Absolute requirement for GTP in activation of human neutrophil NADPH oxidase in a cell-free system: role of ATP in regenerating GTP

Rho GTPases: Non-canonical regulation by cysteine oxidation

Rho GTPases are critically important and are centrally positioned regulators of the actomyosin cytoskeleton. By influencing the organization and architecture of the cytoskeleton, Rho proteins play prominent roles in many cellular processes including adhesion, migration, intra-cellular transportation, and proliferation. The most important method of Rho GTPase regulation is via the GTPase cycle; however, post-translational modifications (PTMs) also play critical roles in Rho protein regulation. Relative to other PTMs such as lipidation or phosphorylation that have been extensively characterized, protein oxidation is a regulatory PTM that has been poorly studied. Protein oxidation primarily occurs from the reaction of reactive oxygen species (ROS), such as hydrogen peroxide (H₂ O₂ ), with amino acid side chain thiols on cysteine (Cys) and methionine (Met) residues. The versatile redox modifications of cysteine residues exemplify their integral role in cell signalling processes. Here we review prominent members of the Rho GTPase family and discuss how lipidation, phosphorylation, and oxidation on conserved cysteine residues affects their regulation and function.

Rac-dependent feedforward autoactivation of NOX2 leads to oxidative burst

NADPH oxidase 2 (NOX2) produces the superoxide anion radical (O₂⁻), which has functions in both cell signaling and immune defense. NOX2 is a multimeric-protein complex consisting of several protein subunits including the GTPase Rac. NOX2 uniquely facilitates an oxidative burst, which is described by initially slow O₂⁻ production, which increases over time. The NOX2 oxidative burst is considered critical to immune defense because it enables expedited O₂⁻ production in response to infections. However, the mechanism of the initiation and progression of this oxidative burst and its implications for regulation of NOX2 have not been clarified. In this study, we show that the NOX2 oxidative burst is a result of autoactivation of NOX2 coupled with the redox function of Rac. NOX2 autoactivation begins when active Rac triggers NOX2 activation and the subsequent production of O₂⁻, which in turn activates redox-sensitive Rac. This activated Rac further activates NOX2, amplifying the feedforward cycle and resulting in a NOX2-mediated oxidative burst. Using mutagenesis-based kinetic and cell analyses, we show that enzymatic activation of Rac is exclusively responsible for production of the active Rac trigger that initiates NOX2 autoactivation, whereas redox-mediated Rac activation is the main driving force of NOX2 autoactivation and contributes to generation of ∼98% of the active NOX2 in cells. The results of this study provide insight into the regulation of NOX2 function, which could be used to develop therapeutics to control immune responses associated with dysregulated NOX2 oxidative bursts.

Mechanistic computational modeling of the kinetics and regulation of NADPH oxidase 2 assembly and activation facilitating superoxide production

Reactive oxygen species (ROS) play a crucial role in many physiological processes. However, ROS overproduction leads to oxidative stress, which plays a critical role in cell injury/death and the pathogenesis of many diseases. Members of NADPH oxidase (NOX) family, most of which are comprised of membrane and cytosolic components, are known to be the major nonmitochondrial sources of ROS in many cells. NOX2 is a widely-expressed and well-studied NOX family member, which is activated upon assembly of its membrane subunits gp91 ^phox and p22 ^phox with its cytosolic subunits p40 ^phox , p47 ^phox , p67 ^phox , and Rac, facilitating ROS production. NOX2 activation is also enhanced by GTP and inhibited by GDP. However, there remains a lack of a mechanistic, quantitative, and integrated understanding of the kinetics and regulation of the assembly of these subunits and their relative contributions toward NOX2 activation and ROS production. Toward this end, we have developed a mechanistic computational model, which incorporates a generalized random rapid equilibrium binding mechanism for NOX2 assembly and activation as well as regulations by GTP (activation), GDP (inhibition), and individual subunits enhancing the binding of other subunits (mutual binding enhancement). The resulting model replicates diverse published kinetic data, including subunit concentration-dependent NOX2 activation and ROS production, under different assay conditions, with appropriate estimates of the unknown model parameters. The model provides a mechanistic, quantitative, and integrated framework for investigating the critical roles of NOX2 subunits in NOX2 assembly and activation facilitating ROS production in a variety of physiological and pathophysiological conditions. However, there is also a need for better quantitative kinetic data based on current understanding of NOX2 assembly and activation in order to test and further develop this model.

Role of the Rho GTPase Rac in the activation of the phagocyte NADPH oxidase: outsourcing a key task

The superoxide-generating NADPH oxidase of phagocytes consists of the membrane-associated cytochrome b 558 (a heterodimer of Nox2 and p22(phox)) and 4 cytosolic components: p47(phox), p67(phox), p40(phox), and the small GTPase, Rac, in complex with RhoGDI. Superoxide is produced by the NADPH-driven reduction of molecular oxygen, via a redox gradient located in Nox2. Electron flow in Nox2 is initiated by interaction with cytosolic components, which translocate to the membrane, p67(phox) playing the central role. The participation of Rac is expressed in the following sequence: (1) Translocation of the RacGDP-RhoGDI complex to the membrane; (2) Dissociation of RacGDP from RhoGDI; (3) GDP to GTP exchange on Rac, mediated by a guanine nucleotide exchange factor; (4) Binding of RacGTP to p67(phox); (5) Induction of a conformational change in p67(phox), promoting interaction with Nox2. The particular involvement of Rac in NADPH oxidase assembly serves as a paradigm for signaling by Rho GTPases, in general.

