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Biology LibreTexts

Cancer

Introduction

Cancer is the uncontrolled growth of cells (mitosis). It ultimately kills the organism.

One out of three Americans will develop cancer. One out of four males and one out of three females will die of cancer.

Characteristics of Cancer

Exhibit Uncontrolled Growth (Immortality)

Most cells have a fixed number of divisions (approximately 50) before they die. Cancer cells do not have a limit.

Abnormal DNA (Mutations)

The nuclei of cancer cells are enlarged.

Some cancers have chromosomal mutations, either extra or missing chromosomes or parts of chromosomes.

Cancer cells frequently have extra copies of certain genes, a phenomenon called gene amplification.

Cancer is also associated with gene mutations.

Apoptosis

Cells with damaged DNA that cannot be repaired normally undergo apoptosis, a process in which the cell kills itself. Tumor cells have high levels of a protein (survivin) that inhibits apoptosis.

Lack Differentiation

Normally, cells become differentiated and become capable of specific functions.  For example, muscle cells are specialized for contraction, nerve cells (neurons) are specialized for conducting signals, etc.  Cancer cells are not specialized.

Lack Contact Inhibition

Normal cells stop dividing when they become crowded because mitosis is inhibited when cells contact nearby cells. Cancer cells continue to divide and produce a mass of cells called a tumor.

In a laboratory culture dish, normal cells grow in a single layer on the bottom but cancer cells continue to grow and produce a thick mass of cells.

Lack Anchorage Dependence

Normal cells cling to neighboring cells, cancer cells do not.

Metastasis occurs when cells break away and travel through the body via the circulatory and lymphatic systems. These cells form secondary tumors at other sites in the body.

Malignancy refers to the ability of a tumor to metastasize, that is, to produce a new tumor at a different location. A malignant tumor is dangerous because it can metastasize (spread). A benign tumor does not spread.

Have a Reduced Need for Growth Factors

Growth factors are chemicals that stimulate cell division in normal cells by causing certain metabolic reactions to occur. Normally, cyclin combines with kinase only in the presence of growth factors. In cancer cells, cyclin may combine with kinase in the absence of growth factors.

Some genetic mutations (called oncogenes, discussed below) allow these reactions to occur with reduced amounts of growth factors.

The reduced need for growth factors may be responsible for the lack of contact inhibition and anchorage dependence discussed above.

Ability to Penetrate the Lamina

The lamina is a noncellular barrier that is attached to cells that line the surfaces, internal cavities, and organs (epithelial tissue). Normally, cells cannot penetrate this barrier and therefore cannot invade neighboring tissues and organs.

Cancer cells have the ability to penetrate the lamina. The cells are able to invade and destroy neighboring tissue.

A benign tumor does not invade neighboring tissue.

Angiogenesis

Tumors need a blood supply for food and oxygen. Cancer cells release growth factors that cause nearby blood vessels to produce branches that grow into the cancerous tissue.

Immune System

The immune system can recognize foreign cells and invaders (bacteria, viruses, etc.) because they have proteins and other structures that are different from the body’s "self" markers.

Cancer cells may have mutated "self" markers and are often destroyed by the immune system.

Prognosis

A persons prognosis is less favorable with the occurrence of each step below:

  1. the tumor has invaded surrounding tissues
  2. there is lymph node involvement
  3. malignant tumors occur in other parts of the body

Genes and Cancer

Oncogenes

Proto-oncogenes are normal genes that are somehow involved in this process of cell division. Their DNA may contain the code for growth factors or some chemical that needs to be activated in order to stimulate cell division.

When a proto-oncogene becomes mutated, it may no longer function properly, leading to uncontrolled cell growth.

Mutated proto-oncogenes that cause cancer are called oncogenes.

Approximately 100 oncogenes have been identified.

Examples of Oncogenes

The rasK oncogene is found in 25% of lung cancers, 50% of colon cancers, and 90% of pancreatic cancers. The rasN oncogene is associated with leukemias and lymphomas. These oncogenes stimulate growth even when there is no growth factor produced.

Tumor-Suppressor Genes

Damaged or mutated DNA may result in uncontrolled cell growth. 

Proteins produced by some tumor-suppressor genes stop the cell cycle when DNA becomes damaged. If the DNA cannot be repaired, these proteins cause the cell to kill itself (called apoptosis). 

