Insulin-stimulated hydrogen peroxide reversibly inhibits protein-tyrosine phosphatase 1b in vivo and enhances the early insulin action cascade

The role of peroxidases in the pathogenesis of atherosclerosis

Reactive oxygen species (ROS), which include superoxide anions and peroxides, induce oxidative stress, contributing to the initiation and progression of cardiovascular diseases involving atherosclerosis. The endogenous and exogenous factors hypercholesterolemia, hyperglycemia, hypertension, and shear stress induce various enzyme systems such as nicotinamide adenine dinucleotide (phosphate) oxidase, xanthine oxidase, and lipoxygenase in vascular and immune cells, which generate ROS. Besides inducing oxidative stress, ROS mediate signaling pathways involved in monocyte adhesion and infiltration, platelet activation, and smooth muscle cell migration. A number of antioxidant enzymes (e.g., superoxide dismutases, catalase, glutathione peroxidases, and peroxiredoxins) regulate ROS in vascular and immune cells. Atherosclerosis results from a local imbalance between ROS production and these antioxidant enzymes. In this review, we will discuss 1) oxidative stress and atherosclerosis, 2) ROS-dependent atherogenic signaling in endothelial cells, macrophages, and vascular smooth muscle cells, 3) roles of peroxidases in atherosclerosis, and 4) antioxidant drugs and therapeutic perspectives.

Conformation-sensing antibodies stabilize the oxidized form of PTP1B and inhibit its phosphatase activity

Protein tyrosine phosphatase 1B (PTP1B) plays important roles in downregulation of insulin and leptin signaling and is an established therapeutic target for diabetes and obesity. PTP1B is regulated by reactive oxygen species (ROS) produced in response to various stimuli, including insulin. The reversibly oxidized form of the enzyme (PTP1B-OX) is inactive and undergoes profound conformational changes at the active site. We generated conformation-sensor antibodies, in the form of single-chain variable fragments (scFvs), that stabilize PTP1B-OX and thereby inhibit its phosphatase function. Expression of conformation-sensor scFvs as intracellular antibodies (intrabodies) enhanced insulin-induced tyrosyl phosphorylation of the β subunit of the insulin receptor and its substrate IRS-1 and increased insulin-induced phosphorylation of PKB/AKT. Our data suggest that stabilization of the oxidized, inactive form of PTP1B with appropriate therapeutic molecules may offer a paradigm for phosphatase drug development.

The biological buffer bicarbonate/CO2 potentiates H2O2-mediated inactivation of protein tyrosine phosphatases

Hydrogen peroxide is a cell signaling agent that inactivates protein tyrosine phosphatases (PTPs) via oxidation of their catalytic cysteine residue. PTPs are inactivated rapidly during H(2)O(2)-mediated cellular signal transduction processes, but, paradoxically, hydrogen peroxide is a rather sluggish PTP inactivator in vitro. Here we present evidence that the biological buffer bicarbonate/CO(2) potentiates the ability of H(2)O(2) to inactivate PTPs. The results of biochemical experiments and high-resolution crystallographic analysis are consistent with a mechanism involving oxidation of the catalytic cysteine residue by peroxymonocarbonate generated via the reaction of H(2)O(2) with HCO(3)(-)/CO(2).

Regulation of protein tyrosine phosphatases by reversible oxidation

Oxidation of the catalytic cysteine of protein-tyrosine phosphatases (PTP), which leads to their reversible inactivation, has emerged as an important regulatory mechanism linking cellular tyrosine phosphorylation and signalling by reactive-oxygen or -nitrogen species (ROS, RNS). This review focuses on recent findings about the involved pathways, enzymes and biochemical mechanisms. Both the general cellular redox state and extracellular ligand-stimulated ROS production can cause PTP oxidation. Members of the PTP family differ in their intrinsic susceptibility to oxidation, and different types of oxidative modification of the PTP catalytic cysteine can occur. The role of PTP oxidation for physiological signalling processes as well as in different pathologies is described on the basis of well-investigated examples. Criteria to establish the causal involvement of PTP oxidation in a given process are proposed. A better understanding of mechanisms leading to selective PTP oxidation in a cellular context, and finding ways to pharmacologically modulate these pathways are important topics for future research.

SSAO substrates exhibiting insulin-like effects in adipocytes as a promising treatment option for metabolic disorders

BACKGROUND

Benzylamine exerts insulin-like effects in adipocytes (e.g., glucose uptake and antilipolysis) and improves glucose handling in rodents.

RESULTS

In murine adipocytes, benzylamine mimics another insulin action: it enhances apelin expression in a manner that is blocked by the semicarbazide-sensitive amine oxidase/vascular adhesion protein-1 (SSAO/VAP-1) inhibitor semicarbazide. It is shown that in human adipocytes, benzylamine activates glucose transport, but its effects are not additive to maximal insulin stimulation. Benzylamine effects are hydrogen peroxide dependent. They can be reproduced by novel substrates, but not by benzaldehyde.

CONCLUSION

Owing to the parallelism between the in vitro insulin mimicry and the in vivo improvement of glucose handling elicited by benzylamine in rodents, the SSAO/VAP-1 substrates, with stronger effects on human adipocytes than benzylamine, show promising applications for the treatment of insulin resistance.

Mitochondria and redox signaling in steatohepatitis

Alcoholic and nonalcoholic fatty liver diseases are potentially pathological conditions that can progress to steatohepatitis, fibrosis, and cirrhosis. These conditions affect millions of people throughout the world in part through poor lifestyle choices of excess alcohol consumption, overnutrition, and lack of regular physical activity. Abnormal mitochondrial and cellular redox homeostasis has been documented in steatohepatitis and results in alterations of multiple redox-sensitive signaling cascades. Ultimately, these changes in signaling lead to altered enzyme function and transcriptional activities of proteins critical to mitochondrial and cellular function. In this article, we review the current hypotheses linking mitochondrial redox state to the overall pathophysiology of alcoholic and nonalcoholic steatohepatitis and briefly discuss the current therapeutic options under investigation.

