Lecture 10.1 - Neoplasia 3 PDF

Hallmarks of cancer: The cause of neoplasia is multifactorial: ◦A critical combination of intrinsic host factors such as heredity, age and sex (especially hormonal), and extrinsic factors related to the environment and behaviour account for cancer risk. Much of the increased cancer...

Hallmarks of cancer: The cause of neoplasia is multifactorial: ◦A critical combination of intrinsic host factors such as heredity, age and sex (especially hormonal), and extrinsic factors related to the environment and behaviour account for cancer risk. Much of the increased cancer incidence over the last century is due to prolonged life- span. ◦Intrinsic = heredity, age, sex ◦Extrinsic = environment, lifestyle Estimated percentage of cancer deaths attributed to different factors: ◦Diet - 41% ◦Tobacco - 24% ◦Infection - 10% ◦Ultraviolet light - 10% ◦Sexual factors - 7% ◦Occupation - 5% ◦Alcohol - 3% ◦Pollution - 1% ◦Medical procedures - 1% ◦Ionising radiation - 1% How do we link environmental factors to carcinogenesis?: ◦Observation: ‣ In 1567, Paracelsus suggested that the "wasting disease of miners" might be attributed to exposure to realgar (arsenic sulfide) ‣ In 1761, John Hill noted that nasal cancer occurred in some people who used snuff excessively and in 1859 Bouisson described oral cancer in tobacco smokers. ‣ The London surgeon Percival Pott reported in 1775 that cancer of the scrotum sometimes developed in men after being exposed in childhood when they worked as chimney sweeps ‣ Work with radium suggested the induction of skin cancer by repeated X-ray burns ◦Epidemiology: ‣ Epidemiological evidence has been important in detecting carcinogenic substances. Rehn (1895) reported an increased incidence of bladder cancer in aniline dye workers in Germany. The major carcinogen involved is now believed to be 2-naphthylamine. ‣ Almost 50 years ago, studies of cancer mortality among Japanese immigrants to Hawaii and the US mainland reported remarkable reductions in stomach cancer deaths with concomitant increases in breast and colorectal cancer mortality. Since then, numerous studies of immigrants to the United States have reported the common phenomenon of decreasing incidence rates of cancers of infectious origin, such as liver (linked to hepatitis B virus), stomach (associated with Helioobacter pylori infection), and cervix (caused  by human papilloma virus), common in the countries of origin, whereas incidence rates of breast, colon, prostate, and lung cancer have increased despite remaining relatively low in the host nation. ◦Experiment: ‣ The first chemical induction of cancer in laboratory animals was achieved by Yamagiwa and Ichikawa (1915) by painting coal tar on the ears of rabbits every 2-3 days for more than a year. ‣ The first pure carcinogen, 1,2,5,6-dibenzanthracene, was synthesized in 1929 and in the 1930s Kenneway and Cook and their associates isolated carcinogenic polycyclic aromatic hydrocarbons including benzo(a)pyrene from coal tar. ‣ In the early 1900s, Boveri proposed a mutation theory of carcinogenesis but at that time it was not amenable to chemical investigation. Testing of chemical carcinogens: ◦It has not been economically feasible to test all the compounds to which people may be exposed. Criteria for selection includes: ‣ Compounds related to known carcinogens ‣ New compounds that are to be placed in the environment ‣ Compounds that are indicated by surveys to be associated with an increased incidence of cancer In vitro testing of chemical carcinogens: ◦The high cost of animal screening has driven the search for short-term in vitro tests. The best known in vitro test is that devised by Bruce Ames which measure mutagenicity in a Salmonella strain that requires histidine for growth. Mutation can result in a reversion to the wild type phenotype that permits growth in the absence of histidine ◦Because many carcinogens require metabolic activation, the bacteria are incubated with a rat liver S9 fraction ◦The theoretical basis for tests of this type is the good but not perfect correlation between mutagenic and carcinogenic activity. For some studies this has been about 90% for large numbers of compounds but other studies have seen a correlation of about 75% The Ames test for mutagenicity: Mutagenic versus carcinogenic potency:  The diversity of chemical carcinogens: ◦The number of known carcinogens in experimental animals is large. It is suspected that most of these are potentially carcinogenic in humans but documentation is lacking in most cases. The following list includes substances for which there is good evidence of carcinogenicity in humans. Carcinogen metabolism and activation: ◦Chemical