Receptor Tyrosine Kinases Flashcards
How many kinases are there in the human genome, and how are they categorized?
There are approximately 500 kinases in the human genome, Serine-Threonine kinases, Receptor Tyrosine Kinases, non-Receptor Tyrosine Kinases.
How many receptor tyrosine kinases are there and how are they categorized?
There are approximately 60 different receptor tyrosine kinases, and they fall into 15 “families” (VEGFR, PDGFR, IGFR, EGFR, etc.) These function in cell growth, metabolism, motility, cell survival, and differentiation, and are involved in many cancers. These slide focus on the EGFR receptor kinases.
What are the three most common and important phosphorylated amino acids?
Phospho-Serine, Phospho-Threonine, and Phospho-Tyrosine
What is the overall structure and function of the Receptor Tyrosine Kinase?
Each RTK has an extracellular receptor region, a transmembrane domain, and an intracellular region containing an N-terminal domain near the plasma membrane and a C-terminal domain distal in the cytoplasm. Polypeptide loops mix in the active site until the extracellular domains bind the ligand signal. Internal loops are then phosphorylated to dimerize the RTK and open the active site.
How can RTKs be modified to affect many other pathways?
Some ligands can cause heterodimerization of RTKs, leading to different signaling pathways. RTKs may also form multimers of greater than 2 RTK molecules, though these have not been isolated. Phosphorilization of tyrosine residues also shows preferential patterns, where some tyrosines phosphorilated more readily that others.
What purpose do phospho-tyrosine residues serve?
They act as “docking sites” for secondary signaling molecules
What occurs when secondary messengers “dock” with the phospho-tyrosine residues?
Some, like PI3, get phosphorylated and change their conformations to become activated.
Some, such as Grb2/SOS and Shc/Grb2/SOS bind and become localized near the plasma membrane to enhance interactions with their substrates.
What do the Grb2/SOS pathway and PI3 pathway lead to?
Grb2/SOS leads to activation of the Ras/Raf/Mek1-2 cell proliferation pathway
PI3 phosphorylates ATK which activates mTOR
What sort of cellular processes do most RTKs stimulate?
Generally stimulate cell proliferation, survival, and biomass increases.
How do SOS and RAS interact?
RAS is usually bound to GDP and inactive. SOS acts as a GTP Exchange Factor for RAS, replacing its GDP for GTP and activating it to activate its downstream messengers. RAS localizes to the plasma membrane, thus SOS must bind to TKRs via Grb2 to interact with RAS. Experimentally, anything that causes SOS to localize on the plasma membrane causes the activation of the RAS pathway due to interaction with SOS.
What creates specificity in the binding locations for each secondary signaling molecules on RTKs?
Each 2nd signal molecule has SH2 or SH3 domains (or both) that binds to Phospho-Tyr residues that have specific sequences in the downstream (C-Terminal) AAs. Many different signaling molecules may bind to an activated RTK, all at different Phospho-Tyr residues determined via their SH2/3 domain specificities.
What are four reasons EGFR RTKs are of therapeutic interest?
1 They are over expressed in many tumors.
2 They are mutated in glioblastomas, causing constitutive activation
3 Increased expression of EGFR correlates with poorer clinical outcomes in many cancers
4 Increased expression of EGFR is also associated with increased production of ligand in an autocrine stimulatory pathway (cancer cell can trigger its own growth)
What are two classes of drugs that can target EGFR?
Monoclonal antibodies originally designed to purify EGFR have been shown to inhibit the proliferation of EGFR mutant cells.
Small molecule inhibitors identified to be unique to each TKR.
How do the monoclonal antibodies affect the function of the EGFRs?
The antibodies bind to the cleft between two parts (I & III) of the ligand binding domain and interrupt its conformation, inhibiting dimerization. They also likely stimulate the immune system to target and kill the cells they are bound to. However, they cannot be produced by the host organism because TKRs are normal “self” structures, immune system cells with these antibodies would be destroyed.
How to small molecule RTK inhibitors work and what problems are associated with them.
Small molecule inhibitors bind to the ATP binding site on the RTK. They are highly specific to individual sites, and this specificity makes them ineffective if a patient does not have the exact mutation they are effective against or if the patient develops another mutation.
