Peroxisome proliferator-activated receptor-alpha deficiency does not alter insulin sensitivity in mice maintained on regular or high-fat diet: hyperinsulinemic-euglycemic clamp studies

Ling-gui-zhu-gan granules reduces obesity and ameliorates metabolic disorders by inducing white adipose tissue browning in obese mice

Background

Ling-gui-zhu-gan (LGZG) formula has been demonstrated to effectively ameliorate the clinical symptoms of patients with obesity or metabolic syndrome. This study aimed to explore both the effect and the underlying mechanisms of LGZG against obesity.

Methods

Male C57BL/6N mice were randomized into four groups (n = 8): normal control (NC), obese (OB), metformin (Met), and LGZG. After 8 weeks of gavage administration, the pharmacological effects of LGZG on obesity and metabolism were investigated using biochemical parameters, histomorphological examination, and lipidomics techniques. Pivotal factors associated with white adipose tissue browning were evaluated using quantitative real-time polymerase chain reaction and western blotting.

Results

The results revealed that LGZG reduced the levels of obesity markers, including body weights, body fat mass and food intake in obese mice. Further evaluations highlighted that LGZG restored glucose homeostasis and significantly improved insulin sensitivity in obese mice. Importantly, LGZG could adjust serum lipid profiles and regulate the lipidomic spectrum of intestinal contents, with noticeable shifts in the levels of certain lipids, particularly diacylglycerols and monoacylglycerols. Histopathological examinations of LGZG-treated mice also revealed more favorable adipose tissue structures than their obese counterparts. Furthermore, we found that LGZG upregulated the expression of several key thermogenesis-related factors, such as UCP1, PRDM16, PGC-1α, PPARα, PPARγ, CTBP1, and CTBP2 in white adipose tissues.

Conclusion

Our findings position LGZG as a novel strategy for preventing obesity and improving metabolic health.

Bilirubin deficiency renders mice susceptible to hepatic steatosis in the absence of insulin resistance

Background & aims

Plasma concentrations of bilirubin, a product of heme catabolism formed by biliverdin reductase A (BVRA), inversely associate with the risk of metabolic diseases including hepatic steatosis and diabetes mellitus in humans. Bilirubin has antioxidant and anti-inflammatory activities and may also regulate insulin signaling and peroxisome proliferator-activated receptor alpha (PPARα) activity. However, a causal link between bilirubin and metabolic diseases remains to be established. Here, we used the global Bvra gene knockout (Bvra^–/–) mouse as a model of deficiency in bilirubin to assess its role in metabolic diseases.

Approach & results

We fed mice fat-rich diets to induce hepatic steatosis and insulin resistance. Bile pigments were measured by LC-MS/MS, and hepatic lipids by LC-MS/MS (non-targeted lipidomics), HPLC-UV and Oil-Red-O staining. Oxidative stress was evaluated measuring F₂-isoprostanes by GC-MS. Glucose metabolism and insulin sensitivity were verified by glucose and insulin tolerance tests, ex vivo and in vivo glucose uptake, and Western blotting for insulin signaling. Compared with wild type littermates, Bvra^–/– mice contained negligible bilirubin in plasma and liver, and they had comparable glucose metabolism and insulin sensitivity. However, Bvra^–/– mice exhibited an inflamed and fatty liver phenotype, accompanied by hepatic accumulation of oxidized triacylglycerols and F₂-isoprostanes, in association with depletion of α-tocopherol. α-Tocopherol supplementation reversed the hepatic phenotype and observed biochemical changes in Bvra^–/– mice.

Conclusions

Our data suggests that BVRA deficiency renders mice susceptible to oxidative stress-induced hepatic steatosis in the absence of insulin resistance.

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Low plasma levels of bilirubin associate with increased metabolic disease risk.

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A direct link between bilirubin and metabolic disease remains to be established.

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Global BVRA deficiency causes global bilirubin deficiency and a fatty, inflamed liver.

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This hepatic phenotype is linked to decreased vitamin E and increased lipid oxidation.

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Vitamin E supplements restore normal liver phenotype in BVRA deficiency.

The transcription factors CREBH, PPARa, and FOXO1 as critical hepatic mediators of diet-induced metabolic dysregulation

The liver is a critical mediator of lipid and/or glucose homeostasis and is a primary organ involved in dynamic changes during feeding and fasting. Additionally, hepatic-centric pathways are prone to dysregulation during pathophysiological states including metabolic syndrome (MetS) and non-alcoholic fatty liver disease. Omics platforms and GWAS have elucidated genes related to increased risk of developing MetS and related disorders, but mutations in these metabolism-related genes are rare and cannot fully explain the increasing prevalence of MetS-related pathologies worldwide. Complex interactions between diet, lifestyle, environmental factors, and genetic predisposition jointly determine inter-individual variability of disease risk. Given the complexity of these interactions, researchers have focused on master regulators of metabolic responses incorporating and mediating the impact of multiple environmental cues. Transcription factors are DNA binding, terminal executors of signaling pathways that modulate the cellular responses to complex metabolic stimuli and are related to the control of hepatic lipid and glucose homeostasis. Among numerous hepatic transcription factors involved in regulating metabolism, three emerge as key players in transducing nutrient sensing, which are dysregulated in MetS-related perturbations in both clinical and preclinical studies: cAMP Responsive Element Binding Protein 3 Like 3 (CREB3L3), Peroxisome Proliferator Activated Receptor Alpha (PPAR), and Forkhead Box O1 (FOXO1). Additionally, these three transcription factors appear to be amenable to dietary and/or nutrient-based therapies, being potential targets of nutritional therapy. In this review we aim to describe the activation, regulation, and impact of these transcription factors in the context of metabolic homeostasis. We also summarize their perspectives in MetS and nutritional therapies.

