Objective: Activation of Smad1 signaling has recently been implicated in the development of fibrosis. The goal of the present study was to gain further insights into activation of the Smad1 pathway in fibrosis in systemic sclerosis (SSc) and to determine whether this pathway is targeted by the antifibrotic drug imatinib mesylate.
Methods: Levels of phosphorylated Smad1 and total Smad1 were examined in SSc and control skin biopsy samples by immunohistochemistry and in cultured fibroblasts by Western blotting. Activity of the CCN2 promoter was examined by a luciferase reporter gene assay. Interactions of Smad1 with the CCN2 promoter were examined by in vitro and in vivo DNA binding assays. Expression of the nonreceptor tyrosine kinase c-Abl and Smad1 was blocked using respective small interfering RNA.
Results: Total and phosphorylated Smad1 levels were significantly elevated in SSc skin biopsy samples and in cultured SSc fibroblasts and correlated with elevated CCN2 protein and CCN2 promoter activity. DNA binding assays demonstrated that Smad1 was a direct activator of the CCN2 gene. Small interfering RNA-mediated depletion of Smad1 in SSc fibroblasts normalized the production of CCN2 and collagen. Imatinib mesylate blocked activation of the Smad1 pathway in transforming growth factor beta-stimulated control fibroblasts and reversed activation of this pathway in SSc fibroblasts. Likewise, blockade of c-Abl abrogated activation of the Smad1 pathway in SSc fibroblasts.
Conclusion: Our findings demonstrate that activation of Smad1 signaling occurs in a subset of SSc patients and contributes to persistent activation of SSc fibroblasts. Demonstration that the Smad1/CCN2 pathway is blocked by imatinib mesylate further clarifies the mechanism of the antifibrotic effects of this compound. This study suggests that SSc patients with activated Smad1 signaling may benefit from imatinib mesylate treatment.