Originally published online as doi:10.1189/jlb.0404216 on July 7, 2004

Published online before print July 7, 2004

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(Journal of Leukocyte Biology. 2004;76:760-781.)
© 2004 by Society for Leukocyte Biology

Structure and regulation of the neutrophil respiratory burst oxidase: comparison with nonphagocyte oxidases

Mark T. Quinn¹ and Katherine A. Gauss

Department of Veterinary Molecular Biology, Montana State University, Bozeman

¹Correspondence: Department of Veterinary Molecular Biology, Montana State University, Bozeman, MT 59717-3610. E-mail: mquinn{at}montana.edu

TOP ABSTRACT INTRODUCTION PHAGOCYTE NADPH OXIDASE... NONPHAGOCYTE NADPH OXIDASE... MODEL OF NADPH OXIDASE... TRANSCRIPTIONAL REGULATION FUNCTIONAL ASPECTS SUMMARY REFERENCES

 
Neutrophils play an essential role in the body’s innate defense against pathogens and are one of the primary mediators of the inflammatory response. To defend the host, neutrophils use a wide range of microbicidal products, such as oxidants, microbicidal peptides, and lytic enzymes. The generation of microbicidal oxidants by neutrophils results from the activation of a multiprotein enzyme complex known as the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, which is responsible for transferring electrons from NADPH to O₂, resulting in the formation of superoxide anion. During oxidase activation, cytosolic oxidase proteins translocate to the phagosome or plasma membrane, where they assemble around a central membrane-bound component known as flavocytochrome b. This process is highly regulated, involving phosphorylation, translocation, and multiple conformational changes. Originally, it was thought that the NADPH oxidase was restricted to phagocytes and used solely in host defense. However, recent studies indicate that similar NADPH oxidase systems are present in a wide variety of nonphagocytic cells. Although the nature of these nonphagocyte NADPH oxidases is still being defined, it is clear that they are functionally distinct from the phagocyte oxidases. It should be noted, however, that structural features of many nonphagocyte oxidase proteins do seem to be similar to those of their phagocyte counterparts. In this review, key structural and functional features of the neutrophil NADPH oxidase and its protein components are described, including a consideration of transcriptional and post-translational regulatory features. Furthermore, relevant details about structural and functional features of various nonphagocyte oxidase proteins will be included for comparison.

Key Words: phagocyte NADPH oxidase • Nox • nonphagocyte NADPH oxidase • superoxide anion • oxidants • free radicals • chronic granulomatous disease

TOP ABSTRACT INTRODUCTION PHAGOCYTE NADPH OXIDASE... NONPHAGOCYTE NADPH OXIDASE... MODEL OF NADPH OXIDASE... TRANSCRIPTIONAL REGULATION FUNCTIONAL ASPECTS SUMMARY REFERENCES

 
The innate immune response represents a highly conserved strategy used by the host in defense against a wide array of bacterial, fungal, and viral pathogens [¹ ]. Activation of the innate immune system results in an inflammatory response, which is essential for rapidly controlling infections before they can spread. In addition, it is now clear that cells of the innate immune system contribute to the initiation and subsequent focus of the ensuing adaptive immune response [¹ ]. Phagocytes are especially critical to the acute inflammatory response as a result of their capacity to efficiently engulf and destroy a variety of pathogens. These cells are also known as professional phagocytes and are comprised of neutrophils, monocytes, macrophages, and eosinophils. Among this group, neutrophils are the most numerous, are usually the first cell to arrive at sites of inflammation, and are possibly the most important cellular component of the innate response during acute infection [² ].

Neutrophils (a.k.a., polymorphonuclear leukocytes) are normally found circulating in the bloodstream (circulating half-life of 7 h) and migrating through tissues (2–3 days) and devote their short lifetime to surveillance [³ ]. However, during an infection, the neutrophil lifespan is increased, and large numbers of neutrophils are rapidly recruited to the site(s) of infection where they function to destroy invading pathogens. In this capacity, neutrophils serve as one of the body’s first lines of defense against infection. These cells use an extraordinary array of oxygen-dependent and oxygen-independent microbicidal weapons to destroy and remove infectious agents [⁴ ]. Oxygen-dependent mechanisms involve the production of reactive oxygen species (ROS), which can be microbicidal [⁵ ], and oxygen-independent mechanisms include most other neutrophil functions, such as chemotaxis, phagocytosis, degranulation, and release of lytic enzymes and bactericidal peptides (reviewed in ref. [⁴ ]).

The generation of microbicidal oxidants by neutrophils results from the activation of a multiprotein enzyme complex known as the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, which catalyzes the formation of superoxide anion (O₂^·–; reviewed in ref. [⁶ ]). The classical NADPH oxidase was first described and characterized in neutrophils and other phagocytes, and it was originally thought that this system was restricted only to phagocytes and used solely in host defense. However, recent studies in the last 10–15 years indicate that similar NADPH oxidase systems are present in a wide variety of nonphagocytic cells of leukocyte and nonleukocyte origin (reviewed in refs. [⁷ , ⁸ ]). These oxidase systems are functionally distinct from the phagocyte oxidases, as they produce much lower levels of ROS and appear to play distinct roles in inter- and intracellular signaling events. Conversely, structural features of many nonphagocyte oxidase proteins do seem to be similar or even identical to those of their phagocyte counterparts. Therefore, although the primary focus of this review will center on the neutrophil/phagocyte NADPH oxidase, relevant details about structural and functional features of various nonphagocyte oxidase proteins will be included for comparison.

TOP ABSTRACT INTRODUCTION PHAGOCYTE NADPH OXIDASE... NONPHAGOCYTE NADPH OXIDASE... MODEL OF NADPH OXIDASE... TRANSCRIPTIONAL REGULATION FUNCTIONAL ASPECTS SUMMARY REFERENCES

 
The importance of the phagocyte NADPH oxidase to host immunity is clearly demonstrated by a rare genetic disorder known as chronic granulomatous disease (CGD; reviewed in ref. [⁹ ]), which is characterized by various genetic defects in essential NADPH oxidase components and results in an inactive oxidase. Patients with CGD experience severe, recurrent bacterial and fungal infections and often develop granulomas formed by the fusion of monocytes and macrophages, which have phagocytosed bacteria but are unable to destroy them as a result of a defective NADPH oxidase [⁹ ].

It is now generally accepted that the core NADPH oxidase enzyme is composed of four oxidase-specific proteins (p22^phox, p47^phox, p67^phox, and gp91^phox) and a GTPase (Rac1/2). One other oxidase-specific protein (p40^phox) and a second GTPase (Rap1A) have also been shown to play roles in regulating oxidase activity; however, their specific functions are still not well understood. Originally, the nomenclature for the various components differed throughout the literature; however, the generally accepted nomenclature for the phagocyte oxidase-specific components now includes the suffix phox, which refers to phagocyte oxidase [¹⁰ ]. The one exception is gp91^phox, which has also been named NADPH oxidase 2 (Nox2) [¹¹ ]. Overall, the phox proteins are highly conserved throughout the various species studied to date, confirming the absolute requirement for this system in mammalian host defense (e.g., see ref. [¹² ]; Table 1 ). Details of each of the individual component are summarized below.

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Table 1. Comparison of Human and Other Known phox Protein Sequences

Flavocytochrome b
The first NADPH oxidase component to be identified was a nonmitochondrial cytochrome b, which was shown by Segal and Jones [¹³ ] to be present in phagocytic vacuoles of granulocytes. This molecule was designated as cytochrome b₅₅₈/b₅₅₉ as a result of its -band absorption maximum of 558–559 nm or cytochrome b_–245 because of its unusually low midpoint reduction potential of –245 mV (reviewed in ref. [¹⁴ ]). Initially, it was thought that phagocyte cytochrome b was comprised of a single protein; however, subsequent analyses demonstrated it was actually a heterodimer of a 91-kDa glycoprotein (a.k.a., ß-chain) and a 22-kDa nonglycosylated protein (a.k.a., -chain) [¹⁵ ]. These proteins are now known universally as gp91^phox and p22^phox, respectively. Hydrodynamic analyses of purified cytochrome b were consistent with the complex being an -, ß-type heterodimer or an , ß, ß-type hetero-oligomer [¹⁶ ], and early structural models of cytochrome b were developed around an -, ß-, ß-type complex [¹⁷ ]. However, this paradigm has been subsequently revised, as later studies indicated cytochrome b was actually a 1:1 -, ß-type heterodimer of p22^phox and gp91^phox [¹⁸ ].

Hydropathy analyses of p22^phox and gp91^phox indicated the presence of two to three and four to six transmembrane regions in these proteins, respectively [¹⁹ , ²⁰ ]. Based on the experimentally determined heme content of purified cytochrome b, Parkos et al. [²⁰ ] concluded that more than one heme was present per cytochrome b molecule. Subsequent studies of cytochrome b confirmed this conclusion, indicating a bihistidinyl, multiheme cytochrome with closely spaced hemes [^{21 22 23} ]. This concept was further refined when re-evaluation of the averaged heme potential of –245 mV, using higher resolution methods, showed that cytochrome b contained two nonidentical hemes with midpoint redox potentials of –225 and –265 mV, respectively [²⁴ ]. Based on this and other evidence, it is now generally accepted that cytochrome b contains two hemes.

The exact location of the hemes associated with cytochrome b has been difficult to determine, and two basic models of heme placement are currently under consideration (Fig. 1 ). According to the shared-heme model, one heme is coordinated within gp91^phox, and the second heme is coordinately shared between gp91^phox and the invariant histidine of p22^phox [²⁵ ] (Fig. 1A) . In an alternative model, which is based on sequence similarities between gp91^phox and yeast iron reductase Fre1, both hemes are stacked within gp91^phox between transmembrane helices III and V and are coordinated by histidines 101, 115, 209, and 222 [²⁶ ] (Fig. 1B) . Presently, there are direct and indirect experimental data to support both models.

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Figure 1. Models of flavocytochrome b showing proposed transmembrane helices and heme placement. (A) Shared heme model with one heme coordinated by gp91phox helices III and V (solid diamond) and one shared between gp91phox helix V and p22phox helix III (hatched diamond). (B) Nonshared heme model with both hemes coordinated by gp91phox helices III and V. Proposed sites of glycosylation (Y) and location of binding domains for other redox components [flavin adenine dinucleotide (FAD) and NADPH] are indicated.