MicroRNA-184 alleviates insulin resistance in cardiac myocytes and high fat diet-induced cardiac dysfunction in mice through the LPP3/DAG pathway

AIM

Cardiovascular complication is a major cause of mortality and morbidity in patients with diabetes. Insulin sensitivity loss is a major contributor to the pathogenesis of cardiovascular diseases in diabetes. Based on our previous research, diacylglycerol (DAG) levels play an important role in high saturated fatty acid-induced insulin resistance. Phosphatidic acid phosphatase (LPP3), a key enzyme for synthesizing DAG, is indispensable for normal cardiac functions and vascular health. However, adipose knockdown of LPP3 increases insulin sensitivity, suggesting that LPP3 regulation may be complicated in hearts. The aim of this study was to investigate LPP3 roles in diabetic cardiac insulin sensitivity and to identify potential upstream targets implicated in diabetic cardiomyopathy.

METHODS AND RESULTS

Mice were fed a high fat diet (HF) or a low fat diet (control) for up to 24 weeks. After 24 weeks, we found that high fat diet-induced cardiac dysfunction is linked to elevated LPP3 compared to the control group (P < 0.05). In addition, knockdown of LPP3 rescued the glucose uptake that was impaired by palmitate treatment alone in cardiomyoblasts (P < 0.05). Furthermore, we identified miR-184 as an upstream regulator targeting LPP3 and further confirmed the link between DAG and insulin sensitivity. MiR-184 mimic transfection rescued the glucose uptake and glucose consumption that had been impaired by palmitate treatment alone (P < 0.05).

CONCLUSION

In hearts of high fat diet-fed mice, increased LPP3 contributes to insulin resistance via increased DAG levels. A small non-coding RNA, miR-184, at least partially regulates this signal pathway to alleviate insulin resistance.

Bilirubin remodels murine white adipose tissue by reshaping mitochondrial activity and the coregulator profile of peroxisome proliferator-activated receptor α

Activation of lipid-burning pathways in the fat-storing white adipose tissue (WAT) is a promising strategy to improve metabolic health and reduce obesity, insulin resistance, and type II diabetes. For unknown reasons, bilirubin levels are negatively associated with obesity and diabetes. Here, using mice and an array of approaches, including MRI to assess body composition, biochemical assays to measure bilirubin and fatty acids, MitoTracker-based mitochondrial analysis, immunofluorescence, and high-throughput coregulator analysis, we show that bilirubin functions as a molecular switch for the nuclear receptor transcription factor peroxisome proliferator-activated receptor α (PPARα). Bilirubin exerted its effects by recruiting and dissociating specific coregulators in WAT, driving the expression of PPARα target genes such as uncoupling protein 1 (Ucp1) and adrenoreceptor β 3 (Adrb3). We also found that bilirubin is a selective ligand for PPARα and does not affect the activities of the related proteins PPARγ and PPARδ. We further found that diet-induced obese mice with mild hyperbilirubinemia have reduced WAT size and an increased number of mitochondria, associated with a restructuring of PPARα-binding coregulators. We conclude that bilirubin strongly affects organismal body weight by reshaping the PPARα coregulator profile, remodeling WAT to improve metabolic function, and reducing fat accumulation.

Loss of hepatic PPARα promotes inflammation and serum hyperlipidemia in diet-induced obesity

Agonists for PPARα are used clinically to reduce triglycerides and improve high-density lipoprotein (HDL) cholesterol levels in patients with hyperlipidemia. Whether the mechanism of PPARα activation to lower serum lipids occurs in the liver or other tissues is unknown. To determine the function of hepatic PPARα on lipid profiles in diet-induced obese mice, we placed hepatocyte-specific peroxisome proliferator-activated receptor-α (PPARα) knockout (Ppara^HepKO) and wild-type (Ppara^fl/fl) mice on high-fat diet (HFD) or normal fat diet (NFD) for 12 wk. There was no significant difference in weight gain, percent body fat mass, or percent body lean mass between the groups of mice in response to HFD or NFD. Interestingly, the Ppara^HepKO mice on HFD had worsened hepatic inflammation and a significant shift in the proinflammatory M1 macrophage population. These changes were associated with higher hepatic fat mass and decreased hepatic lean mass in the Pparα^HepKO on HFD but not in NFD as measured by Oil Red O and noninvasive EchoMRI analysis (31.1 ± 2.8 vs. 20.2 ± 1.5, 66.6 ± 2.5 vs. 76.4 ± 1.5%, P < 0.05). We did find that this was related to significantly reduced peroxisomal gene function and lower plasma β-hydroxybutyrate in the Ppara^HepKO on HFD, indicative of reduced metabolism of fats in the liver. Together, these provoked higher plasma triglyceride and apolipoprotein B100 levels in the Ppara^HepKO mice compared with Ppara^fl/fl on HFD. These data indicate that hepatic PPARα functions to control inflammation and liver triglyceride accumulation that prevent hyperlipidemia.

TLR4 Signaling via NANOG Cooperates With STAT3 to Activate Twist1 and Promote Formation of Tumor-Initiating Stem-Like Cells in Livers of Mice

BACKGROUND & AIMS

Obesity and alcohol consumption contribute to steatohepatitis, which increases the risk for hepatitis C virus (HCV)-associated hepatocellular carcinomas (HCCs). Mouse hepatocytes that express HCV-NS5A in liver up-regulate the expression of Toll-like receptor 4 (TLR4), and develop liver tumors containing tumor-initiating stem-like cells (TICs) that express NANOG. We investigated whether the TLR4 signals to NANOG to promote the development of TICs and tumorigenesis in mice placed on a Western diet high in cholesterol and saturated fat (HCFD).

METHODS

We expressed HCV-NS5A from a transgene (NS5A Tg) in Tlr4-/- (C57Bl6/10ScN), and wild-type control mice. Mice were fed a HCFD for 12 months. TICs were identified and isolated based on being CD133+, CD49f+, and CD45-. We obtained 142 paraffin-embedded sections of different stage HCCs and adjacent nontumor areas from the same patients, and performed gene expression, immunofluorescence, and immunohistochemical analyses.

