TGR5 (Gpbar-1) is a plasma membrane-bound,
G protein-coupled receptor for
bile acids. TGR5
messenger RNA (
mRNA) has been detected in many tissues, including rat cholangiocytes and mouse gallbladder. A role for TGR5 in
gallstone formation has been suggested, because TGR5 knockout mice did not develop
gallstones when fed a lithogenic diet. In this study, expression and localization of TGR5 was studied in human gallbladders. TGR5
mRNA and
protein were detected in all 19 gallbladders. Although TGR5
mRNA was significantly elevated in the presence of
gallstones, no such relation was found for TGR5
protein levels. In order to study the localization of TGR5 in human gallbladders, a novel antibody was generated. The receptor was localized in the apical membrane and the rab11-positive recycling endosome of gallbladder epithelial cells. Furthermore, the TGR5 staining colocalized with the cyclic
adenosine monophosphate-regulated
chloride channel cystic fibrosis transmembrane conductance regulator (CFTR) and the apical
sodium-dependent
bile salt uptake transporter, suggesting a functional coupling of TGR5 to
bile acid uptake and
chloride secretion. Stimulation with
bile acids significantly increased cyclic
adenosine monophosphate concentration in human gallbladder tissue. Incubation of gallbladder epithelial cells with a TGR5 agonist led to a rise of
N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (MQAE)-fluorescence, suggestive of a decrease in intracellular
chloride concentration. The TGR5 agonist-dependent increase in MQAE-fluorescence was absent in TGR5 knockout mice or in the presence of a CFTR inhibitor, indicating that TGR5 mediates
chloride secretion via activation of CFTR. The presence of the receptor in both the plasma membrane and the recycling endosome indicate that TGR5 can be regulated by translocation.
CONCLUSION: