14.1 Apoptosis II Flashcards
What regulates the release of cytochrome c from mitochonrdia?
members of the B-cell leukemia/lymphoma 2 (Bcl-2) protein family.
What does Bcl stand for?
B-cell leukemia/lymphoma
What are the Bcl-2 homology (BH) domains?
BH1 BH2 BH3 BH4 The BH domains are anti-apoptotic proteins and they crucial for function as deletion of these domains affects survival/apoptosis.
Describe the difference between BH proteins that have 4 BH domains and BH proteins that have less than 4 domains
4 domains means they are anti-apoptotic (prevent apoptosis)
Less then 4 domains promote apoptosis
What is the effect of pro-apoptotic Bcl-2 proteins on BH123?
The major antagonists of the BH123 signaling proteins are Bcl2 and Bcl-X, which multimerize
with BH123 proteins and thereby prevent their activity.
What happens when Bak and Bax are activated
when these proteins are activated, they multimerize and form pores in the mitochondrial membrane, which allows cytochrome c to escape (as well as other proteins)
This triggers the apoptotic signaling pathway through the intrinsic mechanisms
What is the difference between Bax and Bak?
Bax is found in the cytoplasm, so it has to translocate to the mitochondrial membrane when it is activated
Bak is always associated w/the mitochondrial membrane and it’s always present and ready to go.
What down regulates the pro-apoptotic effects of the BH123 proteins (Bak and Bax)? Why is down regulation important
They are down regulated by the effects of the anti-apoptotic Bcl2 proteins, Bcl2 and Bcl-X.
Bcl2 and Bcl-X are present in order to prevent unintended activation of apoptosis, which could lead to cell death for random reasons. They can also be stimulated by survival factors that prevent programmed cell death that might otherwise occur.
How do Bcl2 and Bcl-X act as antagonist to BH123 signaling proteins?
Bcl2 and Bcl-X multimerize w/BH123 proteins and prevent their activity
Bcl2 is associated w/the mitochondrial membrane in order to couteract any accidental apoptotic triggering by Bak.
Why does the activity of Bcl2 have to be turned off at some point? What turns it off?
Since Bcl2 is always present at the mitochondrial membrane to prevent any unwanted apoptosis, the activity of Bcl2 has to be turned off when apoptosis is really needed.
A third group of Bcl2 family proteins called BH3 (Bid and Bad) turns off/inactivates Bcl2 at the mitochondrial membrane and then allow Bak to do its thing and promote apoptosis
How many domains are in BH3 proteins and what are the two that turn off Bcl2?
Bid and Bad inactivate Bcl2 and they only have one of the BH domains.
What are IAP’s? What is their role?
Inhibitors of apoptosis
- Can rescue cells from cell-death
- bind to and inhibit caspases (initiators and executioner)
- prevent accidental apoptosis caused by spontaneous activation of procaspases by inhibiting procaspase activation
Why must each cell have at least 1 IAP or one kind of IAP in order to survive?
B/c if it didn’t when one of the apoptotic promoting factors gets started there would be nothing to prevent it from killing off the cell.
what is the main structural feature shared by IAP’s
a domain called BIR (baculovirus IAP repeat)
How do viruses use IAPs to propagate?
cells that are infected w/a virus will try to go through apoptosis as a way to prevent the spread of the virus in an organism. However, some viruses have developed counter strategies to apoptosis by host cell, such as causing cells to express IAPs
In addition to binding and inactivating caspases, what else do IAPs do with caspases?
they might also target caspases for degradation through polyubiquitination
How are the functions of IAPs inhibited when cells are supposed to go through cell death?
by inter-membrane mitochondrial proteins called Reaper, Grim, and Hid (in drosophila)
in mammals the protein is called Smac or Diablo
What happens when reaper and hid are mutated or overexpressed?
they cause cell death in various tissues.
List all anti-IAPs
Reaper
Grim
Hid
Smac/Diablo
Gain of function of mutants in anti-IAPs Reaper and grim cause what?
eye defects in flies. Flies will have smaller eyes because many cells have died through apoptotic cell death. This is happening b/c the anti-apoptotic signaling mechanisms of IAPs that are present are overwhelmed by the anti-IAPs, Grim and Reaper
Which apoptotic proteins are up regulated as a consequence of p53 gene regulation?
Fas Ligand and proteins Bid and Bax
cells that are always programmed to die require what?
their survival requires rescue factors
What is an example of cells programmed to die using a rescue factor?
Some cells are always programmed to die: their survival requires rescue factors
A major example of this type of cell survival
signaling is found in the developing nervous system. During fetal development, many nerve
cells are produced and this large number of nerve cells is produced in order to ensure that targeting cells which have to be innervated will be contacted by at least one neuron. The interaction between the nerve cell and a target cell promotes anti-apoptotic signaling pathways in the nerve cell that allow it to continue to live. However, any nerve cell that was produced yet
failed to find an appropriate target cell within some period of time then undergo apoptotic cell death, so they are pruned back, leaving only the neurons that have actually managed to make a connection. This mechanism ensures that every target cell has a neuron associated with it, but that there’s also just exactly one nerve cell for each target cel
How do survival factors suppress apoptosis? What are the three mechanisms?
often by regulating activity of members of the Bcl2 family of proteins (mechanism 1)
1) Increased production of the anti-apoptotic Bcl2 protein
2) Inactivation of pro-apoptotic BH3-only Bcl2 protein
3) Inactivation of anti-IAPs
What is the Akt cell signaling? How is apoptosis blocked in this signal?
1) Survival factor binds to activated receptor initiation pi-3 kinase signaling pathway
2) Pi-3 Kinase signaling pathway activates Akt kinase
3) Bad is phosphorylated by Akt, and deactivates Bad
4) Deactivation of Bad releases Bcl2 to be functional
what is the MAP kinase signaling? How does it block apoptosis?
1) survival factor binds to activated receptor
2) Map-kinase becomes activated and phosphorylates anti-IAP Hid
3) IAP is released from Hid and IAP becomes functional
What activates Akt?
Pi-3 kinase signaling pathway
What is Akt?
a major regulator of cell survival and cell growth w/many downstream targets like BH3-only protein Bad
(Bad inactivates Bcl2)
What happens when Bad is phosphorylated by Akt?
it gets deactivated and this releases Bcl2 to be functional
how does the mitogen activated protein in kinase signaling pathway regulate many pro survival pathways?
one way is by phosphorylation of the anti-IAP HID, which inactivates it.
How can the extrinsic apoptotic signaling pathway activate the intrinsic pathway?
through activity of BID (a BH3-only protein) that is expressed in an inactive form in the cytoplasm.
BID gets cleaved by initiator caspases, resulting in a smaller active form called tBid.
tBid inactivates Bcl2, and that leads to the release of cytochrome c from mitochondria through the function of proteins like Bak
How can p53 cause apoptosis?
p53 can cause the expression of the Fas ligand which binds to death receptor on extrinsic pathway. This can cause apoptosis through autocrine stimulation of the extrinsic apoptotic signaling pathway. This is one way a cell would signal itself to die
Anti-apoptotic protein families with 4 BH domains are known as
Bcl2 (4 domains)
Pro-apoptotic protein families with 3 domains are known as? What are examples
BH123; Bax and Bak
Pro-apoptotic protein with one domain are known as ? what are examples?
BH3; Bad, Bim, Puma, Noxa
