Bile acid metabolism is intimately linked to the control of energy homeostasis and glucose and lipid metabolism. The nuclear receptor farnesoid X receptor (FXR) plays a major role in the enterohepatic cycling of bile acids, but the impact of nutrients on bile acid homeostasis is poorly characterized. Metabolically active hepatocytes cope with increases in intracellular glucose concentrations by directing glucose into storage (glycogen) or oxidation (glycolysis) pathways, as well as to the pentose phosphate shunt and the hexosamine biosynthetic pathway. Here we studied whether the glucose nonoxidative hexosamine biosynthetic pathway modulates FXR activity. Our results show that FXR interacts with and is OGlcNAcylated by O-GlcNAc transferase in its N-terminal AF1 domain. Increased FXR OGlcNAcylation enhances FXR gene expression and protein stability in a cell type-specific manner. High glucose concentrations increased FXR O-GlcNAcylation, hence its protein stability and transcriptional activity by inactivating corepressor complexes, which associate in a ligand-dependent manner with FXR, and increased FXR binding to chromatin. Finally, in vivo fasting-refeeding experiments show that FXR undergoes O-GlcNAcylation in fed conditions associated with increased direct FXR target gene expression and decreased liver bile acid content. Conclusion: FXR activity is regulated by glucose fluxes in hepatocytes through a direct posttranslational modification catalyzed by the glucose-sensing hexosamine biosynthetic pathway. (HEPATOLOGY 2014;59:2023-2034 See Editorial on Page 1665 T he liver plays an important role in energy homeostasis by constantly tuning metabolic networks in response to varying nutrient fluxes. Marked variations in blood glucose concentrations occur daily, which are rapidly compensated by an integrated response of several tissues, such as the liver, which reacts by adjusting the relative activity of gluconeogenic, glycogenogenic and glycolytic pathways, hence contributing to the maintenance of glucose homeostasis. Hepatocytes also direct glucose into the pentose phosphate shunt and the hexosamine biosynthetic pathways (HBP), the latter consuming up to 5% of the cellular glucose. HBP generates uridine diphosphate N-acetyl-glucosamine (UDP-GlcNAc) from glucose, glutamine, acetyl-coenzyme A, uridine, and adenosine triphosphate (ATP). UDP-GlcNAc synthesis is catalyzed by Abbreviations: BA, bile acid; DON,6

a b s t r a c tNuclear receptors (NRs) are molecular transducers of endocrine and dietary signals allowing tissues to adapt their transcriptional responses to endogenous or exogenous cues. These signals act in many cases as specific ligands, converting of NRs into transcriptionally active molecules. This on-off mechanism needs, however, to be finely tuned with respect to the tissue environment and adjusted to the organism needs. These subtle adjustments of NR transcriptional activity are brought about by posttranslational modifications (PTMs), which can be, in the case of orphan NRs, the sole regulatory mechanism. The role of PTMs, with a more specific focus on phosphorylation, affecting the functions of NR controlling metabolic events is described in this review.