Activation of the leukocyte NADPH oxidase by phorbol ester requires the phosphorylation of p47PHOX on serine 303 or 304

Structural profiles of the full phagocyte NADPH oxidase unveiled by combining computational biology and experimental knowledge

The phagocyte NADPH oxidase (NOX2) is an enzyme, crucial for innate immune defense, producing reactive oxygen species necessary for pathogen destruction. Its activation requires the assembly of soluble proteins (p47phox, p40phox, p67phox, and Rac) with the membrane-bound flavocytochrome b558 (cytb558). We combined circular-dichroism analyses, with decades of experimental data, to filter structural models of the NADPH oxidase complex generated by the artificial intelligence program AlphaFold2 (AF2). The predicted patterns tend to closely resemble the active states of the proteins, as shown by the compact structure of the cytb558, whose dehydrogenase domain is stabilized closer to the membrane. The modeling of the interaction of p47phox with cytb558, which is the initial assembly and activation steps of the NADPH oxidase, enables us to describe how the C-terminus of p47phox interacts with the cytb558. Combining the AF2 cytb558 -p47phox model and its classical molecular dynamics simulations, we highlighted new hydrophobic lipid insertions of p47phox, particularly at residues Trp80-Phe81 of its PX domain. The AF2 models also revealed the implications of intrinsically disordered regions, such as the fragment between the PX domain and the SH3 regions of p47phox, in ensuring distant protein-protein and membrane-protein interactions. Finally, the AF2 prediction of the cytb558-Trimera model highlighted the importance of leaving Rac1 as a separate protein to reach an active state of the NADPH oxidase complex. Altogether, our step-by-step approach provides a structural model of the active complex showing how disordered regions and specific lipid and protein interactions can enable and stabilize the multi-subunit assembly.

NADPH Oxidase 3: Beyond the Inner Ear

Reactive oxygen species (ROS) were formerly known as mere byproducts of metabolism with damaging effects on cellular structures. The discovery and description of NADPH oxidases (Nox) as a whole enzyme family that only produce this harmful group of molecules was surprising. After intensive research, seven Nox isoforms were discovered, described and extensively studied. Among them, the NADPH oxidase 3 is the perhaps most underrated Nox isoform, since it was firstly discovered in the inner ear. This stigma of Nox3 as "being only expressed in the inner ear" was also used by me several times. Therefore, the question arose whether this sentence is still valid or even usable. To this end, this review solely focuses on Nox3 and summarizes its discovery, the structural components, the activating and regulating factors, the expression in cells, tissues and organs, as well as the beneficial and detrimental effects of Nox3-mediated ROS production on body functions. Furthermore, the involvement of Nox3-derived ROS in diseases progression and, accordingly, as a potential target for disease treatment, will be discussed.

Role of NADPH Oxidases in Blood-Brain Barrier Disruption and Ischemic Stroke

NADPH oxidases (Nox) are one of the main sources of reactive oxygen species (ROS) in the central nervous system (CNS). While these enzymes have been shown to be involved in physiological regulation of cerebral vascular tone, excessive ROS produced by Nox1-5 play a critical role in blood–brain barrier (BBB) dysfunction in numerous neuropathologies. Nox-derived ROS have been implicated in mediating matrix metalloprotease (MMP) activation, downregulation of junctional complexes between adjacent brain endothelial cells and brain endothelial cell apoptosis, leading to brain microvascular endothelial barrier dysfunction and consequently, increases in BBB permeability. In this review, we will highlight recent findings on the role played by these enzymes in BBB disruption induced by ischemic stroke.

NADPH Oxidases in Aortic Aneurysms

Abdominal aortic aneurysms (AAAs) are a progressive dilation of the infrarenal aorta and are characterized by inflammatory cell infiltration, smooth muscle cell migration and proliferation, and degradation of the extracellular matrix. Oxidative stress and the production of reactive oxygen species (ROS) have been shown to play roles in inflammatory cell infiltration, and smooth muscle cell migration and apoptosis in AAAs. In this review, we discuss the principles of nicotinamide adenine dinucleotide phosphate oxidase (NADPH oxidase/NOX) signaling and activation. We also discuss the effects of some of the major mediators of NOX signaling in AAAs. Separately, we also discuss the influence of genetic or pharmacologic inhibitors of NADPH oxidases on experimental pre-clinical AAAs. Experimental evidence suggests that NADPH oxidases may be a promising future therapeutic target for developing pharmacologic treatment strategies for halting AAA progression or rupture prevention in the management of clinical AAAs.

SLAMF8 Downregulates Mouse Macrophage Microbicidal Mechanisms via PI3K Pathways

Signaling lymphocytic activation molecule family 8 (SLAMF8) is involved in the negative modulation of NADPH oxidase activation. However, the impact of SLAMF8 downregulation on macrophage functionality and the microbicide mechanism remains elusive. To study this in depth, we first analyzed NADPH oxidase activation pathways in wild-type and SLAMF8-deficient macrophages upon different stimulus. Herein, we describe increased phosphorylation of the Erk1/2 and p38 MAP kinases, as well as increased phosphorylation of NADPH oxidase subunits in SLAMF8-deficient macrophages. Furthermore, using specific inhibitors, we observed that specific PI3K inhibition decreased the differences observed between wild-type and SLAMF8-deficient macrophages, stimulated with either PMA, LPS, or Salmonella typhimurium infection. Consequently, SLAMF8-deficient macrophages also showed increased recruitment of small GTPases such as Rab5 and Rab7, and the p47^phox subunit to cytoplasmic Salmonella, suggesting an impairment of Salmonella-containing vacuole (SCV) progression in SLAMF8-deficient macrophages. Enhanced iNOS activation, NO production, and IL-6 expression were also observed in the absence of SLAMF8 upon Salmonella infection, either in vivo or in vitro, while overexpression of SLAMF8 in RAW264.7 macrophages showed the opposite phenotype. In addition, SLAMF8-deficient macrophages showed increased activation of Src kinases and reduced SHP-1 phosphate levels upon IFNγ and Salmonella stimuli in comparison to wild-type macrophages. In agreement with in vitro results, Salmonella clearance was augmented in SLAMF8-deficient mice compared to that in wild-type mice. Therefore, in conclusion, SLAMF8 intervention upon bacterial infection downregulates mouse macrophage activation, and confirmed that SLAMF8 receptor could be a potential therapeutic target for the treatment of severe or unresolved inflammatory conditions.

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 Role of NADPH Oxidase in Neuronal Death and Neurogenesis after Acute Neurological Disorders

Oxidative stress is a well-known common pathological process involved in mediating acute neurological injuries, such as stroke, traumatic brain injury, epilepsy, and hypoglycemia-related neuronal injury. However, effective therapeutic measures aimed at scavenging free reactive oxygen species have shown little success in clinical trials. Recent studies have revealed that NADPH oxidase, a membrane-bound enzyme complex that catalyzes the production of a superoxide free radical, is one of the major sources of cellular reactive oxygen species in acute neurological disorders. Furthermore, several studies, including our previous ones, have shown that the inhibition of NADPH oxidase can reduce subsequent neuronal injury in neurological disease. Moreover, maintaining appropriate levels of NADPH oxidase has also been shown to be associated with proper neurogenesis after neuronal injury. This review aims to present a comprehensive overview of the role of NADPH oxidase in neuronal death and neurogenesis in multiple acute neurological disorders and to explore potential pharmacological strategies targeting the NADPH-related oxidative stress pathways.