RESULTS

A higher proportion of NS5A Tg mice developed liver tumors (39%) than mice that did not express HCV NS5A after the HCFD (6%); only 9% of Tlr4-/- NS5A Tg mice fed HCFD developed liver tumors. Livers from NS5A Tg mice fed the HCFD had increased levels of TLR4, NANOG, phosphorylated signal transducer and activator of transcription (pSTAT3), and TWIST1 proteins, and increases in Tlr4, Nanog, Stat3, and Twist1 messenger RNAs. In TICs from NS5A Tg mice, NANOG and pSTAT3 directly interact to activate expression of Twist1. Levels of TLR4, NANOG, pSTAT3, and TWIST were increased in HCC compared with nontumor tissues from patients.

CONCLUSIONS

HCFD and HCV-NS5A together stimulated TLR4-NANOG and the leptin receptor (OB-R)-pSTAT3 signaling pathways, resulting in liver tumorigenesis through an exaggerated mesenchymal phenotype with prominent Twist1-expressing TICs.

NANOG Metabolically Reprograms Tumor-Initiating Stem-like Cells through Tumorigenic Changes in Oxidative Phosphorylation and Fatty Acid Metabolism

Stem cell markers, including NANOG, have been implicated in various cancers; however, the functional contribution of NANOG to cancer pathogenesis has remained unclear. Here, we show that NANOG is induced by Toll-like receptor 4 (TLR4) signaling via phosphorylation of E2F1 and that downregulation of Nanog slows down hepatocellular carcinoma (HCC) progression induced by alcohol western diet and hepatitis C virus protein in mice. NANOG ChIP-seq analyses reveal that NANOG regulates the expression of genes involved in mitochondrial metabolic pathways required to maintain tumor-initiating stem-like cells (TICs). NANOG represses mitochondrial oxidative phosphorylation (OXPHOS) genes, as well as ROS generation, and activates fatty acid oxidation (FAO) to support TIC self-renewal and drug resistance. Restoration of OXPHOS activity and inhibition of FAO renders TICs susceptible to a standard care chemotherapy drug for HCC, sorafenib. This study provides insights into the mechanisms of NANOG-mediated generation of TICs, tumorigenesis, and chemoresistance through reprogramming of mitochondrial metabolism.

Molecular mechanism of PPARα action and its impact on lipid metabolism, inflammation and fibrosis in non-alcoholic fatty liver disease

Peroxisome proliferator-activated receptor α (PPARα) is a ligand-activated transcription factor belonging, together with PPARγ and PPARβ/δ, to the NR1C nuclear receptor subfamily. Many PPARα target genes are involved in fatty acid metabolism in tissues with high oxidative rates such as muscle, heart and liver. PPARα activation, in combination with PPARβ/δ agonism, improves steatosis, inflammation and fibrosis in pre-clinical models of non-alcoholic fatty liver disease, identifying a new potential therapeutic area. In this review, we discuss the transcriptional activation and repression mechanisms by PPARα, the spectrum of target genes and chromatin-binding maps from recent genome-wide studies, paying particular attention to PPARα-regulation of hepatic fatty acid and plasma lipoprotein metabolism during nutritional transition, and of the inflammatory response. The role of PPARα, together with other PPARs, in non-alcoholic steatohepatitis will be discussed in light of available pre-clinical and clinical data.

Role of forkhead box protein A3 in age-associated metabolic decline

Aging is associated with increased adiposity and diminished thermogenesis, but the critical transcription factors influencing these metabolic changes late in life are poorly understood. We recently demonstrated that the winged helix factor forkhead box protein A3 (Foxa3) regulates the expansion of visceral adipose tissue in high-fat diet regimens; however, whether Foxa3 also contributes to the increase in adiposity and the decrease in brown fat activity observed during the normal aging process is currently unknown. Here we report that during aging, levels of Foxa3 are significantly and selectively up-regulated in brown and inguinal white fat depots, and that midage Foxa3-null mice have increased white fat browning and thermogenic capacity, decreased adipose tissue expansion, improved insulin sensitivity, and increased longevity. Foxa3 gain-of-function and loss-of-function studies in inguinal adipose depots demonstrated a cell-autonomous function for Foxa3 in white fat tissue browning. Furthermore, our analysis revealed that the mechanisms of Foxa3 modulation of brown fat gene programs involve the suppression of peroxisome proliferator activated receptor γ coactivtor 1 α (PGC1α) levels through interference with cAMP responsive element binding protein 1-mediated transcriptional regulation of the PGC1α promoter. Overall, our data demonstrate a role for Foxa3 in energy expenditure and in age-associated metabolic disorders.

Dietary modification of metabolic pathways via nuclear hormone receptors

Nuclear hormone receptors (NHRs), as ligand-dependent transcription factors, have emerged as important mediators in the control of whole body metabolism. Because of the promiscuous nature of several members of this superfamily that have been found to bind ligand with lower affinity than the classical steroid NHRs, they consequently display a broader ligand selectivity. This promiscuous nature has facilitated various bioactive dietary components being able to act as agonist ligands for certain members of the NHR superfamily. By binding to these NHRs, bioactive dietary components are able to mediate changes in various metabolic pathways, including, glucose, cholesterol and triglyceride homeostasis among others. This review will provide a general overview of the nuclear hormone receptors that have been shown to be activated by dietary components. The physiological consequences of such receptor activation by these dietary components will then be discussed in more detail.

