Abstract 2317: Targeting prostate cancer super enhancers by co-inhibiting LSD1 and BRD4

In prostate cancer (PCa) cells, “pioneer” factor FOXA1 resides at distal enhancers enriched with histone 3 lysine 4 methylation (H3K4me) and lysine 27 acetylation (H3K27ac). FOXA1 is capable of remodeling the compact chromatin to facilitate androgen receptor (AR) recruitment. Recent studies revealed overexpression of FOXA1 caused by frequent structural alterations in the lethal stage of PCa, namely castration-resistant PCa(CRPC), suggesting FOXA1 as a critical target in CRPC. However, a lack of targetable domain hinders the effective targeting of FOXA1. Our recent (epi)genomic profiling study revealed that lysine-specific demethylase (LSD1) and FOXA1 co-occupy at active enhancers in PCa cells and LSD1 demethylase activity is required for FOXA1 chromatin binding. LSD1 inhibition can quickly and drastically reduce FOXA1 global binding and concurrently impair the “openness” of enhancers, leading to disrupted AR cistrome. Mechanistically, we demonstrated that this activator function of LSD1 is mediated through directly demethylating K270 of FOXA1. By testing an array of adenocarcinoma CRPC in vivo models, we further showed that the efficacy of LSD1 inhibitors may be correlated with FOXA1 levels. Collectively, these results provide the molecular reasoning on co-targeting the epigenetic regulators that are important for promoting AR activity in treating CRPC. Furthermore, we have found that LSD1, FOXA1, and BRD4 (a Bromodomain and Extra-Terminal motif (BET) protein) co-occupy the super enhancers characterized by distinguishable high levels of H3K4me2 and H3K27ac in PCa cells. Super enhancers have been found to drive oncogenic transcription program in PCa. As an active enhancer marker and an important AR coactivator, BRD4 recognizes H3K27ac signal and promotes the initiation and elongation steps in gene transcription process. Herein, we focus on the potential synergism of combined targeting of LSD1 and BRD4. We found that FOXA1 interacted with BRD4 in different cell lines and patient-derived xenograft (PDX) tumor tissues. In line with this association, either LSD1 inhibitors or BET inhibitors could decrease FOXA1 and BRD4 chromatin binding. Characterization of the physical interaction is an ongoing study. We found that the combinational treatment lead to a more effective reduction of AR transcriptional activity than either single treatment. The PCa cell proliferation in vitro was also more profoundly inhibited by combined therapy. More importantly, we demonstrated this synergism in both 22RV1 xenograft and PDX LuCaP35CR in the castrated mice, with a lower dose of LSD1 inhibitor and BET inhibitor (both in clinical trials) which did not induce overt toxicity in the animals. Currently, we are performing RNA-seq studies to identify the convergent downstream pathways by this co-treatment. In summary, our study provides important pre-clinical insights for targeting the CRPC enhancers by combining inhibitors of LSD1 and BRD4. Citation Format: Shuai Gao, Muqing Li, Mingyu Liu, Dong Han, Zifeng Wang, Wanting Han, Changmeng Cai. Targeting prostate cancer super enhancers by co-inhibiting LSD1 and BRD4 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2317.