Exercise training and the antioxidant alpha-lipoic acid in the treatment of insulin resistance and type 2 diabetes

One hallmark of the insulin-resistant state of prediabetes and overt type 2 diabetes is an impaired ability of insulin to activate glucose transport in skeletal muscle, due to defects in IRS-1-dependent signaling. An emerging body of evidence indicates that one potential factor in the multifactorial etiology of skeletal muscle insulin resistance is oxidative stress, an imbalance between the cellular exposure to an oxidant stress and the cellular antioxidant defenses. Exposure of skeletal muscle to an oxidant stress leads to impaired insulin signaling and subsequently to reduced glucose transport activity. Numerous studies have demonstrated that treatment of insulin-resistant animals and type 2 diabetic humans with antioxidants, including alpha-lipoic acid (ALA), is associated with improvements in skeletal muscle glucose transport activity and whole-body glucose tolerance. An additional intervention that is effective in ameliorating the skeletal muscle insulin resistance of prediabetes and type 2 diabetes is endurance exercise training. Recent investigations have demonstrated that the combination of exercise training and antioxidant treatment using ALA in an animal model of obesity-associated insulin resistance provides a unique interactive effect resulting in a greater improvement in insulin action on skeletal muscle glucose transport than either intervention individually. Moreover, this interactive effect of exercise training and ALA is due in part to improvements in IRS-1-dependent insulin signaling. These studies highlight the effectiveness of combining endurance exercise training and antioxidants in beneficially modulating the molecular defects in insulin action observed in insulin-resistant skeletal muscle.     

Glutathione peroxidase 3 mediates the antioxidant effect of peroxisome proliferator-activated receptor gamma in human skeletal muscle cells

Oxidative stress plays an important role in the pathogenesis of insulin resistance and type 2 diabetes mellitus and in diabetic vascular complications. Thiazolidinediones (TZDs), a class of peroxisome proliferator-activated receptor gamma (PPARgamma) agonists, improve insulin sensitivity and are currently used for the treatment of type 2 diabetes mellitus. Here, we show that TZD prevents oxidative stress-induced insulin resistance in human skeletal muscle cells, as indicated by the increase in insulin-stimulated glucose uptake and insulin signaling. Importantly, TZD-mediated activation of PPARgamma induces gene expression of glutathione peroxidase 3 (GPx3), which reduces extracellular H(2)O(2) levels causing insulin resistance in skeletal muscle cells. Inhibition of GPx3 expression prevents the antioxidant effects of TZDs on insulin action in oxidative stress-induced insulin-resistant cells, suggesting that GPx3 is required for the regulation of PPARgamma-mediated antioxidant effects. Furthermore, reduced plasma GPx3 levels were found in patients with type 2 diabetes mellitus and in db/db/DIO mice. Collectively, these results suggest that the antioxidant effect of PPARgamma is exclusively mediated by GPx3 and further imply that GPx3 may be a therapeutic target for insulin resistance and diabetes mellitus.     

Oxidative stress and the etiology of insulin resistance and type 2 diabetes

The condition of oxidative stress arises when oxidant production exceeds antioxidant activity in cells and plasma. The overabundance of oxidants is mechanistically connected to the multifactorial etiology of insulin resistance, primarily in skeletal muscle tissue, and the subsequent development of type 2 diabetes. Two important mechanisms for this oxidant excess are (1) the mitochondrial overproduction of hydrogen peroxide and superoxide ion under conditions of energy surplus and (2) the enhanced activation of cellular NADPH oxidase via angiotensin II receptors. Several recent studies are reviewed that support the concept that direct exposure of mammalian skeletal muscle to an oxidant stress (including hydrogen peroxide) results in stimulation of the serine kinase p38 mitogen-activated protein kinase (p38 MAPK), and that the engagement of this stress-activated p38 MAPK signaling is mechanistically associated with diminished insulin-dependent stimulation of insulin signaling elements and glucose transport activity. The beneficial interactions between the antioxidant α-lipoic acid and the advanced glycation end-product inhibitor pyridoxamine that ameliorate oxidant stress-associated defects in whole-body and skeletal-muscle insulin action in the obese Zucker rat, a model of prediabetes, are also addressed. Overall, this review highlights the importance of oxidative stress in the development of insulin resistance in mammalian skeletal muscle tissue, at least in part via a p38-MAPK-dependent mechanism, and indicates that interventions that reduce this oxidative stress and oxidative damage can improve insulin action in insulin-resistant animal models. Strategies to prevent and ameliorate oxidative stress remain important in the overall treatment of insulin resistance and type 2 diabetes.     

