsic negative-feedback mechanism that normally
ensures that active signal transmission is transitory.
Analogous negative-feedback mechanisms operate at
multiple nodes within the proliferative signaling circuitry. A prominent
example involves the PTEN phosphatase, which counteracts
PI3-kinase by degrading its product, phosphatidylinositol
(3,4,5) trisphosphate (PIP3). Loss-of-function mutations in PTEN
amplify PI3K signaling and promote tumorigenesis in a variety
of experimental models of cancer; in human tumors, PTEN
expression is often lost by promoter methylation (Jiang and
Liu, 2009; Yuan and Cantley, 2008).
Yet another example involves the mTOR kinase, a coordinator
of cell growth and metabolism that lies both upstream and downstream
of the PI3K pathway. In the circuitry of some cancer cells,
mTOR activation results, via negative feedback, in the inhibition
of PI3K signaling. Thus, when mTOR is pharmacologically
inhibited in such cancer cells (such as by the drug rapamycin),
the associated loss of negative feedback results in increased
activity of PI3K and its effector Akt/PKB, thereby blunting the
antiproliferative effects of mTOR inhibition (Sudarsanam and
Johnson, 2010; O’Reilly et al., 2006). It is likely that compromised
negative-feedback loops in this and other signaling pathways
will prove to be widespread among human cancer cells and
serve as an important means by which these cells can achieve
proliferative independence. Moreover, disruption of such selfattenuating
signaling may contribute to the development of
adaptive resistance toward drugs targeting mitogenic signaling.
Excessive Proliferative Signaling Can Trigger Cell
Senescence
Early studies of oncogene action encouraged the notion that
ever-increasing expression of such genes and the signals manifested
in their protein products would result in correspondingly
increased cancer cell proliferation and thus tumor growth. More
recent research has undermined this notion, in that excessively
elevated signaling by oncoproteins such as RAS, MYC, and
RAF can provoke counteracting responses from cells, specifically
induction of cell senescence and/or apoptosis (Collado
and Serrano, 2010; Evan and d’Adda di Fagagna, 2009; Lowe
et al., 2004). For example, cultured cells expressing high levels
of the Ras oncoprotein may enter into the nonproliferative but
viable state called senescence; in contrast, cells expressing
lower levels of this protein may avoid senescence and proliferate.
Cells with morphological features of senescence, including
enlarged cytoplasm, the absence of proliferation markers, and
expression of the senescence-induced b-galactosidase
enzyme, are abundant in the tissues of mice engineered to overexpress
certain oncogenes (Collado and Serrano, 2010; Evan
and d’Adda di Fagagna, 2009) and are prevalent in some cases
of human melanoma (Mooi and Peeper, 2006). These ostensibly
paradoxical responses seem to reflect intrinsic cellular defense
mechanisms designed to eliminate cells experiencing excessive
levels of certain types of signaling. Accordingly, the relative
intensity of oncogenic signaling in cancer cells may represent
compromises between maximal mitogenic stimulation and
avoidance of these antiproliferative defenses. Alternatively,
some cancer cells may adapt to high levels of oncogenic
signaling by disabling their senescence- or apoptosis-inducing
circuitry.
Evading Growth Suppressors
In addition to the hallmark capability of inducing and sustaining
positively acting growth-stimulatory signals, cancer cells must
also circumvent powerful programs that negatively regulate
cell proliferation; many of these programs depend on the actions
of tumor suppressor genes. Dozens of tumor suppressors that
operate in various ways to limit cell growth and proliferation
have been discovered through their characteristic inactivation
in one or another form of animal or human cancer; many of these
genes have been validated as bona fide tumor suppressors
through gain- or loss-of-function experiments in mice. The two
prototypical tumor suppressors encode the RB (retinoblastoma-associated)
and TP53 proteins; they operate as central
control nodes within two key complementary cellular regulatory
circuits that govern the decisions of cells to proliferate or, alternatively,
activate senescence and apoptotic programs.
The RB protein integrates signals from diverse extracellular
and intracellular sources and, in response, decides whether or
not a cell should proceed through its growth-and-division cycle
(Burkhart and Sage, 2008; Deshpande et al., 2005; Sherr and
McCormick, 2002). Cancer cells with defects in RB pathway
function are thus missing the services of a critical gatekeeper
of cell-cycle progression whose absence permits persistent
cell proliferation. Whereas RB transduces growth-inhibitory
signals that originate largely outside of the cell, TP53 receives
inputs from stress and abnormality sensors that function within
the cell’s intracellular operating systems: if the degree of
damage to the genome is excessive, or if the levels of nucleotide
pools, growth-promoting signals, glucose, or oxygenation are
suboptimal, TP53 can call a halt to further cell-cycle progression
until these conditions have been normalized. Alternatively, in the
face of alarm signals indicating overwhelming or irreparable
damage to such cellular subsystems, TP53 can trigger
apoptosis. Notably, the various effects of activated TP53 are
complex and highly context dependent, varying by cell type as
well as by the severity and persistence of conditions of cell stress
and genomic damage.
Although the two canonical suppressors of proliferation—
TP53 and RB—have preeminent importance in regulating cell
proliferation, various lines of evidence indicate that each operates
as part of a larger network that is wired for functional redundancy.
For example, chimeric mice populated throughout their
bodies with individual cells lacking a functional Rb gene are
surprisingly free of proliferative abnormalities, despite the expectation
that loss of RB function would allow continuous firing of the
cell division cycle in these cells and their lineal descendants;
some of the resulting clusters of Rb null cells should, by all rights,
progress to neoplasia. Instead, the Rb null cells in such chimeric
mice have been found to participate in relatively normal tissue
morphogenesis throughout the body; the only neoplasia
observed was in the development of pituitary tumors late in life
(Lipinski and Jacks, 1999). Similarly, TP53 null mice develop normally,
show largely proper cell and tissue homeostasis, and
again develop abnormalities later in life, in the form of leukemias
and sarcomas (Ghebranious and Donehower, 1998). Both examples
must reflect the operations of redundantly acting mechanisms
that serve to constrain inappropriate replication of cells
lacking these key proliferation suppressors.
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Mechanisms of Contact Inhibition and Its Evasion
Four decades of research have demonstrated that the cell-tocell
contacts formed by dense populations of normal cells propagated
in two-dimensional culture operate to suppress further
cell proliferation, yielding confluent cell monolayers. Importantly,
such ‘‘contact inhibition’’ is abolished in various types of cancer
cells in culture, suggesting that contact inhibition is an in vitro
surrogate of a mechanism that operates in vivo to ensure normal
tissue homeostasis, one that is abrogated during the course of
tumorigenesis. Until recently, the mechanistic basis for this
mode of growth control remained obscure. Now, however,
mechanisms of contact inhibition are beginning to emerge.
One mechanism involves the product of the NF2 gene, long
implicated as a tumor suppressor because its loss triggers
a form of human neurofibromatosis. Merlin, the cytoplasmic
NF2 gene product, orchestrates contact inhibition via coupling
cell-surface adhesion molecules (e.g., E-cadherin) to transmembrane
receptor tyrosine kinases (e.g., the EGF receptor). In so
doing, Merlin strengthens the adhesivity of cadherin-mediated
cell-to-cell attachments. Additionally, by sequestering growth
factor receptors, Merlin limits their ability to efficiently emit mitogenic
signals (Curto et al., 2007; Okada et al., 2005).
A second mechanism of contact inhibition involves the LKB1
epithelial polarity protein, which organizes epithelial structure
and helps maintain tissue integrity. LKB1 can, for example,
overrule the mitogenic effects of the powerful Myc oncogene
when the latter is upregulated in organized, quiescent epithelial
structures; in contrast, when LKB1 expression is suppressed,
epithelial integrity is destabilized, and epithelial cells become
susceptible to Myc-induced transformation (Partanen et al.,
2009; Hezel and Bardeesy, 2008). LKB1 has also been identified
as a tumor suppressor gene that is lost in certain human malignancies
(Shaw, 2009), possibly reflecting its normal function as
a suppressor of inappropriate proliferation. It remains to be
seen how frequently these two mechanisms of contact-mediated
growth suppression are compromised in human cancers;
no doubt yet other contact-induced proliferative barriers are
yet to be discovered. Clearly mechanisms like these that enable
cells to construct and maintain architecturally complex tissues
represent important means of suppressing and counterbalancing
inappropriate proliferative signals.
Corruption of the TGF-b Pathway Promotes Malignancy
TGF-b is best known for its antiproliferative effects, and evasion
by cancer cells of these effects is now appreciated to be far more
elaborate than simple shutdown of its signaling circuitry (Ikushima
and Miyazono, 2010; Massague´ , 2008; Bierie and Moses,
2006). In many late-stage tumors, TGF-b signaling is redirected
away from suppressing cell proliferation and is found instead
to activate a cellular program, termed the epithelial-to-mesenchymal
transition (EMT), that confers on cancer cells traits associated
with high-grade malignancy, as discussed in further detail
below.
Resisting Cell Death
The concept that programmed cell death by apoptosis serves as
a natural barrier to cancer development has been established by
compelling functional studies conducted over the last two
decades (Adams and Cory, 2007; Lowe et al., 2004: Evan and
Littlewood, 1998). Elucidation of the signaling circuitry governing
the apoptotic program has revealed how apoptosis is triggered
in response to various physiologic stresses that cancer cells
experience during the course of tumorigenesis or as a result of
anticancer therapy. Notable among the apoptosis-inducing
stresses are signaling imbalances resulting from elevated levels
of oncogene signaling, as mentioned earlier, and DNA damage
associated with hyperproliferation. Yet other research has revealed
how apoptosis is attenuated in those tumors that
succeed in progressing to states of high-grade malignancy and
resistance to therapy (Adams and Cory, 2007; Lowe et al., 2004).
The apoptotic machinery is composed of both upstream regulators
and downstream effector components (Adams and Cory,
2007). The regulators, in turn, are divided into two major circuits,
one receiving and processing extracellular death-inducing
signals (the extrinsic apoptotic program, involving for example
the Fas ligand/Fas receptor), and the other sensing and integrating
a variety of signals of intracellular origin (the intrinsic
program). Each culminates in activation of a normally latent
protease (caspases 8 and 9, respectively), which proceeds to
initiate a cascade of proteolysis involving effector caspases
responsible for the execution phase of apoptosis, in which the
cell is progressively disassembled and then consumed, both
by its neighbors and by professional phagocytic cells. Currently,
the intrinsic apoptotic program is more widely implicated as
a barrier to cancer pathogenesis.
The ‘‘apoptotic trigger’’ that conveys signals between the regulators
and effectors is controlled by counterbalancing pro- and
antiapoptotic members of the Bcl-2 family of regulatory proteins
(Adams and Cory, 2007). The archetype, Bcl-2, along with its
closest relatives (Bcl-xL, Bcl-w, Mcl-1, A1) are inhibitors of
apoptosis,acting inlargepartbybinding toandtherebysuppressing
two proapoptotic triggering proteins (Bax and Bak); the latter
are embedded in the mitochondrial outer membrane. When
relieved of inhibition by their antiapoptotic relatives, Bax and
Bak disrupt the integrity of the outer mitochondrial membrane,
causing the release of proapoptotic signaling proteins, the most
important of which is cytochrome c. The released cytochrome c
activates, in turn, a cascade of caspases that act via their proteolytic
activities to induce the multiple cellular changes associated
with the apoptotic program. Bax and Bak share protein-protein
interaction domains, termed BH3 motifs, with the antiapoptotic
Bcl-2-like proteins that mediate their various physical interactions.
The activities of a subfamily of related proteins, each of
which contains a single such BH3 motif, are coupled to a variety
of sensors of cellular abnormality; these ‘‘BH3-only’’ proteins
act either by interfering with antiapoptotic Bcl-2 proteins or by
directly stimulating the proapoptotic members of this family
(Adams and Cory, 2007; Willis and Adams, 2005).
Although the cellular conditions that trigger apoptosis remain
to be fully enumerated, several abnormality sensors that play
key roles in tumor development have been identified (Adams
and Cory, 2007; Lowe et al., 2004). Most notable is a DNAdamage
sensor that functions via the TP53 tumor suppressor
(Junttila and Evan, 2009); TP53 induces apoptosis by upregulating
expression of the Noxa and Puma BH3-only proteins, doing
so in response to substantial levels of DNA breaks and other
chromosomal abnormalities. Alternatively, insufficient survival
Cell 144, March 4, 2011 ª2011 Elsevier Inc. 649
factor signaling (for instance inadequate levels of interleukin-3 in
lymphocytes or of insulin-like growth factor 1/2 [Igf1/2] in epithelial
cells) can elicit apoptosis through a BH3-only protein called
Bim. Yet another condition leading to cell death involves hyperactive
signaling by certain oncoproteins, such as Myc, which
triggers apoptosis (in part via Bim and other BH3-only proteins)
unless counterbalanced by antiapoptotic factors (Junttila and
Evan, 2009; Lowe et al., 2004).
Tumor cells evolve a variety of strategies to limit or circumvent
apoptosis. Most common is the loss of TP53 tumor suppressor
function, which eliminates this critical damage sensor from the
apoptosis-inducing circuitry. Alternatively, tumors may achieve
similar ends by increasing expression of antiapoptotic regulators
(Bcl-2, Bcl-xL) or of survival signals (Igf1/2), by downregulating
proapoptotic factors (Bax, Bim, Puma), or by short-circuiting
the extrinsic ligand-induced death pathway. The multiplicity of
apoptosis-avoiding mechanisms presumably reflects the diversity
of apoptosis-inducing signals that cancer cell populations
encounter during their evolution to the malignant state.
The structure of the apoptotic machinery and program, and
the strategies used by cancer cells to evade its actions, were
widely appreciated by the beginning of the last decade. The
most notable conceptual advances since then have involved
other forms of cell death that broaden the scope of ‘‘programmed
cell death’’ as a barrier to cancer.
Autophagy Mediates Both Tumor Cell Survival and Death
Autophagy represents an important cell-physiologic response
that, like apoptosis, normally operates at low, basal levels in cells
but can be strongly induced in certain states of cellular stress,
the most obvious of which is nutrient deficiency (Levine and
Kroemer, 2008; Mizushima, 2007). The autophagic program
enables cells to break down cellular organelles, such as ribosomes
and mitochondria, allowing the resulting catabolites to
be recycled and thus used for biosynthesis and energy metabolism.
