Abstract 1812: Mechanism of telapristone acetate (CDB4124) on progesterone receptor action in breast cancer cells

Progesterone is a steroid hormone that plays an important role in the breast. Progesterone exerts its action through binding to Progesterone Receptor (PR), a transcription factor that belongs to the nuclear receptor family. Deregulation of the progesterone signaling pathway is implicated in the formation, development and progression of breast cancer. Next generation selective progesterone receptor modulators (SPRMs) have potent anti-progestin activity and are selective for PR, reducing the off-target effects on other nuclear receptors. To date, there is limited information on how the newer generation of SPRM9s such as Telapristone acetate (TPA), affect PR function at the molecular level. In this study we investigated the molecular mechanisms by which TPA antagonizes PR action in T47D breast cancer cells. First, we performed ChiP sequencing to obtain a global analysis of the effect of TPA on the PR cistrome. We observed that TPA decreases PR genome wide recruitment to the chromatin. Approximately 19,865 and 19,892 PR binding peaks were identified in the progestin, R5020 or R5020 + TPA data sets, respectively, with 83% (18,113) of overlapping PR binding regions. HOMER motif analysis indicated similar enriched motifs between R5020 and R5020+TPA groups, identifying progesterone response elements as the most significant motif. We validated our ChiP-seq data with PR ChIP by examining the promoter regions of known PR target genes. PR occupancy was decreased at these sites with the addition of TPA. In addition, TPA decreased R5020- driven expression of these genes. To further investigate the mechanism by which TPA inhibits PR transcriptional activity we performed Rapid immunoprecipitation mass spectrometry of endogenous proteins (RIME). TPA significantly influenced co-regulator recruitment to PR. After applying a stringent cut-off to our data set, 3 unique proteins were identified in the R5020+TPA group: TRPS1, LASP1 and AP1G1. TRPS1 has been shown to be a transcriptional corepressor and is overexpressed in majority of human breast cancer. Follow-up studies revealed an increase in the levels of PR-TRPS1 complex with TPA treatment. In addition, silencing TRPS1 with siRNA increased PR occupancy to the known PR gene promoters, suggesting that TRPS1 inhibited PR binding to these regions upon TPA treatment. In addition, knockdown of TRPS1 attenuated the inhibition of ACSL1, NOTCH2, PACSIN1 and GREB1 gene expression after TPA treatment. Taken together our results show that TPA decreases recruitment of PR to chromatin, recruits co-repressors such as TRPS1 to the PR complex thereby regulates PR target gene expression. This is the first study showing TRPS1 complexes with PR and regulates its activity. Ongoing studies are focused on the potential role of TRPS1 as a predictive biomarker of SPRM response in breast cancer. Citation Format: Batzaya Davaadelger, Seema A. Khan, J Julie Kim. Mechanism of telapristone acetate (CDB4124) on progesterone receptor action in breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1812.