Cell Death and Proliferation Flashcards
1
Q
What is cell death and when does it occur
A
- Cell often undergoes reversible cell injury (stress)
- Able to return to normal cell functioning
- Stress can lead to irreversible injury, irreversible mitochondrial dysfunction, profound ultrastructural changes, light microscope / gross morphologic disturbances
- This leads to apoptosis or necrosis
2
Q
What is apoptosis
A
- Programmed cell death
- Counters cell production and maintains appropriate cell number
- Aids maintenance of size and shape, removal of immature B cells, protection against cancer
- Undergoes change in shape, loss of integrity, shrinkage
- Membrane / nucleus fragments into vesicles (blebbing)
- Removed by phagocytosis by macrophages
3
Q
Briefly compare and contrast apoptosis and necrosis
A
- Apoptosis: Blebbing of membrane, shrinkage, DNA degradation, controlled
- Necrosis: Cell swells, lysis and release of cytoplasmic contents, uncontrolled
4
Q
What are caspases (C. elegans)
A
- Cysteine (catalytic site) aspartate (target site) proteases
- Target aspartate residues
- Active towards end of apoptosis
- Stored as zymogens (inactive)
- CED3 and 4 are pro apoptotic genes
- CED9 is anti-apoptotic, blocks the functions of 3 and 4
5
Q
What is Bcl2 / BclXL, BAD and BAX
A
- BCL: Prevents apoptosis, prevents leakage of CytC from mitochondria
- BAD: Promotes apoptosis, complexes with Bcl-2 and prevents it from stopping leakage of CytC
- BAX: Triggers release of CytC from mitochondrion to start apoptosis
6
Q
How does induction of apoptosis occur (BAD / BAX)
A
- Binding of BAD to Bcl-2 / Bcl-XL dimer complex (contains Apaf-1)
- Allows ion influx through Bax dimer into mitochondria
- Influx of ions through Bax dimer causes CytC leakage into cytoplasm
- Presence of CytC activates Apaf-1 which induces apoptosis through caspase activation
7
Q
How does inhibition of apoptosis occur (BAD / BAX)
A
- Distraction / phosphorylation of BAD leads to an absence of binding to Bcl-2 / Bcl-XL
- This inhibits Bax dimer function
- CytC remains in the mitochondria
- Apaf-1 remains inactive
- Activation of caspases does not occur (no induction of apoptosis)
8
Q
How does phosphorylation of BAD occur
A
- Trophic factors (ligands) are recognised by receptors on cell surface
- Binding causes activation of tyrosine kinases, signal transmitted by a cascade of kinases
- Phosphorylation and activation of Akt by PDK1
- Bad is phosphorylated by active Akt leading to removal of inactive apoptosis inhibitory protein from Bad
- Bad binds 14-3-3 protein and apoptosis inhibitory protein becomes active
- Apoptosis inhibited
9
Q
How is apoptosis regulated
A
- GF and receptor activation (original stimulus)
- Signal transduction pathways / molecules (regulation of gene expression)
- Type of ligand, regulation of cell cycle through cyclins / cyclin dependent kinases (CDKs)
- CDKs drive growth / proliferation of cells (linked to original stimulus)
10
Q
What mutations can affect regulating apoptosis
A
- Mutations along signal transduction pathways
- Mutations at the original stimulus (GF), signal transducer, secondary messenger or TFs
- Can be affected by both up and down stream events / mutations
11
Q
How do excess GFs affect apoptosis
A
- Involves self-stimulation (autocrine function)
- Prolonged response to presence of GF (prior to deactivation)
- Regulation of TFs and gene expression is unrestrained
12
Q
How do excess signal transducers affect apoptosis
A
- Mutated ras results in constitutive activation of kinases and TFs
- 30% of human tumours
- Can no longer be switched off
13
Q
How do excess TFs affect apoptosis
A
- Mutated TFs lead to continuous transcription and consequent gene expression that is out of control
- Activation of growth
- Encourage entry to S-phase and consequent division
14
Q
What is Apaf-1
A
- Equivalent to CED4
- Activates caspase activity after activation of Bcl, Bad and Bax
