BASICS OF
CARCINOGENESIS

INTRODUCTION

Cancer is the second among fatal diseases, next to cardiovascular diseases, in industrialized countries. It’s estimated that in the next quarter of a century the number of new cancer cases globally is going to double, half of them in the developing countries (WHO 1991). The World Health Organization has launched a campaign against cancer, with a three-fold strategy: prevent all the preventable cancers, cure all that can be cured, and reduce pain and discomfort where cure is not possible (WHO 1995). In this context it may be worthwhile to examine the basic cellular changes leading to cancer development and to discuss some of the areas where strategies for prevention can be implemented.

Cancer is a broad term used for identifying a large number of diseases. Perhaps the only common feature of these diseases is the ability of uncontrolled cell proliferation that cannot be checked by the normal cell kinetics regulators. A normal cell suddenly turns into a rogue cell and start dividing continuously without check, leading to the development of solid lumps (tumors) or an abnormal rise in the number of dispersed cells like the blood corpuscles. Cancer can occur in any part of the body and in any organ or tissue. Cancer originates in our own cells, but several factors, both internal and external can add to the life time cancer risk. While cancer, as such, is not infectious, some infections can act as a stimulus to induce and promote cancer development. In addition, environmental pollutants like many chemicals, industrial effluents, some therapeutic drugs, and mutagenic agents, including ionizing radiation, can increase the incidence of cancer. About 90% or more of all cancers are attributed to life style, eg. diet, tobacco habits and alcohol consumption, and exposure to industrial toxins. All of these factors tend to create a toxic environment that tends to induce cancer due primarily to the nature of chronic inflammation.

PROCESS OF CARCINOGENESIS

Cancer development is understood to be a multistep process. The concept of multi-stage carcinogenesis was first proposed by Berenblum and Schubik in 1948 and supported by later studies. Present day oncology recognizes three main phases: initiation, promotion and progression.

Initiation: Neoplasia initiation is essentially irreversible changes in appropriate target somatic cells. In the simplest terms, initiation involves one or more stable cellular changes arising spontaneously, induced by exposure to a carcinogen, or due to chronic inflammation. This is considered to be the first step in carcinogenesis, where the cellular

genome undergoes mutations, creating the potential for neoplastic development (UNSCEAR 1993, Cox 1994), which predisposes the affected cell and its progeny to subsequent neoplastic transformation. The human DNA sequences responsible for transformation are called oncogenes. Many of the active oncogenes have been isolated by molecular cloning, eg. Human bladder carcinoma, Burkitt’s lymphoma, lung carcinoma, carcinoma of the breast and several others. Although the activation of more than one oncogene appears to be necessary for neoplastic transformation, the data imply that initiation may be induced with one hit kinetics (Bishop 1982). For example, in the human bladder carcinoma, a single point mutation converting the Ha-ras proto-oncogene into a potent oncogene was the first identified mutation in a human oncogene (Tabin et al. 1982). Such tumor gene mutations can have profound effects on cellular behavior and response, and can lead to dysregulation of genes involved in biochemical signaling pathways associated with control of cell proliferation and/or disruption of the natural processes of cellular communication, development and differentiation. However, the full expression of such neoplasia initiating mutations invariably requires interaction with other later arising gene mutations and/or changes to the cellular environment, but the initiating mutation creates the stable potential for pre-neoplastic cellular development in cells with proliferative capacity (UNSCEAR 2000). The transformed cell undergoes continuous division with fidelity to the transformed karyotype and, possibly, with further mutations, before a malignant lesion is manifested.

MECHANISMS OF ONCOGENE ACTIVATION

Each oncogene is closely associated with a normal DNA sequence present in the cellular genome, the proto-oncogene. At least five different mechanisms are considered for the conversion of proto-oncogenes to active oncogenes (Land et al. 1983):

(1) Overexpression of proto-oncogene following acquisition of a novel transcriptional promoter. The oncogene then acquires activity because their transcripts are produced at much higher levels than those of the related normal proto-oncogene.

(2) Over-expression due to amplification of the proto-oncogene or oncogene. The increased gene copies cause     corresponding increases in transcript and gene product.

(3) Influences on the levels of transcription and, in turn, the amount of gene product.

(4) Juxtaposition of the oncogene and immunoglobulin domains, following chromosomal translocations, that     appears to result in deregulation of the gene.

(5) Alteration in the structure of the oncogene protein. This is the most well documented mechanism in the case     of the oncogene proteins encoded by the ras genes.
The fourth and fifth mechanisms seem to be inter-related. A translocation can disturb the regulation of an oncogene (Rabbitts 1994) by:

a) providing a new promoter region or some other control element that would activate the oncogene; or

b) altering the coding sequence of a gene, changing its protein product from a benign to a malignant form.

A close association between specific chromosomal translocations and certain human neoplasms has been demonstrated (Rowley and Mitelman 1993).

Promotion: The transformed (initiated) cell can remain harmless, unless and until it is stimulated to undergo further proliferation, upsetting the cellular balance. The subsequent changes of an initiated cell leading to neoplastic transformation may involve more than one step and requires repeated and prolonged exposures to promoting stimuli (Upton et al. 1986). Thus, in contrast to initiation which is induced at a rate of 0.1-1.0 per cell/Gy of radiation, the subsequent transforming event in the initiated cells occurs at a rate of only 10-6 to 10-7 per cell generation (Kennedy and Little 1984). Neoplastic development is influenced by the intra- and extracellular environment. Expression of the initial mutation will depend not only on interaction with other oncogenic mutations but also on factors that may temporarily change the patterns of specific gene expression, eg. cytokines, lipid metabolites, and certain phorbol esters. This may result in an enhancement of cellular growth potential and/or an uncoupling of the intercellular communication processes that restrict cellular autonomy and thereby coordinate tissue maintenance and development (Trosko et al. 1992, UNSCEAR 1993).

