Nongenotoxic carcinogenesis

1
Non-genotoxic carcinogenesis

• Chemicals, Risk Cancer 8
• David R. Bell

2
Rodent Bioassay Bias

• 60 of chemicals are carcinogenic
• Liver Bias
• Mouse liver tumours are 45-60 of mouse
  carcinogens
• Rat liver tumours are 20-30 of rat carcinogens
• Chemicals which induce mouse liver tumours only
  are 30 Ames positive
• Non-carcinogens are 30 Ames

3
Thyroid

• Agents which cause functional thyroid
  insufficiency
• Hepatic inducing agents
• Induction of UDPGT and metabolism of T4
• Directly thyrotoxic compounds (thioureas)
• Iodine radioisotopes

4
Feedback control of thyroxine

• Thyroid produces thyroxine (T4)
• The level of T4 is under feedback control from
  the hypothalamus
• Thyroid Stimulating Hormone
• Causes the thyroid to grow
• More T4

5
Thyroid tumours

• Require continued thyroid insufficiency for
  tumour growth
• Removal of TSH causes regression of tumours which
  have progressed to frank carcinoma
• Characteristic of a tumour promoter

6
Rodent thyroid carcinogens

• e.g. thioureas, barbiturates, peroxisome
  proliferators, dioxin
• Requires continued presence of agent reversible
• The mechanism postulates a threshold
• No human cancer risk below a certain amount

7
Liver

• Four types of P450 inducing agent
• Dioxin- AhR- CYP1A
• Barbiturates- CAR- CYP2B
• Steroids- PXR- CYP3A
• Peroxisome proliferators- PPAR- CYP4A
• Members of the steroid hormone receptor
  superfamily

8
Steroid Hormone Receptors
Ligand
RXR
SHR
AGGTCA(N)xAGGTCA
hsp90
Bind to DNA Transcription
Nuclear translocation is variable
9
CAR

• Prototypical inducers
• Phenobarbital, TCBOPOP (mouse only)
• Effective at mM concentration
• Hepatic enlargement and hyperplasia
• Strong promoting ability
• Carcinogens
• CARko.pdf

10
PXR

• Prototypical inducers
• Pregnenolone 16a-carbonitrile, steroids
• Frequently mixed agonists
• Carcinogens
• Liver growth agents
• SXR.pdf, chickCXR.pdf

11
PPARa

• Peroxisome Proliferator Activated Receptor a
• Agrochemicals, e.g. fomesafem
• Pharmaceuticals, e.g. clofibrate, aspirin
• Industrial intermediates, e.g. trichloroethylene,
  trichloroacetic acid
• Fatty acids, especially long chain
• Diabetes, starvation
• Widespread human exposure to these agents

12
Non-genotoxic carcinogens

• Peroxisome proliferators cause liver cancer, but
  do not react with DNA
• Do not cause mutations in bacteria (Ames test)
• Do not cause unscheduled DNA synthesis in liver
  cells (i.e. DNA repair)
• Do not cause DNA adducts

13
Peroxisome proliferation

• Liver growth
• hypertrophy
• hyperplasia
• Induction of liver enzymes
• peroxisomal enzymes (peroxisome proliferation)
• P450 - the CYP4 genes
• Proliferation of the Endoplasmic Reticulum and
  peroxisomes
• Hypolipidaemia

14
Peroxisomes

• Peroxisomes 0.5?M diameter, contribute to 20
  cytoplasmic volume.
• Enzymes for oxidation of fatty acids
• Oxidation produces H2O2
• Increase in peroxisomal number and volume

15
Oxidative Theory
Fatty acids
increase in peroxisomal B-oxidation without
catalase (lt2 fold)
Increase H2O2
genotoxic DNA damage
Cancer
16
Problems with H2O2 theory

• No evidence for large amounts of DNA damage by
  peroxisome proliferator treatment
• Two fold increase in DNA-oxygen adducts
• No increase in unscheduled DNA synthesis

17
DNA synthesis

• Potent and weak peroxisome proliferator
  carcinogens cause equivalent levels of peroxisome
  proliferation and presumably H2O2.
• Potent carcinogens cause higher levels of chronic
  DNA synthesis
• Potent carcinogens prevent cell death

18
Promotion

• Peroxisome proliferators promote pre-neoplastic
  cells by
• increased fixation of mutations
• decreased time for DNA repair
• decreased cell death
• Older animals treated with PPs get more tumours
  than younger animals.

19
Species Differences

• Mouse and rat highly responsive to PPs
• Marmoset does not respond
• Guinea Pig does not undergo peroxisome
  proliferation, but undergo hypolipidaemia
• Humans are believed to be unresponsive, but
  undergo hypolipidaemia.

20
Species Differences

• PPARa exists in mouse, rat, guinea pig and human
• Low hepatic levels in human and g-pig
• If PPARs cause cancer in rats, do they cause
  cancer in humans ?
• Guinea Pig offers a model system for
  understanding induction of hypolipidaemia and
  responsiveness in humans

21
Human risk assessment
Chemical
Peroxisome proliferation
DNA synthesis/ Suppression of apoptosis
Oxidative stress
Cancer
22
Human Risk Assessment

• Human liver
• No peroxisome proliferation
• No induction of liver growth
• Therefore, no risk of cancer
• Why do we see PPARa-mediated hypolipidaemia in
  humans and guinea pig ?