Oxidative stress--mediated alterations in glucose dynamics in a genetic animal model of type II diabetes

Insulin resistance, characterized by an inexorable decline in skeletal muscle glucose utilization and/or an excessive hepatic glucose production, constitutes a major pathogenic importance in a cluster of clinical disorders including diabetes mellitus, hypertension, dyslipidemia, central obesity and coronary artery disease. A novel concept suggests that heightened state of oxidative stress during diabetes contributes, at least in part, to the development of insulin resistance. Several key predictions of this premise were subjected to experimental testing using Goto-Kakizaki (GK) rats as a genetic animal model for non-obese type II diabetes. Euglycemic-hyperinsulinemic clamp studies with an insulin infusion index of 5 mU/kg bw/min were used to measure endogenous glucose production (EGP), glucose infusion rate (GIR), glucose disposal rate (GDR) and skeletal muscle glucose utilization index (GUI). Moreover, the status of oxidative stress as reflected by the urinary levels of isoprostane and protein carbonyl formation were also assessed as a function of diabetes. Post-absorptive basal EGP and circulating levels of insulin, glucose and free fatty acid (FFA) were elevated in GK rats, compared to their corresponding control values. In contrast, steady state GIR and GDR of the hyperglycemic/hyperinsulinemic animals were reduced, concomitantly with impaired insulin's ability to suppress EGP. Insulin stimulated [3H]-2-deoxyglucose (2-DG) uptake (a measure of glucose transport activity) by various types of skeletal muscle fibers both in vivo and in vitro (isolated muscle, cultured myoblasts) was diminished in diabetic GK rats. This diabetes-related suppression of skeletal muscle glucose utilization was associated with a decrease in insulin's ability to promote the phosphorylation of tyrosine residues of insulin receptor substrate-1 (IRS-1). Similarly, the translocation of GLUT-4 from intracellular compartment to plasma membrane in response to insulin was also reduced in these animals. Oxidative stress-based markers (e.g. urinary isoprostane, carbonyl-bound proteins) were elevated as a function of diabetes. Nullification of the heightened state of oxidative stress in the GK rats with alpha-lipoic acid resulted in a partial amelioration of the diabetes-related impairment of the in vivo and in vitro insulin actions. Collectively, the above data suggest that 1) insulin resistance in GK rats occurs at the hepatic and skeletal muscle levels, 2) muscle cell glucose transport exhibited a blunted response to insulin and it is associated with a major defect in key molecules of both GLUT-4 trafficking and insulin signaling pathways, 3) skeletal muscle insulin resistance in GK rats appears to be of genetic origin and not merely related to a paracrine or autocrine effect, since this phenomenon is also observed in cultured myoblasts over several passages and finally heightened state of oxidative stress may mediate the development of insulin resistance during diabetes.     

