Deletion of tumor necrosis factor-α receptor type 1 exacerbates insulin resistance and hepatic steatosis in aromatase knockout mice

The relevance of estrogen functions in lipid metabolism has been suggested in patients with estrogen-signaling deficiencies. Their importance was further implied by studies in estrogen-deficient mice (ArKO mice), which progressively developed hepatic steatosis. As circulating tumor necrosis factor (TNF)-α levels are known to positively correlate with disturbances in lipid metabolism, we investigated the impact of the loss of TNF-α signaling on carbohydrate and lipid metabolism in ArKO mice. Histological examinations of the livers of mice at 5 months of age revealed that ArKO male mice lacking the TNF-α receptor type 1 (TNFR1) gene (ArKO/TNFR1KO) or both the TNFR 1 and 2 genes (ArKO/TNFR1&2KO) developed more severe hepatic steatosis than ArKO or ArKO/TNFR2KO mice. Serum analyses demonstrated a clear increase in cholesterol and insulin levels in the ArKO/TNFR1KO mice compared with the ArKO mice. Glucose- and insulin-tolerance tests further revealed exacerbation of the systemic insulin resistant phenotype in the ArKO/TNFR1KO mice. Hepatic expression of lipogenic genes including fatty-acid synthase and stearoyl-Coenzyme A desaturase 1 were more markedly upregulated in the ArKO/TNFR1KO mice than the ArKO mice. These findings indicate that under estrogen-deficient physiological conditions, hepatic lipid metabolism would benefit from TNF-α mediated signaling via TNFR1.     

Age dependency of estrogen responsiveness in the uterus and adipose tissue of aromatase-knockout (ArKO) mice

Aging is often associated with weight gain caused by metabolic changes including an increase of body fat. In this study we assessed the impact of age on estrogen responsiveness in the uterus and adipose tissue (AT) in aromatase-knockout (ArKO) mice. ArKO mice at the age of three or twelve months respectively were treated s.c. with vehicle, E(2) (10 μg/kg BW/d) or genistein (15 mg/kg BW/d) for three days. In the ArKO mouse model we were able to demonstrate that estrogen treatment resulted in an age specific response pattern both on a physiological and molecular level. Assessment of basal gene expression levels revealed significant age dependent differences only for elevated Esr1 levels in the uterus and leptin levels in infrarenal fat as well as lower levels of Pparg in the gonadal fat tissue. Investigating age dependency of estrogen responsiveness we were able to show that the E(2) and genistein resulted in age related pattern of regulation of expression of Esr1 and Lep in infrarenal and gonadal AT as well as the uterine expression of Pgr, Ltf and Pparg. In conclusion, evidence is provided that aging has an impact on the effectiveness of estrogen regulated processes in uterus and AT of ArKO mice. It remains to be elucidated whether or not this is associated with weight gain caused by an increase in body fat mass.     

Aromatase-deficient (ArKO) mice accumulate excess adipose tissue

Aromatase is the enzyme which catalyses the conversion of C₁₉ steroids into C₁₈ estrogens. We have generated a mouse model wherein the Cyp19 gene, which encodes aromatase, has been disrupted, and hence, the aromatase knockout (ArKO) mouse cannot synthesise endogenous estrogens. We examined the consequences of estrogen deficiency on accumulation of adipose depots in male and female ArKO mice, observing that these animals progressively accrue significantly more intra-abdominal adipose tissue than their wildtype (WT) litter mates, reflected in increased adipocyte volume and number. This increased adiposity was not due to hyperphagia or reduced resting energy expenditure, but was associated with reduced spontaneous physical activity levels, reduced glucose oxidation, and a decrease in lean body mass. Elevated circulating levels of leptin and cholesterol were present in 1-year-old ArKO mice compared to WT controls, as were elevated insulin levels, although blood glucose was unchanged. Associated with these changes, the livers of ArKO animals were characterised by a striking accumulation of lipid droplets. Our findings demonstrate an important role for estrogen in the maintenance of lipid homeostasis in both males and females.     

