Granulopoiesis Requires Increased C/EBPa Compared to
Monopoiesis, Correlated with Elevated Cebpa in
Immature G-CSF Receptor versus M-CSF Receptor
Expressing Cells
Ou Ma1
, SunHwa Hong1
, Hong Guo1
, Gabriel Ghiaur2
, Alan D. Friedman1
*
1 Division of Pediatric Oncology, Johns Hopkins University, Baltimore, Maryland, United States of America, 2 Division of Hematologic Malignancies, Johns Hopkins
University, Baltimore, Maryland, United States of America
Abstract
C/EBPa is required for the formation of granulocyte-monocyte progenitors; however, its role in subsequent myeloid lineage
specification remains uncertain. Transduction of murine marrow with either of two Cebpa shRNAs markedly increases
monocyte and reduces granulocyte colonies in methylcellulose or the monocyte to neutrophil ratio in liquid culture. Similar
findings were found after marrow shRNA transduction and transplantation and with CEBPA knockdown in human marrow
CD34+
cells. These results apparently reflect altered myeloid lineage specification, as similar knockdown allowed nearly
complete 32Dcl3 granulocytic maturation. Cebpa knockdown also generated lineage-negative blasts with increased colony
replating capacity but unchanged cell cycle parameters, likely reflecting complete differentiation block. The shRNA having
the greatest effect on lineage skewing reduced Cebpa 3-fold in differentiating cells but 6-fold in accumulating blasts.
Indicating that Cebpa is the relevant shRNA target, shRNA-resistant C/EBPa-ER rescued marrow myelopoiesis. Cebpa
knockdown in murine marrow cells also increased in vitro erythropoiesis, perhaps reflecting 1.6-fold reduction in PU.1
leading to GATA-1 derepression. Global gene expression analysis of lineage-negative blasts that accumulate after Cebpa
knockdown demonstrated reduction in Cebpe and Gfi1, known transcriptional regulators of granulopoiesis, and also
reduced Ets1 and Klf5. Populations enriched for immature granulocyte or monocyte progenitor/precursors were isolated by
sorting Lin2
Sca-12
c-Kit+
cells into GCSFR+
MCSFR2
or GCSFR2
MCSFR+
subsets. Cebpa, Cebpe, Gfi1, Ets1, and Klf5 RNAs were
increased in the c-Kit+
GCSFR+
and Klf4 and Irf8 in the c-Kit+
MCSFR+
populations, with PU.1 levels similar in both. In summary,
higher levels of C/EBPa are required for granulocyte and lower levels for monocyte lineage specification, and this myeloid
bifurcation may be facilitated by increased Cebpa gene expression in granulocyte compared with monocyte progenitors.
Citation: Ma O, Hong S, Guo H, Ghiaur G, Friedman AD (2014) Granulopoiesis Requires Increased C/EBPa Compared to Monopoiesis, Correlated with Elevated
Cebpa in Immature G-CSF Receptor versus M-CSF Receptor Expressing Cells. PLoS ONE 9(4): e95784. doi:10.1371/journal.pone.0095784
Editor: Amit Verma, Albert Einstein College of Medicine, United States of America
Received January 20, 2014; Accepted March 30, 2014; Published April 21, 2014
Copyright: ß 2014 Ma et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was supported by National Institutes of Health grants R01 HL089176 and U01 HL099775 and the Giant Food Children’s Cancer Research
Fund. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: afriedm2@jhmi.edu
Introduction
CCAAT/enhancer binding protein a (C/EBPa) is a basic
region-leucine zipper transcription factor expressed within granulocytic
and monocytic myeloid cells during hematopoiesis; C/
EBPa is the predominant C/EBP protein in immature myeloid
cells [1,2]. Newborn C/EBPa (2/2) mice lack granulocytes but
retain monocytes; however, marrow from adult C/EBPa (flox/
flox);Mx1-Cre mice exposed to pIpC to induce Cebpa gene deletion
have markedly reduced numbers of granulocyte-monocyte progenitors
(GMP), leading to impairment of both granulopoiesis and
monopoiesis, with increased numbers of preceding common
myeloid progenitors (CMP) [3,4]. In addition, exogenous C/
EBPa directs granulocytic maturation of the U937, HL-60, or
32Dcl3 myeloid cell lines but induces monocytic maturation of
murine marrow myeloid progenitors or lymphoid cells [2,5–8].
The role of C/EBPa beyond the GMP in directing myeloid
lineage specification thus remains uncertain.
To gain further insight into the regulation of myelopoiesis by C/
EBPa, we have investigated the consequences of reducing but not
eliminating C/EBPa expression. We find that two different Cebpa
shRNAs impair murine marrow granulopoiesis and enable
increased monopoiesis. Cebpa knockdown also led to accumulation
of an immature population apparently unable to commit to either
lineage, with increased in vitro growth and replating capacity, a
preleukemic phenotype. Cebpa RNA was reduced 3-fold in the bulk
population that retains myeloid differentiation capacity but 6-fold
in lineage-negative cells unable to mature along either lineage. In
addition to use of two independent shRNAs, we further support
the conclusion that Cebpa is the relevant shRNA target by
demonstrating that shRNA-resistant C/EBPa-ER overcomes the
block to marrow cell myeloid development. Supporting the idea
that Cebpa knockdown blocks granulocyte lineage commitment and
not simply granulocytic maturation, we show that 5-fold Cebpa
reduction in the 32Dcl3 cells line allows maturation to the
metamyelocyte/band stage in response to G-CSF. Cebpa knockPLOS
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down also unexpectedly increased marrow cell erythropoiesis,
potentially reflecting reduced Sfpi1/PU.1 expression and thereby
GATA-1 derepression [9,10].
To gain insight into the mechanism underlying their impeded
myelopoiesis, we conducted global RNA expression analysis of
control versus Cebpa knockdown lineage-negative cells, revealing
multiple changes including reduction in Cebpe and Gfi1, known C/
EBPa targets and regulators of granulopoiesis, and also reduced
Ets1 and Klf5. Having observed reduced granulopoiesis but
preserved monopoiesis upon partial Cebpa knockdown, we also
sought to determine whether Cebpa RNA levels are actually
elevated in marrow granulocyte compared with monocyte
progenitors/precursors. Immature Lin2
Sca-12
c-Kit+
cells were
sorted into populations exclusively expressing the G-CSF receptor
(GCSFR) or the M-CSF receptor (MCSFR). CFU-G were mainly
found in the former and CFU-M in the latter subset, and there was
significantly higher Cebpa mRNA levels in the c-Kit+
GCSFR+
population. Notably, Cebpe, Gfi1, Ets1, and Klf5 RNAs were also
increased in this population and Klf4 and Irf8 in the c-
Kit+
MCSFR+
subset, with PU.1 similar in both. The relevance
of our findings to normal and malignant myelopoiesis will be
further discussed.
Methods
Ethics Statement
This study was carried out in strict accordance with the
recommendations in the Guide for the Care and Use of
Laboratory Animals of the National Institutes of Health. The
protocol (M013M116) was approved by the Johns Hopkins
University Animal Care and Use Committee. All efforts were
made to minimize suffering. Use of human marrow samples from
donors who gave written informed consent was approved by the
Johns Hopkins University Institutional Review Board.
shRNAs and Plasmids
To target murine Cebpa, we utilized shRNAs TRCN9502–9504
(B9–B11) in the pLKO.1-Puro lentiviral vector (Open Biosystems).
Human CEBPA was targeted with TRCN7305–6 (G5, G6). Empty
vector and mammalian shRNA SHC002 non-targeting vector
(NTV, Sigma Aldrich) were employed as controls. To rescue C/
EBPa expression we modified the C/EBPa-ER cDNA to be
resistant to B9 shRNA by changing the targeted site AGC-CGAGAT-AAA-GCC-AAA-CAG
to TCT-AGG-GAC-AAG-GCTAAG-CAG,
with synonymous changes italicized.
