Association of Serum Retinol Binding Protein 4 with Atherogenic Dyslipidemia in Morbid Obese Patients

Retinol binding protein 4 (RBP4) is an adipokine that may contribute to the development of insulin resistance. However, how this adipokine is affected and its possible involvement in lipid metabolism in obese patients with varying degrees of insulin resistance is yet to be determined. A total of 299 middle-aged morbid obese patients (BMI>40 kg/m²) were divided in euglycemic, metabolic syndrome or type 2 diabetic. Anthropometric measurements, biochemical variables and systemic RBP4 levels were determined. RBP4 levels were significantly higher in patients with metabolic syndrome and type 2 diabetes than in euglycemic subjects (42.9±14.6; 42.3±17.0 and 37.4±11.7 µg/ml, respectively) and correlated with triglycerides but not with those of HOMA-IR in the whole population. The multivariate regression model revealed that triglycerides were the strongest predictor of systemic RBP4 levels. Analysis of lipoprotein subfractions in a subpopulation of 80 subjects showed an altered profile of insulin resistant states characterized by higher VLDL, sdLDL and small HDL percentages and lower large HDL percentage. Although RBP4 levels correlated significantly with LDL particle size and small HDL percentage, the latter parameter was independently associated only with RBP4. Our study reveals that systemic RBP4 levels could play an important role in lipid metabolism in morbid obesity, increasing triglyceride levels and contributing to the formation of small HDL.     

基于肠道微生态探讨黄连素在代谢性疾病中的抗炎作用

肠道菌群与能量代谢密切相关,其组成和代谢紊乱可通过多种途径导致胰岛素抵抗,肥胖和2型糖尿病。黄连素因具有减重、降糖、调脂等作用被广泛用于肥胖、2型糖尿病及非酒精性脂肪性肝病等代谢性疾病的辅助治疗;研究表明,黄连素可调节肠道菌群的组成和代谢,改善肠道微生态环境,从而改善胰岛素抵抗和代谢。本文综述了黄连素通过肠道菌群-炎症轴在干预代谢性疾病的研究进展,以期为代谢性疾病的治疗寻找新的策略,并为今后该领域的深入研究提供指导意义。     

Impact of heart-specific disruption of the circadian clock on systemic glucose metabolism in mice

The daily rhythm of glucose metabolism is governed by the circadian clock, which consists of cell-autonomous clock machineries residing in nearly every tissue in the body. Disruption of these clock machineries either environmentally or genetically induces the dysregulation of glucose metabolism. Although the roles of clock machineries in the regulation of glucose metabolism have been uncovered in major metabolic tissues, such as the pancreas, liver, and skeletal muscle, it remains unknown whether clock function in non-major metabolic tissues also affects systemic glucose metabolism. Here, we tested the hypothesis that disruption of the clock machinery in the heart might also affect systemic glucose metabolism, because heart function is known to be associated with glucose tolerance. We examined glucose and insulin tolerance as well as heart phenotypes in mice with heart-specific deletion of Bmal1, a core clock gene. Bmal1 deletion in the heart not only decreased heart function but also led to systemic insulin resistance. Moreover, hyperglycemia was induced with age. Furthermore, heart-specific Bmal1-deficient mice exhibited decreased insulin-induced phosphorylation of Akt in the liver, thus indicating that Bmal1 deletion in the heart causes hepatic insulin resistance. Our findings revealed an unexpected effect of the function of clock machinery in a non-major metabolic tissue, the heart, on systemic glucose metabolism in mammals.     

