Regulation of the neutrophil respiratory burst oxidase. Identification of an activation domain in p67(phox)

Regulation of ion transport by oxidants

Ion channels perform a variety of cellular functions in lung epithelia. Oxidant- and antioxidant-mediated mechanisms (that is, redox regulation) of ion channels are areas of intense research. Significant progress has been made in our understanding of redox regulation of ion channels since the last Experimental Biology report in 2003. Advancements include: 1) identification of nonphagocytic NADPH oxidases as sources of regulated reactive species (RS) production in epithelia, 2) an understanding that excessive treatment with antioxidants can result in greater oxidative stress, and 3) characterization of novel RS signaling pathways that converge upon ion channel regulation. These advancements, as discussed at the 2013 Experimental Biology Meeting in Boston, MA, impact our understanding of oxidative stress in the lung, and, in particular, illustrate that the redox state has profound effects on ion channel and cellular function.

C-terminal truncation of Noxa1 greatly enhances its ability to activate Nox2 in a pure reconstitution system

Noxa1 activates Nox2 together with Noxo1 and Rac in a pure reconstitution system, but the resulting activity is considerably lower than that induced by p67(phox) and p47(phox). In this study, we found that C-terminal-truncated forms of Noxa1 exhibited higher activities than full-length Noxa1. Of the truncations examined, Noxa1(1-225) showed the highest ability for activation. Kinetic studies revealed that Noxa1(1-225) had a threefold higher Vmax value than full-length Noxa1 with a similar EC50 value. The affinities of Noxo1 and RacQ61L were not much altered by the truncation. Conversely, the affinity of FAD for the Nox2 complex was enhanced after the truncation. In the absence of Noxo1, Noxa1(1-225) showed much higher activity with a lower EC50 than full-length Noxa1. Noxa1(1-225) showed comparable activity to that of p67(phox) with either Noxo1 or p47(phox), although the stability was lower than that with p67(phox) and p47(phox). These findings indicate that the role of the C-terminal half of Noxa1 is autoinhibition. The data suggest a two-step autoinhibition mechanism, comprising self-masking to interrupt the binding to the oxidase, and holding of the activation domain in a suboptimal position to the oxidase. This study reveals that when both types of inhibition are released, Noxa1 achieves high-level superoxide production.

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.

Rational design of small molecule inhibitors targeting the Rac GTPase-p67(phox) signaling axis in inflammation

The NADPH oxidase enzyme complex, NOX2, is responsible for reactive oxygen species production in neutrophils and has been recognized as a key mediator of inflammation. Here, we have performed rational design and in silico screen to identify a small molecule inhibitor, Phox-I1, targeting the interactive site of p67(phox) with Rac GTPase, which is a necessary step of the signaling leading to NOX2 activation. Phox-I1 binds to p67(phox) with a submicromolar affinity and abrogates Rac1 binding and is effective in inhibiting NOX2-mediated superoxide production dose-dependently in human and murine neutrophils without detectable toxicity. Medicinal chemistry characterizations have yielded promising analogs and initial information of the structure-activity relationship of Phox-I1. Our studies suggest the potential utility of Phox-I class inhibitors in NOX2 oxidase inhibition and present an application of rational targeting of a small GTPase-effector interface.

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.

Biochemistry, physiology, and pathophysiology of NADPH oxidases in the cardiovascular system

The NADPH oxidase (Nox) enzymes are critical mediators of cardiovascular physiology and pathophysiology. These proteins are expressed in virtually all cardiovascular cells, and regulate such diverse functions as differentiation, proliferation, apoptosis, senescence, inflammatory responses and oxygen sensing. They target a number of important signaling molecules, including kinases, phosphatases, transcription factors, ion channels, and proteins that regulate the cytoskeleton. Nox enzymes have been implicated in many different cardiovascular pathologies: atherosclerosis, hypertension, cardiac hypertrophy and remodeling, angiogenesis and collateral formation, stroke, and heart failure. In this review, we discuss in detail the biochemistry of Nox enzymes expressed in the cardiovascular system (Nox1, 2, 4, and 5), their roles in cardiovascular cell biology, and their contributions to disease development.

Identification and functional characterization of two novel mutations in the α-helical loop (residues 484-503) of CYBB/gp91(phox) resulting in the rare X91(+) variant of chronic granulomatous disease

Chronic granulomatous disease (CGD) is mainly caused by mutations in X-linked CYBB that encodes gp91. We have identified two novel mutations in CYBB resulting in the rare X91(+)-CGD variant, c.1500T>G (p.Asp500Glu) in two male siblings and c.1463C>A (p.Ala488Asp) in an unrelated male. Zymosan and/or PMA (Phorbol 12-myristate 13-acetate)-induced recruitment of p47(phox) and p67(phox) to the membrane fraction was normal for both mutants. Cell-free assays using recombinant wild-type and the mutant proteins revealed that these mutants were not activated by NADPH (nicotinamide adenine dinucleotide phosphate). Interestingly, the Ala488Asp mutant was activated by NADPH in the presence of glutathione. These data suggest that the mutations prevented NADPH from binding to gp91(phox) and the requirement of a negative charge at residue 500 in gp91(phox) for NADPH oxidase assembly, in contrast to a previously described Asp500Gly change. These mutations and the effect of glutathione provide a unique insight into disease pathogenesis and potential therapy in variant X91(+)-CGD.

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.

The NADPH oxidase cytosolic component p67phox is constitutively phosphorylated in human neutrophils: Regulation by a protein tyrosine kinase, MEK1/2 and phosphatases 1/2A

Neutrophils play a key role in host defense and inflammation through the production of superoxide anion and other reactive oxygen species (ROS) by the enzyme complex NADPH oxidase. The cytosolic NADPH oxidase component, p67phox, has been shown to be phosphorylated in human neutrophils but the pathways involved in this process are largely unknown. In this study, we show that p67phox is constitutively phosphorylated in resting human neutrophils and that neutrophil stimulation with PMA further enhanced this phosphorylation. Inhibition of the constitutively active serine/threonine phosphatases type 1 and type 2A (PP1/2A) by calyculin A resulted in the enhancement of p67phox phosphorylation. Constitutive and calyculin A-induced phosphorylation of p67phox was completely inhibited by the protein tyrosine kinase inhibitor genistein and partially inhibited by the MEK1/2 inhibitor PD98059, but was unaffected by GF109203X, wortmannin and SB203580, inhibitors of PKC, PI3K and p38MAP kinase, respectively. Two-dimensional phosphopeptide mapping revealed that constitutive and calyculin A-induced p67phox phosphorylation occurred on the same major sites. Interestingly, calyculin A enhanced formyl-Met-Leu-Phe (fMLP)-induced superoxide production, while genistein inhibited this process. Taken together, these results suggest that (i) p67phox undergoes a continual cycle of phosphorylation/dephosphorylation in resting cells; (ii) p67phox phosphorylation is controlled by MEK1/2 and an upstream tyrosine kinase; (iii) PP1/2A directly or indirectly antagonize this process. Thus, these pathways could play a role in regulating ROS production by human neutrophils at inflammatory sites.

