Apoptosis lecture 13 Flashcards
Summarise apoptosis
- DNA fragmentation and protein degradation
- Endonucleases fragment DNA allowing enzymes access between nucleosomes
- Cell cycle and DDR proteins degraded by proteases - Cell shrinkage and chromatin condensation
- Cytoplasm shrinks following cleavage of actin
- Organelles and chromatin become more tightly packed - Membrane fragmentation
- Translocation of phosphatidylserine to outer leaflet promotes phagocytosis by macrophages - Apoptotic bodies
- Cell fragments separate into apoptotic bodies containing cytoplasm, organelles and DNA fragments - Phagocytosis
- Apoptotic bodies allow for easy clearance by macrophages. Once engulfed they are degraded within phagolysosomes
No inflammatory response as cells contents not released into tissue
Describe the mechanism of apoptosis
Bcl2 is repressed, allowing the release of pro-apoptotic activators - caspases (cysteine proteases)
Bcl2 is overexpressed in many tumours and can decrease permeability of mitochondria
Extrinsic pathway (i by FLIP)- ligand binds to and activate death R. FADD adapter protein activated. Activation of caspsases.
Intrinsic pathway - Intracellular stimuli caused by stress. Mitochondrial membrane potential change allows release of pro-apoptotic proteins into the cytosol: cytochrome C, Diablo and Omi.
Activation of caspases.
Cytochrome C release activates the scaffold protein Apaf1. Procaspase-9 clusters with them to form the apoptosome. This causes activation of Caspase-9. Caspase-8 activation causes cleavage of BID protein, which is pro-apoptotic. Cleaved BID causes mitochondrial damage and activation of the intrinsic pathway.
Granzyme pathway - activated by granules from Tc. Perforin and granzyme degrade cell.
Activated caspases activates more caspases to amplify the apoptotic signal, they have prolytic activty and cause degredation of various substances
Describe the extrinsic pathway
- Ligand binds to activate death receptor
- Death domain interacts with adaptor protein (FADD) to activate it
- The adaptor protein interacts with procaspase-8
- DISC then activates procaspase-8 by cleavage of its N-terminal end
This activation can be inhibited by FLIP protein
Describe the intrinsic pathwya
Itracelllular stimuli causes by stress causes changes in mitochondira membrane potential to allow release of pro-apototic proteins into the cytosol: Cytochrome C, Diablo and Omi
Bid/BAX is active if BH3/Bcl2 is swtiched off, and can reside on mitochondrial membrane allowing changes in it and relase of apoptotic proteins. This is done by BAX forming a dimer which allows apoptotic protein to be released
These cause activation of caspase-9 and effector caspases
Cytochrome C release activates the scaffold protein Apaf1. Procapsase-9 clusters with them to form the apoptosome, causing the activation of caspase-9.
Cross talk between the intrinsic and extrinsic pathways. Caspase-8 activation causes cleavage of BID protein, which is pro-apototic
Cleaved BID causes mitchonsdrial damanfe and activation of the intrinsic pathway.
IAP is an inhibitor of apoptosis proteins
Desrbie the granzyme pathway
Activated by cytotoxic T cells, which releaase granules containing perforin and granzyme
Perforin creates porres in the target cell membrane
Granzyme (A or B) then moves into the cell and degrades proteins/activates caspases
ICAD - inhibitor of caspase activated DNAase
There is cross talk between the intrinsic and granzyme pathways. Granzye causess cleavage of BID proteins which is pro-apototic
Cleaved BID causes mitchondiral damage and activation of the intrinsc pathway
What is the difference between the intrinsic and extrinsic pathways to apoptosis?
Intrinsic- mitochondrial response to apoptotic signal
Extrinsic- death ligand binds to death receptor to activate the caspase cascade eg. Fas and FasL recruit FADD activate caspase 8 and 10
How do cell stress signals activate apoptotic pathways?
DNA damage activates p53 which can lead to the activation of the caspases cascade by the mitochondria and cytochrome C
p53 promotes apoptosis by activating pro-apoptotic genes : PUMA, BAX, FAS receptor
p53 activity is blokced by MDM2
What is the relationship between cancer and apoptosis?
Apoptotic pathways are suppressed in cancer and survival pathways are activated
Cancers can be treated by activating apoptosis
Compare necrosis to apoptosis
Apoptosis - from external or internal signals, energy dependent, single cells, cells shrink and are engulfed, normally no inflammation
Necrosis- ischemia/toxins/radiation, groups if cells, cells swell and burst, marked inflammation
What is autophagy?
-Recycles cellular components during stress, providing energy to the cell. However over-activation leads to death
Mechanism:
- mTORC1 is activated in nutrient-rich conditions to promote cell growth through synthesis of proteins/lipids/nucleotides
- In starvation conditions mTORC1 is inhibited and Atg proteins activated
- Atg work to assemble the phagophore, which entraps cellular components to become the autophagosome
- This fuses with lysosome and hydrolase enzymes degrade trapped cellular components
- Macromolecules are released and free nutrient levels reach a level to activate mTORC1 again and switch off autophagy
Describe autophagy in cancer
Pro-tumour- escape from stress, tumour cell fitness, treatment resistance, dormancy
Anti-tumour- inhibits chromosomal instability, oxidative stress, inflammation and promotes senescence
Describe cancer and apoptosis
Leads to faulty apoptosis - overactivation of anti-apoptotic genes and overactication of pro-survival genes, down regulation of death receeptrs, mutation in p53.
Lots of chemotherapy agents aim to cause DNA damage and induce apoptosis by p53
Could also stimulate death receptors, down regualte Bcl2, introduce wild type p53 into cells
