Activation Of Insulin Receptor Substrate Research Articles

Stimulatory Effect of Balanced Deep-Sea Water Containing Chitosan Oligosaccharides on Glucose Uptake in C2C12 Myotubes.

Deep-sea water (DSW) and chitosan oligosaccharides (COS) have recently drawn much attention because of their potential medical and pharmaceutical applications. Balanced DSW (BDSW) was prepared by mixing DSW mineral extracts and desalinated water. This study investigated the effects of BDSW, COS, and BDSW containing COS on glucose uptake and their mode of action in mature C2C12 myotubes. BDSW and COS increased glucose uptake in a dose-dependent manner. BDSW containing COS synergistically increased glucose uptake; this was dependent on the activation of insulin receptor substrate 1 and protein kinase C in insulin-dependent signaling pathways as well as liver kinase B1, AMP-activated protein kinase, and mammalian target of rapamycin in insulin-independent signaling pathways. Quantitative real-time polymerase chain reaction revealed that the expressions of the following genes related to glucose uptake were elevated: glucose transporter 4 (GLUT4), insulin-responsive aminopeptidase, and vesicle-associated membrane protein 2 for abundant proteins of GLUT4 storage vesicles (GSVs); syntaxin 4 and soluble N-ethylmaleimide-sensitive factor attachment protein 23 for trafficking between the plasma membrane and GSVs; and syntaxin 6 and syntaxin 16 for trafficking between GSVs and the trans-Golgi network. Taken together, these results suggest BDSW containing COS has a greater stimulatory effect on glucose uptake than BDSW or COS alone. Moreover, this effect is mediated by the stimulation of diverse signaling pathways via the activation of main signaling molecules related to GSV trafficking.

NOD2 activation induces oxidative stress contributing to mitochondrial dysfunction and insulin resistance in skeletal muscle cells

Nucleotide-binding oligomerization domain protein-2 (NOD2) activation in skeletal muscle cells has been associated with insulin resistance, but the underlying mechanisms are not yet clear. Here we demonstrate the implication of oxidative stress in the development of mitochondrial dysfunction and insulin resistance in response to NOD2 activation in skeletal muscle cells. Treatment with the selective NOD2 ligand muramyl dipeptide (MDP) increased mitochondrial reactive oxygen species (ROS) generation in L6 myotubes. MDP-induced ROS production was associated with increased levels of protein carbonyls and reduction in citrate synthase activity, cellular ATP level, and mitochondrial membrane potential, as well as altered expression of genes involved in mitochondrial function and metabolism. Antioxidant treatment attenuated MDP-induced ROS production and restored mitochondrial functions. In addition, the presence of antioxidant prevented NOD2-mediated activation of MAPK kinases and the inflammatory response. This was associated with reduced serine phosphorylation of insulin receptor substrate-1 (IRS-1) and improved insulin-stimulated tyrosine phosphorylation of IRS-1 and downstream activation of Akt phosphorylation. These data indicate that oxidative stress plays a role in NOD2 activation-induced inflammatory response and that MDP-induced oxidative stress correlates with impairment of mitochondrial functions and induction of insulin resistance in skeletal muscle cells.

Type 1 Insulin-Like Growth Factor Receptor/Insulin Receptor Substrate 1 Signaling Confers Pathogenic Activity on Breast Tumor Cells Lacking REST.

Loss of repressor element 1 silencing transcription factor (REST) occurs in 20% of breast cancers and correlates with a poor patient prognosis. However, the molecular basis for enhanced malignancy in tumors lacking REST (RESTless) is only partially understood. We used multiplatform array data from the Cancer Genome Atlas to identify consistent changes in key signaling pathways. Of the proteins screened in the reverse-phase protein array, we found that insulin receptor substrate 1 (IRS1) is the most highly upregulated protein in RESTless breast tumors. Analysis of breast tumor cell lines showed that REST directly represses IRS1, and cells lacking REST have increased levels of IRS1 mRNA and protein. We find that the upregulation of IRS1 function is both necessary and sufficient for enhanced signaling and growth in breast cancer cells lacking REST. IRS1 overexpression is sufficient to phenocopy the enhanced activation of the signaling hubs AKT and mitogen-activated protein kinase (MAPK) of MCF7 cells lacking REST. Loss of REST renders MCF7 and MDA-MB-231 breast tumor cells dependent on IRS1 activity for colony formation in soft agar. Inhibition of the type 1 insulin-like growth factor receptor (IGF1R) reduces the enhanced signaling, growth, and migration in breast tumor cells that occur upon REST loss. We show that loss of REST induces a pathogenic program that works through the IGF1R/IRS1 pathway.

