Current molecular models for NADPH oxidase regulation by Rac GTPase

Regulation of signal transduction through protein cysteine oxidation

The production of reactive oxygen species (ROS) accompanies many signaling events. Antioxidants and ROS scavenging enzymes in general have effects that indicate a critical role for ROS in downstream signaling, but a mechanistic understanding of the contribution of ROS as second messengers is incomplete. Here, the role of reactive oxygen species in cell signaling is discussed, emphasizing the ability of ROS to directly modify signaling proteins through thiol oxidation. Examples are provided of protein thiol modifications that control signal transduction effectors that include protein kinases, phosphatases, and transcription factors. Whereas the effects of cysteine oxidation on these proteins in experimental systems is clear, it has proven more difficult to demonstrate these modifications in response to physiologic stimuli. Improved detection methods for analysis of thiol modification will be essential to define these regulatory mechanisms. Bridging these two areas of research could reveal new regulatory mechanisms in signaling pathways, and identify new therapeutic targets.

Endosomal Acidification and Activation of NADPH Oxidase Isoforms Are Upstream Events in Hyperosmolarity-induced Hepatocyte Apoptosis

Hyperosmotic exposure of rat hepatocytes induced a rapid oxidative-stress(ROS) response as an upstream signal for proapoptotic CD95 activation. This study shows that hyperosmotic ROS formation involves a rapid ceramide- and protein kinase Czeta (PKCzeta)-dependent serine phosphorylation of p47phox and subsequent activation of NADPH oxidase isoforms. Hyperosmotic p47phox phosphorylation and ROS formation were sensitive to inhibition of sphingomyelinases and were strongly blunted after knockdown of acidic sphingomyelinase (ASM) or of p47phox protein. Hyperosmolarity induced a rapid bafilomycin- and 4,4 '-diisothiocyanostilbene-2,2 '-disulfonic acid disodium salt (DIDS)-sensitive acidification of a vesicular compartment, which was accessible to endocytosed fluorescein isothiocyanate-dextran and colocalized with ASM, PKCzeta, and the NADPH oxidase isoform Nox 2 (gp91phox). Bafilomycin and DIDS prevented the hyperosmolarity-induced increase in ceramide formation, p47phox phosphorylation, and ROS formation. As shown recently (Reinehr, R., Becker, S., Höngen, A., and Häussinger, D. (2004) J. Biol. Chem. 279, 23977-23987), hyperosmolarity induced a Yes-dependent activation of JNK and the epidermal growth factor receptor (EGFR), followed by EGFR-CD95 association, EGFR-catalyzed CD95-tyrosine phosphorylation, and translocation of the EGFR-CD95 complex to the plasma membrane, where formation of the deathinducing signaling complex occurs. These proapoptotic responses were not only sensitive to inhibitors of sphingomyelinase, PKCzeta, or NADPH oxidases but also to ASM knockdown, bafilomycin, and DIDS, i.e. maneuvers largely preventing hyperosmolarity-induced endosomal acidification and/or ceramide formation. In hepatocytes from p47phox knock-out mice, hyperosmolarity failed to activate the CD95 system. The data suggest that hyperosmolarity induces endosomal acidification as an important upstream event for CD95 activation through stimulation of ASM-dependent ceramide formation and activation of NADPH oxidase isoforms.

Expression of Nox genes in rat organs, mouse oocytes, and sea urchin eggs

Degenerate primers were designed to isolate new homologs of Nox family genes in rat organs and sea urchin eggs. The primers were capable of amplifying Nox1, Nox2, Nox3, Nox4, Duox1 and Duox2 but not Nox5, and failed to isolate novel homologs in rat. However, a novel homolog (named as Nox-U1) was identified in sea urchin eggs. In the most conserved region (amino acid 336--417 in human Nox2) Nox-U1 has the highest identity with Nox2, which appears to be abundant in mouse oocytes. However, phylogenetic analysis of the entire sequence has revealed that Nox-U1 is closer to Nox4 or Nox5 than Nox2 or Nox3. Histidine residues assumed to be responsible for heme ligation, motifs for FAD- and NADPH-binding, and two asparagine-linked glycosylation sites are conserved.

Assembly of the phagocyte NADPH oxidase complex: chimeric constructs derived from the cytosolic components as tools for exploring structure-function relationships

Phagocytes generate superoxide (O2*-) by an enzyme complex known as reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Its catalytic component, responsible for the NADPH-driven reduction of oxygen to O2*-, is flavocytochrome b559, located in the membrane and consisting of gp91phox and p22phox subunits. NADPH oxidase activation is initiated by the translocation to the membrane of the cytosolic components p47phox, p67phox, and the GTPase Rac. Cytochrome b559 is converted to an active form by the interaction of gp91phox with p67phox, leading to a conformational change in gp91phox and the induction of electron flow. We designed a new family of NADPH oxidase activators, represented by chimeras comprising various segments of p67phox and Rac1. The prototype chimera p67phox (1-212)-Rac1 (1-192) is a potent activator in a cell-free system, also containing membrane p47phox and an anionic amphiphile. Chimeras behave like bona fide GTPases and can be prenylated, and prenylated (p67phox -Rac1) chimeras activate the oxidase in the absence of p47phox and amphiphile. Experiments involving truncations, mutagenesis, and supplementation with Rac1 demonstrated that the presence of intrachimeric bonds between the p67phox and Rac1 moieties is an absolute requirement for the ability to activate the oxidase. The presence or absence of intrachimeric bonds has a major impact on the conformation of the chimeras, as demonstrated by fluorescence resonance energy transfer, small angle X-ray scattering, and gel filtration. Based on this, a "propagated wave" model of NADPH oxidase activation is proposed in which a conformational change initiated in Rac is propagated to p67phox and from p67phox to gp91phox.

EGF blocks NADPH oxidase activation by TGF-beta in fetal rat hepatocytes, impairing oxidative stress, and cell death

Epidermal growth factor (EGF) is a survival signal for transforming growth factor-beta (TGF-beta)-induced apoptosis in hepatocytes, phosphatidylinositol 3-kinase (PI 3-K) being involved in this effect. Here, we analyze the possible cross talks between EGF and TGF-beta signals to understand how EGF impairs the early pro-apoptotic events induced by TGF-beta. Data have indicated that neither SMAD nor c-Jun NH2 Terminal Kinase (JNK) activations are altered by EGF, which clearly interferes with events directly related to the radical oxygen species (ROS) production, impairing oxidative stress, p38 MAP kinase activation, and cell death. Activation of a NADPH-oxidase-like system, which is responsible for the early ROS production by TGF-beta, is completely inhibited by EGF, through a PI 3-K-dependent mechanism. Activity of RAC1 increases by TGF-beta, but also by EGF, and both act synergistically to get maximum effects. Fetal rat hepatocytes express nox4, in addition to nox1 and nox2, and TGF-beta clearly upregulates nox4. EGF blocks up-regulation of nox4 by TGF-beta. Interestingly, in the presence of PI 3-K inhibitors, EGF is not able to counteract the nox4 upregulation by TGF-beta. Taking together these results indicate that impairment of TGF-beta-induced NADPH oxidase activation by EGF is a RAC1-independent process and correlates with an inhibition of the mechanisms that address the increase of nox4 mRNA levels by TGF-beta.

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

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

Lovastatin-Induced Apoptosis in Macrophages through the Rac1/Cdc42/JNK Pathway

Statins, inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A reductase, have been used successfully in the treatment of hypercholesterolemia for more than a decade. Statins also exhibit overall clinical benefits on cardiovascular diseases independent of their effects on lowering serum cholesterol levels. These beneficial effects of statin therapy are believed to be due, at least in part, to the anti-inflammatory and immunomodulatory roles of statins. Statin treatment reduces the levels of inflammatory markers, decreases the activation and recruitment of immune cells, and delays the progression of atherosclerosis, a chronic inflammatory disease. However, little is known about the direct impact of statins on immune cells, particularly on macrophages. We report that lovastatin, a member of the statin family, effectively induces apoptosis in macrophages. Further investigation of the molecular mechanism has revealed that Rac1 and Cdc42, the small GTPase family members, may play an important role in lovastatin-induced macrophage apoptosis. Moreover, the activation of the JNK pathway may contribute to this event. Our findings provide a better understanding of the molecular basis underlying the anti-inflammatory clinical benefits of statin therapy in cardiovascular diseases.

