Abstract A30: Epigenetic regulation of Wnt/β-catenin signaling in vivo by PRC2 protein EZH2

Abstract

Abstract Introduction: Colon cancer results from the accumulation of both acquired genetic and epigenetic changes that transform normal glandular epithelium into invasive adenocarcinomas. The Enhancer of Zeste Homolog 2 (EZH2) protein is a core component of the Polycomb Repressive Complex 2 (PRC2) that represses gene transcription through histone H3 lysine 27 trimethylation (H3K27me3). EZH2 is overexpressed in several malignancies including colon cancer; EZH2's role in tumor initiation however, is less clear. Citrobacter rodentium (CR)-induced transmissible murine colonic hyperplasia (TMCH) that utilizes a type-three secretion system (T3SS) to activate Wnt/β-catenin signaling, represents the earliest molecular and functional changes associated with colon carcinogenesis; the increases in β-catenin recorded during TMCH however, are neither due to mutations in Adenomatous polyposis coli (Apc) or β-catenin (CTNNB1) gene. Aim: To investigate if EZH2 is critical in epigenetically regulating Wnt/β-catenin signaling in response to bacterial infection. Experimental Procedures: TMCH or tumorigenesis were induced by CR infection (108cfu) in wild type and ApcMin/+ mice, respectively. AOM (@10mg/Kg body wt.)/DSS (2.5%; 3 cycles) model of colitis-associated cancer and de-identified human adenoma and adenocarcinoma samples were used to further examine evidence of epigenetically-regulated Wnt/β-catenin signaling. Results: Following CR infection, EZH2 protein increased significantly in the crypt at day 6 and particularly at day 12 (peak hyperplasia) and coincided with expression of EZH2 target protein H3K27me3. The changes accrued were specific to CR as T3SS mutant escV failed to elicit alterations in either EZH2, H3K27me3 or crypt hyperplasia. Mechanistically, siRNA-mediated knockdown of EZH2 led to: i) attenuated cell migration, ii) reduced colonosphere formation and, iii) decreased β-catenin/TCF4-dependent TOPflash reporter activity due to loss of β-catenin. Chromatin immunoprecipitation revealed EZH2's occupancy on WIF1 (Wnt Inhibitory Factor 1) promoter resulting in reduced WIF1 mRNA and protein expression. Following EZH2 knockdown either via siRNA or EZH2-inhibitor DZNep (5μM), relative levels of H3K27me3 decreased which led to the transcriptional upregulation of WIF1 as was evidenced by WIF1 promoter activity. Besides EZH2, we also discovered increases in miR-203 expression in the crypts at days-6 and 12 post-infection that correlated with reduced levels of its target WIF1, at these time points; overexpression of miR-203 in Young Adult Mouse Colon primary cells resulted in decreased WIF1 transcription. Tumors in the distal colons of ApcMin/+ mice following CR infection expressed high levels of EZH2 and β-catenin and a concomitant decrease in WIF1. In response to AOM/DSS, colonic tumors at 24 weeks expressed high levels of EZH2 and β-catenin and a concomitant decrease in WIF1. Pharmacological intervention with honokiol (HNK; @5mg/Kg body wt. p.o.) a week prior to and during AOM/DSS-induced colon carcinogenesis reduced both tumor burden and tumor size. Immunohistochemical analysis of HNK-treated tissues revealed significant reduction in EZH2 and β-catenin staining while WIF1 expression was dramatically upregulated. In human adenomas/adenocarcinomas, simultaneous increases in EZH2 and β-catenin staining coincided with a corresponding decrease in WIF1 positivity. Conclusions: 1. CR infection induces EZH2 overexpression that correlates with aberrant Wnt/β-catenin signaling in the colonic crypts.2. EZH2 downregulates WIF1 expression which may correlate with earliest stages of colon tumorigenesis. 3. Honokiol-induced upregulation of WIF1 to block aberrant Wnt/β-catenin signaling may be a novel strategy to target epigenetic abnormalities for colon cancer prevention and therapeutics. Citation Format: Badal C. Roy, Dharmalingam Subramaniam, Ishfaq Ahmed, Shrikant Anant, Shahid Umar. Epigenetic regulation of Wnt/β-catenin signaling in vivo by PRC2 protein EZH2. [abstract]. In: Proceedings of the Twelfth Annual AACR International Conference on Frontiers in Cancer Prevention Research; 2013 Oct 27-30; National Harbor, MD. Philadelphia (PA): AACR; Can Prev Res 2013;6(11 Suppl): Abstract nr A30.