Redox regulation of protein tyrosine phosphatases: structural and chemical aspects

Protein tyrosine phosphatases (PTPs) are important targets of the H(2)O(2) that is produced during mammalian signal transduction. H(2)O(2)-mediated inactivation of PTPs also may be important in various pathophysiological conditions involving oxidative stress. Here we review the chemical and structural biology of redox-regulated PTPs. Reactions of H(2)O(2) with PTPs convert the catalytic cysteine thiol to a sulfenic acid. In PTPs, the initially generated sulfenic acid residues have the potential to undergo secondary reactions with a neighboring amide nitrogen or cysteine thiol residue to yield a sulfenyl amide or disulfide, respectively. The chemical mechanisms by which formation of sulfenyl amide and disulfide linkages can protect the catalytic cysteine residue against irreversible overoxidation to sulfinic and sulfonic oxidation states are described. Due to the propensity for back-door and distal cysteine residues to engage with the active-site cysteine after oxidative inactivation, differences in the structures of the oxidatively inactivated PTPs may stem, to a large degree, from differences in the number and location of cysteine residues surrounding the active site of the enzymes. PTPs with key cysteine residues in structurally similar locations may be expected to share similar mechanisms of oxidative inactivation.

Assessment of redox changes to hydrogen peroxide-sensitive proteins during EGF signaling

Hydrogen peroxide acts as a second messenger in growth factor signaling where it can oxidize and modify the function of redox-sensitive proteins. While selective thiol oxidation has been measured, there has been no global assessment of protein oxidation following growth factor activation. Significant changes to the abundant and widely distributed redox sensitive thiol proteins were observed in A431 epidermoid carcinoma cells exposed to hydrogen peroxide, but no changes were observed following treatment with epidermal growth factor (EGF). This included members of the peroxiredoxin family, which were also monitored in the presence of the thioredoxin reductase inhibitor auranofin to limit their capacity to recycle to the reduced form. We conclude that widespread thiol oxidation does not occur in cells during EGF signaling, and that hydrogen peroxide must act in a highly localized or selective manner.

The peroxide dilemma: opposing and mediating insulin action

Recent compelling data show that reactive oxygen species (ROS) not only are a harmful by-product of aerobic metabolism, but also are used as signaling molecules to regulate various cellular processes. In mammalian cells, ROS are produced transiently in response to many extracellular stimuli, including insulin, and specific inhibition of the ROS suppresses insulin-dependent signaling. Initially, this finding rationalized the concept of ROS acting as insulin mimetics. However, it is becoming evident that ROS are also causal to diabetes, a metabolic disorder characterized by insufficiency of secretion of, or receptor insensitivity to, endogenous insulin. This notion underlines a dual role for ROS in insulin signaling as both deleterious and beneficiary. Moreover, it strongly suggests that a delicate redox balance is required for insulin signaling to remain "healthy" for an organism.

Increased serum IGF-1 levels protect the musculoskeletal system but are associated with elevated oxidative stress markers and increased mortality independent of tissue igf1 gene expression

Although the literature suggests a protective (anabolic) effect of insulin-like growth factor-1 (IGF-1) on the musculoskeletal system during growth and aging, there is evidence that reductions in IGF-1 signaling are advantageous for promoting an increase in life span through reduction in oxidative stress-induced tissue damage. To better understand this paradox, we utilized the hepatocyte-specific IGF-1 transgenic (HIT) mice, which exhibit 3-fold increases in serum IGF-1, with normal IGF-1 expression in other tissues, and mice with an IGF-1 null background that exclusively express IGF-1 in the liver, which thereby deliver IGF-1 by the endocrine route only (KO-HIT mice). We found that in the total absence of tissue igf1 gene expression (KO-HIT), increases in serum IGF-1 levels were associated with increased levels of lipid peroxidation products in serum and increased mortality rate at 18 months of age in both genders. Surprisingly, however, we found that in female mice, tissue IGF-1 plays an important role in preserving trabecular bone architecture as KO-HIT mice show bone loss in the femoral distal metaphysis. Additionally, in male KO-HIT mice, increases in serum IGF-1 levels were insufficient to protect against age-related muscle loss.

Enteric commensal bacteria potentiate epithelial restitution via reactive oxygen species-mediated inactivation of focal adhesion kinase phosphatases

The mechanisms by which enteric commensal microbiota influence maturation and repair of the epithelial barrier are relatively unknown. Epithelial restitution requires active cell migration, a process dependent on dynamic turnover of focal cell-matrix adhesions (FAs). Here, we demonstrate that natural, commensal bacteria stimulate generation of reactive oxygen species (ROS) in intestinal epithelia. Bacteria-mediated ROS generation induces oxidation of target cysteines in the redox-sensitive tyrosine phosphatases, LMW-PTP and SHP-2, which in turn results in increased phosphorylation of focal adhesion kinase (FAK), a key protein regulating the turnover of FAs. Accordingly, phosphorylation of FAK substrate proteins, focal adhesion formation, and cell migration are all significantly enhanced by bacterial contact in both in vitro and in vivo models of wound closure. These results suggest that commensal bacteria regulate cell migration via induced generation of ROS in epithelial cells.

Native low-density lipoprotein-dependent interleukin-8 production through pertussis toxin-sensitive G-protein coupled receptors and hydrogen peroxide generation contributes to migration of human aortic smooth muscle cells

PURPOSE

Stimulation of human aortic smooth muscle cells (hAoSMCs) with native low-density lipoprotein (nLDL) induced the production of interleukin-8 (IL-8) that is involved in the pathogenesis of cardiovascular diseases. However, the process of signal transduction of nLDL was currently uncharacterized. Therefore, the aim of this study was to investigate the signal transduction pathway of nLDL-dependent IL-8 production and the effect of IL-8 on hAoSMCs migration.

MATERIALS AND METHODS

nLDL was prepared by ultracentrifugation with density-adjusted human serum of normocholesterolemia. In hAoSMCs, IL-8 secreted to medium was measured using ELISA assay, and Western blot analysis was performed to detect p38 MAPK activation as a key regulator of IL-8 production. nLDL-dependent H₂O₂ generation was determined by microscopic analysis using 2',7'-dichlorofluoroscein diacetate (DCF-DA). IL-8-induced migration of hAoSMCs was evaluated by counting the cell numbers moved to lower chamber using Transwell plates.

RESULTS

nLDL-induced IL-8 production was completely blocked by preincubation of hAoSMCs with pertussis toxin (PTX), which inhibited nLDL-dependent p38 MAPK phosphorylation. PTX-sensitive G-protein coupled receptor was responsible for nLDL-dependent H₂O₂ generation that was abrogated with preincubation of the cells with of polyethylene glycol-conjugated catalase (PEG-Cat). Pretreatment of PEG-Cat prevented nLDL-induced p38 MAPK phosphorylation and IL-8 production, which was partly mimicked by treatment with exogenous H₂O₂2. Finally, IL-8 increased hAoSMCs migration that was completely blocked by incubation with IL-8 neutralizing antibody.

