The Rac target NADPH oxidase p67phox interacts preferentially with Rac2 rather than Rac1

Theranostic imaging and multimodal photodynamic therapy and immunotherapy using the mTOR signaling pathway

Tumor metastases are considered the leading cause of cancer-associated deaths. While clinically applied drugs have demonstrated to efficiently remove the primary tumor, metastases remain poorly accessible. To overcome this limitation, herein, the development of a theranostic nanomaterial by incorporating a chromophore for imaging and a photosensitizer for treatment of metastatic tumor sites is presented. The mechanism of action reveals that the nanoparticles are able to intervene by local generation of cellular damage through photodynamic therapy as well as by systemic induction of an immune response by immunotherapy upon inhibition of the mTOR signaling pathway which is of crucial importance for tumor onset, progression and metastatic spreading. The nanomaterial is able to strongly reduce the volume of the primary tumor as well as eradicates tumor metastases in a metastatic breast cancer and a multi-drug resistant patient-derived hepatocellular carcinoma models in female mice.

HIF-1α promotes paraquat induced acute lung injury and implicates a role NF-κB and Rac2 activity

Paraquat (PQ) is a bipyridine herbicide and oral exposure is the main way of PQ exposure with a very high mortality. At present, it is believed that large number of oxygen free radicals are generated and cause lipid peroxidation of tissue and organ cell membranes after PQ is absorbed. PQ exposure could cause multiple organ dysfunction, among which acute lung injury is the most common and most serious. However, its specific mechanism is still unclear. In this study, the C57BL/6J mouse (alveolar epithelial cell-specific knockout HIF-1α) model of acute lung injury (40 mg/kg PQ) at several time pointes and a model of acute type II alveolar epithelial cell (A549, 800 μM PQ) injury constructed. The oxidative stress (ROS, MDA) and inflammatory response (IL-1β, IL-6, TNF-α) were significantly inhibited in the alveolar epithelial cell-specific knockout of HIF-1α mice and siRNA technology to inhibit HIF-1α in alveolar epithelial cells. Further proteomic analysis showed that the expression of Rac2 protein, which is closely related to oxidative stress, was significantly increased after PQ exposure. And the inhibition of Rac2 expression in vitro significantly alleviated PQ-induced oxidative stress and inflammatory response. The expression of Rac2 protein was regulated by HIF-1α. The above suggests that HIF-1α may promote oxidative stress and inflammatory response in alveolar epithelial cells by regulating the expression of Rac2, and then participate in the promotion of PQ exposure-induced acute lung injury.

Identification of Shared Genes and Pathways in Periodontitis and Type 2 Diabetes by Bioinformatics Analysis

INTRODUCTION

It is well known that the presence of diabetes significantly affects the progression of periodontitis and that periodontitis has negative effects on diabetes and diabetes-related complications. Although this two-way relationship between type 2 diabetes and periodontitis could be understood through experimental and clinical studies, information on common genetic factors would be more useful for the understanding of both diseases and the development of treatment strategies.

MATERIALS AND METHODS

Gene expression data for periodontitis and type 2 diabetes were obtained from the Gene Expression Omnibus database. After preprocessing of data to reduce heterogeneity, differentially expressed genes (DEGs) between disease and normal tissue were identified using a linear regression model package. Gene ontology and Kyoto encyclopedia of genes and genome pathway enrichment analyses were conducted using R package 'vsn'. A protein-protein interaction network was constructed using the search tool for the retrieval of the interacting genes database. We used molecular complex detection for optimal module selection. CytoHubba was used to identify the highest linkage hub gene in the network.

RESULTS

We identified 152 commonly DEGs, including 125 upregulated and 27 downregulated genes. Through common DEGs, we constructed a protein-protein interaction and identified highly connected hub genes. The hub genes were up-regulated in both diseases and were most significantly enriched in the Fc gamma R-mediated phagocytosis pathway.

DISCUSSION

We have identified three up-regulated genes involved in Fc gamma receptor-mediated phagocytosis, and these genes could be potential therapeutic targets in patients with periodontitis and type 2 diabetes.

When Actin is Not Actin' Like It Should: A New Category of Distinct Primary Immunodeficiency Disorders

An increasing number of primary immunodeficiencies (PIDs) have been identified over the last decade, which are caused by deleterious mutations in genes encoding for proteins involved in actin cytoskeleton regulation. These mutations primarily affect hematopoietic cells and lead to defective function of immune cells, such as impaired motility, signaling, proliferative capacity, and defective antimicrobial host defense. Here, we review several of these immunological "actinopathies" and cover both clinical aspects, as well as cellular mechanisms of these PIDs. We focus in particular on the effect of these mutations on human neutrophil function.

Overexpression of Ras-Related C3 Botulinum Toxin Substrate 2 Radiosensitizes Melanoma Cells In Vitro and In Vivo

Radioresistance is the major obstacle in the radiotherapy of the malignant melanoma. Thus, it is of importance to increase the radiosensitivity of melanoma cells. In the present study, the radioresistant melanoma cell line OCM-1 with inducible overexpression of Ras-related C3 botulinum toxin substrate 2 was established based on a radiation-inducible early growth response gene (Egr-1) promoter. The effects of Ras-related C3 botulinum toxin substrate 2 overexpression on the radiosensitivity of melanoma cells exposed to either X-rays or carbon ion beams were evaluated in cultured cells as well as xenograft tumor models. In addition, both reactive oxygen species yield and the NADPH oxidase activity were measured in the irradiated melanoma cells. It was found that the radiation-inducible overexpression of Ras-related C3 botulinum toxin substrate 2 sensitized the melanoma cells to both X-rays and carbon ion irradiation by enhancing the NADPH oxidase activity and the subsequent reactive oxygen species production. Besides, the overexpression of Ras-related C3 botulinum toxin substrate 2 enhanced the tumor-killing effect of radiotherapy in xenograft tumors significantly. The results of this study indicate that Ras-related C3 botulinum toxin substrate 2 is promising in increasing the radiosensitivity of melanoma cells, which provides experimental evidence and theoretical basis for clinical radiosensitization of the malignant melanoma.

Gastrointestinal Complications in Chronic Granulomatous Disease

Almost half of patients with chronic granulomatous disease (CGD) suffer from gastrointestinal (GI) inflammation, the pathogenesis of which is complex and multifactorial. As a result, the management of CGD-associated GI inflammation remains challenging due to its chronicity and difficulty in managing the simultaneous need for immunomodulation with increased susceptibility to infection. In order to contextualize prospective treatment interventions for CGD-associated GI inflammation, we have reviewed the clinical presentation, pathogenesis and current management of this disease. Increased understanding of the role of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex 2 (NOX2)-derived reactive oxygen species (ROS) in inflammatory bowel disease (IBD) will likely reveal novel targets for therapeutic intervention.

Macrophage Rac2 Is Required to Reduce the Severity of Cigarette Smoke-induced Pneumonia

RATIONALE

Cigarette smoking is prevalent in the United States and is the leading cause of preventable diseases. A prominent complication of smoking is an increase in lower respiratory tract infections (LRTIs). Although LRTIs are known to be increased in subjects that smoke, the mechanism(s) by which this occurs is poorly understood.

