Abstract P3-04-06: Inhibiting androgen receptor nuclear localization decreases estrogen receptor (ER) activity and tumor growth in ER+ breast cancer

Background: Androgen receptor (AR) is widely expressed in breast tumors, but the role of AR in estrogen receptor (ER)+ tumors is controversial. In the absence of estradiol (E2), dihydrotestosterone (DHT) increases growth of ER+ breast cancer cells in vitro and in vivo. Anti-androgens such as bicalutamide (Bic) and enzalutamide (ENZ) inhibit this DHT-mediated proliferation. Surprisingly we have found that ENZ, which impairs nuclear entry of liganded AR, also inhibits E2-mediated proliferation of ER+ breast cancer cells, while Bic does not. Hypothesis: We hypothesize that nuclear localization of AR is necessary for maximal E2-mediated proliferation in ER+/AR+ breast cancer cells, and targeting AR with ENZ or other agents that impede AR nuclear entry or cause AR degradation will inhibit growth of ER+/AR+ human breast cancer cell lines and decrease tumor burden in preclinical models. Methods: ER+/AR+ MCF7, BCK4, and ZR-75-1 cells were treated with E2 plus or minus ER and AR antagonists and proliferation was measured by crystal violet staining or Incucyte live cell imaging. Nuclear AR was assessed by immunocytochemistry or nuclear/cytoplasmic fractionation. For in vivo experiments, 1x10^6 luciferase-expressing MCF7 cells were injected into the mammary fat pad of nu/nu mice and tumor growth monitored by caliper and IVIS imaging. Results: ENZ blocked E2-induced proliferation and showed synergistic activity with the ER antagonists 4-hydroxy-Tamoxifen (OH-Tam) and Fulvestrant (Fulv) in vitro. E2-induced expression of ER target genes including PR and SDF1 was also inhibited by ENZ, but not by Bic. Similarly, AR knockdown decreased baseline and E2-induced proliferation of MCF7 cells and E2-induced ER target gene expression. Both DHT and E2 treatment induced nuclear translocation of AR, which was decreased by ENZ. Nuclear translocation of AR in response to E2 occurs only in ER+ cell lines, further supporting a role for nuclear AR in E2-induced ER activity. In vivo, ENZ inhibited E2-induced growth of MCF7 tumors as effectively as Tamoxifen (Tam), and the combination of ENZ plus Tam was more effective than either drug alone. ENZ also inhibited growth of Tam-resistant MCF7 cells in vitro. Conclusions: Our results suggest that nuclear localization of AR plays a previously-unrecognized role in E2-mediated ER activity in ER+/AR+ breast cancer cells. Because of its ability to inhibit nuclear entry of liganded AR, ENZ may serve as an effective therapeutic in ER+/AR+ breast cancers. Importantly, ENZ may be particularly useful in combination with current anti-estrogen therapies (Tam or Fulv) since it affects ER, but via an indirect mechanism acting through AR. ENZ may also be effective in tumors resistant to ER-directed therapy based on in vitro data and published clinical data indicating that many such tumors express more AR protein than ER. Funded by: DOD BCRP Clinical Translational Award BC120183 to JKR, American Cancer Society Postdoctoral Fellowship PF-13-314-01 – CDD to NCD. Citation Format: Nicholas C D9Amato, Britta M Jacobsen, Dawn R Cochrane, Nicole S Spoelstra, Beatrice L Babbs, Anthony Elias, Jennifer K Richer. Inhibiting androgen receptor nuclear localization decreases estrogen receptor (ER) activity and tumor growth in ER+ 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 P3-04-06.