Lecture 24 Apoptosis and Cancer I Flashcards
What are the phenotypes of apoptosis?
Overall shrinkage in vol of cell and its nucleus
Loss of adhesion to neighboring cells
Formation of blebs on surface
DNA fragmentation
Cytoskeleton collapses
Nuclear envelope disassembles
Rapid engulfment of dying cell by phagocytosis
Cytochrome C released from mitochondria is another marker
What is apoptosis?
Cells undergoing cell death under physiological conditions
Clean way of dying compared to necrosis
It is a programmed cell death
Most common cell death
Done to minimize spread of damage and/or inflammation
Cells shrink and condense and contents never spill out
Caspases:
Activation of caspases is a key event in apoptosis
=Cystein ASPartyl specific proteASE
Has a cysteine in active site
Targets proteins and cleaves them in their sequence where an aspartic aa residue occurs
Procaspase:
Inactive precursor form of caspases
Becomes activated by protease cleavage
Procaspases cleaved at specific sites to form a large and small subunit which forms a heterodimer
Initiator caspases:
Initiates apoptosis (includes caspase-8 and caspase-9) These activate executioner caspases
Executioner caspases:
Destroys actual targets Executes apoptosis Caspase-3 Cleaves downstream proteins Cleaves inactive endonuclease Targets cytoskeleton Attacks cell adhesion proteins-cells roll up into a ball
*CASPASE CASCADE IS IRREVERSIBLE
How are initiator caspases activated?
they auto-activate themselves
The machinery for apoptosis is always in place
Describe internal pathway
Internal stimuli: abnormalities in DNA
Mitochondrial dependent
Describe external pathway
External stimuli: removal for survival factors and proteins of tumor necrosis factor family
Mitochondrial independent
Extracellular signals bind to death receptors and trigger pathway
What are death receptors
Transmembrane proteins with 3 domains -extracellular binding domain -single transmembrane domain -intracellular death domain Receptors are homotrimers: three proteins of same type
Describe the Extrinsic pathway cascade
Fas binds to Fas death receptor
Adaptor proteins recruited: FADD and procaspase-8 with death effector domain
Activates caspase-8 or -10 (forms DISC)
Activates downstream executioner caspases - caspase-3
What are inhibitory proteins for extrinsic pathway?
Decoy receptors: have ligand binding domain but no death domain, can bind death ligand but does not activate pathway
FLIP: protein resembling initiator procaspase with no proteolytic domain; competitive inhibitor against procasp-8 and -10: prevents apoptosis
Act as sponges
Describe the intrinsic pathway cascade
Cytochrome C is released from mitochondria
Binds to Apaf1
Apaf1 forms apoptosome which activates casp-9
Casp-9 activates downstream executioner casps - casp - 3
What controls the release of cytoc C into cytosol?
Bcl2
What are the two types of Bcl proteins?
Pro-apoptotic and Anti-apoptotic
Describe anti-apoptotic protein Bcl2
Has 4 distinctive domains: called Bcl homology domains or BH
Blocks release of cytoc C
Describe pro-apoptotic proteins
BH123 + BH-3 only protein
Promotes release of CytocC
What is the function of BH123 protein?
It is a pro-apoptotic Bcl protein that once activated by stimulus, it forms an aggregation in mitochondrial outer membrane and induce release of cytoc C
What is the role of Bcl2 and Bcl-XL?
They are both BH1234
Anti-apoptotic Bcl protein that is mainly located on cytosolic surface of outer mitochondrial membrane
Prevent apoptosis by binding to pro-apoptotic proteins and prevent aggregation into active form
What is the role of BH3 - only protein?
pro-apoptotic
An activated BH3-only protein is cytosolic and translocates to mitochondria after apoptotic signal activates it
Inhibits anti-apoptotic Bcl2 protein from inhibiting aggregation to release cytochrome C
IAPs:
Inhibitors of apoptosis
Bind and inhibit caspases
Some add ubiquitin to caspases to mark for destruction
IAPs block apoptosis by binding to caspases
IAPs are there for when initiator caspases spontaneously activate but the cell does not need to die
If there is apoptotic stimuli or apoptosis signals, how are IAPs surpassed?
There is a release of anti-IAPs when apoptosis needs to occur from the mitochondria to block activity of IAPs
Executioner caspases can therefore be activated.
How is cancer caused by excess Bcl2?
if there is always inhibition of apoptosis, then DNA-damaged cells will not be programmed for cell death but can go on to cause cancer
How does p53 mutation cause cancer
Mutated p53 can no longer cause cell cycle arrest
No longer promotes apoptosis
Cells with DNA damage sticks around
