Abstract 2012: SPDEF enforces tumor quiescence by shifting the transcriptional targets of activated β-catenin
Authors: Lo Yuan-Hung , Noah Taeko , Chen Min-Shan , Zou Winnie , Shroyer Noah 
Published: July 2016

Background: Colorectal cancer (CRC) carcinogenesis is driven by a series of genetic and epigenetic changes that results in the oncogenic transformation of normal colonic mucosa. Canonical Wnt/β-catenin signaling pathway activation, with resultant high β-catenin transcriptional activity, is frequently implicated human CRC (∼ 90% of CRC); however, there are currently no treatments targeting this pathway. We have previously reported that SAM Pointed Domain Ets transcription Factor (SPDEF) is a colonic tumor suppressor that negatively regulates canonical Wnt/β-catenin signaling. In agreement with the tumor repressor role of SPDEF, the absence of SPDEF enhances intestinal tumor formation, while re-expression of SPDEF inhibits colon adenocarcinoma proliferation in both genetic (Apcmin/+) and chemically colitis-associated (AOM/DSS) CRC models. However, the molecular mechanism by which SPDEF mediates colorectal tumor repression is still largely unknown. Here we aim to elucidate the molecular mechanism by which SPDEF mediates repression of canonical Wnt/β-catenin signaling in CRCs.

Material and Methods: To achieve our goal, we directly analyzed the effects of SPDEF expression in β-catenin-driven intestinal tumors in vivo using a new inducible mouse model (Lgr5CreERT2; β-cateninexon3; Rosa26rtta-ires-EGFP; TRE-Spdef) and human colon cancer xenografts. Moreover, wildtype or truncated SPDEF mutants were used for β-catenin transcriptional activity assay, co-immunoprecipitation, and chromatin immunoprecipitation in human colon cancer cell lines.

Results: In this study, we find that SPDEF is sufficient to inhibit β-catenin-driven intestinal tumorigenesis and shrink established tumors in both transgenic mice and xenografted human colon cancer cells. SPDEF inhibits canonical β-catenin transcriptional activity through protein-protein interaction, independent of its DNA binding capacity. We find that SPDEF disrupts the binding between β-catenin and its DNA binding partners TCF1 and TCF3, but not LEF1 and TCF4, selectively displacing β-catenin from the promoter/enhancer regions of cell cycle genes without affecting stem cell signature genes. Consistent with this observation, re-expression of SPDEF enforces a quiescent state on β-catenin-driven tumor cells in vivo. Taken together, for the first time, we unveil a novel mechanism in which SPDEF shifts the transcriptional targets of activated β-catenin to regulate the active to quiescent switch in tumor initiating cells. These findings provide insights into the mechanisms that can regulate tumor cell quiescence, and offer a novel approach for targeting canonical Wnt/β-catenin transcriptional machinery as a therapeutic strategy.

Citation Format: Yuan-Hung Lo, Taeko Noah, Min-Shan Chen, Winnie Zou, Noah Shroyer. SPDEF enforces tumor quiescence by shifting the transcriptional targets of activated β-catenin. [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 2012.