Signaling from the Human Melanocortin 1 Receptor to ERK1 and ERK2 Mitogen-Activated Protein Kinases Involves Transactivation of cKIT

Melanocortin 1 receptor (MC1R), a Gs protein-coupled receptor expressed in melanocytes, is a major determinant of skin pigmentation, phototype and cancer risk. Upon stimulation by MSH, MC1R triggers the cAMP and ERK1/ERK2 MAPK pathways. In mouse melanocytes, ERK activation by MSH binding to Mc1r depends on cAMP, and melanocytes are considered a paradigm for cAMP-dependent ERK activation. However, human MC1R variants associated with red hair, fair skin red hair color (RHC) phenotype, and increased skin cancer risk display reduced cAMP signaling but activate ERKs as efficiently as wild type in heterologous cells, suggesting independent signaling to ERKs and cAMP in human melanocytes. We show that MC1R signaling activated the ERK pathway in normal human melanocytes and melanoma cells expressing physiological levels of endogenous RHC variants. ERK activation was comparable for wild-type and mutant MC1R and was independent on cAMP because it was neither triggered by stimulation of cAMP synthesis with forskolin nor blocked by the adenylyl cyclase inhibitor 2,5-dideoxyadenosine. Stimulation of MC1R with MSH did not lead to protein kinase C activation and ERK activation was unaffected by protein kinase C inhibitors. Conversely, pharmacological interference, small interfering RNA studies, expression profiles, and functional reconstitution experiments showed that MSH-induced ERK activation resulted from Src tyrosine kinase-mediated transactivation of the stem cell factor receptor, a receptor tyrosine kinase essential for proliferation, differentiation, and survival of melanocyte precursors, thus demonstrating a functional link between the stem cell factor receptor and MC1R. Moreover, this transactivation phenomenon is unique because it is unaffected by natural mutations impairing canonical MC1R signaling through the cAMP pathway. (Molecular Endocrinology 25: 138 -156, 2011) M elanocytes are skin cells specialized in the biosyn- thesis of photoprotective melanin pigments. Their proliferation and the synthesis of melanins are tightly controlled by interacting chemical and physical cues (1, 2). Among the signaling cascades triggered by these signals, the cAMP and the MAPK ERK1 and ERK2 pathways have been intensively analyzed. cAMP induces melanocyte differentiation in vitro (3) and in vivo (4), and ERK signaling is crucial for the control of both proliferation (5) and melanogenesis, through the activation of the cAMP response element binding protein (CREB) (6) and the microphthalmia-associated transcription factor (MITF), a master regulator of melanocyte development, differentiation, and proliferation (7).