A transverse tubule NADPH oxidase activity stimulates calcium release from isolated triads via ryanodine receptor type 1 S -glutathionylation

We report here the presence of an NADPH oxidase (NOX) activity both in intact and in isolated transverse tubules and in triads isolated from mammalian skeletal muscle, as established by immunochemical, enzymatic, and pharmacological criteria. Immunohistochemical determinations with NOX antibodies showed that the gp91(phox) membrane subunit and the cytoplasmic regulatory p47(phox) subunit co-localized in transverse tubules of adult mice fibers with the alpha1s subunit of dihydropyridine receptors. Western blot analysis revealed that isolated triads contained the integral membrane subunits gp91(phox) and p22(phox), which were markedly enriched in isolated transverse tubules but absent from junctional sarcoplasmic reticulum vesicles. Isolated triads and transverse tubules, but not junctional sarcoplasmic reticulum, also contained varying amounts of the cytoplasmic NOX regulatory subunits p47(phox) and p67(phox). NADPH or NADH elicited superoxide anion and hydrogen peroxide generation by isolated triads; both activities were inhibited by NOX inhibitors but not by rotenone. NADH diminished the total thiol content of triads by one-third; catalase or apocynin, a NOX inhibitor, prevented this effect. NADPH enhanced the activity of ryanodine receptor type 1 (RyR1) in triads, measured through [3H]ryanodine binding and calcium release kinetics, and increased significantly RyR1 S-glutathionylation over basal levels. Preincubation with reducing agents or NOX inhibitors abolished the enhancement of RyR1 activity produced by NADPH and prevented NADPH-induced RyR1 S-glutathionylation. We propose that reactive oxygen species generated by the transverse tubule NOX activate via redox modification the neighboring RyR1 Ca2+ release channels. Possible implications of this putative mechanism for skeletal muscle function are discussed.

RhoG regulates the neutrophil NADPH oxidase

RhoG is a Rho family small GTPase implicated in cytoskeletal regulation, acting either upstream of or in parallel to Rac1. The precise function(s) of RhoG in vivo has not yet been defined. We have identified a novel role for RhoG in signaling the neutrophil respiratory burst stimulated by G protein-coupled receptor agonists. Bone marrow-derived neutrophils from RhoG knockout (RhoG(-/-)) mice exhibited a marked impairment of oxidant generation in response to C5a or fMLP, but normal responses to PMA or opsonized zymosan and normal bacterial killing. Activation of Rac1 and Rac2 by fMLP was diminished in RhoG(-/-) neutrophils only at very early (5 s) time points (by 25 and 32%, respectively), whereas chemotaxis in response to soluble agonists was unaffected by lack of RhoG. Additionally, fMLP-stimulated phosphorylation of protein kinase B and p38MAPK, activation of phospholipase D, and calcium fluxes were equivalent in wild-type and RhoG(-/-) neutrophils. Our results define RhoG as a critical component of G protein-coupled receptor-stimulated signaling cascades in murine neutrophils, acting either via a subset of total cellular Rac relevant to oxidase activation and/or by a novel and as yet undefined interaction with the neutrophil NADPH oxidase.

Sphingolipid signaling and redox regulation

Sphingolipids including ceramide and its derivatives such as ceramide-1-phosphate, glycosyl-ceramide, and sphinogosine (-1-phosphate) are now recognized as novel intracellular signal mediators for regulation of inflammation, apoptosis, proliferation, and differentiation. One of the important and regulated steps in these events is the generation of these sphingolipids via hydrolysis of sphingomyelin through the action of sphingomyelinases (SMase). Several lines of evidence suggest that reactive oxygen species (ROS; O2-, H2O2, and OH-,) and reactive nitrogen species (RNS; NO, and ONOO-) and cellular redox potential, which is mainly regulated by cellular glutathione (GSH), are tightly linked to the regulation of SMase activation. On the other hand, sphingolipids are also known to play an important role in maintaining cellular redox homeostasis through regulation of NADPH oxidase, mitochondrial integrity, and antioxidant enzymes. Therefore, this paper reviews the relationship between cellular redox and sphingolipid metabolism and its biological significance.

TcRho1, the Trypanosoma cruzi Rho homologue, regulates cell-adhesion properties: Evidence for a conserved function

Rho proteins are members of the Ras superfamily of small GTPases. In higher eukaryotes these proteins play pivotal role in cell movement, phagocytosis, intracellular transport, cell-adhesion, and maintenance of cell morphology, mainly through the regulation of actin microfilaments. The GTPase TcRho1 is the only member of the Rho family described in human protozoan parasite Trypanosoma cruzi. We previously demonstrated that TcRho1 is actually required for differentiation of epimastigote to trypomastigote forms during the parasite cell cycle. In the present work, we describe cellular phenotypes induced by TcRho1 heterologous expression in NIH 3T3 fibroblasts. The NIH-3T3 lineages expressing the TcRho1-G15V and TcRho1-Q76L mutants displayed decreased levels of migration compared to the control lineage NIH-3T3 pcDNA3.1, a phenotype probably due to distinct cell-substrate adhesion properties expressed by the mutant cell lines. Accordingly, cell-substrate adhesion assays revealed that the mutant cell lines of NIH-3T3 expressing TcRho1-positive dominants constructions present enhanced substrate-adhesion phenotype. Furthermore, similar experiments with T. cruzi expressing TcRho1 mutants also revealed an enhancement of cell attachment. These results suggest that TcRho1 plays a conserved regulatory role in cell-substrate adhesion in both NIH-3T3 fibroblasts and T. cruzi epimastigotes. Taken together, our data corroborate the notion that TcRho1 may regulate the substrate-adhesion in T. cruzi, a critical step for successful progression of the parasite life cycle.

Inhibition of ceramide-redox signaling pathway blocks glomerular injury in hyperhomocysteinemic rats

Ceramide-activated NAD(P)H oxidase has been reported to participate in homocysteine (Hcys)-induced abnormal metabolism of the extracellular matrix (ECM) in rat glomerular mesangial cells. However, it remains unknown whether this ceramide-redox signaling pathway contributes to glomerular injury induced by hyperhomocysteinemia (hHcys) in vivo. The present study was designed to address this question, defining the role of ceramide and activated NAD(P)H oxidase in the development of hHcys-induced glomerular injury. Uninephrectomized Sprague-Dawley rats were fed a folate-free diet for 8 weeks to produce hHcys and the de novo ceramide synthesis inhibitor myriocin or the NAD(P)H oxidase inhibitor apocynin was administrated. Rats with folate-free diet significantly increased plasma Hcys levels, renal ceramide levels, and NAD(P)H oxidase activity accompanied by marked glomerular injury. Treatment of rats with myriocin significantly reduced ceramide levels and improved glomerular injury, as shown by decreased urinary albumin excretion and reduced glomerular damage index. ECM components changed towards to normal levels with decreased tissue inhibitor of metalloproteinase-1 and increased matrix metalloproteinase-1 activity. NAD(P)H oxidase activity and Rac GTPase activity were reduced by 69 and 66%, respectively. In rats treated with apocynin, similar beneficial effects in protecting glomeruli from hHcys-induced injury were observed. These results support the view that de novo ceramide production is involved in Hcys-induced NAD(P)H oxidase activity in the kidney of hHcys rats and indicate the important role of ceramide-mediated redox signaling in hHcys-induced glomerular injury in rats.