Improvement of insulin sensitivity by antagonism of the renin-angiotensin system

The reduced capacity of insulin to stimulate glucose transport into skeletal muscle, termed insulin resistance, is a primary defect leading to the development of prediabetes and overt type 2 diabetes. Although the etiology of this skeletal muscle insulin resistance is multifactorial, there is accumulating evidence that one contributor is overactivity of the renin-angiotensin system (RAS). Angiotensin II (ANG II) produced from this system can act on ANG II type 1 receptors both in the vascular endothelium and in myocytes, with an enhancement of the intracellular production of reactive oxygen species (ROS). Evidence from animal model and cultured skeletal muscle cell line studies indicates ANG II can induce insulin resistance. Chronic ANG II infusion into an insulin-sensitive rat produces a markedly insulin-resistant state that is associated with a negative impact of ROS on the skeletal muscle glucose transport system. ANG II treatment of L6 myocytes causes impaired insulin receptor substrate (IRS)-1-dependent insulin signaling that is accompanied by augmentation of NADPH oxidase-mediated ROS production. Further critical evidence has been obtained from the TG(mREN2)27 rat, a model of RAS overactivity and insulin resistance. The TG(mREN2)27 rat displays whole body and skeletal muscle insulin resistance that is associated with local oxidative stress and a significant reduction in the functionality of the insulin receptor (IR)/IRS-1-dependent insulin signaling. Treatment with a selective ANG II type 1 receptor antagonist leads to improvements in whole body insulin sensitivity, enhanced insulin-stimulated glucose transport in muscle, and reduced local oxidative stress. In addition, exercise training of TG(mREN2)27 rats enhances whole body and skeletal muscle insulin action. However, these metabolic improvements elicited by antagonism of ANG II action or exercise training are independent of upregulation of IR/IRS-1-dependent signaling. Collectively, these findings support targeting the RAS in the design of interventions to improve metabolic and cardiovascular function in conditions of insulin resistance associated with prediabetes and type 2 diabetes.     

Alpha-lipoic acid mitigates insulin resistance in Goto-Kakizaki rats.

Impaired glucose uptake and metabolism by peripheral tissues is a common feature in both type I and type II diabetes mellitus. This phenomenon was examined in the context of oxidative stress and the early events within the insulin signalling pathway using soleus muscles derived from non-obese, insulin-resistant type II diabetic Goto-Kakizaki (GK) rats, a well-known genetic rat model for human type II diabetes. Insulin-stimulated glucose transport was impaired in soleus muscle from GK rats. Oxidative and non-oxidative glucose disposal pathways represented by glucose oxidation and glycogen synthesis in soleus muscles of GK rats appear to be resistant to the action of insulin when compared to their corresponding control values. These diabetes-related abnormalities in glucose disposal were associated with a marked diminution in the insulin-mediated enhancement of protein kinase B (Akt/PKB) and insulin receptor substrate-1 (IRS-1)-associated phosphatidylinostol 3-kinase (PI 3-kinase) activities; these two kinases are key elements in the insulin signalling pathway. Moreover, heightened state of oxidative stress, as indicated by protein bound carbonyl content, was evident in soleus muscle of GK diabetic rats. Chronic administration of the hydrophobic/hydrophilic antioxidant alpha -lipoic-acid (ALA, 100 mg/kg, i.p.) partly ameliorated the diabetes-related deficit in glucose metabolism, protein oxidation as well as the activation by insulin of the various steps of the insulin signalling pathway, including the enzymes Akt/PKB and PI-3 kinase. Overall, the current investigation illuminates the concept that oxidative stress may indeed be involved in the pathogenesis of certain types of insulin resistance. It also harmonizes with the notion of including potent antioxidants such as ALA in the armamentarium of antidiabetic therapy.     

Hypoxic preconditioning increases survival and angiogenic potency of peripheral blood mononuclear cells via oxidative stress resistance

