Lecture 11: Apoptosis I Flashcards
Cell death mechanisms
Apoptosis, Autophaghy, Pyroptosis, necroptosis and necrosis
Apoptosis in normal function
- Organism development (e.g. neural development, tadpole to frog)
- Regulation of damaged/aged cell population
- Immune response to infection (virus infections, cell death without
inflammation)
Hallmarks of apoptosis
- Apoptosis is “programmed cell death”
- Highly regulated
- Apoptotic cells undergo key morphological
changes – cell shrinkage, blebbing, nuclear
fragmentation, formation of apoptotic bodies
Apoptosis vs. Necrosis
Apoptosis: Small blebs form– the nucleus, DNA and organelles break off within blebs– the cell breaks into several apoptotic bodies and the organelles are still functional
Necrosis: Small blebs form and the structure of the nucleus changes– the blebs fuse and become larger, while no organelles are locates in the blebs– cell lyses and releases content, while the organelles are not functional
Morphological steps of apoptosis
Stage 1: Apoptotic signal
Stage 2: Cell shrinkage and chromatin condensation
Stage 3: Nuclear fragmentation and membrane blebbing
Stage 4: Apoptotic body formation
Triggers of apoptosis
- Radiation
- Cytotoxic agents
- Toxins
- Growth factor/nutrient/oxygen
deprivation
Two major apoptotic pathways
- Intrinsic pathway
- Extrinsic pathway
Caspases – the enzymatic mediators of apoptosis
- Enzymatic cascade of apoptosis
- Expressed as larger inactive pro-enzymes (pro-caspases)
- Cysteine protease that cleaves after aspartic acid residues
- Composed of a large and small subunit
- Initiator and executioner caspases
Initiator caspases
(Caspase-8, -9, and -10)
* Prodomain has DED (death effector domain) in procaspase 8 or 10 and
CARD (caspase recruitment domain) in procaspase 9.
* DED or CARD domains allow for homophilic interactions (DED-DED or
CARD-CARD) with adaptor proteins
Executioner caspases are pivotal in both intrinsic
and extrinsic pathways
- Executioner/effector caspases
carry out regulated disassembly
of the cell - Caspase-3 and -7
- Activation by proteolytic
cleavage, mediated by initiator
caspases - Procaspase cleavage leads to
tetramer of small and large
heterodimers
How do executioner caspases kill the cell?
- Disassembly of cell structures –
degradation of nuclear lamina - Inactivation of apoptotic
inhibitors e.g. i-CAD - Deregulation of other protein
activity e.g. gelsolin -> actin
depolymerisation
The Intrinsic Pathway of Apoptosis
- Intrinsic or mitochondrial pathway
- Activated by cellular stresses e.g. DNA damage,
growth factor depletion, hypoxia - Stress signal relayed to mitochondria, leading to
mitochondrial outer membrane permeabilisation
(MOMP), which releases apoptotic proteins into
the cytoplasm
The Intrinsic Pathway of Apoptosis
* Pro-apoptotic proteins released
include
cytochrome c and Smac
(Diablo)
* Cytochrome c associates with other
proteins like Apaf-1, which allows
for the formation of apoptosome
* Smac inactivates a group of antiapoptotic proteins called IAPs
(inhibitors of apoptosis)
The Intrinsic Pathway of Apoptosis – Cytochrome c
- Cytosolic cytochrome c activates
the apoptosome - Cytochome c + Apaf-1 +
procaspase 9 = activation of
caspase 9 - Apaf-1 (apoptosis protease
activating factor 1) scaffold protein - Caspase-9 leads to activation of
effector caspases
Bcl-2 protein family (B cell lymphoma-2)
-Pro-apoptotic (BAX subfamily): BAX and BAK Essential for MOMP
-Proapoptotic (BH3 only): BIM, BAD, NOXA, PUMA, BID Activate Bax/Bak
-Anti-apoptotic: Bcl-2, Bcl-XL, MCL-1, Bcl-w Cell survival
Bcl-2 family
- BH3-only proteins are thought to activate BAX/BAK either through direct
binding [tBID, BIM,PUMA] and/or indirectly by binding to their repressors, prosurvival BCL2 proteins - BH3 proteins are regulated at the transcriptional and post-translational level
and activated in response to given stimuli (eg UV (Bim), genotoxic damage
(Noxa, PUMA), cytokine deprivation (Bad), death receptors (tBid) - The relative levels of pro- and anti-apoptotic proteins within each channel
determine whether cytochrome c will be released from the mitochondrion
Inhibitors of apoptosis (IAPs)
- 8 proteins including c-IAP1, c-IAP2, XIAP, and surviving
- X-IAP inhibits caspase-3 and -9
- cIAP1 ubiquitinates caspase-3 and -7
Why is apoptosis dysregulated in cancer cells?Bcl-2 family and cancer
- Upregulation of anti-apoptotic proteins is common in cancer
(e.g. Bcl-2 and Mcl-1) - Inactivation of pro-apoptotic proteins (e.g. Bax is inactivated
in the majority of colon cancers)
