Cellular Signaling III: Growth Factors and Cytokines

Question 1

What is unique about the receptors of growth factors

About their structure and properties

Answer 1

These receptors are usually 2 molecules that undergo nucleation event when the ligand binds to them.

Regarding the properties these receptors have catalytic activity associated with in their intercellular transmembrane domain. These are tyrosine kinase domain which can p’s stuff

Question 2

What receptor did he show us

Answer 2

EGF receptor

Question 3

What happens when the EGF ligand binds to the receptor

Answer 3

It leads to autop’s or cross p’s so now the receptor has phosphate groups on themselves

Question 4

What are the other protein associated with growth factor receptor

Answer 4

A GDP binding protein called RAS, an adapter called GRB2-SOS and then a cascade of myogen activated protein kinases

1. RAF
2. MEK
3. ERK

Ultimately what happens is that the signal is transmitted from the receptor to the myogen activated protein receptors

Question 5

How is the singal transmitted between the receptor and myogen activated proteins

Answer 5

Grb2 is binds to the phosphate group on the tyrosine kinase part of the receptor at one end and to the SOS at the other end

Question 6

What is RAS

Answer 6

It is a small G protein

Question 7

What happens next

Answer 7

SOS if a GEF so it makes RAS loose its GDP and then RAS binds to GTP. This leads to tha activation of MAP kinases.

These MAP kinases ps in turn so RAF gets ps then MEK gets ps and finally ERK get ps which leads to the formation of trasncription facts which then go to the nucleus and regulates trasncription

Question 8

What are some of the other proteins similar to ERK that regulate TFs p’s

Answer 8

1. ERK regulates TF for proliferation and differentiation
2. JNK regulates TF for stress response
3. p38 also does stress response and apoptosis
4. ERK5 does proliferation, differentiation and development

Each of these responses are isolated from each other

Question 9

What is similar and different between growth factors and cytokine receptors

Answer 9

Cytokine receptors are pretty much like growth factors (are 2 molecules, undergo nucleation) but they dont have tyrosine kinase activity.

However cytokine receptors are associated with catalytic kinases called JAK kinases

Question 10

How are the cytokine receptors activated

Answer 10

Interferon ligand attaches itself to the receptor, this leads to nucleation, this leads to a conformational change in JAK, JAK p’s the intracellular domain of the receptor, this leads to the binding of STAT protein, STAT is also a TF, STAT then dimerizes and goes to the nucleus to regulate transcription

Question 11

How is the production of iNOS regulated

Answer 11

By cytokines. It is the same mechanism as described before

Question 12

How are the growth factor signals terminated

Answer 12

There are a bunch os phosphatases that run around and dep’s the catalytic site of the receptors and the MAP kinases. However this is not enough since RAS with GTP is still bound to RAF and we also have the ligand bound to the receptor.

So there are GAP proteins that bind to RAS and causes the GTP to form GDP and inactvate RAS.

The receptor itself can be internalized and made to dissciate from the ligand. Receptor can also be degraded.

Question 13

How are cytokines inhibited

Answer 13

PIAS, it stands for protein inhibitor of activated STAT. It has a binding site for STAT dimer so it doesnt let STAT to go to the nucleus.

Then the SOCS kick in and they bind to the p’s groups on the intermembrane domain, inhibiting more STATs to bind to these sites..

Then SOCS also have ubiqutinylation ligase activity so it will put ubiquitin markers on JAK. This is then spit out of the receptor and chewed up by proteasome.

Question 14

What is inflammatory bowel disease

Answer 14

It has several diseases in itself like Crohn’s disease and Ulcerative colitis. The UC there is overexpression of the interferon gamma and there are elevated levels of STAT1. This leads to overexpression of genes that make NO so people with this disease pour out alot of NO into their gut which causes them to have this disease.

Question 15

What are myeloproliferative disorders

Answer 15

There is a mutation in the JAK causing the JAK to be active without the nucleation event without the ligand cytokine, it will p’s the receptor domain causing the STAT to bind to the domain. This leads to a pannel of disorders.

Question 16

What is FGF involved in

Answer 16

FGF signalling makes p21 which is a potent inhibitor of cell cycle.

Question 17

What is the biochemical mechanism underlying FGF

Answer 17

The bone undergoes growth and after it reaches a certain length, FGF is expressed which causes a stop in the bone lengthening and leads to bone maturation

Question 18

What is the disease associated with misregulation of FGF singaling

Answer 18

Achondroplasia, there is mutation in the FGF receptor, triggers FGF signaling in the absence of FGF, which makes p21 which antoagonizes the cell cycle.

Leads to premature arrest of cell cycle

Question 19

What is an oncogene

Answer 19

Protooncogene is to control cell proliferation, oncogene is defected form of protooncogene.

If a mutation targets protooncogene then it can result in cancer. Only one of the copy needs to be mutated since there will be a gai of function

Question 20

How is RAS associated with tumors

Answer 20

Mutated RAS might be such that it is not inactivated by GAP leading to continuous transcription and perhaps leading to development of a tumor.

Question 21

How are some of the other ways oncogenic mutations can arise

Answer 21

We can have mutation like in receptor that affects the overall levels of the protein which sometimes happens in breast cancer

Question 22

What is the mechanism for this

Answer 22

ERB3 and ERB 2 are receptor, ERB3 have weak tyrosine kinase activity, ERB2 have strong, when they combine they lead to normal cell signaling.

If the signaling of ERB2 becomes aberrant so there is aot of ERB2 (even though the protein is normal) this will increase the kinase activity of ERB 2 leading to cell proliferation and breast cancer

Question 23

How can RAF lead to a mutation

Answer 23

BRaf can cause the p’s of MAP proteins even in the absence of a ligand and in the absence of activated RAS leading to increase transcription which can lead to development of cancer. This is common in moles and melanoma

Question 24

What doesnt BRaf turn off

Answer 24

It has a glutamic acid side chain mutation which has a negative charge like a P group, phosphatases cannot remove this negative charge leading to contunous signaling

Question 25

What was the last topic we discussed in this lecture

Answer 25

He talked about a drug that can target these melonomas as the wild type BRAF is not affected by this drug. This drug has a huge success rate as it doesnt have as many side effects as it only targets the mutant BRef. However this drug was not very effective in the treatment of colorectal cancer (which also have the same BRaf mutation) as they have high levels of EGFR (epidermal frowth factor receptor), whereas melanomas have very few of these receptors. Inhibition of Braf causes a feedback loop causing the activation of EGFR which drives the cellular proliferation. However, since melanomas have low levels of EGFR there is no activation of this signal, but in colorectal cancer this signal activation happens and the cells keep on dividing