A G Protein-coupled Receptor Responsive to Bile Acids

Abstract

So far some nuclear receptors for bile acids have been identified. However, no cell surface receptor for bile acids has yet been reported. We found that a novel G protein-coupled receptor, TGR5, is responsive to bile acids as a cell-surface receptor. Bile acids specifically induced receptor internalization, the activation of extracellular signal-regulated kinase mitogen-activated protein kinase, the increase of guanosine 5-O-3-thiotriphosphate binding in membrane fractions, and intracellular cAMP production in Chinese hamster ovary cells expressing TGR5. Our quantitative analyses for TGR5 mRNA showed that it was abundantly expressed in monocytes/macrophages in human and rabbit. Treatment with bile acids was found to suppress the functions of rabbit alveolar macrophages including phagocytosis and lipopolysaccharide-stimulated cytokine productions. We prepared a monocytic cell line expressing TGR5 by transfecting a TGR5 cDNA into THP-1 cells that did not express TGR5 originally. Treatment with bile acids suppressed the cytokine productions in the THP-1 cells expressing TGR5, whereas it did not influence those in the original THP-1 cells, suggesting that TGR5 is implicated in the suppression of macrophage functions by bile acids.Bile acids are not simply byproducts of cholesterol metabolism but play essential roles in the absorption of dietary lipids and in the regulation of bile acid synthesis (1). Farnesoid X receptor and pregnane X receptor have been recently identified as specific nuclear receptors for bile acids (2-5). Through the activation of farnesoid X receptor bile acids repress the expression of cholesterol 7␣-hydroxylase, the rate-limiting enzyme in bile acid synthesis (2, 3). The activation of pregnane X receptor by bile acids results in both the repression of cholesterol 7␣-hydroxylase and the transcriptional induction of cytochrome P450 3a, the bile acid-metabolizing enzyme (4, 5). However, no cell surface receptor for bile acids has yet been identified. In hepatobiliary diseases including obstructive jaundice, viral hepatitis, and primary biliary cirrhosis, the mean serum concentration of bile acids exceeds 100 M (range, 70 -400 M), whereas normally this remains below 10 M (6). At such high concentrations, bile acids are known to exhibit immunosuppressive effects on cell-mediated immunity and macrophage functions (6 -8). The phagocytic capacity of the reticuloendothelial system including Kupffer cells is depressed in cholestasis or obstructive jaundice (8). Cholestatic jaundice frequently causes infectious complications and endotoxemia, which are closely related to elevated serum bile acid levels (7, 9). Furthermore, bile acids including deoxycholic acid (DCA) 1 and chenodeoxycholic acid (CDCA) have been demonstrated to have inhibitory activities on the lipopolysaccharide (LPS)-induced production of cytokines in macrophages, including interleukin (IL)-1, IL-6, and tumor necrosis factor ␣ (TNF␣) (10, 11). However, the precise mechanisms involved have remained unclear. Here we show that a novel G prot...

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Clearance of bile and trypsin in rat lungs following aspiration of human gastric fluid

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