Kinetic study of the activation of the neutrophil NADPH oxidase by arachidonic acid. Antagonistic effects of arachidonic acid and phenylarsine oxide

Cell-Free NADPH Oxidase Activation Assays: A Triumph of Reductionism

The superoxide (O₂^·-)-generating NADPH oxidase complex of phagocytes comprises a membrane-associated heterodimeric flavocytochrome, known as cytochrome b ₅₅₈ (consisting of NOX2 and p22^phox) and four cytosolic regulatory proteins, p47^phox, p67^phox, p40^phox, and the small GTPase Rac. Under physiological conditions, in the resting phagocyte, O₂^·- generation is initiated by engagement of membrane receptors by a variety of stimuli, followed by signal transduction sequences leading to the translocation of the cytosolic components to the membrane and their association with the cytochrome, a process known as NADPH oxidase assembly. A consequent conformational change in NOX2 initiates the electron flow along a redox gradient, from NADPH to molecular oxygen (O₂), leading to the one-electron reduction of O₂ to O₂^·-. Historically, methodological difficulties in the study of the assembled complex derived from stimulated cells, due to its lack of stability, led to the design of "cell-free" systems (also known as "broken cells" or in vitro systems). In a major paradigm shift, the cell-free systems have as their starting point NADPH oxidase components derived from resting (unstimulated) phagocytes, or as in the predominant method at present, recombinant proteins representing the components of the NADPH oxidase complex. In cell-free systems, membrane receptor stimulation and the signal transduction sequence are absent, the accent being placed on the actual process of assembly, all of which takes place in vitro. Thus, a mixture of the individual components of the NADPH oxidase is exposed in vitro to an activating agent, the most common being anionic amphiphiles, resulting in the formation of a complex between cytochrome b ₅₅₈ and the cytosolic components and O₂^·- generation in the presence of NADPH. Alternative activating pathways require posttranslational modification of oxidase components or modifying the phospholipid milieu surrounding cytochrome b ₅₅₈. Activation is commonly quantified by measuring the primary product of the reaction, O₂^·-, trapped immediately after its generation by an appropriate acceptor in a kinetic assay, permitting the calculation of rates of O₂^·- production, but numerous variations exist, based on the assessment of reaction products or the consumption of substrates. Cell-free assays played a paramount role in the identification and characterization of the components of the NADPH oxidase complex, the performance of structure-function studies, the deciphering of the mechanisms of assembly, the search for inhibitory drugs, and the diagnosis of various forms of chronic granulomatous disease (CGD).

Conversion of NOX2 into a constitutive enzyme in vitro and in living cells, after its binding with a chimera of the regulatory subunits

During the phagocytosis of pathogens by phagocyte cells, the NADPH oxidase complex is activated to produce superoxide anion, a precursor of microbial oxidants. The activated NADPH oxidase complex from phagocytes consists in two transmembrane proteins (Nox2 and p22^phox) and four cytosolic proteins (p40^phox, p47^phox, p67^phox and Rac1-2). In the resting state of the cells, these proteins are dispersed in the cytosol, the membrane of granules and the plasma membrane. In order to synchronize the assembly of the cytosolic subunits on the membrane components of the oxidase, a fusion of the cytosolic proteins p47^phox, p67^phox and Rac1 named trimera was constructed. The trimera investigated in this paper is composed of the p47^phox segment 1-286, the p67^phox segment 1-212 and the mutated Rac1(Q61L). We demonstrate that the complex trimera-cyt b₅₅₈ is functionally comparable to the one containing the separated subunits. Each of the subunits p47^phox, p67^phox and Rac1Q61L has kept its own activating property. The trimera is produced in an activated conformation as seen by circular dichroism. However, the presence of amphiphile is still necessary in a cell-free system to trigger superoxide anion production. The COS7^gp91-p22 cells expressing the trimera produce continuously superoxide anion at high rate. This constitutive activity in cells can be of particular interest for understanding the NADPH oxidase functioning independently of signaling pathways.

Inhibition of NOX2 reduces locomotor impairment, inflammation, and oxidative stress after spinal cord injury

Background

Spinal cord injury (SCI) results in the activation of the NADPH oxidase (NOX) enzyme, inducing production of reactive oxygen species (ROS). We hypothesized that the NOX2 isoform plays an integral role in post-SCI inflammation and functional deficits.

Methods

Moderate spinal cord contusion injury was performed in adult male mice, and flow cytometry, western blot, and immunohistochemistry were used to assess NOX2 activity and expression, inflammation, and M1/M2 microglia/macrophage polarization from 1 to 28 days after injury. The NOX2-specific inhibitor, gp91ds-tat, was injected into the intrathecal space immediately after impact. The Basso Mouse Scale (BMS) was used to assess locomotor function at 24 h post-injury and weekly thereafter.

Results

Our findings show that gp91ds-tat treatment significantly improved functional recovery through 28 days post-injury and reduced inflammatory cell concentrations in the injured spinal cord at 24 h and 7 days post-injury. In addition, a number of oxidative stress markers were reduced in expression at 24 h after gp91ds-tat treatment, which was accompanied by a reduction in M1 polarization marker expression.

Conclusion

Based on our findings, we now conclude that inhibition of NOX2 significantly improves outcome after SCI, most likely via acute reductions in oxidative stress and inflammation. NOX2 inhibition may therefore have true potential as a therapy after SCI.

