Endogenous hydrogen peroxide and peroxidative metabolism in adipocytes in response to insulin and sulfhydryl reagents

The dynamic clustering of insulin receptor underlies its signaling and is disrupted in insulin resistance

Insulin receptor (IR) signaling is central to normal metabolic control and is dysregulated in metabolic diseases such as type 2 diabetes. We report here that IR is incorporated into dynamic clusters at the plasma membrane, in the cytoplasm and in the nucleus of human hepatocytes and adipocytes. Insulin stimulation promotes further incorporation of IR into these dynamic clusters in insulin-sensitive cells but not in insulin-resistant cells, where both IR accumulation and dynamic behavior are reduced. Treatment of insulin-resistant cells with metformin, a first-line drug used to treat type 2 diabetes, can rescue IR accumulation and the dynamic behavior of these clusters. This rescue is associated with metformin's role in reducing reactive oxygen species that interfere with normal dynamics. These results indicate that changes in the physico-mechanical features of IR clusters contribute to insulin resistance and have implications for improved therapeutic approaches.

The interplay among oxidative stress, brain insulin resistance and AMPK dysfunction contribute to neurodegeneration in type 2 diabetes and Alzheimer disease

Alzheimer's disease (AD) is the most common form of dementia in the elderly followed by vascular dementia. In addition to clinically diagnosed dementia, cognitive dysfunction has been reported in diabetic patients. Recent studies are now beginning to recognize type 2 diabetes mellitus (T2DM), characterized by chronic hyperglycemia and insulin resistance, as a risk factor for AD and other cognitive disorders. While studies on insulin action have remained traditionally in the domain of peripheral tissues, the detrimental effects of insulin resistance in the central nervous system on cognitive dysfunction are increasingly being reported in recent clinical and preclinical studies. Brain functions require continuous supply of glucose and oxygen and a tight regulation of metabolic processes. Loss of this metabolic regulation has been proposed to be a contributor to memory dysfunction associated with neurodegeneration. Within the above scenario, this review will focus on the interplay among oxidative stress (OS), insulin resistance and AMPK dysfunctions in the brain by highlighting how these neurotoxic events contribute to neurodegeneration. We provide an overview on the detrimental effects of OS on proteins regulating insulin signaling and how these alterations impact cell metabolic dysfunctions through AMPK dysregulation. Such processes, we assert, are critically involved in the molecular pathways that underlie AD.

Brain Insulin Resistance: Focus on Insulin Receptor-Mitochondria Interactions

Current hypotheses implicate insulin resistance of the brain as a pathogenic factor in the development of Alzheimer’s disease and other dementias, Parkinson’s disease, type 2 diabetes, obesity, major depression, and traumatic brain injury. A variety of genetic, developmental, and metabolic abnormalities that lead to disturbances in the insulin receptor signal transduction may underlie insulin resistance. Insulin receptor substrate proteins are generally considered to be the node in the insulin signaling system that is critically involved in the development of insulin insensitivity during metabolic stress, hyperinsulinemia, and inflammation. Emerging evidence suggests that lower activation of the insulin receptor (IR) is another common, while less discussed, mechanism of insulin resistance in the brain. This review aims to discuss causes behind the diminished activation of IR in neurons, with a focus on the functional relationship between mitochondria and IR during early insulin signaling and the related roles of oxidative stress, mitochondrial hypometabolism, and glutamate excitotoxicity in the development of IR insensitivity to insulin.

Regulation of Metabolic Processes by Hydrogen Peroxide Generated by NADPH Oxidases

Hydrogen peroxide (H2O2) is an important oxidizing molecule that regulates the metabolisms of aerobic organisms. Redox signaling comprises physiological oxidative stress (eustress), while excessive oxidative stress causes damage to molecules. The main enzymatic generators of H2O2 are nicotinamide adenine dinucleotide phosphate oxidases or NADPH oxidases (NOXs) and mitochondrial respiratory chains, as well as various oxidases. The NOX family is constituted of seven enzyme isoforms that produce a superoxide anion (O2−), which can be converted to H2O2 by superoxide dismutase or spontaneously. H2O2 passes through the membranes by some aquaporins (AQPs), known as peroxyporins. It diffuses through cells and tissues to initiate cellular effects, such as proliferation, the recruitment of immune cells, and cell shape changes. Therefore, it has been proposed that H2O2 has the same importance as Ca2+ or adenosine triphosphate (ATP) to act as modulators in signaling and the metabolism. The present overview focuses on the metabolic processes of liver and adipose tissue, regulated by the H2O2 generated by NOXs.

The Emerging Roles of Nicotinamide Adenine Dinucleotide Phosphate Oxidase 2 in Skeletal Muscle Redox Signaling and Metabolism

Significance: Skeletal muscle is a crucial tissue to whole-body locomotion and metabolic health. Reactive oxygen species (ROS) have emerged as intracellular messengers participating in both physiological and pathological adaptations in skeletal muscle. A complex interplay between ROS-producing enzymes and antioxidant networks exists in different subcellular compartments of mature skeletal muscle. Recent evidence suggests that nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs) are a major source of contraction- and insulin-stimulated oxidants production, but they may paradoxically also contribute to muscle insulin resistance and atrophy. Recent Advances: Pharmacological and molecular biological tools, including redox-sensitive probes and transgenic mouse models, have generated novel insights into compartmentalized redox signaling and suggested that NOX2 contributes to redox control of skeletal muscle metabolism. Critical Issues: Major outstanding questions in skeletal muscle include where NOX2 activation occurs under different conditions in health and disease, how NOX2 activation is regulated, how superoxide/hydrogen peroxide generated by NOX2 reaches the cytosol, what the signaling mediators are downstream of NOX2, and the role of NOX2 for different physiological and pathophysiological processes. Future Directions: Future research should utilize and expand the current redox-signaling toolbox to clarify the NOX2-dependent mechanisms in skeletal muscle and determine whether the proposed functions of NOX2 in cells and animal models are conserved into humans.

Insulin receptor in the brain: Mechanisms of activation and the role in the CNS pathology and treatment

While the insulin receptor (IR) was found in the CNS decades ago, the brain was long considered to be an insulin-insensitive organ. This view is currently revisited, given emerging evidence of critical roles of IR-mediated signaling in development, neuroprotection, metabolism, and plasticity in the brain. These diverse cellular and physiological IR activities are distinct from metabolic IR functions in peripheral tissues, thus highlighting region specificity of IR properties. This particularly concerns the fact that two IR isoforms, A and B, are predominantly expressed in either the brain or peripheral tissues, respectively, and neurons express exclusively IR-A. Intriguingly, in comparison with IR-B, IR-A displays high binding affinity and is also activated by low concentrations of insulin-like growth factor-2 (IGF-2), a regulator of neuronal plasticity, whose dysregulation is associated with neuropathologic processes. Deficiencies in IR activation, insulin availability, and downstream IR-related mechanisms may result in aberrant IR-mediated functions and, subsequently, a broad range of brain disorders, including neurodevelopmental syndromes, neoplasms, neurodegenerative conditions, and depression. Here, we discuss findings on the brain-specific features of IR-mediated signaling with focus on mechanisms of primary receptor activation and their roles in the neuropathology. We aimed to uncover the remaining gaps in current knowledge on IR physiology and highlight new therapies targeting IR, such as IR sensitizers.

