BETA-CATENIN IS FREQUENTLY MUTATED AND DEMONSTRATES ALTERED CELLULAR LOCATION IN AZOXYMETHANE-INDUCED RAT COLON TUMORS

beta-Catenin is a key regulator of the cadherin-mediated cell-fell adh esion system and an important element in the Wnt signal transduction p athway. Stabilization and accumulation of cytoplasmic beta-catenin, wh ich result from mutations in either the adenomatous polyposis coli or beta-catenin genes, are causatively associated with colon carcinogenes is. In the present study, we examined the expression of beta-catenin i n 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 ca rcinoma tissues, whereas antibody binding was limited to membranes at the intercellular borders in normal colon epithelial cells. Increase o f the free beta-catenin fraction in tumor cells was also indicated by immunoblot analysis of fractionated tissue lysates. Investigation of m utations in the glycogen synthase kinase-3 beta phosphorylation consen sus moth of the beta-catenin gene by PCR-single strand conformation po lymorphism methods and direct sequencing revealed eight mutations in s ix of the eight colon carcinomas, and seven of these were shown to be G:C to A:T transitions, with five being CTG (G) under bar A to CTG (A) under bar A. Such frequent mutations of the beta-catenin gene in azox ymethane-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.