Glutathione Reductase Promotes Fungal Clearance and Suppresses Inflammation during Systemic Candida albicans Infection in Mice

Glutathione reductase (Gsr) catalyzes the reduction of glutathione disulfide to glutathione, which plays an important role in redox regulation. We have previously shown that Gsr facilitates neutrophil bactericidal activities and is pivotal for host defense against bacterial pathogens. However, it is unclear whether Gsr is required for immune defense against fungal pathogens. It is also unclear whether Gsr plays a role in immunological functions outside of neutrophils during immune defense. In this study, we report that Gsr^-/- mice exhibited markedly increased susceptibility to Candida albicans challenge. Upon C. albicans infection, Gsr^-/- mice exhibited dramatically increased fungal burden in the kidneys, cytokine and chemokine storm, striking neutrophil infiltration, histological abnormalities in both the kidneys and heart, and substantially elevated mortality. Large fungal foci surrounded by massive numbers of neutrophils were detected outside of the glomeruli in the kidneys of Gsr ^-/- mice but were not found in wild-type mice. Examination of the neutrophils and macrophages of Gsr^-/- mice revealed several defects. Gsr ^-/- neutrophils exhibited compromised phagocytosis, attenuated respiratory burst, and impaired fungicidal activity in vitro. Moreover, upon C. albicans stimulation, Gsr ^-/- macrophages produced increased levels of inflammatory cytokines and exhibited elevated p38 and JNK activities, at least in part, because of lower MAPK phosphatase (Mkp)-1 activity and greater Syk activity. Thus, Gsr-mediated redox regulation is crucial for fungal clearance by neutrophils and the proper control of the inflammatory response by macrophages during host defense against fungal challenge.

NADPH oxidase activation in neutrophils: Role of the phosphorylation of its subunits

Neutrophils are key cells of innate immunity and during inflammation. Upon activation, they produce large amounts of superoxide anion (O₂ ^-. ) and ensuing reactive oxygen species (ROS) to kill phagocytized microbes. The enzyme responsible for O₂ ^-. production is called the phagocyte NADPH oxidase. This is a multicomponent enzyme system that becomes active after assembly of four cytosolic proteins (p47^phox , p67^phox , p40^phox and Rac2) with the transmembrane proteins (p22^phox and gp91^phox , which form the cytochrome b₅₅₈ ). gp91^phox represents the catalytic subunit of the NADPH oxidase and is also called NOX2. NADPH oxidase-derived ROS are essential for microbial killing and innate immunity; however, excessive ROS production induces tissue injury and prolonged inflammatory reactions that contribute to inflammatory diseases. Thus, NADPH oxidase activation must be tightly regulated in time and space to limit ROS production. NADPH oxidase activation is regulated by several processes such as phosphorylation of its components, exchange of GDP/GTP on Rac2 and binding of p47^phox and p40^phox to phospholipids. This review aims to provide new insights into the role of the phosphorylation of the NADPH oxidase components, that is gp91^phox , p22^phox , p47^phox , p67^phox and p40^phox , in the activation of this enzyme.

PKCδ-Mediated Nox2 Activation Promotes Fluid-Phase Pinocytosis of Antigens by Immature Dendritic Cells

Aims

Macropinocytosis is a major endocytic pathway by which dendritic cells (DCs) internalize antigens in the periphery. Despite the importance of DCs in the initiation and control of adaptive immune responses, the signaling mechanisms mediating DC macropinocytosis of antigens remain largely unknown. The goal of the present study was to investigate whether protein kinase C (PKC) is involved in stimulation of DC macropinocytosis and, if so, to identify the specific PKC isoform(s) and downstream signaling mechanisms involved.

Methods

Various cellular, molecular and immunological techniques, pharmacological approaches and genetic knockout mice were utilized to investigate the signaling mechanisms mediating DC macropinocytosis.

Results

Confocal laser scanning microscopy confirmed that DCs internalize fluorescent antigens (ovalbumin) using macropinocytosis. Pharmacological blockade of classical and novel PKC isoforms using calphostin C abolished both phorbol ester- and hepatocyte growth factor-induced antigen macropinocytosis in DCs. The qRT-PCR experiments identified PKCδ as the dominant PKC isoform in DCs. Genetic studies demonstrated the functional role of PKCδ in DC macropinocytosis of antigens, their subsequent maturation, and secretion of various T-cell stimulatory cytokines, including IL-1α, TNF-α and IFN-β. Additional mechanistic studies identified NADPH oxidase 2 (Nox2) and intracellular superoxide anion as important players in DC macropinocytosis of antigens downstream of PKCδ activation.

Conclusion

The findings of the present study demonstrate a novel mechanism by which PKCδ activation via stimulation of Nox2 activity and downstream redox signaling promotes DC macropinocytosis of antigens. PKCδ/Nox2-mediated antigen macropinocytosis stimulates maturation of DCs and secretion of T-cell stimulatory cytokines. These findings may contribute to a better understanding of the regulatory mechanisms in DC macropinocytosis and downstream regulation of T-cell-mediated responses.

Turning off NADPH oxidase-2 by impeding p67phox activation in infected mouse macrophages reduced viral entry and inflammation

BACKGROUND

Targeting cells of the host immune system is a promising approach to fight against Influenza A virus (IAV) infection. Macrophage cells use the NADPH oxidase-2 (NOX2) enzymatic complex as a first line of defense against pathogens by generating superoxide ions O₂^- and releasing H₂O₂. Herein, we investigated whether targeting membrane -embedded NOX2 decreased IAV entry via raft domains and reduced inflammation in infected macrophages.

METHODS

Confocal microscopy and western blots monitored levels of the viral nucleoprotein NP and p67^phox, NOX2 activator subunit, Elisa assays quantified TNF-α levels in LPS or IAV-activated mouse or porcine alveolar macrophages pretreated with a fluorescent NOX inhibitor, called nanoshutter NS1.

RESULTS

IAV infection in macrophages promoted p67^phox translocation to the membrane, rafts clustering and activation of the NOX2 complex at early times. Disrupting rafts reduced intracellular viral NP. NS1 markedly reduced raft clustering and viral entry by binding to the C-terminal of NOX2 also characterized in vitro. NS1 decrease of TNF-α release depended on the cell type.

CONCLUSION

NOX2 participated in IAV entry and raft-mediated endocytosis. NOX2 inhibition by NS1 reduced viral entry. NS1 competition with p67^phox for NOX2 binding shown by in silico models and cell-free assays was in agreement with NS1 inhibiting p67^phox translocation to membrane-embedded NOX2 in mouse and porcine macrophages.

GENERAL SIGNIFICANCE

We introduce NS1 as a compound targeting NOX2, a critical enzyme controlling viral levels and inflammation in macrophages and discuss the therapeutic relevance of targeting the C-terminal of NADPH oxidases by probes like NS1 in viral infections.

Nox2-Mediated PI3K and Cofilin Activation Confers Alternate Redox Control of Macrophage Pinocytosis

AIMS

Internalization of extracellular fluid and its solute by macropinocytosis requires dynamic reorganization of actin cytoskeleton, membrane ruffling, and formation of large endocytic vacuolar compartments, called macropinosomes, inside the cell. Although instigators of macropinocytosis, such as growth factors and phorbol esters, stimulate NADPH oxidase (Nox) activation and signal transduction mediators upstream of Nox assembly, including Rac1 and protein kinase C (PKC), are involved in macropinocytosis, the role of Nox enzymes in macropinocytosis has never been investigated. This study was designed to examine the role of Nox2 and the potential downstream redox signaling involved in macropinocytosis.

