The impact of the reactive oxygen species scavenger N-acetyl-L-cysteine for PDGF-BB signaling in cultured hepatic stellate cells

Aims: Oxidative stress is important in the pathogenesis of liver fibrosis through its induction of hepatic stellate cell (HSC) proliferation and enhancement of collagen synthesis [1, 2]. 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. We have previously shown that NAC is capable of reducing the effects of TGF-β in cultured HSC through its direct reducing activity upon TGF-β molecules and monomerisation of its type III receptor endoglin [3, 4]. Methods: We here analyzed the effects of NAC on PDGF integrity, receptor binding, and downstream signaling in culture-activated HSC. The PDGF-BB signaling pathways were evaluated by autophosphorylation of the Tyr residues of PDGF-activated PDGFR, phosphorylated ERK1/2 MAPK and phospho Akt of PI3K. Results: 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 kinases (ERK1/2), or protein kinase B (PKB/Akt). Conclusions: We conclude that NAC inhibits PDGF signaling in cultured HSC through a mechanism involving the cellular redox status, rather than through a direct reducing effect on PDGF or its signaling components. Furthermore, antifibrogenic effects induced by NAC and other thiols in experimental liver fibrosis might be the result of intracellular redox status that inhibits several signaling pathways, especially in both major growth factors PDGF and TGF-β.