The MAPK pathway functions as a redundant survival signal that reinforces the PI3K cascade in c-Kit mutant melanoma

Stimulation of the c-Kit receptor tyrosine kinase has a critical role in the development and migration of melanocytes, and oncogenic c-Kit mutants contribute to the progression of some melanomas. c-Kit signalling activates the mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) pathways and their relative contribution to the activities of oncogenic and ligand-dependent c-Kit remains uncertain. We show that PI3K is a major regulator of MAPK activation in response to c-Kit activity and the dominant effector of c-Kit-driven melanocyte proliferation and melanoma survival. Nevertheless, inhibition of the PI3K pathway in c-Kit mutant melanoma cells did not replicate the apoptotic efficacy of the c-Kit inhibitor, imatinib mesylate. Instead, the simultaneous suppression of the PI3K and MAPK pathways promoted a strong synergistic apoptotic effect. These data indicate that MAPK functions as a redundant survival signal that reinforces the PI3K cascade in c-Kit mutant melanoma. Thus, the concurrent inhibition of PI3K and MAPK signalling is required to suppress oncogenic c-Kit activity and may provide an effective therapeutic strategy in c-Kit mutant melanomas.