Cytochrome P-450 CYP2E1 knockout mice are protected against high-fat diet-induced obesity and insulin resistance

Conventional (whole body) CYP2E1 knockout mice displayed protection against high-fat diet-induced weight gain, obesity, and hyperlipidemia with increased energy expenditure despite normal food intake and spontaneous locomotor activity. In addition, the CYP2E1 knockout mice displayed a marked improvement in glucose tolerance on both normal chow and high-fat diets. Euglycemic-hyperinsulinemic clamps demonstrated a marked protection against high-fat diet-induced insulin resistance in CYP2E1 knockout mice, with enhanced adipose tissue glucose uptake and insulin suppression of hepatic glucose output. In parallel, adipose tissue was protected against high-fat diet-induced proinflammatory cytokine production. Taken together, these data demonstrate that the CYP2E1 deletion protects mice against high-fat diet-induced insulin resistance with improved glucose homeostasis in vivo.     

Alternations in hepatic expression of fatty-acid metabolizing enzymes in ArKO mice and their reversal by the treatment with 17beta-estradiol or a peroxisome proliferator

We generated aromatase gene knockout mice (ArKO mice) by targeting disruption of Cyp19, which encodes an enzyme responsible for conversion of androgens to estrogens. We found that ArKO males developed hepatic steatosis spontaneously with aging, indicating that the function of Cyp19 is required to maintain constitutive lipid metabolism in male mice. Plasma lipoprotein analysis using a gel permeation chromatography revealed that high density lipoprotein (HDL)-cholesterol levels were slightly higher in ArKO males than in wild-type males, whereas no other obvious alternations in the profiles were detected. Nevertheless, analysis of lipoprotein compositions by SDS-polyacrylamide gel electrophoresis demonstrated apparent reduction in the amounts of apolipoprotein E, functioning in receptor-mediated clearance of lipoproteins in the liver, in the IDL/LDL fraction of ArKO males as compared with that of wild-type males. Biochemical analysis on the ArKO livers revealed suppression of mRNA expression and activity of enzymes involved in fatty acid beta-oxidation. The impairment was reversed to the wild-type levels by treatment with 17beta-estradiol or bezafibrate, the latter is a synthetic peroxisome proliferator. These findings indicated a pivotal role of estrogen in supporting constitutive hepatic expression of genes involved in fatty acid beta-oxidation and in maintaining lipid homeostasis.     

Development of insulin resistance preceded major changes in iron homeostasis in mice fed a high-fat diet

Type 2 diabetes mellitus (T2DM) and insulin resistance (IR) have been associated with dysregulation of iron metabolism. The basis for this association is not completely understood. To attempt to investigate this, we studied temporal associations between onset of insulin resistance (IR) and dysregulated iron homeostasis, in a mouse model of T2DM.Male C57Bl/6 mice (aged 8 weeks) were fed a high-fat diet (HFD; 60% energy from fat) or a control diet (CD; 10% energy from fat) for 4, 8, 12, 16, 20 and 24 weeks. Development of IR was documented, and various metabolic, inflammatory and iron-related parameters were studied in these mice.HFD-feeding induced weight gain, hepato-steatosis and IR in the mice. Onset of IR occurred from 12 weeks onwards. Hepatic iron stores progressively declined from 16 weeks onwards. Accompanying changes included a decrease in hepatic hepcidin (Hamp1) mRNA expression and serum hepcidin levels and an increase in iron content in the epididymal white adipose tissue (eWAT). Iron content in the liver negatively correlated with that in the eWAT. Factors known to regulate hepatic Hamp1 expression (such as serum iron levels, systemic inflammation, and bone marrow-derived erythroid regulators) were not affected by HFD-feeding. In conclusion, the results show that the onset of IR in HFD-fed mice preceded dysregulation of iron homeostasis, evidence of which were found both in the liver and visceral adipose tissue.     