Versatile TPR domains accommodate different modes of target protein recognition and function

The tetratricopeptide repeat (TPR) motif is one of many repeat motifs that form structural domains in proteins that can act as interaction scaffolds in the formation of multi-protein complexes involved in numerous cellular processes such as transcription, the cell cycle, protein translocation, protein degradation and host defence against invading pathogens. The crystal structures of many TPR domain-containing proteins have been determined, showing TPR motifs as two anti-parallel α-helices packed in tandem arrays to form a structure with an amphipathic groove which can bind a target peptide. This is however not the only mode of target recognition by TPR domains, with short amino acid insertions and alternative TPR motif conformations also shown to contribute to protein interactions, highlighting diversity in TPR domains and the versatility of this structure in mediating biological events.

Granulomatous colitis of boxer dogs

Granulomatous colitis (GC) is a rare, breed-specific inflammatory bowel disease of young Boxer dogs. GC has been refractory to treatment and associated with high mortality rates, but culture-independent molecular analysis has transformed therapy and prognosis by uncovering a correlation between GC and Escherichia coli invasion within colonic mucosal macrophages. GC-associated invasive E coli are similar to a newly identified E coli pathotype, "adherent and invasive E coli," that are increasingly associated with Crohn's disease in humans. Successful treatment of GC requires antimicrobials that are effective against E coli and penetrate intracellularly. Enrofloxacin is widely regarded as the antibiotic of choice.

Modulation of the activity of the NADPH oxidase system by reactive oxygen species: influence of catalase

The nicotinamide adenine dinucleotide phosphate oxidase complex (Nox) is a major source of non-mitochondrial reactive oxygen species in cells. Nox contains both membrane (Cytb(558)) and cytosolic (p40(phox), p47(phox), p67(phox) and Rac) components. Nox has been submitted to a combination of oxygen free radicals produced by irradiation and to hydrogen peroxide. Irradiation of a single component with high doses led to partial inactivation; however, the irradiation of the whole system during its assembly phase with lower doses (2-10 Gy) led either to activation (2.7 Gy) or to strong inactivation if irradiation took place during the first minute of the assembly. Incubation of the membrane fractions or of p67(phox) with H(2)O(2) led to fast inactivation. Catalase protected weakly p67(phox) from H(2)O(2). Conversely, incubation of the membrane fractions with catalase led to over-activation of the system.

Low energy laser light (632.8 nm) suppresses amyloid-β peptide-induced oxidative and inflammatory responses in astrocytes

Oxidative stress and inflammation are important processes in the progression of Alzheimer's disease (AD). Recent studies have implicated the role of amyloid β-peptides (Aβ) in mediating these processes. In astrocytes, oligomeric Aβ induces the assembly of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complexes resulting in its activation to produce anionic superoxide. Aβ also promotes production of pro-inflammatory factors in astrocytes. Since low energy laser has previously been reported to attenuate oxidative stress and inflammation in biological systems, the objective of this study was to examine whether this type of laser light was able to abrogate the oxidative and inflammatory responses induced by Aβ. Primary rat astrocytes were exposed to Helium-Neon laser (λ=632.8 nm), followed by the treatment with oligomeric Aβ. Primary rat astrocytes were used to measure Aβ-induced production of superoxide anions using fluorescence microscopy of dihydroethidium (DHE), assembly of NADPH oxidase subunits by the colocalization between the cytosolic p47(phox) subunit and the membrane gp91(phox) subunit using fluorescent confocal microscopy, phosphorylation of cytosolic phospholipase A(2) cPLA(2) and expressions of pro-inflammatory factors including interleukin-1β (IL-1β) and inducible nitric-oxide synthase (iNOS) using Western blot Analysis. Our data showed that laser light at 632.8 nm suppressed Aβ-induced superoxide production, colocalization between NADPH oxidase gp91(phox) and p47(phox) subunits, phosphorylation of cPLA(2,) and the expressions of IL-1β and iNOS in primary astrocytes. We demonstrated for the first time that 632.8 nm laser was capable of suppressing cellular pathways of oxidative stress and inflammatory responses critical in the pathogenesis in AD. This study should prove to provide the groundwork for further investigations for the potential use of laser therapy as a treatment for AD.

A conserved region between the TPR and activation domains of p67phox participates in activation of the phagocyte NADPH oxidase

The phagocyte NADPH oxidase, dormant in resting cells, is activated during phagocytosis to produce superoxide, a precursor of microbicidal oxidants. The membrane-integrated protein gp91(phox) serves as the catalytic core, because it contains a complete electron-transporting apparatus from NADPH to molecular oxygen for superoxide production. Activation of gp91(phox) requires the cytosolic proteins p67(phox), p47(phox), and Rac (a small GTPase). p67(phox), comprising 526 amino acids, moves upon cell stimulation to the membrane together with p47(phox) and there interacts with Rac; these processes are prerequisite for gp91(phox) activation. Here we show that a region of p67(phox) (amino acids 190-200) C-terminal to the Rac-binding domain is evolutionarily well conserved and participates in oxidase activation at a later stage in conjunction with an activation domain. Alanine substitution for Tyr-198, Leu-199, or Val-204 abrogates the ability of p67(phox) to support superoxide production by gp91(phox)-based oxidase as well as its related oxidases Nox1 and Nox3; the activation also involves other invariant residues such as Leu-193, Asp-197, and Gly-200. Intriguingly, replacement of Gln-192 by alanine or that of Tyr-198 by phenylalanine or tryptophan rather enhances superoxide production by gp91(phox)-based oxidase, suggesting a tuning role for these residues. Furthermore, the Y198A/V204A or L199A/V204A substitution leads to not only a complete loss of the activity of the reconstituted oxidase system but also a significant decrease in p67(phox) interaction with the gp91(phox) NADPH-binding domain, although these mutations affect neither the protein integrity nor the Rac binding activity. Thus the extended activation domain of p67(phox) (amino acids 190-210) containing the D(Y/F)LGK motif plays an essential role in oxidase activation probably by interacting with gp91(phox).

A prenylated p47phox-p67phox-Rac1 chimera is a Quintessential NADPH oxidase activator: membrane association and functional capacity

The superoxide-generating NADPH oxidase complex of resting phagocytes includes cytochrome b(559), a membrane-associated heterodimer composed of two subunits (Nox2 and p22(phox)), and four cytosolic proteins (p47(phox), p67(phox), Rac, and p40(phox)). Upon stimulation, the cytosolic components translocate to the membrane, as the result of a series of interactions among the cytosolic components and among the cytosolic components and cytochrome b(559) and its phospholipid environment. We described the construction of a tripartite chimera (trimera) consisting of strategic domains of p47(phox), p67(phox), and Rac1, in which interactions among cytosolic components were replaced by fusion (Berdichevsky, Y., Mizrahi, A., Ugolev, Y., Molshanski-Mor, S., and Pick, E. (2007) J. Biol. Chem. 282, 22122-22139). We now fused green fluorescent protein (GFP) to the N terminus of the trimera and found the following. 1) The GFP-p47(phox)-p67(phox)-Rac1 trimera activates the oxidase in amphiphile-dependent and -independent (anionic phospholipid-enriched membrane) cell-free systems. 2) Geranylgeranylation of the GFP-trimera makes it a potent oxidase activator in unmodified (native) membranes and in the absence of amphiphile. 3) Prenylated GFP-trimera binds spontaneously to native membranes (as assessed by gel filtration and in-line fluorometry), forming a tight complex capable of NADPH-dependent, activator-independent superoxide production at rates similar to those measured in canonical cell-free systems. 4) Prenylation of the GFP-trimera supersedes completely the dependence of oxidase activation on the p47(phox) phox homology domain and, partially, on the Rac1 polybasic domain, but the requirement for Trp(193) in p47(phox) persists. Prenylated GFP-p47(phox)-p67(phox)-Rac1 trimera acts as a quintessential single molecule oxidase activator of potential use in high throughput screening of inhibitors.

