Cyclin D1 over-expression correlates with β-catenin activation, but not with H-ras mutations, and phosphorylation of Akt, GSK3β and ERK1/2 in mouse hepatic carcinogenesis

Mutational activation of beta-catenin and cyclin D1 overexpression are a frequent change in mouse hepatic tumors. Although activated beta-catenin may bind to T cell factor (TCF) family members and transcriptionally activate the cyclin D1 gene, either beta-catenin or cyclin D1 may be activated by various pathways independently of beta-catenin mutations. In this study, we investigated beta-catenin activation and mutations, cyclin D1 expression, H-ras mutations and phosphorylation of extracellular signal regulated protein kinases 1/2 (ERK1/2), Akt and glycogen synthetase kinase 3beta (GSK3beta) in mouse hepatic carcinogenesis. Nuclear/cytoplasmic staining of beta-catenin, a sign of beta-catenin activation, was frequently observed in association with the high nuclear cyclin D1 labeling index in the hepatic tumors at the late stage of carcinogenesis. The beta-catenin activation was further suggested by the fact that all hepatocellular carcinoma (HCC) cell lines examined showed the nuclear beta-catenin/TCF4 complex together with cyclin D1 overexpression. However, the fact that only 31.8% (7/22) of the lesions with the nuclear/cytoplasmic beta-catenin staining showed beta-catenin mutations indicated that beta-catenin was activated not only by its own mutations but also by other reason(s). On the other hand, there was no correlation between the beta-catenin/cyclin D1 activation and the H-ras mutations or phosphorylation. of Akt, GSK3beta and ERK1/2, although GSK3beta was frequently over-expressed in the tumors. These results indicate that, although P-catenin and cyclin D1 activation are well correlated, the Akt/GSK3beta and ras/ERK1/2 pathways may not play a major role in the beta-catenin/cyclin D1 activation.