Homeostatic generation of reactive oxygen species protects the zebrafish liver from steatosis

Nonalcoholic fatty liver disease is the most common liver disease in both adults and children. The earliest stage of this disease is hepatic steatosis, in which triglycerides are deposited as cytoplasmic lipid droplets in hepatocytes. Through a forward genetic approach in zebrafish, we found that guanosine monophosphate (GMP) synthetase mutant larvae develop hepatic steatosis. We further demonstrate that activity of the small GTPase Rac1 and Rac1-mediated production of reactive oxygen species (ROS) are down-regulated in GMP synthetase mutant larvae. Inhibition of Rac1 activity or ROS production in wild-type larvae by small molecule inhibitors was sufficient to induce hepatic steatosis. More conclusively, treating larvae with hydrogen peroxide, a diffusible ROS that has been implicated as a signaling molecule, alleviated hepatic steatosis in both GMP synthetase mutant and Rac1 inhibitor-treated larvae, indicating that homeostatic production of ROS is required to prevent hepatic steatosis. We further found that ROS positively regulate the expression of the triglyceride hydrolase gene, which is responsible for the mobilization of stored triglycerides in hepatocytes. Consistently, inhibition of triglyceride hydrolase activity in wild-type larvae by a small molecule inhibitor was sufficient to induce hepatic steatosis.

CONCLUSION

De novo GMP synthesis influences the activation of the small GTPase Rac1, which controls hepatic lipid dynamics through ROS-mediated regulation of triglyceride hydrolase expression in hepatocytes.

Cigarette smoke affects keratinocytes SRB1 expression and localization via H2O2 production and HNE protein adducts formation

Scavenger Receptor B1 (SR-B1), also known as HDL receptor, is involved in cellular cholesterol uptake. Stratum corneum (SC), the outermost layer of the skin, is composed of more than 25% cholesterol. Several reports support the view that alteration of SC lipid composition may be the cause of impaired barrier function which gives rise to several skin diseases. For this reason the regulation of the genes involved in cholesterol uptake is of extreme significance for skin health. Being the first shield against external insults, the skin is exposed to several noxious substances and among these is cigarette smoke (CS), which has been recently associated with various skin pathologies. In this study we first have shown the presence of SR-B1 in murine and human skin tissue and then by using immunoblotting, immunoprecipitation, RT-PCR, and confocal microscopy we have demonstrated the translocation and the subsequent lost of SR-B1 in human keratinocytes (cell culture model) after CS exposure is driven by hydrogen peroxide (H₂O₂) that derives not only from the CS gas phase but mainly from the activation of cellular NADPH oxidase (NOX). This effect was reversed when the cells were pretreated with NOX inhibitors or catalase. Furthermore, CS caused the formation of SR-B1-aldheydes adducts (acrolein and 4-hydroxy-2-nonenal) and the increase of its ubiquitination, which could be one of the causes of SR-B1 loss. In conclusion, exposure to CS, through the production of H₂O₂, induced post-translational modifications of SR-B1 with the consequence lost of the receptor and this may contribute to the skin physiology alteration as a consequence of the variation of cholesterol uptake.

The effect of ligand efficacy on the formation and stability of a GPCR-G protein complex

G protein-coupled receptors (GPCRs) mediate the majority of physiologic responses to hormones and neurotransmitters. However, many GPCRs exhibit varying degrees of agonist-independent G protein activation. This phenomenon is referred to as basal or constitutive activity. For many of these GPCRs, drugs classified as inverse agonists can suppress basal activity. There is a growing body of evidence that basal activity is physiologically relevant, and the ability of a drug to inhibit basal activity may influence its therapeutic properties. However, the molecular mechanism for basal activation and inhibition of basal activity by inverse agonists is poorly understood and difficult to study, because the basally active state is short-lived and represents a minor fraction of receptor conformations. Here, we investigate basal activation of the G protein Gs by the beta(2) adrenergic receptor (beta(2)AR) by using purified receptor reconstituted into recombinant HDL particles with a stoichiometric excess of Gs. The beta(2)AR is site-specifically labeled with a small, environmentally sensitive fluorophore enabling direct monitoring of agonist- and Gs-induced conformational changes. In the absence of an agonist, the beta(2)AR and Gs can be trapped in a complex by enzymatic depletion of guanine nucleotides. Formation of the complex is enhanced by the agonist isoproterenol, and it rapidly dissociates on exposure to concentrations of GTP and GDP found in the cytoplasm. The inverse agonist ICI prevents formation of the beta(2)AR-Gs complex, but has little effect on preformed complexes. These results provide insights into G protein-induced conformational changes in the beta(2)AR and the structural basis for ligand efficacy.

Development of an ion-pair HPLC method for investigation of energy charge changes in cerebral ischemia of mice and hypoxia of Neuro-2a cell line

The determination of adenine nucleotides and energy charge (EC) has great importance in the characterization of cerebral ischemic injury and post-ischemic recovery. An IP-HPLC method was developed for the quantification of AMP, ADP, ATP and EC in cerebral ischemia and hypoxia of the Neuro-2a cell line. The chromatographic conditions were: a Zorbax SB-C18 reversed-phase column; mobile phase 100 mM KH(2)PO(4), 1 mM tetrabutylammonium hydroxide, and 2.5% acetonitrile, brought to pH 7.0 with potassium hydroxide (4 M), filtered through a 0.45 microm Millipore filter and degassed prior to use. The flow-rate was 1.0 mL/min. The injection volume was 20 microL. Detection was performed at a wavelength of 254 nm under a constant temperature (27 +/- 1 degrees C). The method was validated by means of linearity, using calibration curves constructed with five concentration levels of each compound. The limit of detection was also determined. The system precision was calculated as the coefficient of variation for five injections for each compound tested. Cerebral tissue was homogenized (4 degrees C) in 1 mL of an ice-cold 6% trichloroacetic acid that contained ATPase inhibitor and obtained good recovery (>90%). The results show that the described method for the determination of adenine nucleotides by HPLC has good linearity, limit of detection, precision and specificity, and is simple and rapid to perform.