OBJECTIVES

Determine how cigarette smoke (CS) reduces reactive oxygen species (ROS) production by the phagocytic NOX2 (NADPH oxidase 2), which is essential for innate immunity in lung macrophages.

METHODS

NOX2-derived ROS and Rac2 (Ras-related C3 botulinum toxin substrate 2) activity were determined in BAL cells from wild-type and Rac2-/- mice exposed to CS or cadmium and in BAL cells from subjects that smoke. Host defense to respiratory pathogens was analyzed in mice infected with Streptococcus pneumoniae.

MEASUREMENTS AND MAIN RESULTS

NOX2-derived ROS in BAL cells was reduced in mice exposed to CS via inhibition of the small GTPase Rac2. These mice had greater bacterial burden and increased mortality compared with air-exposed mice. BAL fluid from CS-exposed mice had increased levels of cadmium, which mediated the effect on Rac2. Similar observations were seen in human subjects that smoke. To support the importance of Rac2 in the macrophage immune response, overexpression of constitutively active Rac2 by lentiviral administration increased NOX2-derived ROS, decreased bacterial burden in lung tissue, and increased survival compared with CS-exposed control mice.

CONCLUSIONS

These observations suggest that therapies to maintain Rac2 activity in lung macrophages restore host defense against respiratory pathogens and diminish the prevalence of LRTIs in subjects that smoke.

Crosstalk between Rac1-mediated actin regulation and ROS production

The small RhoGTPase Rac1 is implicated in a variety of events related to actin cytoskeleton rearrangement. Remarkably, another event that is completely different from those related to actin regulation has the same relevance; the Rac1-mediated production of reactive oxygen species (ROS) through NADPH oxidases (NOX). Each outcome involves different Rac1 downstream effectors; on one hand, events related to the actin cytoskeleton require Rac1 to bind to WAVEs proteins and PAKs that ultimately promote actin branching and turnover, on the other, NOX-derived ROS production demands active Rac1 to be bound to a cytosolic activator of NOX. How Rac1-mediated signaling ends up promoting actin-related events, NOX-derived ROS, or both is poorly understood. Rac1 regulators, including scaffold proteins, are known to exert tight control over its functions. Hence, evidence of Rac1 regulatory events leading to both actin remodeling and NOX-mediated ROS generation are discussed. Moreover, cellular functions linked to physiological and pathological conditions that exhibit crosstalk between Rac1 outcomes are analyzed, while plausible roles in neuronal functions (and dysfunctions) are highlighted. Together, discussed evidence shed light on cellular mechanisms which requires Rac1 to direct either actin- and/or ROS-related events, helping to understand crucial roles of Rac1 dual functionality.

Temporal gene profiling of the 5XFAD transgenic mouse model highlights the importance of microglial activation in Alzheimer's disease

BACKGROUND

The 5XFAD early onset mouse model of Alzheimer's disease (AD) is gaining momentum. Behavioral, electrophysiological and anatomical studies have identified age-dependent alterations that can be reminiscent of human AD. However, transcriptional changes during disease progression have not yet been investigated. To this end, we carried out a transcriptomic analysis on RNAs from the neocortex and the hippocampus of 5XFAD female mice at the ages of one, four, six and nine months (M1, M4, M6, M9).

RESULTS

Our results show a clear shift in gene expression patterns between M1 and M4. At M1, 5XFAD animals exhibit region-specific variations in gene expression patterns whereas M4 to M9 mice share a larger proportion of differentially expressed genes (DEGs) that are common to both regions. Analysis of DEGs from M4 to M9 underlines the predominance of inflammatory and immune processes in this AD mouse model. The rise in inflammation, sustained by the overexpression of genes from the complement and integrin families, is accompanied by an increased expression of transcripts involved in the NADPH oxidase complex, phagocytic processes and IFN-γ related pathways.

CONCLUSIONS

Overall, our data suggest that, from M4 to M9, sustained microglial activation becomes the predominant feature and point out that both detrimental and neuroprotective mechanisms appear to be at play in this model. Furthermore, our study identifies a number of genes already known to be altered in human AD, thus confirming the use of the 5XFAD strain as a valid model for understanding AD pathogenesis and for screening potential therapeutic molecules.

Rho GTPases control specific cytoskeleton-dependent functions of hematopoietic stem cells

The Rho family of guanosine triphosphatases (GTPases) is composed of members of the Ras superfamily of proteins. They are GTP-bound molecules with a modest intrinsic GTPase activity that can be accelerated upon activation/localization of specialized guanine nucleotide exchange factors. Members of this family act as molecular switches and are required for coordinated cytoskeletal rearrangements that are crucial in a set of specialized functions of mammalian stem cells. These functions include self-renewal, adhesion, and migration. Mouse gene-targeting studies have provided convincing evidence of the indispensable and dispensable roles of individual members of the Rho GTPase family and the putative upstream and downstream mediators in stem cell-specific functions. The role of Rho GTPases and related signaling pathways previously seen in other cell types and organisms have been confirmed in mammalian hematopoietic stem cells (HSCs), and new signaling pathways and unexpected functions unique to HSCs have been identified and dissected. This review summarizes our current understanding of the role of Rho family of GTPases on HSC and progenitor activity through cytoskeleton-mediated signaling pathways, providing insight about relevant signaling pathways that regulate mammalian stem cell self-renewal, adhesion, and migration.

P21-activated kinase in inflammatory and cardiovascular disease

P-21 activated kinases, or PAKs, are serine-threonine kinases that serve a role in diverse biological functions and organ system diseases. Although PAK signaling has been the focus of many investigations, still our understanding of the role of PAK in inflammation is incomplete. This review consolidates what is known about PAK1 across several cell types, highlighting the role of PAK1 and PAK2 in inflammation in relation to NADPH oxidase activation. This review explores the physiological functions of PAK during inflammation, the role of PAK in several organ diseases with an emphasis on cardiovascular disease, and the PAK signaling pathway, including activators and targets of PAK. Also, we discuss PAK1 as a pharmacological anti-inflammatory target, explore the potentials and the limitations of the current pharmacological tools to regulate PAK1 activity during inflammation, and provide indications for future research. We conclude that a vast amount of evidence supports the idea that PAK is a central molecule in inflammatory signaling, thus making PAK1 itself a promising prospective pharmacological target.

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

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

The 3BP2 adapter protein is required for chemoattractant-mediated neutrophil activation

3BP2 is a pleckstrin homology and Src homology 2 domain-containing adapter protein mutated in cherubism, a rare autosomal-dominant human bone disorder. Previously, we have demonstrated a functional role for 3BP2 in peripheral B cell development and in peritoneal B1 and splenic marginal zone B cell-mediated Ab responses. In this study, we show that 3BP2 is required for G protein-coupled receptor-mediated neutrophil functions. Neutrophils derived from 3BP2-deficient (Sh3bp2-/-) mice failed to polarize their actin cytoskeleton or migrate in response to a gradient of chemotactic peptide, fMLF. Sh3bp2-/- neutrophils failed to adhere, crawl, and emigrate out of the vasculature in response to fMLF superfusion. 3BP2 is required for optimal activation of Src family kinases, small GTPase Rac2, neutrophil superoxide anion production, and for Listeria monocytogenes bacterial clearance in vivo. The functional defects observed in Sh3bp2-/- neutrophils may partially be explained by the failure to fully activate Vav1 guanine nucleotide exchange factor and properly localize P-Rex1 guanine nucleotide exchange factor at the leading edge of migrating cells. Our results reveal an obligate requirement for the adapter protein 3BP2 in G protein-coupled receptor-mediated neutrophil function.

