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

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).

Role of PLD-PKCζ signaling axis in p47phox phosphorylation for activation of NADPH oxidase by angiotensin II in pulmonary artery smooth muscle cells

We sought to determine the mechanism by which angiotensin II (ANGII) stimulates NADPH oxidase-mediated superoxide (O₂ ^.- ) production in bovine pulmonary artery smooth muscle cells (BPASMCs). ANGII-induced increase in phospholipase D (PLD) and NADPH oxidase activities were inhibited upon pretreatment of the cells with chemical and genetic inhibitors of PLD2, but not PLD1. Immunoblot study revealed that ANGII treatment of the cells markedly increases protein kinase C-α (PKC-α), -δ, -ε, and -ζ levels in the cell membrane. Pretreatment of the cells with chemical and genetic inhibitors of PKC-ζ, but not PKC-α, -δ, and -ε, attenuated ANGII-induced increase in NADPH oxidase activity without a discernible change in PLD activity. Transfection of the cells with p47phox small interfering RNA inhibited ANGII-induced increase in NADPH oxidase activity without a significant change in PLD activity. Pretreatment of the cells with the chemical and genetic inhibitors of PLD2 and PKC-ζ inhibited ANGII-induced p47phox phosphorylation and subsequently translocation from cytosol to the cell membrane, and also inhibited its association with p22phox (a component of membrane-associated NADPH oxidase). Overall, PLD-PKCζ-p47phox signaling axis plays a crucial role in ANGII-induced increase in NADPH oxidase-mediated O₂ ^.- production in the cells.

The intimate and controversial relationship between voltage-gated proton channels and the phagocyte NADPH oxidase

One of the most fascinating and exciting periods in my scientific career entailed dissecting the symbiotic relationship between two membrane transporters, the Nicotinamide adenine dinucleotide phosphate reduced form (NADPH) oxidase complex and voltage-gated proton channels (HV 1). By the time I entered this field, there had already been substantial progress toward understanding NADPH oxidase, but HV 1 were known only to a tiny handful of cognoscenti around the world. Having identified the first proton currents in mammalian cells in 1991, I needed to find a clear function for these molecules if the work was to become fundable. The then-recent discoveries of Henderson, Chappell, and colleagues in 1987-1988 that led them to hypothesize interactions of both molecules during the respiratory burst of phagocytes provided an excellent opportunity. In a nutshell, both transporters function by moving electrical charge across the membrane: NADPH oxidase moves electrons and HV 1 moves protons. The consequences of electrogenic NADPH oxidase activity on both membrane potential and pH strongly self-limit this enzyme. Fortunately, both consequences specifically activate HV 1, and HV 1 activity counteracts both consequences, a kind of yin-yang relationship. Notwithstanding a decade starting in 1995 when many believed the opposite, these are two separate molecules that function independently despite their being functionally interdependent in phagocytes. The relationship between NADPH oxidase and HV 1 has become a paradigm that somewhat surprisingly has now extended well beyond the phagocyte NADPH oxidase - an industrial strength producer of reactive oxygen species (ROS) - to myriad other cells that produce orders of magnitude less ROS for signaling purposes. These cells with their seven NADPH oxidase (NOX) isoforms provide a vast realm of mechanistic obscurity that will occupy future studies for years to come.

Protective effect of HDL on NADPH oxidase-derived super oxide anion mediates hypoxia-induced cardiomyocyte apoptosis

Cardiovascular diseases are the leading cause of death of death in Taiwan. Atherosclerosis can lead to serious problems, including heart attack, stroke, or even death. Coronary heart disease (CHD) occurs when plaque builds up in the coronary arteries to cause the ischemic heart disease which will enhance myocardial remodeling and also induce myocardial hypoxia. High density lipoprotein (HDL) has been proposed to have cardio-protective effects. Under hypoxic conditions (1%O₂ for 24hr), in H9c2 cells, reactive oxygen species (ROS) is induced which leads to cardiomyocyte apoptosis and cardiac dysfunction. Therefore, the present study described the protective effect of HDL on hypoxia-induced cardiomyocyte damage. We investigated the NADPH oxidase-produced ROS-related signaling pathways and apoptosis in cardiomyocytes under hypoxia conditions. Results showed that the ROS mediated cardiac damage might occur via AT1 and PKC activation. Furthermore, hypoxia downregulated the survival protein (p-AKT^ser473) and anti-apoptotic protein (BCL₂), whereas pro-apoptotic protein, Bax and caspase 3 were upregulated. These detrimental effects by ROS and apoptosis were prevented by HDL pretreatment. Our findings revealed the underlying molecular mechanism by which HDL suppresses the hypoxia-induced cardiomyocyte dysfunction. Further, we elucidated the role of HDL on preventing hypoxia induced cardiomyocyte apoptosis is mediated through the inhibition of NADPH oxidase-derived ROS.

The physicochemical properties of membranes correlate with the NADPH oxidase activity

BACKGROUND

Phagocytes kill ingested microbes by exposure to high concentrations of toxic reactive species generated by NADPH-oxidases. This membrane-bound electron-transferring enzyme is tightly regulated by cellular signaling cascades. So far, molecular and biophysical studies of the NADPH-oxidase were performed over limited temperature ranges, which weaken our understanding of immune response or inflammatory events. In this work, we have inspected the influence of temperature and lipid membrane properties on the NADPH-oxidase activity using a system free of cell complexity.

METHODS

We have extended the experimental conditions of the accepted model for NADPH-oxidase activity, the so-called cell-free assay, to a large temperature range (10-40°C) using different membrane compositions (subcellular compartments or liposomes).

RESULTS

A remarkable increase of superoxide production rate was observed with rising temperature. Synchrotron radiation circular dichroism data showed that this is not correlated with protein secondary structure changes. When lipid bilayers are in fluid phase, Arrhenius plots of the oxidase activity showed linear relationships with small activation energy (Ea), while when in solid phase, high Ea was found. The sterol content modulates kinetic and thermodynamic parameters.

CONCLUSION

High temperature promotes the rate of superoxide production. The key element of this enhancement is related to membrane properties such as thickness and viscosity and not to protein structural changes. Membrane viscosity that can be driven by sterols is a paramount parameter of Ea of NADPH oxidase activity. The membrane bilayer state modulated by its sterol content may be considered locally as an enzyme regulator. This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo.

Phospholipase D signaling pathways and phosphatidic acid as therapeutic targets in cancer

Phospholipase D is a ubiquitous class of enzymes that generates phosphatidic acid as an intracellular signaling species. The phospholipase D superfamily plays a central role in a variety of functions in prokaryotes, viruses, yeast, fungi, plants, and eukaryotic species. In mammalian cells, the pathways modulating catalytic activity involve a variety of cellular signaling components, including G protein-coupled receptors, receptor tyrosine kinases, polyphosphatidylinositol lipids, Ras/Rho/ADP-ribosylation factor GTPases, and conventional isoforms of protein kinase C, among others. Recent findings have shown that phosphatidic acid generated by phospholipase D plays roles in numerous essential cellular functions, such as vesicular trafficking, exocytosis, autophagy, regulation of cellular metabolism, and tumorigenesis. Many of these cellular events are modulated by the actions of phosphatidic acid, and identification of two targets (mammalian target of rapamycin and Akt kinase) has especially highlighted a role for phospholipase D in the regulation of cellular metabolism. Phospholipase D is a regulator of intercellular signaling and metabolic pathways, particularly in cells that are under stress conditions. This review provides a comprehensive overview of the regulation of phospholipase D activity and its modulation of cellular signaling pathways and functions.

