Cancer Biology 5.2 Completed PDF

5. CANCER BIOLOGY 1 THE CELL CYCLE 5.2.1 The Normal Cell Cycle 5.2.1a Describe in detail the phases of the cell cycle and the cellular events characterizing each phase of the cycle. (done) 1. G1 PHASE (GAP 1 PHASE) Definition: The int...

5. CANCER BIOLOGY 1 THE CELL CYCLE 5.2.1 The Normal Cell Cycle 5.2.1a Describe in detail the phases of the cell cycle and the cellular events characterizing each phase of the cycle. (done) 1. G1 PHASE (GAP 1 PHASE) Definition: The interval between the mitosis of the preceding cell cycle and the initiation of DNA synthesis in the current cycle. Key Features: o Restriction Point: A critical checkpoint at which the cell commits to the cycle if favourable conditions exist. Before passing this point, cells are responsive to growth signals (mitogenic signals), but once they have passed a restriction point, they are committed to enter S phase. o Arrest and Response: Cells may arrest at different points in G1 in response to inhibitory growth signals (e.g., nutrient scarcity, DNA damage). o Mitogenic Signals: Progression to the S phase is promoted by mitogenic signals phosphorylation of the retinoblastoma gene product protein (pRb) by cyclin-dependent kinases (CDKs). Outcome: Once past the restriction point, the cell is committed to entering the S phase. 2. S PHASE (SYNTHESIS PHASE) Definition: The phase in which DNA replication occurs. Key Features: The chromosomes are replicated, resulting in the duplication of genetic material. The DNA content transitions from 2n (diploid) to 4n (tetraploid). This ensures that each daughter cell will receive a complete set of chromosomes during division. 3. G2 PHASE (GAP 2 PHASE) Definition: The interval between the completion of DNA synthesis and the start of mitosis. Key Features: o Error Correction: The cell repairs any errors or damage that occurred during DNA replication, preventing the propagation of mutations. o Preparation for Mitosis: Spindle assembly begins during this phase, laying the groundwork for proper chromosome segregation in mitosis. 4. M PHASE (MITOSIS PHASE) Definition: The cell division phase where one parent cell divides into two genetically identical daughter cells. Key Events: o Nuclear envelope breakdown. o Chromosome condensation and alignment on the metaphase plate. o Mitotic spindle assembly (spindle assembly begins in G2, before mitosis) for chromosome segregation. o Disassembly of the Golgi apparatus and nucleoli. o Cell rounding as the division progresses. Role of CDK1—Cyclin B Complex (CDC2): Also known as the “maturation-promoting factor,” this complex drives the entry into mitosis phase (M Phase) by regulating essential events like chromosome condensation and spindle assembly. Interphase: G1, S and G2 together. Most adult cells are in G0, a resting state outside of the active cell cycle, where they perform specialized functions but do not divide. Regulation of the Cell Cycle: Cyclins and CDKs: Cyclins are proteins that regulate transitions between cell cycle phases by activating cyclin- dependent kinases (CDKs). CDKs are highly conserved eukaryotic regulators that ensure the cycle progresses in a controlled manner. E.g. The CDK1-cyclin B heterodimer specifically regulates the G2-to-M transition and drives activity into the M phase.  5. CANCER BIOLOGY 2 5.2.2 Regulation of the Cell Cycle, Cell Death and Differentiation 5.2.2a Explain the role of mitogens (and inhibitory factors) in the regulation of the normal cell cycle (done) Summary Mitogens and inhibitory factors regulate the normal cell cycle by controlling cyclin-dependent kinases (CDKs) and their associated pathways, ensuring precise timing and conditions for cell cycle progression. ROLE OF MITOGENS: Mitogens are peptide hormones that stimulate cell growth and division by triggering signalling pathways leading to cell cycle progression. Their key roles include: Mitogen Signalling Pathway 1. Mitogens bind to surface receptors with tyrosine kinase activity. 2. Activation of downstream Ras proteins (GTPase activity) acts as a molecular switch, relaying the signal. 