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Abstract
Cdc2-like kinase 2 (CLK2) is known as a regulator of RNA splicing that ultimately controls multiple physiological processes. However, the function of CLK2 in glioblastoma progression has not been described. Reverse-phase protein array (RPPA) was performed to identify proteins differentially expressed in CLK2 knockdown cells compared to controls. The RPPA results indicated that CLK2 knockdown influenced the expression of survival-, proliferation-, and cell cycle-related proteins in GSCs. Thus, knockdown of CLK2 expression arrested the cell cycle at the G1 and S checkpoints in multiple GSC lines. Depletion of CLK2 regulated the dephosphorylation of AKT and decreased phosphorylation of Forkhead box O3a (FOXO3a), which not only translocated to the nucleus but also increased p27 expression. In two glioblastoma xenograft models, the survival duration of mice with CLK2-knockdown GSCs was significantly longer than mice with control tumors. Additionally, tumor volumes were significantly smaller in CLK2-knockdown mice than in controls. Knockdown of CLK2 expression reduced the phosphorylation of FOXO3a and decreased Ki-67 in vivo. Finally, high expression of CLK2 protien was significantly associated with worse patient survival. These findings suggest that CLK2 plays a critical role in controlling the cell cycle and survival of glioblastoma via FOXO3a/p27.
Highlights
Glioblastoma is a highly malignant, aggressive disease that is the most common primary intrinsic brain tumor in adults
Regulation of mRNA splicing is recognized as a dynamic processing event, and Cdc2-like kinases (CLKs) kinases identified as splicing factors in vivo [29, 30]
We report the identification of a function of Cdc2-like kinase 2 (CLK2) in glioblastoma stem cells
Summary
Glioblastoma is a highly malignant, aggressive disease that is the most common primary intrinsic brain tumor in adults. CSNK2A1, the gene that encodes for CK2a, is amplified in glioblastoma [1]. CK2 can positively regulate phosphoinositide 3-kinase (PI3K)/AKT activity by directly phosphorylating AKT at Ser129 [2]. Kim et al [3] found that CX-4945 is a potent, selective inhibitor of CK2 that modulates serine/arginine-rich protein phosphorylation by directly inhibiting Cdc2-like kinases (CLKs). The CLKs, including CLK1, CLK2, and CLK3 [5,6,7], phosphorylate serine/arginine-rich proteins, which regulate RNA splicing [3]. Inhibitors of CLKs suppress cell growth and induce apoptosis by modulating pre-mRNA splicing [8]. Hepatic CLK2 protein level and kinase activity is induced by insulin/AKT signaling. AKT directly binds to CLK2 and phosphorylates CLK2 [10]
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