Fig. 3.

Summary of actions of bile acids (BA) mediated by farnesoid X receptor (FXR) and TGR5 (G protein-coupled receptor). Bile acids are actively transported into ileal enterocytes by ASBT [apical sodium-coupled bile acid transporter, also known as the ileal bile acid transporter (IBAT)]. Bile acids are agonists at the nuclear FXR activating the synthesis of the hormone, fibroblast growth factor-19 (FGF-19), which has multiple effects including reduced glycolysis and lipogenesis, improved insulin sensitivity, and reduced bile acid synthesis. Independently of FXR receptor actions, the absorbed bile acids increase energy expenditure in muscle and fat. Bile acids also bind the G protein-coupled bile acid receptor TGR5 (triglyceride receptor 5) on enteroendocrine L cells to activate release of glucagon-like peptides (GLP)-1 and -2, which results in retardation of gastric emptying, inhibition of appetite, and improved glucose metabolism. Other TGR5 receptors on intrinsic nerves result in effects of bile acids to increase colonic fluid and electrolyte secretion and motility. G, G protein; AC, adenylate cyclase; ILBP, ileal lipid binding protein; OST, organic solute transporter.