3. Signal transduction involves BRAF, which carries the signal from Ras to the nucleus, culminating in the transcription of D-type cyclins. D-type Cyclins and Cell Cycle Commitment o D-type cyclins are crucial for cell cycle commitment: They activate CDK4/6, driving progression through the restriction point (R-point). R-point marks the transition beyond which cells no longer rely on mitogenic signals to proceed with the cycle. o Cyclins are inherently unstable and degrade through hydrolysis unless there is sufficient mitogenic signals to stabilize their levels. o Their function includes activating CDK activity and ensuring cell cycle progression. R-Point and E2F Activation o Retinoblastoma (RB) family proteins act as transcriptional repressors by sequestering E2F transcription factors. o When RB proteins are phosphorylated by CDK-cyclin D complexes, the RB proteins release E2F. o Released E2F activates genes required for S-phase entry, committing the cell to the cycle. Physiological Cell Proliferation o Mitogenic signalling must also provide survival signals to suppress the default p53 response, which would otherwise cause cell cycle arrest. o This ensures controlled and appropriate proliferation.  5. CANCER BIOLOGY 3 Note: The restriction point is the point beyond which mammalian cells become relatively independent of mitogenic signalling, and are therefore committed to cell cycle ROLE OF INHIBITORY FACTORS: Inhibitory factors maintain control over the cell cycle, preventing unscheduled or inappropriate progression. Regulation of CDKs o Activation: CDK-activating kinase (CAK) activates CDKs by opening the ATP binding site. o Inhibition by Phosphorylation: WEE1 protein kinase phosphorylates the ATP binding site, inhibiting CDK activity. This inhibition can be reversed by CDC25 phosphatase, which dephosphorylates the ATP binding site. o Mutational Impacts: Loss-of-function (LOF) in WEE1 leads to unregulated cell cycle progression. LOF in CDC25 results in cell cycle arrest. o At the end of mitosis, cyclin is destroyed by ubiquitin-proteasome-dependent cyclin destruction, this ensures unidirectional progression through mitosis o Examples: CDK inhibitors include p16, p15, p18, p19, p21, p27, p57. p16, 15, 18 and 19 target specifically CDK4 and 6, with no effect on CDK1 and CDK12. p21, p27 & p57 are more widely acting and able to inhibit all types o CDK inhibition: p21 is transcriptionally regulated by p53, enabling p53 to inhibit CKI activity and arrest the cell cycle under stress or DNA damage conditions. CKI activity is critical for maintaining cell cycle checkpoints and preventing inappropriate proliferation. Note: Normal physiological cell proliferation requires not only mitogenic signally, but additional survival signals to block the default p53 response that would otherwise arrest the cell at an early stage, since p53 also activated by mitogenic signalling to tightly control & prevent inappropriate proliferation R POINT DEREGULATION IN CANCER R-point deregulation is a hallmark of cancer and can result from: 1. RB mutation: familial (bilateral) retinoblastoma; diverse sporadic cancers. 2. Overexpression of Cyclin D/E: This can be due to 1. gene amplification, 2. increased synthesis in response. to oncogenic signalling (e.g. RAS mutation). 3. defects in ubiquitin-dependent proteolysis of cyclin E. 3. Mutation or epigenetic silencing of (CKI’s such as p16 e.g. by promoter methylation. 4. Enhanced degradation of CKS’s such as p27. 5. CDK4 mutation: CDK4 mutation disrupts interaction with p16 rendering it insensitive to inhibition. 5.2.2b Outline the role of apoptosis in the regulation of cellular behaviour, including factors that promote and inhibit apoptosis (done) THE ROLE OF APOPTOSIS IN THE REGULATION OF CELLULAR BEHAVIOUR: Apoptosis, or programmed cell death, is a critical process for maintaining tissue homeostasis and proper cellular function. It enables the removal of damaged, dysfunctional, or unnecessary cells without causing inflammation. Key Features of Apoptosis Include: o Contents Recycling: Cellular components are recycled through phagocytosis.  5. CANCER BIOLOGY 4 o Regulatory and Effector Components: 1. Upstream Regulatory elements, sense intrinsic and extrinsic pro-apoptotic signals. 2. Downstream Effector components, in response to signalling from regulatory elements downstream effector components execute apoptosis via a cascade of proteolytic events and cell disassembly. o Morphological Changes: Apoptosis leads to nuclear fragmentation, chromosomal condensation, and cellular shrinking with loss of intracellular contact. This is followed by membrane blebbing, cellular fragmentation, and the formation of apoptotic bodies, which are subsequently phagocytosed by neighbouring cells. FACTORS THAT PROMOTE APOPTOSIS: 1. Proteolytic Caspases: These enzymes mediate apoptosis by cleaving specific proteins in the cytoplasm and nucleus. 