Abstract 4106: Cool-1-mediated inhibition of c-Cbl as a therapuetic target, which modulates multiple critical properties of glioblastomas.
Authors: Stevens Brett M. , Folts Christopher J. , Chen Warner , Bardin Addie L. , Noble Mark 
Published: August 2014

Abstract
Glioblastomas (GBMs), the most malignant glial tumors, differ from normal glial progenitor cells in many ways, but little is known about the molecular circuitry underlying these differences.
We have discovered a novel means by which GBMs become chemo-resistant, based on inhibition of the redox/Fyn/c-Cbl pathway by overexpression of Cool-1(Beta-Pix). The protein c-Cbl, an E3 ubiquitin ligase, is responsible for the ubiquitination and degradation of multiple receptor tyrosine kinases critical for cell division and cell survival. In normal glial progenitor cells of the CNS, exposure to chemotherapy oxidizes cells, leading to sequential activation of Fyn and c-Cbl. However in GBM cells, exposure to BCNU or other chemical pro-oxidants does not lead to c-Cbl activation. Further studies demonstrated that these agents do cause Fyn activation, suggesting that GBMs inhibit c-Cbl phosphorylation leading to decreased degradation of EGFR, accounting for increased EGFR signaling. We next discovered that the decrease in c-Cbl activation via the redox/Fyn/c-Cbl pathway was due to c-Cbl sequestration by Cool-1, which is overexpressed in GBM cells and samples of tumors. We found that oxidant-associated c-Cbl activation can be restored by shRNA-mediated inhibition of Cool-1 expression, leading to restoration of normal c-Cbl-mediated degradation of RTKs (including EGFR). We further found that restoration of Cool-1 activity modulated multiple other hallmarks of GBM, including changes in migration, cell cycle parameters, and altering the cancer stem cell phenotype of our cells. In contrast, Cool-1/c-Cbl complexes were not found in normal brain or normal glial progenitors and Cool-1 knockdown did not reduce progenitor cell division. The translation of these findings into an in vivo intracranial xenograft model of GBM showed that Cool-1 is essential for tumorigenesis and decreasing Cool-1 levels leads to decreased tumor take, decreased tumor growth, and increased survival.
In sum, the ability of Cool-1/c-Cbl interactions to modulate a variety of tumor cell properties suggests that these interactions offer an attractive target for modulating GBM growth, particularly due to the absence of Cool-1/c-Cbl complexes in normal brain tissue and to the lack of effects of Cool-1 knockdown on division of normal glial progenitor cells. The activation of c-Cbl activity through pharmacological agents is a potentially viable strategy for the specific eradication of GBM cells.
Citation Format: Brett M. Stevens, Christopher J. Folts, Warner Chen, Addie L. Bardin, Mark Noble. Cool-1-mediated inhibition of c-Cbl as a therapuetic target, which modulates multiple critical properties of glioblastomas. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4106. doi:10.1158/1538-7445.AM2013-4106