Oxidative stress and stress-activated signaling pathways: a unifying hypothesis of type 2 diabetes

Reactive oxygen species-induced oxidative stress in the development of insulin resistance and hyperandrogenism in polycystic ovary syndrome

CONTEXT

Insulin resistance and chronic low level inflammation are often present in women with polycystic ovary syndrome (PCOS).

OBJECTIVE

The purpose of this study was to determine the effects of hyperglycemia on reactive oxygen species (ROS) generation from mononuclear cells (MNCs) in PCOS.

DESIGN

This was a prospective controlled study.

SETTING

The study was conducted at an academic medical center.

PATIENTS

The study population consisted of 16 women with PCOS (eight lean, eight obese) and 15 age- and body composition-matched controls (eight lean, seven obese).

MAIN OUTCOME MEASURES

Insulin sensitivity was derived from a 2-h, 75-g oral glucose tolerance test (IS(OGTT)). ROS generation and p47(phox) protein expression were quantitated from MNCs obtained from blood drawn fasting and 2 h after glucose ingestion.

RESULTS

IS(OGTT) was lower in PCOS, compared with controls (3.1 +/- 0.3 vs. 6.3 +/- 0.9, P < 0.003). The percent change in ROS generation from MNCs was higher in lean and obese PCOS, compared with lean controls (138.8 +/- 21.3 and 154.2 +/- 49.1 vs. 0.6 +/- 12.7, P < 0.003). The percent change in ROS generation from MNCs correlated positively with glucose area under the curve (r = 0.38, P < 0.05), and plasma levels of testosterone (r = 0.59, P < 0.002) and androstenedione (r = 0.50, P < 0.009). The percent change in p47(phox) from MNCs was also higher in lean and obese PCOS, compared with lean controls (36.2 +/- 18.2 and 39.1 +/- 8.0 vs. -13.7 +/- 8.7, P < 0.02), and correlated negatively with IS(OGTT) (r = -0.39, P < 0.05).

CONCLUSION

ROS generation from MNCs in response to hyperglycemia is increased in PCOS independent of obesity. The resultant oxidative stress may contribute to a proinflammatory state that induces insulin resistance and hyperandrogenism in women with this disorder.

Chronic Administration of Satsuma Mandarin Fruit (Citrus unshiu MARC.) Improves Oxidative Stress in Streptozotocin-Induced Diabetic Rat Liver

Fruits and vegetables contain numerous antioxidants such as carotenoids, vitamins, and phenolic phytochemicals. Recent studies have demonstrated that antioxidants may reduce the risk for diabetes or its complications. In this study, we investigated the effects of the chronic administration of Satsuma mandarin fruit on an antioxidant defense system in streptozotocin (STZ)-induced diabetic rat liver. After a ten-week administration of Satsuma mandarin, antioxidant enzymes and glutathione levels in the liver were evaluated. The superoxide dismutase (SOD), catalase, and glutathione-peroxidase (GPx) activities, and glutathione level in the STZ-induced diabetic rats liver decreased significantly compared with those in the age-matched normal rats. The glutathione-reductase (GR) activities did not differ significantly between these two groups. In contrast, the SOD, GR, and glutathione levels in the Satsuma mandarin (1% or 3%) diet-fed STZ-diabetic rat livers were significantly higher than those in the normal diet-fed STZ-diabetic rat livers. In addition, although the serum alanine aminotransferase and gamma-glutamyl-aminotransferase concentrations of normal diet-fed STZ-diabetic rats were significantly higher than those of the age-matched normal rats, these increments of serum liver enzymes were diminished by the chronic administration of Satsuma mandarin. These results suggest that Satsuma mandarin may act as a suppressor against liver cell damage and inhibit the progression of liver dysfunction induced by chronic hyperglycemia.

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.

Palmitate induces tumor necrosis factor-alpha expression in C2C12 skeletal muscle cells by a mechanism involving protein kinase C and nuclear factor-kappaB activation

The mechanisms responsible for increased expression of TNF-alpha in skeletal muscle cells in diabetic states are not well understood. We examined the effects of the saturated acid palmitate on TNF-alpha expression. Exposure of C2C12 skeletal muscle cells to 0.75 mm palmitate enhanced mRNA (25-fold induction, P < 0.001) and protein (2.5-fold induction) expression of the proinflammatory cytokine TNF-alpha. This induction was inversely correlated with a fall in GLUT4 mRNA levels (57% reduction, P < 0.001) and glucose uptake (34% reduction, P < 0.001). PD98059 and U0126, inhibitors of the ERK-MAPK cascade, partially prevented the palmitate-induced TNF-alpha expression. Palmitate increased nuclear factor (NF)-kappaB activation and incubation of the cells with the NF-kappaB inhibitors pyrrolidine dithiocarbamate and parthenolide partially prevented TNF-alpha expression. Incubation of palmitate-treated cells with calphostin C, a strong and specific inhibitor of protein kinase C (PKC), abolished palmitate-induced TNF-alpha expression, and restored GLUT4 mRNA levels. Palmitate treatment enhanced the expression of phospho-PKCtheta, suggesting that this PKC isoform was involved in the changes reported, and coincubation of palmitate-treated cells with the PKC inhibitor chelerythrine prevented the palmitate-induced reduction in the expression of IkappaBalpha and insulin-stimulated Akt activation. These findings suggest that enhanced TNF-alpha expression and GLUT4 down-regulation caused by palmitate are mediated through the PKC activation, confirming that this enzyme may be a target for either the prevention or the treatment of fatty acid-induced insulin resistance.

Energy restriction prevents the development of type 2 diabetes in Zucker diabetic fatty rats: coordinated patterns of gene expression for energy metabolism in insulin-sensitive tissues and pancreatic islets determined by oligonucleotide microarray analysis

Energy restriction (ER) causes metabolic improvement in the prediabetic and diabetic state. Little information exists on the mechanism of action of ER, for example, on the changes at the transcriptional gene level in insulin-sensitive tissues. To gain further insight, we have investigated changes in gene expressions in skeletal muscle, liver, fat, and pancreatic islets after ER in male Zucker diabetic fatty rats. Eighteen Zucker diabetic fatty rats were divided at the age of 7 weeks into a control group (ad libitum diet) and an ER group (30% ER compared with the control group). Blood glucose, weight, and food intake were measured weekly. After 5 weeks, blood samples, and skeletal muscle, liver, visceral fat (epididymal fat pads), and islets tissue were collected. Gene expression was quantified with high-density oligonucleotide, microarray GeneChip technology. ER ameliorated the development of hyperglycemia, increased the levels of plasma insulin, and reduced plasma total cholesterol and the glucagon-insulin ratio (P < .05). In skeletal muscle, the expression of 55 genes increased and 245 decreased involving genes related to glucose metabolism (eg, phosphorylase kinase, pyruvate dehydrogenase kinase 4), lipid metabolism (eg, carnitine palmitoyltransferase 1, fatty acid transporter), and signaling pathways (eg, mitogen-activated protein kinases, protein kinase C). In the liver, the expression of 123 genes increased and 103 decreased involving genes related primarily to lipid metabolism. In pancreatic islets, the expression of 110 genes increased and that of 127 decreased, whereas in visceral fat, the expression of 279 genes increased and that of 528 decreased. ER counteracts the development of diabetes and causes changes in the expression of multiple genes involved in glucose and lipid metabolism in skeletal muscle, liver, and pancreatic islets, which may play an important role for the prevention of diabetes.

The metabolic syndrome: role of skeletal muscle metabolism

Skeletal muscle constitutes the largest insulin-sensitive tissue in the body and is the primary site for insulin-stimulated glucose utilization. Skeletal muscle resistance to insulin is fundamental to the metabolic dysregulation associated with obesity and physical inactivity, and contributes to the development of the metabolic syndrome (MS). The inability to efficiently take up and store fuel, and to transition from fat to glucose as the primary source of fuel during times of caloric abundance (high insulin) or scarcity (low insulin) has been termed metabolic inflexibility which contributes to a whole body metabolic dysregulation and cardiovascular risk. Potential mechanisms contributing to reduced insulin signaling and action in skeletal muscle includes adipose tissue expansion and increased inflammatory adipokines, increased renin-angiotensin-aldosterone system (RAAS) activity, decreases in muscle mitochondrial oxidative capacity, increased intramuscular lipid accumulation, and increased reactive oxygen species. Future research is focused upon understanding these and other potential mechanisms in order to identify therapeutic targets for reducing MS risk. Strategies will include adequate physical activity and maintaining a healthy weight, but may also require specific pharmacologic interventions.

Ginseng and diabetes

Ginseng is a well-known medicinal plant used in traditional Oriental medicine. In recent decades, ginseng root has gained popularity as a dietary supplement in the United States. Ginseng has also been commonly used in Oriental medicine to treat diabetes-like conditions. The present review discusses the research on the anti-diabetic effects of ginseng and the possible mechanisms of its anti-diabetic actions.

