A novel interaction of PAK4 with PPARγ to regulate Nox1 and radiation-induced epithelial-to-mesenchymal transition in glioma

Tumor recurrence in glioblastoma (GBM) is, in part, attributed to increased epithelial-to-mesenchymal transition (EMT) and enhanced tumor cell dissemination in adjacent brain parenchyma after ionizing radiation (IR). EMT is associated with aggressive behavior, increased stem-like characteristics and treatment resistance in malignancies; however, the underlying signaling mechanisms that regulate EMT are poorly understood. We identified grade-dependent p21-activated kinases 4 (PAK4) upregulation in gliomas and further determined its role in mesenchymal transition and radioresistance. IR treatment significantly elevated expression and nuclear localization of PAK4 in correlation with induction of reactive oxygen species (ROS) and mesenchymal transition in GBM cells. Stable PAK4 overexpression promoted mesenchymal transition by elevating EMT marker expression in these cells. Of note, transcription factor-DNA-binding arrays and chromatin immunoprecipitation experiments identified the formation of a novel nuclear PAK4/PPAR gamma complex which was recruited to the promoter of Nox1, a peroxisome proliferator-activated receptor gamma (PPAR gamma) target gene. In addition, IR further elevated PAK4/PPAR gamma complex co-recruitment to Nox1 promoter, and increased Nox1 expression and ROS levels associated with mesenchymal transition in these cells. Conversely, specific PAK4 downregulation decreased PPAR gamma-mediated Nox1 expression and suppressed EMT in IR-treated cells. In vivo orthotopic tumor experiments showed inhibition of growth and suppression of IR-induced PPAR gamma and Nox1 expression by PAK4 downregulation in tumors. Our results provide the first evidence of a novel role for PAK4 in IR-induced EMT and suggest potential therapeutic efficacy of targeting PAK4 to overcome radioresistance in gliomas.