Hyperinsulinemic-euglycemic clamps in conscious, unrestrained mice

Type 2 diabetes is characterized by a defect in insulin action. The hyperinsulinemic-euglycemic clamp, or insulin clamp, is widely considered the "gold standard" method for assessing insulin action in vivo. During an insulin clamp, hyperinsulinemia is achieved by a constant insulin infusion. Euglycemia is maintained via a concomitant glucose infusion at a variable rate. This variable glucose infusion rate (GIR) is determined by measuring blood glucose at brief intervals throughout the experiment and adjusting the GIR accordingly. The GIR is indicative of whole-body insulin action, as mice with enhanced insulin action require a greater GIR. The insulin clamp can incorporate administration of isotopic 2[¹⁴C]deoxyglucose to assess tissue-specific glucose uptake and [3-³H]glucose to assess the ability of insulin to suppress the rate of endogenous glucose appearance (endoRa), a marker of hepatic glucose production, and to stimulate the rate of whole-body glucose disappearance (Rd).

The miniaturization of the insulin clamp for use in genetic mouse models of metabolic disease has led to significant advances in diabetes research. Methods for performing insulin clamps vary between laboratories. It is important to note that the manner in which an insulin clamp is performed can significantly affect the results obtained. We have published a comprehensive assessment of different approaches to performing insulin clamps in conscious mice¹ as well as an evaluation of the metabolic response of four commonly used inbred mouse strains using various clamp techniques². Here we present a protocol for performing insulin clamps on conscious, unrestrained mice developed by the Vanderbilt Mouse Metabolic Phenotyping Center (MMPC; URL: www.mc.vanderbilt.edu/mmpc). This includes a description of the method for implanting catheters used during the insulin clamp. The protocol employed by the Vanderbilt MMPC utilizes a unique two-catheter system³. One catheter is inserted into the jugular vein for infusions. A second catheter is inserted into the carotid artery, which allows for blood sampling without the need to restrain or handle the mouse. This technique provides a significant advantage to the most common method for obtaining blood samples during insulin clamps which is to sample from the severed tip of the tail. Unlike this latter method, sampling from an arterial catheter is not stressful to the mouse¹. We also describe methods for using isotopic tracer infusions to assess tissue-specific insulin action. We also provide guidelines for the appropriate presentation of results obtained from insulin clamps.

Genetic manipulations of PPARs: effects on obesity and metabolic disease

The interest in genetic manipulations of PPARs is as old as their discovery as receptors of ligands with beneficial clinical activities. Considering the effects of PPAR ligands on critical aspects of systemic physiology, including obesity, lipid metabolism, insulin resistance, and diabetes, gene knockout (KO) in mice is the ideal platform for both hypothesis testing and discovery of new PPAR functions in vivo. With the fervent pursuit of the magic bullet to eradicate the obesity epidemic, special emphasis has been placed on the impacts of PPARs on obesity and its associated diseases. As detailed in this review, understanding how PPARs regulate gene expression and basic metabolic pathways is a necessary intermediate en route to deciphering their effects on obesity. Over a decade and dozens of genetic modifications of PPARs into this effort, valuable lessons have been learned, but we are left with more questions to be answered. These lessons and future prospects are the subject of this review.

Energy intake and adiponectin gene expression

Hypoadiponectinemia and decreased adiponectin gene expression in white adipose tissue (WAT) have been well observed in obese subjects and animal models. However, the mechanism for obesity-associated hypoadiponectinemia is still largely unknown. To investigate the regulatory role of energy intake, dietary fat, and adiposity in adiponectin gene expression and blood adiponectin level, a series of feeding regimens was employed to manipulate energy intake and dietary fat in obese-prone C57BL/6, genetically obese ob/ob, obese-resistant A/J and peroxisome proliferator-activated receptor-α gene knockout (PPARα KO) mice. Adiponectin gene expression in WAT and circulating adiponectin levels were studied in these dietary intervention-treated mice. Our study showed that calorie restriction (CR) robustly increased adiponectin gene expression in epididymal fat and blood adiponectin levels in both low-fat (LF) and high-fat (HF) diet-fed C57BL/6 mice. Although HF pair-fed C57BL/6 mice received the same amount of calories as LF ad libitum-fed mice, HF diet clearly increased adiposity but showed no significant effects on adiponectin gene expression and blood adiponectin level. CR also significantly increased blood adiponectin levels in ob/ob and A/J mice. Neither CR nor HF feeding displayed any significant effect on blood adiponectin half-life in C57BL/6 mice. Interestingly, CR increased PPARα expression in epididymal fat of C57BL/6 mice. Low levels of blood adiponectin and adiponectin gene expression in WAT were observed in PPARα KO mice. PPARα agonist treatment increased adiponectin mRNA levels in 3T3-L1 adipocytes. Furthermore, CR failed to increase adiponectin gene expression and blood adiponectin levels in PPARα KO mice. Therefore, our study demonstrated that energy intake, not dietary fat, plays an important role in regulating adiponectin gene expression and blood adiponectin level. PPARα mediates CR-enhanced adiponectin gene expression in WAT.

MKR mice have increased dynamic glucose disposal despite metabolic inflexibility, and hepatic and peripheral insulin insensitivity

AIMS/HYPOTHESIS

Recent work has shown that there can be significant differences when glucose disposal is assessed for high-fat induced insulin resistance by static clamp methods vs dynamic assessment during a stable isotope i.p. glucose tolerance test. MKR mice, though lean, have severe insulin resistance and decreased muscle fatty acid oxidation. Our goal was to assess dynamic vs static glucose disposal in MKR mice, and to correlate glucose disposal and muscle-adipose-liver flux interactions with metabolic flexibility (indirect calorimetry) and muscle characteristics.

METHODS

Stable isotope flux phenotyping was performed using [6,6-(2)H(2)]glucose, [U-(13)C(6)]glucose and [2-(13)C]glycerol. Muscle triacylglycerol (TAG) and diacylglycerol (DAG) content was assessed by thin layer chromatography, and histological determination of fibre type and cytochrome c activity performed. Metabolic flexibility was assessed by indirect calorimetry.

