Sphingosine-1-phosphate decreases melanin synthesis via sustained ERK activation and subsequent MITF degradation

Sphingosine-1-phosphate (S1P) has emerged as a bioactive lipid modulator that mediates a variety of cell functions. However, the effects of S1P on melanogenesis are not well known. Therefore, we investigated the actions of S1P on melanin synthesis using a spontaneously immortalized mouse melanocyte cell line, Mel-Ab. This study shows that S1P significantly inhibits melanin synthesis in a concentration-dependent manner, and also that the activity of tyrosinase was reduced in S1P-treated cells. In contrast, a specific extracellular signal-regulated protein kinase (ERK) pathway inhibitor, PD98059, increased tyrosinase activity and melanin production, and PD98059 also restored the S1P-induced reduction of tyrosinase activity and pigmentation. In addition, we found that S1P induces the sustained activation of ERK and the subsequent degradation of microphthalmia-associated transcription factor (MITF), which plays a key role in melanogenesis. Thus, we further studied the relationship between the ERK pathway and melanin synthesis. PD98059 was found to prevent the S1P-induced MITF phosphorylation and degradation and to abrogate the S1P-induced downregulation of tyrosinase and of tyrosinase-related protein 1 (TRP1) production. These results indicate that the ERK pathway is potently involved in the melanogenic signaling cascade, and that S1P-induced ERK activation contributes to reduced melanin synthesis via MITF degradation. Therefore, we suggest that S1P reduces melanin synthesis by ERK activation, MITF phosphorylation and degradation, and by the subsequent downregulation of tyrosinase and TRP-1 production.