While bile acids (BAs) have long been known to be essential in dietary lipid absorption and cholesterol catabolism, in recent years an important role for BAs as signalling molecules has emerged. BAs activate mitogen-activated protein kinase pathways, are ligands for the G-protein-coupled receptor (GPCR) TGR5 and activate nuclear hormone receptors such as farnesoid X receptor alpha (FXR-alpha; NR1H4). FXR-alpha regulates the enterohepatic recycling and biosynthesis of BAs by controlling the expression of genes such as the short heterodimer partner (SHP; NR0B2) that inhibits the activity of other nuclear receptors. The FXR-alpha-mediated SHP induction also underlies the downregulation of the hepatic fatty acid and triglyceride biosynthesis and very-low-density lipoprotein production mediated by sterol-regulatory-element-binding protein 1c. This indicates that BAs might be able to function beyond the control of BA homeostasis as general metabolic integrators. Here we show that the administration of BAs to mice increases energy expenditure in brown adipose tissue, preventing obesity and resistance to insulin. This novel metabolic effect of BAs is critically dependent on induction of the cyclic-AMP-dependent thyroid hormone activating enzyme type 2 iodothyronine deiodinase (D2) because it is lost in D2-/- mice. Treatment of brown adipocytes and human skeletal myocytes with BA increases D2 activity and oxygen consumption. These effects are independent of FXR-alpha, and instead are mediated by increased cAMP production that stems from the binding of BAs with the G-protein-coupled receptor TGR5. In both rodents and humans, the most thermogenically important tissues are specifically targeted by this mechanism because they coexpress D2 and TGR5. The BA-TGR5-cAMP-D2 signalling pathway is therefore a crucial mechanism for fine-tuning energy homeostasis that can be targeted to improve metabolic control.

TGR5, a metabotropic receptor that is G-protein-coupled to the induction of adenylate cyclase, has been recognized as the molecular link connecting bile acids to the control of energy and glucose homeostasis. With the aim of disclosing novel selective modulators of this receptor and at the same time clarifying the molecular basis of TGR5 activation, we report herein the biological screening of a collection of natural occurring bile acids, bile acid derivatives, and some steroid hormones, which has resulted in the discovery of new potent and selective TGR5 ligands. Biological results of the tested collection of compounds were used to extend the structure-activity relationships of TGR5 agonists and to develop a binary classification model of TGR5 activity. This model in particular could unveil some hidden properties shared by the molecular shape of bile acids and steroid hormones that are relevant to TGR5 activation and may hence be used to address the design of novel selective and potent TGR5 agonists.