Protein-tyrosine phosphatase 1B deficiency reduces insulin resistance and the diabetic phenotype in mice with polygenic insulin resistance

Spilanthes filicaulis (Schumach. & Thonn.) C. D Adam leaf extract prevents assault of streptozotocin on liver cells via inhibition of oxidative stress and activation of the NrF2/Keap1, PPARγ, and PTP1B signaling pathways

BACKGROUND

Spilanthes filicaulis (Schumach. & Thonn.) C. D Adam is a shrubby plant of the Asteraceae family that has medicinal benefits for the pharmaceutical and cosmetic industries.

PURPOSE

The purpose of this study was to assess the effectiveness of Spilanthes filicaulis leaf extract in a streptozotocin (STZ)-induced rat model and the associated signaling pathways.

METHODS

A sample of 25 male Wistar rats was randomly assigned to groups I, II, III, IV, and V. Each group included five animals, i.e., control rats, diabetic control rats, diabetic rats treated with metformin, and diabetic rats treated with 150 mg/kg/bw and 300 mg/kg/bw of the methanolic extract of S. filicaulis leaves (MESFL). Treatment was administered for 15 successive days via oral gavage. After 15 days, the rats were evaluated for fasting blood glucose (FBG), glycated hemoglobin (HbA1c), alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), triglyceride (TG), total cholesterol (TC), low-density lipoprotein (LDL), high-density lipoprotein (HDL), reduced glutathione (GSH), glutathione-S-transferase (GST), superoxide dismutase (SOD), catalase (CAT), lipid peroxidation (MDA), hexokinase, and glucose-6-phosphatase activities. Gene expression levels of nuclear factor erythroid 2-related factor 2 (Nrf2), peroxisome proliferator-activated receptor gamma (PPAR-γ), kelch-like ECH-associated protein 1 (Keap1), protein tyrosine phosphatase 1B (PTP1B) and the antiapoptotic protein caspase-3 were examined.

RESULTS

MESFL was administered to diabetic rats, and changes in body weight, fasting blood glucose (FBG) and HbA1c were restored. Furthermore, in diabetic rats, S. filicaulis significantly reduced the levels of triglycerides (TGs), total cholesterol (TC), low-density lipoprotein (LDL), and very low-density lipoprotein (VLDL) and significantly increased HDL. S. filicaulis improved ALT, AST, and ALP enzyme activity in diabetic rats. MDA levels decreased considerably with increasing activity of antioxidant enzymes, such as GST, SOD, CAT and GSH, in diabetic liver rats treated with S. filicaulis. Diabetic rats treated with MESFL and metformin exhibited upregulated mRNA expression levels of nuclear factor erythroid 2-related factor 2 (Nrf2) and peroxisome proliferator-activated receptor gamma (PPAR-γ). Kelch-like ECH-associated protein 1 (Keap1) and protein tyrosine phosphatase 1B (PTP1B) mRNA expression in the liver was downregulated in diabetic rats treated with MESFL and metformin. In addition, MESFL downregulated the mRNA expression of caspase-3 in diabetic rats.

CONCLUSION

It can be concluded from the data presented in this study that MESFL exerts a protective effect on diabetic rats due to its antidiabetic, antioxidant, antihyperlipidemic and antiapoptotic effects and may be considered a treatment for T2DM.

The significance of glutaredoxins for diabetes mellitus and its complications

Diabetes mellitus is a non-communicable metabolic disease hallmarked by chronic hyperglycemia caused by beta-cell failure. Diabetic complications affect the vasculature and result in macro- and microangiopathies, which account for a significantly increased morbidity and mortality. The rising incidence and prevalence of diabetes is a major global health burden. There are no feasible strategies for beta-cell preservation available in daily clinical practice. Therefore, patients rely on antidiabetic drugs or the application of exogenous insulin. Glutaredoxins (Grxs) are ubiquitously expressed and highly conserved members of the thioredoxin family of proteins. They have specific functions in redox-mediated signal transduction, iron homeostasis and biosynthesis of iron-sulfur (FeS) proteins, and the regulation of cell proliferation, survival, and function. The involvement of Grxs in chronic diseases has been a topic of research for several decades, suggesting them as therapeutic targets. Little is known about their role in diabetes and its complications. Therefore, this review summarizes the available literature on the significance of Grxs in diabetes and its complications. In conclusion, Grxs are differentially expressed in the endocrine pancreas and in tissues affected by diabetic complications, such as the heart, the kidneys, the eye, and the vasculature. They are involved in several pathways essential for insulin signaling, metabolic inflammation, glucose and fatty acid uptake and processing, cell survival, and iron and mitochondrial metabolism. Most studies describe significant changes in glutaredoxin expression and/or activity in response to the diabetic metabolism. In general, mitigated levels of Grxs are associated with oxidative distress, cell damage, and even cell death. The induced overexpression is considered a potential part of the cellular stress-response, counteracting oxidative distress and exerting beneficial impact on cell function such as insulin secretion, cytokine expression, and enzyme activity.

Syzygium cumini ameliorates high fat diet induced glucose intolerance, insulin resistance, weight gain, hepatic injury and nephrotoxicity through modulation of PTP1B and PPARγ signaling

Metabolic disorders are majorly associated with insulin resistance and an impaired glucose tolerance. Since, many of the currently available drugs exhibit adverse effects and are resistant to therapies, natural products are a promising alternate in the alleviation of complex metabolic disorders. In the current study, Syzygium cumini methanolic extract (SCE) was investigated for its anti-diabetic and anti-adipogenic potential using C57BL/6 mice fed on high fat diet (HFD). The HFD fed obese mice were treated with 200 mg/kg SCE and compared with positive controls Metformin, Pioglitazone and Sodium Orthovanadate. The biometabolites in SCE were characterized using Fourier transform infrared and gas chromatography and mass spectroscopy. A reduction in blood glucose levels with improved insulin sensitivity and glucose tolerance was observed in SCE-treated HFD obese mice. Histopathological and biochemical investigations showed a reduction in hepatic injury and nephrotoxicity in SCE-administered HFD mice. Results showed inhibition of PTP1B and an upregulation of IRS1 and PKB-mediated signaling in skeletal muscle. A significant decrease in lipid markers such as TC, TG, LDL-c and VLDL-c levels were observed with increased HDL-c in SCE-treated HFD mice. A significant decrease in weight and adiposity was observed in SCE-administered HFD mice in comparison to controls. This decrease could be due to the partial agonism of PPARγ and an increased expression of adiponectin, an insulin sensitizer. Hence, the dual-modulatory effect of SCE, partly due to the presence of 26% Pyrogallol, could be useful in the management of diabetes and its associated maladies.

Analogues of Dihydroflavonol and Flavone as Protein Tyrosine Phosphatase 1B Inhibitors from the Leaves of Artocarpus elasticus

Protein tyrosine phosphatase 1B (PTP1B) is one of the target enzymes whose disruption leads to obesity and diabetes. A series of PTP1B inhibitors were isolated from the leaves of Artocarpus elasticus, used in traditional medicines for diabetes. The isolated inhibitors (1-13), including two new compounds (1 and 2), consisted of dihydroflavonols and flavones. The structural requirements for the PTP1B inhibitory mode and potency were revealed in both skeletons. The two highest PTP1B inhibitory properties were dihydroflavonol 1 and flavone 6 analogs with IC50 values of 0.17 and 0.79 μM, respectively. The stereochemistry also affected inhibitory potencies: trans isomer 1 (IC50= 0.17 μM) vs cis isomer 2 (IC50= 2.24 μM). Surprisingly, the dihydroflavonol and flavone glycosides (11 and 13) displayed potent inhibition with IC50s of 2.39 and 0.22 μM, respectively. Furthermore, competitive inhibitor 1 was applied to time-dependence experiments as a simple slow-binding inhibitor with parameters of Kiapp = 0.064103 μM, k3 = 0.2262 μM-1 min-1, and k4 = 0.0145 min-1. The binding affinities by using the fluorescence quenching experiment were highly correlated with inhibitory potencies: 1 (IC50= 0.17 μM, KSV = 0.4375 × 105 L·mol-1) vs 3 (IC50= 17.79 μM, KSV = 0.0006 × 105 L·mol-1). The specific binding interactions were estimated at active and allosteric sites according to the inhibitory mode by molecular docking.

CD248 promotes insulin resistance by binding to the insulin receptor and dampening its insulin-induced autophosphorylation

BACKGROUND

In spite of new treatments, the incidence of type 2 diabetes (T2D) and its morbidities continue to rise. The key feature of T2D is resistance of adipose tissue and other organs to insulin. Approaches to overcome insulin resistance are limited due to a poor understanding of the mechanisms and inaccessibility of drugs to relevant intracellular targets. We previously showed in mice and humans that CD248, a pre/adipocyte cell surface glycoprotein, acts as an adipose tissue sensor that mediates the transition from healthy to unhealthy adipose, thus promoting insulin resistance.