CONCLUSION

PTX-sensitive G-protein coupled receptor-dependent H₂O₂ generation by nLDL plays a critical role in IL-8 production in hAoSMC, and IL-8 may contribute to atherogenesis through increased migration of hAoSMCs.

Reactive oxygen species: are they important for haematopoiesis?

The production of reactive oxygen species (ROS) has traditionally been related to deleterious effects for cells. However, it is now widely accepted that ROS can play an important role in regulating cellular signalling and gene expression. NADPH oxidase ROS production seems to be especially important in this regard. Some lines of evidence suggest that ROS may be important modulators of cell differentiation, including haematopoietic differentiation, in both physiologic and pathologic conditions. Here we shall review how ROS can regulate cell signalling and gene expression. We shall also focus on the importance of ROS for haematopoietic stem cell (HSC) biology and for haematopoietic differentiation. We shall review the involvement of ROS and NADPH oxidases in cancer, and in particular what is known about the relationship between ROS and haematological malignancies. Finally, we shall discuss the use of ROS as cancer therapeutic targets.

Delaying of insulin signal transduction in skeletal muscle cells by selenium compounds

Supranutritional selenium (Se) intake and high serum Se levels have been associated epidemiologically with increased risk for type 2 diabetes, suggesting adverse effects of dietary Se compounds and/or antioxidant selenoenzymes on the sensitivity of target tissues for insulin. Here, we compared the capability of inorganic (sodium selenite and sodium selenate) and organic (selenomethionine and methylseleninic acid (MSeA)) Se compounds to interfere with insulin signaling in rat L6 myotubes, differentiated skeletal muscle cells. When applied at doses of 1 μM, only selenite and MSeA were capable of delaying insulin-induced phosphorylation of protein kinase B (Akt) and attenuating insulin-induced phosphorylation of forkhead box class O transcription factors FoxO1a and FoxO3. Insulin-stimulated glucose uptake was lowered by selenite and MSeA as well. Even though all tested Se compounds strongly stimulated expression/activity of the cellular selenoproteins glutathione peroxidase 1 and selenoprotein W, selenite and MSeA were the most efficiently utilized Se donors. Moreover, at doses of 1 μM, only selenite and MSeA had a significant inhibitory effect on generation of intracellular reactive oxygen species (ROS). These results suggest that the Se(IV) compounds selenite and MSeA may impair the insulin sensitivity of myocytes by influencing cellular redox homeostasis.

Reactive oxygen species play an essential role in IGF-I signaling and IGF-I-induced myocyte hypertrophy in C2C12 myocytes

IGF-I induces skeletal muscle hypertrophy by stimulating protein synthesis and suppressing the protein degradation pathway; the downstream signaling pathways Akt-mammalian target of rapamycin (mTOR)-p70-kDA-S6-kinase (p70S6K), and Forkhead box O1 (FoxO1) play essential roles in this regulation. Reactive oxygen species (ROS) modulate the signaling of various growth factors via redox regulation. However, the role of ROS in IGF-I signaling is not fully understood. In this study, we investigated whether ROS regulate the signaling and biological action of IGF-I in C2C12 myocytes. We found that IGF-I induces ROS in C2C12 myocytes. While treatment with H(2)O(2) significantly enhanced IGF-I-induced phosphorylation of the IGF-I receptor (IGF-IR), IGF-IR phosphorylation was markedly attenuated when cells were treated with antioxidants. The downstream signaling pathway, Akt-mTOR-p70S6K was subsequently down-regulated. Furthermore, the phosphorylation of FoxO1 by IGF-I decreased concomitantly with the restoration of the expression of its target genes, Atrogin-1 and muscle RING finger 1, which are related to muscle atrophy. Nox4 knockdown, which is reportedly to produce ROS in insulin signaling, attenuated IGF-I-induced IGF-IR phosphorylation, indicating that Nox4 is involved in the regulation of IGF-I signaling. Importantly, antioxidant treatments inhibited IGF-I-induced myocyte hypertrophy, demonstrating that ROS are necessary for IGF-I-induced myocyte hypertrophy in vitro. These results indicate that ROS play an essential role in the signaling and biological action of IGF-I in C2C12 myocytes.

Zinc and cardiovascular disease

Zinc is a vital element in maintaining the normal structure and physiology of cells. The fact that it has an important role in states of cardiovascular diseases has been studied and described by several research groups. It appears to have protective effects in coronary artery disease and cardiomyopathy. Intracellular zinc plays a critical role in the redox signaling pathway, whereby certain triggers such as ischemia and infarction lead to release of zinc from proteins and cause myocardial damage. In such states, replenishing with zinc has been shown to improve cardiac function and prevent further damage. Thus, the area of zinc homeostasis is emerging in cardiovascular disease research. The goal of this report is to review the current knowledge and suggest further avenues of research.

Inhibition of RhoA GTPase and the subsequent activation of PTP1B protects cultured hippocampal neurons against amyloid β toxicity

Background

Amyloid beta (Aβ) is the main agent responsible for the advent and progression of Alzheimer's disease. This peptide can at least partially antagonize nerve growth factor (NGF) signalling in neurons, which may be responsible for some of the effects produced by Aβ. Accordingly, better understanding the NGF signalling pathway may provide clues as to how to protect neurons from the toxic effects of Aβ.

Results

We show here that Aβ activates the RhoA GTPase by binding to p75^NTR, thereby preventing the NGF-induced activation of protein tyrosine phosphatase 1B (PTP1B) that is required for neuron survival. We also show that the inactivation of RhoA GTPase and the activation of PTP1B protect cultured hippocampal neurons against the noxious effects of Aβ. Indeed, either pharmacological inhibition of RhoA with C3 ADP ribosyl transferase or the transfection of cultured neurons with a dominant negative form of RhoA protects cultured hippocampal neurons from the effects of Aβ. In addition, over-expression of PTP1B also prevents the deleterious effects of Aβ on cultured hippocampal neurons.

Conclusion

Our findings indicate that potentiating the activity of NGF at the level of RhoA inactivation and PTP1B activation may represent a new means to combat the noxious effects of Aβ in Alzheimer's disease.

