Sandrine Caron

The Farnesoid X Receptor Modulates Adiposity and Peripheral Insulin Sensitivity in Mice

The farnesoid X receptor (FXR) is a bile acid (BA)-activated nuclear receptor that plays a major role in the regulation of BA and lipid metabolism. Recently, several studies have suggested a potential role of FXR in the control of hepatic carbohydrate metabolism, but its contribution to the maintenance of peripheral glucose homeostasis remains to be established. FXR-deficient mice display decreased adipose tissue mass, lower serum leptin concentrations, and elevated plasma free fatty acid levels. Glucose and insulin tolerance tests revealed that FXR deficiency is associated with impaired glucose tolerance and insulin resistance. Moreover, whole-body glucose disposal during a hyperinsulinemic euglycemic clamp is decreased in FXR-deficient mice. In parallel, FXR deficiency alters distal insulin signaling, as reflected by decreased insulin-dependent Akt phosphorylation in both white adipose tissue and skeletal muscle. Whereas FXR is not expressed in skeletal muscle, it was detected at a low level in white adipose tissue in vivo and induced during adipocyte differentiation in vitro. Moreover, mouse embryonic fibroblasts derived from FXR-deficient mice displayed impaired adipocyte differentiation, identifying a direct role for FXR in adipocyte function. Treatment of differentiated 3T3-L1 adipocytes with the FXR-specific synthetic agonist GW4064 enhanced insulin signaling and insulin-stimulated glucose uptake. Finally, treatment with GW4064 improved insulin resistance in genetically obese ob/ob mice in vivo. Although the underlying molecular mechanisms remain to be unraveled, these results clearly identify a novel role of FXR in the regulation of peripheral insulin sensitivity and adipocyte function. This unexpected function of FXR opens new perspectives for the treatment of type 2 diabetes.

Genome-Wide Profiling of Liver X Receptor, Retinoid X Receptor, and Peroxisome Proliferator-Activated Receptor α in Mouse Liver Reveals Extensive Sharing of Binding Sites

ABSTRACT The liver X receptors (LXRs) are nuclear receptors that form permissive heterodimers with retinoid X receptor (RXR) and are important regulators of lipid metabolism in the liver. We have recently shown that RXR agonist-induced hypertriglyceridemia and hepatic steatosis in mice are dependent on LXRs and correlate with an LXR-dependent hepatic induction of lipogenic genes. To further investigate the roles of RXR and LXR in the regulation of hepatic gene expression, we have mapped the ligand-regulated genome-wide binding of these factors in mouse liver. We find that the RXR agonist bexarotene primarily increases the genomic binding of RXR, whereas the LXR agonist T0901317 greatly increases both LXR and RXR binding. Functional annotation of putative direct LXR target genes revealed a significant association with classical LXR-regulated pathways as well as peroxisome proliferator-activated receptor (PPAR) signaling pathways, and subsequent chromatin immunoprecipitation-sequencing (ChIP-seq) mapping of PPARα binding demonstrated binding of PPARα to 71 to 88% of the identified LXR-RXR binding sites. The combination of sequence analysis of shared binding regions and sequential ChIP on selected sites indicate that LXR-RXR and PPARα-RXR bind to degenerate response elements in a mutually exclusive manner. Together, our findings suggest extensive and unexpected cross talk between hepatic LXR and PPARα at the level of binding to shared genomic sites.

Farnesoid X Receptor Deficiency Improves Glucose Homeostasis in Mouse Models of Obesity

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.

Bile acid metabolism and the pathogenesis of type 2 diabetes.

Type 2 diabetes (T2D) is a growing health problem worldwide, but the currently available strategies for therapy and prevention are insufficient. Recent observations indicate that bile acid homeostasis is altered in T2D. Bile acids are metabolic regulators that act as signaling molecules through receptor-dependent and -independent pathways. The most prominent signaling molecules mediating bile acid signaling are the nuclear receptor farnesoid X receptor (FXR) and the membrane receptor TGR5. Both are implicated in the regulation of lipid, glucose, and energy metabolism. Dysregulation of these pathways might contribute to the development of T2D and associated complications. Interestingly, data from studies with bile acids or bile acid sequestrants indicate that the manipulation of bile acid homeostasis might be an attractive approach for T2D therapy. In this review, we summarize the mechanisms of bile acid–mediated metabolic control that might be relevant in the pathogenesis of T2D.

