The transforming growth factor-β1 (TGF-β1) plays a key role in lung fibrosis. However, the molecular mechanisms involved in TGF-β1-induced lung fibrosis are unclear. TGF-β1 is the key inducer of myofibroblast transdifferentiation via de novo synthesis of α-smooth muscle actin (α-SMA). Since TGF-β1 signals through reactive oxygen species (ROS) and ROS have been shown to induce accumulation of extracellular matrix (ECM) in various tissues, this study examined if ROS play a role in TGF-β1-induced fibronectin secretion and α-SMA expression in human lung fibroblasts, MRC-5 cells.

Growth arrested and synchronized MRC-5 cells were stimulated with TGF-β1 (0.2-10 ng/ml) in the presence or absence of N-acetylcysteine (NAC) or diphenyleneiodonium (DPI) for up to 96 hours. Dichlorofluorescein (DCF)-sensitive cellular ROS were measured by FACScan and secreted fibronectin and cellular α-SMA by Western blot analysis.

TGF-β1 increased the level of fibronectin secretion and α-SMA expression in MRC-5 cells in a dose-dependent manner. Both NAC (20 and 30 mM) and DPI (1 and 5 µM significantly inhibited TGF-β1-induced fibronectin and α-SMA upregulation. The TGF-β1-induced cellular ROS level was also significantly reduced by NAC and DPI.

The results suggest that NADPH oxidase-dependent ROS play an important role in TGF-β1-induced fibronectin secretion and α-SMA expression in MRC-5 cells, which leads to myofibroblast transdifferentiation and progressive lung fibrosis.