Transforming growth factor β1 (TGFβ1) promotes fibrosis by, among other mechanisms, activating quiescent fibroblasts into myofibroblasts and increasing the expression of extracellular matrices. Recent work suggests that peroxisome proliferator-activated receptor γ (PPARγ) is a negative regulator of TGFβ1-induced fibrotic events. We, however, hypothesized that antifibrotic pathways mediated by PPARγ are influenced by TGFβ1, causing an imbalance towards fibrogenesis. Consistent with this, primary murine primary lung fibroblasts responded to TGFβ1 with a sustained downregulation of PPARγ transcripts. This effect was dampened in lung fibroblasts deficient in Smad3, a transcription factor that mediates many of the effects of TGFβ1. Paradoxically, TGFβ1 stimulated the activation of the PPARγ gene promoter and induced the phosphorylation of PPARγ in primary lung fibroblasts. The ability of TGFβ1 to modulate the transcriptional activity of PPARγ was then tested in NIH/3T3 fibroblasts containing a PPARγ-responsive luciferase reporter. In these cells, stimulation of TGFβ1 signals with a constitutively active TGFβ1 receptor transgene blunted PPARγ-dependent reporter expression induced by troglitazone, a PPARγ activator. Overexpression of PPARγ prevented TGFβ1 repression of troglitazone-induced PPARγ-dependent gene transcription, whereas coexpression of PPARγ and Smad3 transgenes recapitulated the TGFβ1 effects. We conclude that modulation of PPARγ is controlled by TGFβ1, in part through Smad3 signals, involving regulation of PPARγ expression and transcriptional potential. 1. Introduction Transforming growth factor β1 (TGFβ1) is a pleomorphic growth factor with anti-inflammatory and profibrotic properties that has been implicated in many forms of natural and experimental tissue fibrosis [1]. In lung, TGFβ1 is produced by epithelial cells, alveolar and tissue macrophages, and fibroblasts after exposure to injurious agents such as silica, bleomycin, hyperoxia, and paraquat among others [2]. A key role for TGFβ1 in lung fibrosis has been confirmed in studies showing the development of lung fibrosis in animals transfected with TGFβ1-producing adenovirus, and by work demonstrating inhibition of experimental lung fibrosis by interventions targeting TGFβ1 or its downstream signals [3, 4]. The profibrotic effects of TGFβ1 are mostly, but not entirely, mediated by intracellular signals triggered by the transcription factor Smad3 [5]. TGFβ1/Smad3 signaling stimulates connective tissue expression and epithelial-mesenchymal transition, events considered key to the development
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