Regulation of μ-opioid receptors: desensitization, phosphorylation, internalization, and tolerance

Morphine and related µ-opioid receptor (MOR) agonists remain among the most effective drugs known for acute relief of severe pain. A major problem in treating painful conditions is that tolerance limits the long-term utility of opioid agonists. Considerable effort has been expended on developing an understanding of the molecular and cellular processes that underlie acute MOR signaling, short-term receptor regulation, and the progression of events that lead to tolerance for different MOR agonists. Although great progress has been made in the past decade, many points of contention and controversy cloud the realization of this progress. This review attempts to clarify some confusion by clearly defining terms, such as desensitization and tolerance, and addressing optimal pharmacological analyses for discerning relative importance of these cellular mechanisms. Cellular and molecular mechanisms regulating MOR function by phosphorylation relative to receptor desensitization and endocytosis are comprehensively reviewed, with an emphasis on agonist-biased regulation and areas where knowledge is lacking or controversial. The implications of these mechanisms for understanding the substantial contribution of MOR signaling to opioid tolerance are then considered in detail. While some functional MOR regulatory mechanisms contributing to tolerance are clearly understood, there are large gaps in understanding the molecular processes responsible for loss of MOR function after chronic exposure to opioids. Further elucidation of the cellular mechanisms that are regulated by opioids will be necessary for the successful development of MOR-based approaches to new pain therapeutics that limit the development of tolerance.

Effects of reactive oxygen species and neutrophils on endothelium-dependent relaxation of rat thoracic aorta

Reactive oxygen species (ROS) are produced in different metabolic processes including the respiratory burst of neutrophils accompanying local inflammation. The aim of this study was to analyze the effects of N-formyl-methionyl-leucyl-phenylalanine (FMLP)-activated neutrophils, isolated from the guinea pig peritoneal cavity, on isolated rings of a large (conduit) artery, the rat thoracic aorta. FMLP-activated neutrophils enhanced the basal tension increased by α(1)-adrenergic stimulation. In phenylephrine-precontracted aortae, they elicited marked contraction, while in noradrenaline-precontracted rat aortal rings they caused a biphasic response (contraction-relaxation). To eliminate interaction of activated neutrophils with catecholamines, in the subsequent experiments the basal tension was increased by KCl-induced depolarization. Activated neutrophils evoked a low-amplitude biphasic response (relaxation-contraction) on the KCl-induced contraction. Not only the acetylcholine- and A(23187)-induced relaxations but also the catalase sensitive hydrogen peroxide (H(2)O(2)) elicited contractions were endothelium-dependent. Even though the acetylcholine-induced relaxation was changed by activated neutrophils and by the ROS studied, their effects differed significantly, yet none of them did eliminate fully the endothelium-dependent acetylcholine relaxation. The effect of activated neutrophils resembled the effect of superoxide anion radical (O(2) (•-)) produced by xanthine/xanthine oxidase (X/XO) and differed from the inhibitory effects of Fe(2)SO(4)/H(2)O(2)-produced hydroxyl radical ((•)OH) and H(2)O(2). Thus O(2) (•-) produced either by activated neutrophils or X/XO affected much less the endothelium-dependent acetylcholine-activated relaxation mechanisms than did (•)OH and H(2)O(2). In the large (conduit) artery, the effects of activated neutrophils and various ROS (O(2) (•-), (•)OH and H(2)O(2)) seem to be more dependent on muscle tension than on endothelial mechanisms.

mu-opioid receptor-stimulated synthesis of reactive oxygen species is mediated via phospholipase D2

We have recently shown that the activation of the rat mu-opioid receptor (MOPr, also termed MOR1) by the mu-agonist [D-Ala(2), Me Phe(4), Glyol(5)]enkephalin (DAMGO) leads to an increase in phospholipase D2 (PLD2) activity and an induction of receptor endocytosis, whereas the agonist morphine which does not induce opioid receptor endocytosis fails to activate PLD2. We report here that MOPr-mediated activation of PLD2 stimulates production of reactive oxygen molecules via NADH/NADPH oxidase. Oxidative stress was measured with the fluorescent probe dichlorodihydrofluorescein diacetate and the role of PLD2 was assessed by the PLD inhibitor D-erythro-sphingosine (sphinganine) and by PLD2-small interfering RNA transfection. To determine whether NADH/NADPH oxidase contributes to opioid-induced production of reactive oxygen species, mu-agonist-stimulated cells were pre-treated with the flavoprotein inhibitor, diphenylene iodonium, or the specific NADPH oxidase inhibitor, apocynin. Our results demonstrate that receptor-internalizing agonists (like DAMGO, beta-endorphin, methadone, piritramide, fentanyl, sufentanil, and etonitazene) strongly induce NADH/NADPH-mediated ROS synthesis via PLD-dependent signaling pathways, whereas agonists that do not induce MOPr endocytosis and PLD2 activation (like morphine, buprenorphine, hydromorphone, and oxycodone) failed to activate ROS synthesis in transfected human embryonic kidney 293 cells. These findings indicate that the agonist-selective PLD2 activation plays a key role in the regulation of NADH/NADPH-mediated ROS formation by opioids.

Inhibitory effect of stobadine on FMLP-induced chemiluminescence in human whole blood and isolated polymorphonuclear leukocytes

The chemiluminescence (CL) technique with luminol and isoluminol was used to characterize the effect of stobadine on reactive oxygen metabolites (ROM) generation in human whole blood and in isolated polymorphonuclear leukocytes (PMNL) stimulated with N-formyl-methionyl-leucyl-phenyl-alanine (FMLP). In whole blood and in isolated PMNL, stobadine in the concentrations of 1, 10 and 100 micromol/L significantly inhibited the CL signal after FMLP, which activated predominantly extracellular generation of ROM. The same concentrations of stobadine were effective on CL in a cell-free system. On the other hand, myeloperoxidase (MPO) liberation was decreased by stobadine only in the concentration of 100 micromol/L. The results showed stobadine to act as a potent inhibitor/scavenger of extracellularly produced ROM in human PMNL and indicated interference of stobadine with ROM as well as with signalling events resulting in NADPH-oxidase activation and MPO liberation.