The shared-heme model (Fig. 1A) is indirectly supported by studies showing that incorporation of heme by gp65 (the unglycosylated precursor of gp91^phox) precedes and is required for heterodimer formation, suggesting the possibility that the heme might serve as a dimerizing agent, linking gp65 and p22^phox [²⁷ ]. In addition, peptides corresponding to regions adjacent to the invariant histidine of p22^phox (His94) inhibited O₂^·– generation in a cell-free system [²⁸ ]. Furthermore, missense mutations in His94 (the only invariant histidine in p22^phox) result in an autosomal form of CGD and absence of cytochrome b, providing strong support for a functional role of this histidine in neutrophil cytochrome b [²⁹ ]. Finally, recent studies of Foubert et al. [³⁰ ] showed that a spectrally stable, proteolytic product of neutrophil cytochrome b contained fragments of gp91^phox and p22^phox, which is also consistent with the presence of a shared heme.

The nonshared heme model (Fig. 1B) is also supported by substantial experimental evidence. For example, transgenic cell lines expressing gp91^phox alone exhibit a heme spectrum very similar to neutrophil cytochrome b, whereas cells expressing p22^phox alone lack a heme spectrum, indicating that at least in this system, gp91^phox is able to coordinate both hemes [³¹ ]. In addition, replacement of gp91^phox histidines 101, 115, 209, and 222 with leucine or arginine results in lost or significantly decreased heme spectrum [³² ], and missense mutations in any of these residues have been associated with X-linked CGD [³³ ]. Furthermore, mutagenesis of the p22^phox His94 suggested, in contrast to the CGD mutation described above, that a histidine at this position was not required for flavocytochrome b function and that heme was not shared between subunits [³⁴ ]. Thus, the role of the two cytochrome b subunits in heme coordination remains a matter of debate, and further studies will be necessary to resolve this issue.

Initially, it was thought that the NADPH oxidase was composed only of cytochrome b and an unknown flavoprotein [³⁵ ]. The search for the putative flavoprotein led to the identification of several candidate proteins (reviewed in ref. [⁶ ]); however, many discrepancies remained, and ongoing searches led to the consideration that the FAD-binding moiety might actually be cytochrome b itself. This issue was independently resolved by three groups, which concurrently provided data demonstrating that cytochrome b was indeed a flavocytochrome and led to the renaming of cytochrome b₅₅₈ to the currently accepted nomenclature of flavocytochrome b₅₅₈ (a.k.a., flavocytochrome b) [^{36 37 38} ]. Although sequence homology with the ferridoxin-NADP⁺ reductase (FNR) family of reductases suggested that the putative FAD-binding region involved gp91^phox residues 214–246, 335–345, and 350–360 [^{36 37 38} ], more recent information indicates that one of these regions (residues 214–246) falls within an extracellular loop and cannot participate in FAD binding [³⁹ ].

As NADPH is the electron source for the catalysis of O₂ to O₂^·–, a NADPH-binding component must be present in or near the enzyme complex. As with the FAD-binding protein, the search for the NADPH-binding moiety also resulted in the identification of several candidates, including cytosolic proteins (reviewed in ref. [⁶ ]). Based on amino acid sequence comparison between gp91^phox and FNR, it was proposed that flavocytochrome b might also contain a NADPH-binding domain [^{36 37 38} ], and this idea was subsequently confirmed by photoaffinity-labeling studies [⁴⁰ ]. Furthermore, Koshkin and Pick [⁴¹ ] showed that relipidated flavocytochrome b alone could generate O₂^·–, providing direct evidence that flavocytochrome b was able to functionally bind FAD and NADPH. It should be noted, however, that several recent studies suggest flavocytochrome b may not be the sole NADPH-binding component and that p67^phox may also contribute in some way to this process (see below).

Based on the information summarized above, it can be concluded that flavocytochrome b contains all redox components used by the NADPH oxidase for transmembrane electron transport (reviewed in ref. [¹⁴ ]). Conversely, flavocytochrome b cannot and does not function independently in the cell, thereby insuring against inappropriate activation. Instead, a number of oxidase protein cofactors (described below) are absolutely required for enzymatic activity, presumably to initiate or facilitate the electron transfer process. Regardless of whether the hemes are shared between subunits or not, the currently accepted electron transport pathway involves sequential and step-wise transfer of two electrons from NADPH via FAD and two hemes to ultimately reduce O₂ (Fig. 2 ). It should be noted, however, that most evidence suggests the final step in this process occurs via a peripheral mechanism, whereby electrons are transferred from the outer heme to O₂ in a pocket near the heme edge, without actual binding of O₂ to the heme [¹⁴ , ⁴² ] (Fig. 2) . This type of scheme would be analogous to that of other heme proteins involved in long-range electron transfer [⁴³ ].

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Figure 2. Schematic representation of electron flow via flavocytochrome b redox components. Both hemes (H1, H2) are coordinated by flavocytochrome b transmembrane helices, resulting in a pathway for electron transfer across the membrane. Mid-point potentials at pH 7.0 are indicated for each step in the pathway. FADH2, Reduced FAD.

p47^phox
The availability of neutrophils from patients with CGD has been instrumental in research efforts focused on the identification of NADPH oxidase component proteins. Early on, Segal et al. [⁴⁴ ] observed that neutrophils from patients with autosomal, recessive CGD failed to phosphorylate a 44-kDa protein and suggested this protein could be oxidase-related. Through genetic analyses, complementation experiments, and cell-free assays, two groups concurrently identified this protein, which was known previously as neutrophil cytosolic factor 1 (NCF1) but is now called p47^phox [^{45 46} ]. p47^phox is a highly basic protein, containing a number of potential phosphorylation sites (residues 314–347), tandem Src homology 3 (SH3) domains (residues 163–211 and 227–281), a C-terminal proline-rich domain (residues 360–371), and an N-terminal phox homology (PX) domain (residues 4–125), which may play a role in phosphoinositide binding [^{47 48 49} ] (Fig. 3 ).

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Figure 3. Structural comparison of phagocyte and nonphagocyte oxidase cytosolic proteins. Major functional domains are indicated on these scale models (see text for details). PC, Phox and Cdc; NOXO1, Nox organizer 1; NOXA1, Nox activator 1; TPR, tetratricopeptide repeat; PB1, Phox and Bem.

In resting neutrophil cytosol, p47^phox exists in a free form, as well as in a multimeric complex consisting of equimolar (1:1:1) amounts of p47^phox, p67^phox, and p40^phox [^{50 51 52} ]. Following neutrophil activation, the entire complex apparently translocates en masse to the membrane [⁵⁰ , ⁵¹ ]. In addition, free p47^phox can also translocate to the membrane by itself [⁵³ ]. Although the exact role of p47^phox in NADPH oxidase assembly is still debated, it appears to function in a regulatory role during oxidase activation and deactivation. One possibility suggested by Cross and Curnutte [⁵⁴ ] is that p47^phox facilitates electron transfer from FAD to the heme center of flavocytochrome b, although this interpretation has been questioned [⁵⁵ ]. In any case, p47^phox seems to be the first cytosolic component to interact with flavocytochrome b during oxidase assembly [⁵⁶ , ⁵⁷ ], and its association with flavocytochrome b is a prerequisite for translocation of p67^phox and/or p40^phox [^{56 57 58} ]. The kinetics of p47^phox (and p67^phox) translocation closely resembles the kinetics of oxidase activation, and continuous translocation/association of cytosolic components maintains respiratory burst activity, possibly to facilitate assembly of new enzyme complexes [⁵⁹ , ⁶⁰ ]. In support of this idea, van Bruggen et al. [⁶¹ ] recently reported that in addition to p67^phox, Rac2 was continuously translocated during phagocytosis in living cells and concluded that flavocytochrome b-bound p67^phox and Rac2 were continuously exchanged with cytosolic p67^phox and Rac2.

Phosphorylation of p47^phox is one of the key intracellular events associated with NADPH oxidase activation, and it has been proposed that phosphorylation causes a conformational change in p47^phox and/or neutralizes the cationic domain of the protein (see Fig. 3 ) in vivo so that it can interact with the membrane and other oxidase proteins (reviewed in ref. [⁶² ]). Indeed, Ago et al. [⁶³ ] found that phosphorylation of only three serines in p47^phox could mimic this conformational change. In the cell-free assay system, amphiphilic agents such as sodium dodecyl sulfate (SDS) or arachidonic acid (AA) serve a similar role, enabling p47^phox to undergo analogous, conformational changes in vitro that are likely to occur during phosphorylation in vivo [⁶⁴ ]. Recent studies by Peng et al. [⁶⁵ ], showing that p47^phox mutants with unmasked SH3 domains are able to fully reconstitute oxidase activity in a cell-free assay lacking AA, and studies by Ago et al. [⁶⁶ ], showing that p47^phox phosphorylation caused unmasking of phosphoinositide-binding domains, further support a requirement for activation-induced, conformational changes in p47^phox. A number of kinases have been proposed to participate in p47^phox phosphorylation, including protein kinase C (PKC; e.g., ref. [⁶⁷ ]), p38 mitogen-activated protein kinases [MAPK; and extracellular signal-regulated kinase (ERK)1/2; refs. ^{67 68 69} ], p21-activated kinase (PAK) [⁷⁰ ], Akt [⁷¹ ], casein kinase 2 [⁷² ], and a phosphatidic acid-activated kinase [⁷³ ]. PKC plays a dominant role in this process, and it has been shown that PKC-, -ßII, -, and - isoforms can individually phosphorylate p47^phox to varying degrees, resulting in oxidase activation [^{74 75 76} ]. The physiological role of various other protein kinases in p47^phox activation is not as well understood. Consensus MAPK substrate sequences have been identified in p47^phox, although phosphorylation at these sites does not appear to be critical for O₂^·– generation in phorbol myristate acetate (PMA)-stimulated cells [⁶⁷ , ⁶⁸ ]. In contrast, recent studies using physiological activators suggest that ERK1/2 and PKC actually cooperate in p47^phox phosphorylation [⁶⁹ ]. PAKs have also been implicated in p47^phox phosphorylation, and Knaus et al. [⁷⁰ ] suggested these kinases may participate in linking chemoattractant receptor stimulation with oxidase activation. Additionally, Akt is rapidly activated when neutrophils are treated with oxidase-activating agents, and p47^phox phosphorylation is enhanced in cells expressing membrane-targeted phosphoinositide-3 kinase (PI-3K), which constitutively activates Akt [⁷¹ ]. Recent studies showing that Akt can phosphorylate p47^phox further support a role for Akt in mediating PI-3K-dependent phosphorylation of p47^phox [⁷⁷ , ⁷⁸ ]. Thus, depending on the nature of the activation conditions, a variety of kinases seems to cooperate in mediating p47^phox phosphorylation events.