Knockouts of SOD1 and GPX1 exert different impacts on murine islet function and pancreatic integrity

Metabolic subtlety and clinical relevance of different forms of reactive oxygen species in diabetes remain unclear. Using single knockout of Cu,Zn-superoxide dismutase (SOD1(-/-)) or Se-glutathione peroxidase-1 (GPX1(-/-)) and their double-knockout (DKO) mouse models, we determined if elevating endogenously-derived superoxide and hydroperoxide exerted distinct impacts and mechanisms on body glucose homeostasis. Whereas the three knockout groups displayed decreased plasma insulin concentrations and islet β-cells mass, only SOD1(-/-) showed decreased body weight, increased blood glucose, and blocked glucose-stimulated insulin secretion. Null of SOD1 and GPX1 elevated respective islet superoxide and hydroperoxide production, and upregulated p53 phosphorylation. Knockout of SOD1 downregulated the foxhead box A2/pancreatic and duodenal homeobox 1 pathway in a superoxide-dependent fashion at epigenetic, mRNA, and protein levels in islets, but improved insulin signaling in liver and muscle. The SOD1(-/-) mice showed more apparent pancreatitis than the GPX1(-/-) mice that were more susceptible to the cerulein-induced amylase increase. Knockout of SOD1 impaired islet function, pancreas integrity, and body glucose homeostasis more than that of GPX1. Simultaneous ablation of both enzymes did not result in additive or aggravated metabolic outcomes.

First in class, potent, and orally bioavailable NADPH oxidase isoform 4 (Nox4) inhibitors for the treatment of idiopathic pulmonary fibrosis

We describe the design, synthesis, and optimization of first-in-class series of inhibitors of NADPH oxidase isoform 4 (Nox4), an enzyme implicated in several pathologies, in particular idiopathic pulmonary fibrosis, a life-threatening and orphan disease. Initially, several moderately potent pyrazolopyridine dione derivatives were found during a high-throughput screening campaign. SAR investigation around the pyrazolopyridine dione core led to the discovery of several double-digit nanomolar inhibitors in cell free assays of reactive oxygen species (ROS) production, showing high potency on Nox4 and Nox1. The compounds have little affinity for Nox2 isoform and are selective for Nox4/1 isoforms. The specificity of these compounds was confirmed in an extensive in vitro pharmacological profile, as well as in a counterscreening assay for potential ROS scavenging. Concomitant benefits are good oral bioavailability and high plasma concentrations in vivo, allowing further clinical trials for the potential treatment of fibrotic diseases, cancers, and cardiovascular and metabolic diseases.

High selenium intake and increased diabetes risk: experimental evidence for interplay between selenium and carbohydrate metabolism

The essential trace element selenium has long been considered to exhibit anti-diabetic and insulin-mimetic properties, but recent epidemiological studies indicated supranutritional selenium intake and high plasma selenium levels as possible risk factors for development of type 2 diabetes, pointing to adverse effects of selenium on carbohydrate metabolism in humans. However, increased plasma selenium levels might be both a consequence and a cause of diabetes. We summarize current evidence for an interference of selenium compounds with insulin-regulated molecular pathways, most notably the phosphoinositide-3-kinase/protein kinase B signaling cascade, which may underlie some of the pro- and anti-diabetic actions of selenium. Furthermore, we discuss reports of hyperinsulinemia, hyperglycemia and insulin resistance in mice overexpressing the selenoenzyme glutathione peroxidase 1. The peroxisomal proliferator-activated receptor gamma coactivator 1α represents a key regulator for biosynthesis of the physiological selenium transporter, selenoprotein P, as well as for hepatic gluconeogenesis. As proliferator-activated receptor gamma coactivator 1α has been shown to be up-regulated in livers of diabetic animals and to promote insulin resistance, we hypothesize that dysregulated pathways in carbohydrate metabolism and a disturbance of selenium homeostasis are linked via proliferator-activated receptor gamma coactivator 1α.

Chronic ethanol consumption up-regulates protein-tyrosine phosphatase-1B (PTP1B) expression in rat skeletal muscle

AIM

to investigate the potential effects of chronic ethanol intake on protein-tyrosine phosphatase-1B (PTP1B) and the insulin receptor signaling pathway in rat skeletal muscle.

METHODS

rats received ethanol treatment at a daily dose of 0 (control), 0.5 (group L), 2.5 (group M) or 5 gxkg(-1) (group H) via gastric gavage for 22 weeks. In vivo insulin sensitivity was measured using a hyperinsulinemic-euglycemic clamp. Expression of PTP1B in skeletal muscles was examined at both the mRNA (real-time PCR) and protein (Western blot) levels. PTP1B activity was assayed with a p-nitrophenol phosphate (PNPP) hydrolysis method. Changes of insulin signaling in skeletal muscle were analyzed with Western blotting.

RESULTS

the activity and expression of PTP1B were dose-dependently elevated 1.6 and 2.0 fold in the skeletal muscle by ethanol, resepctively, at the doses of 2.5 and 5 gxkg(-1)xd(-1). Total IRβ and IRS-1, as well as their phosphorylated forms, were decreased by ethanol at the two higher doses. Moreover, chronic ethanol consumption resulted in a significant inhibition of the association between IRS-1 and the p85 subunit of phosphatidylinositol 3-kinase, inhibition of Akt phosphorylation and reduced levels of mitogen-activated protein kinase phosphorylation.

CONCLUSION

chronic ethanol intake at 2.5 and 5 xkg(-1)xd(-1) sufficient doses can down-regulate the expression of IRβ, P-IRβ, and IRS-1, as well as the phosphorylated forms of IRS-1 and Akt, in rat skeletal muscle, possibly through increased PTP1B activity.

Clinical review: oxygen as a signaling molecule

Molecular oxygen is obviously essential for conserving energy in a form useable for aerobic life; however, its utilization comes at a cost--the production of reactive oxygen species (ROS). ROS can be highly damaging to a range of biological macromolecules, and in the past the overproduction of these short-lived molecules in a variety of disease states was thought to be exclusively toxic to cells and tissues such as the lung. Recent basic research, however, has indicated that ROS production--in particular, the production of hydrogen peroxide--plays an important role in both intracellular and extracellular signal transduction that involves diverse functions from vascular health to host defense. The present review summarizes oxygen's capacity, acting through its reactive intermediates, to recruit the enzymatic antioxidant defenses, to stimulate cell repair processes, and to mitigate cellular damage.