Phospholipase D2‐derived phosphatidic acid binds to and activates ribosomal p70 S6 kinase independently of mTOR

The product of phospholipase D (PLD) enzymatic action in cell membranes, phosphatidic acid (PA), regulates kinases implicated in NADPH oxidase activation, as well as the mammalian target of rapamycin (mTOR) kinase. However, other protein targets for this lipid second messenger must exist in order to explain other key PA-mediated cellular functions. In this study, PA was found to specifically and saturably bind to and activate recombinant and immunoprecipitated endogenous ribosomal S6 kinase (S6K) with a stoichiometry of 94:1 lipid/protein. Polyphosphoinositides PI4-P and PI4,5P2 and cardiolipin could also bind to and activate S6K, albeit with different kinetics. Conversely, PA with at least one acyl side chain saturated (10:0) was ineffective in binding or activating the enzyme. Transfection of COS-7 cells with a wild-type myc-(pcDNA)-PLD2 construct resulted in high PLD activity, concomitantly with an increase in ribosomal p70S6K enzyme activity and phosphorylation in T389 and T421/S424 as well as phosphorylation of p70S6K's natural substrate S6 protein in S235/S236. Overexpression of a lipase inactive mutant (K758R), however, failed to induce an increase in both PLD and S6K activity or phosphorylation, indicating that the enzymatic activity of PLD2 (i.e., synthesis of PA) must be present to affect S6K. Neither inhibiting mTOR kinase activity with rapamycin nor silencing mTOR gene expression altered the augmentative effect of PLD2 exerted on p70S6K activity. This finding indicates that PA binds to and activates p70S6K, even in the absence of mTOR. Lastly, COS-7 transfection with PLD2 changed the pattern of subcellular expression, and a colocalization of S6K and PLD2 was observed by immunofluorescence microscopy. These results show for the first time a direct (mTOR-independent) participation of PLD in the p70S6K pathway and implicate PA as a nexus that brings together cell phospholipases and kinases.

Protein kinase C-independent pathway for NADPH oxidase activation in guinea pig peritoneal polymorphonuclear leukocytes by cytochalasin D

Cytochalasin D (CD) induced production of the superoxide radical (O(2)(-)) in guinea pig polymorphonuclear leukocytes (PMNs). The protein kinase C (PKC) inhibitor GF109203X (GFX) was rarely without effect on CD-induced O(2)(-) production. CD as well as PMA induced the translocation of p47(phox) to the membrane fraction, and this translocation was slightly decreased by GFX. Moreover, the inhibitory effect of a PKCzeta antagonist with sequences based on the endogenous PKCzeta pseudosubstrate region was weaker than the inhibitory effect on N-formyl-methionyl-leucyl-phenylalanine (fMLP)-induced O(2)(-) production. On the other hand, the production of O(2)(-) induced by CD was more strongly suppressed by the PLD inhibitor ethanol and phosphatidylinositol 3-kinase (PI3-K) inhibitor wortmannin than that induced by fMLP, and the activation of phospholipase D (PLD) by CD was restrained by wortmannin. These findings suggest that NADPH oxidase is activated by CD through a PKC-independent signaling pathway in PMNs, and this pathway involves the activation of PLD through PI3-K.

Lindane stimulates neutrophils by selectively activating phospholipase C and phosphoinositide-kinase activity

The organochlorine insecticide lindane is a known activator of neutrophil responses. In this work we delineated the biochemical pathways by which lindane stimulates neutrophil oxidant production. Plasma membrane GTPase activity was not stimulated by lindane, ruling out a role for lindane-induced activation of G-proteins or G-protein coupled receptors, whereas inhibition of phospholipase C inhibited lindane-induced oxidant production. Together these data pointed to phospholipase C as the direct target of lindane activation. Type I phosphoinositide 3-kinase was not significantly activated by lindane and an inhibitor of phosphoinositide 3-kinases inhibited oxidant production by only 40%. Thus, Type I phosphoinositide 3-kinase played a minor role, if any, in lindane-induced oxidant production. Lindane stimulated an increase in phosphatidylinositol phosphate suggesting a Type II or III phosphotidylinositol 3-kinase or phosphatidylinositol 4-kinase activity was also stimulated.

Assays to Study Phospholipase D Regulation by Inositol Phospholipids and ADP‐Ribosylation Factor 6

Phospholipase D (PLD) is an enzyme implicated in the regulation of both exocytic and endocytic vesicle trafficking as well as many other processes. Consistent with this, the small GTPase Arf6 and regulated changes in inositol phospholipids levels are two factors that regulate both PLD and vesicle trafficking. Here we describe three methodologies through which the activation of PLD by Arf6 and inositol phospholipids may be investigated. The first method described is an in vitro protocol that allows the analysis of purified proteins or cell lysates. Furthermore, this protocol can be used to analyze the effects of different inositol phospholipids by changing the composition of the substrate vesicle. The major advantage of this protocol lies in the ability to analyze the effects of direct interactions on PLD activation by using pure proteins and lipids. The other two methods are in vivo protocols for the analysis of PLD activation in response to extracellular stimuli. Modification of cellular composition using overexpression/deletion or knockout of specific genes can be utilized with these protocols to characterize PLD activation pathways. The first of these methods uses the detection of radiolabeled PLD products and can be used for most cell types whereas the second of these two protocols is used to measure PLD products when radiolabeling of cells is not possible, such as freshly isolated cells that will not survive long enough to attain radiochemical equilibrium.

Protein kinase Cdelta regulates p67phox phosphorylation in human monocytes

Phosphorylation of the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase components p67phox and p47phox accompanies the assembly and activation of this enzyme complex. We have previously reported that activation of human monocytes with opsonized zymosan (ZOP), a potent stimulator of NADPH oxidase activity, results in the phosphorylation of p67phox and p47phox. In this study, we investigated the regulation of p67phox phosphorylation. Although protein kinase C (PKC)alpha has previously been shown to regulate NADPH oxidase activity, we found that inhibition of PKCalpha had no effect on p67phox phosphorylation. Our studies demonstrate that pretreatment of monocytes with antisense oligodeoxyribonucleotides specific for PKCdelta or rottlerin, a selective inhibitor for PKCdelta, inhibited the phosphorylation of p67phox in monocytes, and Go6976, a specific inhibitor for conventional PKCs, PKCalpha and PKCbeta, had no such inhibitory effect. Additional studies indicate that ZOP stimulation of monocytes induces PKCdelta and p67phox to form a complex. We also demonstrate that lysates from activated monocytes as well as PKCdelta immunoprecipitates from activated monocytes can phosphorylate p67phox in vitro and that pretreatment of monocytes with rottlerin blocked the phosphorylation in each case. We further show that recombinant PKCdelta can phosphorylate p67phox in vitro. Finally, we show that PKCdelta-deficient monocytes produce significantly less superoxide anion in response to ZOP stimulation, thus emphasizing the functional significance of the PKCdelta regulation of p67phox phosphorylation. Taken together, this is the first report to describe the requirement of PKCdelta in regulating the phosphorylation of p67phox and the related NADPH oxidase activity in primary human monocytes.