Activation of the farnesoid X receptor represses PCSK9 expression in human hepatocytes

The purpose of this study was to determine whether bile acids (BAs) modulate hepatic pro‐protein convertase subtilisin/kexin 9 (PCSK9) gene expression. Immortalized human hepatocytes were treated with various BAs. Chenodeoxycholic acid (CDCA) treatment specifically decreased both PCSK9 mRNA and protein contents. Moreover, activation of the BA‐activated farnesoid X receptor (FXR) by its synthetic specific agonist GW4064 also decreased PCSK9 expression. Of functional relevance, coadministration of CDCA counteracted the statin‐induced PCSK9 expression, leading to a potentiation of LDL receptor activity. This study suggests that a transcriptional repression of PCSK9 by CDCA or FXR agonists may potentiate the hypolipidemic effect of statins.

Transcriptional Activation of Apolipoprotein CIII Expression by Glucose May Contribute to Diabetic Dyslipidemia

Objective—Hypertriglyceridemia and fatty liver are common in patients with type 2 diabetes, but the factors connecting alterations in glucose metabolism with plasma and liver lipid metabolism remain unclear. Apolipoprotein CIII (apoCIII), a regulator of hepatic and plasma triglyceride metabolism, is elevated in type 2 diabetes. In this study, we analyzed whether apoCIII is affected by altered glucose metabolism. Methods and Results—Liver-specific insulin receptor–deficient mice display lower hepatic apoCIII mRNA levels than controls, suggesting that factors other than insulin regulate apoCIII in vivo. Glucose induces apoCIII transcription in primary rat hepatocytes and immortalized human hepatocytes via a mechanism involving the transcription factors carbohydrate response element–binding protein and hepatocyte nuclear factor-4&agr;. ApoCIII induction by glucose is blunted by treatment with agonists of farnesoid X receptor and peroxisome proliferator-activated receptor-&agr; but not liver X receptor, ie, nuclear receptors controlling triglyceride metabolism. Moreover, in obese humans, plasma apoCIII protein correlates more closely with plasma fasting glucose and glucose excursion after oral glucose load than with insulin. Conclusion—Glucose induces apoCIII transcription, which may represent a mechanism linking hyperglycemia, hypertriglyceridemia, and cardiovascular disease in type 2 diabetes.

Farnesoid X receptor inhibits glucagon-like peptide-1 production by enteroendocrine L cells

Bile acids (BA) are signalling molecules which activate the transmembrane receptor TGR5 and the nuclear receptor FXR. BA sequestrants (BAS) complex BA 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 β-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 glycemia 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.

STAT5 and Oct-1 form a stable complex that modulates cyclin D1 expression.

ABSTRACT Signal transducer and activator of transcription 5 (STAT5) is activated by numerous cytokines that control blood cell development. STAT5 was also shown to actively participate in leukemogenesis. Among the target genes involved in cell growth, STAT5 had been shown to activate cyclin D1 gene expression. We now show that thrombopoietin-dependent activation of the cyclin D1 promoter depends on the integrity of a new bipartite proximal element that specifically binds STAT5A and -B transcription factors. We demonstrate that the stable recruitment of STAT5 to this element in vitro requires the integrity of an adjacent octamer element that constitutively binds the ubiquitous POU homeodomain protein Oct-1. We observe that cytokine-activated STAT5 and Oct-1 form a unique complex with the cyclin D1 promoter sequence. We find that STAT5 interacts with Oct-1 in vivo, following activation by different cytokines in various cellular contexts. This interaction involves a small motif in the carboxy-terminal region of STAT5 which, remarkably, is similar to an Oct-1 POU-interacting motif present in two well-known partners of Oct-1, namely, OBF-1/Bob and SNAP190. Our data offer new insights into the transcriptional regulation of the key cell cycle regulator cyclin D1 and emphasize the active roles of both STAT5 and Oct-1 in this process.