Role of nucleotides and phosphoinositides in the stability of electron and proton currents associated with the phagocytic NADPH oxidase

The phagocytic NADPH oxidase (phox) moves electrons across cell membranes to kill microbes. The activity of this lethal enzyme is tightly regulated, but the mechanisms that control phox inactivation are poorly understood for lack of appropriate assays. The phox generates measurable electron currents, I(e), that are associated with inward proton currents, I(H). To study the inactivation of the phox and of its associated proton channel, we determined which soluble factors can stabilize I(e) (induced by the addition of NADPH) and I(H) (initiated by small depolarizing voltage steps) in inside-out patches from PMA-activated human eosinophils. I(e) decayed rapidly in the absence of nucleotides (tau approximately 6 min) and was maximally stabilized by the combined addition of 5 mM ATP and 50 microM of the non-hydrolysable GTP analogue GTP[S] (guanosine 5'-[gamma-thio]triphosphate) (tau approximately 57 min), but not by either ATP or GTP[S] alone. I(H) also decayed rapidly and was stabilized by the ATP/GTP[S] mixture, but maximal stabilization of I(H) required further addition of 25 muM PI(3,4)P2 (phosphoinositide 3,4-bisphosphate) to the cytosolic side of the patch. PI(3,4)P2 had no effect on I(e) and its stabilizing effect on I(H) could not be mimicked by other phosphoinositides. Reducing the ATP concentration below millimolar levels decreased I(H) stability, an effect that was not prevented by phosphatase inhibitors but by the non-hydrolysable ATP analogue ATP[S] (adenosine 5'-[gamma-thio]triphosphate). Our data indicate that the assembled phox complex is very stable in eosinophil membranes if both ATP and GTP[S] are present, but inactivates within minutes if one of the nucleotides is removed. Stabilization of the phox-associated proton channel in a highly voltage-sensitive conformation does not appear to involve phosphorylation but ATP binding, and requires not only ATP and GTP[S] but also PI(3,4)P2, a protein known to anchor the cytosolic phox subunit p47(phox) to the plasma membrane.

Development of an HPLC method for determination of metabolic compounds in myocardial tissue

The determination of adenine nucleotides and creatine compounds has great importance in the characterization of ischemic myocardial injury and post-ischemic recovery. It was developed by an HPLC method for the quantification of creatine (Cr), creatine phosphate (CrP), hypoxanthine (HX), AMP, adenosine (Ad), ADP and ATP in isolated perfused rat hearts. The chromatographic conditions were: RP 18 column; mobile phase composed by KH(2)PO(4) (215 mM), tetrabutylammonium hydrogen sulfate (2.3mM), acetonitrile (4%) and KOH (1M 0.4%); flow rate 1 ml min(-1); temperature 25 degrees C; injection volume 20 microl; detection at 220 nm and height peak (HP) as the integration parameter. The method was validated by means of linearity and sensitivity evaluations, using calibration curves done with five concentration levels of each compound. The limits of quantification (LOQ) were also determined. The system precision was calculated as the coefficient of variation for five injections for each compound tested. The purity of the peaks was established using enzymatic peak shift analysis with hexokinase and creatine kinase and also comparing HP at various wavelengths. Frozen hearts were homogenized with a mechanical homogenizer for 3 min at 0 degrees C added with 5 ml of 0.4N HCLO(4). After precipitation with 0.8 ml of 2M KOH the extract was shaked for 2 min and later centrifuged at 0 degrees C for 10 min. The supernatant was kept on ice, filtrated and injected into the HPLC system. The results show that the method for the determination of Cr, CrP, HX, AMP, Ad, ADP and ATP by HPLC here described has good linearity, LOQ, precision, specificity and is simple and rapid to perform.

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.

A constitutive NADPH oxidase-like system containing gp91phox homologs in human keratinocytes

In non-phagocytic cells, superoxide has been implicated in physiological and pathological cellular functions in the skin and mucosa, such as, host defense, mitogenic responses, and malignant conversion. Here, we identify a constitutively expressed heme-flavoprotein NADPH oxidase (Nox) system as a source of superoxide in human skin (HaCaT) and gingival mucosal (GM16) keratinocyte cell lines. Western blot analysis showed that both cell lines expressed the phagocyte oxidase (phox) cytosolic proteins Rac1, p40phox, and p67phox. With respect to the catalytic flavoheme protein subunit, HaCaT membranes, which expressed p22phox, showed an absorbance peak at 558 nm indicative of a b-type cytochrome. At mRNA levels, both GM16 and HaCaT cells expressed gp91phox homologs Nox1, Nox2, and Nox4, however, HaCaT cells expressed very low levels of Nox1 mRNA. At protein levels, Nox1 was readily detected in HaCaT but was nearly undetectable in GM16 cells. Consistently, Nox activity of HaCaT membranes was demonstrated by electron paramagnetic resonance spin-trapping and cytochrome c reduction, and the activity was sensitive to the flavoprotein inhibitor diphenylene iodonium. V(max) values were 20-fold lower than those reported for phagocytic oxidase. In conclusion, keratinocytes expressed a Nox distinct from the phox isoform of phagocytes providing molecular evidence for a source of superoxide that may regulate cell proliferation and host defense in skin and oral mucosa.

The guanine nucleotide exchange factor trio activates the phagocyte NADPH oxidase in the absence of GDP to GTP exchange on Rac. "The emperor's nw clothes"

The superoxide-generating NADPH oxidase complex of phagocytes consists of a membrane-associated flavocytochrome b(559) and four cytosolic components as follows: p47(phox), p67(phox), p40(phox), and the small GTPase Rac (1 or 2). Activation of the oxidase is the result of assembly of the cytosolic components with cytochrome b(559) and can be mimicked in vitro by mixtures of membrane and cytosolic components exposed to an anionic amphiphile, serving as activator. We reported that prenylation of Rac1 endows it with the ability to support oxidase activation in conjunction with p67(phox) but in the absence of amphiphile and p47(phox). We now show the following 6 points. 1) The Rac guanine nucleotide exchange factor Trio markedly potentiates oxidase activation by prenylated Rac1-GDP. 2) This occurs in the absence of exogenous GTP or any other source of GTP generation, demonstrating that the effect of Trio does not involve GDP to GTP exchange on Rac1. 3) Trio does not potentiate oxidase activation by prenylated Rac1-GTP, by nonprenylated Rac1-GDP in the presence or absence of amphiphile, and by a prenylated [p67(phox)-Rac1] chimera in GDP-bound form. 4) Rac1 mutants defective in the ability to bind Trio or to respond to Trio by nucleotide exchange fail to respond to Trio by enhanced oxidase activation. 5) A Trio mutant with conserved Rac1-binding ability but lacking nucleotide exchange activity fails to enhance oxidase activation. 6) The effect of Trio is mimicked by displacement of Mg(2+) from Rac1-GDP. These results reveal the existence of a novel mechanism of Rac activation by a guanine nucleotide exchange factor and suggest that the induction by Trio of a conformational change in Rac1, in the absence of nucleotide exchange, is sufficient for enhancing its effector function.