RESULTS

Indirect calorimetry showed that MKR mice used more glucose than FVB/N mice during fasting (respiratory exchange ratio [RER] 0.88 vs 0.77, respectively). Compared with FVB/N mice, MKR mice had faster dynamic glucose disposal, despite increased whole-muscle DAG and TAG, and similar hepatic glucose production with higher fasting insulin and unchanged basal glucose. Fed MKR muscle had more glycogen, and increased levels of GLUT1 and GLUT4 than FVB/N muscle. Histology indicated that MKR soleus had mildly decreased cytochrome c activity overall and more type II (glycolytic) fibres compared with that in FVB/N mice.

CONCLUSIONS/INTERPRETATION

MKR muscle adapts to using glucose, with more type II fibres present in red muscle. Fasting RER is elevated and glucose disposal during an i.p. glucose tolerance test is accelerated despite increased muscle DAG and TAG. Metabolic inflexibility may result from the compensatory use of fuel that can be best utilised for energy requirements; static vs dynamic glucose disposal assessments may measure complementary aspects of metabolic flexibility and insulin sensitivity.

Standard operating procedures for describing and performing metabolic tests of glucose homeostasis in mice

The Mouse Metabolic Phenotyping Center (MMPC) Consortium was established to address the need to characterize the growing number of mouse models of metabolic diseases, particularly diabetes and obesity. A goal of the MMPC Consortium is to propose standard methods for assessing metabolic phenotypes in mice. In this article, we discuss issues pertaining to the design and performance of various tests of glucose metabolism. We also propose guidelines for the description of methods, presentation of data and interpretation of results. The recommendations presented in this article are based on the experience of the MMPC Consortium and other investigators.

Advantages of dynamic "closed loop" stable isotope flux phenotyping over static "open loop" clamps in detecting silent genetic and dietary phenotypes

In vivo insulin sensitivity can be assessed using "open loop" clamp or "closed loop" methods. Open loop clamp methods are static, and fix plasma glucose independently from plasma insulin. Closed loop methods are dynamic, and assess glucose disposal in response to a stable isotope labeled glucose tolerance test. Using PPARalpha(-/-) mice, open and closed loop assessments of insulin sensitivity/glucose disposal were compared. Indirect calorimetry done for the assessment of diurnal substrate utilization/metabolic flexibility showed that chow fed PPARalpha(-/-) mice had increased glucose utilization during the light (starved) cycle. Euglycemic clamps showed no differences in insulin stimulated glucose disposal, whether for chow or high fat diets, but did show differences in basal glucose clearance for chow fed PPARalpha(-/-) versus SV129J-wt mice. In contrast, the dynamic stable isotope labeled glucose tolerance tests reveal enhanced glucose disposal for PPARalpha(-/-) versus SV129J-wt, for chow and high fat diets. Area under the curve for plasma labeled and unlabeled glucose for PPARalpha(-/-) was approximately 1.7-fold lower, P < 0.01 during the stable isotope labeled glucose tolerance test for both diets. Area under the curve for plasma insulin was 5-fold less for the chow fed SV129J-wt (P < 0.01) but showed no difference on a high fat diet (0.30 +/- 0.1 for SV129J-wt vs. 0.13 +/- 0.10 for PPARalpha(-/-), P = 0.28). This study demonstrates that dynamic stable isotope labeled glucose tolerance test can assess "silent" metabolic phenotypes, not detectable by the static, "open loop", euglycemic or hyperglycemic clamps. Both open loop and closed loop methods may describe different aspects of metabolic inflexibility and insulin sensitivity.

Absence of peroxisome proliferator-activated receptor-alpha abolishes hypertension and attenuates atherosclerosis in the Tsukuba hypertensive mouse

Peroxisome proliferator-activated receptor-alpha is widely distributed in the vasculature where it is believed to exert pleiotropic antiatherogenic effects. Its role in the regulation of blood pressure is still unresolved; however, some evidence suggests that it may affect the renin-angiotensin system. We investigated its role in angiotensin II-induced hypertension in the Tsukuba hypertensive mouse (THM). This is a model of hypertension and atherosclerosis because of high angiotensin II and aldosterone levels as a result of the transgenic expression of the entire human renin-angiotensin system. Making the THM animals deficient in Peroxisome proliferator-activated receptor-alpha (THM/PPARKO) totally abolished hypertension and myocardial hypertrophy. This was accompanied by a reduction in plasma human active renin in THM/PPARKO mice compared with THM animals from 3525+/-128 mU/L to 1910+/-750 mU/L (P<0.05) and by a normalization of serum aldosterone (1.6+/-0.29 nmol/L versus 3.4+/-0.69 nmol/L; P=0.003). In the THM/PPARKO mice, the extent of atherosclerosis at the aortic sinus after a 12-week period on an atherogenic diet was decreased by >80%. In addition, the spontaneous formation of foam cells from peritoneal macrophages, a blood pressure-independent event, was reduced by 92% in the THM/PPARKO mice, suggesting protection from the usual oxidative stress in these animals, possibly because of lower prevailing angiotensin II levels. Finally, chronic fenofibrate treatment further elevated blood pressure in THM animals but not in THM/PPARKO animals. Taken together, these data indicate that peroxisome proliferator-activated receptor-alpha may regulate the renin-angiotensin system. They raise the possibility that its activation may aggravate hypertension and hasten atherosclerosis in the context of an activated renin-angiotensin system.

Peroxisome proliferator-activated receptor-alpha deficiency protects aged mice from insulin resistance induced by high-fat diet

BACKGROUND/AIMS

Insulin resistance is a central feature of the metabolic syndrome and progressively increases with age, resulting in excessively high incidence of type II diabetes in the elderly population. Peroxisome proliferator-activated receptor-alpha (PPARalpha) is widely expressed in insulin target tissues, including those of the liver, kidney, and muscle, where it mediates expression of genes promoting fatty acid beta-oxidation. The aim of this study was to evaluate the potential role of PPARalpha in insulin resistance in aging mice induced by a high-fat diet.

