Smad4/DPC4 and Smad3 Mediate Transforming Growth Factor-β (TGF-β) Signaling through Direct Binding to a Novel TGF-β-responsive Element in the Human Plasminogen Activator Inhibitor-1 Promoter

The transforming growth factor-β (TGF-β) family of cytokines mediates multiple biological effects by regulating the expression of target genes. The Smad family proteins function as intracellular signal transducers downstream of the receptors to transmit the TGF-β signal from cell membrane to nucleus. The mechanisms by which Smads mediate transcriptional activation of target genes is largely unknown. Here we report the identification of a novel TGF-β-responsive element in the human type 1 plasminogen activator inhibitor promoter that is required for mediating strong transcriptional activation of this gene by TGF-β. Smad3 and Smad4 are incorporated into a TGF-β-inducible complex formed on this element following TGF-β stimulation of human hepatoma cells. Both Smad3 and Smad4 bind directly to this TGF-β-responsive element through their conserved MH1 domains. These results indicate that Smad3 and Smad4 mediate TGF-β signaling by directly interacting with a specific response element in a physiological target gene.