Creation of a genetic system for analysis of the phagocyte respiratory burst: high-level reconstitution of the NADPH oxidase in a nonhematopoietic system

The phagocyte nicotinamide adenine dinucleotide phosphate (reduced form) (NADPH) oxidase was functionally reconstituted in monkey kidney COS-7 cells by transfection of essential subunits, gp91(phox), p22(phox), p47(phox), and p67(phox). COS-7 cells express the essential small guanosine 5'-triphosphatase, Rac1. Transgenic COS-phox cells were capable of arachidonic acid-induced NADPH oxidase activity up to 80% of that of human neutrophils, and of phorbol myristate acetate (PMA)-induced activity up to 20% of that of neutrophils. Expression of all 4 phox components was required for enzyme activity, and enzyme activation was associated with membrane translocation of p47(phox), p67(phox), and Rac1. Expression of p47(phox) Ser303Ala/Ser304Ala or Ser379Ala phosphorylation-deficient mutants resulted in significantly impaired NAPDH oxidase activity, compared with expression of wild-type p47(phox) or the p47(phox) Ser303Glu/Ser304Glu phosphorylation mimic, suggesting that p47(phox) phosphorylation contributes to enzyme activity in the COS system, as is the case in neutrophils. Hence, COS-phox cells should be useful as a new whole-cell model that is both capable of high-level superoxide production and readily amenable to genetic manipulation for investigation of NADPH oxidase function. PMA-elicited superoxide production in COS-phox cells was regulated by activation of protein kinase C (PKC) and Rac. Although COS-7 cells differ from human neutrophils in PKC isoform expression, transient expression of major neutrophil isoforms in COS-phox cells did not increase PMA-induced superoxide production, suggesting that endogenous isoforms were not rate limiting. Val204 in p67(phox), previously shown to be required for NADPH oxidase activity under cell-free conditions, was found to be essential for superoxide production by intact COS-phox cells, on the basis of transfection studies using a p67(phox) (Val204Ala) mutant.

ATP-dependent membrane assembly of F-actin facilitates membrane fusion

We recently established an in vitro assay that monitors the fusion between latex-bead phagosomes and endocytic organelles in the presence of J774 macrophage cytosol (). Here, we show that different reagents affecting the actin cytoskeleton can either inhibit or stimulate this fusion process. Because the membranes of purified phagosomes can assemble F-actin de novo from pure actin with ATP (), we focused here on the ability of membranes to nucleate actin in the presence of J774 cytosolic extracts. For this, we used F-actin sedimentation, pyrene actin assays, and torsional rheometry, a biophysical approach that could provide kinetic information on actin polymerization and gel formation. We make two major conclusions. First, under our standard in vitro conditions (4 mg/ml cytosol and 1 mM ATP), the presence of membranes actively catalyzed the assembly of cytosolic F-actin, which assembled into highly viscoelastic gels. A model is discussed that links these results to how the actin may facilitate fusion. Second, cytosolic actin paradoxically polymerized more under ATP depletion than under high-ATP conditions, even in the absence of membranes; we discuss these data in the context of the well described, large increases in F-actin seen in many cells during ischemia.

Genetic, biochemical, and clinical features of chronic granulomatous disease

The reduced nicotinamide dinucleotide phosphate (NADPH) oxidase complex allows phagocytes to rapidly convert O2 to superoxide anion which then generates other antimicrobial reactive oxygen intermediates, such as H2O2, hydroxyl anion, and peroxynitrite anion. Chronic granulomatous disease (CGD) results from a defect in any of the 4 subunits of the NADPH oxidase and is characterized by recurrent life-threatening bacterial and fungal infections and abnormal tissue granuloma formation. Activation of the NADPH oxidase requires translocation of the cytosolic subunits p47phox (phagocyte oxidase), p67phox, and the low molecular weight GT-Pase Rac, to the membrane-bound flavocytochrome, a heterodimer composed of the heavy chain gp91phox and the light chain p22phox. This complex transfers electrons from NADPH on the cytoplasmic side to O2 on the vacuolar or extracellular side, thereby generating superoxide anion. Activation of the NADPH oxidase requires complex rearrangements between the protein subunits, which are in part mediated by noncovalent binding between src-homology 3 domains (SH3 domains) and proline-rich motifs. Outpatient management of CGD patients relies on the use of prophylactic antibiotics and interferon-gamma. When infection is suspected, aggressive effort to obtain culture material is required. Treatment of infections involves prolonged use of systemic antibiotics, surgical debridement when feasible, and, in severe infections, use of granulocyte transfusions. Mouse knockout models of CGD have been created in which to examine aspects of pathophysiology and therapy. Gene therapy and bone marrow transplantation trials in CGD patients are ongoing and show great promise.

