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Carcinogen

Carcinogen

A carcinogen is a substance that causes a normal cell to change into a cancerous cell, resulting in uncontrolled cell growth. Cancer cells can multiply unchecked, forming a mass of tissue called a tumor. Sometimes cancerous cells "break off" from a tumor, traveling through the body and multiplying in other organs and tissues.

How carcinogens cause cancer

Carcinogens cause cancer by producing changes (or mutations) in the genetic material, or DNA, of a cell. These mutations result in uncontrolled cell division. A cancer-causing substance can alter the DNA of a cell directly or it can react with other chemicals in the body to form substances that cause gene mutations. (Genes are sections of DNA that serve as units of hereditary information.) A transformed cell may continue to function normally and not begin to multiply and develop into a tumor until many months or years later. It is not unusual for cancer to appear 20 to 25 years after initial exposure to a carcinogen.

Types of carcinogens

Carcinogens include both naturally occurring and artificially produced chemicals, ultraviolet light, and radioactive substances such as radon (a radioactive gas that is present in rock).

Words to Know

Aflatoxin: A carcinogenic poison produced by a mold that grows on peanuts and grains.

Cancer: A disease of uncontrolled cell growth.

DNA (deoxyribonucleic acid): The genetic material in the nucleus of cells that contains information for an organism's development.

Mutation: A change in the genetic material of a cell.

Radiation: Energy that is sent out as waves or particles.

Tumor: A mass of tissue formed by abnormal cell growth.

About 23 chemicals have been identified as carcinogens in humans, with many more shown to cause cancer in laboratory animals. Tobacco smoke contains several carcinogenic substances and is the major cause of lung cancer. Some drugs used in the treatment of cancer are themselves cancer-causing. In addition, various chemicals used in industrial processes, such as vinyl chloride and certain dyes, are known human carcinogens. Nitriteschemicals added to processed meats such as bacon, sausage, and bologna to prevent spoilagereact with substances in the digestive tract to form carcinogenic chemicals called nitrosamines. Even synthetic dyes added to food for coloring are potential mutagens (substances that cause mutations in the genetic material of cells).

Sunlight is a well-known carcinogen that can cause changes in skin cells that may lead to skin cancer. Radiation emitted from an atomic bomb or released in nuclear power accidents can result in cancer in people exposed to it. Repeated exposure to radiation from medical X rays or other sources also may increase a person's risk of developing genetic mutations.

Some foods, such as celery, black pepper, white mushrooms, and mustard contain naturally occurring carcinogens. Aflatoxin is a cancer-causing chemical produced by molds on peanuts. However, these foods must be consumed in large quantities over a long period to initiate cancer.

Ames Test

The Ames test is a quick method of determining if a substance is capable of producing mutations. A culture of a strain of Salmonella bacteria that lacks an enzyme needed for growth is exposed to possible carcinogens. If the substance added to the culture is carcinogenic, it will cause mutations in the bacteria that allow the bacteria to grow. The Ames test has positively identified many carcinogens. It is used by cosmetic companies, drug manufacturers, and other industries that must prove that their products will not cause cancer in humans.

[See also Cancer; Cigarette smoke; Mutation; Nucleic acid; Radiation exposure; Virus ]

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Carcinogen

CARCINOGEN

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.

Gail Charnley

(see also: Ames Test; Cancer; Carcinogen Assessment Groups; Carcinogenesis; Toxicology )

Bibliography

Weinberg, R. A. (1996). Racing to the Beginning of the Road: The Search for the Origin of Cancer. New York: W. H. Freeman.

Zuddon, R. W. (1987). Cancer Biology. New York: Oxford University Press.

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Carcinogen

Carcinogen


A carcinogen is a substance that can cause cancer in humans or animals. Carcinogens bring about molecular and biochemical disturbances in cells, resulting in dedifferentiation (the loss of cells' morphological and functional specializations, such that they behave like immature cells capable of resuming cell division) and uncontrolled growth (neoplasia).

Some common substances that are known to be carcinogenic are asbestos, pesticides, lead, cadmium, arsenic , benzene, polyvinyl chlorides (PVC), soot, crystallized silica, glass wool (often a component of fiberglass), tobacco smoke, and smokeless tobacco. Most of these chemical carcinogens are called "procarcinogens," requiring metabolic conversion into "ultimate carcinogens" capable of damaging deoxyribonucleic acid (DNA , the genetic material in cells). Ultraviolet radiation (UVA, UVB, and UVC) from the Sun is also carcinogenic and can lead to different types of skin cancer.