carcinogenesis appears to be associated with reaction with cellular nucleophiles (electron donor) ◦Many carcinogens must be metabolised to form electrophilic species (electron acceptor) ◦Organic compounds with double bonds may be metabolised to form reactive epoxides e.g. with benzo(a)pyrene, vinyl chloride and aflatoxin ◦Nitrosamines can be metabolised to form carbonium ions that react with guanine to give an O6-methyl derivative Benzo(a)pyrene activation: Different steps of carcinogenesis: ◦Initiation: ‣ Mutation in one or more cellular genes controlling key regulatory pathways of the cell (irreversible) - must be a heritable DNA alteration ◦Promotion: ‣ Selective growth enhancement induced in the initiated cell and its progeny by the continuous exposure to a promoting agent ◦Progression: ‣ Results from continuing evolution of unstable chromosomes; further mutations from genetic instability during promotion - results in further degrees of independence, invasiveness, metastasis etc Initiation: ◦Initiation is the induction of a mutation in a critical gene involved in the control of cell proliferation and/or apoptosis ◦As with mutational events, initiation requires one or more rounds of cell division for the "fixation" of the process ◦The metabolism of initiating agents to non-reactive forms and the high efficiency of DNA repair of the tissue can alter the process of initiation ◦Initiation is irreversible although the initiated cell may eventually die during the development of the neoplasm Mutational targets of initiation:  ◦Chemical and physical carcinogens initiate cells via: ‣ Mutational activation of oncogenic (proliferative) pathways e.g. growth factor receptors and downstream signalling proteins, proteins involved in cell cycle checkpoints ‣ Mutational inactivation of apoptotic (cell death) pathways (e.g. growth inhibitory receptors, proteins involved in apoptosis, tumour suppressors) ‣ Mutational inactivation of DNA repair mechanisms (e.g. BER, NER) ‣ Mutational inactivation of antioxidant response (e.g. Super Oxide Dismutase) p53 protein - the Guardian of the Genome: ◦p53 is mutated in most cancers ◦p53 is a transcriptional factor that controls cell cycle, apoptosis, DNA repair mechanisms ◦Mdm2 is a negative regulator of p53 that functions both as an E3 ubiquitin ligase and an inhibitor of p53 transcriptional activation ◦Carcinogens often inactivate p53 as well as proteins that control p53 function (e.g. Mdm2, p14) p53 signalling in unstressed cell vs stressed cell: Ras gene and protein: ◦RAS gene is mutated in approximately a third of all human malignant neoplasias ◦The RAS gene encodes a small G protein that relays growth promoting signals into the cell that release the cell cycle restriction points ◦Mutant RAS protein is always active ◦As for p52, carcinogens often inactivate RAS itself as well as proteins that control Ras function Ras - GTP - control of cell cycle and apoptosis: K-RAS and p53: ◦K-Ras and p53 are the two oncogenes most frequently mutated in smoking-related lung cancers  ◦Benzo(a)pyrene leads to mutations in K-Ras and p53 in the genomic loci found to be mutated in smoking-induced lung cancer ◦If not corrected by the cell’s DNA repair mechanism, this guanine “adduct” is misread as a thymine by the DNA polymerase that copies chromosomes during replication ◦Ultimately, the original G-C base pair may be replaced by a T-A base pair, a mutation called a transversion ◦Cells treated with Benzo(a)pyrene show the same spectrum of G-T transversions as found in the K-RAS and p53 of smokers. ◦These mutational “hot spots” map well to the guanine binding sites of BaP epoxide Tumour suppressor genes and oncogenes: ◦TP53 - a tumour suppressor gene ◦RAS - an oncogene ◦Genes that inhibit neoplastic growth are known as tumour suppressor genes. Because they act like brakes on tumour growth, both alleles must be inactivated. ◦In contrast, genes that enhance neoplastic growth are known oncogenes and are abnormally activated versions of normal genes called proto- oncogenes. Only one allele of each proto-oncogene needs to be activated to favour neoplastic growth. ◦Proto-oncogenes can encode growth factors (e.g. PDGF), growth factor receptors (e.g. HER2), plasma membrane signal transducers (e.g. RAS), intracellular kinases (e.g. BRAF), transcription factors (e.g. MYC), cell cycle regulators (e.g. CYCLIN D1) or apoptosis regulators (e.g. BCL2). ◦TS genes encode proteins in the same pathways but with anti- growth effects (e.g. TP53). Promotion: ◦Epigenetic event - change in gene expression without change in DNA ◦Mitogenic (not mutagenic) stimulates proliferation. Causes both mutated and normal cells to proliferate ◦Enhances the effect of