Hyperosmotic activation of the CD95 death receptor system

Apoptosis is characterized by cell shrinkage, nuclear condensation, DNA fragmentation and apoptotic body formation. These features distinguish apoptosis from other types of cell death, such as necrosis. Whereas some signs of apoptosis, such as externalization of phosphatidylserine, altered mitochondrial function or activation of caspases are cell type- and death signal-dependent, apoptotic cell volume decrease (AVD) is an early and ubiquitous event and little is known about the signalling events, which are localized upstream of the plasma membrane transport steps leading to AVD and the proapoptotic events, which are induced by osmolyte loss and cell shrinkage. In hepatocytes hyperosmotic shrinkage sensitizes the cells towards CD95 ligand-induced apoptosis by activating the CD95 system. This complex process with a NADPH oxidase-derived reactive oxygen species signal as an important upstream event, allows via Yes, JNK and epidermal growth factor-receptor activation for CD95 tyrosine phosphorylation as a prerequisite for CD95 targeting to the plasma membrane and formation of the death inducing signalling complex. Other covalent modifications such as CD95-tyrosine-nitration or CD95-serine/threonine-phosphorylation can interfere with the CD95 activation process. The findings not only provide a mechanistic explanation for the high susceptibility of dehydrated cells for apoptosis, but also give insight into the role of AVD.

Inactivation of NADPH oxidase organizer 1 results in severe imbalance

Otoconia are biominerals of the vestibular system that are indispensable for the perception of gravity. Despite their importance, the process of otoconia genesis is largely unknown. Reactive oxygen species (ROS) have been recognized for their toxic effects in antimicrobial host defense as well as in aging and carcinogenesis. Enzymes evolved for ROS production belong to the recently discovered NADPH oxidase (Nox) enzyme family . Here we show that the inactivation of a regulatory subunit, NADPH oxidase organizer 1 (Noxo1), resulted in the severe balance deficit seen in the spontaneous mutant "head slant" (hslt) mice whose phenotype was rescued by Noxo1 transgenes. Wild-type Noxo1 was expressed in the vestibular and cochlear epithelia and was required for ROS production by an oxidase complex. In contrast, the hslt mutation of Noxo1 was biochemically inactive and led to an arrest of otoconia genesis, characterized by a complete lack of calcium carbonate mineralization and an accumulation of otoconial protein, otoconin-90/95 (OC-90/95). These results suggest that ROS generated by a Noxo1-dependent vestibular oxidase are critical for otoconia formation and may be required for interactions among otoconial components. Noxo1 mutants implicate a constructive developmental role for ROS, in contrast to their previously described toxic effects.

Rho GTPases and Leukocyte Adhesion Receptor Expression and Function in Endothelial Cells

Rho family GTPases are key signal transducers that regulate cell adhesion and migration and a variety of other cellular responses, including changes in gene expression. In this review, we discuss how Rho GTPases regulate signaling by endothelial cell receptors involved in leukocyte extravasation. First, Rho GTPases affect the expression of some leukocyte adhesion molecules on endothelial cells, such as intracellular adhesion molecule-1 and E-selectin, that can be induced by proinflammatory mediators, hypoxia, or shear stress. Second, Rho GTPases are activated by engagement of several leukocyte adhesion receptors and contribute to both early morphological changes and subsequent alterations in gene expression. Rho GTPases are therefore candidate targets for inhibiting leukocyte transendothelial migration in heart disease and chronic inflammatory disorders.

FcgammaR-induced production of superoxide and inflammatory cytokines is differentially regulated by SHIP through its influence on PI3K and/or Ras/Erk pathways

Phagocytosis of IgG-coated particles via FcgammaR is accompanied by the generation of superoxide and inflammatory cytokines, which can cause collateral tissue damage in the absence of regulation. Molecular mechanisms regulating these phagocytosis-associated events are not known. SHIP is an inositol phosphatase that downregulates PI3K-mediated activation events. Here, we have examined the role of SHIP in FcgammaR-induced production of superoxide and inflammatory cytokines. We report that primary SHIP-deficient bone marrow macrophages produce elevated levels of superoxide upon FcgammaR clustering. Analysis of the molecular mechanism revealed that SHIP regulates upstream Rac-GTP binding, an obligatory event for superoxide production. Likewise, SHIP-deficient macrophages displayed enhanced IL-1beta and IL-6 production in response to FcgammaR clustering. Interestingly, whereas IL-6 production required activation of both PI3K and Ras/Erk pathways, IL-1beta production was dependent only on Ras/Erk activation, suggesting that SHIP may also regulate the Ras/Erk pathway in macrophages. Consistently, SHIP-deficient macrophages displayed enhanced activation of Erk upon FcgammaR clustering. Inhibition of Ras/Erk or PI3K suppressed the enhanced production of IL-6 in SHIP-deficient macrophages. In contrast, inhibition of Ras/Erk, but not PI3K, suppressed IL-1beta production in these cells. Together, these data demonstrate that SHIP regulates phagocytosis-associated events through the inhibition of PI3K and Ras/Erk pathways.

Regulation of NADPH oxidases: the role of Rac proteins

The role for reactive oxygen species (ROS) in cellular (patho)physiology, in particular in signal transduction, is increasingly recognized. The family of NADPH oxidases (NOXes) plays an important role in the production of ROS in response to receptor agonists such as growth factors or inflammatory cytokines that signal through the Rho-like small GTPases Rac1 or Rac2. The phagocyte oxidase (gp91phox/NOX2) is the best characterized family member, and its mode of activation is relatively well understood. Recent work has uncovered novel and increasingly complex modes of control of the NOX2-related proteins. Some of these, including NOX2, have been implicated in various aspects of (cardio)vascular disease, including vascular smooth muscle and endothelial cell hypertrophy and proliferation, inflammation, and atherosclerosis. This review focuses on the role of the Rac1 and Rac2 GTPases in the activation of the various NOX family members.

Physiological and pathophysiological aspects of ceramide

Activation of cells by receptor- and nonreceptor-mediated stimuli not only requires a change in the activity of signaling proteins but also requires a reorganization of the topology of the signalosom in the cell. The cell membrane contains distinct domains, rafts that serve the spatial organization of signaling molecules in the cell. Many receptors or stress stimuli transform rafts by the generation of ceramide. These stimuli activate the acid sphingomyelinase and induce a translocation of this enzyme onto the extracellular leaflet of the cell membrane. Surface acid sphingomyelinase generates ceramide that serves to fuse small rafts and to form large ceramide-enriched membrane platforms. These platforms cluster receptor molecules, recruit intracellular signaling molecules to aggregated receptors, and seem to exclude inhibitory signaling factors. Thus ceramide-enriched membrane platforms do not seem to be part of a specific signaling pathway but may facilitate and amplify the specific signaling elicited by the cognate stimulus. This general function may enable these membrane domains to be critically involved in the induction of apoptosis by death receptors and stress stimuli, bacterial and viral infections of mammalian cells, and the regulation of cardiovascular functions.

Sphingosine kinase functionally links elevated transmural pressure and increased reactive oxygen species formation in resistance arteries

Myogenic vasoconstriction, an intrinsic response to elevated transmural pressure (TMP), requires the activation of sphingosine kinase (Sk1) and the generation of reactive oxygen species (ROS). We hypothesized that pressure-induced Sk1 signaling and ROS generation are functionally linked. Using a model of cannulated resistance arteries isolated from the hamster gracilis muscle, we monitored vessel diameter and smooth muscle cell (SMC) Ca2+i (Fura-2) or ROS production (dichlorodihydrofluorescein). Elevation of TMP stimulated the translocation of a GFP-tagged Sk1 fusion protein from the cytosol to the plasma membrane, indicative of enzymatic activation. Concurrently, elevation of TMP initiated a rapid and transient production of ROS, which was enhanced by expression of wild-type Sk1 (hSk(wt)) and inhibited by its dominant-negative mutant (hSk(G82D)). Exogenous sphingosine-1-phosphate (S1P) also stimulated ROS generation is isolated vessels. Chemical (1 micromol/L DPI), peptide (gp91ds-tat/gp91ds), and genetic (N17Rac) inhibition strategies indicated that NADPH oxidase was the source of the pressure-induced ROS. NADPH oxidase inhibition attenuated myogenic vasoconstriction and reduced the apparent Ca2+ sensitivity of the SMC contractile apparatus, without affecting Ca2+-independent, RhoA-mediated vasoconstriction in response to exogenous S1P. Our results indicate a mandatory role for Sk1/S1P in mediating pressure-induced, NADPH oxidase-derived ROS formation. In turn, ROS generation appears to increase Ca2+ sensitivity, necessary for full myogenic vasoconstriction.

