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

ABSTRACT

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.

ACKNOWLEDGMENTS

We thank Eugene Marcantonio, Frank Costantini, Rudy Grosschedl, and Martin Julius for discussions and critical reading of the manuscript, and we thank Martin Julius and Qingyou Yan for construction of retroviral vectors. pTOPFLASH and pFOPFLASH reporter constructs were generously provided by Hans Clevers, β-catenin (S37A) cDNA was generously provided by Steven Byers, and Rat-1 fibroblasts were generously provided by Riccardo Dalla-Favera.

This work was supported by grants to J.K. from the U.S. Army Medical Research and Material Command (USAMRMC) under grant DAMD17-94-J-4069, the American Cancer Society (ACS DB-81), and the Marilyn Bokemeier Sperry Fund and by a predoctoral fellowship to C.S.Y. from the NIH (T32 CA09503).

ADDENDUM IN PROOF

Since submission of this paper, other groups have reported that Wnt-1 can induce Tcf reporter gene transcription (V. Korinek, N. Barker, K. Willert, M. Molenaar, J. Roose, G. Wagenaar, M. Markman, W. Lamers, O. Destree, and H. Clevers, Mol. Cell. Biol.18:1248–1256, 1998) or a β-catenin–LEF-1 complex (E. Porfiri, B. Rubinfeld, I. Albert, K. Hovanes, M. Waterman, and P. Polakis, Oncogene 15:2833–2839, 1997).