Cell-based angiogenesis is a promising treatment for ischemic diseases; however, the survival of implanted cells is impaired by oxidative stress in the ischemic microenvironment. We tested the hypothesis that hypoxic preconditioning of implanted cells enhances their resistance against oxidative stress, increasing cell survival and angiogenic potency after implantation into ischemic tissue. Mouse peripheral blood mononuclear cells (PBMNCs) were collected and subjected to hypoxic preconditioning by culture for 24 h in 2% O(2) at 33 degrees C. Hypoxic preconditioning of PBMNCs increased the expression of various genes related to antioxidant and survival signals remarkably. Compared with cells cultured under normoxia, the hypoxia-preconditioned PBMNCs showed significantly lower reactive oxygen species (ROS) accumulation and higher cell survival under oxidative stress induced by LY-83583 (a superoxide generator). Three days after intramuscular implantation into the ischemic hindlimbs of mice, survival of the hypoxia-preconditioned PBMNCs was high, whereas that of the normoxia-cultured PBMNCs was relatively low. Furthermore, 28 days after treatment microvessel density and blood flow in the ischemic hindlimbs were significantly better in the mice implanted with hypoxia-preconditioned PBMNCs than in those implanted with normoxia-cultured PBMNCs. Hypoxic preconditioning increased the survival and angiogenic potency of PBMNCs, through oxidative stress resistance mechanisms.     

N-acetylcysteine and taurine prevent hyperglycemia-induced insulin resistance in vivo: possible role of oxidative stress

Exposure to high concentrations of glucose and insulin results in insulin resistance of metabolic target tissues, a characteristic feature of type 2 diabetes. High glucose has also been associated with oxidative stress, and increased levels of reactive oxygen species have been proposed to cause insulin resistance. To determine whether oxidative stress contributes to insulin resistance induced by hyperglycemia in vivo, nondiabetic rats were infused with glucose for 6 h to maintain a circulating glucose concentration of 15 mM with and without coinfusion of the antioxidant N-acetylcysteine (NAC), followed by a 2-h hyperinsulinemic-euglycemic clamp. High glucose (HG) induced a significant decrease in insulin-stimulated glucose uptake [tracer-determined disappearance rate (Rd), control 41.2 +/- 1.7 vs. HG 32.4 +/- 1.9 mg. kg-1. min-1, P < 0.05], which was prevented by NAC (HG + NAC 45.9 +/- 3.5 mg. kg-1. min-1). Similar results were obtained with the antioxidant taurine. Neither NAC nor taurine alone altered Rd. HG caused a significant (5-fold) increase in soleus muscle protein carbonyl content, a marker of oxidative stress that was blocked by NAC, as well as elevated levels of malondialdehyde and 4-hydroxynonenal, markers of lipid peroxidation, which were reduced by taurine. In contrast to findings after long-term hyperglycemia, there was no membrane translocation of novel isoforms of protein kinase C in skeletal muscle after 6 h. These data support the concept that oxidative stress contributes to the pathogenesis of hyperglycemia-induced insulin resistance.     

Increased lipid accumulation and insulin resistance in transgenic mice expressing DGAT2 in glycolytic (type II) muscle

Insulin resistance and type 2 diabetes are frequently accompanied by lipid accumulation in skeletal muscle. However, it is unknown whether primary lipid deposition in skeletal muscle is sufficient to cause insulin resistance or whether the type of muscle fiber, oxidative or glycolytic fiber, is an important determinant of lipid-mediated insulin resistance. Here we utilized transgenic mice to test the hypothesis that lipid accumulation specifically in glycolytic muscle promotes insulin resistance. Overexpression of DGAT2, which encodes an acyl-CoA:diacylglycerol acyltransferase that catalyzes triacylglycerol (TG) synthesis, in glycolytic muscle of mice increased the content of TG, ceramides, and unsaturated long-chain fatty acyl-CoAs in young adult mice. This lipid accumulation was accompanied by impaired insulin signaling and insulin-mediated glucose uptake in glycolytic muscle and impaired whole body glucose and insulin tolerance. We conclude that DGAT2-mediated lipid deposition specifically in glycolytic muscle promotes insulin resistance in this tissue and may contribute to the development of diabetes.     

