Endothelin-1 as a new melanogen: coordinated expression of its gene and the tyrosinase gene in UVB-exposed human epidermis.

We previously demonstrated that human keratinocytes produce and secrete endothelins (ET), which can be strong mitogens for human melanocytes. Ultraviolet B (UVB) exposure highly stimulated the paracrine linkage of endothelins between keratinocytes and melanocytes, indicating that they are keratinocyte-derived intrinsic mitogens in UVB-induced pigmentation. In this study, the role of ET-1 as a melanogen in UVB melanogenesis was investigated in vitro and in vivo. In the conditioned medium of keratinocytes exposed to UVB, melanin synthesis by human melanocytes, as measured by 14C-thiouracil incorporation, was significantly accentuated. This stimulatory effect was reduced by anti-ET-1 to the level of that in the non-UVB-exposed control, suggesting an essential role of ET-1 as an intrinsic melanogen in UVB-induced melanogenesis. In a parallel study, the addition of 10 nM ET-1 induced an increase in tyrosinase activity in cultured human melanocytes and was accompanied by elevated levels of tyrosinase and tyrosinase-related protein-1 mRNA expression as shown by Northern blotting. Reverse transcription-polymerase chain reaction of RNA isolated from the epidermis of human skin exposed to UVB revealed that, whereas in non-exposed sites ET-1, IL-1 alpha, and tyrosinase mRNA signals were scarcely detected, UVB-irradiation, with a dose of twice the minimal erythema dose, caused a significant increase in the expressions of the three genes 5 d after irradiation. These findings suggest that ET-1 is an important mediator for UVB-induced pigmentation in the epidermis in vivo.