Characterization of neutrophil NADPH oxidase factors p47-phox and p67-phox from recombinant baculoviruses

INF-γ Enhances Nox2 Activity by Upregulating phox Proteins When Applied to Differentiating PLB-985 Cells but Does Not Induce Nox2 Activity by Itself

Background

The cytokine and drug interferon-γ enhances superoxide anion production by the antimicrobicidal Nox2 enzyme of neutrophils. Because mature neutrophils have a short lifespan, we hypothesized that the effects of interferon-γ on these cells might be mediated by its prolonged exposure to differentiating neutrophil precursors in the bone marrow rather than its brief exposure to mature circulating neutrophils.

Effects of INF-γ on Nox2 Activity

To address this possibility we exposed the myeloid PLB-985 cell line to interferon-γ for 3 days in the presence of dimethyl sulfoxide which induces terminal differentiation of these cells. Interferon-γ was found to enhance superoxide production by Nox2 in a concentration dependent manner. In contrast, application of interferon-γ alone for 3 days failed to induce detectible Nox2 activity. Additionally, application of interferon-γ for 3 hours to pre-differentiated PLB-985 cells, which models studies using isolated neutrophils, was much less effective at enhancing superoxide anion production.

Effects of INF-γ on phox Protein Levels

Addition of interferon-γ during differentiation was found to upregulate the Nox2 proteins gp91phox and p47phox in concert with elevated transcription of their genes. The p22phox protein was upregulated in the absence of increased transcription presumably reflecting stabilization resulting from binding to the elevated gp91phox. Thus, increased levels of gp91phox, p47phox and p22phox likely account for the interferon-γ mediated enhancement of dimethyl sulfoxide-induced Nox2 activity. In contrast, although interferon-γ alone also increased various phox proteins and their mRNAs, the pattern was very different to that seen with interferon-γ plus dimethyl sulfoxide. In particular, p47phox was not induced thus explaining the inability of interferon -γ alone to enhance Nox2 activity. Short application of interferon-γ to already differentiated cells failed to increase any phox proteins.

Conclusions

Our findings indicate that interferon-γ has complex effects on phox protein expression and that these are different in cells undergoing terminal differentiation. Understanding these changes may indicate additional therapeutic uses for this cytokine in human disorders.

Selective recapitulation of conserved and nonconserved regions of putative NOXA1 protein activation domain confers isoform-specific inhibition of Nox1 oxidase and attenuation of endothelial cell migration

Excessive vascular and colon epithelial reactive oxygen species production by NADPH oxidase isoform 1 (Nox1) has been implicated in a number of disease states, including hypertension, atherosclerosis, and neoplasia. A peptide that mimics a putative activation domain of the Nox1 activator subunit NOXA1 (NOXA1 docking sequence, also known as NoxA1ds) potently inhibited Nox1-derived superoxide anion (O2·-) production in a reconstituted Nox1 cell-free system, with no effect on Nox2-, Nox4-, Nox5-, or xanthine oxidase-derived reactive oxygen species production as measured by cytochrome c reduction, Amplex Red fluorescence, and electron paramagnetic resonance. The ability of NoxA1ds to cross the plasma membrane was tested by confocal microscopy in a human colon cancer cell line exclusively expressing Nox1 (HT-29) using FITC-labeled NoxA1ds. NoxA1ds significantly inhibited whole HT-29 carcinoma cell-derived O2·- generation. ELISA and fluorescence recovery after photobleaching experiments indicate that NoxA1ds, but not its scrambled control, binds Nox1. FRET experiments conducted using Nox1-YFP and NOXA1-CFP illustrate that NoxA1ds disrupts the binding interaction between Nox1 and NOXA1, whereas a control peptide did not. Moreover, hypoxia-induced human pulmonary artery endothelial cell O2·- production was completely inhibited by NoxA1ds. Human pulmonary artery endothelial cell migration under hypoxic conditions was also reduced by pretreatment with NoxA1ds. Our data indicate that a peptide recapitulating a putative activation subdomain of NOXA1 (NoxA1ds) is a highly efficacious and selective inhibitor of Nox1 activity and establishes a critical interaction site for Nox1-NOXA1 binding required for enzyme activation.

Phox activity of differentiated PLB-985 cells is enhanced, in an agonist specific manner, by the PLA2 activity of Prdx6-PLA2

Peroxiredoxin 6-phospholipase A(2) (Prdx6-PLA(2) ) is a bi-functional enzyme with peroxi-redoxin (Prdx) and phospholipase A(2) (PLA(2) ) activities. To investigate its impact on phagocyte NADPH oxidase (phox) activity in a neutrophil model, the protein was knocked down in PLB-985 cells using stable expression of a small hairpin RNA (shRNA) and phox activity was monitored after cell differentiation. The knockdown cells had reduced oxidase activity in response to stimulation with the formylated peptide fMLF, but the response to the phorbol ester PMA was unchanged. Reintroduction of shRNA-resistant Prdx6-PLA(2) into the knockdown cells by stable transfection with a Prdx6-PLA(2) expression plasmid restored the fMLF response, as did reintroduction of Prdx6-PLA(2) mutated in the Prdx active site; reintroduction of PLA(2) active site mutants, however, failed to restore the response. Thus, the PLA(2) activity of Prdx6-PLA(2) in intact cells mediates its ability to enhance phox activity in response to fMLF. In combination with previous publications by other groups, our work indicates that various PLA(2) isoforms can enhance oxidase activity but they are differentially important in different cell types and in the response to different agonists.

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.

Regulatory role of cytosolic phospholipase A2alpha in NADPH oxidase activity and in inducible nitric oxide synthase induction by aggregated Abeta1-42 in microglia

In Alzheimer's disease, extracellular deposits of amyloid beta(1-42) (Abeta(1-42)) may induce activation of microglial cells by releasing proinflammatory factors that contribute to the neurodegeneration process. Since the activation of cytosolic phospholipase A(2)alpha (cPLA(2)alpha) has been reported in inflammatory conditions, its role in primary rat microglial cell activated by aggregated Abeta(1-42) was elucidated. The results of the present study show that activation of microglia by 5 microM aggregated Abeta(1-42) (as evident by the amoeboid morphology and increased CD68 immunofluorescence reactivity) caused an immediate activation of cPLA(2)alpha, measured by its phosphorylated form and its specific activity, followed by a gradual elevation of its expression and activity during 24 h. Inhibition of cPLA(2)alpha expression and activity by the presence of 1 microM specific antisense resulted in a significant decrease in NADPH oxidase activity that releases superoxides, PGE(2) formation, iNOS expression, and NO production, indicating a major role for cPLA(2)alpha in the regulation of these inflammatory processes. NADPH oxidase activity, which is under cPLA(2)alpha regulation, was found to upregulate cPLA(2)alpha and COX-2 protein expression through the redox-sensitive NFkappaB activation as evident by its phosphorylation on Ser-536, resulting in increased PGE(2) formation. The secreted PGE(2) induced the synthesis of iNOS and the production of NO through the PKA-CREB pathway. Taken together, our results suggest that the response of cPLA(2)alpha to aggregated Abeta(1-42) is probably a key player in the oxidative stress present in AD, regulating potent oxidative agents: the production of superoxides by NADPH oxidase and NO formation by iNOS.

Phosphorylation of p22phox on threonine 147 enhances NADPH oxidase activity by promoting p47phox binding

NADPH oxidase comprises both cytosolic and membrane-bound subunits, which, when assembled and activated, initiate the transfer of electrons from NADPH to molecular oxygen to form superoxide. This activity, known as the respiratory burst, is extremely important in the innate immune response as indicated by the disorder chronic granulomatous disease. The regulation of this enzyme complex involves protein-protein and protein-lipid interactions as well as phosphorylation events. Previously, our laboratory demonstrated that the small membrane subunit of the oxidase complex, p22(phox), is phosphorylated in neutrophils and that its phosphorylation correlates with NADPH oxidase activity. In this study, we utilized site-directed mutagenesis in a Chinese hamster ovarian cell system to determine the phosphorylation sites within p22(phox). We also explored the mechanism by which p22(phox) phosphorylation affects NADPH oxidase activity. We found that mutation of threonine 147 to alanine inhibited superoxide production in vivo by more than 70%. This mutation also blocked phosphorylation of p22(phox) in vitro by both protein kinase C-alpha and -delta. Moreover, this mutation blocked the p22(phox)-p47(phox) interaction in intact cells. When phosphorylation was mimicked in vivo through mutation of Thr-147 to an aspartyl residue, NADPH oxidase activity was recovered, and the p22(phox)-p47(phox) interaction in the membrane was restored. Maturation of gp91(phox) was not affected by the alanine mutation, and phosphorylation of the cytosolic component p47(phox) still occurred. This study directly implicates threonine 147 of p22(phox) as a critical residue for efficient NADPH oxidase complex formation and resultant enzyme activity.

