Mapping sites of interaction of p47-phox and flavocytochrome b with random-sequence peptide phage display libraries

NADPH Oxidases in the Central Nervous System: Regional and Cellular Localization and the Possible Link to Brain Diseases

Significance: The significant role of reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (Nox) in signal transduction is mediated by the production of reactive oxygen species (ROS), especially in the central nervous system (CNS). The pathogenesis of some neurologic and psychiatric diseases is regulated by ROS, acting as a second messenger or pathogen. Recent Advances: In the CNS, the involvement of Nox-derived ROS has been implicated in the regulation of multiple signals, including cell survival/apoptosis, neuroinflammation, migration, differentiation, proliferation, and synaptic plasticity, as well as the integrity of the blood/brain barrier. In these processes, the intracellular signals mediated by the members of the Nox family vary among different tissues. The present review illuminates the regions and cellular, subcellular localization of Nox isoforms in the brain, the signal transduction, and the role of NOX enzymes in pathophysiology, respectively. Critical Issues: Different signal transduction cascades are coupled to ROS derived from various Nox homologues with varying degrees. Therefore, a critical issue worth noting is the varied role of the homologues of NOX enzymes in different signaling pathways and also they mediate different phenotypes in the diverse pathophysiological condition. This substantiates the effectiveness of selective Nox inhibitors in the CNS. Future Directions: Further investigation to elucidate the role of various homologues of NOX enzymes in acute and chronic brain diseases and signaling mechanisms, and the development of more specific NOX inhibitors for the treatment of CNS disease are urgently needed. Antioxid. Redox Signal. 35, 951-973.

NADPH Oxidases (NOX): An Overview from Discovery, Molecular Mechanisms to Physiology and Pathology

The reactive oxygen species (ROS)-producing enzyme NADPH oxidase (NOX) was first identified in the membrane of phagocytic cells. For many years, its only known role was in immune defense, where its ROS production leads to the destruction of pathogens by the immune cells. NOX from phagocytes catalyzes, via one-electron trans-membrane transfer to molecular oxygen, the production of the superoxide anion. Over the years, six human homologs of the catalytic subunit of the phagocyte NADPH oxidase were found: NOX1, NOX3, NOX4, NOX5, DUOX1, and DUOX2. Together with the NOX2/gp91phox component present in the phagocyte NADPH oxidase assembly itself, the homologs are now referred to as the NOX family of NADPH oxidases. NOX are complex multidomain proteins with varying requirements for assembly with combinations of other proteins for activity. The recent structural insights acquired on both prokaryotic and eukaryotic NOX open new perspectives for the understanding of the molecular mechanisms inherent to NOX regulation and ROS production (superoxide or hydrogen peroxide). This new structural information will certainly inform new investigations of human disease. As specialized ROS producers, NOX enzymes participate in numerous crucial physiological processes, including host defense, the post-translational processing of proteins, cellular signaling, regulation of gene expression, and cell differentiation. These diversities of physiological context will be discussed in this review. We also discuss NOX misregulation, which can contribute to a wide range of severe pathologies, such as atherosclerosis, hypertension, diabetic nephropathy, lung fibrosis, cancer, or neurodegenerative diseases, giving this family of membrane proteins a strong therapeutic interest.

The NADPH Oxidase Family and Its Inhibitors

Significance: The oxidative stress, resulting from an imbalance in the production and scavenging of reactive oxygen species (ROS), is known to be involved in the development and progression of several pathologies. The excess of ROS production is often due to an overactivation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOX) and for this reason these enzymes became promising therapeutic targets. However, even if NOX are now well characterized, the development of new therapies is limited by the lack of highly isoform-specific inhibitors. Recent Advances: In the past decade, several groups and laboratories have screened thousands of molecules to identify new specific inhibitors with low off-target effects. These works have led to the characterization of several new potent NOX inhibitors; however, their specificity varies a lot depending on the molecules. Critical Issues: Here, we are reviewing more than 25 known NOX inhibitors, focusing mainly on the newly identified ones such as APX-115, NOS31, Phox-I1 and 2, GLX7013114, and GSK2795039. To have a better overall view of these molecules, the inhibitors were classified according to their specificity, from pan-NOX inhibitors to highly isoform-specific ones. We are also presenting the use of these compounds both in vitro and in vivo. Future Directions: Several of these new molecules are potent and very specific inhibitors that could be good candidates for the development of new drugs. Even if the results are very promising, most of these compounds were only validated in vitro or in mice models and further investigations will be required before using them as potential therapies.

NADPH oxidases: Current aspects and tools

Reactive oxygen species (ROS) have been shown or at least suggested to play an essential role for cellular signaling as second messengers. NADPH oxidases represent a source of controlled ROS formation. Accordingly, understanding the role of individual NADPH oxidases bears potential to interfere with intracellular signaling cascades without disturbing the signaling itself. Many tools have been developed to study or inhibit the functions and roles of the NADPH oxidases. This short review summarizes diseases, potentially associated with NADPH oxidases, genetically modified animals, and inhibitors.

Rational Design and Delivery of NOX-Inhibitory Peptides

A growing appreciation of NADPH oxidases (NOXs) as mediators of fundamental physiological processes and as important players in myriad diseases has led many laboratories on a search for specific inhibitors to help dissect the role in a given pathway or pathological condition. To date, there are only a few available inhibitors with a demonstrated specificity for a given isozyme. Among those, peptidic inhibitors have the advantage of being designed to target very specific protein-protein interactions that are essential for NOX activity. Herein, we provide the techniques to deliver these inhibitors both in cell culture as well as in vivo.

Design of a mimotope-peptide based double epitope vaccine against disseminated candidiasis

Hematogenously disseminated candidiasis in humans is the third leading cause of nosocomial bloodstream infections in the US. There is no FDA approved antifungal vaccine or prophylactic/therapeutic antibody for use in humans. We first reported novel synthetic peptide and glycopeptide vaccines against Candida albicans cell surface epitopes that protect mice against disseminated candidiasis. We showed that antibodies specific for the peptide Fba (derived from C. albicans cell surface protein fructose bisphosphate aldolase) or for C. albicans cell surface glycan epitope β-1, 2-mannotriose [β-(Man)₃]) are both protective. This is an important step forward in vaccine design against disseminated candidiasis in humans. However, given the complexity of oligosaccharide synthesis, in this study we performed a new strategy for use of peptide mimotopes that structurally mimic the protective glycan epitope β-(Man)₃ as surrogate immunogens that substitute for the glycan part of glycopeptide [β-(Man)₃-Fba] vaccine. All five selected mimotopes are immunogenic in mice and three mimotopes were able to induce protection in mice against disseminated candidiasis. Furthermore, immunization with three mimotope-peptide conjugate vaccines was also able to induce specific antibody responses, and importantly, protection against disseminated candidiasis in mice. Therefore, our new design of a mimotope-peptide based double epitope vaccine against candidiasis is a potential vaccine candidate that is economical to produce, highly efficacious and safe for use in humans.