Reactive oxygen species in haematopoiesis: leukaemic cells take a walk on the wild side

Oxidative stress is related to ageing and degenerative diseases, including cancer. However, a moderate amount of reactive oxygen species (ROS) is required for the regulation of cellular signalling and gene expression. A low level of ROS is important for maintaining quiescence and the differentiation potential of haematopoietic stem cells (HSCs), whereas the level of ROS increases during haematopoietic differentiation; thus, suggesting the importance of redox signalling in haematopoiesis. Here, we will analyse the importance of ROS for haematopoiesis and include evidence showing that cells from leukaemia patients live under oxidative stress. The potential sources of ROS will be described. Finally, the level of oxidative stress in leukaemic cells can also be harnessed for therapeutic purposes. In this regard, the reliance of front-line anti-leukaemia chemotherapeutics on increased levels of ROS for their mechanism of action, as well as the active search for novel compounds that modulate the redox state of leukaemic cells, will be analysed.

Bidirectional modulation of insulin action by reactive oxygen species in 3T3‑L1 adipocytes

Reactive oxygen species (ROS) serve an important role in glucose‑lipid metabolic regulation. In the present study, the results demonstrated that there was bidirectional regulation of insulin action in 3T3‑L1 adipocytes treated with ROS. Transient and acute ROS exposure improved insulin‑induced metabolic effects in 3T3‑L1 adipocytes. Hydrogen peroxide (H2O2), as a stable and diffusible ROS, diffused into adipocytes and altered intracellular redox homeostasis, resulting in oxidation and inactivation of phosphatase and tensin homologue deleted on chromosome 10 (PTEN). Inactivation of PTEN enhanced the activation of insulin‑induced protein kinase B (AKT), leading to increased glucose transporter 4 (GLUT4) redistribution and glucose uptake in 3T3‑L1 adipocytes. However, chronic ROS treatment induced insulin resistance in 3T3‑L1 adipocytes. It was also revealed that insulin‑induced AKT activation, GLUT4 translocation to cell membrane and glucose uptake were significantly inhibited in chronic ROS‑treated 3T3‑L1 adipocytes. Taken together, the present study provided further demonstration that transient ROS treatment improved insulin sensitivity; however, chronic ROS exposure induced insulin resistance in 3T3‑L1 adipocytes.

The Importance of NADPH Oxidases and Redox Signaling in Angiogenesis

Eukaryotic cells have to cope with the constant generation of reactive oxygen species (ROS). Although the excessive production of ROS might be deleterious for cell biology, there is a plethora of evidence showing that moderate levels of ROS are important for the control of cell signaling and gene expression. The family of the nicotinamide adenine dinucleotide phosphate oxidases (NADPH oxidases or Nox) has evolved to produce ROS in response to different signals; therefore, they fulfil a central role in the control of redox signaling. The role of NADPH oxidases in vascular physiology has been a field of intense study over the last two decades. In this review we will briefly analyze how ROS can regulate signaling and gene expression. We will address the implication of NADPH oxidases and redox signaling in angiogenesis, and finally, the therapeutic possibilities derived from this knowledge will be discussed.

Prolonged insulin treatment sensitizes apoptosis pathways in pancreatic β cells

Insulin resistance results from impaired insulin signaling in target tissues that leads to increased levels of insulin required to control plasma glucose levels. The cycle of hyperglycemia and hyperinsulinemia eventually leads to pancreatic cell deterioration and death by a mechanism that is yet unclear. Insulin induces ROS formation in several cell types. Furthermore, death of pancreatic cells induced by oxidative stress could be potentiated by insulin. Here, we investigated the mechanism underlying this phenomenon. Experiments were done on pancreatic cell lines (Min-6, RINm, INS-1), isolated mouse and human islets, and on cell lines derived from nonpancreatic sources. Insulin (100nM) for 24h selectively increased the production of ROS in pancreatic cells and isolated pancreatic islets, but only slightly affected the expression of antioxidant enzymes. This was accompanied by a time- and dose-dependent decrease in cellular reducing power of pancreatic cells induced by insulin and altered expression of several ER stress response elements including a significant increase in Trb3 and a slight increase in iNos The effect on iNos did not increase NO levels. Insulin also potentiated the decrease in cellular reducing power induced by H2O2 but not cytokines. Insulin decreased the expression of MCL-1, an antiapoptotic protein of the BCL family, and induced a modest yet significant increase in caspase 3/7 activity. In accord with these findings, inhibition of caspase activity eliminated the ability of insulin to increase cell death. We conclude that prolonged elevated levels of insulin may prime apoptosis and cell death-inducing mechanisms as a result of oxidative stress in pancreatic cells.

The paracrine control of vascular motion. A historical perspective

During the last quarter of the past century, the leading role the endocrine and nervous systems had on the regulation of vasomotion, shifted towards a more paracrine-based regulation. This begun with the recognition of endothelial cells as active players of vascular control, when the vessel's intimal layer was identified as the main source of prostacyclin and was followed by the discovery of an endothelium-derived smooth muscle cell relaxing factor (EDRF). The new position acquired by endothelial cells prompted the discovery of other endothelium-derived regulatory products: vasoconstrictors, generally known as EDCFs, endothelin, and other vasodilators with hyperpolarizing properties (EDHFs). While this research was taking place, a quest for the discovery of the nature of EDRF carried back to a research line commenced a decade earlier: the recently found intracellular messenger cGMP and nitrovasodilators. Both were smooth muscle relaxants and appeared to interact in a hormonal fashion. Prejudice against an unconventional gaseous molecule delayed the acceptance that EDRF was nitric oxide (NO). When this happened, a new era of research that exceeded the vascular field commenced. The discovery of the pathway for NO synthesis from L-arginine involved the clever assembling of numerous unrelated observations of different areas of knowledge. The last ten years of research on the paracrine regulation of the vascular wall has shifted to perivascular fat (PVAT), which is beginning to be regarded as the fourth layer of the vascular wall. Starting with the discovery of an adipose-derived relaxing substance (ADRF), the role that different adipokines have on the paracrine control of vasomotion is now filling the research activity of many vascular pharmacology labs, and surprising interactions between the endothelium, PVAT and smooth muscle are being unveiled.

Redox signaling: An evolution from free radicals to aging

Redox biology has evolved from studies of the pathology that involves oxidants to an understanding of how oxidants participate in normal as well as aberrant signal transduction. Although the concept that signal transduction involved changes in the redox state dates from the 1930s, the modern history of redox biology began with the discovery of superoxide dismutase by McCord and Fridovich. The initial focus was on free radicals and damage of macromolecules, which remains an important topic. But, over time it was realized that hydroperoxides, especially H2O2 produced by NADPH oxidases, and electrophiles derived from lipid peroxidation or metabolism, played essential roles in physiologically relevant signaling. The mechanisms through which H2O2 and other electrophiles signal became an important area of study that provided insight into how these reactive molecules were involved in major signaling pathways and regulation of transcription factors. Thus, the field of redox signaling that is the overlap of signal transduction with redox biology was established. Alterations in redox signaling are observed in aging, but we also now know that redox signaling is essential in physiological homeostasis and that sustained deviation from redox homeostasis results in disease. This is a review of the history of redox biology from a personal perspective of nearly fifty years working in this field that hopefully provides some insights for the reader.

