Objective: We tested a hypothesis that PPARgamma inhibits TGFbeta1-activation of TGFbeta receptor (TGFbetaR)-1 signaling in quiescent stellate cells, thereby abrogating Smad3 phosphorylation and inhibiting PAI-1 and collagen expressions.
Methods: Human stellate cells (HSC) were cultured in a medium containing isobutylmethylxanthine, dexamethasone and insulin (MDI) to induce a quiescent adipocytic phenotype one, and then they were treated with TGFbeta1 with or without SB431542, a TGFbetaR1 kinase inhibitor, or the PPARgamma agonist ciglitazone. Effects on Smad 3 phosphorylation, TGFbeta-responsive transcriptional activity, and expressions of collagen and PAI-1 were assessed.
Results: Culturing HSC in MDI induced an adipocytic phenotype characterized by lipid accumulation and increased PPARgamma expression and transcriptional activity. TGFbeta1 treatment caused dose- and time-dependent increases in ECM gene expression, increasing collagen and PAI-1 mRNAs by 3 fold within 3 h and increasing PAI-1 protein levels by 8 fold within 6 h. Treatment with the TGFbetaR1 kinase inhibitor, SB431542, inhibited all of these responses. The PPARbeta agonist ciglitazone also caused a dose-dependent inhibition of TGFbeta1's fibrogenic actions. 1 mmol/L ciglitazone blocked TGFbeta1-transcriptional activity and abolished TGFbeta-mediated induction of collagen and PAI-1 expressions.
Conclusion: The anti-fibrotic ability of PPARgamma agonist ciglitazone may be related to its ability to inhibit TGFbeta1-TGFbetaR1 signaling and blocking pSmad3-dependent induction of PAI-1 and collagen expression.