Anti-angiogenic Resistance in Glioblastoma Through a Mesenchymal Phenotype Shift
Introduction. Glioblastoma multiforme (GBM) is a rare tumor characterized by a high mortality with a 5-year survival rate of 5%1. GBM is a highly vascular tumor and angiogenesis is correlated with its rapid progression1. Anti-vascular endothelial growth factor (VEGF) therapy continues to be the first line treatment for GBM despite no improvements in mortality2. Patients commonly present tumor resurgence after treatment, often with anti- VEGF therapy resistance. The resistant tumors have been associated with a mesenchymal phenotype shift of GBM cells1,3. Understanding the mechanisms behind the epithelial to mesenchymal shift (EMT) of GBM may provide insight in new pharmacotherapies to prevent GBM resistance to anti-VEGF therapy. Methods. GBM cells from human biopsy and mouse models were stratified into pro-neural (PN) and mesenchymal (MES) populations. Western blots and transcriptome analysis were then used to identify mechanisms behind anti- VEGF therapy resistance in the MES cell population3. Histological slides were stained with Ki-67, a proliferation marker, and CD34, an endothelial marker, to quantify microvascular density. Shifts from a PN dominant phenotype and MES dominant phenotype were then correlated with IL-8, PDGF and DGKa3,4,5. Mouse models were then treated with anti-IL-8, PDGF, and DGKa antibodies to show reduced tumor proliferation via fluorescent imaging. Results. Anti-VEGF therapy resulted in the downregulation of VEGFR2 receptors and upregulation of VEGFR1 receptors. VEGFR2 receptors in the PN cell population mediated most angiogenic activity in primary GBM, whereas VEGFR1 was dominant on the MES phenotype in recurrent tumors3. To identify drivers in the PN and EMT shift, glioma conditioned medium was treated with IL-8, PDGF, and DGKa antibodies and exhibited decreased expression of proliferation and endothelial markers3,4. PDGF and DGKa antibodies were then introduced in a mouse model and demonstrated decreased tumor mass. Dual treatment of anti-PDGF and anti-VEGF therapy resulted in a significant increase in mouse survival, +34 days4. Conclusion. Interleukin-8, PDGF, and DGKa all exhibited casual effects on PN to MES phenotype shift. Inhibition of these pathways resulted in decreased angiogenesis while maintaining sensitivity to anti-VEGF therapy in vitro and in mouse models. While targeting one of these EMT pathways or VEGF did not increase survival, dual therapy resulted in significantly reduced mortality. These results suggest that EMT is a key driver in anti-VEGF therapy resistance and may provide a pathway to maintain sensitivity and prevent recurrence of GBM.
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