Download Non-Genotoxic carcinogens Cell proliferation

Chemical Carcinogenesis:
GENOTOXIC and NON-GENOTOXIC
carcinogens
Classification of Carcinogens According
to the Mode of Action
GENOTOXIC
NON-GENOTOXIC
Stages of Carcinogenesis
Initiation
Initiating
Event
Cell Proliferation
(clonal expansion)
ng
i
t
a
ut
M
n d nt
o
c
Se
Eve
Promotion
Cell Proliferation
g
n
i
t
uta
M
t
rd
Thi Even
Cell Proliferation
Progression
Malignancy
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
Mechanism of Carcinogenesis:
Genotoxic Carcinogens
1. Carcinogen activation
Chemical
“inactivated“
carcinogen
"Activated“
carcinogen
2. DNA binding
4. Gene mutation
3. Cell proliferation
(fix mutation)
DNA Repair
APOPTOSIS
Interaction of the exo-epoxide of
aflatoxin B1 with DNA
Smela et al., Carcinogenesis 22:535-45 (2001)
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
Non-Genotoxic Carcinogens
1) Mitogens:
•
•
•
stimulation of proliferation
mutations may occur secondarily to cell proliferation
may cause preferential growth of preneoplastic cells
2) Cytotoxicants:
•
•
•
cytolethal
induce regenerative growth
mutations may occur secondarily to cell proliferation
Tissue Changes with Mitogenic and
Cytotoxic Agents
Mitogenic
Agent
Proliferation
Tissue
Cell Death
Proliferation
Cytotoxic
Agent
Mechanism of Carcinogenesis:
Non-Genotoxic carcinogens
Cell proliferation (to fix “spontaneous” mutation)
CANCER
X
APOPTOSIS
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
 ???
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
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
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
Mutagenesis  Carcinogenesis
Cell Proliferation  Carcinogenesis
Toxicity  Cell Proliferation
Apoptosis
Programmed Cell Death (Apoptosis): Active, orderly and celltype-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
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)
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
Nature 407, 920 - 923 (2000) ©
The nuclear receptor CAR mediates specific
xenobiotic induction of drug metabolism
XB
CAR
ATCGGTTA……
CYP 2b10
Oxidative Stress

Indirect DNA damage

Induction of cell proliferation/apoptosis signaling
cascades
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
PEROXISOME
•
•
•
•
•
b-oxidation of fatty acids
bile acid synthesis
purine and polyamine catabolism
amino acid catabolism
oxygen metabolism
Fatty Acid
Fatty acyl-CoA
synthetase
Acyl-CoA
H2O2
Acyl-CoA oxidase
Enoyl-CoA
Enoyl-CoA hydrolase
Hydroxyacyl-CoA
Hydroxyacyl-CoA
dehydrogenase
Ketoacyl-CoA
Thiolase
Acetyl-CoA
Acyl-CoA shortened
by two carbons
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
Peroxisome Proliferator - Activated Receptors
FA
Fat Stores
Diet
FABP
FA Storage
Adipocyte Differentiation
Glucose Homeostasis
Macrophage Function
?
PPRE
Peroxisome Proliferation
Lipid Homeostasis
Liver Carcinogenesis
?
PPRE
PP
PG
LT
FA
FA
FA Metabolism
PPRE
Lipid Homeostasis
Skin proliferation
PPARa agonist-induced hepatocarcinogenesis mode of action
Peters& Gonzalez, J. Mol. Med., 2005
Klaunig et al., Crit. Rev. Tox., 2003
Klaunig et al., Crit. Rev. Tox., 2003
PPARa (+/+)
PPARa (-/-)
+ WY-14,643 (11 months)
+ WY-14,643 (11 months)
Peters et al., Carcinogenesis, 1997
Peroxisome Proliferators: Species Differences
•
•
•
•
Mouse and rat:
Marmoset:
Guinea Pig:
Humans:
highly responsive
does not respond
no peroxisome proliferation, but have hypolipidaemia
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
So, we have a chemical that
is a non-genotoxic RODENT
carcinogen!
If we would regulate this chemical,
would it help to improve the quality
of HUMAN life?
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%
702/1348
52%
Natural pesticides
37/71
52%
Mold toxins
14/23
61%
Chemicals in roasted coffee
21/30
70%
17/34
50%
117/241
49%
125/282
44%
Chemicals tested in rats and/or mice
Chem. in Carcinogen. Potency Database
Innes negative chemicals retested
Physician’s desk reference PDR
Drugs with reported cancer tests
FDA database of drug submissions
Ames and Gold Mutat Res 447:3-13, 2000
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