12/15-lipoxygenase-derived lipid peroxides control receptor tyrosine kinase signaling through oxidation of protein tyrosine phosphatases

Protein tyrosine phosphatases (PTPs) are regulated through reversible oxidation of the active-site cysteine. Previous studies have implied soluble reactive oxygen species (ROS), like H(2)O(2), as the mediators of PTP oxidation. The potential role(s) of peroxidized lipids in PTP oxidation have not been described. This study demonstrates that increases in cellular lipid peroxides, induced by disruption of glutathione peroxidase 4, induce cellular PTP oxidation and reduce the activity of PDGF receptor targeting PTPs. These effects were accompanied by site-selective increased PDGF beta-receptor phosphorylation, sensitive to 12/15-lipoxygenase (12/15-LOX) inhibitors, and increased PDGF-induced cytoskeletal rearrangements. Importantly, the 12/15-LOX-derived 15-OOH-eicosatetraenoic acid lipid peroxide was much more effective than H(2)O(2) in induction of in vitro PTP oxidation. Our study thus establishes that lipid peroxides are previously unrecognized inducers of oxidation of PTPs. This identifies a pathway for control of receptor tyrosine kinase signaling, which might also be involved in the etiology of diseases associated with increased lipid peroxidation.

Receptor activation of NADPH oxidases

Reactive oxygen species (ROS) have been implicated in many intra- and intercellular processes. High levels of ROS are generated as part of the innate immunity in the respiratory burst of phagocytic cells. Low levels of ROS, however, are generated in a highly controlled manner by various cell types to act as second messengers in redox-sensitive pathways. A NADPH oxidase has been initially described as the respiratory burst enzyme in neutrophils. Stimulation of this complex enzyme system requires specific signaling cascades linking it to membrane-receptor activation. Subsequently, a family of NADPH oxidases has been identified in various nonphagocytic cells. They mainly differ in containing one out of seven homologous catalytic core proteins termed NOX1 to NOX5 and DUOX1 or 2. NADPH oxidase activity is controlled by regulatory subunits, including the NOX regulators p47phox and p67phox, their homologs NOXO1 and NOXA1, or the DUOX1 or 2 regulators DUOXA1 and 2. In addition, the GTPase Rac modulates activity of several of these enzymes. Recently, additional proteins have been identified that seem to have a regulatory function on NADPH oxidase activity under certain conditions. We will thus summarize molecular pathways linking activation of different membrane-bound receptors with increased ROS production of NADPH oxidases.

The nonphagocytic NADPH oxidase Duox1 mediates a positive feedback loop during T cell receptor signaling

Production of reactive oxygen species, often by NADPH (reduced form of nicotinamide adenine dinucleotide phosphate) oxidases, plays a role in the signaling responses of cells to many receptor stimuli. Here, we describe the function of the calcium-dependent, nonphagocytic NADPH oxidase Duox1 in primary human CD4(+) T cells and cultured T cell lines. Duox1 bound to inositol 1,4,5-trisphosphate receptor 1 and was required for early T cell receptor (TCR)-stimulated production of hydrogen peroxide (H(2)O(2)) through a pathway that was dependent on TCR-proximal kinases. Transient or stable knockdown of Duox1 inhibited TCR signaling, especially phosphorylation of tyrosine-319 of zeta chain-associated protein kinase of 70 kilodaltons (ZAP-70), store-operated entry of calcium ions (Ca(2+)), and activation of extracellular signal-regulated kinase. The production of cytokines was also inhibited by knockdown of Duox1. Duox1-mediated inactivation of Src homology 2 domain-containing protein tyrosine phosphatase 2 promoted the phosphorylation of ZAP-70 and its association with the Src family tyrosine kinase Lck and the CD3zeta chain of the TCR complex. Thus, we suggest that activation of Duox1, downstream of proximal TCR signals, generates H(2)O(2) that acts in a positive feedback loop to enhance and sustain further TCR signaling.

PTP1B: a double agent in metabolism and oncogenesis

PTP1B, a non-transmembrane protein tyrosine phosphatase that has long been studied as a negative regulator of insulin and leptin signaling, has received renewed attention as an unexpected positive factor in tumorigenesis. Here, we highlight how views of this enzyme have evolved from regarding it as a simple metabolic off-switch to a more complex view of PTP1B as an enzyme that can play both negative and positive roles in diverse signaling pathways. These dual characteristics make PTP1B a particularly attractive therapeutic target for diabetes, obesity, and perhaps breast cancer.

Analysis of the redox regulation of protein tyrosine phosphatase superfamily members utilizing a cysteinyl-labeling assay

The catalytic activity of protein tyrosine phosphatase (PTP) superfamily members is regulated by the reversible oxidation of their invariant catalytic Cys residue in vivo. Transient and specific regulation of PTP activity by reactive oxygen species (ROS) attenuates dephosphorylation and, thereby, promotes phosphorylation, hence facilitating signal transduction. We have recently developed a modified cysteinyl-labeling assay [Boivin, B., Zhang, S., Arbiser, J. L., Zhang, Z. Y., and Tonks, N. K. (2008). Proc. Natl. Acad. Sci. USA105, 9959-9964.] that showed broad selectivity in detecting reversible oxidation of members from different PTP subclasses in platelet-derived growth factor (PDGF)-BB overexpressing cells. Herein, we applied this assay, which utilizes the unique chemistry of the invariant catalytic Cys residue to enrich and identify PTPs that are reversibly oxidized upon acute growth factor stimulation. Performing the cysteinyl-labeling assay with Rat-1 fibroblasts enabled us to capture both PTEN and SHP-2 as a consequence to acute PDGF-BB stimulation. Given the ability of this assay to detect reversible oxidation of a broad array of members of the PTP family, we anticipate that it should permit profiling of the entire ROS-regulated PTPome in a wide array of signaling paradigms.

IGF-I stimulates reactive oxygen species (ROS) production and inhibits insulin-dependent glucose uptake via ROS in 3T3-L1 adipocytes

OBJECTIVE

IGF-I is known to enhance insulin sensitivity in whole body mainly via the IGF-I receptors in muscles. However, the effect of IGF-I on the regulation of insulin sensitivity in the adipose tissue is yet unclear. Insulin sensitivity was found to be higher in the IGF-I receptor-deficient adipocytes than that in wild-type adipocytes, suggesting that IGF-I signaling induces insulin resistance in adipocytes. However, the underlying mechanism has not yet been elucidated. In addition, the effect of superphysiological levels of IGF-I, as is observed in patients with acromegaly, on insulin sensitivity remains unclear.