NOX2 As a Target for Drug Development: Indications, Possible Complications, and Progress

SIGNIFICANCE

NOX2 is important for host defense, and yet is implicated in a large number of diseases in which inflammation plays a role in pathogenesis. These include acute and chronic lung inflammatory diseases, stroke, traumatic brain injury, and neurodegenerative diseases, including Alzheimer's and Parkinson's Diseases.

RECENT ADVANCES

Recent drug development programs have targeted several NOX isoforms that are implicated in a variety of diseases. The focus has been primarily on NOX4 and NOX1 rather than on NOX2, due, in part, to concerns about possible immunosuppressive side effects. Nevertheless, NOX2 clearly contributes to the pathogenesis of many inflammatory diseases, and its inhibition is predicted to provide a novel therapeutic approach.

CRITICAL ISSUES

Possible side effects that might arise from targeting NOX2 are discussed, including the possibility that such inhibition will contribute to increased infections and/or autoimmune disorders. The state of the field with regard to existing NOX2 inhibitors and targeted development of novel inhibitors is also summarized.

FUTURE DIRECTIONS

NOX2 inhibitors show particular promise for the treatment of inflammatory diseases, both acute and chronic. Theoretical side effects include pro-inflammatory and autoimmune complications and should be considered in any therapeutic program, but in our opinion, available data do not indicate that they are sufficiently likely to eliminate NOX2 as a drug target, particularly when weighed against the seriousness of many NOX2-related indications. Model studies demonstrating efficacy with minimal side effects are needed to encourage future development of NOX2 inhibitors as therapeutic agents.

Cross-Presentation of Cell-Associated Antigens by MHC Class I in Dendritic Cell Subsets

Dendritic cells (DCs) have the unique ability to pick up dead cells carrying antigens in tissue and migrate to the lymph nodes where they can cross-present cell-associated antigens by MHC class I to CD8⁺ T cells. There is strong in vivo evidence that the mouse XCR1⁺ DCs subset acts as a key player in this process. The intracellular processes underlying cross-presentation remain controversial and several pathways have been proposed. Indeed, a wide number of studies have addressed the cellular process of cross-presentation in vitro using a variety of sources of antigen and antigen-presenting cells. Here, we review the in vivo and in vitro evidence supporting the current mechanistic models and disscuss their physiological relevance to the cross-presentation of cell-associated antigens by DCs subsets.

The dehydrogenase region of the NADPH oxidase component Nox2 acts as a protein disulfide isomerase (PDI) resembling PDIA3 with a role in the binding of the activator protein p67 (phox.)

The superoxide (O(·-) 2)-generating NADPH oxidase of phagocytes consists of a membrane component, cytochrome b 558 (a heterodimer of Nox2 and p22 (phox) ), and four cytosolic components, p47 (phox) , p67 (phox) , p40 (phox) , and Rac. The catalytic component, responsible for O(·-) 2 generation, is Nox2. It is activated by the interaction of the dehydrogenase region (DHR) of Nox2 with the cytosolic components, principally with p67 (phox) . Using a peptide-protein binding assay, we found that Nox2 peptides containing a (369)CysGlyCys(371) triad (CGC) bound p67 (phox) with high affinity, dependent upon the establishment of a disulfide bond between the two cysteines. Serially truncated recombinant Nox2 DHR proteins bound p67 (phox) only when they comprised the CGC triad. CGC resembles the catalytic motif (CGHC) of protein disulfide isomerases (PDIs). This led to the hypothesis that Nox2 establishes disulfide bonds with p67 (phox) via a thiol-dilsulfide exchange reaction and, thus, functions as a PDI. Evidence for this was provided by the following: (1) Recombinant Nox2 protein, which contained the CGC triad, exhibited PDI-like disulfide reductase activity; (2) Truncation of Nox2 C-terminal to the CGC triad or mutating C369 and C371 to R, resulted in loss of PDI activity; (3) Comparison of the sequence of the DHR of Nox2 with PDI family members revealed three small regions of homology with PDIA3; (4) Two monoclonal anti-Nox2 antibodies, with epitopes corresponding to regions of Nox2/PDIA3 homology, reacted with PDIA3 but not with PDIA1; (5) A polyclonal anti-PDIA3 (but not an anti-PDIA1) antibody reacted with Nox2; (6) p67 (phox) , in which all cysteines were mutated to serines, lost its ability to bind to a Nox2 peptide containing the CGC triad and had an impaired capacity to support oxidase activity in vitro. We propose a model of oxidase assembly in which binding of p67 (phox) to Nox2 via disulfide bonds, by virtue of the intrinsic PDI activity of Nox2, stabilizes the primary interaction between the two components.

Vicinal thiols are required for activation of the αIIbβ3 platelet integrin

BACKGROUND

 Closely spaced thiols in proteins that interconvert between the dithiol form and disulfide bonds are called vicinal thiols. These thiols provide a mechanism to regulate protein function. We previously found that thiols in both αIIb and β3 of the αIIbβ3 fibrinogen receptor were required for platelet aggregation.

