Abstract B37: Tumor heterogeneity contributes to anti-EGFR therapy resistance in glioblastoma

Abstract

Abstract Introduction: Glioblastoma is the most aggressive tumor type affecting the adult brain, with an overall patient survival from the time of diagnosis of about 15 months. Tumor promoting proteins such as EGFR (Epidermal Growth Factor Receptor) and its mutant form, EGFRvIII, are amplified in a subgroup of patients and have attracted attention as potential therapeutic targets. Unfortunately, kinase inhibition of EGFR or EGFRvIII has been proven to be ineffective due to resistance mechanisms that prevent the onset of cell death. Here we hypothesize that the presence of cytokines in the tumor microenvironment, in addition to their previously shown promotion of glioblastoma growth, also promote resistance to EGFR-directed therapies. To address this, we investigated core pathways known to protect cancer cells from induced cell death and tested novel treatment strategies of EGFR blockade in combination with inhibitors of cytokine-induced signaling effectors, activated by inter-clonal communication between heterogeneous tumor cell populations expressing EGFRvIII and EGFR. Experimental procedures: For these studies we used human and mouse glioma cell lines engineered to express wild type (wt) EGFR, EGFRvIII, or an analog-sensitive (as)-allele of EGFRvIII, as well as primary human glioma neurospheres expressing amplified EGFRvIII. Expression of cell death-related molecules was determined by RT-qPCR to identify potential survival factors regulated by conditioned medium (CM) produced from EGFRvIII-expressing glioma cells. Findings were validated in vivo by orthotopic engraftment of heterogeneous tumors and measurement of tumor burden by fluorescence molecular tomography technology during single and combined drug therapy regimens. Results and discussion: Previous results demonstrated that CM from EGFRvIII-expressing glioma cells enhances in vitro and in vivo growth of wtEGFR cells through an EGFRvIII-prompted IL-6/LIF paracrine mechanism. Here, our data indicate that EGFRvIII-CM additionally promotes resistance to EGFR inhibition in cells and tumors expressing wtEGFR. We show that IL-6/gp130-mediated signaling specifically promotes this resistance through mTORC2/NF-κB upregulation of the anti-apoptotic protein, Survivin (BIRC5). Blocking Survivin expression or pharmacologically inhibiting mTORC2 restores sensitivity to EGFR inhibitors. Conclusion: The mTORC2/NF-κB/Survivin axis promotes resistance to EGFR inhibitors in glioblastoma and nominates targeting this pathway as a means to enhance efficacy of EGFR blockade. Citation Format: Ciro Zanca, Webster Cavenee, Frank Furnari. Tumor heterogeneity contributes to anti-EGFR therapy resistance in glioblastoma. [abstract]. In: Proceedings of the AACR Special Conference: Advances in Brain Cancer Research; May 27-30, 2015; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2015;75(23 Suppl):Abstract nr B37.