Rac regulates PtdInsP₃ signaling and the chemotactic compass through a redox-mediated feedback loop

Directional cell migration is an essential requirement for efficient neutrophil translocation to sites of infection and requires the establishment of a polarized cell characterized by an actin-rich leading edge facing the chemoattractant gradient. The asymmetrical accumulation of phosphatidylinositol(3,4,5)-trisphosphate [PtdIns(3,4,5)P(3)] in the up-gradient leading edge is a hallmark of polarization and regulates the recruitment and localization of various effector proteins at the leading-edge plasma membrane. How shallow gradients of chemoattractants trigger and maintain a much steeper intracellular gradient of PtdIns(3,4,5)P(3) is a critical question in the study of leukocyte chemotaxis. Our data demonstrate that the migration of neutrophils toward the chemoattractant N-formyl-L-methionyl-L-leucyl-L-phenylalanine depends on the generation of reactive oxygen species by the phagocytic NADPH oxidase (NOX2) and subsequent oxidation and inhibition of phosphatase and tensin homolog. Moreover, we show that events downstream of PtdIns(3,4,5)P(3), including phosphorylation of AKT, Rac activation, uncapping of actin filaments, and directional migration, can be attenuated by ROS scavengers or genetic ablation of NOX2. Using Rac mutants that are defective in their ability to activate NOX2, we show that Rac regulates a redox-mediated feedback loop that mediates directional migration of neutrophils.

Specific activation of human neutrophils by scorpion venom: a flow cytometry assessment

Acute lung injury following envenomation by Tityus scorpion species is due in part to activation of the inflammatory response leading to release of cytotoxic leukocyte-derived products, including cytokines and possibly reactive oxygen species (ROS). Tityus zulianus envenomation in Venezuela produces cardiorespiratory complications and death by lung injury whereas stings by Tityus discrepans produce mainly gastrointestinal and pancreatic alterations. To ascertain the role played by granulocytes in the envenomation by T. zulianus (TzV) and T. discrepans (TdV), human peripheral blood neutrophils, eosinophils, and monocytes were exposed to scorpion venoms (0.001-5 μg/mL) and the kinetics (5-15 min) of peroxide production determined by flow cytometry, using 2',7'-dichlorodihydrofluorescein diacetate (succinimidyl ester) as a fluorescent substrate. TzV induced a significantly (p<0.01) more potent increase in peroxide production in neutrophils (for 5 and 10 min of incubation), and to a lesser extent in monocytes (5-15 min), compared to TdV. TzV induced necrosis in neutrophils at doses higher than 5 μg/mL. No effect was observed on eosinophils, suggesting that TzV specifically targets neutrophil intracellular ROS production. The TzV-stimulated pathway is protein kinase C-dependent because it was almost completely (>90%) abolished by staurosporine. The stimulatory effect is associated with the lowest molecular mass venom peptides as gel filtration fractions TzII and TzIII significantly enhanced peroxide production. The combined used of the intracellular ROS agonist, phorbol myristate acetate (PMA), and TzV produced a modest but significant increase in peroxide production suggesting the possibility of overlapping signaling cascades amongst PMA and TzV. Up-regulation of intracellular neutrophil ROS production may be an important in vivo target for TzV which could have a role to play in the cardiorespiratory complications elicited after envenomation by this species.

Regulation of Nox and Duox enzymatic activity and expression

In recent years, it has become clear that reactive oxygen species (ROS, which include superoxide, hydrogen peroxide, and other metabolites) are produced in biological systems. Rather than being simply a by-product of aerobic metabolism, it is now recognized that specific enzymes--the Nox (NADPH oxidase) and Duox (Dual oxidase) enzymes--seem to have the sole function of generating ROS in a carefully regulated manner, and key roles in signal transduction, immune function, hormone biosynthesis, and other normal biological functions are being uncovered. The prototypical Nox is the respiratory burst oxidase or phagocyte oxidase, which generates large amounts of superoxide and other reactive species in the phagosomes of neutrophils and macrophages, playing a central role in innate immunity by killing microbes. This enzyme system has been extensively studied over the past two decades, and provides a basis for comparison with the more recently described Nox and Duox enzymes, which generate ROS in a variety of cells and tissues. This review first considers the structure and regulation of the respiratory burst oxidase, and then reviews recent studies relating to the regulation of the activity of the novel Nox/Duox enzymes. The regulation of Nox and Duox expression in tissues and by specific stimuli is also considered here. An accompanying review considers biological and pathological roles of the Nox family of enzymes.

Cytokine expression and AIF-1-mediated activation of Rac2 in vascular smooth muscle cells: a role for Rac2 in VSMC activation

Allograft inflammatory factor-1 (AIF-1) is a cytoplasmic, calcium-binding, inflammation-responsive scaffold protein involved in vascular smooth muscle cell (VSMC) migration and proliferation. The objective of this study is to characterize AIF-1 functional protein interactions that may regulate VSMC activation. Through use of a bacterial two-hybrid screen, we identified a molecular interaction between AIF-1 and the small GTPase, Rac2, which was verified by pull-down and colocalization experiments. This was unexpected in that Rac2 expression had been considered to be restricted to hematopoietic cells. The Rac2/AIF-1 interaction is functional, in that a loss-of-function, point-mutated AIF-1 does not interact with Rac2; Rac2 colocalizes with AIF-1 in the cytoplasm of VSMC and cotranslocates to lamellopodia upon platelet-derived growth factor stimulation; and AIF-1 expression in VSMC leads to Rac2 activation. Because Rac2 function in VSMC had not been described, we focused on characterization of its function in these cells. Rac2 protein expression in VSMC is inducible by inflammatory cytokines, and Rac2 activation in VSMC is also responsive to inflammatory cytokines. Rac2 expression and activation patterns differ from the ubiquitously expressed Rac1. We hypothesized that Rac2 participates in VSMC activation. Retroviral overexpression of Rac2 in primary VSMC leads to increased migration, activation of the NADPH oxidation cascade, and increased activation of the Rac2 effector protein Pak1 and its proximal effectors, ERK1/2, and p38 (P < 0.05 for all). The major points of this study indicate a functional interaction between AIF-1 and Rac2 in VSMC leading to Rac2 activation and a potential function for Rac2 in inflammation-driven VSMC response to injury.

Fcgamma receptor signaling in phagocytes

Fcgamma receptors are among the best-studied phagocytic receptors. The key features of Fcgamma receptor-mediated phagocytosis include phagocytic cup formation by extensive actin cytoskeletal rearrangements, particle engulfment, and the release of proinflammatory mediators such as cytokines and reactive oxygen species. These events are elegantly regulated by the simultaneous engagement of activating and inhibitory Fcgamma receptors and by intracellular signaling molecules. Extensive studies in the past several years have defined the molecular mechanisms of the phagocytic process. The purpose of this review is to revisit some of the well-established signaling pathways as well as to summarize the new findings in this field.

