Molecular Biology of Cancer BC.504 Lecture Notes PDF

# Molecular Biology of Cancer ## BC.504 Presented by: Dr. Amany H. Adam, Ph.D. Lecturer of Biochemistry, Faculty of Science, Damanhour University. ## Risk Factors for Development of Cancer | **Risk Factors** | **Intrinsic** | **Non-Intrinsic** | **Endogenous** | **Exogenous** | **Modifiable** |...

# Molecular Biology of Cancer ## BC.504 Presented by: Dr. Amany H. Adam, Ph.D. Lecturer of Biochemistry, Faculty of Science, Damanhour University. ## Risk Factors for Development of Cancer | **Risk Factors** | **Intrinsic** | **Non-Intrinsic** | **Endogenous** | **Exogenous** | **Modifiable** | |---|---|---|---|---|---| | **Random errors in DNA replication** | ✓ | | | | | | **Biologic aging** | | ✓ | ✓ | | Partially | | **Genetic susceptibility** | | ✓ | ✓ | | Partially | | **DNA repair machinery** | | ✓ | ✓ | | Partially | | **Hormones** | | ✓ | ✓ | | Partially | | **Growth factors** | | ✓ | ✓ | | Partially | | **Inflammation** | | ✓ | ✓ | | Partially | | **Radiation ** | | ✓ | | ✓ | ✓ | | **Chemical carcinogens** | | ✓ | | ✓ | ✓ | | **Tumor causing viruses** | | ✓ | | ✓ | ✓ | | **Bad Life Style** | | ✓ | | ✓ | ✓ | ### Common Risk Factors for Cancer - Obesity - Tobacco - Alcohol - Sedentary lifestyle - Poor diet - Excessive UV exposure - Carcinogenic chemicals - Age - Genetics - Radiation The backbone of cancer biology has been the identification of carcinogens and the identification of specific mutations as causative factors of carcinogenesis along with the elucidation of the pathways they affect. ## Radiation as a Carcinogen - Over the past 10 years, increased utilization of X-ray examinations and computed tomography (CT) has led to corresponding increases in patient exposure to ionizing radiation raising awareness of the public to its deleterious effects. - Despite notable decreases in the radiation dose associated with individual scans, increased utilization of medical imaging is a major contributor to radiation exposure and radiation-associated pathology. - It has been shown that there is a dose-dependent relationship between radiation exposure and leukemia, breast cancer, thyroid cancer, and other solid tumors. - Ionizing radiation has immediate, measurable deleterious effects on cells, including increasing reactive oxygen species (ROS), generation of single-stranded DNA breaks (SSBs), and double-stranded DNA breaks (DSBs). ### Generation of Reactive Oxygen Species (ROS) in Response to Ionizing Radiation - Ionizing radiation induces damage of cellular structures in two primary ways: direct damage to DNA and generation of free radical-containing reactive molecules. ## Downstream Molecules and Effects Following DNA Damage Due to Ionizing Radiation - Ionizing radiation causes damage to DNA both directly and indirectly. - Indirect damage occurs through the radiation-associated formation of free radicals. - Double-stranded breaks (DSBs) are the most common form of DNA damage associated with ionizing radiation. - After DSBs are generated, a cascade of enzymatic processes is triggered to allow for DNA repair or to induce apoptosis. This process includes the activation of p53 and the induction of cell cycle arrest. - If the damage exceeds the cell's ability to repair itself, either apoptosis or necrosis will occur. Alternatively, there are two common mechanisms of DSB repair: Non-homologous end joining and homologous recombination. - In homologous recombination, the enzymes BRCA 1 and BRCA 2 are activated and initiate repair. If repair is successful, the cell cycle can resume. - If homologous recombination is unsuccessful the cell will likely undergo apoptosis. - Importantly, failure of these processes in the setting of significant mutations in cell cycle regulation or the apoptotic pathway can lead to carcinogenic transformation. - In non-homologous end joining, as the name suggests, non-homologous ends are joined together to mitigate DNA damage. This can lead to significant mutations in cell cycle regulation and result in carcinogenic transformation. ## UV Radiation as a Carcinogen - Ultraviolet radiation (UV) from the sun is also carcinogenic and is a principal cause of skin cancer. - Of the 3 types of UV light - UVA, UVB, and UVC - UVB is the most effective carcinogen. - The conjugated double bonds in the rings of the nitrogenous bases of DNA absorb UV radiation. - UVB directly and uniquely causes characteristic UV photoproducts: cyclobutane pyrimidine dimers and pyrimidine-pyrimidone photoproducts. - Cyclobutane pyrimidine dimers are the most prevalent, formed at least 20-40 times more frequently than other UV photoproducts. - The formation of a pyrimidone (6-4) photoproduct mimics an abasic site (a nucleotide minus a base) and is more efficiently repaired than cyclobutane pyrimidine dimers. - The formation of a pyrimidine dimer causes a bend in the DNA helix and, as a result, DNA polymerase cannot read the DNA template. - Under these conditions DNA polymerase preferentially incorporates an “A” residue. Consequently, TT dimers are often restored but TC and CC dimers result in transitions (TC→TT and CC→TT). - Cyclobutane pyrimidine dimers are responsible for at least 80% of UVB-induced mutations in Skin Cancer cases. - UVA indirectly damages DNA via free radical-mediated damage. - Water is fragmented by UVA, generating electron-seeking ROS (such the hydroxyl radical) that cause DNA damage (e.g. oxidation of bases). - G→T transversions are characteristic of UVA damage. ## Carcinogenic Chemicals - Many chemicals in our environment and in our diet play a role in human carcinogenesis. - The common mechanism of action of chemical carcinogenesis that an electrophilic (electron-deficient) form reacts with nucleophilic sites (sites that can donate electrons) in the purine and pyrimidine rings of nucleic acids. - Some chemical carcinogens can act directly on DNA but others become active only after they are metabolized in the body, forming what are called ultimate carcinogens, the molecules that execute the damage. - A family of enzymes called the cytochrome P450 enzymes (Drug metabolizing enzymes) is involved in the metabolism of chemicals in the liver and is important in the activation of carcinogens to ultimate carcinogens. - Genetic polymorphisms and variable expression account for differences in responses to chemical carcinogens among individuals. ### Carcinogens can be segregated into 10 groups: 1. Polycyclic aromatic hydrocarbons (PAHs) 2. Aromatic amines 3. Azo dyes 4. Nitrosamines and Nitrosamides 5. Hyrazo and Azoxy compounds 6. Carbamates 7. Halogenated compounds 8. Natural products 9. Inorganic carcinogens 10. Miscellaneous compounds (alkylating agents, aldehydes, phenolics). - These carcinogens exert their effects by adding functional groups covalently to DNA. Chemically modified bases, called DNA adducts, distort the DNA helix causing errors to be made during replication. The resulting mutations initiate cell carcinogenesis. ## Polycyclic Aromatic Hydrocarbons (PAHs) - The first demonstration that chemicals could be used to induce cancer in animals was carried out in 1915. Coal tar, containing carcinogenic PAHs, induced skin carcinomas on the ears of rabbits. - Carcinogenic PAHs are derived from phenanthrene. Additional rings and/or methyl groups in the bay region of the three aromatic rings can convert inactive phenanthrene into an active carcinogen. - High levels of PAHs are present in cigarette smoke, vehicle exhaust, and charcoal-grilled foods. - PAHs must be metabolized further in order to give the ultimate carcinogen that will form adducts with purine bases of DNA. - The P450 enzyme, CYP1A1, is the predominant enzyme that metabolizes BP to the highly reactive mutagenic BP diol epoxides. - BP results mainly in G→T transversions. ## Polycyclic Aromatic Hydrocarbons (PAHs) - Mammary tumors can be produced in rodents following administration of DMBA by oral gavage, leading to up-regulation of the cellular cytosolic receptor for DMBA, the aryl hydrocarbon receptor (AhR). Upon ligand activation, the AhR translocates into the nucleus and associates with the cofactor ARNT, the AhR nuclear translocation protein. - This activated AhR/ARNT complex then binds to specific DNA recognition sites upstream of AhR responsive genes and induces gene transcription. - The early steps in tumorigenesis involve AhR-dependent up-regulation of cytochrome P450 enzymes, which metabolize DMBA into a mutagenic epoxide intermediate that readily forms DNA adducts. - These adducts are associated with DNA mutations and the malignant transformation that is thought to be involved in PAH-mediated carcinogenesis. ## Tobacco Smoking - At least 60 known or suspected carcinogens are present in cigarette smoke, including PAHs and nitrosamines; such as 4-(methylnitrosamino)-1-( 3-pyridyl)-1-butanone (NNK). - These and other compounds are processed by enzymes of the cytochrome P450 system as part of a detoxification process to generate more water-soluble metabolites that can be secreted. - Some of these metabolites are highly reactive due to the addition of electrophilic moieties and form specific types of DNA adducts. - Errors during repair of the adducts by the DNA repair machinery generate base changes, with C > A/G > T transversions most strongly associated with smoking-associated lung cancer and responsible for the high mutational burden of these cancers as well as specific hot spot mutations in the TP53 tumor suppressor gene that are more common in lung cancer. - (A) Tobacco smoke contains >70 classified carcinogens; shown are five compounds strongly associated with mutagenesis: benzo(a)pyrene (BaP), nicotine-derived NNK, N-nitrosodimethylamine (NDMA), 4-aminobiphenyl (4-ABP), and N-nitrosonornicotine (NNN). - Many of the compounds in tobacco smoke are metabolized by cytochrome P450, resulting in molecules with highly reactive electrophilic moieties. (Black bar) Representative molecular structures with electrophilic moieties produced from the chemicals metabolized by P450. Electrophilic