Abstract 1388: Acquired resistance to anti-VEGF therapy in glioblastoma is associated with an inflammatory and mesenchymal phenotype
Authors: Piao Yuji , Liang Ji , Holmes Lindsay , Henry Verlene , de Groot John F. 
Published: August 2012

Abstract
Glioblastoma is the most common malignant brain tumor and is characterized by rapid angiogenesis-dependent (re)growth, cell heterogeneity, and extensive local tissue infiltration. Antiangiogenic therapy, among the most promising treatment developments, rapidly reduces vascular permeability and delays glioblastoma progression but the therapy itself ultimately promotes an aggressive treatment-resistant phenotype. The aim of the present study was to identify mechanisms responsible for glioblastoma resistance to antiangiogenic therapy. When mice bearing U87 and NSC11 xenografts treated with bevacizumab became moribund, tumor tissue was extracted to obtain tumor cells with acquired resistance to bevacizumab. U87 and NSC11 cells were dissociated and cultured in serum-containing and stem cell media, respectively. To verify resistance, the resistant cells were expanded in vitro and then re-implanted orthotopically in nude mice. Mice were randomized to receive bevacizumab or vehicle and followed for survival. As anticipated, tumors formed by parental U87 and NSC11 cells were sensitive, whereas tumors obtained at the time of acquired resistance to anti-VEGF therapy were resistant to bevacizumab and did not have prolongation of survival compared to untreated controls. To elucidate the molecular mechanisms underlying resistance to antiangiogenic treatment, we performed gene expression profiling using Affymetrix U133Plus_2.0 arrays comparing untreated and bevacizumab-treated tumors. Comparison of anti-VEGF therapy-resistant cell lines to untreated controls demonstrated an increase in genes associated with a mesenchymal phenotype. Antiangiogenic therapy increased the expression of genes of mesenchymal origin, cellular migration/invasion, and inflammation including chemokine secretion, myeloid cell chemotaxis and multiple markers reflecting a pro-inflammatory environment. Consistent with these data, mice bearing U87 resistant tumors showed significantly greater infiltration of F4/80+ myeloid cells compared to wild-type tumors. Comparing the mesenchymal gene signature from Phillips et al with the gene changes in our experiments using Gene Set Enrichment Analysis (GSEA v2.07, Broad Institute, MIT), we show that NSC11 tumors treated with bevacizumab showed a highly significant correlation to the published mesenchymal gene signature. In addition to the pro-inflammatory changes, invasion-related genes were also upregulated. We found the U87 and NSC11 resistant cells have significantly higher invasion rates in vitro compared with U87 and NSC11 parental cell lines. Our studies identify prominent mechanisms of glioblastoma resistance to antiangiogenic therapy. A better understanding of resistance may provide new strategies to prolong the efficacy of this therapy and accelerate the integration of combination therapies into clinical trials.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1388. doi:1538-7445.AM2012-1388