Inhibition of myocardin-related transcription factor/serum response factor signaling decreases lung fibrosis and promotes mesenchymal cell apoptosis

Myofibroblasts are crucial to the pathogenesis of tissue fibrosis. Their formation of stress fibers results in the release of myocardin-related transcription factor (MRTF), a transcriptional coactivator of serum response factor (SRF). MRTF-A (Mkl1)-deficient mice are protected from lung fibrosis. We hypothesized that the SRF/MRTF pathway inhibitor CCG-203971 would modulate myofibroblast function in vitro and limit lung fibrosis in vivo. Normal and idiopathic pulmonary fibrosis lung fibroblasts were treated with/without CCG-203971 (N-[4-chlorophenyl]-1-[3-(2-furanyl)benzoyl]-3-piperidine carboxamide) and/or Fas-activating antibody in the presence/absence of transforming growth factor (TGF)-β1, and apoptosis was assessed. In vivo studies examined the effect of therapeutically administered CCG-203971 on lung fibrosis in two distinct murine models of fibrosis induced by bleomycin or targeted type II alveolar epithelial injury. In vitro, CCG-203971 prevented nuclear localization of MRTF-A; increased the apoptotic susceptibility of normal and idiopathic pulmonary fibrosis fibroblasts; blocked TGF-β1-induced myofibroblast differentiation; and inhibited TGF-β1-induced expression of fibronectin, X-linked inhibitor of apoptosis, and plasminogen activator inhibitor-1. TGF-β1 did not protect fibroblasts or myofibroblasts from apoptosis in the presence of CCG-203971. In vivo, CCG-203971 significantly reduced lung collagen content in both murine models while decreasing alveolar plasminogen activator inhibitor-1 and promoting myofibroblast apoptosis. These data support a central role of the SRF/MRTF pathway in the pathobiology of lung fibrosis and suggest that its inhibition can help resolve lung fibrosis by promoting fibroblast apoptosis.