β-Catenin is a key regulator of the cadherin-mediated cell-cell adhesion system and an important element in the Wnt signal transduction pathway. Stabilization and accumulation of cytoplasmic β-catenin, which result from mutations in either the adenomatous polyposis coli or β-catenin genes, are causatively associated with colon carcinogenesis. In the present study, we examined the expression of β-catenin in rat colon tumors induced by azoxymethane in comparison with adjacent normal colon mucosa by immunostaining and immunoblotting. Cytoplasmic and nuclear immunostaining was pronounced in all colon adenoma and carcinoma tissues, whereas antibody binding was limited to membranes at the intercellular borders in normal colon epithelial cells. Increase of the free β-catenin fraction in tumor cells was also indicated by immunoblot analysis of fractionated tissue lysates. Investigation of mutations in the glycogen synthase kinase-3β phosphorylation consensus motif of the β-catenin gene by PCR-single strand conformation polymorphism methods and direct sequencing revealed eight mutations in six of the eight colon carcinomas, and seven of these were shown to be G:C to A:T transitions, with five being CTGGA to CTGAA. Such frequent mutations of the β-catenin gene in azoxymethane-induced rat colon tumors suggest that consequent alterations in the stability and localization of the protein may play an important role in this colon carcinogenesis model.
This work was supported in part by a Grant-in-Aid for Cancer Research from the Ministry of Health and Welfare, Japan, and a grant from the Program for Promotion of Fundamental Studies in Health Sciences of the Organization for Drug ADR Relief, R&G Promotion, and Product Review of Japan.