Metabolic consequences of ENPP1 overexpression in adipose tissue

Ectonucleotide pyrophosphate phosphodiesterase (ENPP1) has been shown to negatively modulate insulin receptor and to induce cellular insulin resistance when overexpressed in various cell types. Systemic insulin resistance has also been observed when ENPP1 is overexpressed in multiple tissues of transgenic models and attributed largely to tissue insulin resistance induced in skeletal muscle and liver. Another key tissue in regulating glucose and lipid metabolism is adipose tissue (AT). Interestingly, obese patients with insulin resistance have been reported to have increased AT ENPP1 expression. However, the specific effects of ENPP1 in AT have not been studied. To better understand the specific role of AT ENPP1 on systemic metabolism, we have created a transgenic mouse model (C57/Bl6 background) with targeted overexpression of human ENPP1 in adipocytes, using aP2 promoter in the transgene construct (AdiposeENPP1-TG). Using either regular chow or pair-feeding protocol with 60% fat diet, we compared body fat content and distribution and insulin signaling in adipose, muscle, and liver tissues of AdiposeENPP1-TG and wild-type (WT) siblings. We also compared response to intraperitoneal glucose tolerance test (IPGTT) and insulin tolerance test (ITT). Our results show no changes in Adipose ENPP1-TG mice fed a regular chow diet. After high-fat diet with pair-feeding protocol, AdiposeENPP1-TG and WT mice had similar weights. However, AdiposeENPP1-TG mice developed fatty liver in association with changes in AT characterized by smaller adipocyte size and decreased phosphorylation of insulin receptor Tyr(1361) and Akt Ser(473). These changes in AT function and fat distribution were associated with systemic abnormalities of lipid and glucose metabolism, including increased plasma concentrations of fatty acid, triglyceride, plasma glucose, and insulin during IPGTT and decreased glucose suppression during ITT. Thus, our results show that, in the presence of a high-fat diet, ENPP1 overexpression in adipocytes induces fatty liver, hyperlipidemia, and dysglycemia, thus recapitulating key manifestations of the metabolic syndrome.     

A liver-derived secretory protein, selenoprotein P, causes insulin resistance

The liver may regulate glucose homeostasis by modulating the sensitivity/resistance of peripheral tissues to insulin, by way of the production of secretory proteins, termed hepatokines. Here, we demonstrate that selenoprotein P (SeP), a liver-derived secretory protein, causes insulin resistance. Using serial analysis of gene expression (SAGE) and DNA chip methods, we found that hepatic SeP mRNA levels correlated with insulin resistance in humans. Administration of purified SeP impaired insulin signaling and dysregulated glucose metabolism in both hepatocytes and myocytes. Conversely, both genetic deletion and RNA interference-mediated knockdown of SeP improved systemic insulin sensitivity and glucose tolerance in mice. The metabolic actions of SeP were mediated, at least partly, by inactivation of adenosine monophosphate-activated protein kinase (AMPK). In summary, these results demonstrate a role of SeP in the regulation of glucose metabolism and insulin sensitivity and suggest that SeP may be a therapeutic target for type 2 diabetes.     

Alternative M2 activation of Kupffer cells by PPARdelta ameliorates obesity-induced insulin resistance

Macrophage infiltration and activation in metabolic tissues underlie obesity-induced insulin resistance and type 2 diabetes. While inflammatory activation of resident hepatic macrophages potentiates insulin resistance, the functions of alternatively activated Kupffer cells in metabolic disease remain unknown. Here we show that in response to the Th2 cytokine interleukin-4 (IL-4), peroxisome proliferator-activated receptor delta (PPARdelta) directs expression of the alternative phenotype in Kupffer cells and adipose tissue macrophages of lean mice. However, adoptive transfer of PPARdelta(-/-) (Ppard(-/-)) bone marrow into wild-type mice diminishes alternative activation of hepatic macrophages, causing hepatic dysfunction and systemic insulin resistance. Suppression of hepatic oxidative metabolism is recapitulated by treatment of primary hepatocytes with conditioned medium from PPARdelta(-/-) macrophages, indicating direct involvement of Kupffer cells in liver lipid metabolism. Taken together, these data suggest an unexpected beneficial role for alternatively activated Kupffer cells in metabolic syndrome and type 2 diabetes.     