Effect of adipocyte beta3-adrenergic receptor activation on the type 2 diabetic MKR mice

The antiobesity and antidiabetic effects of the beta3-adrenergic agonists were investigated on nonobese type 2 diabetic MKR mice after injection with a beta3-adrenergic agonist, CL-316243. An intact response to acute CL-316243 treatment was observed in MKR mice. Chronic intraperitoneal CL-316243 treatment of MKR mice reduced blood glucose and serum insulin levels. Hyperinsulinemic euglycemic clamps exhibited improvement of the whole body insulin sensitivity and glucose homeostasis concurrently with enhanced insulin action in liver and adipose tissue. Treating MKR mice with CL-316243 significantly lowered serum and hepatic lipid levels, in part due to increased whole body triglyceride clearance and fatty acid oxidation in adipocytes. A significant reduction in total body fat content and epididymal fat weight was observed along with enhanced metabolic rate in both wild-type and MKR mice after treatment. These data demonstrate that beta3-adrenergic activation improves the diabetic state of nonobese diabetic MKR mice by potentiation of free fatty acid oxidation by adipose tissue, suggesting a potential therapeutic role for beta3-adrenergic agonists in nonobese diabetic subjects.     

Alternations in hepatic expression of fatty-acid metabolizing enzymes in ArKO mice and their reversal by the treatment with 17β-estradiol or a peroxisome proliferator

We generated aromatase gene knockout mice (ArKO mice) by targeting disruption of Cyp19, which encodes an enzyme responsible for conversion of androgens to estrogens. We found that ArKO males developed hepatic steatosis spontaneously with aging, indicating that the function of Cyp19 is required to maintain constitutive lipid metabolism in male mice. Plasma lipoprotein analysis using a gel permeation chromatography revealed that high density lipoprotein (HDL)-cholesterol levels were slightly higher in ArKO males than in wild-type males, whereas no other obvious alternations in the profiles were detected. Nevertheless, analysis of lipoprotein compositions by SDS-polyacrylamide gel electrophoresis demonstrated apparent reduction in the amounts of apolipoprotein E, functioning in receptor-mediated clearance of lipoproteins in the liver, in the IDL/LDL fraction of ArKO males as compared with that of wild-type males. Biochemical analysis on the ArKO livers revealed suppression of mRNA expression and activity of enzymes involved in fatty acid β-oxidation. The impairment was reversed to the wild-type levels by treatment with 17β-estradiol or bezafibrate, the latter is a synthetic peroxisome proliferator. These findings indicated a pivotal role of estrogen in supporting constitutive hepatic expression of genes involved in fatty acid β-oxidation and in maintaining lipid homeostasis.     

Estrogen and adiposity--utilizing models of aromatase deficiency to explore the relationship

Estrogen has an important role to play in energy homeostasis in both men and mice. Lack of estrogen results in the development of a metabolic syndrome in humans and rodents, including excess adiposity, hepatic steatosis (in male but not female aromatase knockout (ArKO) mice) and insulin resistance. Estrogen replacement results in a prompt reversal of the energy imbalance symptoms associated with estrogen deficiency. A corollary to the perturbed energy balance observed in the ArKO mouse is the death by apoptosis of dopaminergic neurons in the hypothalamic arcuate nucleus of male ArKO mice, an area of the brain pivotal to the regulation of energy uptake, storage, and mobilisation. An extension of our work exploring the relationship between estrogen and adiposity has been to examine the role played by androgens in energy balance. We have demonstrated that an increased androgen to estrogen ratio can promote visceral fat accumulation in the rodent by inhibiting AMPK activation and stimulating lipogenesis. Therefore, understanding the regulation of energy homeostasis is becoming an increasingly fascinating challenge, as the number of contributors, their communications, and the complexity of their interactions, involved in the preservation of this equilibrium continues to increase. Models of aromatase deficiency, both naturally occurring and engineered, will continue to provide valuable insights into energy homeostasis.     