Imbalanced insulin action in chronic over nutrition: Clinical harm, molecular mechanisms, and a way forward

The growing worldwide prevalence of overnutrition and underexertion threatens the gains that we have made against atherosclerotic cardiovascular disease and other maladies. Chronic overnutrition causes the atherometabolic syndrome, which is a cluster of seemingly unrelated health problems characterized by increased abdominal girth and body-mass index, high fasting and postprandial concentrations of cholesterol- and triglyceride-rich apoB-lipoproteins (C-TRLs), low plasma HDL levels, impaired regulation of plasma glucose concentrations, hypertension, and a significant risk of developing overt type 2 diabetes mellitus (T2DM). In addition, individuals with this syndrome exhibit fatty liver, hypercoagulability, sympathetic overactivity, a gradually rising set-point for body adiposity, a substantially increased risk of atherosclerotic cardiovascular morbidity and mortality, and--crucially--hyperinsulinemia. Many lines of evidence indicate that each component of the atherometabolic syndrome arises, or is worsened by, pathway-selective insulin resistance and responsiveness (SEIRR). Individuals with SEIRR require compensatory hyperinsulinemia to control plasma glucose levels. The result is overdrive of those pathways that remain insulin-responsive, particularly ERK activation and hepatic de-novo lipogenesis (DNL), while carbohydrate regulation deteriorates. The effects are easily summarized: if hyperinsulinemia does something bad in a tissue or organ, that effect remains responsive in the atherometabolic syndrome and T2DM; and if hyperinsulinemia might do something good, that effect becomes resistant. It is a deadly imbalance in insulin action. From the standpoint of human health, it is the worst possible combination of effects. In this review, we discuss the origins of the atherometabolic syndrome in our historically unprecedented environment that only recently has become full of poorly satiating calories and incessant enticements to sit. Data are examined that indicate the magnitude of daily caloric imbalance that causes obesity. We also cover key aspects of healthy, balanced insulin action in liver, endothelium, brain, and elsewhere. Recent insights into the molecular basis and pathophysiologic harm from SEIRR in these organs are discussed. Importantly, a newly discovered oxide transport chain functions as the master regulator of the balance amongst different limbs of the insulin signaling cascade. This oxide transport chain--abbreviated 'NSAPP' after its five major proteins--fails to function properly during chronic overnutrition, resulting in this harmful pattern of SEIRR. We also review the origins of widespread, chronic overnutrition. Despite its apparent complexity, one factor stands out. A sophisticated junk food industry, aided by subsidies from willing governments, has devoted years of careful effort to promote overeating through the creation of a new class of food and drink that is low- or no-cost to the consumer, convenient, savory, calorically dense, yet weakly satiating. It is past time for the rest of us to overcome these foes of good health and solve this man-made epidemic.

Suppressive effects of the NADPH oxidase inhibitor apocynin on intestinal tumorigenesis in obese KK-A(y) and Apc mutant Min mice

Obesity is a risk factor for colorectal cancer. The accumulation of abdominal fat tissue causes abundant reactive oxygen species production through the activation of NADPH oxidase due to excessive insulin stimulation. The enzyme NADPH oxidase catalyzes the production of reactive oxygen species and evokes the initiation and progression of tumorigenesis. Apocynin is an NADPH oxidase inhibitor that blocks the formation of the NADPH oxidase complex (active form). In this study, we investigated the effects of apocynin on the development of azoxymethane‐induced colonic aberrant crypt foci in obese KK‐A^y mice and on the development of intestinal polyps in Apc mutant Min mice. Six‐week‐old KK‐A^y mice were injected with azoxymethane (200 μg/mouse once per week for 3 weeks) and given 250 mg/L apocynin or 500 mg/L apocynin in their drinking water for 7 weeks. Six‐week‐old Min mice were also treated with 500 mg/L apocynin for 6 weeks. Treatment with apocynin reduced the number of colorectal aberrant crypt foci in KK‐A^y mice by 21% and the number of intestinal polyps in Min mice by 40% compared with untreated mice. Both groups of mice tended to show improved oxidation of serum low‐density lipoprotein and 8‐oxo‐2′‐deoxyguanosine adducts in their adipose tissues. In addition, the inducible nitric oxide synthase mRNA levels in polyp tissues decreased. Moreover, apocynin was shown to suppress nuclear factor‐κB transcriptional activity in vitro. These results suggest that apocynin and other NADPH oxidase inhibitors may be effective colorectal cancer chemopreventive agents.

α6-Containing GABAA Receptors Are the Principal Mediators of Inhibitory Synapse Strengthening by Insulin in Cerebellar Granule Cells

Activity-dependent strengthening of central synapses is a key factor driving neuronal circuit behavior in the vertebrate CNS. At fast inhibitory synapses, strengthening is thought to occur by increasing the number of GABAA receptors (GABARs) of the same subunit composition to preexisting synapses. Here, we show that strengthening of mouse cerebellar granule cell GABAergic synapses occurs by a different mechanism. Specifically, we show that the neuropeptide hormone, insulin, strengthens inhibitory synapses by recruiting α6-containing GABARs rather than accumulating more α1-containing receptors that are resident to the synapse. Because α6-receptors are targeted to functionally distinct postsynaptic sites from α1-receptors, we conclude that only a subset of all inhibitory synapses are strengthened. Together with our recent findings on stellate cells, we propose a general mechanism by which mature inhibitory synapses are strengthened. In this scenario, α1-GABARs resident to inhibitory synapses form the hardwiring of neuronal circuits with receptors of a different composition fulfilling a fundamental, but unappreciated, role in synapse strengthening.

Involvement of NADPH oxidases in suppression of cyclooxygenase-2 promoter-dependent transcriptional activities by sesamol

Cyclooxygenase-2 (COX-2) has been shown to play an important role in colon carcinogenesis. Moreover, one of the components of reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, NADPH oxidase 1 (NOX1), dominantly expressed in the colon, is implicated in the pathogenesis of colon cancer. We have reported that sesamol, one of the lignans in sesame seeds, suppressed COX-2 gene transcriptional activity in human colon cancer cells, and also suppressed intestinal polyp formation in Apc-mutant mice. In the present study, we investigated the involvement of NADPH oxidase in the inhibition of COX-2 transcriptional activity by sesamol. We found that several NADPH oxidase inhibitors, such as apocynin, showed suppressive effects on COX-2 transcriptional activity. Moreover, sesamol significantly suppressed NOX1 mRNA levels in a dose-dependent manner. In addition, we demonstrated that knockdown of NOX1 successfully suppressed COX-2 transcriptional activity. These results suggest that inhibition of NADPH oxidase, especially NOX1, may be involved in the mechanism of the suppression of COX-2 transcriptional activity by sesamol.

Oxidative stress as a mechanism of added sugar-induced cardiovascular disease

Added sugars comprising of table sugar, brown sugar, corn syrup, maple syrup, honey, molasses, and other sweeteners in the prepared processed foods and beverages have been implicated in the pathophysiology of cardiovascular diseases. This article deals with the reactive oxygen species (ROS) as a mechanism of sugar-induced cardiovascular diseases. There is an association between the consumption of high levels of serum glucose with cardiovascular diseases. Various sources of sugar-induced generation of ROS, including mitochondria, nicotinamide adenine dinucleotide phosphate-oxidase, advanced glycation end products, insulin, and uric acid have been discussed. The mechanism by which ROS induce the development of atherosclerosis, hypertension, peripheral vascular disease, coronary artery disease, cardiomyopathy, heart failure, and cardiac arrhythmias have been discussed in detail. In conclusion, the data suggest that added sugars induce atherosclerosis, hypertension, peripheral vascular disease, coronary artery disease, cardiomyopathy, heart failure, and cardiac arrhythmias and that these effects of added sugars are mediated through ROS.