Activation of human neutrophil NADPH oxidase by phosphatidic acid or diacylglycerol in a cell-free system. Activity of diacylglycerol is dependent on its conversion to phosphatidic acid

The superoxide-generating neutrophil NADPH oxidase can be activated in cell-free reconstitution systems by several agonists, most notably arachidonic acid and the detergent sodium dodecyl sulfate. In this study, we show that both phosphatidic acids and diacylglycerols can serve separately as potent, physiologic activators of NADPH oxidase in a cell-free system. Stimulation of superoxide generation by these lipids was dependent upon both Mg(2+) and agonist concentration. Activation of NADPH oxidase by phosphatidic acids did not appear to require their conversion to corresponding diacylglycerols by phosphatidate phosphohydrolase, since diacylglycerols were much slower than phosphatidic acids to activate the system and required the presence of ATP. Stimulation of the oxidase by dioctanoylglycerol proved to be by a means other than the activation of protein kinase C. Instead, dioctanoylglycerol was converted to dioctanoylphosphatidic acid by an endogenous diacylglycerol kinase present in the cell-free reaction system. This conversion was sensitive to the diacylglycerol kinase inhibitor R59949 and explains the markedly slower kinetics of activation and the novel ATP requirement seen with dioctanoylglycerol. The level of dioctanoylphosphatidic acid formed was suboptimal for NADPH oxidase activation but could synergize with the unmetabolized dioctanoylglycerol to activate superoxide generation.

Stimulation of glycosylphosphatidylinositol biosynthesis in mammalian cell-free systems by GTP hydrolysis: evidence for the involvement of membrane fusion

The second step in glycosylphosphatidylinositol (GPI) biosynthesis, the deacetylation of GlcNAc-phosphatidylinositol (GlcNAc-PI), has been shown to be stimulated by GTP hydrolysis [Stevens (1993) J. Biol. Chem. 268, 9718-9724]. We have now developed a system to study this regulation that uses microsomes from cells defective in the first step in GPI biosynthesis (class A, C and H lymphoma mutants) and the second reaction in the pathway (G9PLAP.85). With this mixed-microsome system, the deacetylation of GlcNAc-PI was almost completely dependent on GTP hydrolysis. Because GlcNAc-PI synthesized by the G9PLAP.85 microsomes cannot readily move to the first-step-mutant microsomes to be deacetylated, this result indicated that the role of GTP was to facilitate the 'apparent' transfer of this substrate between membrane vesicles. The microsomes could be stably preactivated by pretreatment with GTP before GPI biosynthesis was initiated, indicating that fusion was the most likely mechanism for this regulation. GlcNAc-PI deacetylation could also be stably preactivated in EL4 microsomes, suggesting that fusion also occurred in wild-type membranes. Some differential localization of the GlcNAc-PI synthetic and deacetylation activities with the endoplasmic reticulum was found. Therefore fusion seems to stimulate GPI biosynthesis in mammalian microsomes by bringing together the first two enzymes in the pathway in the same membrane vesicle.

NAD(P)H oxidase activity in cultured human podocytes: effects of adenosine triphosphate

Reactive oxygen species contribute to glomerular damage and proteinuria. In this study, we show that cultured human podocytes produce superoxide in response to extracellular adenosine triphosphate (ATP), and we identified the oxidases involved in this process. Adenosine triphosphate (10-4 M for 4 hr) raised superoxide production from 1.28 +/- 0.15 to 2.67 &/- 0.34 nmol/mg protein/min. Studies with podocyte homogenates revealed activation of both nicotinamide adenine dinucleotide (NADH; from 2.65 +/- 0.23 to 7.43 +/- 0.57) and nicotinamide adenine dinucleotide phosphate (NADPH) dependent oxidases [from 1.74 +/- 0.13 to 4.05 +/- 0.12 (nmol O2/mg protein/min)] by ATP. Activity of xanthine-oxidases was low and unchanged by ATP. Activation of the plasma-membrane bound NAD(P)H oxidases by ATP was time and dose dependent. Reverse transcribed-polymerase chain reaction (RT-PCR) studies with primers derived from monocyte sequences amplified mRNA for the NADPH oxidase subunits p22phox, p47phox, gp91phox, and p67phox, and the latter was transiently increased by ATP. Experiments with actinomycin D and cycloheximide suggested that ATP modulates enzyme activity at the transcriptional and translational levels. In conclusion, NAD(P)H dependent, membrane associated oxidases represent the major superoxide source in human podocytes. Activation of NAD(P)H oxidase by ATP might be secondary to increased mRNA expression of the NADPH oxidase subunit gp67phox.

Membrane fusion protein synexin (annexin VII) as a Ca2+/GTP sensor in exocytotic secretion

Exocytotic membrane fusion and secretion are promoted by the concerted action of GTP and Ca2+, although the precise site(s) of action in the process are not presently known. However, the calcium-dependent membrane fusion reaction driven by synexin (annexin VII) is an in vitro model for this process, which we have now found to be further activated by GTP. The mechanism of fusion activation depends on the unique ability of synexin to bind and hydrolyze GTP in a calcium-dependent manner, both in vitro and in vivo in streptolysin O-permeabilized chromaffin cells. The required [Ca2+] for GTP binding by synexin is in the range of 50-200 microM, which is known to occur at exocytotic sites in chromaffin cells, neurons, and other cell types. Previous immunolocalization studies place synexin at exocytotic sites in chromaffin cells, and we conclude that synexin is an atypical G protein that may be responsible for both detecting and mediating the Ca2+/GTP signal for exocytotic membrane fusion.

