The role of c-Kit in signalling and drug resistance in elanocytes and melanoma

Abstract

Activation of the c-Kit receptor tyrosine kinase (RTK) by amplification or oncogenic mutation occurs in 20-40% of melanoma arising on mucosal membranes, acral skin, and skin with chronic sun-induced damage. Many activating c-Kit mutations have been shown to be highly sensitive to the RTK inhibitor imatinib mesylate, although the majority of c-Kit mutant melanoma patients eventually progress on this inhibitor. This thesis examines the critical effector pathways of wild-type and oncogenic c-Kit using human melanocyte and melanoma cell models. We show that both ligand-dependent and oncogenic mutant c-Kit activity requires both the mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) cascades to mediate downstream proliferative and survival effects in melanocytes and melanoma. We also investigated mechanisms of drug resistance in c-Kit mutant melanoma by generating melanoma cell sublines resistant to RTK inhibitors imatinib and nilotinib. Two functionally-distinct secondary c-Kit mutations developed in separate subpopulations, and these mutations promoted survival by blocking inhibitor binding to the receptor. Importantly, these secondary mutations conferred differential resistance to RTK inhibitors nilotinib, dasatinib and sunitinib. A third imatinib-resistant subpopulation developed c-Kit-independent activation of the MAPK and PI3K cascades. Inhibition of the MAPK cascade, but not the PI3K cascade, resensitised these cells to imatinib.