Tyrosine Receptor Kinases Flashcards
- Describe mechanism of receptor tyrosine kinase (RTK) activation.
Receptor Tyrosine Kinsase (RTK):
-Activation driven by dimerization –> 2 kinase domains (catalytically active when bound to ATP, to phosphorylate Tyr) –> ligand in ECF promotes dimerization –> causes a conformational change in the intracellular domain that opens the active site.
- Then ATP can bind to phosphorylate.
- Most often, RTKs phosphorylate themselves.
- RTKs can homodimerize or heterodimerize to activate signaling –> this increases the spectrum of stimuli that can activate downstream signaling events (and cause different ones?).
- Generally RTKs autophosphorylate –> their main target is themselves.
His Answers from end of PP:
Ligand binding drives dimerization, which activates catalytic activity of the kinase resulting in tyrosine autophosphorylation at specific sites.
- Explain molecular mechanism of stimulation of ras GTPase by RTKs.
- Ras proteins are membrane-bound switches regulated by GAPs and GEFs.
- GAPs help the GTPase activity of Ras by taking a phosphate and making it go from active (GTP bound) to inactive (GDP bound).
- GEF helps take GDP off inactive Ras so GTP can bind again to activate it.
- When Tyr phosphorylation happens, Ras activation involves Grb2 (adapter protein that consists of SH2 and SH3) and a GEF (Sos).
- SH2 domain binds to phosphor-tyr containing peptides and
- SH3 domains bind pro-containing peptides.
- Sos is a Ras GEF that contains a pro-rich region SH3 can bind to –>recruitment of Sos to receptor brings it in close contact with Ras, which can get turned on.
- Sos can turn on Ras in absence of RTK stimulation?
His Answers from end of PP:
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.
- Describe mechanism of action of two main classes of RTK-targeted anti-cancer agents (antibodies and TKI’s).
-Antibodies used to block ligand binding to the receptor.
-TKIs inhibit catalytic activity (generally) by binding in substrate-binding site of kinase.
-Antibodies have to work outside the cell and recognize the extracellular domain of the receptor –> block the ability of receptor to bind ligand and dimerize (which is necessary for activation).
(Ex = cetuximab –> anti-EGFR.)
- Inhibitors, on the other hand, can hit lots of targets –> small molecules can inhibit many kinases, while others are more specific.
- EGFR inhibitors block ATP binding in the catalytic site of the RTK and stops downstream signaling.
His Answers from end of PP:
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. usually does not prevent dimerization, rather the auto-phos.
- List tumor cell characteristics that predict clinical response to EGFR-targeted therapeutics.
-Response to EGFR TKI correlated with receptor mutations in the EGFR that may ‘activate’ the receptor.
-EGFR amplification or overexpression can be determined by FISH or immunohistochemistry.
Ex: Gefitinib works in about 20% of people (never smokers, Asian/Japanese, women) –> due to point mutations that occur that allow for a small benefit (but eventually build resistance).
His Answers from end of PP:
Response to EGFR TKI correlated with receptor mutations that may “activate” the receptor, EGFR amplification or over-expression as determined by FISH or immunohistochemistry.
- Describe mechanism of resistance to TKI’s such as EGFR inhibitors.
- It’s generally an acquired resistance –> secondary EGFR mutations can arise in patients who initially benefitted from the therapy but then acquired a secondary mutation that allowed for resistance and disease progression.
- Most common mutation = T790M.
- Think in terms of natural selection –> tumors with advantage are the ones that grow back.
- These mutations block inhibitor from binding to the kinase active site.
- While we may be able to design new inhibitors to avoid the problem of resistance, can we combine inhibitors or make dual specificity inhibitors?
- We can make drugs that inhibit resistance as well = combo therapy.
- If you activate another RTK –> you need a drug that targets a new receptor (maybe use one of those less specific TKIs).
- If the oncogenic event is downstream of the receptor, like a Ras mutation getting locked in the on position, then inhibiting the EGFR upstream won’t work.
- This is the epitome of personalized medicine –> we can sequence your tumors and see which treatment is best for you.
His Answers from end of PP:
Acquired resistance-second site of mutations in EGFR arising or seected in patients who initally benefit from therapy but then acquire resistance and diesease progression. These mutations block inhibitor binding to kinase active site. May be able to design new inhibitors to avoid this problem. Activation of other receptors like Met or ErbB2 Comine inhibitors or make dual specificity inhibitors. Primary resistance–> if the tumor has a Ras mutation inhibiting the receptor further up the pathway will not do jack shit.