Sources of Vascular Nitric Oxide and Reactive Oxygen Species and Their Regulation

Nitric oxide (NO) is a small free radical with critical signaling roles in physiology and pathophysiology. The generation of sufficient NO levels to regulate the resistance of the blood vessels and hence the maintenance of adequate blood flow is critical to the healthy performance of the vasculature. A novel paradigm indicates that classical NO synthesis by dedicated NO synthases is supplemented by nitrite reduction pathways under hypoxia. At the same time, reactive oxygen species (ROS), which include superoxide and hydrogen peroxide, are produced in the vascular system for signaling purposes, as effectors of the immune response, or as byproducts of cellular metabolism. NO and ROS can be generated by distinct enzymes or by the same enzyme through alternate reduction and oxidation processes. The latter oxidoreductase systems include NO synthases, molybdopterin enzymes, and hemoglobins, which can form superoxide by reduction of molecular oxygen or NO by reduction of inorganic nitrite. Enzymatic uncoupling, changes in oxygen tension, and the concentration of coenzymes and reductants can modulate the NO/ROS production from these oxidoreductases and determine the redox balance in health and disease. The dysregulation of the mechanisms involved in the generation of NO and ROS is an important cause of cardiovascular disease and target for therapy. In this review we will present the biology of NO and ROS in the cardiovascular system, with special emphasis on their routes of formation and regulation, as well as the therapeutic challenges and opportunities for the management of NO and ROS in cardiovascular disease.

CRISPR/Cas9-mediated modification of the extreme C-terminus impairs PDGF-stimulated activity of Duox2

Duox2 belongs to the large family of NADPH-oxidase enzymes that are implicated in immune response, vasoregulation, hormone synthesis, cell growth and differentiation via the regulated synthesis of H2O2 and reactive oxygen species. We and others have shown that Duox2 and H2O2 are involved in platelet-derived growth factor (PDGF) induced migration of fibroblasts. Now, using the CRISPR/Cas9-mediated genome editing we demonstrate that the extreme C-terminal region of Duox2 is required for PDGF-stimulated activity of Duox2 and H2O2 production. We generated the fibroblast cells that stably co-express the wild-type or C-terminally modified Duox2 and fluorescent H2O2 probe Hyper. We found that nonsense substitution of the last 23 amino acids in Duox2 results in complete loss of PDGF stimulation of intracellular H2O2 and fibroblast migration, yet these mutations have no effects on the expression of Duox2 and other NADPH-oxidases in cells. These findings illustrate for the first time that the extreme C-terminus of Duox2 is required for the functional activity of the enzyme. Furthermore, the conservative nature of the C-terminus suggests its role for activity in other NADPH-oxidases.

Characterization of CD38 in the major cell types of the heart: endothelial cells highly express CD38 with activation by hypoxia-reoxygenation triggering NAD(P)H depletion

The NAD(P)⁺-hydrolyzing enzyme CD38 is activated in the heart during the process of ischemia and reperfusion, triggering NAD(P)(H) depletion. However, the presence and role of CD38 in the major cell types of the heart are unknown. Therefore, we characterize the presence and function of CD38 in cardiac myocytes, endothelial cells, and fibroblasts. To comprehensively evaluate CD38 in these cells, we measured gene transcription via mRNA, as well as protein expression and enzymatic activity. Endothelial cells strongly expressed CD38, while only low expression was present in cardiac myocytes with intermediate levels in fibroblasts. In view of this high level expression in endothelial cells and the proposed role of CD38 in the pathogenesis of endothelial dysfunction, endothelial cells were subjected to hypoxia-reoxygenation to characterize the effect of this stress on CD38 expression and activity. An activity-based CD38 imaging method and CD38 activity assays were used to characterize CD38 activity in normoxic and hypoxic-reoxygenated endothelial cells, with marked CD38 activation seen following hypoxia-reoxygenation. To test the impact of hypoxia-reoxygenation-induced CD38 activation on endothelial cells, NAD(P)(H) levels and endothelial nitric oxide synthase (eNOS)-derived NO production were measured. Marked NADP(H) depletion with loss of NO and increase in superoxide production occurred following hypoxia-reoxygenation that was prevented by CD38 inhibition or knockdown. Thus, endothelial cells have high expression of CD38 which is activated by hypoxia-reoxygenation triggering CD38-mediated NADP(H) depletion with loss of eNOS-mediated NO generation and increased eNOS uncoupling. This demonstrates the importance of CD38 in the endothelium and explains the basis by which CD38 triggers post-ischemic endothelial dysfunction.

NOX Activation by Subunit Interaction and Underlying Mechanisms in Disease

Nicotinamide adenine dinucleotide phosphate (NAPDH) oxidase (NOX) is an enzyme complex with the sole function of producing superoxide anion and reactive oxygen species (ROS) at the expense of NADPH. Vital to the immune system as well as cellular signaling, NOX is also involved in the pathologies of a wide variety of disease states. Particularly, it is an integral player in many neurological diseases, including stroke, TBI, and neurodegenerative diseases. Pathologically, NOX produces an excessive amount of ROS that exceed the body’s antioxidant ability to neutralize them, leading to oxidative stress and aberrant signaling. This prevalence makes it an attractive therapeutic target and as such, NOX inhibitors have been studied and developed to counter NOX’s deleterious effects. However, recent studies of NOX have created a better understanding of the NOX complex. Comprised of independent cytosolic subunits, p47-phox, p67-phox, p40-phox and Rac, and membrane subunits, gp91-phox and p22-phox, the NOX complex requires a unique activation process through subunit interaction. Of these subunits, p47-phox plays the most important role in activation, binding and translocating the cytosolic subunits to the membrane and anchoring to p22-phox to organize the complex for NOX activation and function. Moreover, these interactions, particularly that between p47-phox and p22-phox, are dependent on phosphorylation initiated by upstream processes involving protein kinase C (PKC). This review will look at these interactions between subunits and with PKC. It will focus on the interaction involving p47-phox with p22-phox, key in bringing the cytosolic subunits to the membrane. Furthermore, the implication of these interactions as a target for NOX inhibitors such as apocynin will be discussed as a potential avenue for further investigation, in order to develop more specific NOX inhibitors based on the inhibition of NOX assembly and activation.

Nox Inhibitors & Therapies: Rational Design of Peptidic and Small Molecule Inhibitors

Oxidative stress-related diseases underlie many if not all of the major leading causes of death in United States and the Western World. Thus, enormous interest from both academia and pharmaceutical industry has been placed on the development of agents which attenuate oxidative stress. With that in mind, great efforts have been placed in the development of inhibitors of NADPH oxidase (Nox), the major enzymatic source of reactive oxygen species and oxidative stress in many cells and tissue. The regulation of a catalytically active Nox enzyme involves numerous protein-protein interactions which, in turn, afford numerous targets for inhibition of its activity. In this review, we will provide an updated overview of the available Nox inhibitors, both peptidic and small molecules, and discuss the body of data related to their possible mechanisms of action and specificity towards each of the various isoforms of Nox. Indeed, there have been some very notable successes. However, despite great commitment by many in the field, the need for efficacious and well-characterized, isoform-specific Nox inhibitors, essential for the treatment of major diseases as well as for delineating the contribution of a given Nox in physiological redox signalling, continues to grow.

Regulation of NADPH oxidases in skeletal muscle

The only known function of NAD(P)H oxidases is to produce reactive oxygen species (ROS). Skeletal muscles express three isoforms of NAD(P)H oxidases (Nox1, Nox2, and Nox4) that have been identified as critical modulators of redox homeostasis. Nox2 acts as the main source of skeletal muscle ROS during contractions, participates in insulin signaling and glucose transport, and mediates the myocyte response to osmotic stress. Nox2 and Nox4 contribute to skeletal muscle abnormalities elicited by angiotensin II, muscular dystrophy, heart failure, and high fat diet. Our review addresses the expression and regulation of NAD(P)H oxidases with emphasis on aspects that are relevant to skeletal muscle. We also summarize: i) the most widely used NAD(P)H oxidases activity assays and inhibitors, and ii) studies that have defined Nox enzymes as protagonists of skeletal muscle redox homeostasis in a variety of health and disease conditions.