Progression: is the process through which successive changes in the neoplasm give rise to increasingly malignant sub-populations. Molecular mechanisms of tumor progression are not fully understood, but mutations and chromosomal aberrations are thought to be involved. The process may be accelerated by repeated exposures to carcinogenic stimuli, chronic inflammation or by selection pressures favoring the autonomous clonal derivatives. The initiated cells proliferate causing a fast increase in the tumor size. As the tumor grows in size, the cells may undergo further mutations, leading to increasing heterogeneity of the cell population.

In the first phase of progression, sometimes referred to as neoplastic conversion, the pre-neoplastic cells are transformed to a state in which they are more committed to malignant development. This may involve further gene mutations accumulating within the expanding pre-neoplastic cell clone (UNSCEAR 2000). The dynamic cellular heterogeneity, a feature of malignant development, may, in many instances, be a consequence of the early acquisition of genespecific mutations that destabilize the genome. Examples are mutations of the p53 gene (Hartwell and Kastan 1994) or DNA mismatch repair genes (Fishel and Kolodner 1995). Many tumor types develop transforming sequences in their DNA during their progression from the normal to the cancerous state. An elevated mutation rate established relatively early in tumor development may, therefore, provide for the high-frequency generation of variant cells within a premalignant cell population. Such variant cells, having the capacity to evade the constraints that act to restrict proliferation of aberrant cells, will tend to be selected during tumorigenesis (UNSCEAR 2000).

Tumor metastasis: As the tumor progression advances, the cells lose their adherence property, detach from the tumor mass, invade neighboring tissues, as well as enter the circulating blood and lymph systems. Once circulating, the (CTC) circulating tumor cells are transported to other organs/tissues away from the site of the primary growth and develop into secondary tumors at the new sites. These form the distant metastases, resulting in widely spread cancers. Cancer metastasis consists of a number of steps; the main steps are common for all tumors. The progress of the neoplastic disease depends on metastatic changes that facilitate:

(a) invasion of local normal tissues,

(b) entry and transit of neoplastic cells in the blood and lymphatic systems, and

(c) the subsequent establishment of secondary tumor growth at distant sites (Hart and Saini 1992, Takeichi 1993).

Many of the steps in tumor metastasis involve cell -cell and cell-matrix interactions, involving specific cell surface molecules. Malignant cells are thought to have reduced ability to adhere to each other, so that they detach from the primary tumor and invade the surrounding tissues. The behavior of tumor is influenced by the cell adhesion molecules, one of the most important of which are cadherins (Takeichi 1991). Animal studies have shown that a down-regulation of E-cadherin expression, resulting in lower levels, correlated with metastatic behavior in vivo, suggesting that cadherins function as invasion suppressor gene products (Vleminckx et al. 1991).

It is the metastatic process and tumor spreading that are mainly responsible for the lethal effects of many common human tumors. In many cases gene mutations are believed to be the driving force for tumor metastasis, with the development of tumor vasculature playing an important role in the disease progression (Folkman 1995).

Tumor angiogenesis: Tumor growth depends on the supply of growth factors and efficient removal of toxic molecules, which comes through an adequate blood supply. In solid tumors, efficient oxygen diffusion from capillaries occurs to a radius of 150-200(m, beyond which the cells become anoxic and die. Therefore, increase in tumor mass to more than 1-2 mm will depend on adequate blood supply through development of blood capillaries (angiogenesis). Schubik was the first to coin the term ‘tumor angiogenesis’ (Shubik 1982). But it was Judah Folkman who hypothesized the importance of tumor angiongenesis in thedevelopment and metastasis of solid tumors. His theories are widely accepted today. Folkman and colleagues established that tumor growth beyond about 2mm size could proceed only if a vascular supply is established (Folkman 1985). A number of tissue factors have been identified, which stimulate endothelial cell proliferation. These include the tumor angiogenesis factor (TAF, Folkman 1974), the vascular endothelial growth factor (VEGF, Dvorak et al 1995), angioproteins – ang-1 and ang – 2 (Davis and Yancopoulos 1999), transforming growth factors (TGFs) (Leibovich et al. 1987), interleukin – 1 ( (Mahadevan et al 1989), and platelet-derived endothelial cell growth factor (PD-ECGF, Ishikawa et al. 1989).

Although the blood vessels that supply the developing tumors are derived from the host vasculature, their architecture differs considerably from that in the normal tissue. Tumor vessels are often dilated, saccular and tortuous and may contain tumor cells within the endothelial lining of the vessel (Jain 1989). Therefore, the blood flow in the tumor may be sluggish compared to that in the adjacent normal tissues and the tumor microvasculature may show hyperpermeability to plasma proteins.

CANCER GENES

Somatic gene mutations are widely accepted as the basic event in the conversion of a normal cell into cancer cell. Many different genes are demonstrated to be involved in carcinogenesis. The gene mutation theory of oncogenesis maintains that carcinogens interact with DNA resulting in irreversible changes in the gene (point mutations), which predispose the cells to malignant transformation. The somatic genetic changes in cells that contribute to multistage tumor development potentially involve sequential mutation of different classes of genes, ie. Proto-oncogenes, tumor suppressor genes, genes involved in cell cycle regulation, and genes that play roles in maintaining normal genomic stability. Biochemical interactions between tumor gene mutations may destabilize the genome, compromise control of cell signaling, proliferation, and differentiation, and interfere with the normal interaction of cells in tissues (Karp and Broder 1995; Skuse and Ludlow 1995).

Two classes of regulatory genes are directly involved in carcinogenesis, the oncogenes and the antioncogenes (Vogelstein and Kinzler 1998).

Oncogenes: They are positive regulators of carcinogenesis. In non-transformed cells, they are inactive (proto oncogenes). Gene mutations can activate proto-oncogenes, resulting in a gain of function. viral transformation of cellular genome, eg. c-erbB, cmos, c-myc, c-myb, C-H-ras (reviewed by Bishop 1987). A large number of mutations in specific oncogenes – eg. ras, myc, etc. – have been found to be closely associated with different types of cancers.

