CSIG-09NOVEL PAK4 REGULATION OF PPARγ-MEDIATED EPITHELIAL-MESENCHYMAL TRANSITION IN GLIOBLASTOMA

Abstract

Tumor recurrence is inevitable despite using the post-surgical chemo- and radiotherapies for Glioblastomas (GB), the most aggressive of primary brain tumors associated with poor prognosis. Epithelial-mesenchymal transition (EMT) is the dynamic transdifferentiation process which potentially leads to acquisition of invasiveness, stemness and therapy resistance in tumors. Recent studies focused to develop the novel targets against EMT and stemness in tumors; however, key factors which regulate EMT signaling and radioresistance in GBs still remains to be identified. Our studies indicated p21-activated kinase 4 (PAK4) deregulation in positive correlation with glioma pathological grades, and showed its role in proliferation and invasion in glioma cell lines. Here, we aimed to study whether PAK4-regulated invasiveness is associated with EMT of glioma cells and to delineate the essential signaling events that govern EMT and maintenance of stemness in GBs. PAK4 overexpression increased EMT in glioma cells by elevating N-cadherin and vimentin levels. Conversely, specific shRNA-mediated PAK4 downregulation led to loss of mesenchymal phenotype in these cells. Further, irradiation (IR, 8 Gy) treatment significantly increased PAK4 nuclear localization in correlation with increased EMT in IR-treated cells. Transcription factor (TF) interaction arrays with immunoprecipitated PAK4 as a bait revealed the novel nuclear-PAK4 association with TFs, including PPARγ. Moreover, ChIP and EMSA assays showed IR-induced PAK4 binding with PPARγ-DNA binding complex and recruitment on PPREs of Nox-1 promoter. PAK4-knockdown suppressed the IR-induced Nox1 levels and EMT in glioma cells. In line with these in vitro results, mouse orthotopic intracranial xenograft experiments showed decreased tumor growth in PAK4-shRNA tumors when compared with controls. These results strongly support a novel PAK4 regulation of IR-induced EMT via PPARγ/Nox1 signaling and provide a rationale for PAK4 as a new therapeutic target against radioresistance in glioblastoma.