RESULTS

Phorbol myristate acetate activation of human and murine macrophages stimulated membrane ruffling, macropinosome formation, and subsequent uptake of macromolecules by macropinocytosis. Mechanistically, we found that pharmacological blockade of PKC, transcriptional knockdown of Nox2, and scavenging of intracellular superoxide anion abolished phorbol ester-induced macropinocytosis. We observed that Nox2-derived reactive oxygen species via inhibition of phosphatase and tensin homolog and activation of the phosphoinositide-3-kinase (PI3K)/Akt pathway lead to activation of actin-binding protein cofilin, membrane ruffling, and macropinocytosis. Similarly, activation of macropinocytosis by macrophage colony-stimulating factor involves Nox2-mediated cofilin activation. Furthermore, peritoneal chimera experiments indicate that macropinocytotic uptake of lipids in hypercholesterolemic ApoE^-/- mice was attenuated in Nox2^y/- macrophages compared with wild-type controls. Innovation and Conclusion: In summary, these findings demonstrate a novel Nox2-mediated mechanism of solute uptake via macropinocytosis, with broad implications for both general cellular physiology and pathological processes. The redox mechanism described here may also identify new targets in atherosclerosis and other disease conditions involving macropinocytosis. Antioxid. Redox Signal. 26, 902-916.

NADPH oxidases as novel pharmacologic targets against influenza A virus infection

Influenza A viruses represent a major global health care challenge, with imminent pandemics, emerging antiviral resistance, and long lag times for vaccine development, raising a pressing need for novel pharmacologic strategies that ideally target the pathology irrespective of the infecting strain. Reactive oxygen species (ROS) pervade all facets of cell biology with both detrimental and protective properties. Indeed, there is compelling evidence that activation of the NADPH oxidase 2 (NOX2) isoform of the NADPH oxidase family of ROS-producing enzymes promotes lung oxidative stress, inflammation, injury, and dysfunction resulting from influenza A viruses of low to high pathogenicity, as well as impeding virus clearance. By contrast, the dual oxidase isoforms produce ROS that provide vital protective antiviral effects for the host. In this review, we propose that inhibitors of NOX2 are better alternatives than broad-spectrum antioxidant approaches for treatment of influenza pathologies, for which clinical efficacy may have been limited owing to poor bioavailability and inadvertent removal of beneficial ROS. Finally, we briefly describe the current suite of NADPH oxidase inhibitors and the molecular features of the NADPH oxidase enzymes that could be exploited by drug discovery for development of more specific and novel inhibitors to prevent or treat disease caused by influenza.

Arachidonic acid induces direct interaction of the p67(phox)-Rac complex with the phagocyte oxidase Nox2, leading to superoxide production

The phagocyte NADPH oxidase Nox2, heterodimerized with p22(phox) in the membrane, is dormant in resting cells but becomes activated upon cell stimulation to produce superoxide, a precursor of microbicidal oxidants. Nox2 activation requires two switches to be turned on simultaneously: a conformational change of the cytosolic protein p47(phox) and GDP/GTP exchange on the small GTPase Rac. These proteins, in an active form, bind to their respective targets, p22(phox) and p67(phox), leading to productive oxidase assembly at the membrane. Although arachidonic acid (AA) efficiently activates Nox2 both in vivo and in vitro, the mechanism has not been fully understood, except that AA induces p47(phox) conformational change. Here we show that AA elicits GDP-to-GTP exchange on Rac at the cellular level, consistent with its role as a potent Nox2 activator. However, even when constitutively active forms of p47(phox) and Rac1 are both expressed in HeLa cells, superoxide production by Nox2 is scarcely induced in the absence of AA. These active proteins also fail to effectively activate Nox2 in a cell-free reconstituted system without AA. Without affecting Rac-GTP binding to p67(phox), AA induces the direct interaction of Rac-GTP-bound p67(phox) with the C-terminal cytosolic region of Nox2. p67(phox)-Rac-Nox2 assembly and superoxide production are both abrogated by alanine substitution for Tyr-198, Leu-199, and Val-204 in the p67(phox) activation domain that localizes the C-terminal to the Rac-binding domain. Thus the "third" switch (AA-inducible interaction of p67(phox)·Rac-GTP with Nox2) is required to be turned on at the same time for Nox2 activation.

P21-activated kinase in inflammatory and cardiovascular disease

P-21 activated kinases, or PAKs, are serine-threonine kinases that serve a role in diverse biological functions and organ system diseases. Although PAK signaling has been the focus of many investigations, still our understanding of the role of PAK in inflammation is incomplete. This review consolidates what is known about PAK1 across several cell types, highlighting the role of PAK1 and PAK2 in inflammation in relation to NADPH oxidase activation. This review explores the physiological functions of PAK during inflammation, the role of PAK in several organ diseases with an emphasis on cardiovascular disease, and the PAK signaling pathway, including activators and targets of PAK. Also, we discuss PAK1 as a pharmacological anti-inflammatory target, explore the potentials and the limitations of the current pharmacological tools to regulate PAK1 activity during inflammation, and provide indications for future research. We conclude that a vast amount of evidence supports the idea that PAK is a central molecule in inflammatory signaling, thus making PAK1 itself a promising prospective pharmacological target.

Toona sinensis inhibits LPS-induced inflammation and migration in vascular smooth muscle cells via suppression of reactive oxygen species and NF-κB signaling pathway

Toona sinensis is one of the most popular vegetarian cuisines in Taiwan and it has been shown to possess antioxidant, antiangiogenic, and anticancer properties. In this study, we investigated the antiatherosclerotic potential of aqueous leaf extracts from Toona sinensis (TS; 25–100 μg/mL) and its major bioactive compound, gallic acid (GA; 5 μg/mL), in LPS-treated rat aortic smooth muscle (A7r5) cells. We found that pretreatment with noncytotoxic concentrations of TS and GA significantly inhibited inflammatory NO and PGE₂ production by downregulating their precursors, iNOS and COX-2, respectively, in LPS-treated A7r5 cells. Furthermore, TS and GA inhibited LPS-induced intracellular ROS and their corresponding mediator, p47^phox. Notably, TS and GA pretreatment significantly inhibited LPS-induced migration in transwell assays. Gelatin zymography and western blotting demonstrated that treatment with TS and GA suppressed the activity or expression of MMP-9, MMP-2, and t-PA. Additionally, TS and GA significantly inhibited LPS-induced VEGF, PDGF, and VCAM-1 expression. Further investigation revealed that the inhibition of iNOS/COX-2, MMPs, growth factors, and adhesion molecules was associated with the suppression of NF-κB activation and MAPK (ERK1/2, JNK1/2, and p38) phosphorylation. Thus, Toona sinensis may be useful for the prevention of atherosclerosis.

Redox-regulated growth factor survival signaling

SIGNIFICANCE

Once the thought of as unwanted byproducts of cellular respiration in eukaryotes, reactive oxygen species (ROS) have been shown to facilitate essential physiological roles. It is now understood that ROS are critical mediators of intracellular signaling. Control of signal transduction downstream of growth factor receptors by ROS is a complex process whose details are only recently coming to light.

RECENT ADVANCES

Indeed, recent evidence points to control of signal propagation by ROS at multiple levels in the typical cascade. Growth factor stimulation activates nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (Noxs) at the membrane, producing superoxide in the extracellular matrix, which is catalyzed to the membrane-permeable hydrogen peroxide (H2O2) that mediates intracellular signaling events.