Modulation of NADPH-oxidase gene expression in rolB-transformed calli of Arabidopsis thaliana and Rubia cordifolia

Expression of rol genes from Agrobacterium rhizogenes induces reprogramming of transformed plant cells and provokes pleiotropic effects on primary and secondary metabolism. We have previously established that the rolB and rolC genes impair reactive oxygen species (ROS) generation in transformed cells of Rubia cordifolia and Arabidopsis thaliana. In the present investigation, we tested whether this effect is associated with changes in the expression levels of NADPH oxidases, which are considered to be the primary source of ROS during plant-microbe interactions. We identified two full-length NADPH oxidase genes from R. cordifolia and examined their expression in non-transformed and rolB-transformed calli. In addition, we examined the expression of their homologous genes from A. thaliana in non-transformed and rolB-expressing cells. The expression of Rboh isoforms was 3- to 7-fold higher in both R. cordifolia and A. thaliana rolB-transformed cells compared with non-transformed cells. Our results for the first time show that Agrobacterium rolB gene regulates particular NADPH oxidase isoforms.

NADPH oxidases: an overview from structure to innate immunity-associated pathologies

Oxygen-derived free radicals, collectively termed reactive oxygen species (ROS), play important roles in immunity, cell growth, and cell signaling. In excess, however, ROS are lethal to cells, and the overproduction of these molecules leads to a myriad of devastating diseases. The key producers of ROS in many cells are the NOX family of NADPH oxidases, of which there are seven members, with various tissue distributions and activation mechanisms. NADPH oxidase is a multisubunit enzyme comprising membrane and cytosolic components, which actively communicate during the host responses to a wide variety of stimuli, including viral and bacterial infections. This enzymatic complex has been implicated in many functions ranging from host defense to cellular signaling and the regulation of gene expression. NOX deficiency might lead to immunosuppression, while the intracellular accumulation of ROS results in the inhibition of viral propagation and apoptosis. However, excess ROS production causes cellular stress, leading to various lethal diseases, including autoimmune diseases and cancer. During the later stages of injury, NOX promotes tissue repair through the induction of angiogenesis and cell proliferation. Therefore, a complete understanding of the function of NOX is important to direct the role of this enzyme towards host defense and tissue repair or increase resistance to stress in a timely and disease-specific manner.

The quest for selective nox inhibitors and therapeutics: challenges, triumphs and pitfalls

SIGNIFICANCE

Numerous studies in animal models and human subjects corroborate that elevated levels of reactive oxygen species (ROS) play a pivotal role in the progression of multiple diseases. As a major source of ROS in many organ systems, the NADPH oxidase (Nox) has become a prime target for therapeutic development.

RECENT ADVANCES

In recent years, intense efforts have been dedicated to the development of pan- and isoform-specific Nox inhibitors as opposed to antioxidants that proved ineffective in clinical trials. Over the past decade, an array of compounds has been proposed in an attempt to fill this void.

CRITICAL ISSUES

Although many of these compounds have proven effective as Nox enzyme family inhibitors, isoform specificity has posed a formidable challenge to the scientific community. This review surveys the most prominent Nox inhibitors, and discusses potential isoform specificity, known mechanisms of action, and shortcomings. Some of these inhibitors hold substantial promise as targeted therapeutics.

FUTURE DIRECTIONS

Increased insight into the mechanisms of action and regulation of this family of enzymes as well as atomic structures of key Nox subunits are expected to give way to a broader spectrum of more potent, efficacious, and specific molecules. These lead molecules will assuredly serve as a basis for drug development aimed at treating a wide array of diseases associated with increased Nox activity.

NADPH oxidase as a therapeutic target for oxalate induced injury in kidneys

A major role of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase family of enzymes is to catalyze the production of superoxides and other reactive oxygen species (ROS). These ROS, in turn, play a key role as messengers in cell signal transduction and cell cycling, but when they are produced in excess they can lead to oxidative stress (OS). Oxidative stress in the kidneys is now considered a major cause of renal injury and inflammation, giving rise to a variety of pathological disorders. In this review, we discuss the putative role of oxalate in producing oxidative stress via the production of reactive oxygen species by isoforms of NADPH oxidases expressed in different cellular locations of the kidneys. Most renal cells produce ROS, and recent data indicate a direct correlation between upregulated gene expressions of NADPH oxidase, ROS, and inflammation. Renal tissue expression of multiple NADPH oxidase isoforms most likely will impact the future use of different antioxidants and NADPH oxidase inhibitors to minimize OS and renal tissue injury in hyperoxaluria-induced kidney stone disease.

Effects of angiotensin II on the cerebral circulation: role of oxidative stress

Oxidative stress has emerged as a key component of many diseases that affect the vasculature. Oxidative stress is characterized as a cellular environment where the generation of oxidant molecules overwhelms endogenous anti-oxidant defense mechanisms. NADPH oxidases are a family of enzymes whose primary purpose is generation of reactive oxygen species (oxidant molecules) and therefore are likely to be key contributors to oxidative stress. Hypertension is associated with oxidative stress in the vasculature and is a major risk factor for stroke and cognitive abnormalities. Angiotensin II (Ang II) is the main effector peptide of the renin-angiotensin system (RAS) and plays a critical role in promoting oxidative stress in the vasculature. In the cerebral circulation, Ang II has been implicated in reactive oxygen species generation, alterations to vasomotor function, impaired neurovascular coupling, inflammation, and vascular remodeling. Furthermore, studies in humans have shown that cerebral blood flow is altered during hypertension and therapeutically targeting the RAS improves cerebral blood flow. Importantly, many of the aforementioned effects have been shown to be dependent on NADPH oxidases. Thus, Ang II, NADPH oxidases and oxidative stress are likely to play key roles in the pathogenesis of hypertension and associated cerebrovascular disease. This review will focus on our current understanding of the contribution of Ang II and NADPH oxidases to oxidative stress in the cerebral circulation.

NOX2β: A novel splice variant of NOX2 that regulates NADPH oxidase activity in macrophages

Nox2 oxidase is one isoform in a family of seven NADPH oxidases that generate reactive oxygen species (ROS) and thereby contribute to physiological and pathological processes including host defense, redox signaling and oxidative tissue damage. While alternative mRNA splicing has been shown to influence the activity of several Nox-family proteins, functionally relevant splice variants of Nox2 have not previously been identified. We immunoscreened several mouse tissues and cells for the presence of truncated Nox2 proteins and identified a 30 kDa protein in lung, spleen and macrophages. RT-PCR analysis of mRNA from primary and immortalised (RAW264.7) mouse macrophages, and from human alveolar macrophages, identified a truncated Nox2 transcript which, upon sequence analysis, was found to be a product of the ‘exon skipping’ mode of alternative splicing, lacking exons 4–10 of the Nox2 gene. The predicted protein is comparable in size to that identified by immunoscreening and contains two transmembrane helices and an extended cytosolic C-terminus with binding sites for NADPH and the Nox organiser protein p47phox. Importantly, selective siRNA-mediated knockdown of the transcript reduced expression of the 30 kDa protein in macrophages, and suppressed phorbol ester-stimulated ROS production by 50%. We thus provide the first evidence that Nox2 undergoes alternative mRNA splicing to yield a 30 kDa protein – herein termed Nox2β – that regulates NADPH oxidase activity in macrophages from mice and humans. The discovery of Nox2β paves the way for future examination of its role in physiological and pathological processes.

