Novel Fate-Tracing Strategies Show that Hepatic Stellate Cells Mediate Fibrosis In Vivo

Mederacke I, Hsu CC, Troeger JS, et al. Fate tracing reveals hepatic stellate cells as dominant contributors to liver fibrosis independent of its aetiology. Nat Commun 2013;4:2823. Liver fibrosis often arises as a result of chronic liver damage and is characterized by the deposition of extracellular matrix (ECM) proteins, including collagen at injury sites, which can impair normal liver function and may lead to portal hypertension and hepatic failure (N Engl J Med 2004;350:1646–1654). Clinically, liver fibrosis leads to severe morbidity and mortality in patients (J Clin Invest 2005;115:209–218). Myofibroblast, a cell population that produce ECM proteins, has been shown as the mediator for liver fibrogenesis. However, the identity of cellular sources that give rise to myofibroblast remains elusive. In a recent article published in Nature Communications, Mederacke et al discovered lecithin-retinol acyltrasferase (Lrat) as a novel marker for hepatic stellate cells (HSCs). The expression of Lrat was distinctively high in HSCs, whereas other cells, such as hepatocytes, Kupffer cells, cholangiocytes, or endothelial cells, had undetectable Lrat expression. Consistent with its unique expression pattern, Lrat has been shown to play a pivotal role in regulating the formation of retinyl ester-containing lipid droplet, the signature for quiescent HSCs. With the help of this newly discovered marker for HSCs, Mederacke et al studied the precise contribution of HSCs in promoting liver fibrosis in vivo. Briefly, they designed a novel fate-tracing strategy by generating transgenic mice that have Cre recombinase expression cassette under the control of murine Lrat promoter. These animals were then crossed with floxed Zs-Green fluorescent mice; therefore, HSCs in the progeny mice would be labeled by Zs-Green fluorescence. Mederacke et al first confirmed that Lrat-Cre-labeled cells are mostly (>99%) HSCs, because Lrat-Cre expression and known markers of HSCs (ie, desmin and Pdgfrβ) almost completely overlapped. To investigate the precise role of HSCs in promoting liver fibrogenesis in vivo, they performed fate-tracing experiments in several different fibrotic animal models, including toxin-induced liver fibrosis models (ie, carbon tetrachloride-induced and thioacetamide-induced liver fibrosis models) and biliary liver fibrosis models (ie, bile duct ligation model, 3,5-diethoxycarbonyl-1,4-dihydrocollidine-containing diet model and Mdr2KO mouse model). Strikingly, Mederacke et al found that HSCs are the dominant and universal cell population that gives rise to collagen-secreting myofibroblasts in various murine liver fibrosis models. This was also confirmed by the reduction of liver fibrosis in mice with depletion of HSCs using Lrat-induced diphthelia toxin receptor, which further strengthens the concept that HSCs are the responsible cell types in hepatic fibrosis in general. Another important finding by Mederacke et al is that HSCs do not give rise to epithelial cells after liver damage. They found no increase in the numbers of Lrat-Cre and HNF4a (a hepatocyte marker) double-positive cells during liver regeneration after liver damage caused by 7 different hepatic injury models, including 70% partial hepatectomy model. In addition, by employing series of experiments using bone marrow transplantation, Mederacke et al further concluded that HSCs are endogenous to liver, rather than a bone marrow-derived cell population.