Cellular Signalling III: Growth Factors and Cytokine Receptors

Question 1

Compare and Contrast Growth Factors and Cytokines

Answer 1

Both Growth Factors and Cytokines:
1, are secreted proteins
2. bind to transmembrane receptors

Only Growth Factors:
1. Stimulate Cell proliferation and differentiation

Only Cytokines:

1. illicit immune response
2. (rarely can act as GF)
3. DO NOT possess tyrosine kinase activity!

Question 2

Describe the basic 5-step mechanism of Growth Factors

Answer 2

1. Extracellular receptor binds to ligand, creating dimer
2. Kinases phosphorylate Tyrosine domain on intracellular GF/cytokine receptor
3. Proteins (with positively charged domain) bind to phosphorylated tyrosine domain
4. MAPK cascade
5. Altered Gene Transcription

Question 3

Explain a MAPK Cascade

Answer 3

1. Active (Phosphorylated) Guanine Nucleotide Exchange Factor (GEF) i.e. SOS activates G-protein (Ras) by GTP
2. Active (phosphorylated) G-protein (Ras) activates a protein Kinase (MEK)
3. Protein Kinase activates another Protein Kinase (ERK)
4. This protein Kinase phosphorylates a Transcription Factor which alters genes.

Question 4

MAPK ERK responsible for:

Answer 4

proliferation, differentiation

Question 5

MAPK JNK responsible for:

Answer 5

stress response

Question 6

MAPK p38:

Answer 6

stress and apoptosis

Question 7

MAPK ERK5

Answer 7

proliferation, differentiation, development

Question 8

5 step mechanism of Cytokine Receptor Signaling:

Answer 8

1. Extracellular receptor binds to ligand, creating dimer
2. JAK domain on receptor is activated and phosphorylates receptor
3. Signal Transducers and Activators of Transcription proteins bind to receptor to get phosphorylated by JAK
4. STAT proteins dimerize once phosphorylated
5. Genes transcribed in nucleus

Question 9

Deactivating Growth Factors via phosphatases:

Answer 9

Phosphatases dephosphorylate protein kinases so they can’t bind to receptor.

Then, to prevent G-protein cascade reactivating, GAP hydrolizes the GTP back to GDP.

Question 10

Deactivating Growth Factor RECEPTORS:

Answer 10

phosphatase inhibition signals for the receptor to be internalized and degraded
OR
to undimerize and be recycled

Question 11

PIAS

Answer 11

protein inhibitors of activated STAT bind to STAT -blocking STAT from binding to DNA for transcription

Question 12

SOCS

Answer 12

Suppressor of Cytokine Signaling. “puts a sock on the receptor” AKA binds to receptor -preventing it from phosphorylating STATs

ALSO thought to ubiquitinate JAK to be eaten by proteosome

Question 13

3 Cytokine signalling mediated pathologies:

Answer 13

1. Crohns
2. Ulcerative colitis
   - ->elevated levels of iNOS due to elevated levels of STAT1
3. Myeloproliferative disorders:
   - ->mutation in JAK causes it to always appear phosphorylated, therefore activating STAT.
   - —–>this causes over production of RBCs and platelets etc.

Question 14

Achondroplasia:

Answer 14

Fibroblast Growth Factor Receptor Signalling error.

• ->Receptor is active w/o GF.
• ->Receptor causes cell cycle to stop, resulting in premature arrest of bone formation

Question 15

Mutations in oncogenes often mutate this protein and GF Signals?

Answer 15

Often, mutations in Raf (GEF,) prevent GAP from working. This causes phosphorylation cascade to be turned on resulting in overgrowth. aka tumor.

Question 16

ErbB2 receptor pathology:

Answer 16

over-expression of ERbB2 causes too strong of tyrosine kinase activity, leading to cancer

Question 17

BRaf V600E

Answer 17

This is a mutated protein kinase that appears to always be active because it has a negative charge. It has a negative charge because it has a substituted amino acid.

Question 18

PLX4032

Answer 18

Inhbits BRAF V600 E mutant protein kinase. Only really applicable to melanomas because melanomas (not colorectal etc.) are caused by BRAF V600E mutation