OBJECTIVEBile acids (BA) participate in the maintenance of metabolic homeostasis acting through different signaling pathways. The nuclear BA receptor farnesoid X receptor (FXR) regulates pathways in BA, lipid, glucose, and energy metabolism, which become dysregulated in obesity. However, the role of FXR in obesity and associated complications, such as dyslipidemia and insulin resistance, has not been directly assessed.RESEARCH DESIGN AND METHODSHere, we evaluate the consequences of FXR deficiency on body weight development, lipid metabolism, and insulin resistance in murine models of genetic and diet-induced obesity.RESULTSFXR deficiency attenuated body weight gain and reduced adipose tissue mass in both models. Surprisingly, glucose homeostasis improved as a result of an enhanced glucose clearance and adipose tissue insulin sensitivity. In contrast, hepatic insulin sensitivity did not change, and liver steatosis aggravated as a result of the repression of β-oxidation genes. In agreement, liver-specific FXR deficiency did not protect from diet-induced obesity and insulin resistance, indicating a role for nonhepatic FXR in the control of glucose homeostasis in obesity. Decreasing elevated plasma BA concentrations in obese FXR-deficient mice by administration of the BA sequestrant colesevelam improved glucose homeostasis in a FXR-dependent manner, indicating that the observed improvements by FXR deficiency are not a result of indirect effects of altered BA metabolism.CONCLUSIONSOverall, FXR deficiency in obesity beneficially affects body weight development and glucose homeostasis.

The nuclear bile acid receptor farnesoid X receptor (FXR) is an important transcriptional regulator of bile acid, lipid, and glucose metabolism. FXR is highly expressed in the liver and intestine and controls the synthesis and enterohepatic circulation of bile acids. However, little is known about FXR-associated proteins that contribute to metabolic regulation. Here, we performed a mass spectrometry-based search for FXR-interacting proteins in human hepatoma cells and identified AMPK as a coregulator of FXR. FXR interacted with the nutrient-sensitive kinase AMPK in the cytoplasm of target cells and was phosphorylated in its hinge domain. In cultured human and murine hepatocytes and enterocytes, pharmacological activation of AMPK inhibited FXR transcriptional activity and prevented FXR coactivator recruitment to promoters of FXR-regulated genes. Furthermore, treatment with AMPK activators, including the antidiabetic biguanide metformin, inhibited FXR agonist induction of FXR target genes in mouse liver and intestine. In a mouse model of intrahepatic cholestasis, metformin treatment induced FXR phosphorylation, perturbed bile acid homeostasis, and worsened liver injury. Together, our data indicate that AMPK directly phosphorylates and regulates FXR transcriptional activity to precipitate liver injury under conditions favoring cholestasis.

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The CDKN2A locus, which contains the tumor suppressor gene p16 INK4a , is associated with an increased risk of agerelated inflammatory diseases, such as cardiovascular disease and type 2 diabetes, in which macrophages play a crucial role. Monocytes can polarize toward classically (CAM) or alternatively (AAM) activated macrophages. However, the molecular mechanisms underlying the acquisition of these phenotypes are not well defined. Here, we show that p16 INK4a IntroductionThe tumor suppressor p16 INK4a is encoded by the CDKN2A locus on the human chromosome 9p21 and on the murine chromosome 4. p16 INK4a belongs to the INK4 family of cyclin-dependent kinase (CDK) inhibitors, also including p15 INK4b , p18 INK4c , and p19 INK4d . [1][2][3][4][5] p16 INK4a inhibits cell-cycle progression by preventing cyclin D-CDK 4/6 complex formation. As a consequence, pRb hyperphosphorylation and its association with E2F, which induces transcription of S-phase genes, are inhibited. p16 INK4a inactivation by deletion, point mutation, or promoter methylation, occurs frequently in most tumors. 6 Besides its role in cancer as an inhibitor of cell-cycle progression, p16 INK4a plays a crucial role in senescence and aging. 7,8 Indeed, expression of p16 INK4a increases with age in various tissues from several species. 9-11 A genome-wide association study has shown association of the CDKN2A locus with an increased risk of the age-related frailty syndrome. 12 In addition, increased p16 INK4a expression causes the age-dependent decline in proliferation of self-renewing cellular compartments such as HSCs, 13 which give rise to immune cells.Although the role of p16 INK4a in mature immune cells has not yet been investigated, several studies has shown that the CDKN2A locus is associated with an increased risk for coronary heart disease, 14 atherosclerosis, 15 and type 2 diabetes (T2D). 16 In these pathologies, immune cells, such as macrophages, play a crucial role. Besides their pleiotropic immune functions, macrophages also play a role in the development and homeostasis of several tissues, such as adipose tissue 17 and liver. 18 Depending on the cytokine environment, macrophages differentiate into distinct subclasses with specific characteristics. Classically activated macrophages (CAM) differentiate in presence of Th1 cytokines, such as IFN␥, or in presence of bacterial products such as lipopolysaccharide (LPS). CAM trigger proinflammatory responses required to kill intracellular pathogens. 19 Alternatively activated macrophages (AAM), induced by Th2 cytokines such as IL-4 and IL-13, are associated with Th2-type immune responses as seen in helminth parasite infections. 19 During inflammation, AAM play a key role in protecting the organism against tissue damage. 20 However, little is known about the mechanisms underlying the acquisition of the AAM phenotype.In the present study, we investigated whether p16 INK4a deficiency influences macrophage activation in vitro, using BMderived macrophages (BMDMs), and in vivo by infection with the pa...

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The farnesoid X receptor (FXR) has been suggested to play a role in gluconeogenesis. To determine whether FXR modulates the response to fasting in vivo, FXR-deficient (FXR À/À ) and wild-type mice were submitted to fasting for 48 h. Our results demonstrate that FXR modulates the kinetics of alterations of glucose homeostasis during fasting, with FXR À/À mice displaying an early, accelerated hypoglycaemia response. Basal hepatic glucose production rate was lower in FXR À/À mice, together with a decrease in hepatic glycogen content. Moreover, hepatic PEPCK gene expression was transiently lower in FXR À/À mice after 6 h of fasting and was decreased in FXR À/À hepatocytes. FXR therefore plays an unexpected role in the control of fuel availability upon fasting.

Farnesoid X receptor (FXR) is highly expressed in liver and intestine where it controls bile acid (BA), lipid and glucose homeostasis. Here we show that FXR is expressed and functional, as assessed by target gene expression analysis, in human islets and beta-cell lines. FXR is predominantly cytosolic-localized in the islets of lean mice, but nuclear in obese mice. Compared to FXR+/+ mice, FXR-/- mice display a normal architecture and beta-cell mass but the expression of certain islet-specific genes is altered. Moreover, glucose-stimulated insulin secretion (GSIS) is impaired in the islets of FXR-/- mice. Finally, FXR activation protects human islets from lipotoxicity and ameliorates their secretory index.

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