METHODS AND RESULTS

 Using p-chloromercuribenzene sulfonate (pCMBS) we provide evidence that surface thiols in αIIbβ3 are exposed during platelet activation. Phenylarsine oxide (PAO), a reagent that binds vicinal thiols, inhibits platelet aggregation and labeling of sulfhydryls in both αIIb and β3. For the aggregation and labeling studies, binding of PAO to vicinal thiols was confirmed by reversal of PAO binding with the dithiol reagent 2,3-Dimercapto-1-propanesulfonic acid (DMPS). In contrast, the monothiol β-mercaptoethanol did not reverse the effects of PAO. Additionally, PAO did not inhibit sulfhydryl labeling of the monothiol protein albumin, confirming the specificity of PAO for vicinal thiols in αIIbβ3. As vicinal thiols represent redox sensitive sites that can be regulated by reducing equivalents from the extracellular or cytoplasmic environment, they are likely to be important in regulating activation of αIIbβ3. Additionally, when the labeled integrin was passed though a lectin column containing wheat germ agglutinin and lentil lectin a substantial amount of non-labeled αIIbβ3 eluted separately from the labeled receptor. This suggests that two populations of integrin exist on platelets that can be distinguished by thiol labeling.

CONCLUSION

 A vicinal thiol-containing population of αIIbβ3 provides redox sensitive sites for regulation of αIIbβ3.

S100A8/A9: a Janus-faced molecule in cancer therapy and tumorgenesis

Correlations exist between the abundance of S100 proteins and disease pathologies. Indeed, this is evidenced by the heterodimeric S100 protein complex S100A8/A9 which has been shown to be involved in inflammatory and neoplastic disorders. However, S100A8/A9 appears as a Janus-faced molecule in this context. On the one hand, it is a powerful apoptotic agent produced by immune cells, making it a very fascinating tool in the battle against cancer. It spears the risk to induce auto-immune response and may serve as a lead compound for cancer-selective therapeutics. In contrast, S100A8/A9 expression in cancer cells has also been associated with tumor development, cancer invasion or metastasis. Clearly, there is a dichotomy and future investigations into the role of S100A8/A9 in cancer biology need to consider both sides of the same coin.

Redox control of platelet function

There has recently been a dramatic expansion in research in the area of redox biology with systems that utilize thiols to perform redox chemistry being central to redox control. Thiol-based reactions occur in proteins involved in platelet function, including extracellular platelet proteins. The alphaIIbbeta3 fibrinogen receptor contains free thiols that are required for the activation of this receptor to a fibrinogen-binding conformation. This process is under enzymatic control, with protein disulfide isomerase playing a central role in the activation of alphaIIbbeta3. Other integrins, such as the alpha2beta1 collagen receptor on platelets, are also regulated by protein disulfide isomerase and thiol metabolism. Low molecular weight thiols that are found in blood regulate these processes by converting redox sensitive disulfide bonds to thiols and by providing the appropriate redox potential for these reactions. Additional mechanisms of redox control of platelets involve nitric oxide that inhibits platelet responses, and reactive oxygen species that potentiate platelet thrombus formation. Specific nitrosative or oxidative modifications of thiol groups in platelets may modulate platelet function. Since many biologic processes are regulated by redox reactions that involve surface thiols, the extracellular redox state can have an important influence on health and disease status and may be a target for therapeutic intervention.

Free Fatty Acids and Methyl Jasmonate Trigger Defense Reactions in Laminaria digitata

Arachidonic acid, linolenic acid and methyl jasmonate (MeJA) were found to be strong triggers of an oxidative burst in the kelp Laminaria digitata. These findings constitute the first report of an oxidative burst in an algal system induced by free fatty acids. The source of reactive oxygen species can be at least partially inhibited by diphenylene iodonium (DPI). Treatment with arachidonic acid increases the levels of a number of free fatty acids [including myristic (C14:0), linoleic (C18:2), linolenic (C18:3) and eicosapentaeneoic (C20:5) acids] and hydroxylated derivatives [such as 15-hydroxyeicosatetraenoic acid (15-HETE), 13-hydroxyoctadecatrienoic acid (13-HOTE) and 15-hydroxyeicosapentaenoic acid (15-HEPE)]. Similar to a previous report of the function of an alginate oligosaccharide-triggered oxidative burst in the establishment of resistance in L. digitata against infection by its brown algal endophyte Laminariocolax tomentosoides, C20:4- and MeJA-induced oxidative bursts seem to be involved in establishing the same protection in L. digitata. Altogether, this study supports the notion that lipid oxidation signaling plays a key role in defense induction in marine brown algae.

Fatty acids as modulators of the cellular production of reactive oxygen species

Long-chain nonesterified ("free") fatty acids (FFA) and some of their derivatives and metabolites can modify intracellular production of reactive oxygen species (ROS), in particular O(2)(-) and H(2)O(2). In mitochondria, FFA exert a dual effect on ROS production. Because of slowing down the rate of electron flow through Complexes I and III of the respiratory chain due to interaction within the complex subunit structure, and between Complexes III and IV due to release of cytochrome c from the inner membrane, FFA increase the rate of ROS generation in the forward mode of electron transport. On the other hand, due to their protonophoric action on the inner mitochondrial membrane ("mild uncoupling effect"), FFA strongly decrease ROS generation in the reverse mode of electron transport. In the plasma membrane of phagocytic neutrophils and a number of other types of cells, polyunsaturated FFA stimulate O(2)(-) generation by NADPH oxidase. These effects of FFA can modulate signaling functions of ROS and be, at least partly, responsible for their proapoptotic effects in several types of cells.

