Chapter 2 – Part II
Chemical- and Radiation-induced
Carcinogenesis
- 2.8 - 2.9 - 2.10 -
Mar 8 & 13, 2007
Causes of cancer?
Causes of cancer
Physical → radiation
Chemical → chemicals
Biological → microorganisms, especially viruses
2.8 Specific chemical agents can induce
cancer
[historical perspectives]
The evidence that chemicals can induce cancer
in humans has been accumulating for more than
two centuries.
Hill (1761) – Nasal cancers occurred in people
who used snuff excessively.
Pott (1775) – a high incidence of scrotal (陰囊)
skin cancer among men who had
spent their childhood as chimney
sweeps
von Volkman & Bell (~ 1875)
– Skin cancers occurred in workers whose skin
was in continuous contact with tar (焦油) and
paraffin oils (石蠟油), (now known as
polycyclic aromatic hydrocarbons, PAH)
Rehn (1895)
– development of urinary bladder cancer in
aniline (苯胺) dye workers
Similar observations were later made in many
laboratories and established a relationship between
heavy exposure to 2-naphthylamine, benzidine, or
4-amino-biphenyl and bladder cancer.
Yamagiwa & Ichikawa (1915)
induced skin carcinomas by the
repeated application of coal tar
to the ears of rabbits. – the 1st
animal experiment demonstrating
chemical carcinogenesis.
Yamagiwa 山極
Direct evidence for a carcinogenic effect
of the polycyclic aromatic hydrocarbons in
tars (1930s):
Kennaway & Hieger – synthetic 1,2,5,6dibenzanthracene was a
carcinogen.
Cook, Hewitt & Hieger – identification of the
carcinogen 3,4benzpyrene in coal tar
Since 1940s, the list of known carcinogenic
chemicals has been expanded tremendously.
Carcinogenic hydrocarbons
PAH
Chemical carcinogens: chemicals that cause tumor
formation
1. have a very broad range of structures with no
obvious unifying features
2. are genotoxic and can be classified into two
broad categories based on their action mechanisms:
a. Direct-acting carcinogens
- react with nitrogen and/or oxygen atoms in DNA
example: ethylmethane sulfonate (EMS)
b. Indirect-acting carcinogens
- become reactive after metabolic activation
examples: aflatoxin, benzo[a]pyrene
*genotoxic: an agent or process that interacts with cellular DNA,
resulting in alteration of DNA structure
The direct-acting carcinogens interact with
macromolecules through the covalent bond
formation between an electrophilic form of the
carcinogen and the nucleophilic sites in proteins
(e.g., S, O, and N atoms in cysteine, tyrosine, and
histidine, respectively) and nucleic acids (e.g., N
and O atoms in purine or pyrimidine), such as
N-methyl-N-nitrosourea, a chemically-reactive
alkylating agent.
Some agents can intercalate (嵌入) into the
DNA double helix by forming tight noncovalent
bonds (e.g., daunorubicin).
Most of carcinogens are indirectly-acting;
they do not interact in vitro with macromolecules until it has been incubated with
liver homogenates or liver microsomal
fractions. Thus, metabolic activation of
certain carcinogenic agents is necessary
to produce the “ultimate carcinogen” that
actually reacts with crucial molecules in
target cells.
Metabolic activation of benzo[a]pyrene
guanine
electrophilic
DNA
adducts
1. Cytochrome P450 catalyses initial epoxidation.
2. With the exception of the 1 - 2 and 2 - 3 oxides that convert to phenols, epoxide hydrolase may
catalyze the formation of dihydrodiols.
3. Benzo[a]pyrene-7, 8-dihydrodiol is further metabolized at the olefinic double bond by cytochrome
P450 to form a vicinal diol-epoxide (r7, t8-dihydroxy-c9, 10 epoxy-7,8,9,10tetrahydroxybenzo[a]pyrene).
4. The highly unstable arene ring opens spontaneously to form a carbocation.
5. This electrophilic species forms a covalent bond between the 10 position of the hydrocarbon and the
exocyclic amino group of deoxyguanosine.
Metabolic activation of aflatoxin
epoxide
(procarcinogen)
(ultimate carcinogen)
Aflatoxin B1, a toxin from a mold (Aspergillus flavus oryzae) that grows
on grain and peanuts when they are stored under humid tropical conditions.