Effect of ovariectomy on adipose tissue of mice in the absence of estrogen receptor alpha (ERalpha): a potential role for estrogen receptor beta (ERbeta).

Adipose tissue deposition is highly responsive to estrogen; ovariectomy increases adipose deposition, and estrogen replacement reverses this. Estrogen receptor alpha (ERalpha) plays a major role in adipose tissue. ERalpha knockout (alphaERKO) mice show an increase in adipose tissue of over a 100 % compared to wild-type mice. However, alphaERKO mice undergo a 10-fold increase in 17beta-estradiol (E2), and persistent or even increased signaling through ERbeta could be a factor in obesity of alphaERKO mice. To test the hypothesis that ERbeta plays a role in adipose tissue, adult female alphaERKO mice were ovariectomized or sham-ovariectomized and fed a phytoestrogen-free diet. Ovariectomized mice were treated with vehicle or E2, and bodyweights and food consumption were measured. Mice were killed after 28 days and inguinal and parametrial fat pads collected. Sham-ovariectomized alphaERKO mice had increased body weight, ovariectomized alphaERKO mice showed a 6 % decrease, and E2 replacement restored body weight to sham levels. Fat pads of ovariectomized alphaERKO mice showed 45 % and 16 % decreases in weight and adipocyte circumference, respectively, compared to sham-ovariectomized or E2-replaced ovariectomized alphaERKO mice. Ovariectomized alphaERKO mice showed a trend towards decreased feed consumption that did not reach significance. Blood glucose levels were lower both before and after glucose injection in ovariectomized compared to sham alphaERKO mice, and E2 treatment reversed this. Insulin levels following glucose challenge were lower in ovariectomized compared to sham-ovariectomized alphaERKO mice, indicating that ovariectomy ameliorated the glucose intolerance and insulin resistance in alphaERKO mice. Immunohistochemical analysis revealed strong staining for ERbeta in adipose tissue. These observations indicate that removing E2/ERbeta signaling in alphaERKO mice by ovariectomy decreases body and fat-pad weights and adipocyte size, while improving insulin and glucose metabolism. ERbeta mediated effects on adipose tissue are opposite those of ERalpha, although E2 effects on adipose tissue are predominately through ERalpha.     

Caveolin-3 knockout mice show increased adiposity and whole body insulin resistance, with ligand-induced insulin receptor instability in skeletal muscle

Caveolin-3 (Cav-3) is expressed predominantly in skeletal muscle fibers, where it drives caveolae formation at the muscle cell's plasma membrane. In vitro studies have suggested that Cav-3 may play a positive role in insulin signaling and energy metabolism. We directly address the in vivo metabolic consequences of genetic ablation of Cav-3 in mice as it relates to insulin action, glucose metabolism, and lipid homeostasis. At age 2 mo, Cav-3 null mice are significantly larger than wild-type mice, and display significant postprandial hyperinsulinemia, whole body insulin resistance, and whole body glucose intolerance. Studies using hyperinsulinemic-euglycemic clamps revealed that Cav-3 null mice exhibited 20% and 40% decreases in insulin-stimulated whole body glucose uptake and whole body glycogen synthesis, respectively. Whole body insulin resistance was mostly attributed to 20% and 40% decreases in insulin-stimulated glucose uptake and glucose metabolic flux in the skeletal muscle of Cav-3 null mice. In addition, insulin-mediated suppression of hepatic glucose production was significantly reduced in Cav-3 null mice, indicating hepatic insulin resistance. Insulin-stimulated glucose uptake in white adipose tissue, which does not express Cav-3, was decreased by 70% in Cav-3 null mice, suggestive of an insulin-resistant state for this tissue. During fasting, Cav-3 null mice possess normal insulin receptor protein levels in their skeletal muscle. However, after 15 min of acute insulin stimulation, Cav-3 null mice show dramatically reduced levels of the insulin receptor protein, compared with wild-type mice treated identically. These results suggest that Cav-3 normally functions to increase the stability of the insulin receptor at the plasma membrane, preventing its rapid degradation, i.e., by blocking or slowing ligand-induced receptor downregulation. Thus our results demonstrate the importance of Cav-3 in regulating whole body glucose homeostasis in vivo and its possible role in the development of insulin resistance. These findings may have clinical implications for the early diagnosis and treatment of caveolinopathies. limb girdle muscular dystrophy; glucose intolerance; hyperinsulinemia; insulin receptor degradation     

