Abstract
Considerable recent interest has focused on the stabilization and accumulation of beta-catenin protein in human and animal tumors and the corresponding activation of downstream beta-catenin/TCF/LEF target genes. However, there is only sparse information on the regulation of beta-catenin expression at the transcriptional level and its possible involvement in physiological and pathophysiological processes. We report here the cloning and characterization of a 3.6-kb promoter fragment from the rat beta-catenin gene, Ctnnb1, and its comparison with corresponding promoters from the mouse and human genes, Catnb and CTNNB1. Several AP1 binding sites were confirmed in the promoters of all three species using mobility shift and reporter assays, and one putative TCF/LEF site also was observed in the promoter of CTNNB1. Subsequently, protein/DNA array analyses identified numerous other transcription factors through their high-affinity binding to the Ctnnb1 promoter, including E2F1, NFkappaB, MEF1, Pax5, ISRE2, Smad3/4, GATA, and ZIC. The strong binding of E2F1 and NFkappaB is especially noteworthy, because the former transcription factor is regulated by cyclin D1, a downstream target of beta-catenin/TCF/LEF signaling, whereas the latter transcription factor has been implicated in "cross talk" between the Wnt and the NFkappaB signaling pathways. These results are discussed in terms of their implications for human cancer development and specifically the various tumors in which beta-catenin mRNA is overexpressed, as well as for embryonic development, when reversible changes in beta-catenin expression occur in response to secreted Wnt ligands. The findings reported here should provide important avenues for further research focused on the regulation of Ctnnb1 activity, including the ability of beta-catenin/Tcf downstream targets to modulate beta-catenin expression at the transcriptional level.