Murine Marrow Culture, Transduction, FACS Analysis, and
Transplantation
Marrow isolated from 8–16 week old C57BL/6 female mice was
subjected to red cell lysis with NH4Cl, and the cells were lineagedepleted
using biotin-conjugated B220, Gr-1, Mac-1, Ter119, and
CD3 mouse Lineage Cocktail (BD Pharmingen), anti-biotin
microbeads, and MACS columns (Miltenyi Biotec). The cells
were then cultured in Iscove’s modified Dulbecco medium
(IMDM) with 10% heat-inactivated fetal bovine serum (HI-FBS)
containing 50 ng/mL murine stem cell factor (SCF), 100 ng/mL
murine FLT3 ligand (FL), and 10 ng/mL murine thrombopoietin
(TPO, Peprotech) for 24 hrs. Lentiviral particles were generated
by transient transfection of 293T cells with plasmids at a ratio of
3.75 mg pLKO.1 vector: 5 mg pCMV-DR8.91: 1.25 mg
pMD.G(VSV.G) per 100 mm dish using 20 mL Lipofectamine
2000 (Invitrogen). Lentiviral supernatants collected at 48 hr were
concentrated with AmiconUltra filters (Millipore). Retroviral
particles were packaged similarly at a ratio of 8 mg MIG or
pBabe vector: 2 mg pkat2ecopac. The cells were transduced by
spinoculation at 1500 g for 2 hours after addition of lentiviral
supernatant alone or with retroviral supernatant and 1 mg/mL
Polybrene. Co-culture with viral supernatant and 0.5 mg/mL
Polybrene was then continued for 2 days, followed by addition of
Figure 1. Cebpa knockdown increases murine marrow cell monopoiesis relative to granulopoiesis and generates a blast population.
A) Diagram of marrow transduction protocol. B) Western blot for C/EBPa and b-actin after transduction and puromycin selection with the pLKO.1
vector (Vec) or with Cebpa shRNAs B9 or B11. C) Number of CFU-M, CFU-G, or CFU-GM per 1E4 cells plated in methylcellulose culture with IL-3, IL-6,
and SCF (n = 6–9). D) FACS analysis for Mac-1 and Gr-1 on pooled CFUs. Arrows indicate immature, lineage-negative cells (top). Morphology of pooled
CFU cells (bottom). *p,0.05, **p,0.01, ***p,0.001.
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2.5 mg/mL puromycin for 2 additional days. Viable cells were
collected using Lympholyte-M polysucrose (Cedarlane Labs) and
transferred to IMDM with 10% HI-FBS with 10 ng/mL murine
IL-3, 10 ng/mL murine IL-6, and 50 ng/mL murine SCF in
liquid culture or to Methocult M3231 (StemCell Technologies)
containing methylcellulose with IMDM, HI-FBS, and these same
cytokines at 1E4 cells/mL for myeloid colony analysis. Erythroid
colonies were obtained by plating 1.6E5 cells/mL in Methocult
3120 (1% final concentration) with IMDM, 2 mM glutamine,
55 nM b-mercaptoethanol, 10% plasma-derived serum (Animal
Technologies), 20% BIT (Stem Cell Technologies), 5% PFHM-II
(Invitrogen), and 2 U/mL (20 ng/mL) murine erythropoietin
(EPO). Colony forming units (CFUs) were enumerated 7 days
later. Colony replating was done weekly also at 1E4 cells/mL.
Cells were subjected to morphologic analysis via cytospin and
Wright-Giemsa staining, to cell cycle analysis via propidium iodide
(PI) staining after fixation in 70% methanol and treatment with
DNase-free RNase, and to surface marker analysis via fluorescent
activated cell sorting (FACS) using PE-anti-Mac-1, FITC-anti-Gr-
1, PerCP-Cy5.5-anti-Sca-1, APC-anti-c-Kit, and PE-anti-Ter119.
For CMP/GMP/MEP analysis, cells were stained with biotinanti-lineage
markers, PerCP-Cy5.5-streptavidin, PE-Cy7-anti-Sca-
1, APC-anti-c-Kit, FITC-anti-CD34, and PE-anti-FccR. Photomicrographs
were taken using a Zeiss Axiophot microscope (Carl
Zeiss), a Kontron Electronik Progress 3012 camera (Kontron), and
a 63X/1.40 NA oil objective. Viable cell counts were enumerated
using Trypan blue dye and a hemocytometer.
For in vivo analysis, 8E5 puromycin-resistant CD45.2+
cells
combined with 2E5 CD45.1+
nucleated marrow cells were
transplanted via tail vein injection into syngeneic CD45.1
recipients irradiated to 950 cGy. At day 28 marrow was analyzed
by FACS and sorted for CD45.2+
CD45.12
cells for CFU assay via
use of a FACSAria II cell sorter (BD Biosciences).
Human Marrow Culture, Transduction, and FACS Analysis
Cryopreserved human bone marrow CD34+
cells were obtained
from the Johns Hopkins cell bank. Cells were placed directly in
serum-free Stemline II Hematopoietic Stem Cell Expansion
Medium (Sigma-Aldrich) with 100 ng/mL FLT3 ligand,
100 ng/mL human SCF, and 20 ng/mL human thrombopoietin
(TPO) and recovered for 2 days, followed by transduction with
lentiviral vectors in the presence of 2 mg/ml Polybrene for 2
consecutive days. On the third day, transduced cells were subject
to 2 mg/ml puromycin selection for 2 days, and viable cells were
separated by density gradient centrifugation (Ficoll-Paque Plus;
GE Healthcare Life Sciences) at 1,500 rpm for 30 minutes, and
cultured either in methylcellose (Methocult H4230, StemCell
Technologies) with IMDM at 2E4 cells/mL or in liquid culture
with RPMI and 10% HI-FBS at 2E5 cells/ml, in either case with
50 ng/ml human GM-CSF. Myeloid colonies were enumerated
Figure 2. CEBPA knockdown increases human marrow cell monopoiesis relative to granulopoiesis. A) Diagram of marrow transduction
protocol (top) and CEBPA mRNA expression normalized to b-Actin on D0 after transduction with non-targeting vector (NTV) or CEBPA shRNAs G5 or
G6. B) FACS analysis for the CD14 (monocytic) and CD15 (granulocytic) markers at D7 for liquid cultured cells. C) Number of CFU-M, CFU-G, and CFUGM
per 5E4 cells plated (n = 3). D) Representative morphology of pooled CFU cells. Note large monocytes with abundant basophilic cytoplasm and
smaller granulocytic cells with eosinophilic cytoplasm and granules. The proportion of monocytes (M) and granulocytes (G) amongst 100 cell counted
are shown.
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on day 14. For FACS, cells were stained with hIgG Fc Receptor
Block (Biolegend), FITC-anti-CD14, and PE-anti-CD15.
32Dcl3 Cell Culture and Transduction
32Dcl3 cells [11] were maintained in IMDM with 10% HI-FBS
with 1 ng/mL murine IL-3. To induce differentiation, cells were
washed twice with PBS and transferred to IMDM with 10% HIFBS
and 20 ng/mL human G-CSF (Amgen). 32Dcl3 cells were
transduced for 2 days by addition of lentiviral or retroviral
supernatant in the presence of 4 mg/mL Polybrene, followed by
selection with 2 mg/mL puromycin or 1.2 mg/mL G418 (total).
Subclones were obtained by limiting dilution. G-CSF receptor
(GCSFR) surface expression was evaluated using biotin-G-CSF
and streptavidin-APC +/2100-fold excess G-CSF. Biotin G-CSF
was generated using human G-CSF (Amgen) and a biotinylation
kit (Pierce).
