Abstract LB-A14: Combinational therapy by targeting LSD1 and BRD4 in prostate cancer

As a nuclear transcriptional factor, androgen receptor (AR) plays a significant role in prostate cancer (PCa) development. Androgen deprivation therapy (ADT) blunting the AR activity is the mainstream strategy for treating PCa. Despite the initial effectiveness of ADTs, most cases will relapse to a lethal stage termed castration-resistant prostate cancer (CRPC), where AR regains activity and drives tumor growth. This unmet clinical challenge to further target AR has led to a number of clinical investigations of targeting some of the key components in AR signaling. Epigenetic regulators are promising targets as they are critical players for rewiring the AR network during PCa progression. Lysine specific demethylase 1 (LSD1), an important epigenetic modifier that is well known for its repressor function through demethylating histone 3 lysine 4 (H3K4), can also function as an activator. In PCa cells, our epigenomic profiling study showed that LSD1 resides at active chromatin regions marked by high H3K27ac and FOXA1. Further genome-wide studies showed that inhibiting LSD1 disrupts AR cistrome and impairs its transcriptional output. Our mechanistic studies show that the activator function of LSD1 is through controlling the chromatin binding of FOXA1, which is a critical pioneer factor for AR. Importantly, at the AR-regulated enhancers, we observed that LSD1 inhibition also caused a rapid and broad decrease of H3K27ac signal, which directs Bromodomain-containing protein 4 (BRD4) chromatin binding. BRD4 belongs to BET (bromodomain and extra terminal domain) family, which plays an essential role in promoting the initiation and elongation steps in gene transcription process. The most credited function of BRD4 is its regulation of oncogene c-MYC via binding to the highly active super enhancers. Moreover, the oncogenic function of BRD4 in PCa has been well attributed to its coactivator role with AR. Multiple inhibitors targeting BRD4 have been developed and shown early promising clinical outcomes, however the drug resistance also emerges in the preclinical models. Herein, we reason that targeting LSD1 and BRD4 together could further impair the AR action and achieve better anti-tumor effect. To examine whether there is a synergistic effect, we employed inhibitors that have been tested in clinical trials: ORY-1001 and I-BET762, which target LSD1 and BRD4 respectively. We found that the combinational treatment lead to a more effective reduction of AR transcriptional activity than either single treatment. The proliferation of AR-positive LNCaP PCa cell line and CWR22RV1 CRPC cell line which expresses high levels of C-terminal truncated AR splicing variants (AR-Vs) was also more profoundly inhibited by combined therapy. Besides disrupting the full-length AR cistrome, LSD1 inhibition also impairs the activity of AR-V7, a major form of AR-Vs that drives developing ADT resistance in CRPC. However, the function of BRD4 in regulating AR-V7 activity remains to be elucidated. Therefore, we tested the combined treatment in CWR22RV1 cells under hormone-depleted condition. By targeting LSD1 and BRD4 together, AR-V7 regulated gene expression was significantly reduced. Currently, we are investigating the genome-wide effect of targeting both epigenetic regulators on AR and AR-V7. To provide clinical evidence, we are testing this combined treatment in patient-derived xenograft models, which recapitulate the CRPC by harboring high levels of AR and AR-Vs and ETS gene translocation. Citation Format: Muqing Li, Dong Han, zifeng wang, shuai gao, Changmeng Cai. Combinational therapy by targeting LSD1 and BRD4 in prostate cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr LB-A14. doi:10.1158/1535-7163.TARG-19-LB-A14