The nucleoside diphosphate kinase of human neutrophils

Nucleoside diphosphate kinase (NDP kinase) catalyses the phosphate transfer between nucleoside triphosphates and nucleoside diphosphates. As formation of guanosine triphosphate could be dependent on ATP in neutrophils, the presence of NDP kinase was tested in these phagocytic cells. Both membrane and cytosolic fractions of human neutrophils were found to contain NDP kinase activity. The specific activity measured in the cytosol appeared 10-fold higher than in the membrane and was not modified when the cells were activated with phorbol 12-myristate 13-acetate. Interestingly, stimulation with N-formylmethionyl leucylphenylalanine in the presence of cytochalasin B showed an increase in membrane NDP kinase activity together with the translocation of the enzyme from the cytosol to the membrane, suggesting a possible role of NDP kinase in regulating G-proteins as previously reported. In addition, activation with opsonized zymosan induced an increase in cytosolic activity, suggesting different regulation depending on the signal transduction pathway. The neutrophil enzyme consisted of two subunits of 21 kDa (NDPKA) and 18 kDa (NDPKB) again essentially present in the cytosol of the cell. Separation of proteins by two-dimensional PAGE demonstrated that each subunit consisted of at least four isoforms, indicating post translational modifications. A characteristic of this family of enzymes is the stability of the phosphorylated intermediate. In neutrophils, only one acidic isoform of each NDPKA and NDPKB was labelled in the presence of EDTA. In addition, non-denatured complexes were apparent between 91 and 130 kDa, suggesting a hexameric structure as was also proposed for NDP kinases from other eukaryotic cells. These complexes were found to differ in their isoelectric points, indicating the existence of various isoenzymes probably resulting from combination between several isoforms of each subunit.

Naphthalenesulphonamides block neutrophil superoxide production by intact cells and in a cell-free system: is myosin light chain kinase responsible for these effects?

Selective antagonists of myosin light chain kinase (MLCK) [e.g. ML-7; 1-(5-iodonaphthalene-1-sulphonyl)-1H-hexahydro-1,4-diazepine hydrochloride] were found to inhibit superoxide (O2-) release from stimulated neutrophils. The concentrations of ML-7 that were inhibitory were substantially lower than those reported for a selective antagonist of protein kinase C [i.e. H-7; 1-(5-isoquinolinesulphonyl)-2-methylpiperazine dihydrochloride]. ML-7 also reduced the phosphorylation of the 47 kDa subunit of the NADPH-oxidase system (p47-phox) and blocked translocation of this protein to the Triton X-100-insoluble fraction in stimulated cells. Interestingly, ML-7 also inhibited O2- production in a cell-free system derived from neutrophils at concentrations similar to those that were effective in vivo. This cell-free system does not require ATP and is insensitive to all other inhibitors of protein kinases tested, including some highly effective against MLCK (i.e. staurosporine). Thus, the data suggest that ML-7 does not block O2- release by inhibiting a protein kinase but instead may interact directly with a subunit of the oxidase. The binding site for ML-7 may provide a valuable target for inhibiting the inflammatory properties of phagocytic leucocytes by naphthalenesulphonamides designed to lack activity against protein kinases.

Cell-free activation of neutrophil NADPH oxidase by a phosphatidic acid-regulated protein kinase

The phosphorylation-dependent mechanisms regulating activation of the human neutrophil respiratory-burst enzyme, NADPH oxidase, have not been elucidated. We have shown that phosphatidic acid (PA) and diacylglycerol (DG), products of phospholipase activation, synergize to activate NADPH oxidase in a cell-free system. We now report that activation by PA plus DG involves protein kinase activity, unlike other cell-free system activators. NADPH oxidase activation by PA plus DG is reduced approximately 70% by several protein kinase inhibitors [1-(5-isoquinolinesulfonyl)piperazine, staurosporine, GF-109203X]. Similarly, depletion of ATP by dialysis reduces PA plus DG-mediated NADPH oxidase activation by approximately 70%. Addition of ATP, but not a nonhydrolyzable ATP analog, to the dialyzed system restores activation levels to normal. In contrast, these treatments have little effect on NADPH oxidase activation by arachidonic acid or SDS plus DG. PA plus DG induces the phosphorylation of a number of endogenous proteins. Phosphorylation is largely mediated by PA, not DG. A predominant substrate is p47-phox, a phosphoprotein component of NADPH oxidase. Phosphorylation of p47-phox precedes activation of NADPH oxidase and is markedly reduced by the protein kinase inhibitors. In contrast, arachidonic acid alone or SDS plus DG is a poor activator of protein phosphorylation in the cell-free system. Thus, PA induces activation of one or more protein kinases that regulate NADPH oxidase activation in a cell-free system. This cell-free system will be useful for identifying a functionally important PA-activated protein kinase(s) and for dissecting the phosphorylation-dependent mechanisms responsible for NADPH oxidase activation.

Regulation of human leukocyte p21-activated kinases through G protein--coupled receptors

The Rac guanosine 5'-triphosphate (GTP)-binding proteins regulate oxidant production by phagocytic leukocytes. Two Ste20-related p21-activated kinases (PAKs) were identified as targets of Rac in human neutrophils. Activity of the approximately 65- and approximately 68-kilodalton PAKs was rapidly stimulated by chemoattractants acting through pertussis toxin-sensitive heterotrimeric GTP-binding proteins (G proteins). Native and recombinant PAKs phosphorylated the p47phox reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase component in a Rac-GTP-dependent manner. The action of PAKs during phagocyte activation by G protein-coupled pathways may contribute to regulation of NADPH oxidase activity.

Translocation of p21rac2 from cytosol to plasma membrane is neither necessary nor sufficient for neutrophil NADPH oxidase activity

Activation of the membrane-associated NADPH oxidase of neutrophils requires several cytosolic factors including p47phox, p67phox and p21rac2. We compared NADPH oxidase activity with the membrane translocation of p47phox, p67phox, and p21rac2. In a cell-free system, GTP gamma S stimulated translocation of p47phox and p67phox to the plasma membrane only in the presence of arachidonate, and this translocation correlated with NADPH oxidase activity of the reisolated plasma membranes (R = 0.94 and 0.97, respectively). In contrast, GTP gamma S-stimulated p21rac2 translocation with or without arachidonate, and the extent of translocation did not correlate with oxidase activity (R = 0.17). Neutrophil cytoplasts were used to relate membrane translocation of p47phox, p67phox and p21rac2 to membrane oxidase activity in response to the inflammatory agonists. Whereas N-formyl-methionyl-leucyl-phenylalanine stimulated equimolar, transient membrane translocation of p47phox and p67phox which kinetically paralleled NADPH oxidase activity, relatively little p21rac2 translocated (moles of p47phox/p21rac2 = 16.6). Moreover, although phorbol 12-myristate 13-acetate stimulated both the stable translocation of p47phox and p67phox and sustained NADPH oxidase activity, it did not stimulate p21rac2 translocation. From these data we conclude that membrane translocation of p21rac2 does not regulate NADPH oxidase activity stoichiometrically.