The association of phosphoinositide 3-kinase enhancer A with hepatic insulin receptor enhances its kinase activity

Dysfunction of hepatic insulin receptor tyrosine kinase (IRTK) causes the development of type 2 diabetes. However, the molecular mechanism regulating IRTK activity in the liver remains poorly understood. Here, we show that phosphoinositide 3-kinase enhancer A (PIKE-A) is a new insulin-dependent enhancer of hepatic IRTK. Liver-specific Pike-knockout (LPKO) mice display glucose intolerance with impaired hepatic insulin sensitivity. Specifically, insulin-provoked phosphoinositide 3-kinase/Akt signalling is diminished in the liver of LPKO mice, leading to the failure of insulin-suppressed gluconeogenesis and hyperglycaemia. Thus, hepatic PIKE-A has a key role in mediating insulin signal transduction and regulating glucose homeostasis in the liver.     

Mechanism of glucose intolerance in mice with dominant negative mutation of CEACAM1

Mice with liver-specific overexpression of dominant negative phosphorylation-defective S503A-CEACAM1 mutant (L-SACC1) developed chronic hyperinsulinemia resulting from blunted hepatic clearance of insulin, visceral obesity, and glucose intolerance. To determine the underlying mechanism of altered glucose homeostasis, a 2-h hyperinsulinemic euglycemic clamp was performed, and tissue-specific glucose and lipid metabolism was assessed in awake L-SACC1 and wild-type mice. Inactivation of carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) caused insulin resistance in liver that was mostly due to increased expression of fatty acid synthase and lipid metabolism, resulting in elevated intrahepatic levels of triglyceride and long-chain acyl-CoAs. Whole body insulin resistance in the L-SACC1 mice was further attributed to defects in insulin-stimulated glucose uptake in skeletal muscle and adipose tissue. Insulin resistance in peripheral tissues was associated with significantly elevated intramuscular fat contents that may be secondary to increased whole body adiposity (assessed by (1)H-MRS) in the L-SACC1 mice. Overall, these results demonstrate that L-SACC1 is a mouse model in which chronic hyperinsulinemia acts as a cause, and not a consequence, of insulin resistance. Our findings further indicate the important role of CEACAM1 and hepatic insulin clearance in the pathogenesis of obesity and insulin resistance.     

Omija fruit ethanol extract improves adiposity and related metabolic disturbances in mice fed a high-fat diet

This study investigated the biological and molecular mechanisms underlying the antiobesity effect of omija fruit ethanol extract (OFE) in mice fed a high-fat diet (HFD). C57BL/6J mice were fed an HFD (20% fat, w/w) with or without OFE (500 mg/kg body weight) for 16 weeks. Dietary OFE significantly increased brown adipose tissue weight and energy expenditure while concomitantly decreasing white adipose tissue (WAT) weight and adipocyte size by up-regulating the expression of brown fat-selective genes in WAT. OFE also improved hepatic steatosis and dyslipidemia by enhancing hepatic fatty acid oxidation-related enzymes activity and fecal lipid excretion. In addition to steatosis, OFE decreased the expression of pro-inflammatory genes in the liver. Moreover, OFE improved glucose tolerance and lowered plasma glucose, insulin and homeostasis model assessment of insulin resistance, which may be linked to decreases in the activity of hepatic gluconeogenic enzymes and the circulating level of gastric inhibitory polypeptide. These findings suggest that OFE may protect against diet-induced adiposity and related metabolic disturbances by controlling brown-like transformation of WAT, fatty acid oxidation, inflammation in the liver and fecal lipid excretion. Improved insulin resistance may be also associated with its antiobesity effects.     

