Peroxisome proliferator-activated receptor gamma inhibits transforming growth factor beta-induced connective tissue growth factor expression in humanaortic smooth muscle cells by interfering with Smad3

Activation of peroxisome proliferator-activated receptor gamma (PPAR gamma) after balloon injury significantly inhibits VSMC proliferation and neointi ma formation. However, the precise mechanisms of this inhibition have not b een determined. We hypothesized that activation of PPAR gamma in vascular i njury could attenuate VSMC growth and matrix production during vascular les ion formation. Since connective tissue growth factor (CTGF) is a key factor regulating extracellular matrix production, abrogation of transforming gro wth factor beta (TGF-beta) -induced CTGF production by PPAR gamma activatio n may be one of the mechanisms through which PPAR gamma agonists inhibit ne ointima formation after vascular injury. In this study, we demonstrate that the PPAR gamma natural ligand (15-deoxyprostaglandin J(2)) and a synthetic ligand (GW7845) significantly inhibit TGF-beta -induced CTGF production in a dose-dependent manner in HASMCs. In addition, suppression of CTGF mRNA e xpression is relieved by pretreatment with an antagonist of PPAR gamma (GW9 662), suggesting that the inhibition of CTGF expression is mediated by PPAR gamma. To elucidate further the molecular mechanism by which PPAR gamma in hibits CTGF expression, an similar to2-kilobase pair CTGF promoter was clon ed. We found that PPAR gamma activation inhibits TGF-beta -induced CTGF pro moter activity in a dose-dependent manner, and suppression of CTGF promoter activity by PPAR gamma activation is completely rescued by overexpression of Smad3, but not by Smad4. Furthermore, PPAR gamma physically interacts wi th Smad3 but not Smad4 in vitro in glutathione S-transferase pull-down expe riments. Taken together, the data suggest that PPAR gamma inhibits TGF-beta -induced CTGF expression in HASMCs by directly interfering with the Smad3 signaling pathway.