As part of this program, intracellular vesicles termed autophagosomes
envelope intracellular organelles and then fuse with
lysosomes wherein degradation occurs. In this fashion, lowmolecular-weight
metabolites are generated that support
survival in the stressed, nutrient-limited environments experienced
by many cancer cells.
Like apoptosis, the autophagy machinery has both regulatory
and effector components (Levine and Kroemer, 2008; Mizushima,
2007). Among the latter are proteins that mediate autophagosome
formation and delivery to lysosomes. Of note, recent
research has revealed intersections between the regulatory
circuits governing autophagy, apoptosis, and cellular homeostasis.
For example, the signaling pathway involving the PI3kinase,
AKT, and mTOR kinases, which is stimulated by survival
signals to block apoptosis, similarly inhibits autophagy; when
survival signals are insufficient, the PI3K signaling pathway is
downregulated, with the result that autophagy and/or apoptosis
may be induced (Levine and Kroemer, 2008; Sinha and Levine,
2008; Mathew et al., 2007).
Another interconnection between these two programs resides
in the Beclin-1 protein, which has been shown by genetic studies
to be necessary for induction of autophagy (Levine and Kroemer,
2008; Sinha and Levine, 2008; Mizushima, 2007). Beclin-1 is
a member of the BH3-only subfamily of apoptotic regulatory
proteins, and its BH3 domain allows it to bind the Bcl-2/Bcl-xL
proteins. Stress-sensor-coupled BH3 proteins can displace Beclin-1
from its association with Bcl-2/Bcl-xL, enabling the liberated
Beclin-1 to trigger autophagy, much as they can release
proapoptotic Bax and Bak to trigger apoptosis. Hence, stresstransducing
BH3 proteins (e.g., Bid, Bad, Puma, et al.) can
induce apoptosis and/or autophagy depending on the physiologic
state of the cell.
Mice bearing inactivated alleles of the Beclin-1 gene or of
certain other components of the autophagy machinery exhibit
increased susceptibility to cancer (White and DiPaola, 2009:
Levine and Kroemer, 2008). These results suggest that induction
of autophagy can serve as a barrier to tumorigenesis that may
operate independently of or in concert with apoptosis. Accordingly,
autophagy appears to represent yet another barrier that
needs to be circumvented during tumor development (White
and DiPaola, 2009).
Perhaps paradoxically, nutrient starvation, radiotherapy, and
certain cytotoxic drugs can induce elevated levels of autophagy
that are apparently cytoprotective for cancer cells, impairing
rather than accentuating the killing actions of these stressinducing
situations (White and DiPaola, 2009; Apel et al., 2009;
Amaravadi and Thompson, 2007; Mathew et al., 2007). Moreover,
severely stressed cancer cells have been shown to shrink
via autophagy to a state of reversible dormancy (White and
DiPaola, 2009; Lu et al., 2008). This survival response may
enable the persistence and eventual regrowth of some latestage
tumors following treatment with potent anticancer agents.
Thus, in analogy to TGF-b signaling, which can be tumor suppressing
at early stages of tumorigenesis and tumor promoting
later on, autophagy seems to have conflicting effects on tumor
cells and thus tumor progression (Apel et al., 2009; White and
DiPaola, 2009). An important agenda for future research will
involve clarifying the genetic and cell-physiologic conditions
that dictate when and how autophagy enables cancer cells to
survive or causes them to die.
Necrosis Has Proinflammatory and Tumor-Promoting
Potential
In contrast to apoptosis, in which a dying cell contracts into an
almost-invisible corpse that is soon consumed by neighbors,
necrotic cells become bloated and explode, releasing their
contents into the local tissue microenvironment. Although
necrosis has historically been viewed much like organismic
death, as a form of system-wide exhaustion and breakdown,
the conceptual landscape is changing: cell death by necrosis
is clearly under genetic control in some circumstances, rather
than being a random and undirected process (Galluzzi and
Kroemer, 2008; Zong and Thompson, 2006).
Perhaps more important, necrotic cell death releases proinflammatory
signals into the surrounding tissue microenvironment,
in contrast to apoptosis and autophagy, which do not.
As a consequence, necrotic cells can recruit inflammatory cells
of the immune system (Grivennikov et al., 2010; White et al.,
2010; Galluzzi and Kroemer, 2008), whose dedicated function
is to survey the extent of tissue damage and remove associated
necrotic debris. In the context of neoplasia, however, multiple
lines of evidence indicate that immune inflammatory cells can
be actively tumor promoting, given that such cells are capable
650 Cell 144, March 4, 2011 ª2011 Elsevier Inc.
of fostering angiogenesis, cancer cell proliferation, and invasiveness
(see below). Additionally, necrotic cells can release bioactive
regulatory factors, such as IL-1a, which can directly stimulate
neighboring viable cells to proliferate, with the potential,
once again, to facilitate neoplastic progression (Grivennikov
et al., 2010). Consequently, necrotic cell death, while seemingly
beneficial in counterbalancing cancer-associated hyperproliferation,
may ultimately do more damage than good. Accordingly,
incipient neoplasias and potentially invasive and metastatic
tumors may gain an advantage by tolerating some degree of
necrotic cell death, doing so in order to recruit tumor-promoting
inflammatory cells that bring growth-stimulating factors to the
surviving cells within these growths.
Enabling Replicative Immortality
By 2000, it was widely accepted that cancer cells require unlimited
replicative potential in order to generate macroscopic
tumors. This capability stands in marked contrast to the behavior
of the cells in most normal cell lineages in the body, which are
able to pass through only a limited number of successive cell
growth-and-division cycles. This limitation has been associated
with two distinct barriers to proliferation: senescence, a typically
irreversible entrance into a nonproliferative but viable state, and
crisis, which involves cell death. Accordingly, when cells are
propagated in culture, repeated cycles of cell division lead first
to induction of senescence and then, for those cells that succeed
in circumventing this barrier, to a crisis phase, in which the great
majority of cells in the population die. On rare occasion, cells
emerge from a population in crisis and exhibit unlimited replicative
potential. This transition has been termed immortalization,
a trait that most established cell lines possess by virtue of their
ability to proliferate in culture without evidence of either senescence
or crisis.
Multiple lines of evidence indicate that telomeres protecting
the ends of chromosomes are centrally involved in the capability
for unlimited proliferation (Blasco, 2005; Shay and Wright, 2000).
The telomeres, composed of multiple tandem hexanucleotide
repeats, shorten progressively in nonimmortalized cells propagated
in culture, eventually losing the ability to protect the
ends of chromosomal DNAs from end-to-end fusions; such
fusions generate unstable dicentric chromosomes whose resolution
results in a scrambling of karyotype that threatens cell
viability. Accordingly, the length of telomeric DNA in a cell
dictates how many successive cell generations its progeny can
pass through before telomeres are largely eroded and have
consequently lost their protective functions, triggering entrance
into crisis.
Telomerase, the specialized DNA polymerase that adds telomere
repeat segments to the ends of telomeric DNA, is almost
absent in nonimmortalized cells but expressed at functionally
significant levels in the vast majority (90%) of spontaneously
immortalized cells, including human cancer cells. By extending
telomeric DNA, telomerase is able to counter the progressive
telomere erosion that would otherwise occur in its absence.
The presence of telomerase activity, either in spontaneously
immortalized cells or in the context of cells engineered to
express the enzyme, is correlated with a resistance to induction
of both senescence and crisis/apoptosis; conversely, suppression
of telomerase activity leads to telomere shortening and to
activation of one or the other of these proliferative barriers.
The two barriers to proliferation—senescence and crisis/
apoptosis—have been rationalized as crucial anticancer
defenses that are hard-wired into our cells, being deployed to
impede the outgrowth of clones of preneoplastic and frankly
neoplastic cells. According to this thinking, most incipient
neoplasias exhaust their endowment of replicative doublings
and are stopped in their tracks by one or the other of these
barriers. The eventual immortalization of rare variant cells that
proceed to form tumors has been attributed to their ability to
maintain telomeric DNA at lengths sufficient to avoid triggering
senescence or apoptosis, achieved most commonly by upregulating
expression of telomerase or, less frequently, via an
alternative recombination-based telomere maintenance mechanism.
Hence, telomere shortening has come to be viewed as
a clocking device that determines the limited replicative potential
of normal cells and thus one that must be overcome by
cancer cells.
Reassessing Replicative Senescence
Whereas telomere maintenance has been increasingly substantiated
as a condition critical to the neoplastic state, the concept
of replication-induced senescence as a general barrier requires
refinement and reformulation. (Differences in telomere structure
and function in mouse versus human cells have also complicated
investigation of the roles of telomeres and telomerase in replicative
senescence.) Recent experiments have revealed that the
induction of senescence in certain cultured cells can be delayed
and possibly eliminated by the use of improved cell culture
conditions, suggesting that recently explanted primary cells
may be able to proliferate unimpeded in culture up the point of
crisis and the associated induction of apoptosis triggered by critically
shortened telomeres (Ince et al., 2007; Passos et al., 2007;
Zhang et al., 2004; Sherr and DePinho, 2000). In contrast, experiments
in mice engineered to lack telomerase indicate that the
consequently shortened telomeres can shunt premalignant cells
into a senescent state that contributes (along with apoptosis) to
attenuated tumorigenesis in mice genetically destined to
develop particular forms of cancer (Artandi and DePinho,
2010). Such telomerase null mice with highly eroded telomeres
exhibit multiorgan dysfunction and abnormalities that include
evidence for both senescence and apoptosis, perhaps analogous
to the senescence and apoptosis observed in cell culture
(Artandi and DePinho, 2010; Feldser and Greider, 2007).
Of note, and as discussed earlier, a morphologically similar
form of cell senescence induced by excessive or unbalanced
oncogene signaling is now well documented as a protective
mechanism against neoplasia; the possible interconnections of
this form of senescence with telomerase and telomeres remain
to be ascertained. Thus, cell senescence is emerging conceptually
as a protective barrier to neoplastic expansion that can be
triggered by various proliferation-associated abnormalities,
including high levels of oncogenic signaling and, apparently,
subcritical shortening of telomeres.
Delayed Activation of Telomerase May Both Limit
and Foster Neoplastic Progression
There is now evidence that clones of incipient cancer cells often
experience telomere loss-induced crisis relatively early during
Cell 144, March 4, 2011 ª2011 Elsevier Inc. 651
the course of multistep tumor progression due to their inability to
express significant levels of telomerase. Thus, extensively
eroded telomeres have been documented in premalignant
growths through the use of fluorescence in situ hybridization
(FISH), which has also revealed the end-to-end chromosomal
fusions that signal telomere failure and crisis (Kawai et al.,
2007; Hansel et al., 2006). These results also suggest that such
cells have passed through a substantial number of successive
telomere-shortening cell divisions during their evolution from
fully normal cells-of-origin. Accordingly, the development of
some human neoplasias may be aborted by telomere-induced
crisis long before they succeed in becoming macroscopic,
frankly neoplastic growths.
In contrast, the absence of TP53-mediated surveillance of
genomic integrity may permit other incipient neoplasias to
survive initial telomere erosion and attendant chromosomal
breakage-fusion-bridge (BFB) cycles. The genomic alterations
resulting from these BFB cycles, including deletions and amplifications
of chromosomal segments, evidently serve to increase
the mutability of the genome, thereby accelerating the acquisition
of mutant oncogenes and tumor suppressor genes. The realization
that impaired telomere function can actually foster tumor
progression has come from the study of mutant mice that lack
both p53 and telomerase function (Artandi and DePinho, 2010,
2000). The proposition that these two defects can cooperatively
enhance human tumorigenesis has not yet been directly docu-
mented.
Circumstantial support for the importance of transient telomere
deficiency in facilitating malignant progression has come,
in addition, from comparative analyses of premalignant and
malignant lesions in the human breast (Raynaud et al., 2010;
Chin et al., 2004). The premalignant lesions did not express
significant levels of telomerase and were marked by telomere
shortening and nonclonal chromosomal aberrations. In contrast,
overt carcinomas exhibited telomerase expression concordantly
with the reconstruction of longer telomeres and the fixation (via
clonal outgrowth) of the aberrant karyotypes that would seem
to have been acquired after telomere failure but before the acquisition
of telomerase activity. When portrayed in this way, the
delayed acquisition of telomerase function serves to generate
tumor-promoting mutations, whereas its subsequent activation
stabilizes the mutant genome and confers the unlimited replicative
capacity that cancer cells require in order to generate clinically
apparent tumors.
New Functions of Telomerase
Telomerase was discovered because of its ability to elongate
and maintain telomeric DNA, and almost all telomerase research
has been posited on the notion that its functions are confined to
this crucial function. However, in recent years it has become
apparent that telomerase exerts functions that are relevant to
cell proliferation but unrelated to telomere maintenance. The
noncanonical roles of telomerase, and in particular its protein
subunit TERT, have been revealed by functional studies in
mice and cultured cells; in some cases novel functions have
been demonstrated in conditions where the telomerase enzymatic
activity has been eliminated (Cong and Shay, 2008).
Among the growing list of telomere-independent functions of
TERT/telomerase is the ability of TERT to amplify signaling by
the Wnt pathway, by serving as a cofactor of the b-catenin/LEF
transcription factor complex (Park et al., 2009). Other ascribed
telomere-independent effects include demonstrable enhancement
of cell proliferation and/or resistance to apoptosis (Kang
et al., 2004), involvement in DNA-damage repair (Masutomi
et al., 2005), and RNA-dependent RNA polymerase function
(Maida et al., 2009). Consistent with these broader roles, TERT
can be found associated with chromatin at multiple sites along
the chromosomes, not just at the telomeres (Park et al., 2009;
Masutomi et al., 2005). Hence, telomere maintenance is proving
to be the most prominent of a diverse series of functions to which
TERT contributes. The contributions of these additional functions
of telomerase to tumorigenesis remain to be fully eluci-
dated.
