7T: Signaling Through Growth Factor and Cytokine Receptors

Question 1

Growth Factors vs. Cytokines

Answer 1

Growth Factors: stimulate cell proliferation, stimulate cell differentiation (Ex. EGF, VEGF, FGF)
Cytokines: elicit various cellular effects, usually associated with an immune response, a cytokine can function as a growth factor (Ex. ILs, IFN)
Both: are secreted proteins, bind transmembrane receptors

Question 2

Growth factor receptor

Answer 2

• bound at the cellular membrane, with a single transmembrane domain
• intercellular domain is a tyrosine kinase domain

Question 3

EGF signaling

Answer 3

• ligand binds
• brings the 2 receptors close to each other
• leads to autophosphorylation of the EGF receptor
• Grb2-SOS binds to activated receptor and to Ras
• binding of Ras leads to phosphorylation of RAF –> MEK —> ERK —> TF (phosphorylation cascade)
• eventually a transcription factor is phosphorylated and then transcription is increased

Question 4

Cytokine receptors

Answer 4

• bound at the cellular membrane, with a single transmembrane domain
• does NOT have a tyrosine kinase domain (no catalytic activity themselves)
• associated with JAK, which is a catalytic kinase

Question 5

Cytokine receptor signaling

Answer 5

• ligand binds and brings to 2 receptors together
• JAK is activated
• JAK phosphorylated the receptors
• STAT binds to the phosphorylated receptor
• JAK phosphorylates STAT, which dimerizes the two molecules
• STAT is a transcription factor that can now do its job
  (NOS is one gene that is increased through this signaline)

Question 6

Phosphatase activity in growth factor signaling

Answer 6

• Phosphatases take off the phosphate group of the kinases and the transcription factors (Raf, MEK, ERK, and the TF)

Question 7

GAP activity in growth factor signaling

Answer 7

• GTPase activated proteins promote the intrinsic GTPase activity of Ras, and hydrolyze it’s GTP back to GDP which inactivates it

Question 8

Growth factor receptor inactivation

Answer 8

• can be internalized, which leads to dissociation of the complex
• can be degraded

Question 9

PIAS and SOCS activity in cytokine receptor signaling

Answer 9

• PIAS antagonizes cytokine signaling
• has a binding site for the STAT dimer on the STAT DNA binding domain, and blocks it from binding to increase transcription
• SOCS prevents interaction of STAT with the receptor
• SOCS is upregulated in response to cytokine signaling
• SOCS binds the phosphorylated receptor in the STAT binding site and doesn’t allow STAT to bind
• SOCS also has an intrinsic Ubiquitine ligase activity
• Ubiquitinates JAK so that it is degraded which then inactivates the receptor

Question 10

Inflammatory bowel disease

Answer 10

• Crohn’s disease

- Ulcerative colitis: over-expression of interferon gamma. Elevated activation of STAT1

Question 11

Myeloproliferative disorders

Answer 11

• activating mutation in JAK that leads to constitutive activation of JAK
• This leads to increased phosphorylation of STAT, and elevated STAT mediated signaling (increased transcription)

Question 12

Achondroplasia

Answer 12

• one of the downstream activated genes of FGF is p21
• p21 is a cell cycle antagonist
• caused by a mutation in the FGF receptor that triggers the receptor with no ligand
• which leads to premature arrest of the cell cycle (extremely decreased growth)

Question 13

Oncogenes

Answer 13

• mutant form of a proto-oncogene (normal fx is in normal cell proliferation or cell cycle)
• mutations can arise spontaneously, and if the mutation is in the proto-oncogene and results in an activating mutation (over-expression or gain-of-fx), then it is now an oncogene
• only need 1 mutated copy

Question 14

What mutations can occur to create oncogenes?

Answer 14

• mutations of the protein itself
  Ex. Mutant Ras. Certain mutations can lead to the inability of Ras to hydrolyze the GTP (so continual active Ras)
• mutations that effect overall levels of the protein
  Ex. normal ErbB3 and ErbB2 come together to produce tyrosine kinase activity. Certain mutations can increase ErbB2 production which leads to breast cancer (normal protein function, just too much of it)

Question 15

BRaf mutations

Answer 15

• Can have a mutation that leads to the constitutive activation of BRaf without a ligand binding (inactive Ras and no GF)
• These mutations are common in melanoma and moles
• This happens because the mutation is Val600Glu and the Glu is negatively charged. This mimics the phosphate group “activation” and can’t be turned off because phophatases can’t cleave that AA.