Elevated expression of cav‐1 in a subset of SS c fibroblasts contributes to constitutive Alk1/Smad1 activation

Previous studies have shown that the transforming growth factor ( TGF )β/Alk1/Smad1 signaling pathway is constitutively activated in a subset of systemic sclerosis ( SS c) fibroblasts and this pathway is a critical regulator of CCN 2 gene expression. Caveolin‐1 (cav‐1), an integral membrane protein and the main component of caveolae, has also been implicated in SS c pathogenesis. This study was undertaken to evaluate the role of caveolin‐1 in Smad1 signaling and CCN 2 expression in healthy and SS c dermal fibroblasts. We show that a significant subset of SS c dermal fibroblasts has up‐regulated cav‐1 expression in vitro , and that cav‐1 up‐regulation correlates with constitutive Smad1 phosphorylation. In addition, basal levels of phospho‐Smad1 were down‐regulated after inhibition of cav‐1 in SS c dermal fibroblasts. Caveolin‐1 formed a protein complex with Alk1 in dermal fibroblasts, and this association was enhanced by TGF β. By using si RNA against cav‐1 and adenoviral cav‐1 overexpression we demonstrate that activation of Smad1 in response to TGF β requires cav‐1 and that cav‐1 is sufficient for Smad‐1 phosphorylation. We also show that cav‐1 is a positive regulator of CCN 2 gene expression, and that it is required for the basal and TGF β‐induced CCN 2 levels. In conclusion, this study has revealed an important role of cav‐1 in mediating TGF β/Smad1 signaling and CCN 2 gene expression in healthy and SS c dermal fibroblasts.