Comparison of macrophage antimicrobial responses induced by type II interferons of the goldfish (Carassius auratus L.)

Unlike mammals, bony fish have two type II interferons, IFNgamma and IFNgammarel, whose pro-inflammatory functions have not been fully characterized. To elucidate the distinct roles of these type II interferons of bony fish, we examined the effects of recombinant goldfish (rg) IFNgamma and IFNgammarel on the macrophage antimicrobial responses, immune gene expression, and their signaling pathways. Our findings indicate that rgIFNgamma and rgIFNgammarel possess unique capacities to mediate each of the above processes. Q-PCR analysis revealed similar expression of both cytokines in tissues and immune cell populations of the goldfish, although IFNgamma mRNA levels were generally higher in most tissues and cell types. Whereas rgIFNgamma had long-lasting effects on the priming of goldfish monocyte ROI production, the rgIFNgammarel had relatively short-lived ROI priming potential and eventually down-regulated the priming of ROI production induced by rgIFNgamma or rgTNFalpha2. Whereas rgIFNgamma induced relatively modest phagocytic and nitric oxide responses of goldfish macrophages, rgIFNgammarel induced significantly higher phagocytosis, iNOSA and iNOSB gene expression and nitric oxide production compared with rgIFNgamma. The rgIFNgamma and rgIFNgammarel induced different gene expression profiles in goldfish monocytes. These differences included significantly higher induction of TNFalpha2, CXCL8, ceruloplasmin, and interferon regulatory factor (IRFs) expression after activation of monocytes with rgIFNgammarel. The rgIFNgammarel was more abundant in whole cell lysates compared with rgIFNgamma. Both cytokines induced the phosphorylation of Stat1, while the nuclear localization of Stat1 was only observed following treatment of monocytes with rgIFNgamma. Our findings suggest the presence of functional segregation of the induction of macrophage antimicrobial functions by type II interferons of bony fish.

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.

Constitutive NADPH-dependent electron transferase activity of the Nox4 dehydrogenase domain

NADPH oxidase 4 (Nox4) is constitutively active, while Nox2 requires the cytosolic regulatory subunits p47(phox) and p67(phox) and activated Rac with activation by phorbol 12-myristate 13-acetate (PMA). This study was undertaken to identify the domain on Nox4 that confers constitutive activity. Lysates from Nox4-expressing cells exhibited constitutive NADPH- but not NADH-dependent hydrogen peroxide production with a K(m) for NADPH of 55 +/- 10 microM. The concentration of Nox4 in cell lysates was estimated using Western blotting and allowed calculation of a turnover of approximately 200 mol of H(2)O(2) min(-1) (mol of Nox4)(-1). A chimeric protein (Nox2/4) consisting of the Nox2 transmembrane (TM) domain and the Nox4 dehydrogenase (DH) domain showed H(2)O(2) production in the absence of cytosolic regulatory subunits. In contrast, chimera Nox4/2, consisting of the Nox4 TM and Nox2 DH domains, exhibited PMA-dependent activation that required coexpression of regulatory subunits. Nox DH domains from several Nox isoforms were purified and evaluated for their electron transferase activities. Nox1 DH, Nox2 DH, and Nox5 DH domains exhibited barely detectable activities toward artificial electron acceptors, while the Nox4 DH domain exhibited significant rates of reduction of cytochrome c (160 min(-1), largely superoxide dismutase-independent), ferricyanide (470 min(-1)), and other electron acceptors (artificial dyes and cytochrome b(5)). Rates were similar to those observed for H(2)O(2) production by the Nox4 holoenzyme in cell lysates. The activity required added FAD and was seen with NADPH but not NADH. These results indicate that the Nox4 DH domain exists in an intrinsically activated state and that electron transfer from NADPH to FAD is likely to be rate-limiting in the NADPH-dependent reduction of oxygen by holo-Nox4.

Regulation of phagocyte NADPH oxidase by hydrogen peroxide through a Ca(2+)/c-Abl signaling pathway

The importance of H(2)O(2) as a cellular signaling molecule has been demonstrated in a number of cell types and pathways. Here we explore a positive feedback mechanism of H(2)O(2)-mediated regulation of the phagocyte respiratory burst NADPH oxidase (NOX2). H(2)O(2) induced a dose-dependent stimulation of superoxide production in human neutrophils, as well as in K562 leukemia cells overexpressing NOX2 system components. Stimulation was abrogated by the addition of catalase, the extracellular Ca(2+) chelator BAPTA, the T-type Ca(2+) channel inhibitor mibefradil, the PKCdelta inhibitor rottlerin, or the c-Abl nonreceptor tyrosine kinase inhibitor imatinib mesylate or by overexpression of a dominant-negative form of c-Abl. H(2)O(2) induced phosphorylation of tyrosine 311 on PKCdelta and this activating phosphorylation was blocked by treatment with rottlerin, imatinib mesylate, or BAPTA. Rac GTPase activation in response to H(2)O(2) was abrogated by BAPTA, imatinib mesylate, or rottlerin. In conclusion, H(2)O(2) stimulates NOX2-mediated superoxide generation in neutrophils and K562/NOX2 cells via a signaling pathway involving Ca(2+) influx and c-Abl tyrosine kinase acting upstream of PKCdelta. This positive feedback regulatory pathway has important implications for amplifying the innate immune response and contributing to oxidative stress in inflammatory disorders.

Phosphorylation of serine282 in NADPH oxidase activator 1 by Erk desensitizes EGF-induced ROS generation

Accumulating evidence indicates that protein phosphorylation regulates Nox activity. In this report, we show that serine282 residue of Nox activator 1 (NoxA1) is phosphorylated by Erk in response to EGF resulting in desensitization of Nox1 activity. Specifically, murine NoxA1 is detected as two independent protein bands in SDS PAGE, and the form of protein with higher mobility shifted to and merged with the one with lower mobility in response to EGF treatment. Pretreatment with PD98059 resulted in inhibition of NoxA1 migration in response to EGF indicating that Erk was involved in the process. Site-directed mutagenesis showed that S282A mutant but not S239A mutant failed to respond to EGF, demonstrating that serine282 is the target amino acid of Erk. Expression of S282A mutant of NoxA1 in these cells led to increased superoxide anion production in response to EGF compared to expression of the wild type, whereas the expression of S282E, a phosphomimetic mutant, resulted in significantly decreased superoxide anion generation. We also tested whether the phosphorylation of serine282 of NoxA1 affects Rac activation. Expression of S282A mutant NoxA1 up-regulated the Rac activity, whereas expression of S282E mutant led to the abrogation of Rac activation. Taken together, these results demonstrate that phosphorylation of NoxA1 is a part of the feedback mechanism that functions through activation of Rac with a net outcome of negative modulation of Nox1 activity.

