Overcoming EZH2 Inhibitor Resistance by Taxane in PTEN-Mutated Cancer

Rationale: The Polycomb group (PcG) protein EZH2 is implicated in cancer progression due to its frequent overexpression in many cancer types and therefore is a promising therapeutic target. Forkhead box transcription factor-1 (FOXO1) is a tumor suppressor that is often transcriptionally downregulated in human cancers such as prostate cancer although the underlying regulatory mechanisms remain elusive. Methods: Analysis of EZH2 ChIP-seq and ChIP-on-chip data in various cell types was performed. ChIP-qPCR, RT-qPCR, and western blot analyses were conducted to determine the mechanism by which EZH2 represses FOXO1 expression. Immunohistochemistry was employed to assess the correlation between EZH2 and FOXO1 protein expression in prostate cancer patient specimens. In vitro MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) and animal experiments were performed to determine the anti-cancer efficacy of EZH2 inhibitor alone or in combination of docetaxel, a chemotherapy agent of the taxane family, and dependency of the efficacy on FOXO1 expression. Results: We demonstrated that EZH2 binds to the FOXO1 gene promoter. EZH2 represses FOXO1 gene expression at the transcriptional level. EZH2 protein level inversely correlated with FOXO1 protein expression in prostate cancer patient specimens. This repression requires the methyltransferase activity and the functional PRC2 complex. While effectively inducing loss of viability of PTEN-positive 22Rv1 prostate cancer cells, EZH2 inhibitor failed to inhibit growth of PTEN-negative C4-2 prostate cancer cells. Co-treatment with docetaxel overcame EZH2 inhibitor resistance in PTEN-negative cancer cells in vitro and in mice. This effect was largely mediated by docetaxel-induced nuclear localization and activation of FOXO1. Conclusions: This study identifies FOXO1 as a bona fide repression target of EZH2 and an essential mediator of EZH2 inhibition-induced cell death. Our findings suggest that EZH2 repression of FOXO1 can be targeted by EZH2 inhibitor as a monotherapy for PTEN-proficient cancers or in combination with taxane for treatment of cancers with PTEN mutation or deletion.