METHODS

Molecular mechanisms by which CD248 regulates insulin signaling were explored using in vivo insulin clamp studies and biochemical analyses of cells/tissues from CD248 knockout (KO) and wild-type (WT) mice with diet-induced insulin resistance. Findings were validated with human adipose tissue specimens.

FINDINGS

Genetic deletion of CD248 in mice, overcame diet-induced insulin resistance with improvements in glucose uptake and lipolysis in white adipose tissue depots, effects paralleled by increased adipose/adipocyte GLUT4, phosphorylated AKT and GSK3β, and reduced ATGL. The insulin resistance of the WT mice could be attributed to direct interaction of the extracellular domains of CD248 and the insulin receptor (IR), with CD248 acting to block insulin binding to the IR. This resulted in dampened insulin-mediated autophosphorylation of the IR, with reduced downstream signaling/activation of intracellular events necessary for glucose and lipid homeostasis.

INTERPRETATION

Our discovery of a cell-surface CD248-IR complex that is accessible to pharmacologic intervention, opens research avenues toward development of new agents to prevent/reverse insulin resistance.

FUNDING

Funded by Canadian Institutes of Health Research (CIHR), Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundations for Innovation (CFI), the Swedish Diabetes Foundation, Family Ernfors Foundation and Novo Nordisk Foundation.

The importance of including the C-terminal domain of PTP1B1-400 to identify potential antidiabetic inhibitors

In the present work, we studied the inhibitory and kinetic implications of classical PTP1B inhibitors (chlorogenic acid, ursolic acid, suramin) using three enzyme constructs (hPTP1B_1-285, hPTP1B_1-321, and hPTP1B_1-400). The results indicate that the unstructured region of PTP1B (300-400 amino acids) is very important both to obtain optimal inhibitory results and propose classical inhibition mechanisms (competitive or non-competitive) through kinetic studies. The IC₅₀ calculated for ursolic acid and suramin using hPTP1B_1-400 are around four and three times lower to the short form of the enzyme, the complete form of PTP1B, the one found in the cytosol (in vivo). On the other hand, we highlight the studies of enzymatic kinetics using the hPTP1B_1-400 to know the type of enzymatic inhibition and to be able to direct docking studies, where the unstructured region of the enzyme can be one more option for binding compounds with inhibitory activity.

In Vitro and In Vivo Evaluation of the Antidiabetic Activity of Solidago virgaurea Extracts

Background

Solidago virgaurea (Asteraceae) has been used for more than 700 years for treating cystitis, chronic nephritis, urolithiasis, rheumatism, and inflammatory diseases. However, the antidiabetic activity of Solidago virgaurea has been rarely studied.

Methods

Three extracts of Solidago virgaurea were prepared, and their antidiabetic potentials were evaluated by various cell-free, cell-based, and in vivo studies.

Results

We found that the Solidago virgaurea contained multiple bioactive phytochemicals based on the GC-MS analysis. The Solidago virgaurea extracts effectively inhibited the functions of the carbohydrate digestive enzyme (α-glucosidase) and protein tyrosine phosphatase 1B (PTP1B), as well as decreased the amount of advanced glycation end products (AGEs). In the L6 myotubes, the Solidago virgaurea methanolic extract remarkably enhanced the glucose uptake via the upregulation of glucose transporter type 4 (GLUT4). The extract also significantly downregulated the expression of PTP1B. In the streptozotocin-nicotinamide induced diabetic mice, the daily intraperitoneal injection of 100 mg/kg Solidago virgaurea methanolic extract for 24 days, substantially lowered the postprandial blood glucose level with no obvious toxicity. The extract's anti-hyperglycemic effect was comparable to that of the glibenclamide treatment.

Conclusion

Our findings suggested that the Solidago virgaurea extract might have great potential in the prevention and treatment of diabetes.

Computational Methods in Cooperation with Experimental Approaches to Design Protein Tyrosine Phosphatase 1B Inhibitors in Type 2 Diabetes Drug Design: A Review of the Achievements of This Century

Protein tyrosine phosphatase 1B (PTP1B) dephosphorylates phosphotyrosine residues and is an important regulator of several signaling pathways, such as insulin, leptin, and the ErbB signaling network, among others. Therefore, this enzyme is considered an attractive target to design new drugs against type 2 diabetes, obesity, and cancer. To date, a wide variety of PTP1B inhibitors that have been developed by experimental and computational approaches. In this review, we summarize the achievements with respect to PTP1B inhibitors discovered by applying computer-assisted drug design methodologies (virtual screening, molecular docking, pharmacophore modeling, and quantitative structure–activity relationships (QSAR)) as the principal strategy, in cooperation with experimental approaches, covering articles published from the beginning of the century until the time this review was submitted, with a focus on studies conducted with the aim of discovering new drugs against type 2 diabetes. This review encourages the use of computational techniques and includes helpful information that increases the knowledge generated to date about PTP1B inhibition, with a positive impact on the route toward obtaining a new drug against type 2 diabetes with PTP1B as a molecular target.

Evaluation of Biological Activity of Natural Compounds: Current Trends and Methods

Natural compounds have diverse structures and are present in different forms of life. Metabolites such as tannins, anthocyanins, and alkaloids, among others, serve as a defense mechanism in live organisms and are undoubtedly compounds of interest for the food, cosmetic, and pharmaceutical industries. Plants, bacteria, and insects represent sources of biomolecules with diverse activities, which are in many cases poorly studied. To use these molecules for different applications, it is essential to know their structure, concentrations, and biological activity potential. In vitro techniques that evaluate the biological activity of the molecules of interest have been developed since the 1950s. Currently, different methodologies have emerged to overcome some of the limitations of these traditional techniques, mainly via reductions in time and costs. These emerging technologies continue to appear due to the urgent need to expand the analysis capacity of a growing number of reported biomolecules. This review presents an updated summary of the conventional and relevant methods to evaluate the natural compounds' biological activity in vitro.

Current Studies on Molecular Mechanisms of Insulin Resistance

Diabetes is a metabolic disease that raises the risk of microvascular and neurological disorders. Insensitivity to insulin is a characteristic of type II diabetes, which accounts for 85-90 percent of all diabetic patients. The fundamental molecular factor of insulin resistance may be impaired cell signal transduction mediated by the insulin receptor (IR). Several cell-signaling proteins, including IR, insulin receptor substrate (IRS), and phosphatidylinositol 3-kinase (PI3K), have been recognized as being important in the impaired insulin signaling pathway since they are associated with a large number of proteins that are strictly regulated and interact with other signaling pathways. Many studies have found a correlation between IR alternative splicing, IRS gene polymorphism, the complicated regulatory function of IRS serine/threonine phosphorylation, and the negative regulatory role of p85 in insulin resistance and diabetes mellitus. This review brings up-to-date knowledge of the roles of signaling proteins in insulin resistance in order to aid in the discovery of prospective targets for insulin resistance treatment.

The down regulation of PTP1B expression and attenuation of disturbed glucose and lipid metabolism using Borassus flabellifer (L) fruit methanol extract in high fat diet and streptozotocin induced diabetic rats

Borassus flabellifer L. is a tall palm traditionally used for its stimulating, diuretic and anti-inflammatory activities; it is rich in fibers and various pharmacologically important secondary metabolites. This study was undertaken to evaluate the antidiabetic effects of Borassus flabellifer fruit methanol extract (BF-M) on diabetic rats induced with High Fat Diet (HFD)/streptozotocin (STZ). When BF-M (100 or 200 mg/kg) was administered for 21 days orally it led to a sharp decline in triglycerides, total cholesterol, free unsaturated fat, glucose-6-phosphate, fasting blood glucose and fructose 1,6 bisphosphatase in contrast to diabetic control. BF-M also downregulated Protein Tyrosine Phosphatase 1B. In vitro study showed the IC₅₀ value to be 23.98 μg/mL. BF-M significantly increased serum insulin, glycogen content, and body weight. Western blot analysis exhibited significant inhibition of PTP1B in pancreatic tissue which was confirmed by histology and immunohistological studies. GC-MS analysis revelaled that the presence of major compounds such as 5-hydroxymethylfurfural (47.56%), Guanosine (21.01%) and n-hecxadeconoic acid (25.14%) in BF-M. In short, BF-M exerted antidiabetic property by down regulating PTP1B expression, and eventually enhancing glucose stimulated insulin release; it also exhibited favorable effects in diabetes and its secondary complications.