Correction of nitrergic neurovascular dysfunction in diabetic mouse corpus cavernosum by p38 mitogen-activated protein kinase inhibition

Increased p38 mitogen-activated protein kinase (MAPK) in response to stress stimuli, including hyperglycemia, contributes to diabetic somatic neuropathy. However, effects on autonomic nerve and vascular function have not been determined. The aim of this study was to investigate the effects of the p38 MAPK inhibitor, LY2161793, on penile neurovascular function in streptozotocin-induced diabetic mice. Diabetes duration was 6 weeks and intervention LY2161793 treatment was given for the final 2 weeks. In vitro measurements on phenylephrine-precontracted corpus cavernosum revealed a 32% reduction in maximum nitrergic nerve-mediated relaxation with diabetes that was 74% corrected by LY2161793 treatment. Maximum nitric oxide-mediated endothelium-dependent relaxation to acetylcholine was 42% attenuated by diabetes and 88% restored by LY2161793. Moreover, treatment partially corrected a diabetic deficit in endothelium-independent relaxation to a nitric oxide donor. Thus, p38 MAPK inhibition corrects nitric oxide-dependent indices of diabetic erectile autonomic neuropathy and vasculopathy, a therapeutic approach potentially worthy of consideration for clinical trials.

AMPK, the metabolic syndrome and cancer

The fuel-sensing enzyme 5'-AMP-activated protein kinase (AMPK) has a major role in the regulation of cellular lipid and protein metabolism in response to stimuli such as exercise, changes in fuel availability and the adipocyte-derived hormones leptin and adiponectin. Recent studies indicate that abnormalities in cellular lipid metabolism are involved in the pathogenesis of the metabolic syndrome, possibly because of dysregulation of AMPK and malonyl-CoA, a closely related molecule. As we discuss in this article, several findings also point to a link between AMPK and the growth and/or survival of some cancer cells. Thus, it has been demonstrated recently that the tumor suppressor LKB1 is a kinase that has a major role in phosphorylating and activating AMPK, and that another tumor suppressor, tuberous sclerosis complex 2, is phosphorylated and activated by AMPK. In addition, other studies indicate that mammalian homolog of target of rapamycin (mTOR), which has been implicated in the pathogenesis of insulin resistance and many types of cancer, is inhibited by AMPK.

Mechanisms of endothelial dysfunction in obesity

Obesity is a chronic disease, whose incidence is alarmingly growing, affecting not only adults but also children and adolescents. It is associated with severe metabolic abnormalities and increased cardiovascular morbidity and mortality. Adipose tissue secretes a great number of hormones and cytokines that not only regulate substrate metabolism but may deeply and negatively influence endothelial physiology, a condition which may lead to the formation of the atherosclerotic plaque. In this review, the physiology of the endothelium is summarised and the mechanisms by which obesity, through the secretory products of adipose tissue, influences endothelial function are explained. A short description of methodological approaches to diagnose endothelial dysfunction is presented. The possible pathogenetic links between obesity and cardiovascular disease, mediated by oxidative stress, inflammation and endothelial dysfunction are described as well.

[Endothelial dysfunction in patients with diabetes: identification, pathogenesis and treatment]

Cardiovascular diseases are the leading cause of morbidity and mortality in people with diabetes. Vascular abnormalities can be observed long before atherosclerosis develops and in sites not usually prone to atherosclerosis. These vascular abnormalities are known to be due to endothelial dysfunctions, one of the most frequent of which is depressed endothelium-dependent dilation. In patients with diabetes, this is mainly linked to decreased bioavailability of nitric oxide. Although inactivation of tetrahydrobiopterin, a co-factor of NO-synthase, may depress nitric oxide production, the latter is more likely due to the inactivation of nitric oxide by superoxide anions: enhanced oxidative stress increases their production in people with diabetes. Moreover, hyperglycemia directly activates oxidative stress, which in turn depresses endothelium-dependent vasodilation. Glycemia and oxidative stress are positively correlated in people with diabetes. However, while depression of endothelium-dependent dilation may be a visible functional manifestation of oxidative stress, the oxidative stress itself is mainly responsible for the cascade of endothelial events that play a key role in development of vascular atherosclerosis and its complications. Especially important among these events are the activation of NF-kappaB and the oxidation of LDL-cholesterol. Although antioxidants provide short-term improvement of endothelial function in humans, all studies of the effectiveness of preventive antioxidant therapy have been disappointing. Control of hyperglycemia thus remains the best way to improve endothelial function and to prevent atherosclerosis and other cardiovascular complications of diabetes.

Vascular dementia prevention: a risk factor analysis

Brain injury from ischemic or hemorrhagic cerebrovascular disease (CVD) produces decline in cognitive functions and vascular dementia (VaD). Likewise, CVD may cause VaD from hypoperfusion of susceptible brain areas. CVD may also worsen degenerative dementias such as Alzheimer's disease. Significant advances have been made in the identification and control of risk factors for stroke and cardiovascular disease. The main risk factors for VaD include age, hypertension and absence of antihypertensive medication, diabetes, cigarette smoking, history of cardiovascular disease (coronary heart disease, congestive heart failure, peripheral vascular disease), atrial fibrillation, left ventricular hypertrophy, hyperhomocysteinemia, orthostatic hypotension, cardiac arrhythmias, hyperfibrinogenemia, and sleep apnea. Recently identified risk factors include chronic infection and elevation of C-reactive protein, particularly in patients with diabetes. Evidence from controlled clinical trials strongly suggests that control of vascular risk factors, in particular hypertension, could prevent the development of dementia.

Mechanisms of beta-cell death in type 2 diabetes

A decrease in the number of functional insulin-producing beta-cells contributes to the pathophysiology of type 2 diabetes. Opinions diverge regarding the relative contribution of a decrease in beta-cell mass versus an intrinsic defect in the secretory machinery. Here we review the evidence that glucose, dyslipidemia, cytokines, leptin, autoimmunity, and some sulfonylureas may contribute to the maladaptation of beta-cells. With respect to these causal factors, we focus on Fas, the ATP-sensitive K+ channel, insulin receptor substrate 2, oxidative stress, nuclear factor-kappaB, endoplasmic reticulum stress, and mitochondrial dysfunction as their respective mechanisms of action. Interestingly, most of these factors are involved in inflammatory processes in addition to playing a role in both the regulation of beta-cell secretory function and cell turnover. Thus, the mechanisms regulating beta-cell proliferation, apoptosis, and function are inseparable processes.

Up-regulation of advanced glycated products receptors in the brain of diabetic rats is prevented by antioxidant treatment

Diabetics have at least twice the risk of stroke and may show performance deficit in a wide range of cognitive domains. The mechanisms underlying this gradually developing end-organ damage may involve both vascular changes and direct damage to neuronal cells as a result of overproduction of superoxide by the respiratory chain and consequent oxidative stress. The study aimed to assess the role of oxidative stress on the aldose reductase-polyol pathway, on advanced glycated end-product (AGE)/AGE-receptor interaction, and on downstream signaling in the hippocampus of streptozotocin-treated rats. Data show that, in diabetic rats, levels of prooxidant compounds increase, whereas levels of antioxidant compounds fall. Receptor for AGE and galectin-3 content and polyol flux increase, whereas glyceraldehyde-3-phosphate dehydrogenase activity is impaired. Moreover, nuclear factor kappaB (p65) transcription factor levels and S-100 protein are increased in the hippocampus cytosol, suggesting that oxidative stress triggers the cascade of events that finally leads to neuronal damage. Dehydroepiandrosterone, the most abundant hormonal steroid in the blood, has been reported to possess antioxidant properties. When dehydroepiandrosterone was administered to diabetic rats, the improved oxidative imbalance and the marked reduction of AGE receptors paralleled the reduced activation of nuclear factor kappaB and the reduction of S-100 levels, reinforcing the suggestion that oxidative stress plays a role in diabetes-related neuronal damage.

Prevention of hyperglycemia in Zucker diabetic fatty rats by exercise training: effects on gene expression in insulin-sensitive tissues determined by high-density oligonucleotide microarray analysis

Exercise training (ET) causes metabolic improvement in the prediabetic and diabetic states. However, only little information exists on the changes to ET at the transcriptional level in insulin-sensitive tissues. We have investigated the gene expression changes in skeletal muscle, liver, fat, and pancreatic islets after ET in male Zucker diabetic fatty (ZDF) rats. Eighteen ZDF rats (7 weeks old) were divided in a control and ET group. Exercise was performed using a motorized treadmill (20 m/min 1 hour daily for 6 days a week). Blood glucose, weight, and food intake were measured weekly. After 5 weeks, blood samples, soleus muscle, liver, visceral fat (epididymal fat pads), and islet tissue were collected. Gene expression was quantified with Affymetrix RG-U34A array (16 chips). Exercise training ameliorates the development of hyperglycemia and reduces plasma free fatty acid and the level of glucagon-insulin ratio (P < .05). In skeletal muscle, the expression of 302 genes increased, whereas that of 119 genes decreased. These changes involved genes related to skeletal muscle plasticity, Ca(2+) signals, energy metabolism (eg, glucose transporter 1, phosphorylase kinase), and other signaling pathways as well as genes with unknown functions (expressed sequence tags). In the liver, expression of 148 genes increased, whereas that of 199 genes decreased. These were primarily genes involved in lipogenesis and detoxification. Genes coding for transcription factors were changed in parallel in skeletal muscle and liver tissue. Training did not markedly influence the gene expression in islets. In conclusion, ET changes the expression of multiple genes in the soleus muscle and liver tissue and counteracts the development of diabetes, indicating that ET-induced changes in gene transcription may play an important role en the prevention of diabetes.