Comparative assessment of estrogenic responses with relevance to the metabolic syndrome and to menopausal symptoms in wild-type and aromatase-knockout mice

Knockout of the Cyp-19 gene (aromatase) renders mice to have insufficient endogenous estrogen production and contributes to the development of symptoms related the metabolic syndrome, including excess adiposity and insulin resistance. This study comparatively assessed the estrogen responsiveness in animal models of genetical versus surgical (ovariectomy) origin of estrogen deficiency. Evaluation of physiological parameters and gene expression pattern in response to estrogens revealed differences in estrogen responsiveness between aromatase deficient and castrated or intact wild-type mice. ArKO mice had a significantly higher bodyweight than matched ovariectomized wild-type mice. The weight of the completely regressed uterus following ovariectomy was higher than the uterine weight of ArKO mice. Further, alterations in metabolic parameters like increased serum leptin levels and decreased plasma glucose levels in genetically deficient mice became apparent. Finally, expression pattern of estrogen responsive genes differed in the two experimental models of estrogen deficiency. Both, in uterine and adipose tissues the regulation of expression of some genes either was inversed of regulation or considerably differed in the magnitude of the response in the two models. Our studies demonstrate that the cause of estrogen deficiency significantly impacts on estrogen responsiveness and may be of relevance for investigations on aspects of estrogen deficiency and metabolic and/or menopausal symptoms.     

12/15-Lipoxygenase Is Required for the Early Onset of High Fat Diet-Induced Adipose Tissue Inflammation and Insulin Resistance in Mice

Background

Recent understanding that insulin resistance is an inflammatory condition necessitates searching for genes that regulate inflammation in insulin sensitive tissues. 12/15-lipoxygenase (12/15LO) regulates the expression of proinflammatory cytokines and chemokines and is implicated in the early development of diet-induced atherosclerosis. Thus, we tested the hypothesis that 12/15LO is involved in the onset of high fat diet (HFD)-induced insulin resistance.

Methodology/Principal Findings

Cells over-expressing 12/15LO secreted two potent chemokines, MCP-1 and osteopontin, implicated in the development of insulin resistance. We assessed adipose tissue inflammation and whole body insulin resistance in wild type (WT) and 12/15LO knockout (KO) mice after 2–4 weeks on HFD. In adipose tissue from WT mice, HFD resulted in recruitment of CD11b⁺, F4/80⁺ macrophages and elevated protein levels of the inflammatory markers IL-1β, IL-6, IL-10, IL-12, IFNγ, Cxcl1 and TNFα. Remarkably, adipose tissue from HFD-fed 12/15LO KO mice was not infiltrated by macrophages and did not display any increase in the inflammatory markers compared to adipose tissue from normal chow-fed mice. WT mice developed severe whole body (hepatic and skeletal muscle) insulin resistance after HFD, as measured by hyperinsulinemic euglycemic clamp. In contrast, 12/15LO KO mice exhibited no HFD-induced change in insulin-stimulated glucose disposal rate or hepatic glucose output during clamp studies. Insulin-stimulated Akt phosphorylation in muscle tissue from HFD-fed mice was significantly greater in 12/15LO KO mice than in WT mice.

Conclusions

These results demonstrate that 12/15LO mediates early stages of adipose tissue inflammation and whole body insulin resistance induced by high fat feeding.     

