Tamoxifen, an estrogen receptor (ER) antagonist, is the mainstay treatment of breast cancer and the development of resistance represents a major obstacle for a cure. Although lncRNAs such as HOTAIR have been implicated in breast tumorigenesis, their roles in chemotherapy resistance remain largely unknown. In this study, we report that HOTAIR is up-regulated in tamoxifen-resistant breast cancer tissues compared to their primary counterparts. Mechanistically, HOTAIR is a direct target of ER-mediated transcriptional repression and is thus restored upon the blockade of ER signaling, either by hormone deprivation or tamoxifen treatment. Interestingly, this elevated HOTAIR increases ER protein level and thus enhances ER occupancy on the chromatin and potentiates its downstream gene regulation. HOTAIR overexpression is sufficient to activate the ER transcriptional program even under hormone-deprived conditions. Functionally, we found that HOTAIR overexpression increases breast cancer cell proliferation, whereas its depletion significantly impairs cell survival and abolishes tamoxifen-resistant cell growth. In conclusion, the lncRNA HOTAIR is directly repressed by ER and its up-regulation promotes ligand-independent ER activities and contributes to tamoxifen resistance.

SUMMARY Understanding the mechanisms of androgen receptor (AR) activation in the milieu of low androgen is critical to effective treatment of castration-resistant prostate cancer (CRPC). Here, we report HOTAIR as an androgen-repressed lncRNA, and, as such, it is markedly upregulated following androgen deprivation therapies and in CRPC. We further demonstrate a distinct mode of lncRNA-mediated gene regulation, wherein HOTAIR binds to the AR protein to block its interaction with the E3 ubiquitin ligase MDM2, thereby preventing AR ubiquitination and protein degradation. Consequently, HOTAIR expression is sufficient to induce androgen-independent AR activation and drive the AR-mediated transcriptional program in the absence of androgen. Functionally, HOTAIR overexpression increases, whereas HOTAIR knockdown decreases, prostate cancer cell growth and invasion. Taken together, our results provide compelling evidence of lncRNAs as drivers of androgen-independent AR activity and CRPC progression, and they support the potential of lncRNAs as therapeutic targets.

The pioneering factor FOXA1 opens chromatin to facilitate androgen receptor (AR) binding to prostate-specific genes. How FOXA1 controls the AR cistrome, however, is incompletely understood. Here we show that AR directly binds chromatin through the androgen-response elements (AREs). FOXA1 is not required for AR-chromatin interaction, but instrumental in recruiting AR to low-affinity half-AREs by opening local chromatin around adjacent FKHD sites. Too much FOXA1 creates excessive open chromatin regions, which serve as reservoirs that retain AR via abundant half-AREs, thereby reducing its availability for specific sites. FOXA1 down-regulation, by contrast, relinquishes AR to permissively bind AREs across the genome, resulting in substantial AR binding events and AR-target gene expression even in the absence of androgen. Taken together, our data illustrate the mechanistic details by which cooperativity and equilibrium with FOXA1 define AR cistrome and reveal a previously unknown function of FOXA1 in inhibiting AR signaling and castration-resistant prostate cancer growth.

SUMMARY Enhancer of Zeste 2 (EZH2) is the enzymatic subunit of Polycomb Repressive Complex 2 (PRC2), which catalyzes histone H3 lysine 27 trimethylation (H3K27me3) at target promoters for gene silencing. Here, we report that EZH2 activates androgen receptor (AR) gene transcription through direct occupancy at its promoter. Importantly, this activating role of EZH2 is independent of PRC2 and its methyltransferase activities. Genome-wide assays revealed extensive EZH2 occupancy at promoters marked by either H3K27ac or H3K27me3, leading to gene activation or repression, respectively. Last, we demonstrate enhanced efficacy of enzymatic EZH2 inhibitors when used in combination with AR antagonists in blocking the dual roles of EZH2 and suppressing prostate cancer progression in vitro and in vivo. Taken together, our study reports EZH2 as a transcriptional activator, a key target of which is AR, and suggests a drug-combinatory approach to treat advanced prostate cancer.

Neuroendocrine prostate cancer (NEPC) has increasingly become a clinical challenge. The mechanisms by which neuroendocrine (NE) cells arises from prostate adenocarcinoma cells are poorly understood. FOXA1 is a transcription factor of the forkhead family that is required for prostate epithelial differentiation. In this study, we demonstrated that FOXA1 loss drives neuroendocrine (NE) differentiation, demarcated by phenotypical changes and NEPC marker expressions. Mechanistically, this is mediated by FOXA1 binding to the promoter of IL-8, a chemokine previously shown elevated in NEPC, to directly inhibit its expression. Further, IL-8 up-regulation activates the MAPK/ERK pathway, leading to ERK phosphorylation and ENO2 expression. IL-8 knockdown or ERK inhibition, on the other hand, abolished FOXA1 loss-induced NE differentiation. Analysis of xenograft mouse models confirmed FOXA1 loss in NEPC tumors relative to its adenocarcinoma counterparts. Importantly, FOXA1 is down-regulated in human NEPC tumors compared to primary and castration-resistant prostate cancers, and its expression is negatively correlated with that of ENO2. These findings indicate that FOXA1 transcriptionally suppresses IL-8, the expression of which would otherwise stimulate MAPK/ERK pathway to promote NE differentiation of prostate cancer cells. Our data strongly suggest that FOXA1 loss may play a significant role in enabling prostate cancer progression to NEPC, while IL-8 and MAPK/ERK pathways may be promising targets for therapeutic intervention.