Dual role of the bile acid receptor Takeda G‐protein‐coupled receptor 5 for hepatic lipid metabolism in feast and famine
Authors: Dieter Häussinger, Verena Keitel
Journal: Hepatology (2016)

Potential conflict of interest: Nothing to report. See Article ON PAGE 813 Bile acids (BAs) not only facilitate the solubilization and absorption of dietary lipids from the intestine, but are signaling molecules and important regulators of BA, lipid, and glucose homeostasis as well as of energy expenditure and immune functions. BA signaling is mediated through different types of BA receptors, comprising nuclear receptors, G‐protein‐coupled receptors and other BA‐responsive molecules, such as integrins or ion channels.1 TGR5 (GPBAR1) is a G‐protein‐coupled receptor that is activated by various BAs with decreasing potency from the secondary BAs, lithocholic acid (LCA) and deoxycholic acid (DCA), to the primary BAs, chenodeoxycholic acid (CDCA) and cholic acid (CA).4 Conjugation of BAs with taurine increases their TGR5 agonistic activity, rendering taurolithocholic acid (TLCA) and taurodeoxycholic acid (TDCA) the most potent TGR5 ligands.4 BA binding to TGR5 through stimulation of adenylate cyclase triggers an increase in intracellular cAMP and calcium concentrations and activates further downstream signaling pathways.1 TGR5 mRNA is detected in a wide variety of tissues, including liver, gallbladder, intestine, and adipose tissue, as well as central and peripheral nervous system.6 In liver, TGR5 is not expressed in hepatocytes, but in different nonparenchymal cells such as sinusoidal endothelial cells, Kupffer cells, activated stellate cells, and cholangiocytes.1 Here, activation of TGR5 modulates sinusoidal blood flow, promotes biliary fluid secretion, and has cell‐protective, antiapoptotic, and anti‐inflammatory effects, thereby attenuating liver injury in animal models of BA overload and inflammation.1 In the intestine, TGR5 is expressed in epithelial cells, enteroendocrine L‐cells, macrophages, and cells of the enteric nervous system. Stimulation of TGR5 in L‐cells promotes secretion of glucagon‐like peptide‐1 (GLP‐1), thereby modulating glucose homeostasis.2 Furthermore, TGR5 is localized in brown adipose tissue and skeletal muscle, where activation of the receptor has been linked to increased energy expanditure.2 TGR5 knockout mice are viable and develop no obvious disease phenotype when kept on a standard rodent chow diet.6 When challenged with a high‐fat diet (HFD), male TGR5 knockout mice displayed a more pronounced liver steatosis as compared to their male wild‐type littermates or female TGR5 knockout mice.1 Treatment of wild‐type mice with either CA or a specific TGR5 agonist promoted increased fat burning and energy expenditure, thereby attenuating HFD‐induced obesity.2 Moreover, administration of a TGR5 agonist to wild‐type mice on HFD resulted in reduced liver fatty acid and triglyceride concentrations and a significant improvement of steatohepatitis.5 The molecular mechanism underlying the TGR5‐dependent changes in lipid metabolism and improvement of steatohepatitis are not fully understood. Potential mechanisms contributing to the beneficial effects of TGR5 activation on HFD‐induced liver steatosis include the anti‐inflammatory actions of the receptor in liver and adipose tissue, the TGR5‐mediated increase in energy expenditure, and the TGR5‐dependent rise in GLP‐1 secretion from intestinal L‐cells, which may stimulate GLP‐1 signaling in hepatocytes.1 Thus, in the situation of overnutrition, activation of TGR5 improves glucose homeostasis, reduces hepatic steatosis, and attenuates weight gain. In contrast, in lean wild‐type mice fed with standard rodent chow diet, supplementation with CA did not affect body weight.2 Although the role of TGR5 in obesity, glucose, and lipid metabolism has been studied in models of chronic overnutrition using both pharmacological activation of the receptor as well as genetic approaches, data on the role of TGR5 in lipid metabolism under physiological conditions or after fasting are lacking. These open questions have now been addressed by Donepudi et al. in the current issue of Hepatology.10 The authors studied the role of TGR5 for hepatic lipid metabolism in ad libitum fed and fasted wild‐type and TGR5 knockout mice. Targeted deletion of TGR5 attenuated fasting‐induced hepatic accumulation of triglycerides, free fatty acids (FFAs), and cholesterol and thus steatosis development. The resistance against fasting‐induced steatosis in TGR5 knockout mice was attributed to a significant increase in physical activity and energy expenditure as well as an increased usage of lipids as an energy source, indicating involvement of extrahepatic tissues such as skeletal muscle. In liver, fasted TGR5 knockout mice displayed reduced hepatic fatty acid uptake as well as increased hepatic lipolysis and very‐low‐density lipoprotein (VLDL) secretion, which together prevented the development of fasting‐induced steatosis. At the molecular level, these changes were attributed to higher serum levels of growth hormone (GH) and subsequent increased GH signaling in hepatocytes. GH activates its surface receptor, which triggers phosphorylation and nuclear translocation of signal transducer and activator of transcription (STAT) 5. This is associated with increased expression of genes relevant for autophagy as well as reduced expression of CD36, resulting