Inhibition of TGFβ1-mediated PAI-1 induction by oltipraz through selective interruption of Smad 3 activation

Transforming growth factor-β1 (TGFβ1) induces plasminogen activator inhibitor-1 (PAI-1) as a major target protein. PAI-1 is associated with fibrosis, thrombosis, and metabolic disorders. TGFβ1 induces PAI-1 via phosphorylation and nuclear translocation of Smads. Oltipraz inhibits TGFβ1 expression and also regenerates cirrhotic liver. Nevertheless, whether oltipraz modulates TGFβ1-mediated cell signaling is unclear. First, this study examined the effect of oltipraz on PAI-1 expression in cirrhotic rat liver. The cells immunochemically stained with anti-PAI-1 antibody accumulated around and within fibrous nodules in cirrhotic liver, which was notably decreased by oltipraz treatment. Next, whether oltipraz inhibits TGFβ1-mediated Smads activation or Smad-mediated PAI-1 induction was determined in L929 fibroblasts. Oltipraz inhibited the ability of TGFβ1 to induce PAI-1, as indicated by repression of TGFβ1-mediated luciferase induction from the plasmid comprising the human PAI-1 promoter and of TGFβ1-induced Smad-DNA-binding activity. TGFβ1 induced nuclear transport of receptor-regulated Smad 2 and Smad 3, of which oltipraz selectively inhibited the transport and phosphorylation of Smad 3, thereby reducing formation of Smad 3/4 complex in the nucleus. In summary, oltipraz inhibits PAI-1 induction via a decrease in the formation of Smad 3/4 complex due to selective interruption of Smad 3 activation, indicating that oltipraz regulates the cellular responses downstream of ligand-activated TGFβ1 receptor.