Ras effector-homologue region on Rac regulates protein associations in the neutrophil respiratory burst oxidase complex

Rac, a small molecular weight GTPase in the Ras superfamily, participates in the activation of the multicomponent superoxide-generating NADPH oxidase of human neutrophils. Rac is 30% identical to Ras overall, but is 75% identical within the sequence corresponding to the effector region of Ras, which regulates mitogenesis through interactions with the protein kinase Raf1. We investigated the role of this region in Rac1 using site-directed mutagenesis. In a cell-free semirecombinant NADPH oxidase system, mutants in the 26, 33, 38, and 45 amino acids showed 20-110-fold reduced binding to the oxidase complex as judged by EC50 values and reduced (44-80%) maximal activities in superoxide generation. Only the GTP gamma S-bound form associated, since the GDP-bound form of Rac neither activated alone nor competed with GTP gamma S-Rac. EC50 values for neither p47-phox nor p67-phox were affected when mutant Racs were used in place of Rac. Data indicate direct binding of the Rac effector region to one or more components of the respiratory burst oxidase. Results indicate a general role for conserved effector-equivalent regions in small GTPases in the regulation of protein-protein interactions.

Aminoacyl chloromethanes as tools to study the requirements of NADPH oxidase activation in human neutrophils

Previous studies from this laboratory described the kinetic characteristics of the inhibition by tosylphenylalanine chloromethane (TosPheCH2Cl) on superoxide anion production by human neutrophils (PMN) stimulated with a phorbol ester (PMA). In this study we present further evidence concerning the potential role of the chloromethane target in the normal cellular activation of NADPH oxidase. When PMN are treated with TosPheCH2Cl and subsequently PMA, or with the two reagents in the reverse order, the inhibition of superoxide production by the intact cells is still present in a particulate NADPH oxidase fraction prepared from these cells. Nevertheless, when cells incubated only with the chloromethane and not with PMA are disrupted, both their cytosolic and membrane fractions are fully competent in the cell-free activation assay. Thus, the chloromethane target has a role in NADPH oxidase activation exclusively at the cellular level. This observation constitutes additional evidence in favour of the idea that activation in the cell-free system reflects only partially the events which occur in the cells. When cells are activated with PMA, their cytosol displays a loss of activating capacity in the cell-free activation assay in the presence of arachidonate, as was shown before with SDS as activator [Ambruso, D. R., Bolsher, B. G. J. M., Stockman, P. M., Verhoeven, A. J. & Roos, D. (1990) J. Biol. Chem. 265, 924-930]. This phenomenon was shown to arise most probably from the translocation of cytosolic factors to the membrane, resulting in a depleted cytosol. When superoxide production was inhibited by cell treatment with TosPheCH2Cl, either before or after activation with PMA, the cytosol from inhibited cells showed a recovery of activation capacity in the cell-free system. This effect probably results from TosPheCH2Cl inhibiting the translocation of the cytosolic factors when added before PMA. This results in an insufficient activation at the membrane level, which was previously considered as an inhibition. The effect of TosPheCH2Cl, when added after PMA, can best be explained again as an inhibition of translocation in the frame of the continuous replenishment-deactivation hypothesis proposed by Akard et al. [Akard, L. P., English, D. & Gabig, T. G. (1988) Blood 72, 322-327]. Thus, TosPheCH2Cl is apparently a promising new tool for studying the activation of NADPH oxidase at the cellular level.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of NADPH oxidase activity by Rac GTPase activating protein(s)

Activation of the NADPH oxidase of phagocytic cells requires the action of Rac2 or Rac1, members of the Ras superfamily of GTP-binding proteins. Rac proteins are active when in the GTP-bound form and can be regulated by a variety of proteins that modulate the exchange of GDP for GTP and/or GTP hydrolysis. The p190 Rac GTPase Activating Protein (GAP) inhibits human neutrophil NADPH oxidase activity in a cell-free assay system with a K1 of approximately 100 nM. Inhibition by p190 was prevented by GTP gamma S, a nonhydrolyzable analogue of GTP. Similar inhibition was seen with a second protein exhibiting Rac GAP activity, CDC42Hs GAP. The effect of p190 on superoxide (O2-) formation was reversed by the addition of a constitutively GTP-bound Rac2 mutant or Rac1-GTP gamma S but not by RhoA-GTP gamma S. Addition of p190 to an activated oxidase produced no inhibitory effect, suggesting either that p190 no longer has access to Rac in the assembled oxidase or that Rac-GTP is not required for activity once O2- generation has been initiated. These data confirm the role of Rac in NADPH oxidase regulation and support the view that it is the GTP form of Rac that is necessary for oxidase activation. Finally, they raise the possibility that NADPH oxidase may be regulated by the action of GAPs for Rac proteins.

Regulation of phagocyte function by low molecular weight GTP-binding proteins

The mechanisms used by phagocytic leukocytes in the process of bacterial killing are regulated by GTP-binding proteins of the Ras superfamily. In particular, the formation of toxic oxygen metabolites via the NADPH oxidase requires the action of both Rac and Rap1A proteins. Rac2 forms a third cytosolic component of the human neutrophil NADPH oxidase. Rac2 is active in its GTP-bound form, and requires post-translational processing (isoprenylation) in order to interact with regulatory proteins which stimulate the exchange of GTP for GDP. In the resting neutrophil, Rac is localized to the cytosol in the form of a complex with a GDP dissociation inhibitor (GDI) protein. Upon cell activation, this complex is disrupted to enable Rac to translocate to the active oxidase at the plasma membrane. The Rac-GDI complex may be regulated by the release of specific lipids known to be generated during phagocyte activation.