Isolation of Populations Enriched for CFU-G Or CFU-M
Marrow isolated from C57BL/6 mice was subjected to red cell
lysis with NH4Cl and then stained with FITC-anti-Lineage (Lin)
Markers (CD3, B220, Ter119, Gr-1, Mac-1; BioLegend). PerCPCy5.5-anti-c-Kit
(2B8; BioLegend), PE-Cy7-anti-Sca-1 (D7;
eBioscience), PE-anti-MCSFR (AFS98; BioLegend), biotin-GCSF,
and streptavidin-APC. The Lin2
c-Kit+
Sca-
12
GCSFR+
MCSFR2
and Lin2
c-Kit+
Sca-12
GCSFR2
MCSFR+
populations were then isolated by flow cytometry. GMPs were
isolated by flow cytometry as described [12].
Western Blot and RNA Analysis
For Western blotting, samples in Laemmli sample buffer (LSB)
corresponding to 5E5 cells were subjected to PAGE using 10%
gels and transferred to Hybond-P membrane (Amersham).
Primary antibodies used were C/EBPa (14AA, Santa Cruz
Biotechnology) and b-actin (AC-15; Sigma-Aldrich). After addition
of HRP-conjugated secondary antibody and subsequent washes, a
signal was generated using HyGlo chemiluminescence reagents
(Denville Scientific) and detected by autoradiography.
RNA was prepared from 1E6–1E7 cells using the NucleoSpin
RNA II kit, with use of RNase-free DNase (Machery-Nagel). First
strand cDNA was prepared using AMV reverse transcriptase
(Promega) and oligo(dT) primer. Quantitative PCR was carried
out using 50 ng of each cDNA using iQ SYBR Green supermix
(Bio-Rad). Oligonucleotides used are listed in Table S1.
Global Gene Expression Analysis
Mouse WG-6 v2.0 Expression BeadChips (Illumina) were used
for microarray hybridizations. Total RNA was prepared using the
NucleoSpin RNA II kit. RNA quality was confirmed via
Nanodrop-1000 spectrometric analysis and via a Bioanalyzer
(Agilent Technologies). 500 ng total RNA from each sample was
amplified and labeled using the Illumina TotalPrep RNA
Amplification Kit with oligo(dT) priming (Ambion). 750 ng
biotin-labeled cRNA was combined with hybridization buffer
and hybridized to the array at 58uC for 16–20 hours. After
hybridization, the array was washed with buffer at 55uC and
blocked at room temperature. Bound, biotinylated cRNA was
stained with streptavidin-Cy3 and then washed. Dried arrays were
scanned with the iScan System, and data with quantile normalization
was exported from GenomeStudio v2011.1 Gene Expression
Module 1.9.0 and imported to GeneSpring. Differentially
expressed genes with .1.5-fold change were subjected to pathway
analysis using Ingenuity Systems software. Analysis of fold-change
between paired samples was conducted without subtraction of
background signal. Primary microarray data is available from
GEO (accession number GSE55760).
Statistical Analysis
Means and standard deviations are shown. The Student t test
was used for statistical comparisons. Band intensities were
quantified using National Institutes of Health ImageJ 1.38
software.
Results
Cebpa Knockdown Reduces CFU-G While Increasing CFUM
and a Population of Immature Blasts
Murine lineage-negative marrow cells were transduced with
pLKO.1 lentiviral vectors expressing one of several Cebpa shRNAs
(B9, B10, or B11) or the empty pLKO.1 vector for 2 days followed
by puromycin selection for 2 additional days. This was done in
Figure 3. Cebpa knockdown increases myeloid colony replating.
A) Myeloid CFUs obtained after transduction with the pLKO.1
vector or with Cebpa shRNAs B9 or B11 were replated each 7 days. The
average number of generations for which CFUs were obtained is
plotted (top, n = 4–5). G4 indicates that initial CFUs could be replated 3
times to yield CFUs. CFU-G and CFU-M obtained at each generation
from a typical experiment done in triplicate, 1E4 cell/plate (bottom). B)
FACS analysis for Mac-1 and Gr-1 from G3 CFUs. Arrows indicate
immature, lineage-negative cells (top). Morphology of G4 and G6 CFUs
from the same experiment (bottom).
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TPO, SCF, and FL, cytokines that maintain the majority of cells
undifferentiated [13]. Viable cells were then transferred to IL-3,
IL-6, and SCF to allow myeloid maturation (Figure 1A). The B9
or B11 shRNAs reduced C/EBPa protein 2-fold relative to the
pLKO.1 vector, based on image scanning, in cells collected on D0
directly after drug selection (Figure 1B). B9 or B11 reduced CFUG
while increasing CFU-M relative to empty vector (Figure 1C).
Typical murine CFU-G, CFU-M, and CFU-GM colony morphologies
are shown (Figure S1A). FACS analysis of pooled CFUs
to enumerate Mac-1+
Gr-1+
granulocytes and Mac-1+
Gr-12
monocytes confirmed reduced granulopoiesis relative to monopoiesis
(Figure 1D, top). We previously demonstrated that Mac-
1+
Gr-12
cell in similar analyses express both CD115/MCSFR
and F4/80, confirming their identity as monocytes [12]. FACS
analyses herein also revealed accumulation of an immature Mac-
12
Gr-12
population, evident in this representative analysis as 1%
of vector-transduced but 44% or 12% of B9 or B11-transduced
CFU cells, respectively (arrows). Morphologic analysis revealed
both macrophages and neutrophils after vector-transduction, but
predominantly macrophages and cells with blast morphology in
CFUs derived from B9- or B11-transduced marrow (Figure 1D,
bottom). Together, these data indicate that graded C/EBPa levels
guide myeloid lineage specification and that sufficient reduction of
C/EBPa prevents progression along either lineage.
Human bone marrow CD34+
cells were transduced in TPO/
FL/SCF with non-targeting lentiviral vector (NTV) or with vectors
expressing human CEBPA shRNAs G5 or G6, followed by
puromycin selection in these same cytokines and then transfer
on D0 to GM-CSF to induce myeloid differentiation, as
diagrammed (Figure 2A, top). Quantitative RT-PCR analysis
demonstrated approximately 3-fold CEBPA RNA knockdown by
G5 and 2.5-fold knockdown by G6 on D0 (Figure 2A, bottom).
After 7 days in liquid culture, cells transduced with NTV or G5
were subjected to CD14/CD15 FACS, with G5 transduction
reducing CD15+
granulocytic cells and increasing CD14+
monocytic
cells (Figure 2B). CEBPA knockdown by either of two
shRNAs also increased CFU-M and reduced CFU-G relative to
NTV, without diminishing total CFU numbers (Figure 2C).
Morphologies of typical human CFUs obtained are shown (Figure
S1B). Pooled CFUs transduced with NTV or G6 were cytospun,
stained with Wright-Giemsa stain, and myeloid cells were
Figure 4. Cebpa knockdown reduces murine granulopoiesis relative to monopoiesis and increases immature blasts in liquid culture.
A) Transduced, puromycin selected marrow cells were placed in liquid culture with IL-3, IL-6, and SCF on D0 and viable cell counts were enumerated
on days 0, 2, 3, 4, and 7. Representative data is shown. B) Cell cycle analysis was conducted using PI staining on D0 or D3. C) FACS analysis for Mac-1
and Gr-1 on D2. Arrows indicate immature, Mac-12
Gr-12
cells (top). FACS analysis of Mac-12
Gr-12
cells stained for Sca-1 and c-Kit (middle). FACS
analysis of D2 cells for FccR and CD34, gating on Lin2
Sca-12
c-Kit+
progenitors (bottom). Representative data are shown. D) Total cellular RNAs
collected on day 3 (D3) after lentiviral transduction with the pLKO.1 vector or with Cebpa shRNAs B9 or B11, puromycin selection, and transfer to IL-3,
IL-6, and SCF were subjected to quantitative RT-PCR for indicated mRNAs (n = 3).