Towards specific NADPH oxidase inhibition by small synthetic peptides

Reactive oxygen species (ROS) production by the phagocyte NADPH oxidase is essential for host defenses against pathogens. ROS are very reactive with biological molecules such as lipids, proteins and DNA, potentially resulting in cell dysfunction and tissue insult. Excessive NADPH oxidase activation and ROS overproduction are believed to participate in disorders such as joint, lung, vascular and intestinal inflammation. NADPH oxidase is a complex enzyme composed of six proteins: gp91phox (renamed NOX2), p22phox, p47phox, p67phox, p40phox and Rac1/2. Inhibitors of this enzyme could be beneficial, by limiting ROS production and inappropriate inflammation. A few small non-peptide inhibitors of NADPH oxidase are currently used to inhibit ROS production, but they lack specificity as they inhibit NADPH oxidase homologues or other unrelated enzymes. Peptide inhibitors that target a specific sequence of NADPH oxidase components could be more specific than small molecules. Here we review peptide-based inhibitors, with particular focus on a molecule derived from gp91phox/NOX2 and p47phox, and discuss their possible use as specific phagocyte NADPH oxidase inhibitors.

Strategies for identifying synthetic peptides to act as inhibitors of NADPH oxidases, or "all that you did and did not want to know about Nox inhibitory peptides"

Phagocytes utilize reactive oxygen species (ROS) to kill pathogenic microorganisms. The source of ROS is an enzymatic complex (the NADPH oxidase), comprising a membrane-associated heterodimer (flavocytochrome b (558)), consisting of subunits Nox2 and p22(phox), and four cytosolic components (p47(phox), p67(phox), p40(phox), and Rac). The primordial ROS (superoxide) is generated by the reduction of molecular oxygen by NADPH via redox centers located on Nox2. This process is activated by the translocation of the cytosolic components to the membrane and their assembly with Nox2. Membrane translocation is preceded by interactions among cytosolic components. A number of proteins structurally and functionally related to Nox2 have been discovered in many cells (the Nox family) and these have pleiotropic functions related to the production of ROS. An intense search is underway to design therapeutic means to modulate Nox-dependent overproduction of ROS, associated with diseases. Among drug candidates, a central position is held by synthetic peptides reflecting domains in oxidase components involved in NADPH oxidase assembly. Peptides, corresponding to domains in Nox2, p22(phox), p47(phox), and Rac, found to be oxidase activation inhibitory in vitro, are reviewed. Usually, peptides are inhibitory only when added preceding assembly of the complex. Although competition with intact components seems most likely, less obvious mechanisms are, sometimes, at work. The use of peptides as inhibitory drugs in vivo requires the development of methods to assure cell penetration, resistance to degradation, and avoidance of toxicity, and modest successes have been achieved. The greatest challenge remains the discovery of peptide inhibitors acting specifically on individual Nox isoforms.

The NADPH oxidase family and its inhibitors

The classical nicotinamide adenine dinucleotide phosphate (NADPH) oxidase was originally detected in neutrophils as a multicomponent enzyme that catalyzes the generation of superoxide from oxygen and the reduced form of NADPH. This enzyme is composed of two membrane-bound subunits (p22phox and gp91phox), three cytosolic subunits (p67phox, p47phox, and p40phox) and a small G-protein Rac (Rac1 and Rac2). Recently, it has been demonstrated that there are several isoforms of nonphagocytic NADPH oxidase. Endothelial cells, vascular smooth muscle cells or adventitial fibroblasts possess multiple isoforms of this enzyme. The new homologs, along with gp91phox are now designated the Nox family of NADPH oxidases and are key sources of reactive oxygen species in the vasculature. Reactive oxygen species play a significant role in regulating endothelial function and vascular tone. However, besides the participation in the processes of physiological cell, these enzymes can also be the perpetrator of oxidative stress that causes endothelial dysfunction. This review summarizes the current state of knowledge of the structure and functions of NADPH oxidase and NADPH oxidase inhibitors in the treatment of disorders with endothelial damage.

NADPH oxidases as regulators of tumor angiogenesis: current and emerging concepts

SIGNIFICANCE

Reactive oxygen species (ROS) such as superoxide, hydrogen peroxide, and peroxynitrite are generated ubiquitously by all mammalian cells and have been understood for many decades as inflicting cell damage and as causing cancer by oxidation and nitration of macromolecules, including DNA, RNA, proteins, and lipids.

RECENT ADVANCES

A current concept suggests that ROS can also promote cell signaling pathways triggered by growth factors and transcription factors that ultimately regulate cell proliferation, differentiation, and apoptosis, all of which are important hallmarks of tumor cell proliferation and angiogenesis. Moreover, an emerging concept indicates that ROS regulate the functions of immune cells that infiltrate the tumor environment and stimulate angiogenesis, such as macrophages and specific regulatory T cells.

CRITICAL ISSUES

In this article, we highlight that the NADPH oxidase family of ROS-generating enzymes are the key sources of ROS and, thus, play an important role in redox signaling within tumor, endothelial, and immune cells thereby promoting tumor angiogenesis.

FUTURE DIRECTIONS

Knowledge of these intricate ROS signaling pathways and identification of the culprit NADPH oxidases is likely to reveal novel therapeutic opportunities to prevent angiogenesis that occurs during cancer and which is responsible for the revascularization after current antiangiogenic treatment.

NADPH oxidase inhibitors: a decade of discovery from Nox2ds to HTS

NADPH oxidases (Nox) are established as major sources of reactive oxygen species (ROS). Over the past two decades, Nox-derived ROS have emerged as pivotal in the development of myriad diseases involving oxidative stress. In contrast, Nox are also involved in signaling mechanisms necessary for normal cell function. The study of these enzymes in physiological and pathophysiological conditions is made considerably more complex by the discovery of 7 isoforms: Nox1 through 5 as well as Duox1 and 2, each with its own specific cytosolic components, regulatory control mechanisms, subcellular localization and/or tissue distribution. A clear understanding of the role individual isoforms play in a given system is hindered by the lack of isoform-specific inhibitors. In animal models, knockdown or knockout methodologies are providing definitive answers to perplexing questions of the complex interplay of multiple Nox isoforms in cell and tissue signaling. However, the complex structures and interactions of these heteromeric isozymes predict pleiotropic actions of the Nox subunits and thus suppression of these proteins is almost certain to have untoward effects. Thus, as both therapies and pharmacological tools, molecule-based inhibitors continue to prove extremely useful and rational in design. Unfortunately, many of the available inhibitors have proven non-specific, falling into the category of scavengers or inhibitors of more than one source of ROS. Here, we will review some of the efforts that have been undertaken to develop specific inhibitors of NADPH oxidase over the past decade, from the peptidic inhibitor Nox2ds-tat to more recent small molecule inhibitors that have emerged from high-throughput screening campaigns.