Regulation of the phagocyte NADPH oxidase by Rac GTPase

Phagocytic leukocytes generate reactive oxygen species important for the killing of invading microorganisms. The source of these oxidants is the NADPH oxidase, a tightly controlled multicomponent enzyme made up of a membrane-associated catalytic moiety and cytosolic regulatory components that must assemble to form the active oxidase. The phagocyte NADPH oxidase was the first mammalian system shown to be directly regulated by a Rac GTPase. We review here our understanding of NADPH oxidase regulation by Rac, as well as the regulation of Rac itself, in phagocytic leukocytes. Rather than viewing Rac as a "cog" in the NADPH oxidase machinery, we argue for a view of Rac GTPases as critical "molecular switches" regulating the formation of ROS by phagocytic leukocytes under physiologic and pathologic conditions.

Assays and properties of arfaptin 2 binding to Rac1 and ADP-ribosylation factors (Arfs)

Arfaptin 1 and 2 were identified as targets for GTP bound ADP-ribosylation factors (Arfs). Arfaptin 1 had no significant effects on guanine nucleotide binding to Arfs, nor enzymatic activities of guanine nucleotide exchange factor (GEF) and GTPase activating protein (GAP) acting on Arfs. However, arfaptin 1 inhibited Arf activation of cholera toxin and phospholipase D (PLD) in a dose-dependent manner. Only GTP-bound forms of Arf1, 5, and 6 interacted with arfaptin 1 and 2, but GTP-Arf1 showed the strongest binding to the arfaptins. In contrast to the binding of Arfs to arfaptins, GDP-Rac1 or dominant negative Rac1-N17N bound to arfaptin 2, whereas GTP-Rac1 or dominant active Rac1-Q61L did not bind to arfaptin 2. Neither GTP-Rac1 nor GDP-Rac1 bound to arfaptin 1. Based on our observation, we propose that arfaptin 2 is a target for GDP-Rac1 and for GTP-Arf1, and is involved in interactions between the Rac1 and Arfs signaling pathways. This chapter describes methods for investigating the interactions of arfaptins 1 and 2 with GTP- or GDP-liganded Arfs and Rac1.

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.

Impaired NADPH oxidase activity in Rac2-deficient murine neutrophils does not result from defective translocation of p47phox and p67phox and can be rescued by exogenous arachidonic acid

Rac2 is a hematopoietic-specific Rho-GTPase that plays a stimulus-specific role in regulating reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activation and other functional responses in neutrophils. In this study, rac2-/- neutrophils were shown to have significantly decreased NADPH oxidase activity and actin remodeling in response to exogenous arachidonic acid (AA), as previously observed for phorbol 12-myristate 13-acetate (PMA) or formyl-Met-Leu-Phe (fMLP) as agonists. PMA-, fMLP-, or AA-induced translocation of p47phox and p67phox to the plasma membrane was not impaired in rac2-/- neutrophils. Combined stimulation of rac2-/- neutrophils with exogenous AA and PMA had a synergistic effect on NADPH oxidase activity, and superoxide production increased to a level that was at least as high as wild-type cells and had no effect on fMLP-elicited enzyme activity. Membrane translocation of p47phox and p67phox as well as Rac1 activation was not increased further by combined PMA and AA stimulation. Inhibitor studies were consistent with important roles for phorbol ester-activated protein kinase C (PKC) isoforms and an atypical isoform, PKCzeta, in superoxide production by wild-type and rac2-/- neutrophils stimulated with AA and PMA. In addition, PMA-stimulated release of AA and cytoplasmic phospholipase A2 expression in rac2-/- neutrophils were similar to wild-type, suggesting that deficient AA production by PMA-stimulated rac2-/- neutrophils does not explain the effect of exogenous AA on oxidase activity. Although not required for translocation of p47phox and p67phox, Rac2 is necessary for optimal activity of the assembled oxidase complex, an effect that can be replaced by exogenous AA, which may act directly or via an exogenous AA-induced mediator.

The role of Rho GTPases and SNAREs in mediator release from granulocytes

Granulocytes are defined as the population of granulated white blood cells (eosinophils, neutrophils, and basophils). These cells are involved in inflammation and contribute to the pathogenesis of allergic and inflammatory diseases. Inflammation is induced by the release of mediators from granulocytes recruited to or resident within tissues, resulting in edema, leukocyte recruitment, and tissue injury. Eosinophils and neutrophils express Rac1 and Rac2 guanosine triphosphatases (GTPases), 2 members of the Rho GTPase subfamily of ras-related GTPases. Rho GTPases are activated by receptors in the cell membrane and are proposed to function as intracellular molecular switches to regulate mediator release, including exocytosis, from granulocytes. Exocytosis involves granule fusion, which requires the binding of intracellular membrane receptors known as SNAP receptor (SNAREs; soluble N-ethylmaleimide-sensitive factor [NSF] attachment protein [SNAP] receptors). Eosinophils and neutrophils express similar SNARE isoforms that are important in granule fusion events. Together, these molecules link together to form a common signaling pathway for mediator release from granulocytes. Identifying these molecules and their function may provide novel targets for the prevention of inflammatory reactions.

Rho-family GTPases: it's not only Rac and Rho (and I like it)

The Rho-family proteins make up a major branch of the Ras superfamily of small GTPases. To date, 22 human genes encoding at least 25 proteins have been described. The best known 'classical' members are RhoA, Rac1 and Cdc42. Highly related isoforms of these three proteins have not been studied as intensively, in part because it has been assumed that they are functionally identical to their better-studied counterparts. This now appears not to be the case. Variations in C-terminal-signaled modifications and subcellular targeting cause otherwise highly biochemically related isoforms (e.g. RhoA, RhoB and RhoC) to exhibit surprisingly divergent biological activities. Whereas the classical Rho GTPases are regulated by GDP/GTP cycling, other Rho GTPases are also regulated by other mechanisms, particularly by transcriptional regulation. Newer members of the family possess additional sequence elements beyond the GTPase domain, which suggests they exhibit yet other mechanisms of regulation.

Important role for Rac1 in regulating reactive oxygen species generation and pulmonary arterial smooth muscle cell growth

Vascular NADPH oxidases have been shown to be a major source of reactive oxygen species (ROS). Recent studies have also implicated ROS in the proliferation of vascular smooth muscle cells. However, the components required for activation of the NADPH oxidase complex have not been clearly elucidated. Here we demonstrate that ROS generation in ovine pulmonary arterial smooth muscle cells (PASMCs) requires the activation of Rac1, implicating this protein as an important subunit of the NADPH oxidase complex. Our results, using a geranylgeranyl transferase inhibitor (GGTI-287), demonstrated a dose-dependent inhibition of Rac1 activity and ROS production. This was associated with an inhibition of PASMC proliferation with an arrest at G2/M. The inhibition of Rac1 by GGTI-287 led us to more specifically target Rac1 to investigate its role in the generation of ROS and cellular proliferation. To accomplish this, we utilized a dominant negative Rac1 (N17Rac1) and a constitutively active Rac1 (V12Rac1). These two forms of Rac1 were transiently expressed in PASMCs using adenovirus-mediated gene transfer. N17Rac1 expression resulted in decreased cellular Rac1 activity, whereas V12Rac1 infection showed increased activity. Compared with controls, the V12Rac1-expressing cells had higher levels of ROS production and increased proliferation, whereas the N17Rac1-expressing cells had decreased ROS generation and proliferation and cell cycle arrest at G2/M. However, the inhibition of cell growth produced by N17Rac1 overexpression could be overcome if cells were co-incubated with the Cu,Zn superoxide dismutase inhibitor DETC. These results indicate the importance of Rac1 in ROS generation and proliferation of vascular smooth muscle cells.

