Sphingosine-1-phosphate promotes mouse melanocyte survival via ERK and Akt activation.

In this study, we investigated the signalling pathways induced by ultraviolet B (UVB) and the effects of sphingosine-1-phosphate on UVB-induced apoptosis of mouse melanocytes, Mel-Ab, and observed the cytoprotective effects of sphingosine-1-phosphate on UVB-induced apoptosis. Since sphingosine-1-phosphate is a well-known mitogenic agent, we thought it possible that the mitogenic effect of sphingosine-1-phosphate might contribute to cell survival. However, we found that sphingosine-1-phosphate significantly inhibits DNA synthesis. We next examined the regulation of the three major subfamilies of mitogen-activated protein (MAP) kinases and of the Akt pathway by sphingosine-1-phosphate against UVB-induced apoptosis. UVB irradiation resulted in the remarkable and sustained activation of c-Jun N-terminal kinase (JNK), while p38 MAP kinase was only transiently activated. The basal level of extracellular signal-regulated protein kinase (ERK) phosphorylation decreased 30 min after UVB irradiation, whereas the basal level of Akt phosphorylation was unaffected by UVB. We also found that sphingosine-1-phosphate potently stimulates the phosphorylation of both ERK and Akt, which are involved in the cell survival-signalling cascade. Furthermore, the specific inhibition of the ERK and Akt pathways by PD98059 and LY294002, respectively, restored the cytoprotective effect induced by sphingosine-1-phosphate. On the other hand, the p38 inhibitor SB203580 additively enhanced the cytoprotective effect on sphingosine-1-phosphate. Based on these results, we conclude that the activation of p38 MAP kinase plays an important role in UVB-induced apoptosis, and that sphingosine-1-phosphate probably exert its cytoprotective effect in Mel-Ab cells through ERK and Akt activation.

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.