Apoptosis Flashcards
Apoptosis
Programmed cell death
- controlled disassembly of cellular contents without disruption
- no inflammatory response
Why programmed cell death?
- Harmful cells
- Developmentally defective cells
- Excess/unnecessary cells
- Obsolete cells
- Exploitation
Necrosis
-unregulated cell death associated with trauma, cellular disruption and an inflammatory response
Features of necrosis
- plasma membrane becomes permeable
- cell swelling and rupture of cellular membranes
- release of proteases leading to autodigestion and dissolution of the cell
- localised inflammation
Apoptosis phases
- latent phase
- execution phase
Apoptosis latent phase
-death pathways are activated, but cells appear morphologically the same
Apoptosis execution phase
-loss of microvilli and intercellular junctions
-cell shrinkage
-loss of plasma membrane asymmetry (phosphatidylserine lipid appears in outer leaflet)
-chromatin and nuclear condensation
-DNA fragmentation
-formation of membrane blebs
-fragmentation into membrane-enclosed apoptotic bodies
PLASMA MEMBRANE REMAINS INTACT=NO INFLAMMATION
DNA modification in apoptosis
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Apoptosis-like PCD
- some, but not all, features of apoptosis
- display of phagocytic recognition molecules before plasma membrane lysis
Necrosis-like PCD
- variable features of apoptosis before cell lysis
- ‘aborted apoptosis’
Apoptotic cell death mechanism
- The executioners (Caspases)
- Initiating the death programme (death receptors, mitochondria)
- The Bcl-2 family
- Stopping the death programme
Caspases
Cysteine-dependent aspartate-directed proteases
- executioners of apoptosis
- activated by proteolysis
- cascade of activation
Initiator caspases
-trigger apoptosis by cleaving and activating
Effector caspases
- carry out the apoptotic programme
- cleave and inactive proteins or complexes (eg: nuclear lamins leading to nuclear breakdown)
- activate enzymes (including proteins kinases, nucleases eg: CAD) by direct cleavage, or cleavage of inhibitory molecules
Caspase maturation
-cleavage of the inactive procaspase precursor is followed by folding of 2 large and 2 small chains to form an active L2S2 heterotetramer
Caspase cascades
- amplification
- divergent responses
- regulation
Caspase activation mechanisms
- death by design (receptor-mediated pathways)
- death by default (mitochondrial death pathway)
Death receptors
-secreted or transmembrane ligands (trimeric)
Adaptor proteins in receptor-mediated apoptosis
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Signalling through death receptors
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Initiator procaspase 8 oligomerisation results in cleavage and activation
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Death receptor activation of caspase 8 is inhibited by FLIP
- competes for binding to receptor tails/FADD via DED domains
- incorporates into receptor-procaspase complexes and interferes with transcleavage
Caspase 8 activation of downstream effector caspases
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Mitochondrial regulation of apoptosis
- cellular stresses activate (eg: lack of or overstimulation by growth factors, DNA damage (p53), ROS)
- loss of mitochondrial membrane potential
- release of cytochrome c
- release of other apoptosis-inducing factors
- formation of the apoptosome complex
The apoptosome
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Apoptosis energy requirement
- the apoptosome requires ATP
- energy levels in the cell may determine whether death is by necrosis or apoptosis
Bid links receptor and mitochondrial death pathways
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Bcl-2 family proteins as the modulators of apoptosis
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Anti-apoptotic Bcl-2 family proteins
Bcl-2
Bcl-xL
MITOCHONDRIAL
Pro-apoptotic Bcl-2 family proteins
Bid Bad Bax Bak MOVE BETWEEN CYTOSOL AND MITOCHONDRIA
PI3’-Kinase signalling pathway in cell cycle and apoptosis regulation
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PKB/Akt inducing cell survival by blocking apoptosis
- phosphorylates and inactivates Bad
- phosphorylates and inactivates caspase 9
- inactivates FOXO transcription factors (FOXOs promote expression of apoptosis-promoting genes)
- Other eg: stimulates ribosome production and protein synthesis
BH3 heterodimerisation
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PTEN (lipid phosphatase) counteracts PI3’-K signalling
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Inhibitor of Apoptosis Proteins (IAPs) regulate apoptosis
EXTRINSIC PATHWAY
- bind to procaspases and prevent activation
- bind to active caspases and inhibit their activity
Cytoprotective/anti-apoptotic pathways
- INTRINSIC PATHWAY: Bcl-2, Bcl-xL
- EXTRINSIC PATHWAY: FLIP, IAPs
- GROWTH FACTOR PATHWAYS via PI3’-K and PKB/Akt
How can cancer cells avoid apoptosis?
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Apoptosis therapeutic uses
- For harmful (oncogenic) cells eg: cells with viral infection, DNA damage
- Chemotherapeutic killing of tumour cells eg: Dexamethasone stimulates DNA cleavage
