Question

Many cancer drugs have been developed that inhibit or activate enzyme function.

a. Geftinib is a tyrosine kinase inhibitor that has been shown to specifically target EGFR. EGFR is a receptor tyrosine kinase (RTK) and can become constitutively active in a cancer cell. Before we continue, explain how a RTKs function to transmit a signal (EGF) from the outside of the cell across the membrane to the inside of the cell. Be specific, what changes occur on the outside of the cell when ligand is bound? What changes occur on the inside of the cell that pass that information from the receptor to the first downstream target? I don’t need the entire signal transduction cascade. Rubric (4): correct explanation with the effect of ligand binding by the receptor on outside of the cell (2) and effect of ligand binding on inside of the cell (2).

b. Geftinib has been shown to occupy the ATP binding site of the tyrosine kinase domain of the EGFR, therefore we can hypothesize that it acts as a(n) __________________ inhibitor. (2pts)

c. Let’s think about a specific mutation in the EGFR (L858R).   The Kd for ATP binding to the tyrosine kinase domain of the EGFR (L858R) mutation is 148mM while the Kd of Geftinib for the L858R mutation of the EGFR is 2.4nM. Which binds more tightly (ATP or Geftinib) to EGFR (L858R) and how do these numbers confirm your conclusion in “b”? Rubric (4): correct analysis of binding (2) and correct explanation that relates analysis to conclusion in “b” (2).

d. Researchers determined proteins downstream of activated EGFR that were affected by Geftinib. We know some of these proteins! In the experiment, cancer cells with constitutively active EGFR are treated with or without Geftinib. In these experiments, Raf was downregulated in the cells treated with Geftinib when compared to untreated. Why does this make sense?   Make sure you substantiate your answer with concepts covered this week! Rubric (4): correct correlation is made between specific effect of the drug (1) and obvious application of concepts taught this week is applied to the answer, and they are correct (3).

Answer 1

a.

Extracellular ligand binding will cause or stabilize receptor dimerization. This allows a tyrosine in the cytoplasmic domain of the receptor monomer to be trans phosphorylated by the other receptor by which the signal is transmitted into the cell. The activation of the receptor will induce the binding of Src homology 2 domain and PTB domain containing proteins. This will lead to initiation of signal transduction inside the cell.

b. By binding to the EGFR receptor tyrosine kinase domain, it acts as a EGFR inhibitor.

c. Kd the equilibrium dissociation constant between the receptor and a substrate. Kd and affinity are inversely related. So lower the Kd value and thus the higher affinity for the substrate towards the receptor. According to the values, Geftinib has concentration in nM range which will lead to higher affinity to the EGFR mutation compared to the ATP.

d. The main action of EGFR mutation is through constitutive activation of Raf, while in the cells treated with Geftinib have downregulated Raf which means that mode of action of Geftinib is by reducing the activation of Raf in the cells.