Phosphorylation of NADPH oxidase activator 1 (NOXA1) on serine 282 by MAP kinases and on serine 172 by protein kinase C and protein kinase A prevents NOX1 hyperactivation

NADPH oxidase activator 1 (NOXA1) together with NADPH oxidase organizer 1 (NOXO1) are key regulatory subunits of the NADPH oxidase NOX1. NOX1 is expressed mainly in colon epithelial cells and could be involved in mucosal innate immunity by producing reactive oxygen species (ROS). Contrary to its phagocyte counterpart NOX2, the mechanisms involved in NOX1 activation and regulation remain unclear. Here we report that NOX1 activity is regulated through MAP kinase (MAPK), protein kinase C (PKC), and protein kinase A (PKA)-dependent phosphorylation of NOXA1. We identified Ser-282 as target of MAPK and Ser-172 as target of PKC and PKA in vitro and in a transfected human embryonic kidney 293 (HEK293) cell model using site directed mutagenesis and phosphopeptide mapping analysis. In HEK293 cells, phosphorylation of these sites occurred at a basal level and down-regulated constitutive NOX1 activity. Indeed, S172A and S282A single mutants of NOXA1 significantly up-regulated constitutive NOX1-derived ROS production, and S172A/S282A double mutant further increased it, as compared to wild-type NOXA1. Furthermore, phosphorylation of NOXA1 on Ser-282 and Ser-172 decreased its binding to NOX1 and Rac1. These results demonstrated a critical role of NOXA1 phosphorylation on Ser-282 and Ser-172 in preventing NOX1 hyperactivation through the decrease of NOXA1 interaction to NOX1 and Rac1.

Emerging evidence for the importance of phosphorylation in the regulation of NADPH oxidases

The NADPH oxidase (Nox) enzyme family generates reactive oxygen species (ROS) that contribute to cell signaling, innate immune responses, proliferation, and transcription. The signaling mechanisms that regulate this important enzyme family are only beginning to be understood. Evidence is emerging which suggests that phosphorylation of Nox and/or their regulatory components may be important means of modulating their activity. We describe here the evidence for Nox regulation through the action of kinases, and speculate on how such regulatory mechanisms might contribute to the development of pathological disease states.

Four different NCF2 mutations in six families from Turkey and an overview of NCF2 gene mutations

BACKGROUND

One of the rarest forms of autosomal recessive chronic granulomatous disease (AR-CGD) is attributable to mutations in the NCF2 gene, which encodes the polypeptide p67(phox), a key cytoplasmic protein in the phagocyte NADPH oxidase system. NCF2 is localized on chromosome 1q25, encompasses 40 kb and contains 16 exons.

MATERIALS AND METHODS

We report here the clinical and molecular characterization of six patients with CGD from six consanguineous Turkish families. The ages of the five female patients were between 3 and 22 years and a male patient was 2 years old; all patients showed clear clinical symptoms of CGD.

RESULTS

The mothers of the patients did not show a bimodal histogram pattern specific for X-CGD in the dihydrorhodamine-1,2,3 (DHR) assay. Moreover, p67(phox) protein expression was not detectable using flow cytometric analysis of the patients' neutrophils except in those from patient 6, which had a diminished expression. Mutation analysis of NCF2 revealed four different homozygous mutations: a novel nonsense mutation in exon 3 c.229C>T, p.Arg77X; a novel missense mutation in exon 4 c.279C>G, p.Asp93Glu; a nonsense mutation in exon 4 c.304C>T, p.Arg102X; and a novel missense mutation in exon 6 c.605C>T, p.Ala202Val. The parents were found to be heterozygotes for these mutations.

CONCLUSIONS

The prevalence of NCF2 mutant families is approximately 15% in our series of 40 CGD families. This high incidence of A67 CGD in Turkey is undoubtedly caused by the high incidence of consanguineous marriages. We found three new mutations in NCF2 and one previously described. These are presented together with an overview of all NCF2 mutations now known.

Identification and expression profile of multiple genes in the anterior kidney of channel catfish induced by modified live Edwardsiella ictaluri vaccination

Using PCR-select subtractive cDNA hybridization technique, 57 expressed sequence tags (ESTs) were isolated from 240 clones of a modified live Edwardsiella ictaluri vaccinated vs. sham-vaccinated channel catfish anterior kidney subtractive library. The transcription levels of the 57 ESTs in response to E. ictaluri vaccination were then evaluated by quantitative PCR (QPCR). Of the 57 ESTs, 43 were induced at least 2-fold higher in all three vaccinated fish compared to unvaccinated control fish. Of the 43 upregulated genes, five were consistently upregulated greater than 10-fold, including two highly upregulated (>20-fold) glycosyltransferase and Toll-like receptor 5. The transcriptional levels of GTPase 1, coatomer protein complex zeta 1, and type II arginine deiminase were consistently induced greater than 10-fold. MHC class I alpha chain and transposase were upregulated greater than 10-fold in two of the three vaccinated fish. The 43 upregulated genes also included 19 moderately upregulated (3-10-fold) and 17 slightly upregulated (2-3-fold). Our results suggest that subtractive cDNA hybridization and QPCR are powerful cost-effective techniques to identify differentially expressed genes in response to modified live E. ictaluri vaccination.

Delayed association of the NADPH oxidase complex with macrophage vacuoles containing the opportunistic pathogen Burkholderia cenocepacia

Burkholderia cenocepacia causes chronic lung infections in patients suffering from cystic fibrosis and chronic granulomatous disease. We have previously shown that B. cenocepacia survives intracellularly in macrophages within a membrane vacuole (BcCV) that delays acidification. Here, we report that after macrophage infection with live B. cenocepacia there is a approximately 6 h delay in the association of NADPH oxidase with BcCVs, while heat-inactivated bacteria are normally trafficked into NADPH oxidase-positive vacuoles. BcCVs in macrophages treated with a functional inhibitor of the cystic fibrosis transmembrane conductance regulator exhibited a further delay in the assembly of the NADPH oxidase complex at the BcCV membrane, but the inhibitor did not affect NADPH oxidase complex assembly onto vacuoles containing heat-inactivated B. cenocepacia or live Escherichia coli. Macrophages produced less superoxide following B. cenocepacia infection as compared to heat-inactivated B. cenocepacia and E. coli controls. Reduced superoxide production was associated with delayed deposition of cerium perhydroxide precipitates around BcCVs of macrophages infected with live B. cenocepacia, as visualized by transmission electron microscopy. Together, our results demonstrate that intracellular B. cenocepacia resides in macrophage vacuoles displaying an altered recruitment of the NADPH oxidase complex at the phagosomal membrane. This phenomenon may contribute to preventing the efficient clearance of this opportunistic pathogen from the infected airways of susceptible patients.