Recently, a novel phosphatidylinositol (PI)-targeting module was identified in p40^phox and p47^phox, and this conserved, proline-rich motif was designated as the PX domain because of its presence in these phox proteins [⁷⁹ ] (Fig. 3) . However, it is now known that PX motifs serve as phosphoinositide-binding modules in many different proteins [⁸⁰ ]. The p47^phox PX domain is located within an 120 amino acid module encompassing residues 4–125 [⁴⁹ ] and has been shown to bind phosphoinositides with apparent preference for PI 3,4-bisphosphate [PI(3,4)P2; refs. ⁸¹ , ⁸² ], although the relative selectivity for various phosphoinositides is not completely resolved [⁸³ ]. The ability to bind phosphoinositides suggests that the PX domain plays a role in targeting p47^phox to membranes, and PX domain-mediated targeting of this protein has been demonstrated [^{82 83 84} ]. In addition, analysis of the crystallized p47^phox PX domain by Karathanassis et al. [⁸⁵ ] showed that it actually contained two binding pockets, one for PI(3,4)P2 and the other for anionic phospholipids, such as phosphatidic acid or phosphatidylserine. They also confirmed previous studies of Hiroaki et al. [⁴⁹ ], who reported that the PX domain was masked by an intramolecular interaction with the C-terminal SH3 domain of p47^phox. These findings suggest an additional role for PX modules in protein–protein binding, and this idea is supported by studies showing that binding p47^phox to moesin is mediated by the PX domain, although in a phosphoinositide-dependent process [⁸⁶ ]. Furthermore, the identification of a putative phosphatidic acid-binding region is consistent with the ability of phosphatidic acid to activate the oxidase [⁸⁷ ].

p67^phox
Like p47^phox, p67^phox was initially identified as a missing cystolic factor (NCF2) in patients with autosomal, recessive CGD [⁴⁵ , ⁴⁶ , ⁸⁸ ]. p67^phox contains two SH3 domains (residues 245–295 and 458–517, respectively), a proline-rich domain (residues 219–231), and four N-terminal TPR motifs (within residues 6–154) [⁸⁸ , ⁸⁹ ] (Fig. 3) . Complementation studies with neutrophil fractions obtained from p67^phox-deficient CGD neutrophils suggest that p67^phox is the limiting oxidase cofactor [⁹⁰ ], and there appears to be two to three times less p67^phox in neutrophil cytosol compared with p47^phox [⁵³ , ⁹¹ ], indicating most or all of the p67^phox must be complexed with p47^phox. In support of this conclusion, addition of exogenous p67^phox has been reported to enhance binding of p47^phox to the membrane in vitro, possibly by associating with free p47^phox [⁵⁸ ]. This would be expected if there was always more p47^phox available for active complex formation at the membrane.

Most reports agree that in vivo translocation and membrane association of p67^phox are dependent on cotranslocation of p47^phox to the plasma membrane and prior interaction of p47^phox with flavocytochrome b [^{56 57 58} ]. This conclusion is supported by recent work of Paclet et al. [⁹² ], who used atomic force microscopy to verify that p47^phox preceded p67^phox and enhanced the affinity of p67^phox for binding to flavocytochrome b during oxidase activation. It should be noted, however, that in vitro reconstitution of NADPH oxidase activity in the absence of p47^phox is possible when relipidated flavocytochrome b and high concentrations of p67^phox and Rac are combined [⁹³ , ⁹⁴ ], supporting the idea that p67^phox and Rac play more direct roles in electron transport and suggesting a possible direct interaction between p67^phox and flavocytochrome b. Subsequent studies showing that chimeric proteins containing truncated p67^phox fused to Rac were also able to support p47^phox-independent oxidase activity in vitro [⁹⁵ , ⁹⁶ ] provide further support for this interaction. Ultimately, direct binding between p67^phox and flavocytochrome b was demonstrated by Dang et al. [⁹⁷ ], who also observed that binding was enhanced by the presence of Rac, which is consistent with the chimeric protein studies.

The possibility that p67^phox is the NADPH-binding protein of the oxidase or at least participates in NADPH binding is still a matter of debate. Based on a number of studies, Smith et al. [⁹⁸ ] concluded that p67^phox was an NADPH-binding protein and that the oxidase actually contained two NADPH-binding sites, one low-affinity site in gp91^phox and one of higher affinity in p67^phox. Furthermore, Dang et al. [⁹⁹ , ¹⁰⁰ ] recently showed that p67^phox was able to bind NADPH directly via the TPR domains and could catalyze pyridine nucleotide dehydrogenation. Thus, it is possible that p67^phox and gp91^phox may be involved in NADPH binding, perhaps to create a shared or cooperative NADPH-binding site involving both proteins. This possibility might explain the apparent contradictions between various reports.

The interaction between p67^phox and Rac is essential for NADPH oxidase activation and appears to be mediated primarily by binding Rac to the N-terminal region of p67^phox (residues 1–200) [¹⁰¹ ], although Faris et al. [¹⁰² ] found that the C terminus of p67^phox may also help to stabilize Rac binding. Detailed analysis of the p67^phox N terminus resulted in the identification of an array of four tandem TPR motifs (TPR1–4), which mediate Rac binding [⁸⁹ ] (Fig. 3) . Furthermore, crystallization of the TPR domain bound to Rac demonstrated that this interaction represents a novel mode of TPR domain-mediated binding [¹⁰³ , ¹⁰⁴ ]. Analysis of the p67^phox N-terminus also resulted in the identification of an activation domain encompassing residues 199–210, and Han et al. [¹⁰⁵ ] showed that this domain was required for oxidase activation (Fig. 3) . Furthermore, this domain appears to play a role in regulating electron flow from NADPH to flavocytochrome b-associated FAD [¹⁰⁶ ].

Although it is now evident that p67^phox can be phosphorylated, the physiological role of this event in NADPH oxidase assembly/activation is still unclear. El Benna et al. [¹⁰⁷ ] used immunoprecipitation to show that p67^phox was phosphorylated in neutrophils stimulated with N-formyl-methionyl-leucyl-phenyalanine (fMLF) or PMA and that phosphorylation occurred by PKC-dependent and independent pathways. Indeed, the actual phosphorylation site was mapped to Thr233, which is in a consensus MAPK substrate sequence [¹⁰⁸ ], and it was also found that p67^phox phosphorylation occurred in the cytosol and was independent of any interaction with p47^phox [¹⁰⁹ ]. Recently, Dang et al. [¹¹⁰ ] showed that p67^phox was also phosphorylated by ERK2 and p38 MAPK in vitro and in intact neutrophils. Phosphorylation occurred at several sites, with the primary ERK2 target localized to the N-terminal fragment and MAPK-mediated phosphorylation primarily in the C terminus. It is interesting that the C-terminal phosphorylation site(s) appears to be masked in the intact protein and may become accessible only after a conformational change [¹¹⁰ ], suggesting phosphorylation could be regulating an intramolecular interaction between the p67^phox C terminus and the N-terminal TPR motifs.

p40^phox
Compared with the other oxidase proteins, much less is known about the function of p40^phox, which was identified in a fractionated, resting neutrophil cytosol via its binding to immunoprecipitated p67^phox and contains a single SH3 domain (residues 175–226) [¹¹¹ ], an N-terminal PX domain (residues 24–143) [⁷⁹ ], and a C-terminal PC motif (residues 283–310) [¹¹² ] (Fig. 3) . p40^phox resides within the cytosolic complex [¹¹¹ ] and seems to bind preferentially to p67^phox [¹¹³ ]. As with the other cytosolic phox proteins, p40^phox is phosphorylated during NADPH oxidase activation, and mapping studies indicate phosphorylation occurs at Thr154 and Ser315 [¹¹⁴ ].

p40^phox has been proposed to play a role in stabilization of the cytosolic phox protein complex and can cotranslocate to the membrane during neutrophil activation [¹¹¹ ]. However, the actual role of p40^phox in oxidase function is still not well understood. Tsunawaki et al. [¹¹⁵ ] found that dissociation of p40^phox from the cytosolic complex inhibited the cell-free assay, suggesting that p40^phox was a positive regulator of the oxidase. In contrast, Sathyamoorthy et al. [¹¹⁶ ] reported that p40^phox inhibited oxidase activity in vitro and in transfected K562 cells, albeit at relatively high p40^phox concentrations, and proposed that it functioned in down-regulating the oxidase by competing with SH3 domain interactions between other oxidase components. More recently, Cross [¹¹⁷ ] reported that p40^phox promotes oxidase activation by increasing the affinity of p40^phox for flavocytochrome b by approximately threefold. In support of the latter observation, Kuribayashi et al. [¹¹⁸ ] recently used a number of approaches to show that p40^phox is probably a positive regulator of the oxidase and enhances translocation of p47^phox and p67^phox in stimulated cells.

In contrast to the p47^phox PX domain, the p40^phox PX domain preferentially binds PI 3-phosphate [PI(3)P; refs. ⁸¹ , ⁸² ], suggesting differential membrane targeting of these proteins based on phosphoinositide specificity. Crystallization of the p40^phox PX domain bound to PI(3)P showed that it contains a phosphoinositide-binding pocket similar to that of p47^phox; however, unique binding constraints present in the p40^phox PX domain influence its phosphoinositide-binding specificity [¹¹⁹ ]. The physiological role of the p40^phox PX domain is not well understood; however, it is thought that participation in intracellular targeting is likely [¹²⁰ ]. PI(3)P accumulates in phagosomal membranes [¹²¹ ], and Ellson et al. [¹²² ] suggested that PI(3)P could facilitate oxidase assembly by binding to the p40^phox PX domain, thereby recruiting p67^phox (and possibly other components) to the assembling oxidase complex via association with p40^phox.

Analysis of the interaction between p40^phox and p67^phox resulted in the identification of a novel, protein–protein-binding motif (residues 282–309), and because of its presence in p40^phox and Cdc24p (a guanine nucleotide exchange factor in yeast), this motif was designated as the PC motif [¹¹² ]. The PC motif-mediated interaction is not dissociated by anionic amphiphiles in vitro, suggesting that this interaction might be maintained throughout the activation process [¹¹² ]; however, further studies are necessary to verify this conclusion in intact cells. The PC motif target in p67^phox also appears to be a novel modular domain (residues 345–427), which is located between the two SH3 domains and is designated the PB1 domain because of its presence in p67^phox and Bem1p, a yeast scaffold protein involved in cell polarity [¹¹² ] (Fig. 3) . PB1 domains exist in a variety of proteins and appear to provide a scaffold for PC motif binding, facilitating protein–protein interactions in a number of biological processes [¹²³ ].

