Major Causes of Cancer

The following points highlight the three major causes of cancer. The causes are: 1. Radiant Energy 2. Chemical Compounds 3. Oncogenic Viruses.

Cause # 1. Radiant Energy:

(a) Ultraviolet rays, X-rays, and y-rays dam­age DNA in several ways.

(b) Ultraviolet radiation may cause pyrimidine dimers to form.

(c) Apurinic or apyrimidinic sites may form by elimination of corresponding bases. Single and double-strand breaks or cross- linking of strands may occur.

(d) The basic mechanism of carcinogenicity with radiant energy is to cause damage to DNA.

(e) Free radicals are formed in tissues by X- rays and y-rays. The resultant OH, superoxide, and other radicals can inter­act with DNA and other macromolecules, leading to molecular damage and thereby probably contributing to carcinogenic ef­fects of radiant energy.

Cause # 2. Chemical Compounds:

(a) It has been estimated that environmental factors, principally chemicals, can cause up to 80 per cent of human cancers.

(b) Exposure to such compounds can occur because of a person’s occupation (e.g., ben­zene, asbestos); diet (e.g., aflatoxin B₁ which is produced by the mold Aspergil­lus flavus and sometimes found as a con­taminant of peanuts and other foodstuffs); Life style (e.g., cigarette smoking); or in other ways (e.g., certain therapeutic drugs can be carcinogenic).

(c) The carcinogenic substances may be both organic and inorganic molecules.

The fol­lowing table shows some chemical com­pounds which may cause cancer:

Action:

The organic carcinogens, such as nitrogen mustard and β-propiolactone, interact di­rectly with target molecules (direct car­cinogens), but others require prior metabo­lism to become carcinogenic (Pro-carcinogens). The process by which one or more enzyme-catalyzed reactions convert pro-carcinogens to active carcinogens is called metabolic activation.

Any interme­diate compound formed is proximate car­cinogens, and the final compound that reacts with cellular components (e.g., DNA) is the ultimate carcinogen. Pro-carcinogen → Proximate carcinogen A → Proximate carcinogen B → Ultimate carcinogen.

(b) The ultimate carcinogen is highly reac­tive and is usually electrophile (molecule deficient in electron). This readily attacks nucleophilic (electron-rich) group in DNA, RNA, and protein.

Mono-oxygenases and Transferases:

(a) Mono oxygenases and transferases cause the metabolism of pro-carcinogens and other xenobiotics.

(b) The heme-containing mono-oxygenases of the cytochrome P-450 type located in the endoplasmic reticulum are mainly re­sponsible for the metabolic activation of pro-carcinogens.

(c) The mono-oxygenases catalyze the hydroxylation of various pro-carcinogens and other xenobiotics using molecular oxygen as the source of oxygen and NADPH as a reducing source.

R − H + O₂ + NADPH + H → R 0H + H₂O+ NADP⁺

(d) At least 6 or many more such monoxygenases are present in the endoplasmic reticulum of human liver.

(e) The specific mono-oxygenase responsible for the metabolism of polycyclic aromatic hydrocarbons is named cytochrome P-448 or aromatic hydrocarbon hydroxylase. The reactions catalyzed by these mono-oxygenases are called phase I reactions of xenobiotic metabolism.

(f) In phase 2 reactions of xenobiotic metabo­lism, the hydroxylated xenobiotics are conjugated with various moieties (e.g., Glucuronate, sulfate, acetate, glutathione). These reactions usually detoxify the reac­tivity of the compounds involved and make them ready for excretion, mainly in the urine.

(g) In some cases, conjugation actually in­creases the biologic activity or chemical reactivity of a molecule. The enzymes catalysing the above conjugation reac­tions are usually cytosolic in location, al­though some are also present in the endo­plasmic reticulum. The various glutath­ione transferases use glutathione transferases utilizing glutathione itself as the donor.

Factors affecting enzymes metabolizing xenobiotics:

The following factors affect the activities of the enzymes metabolizing xenobiotics:

(i) The activities of these enzymes may dif­fer substantially among species.

(ii) Significant differences are found in en­zyme activities among individuals, many of which are due to genetic factors.

(iii) The activities of some of the enzymes vary according to age and sex.

(iv) Intake of phenobarbital, PCBS, or certain hydrocarbons can also increase the activi­ties of many enzymes by a process known as enzyme induction. Hydrocarbon inha­lation from cigarette smoking during preg­nancy induces the activity of cytochrome P-448 in the placenta altering the amounts of certain metabolites of hydrocarbons to which the fetus is exposed.

(v) The metabolites of certain drugs can in­hibit the activities of xenobiotic-metabolizing enzymes.

Mutagens:

(a) Most of the chemical carcinogens are mutagens.

(b) The use of bacteria is mutagenicity tests creates a problem that they do not contain the spectrum of mono-oxygenases found in higher animals.

Initiation and Promotion:

(a) The stage of carcinogenesis of the skin of experimental mice caused by the applica­tion of benzo [a] pyrene is called initia­tion and this stage is rapid and irrevers­ible. It is supposed to involve an irrevers­ible modification of DNA resulting in one or more mutations. Benzo [a] pyrene is thus called an initiating agent.