Potential role of NADPH-oxidase in early steps of lead-induced oxidative burst in Vicia faba roots

The mechanism of oxidative burst induced by lead in Vicia faba excised roots was investigated by luminol-dependent chemiluminescence. Results showed that lead triggered a rapid and dose-dependent increase in chemiluminescence production. In this study, specific inhibitors of putative reactive oxygen species (ROS) sources were used to determine the mechanism of lead-induced ROS generation. This generation was sensitive to dephenylene iodonium (DPI), quinacrine and imidazole, some inhibitors of the NADPH-oxidase and not inhibited by other putative ROS sources inhibitors. Data reported in this work clearly demonstrated the pivotal role of NADPH-oxidase-like enzyme in early steps of lead-induced oxidative burst. To investigate the respective implication of calmodulin and protein kinase (PK) in lead-induced NADPH-oxidase activation, excised roots were treated with the calmodulin inhibitor W7 or with the PK inhibitor staurosporine. The chemiluminescence generation inhibition by these inhibitors illustrated the role of PK in lead-induced NADPH-oxidase activation and revealed a calmodulin-dependent step. Using the calcium entry blocker La(3+) or different concentrations of calcium in the extra-cellular medium, our data highlighted the implication of Ca(2+) channel in lead-induced oxidative burst.

Lipid signaling and the modulation of surface charge during phagocytosis

Phagocytosis is an important component of innate and adaptive immunity. The formation of phagosomes and the subsequent maturation that capacitates them for pathogen elimination and antigen presentation are complex processes that involve signal transduction, cytoskeletal reorganization, and membrane remodeling. Lipids are increasingly appreciated to play a crucial role in these events. Sphingolipids, cholesterol, and glycerophospholipids, notably the phosphoinositides, are required for the segregation of signaling microdomains and for the generation of second messengers. They are also instrumental in the remodeling of the actin cytoskeleton and in directing membrane traffic. They accomplish these feats by congregating into liquid-ordered domains, by generating active metabolites that activate receptors, and by recruiting and anchoring specific protein ligands to the membrane, often altering their conformation and catalytic activity. A less appreciated role of acidic phospholipids is their contribution to the negative surface charge of the inner leaflet of the plasmalemma. The unique negativity of the inner aspect of the plasma membrane serves to attract and anchor key signaling and effector molecules that are required to initiate phagosome formation. Conversely, the loss of charge that accompanies phospholipid metabolism as phagosomes seal facilitates the dissociation of proteins and the termination of signaling and cytoskeleton assembly. In this manner, lipids provide a binary electrostatic switch to control phagocytosis.

HaCaT keratinocytes overexpressing the S100 proteins S100A8 and S100A9 show increased NADPH oxidase and NF-kappaB activities

The calcium- and arachidonic acid (AA)-binding proteins S100A8 and S100A9 are involved in nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activation in phagocytes. They are specifically expressed in myeloid cells, and are also found in epithelial cells in various (patho)physiological conditions. We have investigated the consequences of S100A8/A9 overexpression in epithelial cell lines on reactive oxygen species (ROS) generation and downstream signaling. Epithelial carcinoma HeLa cells, which exclusively express Nox2, showed dramatically increased activation of NADPH oxidase by phorbol 12-myristate 13-acetate after S100A8/A9 gene transfection. HaCaT keratinocytes overexpressing S100A8/A9 showed enhanced, transient ROS generation in response to the calcium ionophore A23187 compared to mock-transfected cells. Polymerase chain reaction analysis revealed mRNA transcripts for Nox1, Nox2, and Nox5 in HaCaT keratinocytes. Detailed transfection studies confirmed that NADPH oxidase activities in Nox1- and Nox5-transfected HeLa cells were enhanced after S100A8/A9 gene complementation. Furthermore, mutational analysis revealed that AA binding and Thr113 phosphorylation are important for S100A8/A9-enhanced activation of NADPH oxidase. Nuclear factor-kappaB (NF-kappaB) activation and interleukin-8 mRNA levels were increased in S100A8/A9-HaCaT keratinocytes, consistent with the view that NF-kappaB is a redox-sensitive transcription factor. Because they are expressed in epithelia under specific conditions, S100A8 and S100A9 might be involved in skin pathogenesis by modulating aspects of downstream signaling.

Cis-unsaturated fatty acids stimulate reactive oxygen species generation and lipid peroxidation in human spermatozoa

CONTEXT

Defective sperm function is the largest defined cause of human infertility; however, the etiology of this condition is poorly understood. Although oxidative stress is acknowledged as a key contributor to this pathology, there are also data indicating that defective human spermatozoa contain abnormally high amounts of cis-unsaturated fatty acids. This study investigated whether a causative relationship exists between these two attributes of impaired semen quality.

OBJECTIVE

The objective of this study was to determine whether polyunsaturated fatty acids can induce oxidative stress in human spermatozoa.

METHOD

Dihydroethidium and SYTOX Green were used in conjunction with flow cytometry and HPLC to investigate reactive oxygen species (ROS) generation by human spermatozoa after fatty acid exposure.

