Src kinases in systemic sclerosis: Central roles in fibroblast activation and in skin fibrosis

Objective Src kinases are nonreceptor tyrosine kinases, which have been implicated in cytoskeletal organization and cell mobility. This study was undertaken to evaluate the potential of Src kinases as novel targets of antifibrotic therapies. Methods Fibroblast cultures were obtained from 10 patients with systemic sclerosis (SSc) and 5 healthy subjects. Src signaling was inhibited using small‐molecule inhibitors and overexpression of a dominant‐negative mutant of Src and of the endogenous inhibitor Csk. The expression of extracellular matrix proteins was analyzed by real‐time polymerase chain reaction and by SirCol collagen assay. Toxic effects were excluded by MTT assay and staining for annexin V and propidium iodide. The mouse model of bleomycin‐induced dermal fibrosis was used to assess the role of Src kinases in dermal fibrosis in vivo. Results Stimulation with transforming growth factor β and platelet‐derived growth factor activated Src signaling in dermal fibroblasts from patients with SSc and healthy donors. Incubation with the Src kinase inhibitors or overexpressed mutant Src or Csk reduced the synthesis of messenger RNA for COL1A1, COL1A2, and fibronectin 1. A dose‐dependent reduction in collagen release was also observed at the protein level. No inhibitory effects on proliferation and no increase in the number of apoptotic or necrotic fibroblasts were observed. Consistent with the in vitro data, inhibition of Src kinases prevented experimental dermal fibrosis. Dermal thickness, the amount of collagen protein, and the number of myofibroblasts were reduced in a dose‐dependent manner. Conclusion These findings indicate that Src kinases play important roles in the activation of fibroblasts and in the development of experimental fibrosis. Thus, Src kinases might be interesting targets for novel antifibrotic therapies in SSc.