Title: Chemical Carcinogenesis
1Chemical Carcinogenesis
- David.bell_at_nottingham.ac.uk
- http//dmg.nott.ac.uk/teaching/
2Do chemicals cause cancer ?
- Epidemiology
- The study of illness in populations
- Correlate with influences on the population, eg
diet, work, geography - 1775
- The surgeon, Percival Potts, noted that coal
sweeps had a high incidence of cancer, scrotal
cancer. He hypothesised that the cancer was due
to their intimate and prolonged exposure to coal
dust.
3Latency
Cancer takes 10-50 years to develop in man
1900- Cigarette smoking an accepted habit
1940- rise in male lung cancer- alarm
1960- smoking is shown to be the cause of lung
cancer
1960-2000. People who started smoking before
1960 die of lung cancer Lung cancer is the most
frequent site of cancer in UK men
4Causes of cancer
Note that diet, tobacco, occupation, alcohol and
pollution are principally chemically-induced
cancers.
5Screening for carcinogens
- Animal tests
- 50 animals per group
- Maximal tolerated dose (MTD), and 25
- Cost 250 000
- Time 2 years (rat or mouse)
6Problems with bioassay
- The US National Toxicology Programme has tested gt
300 compounds, with selection based solely on
environmental relevance - Approx 50-66 of all chemicals cause cancer
7Nitrosamines
- Cause cancer in mouse, rat, monkey, guinea-pig,
rabbit, fowl, newt, trout - Wide range of organs affected
- Formed in food in the presence of nitrate, and
acid, ie pickles - Potent
R1
N
NO
R2
8Nitrosamines
O-H
CH2
CH3
Oxidation
N
NO
N
NO
CH3
CH3
The methyl carbonium ion is a powerful
electrophile, and reacts quickly with
macromolecules.
H2-CO
CH3-NN-OH
N2
CH3-N2
CH3
OH-
9Electrophiles
- Chemical entities which react with centres having
a surplus of electrons, or nucleo-philes. - Protein, RNA and DNA contain nucleophilic sites
10Electrophiles and DNA 2
H
- There are multiple reactions of electrophiles
with DNA - O6 methyl guanine is promutagenic
- The alkylated base mispairs G-T, instead of G C
- This leads to inaccurate repair, and mutations in
DNA - N7-G also promutagenic
N7
O
N
N
N
N
H2N
Guanine
dR
11Genotoxicity
- Toxicity to the genome- hence genotoxicity
12Benzo(a)pyrene
1
Bay region
2
10
3
9
4
8
5
7
6
K region
High electron density at Bay and K regions Planar
molecule
13Benzo(a)pyrene
- Typical PAH
- Ubiquitous
- Coal tar, organic matter
- Formed during combustion of organics
- Cooking !
- Oils, especially used at high temperature
- Chemically inert
- Principal active agent in coal tar
14B(a)P metabolism
Multiple sites of metabolism combination of
metabolism at multiple sites. Highly
complex metabolic pathways.
2
10
3
9
4
8
5
7
6
Hydroxylation- reactive phenols and quinones
15The ultimate carcinogen
Cytochrome P450
O
B(a)P 7,8 epoxide
Epoxide hydrolase
P450
O
OH
OH
OH
OH
((anti))B(a)P 7,8 dihydrodiol 9,10 epoxide
B(a)P 7,8 dihydrodiol
16Is BPDE important ?
- Analysis of B(a)P- DNA adducts
- BPDE should be more carcinogenic than BP
- Dose mice at birth and autopsy at 7 months. Lung
tumours/mouse are shown.
17Adducts from B(a)P
- Adducts form on C10- N2 of dG using the BPDE on
DNA in vitro - The same adducts are formed in vivo on DNA
- Mutations in the ras oncogene with BaP are
typified by G-T transitions at codon 12 or 13
18DNA repair
- Methylnitrosourea (MNU) is a direct alkylating
agent - In rats, it has tissue-specificity, with
brain-specific tumorigenesis - DNA-adduct levels are similar in brain in liver
- Adducts are removed rapidly in liver, but persist
in brain
19DNA damage ? cancer
DNA damage is similar between the two
strains Tumour development is markedly
different Therefore other factors control the
development of cancer
20Do adducts cause cancer ?
- Chemically synthesise DNA-adducts, insert into
plasmid and transform into E. coli - Measure amount of mutations in prokaryotic DNA
- Treat a cellular proto-oncogene with carcinogen
in vitro - Transform DNA into cells, and look for cellular
transformation
21How many adducts for cancer ?
- For a 50 incidence of tumours in rat liver
- Estimate from 50 to 2000 adducts/ 108 nucleotides
(2 per 100 000) - Variation
- Measurement
- Repair
- Different compounds and adducts
22The numbering of the beast
4
6
3
5
7
8
2
10
9
1
2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD) One of
the most potent of a variety of related
compounds Long half-life in humans (6 years)
23TCDD and cancer
- Tumorigenic in rat, mouse, guinea pig and fish
(everything tested) - Primate study aborted when total dose of 2 mg/kg
lead to death - In rat, the lowest carcinogenic dose was 1.4
ng/kg/day (Plt0.05) for thyroid tumours - MTD 71ng/kg/day
24A prototypical non-genotoxin
- TCDD is a potent carcinogen
- Very weak/ no mutagenicity in Ames test
- No activity in tests of initiation activity
- Maximum estimate for binding of TCDD to rat liver
DNA 1 molecule per 1011 bases - 1 per 107- 105 for known carcinogens
- TCDD is a non-genotoxic carcinogen
25Chemical carcinogenesis
- Chemicals are involved in 60-80 of human cancers
- Reaction of chemical with DNA
- Some chemicals require metabolic activation
- Non-genotoxic chemicals
- Activation of cellular receptors