Abstract 2912: Inhibition of redox/Fyn/c-Cbl pathway function by Cdc42 controls tumor initiation capacity and tamoxifen sensitivity in basal-like breast cancer cells.

Abstract

Two of the substantial challenges in cancer research are to identify means overcoming resistance of malignant cells to existing therapies and to eliminate the unique small population within bulk tumors, named tumor initiating cells or cancer stem cells which have been proposed to regenerate resistant relapsed tumors. In this study, we offer a novel therapeutic target relevant to both of these goals on basal-like (triple-negative) breast cancer (BLBC) cells, one of the most challenging cancers to treat with current regimens. Our findings emerged from examining the puzzling differences in regulation of activation of the E3 ubiqutin ligase c-Cbl in normal cells and in cancer cells by low microM concentrations of tamoxifen (TMX). In normal progenitor cells, TMX causes oxidation and sequential activation of Fyn kinase and the c-Cbl ubiquitin ligase. Activation of c-Cbl via this redox/Fyn/c-Cbl (RFC) pathway leads to accelerated degradation of receptor tyrosine kinases that are critical in cell survival and division (such as the epidermal growth factor (EGFR)). In BLBC cells, in contrast, TMX does not activate c-Cbl despite making the cells more oxidized and causing activation of Fyn. Moreover, BLBC cells exposed to TMX show no reduction in levels of EGFR. We found that TMX-induced activation of c-Cbl in BLBC cells was inhibited due to expression of Cdc42. Cdc42 sequestered c-Cbl, prevented its activation and prevented EGFR from being degraded. Restoration of c-Cbl function, by inhibiting activation of Cdc42, reduced EGFR levels in these cells. More critically, restoring c-Cbl function sensitized these cells to TMX both in vitro and in vivo through estrogen receptor-independent mechanisms. Analysis of tumor growth and formation in vivo showed that reducing the levels of Cdc42 reduced the size of tumors, increased sensitivity to TMX and decreased tumor forming capacity of these cells. The results provide a novel defense mechanism that BLBC cells utilize to prevent EGFR degradation, which may have high relevance to treatment of these tumors. Of particular importance is the ability of cdc42 to confer TMX sensitivity on these otherwise resistant tumor cells. Moreover, as use of low microM levels of TMX has been attempted for over a dozen different kinds of cancers, at least some of which express increased levels of Cdc42, this strategy may be relevant to the treatment of multiple different cancers. Citation Format: Hsing-Yu Chen, Yin Yang, Brett Stevens, Mark Noble. Inhibition of redox/Fyn/c-Cbl pathway function by Cdc42 controls tumor initiation capacity and tamoxifen sensitivity in basal-like breast cancer cells. [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 2912. doi:10.1158/1538-7445.AM2013-2912