Lecture #8 - Growth Factor Signaling

Front 1

What is a growth factor?

Back 1

Proteins that bind cell surface receptors and cause cell proliferation or cell differentiation.

Front 2

Growth factors are very dependent on this

Back 2

Phosphorylation

Front 3

What are the 3 most commonly phosphorylated amino acids?

Back 3

Threonine
Serine
Tyrosine
Histidine

***Thr/Ser kinases
Tyr kinases
MAP kinases = Thr/Tyr

Front 4

What are the 6 classes of enzyme linked cell surface receptors?

Back 4

a - Receptor Tyrosine kinases = phosphorylate tyrosine
b - Tyrosine - kinase associated receptors = recruit a cytosolic tyrosine kinase to relay the signal
c - Receptor serine/theronine kinases = phosphorylate Ser and/or Thr residues
d - Histidine kinase - associated receptors = activate an associated histidine kinase
e - Receptor guanylyl cyclases = Catalyze the formation of cGMP
f - Receptor-like protein phosphatases = dephosphorylate P-tyrosine residues but do not have known agonists

Front 5

What does the addition of a phosphate do to a protein?

Back 5

Changes the charge and conformation by two negative charges for every phosphate added.

Front 6

How are Receptor Tyrosine Kinases structurally different from GPCR's?

Back 6

RTK's only span the membrane once.

It's intracellular domain exhibits kinase activity

Front 7

The ligands involved with RTK's belong to the family of...

Back 7

Growth Factors

Front 8

What occurs during ligand binding to RTK's?

Back 8

Either a dimerization or conformational change if the dimer is already assembled.
Trans-autophosphorylation is then allowed to occur on certain tyrosines. This increases the enzymatic activity in the kinase domain and creates docking sites for effectors and scaffold proteins outside the kinase domain.

Front 9

What are SH2 and PTB?

Back 9

They are domains on adapter proteins that bind phosphotyrosine sequences on RTK's. They are phosphotyrosine binding motif's.
They have a special binding site for the phosphotyrosine and a special binding site for the amino acid side chain.

Front 10

Describe the structure of the insulin receptor

Back 10

A stable tetramer that does not come together once bound to insulin and are bound together by disulfide bonds.
They are composed of 2 alpha and 2 beta subunits.

Front 11

What occurs once the insulin receptor binds insulin?

Back 11

Binding of the ligand causes a conformational change that activates trans-autophosphorylation and tyrosine phosphorylation of IRS-1 (Insulin receptor substrate 1), which becomes a docking site for multiple downstream effectors.

Front 12

What is the MAPK cascade?

Back 12

Mitogen-activated protein kinase cascade

Front 13

Describe how this pathway works

Back 13

An adaptor protein called Grab2 binds an activated RTK. A RAS-GEF called Sos then catalyzes the exchange of GDP for GTP on RAS , a membrane bound protein. Once RAS is bound by GTP, it recruits and activates members of the MAPK cascade.

Front 14

How does the activated RAS protein continue the signal?

Back 14

It phosphorylates MAPKKK, which phosphorylates MAPKK, which phosphorylates MAPK.

Front 15

Once MAPK is phosphorylated, what happens?

Back 15

MAPK directly phosphorylates transcription factors that modulate gene expression and other enzymes involved in proliferation.

Front 16

There are several MAPK modules in mammals. What is the most commonly activated?

Back 16

Insulin and other growth factors and RTKs via Grb2/Sos/Ras

Front 17

Alternate names for the three MAP kinase enzymes are Raf, Mek, and Erk respectively. Describe the feedback loops that exist in this pathway.

Back 17

Once gene expression is altered by MAPK, a phosphatase is upregulated that deactivates MAPK. An additional feedback loop exists where MAPK phosphorylates MAPKKK and in this case deactivates it.

Front 18

What kind of Kinase is MAPKKK? MAPKK? MAPK?

Back 18

Serine/Threonine
Threonine/Tyrosine
Serine/Threonine

Front 19

Two more important MAPK modules are JNK and p38. How are they activated?

Back 19

In response to UV, lipopolysaccarhides and cytokines. They induce cellular responses such as inflammation, apoptosis, and in some cases growth and differentiation.

Front 20

What is the PI3K pathway?

Back 20

Phosphoinositide 3 kinase pathway

Front 21

How does this pathway work?

Back 21

An RTK is stimulated and activates a lipid kinase. This kinase phosphorylates the 3 position of the inositol ring of PI, PI4P and PI4.5bP. Once phosphorylated, they are referred to as PIP3 lipids.

Front 22

What is the function of PIP3 lipids?

Back 22

They are membrane docking signals for several proteins that contain Pleckstrin homology. This means that these proteins can bind PIP3 lipids.

Front 23

What are two proteins essential for insulin signaling and cell survival?

Back 23

PDK-1 and Akt

Front 24

How do these proteins propogate the signal?

Back 24

They colocalize (come together), and this induces Akt phosphorylation and activation.

Front 25

Once Akt is activated, what occurs?

Back 25

The Akt-PDK-1 complex is released from the membrane and in an activated state, phosphorylates itself.

Front 26

What are the 4 ultimate cell responses to Akt activation by insulin binding?

Back 26

Glucose uptake via mobilization of GLUT4 transporters to the membrane of skeletal muscle and adipose tissue.
As well as glycogen synthesis via phosphorylation of GSK-3 (Glycogen Synthase Kinase).
Protein synthesis by signaling through the mTOR cascade
Promotes cell survival via inhibition of pro-apoptotic proteins.

Front 27

What is the name of the phosphatase that removes the phosphate from the PI's? What is it's significance?

Back 27

PTEN
It is a tumor suppressor gene
Many of those with cancer have mutated versions of this gene and since this pathway is critical to cells survival. They are targets for cancer therapeutics

Front 28

What are cytokines?

Back 28

They are a type of growth factor that is historically associated with hematopoietic and immune system cells. They bind to their cognate receptors and mediate intracellular signaling events that result in rapid changes in gene expression. Two examples include erythropoietin and interleukins.

Front 29

What type of receptor are cytokine receptors?

Back 29

Tyrosine kinase associated receptors. They do not possess intrinsic kinase activity.

Front 30

When a cytokine binds its receptor, what occurs?

Back 30

Dimerization of the receptors, which bring together two or more Janus Kinases (JAKs). They JAKs transphosphorlate each other and then phosphorylate the receptors creating docking sites for STATs (signal transducers and activators for transcription). STATs are then phosphorylated whcih causes their dissociation from the cytokine receptor, homodimerization, nuclear translocation and stimulation of gene transcription.

Front 31

What growth factor is binds Ser/Thr kinase receptors?

Back 31

Transforming Growth Factor -beta (TGF-beta)

Front 32

What three molecules comprise the TGF-beta superfamily?

Back 32

TGF-beta
activins
bone morphogenetic proteins (BMPs)

Front 33

What are the three steps that occur upon ligand binding to the Ser/Thr kinase receptors?

Back 33

A sequential phosphorylation similar to the JAK/STAT pathway
1) The type II receptor phosphorylates the type I subunit
2) The type I receptor recruits and phosphorylates a Smad protein
3) Phospho-Smad dissociates from the receptor, associates with a co-Smad and translocates to the nucleus to target TGFbeta responsive genes