You are what you breathe: Evidence linking air pollution and blood pressure

Exposure to particulate matter (PM) air pollution increases the risk for myocardial infarctions, strokes, and cardiovascular mortality. A variety of responsible mechanisms have been described, including PM-induced elevations in blood pressure. Observational studies and controlled experiments have provided evidence that PM is capable of acutely increasing blood pressure in certain scenarios. Enhanced sympathetic tone and vascular dysfunction due to PM-induced systemic oxidative stress/inflammation are leading explanations. The hemodynamic responses to air pollution may be altered by underlying cardiovascular risk factors and the chemical composition of the PM. However, even the small elevations in blood pressure observed following certain exposures to PM have tremendous public health implications, due to the ubiquitous nature of air pollution.

Effects of antioxidant supplementation on postprandial oxidative stress and endothelial dysfunction: A single-blind, 15-day clinical trial in patients with untreated type 2 diabetes, subjects with impaired glucose tolerance, and healthy controls

BACKGROUND

Increased generation of reactive oxygen species (ROS) and oxidative stress may be of crucial importance in the pathogenesis of endothelial damage. Furthermore, there is understood to be a relationship between endothelial damage, glycemic control, disorders of lipid metabolism, and coagulative hemostatic disorders.

OBJECTIVE

This study investigated within- and between-group changes in various circulating markers of oxidation-reduction balance and endothelial function after a balanced moderate-fat meal with and without antioxidant supplementation in patients with early-stage, untreated type 2 diabetes mellitus; subjects with impaired glucose tolerance (IGT); and healthy controls.

METHODS

In this single-blind, controlled clinical study, groups of patients with type 2 diabetes and subjects with IGT were identified and compared with a group of healthy controls. All groups followed a controlled, well-balanced diet for 10 days before and throughout the study. Before and after consumption of a standardized moderate-fat meal, plasma levels of oxidants (malondialdehyde, 4-hydroxynonenal, oxidized low-density lipoprotein), the antioxidant glutathione peroxidase, and markers of endothelial function (NO, endothelin-1, von Willebrand factor [vWF], vascular cell adhesion molecule-1 [VCAM-1]) were determined. These measures were then reassessed after 15 days of standard antioxidant treatment consisting of a thiol-containing antioxidant (N-acetylcysteine 600 g/d), a bound antioxidant (vitamin E 300 g/d), and an aqueous phase antioxidant (vitamin C 250 mg/d). The efficacy of antioxidant treatment in reversing abnormalities in oxidation-reduction balance after a moderate-fat meal was assessed by evaluating changes in plasma levels of ROS on the morning of the 16th day following an overnight fast. Safety was monitored in terms of adverse events, vital signs, physical findings, and laboratory values.

RESULTS

The study included 46 patients with type 2 diabetes (23 men, 23 women; mean [SD] age, 41 [3] years; mean body mass index [BMI], 24 [2] kg/m(2)), 46 with IGT (23 men, 23 women; mean age, 39 [3] years; mean BMI, 23 [3] kg/m(2)), and 46 control subjects (23 men, 23 women; mean age, 40 [1] years; mean BMI, 22 [1] kg/m(2)). Before supplementation, all 3 groups had significantly increased levels of oxidants, vWF, and VCAM-1 (all, P < 0.001) and significantly decreased levels of antioxidants and NO (both, P < 0.001) after consumption of a moderate-fat meal. After 15 days of antioxidant treatment, significant improvements in these measures were seen in all groups (P < 0.05).

CONCLUSIONS

This study showed changes in oxidation-reduction balance, NO bioavailability, and nonthrombogenic endothelial factors after a moderate-fat meal in patients with type 2 diabetes and those with IGT, but these postprandial changes were reverse in all subjects after 15 days of standard antioxidant supplementation. These findings suggest that the use of anti-oxidants may have decreased oxidative stress in these subjects.

Diabetic neuropathy is associated with activation of the TNF-alpha system in subjects with type 1 diabetes mellitus

OBJECTIVE

The development of diabetic neuropathy (DN) is predicted by cardiovascular risk factors associated with insulin resistance. As inflammation seems to be implicated in the pathogenesis of insulin resistance, we investigated whether subjects with type 1 diabetes mellitus (T1DM) and DN have an increase in plasma concentrations of inflammatory proteins involved in insulin resistance.

DESIGN

Cross-sectional. Patients One hundred twenty subjects, all diagnosed with T1DM 14 years before.

MEASUREMENTS

(1) Sex, age, body mass index, waist-to-hip ratio (WHR), blood pressure, smoking, alcohol intake, insulin dose, HbA1c and lipid profile; (2) DN (peripheral and cardiac autonomic), retinopathy and nephropathy; (3) plasma concentrations of soluble fractions of tumour necrosis factor alpha receptors 1 and 2 (sTNFR1 and sTNFR2), interleukin-6, high-sensitive C-reactive protein, adiponectin and leptin; and (4) insulin resistance (by way of a mathematical estimation of the glucose disposal rate - eGDR-).

RESULTS

Thirty-six subjects had DN and 84 did not. Subjects with DN received higher insulin doses (57.6 +/- 16.7 vs. 49.2 +/- 15.0 IU/day; P = 0.008) and had higher WHR (0.85 +/- 0.07 vs. 0.81 +/- 0.10; P = 0.007) and HbA1c values (8.5 (7.6-9.6) vs. 7.7 (7.3-8.9)%; P = 0.049) than subjects without DN. They also had higher values of sTNFR1 (2.42 +/- 0.60 vs. 1.96 +/- 0.66 microg/l; P = 0.001) and sTNFR2 (4.73 +/- 1.33 vs. 4.14 +/- 1.09 microg/l; P = 0.015), and were more insulin resistant (eGDR values: 7.28 (5.83-8.03) vs. 8.30 (7.17-9.03) mg kg(-1) min(-1); P = 0.003). The relationship between DN and either sTNFR1 or sTNFR2 remained essentially unchanged after adjusting for several confounders, including glycaemic control, WHR, lipid profile, blood pressure and other microvascular complications (OR for sTNFR1: 2.592 (1.222-5.498), P = 0.013; OR for sTNFR2: 2.124 (1.258-3.587), P = 0.005).

CONCLUSIONS

The activity of the TNF-alpha system is increased in subjects with type 1 diabetes mellitus and diabetic neuropathy, regardless of their glycaemic control and cardiovascular risk factors associated with insulin resistance. These results suggest that TNF-alpha may play a pathogenic role in the development of diabetic neuropathy.

Dietary fructose: implications for dysregulation of energy homeostasis and lipid/carbohydrate metabolism

Fructose intake and the prevalence of obesity have both increased over the past two to three decades. Compared with glucose, the hepatic metabolism of fructose favors lipogenesis, which may contribute to hyperlipidemia and obesity. Fructose does not increase insulin and leptin or suppress ghrelin, which suggests an endocrine mechanism by which it induces a positive energy balance. This review examines the available data on the effects of dietary fructose on energy homeostasis and lipid/carbohydrate metabolism. Recent publications, studies in human subjects, and areas in which additional research is needed are emphasized.

Quercetin decreases oxidative stress, NF-kappaB activation, and iNOS overexpression in liver of streptozotocin-induced diabetic rats

Increasing evidence in both experimental and clinical studies suggests that oxidative stress is involved in the pathogenesis and progression of diabetic tissue damage. This study investigated the protective effects of quercetin treatment on oxidative stress, nuclear factor (NF)-kappaB activation and expression of inducible nitric oxide synthase (iNOS) in streptozotocin-induced diabetic rats. Male Wistar rats were divided into 4 groups: control rats, control rats treated daily with quercetin (150 micromol/kg, i.p.), untreated diabetic rats, and diabetic rats treated with quercetin. Diabetes was induced by a single i.p. injection of streptozotocin (70 mg/kg). Eight weeks later we measured TBARS and hydroperoxide-initiated chemiluminescence (QL) in liver as markers of oxidative stress, and activities of the antioxidant enzymes catalase, superoxide dismutase (SOD), and glutathione peroxidase, NF-kappaB activation by an electrophoretic mobility shift assay and expression of IkappaB kinases (IKKalpha and IKKbeta), the inhibitor IkappaB (IkappaBalpha and IkappaBbeta), and iNOS by Western blot. The plasma glucose concentration was significantly increased in diabetic rats and was not changed by quercetin. Streptozotocin administration induced significant increases in hepatic TBARS concentration, QL, and SOD and catalase activities that were prevented by quercetin. Activation of NF-kappaB, induction of IKKalpha and iNOS protein levels, and increased degradation of IkappaBalpha were also observed in streptozotocin-treated rats. All of those effects were abolished by quercetin. These findings suggest that quercetin treatment, by abolishing the IKK/NF-kappaB signal transduction pathway, may block the production of noxious mediators involved in the development of early diabetes tissue injury and in the evolution of late complications.