Rac
Early on, it was suggested that a cytosolic guanosine triphosphate (GTP)-binding factor participated in the NADPH oxidase, and this factor was concurrently identified by two groups as the small GTP-binding protein Rac (Rac1 or Rac2, depending on the cell type and species; reviewed in ref. [¹²⁴ ]). Furthermore, Dorseuil et al. [¹²⁵ ] showed that Rac antisense oligonucleotides caused dose-dependent inhibition of O₂^·– production in transformed B lymphocytes, demonstrating a requirement for Rac in vivo. Subsequently, analysis of Rac-deficient mice showed that neutrophils from these mice had diminished O₂^·– production; however, the defect could be partially corrected by treating with tumor necrosis factor (TNF-) prior to stimulation with PMA [¹²⁶ ]. Further studies in Rac2-deficient mice showed that the requirement for Rac2 in oxidase activation may be stimulus-specific and that Rac2 was essential when cells were activated with physiologically relevant agents such as fMLF or immunoglobulin G (IgG)-opsonized particles [¹²⁷ ]. As Rac1 and Rac2 are capable of reconstituting oxidase activity in cell-free assays, it was suggested that the ability to achieve partial oxidase activity in neutrophils from Rac2-deficient mice may be a result of substitution by Rac1. However, Rac1 cannot substitute completely for Rac2, as shown recently by Glogauer et al. [¹²⁸ ] and Gu et al. [¹²⁹ ], who found that oxidase activity was normal in Rac1-deficient cells but was markedly diminished in Rac2-deficient cells, concluded that Rac2 was physiologically critical for oxidase activation, and that Rac1 was more important in other neutrophil functions. The physiological importance of Rac2 in the human neutrophil oxidase was substantiated recently when a patient with abnormal neutrophil function was shown to have an inhibitory (dominant-negative) mutation in Rac2, resulting in decreased oxidase activity and other neutrophil-functional defects [¹³⁰ , ¹³¹ ].

In resting cells, Rac is maintained in a soluble, cytosolic complex with a guanine nucleotide exchange protein known as guanosine diphosphate (GDP) dissociation inhibitor (GDI; reviewed in ref. [¹²⁴ ]). However, Rac dissociates from GDI during phagocyte activation, allowing GTP-bound (active) Rac to translocate to the membrane and/or interact with its downstream oxidase targets [⁶⁰ , ¹³² ]. Rac translocation corresponds temporally and quantitatively with p47^phox/p67^phox translocation and with oxidase activation [⁶⁰ , ¹³² ], although Rac seems to translocate independently of the other cytosolic oxidase components, suggesting it does not directly mediate phox protein translocation [⁹¹ , ¹³³ ]. Nevertheless, recent studies suggest Rac activation can induce oxidase assembly [¹³⁴ ], possibly through an indirect mechanism involving activation of PAK, thereby leading to p47^phox phosphorylation [⁷⁰ ]. Rac has been shown to interact with p67^phox [¹⁰¹ ], as well as with flavocytochrome b [⁹¹ ]. Thus, it is likely that Rac can modulate the function of more than one NADPH oxidase protein. Furthermore, studies showing that components of the oxidase are associated with the actin cytoskeleton [¹³⁵ ] suggest the possibility that Rac may play an additional role related to oxidase function via its ability to regulate cytoskeletal structure [¹³⁶ ].

Rap1A
The first GTPase to be identified in association with the neutrophil NADPH oxidase was Rap1A, which is a member of the Ras superfamily of GTP-binding proteins and was shown to be associated with flavocytochrome b [¹³⁷ ]. The association between flavocytochrome b and Rap1A was confirmed using reconstitution procedures in vitro, and Rap1A was found to form stoichiometric complexes with flavocytochrome b, indicating a direct binding of Rap1A to flavocytochrome b [¹³⁸ ]. Although Rap1A is not required for cell-free oxidase reconstitution [³⁶ ], several studies suggest it may play an important regulatory function in intact cells. For example, transfection of transformed B lymphocytes or differentiated HL-60 cells with dominant inhibitory (17N) mutants of Rap1A resulted in significant inhibition of NADPH oxidase activity, directly supporting a role for Rap1A in the regulation of the oxidase [¹³⁹ , ¹⁴⁰ ]. Nevertheless, further studies are necessary to define the exact role of Rap1A in this complex system.

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Recently, a number of studies demonstrated the presence of analogous NADPH oxidase systems in nonphagocytic cells, including fibroblasts, endothelial cells, and epithelial cells (reviewed in refs. [⁷ , ⁸ , ¹⁴¹ ]). In many tissues, the components of these oxidases appear to be structurally similar or even identical to phagocyte oxidase proteins; however, recent studies in the last few years have also identified and characterized unique, nonphagocyte oxidase proteins. As this area of investigation is unfolding rapidly, a complete understanding of the relative roles of these proteins and their interactions with the phox proteins in tissues containing both is forthcoming. In any case, details of what is currently known about the structure of these proteins are summarized below.

Nox
Based on the demonstrated presence of NADPH oxidase-like activity in a number of tissues [¹⁴² ] and the key importance of gp91^phox in electron transport [¹⁴ ], the search for nonphagocyte oxidase homologs initially focused on identifying gp91^phox-related species. The first of these homologs to be cloned was designated as mitogenic oxidase 1 because of its postulated role in regulating fibroblast growth and transformation [¹⁴³ ], although it is now thought that transformation was a result of contamination with mutated V12 Ras in these cells [⁸ ]. Concurrent with these studies, Bánfi et al. [¹⁴⁴ ] identified the same protein and named it NADPH oxidase homolog 1 (NOH-1). To alleviate confusion, a common nomenclature was subsequently adopted, designating this protein as Nox1 [¹¹ ], which is 564 amino acids long and exhibits 56% sequence identity with gp91^phox (a.k.a., Nox2), including strikingly homologous, functional domains [¹⁴³ ] (Fig. 4 ). Conversely, the Nox1 message is not present in phagocytes but is found primarily in colon epithelium and at lower levels in prostate, uterus, and vascular smooth muscle cells [¹⁴³ ]. Alternative splice forms of Nox1 have been reported and were designated as NOH-1S and NOH-1L and NOH-1Lv [¹⁴⁴ ]. Whereas NOH-1L and NOH-1Lv messages code for Nox1, NOH-1S appeared to represent a significantly smaller species, missing several functional domains [¹⁴⁴ ]. However, recent studies suggest NOH-1S is actually a cloning artifact and does not exist in nature.

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Figure 4. Structural comparison of mammalian Nox proteins. Major functional domains are indicated on these scale models (see text for details). NOH-1L, 1Lv, Long and long-variant forms of NOH-1, respectively; Nox5-S, short form of Nox5; DUOX, dual oxidase; ThOX, human thyroid oxidase.

Shortly after the identification of Nox1, Dupuy et al. [¹⁴⁵ ] purified a novel flavoprotein involved in the thyroid NADPH oxidase, and cloning of this entity revealed a unique gp91^phox homologue, which they named p138^Tox. Subsequently, De Deken et al. [¹⁴⁶ ] used cDNA library screening to clone ThOX1 and ThOX2 and showed that p138^Tox actually represented a C-terminal fragment of ThOX2. The C-terminal regions of these proteins are similar in structure to Nox1; however, the ThOX proteins are three times larger than Nox1 as a result of N-terminal extensions containing two proximal EF-hand motifs that function in Ca²⁺ binding, an additional transmembrane helix, and a distal peroxidase homology domain [⁸ ] (Fig. 4) . Based on the presence of NADPH oxidase and peroxidase homology domains, these proteins are now known as DUOX1 and -2 [¹¹ ]. Concurrent with cloning of the DUOX genes, Geiszt et al. [¹⁴⁷ ] and Shiose et al. [¹⁴⁸ ] identified and cloned a gp91^phox homologue in the kidney. Originally named Renox [¹⁴⁷ ] but now also known as Nox4, this homologue is highly expressed in the proximal tubules of the renal cortex and is suggested to play a possible role in oxygen sensing or regulating cell growth in the kidney [¹⁴⁷ , ¹⁴⁸ ] (Fig. 4) . Subsequently, Cheng et al. [¹⁴⁹ ] also reported the cloning and characterization of Nox4, as well as two additional gp91^phox homologs, designated as Nox3 and Nox5 (Fig. 4) . Like Nox1 and -4, Nox3 is homologous in size and domain structure to gp91^phox; however, its message seems to be present primarily in fetal tissues and certain cancer cell lines [¹⁴⁹ ]. It is interesting that Paffenholz et al. [¹⁵⁰ ] recently reported Nox3 message was present in the inner ear of embryonic and adult mice and that mutations in the otoconia-deficient head tilt (het) locus affected the Nox3 gene, suggesting a novel role for Nox3 in otoconia morphogenesis. Nox5 seems to be distinct in structure from Nox1–4, containing an N-terminal extension encompassing three EF-hand-like, Ca²⁺-binding motifs, and can be activated in a Ca²⁺-dependent manner [¹⁵¹ ] (Fig. 4) . Apparently, several splice variants of the Nox5 message can occur, with a short form found only in fetal kidney [¹⁴⁹ ] and two long isoforms (Nox5 and -ß) found in adult spleen (), lymph nodes (), and testis (ß) [¹⁵¹ ].

As summarized above, p22^phox and gp91^phox (Nox2) are required in phagocytes for normal flavocytochrome b function. Conversely, p22^phox is expressed in a variety of nonphagocytic cells, some expressing Nox2 and others expressing different Nox homologs (reviewed in ref. [¹⁵² ]). Although it is still not clear if the p22^phox interaction occurs with all Nox homologs, recent studies suggest that Nox1 can interact with p22^phox in vascular smooth muscle cells [¹⁵³ ] and transfected Chinese hamster ovary cells [¹⁵⁴ ]. Conversely, obvious vascular defects have not been noted in CGD patients with p22^phox mutations; thus, the physiological relevance of these interactions remains to be elucidated.

NOXO1/NOXA1
Based on analogy with the phagocyte NADPH oxidase, it seemed reasonable that other Nox-based systems might require the participation of additional cofactors for proper function. In support of this idea, cells transfected with Nox1 alone produced very low levels of O₂^·– [¹⁴³ ], whereas activity could be substantially enhanced by coexpression of p47^phox and p67^phox, indicating that these phox proteins could functionally associate with Nox1 [¹⁵⁵ ]. Nevertheless, it was clear that other Nox-related cofactors must exist, as various combinations of phox proteins are coexpressed with various Nox proteins in nonphagocytic cells, and some Nox-expressing cells apparently don’t express any classical phox proteins (reviewed in ref. [¹⁴² ]). This issue was recently addressed by three groups, which concurrently identified homologs of p47^phox and p67^phox in mouse [¹⁵⁶ ] and human [¹⁵⁴ , ¹⁵⁷ ] colons.

