Tumor etiology and historical examples
The development of malignant neoplasms is always associated with damage to the cellular apparatus. Carcinogenic agents target the cell genome. As a result of this exposure, a fatal error occurs: the normal genetic life program is completely replaced by the pathological program of tumor formation. This process is called malignization.
According to their origin, all carcinogenic factors are divided into three main groups: chemical, physical, and biological (the latter include oncogenic viruses). At the same time, a major risk factor that significantly increases the body's susceptibility to carcinogens is various hereditary defects.
Many chemical substances are capable of triggering malignization under certain conditions. This category even includes certain medications. Historically, the detrimental effect of chemical compounds was noted by physicians long before the detailed study of DNA. Classic clinical examples include:
- Soot (coal combustion components): contact with it caused occupational scrotal cancer in chimney sweeps (described by Pott in 1775).
- Asbestos dust: upon inhalation, it specifically damages respiratory tract tissues, causing pleural mesothelioma.
- Tobacco combustion products: are a proven cause of lung cancer.
Classification by chemical structure
Chemical carcinogens are traditionally divided into two large groups: organic and inorganic compounds.
1. Organic compounds This is the most extensive group, which includes several classes of substances with completely different mechanisms of action:
- Polycyclic aromatic hydrocarbons (PAHs): have a pronounced local effect, meaning they cause tumor development directly at the site of tissue exposure. Maximum carcinogenic activity is exhibited by 3,4-benzopyrene, as well as 20-methylcholanthrene and dimethylbenzanthracene. The main sources of PAHs are tobacco smoke, automobile exhaust, and smoked foods.
- Aromatic amines and amides: unlike PAHs, these are carcinogens with a resorptive effect. They begin to act only after absorption into the bloodstream, causing tumors remotely from the initial site of penetration. Prominent representatives: 2-naphthylamine, benzidine, 2-aminofluorene. An important feature: 2-naphthylamine, benzidine, and 2-aminothiophenyl exhibit strict specificity and cause urinary bladder cancer.
- Nitroso compounds: include substances most hazardous to humans (diethylnitrosamine, dimethylnitrosamine, and nitrosomethylurea).
- Aminoazo compounds: possess potent carcinogenic properties (4-dimethylaminoazobenzene, ortho-aminoazotoluene).
- Aflatoxins: chemically derived from coumarins. These are toxic metabolic products of molds, predominantly of the Aspergillus flavus species.
- Heterocyclic aromatic hydrocarbons: represented by dibenzacridine and dibenzcarbazole.
- Other organic substances: epoxides, various plastics, urethane, carbon tetrachloride, and chloroethylamines.
2. Inorganic compounds Inorganic chemical carcinogens include heavy metals and their derivatives: arsenic, beryllium oxide, chromates, and cobalt.
Classification by origin
Depending on where carcinogenic substances originate in the body, they are classified into two groups: exogenous and endogenous.
Exogenous chemical carcinogens These are substances that enter the body exclusively from the external environment. This group includes asbestos, as well as inorganic compounds: chromates, arsenic and its compounds, cobalt, and beryllium oxide.
Endogenous carcinogens These compounds do not enter from the outside, but are formed directly within the body itself. The process of their formation is associated with the physicochemical modification of normal metabolic products. Potential endogenous carcinogens include:
- Hormones (specifically, estrogens).
- Bile acids.
- Certain amino acids (tyrosine, tryptophan).
- Lipoperoxide compounds.
Briefly: answers to topical questions
What is the difference between direct chemical carcinogens and indirect ones (procarcinogens)?
The main difference lies in the need for metabolic activation in order to acquire the ability to interact with genetic material.
| Characteristic | Direct carcinogens | Indirect carcinogens (procarcinogens) |
|---|---|---|
| Interaction with DNA | Interact with DNA directly | Interact after chemical transformations |
| Metabolic activation | Do not require metabolic activation | Require activation by enzyme systems (monooxygenase, epoxide hydrolases, transferases) |
What is the molecular mechanism of interaction between chemical carcinogens and the DNA molecule?
The molecular mechanism consists of the alkylation of DNA nitrogenous bases by chemical substances or their activated metabolites, leading to covalent cross-links and mutations.
- DNA alkylation — attachment of an alkyl group of chemical substances to nucleotides (most commonly to the N7 position of the guanine purine ring).
- Impaired complementarity — the ionization of guanine changes, causing thymine to pair with it instead of cytosine during replication.
- Point mutations — as a result of replication errors in the next cell generation, the normal G—C pair is replaced by A—T.
Additionally, chemical substances can cause covalent cross-links between DNA strands or between DNA and proteins.
To remember the specificity of aromatic amines (benzidine, 2-naphthylamine), use the mnemonic: "Amines were absorbed and flowed down" — they have a resorptive effect and specifically cause urinary bladder cancer.
Class and exam questions
What is the difference between local and resorptive action of carcinogens?
Local-action carcinogens (e.g., PAHs) cause a tumor directly at the site of contact with tissue. Resorptive-action substances (aromatic amines) are absorbed, metabolized, and provoke cancer remotely from the site of entry.
What substances are produced by molds and to what class do they belong?
Molds (predominantly Aspergillus flavus) produce aflatoxins. Chemically, these are organic compounds derived from coumarins.
Can the body's natural metabolites cause cancer?
Yes, they can. Through physicochemical modification, normal metabolic products such as estrogens, bile acids, or amino acids (tyrosine, tryptophan) are converted into endogenous carcinogens.
What else to study on this topic
- Physical carcinogens in tumor etiology
- Biological carcinogens: oncogenic viruses
- The role of hereditary defects in the development of neoplasms
- Molecular mechanisms of cell malignization
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