Apoptosis

Question 1

How do anti-apoptotic factors via receptors promote cell survival? Describe one mechanism in more detail.

Answer 1

For example by survival factor binding activating receptor which stimulates the transcription of anti-apoptotic Bcl2 family proteins (bcl2 and BclXl). These are present on cytosolic surface of outer mitochondrial membrane where they help prevent inappropriate release of intermembrane proteins such as cytochrome C. They bind and inhibit pro-apoptotic Bcl2 family proteins, eg binding Bak preventing oligomerization, inhibiting release cytochrome C.

Question 2

When a cell is no longer able to produce the Akt protein, will this cell be able to resist apoptosis? Explain your answer.

Answer 2

A deficit of Akt results in the inability to split Bad from the anti-apoptotic Bcl2 family protein. Since anti-apoptotic proteins can not be activated, resisting apoptosis becomes harder.

Question 3

Describe the intrinsic pathway of apoptosis.

Answer 3

Activated from inside the cell, often in response to stresses, eg DNA damage. ATM binds damaged DNA and recruits Chk1 and Chk2, which activate p53 and inhibit MDM2 mediated p53 degradation. P53 activates pro-apoptotic Bcl2 protein and transactivates eg apaf1. Bcl2 proteins aggregate, oligomerizing in mitcohondrial membrane. Promoting release of mitochondiral proteins, such as Cytochrome C, when released binds Apaf1, which oligomerizes into an apoptosome. Apoptosome recruits initiator caspase-9 proteins, which activate downstream executioner caspases to induce apoptosis.

Question 4

Which three proteins are involved in the release of cytochrome C and how do they regulate it? Mention their domains.

Answer 4

Apoptotic stimulus triggers the intrinsic pathway. Pro apoptotic effector Bcl2 family proteins are activated and aggregate, forming oligomers in mitochondrial outer membrane, inducing the release of cytochrome C. Bax and Bak are maineffector Bcl2 proteins, with BH domains 1-3.
Third pro-apoptotic Bcl2 family proteins are the BH3-only proteins. BH3 only proteins promote apoptosis by inhibiting anit-apoptotic Bcl2 family proteins, enabling aggregation of Bax and Bak, triggering release of Cytochrome C.

Question 5

How do BH3-only proteins promote apoptosis in response to an apoptotic stimulant?

Answer 5

BH3-ony proteins promote apoptosis by inhibiting anti-apoptotic Bcl2 family proteins.
BH3 domain binds to a hydrophobic groove on anti-apoptotic Bcl2 family proteins, neutralizing their activities.
Binding and inhibition enables aggregation of Bax and Bak on the mitochondrial surface. Triggers the release of intermembrane mitochondrial proteins (cytochrome C), inducing apoptosis (intrinsic pathway).

Question 6

Mention and briefly explain the 3 mechanisms by which apoptosis can be blocked by extracellular survival factors.

Answer 6

1. By iPI 3-kinase pathway:
   Survival factor binds and activates RTK, which activates PI 3-kinase. Which produces PI(3,4,5)P3, activating PDK1 and Akt. Akt phosphorylates and ianctivates pro-apoptotic Bad. Bad dissociates from Bcl2, which is then free to supress apoptosis.
   (akt also inactivates transcription regulatory proteins what stimulate transcription of apoptosis promoting proteins).
2. By stimulating transcription o f genes encoding anti-apoptotic Bcl2 family proteins (Bcl2 and BclXl)
3. By inactivation of anti-IAPs (inhibitors of apoptosis).
   survival factor activates receptor, which activates MAP kinase. Inactivation of Hid, Hid no longer inhibits IAPs.

Question 7

Which proteins belong to the anti-apoptotic Bcl2 family?

Answer 7

Bcl2, BclXl

Question 8

What is the role of cytochrome C in apoptosis?

Answer 8

Release of Cytochrome C, in response of apoptotic factors, induces the intrinsic pathway of apoptosis. Cytochrome C binds Apaf1, and oligomerizes into an apoptosome. Apoptosome recruits initiator caspase-9 proteins, which activate downstream executioner caspases to induce apoptosis.

Question 9

What are the 3 main types of p53 inactivation in cancer cells?

Answer 9

1. Direct inactivation of p53 as a result of mutations in the p53 gene. (Change in amino acid binding domain prevents binding to DNA and activation of genes)
2. Indirect activation through binding to viral proteins. (products of viral oncogenes bind and inactivate p52, sometimes stimulating p53 degradation)
3. Indirect activation as a result of alterations in genes whose products interact with p53 or transmit information to ro from p53. (Multiplication of MDM2 gene, extra mdm2 stimulates p53 degradation)