BMI-related progression of atypical PKC-dependent aberrations in insulin signaling through IRS-1, Akt, FoxO1 and PGC-1α in livers of obese and type 2 diabetic humans

Information on insulin resistance in human liver is limited. In mouse diet-induced obesity (DIO), hepatic insulin resistance initially involves: lipid+insulin-induced activation of atypical protein kinase C (aPKC); elevated Akt activity/activation but selective impairment of compartmentalized Akt-dependent FoxO1 phosphorylation; and increases in gluconeogenic and lipogenic enzymes. In advanced stages, e.g., in hepatocytes of type 2 diabetes (T2D) humans, insulin activation of insulin receptor substrate-1(IRS-1) and Akt fails, further increasing FoxO1-dependent gluconeogenic/lipogenic enzyme expression. Increases in hepatic PGC-1α also figure prominently, but uncertainly, in this scheme. Here, we examined signaling factors in liver samples harvested from human transplant donors with increasing BMI, 20→25→30→35→40→45.We found, relative to lean (BMI=20–25) humans, obese (BMI>30) humans had all abnormalities seen in early mouse DIO, but, surprisingly, at all elevated BMI levels, had decreased insulin receptor-1 (IRS-1) levels, decreased Akt activity, and increased expression/abundance of aPKC-ι and PGC-1α. Moreover, with increasing BMI, there were: progressive increases in aPKC activity and PKC-ι expression/abundance; progressive decreases in IRS-1 levels, Akt activity and FoxO1 phosphorylation; progressive increases in expression/abundance of PGC-1α; and progressive increases in gluconeogenic and lipogenic enzymes. Remarkably, all abnormalities reached T2D levels at higher BMI levels. Most importantly, both “early” and advanced abnormalities were largely reversed by 24-hour treatment of T2D hepatocytes with aPKC inhibitor. We conclude: hepatic insulin resistance in human obesity is: advanced; BMI-correlated; and sequentially involves increased aPKC-activating ceramide; increased aPKC levels and activity; decreases in IRS-1 levels, Akt activity, and FoxO1 phosphorylation; and increases in expression/abundance of PGC-1α and gluconeogenic and lipogenic genes.

Duodenal-Jejunal Bypass Surgery Ameliorates Glucose Homeostasis and Reduces Endoplasmic Reticulum Stress in the Liver Tissue in a Diabetic Rat Model.

Duodenal-jejunal bypass (DJB) has been shown to be an effective surgical treatment for type 2 diabetes mellitus (T2DM). However, the underlying mechanisms are poorly understood. Recently, accumulating evidences suggest that endoplasmic reticulum (ER) stress plays an important role in the development of insulin resistance in T2DM. The present study was designed to investigate the effect of DJB on glucose homeostasis, the ER stress state in the liver tissue, and the involving signaling independently of weight loss. Thirty adult male T2DM Sprague-Dawley (SD) rats induced by high-fat diet and low dose of streptozotocin (STZ) were randomly divided into DJB and sham groups. Ten age-matched male SD rats were assigned as the control group. The parameters of body weight and calorie intake were measured at indicated time points. The glucose tolerance and insulin resistance were detected to evaluate the glucose homeostasis. Serum insulin was determined by enzyme-linked immunosorbent assay (ELISA). The markers of ER stress, the activity of c-Jun N-terminal kinase (JNK) and serine phosphorylation of insulin receptor substrate 1 (IRS-1) in the liver tissue, were determined by Western blotting. DJB induced significant improvements in glucose homeostasis and insulin sensitivity, but without weight loss. DJB improved the ER stress state indicated by decreased protein kinase RNA (PKR)-like ER protein kinase (PERK) and inositol-requiring enzyme 1 (IRE-1) phosphorylation in the liver tissue. The JNK activity and serine phosphorylation of IRS-1 in the liver tissue were significantly reduced after DJB. DJB ameliorates glucose homeostasis. Meanwhile, our study helps to reveal that the reduced hepatic ER stress and the decreased JNK activity may contribute to the improved glucose homeostasis after DJB.

