Angara, Kartik Prasad; Department of Biochemistry and Molecular Biology (Augusta University, 7/20/2018)
      Glioblastoma (GBM) is a hypervascular and hypoxic neoplasia of the central nervous system with an extremely high rate of mortality. Owing to its hypervascularity, anti-angiogenic therapies (AAT) have been used as an adjuvant to the traditional surgical resection, chemotherapy, and radiation to normalize blood vessels, control abnormal vasculatures and prevent recurrence. The benefits of AAT have been transient and the tumors were shown to relapse faster and demonstrated particularly high rates of AAT-induced therapy resistance due to activation of alternative neovascularization mechanisms. Vascular Mimicry (VM) is the uncanny ability of tumor cells to acquire endothelial-like properties, lay down vascular patterned networks reminiscent of host endothelial blood vessels and served as an irrigation system for the tumors to meet with the increasing metabolic and nutrient demands in the event of the ensuing hypoxia resulting from AAT. In our studies, we have demonstrated that AAT accelerates VM. We observed that Vatalanib (a VEGFR2 tyrosine kinase inhibitor) induced VM vessels are positive for periodic acid-Schiff (PAS) matrix but devoid of any endothelium on the inner side and lined by tumor cells on the outer side. Interestingly, 20-HETE synthesis inhibitor HET0016 significantly decreased GBM tumors through decreasing VM structures both at the core and at the periphery of the tumors. During our extensive studies to understand the tumor-inherent mechanisms of AAT-induced resistance, we identified a crucial chemokine, CXCL8 or IL-8, to be highly upregulated in the GBM tumors treated with AAT. IL-8 has been well established as a highly prevalent cytokine in GBM with potent pro-migratory and pro-angiogenic functions. AAT-treated groups had significantly higher populations of CXCR2+ glioma stem cells and endothelial-like subpopulations and these populations were decreased following treatment with HET0016 and SB225002 (a CXCR2 antagonist). CXCR2+ GBM tumor cells were shown to form VM-like vascular channels carrying functional RBCs. Knocking down CXCR2 led to smaller tumor size in the animals and improperly developed vascular structures without CXCR2+ GBM cells lining them. This confirms our hypothesis that CXCR2+ GBM cells initiate VM and contribute to AAT resistance in GBM. Our present study suggests that HET0016 and SB225002 have potential to target therapeutic resistance and can be combined with other antitumor agents in preclinical and clinical trials.