RESULTS

Arachidonic acid (AA) induced a time- and dose-dependent increase in ROS generation by human spermatozoa that led to the promotion of peroxidative damage and a loss of sperm motility. This effect could not be blocked with inhibitors of the cyclooxygenase or lipoxygenase pathways of AA metabolism, rotenone, protein kinase C antagonists, or known inhibitors of plasma membrane redox systems. However, ROS generation could be triggered with other cis-unsaturated fatty acids including linoleic and docosahexaenoic acids. Saturated fatty acids, methyl esters of unsaturated fatty acids, or other amphiphiles were all ineffective. However in a cell-free system, AA could trigger a redox signal via mechanisms that were profoundly disrupted by diphenylene iodonium, a flavoprotein inhibitor.

CONCLUSIONS

The presence of excess unsaturated fatty acids in defective human spermatozoa may precipitate the oxidative stress encountered in male infertility.

The role of cytosolic phospholipase A2-alfa in regulation of phagocytic functions

Phospholipase A2(s) (PLA2(s)) are a family of enzymes that is present in a variety of mammalian and nonmammalian sources. Phagocytic cells contain cytosolic PLA2 (cPLA2) as well as several types of secreted PLA2, all of which have the potential to produce proinflammatory lipid mediators. The role of the predominant form of cPLA2 present in neutrophils is cPLA2alpha was studied by many groups. By modulating its expression in a variety of phagocytes it was found that it plays a major role in formation of eicosanoids. In addition, it was reported that cPLA2alpha also regulates the NADPH oxidase activation. The specificity of its effect on the NADPH oxidase is evident by results demonstrating that the differentiation process as well as other phagocytic functions are normal in cPLA2alpha-deficient PLB cell model. The novel dual subcellular localization of cPLA2alpha in different compartments, in the plasma membranes and in the nucleus, provides a molecular mechanism for the participation of cPLA2alpha in different processes (stimulation of NADPH oxidase and formation of eicosanoids) in the same cells.

Single-cell Raman and fluorescence microscopy reveal the association of lipid bodies with phagosomes in leukocytes

Cellular imaging techniques based on vibrational spectroscopy have become powerful tools in cell biology because the molecular composition of subcellular compartments can be visualized without the need for labeling. Using high-resolution, nonresonant confocal Raman microscopy on individual cells, we demonstrate here that lipid bodies (LBs) rich in arachidonate as revealed by their Raman spectra associate with latex bead-containing phagosomes in neutrophilic granulocytes. This finding was corroborated in macrophages and in PLB-985 cells, which can be induced to differentiate into neutrophil-like cells, by selective staining of LBs and visualization by confocal fluorescence microscopy. We further show that the accumulation of LBs near phagosomes is mediated at least in part by the flavohemoprotein gp91phox (in which "phox" is phagocyte oxidase), because different LB distributions around phagocytosed latex beads were observed in WT and gp91phox-deficient PLB-985 cells. gp91phox, which accumulates in the phagosomal membrane, is the catalytic subunit of the leukocyte NADPH oxidase, a critical enzyme in the innate immune response. Finally, time-lapse fluorescence microscopy experiments on neutrophils revealed that the LB-phagosome association is transient, similar to the "kiss-and-run" behavior displayed by endosomes involved in phagosome maturation. Because arachidonic acid (AA) has been shown to be involved in NADPH oxidase activation and phagosome maturation in neutrophils and macrophages, respectively, the findings reported here suggest that LBs may provide a reservoir of AA for local activation of these essential leukocyte functions.

The arachidonic acid-binding protein S100A8/A9 promotes NADPH oxidase activation by interaction with p67phox and Rac-2

The Ca2+- and arachidonic acid-binding S100A8/A9 protein complex was recently identified by in vitro studies as a novel partner of the phagocyte NADPH oxidase. The present study demonstrated its functional relevance by the impaired oxidase activity in neutrophil-like NB4 cells, after specific blockage of S100A9 expression, and bone marrow polymorphonuclear neutrophils from S100A9-/- mice. The impaired oxidase activation could also be mimicked in a cell-free system by pretreatment of neutrophil cytosol with an S100A9-specific antibody. Further analyses gave insights into the molecular mechanisms by which S100A8/A9 promoted NADPH oxidase activation. In vitro analysis of oxidase activation as well as protein-protein interaction studies revealed that S100A8 is the privileged interaction partner for the NADPH oxidase complex since it bound to p67phox and Rac, whereas S100A9 did interact with neither p67phox nor p47phox. Moreover, S100A8/A9 transferred the cofactor arachidonic acid to NADPH oxidase as shown by the impotence of a mutant S100A8/A9 complex unable to bind arachidonic acid to enhance NADPH oxidase activity. It is concluded that S100A8/A9 plays an important role in phagocyte NADPH oxidase activation.

Lipids, lipases, and lipid-modifying enzymes in plant disease resistance

Lipids and lipid metabolites influence pathogenesis and resistance mechanisms associated with plant-microbe interactions. Some microorganisms sense their presence on a host by perceiving plant surface waxes, whereas others produce toxins that target plant lipid metabolism. In contrast, plants have evolved to recognize microbial lipopolysaccharides (LPSs), sphingolipids, and lipid-binding proteins as elicitors of defense response. Recent studies have demonstrated that the plasma membrane provides a surface on which some plant resistance (R) proteins perceive pathogen-derived effectors and thus confer race-specific resistance. Plant cell membranes also serve as reservoirs from which biologically active lipids and precursors of oxidized lipids are released. Some of these oxylipins, for example jasmonic acid (JA), are important signal molecules in plant defense. Arabidopsis thaliana is an excellent model plant to elucidate the biosynthesis and metabolism of lipids and lipid metabolites, and the characterization of signaling mechanisms involved in the modulation of plant defense responses by phytolipids. This review focuses on recent studies that highlight the involvement of lipids and lipid metabolites, and enzymes involved in lipid metabolism and modification in plant disease resistance.