CRITICAL ISSUES

The potential for H2O2, however, to disrupt cellular functions by damaging proteins and nucleic acids demands that its levels are kept in check by receptor-associated peroxiredoxins. This interplay of Nox and peroxiredoxin activity moderates levels of H2O2 sufficiently to modify signaling partners locally. Among the best studied of these partners are redox-controlled phosphatases that are inactivated by H2O2. Phosphatases regulate signal propagation downstream of receptors, and thus their inactivation allows a further level of control. Transmission of information further downstream to targets such as transcription factors, themselves regulated by ROS, completes this pathway.

FUTURE DIRECTIONS

Thus, signal propagation or attenuation can be dictated by ROS at multiple points. Given the complex nature of these processes, we envisage the emerging trends in the field of redox signaling in the context of growth factor stimulation.

Phosphorylation of Noxo1 at threonine 341 regulates its interaction with Noxa1 and the superoxide-producing activity of Nox1

Superoxide production by Nox1, a member of the Nox family NAPDH oxidases, requires expression of its regulatory soluble proteins Noxo1 (Nox organizer 1) and Noxa1 (Nox activator 1) and is markedly enhanced upon cell stimulation with phorbol 12-myristate 13-acetate (PMA), a potent activator of protein kinase C (PKC). The mechanism underlying PMA-induced enhancement of Nox1 activity, however, remains to be elucidated. Here we show that, in response to PMA, Noxo1 undergoes phosphorylation at multiple sites, which is inhibited by the PKC inhibitor GF109203X. Among them, Thr341 in Noxo1 is directly phosphorylated by PKC in vitro, and alanine substitution for this residue reduces not only PMA-induced Noxo1 phosphorylation but also PMA-dependent enhancement of Nox1-catalyzed superoxide production. Phosphorylation of Thr341 allows Noxo1 to sufficiently interact with Noxa1, an interaction that participates in Nox1 activation. Thus phosphorylation of Noxo1 at Thr341 appears to play a crucial role in PMA-elicited activation of Nox1, providing a molecular link between PKC-mediated signal transduction and Nox1-catalyzed superoxide production. Furthermore, Ser154 in Noxo1 is phosphorylated in both resting and PMA-stimulated cells, and the phosphorylation probably participates in a PMA-independent constitutive activity of Nox1. Ser154 may also be involved in protein kinase A (PKA) mediated regulation of Nox1; this serine is the major residue that is phosphorylated by PKA in vitro. Thus phosphorylation of Noxo1 at Thr341 and at Ser154 appears to regulate Nox1 activity in different manners.

STRUCTURED DIGITAL ABSTRACT

Noxo1 binds to p22phox by pull down (1, 2, 3) Noxo1 binds to Noxo1 by pull down (View interaction) Noxa1 binds to Noxo1 by pull down (1, 2, 3, 4, 5).

Protein Kinase C Epsilon Contributes to NADPH Oxidase Activation in a Pre-Eclampsia Lymphoblast Cell Model

Pre-eclampsia is a pregnancy-specific disorder characterised by hypertension and proteinuria, which in severe cases results in multi-system disturbances. The maternal syndrome is associated with a pro-inflammatory state, consisting of leukocyte activation, which is thought to contribute to the widespread endothelial dysfunction. We previously showed increased activation of NADPH oxidase in pre-eclampsia, in both neutrophils and B-lymphoblast cell lines (B-LCLs). In this study, the mechanism by which NADPH oxidase activity is increased in pre-eclampsia was further investigated. NADPH oxidase activity was found to be increased in phorbol-12-myristate-13-acetate (PMA) stimulated B-LCLs isolated from women with pre-eclampsia. This correlated with an increase in protein kinase C (PKC) substrate phosphorylation, p47-phox phosphorylation (a regulatory component of NADPH oxidase) and p47-phox directed-kinase activity. Using ion exchange and hydroxyapatite chromatography we identified a major peak of PMA regulated p47-phox kinase activity. Chromatography fractions were probed for PKC isoforms. We found the major peak of p47-phox kinase activity could not be separated from the elution profile of PKC epsilon. Using a peptide inhibitor of PKC epsilon, PMA-induced reactive oxygen species (ROS) production could be reduced to that of a normal B-LCL. These data suggest a pro-inflammatory role for PKC epsilon in the pathogenesis of pre-eclampsia.

Redox signaling in cardiovascular health and disease

Spatiotemporal regulation of the activity of a vast array of intracellular proteins and signaling pathways by reactive oxygen species (ROS) governs normal cardiovascular function. However, data from experimental and animal studies strongly support that dysregulated redox signaling, resulting from hyperactivation of various cellular oxidases or mitochondrial dysfunction, is integral to the pathogenesis and progression of cardiovascular disease (CVD). In this review, we address how redox signaling modulates the protein function, the various sources of increased oxidative stress in CVD, and the labyrinth of redox-sensitive molecular mechanisms involved in the development of atherosclerosis, hypertension, cardiac hypertrophy and heart failure, and ischemia-reperfusion injury. Advances in redox biology and pharmacology for inhibiting ROS production in specific cell types and subcellular organelles combined with the development of nanotechnology-based new in vivo imaging systems and targeted drug delivery mechanisms may enable fine-tuning of redox signaling for the treatment and prevention of CVD.

Diacylglycerol kinases terminate diacylglycerol signaling during the respiratory burst leading to heterogeneous phagosomal NADPH oxidase activation

It is commonly assumed that all phagosomes have identical molecular composition. This assumption has remained largely unchallenged due to a paucity of methods to distinguish individual phagosomes. We devised an assay that extends the utility of nitro blue tetrazolium for detection and quantification of NAPDH oxidase (NOX) activity in individual phagosomes. Implementation of this assay revealed that in murine macrophages there is heterogeneity in the ability of individual phagosomes to generate superoxide, both between and within cells. To elucidate the molecular basis of the variability in NOX activation, we employed genetically encoded fluorescent biosensors to evaluate the uniformity in the distribution of phospholipid mediators of the oxidative response. Despite variability in superoxide generation, the distribution of phosphatidylinositol 3,4,5-trisphosphate, phosphatidylinositol 3-phosphate, and phosphatidic acid was nearly identical in all phagosomes. In contrast, diacylglycerol (DAG) was not generated uniformly across the phagosomal population, varying in a manner that directly mirrored superoxide production. Modulation of DAG levels suggested that NOX activation is precluded when phagosomes fail to reach a critical DAG concentration. In particular, forced expression of diacylglycerol kinase β abrogated DAG accumulation at the phagosome, leading to impaired respiratory burst. Conversely, pharmacological inhibition of DAG kinases or expression of an inactive diacylglycerol kinase β mutant increased the proportion of DAG-positive phagosomes, concomitantly potentiating phagosomal NOX activity. Our data suggest that diacylglycerol kinases limit the extent of NADPH oxidase activation, curtailing the production of potentially harmful reactive oxygen species. The resulting heterogeneity in phagosome responsiveness could enable the survival of a fraction of invading microorganisms.

p38 Mitogen-activated protein kinase and extracellular signal-regulated kinase signaling pathways are not essential regulators of formyl peptide-stimulated p47(phox) activation in neutrophils