Treatments with sodium selenate or doxycycline offset diabetes-induced perturbations of thioredoxin-1 levels and antioxidant capacity

Diabetes is associated with increased oxidative stress and impaired antioxidant defenses. Thioredoxin-1 (TRX-1) is a cytosolic thiol antioxidant and redox-active protein which plays a vital role in the maintenance of reduced intracellular redox state. In this study, the authors examined whether 4-week treatments with sodium selenate and doxycycline--a metalloproteinase-2 inhibitor which also has antioxidant-like effects--offset perturbations in oxidative stress and antioxidant protection in rat liver and skeletal muscle in streptozotocin-induced diabetes (SID) model. Experimental diabetes decreased TRX-1 levels in skeletal muscle and liver. On the other hand, SID increased oxidative stress marker protein carbonyl levels and decreased oxygen radical absorbance capacity (ORAC), an indicator of antioxidant capacity, in liver. A 4-week treatment of sodium selenate to diabetic rats decreased blood glucose levels moderately, while doxycycline treatment caused a reduction in weight loss of diabetic rats. Both doxycycline and sodium selenate prevented diabetes-induced decrease of TRX-1 levels in skeletal muscle, whereas only doxyxycline was effectively preventing diabetes-induced decrease of TRX-1 in liver. Furthermore, both treatments prevented diabetes-induced altered levels of protein carbonyls and ORAC in liver, and restored free and total protein thiol levels in both skeletal muscle and liver. In conclusion, the data of this study provides further evidence that sodium selenate and doxycycline treatments may control oxidative stress and improve antioxidant defense in diabetes.     

Dietary restriction improves systemic and muscular oxidative stress in type 2 diabetic Goto–Kakizaki rats

Type 2 diabetes is a heterogeneous metabolic disease characterized by insulin resistance and β-cell dysfunction leading to hyperglycaemia and dyslipidaemia. Dietary intervention seems to improve some of these cellular complications, namely insulin resistance. Our aim was to evaluate the effects of dietary restriction on systemic and skeletal muscle oxidative stress and insulin resistance in normal Wistar rats and Goto–Kakizaki (GK) rats, a non-obese type 2 diabetic animal model. Four-month-old normal and diabetic rats were separated in four groups. One group of each strain was maintained with ad libitum standard diet, and the other group was submitted to a dietary restriction (50% of control animals daily food intake), during 2 months. Metabolic profile, insulin resistance indexes and muscle lipids were determined. Oxidative stress parameters were also measured at systemic and muscle levels: protein carbonyl, 8-hydroxy-2′-deoxyguanosine and free 8-isoprostane. Dietary restriction improved lipid profile in both strains and urinary free 8-isoprostane and plasma carbonyl compounds in diabetic rats. An improvement of muscle triglycerides accumulation and 8-isoprostane concentration and a reduction of insulin resistance were also observed in GK rats. Our data show that dietary restriction ameliorates systemic and skeletal muscle oxidative stress state in type 2 diabetes, which is associated with improved insulin resistance.     

Skeletal muscle insulin resistance: role of inflammatory cytokines and reactive oxygen species

The cardiometabolic syndrome (CMS), with its increased risk for cardiovascular disease (CVD), nonalcoholic fatty liver disease (NAFLD), and chronic kidney disease (CKD), has become a growing worldwide health problem. Insulin resistance is a key factor for the development of the CMS and is strongly related to obesity, hyperlipidemia, hypertension, type 2 diabetes mellitus (T2DM), CKD, and NAFLD. Insulin resistance in skeletal muscle is particularly important since it is normally responsible for more than 75% of all insulin-mediated glucose disposal. However, the molecular mechanisms responsible for skeletal muscle insulin resistance remain poorly defined. Accumulating evidence indicates that low-grade chronic inflammation and oxidative stress play fundamental roles in the development of insulin resistance, and inflammatory cytokines likely contribute to the link between inflammation, oxidative stress, and skeletal muscle insulin resistance. Understanding the mechanisms by which skeletal muscle tissue develops resistance to insulin will provide attractive targets for interventions, which may ultimately curb this serious problem. This review is focused on the effects of inflammatory cytokines and oxidative stress on insulin signaling in skeletal muscle and consequent development of insulin resistance.     