Role of Reactive Oxygen Species in Tumor Necrosis Factor-alpha Induced Endothelial Dysfunction

Endothelial cell injury and dysfunction are the major triggers of pathophysiological processes leading to cardiovascular disease. Endothelial dysfunction (ED) has been implicated in atherosclerosis, hypertension, coronary artery disease, vascular complications of diabetes, chronic renal failure, insulin resistance and hypercholesterolemia. Although now recognized as a class of physiological second messengers, reactive oxygen species (ROS) are important mediators in cellular injury, specifically, as a factor in endothelial cell damage. Uncontrolled ROS production and/or decreased antioxidant activity results in a deleterious state referred to as 'oxidative stress'. A candidate factor in causing ROS production in endothelial cells is tumor necrosis factor alpha (TNF-α), a pleiotropic inflammatory cytokine. TNF-α has been shown to both be secreted by endothelial cells and to induce intracellular ROS formation. These observations provide a potential mechanism by which TNF-α may activate and injure endothelial cells resulting in ED. In this review, we focus on the relationship between intracellular ROS formation and ED in endothelial cells or blood vessels exposed to TNF-α to provide insight into the role of this important cytokine in cardiovascular disease.

Inhibitory action of NoxA1 on dual oxidase activity in airway cells

Imbalance between pro- and antioxidant mechanisms in the lungs can compromise pulmonary functions, including blood oxygenation, host defense, and maintenance of an anti-inflammatory environment. Thus, tight regulatory control of reactive oxygen species is critical for proper lung function. Increasing evidence supports a role for the NADPH oxidase dual oxidase (Duox) as an important source for regulated H2O2 production in the respiratory tract epithelium. In this study Duox expression, function, and regulation were investigated in a fully differentiated, mucociliary airway epithelium model. Duox-mediated H2O2 generation was dependent on calcium flux, which was required for dissociation of the NADPH oxidase regulatory protein Noxa1 from plasma membrane-bound Duox. A functional Duox1-based oxidase was reconstituted in model cell lines to permit mutational analysis of Noxa1 and Duox1. Although the activation domain of Noxa1 was not required for Duox function, mutation of a proline-rich domain in the Duox C terminus, a potential interaction motif for the Noxa1 Src homology domain 3, caused up-regulation of basal and stimulated H2O2 production. Similarly, knockdown of Noxa1 in airway cells increased basal H2O2 generation. Our data indicate a novel, inhibitory function for Noxa1 in Duox regulation. This represents a new paradigm for control of NADPH oxidase activity, where second messenger-promoted conformational change of the Nox structure promotes oxidase activation by relieving constraint induced by regulatory components.

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.

NADPH oxidase: recent evidence for its role in erectile dysfunction

Important roles for reactive oxygen species (ROS) in physiology and pathophysiology have been increasingly recognized. Under normal conditions, ROS serve as signaling molecules in the regulation of cellular functions. However, enhanced ROS production as a result of the activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase contributes significantly to the pathogeneses of vascular diseases. Although it has become evident that increased ROS is associated with erectile dysfunction (ED), the sources of ROS in the penis remain largely unknown. In recent years, emergent evidence suggests the possible role of NADPH oxidase in inducing ED. In this review, we examine the relationship between ROS and ED in different disease models and discuss the current evidence basis for NADPH oxidase-derived ROS in ED.

Inhibitory effects of ginsenosides from the root of Panax ginseng on stimulus-induced superoxide generation, tyrosyl or serine/threonine phosphorylation, and translocation of cytosolic compounds to plasma membrane in human neutrophils

The effects of five ginsenosides (G-Rh2, -Rd, -Rb1, -Rb2, -Rh1) isolated from the root of Panax gingseng on stimulus-induced superoxide generation in human neutrophils were evaluated by measuring the reduction of ferricytochrome c. The tyrosyl or serine/threonine phosphorylation of neutrophil proteins and translocation of p47phox, p67phox, and Rac to the plasma membrane were detected using specific monoclonal antibodies. G-Rh2 significantly suppressed superoxide generation induced by N-formylmethionyl-leucylphenylalanine (fMLP), phorbol 12-myristate 13-acetate (PMA), and arachidonic acid (AA) in a concentration-dependent manner. G-Rh1 showed a comparably lower suppression on fMLP-induced superoxide generation. G-Rd, -Rb1, and -Rb2 also suppressed AA-induced superoxide generation in high concentrations. G-Rd and G-Rb1 showed no effect on fMLP- and PMA-induced superoxide generation. FMLP-, PMA-, and AA-induced tyrosyl or serine/threonine phosphorylation and translocation of p47phox, p67phox, and Rac to the plasma membrane were in parallel with the suppression of the stimulus-induced superoxide generation.

Identification of a conserved Rac-binding site on NADPH oxidases supports a direct GTPase regulatory mechanism

The NADPH oxidases (Noxs) are a family of superoxide-generating enzymes implicated in a variety of biological processes. Full activity of Nox1, -2, and -3 requires the action of a Rac GTPase. A direct regulatory interaction of Rac with Nox2 has been proposed as part of a two-step mechanism for regulating electron transfer during superoxide formation. Using truncation analysis of Rac binding to the cytoplasmic tail of Nox2, along with peptides derived from this region in cell-free assays, we identify a Rac interaction site within amino acids 419-430 of Nox2. This region is required for binding Rac2 but not p47(phox) or p67(phox) cytosolic regulatory factors. A cell-permeant version of the peptide encompassing amino acids 419-430 specifically inhibits NADPH oxidase activation in intact human neutrophils. Mutational analysis of the putative Rac-binding site revealed specific residues, particularly Lys-421, Tyr-425, and Lys-426, individually required for Rac-dependent NADPH oxidase activity that are conserved in the Rac-regulated Nox1, Nox2, and Nox3 enzymes but not in Nox4 or Nox5. Mutation of the conserved residues in the Rac-binding site of Nox1 also result in the loss of Rac-dependent activity. Our data identify a functional Rac interaction site conserved in Rac-dependent Noxs and support a direct regulatory interaction of Rac GTPases to promote activation of these NADPH oxidases.

Molecular evolution of Phox-related regulatory subunits for NADPH oxidase enzymes

Background

The reactive oxygen-generating NADPH oxidases (Noxes) function in a variety of biological roles, and can be broadly classified into those that are regulated by subunit interactions and those that are regulated by calcium. The prototypical subunit-regulated Nox, Nox2, is the membrane-associated catalytic subunit of the phagocyte NADPH-oxidase. Nox2 forms a heterodimer with the integral membrane protein, p22phox, and this heterodimer binds to the regulatory subunits p47phox, p67phox, p40phox and the small GTPase Rac, triggering superoxide generation. Nox-organizer protein 1 (NOXO1) and Nox-activator 1 (NOXA1), respective homologs of p47phox and p67phox, together with p22phox and Rac, activate Nox1, a non-phagocytic homolog of Nox2. NOXO1 and p22phox also regulate Nox3, whereas Nox4 requires only p22phox. In this study, we have assembled and analyzed amino acid sequences of Nox regulatory subunit orthologs from vertebrates, a urochordate, an echinoderm, a mollusc, a cnidarian, a choanoflagellate, fungi and a slime mold amoeba to investigate the evolutionary history of these subunits.