For example, a peeling sunburn is due to apoptosis. The DNA has become too badly damaged to be repaired, so the cells kill themselves. This prevents cancer from developing from the mutations.

A tumor-suppressor gene called p53 stops the cell cycle when damage has occurred to DNA and it stimulates repair enzymes to repair the DNA. It produces a protein that combines with the cyclin-kinase complex and inactivates it, thus preventing the cell from dividing.

If the DNA cannot be repaired, the p53 protein stimulates apoptosis.

When tumor-suppressor genes don't function properly, they may not be able to prevent abnormal cell division in mutated cells and the result could be cancer. The most frequent cause of new cancer is a mutation in the p53 gene. It is involved in an estimated 60% of all cancers including cancers of the breast, lung, liver, skin, prostate, bladder, cervix, and colon.

Breast cancer prognosis is associated with a tumor suppressor gene called p27.

Many cancers are the result of tumor suppressor genes that have become "turned off" so that they do not function properly. Therapies are being developed to turn these genes back on.

Carcinogens

Carcinogens are chemicals that cause cancer by causing mutations.

Cancer may result if the mutations produce some of the characteristics described above.

Generally, chemicals that produce mutations are more likely to cause cancer than those that do not produce mutations. The likelihood that any particular chemical will cause cancer can be determined by observing the rate at which it causes mutations in bacteria. This is the basis of the Ames test.

Prevention of Cancer

Behavior patterns contribute to approximately 40% of all cancers.

Diet plays role in 35% of all cancer cases.

The environment, workplace pollutants and natural causes play a role in the rest.

60% of cancer deaths can be prevented by not smoking and with diet, exercise, and screening.

Smoking

Smoking is responsible for 30% of all cancer deaths and 87% of all lung cancer cases.  It causes 90% of lung cancer cases in men and 79% of lung cancer cases in women.

Sun

Ultraviolet radiation from the sun causes almost all cases of basal and squamous cell skin cancers.  It is also a major factor in the incidence of melanoma.

Alcohol

Alcohol is associated with cancers of the mouth, throat, esophagus, larynx, and liver.

Alcohol is even more dangerous if drinking is accompanied by tobacco use (smoking or chewing).

Radiation

Radiation such as X-rays causes cancer.

Radon is a colorless, odorless gas that reaches high concentrations in some houses.   It causes lung cancer, especially in smokers.

Regular Examination

Regular self-examination of the breasts and testes can help detect cancers in these areas at an early stage.

Occupational Hazards

Industrial chemicals such as nickel, chromate, asbestos, and vinyl chloride cause cancer.

Smoking increases the risk associated with exposure to asbestos.

Exposure to radiation is a significant threat in some occupations.

Hormone Therapy

Estrogen increases the risk of endometrial cancer but progesterone reduces the risk.

Diet

Obesity is associated with cancer, particularly those of the colon, breast, and uterus.  The risk of cancer is 55% greater in obese women and 33% greater in obese men.

Diets high in fat are associated with cancer of the colon, prostate, and possibly the breast.

Salt-Cured, Smoked, or Nitrite-Cured Foods

Salt-cured or pickled foods cause cancers in the stomach and esophagus.

Smoked foods such as ham and sausage contain carcinogens similar to those in tobacco smoke.

Hot dogs and cold cuts contain nitrites, which are converted to nitrosamines.

Antioxidants

Free radicals organic ions with unpaired electrons. These molecules are destructive because they steal electrons from DNA and other molecules. Antioxidants are chemicals that destroy free radicals and other oxidants (peroxides, superoxides), thus preventing mutation and other damage.

Examples of Antioxidants

Vitamin A is found in dark green, leafy vegetables, carrots, and some fruits.

Vitamin C is found in citrus fruits.  In addition to its antioxidant properties, it also prevents the conversion of nitrates to a carcinogenic form called nitrosamines.

Cabbage Family

Cabbage, broccoli, brussel sprouts, kohlrabi, and cauliflower reduce risk of gastrointestinal and respiratory tract cancers.

How to Eat to Prevent Cancer

The Medical News Today website has a video on how to eat to prevent cancer. A link to the page containing the video is below. Scroll down to "How to Eat to Prevent Cancer."

http://www.medicalnewstoday.com/info/cancer-oncology/whatiscancer.php