Mitochondrial ion channels/transporters as sensors and regulators of cellular redox signaling

SIGNIFICANCE

Mitochondrial ion channels/transporters and the electron transport chain (ETC) serve as key sensors and regulators for cellular redox signaling, the production of reactive oxygen species (ROS) and nitrogen species (RNS) in mitochondria, and balancing cell survival and death. Although the functional and pharmacological characteristics of mitochondrial ion transport mechanisms have been extensively studied for several decades, the majority of the molecular identities that are responsible for these channels/transporters have remained a mystery until very recently.

RECENT ADVANCES

Recent breakthrough studies uncovered the molecular identities of the diverse array of major mitochondrial ion channels/transporters, including the mitochondrial Ca2+ uniporter pore, mitochondrial permeability transition pore, and mitochondrial ATP-sensitive K+ channel. This new information enables us to form detailed molecular and functional characterizations of mitochondrial ion channels/transporters and their roles in mitochondrial redox signaling.

CRITICAL ISSUES

Redox-mediated post-translational modifications of mitochondrial ion channels/transporters and ETC serve as key mechanisms for the spatiotemporal control of mitochondrial ROS/RNS generation.

FUTURE DIRECTIONS

Identification of detailed molecular mechanisms for redox-mediated regulation of mitochondrial ion channels will enable us to find novel therapeutic targets for many diseases that are associated with cellular redox signaling and mitochondrial ion channels/transporters.

Mitochondrial H2O2 as an enable signal for triggering autophosphorylation of insulin receptor in neurons

Background

Insulin receptors are widely distributed in the brain, where they play roles in synaptic function, memory formation, and neuroprotection. Autophosphorylation of the receptor in response to insulin stimulation is a critical step in receptor activation. In neurons, insulin stimulation leads to a rise in mitochondrial H₂O₂ production, which plays a role in receptor autophosphorylation. However, the kinetic characteristics of the H₂O₂ signal and its functional relationships with the insulin receptor during the autophosphorylation process in neurons remain unexplored to date.

Results

Experiments were carried out in culture of rat cerebellar granule neurons. Kinetic study showed that the insulin-induced H₂O₂ signal precedes receptor autophosphorylation and represents a single spike with a peak at 5–10 s and duration of less than 30 s. Mitochondrial complexes II and, to a lesser extent, I are involved in generation of the H₂O₂ signal. The mechanism by which insulin triggers the H₂O₂ signal involves modulation of succinate dehydrogenase activity. Insulin dose–response for receptor autophosphorylation is well described by hyperbolic function (Hill coefficient, n_H, of 1.1±0.1; R²=0.99). N-acetylcysteine (NAC), a scavenger of H₂O₂, dose-dependently inhibited receptor autophosphorylation. The observed dose response is highly sigmoidal (Hill coefficient, n_H, of 8.0±2.3; R²=0.97), signifying that insulin receptor autophosphorylation is highly ultrasensitive to the H₂O₂ signal. These results suggest that autophosphorylation occurred as a gradual response to increasing insulin concentrations, only if the H₂O₂ signal exceeded a certain threshold. Both insulin-stimulated receptor autophosphorylation and H₂O₂ generation were inhibited by pertussis toxin, suggesting that a pertussis toxin-sensitive G protein may link the insulin receptor to the H₂O₂-generating system in neurons during the autophosphorylation process.

Conclusions

In this study, we demonstrated for the first time that the receptor autophosphorylation occurs only if mitochondrial H₂O₂ signal exceeds a certain threshold. This finding provides novel insights into the mechanisms underlying neuronal response to insulin. The neuronal insulin receptor is activated if two conditions are met: 1) insulin binds to the receptor, and 2) the H₂O₂ signal surpasses a certain threshold, thus, enabling receptor autophosphorylation in all-or-nothing manner. Although the physiological rationale for this control remains to be determined, we propose that malfunction of mitochondrial H₂O₂ signaling may lead to the development of cerebral insulin resistance.

H2O2 Signalling Pathway: A Possible Bridge between Insulin Receptor and Mitochondria

This review is focused on the mechanistic aspects of the insulin-induced H2O2 signalling pathway in neurons and the molecules affecting it, which act as risk factors for developing central insulin resistance. Insulin-induced H2O2 promotes insulin receptor activation and the mitochondria act as the insulin-sensitive H2O2 source, providing a direct molecular link between mitochondrial dysfunction and irregular insulin receptor activation. In this view, the accumulation of dysfunctional mitochondria during chronological ageing and Alzheimer's disease (AD) is a risk factor that may contribute to the development of dysfunctional cerebral insulin receptor signalling and insulin resistance. Due to the high significance of insulin-induced H2O2 for insulin receptor activation, oxidative stress-induced upregulation of antioxidant enzymes, e.g., in AD brains, may represent another risk factor contributing to the development of insulin resistance. As insulin-induced H2O2 signalling requires fully functional mitochondria, pharmacological strategies based on activating mitochondria biogenesis in the brain are central to the treatment of diseases associated with dysfunctional insulin receptor signalling in this organ.

Non-steroidal anti-inflammatory drugs activate NADPH oxidase in adipocytes and raise the H2O2 pool to prevent cAMP-stimulated protein kinase a activation and inhibit lipolysis

BACKGROUND

Non-steroidal anti-inflammatory drugs (NSAIDs) -aspirin, naproxen, nimesulide, and piroxicam- lowered activation of type II cAMP-dependent protein kinase A (PKA-II) in isolated rat adipocytes, decreasing adrenaline- and dibutyryl cAMP (Bt2cAMP)-stimulated lipolysis. The molecular bases of insulin-like actions of NSAID were studied.

RESULTS

Based on the reported inhibition of lipolysis by H2O2, catalase was successfully used to block NSAID inhibitory action on Bt2cAMP-stimulated lipolysis. NSAID, at (sub)micromolar range, induced an H2O2 burst in rat adipocyte plasma membranes and in whole adipocytes. NSAID-mediated rise of H2O2 was abrogated in adipocyte plasma membranes by: diphenyleneiodonium, an inhibitor of NADPH oxidase (NOX); the NOX4 antibody; and cytochrome c, trapping the NOX-formed superoxide. These three compounds prevented the inhibition of Bt2cAMP-stimulated lipolysis by NSAIDs. Inhibition of aquaporin-mediated H2O2 transport with AgNO3 in adipocytes allowed NOX activation but prevented the lipolysis inhibition promoted by NSAID: i.e., once synthesized, H2O2 must reach the lipolytic machinery. Since insulin inhibits adrenaline-stimulated lipolysis, the effect of aspirin on isoproterenol-stimulated lipolysis in rat adipocytes was studied. As expected, isoproterenol-mediated lipolysis was blunted by both insulin and aspirin.

CONCLUSIONS

NSAIDs activate NOX4 in adipocytes to produce H2O2, which impairs cAMP-dependent PKA-II activation, thus preventing isoproterenol-activated lipolysis. H2O2 signaling in adipocytes is a novel and important cyclooxygenase-independent effect of NSAID.