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

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

X-linked chronic granulomatous disease secondary to skewed X chromosome inactivation in a female with a novel CYBB mutation and late presentation

Chronic granulomatous disease (CGD) is characterized by defects in the superoxide producing enzyme NADPH oxidase causing phagocytes to improperly clear invading pathogens. Here we report findings of a late presenting 16-year-old female with X-linked CGD. The patient presented with community-acquired pneumonia, but symptoms persisted for 2 weeks during triple antimicrobial coverage. Cultures revealed Aspergillus fumigatus which was resolved through aggressive voriconazole treatment. Neutrophil studies revealed NADPH oxidase activity and flavocytochrome b(558) levels that were 4-8% of controls and suggested carrier status of the mother. We found a null mutation in the CYBB gene (c.252insAG) predicting an aberrant gp91(phox) protein (p.Cys85fsX23) in the heterozygous state. Methylation analysis demonstrated extremely skewed X chromosome inactivation favoring the maternally inherited defective gene. In conclusion, a novel mutation in the CYBB gene and an extremely skewed X-inactivation event resulted in the rare expression of the CGD phenotype in a carrier female.

Sequential binding of cytosolic Phox complex to phagosomes through regulated adaptor proteins: evaluation using the novel monomeric Kusabira-Green System and live imaging of phagocytosis

We engineered a method for detecting intramolecular and intermolecular phox protein interactions in cells by fluorescence microscopy using fusion proteins of complementary fragments of a coral fluorescent reporter protein (monomeric Kusabira-Green). We confirmed the efficacy of the monomeric Kusabira-Green system by showing that the PX and PB1 domains of p40phox interact in intact cells, which we suggested maintains this protein in an inactive closed conformation. Using this system, we also explored intramolecular interactions within p47phox and showed that the PX domain interacts with the autoinhibited tandem Src homology 3 domains maintained in contact with the autoinhibitory region, along with residues 341-360. Furthermore, we demonstrated sequential interactions of p67phox with phagosomes involving adaptor proteins, p47phox and p40phox, during FcgammaR-mediated phagocytosis. Although p67phox is not targeted to phagosomes by itself, p47phox functions as an adaptor for the ternary complex (p47phox-p67phox-p40phox) in early stages of phagocytosis before phagosome closure, while p40phox functions in later stages after phagosomal closure. Interestingly, a mutated "open" form of p40phox linked p47phox to closed phagosomes and prolonged p47phox and p67phox retention on phagosomes. These results indicate that binding of the ternary complex to phagosomes can be temporally regulated by switching between adaptor proteins that have PX domains with distinct lipid-binding specificities.

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.

The K-562 cell model for analysis of neutrophil NADPH oxidase function

Polymorphonuclear neutrophils (PMN) have a remarkable capacity for generation of large amounts of reactive oxygen species in response to a variety of infectious or inflammatory stimuli, a process known as the respiratory burst that involves activation of a multicomponent NADPH oxidase. Given their short life span, PMN are not amenable to most molecular biology methods for studying activation of this oxidant-generating system. We have explored a variety of methods for introduction of components of the phagocytic oxidase (phox system) into the promyelocytic erythroleukemia cell line, K-562. Here, we describe a series of cloned K-562 cell lines that were retrovirally transduced for stable production of one or more essential components of the phagocytic oxidase (phox) complex. We outline methods for the use of these transfectable cells for investigating structure, function, and signaling requirements for assembly and activation of the phox system. These versatile lines can be used to examine effects of genetic polymorphisms or mutations in phox components associated with chronic granulomatous disease, to serve as a system for testing gene therapy vectors designed to correct the defective oxidase, to study cross-functioning with recently described phox component homologs, or to explore signaling components involved in regulation of the respiratory burst.

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.

Involvement of Rac1 in activation of multicomponent Nox1- and Nox3-based NADPH oxidases

Several Nox family NADPH oxidases function as multicomponent enzyme systems. We explored determinants of assembly of the multicomponent oxidases Nox1 and Nox3 and examined the involvement of Rac1 in their regulation. Both enzymes are supported by p47phox and p67phox or homologous regulators called Noxo1 and Noxa1, although Nox3 is less dependent on these cofactors for activity. Plasma membrane targeting of Noxa1 depends on Noxo1, through tail-to-tail interactions between these proteins. Noxa1 can support Nox1 without Noxo1, when targeted to the plasma membrane by fusing membrane-binding sequences from Rac1 (amino acids 183 to 192) to the C terminus of Noxa1. However, membrane targeting of Noxa1 is not sufficient for activation of Nox1. Both the Noxo1-independent and -dependent Nox1 systems involve Rac1, since they are affected by Rac1 mutants or Noxa1 mutants defective in Rac binding or short interfering RNA-mediated Rac1 silencing. Nox1 or Nox3 expression promotes p22phox transport to the plasma membrane, and both oxidases are inhibited by mutations in the p22phox binding sites (SH3 domains) of the Nox organizers (p47phox or Noxo1). Regulation of Nox3 by Rac1 was also evident from the effects of mutant Rac1 or mutant Nox3 activators (p67phox or Noxa1) or Rac1 silencing. In the absence of Nox organizers, the Nox activators (p67phox or Noxa1) colocalize with Rac1 within ruffling membranes, independently of their ability to bind Rac1. Thus, Rac1 regulates both oxidases through the Nox activators, although it does not appear to direct the subcellular localization of these activators.

Identification of an actin-binding site in p47phoxan organizer protein of NADPH oxidase

Actin has been reported to enhance the superoxide-generating activity of neutrophil NADPH oxidase in a cell-free system and to interact with p47phox, a regulatory subunit of the oxidase. In the present study, we searched for an actin-binding site in p47phox by far-western blotting and blot-binding assays using truncated forms of p47phox. The amino-acid sequence 319-337 was identified as an actin-binding site, and a synthetic peptide of this sequence bound to actin. The sequence shows no homology to other known actin-binding motifs. It is located in the autoinhibitory region of p47phox and includes Ser-328, a phosphorylation site essential for unmasking. Although a phosphorylation-mimetic p47phox mutant bound to actin with a lower affinity than the wild type, the same mutant interacted with filamentous actin more efficiently than the wild type. A mutant peptide p47phox (319-337, Ser328Glu) bound to filamentous actin more tightly than to monomer actin. These results suggest that p47phox moves to cortical actin when it becomes unmasked in the cells.

Oxidase activity in cord blood neutrophils: a balance between increased membrane associated cytochrome b558 and deficient cytosolic components

INTRODUCTION

Newborn infants are prone to develop life-threatening pyogenic infections. Alterations in the function of neonatal phagocytes, including the activity of the neutrophil NADPH oxidase, have been suggested as one cause of increased susceptibility to such infections.

METHODS

In the present study, comprehensive analysis of NADPH oxidase enzyme system was performed in cord blood neutrophils from vaginally and cesarean section (CS) delivered, healthy, full-term infants.

RESULTS

Superoxide anion (O(2) (-)) production by intact neutrophils from cord blood in response to soluble stimuli was equal to or increased compared to levels generated by cells from adult controls. In the sodium dodecyl sulfate (SDS) cell-free system, cytosol and plasma membrane from cord blood neutrophils generated O(2) (-) at comparable rates to subcellular fractions from healthy adults. However, mixing experiments demonstrated higher O(2) (-) generation with combination of cytosol from adult controls and membrane from cord blood neutrophils and lower O(2) (-) production with combination of cytosol from cord blood neutrophils and membrane from adult controls. Kinetic parameters for cord blood specimens were no different from those obtained for fractions from adult controls. Quantitative analysis of cytosolic components showed moderately reduced amount of p40-phox, p47-phox, and p67-phox in neutrophils from cord blood. In contrast, cytochrome b(558) content of plasma membrane of cord blood neutrophils was approximately 2-fold higher compared to adult controls.