Anti-oncogenes or tumor suppressor genes: They are negative growth regulators. Many human tumors, eg. retinoblastoma, Wilm’s tumor, colon carcinoma, result from recessive mutation, which cause cancer when present on both homologues (Knudson 1993). These genes function as anti-oncogenes or tumor suppressor genes. In normal cells they regulate cell proliferation by checking cell cycle progression. Mutation in these genes results in a loss of gene function (the protein product will not be produced), which promotes carcinogenesis (Perkins and Stern 1997). Such gene mutations have been detected in several solid tumors, eg. cancers of breast, lung, rectum, etc., but only few such mutations have been seen in leukemias.

The two most widely studied tumor suppressor genes are the Rb gene and p53 gene. The proteins encoded by these genes inhibit cell cycle progression by blocking transcription of gene products necessary for transition from G1 to S phase. Mutation in the Rb gene could lead to loss of normal inhibitory control of cell cycle progression and, thereby, increase cell proliferation. This effect, coupled with genetic changes that cause loss of apoptotic signals, would enhance malignant transformation (Symonds 1994).

p53 has a major role in maintaining the genomic stability and cellular equilibrium. In normal cells, this gene promotes apoptosis, regulates cell cycle through G1 – S checkpoint control and induces cell differentiation. p53 participates in a cell cycle checkpoint signal transduction pathway that causes either a G1 arrest or apoptotic cell death after DNA damage (Kastan 1997). Mutations in p53, resulting in loss of function, will cause suppression of apoptosis, promote cell division by releasing the G1-S block and prevent differentiation of the cells, leading to neoplasm development (Curtis 1993). Mutations in the p53 gene are the most common genetic change observed in a large number of human malignancies; at least 50% of all human cancers have been found to contain p53 abnormality (Hollstein et al. 1991). Mutations in this gene have been observed in a wide range of human cancers like cancers of the breast, lung, colon, skin, urinary bladder, ovary and lymphoid organs. More than 500 mutations of this gene have been documented in breast cancer (Hartmann et al. 1997).

THEORIES OF CARCINOGENESIS

Gene mutation theory: This theory maintains that somatic gene mutations form the basis of neoplastic transformation and their clonal expansion leading to carcinogenesis. It is the most widely accepted and is supported by a large volume of experimental data (review by Bishop 1987). However, it does not explain tumor heterogeneity and aneuploidy and also the long latent periods between exposure to carcinogens and the development of tumors, which is actually made clear by chronic inflammation and accumulated toxic load.

Aneuploidy theory: Another theory that is currently gaining momentum is the aneuploidy hypothesis. According to this hypothesis, a carcinogen initiates carcinogenesis by a preneoplastic aneuploidy, which destabilizes mitosis. This initiates an autocatalytic karyotype evolution that generates new chromosomal variants, including rare neoplastic aneuploidy (Duesberg et al. 2001). The aneuploidy hypothesis provides a plausible explanation for the long latent periods from carcinogen treatment to cancer development and the clonality.

Epigenetic theory: It has been recognized that non-mutational stable changes occur in cellular genome, which can contribute to carcinogenesis (Feinberg 1993 Cross and Bird 1995). Such events are broadly termed epigenetic and are thought to involve DNA methylation, genome imprinting and changes in DNA – nucleoprotein structure. Increased levels of methylated cytosine (one of the pyrimidine bases in DNA) results in the elevation of spontaneous mutation rates in the affected genome (Balmain 1995).

Chronic Inflammation theory: This increasingly accepted theory is that all lifestyle factors that cause cancer (carcinogenic agents) and all agents that prevent cancer (chemopreventive agents) are linked through chronic inflammation. The fact that chronic inflammation is closely linked to the tumorigenic pathway is evident from numerous lines of evidence.

First, inflammatory markers such as cytokines (such as TNF, IL-1, IL-6, and chemokines), enzymes (such as COX-2, 5-LOX, and matrix metalloproteinase-9 [MMP-9]), and adhesion molecules (such as intercellular adhesion molecule 1, endothelium leukocyte adhesion molecule 1, and vascular cell adhesion molecule 1) have been closely linked with tumorigenesis.

Second, all of these inflammatory genenproducts have been shown to be regulated by the nuclear transcription factor, NF-κB.

Third, NF-κB has been shown to control the expression of other gene products linked with tumorigenesis such as tumor cell survival or antiapoptosis (Bcl-2, Bcl-xL, IAP-1, IAP-2, XIAP, survivin, cFLIP, and TRAF-1), proliferation (such as c-myc and cyclin D1), invasion (MMP-9), and angiogenesis (vascular endothelial growth factor).

Fourth, in most cancers, chronic inflammation precedes tumorigenesis.

Fifth, most carcinogens and other risk factors for cancer, including cigarette smoke, obesity, alcohol, hyperglycemia, infectious agents, sunlight, stress, food carcinogens, and environmental pollutants, have been shown to activate NF-κB.

Sixth, constitutive NF-κB activation has been encountered in most types of cancers.

Seventh, most chemotherapeutic agents and γ-radiation, used for the treatment of cancers, lead to activation of NF-κB.

Eighth, activation of NF-κB has been linked with chemoresistance and radioresistance.

Ninth, suppression of NF-κB inhibits the proliferation of tumors, leads to apoptosis, inhibits invasion, and suppresses angiogenesis.

Tenth, polymorphisms of TNF, IL-1, IL-6, and cyclin D1 genes encountered in various cancers are all regulated by NF-κB. Also, mutations in genes encoding for inhibitors of NF-κB have been found in certain cancers.

Eleventh, almost all chemopreventive agents described above have been shown to suppress NF-κB activation.

While each theory has its own merits, it may not be possible to assign an exclusive role to a single process alone in carcinogenesis. In many cases, a combination of multiple processes may work in cooperation. An initiating somatic gene mutation can destabilize the genome and lead to aneuploidy and chromosome heterogeneity, characteristic of solid tumors, while epigenetic events can contribute to the neoplastic cell transformation and also facilitate promotional changes.

