Bile acid receptors and gastrointestinal functions

Abstract

Bile acids modulate several gastrointestinal (GI) functions including electrolyte secretion and absorption, gastric emptying, and small intestinal and colonic motility. High concentrations of bile acids lead to diarrhea and are implicated in the development of esophageal, gastric and colonic cancer. Alterations in bile acid homeostasis are also implicated in the pathophysiology of irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD). Our understanding of the mechanisms underlying these effects of bile acids on gut functions has been greatly enhanced by the discovery of bile acid receptors, including the nuclear receptors: farnesoid X receptor (FXR), vitamin D receptor (VDR), pregnane X receptor (PXR), and constitutive androstane receptor (CAR); and G protein-coupled receptors (GPCRs): Takeda G protein-coupled receptor 5 (TGR5), sphingosine-1-phosphate receptor 2 (S1PR2), and muscarinic acetylcholine receptor M3 (M3R). For example, various studies provided evidence demonstrating the anti-inflammatory effects of FXR and TGR5 activation in models of intestinal inflammation. In addition, the activation of TGR5 in enteric neurons was recently shown to increase colonic motility, which may lead to bile acid-induced diarrhea (BAD). Interestingly, TGR5 induces the secretion of glucagon-like peptide-1 (GLP-1) from L-cells to enhance insulin secretion and modulate glucose metabolism. Because of the importance of these receptors, agonists of TGR5 and intestine-specific FXR agonists are currently being tested as an option for the treatment of diabetes mellitus and primary bile acid diarrhea, respectively. This review summarizes current knowledge of the functional roles of bile acid receptors in the GI tract. Keywords: Bile acids, Nuclear receptors, Farnesoid X receptor (FXR), Takeda G protein-coupled receptor 5 (TGR5), Gastrointestinal function