Chemical carcinogenesis and classification of agents
There are thousands of potentially hazardous compounds worldwide, but the role of only about twenty of them has been proven in humans. Striking examples include occupational tumors: contact with aniline dyes causes bladder cancer, working with asbestos leads to lung cancer, and polyvinyl chloride production is associated with liver cancer.
Chemical carcinogens are divided into two large groups based on their mechanism of interaction with the cell:
- Genotoxic. They damage DNA directly or following metabolic activation. These include polycyclic aromatic hydrocarbons, aromatic amines, and nitroso compounds.
- Non-genotoxic (epigenetic). They do not yield positive results in mutagenicity tests, but their introduction into the organism induces tumor development (e.g., organochlorine compounds and immunosuppressants).
Genotoxic agents can be direct (interacting with DNA directly without requiring modifications) and indirect (procarcinogens). The latter require metabolic activation involving the monooxygenase system (cytochrome P450), epoxide hydrolases, and transferases. This process is most active in the liver, as well as in the epithelium of the bronchi, kidneys, and gastrointestinal tract. As a result, active metabolites are formed, which cause alkylation of DNA bases (adenine, guanine, etc.) and point mutations.
Stages of chemically induced tumor development
The process does not occur instantaneously and involves sequential stages, each requiring its own factors:
- Initiation. The carcinogen interacts with the genome, causing an irreversible alteration. However, initiation alone is insufficient for malignant transformation.
- Promotion. An additional agent comes into play — a promoter (often an epigenetic carcinogen or a substance that is not a carcinogen by itself). It provokes further genetic lesions, after which the cell undergoes malignant transformation and begins uncontrolled division.
It is important to remember that the effect of carcinogens depends on the dose and duration of exposure, with no safe threshold, and the effects of different substances can be cumulative.
Endogenous chemical carcinogens
Not all hazardous substances come from the outside (exogenously). Some are formed inside the body against the background of chronic hypoxia or prolonged diseases. Internal carcinogens include:
- Bile acids and cholesterol;
- Tryptophan;
- Lipid peroxides;
- Certain steroid hormones.
Physical carcinogenesis (Radiation)
Physical factors also exert their effect through DNA damage. Three main groups are distinguished:
- Ultraviolet (UV) radiation. The carcinogenic effect of solar radiation has been proven, which is the cause of melanoma development. Residents of near-equatorial regions are at the highest risk, especially fair-skinned individuals. The pigment melanin performs a protective function here, shielding cells from the mutagenic effects of UV rays.
- Ionizing radiation. The problem became particularly acute in the second half of the 20th century due to atomic bombings, nuclear tests, nuclear power plant accidents, and medical X-rays.
- Radioactive substances.
Classic examples of radiation carcinogenesis include occupational diseases: lung cancer in uranium mine workers (due to inhalation of radioactive radon gas) and bone sarcoma in workers who applied luminous paints to watch dials (accumulation of radioactive particles in bone tissue). Large-scale radiation disasters, such as the Chornobyl disaster, lead to a sharp increase in cancer incidence, primarily leukemias and solid tumors.
Briefly: answers to topical questions
What is the molecular mechanism of action of non-genotoxic (epigenetic) carcinogens?
The molecular mechanism of action of non-genotoxic (epigenetic) carcinogens consists of altering gene expression without changing the DNA sequence. These substances do not yield positive results in mutagenicity tests, but their administration into the organism induces tumor development. The main epigenetic mechanisms include:
- DNA methylation — a stable epigenetic intervention that is consolidated in most cases.
- Chemical modification of histone proteins — includes acetylation and phosphorylation. Acetylation weakens the bond between DNA and histones, making chromatin accessible to transcription factors, whereas deacetylation represses transcription.
Through what pathogenetic mechanisms does ionizing radiation cause DNA damage?
Ionizing radiation causes DNA damage through direct impact on biological molecules and the ionization of atoms. The main pathogenetic mechanisms include:
- Ionization of atoms and molecules — an electron is knocked out under the action of a particle stream or high-energy quanta, resulting in the formation of a free electron and a positively charged ion.
- Direct action — radiation exerts a direct influence on biological molecules, with DNA being the primary target.
These processes lead to damage to the cellular genomic DNA, which forms the basis of physical carcinogenesis.
What historical theories of tumor origin exist in oncomorphology?
In oncomorphology, there are several main historical theories of tumor origin explaining the etiology of carcinogenesis. These include:
- Theory of chemical carcinogens — links cancer development to chemical agents.
- Theory of physical carcinogens — explains the occurrence of tumors by the influence of radiation.
- Viral-genetic theory — postulates the need for integration of the virus genome and the cell genome (includes L.A. Zilber's viral-genetic theory).
- Infectious theory — views infectious agents as the cause of tumors.
- J. Cohnheim's dysontogenetic theory — the theory of embryonic rests.
- Polyetiological theory — unifies all existing theories, asserting that various factors may participate in tumor development.
To remember the stages of chemical carcinogenesis, use the abbreviation I-P-P: Initiation (irreversible DNA lesion), Promotion (malignant transformation under the action of a promoter), Progression (further tumor development).
Class and exam questions
Is there a safe dose of a chemical carcinogen?
No, chemical carcinogens are characterized by the absence of a threshold dose. Any amount of the substance, even the smallest, carries a risk of mutations.
Why did thyroid cancer incidence spike among children after the Chernobyl accident?
Radioactive iodine is formed during nuclear chain reactions. Thyroid tissue selectively accumulates it, leading to local irradiation and damage to cellular DNA.
What is cocarcinogenesis?
This is the summation of the effects of various factors. For example, radiation may not only damage DNA independently but also activate latent oncogenic viruses (as proven by the example of the mouse myeloid leukemia virus).
Can the body produce its own carcinogens?
Yes, endogenous carcinogens exist. These include certain bile acids, cholesterol, lipid peroxides, tryptophan, and a number of steroid hormones. They accumulate during chronic diseases and hypoxic states.
What else to study on this topic
- Viral-genetic and infectious theories of carcinogenesis
- J. Cohnheim's dysontogenetic theory (theory of embryonic rests)
- Mechanisms of DNA base alkylation
- The role of cytochrome P450 in the metabolic activation of procarcinogens
- Occupational tumors: pathogenesis of lesions caused by asbestos, vinyl chloride, and aniline dyes
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