DESIGN

To clarify the role of IGF-I on insulin sensitivity in adipocytes, we determined insulin-induced glucose uptake and IRS-1 status in 3T3-L1 adipocytes treated with IGF-I. Since reactive oxygen species (ROS) are causally related to insulin resistance, we investigated the effect of IGF-I on ROS production to elucidate the molecular mechanism underlying insulin resistance.

RESULTS

Preincubation of the adipocytes with IGF-I attenuated insulin-dependent glucose uptake. Interestingly, we found that IGF-I significantly stimulated ROS production. Furthermore, preincubation of adipocytes with an antioxidant, N-acetyl-cysteine (NAC) restored the IGF-I-induced attenuation of insulin-dependent glucose uptake; this indicates that IGF-I induces insulin resistance via ROS. Serine phosphorylation of IRS-1 was strongly induced and the insulin-dependent tyrosine phosphorylation of IRS-1 was suppressed by preincubating the adipocytes with IGF-I. Further, NAC restored these changes induced by IGF-I on both serine and tyrosine phosphorylation of IRS-1.

CONCLUSIONS

These data indicate that IGF-I inhibited insulin activity in the 3T3-L1 adipocytes via ROS production, which affects IRS-1 phosphorylation status.

Decrease in dietary K intake stimulates the generation of superoxide anions in the kidney and inhibits K secretory channels in the CCD

We previously demonstrated that K depletion inhibited ROMK-like small-conductance K channels (SK) in the cortical collecting duct (CCD) and that the effect was mediated by superoxide anions that stimulated Src family protein tyrosine kinase (PTK) and mitogen-activated protein kinase (MAPK) (51). However, because animals on a K-deficient diet had a severe hypokalemia, superoxide-dependent signaling may not regulate ROMK channels under physiological conditions with a normal plasma K concentration. In the present study, we used the patch-clamp technique and Western blot to examine the effect of a moderate K restriction on ROMK-like SK channels and the role of PTK and MAPK in regulating apical K channels in the CCD of animals on a low-K diet (LK; 0.1% K). Rats and mice fed a LK diet for 7 days had a normal plasma K concentration. However, a LK intake increased the expression of angiotensin II type 1 receptor in the kidney. Moreover, patch-clamp experiments demonstrated that LK intake decreased the probability finding SK channels and channel activity defined by NP(o) (a product of channel number and open probability) in the CCD of both rat and mouse kidneys. Also, LK intake significantly stimulated the production of superoxide anions in the renal cortex and outer medulla in both rats and mice and increased superoxide level in the rat CCD. Moreover, LK intake augments the phosphorylation of p38 and ERK MAPK, the expression of c-Src and tyrosine phosphorylation of ROMK channels. However, treatment of animals with tempol abolished the effect of LK intake on MAPK and c-Src and increased ROMK channel activity in comparing with those of nontreated rats on a LK diet. Inhibiting p38 and ERK with SB202190 and PD98059 significantly stimulated SK in the CCD in rats on a LK diet. In addition, inhibition of PTK with herbimycin A activated SK channels in the CCD from rats on a LK diet. We conclude that LK intake stimulates the generation of superoxide anion and related products and that MAPK and Src family PTK play a physiological role in inhibiting apical K channels in the principal cells in response to LK intake.

Effect of paraquat-induced oxidative stress on insulin regulation of insulin-like growth factor-binding protein-1 gene expression

Oxidative stress is thought to play a role in the development of insulin resistance. In order to elucidate the molecular effect of oxidative stress on liver insulin signaling, we analyzed the effect of paraquat (1,1-dimethyl-4,4-dipyridynium; PQ)-derived oxidative stress on the expression of insulin-dependent genes and activation of liver insulin signaling pathway. Incubation of primary cultured rat hepatocytes with 2 mM PQ for 6 h impaired the suppressive effect of insulin on insulin-like growth factor-binding protein-1 (IGFBP-1) gene expression, but did not influence glucose-6-phosphatase gene expression. Insulin-dependent phosphorylation or activation of insulin receptor, insulin receptor substrate-1 and -2, phosphatidylinositol 3-kinase, Akt and forkhead in rhabdomyosarcoma were not affected by PQ pre-treatment. In contrast, PQ treatment impaired insulin-dependent phosphorylation of mammalian target of rapamycin (mTOR). These results indicate that PQ-induced oxidative stress impairs insulin-dependent mTOR activation and that this impairment probably causes inhibition of insulin-dependent repression of IGFBP-1 expression.

Protection of a single-cysteine redox switch from oxidative destruction: On the functional role of sulfenyl amide formation in the redox-regulated enzyme PTP1B

Model reactions offer a chemical mechanism by which formation of a sulfenyl amide residue at the active site of the redox-regulated protein tyrosine phosphatase PTP1B protects the cysteine redox switch in this enzyme against irreversible oxidative destruction. The results suggest that 'overoxidation' of the sulfenyl amide redox switch to the sulfinyl amide in proteins is a chemically reversible event, because the sulfinyl amide can be easily returned to the native cysteine thiol residue via reactions with cellular thiols.

Enhancement of insulin responsiveness by nitric oxide-mediated inactivation of protein-tyrosine phosphatases

NO synthesis is a prerequisite for proper insulin sensitivity in insulin-targeted tissues; however, the molecular basis for this process remains unclear. Using a gain-of-function model of endothelial nitric-oxide synthase (eNOS)-transfected COS-7 cells, we have shown a critical role of NO in insulin responsiveness, as evidenced by an NO-dependent increase of tyrosine phosphorylation levels of the insulin receptor and its downstream effectors insulin receptor substrate-1 and PKB/AKT. We hypothesized that NO-induced inactivation of endogenous protein-tyrosine phosphatases (PTPs) would enhance insulin receptor-mediated signaling. To test this hypothesis, we devised a new method of the PTP labeling using a cysteine sulfhydryl-reacted probe. Under the acidic conditions employed in this study, the probe recognized the reduced and active forms but not the S-nitrosylated and inactive forms of endogenous PTPs. Our data suggest that phosphatases SHP-1, SHP-2, and PTP1B, but not TC-PTP, are likely S-nitrosylated at the active site cysteine residue concomitantly with a burst of NO production in signaling response to insulin stimulation. These results were further confirmed by phosphatase activity assays. We investigated further the role of NO as a regulator of insulin signaling by RNA interference that ablates endogenous eNOS expression in endothelial MS-1 cells. We have shown that eNOS-dependent NO production is essential for the activation of insulin signaling. Our findings demonstrate that NO mediates enhancement of insulin responsiveness via the inhibition of insulin receptor phosphatases.