Oxidative stress, insulin signaling, and diabetes

Oxidative stress has been implicated as a contributor to both the onset and the progression of diabetes and its associated complications. Some of the consequences of an oxidative environment are the development of insulin resistance, β-cell dysfunction, impaired glucose tolerance, and mitochondrial dysfunction, which can lead ultimately to the diabetic disease state. Experimental and clinical data suggest an inverse association between insulin sensitivity and ROS levels. Oxidative stress can arise from a number of different sources, whether disease state or lifestyle, including episodes of ketosis, sleep restriction, and excessive nutrient intake. Oxidative stress activates a series of stress pathways involving a family of serine/threonine kinases, which in turn have a negative effect on insulin signaling. More experimental evidence is needed to pinpoint the mechanisms contributing to insulin resistance in both type 1 diabetics and nondiabetic individuals. Oxidative stress can be reduced by controlling hyperglycemia and calorie intake. Overall, this review outlines various mechanisms that lead to the development of oxidative stress. Intervention and therapy that alter or disrupt these mechanisms may serve to reduce the risk of insulin resistance and the development of diabetes.     

Acute Oxidative Stress Can Reverse Insulin Resistance by Inactivation of Cytoplasmic JNK

Chronic oxidative stress results in decreased responsiveness to insulin, eventually leading to diabetes and cardiovascular disease. Activation of the JNK signaling pathway can mediate many of the effects of stress on insulin resistance through inhibitory phosphorylation of insulin receptor substrate 1. By contrast, exercise, which acutely increases oxidative stress in the muscle, improves insulin sensitivity and glucose tolerance in patients with Type 2 diabetes. To elucidate the mechanism underlying the contrasting effects of acute versus chronic oxidative stress on insulin sensitivity, we used a cellular model of insulin-resistant muscle to induce either chronic or acute oxidative stress and investigate their effects on insulin and JNK signaling. Chronic oxidative stress resulted in increased levels of phosphorylated (activated) JNK in the cytoplasm, whereas acute oxidative stress led to redistribution of JNK-specific phosphatase MKP7 from the nucleus into the cytoplasm, reduction in cytoplasmic phospho-JNK, and a concurrent accumulation of phospho-JNK in the nucleus. Acute oxidative stress restored normal insulin sensitivity and glucose uptake in insulin-resistant muscle cells, and this effect was dependent on MKP7. We propose that the contrasting effects of acute and chronic stress on insulin sensitivity are driven by changes in subcellular distribution of MKP7 and activated JNK.     

Enhanced autophagy plays a cardinal role in mitochondrial dysfunction in type 2 diabetic Goto-Kakizaki (GK) rats: ameliorating effects of (-)-epigallocatechin-3-gallate

Oxidative stress and mitochondrial dysfunction are known to play important roles in type 2 diabetes mellitus (T2DM) and insulin resistance. However, the pathology of T2DM remains complicated; in particular, the mechanisms of mitochondrial dysfunction in skeletal muscle and other insulin-sensitive tissues are as yet unclear. In the present study, we investigated the underlying mechanisms of oxidative stress and mitochondrial dysfunction by focusing on mitochondrial dynamics, including mitochondrial biogenesis and autophagy, in skeletal muscle of a nonobese diabetic animal model--the Goto-Kakizaki (GK) rat. The results showed that GK rats exhibited impaired glucose metabolism, increased oxidative stress and decreased mitochondrial function. These dysfunctions were found to be associated with induction of LC3B, Beclin1 and DRP1 (key molecules mediating the autophagy pathway), while they appeared not to affect the mitochondrial biogenesis pathway. In addition, (-)-epigallocatechin-3-gallate (EGCG) was tested as a potential autophagy-targeting nutrient, and we found that EGCG treatment improved glucose tolerance and glucose homeostasis in GK rats, and reduced oxidative stress and mitochondrial dysfunction in skeletal muscle. Amelioration of excessive muscle autophagy in GK rats through the down-regulation of the ROS-ERK/JNK-p53 pathway leads to improvement of glucose metabolism, reduction of oxidative stress and inhibition of mitochondrial loss and dysfunction. These results suggest (a) that hyperglycemia-associated oxidative stress may induce autophagy through up-regulation of the ROS-ERK/JNK-p53 pathway, which may contribute to mitochondrial loss in soleus muscle of diabetic GK rats, and (b) that EGCG may be a potential autophagy regulator useful in treatment of insulin resistance.     

