Microbially-conjugated Bile Salts Found in Human Bile Activate the Bile Salt Receptors TGR5 and FXR.
Background & AimsBile salts of hepatic and microbial origin mediate inter-organ crosstalk in the gut-liver axis. Here, we assessed whether the newly discovered class of microbial bile salt conjugates (MBSCs), activate the main host bile salt receptors (TGR5 and FXR) and enter the human systemic and enterohepatic circulation. Approach & ResultsN-amidates of (chenodeoxy)cholic acid and leucine, tyrosine and phenylalanine were synthesized. Receptor activation was studied in cell-free and cell-based assays. MBSCs were quantified in mesenteric and portal blood and bile of patients undergoing pancreatic surgery. MBSCs were activating ligands of TGR5 as evidenced by recruitment of Gs protein, activation of a cAMP-driven reporter, and diminution of LPS-induced cytokine release from macrophages. Intestine- and liver-enriched FXR isoforms were both activated by MBSCs, provided that a bile salt importer was present. Affinity of MBSCs for TGR5 and FXR was not superior to host-derived bile salt conjugates. Individual MBSCs were generally not detected (i.e. <2.5 nmol/L) in human mesenteric or portal blood, but Leu- and Phe-variants were readily measurable in bile, where MBSCs comprised up to 213 ppm of biliary bile salts. ConclusionsMBSCs activate the cell surface receptor TGR5 and the transcription factor FXR, and are substrates for intestinal (ASBT) and hepatic (NTCP) transporters. Their entry into the human circulation is, however, non-substantial. Given low systemic levels and surplus of other equipotent bile salt species, the studied MBSCs are unlikely to have an impact on enterohepatic TGR5/FXR signaling in humans. Origin and function of biliary MBSCs remain to be determined. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=106 SRC="FIGDIR/small/556292v1_ufig1.gif" ALT="Figure 1"> View larger version (25K): org.highwire.dtl.DTLVardef@72f57borg.highwire.dtl.DTLVardef@1526fe8org.highwire.dtl.DTLVardef@130fd89org.highwire.dtl.DTLVardef@155603f_HPS_FORMAT_FIGEXP M_FIG C_FIG Created with BioRender.com