Tokoronin Contained in Dioscorea tokoro Makino ex Miyabe Suppressed α-MSH-Induced Melanogenesis in B16 Cells via Suppression of Classical MAPK Pathway Activation.

In this study, melanogenesis inhibition in B16 cells by eight compounds, namely, tokorogenin, tokoronin, yononin, gracillin, proto-yonogenin, proto-tokoronin, proto-yononin, and proto-gracillin, isolated from Dioscorea tokoro Makino ex Miyabe were evaluated. The results of the cytotoxicity and alpha-MSH-induced melanogenesis inhibition effects of the eight compounds revealed that tokoronin was the most effective in terms of low-cytotoxicity and melanogenesis inhibition. Tokoronin downregulated alpha-MSH-induced melanogenesis via suppression of the expression of the three types of melanogenesis-related enzymes [tyrosinase, tyrosinase-related protein-1 (TRP-1), TRP-2] by the inhibition of phospho-microphthalmia-associated transcription factor (p-MITF) and cAMP response element binding protein (CREB) levels. p-MITF and CREB are regulated by various kinases [Akt, mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase (MAPK), and c-jun N-terminal kinase (JNK)]. As the results of measurement of the combined effects of tokoronin with inhibitors or promoters of these kinases, no change in the biological activity of tokoronin by Akt inhibitor (wortmannin) or p38 MAPK inhibitor (SB202190) was observed, however, the effect of tokoronin was reduced by the MEK/ERK inhibitor (U0126) and promoted by the MEK/ERK activator (FGF2). Therefore, it was deduced that tokoronin first inactivated ERK; then, it suppressed p-MITF and CREB levels; and finally, alpha-MSH-induced melanogenesis was suppressed.