Abstract 923: FOXA1 chromatin binding is regulated by LSD1-mediated methylation in prostate cancer cells

Androgen receptor (AR) plays a critical function in driving prostate cancer (PCa) development and androgen deprivation therapy (ADT) is standard of care in treating PCa patients. Although patients generally response well, the cancer eventually relapse and progress to the castration-resistant stage of PCa (called CRPC), which can be further treated with more aggressive ADT. However, the tumors still develop resistance within a year and restored AR activity is found in the majority of relapsed tumors, indicating a pressing need for developing novel AR-targeted therapies. In PCa cells, AR activity is dependent on the chromatin binding of the pioneer factor, FOXA1. In particular, FOXA1 recognizes the H3K4me1,2 marked enhancers and its binding opens the chromatin for subsequent AR recruitment. Although the role of FOXA1 on AR signaling has been intensively studied, the mechanisms regulating FOXA1 binding to chromatin remain elusive. LSD1/KDM1A was initially identified as a transcription repressor through demethylation of H3K4me1,2. In contrast to its well-established corepressor function, LSD1 has been found to coactivate several transcription factors including AR. This AR coactivator function has been attributed to the phosphorylation of histone 3 on threonine 6 and threonine 11, which may switch LSD1 substrate specificity from H3K4me1,2 to the repressive mark H3K9me1,2. However, we have recently reported that the H3K4 demethylase activity of LSD1 persists at AR-regulated enhancer sites, including sites marked by H3T6ph, arguing against the coactivator function of LSD1 being generally controlled by this mechanism. Importantly, we have shown in a recent study that LSD1 associates with FOXA1 and active enhancer markers, and that LSD1 silencing impaired FOXA1 binding at AR-regulated enhancers, suggesting a role of LSD1 in regulating enhancer availability to AR. In the current study, we found that LSD1 inhibition globally disrupts FOXA1 chromatin binding prior to androgen stimulation and thus impairs further AR recruitment, resulting in the global inhibition of AR transcriptome. Using immunoprecipitation and mass-spectrometry assays, we discovered that LSD1 maintains FOXA1 binding by demethylating lysine K270, adjacent to its DNA binding domain. We further found that the expression of a methylation-deficient FOXA1 mutant (K270R) stabilizes AR chromatin binding and allows PCa cells resistant to enzalutamide treatment. Using a clinical tested LSD1 inhibitor, we have shown in vivothat LSD1 inhibition dramatically decreases PCa growth alone and in synergy with enzalutamide in AR/AR variants-positive CRPC models. These findings provide fundamental new insights into FOXA1, LSD1, and mechanisms that regulate the activity of AR in PCa cells. As LSD1 inhibitors are moving into the clinic, these new insights into LSD1 function strongly support LSD1 as a therapeutic target in CRPC. Citation Format: Shuai Gao, Sujun Chen, Dong Han, Wanting Han, Jude N. Owiredu, Steven P. Balk, Housheng H. He, Changmeng Cai. FOXA1 chromatin binding is regulated by LSD1-mediated methylation in prostate cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 923.