Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

CANCER PREVENTION AND TREATMENT: PRESENT AND FUTURE
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cytochrome P-450
enzymes
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DNA
(A)
AFLATOXIN
AFLATOXIN-2,3-EPOXIDE
CARCINOGEN BOUND TO GUANINE IN DNA
Figure 20–38 Some known carcinogens. (A) Carcinogen activation.
A metabolic transformation must activate many chemical carcinogens before
they will cause mutations by reacting with DNA. The compound illustrated
here is aflatoxin B1, a toxin from a mold (Aspergillus flavus oryzae) that grows
on grain and peanuts when they are stored under humid tropical conditions.
Aflatoxin is an important cause of liver cancer in the tropics. (B) Different
carcinogens cause different types of cancer. (B, data from Cancer and the
Environment: Gene Environment Interactions, National Academies Press, 2002.)
Most of the carcinogenic factors that are known to be significant are by no
means specific to the modern world. The most potent known carcinogen, by certain
assays at least, is aflatoxin B1 (see Figure 20–38). It is produced by fungi that
naturally contaminate foods such as tropical peanuts and is an important cause of
liver cancer in Africa and Asia.
Except for tobacco, chemical toxins and mutagens are of lesser importance
as contributory causes of cancer than other factors that are more a matter of personal
choice. One important factor is the quantity of food we eat: as mentioned
earlier, the risk of cancer is greatly increased in people who are obese. In fact, it is
estimated that as many as 50% of all cancers could be avoided by simple, identifiable
changes in lifestyle (see Figure 20–37B).
Viruses and Other Infections Contribute to a Significant Proportion
of Human Cancers
MBoC6 m20.22/20.38
Cancer in humans is not an infectious disease, and most human cancers do not
have any infectious cause. However, a small but significant proportion of human
cancers, perhaps 15% in the world as a whole, are thought to arise by mechanisms
that involve viruses, bacteria, or parasites. Evidence for their involvement comes
partly from the detection of viruses in cancer patients and partly from epidemiology.
Thus, cancer of the uterine cervix is associated with infection with a papillomavirus,
while liver cancer is very common in parts of the world (Africa and
Southeast Asia) where hepatitis-B viral infections are common. Chronic infection
deaths per 100,000 population
(A)
100
80
60
40
20
0
1930
FEMALES
colon and rectum
lung and bronchus
stomach
breast
pancreas
uterus
ovary
deaths per 100,000 population
100
0
1945 1960 1975 1990 2005 1930
(B)
80
60
40
20
MALES
colon and rectum
lung and bronchus
stomach
prostate
pancreas
leukemia
liver
1945 1960 1975 1990 2005
• VINYL CHLORIDE:
(B)
liver angiosarcoma
• BENZENE:
acute leukemias
• ARSENIC:
skin carcinomas, bladder cancer
• ASBESTOS:
mesothelioma
• RADIUM:
osteosarcoma
Figure 20–39 Age-adjusted cancer
death rates, United States, 1930–2008.
Selected death rates, adjusted to the
age distribution of the US population,
are plotted for (A) females and (B) males.
Note the dramatic rise in lung cancer
for both sexes, following the pattern of
tobacco smoking, and the fall in deaths
from stomach cancer, thought to be
related to a fall in rates of infection with
Helicobacter pylori. Recent reductions in
other cancer death rates may correspond
to improvements in detection and
treatment. Age-adjusted data like these are
needed to compensate for the inevitable
increase in cancer as people live longer,
on average. (Adapted from Cancer Facts
and Figures, 2012. Data from U.S. Mortality
Volumes 1930 to 1959, U.S. Mortality Data
1960 to 2008, National Center for Health
Statistics, Centers for Disease Control and
Prevention. © 2012, American Cancer
Society, Inc., Surveillance Research.)