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Background and Prupose: The nuclear bile acid (BA) receptor farnesoid X receptor (FXR) is a major regulator of metabolic/energy homeostasis in peripheral organs. Indeed, enterohepatic-expressed FXR controls metabolic processes (BA, glucose and lipid metabolism, fat mass, body weight). The central nervous system (CNS) regulates energy homeostasis in close interaction with peripheral organs. While FXR has been reported to be expressed in the brain, its function has not been studied so far. Experimental Approach: We studied the role of FXR in brain control of energy homeostasis by treating wild-type and FXR-deficient mice by intracerebroventricular (ICV) injection with the reference FXR agonist GW4064. Key Results: Here we show that pharmacological activation of brain FXR modifies energy homeostasis by affecting brown adipose tissue (BAT) function. Brain FXR activation decreases the rate-limiting enzyme in catecholamine synthesis, tyrosine hydroxylase (TH), and consequently the sympathetic tone. FXR activation acts by inhibiting hypothalamic PKA-CREB induction of TH expression. Conclusions and Implication: These findings identify a function of brain FXR in the control of energy homeostasis and shed new light on the complex control of energy homeostasis by BA through FXR.

The gut microbiota participates in the control of energy homeostasis partly through fermentation of dietary fibers hence producing short-chain fatty acids (SCFAs), which in turn promote the secretion of the incretin Glucagon-Like Peptide-1 (GLP-1) by binding to the SCFA receptors FFAR2 and FFAR3 on enteroendocrine L-cells. We have previously shown that activation of the nuclear Farnesoid X Receptor (FXR) decreases the L-cell response to glucose. Here, we investigated whether FXR also regulates the SCFA-induced GLP-1 secretion. GLP-1 secretion in response to SCFAs was evaluated ex vivo in murine colonic biopsies and in colonoids of wild-type (WT) and FXR knock-out (KO) mice, in vitro in GLUTag and NCI-H716 L-cells activated with the synthetic FXR agonist GW4064 and in vivo in WT and FXR KO mice after prebiotic supplementation. SCFA-induced GLP-1 secretion was blunted in colonic biopsies from GW4064-treated mice and enhanced in FXR KO colonoids. In vitro FXR activation inhibited GLP-1 secretion in response to SCFAs and FFAR2 synthetic ligands, mainly by decreasing FFAR2 expression and downstream Gαq-signaling. FXR KO mice displayed elevated colonic FFAR2 mRNA levels and increased plasma GLP-1 levels upon local supply of SCFAs with prebiotic supplementation. Our results demonstrate that FXR activation decreases L-cell GLP-1 secretion in response to inulin-derived SCFA by reducing FFAR2 expression and signaling. Inactivation of intestinal FXR using bile acid sequestrants or synthetic antagonists in combination with prebiotic supplementation may be a promising therapeutic approach to boost the incretin axis in type 2 diabetes.

Objective— Although initially seemingly paradoxical because of the lack of nucleus, platelets possess many transcription factors that regulate their function through DNA-independent mechanisms. These include the farnesoid X receptor (FXR), a member of the superfamily of ligand-activated transcription factors, that has been identified as a bile acid receptor. In this study, we show that FXR is present in human platelets and FXR ligands, GW4064 and 6α-ethyl-chenodeoxycholic acid, modulate platelet activation nongenomically. Approach and Results— FXR ligands inhibited the activation of platelets in response to stimulation of collagen or thrombin receptors, resulting in diminished intracellular calcium mobilization, secretion, fibrinogen binding, and aggregation. Exposure to FXR ligands also reduced integrin α IIb β 3 outside-in signaling and thereby reduced the ability of platelets to spread and to stimulate clot retraction. FXR function in platelets was found to be associated with the modulation of cyclic guanosine monophosphate levels in platelets and associated downstream inhibitory signaling. Platelets from FXR-deficient mice were refractory to the actions of FXR agonists on platelet function and cyclic nucleotide signaling, firmly linking the nongenomic actions of these ligands to the FXR. Conclusions— This study provides support for the ability of FXR ligands to modulate platelet activation. The atheroprotective effects of GW4064, with its novel antiplatelet effects, indicate FXR as a potential target for the prevention of atherothrombotic disease.

