Receptor tyrosine kinases

Question 1

What is the mechanism of receptor tyrosine kinase activation?

Answer 1

Ligand binding causes dimerization -> activates catalytic activity of kinase => tyrosine autophosphorylation at specific sites

Question 2

Explain the molecular mechanismof stimulation of ras GTPase by RTKs

Answer 2

Tyrosine phosphorylation @ receptor -> binding by SH2 domain -> Grb2 (part of SH2 domain) binds a Ras GEF (Sos)

=> proximity of Sos to membrane bound ras = guanine nucleotide exchange

Question 3

What is the mechanism of action of the two main classes of RTK targeted anti-cancer agents?

Answer 3

Antibodies:
prevent ligand from binding to receptor

TKIs:
inhibit catalytic activity
often bind in substrate binding site of kinases

Question 4

What tumor characteristics predict a clinical response to EGFR- targeted therapeutics?

Answer 4

EGFR targeted therapeutics work best when the EGFR has mutated and is now amplified or overexpressed

Question 5

What is the mechanism of resistance to TKI’s?

Answer 5

Acquired resistance:
2nd site mutation - block inhibitor from binding to kinase active site (use drug cocktail?)

Primary resistance:
tumor has a Ras mutation that affects pathway further up -> TKIs are ineffective

Question 6

What is the active form of Ras

Answer 6

Active: GTP
Inactive: GDP

Question 7

What do SH2 domains bind?

Answer 7

Phosphorylated-Tyr containing domains in peptides

Question 8

What do SH3 domains bind?

Answer 8

Pro containing peptides

Question 9

Do Receptor Tyrosine Kinases homodimerize or heterodimerize to activate signaling?

Answer 9

both

Question 10

Where does Grb2 bind GEF?

Answer 10

at Sos - the Pro rich region binds Grb2’s SH3 domain

Question 11

What is Grb2?

Answer 11

Growth factor receptor bound protein 2

• adaptor protein
• involved in signal transduction

Question 12

What is GEF?

Answer 12

guanine nucleotide exchange factor

• activate monomeric GTPases
• Stimulate the release of GDP to allow the binding of GTP
• Involved in the activation of small GTPases in intracellular signalling pathways (many downstream targets)
• Ras superfamily is most well known
• active when bound to GTP

Question 13

What is EGFR?

Answer 13

epidermal growth factor receptor

Question 14

How is EGFR related to cancer?

Answer 14

• over expressed in many tumors (breast, lung, glioblastoma, head & neck, bladder, colorectal, ovarian, prostate
• mutations leading to constitutive activation
• increased EGFR = poorer clinical outcome
• Increased EGFR = increased ligand production

Question 15

How do antibody treatments target EGFR?

Answer 15

bind to EGFR, blocking ligand binding site => prevent dimerization

Question 16

What is a TKI?

Answer 16

Tyrosine-Kinase inhibitor

• inhibit T-K enzymes which are important in intracellular signaling pathways
• can have large degree of specificity

Question 17

How do TKI treatments target EGFR?

Answer 17

• Compete with ATP at active site
• Block substrate
• Block both ATP and substrate
• Act as allosteric inhibitor

Question 18

How does Gefitinib work?

Answer 18

Blocks EGFR kinase activity => stops downstream signaling

Question 19

What is NSCLC?

Answer 19

Non-small cell lung cancer

Question 20

What predicts a clinical response to EGFR targeted drugs?

Answer 20

tumors with EGFR mutations that make them more active

Question 21

How can tumors become resistant to TKIs?

Answer 21

caused especially by 1 mutation: T790M

- “gatekeeper” mutation that stops drug and allows ATP to continue binding

Question 22

What is the best treatment to prevent resistance?

Answer 22

Treat with combinations of drugs:

Gefitinib + Rx that inhibits EGFR resistant mut.

Question 23

How will the tumor respond to EGFR tx (antigen or TKI) if the ocogenic even is downstream from EGFR?

Answer 23

ZERO RESPONSE => UNAFFECTED