Sphingosine-1-phosphate decreases melanin synthesis via microphthalmia-associated transcription factor phosphorylation through the S1P3 receptor subtype.

Objectives Previously, we reported that sphingosine-1-phosphate (S1P) reduced melanin synthesis. In this study we have investigated S1P receptor-mediated extracellular signal-regulated protein kinase (ERK) activation and microphthalmia-associated transcription factor (MITF) phosphorylation. Methods To examine S1P-induced signalling pathways, electron and confocal microscopic studies, reverse transcription-polymerase chain reaction and Western blot analysis were performed. Key findings S1P phosphorylated MITF at Ser73, which may have resulted in a MITF mobility shift. Furthermore, 90 kDa ribosomal S6 kinase-1 (RSK-1) phosphorylation was observed after S1P treatment. In addition, PD98059 abrogated the S1P-induced MITF mobility shift and RSK-1 activation. In experiments with MITF mutants, it was shown that dual phosphorylation at Ser73 and Ser409 was indispensable for MITF degradation. We investigated further the actions of S1P on its specific receptors. The results showed that pertussis toxin completely abolished the hypopigmentary effects and ERK pathway activation by S1P, suggesting that S1P regulated melanogenesis via its receptor. The use of specific receptor antagonists indicated that the S1P3 receptor was dominantly involved in S1P-induced ERK activation and hypopigmentation. Conclusions The results suggested that S1P reduced melanin synthesis via S1P3 receptor-mediated ERK and RSK-1 activation, and subsequent MITF dual phosphorylation and degradation. [ABSTRACT FROM AUTHOR]