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Figure 5. Cebpa knockdown reduces murine granulopoiesis and increases monopoiesis and immature cells in vivo. A) Diagram of
transduction and transplantation protocol using the pLKO.1 vector or Cebpa shRNAs B9 or B11. B) Marrow cells isolated 28 days after transplantation
(D28) were subjected to CD45.2/CD45.1 or Mac-1/Gr-1 FACS analysis. Representative data are shown. C) Summary of Sca-1/c-Kit and Mac-1/Gr-1 FACS
analyses (vector and B9, n = 4; B11, n = 3). D) On day 28, CD45.2+
CD45.12
marrow cells isolated by flow cytometry were plated in methylcellulose with
IL-3, IL-6, and SCF. The number of CFU-M, CFU-G, or CFU-GM obtained per 1E4 cells plated is shown (n = 3).
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Figure 6. 32Dcl3 cells with partial Cebpa knockdown retain capacity for nearly complete granulocytic maturation. A) Parental 32Dcl3
cells, pools of 32Dcl3 cells stably transduced with the pLKO.1 vector (Vec) or Cebpa B9 or B11 shRNAs, or subclones of the B9 or B11 pools were
subjected to Western blotting for C/EBPa and b-actin. B) Indicated 32Dcl3 lines were subjected to FACS analysis for GCSFR using biotin-G-CSF in the
absence (heavy line) or presence (light line) of excess unbiotinylated G-CSF; x-axis is log scale (top). RNAs prepared from indicated 32Dcl3 lines
cultured in IL-3 or after one or four days in G-CSF (G1, G4) were subjected to quantitative RT-PCR for Mpo (bottom, n = 3). C) Vector, B11-1, or B9-2
cells cultured in GCSF were subjected to Wright-Giemsa staining on the days of maximal differentiation, G6, G8, and G4 respectively (top row). Vector/
GR, B11-1/GR, or B9-2/GR, G14, G14, and G6, respectively, were analyzed similarly (bottom row). D) Pools of vector, B11-1, or B9-2 lines transduced
with pBabeNeo or pBabeNeo-GCSFR were subjected to FACS analysis for GCSFR (top). RNAs prepared from these lines in IL-3 or G-CSF were analyzed
for Mpo expression (bottom, n = 3).
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enumerated, revealing a reduced proportion of granulocytes and a
corresponding increased proportion of monocytes (Figure 2D).
Cebpa Knockdown Increases Myeloid Colony Replating
To determine whether the blastic, lineage-negative cells that
accumulate after transduction of murine marrow with Cebpa
shRNAs have increased proliferative capacity, CFUs were replated
weekly. CFUs derived from pLKO.1-transduced marrow cells lost
replating capacity after an average of 2.2 generations, and CFUs
expressing a non-targeting shRNA stopped replating after 2.0
generations (not shown). In contrast, CFUs obtained after B11transduction
replated an average of 3.75 generations (p = 0.04),
and those obtained from B9-transduced cells replated for at least 8
generations (p,0.001, Figure 3A, top), with CFU-M predominating
(Figure 3A, bottom). Although neither B9- nor B11-transduced
cells gained cytokine independence, in one experiment we
continued replating B9-transduced cells for 16 generations before
stopping. FACS analysis of third generation CFUs demonstrated
strong accumulation of Mac-12
Gr-12
cells in both the B9 and
B11 cultures, with shift of the remaining cells towards monopoiesis
(Figure 3B, top). Morphologic analysis demonstrated macrophages
and blasts but no granulocytes in B9 or B11 CFUs (Figure 3B,
bottom).
Cebpa Knockdown Impairs Granulopoiesis and Increases
Monopoiesis in Liquid Culture without Altering the Cell
Cycle
To investigate the early effects of Cebpa knockdown, transduced
marrow cells were placed in liquid culture with IL-3, IL-6, and
SCF. B9- or B11-transduced cells expanded more rapidly, as
shown for a representative experiment (Figure 4A). Although
exogenous C/EBPa inhibits G1 to S cell cycle progression in
myeloid cells [5], cell cycle analysis did not uncover a significant
difference in the proportion of cells in the G1, S, or G2/M cell
cycle phases due to Cebpa knockdown, either on D0 or D3 after
completion of puromycin selection, and there was no difference in
the small number of sub-G1 apoptotic cells (Figure 4B).
FACS analysis revealed that as early as D2 there was reduction
in the proportion of granulocytes, increased monocytes, and
accumulation of immature Mac-12
Gr-12
cells; these changes
were most evident with B9 but also occurred with B11 Cebpa
shRNA transduction (Figure 4C, top). As a control, we transduced
marrow cells with non-targeting shRNA in comparison to empty
pLKO.1 or the B9 shRNA. After 2 days in liquid culture FACS
analysis demonstrated near equivalence of the two control cultures
and again showed reduced granulocytes, increased monocytes,
and striking accumulation of Mac-12
Gr-12
cells with B9 but not
the two control vectors (Figure S2).
FACS analysis also revealed that the majority of Mac-12
Gr-12
(Lin2
) cells are Lin2
Sca-12
c-Kit+
progenitors or Lin2
Sca-1+
c-
Kit+
(LSK) stem cells, with the stem cell subset increased by Cebpa
knockdown (Figure 4C, middle). When the Lin2
Sca-12
c-Kit+
progenitor cells were further analyzed using FccR and CD34
antibodies, the B9 and B11 cultures had diminished GMP relative
to CMP in comparison to the control culture, consistent with
results obtained upon deletion of the Cebpa genes in adult mice [4],
and unexpectedly there was increased megakaryocyte/erythroid
progenitors (MEP) with B9 (Figure 4C, bottom).
To further confirm that Cebpa knockdown reduces granulopoiesis
while increasing monopoiesis, total cellular RNAs prepared
from vector-, B9-, of B11-transduced marrow cells on D3 after
transfer to IL-3, IL-6, and SCF were analyzed for several myeloid
transcription factors and lineage markers (Figure 4D). Cebpa was
reduced 3-fold by B9 and 2-fold by B11. Even though increased
PU.1 levels favor monopoiesis, PU.1 mRNA was reduced 1.6-fold
by B9 or B11. Runx1 was unaffected. Egr1 and Irf8, encoding
proteins that direct monopoiesis, were increased. The granulocytic
Figure 7. C/EBPa-ER rescues the differentiation blocked induced in murine marrow or 32Dcl3 cells by Cebpa knockdown. A) Diagram
of marrow transduction protocol with lentiviral pLKO.1(puro)-B9 shRNA and MIG or MIG-C/EBPa-ER(B9res) retrovirus (top). 293T whole cell extracts
obtained 2 days after transfection with MIG or MIG-C/EBPa-ER(B9res) DNAs were subjected to Western blotting with C/EBPa or actin antibodies
(center, right). Representative Mac-1;Gr-1 FACS analysis of marrow cells 2 days after transfer to IL-3, IL-6, SCF +/2 E2 (bottom, left). Average reduction
in Mac-12
Gr-12
(Lin2
) cells or increase in percent monocytes compared to total monocytes + granulocytes (n = 3, bottom, right). B) Two subclones of
B9-2 cells transduced with pBabeNeo-C/EBPa-ER(B9res), B9-2/Neo and Vec/Neo cells were subjected to Western blotting for C/EBPa and b-actin (left).
B9-2/Neo, B9-2/aER-1, and B9-2/aER-2 cells cultured in G-CSF with estradiol were subjected to Wright-Giemsa staining on G6, the day of maximal
differentiation (center). RNAs prepared in IL-3 or G-CSF with simultaneous addition of estradiol were analyzed for Mpo expression (right, n = 3).
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markers Mpo, Ela2/NE, and Csf3r/Gcsfr were reduced, more
prominently by B9 compared with B11, whereas B9 but not B11
led to increased expression of the monocytic markers Csf1r/Mcsfr
and Cd14. C/EBPa activates the gene encoding c-Myc via DNAbound
E2F1 and the gene encoding Bcl-2 via DNA-bound NF-kB
p50 [14,15]; however, B9 had no effect and B11 only minimal
effect on expression of their corresponding mRNAs.