Results

Ancestral p47phox, p67phox, and p22phox genes are broadly seen in the metazoa, except for the ecdysozoans. The choanoflagellate Monosiga brevicollis, the unicellular organism that is the closest relatives of multicellular animals, encodes early prototypes of p22phox, p47phox as well as the earliest known Nox2-like ancestor of the Nox1-3 subfamily. p67phox- and p47phox-like genes are seen in the sea urchin Strongylocentrotus purpuratus and the limpet Lottia gigantea that also possess Nox2-like co-orthologs of vertebrate Nox1-3. Duplication of primordial p47phox and p67phox genes occurred in vertebrates, with the duplicated branches evolving into NOXO1 and NOXA1. Analysis of characteristic domains of regulatory subunits suggests a novel view of the evolution of Nox: in fish, p40phox participated in regulating both Nox1 and Nox2, but after the appearance of mammals, Nox1 (but not Nox2) became independent of p40phox. In the fish Oryzias latipes, a NOXO1 ortholog retains an autoinhibitory region that is characteristic of mammalian p47phox, and this was subsequently lost from NOXO1 in later vertebrates. Detailed amino acid sequence comparisons identified both putative key residues conserved in characteristic domains and previously unidentified conserved regions. Also, candidate organizer/activator proteins in fungi and amoeba are identified and hypothetical activation models are suggested.

Conclusion

This is the first report to provide the comprehensive view of the molecular evolution of regulatory subunits for Nox enzymes. This approach provides clues for understanding the evolution of biochemical and physiological functions for regulatory-subunit-dependent Nox enzymes.

Structure and Function of the PB1 Domain, a Protein Interaction Module Conserved in Animals, Fungi, Amoebas, and Plants

Proteins containing the PB1 domain, a protein interaction module conserved in animals, fungi, amoebas, and plants, participate in diverse biological processes. The PB1 domains adopt a ubiquitin-like beta-grasp fold, containing two alpha helices and a mixed five-stranded beta sheet, and are classified into groups harboring an acidic OPCA motif (type I), the invariant lysine residue on the first beta strand (type II), or both (type I/II). The OPCA motif of a type I PB1 domain forms salt bridges with basic residues, especially the conserved lysine, of a type II PB1 domain, thereby mediating a specific PB1-PB1 heterodimerization, whereas additional contacts contribute to high affinity and specificity of the modular interaction. The canonical PB1 dimerization is required for the formation of complexes between p40(phox) and p67(phox) (for activation of the NADPH oxidase crucial for mammalian host defense), between the scaffold Bem1 and the guanine nucleotide exchange factor Cdc24 (for polarity establishment in yeasts), and between the polarity protein Par6 and atypical protein kinase C (for cell polarization in animal cells), as well as for the interaction between the mitogen-activated protein kinase kinase kinases MEKK2 or MEKK3 and the downstream target mitogen-activated protein kinase kinase MEK5 (for early cardiovascular development in mammals). PB1 domains can also mediate interactions with other protein domains. For example, an intramolecular interaction between the PB1 and PX domains of p40(phox) regulates phagosomal targeting of the microbicidal NADPH oxidase; the PB1 domain of MEK5 is likely responsible for binding to the downstream kinase ERK5, which lacks a PB1 domain; and the scaffold protein Nbr1 associates through a PB1-containing region with titin, a sarcomere protein without a PB1 domain. This Review describes various aspects of PB1 domains at the molecular and cellular levels.

Tripartite chimeras comprising functional domains derived from the cytosolic NADPH oxidase components p47phox, p67phox, and Rac1 elicit activator-independent superoxide production by phagocyte membranes: an essential role for anionic membrane phospholipids

The superoxide-generating NADPH oxidase is converted to an active state by the assembly of a membrane-localized cytochrome b(559) with three cytosolic components: p47(phox), p67(phox), and GTPase Rac1 or Rac2. Assembly involves two sets of protein-protein interactions: among cytosolic components and among cytosolic components and cytochrome b(559) within its lipid habitat. We circumvented the need for interactions among cytosolic components by constructing a recombinant tripartite chimera (trimera) consisting of the Phox homology (PX) and Src homology 3 (SH3) domains of p47(phox), the tetratricopeptide repeat and activation domains of p67(phox), and full-length Rac1. Upon addition to phagocyte membrane, the trimera was capable of oxidase activation in vitro in the presence of an anionic amphiphile. The trimera had a higher affinity (lower EC(50)) for and formed a more stable complex (longer half-life) with cytochrome b(559) compared with the combined individual components, full-length or truncated. Supplementation of membrane with anionic but not neutral phospholipids made activation by the trimera amphiphile-independent. Mutagenesis, truncations, and domain replacements revealed that oxidase activation by the trimera was dependent on the following interactions: 1) interaction with anionic membrane phospholipids via the poly-basic stretch at the C terminus of the Rac1 segment; 2) interaction with p22(phox) via Trp(193) in the N-terminal SH3 domain of the p47(phox) segment, supplementing the electrostatic attraction; and 3) an intrachimeric bond among the p67(phox) and Rac1 segments complementary to their physical fusion. The PX domain of the p47(phox) segment and the insert domain of the Rac1 segment made only minor contributions to oxidase assembly.

NOX in liver fibrosis

NADPH oxidase is a multi-protein complex producing reactive oxygen species (ROS) both in phagocytic cells, being essential in host defense, and in non-phagocytic cells, regulating intracellular signalling. In the liver, NADPH oxidase plays a central role in fibrogenesis. A functionally active form of the NADPH oxidase is expressed not only in Kupffer cells (phagocytic cell type) but also in hepatic stellate cells (HSCs) (non-phagocytic cell type), suggesting a role of the non-phagocytic NADPH oxidase in HSC activation. Consistent with this concept, profibrogenic agonists such as Angiotensin II (Ang II) and platelet derived growth factor (PDGF), or apoptotic bodies exert their activity through NADPH oxidase-activation in HSCs. Both pharmacological inhibition with DPI and genetic studies using p47(phox) knockout mice provided evidence for a central role of NADPH oxidase in the regulation of HSC-activity and liver fibrosis. In addition to the p47(phox) component, only Rac1 has been identified as a functional active component of the NADPH oxidase complex in HSCs.

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.