Fibroblast Growth Factor 21 Improves Insulin Sensitivity and Synergizes with Insulin in Human Adipose Stem Cell-Derived (hASC) Adipocytes

Fibroblast growth factor 21 (FGF21) has evolved as a major metabolic regulator, the pharmacological administration of which causes weight loss, insulin sensitivity and glucose control in rodents and humans. To understand the molecular mechanisms by which FGF21 exerts its metabolic effects, we developed a human in vitro model of adipocytes to examine crosstalk between FGF21 and insulin signaling. Human adipose stem cell-derived (hASC) adipocytes were acutely treated with FGF21 alone, insulin alone, or in combination. Insulin signaling under these conditions was assessed by measuring tyrosine phosphorylation of insulin receptor (InsR), insulin receptor substrate-1 (IRS-1), and serine 473 phosphorylation of Akt, followed by a functional assay using 14C-2-deoxyglucose [14C]-2DG to measure glucose uptake in these cells. FGF21 alone caused a modest increase of glucose uptake, but treatment with FGF21 in combination with insulin had a synergistic effect on glucose uptake in these cells. The presence of FGF21 also effectively lowered the insulin concentration required to achieve the same level of glucose uptake compared to the absence of FGF21 by 10-fold. This acute effect of FGF21 on insulin signaling was not due to IR, IGF-1R, or IRS-1 activation. Moreover, we observed a substantial increase in basal S473-Akt phosphorylation by FGF21 alone, in contrast to the minimal shift in basal glucose uptake. Taken together, our data demonstrate that acute co-treatment of hASC-adipocytes with FGF21 and insulin can result in a synergistic improvement in glucose uptake. These effects were shown to occur at or downstream of Akt, or separate from the canonical insulin signaling pathway.

High uric acid directly inhibits insulin signalling and induces insulin resistance

Background and aimAccumulating clinical evidence suggests that hyperuricemia is strongly associated with abnormal glucose metabolism and insulin resistance. However, how high uric acid (HUA) level causes insulin resistance remains unclear. We aimed to determine the direct role of HUA in insulin resistance in vitro and in vivo in mice. MethodsAn acute hyperuricemia mouse model was created by potassium oxonate treatment, and the impact of HUA level on insulin resistance was investigated by glucose tolerance test, insulin tolerance test and insulin signalling, including phosphorylation of insulin receptor substrate 1 (IRS1) and Akt. HepG2 cells were exposed to HUA treatment and N-acetylcysteine (NAC), reactive oxygen species scavenger; IRS1 and Akt phosphorylation was detected by Western blot analysis after insulin treatment. ResultsHyperuricemic mice showed impaired glucose tolerance with insulin resistance. Hyperuricemia inhibited phospho-Akt (Ser473) response to insulin and increased phosphor-IRS1 (Ser307) in liver, muscle and fat tissues. HUA induced oxidative stress, and the antioxidant NAC blocked HUA-induced IRS1 activation and Akt inhibition in HepG2 cells. ConclusionThis study supplies the first evidence of HUA directly inducing insulin resistance in vivo and in vitro. Increased uric acid level may inhibit IRS1 and Akt insulin signalling and induce insulin resistance. The reactive oxygen species pathway plays a key role in HUA-induced insulin resistance.

