ANGI-06PAK4 GOVERNS RADIATION-INDUCED ENDOTHELIAL Gro-1α-CXCR2 SIGNALING AND ANGIOGENESIS IN GLIOMA

Abstract

Aberrant neovascularization is a pathological hallmark of glioblastoma that correlates with poor clinical outcome. Factors that drive endothelial proliferation, recruitment into the tumor and angiogenesis in the context of radiotherapy are potentially novel therapeutic targets for glioma. Our studies showed high expression of p21-activated kinase 4 (PAK4) in gliomas; analyses of conditioned medium (CM) prepared from shRNA-mediated PAK4-knockdown (PAK4.sh) glioma cells using a human angiogenesis array showed a significant downregulation in key angiogenic factors including Gro-1α. Treatment of human brain microvascular endothelial cells (HBMECs) with CM from PAK4.sh-cells significantly inhibited endothelial proliferation, migration and microtubule formation compared with controls. PAK4.sh-CM treatment also suppressed CXCR2 receptor activation and decreased the expression levels of p-Stat3, MMP-2 and cyclin D1 in HBMECs. Ionizing radiation (IR, 8 Gy) activated Gro-1α-CXCR2 signaling in HBMECs, and induced high levels of endothelial capillary network formation on matrigel. On the other hand, PAK4.sh inhibited IR-induced angiogenesis by decreasing Gro-1α and CXCR2 levels in HBMECS. Further, in vivo experiments with orthotopic intracranial tumors in nude mice showed growth suppression in PAK4.sh tumors compared to control tumors. Immunohistochemical analyses of brain tumor sections showed a significant decrease in MMP-2 in PAK4.sh tumors. In summary, our studies emphasize a novel role of PAK4 as a critical mediator of tumor-endothelial cell interactions via Gro-1α-CXCR2 signaling, and in vivo tumor growth suppression by PAK4 depletion indicates its therapeutic potential in the treatment of glioma.