CONCLUSION

The normal to increased respiratory burst of intact cord blood neutrophils is the result of alterations to oxidase components: increased content of cytochrome b(558) in the plasma membrane and decreased levels of cytosolic components p47-phox, p67-phox, and p40-phox.

Calcium-sensing soluble adenylyl cyclase mediates TNF signal transduction in human neutrophils

Through chemical screening, we identified a pyrazolone that reversibly blocked the activation of phagocyte oxidase (phox) in human neutrophils in response to tumor necrosis factor (TNF) or formylated peptide. The pyrazolone spared activation of phox by phorbol ester or bacteria, bacterial killing, TNF-induced granule exocytosis and phox assembly, and endothelial transmigration. We traced the pyrazolone's mechanism of action to inhibition of TNF-induced intracellular Ca2+ elevations, and identified a nontransmembrane ("soluble") adenylyl cyclase (sAC) in neutrophils as a Ca2+-sensing source of cAMP. A sAC inhibitor mimicked the pyrazolone's effect on phox. Both compounds blocked TNF-induced activation of Rap1A, a phox-associated guanosine triphosphatase that is regulated by cAMP. Thus, TNF turns on phox through a Ca2+-triggered, sAC-dependent process that may involve activation of Rap1A. This pathway may offer opportunities to suppress oxidative damage during inflammation without blocking antimicrobial function.

Src-mediated Tyrosine Phosphorylation of p47 in Hyperoxia-induced Activation of NADPH Oxidase and Generation of Reactive Oxygen Species in Lung Endothelial Cells

Superoxide (O(2)(-)) production by nonphagocytes, similar to phagocytes, is by activation of the NADPH oxidase multicomponent system. Although activation of neutrophil NADPH oxidase involves extensive serine phosphorylation of p47(phox), the role of tyrosine phosphorylation of p47(phox) in NADPH oxidase-dependent O(2)(-) production is unclear. We have shown recently that hyperoxia-induced NADPH oxidase activation in human pulmonary artery endothelial cells (HPAECs) is regulated by mitogen-activated protein kinase signal transduction. Here we provided evidence on the role of nonreceptor tyrosine kinase, Src, in hyperoxia-induced tyrosine phosphorylation of p47(phox) and NADPH oxidase activation in HPAECs. Exposure of HPAECs to hyperoxia for 1 h resulted in increased O(2)(-) and reactive oxygen species (ROS) production and enhanced tyrosine phosphorylation of Src as determined by Western blotting with phospho-Src antibodies. Pretreatment of HPAECs with the Src kinase inhibitor PP2 (1 mum) or transient expression of a dominant-negative mutant of Src attenuated hyperoxia-induced tyrosine phosphorylation of Src and ROS production. Furthermore, exposure of cells to hyperoxia enhanced tyrosine phosphorylation of p47(phox) and its translocation to cell peripheries that were attenuated by PP2. In vitro, Src phosphorylated recombinant p47(phox) in a time-dependent manner. Src immunoprecipitates of cell lysates from control cells revealed the presence of immunodetectable p47(phox) and p67(phox), suggesting the association of oxidase components with Src under basal conditions. Moreover, exposure of HPAECs to hyperoxia for 1 h enhanced the association of p47(phox), but not p67(phox), with Src. These results indicated that Src-dependent tyrosine phosphorylation of p47(phox) regulates hyperoxia-induced NADPH oxidase activation and ROS production in HPAECs.

Fungal metabolite gliotoxin targets flavocytochrome b558 in the activation of the human neutrophil NADPH oxidase

Fungal gliotoxin (GT) is a potent inhibitor of the O(2)(-)-generating NADPH oxidase of neutrophils. We reported that GT-treated neutrophils fail to phosphorylate p47(phox), a step essential for the enzyme activation, because GT prevents the colocalization of protein kinase C betaII with p47(phox) on the membrane. However, it remains unanswered whether GT directly affects any of NADPH oxidase components. Here, we examine the effect of GT on the NADPH oxidase components in the cell-free activation assay. The O(2)(-)-generating ability of membranes obtained from GT-treated neutrophils is 40.0 and 30.6% lower, respectively, than the untreated counterparts when assayed with two distinct electron acceptors, suggesting that flavocytochrome b(558) is affected in cells by GT. In contrast, the corresponding cytosol remains competent for activation. Next, GT addition in vitro to the assay consisting of flavocytochrome b(558) and cytosolic components (native cytosol or recombinant p67(phox), p47(phox), and Rac2) causes a striking inhibition (50% inhibitory concentration = 3.3 microM) when done prior to the stimulation with myristic acid. NADPH consumption is also prevented by GT, but the in vitro assembly of p67(phox), p47(phox), and Rac2 with flavocytochrome b(558) is normal. Posterior addition of GT to the activated enzyme is ineffective. The separate treatment of membranes with GT also causes a marked loss of flavocytochrome b(558)'s ability to reconstitute O(2)(-) generation, supporting the conclusion at the cellular level. The flavocytochrome b(558) heme spectrum of the GT-treated membranes stays, however, unchanged, showing that hemes remain intact. These results suggest that GT directly harms site(s) crucial for electron transport in flavocytochrome b(558), which is accessible only before oxidase activation.

Superoxide generation by Nox1 in guinea pig gastric mucosal cells involves a component with p67(phox)-ability

Nox1, a homologue of gp91(phox) subunit of the phagocyte NADPH oxidase, is responsible for spontaneous superoxide (O(2)(-)) generation in guinea pig gastric mucosal cells (GMC), but involvement of regulatory components (p67(phox), p47(phox), and Rac) which are essential in phagocytes remains unknown. Here, we aimed to figure out how Nox1 of GMC achieves an active oxidase status. GMC in primary culture show low O(2)(-) generation but acquire a 9-fold higher activity when cultured with Helicobacter pylori lipopolysaccharide (LPS), in correlation with a far increased Nox1 expression. Investigation into the O(2)(-)-generating ability of LPS-induced Nox1 in cell-free reconstitution assays showed that: 1) Nox1 is unable to generate O(2)(-) per se; 2) the combination of Nox1 with GMC cytosol is insufficient for a significant O(2)(-) generation; 3) the combination with neutrophil cytosol enables Nox1 to act like gp91(phox), i.e., to produce O(2)(-) appreciably in response to myristate stimulation; 4) Nox1 prefers NADPH to NADH under the in vitro assay with neutrophil cytosol plus myristate (K(m)=10.4 microM); 5) substitution of neutrophil cytosol by a set of recombinant cytosolic components (rp67(phox), rp47(phox), Rac2) is, however, ineffective and still requires GMC cytosol. Thus, Nox1 probably requires an additional cytosolic factor(s). In contrast, GMC cytosol enables cytochrome b(558) to generate plenty of O(2)(-), on condition that rp47(phox) is added. This result suggests that GMC cytosol contains a component with p67(phox)-ability, and also Rac, but lacks p47(phox). These data indicate that GMC Nox1 requires at least a p67(phox) counterpart and Rac to acquire NADPH oxidase activity.

Activation of the phagocyte NADPH oxidase by Rac Guanine nucleotide exchange factors in conjunction with ATP and nucleoside diphosphate kinase

Activation of the phagocyte NADPH oxidase is the consequence of the assembly of membranal cytochrome b559 with the cytosolic components p47phox, p67phox, and the GTPase Rac and is mimicked by a cell-free system comprising these components and an activator. We designed a variant of this system, consisting of membranes, p67phox) prenylated Rac1-GDP, and the Rac-specific guanine nucleotide exchange factor (GEF) Trio, in which oxidase activation is induced in the absence of an activator and p47phox. We now show that: 1) Trio and another Rac GEF (Tiam1) act by inducing GDP to GTP exchange on prenylated Rac1-GDP and that our earlier assertion that activation is GTP-independent is explained by contamination of p67phox preparations with GTP and/or ATP. 2) Oxidase activation by Rac GEFs is supported not only by GTP but also by ATP. 3) Non-hydrolysable GTP analogs are active, whereas ATP analogs, incapable of gamma-phosphoryl transfer, are inactive. 4) The ability of ATP to support GEF-induced oxidase activation is explained by ATP serving as a gamma-phosphoryl donor for a membrane-localized nucleoside diphosphate kinase (NDPK), converting GDP to GTP. 5) The existence of a NDPK in macrophage membranes is proven by functional, enzymatic, and immunologic criteria. 6) NDPK acts on free GDP, and the newly formed GTP is bound again to Rac. 7) Free GDP is derived exclusively by dissociation from prenylated Rac1-GDP, mediated by GEF. NDPK and GEF appear to be functionally linked in the sense that the availability of GDP, serving as substrate for NDPK, is dependent on the level of activity of GEF.