For a substance to be declared a carcinogen there must exist sufficient evidence of a relationship between exposure to that substance and cancer in humans or animals. Because the use of human test subjects is deemed unethical, testing is done on animals (e.g., mice and rats) and on animal and human cell cultures (specialized nutrient-rich growth media); the data are then extrapolated to humans. An agent's potential carcinogenicity in humans is also determined from what is known of that agent's effect at the molecular level (i.e., damage to DNA/protein) or from anecdotal evidence(e.g., UV exposure and cancer).

The potency of a carcinogen is expressed as the dose rate that, when administered chronically throughout the standard life span of a test species, will reduce the probability of the population remaining tumorless for that period by 50 percent. Called the TD50 value, the rate is measured in milligrams per kilogram [mg/kg] body weight per day. Hence, the lower the TD50 value, the more potent the carcinogen. Caution is necessary in extrapolating TD50 values from animal systems to humans, however, because the pathway through which that substance becomes carcinogenic may be absent in humans.

see also Mutagen; Teratogen.

Hiranya S. Roychowdhury

Bibliography

Pohanish, Richard P., and Sittig, Marshall, eds. (2002). Handbook of Toxic and Hazardous Chemicals and Carcinogens. Norwich, NY: William Andrew Inc.

Internet Resources

"National Toxicological Program Report on Carcinogens." National Toxicology Program. Updated December 11, 2002. Available at <http://ntp-server.niehs.nih.gov/NewHomeRoC/AboutRoC.html>.

"Safety and Health Topics: Carcinogens." U.S. Department of Labor, Occupational Safety & Health Administration. Available at <http://www.osha.gov/SLTC/carcinogens>.

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carcinogen

carcinogen External substance or agent that causes cancer, including chemicals, such as the tar present in cigarette smoke, large doses of radiation and some viruses, such as polyoma. Many different chemical carcinogens have been found to cause cancer in animals, but more research is needed to confirm their ability to cause cancer in humans. It is thought that tobacco smoke causes lung cancer because of carcinogenic hydrocarbons.

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carcinogen

car·cin·o·gen / kärˈsinəjən; ˈkärsənəˌjen/ • n. a substance capable of causing cancer in living tissue.

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carcinogen

carcinogen (kar-sin-ŏ-jin) n. any substance that, when exposed to living tissue, may cause the production of cancer.
carcinogenic (kar-sin-ŏ-jen-ik) adj.

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carcinogen

carcinogen Any agent that produces cancer, e.g. tobacco smoke, certain industrial chemicals, and ionizing radiation (such as X-rays and ultraviolet rays).

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carcinogen

carcinogen A substance that can induce cancer; carcinogenesis is the process of induction of cancer.

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carcinogen

carcinogen A substance that can cause cancer.

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carcinogen

carcinogen: see cancer.

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carcinogen

carcinogenabrasion, Australasian, equation, Eurasian, evasion, invasion, occasion, persuasion, pervasion, suasion, Vespasianadhesion, cohesion, Friesian, lesion •circumcision, collision, concision, decision, derision, division, elision, envision, excision, imprecision, incision, misprision, precisian, precision, provision, scission, vision •subdivision • television • Eurovision •LaserVision •corrosion, eclosion, erosion, explosion, implosion •allusion, collusion, conclusion, confusion, contusion, delusion, diffusion, effusion, exclusion, extrusion, fusion, illusion, inclusion, interfusion, intrusion, obtrusion, occlusion, preclusion, profusion, prolusion, protrusion, reclusion, seclusion, suffusion, transfusion •Monaghan • Belgian •Bajan, Cajun, contagion, TrajanGlaswegian, legion, Norwegian, region •irreligion, religion •Injun • Harijan • oxygen • antigen •sojourn • donjon • Georgian •theologian, Trojan •Rügen •bludgeon, curmudgeon, dudgeon, gudgeon, trudgen •dungeon • glycogen • halogen •collagen • Imogen • carcinogen •hallucinogen • androgen •oestrogen (US estrogen) •hydrogen • nitrogen •burgeon, sturgeon, surgeon

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