Insulin neuroprotection against oxidative stress in cortical neurons--involvement of uric acid and glutathione antioxidant defenses

In this study we investigated the effect of insulin on neuronal viability and antioxidant defense mechanisms upon ascorbate/Fe2+-induced oxidative stress, using cultured cortical neurons. Insulin (0.1 and 10 microM) prevented the decrease in neuronal viability mediated by oxidative stress, decreasing both necrotic and apoptotic cell death. Moreover, insulin inhibited ascorbate/Fe2+-mediated lipid and protein oxidation, thus decreasing neuronal oxidative stress. Increased 4-hydroxynonenal (4-HNE) adducts on GLUT3 glucose transporters upon exposure to ascorbate/Fe2+ were also prevented by insulin, suggesting that this peptide can interfere with glucose metabolism. We further analyzed the influence of insulin on antioxidant defense mechanisms in the cortical neurons. Oxidative stress-induced decreases in intracellular uric acid and GSH/GSSG levels were largely prevented upon treatment with insulin. Inhibition of phosphatidylinositol-3-kinase (PI-3K) or mitogen-induced extracellular kinase (MEK) reversed the effect of insulin on uric acid and GSH/GSSG, suggesting the activation of insulin-mediated signaling pathways. Moreover, insulin stimulated glutathione reductase (GRed) and inhibited glutathione peroxidase (GPx) activities under oxidative stress conditions, further supporting that insulin neuroprotection was related to the modulation of the glutathione redox cycle. Thus, insulin may be useful in preventing oxidative stress-mediated injury that occurs in several neurodegenerative disorders.

Inter-relationships between DNA damage, ascorbic acid and glycaemic control in Type 2 diabetes mellitus

AIMS

The onset of complications in Type 2 diabetes mellitus (DM) patients cannot be predicted in individuals. Evidence suggests a link between complications and hyperglycaemia, oxidative stress and antioxidants, but causality is unclear. This study investigated baseline (entry) fasting plasma ascorbic acid, lymphocytic DNA damage and glycaemic control in Type 2 DM as part of a long-term study, the aim of which is to explore a biomarker profiling approach to identify and improve outcome in high-risk subjects.

METHODS

A cross-sectional study, in which DNA damage, glycated haemoglobin (HbA(1c)), fasting plasma glucose (FPG) and ascorbic acid (AA) were measured on fasting blood samples collected from 427 Type 2 DM subjects.

RESULTS

DNA damage was significantly (P < 0.0001) and directly correlated to both FPG (r = 0.540) and HbA(1c) (r = 0.282), and was significantly (P < 0.0001), independently and inversely correlated to plasma AA (r = -0.449). In those subjects with both poor glycaemic control and low AA (< 48 microm, the overall mean value for the study group), DNA damage was significantly (P < 0.005) higher compared with those subjects with a similar degree of hyperglycaemia but with AA above the mean.

CONCLUSIONS

The novel finding of a significant inverse relationship between plasma AA and DNA damage in Type 2 DM indicates that poorly controlled diabetic subjects might benefit from increased dietary vitamin C. The data also have important implications for biomarker profiling to identify those subjects who might benefit most from intensive therapy. Longer-term follow-up is underway.

In vivo misregulation of genes involved in apoptosis, development and oxidative stress in mice lacking both functional Werner syndrome protein and poly(ADP-ribose) polymerase-1

Werner syndrome (WS) is a rare disorder characterized by the premature onset of a number of age-related diseases. The gene responsible for WS is believed to be involved in different aspects of transcription, replication and/or DNA repair. The poly(ADP-ribose) polymerase-1 (PARP-1) enzyme is also involved in DNA repair and is known to affect transcription of several genes. In this study, we examined the expression profile of cells lacking the normal function of either or both enzymes. All mutant cells exhibited altered expression of genes normally responding to oxidative stress. Interestingly, more than 58% of misregulated genes identified in double mutant cells were not altered in cells with either the Wrn or PARP-1 mutation alone. So, the impact on gene expression profile when both Wrn and PARP-1 are mutated was greater than a simple addition of individual mutant genotype. In addition, double mutant cultured cells showed major misregulation of genes involved in apoptosis, cell cycle control, embryonic development, metabolism and signal transduction. More importantly, in vivo analyses of double mutant mice have confirmed the increased apoptosis and the developmental defects in embryos as well as the major increase in intracellular phosphorylation and oxidative DNA damage in adult tissues. They also exhibited a progressive increase in oxidative stress with age. Thus, a major result of this study is that changes in expression of several genes and physiological functions identified in vitro were confirmed in mouse embryonic and adult tissues.

Mechanisms of antioxidant and pro-oxidant effects of alpha-lipoic acid in the diabetic and nondiabetic kidney

BACKGROUND

alpha-Lipoic acid is a potent antioxidant that improves renal function in diabetes by lowering glycemia, however, the mechanisms by which alpha-lipoic acid exerts its antioxidant effects are not completely understood.

METHODS

Metabolic parameters, renal function, and morphology, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity and subunit expression were analyzed in nondiabetic and streptozotocin-induced diabetic rats fed normal rat chow (control) with or without alpha-lipoic acid (30 mg/kg body weight) for 12 weeks.

RESULTS

Blood glucose was increased with diabetes (nondiabetic + control 89 +/- 3 mg/dL and diabetic + control 336 +/- 28 mg/dL) and was similar with alpha-lipoic acid treatment (diabetic +alpha-lipoic acid 351 +/- 14 mg/dL). In contrast, alpha-lipoic acid attenuated albuminuria (nondiabetic + control 8.9 +/- 1.3 mg/day; diabetic + control 28.1 +/- 4.6 mg/day; and diabetic +alpha-lipoic acid 17.8 +/- 1.2 mg/day) associated with diabetes. Similarly, alpha-lipoic acid attenuated glomerulosclerosis (nondiabetic + control 0.22 +/- 0.01; diabetic + control 0.55 +/- 0.04; diabetic +alpha-lipoic acid 0.36 +/- 0.03), tubulointerstitial fibrosis (nondiabetic + control 0.42 +/- 0.18; diabetic + control 1.52 +/- 0.05; diabetic +alpha-lipoic acid 1.10 +/- 0.05), superoxide anion (O(.-) (2)) generation (nondiabetic +control 15.8 +/- 1.7; diabetic +control 87.1 +/- 3.5; diabetic +alpha-lipoic acid 25.5 +/- 3.3 RLU/mg protein), and urine 8-isoprostane (8-iso) excretion (nondiabetic + control 7.4 +/- 1.4; diabetic + control 26.0 +/- 4.5; diabetic +alpha-lipoic acid 19.6 +/- 5.6 ng/day) associated with diabetes. alpha-Lipoic acid also reduced kidney expression of NADPH oxidase subunits p22phox and p47phox. Surprisingly, alpha-lipoic acid appears to cause pro-oxidant effects in nondiabetic animals, resulting in increased albuminuria (nondiabetic +alpha-lipoic acid 14.2 +/- 1.2 mg/day), increase in plasma creatinine levels (nondiabetic + control 59 +/- 6; diabetic + control 68 +/- 6; nondiabetic +alpha-lipoic acid 86 +/- 9; diabetic +alpha-lipoic acid 69 +/- 7 mumol/L), exacerbated glomerulosclerosis and tubulointerstitial fibrosis, increased O(.-) (2) generation, up-regulated p22phox and p47phox expression and increased 8-iso excretion.

CONCLUSION

We conclude that alpha-lipoic acid improves albuminuria and pathology in diabetes by reducing oxidative stress, while in healthy animals, alpha-lipoic acid may act as a pro-oxidant, contributing to renal dysfunction.

