The RET proto-oncogene is implicated in medullary thyroid cancer (MTC) and has been shown to signal indirectly to focal adhesion kinase (FAK) in cell types other than MTC. We have previously shown that FAK is phosphorylated in MTC cells. We hypothesized that inhibition of RET with pharmacologic inhibitors or by depletion with siRNA would decrease FAK phosphorylation in MTC cells, thereby implicating a RET-FAK signaling pathway. Human MTC cells (TT cells) were treated with pharmacologic inhibitors or transfected with RET siRNA. Total protein was detected by immunoblotting. Phosphorylated FAK was detected by immunoprecipitating total FAK and immunoblotting with antiphosphotyrosine. Treatment of MTC cells with the inhibitor PP2 significantly inhibited RET phosphorylation and, to a lesser extent, FAK phosphorylation. Imatinib mesylate inhibited FAK phosphorylation only at high doses. RET siRNA significantly decreased RET expression and FAK phosphorylation. RET signals through FAK in MTC cells. Whether this is due to a direct or indirect interaction is not yet clear. PP2 or a similar inhibitor might be a useful treatment for MTC.
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