Abstract 1007: The Redox/Fyn/c-Cbl pathway and its interaction with Cool-1: A novel pathway that regulates chemosensitivity in glioblastoma

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL Glioblastomas (GBMs) are the most common primary brain tumor, accounting for 52 percent of all diagnosed brain tumors. They are also the most devastating of all brain tumors, with a five-year survival rate of 3.3 percent. One of the reasons for this poor prognosis is resistance to treatment with chemotherapy. We have discovered a novel means by which GBMs become chemoresistant, based on inhibition of the redox/Fyn/c-Cbl pathway by overexpression of Cool-1. The protein c-Cbl, a receptor tyrosine kinase adaptor protein and E3 ubiquitin ligase, is responsible for the ubiquitination and degradation of epidermal growth factor receptor (EGFR) and other receptor tyrosine kinases critical for cell division and cell survival. c-Cbl is activated by Fyn kinase, which itself can be activated by pro-oxidants. In normal glial progenitor cells of the CNS, exposure to chemotherapy oxidizes cells, leading to sequential activation of Fyn and c-Cbl. With GBM cells, in contrast, exposure to BCNU or to chemical pro-oxidants does not lead to c-Cbl activation. Further studies have demonstrated that these agents do cause Fyn activation, suggesting that GBMs inhibit Fyn-mediated c-Cbl phosphorylation. This would lead to decreased degradation of EGFR, thus enabling continued cell proliferation and providing resistance to chemotherapy. We therefore tested the hypothesis that reversal of this inhibition would decrease cell division and enhance susceptibility to chemotherapy-induced cell death. In this work we discovered that oxidant-associated c-Cbl activation can be restored by suppressing expression of the protein Cool-1, which is over-expressed in many cancers including GBM. Our data further demonstrate that Cool-1 and c-Cbl interact, that genetic reduction in Cool-1 levels with small inhibitory RNAs restores normal c-Cbl-mediated degradation of RTKs (including EGFR) in response to exposure to pro-oxidants and BCNU, and that restoration of normal c-Cbl activation also increases sensitivity to this and other chemotherapeutic agents. The translation of these findings into an in vivo xenograft model shows that reduction in Cool-1 levels leads to decreased tumor take, decreased tumor growth, and increased survival. Specific pharmacological inhibitors of Cool-1 are not available and it was therefore necessary to explore other means of chemically inhibiting Cool-1 activation. We have data suggesting that pharmacological inhibition of Focal Adhesion Kinase leads to decreases in Cool-1 activation and a rescue of oxidant-induced c-Cbl activation and subsequent RTK degradation. Together these results support c-Cbl and Cool-1's roles in tumor development and increased resistance to chemotherapeutics. Rescuing the activation of c-Cbl mediated RTK degradation through Cool-1 inhibition is a novel method of treating GBMs that increases both sensitivity to chemotherapeutics and animal survival. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1007. doi:10.1158/1538-7445.AM2011-1007