Variation in the eNOS gene modifies the association between total energy expenditure and glucose intolerance

Endothelium-derived nitric oxide (NO) facilitates skeletal muscle glucose uptake. Energy expenditure induces the endothelial NO synthase (eNOS) gene, providing a mechanism for insulin-independent glucose disposal. The object was to test 1) the association of genetic variation in eNOS, as assessed by haplotype-tagging single nucleotide polymorphisms (htSNPs) with type 2 diabetes, and 2) the interaction between eNOS haplotypes and total energy expenditure on glucose intolerance. Using multivariate models, we tested associations between eNOS htSNPs and diabetes (n = 461 and 474 case and control subjects, respectively) and glucose intolerance (two cohorts of n = 706 and 738 U.K. and Spanish Caucasians, respectively), and we tested eNOS x total energy expenditure interactions on glucose intolerance. An overall association between eNOS haplotype and diabetes was observed (P = 0.004). Relative to the most common haplotype (111), two haplotypes (121 and 212) tended to increase diabetes risk (OR 1.22, 95% CI 0.96-1.55), and one (122) was associated with decreased risk (0.58, 0.39-0.86). In the cohort studies, no association was observed between haplotypes and 2-h glucose (P > 0.10). However, we observed a significant total energy expenditure-haplotype interaction (P = 0.007). Genetic variation at the eNOS locus is associated with diabetes, which may be attributable to an enhanced effect of total energy expenditure on glucose disposal in individuals with specific eNOS haplotypes. Gene-environment interactions such as this may help explain why replication of genetic association frequently fails.

Decreased circulating levels of active ghrelin are associated with increased oxidative stress in obese subjects

OBJECTIVE

To investigate the relationship between active ghrelin and oxidative stress in obese subjects.

DESIGN

We measured the plasma levels of free 8-epi-prostaglandin F(2alpha) (8-epi-PGF(2alpha), a reliable and systemic marker of oxidative stress) and the active form of ghrelin in 17 obese and 17 normal subjects. The biologically active forms of ghrelin were measured using a commercially available radio-immunoassay kit and free 8-epi-PGF(2alpha) was measured using an enzyme immunoassay kit.

RESULTS

The circulating level of active ghrelin was significantly decreased (20.4 +/- 2.6 vs 40.9 +/- 3.9 fmol/ml, P < 0.01) while that of 8-epi-PGF(2alpha) was significantly increased (61.5 +/- 9.6 vs 17.3 +/- 3.4 pg/ml, P < 0.01) in obese subjects compared with normal subjects. The plasma levels of active ghrelin and 8-epi-PGF(2alpha) were significantly correlated in obese (r = -0.507, P < 0.05) and in all (r = -0.577, P < 0.01) subjects. Multivariate analysis showed that the plasma levels of active ghrelin and 8-epi-PGF(2alpha) were significantly and independently correlated in all subjects (F = 7.888, P < 0.01).

CONCLUSIONS

There is an inverse correlation between circulating levels of active ghrelin and oxidative stress in obesity. Low circulating levels of active ghrelin may enhance oxidative stress and the process of atherosclerosis in obese subjects.

Hyperglycemia alters tumor necrosis factor-alpha release from mononuclear cells in women with polycystic ovary syndrome

CONTEXT

Women with polycystic ovary syndrome (PCOS) are often insulin resistant and have chronic low-level inflammation.

OBJECTIVE

The purpose of this study was to determine the effects of hyperglycemia on lipopolysaccharide (LPS)-stimulated TNFalpha release from mononuclear cells (MNC) in PCOS.

DESIGN

The study was designed as a prospective controlled study.

SETTING

The study was carried out at an academic medical center.

PATIENTS

Sixteen reproductive age women with PCOS (eight lean, eight obese) and 14 age-matched controls (eight lean, six obese) participated in the study.

MAIN OUTCOME MEASURES

Insulin sensitivity (IS) was derived from a 2-h 75-g oral glucose tolerance test (IS(OGTT)). Percentage of truncal fat was determined by dual-energy absorptiometry. TNFalpha release was measured from MNC cultured in the presence of LPS from blood samples drawn fasting and 2 h after glucose ingestion.

RESULTS

IS(OGTT) was lower in women with PCOS compared with controls (3.9 +/- 0.4 vs. 6.3 +/- 1.0; P < 0.03) and was negatively correlated with percentage of truncal fat (r = 0.56; P < 0.002). Truncal fat was greater in lean women with PCOS compared with lean controls (29.8 +/- 2.6 vs. 23.8 +/- 2.5%; P < 0.04). The TNFalpha response was different between obese and lean controls (-96.9 +/- 21.2 vs. 24.4 +/- 21.6 pg/ml; P < 0.03) and obese and lean women with PCOS (-94.1 +/- 34.5 vs. 30.4 +/- 17.6 pg/ml; P < 0.002). Fasting plasma C-reactive protein was elevated (P < 0.003) in obese PCOS and obese controls compared with lean controls.

CONCLUSION

An increase in abdominal adiposity and increased TNFalpha release from MNC after hyperglycemia may contribute to insulin resistance in lean PCOS patients. In contrast, obese PCOS patients have more profound chronic inflammation, and thus may have LPS tolerance that protects them from relatively mild excursions in blood glucose.

Role of oxidative stress, endoplasmic reticulum stress, and c-Jun N-terminal kinase in pancreatic β-cell dysfunction and insulin resistance

Type 2 diabetes is the most prevalent and serious metabolic disease affecting people all over the world. Pancreatic beta-cell dysfunction and insulin resistance are the hallmark of type 2 diabetes. Normal beta-cells can compensate for insulin resistance by increasing insulin secretion and/or beta-cell mass, but insufficient compensation leads to the onset of glucose intolerance. Once hyperglycemia becomes apparent, beta-cell function gradually deteriorates and insulin resistance aggravates. Under diabetic conditions, oxidative stress and endoplasmic reticulum stress are induced in various tissues, leading to activation of the c-Jun N-terminal kinase pathway. The activation of c-Jun N-terminal kinase suppresses insulin biosynthesis and interferes with insulin action. Indeed, suppression of c-Jun N-terminal kinase in diabetic mice improves insulin resistance and ameliorates glucose tolerance. Thus, the c-Jun N-terminal kinase pathway plays a central role in pathogenesis of type 2 diabetes and could be a potential target for diabetes therapy.

High prevalence of glucose-6-phosphate dehydrogenase deficiency without gene mutation suggests a novel genetic mechanism predisposing to ketosis-prone diabetes

CONTEXT

Ketosis-prone diabetes (KPD) is mostly observed in males of West African descent and is characterized by phasic or permanent insulin dependence without apparent autoimmune process.

OBJECTIVE

KPD subjects display a propensity to hyperglycemia-induced acute insulin deficiency, suggesting that they exhibit a propensity to oxidative stress in beta-cells. The enzyme glucose-6-phosphate dehydrogenase (G6PD) is a defense mechanism against oxidative stress, and G6PD deficiency, an X-linked genetic disorder with male predominance, is frequent in West Africans. We hypothesized that mutations in the G6PD gene could predispose to KPD.

DESIGN

We studied G6PD erythrocyte enzyme activity and the insulin secretory reserve (glucagon-stimulated C peptide) in a cohort of hospitalized West Africans with KPD (n = 59) or type 2 diabetes (T2DM; n = 59) and in normoglycemic controls (n = 55). We also studied the G6PD gene in an extended population of KPD patients (n = 100), T2DM patients (n = 59), and controls (n = 85).

RESULTS

The prevalence of G6PD deficiency was higher in KPD than in T2DM and controls (42.3%; 16.9%; 16.4%; P = 0.01). In KPD, but not in T2DM, insulin deficiency was proportional to the decreased G6PD activity (r = 0.33; P = 0.04). We found no increase in the prevalence of G6PD gene mutations in KPD compared with T2DM and controls. Rather, we found a 20.3% prevalence of G6PD deficiency in KPD without gene mutation.

CONCLUSIONS

This study suggests that 1) G6PD deficiency alone is not causative of KPD; and 2) alterations in genes controlling both insulin secretion and G6PD-mediated antioxidant defenses may contribute to the predisposition to KPD in West Africans.

Catalase enzyme mutations and their association with diseases

Enzyme catalase seems to be the main regulator of hydrogen peroxide metabolism. Hydrogen peroxide at high concentrations is a toxic agent, while at low concentrations it appears to modulate some physiological processes such as signaling in cell proliferation, apoptosis, carbohydrate metabolism, and platelet activation. Benign catalase gene mutations of 5' noncoding region (15) and intron 1 (4) have no effect on catalase activity and are not associated with disease. Catalase gene mutations have been detected in association with diabetes mellitus, hypertension, and vitiligo. Decreases in catalase activity in patients with tumors is more likely to be due to decreased enzyme synthesis rather than to catalase mutations.Acatalasemia, the inherited deficiency of catalase has been detected in 11 countries. Its clinical features might be oral gangrene, altered lipid, carbohydrate, homocysteine metabolism and the increased risk of diabetes mellitus. The Japanese, Swiss, and Hungarian types of acatalasemia display differences in biochemical and genetic aspects. However, there are only limited reports on the syndrome causing these mutations. These data show that acatalasemia may be a syndrome with clinical, biochemical, genetic characteristics rather than just a simple enzyme deficiency.