Phosphatidic acid positively regulates LPS-induced differentiation of RAW264.7 murine macrophage cell line into dendritic-like cells

Phosphatidic acid (PA) is an important second messenger produced by the activation of numerous cell surface receptors. Recent data have suggested that PA regulates multiple cellular processes. In this study, we found that PA positively regulates the lipopolysaccharide (LPS)-induced differentiation of RAW264.7 murine macrophage cells into dendritic-like cells. Co-treatment of PA with LPS further increased dendritic cell surface marker expressions (CD80, CD86, CD40, MHC class I, and class II antigens) and reduced the phagocytic activity of LPS-treated cells. Moreover, PA up regulated allostimulatory activity and the secretion of IL-12 in LPS-treated RAW264.7 cells. Taken together, these data indicate that PA might play a role in the LPS-mediated differentiation of macrophage cells into dendritic-like cells.

Phosphatidic acid regulates systemic inflammatory responses by modulating the Akt-mammalian target of rapamycin-p70 S6 kinase 1 pathway

Macrophages are pivotal effector cells in the innate immune system. When microbial products bind to pathogen recognition receptors, macrophages are activated and release a broad array of mediators, such as cytokines, that orchestrate the inflammatory responses of the host. Phosphatidic acid (PA) has been implicated as an important metabolite of phospholipid biosynthesis and in membrane remodeling and has been further suggested to be a crucial second messenger in various cellular signaling events. Here we show that PA is an essential regulator of inflammatory response. Deleterious effects of PA are associated with the secretion of proinflammatory cytokines, such as tumor necrosis factor-alpha, interleukin-1beta, interleukin-6, and the production of nitric oxide, prostaglandin E2, which are predominantly released by macrophage Raw264.7 cells. Furthermore, the administration of PA to mice increased the serum cytokine level. Moreover, direct or lipopolysaccharide-induced PA accumulation by macrophages led to the Akt-dependent activation of the mammalian target of rapamycin-p70 S6 kinase 1, a process required for the induction of inflammatory mediators. These findings demonstrate the importance of the role of PA in systemic inflammatory responses, and provide a potential usefulness as specific targets for the development of therapies.

High sensitivity of plasma membrane ion transport ATPases from human neutrophils towards 4-hydroxy-2,3-trans-nonenal

Lipid peroxidation results in release of 4-hydroxy-2,3-trans-nonenal (HNE), which is known to conjugate to specific amino acids of proteins and may alter their function. The effect of HNE on the activities of Na(+)/K(+)-ATPase, Mg(2+)-ATPase, Ca(2+)-ATPase, and calmodulin-stimulated Ca(2+)-ATPase has been studied both in erythrocyte ghosts and in neutrophil membrane preparations. Neutrophil Ca(2+)-ATPase was strongly inhibited by micromolar concentrations of HNE (IC(50) = 12 microM), that means in the range of pathophysiologically relevant HNE levels. The IC(50) value for neutrophil Na(+)/K(+)-ATPase was about 40 microM. HNE was considerably less effective against neutrophil Mg(2+)-ATPase and the erythrocyte ghost enzymes (IC(50) values range from 91 to 240 microM). The data suggest that HNE may play a specific role in the regulation of neutrophil calcium homeostasis in response to oxidative stress.

Pulmonary phosphatidic acid phosphatase and lipid phosphate phosphohydrolase

The lung contains two distinct forms of phosphatidic acid phosphatase (PAP). PAP1 is a cytosolic enzyme that is activated through fatty acid-induced translocation to the endoplasmic reticulum, where it converts phosphatidic acid (PA) to diacylglycerol (DAG) for the biosynthesis of phospholipids and neutral lipids. PAP1 is Mg(2+) dependent and sulfhydryl reagent sensitive. PAP2 is a six-transmembrane-domain integral protein localized to the plasma membrane. Because PAP2 degrades sphingosine-1-phosphate (S1P) and ceramide-1-phosphate in addition to PA and lyso-PA, it has been renamed lipid phosphate phosphohydrolase (LPP). LPP is Mg(2+) independent and sulfhydryl reagent insensitive. This review describes LPP isoforms found in the lung and their location in signaling platforms (rafts/caveolae). Pulmonary LPPs likely function in the phospholipase D pathway, thereby controlling surfactant secretion. Through lowering the levels of lyso-PA and S1P, which serve as agonists for endothelial differentiation gene receptors, LPPs regulate cell division, differentiation, apoptosis, and mobility. LPP activity could also influence transdifferentiation of alveolar type II to type I cells. It is considered likely that these lipid phosphohydrolases have critical roles in lung morphogenesis and in acute lung injury and repair.

Regulation of phospholipase D activity by actin. Actin exerts bidirectional modulation of Mammalian phospholipase D activity in a polymerization-dependent, isoform-specific manner

Many critical cellular processes, including proliferation, vesicle trafficking, and secretion, are regulated by both phospholipase D (PLD) and the actin microfilament system. Stimulation of human PLD1 results in its association with the detergent-insoluble actin cytoskeleton, but the molecular mechanisms and functional consequences of PLD-actin interactions remain incompletely defined. Biochemical and pharmacologic modulation of actin polymerization resulted in complex bidirectional effects on PLD activity, both in vitro and in vivo. Highly purified G-actin inhibited basal and stimulated PLD activity, whereas F-actin produced the opposite effects. Actin-induced modulation of PLD activity was independent of the activating stimulus. The efficacy and potency of the effects of actin were isoform-specific but broadly conserved among actin family members. Human betagamma-actin was only 45% as potent and 40% as efficacious as rabbit skeletal muscle alpha-actin, whereas its inhibitory profile was similar to the single actin species from the yeast, Saccharomyces cerevisiae. Use of actin polymerization-specific reagents indicated that PLD1 binds both monomeric G-actin, as well as actin filaments. These data are consistent with a model in which the physical state of the actin cytoskeleton is a critical determinant of its regulation of PLD activity.

The role of phosphatidic acid in the regulation of the Ras/MEK/Erk signaling cascade

Phosphatidic acid (PA) is an important second messenger produced by the activation of numerous cell surface receptors. Recent data have suggested that PA regulates multiple cellular processes. This review addresses primarily the role of PA in the regulation of the Erk1/2 cascade pathway. A model for the regulation of Erk1/2 phosphorylation by cell surface receptors is presented. According to this model, agonists stimulate the binding of GTP to Ras and the activation of phospholipase D to generate phosphatidic acid. PA promotes the binding of cRaf-1 kinase to the membrane, where it interacts with Ras.GTP and other regulatory components of the pathway. Ras-Raf complexes remain bound to the surface of endosomes, where scaffolding complexes involving Ras, cRaf-1, MEK and Erk are formed. Complete activation and coupling of the cascade requires endocytosis, a process that is also modulated by PA.