Differential effects of pharmacological liver X receptor activation on hepatic and peripheral insulin sensitivity in lean and ob/ob mice

Liver X receptor (LXR) agonists have been proposed to act as anti-diabetic drugs. However, pharmacological LXR activation leads to severe hepatic steatosis, a condition usually associated with insulin resistance and type 2 diabetes mellitus. To address this apparent contradiction, lean and ob/ob mice were treated with the LXR agonist GW-3965 for 10 days. Insulin sensitivity was assessed by hyperinsulinemic-euglycemic clamp studies. Hepatic glucose production (HGP) and metabolic clearance rate (MCR) of glucose were determined with stable isotope techniques. Blood glucose and hepatic and whole body insulin sensitivity remained unaffected upon treatment in lean mice, despite increased hepatic triglyceride contents (61.7 +/- 7.2 vs. 12.1 +/- 2.0 nmol/mg liver, P < 0.05). In ob/ob mice, LXR activation resulted in lower blood glucose levels and significantly improved whole body insulin sensitivity. GW-3965 treatment did not affect HGP under normo- and hyperinsulinemic conditions, despite increased hepatic triglyceride contents (221 +/- 13 vs. 176 +/- 19 nmol/mg liver, P < 0.05). Clamped MCR increased upon GW-3965 treatment (18.2 +/- 1.0 vs. 14.3 +/- 1.4 ml x kg(-1) x min(-1), P = 0.05). LXR activation increased white adipose tissue mRNA levels of Glut4, Acc1 and Fasin ob/ob mice only. In conclusion, LXR-induced blood glucose lowering in ob/ob mice was attributable to increased peripheral glucose uptake and metabolism, physiologically reflected in a slightly improved insulin sensitivity. Remarkably, steatosis associated with LXR activation did not affect hepatic insulin sensitivity.     

Acute activation of central GLP-1 receptors enhances hepatic insulin action and insulin secretion in high-fat-fed, insulin resistant mice

Glucagon-like peptide-1 (GLP-1) receptor knockout (Glp1r(-/-)) mice exhibit impaired hepatic insulin action. High fat (HF)-fed Glp1r(-/-) mice exhibit improved, rather than the expected impaired, hepatic insulin action. This is due to decreased lipogenic gene expression and triglyceride accumulation. The present studies overcome these secondary adaptations by acutely modulating GLP-1R action in HF-fed wild-type mice. The central GLP-1R was targeted given its role as a regulator of hepatic insulin action. We hypothesized that acute inhibition of the central GLP-1R impairs hepatic insulin action beyond the effects of HF feeding. We further hypothesized that activation of the central GLP-1R improves hepatic insulin action in HF-fed mice. Insulin action was assessed in conscious, unrestrained mice using the hyperinsulinemic euglycemic clamp. Mice received intracerebroventricular (icv) infusions of artificial cerebrospinal fluid, GLP-1, or the GLP-1R antagonist exendin-9 (Ex-9) during the clamp. Intracerebroventricular Ex-9 impaired the suppression of hepatic glucose production by insulin, whereas icv GLP-1 improved it. Neither treatment affected tissue glucose uptake. Intracerebroventricular GLP-1 enhanced activation of hepatic Akt and suppressed hypothalamic AMP-activated protein kinase. Central GLP-1R activation resulted in lower hepatic triglyceride levels but did not affect muscle, white adipose tissue, or plasma triglyceride levels during hyperinsulinemia. In response to oral but not intravenous glucose challenges, activation of the central GLP-1R improved glucose tolerance. This was associated with higher insulin levels. Inhibition of the central GLP-1R had no effect on oral or intravenous glucose tolerance. These results show that inhibition of the central GLP-1R deteriorates hepatic insulin action in HF-fed mice but does not affect whole body glucose homeostasis. Contrasting this, activation of the central GLP-1R improves glucose homeostasis in HF-fed mice by increasing insulin levels and enhancing hepatic insulin action.