Atypical diabetes associated with inclusion formation in the R6/2 mouse model of Huntington's disease is not improved by treatment with hypoglycaemic agents

The R6/2 transgenic mouse model of Huntington's disease (HD) develops a progressive neurological phenotype that involves severe motor and cognitive dysfunctions. Although not a cardinal sign, diabetes has been described in R6/2 mice. It is not clear, however, whether the diabetes contributes to the HD-like phenotype of R6/2 mice. In our study we found that the severity of diabetes in R6/2 mice was associated with the progressive formation of ubiquinated inclusions in pancreatic beta cells. Diabetes is dissociated from early motor and cognitive dysfunctions and did not correlate with motor impairment and survival of R6/2 mice. However, chronic behavioural testing (at a level higher than that which is reported to improve several aspects of the R6/2 phenotype) exacerbated the onset of diabetes. Pharmacological treatment of the diabetes was attempted using two oral hypoglycaemic agents commonly used by diabetics. The mice responded acutely to glibenclamide (which induces exocytosis of insulin) but not to rosiglitazone (which induces sensitization to insulin). This supports the suggestion that the diabetes in R6/2 mice is caused by an impairment in insulin release rather than insulin insensitivity. However, chronic treatment with these hypoglycaemic agents had no effect on either the course of the diabetes or the disease in R6/2 mice.

The molecular basis for oxidative stress-induced insulin resistance

Reactive oxygen and nitrogen molecules have been typically viewed as the toxic by-products of metabolism. However, accumulating evidence has revealed that reactive species, including hydrogen peroxide, serve as signaling molecules that are involved in the regulation of cellular function. The chronic and/or increased production of these reactive molecules or a reduced capacity for their elimination, termed oxidative stress, can lead to abnormal changes in intracellular signaling and result in chronic inflammation and insulin resistance. Inflammation and oxidative stress have been linked to insulin resistance in vivo. Recent studies have found that this association is not restricted to insulin resistance in type 2 diabetes, but is also evident in obese, nondiabetic individuals, and in those patients with the metabolic syndrome. An increased concentration of reactive molecules triggers the activation of serine/threonine kinase cascades such as c-Jun N-terminal kinase, nuclear factor-kappaB, and others that in turn phosphorylate multiple targets, including the insulin receptor and the insulin receptor substrate (IRS) proteins. Increased serine phosphorylation of IRS reduces its ability to undergo tyrosine phosphorylation and may accelerate the degradation of IRS-1, offering an attractive explanation for the molecular basis of oxidative stress-induced insulin resistance. Consistent with this idea, studies with antioxidants such as vitamin E, alpha-lipoic acid, and N-acetylcysteine indicate a beneficial impact on insulin sensitivity, and offer the possibility for new treatment approaches for insulin resistance.

Tempol reduces oxidative stress, improves insulin sensitivity, decreases renal dopamine D1 receptor hyperphosphorylation, and restores D1 receptor-G-protein coupling and function in obese Zucker rats

Oxidative stress plays a pathogenic role in hypertension, particularly the one associated with diabetes and obesity. Here, we test the hypothesis that renal dopamine D1 receptor dysfunction in obese Zucker rats is caused by oxidative stress. One group each from lean and obese Zucker rats received tempol, a superoxide dismutase mimetic in drinking water for 2 weeks. Obese animals were hypertensive, hyperglycemic, and hyperinsulinemic, exhibited renal oxidative stress, and increased protein kinase C activity. Also, there was hyperphosphorylation of D1 receptor, defective receptor-G-protein coupling, blunted dopamine-induced Na+-K+-ATPase inhibition, and diminished natriuretic response to D1 receptor agonist, SKF-38393. However, obese animals had elevated levels of plasma nitric oxide and urinary cGMP. In addition, L-N-nitroarginine and sodium nitroprusside showed similar effect on blood pressure in lean and obese rats. In obese animals, tempol reduced blood pressure, blood glucose, insulin, renal oxidative stress, and protein kinase C activity. Tempol also decreased D1 receptor phosphorylation and restored receptor G-protein coupling. Dopamine inhibited Na+-K+-ATPase activity, and SKF-38393 elicited a natriuretic response in tempol-treated obese rats. Thus in obese Zucker rats, tempol ameliorates oxidative stress and improves insulin sensitivity. Consequently, hyperphosphorylation of D1 receptor is reduced, leading to restoration of receptor-G-protein coupling and the natriuretic response to SKF-38393.

Genetic variability in the RAGE gene: Possible implications for nutrigenetics, nutrigenomics, and understanding the susceptibility to diabetic complications

Complex chemical processes called nonenzymatic glycation and glycoxidation are one of the interesting examples of potentially harmful interaction between nutrition and disease. This review summarizes factors influencing the extent of glycoxidation in health and disease and especially focuses on the role of genetic variability in "glycoxidation-related genes" in a disease and diet-related pathogenesis. Possible interaction between genetic variability in relevant loci and dietary advanced glycation end products (AGEs) is considered. As AGEs possess a wide range of chemical and biological effects, the interindividual functional variability in systems dealing with glycoxidation could have a significant nutrigenomic and nutrigenetic consequences.

Utilization of the insulin-signaling network in the metabolic actions of alpha-lipoic acid-reduction or oxidation?

Alpha-lipoic acid is a naturally occurring cofactor of mitochondrial dehydrogenase complexes and a potent antioxidant. It can interchange between a reduced form and an oxidized form, thereby displaying reducing (antioxidant) and prooxidant properties, respectively. It is suggested that alpha-lipoic acid through its prooxidant properties acutely stimulates the insulin-signaling cascade, thereby increasing glucose uptake in muscle and fat cells. On the other hand, alpha-lipoic acid appears to protect the insulin-signaling cascade from oxidative stress-induced insulin resistance through its reducing capacities. In addition, alpha-lipoic acid seems to inhibit hepatic gluconeogenesis by interfering with fatty acid oxidation, as well as to increase peripheral glucose utilization by activating pyruvate dehydrogenase resulting in increased glucose oxidation. These different properties render alpha-lipoic acid a potentially attractive therapeutic agent for the treatment of insulin resistance. Moreover, given the potential role of oxidative stress in the pathogenesis of secondary complications in diabetes, alpha-lipoic acid might be beneficial in the prevention/treatment of these complications as was recently shown for diabetic neuropathy.

High glucose evokes an intrinsic proapoptotic signaling pathway in mesangial cells

BACKGROUND

In response to chronic hyperglycemia, microvascular cells undergo stress and injury, which can lead to cell death. We characterized a proapoptotic signaling pathway whereby high glucose evokes an intrinsic, caspase-9-dependent mechanism of cell death in human mesangial cells.

METHODS

Biochemical (caspase activity, cytochrome-c release, etc.) and morphologic (chromatin condensation and nuclear segmentation) features of apoptotic cell death were assessed in cultured human mesangial cells exposed to high glucose, a risk factor for mesangial cell injury and diabetic glomerulosclerosis. Proapoptotic signaling was also analyzed in the db/db murine model of kidney injury in diabetes.

RESULTS

Incubation in high glucose caused cytotoxicity and apoptosis in mesangial cells. High glucose stimulated mitochondrial release of cytochrome-c, cleavage of procaspase-9, and caspase-9 enzyme activity, suggesting an intrinsic pathway of proapoptotic signaling. In contrast, caspase-8 was unaffected by high glucose. A cell-permeable, caspase-9-selective inhibitor blocked caspase-3 activation and prevented chromatin condensation and nuclear segmentation in cells treated with high glucose. To determine whether an intrinsic signaling pathway occurs in the diabetic kidney in vivo, apoptosis was investigated in diabetic 8- and 16-week db/db murine kidneys. Effector caspases-3 and -7 were activated in diabetic db/db kidneys but not in age-matched nondiabetic db/m controls. At 16 weeks, apoptotic cells in db/db glomeruli were identified on the basis of nuclear segmentation and DNA fragmentation. Apoptosis of glomerular cells correlated with expansion of the mesangial matrix and with worsening of albuminuria. Consistent with an intrinsic signaling pathway, caspase-9 cleavage was elevated only in db/db kidneys, whereas activation of caspase-8 and caspase-12 was undetectable.

CONCLUSION

These findings support the hypothesis that hyperglycemia evokes an intrinsic pathway of proapoptotic signaling in mesangial cells. In addition, these results point to an important role for the intrinsic pathway in microvascular injury in the diabetic kidney in vivo.