FACTORS INFLUENCING CANCER DEVELOPMENT

A number of intrinsic (biological) and external factors are associated with the development of cancers. The intrinsic factors include the age and hormonal status of the individual, familial history and genetic predisposition. The extraneous factors include diet and life style, individuals habits like smoking and alcohol use, exposure to toxic chemicals and radiation, some infections, etc. Countless external factors, including asbestos, many chemicals, dyes, food additives, vehicular emissions, numerous household cleaning products, and hydrogenated oils, just to name a few can act as significant promoters in carcinogenesis.

Biological factors:

Age and hormonal status: Cancer has long been considered to be an old age disease, since some types of cancers are almost entirely found in people above 50-55 years, eg. prostate cancer. Similarly cervix cancer in women are more commonly detected at the peri- or post-menopausal ages. However, no age group is immune to this disease, and what were once considered cancers of the aged are now becoming increasingly common in the relatively young. Hormonal factors play an important role in the development of gender-specific cancers, eg. Estrogen in cancers of ovary and uterus in female (Henderson et al. 1988).

Family history: Some cancers are indicated to have a small link with familial occurrence. For example, women whose close relatives like grandmother, mother, maternal aunt or sister has suffered from breast cancer, are found to run about 3 times higher risk of developing breast cancer than those who do not have such a family history. Similarly, cancers of the uterine cervix (females) and of prostate (males) are also thought to have a minor familial connection.

Genetic predisposition: Certain rare genetic conditions are known to predispose the individual to cancer. For example, individuals with genetic conditions like xeroderma pigmentosum, ataxia telangiectasia, Bloom’s syndrome, and Fanconi’s anaemia are found to be highly susceptible to different types of cancer (Bale and Li 1997).

External factors:

Diet, alcohol, and tobacco use: More than 90% of all cancers are beleieved to be related to the diet, individual habits like alcoholism, tobacco chewing and smoking, and environmental exposures. A high saturated fat diet and obesity are associated with breast cancer. A positive correlation has been reported between age-adjusted breast cancer mortality rates and the average per capita fat consumption in a given nation on a daily basis (Carroll et al. 1975). Similarly, deep-fried and burnt food and preserved (high salt) food are associated with increase in gastric cancer incidence. Regular consumption of food low in fibre content and rich in animal fat increased the risk of cancers of stomach and oesophagus. High intake of red meat and low fibre diet has been considered to be the cause of the high incidence of gastric cancer in the USA.

The role of cigarette smoking in lung cancer is established. Tobacco smoke contains a chemical, nitrosamine, which can induce neoplastic changes in the lung cells. Non-smoking tobacco habits, like chewing, are found to greatly increase the cancers of the upper alimentary tract and buccal mucosa. For example, India has the highest incidence of oral cancers in the world, which is correlated with the widespread tobacco chewing habit. Alcoholism is found to increase the risk of liver and bladder cancers. Smoking combined with alcohol consumption poses a higher risk of cancers of the breast, oesophagus, liver, stomach and urinary bladder. Alcoholism along with hepatitis B virus infection is a more serious risk factor in liver cancer.

Radiation and cancer:

Ionizing radiation is an established carcinogen, having both initiating and promoting effects. The positive correlation between ionizing radiation and carcinogenesis has been established from the studies on the early radiologists, radium dial painters and atomic bomb victims of Japan. A positive association has been seen in the increase in childhood cancers and obstetric X-ray exposures of the mother (Knox et al. 1987). Tumors induced by radiation have relatively long latencies, which vary in different species as a more or less constant function. Within a given species the latency varies also with age at the time of irradiation and with the type of neoplasm induced. The age differences in latencies appear to be related to similar age differences in the rates of corresponding spontaneous leukemias (Upton et al. 1964). The risk of adult type of malignancies tend to increase progressively with time after irradiation, in parallel with the age-dependent increase in the underlying base-line incidence (UNSCEAR 1993).

Viruses and cancer:

Oncoviruses play an important role in specific human cancers, eg. human papilloma virus in cervix cancer (zur Hausen 1994), and certain skin cancers (Mc Grae et al. 1993); Epstein-Barr virus in Burkitt lymphoma (Tosato et al. 1994) and nasopharyngeal carcinoma (Fandi and Cvitkovoc 1995); hepatitis B virus in hepatocellular carcinoma (Robinson 1994); human T-cell leukemia virus in leukemia (Feuer and Chen 1992). The viruses are of two types: DNA viruses which incorporate into the cellular genome and the retroviruses (RNA viruses) which cause transformation of cellular genome, leading to malignant changes in the infected cell.

Role of free radicals:

Reactive oxygen species (ROS) and other free radicals are produced in the body, both during the normal metabolic process as well as by interaction with external toxic agents, for example, radiation, toxic chemicals, and hydrogenated oils. They include superoxide anions, hydroxyl radicals, peroxy radicals and hydroperoxides. These interact with DNA and produce gene mutations and chromosomal aberrations, leading to cell transformation. Free radicals are considered to have a major role in the induction of cancers by chemicals and radiation (Clayson et al. 1994). Several factors of our modern life style, eg. excess alcohol consumption, tobacco chewing and smoking habits, exposure to toxic chemicals and radiations, all add to the free radical production in the body and increase the risk of cancer.

CELLULAR DEFENSE MECHANISMS IN RELATION TO CANCER PREVENTION AND CARCINOGENESIS

Normal cells are naturally equipped with efficient defense mechanisms that work at different levels.

Antioxidants: The cells synthesize their own defense molecules, which include the non-protein thiol gluthathione, and antioxidant enzymes like superoxide dismutase, catalase, glutathione peroxidase, reductase and S-transferases. These scavenge the ROS before they can reach the target molecules in the cell (Halliwell et al. 1992) and thus protect against their attack on the vital molecules like DNA. Thus they serve as the biological watchdogs in safeguarding against free radical induced initiating changes, mutations and chromosomal aberrations. Many dietary ingredients like green vegetables, fruits, tea, spices and some diet supplements contain antioxidants. These include the vitamins A, C, and E, beta-carotene, alpha-tocopherol, ascorbic acid, flavonoids, lycopenes, curcumins and enzymes like caspasine. They act as chemo-preventers by scavenging free radicals and enhancing cellular defense through their adaptogenic properties.