Inhibitory effects of a dominant-interfering form of the Rho-GTPase Cdc42 in the chemoattractant-elicited signaling pathways leading to NADPH oxidase activation in differentiated HL-60 cells

A tetracycline-controlled expression system was adapted to the human promyelocytic HL-60 cell line by placement of the transactivator (tTA-off) sequence under the control of the human EF-1alpha promoter region. Constitutively active and dominant-inhibitory forms of Cdc42 (Cdc42V12 and Cdc42N17, respectively) were conditionally expressed in this system. The expression of Cdc42V12 had no marked effect on chemoattractant-mediated superoxide production, corroborating previous results indicating that the guanosine 5'-triphosphate (GTP)-bound form of Cdc42 is ineffective in directly activating nicotinamide adenine dinucleotide phosphate (NADPH) oxidase in a cell-free system. However, the N17 mutant potently inhibited chemoattractant-induced superoxide production. The expression of Cdc42N17 interfered with the GTP-loading of Rac and Ras and with the activation of the MAP-kinase pathway. A drastic reduction of chemoattractant-induced inositol-1,4,5-trisphosphate formation and calcium mobilization was observed, corroborating previous in vitro study results identifying PLCbeta2 as a Rac/Cdc42 effector. Cdc42N17 was also found to inhibit the translocation of Ras-GRF2, a guanine nucleotide exchange factor for Ras and Rac but not for Cdc42. Thus, the dominant-inhibitory mutant Cdc42N17 was found to interfere at multiple levels in the signaling pathways. The pleiotropic inhibitory effects of Cdc42N17 illustrate the potential pitfalls of using dominant-inhibitory proteins to study the function of Ras-family GTPases. In this regard, a number of conclusions drawn from the use of dominant-inhibitory mutants in myeloid cells might have to be reconsidered.

Involvement of protein kinase D in Fc gamma-receptor activation of the NADPH oxidase in neutrophils

Protein kinases involved in the activation of the NADPH oxidase by Fc gamma receptors in neutrophils were studied. Of three different protein kinase C (PKC) inhibitors, Gö 6976 inhibited the NADPH oxidase completely, whereas bisindolylmaleimide I and Ro 31-8220 caused a 70-80% inhibition. Thus a Gö 6976-sensitive, bisindolylmaleimide I/Ro 31-8220-insensitive component contributes to NADPH oxidase activation induced by Fc gamma receptors. Down-regulation of PKC isotypes resulted in inhibition of Fc gamma-receptor-activated NADPH oxidase, but a down-regulation-insensitive component was still present. This component was sensitive to Gö 6976, but insensitive to Ro 31-8220. It has been shown previously that protein kinase D/PKC-mu (PKD) shows this same pharmacology in vitro. We show that PKD is present in neutrophils and that, in contrast with PKC isotypes, PKD is not down-regulated. Therefore PKD may participate in NADPH oxidase activation. To obtain direct evidence for this we adopted an antisense approach. Antisense PKD inhibited NADPH oxidase induced by Fc gamma-receptor stimulation by 50% and the Ro 31-8220-insensitive component in the activation was inhibited by antisense PKD. In vitro kinase assays showed that PKD is activated by presenting IgG-opsonized particles to neutrophils. Furthermore, PKD localizes to the area of particle intake in the cell and phosphorylates two of the three cytosolic components of the NADPH oxidase, p40(phox) and p47(phox). Taken together, these data indicate that Fc gamma receptors engage PKD in the regulation of the NADPH oxidase.

Antigen-stimulated activation of phospholipase D1b by Rac1, ARF6, and PKCalpha in RBL-2H3 cells

Phospholipase D (PLD) activity can be detected in response to many agonists in most cell types; however, the pathway from receptor occupation to enzyme activation remains unclear. In vitro PLD1b activity is phosphatidylinositol 4,5-bisphosphate dependent via an N-terminal PH domain and is stimulated by Rho, ARF, and PKC family proteins, combinations of which cooperatively increase this activity. Here we provide the first evidence for the in vivo regulation of PLD1b at the molecular level. Antigen stimulation of RBL-2H3 cells induces the colocalization of PLD1b with Rac1, ARF6, and PKCalpha at the plasma membrane in actin-rich structures, simultaneously with cooperatively increasing PLD activity. Activation is both specific and direct because dominant negative mutants of Rac1 and ARF6 inhibit stimulated PLD activity, and surface plasmon resonance reveals that the regulatory proteins bind directly and independently to PLD1b. This also indicates that PLD1b can concurrently interact with a member from each regulator family. Our results show that in contrast to PLD1b's translocation to the plasma membrane, PLD activation is phosphatidylinositol 3-kinase dependent. Therefore, because inactive, dominant negative GTPases do not activate PLD1b, we propose that activation results from phosphatidylinositol 3-kinase-dependent stimulation of Rac1, ARF6, and PKCalpha.

Oxidant-dependent phosphorylation of p40phox in B lymphocytes

As with the neutrophil NADPH oxidase, the B lymphocyte NADPH oxidase consists of a membrane-bound flavocytochrome b and regulatory factors including Rac and the cytosolic phox protein triad p67phox, p47phox, and p40phox. Here we demonstrate by phosphoamino acid analysis and the use of the potent PKC inhibitor GFX that, in response to stimulation of B lymphocytes with sodium orthovanadate and H(2)O(2), the p40phox component of the cytosolic phox triad is selectively phosphorylated on serine and threonine residues by a PKC-type protein kinase. The pattern of p40phox phosphorylation was closely related to the kinetics of tyrosine phosphorylation of PKC-delta, the main PKC isotype of B lymphocytes. Blocking H(2)O(2)-dependent tyrosine phosphorylation of PKC by genistein resulted in inhibition of p40phox phosphorylation. The correlation between the tyrosine phosphorylation of PKC-delta and the serine/threonine phosphorylation of p40phox, together with the inhibition of p40phox phosphorylation by rottlerin, a selective inhibitor of PKC-delta, makes the activated PKC-delta a likely candidate in the process of the oxidant-dependent phosphorylation of p40phox in B cells.

Increased generation of superoxide by angiotensin II in smooth muscle cells from resistance arteries of hypertensive patients: role of phospholipase D-dependent NAD(P)H oxidase-sensitive pathways

OBJECTIVE

We tested the hypothesis that increased responsiveness of phospholipase D (PLD) to angiotensin II (Ang II) is associated with increased oxidative stress and exaggerated growth responses in vascular smooth muscle cells (VSMC) from untreated essential hypertensive patients.

DESIGN

VSMCs from peripheral resistance arteries of normotensive and hypertensive subjects were studied. Production of reactive oxygen species (ROS) was measured with the fluoroprobe 5-(and 6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate (CM-H2DCFDA). PLD and reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) oxidase were assessed with the inhibitors, dihydro-D-erythro-sphingosine (sphinganine) and diphenylene iodinium (DPI), respectively, and protein kinase C (PKC) effects were determined using chelerythrine chloride and calphostin C. PLD activity was measured by the transphosphatidylation assay.

RESULTS

Ang II increased the CM-H2DCFDA fluorescence signal, derived predominantly from H2O2. Ang II-induced generation of DPI-inhibitable ROS was significantly enhanced in cells from hypertensives compared with normotensives (Emax = 72 +/- 2 versus 56.9 +/- 1.8 fluorescence units, P< 0.01). PLD inhibition attenuated Ang II-induced ROS generation, with greater effects in the hypertensive group than the normotensive group (delta = 42 +/- 3.3 versus 21 +/- 2 units). PKC inhibition partially decreased Ang II-elicited signals. Ang II-stimulated PLD activity and DNA and protein synthesis were significantly greater in cells from hypertensives than normotensives. These effects were normalized by DPI and sphinganine.

