TGR5-dependent hepatoprotection through the regulation of biliary epithelium barrier function

Abstract

ObjectiveWe explored the hypothesis that TGR5, the bile acid (BA) G-protein-coupled receptor highly expressed in biliary epithelial cells, protects the liver against BA overload through the regulation of biliary epithelium permeability.DesignExperiments were performed under basal and TGR5 agonist treatment. In vitro transepithelial electric resistance (TER) and FITC-dextran diffusion were measured in different cell lines. In vivo FITC-dextran was injected in the gallbladder (GB) lumen and traced in plasma. Tight junction proteins and TGR5-induced signalling were investigated in vitro and in vivo (wild-type [WT] and TGR5-KO livers and GB). WT and TGR5-KO mice were submitted to bile duct ligation or alpha-naphtylisothiocyanate intoxication under vehicle or TGR5 agonist treatment, and liver injury was studied.ResultsIn vitro TGR5 stimulation increased TER and reduced paracellular permeability for dextran. In vivo dextran diffusion after GB injection was increased in TGR5-knock-out (KO) as compared with WT mice and decreased on TGR5 stimulation. In TGR5-KO bile ducts and GB, junctional adhesion molecule A (JAM-A) was hypophosphorylated and selectively downregulated among TJP analysed. TGR5 stimulation induced JAM-A phosphorylation and stabilisation both in vitro and in vivo, associated with protein kinase C-ζ activation. TGR5 agonist-induced TER increase as well as JAM-A protein stabilisation was dependent on JAM-A Ser285 phosphorylation. TGR5 agonist-treated mice were protected from cholestasis-induced liver injury, and this protection was significantly impaired in JAM-A-KO mice.ConclusionThe BA receptor TGR5 regulates biliary epithelial barrier function in vitro and in vivo through an impact on JAM-A expression and phosphorylation, thereby protecting liver parenchyma against bile leakage.

Bile Acid pool composition and Gallbladder function are controlled by TGR5 to protect the liver against Bile Acid overload

“…We recently reported that TGR5-mediated BA signaling in the biliary epithelium strengthened paracellular barrier function, protecting the liver parenchyma against BA-induced injury during cholestasis [6]. The present study provides evidence that TGR5 contributes to build a more hydrophilic BA pool at least through targeting the GB compartment in the enterohepatic cycle, by modulating GB function (dilatation) (Supplementary Figure 12).…”

“…Indeed, we and others previously reported that TGR5-KO mice were more sensitive to BDL-or CA-enriched diet-induced liver injury [5,6,8]. We recently reported that TGR5-mediated signaling operated a control on the biliary epithelial barrier function, explaining at least in part why TGR5-KO mice were more prone to BA-induced parenchymal injury [6]. However, TGR5 impact on GB function and/or BA pool composition might also contribute to this phenotype.…”

“…This effect was also observed in more prolonged RO treatment, and did not result from any bile flow stimulation (Figure 5B, right). We further built a volume-pressure curve after sequential GB lumen injections while a continuous monitoring of intra-luminal pressure was performed as previously described [6]. We found that TGR5-KO had strikingly lower capacitance than WT GB, as reflected by the slope values extracted from these curves (Supplementary Figure 9).…”

“…BA signal through both nuclear (mainly Farnesoid X Receptor, FXR) and membrane (mainly S1PR2 and the G Protein-coupled BA Receptor 1, GPBAR-1 or TGR5) receptors, whose distributions are large in the organism, and whose activation elicits a wide array of biological responses. While FXR is well reported to orchestrate adaptive responses protecting the liver from BA overload, TGR5 has been less explored in the liver repair field [5,6]. TGR5 is poorly expressed in hepatocytes but is highly enriched in the biliary tract in which it has been proposed to control chloride (Cl -) secretion [7] , cholangiocyte proliferation [8], biliary epithelial paracellular permeability [6] and gallbladder (GB) filling [9,10].…”

“…While FXR is well reported to orchestrate adaptive responses protecting the liver from BA overload, TGR5 has been less explored in the liver repair field [5,6]. TGR5 is poorly expressed in hepatocytes but is highly enriched in the biliary tract in which it has been proposed to control chloride (Cl -) secretion [7] , cholangiocyte proliferation [8], biliary epithelial paracellular permeability [6] and gallbladder (GB) filling [9,10]. We previously demonstrated that two-thirds partial hepatectomy (PH) was followed by an immediate and massive BA overload in rats, mice and humans and that the lack of TGR5 in mice resulted in impaired liver regeneration mainly through an alteration of BA homeostasis [1,2,4,5].…”