Binding of arachidonic acid to myeloid-related proteins (S100A8/A9) enhances phagocytic NADPH oxidase activation

Activation of the O(2)(-) generating NADPH oxidase of phagocytes results from the assembly of the membrane-bound flavocytochrome b(558) with cytosolic proteins, p67(phox), p47(phox), and Rac. However, it has been recently reported that the arachidonic acid- and calcium-binding heterodimer S100A8/A9, abundant in neutrophil cytosol, influences the activation process. In a semi-recombinant system comprising neutrophil membranes, recombinant proteins, p67(phox), p47(phox), GTPgamma S-loaded Rac2, and arachidonic acid (AA), both the rate and the extent of the oxidase activation were increased by S100A8/A9, provided it was preloaded with AA. Binding of [(14)C]AA to S100A8/A9 was potentiated by recombinant cytosolic phox proteins and GTPgammaS, suggesting the formation of a complex, comprising oxidase activating proteins and S100A8/A9, with a greater affinity for AA. The rate constant of oxidase activation was not increased by AA-loaded S100A8/A9, whereas the maximal oxidase activity elicited was twice as high. AA-loaded S100A8/A9 increases oxidase activation probably by decreasing the deactivation rate.

Role of chloroplast trienoic fatty acids in plant disease defense responses

Trienoic fatty acids (TAs) are the major polyunsaturated fatty acid species in the membrane lipids in plant cells. TAs are crucial for the adaptation to abiotic stresses, especially low- or high-temperature stress. We show that TAs in chloroplast membrane lipids are involved in defense responses against avirulent bacterial pathogens. Avirulent pathogen invasion of plants induces a transient production of reactive oxygen intermediates (ROI), programmed cell death and subsequent disease resistance. The Arabidopsis fad7fad8 mutation, which prevents the synthesis of TAs in chloroplast lipids, caused the reduction in ROI accumulation in leaves inoculated with Pseudomonas syringae pv. tomato DC3000 (avrRpm1). Linolenic acid, the most abundant TA, activated the NADPH oxidase that is responsible for ROI generation. TAs were transferred from chloroplast lipids to extrachloroplast lipids coincident with ROI accumulation after inoculation with Pst DC3000 (avrRpm1). Furthermore, the fad7fad8 mutant exhibited reduced cell death and was compromised in its resistance to several avirulent P. syringae strains. These results suggest that TAs derived from chloroplast lipids play an important role in the regulation of plant defense responses.

The NADPH oxidase of professional phagocytes--prototype of the NOX electron transport chain systems

The NADPH oxidase is an electron transport chain in "professional" phagocytic cells that transfers electrons from NADPH in the cytoplasm, across the wall of the phagocytic vacuole, to form superoxide. The electron transporting flavocytochrome b is activated by the integrated function of four cytoplasmic proteins. The antimicrobial function of this system involves pumping K+ into the vacuole through BKCa channels, the effect of which is to elevate the vacuolar pH and activate neutral proteases. A number of homologous systems have been discovered in plants and lower animals as well as in man. Their function remains to be established.

Anionic amphiphile and phospholipid-induced conformational changes in human neutrophil flavocytochrome b observed by fluorescence resonance energy transfer

The integral membrane protein flavocytochrome b (Cyt b) comprises the catalytic core of the human phagocyte NADPH oxidase complex and serves to initiate a cascade of reactive oxygen species that participate in the elimination of infectious agents. Superoxide production by the NADPH oxidase complex has been shown to be specifically regulated by the enzymatic generation of lipid second messengers following phagocyte activation. In the present study, a Cyt b-specific monoclonal antibody (mAb 44.1) was labeled with Cascade Blue (CCB) and used in resonance energy transfer (RET) studies probing the effects of a panel of lipid species on the structure of Cyt b. The binding of CCB-mAb 44.1 to immunoaffinity-purified Cyt b was both highly specific and resulted in significant quenching of the steady state donor fluorescence. Titration of the CCB-mAb 44.1:Cyt b complex with the anionic amphiphile lithium dodecyl sulfate (LDS) resulted in a saturable relaxation of fluorescence quenching due to conformational changes in Cyt b at concentrations of the amphiphile required for maximum rates of superoxide production by Cyt b in cell-free assays. Similar results were observed for the anionic amphiphile arachidonic acid (AA), although no relaxation of fluorescence quenching was observed for arachidonate methyl ester (AA-ME). Saturable relaxation of fluorescence quenching was also observed with the anionic, 18:1 phospholipids phosphatidic acid (DOPA) and phosphatidylserine (DOPS), while no relaxation was observed upon addition of the neutral 18:1 lipids phosphatidylcholine (DOPC), phosphatidylethanolamine (DOPE) or diacylglycerol (DAG) at similar levels. Further examination of a variety of phosphatidic acid (PA) species demonstrated DOPA to both potently induce conformational changes in Cyt b and to cause more dramatic conformational changes than PA species with shorter, saturated acyl chains. The data presented in this study support the hypothesis that second messenger lipids, such as AA and PA, directly bind to flavocytochrome b and modulate conformational states relevant to the activation of superoxide production.

