Carcinogenesis is the process of formation of cancer cells by  carcinogens. Carcinogens are the provocative factors that trigger the cancer process , some of them can be viruses, bacteria, physical agents, chemical agents, genetic or familial factors, dietary factors, and hormonal agents. Carcinogenesis has gained the attention of many doctors as the number of cancer-related hospital admissions are rising each day.  While in most of the cases, carcinogenesis is behaviorally acquired among smokers and alcoholics. This article is designed to understand the underlying causes and mechanism of carcinogenesis. 



Phases of carcinogenesis 

The 3 steps in cellular carcinogenesis are initiation, promotion, and progression. Each phase varies and it is important to recognize in which state the patient is in. Earlier the detection, better the outcome.

  1. During the initiation, carcinogens such as cigarette smoke, tobacco,  industrial chemicals, physical factors, and biologic agents come in contact with the normal cells. These triggers enter into the DNA of the cell, as a result, DNA is seriously damaged which in turn result in mutation. Mutation can significantly affect the normal physiology of our body.

  2. During the promotion, the mutant cells undergo a repeated exposure to the carcinogens. This can further complicate the issue as the prolonged exposure of mutant genes to the carcinogens results in the faulty expression of genetic information. Furthermore, the altered DNA may result in abnormal growth and development of normal cells by inducing an uncontrolled multiplication. Consequently, the mass of cells is formed in a specified location of the body. Most common sites of cancer are lungs, breast, stomach, testicles, bones etc.

  3. Progression is the third stage in which the damaged cells further extend the process of malignant behaviour resulting in the exchange of genetic material - DNA between the normal and abnormal cells.


Common carcinogenic agents

  1. Viruses and Bacterium

Viruses can cause an irreversible aberration and derangement of the genetic composition of cells. Some of the most common carcinogenic viruses are, Epstein-Barr, Herpes simplex virus type II, cytomegalovirus, and human papillomavirus types 16, 18, 31, and 33. Some of the bacteria such as Helicobacter pylori have been identified as the cause of gastric cancer.

  1. Physical Agents

Chronic exposure to sunlight, radiation, irritants, and some inflammatory agents such as tobacco can result in carcinogenesis. The risk doubles if the skin is constantly exposed to the UV rays and nuclear material hence it is linked with poor clothing  (sleeveless shirts or shorts).

  1. Chemical Agents

Tobacco smoking is the most common well-known carcinogen. Tobacco contains nearly 70,000 varieties of chemicals with Nicotine being the most significant cause  cancer of lungs,  head and neck, oesophagus, pancreas, cervix, and the urinary bladder. The prolonged usage of chemical substances such as alcohol, amines, aniline dyes, formaldehyde, arsenic, plastic, polyvinyl compounds, lead and some radioactive agents can further intensify the risk of cancer.

  1. Genetic and Familial Factors

Genetic factors can contribute to about 40% of new cancer cases and it is increasing every 10 years. Genetic influence of cancer is high if both the parents of a patient had a carcinogenic tendency.

  1. Dietary Factors

About 35% of all cancers are due to high-risk dietary intake. Dietary substances including excessive use of high-density level lipids, bad fats, alcohol, frequent consumption of salted and smoked meats and partly cooked meats and also the diet low in fibre can induce carcinogenesis. However, dietary substances that seem to reduce cancer risk are high-fibre foods, vegetables like cabbage, broccoli, cauliflower, Brussels sprouts, kohlrabi, carotenoids as they can prevent the growth of cancer cells. Obesity can also further increase the risk of inducing cancer process.

  1. Hormonal Agents

Androgens; estrogen and progesterone are responsible for many types of cancer in women. These hormones can cause, cancer of breast, vagina, ovaries, prostate, and uterus. Breast cancer is the most common cancers in females caused by birth control agents - oral contraceptives, prolonged estrogen replacement therapy may result in endometrial and ovarian cancers. In men testicular or scrotal cancer is seen rarely, nevertheless, women are more prone for the cancers of reproductive system.

  1. Failure of Immune System

The immune system plays a critical role in causing cancer. Cancer cells can easily attack the body cells that are immunocompromised. Our immune system is led by T-lymphocytes, B-Lymphocytes, WBCs and some of the glandular secretions. If these cells undergo stress, they can induce the abnormal growth pattern of cells resulting in the chronic condition. They are manifested by chronic infections, bleeding, swelling and anaemia. However, a healthy immune system is able to fight against the cancer cells in a number of ways. When a cancer cell enters into one of our body systems cells releases antibodies. Fo this, WBCs must recognize the antigens released by the cancer cells to initiate the defence mechanism against carcinogenic factors.  Rarely, the immune system may fail to perform its duty allowing the domination from cancer cells.



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Cell cycle and cell division                    







Check your understanding

  1. Define cancer.

  2. What are carcinogens?

  3. Name any 5 carcinogens.

  4. Explain the 3 phases of carcinogenesis.





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