III. Signal transduction and cell cycle | 45. Insulin signaling

Question 1

I. Insulin and its receptors
1. What is insulin?

Answer 1

Insulin is a hormone released into the bloodstream from pancreatic beta-cells in response to hyperglycemia.

Question 2

I. Insulin and its receptors
2. What is the role of insulin?

Answer 2

Contributes to storage of glucose in the liver, adipose tissue and muscles.

Question 3

I. Insulin and its receptors
3. How does insulin work?

Answer 3

Insulin will bind to insulin receptors, that will result in a cascade of intracellular events.

Question 4

I. Insulin and its receptors
4. What are insulin receptors?

Answer 4

Insulin receptors are tyrosine kinase receptors that will bind insulin on its extracellular ligand-binding domain, which will lead to autophosphorylation of the tyrosine residues.

Question 5

I. Insulin and its receptors
5. What is the mechanism of insulin receptors?

Answer 5

• When activated, the insulin receptor substrate (IRS) [with its PTB domain] will bind to the phosphorylated tyrosine residues in the insulin receptor.
• Once IRS is phosphorylated by RTK, it can function as a docking site for different signaling molecules.
• By this, we can distinguish between 2 main pathways: MAP kinase pathway and PI 3-kinase pathway.

Question 6

I. Insulin and its receptors
6. What are the 2 main pathways of insulin signaling?

Answer 6

MAP kinase pathway and PI 3-kinase pathway.

Question 7

II. MAP kinase pathway
1. What are the 5 steps of MAP kinase pathway?

Answer 7

1. Insulin binds to receptor
   => receptor undergoes autophosphorylation
2. IRS-1 binds to the phosphorylated on the tyrosine residues. RTK will then phosphorylate IRS-1.
3. IRS-1 will bind to the SH2 domain of Grb2
   -> Grb2 will bind to SOS -> SOS binds to Ras, and exchange GDP with GTP -> activation of Raf (MAP3K)
4. Raf (MAP3K) will phosphorylate MEK (MAP2K), which will phosphorylate Erk (MAPK) and activate it
5. Erk (MAPK) will move into the nucleus and phosphorylate nuclear transcription factors, eventually activating them
   => mainly initiates cell growth (cell division etc.)

Question 8

II. MAP kinase pathway
2. What happen if IRS-1 bind to the SH2 domain of Grb2?

Answer 8

IRS-1 will bind to the SH2 domain of Grb2
-> Grb2 will bind to SOS
-> SOS binds to Ras, and exchange GDP with GTP -
> activation of Raf (MAP3K)

Question 9

II. MAP kinase pathway
3. Make a schematic diagram of MAP kinase pathway

Answer 9

Question 10

III. PI 3-kinase signaling pathway
1. What are the steps of PI 3-kinase signaling pathway?

Answer 10

• IRS can also bind PI 3-kinase. Ras will contribute to the activation of PI 3-kinase
• Activation of PI 3-kianse results in phosphorylation of PIP2 into PIP3.
• When concentration of PIP3 is high enough, it will bind to PDK1 (3-phosphoinositide-dependent protein kinase-1)
• PDK1 can activate PKB (Akt) by phosphorylation of serine/threonine residues.
• The activated PKB can further activate many different processes

Question 11

III. PI 3-kinase signaling pathway
2. What is the consequence of Activation of PI 3-kinase?

Answer 11

Activation of PI 3-kinase results in phosphorylation of PIP2 into PIP3

Question 12

III. PI 3-kinase signaling pathway
3. How can PDK1 activate PKB (Akt)?

Answer 12

• PDK1 can activate PKB (Akt) by phosphorylation of serine/threonine residues.
• The activated PKB can further activate many different processes

Question 13

IV. PKB/Akt
1. What is PKB/Akt?

Answer 13

It is a serine/threonine kinase (means it phosphorylates serine and threonine residues on target proteins)

Question 14

IV. PKB/Akt
2. PKB/Akt has may different effects
=> What are the affected substrates?

Answer 14

1. AS160
2. GSK3 (glycogen synthase kinase 3)
3. FOX01 (TF)
4. SREBP (TF)

Question 15

IV. PKB/Akt
3. What is the effect of PKB/Akt on AS160?

Answer 15

• PKB will phosphorylate AS160 and thereby inactive it.
• AS160 is the inactivator of Rab small GTPase.
• But since AS160 is now inactivated, Rab will be active and it will induce synthesis of GLUT4 (glucose transporter) and their translocation to the PM => increased glucose uptake

Question 16

IV. PKB/Akt
4. What is the effect of PKB/Akt on GSK3?

Answer 16

• GSK3 is a protein kinase that phosphorylates and inhibits glycogen synthase, and thereby glycogen synthesis.
• PKB will inhibit GSK3 through phosphorylation, keeping glycogen synthesis active in response to insulin

Question 17

IV. PKB/Akt
5. What is the effect of PKB/Akt on FOX01 (TF)?

Answer 17

PKB will phosphorylate FOX01 and inhibit it
=> decrease gluconeogenesis (less glucose produced)

Question 18

IV. PKB/Akt
6. What is the effect of PKB/Akt on SREBP (TF)?

Answer 18

SREBP (TF) will be activated by PKB, stimulating cholesterol uptake and synthesis

Question 19

IV. PKB/Akt
7. What is the consequence of Serine phosphorylation of IRS proteins?

Answer 19

• Serine phosphorylation of IRS proteins can uncouple it from the receptor, thereby leading to impaired signaling and eventually insulin resistance.
• Serine phosphorylation can also lead to an accelerated degradation of IRS-1 protein.
• The mediators that cause the phosphorylation can be: TNFα, free fatty acids and stress

Question 20

IV. PKB/Akt
8. What are tmediators that cause the phosphorylation?

Answer 20

TNFα, free fatty acids and stress