Liver-Specific Peroxisome Proliferator-Activated Receptor α Target Gene Regulation by the Angiotensin Type 1 Receptor Blocker Telmisartan

OBJECTIVE—The angiotensin type 1 receptor blocker (ARB) and peroxisome proliferator–activated receptor (PPAR) γ modulator telmisartan has been recently demonstrated to reduce plasma triglycerides in nondiabetic and diabetic hypertensive patients. The present study investigates the molecular mechanisms of telmisartans hypolipidemic actions, in particular its effect on the PPARα pathway. RESEARCH DESIGN AND METHODS—Regulation of PPARα target genes by telmisartan was studied by real-time PCR and Western immunoblotting in vitro and in vivo in liver/skeletal muscle of mice with diet-induced obesity. Activation of the PPARα ligand binding domain (LBD) was investigated using transactivation assays. RESULTS—Telmisartan significantly induced the PPARα target genes carnitine palmitoyl transferase 1A (CPT1A) in human HepG2 cells and acyl-CoA synthetase long-chain family member 1 (ACSL1) in murine AML12 cells in the micromolar range. Telmisartan-induced CPT1A stimulation was markedly reduced after small interfering RNA–mediated knockdown of PPARα. Telmisartan consistently activated the PPARα-LBD as a partial PPARα agonist. Despite high in vitro concentrations required for PPARα activation, telmisartan (3 mg · kg−1 · day−1) potently increased ACSL1 and CPT1A expression in liver from diet-induced obese mice associated with a marked decrease of hepatic and serum triglycerides. Muscular CPT1B expression was not affected. Tissue specificity of telmisartan-induced PPARα target gene induction may be the result of previously reported high hepatic concentrations of telmisartan. CONCLUSIONS—The present study identifies the ARB/PPARγ modulator telmisartan as a partial PPARα agonist. As a result of its particular pharmacokinetic profile, PPARα activation by telmisartan seems to be restricted to the liver. Hepatic PPARα activation may provide an explanation for telmisartans antidyslipidemic actions observed in recent clinical trials.

Prothrombotic factors in histologically proven nonalcoholic fatty liver disease and nonalcoholic steatohepatitis

An independent role of nonalcoholic fatty liver disease (NAFLD) in the development of cardiovascular disease has been suggested, probably mediated through increased levels of prothrombotic factors. Therefore, we examined whether NAFLD is linked to a prothrombotic state, independently of metabolic risk factors in a large single‐center cohort of overweight/obese patients. Patients presenting to the obesity clinic underwent a detailed metabolic and liver assessment, including an extensive panel of coagulation factors. If NAFLD was suspected, a liver biopsy was proposed. A series of 273 consecutive patients (65% female) with a liver biopsy were included (age, 44 ± 0.76 years; body mass index: 39.6 ± 0.40 kg/m2). Increase in fibrinogen, factor VIII, and von Willebrand factor and decrease in antithrombin III correlated with metabolic features, but not with liver histology. Levels of plasminogen activator inhibitor‐1 (PAI‐1) increased significantly with increasing severity of steatosis (P < 0.001), lobular inflammation (P < 0.001), ballooning (P = 0.002), and fibrosis (P < 0.001). Patients with nonalcoholic steatohepatitis had significantly higher PAI‐1 values than those with normal liver (P < 0.001). In multiple regression, including anthropometric and metabolic parameters, steatosis remained an independent predictor of PAI‐1 levels, explaining, together with fasting C‐peptide and waist circumference, 21% of the variance in PAI‐1. No consistent correlations with histology were found for the other coagulation factors. Conclusion: In obesity, NAFLD severity independently contributes to the increase in PAI‐1 levels, whereas other coagulation factors are unaltered. This finding might, in part, explain the increased cardiovascular risk associated with NAFLD. (Hepatology 2014;58:121–129)