Glabridin Inhibits Melanogenesis and Melanin Transfer via Wnt/β-Catenin Pathway and Rho Family GTPase-Mediated Dendritic Formation Suppression

Background: Glabridin, a natural compound derived from Glycyrrhiza glabra L., possesses skin-lightening effects. This study aims to further elucidate the depigmentation mechanism of glabridin by investigating its effects on melanogenesis and melanin transfer. Methods: We initially confirmed the anti-melanogenic effects of glabridin in MNT-1 human melanoma cells. Then, we investigated the mechanism of its anti-melanogenic effect by evaluating the protein expression of β-catenin and MITF via Western blot. To investigate melanin transfer, we compared glabridin’s efficacy with that of niacinamide, a recognized inhibitor of melanosome transfer and employed two complementary experimental models: (1) α-melanocyte-stimulating hormone (α-MSH)-stimulated MNT-1 cells to analyze dendrite formation, and (2) a UVB-irradiated co-culture system of MNT-1 cells and HaCaT keratinocytes to evaluate melanin transfer. Results: By measuring glabridin’s effects on melanin content, tyrosinase activity, and melanogenesis-related protein expression confirmed its inhibition of melanin synthesis. Further investigation demonstrated that glabridin suppresses melanogenesis by downregulating β-catenin and MITF, indicating inhibition of the Wnt/β-catenin pathway. Furthermore, in α-MSH-treated MNT-1 cells, both glabridin and niacinamide were found to suppress dendrite formation and elongation. In a UVB-exposed co-culture system, both glabridin and niacinamide inhibited melanin transfer to keratinocytes. Mechanistically, these effects were linked to the regulation of Rho GTPases (Rac1, RhoA, Cdc42) and suppression of F-actin reorganization. Conclusions: This study provides, for the first time, evidence that the skin-lightening effect of glabridin involves two complementary mechanisms: inhibition of melanogenesis through suppression of the Wnt/β-catenin pathway, and attenuation of both dendricity and melanin transfer via the influence of Rho family GTPases expression.