TMIC-63. ENDOCAN BRIDGING THE GAP: TUMOR ENDOTHELIAL CELL SECRETOME AND THE VASCULATURE-PROXIMAL TUMOR EDGE

BACKGROUND Glioblastoma (GBM) is a highly invasive, angiogenic and therapeutically resistant primary brain tumor. It is impossible to completely resect the tumor, which subsequently leaves behind tumor “edge” that can act as seeds for recurrence. We recently reported the leading “edge” and tumor core have the presence of two distinct types of glioma stem-like cells (GSCs) that resemble proneural (PN) and mesenchymal (MES) subtypes, respectively. GSCs are enriched in perivascular niches, and vascular endothelial (VE) cells are known to provide paracrine factors, radioresistance properties to GSCs. Therapeutically, targeting GBM using an anti-angiogenic anti-VEGF inhibitor, Bevacizumab, did not aid in overall survival and instead resulted in aggressive MES phenotype. Hence, it is imperative to study this nexus in greater detail to develop novel targeted therapies for VE and GBM cells. The purpose of this study was to understand the molecular mechanisms emanating from VE cells that cause the invading “edge” to recur and determine if targeting these molecular cascades can provide therapeutic benefit. RESULTS Using unbiased approach to identify paracrine factors from GBM patient derived VE cells, we identified endocan, a vascular endothelial cell secreted molecule, as a pro-tumorigenic factor essential for neovascularization, radioresistance, and stemness of GBM edge tumor cells. Utilizing patient-derived and mouse glioblastoma sphere models and the corresponding intracranial tumor models, we demonstrate that endocan maintains the “edge”-located glioma stem cell signature and protects glioblastoma cells from radiation-induced differentiation and apoptosis. Mechanistically, we show that endocan is a novel ligand that interacts with PDGFRa and induce its phosphorylation, leading to phosphorylation of the downstream targets PI3K and MAPK in GBM. CONCLUSIONS Collectively, these findings demonstrate a novel signaling axis of endocan-PDGFRa in GBM that is activated by tumor endothelial cell and plays an essential role in glioma stem cells maintenance and radioresistance in tumor “edge”.