2. TNF Receptor Gene Superfamily: Notable members include: TNFα (Tumour Necrosis Factor-alpha). FasL (Fas Ligand) 3. p53 Transcription Factor: p53 is a transcription factor capable of promoting differentiation, cell cycle arrest or apoptosis. Activated by stresses like DNA damage, hypoxia, and oncogene activation. Promotes apoptosis by blocking CDK (cyclin-dependent kinases) or directly inducing apoptotic pathways. 4. Removal of Survival Factors: Removal/lack of survival factors like IL-3 (Interleukin-3) or IGF-3 (Insulin-like Growth Factor-3) can trigger apoptosis by removing inhibitory signals that block p53. FACTORS THAT INHIBIT APOPTOSIS: 1. BCL-2 Family Proteins: The BCL-2 gene is classified as an oncogene and inhibits apoptosis. 2. Viral Inhibitors: Example: crmA, which inhibits caspases 1 and 8. 3. Inhibitors of Apoptosis Proteins (IAPs): These proteins block various caspases, preventing apoptosis from being executed. 5.2.2c Discuss the differences between apoptosis and necrosis as mechanisms of cell death (done) ASPECT APOPTOSIS NECROSIS Definition: Programmed cell death involving a Premature cell and tissue death caused by controlled sequence of events. external factors which can cause membrane damage and lysis. Causes: Intrinsic (internal) cellular mechanisms. Infection, toxins, trauma, respiratory poisons, or hypoxia. Effects: o Beneficial for maintaining cell balance o Always detrimental to the organism. and removing damaged or unnecessary o Leads to inflammation. cells. o Does not cause inflammation. Process: 1. Membrane blebbing. 1. Membrane damage and disruption. 2. Cell shrinkage. 2. ATP depletion. 3. Nuclear collapse 3. Metabolic collapse. 4. Formation of apoptotic bodies, which are 4. Cell swelling and rupture. phagocytosed by white blood cells. Energy Requires ATP for the controlled process. ATP depletion contributes to cell death Use: mechanisms.  5. CANCER BIOLOGY 5 Summary: Apoptosis is a programmed and beneficial process of cell death characterized by membrane blebbing, nuclear collapse, and formation of apoptotic bodies, which are cleared without inflammation. In contrast, necrosis is a detrimental, premature cell death caused by external factors like toxins or trauma, resulting in membrane rupture, ATP depletion, cell swelling, and inflammation. 5.2.2d Outline the concept of cellular transformation (with independence from mitogens and the potential for unlimited cell division) (done) DEFINITION: Cellular transformation refers to the transition of normal cells into a tumorigenic state involving both changes in cell morphology and function. CHARACTERISTICS: o Cancer cells maintain reasonably efficient DNA replication and mitosis. o However deregulated cell cycle commitment is a key feature in most cancers. o Defects in cell cycle checkpoints contribute to genomic instability, allowing uncontrolled proliferation. o Normal cells rely on mitogens to progress through the cell cycle. Cancer cells bypass this dependency, often due to mutations in the Ras signalling pathway. o Healthy tissues regulate mitogenic pathways tightly to maintain homeostasis; mitogenic pathways are responsible for driving progression through the cell cycle. o Mitogens typically bind to receptors and activate intracellular tyrosine-kinase signalling cascades, altering gene expression to promote proliferation and growth. INDEPENDENCE FROM MITOGENS: Signalling mechanisms are deregulated in cancerous cells; cancer cells achieve independence through the following mechanisms o Overproduction of growth factor ligands o Overproduction of growth factor receptors o Expression of structurally altered receptors that signal without ligand binding. o Activation of intracellular signalling pathway components so that signalling is no longer ligand-dependent. THE POTENTIAL FOR UNLIMITED CELL DIVISION The potential for unlimited cell division is due to the unlimited proliferative capacity via telomerase activity 4. Telomeres and Cell Division Limits: In normal cells, telomeres shorten with each cell division. This shortening process functions as a "mitotic clock" to progressively limit cellular replication; telomeres shorten as small fragments of telomeric DNA are lost within successive cycles of replication. When telomeres become critically short, the p53 and RB pathways induce senescence, preventing the cell dividing further. 5. Telomerase Enzyme Function: Telomerase, a specialised polymerase enzyme, adds nucleotides to telomeres, counteracting their shortening and enabling continued division. Telomerase is normally absent or minimally expressed in healthy cells, however telomerase is significantly upregulated in many cancers.  