Redox regulation of protein kinases

Protein kinases represent one of the largest families of genes found in eukaryotes. Kinases mediate distinct cellular processes ranging from proliferation, differentiation, survival, and apoptosis. Ligand-mediated activation of receptor kinases can lead to the production of endogenous hydrogen peroxide (H₂O₂) by membrane-bound NADPH oxidases. In turn, H₂O₂ can be utilized as a secondary messenger in signal transduction pathways. This review presents an overview of the molecular mechanisms involved in redox regulation of protein kinases and its effects on signaling cascades. In the first half, we will focus primarily on receptor tyrosine kinases (RTKs), whereas the latter will concentrate on downstream non-receptor kinases involved in relaying stimulant response. Select examples from the literature are used to highlight the functional role of H₂O₂ regarding kinase activity, as well as the components involved in H₂O₂ production and regulation during cellular signaling. In addition, studies demonstrating direct modulation of protein kinases by H₂O₂ through cysteine oxidation will be emphasized. Identification of these redox-sensitive residues may help uncover signaling mechanisms conserved within kinase subfamilies. In some cases, these residues can even be exploited as targets for the development of new therapeutics. Continued efforts in this field will further basic understanding of kinase redox regulation, and delineate the mechanisms involved in physiological and pathological H₂O₂ responses.

NOX4 pathway as a source of selective insulin resistance and responsiveness

OBJECTIVE

Type 2 diabetes mellitus and related syndromes exhibit a deadly triad of dyslipoproteinemia, which leads to atherosclerosis; hyperglycemia, which causes microvascular disease; and hypertension. These features share a common, but unexplained, origin-namely, pathway-selective insulin resistance and responsiveness. Here, we undertook a comprehensive characterization of pathway-selective insulin resistance and responsiveness in liver and hepatocytes by examining 18 downstream targets of the insulin receptor, surveying the AKT, ERK, and NAD(P)H oxidase 4 pathways.

METHODS AND RESULTS

Injection of insulin into hyperphagic, obese type 2 diabetic db/db mice failed to inactivate hepatic protein tyrosine phosphatase gene family members, a crucial action of NAD(P)H oxidase 4 previously thought to be required for all signaling through AKT and ERK. Insulin-stimulated type 2 diabetic livers unexpectedly produced an unusual form of AKT that was phosphorylated at Thr308 (pT308), with only weak insulin-stimulated phosphorylation at Ser473. Remarkably, knockdown or inhibition of NAD(P)H oxidase 4 in cultured hepatocytes recapitulated the entire complicated pattern of pathway-selective insulin resistance and responsiveness seen in vivo-namely, monophosphorylated pT308-AKT, impaired insulin-stimulated pathways for lowering plasma lipids and glucose, but continued lipogenic pathways and robust ERK activation.

CONCLUSIONS

Functional disturbance of a single molecule, NAD(P)H oxidase 4, is sufficient to induce the key harmful features of deranged insulin signaling in type 2 diabetes mellitus, obesity, and other conditions associated with hyperinsulinemia and pathway-selective insulin resistance and responsiveness.

Lipid raft redox signaling: molecular mechanisms in health and disease

Lipid rafts, the sphingolipid and cholesterol-enriched membrane microdomains, are able to form different membrane macrodomains or platforms upon stimulations, including redox signaling platforms, which serve as a critical signaling mechanism to mediate or regulate cellular activities or functions. In particular, this raft platform formation provides an important driving force for the assembling of NADPH oxidase subunits and the recruitment of other related receptors, effectors, and regulatory components, resulting, in turn, in the activation of NADPH oxidase and downstream redox regulation of cell functions. This comprehensive review attempts to summarize all basic and advanced information about the formation, regulation, and functions of lipid raft redox signaling platforms as well as their physiological and pathophysiological relevance. Several molecular mechanisms involving the formation of lipid raft redox signaling platforms and the related therapeutic strategies targeting them are discussed. It is hoped that all information and thoughts included in this review could provide more comprehensive insights into the understanding of lipid raft redox signaling, in particular, of their molecular mechanisms, spatial-temporal regulations, and physiological, pathophysiological relevances to human health and diseases.

Never-ageing cellular senescence

Cellular senescence was historically discovered as a form of cellular ageing of in vitro cultured cells. It has been under the spotlight following the evidence of oncogene-induced senescence in vivo and its role as a potent tumour suppressor mechanism. Presently, a PubMed search using keywords ‘cellular senescence and cancer’ reveals 8398 number of references (by April 2011) showing that while our knowledge of senescence keeps expanding, the complexity of the phenomenon keeps us – researchers in the field of cancer biology – fascinated and busy. In this short review, we summarise the many cellular pathways leading to cellular senescence and we discuss the latest experimental evidence and the questions emerging in the field.

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.

Signaling functions of reactive oxygen species

We review signaling by reactive oxygen species, which is emerging as a major physiological process. However, among the reactive oxygen species, H(2)O(2) best fulfills the requirements of being a second messenger. Its enzymatic production and degradation, along with the requirements for the oxidation of thiols by H(2)O(2), provide the specificity for time and place that are required in signaling. Both thermodynamic and kinetic considerations suggest that among possible oxidation states of cysteine, formation of sulfenic acid derivatives or disulfides can be relevant as thiol redox switches in signaling. In this work, the general constraints that are required for protein thiol oxidation by H(2)O(2) to be fast enough to be relevant for signaling are discussed in light of the mechanism of oxidation of the catalytic cysteine or selenocysteine in thiol peroxidases. While the nonenzymatic reaction between thiol and H(2)O(2) is, in most cases, too slow to be relevant in signaling, the enzymatic catalysis of thiol oxidation by these peroxidases provides a potential mechanism for redox signaling.

Signaling functions of reactive oxygen species

We review signaling by reactive oxygen species, which is emerging as a major physiological process. However, among the reactive oxygen species, H(2)O(2) best fulfills the requirements of being a second messenger. Its enzymatic production and degradation, along with the requirements for the oxidation of thiols by H(2)O(2), provide the specificity for time and place that are required in signaling. Both thermodynamic and kinetic considerations suggest that among possible oxidation states of cysteine, formation of sulfenic acid derivatives or disulfides can be relevant as thiol redox switches in signaling. In this work, the general constraints that are required for protein thiol oxidation by H(2)O(2) to be fast enough to be relevant for signaling are discussed in light of the mechanism of oxidation of the catalytic cysteine or selenocysteine in thiol peroxidases. While the nonenzymatic reaction between thiol and H(2)O(2) is, in most cases, too slow to be relevant in signaling, the enzymatic catalysis of thiol oxidation by these peroxidases provides a potential mechanism for redox signaling.

Reactive oxygen species mediate mitogenic growth factor signaling pathways in human leiomyoma smooth muscle cells

Uterine leiomyomas are benign uterine tumors characterized by extracellular matrix remodeling, increased collagen deposition, and increased smooth muscle cell (SMC) proliferation. The reactive oxygen species (ROS) producing NADPH oxidase complex has been shown to be involved in the signaling pathways of several growth factors, cytokines, and vasoactive agents that stimulate proliferation of a variety of cell types. Our objective was to test the hypothesis that ROS derived from NADPH oxidase is a necessary component of the MAP kinase mitogenic pathway activated by platelet derived growth factor (PDGF) and epidermal growth factor (EGF) in leiomyoma SMCs (LSMCs). Primary cell cultures of LSMCs were used as our experimental model. Our results showed that stimulation of these cells with PDGF or EGF caused a marked increase in intracellular ROS production and that the NADPH oxidase inhibitor, DPI, blocks ROS production. In addition, inhibition of ROS production by NADPH oxidase inhibitors blocked, in a dose-dependent manner, the EGF- and PDGF-induced increase in [(3)H]thymidine incorporation by LSMCs. Furthermore, an exogenous source of ROS, hydrogen peroxide, was sufficient to stimulate [(3)H]thymidine incorporation in LSMCs but did not affect COL1A2 and COL3A1 mRNA levels. Inhibition of the NADPH oxidase complex decreased PDGF-induced MAPK1/MAPK3 activation, whereas exogenous hydrogen peroxide induced MAPK1/MAPK3 activation. This article is the first report suggesting the presence of the NADPH oxidase system and its importance in mitogenic signaling pathways in LSMCs. The necessity of NADPH oxidase-derived ROS for EGF and PDGF signaling pathways leading to cell proliferation points to another potential therapeutic target for treatment and/or prevention of uterine leiomyomas.

