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Cell communication & regulation
- target of toxicants
Cell
communication
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
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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
1: Membrane receptors (PKs)
-> Adenylate cyclase -> cAMP -> PKA ­ modulation
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Mitogen Activated Protein Kinases
(MAPK) ­ dependent effects
2: Membrane receptors
-> Phospholipase C:
PIPs -> DAG -> PKC / arachidonic acid
+ IP3 -> Ca2+
Crosstalk
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Examples
ER-dependent estrogenicity (DDE) [other lecture]
xenoestrogenicity, binding to ER + activation
ER-independent estrogenicity (PAHs)
modulation of PKs/PPases: phosphorylation
-> activation of ER-dependent genes
AhR-dependent anti-estrogenicity, retinoid toxicity
modulation of estrogen / retinoid levels
[other lectures]
AhR -> CYPs -> steroid-metabolism
PAHs/POPs -> inhibition of Aromatase (CYP19)
PAHs significantly potentiate the effect
of 17-estradiol (via increased phosporylation of ER)
Vondráček et al. 2002 Toxicol Sci 70(2) 193
Examples
Microcystins -> liver tumor promotion
inhibition of PPases [other lecture]
Immunotoxicity
- (Cyano)bacterial lipopolysaccharides,
heavy metals ...
- Cholera toxin
- AC: cAMP -> effects
PAHs -> Inhibition of Gap-junctions
- Gap-junctional intercellular communication
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Inhibition of GJIC - biomarker of tumor promotion
from Trosko and Ruch 1998, Frontiers in Bioscience 3:d208
ˇ gap-junctional intercellular communication (GJIC)
- transfer of signalling molecules via protein
channels (gap junctions)
ˇ regulation of proliferation, differentiation,
apoptosis
ˇ inhibition of GJIC -> proliferation ~ tumor
promotion
ˇ relevance: tumors in vivo have inhibited gap-
junctions
Rat liver WB-F344 (normal stem-like cells)
Exposure (30 min)
standards, samples
- Wash
- Fluorescence dye (lucifer yellow)
- Scrape of the cells with the razor blade
Fluorescence microscopy
diffusion length ~ level of GJIC
Scrape loading / dye transfer assay (GJIC inhibition)
Control cells
Inhibition of GJIC
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
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Membrane gradient
disruption
Ion transfer ("ionofors")
antibiotics
(K+, Ca2+, Mg2+)
Ion Channel BLOCKERS / ACTIVATORS
Neuromodulators (drugs) Neurotoxins (cyanobacterial)
Botulotoxin, Tetanotoxin
- proteases (!)
- selective inhibition of neutrotransmitter release
(membrane vesicles)
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Cytoskeleton as target of toxicants
microtubules / actin-myosin
Cytoskeleton ­ function
- intracellular transport
- cell replication and division (mitotic poisons)
- muscle movement
- membrane (vesicles) fusion
TOXINS: effects on (DE)POLYMERIZATION
cytochalasin D
phalloidin
TOXINS: effects on (DE)POLYMERIZATION
colchicine
taxol