Disruption of intermolecular disulfide bonds in PDGF-BB dimers by N-acetyl-l-cysteine does not prevent PDGF signaling in cultured hepatic stellate cells

Oxidative stress is important in the pathogenesis of liver fibrosis through its induction of hepatic stellate cell (HSC) proliferation and enhancement of collagen synthesis. Reactive oxygen species have been found to be essential second messengers in the signaling of both major fibrotic growth factors, platelet-derived growth factor (PDGF) and transforming growth factor-β (TGF-β), in cultured HSC and liver fibrosis. The non-toxic aminothiol N-acetyl-l-cysteine (NAC) inhibits cellular activation and attenuates experimental fibrosis in liver. Prior reports show that NAC is capable of reducing the effects of TGF-β in biological systems, in cultured endothelial cells, and HSC through its direct reducing activity upon TGF-β molecules. We here analyzed the effects of NAC on PDGF integrity, receptor binding, and downstream signaling in culture-activated HSC. We found that NAC dose-dependently induces disintegration of PDGF in vitro. However, even high doses (>20 mM) were not sufficient to prevent the phosphorylation of the PDGF receptor type β, extracellular signal-regulated kinase, or protein kinase B (PKB/Akt). Therefore, we conclude that the PDGF monomer is still active. The described antifibrotic effects are therefore mainly attributable to the structural impairment of TGF-β signaling components reported previously.