Peroxisome Proliferator-activated Receptor γ Inhibits Transforming Growth Factor β-induced Connective Tissue Growth Factor Expression in Human Aortic Smooth Muscle Cells by Interfering with Smad3

Abstract

Activation of peroxisome proliferator-activated receptor γ (PPARγ) after balloon injury significantly inhibits VSMC proliferation and neointima formation. However, the precise mechanisms of this inhibition have not been determined. We hypothesized that activation of PPARγ in vascular injury could attenuate VSMC growth and matrix production during vascular lesion formation. Since connective tissue growth factor (CTGF) is a key factor regulating extracellular matrix production, abrogation of transforming growth factor β (TGF-β)-induced CTGF production by PPARγ activation may be one of the mechanisms through which PPARγ agonists inhibit neointima formation after vascular injury. In this study, we demonstrate that the PPARγ natural ligand (15-deoxyprostaglandin J 2) and a synthetic ligand (GW7845) significantly inhibit TGF-β-induced CTGF production in a dose-dependent manner in HASMCs. In addition, suppression of CTGF mRNA expression is relieved by pretreatment with an antagonist of PPARγ (GW9662), suggesting that the inhibition of CTGF expression is mediated by PPARγ. To elucidate further the molecular mechanism by which PPARγ inhibits CTGF expression, an ∼2-kilobase pair CTGF promoter was cloned. We found that PPARγ activation inhibits TGF-γ-induced CTGF promoter activity in a dose-dependent manner, and suppression of CTGF promoter activity by PPARγ activation is completely rescued by overexpression of Smad3, but not by Smad4. Furthermore, PPARγ physically interacts with Smad3 but not Smad4 in vitro in glutathione S-transferase pull-down experiments. Taken together, the data suggest that PPARγ inhibits TGF-β-induced CTGF expression in HASMCs by directly interfering with the Smad3 signaling pathway.