moieties can readily interact with DNA to form DNA adducts. - DNA adducts can be repaired to correct the obstacle and re-establish "normal" DNA; this is frequently achieved by the cell's repair machinery through a process called nucleotide excision repair (NER). However, if repair is unsuccessful and cells do not undergo apoptosis, permanent procancerous mutations may be established. -(B) Epigenetic modification commonly refers to processes that do not directly alter genetic information encoded by DNA but rather alter availability of genes for transcription; for instance, by addition of reversible methyl or acetyl modifications to DNA or histones. - Chronic exposure to tobacco smoke extensively modifies the epigenome of cells in the affected tissue, with characteristic modifications, including hypermethylation of CpG islands (regions with high occurrence of cytosine and guanine separated by only one phosphate group, frequently found near gene promoters). - This hypermethylation, generally in the context of tobacco-induced mutations, leads to reduced expression of genes important for tumor suppression and has been shown to significantly contribute to lung tumor formation. - Methylated residues (filled black circles) are typically generated by the action of methyltransferase enzymes (e.g., DNMT1 and EZH2) and limit transcription of growth inhibitory proteins. -(C) Tobacco smoke also induces an inflammatory response that involves both epithelial and immune cells. - Chemicals in the smoke induce production of fibrosis-associated proteins, most prominently TGF-ẞ (transforming growth factor ẞ); a number of highly active cytokines and regulators of the immune system (e.g., IL-8, C-X-C motif chemokine proteins [CXC], TNF-a, and others); and the release of nitric oxide (NO). - This induces fibrosis and remodeling of the extracellular matrix (ECM), creating a more favorable microenvironment for tumorigenesis. (MMPs)Matrix metalloproteinases; (LTB4) leukotriene B4. ## Alcohol-Induced Carcinogenesis - Chronic alcohol consumption is a major health concern worldwide, and may lead to damage of almost every organ of the body. - Globally, in 2012, 3.3 million deaths, or 5.9% of total deaths, were attributable to alcohol consumption. In 2014, about 16.3 million adults (ages 18 and older) had an alcohol use disorder in the United States. - Ethanol metabolism plays an important role in carcinogenesis. Ethanol is absorbed by the small intestine and later metabolized by alcohol dehydrogenases (ADH) into acetaldehyde in the liver. - When alcohol consumption is high, cytochrome P450 2E1 (CYP2E1) can also catalyze ethanol into acetaldehyde while producing reactive oxygen species (ROS). - Emerging evidence indicates that the imbalance in ethanol metabolism may markedly involved in alcohol-associated carcinogenesis. ## Effect of Physical Activity, Excess Body Fat, and Sedentary Behavior to Cancer Risk - Insulin and insulin-like growth factor (IGF)-I are anabolic endocrine hormones with important physiological roles in glucose metabolism, cell proliferation, cell death, and angiogenesis. Overstimulation of these biomarkers, their related binding proteins [i.e., insulin like growth factor-binding protein (IGFBP)-1], and their signaling pathways have been associated with increased risk of several malignancies. - It is well established that excess body fat, particularly abdominal fat, is positively correlated with insulin resistance. When there are consistently high levels of blood glucose, excess insulin is secreted from the pancreas and commonly results in hyperinsulinemia leading to decreased IGFBP-3 and subsequently increased levels of free IGF-I, which may promote tumorigenesis. - Prolonged hyperinsulinemia reduces bioavailable sex hormone-binding globulin (SHBG) and increases circulating estrogens and androgens, which may further contribute to tumorigenesis. - SHBG regulates the bioavailability of free estrogens, which if unbound, are considered to be highly active and associated with increased risk of some hormone sensitive cancers, particularly breast cancer. - Modifiable lifestyle factors such as caloric restriction and physical activity are effective interventions for reducing adipose tissue and correcting metabolic abnormalities, thereby lowering the risk of certain cancers. - Observational studies have also supported the hypothesis that physical activity lowers IGF-1 levels and raises IGFBP-3 levels. ## Age - Advancing age is the most important risk factor for cancer overall and for many individual cancer types. The incidence rates for cancer overall climb steadily as age increases, from fewer than 25 cases per 100,000 people in age groups under age 20, to about 350 per 100,000 people among those aged 45-49, to more than 1,000 per 100,000 people in age groups 60 years and older. ## Carcinogenesis - Carcinogenesis is a multistage process characterized by multiple defects in inter- and intra-cellular signaling mechanisms governing cell growth, survival and apoptosis.