Two citrus polymethoxyflavones, nobiletin and sudachitin, suppress melanogenesis through distinct molecular mechanisms.

Polymethoxyflavones (PMFs), a subclass of flavonoids abundant in citrus peels, exhibit diverse biological activities. However, their effects on melanogenesis remain poorly understood. In this study, we investigated the effects of two PMFs, nobiletin (5,6,7,8,3',4'-hexamethoxyflavone) and sudachitin (5,7,4'-trihydroxy-6,8,3'-trimethoxyflavone), on melanogenesis in murine B16 melanoma cells. Both PMFs significantly reduced α-melanocyte-stimulating hormone- or forskolin/3-isobutyl-1-methylxanthine-induced melanin production, comparable to that observed with the tyrosinase inhibitor glabridin. However, neither PMF directly inhibited cell-free tyrosinase activity, unlike glabridin. Instead, both PMFs suppressed the expression of tyrosinase and microphthalmia-associated transcription factor. Mechanistic analyses revealed that sudachitin inhibited melanogenesis by attenuating CREB activation, whereas nobiletin suppressed melanogenesis, accompanied by activation of the ERK signaling pathway. Additionally, both PMFs induced autophagy in B16 melanoma cells; notably, nobiletin-induced autophagy partially contributed to its anti-melanogenic effects. These findings demonstrate that nobiletin and sudachitin suppress melanogenesis via distinct molecular mechanisms and identify autophagy as a contributing factor in nobiletin-mediated inhibition of melanogenesis.