The DNA methylation inhibitor 5-azacytidine decreases melanin synthesis by inhibiting CREB phosphorylation.

Here we examined the effects of a DNA methylation inhibitor, 5-azacytidine, on melanogenesis in Mel-Ab cells. We found that 5-azacytidine decreased the melanin content and tyrosinase activity in these cells in a dose-dependent manner; importantly, 5-azacytidine was not cytotoxic at the concentrations used in these experiments. On the other hand, 5-azacytidine did not affect tyrosinase activity in a cell-free system, indicating that 5-azacytidine is not a direct tyrosinase inhibitor. Instead, 5-azacytidine decreased the protein levels of microphthalmia-associated transcription factor (MITF) and tyrosinase. Thus, we investigated the effects of 5-azacytidine on signal transduction pathways related to melanogenesis. However, 5-azacytidine did not have any effect on either Akt or glycogen synthase kinase 3ß (GSK3ß) phosphorylation. The phosphorylation of cAMP response element-binding protein (CREB) is well known to regulate MITF expression, thereby also regulating tyrosinase expression. We found that 5-azacytidine decreased the phosphorylation of CREB. Therefore, we propose that 5-azacytidine may decrease melanin synthesis by downregulating MITF and tyrosinase via CREB inactivation.

A derivative of imidazobenzimidazole, ML106, inhibits melanin synthesis via p38 MAPK activation.

We investigated the effects of ML106 on melanogenesis in B16F10 melanoma cells. Our results showed that ML106 decreased melanin content and tyrosinase activity in a dose-dependent manner. Interestingly, ML106 did not inhibit microphthalmia-associated transcription factor (MITF) expression, but did decrease tyrosinase expression. Thus, we further investigated the expression and degradation of tyrosinase and related signal transduction pathways. Although ML106 increased glycogen synthase kinase 3beta (GSK3beta) activation, the level of beta-catenin level was not affected. Thus, we excluded the involvement of GSK3beta and beta-catenin in ML106-induced hypopigmentation. However, ML106 induced the phosphorylation of p38 mitogen-activated protein kinase (MAPK), causing down-regulation of tyrosinase. Thus, we next investigated whether tyrosinase down-regulation was due to proteasomal degradation by p38 MAPK activation. We found that ML106-induced tyrosinase down-regulation was restored by MG132, a proteasome inhibitor. Thus, we propose that ML106 has hypopigmentary activity through tyrosinase degradation via p38 MAPK phosphorylation.

Scopoletin from Cirsium setidens Increases Melanin Synthesis via CREB Phosphorylation in B16F10 Cells.

In this study, we isolated scopoletin from Cirsium setidens Nakai (Compositae) and tested its effects on melanogenesis. Scopoletin was not toxic to cells at concentrations less than 50 µM and increased melanin synthesis in a dose-dependent manner. As melanin synthesis increased, scopoletin stimulated the total tyrosinase activity, the rate-limiting enzyme of melanogenesis. In a cell-free system, however, scopoletin did not increase tyrosinase activity, indicating that scopoletin is not a direct activator of tyrosinase. Furthermore, Western blot analysis showed that scopoletin stimulated the production of microphthalmia-associated transcription factor (MITF) and tyrosinase expression via cAMP response element-binding protein (CREB) phosphorylation in a dose-dependent manner. Based on these results, preclinical and clinical studies are needed to assess the use of scopoletin for the treatment of vitiligo.

Dual hypopigmentary effects of punicalagin via the ERK and Akt pathways.

Punicalagin is a phenolic compound with antioxidant properties. However, the effects of punicalagin on melanin synthesis have been poorly evaluated. Therefore, we investigated the effects of punicalagin on melanogenesis in Mel-Ab cells. Punicalagin significantly inhibited melanin synthesis in a dose-dependent manner. In accordance with the melanin content, punicalagin also dose-dependently decreased tyrosinase activity. Punicalagin did not directly inhibit tyrosinase in a cell-free system but did downregulate the expression of microphthalmia-associated transcription factor (MITF) and tyrosinase. Therefore, we examined the effects of punicalagin on melanogenesis-related signaling pathways. Punicalagin induced extracellular signal-regulated kinase (ERK) and Akt phosphorylation but had no effect on ß-catenin level. We measured melanin content and MITF expression in the presence of the ERK pathway inhibitor PD98059 and/or the Akt pathway inhibitor LY294002. Cotreatment with PD98059 and LY294002 almost completely restored punicalagin-induced hypopigmentation. These data indicate that punicalagin inhibits melanin synthesis through ERK and Akt phosphorylation, with subsequent downregulation of MITF and tyrosinase.