A prominent pathway of transforming growth factor (TGF)-β signaling involves receptor-dependent phosphorylation of Smad2 and Smad3, which then translocate to the nucleus to activate transcription of target genes. To investigate the relative importance of these two Smad proteins in TGF-β1 signal transduction, we have utilized a loss of function approach, based on analysis of the effects of TGF-β1 on fibroblasts derived from mouse embryos deficient in Smad2 (S2KO) or Smad3 (S3KO). TGF-β1 caused 50% inhibition of cellular proliferation in wild-type fibroblasts as assessed by [3H]thymidine incorporation, whereas the growth of S2KO or S3KO cells was only weakly inhibited by TGF-β1. Lack of Smad2 or Smad3 expression did not affect TGF-β1-induced fibronectin synthesis but resulted in markedly suppressed induction of plasminogen activator inhibitor-1 by TGF-β1. Moreover, TGF-β1-mediated induction of matrix metalloproteinase-2 was selectively dependent on Smad2, whereas induction of c-fos, Smad7, and TGF-β1 autoinduction relied on expression of Smad3. Investigation of transcriptional activation of TGF-β-sensitive reporter genes in the different fibroblasts showed that activation of the (Smad binding element)4-Lux reporter by TGF-β1 was dependent on expression of Smad3, but not Smad2, whereas activation of the activin response element-Lux reporter was strongly suppressed in S2KO fibroblasts but, on the contrary, enhanced in S3KO cells. Our findings indicate specific roles for Smad2 and Smad3 in TGF-β1 signaling.
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Published, JBC Papers in Press, March 21, 2001, DOI 10.1074/jbc.M102382200
This work was supported by National Institutes of Health Grant DK-56077-01 (to E. P. B.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.