Downregulation of TGR5 (GPBAR1) in biliary epithelial cells contributes to the pathogenesis of sclerosing cholangitis.

Abstract Background&Aims Primary sclerosing cholangitis (PSC) is characterized by chronic inflammation and progressive fibrosis of the biliary tree. The bile acid receptor TGR5 is found on biliary epithelial cells (BECs), where it promotes secretion, proliferation and tight junction integrity. Thus, we speculated that changes in TGR5-expression in BECs may contribute to PSC pathogenesis. Methods TGR5-expression and -localization were analyzed in PSC livers and liver tissue, isolated bile ducts and BECs from Abcb4-/-, Abcb4-/-/Tgr5Tg and UDCA-as well as norUDCA-fed Abcb4-/- mice. Effects of IL8/IL8 homologues on TGR5 mRNA and protein levels were studied. BEC gene expression was analyzed by single-cell transcriptomics (scRNA-seq) from distinct mouse models. Results TGR5 mRNA expression and immunofluorescence staining intensity were reduced in BECs of PSC and Abcb4-/- livers, in Abcb4-/- extrahepatic bile ducts, but not in intrahepatic macrophages. No changes in TGR5 BEC fluorescence intensity were detected in liver tissue of other liver diseases, including primary biliary cholangitis (PBC). Incubation of BECs with IL8/IL8 homologues, but not with other cytokines, reduced TGR5 mRNA and protein levels. BECs from Abcb4-/- mice had lower levels of phosphorylated Erk and higher expression levels of Icam1, Vcam1 and Tgfβ2. Overexpression of Tgr5 abolished the activated inflammatory phenotype characteristic of Abcb4-/- BECs. NorUDCA-feeding restored TGR5-expression levels in BECs in Abcb4-/- livers. Conclusions Reduced TGR5 levels in BECs of PSC and Abcb4-/- deficiency promote development of a reactive BEC phenotype, aggravate biliary injury and thus contribute to sclerosing cholangitis pathogenesis. Restoration of biliary TGR5-expression levels represents a previously unknown mechanism of action of norUDCA. Lay Summary Primary sclerosing cholangitis (PSC) is a chronic cholestatic liver disease-associated with progressive inflammation of the bile duct, leading to fibrosis and end-stage liver disease. Bile acid (BA) toxicity may contribute to the development and disease progression of PSC. TGR5 is a membrane-bound receptor for BAs, which is found on bile ducts and protects bile ducts from BA toxicity. In this study, we investigated whether changes in TGR5 levels occur on bile duct cells. TGR5 levels were reduced in bile ducts from PSC livers and also in bile ducts from a genetic mouse model of PSC. Genetically modified mice with high quantities of TGR5 in their biliary epithelial cells only developed a milder form of sclerosing cholangitis as compared to their control littermates. A similar beneficial effect could be obtained by norUDCA-feeding to the PSC mice. These findings indicate that lower levels of TGR5 in bile ducts may contribute to PSC development and progression. Furthermore, treatment with norUDCA, a drug currently being tested in a phase III trial for PSC, restored TGR5-levels in biliary epithelial cells.