FOXA1 inhibits hypoxia programs through transcriptional repression of HIF1A.

Intratumoral hypoxia is associated with castration-resistant prostate cancer (CRPC), a lethal disease. FOXA1 is an epithelial transcription factor that is down-regulated in CRPC. We have previously reported that FOXA1 loss induces epithelial-mesenchymal transition (EMT) and cell motility through elevated TGFß signaling. However, whether FOXA1 directly regulates hypoxia pathways of CRPC tumors has not been previously studied. Here we report that FOXA1 down-regulation induces hypoxia transcriptional programs, and FOXA1 level is negatively correlated with hypoxia markers in clinical prostate cancer (PCa) samples. Mechanistically, FOXA1 directly binds to an intragenic enhancer of HIF1A to inhibit its expression, and HIF1A, in turn, is critical in mediating FOXA1 loss-induced hypoxia gene expression. Further, we identify CCL2, a chemokine ligand that modulates tumor microenvironment and promotes cancer progression, as a crucial target of the FOXA1-HIF1A axis. We found that FOXA1 loss leads to immunosuppressive macrophage infiltration and increased cell invasion, dependent on HIF1A expression. Critically, therapeutic targeting of HIF1A-CCL2 using pharmacological inhibitors abolishes FOXA1 loss-induced macrophage infiltration and PCa cell invasion. In summary, our study reveals an essential role of FOXA1 in controlling the hypoxic tumor microenvironment and establishes the HIF1A-CCL2 axis as one mechanism of FOXA1 loss-induced CRPC progression.

Going beyond Polycomb: EZH2 functions in prostate cancer.

The Polycomb group (PcG) protein Enhancer of Zeste Homolog 2 (EZH2) is one of the three core subunits of the Polycomb Repressive Complex 2 (PRC2). It harbors histone methyltransferase activity (MTase) that specifically catalyze histone 3 lysine 27 (H3K27) methylation on target gene promoters. As such, PRC2 are epigenetic silencers that play important roles in cellular identity and embryonic stem cell maintenance. In the past two decades, mounting evidence supports EZH2 mutations and/or over-expression in a wide array of hematological cancers and solid tumors, including prostate cancer. Further, EZH2 is among the most upregulated genes in neuroendocrine prostate cancers, which become abundant due to the clinical use of high-affinity androgen receptor pathway inhibitors. While numerous studies have reported epigenetic functions of EZH2 that inhibit tumor suppressor genes and promote tumorigenesis, discordance between EZH2 and H3K27 methylation has been reported. Further, enzymatic EZH2 inhibitors have shown limited efficacy in prostate cancer, warranting a more comprehensive understanding of EZH2 functions. Here we first review how canonical functions of EZH2 as a histone MTase are regulated and describe the various mechanisms of PRC2 recruitment to the chromatin. We further outline non-histone substrates of EZH2 and discuss post-translational modifications to EZH2 itself that may affect substrate preference. Lastly, we summarize non-canonical functions of EZH2, beyond its MTase activity and/or PRC2, as a transcriptional cofactor and discuss prospects of its therapeutic targeting in prostate cancer.

Posttranslational regulation of FOXA1 by Polycomb and BUB3/USP7 deubiquitin complex in prostate cancer.

Forkhead box protein A1 (FOXA1) is essential for androgen-dependent prostate cancer (PCa) growth. However, how FOXA1 levels are regulated remains elusive and its therapeutic targeting proven challenging. Here, we report FOXA1 as a nonhistone substrate of enhancer of zeste homolog 2 (EZH2), which methylates FOXA1 at lysine-295. This methylation is recognized by WD40 repeat protein BUB3, which subsequently recruits ubiquitin-specific protease 7 (USP7) to remove ubiquitination and enhance FOXA1 protein stability. They functionally converge in regulating cell cycle genes and promoting PCa growth. FOXA1 is a major therapeutic target of the inhibitors of EZH2 methyltransferase activities in PCa. FOXA1-driven PCa growth can be effectively mitigated by EZH2 enzymatic inhibitors, either alone or in combination with USP7 inhibitors. Together, our study reports EZH2-catalyzed methylation as a key mechanism to FOXA1 protein stability, which may be leveraged to enhance therapeutic targeting of PCa using enzymatic EZH2 inhibitors.