It is defined as any substance or radiation that promotes cancer formation or carcinogenesis.
Chemical carcinogens can be natural or synthetic, toxic or non-toxic. Many carcinogens are organic in nature, such as benzopyrene and viruses. An example of carcinogenic radiation is ultraviolet light.
How do carcinogens work?
Carcinogens prevent normal cell death ( apoptosis ), so cell division is not controlled. This results in a tumor. If the tumor develops the ability to spread or metastasize (become malignant), cancer occurs.
Some carcinogens damage DNA, however, if significant genetic damage occurs, usually a cell simply dies.
Carcinogens alter cell metabolism in other ways, causing affected cells to become less specialized and hide them from the immune system or prevent the immune system from killing them.
Everyone is exposed to carcinogens every day, however, not all exposures lead to cancer. The body uses various mechanisms to eliminate carcinogens or repair / eliminate damaged cells.
Cells recognize many carcinogens and attempt to render them harmless through biotransformation. Biotransformation increases the solubility of a carcinogen in water, which facilitates the washing of the body. However, biotransformation sometimes increases the carcinogenicity of a chemical.
DNA repair genes repair damaged DNA before it can replicate. Usually the mechanism works, but sometimes the damage is not fixed or too extensive for the system to repair.
Tumor suppressor genes ensure that cell growth and division behave normally.
If a carcinogen affects a proto-oncogene (a gene involved in normal cell growth), the change can allow cells to divide and live when they normally would not. Genetic changes or inherited predisposition play a role in carcinogenic activity.
Examples of carcinogens
Radionuclides are carcinogens, toxic or non-toxic, as they emit alpha, beta, gamma, or neutron radiation that can ionize tissues. Many types of radiation are carcinogenic, such as ultraviolet light (including sunlight), X-rays, and gamma rays.
Microwaves, radio waves, infrared light, and visible light are generally not considered carcinogenic because photons do not have enough energy to break chemical bonds.
However, there are documented cases of usually “safe” forms of radiation that are associated with an increased rate of cancer with prolonged high intensity exposure. Food and other materials that have been irradiated with electromagnetic radiation (eg, X-rays, gamma rays) are not carcinogenic.
Neutron irradiation, on the other hand, can make substances carcinogenic through secondary radiation.
Chemical carcinogens include carbon electrophiles, which attack DNA. Examples of carbon electrophiles are mustard gas, some alkenes, aflatoxins, and benzopyrene.
Cooking and processing food can produce carcinogens. Grilling or deep-frying foods, in particular, can produce carcinogens such as acrylamide (in french fries and potato chips) and polynuclear aromatic hydrocarbons (in grilled meats).
Some of the main carcinogens in cigarette smoke are benzene, nitrosamine, and polycyclic aromatic hydrocarbons. Many of these compounds are found in other types of smoke, too. Other important chemical carcinogens are formaldehyde, asbestos, and vinyl chloride.
Natural carcinogens include aflatoxins (found in grains and peanuts), hepatitis B and human papilloma viruses, Helicobacter pylori bacteria, and the liver parasites Clonorchis sinensis and Oposthorchis veverrini.
How are they classified?
There are many different classification systems for carcinogens, generally based on whether a substance is known to be a human carcinogen, a suspected carcinogen, or an animal carcinogen.
Some classification systems also allow a chemical to be labeled as unlikely to be a human carcinogen.
One system is the one used by the International Agency for Research on Cancer (IARC), which is part of the World Health Organization (WHO).
- Group 1: known human carcinogen, which can cause cancer under typical exposure circumstances.
- Group 2A: Probably a human carcinogen.
- Group 2B: Possibly a human carcinogen.
- Group 3: not classifiable.
- Group 4: Probably not a human carcinogen.
Carcinogens can be categorized based on the type of damage they cause. Genotoxins are carcinogens that bind to DNA, mutate it, or cause irreversible damage.
Examples of genotoxins include ultraviolet light, other ionizing radiation, some viruses, and chemicals such as N-nitroso-N-methylurea (NMU).
Nongenotoxins do not damage DNA, but they promote cell growth and / or prevent programmed cell death. Examples of non-genotoxic carcinogens are some hormones and other organic compounds.