Three structurally unrelated p38 mitogen-activated protein kinase (MAPK) inhibitors, (4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)imidazole (SB203580), 1-5-tert-butyl-2-p-tolyl-2H-pyrazol-3-yl)-3-[4-(2-morpholin-4-yl-ethoxy)naphthalen-1-yl] urea (BIRB 796) and 5-(2,6-dichlorophenyl)-2-[2,4-difluorophenyl]thio]-6H-pyrimido[1,6-b]pyridazin-6-one (VX 745) showed approximately 40% inhibition of formyl-Met-Leu-Phe (fMLP)-stimulated neutrophil superoxide anion (O2(•-)) generation at concentrations that greatly diminished p38 MAPK activity. However, a significant inhibition of p47(phox) activation occurred at concentrations much higher than the corresponding IC50 values of these inhibitors in blocking p38 MAPK activity. 4-Ethyl-2(p-methoxyphenyl)-5-(4'-pyridyl)-IH-imidazole (SB202474), an inactive analogue of SB203580, at a concentration (30μM) which significantly attenuated p38 MAPK activity, had no effect on p47(phox) activation, whereas it inhibited O2(•-) generation with an IC50 value of approximately 16μM. Moreover, both SB203580 and BIRB 796 had no effect on protein kinase B (PKB)/Akt Ser473 phosphorylation and S100A9 protein membrane translocation at concentrations that effectively blocked p38 MAPK activity. Pretreatment of cells with two structurally unrelated MAPK/extracellular signal-regulated kinase (ERK) kinase (MEK) inhibitors, 2-(2-amino-3-methoxy-phenyl)-chromen-4-one (PD 98059) and 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene (U0126), at concentrations that effectively blocked MEK activity, attenuated p47(phox) phosphorylation but did not affect the recruitment of p47(phox) to p22(phox) or O2(•-) generation. Both p47(phox) activation and O2(•-) generation were attenuated by a protein kinase C (PKC) inhibitor 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide (GF 109203X) in the concentration range that effectively blocked PKC activity. Taken together, these results suggest that the ERK-mediated Ser phosphorylation of p47(phox) is not implicated in the assembly of NADPH oxidase or O2(•-) generation, and that O2(•-) generation is partly attributable to p38 MAPK signaling through mechanisms other than p47(phox) activation, Akt activation and S100A9 membrane recruitment in fMLP-stimulated neutrophils.

Targeting microglia-mediated neurotoxicity: the potential of NOX2 inhibitors

Microglia are key sentinels of central nervous system health, and their dysfunction has been widely implicated in the progressive nature of neurodegenerative diseases. While microglia can produce a host of factors that are toxic to neighboring neurons, NOX2 has been implicated as a common and essential mechanism of microglia-mediated neurotoxicity. Accumulating evidence indicates that activation of the NOX2 enzyme complex in microglia is neurotoxic, both through the production of extracellular reactive oxygen species that damage neighboring neurons as well as the initiation of redox signaling in microglia that amplifies the pro-inflammatory response. More specifically, evidence supports that NOX2 redox signaling enhances microglial sensitivity to pro-inflammatory stimuli, and amplifies the production of neurotoxic cytokines, to promote chronic and neurotoxic microglial activation. Here, we describe the evidence denoting the role of NOX2 in microglia-mediated neurotoxicity with an emphasis on Alzheimer's and Parkinson's disease, describe available inhibitors that have been tested, and detail evidence of the neuroprotective and therapeutic potential of targeting this enzyme complex to regulate microglia.

Divergent effects of p47(phox) phosphorylation at S303-4 or S379 on tumor necrosis factor-α signaling via TRAF4 and MAPK in endothelial cells

OBJECTIVE

To define the mechanism of p47(phox) phosphorylation in regulating endothelial cell response to tumor necrosis factor-α (TNFα) stimulation.

METHODS AND RESULTS

We replaced 11 serines (303-4, 310, 315, 320, 328, 345, 348, 359, 370, and 379) with alanines and investigated their effects on TNFα (100 U/mL, 30 minutes)-induced acute O(2)(.-) production and mitogen-activated protein kinase phosphorylation in endothelial cells. Seven constructs, S303-4A (double), S310A, S315A, S328A, S345A, S370A, and S379A, significantly reduced the O(2)(.-) production, and 4 of them (S328A, S345A, S370A, and S379A) also inhibited TNFα-induced extracellular-signal-regulated kinase (ERK) 1/2 phosphorylation. Blocking the phosphorylation of S303-4 and S379 inhibited most effectively TNFα-induced O(2)(.-) production. However, phosphorylation of S303-4 was not required for TNFα-induced p47(phox) membrane translocation and binding to TNF receptor-associated factor 4, ERK1/2 activation, and subsequent vascular cell adhesion molecule-1 expression. Knockout of p47(phox) or knockdown of TNF receptor-associated factor 4 using siRNA abolished TNFα-induced ERK1/2 phosphorylation, and inhibition of ERK1/2 activation significantly reduced the TNFα-induced vascular cell adhesion molecule-1 expression.

CONCLUSIONS

Phosphorylation of p47(phox) at different serine sites plays distinct roles in endothelial cell response to TNFα stimulation. Double serine (S303-4) phosphorylation is crucial for acute O(2)(.-) production, but is not involved in TNFα signaling through TNF receptor-associated factor 4 and ERK1/2. p47(phox) requires serine phosphorylation at distinct sites to support specific signaling events in response to TNFα.

Glutathione reductase facilitates host defense by sustaining phagocytic oxidative burst and promoting the development of neutrophil extracellular traps

Glutathione reductase (Gsr) catalyzes the reduction of glutathione disulfide to glutathione, which plays an important role in the bactericidal function of phagocytes. Because Gsr has been implicated in the oxidative burst in human neutrophils and is abundantly expressed in the lymphoid system, we hypothesized that Gsr-deficient mice would exhibit marked defects during the immune response against bacterial challenge. We report in this study that Gsr-null mice exhibited enhanced susceptibility to Escherichia coli challenge, indicated by dramatically increased bacterial burden, cytokine storm, striking histological abnormalities, and substantially elevated mortality. Additionally, Gsr-null mice exhibited elevated sensitivity to Staphylococcus aureus. Examination of the bactericidal functions of the neutrophils from Gsr-deficient mice in vitro revealed impaired phagocytosis and defective bacterial killing activities. Although Gsr catalyzes the regeneration of glutathione, a major cellular antioxidant, Gsr-deficient neutrophils paradoxically produced far less reactive oxygen species upon activation both ex vivo and in vivo. Unlike wild-type neutrophils that exhibited a sustained oxidative burst upon stimulation with phorbol ester and fMLP, Gsr-deficient neutrophils displayed a very transient oxidative burst that abruptly ceased shortly after stimulation. Likewise, Gsr-deficient neutrophils also exhibited an attenuated oxidative burst upon encountering E. coli. Biochemical analysis revealed that the hexose monophosphate shunt was compromised in Gsr-deficient neutrophils. Moreover, Gsr-deficient neutrophils displayed a marked impairment in the formation of neutrophil extracellular traps, a bactericidal mechanism that operates after neutrophil death. Thus, Gsr-mediated redox regulation is crucial for bacterial clearance during host defense against massive bacterial challenge.

Redox signaling at invasive microdomains in cancer cells

Redox signaling contributes to the regulation of cancer cell proliferation, survival, and invasion and participates in the adaptation of cancer cells to their microenvironment. NADPH oxidases are important mediators of redox signaling in normal and cancer cells. Redox signal specificity in normal cells is in part achieved by targeting enzymes that generate reactive oxygen species to specific subcellular microdomains such as focal adhesions, dorsal ruffles, lipid rafts, or caveolae. In a similar fashion, redox signal specificity during cancer cell invasion can be regulated by targeting reactive oxygen generation to invasive microdomains such as invadopodia. Here we summarize recent advances in the understanding of the redox signaling processes that control the cancer cell proinvasive program by modulating cell adhesion, migration, and proteolysis as well as the interaction of cancer cells with the tumor microenvironment. We focus on redox signaling events mediated by invadopodia NADPH oxidase complexes and their contribution to cancer cell invasion.