Model-Based Quantification of the Systemic Interplay between Glucose and Fatty Acids in the Postprandial State

In metabolic diseases such as Type 2 Diabetes and Non-Alcoholic Fatty Liver Disease, the systemic regulation of postprandial metabolite concentrations is disturbed. To understand this dysregulation, a quantitative and temporal understanding of systemic postprandial metabolite handling is needed. Of particular interest is the intertwined regulation of glucose and non-esterified fatty acids (NEFA), due to the association between disturbed NEFA metabolism and insulin resistance. However, postprandial glucose metabolism is characterized by a dynamic interplay of simultaneously responding regulatory mechanisms, which have proven difficult to measure directly. Therefore, we propose a mathematical modelling approach to untangle the systemic interplay between glucose and NEFA in the postprandial period. The developed model integrates data of both the perturbation of glucose metabolism by NEFA as measured under clamp conditions, and postprandial time-series of glucose, insulin, and NEFA. The model can describe independent data not used for fitting, and perturbations of NEFA metabolism result in an increased insulin, but not glucose, response, demonstrating that glucose homeostasis is maintained. Finally, the model is used to show that NEFA may mediate up to 30–45% of the postprandial increase in insulin-dependent glucose uptake at two hours after a glucose meal. In conclusion, the presented model can quantify the systemic interactions of glucose and NEFA in the postprandial state, and may therefore provide a new method to evaluate the disturbance of this interplay in metabolic disease.     

Uncarboxylated osteocalcin decreases insulin-stimulated glucose uptake without affecting insulin signaling and regulators of mitochondrial biogenesis in myotubes

Uncarboxylated osteocalcin (uOC) is a circulating bone matrix protein, which has previously been shown to regulate glucose uptake and systemic metabolism. However, the cellular mechanism by which uOC acts has yet to be elucidated. C2C12 mouse myotubes were treated for 72 h with uOC (1–100 ng/mL). Cellular metabolism was analyzed using oxygen consumption and extracellular acidification rate. Metabolic gene and protein expression were measured via quantitative real-time polymerase chain reaction and Western blot, respectively. Additionally, C2C12 myotubes were treated with 10 ng/mL uOC to examine glucose uptake and activation of insulin signaling with or without insulin resistance. Finally, cellular lipid content was measured via Oil Red O and Nile Red staining. uOC treatment resulted in dose-dependent alterations of oxygen consumption with little effect on regulators of mitochondrial metabolism. Basal expression of regulators of glucose uptake were unaffected by uOC treatment. However, insulin-stimulated glucose uptake was blunted by uOC treatment with no concurrent alterations in insulin signaling. While chronic insulin treatment resulted in suppressed activation of Akt, concurrent uOC treatment was unable to prevent these detrimental effects on insulin signaling. uOC treatment had no effect on markers of lipogenesis and cellular lipid content. These findings suggest that 72-h uOC treatment may alter oxygen consumption without effect on regulators of mitochondrial biogenesis. Additionally, uOC treatment suppressed insulin-stimulated glucose uptake in cultured myotubes but had little effect on insulin signaling or regulators of cellular metabolism and was unable to mitigate insulin resistance.     

Enhanced Lipid Oxidation and Maintenance of Muscle Insulin Sensitivity Despite Glucose Intolerance in a Diet-Induced Obesity Mouse Model

Background

Diet-induced obesity is a rising health concern which can lead to the development of glucose intolerance and muscle insulin resistance and, ultimately, type II diabetes mellitus. This research investigates the associations between glucose intolerance or muscle insulin resistance and tissue specific changes during the progression of diet-induced obesity.

Methodology

C57BL/6J mice were fed a normal or high-fat diet (HFD; 60% kcal fat) for 3 or 8 weeks. Disease progression was monitored by measurements of body/tissue mass changes, glucose and insulin tolerance tests, and ex vivo glucose uptake in intact muscles. Lipid metabolism was analyzed using metabolic chambers and ex vivo palmitate assays in intact muscles. Skeletal muscle, liver and adipose tissues were analyzed for changes in inflammatory gene expression. Plasma was analyzed for insulin levels and inflammatory proteins. Histological techniques were used on muscle and liver cryosections to assess metabolic and morphological changes.

