GRK2 promotes activation of lung fibroblast cells and contributes to pathogenesis of pulmonary fibrosis through increasing Smad3 expression.

Pulmonary fibrosis is a chronic, progressive, and irreversible interstitial lung disease. Transforming growth factor-β1 (TGF-β1) plays a major role in lung fibroblast cell differentiation to myofibroblast cells and production of extracellular matrix, which are hallmarks of pulmonary fibrosis. G protein-coupled receptor kinase-2 (GRK2) has been shown to play controversial roles in TGF-β1-induced signal transduction in different cell types; however, the role of GRK2 in TGF-β1-induced activation of lung fibroblast cells and development of pulmonary fibrosis has not been revealed. In this study, we found that GRK2 levels were increased in lungs and isolated fibroblast cells in a murine model of pulmonary fibrosis, as well as TGF-β1-treated lung fibroblasts. GRK2 levels were not changed in lungs in the injury phase of pulmonary fibrosis. Posttreatment with GRK2 inhibitor reduced extracellular matrix (ECM) accumulation in lungs in bleomycin-challenged mice, suggesting that GRK2 activation contributes to the progressive phase of pulmonary fibrosis. Inhibition or downregulation of GRK2 attenuates fibronectin, collagen, and α-smooth muscle actin expression in TGF-β1-induced lung fibroblast cells or myofibroblast cells isolated from patients with pulmonary fibrosis. Furthermore, we showed that GRK2 regulates Smad3 expression, indicating that inhibition of GRK2 attenuates ECM accumulation through downregulation of Smad3 expression. This study reveals that GRK2 is a therapeutic target in treating pulmonary fibrosis and inhibition of GRK2 dampens pulmonary fibrosis by suppression of Smad3 expression, eventually attenuating TGF-β1 signal pathway and ECM accumulation.