METHODS

We used male PPARalpha knockout (KO) mice and wild-type (WT) littermates that were 18 months old. Animals were fed with a high-fat diet (HFD) for 4 weeks, and metabolic parameters associated with insulin sensitivity were assessed.

RESULTS

Following HFD treatment, WT mice showed more severe insulin resistance than did mice lacking the PPARalpha gene, as assessed by both the glucose tolerance test (GTT) and insulin tolerance test (ITT). In addition, WT mice exhibited significantly higher HOMA-IR, plasma total cholesterol levels and urinary albumin-creatinine ratio but less liver weight than did PPARalpha KO mice.

CONCLUSION

These data suggest that PPARalpha gene deficiency may protect aged mice from developing insulin resistance and albuminuria induced by a HFD.

Artificial selection for high-capacity endurance running is protective against high-fat diet-induced insulin resistance

Elevated oxidative capacity, such as occurs via endurance exercise training, is believed to protect against the development of obesity and diabetes. Rats bred both for low (LCR)- and high (HCR)-capacity endurance running provide a genetic model with inherent differences in aerobic capacity that allows for the testing of this supposition without the confounding effects of a training stimulus. The purpose of this investigation was to determine the effects of a high-fat diet (HFD) on weight gain patterns, insulin sensitivity, and fatty acid oxidative capacity in LCR and HCR male rats in the untrained state. Results indicate chow-fed LCR rats were heavier, hypertriglyceridemic, less insulin sensitive, and had lower skeletal muscle oxidative capacity compared with HCR rats. Upon exposure to an HFD, LCR rats gained more weight and fat mass, and their insulin resistant condition was exacerbated, despite consuming similar amounts of metabolizable energy as chow-fed controls. These metabolic variables remained unaltered in HCR rats. The HFD increased skeletal muscle oxidative capacity similarly in both strains, whereas hepatic oxidative capacity was diminished only in LCR rats. These results suggest that LCR rats are predisposed to obesity and that expansion of skeletal muscle oxidative capacity does not prevent excess weight gain or the exacerbation of insulin resistance on an HFD. Elevated basal skeletal muscle oxidative capacity and the ability to preserve liver oxidative capacity may protect HCR rats from HFD-induced obesity and insulin resistance.

Thermogenic and metabolic antiobesity drugs: rationale and opportunities

Antiobesity drugs that target peripheral metabolism may avoid some of the problems that have been encountered with centrally acting anorectic drugs. Moreover, if they cause weight loss by increasing fat oxidation, they not only address a cause of obesity but also should promote loss of fat rather than lean tissue and improve insulin sensitivity. Weight loss may be slow but more sustained than with anorectic drugs, and thermogenesis may be insufficient to cause any discomfort. Some thermogenic approaches are the activation of adrenergic, thyroid hormone or growth hormone receptors and the inhibition of glucocorticoid receptors; the modulation of transcription factors [e.g. peroxisome proliferator-activated receptor delta (PPARdelta) activators] or enzymes [e.g. glutamine fructose-6-phosphate amidotransferase (GFAT) inhibitors] that promote mitochondrial biogenesis, and the modulation of transcription factors (PPAR alpha activators) or enzymes (AMP-activated protein kinase) that promote fatty acid oxidation. More surprisingly, studies on genetically modified animals and with enzyme inhibitors suggest that inhibitors of fatty acid synthesis [e.g. ATP citrate lyase, fatty acid synthase, acetyl-CoA carboxylase (ACC)], fatty acid interconversion [stearoyl-CoA desaturase (SCD)] and triglyceride synthesis (e.g. acyl-CoA : diacylglycerol acyltransferase) may all be thermogenic. Some targets have been validated only by deleting genes in the whole animal. In these cases, it is possible that deletion of the protein in the brain is responsible for the effect on adiposity, and therefore a centrally penetrant drug would be required. Moreover, whilst a genetically modified mouse may display resistance to obesity in response to a high fat diet, it requires a tool compound to demonstrate that a drug might actually cause weight loss. Even then, it is possible that differences between rodents and humans, such as the greater thermogenic capacity of rodents, may give a misleading impression of the potential of a drug.

High-fat diets: modeling the metabolic disorders of human obesity in rodents

RESEARCH METHODS AND PROCEDURES

High-fat (HF) diet feeding can induce obesity and metabolic disorders in rodents that resemble the human metabolic syndrome. However, this dietary intervention is not standardized, and the HF-induced phenotype varies distinctly among different studies. The question which HF diet type is best to model the metabolic deterioration seen in human obesity remains unclear. Therefore, in this review, metabolic data obtained with different HF diet approaches are compiled. Both whole-body and organ-specific diet effects are analyzed.

RESULTS

On the basis of these results, we conclude that animal fats and omega-6/omega-9-containing plant oils can be used to generate an obese and insulin-resistant phenotype in rodents, whereas fish oil-fed animals do not develop these disorders.

DISCUSSION

Looking at the present data, it does not seem possible to define an ideal HF diet, and an exact definition of diet composition and a thorough metabolic characterization of the HF diet effects in a researcher's specific laboratory setting remains essential for metabolic studies with this model.