Activation of the flavoprotein domain of gp91phox upon interaction with N-terminal p67phox (1-210) and the Rac complex

A series of truncated forms of His(6)-tagged gp91phox were expressed, solubilized, and purified in the presence of 30 microM FAD. The truncated gp91phox with the longest sequence in the C-terminal region (221-570) (gp91C) showed the highest activity (turnover rate, 0.92) for NADPH diaphorase in the presence of either 0.3% Triton X-100 or 0.5% Genapol X-80. Activity was not inhibited by superoxide dismutase but was blocked by an inhibitor of the respiratory burst oxidase, diphenylene iodonium. The flavinated gp91C contained approximately 0.9 mol of FAD/mol of protein (MW 46 kDa) and 12% alpha-helix content. In the absence of p47phox, p67phox showed considerable activation of gp91C in the presence of Rac. Carboxyl-terminal truncated p67phox (1-210) (p67N), which is the minimal active fragment, was fused with Rac or Q61LRac. The fusion protein p67N-Rac (or p67N-Q61LRac) showed a 2-fold higher stimulatory effect on NBT reductase activity of gp91C than the combination of the individual cytosolic p67N and Rac proteins. In contrast, Rac-p67N, a fusion with the opposite orientation, showed a smaller significant effect on the enzyme activity. The EC(50) values for p67phox, p67N, p67N-Rac, and Rac-p67N were 8.00. 4.35, 2.56, and 15.2 microM, respectively, while the K(m) value for NADPH in the presence and absence of the cytosolic components was almost the same (40-55 microM). In the presence of Rac, p67N or p67phox bound to gp91C with a molar ratio of approximately 1:1 but neither p67N nor Rac alone showed significant binding.

The polybasic region of Ras and Rho family small GTPases: a regulator of protein interactions and membrane association and a site of nuclear localization signal sequences

Many small GTPases in the Ras and Rho families have a C-terminal polybasic region (PBR) comprised of multiple lysines or arginines. The PBR controls diverse functions of these small GTPases, including their ability to associate with membranes, interact with specific proteins, and localize in subcellular compartments. Different signaling pathways mediated by Ras and Rho family members may converge when the small GTPases are directed by their PBRs to shared binding sites in specific proteins or at cell membranes. The PBR promotes the interactions of small GTPases with SmgGDS, which is a nucleocytoplasmic shuttling protein that stimulates guanine nucleotide exchange by small GTPases. The PBR of Rac1 was recently found to have a functional nuclear localization signal (NLS) sequence, which enhances the nuclear accumulation of protein complexes containing SmgGDS and Rac1. Sequence analysis demonstrates that canonical NLS sequences (K-K/R-x-K/R) are present in the PBRs of additional Ras and Rho family members, and are evolutionarily conserved across several phyla. These findings suggest that the PBR regulates the nucleocytoplasmic shuttling of some Ras and Rho family members when they are in protein complexes that are too large to diffuse through nuclear pores. These diverse functions of the PBR indicate its critical role in signaling by Ras and Rho family GTPases.

Human monocytes use Rac1, not Rac2, in the NADPH oxidase complex

Phagocyte NADPH oxidase is critical for defense against pathogens and contributes to inflammatory tissue injury. One component of the NADPH oxidase complex is the small GTP-binding protein Rac. There are two isoforms of Rac, and Rac2 is the predominant isoform in neutrophils and has been shown to be essential for NADPH oxidase activity. In primary human monocytes we report that in contrast to neutrophils, Rac1 is the predominantly expressed isoform. Upon monocyte activation by a variety of agents, we found that Rac1 dissociates from Rho GDP dissociation inhibitor (RhoGDI) and translocates to the membrane. We also found that Rac1 interacts with two other NADPH oxidase components, p67phox and p47phox, upon monocyte activation. These data indicate that Rac1, and not Rac2, is a component of the activated NADPH oxidase in monocytes. This finding suggests that it may be possible to selectively interfere with either monocyte or neutrophil NADPH oxidase activity, thereby selectively targeting chronic versus acute inflammatory processes.

Transplasma membrane electron transport: enzymes involved and biological function

The notion of transmembrane electron transport is usually associated with mitochondria and chloroplasts. However, since the early 1970s, it has been known that this phenomenon also occurs at the level of the plasma membrane. Ever since, evidence has accumulated for the existence of a plethora of transplasma membrane electron transport enzymes. In this review, we discuss the various enzymes known, their molecular characteristics and their biological functions.

Reduced expression of flavocytochrome b558, a component of the NADPH oxidase complex, in neutrophils from patients with myelodysplasia

OBJECTIVE

Patients with myelodysplasia (MDS) show a disturbed production of ROS in response to N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP) in granulocyte-macrophage colony-stimulating factor (GM-CSF)-primed neutrophils. Because generation of ROS is mediated by the NADPH oxidase complex, a component of which is flavocytochrome b558, we investigated whether the expression of flavocytochrome b558 in neutrophils from MDS patients is affected.

MATERIAL AND METHODS

Neutrophils were stimulated with fMLP and GM-CSF, and plasma membrane expression of flavocytochrome b558 and specific granule markers were assessed by fluorescence-activated cell sorting analysis. Protein levels of the flavocytochrome b558 subunits gp91phox and p22phox in whole neutrophil lysates were detected by Western blotting.

RESULTS

Stimulation of neutrophils with GM-CSF and fMLP increased the flavocytochrome b558 plasma membrane expression. The fMLP-induced translocation of flavocytochrome b558 was reduced in neutrophils from MDS patients (140%+/-9% vs 180%+/-13%, p<0.05). Analysis of cell surface expression of markers of flavocytochrome b558 containing granules (CD35 and CD66b) indicated that exocytosis of these granules in response to fMLP stimulation was not affected in MDS patients. Western blot analysis demonstrated a decreased protein expression level of the flavocytochrome b558 subunits gp91phox and p22phox in neutrophils from MDS patients.

CONCLUSION

Our results indicate both a lower basal protein level and a disturbed fMLP-induced increase in plasma membrane expression of flavocytochrome b558 in neutrophils from MDS patients, which together might play a role in decreased ROS production.