The p47^phox homologue was originally designated as p41 or p41^nox, based on its predicted molecular weight, but is now known as NOXO1 as a result of its putative role in organizing oxidase proteins during oxidase assembly [¹⁵⁴ , ¹⁵⁶ , ¹⁵⁷ ]. NOXO1 is expressed primarily in colon and testis but is also found at low levels in pancreas, liver, thymus, and small intestine [¹⁵⁴ , ¹⁵⁶ , ¹⁵⁷ ]. Although it exhibits only 23% amino acid sequence identity with p47^phox, NOXO1 is structurally similar to p47^phox and contains analogously spaced PX, tandem SH3, and proline-rich domains (Fig. 3) . In contrast, NOXO1 lacks the polybasic autoinhibitory domain, which becomes multiply phosphorylated during p47^phox activation [¹⁵⁴ , ¹⁵⁶ , ¹⁵⁷ ]. Thus, it appears that the NOXO1 may be constitutively active with its SH3 domains always accessible for binding to Nox1, which would be consistent with the constitutive nature of the Nox1-based enzyme [¹⁴³ ].

The p67^phox homologue was originally designated as p51 or p51^nox, based on its predicted molecular weight, but is now known as NOXA1 because of its putative role in oxidase activation [¹⁵⁴ , ¹⁵⁶ , ¹⁵⁷ ]. Like NOXO1, NOXA1 is highly expressed in the colon, although the relative levels vary widely between studies [¹⁵⁴ , ¹⁵⁶ , ¹⁵⁷ ]. In addition NOXA1 seems to be expressed in a wider range of tissues than NOXO1 [¹⁵⁴ ]. NOXA1 exhibits 28% amino acid sequence identity with p67phox, although it is significantly shorter as a result of the absence of the first SH3 domain and a small region near the second SH3 domain [¹⁵⁴ , ¹⁵⁶ , ¹⁵⁷ ] (Fig. 3) . Conversely, NOXA1 contains an analogous TPR motif, activation domain, PB1 domain, and C-terminal SH3 domain [¹⁵⁴ , ¹⁵⁶ , ¹⁵⁷ ].

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Phagocyte oxidase
As summarized in the previous sections, the neutrophil NADPH oxidase is composed of multiple proteins that associate with each other through a temporal and spatial array of protein–protein-binding interactions, resulting in an active, O₂^·–-generating complex (see Figs. 5 and 6 ).

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Figure 5. Activation-induced, conformational changes expose p47phox-binding domains. Phosphorylation (P)/amphiphiles help to neutralize charge in the cationic/autoinhibitory domain, resulting in release of the PX-SH3 domain interaction and exposure of all three domains for subsequent binding events. The C-terminal, praline-rich region presumably remains bound to p67phox throughout these events.

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Figure 6. Model of NADPH oxidase assembly. Activation/phosphorylation (P)-induced conformational changes in p47phox release autoinhibitory interactions to unmask essential binding domains and exposure of PX domains that facilitate membrane targeting and binding of SH3- and non-SH3-mediated, binding events. Final interaction of the p67phox and Rac with flavocytochrome b induces conformational change, resulting in electron flow. See text for a detailed description of the assembly events.

In resting cells, p40^phox, p47^phox, and p67^phox exist in a stoichiometric, cytosolic complex that is stabilized, in part, by SH3 domain interactions [⁵² , ¹⁵⁸ ]. Intramolecular, autoinhibitory interactions maintain p47^phox in a closed conformation and include association of both SH3 domains with the polybasic domain and concurrent binding of the N-terminal PX domain binding to the C-terminal SH3 domain [⁴⁹ , ^{52 159} ] (Fig. 5 ). This charge-mediated, autoinhibitory interaction appears to prevent binding of p47^phox to p22^phox in resting cells [⁶³ , ¹⁵⁹ , ¹⁶⁰ ]. Additional interactions that seem to be present in the resting cytosolic complex include high-affinity interactions between the C-terminal, proline-rich region of p47^phox and the C-terminal SH3 domain of p67^phox [⁵² , ¹⁶¹ , ¹⁶² ] and possibly the SH3 domain of p40^phox [¹⁶³ , ¹⁶⁴ ], although the relative importance of the latter interaction has been debated [⁵² ]. Instead, the most relevant interaction involving p40^phox appears to be a non-SH3-mediated interaction between the p40^phox C-terminal PC motif and the p67^phox PB1 domain [¹²³ ]. Together, these interactions function to stabilize the multiprotein cytosolic complex as well as any free p47^phox in a resting state. At the same time, flavocytochrome b is also present in an inactive state, which seems to be a result of undefined, conformational constraints that hinder electron transfer [⁹² , ¹⁶⁵ ].

As discussed above, multiple phosphorylation events are associated with NADPH oxidase activation. Phosphorylation of p47^phox, primarily in the polybasic domain and C terminus, induces conformational changes that induce release of the intramolecular, autoinhibitory interaction [⁶³ , ¹⁵⁹ , ¹⁶⁶ ] (Fig. 5) . However, phosphorylation alone is apparently not sufficient to activate p47^phox in vivo, as Dana et al. [¹⁶⁷ ] showed that cytosolic phospholipase A₂-generated AA was essential for complete activation of the oxidase in vivo. In support of this concept, Shiose and Sumimoto [¹⁶⁰ ] found that p47^phox activation in vivo required the synergistic action of phosphorylation and AA. Additionally, other lipids known to accumulate during oxidase activation, such as phosphatidic acid, may also contribute to this process [¹⁶⁸ ]. In any case, unmasking of the p47^phox autoinhibitory conformation is essential and results in exposure of both SH3 domains, the polybasic region and the PX domain [⁶³ , ⁶⁶ , ¹⁶⁶ ]. Apparently, p47^phox and p67^phox remain associated throughout these conformational changes via association of the p47^phox proline-rich C-terminus with the p67^phox C-terminal SH3 domain [⁵² , ⁶⁵ , ¹⁶¹ ] (Fig. 6 ). Phosphorylation may help to neutralize charge within the exposed polybasic region, thereby facilitating interactions with p67^phox and possibly stabilizing the active, open conformation of the tandem SH3 domains [¹⁶⁹ ].

During oxidase activation, cytosolic components translocate and eventually bind to flavocytochrome b (Fig. 6) ; however, it is not clear at what point in this process full activation of the cytosolic proteins is achieved. Phosphorylation occurs in a step-wise manner, and apparently, only the most highly phosphorylated forms of p47^phox associate with the membrane [¹⁷⁰ ]. Indeed, two of the most acidic forms (those containing the greatest amount of phosphorylation) are not present in X-linked CGD cells, indicating that some phosphorylation occurs at the membrane following translocation and/or requires flavocytochrome b interaction [¹⁷⁰ , ¹⁷¹ ]. Thus, it is plausible that activation of p47^phox and the other factors of the cytosolic complex is a cumulative process occurring throughout translocation and that arrival at the membrane is nearly coincidental with full exposure of the tandem p47^phox SH3 domains, which are essential for binding to their proline-rich target in p22^phox [¹⁷² , ¹⁷³ ] and exposure of the p40^phox and p47^phox PX domains involved in binding to membrane phospholipids [⁶⁵ , ⁶⁶ , ¹¹⁸ ]. In addition, phosphorylation of p22^phox during activation may contribute to this process [¹⁷⁴ ]. The association of the p47^phox SH3 domains with p22^phox is a critical step in the assembly process and may facilitate initial docking of p47^phox and associated oxidase proteins with flavocytochrome b (Fig. 6) . Indeed, p67^phox seems to depend on p47^phox for binding to the oxidase assembly [⁵⁸ ], although membrane-targeted Rac can serve as a surrogate p67^phox target in a modified, cell-free assay [¹⁷⁵ ]. In any case, correct alignment of p47^phox would then insure accurate proximity for binding of the remaining sites of interaction between p47^phox/p67^phox and flavocytochrome b, including binding of p47^phox to multiple sites within gp91^phox [¹⁷⁶ , ¹⁷⁷ ] and binding of the p67^phox N terminus to gp91^phox [¹⁷⁸ ], possibly via the activation domain [¹⁰⁵ ]. It is interesting that p47^phox binding also appears to be enhanced by the presence of p67^phox [⁵⁸ ], suggesting the possibility that the binding of p47^phox helps chaperone p67^phox to a zone where it can interact with gp91^phox but also that this subsequent interaction may form a "sandwich" that further anchors p47^phox (Fig. 6) . During p47^phox/p67^phox binding with flavocytochrome b, regulatory input seems to be required to initiate release of p67^phox from the cationic domain of p47^phox so that this region is accessible for subsequent high-affinity interactions with flavocytochrome b [¹⁶⁹ ].

The most likely candidate for regulatory input at this stage of oxidase activation appears to be Rac, and several models of oxidase regulation by Rac have recently been proposed [¹²⁴ ]. As discussed above, Rac translocates independently of the other cytosolic factors [⁹¹ , ¹³³ ] and can bind, via the switch I region, to the p67^phox N-terminal TPR motifs [⁸⁹ , ¹⁷⁹ ]. Rac also appears to associate with flavocytochrome b [⁹¹ , ¹⁸⁰ ], possibly via the Rac insert region [¹⁷⁹ ]. The relative timing or sequence of Rac-binding interactions during oxidase assembly is unclear; however, Diebold and Bokoch [¹⁸⁰ ] recently provided evidence that the Rac:flavocytochrome b interaction may occur prior to the Rac:p67^phox interaction and that two distinct, Rac-dependent steps participate in the electron transfer process. Step 1 involves direct binding of the Rac insert domain to flavocytochrome b [¹⁸⁰ ], which is independent of p67^phox but seems to facilitate subsequent targeting or modulation of p67^phox [¹⁷⁹ , ¹⁸¹ ], initiating electron flow from NADPH to FAD. In step 2, interaction between the switch I domain of Rac and p67^phox is proposed to induce a conformational change in p67^phox, thereby allowing electron flow from FAD to the hemes. Note that this two-step model is consistent with kinetics of oxidase activation described previously [¹⁸² ].

Rac has been shown to disrupt the p40^phox–p67^phox interaction, and this event seems to be essential for efficient oxidase activation [⁹⁰ , ¹⁸³ ]. Thus, although p40^phox is required early on to modulate recruitment of p47^phox/p67^phox to the membrane via PB1–PC interactions with p67^phox and PX domain interactions with the membrane [¹¹⁸ ], it subsequently becomes displaced by Rac binding. The binding of Rac and/or release of p40^phox from p67^phox could then induce conformational changes in p67^phox, thereby liberating the p47^phox cationic region for gp91^phox binding [¹⁶⁹ ] and/or positioning the p67^phox activation domain correctly for imminent binding to gp91^phox [¹⁰⁵ ]. Apparently, one or more of these events induces conformational changes in flavocytochrome b, thereby permitting completion of electron transfer from FAD to heme and ultimately, to O₂ [¹⁸² ]. In support of this conclusion, atomic force microscopy has demonstrated measurable changes in flavocytochrome b during transition to the active state [⁹² , ¹⁸⁴ ]. Furthermore, Foubert et al. [¹⁶⁵ ] recently found that amphiphilic agents used in cell-free oxidase activation induced significant conformational changes in flavocytochrome b, as determined by monitoring resonance energy transfer from an external fluorescent probe to the heme, and suggested that these changes in structure may facilitate heme alignment for efficient electron transfer.