Lipopolysaccharide impairs insulin sensitivity via activation of phosphoinositide 3-kinase in adipocytes

The effect of lipopolysaccharide (LPS) on insulin sensitivity in adipocytes were examined by using differentiated 3T3-L1 adipocytes. Insulin-mediated activation of insulin receptor substrate (IRS) 1/2 was inhibited in LPS-pretreated adipocytes and IRS1/2-mediated Akt activation was also attenuated in those cells. LPS inhibited activation of glycogen synthase kinase 3 as a negative regulator of glycogenesis and impaired the glycogen synthesis in response to insulin. LPS-induced activation of phosphoinositide 3-kinase (PI3K) in adipocytes. Involvement of suppressor of cytokine signaling 3 (SOCS3) in LPS-induced IRS1/2 inhibition was excluded. Considering that both insulin and LPS were able to activate the PI3K/Akt signaling pathway, LPS was suggested to impair insulin sensitivity of adipocytes through down-regulating insulin-mediated PI3K/Akt activation.

Hydroxylamine enhances glucose uptake in C2C12 skeletal muscle cells through the activation of insulin receptor substrate 1

Diabetes mellitus is a global disease, and the number of patients with it is increasing. Of various agents for treatment, those that directly act on muscle are currently attracting attention because muscle is one of the main tissues in the human body, and its metabolism is decreased in type II diabetes. In this study, we found that hydroxylamine (HA) enhances glucose uptake in C2C12 myotubes. Analysis of HA’s mechanism revealed the involvement of IRS1, PI3K and Akt that is related to the insulin signaling pathway. Further investigation about the activation mechanism of insulin receptor or IRS1 by HA may provide a way to develop a novel anti-diabetic agent alternating to insulin.

Suppression of Grb2 expression improved hepatic steatosis, oxidative stress, and apoptosis induced by palmitic acid in vitro partly through insulin signaling alteration

In this study, we aimed to study the role of growth factor receptor-bound protein 2 (Grb2) in palmitic acid-induced steatosis and other "fatty liver" symptoms in vitro. HepG2 cells, with or without stably suppressed Grb2 expression, were incubated with palmitic acid for 24h to induce typical clinical "fatty liver" features, including steatosis, impaired glucose metabolism, oxidative stress, and apoptosis. MTT and Oil Red O assays were applied to test cell viability and fat deposition, respectively. Glucose uptake assay was used to evaluate the glucose utilization of cells. Quantitative polymerase chain reaction and Western blot were used to measure expressional changes of key markers of insulin signaling, lipid/glucose metabolism, oxidative stress, and apoptosis. After 24-h palmitic acid induction, increased fat accumulation, reduced glucose uptake, impaired insulin signaling, enhanced oxidative stress, and increased apoptosis were observed in HepG2 cells. Suppression of Grb2 in HepG2 significantly reduced fat accumulation, improved glucose metabolism, ameliorated oxidative stress, and restored the activity of insulin receptor substrate-1/Akt and MEK/ERK pathways. In addition, Grb2 deficiency attenuated hepatic apoptosis shown by reduced activation of caspase-3 and fluorescent staining. Modulation of Bcl-2 and Bak1 also contributed to reduced apoptosis. In conclusion, suppression of Grb2 expression in HepG2 cells improved hepatic steatosis, glucose metabolism, oxidative stress, and apoptosis induced by palmitic acid incubation partly though modulating the insulin signaling pathway.

Cardamonin Ameliorates Insulin Resistance Induced by High Insulin and High Glucose through the mTOR and Signal Pathway