Anti-diabetic study of vitamin B6 on hyperglycaemia induced protein carbonylation, DNA damage and ROS production in alloxan induced diabetic rats

Oxidative stress performs an imperative role in the onset and progression of diabetes. Metabolic enzymes and cellular organelles are detrimental to increased levels of free radicals and the subsequent reduction in anti-oxidant defence. Pyridoxamine (vitamin B6) is an indispensible nutrient for humans and is considered to be an important food additive too. The aim of this research was to examine the effect of vitamin B6 in a diabetic environment. This study reports the effects of pyridoxamine supplementation in alloxan induced diabetic rats. Diabetes was induced by the single intra peritoneal dose of alloxan (120 mg per kg body weight). Diabetic rats were treated with pyridoxamine (10 and 15 mg per kg body weight) and compared with a control set of diabetic rats without supplementation. Pyridoxamine treatment showed dose dependent recovery in all parameters. A notable decline in oxidative stress parameters and ROS production with reductions in fasting blood glucose levels along with normal patterns of the glucose tolerance test has been reported here. Histological studies reveal damage recovery in the liver as well as kidney tissues. A notable amount of recovery was observed in cellular DNA distortion and damage. It is thus advocated that pyridoxamine might help in reducing problems associated with diabetes. A probable mechanism pertaining to the action of pyridoxamine is proposed as well.

Regulation of platelet-activating factor-induced interleukin-8 expression by protein tyrosine phosphatase 1B

Background

Platelet-activating factor (PAF) is a potent lipid mediator whose involvement in the onset and progression of atherosclerosis is mediated by, among others, the modulation of cytokine expression patterns. The presence of multiple potential protein-tyrosine phosphatase (PTP) 1B substrates in PAF receptor signaling pathways brought us to investigate its involvement in PAF-induced cytokine expression in monocyte-derived dendritic cells (Mo-DCs) and to study the pathways involved in this modulation.

Methods

We used in-vitro-matured human dendritic cells and the HEK-293 cell line in our studies. PTP1B inhibition was though siRNAs and a selective inhibitor. Cytokine expression was studied with RT-PCR, luciferase assays and ELISA. Phosphorylation status of kinases and transcription factors was studied with western blotting.

Results

Here, we report that PTP1B was involved in the modulation of cytokine expression in PAF-stimulated Mo-DCs. A study of the down-regulation of PAF-induced IL-8 expression, by PTP1B, showed modulation of PAF-induced transactivation of the IL-8 promoter which was dependent on the presence of the C/EBPß -binding site. Results also suggested that PTP1B decreased PAF-induced IL-8 production by a glycogen synthase kinase (GSK)-3-dependent pathway via activation of the Src family kinases (SFK). These kinases activated an unidentified pathway at early stimulation times and the PI3K/Akt signaling pathway in a later phase. This change in GSK-3 activity decreased the C/EBPß phosphorylation levels of the threonine 235, a residue whose phosphorylation is known to increase C/EBPß transactivation potential, and consequently modified IL-8 expression.

Conclusion

The negative regulation of GSK-3 activity by PTP1B and the consequent decrease in phosphorylation of the C/EBPß transactivation domain could be an important negative feedback loop by which cells control their cytokine production after PAF stimulation.

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0334-6) contains supplementary material, which is available to authorized users.

Isolation and Characterization of Protein Tyrosine Phosphatase 1B (PTP1B) Inhibitory Polyphenolic Compounds From Dodonaea viscosa and Their Kinetic Analysis

Diabetes mellitus is one of a major worldwide concerns, regulated by either defects in secretion or action of insulin, or both. Insulin signaling down-regulation has been related with over activity of protein tyrosine phosphatase 1B (PTP1B) enzyme, which has been a promising target for the treatment of diabetes mellitus. Herein, activity guided separation of methanol extract (95%) of Dodonaea viscosa aerial parts afforded nine (1-9) polyphenolic compounds, all of them were identified through spectroscopic data including 2D NMR and HREIMS. Subsequently, their PTP1B inhibitory potentials were evaluated, in which all of the isolates exhibited significant dose-dependent inhibition with IC₅₀ 13.5–57.9 μM. Among them, viscosol (4) was found to be the most potent compound having IC₅₀ 13.5 μM. In order to unveil the mechanistic behavior, detailed kinetic study was carried out, in which compound 4 was observed as a reversible, and mixed type I inhibitor of PTP1B with inhibitory constant (K_i) value of 4.6 μM. Furthermore, we annotated the major metabolites through HPLC-DAD-ESI/MS analysis, in which compounds 3, 6, 7, and 9 were found to be the most abundant metabolites in D. viscosa extract.

Recent advances in understanding the role of protein-tyrosine phosphatases in development and disease

Protein-tyrosine phosphatases (PTPs) remove phosphate groups from tyrosine residues, and thereby propagate or inhibit signal transduction, and hence influence cellular processes such as cell proliferation and differentiation. The importance of tightly controlled PTP activity is reflected by the numerous mechanisms employed by the cell to control PTP activity, including a variety of post-translational modifications, and restricted subcellular localization. This review highlights the strides made in the last decade and discusses the important role of PTPs in key aspects of embryonic development: the regulation of stem cell self-renewal and differentiation, gastrulation and somitogenesis during early embryonic development, osteogenesis, and angiogenesis. The tentative importance of PTPs in these processes is highlighted by the diseases that present upon aberrant activity.

BPN, a marine-derived PTP1B inhibitor, activates insulin signaling and improves insulin resistance in C2C12 myotubes

Insulin resistance is a key feature of type 2 diabetes mellitus (T2DM) and is characterized by defects in insulin signaling. Protein tyrosine phosphatase 1B (PTP1B) is a major negative regulator of insulin signaling cascade and has attracted intensive investigation in recent T2DM therapy study. BPN, a marine-derived bromophenol compound, was isolated from the red alga Rhodomela confervoides. This study investigated the effects of BPN on the insulin signaling pathway in insulin-resistant C2C12 myotubes by inhibiting PTP1B. Molecular docking study and analysis of small- molecule interaction with PTP1B all showed BPN inhibited PTP1B activity via binding to the catalytic site through hydrogen bonds. We then found that BPN permeated into C2C12 myotubes, on the one hand, activated insulin signaling in an insulin-independent manner in C2C12 cells; on the other hand, ameliorated palmitate-induced insulin resistance through augmenting insulin sensitivity. Moreover, our studies also showed that PTP1B inhibition by BPN increased glucose uptake in normal and insulin-resistant C2C12 myotubes through glucose transporter 4 (GLUT4) translocation. Taken together, BPN activates insulin signaling and alleviates insulin resistance and represents a potential candidate for further development as an antidiabetic agent.

Impaired Insulin Signaling is Associated with Hepatic Mitochondrial Dysfunction in IR+/--IRS-1+/- Double Heterozygous (IR-IRS1dh) Mice

Mitochondria play a pivotal role in energy metabolism, but whether insulin signaling per se could regulate mitochondrial function has not been identified yet. To investigate whether mitochondrial function is regulated by insulin signaling, we analyzed muscle and liver of insulin receptor (IR)^+/−-insulin receptor substrate-1 (IRS-1)^+/− double heterozygous (IR-IRS1dh) mice, a well described model for insulin resistance. IR-IRS1dh mice were studied at the age of 6 and 12 months and glucose metabolism was determined by glucose and insulin tolerance tests. Mitochondrial enzyme activities, oxygen consumption, and membrane potential were assessed using spectrophotometric, respirometric, and proton motive force analysis, respectively. IR-IRS1dh mice showed elevated serum insulin levels. Hepatic mitochondrial oxygen consumption was reduced in IR-IRS1dh animals at 12 months of age. Furthermore, 6-month-old IR-IRS1dh mice demonstrated enhanced mitochondrial respiration in skeletal muscle, but a tendency of impaired glucose tolerance. On the other hand, 12-month-old IR-IRS1dh mice showed improved glucose tolerance, but normal muscle mitochondrial function. Our data revealed that deficiency in IR/IRS-1 resulted in normal or even elevated skeletal muscle, but impaired hepatic mitochondrial function, suggesting a direct cross-talk between insulin signaling and mitochondria in the liver.