Principal Findings/Conclusions

A rapid shift in whole body metabolism towards lipids was observed with HFD. Following 3 weeks of HFD, elevated total lipid oxidation and an oxidative fiber type shift had occurred in the skeletal muscle, which we propose was responsible for delaying intramyocellular lipid accumulation and maintaining muscle’s insulin sensitivity. Glucose intolerance was present after three weeks of HFD and was associated with an enlarged adipose tissue depot, adipose tissue inflammation and excess hepatic lipids, but not hepatic inflammation. Furthermore, HFD did not significantly increase systemic or muscle inflammation after 3 or 8 weeks of HFD suggesting that early diet-induced obesity does not cause inflammation throughout the whole body. Overall these findings indicate skeletal muscle did not contribute to the development of HFD-induced impairments in whole-body glucose tolerance following 3 weeks of HFD.     

Cytoplasmic fatty acid-binding proteins: emerging roles in metabolism and atherosclerosis

Cytoplasmic fatty acid-binding proteins (FABPs) are a family of proteins, expressed in a tissue-specific manner, that bind fatty acid ligands and are involved in shuttling fatty acids to cellular compartments, modulating intracellular lipid metabolism, and regulating gene expression. Several members of the FABP family have been shown to have important roles in regulating metabolism and have links to the development of insulin resistance and the metabolic syndrome. Recent studies demonstrate a role for intestinal FABP in the control of dietary fatty acid absorption and chylomicron secretion. Heart FABP is essential for normal myocardial fatty acid oxidation and modulates fatty acid uptake in skeletal muscle. Liver FABP is directly involved in fatty acid ligand signaling to the nucleus and interacts with peroxisome proliferator-activated receptors in hepatocytes. The adipocyte FABP (aP2) has been shown to affect insulin sensitivity, lipid metabolism and lipolysis, and has recently been shown to play an important role in atherosclerosis. Interestingly, expression of aP2 by the macrophage promotes atherogenesis, thus providing a link between insulin resistance, intracellular fatty acid disposition, and foam cell formation. The FABPs are promising targets for the treatment of dyslipidemia, insulin resistance, and atherosclerosis in humans.     

Nonobese, insulin-deficient Ins2Akita mice develop type 2 diabetes phenotypes including insulin resistance and cardiac remodeling

Although insulin resistance has been traditionally associated with type 2 diabetes, recent evidence in humans and animal models indicates that insulin resistance may also develop in type 1 diabetes. A point mutation of insulin 2 gene in Ins2(Akita) mice leads to pancreatic beta-cell apoptosis and hyperglycemia, and these mice are commonly used to investigate type 1 diabetes and complications. Since insulin resistance plays an important role in diabetic complications, we performed hyperinsulinemic-euglycemic clamps in awake Ins2(Akita) and wild-type mice to measure insulin action and glucose metabolism in vivo. Nonobese Ins2(Akita) mice developed insulin resistance, as indicated by an approximately 80% reduction in glucose infusion rate during clamps. Insulin resistance was due to approximately 50% decreases in glucose uptake in skeletal muscle and brown adipose tissue as well as hepatic insulin action. Skeletal muscle insulin resistance was associated with a 40% reduction in total GLUT4 and a threefold increase in PKCepsilon levels in Ins2(Akita) mice. Chronic phloridzin treatment lowered systemic glucose levels and normalized muscle insulin action, GLUT4 and PKCepsilon levels in Ins2(Akita) mice, indicating that hyperglycemia plays a role in insulin resistance. Echocardiography showed significant cardiac remodeling with ventricular hypertrophy that was ameliorated following chronic phloridzin treatment in Ins2(Akita) mice. Overall, we report for the first time that nonobese, insulin-deficient Ins2(Akita) mice develop type 2 diabetes phenotypes including peripheral and hepatic insulin resistance and cardiac remodeling. Our findings provide important insights into the pathogenesis of metabolic abnormalities and complications affecting type 1 diabetes and lean type 2 diabetes subjects.     

Portal systemic shunting of insulin does not lead to insulin resistance in patients with extrahepatic portal vein obstruction.