Stimulators of AMP-activated protein kinase inhibit the respiratory burst in human neutrophils

In the present study, we have examined the potential ability of 5'-AMP-activated protein kinase (AMPK) to modulate NADPH oxidase activity in human neutrophils. AMPK activated with either 5'-aminoimidazole-4-carboxamide ribonucleoside (AICAR) or with 5'-AMP significantly attenuated both phorbol 12-myristate 13-acetate (PMA) and formyl methionyl leucyl phenylalanine-stimulated superoxide anion O2- release by human neutrophils, consistently with a reduced translocation to the cell membrane and phosphorylation of a cytosolic component of NADPH oxidase, namely p47phox. AMPK was found to be present in human neutrophils and to become phosphorylated in response to either AICAR or other stimulators of its enzyme activity. Furthermore, AICAR also strongly reduced PMA-dependent H2O2 release, and induced the phosphorylation of c-jun N-terminal kinase 1 (p46), p38 mitogen-activated protein kinase and extracellular signal-regulated kinase. Present data demonstrate for the first time that the activation of AMPK, in states of low cellular energy charge (such as under high levels of 5'-AMP) or other signals, could be a factor contributing to reduce the host defense mechanisms.

Unique targeting of cytosolic phospholipase A2 to plasma membranes mediated by the NADPH oxidase in phagocytes

Cytosolic phospholipase A2 (cPLA2)-generated arachidonic acid (AA) has been shown to be an essential requirement for the activation of NADPH oxidase, in addition to its being the major enzyme involved in the formation of eicosanoid at the nuclear membranes. The mechanism by which cPLA2 regulates NADPH oxidase activity is not known, particularly since the NADPH oxidase complex is localized in the plasma membranes of stimulated cells. The present study is the first to demonstrate that upon stimulation cPLA2 is transiently recruited to the plasma membranes by a functional NADPH oxidase in neutrophils and in granulocyte-like PLB-985 cells. Coimmunoprecipitation experiments and double labeling immunofluorescence analysis demonstrated the unique colocalization of cPLA2 and the NADPH oxidase in plasma membranes of stimulated cells, in correlation with the kinetic burst of superoxide production. A specific affinity in vitro binding was detected between GST-p47phox or GST-p67phox and cPLA2 in lysates of stimulated cells. The association between these two enzymes provides the molecular basis for AA released by cPLA2 to activate the assembled NADPH oxidase. The ability of cPLA2 to regulate two different functions in the same cells (superoxide generation and eicosanoid production) is achieved by a novel dual subcellular localization of cPLA2 to different targets.

Oxidative stress is a critical mediator of the angiotensin II signal in human neutrophils: involvement of mitogen-activated protein kinase, calcineurin, and the transcription factor NF-kappaB

Neutrophils are mobilized to the vascular wall during vessel inflammation. Published data are conflicting on phagocytic nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase activation during the hypertensive state, and the capacity of angiotensin II (Ang II) to modulate the intracellular redox status has not been analyzed in neutrophils. We here describe that Ang II highly stimulates endogenous and extracellular O2- production in these cells, consistent with the translocation to the cell membrane of the cytosolic components of NADPH oxidase, p47phox, and p67phox. The Ang II-dependent O2- production was suppressed by specific inhibitors of AT1 receptors, of the p38MAPK and ERK1/2 pathways, and of flavin oxidases. Furthermore, Ang II induced a robust phosphorylation of p38MAPK, ERK1/2, and JNK1/2 (particularly JNK2), which was hindered by inhibitors of NADPH oxidase, tyrosine kinases, and ROS scavengers. Ang II increased cytosolic Ca2+ levels-released mainly from calcium stores-enhanced the synthesis de novo and activity of calcineurin, and stimulated the DNA-binding activity of the transcription factor NF-kappaB in cultured human neutrophils. Present data demonstrate for the first time a stimulatory role of Ang II in the activation of phagocytic cells, underscore the relevant role of ROS as mediators in this process, and uncover a variety of signaling pathways by which Ang II operates in human neutrophils.

Expression of a p67(phox) homolog in Caco-2 cells giving O(2)(-)-reconstituting ability to cytochrome b(558) together with recombinant p47(phox)

Human normal and transformed (Caco-2) colon tissues as well as guinea pig gastric mucosal cells express Nox1, which is a homolog of the phagocyte NADPH oxidase subunit, gp91(phox) of membrane-bound cytochrome b(558). It was reported that Nox1-transfection to NIH 3T3 cells could provide O(2)(-)-generating ability, independently of regulatory cytosolic factors (Rac2, p67(phox), and p47(phox)) that are obligatory in the phagocyte oxidase system. Here, we detected and sequenced a p67(phox) homolog in Caco-2 almost identical to the neutrophil sequence, except for three nucleotide substitutions, two of which changed lysines 181 and 328 to arginines. Investigation of its ability to support O(2)(-)-generation in cell-free reconstitution experiments combining with neutrophil cytochrome b(558) showed O(2)(-)-generation, provided that recombinant p47(phox) was added. This result demonstrates that the intrinsic p67(phox) homolog of Caco-2 was able to function as a phagocyte p67(phox) for cytochrome b(558). The requirement of p47(phox) addition suggested that this component was absent in Caco-2 cells. Caco-2 membranes, used as a source of Nox1 in place of cytochrome b(558), did not show significant O(2)(-)-generation, which was mainly explained by their very little Nox1 expression.

Increased free radical production in hypertension due to increased expression of the NADPH oxidase subunit p22(phox) in lymphoblast cell lines

OBJECTIVES

To confirm increased production of reactive oxygen species (ROS) in hypertension, to demonstrate the source of ROS and to analyse NADPH oxidase subcomponent expression in hypertension.

DESIGN

A lymphoblast model was used, as this has previously been used in the study of hypertension and of NADPH oxidase. Chemiluminescence (CL) was chosen to assay ROS production, as it is simple and sensitive.

METHODS

Lymphocytes from 12 hypertensive patients (HT), and 12 age- and sex-matched normotensive (NT) subjects, were immortalized. Luminol, isoluminol and Cypridina luciferin analogue (CLA) CL were used to assay ROS production. NADPH oxidase subunits were measured by Western blot analysis.

RESULTS

Stimulation with 50 micromol/l arachidonic acid (AA) resulted in increased ROS production in HT cell lines with luminol, CLA and isoluminol CL. Stimulation with 500 nmol/l 12-O-tetradecanoylphorbol-13-acetate (TPA) produced a detectable increase in HT ROS production with luminol and with CLA, whereas there was no significant difference with isoluminol. The ROS production was abolished by diphenyleneiodonium chloride (DPI) but not by rotenone, indicating that a non-mitochondrial flavoprotein such as NADPH oxidase is the source of ROS. Analysis of NADPH oxidase subcomponents revealed an increase in p22(phox) in HT subjects.

CONCLUSIONS

We have shown there is increased ROS production in lymphoblasts derived from hypertensive subjects, probably originating from NADPH oxidase. As the ROS production persists in transformed cells, this suggests a genetic predisposition to increased ROS production. Increased expression of p22(phox) in HT lymphoblasts may account for some of the increased ROS.