Peroxisome proliferator-activated receptor-alpha-null mice have increased white adipose tissue glucose utilization, GLUT4, and fat mass: Role in liver and brain

Activation of the peroxisome proliferator-activated receptor (PPAR)-alpha increases lipid catabolism and lowers the concentration of circulating lipid, but its role in the control of glucose metabolism is not as clearly established. Here we compared PPARalpha knockout mice with wild type and confirmed that the former developed hypoglycemia during fasting. This was associated with only a slight increase in insulin sensitivity but a dramatic increase in whole-body and adipose tissue glucose use rates in the fasting state. The white sc and visceral fat depots were larger due to an increase in the size and number of adipocytes, and their level of GLUT4 expression was higher and no longer regulated by the fed-to-fast transition. To evaluate whether these adipocyte deregulations were secondary to the absence of PPARalpha from liver, we reexpresssed this transcription factor in the liver of knockout mice using recombinant adenoviruses. Whereas more than 90% of the hepatocytes were infected and PPARalpha expression was restored to normal levels, the whole-body glucose use rate remained elevated. Next, to evaluate whether brain PPARalpha could affect glucose homeostasis, we activated brain PPARalpha in wild-type mice by infusing WY14643 into the lateral ventricle and showed that whole-body glucose use was reduced. Hence, our data show that PPARalpha is involved in the regulation of glucose homeostasis, insulin sensitivity, fat accumulation, and adipose tissue glucose use by a mechanism that does not require PPARalpha expression in the liver. By contrast, activation of PPARalpha in the brain stimulates peripheral glucose use. This suggests that the alteration in adipocyte glucose metabolism in the knockout mice may result from the absence of PPARalpha in the brain.

Effects of PPARalpha on cardiac glucose metabolism: a transcriptional equivalent of the glucose-fatty acid cycle?

Cardiovascular disease is exceptionally prevalent in patients with diabetes mellitus and is the most common cause of death. With the emerging pandemic of obesity and resulting metabolic abnormalities, the occurrence of cardiovascular disease is almost nearly certain to increase at a remarkable rate in the near future. Currently, several ligands for the peroxisome proliferator-activated receptor (PPAR) family of nuclear receptors are prescribed as lipid-lowering and insulin-sensitizing drugs. The PPARs are ligand-activated transcription factors that influence the expression of the entire program of fatty acid utilization enzymes. It is believed that these compounds remedy glucose homeostasis and cardiovascular disease by lowering circulating lipid levels, improving the profile of secreted adipokines, as well as via their anti-inflammatory properties. Conversely, overexpression of the PPARalpha isoform in the muscle or heart of mice drives diminished glucose transporter gene expression and glucose uptake into those insulin target tissues. Although the effects of overexpressing PPARalpha in a specific tissue obviously differ from activating PPARalpha in a systemic manner, studies such as this may influence the development of the next generation of PPAR ligands.

Considerations in the design of hyperinsulinemic-euglycemic clamps in the conscious mouse

Despite increased use of the hyperinsulinemic-euglycemic clamp to study insulin action in mice, the effects of experimental parameters on the results obtained have not been addressed. In our studies, we determined the influences of sampling sites, fasting duration, and insulin delivery on results obtained from clamps in conscious mice. Carotid artery and jugular vein catheters were implanted in C57BL/6J mice (n = 6-10/group) fed a normal diet for sampling and infusions. After a 5-day recovery period, mice underwent a 120-min clamp (2.5-mU . kg(-1) . min(-1) insulin infusion; approximately 120-130 mg/dl glucose) while receiving [3-(3)H]glucose to determine glucose appearance (endoR(a)) and disappearance (R(d)). Sampling large volumes (approximately 100 mul) from the cut tail resulted in elevated catecholamines and basal glucose compared with artery sampling. Catecholamines were not elevated when taking small samples ( approximately 5 mul) from the cut tail. Overnight (18-h) fasting resulted in greater loss of total body, lean, and fat masses and hepatic glycogen but resulted in enhanced insulin sensitivity compared with 5-h fasting. Compared with a 16-mU/kg insulin prime, a 300-mU/kg prime resulted in hepatic insulin resistance and slower acquisition of steady-state glucose infusion rates (GIR) after a 5-h fast. The steady-state GIR was expedited after the 300-mU/kg prime in 18-h-fasted mice. The GIR and R(d) rose with increasing insulin infusions (0.8, 2.5, 4, and 20 mU . kg(-1) . min(-1)), but endoR(a) was fully suppressed with doses higher than 0.8 mU . kg(-1) . min(-1). Thus, common variations in experimental factors yield different results and should be considered in designing and interpreting clamps.

Effects of fenofibrate and insulin on the biosynthesis of unsaturated fatty acids in streptozotocin diabetic rats

Both insulin and PPAR-alpha up-modulate hepatic Delta9, Delta6 and Delta5 desaturating enzymes involved in the biosynthesis of mono- and polyunsaturated fatty acids. Currently, we have examined for 9 days the independent and simultaneous effects of daily glargine insulin and fenofibrate administration on the insulinemia, glycemia, hepatic acyl-CoA oxidase activity and mRNAs and enzymatic activities of stearoyl-CoA desaturase-1 (SCD-1) and Delta5 desaturase in streptozotocin diabetic rats. Glargine insulin depressed the hyperglycemia of diabetic rats at 4h, but not after 24h of injection. Fenofibrate increased the radioimmunoreactive insulinemia in non-diabetic rats without changing the glycemia. Insulin increased the mRNAs and activities of SCD-1 and Delta5 desaturase depressed in diabetic rats. Fenofibrate increased acyl-CoA oxidase activity, and the mRNAs and activities of both desaturating enzymes in non-diabetic, diabetic and insulin-treated diabetic rats, but was less effective in the mRNAs modification of diabetic animals. Therefore, insulin, and fenofibrate through PPAR-alpha activation, enhance liver mRNAs and activities of SCD-1 and Delta5 desaturases independently and synergistically through different mechanisms. Insulin and fenofibrate independently increased the 18:1/18:0 ratio in liver lipids, increasing the fluidity of the membranes. The 20:4/18:2 ratio was maintained. Fenofibrate increased palmitic acid, but decreased stearic acid percentage in liver lipids.