Rac1-NADPH oxidase-regulated generation of reactive oxygen species mediates glutamate-induced apoptosis in SH-SY5Y human neuroblastoma cells

Reactive oxygen species (ROS) are known to play an important role in glutamate-induced neuronal cell death. In the present study, we examined whether NADPH oxidase serves as a source of ROS production and plays a role in glutamate-induced cell death in SH-SY5Y human neuroblastoma cells. Stimulation of the cells with glutamate (100 mM) induced apoptotic cell death and increase in the level of ROS, and these effects of glutamate were significantly suppressed by the inhibitors of the NADPH oxidase, diphenylene iodonium, apocynin, and neopterine. In addition, RT-PCR revealed that SH-SY5Y cells expressed mRNA of gp91phox, p22phox and cytosolic p47phox, p67phox and p40phox, the components of the plasma membrane NADPH oxidase. Treatment with glutamate also resulted in activation and translocation of Rac1 to the plasma membrane. Moreover, the expression of Rac1N17, a dominant negative mutant of Rac1, significantly blocked the glutamate-induced ROS generation and cell death. Collectively, these results suggest that the plasma membrane-bound NADPH oxidase complex may play an essential role in the glutamate-induced apoptotic cell death through increased production of ROS.

Recognition of apoptotic cells by macrophages activates the peroxisome proliferator-activated receptor-gamma and attenuates the oxidative burst

It is appreciated that phagocytosis of apoptotic cells (AC) is an immunological relevant process that shapes the pro- versus anti-inflammatory macrophage phenotype. It was our intention to study the respiratory burst, a prototype marker of macrophage activation, under the impact of AC. Following incubation of RAW264.7 macrophages with AC, we noticed attenuated production of reactive oxygen species (ROS) in response to PMA treatment, and observed a correlation between attenuated ROS formation and suppression of protein kinase Calpha (PKCalpha) activation. EMSA analysis demonstrated an immediate activation of peroxisome proliferator-activated receptor-gamma (PPARgamma) following supplementation of AC to macrophages. In macrophages carrying a dominant-negative PPARgamma mutant, recognition of AC no longer suppressed PKCalpha activation, and the initial phase of ROS formation was largely restored. Interference with actin polymerization and transwell experiments suggest that recognition of AC by macrophages suffices to attenuate the early phase of ROS formation that is attributed to PPARgamma activation.

Expression and Activity of NOX5 in the Circulating Malignant B Cells of Hairy Cell Leukemia

Hairy cells (HCs) are mature malignant B cells that contain a number of constitutively active signaling molecules including GTP-bound Rac1, protein kinase C, and Src family kinases. Because Rac1 is a component of the reactive oxidant species (ROS)-generating NADPH oxidase system, we investigated the role of this GTPase in ROS production in HCs. In this study, we show that ROS production in HCs involves a flavin-containing oxidase dependent on Ca2+, but not on GTP-Rac1 or protein kinase C. This suggests the involvement of the nonphagocytic NADPH oxidase NOX5, an enzyme found in lymphoid tissues, but not in circulating lymphocytes. By using RT-PCR and Southern and Western blotting and by measuring superoxide anion production in membrane fractions in the absence of cytosolic components, we demonstrate for the first time that HCs (but not circulating normal B cells or some other lymphoid cell types) express NOX5. We also demonstrate that inhibition of NADPH oxidase in HCs results in a selective increase in the activity of Src homology region 2 domain-containing phosphatase 1 (SHP-1). Furthermore, SHP-1 in HCs coimmunoprecipitates with tyrosine phosphorylated CD22 and localizes in the same cellular compartment as NOX5. This allows the inactivation of SHP-1 by NOX5-generated ROS and contributes to the maintenance of the constitutive activation of HCs.

Bile salt-induced apoptosis involves NADPH oxidase isoform activation

BACKGROUND & AIMS

Hydrophobic bile salts trigger a rapid oxidative stress response as an upstream event of CD95 activation and hepatocyte apoptosis.

METHODS

The underlying mechanisms were studied by Western blot, immunocytochemistry, protein knockdown, and fluorescence resonance energy transfer microscopy in rat hepatocytes and human hepatoma cell line 7 (Huh7).

RESULTS

The rapid oxidative stress formation in response to taurolithocholate-3-sulfate (TLCS) was inhibited by diphenyleneiodonium, apocynin, and neopterin, suggestive for the involvement of nicotinamide adenine dinucleotide phosphate (NADPH) oxidases. TLCS induced a rapid serine phosphorylation of the regulatory subunit p47phox, which was sensitive to inhibition of sphingomyelinase and protein kinase Czeta (PKCzeta). Inhibitors of p47phox phosphorylation and p47phox protein knockdown abolished the TLCS-induced oxidative stress response and blunted subsequent CD95 activation. Consequences of TLCS-induced oxidative stress were c-Jun-N-terminal kinase activation and Yes-dependent activation of the epidermal growth factor receptor (EGFR), followed by EGFR-catalyzed CD95 tyrosine phosphorylation, formation of the death-inducing signaling complex, and execution of apoptosis. As shown by fluorescence resonance energy transfer experiments in Huh7 cells, TLCS induced a c-Jun-N-terminal kinase-dependent EGFR/CD95 association in the cytosol and trafficking of this protein complex to the plasma membrane. Inhibition of EGFR tyrosine kinase activity by AG1478 allowed for cytosolic EGFR/CD95 association, but prevented targeting of the EGFR/CD95 complex to the plasma membrane. Both processes, and TLCS-induced Yes and EGFR activation, were sensitive to inhibition of sphingomyelinase, PKCzeta, or NADPH oxidases.

CONCLUSIONS

The data suggest that hydrophobic bile salts activate NADPH oxidase isoforms with the resulting oxidative stress response triggering activation of the CD95 system and apoptosis.

Leishmania donovani promastigotes induce periphagosomal F-actin accumulation through retention of the GTPase Cdc42

Leishmania donovani promastigotes inhibit phagosome maturation and induce the accumulation of periphagosomal F-actin during the establishment of infection within macrophages. These events are mediated by the surface glycolipid lipophosphoglycan (LPG), but the underlying mechanisms remain to be elucidated. In this study, we addressed the role of the Rho-family GTPases RhoA, Rac1 and Cdc42 in the uptake of L. donovani promastigotes and in the accumulation of periphagosomal F-actin. Confocal microscopy analyses revealed that association of both Rac1 and RhoA to phagosomes containing L. donovani promastigotes was independent of the presence of LPG. In contrast, Cdc42 and proteins required for F-actin assembly (Arp2/3, WASP, Myosin, alpha-actinin) were retained on phagosomes in a LPG-dependent manner. Expression of the RhoA inhibitor C3-transferase blocked the internalization of complement-opsonized promastigotes, whereas the dominant-negative Rac1N17 blocked the uptake of unopsonized promastigotes. The dominant-negative Cdc42N17 inhibited LPG-mediated phagosomal accumulation of F-actin and retention of Arp2/3 and Myosin. Thus, our data suggest that the effect of LPG on the accumulation of periphagosomal F-actin is the consequence of an abnormal retention or activation of Cdc42 at the phagosome.

Interleukin-8-induced priming of neutrophil oxidative burst requires sequential recruitment of NADPH oxidase components into lipid rafts

The superoxide-producing phagocyte NADPH oxidase consists of a membrane-bound flavocytochrome b(558), the cytosol factors p47(phox), p67(phox), p40(phox), and the small GTPase Rac2, which translocate to the membrane to assemble the active complex following neutrophil activation. Interleukin-8 (IL-8) does not activate NADPH oxidase, but potentiates the oxidative burst induced by stimuli such as formyl-methionyl-leucyl-phenylalanine (fMLP) via a priming mechanism. The effect of IL-8 on the components of NADPH oxidase during the priming process has never been investigated in human neutrophils. Here we showed that within 3 min, IL-8 treatment enhanced the Btk- and ERK1/2-dependent phosphorylation of p47(phox), as well as the recruitment of flavocytochrome b(558), p47(phox), and Rac2 into cholesterol-enriched detergent-resistant microdomains (or lipid rafts). Conversely, IL-8 treatment lasting 15 min failed to recruit flavocytochrome b(558), p47(phox), or Rac2, but did enhance the Btk- and p38 MAPK-dependent phosphorylation and the translocation of p67(phox) into detergent-resistant microdomains. Moreover, methyl-beta-cyclodextrin, which disrupts lipid rafts, inhibited IL-8-induced priming in response to fMLP. Our findings indicate that IL-8-induced priming of the oxidative burst in response to fMLP involves a sequential assembly of the NADPH oxidase components in the lipid rafts of neutrophils.