Inducing Angiogenesis
Like normal tissues, tumors require sustenance in the form of
nutrients and oxygen as well as an ability to evacuate metabolic
wastes and carbon dioxide. The tumor-associated neovasculature,
generated by the process of angiogenesis, addresses these
needs. During embryogenesis, the development of the vasculature
involves the birth of new endothelial cells and their assembly
into tubes (vasculogenesis) in addition to the sprouting (angiogenesis)
of new vessels from existing ones. Following this
morphogenesis, the normal vasculature becomes largely quiescent.
In the adult, as part of physiologic processes such as
wound healing and female reproductive cycling, angiogenesis
is turned on, but only transiently. In contrast, during tumor
progression, an ‘‘angiogenic switch’’ is almost always activated
and remains on, causing normally quiescent vasculature to
continually sprout new vessels that help sustain expanding
neoplastic growths (Hanahan and Folkman, 1996).
A compelling body of evidence indicates that the angiogenic
switch is governed by countervailing factors that either induce
or oppose angiogenesis (Baeriswyl and Christofori, 2009; Bergers
and Benjamin, 2003). Some of these angiogenic regulators
are signaling proteins that bind to stimulatory or inhibitory cellsurface
receptors displayed by vascular endothelial cells. The
well-known prototypes of angiogenesis inducers and inhibitors
are vascular endothelial growth factor-A (VEGF-A) and thrombospondin-1
(TSP-1), respectively.
The VEGF-A gene encodes ligands that are involved in orchestrating
new blood vessel growth during embryonic and postnatal
development, and then in homeostatic survival of endothelial
cells, as well as in physiological and pathological situations in
the adult. VEGF signaling via three receptor tyrosine kinases
(VEGFR-1–3) is regulated at multiple levels, reflecting this
complexity of purpose. Thus, VEGF gene expression can by
upregulated both by hypoxia and by oncogene signaling (Ferrara,
2009; Mac Gabhann and Popel, 2008; Carmeliet, 2005).
Additionally, VEGF ligands can be sequestered in the extracellular
matrix in latent forms that are subject to release and activation
by extracellular matrix-degrading proteases (e.g., MMP-9;
Kessenbrock et al., 2010). In addition, other proangiogenic
signals, such as members of the fibroblast growth factor (FGF)
family, have been implicated in sustaining tumor angiogenesis
when their expression is chronically upregulated (Baeriswyl
and Christofori, 2009). TSP-1, a key counterbalance in the
652 Cell 144, March 4, 2011 ª2011 Elsevier Inc.
angiogenic switch, also binds transmembrane receptors displayed
by endothelial cells and thereby evokes suppressive
signals that can counteract proangiogenic stimuli (Kazerounian
et al., 2008).
The blood vessels produced within tumors by chronically activated
angiogenesis and an unbalanced mix of proangiogenic
signals are typically aberrant: tumor neovasculature is marked
by precocious capillary sprouting, convoluted and excessive
vessel branching, distorted and enlarged vessels, erratic blood
flow, microhemorrhaging, leakiness, and abnormal levels of
endothelial cell proliferation and apoptosis (Nagy et al., 2010;
Baluk et al., 2005).
Angiogenesis is induced surprisingly early during the multistage
development of invasive cancers both in animal models
and in humans. Histological analyses of premalignant, noninvasive
lesions, including dysplasias and in situ carcinomas arising
in a variety of organs, have revealed the early tripping of the
angiogenic switch (Raica et al., 2009; Hanahan and Folkman,
1996). Historically, angiogenesis was envisioned to be important
only when rapidly growing macroscopic tumors had formed, but
more recent data indicate that angiogenesis also contributes to
the microscopic premalignant phase of neoplastic progression,
further cementing its status as an integral hallmark of cancer.
The past decade has witnessed an astonishing outpouring of
research on angiogenesis. Amid this wealth of new knowledge,
we highlight several advances of particular relevance to tumor
physiology.
Gradations of the Angiogenic Switch
Once angiogenesis has been activated, tumors exhibit diverse
patterns of neovascularization. Some tumors, including such
highly aggressive types as pancreatic ductal adenocarcinomas,
are hypovascularized and replete with stromal ‘‘deserts’’ that are
largely avascular and indeed may even be actively antiangiogenic
(Olive et al., 2009). Many other tumors, including human
renal and pancreatic neuroendocrine carcinomas, are highly
angiogenic and consequently densely vascularized (Zee et al.,
2010; Turner et al., 2003).
Collectively, such observations suggest an initial tripping of
the angiogenic switch during tumor development that is followed
by a variable intensity of ongoing neovascularization, the latter
being controlled by a complex biological rheostat that involves
both the cancer cells and the associated stromal microenvironment
(Baeriswyl and Christofori, 2009; Bergers and Benjamin,
2003). Of note, the switching mechanism can vary in its form,
even though the net result is a common inductive signal (e.g.,
VEGF). In some tumors, dominant oncogenes operating within
tumor cells, such as Ras and Myc, can upregulate expression
of angiogenic factors, whereas in others, such inductive signals
are produced indirectly by immune inflammatory cells, as discussed
below. The direct induction of angiogenesis by oncogenes
that also drive proliferative signaling illustrates the important
principle that distinct hallmark capabilities can be
coregulated by the same transforming agents.
Endogenous Angiogenesis Inhibitors Present Natural
Barriers to Tumor Angiogenesis
Research in the 1990s revealed that TSP-1 as well as fragments
of plasmin (angiostatin) and type 18 collagen (endostatin) can
act as endogenous inhibitors of angiogenesis (Ribatti, 2009;
Kazerounian, et al., 2008; Folkman, 2006, 2002; Nyberg et al.,
2005). The last decade has seen reports of another dozen
such agents (Ribatti, 2009; Folkman, 2006; Nyberg et al.,
2005). Most are proteins, and many are derived by proteolytic
cleavage of structural proteins that are not themselves angiogenic
regulators. A number of these endogenous inhibitors of
angiogenesis can be detected in the circulation of normal
mice and humans. The genes encoding several endogenous
angiogenesis inhibitors have been deleted from the mouse
germline without untoward physiological effects; the growth of
autochthonous and implanted tumors, however, is enhanced
as a consequence (Ribatti, 2009; Nyberg et al., 2005). By
contrast, if the circulating levels of an endogenous inhibitor
are genetically increased (e.g., via overexpression in transgenic
mice or in xenotransplanted tumors), tumor growth is impaired
(Ribatti, 2009; Nyberg et al., 2005); interestingly, wound healing
and fat deposition are impaired or accelerated by elevated or
ablated expression of such genes (Cao, 2010; Seppinen et al.,
2008). The data suggest that such endogenous angiogenesis
inhibitors serve under normal circumstances as physiologic
regulators that modulate transitory angiogenesis during tissue
remodeling and wound healing; they may also act as intrinsic
barriers to induction and/or persistence of angiogenesis by
incipient neoplasias.
Pericytes Are Important Components
of the Tumor Neovasculature
Pericytes have long been known as supporting cells that are
closely apposed to the outer surfaces of the endothelial tubes
in normal tissue vasculature, where they provide important
mechanical and physiologic support to the endothelial cells.
Tumor-associated vasculature, in contrast, was portrayed as
lacking appreciable coverage by these auxiliary cells. However,
careful microscopic studies conducted in recent years have revealed
that pericytes are associated, albeit loosely, with the neovasculature
of most if not all tumors (Raza et al., 2010; Bergers
and Song, 2005). More importantly, mechanistic studies discussed
below have revealed that pericyte coverage is important
for the maintenance of a functional tumor neovasculature.
A Variety of Bone Marrow-Derived Cells Contribute
to Tumor Angiogenesis
It is now clear that a repertoire of cell types originating in the bone
marrow play crucial roles in pathological angiogenesis (Qian and
Pollard, 2010; Zumsteg and Christofori, 2009; Murdoch et al.,
2008; De Palma et al., 2007). These include cells of the innate
immune system—notably macrophages, neutrophils, mast cells,
and myeloid progenitors—that infiltrate premalignant lesions
and progressed tumors and assemble at the margins of such
lesions; the peri-tumoral inflammatory cells help to trip the angiogenic
switch in previously quiescent tissue and to sustain
ongoing angiogenesis associated with tumor growth, in addition
to facilitating local invasion, as noted below. In addition, they can
help protect the vasculature from the effects of drugs targeting
endothelial cell signaling (Ferrara, 2010). Additionally, several
types of bone marrow-derived ‘‘vascular progenitor cells’’ have
been observed in certain cases to have migrated into neoplastic
lesions and become intercalated into the neovasculature as pericytes
or endothelial cells (Patenaude et al., 2010; Kovacic and
Boehm, 2009; Lamagna and Bergers, 2006).
Cell 144, March 4, 2011 ª2011 Elsevier Inc. 653
Activating Invasion and Metastasis
In 2000, the mechanisms underlying invasion and metastasis
were largely an enigma. It was clear that as carcinomas arising
from epithelial tissues progressed to higher pathological grades
of malignancy, reflected in local invasion and distant metastasis,
the associated cancer cells typically developed alterations in
their shape as well as in their attachment to other cells and to
the extracellular matrix (ECM). The best characterized alteration
involved the loss by carcinoma cells of E-cadherin, a key cell-tocell
adhesion molecule. By forming adherens junctions with
adjacent epithelial cells, E-cadherin helps to assemble epithelial
cell sheets and maintain the quiescence of the cells within these
sheets. Increased expression of E-cadherin was well established
as an antagonist of invasion and metastasis, whereas reduction
of its expression was known to potentiate these phenotypes. The
frequently observed downregulation and occasional mutational
inactivation of E-cadherin in human carcinomas provided strong
support for its role as a key suppressor of this hallmark capability
(Berx and van Roy, 2009; Cavallaro and Christofori, 2004).
Additionally, expression of genes encoding other cell-to-cell
and cell-to-ECM adhesion molecules is demonstrably altered
in some highly aggressive carcinomas, with those favoring cytostasis
typically being downregulated. Conversely, adhesion
molecules normally associated with the cell migrations that
occur during embryogenesis and inflammation are often upregulated.
For example, N-cadherin, which is normally expressed in
migrating neurons and mesenchymal cells during organogenesis,
is upregulated in many invasive carcinoma cells. Beyond
the gain and loss of such cell-cell/matrix attachment proteins,
the master regulators of invasion and metastasis were largely
unknown or, when suspected, lacking in functional validation
(Cavallaro and Christofori, 2004).
The multistep process of invasion and metastasis has been
schematized as a sequence of discrete steps, often termed the
invasion-metastasis cascade (Talmadge and Fidler, 2010; Fidler,
2003). This depiction envisions a succession of cell-biologic
changes, beginning with local invasion, then intravasation by
cancer cells into nearby blood and lymphatic vessels, transit of
cancer cells through the lymphatic and hematogenous systems,
followed by escape of cancer cells from the lumina of such
vessels into the parenchyma of distant tissues (extravasation),
the formation of small nodules of cancer cells (micrometastases),
and finally the growth of micrometastatic lesions into
macroscopic tumors, this last step being termed ‘‘colonization.’’
Research into the capability for invasion and metastasis has
accelerated dramatically over the past decade as powerful
new research tools and refined experimental models have
become available, and as critical regulatory genes were identified.
While still an emerging field replete with major unanswered
questions, significant progress has been made in delineating
important features of this complex hallmark capability. An admittedly
incomplete representation of these advances is highlighted
below.
The EMT Program Broadly Regulates Invasion
and Metastasis
A developmental regulatory program, referred to as the ‘‘epithelial-mesenchymal
transition’’ (EMT), has become prominently
implicated as a means by which transformed epithelial cells
can acquire the abilities to invade, to resist apoptosis, and to
disseminate (Klymkowsky and Savagner, 2009; Polyak and
Weinberg, 2009; Thiery et al., 2009; Yilmaz and Christofori,
2009; Barrallo-Gimeno and Nieto, 2005). By co-opting a process
involved in various steps of embryonic morphogenesis and
wound healing, carcinoma cells can concomitantly acquire
multiple attributes that enable invasion and metastasis. This
multifaceted EMT program can be activated transiently or stably,
and to differing degrees, by carcinoma cells during the course of
invasion and metastasis.
A set of pleiotropically acting transcriptional factors, including
Snail, Slug, Twist, and Zeb1/2, orchestrate the EMT and related
migratory processes during embryogenesis; most were initially
identified by developmental genetics. These transcriptional
regulators are expressed in various combinations in a number
of malignant tumor types and have been shown in experimental
models of carcinoma formation to be causally important for
programming invasion; some have been found to elicit metastasis
when ectopically overexpressed (Micalizzi et al., 2010;
Taube et al., 2010; Schmalhofer et al., 2009; Yang and Weinberg,
2008). Included among the cell-biological traits evoked by such
transcription factors are loss of adherens junctions and associated
conversion from a polygonal/epithelial to a spindly/fibroblastic
morphology, expression of matrix-degrading enzymes,
increased motility, and heightened resistance to apoptosis—all
traits implicated in the processes of invasion and metastasis.
Several of these transcription factors can directly repress E-cadherin
gene expression, thereby depriving neoplastic epithelial
cells of this key suppressor of motility and invasiveness (Peinado
et al., 2004).
The available evidence suggests that these transcription
factors regulate one another as well as overlapping sets of target
genes. No rules have yet been established to describe their interactions
and the conditions that govern their expression.
Evidence from developmental genetics indicates that contextual
signals received from neighboring cells in the embryo are
involved in triggering expression of these transcription factors
in those cells destined to pass through an EMT (Micalizzi et al.,
2010); in an analogous fashion, increasing evidence suggests
that heterotypic interactions of cancer cells with adjacent
tumor-associated stromal cells can induce expression of the
malignant cell phenotypes that are known to be choreographed
by one or more of these transcriptional regulators (Karnoub and
Weinberg, 2006–2007; Brabletz et al., 2001). Moreover, cancer
cells at the invasive margins of certain carcinomas can be
seen to have undergone an EMT, suggesting that these cancer
cells are subject to microenvironmental stimuli distinct from
those received by cancer cells located in the cores of these
lesions (Hlubek et al., 2007).
