Abstract P6-03-06: Therapeutic targeting of ERβ in triple negative breast cancer

Abstract

Abstract Background: While the biological functions and clinical importance of ERα are well understood in breast cancer, much less is known about its most closely related family member, ERβ, particularly in the setting of triple negative disease. Additionally, the ability to therapeutically target ERβ in triple negative breast cancer (TNBC) has not been fully explored. Methods: Expression of ERβ protein was determined using a well characterized and validated ERβ specific monoclonal antibody that only recognizes the full-length form of this receptor (PPG5/10) in a cohort of 71 TNBCs. To further define the biological functions of ERβ in TNBC, novel ERβ expressing triple negative cell lines (MDA-MB-231 and Hs578T) were developed and comprehensively characterized at the level of global gene expression profiling, modulation of important biological pathways, cellular proliferation and response to targeted therapies. Results: In TNBCs from 71 patients, nuclear and cytoplasmic ERβ was detected at moderate to high levels in 24% and 32% of cases respectively. This moderate to high expression of both nuclear and cytoplasmic ERβ was associated with higher levels of Ki67. Of the 17 tumors expressing ERβ, 13 (76%) were negative for androgen receptor expression. In the triple negative MDA-MB-231-ERβ and Hs578T-ERβ cell lines, expression of ERβ led to inhibition of proliferation in response to both estrogen and multiple ERβ specific agonists. Microarray analysis and RT-PCR profiling of these cells revealed that estrogen and ERβ agonists highly induced the expression of multiple cystatins, a family of small secreted cysteine protease inhibitors, while suppressing the expression of many interleukins. Conditioned media isolated from estrogen or ERβ agonist treated MDA-MB-231-ERβ cells inhibited the proliferation rates and blocked TGFβ signaling in non-ERβ expressing TNBC cells, effects that were completely reversed following depletion of cystatins from the conditioned media. Conclusions: ERβ is expressed in a substantial proportion of TNBCs, and most do not express the androgen receptor. In TNBC, where targeted therapies are lacking, our data suggest that estrogen or ERβ specific agonists would be expected to elicit anti-tumor effects when ERβ is expressed. These tumor suppressive effects of ERβ appear to be mediated in part through the actions of cystatins, and their inhibition of TGFβ signaling, suggesting that this family of secreted proteins may represent novel biomarkers for monitoring ERβ specific drug responsiveness and/or patient outcomes. Citation Format: Jordan M Reese, Malayannan Subramaniam, Vera J Suman, Xianglin Wu, Vivian Negron, Anne Gingery, Kevin S Pitel, Sejal S Shah, Heather E Cunliffe, Ann E McCullough, Barbara A Pockaj, Thomas C Spelsberg, Matthew P Goetz, James N Ingle, John R Hawse. Therapeutic targeting of ERβ in triple negative breast cancer [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P6-03-06.