Histone Deacetylase 1 (HDAC1) Negatively Regulates Thermogenic Program in Brown Adipocytes via Coordinated Regulation of Histone H3 Lysine 27 (H3K27) Deacetylation and Methylation

Inhibiting class I histone deacetylases (HDACs) increases energy expenditure, reduces adiposity, and improves insulin sensitivity in obese mice. However, the precise mechanism is poorly understood. Here, we demonstrate that HDAC1 is a negative regulator of the brown adipocyte thermogenic program. The Hdac1 level is lower in mouse brown fat (BAT) than white fat, is suppressed in mouse BAT during cold exposure or β3-adrenergic stimulation, and is down-regulated during brown adipocyte differentiation. Remarkably, overexpressing Hdac1 profoundly blocks, whereas deleting Hdac1 significantly enhances, β-adrenergic activation-induced BAT-specific gene expression in brown adipocytes. β-Adrenergic activation in brown adipocytes results in a dissociation of HDAC1 from promoters of BAT-specific genes, including uncoupling protein 1 (Ucp1) and peroxisome proliferator-activated receptor γ co-activator 1α (Pgc1α), leading to increased acetylation of histone H3 lysine 27 (H3K27), an epigenetic mark of gene activation. This is followed by dissociation of the polycomb repressive complexes, including the H3K27 methyltransferase enhancer of zeste homologue (EZH2), suppressor of zeste 12 (SUZ12), and ring finger protein 2 (RNF2) from (and concomitant recruitment of H3K27 demethylase ubiquitously transcribed tetratricopeptide repeat on chromosome X (UTX) to) Ucp1 and Pgc1α promoters, leading to decreased H3K27 trimethylation, a histone transcriptional repression mark. Thus, HDAC1 negatively regulates the brown adipocyte thermogenic program, and inhibiting Hdac1 promotes BAT-specific gene expression through a coordinated control of increased acetylation and decreased methylation of H3K27, thereby switching the transcriptional repressive state to the active state at the promoters of Ucp1 and Pgc1α. Targeting HDAC1 may be beneficial in prevention and treatment of obesity by enhancing BAT thermogenesis.

Ceramide as a mediator of non-alcoholic Fatty liver disease and associated atherosclerosis

Cardiovascular disease (CVD) is a serious comorbidity in nonalcoholic fatty liver disease (NAFLD). Since plasma ceramides are increased in NAFLD and sphingomyelin, a ceramide metabolite, is an independent risk factor for CVD, the role of ceramides in dyslipidemia was assessed using LDLR-/- mice, a diet-induced model of NAFLD and atherosclerosis. Mice were fed a standard or Western diet (WD), with or without myriocin, an inhibitor of ceramide synthesis. Hepatic and plasma ceramides were profiled and lipid and lipoprotein kinetics were quantified. Hepatic and intestinal expression of genes and proteins involved in insulin, lipid and lipoprotein metabolism were also determined. WD caused hepatic oxidative stress, inflammation, apoptosis, increased hepatic long-chain ceramides associated with apoptosis (C16 and C18) and decreased very-long-chain ceramide C24 involved in insulin signaling. The plasma ratio of ApoB/ApoA1 (proteins of VLDL/LDL and HDL) was increased 2-fold due to increased ApoB production. Myriocin reduced hepatic and plasma ceramides and sphingomyelin, and decreased atherosclerosis, hepatic steatosis, fibrosis, and apoptosis without any effect on oxidative stress. These changes were associated with decreased lipogenesis, ApoB production and increased HDL turnover. Thus, modulation of ceramide synthesis may lead to the development of novel strategies for the treatment of both NAFLD and its associated atherosclerosis.

The evaluation of serum levels of testosterone in type 2 diabetic men and its relation with lipid profile

INTRODUCTION AND AIM

Type 2 diabetes mellitus (T2DM) is the predominant form of diabetes worldwide and much is known about its patho-physiology. Yet, newer aspects related to it are being constantly explored. For ages, testosterone has been known to men as the male sex hormone but now it has been shown by certain studies that it might have a role in the development of metabolic disorders like type 2 diabetes. This study was carried out to determine the relation of testosterone levels with type 2 diabetes mellitus and lipid profile in North East Indian men aged 31 to 73 years.

MATERIALS AND METHODS

This case control study comprised of 40 type 2 diabetic men and 40 age matched non diabetic healthy men. Testosterone, SHBG levels and lipid profile were evaluated in both the groups along with anthropometric measurements and were statistically analysed.

RESULTS

Serum total and free testosterone and Sex Hormone Binding Globulin were significantly lower in the test group than in the control group. Prevalence of type 2diabetes was five times higher in men having a total testosterone less than 8nmol/L and 5.57 times higher in those having a free testosterone of less than 0.225nmol/L. Fasting blood glucose showed a strong negative correlation with total and free testosterone. Glycated haemoglobin correlated negatively with SHBG but no such correlation was seen with total or free testosterone. Serum total and LDL cholesterol showed significant negative correlation with total testosterone and SHBG but no significant correlation was found with free testosterone. Serum VLDL, HDL and triglycerides did not show any significant correlation with total or free testosterone and SHBG levels.

CONCLUSION

Low testosterone might have a role in the development of type 2 DM and to the associated altered lipid profile. This study, though a small one is among the few of its kind in India and it thrives to assist other studies related to the matter.

Age- and gender-related changes in glucose homeostasis in glucocorticoid-treated rats

The disruption to glucose homeostasis upon glucocorticoid (GC) treatment in adult male rats has not been fully characterized in older rats or in females. Thus, we evaluated the age- and gender-related changes in glucose homeostasis in GC-treated rats. We injected male and female rats at 3 months and 12 months of age with either dexamethasone (1.0 mg/kg body mass, intraperitoneally) or saline, daily for 5 days. All of the GC-treated rats had decreased body mass and food intake, and adrenal hypotrophy. Increased glycemia was observed in all of the GC-treated groups and only the 3-month-old female rats were not glucose intolerant. Dexamethasone treatment resulted in hyperinsulinemia and hypertriacylglyceridemia in all of the GC-treated rats. The glucose-stimulated insulin secretion (GSIS) was higher in all of the dexamethasone-treated animals, but it was less pronounced in the older animals. The β-cell mass was increased in the younger male rats treated with dexamethasone. We conclude that dexamethasone treatment induces glucose intolerance in both the 3- and 12-month-old male rats as well as hyperinsulinemia and augmented GSIS. Three-month-old female rats are protected from glucose intolerance caused by GC, whereas 12-month-old female rats developed the same complications that were present in 3- and 12-month-old male rats.

Protein tyrosine phosphatase-1B modulates pancreatic β-cell mass

Protein tyrosine phosphatase 1B (PTP1B) is a negative regulator of the insulin signalling pathway. It has been demonstrated that PTP1B deletion protects against the development of obesity and Type 2 Diabetes, mainly through its action on peripheral tissues. However, little attention has been paid to the role of PTP1B in β-cells. Therefore, our aim was to study the role of PTP1B in pancreatic β-cells. Silencing of PTP1B expression in a pancreatic β-cell line (MIN6 cells) reveals the significance of this endoplasmic reticulum bound phosphatase in the regulation of cell proliferation and apoptosis. Furthermore, the ablation of PTP1B is able to regulate key proteins involved in the proliferation and/or apoptosis pathways, such as STAT3, AKT, ERK1/2 and p53 in isolated islets from PTP1B knockout (PTP1B ⁻/⁻) mice. Morphometric analysis of pancreatic islets from PTP1B ⁻/⁻ mice showed a higher β-cell area, concomitantly with higher β-cell proliferation and a lower β-cell apoptosis when compared to islets from their respective wild type (WT) littermates. At a functional level, isolated islets from 8 weeks old PTP1B ⁻/⁻ mice exhibit enhanced glucose-stimulated insulin secretion. Moreover, PTP1B ⁻/⁻ mice were able to partially reverse streptozotocin-induced β-cell loss. Together, our data highlight for the first time the involvement of PTP1B in β-cell physiology, reinforcing the potential of this phosphatase as a therapeutical target for the treatment of β-cell failure, a central aspect in the pathogenesis of Type 2 Diabetes.

Endothelial cell PTP1B regulates leukocyte recruitment during allergic inflammation

Pulmonary eosinophilia is a consistent hallmark of allergic lung inflammation. Infiltration of eosinophils into ovalbumin (OVA)-challenged lungs is dependent on the adhesion molecule vascular cell adhesion molecule-1 (VCAM-1) on endothelial cells. Ligation of VCAM-1 activates endothelial cell protein tyrosine phosphatase 1B (PTP1B), which is required for VCAM-1-dependent leukocyte migration in vitro. To examine whether nonhematopoietic PTP1B modulates eosinophil recruitment in vivo, mice deficient in PTP1B were irradiated and received wild-type hematopoietic cells to generate chimeric PTP1B-/- mice. In response to OVA challenge, the chimeric PTP1B-/- mice had reduced eosinophilia in the lung tissue and bronchoalveolar lavage, indicating a role for PTP1B in nonhematopoietic cells during leukocyte recruitment. To determine whether endothelial cell PTP1B modulates eosinophil recruitment, mice with an inducible endothelial cell-specific PTP1B deletion (iePTP1B mice) were generated and the PTP1B deletion was induced after antigen sensitization before antigen challenge. In response to OVA challenge, the iePTP1B mice with the endothelial cell PTP1B deletion had an increased accumulation of eosinophils bound to the luminal surface of the endothelium in the lung vasculature and had a decrease in leukocyte recruitment into the lung tissue. In the iePTP1B mice, expression of adhesion molecules, cytokines, or chemokines that regulate leukocyte recruitment during inflammation was not altered, consistent with other studies that deletion of endothelial adhesion molecule signals does not alter lung cytokines and chemokines. In summary, these data suggest that VCAM-1 activation of PTP1B in the endothelium is necessary for eosinophil recruitment during allergic inflammation. Moreover, these studies provide a basis for targeting VCAM-1-dependent signaling pathways in allergy therapies.