Role of Rac-GTPase and reactive oxygen species in cardiac differentiation of stem cells

In the life of a cell, there is a constant balance between generation of reactive oxygen species (ROS) and activity of antioxidant defense mechanisms. Besides the damaging effects of ROS on many biomolecules, ROS also play a significant role in signal transduction pathways of growth factors suggesting a role of oxidative species in cell differentiation. ROS have recently been involved in the process of cardiac differentiation of stem cells. Several molecular mechanisms, including ones mediated by the GTPase Rac that underlie the regulatory role of ROS in the process of stem cell differentiation toward a cardiac lineage, are reviewed.

TRAF4 acts as a silencer in TLR-mediated signaling through the association with TRAF6 and TRIF

Toll-like receptors (TLR) and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase play an essential role in intracellular eradication of engulfed pathogens. Here, we demonstrate the physical and functional association between components of the cytosolic NADPH oxidase and TLR-mediated signaling molecules. Cytosolic components of NADPH oxidase suppressed TLR-mediated NF-kappaB activation as well as IFN-beta promoter activation. We demonstrate that TNF-associated factor (TRAF) 4 associates with p47(phox), a component of cytosolic NADPH oxidase, and physically interacts and functionally counteracts with TRAF6 and Toll-IL-1 receptor (TIR) domain-containing adaptor-inducing IFN-beta (TRIF) molecules that critically regulate TLR-mediated signaling. TRAF4 mRNA expression was elicited in RPMI 8226 cells following LPS or CpG DNA treatment. These results suggest that TRAF4 participates in the molecular mechanism underlying silencing of TLR-mediated signaling through the interaction with molecules harboring phagosome/endosome membrane.

Angiotensin II-induced oxidative burst is fluvastatin sensitive in neutrophils of patients with hypercholesterolemia

The aim of this study was to investigate the effect of the 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor fluvastatin (Flu) on angiotensin II (AII)-stimulated neutrophils of patients with hypercholesterolemia. Results suggest that a 6-week-long Flu administration completely counteracted the AII-induced increase in superoxide anion and leukotriene C4 production of the neutrophils of patients with hypercholesterolemia. However, the failure of signal processing through pertussis toxin-sensitive G protein, the increase in [Ca2+]i in membrane-bound protein kinase C activity, and the increase in neutrophil-bound cholesterol content were only partially restored by Flu. In addition, Flu had no effect on the increased membrane rigidity of the neutrophils of patients with hypercholesterolemia. To sum it up, Flu administration had a beneficial effect on AII-triggered reactive oxygen species generation; it resulted in partial restoration of signaling processes and of membrane composition, but membrane fluidity remained unchanged.

Adhesion and Rac1-dependent Regulation of Biglycan Gene Expression by Transforming Growth Factor-β

Both transforming growth factor-beta (TGF-beta)-induced expression of biglycan (BGN) and activation of p38 MAPK have been implicated in cellular adhesion and migration. Here, we analyzed the role of adhesive events and the small GTPase Rac1 in TGF-beta regulation of BGN. TGF-beta1 induction of BGN expression and activation of p38 was abolished or strongly reduced when cells were kept in suspension or exposed to either the actin cytoskeleton-disrupting agent cytochalasin D or a specific chemical Rac1 inhibitor. Ectopic expression of a dominant negative mutant (T17N) of Rac1 abrogated both TGF-beta-induced p38 MAPK activation and BGN up-regulation but did not affect TGF-beta-induced phosphorylation of Smad3 or transcriptional induction of Growth Arrest DNA Damage 45beta, previously shown to be crucial for TGF-beta regulation of BGN. Overexpression of wild type Rac1 greatly enhanced the TGF-beta effect on BGN in adherent cells, whereas ectopic expression of constitutively active Rac1 (Q61L) activated p38 and in the presence of exogenous TGF-beta was able to rescue BGN expression in nonadherent cells. Endogenous Rac1 was activated by TGF-beta treatment in PANC-1 cells in an adhesion-dependent fashion. Like Rac1-T17N, the NADPH oxidase inhibitor diphenylene iodonium and the tyrosine kinase inhibitor herbimycin A blocked TGF-beta-induced p38 activation and BGN expression, suggesting that Rac1 exerts its effect on BGN and p38 through increasing NADPH oxidase activity and subsequent production of reactive oxygen species. These results show that the TGF-beta effect on BGN is dependent on cell adhesion and that activated Rac1, presumably acting through NADPH oxidase(s), is necessary but not sufficient for TGF-beta-induced BGN expression.

Cytotoxic necrotizing factor type 1 production by uropathogenic Escherichia coli modulates polymorphonuclear leukocyte function

Many strains of uropathogenic Escherichia coli (UPEC) produce cytotoxic necrotizing factor type 1 (CNF1), a toxin that constitutively activates the Rho GTPases RhoA, Rac1, and Cdc42. We previously showed that CNF1 contributes to the virulence of UPEC in a mouse model of ascending urinary tract infection and a rat model of acute prostatitis and that a striking feature of the histopathology of the mouse bladders and rat prostates infected with CNF1-positive strains is an elevation in levels of polymorphonuclear leukocytes (PMNs). We also found that CNF1 synthesis leads to prolonged survival of UPEC in association with human neutrophils. Here, we tested the hypothesis that CNF1 production by UPEC diminishes the antimicrobial capacity of mouse PMNs by affecting phagocyte function through targeting Rho family GTPases that are critical to phagocytosis and the generation of reactive oxygen species. We found that, as with human neutrophils, CNF1 synthesis provided a survival advantage to UPEC incubated with mouse PMNs. We also observed that CNF1-positive UPEC down-regulated phagocytosis, altered the distribution of the complement receptor CR3 (CD11b/CD18), enhanced the intracellular respiratory burst, and increased levels of Rac2 activation in PMNs. From these results, we conclude that modulation of PMN function by CNF1 facilitates UPEC survival during the acute inflammatory response.

Point Mutations in the Proline-rich Region of p22 Are Dominant Inhibitors of Nox1- and Nox2-dependent Reactive Oxygen Generation

The integral membrane protein p22phox is an indispensable component of the superoxide-generating phagocyte NADPH oxidase, whose catalytic core is the membrane-associated gp91phox (also known as Nox2). p22phox associates with gp91phox and, through its proline-rich C terminus, provides a binding site for the tandem Src homology 3 domains of the activating subunit p47phox. Whereas p22phox is expressed ubiquitously, its participation in regulating the activity of other Nox enzymes is less clear. This study investigates the requirement of p22phox for Nox enzyme activity and explores the role of its proline-rich region (PRR) for regulating activity. Coexpression of specific Nox catalytic subunits (Nox1, Nox2, Nox3, Nox4, or Nox5) along with their corresponding regulatory subunits (NOXO1/NOXA1 for Nox1; p47phox/p67phox/Rac for Nox2; NOXO1 for Nox3; no subunits for Nox4 or Nox5) resulted in marked production of reactive oxygen. Small interfering RNAs decreased endogenous p22phox expression and inhibited reactive oxygen generation from Nox1, Nox2, Nox3, and Nox4 but not Nox5. Truncated forms of p22phox that disrupted the PRR-inhibited reactive oxygen generation from Nox1, Nox2, and Nox3 but not from Nox4 and Nox5. Similarly, p22phox (P156Q), a mutation that disrupts Src homology 3 binding by the PRR, potently inhibited reactive oxygen production from Nox1 and Nox2 but not from Nox4 and Nox5. Expression of p22phox (P156Q) inhibited NOXO1-stimulated Nox3 activity, but co-expression of NOXA1 overcame the inhibitory effect. The P157Q and P160Q mutations of p22phox showed selective inhibition of Nox2/p47phox/p67phox, and selectivity was specific for the organizing subunit (p47phox or NOXO1) rather than the Nox catalytic subunit. These studies stress the importance of p22phox for the function of Nox1, Nox2, Nox3, and Nox4, and emphasize the key role of the PRR for regulating Nox proteins whose activity is dependent upon p47phox or NOXO1.