A regulated adaptor function of p40phox: distinct p67phox membrane targeting by p40phox and by p47phox

In the phagocytic cell, NADPH oxidase (Nox2) system, cytoplasmic regulators (p47(phox), p67(phox), p40(phox), and Rac) translocate and associate with the membrane-spanning flavocytochrome b(558), leading to activation of superoxide production. We examined membrane targeting of phox proteins and explored conformational changes in p40(phox) that regulate its translocation to membranes upon stimulation. GFP-p40(phox) translocates to early endosomes, whereas GFP-p47(phox) translocates to the plasma membrane in response to arachidonic acid. In contrast, GFP-p67(phox) does not translocate to membranes when expressed alone, but it is dependent on p40(phox) and p47(phox) for its translocation to early endosomes or the plasma membrane, respectively. Translocation of GFP-p40(phox) or GFP-p47(phox) to their respective membrane-targeting sites is abolished by mutations in their phox (PX) domains that disrupt their interactions with their cognate phospholipid ligands. Furthermore, GFP-p67(phox) translocation to either membrane is abolished by mutations that disrupt its interaction with p40(phox) or p47(phox). Finally, we detected a head-to-tail (PX-Phox and Bem1 [PB1] domain) intramolecular interaction within p40(phox) in its resting state by deletion mutagenesis, cell localization, and binding experiments, suggesting that its PX domain is inaccessible to interact with phosphatidylinositol 3-phosphate without cell stimulation. Thus, both p40(phox) and p47(phox) function as diverse p67(phox) "carrier proteins" regulated by the unmasking of membrane-targeting domains in distinct mechanisms.

Pollen NAD(P)H Oxidases and Their Contribution to Allergic Inflammation

This article provides an overview of NADPH oxidase and its role in allergic inflammation. A background and historical perspectives of NADPH oxidase are first provided, followed by a detailed overview of mammalian NADPH oxidase subunits and their functional organization. Plant NADPH oxidase, the authors' discovery of NADPH oxidase in pollens, and their contribution to allergic inflammation are then discussed, concluding with a discussion of future directions and outstanding questions that require attention.

Cell-free assays: the reductionist approach to the study of NADPH oxidase assembly, or "all you wanted to know about cell-free assays but did not dare to ask"

The superoxide (O2-)-generating enzyme complex of phagocytes, known as the NADPH oxidase, can be assayed in a number of in vitro cell-free (or broken cell) systems. These consist of a mixture of the individual components of the NADPH oxidase, derived from resting phagocytes or in the form of purified recombinant proteins, exposed to an activating agent (or situation), in the presence of NADPH and oxygen. O2- produced by the mixture is measured by being trapped immediately after its generation with an appropriate acceptor in a kinetic assay, which permits the calculation of the linear rate of O2- production over time. Cell-free assays are distinguished from whole-cell assays or assays performed on membranes derived from stimulated cells by the fact that all components in the reaction are derived from resting, nonstimulated cells and, thus, the steps of NADPH oxidase activation (precatalytic [assembly] and catalytic) occur in vitro. Cell-free assays played a paramount role in the identification of the components of the NADPH oxidase complex, the diagnosis of various forms of chronic granulomatous disease (CGD), and, more recently, the analysis of the domains present on the components of the NADPH oxidase participating in protein-protein interactions leading to the assembly of the active complex.

Leu505 of Nox2 is crucial for optimal p67phox-dependent activation of the flavocytochrome b558 during phagocytic NADPH oxidase assembly

The role of Leu505 of Nox2 on the NADPH oxidase activation process was investigated. An X-CGD PLB-985 cell line expressing the Leu505Arg Nox2 mutant was obtained, exactly mimicking the phenotype of a previously published X91+-CGD case. In a reconstituted cell-free system (CFS), NADPH oxidase and iodonitrotetrazolium (INT) reductase activities were partially maintained concomitantly with a partial cytosolic factors translocation to the plasma membrane. This suggests that assembly and electron transfer from NADPH occurred partially in the Leu505Arg Nox2 mutant. Moreover, in a simplified CFS using purified mutant cytochrome b558 and recombinant p67phox, p47phox, and Rac1proteins, we found that the Km for NADPH and for NADH was about three times higher than those of purified WT cytochrome b558, indicating that the Leu505Arg mutation induces a slight decrease of the affinity for NADPH and NADH. In addition, oxidase activity can be extended by increasing the amount of p67phox in the simplified CFS assay. However, the maximal reconstituted oxidase activity using WT purified cytochrome b558 could not be reached using mutant cytochrome b558. In a three-dimensional model of the C-terminal tail of Nox2, Leu505 appears to have a strategic position just at the entry of the NADPH binding site and at the end of the alpha-helical loop (residues 484-504), a potential cytosolic factor binding region. The Leu505Arg mutation seems to affect the oxidase complex activation process through alteration of cytosolic factors binding and more particularly the p67phox interaction with cytochrome b558, thus affecting NADPH access to its binding site.

A p67Phox-like regulator is recruited to control hyphal branching in a fungal-grass mutualistic symbiosis

Key requirements for microbes to initiate and establish mutualistic symbiotic interactions with plants are evasion of potential host defense responses and strict control of microbial growth. Reactive oxygen species (ROS) produced by a specific NADPH oxidase isoform, NoxA, regulate hyphal growth in the mutualistic interaction between the fungal endophyte Epichloë festucae and its grass host Lolium perenne. Unlike mammalian systems, little is known about the fungal NADPH oxidase complex and its response to differentiation signals. We identify an E. festucae p67(phox)-like regulator, NoxR, dispensable in culture but essential in planta for the symbiotic interaction. Plants infected with a noxR deletion mutant show severe stunting and premature senescence, whereas hyphae in the meristematic tissues show increased branching leading to increased fungal colonization of pseudostem and leaf blade tissue. Inhibition of ROS production or overexpression of noxR recapitulates the hyperbranching phenotype in culture. NoxR interacts in vitro with the small GTP binding protein RacA and requires a functional RacA binding site to complement the noxR mutant and restore the wild-type plant interaction phenotype. These results show that NoxR is a key regulator of NoxA in symbiosis, where it acts together with RacA to spatially regulate ROS production and control hyphal branching and patterning.

Regulation of the phagocyte NADPH oxidase by Rac GTPase

Phagocytic leukocytes generate reactive oxygen species important for the killing of invading microorganisms. The source of these oxidants is the NADPH oxidase, a tightly controlled multicomponent enzyme made up of a membrane-associated catalytic moiety and cytosolic regulatory components that must assemble to form the active oxidase. The phagocyte NADPH oxidase was the first mammalian system shown to be directly regulated by a Rac GTPase. We review here our understanding of NADPH oxidase regulation by Rac, as well as the regulation of Rac itself, in phagocytic leukocytes. Rather than viewing Rac as a "cog" in the NADPH oxidase machinery, we argue for a view of Rac GTPases as critical "molecular switches" regulating the formation of ROS by phagocytic leukocytes under physiologic and pathologic conditions.