Although the evidence is still incomplete, it would appear that
EMT-inducing transcription factors are able to orchestrate most
steps of the invasion-metastasis cascade save the final step of
colonization. We still know rather little about the various manifestations
and temporal stability of the mesenchymal state
produced by an EMT. Although expression of EMT-inducing
transcription factors has been observed in certain nonepithelial
tumor types, such as sarcomas and neuroectodermal tumors,
their roles in programming malignant traits in these tumors are
654 Cell 144, March 4, 2011 ª2011 Elsevier Inc.
presently poorly documented. Additionally, it remains to be
determined whether invasive carcinoma cells necessarily
acquire their capability through activation of parts of the EMT
program, or whether alternative regulatory programs can also
enable this capability.
Heterotypic Contributions of Stromal Cells to Invasion
and Metastasis
It is increasingly apparent that crosstalk between cancer cells
and cells of the neoplastic stroma is involved in the acquired
capability for invasive growth and metastasis (Egeblad et al.,
2010; Qian and Pollard, 2010; Joyce and Pollard, 2009; Kalluri
and Zeisberg, 2006). Such signaling may impinge on carcinoma
cells and act to alter their hallmark capabilities as suggested
above. For example, mesenchymal stem cells (MSCs) present
in the tumor stroma have been found to secrete CCL5/RANTES
in response to signals released by cancer cells; CCL5 then acts
reciprocally on the cancer cells to stimulate invasive behavior
(Karnoub et al., 2007).
Macrophages at the tumor periphery can foster local invasion
by supplying matrix-degrading enzymes such as metalloproteinases
and cysteine cathepsin proteases (Kessenbrock et al.,
2010; Joyce and Pollard, 2009; Palermo and Joyce, 2008; Mohamed
and Sloane, 2006); in one model system, the invasionpromoting
macrophages are activated by IL-4 produced by the
cancer cells (Gocheva et al., 2010). And in an experimental
model of metastatic breast cancer, tumor-associated macrophages
(TAMs) supply epidermal growth factor (EGF) to breast
cancer cells, while the cancer cells reciprocally stimulate the
macrophages with CSF-1; their concerted interactions facilitate
intravasation into the circulatory system and metastatic dissemination
of the cancer cells (Qian and Pollard, 2010; Wyckoff et al.,
2007).
Observations like these indicate that the phenotypes of highgrade
malignancy do not arise in a strictly cell-autonomous
manner, and that their manifestation cannot be understood
solely through analyses of tumor cell genomes. One important
implication, still untested, is that the ability to negotiate most of
the steps of the invasion-metastasis cascade may be acquired
in certain tumors without the requirement that the associated
cancer cells undergo additional mutations beyond those that
were needed for primary tumor formation.
Plasticity in the Invasive Growth Program
The role of contextual signals in inducing an invasive growth
capability (often via an EMT) implies the possibility of reversibility,
in that cancer cells that have disseminated from a primary
tumor to a more distant tissue site may no longer benefit from the
activated stroma and invasion/EMT-inducing signals that they
experienced while residing in the primary tumor; in the absence
of ongoing exposure to these signals, carcinoma cells may revert
in their new homes to a noninvasive state. Thus, carcinoma cells
that have undergone an EMT during initial invasion and metastatic
dissemination may pass through the reverse process,
termed the mesenchymal-epithelial transition (MET). This plasticity
may result in the formation of new tumor colonies of carcinoma
cells exhibiting a histopathology similar to those of carcinoma
cells in the primary tumor that never underwent an EMT
(Hugo et al., 2007). Moreover, the notion that cancer cells
routinely pass through a complete EMT program is likely to be
simplistic; instead, in many cases, cancer cells may enter into
an EMT program only partially, thereby acquiring new mesenchymal
traits while continuing to express residual epithelial traits.
Distinct Forms of Invasion May Underlie Different
Cancer Types
The EMT program regulates a particular type of invasiveness
that has been termed ‘‘mesenchymal.’’ In addition, two other
distinct modes of invasion have been identified and implicated
in cancer cell invasion (Friedl and Wolf, 2008, 2010). ‘‘Collective
invasion’’ involves nodules of cancer cells advancing en masse
into adjacent tissues and is characteristic of, for example,
squamous cell carcinomas; interestingly, such cancers are
rarely metastatic, suggesting that this form of invasion lacks
certain functional attributes that facilitate metastasis. Less clear
is the prevalence of an ‘‘amoeboid’’ form of invasion (Madsen
and Sahai, 2010; Sabeh et al., 2009), in which individual cancer
cells show morphological plasticity, enabling them to slither
through existing interstices in the extracellular matrix rather
than clearing a path for themselves, as occurs in both the mesenchymal
and collective forms of invasion. It is presently unresolved
whether cancer cells participating in the collective and
amoeboid forms of invasion employ components of the EMT
program, or whether entirely different cell-biological programs
are responsible for choreographing these alternative invasion
programs.
Another emerging concept, noted above, involves the facilitation
of cancer cell invasion by inflammatory cells that assemble
at the boundaries of tumors, producing the extracellular
matrix-degrading enzymes and other factors that enable invasive
growth (Kessenbrock et al., 2010; Qian and Pollard, 2010;
Joyce and Pollard, 2009); these functions may obviate the
need of cancer cells to produce these proteins through activation
of EMT programs. Thus, cancer cells may secrete the
chemoattractants that recruit the proinvasive inflammatory cells
rather than producing the matrix-degrading enzymes them-
selves.
The Daunting Complexity of Metastatic Colonization
Metastasis can be broken down into two major phases: the
physical dissemination of cancer cells from the primary tumor
to distant tissues, and the adaptation of these cells to foreign
tissue microenvironments that results in successful colonization,
i.e., the growth of micrometastases into macroscopic tumors.
The multiple steps of dissemination would seem to be in the
purview of the EMT and similarly acting migratory programs.
Colonization, however, is not strictly coupled with physical
dissemination, as evidenced by the presence in many patients
of myriad micrometastases that have successfully disseminated
but never progress to macroscopic metastatic tumors (Talmadge
and Fidler, 2010; McGowan et al., 2009; Aguirre-Ghiso,
2007; Townson and Chambers, 2006; Fidler, 2003).
In some types of cancer, the primary tumor may release
systemic suppressor factors that render such micrometastases
dormant, as revealed clinically by explosive metastatic growth
soon after resection of the primary growth (Demicheli et al.,
2008; Folkman, 2002). In others, however, such as breast cancer
and melanoma, macroscopic metastases may erupt decades
after a primary tumor has been surgically removed or pharmacologically
destroyed; these metastatic tumor growths evidently
Cell 144, March 4, 2011 ª2011 Elsevier Inc. 655
reflectdormantmicrometastasesthathavesolved,aftermuchtrial
and error, the complex problem of tissue colonization (Barkan,
et al., 2010; Aguirre-Ghiso, 2007; Townson and Chambers, 2006).
One can infer from such natural histories that micrometastases
may lack other hallmark capabilities necessary for vigorous
growth, such as the ability to activate angiogenesis; indeed the
inability of certain experimentally generated dormant micrometastases
to form macroscopic tumors has been ascribed to their
failure to activate tumor angiogenesis (Naumov et al., 2008;
Aguirre-Ghiso, 2007). Additionally, recent experiments have
shown that nutrient starvation can induce intense autophagy
that causes cancer cells to shrink and adopt a state of reversible
dormancy; such cells may exit this state and resume active
growth and proliferation when changes in tissue microenvironment,
such as access to more nutrients, permit (Kenific et al.,
2010; Lu et al., 2008). Other mechanisms of micrometastatic
dormancy may involve anti-growth signals embedded in normal
tissue extracellular matrix (Barkan et al., 2010) and tumor-suppressing
actions of the immune system (Teng et al., 2008;
Aguirre-Ghiso, 2007).
Most disseminated cancer cells are likely to be poorly adapted,
at least initially, to the microenvironment of the tissue in
which they have landed. Accordingly, each type of disseminated
cancer cell may need to develop its own set of ad hoc solutions
to the problem of thriving in the microenvironment of one or
another foreign tissue (Gupta et al., 2005). These adaptations
might require hundreds of distinct colonization programs, each
dictated by the type of disseminating cancer cell and the nature
of the tissue microenvironment in which colonization is
proceeding. As further discussed below, however, certain tissue
microenviroments may be preordained to be intrinsically hospitable
to disseminated cancer cells (Peinado et al., 2011;
Talmadge and Fidler, 2010).
Metastatic dissemination has long been depicted as the last
step in multistep primary tumor progression, and indeed for
many tumors that is likely the case, as illustrated by recent
genome sequencing studies that present genetic evidence for
clonal evolution of pancreatic ductal adenocarcinoma to metastasis
(Campbell et al., 2010; Luebeck, 2010; Yachida et al.,
2010). On the other hand, evidence has recently emerged
indicating that cells can disseminate remarkably early,
dispersing from ostensibly noninvasive premalignant lesions in
both mice and humans (Coghlin and Murray, 2010; Klein,
2009). Additionally, micrometastases can be spawned from
primary tumors that are not obviously invasive but possess
a neovasculature lacking in lumenal integrity (Gerhardt and
Semb, 2008). Although cancer cells can clearly disseminate
from such pre-neoplastic lesions and seed the bone marrow
and other tissues, their capability to colonize these sites and
develop into pathologically significant macrometastases
remains unproven. At present, we view this early metastatic
dissemination as a demonstrable phenomenon in mice and humans
whose clinical significance is yet to be established.
Beyond the timing of their dissemination, it also remains
unclear when and where cancer cells develop the ability to colonize
foreign tissues as macroscopic tumors. This capability may
arise during primary tumor formation as a result of a tumor’s
particular developmental path prior to any dissemination, such
that primary tumor cells entering the circulation are fortuitously
endowed with the ability to colonize certain distant tissue sites
(Talmadge and Fidler, 2010). Alternatively, the ability to colonize
specific tissues may only develop in response to the selective
pressure on already disseminated cancer cells to adapt to
growth in foreign tissue microenvironments.
Having developed such tissue-specific colonizing ability, the
cells in metastatic colonies may proceed to disseminate further,
not only to new sites in the body but also back to the primary
tumors in which their ancestors arose. Accordingly, tissuespecific
colonization programs that are evident among cells
within a primary tumor may originate not from classical tumor
progression occurring within the primary lesion but instead
from emigrants that have returned home (Kim et al., 2009).
Such reseeding is consistent with the aforementioned studies
of human pancreatic cancer metastasis (Campbell et al.,
2010; Luebeck, 2010; Yachida et al., 2010). Stated differently,
the phenotypes and underlying gene expression programs of
the populations of cancer cells (and of the cancer stem cells
discussed below) within primary tumors may be significantly
modified by reverse migration of their distant metastatic
progeny.
Implicit in this self-seeding process is another notion: the
supportive stroma that arises in a primary tumor and contributes
to its acquisition of malignant traits may intrinsically provide
a hospitable site for reseeding and colonization by circulating
cancer cells emanating from metastatic lesions.
Clarifying the regulatory programs that enable metastatic
colonization represents an important agenda for future research.
Substantial progress is being made, for example, in defining sets
of genes (‘‘metastatic signatures’’) that correlate with and appear
to facilitate the establishment of macroscopic metastases in
specific tissues (Coghlin and Murray, 2010; Bos et al., 2009;
Olson et al., 2009; Nguyen et al., 2009; Gupta et al., 2005). The
challenge is considerable, given the apparent multitude of
distinct colonization programs cited above. Moreover, colonization
is unlikely to depend exclusively on cell-autonomous
processes. Instead, it almost certainly requires the establishment
of a permissive tumor microenvironment composed of
critical stromal support cells. For these reasons, the process
of colonization is likely to encompass a large number of cellbiological
programs that are, in aggregate, considerably more
complex and diverse than the preceding steps of metastatic
dissemination.
Programming of Hallmark Capabilities
by Intracellular Circuitry
In 2000, we presented a metaphor, in which the numerous
signaling molecules affecting cancer cells operate as nodes
and branches of elaborate integrated circuits that are reprogrammed
derivatives of the circuits operating in normal cells.
The ensuing decade has both solidified the original depiction
of these circuits and expanded the catalog of signals and the
interconnections of their signaling pathways. It is difficult if not
impossible to graphically portray this circuit comprehensively
and coherently, as was already the case in 2000.
We now suggest a portrayal of this circuitry that is aligned with
individual hallmarks of cancer. Thus, the intracellular integrated
656 Cell 144, March 4, 2011 ª2011 Elsevier Inc.
circuit can be segmented into distinct subcircuits, each of which
is specialized to support a discrete cell-biological property in
normal cells and is reprogrammed in order to implement
a hallmark capability in cancer cells (Figure 2). Only a subset of
hallmark capabilities are addressed in this figure, either because
their underlying control circuits remain poorly understood or
because they overlap extensively with those portrayed here.
An additional dimension of complexity involves considerable
interconnections and thus crosstalk between the individual subcircuits.
For example, certain oncogenic events can affect
multiple capabilities, as illustrated by the diverse effects that
prominent oncogenes, such as mutant RAS and upregulated
MYC, have on multiple hallmark capabilities (e.g., proliferative
signaling, energy metabolism, angiogenesis, invasion, and
survival). We anticipate that future renditions of this integrated
circuit will encompass subcircuits and associated hallmark
capabilities that are still not addressed here.
ENABLING CHARACTERISTICS AND EMERGING
HALLMARKS
We have defined the hallmarks of cancer as acquired functional
capabilities that allow cancer cells to survive, proliferate, and
disseminate; these functions are acquired in different tumor
types via distinct mechanisms and at various times during the
course of multistep tumorigenesis. Their acquisition is made
possible by two enabling characteristics. Most prominent is the
development of genomic instability in cancer cells, which
generates random mutations including chromosomal rearrangements;
among these are the rare genetic changes that can
orchestrate hallmark capabilities. A second enabling characteristic
involves the inflammatory state of premalignant and frankly
malignant lesions that is driven by cells of the immune system,
some of which serve to promote tumor progression through
various means.