Bile acids are signalling molecules, which activate the transmembrane receptor TGR5 and the nuclear receptor FXR. BA sequestrants (BAS) complex bile acids in the intestinal lumen and decrease intestinal FXR activity. The BAS-BA complex also induces glucagon-like peptide-1 (GLP-1) production by L cells which potentiates beta-cell glucose-induced insulin secretion. Whether FXR is expressed in L cells and controls GLP-1 production is unknown. Here, we show that FXR activation in L cells decreases proglucagon expression by interfering with the glucose-responsive factor Carbohydrate-Responsive Element Binding Protein (ChREBP) and GLP-1 secretion by inhibiting glycolysis. In vivo, FXR deficiency increases GLP-1 gene expression and secretion in response to glucose hence improving glucose metabolism. Moreover, treatment of ob/ob mice with the BAS colesevelam increases intestinal proglucagon gene expression and improves glycaemia in a FXR-dependent manner. These findings identify the FXR/GLP-1 pathway as a new mechanism of BA control of glucose metabolism and a pharmacological target for type 2 diabetes.

Conditional gene knockout technology is a powerful tool to study the function of a gene in a specific tissue, organ or cell lineage. The most commonly used procedure applies the Cre-LoxP strategy, where the choice of the Cre driver promoter is critical to determine the efficiency and specificity of the system. However, a considered choice of an appropriate promoter does not always protect against the risk of unwanted recombination and the consequent deletion of the gene in other tissues than the desired one(s), due to phenomena of non-specific activation of the Cre transgene. Furthermore, the causes of these phenomena are not completely understood and this can potentially affect every strain of Cre-mice. In our study on the deletion of a same gene in two different tissues, we show that the incidence rate of non-specific recombination in unwanted tissues depends on the Cre driver strain, ranging from 100 %, rendering it useless (aP2-Cre strain), to ~5 %, which is still compatible with their use (RIP-Cre strain). The use of a simple PCR strategy conceived to detect this occurrence is indispensable when producing a tissue-specific knockout mouse. Therefore, when choosing the Cre-driver promoter, researchers not only have to be careful about its tissue-specificity and timing of activation, but should also include a systematical screening in order to exclude mice in which atypical recombination has occurred and to limit the unnecessary use of laboratory animals in uninterpretable experiments.

International audience; The glucose-activated transcription factor carbohydrate response element binding protein (ChREBP) induces the expression of hepatic glycolytic and lipogenic genes. The farnesoid X receptor (FXR) is a nuclear bile acid receptor controlling bile acid, lipid, and glucose homeostasis. FXR negatively regulates hepatic glycolysis and lipogenesis in mouse liver. The aim of this study was to determine whether FXR regulates the transcriptional activity of ChREBP in human hepatocytes and to unravel the underlying molecular mechanisms. Agonist-activated FXR inhibits glucose-induced transcription of several glycolytic genes, including the liver-type pyruvate kinase gene (L-PK), in the immortalized human hepatocyte (IHH) and HepaRG cell lines. This inhibition requires the L4L3 region of the L-PK promoter, known to bind the transcription factors ChREBP and hepatocyte nuclear factor 4α (HNF4α). FXR interacts directly with ChREBP and HNF4α proteins. Analysis of the protein complex bound to the L4L3 region reveals the presence of ChREBP, HNF4α, FXR, and the transcriptional coactivators p300 and CBP at high glucose concentrations. FXR activation does not affect either FXR or HNF4α binding to the L4L3 region but does result in the concomitant release of ChREBP, p300, and CBP and in the recruitment of the transcriptional corepressor SMRT. Thus, FXR transrepresses the expression of genes involved in glycolysis in human hepatocytes.

OBJECTIVE Bile 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 METHODS Here, 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. RESULTS FXR 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. CONCLUSIONS Overall, FXR deficiency in obesity beneficially affects body weight development and glucose homeostasis.