Hyperglycemia-induced oxidative stress in diabetic complications

Reactive oxygen species are increased by hyperglycemia. Hyperglycemia, which occurs during diabetes (both type 1 and type 2) and, to a lesser extent, during insulin resistance, causes oxidative stress. Free fatty acids, which may be elevated during inadequate glycemic control, may also be contributory. In this review, we will discuss the role of oxidative stress in diabetic complications. Oxidative stress may be important in diabetes, not just because of its role in the development of complications, but because persistent hyperglycemia, secondary to insulin resistance, may induce oxidative stress and contribute to beta cell destruction in type 2 diabetes. The focus of this review will be on the role of oxidative stress in the etiology of diabetic complications.     

Angiotensin II-induced skeletal muscle insulin resistance mediated by NF-kappaB activation via NADPH oxidase

Reduced insulin sensitivity is a key factor in the pathogenesis of type 2 diabetes and hypertension. Skeletal muscle insulin resistance is particularly important for its major role in insulin-mediated glucose disposal. Angiotensin II (ANG II) is integral in regulating blood pressure and plays a role in the pathogenesis of hypertension. In addition, we have documented that ANG II-induced skeletal muscle insulin resistance is associated with generation of reactive oxygen species (ROS). However, the linkage between ROS and insulin resistance in skeletal muscle remains unclear. To explore potential mechanisms, we employed the transgenic TG(mRen2)27 (Ren-2) hypertensive rat, which harbors the mouse renin transgene and exhibits elevated tissue ANG II levels, and skeletal muscle cell culture. Compared with Sprague-Dawley normotensive control rats, Ren-2 skeletal muscle exhibited significantly increased oxidative stress, NF-kappaB activation, and TNF-alpha expression, which were attenuated by in vivo treatment with an angiotensin type 1 receptor blocker (valsartan) or SOD/catalase mimetic (tempol). Moreover, ANG II treatment of L6 myotubes induced NF-kappaB activation and TNF-alpha production and decreased insulin-stimulated Akt activation and GLUT-4 glucose transporter translocation to plasma membranes. These effects were markedly diminished by treatment of myotubes with valsartan, the antioxidant N-acetylcysteine, NADPH oxidase-inhibiting peptide (gp91 ds-tat), or NF-kappaB inhibitor (MG-132). Similarly, NF-kappaB p65 small interfering RNA reduced NF-kappaB p65 subunit expression and nuclear translocation and TNF-alpha production but improved insulin-stimulated phosphorylation (Ser(473)) of Akt and translocation of GLUT-4. These findings suggest that NF-kappaB plays an important role in ANG II/ROS-induced skeletal muscle insulin resistance.     

Insulin resistance mediated biochemical alterations in eye lens of neonatal streptozotocin-induced diabetic rat