Table 1. Selected mRNAs decreased or increased in Lin2
marrow cells by Cebpa shRNA*.
Gene name Fold-change Descriptive name Gene name Fold-change Descriptive Name
Aatk 22.8/2.2 Apoptosis-associated tyr kinase Mapk31 21.9/1.7 MAP kinase 11 (MLK3)
Camp 27.7/8.3 Cathelicidin antimicrobial
peptide
Map3k3 21.9/1.6 MAP kinase kinase 3
Cebpa 21.9/1.8 C/EBPa transcription
factor
Med21 22.1/1.6 Mediator complex
subunit 21
Cebpe 24.4/3.4 C/EBPe transcription
factor
Mpo 22.2/1.8 Myeloperoxidase
Csf3r 21.5/1.6 G-CSF receptor Nfe2l2 21.5/1.4 NF-E2 like 2
Csf2ra 21.5/1.7 GM-CSF receptor a subunit Nras 22.3/1.8 N-Ras
Ctsb 21.6/1.6 Cathepsin B Prg2 24.3/3.6 Eosinophil major
basic protein
Ctsg 21.9/1.7 Cathepsin G Prkcb 21.9/1.6 Protein kinase C b
Ctsgh 21.9/1.8 Cathepsin H Prss34 247/219 Mast cell tryptase 34
Cxcr4 21.7/1.6 Cxcr4 chemokine
receptor
Prtn3 22.7/2.2 Proteinase 3
Dgkg 22.0/2.0 DAG kinase c Rab27a 22.6/2.2 Rab27a GTPase
Dnmt31 22.2/2.4 DNA-methyltransferase
3-like
Rab32 21.8/2.0 Rab32 GTPase
Ela2 210/5.6 Neutrophil elastase Sfpi1** 21.6/1.3 PU.1 transcription
factor
Epx 23.2/2.5 Eosinophil peroxidase Tcn2 22.3/2.2 Trans-cobalamin 2
Ets1 23.0/1.8 Ets1 transcription factor Alad +2.0/2.1 Aminolevulinae dehdratase
Fcgr2b 22.0/1.6 Fcc receptor 2b Bcl11 +1.8/1.7 Bcl11A transcription factor
Fcgr3 23.2/2.2 Fcc receptor 3 Ccnd1 +5.0/3.6 Cyclin D1
Fkbp11 22.5/1.7 Peptidyl proline
isomerase
Cd14** +1.7/1.3 CD14
Gadd45a 21.9/1.8 Growth arrest/DNAdamage
45a
Cd34 +1.8/1.7 CD34
Gapt 23.8/2.9 GRB2-binding adaptor
protein
Cd52 +4.1/2.7 CD52
Gfi1 25.0/3.7 Gfi-1 transcription
factor
Dnmt3b +2.5/2.0 DNA-methyltransferase 3b
Hgf 22.5/1.5 Hepatocyte growth
factor
Gfi1b +3.7/3.6 Gfi-1b transcription
factor
Idh1 21.7/1.8 Isocitrate
dehydrogenase I
Hba-a1 +22/16 Hemoglobin a
Ifngr1 21.8/1.9 Interferon c
receptor 1
Hba-b1 +5.2/5.4 Hemoglobin b
Ikbe1 21.7/1.5 IkB kinase e Klf1 +2.6/2.3 Klf1 transcription
factor
Irak3 21.9/1.5 IL-1 receptor
associated kinase
Meis1 +1.7/1.7 Meis1 transcription
factor
Klf5 22.2/1.4 Klf5 transcription
factor
Nfkbia +1.5/1.4 IkBa
Lrb4r1 24.2/3.2 Leukotriene B4
receptor
Pip4k2a +1.9/1.6 PI-5-phosphate-4-kinase
Ltf 23.1/2.0 Lactoferrin Tal1 +1.8/2.2 Tal1/SCL transcription
factor
Lyz 21.5/2.0 Lysozyme Tnf +1.6/1.6 Tumor necrosis factor
*Fold-change at least 1.4-fold, except as marked by **, in two transductions with vector and B9-shRNA.
(2) decreased, (+) increased by shRNA B9.
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Cebpa Knockdown Impairs Granulopoiesis and Increases
Monopoiesis In vivo
To evaluate the effect of Cebpa knockdown in vivo, 8E5
transduced CD45.2+
marrow cells were transplanted with 2E5
CD45.1+
carrier cells into lethally irradiated CD45.1+
recipient
mice (Figure 5A). Compared with vector, marrow reconstituted
with B9-transduced cells had a reproducibly higher proportion of
CD45.2+
cells on day 28 (Figure 5B, top panels), 82+/27% versus
41+/211% (p = 0.002). In addition, marrow derived from B9transduced
cells had .3-fold increased Sca-1+
c-Kit+
cells,
averaging 9% (Figure 5C, left). These data indicate that Cebpa
knockdown leads to a population of immature progenitors that
expand over a 4 week period in vivo, paralleling the increased
in vitro replating capacity of similarly transduced cells. This
expanded, immature population was transient, as only 2 of 7
mice euthanized 20–24 weeks after transplant with B9-transduced
marrow had .10% CD45.2+
CD45.12
marrow cells, and only
0.8% or 1.7% of these cells were Sca-1+
c-Kit+
(not shown).
Cebpa knockdown also increased the proportion of marrow Mac-
1+
Gr-12
monocytes relative to Mac-1+
Gr-1+
granulocytes on day
28 after transplantation, with B9 having a greater effect than B11
(Figure 5B, bottom panels and 5C, right). This increase was no
longer apparent in the two B9-transduced mice with .10%
CD45.2+
cells at weeks 20–24 (not shown), further indicating that
Cebpa knockdown affected the differentiation of a transient myeloid
progenitor. Providing further support for an in vivo effect of Cebpa
knockdown on myeloid specification, CD45.2+
marrow cells sorted
from transplanted mice on day 28 were assessed for formation of
myeloid CFUs in methylcellulose (Figure 5D). Both B9 and B11
increased CFU-M numbers and the CFU-M to CFU-G ratio, with
B9 having a greater effect. Neither B9 nor B11 significantly altered
the proportion of CD45.2+
cells expressing the erythroid marker
Ter119, either on day 28 or at weeks 20–24 (not shown).
Partial Cebpa Knockdown Allows Nearly Complete 32Dcl3
Granulocytic Maturation
Parental 32Dcl3 cells, pooled vector, B9 or B11 transductants,
and several subclones were analyzed for C/EBPa expression by
Western blotting (Figure 6A). Within the pools, B11 modestly and
B9 almost completely reduced C/EBPa levels. Subclones B11-1
and B11-2 have 5-fold reduced C/EBPa protein, based on image
scanning, while B11-3, B11-4, B9-1, and B9-2 have essentially
absent C/EBPa. A similar pattern was seen on RNA analysis (not
shown). The GCSFR promoter is activated by C/EBPa [16], and
GCSFR surface expression is reduced modestly in B11-1 or B11-2
cells and more extensively in the B9-2 subclone (Figure 6B, top).
These three subclones and vector-transduced cells were transferred
to G-CSF and monitored for morphologic maturation and
induction of the Mpo differentiation marker (Figure 6B, bottom
and 6C, upper row). Mpo was induced fully in B11-1 cells and at
mildly reduced levels in B11-2 cells, but only minimally in the B9-2
subclone, which did not survive past day 4 in G-CSF. Vector cells
produced mature neutrophils; B11-1 cells, and B11-2 cells (not
shown), matured to the metamyelocyte/band stage; B9-2 cells did
not show any granulocytic maturation. B11-1 or B9-2 32Dcl3 cells
transduced with pBabeNeo-GCSFR demonstrated rescue of
GCSFR expression (Figure 6D, top). B11-1/GR cells now
demonstrated full neutrophil maturation in G-CSF, whereas B9-
2/GR cells were still incapable of even early morphologic
maturation or substantial induction of Mpo RNA (Figure 6C,
lower row and Figure 6D, bottom). Nearly complete terminal
differentiation of B11-1 and B11-2 32Dcl3 cells despite reduction
in Cebpa to an extent greater than the average 3-fold reduction
seen in marrow cells transduced with Cebpa shRNAs suggests that
impaired marrow granulopoiesis upon B9 or B11 transduction
reflects lack of commitment to the granulocytic lineage rather than
defective granulocytic maturation.