Redox control of the cell cycle in health and disease

The cellular oxidation and reduction (redox) environment is influenced by the production and removal of reactive oxygen species (ROS). In recent years, several reports support the hypothesis that cellular ROS levels could function as ''second messengers'' regulating numerous cellular processes, including proliferation. Periodic oscillations in the cellular redox environment, a redox cycle, regulate cell-cycle progression from quiescence (G(0)) to proliferation (G(1), S, G(2), and M) and back to quiescence. A loss in the redox control of the cell cycle could lead to aberrant proliferation, a hallmark of various human pathologies. This review discusses the literature that supports the concept of a redox cycle controlling the mammalian cell cycle, with an emphasis on how this control relates to proliferative disorders including cancer, wound healing, fibrosis, cardiovascular diseases, diabetes, and neurodegenerative diseases. We hypothesize that reestablishing the redox control of the cell cycle by manipulating the cellular redox environment could improve many aspects of the proliferative disorders.

Insights into the critical role of NADPH oxidase(s) in the normal and dysregulated pancreatic beta cell

It is now widely accepted that reactive oxygen species (ROS) contribute to cell and tissue dysfunction and damage in diabetes. The source of ROS in the insulin secreting pancreatic beta cells has traditionally been considered to be the mitochondrial electron transport chain. While this source is undoubtedly important, we fully describe in this article recent information and evidence of NADPH oxidase-dependent generation of ROS in pancreatic beta cells and identify the various isoforms that contribute to O(2)(*-) and H(2)O(2) production in various conditions. While glucose-stimulated ROS generation may be important for acute regulation of insulin secretion, at higher levels ROS may disrupt mitochondrial energy metabolism. However, ROS may alter other cellular processes such as signal transduction, ion fluxes and/or cell proliferation/death. The various beta cell isoforms of NADPH oxidase (described in this review) may, via differences in the kinetics and species of ROS generated, positively and negatively regulate insulin secretion and cell survival.

Hydrogen peroxide as a cell-survival signaling molecule

Reactive oxygen species (ROS) were seen as destructive molecules, but recently, they have been shown also to act as second messengers in varying intracellular signaling pathways. This review concentrates on hydrogen peroxide (H2O2), as it is a more stable ROS, and delineates its role as a survival molecule. In the first part, the production of H2O2 through the NADPH oxidase (Nox) family is investigated. Through careful examination of Nox proteins and their regulation, it is determined how they respond to stress and how this can be prosurvival rather than prodeath. The pathways on which H2O2 acts to enable its prosurvival function are then examined in greater detail. The main survival pathways are kinase driven, and oxidation of cysteines in the active sites of various phosphatases can thus regulate those survival pathways. Regulation of transcription factors such as p53, NF-kappaB, and AP-1 also are reviewed. Finally, prodeath proteins such as caspases could be directly inhibited through their cysteine residues. A better understanding of the prosurvival role of H2O2 in cells, from the why and how it is generated to the various molecules it can affect, will allow more precise targeting of therapeutics to this pathway.

Oxidative stress in bone remodelling and disease

Oxidative stress is characterised by an increased level of reactive oxygen species (ROS) that disrupts the intracellular reduction-oxidation (redox) balance. Although initially shown to be involved in aging, physiological roles for ROS in regulating cell functions and mediating intracellular signals have emerged. In bone tissues, recent studies have demonstrated that ROS generation is a key modulator of bone cell function and that oxidative status influences the pathophysiology of mineralised tissues. Here, we review the crucial role of oxidative stress in bone pathophysiology, and discuss the possibility that ROS production might be a relevant therapeutic target under certain conditions. Further studies will be needed to investigate whether manipulation of the redox balance in bone cells represents a useful approach in the design of future therapies for bone diseases.

Insulin is a stronger inducer of insulin resistance than hyperglycemia in mice with type 1 diabetes mellitus (T1DM)

Subjects with type 1 diabetes mellitus (T1DM) eventually develop insulin resistance and other features of T2DM such as cardiovascular disorders. The exact mechanism has been not been completely understood. In this study, we tested the hypothesis that excessive or inappropriate exposure to insulin is a primary mediator of insulin resistance in T1DM. We found that continuous exposure of mice with non-obese diabetes to insulin detemir, which is similar to some current conventional treatment of human T1DM, induced severe insulin resistance, whereas untreated hyperglycemia for the same amount of time (2 weeks) did not cause obvious insulin resistance. Insulin resistance was accompanied by decreased mitochondrial production as evaluated by mitochondrial DNA and levels of transcripts and proteins of mitochondrion-associated genes, increased ectopic fat accumulation in liver and skeletal muscle (gastrocnemius) evaluated by measurements of triglyceride content, and elevated oxidative stress detected by the GSH/GSSG ratio. Prolonged exposure of cultured hepatocytes to insulin induced significant insulin resistance, whereas the same length of exposure to a high level of glucose (33 mm) did not cause obvious insulin resistance. Furthermore, our results showed that prolonged exposure to insulin caused oxidative stress, and blockade of mitochondrion-derived oxidative stress by overexpression of manganese-superoxide dismutase prevented insulin resistance induced by the prolonged exposure to insulin. Together, our results show that excessive exposure to insulin is a primary inducer of insulin resistance in T1DM in mice.

Sphingosine 1-phosphate increases glucose uptake through trans-activation of insulin receptor

Sphingosine 1-phosphate (S1P) is a bioactive lipid that acts through a family of G-protein-coupled receptors. Herein, we report evidence of a novel redox-based cross-talk between S1P and insulin signaling pathways. In skeletal muscle cells S1P, through engagement of its S1P(2) receptor, is found to produce a transient burst of reactive oxygen species through a calcium-dependent activation of the small GTPase Rac1. S1P-induced redox-signaling is sensed by protein tyrosine phosphatase-1B, the main negative regulator of insulin receptor phosphorylation, which undergoes oxidation and enzymatic inhibition. This redox-based inhibition of the phosphatase provokes a ligand-independent trans-phosphorylation of insulin receptor and a strong increase in glucose uptake. Our results propose a new role of S1P, recognizing the lipid as an insulin-mimetic cue and pointing at reactive oxygen species as critical regulators of the cross-talk between S1P and insulin pathways. Any possible implication of S1P-directed insulin signaling in diabetes and insulin resistance remains to be established.