The nature of the event that induces flavocytochrome b conformational change directly is still under investigation. In vitro, it has been reported that p47^phox and p67^phox can induce conformational changes in flavocytochrome b [⁹² ]; however, only p67^phox binding induced electron flow and O₂^·– production [⁹² , ¹⁰⁶ , ¹⁸¹ ]. Although not currently included in the various models of oxidase assembly and activation, Rap1A has been shown to play a regulatory role in intact cells [¹³⁹ , ¹⁴⁰ ] and could conceivably be important in this process by virtue of its ability to associate with flavocytochrome b [¹³⁷ ].

Assembly of nonphagocyte oxidases
Currently, not much is known regarding actual binding events during assembly of the nonphagocyte oxidase systems; however, the overall similarity in domain structure between Nox/NOXO1/NOXA1 proteins and their respective phox homologs suggests that some of the interactions described above may be relevant (Figs. 3 and 4) . Indeed, NOXO1 can interact with p22^phox and p67^phox via analogous SH3 domain-mediated binding, and NOXA1 can bind to p47^phox and Rac through interactions similar to those with p67^phox [¹⁵⁴ ]. In addition, NOXO1 and NOXA1 can substitute for and combine with their respective phox homologs in activation of gp91^phox [¹⁵⁴ , ¹⁵⁵ ]. There does appear to be some structural specificity required for the various cofactors to activate Nox1, however, as only NOXO1 and NOXA1 are fully competent in this respect, and substitution with one or both p47^phox and p67^phox results in minimal Nox1 activity [^{154 155 156 157} ]. Conversely, recent studies indicate that Nox3 is more compliant and can be activated by phox and NOX regulatory proteins [¹⁸⁵ ]. Nox3 can also be activated by NOXO1 alone in the absence of activator subunits (NOXA1 or p67^phox) [¹⁸⁵ ].

Nonphagocyte oxidases also appear to lack some key regulatory interactions characteristic of the phagocyte oxidase. For example, the absence of an analogous, autoinhibitory region in NOXO1 suggests that its SH3 domains are constitutively accessible for Nox binding and that phosphorylation may not play as important a role in NOXO1 activation. Indeed, Cheng and Lambeth [¹⁸⁶ ] recently reported that NOXO1 colocalizes with Nox1 in the membranes of resting cells. NOXA1 does not appear to bind p40^phox, which may be a result of the lack of the conserved Lys355 in the NOXA1 PB1 domain [¹⁸⁷ ]. As p40^phox stabilizes the cytosolic phox protein complex in phagocytes [⁹⁰ ], one might also conclude that such a complex is unstable or may not exist in nonphagocytic cells using Nox1. Alternatively, an unidentified homologue of p40^phox could be present in cells expressing NOXA1. Finally, the activation-induced, binding interaction involving the exposed p47^phox polybasic region and p67^phox [¹⁶⁹ ] would be absent in the NOXO1–NOXA1 system. The absence of these regulatory interactions would be consistent with and might help to explain the constitutive oxidase activity observed in many of the nonphagocyte oxidases (reviewed in refs. [⁸ , ¹⁴² ]). Indeed, agonist-independent oxidase activity is observed in cells cotransfected with NOXO1/NOXA1 and Nox1 or Nox3, although this activity can be acutely up-regulated in an agonist-dependent manner [¹⁵⁴ , ¹⁵⁶ , ¹⁵⁷ , ¹⁸⁵ , ¹⁸⁶ ].

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Previously, neutrophil gene expression was assumed to be minimal or absent in mature neutrophils [¹⁸⁸ ]; however, subsequent investigation of this issue indicated that circulating neutrophils actually continue to transcribe and translate certain genes at fairly high levels [¹⁸⁹ ]. More recently, a number of studies have used genomic approaches to show that neutrophils are indeed capable of extensive and rapid changes in gene expression (reviewed in ref. [¹⁹⁰ ]).

During neutrophil maturation, expression of genes coding for NADPH oxidase proteins is highly regulated at the transcriptional level. For example, studies in bone marrow granulocytes and promyelocytic cell lines demonstrated variable timing in gp91^phox, p67^phox, and p47^phox expression patterns during differentiation, indicating a role for transcriptional regulation in this process [¹⁹¹ ]. However, a number of studies indicate that cytokines and inflammatory mediators can also modulate the level of oxidase protein expression in mature phagocytes via transcriptional regulation involving the coordinated activity of a variety of transcription factors (reviewed in ref. [¹⁹² ]; Table 2 ).

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Table 2. Factors Regulating Phagocyte NADPH Oxidase Gene Transcription

Regulation of CYBB, the gp91^phox gene
Transcriptional regulation of CYBB is the best understood of the various oxidase-related genes. CYBB can be induced by a number of cytokines and inflammatory factors, and Skalnik et al. [²⁰⁸ ] showed that as few as 450 bp of the proximal CYBB promoter was sufficient to direct lineage-restricted transcription. However, subsequent analysis of the entire CYBB locus indicated that undefined distal elements were also required for its expression in granulocytes [²⁰⁹ ]. Transcriptional regulation of CYBB seems to the most complex of all oxidase genes involving transcriptional activators and repressors [¹⁹² ] (Table 2) .

Ets factors have been shown to play critical roles in transcriptional regulation of CYBB. A consensus sequence for ets factor-binding is located in the proximal promoter of CYBB and is designated as the Pu box, as it binds the myeloid-specific ets factors PU.1 and Elf-1 [¹⁹⁹ , ²¹⁰ ]. In addition, it appears that PU.1 or Elf-1 may cooperatively bind to the CYBB promoter as a complex comprised of two IRFs, IRF-1 and ICSBP, which are known as the HAF-1 complex [²¹¹ ]. Binding HAF-1 to the CYBB promoter has been reported to recruit the transcriptional coactivator CBP, which is thought to be involved in IFN-induced activation of CYBB and other myeloid genes [¹⁹⁶ ]. Furthermore, Kautz et al. [¹⁹⁸ ] reported that the protein-tyrosine phosphatase (SH2-containing tyrosine phosphatase 1) inhibited gp91^phox and p47^phox expression in undifferentiated myeloid cell lines by keeping IRF-1 and ICSBP dephosphorylated, thereby inhibiting HAF-1 binding. It should be noted, however, that the relative roles of PU.1 and HAF-1 in regulating CYBB transcription are still uncertain, and it has recently been suggested that PU.1 binds with higher affinity and is dominant, and HAF-1 is supplementary for activation of CYBB [²¹² ]. Additionally, YY1 has been shown to bind and activate the CYBB promoter [²⁰⁰ ].

Transcriptional repression has also been proposed to participate in CYBB regulation, with potential involvement of multiple repressors and overlapping binding elements. For example, Skalnik and co-workers [²⁰¹ ] showed that the transcriptional repressor CDP binds to multiple sites in the proximal promoter of CYBB to repress this gene in immature myeloid cells and that down-regulation of CDP during final maturation is required for gp91^phox expression. In addition, it was found that CDP binding repressed transcription by blocking the binding sites for multiple transcriptional activators, including CP1, IRF-1, and IRF-2 [²⁰² , ²⁰³ ]. One of the CDP-binding sites was also found to be a consensus target sequence for binding of the repressor element HoxA10, and phosphorylation of HoxA10 during myeloid differentiation decreased its binding, thereby releasing transcriptional repression of CYBB [²⁰⁴ ]. Recently, seven binding sites for the matrix attachment region-binding protein SATB1 were identified in the CYBB promoter, and it was found that SATB1 also repressed gp91^phox expression [²⁰⁵ ]. Like CDP, SATB1 binding to the CYBB promoter was abundant in immature cells but was down-regulated during terminal phagocyte differentiation, suggesting that the two factors may play similar or complementary roles [²¹⁰ ]. Fujii et al. [²⁰⁶ ] recently found that SATB1 primarily recruited p300 and that binding of the SATB1/p300 complex hindered CDP binding to an adjacent segment of the CYBB promoter. Finally, it has been reported that GATA-3 represses CYBB transcription in eosinophil-committed myeloid cells [²⁰⁷ ]. Thus, transcriptional regulation of CYBB is quite complex and involves the integration of multiple positive and negative inputs to achieve the appropriate response.

Regulation of CYBA, the p22^phox gene
In general, the expression of p22^phox seems to be less restrictive than that of gp91^phox, and Parkos et al. [²⁰ ] originally reported that the p22^phox message was constitutively expressed in a variety of cell types. Unlike gp91^phox, p22^phox is expressed in immature myeloid cells; however, functional flavocytochrome b is only present after the myeloid stage when both subunits are expressed [¹⁹¹ ]. Thus, the role of p22^phox expression in the absence of gp91^phox is unclear. In addition, relatively little is known regarding p22^phox transcriptional regulation. Newburger et al. [²¹³ ] reported that IFN- induced CYBB but had no effect on CYBA transcription. This finding was subsequently confirmed by Cassatella et al. [²¹⁴ ] who also reported that lipopolysaccharide (LPS) caused the same effect. In contrast, Newburger et al. [²¹⁵ ] found that LPS and various cytokines (TNF- and colony-stimulating factor) each induced coordinate up-regulation of CYBA and CYBB, although the effect was greater for CYBB. Thus, it appears that there is some level of transcriptional regulation of CYBA; however, details regarding the nature of this regulation and transcription factors involved are still unknown.

Regulation of NCF1, the p47^phox gene
As with gp91^phox, p47^phox is not expressed in immature myeloid cells but is induced during differentiation [¹⁹² ]. In addition, a number of cytokines and inflammatory factors can modulate p47^phox transcription to regulate NCF1 message levels (e.g., refs. [²¹⁴ , ²¹⁶ ]). To better understand this process, Li et al. [¹⁹⁴ ] sequenced and characterized the NCF1 promoter region and found that the first 86 bp upstream of the transcriptional start site possessed tissue-specific NCF1 promoter activity in myeloid cells. Furthermore, analysis of this region by a number of approaches showed that the promoter element involved in regulating NCF1 transcription corresponded to the PU.1 consensus-binding motif, that it actually bound PU.1, and that PU.1 was essential for basal transcription of NCF1 [¹⁹⁴ ] (Table 2) . More recently, Marden et al. [²¹⁷ ] reported that differentiation-dependent up-regulation of NCF1 transcription is associated with hyperphosphorylation of PU.1, which increases its binding affinity. In addition to the PU.1 element, analysis of the NCF1 promoter suggested consensus-binding elements for other factors [¹⁹⁴ ]; however, it has not yet been determined whether any of these factors bind and play a role in regulating the NCF1 promoter.

