[W5] Pathways of cell survival and growth control - pt 2

Question 1

What two major pathways must integrate to regulate cell fate?

Answer 1

Ras/MAPK and PI 3-kinase/PKB (Akt) pathways

Question 2

What cellular outcomes are controlled by the Ras/MAPK and PI 3-kinase/PKB pathways?

Answer 2

Apoptosis, cell division, and cell survival

Question 3

What activates the Ras/MAPK pathway?

Answer 3

Binding of growth factors to receptor tyrosine kinases (RTKs)

Question 4

How does Ras/MAPK affect the cell cycle?

Answer 4

It increases expression of Myc, Cyclin D, and other regulators of cell cycle progression

Question 5

What is Myc and what does it do?

Answer 5

A transcription factor that promotes cell cycle entry; too much Myc can cause arrest or apoptosis

Question 6

Which Cyclins and CDKs are regulated downstream of Ras?

Answer 6

Cyclin D/CDK4 and Cyclin E/CDK2 complexes

Question 7

What does E2F do in the cell cycle?

Answer 7

Transcribes genes required for entry into S phase

Question 8

Why are survival signals important?

Answer 8

To prevent apoptosis; cells need constant survival input

Question 9

Name sources of survival signals.

Answer 9

• Growth factors
• Cytokines
• Integrins
• T/B cell receptors
• Insulin

Question 10

What is PI 3-kinase (PI3K)?

Answer 10

A lipid kinase that phosphorylates membrane phosphoinositides to produce docking sites for signalling proteins

Question 11

What types of PI3K exist?

Answer 11

• Class IA (RTK-activated)
• Class IB (GPCR-activated)
• Class II
• Class III

Question 12

What lipid does PI3K generate from PIP2?

Answer 12

PI(3,4,5)P₃ (PIP3)

Question 13

What domains bind PIP3?

Answer 13

PH (pleckstrin homology) domains

Question 14

What are some other lipid-binding domains?

Answer 14

• FYVE
• PX
• ENTH
• C2

Question 15

What happens when a survival factor binds its receptor?

Answer 15

The receptor autophosphorylates on tyrosine residues

Question 16

How is PI3K recruited to the receptor?

Answer 16

Via SH2 domains on the PI3K regulatory subunit

Question 17

What does PI3K do at the membrane?

Answer 17

Converts PI(4,5)P₂ to PI(3,4,5)P₃

Question 18

Which two key proteins bind PIP3 via PH domains?

Answer 18

• PDK1
• PKB (Akt)

Question 19

What is PDK1?

Answer 19

A kinase that activates PKB via phosphorylation on Thr308

Question 20

How is PKB fully activated?

Answer 20

By phosphorylation on both Thr308 and Ser473 (by mTORC2 or other kinases)

Question 21

What does activated PKB do?

Answer 21

Phosphorylates target proteins to promote survival and metabolism

Question 22

What is BAD and what happens when PKB phosphorylates it?

Answer 22

A pro-apoptotic protein; phosphorylation inactivates it and promotes survival

Question 23

What is PTEN?

Answer 23

A tumour suppressor that dephosphorylates PIP3 to PIP2, reversing PI3K signalling

Question 24

What happens when PTEN is lost?

Answer 24

Increased cell survival and risk of oncogenesis

Question 25

What other proteins are PKB targets?

Answer 25

* GSK3β * mTOR * Forkhead (FKHR) * NF-κB pathway * S6 kinase

Question 26

How does PKB regulate Forkhead transcription factors (e.g. FKHRL1)?

Answer 26

Phosphorylation causes nuclear export and binding to 14-3-3 proteins, blocking pro-apoptotic gene transcription

Question 27

What does 14-3-3 bind to?

Answer 27

Phosphoserine-containing motifs in proteins like FKHRL1

Question 28

How does PKB affect translation?

Answer 28

Indirectly activates S6 kinase and mTOR to enhance protein synthesis

Question 29

What is mTOR?

Answer 29

A kinase that regulates growth, translation, and metabolism downstream of PI3K/PKB

Question 30

How does mTOR influence translation?

Answer 30

By activating S6 kinase and releasing 4E-BP from eIF4E to initiate cap-dependent translation

Question 31

What is the relationship between PI3K and proliferation?

Answer 31

PI3K signalling contributes to both survival and cell proliferation through mTOR and PKB

Question 32

What are GFP–PH domain fusion proteins used for?

Answer 32

To visualise PI3K activity by binding to PIP3 in live cells

Question 33

What type of inhibition helped define PI3K functions?

Answer 33

Use of small molecule inhibitors like Wortmannin and LY294002

Question 34

What is the general structure of PI3K lipid substrates?

Answer 34

Phosphatidylinositol with a phosphorylated inositol headgroup

Question 35

What’s the overall goal of combining Ras/MAPK and PI3K/PKB pathways?

Answer 35

To ensure cells grow, divide, and survive appropriately by coordinating transcription, translation, and anti-apoptotic signals

Question 36

What reading is recommended for this topic?

Answer 36

Alberts Molecular Biology of the Cell, Ch. 15 (pp. 879–941), and pp. 1060–1067, 1101–1112