Role of the bile acid receptor TGR5 (Gpbar-1) in liver damage and regeneration

Bile acids (BA) are signaling molecules with pleiotropic paracrine and endocrine functions (for recent reviews see [1-4] and references herein). Bile acids are involved in the regulation of bile acid, glucose, lipid and energy homeostasis and can modulate the immune response both in the liver and in the intestine [1,2]. Furthermore, bile acids can promote cell proliferation, cell differentiation and liver regeneration [1-6]. However, especially hydrophobic bile acids may also induce liver damage through induction of programmed cell death (apoptosis) as well as necrosis in hepatocytes [1,2]. Bile acid effects in the liver are cell type and bile acid specific and are facilitated through different bile acid sensing molecules, comprising nuclear hormone receptors, G-protein coupled receptors (GPCRs) as well as integrins and ion channels [1,2]. The farnesoid X receptor (FXR, NR1H4) is a ligand activated transcription factor responsive to different bile acids and highly expressed in hepatocytes. FXR plays an important role in the regulation of bile acid synthesis, detoxification and secretion. Activation of FXR contributes to liver regeneration following partial hepatectomy [5] and alleviates liver injury in different animal models of cholestatic liver disease [1,2]. TGR5 (Gpbar-1, M-Bar) is a GPCR responsive to various unconjugated and conjugated bile acids with taurineconjugated secondary bile acids, such as taurolithocholic acid (TLC) and taurodeoxycholic acid (TDC), being the most potent TGR5 ligands [1,2]. Expression of TGR5 mRNA is almost ubiquitously detected in human and rodent tissues. In liver, TGR5 is found in sinusoidal endothelial cells (SEC), Kupffer cells (KC), cholangiocytes, gallbladder epithelial cells and gallbladder smooth muscle cells [1-3]. Here, activation of TGR5 by bile acids can alter hepatic microcirculation, modulate the immune response, promote cholangiocyte secretion and induce gallbladder filling [3]. The expression pattern in different nonparenchymal cells of the liver as well as the anti-inflammatory, anti-apoptotic, choleretic and proliferative functions of TGR5 suggest a role for this bile acid receptor in liver injury as well as in liver regeneration [1,3]. Mice with a targeted deletion of TGR5 are more susceptible to develop liver steatosis and hepatic inflammation than wildtype littermates, but are protected from the formation of cholesterol gallstones [3]. Using TGR5 specific agonists different groups could demonstrate that activation of the receptor reduces liver inflammation and steatohepatitis, thereby improving liver function tests [3]. Aim of our studies is to elucidate the role of TGR5 in bile acid induced liver damage and regeneration. TGR5 knockout and wildtype mice were fed ad libitum with a bile acid containing diet for 7 days. Liver injury was assessed by serum biochemistry and liver histology. Cellular proliferation was made visible using immunohistochemistry of liver sections and an antibody against proliferating cell nuclear antigen (PCNA). Ductular proliferation was assessed and quantified by cytokeratin (CK)-19 immunofluorescence staining and confocal laser scanning microscopy. Cholangiocytes were cultivated from isolated ducts from livers of TGR5 wildtype and knockout mice and cell proliferation was measured by BrdU incorporation after stimulation with bile acids or specific TGR5 agonists. The role of TGR5 downstream signalling pathways was analyzed with different kinase inhibitors. The generation of reactive oxygen species (ROS) was measured using a fluorescent dye and shedding of EGF was determined by an ELISA assay. Western blotting was carried out to confirm the phosphorylation of the EGFR and ERK1/2. Bile acid feeding resulted in more elevated liver function tests in TGR5 knockout mice as compared to their wildtype littermates on the * Correspondence: [email protected] Clinic of Gastroenterology, Hepatology and Infectious Diseases, Heinrich Heine University, 40225 Düsseldorf, Germany Keitel et al. European Journal of Medical Research 2014, 19(Suppl 1):S21 http://www.eurjmedres.com/content/19/S1/S21 EUROPEAN JOURNAL OF MEDICAL RESEARCH