Carbenoxolone treatment ameliorated metabolic syndrome in WNIN/Ob obese rats, but induced severe fat loss and glucose intolerance in lean rats

BACKGROUND

11beta-hydroxysteroid dehydrogenase type 1 (11β-HSD1) regulates local glucocorticoid action in tissues by catalysing conversion of inactive glucocorticoids to active glucocorticoids. 11β-HSD1 inhibition ameliorates obesity and associated co-morbidities. Here, we tested the effect of 11β-HSD inhibitor, carbenoxolone (CBX) on obesity and associated comorbidities in obese rats of WNIN/Ob strain, a new animal model for genetic obesity.

METHODOLOGY/PRINCIPAL FINDINGS

Subcutaneous injection of CBX (50 mg/kg body weight) or volume-matched vehicle was given once daily for four weeks to three month-old WNIN/Ob lean and obese rats (n = 6 for each phenotype and for each treatment). Body composition, plasma lipids and hormones were assayed. Hepatic steatosis, adipose tissue morphology, inflammation and fibrosis were also studied. Insulin resistance and glucose intolerance were determined along with tissue glycogen content. Gene expressions were determined in liver and adipose tissue. CBX significantly inhibited 11β-HSD1 activity in liver and adipose tissue of WNIN/Ob lean and obese rats. CBX significantly decreased body fat percentage, hypertriglyceridemia, hypercholesterolemia, insulin resistance in obese rats. CBX ameliorated hepatic steatosis, adipocyte hypertrophy, adipose tissue inflammation and fibrosis in obese rats. Tissue glycogen content was significantly decreased by CBX in liver and adipose tissue of obese rats. Severe fat loss and glucose- intolerance were observed in lean rats after CBX treatment.

CONCLUSIONS/SIGNIFICANCE

We conclude that 11β-HSD1 inhibition by CBX decreases obesity and associated co-morbidities in WNIN/Ob obese rats. Our study supports the hypothesis that inhibition of 11β-HSD1 is a key strategy to treat metabolic syndrome. Severe fat loss and glucose -intolerance by CBX treatment in lean rats suggest that chronic 11β-HSD1 inhibition may lead to insulin resistance in normal conditions.

Zinc transporters, mechanisms of action and therapeutic utility: implications for type 2 diabetes mellitus

Zinc is an essential trace element that plays a vital role in maintaining many biological processes and cellular homeostasis. Dysfunctional zinc signaling is associated with a number of chronic disease states including cancer, cardiovascular disease, Alzheimer's disease, and diabetes. Cellular homeostasis requires mechanisms that tightly control the uptake, storage, and distribution of zinc. This is achieved through the coordinated actions of zinc transporters and metallothioneins. Evidence on the role of these proteins in type 2 diabetes mellitus (T2DM) is now emerging. Zinc plays a key role in the synthesis, secretion and action of insulin in both physiological and pathophysiological states. Moreover, recent studies highlight zinc's dynamic role as a "cellular second messenger" in the control of insulin signaling and glucose homeostasis. This suggests that zinc plays an unidentified role as a novel second messenger that augments insulin activity. This previously unexplored concept would raise a whole new area of research into the pathophysiology of insulin resistance and introduce a new class of drug target with utility for diabetes pharmacotherapy.

The full capacity of AICAR to reduce obesity-induced inflammation and insulin resistance requires myeloid SIRT1

Chronic Inflammation is a key link between obesity and insulin resistance. We previously showed that two nutrient sensors AMP-activated protein kinase (AMPK) and SIRT1 interact to regulate macrophage inflammation. AMPK is also a molecular target of 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR), which has been shown to reduce insulin resistance in various animal models. Here we aim to determine whether the therapeutic effects of AICAR against insulin resistance involve its anti-inflammatory function, which requires macrophage SIRT1. Long-term administration of low-dose AICAR significantly suppressed adipose inflammation in established diet-induced obese mice. This was associated with improved glucose homeostasis and insulin sensitivity without changes of body weight. In contrast, SIRT1 deletion in myeloid SIRT1 knockout (MSKO) mice increased infiltration of classically activated M1 macrophages and decreased alternatively activated M2 macrophages in adipose tissue. As a result, MSKO mice on high fat (HF) diets exhibited impaired insulin signaling in skeletal muscle, fat, and liver, and developed systemic insulin resistance in glucose tolerance tests, insulin tolerance tests, and hyperinsulinemic-euglycemic clamp experiments. Interestingly, the beneficial effects of AICAR on adipose inflammation and insulin sensitivity were absent in MSKO mice fed HF diets, suggesting that the full capacity of AICAR to antagonize obesity-induced inflammation and insulin resistance requires myeloid SIRT1. In summary, AICAR negatively regulates HF diet-induced inflammation, which requires myeloid SIRT1, thereby contributing to the protection against insulin resistance. Myeloid SIRT1 is a therapeutic target of the anti-inflammatory and insulin-sensitizing effects of AICAR.

Skeletal muscle protein tyrosine phosphatase 1B regulates insulin sensitivity in African Americans

Protein tyrosine phosphatase 1B (PTP1B) is postulated to modulate insulin action by dephosphorylating the insulin receptor signaling proteins and attenuating insulin signaling. We sought to determine the relationship of skeletal muscle PTP1B to whole-body insulin sensitivity. We studied 17 African Americans with type 2 diabetes mellitus (T2DM) and 16 without diabetes. PTP1B gene expression and protein abundance were determined in the biopsied skeletal muscles at the baseline of a hyperinsulinemic-euglycemic clamp. PTP1B gene expression was significantly higher in subjects with T2DM versus control (P < 0.0001) and remained significantly different after adjusting for age and insulin sensitivity (P = 0.05). PTP1B gene expression was positively related to protein abundance (r(s) = 0.39; P = 0.03; adjusted for age and insulin sensitivity) and negatively related to insulin sensitivity (r(s) = -0.52; P = 0.002; adjusted for age). Overexpression and interference RNA of PTP1B were performed in primary human skeletal muscle culture. PTP1B overexpression resulted in reduction of Akt phosphorylation in the control subjects. Moreover, interference RNA transfection downregulated PTP1B expression and enhanced Akt phosphorylation in subjects with T2DM. These data show that skeletal muscle PTP1B gene expression is increased in African American subjects with T2DM, is negatively associated with whole-body insulin sensitivity, and contributes to modulation of insulin signaling.

PTP1B and TCPTP--nonredundant phosphatases in insulin signaling and glucose homeostasis

Insulin resistance is a key pathological feature of type 2 diabetes and is characterized by defects in signaling by the insulin receptor (IR) protein tyrosine kinase. The inhibition of protein tyrosine phosphatases (PTPs) that antagonize IR signaling may provide a means for enhancing the insulin response and alleviating insulin resistance. The prototypic phosphotyrosine-specific phosphatase PTP1B dephosphorylates the IR and attenuates insulin signaling in muscle and liver. Mice that are deficient for PTP1B exhibit improved glucose homeostasis in diet and genetic models of insulin resistance and type 2 diabetes. The phosphatase TCPTP shares 72% catalytic domain sequence identity with PTP1B and has also been implicated in IR regulation. Despite their high degree of similarity, PTP1B and TCPTP act together in vitro and in vivo to regulate insulin signaling and glucose homeostasis. This review highlights their capacity to act specifically and nonredundantly in cellular signaling, describes their roles in IR regulation and glucose homeostasis, and discusses their potential as drug targets for the enhancement of IR phosphorylation and insulin sensitivity in type 2 diabetes.

Insulin immuno-neutralization in fed chickens: effects on liver and muscle transcriptome

Chickens mimic an insulin-resistance state by exhibiting several peculiarities with regard to plasma glucose level and its control by insulin. To gain insight into the role of insulin in the control of chicken transcriptome, liver and leg muscle transcriptomes were compared in fed controls and "diabetic" chickens, at 5 h after insulin immuno-neutralization, using 20.7K-chicken oligo-microarrays. At a level of false discovery rate <0.01, 1,573 and 1,225 signals were significantly modified by insulin privation in liver and muscle, respectively. Microarray data agreed reasonably well with qRT-PCR and some protein level measurements. Differentially expressed mRNAs with human ID were classified using Biorag analysis and Ingenuity Pathway Analysis. Multiple metabolic pathways, structural proteins, transporters and proteins of intracellular trafficking, major signaling pathways, and elements of the transcriptional control machinery were largely represented in both tissues. At least 42 mRNAs have already been associated with diabetes, insulin resistance, obesity, energy expenditure, or identified as sensors of metabolism in mice or humans. The contribution of the pathways presently identified to chicken physiology (particularly those not yet related to insulin) needs to be evaluated in future studies. Other challenges include the characterization of "unknown" mRNAs and the identification of the steps or networks, which disturbed tissue transcriptome so extensively, quickly after the turning off of the insulin signal. In conclusion, pleiotropic effects of insulin in chickens are further evidenced; major pathways controlled by insulin in mammals have been conserved despite the presence of unique features of insulin signaling in chicken muscle.