[Pathogenesis of primary nonalcoholic fatty liver disease]

The challenges of growing prevalence and evident trend to progressive damage of primary nonalcoholic fatty liver disease confront a poorly understood pathogenesis. It appears to develop in two steps. First, a high adipocyte protein production in the context of a silent inflammatory background causes insulin resistance in adipose tissue. It leads both to lipolysis, with increase of the circulating and hepatic uptake of free fatty acids, and hyperinsulinemia. Within hepatocytes, the subsequent lipogenesis, together with a decreased secretion of lipoproteins, induces an accumulation of excessive hepatic triglycerides (steatosis), impliying some oxidative damage, but it remain balanced by uncoupling protein upregulation and antioxidant systems activation. Second, a more forceful fat catabolism by beta and omega oxidation results in respiratory chain hyperactivity with overproduction of free radicals and reactive oxygen species that exceed the antioxidant capacity. These agents lead to hepatocellular injury and necrosis, inflammatory infiltration and fibrosis (steatohepatitis) through induction of Fas ligand and cytokines (tumor necrosis factor alpha, transforming growth factor beta, interleukin-8), and lipid peroxidation and by-products (malondialdehyde and 4-hydroxynonenal). Other mechanisms (hepatic iron, Kupffer cells dysfunction or endotoxemia) play uncertain roles.

Insulin and leptin resistance with hyperleptinemia in mice lacking androgen receptor

Epidemiological evidence suggests that sex differences exist in type 2 diabetes. Men seem to be more susceptible than women to the consequences of obesity and sedentary lifestyle, possibly because of differences in insulin sensitivity and regional body fat deposition. Thus, lacking androgen receptor (AR) in male individuals may promote insulin resistance. To determine whether lacking AR in male individuals contributes to in vivo insulin resistance, an AR knockout model (AR(-/y)) was used to study the correlation between AR and insulin resistance. Progressive reduced insulin sensitivity and impaired glucose tolerance were seen in AR(-/y) mice with advancing age. Aging AR(-/y) mice displayed accelerated weight gain, hyperinsulinemia, and hyperglycemia, and loss of AR contributes to increased triglyceride content in skeletal muscle and liver. Leptin is higher in serum of AR(-/y) mice. Treatment with exogenous leptin fails to stimulate weight loss in AR(-/y) mice in advanced age, suggesting leptin resistance in the AR(-/y/) mice. Exogenous dihydrotestosterone replacement fails to reverse the metabolic abnormalities and insulin resistance in AR(-/y) mice. Our in vivo studies demonstrate that androgen-AR plays key roles in the development of insulin and leptin resistance, which may contribute to the development of type 2 diabetes and cardiovascular disease.

Regulation of adipocyte differentiation and function by polyunsaturated fatty acids

A diet enriched in PUFAs, in particular of the n-3 family, decreases adipose tissue mass and suppresses development of obesity in rodents. Although several nuclear hormone receptors are identified as PUFA targets, the precise molecular mechanisms underlying the effects of PUFAs still remain to be elucidated. Here we review research aimed at elucidating molecular mechanisms governing the effects of PUFAs on the differentiation and function of white fat cells. This review focuses on dietary PUFAs as signaling molecules, with special emphasis on agonistic and antagonistic effects on transcription factors currently implicated as key players in adipocyte differentiation and function, including peroxisome proliferator activated receptors (PPARs) (alpha, beta and gamma), sterol regulatory element binding proteins (SREBPs) and liver X receptors (LXRs). We review evidence that dietary n-3 PUFAs decrease adipose tissue mass and suppress the development of obesity in rodents by targeting a set of key regulatory transcription factors involved in both adipogensis and lipid homeostasis in mature adipocytes. The same set of factors are targeted by PUFAs of the n-6 family, but the cellular/physiological responses are dependent on the experimental setting as n-6 PUFAs may exert either an anti- or a proadipogenic effect. Feeding status and hormonal background may therefore be of particular importance in determining the physiological effects of PUFAs of the n-6 family.

PPARalpha governs glycerol metabolism

Glycerol, a product of adipose tissue lipolysis, is an important substrate for hepatic glucose synthesis. However, little is known about the regulation of hepatic glycerol metabolism. Here we show that several genes involved in the hepatic metabolism of glycerol, i.e., cytosolic and mitochondrial glycerol 3-phosphate dehydrogenase (GPDH), glycerol kinase, and glycerol transporters aquaporin 3 and 9, are upregulated by fasting in wild-type mice but not in mice lacking PPARalpha. Furthermore, expression of these genes was induced by the PPARalpha agonist Wy14643 in wild-type but not PPARalpha-null mice. In adipocytes, which express high levels of PPARgamma, expression of cytosolic GPDH was enhanced by PPARgamma and beta/delta agonists, while expression was decreased in PPARgamma(+/-) and PPARbeta/delta(-/-) mice. Transactivation, gel shift, and chromatin immunoprecipitation experiments demonstrated that cytosolic GPDH is a direct PPAR target gene. In line with a stimulating role of PPARalpha in hepatic glycerol utilization, administration of synthetic PPARalpha agonists in mice and humans decreased plasma glycerol. Finally, hepatic glucose production was decreased in PPARalpha-null mice simultaneously fasted and exposed to Wy14643, suggesting that the stimulatory effect of PPARalpha on gluconeogenic gene expression was translated at the functional level. Overall, these data indicate that PPARalpha directly governs glycerol metabolism in liver, whereas PPARgamma regulates glycerol metabolism in adipose tissue.

PPARs and LXRs: atherosclerosis goes nuclear

Atherosclerosis is the leading cause of mortality in the Western world, and new therapeutics to target the metabolic and inflammatory factors that underlie its pathogenesis are needed. Peroxisome proliferator-activated receptors and liver X receptors are lipid-activated nuclear receptors that regulate systemic glucose and lipid metabolism, and modulate inflammation within the vascular wall. New understanding of their functions in physiology and the development of high-affinity synthetic ligands highlight their potential as targets for the treatment of cardiovascular disease.