Constitutive NADPH oxidase 4 activity resides in the composition of the B-loop and the penultimate C terminus

Redox regulation of signaling molecules contributes critically to propagation of intracellular signals. The main source providing reactive oxygen species (ROS) for these physiological processes are activated NADPH oxidases (Nox/Duox family). In a pathophysiological context, some NADPH oxidase complexes produce large amounts of ROS either as part of the antimicrobial immune defense or as pathologic oxidative stress in many chronic diseases. Thus, understanding the switch from a dormant, inactive conformation to the active state of these enzymes will aid the development of inhibitors. As exogenously expressed Nox4 represents the only constitutively active enzyme in this family, analysis of structural determinants that permit this active conformation was undertaken. Our focus was directed toward a cell-based analysis of the first intracellular loop, the B-loop, and the C-terminus, two regions of Nox family enzymes that are essential for electron transfer. Mutagenesis of the B-loop identified several unique residues and a polybasic motif that contribute to the catalytic activity of Nox4. By using a multifaceted approach, including Nox4-Nox2 chimeras, mutagenesis, and insertion of Nox2 domains, we show here that the penultimate 22 amino acids of Nox4 are involved in constitutive ROS generation. The appropriate spacing of the C-terminal Nox4 sequence may cooperate with a discrete arginine-based interaction site in the B-loop, providing an intrinsically active interface that could not be disrupted by peptides derived from the Nox4 C-terminus. These results indicate that accessibility for a Nox4-specific peptide inhibitor might be difficult to achieve in vivo.

Inhibition of NADPH oxidase activation by peptides mapping within the dehydrogenase region of Nox2-A "peptide walking" study

In this study, the "peptide walking" approach was applied to the DH region of Nox2 (residues 288-570) with the purpose of identifying domains of functional importance in the assembly and/or catalytic function of the NADPH oxidase complex of phagocytes. Ninety-one overlapping 15-mer peptides were synthesized to cover the full length of the Nox2 DH region, and these were tested for the ability to interfere with the activation of the oxidase in vitro in two semi-recombinant cell-free systems. The first consisted of phagocyte membranes p47(phox), p67(phox), and Rac1 and an amphiphile; the second was p47(phox)- and amphiphile-free and contained prenylated Rac1. We identified 10 clusters of inhibitory peptides with IC(50) values of 10 μM, all of which were inhibitory, also in the absence of p47(phox). Based on the identification of residues shared by peptides in a particular cluster, we defined 10 functional domains in the Nox2 DH region. One domain corresponded to one FAD-binding subdomain, and four domains overlapped parts of three NADPH-binding subdomains. As expected, most inhibitory peptides acted only when added prior to the completion of oxidase assembly, but peptides associated with two NADPH-binding subdomains were also active after assembly. Kinetic analysis demonstrated that inhibition by peptides was not explained by competition for substrates (FAD, NADPH) but was of a more complex nature: noncompetitive with respect to FAD and uncompetitive with respect to NADPH. We conclude that oxidase-inhibitory peptides, in five out of 10 clusters identified, act by interfering with FAD- and NADPH-related redox reactions.

Regulation of reactive oxygen species generation in cell signaling

Reactive oxygen species (ROS) including superoxide anion and hydrogen peroxide (H(2)O(2)) are thought to be byproducts of aerobic respiration with damaging effects on DNA, protein, and lipid. A growing body of evidence indicates, however, that ROS are involved in the maintenance of redox homeostasis and various cellular signaling pathways. ROS are generated from diverse sources including mitochondrial respiratory chain, enzymatic activation of cytochrome p450, and NADPH oxidases further suggesting involvement in a complex array of cellular processes. This review summarizes the production and function of ROS. In particular, how cytosolic and membrane proteins regulate ROS generation for intracellular redox signaling will be detailed.

Nox2 B-loop peptide, Nox2ds, specifically inhibits the NADPH oxidase Nox2

In recent years, reactive oxygen species (ROS) derived from the vascular isoforms of NADPH oxidase, Nox1, Nox2, and Nox4, have been implicated in many cardiovascular pathologies. As a result, the selective inhibition of these isoforms is an area of intense current investigation. In this study, we postulated that Nox2ds, a peptidic inhibitor that mimics a sequence in the cytosolic B-loop of Nox2, would inhibit ROS production by the Nox2-, but not the Nox1- and Nox4-oxidase systems. To test our hypothesis, the inhibitory activity of Nox2ds was assessed in cell-free assays using reconstituted systems expressing the Nox2-, canonical or hybrid Nox1-, or Nox4-oxidase. Our findings demonstrate that Nox2ds, but not its scrambled control, potently inhibited superoxide (O(2)(•-)) production in the Nox2 cell-free system, as assessed by the cytochrome c assay. Electron paramagnetic resonance confirmed that Nox2ds inhibits O(2)(•-) production by Nox2 oxidase. In contrast, Nox2ds did not inhibit ROS production by either Nox1- or Nox4-oxidase. These findings demonstrate that Nox2ds is a selective inhibitor of Nox2-oxidase and support its utility to elucidate the role of Nox2 in organ pathophysiology and its potential as a therapeutic agent.

Combating oxidative stress in vascular disease: NADPH oxidases as therapeutic targets

NADPH oxidases are a family of enzymes that generate reactive oxygen species (ROS). The NOX1 (NADPH oxidase 1) and NOX2 oxidases are the major sources of ROS in the artery wall in conditions such as hypertension, hypercholesterolaemia, diabetes and ageing, and so they are important contributors to the oxidative stress, endothelial dysfunction and vascular inflammation that underlies arterial remodelling and atherogenesis. In this Review, we advance the concept that compared to the use of conventional antioxidants, inhibiting NOX1 and NOX2 oxidases is a superior approach for combating oxidative stress. We briefly describe some common and emerging putative NADPH oxidase inhibitors. In addition, we highlight the crucial role of the NADPH oxidase regulatory subunit, p47phox, in the activity of vascular NOX1 and NOX2 oxidases, and suggest how a better understanding of its specific molecular interactions may enable the development of novel isoform-selective drugs to prevent or treat cardiovascular diseases.

Lipopolysaccharide primes the respiratory burst of Atlantic salmon SHK-1 cells through protein kinase C-mediated phosphorylation of p47phox

The superoxide-producing NADPH oxidase complex of phagocytes plays a crucial role in host defenses against microbial infection. NADPH oxidase consists of a membrane heterodimeric protein, composed of gp91phox and p22phox, and the cytosolic proteins, p40phox, p47phox and p67phox. In the present study, we clone and sequence the full-length cDNAs coding for the Atlantic salmon (Salmo salar) phagocyte NADPH oxidase components, p47phox, p67phox and gp91phox, using a homology cloning approach. The sequences of these cDNAs showed that the S. salar p47phox, p67phox and gp91phox genes contained single open reading frames, which encoded predicted proteins of 413, 504 and 565 amino acids, respectively. Comparison of the deduced amino acid sequences showed that the S. salar p47phox, p67phox and gp91phox sequences shared 51, 45 and 68% identity with those of human components, respectively. Despite this relatively low homology between salmon and mammalian NADPH oxidase subunits, their functional domains are highly conserved. We also found that the mRNA levels of p47phox, p67phox and gp91phox expression were higher in immune-related tissues, such as kidney, spleen and gill. In addition, infection of the salmon macrophage cell line SHK-1 with Piscirickettsia salmonis induced the expression of p47phox, but had no effect on p67phox and gp91phox expression. Finally, we show for the first time in fish that activation of macrophages with lipopolysaccharide promotes the activation of protein kinase C, which in turn phosphorylates p47phox, leading to NADPH oxidase activation and reactive oxygen species generation. Collectively, these results suggest that the mechanisms of activation of phagocyte NADPH oxidase are well conserved from fish to mammals.

