Yun Cheong-Yong, You Soon-Tae, Kim Jin-Hwa, Chung Jin H, Han Sang-Bae, Shin Eun-Young, Kim Eung-Gook
Department of Biochemistry and Medical Research Center, College of Medicine, Chungbuk National University, Cheongju, Korea.
R&D Center, Hanbul Cosmetics Co. Ltd, Chungbuk, Korea.
J Invest Dermatol. 2015 May;135(5):1385-1394. doi: 10.1038/jid.2014.548. Epub 2015 Jan 5.
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 (MITF), a master transcription factor in melanogenesis. We therefore investigated the role of PAK4 in melanogenesis. Melanocytes express both PAK2 and PAK4 isoforms, but only RNA interference knockdown of PAK4 significantly influenced α-melanocyte-stimulating hormone (α-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 α-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, MITF, and tyrosinase in both HRM-2 mice and B16 melanoma cells. Moreover, PAK4 knockdown and inhibition suppressed α-MSH-stimulated β-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 α-MSH/UVB-induced melanogenesis via the CREB and Wnt/β-catenin signaling pathways and suggest that PAK4 may be a potential therapeutic target in pigmentation disorders.
p21激活激酶4（PAK4）调节广泛的细胞活动，包括细胞骨架重塑、细胞生长和存活。我们之前的研究确定PAK4是环磷酸腺苷反应元件结合蛋白（CREB）的关键调节因子，其作用于小眼相关转录因子（MITF）的上游，MITF是黑色素生成中的主要转录因子。因此，我们研究了PAK4在黑色素生成中的作用。黑素细胞同时表达PAK2和PAK4异构体，但只有RNA干扰敲低PAK4会显著影响α-黑素细胞刺激激素（α-MSH）诱导的B16黑色素瘤细胞中的黑色素生成。与该结果一致，PAK的强效ATP竞争性抑制剂PF3758309对PAK4的抑制不仅以剂量依赖性方式抑制了B16黑色素瘤细胞和人上皮黑素细胞中α-MSH诱导的黑色素生成，还抑制了有黑色素的无毛小鼠（HRM-2）皮肤中紫外线B（UVB）诱导的黑色素生成。连续几天抑制PAK4显著降低了HRM-2小鼠和B16黑色素瘤细胞中CREB、MITF和酪氨酸酶的水平。此外，PAK4敲低和抑制抑制了α-MSH刺激的β-连环蛋白丝氨酸675（S675）处的磷酸化，但增强了S33/37处的磷酸化，S33/37是泛素化依赖性蛋白水解的指标。总之，我们的结果提供了证据，表明PAK4通过CREB和Wnt/β-连环蛋白信号通路促进α-MSH/UVB诱导的黑色素生成，并表明PAK4可能是色素沉着障碍的潜在治疗靶点。
