6R: Apoptosis Flashcards
Apoptosis vs. Necrosis
Apoptosis
- chromatin condensation, cytoplasmic shrinkage, membrane blebbing, DNA cleavage, Phosphatidyleserine redistribution (from in to out) in plasma membrane (Macrophage “eat me” signal), tight cellular regulation, no inflammation, energy required
Necrosis
- cytolysis: membrane destruction, unregulated, cellular swelling, random DNA cleavage, no energy requirement, inflammation
two pathways of apoptosis
a. intrinsic pathway: signal from the inside of the cell (because of damage or cellular insult)
b. extrinsic pathway: signal from outside of the cell
- both pathways use caspase activation and MOMP (mitochondrial outer membrane permeability)
Extrinsic pathway steps
- caspase 8 binds the death domain on the death receptor
- DD recruits scaffolding proteins
- scaffolding proteins recruit caspase 8 from cytosol
- increased caspase 8 concentration activates the proteolytic activity of the enzyme inducing self cleavage
- activated caspase 8 then cleaves effector caspases and initiates MOMP
initiator vs. effector caspases
initiator caspases: get recruited to the death receptor. Once activated, it’s job is to cut other caspases
- more regulated
effector caspases: goes out and “chews up” other substrates after being activated (by the initiator caspases)
- less regulated
how are caspases activated?
- they are initially secreted as a zymogen
- they have a dimerized structure that has floppy loops hanging out of it. It is the floppy loops that are cleaved to create the activated caspase
- active site has a catalytic cysteine, and usually cuts at aspartate residues
IAP proteins
suppress apoptosis by binding and inhibiting initiator and effector caspases (closes the active site)
- sometimes over expressed in cancers
Downstream events of caspase activation
- kinases become more activated
- cytoskeleton loses integrity
- PARP destruction
- DNase activation (DNA fragmentation)
- lamin degradation (nuclear envelope breakdown)
- phophatidlyserine from in to out of PM = “eat me” signal
How do we cut DNA?
CAD (caspase activated DNases are turned off when bound to ICAD (inhibitors of caspase activated DNases. When apoptosis begins, caspase 3 cleaves ICAD, freeing CAD leading to DNA degradation
How does phosphatidylserine get to the outer leaflet of the plasma membrane?
Scramblase flips them out there. Flippase puts them back on the inner leaflet
How is the intrinsic pathway activated?
Stress activates p53 (transcription factor) and proteins in the BCL-2 family. When we have the cascade of activating the BCL-2 family. The anti-apoptotic member is bound to the BH-3 activator. The BH-3 sensitizer binds the anti-apoptotic member and frees the BH-3 activator. Now the BH-3 activator can interact with the proapoptotic member to activate it so that it can poke holes in the mitochondrial membrane.
MOMP
mitochondrion outer membrane permeability. Leads to loss of mitochondrion function (because of disruption of H+ gradient) and activation of the apoptosome (because cytochrome c leaks out of the mitochondria and activates the apoptosome)
Apoptosome
“super initiator” comprised of APAF1, cytochrome c, and caspase 9
p53
a transcription factor that controls transcription of genes regulating cell cycle, apoptosis, DNA repair, and more. Tightly regulated by MDM2. Degradation because of binding to MDM2 and being ubiquitinated (then sent to the proteasome)
genes that p53 activates
APAF1, BAX, FAS, KILLER, miR-34, NOXA, p53AIP1, PERP, PIDD, PIG3, PUMA, SIVA, p21, PAI1, PML
Why target apoptosis in cancer?
Because some cancer cells have more death receptors that normal cells. So, if we activate these death receptors, we can selectively initiate apoptosis in cancer cells