Role of Phagocyte Oxidase in UVA-Induced Oxidative Stress and Apoptosis in Keratinocytes

Chronic exposure to ultraviolet radiation including ultraviolet A (315-400 nm) (UVA) may cause photocarcinogenesis and photoaging. The UVA-induced production of reactive oxygen species (ROS) and the resultant oxidative stress exposure play an important role in these biological processes. Here we have investigated the role of phagocyte oxidase (PHOX, gp91phox) in the production of ROS, redox status change, and apoptosis after UVA exposure by using gp91phox-deficient (gp91phox-/-) primary keratinocytes. UVA radiation resulted in increased ROS production and oxidation of reduced glutathione (GSH) to its oxidized form (GSSG). The presence of diphenylene iodonium (DPI) inhibited ROS production by UVA. In comparison with wild-type cells, gp91phox-/- cells produced slightly less ROS and GSH oxidation. UVA radiation induced apoptosis in wild-type keratinocytes as detected by phosphatidylserine (PS) translocation, caspase activation, and DNA fragmentation. As compared with wild-type cells, UVA induced less PS translocation in gp91phox-deficient cells. No difference, however, was observed in caspase activation and DNA fragmentation after UVA exposure in wild-type and gp91phox-/- cells. These findings suggest that gp91phox plays a limited role in the UVA-induced ROS production, oxidative stress, and therefore the PS translocation, but has no effect on UVA-induced caspase activation and DNA fragmentation during apoptosis.

Activation of APE1/Ref-1 is dependent on reactive oxygen species generated after purinergic receptor stimulation by ATP

Apurinic apyrimidinic endonuclease redox effector factor-1 (APE1/Ref-1) is involved both in the base excision repair (BER) of DNA lesions and in the eukaryotic transcriptional regulation. APE1/Ref-1 is regulated at both the transcriptional and post-translational levels, through control of subcellular localization and post-translational modification. In response to stress conditions, several cell types release ATP, which exerts stimulatory effects on eukaryotic cells via the purinergic receptors (P2) family. By using western blot and immunofluorescence analysis on a human tumour thyroid cell line (ARO), we demonstrate that purinergic stimulation by extracellular ATP induces quick cytoplasm to nucleus translocation of the protein at early times and its neosynthesis at later times. Continuous purinergic triggering by extracellular ATP released by ARO cells is responsible for the control of APE1/Ref-1 intracellular level. Interference with intracellular pathways activated by P2 triggering demonstrates that Ca²⁺ mobilization and intracellular reactive oxygen species (ROS) production are responsible for APE1/Ref-1 translocation. The APE1/Ref-1 activities on activator protein-1 (AP-1) DNA binding and DNA repair perfectly match its nuclear enrichment upon ATP stimulation. The biological relevance of our data is reinforced by the observation that APE1/Ref-1 stimulation by ATP protects ARO cells by H₂O₂-induced cell death. Our data provide new insights into the complex mechanisms regulating APE1/Ref-1 functions.

Serine protease cathepsin G regulates adhesion-dependent neutrophil effector functions by modulating integrin clustering

The polymorphonuclear leukocyte (PMN)-derived serine proteases play a key role in immune complex (IC)-mediated inflammation. However, the mechanisms by which these proteases regulate inflammatory response remain largely undefined. Here, we show that IC-activated cathepsin G- and neutrophil elastase-deficient (CG/NE) PMNs adhered normally to IC-coated surfaces but did not undergo CD11b clustering and failed to initiate cytoskeletal reorganization and cell spreading. As a result, CG/NE-deficient PMNs exhibited severe defects in MIP-2 secretion and reactive oxygen intermediates production. Exogenously added CG, but not proteolytically inactive CG, was sufficient to restore these defects. These findings identify an important role for CG in integrin-dependent PMN effector functions that are separate from and downstream of integrin-dependent adhesion.

Peroxiredoxin II functions as a signal terminator for H2O2-activated phospholipase D1

Phospholipase D1 (PLD1) is a signal-transduction regulated enzyme which regulates several cell intrinsic processes including activation of NAPDH oxidase, which elevates intracellular H2O2. Several proteins have been reported to interact with PLD1 in resting cells. We sought to identify proteins that interact with PLD1 after phorbol 12-myristate 13-acetate (PMA) stimulation. A novel interaction with peroxiredoxin II (PrxII), an enzyme that eliminates cellular H2O2, which is a known stimulator of PLD1, was identified by PLD1-affinity pull-down and MS. PMA stimulation was confirmed to promote physical interaction between PLD1 and PrxII and to cause PLD1 and PrxII to colocalize subcellularly. Functional significance of the interaction was suggested by the observation that over-expression of PrxII specifically reduces the response of PLD1 to stimulation by H2O2. These results indicate that PrxII may have a signal-terminating role for PLD1 by being recruited to sites containing activated PLD1 after cellular stimulation involving production of H2O2.

Synaptic localization of a functional NADPH oxidase in the mouse hippocampus

Superoxide has been shown to be critical for hippocampal long-term potentiation (LTP) and hippocampus-dependent memory function. A possible source for the generation of superoxide during these processes is NADPH oxidase. The active oxidase consists of two membrane proteins, gp91phox and p22phox, and four cytosolic proteins, p40phox, p47phox, p67phox, and Rac. Upon stimulation, the cytosolic proteins translocate to the membrane to form a complex with the membrane components, which results in production of superoxide. Here, we determined the presence, localization, and functionality of a NADPH oxidase in mouse hippocampus by examining the NADPH oxidase proteins as well as the production of superoxide. All of the NADPH oxidase proteins were present in hippocampal homogenates and enriched in synaptoneurosome preparations. Immunocytochemical analysis of cultured hippocampal neurons indicated that all NADPH oxidase proteins were localized in neuronal cell bodies as well as dendrites. Furthermore, double labeling analysis using antibodies to p67phox and the presynaptic marker synaptophysin suggest a close association of the NADPH oxidase subunits with synaptic sites. Finally, stimulation of hippocampal slices with phorbol esters triggered translocation of the cytoplasmic NADPH oxidase proteins to the membrane and an increase in superoxide production that was blocked by inhibitors of NADPH oxidase. Taken together, our data suggest that NADPH oxidase is present in mouse hippocampus and might be the source of superoxide production required for LTP and memory function.

Involvement of NADPH Oxidase Isoforms and Src Family Kinases in CD95-dependent Hepatocyte Apoptosis

CD95 ligand (CD95L) triggers a rapid formation of reactive oxygen species (ROS) as an upstream event of CD95 activation and apoptosis induction in rat hepatocytes. This ROS response was sensitive to inhibition by diphenyleneiodonium, apocynin, and neopterin, suggestive of an involvement of NADPH oxidases. In line with this, hepatocytes expressed mRNAs not only of the phagocyte gp91phox (Nox 2), but also of the homologs Nox 1 and 4 and Duox 1 and 2, as well as the regulatory subunit p47phox. gp91phox (Nox 2) and p47phox were also identified at the protein level in rat hepatocytes. CD95L induced within 1 min ceramide formation and serine phosphorylation of p47phox, which was sensitive to inhibitors of sphingomyelinase and protein kinase Czeta (PKCzeta). These inhibitors and p47phox protein knockdown inhibited the early CD95L-induced ROS response, suggesting that ceramide and PKCzeta are upstream events of the CD95L-induced Nox/Duox activation. CD95L also induced rapid activation of the Src family kinase Yes, being followed by activation of c-Src, Fyn, and c-Jun-N-terminal kinases (JNK). Only Yes and JNK activation were sensitive to N-acetylcysteine, inhibitors of NADPH oxidase, PKCzeta, or sphingomyelinase, indicating that the CD95L-induced ROS response is upstream of Yes and JNK but not of Fyn and c-Src activation. Activated Yes rapidly associated with the epidermal growth factor receptor (EGFR), which became phosphorylated at Tyr845 and Tyr1173 but not at Tyr1045. Activated EGFR then triggered an AG1478-sensitive CD95-tyrosine phosphorylation, which was a signal for membrane targeting of the EGFR/CD95 complex, subsequent recruitment of Fas-associated death domain and caspase 8, and apoptosis induction. All of these events were significantly blunted by inhibitors of sphingomyelinase, PKCzeta, NADPH oxidases, Yes, or EGFR-tyrosine kinase activity and after protein knockdown of either p47phox, Yes, or EGFR. The data suggest that CD95L-induced apoptosis involves a sphingomyelinase- and PKCzeta-dependent activation of NADPH oxidase isoforms, which is required for Yes/EGFR/CD95 interactions as upstream events of CD95 activation.

