Involvement of mTOR signaling in sphingosylphosphorylcholine-induced hypopigmentation effects

Melanin production was measured in Mel-Ab cells. A luciferase assay was used to detect transcriptional activity of the MITF promoter. Western blot analysis was performed to examine SPC-induced signaling pathways.SPC produced significant hypopigmentation effects in a dose-dependent manner. It was found that SPC induced not only activation of Akt but also stimulation of mTOR, a downstream mediator of the Akt signaling pathway. Moreover, SPC decreased the levels of LC3 II, which is known to be regulated by mTOR. Treatment with the mTOR inhibitor rapamycin eliminated decreases in melanin and LC3 II levels by SPC. Furthermore, we found that the Akt inhibitor LY294002 restored SPC-mediated downregulation of LC3 II and inhibited the activation of mTOR by SPC.Our data suggest that the mTOR signaling pathway is involved in SPC-modulated melanin synthesis.Melanin, a pigment found in hair, eyes, and skin, is produced by melanocytes and its synthesis is promoted by various stimulators such as UV irradiation, hormones, and cytokines [1-3]. At least 3 enzymes are required for melanin synthesis. Tyrosinase catalyses the first 2 rate-limiting steps of melanogenesis, whereas tyrosinase-related protein 1 (TRP1) and TRP2 convert melanin into different types. Microphthalmia-associated transcription factor (MITF) is a critical factor in melanin synthesis because it modulates the expression of tyrosinase, TRP1, and TRP2 [4,5]. Thus, much attention has been directed toward finding materials that regulate the expression of MITF.It has been reported that several signaling pathways are involved in regulating melanin synthesis. The extracellular signal-regulated kinase (ERK) signaling pathway induces the inhibition of melanin synthesis in mouse B16 melanoma cells [6]. The activation of ERK leads to phosphorylation of MITF at serine 73, which results in MITF ubiquitination and degradation [7-9]. Additionally, LY294002, a specific inhibitor of the Akt pathway, triggers melanogenesis in B16 cell