Figure 2. Intracellular Signaling Networks Regulate the Operations of the Cancer Cell
An elaborate integrated circuit operates within normal cells and is reprogrammed to regulate hallmark capabilities within cancer cells. Separate subcircuits,
depicted here in differently colored fields, are specialized to orchestrate the various capabilities. At one level, this depiction is simplistic, as there is considerable
crosstalk between such subcircuits. In addition, because each cancer cell is exposed to a complex mixture of signals from its microenvironment, each of these
subcircuits is connected with signals originating from other cells in the tumor microenvironment, as outlined in Figure 5.
Cell 144, March 4, 2011 ª2011 Elsevier Inc. 657
Yet other distinct attributes of cancer cells have been
proposed to be functionally important for the development of
cancer and might therefore be added to the list of core hallmarks
(Negrini et al., 2010; Luo et al., 2009; Colotta et al., 2009). Two
such attributes are particularly compelling. The first involves
major reprogramming of cellular energy metabolism in order to
support continuous cell growth and proliferation, replacing the
metabolic program that operates in most normal tissues and
fuels the physiological operations of the associated cells. The
second involves active evasion by cancer cells from attack and
elimination by immune cells; this capability highlights the dichotomous
roles of an immune system that both antagonizes and
enhances tumor development and progression. Both of these
capabilities may well prove to facilitate the development and
progression of many forms of human cancer and therefore can
be considered to be emerging hallmarks of cancer. These
enabling characteristics and emerging hallmarks, depicted in
Figure 3, are discussed individually below.
An Enabling Characteristic: Genome Instability
and Mutation
Acquisition of the multiple hallmarks enumerated above depends
in large part on a succession of alterations in the genomes of
neoplastic cells. Simply depicted, certain mutant genotypes
confer selective advantage on subclones of cells, enabling their
outgrowth and eventual dominance in a local tissue environment.
Accordingly, multistep tumor progression can be portrayed as
a succession of clonal expansions, each of which is triggered
by the chance acquisition of an enabling mutant genotype.
Because heritable phenotypes, e.g., inactivation of tumor
suppressor genes, can also be acquired through epigenetic
mechanisms such as DNA methylation and histone modifications
(Berdasco and Esteller, 2010; Esteller, 2007; Jones and Baylin,
2007), some clonal expansions may well be triggered by nonmutational
changes affecting the regulation of gene expression.
The extraordinary ability of genome maintenance systems to
detect and resolve defects in the DNA ensures that rates of
spontaneous mutation are usually very low during each cell
generation. In the course of acquiring the roster of mutant genes
needed to orchestrate tumorigenesis, cancer cells often
increase the rates of mutation (Negrini et al., 2010; Salk et al.,
2010). This mutability is achieved through increased sensitivity
to mutagenic agents, through a breakdown in one or several
components of the genomic maintenance machinery, or both.
In addition, the accumulation of mutations can be accelerated
by compromising the surveillance systems that normally monitor
genomic integrity and force genetically damaged cells into either
senescence or apoptosis (Jackson and Bartek, 2009; Kastan,
2008; Sigal and Rotter, 2000). The role of TP53 is central here,
leading to its being called the ‘‘guardian of the genome’’ (Lane,
1992).
A diverse array of defects affecting various components of the
DNA-maintenance machinery—often referred to as the ‘‘caretakers’’
of the genome (Kinzler and Vogelstein, 1997)—have
been documented. The catalog of defects in these caretaker
genes includes those whose products are involved in (1) detecting
DNA damage and activating the repair machinery, (2) directly
repairing damaged DNA, and (3) inactivating or intercepting
mutagenic molecules before they have damaged the DNA
(Negrini et al., 2010; Ciccia and Elledge, 2010; Jackson and
Bartek, 2009; Kastan, 2008; Harper and Elledge, 2007; Friedberg
et al., 2006). From a genetic perspective, these caretaker genes
behave much like tumor suppressor genes, in that their functions
can be lost during the course of tumor progression, with such
losses being achieved either through inactivating mutations or
via epigenetic repression. Mutant copies of many of these caretaker
genes have been introduced into the mouse germline and
result, predictably, in increased cancer incidence, supporting
their potential involvement in human cancer development
(Barnes and Lindahl, 2004).
Figure 3. Emerging Hallmarks and Enabling
Characteristics
An increasing body of research suggests that two
additional hallmarks of cancer are involved in the
pathogenesis of some and perhaps all cancers.
One involves the capability to modify, or reprogram,
cellular metabolism in order to most effectively
support neoplastic proliferation. The second
allows cancer cells to evade immunological
destruction, in particular by T and B lymphocytes,
macrophages, and natural killer cells. Because
neither capability is yet generalized and fully validated,
they are labeled as emerging hallmarks.
Additionally, two consequential characteristics of
neoplasia facilitate acquisition of both core and
emerging hallmarks. Genomic instability and thus
mutability endow cancer cells with genetic alterations
that drive tumor progression. Inflammation
by innate immune cells designed to fight infections
and heal wounds can instead result in their inadvertent
support of multiple hallmark capabilities,
thereby manifesting the now widely appreciated
tumor-promoting consequences of inflammatory
responses.
658 Cell 144, March 4, 2011 ª2011 Elsevier Inc.
In the decade since we first enumerated the cancer hallmarks,
another major source of tumor-associated genomic instability
has been uncovered: as described earlier, the loss of telomeric
DNA in many tumors generates karyotypic instability and associated
amplification and deletion of chromosomal segments
(Artandi and DePinho, 2010). When viewed in this light, telomerase
is more than an enabler of the hallmark capability for
unlimited replicative potential and must also be added to the
list of critical caretakers responsible for maintaining genome
integrity.
Advances in the molecular-genetic analysis of cancer cell
genomes have provided the most compelling demonstrations
of function-altering mutations and of ongoing genomic instability
during tumor progression. One type of analysis—comparative
genomic hybridization (CGH)—documents the gains and losses
of gene copy number across the cell genome; in many tumors,
the pervasive genomic aberrations revealed by CGH provide
clear evidence for loss of control of genome integrity. Importantly,
the recurrence of specific aberrations (both amplifications
and deletions) at particular sites in the genome indicates that
such sites are likely to harbor genes whose alteration favors
neoplastic progression (Korkola and Gray, 2010).
More recently, with the advent of efficient and economical
DNA-sequencing technologies, higher-resolution analyses
have become possible. Early studies are revealing distinctive
patterns of DNA mutations in different tumor types (see http://
cancergenome.nih.gov/). In the not-too-distant future, the
sequencing of entire cancer cell genomes promises to clarify
the prevalence of ostensibly random mutations scattered across
cancer cell genomes. Thus, recurring genetic alterations may
point to a causal role of particular mutations in tumor pathogen-
esis.
Although the specifics of genome alteration vary dramatically
between different tumor types, the large number of genome
maintenance and repair defects that have already been documented
in human tumors, together with abundant evidence of
widespread destabilization of gene copy number and nucleotide
sequence, persuade us that instability of the genome is inherent
to the great majority of human cancer cells. This leads, in turn, to
the conclusion that the defects in genome maintenance and
repair are selectively advantageous and therefore instrumental
for tumor progression, if only because they accelerate the rate
at which evolving premalignant cells can accumulate favorable
genotypes. As such, genome instability is clearly an enabling
characteristic that is causally associated with the acquisition of
hallmark capabilities.
An Enabling Characteristic: Tumor-Promoting
Inflammation
Pathologists have long recognized that some tumors are densely
infiltrated by cells of both the innate and adaptive arms of the
immune system and thereby mirror inflammatory conditions
arising in non-neoplastic tissues (Dvorak, 1986). With the advent
of better markers for accurately identifying the distinct cell types
of the immune system, it is now clear that virtually every
neoplastic lesion contains immune cells present at densities
ranging from subtle infiltrations detectable only with cell typespecific
antibodies to gross inflammations that are apparent
even by standard histochemical staining techniques (Page` s
et al., 2010). Historically, such immune responses were largely
thought to reflect an attempt by the immune system to eradicate
tumors, and indeed, there is increasing evidence for antitumoral
responses to many tumor types with an attendant pressure on
the tumor to evade immune destruction, as discussed below.
By 2000, there were already clues that the tumor-associated
inflammatory response had the unanticipated, paradoxical effect
of enhancing tumorigenesis and progression, in effect helping
incipient neoplasias to acquire hallmark capabilities. In the
ensuing decade, research on the intersections between inflammation
and cancer pathogenesis has blossomed, producing
abundant and compelling demonstrations of the functionally
important tumor-promoting effects that immune cells—largely
of the innate immune system—have on neoplastic progression
(DeNardo et al., 2010; Grivennikov et al., 2010; Qian and Pollard,
2010; Colotta et al., 2009). Inflammation can contribute to
multiple hallmark capabilities by supplying bioactive molecules
to the tumor microenvironment, including growth factors that
sustain proliferative signaling, survival factors that limit cell
death, proangiogenic factors, extracellular matrix-modifying
enzymes that facilitate angiogenesis, invasion, and metastasis,
and inductive signals that lead to activation of EMT and other
hallmark-facilitating programs (DeNardo et al., 2010;
Grivennikov et al., 2010; Qian and Pollard, 2010; Karnoub and
Weinberg, 2006–2007).
Importantly, inflammation is in some cases evident at the
earliest stages of neoplastic progression and is demonstrably
capable of fostering the development of incipient neoplasias
into full-blown cancers (Qian and Pollard, 2010; de Visser et al.,
2006). Additionally, inflammatory cells can release chemicals,
notably reactive oxygen species, that are actively mutagenic for
nearby cancer cells, accelerating their genetic evolution toward
states of heightened malignancy (Grivennikov et al., 2010). As
such, inflammation can be considered an enabling characteristic
for its contributions to the acquisition of core hallmark capabilities.
The cells responsible for this enabling characteristic are
described in the section below on the tumor microenvironment.
An Emerging Hallmark: Reprogramming Energy
Metabolism
The chronic and often uncontrolled cell proliferation that represents
the essence of neoplastic disease involves not only
deregulated control of cell proliferation but also corresponding
adjustments of energy metabolism in order to fuel cell growth
and division. Under aerobic conditions, normal cells process
glucose, first to pyruvate via glycolysis in the cytosol and thereafter
to carbon dioxide in the mitochondria; under anaerobic
conditions, glycolysis is favored and relatively little pyruvate is
dispatched to the oxygen-consuming mitochondria. Otto
Warburg first observed an anomalous characteristic of cancer
cell energy metabolism (Warburg, 1930, 1956a, 1956b): even in
the presence of oxygen, cancer cells can reprogram their
glucose metabolism, and thus their energy production, by
limiting their energy metabolism largely to glycolysis, leading to
a state that has been termed ‘‘aerobic glycolysis.’’
The existence of this metabolic switch in cancer cells has been
substantiated in the ensuing decades. Such reprogramming of
Cell 144, March 4, 2011 ª2011 Elsevier Inc. 659
energy metabolism is seemingly counterintuitive, in that cancer
cells must compensate for the 18-fold lower efficiency of
ATP production afforded by glycolysis relative to mitochondrial
oxidative phosphorylation. They do so in part by upregulating
glucose transporters, notably GLUT1, which substantially
increases glucose import into the cytoplasm (Jones and Thompson,
2009; DeBerardinis et al., 2008; Hsu and Sabatini, 2008).
Indeed, markedly increased uptake and utilization of glucose
have been documented in many human tumor types, most
readily by noninvasively visualizing glucose uptake using positron
emission tomography (PET) with a radiolabeled analog of
glucose (18
F-fluorodeoxyglucose, FDG) as a reporter.
Glycolytic fueling has been shown to be associated with
activated oncogenes (e.g., RAS, MYC) and mutant tumor
suppressors (e.g., TP53) (DeBerardinis et al., 2008; Jones and
Thompson, 2009), whose alterations in tumor cells have been
selected primarily for their benefits in conferring the hallmark
capabilities of cell proliferation, avoidance of cytostatic controls,
and attenuation of apoptosis. This reliance on glycolysis can be
further accentuated under the hypoxic conditions that operate
within many tumors: the hypoxia response system acts pleiotropically
to upregulate glucose transporters and multiple
enzymes of the glycolytic pathway (Semenza, 2010a; Jones
and Thompson, 2009; DeBerardinis et al., 2008). Thus, both
the Ras oncoprotein and hypoxia can independently increase
the levels of the HIF1a and HIF2a transcription factors, which
in turn upregulate glycolysis (Semenza, 2010a, 2010b; Kroemer
and Pouyssegur, 2008).
A functional rationale for the glycolytic switch in cancer cells
has been elusive, given the relatively poor efficiency of generating
ATP by glycolysis relative to mitochondrial oxidative phosphorylation.
According to one long-forgotten (Potter, 1958) and
recently revived and refined hypothesis (Vander Heiden et al.,
2009), increased glycolysis allows the diversion of glycolytic
intermediates into various biosynthetic pathways, including
those generating nucleosides and amino acids; this facilitates,
in turn, the biosynthesis of the macromolecules and organelles
required for assembling new cells. Moreover, Warburg-like
metabolism seems to be present in many rapidly dividing embryonic
tissues, once again suggesting a role in supporting the
large-scale biosynthetic programs that are required for active
cell proliferation.
Interestingly, some tumors have been found to contain two
subpopulations of cancer cells that differ in their energy-generating
pathways. One subpopulation consists of glucose-dependent
(‘‘Warburg-effect’’) cells that secrete lactate, whereas cells
of the second subpopulation preferentially import and utilize the
lactate produced by their neighbors as their main energy source,
employingpartofthecitricacidcycletodo so(KennedyandDewhirst,
2010; Feron, 2009; Semenza, 2008). These two populations
evidently function symbiotically: the hypoxic cancer cells depend
on glucose for fuel and secrete lactate as waste, which is imported
and preferentially used as fuel by their better-oxygenated
brethren. Although this provocative mode of intratumoral symbiosis
has yet to be generalized, the cooperation between lactatesecreting
and lactate-utilizing cells to fuel tumor growth is in fact
not an invention of tumors but rather again reflects cooption of
a normal physiological mechanism, in this case one operating in
muscle (Kennedy and Dewhirst, 2010; Feron, 2009; Semenza,
2008). Additionally, it is becoming apparent that oxygenation,
ranging from normoxia to hypoxia, is not necessarily static in
tumors but instead fluctuates temporally and regionally (Hardee
et al., 2009), likely as a result of the instability and chaotic organization
of the tumor-associated neovasculature.