DNA repair: Damage to cellular DNA is the crucial early event in the neoplastic transformation of a cell. The DNA lesions may include altered bases, co-valent binding of bulky adducts, inter- and intra-strand crosslinks and generation of strand breaks. A range of alkylated products is formed in DNA by exposure to nitroso-compounds and other alkylating agents.

Ionizing radiation and many genotoxic chemicals generate free radicals, which interact with DNA and produce different lesions ranging from base damage, deletions and complex and multiple lesions. Most normal cells possess a high capacity for repair of DNA damage. However, efficient repair depends on the type of damage, its severity and the time available for repair. The base damage and single strand breaks are repaired fast and without error, restoring the molecular structure. But double strand breaks and multiple breaks and local cluster lesions are not properly repaired and often contain errors (error-prone repair or misrepair), leading to cell death or cell survival with abnormal gene functions and chromosomal abnormalities which are associated with malignant cell transformation. DNA repair involves a number of genes, the products of which operate in a co-ordinated manner to form repair pathways that control restitution of DNA structure (reviewed by Hall 1993).

Apoptosis or programmed cell death is an important mechanism of cellular defense in reducing the risks of error-prone repair. Cells with DNA damage undergo apoptosis, thus preventing these cells from surviving and entering the proliferating cell pool and, thereby, preventing the possibility of tumor development. Apoptosis is a genetically controlled process involving p53, bcl2 and other genes. Mutations in p53 can block the tumor-suppressive effect by eliminating apoptosis (Curtis 1993), and ,thus, allowing the damaged cells to survive and undergo proliferation (UNSCEAR 2000). Some of the gene products that control cell cycle also influence apoptotic tendencies, eg. c-myc, pRb, Tp53.

ROLE OF DIET IN CANCER CONTROL

Doll and Peto (1981) were the first to point out an association between dietary constituents and cancer. Epidemiological studies have suggested that diets rich in vegetables, and fruits reduces the risk of certain cancers. For example, diets rich in fibre, vitamins A,C, and E, beta-carotene, retinols, alpha-tocopherol, polyphenols, and flavonoids, and minerals like selenium and zinc, have cancer chemopreventive effect. Fruits and vegetables are rich sources of chemopreventive chemicals. These include inhibitors of carcinogen formation, blocking agents (block conversion of procarcinogens to carcinogens), stimulators of detoxifying system, trapping agents (trap and eliminate potential carcinogens) and suppressing agents (suppress the different steps of the metabolic pathway leading to cancer) (Stavric 1994). A study in China showed a high incidence of oesophageal and gastric cancers in a population whose diet is deficient in beta-carotene and vitamins C and E. An interventional program, where the diet was supplemented with beta-carotenes, vitamin E and selenium, produced a 20% reduction in the stomach cancer mortality over a period 5 years (Blot et al. 1993).

WHO has recommended dietary intervention in the cancer control strategy for the new millennium. Dietary intervention follows two approaches:

1. Intervention through supplementing with vitamins, antioxidants and other dietary factors.

2. Intervention through dietary modification in which target levels are established for consumption of meat, fat, fiber, fruits and vegetables (Schatzkin et al. 1995).

CONCLUSIONS

Cancer is a broad term to describe a large variety of diseases, the common feature of which is uncontrolled cell division. The process of carcinogenesis consists of three major steps: initiation, where an irreversible change is affected in the cellular genes; promotion, where the initiated cells expand by self-proliferation leading to abnormal growth and further mutations; and progression, where the cells detach from the primary tumor and invade other organs and tissues, forming metastatic growths. Angiogenesis plays an important role in the tumor metastasis. Different types of cancer genes – oncogenes and antioncogenes (tumor suppressor genes) – are involved in cancer development. Gain of function mutations in the oncogenes, leading to abnormal cell proliferation, and loss of function mutations in the anti-oncogenes leading to suppression of cell differentiation and apoptosis, are the major events leading to cancer development. Chromosomal aneuploidy and epigenetic events are also thought to be important. Several factors like age, sex, genetic predisposition, along with extrinsic factors like diet, environmental pollutants, alcoholism and tobacco habits have a major role in determining the cancer risk. Dietary intervention as a cancer preventive measure is a primary agenda on the WHO program.

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Perkins AS and Stern DF (1997). Molecular biology of cancer. In: VT DeVita, S Hellman and SA Rosenberg (eds.). Cancer: Principles and Practice of Oncology, 5th ed., Lippincott Raven Publ., Philadelphia, pp. 79-102.

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Rowley JD and Mitelman F (1993). Principles of molecular biology of cancer: chromosome abnormalities in human cancer and leukemia. In: DeVita, Hellman S and Rosenberg SA (eds.), Cancer: Principles and Practice of Oncology, 4th ed. JB Lippincott, Philadelphia.

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introduction-to-cancer-biology-part-1-abnormal-signal-transductionIntroduction to Cancer Biology Part 1 Abnormal Signal Transduction

introduction-to-cancer-biology-part-2-loss-of-apoptosisIntroduction to Cancer Biology Part 2 Loss of Apoptosis

introduction-to-cancer-biology-part-3-tissue-invasion-and-metastasisIntroduction to Cancer Biology Part 3 Tissue Invasion and Metastasis

introduction-to-cancer-biology-part-4-angiogenesisIntroduction to Cancer Biology Part 4 Angiogenesis

 

To access the drfarrahcancercenter.com site, you must agree to the following;

Terms of Use

OVERVIEW

Please read these Terms of Use carefully before accessing or using our website. By accessing or using any part of the site, you agree to be bound by these Terms of Use. If you do not agree to all the terms and conditions of this agreement, then you may not access the website or use any services.

This website is operated by private parties in the Republic of Panama. Throughout the site, the terms “we”, “us” and “our” refer to drfarrahcancercenter.com, which offers this website, including all information, tools and services available from this site to you, the user, conditioned upon your acceptance of all terms, conditions, policies and notices stated here.

