Biochemistry of Cancer PDF

Biochemistry of Cancer Assist Prof Dr. Nadya Ghassan  Definition of Cancer The International Union Against Cancer (UICC) has defined cancer as a disturbance of growth characterized by excessive proliferation of cells without apparent relation to the physiological demands of the organs invol...

Biochemistry of Cancer Assist Prof Dr. Nadya Ghassan  Definition of Cancer The International Union Against Cancer (UICC) has defined cancer as a disturbance of growth characterized by excessive proliferation of cells without apparent relation to the physiological demands of the organs involved. Oncology deals with the etiology, diagnosis, treatment, prevention and research aspects of cancer.  Etiology of Cancer All cancers are multifactorial in origin. They include genetic, hormonal, metabolic, physical, chemical and environmental factors. Most human cancers are spontaneous. All cancers originate usually from one aberrant cell, which goes on to multiply and produce a tumor mass. As age advances, the number of mutations accumulate. Cancer is the second most common cause for death in developed countries, second only to cardiovascular diseases. Mutagens Any substance which increases the rate of mutation can also enhance the rate of incidence of cancer. Therefore, all carcinogens are mutagens. Examples are X-rays, gamma-rays, ultraviolet rays.  Chemicals Some human cancers are caused by chemicals. These may be introduced into the body by means of: (a) occupation (aniline, asbestos), (b) diet (aflatoxins) (c) lifestyle (smoking). Chemical carcinogens act cumulatively. Tobacco, food additives, coloring agents, and aflatoxins are common carcinogens in our environment. Thousands of chemicals are known mutagens and carcinogens. Methyl cholanthrene is a powerful carcinogen, only nanograms are sufficient to produce a tumor in a mouse.  Aflatoxins They are a group of chemically related compounds synthesized by the fungi, Aspergillus flavus. The mold grows on rice, wheat and groundnut, when kept in damp conditions. The fungi may grow in cattle fodder, which may enter into human body through the cow’s milk. Aflatoxins are powerful carcinogens, which produce hepatomas.  Cigarette Lung cancer is associated with the habit of cigarette smoking. Cigarette contains many carcinogens, the most important group being benzo(a)pyrenes, nicotine, carbon monoxide, nitrogen dioxide and carbon soot. Moreover, non-smoking spouse of a heavy smoker will have 5 times more probability to get lung cancer than a non-smoker. Oral cancer is strongly associated with chewing of tobacco.  An often asked question is, why only some smokers are getting cancer and not all smokers? Glutathione-S-transferase (GST) is involved in the detoxification of various carcinogens, including cigarette smoke. About 5% of population are lacking in GST. Smokers who are devoid of GST are more prone to develop lung cancer.  Alcohol intake increases the risk of oral, pharyngeal, esophageal and liver cancers. Diet high in total fat and cholesterol, increases the risk of colon, breast and prostate cancers.  Promoters of Cancer Most carcinogens require promoters for the production of a cancer. Benzopyrene applied on skin does not produce cancer. Croton oil application also does not lead to skin cancer. But when benzopyrene application is followed by croton oil, tumor is developed. In this case, croton oil is termed as the promoter.  Promoters of Cancer The active agent in croton oil is a phorbol ester, tetradecanoyl phorbol acetate (TPA). It activates protein kinase-C. This results in phosphorylation of membrane proteins, leading to the triggering of malignancy. The carcinogen produces a mutation, but the promoter gives the drive for unchecked cell division.  Progression The biological history of a tumor shows progression of malignancy. Cells with faster growth rate have a selection advantage. Thus, cells with increased malignant character are progressively selected. Familial adenomatous polyposis is a typical example for multistep progression.  Action of Chemical Carcinogens Chemical carcinogens are usually ingested as procarcinogens. They are metabolized in the body, usually in liver, to become the active carcinogen. e.g. 2-acetyl amino fluorene (AAF) when ingested, is metabolized to produce the ultimate carcinogen, sulfate ester of N-hydroxy-AAF. The enzymes responsible for the activation of procarcinogens are cytochrome P-450 system. On the other hand, direct carcinogens are the ones which interact directly with the target molecules, e.g. methyl cholanthrene.  Action of Chemical Carcinogens Mechanisms of action of chemical carcinogens are: (a) Carcinogens are generally electrophiles (molecules deficient in electrons); they readily attack nucleophilic (electron rich) groups of DNA. (b) Carcinogens may bind covalently to cellular DNA. (c) These changes will lead to DNA alterations with increased probability of mutations. Site of cancers Chemical carcinogens may produce the cancer: (a) At the sit of exposure, e.g. buccal cancer in tobacco chewers, skin cancer in tar workers (b) At the site of metabolism, e.g. liver cancer produced by aflatoxin (c) At the site of elimination, e.g. bladder cancer in persons working with aromatic dyes.  