(b) The second stage of carcinogenesis, result­ing from the application of croton oil, is called promotion and croton oil is, there­fore, a promoter. Promoters can cause ini­tiation.

(c) Most carcinogens can act as both initiat­ing and promoting agents.

(d) A good number of compounds including phenobarbital and saccharin can act as pro inoters in different organs.

(e) The active agent of croton oil is a mixture of phorbol esters. The most active phorbol ester is 12-0-tetra-de-canoyl-phorbol-13-acetate (TPA) which has numerous effects.

(f) Protein Kinase C can act as a receptor for TPA. The enzyme being stimulated by interaction with TPA may result in the phosphorylation of a number of membrane proteins resulting in the effects on trans­port and other functions.

Role of DNA:

DNA is the premier target molecule in carcinogen­esis which is being established by the following facts:

(i) Cancer cells beget cancer cells, i.e., the required changes responsible for cancer are transmitted from mother to daughter cells. This is consistent with the behav­iour of DNA.

(ii) DNA is damaged by both irradiation and chemical carcinogens which are capable of causing mutations in DNA.

(iii) Many tumor cells exhibit abnormal chro­mosomes.

(iv) Transfection experiments show that puri­fied DNA (oncogenes) from cancer cells can transform normal cells into cancer cells. Epigenetic factors may also play a role in carcinogenesis.

Cause # 3. Oncogenic Viruses:

(i) Oncogenic viruses contain either DNA or RNA as their genome.

(ii) Polyomavirus and SV 40 viruses have played an important role in the develop­ment of current ideas about viral onco­genesis. Both of them are small and their circular genomes code for only about 5-6 proteins. Under certain circumstances, ap­propriate cells being infected with these viruses can result in malignant transfor­mation. Specific viral proteins are in­volved too.

(iii) In case of SV 40, these proteins (often called antigens) are known as T and t, and in case of polyomavirus, they are known as T, mid-T, and t (T refers to the first of these proteins detected in a tumor).

(iv) The T antigens are to bind tightly to DNA and cause alteration in gene expression. These proteins show cooperative effects, suggesting that alteration of more than one reaction or process is required for trans­formation.

(v) Transformation of certain animal cells are caused by some types of adenovirus.

(vi) Epstein-Barr virus is associated with Burkitt’s Lymphoma and nasopharyngeal carcinoma in humans.

(vii) Herpes Simplex virus is associated with cancer of the cervix, and hepatitis B virus is also associated with some cases of liver cancer in humans.

Transformation:

The cultured cells undergo malignant transforma­tion when they are infected with certain oncogenic viruses. These changes affect cell shape, motility, growth, and a number of biochemical processes. They reflect the conversion from the normal to the malignant state.

Acquisition by cells of the changes collectively known as transformation does not mean that such cells will display the same biologic properties as tumor cells in vivo; cells must yield tumors when injected into a suitable host animal.

Oncogenes:

Oncogenes are genes capable of causing cancer. These were first recognised as unique genes of tumor-causing viruses that are responsible for the process of transformation (viral oncogenes).

Oncogenes of Rous Sarcoma Virus:

(i) The genome of this retrovirus contains four genes named gag, pol, env, and src.

(ii) The gag gene codes for group-specific an­tigens of the virus, pol for the reverse tran­scriptase that characterizes retroviruses, and env for certain glycoproteins of the viral envelope. A protein-tyrosine kinase was shown to be the product of src (i.e., the sarcoma-causing gene) that is respon­sible for transformation.

(iii) Certain glycolytic enzymes become tar­get proteins for the src protein-tyrosine kinase. This shows that transformed cells often show increased rates of glycolysis. The product of src may also catalyze phos­phorylation of phosphatidylinositol to phosphatidylinositol mono- and bi-phosphate.

(iv) When phosphatidylinositol 4, 5-bi-phosphate is hydrolyzed by the action of phospholipase C, 2 second messengers are re­leased: inositol triphosphate and diacylglycerol. The first compound mediates release of Ca⁺⁺ from intracellular sites of storage (e.g., the endoplasmic reticulum).

(v) Diacylglycerol stimulates the activity of the plasma membrane-bound proteins ki­nase C which in turn phosphorylase a number of proteins, some of which may be components of iron pumps.

(vi) Mild alkalinization of the cell brought about by activation of an Na⁺/H⁺ anti-port system can play a role in stimulating mi­tosis.

The product of src may, therefore, affect a large number of cellular processes by its ability to phosphorylate various target proteins and enzymes and by stimulating the pathway of synthesis of the polyphosphoinositides.

Oncogenes of other Retroviruses:

(i) About 20 oncogenes of other retroviruses have been identified. Almost half of the products are protein kinases, mostly of the tyrosine type.

(ii) Some of these encode protein kinases, the remainder encode various other proteins with interesting biologic activities.