C/Ebpa-ER Rescues the Effect of Cebpa Knockdown in
Murine Marrow or 32Dcl3 Cells
Besides use of two shRNAs, as a further control for shRNA offtarget
effects, we developed a variant of C/EBPa-ER rendered
resistant to B9 shRNA via synonymous codon mutations. Marrow
cells were simultaneously transduced with pLKO.1(puro)-B9
shRNA and MIG or MIG-C/EBPa-ER(B9res) vectors, puromycin
selected, transferred to IL-3, IL-6, and SCF +/2 estradiol (E2),
and subjected to FACS, gating on GFP+
cells (Figure 7A). Western
blotting of transfected 293T cells confirmed expression of C/
EBPa-ER(B9res). The proportion of immature Mac-12
Gr-12
cells
that accumulate with B9 shRNA was reduced more than 2-fold by
activation of C/EBPa-ER(B9res), indicating induction of their
myeloid differentiation. There was some reduction even in the
absence of estradiol likely due to ER leakiness. There was also an
increase in the ratio of Mac-1+
Gr-12
to Mac-1+
Gr-1+
cells, as we
had seen previously with expression of C/EBPa-ER in wild-type
murine marrow cells, potentially reflecting formation of C/EBPaER:AP-1
heterodimers [8,17].
32Dcl3(B9-2) cells were transduced with shRNA-resistant
pBabeNeo-C/EBPa-ER(B9res), and two subclones demonstrating
transgene expression were identified (Figure 7B, top). Addition of
estradiol to these lines rescued full granulocytic differentiation in
G-CSF (Figures 7B, bottom). Notably, the large majority of B9-2/
aER cells died by day 2 in G-CSF in the absence of estradiol
whereas there numbers increased 1.5- to 2-fold and then stabilized
for 6 days in the presence of estradiol.
Global RNA Expression Analysis of Cebpa Knockdown
Lineage-negative Marrow Cells Reveals Known and
Potentially Novel C/Ebpa Targets
Murine marrow was transduced with the pLKO.1 or B9
shRNA for 2 days in TPO/FL/SCF, puromycin-selected in these
same cytokines for 2 additional days, then cultured for 2 days in
Figure 8. Quantitative RT-PCR analyses from RNA samples
used for global gene expression analysis. RNA samples from
replicate pLKO.1 and B9 transductions were subjected to quantitative
RT-PCR in triplicate for indicated mRNAs relative to b-actin RNA.
Expression in pLKO.1 vector samples was set to 1.0 on average in each
experiment, and mean Relative Expression values in the two B9 samples
are shown.
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IL-3/IL-6/SCF, and finally lineage-depleted using Mac-1, Gr-1,
Ter119, B220, and CD3 antibodies. mRNAs were then subjected
to gene expression analysis using Illumina arrays. Biologic
replicate samples were analyzed from independent transductions.
A list of genes increased or decreased at least 1.4-fold in both
experiments was obtained (Table S2). A subset of these genes,
representing lineage markers and regulatory proteins, was also
compiled (Table 1). For RNAs expressed near background the
entries in these tables underestimate the true fold-change. The
same RNA samples were subjected to RT-PCR to validate several
of these results (Figure 8). Each of 6 RNAs decreased and 4 RNAs
increased .1.4-fold in the array were indeed decreased or
increased at least to this extent. Cebpa RNA was decreased 6-fold
by B9 shRNA, on average; PU.1 was decreased 1.6-fold. In
comparable lineage-negative cells, B11 shRNA reduced Cebpa 1.4fold
(not shown).
Granulocytic markers decreased by B9 in the lineage-negative
population included Csf3r, Ela2, Ltf, Mpo, Prtn3, and Tcn2,
whereas, monocytic markers Cd14 and Tnf were increased.
Cathepsins, Fcc receptor, and lysozyme, expressed by both
lineages, were also reduced. Cebpe and Gfi1, known transcriptional
regulators of granulopoiesis, were reduced, as were additional
transcription factors, Ets1, Klf5, and Nfe2l2. Klf4 and Irf8 were not
altered substantially on the array, and RT-PCR analysis revealed
little change in Cebpb, JunB, or c-Fos and only 1.6-fold increase in cJun
(not shown). Ikbe was decreased and Nfkbia was increased by
Cebpa knockdown, suggesting a normal role for C/EBPa in upregulating
NF-kB signaling. Ingenuity pathway analysis (Table 2)
identified several additional pro-inflammatory pathways reduced
by Cebpa knockdown, including those mediated by IL-8, IL-10,
iNOS, or LPS signaling. The specific genes altered in these
pathways are also listed.
Table 2. Top pathways with affected genes altered by Cebpa knockdown*.
Pathway 2logP R Genes
Production of NO
and reactive O2 in
macrophages
4.74 0.076 MAP3K11,MAP3K6,MPO,IKBKE,IFNGR1,TLR4,NFKBIA,
RHOB, RND3,CYBA,RHOU,S100A8,TNF,MAP3K3,SIRPA,PRKCB
Interleukin-10
signaling
3.43 0.103 NFKBIA,FCGR2A,BLVRB,IL1B,IKBKE,LBP,
FCGR2B,TNF
LPS/IL-1
mediated
inhibition of RXR
function
3.20 0.063 ALDH4A1,MGST1,GSTM5,CHST12,PAPSS2,TLR4,HS6ST1
,MGST2, IL1B, ALAS1,ALDH3B1,LBP,CHST13,TNF,ACSL1
Inhibitor of NO
synthase (iNOS)
signaling
3.04 0.113 TLR4,NFKBIA,IKBKE,IFNGR1,LBP,IRAK3
Aryl hydrocarbon
receptor signaling
2.99 0.068 TGM2,ALDH4A1,MGST1,MGST2,GSTM5,CDKN1A,IL1B
,ALDH3B1, CCND1,TNF,NFE2L2
Interleukin-8
signaling
2.97 0.064 VCAM1,NRAS,ANGPT1,MPO,IKBKE,IRAK3,CCND1,
PLD1,CSTB, RND3,RHOB,RHOU,PRKCB
Hepatocyte
growth factor
signaling
2.96 0.086 ETS1,NRAS,MAP3K11,MAP3K6,HGF,CDKN1A,CCND1,
MAP3K3,PRKCB
CD27 signaling in
lymphocytes
2.70 0.105 NFKBIA,MAP3K11,MAP3K6,IKBKE,
CD27,MAP3K3
B cell receptor
signaling
2.66 0.067 ETS1,NRAS,NFKBIA,MAP3K11,MAP3K6,FCGR2A,PIK3AP1,IKB
KE, FCGR2B,MAP3K3,PRKCB
High mobility
group box 1
(HMGB1)
signaling
2.54 0.081 TLR4,VCAM1,NRAS,RND3,RHOB,RHOU,IFNGR1,TNF
Xenobiotic
metabolism
signaling
2.48 0.054 ALDH4A1,MGST1,MAP3K11,NRAS,MAP3K6,GSTM5,CHST12,
HS6ST1,MGST2,IL1B,ALDH3B1,CHST13,NFE2L2,TNF,MAP3K3,
PRKCB
Choline
biosynthesis
2.39 0.136 PCYT1B,CHPT1,PLD1
Vitamin D
Receptor/RXR
activation
2.38 0.086 SERPINB1,SPP1,CAMP,GADD45A,CDKN1A,
CEBPA,PRKCB
TREM1 signaling 2.38 0.085 TLR4,LAT2,MPO,IL1B,FCGR2B,TNF
TNFR2 signaling 2.25 0.121 TANK,NFKBIA,IKBKE,TNF
PI-3-kinase
signaling in B
lymphocytes
2.20 0.064 TLR4,NRAS,C3,NFKBIA,ATF5,PIK3AP1,
IKBKE,FCGR2B,PRKCB
*Top pathways from analysis of RNAs with .1.5-fold change in 2 experiments, with affected genes listed and ordered by –logP values. R values, indicating the
proportion of genes in a pathway affected by Cebpa knockdown, are also shown.