There is no clear relation between portal systemic shunting, reduced hepatic insulin extraction leading to an increased systemic delivery of insulin, and, resultant peripheral hyperinsulinemia and insulin resistance. Extrahepatic portal vein obstruction is a natural human model of portal systemic shunting with essentially normal liver function. To investigate the role of portal systemic shunting of insulin in creating systemic hyperinsulinemia and insulin resistance, we studied nine subjects with portal systemic shunting and nine controls matched for age (+/- 2 years), body weight (+/- 2 kg) and height (+/- 5 cm). We carried out an oral glucose tolerance test and hyperinsulinemic euglycemic clamp study at insulin infusion rate of 40 mU/m2/ min. Comparable (p = 0.61) basal insulin concentrations in the two groups (Mean (SE): 21.0 (3.98) vs. 24.1 (4.28) mU/L) demonstrated a lack of hyperinsulinemia in the presence of portal systemic shunting. The lower (p = 0.03) insulin area under curve on oral glucose tolerance test in presence of portal systemic shunting (7.40 (0.95) vs. 10.83 (1.15) U/L-min) indicated that lower extraction of insulin by the liver leads to a lower requirements in the periphery. The coefficient of variation for plasma glucose between 60 and 120 min of the clamps was 4.44 (0.55)%. Comparable (p = 0.82) M-values (6.21 (0.67) vs. 6.38 (0.45) mg/kg/min) in the two groups proved a lack of significant insulin resistance in the presence of portal systemic shunting. We conclude that isolated portal systemic shunting leads to neither hyperinsulinemia nor insulin resistance.     

Insulin resistance in Cushing's syndrome

Insulin resistance is well established in Cushing's syndrome, but its mechanisms are not completely understood. We performed the euglycemic insulin clamp technique on four patients with Cushing's syndrome, five obese patients and five normal volunteers, in order to determine the role of impairments in insulin responsiveness and insulin clearance in hypercorticism and obesity. Insulin was infused at 0.3, 1, 3 and 10 mU/kg/min, and steady-state glucose-infusion rates required to maintain euglycemia were determined. Glucose disposal at maximal insulin levels was 11.9 +/- 0.4 mg/kg/min in normals, with a 29% decrease in obese and a 42% decrease in Cushing's syndrome patients. Half maximally effective insulin concentrations were increased in both abnormal groups compared to normals. Maximal insulin clearance rates were 1460 +/- 200 ml/min/m2 in normals, not significantly changed in obese and 40% decreased in Cushing's syndrome patients. These results indicate that the insulin resistance in Cushing's syndrome is distinct from that occurring in obesity and is characterized by both decreased insulin responsiveness and decreased insulin clearance. These impairments could be caused by a common defect which may be at or distal to the glucose transport level.     

A predominant role for parenchymal c-Jun amino terminal kinase (JNK) in the regulation of systemic insulin sensitivity

It has been established that c-Jun N-terminal kinase 1 (JNK1) is essential to the pathogenesis of insulin resistance and type 2 diabetes. Although JNK influences inflammatory signaling pathways, it remains unclear whether its activity in macrophages contributes to adipose tissue inflammation and ultimately to the regulation of systemic metabolism. To address whether the action of this critical inflammatory kinase in bone marrow-derived elements regulates inflammatory responses in obesity and is sufficient and necessary for the deterioration of insulin sensitivity, we performed bone marrow transplantation studies with wild type and JNK1-deficient mice. These studies illustrated that JNK1-deficiency in the bone marrow-derived elements (BMDE) was insufficient to impact macrophage infiltration or insulin sensitivity despite modest changes in the inflammatory profile of adipose tissue. Only when the parenchymal elements lacked JNK1 could we demonstrate a significant increase in systemic insulin sensitivity. These data indicate that while the JNK1 activity in BMDE is involved in metabolic regulation and adipose milieu, it is epistatic to JNK1 activity in the parenchymal tissue for regulation of metabolic homeostasis.     