Liposomes comprising anionic but not neutral phospholipids cause dissociation of Rac(1 or 2) x RhoGDI complexes and support amphiphile-independent NADPH oxidase activation by such complexes

Activation of the phagocyte NADPH oxidase involves the assembly of a membrane-localized cytochrome b559 with the cytosolic components p47(phox), p67(phox), p40(phox), and the GTPase Rac (1 or 2). In resting phagocytes, Rac is found in the cytosol as a prenylated protein in the GDP-bound form, associated with the Rho GDP dissociation inhibitor (RhoGDI). In the process of NADPH oxidase activation, Rac is dissociated from RhoGDI and translocates to the membrane, in concert with the other cytosolic components. The mechanism responsible for dissociation of Rac from RhoGDI is poorly understood. We generated Rac(1 or 2) x RhoGDI complexes in vitro from recombinant Rac(1 or 2), prenylated enzymatically, and recombinant RhoGDI, and purified these by anion exchange chromatography. Exposing Rac(1 or 2)(GDP) x RhoGDI complexes to liposomes containing four different anionic phospholipids caused the dissociation of Rac(1 or 2)(GDP) from RhoGDI and its binding to the anionic liposomes. Rac2(GDP) x RhoGDI complexes were more resistant to dissociation, reflecting the lesser positive charge of Rac2. Liposomes consisting of neutral phospholipid did not cause dissociation of Rac(1 or 2) x RhoGDI complexes. Rac1 exchanged to the hydrolysis-resistant GTP analogue, GMPPNP, associated with RhoGDI with lower affinity than Rac1(GDP) and Rac1(GMPPNP) x RhoGDI complexes were more readily dissociated by anionic liposomes. Rac1(GMPPNP) x RhoGDI complexes elicited NADPH oxidase activation in native phagocyte membrane liposomes in the presence of p67(phox), without the need for an anionic amphiphile, as activator. Both Rac1(GDP) x RhoGDI and Rac1(GMPPNP) x RhoGDI complexes elicited amphiphile-independent, p67(phox)-dependent NADPH oxidase activation in phagocyte membrane liposomes enriched in anionic phospholipids but not in membrane liposomes enriched in neutral phospholipids.

Sequencing, expression, and functional analyses support the candidacy of Ncf2 in susceptibility to Salmonella typhimurium infection in wild-derived mice

A recessive Salmonella Typhimurium susceptibility locus (immunity to Typhimurium (Ity3) was reported previously on distal mouse chromosome 1 using a cross between C57BL/6J and wild-derived MOLF/Ei mice. This quantitative trait locus is located in a genomic region spanning 84 Mb, rich in candidate genes for which a role in host resistance to Salmonella infection is either known or can be envisioned. In this study, we report the evaluation of neutrophil cytosolic factor 2 (Ncf2) as a candidate Salmonella susceptibility gene for Ity3. Ncf2 encodes p67phox, a subunit of the multiprotein enzyme complex NADPH oxidase, known to be responsible for the generation of superoxides. Congenic mice carrying the Ity3 region from MOLF/Ei, B6.MOLF-Ity/Ity3 were more susceptible to infection compared with control mice heterozygous at Ity3, B6.MOLF-Ity/Ity3(MOLF/B6), confirming the existence of a recessive Salmonella susceptibility locus on distal chromosome 1. Spleen Ncf2 expression levels were lower in infected congenic mice homozygous for the MOLF/Ei allele at Ity3 compared with mice heterozygous at Ity3. C57BL/6J and MOLF/Ei Ncf2 sequence comparisons revealed one nonconservative amino acid change (R394Q) in the functional and highly conserved Phox and Bem1 domain of the protein. Functional analysis revealed that the MOLF/Ei allele had reduced PMA- and Salmonella-induced superoxide induction as compared with their wild-type counterparts ex vivo. The R394Q substitution seems to occur on an amino acid involved in electrostatic interactions with p40phox, crucial in its activation. Moreover, a human mutation in the corresponding R395W, resulting in chronic granulatomous disease, is known to lead to reduced superoxide levels. These results support the candidacy of Ncf2 as the gene underlying Ity3.

Assembly of the phagocyte NADPH oxidase complex: chimeric constructs derived from the cytosolic components as tools for exploring structure-function relationships

Phagocytes generate superoxide (O2*-) by an enzyme complex known as reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Its catalytic component, responsible for the NADPH-driven reduction of oxygen to O2*-, is flavocytochrome b559, located in the membrane and consisting of gp91phox and p22phox subunits. NADPH oxidase activation is initiated by the translocation to the membrane of the cytosolic components p47phox, p67phox, and the GTPase Rac. Cytochrome b559 is converted to an active form by the interaction of gp91phox with p67phox, leading to a conformational change in gp91phox and the induction of electron flow. We designed a new family of NADPH oxidase activators, represented by chimeras comprising various segments of p67phox and Rac1. The prototype chimera p67phox (1-212)-Rac1 (1-192) is a potent activator in a cell-free system, also containing membrane p47phox and an anionic amphiphile. Chimeras behave like bona fide GTPases and can be prenylated, and prenylated (p67phox -Rac1) chimeras activate the oxidase in the absence of p47phox and amphiphile. Experiments involving truncations, mutagenesis, and supplementation with Rac1 demonstrated that the presence of intrachimeric bonds between the p67phox and Rac1 moieties is an absolute requirement for the ability to activate the oxidase. The presence or absence of intrachimeric bonds has a major impact on the conformation of the chimeras, as demonstrated by fluorescence resonance energy transfer, small angle X-ray scattering, and gel filtration. Based on this, a "propagated wave" model of NADPH oxidase activation is proposed in which a conformational change initiated in Rac is propagated to p67phox and from p67phox to gp91phox.

Direct involvement of the small GTPase Rac in activation of the superoxide-producing NADPH oxidase Nox1

Activation of the non-phagocytic superoxide-producing NADPH oxidase Nox1, complexed with p22(phox) at the membrane, requires its regulatory soluble proteins Noxo1 and Noxa1. However, the role of the small GTPase Rac remained to be clarified. Here we show that Rac directly participates in Nox1 activation via interacting with Noxa1. Electropermeabilized HeLa cells, ectopically expressing Nox1, Noxo1, and Noxa1, produce superoxide in a GTP-dependent manner, which is abrogated by expression of a mutant Noxa1(R103E), defective in Rac binding. Superoxide production in Nox1-expressing HeLa and Caco-2 cells is decreased by depletion or sequestration of Rac; on the other hand, it is enhanced by expression of the constitutively active Rac1(Q61L), but not by that of a mutant Rac1 with the A27K substitution, deficient in binding to Noxa1. We also demonstrate that Nox1 activation requires membrane recruitment of Noxa1, which is normally mediated via Noxa1 binding to Noxo1, a protein tethered to the Nox1 partner p22(phox): the Noxa1-Noxo1 and Noxo1-p22(phox) interactions are both essential for Nox1 activity. Rac likely facilitates the membrane localization of Noxa1: although Noxa1(W436R), defective in Noxo1 binding, neither associates with the membrane nor activates Nox1, the effects of the W436R substitution are restored by expression of Rac1(Q61L). The Rac-Noxa1 interaction also serves at a step different from the Noxa1 localization, because the binding-defective Noxa1(R103E), albeit targeted to the membrane, does not support superoxide production by Nox1. Furthermore, a mutant Noxa1 carrying the substitution of Ala for Val-205 in the activation domain, which is expected to undergo a conformational change upon Rac binding, fully localizes to the membrane but fails to activate Nox1.