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Unexpectedly, several lineage markers and regulators of
erythropoiesis were increased by Cebpa knockdown, including aglobin,
b-globin, aminolevulinate dehydrogenase, and transcriptional
regulators Bcl11A, Gfi1b, Klf1, and Tal1, the latter three
confirmed by RT-PCR analysis (Figure 8). Given these findings
and increased MEP seen by FACS, we compared erythroid colony
formation from pLKO.1 versus B9-transduced marrow cells in
methylcellulose with EPO. Cebpa knockdown increased Ter119+
BFU-E numbers (Figure S3), with hemoglobinization evident in
cell pellets (not shown).
Cebpa RNA Is Elevated in a c-Kit+
GCSFR+
Population
Enriched for CFU-G Compared to a c-Kit+
MCSFR+
Population Enriched for CFU-M
Murine marrow cells were stained for Lineage markers, c-Kit,
Sca-1, GCSFR, and MCSFR (Figure 9A). The immature Lin2
Sca-12
c-Kit+
subset was sorted into GCSFR+
MCSFR2
(GR+
MR2
) and GCSFR2
MCSFR+
(GR2
MR+
) subsets followed
by plating in methylcellulose with IL-3, IL-6, and SCF (Figure 9B).
On average, 86% of the CFUs obtained from the GR+
MR2
population were CFU-G, while 65% of the CFUs obtained from
the GR2
MR+
population were CFU-M. The majority of the
remaining CFUs were CFU-GM. CFUs represented 11% of
sorted GR+
MR2
and 9% of GR2
MR+
cells, indicated that the
remainder where either progenitors that do not form colonies
under these culture conditions or are immature precursors. Under
the same culture conditions, 15% of sorted GMP cells formed
myeloid CFUs (not shown). Quantitative RNA analysis from three
independent experiments confirmed enhanced expression of Gcsfr
in the c-Kit+
GR+
MR2
and Mcsfr in the c-Kit+
GR2
MR+
populations and demonstrated that Cebpa RNA is expressed on
average 2.3-fold higher in the myeloid progenitor population
enriched for CFU-G compared to that enriched for CFU-M
(Figure 9C). PU.1 and Runx1 were mildly enriched in the
GR+
MR2
cells. Cebpe, Gfil1, Ets1, and Klf5 RNAs were expressed
Figure 9. Myeloid CFUs and RNAs expression in Lin2
Sca-12
c-Kit+
murine marrow cells expressing G-CSF versus M-CSF receptor. A)
FACS analyses demonstrating isolation of Lin2
Sca-12
c-Kit+
GCSFR(GR)+
MCSFR(MR)2
and Lin2
Sca-12
c-Kit+
GCSFR(GR)2
MCSFR(MR)+
populations. B)
The number of CFU-M, CFU-G, or CFU-GM obtained per 1E4 cells plated in methylcellulose culture with IL-3, IL-6, and SCF is shown (n = 3). C) Total
cellular RNAs were subjected to quantitative RT-PCR for indicated mRNAs (n = 3).
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at markedly higher levels in the CFU-G-enriched population,
whereas Klf4 and Irf8 were increased in the CFU-M-enriched
subset.
Discussion
Deletion of the Cebpa alleles in adult marrow leads to marked
reduction in GMP, making the subsequent role of C/EBPa in
granulocyte versus monocyte lineage specification uncertain.
Providing new insight into the role of C/EBPa in myeloid lineage
specification, we now demonstrate that partial Cebpa knockdown
impairs granulopoiesis while increasing monopoiesis with further
reduction blocking commitment to either myeloid lineage. The
shRNA producing the most striking phenotypic alteration reduced
Cebpa 3-fold in the total population of differentiating myeloid cells
and 6-fold in accumulating blasts. Blast expansion was transient
in vivo, indicating that further genetic alterations would be required
to avoid quiescence or cell death to obtain indefinite expansion
and perhaps malignant transformation. As controls for off target
effects, a shift from granulopoiesis toward monopoiesis was
obtained with two different Cebpa shRNAs in murine marrow
and with two different CEBPA shRNAs in human CD34+
cells,
and shRNA-resistant C/EBPa-ER overcame the effects of Cebpa
knockdown.
Cebpa knockdown reduced CFU-G while increasing CFU-M.
Increased CFU-M suggests a shift in lineage commitment rather
than simply a defect in granulocytic maturation – in support of this
conclusion, Cebpa knockdown in 32Dcl3 cells to an extent greater
than that obtained in total marrow cells did not prevent their near
terminal maturation in response to G-CSF. Thus, higher levels of
C/EBPa are apparently needed for granulocyte compared to
monocyte lineage specification. Accumulation of immature blasts
in response to Cebpa knockdown suggests that low level C/EBPa
expression is still required for monopoiesis, as these blasts may
represent myeloid progenitors that express C/EBPa below a
requisite threshold. Of note, although exogenous C/EBPa blocks
G1 to S cell cycle progression in myeloid and non-myeloid cells
[8,18], Cebpa knockdown did not alter cell cycle parameters in
transduced marrow cells. Also, reduced Cebpa led to increased
CFU replating but not to cytokine-independence.
C/EBPa expression or activity is reduced by multiple mechanisms
in acute myeloid leukemia (AML) [19]. Accumulation
Lin2
Sca-1+
c-kit+
stem cells in vitro and in vivo and extensive colony
replating upon Cebpa knockdown may represent a preleukemic
state, with additional mutations required for full, cytokineindependent
transformation. Human AMLs or murine marrow
completely lacking C/EBPa have increased C/EBPc [20];
however, Cebpg was not increased in Lin2
marrow or 32Dcl3
lines expressing B9 or B11 Cebpa shRNA (Figure S4). Of note,
RUNX1 is commonly mutated in AML, and Runx1 gene deletion in
adult mice reduces Cebpa mRNA and increases in vitro monopoiesis
while reducing granulopoiesis, similar to the effect of Cebpa
knockdown [12]. The present study indicates that Cebpa may be
the Runx1 target gene most critical for the impaired granulopoiesis
and increased myeloproliferation evident in the absence of Runx1.
In contrast to our findings with Cebpa knockdown, reduced PU.1
expression favors granulopoiesis over monopoiesis [21–23]. C/
EBPa activates the PU.1 promoter and 214 kb distal enhancer
[24,25], and herein Cebpa knockdown reduced PU.1 mRNA 1.6fold,
to about 60% of control levels, in lineage-negative marrow
cells. Yet, monopoiesis was favored by Cebpa shRNA transduction,
potentially reflecting the greater decrease in Cebpa RNA. A similar
explanation may account for our finding that Runx1 gene deletion
mimics Cebpa knockdown. Runx1 activates Cebpa gene expression
via binding sites in the Cebpa promoter and via a +37 kb enhancer
and activates PU.1 transcription via the 214 kb distal enhancer
[13,26]; however, Runx1 gene deletion impairs Cebpa RNA
expression to a greater extent than PU.1 in CMP and GMP [12].
