Inhibition of Transforming Growth Factor-β Signaling by Low Molecular Weight Compounds Interfering with ATP- or Substrate-Binding Sites of the TGFβ Type I Receptor Kinase

Transforming growth factor-beta (TGFbeta) is a potent regulator of cell proliferation, differentiation, apoptosis, and migration. TGF-beta type I receptor (TbetaR-I), which has intrinsic serine/threonine kinase activity, is a key component in activation of intracellular TGFbeta signaling. We studied two different classes of TbetaR-I inhibitors, i.e., compounds interfering with the ATP-binding site of the kinase and substrate-mimicking peptides. We found that pyridinylimidazole compounds inhibited TbetaR-I kinase at micromolar concentration. A representative compound, SB203580, inhibited in vivo Smad2 phosphorylation by TbetaR-I and affected TGFbeta-dependent transcriptional activation. Peptides mimicking the TbetaR-I phosphorylation sites at the C-terminus of Smad2 also inhibited the autophosphorylation of TbetaR-I and phosphorylation of Smad2 by TbetaR-I in vitro and in vivo, whereas a similar peptide from Smad5 was without effect. The substrate-mimicking peptide, fused to penetratin, inhibited a TGFbeta1-dependent transcriptional response in a luciferase reporter assay and ligand-dependent growth inhibition of Mv1Lu cells. Thus, the substrate-mimetic peptide is a new type of specific inhibitor of the TGFbeta signaling in vivo.