Adipocyte/macrophage fatty acid binding proteins control integrated metabolic responses in obesity and diabetes

Fatty acid binding proteins (FABPs) are cytosolic fatty acid chaperones whose biological role and mechanisms of action are not well understood. Here, we developed mice with targeted mutations in two related adipocyte FABPs, aP2 and mal1, to resolve their role in systemic lipid, glucose, and energy metabolism. Mice lacking aP2 and mal1 exhibited a striking phenotype with strong protection from diet-induced obesity, insulin resistance, type 2 diabetes, and fatty liver disease. These mice have altered cellular and systemic lipid transport and composition, leading to enhanced insulin receptor signaling, enhanced muscle AMP-activated kinase (AMP-K) activity, and dramatically reduced liver stearoyl-CoA desaturase-1 (SCD-1) activity underlying their phenotype. Taken together with the previously reported strong protection against atherosclerosis, these results demonstrate that adipocyte/macrophage FABPs have a robust impact on multiple components of metabolic syndrome, integrating metabolic and inflammatory responses in mice and constituting a powerful target for the treatment of these diseases.     

Intestinal lipoprotein overproduction in insulin-resistant states

PURPOSE OF REVIEW: Excessive postprandial lipemia is highly prevalent in obese and insulin-resistant/type 2 diabetic individuals and substantially increases the risk of atherosclerosis and cardiovascular disease. This article will review our current understanding of the link between insulin resistance and intestinal lipoprotein overproduction and highlight some of the key recent findings in the field. RECENT FINDINGS: Emerging evidence from several animal models of insulin resistance as well as insulin-resistant humans clearly supports the link between insulin resistance and aberrant intestinal lipoprotein metabolism. In insulin-resistant states, elevated free fatty acid flux into the intestine, downregulation of intestinal insulin signaling and upregulation of microsomal triglyceride transfer protein all appear to stimulate intestinal lipoprotein production. Gut peptides, GLP-1 and GLP-2, may be important regulators of intestinal lipid absorption and lipoprotein production. SUMMARY: Available evidence in humans and animal models strongly favors the concept that the small intestine is not merely an absorptive organ but rather plays an active role in regulating the rate of production of triglyceride-rich lipoproteins. Metabolic signals in insulin resistance and type 2 diabetes and in some cases an aberrant intestinal response to these factors all contribute to the enhanced formation and secretion of triglyceride-rich lipoproteins.     

Targeted disruption of inducible nitric oxide synthase protects against obesity-linked insulin resistance in muscle

Inducible nitric oxide synthase (iNOS) is induced by inflammatory cytokines in skeletal muscle and fat. It has been proposed that chronic iNOS induction may cause muscle insulin resistance. Here we show that iNOS expression is increased in muscle and fat of genetic and dietary models of obesity. Moreover, mice in which the gene encoding iNOS was disrupted (Nos2-/- mice) are protected from high-fat-induced insulin resistance. Whereas both wild-type and Nos2-/- mice developed obesity on the high-fat diet, obese Nos2-/- mice exhibited improved glucose tolerance, normal insulin sensitivity in vivo and normal insulin-stimulated glucose uptake in muscles. iNOS induction in obese wild-type mice was associated with impairments in phosphatidylinositol 3-kinase and Akt activation by insulin in muscle. These defects were fully prevented in obese Nos2-/- mice. These findings provide genetic evidence that iNOS is involved in the development of muscle insulin resistance in diet-induced obesity.     

Characteristics of lipid composition of blood serum lipoproteins in insulin resistance

The purpose of this investigation was to study lipid composition of apoB- and apoA-containing lipoproteines in conditions of insulin resistance pathology. It was shown that hypertriglycerolemia and hypercholesterolemia were determined by triglycerole and cholesterol surplus in the composition of both apoB- and apoA-containing lipoproteins. Phospholipid deficiency in the composition of all lipoproteins fractions were established. Possible mechanisms of formation and metabolism in circulation of blood lipoproteins with altered lipid composition in the blood flow at insulin resistance is discussed.