5. CANCER BIOLOGY 6 6. Consequences of Telomere Dysfunction Without Telomerase: Absence of functional p53 and RB allows continuous cell division despite critical telomere shortening, and therefore chromosomes get shorter and shorter until now indistinguishable from double-strand DNA breaks anywhere in the genome. DNA repair attempts to break those DNA ends, typically by joining them to each other. These repair mechanisms can result in chromosomal reorganisation and genomic instability, often lethal to the cell. 7. Telomerase Reactivation in Cancer: Activation of telomerase stabilises scrambled chromosomes, potentially enabling proto-oncogene translocations. Such changes promote oncogene activation and a tumorigenic phenotype. 8. Example: Telomerase Activation in Cancer: Mutations in the TERT promoter (telomerase reverse transcriptase) have been seen in cancer. This leads to loss of negative regulation of TERT expression, leading to telomerase reactivation. Observed in conditions such as cervical intraepithelial neoplasia (CIN), this process supports cancer progression. 5.2.2e Explain the concept of cellular differentiation and define the term metaplasia (distinguishing this from dysplasia). (done) Cellular Differentiation: Cellular differentiation is the process in which a cell changes from one type to another, usually transforming into a more specialized type. Differentiation occurs numerous times during the development of a multicellular organism as it changes from a simple zygote to a complex system of tissues and cell types. Differentiation requires differential gene expression, which is regulated by epigenetic changes. Metaplasia: o Metaplasia refers to the change of a fully differentiated cell type into another fully differentiated cell type. o This often occurs as a response to harmful environmental factors e.g. in smokers’ lungs: normal columnar epithelium with ciliary hairs in bronchi becomes damaged and replaced by squamous epithelium (squamous metaplasia) which lacks ciliary hairs, and therefore no longer benefits from ciliary clearance. o Metaplasia is not necessarily a premalignant change, but often represents a response to a deleterious environmental factor, e.g. cigarette smoke, which may itself be carcinogenic. Dysplasia: Dysplasia describes a disorder in cellular maturation or differentiation, typically observed in epithelial surfaces. It is a broad term referring to abnormal growth/development of cells within tissues or organs. Dysplasia can lead to the formation of pre-cancerous cells, making it a potential precursor to malignancy. Table Illustrating the Differences Between Metaplasia and Dysplasia Feature Metaplasia Dysplasia Definition Reversible replacement of one Disordered growth and differentiation of cells, differentiated cell type with another. leading to abnormal tissue architecture. Relation to Not directly precancerous but can Pre-cancerous; often a precursor to malignant Cancer increase cancer risk if persistent. transformation. Cellular Normal cells, but inappropriate for the Abnormal cells with variations in size, shape, Appearance location. and organization.  5. CANCER BIOLOGY 7 Reversibility Generally reversible upon removal of May be reversible early, but often progresses to stimulus. cancer if unchecked. Common Chronic irritation or environmental Persistent genetic mutations and chronic Causes stress (e.g., smoking, acid reflux). inflammation. Examples Barrett's oesophagus (squamous to Cervical dysplasia (abnormal epithelial cells in columnar cells). the cervix). 5.2.2f Outline the pathophysiology of Barrett’s oesophagus. (done) Pathophysiology of Barrett’s Oesophagus o Barrett’s oesophagus is a pre-cancerous condition where the normal squamous epithelial cells lining the oesophagus have been replaced with abnormal columnar epithelial cells a process known as metaplasia. These abnormal columnar epithelial cells like those in the lining of the intestines. Over time these cells may grow abnormally, leading to dysplasia and display abnormal phenotype under the microscope. o Barrett’s oesophagus increases risk of cancer in the oesophagus, although the risk is still small. many people with Barrett’s oesophagus do not develop cancer. 3—13% of people with Barrett’s oesophagus in the UK will develop oesophageal adenocarcinoma in their lifetime. o Risk factors: history of acid reflux, abdominal obesity. It’s more common in men than women as well as more common in older people. Symptoms: no specific symptoms, any symptoms are usually related to the underlying acid reflux, such as heartburn.