The Role of MRGPRX1 in the Melanogenesis of Human Primary Epidermal Melanocytes

MRGPRX1 (mas-related G protein-coupled receptor X1) is an orphan receptor, and its function in the skin cells remains unclear. In this study, we demonstrated MRGPRX1 expression in skin cells, including melanocytes. We also found that MRGPRX1 knockdown using short hairpin RNA increased melanin content, cellular tyrosinase activity, and the expression of melanogenic proteins, including tyrosinase, tyrosinase-related protein 2, and MITF in human epidermal melanocytes. In addition, MRGPRX1 knockdown increased CRE-binding protein and p38 phosphorylation and decreased p44/42 phosphorylation. Furthermore, we demonstrated that chloroquine acts as an MRGPRX1 agonist. Specifically, in silico docking simulation showed that chloroquine binds to MRGPRX1. Chloroquine treatment increased MRGPRX1 expression, activation, and intracellular calcium influx. Unlike MRGPRX1 knockdown, chloroquine treatment had antimelanogenic effects on melanin content, tyrosinase activity, melanogenic genes, and MAPKs. Furthermore, whereas chloroquine treatment increased calcium influx through TRPV1 and TRPA1, TRPA1 operated upstream of CRE-binding protein and p38 and activated them through calcium influx. We also found that tert-butyl hydroperoxide and UVA irradiation increased MRGPRX1 expression, suggesting that environmental factors affect MRGPRX1-mediated signaling. Collectively, these findings indicate that MRGPRX1-mediated signaling contributes to melanogenesis suppression, and chloroquine is a possible MRGPRX1 protein agonistic ligand, suggesting that MRGPRX1 could be a therapeutic target for pigmentary disorders.