You understand and agree that you are accessing a website registered, owned, and hosted in the Republic of Panama. Therefore, all issues of a legal nature fall under the jurisdiction of the Republic of Panama legal system.

You understand and agree that as an authorized user you may only access information to which you have the legal authority to view and use. drfarrahcancercenter.com will monitor and record activity on this system to protect the system and its information and may use that monitoring information for official administrative or legal purposes. You understand and agree that you are not permitted to record, screenshot, or disseminate any information contained on the drfarrahcancercenter.com website to any party for any purpose whatsoever, and as such, you agree not to engage in such banned activities.

You understand and agree that unauthorized use of the system such as gaining unauthorized access to data, changing data, causing harm to the system or its data, or misuse of components is prohibited and may result in criminal, civil, or administrative penalties.

You also understand that drfarrahcancercenter.com can suspend or stop your use of this system if drfarrahcancercenter.com suspects any unauthorized use of the system attributable to you has occurred.

Your access to and use of the drfarrahcancercenter.com site is conditioned on your acceptance of and compliance with these Terms. These terms apply to all visitors, users and others who access or use the drfarrahcancercenter.com site. By accessing or using the drfarrahcancercenter.com site, you agree to be bound by these Terms. If you disagree with any part of the following Terms of Use, then you may not access the drfarrahcancercenter.com site.

By visiting our site and/ or purchasing anything from us, you engage in our “Service” and agree to be bound by the following terms and conditions (“Terms of Use”, “Terms”), including those additional terms and conditions and policies referenced herein and/or available by hyperlink. These Terms of Use apply to all users of the site, including without limitation users who are browsers, vendors, customers, merchants, and/ or contributors of content.

Any new features or tools which are added to the current website shall also be subject to the Terms of Use. You can review the most current version of the Terms of Use at any time on this page. We reserve the right to update, change or replace any part of these Terms of Use by posting updates and/or changes to our website. It is your responsibility to check this page periodically for changes. Your continued use of or access to the website following the posting of any changes constitutes acceptance of those changes.

 

SECTION 1 – ONLINE WEBSITE TERMS

By agreeing to these Terms of Use, you represent that you are at least the age of majority in your state, province, country or city of residence, or that you are the age of majority in your state, province, country or city of residence and you have given us your consent to allow any of your minor dependents to use this site.

You may not use any of our website for any illegal or unauthorized purpose nor may you, in the use of the Service, violate any laws in your jurisdiction (including but not limited to copyright laws).

You must not transmit any worms or viruses or any code of a destructive nature.

A breach or violation of any of the Terms will result in an immediate termination of your Services.

 

SECTION 2 – GENERAL CONDITIONS

We reserve the right to refuse service to anyone for any reason at any time.

You agree not to reproduce, duplicate, copy, sell, resell or exploit any portion of the Service, use of the Service, or access to the Service or any contact on the website through which the service is provided, without express written permission by us.

The headings used in this agreement are included for convenience only and will not limit or otherwise affect these Terms.

You understand and agree that this website is for information purposes only, and that by providing the information contained herein, we are not diagnosing, treating, curing, mitigating, or preventing any type of disease or medical condition. You understand and agree that before beginning any type of natural, integrative or conventional treatment regimen, it is advisable to seek the advice of a licensed healthcare professional.

You understand and agree that the information on the site is presented for the sole purpose of disseminating health information for general educational purposes only. You understand and agree that if you think you may have a medical emergency, you should call your doctor or 911. You understand and agree that the information on this site is not intended or implied to be medical advice, and you understand and agree that the information does not constitute the provision or practice of medical, nursing, or professional health care advice or services. You understand and agree that you will not use the information on this site as a substitute for professional medical advice, diagnosis, or treatment. You understand and agree that you should always seek the advice of your physician or other qualified health care provider prior to starting any treatment or with any questions you may have regarding a medical condition.

You understand and agree that nothing contained on this site is intended to be or will be used by you for medical diagnosis or treatment. You understand and agree that you should never disregard professional medical advice or delay in seeking treatment based on the information contained on this site. You understand and agree that the service and any content or information contained on the site is provided on an ‘as is’ basis. You understand and agree that while efforts have been made to assure the accuracy of the information on this site, there is no guarantee that such information is accurate or up-to-date. You understand and agree that, except for information, products, or services clearly identified as being supplied by the site, the site does not operate, control, or endorse any information, products, or services on the internet, in any way.

In addition, access to this site constitutes acceptance and understanding of the following,

By accessing this system, you hereby acknowledge, consent, and agree to all listed provisions and consent to monitoring.

You understand and agree that any copyright laws pertaining to the ownership and reproduction of all data fall exclusively under Panamanian legal jurisdiction for drfarrahcancercenter.com

If you are a copyright holder, or a director, officer, owner, employee, agent, supplier, licensor, contributor, service provider, website hosting company, trade partner, heir or assign of a copyright holder and feel that you or anyone you are engaged in any form of relationship whatsoever may have rights to data as part of this site or related sites, your agreement with the terms of use constitutes the following agreement, which is mandatory in order to access the data contained on this site.

I do hereby declare, understand, agree and warrant that I am a copyright owner, or an authorized representative of a copyright owner of film, print, slide, movie, video, artwork, digital image, negatives or any other material in any format whatsoever (hereafter referred to as “COPYRIGHTED DATA”) which I believe to be, known to be, or suspect to be contained on the drfarrahcancercenter.com website.

