Abstract 5545: Deguelin selectively inhibits proliferation of luminal androgen receptor (LAR) triple-negative breast cancer cells

Triple negative breast cancers (TNBCs) lack expression of the estrogen and progesterone receptors (ER/PR) and HER2 amplification. These aggressive malignancies have higher mortality rates than other breast cancer subtypes. While targeted therapies for ER+/PR+ and HER2-amplified breast cancers have greatly improved survival for these patients, there are no approved targeted therapies for TNBCs. Development of targeted therapies for TNBC will likely provide a similar survival benefit over traditional cytotoxic therapy. The identification of molecular targets for TNBC has been limited without a better understanding of the subtypes of these heterogeneous diseases. Gene expression profiling recently identified 6 defined subtypes of TNBC and representative cell lines, providing the first opportunity to identify subtype-specific leads for TNBC. In this study, we used high-content phenotypic screening to evaluate novel libraries of plant and fungal extracts for antiproliferative and cytotoxic activity in a panel of cell lines modeling the different TNBC subtypes. The aim was to identify extracts with selective activity in a single cell line with the hypothesis that the selectivity resulted from a target relevant specifically to that subtype. A plant extract selectively inhibited proliferation of MDA-MB-453 cells, which represent the luminal androgen receptor (LAR) subtype. Bioassay-guided fractionation identified deguelin, which showed potent and highly selective activity in MDA-MB-453 cells compared to all the other TNBC cell lines, with 240 to 1000-fold higher potency. Further studies investigated the molecular mechanisms of action of degeulin in MDA-MB-453 cells. Based on the sensitivity of LAR TNBCs to both antiandrogens and Hsp90 inhibitors, as well as prior studies demonstrating Hsp90 inhibition by deguelin, we hypothesized that deguelin9s mechanism in MDA-MB-453 cells involves Hsp90 inhibition, resulting in AR destabilization and reduced proliferation. Immunoblotting of deguelin-treated MDA-MB-453 whole-cell lysates indicated that deguelin reduced cellular levels of AR, which are noticeable 8 h after treatment. Immunofluorescence microscopy studies also suggested reduced nuclear localization of AR 18 h after treatment. Previous studies showed that LAR TNBC cells are highly sensitive to PI3K inhibitors compared to other TNBC subtypes, and thus the effects of deguelin on PI3K-Akt signaling were evaluated in MDA-MB-453 cells. Immunoblotting indicated that deguelin reduced the relative phosphorylation of Akt at T308, but had no effect on phosphorylation of S473, suggesting inhibition of PI3K but not mTORC2. Ongoing studies are aimed at better characterizing the mechanisms of deguelin in MDA-MB-453 cells with the goal of identifying targets to develop better therapies for LAR TNBC patients. Citation Format: Andrew J. Robles, Shengxin Cai, Robert H. Cichewicz, Susan L. Mooberry. Deguelin selectively inhibits proliferation of luminal androgen receptor (LAR) triple-negative breast cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5545. doi:10.1158/1538-7445.AM2015-5545