Altered energy metabolism is proving to be as widespread in
cancer cells as many of the other cancer-associated traits that
have been accepted as hallmarks of cancer. This realization
raises the question of whether deregulating cellular energy
metabolism is therefore a core hallmark capability of cancer cells
that is as fundamental as the six well-established core hallmarks.
In fact, the redirection of energy metabolism is largely orchestrated
by proteins that are involved in one way or another in
programming the core hallmarks of cancer. When viewed in
this way, aerobic glycolysis is simply another phenotype that is
programmed by proliferation-inducing oncogenes.
Interestingly, activating (gain-of-function) mutations in the isocitrate
dehydrogenase 1/2 (IDH) enzymes have been reported in
glioma and other human tumors (Yen et al., 2010). Although
these mutations may prove to have been clonally selected for
their ability to alter energy metabolism, there is confounding
data associating their activity with elevated oxidation and
stability of the HIF-1 transcription factors (Reitman and Yan,
2010), which could in turn affect genome stability and angiogenesis/invasion,
respectively, thus blurring the lines of phenotypic
demarcation. Currently, therefore, the designation of reprogrammed
energy metabolism as an emerging hallmark seems
most appropriate, to highlight both its evident importance as
well as the unresolved issues surrounding its functional independence
from the core hallmarks.
An Emerging Hallmark: Evading Immune Destruction
A second, still-unresolved issue surrounding tumor formation
involves the role that the immune system plays in resisting or
eradicating formation and progression of incipient neoplasias,
late-stage tumors, and micrometastases. The long-standing
theory of immune surveillance proposes that cells and tissues
are constantly monitored by an ever-alert immune system, and
that such immune surveillance is responsible for recognizing
and eliminating the vast majority of incipient cancer cells
and thus nascent tumors. According to this logic, solid tumors
that do appear have somehow managed to avoid detection
by the various arms of the immune system or have been able
to limit the extent of immunological killing, thereby evading
eradication.
The role of defective immunological monitoring of tumors
would seem to be validated by the striking increases of certain
cancers in immunocompromised individuals (Vajdic and van
Leeuwen, 2009). However, the great majority of these are
virus-induced cancers, suggesting that much of the control of
this class of cancers normally depends on reducing viral burden
in infected individuals, in part through eliminating virus-infected
cells. These observations, therefore, seem to shed little light
on the possible role of the immune system in limiting formation
of the >80% of tumors of nonviral etiology. In recent years,
however, an increasing body of evidence, both from genetically
engineered mice and from clinical epidemiology, suggests that
660 Cell 144, March 4, 2011 ª2011 Elsevier Inc.
the immune system operates as a significant barrier to tumor
formation and progression, at least in some forms of non-virusinduced
cancer.
When mice genetically engineered to be deficient for various
components of the immune system were assessed for the development
of carcinogen-induced tumors, it was observed that
tumors arose more frequently and/or grew more rapidly in the
immunodeficient mice relative to immunocompetent controls.
In particular, deficiencies in the development or function of
CD8+
cytotoxic T lymphocytes (CTLs), CD4+
Th1 helper T cells,
or natural killer (NK) cells each led to demonstrable increases
in tumor incidence; moreover, mice with combined immunodeficiencies
in both T cells and NK cells were even more susceptible
to cancer development. The results indicated that, at least in
certain experimental models, both the innate and adaptive
cellular arms of the immune system are able to contribute significantly
to immune surveillance and thus tumor eradication (Teng
et al., 2008; Kim et al., 2007).
In addition, transplantation experiments have shown that
cancer cells that originally arose in immunodeficient mice are
often inefficient at initiating secondary tumors in syngeneic
immunocompetent hosts, whereas cancer cells from tumors
arising in immunocompetent mice are equally efficient at initiating
transplanted tumors in both types of hosts (Teng et al.,
2008; Kim et al., 2007). Such behavior has been interpreted as
follows: Highly immunogenic cancer cell clones are routinely
eliminated in immunocompetent hosts—a process that has
been referred to as ‘‘immunoediting’’—leaving behind only
weakly immunogenic variants to grow and generate solid
tumors; such weakly immunogenic cells can thereafter colonize
both immunodeficient and immunocompetent hosts. Conversely,
when arising in immunodeficient hosts, the immunogenic
cancer cells are not selectively depleted and can, instead,
prosper along with their weakly immunogenic counterparts.
When cells from such nonedited tumors are serially transplanted
into syngeneic recipients, the immunogenic cancer cells are
rejected when they confront, for the first time, the competent
immune systems of their secondary hosts (Smyth et al., 2006).
(Unanswered in these particular experiments is the question of
whether the chemical carcinogens used to induce such tumors
are prone to generate cancer cells that are especially immuno-
genic.)
Clinical epidemiology also increasingly supports the existence
of antitumoral immune responses in some forms of human
cancer (Bindea et al., 2010; Ferrone and Dranoff, 2010; Nelson,
2008). For example, patients with colon and ovarian tumors
that are heavily infiltrated with CTLs and NK cells have a better
prognosis than those that lack such abundant killer lymphocytes
(Page` s et al., 2010; Nelson, 2008); the case for other cancers is
suggestive but less compelling and is the subject of ongoing
investigation. Additionally, some immunosuppressed organ
transplant recipients have been observed to develop donorderived
cancers, suggesting that in the ostensibly tumor-free
donors, the cancer cells were held in check, in a dormant state,
by a fully functional immune system (Strauss and Thomas, 2010).
Still, the epidemiology of chronically immunosuppressed
patients does not indicate significantly increased incidences of
the major forms of nonviral human cancer, as noted above.
This might be taken as an argument against the importance of
immune surveillance as an effective barrier to tumorigenesis
and tumor progression. We note, however, that HIV and pharmacologically
immunosuppressed patients are predominantly
immunodeficient in the T and B cell compartments and thus do
not present with the multicomponent immunological deficiencies
that have been produced in the genetically engineered mutant
mice lacking both NK cells and CTLs; this leaves open the possibility
that such patients still have residual capability for an immunological
defense against cancer that is mounted by NK and
other innate immune cells.
In truth, the above discussions of cancer immunology simplify
tumor-host immunological interactions, as highly immunogenic
cancer cells may well evade immune destruction by disabling
components of the immune system that have been dispatched
to eliminate them. For example, cancer cells may paralyze infiltrating
CTLs and NK cells, by secreting TGF-b or other immunosuppressive
factors (Yang et al., 2010; Shields et al., 2010). More
subtle mechanisms operate through the recruitment of inflammatory
cells that are actively immunosuppressive, including
regulatory T cells (Tregs) and myeloid-derived suppressor cells
(MDSCs). Both can suppress the actions of cytotoxic lymphocytes
(Mougiakakos et al., 2010; Ostrand-Rosenberg and Sinha,
2009).
In light of these considerations and the still-rudimentary
demonstrations of antitumor immunity as a significant barrier
to tumor formation and progression in humans, we present
immunoevasion as another emerging hallmark, whose generality
as a core hallmark capability remains to be firmly estab-
lished.
THE TUMOR MICROENVIRONMENT
Over the past decade, tumors have increasingly been recognized
as organs whose complexity approaches and may even
exceed that of normal healthy tissues. When viewed from this
perspective, the biology of a tumor can only be understood
by studying the individual specialized cell types within it
(Figure 4, upper) as well as the ‘‘tumor microenvironment’’
that they construct during the course of multistep tumorigenesis
(Figure 4, lower). This depiction contrasts starkly with the
earlier, reductionist view of a tumor as nothing more than
a collection of relatively homogeneous cancer cells, whose
entire biology could be understood by elucidating the cellautonomous
properties of these cells. We enumerate here
a set of cell types known to contribute in important ways to
the biology of many tumors and discuss the regulatory signaling
that controls their individual and collective functions. Most of
these observations stem from the study of carcinomas, in which
the neoplastic epithelial cells constitute a compartment (the
parenchyma) that is clearly distinct from the mesenchymal cells
forming the tumor-associated stroma.
Cancer Cells and Cancer Stem Cells
Cancer cells are the foundation of the disease; they initiate
tumors and drive tumor progression forward, carrying the
oncogenic and tumor suppressor mutations that define cancer
as a genetic disease. Traditionally, the cancer cells within tumors
Cell 144, March 4, 2011 ª2011 Elsevier Inc. 661
have been portrayed as reasonably homogeneous cell populations
until relatively late in the course of tumor progression,
when hyperproliferation combined with increased genetic
instability spawn distinct clonal subpopulations. Reflecting
such clonal heterogeneity, many human tumors are histopathologically
diverse, containing regions demarcated by various
degrees of differentiation, proliferation, vascularity, inflammation,
and/or invasiveness. In recent years, however, evidence
has accumulated pointing to the existence of a new dimension
of intratumor heterogeneity and a hitherto-unappreciated
subclass of neoplastic cells within tumors, termed cancer stem
cells (CSCs).
Although the evidence is still fragmentary, CSCs may prove to
be a common constituent of many if not most tumors, albeit
being present with widely varying abundance. CSCs are defined
operationally through their ability to efficiently seed new tumors
upon inoculation into recipient host mice (Cho and Clarke, 2008;
Lobo et al., 2007). This functional definition is often complemented
by including the expression in CSCs of markers that
are also expressed by the normal stem cells in the tissue-oforigin
(Al-Hajj et al., 2003).
CSCs were initially implicated in the pathogenesis of hematopoietic
malignancies (Reya et al., 2001; Bonnet and Dick, 1997)
and then years later were identified in solid tumors, in particular
breast carcinomas and neuroectodermal tumors (Gilbertson and
Rich, 2007; Al-Hajj et al., 2003). Fractionation of cancer cells on
the basis of displayed cell-surface markers has yielded subpopulations
of neoplastic cells with a greatly enhanced ability, relative
to the corresponding majority populations, to seed new
tumors upon implantation in immunodeficient mice. These
Figure 4. The Cells of the Tumor Microenviron-
ment
(Upper) An assemblage of distinct cell types constitutes
most solid tumors. Both the parenchyma and stroma of
tumors contain distinct cell types and subtypes that
collectively enable tumor growth and progression.
Notably, the immune inflammatory cells present in tumors
can include both tumor-promoting as well as tumor-killing
subclasses.
(Lower) The distinctive microenvironments of tumors. The
multiple stromal cell types create a succession of tumor
microenvironments that change as tumors invade normal
tissue and thereafter seed and colonize distant tissues.
The abundance, histologic organization, and phenotypic
characteristics of the stromal cell types, as well as of the
extracellular matrix (hatched background), evolve during
progression, thereby enabling primary, invasive, and then
metastatic growth. The surrounding normal cells of the
primary and metastatic sites, shown only schematically,
likely also affect the character of the various neoplastic
microenvironments. (Not shown are the premalignant
stages in tumorigenesis, which also have distinctive
microenvironments that are created by the abundance
and characteristics of the assembled cells.)
often-rare tumor-initiating cells proved to share
transcriptional profiles with certain normal
tissue stem cell populations, motivating their
designation as stem-like.
The origins of CSCs within a solid tumor have
not been clarified and indeed may well vary from
one tumor type to another. In some tumors, normal tissue stem
cells may serve as the cells-of-origin that undergo oncogenic
transformation to yield CSCs; in others, partially differentiated
transit-amplifying cells, also termed progenitor cells, may suffer
the initial oncogenic transformation thereafter assuming more
stem-like character. Once primary tumors have formed, the
CSCs, like their normal counterparts, may self-renew as well
as spawn more differentiated derivatives; in the case of
neoplastic CSCs, these descendant cells form the great bulk of
many tumors. It remains to be established whether multiple
distinct classes of increasingly neoplastic stem cells form during
inception and subsequent multistep progression of tumors, ultimately
yielding the CSCs that have been described in fully developed
cancers.
Recent research has interrelated the acquisition of CSC traits
with the EMT transdifferentiation program discussed above
(Singh and Settleman, 2010; Mani et al., 2008; Morel et al.,
2008). Induction of this program in certain model systems can
induce many of the defining features of stem cells, including
self-renewal ability and the antigenic phenotypes associated
with both normal and cancer stem cells. This concordance
suggests that the EMT program not only may enable cancer cells
to physically disseminate from primary tumors but also can
confer on such cells the self-renewal capability that is crucial
to their subsequent clonal expansion at sites of dissemination
(Brabletz et al., 2005). If generalized, this connection raises an
important corollary hypothesis: the heterotypic signals that
trigger an EMT, such as those released by an activated, inflammatory
stroma, may also be important in creating and maintaining
CSCs.
662 Cell 144, March 4, 2011 ª2011 Elsevier Inc.
An increasing number of human tumors are reported to
contain subpopulations with the properties of CSCs, as defined
operationally through their efficient tumor-initiating capabilities
upon xenotransplantation into mice. Nevertheless, the importance
of CSCs as a distinct phenotypic subclass of
neoplastic cells remains a matter of debate, as does their oftcited
rarity within tumors (Boiko et al., 2010; Gupta et al., 2009;
Quintana et al., 2008). Indeed, it is plausible that the phenotypic
plasticity operating within tumors may produce bidirectional
interconversion between CSCs and non-CSCs, resulting in
dynamic variation in the relative abundance of CSCs. Such
plasticity could complicate definitive measurement of their prevalence.
Analogous plasticity is already implicated in the EMT
program, which can be engaged reversibly (Thiery and Sleeman,
2006).
These complexities notwithstanding, it is evident that this
new dimension of tumor heterogeneity holds important implications
for successful cancer therapies. Increasing evidence in
a variety of tumor types suggests that cells with properties of
CSCs are more resistant to various commonly used chemotherapeutic
treatments (Singh and Settleman, 2010; Creighton
et al., 2009; Buck et al., 2007). Their persistence may help to
explain the almost-inevitable disease recurrence following
apparently successful debulking of human solid tumors by radiation
and various forms of chemotherapy. Indeed, CSCs may
well prove to underlie certain forms of tumor dormancy,
whereby latent cancer cells persist for years or even decades
after surgical resection or radio/chemotherapy, only to
suddenly erupt and generate life-threatening disease. Hence,
CSCs may represent a double-threat, in that they are more
resistant to therapeutic killing and, at the same time, endowed
with the ability to regenerate a tumor once therapy has been
halted.
