Altered expression of β-catenin, inducible nitric oxide synthase and cyclooxygenase-2 in azoxymethane-induced rat colon carcinogenesis

Abstract

Activation of the β-catenin/T cell factor-mediated transcription pathway through mutations of the APC or β -catenin gene is suggested to play an important role in colon carcinogenesis and there is great interest in the target genes. We have described the frequent mutation and an altered cellular localization of β-catenin in rat colon adenocarcinomas induced by azoxymethane (AOM), along with up-regulation of inducible nitric oxide synthase (iNOS) and cyclooxygenase (COX)-2. In the present study, the relation between β-catenin alteration and expression of iNOS and COX-2 in AOM-induced rat colon carcinogenesis was examined in hyperplastic and dysplastic type aberrant crypt, adenoma and adenocarcinoma samples. K- ras gene mutations were also investigated. Mutation analysis by the PCR–single strand conformation polymorphism method and direct sequencing demonstrated the β -catenin gene to be mutated in two of three dysplastic aberrant crypt foci (ACF), two of six adenomas and 20 of 26 adenocarcinomas, while K- ras was mutated in seven of 10 hyperplastic ACF and seven of 26 adenocarcinomas. Immunohistochemical staining showed an alteration in cellular localization of β-catenin in all dysplastic ACF, adenomas and adenocarcinomas examined. iNOS expression was also observed in all but one of the lesions in which β-catenin alterations were observed. Neither iNOS expression nor β-catenin alterations were observed in any hyperplastic ACF. COX-2 expression in stromal elements was found even in normal colon mucosa and increased in adenomas and adenocarcinomas, while epithelial cells were only positive in large adenocarcinomas. These results show that β-catenin alterations may be related to induction of iNOS expression, these being early events in AOM-induced colon tumorigenesis which may play important roles in causing dysplastic changes.