In silico phosphorylation of the autoinhibited form of p47(phox): insights into the mechanism of activation

Activation of the multicomponent enzyme NADPH oxidase requires the interaction between the tandem SH3 domain of the cytosolic subunit p47(phox) and the cytoplasmic tail of membrane-bound p22(phox). In the resting state, p47(phox) exists in an autoinhibited conformation stabilized by intramolecular contacts between the SH3 domains and an adjacent polybasic region. Phosphorylation of three serine residues, Ser(303), Ser(304), and Ser(328) within this polybasic region has been shown to be sufficient for the disruption of the intramolecular interactions thereby inducing an active state of p47(phox). This active conformation is accessible to the cytoplasmic tail of p22(phox) and initiates the formation of the membrane-bound functional enzyme complex. Molecular dynamics simulations reveal insights in the mechanism of activation of the autoinhibited form of p47(phox) by in silico phosphorylation, of the three serine residues, Ser(303), Ser(304), and Ser(328). The simulations highlight the major collective coordinates generating the opening and the closing of the two SH3 domains and the residues that cause the unmasking of the p22(phox) binding site.

Links between innate immunity and normal tissue radiobiology

The body senses "danger" from "damaged self" molecules through members of the same receptor superfamily it uses for microbial "non-self", triggering canonical signaling pathways that lead to the generation of acute inflammatory responses. For this reason, the biology of normal tissue responses to moderate and clinically relevant doses of radiation is inextricably connected to innate immunity. The complex sequence of inflammatory events that ensues causes further cell and tissue damage to eliminate potential invaders but also leads to cytoprotective responses that limit the spread of damage and to wound healing through tissue regeneration or replacement. These sequential processes are orchestrated through multiple feedback control mechanisms involving cyclical production of free radicals and cytokines that are common to both radiation and immune signaling. This requires a concerted effort by resident tissue and inflammatory cell types, with macrophages apparently leading the way. The initial response to moderate doses of radiation therefore feeds into a pro-inflammatory paradigm whose eventual outcome is critically dependent upon the properties of the immune cells that are involved in tissue damage, regeneration and repair and that are in part under genetic influence. Importantly, these canonical pathways provide targets for interventions aimed at modifying normal tissue radiation responses. In this review, we examine areas of intersection between innate immunity and normal tissue radiobiology.

A functional polymorphism in the NAD(P)H oxidase subunit CYBA is related to gene expression, enzyme activity, and outcome in non-Hodgkin lymphoma

NAD(P)H oxidase is a major endogenous source of reactive oxygen species (ROS). ROS may not only be involved in carcinogenesis but also in efficacy of chemotherapeutic agents like doxorubicin. By a comprehensive genotyping approach covering 48 genetic polymorphisms (single-nucleotide polymorphisms) in five subunits of phagocytic NAD(P)H oxidase, we asked whether they affect gene expression, enzymatic activity, and outcome of CHO(E)P chemotherapy. A highly consistent effect was observed for the CYBA 640A>G variant. In peripheral blood granulocytes of 125 healthy volunteers, the G allele of 640A>G was associated with lower NAD(P)H oxidase activity (P = 0.006). Moreover, the G allele was associated with lower mRNA and protein expression (both P = 0.02). Of clinical importance, the outcome of patients suffering from non-Hodgkin lymphoma and treated with CHO(E)P regimen was dependent on the CYBA 640A>G polymorphism. In an exploratory study (n = 401), carriers of 640GG had an event-free survival (EFS) risk ratio of 1.95 [95% confidence interval (95% CI), 1.31-2.90; P = 0.001] compared with 640AA. In a confirmatory set (n = 477), the risk ratios were 1.53 (1.04-2.25, P = 0.03). The complete set of 878 patients showed a relative risk of 1.72 (1.30-2.26) and 1.59 (1.14-2.21) for EFS and overall survival, respectively. Further molecular-biological experiments showed lower expression and reduced stability of transcripts with the G allele in lymphoblastoid cell lines. Transfection of allele-specific plasmids into HEK293 cells elicited lower activity for the G allele in a luciferase reporter gene construct. Thus, CYBA 640A>G was shown to be a functional polymorphism with possible consequences for patients receiving CHO(E)P chemotherapy and might have further implications for other ROS-mediated modalities.

Hydrogen peroxide as a cell-survival signaling molecule

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

Myristic Acid, A Side Chain of Phorbol Myristate Acetate (PMA), Can Activate Human Polymorphonuclear Leukocytes to Produce Oxygen Radicals More Potently than PMA

Myristic acid (MyA), which is a saturated fatty acid (C14:0) and a side chain of phorbol 12-myristate 13-acetate (PMA), was examined if MyA stimulates human polymorphonuclear leukocytes (PMNs) to release oxygen radicals comparable to PMA by applying electron paramagnetic resonance (EPR)-spin-trapping method. When MyA was added to isolated human PMNs, spin adducts of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO)-OH and DMPO-OOH were time-dependently observed. The amounts of these spin adducts were larger than those of PMNs stimulated by PMA. These results clearly show that MyA is more potent agent to prime human PMNs than PMA, in a point of view of not only O₂^·− but also ·OH production. This fact calls attention that too much intake of MyA that is known to be contained vegetable oils can lead to crippling effect through uncontrolled production of reactive oxygen species.

Aryl hydrocarbon receptor-dependent induction of the NADPH oxidase subunit NCF1/p47 phox expression leading to priming of human macrophage oxidative burst

Polycyclic aromatic hydrocarbons such as benzo(a)pyrene (BaP) are toxic environmental contaminants known to regulate gene expression through activation of the aryl hydrocarbon receptor (AhR). In the present study, we demonstrated that acute treatment by BaP markedly increased expression of the NADPH oxidase subunit gene neutrophil cytosolic factor 1 (NCF1)/p47(phox) in primary human macrophages; NCF1 was similarly up-regulated in alveolar macrophages from BaP-instilled rats. NCF1 induction in BaP-treated human macrophages was prevented by targeting AhR, through its chemical inhibition or small interference RNA-mediated down-modulation of its expression. BaP moreover induced activity of the NCF1 promoter sequence, containing a consensus AhR-related xenobiotic-responsive element (XRE), and electrophoretic mobility shift assays and chromatin immunoprecipitation experiments indicated that BaP-triggered binding of AhR to this XRE. Finally, we showed that BaP exposure resulted in p47(phox) protein translocation to the plasma membrane and in potentiation of phorbol myristate acetate (PMA)-induced superoxide anion production in macrophages. This BaP priming effect toward NADPH oxidase activity was inhibited by the NADPH oxidase specific inhibitor apocynin and the chemical AhR inhibitor alpha-naphtoflavone. These results indicated that BaP induced NCF1/p47(phox) expression and subsequently enhanced superoxide anion production in PMA-treated human macrophages, in an AhR-dependent manner; such an NCF1/NADPH oxidase regulation by polycyclic aromatic hydrocarbons may participate in deleterious effects toward human health triggered by these environmental contaminants, including atherosclerosis and smoking-related diseases.

