**1. Introduction**

TGR5 (Gpbar1) is a G protein-coupled bile acid receptor expressed in various cell types, including macrophages, as well as non-parenchymal liver cells such as activated hepatic stellate cells (HSCs) and liver sinusoidal endothelial cells (LSECs) [1–5]. Activation of TGR5 occurs after binding of bile acids (BAs), leading to an intracellular increase of cyclic AMP (cAMP) as second messenger and to the activation of further downstream signaling [6–8]. TGR5 is known to play an important role in biliary epithelial cell function and has anti-inflammatory as well as cytoprotective properties [2,4,6,9–14]. TGR5 knockout mice (TGR5 KO) have a very mild phenotype without any signs of overt liver disease [15,16]. However, TGR5 KO are more susceptible towards cholestatic and inflammatory liver injury [17–19]. While TGR5 is responsive to all human primary and secondary BAs, irrespective of conjugation state, the secondary BA taurolithocholic acid (TLC) represents the most potent TGR5 BA agonist [6,20]. Interestingly, feeding wildtype (WT) mice a diet enriched in 1% (*w*/*w*) LCA triggers within a few days segmental bile duct obstruction by crystal precipitation leading to the development of focal areas of necrosis (bile infarcts) with subsequent recruitment and accumulation of neutrophils, as well as to the induction of periportal fibrosis [21,22]. Using mice deficient for either the intercellular adhesion molecule-1 (ICAM-1 KO mice) or the catalytic subunit of NADPH oxidase (gp91phox KO mice), it was demonstrated that the contribution of neutrophils to liver injury following LCA feeding is negligible. Thus, the hepatotoxicity stems from LCA and its metabolites directly, and this model can be used to rapidly induce severe cholestatic liver injury, which also shows characteristics of sclerosing cholangitis [21,22].

The burden of liver diseases continues to rise in European countries and accounts for approximately 150,000 deaths per year attributed to liver disease, of which about two-thirds of patients die before the age of 65 years [23]. The most important non-malignant complication of chronic liver disease in humans is the development of portal hypertension (PH). PH results from increased intrahepatic vascular resistance, which is due to structural changes within the sinusoids during fibrosis development, to dysfunction of HSCs and LSECs as well as to microvascular thrombosis and platelet dysfunction [24,25]. Nitric oxide (NO) is an essential regulator of portal pressure. However, NO release from LSECs is decreased in cirrhotic livers, contributing to LSEC dysfunction [24]. We have previously demonstrated that TGR5 is expressed in LSECs, where activation of the receptor induced expression and activation of endothelial NO synthase (eNOS) and subsequent generation of NO [1].

While TGR5 was not expressed in quiescent HSCs in vitro and in vivo, cultivation of isolated HSCs on plastic dishes, which triggers differentiation into a myofibroblast-like phenotype (activated HSCs), led to an upregulation of TGR5 mRNA and protein levels [4,26]. These findings suggest that BA signaling via TGR5 may play a role in the regulation of hepatic vascular tone and portal pressure under physiological but also disease conditions. Since LCA feeding not only resulted in severe cholestatic liver injury and recruitment of inflammatory cells, but also rapid development of periductal fibrosis [21], we studied the impact of short-term LCA diet supplementation in TGR5 WT and KO mice.