Mapping of functional domains in the p22(phox) subunit of flavocytochrome b(559) participating in the assembly of the NADPH oxidase complex by "peptide walking"

The superoxide-generating NADPH oxidase complex of phagocytes consists of a membranal heterodimeric flavocytochrome (cytochrome b(559)), composed of gp91(phox) and p22(phox) subunits, and four cytosolic proteins, p47(phox), p67(phox), p40(phox), and the small GTPase Rac (1 or 2). All redox stations involved in electron transport from NADPH to oxygen are located in gp91(phox). NADPH oxidase activation is the consequence of assembly of cytochrome b(559) with cytosolic proteins, a process reproducible in a cell-free system, consisting of phagocyte membranes, and recombinant cytosolic components, activated by an anionic amphiphile. p22(phox) is believed to act as a linker between the cytosolic components and gp91(phox). We applied "peptide walking" to mapping of domains in p22(phox) participating in NADPH oxidase assembly. Ninety one synthetic overlapping pentadecapeptides, spanning the p22(phox) sequence, were tested for the ability to inhibit NADPH oxidase activation in the cell-free system and to bind individual cytosolic NADPH oxidase components. We conclude the following. 1) The p22(phox) subunit of cytochrome b(559) serves as an anchor for both p47(phox) and p67(phox). 2) p47(phox) binds not only to the proline-rich region, located at residues 151-160 in the cytosolic C terminus of p22(phox), but also to a domain (residues 51-63) located on a loop exposed to the cytosol. 3) p67(phox) shares with p47(phox) the ability to bind to the proline-rich region (residues 151-160) and also binds to two additional domains, in the cytosolic loop (residues 81-91) and at the start of the cytosolic tail (residues 111-115). 4) The binding affinity of p67(phox) for p22(phox) peptides is lower than that of p47(phox). 5) Binding of both p47(phox) and p67(phox) to proline-rich p22(phox) peptides occurs in the absence of an anionic amphiphile. A revised membrane topology model of p22(phox) is proposed, the core of which is the presence of a functionally important cytosolic loop (residues 51-91).

Boundary sequences of the NADPH oxidase p67(phox) C-terminal SH3 domain play on its specificity

SH3 domains are found in many signal transduction proteins where they mediate protein-protein binding by recognizing specific peptides rich in proline. Based on the analysis of sequence alignment data, the NADPH oxidase p67(phox) C-terminal SH3 domain possesses a typical compact beta-barrel consisting of five beta-strands arranged in two antiparallel beta-sheets of three and two beta-strands. Multiple amino acid substitutions were made at beta e and its flanking residues to determine the role of the boundary sequences in binding activity and conformational specificity of the domain. Analysis of amino acid P55 indicated that all mutants were completely abolished in their binding activities. The substitution of F58 with Y58 showed no effect of the binding, whereas substitution with stop codon abolished activity. Furthermore, when amino acid V59 was substituted with stop codon, activity was also completely abolished. Substitution of E60 with stop codon showed no effect of binding. Moreover, our data show that V59 particularly could not be replaced by Leu. Taken together, these data suggest that V59 may not only contribute the exact boundary site but also play on the specificity for protein-protein interactions in phagocyte NADPH oxidase.

Fused p47phox and p67phox truncations efficiently reconstitute NADPH oxidase with higher activity and stability than the individual components

Activation of the neutrophil NADPH oxidase occurs via assembly of the cytosolic regulatory proteins p47(phox), p67(phox), and Rac with the membrane-associated flavocytochrome b(558). Following cell-free activation, enzymatic activity is highly labile (Tamura, M., Takeshita, M., Curnutte, J. T., Uhlinger, D. J., and Lambeth, J. D. (1992) J. Biol. Chem. 267, 7529-7538). To try to stabilize the activity and investigate the nature of the complex, fusion proteins between p47N-(1-286) and p67N-(1-210) were constructed. In a cell-free system, a fusion protein, p67N-p47N, had an 8-fold higher efficiency and produced a higher activity than the individual proteins, and also resulted in an 8-fold improved efficiency for Rac and a lowered K(m) for NADPH. O(2) generating activity was remarkably stabilized by using p67N-p47N. The cytosolic proteins fused in the opposite orientation, p47N-p67N, showed similar activity and stability as individual proteins, but with a 4-fold improved efficiency compared with the individual cytosolic factors. In the system efficiency for Rac and affinity for NADPH were also higher than those with the nonfused components. Interestingly, the p67N-p47N showed nearly full activation in the absence of an anionic amphifile in a cell-free system containing cytochrome b(558) relipidated with phosphatidylinositol- or phosphatidylserine-enriched phospholipid mixtures. From the results we consider multiple roles of anionic amphifiles in a cell-free activation, which could be substituted by our system. The fact that a fusion produces a more stable complex indicates that interactions among components determine the longevity of the complex. Based on the findings we propose a model for the topology among p47N, p67N, and cytochrome b(558) in the active complex.

Structure-function relationship in the interaction of mastoparan analogs with neutrophil NADPH oxidase

Mastoparan, an amphiphilic cationic tetradecapeptide was previously shown to block activation of the NADPH oxidase in the cell-free system presumably by association with a cytosolic component/s of the enzyme. Blockade of oxidase activation was now demonstrated in the semirecombinant NADPH oxidase system. The structural basis of the inhibitory effect of MP on oxidase assembly was explored employing a variety of truncated and specifically substituted synthetic peptide analogs. The data indicated that an alpha helical fold, positive net charge, hydrophobicity and amphiphilicity were essential for the inhibitory potency and that peptide analogs below eleven residues were inactive. To identify the MP-binding oxidase subunit three different binding assays were carried out utilizing free or immobilized recombinant p47-phox, p67-phox, p40-phox and Rac1 in conjunction with immobilized MP or soluble (125)I-tyr-MP, respectively. The data implicated p67-phox as the main MP-binding component. The binding site on the p67-phox was localized to the 1-238 aminoterminal fragment of the molecule. NADPH oxidase activation supported by this fragment was inhibitable by MP. In addition, SH3 domains of p47-phox and p40-phox and the carboxyterminal SH3 domain of p67-phox exhibited a low affinity towards MP.

Phosphatidic acid regulates tyrosine phosphorylating activity in human neutrophils: enhancement of Fgr activity

In human neutrophils, the activation of phospholipase D and the Tyr phosphorylation of proteins are early signaling events upon cell stimulation. We found that the pretreatment of neutrophils with ethanol (0.8%) or 1-butanol (0.3%), which results in the accumulation of phosphatidylalcohol at the expense of phosphatidic acid (PA), decreased the phorbol myristate acetate-stimulated Tyr phosphorylation of endogenous proteins (42, 115 kDa). When neutrophil cytosol was incubated in the presence or absence of PA, these and other endogenous proteins became Tyr-phosphorylated in a PA-dependent manner. In contrast, phosphatidylalcohols exhibited only 25% (phosphatidylethanol) or 5% (phosphatidylbutanol) of the ability of PA to stimulate Tyr phosphorylation in the cell-free assay. Similarly, other phospholipids (phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, phosphatidylserine, phosphatidylinositol, polyphosphoinositides, and sphingosine 1-phosphate) showed little ability to stimulate Tyr phosphorylation. These data suggest that PA can function as an intracellular regulator of Tyr phosphorylating activity. Gel filtration chromatography of leukocyte cytosol revealed a peak of PA-dependent Tyr phosphorylating activity distinct from a previously described PA-dependent phosphorylating activity (Waite, K. A., Wallin, R., Qualliotine-Mann, D., and McPhail, L. C. (1997) J. Biol. Chem. 272, 15569-15578). Among the protein Tyr kinases expressed in neutrophils, only Fgr eluted exclusively in the peak of PA-dependent Tyr phosphorylating activity. Importantly, Fgr isolated from unstimulated neutrophil lysates showed increased activity in the presence of PA but not phosphatidylbutanol. Moreover, the pretreatment of neutrophils with 1-butanol decreased Fgr activity in cells stimulated with formyl-methionyl-leucyl phenylalanine plus dihydrocytochalasin B. Together, these results suggest a new second messenger role for PA in the regulation of Tyr phosphorylation.

