We previously reported that activation of mitogenactivated protein kinase (MAPK) is involved in the mitogenic stimulation of normal human melanocytes (NHMC) by endothelin-1 (ET-1). In the present study, we determined signaling mechanisms upstream of MAPK activation that are involved in ET-1 stimulation and their synergism with stem cell factor (SCF). Pretreatment of cultured NHMC with ET B receptor antagonists, pertussis toxin, a specific phospholipase C inhibitor (U73122), or a protein kinase C inhibitor (calphostine) blocked a transient tyrosine phosphorylation of MAPK induced by ET-1, whereas the addition of a calcium chelator (BAPTA) failed to inhibit that tyrosine phosphorylation of MAPK. Treatment with ET-1 and SCF together synergistically increased DNA synthesis, which was accompanied by synergism for MAPK phosphorylation. The time course of inositol 1,4,5-trisphosphate formation revealed that there is no difference in the level of inositol 1,4,5-trisphosphate stimulated by ET-1 ؉ SCF or by ET-1 alone. Evaluations of the serine phosphorylation of MEK and Raf-1 activity showed a synergistic effect in SCF ؉ ET-1-treated NHMC. Stimulation with SCF ؉ ET-1 induced a more rapid and stronger tyrosyl phosphorylation of proteins corresponding to p52 and p66 Shc than did stimulation with SCF only, and this was accompanied by a stronger association of tyrosinephosphorylated Shc with Grb2. Interestingly, a more rapid and marked tyrosine phosphorylation of c-kit was also detected in NHMC-treated with SCF ؉ ET-1 than NHMC treated with SCF only. These data indicate that the synergistic cross-talk between SCF and ET-1 signaling is initiated through the pathway of tyrosine phosphorylation of c-kit, which results in the enhanced formation of the Shc-Grb 2 complex which leads in turn to the synergistic activation of the Ras/Raf-1/MEK/MAP kinase loop.

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