The mammalian target of rapamycin is crucial in the regulation of cell growth and metabolism. Recent studies suggest that the mammalian target of rapamycin and its downstream 70-kDa ribosomal S6 kinase 1 negatively modulate the insulin-signaling pathway, which is considered the main cause of insulin resistance. The aim of this study is to investigate the effects of cardamonin, a potential inhibitor of the mammalian target of the rapamycin, on insulin-resistant vascular smooth muscle cells and the molecular mechanisms involved. Vascular smooth muscle cells were cultured with high glucose and high insulin to induce insulin resistance. The mammalian target of rapamycin was overstimulated in cells that were incubated with high glucose and high insulin, as reflected by the excessive activation of S6 kinase 1. Insulin-resistant vascular smooth muscle cells displayed hyperphosphorylation of insulin receptor substrate-1 at Ser residues 636/639, which decreased the activity of insulin receptor substrate-1. Also, the activation of protein kinase B and phosphorylation of glycogen synthesis kinase-3β were inhibited. Cardamonin increased the 2-deoxyglucose uptake and glycogen concentration, which was reduced by insulin resistance. As with rapamycin, cardamonin inhibited the activity of the mammalian target of rapamycin and S6 kinase 1, decreased the Ser 636/639 phosphorylation of insulin receptor substrate-1 and increased the activation of protein kinase B. Both of them increased the Ser9 phosphorylation of glycogen synthesis kinase-3β and decreased the expression of glycogen synthesis kinase-3β. However, neither cardamonin nor rapamycin increased the expression of glucose transport 4 which decreased in insulin-resistant vascular smooth muscle cells. This study suggests that cardamonin inhibited the activity of the mammalian target of rapamycin and eliminated the negative feedback of the mammalian target of rapamycin and S6 kinase 1 on the insulin-signaling pathway.

Minireview: Nuclear Insulin and Insulin-like Growth Factor-1 Receptors: A Novel Paradigm in Signal Transduction

The specificity of the insulin receptor (InsR) and insulin-like growth factor-1 receptor (IGF1R) signaling pathways has been the focus of significant debate over the past few years. Recent evidence showing nuclear import and a direct transcriptional role for both InsR and IGF1R adds a new layer of complexity to this dialog. Hence, in addition to the classical roles associated with cell-surface receptors (eg, ligand binding, autophosphorylation of the tyrosine kinase domain, activation of insulin receptor substrate 1 (IRS-1) and additional substrates, protein-protein interactions with membrane and cytoplasm components), new data are consistent with nuclear (genomic) role(s) for both InsR and IGF1R. The present review provides a brief overview of the physical and functional similarities and differences between InsR and IGF1R and describes data from a number of laboratories providing evidence for a new layer of signaling regulation (ie, the ability of InsR and IGF1R to translocate to the cell nucleus and to elicit genomic activities usually associated with transcription factors). The ability of InsR and IGF1R to function as transcription factors, although poorly understood, constitutes a new paradigm in signal transduction. Although research on the role of nuclear InsR/IGF1R is still in its infancy, we believe that this rapidly developing area may have a major basic and translational impact on the fields of metabolism, diabetes, and cancer.

Cardiac PI3K-Akt Impairs Insulin-Stimulated Glucose Uptake Independent of mTORC1 and GLUT4 Translocation

Impaired insulin-mediated glucose uptake characterizes cardiac muscle in humans and animals with insulin resistance and diabetes, despite preserved or enhanced phosphatidylinositol 3-kinase (PI3K) and the serine-threonine kinase, Akt-signaling, via mechanisms that are incompletely understood. One potential mechanism is PI3K- and Akt-mediated activation of mechanistic target of rapamycin (mTOR) and ribosomal protein S6 kinase (S6K), which may impair insulin-mediated activation of insulin receptor substrate (IRS)1/2 via inhibitory serine phosphorylation or proteasomal degradation. To gain mechanistic insights by which constitutive activation of PI3K or Akt may desensitize insulin-mediated glucose uptake in cardiomyocytes, we examined mice with cardiomyocyte-restricted, constitutive or inducible overexpression of a constitutively activated PI3K or a myristoylated Akt1 (myrAkt1) transgene that also expressed a myc-epitope-tagged glucose transporter type 4 protein (GLUT4). Although short-term activation of PI3K and myrAkt1 increased mTOR and S6 signaling, there was no impairment in insulin-mediated activation of IRS1/2. However, insulin-mediated glucose uptake was reduced by 50-80%. Although longer-term activation of Akt reduced IRS2 protein content via an mTORC1-mediated mechanism, treatment of transgenic mice with rapamycin failed to restore insulin-mediated glucose uptake, despite restoring IRS2. Transgenic activation of Akt and insulin-stimulation of myrAkt1 transgenic cardiomyocytes increased sarcolemmal insertion of myc-GLUT4 to levels equivalent to that observed in insulin-stimulated wild-type controls. Despite preserved GLUT4 translocation, glucose uptake was not elevated by the presence of constitutive activation of PI3K and Akt. Hexokinase II activity was preserved in myrAkt1 hearts. Thus, constitutive activation of PI3K and Akt in cardiomyocytes impairs GLUT4-mediated glucose uptake via mechanisms that impair the function of GLUT4 after its plasma-membrane insertion.