Positive and negative regulation of insulin signaling by reactive oxygen and nitrogen species

Regulated production of reactive oxygen species (ROS)/reactive nitrogen species (RNS) adequately balanced by antioxidant systems is a prerequisite for the participation of these active substances in physiological processes, including insulin action. Yet, increasing evidence implicates ROS and RNS as negative regulators of insulin signaling, rendering them putative mediators in the development of insulin resistance, a common endocrine abnormality that accompanies obesity and is a risk factor of type 2 diabetes. This review deals with this dual, seemingly contradictory, function of ROS and RNS in regulating insulin action: the major processes for ROS and RNS generation and detoxification are presented, and a critical review of the evidence that they participate in the positive and negative regulation of insulin action is provided. The cellular and molecular mechanisms by which ROS and RNS are thought to participate in normal insulin action and in the induction of insulin resistance are then described. Finally, we explore the potential usefulness and the challenges in modulating the oxidant-antioxidant balance as a potentially promising, but currently disappointing, means of improving insulin action in insulin resistance-associated conditions, leading causes of human morbidity and mortality of our era.

Mitochondrial respiratory chain is involved in insulin-stimulated hydrogen peroxide production and plays an integral role in insulin receptor autophosphorylation in neurons

Background

Accumulated evidence suggests that hydrogen peroxide (H₂O₂) generated in cells during insulin stimulation plays an integral role in insulin receptor signal transduction. The role of insulin-induced H₂O₂ in neuronal insulin receptor activation and the origin of insulin-induced H₂O₂ in neurons remain unclear. The aim of the present study is to test the following hypotheses (1) whether insulin-induced H₂O₂ is required for insulin receptor autophosphorylation in neurons, and (2) whether mitochondrial respiratory chain is involved in insulin-stimulated H₂O₂ production, thus playing an integral role in insulin receptor autophosphorylation in neurons.

Results

Insulin stimulation elicited rapid insulin receptor autophosphorylation accompanied by an increase in H₂O₂ release from cultured cerebellar granule neurons (CGN). N-acetylcysteine (NAC), a H₂O₂ scavenger, inhibited both insulin-stimulated H₂O₂ release and insulin-stimulated autophosphorylation of insulin receptor. Inhibitors of respiratory chain-mediated H₂O₂ production, malonate and carbonyl cyanide-4-(trifluoromethoxy)-phenylhydrazone (FCCP), inhibited both insulin-stimulated H₂O₂ release from neurons and insulin-stimulated autophosphorylation of insulin receptor. Dicholine salt of succinic acid, a respiratory substrate, significantly enhanced the effect of suboptimal insulin concentration on the insulin receptor autophosphorylation in CGN.

Conclusion

Results of the present study suggest that insulin-induced H₂O₂ is required for the enhancement of insulin receptor autophosphorylation in neurons. The mitochondrial respiratory chain is involved in insulin-stimulated H₂O₂ production, thus playing an integral role in the insulin receptor autophosphorylation in neurons.

Use and abuse of exogenous H2O2 in studies of signal transduction

The goal of this review is to present a rationale for the use of exogenous H(2)O(2), which has been demonstrated to have both toxicological and physiological signaling roles. Reasons for the use of exogenous application of nontoxic concentrations of H(2)O(2) in model systems and caveats for interpretation of the data obtained will both be presented. Briefly, an argument for the cautious use of the addition of exogenous H(2)O(2) is that, because of the permeability of cell membranes to this neutral small molecule, a concentration that is produced locally and that is necessary for the physiological action can be mimicked. On the other hand, it must be recognized that the addition of an agent or its enzymatic generation in the medium may produce reactions that may not normally occur because the total dose of H(2)O(2) and the concentration of H(2)O(2) in some cellular locations will exceed what is normally achieved even under a pathophysiological state. For this reason, this review will try to provide an unbiased balanced pros- and -cons analysis of this issue.

Human Mpv17-like protein is localized in peroxisomes and regulates expression of antioxidant enzymes

M-LP (Mpv17-like protein) is a protein that was initially identified in mouse tissues and shows high sequence homology with Mpv17 protein, a peroxisomal membrane protein involved in the development of early-onset glomerulosclerosis [R. Iida, T. Yasuda, E. Tsubota, H. Takatsuka, M. Masuyama, T. Matsuki, K. Kishi, M-LP, Mpv17-like protein, has a peroxisomal membrane targeting signal comprising a transmembrane domain and a positively charged loop and up-regulates expression of the manganese superoxide dismutase gene, J. Biol. Chem. 278 (2003) 6301-6306]. Here we report the identification and characterization of a human homolog of the M-LP (M-LPH) gene. The M-LPH gene is composed of four exons, extends over 14kb on chromosome 16p13.1, and is expressed as two alternatively spliced variants comprising four and three exons, respectively, which include open-reading frames encoding two distinct isoforms composed of 196 (M-LPH1) and 147 (M-LPH2) amino acids, respectively. These two variants were expressed ubiquitously in human tissues, however only M-LPH1 was detected at the protein level. Dual-color confocal analysis of COS-7 cells transfected with a green fluorescent protein-tagged M-LPH1 demonstrated that M-LPH1 is localized in peroxisomes. In order to elucidate the function of M-LPH1, we examined the mRNA levels of several enzymes involved in the metabolism of reactive oxygen species in COS-7 cells and found that transfection with M-LPH1 down-regulates expression of the plasma glutathione peroxidase and catalase genes. These results show the existence of the human homolog of M-LP and its participation in reactive oxygen species metabolism.

Role of insulin-induced reactive oxygen species in the insulin signaling pathway

Oxidants, including hydrogen peroxide (H2O2), have been recognized for years to mimic insulin action on glucose transport in adipose cells. Early studies also demonstrated the complementary finding that H2O2 was elaborated during treatment of cells with insulin, suggesting that cellular H2O2 generation was integral to insulin signaling. Recently, reactive oxygen species elicited by various hormones and growth factors have been shown to affect signal transduction pathways in various cell types. We recently reported that insulin-stimulated H2O2 modulates proximal and distal insulin signaling, at least in part through the oxidative inhibition of protein tyrosine phosphatases (PTPases) that negatively regulate the insulin action pathway. Nox4, a homologue in the family of NADPH oxidase catalytic subunits, was found to be prominently expressed in insulin-sensitive cells. By various molecular approaches, Nox4 was shown to mediate insulin-stimulated H2O2 generation and impact the insulin signaling cascade. Overexpression of Nox4 also significantly reversed the inhibition of insulin-stimulated receptor tyrosine phosphorylation by PTP1B, a widely expressed PTPase implicated in the negative regulation of insulin signaling, by inhibiting its catalytic activity. These recent studies have provided insight into Nox4 as a novel molecular link between insulin-stimulated reactive oxygen species and mechanisms involved in their modulation of insulin signal transduction.