CONCLUSIONS

Our results suggest that in essential hypertension enhanced oxidative stress and augmented growth-promoting actions of Ang II are associated with increased activation of PLD-dependent pathways. These processes may contribute to vascular remodeling in hypertension.

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

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

Protein kinase C zeta phosphorylates a subset of selective sites of the NADPH oxidase component p47phox and participates in formyl peptide-mediated neutrophil respiratory burst

Generation of superoxide anion by the multiprotein complex NADPH phagocyte oxidase is accompanied by extensive phosphorylation of its 47-kDa protein component, p47(phox), a major cytosolic component of this oxidase. Protein kinase C zeta (PKC zeta), an atypical PKC isoform expressed abundantly in human polymorphonuclear leukocytes (PMN), translocates to the PMN plasma membrane upon stimulation by the chemoattractant fMLP. We investigated the role of PKC zeta in p47(phox) phosphorylation and in superoxide anion production by human PMN. In vitro incubation of recombinant p47(phox) with recombinant PKC zeta induced a time- and concentration-dependent phosphorylation of p47(phox) with an apparent K(m) value of 2 microM. Phosphopeptide mapping analysis of p47(phox) showed that PKC zeta phosphorylated fewer selective sites in comparison to "conventional" PKCs. Serine 303/304 and serine 315 were identified as targets of PKC zeta by site-directed mutagenesis. Stimulation of PMN by fMLP induced a rapid and sustained plasma membrane translocation of PKC zeta that correlated to that of p47(phox). A cell-permeant-specific peptide antagonist of PKC zeta inhibited both fMLP-induced phosphorylation of p47(phox) and its membrane translocation. The antagonist also inhibited the fMLP-induced production of oxidant (IC(50) of 10 microM), but not that induced by PMA. The inhibition of PKC zeta expression in HL-60 neutrophil-like cells using antisense oligonucleotides (5 and 10 microM) inhibited fMLP-promoted oxidant production (27 and 50%, respectively), but not that induced by PMA. In conclusion, p47(phox) is a substrate for PKC zeta and participates in the signaling cascade between fMLP receptors and NADPH oxidase activation.

Phospholipase D (PLD) is present in Leishmania donovani and its activity increases in response to acute osmotic stress

We report here that the signaling molecule phospholipase D (PLD) is present in the parasitic protozoan Leishmania donovani. In vitro enzymatic activity is dependent on Ca2+ and Mg2+ ions, its basal activity is stimulated by phosphatidyl-inositol-4,5-bisphosphate (PIP2) and its pH optima are pH 8.0 and pH 6.0. PLD activity increases 3-fold about 5 min after an abrupt decrease in osmolality from 317 mOsm (isosmotic) to 155 mOsm and increases 1.5-fold in response to an abrupt increase in osmolality to 617 mOsM. Cells grown for > 24 h under the anisosmotic conditions showed only marginal changes in activity compared to the controls grown under isosmotic conditions, indicating an adaptation to long-term exposure to hypo- or hyper-osmolarity. Immunologically, two isoforms, PLD1 and PLD2, are present. An analysis of in vitro PLD activity in anti-PLD immunocomplexes revealed that either hypotonic (cell swelling) or hypertonic stress (cell shrinking) causes an increase in PLD1 activation but a reduction in PLD2 activity. The interplay between these two isoforms results in a predominance for PLD1 in the observed increase when measuring total PLD activity. Finally, the increase in enzymatic activity in acute hyposmotic shock is accompanied by tyrosyl phosphorylation of the PLD1 isoform, suggesting a role for protein tyrosine kinase in the control of PLD activity in response to osmotic stress.

Phosphorylation of p22phox is mediated by phospholipase D-dependent and -independent mechanisms. Correlation of NADPH oxidase activity and p22phox phosphorylation

Human neutrophils participate in the host innate immune response, partly mediated by the multicomponent superoxide-generating enzyme NADPH oxidase. A correlation between phosphorylation of cytosolic NADPH oxidase components and enzyme activation has been identified but is not well understood. We previously showed that p22(phox), the small subunit of the membrane-bound oxidase component flavocytochrome b(558), is an in vitro substrate for both a phosphatidic acid-activated kinase and conventional protein kinase C isoforms (Regier, D. S., Waite, K. A., Wallin, R., and McPhail, L. C. (1999) J. Biol. Chem. 274, 36601-36608). Here we show that several neutrophil agonists (phorbol myristate acetate, opsonized zymosan, and N-formyl-methionyl-leucyl-phenylalanine) induce p22(phox) phosphorylation in intact neutrophils. To determine if phospholipase D (PLD) is needed for p22(phox) phosphorylation, cells were pretreated with ethanol, which reduces phosphatidic acid production by PLD in stimulated cells. Phorbol myristate acetate-induced phosphorylation of p22(phox) and NADPH oxidase activity were not reduced by ethanol. In contrast, ethanol reduced both activities when cells were stimulated by N-formyl-methionyl-leucyl-phenylalanine or opsonized zymosan. Varying the time of stimulation with opsonized zymosan showed that the phosphorylation of p22(phox) coincides with NADPH oxidase activation. GF109203X, an inhibitor of protein kinase C and the phosphatidic acid-activated protein kinase, decreased both p22(phox) phosphorylation and NADPH oxidase activity in parallel in opsonized zymosan-stimulated cells. Stimulus-induced phosphorylation of p22(phox) was on Thr residue(s), in agreement with in vitro results. Overall, these data show that NADPH oxidase activity and p22(phox) phosphorylation are correlated and suggest two mechanisms (PLD-dependent and -independent) by which p22(phox) phosphorylation occurs.

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.

Independent functioning of cytosolic phospholipase A2 and phospholipase D1 in Trp-Lys-Tyr-Met-Val-D-Met-induced superoxide generation in human monocytes

Recently, a novel peptide (Trp-Lys-Tyr-Met-Val-D-Met, WKYMVm) has been shown to induce superoxide generation in human monocytes. The peptide stimulated phospholipase A2 (PLA2) activity in a concentration- and time-dependent manner. Superoxide generation as well as arachidonic acid (AA) release evoked by treatment with WKYMVm could be almost completely blocked by pretreatment of the cells with cytosolic PLA2 (cPLA2)-specific inhibitors. The involvement of cPLA2 in the peptide-induced AA release was further supported by translocation of cPLA2 to the nuclear membrane of monocytes incubated with WKYMVm. WKYMVm-induced phosphatidylbutanol formation was completely abolished by pretreatment with PKC inhibitors. Immunoblot showed that monocytes express phospholipase D1 (PLD1), but not PLD2. GF109203X as well as butan-1-ol inhibited peptide-induced superoxide generation in monocytes. Furthermore, the interrelationship between the two phospholipases, cPLA2 and PLD1, and upstream signaling molecules involved in WKYMVm-dependent activation was investigated. The inhibition of cPLA2 did not blunt peptide-stimulated PLD1 activation or vice versa. Intracellular Ca2+ mobilization was indispensable for the activation of PLD1 as well as cPLA2. The WKYMVm-dependent stimulation of cPLA2 activity was partially dependent on the activation of PKC and mitogen-activated protein kinase, while PKC activation, but not mitogen-activated protein kinase activation, was an essential prerequisite for stimulation of PLD1. Taken together, activation of the two phospholipases, which are absolutely required for superoxide generation, takes place through independent signaling pathways that diverge from a common pathway at a point downstream of Ca2+.

