Cell communication & regulation:
a target for toxicants
Any sensitively regulated process is susceptible to toxicants
! REGULATIONS & SIGNALLING
Hierarchy
- systems: neuronal <----> endocrine
- cell-to-cell
hormonal & neuronal signal transmission
contact channels
- intracellular signal transduction
HORMONES - fate
1. Biosynthesis of a particular hormone in a particular tissue
2. Storage and secretion of the hormone
3. Transport of the hormone to the target cell(s)
4. Recognition of the hormone by an associated cell
membrane or intracellular receptor protein.
5. Relay and amplification of the received hormonal signal
via a signal transduction process -> cellular response.
6. The reaction of the target cells is recognized by the original
hormone-producing cells (negative feedback loop)
7. Degradation and metabolism of the hormone
Endocrine system:
1. Pineal gland, 2. Pituitary
gland, 3. Thyroid gland, 4.
Thymus, 5. Adrenal gland, 6.
Pancreas, 7. Ovary, 8.Testis
Example: feedback loop
HORMONES - actions and controls
* stimulation or inhibition of growth
* mood swings
* induction or suppression of apoptosis
(programmed cell death)
* activation or inhibition of the immune system
* regulation of metabolism
* preparation for fighting, fleeing, mating ...
* preparation for a new phase of life
(puberty, caring for offspring, and menopause)
* control of the reproductive cycle
TOXICITY TO HORMONAL ACTION
= ENDOCRINE DISRUPTION
ED & EDCs - major problem in environmental toxicology
- Effects at all levels of hormonal action
(synthesis, transport, action ....)
- Multiple effects (! Not only ,,xenoestrogenicity" & feminization)
(immunotoxicity, reproduction ...)
(WILL BE DISCUSSED FURTHER)
Intersex roach testis
containing both oocytes and spermatozoa,
caused by exposure to environmental oestrogens
HORMONES - chemicals (vertebrates)
* Amine-derived hormones are derivatives of the amino acids
tyrosine and tryptophan. Examples are catecholamines and thyroxine.
(small molecules - similar to organic toxicants - TOXIC EFFECTS)
Further:
* Peptide hormones
* Lipid and phospholipid-derived hormones
Adrenalin Thyroxin
HORMONES - chemicals (vertebrates)
* Peptide hormones chains of amino acids. - small: TRH and
vasopressin; proteins: insulin, growth hormone, luteinizing hormone,
follicle-stimulating hormone and thyroid-stimulating hormone).
Large molecules; receptors on surfaces of the cells
(Interactions with toxic chemicals less likely)
Example - insulin
HORMONES - chemicals (vertebrates)
Lipid derived hormones (1) (from linoleic acid, arachidonic acid)
- prostaglandins
HORMONES - chemicals (vertebrates)
Lipid derived hormones (2)
(small molecules - similar to organic toxicants - TOXIC EFFECTS)
- steroid hormones
(from cholesterol)
testosterone,
cortisol,
estradiol ...
Cell communication & regulation:
a target for toxicants
Cell communication (1)
Cell communication (2)
Cell communication (3)
Signal transduction - target of toxicants
- Regulation of cell life / death (apoptosis)
- metabolism
- proliferation
- differentiation
- death (apoptosis)
- Signalling
- "network" of general pathways
- similar in all cells / different cell-specific effects
Signalling disruption
- Consequences of signalling disruption
- unwanted changes in
proliferation / differentiation / apoptosis
-> cell transformation (carcinogenicity)
-> embryotoxicity
-> immunotoxicity
-> reproduction toxicity
.... other chronic types of toxicity
Signal transduction - principles
: major processes
­ protein-(de)phosphorylation (PKinases, PPases)
- secondary messengers (cAMP / IP3, PIP2, DAG, Ca2+, AA)
1: Membrane receptors (G-protein, kinases)
-> PKA activation: cAMP
2: Membrane receptors -> PLC / PKC activation
-> PKC activation: IP3, PIP2, DAG, Ca2+, AA
3: Cytoplasmic (nuclear) receptors
Membrane receptors (PKs):
G-proteins (GPCRs)
1: Membrane receptors (PKs)
-> Adenylate cyclase -> cAMP -> PKA ­ modulation
(!!!) Mitogen Activated Protein Kinases
(MAPK) ­ dependent effects
2: Membrane receptors
-> Phospholipase C:
PIPs -> DAG -> PKC / arachidonic acid
+ IP3 -> Ca2+
Signalling crosstalk
Examples
ER-independent estrogenicity (PAHs)
modulation of PKs/PPases: phosphorylation
-> activation of ER-dependent genes
PAHs significantly potentiate the effect
of 17-estradiol (via increased phosporylation of ER)
Vondráček et al. 2002 Toxicol Sci 70(2) 193
Cholera toxin - activation of adenylate cyclase
Lipopolysaccharide (bacteria) - immunotoxicity
Examples - other lectures
ER-dependent estrogenicity (DDE) [other lectures]
xenoestrogenicity, binding to ER + activation
AhR-dependent anti-estrogenicity, retinoid toxicity, modulation
of estrogen / retinoid levels [other lectures]
AhR -> CYPs -> steroid-metabolism
PAHs/POPs -> inhibition of Aromatase (CYP19)
Microcystins -> liver tumor promotion
inhibition of PPases [other lecture]
Gap junctions and cellular continuum
(Gap Junctional Intercellular Communication - GJIC)
Inhibition of GJIC - mechanism of tumor promotion
from Trosko and Ruch 1998,
Frontiers in Bioscience 3:d208
* gap-junctional intercellular communication (GJIC)
- transfer of small signalling molecules via protein
channels (gap junctions)
* regulation of proliferation, differentiation,
apoptosis
* inhibition of GJIC -> proliferation ~ tumor
promotion
* relevance: tumors in vivo have inhibited gap-
junctions
Control cells Inhibition of GJIC
GJIC
AhR
ER
Oxidative Stress
PAHs as tumor promoters
- inhibition of GJIC Bláha
et al. 2002 Toxicol Sci 65: 43
- Several PAHs inhibits GJIC
within 30 min exposure
(IC50 ~ 10-40 M)
- Low MW and bay/bay-like
regions promotes the effect
-Fluoranthene
:non-mutagenic
:non-AhR-inducing
:tumor promoter in vivo (!)
Toxicity to membrane gradients and
transport
- Semipermeability of membranes:
several key functions
- cytoplasmic membrane:
signalling, neural cells Na+/K+ gradient
- mitochondrial membrane:
electrone flow -> ATP synthesis
- endoplasmatic reticulum
Ca2+ signalling
- Membrane fusion / transport
neurotransmitter release
Membrane gradient
disruption
Ion transfer ("ionofors")
antibiotics
(K+, Ca2+, Mg2+)
Ion Channel BLOCKERS / ACTIVATORS
Neuromodulators (drugs) Neurotoxins (cyanobacterial)
Botulinum and Tetanus toxins
(Clostridium botulinum, Clostridium tetani)
Toxins = enzymes - proteases (!)
- cleavage of proteins involved in vesicle formation
- selective inhibition of neutrotransmitter release
neurotoxicity
Cytoskeleton as target of toxicants
microtubules / actin-myosin
Cytoskeleton ­ function
- intracellular transport
- cell replication and division (mitosis:chromosomes)
- muscle movement
- membrane (vesicles) fusion
TOXINS: effects on (DE)POLYMERIZATION
cytochalasin D
(fungal toxin)
Phalloidin
(death cap - Amanita phalloides)
TOXINS: effects on (DE)POLYMERIZATION
Colchicine
taxol