A carcinogen is an agent that can cause cancer. Carcinogens can be chemicals, viruses, hormone, ionizing radiation, or solid materials. Carcinogens produce cancer by changing the information that cells receive from their DNA, causing immature cells to accumulate in the body rather than differentiate into normal functional cells. Carcinogens may be genotoxic, meaning that they interact physically with DNA to damage or change its structure. Ionizing radiation is a genotoxic carcinogen. Other carcinogens may change how DNA expresses its information without changing its structure directly, or may create a situation in a cell or tissue that makes it more susceptible to DNA damage from other sources. These are known as nongenotoxic carcinogens, or promoters. Arsenic and estrogen are nongenotoxic carcinogens. Still other carcinogens, such as nickel, may interfere with cell division, changing the number or structure of chromosomes in new cells after a cell divides.
Several changes in a cell's DNA are usually needed to transform a normal cell into a cancer cell. Such changes can accumulate over time, and can sometimes be repaired. Cells can also die before enough changes occur to turn them cancerous. The places that become altered in the DNA of cancer cells are called oncogenes and tumor suppressor genes. Oncogenes and tumor suppressor genes are particular locations on DNA that control a cell's ability to perform its biological functions and to control its growth.
Susceptibility to the action of carcinogens is very complex and is affected by genetic heritage, behavior, physiology, nutrition, external exposures, and other factors. For example, some chemicals are carcinogenic in their original form (direct carcinogens), while some must be metabolized in the body to their active form (indirect carcinogens). In such cases, individual susceptibility to a chemical carcinogen is affected by the rate at which the chemical metabolizes in the body into a cancer-causing form or into a harmless form. This rate varies from person to person.
Some carcinogens have been identified from studies of people exposed to various substances over time. These include cancer in cigarette smokers and leukemia in people breathing benzene in the workplace. Carcinogens have also been identified using laboratory animals exposed over time, usually to high doses. Saccharin was found to be a carcinogen through experiments to produce bladder cancer in rats, and aflatoxin was found to produce liver cancer in rats. Some substances that are carcinogens in laboratory animals, like saccharin, are not carcinogens in people because of differences in how they are metabolized or differences in how they produce cancer. The International Agency for Research on Cancer (part of the World Health Organization) and the U.S. National Toxicology Program publish documents listing chemicals and other exposures that they believe are known to be carcinogenic to humans and those that are suspected or likely to be carcinogens to humans.
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Zuddon, R. W. (1987). Cancer Biology. New York: Oxford University Press.