Protein kinase C-beta contributes to NADPH oxidase activation in neutrophils

We have analysed the involvement of the beta isotype of the protein kinase C (PKC) family in the activation of NADPH oxidase in primary neutrophils. Using immunofluorescence and cell fractionation, PKC-beta is shown to be recruited to the plasma membrane upon stimulation with phorbol ester and to the phagosomal membrane upon phagocytosis of IgG-coated particles (Fcgamma-receptor stimulus). The time course of recruitment is similar to that of NADPH oxidase activation by these stimuli. The PKC-beta specific inhibitor 379196 inhibits the response to PMA as well as to IgG-coated bacteria. Partial inhibition occurs between 10 and 100 nM of inhibitor, the concentration at which PKC-beta, but not other PKC isotypes, is targeted. Neutrophils isolated from a mouse that lacks PKC-beta also showed an inhibition of NADPH oxidase activation by PMA and IgG-coated particles. The level of inhibition is comparable to that achieved with 379196 in human neutrophils. Thus the PKC-beta isotype mediates activation of NADPH oxidase by PMA and by stimulation of Fcgamma receptors in neutrophils.

Multiple forms of phospholipase D in plants: the gene family, catalytic and regulatory properties, and cellular functions

Multiple Phospholipase D (PLD) genes have been identified in plants and encode isoforms with distinct regulatory and catalytic properties. Elucidation of the genetic and biochemical heterogeneity has provided important clues as to the regulation and function of this family of enzymes. Polyphosphoinositides, Ca(2+), and G-proteins are possible cellular regulators for PLD activation. PLD-mediated hydrolysis of membrane lipids increases in response to various stresses. Recent studies suggest that PLD plays a role in the signaling and production of hormones involved in plant stress responses.

Phospholipase D: molecular and cell biology of a novel gene family

Interaction of extracellular-signal molecules with cell-surface receptors often activates a phospholipase D (PLD)-mediated hydrolysis of phosphatidylcholine and other phospholipids, generating phosphatidic acid. The activation of PLD is believed to play an important role in the regulation of cell function and cell fate. Multiple PLD activities were characterized in eukaryotic cells, and, more recently, several PLD genes have been cloned. A PLD gene superfamily, defined by a number of structural domains and sequence motifs, also includes phosphatidyltransferases and certain phosphodiesterases. Among the eukaryotic PLD genes are those from mammals, nematodes, fungi and plants. The present review focuses on the structure, localization, regulation and possible functions of cloned mammalian and yeast PLDs. In addition, an overview of plant PLD genes, and of several distinct PLD activities that have not yet been cloned, is provided. Emerging evidence from recent work employing new molecular tools indicates that different PLD isoforms are localized in distinct cellular organelles, where they are likely to serve diverse functions in signal transduction, membrane vesicle trafficking and cytoskeletal dynamics.

Free radical mediated photoreceptor damage in uveitis

Uveitis is a major cause of blindness, with the visual loss that occurs being due primarily to retinal tissue damage. The tissue damage is mediated mainly by phagocytic inflammatory cells, such as macrophages, by the release of various proteolytic enzymes, arachidonic acid metabolites, cytokines and free radicals. The latter are found to be potent cytotoxic agents that readily cause tissue damage by peroxidation of lipid cell membranes. Recent studies of experimental uveitis indicate that other potent oxidants are generated in uveitis by macrophages. One of these is ONOO-, which is formed from *NO and O(-)2. The macrophages generate *NO preferentially in the outer retina following iNOS expression. In these phagocytes, outer retinal proteins, especially arrestin, are found to be potent iNOS inducers. Current studies of RPE show that these cells protect the retina from ONOO- mediated damage in uveitis by releasing a novel protein called retinal pigment epithelial protective protein. This protein is found to suppress O(-)2 and *NO generation by the phagocytes, in both in vitro and in vivo uveitis models. The protective protein expression is restricted to RPE, its suppressive effect is a result of the inhibition of the phosphorylation of cytosolic proteins, p47-phox, required for the assembly of NADPH and activation of NFkappaB, which are required for generation of 0(-)2 and expression of iNOS respectively. Either pharmacologically or chemically, up-regulation of RPP generation could help in preventing retinal degeneration in uveitis or other degenerative dis

A phosphatidic acid-activated protein kinase and conventional protein kinase C isoforms phosphorylate p22(phox), an NADPH oxidase component

Using a phosphorylation-dependent cell-free system to study NADPH oxidase activation (McPhail, L. C., Qualliotine-Mann, D., and Waite, K. A. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 7931-7935), we previously showed that p47(phox), a cytosolic NADPH oxidase component, is phosphorylated. Now, we show that p22(phox), a subunit of the NADPH oxidase component flavocytochrome b(558), also is phosphorylated. Phosphorylation is selectively activated by phosphatidic acid (PA) versus other lipids and occurs on a threonine residue in p22(phox). We identified two protein kinase families capable of phosphorylating p22(phox): 1) a potentially novel, partially purified PA-activated protein kinase(s) known to phosphorylate p47(phox) and postulated to mediate the phosphorylation-dependent activation of NADPH oxidase by PA and 2) conventional, but not novel or atypical, isoforms of protein kinase C (PKC). In contrast, all classes of PKC isoforms could phosphorylate p47(phox). In a gel retardation assay both the phosphatidic acid-dependent kinase and conventional PKC isoforms phosphorylated all molecules of p22(phox). These findings suggest that phosphorylation of p22(phox) by conventional PKC and/or a novel PA-activated protein kinase regulates the activation/assembly of NADPH oxidase.

Direct interaction between p47phox and protein kinase C: evidence for targeting of protein kinase C by p47phox in neutrophils

p47(phox) is an essential component of the NADPH oxidase, and phosphorylation of p47(phox) is associated with activation of the enzyme. Here we have used p47(phox) affinity chromatography to extract a p47(phox) kinase from neutrophil cytosol. The kinase activity was purified by gel filtration and Mini Q chromatography and shown to be indistinguishable from the catalytic fragments of protein kinase C (PKC)-beta(I), -beta(II) and -delta. The C-terminus of p47(phox) represented the site of interaction with PKC. Co-immunoprecipitation experiments revealed that the interaction between PKC isotypes and p47(phox) takes place in intact cells. However PKC-beta and -delta showed different time courses of co-immunoprecipitation, suggesting that the interactions may serve different functions for the various PKC isotypes. Using cells lacking p47(phox), we investigated the functional relevance of the interaction between PKC and p47(phox). Subcellular fractionation revealed an abnormal recruitment of PKC-beta(I) and -beta(II), but not PKC-delta, to particulate fractions in p47(phox)-deficient cells. Phosphorylation of cytosolic proteins was generally increased in stimulated p47(phox)-deficient neutrophils as compared with normal neutrophils. Furthermore, the cytoskeletal protein coronin was not phosphorylated upon stimulation of p47(phox)-deficient neutrophils. These findings were confirmed in an in vitro-reconstituted system using rat brain cytosol in which addition of p47(phox) affected phosphorylation by PKC/PKM (PKM is the catalytic fragment of PKC). These results indicate that p47(phox) can act as a regulator of PKC in neutrophils.