Oxidized Low-Density Lipoprotein/β2-Glycoprotein I Complexes and Autoantibodies in Patients with Type 2 Diabetes Mellitus

Diabetes mellitus (DM) is associated with a high incidence of atherosclerotic cardiovascular complications that result from chronic metabolic abnormalities such as hyperglycemia-induced oxidative stress. The oxidative-modification of low-density lipoproteins (oxLDL) and oxLDL/beta(2)-GPI complex formation have been reported in patients with autoimmune disorders. OxLDL/beta(2)-GPI complexes and autoantibodies to these complexes were measured by ELISA in serum samples from 50 type 2 DM patients and 50 age/sex-matched healthy controls. Mean OD for oxLDL/beta(2)-GPI complexes in DM was 0.099 +/- 0.065 with 50% of patients reacting above the assay cutoff (P < 0.001 vs. controls). Mean OD for controls was 0.037 +/- 0.015 with 2% positives. Thirty-six (72%) DM patients were taking cholesterol-lowering statins and had a significantly lower mean OD complex level (0.092 +/- 0.071, P = 0.05) compared to patients not taking statins (0.112 +/- 0.05). Mean OD for IgG anti-oxLDL/beta(2)-GPI antibodies in DM was 0.157 +/- 0.112, similar to the controls (0.146 +/- 0.098, P = 0.328). Increased serum levels of oxLDL/beta(2)-GPI complexes may be a consequence of oxidative stress and LDL modification in DM. Lower levels of oxLDL/beta(2)GPI complexes in DM patients taking statins are in agreement with the antioxidant and antithrombotic properties of these drugs. No significant IgG autoantibody production was observed in this group of DM patients. The interaction of oxLDL with beta(2)-GPI in circulation suggests the intriguing possibility that oxLDL/beta(2)-GPI complexes may also play a role in the development of atherosclerosis and/or cardiovascular complications in DM.

From the periphery of the glomerular capillary wall toward the center of disease: podocyte injury comes of age in diabetic nephropathy

Nephropathy is a major complication of diabetes. Alterations of mesangial cells have traditionally been the focus of research in deciphering molecular mechanisms of diabetic nephropathy. Injury of podocytes, if recognized at all, has been considered a late consequence caused by increasing proteinuria rather than an event inciting diabetic nephropathy. However, recent biopsy studies in humans have provided evidence that podocytes are functionally and structurally injured very early in the natural history of diabetic nephropathy. The diabetic milieu, represented by hyperglycemia, nonenzymatically glycated proteins, and mechanical stress associated with hypertension, causes downregulation of nephrin, an important protein of the slit diaphragm with antiapoptotic signaling properties. The loss of nephrin leads to foot process effacement of podocytes and increased proteinuria. A key mediator of nephrin suppression is angiotensin II (ANG II), which can activate other cytokine pathways such as transforming growth factor-beta (TGF-beta) and vascular endothelial growth factor (VEGF) systems. TGF-beta1 causes an increase in mesangial matrix deposition and glomerular basement membrane (GBM) thickening and may promote podocyte apoptosis or detachment. As a result, the denuded GBM adheres to Bowman's capsule, initiating the development of glomerulosclerosis. VEGF is both produced by and acts upon the podocyte in an autocrine manner to modulate podocyte function, including the synthesis of GBM components. Through its effects on podocyte biology, glomerular hemodynamics, and capillary endothelial permeability, VEGF likely plays an important role in diabetic albuminuria. The mainstays of therapy, glycemic control and inhibition of ANG II, are key measures to prevent early podocyte injury and the subsequent development of diabetic nephropathy.

Metformin decreases intracellular production of reactive oxygen species in aortic endothelial cells

Beyond its antidiabetic activity justifying its use in the treatment of the type 2 diabetes, metformin (MET [dimethylguanidine, Glucophage]) has been shown to exhibit antioxidant properties in vitro, which could contribute to limit the deleterious vascular complications of diabetes. We investigated whether MET, at the pharmacological level of 10 -5 mol/L, was able to modulate intracellular production of reactive oxygen species (ROS) both in quiescent bovine aortic endothelial cells (BAECs) and in BAECs stimulated by a short incubation with high levels of glucose (30 mmol/L, 2 hours) or angiotensin II (10 -7 mol/L, 1 hour). Intracellular ROS production was measured by fluorescence of the DCF (2,7-dichlorodihydrofluorescein) probe. Our results showed that MET was able to reduce the intracellular production of ROS in both nonstimulated BAECs (-20%, P < .05) and BAEC stimulated by high levels of glucose or angiotensin II (-28% and -72%, respectively, P < .01). Experiments performed in the presence of the nicotinamide adenine dinucleotide phosphate [NAD(P)H] oxidase inhibitor apocynin or the respiratory mitochondrial chain inhibitor rotenone indicated that MET exerted its effect partly through an inhibition of the formation of ROS produced mainly by NAD(P)H oxidase and also, to a lesser extent, by the respiratory mitochondrial chain.

Effects of metabolic control and vascular complications on indices of oxidative stress in type 2 diabetic patients

The direct effect and the interaction of diabetic angiopathy and metabolic control on free radical and antioxidant activity indices was investigated in 48 patients with type 2 diabetes mellitus. Conjugated dienes (CD) and thiobarbituric acid-reacting substances (TBARS) levels were 34 and 178% of control values, respectively. An approximate two-fold decrease in plasma thiols (PSH) and erythrocyte lysate thiols (LSH) concentrations, parameters reflecting protein oxidative damage, was found. Impairment of blood antioxidant potential in diabetic patients was reflected by an 81% increase in superoxide dismutase (SOD) activity, a 30% decrease in catalase (CT), 20% decrease in glutathione peroxidase (GPx) and glutathione reductase (GR) activities as well as by lowered total antioxidant status (TAS). CD, TBARS and SOD values were positively correlated with plasma glucose concentration and glycated hemoglobin level. A negative correlation existed between levels of LSH, PSH, CT, GPx or TAS and both glucose and HbA(1c). Blood glucose control and vascular complications had strong independent effects on prooxidant-antioxidant status, apart from blood glucose and GR activity. In addition, glycemic control and diabetic vasculopathy interact in their influence on most of the free radical and antioxidant indices, except for CD, LSH levels and CT activity. Thus, we observed different mechanisms by which vascular complications and glucose control affect blood free radical indices and antioxidant status parameters in type 2 diabetic patients.

Liver alanine aminotransferase, insulin resistance and endothelial dysfunction in normotriglyceridaemic subjects with type 2 diabetes mellitus

BACKGROUND

Plasma levels of liver transaminases, including alanine aminotransferase (ALT), are elevated in most cases of nonalcoholic fatty liver disease (NAFLD). Elevated ALT levels are associated with insulin resistance, and subjects with NAFLD have features of the metabolic syndrome that confer high-risk cardiovascular disease. Alanine aminotransferase predicts the development of type 2 diabetes (DM2) in subjects with the metabolic syndrome. However, the role of elevated ALT levels in subjects with overt DM2 has yet not been explored.

MATERIALS AND METHODS

In a cross-sectional study, 64 normotriglyceridaemic subjects with DM2 were studied with regard to the relation between liver transaminases with whole-body insulin sensitivity, measured with the euglycaemic hyperinsulinaemic clamp and with brachial artery flow-mediated dilation (FMD) as a marker of endothelial dysfunction.

RESULTS

On average, patients were normotriglyceridaemic (plasma triglycerides 1.3 +/- 0.4 mmol L-1) and had good glycaemic control (HbA1c 6.2 +/- 0.8%). The mean ALT level was 15.0 +/- 7.5 U L-1, and the mean aspartate aminotransferase concentration equalled 10.6 +/- 2.6 U L-1. Alanine aminotransferase levels were negatively associated with whole-body insulin sensitivity as well as with FMD (both P = 0.03, in multivariate analyses; regression coefficients beta [95%CI]: -0.76 [-1.4 to -0.08] and -0.31 [-0.58 to -0.03] respectively).

CONCLUSIONS

In metabolically well-controlled patients with DM2, ALT levels are related to decreased insulin-sensitivity and an impaired conduit vessel vascular function.

Insulin protects against amyloid beta-peptide toxicity in brain mitochondria of diabetic rats

This study compared the status of brain mitochondria isolated from 12-week streptozotocin (STZ)-diabetic rats versus STZ-diabetic animals treated with insulin during a period of 4 weeks. Brain mitochondria isolated from 12-week citrate (vehicle)-treated rats were used as control. For that purpose, several mitochondrial parameters were evaluated: respiratory indexes (respiratory control ratio (RCR) and ADP/O ratio), transmembrane potential (DeltaPsim), repolarization lag phase, repolarization level, ATP, glutathione and coenzyme Q (CoQ) contents, production of H2O2, ATPase activity, and the capacity of mitochondria to accumulate Ca2+. Furthermore, the effect of Abeta1-40 was also analyzed. We observed that STZ-induced diabetes promoted a significant decrease in mitochondrial CoQ9, ATPase activity, and a lower capacity of mitochondria to accumulate Ca2+ when compared with control and insulin-treated diabetic rats. The presence of 4 microM Abeta1-40 induced a significant decrease in RCR in the three groups of rats. However, this peptide induced a significant increase in the repolarization lag phase and a significant decrease in the repolarization level in control and diabetic animals without insulin treatment. Furthermore, this peptide exacerbated significantly the production of H2O2 in STZ-diabetic rats, this effect being avoided by insulin treatment. Our data show that although diabetes induces some alterations in brain mitochondrial activity, those alterations do not interfere significantly with mitochondria functional efficiency. Similarly, insulin does not affect basal mitochondria function. However, in the presence of amyloid beta-peptide, insulin seems to prevent the decline in mitochondrial oxidative phosphorylation efficiency and avoids an increase in oxidative stress, improving or preserving the function of neurons under adverse conditions, such as Alzheimer's disease.