Phosphatidic acid and diacylglycerol directly activate NADPH oxidase by interacting with enzyme components

The enzyme NADPH oxidase is regulated by phospholipase D in intact neutrophils and is activated by phosphatidic acid (PA) plus diacylglycerol (DG) in cell-free systems. We showed previously that cell-free NADPH oxidase activation by these lipids involves both protein kinase-dependent and -independent pathways. Here we demonstrate that only the protein kinase-independent pathway is operative in a cell-free system of purified and recombinant NADPH oxidase components. Activation by PA + DG was ATP-independent and unaffected by the protein kinase inhibitor staurosporine, indicating the lack of protein kinase involvement. Both PA and DG were required for optimal activation to occur. The drug reduced activation of NADPH oxidase by either arachidonic acid or PA + DG, with IC(50) values of 46 and 25 microm, respectively. The optimal concentration of arachidonic acid or PA + DG for oxidase activation was shifted to the right with, indicating interference of the drug with the interaction of lipid activators and enzyme components. inhibited the lipid-induced aggregation/sedimentation of oxidase components p47(phox) and p67(phox), suggesting a disruption of the lipid-mediated assembly process. The direct effects of on NADPH oxidase activation complicate its use as a "specific" inhibitor of DG kinase. We conclude that the protein kinase-independent pathway of NADPH oxidase activation by PA and DG involves direct interaction with NADPH oxidase components. Thus, NADPH oxidase proteins are functional targets for these lipid messengers in the neutrophil.

Genetic requirement of p47phox for superoxide production by murine microglia

An NADPH oxidase is thought to function in microglial cells of the central nervous system. These conclusions are based on pharmacological and immunochemical evidence, although these approaches are indirect and raise issues of specificity. For example, diphenyleneiodonium inhibits a variety of flavoenzymes, including xanthine oxidase, NADH dehydrogenase, and NADPH oxidase. Here, we provide genetic evidence that p47phox, an essential component of the phagocyte NADPH oxidase, is required for superoxide anion release from microglia. Microglia derived from newborn wild-type mice, but not from newborn p47phox-deficient (knockout; -/-) mice, produced superoxide after stimulation by opsonized zymosan or phorbol myristate acetate. Endogenous p47phox was detected only in wild-type microglia, consistent with selective superoxide production in these cells. Superoxide release was restored in p47phox-deficient microglia that were retrovirally transduced with human p47phox cDNA. Similar kinetics of superoxide generation were observed, consistent with the same enzyme functioning in wild-type and restored microglia. Immuno-detection of p47phox in transduced cells confirmed that restoration of superoxide release correlated with production of recombinant protein. These data provide genetic proof that p47phox is necessary for superoxide release by microglial cells and indicate that a system related to the phagocyte oxidase is active in these cells.

Direct interaction of actin with p47(phox) of neutrophil NADPH oxidase

The cell-free activation of human neutrophil NADPH oxidase is enhanced by actin, and actin filaments formed during activation are suggested to stabilize the oxidase. In an attempt to elucidate the mechanism, we examined the protein-protein interactions between actin and cytosolic components of the oxidase. Far-Western blotting using recombinant phox proteins showed that both alpha- and beta-actin interacted with p47(phox) and rac1, and weakly with rac2. A deletion mutant of p47(phox) proved that its C-terminal region was essential for the interaction. The dissociation constant (K(d)) for interaction between actin and p47(phox) was estimated to be 0.45 microM by surface plasmon resonance, and that between actin and rac1 or rac2 was 1.7 or 4.6 microM, respectively. Far-Western blotting using cytosol as a target showed an interaction between actin and endogenous p47(phox) and rac proteins. These results suggest that actin can directly interact with p47(phox) and possibly with rac in the cells.

Human neutrophil immunodeficiency syndrome is associated with an inhibitory Rac2 mutation

A 5-week-old male infant presented with severe bacterial infections and poor wound healing, suggesting a neutrophil defect. Neutrophils from this patient exhibited decreased chemotaxis, polarization, azurophilic granule secretion, and superoxide anion (O(2)(-)) production but had normal expression and up-regulation of CD11b. Rac2, which constitutes >96% of the Rac in neutrophils, is a member of the Rho family of GTPases that regulates the actin cytoskeleton and O(2)(-) production. Western blot analysis of lysates from patient neutrophils demonstrated decreased levels of Rac2 protein. Addition of recombinant Rac to extracts of the patient neutrophils reconstituted O(2)(-) production in an in vitro assay system. Molecular analysis identified a point mutation in one allele of the Rac2 gene resulting in the substitution of Asp57 by an Asn (Rac2(D57N)). Asp57 is invariant in all defined GTP-binding proteins. Rac2(D57N) binds GDP but not GTP and inhibits oxidase activation and O(2)(-) production in vitro. These data represent the description of an inhibitory mutation in a member of the Rho family of GTPases associated with a human immunodeficiency syndrome.

Protein Kinase C (PKC) Isoforms Translocate to Triton-Insoluble Fractions in Stimulated Human Neutrophils: Correlation of Conventional PKC with Activation of NADPH Oxidase

The responses of human neutrophils (PMN) involve reorganization and phosphorylation of cytoskeletal components. We investigated the translocation of protein kinase C (PKC) isoforms to PMN cytoskeletal (Triton-insoluble) fractions, in conjunction with activation of the respiratory burst enzyme NADPH oxidase. In resting PMN, PKC-δ (29%) and small amounts of PKC-α (0.6%), but not PKC-βII, were present in cytoskeletal fractions. Upon stimulation with the PKC agonist PMA, the levels of PKC-α, PKC-βII, and PKC-δ increased in the cytoskeletal fraction, concomitant with a decrease in the noncytoskeletal (Triton-soluble) fractions. PKC-δ maximally associated with cytoskeletal fractions at 160 nM PMA and then declined, while PKC-α and PKC-βII plateaued at 300 nM PMA. Translocation of PKC-δ was maximal by 2 min and sustained for at least 10 min. Translocation of PKC-α and PKC-βII was biphasic, plateauing at 2–3 min and then increasing up to 10 min. Under maximal stimulation conditions, PKC isoforms were entirely cytoskeletal associated. Translocation of the NADPH oxidase component p47phox to the cytoskeletal fraction correlated with translocation of PKC-α and PKC-βII, but not with translocation of PKC-δ. Oxidase activity in cytoskeletal fractions paralleled translocation of PKC-α, PKC-βII, and p47phox. Stimulation with 1,2-dioctanoylglycerol resulted in little translocation of PKC isoforms or p47phox, and in minimal oxidase activity. We conclude that conventional PKC isoforms (PKC-α and/or PKC-βII) may regulate PMA-stimulated cytoskeletal association and activation of NADPH oxidase. PKC-δ may modulate other PMN responses that involve cytoskeletal components.

Rac1 disrupts p67phox/p40phox binding: a novel role for Rac in NADPH oxidase activation

Phagocytic cells possess a tightly regulated multicomponent enzyme complex, the NADPH oxidase, which produces superoxide, a reactive oxygen molecule that is an essential component of host defense against infection. Upon stimulation, a functional NADPH oxidase is assembled when the cytosolic proteins, Rac, p67phox, p47phox, and possibly p40phox, associate with the gp91phox and p22phox transmembrane proteins. Rac is a GTPase that in the GTP-bound state binds p67phox to activate NADPH oxidase. The function of p40phox is not known; it is believed to have a regulatory function in sequestering p67phox and p47phox in a cytosolic complex. We investigated binding interactions between p40phox, p67phox, and Rac and found that Rac1-GTP displaced p67phox bound to p40phox. In contrast, Cdc42, a GTPase homologous to Rac, did not displace p67phox from p40phox. A synthetic peptide corresponding to p67phox amino acids 170-199, a region identified previously as a Rac binding domain, significantly reduced the ability of Rac1-GTP to disrupt p67phox/p40phox binding. We hypothesize that Rac-GTP binds the p67phox N-terminal domain encompassing amino acids 170-199 that transmits a conformational change which causes p40phox to dissociate from its binding site in the p67phox C-terminus.