Cataract, the leading cause of blindness worldwide, is associated with many risk factors including diabetes. Impaired glucose tolerance (IGT) and impaired fasting glucose (IFG) states are associated with pre-diabetes and insulin resistance. This condition subsequently leads to the development of type-2 diabetes. Epidemiological studies indicated that not only diabetes but IGT/IFG will also lead to the development of microvascular disorders and cataract. However, there are no studies on the mechanism of insulin resistance induced changes in the eye lens. In the present study, IGT/IFG-induced changes in lens using neonatal-streptozotocin (nSTZ) rat model have been investigated. Though, nSTZ rats showed the signs of IGT and insulin resistance starting from two months old, they did not develop cataract even at the age of 8-months. However, biochemical analysis indicates a three-fold increase in sorbitol levels in nSTZ lens upon prolonged (6-months) IGT and insulin resistance. Also there was an increase in lipid peroxidation and alterations in antioxidant enzymes. Results of this study showed that activation of polyol pathway and increased oxidative stress, commonly associated with long-term complications of diabetes, have been observed in eye lens due to prolonged IGT and insulin resistance which may lead to cataract.     

Mitochondrial dysfunction and inhibition of myoblast differentiation in mice with high-fat-diet-induced pre-diabetes

Pre-diabetes is characterized by impaired glucose tolerance (IGT) and/or impaired fasting glucose. Impairment of skeletal muscle function is closely associated with the progression of diabetes. However, the entire pathological characteristics and mechanisms of pre-diabetes in skeletal muscle remain fully unknown. Here, we established a mouse model of pre-diabetes, in which 6-week-old male C57BL6/J mice were fed either normal diet or high-fat diet (HFD) for 8 or 16 weeks. Both non-fasting and fasting glucose levels and the results of glucose and insulin tolerance tests showed that mice fed an 8-week HFD developed pre-diabetes with IGT; whereas mice fed a 16-week HFD presented with impaired fasting glucose and impaired glucose tolerance (IFG-IGT). Mice at both stages of pre-diabetes displayed decreased numbers of mitochondria in skeletal muscle. Moreover, IFG-IGT mice exhibited decreased mitochondrial membrane potential and ATP production in skeletal muscle and muscle degeneration characterized by a shift in muscle fibers from predominantly oxidative type I to glycolytic type II. Western blotting and histological analysis confirmed that myoblast differentiation was only inhibited in IFG-IGT mice. For primary skeletal muscle satellite cells, inhibition of differentiation was observed in palmitic acid-induced insulin resistance model. Moreover, enhanced myoblast differentiation increased glucose uptake and insulin sensitivity. These findings indicate that pre-diabetes result in mitochondrial dysfunction and inhibition of myoblast differentiation in skeletal muscle. Therefore, interventions that enhance myoblast differentiation may improve insulin resistance of diabetes at the earlier stage.     

Ameliorative effect of vanadyl(IV)–ascorbate complex on high-fat high-sucrose diet-induced hyperglycemia,insulin resistance,and oxidative stress in mice

There is mounting evidence demonstrating causative links between hyperglycemia, oxidative stress, and insulin resistance, the core pathophysiological features of type 2 diabetes mellitus. Using a combinational approach, we synthesized a vanadium–antioxidant (i.e., l-ascorbic acid) complex and examined its effect on insulin resistance and oxidative stress. This study was designed to examine whether vanadyl(IV)-ascorbate complex (VOAsc) would reduce oxidative stress, hyperglycemia, and insulin resistance in high-fat high-sucrose diet (HFSD)-induced type 2 diabetes in mice. Male C57BL/6J mice were fed a HFSD for 12 weeks to induce insulin resistance, rendering them diabetic. Diabetic mice were treated with rosiglitazone, sodium l-ascorbate, or VOAsc. At the end of treatment, fasting blood glucose, fasting serum insulin, homeostasis model assessment-insulin resistance index, and serum adipocytokine levels were measured. Serum levels of nitric oxide (NO) parameters were also determined. The liver was isolated and used for determination of malondialdehyde, reduced glutathione, and catalase levels, and superoxide dismutase and glutathione peroxidase activities. VOAsc groups exhibited significant reductions in serum adipocytokine and NO levels, and oxidative stress parameters compared to the corresponding values in the untreated diabetic mice. The results indicated that VOAsc is non-toxic. In conclusion, we identified VOAsc as a potentially effective adjunct therapy for the management of type 2 diabetes.