Due to its profibrogenic actions, TGF-beta1 and the subsequent steps of the TGF-beta1 induced signaling cascade are prefered targets for antifibrogenic therapies. The fact that Smad-dependent TGF-beta signaling is modulated by reactive oxygen species (ROS) prompted us to investigate the influence of L-cysteine and its derivative N-acetylcysteine (NAC), a scavenger of ROS and precursor of glutathion, on TGF-beta signaling in culture-activated hepatic stellate cells (HSC). In a previous study, we have shown that NAC is able to block TGF-beta? signaling in HSC, but is ineffective when the signal is transduced through a constitutive active TGF-beta? receptor (TbetaR) type I. Analysis of the three known TbetaRs (type I, II and III) in HSC by means of ligand crosslinking and western blot experiments revealed that only receptor type III is affected by NAC treatment. We now show, that in contrast to NAC, L-cysteine exhibits no influence on TGF-beta signaling or TbetaRIII protein (betaglycan) integrity, when applied in concentrations, which do not impair cell viability. In vitro data imply that the sensitivity of TbetaRIII for reducing agents is due to a premodification of this receptor in HSC by limited proteolysis. This proteolytic modification seems to be specific for HSC, since the reductant sensitivity is not observed in cultured cells of various species expressing TbetaRIII heterologously. By further dissecting the NAC sensitivity in the initial step of TGF-beta signaling, we were able to identify a so far unknown member of the TbetaR-family in HSC, i.e. endoglin, an endothelial cell homologue of betaglycan. We currently investigate if this disulfide-linked homodimer is involved in the NAC dependent blockade of TGF-beta signaling in HSC.
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