Chemical Carcinogenesis

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Chemical Carcinogenesis

• David.bell_at_nottingham.ac.uk
• http//dmg.nott.ac.uk/teaching/

2
Do 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.

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Latency
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
4
Causes of cancer
Note that diet, tobacco, occupation, alcohol and
pollution are principally chemically-induced
cancers.
5
Screening for carcinogens

• Animal tests
• 50 animals per group
• Maximal tolerated dose (MTD), and 25
• Cost 250 000
• Time 2 years (rat or mouse)

6
Problems 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

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Nitrosamines

• 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
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Nitrosamines
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-
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Electrophiles

• Chemical entities which react with centres having
  a surplus of electrons, or nucleo-philes.
• Protein, RNA and DNA contain nucleophilic sites

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Electrophiles 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
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Genotoxicity

• Toxicity to the genome- hence genotoxicity

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Benzo(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
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Benzo(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

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B(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
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The 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
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Is 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.

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Adducts 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

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DNA 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

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DNA damage ? cancer
DNA damage is similar between the two
strains Tumour development is markedly
different Therefore other factors control the
development of cancer
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Do 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

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How 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

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The 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)
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TCDD 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

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A 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

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Chemical 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