Regulation of NCF2, the p67^phox gene
The expression of p67^phox seems to be regulated differently than that of other phox proteins during cellular differentiation, and recent studies discussed above suggest that p67^phox may be the rate-limiting cofactor in oxidase activation [^{92 93 94} ]. Consistent with this concept, p67^phox is the last phox protein to be expressed during differentiation, and its expression correlates the closest with the acquisition of oxidase activity [¹⁹¹ ]. As with the CYBB and NCF1 promoters, the NCF2 promoter is also regulated by PU.1 (Table 2) . Based on comparison with the CYBB promoter, Eklund and Kakar [¹⁹⁶ ] identified an IFN--inducible element within the NCF2 promoter and reported that PU.1, IRF-1, and ICSBP bound to this element [designated as PU.1(1)]. However, mutations within this element did not eliminate basal transcription, which is quite different from the CYBB and NCF1 promoters, where PU.1-binding site mutations eliminated basal transcription [¹⁹⁴ ]. In addition, analysis of PU.1 null mice indicated that the p67^phox message was present [²¹⁸ ]. Together, these results indicated that PU.1 could enhance but was not essential for basal transcription of NCF2 and that additional regulatory elements were involved. Indeed, more recent studies characterizing the NCF2 promoter demonstrated the presence of two more PU.1-binding elements in intron 1 [designated as PU.1(2) and -⁽³⁾ ] and two more (apparently nonfunctional) PU.1-binding elements further upstream in exon 1 [¹⁹⁵ , ¹⁹⁷ ]. Comparison of the three PU.1-binding elements in intron 1 showed that they bound PU.1 with differing affinities and that their relative roles in transcriptional activation of NCF2 depended on their location within the promoter [¹⁹⁵ , ¹⁹⁷ ]. Although PU.1(1) is similar in sequence to the PU.1/HAF-1 site in the CYBB promoter (see above), PU.1(2) and PU.1(3) are identical to the PU.1-binding elements in the NCF1 and IgJ promoters, respectively [¹⁹⁴ , ²¹⁹ ].

In addition to PU.1-binding elements, putative-binding domains for AP-1 (intron 1), AP-4 (intron 1), and Sp1/3 (upstream of exon 1) transcription factors were identified in the NCF2 promoter [¹⁹⁵ , ¹⁹⁷ ]. Mutational analysis of the putative AP-1 and AP-4 elements showed that although the AP-4 element was not functional, the AP-1 element was absolutely essential and that mutation of this site resulted in complete loss of basal transcription of NCF2. Furthermore, this element was shown to bind AP-1 nuclear factors Fos and Jun, confirming the functional importance of this site [¹⁹⁵ , ¹⁹⁷ ]. In contrast, analysis of the role of Sp1 showed that it was not essential but that it did cooperate with intron 1 elements in regulating NCF2 [¹⁹⁵ ]. Overall, it is clear that NCF2 transcription is regulated cooperatively by multiple factors (Table 2) .

Regulation of NCF4, the p40^phox gene
Like p22^phox, p40^phox is expressed in immature myeloid cells, although the p40^phox message and protein levels do concomitantly increase during maturation and differentiation [¹⁹¹ ]. Recently, Li et al. [¹⁹³ ] investigated regulation of NCF4 in myeloid cells and showed that a proximal region of the promoter (106 bp of the 5'-flanking sequence) directed this process. Detailed analysis showed that this region contained three functional PU.1-binding elements, which differed in binding affinity, and these three elements acted cooperatively in regulating NCF4 transcription. As with the NCF2 promoter, mutation of each PU.1 site alone decreased promoter activity; however, simultaneous mutation of all three sites was required for complete inhibition [¹⁹³ ]. Thus, PU.1 is implicated in regulating myeloid-specific transcription of four of the five phox proteins (Table 2) .

Regulation of nonphagocyte oxidase genes
Not much is known regarding transcription factors involved in regulating nonphagocyte oxidase genes. In addition, the regulation of phox gene transcription in nonphagocytic cells is also unclear, given that some of the key factors required to regulate these genes in phagocytes are specific to hematopoietic cells (e.g., PU.1) [²²⁰ ]. It is clear, however, that oxidase-related genes, whether phagocyte or nonphagocyte, are regulated at the transcriptional level in nonphagocyte systems and can be induced by a variety of cell-specific stimuli. For example, NADPH oxidase activity can be up-regulated by angiotensin II in adventitial fibroblasts [²²¹ ], endothelial cells [²²² ], and cardiomyocytes [²²³ ]. Concomitant with increased oxidase activity, angiotensin II has been shown to induce up-regulation of the message for many of the phox components in vascular cells [^{224 225 226} ] and in renal cortical cells [²²⁷ ]. Likewise, NADPH oxidase activity, as well as phox mRNA can be up-regulated to varying degrees in different nonphagocytic cells by a variety of biochemical signals (e.g., TNF- [²²⁸ ], IFN- [²²⁹ ], LPS [²³⁰ ]), physical events (e.g., shear stress [²³¹ ], cyclic strain [²³² ]), and disease processes (e.g., atherosclerosis [²³³ ], hypertension [²³⁴ ], restenosis [²³⁵ ]). In addition to the phox proteins, Nox1 and/or Nox4 mRNA levels have also been shown to be up-regulated under various conditions, such as shear stress [²³¹ ], balloon injury [²³⁶ ], restenosis [²³⁵ ], atherosclerosis [²³³ ], and mechanical stretch [²³⁷ ].

Down-regulation of the NADPH oxidase can also be mediated at the transcriptional level in nonphagocytic cells, and it has been reported that treatment with 3-hydroxy-3-methylglutaryl CoA reductase inhibitors [²³⁸ ] or 17ß-estradiol [²³⁹ ] reduces gp91^phox and p22^phox mRNA levels, respectively, in endothelial cells.

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Although the initial product of NADPH oxidase activity is the same (i.e., O₂^·–) regardless of the cellular location, it is clear that oxidases play different functional roles in phagocytic and nonphagocytic cells. As summarized below, the role of the phagocyte oxidase is clearly in host defense, although the actual mechanisms involved in O₂^·–-dependent microbicidal activity have recently been debated [²⁴⁰ ]. In contrast, the majority of nonphagocyte oxidases seems to function in cellular signaling, growth, and metabolism (reviewed in refs. [⁸ , ¹⁴² ]).

Phagocyte oxidase
Based on the susceptibility of CGD patients to bacterial and fungal infections [⁹ ], it is well established that the function of the phagocyte respiratory burst is in host defense. Conversely, there is some controversy regarding the role of O₂^·–-derived metabolites, such as HOCl, as direct microbicidal agents involving mechanisms described above. For example, Segal [²⁴¹ ] proposed that the primary role of ROS was to alkalinize the phagocytic vacuole. In support of this idea, Reeves et al. [²⁴² ] recently provided evidence that O₂^·– accumulation in the phagosome could function to create a charge differential across the phagosomal membrane, thereby inducing K⁺ influx into the vacuole and corresponding pH stabilization to create optimal conditions for granule protease release and activation. Based on these studies, the authors concluded that ROS generation and myeloperoxidase (MPO) activity were not themselves sufficient to kill target microbes, rather it is the proteases activated through this process that are primarily responsible for the destruction of bacteria and that MPO may be more important as a catalase to protect these proteases against oxidative damage [²⁴³ ]. More recently, this group reported that the observed K⁺ influx was mediated via large-conductance Ca²⁺-activated K⁺ channels and that microbicidal killing was inhibited when these channels were blocked [²⁴⁴ ]. Although these novel findings suggest a unique mechanism contributing to neutrophil antimicrobial activity, it is clearly not the only mechanism and does not explain the large amount of evidence implicating direct function of ROS in microbial killing (reviewed in ref. [²⁴⁰ ]).

One of the arguments against HOCl being a primary microbicidal agent is that MPO deficiencies are relatively common, and individuals with MPO deficiency generally do not seem to have an increased incidence of infection (reviewed in ref. [²⁴⁵ ]). Thus, MPO-deficient neutrophils must use compensatory MPO-independent but still oxygen-dependent antimicrobial systems [²⁴⁶ ]. For example, the mechanism described above would be functional in MPO-deficient cells, although protective antioxidant aspects of MPO would be missing [²⁴² ]. Recently, Aratani et al. [²⁴⁷ ] used genetically engineered mice to directly compare the relative importance of the NADPH oxidase and MPO systems in fungicidal activity and found that although MPO-deficient and oxidase-deficient mice showed increased susceptibility to pulmonary infections with Candida albicans and Aspergillus fumigatus compared with normal mice, the oxidase-deficient mice exhibited greater mortality, suggesting that although both are important, O₂^·– is somewhat more important than HOCl. It is interesting that these studies also indicated MPO was unable to contribute to the host defense in the absence of NADPH oxidase activity, suggesting that the H₂O₂ required to generate HOCl is solely derived from oxidase-generated O₂^·– [²⁴⁷ ].

As CGD neutrophils do not generate O₂^·– or H₂O₂, it is surprising that they are still able to kill a number of pathogens [⁹ ]. One theory proposed to explain this discrepancy is that the microbicidal capacity of CGD neutrophils depends, to some degree, on H₂O₂ produced by the pathogen itself, and those bacterial strains that do not produce H₂O₂ would be more resistant to killing [²⁴⁸ ]. Furthermore, as many bacteria express their own catalase and would be able to detoxify H₂O₂, it was suggested that catalase-producing organisms are especially resistant to killing by CGD phagocytes [²⁴⁸ ]. Indeed, catalase-positive organisms cause many of the infections in CGD patients, and catalase-negative organisms rarely infect these individuals [²⁴⁹ ]. However, recent studies suggest that this issue is more complicated than previously thought, as certain catalase-deficient organisms have also been shown to be virulent in mouse models of CGD and therefore, must use alternative, nonoxidative virulence mechanisms to survive [²⁵⁰ ]. In addition, Kottilil et al. [²⁵¹ ] recently reported that Hemophilus spp., which are catalase-negative but also do not produce H₂O₂, caused infections in CGD patients and concluded that catalase should still be considered as a virulence factor but only in bacteria that can produce H₂O₂.