I hereby freely grant a non-exclusive license to drfarrahcancercenter.com and its agents to reproduce these COPYRIGHT DATA in perpetuity, and I represent and warrant that I have the legal right and authority to grant such a license. I may at my discretion ask to be credited for my contribution or the contributor I represent to the drfarrahcancercenter.com site as a contributor whether this occurred with or without my knowledge, but even if I or the contributor I represent remains uncredited, this agreement shall survive since participation in the free flowing of information for the public at large is a paramount responsibility we all should share. Therefore, in the best interest of free-flowing information for the public at large, I am freely undertaking this agreement, and clearly warrant that I have the authority to do so. I agree to indemnify and hold harmless drfarrahcancercenter.com and any of its directors, officers, owners, employees, agents, suppliers, licensors, contributors, service providers, website hosting companies, trade partners, heirs and assigns from any and all liability, damages, and expenses (including reasonable actual attorney’s fees) that may incur as a result of use and publication of said material, including any claims brought by any person claiming an interest in the COPYRIGHTED DATA or their subject matter. I agree and warrant that anyone containing any format of data from drfarrahcancercenter.com contained in any medium outside of the drfarrahcancercenter.com website itself, is bound by this agreement, since acceptance of this agreement is the only way to legally access such data. In addition, I understand and agree that accessing or storing any format of data contained in any medium outside of the drfarrahcancercenter.com website where it is hosted, constitutes a violation of the copyright laws of Panama against the offending party. I understand and agree that if I copy, contain, possess, or transmit any such data from the drfarrahcancercenter.com site in any format outside of the website itself, that my possession of such materials is a violation of Panamanian laws, and I agree to destroy such data forthwith.

 

SECTION 3 – ACCURACY, COMPLETENESS AND TIMELINESS OF INFORMATION

We are not responsible if information made available on this site is not accurate, complete or current. The material on this site is provided for general information only and should not be relied upon or used as the sole basis for making decisions without consulting primary, more accurate, more complete or more timely sources of information. Any reliance on the material on this site is at your own risk.

This site may contain certain historical information. Historical information, necessarily, is not current and is provided for your reference only. We reserve the right to modify the contents of this site at any time, but we have no obligation to update any information on our site. You agree that it is your responsibility to monitor changes to our site.

 

SECTION 4 – MODIFICATIONS TO THE SERVICE

We reserve the right at any time to modify or discontinue the Service (or any part or content thereof) without notice at any time.

We shall not be liable to you or to any third-party for any modification, change, suspension or discontinuance of the Service.

 

SECTION 5 – OPTIONAL TOOLS

Any use by you of optional tools offered through the site is entirely at your own risk and discretion and you should ensure that you are familiar with and approve of the terms on which tools are provided by the relevant third-party provider(s).

We may also, in the future, offer new services and/or features through the website (including, the release of new tools and resources). Such new features and/or services shall also be subject to these Terms of Use.

 

SECTION 6 – THIRD-PARTY LINKS

Certain content, products and services available via our Service may include materials from third-parties.

Third-party links on this site may direct you to third-party websites that are not affiliated with us. We are not responsible for examining or evaluating the content or accuracy and we do not warrant and will not have any liability or responsibility for any third-party materials or websites, or for any other materials, products, or services of third-parties.

We are not liable for any harm or damages related to the purchase or use of goods, services, resources, content, or any other transactions made in connection with any third-party websites. Please review carefully the third-party’s policies and practices and make sure you understand them before you engage in any transaction. Complaints, claims, concerns, or questions regarding third-party products should be directed to the third-party.

 

SECTION 7 – USER COMMENTS, FEEDBACK AND OTHER SUBMISSIONS

If, at our request, you send certain specific submissions (for example contest entries) or without a request from us you send creative ideas, suggestions, proposals, plans, or other materials, whether online, by email, by postal mail, or otherwise (collectively, ‘comments’), you agree that we may, at any time, without restriction, edit, copy, publish, distribute, translate and otherwise use in any medium any comments that you forward to us. We are and shall be under no obligation (1) to maintain any comments in confidence; (2) to pay compensation for any comments; or (3) to respond to any comments.

We may, but have no obligation to, monitor, edit or remove content that we determine in our sole discretion are unlawful, offensive, threatening, libelous, defamatory, pornographic, obscene or otherwise objectionable or violates any party’s intellectual property or these Terms of Use.

You agree that your comments will not violate any right of any third-party, including copyright, trademark, privacy, personality or other personal or proprietary right. You further agree that your comments will not contain libelous or otherwise unlawful, abusive or obscene material, or contain any computer virus or other malware that could in any way affect the operation of the Service or any related website. You may not use a false e-mail address, pretend to be someone other than yourself, or otherwise mislead us or third-parties as to the origin of any comments. You are solely responsible for any comments you make and their accuracy. We take no responsibility and assume no liability for any comments posted by you or any third-party.

 

SECTION 8 – ERRORS, INACCURACIES AND OMISSIONS

Occasionally there may be information on our site or in the Service that contains typographical errors, inaccuracies or omissions that may relate to product descriptions, pricing, promotions, offers, product shipping charges, transit times and availability. We reserve the right to correct any errors, inaccuracies or omissions, and to change or update information or cancel orders if any information in the Service or on any related website is inaccurate at any time without prior notice (including after you have submitted your order).

We undertake no obligation to update, amend or clarify information in the Service or on any related website, including without limitation, pricing information, except as required by law. No specified update or refresh date applied in the Service or on any related website, should be taken to indicate that all information in the Service or on any related website has been modified or updated.

 

SECTION 9 – PROHIBITED USES

In addition to other prohibitions as set forth in the Terms of Use, you are prohibited from using the site or its content: (a) for any unlawful purpose; (b) to solicit others to perform or participate in any unlawful acts; (c) to violate any international, federal, provincial or state regulations, rules, laws, or local ordinances; (d) to infringe upon or violate our intellectual property rights or the intellectual property rights of others; (e) to harass, abuse, insult, harm, defame, slander, disparage, intimidate, or discriminate based on gender, sexual orientation, religion, ethnicity, race, age, national origin, or disability; (f) to submit false or misleading information; (g) to upload or transmit viruses or any other type of malicious code that will or may be used in any way that will affect the functionality or operation of the Service or of any related website, other websites, or the Internet; (h) to collect or track the personal information of others; (i) to spam, phish, pharm, pretext, spider, crawl, or scrape; (j) for any obscene or immoral purpose; or (k) to interfere with or circumvent the security features of the Service or any related website, other websites, or the Internet. We reserve the right to terminate your use of the Service or any related website for violating any of the prohibited uses.

