receptor tyrosine kinase

Question 1

receptor tyrosine kinase function

Answer 1

1. cell growth
2. motility
3. metabolism
4. cell survival
5. differentiation

Question 2

RTKs are activated by _____;

Answer 2

dimerization

Question 3

when two nearby RTKs in the membrane are bound by their ligands, they undergo a

Answer 3

conformational change, bind to each other, and co-activate by phosphorylating each other.

Question 4

for these RTKs, their substrates (PO4 targets) are .

Answer 4

other RTKs

Question 5

Once activated, RTKs can also act as

Answer 5

catalysts for other reactions.

Question 6

two different kinds of RTKs can dimerize to

Answer 6

generate a different signal than a dimer formed of two of either kind.

it doesn’t have to be a homodimerization

Question 7

RTKs generally trigger complex kinase cascades that result in changes in

Answer 7

gene expression.

Question 8

All RTKs activate:

Answer 8

ras signaling

Question 9

Ras proteins:

Answer 9

membrane-bound switches regulated by GAPs and GEFs (see below). Involved with cell proliferation.

Question 10

In the “off” state:

Answer 10

it’s bound to GDP

Question 11

In GTP-bound (active) form, Ras proteins sets ______

Answer 11

off proliferative cascade.

Question 12

Generally, Ras proteins can:

Answer 12

switch themselves off (intrinsic GTP->GDP activity)

Question 13

GAP:

Answer 13

(increases its ability to switch itself off).
GTP activating protein

Inactivating mutations in GAPs can cause cancer since ras proliferation is unchecked.

Question 14

in the active state:

Answer 14

it’s bound to GTP.

Question 15

GEF:

Answer 15

Guanine nucleotide exchange factors:

helps exchange “spent” GDP for fresh GTP (thus activating Ras proteins).

Question 16

Increase-of-function mutations in GEFs can lead to

Answer 16

cancer since Ras protein is constantly being activated again after it inactivates itself.

Question 17

GEFs: turn Ras proteins ____

Answer 17

on

oncogenes

Question 18

GAP: turn Ras proteins____

Answer 18

off (tumor suppressor genes).

Question 19

mechanism of stim of ras GTPase by RTKs

Answer 19

1. Dimerized RTKs bind to and activate Grb2, an adaptor protein; this has a SH3 binding
   domain which binds to Sos, which is a GEF (Ras activator).
2. Sos being bound, it’s brought into proximity with Ras proteins, which are located in the cell membrane like the RTKs.
3. Essentially RTKs activate Ras proteins by changing the location of GEFs, not by catalytic activity. (Movement of proteins into proximity causes proliferation).

Question 20

Dimerized RTKs bind to and activate________; this has a SH3 binding domain which binds to____,

Answer 20

Grb2, an adaptor protein

SOS

Question 21

SOS is a

Answer 21

a GEF (Ras activator).

Question 22

Sos being bound, it’s brought into proximity with _____, which are located ______

Answer 22

Ras proteins

in the cell membrane like the RTKs

Question 23

Essentially RTKs activate Ras proteins by

Answer 23

changing the location of GEFs, not by catalytic activity. (Movement of proteins into proximity causes proliferation).

Question 24

Epidermal growth factor receptor (EGFR): is overexpressed in

Answer 24

a variety of tumors; good drug target for cancer treatment.

Question 25

two main classes of RTK-targeted anti-cancer agents

Answer 25

1. antibodies and

2. TKI’s

Question 26

Antibodies: recognize

Answer 26

Recognize extracellular domain of RTKs.

Question 27

antibodies act as

Answer 27

competitive antagonists: prevent ligand from being able to bind to the extracellular receptor domain.

Question 28

TKIs (tyrosine kinase inhibitors): inhibit

Answer 28

tyrosine kinase activity of RTKs.

Question 29

TKIs (tyrosine kinase inhibitors): works by

Answer 29

competitively binding to the ATP binding pockets of specific RTKs. No ATP, no phosphorylation, no activation of RTK dimers.

Question 30

Recall that the “closed” or un-ATP-bound TK has a specific conformation that allows

Answer 30

more specific drug targeting– however, there’s still enough similarity that TKIs will inevitably have multiple TK targets.

Question 31

Patients with non-small-cell lung cancer show only a 20% response to TKIs.
The 20%: usually


Answer 31

non-smokers, women, Asian descent.

Question 32

When you’ve had a mutation in EGFR to make the activation domain much more active, the response to TKIs tends to get ____

Answer 32

better.

Question 33

why does a EGFR mutation make TKI better response?

Answer 33

it’s thought that with an improved or more efficient EGFR proliferation pathway, the cell may be “addicted” to that pathway– which would make the cell much more vulnerable to TKIs, which target that pathway.
Notice this also applies to patients with abnormally high levels of EGFR in their tumor cells: again, may be “addicted” to this pathway.

Question 34

mechanism of resistance to TKI’s caused by EGFR somatic mutations.

Answer 34

1. There are secondary (after first treatment) mutations in EGFR that cause decreased response after an initial improvement when treated with the TKI.
2. Essentially, seem to be selecting for EGFR proteins that have mutations that block TKI binding.

Question 35

there are drugs that can inhibit these mutant EGFR proteins– since we know what the mutations are, we can use the sequence to target ____

Answer 35

inhibitory drugs.

Question 36

Describe mechanism of receptor tyrosine kinase activation.

Answer 36

• Ligand binding drives dimerization, which activates catalytic activity of the kinase resulting in Tyrosine autophosphorylation at specific sites.

Question 37

Explain molecular mechanism of stimulation of ras GTPase by RTKs

Answer 37

• Tyrosine Phosphorylation of receptor causes binding by SH2-domain-containing proteins including the adaptor protein Grb2, which binds a Ras GEF called Sos.
  Proximity of Sos with membrane-bound Ras results in guanine nucleotide exchange.

Question 38

Describe mechanism of action of two main classes of RTK-targeted anti-cancer agents (antibodies and TKI’s)

Answer 38

• primary role of antibodies is to block ligand binding to the receptor. TKIs inhibit catalytic activity (usually) by binding in substrate-binding site of the kinase.

Question 39

List tumor cell characteristics that predict clinical response to EGFR-targeted therapeutics

Answer 39

-response to EGFR TKI correlated with receptor mutations that may “activate” the receptor, EGFR amplification or overexpression as determined by FISH or immunohistochemistry.

Question 40

Describe mechanism of resistance to TKI’s.

Answer 40

Acquired resistance- second site mutations in EGFR arising or selected in patients who initially benefit from therapy but then acquire resistance and disease progression.
These mutations block inhibitor binding to the kinase active site.
May be able to design new inhibitors to avoid this problem.
Activation of other receptors like Met or ErbB2.
Combine inhibitors or make dual specificity inhibitors?
Primary resistance- if the tumor has a Ras mutation inhibiting the receptor further up the pathway will not be effective