Inhibition of the Phosphatidylinositol 3-Kinase/p70S6-Kinase Pathway Induces B16 Melanoma Cell Differentiation*

α-Melanocyte-stimulating hormone and cAMP-elevating agents are known to induce B16 cell differentiation, characterized by increased melanin synthesis and dendrite outgrowth. In order to elucidate intracellular signaling pathways involved in this differentiation process, we focused our interest on the phosphatidylinositol 3-kinase/p70S6-kinase pathway. The specific inhibition of phosphatidylinositol 3-kinase by LY294002 markedly stimulated dendrite outgrowth, thus mimicking the action of cAMP-elevating agents on B16 cell morphology. In addition, LY294002 and rapamycin, a specific p70S6-kinase inhibitor, were found to independently stimulate tyrosinase expression, thus increasing melanin synthesis. In an attempt to better dissect the molecular mechanisms triggered by cAMP to induce melanoma cell differentiation, we examined the effects of a cAMP-elevating agent forskolin, on both phosphatidylinositol 3-kinase and p70S6-kinase activities. Specific kinase assays revealed that forskolin partially inhibited phosphatidylinositol 3-kinase activity and completely blocked p70S6-kinase activity and phosphorylation. In conclusion, our results clearly demonstrate that the inhibition of phosphatidylinositol 3-kinase and p70S6-kinase is involved in the regulation of B16 cell differentiation. Furthermore, we provide evidence which suggests that cAMP-induced melanogenesis and dendricity are, at least partially, mediated by the cAMP inhibition of the phosphatidylinositol 3-kinase/p70S6-kinase signaling pathway.