AI-10 * DISTINCT EGFR SIGNALING IN GLIOBLASTOMA: WILD-TYPE EGFR PROMOTES INVASION WHILE EGFRvIII DRIVES PROTOTYPICAL SFK c-SRC ACTIVATION TO FOSTER ANGIOGENESIS

EGFR gene amplification is observed in upwards of 40% of glioblastoma (GBM) biopsies, and genetic aberration to EGFR at large has been described to occur in excess of 50%. Our lab has developed a clinically relevant in vivo xenograft model derived from patient biopsies in which we are able to accurately study EGFR signaling in human GBM. We previously reported that wild-type EGFR amplification and activation promotes invasion and development of GBM independent of angiogenesis. Importantly, detected in half of GBMs harboring EGFR amplification, the most prominent EGFR mutation arises upon deletion of exons 2 through 7 to generate the constitutively active truncation mutant EGFRvIII. Here we report that while wild-type EGFR overexpression sustains invasion, EGFRvIII, in contrast, induces more aggressive and angiogenic tumor growth. We further performed signaling complex immunocapture mass spectrometry which revealed that EGFRvIII-mediated signaling is divergent from that of its wild-type counterpart. In particular the non-receptor tyrosine kinase c-SRC was highly upregulated and activated in EGFRvIII tumors as verified on western blots and by immunohistochemistry. Inhibition of c-SRC activity by shRNA and Src inhibitors decreased VEGF levels in EGFRvIII tumor cells. Furthermore, EGFRvIII+ xenograft tumors expressing c-SRC shRNA became less angiogenic and aggressive - indeed similar to control tumors lacking EGFRvIII. When overexpressing constitutively activated Src in control tumor cells, we could recapitulate the oncogenic phenotype of EGFRvIII as the tumor cells secreted higher levels of VEGF and became more aggressive and angiogenic in vivo. We hereby report evidence that the EGFRvIII-driven oncogenic activity and angiogenic tumor growth is mediated by c-SRC. In conclusion, our results suggest that the EGFRvIII-SRC-VEGF axis represents an important mechanism of tumor angiogenesis in GBM.