This phenotypic plasticity implicit in CSC state may also
enable the formation of functionally distinct subpopulations
within a tumor that support overall tumor growth in various
ways. For example, an EMT can convert epithelial carcinoma
cells into mesenchymal, fibroblast-like cancer cells that may
well assume the duties of cancer-associated fibroblasts (CAFs)
in some tumors. Remarkably, several recent reports have
documented the ability of glioblastoma cells (or possibly their
associated CSC subpopulations) to transdifferentiate into endothelial-like
cells that can substitute for bona fide host-derived
endothelial cells in forming a tumor-associated neovasculature
(Soda et al., 2011; El Hallani et al., 2010; Ricci-Vitiani et al.,
2010; Wang et al., 2010). Observations like these indicate that
certain tumors may acquire stromal support by inducing some
of their own cancer cells to undergo various types of metamorphosis
to produce stromal cell types rather than relying on
recruited host cells to provide their functions.
The discovery of CSCs and biological plasticity in tumors
indicates that a single, genetically homogeneous population of
cells within a tumor may nevertheless be phenotypically heterogeneous
due to the presence of cells in distinct states of differentiation.
However, an equally important source of phenotypic
variability may derive from the genetic heterogeneity within
a tumor that accumulates as cancer progression proceeds.
Thus, elevated genetic instability operating in later stages of
tumor progression may drive rampant genetic diversification
that outpaces the process of Darwinian selection, generating
genetically distinct subpopulations far more rapidly than they
can be eliminated.
Such thinking is increasingly supported by in-depth sequence
analysis of tumor cell genomes, which has become practical due
to recent major advances in DNA (and RNA) sequencing technology.
Thus the sequencing of the genomes of cancer cells
microdissected from different sectors of the same tumor
(Yachida et al., 2010) has revealed striking intratumoral genetic
heterogeneity. Some of this genetic diversity may be reflected
in the long-recognized histological heterogeneity within individual
human tumors. Alternatively, this genetic diversification
may enable functional specialization, producing subpopulations
of cancer cells that contribute distinct, complementary capabilities,
which then accrue to the common benefit of overall tumor
growth as described above.
Endothelial Cells
Much of the cellular heterogeneity within tumors is found in
their stromal compartments. Prominent among the stromal
constituents are the cells forming the tumor-associated vasculature.
Mechanisms of development, differentiation, and
homeostasis of endothelial cells composing the arteries, veins,
and capillaries were already well understood in 2000. So too
was the concept of the ‘‘angiogenic switch,’’ which activates
quiescent endothelial cells, causing them to enter into a cellbiological
program that allows them to construct new blood
vessels (see above). Over the last decade, a network of interconnected
signaling pathways involving ligands of signal-transducing
receptors displayed by endothelial cells (e.g., Notch,
Neuropilin, Robo, and Eph-A/B) has been added to the
already-prominent VEGF, angiopoietin, and FGF signals. These
newly characterized pathways have been functionally implicated
in developmental and tumor-associated angiogenesis
and illustrate the complex regulation of endothelial cell phenotypes
(Pasquale, 2010; Ahmed and Bicknell, 2009; Dejana
et al., 2009; Carmeliet and Jain, 2000).
Other avenues of research are revealing distinctive gene
expression profiles of tumor-associated endothelial cells and
identifying cell-surface markers displayed on the lumenal
surfaces of normal versus tumor endothelial cells (Nagy et al.,
2010; Ruoslahti et al., 2010; Ruoslahti, 2002). Differences in
signaling, in transcriptome profiles, and in vascular ‘‘ZIP codes’’
will likely prove to be important for understanding the conversion
of normal endothelial cells into tumor-associated endothelial
cells. Such knowledge may lead, in turn, to opportunities to
develop novel therapies that exploit these differences in order
to selectively target tumor-associated endothelial cells.
Closely related to the endothelial cells of the general circulation
are those forming lymphatic vessels (Tammela and Alitalo,
2010). Their role in the tumor-associated stroma, specifically in
supporting tumor growth, is poorly understood. Indeed, because
of high interstitial pressure within solid tumors, intratumoral
lymphatic vessels are typically collapsed and nonfunctional; in
contrast, however, there are often functional, actively growing
(‘‘lymphangiogenic’’) lymphatic vessels at the peripheries of
tumors and in the adjacent normal tissues that cancer cells
Cell 144, March 4, 2011 ª2011 Elsevier Inc. 663
invade. These associated lymphatics likely serve as channels for
the seeding of metastases in the draining lymph nodes that are
commonly observed in a number of cancer types.
Pericytes
As noted earlier, pericytes represent a specialized mesenchymal
cell type (related to smooth muscle cells) with finger-like projections
that wrap around the endothelial tubing of blood vessels. In
normal tissues, pericytes are known to provide paracrine
support signals to the normally quiescent endothelium. For
example, Ang-1 secreted by pericytes conveys antiproliferative
stabilizing signals that are received by the Tie2 receptors
expressed on the surface of endothelial cells; some pericytes
also produce low levels of VEGF that serve a trophic function
in endothelial homeostasis (Gaengel et al., 2009; Bergers and
Song, 2005). Pericytes also collaborate with the endothelial cells
to synthesize the vascular basement membrane that anchors
both pericytes and endothelial cells and helps vessel walls to
withstand the hydrostatic pressure of blood flow.
Genetic and pharmacological perturbation of the recruitment
and association of pericytes has demonstrated the functional
importance of these cells in supporting the tumor endothelium
(Pietras and Ostman, 2010; Gaengel et al., 2009; Bergers and
Song, 2005). For example, pharmacological inhibition of
signaling through the PDGF receptor expressed by tumor pericytes
and bone marrow-derived pericyte progenitors results in
reduced pericyte coverage of tumor vessels, which in turn destabilizes
vascular integrity and function (Pietras and Ostman, 2010;
Raza et al., 2010; Gaengel et al., 2009); interestingly, and in
contrast, the pericytes of normal vessels are not prone to such
pharmacological disruption, providing another example of the
differences in regulation of normal quiescent and tumor vasculature.
An intriguing hypothesis, still to be fully substantiated, is
that tumors with poor pericyte coverage of their vasculature
may be more prone to permit cancer cell intravasation into the
circulatory system, enabling subsequent hematogenous
dissemination (Raza et al., 2010; Gerhardt and Semb, 2008).
Immune Inflammatory Cells
As also discussed above, infiltrating cells of the immune system
are increasingly accepted to be generic constituents of tumors.
These inflammatory cells operate in conflicting ways: both
tumor-antagonizing and tumor-promoting leukocytes can be
found, in various proportions, in most if not all neoplastic lesions.
Although the presence of tumor-antagonizing CTLs and NK cells
is not surprising, the prevalence of immune cells that functionally
enhance hallmark capabilities was largely unanticipated.
Evidence began to accumulate in the late 1990s that the infiltration
of neoplastic tissues by cells of the immune system serves,
perhaps counterintuitively, to promote tumor progression. Such
work traced its conceptual roots back to the association of sites
of chronic inflammation with tumor formation, and to the observation
that tumors could be portrayed as wounds that never heal
(Scha¨ fer and Werner, 2008: Dvorak, 1986). In the course of
normal wound healing and fighting infections, immune inflammatory
cells appear transiently and then disappear, in contrast to
their persistence in sites of chronic inflammation, where their
presence has been associated with various tissue pathologies,
including fibrosis, aberrant angiogenesis, and neoplasia (Grivennikov
et al., 2010; Karin et al., 2006).
Over the past decade, the manipulation of genes involved in
the determination or effector functions of various immune cell
types, together with pharmacological inhibitors of such cells or
their functions, has shown them to play diverse and critical roles
in fostering tumorigenesis. The roster of tumor-promoting
inflammatory cells now includes macrophage subtypes, mast
cells, and neutrophils, as well as T and B lymphocytes (Coffelt
et al., 2010; DeNardo et al., 2010; Egeblad et al., 2010; Johansson
et al., 2008; Murdoch et al., 2008; DePalma et al., 2007).
Such studies are yielding a growing list of signaling molecules
released by inflammatory cells that serve as effectors of their
tumor-promoting actions. These include the tumor growth factor
EGF, the angiogenic growth factor VEGF, other proangiogenic
factors such as FGF2, chemokines, and cytokines that amplify
the inflammatory state; in addition, these cells may produce
proangiogenic and/or proinvasive matrix-degrading enzymes,
including MMP-9 and other matrix metalloproteinases, cysteine
cathepsin proteases, and heparanase (Qian and Pollard, 2010;
Murdoch et al., 2008). Consistent with their expression of these
diverse effectors, tumor-infiltrating inflammatory cells have been
shown to induce and help sustain tumor angiogenesis, to stimulate
cancer cell proliferation, to facilitate, via their presence at the
margins of tumors, tissue invasion, and to support the metastatic
dissemination and seeding of cancer cells (Coffelt et al., 2010;
Egeblad et al., 2010; Qian and Pollard, 2010; Mantovani, 2010;
Joyce and Pollard, 2009; Mantovani et al., 2008; Murdoch
et al., 2008; DePalma et al., 2007).
In addition to fully differentiated immune cells present in tumor
stroma, a variety of partially differentiated myeloid progenitors
have been identified in tumors (Murdoch et al., 2008). Such cells
represent intermediaries between circulating cells of bone
marrow origin and the differentiated immune cells typically found
in normal and inflamed tissues. Importantly, these progenitors,
like their more differentiated derivatives, have demonstrable
tumor-promoting activity. Of particular interest, a class of
tumor-infiltrating myeloid cells (defined as coexpressing the
macrophage marker CD11b and the neutrophil marker Gr1)
has been shown to suppress CTL and NK cell activity, having
been independently identified as MDSCs (Qian and Pollard,
2010; Ostrand-Rosenberg and Sinha, 2009). This attribute raises
the possibility that recruitment of certain myeloid cells may be
doubly beneficial for the developing tumor, by directly promoting
angiogenesis and tumor progression while at the same time
affording a means to evade immune destruction.
The counterintuitive existence of both tumor-promoting and
tumor-antagonizing immune cells can be rationalized by
invoking the diverse roles of the immune system: On the one
hand, the immune system specifically detects and targets infectious
agents with the adaptive immune response, which is supported
by cells of the innate immune system. On the other, the
innate immune system is involved in wound healing and clearing
dead cells and cellular debris. These specialized tasks are
accomplished by distinct subclasses of inflammatory cells,
namely a class of conventional macrophages and neutrophils
(engaged in supporting adaptive immunity), and subclasses of
‘‘alternatively activated’’ macrophages, neutrophils, and
664 Cell 144, March 4, 2011 ª2011 Elsevier Inc.
myeloid progenitors that are engaged in wound healing and
tissue housecleaning (Egeblad et al., 2010; Mantovani, 2010;
Qian and Pollard, 2010; Johansson et al., 2008). The latter
subtypes of immune cells are one of the major sources of the
angiogenic, epithelial, and stromal growth factors and matrix-remodeling
enzymes that are needed for wound healing, and it is
these cells that are recruited and subverted to support
neoplastic progression. Similarly, subclasses of B and T
lymphocytes may facilitate the recruitment, activation, and
persistence of such wound-healing and tumor-promoting
macrophages and neutrophils (DeNardo et al., 2010; Egeblad
et al., 2010; Biswas and Mantovani, 2010). Of course, other
subclasses of B and T lymphocytes and innate immune cell
types can mount demonstrable tumor-killing responses. The
balance between the conflicting inflammatory responses in
tumors is likely to prove instrumental in prognosis and, quite
possibly, in therapies designed to redirect these cells toward
tumor destruction.
Cancer-Associated Fibroblasts
Fibroblasts are found in various proportions across the spectrum
of carcinomas, constituting in many cases the preponderant cell
population of the tumor stroma. The term ‘‘cancer-associated
fibroblast’’ subsumes at least two distinct cell types: (1) cells
with similarities to the fibroblasts that create the structural foundation
supporting most normal epithelial tissues and (2) myofibroblasts,
whose biological roles and properties differ markedly
from those of tissue-derived fibroblasts. Myofibroblasts are
identifiable by their expression of a-smooth muscle actin
(SMA). They are rare in most healthy epithelial tissues, although
certain tissues, such as the liver and pancreas, contain appreciable
numbers of a-SMA-expressing cells. Myofibroblasts transiently
increase in abundance in wounds and are also found in
sites of chronic inflammation. Although beneficial to tissue
repair, myofibroblasts are problematic in chronic inflammation,
contributing to the pathological fibrosis observed in tissues
such as lung, kidney, and liver.
Recruited myofibroblasts and reprogrammed variants of
normal tissue-derived fibroblastic cells have been demonstrated
to enhance tumor phenotypes, notably cancer cell proliferation,
angiogenesis, and invasion and metastasis; their tumorpromoting
activities have largely been defined by transplantation
of cancer-associated fibroblasts admixed with cancer cells into
mice, and more recently by genetic and pharmacologic perturbation
of their functions in tumor-prone mice (Dirat et al., 2010;
Pietras and Ostman, 2010; Ra¨ sa¨ nen and Vaheri, 2010; Shimoda
et al., 2010; Kalluri and Zeisberg, 2006; Bhowmick et al., 2004).
Because they secrete a variety of extracellular matrix components,
cancer-associated fibroblasts are implicated in the formation
of the desmoplastic stroma that characterizes many
advanced carcinomas. The full spectrum of functions contributed
by both subtypes of cancer-associated fibroblasts to tumor
pathogenesis remains to be elucidated.
Stem and Progenitor Cells of the Tumor Stroma
The various stromal cell types that constitute the tumor microenvironment
may be recruited from adjacent normal tissue—the
most obvious reservoir of such cell types. However, in recent
years, the bone marrow has increasingly been implicated as
a key source of tumor-associated stromal cells (Bergfeld and
DeClerck, 2010; Fang and Salven, 2011; Giaccia and Schipani,
2010; Patenaude et al., 2010; Lamagna and Bergers, 2006).