Cytosolic oxidase factors in the NADPH-dependent oxidase of human neutrophils

The NADPH-dependent superoxide generating system of human PMNs is a complex, multicomponent system. Studies over the past two decades have identified some of the various components both in the membrane and in the cytosol. The cytosolic factors p47 phox and p67 phox are clearly essential components of the oxidase, as evidenced by their absence producing autosomal CGD. Despite this, the specific function of each of these factors in the assembled oxidase remains unknown. In the case of p47 phox, determinants for translocation are multifactorial, depending in part on phosphorylation and in part on the participation of a functional domain at p47 phox(323-332). The importance of SH3 regions and proline-rich domains in intramolecular interactions and associations with the membrane skeleton remain to be defined. In addition, factors which modulate the assembly of this oxidase are largely unknown and their elucidation may provide insights into novel means by which to modify the inflammatory response.

Role of the rac1 p21-GDP-dissociation inhibitor for rho heterodimer in the activation of the superoxide-forming NADPH oxidase of macrophages

Activation of the superoxide (O2-)-generating NADPH oxidase of phagocytes requires the interaction of membrane-associated cytochrome b559 with three cytosolic components; p47-phox, p67-phox and sigma 1. We proposed that sigma 1 was a heterodimer composed of proteins of 22 kDa and 24 kDa that were tentatively identified as the small GTP-binding protein (G protein) rac1 p21 and GDP-dissociation inhibitor for rho (rho GDI). We now describe a modified procedure for the rapid purification of sigma 1 and demonstrate that the NADPH-oxidase-activating capacity is associated, throughout the purification sequence, with a protein binding 35S-labelled guanosine 5'-[3-O-thio]triphosphate. SDS/PAGE analysis confirmed the absolute association of sigma 1 activity with the presence of both the 22 kDa and 24 kDa proteins. Immunoblotting with a battery of antibodies against the small G proteins demonstrated that the 22-kDa protein was only recognized by antibodies reacting with rac1 p21; no reaction was found with anti-(rac2 p21), anti-[v-ras(H) p21] and anti anti-(rap1 p21). Free rac1 p21 (not in complex with rho GDI) was not detected at any stage of cytosol fractionation. The proteins comprising the sigma 1 heterodimer could be separated by reverse-phase chromatography and amino acid sequencing was performed on peptides derived by trypsin digestion of each of the isolated proteins. This demonstrated the identity of the 22-kDa protein with rac1 p21 and that of the 24-kDa protein with rho GDI. Purified heterodimeric sigma 1 did not require exogenous GTP for activity under conditions that assured the absence of free nucleotides. Treatment of the sigma 1 heterodimer with 1% sodium cholate, followed by gel filtration or anion-exchange chromatography in the presence of 1% sodium cholate, effectively separated rac1 p21 from rho GDI. Monomeric rac1 p21, obtained by these procedures, was able to stimulate cell-free O2- generation. Artificial heterodimeric sigma 1, capable of NADPH oxidase activation, could be reconstituted in vitro by recombining purified monomeric rac1 p21 and rho GDI and removing the sodium cholate used to dissociate the native sigma 1 dimer. Monomeric rac1 p21 exhibited an almost absolute dependence on exogenous GTP following removal of the endogenous nucleotide in low Mg2+ solution. Under similar conditions, heterodimeric sigma 1 was resistant to nucleotide exchange.(ABSTRACT TRUNCATED AT 400 WORDS)

Requirement for posttranslational processing of Rac GTP-binding proteins for activation of human neutrophil NADPH oxidase

Rac1 and Rac2 are closely related, low molecular weight GTP-binding proteins that have both been implicated in regulation of phagocyte NADPH oxidase. This enzyme system is composed of multiple membrane-bound and cytosolic subunits and when activated catalyzes the one-electron reduction of oxygen to superoxide. Superoxide and its highly reactive derivatives are essential for killing microorganisms. Rac proteins undergo posttranslational processing, primarily the addition of an isoprenyl group to a carboxyl-terminal cysteine residue. We directly compared recombinant Rac1 and Rac2 in a human neutrophil cell-free NADPH oxidase system in which cytosol was replaced by purified recombinant cytosolic components (p47-phox and p67-phox). Processed Rac1 and Rac2 were both highly active in this system and supported comparable rates of superoxide production. Under different cell-free conditions, however, in which suboptimal amounts of cytosol were present in the assay mixture, processed Rac2 worked much better than Rac1 at all but the lowest concentrations. This suggests that a factor in the cytosol may suppress the activity of Rac1 but not of Rac2. Unprocessed Rac proteins were only weakly able to support superoxide generation in either system, but preloading of Rac1 or Rac2 with guanosine 5'-O-(3-thio-triphosphate) (GTP gamma S) restored activity. These results indicate that processing is required for nucleotide exchange but not for interaction with oxidase components.

The respiratory burst oxidase and the molecular genetics of chronic granulomatous disease

The phagocyte respiratory burst oxidase plays a central role in the inflammatory response. This membrane-bound enzyme complex is comprised of both integral membrane and cytosolic proteins and catalyzes the formation of large quantities of superoxide in response to inflammatory stimuli. While superoxide and its oxidant derivatives normally serve a microbicidal function, excessive or inappropriate release of these products contribute to inflammatory tissue injury. Chronic granulomatous disease (CGD) is a group of inherited disorders characterized by an absent neutrophil respiratory burst, which leads to recurrent and often life-threatening infections in affected patients. The analysis of the specific cellular defects in CGD has been instrumental in the identification and characterization of individual oxidase components. Four distinct genetic subgroups are presently recognized, each involving a different protein essential for respiratory burst oxidase function. This article summarizes recent advances in the characterization of the protein components and cellular biochemistry of the respiratory burst oxidase and reviews the classification and molecular genetics of CGD. The application of these findings to new approaches to the diagnosis and treatment of CGD are also reviewed.

Biology of the Rap proteins, members of the ras superfamily of GTP-binding proteins

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