Apoptosis Resistance in cancer

Question 1

What is the death receptor pathway of apoptosis?

Answer 1

• ligand binds to death receptors and molecules such as FADD are recruited
• recruits caspses such as 9+8 that are recruited into a complex inside the cell to form a death inducing signalling complex (DISC)

Question 2

Name some death receptors, their ligands and their roles

Answer 2

Question 3

Describe the premise of death receptor signalling

Answer 3

• death ligand binds death receptor
• recruits adapter proteins
• recruit initiator and effector caspases
• apoptosis or activation of signalling molecules susch as NFkB and MAPK lead to survival

Question 4

Describe fas induced apoptosis

Answer 4

• fas ilgand binds best as a timer
• activates intracellular death domain
• recruits FADD due to their shared homology
• this recruits procaspase 8 that activates effector caspases in type I cells or cleaves Bid in type II cells

Question 5

What are type I cells in the context of fas induced apoptosis?

Answer 5

• cells that do not need Bid for fas mediated cell death
• such as lymphocytes

Question 6

What are type II cells in the contect of fas induced apoptosis?

Answer 6

• cells that use BID to amplify fas induced death signals by release of cytochrome C
• Bcl-2 can block death rceptor induced apoptosis in type II cells only
• hepatocytes

Question 7

What kinds of cancers are caspase 8 mutations commonly seen in?

Answer 7

• gastric and hepatocellular
• C8 mutant cells are more resistant to apoptosis
• those with these mutations have lower survival

Question 8

What is ALPS?

Answer 8

• autoimmune lymphoproliferative syndrome
• point mutations in death domain leads to a truncated fas receptor that cant recruit DISC components
• leads to increased risk of lymphomas

Question 9

What are decoy receptors?

Answer 9

• death receptors with no intracellular death domain
• mop up death ligand and stops it binding functional receptors
• can be solubule
• can be overexpressed in cancer such as colon cancer

Question 10

How does fas ligand expression change as cells move towards a cancer phenotype?

Answer 10

expression increases during progression from adenoma to carcinoma to metastasis

Question 11

Describe fas in colon tumours

Answer 11

• tumours usually resistant to induction of fas-mediated apoptosis
• but produce their own soluble fas ligans during progression that harms the cells around them
• (cell competition)

Question 12

What is cFLIP?

Answer 12

• structurally similar to caspase 8 but no proteolytic activity
• blocks death receptor signalling by docking onto adapter molecules and integrating into the DISC to reduce caspase recruitment
• high levels in malignant melanoma and colon cancer

Question 13

What is the benefit to cancer cells in producing high levels of fas ligand?

Answer 13

• allows them to develop as immune privileged sites by inducing apoptosis of activated infiltrating T cells that express fas
• counter attack

Question 14

What are the consequences of fas ligand expression by tumours?

Answer 14

• facilitates metastatic colonisation of fas-sensitive organs by inducing apoptosis of these cells
• enhances cancer cell proliferation and invasion as they have more space to move into

Question 15

Give an example of a fas-sensitive organ

Answer 15

liver

Question 16

How can the immune system induce apoptosis?

Answer 16

• CD8+ T cells in adaptive
• NK cells in innate
• kill cells through the fas pathway or through the granule exocytosis pathway

Question 17

What is the calcium dependent granule exocytosis pathway?

Answer 17

• lumphocytes secrete perforin that creates pores in the plasma membrane of the target cell
• granzymes (serine proteases) can then produce tBid and induce apoptosis

Question 18

In summary, name 4 ways in which tumour cells can become resistant to fas mediated apoptosis

Answer 18

• receptor mutation
• reduced receptor expression
• overexpression of cFLIP
• overexpression of Bcl-2 in type II cells

Question 19

How can proapoptotic BH3 only proteins be regulated?

Answer 19

• by cleavage into tBID
• regulated by transcriptional mechanisms, phosphorylation or cell matrix detacthment

Question 20

What happens in the cell when survival factors are present?

Answer 20

• PI3K activates Akt
• Akt phosphorylates Bad, inactivating it and allowing it to be sequestered by the 14-3-3 protein

Question 21

What happens to Bad when survival factors are depleted?

Answer 21

• Akt is no longer activated
• Bad isnt phosphorylated and remains active to act as a BH3 protein
• Bad displaces Bcl2 family members leading to apoptosis

Question 22

What are Bmf and Bim?

Answer 22

• BH3 only proteins
• at high levels in normal cells but sequestered
• recognise when cells detach from the ECM due to their links with microtubules
• triggers apoptosis
• Bmf is expressed in non-invasive breast tumours but commonly lost when they become metastatic

Question 23

What is BIM and what its isoforms?

Answer 23

• Bcl-2 family member
• BIM-EL is most common
• BIM-S least common

Question 24

What does BIM do?

Answer 24

• normally proteasomally degraded in the ras pathway
• when growth factors are depleted, BIM binds MCL-1, displacing Bax and allowing apoptosis
• mutations in ras - always displacing Bax

Question 25

What are direct activators?

Answer 25

• BH3 only family proteins that can bind directly to Bax, Bak and initiate their oligomerization without having to displace it from Bcl-2
• Bad can only activate them with this displacement and is known as a sensitiser

Question 26

What are the different antiapoptotic potencies?

Answer 26

• Bcl-2, XL have greater antiapoptotic capcity then others due to their greater stability
• these have half lives of ~20 hours
• other proteins may only last 1-4 hours

Question 27

What else is released from the mitochondria to trigger apoptosis?

Answer 27

• apoptotis inducing factor and enconuclease A
• these cause DNA fragmentation when other pathways are lost
• SMAC/DIABLO + OMI inhibit XIAP and release proapoptotic caspases

Question 28

What are IAPs?

Answer 28

• human inhibitor of apoptosis proteins
• XIAP, survivin etc

Question 29

What is the role of XIAP?

Answer 29

• IAP that binds caspases and inhibits them by targeting them for proteosomal degradation
• can also promote NFkB activation

Question 30

What is survivin?

Answer 30

• IAP overexpressed in almost all human cancers
• low/undetectable levels in normal tissues
• promotes cell survival and affects mitotic transition
• increased in tumours cells by PI3K and B-catenin pathways