TGR5 in the biliary tree

TGR5 is a plasma membrane-bound, G-protein-coupled receptor for bile acids. TGR5 mRNA has been detected in a variety of tissues, including liver. The aim of the present study was to determine the localization and function of the receptor in biliary epithelial cells.

Liver and gallbladder tissue from humans and rodents were analyzed for TGR5 expression and localization by real-time PCR, Western blot and immunofluorescence microscopy. Cholangiocytes and gallbladder epithelial cells were isolated from wild-type and TGR5 knockout mice. Cyclic AMP (cAMP) was measured using a radioimmunoassay and chloride concentrations were analyzed using the chloride-sensitive dye N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (MQAE). Cell proliferation was determined by bromodeoxyuridine incorporation.

TGR5 is localized in the apical membrane and the primary cilium of cholangiocytes and gallbladder epithelial cells. Activation of the receptor by bile acids led to a rise in intracellular cAMP concentrations and a decrease in intracellular chloride concentrations as measured by MQAE fluorescence, indicating increased chloride secretion. This effect could be abolished in the presence of an inhibitor of the cAMP-dependent chloride channel cystic fibrosis transmembrane conductance regulator. Furthermore, activation of TGR5 by bile acids induced cholangiocyte proliferation, which was not observed in cells derived from TGR5 knockout mice.

In biliary epithelial cells, TGR5 acts as a bile acid sensor coupling biliary bile acid concentrations to bile formation. Furthermore, the receptor may play a role in bile acid-dependent cholangiocyte proliferation and may protect biliary epithelial cells from bile acid-induced cell death.