p21-Activated Kinase 4 Critically Regulates Melanogenesis via Activation of the CREB/MITF and β-Catenin/MITF Pathways

p21-activated kinase 4 (PAK4) regulates a wide range of cellular events, including cytoskeletal remodeling, cell growth, and survival. Our previous study identified PAK4 as a key regulator of cAMP-response element-binding protein (CREB) that acts upstream of microphthalmia-associated transcription factor (MITE), a master transcription factor in melanogenesis. We therefore investigated the role of PAK4 in melanogenesis. Melanocytes express both PAK2 and PAK4 isofornns, but only RNA interference knockdown of PAK4 significantly influenced alpha-melanocyte-stimulating hormone (alpha-MSH) induced melanogenesis in B16 melanoma cells. Consistent with this result, PAK4 inhibition by PF3758309, a potent ATP-competitive inhibitor of PAKs, suppressed not only alpha-MSH induced melanogenesis in B16 melanoma and human epithelial melanocyte cells but also UVB-induced melanogenesis in the skin of melanin-possessing hairless mice (HRM-2) in a dose-dependent manner. Inhibition of PAK4 over several days markedly decreased the levels of CREB, MITE, and tyrosinase in both HRM-2 mice and B16 melanoma cells. Moreover, PAK4 knockdown and inhibition suppressed alpha-MSH-stimulated beta-catenin phosphorylation at serine 675 (S675) but enhanced phosphorylation at S33/37, an indicator for ubiquitination-dependent proteolysis. Together, our results provide evidence that PAK4 promotes alpha-MSH/UVB-induced melanogenesis via the CREB and Wnt/beta-catenin signaling pathways and suggest that PAK4 may be a potential therapeutic target in pigmentation disorders.