Wnt-1 Induces Growth, Cytosolic β-Catenin, and Tcf/Lef Transcriptional Activation in Rat-1 Fibroblasts

Genetic evidence suggests that regulation of β-catenin and regulation of Tcf/Lef family transcription factors are downstream events of the Wnt signal transduction pathway. However, a direct link between Wnt activity and Tcf/Lef transcriptional activation has yet to be established. In this study, we show that Wnt-1 induces a growth response in a cultured mammalian cell line, Rat-1 fibroblasts. Wnt-1 induces serum-independent cellular proliferation of Rat-1 fibroblasts and changes in morphology. Rat-1 cells stably expressing Wnt-1 (Rat-1/Wnt-1) show a constitutive up-regulation of cytosolic β-catenin, while membrane-associated β-catenin remains unaffected. Induction of cytosolic β-catenin in Rat-1/Wnt-1 cells is correlated with activation of a Tcf-responsive transcriptional element. We thus provide evidence that Wnt-1 induces Tcf/Lef transcriptional activation in a mammalian system. Expression of a mutant β-catenin (β-CatS37A) in Rat-1 cells does not result in a proliferative response or a detectable change in the cytosolic β-catenin protein level. However, β-CatS37A expression in Rat-1 cells results in strong Tcf/Lef transcriptional activation, comparable to that seen in Wnt-1-expressing cells. These results suggest that Wnt-1 induction of cytosolic β-catenin may have functions in addition to Tcf/Lef transcriptional activation.