Effects of local anesthetics on phospholipase D activity in differentiated human promyelocytic leukemic HL60 cells

Local anesthetics impair certain functions of neutrophils, and phospholipase D (PLD) is considered to play an important role in the regulation of these functions. To understand the mechanisms by which local anesthetics suppress the functions of neutrophils, we examined the effects of local anesthetics on PLD in neutrophil-like differentiated human promyelocytic leukemic HL60 cells. Tetracaine, a local anesthetic, inhibited formyl-methionyl-leucyl-phenylalanine (fMLP)- and 4beta-phorbol 12-myristate 13-acetate (PMA)-induced PLD activation, but potentiated fMLP-stimulated phospholipase C activity. All four local anesthetics tested suppressed PMA-induced PLD activation to different extents, and the order of their potency was tetracaine > bupivacaine > lidocaine > procaine. In a cell-free system, tetracaine suppressed guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS)-induced PLD activation as well as PMA-induced PLD activation. Western blot analysis revealed that tetracaine prevented the membrane translocation of PLD-activating factors, ADP-ribosylation factor, RhoA, and protein kinase Calpha. Tetracaine also inhibited the activity of recombinant hPLD1a in vitro. These results suggest that local anesthetics suppress PLD activation in differentiated HL60 cells by preventing the membrane translocation of PLD-activating factors, and/or by directly inhibiting the enzyme per se. Therefore, it could be assumed that local anesthetics would suppress the functions of neutrophils by inhibition of PLD activation.

NADPH Oxidase Activation and Assembly During Phagocytosis

Generation of superoxide (O2−) by the NADPH-dependent oxidase of polymorphonuclear leukocytes is an essential component of the innate immune response to invading microorganisms. To examine NADPH oxidase function during phagocytosis, we evaluated its activation and assembly following ingestion of serum-opsonized Neisseria meningitidis, serogroup B (NMB), and compared it with that elicited by serum-opsonized zymosan (OPZ). Opsonized N. meningitidis- and OPZ-dependent generation of reactive oxygen species by polymorphonuclear leukocytes peaked early and then terminated. Phosphorylation of p47phox coincided with peak generation of reactive oxygen species by either stimulus, consistent with a role for p47phox phosphorylation during NADPH oxidase activation, and correlated with phagosomal colocalization of flavocytochrome b558 (flavocytochrome b) and p47phox and p67phox (p47/67phox). Termination of respiratory burst activity did not reflect dephosphorylation of plasma membrane- and/or phagosome-associated p47phox; in contrast, the specific activity of phosphorylated p47phox at the phagosomal membrane increased. Most significantly, termination of oxidase activity paralleled the loss of p47/67phox from both NMB and OPZ phagosomes despite the continued presence of flavocytochrome b. These data suggest that 1) the onset of respiratory burst activity during phagocytosis is linked to the phosphorylation of p47phox and its translocation to the phagosome; and 2) termination of oxidase activity correlates with loss of p47/67phox from flavocytochrome b-enriched phagosomes and additional phosphorylation of membrane-associated p47phox.

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.

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

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

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

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

Characterization of cellular response to thiol-modified gold surfaces implanted in mouse peritoneal cavity

The early inflammatory reaction in vivo to three well defined surfaces-gold, gold coated with glutathione (GSH), and 3-mercapto-1, 2-propanediol (MG)-was assessed as manifested by the adherence and activation of inflammatory cells during implantation intraperitoneally in mice. Evaluation of cell adhesion and activation was done by immunohistochemistry using specific monoclonal antibodies directed against cell differentiation antigens CD11b/CD18, CD74, and CD25 or by measurement by chemoluminescence of reactive oxygen radical species produced by adhering cells. Cell recruitment and activation was slow on the GSH-coated gold surfaces. These surfaces also had the highest percentage of adhering cells with an intact cell membrane. The MG-coated surfaces, on the other hand, rapidly recruited and activated cells and also caused cell membrane leakage to propidium iodide, suggesting cell membrane damage or cell death. The respiratory burst of adhering cells was stimulated by phorbol-myristate acetate on the GSH-coated surface but not on the MG-coated surface and by opsonized zymosan on the Mg-coated surface but only to a small degree on the GSH-coated surface. The respiratory burst following zymosan activation of cells adhering to the MG-coated surface was inhibited by treatment with 2. 3-diphosphoglycerate, a phospholipase D inhibitor. The presented data suggest that peritoneal leukocytes adhering to foreign materials may raise a respiratory burst response via a phospholipase D-dependent and protein kinase C-independent pathway.

Modulation of luminol chemiluminescence of fMet-Leu-Phe-stimulated neutrophils by affecting dephosphorylation and the metabolism of phosphatidic acid

This paper is addressed to study how PKC-mediated effects and phosphatidic acid interact together in activation of NADPH-oxidase in formyl-methionyl-leucyl-phenylalanine (fMet-Leu-Phe) stimulated neutrophils as detected by luminol chemiluminescence. The early luminescence response in fMet-Leu-Phe-stimulated cells (up to 5 min after stimulation) depends mainly on reactive oxygen species generated extracellularly, whereas all later events are caused by oxidation of luminol inside the cells. The two protein phosphatase inhibitors, okadaic acid and calyculin A, dramatically increased the late luminescence of cells. This enhancement was totally inhibited by the phospholipase D modulator butanol, while the protein kinase C (PKC) inhibitor bisindolylmaleimide I was insensitive. The early luminescence response of the cells was slightly inhibited by both protein phosphatase inhibitors and depended on protein kinase C as well as on phospholipase D activities. Propranolol, an inhibitor of phosphatidate phosphohydrolase, enhanced all parts of luminescence response of fMet-Leu-Phe-stimulated neutrophils at concentrations up to 2.5 x 10(-5) mol/L. While the late luminescence response of propranolol-treated cells was not inhibited by the PKC inhibitor bisindolylmaleimide I, the first response depended on protein kinase C. The inhibitor of diacylglycerol kinase R59949 enhanced the luminescence signal only during the first 4 min in fMet-Leu-Phe-stimulated cells. Only diacylglycerols derived from phospholipase C, such as 1-stearoyl-2-arachidonoyl-sn-glycerol, were able to initiate an oxidative burst in cells. Saturated diacylglycerols (e.g. 1,2-dipalmitoyl-sn-glycerol or 1,2-distearoyl-sn-glycerol) did not yield any luminol chemiluminescence, although they were incorporated into the plasma membrane, as evidenced by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Our results demonstrate that phosphatidic acid produced by phospholipase D is responsible for NADPH-oxidase activity in fMet-Leu-Phe-stimulated neutrophils over the entire measuring time, whereas PKC-mediated processes are only involved during the first 5 min.