Calpain-mediated degradation of reversibly oxidized protein-tyrosine phosphatase 1B

Protein-tyrosine phosphatases (PTPs) are regulated by reversible inactivating oxidation of the catalytic-site cysteine. We have previously shown that reversible oxidation upon UVA irradiation is followed by calpain-mediated PTP degradation. Here, we address the mechanism of regulated cleavage and the physiological function of PTP degradation. Reversible oxidation of PTP1B in vitro strongly facilitated the association with calpain and led to greatly increased calpain-dependent inactivating cleavage. Both oxidation-induced association and cleavage depended exclusively on the presence of the catalytic (reversibly oxidized) cysteine residue 215. A major cleavage site was identified preceding amino acid position Ala77. In calpain-deficient cells, insulin signaling was apparently diminished, consistent with a possible role for calpain in removing a negative regulator of insulin signaling. Reversibly oxidized PTP1B may be a target of calpain in this context.

Long-term impact of an antioxidant-deficient neonatal diet on lipid and glucose metabolism

Newborn infants are at risk for oxidative stress leading to metabolic syndrome features. Oxidative stress can be induced by oxidant load such as oxygen supplementation, peroxides from intravenous nutrition, or low antioxidant defenses. We hypothesize that a modulation of antioxidant defenses during the neonatal period, without external oxidant challenge, will have a long-term influence on energy metabolism. Guinea pigs were fed between their third and their seventh day of life a regular chow leading to "mature" antioxidant defenses or a deficient chow leading to lower antioxidant defenses. Between weeks 1 and 14, the animals were fed regular chow. The hepatic oxidized redox status of glutathione associated with the deficient diet (-221 +/- 2 vs -228 +/- 1 mV, p < 0.01) was maintained until 14 weeks. At 13-14 weeks, animals fed the deficient diet presented lower plasma TG (479 +/- 57 vs 853 +/- 32 microM, p < 0.01), lower blood glucose (5.8 +/- 0.3 vs 6.9 +/- 0.3 mM, p < 0.05), and better tolerance to glucose (p < 0.05). Blood glucose correlated negatively with the redox status (r2 = 0.47, p < 0.01). Low antioxidant defenses during the neonatal period induce a better energy substrate profile associated with an oxidized redox status later in life. These findings suggest being aware of negative consequences when adopting "aggressive" antioxidant therapies in newborn infants.

Sac3 is an insulin-regulated phosphatidylinositol 3,5-bisphosphate phosphatase: gain in insulin responsiveness through Sac3 down-regulation in adipocytes

Insulin-regulated stimulation of glucose entry and mobilization of fat/muscle-specific glucose transporter GLUT4 onto the cell surface require the phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P(2)) pathway for optimal performance. The reduced insulin responsiveness observed under ablation of the PtdIns(3,5)P(2)-synthesizing PIKfyve and its associated activator ArPIKfyve in 3T3L1 adipocytes suggests that dysfunction of the PtdIns(3,5)P(2)-specific phosphatase Sac3 may yield the opposite effect. Paradoxically, as uncovered recently, in addition to turnover Sac3 also supports PtdIns(3,5)P(2) biosynthesis by allowing optimal PIKfyve-ArPIKfyve association. These opposing inputs raise the key question as to whether reduced Sac3 protein levels and/or hydrolyzing activity will produce gain in insulin responsiveness. Here we report that small interfering RNA-mediated knockdown of endogenous Sac3 by approximately 60%, which resulted in a slight but significant elevation of PtdIns(3,5)P(2) in 3T3L1 adipocytes, increased GLUT4 translocation and glucose entry in response to insulin. In contrast, ectopic expression of Sac3(WT), but not phosphatase-deficient Sac3(D488A), reduced GLUT4 surface abundance in the presence of insulin. Endogenous Sac3 physically assembled with PIKfyve and ArPIKfyve in both membrane and soluble fractions of 3T3L1 adipocytes, but this remained insulin-insensitive. Importantly, acute insulin markedly reduced the in vitro C8-PtdIns(3,5)P(2) hydrolyzing activity of Sac3. The insulin-sensitive Sac3 pool likely controls a discrete PtdIns(3,5)P(2) subfraction as the high pressure liquid chromatography-measurable insulin-dependent elevation in total [(3)H]inositol-PtdIns(3,5)P(2) was minor. Together, our data identify Sac3 as an insulin-sensitive phosphatase whose down-regulation increases insulin responsiveness, thus implicating Sac3 as a novel drug target in insulin resistance.

Hypothalamic reactive oxygen species are required for insulin-induced food intake inhibition: an NADPH oxidase-dependent mechanism

OBJECTIVE

Insulin plays an important role in the hypothalamic control of energy balance, especially by reducing food intake. Emerging data point to a pivotal role of reactive oxygen species (ROS) in energy homeostasis regulation, but their involvement in the anorexigenic effect of insulin is unknown. Furthermore, ROS signal derived from NADPH oxidase activation is required for physiological insulin effects in peripheral cells. In this study, we investigated the involvement of hypothalamic ROS and NADPH oxidase in the feeding behavior regulation by insulin.

RESEARCH DESIGN AND METHODS

We first measured hypothalamic ROS levels and food intake after acute intracerebroventricular injection of insulin. Second, effect of pretreatment with a ROS scavenger or an NADPH oxidase inhibitor was evaluated. Third, we examined the consequences of two nutritional conditions of central insulin unresponsiveness (fasting or short-term high-fat diet) on the ability of insulin to modify ROS level and food intake.

RESULTS

In normal chow-fed mice, insulin inhibited food intake. At the same dose, insulin rapidly and transiently increased hypothalamic ROS levels by 36%. The pharmacological suppression of this insulin-stimulated ROS elevation, either by antioxidant or by an NADPH oxidase inhibitor, abolished the anorexigenic effect of insulin. Finally, in fasted and short-term high-fat diet-fed mice, insulin did not promote elevation of ROS level and food intake inhibition, likely because of an increase in hypothalamic diet-induced antioxidant defense systems.

CONCLUSIONS

A hypothalamic ROS increase through NADPH oxidase is required for the anorexigenic effect of insulin.