Physical Carcinogens X-ray, gamma-ray and UV-ray may cause: (a) formation of pyrimidine dimers (b) sites with consequent break in DNA (c) formation of free radicals and superoxides which cause DNA break, leading to somatic mutations. Exposure of X-ray in fetal life will increase the risk of leukemia in childhood.  Antimutagens i. These are substances which will interfere with tumor promotion. Vitamin A and carotenoids are shown to reverse precancerous conditions. ii. Vitamin E acts as an antioxidant, preventing the damage made by free radicals and superoxides. iii. Vitamin C regularly given to persons working with aniline prevented the production of new cancer cases. iv. Tubers, beans and leafy vegetables are shown to interrupt tumor promotion. v. Curcumin, the yellow substance in Turmeric is known to prevent mutations.  Antimutagens vi. High fiber content of the diet, low protein, low fat, are diet decreases the risk of cancer in animal studies. vii. Flavonoids are phytochemicals that possess antimutagenic properties. Phenolic compounds found in fruits like grapes, strawberries, walnuts, etc. are found to be antimutagenic. Green tea is shown to be effective against smoke induced mutations. Yeast cell wall polysaccharides like beta glycan inhibits lipid peroxidation and protects DNA from oxidative damage. ONCOGENIC VIRUSES Another etiological factor of carcinogenesis is the integration of viral genes into the host DNA. So, there is uncontrolled multiplication of the cells. This is called transformation by oncogenic virus. Functions of Antimutagens a. Prevent transformation of a mutagenic compound into mutagens. b. Inactivate the mutagens. c. Prevent the interaction between mutagens and DNA.  Oncogenes Oncogenes are Normal Constituents of Cells: i. These are genes capable of causing cancer. ii. normal cells do contain DNA sequences similar to viral oncogenes. iii. To distinguish these two genes, they are denoted as V-src (viral gene) and C-src (cellular gene). The oncogenes present in normal cells are also called as proto-oncogenes. iv. Today, more than 100 human proto--oncogenes are known. They are located on specific chromosomes. v. Proto-oncogenes are important regulatory genes of the cells. In fact, viruses carry these genes accidentally picking them from the host cells.  Proto-oncogenes They are Regulatory Genes: i. Products of many oncogenes are polypeptide growth factors, e.g. sis gene produces platelet derived growth factor (PDGF). This factor is required for normal wound healing. ii. Some of the products act as receptors for growth factor, epithelial growth factor (EGF). iii. Some other oncogene products act on key intracellular pathways involved in growth control, e.g. a membrane-bound enzyme, phosphorylates a specific tyrosine residue, leading to cascade activation of cellular events. Receptors for EGF, insulin, PDGF, etc. are also activated by src-product protein. iv. The c-oncogenes are under the control of regulatory genes, and expressed only when required. When virus enters, an extra oncogene is inserted so as to produce continuous expression of the gene leading to uncontrolled cellular activity and malignant transformation. Proto-oncogene activation has been demonstrated in different types of human tumors.  Oncogens and oncogenes are different 1. Oncogens are the chemicals that produce cancer. 2. Oncogenes are the genes causing cancer. 3. Oncogenes are written with small letters, and antioncogenes are written with capital letters. 4. The gene present in normal cell is named with prefix c- (to show that it is in the cell), whereas the corresponding gene present in the virus is denoted with prefix v- (standing for virus).  Factors Activate Oncogenes The oncogenes also provide an explanation for the multifactorial origin of cancer. Thus viruses, chemical carcinogens, chromosome translocations, gamma-rays, spontaneous mutations, and all such factors may converte into one biochemical abnormality, the activation of oncogenes. This would lead to malignancy.  Antioncogenes or Oncosuppressor Genes These are the genes, which normally protect the individual from getting the cancer. When the gene is deleted or mutated, cancer results. A part of short arm of chromosome 17 was shown to be deleted in various human cancers. This region is now known to contain an oncosuppressor gene, called p53. It is so called because the gene encodes a phosphoprotein with molecular weight 53,000. Normal p53 can suppress transforming ability of oncogenic viruses in vitro. It is also seen that p53 activates the expression of genes that suppress cell proliferation.  Oncosuppressor gene, p53, blocks the cells that have damaged DNA by triggering the production of another protein p21, which blocks cell division until the damage is repaired. If the DNA damage is severe, p53 directs the cell to commit suicide by apoptosis. It can complex with other transforming proteins generated by other oncogenic viruses. Most tumors have a complete absence of p53, whereas others show mutant nonfunctional p53.  Growth Factors Many of the oncogenes act through the production of growth factors. The growth factors generally cause mitosis or differentiation of target cells. These may be considered as local hormones. There are more than 100 growth factors. Fibroblast growth factor (FGF), hepatocyte growth factor (HGF), keratinocyte growth factor (KGF), vascular endothelial growth factor or vasculotropin (VEGF) are some other well-characterized growth factors. Interleukins and interferons are growth factors released by lymphocytes/macrophages.  