1. Chemical Carcinogenesis: GENOTOXIC and NON-GENOTOXIC carcinogens

2. Classification of Carcinogens According to the Mode of Action
GENOTOXIC NON-GENOTOXIC

3. Classification of Carcinogens According to the Mode of Action
GENOTOXIC:
DNA-reactive or DNA-reactive metabolites
Direct interaction to alter chromosomal number/integrity
May be mutagenic or cytotoxic
Usually cause mutations in simple systems
DNA Adduct
Mutation
Cancer

4. Mechanism of Carcinogenesis: Genotoxic Carcinogens
1. Carcinogen activation
2. DNA binding
4. Gene mutation
3. Cell proliferation
(fix mutation)
CYP450s
Chemical
"Activated“
carcinogen
DNA Repair
APOPTOSIS
“inactivated“
carcinogen

5. Interaction of the exo-epoxide of aflatoxin B1 with DNA
Smela et al., Carcinogenesis 22: (2001)

6. Classification of Carcinogens According to the Mode of Action
NON-GENOTOXIC:
Do not directly cause DNA mutation
Mechanism of action is not completely understood
Difficult to detect - requires rodent carcinogen bioassay ?
Mutation
Cancer

7. Non-Genotoxic Carcinogens
Mitogens:
stimulation of proliferation
mutations may occur secondarily to cell proliferation
may cause preferential growth of preneoplastic cells
2) Cytotoxicants:
cytolethal
induce regenerative growth

8. Tissue Changes with Mitogenic and Cytotoxic Agents
Proliferation
Tissue
Cell Death
Proliferation
Cytotoxic Agent

9. Mechanism of Carcinogenesis: Non-Genotoxic carcinogens
Cell proliferation (to fix “spontaneous” mutation)
CANCER X
APOPTOSIS

10. Mechanisms of Non-Genotoxic Carcinogenesis (what’s in a “black box” ?)
Increased cell proliferation
Decreased apoptosis
Changes in gene expression
Induction of metabolizing enzymes
Activation of receptors (signaling)
Oxidative stress
???

11. Cell Replication is Essential for Multistage Carcinogenesis
Decreases time available for DNA repair
Converts repairable DNA damage into non-repairable mutations
Necessary for chromosomal aberrations, insertions, deletions and gene amplification
Clonally expands existing cell populations

12. Mitogenic Cytokines and Induction of Cell Proliferation
Complete Mitogens:
Epidermal Growth Factor, Tumor Necrosis Factor a, Hepatocyte Growth Factor, etc.
Co-Mitogens:
Insulin, glucagon, norepinephrin, estrogens
Growth Inhibitors:
Transforming Growth Factor b, InterLeukin 1b

13. Reasons That Not All Agents That Increase Cell Proliferation are Carcinogens
Quality of the data
Temporal association of the increase in cell proliferation
Selective cytotoxicity for initiated cells
Terminal differentiation of proliferating cells

14. Mutagenesis  Carcinogenesis Cell Proliferation  Carcinogenesis
Toxicity  Cell Proliferation

15. Apoptosis
Programmed Cell Death (Apoptosis): Active, orderly and cell-type-specific death distinguishable from necrotic cell death (passive process):
Induced in normal and cancer cells
Non-random event
Result of activation of a cascade of biochemical, gene expression and morphological events
tissue and cell specific
Growth factors and mitogens inhibit apoptosis

16. Alteration of Gene Expression
Nuclear (hormone-like) receptors
Kinase cascades
Calcium-, nitric oxide-mediated signaling
Transcription factors
Gene methylation status (hypo -> enhanced gene expression; hyper -> gene silencing)

17. Induction of Metabolizing Enzymes
May be a reason for tissue-, and/or species-selectivity of carcinogens
Metabolites may be ligands for receptors
Production of reactive oxygen species

18. Nature 407, (2000) ©
 The nuclear receptor CAR mediates specific xenobiotic induction of drug metabolism XB
CAR
ATCGGTTA……
CYP 2b10

19. Oxidative Stress Indirect DNA damage
Induction of cell proliferation/apoptosis signaling cascades

20. Peroxisome Proliferators
A wide range of classes of chemicals: lipid lowering drugs, plasticizers,
food flavors, industrial solvents, herbicides
Cause marked increases in size and number of peroxisomes
Potent rodent liver carcinogens
Human exposure is from therapeutic, environmental, industrial and other sources
No clear epidemiological evidence for or against carcinogenicity in humans

21. PEROXISOME H2O2 b-oxidation of fatty acids bile acid synthesis
purine and polyamine catabolism
amino acid catabolism
oxygen metabolism
Fatty Acid
Acyl-CoA
Enoyl-CoA
Hydroxyacyl-CoA
Ketoacyl-CoA
Acetyl-CoA Acyl-CoA shortened by two carbons
Fatty acyl-CoA synthetase
Acyl-CoA oxidase
Enoyl-CoA hydrolase
Hydroxyacyl-CoA
dehydrogenase
Thiolase
H2O2

22. 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

23. Adipocyte Differentiation Peroxisome Proliferation
Peroxisome Proliferator - Activated Receptors
Adipocyte Differentiation
Glucose Homeostasis
Macrophage Function
Peroxisome Proliferation
Lipid Homeostasis
Liver Carcinogenesis
Lipid Homeostasis
Skin proliferation

24. PPARa agonist-induced hepatocarcinogenesis mode of action
Peters& Gonzalez, J. Mol. Med., 2005

25. Klaunig et al., Crit. Rev. Tox., 2003

26. Klaunig et al., Crit. Rev. Tox., 2003

27. PPARa (+/+) + WY-14,643 (11 months) PPARa (-/-)
Peters et al., Carcinogenesis, 1997

28. Peroxisome Proliferators: Species Differences
Mouse and rat: highly responsive
Marmoset: does not respond
Guinea Pig: no peroxisome proliferation, but have hypolipidaemia
Humans: believed to be unresponsive, but have hypolipidaemia
PPARa exists in mouse, rat, guinea pig and human
In humans: Lower hepatic levels of PPARa
Lower ligand binding activity
Different structure (polymorphisms)
Different PP Response Elements in DNA
Presence of competing proteins for PPRE
Expression of dominant-negative form of PPARa

29. So, we have a chemical that is a non-genotoxic RODENT carcinogen!

30. If we would regulate this chemical, would it help to improve the quality of HUMAN life?

31. Proportion of chemicals evaluated as carcinogenic
Proportion
Percentage
Chemicals tested in both rats and mice
350/590
59%
Naturally occurring chemicals
79/139
57%
Synthetic chemicals
271/451
60%
Chemicals tested in rats and/or mice
Chem. in Carcinogen. Potency Database
702/1348
52%
Natural pesticides
37/71
Mold toxins
14/23
61%
Chemicals in roasted coffee
21/30
70%
Innes negative chemicals retested
17/34
50%
Physician’s desk reference PDR
Drugs with reported cancer tests
117/241
49%
FDA database of drug submissions
125/282
44%
Ames and Gold Mutat Res 447:3-13, 2000

32. What do we do now? Look at the “larger picture”
Probe human relevance of animal data
Continue research on the mechanisms
Change/improve current test used for detection of carcinogenicity