17β-estradiol induces an interaction between adenosine monophosphate-activated protein kinase and the insulin signaling pathway in 3T3-L1 adipocytes

Estrogen (17β-estradiol) has been implicated in maintaining insulin sensitivity. It is thought to act predominantly through genomic pathways and regulate the expression of various genes via binding to estrogen receptors (ERs)-α and -β. 17β-estradiol has been reported to simultaneously stimulate protein kinase B (Akt) and adenosine monophosphate-activated protein kinase (AMPK) in ex vivo skeletal muscle. Since data regarding the interaction between AMPK and the insulin receptor substrate-1 (IRS-1)/Akt pathway are controversial, the correlation between AMPK activation and insulin signaling remains unclear. In this study, we examined whether 17β-estradiol simultaneously stimulates the activation of AMPK and IRS-1/Akt in 3T3-L1 adipocytes as well as the 17β-estradiol-ER-induced interaction between the AMPK and IRS-1/Akt pathway in 3T3-L1 adipocytes not exposed to insulin. 17β-estradiol (10⁻⁷ M) rapidly activated AMPK and IRS-1/Akt in 3T3-L1 adipocytes, while the ER-α/β non-specific antagonist, ICI 182.780 (10 µM), and the AMPK antagonist compound C (20 µM) reversed the estrogen-induced activation of AMPK and tyrosine (Tyr)-IRS-1/Akt in these cells. Moreover, 17β-estradiol increased the expression of the peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α), adiponectin, uncoupling protein 2 (UCP2) and glucose transporter 4 (GLUT4) genes 24 h after treatment, whereas the ER-α/β non-specific antagonist, ICI 182.780 (10 µM), and the AMPK antagonist compound C (20 µM) reversed the estrogen-induced increase in the expression of these genes. These results indicate that 17β-estradiol activates AMPK through an ER and activates Akt through AMPK activation in 3T3-L1 adipocytes, despite the absence of insulin. Furthermore, 17β-estradiol regulates the expression of genes related to glucose metabolism through ER-AMPK activation in these cells.

Diosgenin ameliorates palmitate-induced endothelial dysfunction and insulin resistance via blocking IKKβ and IRS-1 pathways

ObjectiveWe investigated whether diosgenin, a widely used steroidal sapogenin, exerted protection against palmitate (PA)-induced inflammation and insulin resistance in the endothelium. MethodsHuman umbilical vein endothelial cells (HUVECs) were pretreated with diosgenin for 30 min, and then incubated with 100 μmol/L PA for 30 min or 24 h with or without insulin. IKKβ, p65 phosphorylation, serine phosphorylation of insulin receptor substrate-1 (IRS-1) at S307, tyrosine phosphorylation of IRS-1, Akt and eNOS activation were determined by Western blot analysis. Levels of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), endothelin-1 (ET-1) and plasminogen activator inhibitor-1 (PAI-1) were measured with ELISA Kits. Intracellular nitric oxide (NO) was viewed with fluorescence microscopy. Effects of diosgenin on insulin-mediated vasodilation was investigated in the isolated rat aortic rings. ResultsDiosgenin significantly reduced PA-enhanced IKKβ and NF-κB phosphorylation with inhibition of TNF-α and IL-6 production in endothelial cells at the concentrations of 0.1, 1 and 10 μmol/L, well demonstrating its anti-inflammatory activity in an IKKβ/NF-κB-dependent fashion. Meanwhile, diosgenin attenuated PA-induced serine phosphorylation (S307) of IRS-1 and restored IRS-1 tyrosine phosphorylation in response to insulin. The beneficial modulation of serine/tyrosine phosphorylation of IRS-1 by diosgenin contributed to the improvement of insulin signaling along PI3K/Akt/eNOS pathways and thereby increased insulin-mediated NO production. Salicylate (5 mmol/L), an inhibitor of IKKβ, showed similar activities as diosgenin. Diosgenin also remarkably inhibited ET-1 and PAI-1 production in the endothelial cells, and markedly restored the loss of insulin-mediated vasodilation in the presence of PA. ConclusionThe above-mentioned evidence suggests that diosgenin ameliorated endothelial dysfunction involved in insulin resistance through an IKKβ/IRS-1-dependent manner, shows potential application in the treatment for the cardiovascular diseases including atherosclerosis.

