GSK3beta inhibition promotes melanogenesis in mouse B16 melanoma cells and normal human melanocytes

Glycogen synthase kinase 3beta (GSK3beta) is implicated in many biological events, including embryonic development, cell differentiation, apoptosis, and the insulin response. GSK3beta also plays a key role in the Wnt/beta-catenin pathway. The master regulator of the pigmentation microphthalmia-associated transcription factor (MITF) is a target for the Wnt pathway, however, to date, the regulatory role of GSK3beta in the control of melanogenesis has not been elucidated. In this study, we evaluated the effect of inhibiting GSK3beta activity on the regulation of melanocyte differentiation. Exposure of the murine melanoma cell line B16 and normal human melanocytes to GSK3beta specific inhibitors (SB216763, SB415286, BIO, and LiCl) resulted in a dose-dependent accumulation of beta-catenin. This is associated with the induction of melanocyte differentiation-associated markers such as melanin synthesis, tyrosinase activity, and expression of tyrosinase and the microphthalmia-associated transcription factor. Attenuation of GSK3beta activity has an inhibitory effect on cell growth, and this was accompanied by morphological changes. Moreover, treatment of B16 cells with a siRNA targeted against beta-catenin completely abolished the promelanogenic effect of GSK3beta inhibition, however, the overexpression of a constitutively active mutant form of beta-catenin (pCS2beta-cat-mut) only slightly increased the degree of pigmentation. These results demonstrated that GSK3beta is implicated in the regulation of melanogenesis and that pharmacological inhibition of its activity could increase melanin synthesis through mechanisms probably not restricted to Wnt/beta-catenin pathway activation.