Peptide-based inhibitors of the phagocyte NADPH oxidase

Phagocytes such as neutrophils, monocytes and macrophages play an essential role in host defenses against pathogens. To kill these pathogens, phagocytes produce and release large quantities of antimicrobial molecules such as reactive oxygen species (ROS), microbicidal peptides, and proteases. The enzyme responsible for ROS generation is called NADPH oxidase, or respiratory burst oxidase, and is composed of six proteins: gp91phox, p22phox, p47phox, p67phox, p40phox and Rac1/2. The vital importance of this enzyme in host defenses is illustrated by a genetic disorder called chronic granulomatous disease (CGD), in which the phagocyte NADPH oxidase is dysfunctional, leading to life-threatening recurrent bacterial and fungal infections. However, excessive NADPH oxidase activation and ROS over-production can damage surrounding tissues and participate in exaggerated inflammatory processes. As ROS production is believed to be involved in several inflammatory diseases, specific phagocyte NADPH oxidase inhibitors might have therapeutic value. In this commentary, we summarize the structure and activation of the phagocyte NADPH oxidase, and describe pharmacological inhibitors of this enzyme, with particular emphasis on peptide-based inhibitors derived from gp91phox, p22phox and p47phox.

Nox4 B-loop creates an interface between the transmembrane and dehydrogenase domains

By targeting redox-sensitive amino acids in signaling proteins, the NADPH oxidase (Nox) family of enzymes link reactive oxygen species to physiological processes. We previously analyzed the sequences of 107 Nox enzymes and identified conserved regions that are predicted to have important functions in Nox structure or activation. One such region is the cytosolic B-loop, which in Nox1-4 contains a conserved polybasic region. Previous studies of Nox2 showed that certain basic residues in the B-loop are important for activity and translocation of p47(phox)/p67(phox), suggesting this region participates in subunit assembly. However, conservation of this region in Nox4, which does not require p47(phox)/p67(phox), suggested an additional role for the B-loop in Nox function. Here, we show by mutation of Nox4 B-loop residues that this region is important for Nox4 activity. Fluorescence polarization detected binding between Nox4 B-loop peptide and dehydrogenase domain (K(d) = 58 +/- 12 nm). This interaction was weakened with Nox4 R96E B-loop corresponding to a mutation that also markedly decreases the activity of holo-Nox4. Truncations of the dehydrogenase domain localize the B-loop-binding site to the N-terminal half of the NADPH-binding subdomain. Similarly, the Nox2 B-loop bound to the Nox2 dehydrogenase domain, and both the Nox2 and Nox4 interactions were dependent on the polybasic region of the B-loop. These data indicate that the B-loop is critical for Nox4 function; we propose that the B-loop, by binding to the dehydrogenase domain, provides the interface between the transmembrane and dehydrogenase domains of Nox enzymes.

GTPase-Rac enhances depolarization-induced superoxide production by the macula densa during tubuloglomerular feedback

Superoxide (O(2)(-) ) enhances tubuloglomerular feedback (TGF) by scavenging nitric oxide at the macula densa (MD). The primary source of O(2)(-) in the MD during TGF is NADPH oxidase, which is activated by membrane depolarization. While Rac, a small GTP-binding protein, has been shown to enhance NADPH oxidase activity, its role in O(2)(-) generation by the MD is unknown. We hypothesized that depolarization of the MD leads to translocation of Rac to the apical membrane, and its activation, in turn, augments O(2)(-) generation during TGF. We tested this by measuring membrane potential and increased O(2)(-) levels during TGF responses in isolated, perfused tubules containing the intact MD plaque. Switching tubular NaCl from 10 to 80 mM, which induces TGF, depolarized membrane potential by 28.4 + or - 4.5% from control (P < 0.05) and O(2)(-) levels from 124 + or - 19 to 361 + or - 27 U/min. This NaCl-induced depolarization and O(2)(-) generation were blocked by a Cl(-) channel blocker, 5-nitro-2(3-phenylpropylamino) benzoic acid (NPPB; 10(-6) M). Inhibition of Rac blunted NaCl-induced O(2)(-) generation by 47%. When the NaCl content of the MD perfusate was increased from 10 to 80 mM, immunointensity of Rac on the apical side increased from 32 + or - 3.1 to 46 + or - 2.5% of the total immunofluorescence in the MD, indicating that high NaCl induces the translocation of Rac to the apical membrane. This NaCl-induced Rac translocation was blocked by a Cl(-) channel blocker, NPPB, indicating that depolarization of the MD induced Rac translocation. In conclusion, we found that depolarization of the MD during TGF leads to translocation of Rac to the apical membrane, which enhances O(2)(-) generation by the MD.

NOX2 is the primary source of angiotensin II-induced superoxide in the macula densa

Macula densa (MD)-mediated regulation of renal hemodynamics via tubuloglomerular feedback is regulated by interactions between factors such as superoxide (O(2)(-)) and angiotensin II (ANG II). We have reported that NaCl-induced O(2)(-) in the MD is produced by the NOX2 isoform of NADPH oxidase (NOX); however, the source of ANG II-induced O(2)(-) in MD is unknown. Thus we determined the pathways by which ANG II increased O(2)(-) in the MD by measuring O(2)(-) in ANG II-treated MMDD1 cells, a MD-like cell line. ANG II caused MMDD1 O(2)(-) levels to increase by more than twofold (P < 0.01). This increase was blocked by losartan (AT(1) receptor blocker) but not PD-123319 (AT(2) receptor antagonist). Apocynin (a NOX inhibitor) decreased O(2)(-) by 86% (P < 0.01), whereas oxypurinol (a xanthine oxidase inhibitor) and NS-398 (a cyclooxygenase-2 inhibitor) had no significant effect. The NOX-dependent increase in O(2)(-) was due to the NOX2 isoform; a short interfering (si)RNA against NOX2 blunted ANG II-induced increases in O(2)(-), whereas the NOX4/siRNA did not. Finally, we found that inhibiting the Rac1 subunit of NOX blunted ANG II-induced O(2)(-) production in NOX4/siRNA-treated cells but did not further decrease it in NOX2/siRNA-treated cells. Our results indicate that ANG II stimulates O(2)(-) production in the MD primarily via AT(1)-dependent activation of NOX2. Rac1 is required for the full activation of NOX2. This pathway may be an important component of ANG II enhancement of tubuloglomerular feedback.

Structural insights into Nox4 and Nox2: motifs involved in function and cellular localization

Regulated generation of reactive oxygen species (ROS) is primarily accomplished by NADPH oxidases (Nox). Nox1 to Nox4 form a membrane-associated heterodimer with p22(phox), creating the docking site for assembly of the activated oxidase. Signaling specificity is achieved by interaction with a complex network of cytosolic components. Nox4, an oxidase linked to cardiovascular disease, carcinogenesis, and pulmonary fibrosis, deviates from this model by displaying constitutive H(2)O(2) production without requiring known regulators. Extensive Nox4/Nox2 chimera screening was initiated to pinpoint structural motifs essential for ROS generation and Nox subcellular localization. In summary, a matching B loop was crucial for catalytic activity of both Nox enzymes. Substitution of the carboxyl terminus was sufficient for converting Nox4 into a phorbol myristate acetate (PMA)-inducible phenotype, while Nox2-based chimeras never gained constitutive activity. Changing the Nox2 but not the Nox4 amino terminus abolished ROS generation. The unique heterodimerization of a functional Nox4/p22(phox) Y121H complex was dependent on the D loop. Nox4, Nox2, and functional Nox chimeras translocated to the plasma membrane. Cell surface localization of Nox4 or PMA-inducible Nox4 did not correlate with O(2)(-) generation. In contrast, Nox4 released H(2)O(2) and promoted cell migration. Our work provides insights into Nox structure, regulation, and ROS output that will aid inhibitor design.