Interferon-alpha2b (IFN-alpha2b)-induced apoptosis is mediated by p38 MAPK in hepatocytes from rat preneoplastic liver via activation of NADPH oxidase

It is still unclear how Interferon-alfa (IFN-alpha) acts on preventing the appearance of hepatocarcinogenesis. We have demonstrated that IFN-alpha2b induces hepatocytic transforming growth factor-beta1 (TGF-beta(1)) production and secretion by inducing reactive oxygen species (ROS) formation through the activation of NADPH oxidase. This TGF-beta(1), alters antioxidant defences and induces programmed cell death. Since it was demonstrated that IFN-alpha induces apoptosis through the activation of p38 mitogen-activated protein kinase (p38 MAPK), this study was aimed to assess the role of this kinase in the IFN-alpha2b-induced apoptosis in rat liver preneoplasia; and to further evaluate the participation of NADPH oxidase. p38 MAPK pathway was activated during the IFN-alpha2b-induced apoptosis in rat liver preneoplasia. This activation was accompanied with phosphorylation of different transcription factors, depending on the time of IFN-alpha2b stimulus. Our data suggest that NADPH oxidase is activated by IFN-alpha2b through p38 MAPK. p38 MAPK-induced activation of NADPH oxidase is accomplished by a two-step pathway: first, ROS-independent and second ROS- and TGF-beta(1)-dependent.

The arachidonic acid effect on platelet nitric oxide level

Arachidonic acid can act as a second messenger regulating many cellular processes among which is nitric oxide (NO) formation. The aim of the present study was to investigate the molecular mechanisms involved in the arachidonic acid effect on platelet NO level. Thus NO, cGMP and superoxide anion level, the phosphorylation status of nitric oxide synthase, the protein kinase C (PKC), and NADPH oxidase activation were measured. Arachidonic acid dose-dependently reduced NO and cGMP level. The thromboxane A(2) mimetic U46619 behaved in a similar way. The arachidonic acid or U46619 effect on NO concentration was abolished by the inhibitor of the thromboxane A(2) receptor SQ29548 and partially reversed by the PKC inhibitor GF109203X or by the phospholipase C pathway inhibitor U73122. Moreover, it was shown that arachidonic acid activated PKC and decreased nitric oxide synthase (eNOS) activities. The phosphorylation of the inhibiting eNOSthr495 residue mediated by PKC was increased by arachidonic acid, while no changes at the activating ser1177 residue were shown. Finally, arachidonic acid induced NADPH oxidase activation and superoxide anion formation. These effects were greatly reduced by GF109203X, U73122, and apocynin. Likely arachidonic acid reducing NO bioavailability through all these mechanisms could potentiate its platelet aggregating power.

NADPH oxidase is the primary source of superoxide induced by NMDA receptor activation

Neuronal NMDA receptor (NMDAR) activation leads to the formation of superoxide, which normally acts in cell signaling. With extensive NMDAR activation, the resulting superoxide production leads to neuronal death. It is widely held that NMDA-induced superoxide production originates from the mitochondria, but definitive evidence for this is lacking. We evaluated the role of the cytoplasmic enzyme NADPH oxidase in NMDA-induced superoxide production. Neurons in culture and in mouse hippocampus responded to NMDA with a rapid increase in superoxide production, followed by neuronal death. These events were blocked by the NADPH oxidase inhibitor apocynin and in neurons lacking the p47(phox) subunit, which is required for NADPH oxidase assembly. Superoxide production was also blocked by inhibiting the hexose monophosphate shunt, which regenerates the NADPH substrate, and by inhibiting protein kinase C zeta, which activates the NADPH oxidase complex. These findings identify NADPH oxidase as the primary source of NMDA-induced superoxide production.

p47phox, the phagocyte NADPH oxidase/NOX2 organizer: structure, phosphorylation and implication in diseases

Phagocytes such as neutrophils play a vital role in host defense against microbial pathogens. The anti-microbial function of neutrophils is based on the production of superoxide anion (O2 -), which generates other microbicidal reactive oxygen species (ROS) and release of antimicrobial peptides and proteins. The enzyme responsible for O2 - production is called the NADPH oxidase or respiratory burst oxidase. This multicomponent enzyme system is composed of two trans- membrane proteins (p22phox and gp91phox, also called NOX2, which together form the cytochrome b558) and four cytosolic proteins (p47phox, p67phox, p40phox and a GTPase Rac1 or Rac2), which assemble at membrane sites upon cell activation. NADPH oxidase activation in phagocytes can be induced by a large number of soluble and particulate agents. This process is dependent on the phosphorylation of the cytosolic protein p47phox. p47phox is a 390 amino acids protein with several functional domains: one phox homology (PX) domain, two src homology 3 (SH3) domains, an auto-inhibitory region (AIR), a proline rich domain (PRR) and has several phosphorylated sites located between Ser303 and Ser379. In this review, we will describe the structure of p47phox, its phosphorylation and discuss how these events regulate NADPH oxidase activation.

Regulation of NADPH oxidase in vascular endothelium: the role of phospholipases, protein kinases, and cytoskeletal proteins

The generation of reactive oxygen species (ROS) in the vasculature plays a major role in the genesis of endothelial cell (EC) activation and barrier function. Of the several potential sources of ROS in the vasculature, the endothelial NADPH oxidase family of proteins is a major contributor of ROS associated with lung inflammation, ischemia/reperfusion injury, sepsis, hyperoxia, and ventilator-associated lung injury. The NADPH oxidase in lung ECs has most of the components found in phagocytic oxidase, and recent studies show the expression of several homologues of Nox proteins in vascular cells. Activation of NADPH oxidase of nonphagocytic vascular cells is complex and involves assembly of the cytosolic (p47(phox), p67(phox), and Rac1) and membrane-associated components (Noxes and p22(phox)). Signaling pathways leading to NADPH oxidase activation are not completely defined; however, they do appear to involve the cytoskeleton and posttranslation modification of the components regulated by protein kinases, protein phosphatases, and phospholipases. Furthermore, several key components regulating NADPH oxidase recruitment, assembly, and activation are enriched in lipid microdomains to form a functional signaling platform. Future studies on temporal and spatial localization of Nox isoforms will provide new insights into the role of NADPH oxidase-derived ROS in the pathobiology of lung diseases.

A region N-terminal to the tandem SH3 domain of p47phox plays a crucial role in the activation of the phagocyte NADPH oxidase

The superoxide-producing NADPH oxidase in phagocytes is crucial for host defence; its catalytic core is the membrane-integrated protein gp91phox [also known as Nox2 (NADPH oxidase 2)], which forms a stable heterodimer with p22phox. Activation of the oxidase requires membrane translocation of the three cytosolic proteins p47phox, p67phox and the small GTPase Rac. At the membrane, these proteins assemble with the gp91phox–p22phox heterodimer and induce a conformational change of gp91phox, leading to superoxide production. p47phox translocates to membranes using its two tandemly arranged SH3 domains, which directly interact with p22phox, whereas p67phox is recruited in a p47phox-dependent manner. In the present study, we show that a short region N-terminal to the bis-SH3 domain is required for activation of the phagocyte NADPH oxidase. Alanine substitution for Ile152 in this region, a residue that is completely conserved during evolution, results in a loss of the ability to activate the oxidase; and the replacement of Thr153 also prevents oxidase activation, but to a lesser extent. In addition, the corresponding isoleucine residue (Ile155) of the p47phox homologue Noxo1 (Nox organizer 1) participates in the activation of non-phagocytic oxidases, such as Nox1 and Nox3. The I152A substitution in p47phox, however, does not affect its interaction with p22phox or with p67phox. Consistent with this, a mutant p47phox (I152A), as well as the wild-type protein, is targeted upon cell stimulation to membranes, and membrane recruitment of p67phox and Rac normally occurs in p47phox (I152A)-expressing cells. Thus the Ile152-containing region of p47phox plays a crucial role in oxidase activation, probably by functioning at a process after oxidase assembly.