 

SECTION 10 – DISCLAIMER OF WARRANTIES; LIMITATION OF LIABILITY

We do not guarantee, represent or warrant that your use of our service will be uninterrupted, timely, secure or error-free.

We do not warrant that the results that may be obtained from the use of the service will be accurate or reliable.

You agree that from time to time we may remove the service for indefinite periods of time or cancel the service at any time, without notice to you.

You expressly agree that your use of, or inability to use, the service is at your sole risk. The service and all products and services delivered to you through the service are (except as expressly stated by us) provided ‘as is’ and ‘as available’ for your use, without any representation, warranties or conditions of any kind, either express or implied, including all implied warranties or conditions of merchantability, merchantable quality, fitness for a particular purpose, durability, title, and non-infringement.

In no case shall anyone affiliated with drfarrahcancercenter.com including our directors, officers, employees, affiliates, agents, contractors, interns, suppliers, service providers or licensors be liable for any injury, loss, claim, or any direct, indirect, incidental, punitive, special, or consequential damages of any kind, including, without limitation lost profits, lost revenue, lost savings, loss of data, replacement costs, or any similar damages, whether based in contract, tort (including negligence), strict liability or otherwise, arising from your use of any of the service or any products procured using the service, or for any other claim related in any way to your use of the service or any product, including, but not limited to, any errors or omissions in any content, or any loss or damage of any kind incurred as a result of the use of the service or any content (or product) posted, transmitted, or otherwise made available via the service, even if advised of their possibility. Because some states or jurisdictions do not allow the exclusion or the limitation of liability for consequential or incidental damages, in such states or jurisdictions, our liability shall be limited to the maximum extent permitted by law.

You understand and agree that no singular individual, group of individuals or entity in any form whatsoever is responsible or liable in any manner for any content generated on, for, or as a result of the existence of this site, since this is a community generated site. You understand and agree that as this is a community generated site, and as a result of this, there is no good way to control what users and contributors post on or through the sites and drfarrahcancercenter.com cannot be responsible for any offensive, inappropriate, obscene, unlawful, infringing or otherwise objectionable or even illegal user generated content you may encounter on the sites or, in connection with your use of the sites.

You understand and agree with the following statement made on behalf of drfarrahcancercenter.com, “We, on behalf of our directors, officers, employees, agents, suppliers, licensors, contributors and service providers, exclude and disclaim liability for any losses and expenses of whatever nature and howsoever arising including, without limitation, any direct, indirect, general, special, punitive, incidental or consequential damages; loss of use: loss of data; loss caused by a virus: loss of income or profit: loss of or damage to property: loss of life: claims of third parties: or other losses of any kind or character, or the inability to use, the site or the content even if we have been advised of the possibility of such damages or losses, arising out of or in connection with the use of this site or any web site with which it is linked.”

 

SECTION 11 – INDEMNIFICATION

You agree to indemnify, defend and hold harmless drfarrahcancercenter.com and our parent, subsidiaries, affiliates, partners, officers, directors, agents, contractors, licensors, service providers, subcontractors, suppliers, interns and employees, harmless from any claim or demand, including reasonable attorneys’ fees, made by any third-party due to or arising out of your breach of these Terms of Use or the documents they incorporate by reference, or your violation of any law or the rights of a third-party.

 

SECTION 12 – SEVERABILITY

In the event that any provision of these Terms of Use is determined to be unlawful, void or unenforceable, such provision shall nonetheless be enforceable to the fullest extent permitted by applicable law, and the unenforceable portion shall be deemed to be severed from these Terms of Use, such determination shall not affect the validity and enforceability of any other remaining provisions.

 

SECTION 13 – TERMINATION

The obligations and liabilities of the parties incurred prior to the termination date shall survive the termination of this agreement for all purposes.

These Terms of Use are effective unless and until terminated by us. You may terminate use of the drfarrahcancercenter.com site, but this Terms of Use shall survive in perpetuity.

If in our sole judgment you fail, or we suspect that you have failed, to comply with any term or provision of these Terms of Use, we also may terminate this agreement at any time without notice and you will remain liable for all amounts due up to and including the date of termination; and/or accordingly may deny you access to our Services (or any part thereof).

 

SECTION 14 – ENTIRE AGREEMENT

The failure of us to exercise or enforce any right or provision of these Terms of Use shall not constitute a waiver of such right or provision.

These Terms of Use and any policies or operating rules posted by us on this site or in respect to The Service constitutes the entire agreement and understanding between you and us and govern your use of the Service, superseding any prior or contemporaneous agreements, communications and proposals, whether oral or written, between you and us (including, but not limited to, any prior versions of the Terms of Use).

Any ambiguities in the interpretation of these Terms of Use shall not be construed against the drafting party.

 

SECTION 15 – GOVERNING LAW

These Terms of Use and any separate agreements whereby we provide you Services shall be governed by and construed in accordance with the laws of the Republic of Panama.

 

SECTION 16 – CHANGES TO TERMS OF USE

You can review the most current version of the Terms of Use at any time at this page.

We reserve the right, at our sole discretion, to update, change or replace any part of these Terms of Use by posting updates and changes to our website. It is your responsibility to check our website periodically for changes. Your continued use of or access to our website or the Service following the posting of any changes to these Terms of Use constitutes acceptance of those changes.

 

SECTION 17 – CONTACT INFORMATION

Questions about the Terms of Use should be sent to us at staff@drfarrahcancercenter.com

Upon agreeing to these terms and conditions, you gain access to the drfarrahcancercenter.com website and assume total responsibility for any and all actions undertaken by you as a result of your access to the drfarrahcancercenter.com website. You agree and understand that the terms of this agreement shall be binding upon you, your respective heirs, successors, assigns and legal representatives. You understand and agree that all provisions of this Terms of Use agreement that by their nature should survive termination shall survive termination, including, without limitation, ownership provisions, warranty disclaimers, indemnity, licensing in perpetuity and limitations of liability.