Agonist-induced activation releases peroxisome proliferator-activated receptor β/δ from its inhibition by palmitate-induced nuclear factor-κB in skeletal muscle cells

The mechanisms by which elevated levels of free fatty acids cause insulin resistance are not well understood, but there is a strong correlation between insulin resistance and intramyocellular lipid accumulation in skeletal muscle. In addition, accumulating evidence suggests a link between inflammation and type 2 diabetes. The aim of this work was to study whether the exposure of skeletal muscle cells to palmitate affected peroxisome proliferator-activated receptor (PPAR) beta/delta activity. Here, we report that exposure of C2C12 skeletal muscle cells to 0.75 mM palmitate reduced (74%, P<0.01) the mRNA levels of the PPARbeta/delta-target gene pyruvatedehydrogenase kinase 4 (PDK-4), which is involved in fatty acid utilization. This reduction was not observed in the presence of the PPARbeta/delta agonist L-165041. This drug prevented palmitate-induced nuclear factor (NF)-kappaB activation. Increased NF-kappaB activity after palmitate exposure was associated with enhanced protein-protein interaction between PPARbeta/delta and p65. Interestingly, treatment with the PPARbeta/delta agonist L-165041 completely abolished this interaction. These results indicate that palmitate may reduce fatty acid utilization in skeletal muscle cells by reducing PPARbeta/delta signaling through increased NF-kappaB activity.

Oxidative stress and the use of antioxidants in diabetes: linking basic science to clinical practice

Cardiovascular complications, characterized by endothelial dysfunction and accelerated atherosclerosis, are the leading cause of morbidity and mortality associated with diabetes. There is growing evidence that excess generation of highly reactive free radicals, largely due to hyperglycemia, causes oxidative stress, which further exacerbates the development and progression of diabetes and its complications. Overproduction and/or insufficient removal of these free radicals result in vascular dysfunction, damage to cellular proteins, membrane lipids and nucleic acids. Despite overwhelming evidence on the damaging consequences of oxidative stress and its role in experimental diabetes, large scale clinical trials with classic antioxidants failed to demonstrate any benefit for diabetic patients. As our understanding of the mechanisms of free radical generation evolves, it is becoming clear that rather than merely scavenging reactive radicals, a more comprehensive approach aimed at preventing the generation of these reactive species as well as scavenging may prove more beneficial. Therefore, new strategies with classic as well as new antioxidants should be implemented in the treatment of diabetes.

Tempol therapy attenuates medial smooth muscle cell apoptosis and neointima formation after balloon catheter injury in carotid artery of diabetic rats

Accumulating data support the hypothesis that reactive oxygen species (ROS) play a critical role in the vascular complications observed in diabetes. However, the mechanisms of ROS-mediated vascular complications in diabetes are not clear. We tested the hypothesis that ROS-mediated increase in proapoptotic factor Bax expression leads to medial smooth muscle cell (SMC) apoptosis that is associated with neointima formation. We used a fructose-rich diet for 4 wk to model Type 2 diabetes in rats. SOD mimetic membrane-permeable 4-hydroxy-2,2,6,6,-tetramethylpiperidine-1-oxyl (Tempol, 1 mM) was administered in drinking water to scavenge superoxide starting 1 day before surgery and continued during the duration of the experiment. Vascular injury resulted in a significant increase in medial SMC apoptosis that was associated with neointima formation. The number of medial SMC positive for Bax immunostaining significantly increased in injured arteries compared with uninjured arteries. Superoxide scavenging by Tempol treatment inhibited both the Bax-positive index as well as the apoptotic index of medial SMC in response to vascular injury. Tempol treatment inhibited apoptotic loss of medial SMC, thus increasing their density in the injured arteries. These alterations in the media were associated with a marked decrease in neointima formation in injured arteries. We conclude that Bax expression may play an important role in vascular SMC apoptosis and, finally, that this regulatory mechanism is redox sensitive.

Glucose-dependent insulinotropic polypeptide (GIP) stimulation of pancreatic beta-cell survival is dependent upon phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB) signaling, inactivation of the forkhead transcription factor Foxo1, and down-regulation of bax expression

The hormone glucose-dependent insulinotropic polypeptide (GIP) potently stimulates insulin secretion and promotes beta-cell proliferation and cell survival. In the present study we identified Forkhead (Foxo1)-mediated suppression of the bax gene as a critical component of the effects of GIP on cell survival. Treatment of INS-1(832/13) beta-cells with GIP resulted in concentration-dependent activation of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB)/Foxo1 signaling module. In parallel studies, GIP decreased bax promoter activity. Serial deletion analysis of the bax promoter demonstrated that the region -682 to -320, containing FHRE-II (5AAAACAAACA), was responsible for GIP-mediated effects. Foxo1 bound to FHRE-II in gel mobility shift assays, and Foxo1-FHRE-II interactions conferred GIP responsiveness to the bax promoter. INS-1 cells incubated under proapoptotic and glucolipotoxic conditions demonstrated increased nuclear localization of Foxo1 and bax promoter activity and decreased cytoplasmic phospho-PKB/Foxo1. GIP partially restored expression PKB/Foxo1 and bax promoter activity. Similar protective effects were found with dispersed islet cells from C57BL/6 mice, but not with those from GIP receptor knock-out (GIPR(-/-)) mice. GIP treatment reduced glucolipotoxicity-induced cell death in C57 BL/6 and Bax(-/-) islets, but not GIPR(-/-) mouse islets. Chronic treatment of Vancouver diabetic fatty Zucker rats with GIP resulted in down-regulation of Bax and up-regulation of Bcl-2 in pancreatic beta-cells. The results show that PI3K/PKB/Foxo1 signaling mediates GIP suppression of bax gene expression and that this module is a key pathway by which GIP regulates beta-cell apoptosis in vivo.

Anti-oxidative effect of fluvastatin in hyperlipidemic type 2 diabetic patients

An open-label prospective cross-over trial was performed to evaluate the antioxidative effect of fluvastatin in Japanese type 2 diabetics with hyperlipidemia. The study subjects were 10 patients who were on pravastatin (10 mg/day) or simvastatin (5 mg/day). After at least 12 weeks of continuous pravastatin or simvastatin therapy, the drugs were washed out for 12 weeks and replaced with fluvastatin (30 mg/day), then the treatment was continued for another 12 weeks. Total cholesterol and LDL cholesterol were efficiently and comparably reduced by all three statin agents. There were no differences in serum parameters of oxidative stress such as malondialdehyde-modified low-density lipoprotein, thiobarbituric acid-reactive substances, and 8-iso-prostaglandin F2alpha between pravastatin/simvastatin and fluvastatin. However, fluvastatin, but not pravastatin/simvastatin, significantly reduced 3,5,7-cholestatriene in erythrocyte membrane, representing the extent of membrane cholesterol peroxidation. Our data demonstrated that fluvastatin has a unique anti-oxidative effect in patients with type 2 diabetes and hyperlipidemia, compared with other statins.

Glucose suppresses superoxide generation in metabolically responsive pancreatic beta cells

High rates of glucose metabolism and mitochondrial electron transport have been associated with increased mitochondrial production of reactive oxygen species (ROS). This mechanism was also proposed as a possible cause for dysfunction and death of pancreatic beta cells exposed to high glucose levels. We examined whether high rates of glucose metabolism increase ROS production in purified rat beta cells. Glucose up to 20 mm did not stimulate H(2)O(2) or superoxide production, whereas it dose-dependently increased cellular NAD(P)H and FADH(2) levels with an EC(50) around 8 mm. On the contrary, glucose concentration-dependently suppressed H(2)O(2) and superoxide formation, with a major effect between 0 and 5 mm, parallel to an increase in cellular NAD(P)H levels. This suppressive effect was more marked in beta cells with higher NAD(P)H responsiveness to glucose; it was not observed in glucagon-containing alpha cells, which lacked a glucose-induced increase in NAD(P)H. Suppression was also induced by the mitochondrial substrates leucine and succinate. Experiments with electron transport chain inhibitors indicate a role of respiratory complex I in ROS production at low mitochondrial activity and low NADH levels. Superoxide production at low glucose is potentially cytotoxic, because scavenging by the superoxide dismutase mimetic agent manganese(III)tetrakis(4-benzoic acid)porphyrin was found to reduce the rate of beta cell apoptosis. Analysis of islets cultured at 20 mm glucose confirmed that this condition does not induce ROS production in beta cells as a result of their increased rates of glucose metabolism. Our study indicates the need of beta cells for basal nutrients maintaining mitochondrial NADH production at levels that suppress ROS accumulation from an inadequate respiratory complex I activity and thus inhibit a potential apoptotic pathway.