Down-regulation of Rac activity during beta 2 integrin-mediated adhesion of human neutrophils

In human neutrophils, beta2 integrin engagement mediated a decrease in GTP-bound Rac1 and Rac2. Pretreatment of neutrophils with LY294002 or PP1 (inhibiting phosphatidylinositol 3-kinase (PI 3-kinase) and Src kinases, respectively) partly reversed the beta2 integrin-induced down-regulation of Rac activities. In contrast, beta2 integrins induced stimulation of Cdc42 that was independent of Src family members. The PI 3-kinase dependence of the beta2 integrin-mediated decrease in GTP-bound Rac could be explained by an enhanced Rac-GAP activity, since this activity was blocked by LY204002, whereas PP1 only had a minor effect. The fact that only Rac1 but not Rac2 (the dominating Rac) redistributed to the detergent-insoluble fraction and that it was independent of GTP loading excludes the possibility that down-regulation of Rac activities was due to depletion of GTP-bound Rac from the detergent-soluble fraction. The beta2 integrin-triggered relocalization of Rac1 to the cytoskeleton was enabled by a PI 3-kinase-induced dissociation of Rac1 from LyGDI. The dissociations of Rac1 and Rac2 from LyGDI also explained the PI 3-kinase-dependent translocations of Rac GTPases to the plasma membrane. However, these accumulations of Rac in the membrane, as well as that of p47phox and p67phox, were also regulated by Src tyrosine kinases. Inasmuch as Rac GTPases are part of the NADPH oxidase and the respiratory burst is elicited in neutrophils adherent by beta2 integrins, our results indicate that activation of the NADPH oxidase does not depend on the levels of Rac-GTP but instead requires a beta2 integrin-induced targeting of the Rac GTPases as well as p47phox and p67phox to the plasma membrane.

Rac1 deletion in mouse neutrophils has selective effects on neutrophil functions

Defects in myeloid cell function in Rac2 knockout mice underline the importance of this isoform in activation of NADPH oxidase and cell motility. However, the specific role of Rac1 in neutrophil function has been difficult to assess since deletion of Rac1 results in embryonic lethality in mice. To elucidate the specific role of Rac1 in neutrophils, we generated mice with a conditional Rac1 deficiency restricted to cells of the granulocyte/monocyte lineage. As observed in Rac2-deficient neutrophils, Rac1-deficient neutrophils demonstrated profound defects in inflammatory recruitment in vivo, migration to chemotactic stimuli, and chemoattractant-mediated actin assembly. In contrast, superoxide production is normal in Rac1-deficient neutrophils but markedly diminished in Rac2 null cells. These data demonstrate that although Rac1 and Rac2 are both required for actin-mediated functions, Rac2 is specifically required for activation of the neutrophil NADPH oxidase.

Phosphorylation of the NADPH oxidase component p67(PHOX) by ERK2 and P38MAPK: selectivity of phosphorylated sites and existence of an intramolecular regulatory domain in the tetratricopeptide-rich region

p67(PHOX), a cytosolic component of the NADPH oxidase complex, is phosphorylated during neutrophil activation by several agonists. The intracellular signaling pathways leading to its phosphorylation in neutrophils may involve a PKC-dependent pathway and a PKC-independent pathway. Here, we analyzed p67(PHOX) phosphorylation by ERK2 and p38MAPK. Both ERK2 and p38MAPK phosphorylated p67(PHOX) in vitro, with similar K(m) values (10 and 9 microM, respectively). Phosphopeptide mapping indicated that ERK2 and p38MAPK phosphorylate different subgroups of peptides. Using truncated forms of p67(PHOX), we found that the major phosphorylation target site of ERK2 was located in the N-terminal fragment (1-243), while the major phosphorylation target sites of p38MAPK were located in the C-terminal fragment (244-526). Furthermore, an additional peptide, which was not phosphorylated in the intact protein, appeared to be phosphorylated in the isolated C-terminal fragment (aa 244-526). This site may not thus be accessible in the intact protein. Indeed, incubation of the C-terminal fragment (244-526) with different N-terminal fragments (1-243, 1-210, or 1-199) containing the tetratricopeptide-rich region prevented phosphorylation of this C-terminal fragment. ERK1/2 and p38MAPK are also involved in p67(PHOX) phosphorylation in intact neutrophils. Indeed, PD98059 and SB203580, two selective inhibitors of MEK1/2 and p38MAPK, respectively, inhibited p67(PHOX) phosphorylation in fMLP- and PMA-stimulated neutrophils, with additive effects, thus suggesting that they also target different sites in vivo. Furthermore, the major peptides phosphorylated by ERK2 and p38MAPK in vitro were also phosphorylated in fMLP-stimulated neutrophils. Taken together, these results suggest not only that p67(PHOX) is phosphorylated by ERK2 and p38MAPK in vitro and in intact neutrophils on several selective sites but also that a C-terminal phosphorylation site may become accessible after a conformational change of the protein.

GTPases and T cell activation

Guanine nucleotide binding proteins rapidly cycle between a guanosine diphosphate (GDP)-bound and guanosine triphosphate (GTP)-bound state, and they operate as binary switches that control cell activation in response to environmental cues. GTPases adopt different conformations when binding GTP vs. GDP. The GTP-bound state is generally considered to be the active conformation that allows GTPases to interact with downstream effectors and thereby initiate downstream signaling pathways, which regulate many important biological processes. Many members of the Ras family of GTPases, notably Ras and Rap1A, and the Rho family GTPases, Cdc42Hs, Rac1, Rac2 and RhoA, are important components of signal transduction pathways used by antigen receptors, costimulatory, cytokine and chemokine receptors to regulate the immune response. This review discusses current knowledge and ideas about the regulation and function of these GTPases in lymphocytes.

Divergence of mechanisms regulating respiratory burst in blood and sputum eosinophils and neutrophils from atopic subjects

Eosinophil respiratory burst is an important event in asthma and related inflammatory disorders. However, little is known concerning activation of the respiratory burst NADPH oxidase in human eosinophils. Conversely, neutrophils are known to assemble NADPH oxidase in intracellular and plasma membranes. We hypothesized that eosinophils and neutrophils translocate NADPH oxidase to distinct intracellular locations, consistent with their respective functions in O(2)(-)-mediated cytotoxicity. PMA-induced O(2)(-) release assayed by cytochrome c was 3.4-fold higher in atopic human eosinophils than in neutrophils, although membrane-permeable dihydrorhodamine-123 showed similar amounts of release. Eosinophil O(2)(-) release was dependent on Rac, in that it was 54% inhibited by Clostridium difficile toxin B (400-800 ng/ml). In eosinophils stimulated with PMA, a pronounced shift of cytosolic Rac to p22(phox)-positive plasma membrane was observed by confocal microscopy, whereas neutrophils directed Rac2 mainly to intracellular sites coexpressing p22(phox). Similarly, ex vivo sputum eosinophils from asthmatic subjects exhibited predominantly plasma membrane-associated immunoreactivity for Rac, whereas sputum neutrophils exhibited cytoplasmic Rac2 staining. Thus, activated sputum eosinophils, rather than neutrophils, may contribute significantly to the pathogenesis of asthma by extracellular release of tissue-damaging O(2)(-). Our findings suggest that the differential modes of NADPH oxidase assembly in these cells may have important implications for oxidant-mediated tissue injury.

