Chapter 7: Apoptosis (Book, main)

Question 1

Name target proteins of the extrinsic pathway.

Answer 1

Nuclear lamin (allowing nuclear shrinkage), cytoskeletal proteins (for rearranging cell structure), specific kinases (for cell signaling) and enzymes such as DNase (for cleavage of chromatin).

Question 2

Caspases also cleave the tumor suppressor protein RB. What does this cleavage result in?

Answer 2

Degradation of RB protein which is required for apoptosis induced by TNF.

Question 3

Just a refresher:

Match Bcl-2 family members (Bcl-2, Bax, Mcl-1, Bak, Bcl-x1) with their function (pro- or anti-apoptotic).

Answer 3

Pro-apoptotic -> Bax and Bak

Anti-apoptotic -> Bcl-2 and Mcl-1, Bcl-x1

Question 4

The activity of the proteins of the Bcl-2 family can also be regulated in a different way. How?

Answer 4

By phosphorylation

Question 5

What is mitochondrial outer membrane permeabilization (MOMP)?

Answer 5

Upon activation by an apoptotic signal, BH3-only proteins, Bid and Bim, bind and activate Bax. Through a cascade of activation, dimerization and oligomerization of Bax monomers in the outer mitochondrial membrane, this will result in an increase in permeability of the outer membrane allowing apopotic mediators to be released.

Question 6

What anti-apoptotic protein causes the dissociation of Bax oligomers?

Answer 6

Bcl-x1

Question 7

What is the function of Apaf-1?

Answer 7

Apaf-1 is a protein co-factor that is required for the activation of procaspase-9

Question 8

Cytochrome c can bind to Apaf-1. If so, procaspase-9 is recruited. What domains in both cytochrome c as Apaf-1 is responsible for this recruitment?

Answer 8

CARD domains

Question 9

The transcription factor NF-kB is a major player in inflammation. What role does it have in apoptosis?

Answer 9

It is a potent inhibitor of apoptosis, it induces the transcription of IAPs.

Question 10

Regulator Smac/DIABLO can also be released from the mitochondria. What is its’ function?

Answer 10

It competes with activated caspase-9 for binding to XIAP. It eliminates inhibition by IAPs.

Question 11

Normal cells contain inactive procaspases that require proteolytoc cleavage. Cancer cells contain activated caspases that are inhibited by upregulated IAPs. What is the result of this?

Answer 11

That cancer cells are closer to triggering an apoptotic response, compared with a normal cell.

Question 12

Sunburn represents skin cells that are undergoing UV-induced apoptosis. Both the intrinsic and the extrinsic pathways are activated during UV exposure. How?

Answer 12

UV causes the clustering of Fas death receptors (extrinsic) and activation of the caspase cascade (intrinsic) -> think of the crosstalking between the extrinsic and intrinsic pathway (where caspase 8 can cleave and activate Bid that can then activate Bak/Bax).

Question 13

True/false:
Mutations in the Fas pathway will not reduce the apoptotic response and will not lead to an increase in the risk of skin cancer.

Answer 13

False, (somatic) mutations will lead to an increase in the risk of skin cancer.

Question 14

What mutations are there found in small-cell lung carcinoma and neuroblastomas?

Answer 14

Suppression of caspase gene expression, mostly caspase-8 deficiency.

Question 15

Why is caspase-8 so important in the caspase-cascade (thus: why is loss of caspase-8 bad)?

Answer 15

It is the first caspase activated by death receptors and maintains a top position in the initiation of the caspase cascade.

Question 16

Are alterations more common in the intrinsic or extrinsic pathway of apoptosis during carcinogenesis?

Answer 16

Alterations in the intrinsic pathway are much more common.

Question 17

What protein is a major contributor for alterations in the intrinsic pathway?

Answer 17

p53

Question 18

Lymphomas typically display p53 mutations. What specific mutations?

Answer 18

Abnormal methylation and loss of heterozygosity (or alternative splicing) for p73, a p53 family member.

Question 19

What mutations is frequent in Bcl-2 in B-cell lymphomas?

Answer 19

Chromosomal translocation t(14;18)

Question 20

What does chromosomal translocation of Bcl-2 lead to?

Answer 20

Bcl-2 gene is translocated to a position juxtaposed to the immunoglobulin heavy chain enhancer. As a consequence of its relocation next to a strong promotor, oncogenic activation of Bcl-2 gene occurs (leads to insufficient apoptotic turnover and accumulation of B-cells).

Question 21

What gene is mutated in over 50% of a specific class of colon tumors?

Answer 21

Bax

Question 22

Many apoptotic stimuli do not require caspases. Besides apoptosis, there are other ways cells can die. How?

Answer 22

Necrosis, autophagy and mitotic catastrophe.

Question 23

What is autophagy?

Answer 23

It means eating oneself and acts as a recycling system for the cell whereby proteins and compoenents of damaged organelles that require degradation are targeted to the lysosomes. Cells can recycle the resulting products of degradation.

Question 24

When is autophagy important?

Answer 24

Under starvation conditions and for ridding the cell of defective organelles.

Question 25

Describe how autophagy occurs.

Answer 25

Under starvation conditions or in the presence of defective organelles, targeted proteins and organelles are surrounded by a double-membraned structure called the autophagosome. When lysosomes fuse with the autophagosome, the content is degraded.

Question 26

What is mitotic catastrophe caused by?

Answer 26

Aberrant mitosis

Question 27

What is often the primary function of chemotherapeutic agents?

Answer 27

Inducing DNA damage which triggers the intrinsic apoptotic pathway.

Question 28

The extrinsic pathway can also be triggered by chemotherapeutic agents. How?

Answer 28

By inducing particular immune cells to produce TNF, thus triggering the extrinsic pathway.

Question 29

What cells with a specific mutation can be highly resistant to drug-induced apoptosis?

Answer 29

Cells that have engineered to have a p53 knock-out (but even gain-of-function p53 mutations can result in resistance to chemotherapy).

Question 30

What can be concluded from the observations about tumor resistance?

Answer 30

That the genotype of a tumor (especially with respect to p53 and Bcl-2 gene families) influences the effectiveness of therapy.

Question 31

What is a result of resistance to chemotherapy on cancer cells?

Answer 31

Chemotherapy induces extensive DNA damage in order to induce apoptosis. But if apoptosis doesn’t occur, mutations will accumulate in these cells. Therefore the risk of carcinogenesis increases.

Question 32

Why are small-molecule inhibitors of Bcl-2/Bcl-x also called BH3 mimetics?

Answer 32

They mimic the action of BH3 proteins binding to the BH3 hydrophobic groove of anti-apoptotic proteins.