Universality of the mechanisms of tumor transformation
In pathological physiology, there is a fundamental rule: despite the enormous variety of etiological factors (chemical, physical, or biological carcinogens) and clinical forms of neoplasms, at the cellular level the mechanism of disease development is always universal.
The ultimate result of any carcinogenic exposure is tumor transformation. It is driven by a gross disruption of interactions within the cellular genome. In a healthy cell, there is strict control, but during oncogenesis this control is lost due to two parallel processes:
- Activation of oncogene expression (genes that stimulate tumor growth).
- Depression or complete suppression of antioncogenes (tumor suppressors that normally inhibit the cell cycle).
This fundamental process is absolutely universal. It proceeds according to a single scenario regardless of the root cause of the mutation, the histological structure of the future tumor, or its specific localization in the body.
Stage I: Effect of carcinogens on the genome
The entire process of oncogenesis is traditionally divided into four sequential stages.
The first stage is the impact of input factors on the genetic apparatus of a normal cell. The essence of this stage lies in the primary interaction of the carcinogen with the cell genome. The main targets for damaging agents are proto-oncogenes (oncogene precursors) and antioncogenes.
Agents that can act as carcinogens include:
- Chemical and physical agents, as well as biological non-viral factors.
- DNA-containing oncogenic viruses that directly introduce their genetic material.
- RNA-containing oncogenic viruses. Their mechanism is more complex: to integrate into the host genome, they use a specific enzyme—reverse transcriptase. With its help, the virus synthesizes a DNA copy (the so-called DNA provirus), which is then integrated into the cell chromosome.
Stage II: Transformation of the proto-oncogene into an oncogene
The second stage is a critical moment of genetic restructuring. Its essence is the conversion of a normal proto-oncogene into a hyperactive oncogene against the background of obligatory suppression of tumor suppressor activity.
This transformation is implemented through a series of specific genetic alterations (mutations):
- Point mutation of the cellular oncogene (c-onc), altering the structure of the encoded protein.
- Amplification of c-onc — a multiple increase in the number of gene copies, leading to an excess of its product.
- Translocation of c-onc — transfer of a chromosome segment containing the oncogene to another position where it begins to be actively transcribed.
- Insertion of a heterotopic promoter, which artificially triggers the operation of the oncogene.
- Depression of the antioncogene — shutdown of defense mechanisms restraining tumor growth.
Stages III and IV: From oncoprotein synthesis to node formation
The third stage represents direct tumor transformation of the cell. It is important to remember the key rule: oncogene expression is a necessary and fully sufficient condition for converting a normal cell into a malignant one.
At this stage, the active oncogene initiates robust synthesis of specific oncoproteins. These proteins exert their pathological effects either directly or indirectly through cellular growth factors and their membrane receptors. Under the influence of oncoproteins, the genotypically altered cell dramatically changes its properties and acquires a characteristic tumor phenotype.
The fourth stage is the final stage of carcinogenesis, at which the process transitions to the tissue level. The essence of the stage is the onset of unrestrained division of the transformed cell. The cell, devoid of control mechanisms, begins to multiply rapidly, ultimately leading to the formation of a visible neoplasm—the tumor node.
Briefly: answers to topical questions
What classes of chemical carcinogens exist?
Chemical carcinogens are divided into two large groups: organic and inorganic compounds.
- Organic compounds — polycyclic aromatic hydrocarbons (PAHs), heterocyclic aromatic hydrocarbons, nitroso compounds, aminoazo compounds, epoxides, aflatoxins, aromatic amines, and amides.
- Inorganic compounds — arsenic, beryllium oxide, chromates, and cobalt.
What are the main known tumor suppressor genes (antioncogenes)?
Key representatives of cancer suppressor genes (antioncogenes) are the p53 and Rb genes.
- The Rb gene (Retinoblastoma) — controls cell proliferation; loss of its function leads to the development of tumors (e.g., retinoblastoma).
- The p53 gene — encodes a cellular nuclear phosphoprotein (DNA-binding protein) that ensures DNA repair and synthesis, cell cycle control, cell differentiation, and programmed cell death (apoptosis).
Loss of function of these genes (depression or mutation) leads to tumor transformation.
What specific functions do specific oncoproteins perform?
The main role of oncoproteins is signal transduction from the cell membrane to the nucleus. Oncoproteins are classified by functional activity:
- Oncoproteins as growth factor analogues: example — c-sis.
- Oncoproteins as receptor analogues: examples — c-mas, c-erb-B, c-fms.
- Oncoproteins with tyrosine kinase activity: examples — c-ros, c-fms, c-src, c-abl, c-fes.
- Oncoproteins with small molecular weight G-protein activity: example — c-ras; interact with GTP, affect the adenylate cyclase system.
- Oncoproteins with serine-threonine kinase activity: examples — c-raf, c-mos; associated with protein kinase C.
- Oncoproteins with nuclear transcription factor activity: examples — c-jun, c-myc, c-fos, c-myb; localize to the nucleus and directly affect DNA.
- Mitochondrial oncoproteins of the Bcl-2 family: example — c-bcl-2; regulate apoptosis.
What specific properties does the acquired tumor phenotype possess?
The acquired tumor phenotype is characterized by the development of qualitative and quantitative cellular atypia that distinguishes the neoplasm from normal tissues.
- High replicative potential — acquisition by the cell of the ability for unlimited proliferation and immortalization.
- Autonomous growth — absence of proliferation and differentiation control by the body, loss of contact inhibition.
- Tumor progression — irreversible alteration of properties leading to increased genotype variability and tumor adaptation to conditions (including drugs).
- Aggressive behavior — evasion of apoptosis, ability to invade surrounding tissues and metastasize.
To easily remember the 4 stages, use the mnemonic KMAU: Kontact (carcinogen hits the genome) → Mutation (proto-oncogene becomes an oncogene) → Activity (synthesis of oncoproteins and phenotype change) → Uzel (node — division and tumor growth).
Class and exam questions
Does the mechanism of carcinogenesis depend on the type of carcinogen involved?
No, at the cellular level the mechanism is completely universal. Any carcinogen ultimately causes an imbalance between oncogenes and antioncogenes, leading to transformation.
What is the role of reverse transcriptase in oncogenesis?
This enzyme is used by RNA-containing oncogenic viruses. With its help, they synthesize a DNA copy (provirus), which then integrates into the host cell genome.
What is a necessary and sufficient condition for a cell to become tumorous?
Such a condition is oncogene expression, which triggers the synthesis of oncoproteins at the third stage of carcinogenesis.
Through what pathways does a proto-oncogene convert into an oncogene?
This occurs due to genetic alterations: point mutations, amplification, cellular oncogene (c-onc) translocation, as well as promoter insertion and tumor suppressor inactivation.
What else to study on this topic
- Mechanisms of genetic changes: translocation, amplification, and point mutation of c-onc.
- The role of oncoproteins and cellular growth factors in phenotype alteration.
- Differences in the mechanisms of action of DNA- and RNA-containing oncogenic viruses.
- Functions of anti-oncogenes (tumor suppressors) in the normal cell.
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