Veena Sangwan for PTP1B siRNA, Bina Kaplan for technical assistance, Dr. also one of the most vascularized and edematous tumors as they express high levels of VEGF (Ferrara et al., 2003; Sundberg et al., 2001; van Bruggen et al., 1999). Encouragingly, bevacizumab, a humanized monoclonal antibody against VEGF, has demonstrated therapeutic benefit in many GBM patients when used alone or in combination with irinotecan (Friedman et al., 2009; Vredenburgh et al., 2007). This led to the accelerated approval of bevacizumab by the US Food and Drug Administration in 2009 2009 for use as a single agent in recurrent GBM, and its use in the frontline setting for newly diagnosed GBM is currently being evaluated. Despite initial responsiveness, however, the beneficial effects of bevacizumab are transient, and GBM inevitably progress during anti-VEGF treatment by adapting and utilizing alternative pathways to sustain tumor growth, all while VEGFR signaling remains inhibited (Bergers and Hanahan, 2008). Clinical evidence suggests that GBM relapse during anti-VEGF therapy can present with at least two differing radiographic patterns representing distinct mechanisms of evasion. While most GBM patients develop characteristic local recurrences that regain the ability to induce neovascularization as observed by increased magnetic resonance imaging (MRI) contrast enhancement, up to 30% of GBM patients demonstrate a more extensive, infiltrative, and distant disease that lacks angiogenic induction and is non-contrast enhancing, but is instead detectable by fluid-attenuated inversion recovery (FLAIR) MRI (de Groot et al., 2010; Iwamoto et al., 2009; Rose and Ionomycin Aghi, 2010). While the incidence of invasion has been a subject of discussion due the current lack of a standardized definition for radiographic relapse (Chamberlain, 2011; Wick et al., 2011), the frequency of invasive non-enhancing tumors nevertheless appears to be higher than would be expected in patients who do not receive bevacizumab (Iwamoto et al., 2009). This pro-invasive recurrence encumbers surgical resection of recurrent GBM and challenges further therapeutic options for patients. Similar to other studies using mouse models of GBM (de Groot Ionomycin et al., 2010; Kunkel et al., 2001; Rubenstein et al., 2000), we have observed that a more perivascular invasive phenotype, in which tumor cells move predominantly along blood vessels deep into the brain parenchyma, was induced when murine GBM were unable to initiate angiogenesis, a phenomenon that predicted the invasive relapse pattern seen in bevacizumab-treated human GBM (Blouw et al., 2003; Du et al., 2008a; Paez-Ribes et al., 2009). The enhanced invasiveness was a result of impairing tumor angiogenesis either through genetic ablation of key angiogenic factors that drive VEGF-dependent neovascularization (HIF-1, VEGF, MMP-9, MMP-2) (Blouw et al., 2003; Du et al., 2008a; Du et al., 2008b) Ionomycin or by pharmacologic targeting of VEGF signaling (Paez-Ribes et al., 2009). We further revealed an unexpected link between HGF and VEGF, in which VEGF reduced the chemotactic activity of GBM cells towards HGF in vitro (Du et al., 2008a). The HGF receptor MET, a receptor tyrosine kinase that is frequently deregulated in many cancers and promotes proliferation, scattering, invasion, survival, and angiogenesis (Birchmeier et al., 2003; Rong et al., 1992; Trusolino et al., 2010; Wang et al., 2001), is correlated with increased tumor invasion and poorer survival in GBM (Abounader and Laterra, 2005; Koochekpour et al., 1997; Lamszus et al., 1998). Given that VEGF inhibition Ionomycin was a common denominator among the various genetic knockout models and pharmacologic treatments described above, we investigated whether VEGF itself might act as a regulatory switch for GBM invasion through regulating MET. Results Intratumoral VEGF Levels Inversely Correlate with MET Rabbit polyclonal to CapG Phosphorylation and Invasion of GBM Tumor Cells To test our hypothesis that VEGF regulates the HGF/MET axis in tumor cells, we used orthotopically implanted SV40 large T-antigen and H-ras transformed mouse astrocytoma cell lines differing only in their VEGF expression levels: VEGFKO GBM, which are.