Oxidative stress and glutathione in TGF-beta-mediated fibrogenesis

Abstract

Transforming growth factor beta (TGF-beta) 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-beta increases reactive oxygen species production and decreases the concentration of glutathione (GSH), the most abundant intracellular free thiol and an important antioxidant, which mediates many of the fibrogenic effects of TGF-beta in various types of cells. 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, 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 fibrogenesis induced by TGF-beta and the potential therapeutic value of antioxidant treatment in fibrotic diseases.

Fig 1. TGF-β increases ROS production and ROS activate/induce TGF-β

TGF-β increases ROS production by Nox4, mitochondria, and microsomes. On the other hand, ROS induce TGF-β gene expression and activate TGF-β through oxidizing latency associated protein (LAP) or activating MMPs, which in turn release LAP.

Fig 2

ROS mediate many of TGF-β’s fibrogenic effects.

Fig 3. ROS and TGF-β signaling

TGF-β increases ROS production, which inactivates MAPK phosphatases (MKPs). Inactivation of MKPs leads to sustained activation of MAPKs, which induces the expression of TGF-β responsive genes by activating (phosphorylating) transcription factors such as AP-1 and SP-1 or by increasing phosphorylation, nuclear translocation, and/or DNA binding of the Smad proteins.

Fig 4. GSH in antioxidant defense and in redox signaling

GSH reduces hydrogen peroxide (H₂O₂) and lipid peroxide (ROOH) through glutathione peroxidase (GPx) catalyzed reactions. Oxidized glutathione (GSSG) formed is then reduced back to GSH by glutathione reductase (GR) catalyzed reaction in consumption of NADPH. GSH also plays a critical role in protein redox signaling through glutaredoxin (Grx) and sulfiredoxin (Srx) catalyzed reactions. During oxidant challenge, protein cysteine residues, especially those in thiolate form (Cys-S⁻), can be oxidized to the sulfenic (RSOH), sulfinic (RSO₂H) and sulfonic (RSO₃H) acid, depending on the intensity of oxidative insults. Although conversion to sulfonic acid form is considered to be irreversible, sulfinic acid can be reduced back to sulfenic acid through Srx catalyzed reaction. Sulfenic acid then reacts with GSH to form protein mixed disulfides (glutathionylation), which can be reduced back to free-thiol form (deglutathionylation) through Grx or Srx catalyzed reactions using GSH as a reductant.

Fig 5. GSH and TGF-β-mediated fibrogenesis

TGF-β decreases intracellular GSH by suppressing the expression of glutamate cysteine ligase (GCL), the rate-limiting enzyme in de novo GSH synthesis. On the other hand, GSH suppresses TGF-β-induced fibrogenic effects by reducing intracellular ROS levels and by preventing oxidative modifications/inactivation of MAPK phosphatases and thus MAPK activation. GSH may also block TGF-β-induced Smad pathway activity indirectly by inhibiting MAPK pathways.