Changing the conformation state of cytochrome b558 initiates NADPH oxidase activation: MRP8/MRP14 regulation

Phagocyte NADPH oxidase generates O2. for defense mechanisms and cellular signaling. Myeloid-related proteins MRP8 and MRP14 of the S100 family are EF-hand calcium-binding proteins. MRP8 and MRP14 were co-isolated from neutrophils on an anti-p47phox matrix with oxidase cytosolic factors and identified by mass spectrometry. MRP8 and MRP14 are absent from Epstein-Barr virus-immortalized B lymphocytes, and, coincidentally, these cells display weak oxidase activity compared with neutrophils. MRP8/MRP14 that was purified from neutrophils enhanced oxidase turnover of B cells in vitro, suggesting that MRP8/MRP14 is involved in the activation process. This was confirmed ex vivo by co-transfection of Epstein-Barr virus-transformed B lymphocytes with genes encoding MRP8 and MRP14. In a semi-recombinant cell-free assay, recombinant MRP8/MRP14 increased the affinity of p67phox for cytochrome b558 synergistically with p47phox. Moreover, MRP8/MRP14 initiated oxidase activation on its own, through a calcium-dependent specific interaction with cytochrome b558 as shown by atomic force microscopy and a structure-function relationship investigation. The data suggest that the change of conformation in cytochrome b558, which initiates the electron transfer, can be mediated by effectors other than oxidase cytosolic factors p67phox and p47phox. Moreover, MRP8/MRP14 dimer behaves as a positive mediator of phagocyte NADPH oxidase regulation.

Structural changes are induced in human neutrophil cytochrome b by NADPH oxidase activators, LDS, SDS, and arachidonate: intermolecular resonance energy transfer between trisulfopyrenyl-wheat germ agglutinin and cytochrome b(558)

Anionic amphiphiles such as sodium- and lithium dodecyl sulfate (SDS, LDS), or arachidonate (AA) initiate NADPH oxidase and proton channel activation in cell-free systems and intact neutrophils. To investigate whether these amphiphiles exert allosteric effects on cytochrome b, trisulfopyrenyl-labeled wheat germ agglutinin (Cascade Blue-wheat germ agglutinin, CCB-WGA) was used as an extrinsic fluorescence donor for resonance energy transfer (RET) to the intrinsic heme acceptors of detergent-solubilized cytochrome b. In solution, cytochrome b complexed with the CCB-WGA causing a rapid, saturable, carbohydrate-dependent quenching of up to approximately 55% of the steady-state fluorescence. Subsequent additions of SDS, LDS, or AA to typical cell-free oxidase assay concentrations completely relaxed the fluorescence quenching. The relaxation effects were specific, and not caused by dissociation of the CCB-WGA-cytochrome b complex or alterations in the spectral properties of the chromophores. In contrast, addition of the oxidase antagonist, arachidonate methyl ester, caused an opposite effect and was able to partially reverse the activator-induced relaxation. We conclude that the activators induce a cytochrome b conformation wherein the proximity or orientation between the hemes and the extrinsic CCB fluorescence donors has undergone a significant change. These events may be linked to NADPH oxidase assembly and activation or proton channel induction.

Role of components of the phagocytic NADPH oxidase in oxygen sensing

It has been hypothesized that O(2) sensing in type I cells of the carotid body and erythropoietin (EPO)-producing cells of the kidney involves protein components identical to the NADPH oxidase system responsible for the respiratory burst of phagocytes. In the present study, we evaluated O(2) sensing in mice with null mutant genotypes for two components of the phagocytic oxidase. Whole body plethysmography was used to study unanesthetized, unrestrained mice. When exposed to an acute hypoxic stimulus, gp91(phox)-null mutant and wild-type mice increased their minute ventilation by similar amounts. In contrast, p47(phox)-null mutant mice demonstrated increases in minute ventilation in response to hypoxia that exceeded that of their wild-type counterparts: 98.0 +/- 18.0 vs. 20.0 +/- 13.0% (n = 11, P = 0.003). In vitro recordings of carotid sinus nerve (CSN) activity demonstrated that resting (basal) neural activity was marginally elevated in p47(phox)-null mutant mice. With hypoxic challenge, mean CSN discharge was 1.5-fold greater in p47(phox)-null mutant than in wild-type mice: 109.61 +/- 13.29 vs. 72.54 +/- 7.65 impulses/s (n = 8 and 7, respectively, P = 0.026). Consequently, the hypoxia-evoked CSN discharge (stimulus-basal) was approximately 58% larger in p47(phox)-null mutant mice. Quantities of EPO mRNA in kidney were similar in gp91(phox)- and p47(phox)-null mutant mice and their respective wild-type controls exposed to hypobaric hypoxia for 72 h. These findings confirm the previous observation that absence of the gp91(phox) component of the phagocytic NADPH oxidase does not alter the O(2)-sensing mechanism of the carotid body. However, absence of the p47(phox) component significantly potentiates ventilatory and chemoreceptor responses to hypoxia. O(2) sensing in EPO-producing cells of the kidney appears to be independent of the gp91(phox) and p47(phox) components of the phagocytic NADPH oxidase.

