INHIBITION OF THE PHOSPHATIDYLINOSITOL 3-KINASE P70(S6)-KINASE PATHWAY INDUCES B16 MELANOMA CELL-DIFFERENTIATION/

alpha-Melanocyte-stimulating hormone and cAMP-elevating agents are kno wn to induce B16 cell differentiation, characterized by increased mela nin synthesis and dendrite outgrowth. In order to elucidate intracellu lar signaling pathways involved in this differentiation process, we fo cused our interest on the phosphatidylinositol 3-kinase/p70(S6)-kinase pathway. The specific inhibition of phosphatidylinositol 3-kinase by LY294002 markedly stimulated dendrite outgrowth, thus mimicking the ac tion of cAMP-elevating agents on B16 cell morphology, In addition, LY2 94002 and rapamycin, a specific p70(S6)-kinase inhibitor, were found t o independently stimulate tyrosinase expression, thus increasing melan in synthesis, In an attempt to better dissect the molecular mechanisms triggered by cAMP to induce melanoma cell differentiation, we examine d the effects of a cAMP-elevating agent forskolin, on both phosphatidy linositol 3-kinase and p70(S6)-kinase activities. Specific kinase assa ys revealed that forskolin partially inhibited phosphatidylinositol 3- kinase activity and completely blocked p70(S6)-kinase activity and pho sphorylation. In conclusion, our results clearly demonstrate that the inhibition of phosphatidylinositol 3-kinase and p70(S6)-kinase is invo lved in the regulation of B16 cell differentiation, Furthermore, we pr ovide evidence which suggests that cAMP-induced melanogenesis and dend ricity are, at least partially, mediated by the cAMP inhibition of the phosphatidylinositol 3-kinase/p70(S6)-kinase signaling pathway.