Fatty acid-binding protein 4 impairs the insulin-dependent nitric oxide pathway in vascular endothelial cells

BackgroundRecent studies have shown that fatty acid-binding protein 4 (FABP4) plasma levels are associated with impaired endothelial function in type 2 diabetes (T2D). In this work, we analysed the effect of FABP4 on the insulin-mediated nitric oxide (NO) production by endothelial cells in vitro.MethodsIn human umbilical vascular endothelial cells (HUVECs), we measured the effects of FABP4 on the insulin-mediated endothelial nitric oxide synthase (eNOS) expression and activation and on NO production. We also explored the impact of exogenous FABP4 on the insulin-signalling pathway (insulin receptor substrate 1 (IRS1) and Akt).ResultsWe found that eNOS expression and activation and NO production are significantly inhibited by exogenous FABP4 in HUVECs. FABP4 induced an alteration of the insulin-mediated eNOS pathway by inhibiting IRS1 and Akt activation. These results suggest that FABP4 induces endothelial dysfunction by inhibiting the activation of the insulin-signalling pathway resulting in decreased eNOS activation and NO production.ConclusionThese findings provide a mechanistic linkage between FABP4 and impaired endothelial function in diabetes, which leads to an increased cardiovascular risk.

Effect of Chronic CPT-1 Inhibition on Myocardial Ischemia-Reperfusion Injury (I/R) in a Model of Diet-Induced Obesity

By increasing circulating free fatty acids and the rate of fatty acid oxidation, obesity decreases glucose oxidation and myocardial tolerance to ischemia. Partial inhibition of fatty acid oxidation may improve myocardial tolerance to ischemia/reperfusion (I/R) in obesity. We assessed the effects of oxfenicine treatment on post ischemic cardiac function and myocardial infarct size in obese rats. Male Wistar rats were fed a control diet or a high calorie diet which resulted in diet induced obesity (DIO) for 16 weeks. Oxfenicine (200 mg/kg/day) was administered to control and DIO rats for the last 8 weeks. Isolated hearts were perfused and infarct size and post ischemic cardiac function was assessed after regional or global ischemia and reperfusion. Cardiac mitochondrial function was assessed and myocardial expression and activity of CPT-1 (carnitine palmitoyl transferase-1) and IRS-1 (insulin receptor substrate-1) was assessed using Western blot analysis. In the DIO rats, chronic oxfenicine treatment improved post ischemic cardiac function and reduced myocardial infarct size after I/R but had no effect on the cardiac mitochondrial respiration. Chronic oxfenicine treatment worsened post ischemic cardiac function, myocardial infarct size and basal mitochondrial respiration in control rat hearts. Basal respiratory control index (RCI) values, state 2 and state 4 respiration rates and ADP phosphorylation rates were compromised by oxfenicine treatment. Chronic oxfenicine treatment improved myocardial tolerance to I/R in the obese rat hearts but decreased myocardial tolerance to I/R in control rat hearts. This decreased tolerance to ischemia of oxfenicine treated controls was associated with adverse changes in basal and reoxygenation mitochondrial function. These changes were absent in oxfenicine treated hearts from obese rats.

