TGF-beta,Smad signaling in hepatic stellate cells and during liver fibrogenesis
Transforming growth factor (TGF)-beta is a key mediator of hepatic stellate cell (HSC) activation and extracellular matrix accumulation leading to fibrosis. Aim of the present study was to perturb the TGF-beta pathway in this process by overexpression of Smad7, an intracellular antagonist of TGF-beta signaling. Moreover, a new TGF-beta response gene was identified, which participates in the control of profibrogenic transdifferentiation to myofibroblast-like cells. Ligation of the common bile duct was used to induce liver fibrosis in rats. Animals received injections of an adenovirus overexpressing Smad7. The effect of Smad7 on TGF-beta signaling and activation of HSC was further analyzed in primary cultured cells. AdSmad7 infected rats displayed reduced collagen deposition and hydroxyproline content in the liver, when compared with animals administered AdLacZ. Such a beneficial effect was also observed, when Smad7 was expressed in animals with established fibrosis. Accordingly, Smad7 arrested transdifferentiation of primary cultured HSC. AdSmad7 infected cells remained in a quiescent stage and retained storage of vitamin A droplets. Smad7 expression totally blocked TGF-beta signal transduction, shown by inhibiting Smad2/3 phosphorylation. In an attempt to elucidate TGF-beta target genes responsible for fibrogenesis, an analysis of Smad7-dependent mRNA expression profiles in HSC was performed, resulting in identification of the inhibitor of differentiation 1 (Id1) gene. Ectopic Smad7 expression reduced Id1 expression. Conversely, Id1 overexpression in HSC enhanced cell activation and circumvented Smad7-dependent inhibition of transdifferentiation. Moreover, knock-down of Id1 in HSC interfered with the transdifferentiation process, indicating a pivotal role of Id1 for fibrogenesis. Additionally, treatment of HSC with TGF-beta led to increased Id1 protein expression, which was dependent on activation of the novel ALK1/Smad1/5/8 pathway. Knocking-down expression of ALK1 in HSC interfered with TGF-beta dependent Smad1 phosphorylation and Id1 upregulation, but did not block the activation of the Smad2/3 pathway. In summary, gene transfer of Smad7 inhibits experimental fibrogenesis in vivo. Studies with isolated HSC suggest that the underlying mechanisms involve inhibition of TGF-beta signaling and HSC transdifferentiation. Moreover, a novel TGF-beta pathway involving ALK1 and Smad1 was shown to mediate HSC activation via Id1 upregulation. These new findings represent valuable information for the development of more efficient antifibrotic strategies to treat liver damage in the future.
info:eu-repo/classification/ddc/570, Biowissenschaften, Biologie
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