Our results predict that granulocytic progenitors have either
increased C/EBPa expression or similar expression but increased
C/EBPa activity, due to protein modifications or interactions,
compared with monocytic progenitors. As a first step to evaluate
these possibilities we developed a FACS protocol sorting Lin2
Sca-
12
c-Kit+
cells based on GCSFR versus MCSFR expression that
allows enrichment for immature granulocytic versus monocytic
progenitors/precursors. Although these sorted populations were
not pure CFU-G or CFU-M, the increased Cebpa RNA seen in the
GR+
MR2
compared with the GR2
MR+
population suggests that
Cebpa RNA is elevated in CFU-G and immature precursors they
generate compared with CFU-M and their immediate progeny. Of
note, the GR2
MR+
population contains more CFU-GM than the
GR+
MR2
population, eliminating CFU-GM as the source of
increased Cebpa RNA in the GR+
MR2
subset. While future efforts
might uncover a means to obtain pure, CFU-G or CFU-M, our
approach is the first to reveal insight into changes in gene
expression between early granulocyte versus monocyte lineagecommitted
cells.
What pathways might direct increased Cebpa expression to
granulocyte progenitors? As discussed, Runx1 activates the Cebpa
promoter and +37 kb enhancer. Runx1 is elevated 1.6-fold in the
GR+
MR2
compared with the GR2
MR+
population. In addition,
Runx1 might be more active in the granulocytic lineage: G-CSF
activates the SHP2 tyrosine phosphatase more potently than MCSF,
SHP2 knockdown reduces Cebpa but not PU.1 to impair
granulopoiesis relative to monopoiesis [12,27], and SHP2 may
directly activate Runx1, at least in the megakaryocyte lineage [28].
Also of potential relevance, SHP2 also activates Src kinases in
hematopoietic cells [29], and Src-mediated tyrosine phosphorylation
of Runx1 increases its activity in 293T cells and favors
granulopoiesis in Runx1-deleted marrow cells (our unpublished
observations).
Elevated levels of C/EBPa might allow formation of C/
EBPa:C/EBPa homodimers capable of activating targets such as
Gfi1 and Cebpe required for granulopoiesis. In monocytic
progenitors, reduced levels of C/EBPa may instead form C/
EBPa:AP-1 heterodimers, mediated by their leucine zipper
domains. Of note, dimerization of C/EBPa with AP-1 proteins
stimulates murine marrow monopoiesis, and C/EBP:AP-1 hybrid
elements are enriched in promoter and enhancer regions of
multiple monocyte-specific genes [17,30,31]. M-CSF activates
ERK more potently than G-CSF, ERK inactivates C/EBPa via
serine phosphorylation, and chemical ERK inhibition favors
granulopoiesis [27,32]. Thus, not only Cebpa expression but also
C/EBPa activity might be regulated during myeloid lineage
specification, with inactive C/EBPa still capable of forming active
complexes with AP-1 proteins to help direct monopoiesis.
Consistent with prior studies showing that the murine
Cebpa(2/2) fetal liver has increased BFU-E and that exogenous
C/EBPa inhibits erythropoiesis in transduced human CD34+
marrow cells [33,34], Cebpa knockdown increased MEP, BFU-E,
and expression of erythroid-specific genes in lineage-negative
marrow progenitors. This effect may in part reflect inhibition of
PU.1 expression, as PU.1 directly binds GATA-1 to inhibit
erythropoiesis [9,10]. On the other hand, the modest reduction in
PU.1 RNA in response to Cebpa knockdown raises the possibility
that C/EBPa impedes erythropoiesis through additional mechanisms
as well.
Graded C/EBPa Levels Regulate Myelopoiesis
PLOS ONE | www.plosone.org 12 April 2014 | Volume 9 | Issue 4 | e95784
Global gene expression analysis of lineage-negative marrow
progenitors after vector or Cebpa shRNA transduction and culture
in myeloid cytokines confirmed reduction in Gfi1 and Cebpe, known
regulators of granulopoiesis and direct targets of C/EBPa [5,35],
and also demonstrated a decrease in Ets1 and Klf5. Strikingly
correlating with these findings, GR+
MR2
cells were not only
enriched for Cebpa but also for Gfi1, Cebpa, Ets1, and Klf5,
underscoring their potential importance as C/EBPa-regulated
targets relevant to granulopoiesis. Of note, exogenous C/EBPa
induces Klf5 in human CD34+
marrow cells, Klf5 is increased in
granulocytes compared with monocytes or erythroblasts, Klf5
knockdown impairs 32Dcl3 or NB4 granulopoiesis, and Klf5
deletion reduces murine myelopoiesis [34,36–38]. Interestingly,
Klf4 antagonizes Klf5 in non-hematopoietic lineages, exogenous
Klf4 directs marrow monopoiesis, and we find Klf4 RNA enriched
in the GR2
MR+
population [39].
Given substantial data, discussed above, indicated that increased
PU.1 favors monopoiesis over granulopoiesis, it is also
noteworthy that PU.1 levels were similar in the GR+
MR2
and
GR2
MR+
subsets. In contrast, Irf8 was markedly elevated in the
GR2
MR+
population. Irf8 forms a complex with PU.1 to direct
binding to composite Ets/IRF DNA elements, and Irf8 is required
for monopoiesis [40,41]. Thus, graded PU.1 activity, rather than
expression, in the two myeloid lineages might be achieved via
graded Irf8 expression.
Cebpa knockdown reduced Ikbke and increased Nfkbia, thereby
predicting impaired NF-kB activation, and indeed pathway
analysis uncovered suppression of several pro-inflammatory
pathways. Mice lacking NF-kB p50 have 3-fold reduced C/EBPa
RNA and protein, 2-fold reduced CFU-G, and mildly increased
CFU-M [42], suggesting that impairment of NF-kB activation
through changes in NF-kB p50, IkB kinase e, and IkBa expression
may contribute to the reduced granulopoiesis observed with Cebpa
knockdown.
Supporting Information
Figure S1 Myeloid colony morphologies. A) Typical
murine CFU-G, CFU-M, and CFU-GM. B) Typical human
CFU-G, CFU-M, and CFU-GM. Images were obtained at 40X
using a Nikon Eclipse Ti microscope. All six CFUs are shown at
the same scale.
(TIF)
Figure S2 Cebpa knockdown increases monocyte relative
to granulocyte formation in comparison with empty
(Vec) or non-targeting vector (NTV) lentiviral controls.
Transduced, puromycin selected marrow cells were placed in
liquid culture with IL-3, IL-6, and SCF. FACS analyses for Mac-1
and Gr-1 on D2 are shown. Arrows indicate immature, Mac-
12
Gr-12
cells.
(TIF)
Figure S3 Cebpa knockdown increases erythroid colonies.
A) Marrow cells transduced with the pLKO.1 vector or with
Cebpa shRNA B9 were selected in puromycin and then plated in
methylcellulose with EPO at 1.6E5 cells/ml, and BFU-E were
enumerated 7 days later (n = 3). B) Pooled CFU cells were
subjected to FACS analysis for Ter119.
(TIF)
Figure S4 High-level Cebpa knockdown does not reduce
Cebpg in murine marrow progenitors or in the 32Dcl3
cell line. A) RNA samples from replicate pLKO.1 (Vec) and B9
transductions were subjected to quantitative RT-PCR in triplicate
for Cebpg relative to b-actin RNA. Expression in pLKO.1 vector
samples was set to 1.0 on average in each experiment, and mean
Relative Expression values in B9 samples are shown. B) Similar
analysis was conducted for RNAs prepared from parental 32Dcl3
cells or the 32Dcl3 pLKO.1, B11-1, or B9-2 lines. Average
expression in the vector line was set to 1.0.
(TIF)
Table S1 RT-PCR primer pairs.
(PDF)
Table S2 Fold-change (FC) of indicated RNAs by Cebpa
B9 shRNA vs vector in lineage-negative marrow cells.
(PDF)
Acknowledgments
We thank Wayne Yu for assistance with microarray analysis.
Author Contributions
Conceived and designed the experiments: OM SH HG GG ADF.
Performed the experiments: OM SH HG GG. Analyzed the data: OM SH
HG GG ADF. Wrote the paper: ADF.
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