P17.30 * ONCOGENIC EGFRVIII DRIVES PROTOTYPICAL SFK C-SRC ACTIVATION TO PROMOTE GLIOBLASTOMA ANGIOGENESIS

Abstract

Displaying both invasive and angiogenic tumor growth, glioblastoma (GBM) is characterized as the most aggressive primary adult brain tumor. And while a number of molecules discerning these growth modalities have been described, the mechanisms driving the pathological growth remain an enigma. Among the most clinically relevant, and perhaps also the most cryptic, is signaling generated by activation of the epidermal growth factor receptor (EGFR) tyrosine kinase. EGFR gene amplification is observed in upwards of 40% of GBM biopsies, and genetic aberration to EGFR at large has been described to occur in excess of 60%. Our lab has developed a clinically relevant in vivo model 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. Expression analysis in combination with signaling complex immunocapture mass spectrometry motivated our hypothesis that EGFRvIII-mediated signaling is divergent from that of its wild-type counterpart. To investigate this, we performed the following animal experiments in accordance with The Norwegian Animal Research Authority's regulations for animal research. We implanted patient GBM tumor cells stably overexpressing EGFRvIII or oncogenic c-Src into the brains of immunodeficient rats. Compared with corresponding xenograft control tumors, those overexpressing EGFRvIII or oncogenic c-Src were substantially more aggressive. In addition, the tumors displayed increased contrast enhancement on MRI, enhanced histological features of angiogenesis, and upregulation of molecular markers for tumor angiogenesis on western blot. Patient GBM cells stably overexpressing wild-type EGFR, meanwhile, failed to promote angiogenesis and instead contributed to a more infiltrative phenotype consistent with our previous findings. Furthermore, tumors overexpressing EGFRvIII and simultaneously depleted of c-Src using specific shRNA were less aggressive and non-angiogenic. We hereby report evidence that EGFRvIII-driven c-Src protein upreguation and subsequent activation intensifies the malignancy of human GBM by facilitating aggressive tumor growth and promoting vascular endothelial growth factor (VEGF) mediated angiogenesis. In conclusion, our results suggest that the EGFRvIII-SRC-VEGF pathway represents a meaningful target in the therapeutic intervention of primary GBM.