Redox paradox: insulin action is facilitated by insulin-stimulated reactive oxygen species with multiple potential signaling targets

Propelled by the identification of a small family of NADPH oxidase (Nox) enzyme homologs that produce superoxide in response to cellular stimulation with various growth factors, renewed interest has been generated in characterizing the signaling effects of reactive oxygen species (ROS) in relation to insulin action. Two key observations made >30 years ago-that oxidants can facilitate or mimic insulin action and that H(2)O(2) is generated in response to insulin stimulation of its target cells-have led to the hypothesis that ROS may serve as second messengers in the insulin action cascade. Specific molecular targets of insulin-induced ROS include enzymes whose signaling activity is modified via oxidative biochemical reactions, leading to enhanced insulin signal transduction. These positive responses to cellular ROS may seem "paradoxical" because chronic exposure to relatively high levels of ROS have also been associated with functional beta-cell impairment and the chronic complications of diabetes. The best-characterized molecular targets of ROS are the protein-tyrosine phosphatases (PTPs) because these important signaling enzymes require a reduced form of a critical cysteine residue for catalytic activity. PTPs normally serve as negative regulators of insulin action via the dephosphorylation of the insulin receptor and its tyrosine-phosphorylated cellular substrates. However, ROS can rapidly oxidize the catalytic cysteine of target PTPs, effectively blocking their enzyme activity and reversing their inhibitory effect on insulin signaling. Among the cloned Nox homologs, we have recently provided evidence that Nox4 may mediate the insulin-stimulated generation of cellular ROS and is coupled to insulin action via the oxidative inhibition of PTP1B, a PTP known to be a major regulator of the insulin signaling cascade. Further characterization of the molecular components of this novel signaling cascade, including the mechanism of ROS generated by insulin and the identification of various oxidation-sensitive signaling targets in insulin-sensitive cells, may provide a novel means of facilitating insulin action in states of insulin resistance.

[Regulation of gene expression by active oxygen species]

Reactive oxygen species (ROS) are generated from cells stimulated by various cytokines, hormones, and stresses, and regulate cellular functions such as gene expression and cell growth. They affect activities of many types of molecular targets, including signaling molecules and transcription factors. Early-response genes (c-fos, egr-I and JE) that encode transcription factors are induced by ROS, and activities of their products are modulated by ROS through redox-based mechanisms. We isolated a novel gene, hic-5, that was induced by hydrogen peroxide and encodes a focal adhesion protein. hic-5 was found to translocate to the nucleus in cells treated with ROS and regulates several cellular genes. We propose that hic-5 is a key element in the transduction of signals from the cell surface to the nucleus under oxidative stress.

Reactive oxygen intermediates involved in cellular regulation

Reactive oxygen intermediates (ROIs) in low concentration, as released permanently by nonphagocytic cells, possess important functions in inter- and intracellular signalling. They lead to alterations in the phosphorylation pattern followed by gene activation, including the expression of proto-oncogenes. Redox-sensitive sites in membrane molecules may trigger adhesion and chemotaxis or open ion channels and activate transport processes across the cytoplasma membrane. ROIs shift the ratio of cyclic GMP to cyclic AMP giving signals to proliferation and differentiation processes. Senescence, apoptosis, and cell death can also be modulated by ROIs, depending on concentration and cell type.

The role of cysteine in the regulation of blood glutathione-protein mixed disulfides in rats treated with diamide

The kinetics of GSH, GSSG, and thiol-protein mixed disulfides (RS-SP) of GSH (GS-SP) and cysteine (CYS-SP) were studied in rat blood and liver in the time range 0-120 min after treatment with 100 and 200 mg/kg i.p. of diamide. Total consumption (10 min) and regeneration (120 min) of blood GSH, matched by parallel increases and decreases in RS-SP, were observed. GSSG did not change appreciably. No dose-effect relationship was obtained with either treatment. On the contrary, in vitro treatment of blood with 0.75 mM diamide provoked the same trends of GSH and RS-SP as in vivo (e.g., reversible modifications), whereas treatment with 1.5 mM caused drops and rises in GSH and RS-SP, respectively, without any subsequent return to control values. The presence of a hematic factor responsible for RS-SP regulation is hypothesized in the in vivo experiment. Successive experiments involving in vitro pretreatment with 2 mM diamide and treatment with 0.5 mM of various thiols indicated that cysteine (CYS), but not GSH or N-acetylcysteine, rapidly restored erythrocyte GSH and RS-SP to their basal levels. No evident sign of hemolysis was observed in these experiments. These results indicate that CYS is a diffusible thiol important for RS-SP regulation. Analysis of whole blood of rats treated with 100 mg/kg i.p. diamide and the presence of two reversible peaks (about 10 times the corresponding control level) of CYS-SP and free CYS confirmed the plausible role of CYS in maintaining the reversibility of the process. Preliminary results in liver of rats treated with 100 mg/kg diamide indicated that CYS may act by metabolic cooperation between organs. We suggest that CYS may have a role in the regulation of the intracellular redox state of rat erythrocytes during oxidative stress.

General method to identify and enrich vicinal thiol proteins present in intact cells in the oxidized, disulfide state

Some 5% of the soluble proteins of L1210 murine leukemia lymphoblasts contain surface vicinal thiols (Kalef, E., Walfish, P. G., and Gitler, C. (1993) Anal. Biochem. 212, 325-334). Redox dithiol to intraprotein disulfide conversion could regulate the cellular function of these proteins. A general method is presented to identify and enrich vicinal thiol proteins existing in cells in their oxidized, disulfide state. The method is based on the in situ blockage by cell permeable N-ethylmaleimide (NEM) of readily accessible cellular protein sulfhydryls. Following removal of the excess NEM, disulfide-containing proteins were identified by reduction with DTT and specific labeling with N-iodoacetyl-[125I]-3-iodotyrosine. The vicinal thiol proteins formed could also be enriched, prior to labeling with [125I]IAIT, by their selective binding to Sepha-rose-aminohexanoyl-4-aminophenylarsine oxide. Exponentially growing L1210 lymphoblasts contain more than 20 proteins with thiols in the oxidized, disulfide state. The majority derive from vicinal thiol proteins. The fraction oxidized, in some proteins, represents almost the totality of the protein present in the cell. Exposure of lymphoblasts to diamide increases the number and concentration of proteins with intraprotein disulfides. This method allows sensitive direct identification of vicinal thiol proteins that participate in redox regulation and those that are targets to oxidative stress conditions.

