Transforming growth factor beta (TGF-β) is the most potent and ubiquitous profibrogenic cytokine and its expression is increased in almost all the fibrotic diseases and in experimental fibrosis models. TGF-β increases ROS production and decreases the concentration of glutathione (GSH), the most abundant intracellular free thiol and an important antioxidant, in various types of cells, which mediates many of TGF-β’s fibrogenic effects. A decreased GSH concentration is also observed in human fibrotic diseases and in experimental fibrosis models. Although the biological significance of GSH depletion in the development of fibrosis remains obscure, GSH and N-acetylcysteine (NAC), a precursor of GSH, have been used in clinics for the treatment of fibrotic diseases. This review summarizes recent findings in the field to address the potential mechanism whereby oxidative stress mediates TGF-β’s fibrogenesis and the potential therapeutic values of antioxidant treatment in fibrotic diseases.

Transforming growth factor ␤ (TGF-␤) stimulates reactive oxygen species (ROS) production in various cell types, which mediates many of the effects of TGF-␤. The molecular mechanisms whereby TGF-␤ increases ROS production and ROS modulate the signaling processes of TGF-␤, however, remain poorly defined. In this study, we show that TGF-␤1 stimulates NADPH oxidase 4 (Nox4) expression and ROS generation in the nucleus of murine embryo fibroblasts (NIH3T3 cells). This is associated with an increase in protein thiol modification and inactivation of MAPK phosphatase 1 (MKP-1), a nuclear phosphatase. Furthermore, knockdown of MKP-1 using small interfering RNA enhances TGF-␤1-induced phosphorylation of JNK and p38 as well as the expression of plasminogen activator inhibitor 1 (PAI-1), a TGF-␤-responsive gene involved in the pathogenesis of many diseases. Knockdown of Nox4 with Nox4 small interfering RNA, on the other hand, reduces TGF-␤1-stimulated ROS production, p38 phosphorylation, and PAI-1 expression. TGF-␤ also increased the nuclear level of Nox4 protein as well as PAI-1 expression in human lung fibroblasts (CCL-210 cells), suggesting that TGF-␤ may induce PAI-1 expression by a similar mechanism in human lung fibroblasts. In summary, in this study we have identified nuclear MAPK phosphatase MKP-1 as a novel molecular target of ROS in TGF-␤ signaling pathways. Our data suggest that increased generation of ROS by Nox4 mediates TGF-␤1-induced PAI-1 gene expression at least in part through oxidative modification and inhibition of MKP-1 leading to a sustained activation of JNK and p38 MAPKs.