A novel protein kinase target for the lipid second messenger phosphatidic acid

Activation of phospholipase D occurs in response to a wide variety of hormones, growth factors, and other extracellular signals. The initial product of phospholipase D, phosphatidic acid (PA), is thought to serve a signaling function, but the intracellular targets for this lipid second messenger are not clearly identified. The production of PA in human neutrophils is closely correlated with the activation of NADPH oxidase, the enzyme responsible for the respiratory burst. We have developed a cell-free system, in which the activation of NADPH oxidase is induced by the addition of PA. Characterization of this system revealed that a multi-functional cytosolic protein kinase was a target for PA, and that two NADPH oxidase components were substrates for the enzyme. Partial purification of the PA-activated protein kinase separated the enzyme from known protein kinase targets of PA. The partially purified enzyme was selectively activated by PA, compared to other phospholipids, and phosphorylated the oxidase component p47-phox on both serine and tyrosine residues. PA-activated protein kinase activity was present in a variety of hematopoietic cells and cell lines and in rat brain, suggesting it has widespread distribution. We conclude that this protein kinase may be a novel target for the second messenger function of PA.

Activation of the phagocyte NADPH oxidase protein p47(phox). Phosphorylation controls SH3 domain-dependent binding to p22(phox)

Activation of phagocyte NADPH oxidase requires interaction between p47(phox) and p22(phox). p47(phox) in resting phagocytes does not bind p22(phox). Phosphorylation of serines in the p47(phox) C terminus enables binding to the p22(phox) C terminus by inducing a conformational change in p47(phox) that unmasks the SH3A domain. We report that an arginine/lysine-rich region in the p47(phox) C terminus binds the p47(phox) SH3 domains expressed in tandem (SH3AB) but does not bind the individual N-terminal SH3A and C-terminal SH3B domains. Peptides matching amino acids 301-320 and 314-335 of the p47(phox) arginine/lysine-rich region block the p47(phox) SH3AB/p22(phox) C-terminal and p47(phox) SH3AB/p47(phox) C-terminal binding and inhibit NADPH oxidase activity in vitro. Peptides with phosphoserines substituted for serines 310 and 328 do not block binding and are poor inhibitors of oxidase activity. Mutated full-length p47(phox) with aspartic acid substitutions to mimic the effects of phosphorylations at serines 310 and 328 bind the p22(phox) proline-rich region in contrast to wild-type p47(phox). We conclude that the p47(phox) SH3A domain-binding site is blocked by an interaction between the p47(phox) SH3AB domains and the C-terminal arginine/lysine-rich region. Phosphorylation of serines in the p47(phox) C terminus disrupts this interaction leading to exposure of the SH3A domain, binding to p22(phox), and activation of the NADPH oxidase.

Helicobacter pylori lipopolysaccharide enhances the expression of NADPH oxidase components in cultured guinea pig gastric mucosal cells

Recently, we showed that cultured guinea pig gastric pit cells possess a phagocyte NADPH oxidase-like activity, which was up-regulated by Helicobacter pylori lipopolysaccharide. We demonstrate here that these cells express all of the phagocyte NADPH oxidase components (gp91-, p22-, p67-, p47-, and p40-phoxes). Treatment with lipopolysaccharide increased the expression of gp91-, p22-, and p67-phoxes, but not that of p47- and p40-phoxes. Intriguingly, the p67-phox expression consistently correlated with up-regulation of superoxide anion-producing ability. Thus, the gastric pit cell NADPH oxidase may play an important role in regulation of the inflammatory response associated with H. pylori infection.

Vestigial respiratory burst activity in wound macrophages

Macrophages from experimental wounds in rats were tested for their capacity to generate reactive oxygen intermediates. Measurements of superoxide and H2O2 release, O-2-dependent lucigenin chemiluminescence, oxygen consumption, hexose monophosphate shunt flux, and NADPH oxidase activity in cell lysates indicated, at best, the presence of a vestigial respiratory burst response in these cells. The inability of wound cells to release O-2 was not rekindled by priming with endotoxin or interferon-gamma in vivo or in vitro. NADPH oxidase activity in a cell-free system demonstrated that wound macrophage membranes, but not their cytosols, were capable of sustaining maximal rates of O-2 production when mixed with their corresponding counterparts from human neutrophils. Immune detection experiments showed wound macrophages to be particularly deficient in the cytosolic component of the NADPH oxidase p47-phox. Addition of recombinant p47-phox to the human neutrophil-cell membrane/wound macrophage cytosol cell-free oxidase assay, however, failed to support O-2 production. Present findings indicate an unexpected deficit of wound macrophages in their capacity to generate reactive oxygen intermediates.

Activation of the leukocyte NADPH oxidase by protein kinase C in a partially recombinant cell-free system

The leukocyte NADPH oxidase is an enzyme present in phagocytes and B lymphocytes that when activated catalyzes the production of O-2 from oxygen at the expense of NADPH. A correlation between the activation of the oxidase and the phosphorylation of p47(PHOX), a cytosolic oxidase component, is well recognized in whole cells, and direct evidence for a relationship between the phosphorylation of this oxidase component and the activation of the oxidase has been obtained in a number of cell-free systems containing neutrophil membrane and cytosol. Using superoxide dismutase-inhibitable cytochrome c reduction to quantify O-2 production, we now show that p47(PHOX) phosphorylated by protein kinase C activates the NADPH oxidase not only in a cell-free system containing neutrophil membrane and cytosol, but also in a system in which the cytosol is replaced by the recombinant proteins p67(PHOX), Rac2, and phosphorylated p47(PHOX), suggesting that neutrophil plasma membrane plus those three cytosolic proteins are both necessary and sufficient for oxidase activation. In both the cytosol-containing and recombinant cell-free systems, however, activation by SDS yielded greater rates of O-2 production than activation by protein kinase C-phosphorylated p47(PHOX), indicating that a system that employs protein kinase C-phosphorylated p47(PHOX) as the sole activating agent, although more physiological than the SDS-activated system, is nevertheless incomplete.

Mapping of functional domains in p47(phox) involved in the activation of NADPH oxidase by "peptide walking"

The superoxide generating NADPH oxidase of phagocytes consists, in resting cells, of a membrane-associated electron transporting flavocytochrome (cytochrome b559) and four cytosolic proteins as follows: p47(phox), p67(phox), p40(phox), and the small GTPase, Rac(1 or 2). Activation of the oxidase is consequent to the assembly of a membrane-localized multimolecular complex consisting of cytochrome b559 and the cytosolic components. We used "peptide walking" (Joseph, G., and Pick, E. (1995) J. Biol. Chem. 270, 29079-29082) for mapping domains in the amino acid sequence of p47(phox) participating in the molecular events leading to the activation of NADPH oxidase. Ninety-five overlapping pentadecapeptides, with a four-residue offset between neighboring peptides, spanning the complete p47(phox) sequence, were tested for the ability to inhibit NADPH oxidase activation in a cell-free system. This consisted of solubilized macrophage membranes, recombinant p47(phox), p67(phox), and Rac1, and lithium dodecyl sulfate, as the activator. Eight functional domains were identified and labeled a-h. These were (N- and C-terminal residue numbers are given for each domain) as follows: a (21-35); b (105-119); c (149-159); d (193-207); e (253-267); f (305-319); g (325-339), and h (373-387). Four of these domains (c, d, e, and g) correspond to or form parts of regions shown before to participate in NADPH oxidase assembly. Thus, domain c corresponds to a region on the N-terminal boundary of the first src homology 3 (SH3) domain, whereas domains d and e represent more precisely defined sites within the full-length first and second SH3 domains, respectively. Domain g overlaps an extensively investigated arginine-rich region. Domains a and b, in the N-terminal half of p47(phox), and domains f and h, in the C-terminal half, represent newly identified entities, for which there is no earlier experimental evidence of involvement in NADPH oxidase activation. "Peptide walking" was also applied to the identification of domains in p47(phox) mediating binding to p67(phox). This was done by quantifying, by enzyme-linked immunosorbent assay, the binding of p67(phox), in solution, to a series of 95 overlapping biotinylated p47(phox) peptides, attached to streptavidin-coated 96-well plates. A single proline-rich domain (residues 357-371) was found to bind p67(phox) in the absence and presence of lithium dodecyl sulfate.