It is thought to be a contributory cause of liver cancer in the tropics.
DNA adduct formation
Since most chemical carcinogens react with
DNA and are mutagenic, interactions with DNA
have been viewed as the most important reactions
of these agents.
The principal reaction products of the
nitrosamines and similar alkylating agents with
DNA are N7 and O6 guanine derivatives.
Reactions also occur with other DNA bases.
Examples of carcinogen- DNA adducts
N7
O6
deoxyguanosine
A. N-7 (benzo[a]pyren-6-yl)guanine
B. N-(deoxyguanosin-8-yl)-{acetyl}aminobiphenyl
C. 8,9-dihydro-8-(N5-formyl- 2 , 5 , 6 -triamino-4 -oxo-N5-pyrimidyl)-9-hydroxy-aflatoxin B1
D. O6-[4-Oxo-4(3-pyridyl)butyl]guanine, a mutagenic lesion formed by the metabolism of the
tobacco-specific nitrosamine, NNK
E. N-7-methyldeoxyguanosine
Potential biological consequences
of DNA-adduct formation
a. An insertion of the flat planar rings of
a polycyclic hydrocarbon between the
stacked bases of double-helical DNA
may distort the helix, leading to a frameshift mutation during DNA replication
past the point of the intercalation.
b. Alkylated bases in DNA can mispair with
the wrong base during DNA replication –
for example, O6 methyguanine pairs with
thymine instead of cytosine during DNA
replication, leading to a base transition
(i.e., GC→AT) type of mutation during
the next round of DNA replication.
c. Many of the base adducts formed by
carcinogens involve modifications of
N-3 or N-7 positions on purines that
induce an instability in the glycosidic
bond between the purine base and
deoxyribose. This destabilized structure
can then undergo cleavage by DNA
glycosylase, resulting in loss of the base.
Purines and Pyrimidines
d. Interaction with some carcinogens has
been shown to favor a conformational
transition of DNA from its usual doublehelical B form to a Z-DNA form. This
could alter the transcribability of certain
genes, since B→Z conformational
transitions are thought to be involved in
regulating chromatin structure.
Radiation-induced Carcinogenesis
Radiations contain energies greater than that
in chemical bonds. Therefore, chemical
bonds can be broken by radiation.
Energy release from various radiations:
Atomic particles > X-rays
> ultraviolet (UV) light
> visible light
Röntgen discovered x-rays in 1895. The
harmful effects of x-rays were observed soon
after their discovery. The first observed
effects were the acute ones, such as reddening
and blistering of the skin within hours or days
after exposure. By 1902, it became apparent
that cancer was one of the possible delayed
effects of x-ray exposure.
*Wilhelm Röntgen (1845 – 1923), a German physicist,
received the Nobel Prize in physics in 1901.
2.9 Both physical and chemical carcinogens
act as mutagens
In 1927, Muller discovered that the phenotypes
of Drosophila were changed by exposing the flies
to X-rays.
*Hermann Muller (1890 – 1967), an American geneticist,
received the Nobel Prize in Medicine & Physiology in 1946.
The ability of radiation to cause human
cancer, especially leukemia, was dramatically
shown by the increased rates of leukemia
among survivors of the atomic bombs dropped
in World War II, and more recently by the
increase in skin cancer in individuals exposed
to too much sunlight (UV radiation).
UV radiation is a low-energy emission
and does not penetrate deeply. Hence, the
skin absorbs most of the radiation and is
the primary carcinogenic target.
A number of the points made about
chemical carcinogenesis are also true for
radiation-induced carcinogenesis. Both
x-rays and ultraviolet (UV) radiation cause
DNA damage.
When cells are exposed to UV light in
the 240- to 300-nm range, nucleic acid
bases acquire excited energy states,
producing photochemical reactions
between DNA bases. The principal
products in DNA at biologically relevant
doses of UV light are cyclobutane dimers
formed between two adjacent pyrimidine
bases in the DNA chain. Both thyminethymine and thymine-cytosine dimers are
formed.
Cyclobutane dimer
Ames test
Bruce Ames (1975)
- measured the ability
of carcinogens to
mutate the bacteria
Chemicals that are potently mutagenic
are also powerful carcinogens
Figure 2.25 The Biology of Cancer (© Garland Science 2007)
Are all mutagens carcinogens?
Are all carcinogens mutagens?