Mesenchymal stem and progenitor cells have been found to
transit into tumors from the marrow, where they may differentiate
into the various well-characterized stromal cell types. Some of
these recent arrivals may also persist in an undifferentiated or
partially differentiated state, exhibiting functions that their more
differentiated progeny lack.
The bone marrow origins of stromal cell types have
been demonstrated using tumor-bearing mice in which the
bone marrow cells and thus their disseminated progeny have
been selectively labeled with reporters such as green fluorescent
protein (GFP). While immune inflammatory cells have been long
known to derive from the bone marrow, more recently the
progenitors of pericytes and of various subtypes of cancer-associated
fibroblasts originating from the bone marrow have been
described in various mouse models of cancer (Bergfeld and
DeClerck, 2010; Fang and Salven, 2011; Giaccia and Schipani,
2010; Lamagna and Bergers, 2006); the prevalence and functional
importance of endothelial progenitors for tumor angiogenesis
is currently unresolved (Fang and Salven, 2011; Patenaude
et al., 2010). Taken together, these various lines of evidence indicate
that tumor-associated stromal cells may be supplied to
growing tumors by proliferation of preexisting stromal cells, by
differentiation in situ of local stem/progenitor cells originating
in the neighboring normal tissue, or via recruitment of bone
marrow-derived stem/progenitor cells.
Heterotypic Signaling Orchestrates
the Cells of the Tumor Microenvironment
Depictions of the intracellular circuitry governing cancer cell
biology (e.g., Figure 2) will need to be complemented by similar
diagrams charting the complex interactions between the
neoplastic and stromal cells within a tumor and the dynamic
extracellular matrix that they collectively erect and remodel (Egeblad
et al., 2010; Kessenbrock et al., 2010; Pietras and Ostman,
2010; Polyak et al., 2009). A reasonably complete, graphic
depiction of the network of microenvironmental signaling interactions
is still far beyond our reach, as the great majority of
signaling molecules and pathways remain to be identified. We
provide instead a hint of such interactions in Figure 5, upper.
These few well-established examples are intended to exemplify
a signaling network of remarkable complexity that is of critical
importance to tumor pathogenesis.
Another dimension of complexity is not represented in this
simple schematic: both neoplastic cells and the stromal cells
around them change progressively during the multistep transformation
of normal tissues into high-grade malignancies. This
histopathological progression must reflect underlying changes
in heterotypic signaling between tumor parenchyma and stroma.
Such stepwise progression is likely to depend on back-andforth
reciprocal interactions between the neoplastic cells and
the supporting stromal cells, as depicted in Figure 5, lower.
Thus, incipient neoplasias begin the interplay by recruiting and
activating stromal cell types that assemble into an initial preneoplastic
stroma, which in turn responds reciprocally by enhancing
Cell 144, March 4, 2011 ª2011 Elsevier Inc. 665
Figure 5. Signaling Interactions in the Tumor Microenvironment during Malignant Progression
(Upper) The assembly and collective contributions of the assorted cell types constituting the tumor microenvironment are orchestrated and maintained by
reciprocal heterotypic signaling interactions, of which only a few are illustrated.
(Lower) The intracellular signaling depicted in the upper panel within the tumor microenvironment is not static but instead changes during tumor progression as
a result of reciprocal signaling interactions between cancer cells of the parenchyma and stromal cells that convey the increasingly aggressive phenotypes that
underlie growth, invasion, and metastatic dissemination. Importantly, the predisposition to spawn metastatic lesions can begin early, being influenced by the
differentiation program of the normal cell-of-origin or by initiating oncogenic lesions. Certain organ sites (sometimes referred to as ‘‘fertile soil’’ or ‘‘metastatic
niches’’) can be especially permissive for metastatic seeding and colonization by certain types of cancer cells, as a consequence of local properties that are either
intrinsic to the normal tissue or induced at a distance by systemic actions of primary tumors. Cancer stem cells may be variably involved in some or all of the
different stages of primary tumorigenesis and metastasis.
666 Cell 144, March 4, 2011 ª2011 Elsevier Inc.
the neoplastic phenotypes of the nearby cancer cells. The
cancer cells, which may further evolve genetically, again feed
signals back to the stroma, continuing the reprogramming of
normal stromal cells to serve the budding neoplasm; ultimately
signals originating in the tumor stroma enable cancer cells to
invade normal adjacent tissues and disseminate.
This model of reciprocal heterotypic signaling must be
extended to encompass the final stage of multistep tumor
progression—metastasis (Figure 5, lower right). The circulating
cancer cells that are released from primary tumors leave a microenvironment
created by the supportive stroma of such tumors.
However, upon landing in a distant organ, these cancer cells
encounter a naive, fully normal, tissue microenvironment.
Consequently, many of the heterotypic signals that shaped their
phenotype while they resided within primary tumors may be
absent in sites of dissemination, constituting a barrier to growth
of the seeded cancer cells. Thus, the succession of reciprocal
cancer cell to stromal cell interactions that defined multistep
progression in the primary tumor now must be repeated anew
in distant tissues as disseminated cancer cells proceed to colonize
their newfound organ sites.
Although this logic applies in some cases of metastasis, in
others, as mentioned earlier, certain tissue microenvironments
may, for various reasons, already be supportive of freshly
seeded cancer cells; such permissive sites have been referred
to as ‘‘metastatic niches’’ (Peinado et al., 2011; Coghlin and
Murray, 2010). Implicit in this term is the notion that cancer cells
seeded in such sites may not need to begin by inducing
a supportive stroma because it already preexists, at least in
part. Such permissivity may be intrinsic to the tissue site
(Talmadge and Fidler, 2010) or preinduced by circulating factors
released by the primary tumor (Peinado et al., 2011). The most
well-documented components of induced premetastatic niches
are tumor-promoting inflammatory cells, although other cell
types and the ECM may well prove to play important roles in
different metastatic contexts.
The likelihood that signaling interactions between cancer cells
and their supporting stroma evolve during the course of multistage
tumor development clearly complicates the goal of fully
elucidating the mechanisms of cancer pathogenesis. For
example, this reality poses challenges to systems biologists
seeking to chart the crucial regulatory networks than orchestrate
malignant progression. Moreover, it seems likely that understanding
these dynamic variations will become crucial to the
development of novel therapies designed to successfully target
both primary and metastatic tumors.
THERAPEUTIC TARGETING
The introduction of mechanism-based targeted therapies to
treat human cancers has been heralded as one of the fruits of
three decades of remarkable progress of research into the
mechanisms of cancer pathogenesis. We do not attempt here
to enumerate the myriad therapies that are under development
or have been introduced of late into the clinic. Instead, we
consider how the description of hallmark principles is beginning
to inform therapeutic development at present and may increasingly
do so in the future.
The rapidly growing armamentarium of targeted therapeutics
can be categorized according to their respective effects on
one or more hallmark capabilities, as illustrated in the examples
presented in Figure 6. Indeed, the observed efficacy of these
drugs represents, in each case, a validation of a particular capability:
if a capability is truly important for the biology of tumors,
then its inhibition should impair tumor growth and progression.
We note that most of the hallmark-targeting cancer drugs
developed to date have been deliberately directed toward
specific molecular targets that are involved in one way or another
in enabling particular capabilities. Such specificity of action has
been considered a virtue, as it presents inhibitory activity against
a target while having, in principle, relatively fewer off-target
effects and thus less nonspecific toxicity. In fact, resulting clinical
responses have generally been transitory, being followed
by almost-inevitable relapses.
One interpretation of this history, supported by growing experimental
evidence, is that each of the core hallmark capabilities is
regulated by partially redundant signaling pathways. Consequently,
a targeted therapeutic agent inhibiting one key pathway
in a tumor may not completely shut off a hallmark capability, allowing
some cancer cells to survive with residual function until
they or their progeny eventually adapt to the selective pressure
imposed by the therapy being applied. Such adaptation, which
can be accomplished by mutation, epigenetic reprogramming,
or remodeling of the stromal microenvironment, can reestablish
the functional capability, permitting renewed tumor growth and
clinical relapse. Given that the number of parallel signaling pathways
supporting a given hallmark must be limited, it may
become possible to target all of these supporting pathways therapeutically,
thereby preventing the development of adaptive
resistance.
In response to therapy, cancer cells may also reduce their
dependence on a particular hallmark capability, becoming
more dependent on another; this represents a quite different
form of acquired drug resistance. This concept is exemplified
by recent discoveries of unexpected responses to antiangiogenic
therapies. Some have anticipated that effective inhibition
of angiogenesis would render tumors dormant and might even
lead to their dissolution (Folkman and Kalluri, 2004). Instead,
the clinical responses to antiangiogenic therapies have been
found to be transitory (Azam et al., 2010; Ebos et al., 2009; Bergers
and Hanahan, 2008).
In certain preclinical models, where potent angiogenesis inhibitors
succeed in suppressing this hallmark capability, tumors
adapt and shift from a dependence upon continuing angiogenesis
to heightening the activity of another instead—invasiveness
and metastasis (Azam et al., 2010: Ebos et al., 2009; Bergers and
Hanahan, 2008). By invading nearby tissues, initially hypoxic
cancer cells evidently gain access to normal, preexisting tissue
vasculature. Initial clinical validation of this adaptive/evasive
resistance is apparent in the increased invasion and local metastasis
seen when human glioblastomas are treated with antiangiogenic
therapies (Ellis and Reardon, 2009; Norden et al.,
2009; Verhoeff et al., 2009). The applicability of this lesson to
other human cancers has yet to be established.
Analogous adaptive shifts in dependence on other hallmark
traits may also limit efficacy of analogous hallmark-targeting
Cell 144, March 4, 2011 ª2011 Elsevier Inc. 667
therapies. For example, the deployment of apoptosis-inducing
drugs may induce cancer cells to hyperactivate mitogenic
signaling, enabling them to compensate for the initial attrition
triggered by such treatments. Such considerations suggest
that drug development and the design of treatment protocols
will benefit from incorporating the concepts of functionally
discrete hallmark capabilities and of the multiple biochemical
pathways involved in supporting each of them. Thus, in particular,
we can envisage that selective cotargeting of multiple
core and emerging hallmark capabilities and enabling characteristics
(Figure 6) in mechanism-guided combinations will result in
more effective and durable therapies for human cancer.
CONCLUSION AND FUTURE VISION
We have sought here to revisit, refine, and extend the concept of
cancer hallmarks, which has provided a useful conceptual
framework for understanding the complex biology of cancer.
The six acquired capabilities—the hallmarks of cancer—have
stood the test of time as being integral components of most
forms of cancer. Further refinement of these organizing principles
will surely come in the foreseeable future, continuing the
remarkable conceptual progress of the last decade.
Looking ahead, we envision significant advances during the
coming decade in our understanding of invasion and metastasis.
Similarly, the role of aerobic glycolysis in malignant growth will
be elucidated, including a resolution of whether this metabolic
reprogramming is a discrete capability separable from the core
hallmark of chronically sustained proliferation. We remain
perplexed as to whether immune surveillance is a barrier that
virtually all tumors must circumvent, or only an idiosyncrasy of
an especially immunogenic subset of them; this issue too will
be resolved in one way or another.
Yet other areas are currently in rapid flux. In recent years, elaborate
molecular mechanisms controlling transcription through
chromatin modifications have been uncovered, and there are
Figure 6. Therapeutic Targeting of the Hallmarks of Cancer
Drugs that interfere with each of the acquired capabilities necessary for tumor growth and progression have been developed and are in clinical trials or in some
cases approved for clinical use in treating certain forms of human cancer. Additionally, the investigational drugs are being developed to target each of the
enabling characteristics and emerging hallmarks depicted in Figure 3, which also hold promise as cancer therapeutics. The drugs listed are but illustrative
examples; there is a deep pipeline of candidate drugs with different molecular targets and modes of action in development for most of these hallmarks.
668 Cell 144, March 4, 2011 ª2011 Elsevier Inc.
clues that specific shifts in chromatin configuration occur during
the acquisition of certain hallmark capabilities (Berdasco and Esteller,
2010). Functionally significant epigenetic alterations seem
likely to be factors not only in the cancer cells but also in the
altered cells of the tumor-associated stroma. It is unclear at
present whether an elucidation of these epigenetic mechanisms
will materially change our overall understanding of the means by
which hallmark capabilities are acquired or simply add additional
detail to the regulatory circuitry that is already known to govern
them.
Similarly, the discovery of hundreds of distinct regulatory microRNAs
has already led to profound changes in our understanding
of the genetic control mechanisms that operate in
health and disease. By now dozens of microRNAs have been
implicated in various tumor phenotypes (Garzon et al., 2010),
and yet these only scratch the surface of the real complexity,
as the functions of hundreds of microRNAs known to be present
in our cells and altered in expression in different forms of cancer
remain total mysteries. Here again, we are unclear as to whether
future progress will cause fundamental shifts in our understanding
of the pathogenetic mechanisms of cancer or only
add detail to the elaborate regulatory circuits that have already
been mapped out.
Finally, the circuit diagrams of heterotypic interactions
between the multiple distinct cell types that assemble and
collaborate to produce different forms and progressively malignant
stages of cancer are currently rudimentary. In another
decade, we anticipate that the signaling circuitry describing
the intercommunication between these various cells within
tumors will be charted in far greater detail and clarity, eclipsing
our current knowledge. And, as before (Hanahan and Weinberg,
2000), we continue to foresee cancer research as an increasingly
logical science, in which myriad phenotypic complexities are
manifestations of a small set of underlying organizing principles.
SUPPLEMENTAL INFORMATION
Supplemental Information includes six figures that are downloadable for
presentations and can be found with this article online at doi:10.1016/j.cell.
2011.02.013.
ACKNOWLEDGMENTS
We thank Terry Schoop (OFC Graphics, Kensington, CA, USA) for exceptional
efforts in preparing the figures. And we thank Gerard Evan (Cambridge, UK),
Erwin Wagner (Madrid, ESP), and Zena Werb (San Francisco, USA) for valuable
comments and suggestions on the manuscript. D.H. and R.A.W. are
American Cancer Society Research Professors. Research in the authors’ laboratories
has been largely supported by the U.S. National Cancer Institute. Due
to space limitations, many primary and historical publications have not been
cited, particularly in cases where topical reviews are available.
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