Lefty inhibits receptor-regulated Smad phosphorylation induced by the activated transforming growth factor-beta receptor.

Transforming growth factor-beta (TGF-beta) is a pleiotropic cytokine that regulates growth and differentiation of diverse types of cells. TGF-beta actions are directed by ligand-induced activation of TGF-beta receptors with intrinsic serine/threonine kinase activity that trigger phosphorylation of receptor-regulated Smad (R-Smad) protein. Phosphorylated R-Smad proteins bind to Smad4, and the complexes formed move into the nucleus, where they act as components of a transcriptional complex. Here, we show that TGF-beta signaling is inhibited by lefty, a novel member of the TGF-beta superfamily. Lefty perturbed TGF-beta signaling by inhibiting the phosphorylation of Smad2 following activation of the TGF-beta receptor. Moreover, lefty inhibited the events that lie downstream from R-Smad phosphorylation, including heterodimerization of R-Smad proteins with Smad4 and nuclear translocation of the R-Smad.Smad4 complex. Lefty repressed TGF-beta-induced expression of reporter genes for the p21, cdc25, and connective tissue growth factor promoters and of a reporter gene driven by the Smad-binding element. Similarly, lefty inhibited both BMP-mediated Smad5 phosphorylation and gene transcription. The action of lefty does not appear to depend on protein synthesis, including synthesis of inhibitory Smad proteins. Thus, lefty provides a repressed state of TGF-beta- or BMP-responsive genes and participates in negative modulation of TGF-beta and BMP signaling by inhibition of phosphorylation of R-Smad proteins.