in decreased fatty acid uptake into hepatocytes. Reduced levels of intracellular FFAs may account for the up‐regulation of peroxisome proliferator‐activated receptor‐gamma coactivator‐1α, which, in turn, suppresses expression of fibroblast growth factor (FGF) 21. FGF21, which is up‐regulated in response to fasting and triggers lipolysis in adipose tissue and ketogenesis in liver, also inhibits GH‐STAT5 signaling in hepatocytes. Thus, reduced FGF21 levels in TGR5 knockout mice may further amplify the GH‐STAT5 signaling pathway. Protein levels of the rate‐limiting enzyme of mitochondrial beta‐oxidation (β‐ox) carnitine palmitoyltransferase 1 (CPT1) as well as oxidation of palmitic acid were also elevated in liver tissue of fasted TGR5 knockout mice. Although these findings shed new light on the role of TGR5 for lipid metabolism and establish a role for the receptor in the adaptation to fasting, it remains unclear how TGR5 affects GH‐STAT5 and FGF21 signaling as well as gene expression in hepatocytes, which lack expression of this BA receptor. Although TGR5 is expressed in astrocytes and neurons of the central nervous system, the link between TGR5 and GH release from the pituitary gland needs further investigation. This will clarify the role of TGR5 signaling within the brain‐liver axis in response to starvation. A detailed analysis of the gallbladder bile acid composition in wild‐type and TGR5 knockout mice by Donepudi et al. revealed significant genotype‐associated differences.10 Whereas CDCA‐derived taurine‐conjugated alpha‐ and beta‐muricholic acid (tauromuricholic acid; TMCA) constitute over 50% of gallbladder BAs in wild‐type mice, these BAs accounted for only 28% of BAs in TGR5 knockout mice. Absence of TGR5 was associated with increased levels of taurocholic acid (TCA; 61% in TGR5 knockout and 39% in wild‐type animals) and TDCA. These changes were attributed to reduced expression of cytochrome P450 family 7 subfamily B member 1 and thus stimulation of the classic BA synthesis pathway in livers from TGR5 knockout mice. Noteworthy, TCA is a potent ligand for the nuclear BA receptor, farnesoid X receptor (FXR; NR1H4), whereas beta‐TMCA exerts FXR antagonistic properties.11 Therefore, this change in BA composition may lead to increased FXR signaling in the absence of TGR5, which seems plausible given that both BA receptors share some functions such as regulation of glucose homeostasis and modulation of inflammatory response. Whether part of the observed effects on lipid metabolism in fasted TGR5 knockout mice may also be explained by increased FXR signaling remains unclear. The present study demonstrates that absence of TGR5 protects mice from fasting‐induced steatosis through increased GH‐STAT5 signaling and enhanced lipolysis in liver as well as enhanced utilization of lipids in extrahepatic tissues such as skeletal muscle.10 This finding may seem surprising at first glance given that activation of TGR5 reverses steatohepatitis and attenuates weight gain in obese mice. However, the beneficial effects of TGR5 agonists in obese mice were, in part, attributed to increased GLP‐1 secretion and subsequent enhanced insulin secretion, which, in turn, improved glucose homeostasis.5 Insulin signaling should have only minor effects in the starvation model chosen in the present study and may explain the differential effects of TGR5 on hepatic steatosis. Furthermore, pharmacological stimulation of TGR5 in obese mice may not result in the opposite phenotype as that of animals with targeted deletion of TGR5.2 In addition, the finding that TGR5 knockout mice have an altered BA composition in bile with an increase in the FXR ligand, TCA, and a decrease in beta‐TMCA, which has FXR antagonistic properties, may reflect enhanced FXR signaling in the absence of TGR5. The contribution of FXR to the changes observed in the present study needs further investigation. A better understanding of the role of TGR5 in hepatic lipid and BA metabolism is of high clinical relevance given that TGR5 represents a potential drug target for treatment of various aspects of metabolic syndrome, including steatohepatitis.Figure 1: (A) Activation of TGR5 in mice on HFD triggers secretion of GLP‐1 from intestinal L‐cells, which induces satiety in brain and promotes insulin secretion from pancreatic beta cells. Furthermore, GLP‐1 may activate signaling in hepatocytes to alleviate hepatic steatosis. Stimulation of TGR5 in macrophages of white adipose tissue and in Kupffer cells in liver suppresses inflammation, thereby contributing to reduced hepatic steatosis. In skeletal muscle and brown adipose tissue, activation of TGR5 enhances energy expenditure. In summary, stimulation of TGR5 in obese animals improves steatohepatitis, glucose homeostasis, and weight gain. (B) Starvation of TGR5 knockout mice triggers increased secretion of GH, which activates the GH receptor (GHR) on hepatocytes resulting in increased phosphorylation of STAT5. Activation of GH‐STAT5 signaling in liver enhances lipolysis through induction of autophagy and β‐ox. Furthermore, secretion of VLDL is increased, whereas uptake of FAs into hepatocytes is reduced by down‐regulation of CD36. TGR5 knockout mice displayed increased uptake of FAs into muscle, increased usage of lipids as energy source, and increased energy expenditure. These mechanisms prevented the development of fasting‐induced hepatic steatosis in TGR5 knockout mice.