ABSTRACT: The mechanisms by which tumors develop resistance to angiogenesis inhibitors, and the relative contributions of tumor cells and stroma to resistance, are not completely understood. We developed three human lung adenocarcinoma murine models of resistance to the VEGF inhibitor bevacizumab and, using species-specific profiling, separately investigated tumor cell and stromal molecules associated with resistance. Gene expression changes associated with acquired resistance occurred predominantly in stromal (mouse) and not tumor (human) cells. Components of the EGFR and FGFR2 pathways were significantly upregulated in stroma, but not in tumor cells. Increased activated EGFR was detected on pericytes of xenografts that acquired resistance and on endothelium of tumors with relative primary resistance. Acquired resistance was associated with a pattern of pericyte-covered, normalized revascularization, whereas tortuous, uncovered vessels were observed in relative primary resistance. Dual targeting of VEGF and EGFR pathways with bevacizumab and erlotinib, or the VEGFR/EGFR inhibitor vandetanib, reduced pericyte coverage and increased progression-free survival. These findings demonstrate that alterations in tumor stromal pathways, including EGFR and FGFR2, are associated with, and may contribute to VEGF inhibitor resistance and that targeting these pathways may improve therapeutic efficacy. Understanding stromal signaling may be critical for developing biomarkers for angiogenesis inhibitors and improving combination regimens. To identify changes in stromal and tumor gene expression associated with acquired resistance to anti-VEGF therapy, we performed RNA microarray analyses comparing H1975 control (vehicle-treated) and BV-resistant (bevacizumab-treated until progression) xenografts (n=3 biologic replicates per group of treatment) using Illumina mouse (WG-6 v2) and human (WG-6 v3)-specific expression arrays. Probes in these arrays have been designed to minimize cross-species reactivity and consistent with this essentially no cross reactivity was observed in experiments mixing human and mouse cell lines.Total RNA was extracted from snap-frozen tissues using the mirVanaTM miRNA Isolation Kit (Ambion, Austin, TX) according to manufacturer’s protocol. Biotin labeled cRNA samples for hybridization were prepared by using Illumina Total Prep RNA Amplification Kit (Ambion Inc., Austin, TX). One microgram of total RNA was used for the synthesis of cDNA and followed by an amplification and biotin labeling. Each of 1.5 μg of biotinylated cRNAs was hybridized to both mouseWG-6 v2 and human WG-6v3 Expression BeadChips (Illumina®, San Diego, CA) at the same time, for analysis of murine and human transcriptomes. Signals were developed by Amersham fluorolink streptavidin-Cy3 (GE Health care Bio-Sciences, Little Chalfont, UK). Gene expression data were collected by using Illumina bead Array Reader confocal scanner (BeadStation 500GXDW; Illumina Inc.). Data were analyzed using the BRB-ArrayTools Version 3.7.0 Beta platform developed by Dr. Richard Simon and the BRB-Array Tools development team. A log base 2 transformation was applied to the data set prior to data normalization. A median array was selected as the reference array for normalization and statistical significance was set using a p<0.01. Genes differentially expressed between groups were determined applying univariate t-test with estimation of the false discovery rate (FDR). Genes were determined using selection criteria of a p<0.005 and a fold-change ≥1.5.
