Induction of retinal‐dependent calcium influx in human melanocytes by UVA or UVB radiation contributes to the stimulation of melanosome transfer

Objectives The transfer of melanosomes from melanocytes to neighbouring keratinocytes is critical to protect the skin from the deleterious effects of ultraviolet A ( UVA ) and ultraviolet B ( UVB ) irradiation; however, the initial factor(s) that stimulates melanosome transfer remains unclear. In this study, we investigated the induction of retinal‐dependent calcium (Ca 2+ ) influx in melanocytes ( MC s) by UVA or UVB irradiation and the effect of transient receptor potential cation channel subfamily M member 1 ( TRPM 1) (melastatin1)‐related Ca 2+ influx on melanosome transfer. Materials and methods Primary human epidermal MC s were exposed to physiological doses of UVB or UVA light and loaded with a calcium indicator Fluo‐4 dye. The change of intracellular calcium of MC s was monitored using a two‐photon confocal fluorescence microscopy. MC s were co‐cultured with human epidermal keratinocytes ( KC s) in the absence or presence of voriconazole (a TRPM 1 blocker) or calcium chelators. MC s were also transfected with TRPM 1 si RNA for silencing the expression of TRPM 1 gene. The melanosome transfer in the co‐cultured cells was quantitatively analysed using flow cytometry and was further confirmed by immunofluorescent double‐staining. The protein levels and distributions of TRPM 1, OPN 3 and OPN 5 in MC s were measured by Western blotting or immunofluorescent staining. Results The retinal‐dependent Ca 2+ influx of UVA ‐exposed melanocytes differed greatly from that of UVB ‐exposed melanocytes in the timing‐phase. The protein expression of TRPM 1 in mono‐ and co‐cultured MC s was dose‐dependently up‐regulated by UVA and UVB . TRPM 1 si RNA ‐mediated knockdown and the blockage of TRPM 1 channel using a putative antagonist (voriconazole) significantly inhibited melanosome transfer in co‐cultures following UVA or UVB exposure. Conclusions The distinct time‐phases of Ca 2+ influx in MC s induced by UVA or UVB contribute to the consecutive stimulation of melanosome transfer, thereby providing a potent photoprotection against harmful UV radiation.