the genotoxic initiating agent by establishing clones of initiated cells ◦Long delay possible between administration of initiating agent and promoting agent ◦Promotion is reversible Promoters: ◦Reactive oxygen species (ROS) and redox active xenobiotics and metals ◦Phorbol esters (e.g. TPA) ◦Polycyclic aromatic compounds (e.g. Dioxin) ◦Peroxisome proliferators (oxidised fats) ◦Endocrine disruptors (estradiol, DES) ‣ Structurally similar to normal signalling and regulatory molecules ‣ Mode of action - mimicry  Endocrine receptors and carcinogenesis: ◦Endocrine disruptors are involved in breast, ovarian, colon, prostate cancers ‣ ERβ/ERα (oestrogen receptors) ratio is decreased in cancers (ligands include estradiol); ERs are transcription factors. ‣ ERβ inhibits ERα ERα-ERα dimerisation (homodimer) leads to mitogenic activation. ERβ-ERα dimerisation (heterodimer) leads to an inactivation. ‣ Androgen Receptor (prostate) (AR) can also homodimerise with AR leading to mitogenic activation; AR can heterodimerise with ERβ to cause growth arrest (prostate also dependent on oestrogenic signals). Oestrogen receptors and endocrine disruptors - interactions: Initiation promotion relationship: Inflammation as a carcinogen: ◦Inflammation acts at all stages of tumorigenesis ◦It may contribute to tumour initiation through mutations and genomic instability ◦Inflammation activates tissue repair responses, induce proliferation of premalignant cells, and enhances their survival ◦Inflammation also stimulates angiogenesis, causes localised immunosuppression, and promotes the formation hospitable microenvironment in which premalignant cells can survive, expand, and accumulate additional mutations ◦Inflammation also promotes metastatic spread.  Infection as a carcinogen: ◦Some infections directly affect genes that control cell growth. Others do so indirectly by causing chronic tissue injury, where the resulting regeneration acts either as a promoter for any pre-existing mutations or else causes new mutations from DNA replication errors. ◦Human Papilloma virus (HPV), which is strongly linked to cervical carcinoma, is a direct carcinogen because it expresses the E6 and E7 proteins that inhibit p53 and pRB protein function respectively, both of which are important in cell proliferation. ◦In contrast, Hepatitis B and C viruses are indirect carcinogens that cause chronic liver cell injury and regeneration. Bacteria and parasites can also indirectly lead to neoplasms. Helicobacter pylori causes chronic gastric inflammation and parasitic flukes cause inflammation in bile ducts and bladder mucosa, increasing the risk for gastric, cholangio- and bladder carcinomas respectively. ◦Human Immunodeficiency virus (HIV) acts indirectly by lowering immunity and allowing other potentially carcinogenic infections to occur. Sporadic, familial and hereditary cancers: ◦People with sporadic cancer typically do not have relatives with the same type of cancer. ◦Familial Cancer – Cancer likely caused by a combination of genetic and environmental risk factors. Cancer that occurs in families more often than would be expected by chance. These cancers often occur at an early age, and may indicate the presence of a gene mutation that increases the risk of cancer. They may also be a sign of shared environmental or lifestyle factors. ◦Hereditary Cancer – Cancer occurs when an altered gene (gene change) is passed down in the family from parent to child. Inherited predisposition to neoplasia: ◦In 1872 a Brazilian ophthalmologist performed an enucleation (a removal of the eye) in a young boy with retinoblastoma. This boy survived and married a woman without any family history of cancer. The couple had two daughters with bilateral retinoblastoma also seen by the same ophthalmologist, Hilário de Gouvêa. This case became the first documented report of a family with retinoblastoma in more than one generation. ◦In the 1970s Knudson postulated a two hit hypothesis to explain the differences between tumours occurring in families and those occurring in the general population Knudson 2 hit hypothesis: ◦For familial cancers, the first hit is delivered through the germline and affected all cells in the body. The second hit is a somatic mutation. In the case of retinoblastoma this was in one of the 10 million+ retinal cells already carrying the first hit. ◦In contrast, sporadic retinoblastoma has no germline mutation and so requires both hits to be somatic mutations and to occur in the same cell. In 1986 the actual gene, RB, was identified. Several other malignant neoplasms are now recognised that have both familial and sporadic counterparts and that follow the same two hit genetic basis as retinoblastoma