Dihydroxyoctadecamonoenoate esters inhibit the neutrophil respiratory burst

The leukotoxins [9(10)-and 12(13)-EpOME] are produced by activated inflammatory leukocytes such as neutrophils. High EpOME levels are observed in disorders such as acute respiratory distress syndrome and in patients with extensive burns.Although the physiological significance of the EpOMEs remains poorly understood,in some systems, the EpOMEs act as a protoxin,with their corresponding epoxide hydrolase metabolites,9,10-and 12,13-DiHOME, specifically exerting toxicity.Both the EpOMEs and the DiHOMEs were also recently shown to have neutrophil chemotactic activity.We evaluated whether the neutrophil respiratory burst,a surge of oxidant production thought to play an important role in limiting certain bacterial and fungal infections,is modulated by members of the EpOME metabolic pathway.We present evidence that the DiHOMEs suppress the neutrophil respiratory burst by a mechanism distinct from that of respiratory burst inhibitors such as cyclosporin H or lipoxin A4,which inhibit multiple aspects of neutrophil activation.

Homocysteine decreases platelet NO level via protein kinase C activation

Hyperhomocysteinaemia has been associated with increased risk of thrombosis and atherosclerosis. Homocysteine produces endothelial injury and stimulates platelet aggregation. Several molecular mechanisms related to these effects have been elucidated. The study aimed to deeply investigate the homocysteine effect on nitric oxide formation in human platelets. The homocysteine-induced changes on nitric oxide, cGMP, superoxide anion levels and nitrotyrosine formation were evaluated. The enzymatic activity and the phosphorylation status of endothelial nitric oxide synthase (eNOS) at thr495 and ser1177 residues were measured. The protein kinase C (PKC), assayed by immunofluorescence confocal microscopy technique and by phosphorylation of p47pleckstrin, and NADPH oxidase activation, tested by the translocation to membrane of the two cytosolic subunits p47(phox) and p67(phox), were assayed. Results show that homocysteine reduces platelet nitric oxide and cGMP levels. The inhibition of eNOS activity and the stimulation of NADPH oxidase primed by PKC appear to be involved. PKC stimulates the eNOS phosphorylation of the negative regulatory residue thr495 and the dephosphorylation of the positive regulatory site ser1177. GF109203X and U73122, PKC and phospholipase Cgamma2 pathway inhibitors, respectively, reverse this effect. Moreover, homocysteine stimulates superoxide anion elevation and NADPH oxidase activation. These effects are significantly decreased by GF109203X and U73122, suggesting the involvement of PKC in NADPH oxidase activation. Homocysteine induces formation of the peroxynitrite biomarker nitrotyrosine. Taken together these results suggest that the homocysteine-mediated responses leading to nitric oxide impairment are mainly coupled to PKC activation. Thus homocysteine stimulates platelet aggregation and decreases nitric oxide bioavailability.

Priming of the neutrophil NADPH oxidase activation: role of p47phox phosphorylation and NOX2 mobilization to the plasma membrane

Neutrophils play an essential role in host defense against microbial pathogens and in the inflammatory reaction. Upon activation, neutrophils produce superoxide anion (O*2), which generates other reactive oxygen species (ROS) such as hydrogen peroxide (H2O2), hydroxyl radical (OH*) and hypochlorous acid (HOCl), together with microbicidal peptides and proteases. The enzyme responsible for O2* production is called the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase or respiratory burst oxidase. This multicomponent enzyme system is composed of two trans-membrane proteins (p22phox and gp91phox/NOX2, which form the cytochrome b558), three cytosolic proteins (p47phox, p67phox, p40phox) and a GTPase (Rac1 or Rac2), which assemble at membrane sites upon cell activation. NADPH oxidase activation in phagocytes can be induced by a large number of soluble and particulate factors. Three major events accompany NAPDH oxidase activation: (1) protein phosphorylation, (2) GTPase activation, and (3) translocation of cytosolic components to the plasma membrane to form the active enzyme. Actually, the neutrophil NADPH oxidase exists in different states: resting, primed, activated, or inactivated. The resting state is found in circulating blood neutrophils. The primed state can be induced by neutrophil adhesion, pro-inflammatory cytokines, lipopolysaccharide, and other agents and has been characterized as a "ready to go" state, which results in a faster and higher response upon exposure to a second stimulus. The active state is found at the inflammatory or infection site. Activation is induced by the pathogen itself or by pathogen-derived formylated peptides and other agents. Finally, inactivation of NADPH oxidase is induced by anti-inflammatory agents to limit inflammation. Priming is a "double-edged sword" process as it contributes to a rapid and efficient elimination of the pathogens but can also induce the generation of large quantities of toxic ROS by hyperactivation of the NADPH oxidase, which can damage surrounding tissues and participate to inflammation. In order to avoid extensive damage to host tissues, NADPH oxidase priming and activation must be tightly regulated. In this review, we will discuss some of the mechanisms of NADPH oxidase priming in neutrophils and the relevance of this process to physiology and pathology.

Structure, regulation and evolution of Nox-family NADPH oxidases that produce reactive oxygen species

NADPH oxidases of the Nox family exist in various supergroups of eukaryotes but not in prokaryotes, and play crucial roles in a variety of biological processes, such as host defense, signal transduction, and hormone synthesis. In conjunction with NADPH oxidation, Nox enzymes reduce molecular oxygen to superoxide as a primary product, and this is further converted to various reactive oxygen species. The electron-transferring system in Nox is composed of the C-terminal cytoplasmic region homologous to the prokaryotic (and organelle) enzyme ferredoxin reductase and the N-terminal six transmembrane segments containing two hemes, a structure similar to that of cytochrome b of the mitochondrial bc(1) complex. During the course of eukaryote evolution, Nox enzymes have developed regulatory mechanisms, depending on their functions, by inserting a regulatory domain (or motif) into their own sequences or by obtaining a tightly associated protein as a regulatory subunit. For example, one to four Ca(2+)-binding EF-hand motifs are present at the N-termini in several subfamilies, such as the respiratory burst oxidase homolog (Rboh) subfamily in land plants (the supergroup Plantae), the NoxC subfamily in social amoebae (the Amoebozoa), and the Nox5 and dual oxidase (Duox) subfamilies in animals (the Opisthokonta), whereas an SH3 domain is inserted into the ferredoxin-NADP(+) reductase region of two Nox enzymes in Naegleria gruberi, a unicellular organism that belongs to the supergroup Excavata. Members of the Nox1-4 subfamily in animals form a stable heterodimer with the membrane protein p22(phox), which functions as a docking site for the SH3 domain-containing regulatory proteins p47(phox), p67(phox), and p40(phox); the small GTPase Rac binds to p67(phox) (or its homologous protein), which serves as a switch for Nox activation. Similarly, Rac activates the fungal NoxA via binding to the p67(phox)-like protein Nox regulator (NoxR). In plants, on the other hand, this GTPase directly interacts with the N-terminus of Rboh, leading to superoxide production. Here I describe the regulation of Nox-family oxidases on the basis of three-dimensional structures and evolutionary conservation.