The S100A8/A9 protein as a partner for the cytosolic factors of NADPH oxidase activation in neutrophils

In a previous study, the S100A8/A9 protein, a Ca2+- and arachidonic acid-binding protein, abundant in neutrophil cytosol, was found to potentiate the activation of the redox component of the O2- generating oxidase in neutrophils, namely the membrane-bound flavocytochrome b, by the cytosolic phox proteins p67phox, p47phox and Rac (Doussière J., Bouzidi F. and Vignais P.V. (2001) Biochem. Biophys. Res. Commun.285, 1317-1320). This led us to check by immunoprecipitation and protein fractionation whether the cytosolic phox proteins could bind to S100A8/A9. Following incubation of a cytosolic extract from nonactivated bovine neutrophil with protein A-Sepharose bound to anti-p67phox antibodies, the recovered immunoprecipitate contained the S100 protein, p47phox and p67phox. Cytosolic protein fractionation comprised two successive chromatographic steps on hydroxyapatite and DEAE cellulose, followed by isoelectric focusing. The S100A8/A9 heterodimeric protein comigrated with the cytosolic phox proteins, and more particularly with p67phox and Rac2, whereas the isolated S100A8 protein displayed a tendancy to bind to p47phox. Using a semirecombinant cell-free system of oxidase activation consisting of recombinant p67phox, p47phox and Rac2, neutrophil membranes and arachidonic acid, we found that the S100A8/A9-dependent increase in the elicited oxidase activity corresponded to an increase in the turnover of the membrane-bound flavocytochrome b, but not to a change of affinity for NADPH or O2. In the absence of S100A8/A9, oxidase activation departed from michaelian kinetics above a critical threshold concentration of cytosolic phox proteins. Addition of S100A8/A9 to the cell-free system rendered the kinetics fully michaelian. The propensity of S100A8/A9 to bind the cytosolic phox proteins, and the effects of S100A8/A9 on the kinetics of oxidase activation, suggest that S100A8/A9 might be a scaffold protein for the cytosolic phox proteins or might help to deliver arachidonic acid to the oxidase, thus favoring the productive interaction of the cytosolic phox proteins with the membrane-bound flavocytochrome b.

Exploration of the diaphorase activity of neutrophil NADPH oxidase

In the O2- generating flavocytochrome b, the membrane-bound component of the neutrophil NADPH oxidase, electrons are transported from NADPH to O2 in the following sequence: NADPH --> FAD --> heme b -->O2. Although p-iodonitrotetrazolium (INT) has frequently been used as a probe of the diaphorase activity of the neutrophil flavocytochrome b, the propensity of its radical to interact reversibly with O2 led us to question its specificity. This study was undertaken to reexamine the interaction of INT with the redox components of the neutrophil flavocytochrome b. Two series of inhibitors were used, namely the flavin analog 5-deaza FAD and the heme inhibitors bipyridyl and benzylimidazole. The following results indicate that INT reacts preferentially with the hemes rather than with the FAD redox center of flavocytochrome b and is not therefore a specific probe of the diaphorase activity of flavocytochrome b. First, in anaerobiosis, reduced heme b in activated membranes was reoxidized by INT as efficiently as by O2 even in the presence of concentrations of 5-deaza FAD which fully inhibited the NADPH oxidase activity. Second, the titration curve of dithionite-reduced heme b in neutrophil membranes obtained by oxidation with increasing amounts of INT was strictly superimposable on that of dithionite-reduced hemin. Third, INT competitively inhibited the O2 uptake by the activated NADPH oxidase in a cell-free system. Finally, the heme inhibitor bipyridyl competitively inhibited the reduction of INT in anaerobiosis, and the oxygen uptake in aerobiosis.

Heme histidine ligands within gp91(phox) modulate proton conduction by the phagocyte NADPH oxidase

The membrane subunit of the phagocyte NADPH oxidase, gp91(phox), possesses a H(+) channel motif formed by membrane-spanning histidines postulated to coordinate the two heme groups forming the redox center of the flavocytochrome. To study the role of heme-binding histidines on proton conduction, we stably expressed the gp91(phox) cytochrome in human embryonic kidney 293 cells and measured proton currents with the patch clamp technique. Similar to its shorter homologue, NADPH oxidase homologue 1, which is predicted not to bind heme, gp91(phox) generated voltage-activated, pH-dependent, H(+)-selective currents that were reversibly blocked by Zn(2+). The gp91(phox) currents, however, activated faster, deactivated more slowly, and were markedly affected by the inhibition of heme synthesis. Upon heme removal, the currents had larger amplitude, activated faster and at lower voltages, and became sensitive to the histidine reagent diethylpyrocarbonate. Mutation of the His-115 residue to leucine abolished both the gp91(phox) characteristic 558-nm absorbance peak and voltage-activated currents, indicating that His-115 is involved in both heme ligation and proton conduction. These results indicate that the gp91(phox) proton channel is activated upon release of heme from its His-115 ligand. During activation of the oxidase complex, changes in heme coordination within the cytochrome might increase the mobility of histidine ligands, thereby coupling electron and proton transport.