Critical role of proline-rich tyrosine kinase 2 in reversion of the adhesion-mediated suppression of reactive oxygen species generation by human neutrophils

Neutrophils act as the first line of innate immune defense against invading microorganisms during infection and inflammation. The tightly regulated production of reactive oxygen species (ROS) through activation of NADPH oxidase is a major weapon used by neutrophils and other phagocytic leukocytes to combat such pathogens. Cellular adhesion signals play important physiological roles in regulating the activation of NADPH oxidase and subsequent ROS formation. We previously showed that the initial suppression of the oxidase response of chemoattractant-stimulated adherent neutrophils is mediated via inhibition of Vav1-induced activation of the NADPH oxidase regulatory GTPase Rac2 by adhesion signals. In this study we show that prior exposure of neutrophils to a number of cytokines and inflammatory mediators, including TNF-alpha, GM-CSF, and platelet-activating factor, overcomes the adhesion-mediated suppression of ROS formation. Proline-rich tyrosine kinase 2 (pyk2) activity is enhanced under these conditions, correlating with the restoration of Vav1 and Rac2 activities. Both dominant negative pyk2 and a pyk2-selective inhibitor prevented restoration of ROS production induced by TNF-alpha, GM-CSF, and platelet-activating factor, and this loss of pyk2 activity resulted in decreased Vav1 tyrosine phosphorylation and subsequent Rac2 activation. Our studies identify pyk2 as a critical regulatory component and a molecular switch to overcome the suppression of leukocyte oxidant generation by cell adhesion. This activity constitutes a mechanism by which cytokines might lead to rapid elimination of invading pathogens by adherent neutrophils under normal conditions or enhance tissue damage in pathological states.

Bacterial cytotoxins: targeting eukaryotic switches

Many bacterial cytotoxins act on eukaryotic cells by targeting the regulators that are involved in controlling the cytoskeleton or by directly modifying actin, with members of the Rho GTPase family being particularly important targets. The actin cytoskeleton, and especially the GTPase 'molecular switches' that are involved in its control, have crucial functions in innate and adaptive immunity, and have pivotal roles in the biology of infection. In this review, we briefly discuss the role of the actin cytoskeleton and the Rho GTPases in host-pathogen interactions, and review the mode of actions of bacterial protein toxins that target these components.

Oxidative stress and cardiovascular risk: the role of vascular NAD(P)H oxidase and its genetic variants

Several risk factors for coronary artery disease (CAD) induce atherosclerosis through endothelial activation and dysfunction, and ample evidence now suggests that the balance between production and removal of reactive oxygen species (ROS) - a condition termed oxidative stress - is implicated in such processes. A main source of ROS in vascular cells is the reduced nicotinamide adenine dinucleotide/nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase system. This is a membrane-associated enzyme, composed of five subunits, catalyzing the one-electron reduction of oxygen, using NADH or NADPH as the electron donor. One of the system subunits, termed p22-phox, has a polymorphic site on exon 4, associated with variable enzyme activity. This polymorphism is generated by a point mutation (C(242)T) producing a substitution of histidine with tyrosine at position 72, which affects one of the heme binding sites essential for the NAD(P)H enzyme activity. The consequent decrease of superoxide production thus characterizes a phenotype candidate for conferring to the carrier a reduced susceptibility to CAD. At present, however, the body of evidence from current literature is not yet sufficient to confirm or exclude the hypothesis that the C(242)T polymorphism protects from CAD. The functional effects of this polymorphism and the potential and its pathophysiological consequences also need further investigation.

Homocysteine activates NADH/NADPH oxidase through ceramide-stimulated Rac GTPase activity in rat mesangial cells

BACKGROUND

We recently demonstrated that homocysteine (Hcys) increases superoxide (O2-) production via NADH/NADPH oxidase in renal mesangial cells. This O2- production contributes to increased expression of tissue inhibitor of metalloproteinase (TIMP-1) and consequent deposition of collagen in response to Hcys. However, the mechanism by which Hcys activates NADH/NADPH oxidase remains unknown. Given that ceramide is an intracellular activator of this oxidase in several cell types, the present study tests the hypothesis that Hcys activates NADH/NADPH oxidase through a ceramide-mediated signaling pathway in rat mesangial (MG) cells, resulting in O2- production.

METHODS

Rat MG cells were incubated with L-homocysteine (L-Hcys) to determine the mechanism by which Hcys activates NADH/NADPH oxidase. Thin layer chromatography (TLC), Western blot analysis, Rac GTPase activity pull down assay, and NADH/NADPH oxidase activity measurements were performed.

RESULTS

TLC analysis demonstrated that L-Hcys increased de novo production of ceramide in MG cells. L-Hcys and increased ceramide did not alter the amount of NADH/NADPH oxidase subunit p47phox and p67phox in both membrane and cytosolic fractions from MG cells. However, L-Hcys or ceramide markedly increased the level of GTP-bound Rac, which was accompanied by enhanced activity of NADH/NADPH oxidase. These Hcys or ceramide-induced actions were substantially blocked by a Rac GTPase inhibitor, GDPbetaS, and a de novo ceramide synthesis inhibitor, fumonisin B1 (FB1).

CONCLUSION

These results indicate that Hcys activates NADH/NADPH oxidase by stimulating de novo ceramide synthesis, and subsequently enhancing Rac GTPase activity in rat MG cells. This ceramide-Rac GTPase signaling pathway may mediate Hcys-induced oxidative stress in these glomerular cells.

Rac2 regulates neutrophil chemotaxis, superoxide production, and myeloid colony formation through multiple distinct effector pathways

Polymorphonuclear neutrophils (PMN) are an important component of the innate immune system. We have shown previously that migration and superoxide (O2*-) production, as well as some kinase signaling pathways are compromised in mice deficient in the Ras-related Rho GTPase Rac2. In this study, we demonstrate that Rac2 controls chemotaxis and superoxide production via distinct pathways and is critical for development of myeloid colonies in vitro. The Rac2 mutants V36A, F37A, and N39A all bind to both Pak1 and p67(phox), yet are unable to rescue superoxide production and chemotaxis when expressed in Rac2-/- PMN. In contrast, the N43A mutant, which binds to Por1 (Arfaptin 2), p67phox, and Pak1, is able to rescue superoxide production but not chemotaxis. The F37A mutant, demonstrated to have reduced binding to Por1, shows reduced rescue of fMLP-induced chemotaxis. Finally, the Rac2Y40C mutant that is defective in binding to all three potential downstream effectors (Pak1, p67phox, and Por1) is unable to rescue chemotaxis, motility, or superoxide production, but is able to rescue defective growth of myeloid colonies in vitro. These findings suggest that binding to any single effector is not sufficient to rescue the distinct cellular phenotypes of Rac2-/- PMN, implicating multiple, distinct, and potentially parallel effector pathways.