Analysis of activation-induced conformational changes in p47phox using tryptophan fluorescence spectroscopy

Activation of the neutrophil NADPH oxidase requires translocation of cytosolic proteins p47(phox), p67(phox), and Rac to the plasma membrane or phagosomal membrane, where they assemble with membrane-bound flavocytochrome b. During this process, it appears that p47(phox) undergoes conformational changes, resulting in the exposure of binding sites involved in assembly and activation of the oxidase. In the present study, we have directly evaluated activation-induced conformational changes in p47(phox) using tryptophan fluorescence and circular dichroism spectroscopy. Treatment of p47(phox) with amphiphilic agents known to activate the NADPH oxidase (SDS and arachidonic acid) caused a dose-dependent quenching in the intrinsic tryptophan fluorescence of p47(phox), whereas treatment with a number of other amphiphilic agents that failed to activate the oxidase had no effect on p47(phox) fluorescence. In addition, the concentration range of activating agents required to induce changes in fluorescence correlated with the concentration range of these agents that induced maximal NADPH oxidase activity in a cell-free assay system. We next determined if activation by phosphorylation caused the same type of conformational changes in p47(phox). Protein kinase C phosphorylation of p47(phox) in vitro resulted in comparable quenching of fluorescence, which also correlated directly with NADPH oxidase activity. Finally, the circular dichroism (CD) spectrum of p47(phox) was significantly changed by the addition of SDS, whereas treatment with a non-activating detergent had no effect on the CD spectrum. These results support the conclusion that activation by amphiphilic agents results in changes in the secondary structure of p47(phox). Thus, our studies provide direct evidence linking conformational changes in p47(phox) to the NADPH oxidase activation/assembly process and also further support the hypothesis that amphiphile-mediated activation of the NADPH oxidase induces changes in p47(phox) that are similar to those mediated by phosphorylation in vivo.

Structure and chromosomal assignment to 22q12 and 17qter of the ras-related Rac2 and Rac3 human genes

Members of the Rho/Rac/Cdc42Hs family of GTPases have been shown to participate in many aspects of the signaling of cell growth and differentiation. Although the biochemical properties of these GTPases have been extensively studied, very little is known about the structure of the corresponding genes. To gain insight on the evolution of the Rho family, we were interested in studying the genomic structure of several members. We report here the structure and the localization to 22q12 of the human Rac2 gene, as well as the localization to 17qter of Rac3, a new member closely related to Rac1 and Rac2. Unlike the structure of its closest relative ARH-G gene, which contains a single intron, Rac2 is made of at least 7 exons, spanning over 18 kb of DNA. Comparison of gene structure and exonic borders suggests that the emergence of the whole superfamily took place early during evolution.

Phosphorylation of the respiratory burst oxidase subunit p67(phox) during human neutrophil activation. Regulation by protein kinase C-dependent and independent pathways

The respiratory burst oxidase of phagocytes and B lymphocytes catalyzes the reduction of oxygen to superoxide anion (O-2) at the expense of NADPH. This multicomponent enzyme is dormant in resting cells but is activated on exposure to an appropriate stimulus. The phosphorylation-dependent mechanisms regulating the activation of the respiratory burst oxidase are unclear, particularly the phosphorylation status of the cytosolic component p67(phox). In this study, we found that activation of human neutrophils with formyl-methionyl-leucyl-phenylalanine (fMLP), a chemotactic peptide, or phorbol myristate acetate (PMA), a stimulator of protein kinase C (PKC), resulted in the phosphorylation of p67(phox). Using an anti-p67(phox) antibody or an anti-p47(phox) antibody, we showed that phosphorylated p67(phox) and p47(phox) form a complex. Phosphoamino acid analysis of the phosphorylated p67(phox) revealed only 32P-labeled serine residues. Two-dimensional tryptic peptide mapping analysis showed that p67(phox) is phosphorylated at the same peptide whether fMLP or PMA is used as a stimulus. In addition, PKC induced the phosphorylation of recombinant GST-p67(phox) in vitro, at the same peptide as that phosphorylated in intact cells. PMA-induced phosphorylation of p67(phox) was strongly inhibited by the PKC inhibitor GF109203X. In contrast, fMLP-induced phosphorylation was minimally affected by this PKC inhibitor. Taken together, these results show that p67(phox) is phosphorylated in human neutrophils by different pathways, one of which involves protein kinase C.

Phosphatidic acid-mediated phosphorylation of the NADPH oxidase component p47-phox. Evidence that phosphatidic acid may activate a novel protein kinase

Phosphatidic acid (PA), generated by phospholipase D activation, has been linked to the activation of the neutrophil respiratory burst enzyme, NADPH oxidase; however, the intracellular enzyme targets for PA remain unclear. We have recently shown (McPhail, L. C., Qualliotine-Mann, D., and Waite, K. A. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 7931-7935) that a PA-activated protein kinase is involved in the activation of NADPH oxidase in a cell-free system. This protein kinase phosphorylates numerous endogenous proteins, including p47-phox, a component of the NADPH oxidase complex. Phospholipids other than PA were less effective at inducing endogenous protein phosphorylation. Several of these endogenous substrates were also phosphorylated during stimulation of intact cells by opsonized zymosan, an agonist that induces phospholipase D activation. We sought to identify the PA-activated protein kinase that phosphorylates p47-phox. The PA-dependent protein kinase was shown to be cytosolic. cis-Unsaturated fatty acids were poor inducers of protein kinase activity, suggesting that the PA-activated protein kinase is not a fatty acid-regulated protein kinase (e.g. protein kinase N). Chromatographic techniques separated the PA-activated protein kinase from a number of other protein kinases known to be activated by PA or to phosphorylate p47-phox. These included isoforms of protein kinase C, p21 (Cdc42/Rac)-activated protein kinase, and mitogen-activated protein kinase. Gel filtration chromatography indicated that the protein kinase has an apparent molecular size of 125 kDa. Screening of cytosolic fractions from several cell types and rat brain suggested the enzyme has widespread cell and tissue distribution. The partially purified protein kinase was sensitive to the same protein kinase inhibitors that diminished NADPH oxidase activation and was independent of guanosine 5'-3-O-(thio)triphosphate and Ca2+. Phosphoamino acid analysis showed that serine and tyrosine residues were phosphorylated on p47-phox by this kinase(s). These data indicate that one or more potentially novel protein kinases are targets for PA in neutrophils and other cell types. Furthermore, a PA-activated protein kinase is likely to be an important regulator of the neutrophil respiratory burst by phosphorylation of the NADPH oxidase component p47-phox.

Inhibition of NADPH oxidase activation by 4-(2-aminoethyl)-benzenesulfonyl fluoride and related compounds

The elicitation of an oxidative burst in phagocytes rests on the assembly of a multicomponental complex (NADPH oxidase) consisting of a membrane-associated flavocytochrome (cytochrome b559), representing the redox element responsible for the NADPH-dependent reduction of oxygen to superoxide (O-2), two cytosolic components (p47(phox), p67(phox)), and the small GTPase Rac (1 or 2). We found that 4-(2-aminoethyl)-benzenesulfonyl fluoride (AEBSF), an irreversible serine protease inhibitor, prevented the elicitation of O-2 production in intact macrophages and the amphiphile-dependent activation of NADPH oxidase in a cell-free system, consisting of solubilized membrane or purified cytochrome b559 combined with total cytosol or a mixture of recombinant p47(phox), p67(phox), and Rac1. AEBSF acted at the activation step and did not interfere with the ensuing electron flow. It did not scavenge oxygen radicals and did not affect assay reagents. Five other serine protease inhibitors (three irreversible and two reversible) were found to lack an inhibitory effect on cell-free activation of NADPH oxidase. A structure-function study of AEBSF analogues demonstrated that the presence of a sulfonyl fluoride group was essential for inhibitory activity and that compounds containing an aminoalkylbenzene moiety were more active than amidinobenzene derivatives. Exposure of the membrane fraction or of purified cytochrome b559, but not of cytosol or recombinant cytosolic components, to AEBSF, in the presence of a critical concentration of the activating amphiphile lithium dodecyl sulfate, resulted in a marked impairment of their ability to support cell-free NADPH oxidase activation upon complementation with untreated cytosol or cytosolic components. Kinetic analysis of the effect of varying the concentration of each of the three cytosolic components on the inhibitory potency of AEBSF indicated that this was inversely related to the concentrations of p47(phox) and, to a lesser degree, p67(phox). AEBSF also prevented the amphiphile-elicited translocation of p47(phox) and p67(phox) to the membrane. These results are interpreted as indicating that AEBSF interferes with the binding of p47(phox) and/or p67(phox) to cytochrome b559, probably by a direct effect on cytochrome b559.