(iii) The product of the ras oncogene of murine sarcoma viruses binds GTP, has GTPase activity, and is related to the proteins that regulate the activity of the important plasma membrane enzyme, adenylate cy­clase.

Mechanism by which proto-oncogenes become Oncogenes:

A. Promoter Insertion:

(i) When the particular viruses infect cells, a DNA copy (cDNA) of their RNA genome is synthesized by reverse transcriptase, and the cDNA is integrated into the host ge­nome. The integrated double-stranded cDNA is called a Provirus.

(ii) Following infection of chicken B lymphocytes by certain avian leukemia viruses, their proviruses become integrated near the myc gene. The myc gene is acti­vated by an upstream, adjacent viral long terminal repeat acting as a promoter, re­sulting in transcription of both the corre­sponding myc mRNA and translation of its product in such cells.

B. Enhancer Insertion:

(i) In certain cases, the provirus is inserted downstream from the myc gene or up­stream from it but oriented in the reverse direction, the myc gene never become activated. Such activation cannot be due to promoter insertion.

(ii) Enhancer sequences present in the long terminal repeat sequences of the retrovi­ruses.

(iii) The above two mechanisms—promoter and enhancer insertion—commonly op­erate in viral oncogenesis.

C. Chromosomal Translocations:

(i) Many tumor cells show chromosomal ab­normalities. Translocation is a type of chromosomal change seen in cancer cells.

(ii) A piece of one chromosome being split off joins to another chromosome and if the second chromosome donates material to the first, the translocation is said to be reciprocal.

(iii) A number of tumor cells show characteris­tic translocations. One important translocation is the Philadelphia chromosome oc­curring in chronic granulocytic leukemia.

(iv) Burkitt’s Lymphoma is a fast-growing cancer of human B Lymphocytes.

(v) Synthesis of greatly increased amounts of the DNA-binding protein coded for by the myc gene acts to “drive” or “force” the cell towards becoming malignant by an effect on the regulation of mitosis.

D. Gene Amplification:

(i) One method is shown in respect of gene amplification in tumors by administration of the anticancer drug methotrexate, an inhibitor of the enzyme dihydrofolate re­ductase. Tumor cells can become resist­ant to the action of this drug.

(ii) Certain cellular oncogenes can also be am­plified and are thus activated.

(iii) Increased amounts of the products of cer­tain oncogenes produced by gene amplification may play a role in the progres­sion of tumor cells to a more malignant state.

E. Single-point Mutation:

(i) The product of murine retroviruses, a pro­tein of MW 21000 is related to the G pro­teins that modulate the activity of ade­nylate cyclase and thus play a key role in cellular responses to many hormones and drugs.

(ii) The lower activity of GTPase can result in chronic stimulation of the activity of ade­nylate cyclase which normally is dimin­ished when GDP is formed from GTP. The resulting stimulation of the activity of ade­nylate cyclase can result in a number of effects on cellular metabolism exerted by the increased amount of cAMP affecting the activities of various cAMP-dependent protein kinases.

General Comments, on Activation of Oncogenes:

(i) Increased amounts of the product of an oncogene may be sufficient to push a cell towards becoming malignant.

(ii) The presence of a structurally abnormal key regulatory protein in a cell may also be sufficient to tip the scales towards can­cer.

(iii) Oncogenes have been isolated from only about 15 per cent of human tumors.

(iv) Recent work has shown that activation of C-ras in rat mammary cancers induced by nitrosomethylurea was apparently due to a specific G → A transition type of muta­tion, demonstrating that oncogenes are probably involved in chemical carcino­genesis.

(v) Further research is essential to examine the possible involvement of oncogenes in the phenomena of initiation, promotion, tumor progression, and metastasis.

Mechanisms of Action of Oncogenes:

(i) They may act on key intracellular path­ways involved in growth control uncou­pling them from the need for an exogenous stimulus.

(ii) The products of oncogenes may also imi­tate the action of a polypeptide growth factor.

(iii) The products may also imitate an occu­pied receptor for a growth factor.

Polypeptide Growth Factors:

(i) The growth factors affect many different types of cells, e.g., cells from the blood, nervous system, mesenchymal tissues, and epithelial tissues.

(ii) They exert a mitogenic response on their target cells.

(iii) Platelet-derived growth factor (PDGF) re­leased from the alpha granules of plate­lets plays a role in normal wound healing. Various growth factors play key roles in regulating differentiation of stem cells to form various types of mature hemato­poietic cells. Growth inhibitory factors also exist. Thus, chronic exposure to in­creased amounts of a growth inhibitory factor can alter the balance of cellular growth.

Endocrine, Paracrine and Autocrine Actions of Growth Factors:

Growth factors may act in the following ways:

(i) Their effects may be endocrine, like hor­mones, they may be synthesized elsewhere in the body and may pass in the circula­tion to their target cells.

(ii) They may be synthesized in certain cells and secreted from them to affect neigh­bouring cells. The cells that synthesize the growth factor are not themselves affected because they lack suitable receptors. This mode of action is called paracrine.

(iii) Some growth factors can affect the cells that synthesize them. This third mode of action is called autocrine.