Identification of structural elements in Nox1 and Nox4 controlling localization and activity

Nox NADPH oxidases differ in their mode of activation, subcellular localization, and physiological function. Nox1 releases superoxide anions (O(2)(-)) and depends on cytosolic activator proteins, whereas Nox4 extracellularly releases hydrogen peroxide (H(2)O(2)), and its activity does not require cotransfection of additional proteins. We constructed chimeric proteins consisting of Nox1 and Nox4 expressed in HEK293 cells. When the cytosolic tail of Nox4 was fused with the transmembrane part of Nox1, Nox1 became constitutively active. The reciprocal construct was inactive, suggesting that cytosolic subunit-dependent activation requires elements in the transmembrane loops. By TIRF-microscopy, Nox1 was observed in the plasma membrane, whereas Nox4 colocalized with proteins of the endoplasmic reticulum. Fusion proteins of Myc and Nox revealed that the N-terminal part of Nox1 but not Nox4 is cleaved. When the potential signal peptide of Nox4 was inserted into Nox1, plasma-membrane localization was lost, and the protein was retained in vesicle-like structures below the plasma membrane. The potential signal peptide of Nox1 failed to translocate Nox4 to the plasma membrane but switched the extracellularly detectable ROS from H(2)O(2) to O(2)(-). Thus, the very N-terminal part of Nox proteins determines subcellular localization and the ROS type released, whereas the cytosolic tail regulates activity.

Characterization and expression of NADPH oxidase in LPS-, poly(I:C)- and zymosan-stimulated trout (Oncorhynchus mykiss W.) macrophages

In vertebrates, the generation of superoxide reactive oxygen species (ROS) via activation of the Nox/Duox family of NADPH oxidases is a prototypical feature of the pathogen-induced defensive responses of activated professional phagocytes. To understand the role of the rainbow trout (Oncorhynchus mykiss) Phox oxidase from a phylogenetic and functional perspective we describe the cloning, sequencing and expression analysis of multiple NADPH components in cultured macrophages. Phylogenetic analyses support the notion of the emergence of Phox-related components before the diversification of basal euteleosts and add to the limited collection of teleost NADPH oxidases. Expression studies using lipopolysaccharide, polyinosine-polycytidylic acid and zymosan to mimic the onset of inflammatory responses in trout macrophages suggest differences in regulation of the NADPH complex throughout the maturation/differentiation period of culture and between different treatments.

Small interfering peptide (siP) for in vivo examination of the developing lung interactonome

To understand the role of reactive oxygen species in mechanosensory control of lung development a new approach to interfere with protein-protein interactions by means of a short interacting peptide was developed. This technology was used in the developing rodent lung to examine the role of NADPH oxidase (NOX), casein kinase 2 (CK2), and the cystic fibrosis transmembrane conductance regulator (CFTR) in stretch-induced differentiation. Interactions between these molecules was targeted in an in utero system with recombinant adeno-associated virus (rAAV) containing inserted DNA sequences that express a control peptide or small interfering peptides (siPs) specific for subunit interaction or phosphorylation predicted to be necessary for multimeric enzyme formation. In all cases only siPs with sequences necessary for a predicted normal function were found to interfere with assembly of the multimeric enzyme. A noninterfering control siP to nonessential regions or reporter genes alone had no effect. Physiologically, it was shown that siPs that interfered with the NOX-CFTR-CK2 complex that we call an "interactonome" affected markers of stretch-induced lung organogenesis including Wnt/beta-catenin signaling.

NOX enzymes as novel targets for drug development

The members of the NOX/DUOX family of NADPH oxidases mediate such physiologic functions as host defense, cell signaling, and thyroid hormone biosynthesis through the generation of reactive oxygen species (ROS), including superoxide anion and hydrogen peroxide. Moreover, ROS are involved in a broad range of fundamental biochemical and cellular processes, and data accumulated in recent years indicate that the NOX enzymes comprise one of the most important biological sources of ROS. Given the high biochemical reactivity of ROS, it is not surprising that they have been implicated in a wide variety of pathologies and diseases. Prominent among the settings that feature ROS-mediated tissue injury are disorders associated with inflammation, aging, and progressive degenerative changes in cells and organ systems, and it appears that essentially no organ system is exempt. Among the disorders currently believed to be mediated at least in part by NOX-derived ROS are hypertension, aortic aneurysm, myocardial infarction (and other ischemia-reperfusion disorders), pulmonary fibrosis and hypertension, amyotropic lateral sclerosis, Alzheimer's disease, Parkinson's disease, ischemic stroke, diabetic nephropathy, and renal cell carcinoma. Several small-molecule and peptide inhibitors of the NOX enzymes have been useful in experimental studies, but issues of specificity, potency, and toxicity militate against any of the existing published compounds as candidates for drug development. Given the broad array of disease targets documented in recent work, the time is here for vigorous efforts to develop clinically useful inhibitors of the NOX enzymes. As most (though not all) NOX-related diseases appear to be mediated by a single member of the NOX family, agents with isoform specificity will be preferred, although broadly active NOX inhibitors may prove to be useful in some settings.

NADPH oxidase inhibitors: new antihypertensive agents?

NADPH oxidases have recently been shown to contribute to the pathogenesis of hypertension. The development of specific inhibitors of these enzymes has focused attention on their potential therapeutic use in hypertensive disease. Two of the most specific inhibitors, gp91ds-tat and apocynin, have been shown to decrease blood pressure in animal models of hypertension. Other inhibitors, including diphenylene iodonium, aminoethyl benzenesulfono fluoride, S17834, PR39, protein kinase C inhibitors, and VAS2870, have shown promise in vitro, but their in vivo specificity, pharmacokinetics, and effectiveness in hypertension remains to be determined. Of importance, the currently available antihypertensive agents angiotensin-converting enzyme inhibitors and angiotensin receptor blockers also effectively inhibit NADPH oxidase activation. Similarly, the cholesterol-lowering agents, statins, have been shown to attenuate NADPH oxidase activation. Although, antioxidants act to scavenge the reactive oxygen species produced by these enzymes, their effectiveness is limited. Targeting NADPH homologues may have a distinct advantage over current therapies because it would specifically prevent the pathophysiological formation of reactive oxygen species that contributes to hypertension.

The NOX family of ROS-generating NADPH oxidases: physiology and pathophysiology

For a long time, superoxide generation by an NADPH oxidase was considered as an oddity only found in professional phagocytes. Over the last years, six homologs of the cytochrome subunit of the phagocyte NADPH oxidase were found: NOX1, NOX3, NOX4, NOX5, DUOX1, and DUOX2. Together with the phagocyte NADPH oxidase itself (NOX2/gp91(phox)), the homologs are now referred to as the NOX family of NADPH oxidases. These enzymes share the capacity to transport electrons across the plasma membrane and to generate superoxide and other downstream reactive oxygen species (ROS). Activation mechanisms and tissue distribution of the different members of the family are markedly different. The physiological functions of NOX family enzymes include host defense, posttranlational processing of proteins, cellular signaling, regulation of gene expression, and cell differentiation. NOX enzymes also contribute to a wide range of pathological processes. NOX deficiency may lead to immunosuppresion, lack of otoconogenesis, or hypothyroidism. Increased NOX activity also contributes to a large number or pathologies, in particular cardiovascular diseases and neurodegeneration. This review summarizes the current state of knowledge of the functions of NOX enzymes in physiology and pathology.