Accelerated atherosclerosis in Apoe-/- mice heterozygous for the insulin receptor and the insulin receptor substrate-1

OBJECTIVE

Prediabetic states are associated with accelerated atherosclerosis, but the availability of mouse models to study connections between these diseases has been limited. The aim of this study was to test the selective role of impaired insulin receptor/insulin receptor substrate-1 signaling on atherogenesis.

METHODS AND RESULTS

To address the effects of impaired insulin signaling associated with hyperinsulinemia on atherosclerosis in the absence of obesity and hyperglycemia, we generated insulin receptor (Insr)/insulin receptor substrate-1 (Insr1) double heterozygous apolipoprotein (Apoe)-knockout mice (Insr(+/-)Irs1(+/-)Apoe(-/-)) mice. Insr(+/-)Irs1(+/-)Apoe(-/-) mice fed a Western diet for 15 weeks showed elevated levels of fasting insulin compared to Insr(+/+)Irs1(+/+)Apoe(-/-) mice. There were no significant differences in glucose, triglyceride, HDL, VLDL, cholesterol levels or free fatty acid in the plasma of Insr(+/-)Irs1(+/-)Apoe(-/-) and Insr(+/+)Irs1(+/+)Apoe(-/-) mice. Atherosclerotic lesions were increased in male (brachiocephalic artery) and female (aortic tree) Insr(+/-)Irs1(+/-)Apoe(-/-) compared to Insr(+/+)Irs1(+/+)Apoe(-/-) mice. Bone marrow transfer experiments demonstrated that nonhematopoietic cells have to be Insr(+/-)Irs1(+/-) to accelerate atherosclerosis. Impaired insulin signaling resulted in decreased levels of vascular phospho-eNOS, attenuated endothelium-dependent vasorelaxation and elevated VCAM-1 expression in aortas of Insr(+/-)Irs1(+/-)Apoe(-/-) mice. In addition, phospho-ERK and vascular smooth muscle cell proliferation were significantly elevated in aortas of Insr(+/-)Irs1(+/-)Apoe(-/-) mice.

CONCLUSIONS

These results demonstrate that defective insulin signaling is involved in accelerated atherosclerosis in Insr(+/-)Irs1(+/-)Apoe(-/-) mice by promoting vascular dysfunction and inflammation.

Dock/Nck facilitates PTP61F/PTP1B regulation of insulin signalling

PTP1B (protein tyrosine phosphatase 1B) is a negative regulator of IR (insulin receptor) activation and glucose homoeostasis, but the precise molecular mechanisms governing PTP1B substrate selectivity and the regulation of insulin signalling remain unclear. In the present study we have taken advantage of Drosophila as a model organism to establish the role of the SH3 (Src homology 3)/SH2 adaptor protein Dock (Dreadlocks) and its mammalian counterpart Nck in IR regulation by PTPs. We demonstrate that the PTP1B orthologue PTP61F dephosphorylates the Drosophila IR in S2 cells in vitro and attenuates IR-induced eye overgrowth in vivo. Our studies indicate that Dock forms a stable complex with PTP61F and that Dock/PTP61F associate with the IR in response to insulin. We report that Dock is required for effective IR dephosphorylation and inactivation by PTP61F in vitro and in vivo. Furthermore, we demonstrate that Nck interacts with PTP1B and that the Nck/PTP1B complex inducibly associates with the IR for the attenuation of IR activation in mammalian cells. Our studies reveal for the first time that the adaptor protein Dock/Nck attenuates insulin signalling by recruiting PTP61F/PTP1B to its substrate, the IR.

IRS2 and PTP1B: Two opposite modulators of hepatic insulin signalling

Type 2 Diabetes mellitus (T2D) is the most common endocrine disorder associated to metabolic syndrome (MS) and occurs when insulin secretion can no compensate peripheral insulin resistance. Among peripheral tissues, the liver controls glucose homeostasis due to its ability to consume and produce glucose. The molecular mechanism underlying hepatic insulin resistance is not completely understood; however, it involves the impairment of the insulin signalling network. Among the critical nodes of hepatic insulin signalling, insulin receptor substrate 2 (IRS2) and protein tyrosine phosphatase 1B (PTP1B) modulate the phosphatidylinositol (PI) 3-kinase/Akt/Foxo1 pathway that controls the suppression of gluconeogenic genes. In this review, we will focus on recent findings regarding the molecular mechanism by which IRS2 and PTP1B elicit opposite effects on carbohydrate metabolism in the liver in response to insulin. Finally, we will discuss the involvement of the critical nodes of insulin signalling in non-alcoholic fatty liver disease (NAFLD) in humans.

Tissue specificity on insulin action and resistance: past to recent mechanisms

Insulin resistance is the most important pathophysiological feature in many pre-diabetic states. Type 2 diabetes mellitus is a complex metabolic disease and its pathogenesis involves abnormalities in both peripheral insulin action and insulin secretion by pancreatic β-cells. The creation of monogenic or polygenic genetically manipulated mice models in a tissue-specific manner was of great help to elucidate the tissue specificity of insulin action and its contribution to the overall insulin resistance. However, a complete understanding of the molecular bases of insulin action and resistance requires the identification of intracellular pathways that regulate insulin-stimulated proliferation, differentiation and metabolism. Accordingly, cell lines derived from insulin target tissues such as brown adipose tissue, liver and beta islets lacking insulin resistance or sensitive candidate genes such as IRS-1, IRS-2, IRS-3, IR and PTP1B have been developed. Indeed, these cell lines have also been very useful to understand the tissue specificity of insulin action and inaction. Obesity is a risk factor for several components of the metabolic syndromes such as type 2 diabetes, dyslipidaemia and systolic hypertension, because white and brown adipose tissues as endocrine organs express and secrete a variety of adipocytokines that can act at both local and systemic levels, modulating the insulin sensitivity. Recent studies revealed that the subjects with the highest transcription rates of genes encoding TNF-α and IL-6 were prone to develop obesity, insulin resistance and type 2 diabetes. Accordingly, we specifically focus in this review on the impact of those adipocytokines on the modulation of insulin action in skeletal muscle.

Activation of the cholinergic antiinflammatory pathway ameliorates obesity-induced inflammation and insulin resistance

Obesity is associated with a chronic inflammatory state characterized by adipose tissue macrophage infiltration and inflammation, which contributes to insulin resistance. The cholinergic antiinflammatory pathway, which acts through the macrophage α7-nicotinic acetylcholine receptor (α7nAChR), is important in innate immunity. Here we show that adipose tissue possesses a functional cholinergic signaling pathway. Activating this pathway by nicotine in genetically obese (db/db) and diet-induced obese mice significantly improves glucose homeostasis and insulin sensitivity without changes of body weight. This is associated with suppressed adipose tissue inflammation. In addition, macrophages from α7nAChR-/- [α7 knockout (α7KO)] mice have elevated proinflammatory cytokine production in response to free fatty acids and TNFα, known agents causing inflammation and insulin resistance. Nicotine significantly suppressed free fatty acid- and TNFα-induced cytokine production in wild type (WT), but not α7KO macrophages. These data suggest that α7nAChR is important in mediating the antiinflammatory effect of nicotine. Indeed, inactivating this pathway in α7KO mice results in significantly increased adipose tissue infiltration of classically activated M1 macrophages and inflammation in α7KO mice than their WT littermates. As a result, α7KO mice exhibit more severely impaired insulin sensitivity than WT mice without changes of body weight. These data suggest that the cholinergic antiinflammatory pathway plays an important role in obesity-induced inflammation and insulin resistance. Targeting this pathway may provide novel therapeutic benefits in the prevention and treatment of obesity-induced inflammation and insulin resistance.