Decreased phosphorylation of protein kinase B and extracellular signal-regulated kinase in neutrophils from patients with myelodysplasia

Neutrophils from patients with myelodysplastic syndrome (MDS) show a disturbed differentiation pattern and are generally dysfunctional. To study these defects in more detail, we investigated reactive-oxygen species (ROS) production and F-actin polymerization in neutrophils from MDS patients and healthy controls and the involvement of N-formyl-L-methionyl-L-lucyl-L-phenylaline (fMLP) and granulocyte macrophage-colony-stimulating factor (GM-CSF)-stimulated signal transduction pathways. Following fMLP stimulation, similar levels of respiratory burst, F-actin polymerization, and activation of the small GTPase Rac2 were demonstrated in MDS and normal neutrophils. However, GM-CSF and G-CSF priming of ROS production were significantly decreased in MDS patients. We subsequently investigated the signal transduction pathways involved in ROS generation and demonstrated that fMLP-stimulated ROS production was inhibited by the phosphatidylinositol 3 kinase (PI3K) inhibitor LY294002, but not by the MAPK/ERK kinase (MEK) inhibitor U0126. In contrast, ROS production induced by fMLP stimulation of GM-CSF-primed cells was inhibited by LY294002 and U0126. This coincides with enhanced protein kinase B (PKB/Akt) phosphorylation that was PI3K dependent and enhanced extracellular signal-regulated protein kinase 1 and 2 (ERK1/2) phosphorylation that was PI3K independent. We demonstrated higher protein levels of the PI3K subunit p110 in neutrophils from MDS patients and found that though the fMLP-induced phosphorylation of PKB/Akt and ERK1/2 could also be enhanced by pretreatment with GM-CSF in these patients, the degree and kinetics of PKB/Akt and ERK1/2 phosphorylation were significantly disturbed. These defects were observed despite a normal GM-CSF-induced signal transducer and activator of transcription 5 (STAT5) phosphorylation. Our results indicate that the reduced priming of neutrophil ROS production in MDS patients might be caused by a disturbed convergence of the fMLP and GM-CSF signaling routes.

The HIV-1 Nef protein and phagocyte NADPH oxidase activation

Nef, a multifunctional HIV protein, activates the Vav/Rac/p21-activated kinase (PAK) signaling pathway. Given the potential role of this pathway in the activation of the phagocyte NADPH oxidase, we have investigated the effect of the HIV-1 Nef protein on the phagocyte respiratory burst. Microglia (cell line and primary culture) were transduced with lentiviral expression vectors. Expression of Nef did not activate the NADPH oxidase by itself but led to a massive enhancement of the responses to a variety of stimuli (Ca(2+) ionophore, formyl peptide, endotoxin). These effects were not caused by up-regulation of phagocyte NADPH oxidase subunits. Nef mutants lacking motifs involved in the interaction with Vav and PAK failed to reproduce the effects of wild type Nef, suggesting a role for the Vav/Rac/PAK signaling pathway. The following results suggest a key role for Rac in the priming effect of Nef. (i) Inactivation of Rac by Clostridium difficile toxin B abolished the Nef effect. (ii) The fraction of activated Rac1 was increased in Nef-transduced cells, and (iii) the dominant positive Rac1(V12) mutant mimicked the effect of Nef. These results are to our knowledge the first analysis of the effect of Rac activation on the NADPH oxidase in intact phagocytes. Rac activation is not sufficient to stimulate the phagocyte NADPH oxidase; however, it markedly enhances the NADPH oxidase response to other stimuli.

Chemoattractant-stimulated Rac activation in wild-type and Rac2-deficient murine neutrophils: preferential activation of Rac2 and Rac2 gene dosage effect on neutrophil functions

The hemopoietic-specific Rho family GTPase Rac2 shares 92% amino acid identity with ubiquitously expressed Rac1. Neutrophils from rac2(-/-) mice have multiple defects, including chemoattractant-stimulated NADPH oxidase activity and chemotaxis, which may result from an overall reduction in cellular Rac or mechanisms that discriminate Rac1 and Rac2. We show that murine neutrophils have similar amounts of Rac1 and Rac2, unlike human neutrophils, which express predominantly Rac2. An affinity precipitation assay for Rac-GTP showed that although FMLP-induced activation of both isoforms in wild-type neutrophils, approximately 4-fold more Rac2-GTP was detected than Rac1-GTP. Wild-type and Rac2-deficient neutrophils have similar levels of total Rac1. FMLP-induced Rac1-GTP in rac2(-/-) neutrophils was approximately 3-fold greater than in wild-type cells, which have similar levels of total Rac1, yet FMLP-stimulated F-actin, chemotaxis, and superoxide production are markedly impaired in rac2(-/-) neutrophils. Heterozygous rac2(+/-) neutrophils, which had intermediate levels of total and FMLP-induced activated Rac2, exhibited intermediate functional responses to FMLP, suggesting that Rac2 was rate limiting for these functions. Thus, phenotypic defects in FMLP-stimulated Rac2-deficient neutrophils appear to reflect distinct activation and signaling profiles of Rac1 and Rac2, rather than a reduction in the total cellular level of Rac.

Participation of Rac GTPase activating proteins in the deactivation of the phagocytic NADPH oxidase

The aim of the present study was to investigate possible mechanisms that could be involved in the deactivation of the assembled, catalytically active NADPH oxidase of phagocytic cells and thereby lead to termination of O(2)(.-) production. Our major findings are the following: (1) Addition of GDP to the active oxidase is able to reduce O(2)(.-) production both in the fully purified and in a semi-recombinant cell-free activation system. (2) p67(phox) inhibits GTP hydrolysis on Rac whereas p47(phox) has no effect on Rac GTPase activity. (3) Soluble regulatory proteins (GTPase activating protein, guanine nucleotide dissociation inhibitor, and the Rac-binding domain of the target protein p21-activated kinase) inhibit activation of the NADPH oxidase but have no effect on electron transfer via the assembled enzyme complex. (4) Membrane-associated GTPase activating proteins (GAPs) have access also to the assembled, catalytically active oxidase. Taken together, we propose that the GTP-bound active form of Rac is required for sustained enzyme activity and that membrane-localized GAPs have a role in the deactivation of NADPH oxidase.

Opsonization modulates Rac-1 activation during cell entry by Leishmania amazonensis

Lesions caused by Leishmania amazonensis normally heal, but relapses occur due to parasite persistence in host tissues. It has been proposed that infection of fibroblasts plays an important role in this process by providing the parasites with a safe haven in which to replicate. However, most previous studies have focused on the entry of Leishmania into macrophages, a process mediated by serum opsonins. To gain insight into a possible role of nonopsonic entry in the intracellular persistence of amastigotes, we examined the invasion of Chinese hamster ovary (CHO) cells. Amastigotes entered CHO cells by a cytochalasin D, genistein, wortmannin, and 2,3-butanedione monoxime-sensitive pathway and replicated within phagolysosomes. However, unlike most phagocytic processes described to date, amastigote internalization in CHO cells involved activation of the GTPases Rho and Cdc42 but not Rac-1. When uptake was mediated by fibronectin or when amastigotes were opsonized with immunoglobulin G and internalized by Fc receptor-expressing CHO cells, Rac-1 activation was restored and found to be required for parasite internalization. Given the essential role of Rac in assembly of the respiratory burst oxidase, invasion through this nonopsonic, Rac-1-independent pathway may play a central role in the intracellular survival of Leishmania in immune hosts.