Introduction L'invalidation conditionnelle in vivo par la strategie Cre-Lox est un outil puissant pour identifier la fonction d'un gene/d'une proteine specifiquement dans un tissu ou un type cellulaire donne. Le choix du promoteur guidant l'expression de la recombinase Cre est fondamental pour determiner la specificite tissulaire de l'invalidation genique. Cependant, le choix d'un promoteur approprie ne permet pas d'empecher les recombinaisons non-specifiques dues au phenomene d'activation non-specifique du gene codant la Cre dans les tissus non cibles. Materiels et methodes Un meme gene d'interet a ete invalide dans 2 types cellulaires differents (cellule beta pancreatique et adipocyte) en croisant une souche de souris floxee pour le gene d'interet avec respectivement une souche transgenique RIP (Rat Insulin Promoter) -Cre (Tg (Ins2-Cre) 23Herr) et une souche transgenique aP2 (adipocyte protein 2) -Cre (aP2-CreSI). Pour detecter une potentielle recombinaison non-specifique, une recherche systematique de l'allele « nul » (allele floxe et recombine par la l'enzyme Cre-recombinase) en plus de l'allele floxe et de l'allele sauvage a ete realisee par PCR sur l'ADN extrait de biopsie caudale et des principaux tissus metaboliques dans les souris issues des deux elevages. Resultats 1) L'utilisation systematique d'une simple PCR permet de detecter les recombinaisons non-specifiques dans le genotypage sur biopsie caudale, et ainsi d'eliminer les souris problematiques de l'elevage. 2) Le taux de recombi-naison non-specifique est dependant de la souche Cre, allant de 5 % (RIP-Cre, la souche est alors utilisable pour generer les souris invalidees pour le gene d'interet) a 100 % (aP2-Cre, rendant la souche inutilisable). 3) L'analyse de l'ADN de differents tissus metabolique (foie, pancreas, tissus adipeux) par la meme procedure de PCR montre que lorsque la recombinaison non-specifique est presente dans l'ADN caudal, elle est retrouvee aussi dans les autres organes analyses. Conclusion Une recherche systematique de recombinaison non-specifique devrait etre mise en œuvre par simple PCR au cours du genotypage dans toutes les souches murines obtenues par ce systeme. Declaration d’interet Les auteurs declarent ne pas avoir d'interet direct ou indirect (financier ou en nature) avec un organisme prive, industriel ou commercial en relation avec le sujet presente.

Introduction Les acides biliaires (ABs) jouent un role fondamental dans l’homeostasie du glucose en se liant a 2 recepteurs : TGR5, recepteur membranaire, exprime en particulier dans les cellules enterointestinales et promouvant la secretion de l’incretine GLP-1, et FXR, recepteur nucleaire, exprime dans differents organes cles du metabolisme, en particulier dans la cellule beta-pancreatique, ou il a ete montre comme regulateur de la secretion d’insuline. Ainsi, les ABs contribueraient au controle de l’homeostasie du glucose en orchestrant la relation entre l’axe entero-hepatique et le pancreas endocrine. Notre objectif est de comprendre la contribution de FXR de la cellule beta-pancreatique dans la reponse aux ABs dans ce systeme integre en caracterisant le modele murin deficient pour FXR dans la cellule beta-pancreatique (FXRbetacell-KO). Materiels et methodes Le phenotype de la souris FXR-betacell-KO est caracterise par analyse des parametres circulants (glycemie, insulinemie), des tests fonctionnels de reponse au glucose chez la souris nourrie par un regime standard ou par un regime riche en lipides (60 %, Research Diet), des tests fonctionnels ex vivo sur les ilots en reponse a differents stimuli insulino-secretagogues (glucose, incretines…), une analyse transcriptomique et metabolomique, et l’analyse du profil des ABs circulants et tissulaires (HPLC-MS/MS). Resultats Sous regime standard, la souris FXR-betacell-KO presente une intolerance au glucose qui apparait avec l’âge, avec une insulinemie plus faible comparees aux controles, a l’etat basal et apres injection de glucose, phenotype exacerbe chez les souris nourries par un regime riche en graisses. Discussion Nos resultats confirment un role regulateur positif de FXR-betapancreatique dans la secretion de l’insuline en reponse au glucose in vivo. Ces resultats seront completes par l’etude de l’impact de FXR-beta-pancreatique sur la reponse aux incretines et les mecanismes moleculaires associes et permettrons de mieux comprendre le lien Acides Biliaires/axe entero-hepatique/pancreas endocrine dans la physiopathologie du diabete.