Immortalized neurons for the study of hypothalamic function

The hypothalamus is a vital part of the central nervous system: it harbors control systems implicated in regulation of a wide range of homeostatic processes, including energy balance and reproduction. Structurally, the hypothalamus is a complex neuroendocrine tissue composed of a multitude of unique neuronal cell types that express a number of neuromodulators, including hormones, classical neurotransmitters, and specific neuropeptides that play a critical role in mediating hypothalamic function. However, neuropeptide and receptor gene expression, second messenger activation, and electrophysiological and secretory properties of these hypothalamic neurons are not yet fully defined, primarily because the heterogeneity and complex neuronal architecture of the neuroendocrine hypothalamus make such studies challenging to perform in vivo. To circumvent this problem, our research group recently generated embryonic- and adult-derived hypothalamic neuronal cell models by utilizing the novel molecular techniques of ciliary neurotrophic factor-induced neurogenesis and SV40 T antigen transfer to primary hypothalamic neuronal cell cultures. Significant research with these cell lines has demonstrated their value as a potential tool for use in molecular genetic analysis of hypothalamic neuronal function. Insights gained from hypothalamic immortalized cells used in conjunction with in vivo models will enhance our understanding of hypothalamic functions such as neurogenesis, neuronal plasticity, glucose sensing, energy homeostasis, circadian rhythms, and reproduction. This review discusses the generation and use of hypothalamic cell models to study mechanisms underlying the function of individual hypothalamic neurons and to gain a more complete understanding of the overall physiology of the hypothalamus.

Insulin signaling meets mitochondria in metabolism

Insulin controls nutrient and metabolic homeostasis via the IRS-PI3K-AKT signaling cascade that targets FOXO1 and mTOR. Mitochondria, as the prime metabolic platform, malfunction during insulin resistance in metabolic diseases. However, the molecular link between insulin resistance and mitochondrial dysfunction remains undefined. Here we review recent studies on insulin action and the mechanistic association with mitochondrial metabolism. These studies suggest that insulin signaling underpins mitochondrial electron transport chain integrity and activity by suppressing FOXO1/HMOX1 and maintaining the NAD(+)/NADH ratio, the mediator of the SIRT1/PGC1α pathway for mitochondrial biogenesis and function. Mitochondria generate moderately reactive oxygen species (ROS) and enhance insulin sensitivity upon redox regulation of protein tyrosine phosphatase and insulin receptor. However, chronic exposure to high ROS levels could alter mitochondrial function and thereby cause insulin resistance.

Niemann-Pick C1-Like 1 deletion in mice prevents high-fat diet-induced fatty liver by reducing lipogenesis

Niemann-Pick C1-Like 1 (NPC1L1) mediates intestinal absorption of dietary and biliary cholesterol. Ezetimibe, by inhibiting NPC1L1 function, is widely used to treat hypercholesterolemia in humans. Interestingly, ezetimibe treatment appears to attenuate hepatic steatosis in rodents and humans without a defined mechanism. Over-consumption of a high-fat diet (HFD) represents a major cause of metabolic disorders including fatty liver. To determine whether and how NPC1L1 deficiency prevents HFD-induced hepatic steatosis, in this study, we fed NPC1L1 knockout (L1-KO) mice and their wild-type (WT) controls an HFD, and found that 24 weeks of HFD feeding causes no fatty liver in L1-KO mice. Hepatic fatty acid synthesis and levels of mRNAs for lipogenic genes are substantially reduced but hepatic lipoprotein-triglyceride production, fatty acid oxidation, and triglyceride hydrolysis remain unaltered in L1-KO versus WT mice. Strikingly, L1-KO mice are completely protected against HFD-induced hyperinsulinemia under both fed and fasted states and during glucose challenge. Despite similar glucose tolerance, L1-KO relative WT mice are more insulin sensitive and in the overnight-fasted state display significantly lower plasma glucose concentrations. In conclusion, NPC1L1 deficiency in mice prevents HFD-induced fatty liver by reducing hepatic lipogenesis, at least in part, through attenuating HFD-induced insulin resistance, a state known to drive hepatic lipogenesis through elevated circulating insulin levels.

Inhibition of PTP1B restores IRS1-mediated hepatic insulin signaling in IRS2-deficient mice

OBJECTIVE

Mice with complete deletion of insulin receptor substrate 2 (IRS2) develop hyperglycemia, impaired hepatic insulin signaling, and elevated gluconeogenesis, whereas mice deficient for protein tyrosine phosphatase (PTP)1B display an opposing hepatic phenotype characterized by increased sensitivity to insulin. To define the relationship between these two signaling pathways in the regulation of liver metabolism, we used genetic and pharmacological approaches to study the effects of inhibiting PTP1B on hepatic insulin signaling and expression of gluconeogenic enzymes in IRS2(-/-) mice.

RESEARCH DESIGN AND METHODS

We analyzed glucose homeostasis and insulin signaling in liver and isolated hepatocytes from IRS2(-/-) and IRS2(-/-)/PTP1B(-/-) mice. Additionally, hepatic insulin signaling was assessed in control and IRS2(-/-) mice treated with resveratrol, an antioxidant present in red wine.

RESULTS

In livers of hyperglycemic IRS2(-/-) mice, the expression levels of PTP1B and its association with the insulin receptor (IR) were increased. The absence of PTP1B in the double-mutant mice restored hepatic IRS1-mediated phosphatidylinositol (PI) 3-kinase/Akt/Foxo1 signaling. Moreover, resveratrol treatment of hyperglycemic IRS2(-/-) mice decreased hepatic PTP1B mRNA and inhibited PTP1B activity, thereby restoring IRS1-mediated PI 3-kinase/Akt/Foxo1 signaling and peripheral insulin sensitivity.

CONCLUSIONS

By regulating the phosphorylation state of IR, PTB1B determines sensitivity to insulin in liver and exerts a unique role in the interplay between IRS1 and IRS2 in the modulation of hepatic insulin action.

Molecular mechanisms involved in obesity-associated insulin resistance: therapeutical approach

Insulin resistance is an important contributor to the pathogenesis of T2D and obesity is a risk factor for its development. It has been demonstrated that these obesity-related metabolic disorders are associated with a state of chronic low-intensity inflammation. Several mediators released from adipocytes and macrophages, such as the pro-inflammatory cytokines TNF-alpha and IL-6, have been suggested to impair insulin action in peripheral tissues, including fat and skeletal muscle. Such insulin resistance can initially be compensated by increased insulin secretion, but the prolonged presence of the hormone is detrimental for insulin sensitivity. Stress and pro-inflammatory kinases as well as more recent players, phosphatases, seem to be involved in the molecular mechanisms by which pro-inflammatory cytokines and hyperinsulinemia disrupt insulin signalling at the level of IRSs. Pharmacological approaches, such as treatment with PPAR and LXR agonists, overcome such insulin resistance, exerting anti-inflammatory properties as well as controlling the expression of cytokines with tissular specificity.

Deletion of protein tyrosine phosphatase 1b improves peripheral insulin resistance and vascular function in obese, leptin-resistant mice via reduced oxidant tone

RATIONALE

Obesity is a risk factor for cardiovascular dysfunction, yet the underlying factors driving this impaired function remain poorly understood. Insulin resistance is a common pathology in obese patients and has been shown to impair vascular function. Whether insulin resistance or obesity, itself, is causal remains unclear.

OBJECTIVE

The present study tested the hypothesis that insulin resistance is the underlying mediator for impaired NO-mediated dilation in obesity by genetic deletion of the insulin-desensitizing enzyme protein tyrosine phosphatase (PTP)1B in db/db mice.

METHODS AND RESULTS

The db/db mouse is morbidly obese, insulin-resistant, and has tissue-specific elevation in PTP1B expression compared to lean controls. In db/db mice, PTP1B deletion improved glucose clearance, dyslipidemia, and insulin receptor signaling in muscle and fat. Hepatic insulin signaling in db/db mice was not improved by deletion of PTP1B, indicating specific amelioration of peripheral insulin resistance. Additionally, obese mice demonstrate an impaired endothelium dependent and independent vasodilation to acetylcholine and sodium nitroprusside, respectively. This impairment, which correlated with increased superoxide in the db/db mice, was corrected by superoxide scavenging. Increased superoxide production was associated with increased expression of NAD(P)H oxidase 1 and its molecular regulators, Noxo1 and Noxa1.

CONCLUSIONS

Deletion of PTP1B improved both endothelium dependent and independent NO-mediated dilation and reduced superoxide generation in db/db mice. PTP1B deletion did not affect any vascular function in lean mice. Taken together, these data reveal a role for peripheral insulin resistance as the mediator of vascular dysfunction in obesity.

Rapid insulin-dependent endocytosis of the insulin receptor by caveolae in primary adipocytes

Background

The insulin receptor is localized in caveolae and is dependent on caveolae or cholesterol for signaling in adipocytes. When stimulated with insulin, the receptor is internalized.