Generation and characterization of monoclonal antibodies against dengue virus type 1 for epitope mapping and serological detection by epitope-based peptide antigens

Dengue virus (DEN), the pathogen behind dengue hemorrhagic fever, remains a public health problem in Asia and South America. In this study, monoclonal antibodies (MAbs) against DEN serotype 1 (DEN-1) were generated by fusing NSI/1-Ag4-1 mouse myeloma cells with lymphocytes from BALB/c mice immunized with DEN-1. Twelve MAbs were found to react specifically to the DENs by enzyme-linked immunosorbent assay, immunofluorescence analysis, and immunoblotting analysis. Five MAbs, namely, DA4-7, DA6-7, DA9-5, DA10-2, and DA11-13, were found to react with envelope proteins of DEN-1. Two serotype-specific MAbs of DEN-1, DA6-7 and DA11-13, were further shown to neutralize DEN-1 infection by a plaque reduction neutralization test. The neutralizing epitopes of these MAbs were further identified from a random peptide library displayed on phage. Immunopositive phage clones reacted specifically with these MAbs and did not react with normal mouse serum. Epitope-based peptide antigens were proved able to detect antibodies in serum samples collected from DEN-1-infected patients but not in those taken from DEN-2-infected patients or healthy controls. We believe that these MAbs and neutralizing epitopes will provide information that will lead to the development of DEN-1 serotype-specific diagnostic reagents and vaccines.

Analysis of human phagocyte flavocytochrome b(558) by mass spectrometry

The catalytic core of the phagocyte NADPH oxidase is a heterodimeric integral membrane protein (flavocytochrome b (Cyt b)) that generates superoxide and initiates a cascade of reactive oxygen species critical for the host inflammatory response. In order to facilitate structural characterization, the present study reports the first direct analysis of human phagocyte Cyt b by matrix-assisted laser desorption/ionization and nanoelectrospray mass spectrometry. Mass analysis of in-gel tryptic digest samples provided 73% total sequence coverage of the gp91(phox) subunit, including three of the six proposed transmembrane domains. Similar analysis of the p22(phox) subunit provided 72% total sequence coverage, including assignment of the hydrophobic N-terminal region and residues that are polymorphic in the human population. To initiate mass analysis of Cyt b post-translational modifications, the isolated gp91(phox) subunit was subject to sequential in-gel digestion with Flavobacterium meningosepticum peptide N-glycosidase F and trypsin, with matrix-assisted laser desorption/ionization and liquid chromatography-mass spectrometry/mass spectrometry used to demonstrate that Asn-132, -149, and -240 are genuinely modified by N-linked glycans in human neutrophils. Since the PLB-985 cell line represents an important model system for analysis of the NADPH oxidase, methods were developed for the purification of Cyt b from PLB-985 membrane fractions in order to confirm the appropriate modification of N-linked glycosylation sites on the recombinant gp91(phox) subunit. This study reports extensive sequence coverage of the integral membrane protein Cyt b by mass spectrometry and provides analytical methods that will be useful for evaluating posttranslational modifications involved in the regulation of superoxide production.

The phosphoinositide-binding protein p40phox activates the NADPH oxidase during FcgammaIIA receptor-induced phagocytosis

Superoxide produced by the phagocyte reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is essential for host defense. Enzyme activation requires translocation of p67^phox, p47^phox, and Rac-GTP to flavocytochrome b₅₅₈ in phagocyte membranes. To examine the regulation of phagocytosis-induced superoxide production, flavocytochrome b₅₅₈, p47^phox, p67^phox, and the FcγIIA receptor were expressed from stable transgenes in COS7 cells. The resulting COS^phoxFcγR cells produce high levels of superoxide when stimulated with phorbol ester and efficiently ingest immunoglobulin (Ig)G-coated erythrocytes, but phagocytosis did not activate the NADPH oxidase. COS7 cells lack p40^phox, whose role in the NADPH oxidase is poorly understood. p40^phox contains SH3 and phagocyte oxidase and Bem1p (PB1) domains that can mediate binding to p47^phox and p67^phox, respectively, along with a PX domain that binds to phosphatidylinositol-3-phosphate (PI(3)P), which is generated in phagosomal membranes. Expression of p40^phox was sufficient to activate superoxide production in COS^phoxFcγR phagosomes. FcγIIA-stimulated NADPH oxidase activity was abrogated by point mutations in p40^phox that disrupt PI(3)P binding, or by simultaneous mutations in the SH3 and PB1 domains. Consistent with an essential role for PI(3)P in regulating the oxidase complex, phagosome NADPH oxidase activation in primary macrophages ingesting IgG-coated beads was inhibited by phosphatidylinositol 3 kinase inhibitors to a much greater extent than phagocytosis itself. Hence, this study identifies a role for p40^phox and PI(3)P in coupling FcγR-mediated phagocytosis to activation of the NADPH oxidase.

Reactive oxygen species play a role in regulating a fungus-perennial ryegrass mutualistic interaction

Although much is known about the signals and mechanisms that lead to pathogenic interactions between plants and fungi, comparatively little is known about fungus-plant mutualistic symbioses. We describe a novel role for reactive oxygen species (ROS) in regulating the mutualistic interaction between a clavicipitaceous fungal endophyte, Epichloë festucae, and its grass host, Lolium perenne. In wild-type associations, E. festucae grows systemically in intercellular spaces of leaves as infrequently branched hyphae parallel to the leaf axis. A screen to identify symbiotic genes isolated a fungal mutant that altered the interaction from mutualistic to antagonistic. This mutant has a single-copy plasmid insertion in the coding region of a NADPH oxidase gene, noxA. Plants infected with the noxA mutant lose apical dominance, become severely stunted, show precocious senescence, and eventually die. The fungal biomass in these associations is increased dramatically, with hyphae showing increased vacuolation. Deletion of a second NADPH oxidase gene, noxB, had no effect on the E. festucae-perennial ryegrass symbiosis. ROS accumulation was detected cytochemically in the endophyte extracellular matrix and at the interface between the extracellular matrix and host cell walls of meristematic tissue in wild-type but not in noxA mutant associations. These results demonstrate that fungal ROS production is critical in maintaining a mutualistic fungus-plant interaction.

Targeting reactive oxygen species in hypertension

PURPOSE OF REVIEW

Hypertension is a major risk factor for vascular diseases such as stroke, myocardial infarction, and renal microvascular disease. The mechanism by which vascular disease develops is complex, and growing evidence suggests that an increase in reactive oxygen species during hypertension is a major contributing factor. NADPH oxidase, the primary source of reactive oxygen species in the cardiovascular system, is a strong candidate for the development of therapeutic agents to ameliorate hypertension and end-organ damage.

RECENT FINDINGS

Various scavengers and inhibitors of reactive oxygen species have been proposed for use in animal as well as human studies. While many of these agents are effective at lowering tissue reactive oxygen species levels, their specificity is a serious concern. Our laboratory has developed cell-permeant peptidic inhibitors targeting key interactions among the different NAD(P)H oxidase homologues. One of these inhibitors targeting nox2 and p47-phox interaction has proven useful in attenuating target neoplasia and hypertrophy.

SUMMARY

Strategies aimed at specifically inhibiting NAD(P)H oxidase have proven effective in attenuating cardiovascular oxidative stress. The development of new inhibitors targeting novel oxidase homologues appears to hold significant promise for clarifying the physiologic role of these homologues as well as for the development of new antioxidant therapies.