Differences between Normal and Tumor cells The cell cycle is divided into G1, S, G2 and M phases, the cycle being completed within 18–24 hours. The cell cycle time is more or less same for normal cells and cancer cells. In a normal tissue, only 1% cells are in the dividing state. In cancer tissues, about 2–5% of cells are in the cell cycle and this number determines a mildly growing tumor (2%) from an aggressive one (5%). This difference is made use of in treatment. Cytotoxic drugs and radiation will kill the cells in the cell cycle, while sparing the resting cells.  Doubling Time Growth The doubling time is the time taken by a tumor to exactly double its mass, and is a constant for a particular growth over a long period. The tumor doubling time in human cancers varies widely between 10 days to 450 days, with a mean of about 100 days. Doubling Time Growth of a tumor mass depends on: (1) Cellular proliferation. The proliferation co-efficient is the ratio of cells in the cycle to the resting cells. The more the ratio, the more aggressive is the cancer. (2) Cell death by apoptosis, lack of oxygen or nutrition and destruction by immunological mechanisms.  Doubling Time Growth Very rapidly growing tumors will need lesser days to double the volume. In the case of tumor with a doubling time of 100 days, the time taken for this growth to reach 1cm size from the initial mutated cell is about 8–10 years. Thus, the tumor was present in the body for a considerable period before the clinical detection. The same fact explains the development of the secondaries several years after the treatment of the primary growth. Malignant Transformation When a normal cell has acquired malignant character, it is said to be transformed. Normal cells form a monolayer, while cancer cells show multilayered appearance.  Contact Inhibition It is a characteristic of normal cells. If a cut is made in the skin, the cells from both sides start to multiply. This multiplication is stopped when the cells come into contact. This is called contact inhibition. Adjacent cells form tight junctions through which cell to cell communication occurs. But in the case of cancer cells, tight junctions are rare, the property of contact inhibition is lost and adjacent cells continue to multiply to form multilayered or jumbled appearance.  Anchorage Dependence Another malignant character is the loss of anchorage dependence in tissue culture system. Normal cells adhere firmly to the surface of the glass bottle, but cancer cells do not. Vinculin is a protein found in the focal adhesion plates, that is, the structures involved in adhesion between the cells as well as the basement in the case of normal cells. Oncogene product, especially tyrosine kinase, causes abnormal phosphorylation of vinculin. So, there is diminished adhesion to substratum as well as the rounded appearance of transformed cells.  Sialic Acid and Sialylation Most cancer cells carry more negative surface charges on their cell surface than their normal counterparts. This abnormality is due to the higher N-acetyl neuraminic acid (NANA) content of the cancer cell membrane. Due to the higher content of negative charges the cancer cells tend to repel each other, resulting in lesser adhesiveness. Altered sialylation of cell surface glycoproteins and glycolipids is closely related to the malignant phenotype of cancer cells, including the metastatic potential and invasiveness. Human sialidases are indeed related to malignancy and may be potential targets for cancer diagnosis and therapy.  Cell Fusion Cell fusion plays an essential role in fertilization, immune response, tissue repair, and regeneration. Cell fusion may contribute to the initiation and progression of cancer.  Metastasis and Secondaries Cancer cells have a tendency to disintegrate from the main mass and to disseminate to nearby or distant organs. This forms metastasis. The cells from the main cancer tissue migrate farther away. The collagenase and stromolysin released by most cancers help in the penetration of cancer cells into surrounding areas. Metabolic Alterations Many cancer cells are shown to delete different enzymes or even whole metabolic pathways. Generally cancer cells thrive on minimal enzymes. A good example is that many tumors prefer anaerobic glycolysis and eliminate citric acid cycle enzymes. Another example is the deletion of asparagine synthetase in certain lymphomas. Why Cancer Cells are Immortal? One reason is that cancer cells have increased and persistent activity of telomerase, the enzyme that maintains the length of telomeres (end region of chromosomes)  Apoptosis Programed cell death is called apoptosis. In normal organs, the number of cells newly produced will be equal to the number of dead cells. In those cells which are progressing to apoptosis, there will be condensation of chromatin, shrinking of cells, DNA fragmentation and finally disintegration of the cell.  Tumor Immunology All forms of treatment of cancer (surgery, radiotherapy and chemotherapy) leave some residual cancer cells in the body. These are crushed by the body’s immune mechanism, including: (a) T cells (b) NK cells (c) antibody dependent complement mediated lysis (d) antibody dependent cell mediated cytolysis (ADCC), and (e) macrophages.