Intermittent Bolus Feeding Has a Greater Stimulatory Effect on Protein Synthesis in Skeletal Muscle Than Continuous Feeding in Neonatal Pigs

Orogastric tube feeding, using either continuous or intermittent bolus delivery, is common in infants for whom normal feeding is contraindicated. To compare the impact of different feeding strategies on muscle protein synthesis, after withholding food overnight, neonatal pigs received a complete formula orally as a bolus feed every 4 h or were continuously fed. Protein synthesis rate and translational mechanisms in skeletal muscle were examined after 0, 24, and 25.5 h. Plasma amino acid and insulin concentrations increased minimally and remained constant in continuously fed compared to feed-deprived pigs; however, the pulsatile meal feeding pattern was mimicked in bolus-fed pigs. Muscle protein synthesis was stimulated by feeding and the greatest response occurred after a bolus meal. Bolus but not continuous feeds increased polysome aggregation, the phosphorylation of protein kinase B, tuberous sclerosis complex 2, proline-rich Akt substrate of 40 kDa, eukaryotic initiation factor (eIF) 4E binding protein (4EBP1), and rp S6 kinase and enhanced dissociation of the 4EBP1 ·eIF4E complex and formation of the eIF4E ·eIF4G complex compared to feed deprivation (P < 0.05). Activation of insulin receptor substrate-1, regulatory associated protein of mammalian target of rapamycin, AMP-activated protein kinase, eukaryotic elongation factor 2, and eIF2α phosphorylation were unaffected by either feeding modality. These results suggest that in neonates, intermittent bolus feeding enhances muscle protein synthesis to a greater extent than continuous feeding by eliciting a pulsatile pattern of amino acid- and insulin-induced translation initiation.

An Early Response Transcription Factor, Egr-1, Enhances Insulin Resistance in Type 2 Diabetes with Chronic Hyperinsulinism

One of the most important characteristics of type 2 diabetes is insulin resistance, during which the patients normally experienced hyperinsulinism stress that would alter insulin signal transduction in insulin target tissues. We have previously found that early growth responsive gene-1 (Egr-1), a zinc finger transcription factor, is highly expressed in db/db mice and in the fat tissue of individuals with type 2 diabetes. In this report, we found that chronic exposure to hyperinsulinism caused persistent Erk/MAPK activity in adipocytes and enhanced insulin resistance in an Egr-1-dependent manner. An elevation in Egr-1 augmented Erk1/2 activation via geranylgeranyl diphosphate synthase (GGPPS). Egr-1-promoted GGPPS transcription increased Ras prenylation and caused Erk1/2 activation. The sustained activation of Erk1/2 resulted in the phosphorylation of insulin receptor substrate-1 at Serine 612. Phosphorylation at this site impaired insulin signaling in adipocytes and reduced glucose uptake. The loss of Egr-1 function, knockdown of GGPPS, or inhibition of Erk1/2 activity in insulin-resistant adipocytes restored insulin receptor substrate-1 tyrosine phosphorylation and increased insulin sensitivity. Our results suggest a new mechanism by which the Egr-1/GGPPS/Erk1/2 pathway is responsible for insulin resistance during hyperinsulinism. This pathway provides a new therapeutic target for increasing insulin sensitivity: inhibiting the function of Egr-1.

Leptin increases motility and integrin up‐regulation in human prostate cancer cells

Prostate cancer is the most commonly diagnosed malignancy in men and shows a predilection for metastasis to distant organs. Leptin, an adipocyte-derived cytokine that is closely associated with obesity, has recently been shown to be involved in carcinogenesis and cancer progression. The aim of this study was to investigate whether leptin is associated with the motility of prostate cancer cells. We found that leptin increased the migration of human prostate cancer cells and expression of αvβ3 integrin on these cells. Leptin-mediated migration and increased integrin expression were attenuated by OBRl receptor antisense oligonucleotide (ODN). Activation of insulin receptor substrate (IRS-1), phosphatidylinositol 3-kinase (PI3K), Akt, and NF-κB pathways after leptin treatment was demonstrated. Furthermore, leptin-induced integrin expression and migration activity were inhibited by specific inhibitors; small interfering RNAs (siRNAs); and mutants of the IRS-1, PI3K, Akt, and NF-κB cascades. Therefore, this study shows that leptin stimulates the migration of human prostate cancer cells, one of the mechanisms underlying leptin-directed migration was transcriptional up-regulation of αvβ3 integrin expression through the OBR1/IRS-1/PI3K/Akt/NF-κB signal transduction pathway.