Apoptosis Flashcards
Define apoptosis
Complex programme of cellular self-destruction triggered by a variety of stimuli, and involving the activation of caspase enzymes that result in quick fragmentation and phagocytosis of a cell
Why is apoptosis neccessary?
Response to unrepairable defects - stops replication of error (DNA damage). Also forming fingers in fetal hands.
What role does p53 have in apoptosis?
Identified irreparable DNA damage and signals to initiate apoptosis. Absent/defective p53 -> evasion of apoptosis -> replicated DNA error -> cancer/increased risk of cancer.
What are the major changes in cells that have initiated apoptosis?
Cell shrinkage, Nuclear condensation & fragmentation, Surface expression of phosphatidylserine (phospholipid), membrane blebbing, mitochondrial depolarisation, DNA fragmentation, Apoptotic body formation.
What is the extrinsic pathway for apoptosis?
Initiated by the extracellular environment and driven by death receptors in the plasma membrane, activated by death ligands.
How do the death ligand and death receptor interact?
The interaction between the death ligand and death receptor causes the assembly of a dynamic multiprotein complex at the cytoplasmic tail of the receptor: Death-inducing signalling complex (DISC). The death domains at the cytoplasmic tails of the receptor play an important role in transmitting the surface signal to intracellular pathways
Explain the intracellular signalling pathway of extrinsic pathway apoptosis
During FasL binding to FAS, FADD (Fas associated protein with dead domain) adaptor proteins are recruited. For TNF and TNFR binding, it’s TRADD and FADD. The adaptor proteins recruit procaspase-8 via dimerisation of the death effector domain. DISC is formed and caspase 8 cleaved and activated, cleaving and activating executioner caspase 3
What is intrinsic apoptosis?
AKA mitochondrial pathway. Initiated by microenvironmental triggers such as DNA damage or withdrawal of GFs. Irreversible and caused by pro apoptotic BCL2 family proteins. Clearance by macrophages takes place (efferocytosis)so cells retain membrane integrity and metabolic processes. At end stages cells can have a complete membrane breakdown and acquire necrotic morphotype (secondary necrosis).
Describe the process of intrinsic apoptosis
Bak in mitochondrial membrane, and Bax in extracellular space. They are trying to aggregate to form channels in the mitochondrial membrane. Bax continuously translocating, but Bcl2 and Bcl-XL bind them to stop aggregation. Bid and Bad sequester the anti apoptotics, allowing cytochrome c to be released and bind APAF1 and procaspase 9 (apoptosome), activating procaspase 3. SMAC releases with cytochrome c and associates with IAP proteins to regulate apoptosis.
How do executioner caspases work?
Activation of the caspases lead to morphological and biological changes like DNA fragmentation, phosphatidylserine exposure, and apoptotic body formation. Caspase 3 catalyses the proteolytic inactivation of DNA fragmentation subunit a (ICAD - inhibitor of CAD) and releasing the catalytic activity of caspase activated DNAse (CAD).
What is anoikis?
Specific type of intrinsic apoptosis initiated by the loss of integrin dependent attachment to the ECM.
List the differences between different types of cell death
Apoptosis - Genetically programmed, caspase dependent
Necrosis - Not genetically programmed, caspase independent
Mitotic catastrophe - Failure to undergo mitosis due to chromosome damage
Senescence - Inability to divide but still metabolically active
Autophagy - Genetically programmed self digestion, caspase & p53 independent
What role does p53 play in apoptosis?
It is a TF governing anti-proliferative programmes such as senescence, cell cycle arrest, and apoptosis. Via these mechanisms, p53 facilitates repair and survival of damaged cells or eliminates severely injured cells
Explain the pathway that p53 is involved in to activate apoptosis
Intrinsic pathway - Acts via cytosolic and mitochondrial p53. For cytosolic, nuclear p53 induces activation of proapoptotic genes like BCL2, BAX, NOXA, PUMA. This releases cytosolic p53 from inactive BCL-XL complex and allows induction of BAX oligomerisation and mitochondrial translocation. In the mitochondria, p53 promotes BAX and BAK activation, and blocks anti-apoptotic BCL-2 and BCL-XL. p53 also complexes with cyclophilin D - mitochondrial permeability. This releases cytochrome C (and other apoptogenic factor) release, activating caspase 9.
In extrinsic pathway, p53 overexpression promotes Fas levels, activating DR5 and promoting apoptosis via caspase 8.
Describe the link between p53 and cancer
Over 50% of cancers have TP53 loss of function mutations. WT p53 is a tumour suppressor, while mutant p53 can act as an oncogene by WT p53 inhibition in a dominant-negative manner. Cancer derived mutant p53 forms can activate various growth-promoting and oncogenic genes, including c-myc, multiple drug resistance gene 1 (MDR1), EGFR, and TERT. Pro-apoptotic p53 function is significantly inhibited by p53 mutants inducing malignant transformation by c-myc and TERT upregulation. Mutated p53 cells are less sensitive to DNA damage induced apoptosis, have chromosome damage, hypoxic protection, anoikis protection.
