β-catenin represses expression of the tumour suppressor 15-prostaglandin dehydrogenase in the normal intestinal epithelium and colorectal tumour cells.

Background Cyclooxygenase-2 (COX-2) overexpression in colorectal cancer increases levels of its pro-tumorigenic product prostaglandin E2 (PGE(2)). The recently identified colorectal tumour suppressor 15-prostaglandin dehydrogenase (15-PGDH) catalyses prostaglandin turnover and is downregulated at a very early stage in colorectal tumorigenesis; however, the mechanism responsible remains unclear. As Wnt/beta-catenin signalling is also deregulated early in colorectal neoplasia, a study was undertaken to determine whether beta-catenin represses 15-PGDH expression. Methods The effect of modulating Wnt/beta-catenin signalling (using beta-catenin siRNA, mutant TCF4, Wnt3A or GSK3 inhibition) on 15-PGDH mRNA, protein expression and promoter activity was determined in colorectal cell lines by immunoblotting, qRT-PCR and reporter assays. The effect of beta-catenin deletion in vivo was addressed by 15-PGDH immunostaining of beta-catenin(-/lox)-villin-creERT2 mouse tissue. 15-PGDH promoter occupancy was determined using chromatin immunoprecipitation and PGE(2) levels by ELISA. Results The study shows for the first time that beta-catenin knockdown upregulates 15-PGDH in colorectal adenoma and carcinoma cells without affecting COX-2 protein levels. A dominant negative mutant form of TCF4 (dnTCF4), unable to bind beta-catenin, also upregulated 15-PGDH; conversely, increasing beta-catenin activity using Wnt3A or GSK3 inhibition downregulated 15-PGDH. Importantly, inducible beta-catenin deletion in vivo also upregulated intestinal epithelial 15-PGDH. 15-PGDH regulation occurred at the protein, mRNA and promoter activity levels and chromatin immunoprecipitation indicated beta-catenin/TCF4 binding to the 15-PGDH promoter. beta-catenin knockdown decreased PGE(2) levels, and this was significantly rescued by 15-PGDH siRNA. Conclusion These data suggest a novel role for beta-catenin in promoting colorectal tumorigenesis through very early 15-PGDH suppression leading to increased PGE(2) levels, possibly even before COX-2 upregulation.