The NADPH oxidase transfers electrons across the membrane in an electrogenic manner, resulting in a charge differential that is balanced by an associated proton flux [²⁵² ]. Henderson and Meech [²⁵² ] proposed the novel idea that a putative NADPH oxidase-associated H⁺ channel was responsible for proton flux and subsequently reported that this channel was gp91^phox. In support of this conclusion, it has been reported that one or more of the heme-coordinating histidines participate in H⁺ flow-through gp91^phox [²⁵³ , ²⁵⁴ ] and that H⁺ flux is absent in gp91^phox-deficient eosinophils [²⁵⁵ ]. However, this issue remains a matter of debate, as there is also substantial evidence indicating the phagocyte oxidase-associated H⁺ channel is distinct from gp91^phox and that a different entity is responsible for this process [²⁵⁶ ]. For example, Nanda et al. [²⁵⁷ ] showed that H⁺ conductance was normal in CGD neutrophils containing nonfunctional or diminished cytochrome b and concluded that the oxidase-associated H⁺ channel was closely associated with and activated by the oxidase but could not be gp91^phox. This conclusion is supported by subsequent studies of DeCoursey and co-workers [²⁵⁸ ], who observed normal H⁺ currents in gp91^phox-deficient CGD neutrophils and PLB-985 cells as well as in gp91^phox-deficient PLB-985 cells engineered to re-express gp91^phox. Furthermore, this group demonstrated the absence of H⁺ currents in COS^phox cells, indicating that gp91^phox does not function as a proton channel in these cells [²⁵⁹ ]. Regardless of the actual entity involved, it is still clear that H⁺ flux maximizes NADPH oxidase function by preventing the membrane from depolarizing to inhibitory levels and is ideally suited for this process, as H⁺ is osmotically neutral [²⁶⁰ ].

Although primarily involved in phagocyte host defense, ROS may also play additional signaling roles in these cells. For example, Forman and Torres [²⁶¹ ] recently reported that ROS play a role in macrophage intracellular signaling and that redox signaling may help to modulate the inflammatory response by inducing these cells to synthesize cytokines, which can feed back to regulate macrophage function as well as induce neutrophil influx.

Nonphagocyte oxidases
In general, ROS generated by nonphagocyte oxidases seem to be primarily involved in intra- and intercellular signaling, rather than in host defense (reviewed in refs. [⁷ , ²⁶² ]). One exception reported to date is the DUOX-based oxidases, which have been shown to play an essential role in thyroid hormone biosynthesis [²⁶³ , ²⁶⁴ ]. Indeed, Moreno et al. [²⁶⁵ ] recently reported that mutations in the DUOX2 gene, resulting in defective DUOX2, disrupted thyroid-hormone synthesis and were associated with severe and permanent congenital hypothyroidism. DUOX-based oxidases also appear to play important roles in mucosal surface host defense, and Geiszt et al. [²⁶⁶ ] recently demonstrated the presence of the DUOX message and enzymatic activity in salivary glands and mucosal epithelium.

Recent studies suggest that a multitude of physiological processes involve nonphagocyte oxidase signaling pathways. As a result of space limitations, only a few representative examples are included here, and the reader is referred to several recent reviews for a more comprehensive coverage of this topic [⁷ , ⁸ , ¹⁴¹ ]. One of the best-studied systems involving nonphagocyte oxidases is the vascular system, and it has been shown that vascular oxidases play important roles in physiological events, such as regulation of blood pressure, and pathological events, such as hypertension and atherosclerosis (recently reviewed in refs. [¹⁴¹ , ²⁶⁷ , ²⁶⁸ ]). In the vascular system, multiple functions have been reported for O₂^·– generated by nonphagocyte oxidases, including directly scavenging nitric oxide (NO), mitogenic signaling, and oxygen sensing, and the various Nox and phox proteins are distributed widely throughout the vascular in cell- and organ-specific patterns (recently reviewed in ref. [¹⁴¹ ]). Furthermore, multiple different Nox systems can be present in the same cell, and Hilenski et al. [²⁶⁹ ] recently reported differential, subcellular distributions of Nox1 and Nox4 in vascular smooth muscle cells and suggested this type of compartmentalization could facilitate their respective functions in various cellular processes.

It has been established that vascular O₂^·– plays a role in the regulation of smooth muscle tone and consequently, blood pressure, although the precise mechanisms involved are still under investigation (reviewed in ref. [²⁷⁰ ]). Indeed, excess production of O₂^·– has been implicated in hypertension, and it has been suggested that effects on vascular tone are partially a result of the ability of O₂^·– to react with NO, resulting in reduced NO bioavailability (e.g., ref. [²⁷¹ ]). A number of studies have demonstrated that superoxide dismutase mimetics and antioxidants can significantly reduce hypertension (reviewed in refs. [¹⁴¹ , ²⁶⁸ ]), and Rey et al. [²⁷² ] recently showed that in vivo administration of a competitive peptide inhibitor of oxidase assembly attenuated vascular ROS production and hypertension in angiotensin II-treated mice.

ROS also play important functions as intracellular messengers during mitogenic stimulation of vascular cells by various hormones and growth factors, e.g., angiotensin II, platelet-derived growth factor, serotonin, and vascular endothelial growth factor (reviewed in ref. [²⁶⁷ ]). Currently, the exact mechanisms involved in mediating ROS signaling are not completely understood; however, Saran [²⁷³ ] recently proposed that ROS modification of membrane phospholipids plays an important role in this process. Conversely, a variety of redox-sensitive intracellular targets have been implicated in ROS signaling, including direct or indirect effects on thioredoxin, protein tyrosine phosphatases, receptor and nonreceptor tyrosine kinases, and transcription factors (reviewed in ref. [²⁶⁷ ]). For example, ROS play a role in Ras-mediated signaling pathways, presumably via regulation of the transcription factor nuclear factor-B, and it has been proposed that this pathway is involved in cell proliferation through inhibition of apoptosis (reviewed in ref. [²⁷⁴ ]). Indeed ROS-induced proliferation of vascular cells has been suggested to contribute to various cardiovascular diseases and may be a general response to vascular inflammatory stress (recently reviewed in refs. [¹⁴¹ , ²⁶⁷ , ²⁶⁸ ]).

There is substantial evidence to indicate that Nox enzymes play a role in O₂ sensing throughout the body, and Nox-based enzymes have been reported to participate in oxygen sensing in the pulmonary chemoreceptors [neuroepithelial bodies (NEB)], carotid body, erythropoietin (EPO)-producing cells in the kidney, and other organ systems (reviewed in ref. [²⁷⁵ ]). Originally, it was proposed that a gp91^phox (Nox2)-based NADPH oxidase was involved in O₂ sensing, based on its presence in carotid body and airway chemoreceptors (reviewed in ref. [²⁷⁶ ]). However, this conclusion has been debated widely. For example, Wenger et al. [²⁷⁷ ] reported normal hypoxia-induced gene expression in a flavocytochrome b-deficient CGD B cell line and concluded that flavocytochrome b was not an O₂ sensor. This finding was supported by subsequent studies of Archer et al. [²⁷⁸ ], who reported that O₂ sensing was normal in pulmonary artery smooth muscle cells isolated from Nox2-deficient CGD mice. In contrast, Fu et al. [²⁷⁹ ] showed that O₂ sensing and regulation of K⁺ currents in pulmonary NEB from the same strain of mice required a functional Nox2-based oxidase. In support of this idea, NADPH oxidase activity was found to be a significant component of the O₂ sensor in airway chemoreceptor-derived cells, although it was also found that other mechanisms contributed to this response [²⁸⁰ ]. In addition, Kazemian et al. [²⁸¹ ] reported that respiratory control was defective in neonatal CGD mice and concluded that a NADPH oxidase-dependent O₂ sensor was involved in neonatal ventilatory control. Conversely, recent studies by Sanders et al. [²⁸² ], analyzing two strains of CGD mice, showed that O₂ sensing in EPO-producing cells of the kidney was independent of Nox2 and p47^phox and that the absence of Nox2 did not alter carotid body O₂ sensing. It is interesting that the absence of p47^phox significantly potentiated ventilatory and chemoreceptor responses to hypoxia, suggesting possible involvement of a different Nox homologue that could interact with carotid body p47^phox. Thus, although the role of Nox-based oxidases in O₂ sensing seems likely, it is clear that a Nox2-based system cannot fully account for the diversity in responses reported and that other Nox homologs may be involved, as well as additional O₂ sensors that can cooperate with the Nox-based systems.

TOP ABSTRACT INTRODUCTION PHAGOCYTE NADPH OXIDASE... NONPHAGOCYTE NADPH OXIDASE... MODEL OF NADPH OXIDASE... TRANSCRIPTIONAL REGULATION FUNCTIONAL ASPECTS SUMMARY REFERENCES

 
NADPH oxidases play important physiological roles in phagocytic and nonphagocytic cells but can also contribute to the pathogenesis of many inflammatory diseases. For example, the phagocyte NADPH oxidase is an essential component of the human cellular immune response; however, oxidants generated by this system can also contribute to the nonspecific tissue damage associated with arthritis and pulmonary diseases. Likewise, nonphagocyte NADPH oxidases play important roles in regulation of vascular tone and blood pressure; however, deregulation of vascular oxidase enzymes can contribute to the pathogenesis of hypertension and atherosclerosis. Because of the potential role in disease pathogenesis, activation and assembly of the NADPH oxidases are highly regulated and involve transcriptional and post-translational control mechanisms. Oxidase activation requires assembly of multiple proteins with membrane-associated flavocytochrome b, which presumably contains all of the required redox components but cannot catalyze the reaction on its own. By segregating these oxidase components into various subcellular locations, neutrophils are able to prevent inappropriate assembly and activation of the oxidase and thereby control the onset and duration of the oxidative burst. Only after highly regulated, intricate events involving phosphorylation, translocation, and multiple conformational changes does the oxidase enzyme acquire that capacity to generate O₂^·–. Conversely, many nonphagocyte oxidase systems seem to be constitutively assembled and may be more dependent on transcriptional up-regulation and/or other regulatory control mechanisms. Overall, a better comprehension of the structure and assembly of the NADPH oxidase protein components is essential to understanding and/or controlling their function in physiological and pathophysiological responses. Although the nature of these interactions is becoming increasingly apparent, further studies in this area will be necessary to define the exact role played by each protein during activation and assembly of the NADPH oxidase in phagocytes and nonphagocytic cells.

 
This work was supported in part by National Institutes of Health Grants AR42426 and HL66575 and the Montana State University Agricultural Experimental Station (Bozeman). K. A. G. is the recipient of an American Heart Association Scientist Development Grant. We thank Drs. Frank DeLeo, Laboratory of Bacterial Pathogenesis, and Thomas Leto, Laboratory of Host Defenses, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, MT, and Bethesda, MD, respectively, for helpful suggestions. We have attempted to cite as much literature as possible, given the page limitations, and apologize if some appropriate studies have been omitted inadvertently.

Received April 1, 2004; accepted May 17, 2004.

TOP ABSTRACT INTRODUCTION PHAGOCYTE NADPH OXIDASE... NONPHAGOCYTE NADPH OXIDASE... MODEL OF NADPH OXIDASE... TRANSCRIPTIONAL REGULATION FUNCTIONAL ASPECTS SUMMARY REFERENCES

 

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