Activation-Induced Depletion of Protein Kinase Cα Provokes Desensitization of Monocytes/Macrophages in Sepsis

Sepsis accounts for the majority of fatal casualties in critically ill patients, because extensive research failed to significantly improve appropriate therapy strategies. Thus, understanding molecular mechanisms initiating the septic phenotype is important. Symptoms of septic disease are often associated with monocyte/macrophage desensitization. In this study, we provide evidence that a desensitized cellular phenotype is characterized by an attenuated oxidative burst. Inhibition of the oxidative burst and depletion of protein kinase C alpha (PKC alpha) were correlated in septic patients. To prove that PKC alpha down-regulation indeed attenuated the oxidative burst, we set up a cell culture model to mimic desensitized monocytes/macrophages. We show that LPS/IFN-gamma-treatment of RAW264.7 and U937 cells lowered PKC alpha expression and went on to confirm these data in primary human monocyte-derived macrophages. To establish a role of PKC alpha in cellular desensitization, we overexpressed PKC alpha in RAW264.7 and U937 cells and tested for phorbolester-elicited superoxide formation following LPS/IFN-gamma-pretreatment. Inhibition of the oxidative burst, i.e., cellular desensitization, was clearly reversed in cells overexpressing PKC alpha, pointing to PKC alpha as the major transmitter in eliciting the oxidative burst in monocytes/macrophages. However, PKC alpha inactivation by transfecting a catalytically inactive PKC alpha mutant attenuated superoxide formation. We suggest that depletion of PKC alpha in monocytes from septic patients contributes to cellular desensitization, giving rise to clinical symptoms of sepsis.

Inflammation induced changes in arachidonic acid metabolism in cat LES circular muscle

Myogenic lower esophageal sphincter (LES) tone is maintained by arachidonic acid metabolites, such as PGF(2alpha) and thromboxane A(2)/B(2). Experimental esophagitis in cat reduces LES in vivo pressure and in vitro tone. Because IL-1beta may mediate esophagitis-associated reduction in ACh release in esophagus, we examined whether IL-1beta may also play a role in esophagitis-induced reduction of LES tone. A cat model of experimental esophagitis was obtained by repeated esophageal perfusion with HCl (Biancani P, Barwick K, Selling J, and McCallum R. Gastreonterology 87: 8-16, 1984 and Sohn UD, Harnett KM, Cao W, Rich H, Kim N, Behar J, and Biancani P. J Pharmacol Exp Ther 283: 1293-1304, 1997.). LES circular muscle strips were examined in muscle chambers as previously described (Biancani P, Billett G, Hillemeier C, Nissenshon M, Rhim BY, Sweczack S, and Behar J. Gastroenterology 103: 1199-1206, 1992). Levels of inflammatory mediators were measured. IL-1beta levels were higher in esophagitis than in normal LES. IL-1beta reduced normal LES tone, and the reduction was reversed by catalase, suggesting a role of H(2)O(2). This was confirmed by IL-1beta-induced production of H(2)O(2) in normal LES and elevated H(2)O(2) levels in esophagitis. H(2)O(2) by itself is sufficient to explain the changes that occur in the muscle, reducing its ability to contract. H(2)O(2) increased PGE(2) in normal LES, and PGE(2) levels were elevated in esophagitis LES, whereas PGF(2alpha) levels were unchanged. H(2)O(2) also increased levels of 8-isoprostanes, stable prostaglandin-like compounds formed by free radical-induced peroxidation of arachidonic acid, and 8-isoprostane levels were elevated in esophagitis. The PGF(2alpha) analog 8-iso-PGF(2alpha) caused little contraction of LES strips but reduced PGF(2alpha) binding and contraction of normal LES. In esophagitis, PGF(2alpha) binding and contraction were reduced in LES, suggesting that isoprostanes may contribute to reduction in tone in esophagitis. The data suggest that, in esophagitis, IL-1beta causes production of H(2)O(2). H(2)O(2) increases PGE(2), which relaxes the LES, and 8-iso-F(2alpha), which blocks PGF(2alpha)-mediated contraction.

Downregulation of Rac1 activation by caffeic acid in aortic smooth muscle cells

Caffeic acid, a dietary phenol from coffee, fruits and vegetables, is an efficient antioxidant. However, little is known about its anti-oxidative mechanism in the modulation of fundamental cellular processes. In this study, we investigated whether caffeic acid regulates Rac1 GTPase activity, a partner of NADPH oxidase. Our results showed that caffeic acid decrease Rac1 protein level under basal conditions and incubation with angiotensin II (ANG II) in vascular smooth muscle cells. In a Rac-bound-to-PAK pull down assay, caffeic acid clearly inhibited Rac1 activity. We also observed that caffeic acid suppressed the generation of superoxide anion stimulated by ANG II that activates NADPH oxidase. On the other hand, co-incubation with caffei caid and cycloheximide significantly accelerated the Rac1 degradation. In addition, pretreatment with caffeic acid for 24 hours was able to prevent phosphorylation of MLC and HSP27, when cells were challenged with ANG II through the redox sensitive pathway. These results support the hypothesis that caffeic acid reduces Rac1 GTPase protein and activity level, followed by a down-regulation of NADPH oxidase activity.

Physical Inactivity Increases Oxidative Stress, Endothelial Dysfunction, and Atherosclerosis

Objective— Sedentary lifestyle is associated with increased cardiovascular events. The underlying molecular mechanisms are incompletely understood. Reactive oxygen species (ROS) contribute to endothelial dysfunction and atherosclerosis. An important source of vascular ROS is the NADPH oxidase.

Methods and Results— C57BL6 mice were subjected to regular housing (physical inactivity) or voluntary training on running wheels (6 weeks). Inactivity increased vascular lipid peroxidation to 148±9% and upregulated superoxide release to 176±17% (L-012 chemiluminescence) and 188±29% (cytochrome C reduction assay), respectively. ROS production was predominantly increased in the endothelium and the media (dihydroethidium fluorescence). Activity of the NADPH oxidase was increased to 154±22% in the sedentary group. Rac1 GST-PAK pull-down assays showed an upregulation of rac1 activity to 161±14%. Expression levels of the subunits nox1, p47phox, and p67phox were increased. To address the significance of the antioxidative effects of running, experiments were repeated in apolipoprotein E–deficient mice treated with a high-cholesterol diet. Inactivity increased vascular superoxide production and impaired endothelium-dependent vasorelaxation. Atherosclerotic lesion formation was significantly accelerated in sedentary mice.

Conclusions— Inactivity increases vascular NADPH oxidase expression and activity and enhances vascular ROS production, which contributes to endothelial dysfunction and atherosclerosis during sedentary as opposed to physically active lifestyle.

Heart failure and statins—Why do we need a clinical trial?

The effect of statins to reduce mortality and morbidity in primary and secondary prevention as well as in acute coronary syndrome is well established. Recent data show that pleiotropic effects might also have direct effects on the myocardial cell. However, in chronic heart failure the outcome is inversely related to LDL-plasma concentrations and other pleiotropic effects might impair mitochondrial function. Since there are no safety data on the use of statins in chronic heart failure, a controlled randomized and placebo-controlled trial is urgently needed.

Activation and assembly of the NADPH oxidase: a structural perspective

The NADPH oxidase of professional phagocytes is a crucial component of the innate immune response due to its fundamental role in the production of reactive oxygen species that act as powerful microbicidal agents. The activity of this multi-protein enzyme is dependent on the regulated assembly of the six enzyme subunits at the membrane where oxygen is reduced to superoxide anions. In the resting state, four of the enzyme subunits are maintained in the cytosol, either through auto-inhibitory interactions or through complex formation with accessory proteins that are not part of the active enzyme complex. Multiple inputs are required to disrupt these inhibitory interactions and allow translocation to the membrane and association with the integral membrane components. Protein interaction modules are key regulators of NADPH oxidase assembly, and the protein-protein interactions mediated via these domains have been the target of numerous studies. Many models have been put forward to describe the intricate network of reversible protein interactions that regulate the activity of this enzyme, but an all-encompassing model has so far been elusive. An important step towards an understanding of the molecular basis of NADPH oxidase assembly and activity has been the recent solution of the three-dimensional structures of some of the oxidase components. We will discuss these structures in the present review and attempt to reconcile some of the conflicting models on the basis of the structural information available.