Methodology/Principal Findings

We examined primary rat adipocytes by subcellular fractionation to examine if the insulin receptor was internalized in a caveolae-mediated process. Insulin induced a rapid, t_1/2<3 min, endocytosis of the insulin receptor in parallel with receptor tyrosine autophosphorylation. Concomitantly, caveolin-1 was phosphorylated at tyrosine(14) and endocytosed. Vanadate increased the phosphorylation of caveolin-1 without affecting insulin receptor phosphorylation or endocytosis. Immunocapture of endosomal vesicles with antibodies against the insulin receptor co-captured caveolin-1 and immunocapture with antibodies against tyrosine(14)-phosphorylated caveolin-1 co-captured the insulin receptor, demonstrating that the insulin receptor was endocytosed together with tyrosine(14)-phosphorylated caveolin-1. By immunogold electron microscopy the insulin receptor and caveolin-1 were colocalized in endosome vesicles that resembled caveosomes. Clathrin was not endocytosed with the insulin receptor and the inhibitor of clathrin-coated pit-mediated endocytosis, chlorpromazine, did not inhibit internalization of the insulin receptor, while transferrin receptor internalization was inhibited.

Conclusion

It is concluded that in response to insulin stimulation the autophosphorylated insulin receptor in primary adipocytes is rapidly endocytosed in a caveolae-mediated process, involving tyrosine phosphorylation of caveolin-1.

Leptin resistance and insulin resistance

Leptin resistance and insulin resistance are the common pathology of various metabolic diseases, and make great contribution to the metabolic syndrome. Many researches have proved that the two processes affected each other, whereas the exploration of the resistance mechanism was often independently carried out. Here, we'd like to make a review focusing on the relationship between leptin resistance and insulin resistance, and their interactions at multiple levels.

Angiotensin II and the development of insulin resistance: implications for diabetes

Angiotensin II (Ang II), the major effector hormone of the renin-angiotensin system (RAS), has an important role in the regulation of vascular and renal homeostasis. Clinical and pharmacological studies have recently shown that Ang II is a critical promoter of insulin resistance and diabetes mellitus type 2. Ang II exerts its actions on insulin-sensitive tissues such as liver, muscle and adipose tissue where it has effects on the insulin receptor (IR), insulin receptor substrate (IRS) proteins and the downstream effectors PI3K, Akt and GLUT4. The molecular mechanisms involved have not been completely identified, but the role of serine/threonine phosphorylation of the IR and IRS-1 proteins in desensitization of insulin action has been well established. The purpose of this review is to highlight recent advances in the understanding of Ang II actions which lead to the development of insulin resistance and its implications for diabetes.

High-fat diet induces apoptosis of hypothalamic neurons

Consumption of dietary fats is amongst the most important environmental factors leading to obesity. In rodents, the consumption of fat-rich diets blunts leptin and insulin anorexigenic signaling in the hypothalamus by a mechanism dependent on the in situ activation of inflammation. Since inflammatory signal transduction can lead to the activation of apoptotic signaling pathways, we evaluated the effect of high-fat feeding on the induction of apoptosis of hypothalamic cells. Here, we show that consumption of dietary fats induce apoptosis of neurons and a reduction of synaptic inputs in the arcuate nucleus and lateral hypothalamus. This effect is dependent upon diet composition, and not on caloric intake, since pair-feeding is not sufficient to reduce the expression of apoptotic markers. The presence of an intact TLR4 receptor, protects cells from further apoptotic signals. In diet-induced inflammation of the hypothalamus, TLR4 exerts a dual function, on one side activating pro-inflammatory pathways that play a central role in the development of resistance to leptin and insulin, and on the other side restraining further damage by controlling the apoptotic activity.

Dual role of interleukin-6 in regulating insulin sensitivity in murine skeletal muscle

OBJECTIVE

Cytokines are elevated in various insulin-resistant states, including type 2 diabetes and obesity, although the contribution of interleukin-6 (IL-6) in the induction of these diseases is controversial.

RESEARCH DESIGN AND METHODS

We analyzed the impact of IL-6 on insulin action in murine primary myocytes, skeletal muscle cell lines, and mice (wild type and protein-tyrosine phosphatase 1B [PTP1B] deficient).

RESULTS

IL-6 per se increased glucose uptake by activating serine/threonine protein kinase 11 (LKB1)/AMP-activated protein kinase/protein kinase B substrate of 160 kDa (AS160) pathway. A dual effect on insulin action was observed when myotubes and mice were exposed to this cytokine: additive with short-term insulin (increased glucose uptake and systemic insulin sensitivity) but chronic exposure produced insulin resistance (impaired GLUT4 translocation to plasma membrane and defects in insulin signaling at the insulin receptor substrate 1 [IRS-1] level). Three mechanisms seem to operate in IL-6-induced insulin resistance: activation of c-Jun NH(2)-terminal kinase 1/2 (JNK1/2), accumulation of suppressor of cytokine signaling 3 (socs3) mRNA, and an increase in PTP1B activity. Accordingly, silencing JNK1/2 with either small interfering RNA or chemical inhibitors impaired phosphorylation of IRS-1 (Ser307), restored insulin signaling, and normalized insulin-induced glucose uptake in myotubes. When using a pharmacological approach, liver X receptor agonists overcome IL-6-induced insulin resistance by producing downregulation of socs3 and ptp1b gene expression. Finally, the lack of PTP1B confers protection against IL-6-induced insulin resistance in skeletal muscle in vitro and in vivo, in agreement with the protection against the IL-6 hyperglycemic effect observed on glucose and insulin tolerance tests in adult male mice.

CONCLUSIONS

These findings indicate the important role of IL-6 in the pathogenesis of insulin resistance and further implicate PTP1B as a potential therapeutic target in the treatment of type 2 diabetes.

Diabetes reduces autophosphorylation of retinal insulin receptor and increases protein-tyrosine phosphatase-1B activity

PURPOSE

Protein-tyrosine phosphatase-1B (PTP1B) has been implicated in the negative regulation of insulin signaling. The expression, activity, and functional role of PTP1B in the retina are unknown. In this study, the authors examined the relationship between the retinal insulin receptor (IR) and PTP1B in normal and diabetic mouse retinas.

METHODS

IR and PTP1B localization was examined by immunohistochemistry. The activation of IR was analyzed using specific antibodies against phosphotyrosine. PTP1B activity was determined in anti-PTP1B immunoprecipitates. Glutathione-S-transferase fusion proteins containing wild-type and catalytically inactive mutant PTP1B was used to study the interaction between IR and PTP1B. Anti-IR immunoprecipitates and the cytoplasmic domain of purified IR were incubated in the presence of ATP, and the autophosphorylation of IR with antiphosphotyrosine antibody was analyzed.

RESULTS

Immunohistochemical analysis of PTP1B shows that it is predominantly expressed in nonphotoreceptor layers of the retina, though it is clearly expressed in the inner segments of the rod photoreceptors. The IR is predominately expressed in rod inner segments. Biochemical analysis of rod outer segments indicates the presence of IR and PTP1B. Retinal IR exhibits a high level of basal autophosphorylation, and this autophosphorylation is reduced in diabetic mouse retinas. In vitro, PTP1B is able to dephosphorylate the autophosphorylated IR. Substrate mutant-trap results indicate a stable interaction between IR and PTP1B. Further, PTP1B activity was increased in diabetic mouse retinas.

CONCLUSIONS

These studies indicate that diabetes reduces the autophosphorylation of retinal IR and increased PTP1B activity. Further, PTP1B regulates the state of IR phosphorylation in the retina.

Insulin resistance associated to obesity: the link TNF-alpha

Adipose tissue secretes proteins which may influence insulin sensitivity. Among them, tumour necrosis factor (TNF)-alpha has been proposed as a link between obesity and insulin resistance because TNF-alpha is overexpressed in adipose tissue from obese animals and humans, and obese mice lacking either TNF-alpha or its receptor show protection against developing insulin resistance. The activation of proinflammatory pathways after exposure to TNF-alpha induces a state of insulin resistance in terms of glucose uptake in myocytes and adipocytes that impair insulin signalling at the level of the insulin receptor substrate (IRS) proteins. The mechanism found in brown adipocytes involves Ser phosphorylation of IRS-2 mediated by TNF-alpha activation of MAPKs. The Ser307 residue in IRS-1 has been identified as a site for the inhibitory effects of TNF-alpha in myotubes, with p38 mitogen-activated protein kinase (MAPK) and inhibitor kB kinase being involved in the phosphorylation of this residue. Moreover, up-regulation of protein-tyrosine phosphatase (PTP)1B expression was recently found in cells and animals treated with TNF-alpha. PTP1B acts as a physiological negative regulator of insulin signalling by dephosphorylating the phosphotyrosine residues of the insulin receptor and IRS-1, and PTP1B expression is increased in peripheral tissues from obese and diabetic humans and rodents. Accordingly, down-regulation of PTP1B activity by treatment with pharmacological agonists of nuclear receptors restores insulin sensitivity in the presence of TNF-alpha. Furthermore, mice and cells deficient in PTP1B are protected against insulin resistance induced by this cytokine. In conclusion, the absence or inhibition of PTP1B in insulin-target tissues could confer protection against insulin resistance induced by cytokines.