Redox regulation of tyrosine phosphorylation in human spermatozoa and its role in the control of human sperm function

The redox status of human spermatozoa was found to have a profound influence on the fertilizing potential of these cells in association with qualitative and quantitative changes in the patterns of tyrosine phosphorylation. In general, oxidizing conditions enhanced tyrosine phosphorylation and stimulated sperm function, whereas reducing conditions had the opposite effect. Unstimulated human spermatozoa exhibited low levels of spontaneous acrosomal exocytosis and sperm-oocyte fusion and minimal reactive oxygen species generation, while phosphotyrosine expression was largely confined to a single protein of 116 kDa. However, if the spermatozoa were exposed to oxidizing conditions through the addition of exogenous H2O2, or the stimulation of endogenous NADPH-dependent reactive oxygen species generation, then a dramatic increase in tyrosine phosphorylation was observed (major phosphotyrosyl bands at 222 kDa, 200 kDa, 159 kDa, 133 kDa, 116 kDa and 82 kDa) in concert with the functional activation of the spermatozoa. A causal association between reactive oxygen species generation, tyrosine phosphorylation and sperm function was indicated by studies with the ionophore, A23187, which induced high rates of spermoocyte fusion together with enhanced rates of reactive oxygen species production and the increased expression of phosphotyrosyl proteins. This functional response to A23187 could be abrogated, without any concomitant change in sperm motility or viability, by using membrane permeant thiols or catalase to suppress the reactive oxygen species-induced increase in phosphotyrosine expression. The fact that the biological responses of human spermatozoa to biological agonists (recombinant human ZP3 and progesterone) could also be inhibited by catalase indicated the general relevance of these findings.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by N-acetyl-L-cysteine of interleukin-6 mRNA induction and activation of NF kappa B by tumor necrosis factor alpha in a mouse fibroblastic cell line, Balb/3T3

Redox-based modulation plays a role in transcriptional control of gene expression. In the present study, we investigated the possible role of reactive oxygen species in the induction of interleukin-6 (IL-6) mRNA and in increases in NF kappa B binding activity by tumor necrosis factor (TNF) alpha using a mouse fibroblastic cell line, Balb/3T3. Expression of IL-6 mRNA is known to be dependent upon NF kappa B that binds to the 5'-flanking region of the IL-6 gene. We found that: (i) TNF alpha increased IL-6 mRNA levels and this increase was inhibited by N-acetyl-L-cysteine (NAC), a scavenger of reactive oxygen species. (ii) NF kappa B binding activity in this cell line was also increased by TNF alpha, and the increase was inhibited in the presence of NAC. (iii) The treatment of cells with low doses of hydrogen peroxide increased the NF kappa B binding activity. (iv) Expression of a reporter gene in which the chloramphenicol acetyltransferase (CAT) gene was under the control of NF kappa B binding sites was induced by hydrogen peroxide. These results suggest that the induction of IL-6 mRNA is regulated by a mechanism involving reactive oxygen species and that NF kappa B, whose activity is sensitive to the cellular redox state, plays an important role in this induction in a fibroblastic cell line, Balb/3T3, stimulated with TNF alpha.

Production of hydrogen peroxide by transforming growth factor-beta 1 and its involvement in induction of egr-1 in mouse osteoblastic cells

TGF-beta 1 controls the expression of numerous genes, including early response and cellular matrix genes. However, the signal-transducing mechanism underlying this regulation of gene expression is not fully understood. In this study, we investigated whether redox regulation plays a role in the TGF-beta 1 signal transduction in the mouse osteoblastic cell line (MC3T3-E1). The overall intracellular oxidized state of the cells, when measured using 2',7'-dichlorofluorescin diacetate by laser-scanning confocal microscopy, was increased transiently after the addition of TGF-beta 1. This increase was abolished by the addition of oxygen radical scavengers such as catalase and N-acetylcysteine. In a variant cell line lacking the TGF-beta 1 receptor, the intracellular oxidized state was not modulated by treatment with TGF-beta 1. We then examined the expression of early growth response-1 (egr-1) gene, which is inducible by TGF-beta 1 and H2O2. Radical scavengers inhibited the induction of egr-1 by TGF-beta 1, but not that by 12-O-tetradecanoylphorbol-13 acetate. A nuclear run-on assay indicated that this inhibition was at the transcriptional level. From transient expression experiments using chloramphenicol acetyltransferase gene linked to serially deleted egr-1 gene 5'-upstream region, the CArG element in the 5' flanking region of egr-1 was identified as an essential sequence in the transcriptional activation for both TGF-beta 1 and H2O2 stimulation. These findings suggest that H2O2 acts as a mediator for the TGF-beta 1-induced transcription of egr-1 gene.

NADH oxidase of plasma membranes

NADH oxidase is a cyanide-resistant and hormone-responsive oxidase intrinsic to the plasma membrane of both plant and animal cells. The activity has many unique characteristics that distinguish it from other oxidases and oxidoreductases of both organelles and internal membranes and from other oxidoreductases of the plasma membrane. Among these are resistance to inhibition by cyanide, catalase, superoxide dismutase, and phenylchloromercuribenzoate. Activity is stimulated by hormones and growth factors and inhibited by quinone analogs such as piercidin, the flavin antagonist atebrin, and growth inhibiting gangliosides such as GM3. In marked contact to the NADH-ferricyanide oxidoreductase of the plasma membrane, the NADH oxidase is activated by lysophospholipids and fatty acids, products of phospholipase A2 action, in a time-dependent manner suggestive of stabilization of an activated form of the enzyme. The hormone-responsive NADH oxidase of the plasma membrane is not a peroxidase and may function as a terminal oxidase to link transfer of electrons from NADH to oxygen at the plasma membrane. The functional significance of the NADH oxidase of the plasma membrane is unknown but some relationship to growth or growth control is indicated. In both animal and plant plasma membranes, the oxidase is activated by growth factors and hormones to which the cells or tissues of origin have functional hormone or growth factor receptors. In addition, substances that inhibit the oxidase, the associated transmembrane reductase or both, inhibit growth. In transformed cells and tissues, the hormone and growth factor responsiveness of the NADH oxidase is reduced or absent. With human keratinocytes which exhibit an increased sensitivity to the antiproliferative action of both retinoic acid and calcitriol, the NADH oxidase of the plasma membrane is strongly inhibited by these agents and shows the same increased sensitivity. If transfer of electrons from NADH to oxygen across or within the eukaryotic plasma membrane is an important aspect of growth or growth control, then the hormone- and growth factor-responsive NADH oxidase associated with the plasma membrane could be of fundamental importance. Because of its low basal activity, stimulation by growth factors and hormones, and the inhibition of growth in direct proportion to inhibition of the oxidase, the activity is a candidate as a rate-limiting step in the growth process. Completely unknown is the mechanism whereby NADH oxidation and growth or growth control may be coupled. This, together with further characterization of the activity and the mechanism of loss of control with neoplastic transformation, represent important challenges for future investigations.

NADH oxidase of liver plasma membrane stimulated by diferric transferrin and neoplastic transformation induced by the carcinogen 2-acetylaminofluorene

NADH oxidase of purified plasma membranes (electron transfer from NADH to oxygen) was stimulated by the growth factor diferric transferrin. This stimulation was of an activity not inhibited by cyanide and was not seen in plasma membranes prepared from hyperplastic nodules from liver of animals fed the hepatocarcinogen, 2-acetylaminofluorene, nor was it due to reduction of iron associated with diferric transferrin. With plasma membranes from nodules, the activity was already elevated and the added transferrin was without effect. The stimulation by diferric transferrin did not correlate with the absence of transferrin receptors which were increased at the nodule plasma membranes. With liver plasma membranes, the stimulation by diferric transferrin raised the plasma membrane NADH oxidase specific activity to approximately that of the nodule plasma membranes. In contrast to NADH oxidase, which was markedly stimulated by the diferric transferrin, NADH ferricyanide oxidoreductase or reduction of ferric ammonium citrate by liver plasma membranes was approximately equal to or slightly greater than that of the nodule plasma membrane and unaffected by diferric transferrin. The results suggest the possibility of coupling of NADH oxidase activity to a growth factor response in mammalian cells as observed previously for this enzyme in another system.