Abstract 4534: Taxol therapy-induced catalytic EZH2 limits therapeutic responses in PTEN null prostate cancer

Abstract Prostate cancer (PCa) is the most common cancer and a major cause of cancer related deaths in men in North America. The phosphatase and tensin homolog gene (PTEN) is frequently inactivated in PCa. Approximately 40% of primary and 70% of metastatic PCa's have genomic alterations in the PI3K-signaling, mostly through the loss of PTEN. Majority of metastatic PCa associated with PTEN loss are treated with docetaxel; an FDA approved therapy for recurrent PCa. However, in PTEN null PCa docetaxel treatment enhances senescence and does not significantly impact tumor volume. These findings were further confirmed in PCa patients with decreased levels of PTEN. In order to understand the global impact of docetaxel in PTEN null PCa we performed an unbiased RNA-seq of prostate tissues harvested from docetaxel treated prostate specific PTEN null mice. Data analysis showed strong enrichment of chromatin modifying enzymes reactome, mainly PRC2 as one of the top ranked categories. Among them EZH2, an oncogenic methyltransferase was found to be significantly upregulated. Quantitative PCR analysis confirmed statistically significant upregulation of EZH2 in docetaxel treated mice compared to vehicle control. EZH2 upregulation was independent of androgen levels as both androgen dependent and independent cells showed striking upregulation of EZH2 when treated with docetaxel. Next, we confirmed the nature of the upregulated EZH2 (catalytic or non-catalytic) by docetaxel. EZH2 methyltransferase activity assay data showed significant induction of methyltransferase activity by docetaxel in both androgen dependent and independent conditions and in PTEN null mouse tissues. Further, we measured other methyltransferase marks i.e. H3K27me1&2, H3K4me3, H3K9me3 in the presence of docetaxel and found only specific upregulation of H3K27me3, confirming upregulation of only catalytic EZH2 by docetaxel. These data suggested a need for co-targeting EZH2 during docetaxel treatment in PTEN null PCa conditions. In order to provide a proof-of-principle we performed a tumor xenograft study in nude mice implanted with PTEN null PC3 cells. Combination of DZNeP (an EZH2 inhibitor) and docetaxel significantly inhibited tumor growth compared to each agent alone. Results obtained from the xenograft studies prompted us to perform a preclinical study in prostate specific PTEN null mice. Data obtained from this study clearly showed a significant reduction in number of high grade lesions in PTEN null tumors in combination group when compared to either docetaxel or DZNeP treated mice. We also observed clearance of senescent cells and accumulation of CD8+ T cells and classical dendritic (CD8+ Clec9a+ positive) cells. Our data suggests the necessity of designing a combination therapy of targeting EZH2 during docetaxel treatment specifically, which will likely increase the efficacy of docetaxel in PTEN null PCa. Citation Format: Mohammad Imran Khan, Abid Hamid, Vaqar Mustafa Adhami, Chloe Lang, Hasan Mukhtar. Taxol therapy-induced catalytic EZH2 limits therapeutic responses in PTEN null prostate cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4534.