Rac activation induces NADPH oxidase activity in transgenic COSphox cells, and the level of superoxide production is exchange factor-dependent

Transient expression of constitutively active Rac1 derivatives, (G12V) or (Q61L), was sufficient to induce phagocyte NADPH oxidase activity in a COS-7 cell model in which human cDNAs for essential oxidase components, gp91(phox), p22(phox), p47(phox), and p67(phox), were expressed as stable transgenes. Expression of constitutively active Rac1 in "COS(phox)" cells induced translocation of p47(phox) and p67(phox) to the membrane. Furthermore, translocation of p47(phox) was induced in the absence of p67(phox) expression, even though Rac does not directly bind p47(phox). Rac effector domain point substitutions (A27K, G30S, D38A, Y40C), which can selectively eliminate interaction with different effector proteins, impaired Rac1V12-induced superoxide production. Activation of endogenous Rac1 by expression of constitutively active Rac-guanine nucleotide exchange factor (GEF) derivatives was sufficient to induce high level NADPH oxidase activity in COS(phox) cells. The constitutively active form of the hematopoietic-specific GEF, Vav1, was the most effective at activating superoxide production, despite detection of higher levels of Rac1-GTP upon expression of constitutively active Vav2 or Tiam1 derivatives. These data suggest that Rac can play a dual role in NADPH oxidase activation, both by directly participating in the oxidase complex and by activating signaling events leading to oxidase assembly, and that Vav1 may be the physiologically relevant GEF responsible for activating this Rac-regulated complex.

Differential binding of arfaptin 2/POR1 to ADP-ribosylation factors and Rac1

We first identified arfaptin as a protein that bound to GTP-ARFs (especially ARF1). However, a second group reported that POR1, a truncated form of arfaptin, bound to GTP-Rac1. Therefore, we examined the possibility that arfaptin 2/POR1 was a common downstream effector for both ARF1 and Rac1. In this study, we found that constitutively active Rac1 or GTP-Rac1 showed negligible or no binding to arfaptin 2/POR1 in a yeast two-hybrid assay or a GST pull-down assay. However, wild-type or dominant negative Rac1 or Rac1 liganded to GDP showed strong binding. In contrast, constitutively active ARFs1, 5, and 6 showed binding, whereas the wild-type and dominant negative forms did not. Furthermore, the GTP-liganded ARFs bound arfaptin 2, whereas the GDP-bound forms showed little or no binding. Based on these observations, we suggest that arfaptin 2/POR1 is a target protein for GTP-ARFs and for GDP-Rac1, and that it may be involved in interactions between the Rac and ARF signaling pathways.

Biochemical and biological characterization of a human Rac2 GTPase mutant associated with phagocytic immunodeficiency

The Rho GTPase, Rac2, is expressed only in hematopoietic cell lineages, suggesting a specific cellular function in these cells. Genetic targeting studies in mice showed that Rac2 is an essential regulator of neutrophil chemotaxis, L-selectin capture and rolling, and superoxide production. Recently, a dominant negative mutation of Rac2, D57N, has been reported to be associated with a human phagocytic immunodeficiency. To understand further the cellular phenotypes associated with this D57N Rac2 mutant we examined its biochemical characteristics and functional effects when expressed in primary murine bone marrow cells. When compared with wild type (WT) Rac2, D57N Rac2 displayed approximately 10% GTP binding ability resulting from a markedly enhanced rate of GTP dissociation and did not respond to the guanine nucleotide exchange factors. These results suggest that D57N Rac2 may act in a dominant negative fashion in cells by sequestering endogenous guanine nucleotide exchange factors. When expressed in hematopoietic cells, D57N Rac2 reduced endogenous activities of not only Rac2, but also Rac1 and decreased cell expansion in vitro in the presence of growth factors due to increased cell apoptosis. Unexpectedly, D57N expression had no effect on proliferation. In contrast, expansion of cells transduced with WT Rac2 and a dominant active mutant, Q61L, was associated with significantly increased proliferation. Transplantation of transduced bone marrow cells into lethally irradiated recipients showed that the percentage of D57N-containing peripheral blood cells decreased markedly from 40% at 1 month to <5% by 3 months postinjection. Neutrophils derived in vitro from the transduced progenitor cells containing D57N demonstrated markedly impaired migration and O(2)(-) responses to formyl-methionyl-leucyl-phenylalanine, reflecting the same cellular phenotype in these differentiated cells as those described previously in patient cells. These data suggest that the phenotypic abnormalities associated with D57N Rac2 may involve not only neutrophil cellular functions, but also abnormal cell survival in other hematopoietic cells and that overexpression of Rac leads to increased proliferation of normal cells in vitro, whereas deficiency of Rac leads to increased apoptosis.

Characterization of membrane-localized and cytosolic Rac-GTPase-activating proteins in human neutrophil granulocytes: contribution to the regulation of NADPH oxidase

We have investigated the intracellular localization and molecular identity of Rac-GTPase-activating proteins (Rac-GAPs) in human neutrophils. Immunoblot analysis detected the presence of both p190RhoGAP and Bcr mainly in the cytosol. An overlay assay performed with [gamma-(32)P]GTP-bound Rac revealed dominant GAP activity related to a 50 kDa protein both in the membrane and cytosol. This activity could be identified by Western blotting and immunoprecipitation with specific antibody directed against the GAP domain of p50RhoGAP. Using a semirecombinant or fully purified cell-free activation assay of the Rac-activated enzyme NADPH oxidase, we demonstrated the regulatory effect of both the membrane-localized and soluble GAPs. We suggest that in neutrophil granulocytes Rac-GAPs have redundant function and represent suitable targets for both the up-regulation and down-regulation of the NADPH oxidase.

Distinct amplification of the C5a-receptor pathways in normodense and hypodense eosinophils of patients with atopic dermatitis

In patients with atopic dermatitis two different types of blood eosinophils with distinct density can be isolated. The normodense cells represent the huge majority in count, whereas the hypodense eosinophils are characterized by higher effector activity. To understand the altered functional responsiveness of these two cell subtypes, the expression of C5a receptors as well as C5a-induced signal pathways and the production of reactive oxygen metabolites have been analyzed. Chemiluminescence measurements revealed significant higher production of reactive oxygen metabolites in hypodense eosinophils in comparison to normodense cells. However, no difference in the expression level of C5a receptors as well as in the C5a-induced Ca2+-transients between normodense and hypodense eosinophils were found. In contrast, hypodense eosinophils showed a significantly higher actin polymerization response and phosphatidylinositol 4,5 bisphosphate 3-kinase activation after stimulation with C5a than normodense eosinophils. Therefore, normodense and hypodense eosinophils from the blood of patients with atopic dermatitis are characterized by differential amplification of C5a-receptor signal pathways, which might explain the differences in their proinflammatory activity.

Dual role of plasma membrane electron transport systems in defense

Because oxidative stress is one of the main sources of severe cellular damage, cells have different defense weapons against reactive oxygen species. Ubiquitous plasma membrane redox systems play a role in defense against oxidative stress damage. On the other hand, a tightly controlled and localized production of reactive oxygen species by a plasma membrane NADPH oxidase can be used as a potent microbicidal weapon. This dual, prooxidant and antioxidant role of plasma membrane electron transport systems in defense is studied and discussed.

Rac2 is an essential regulator of neutrophil nicotinamide adenine dinucleotide phosphate oxidase activation in response to specific signaling pathways

Rac2 is a hematopoietic-specific Rho family GTPase implicated as an important constituent of the NADPH oxidase complex and shares 92% amino acid identity with the ubiquitously expressed Rac1. In bone marrow (BM) neutrophils isolated from rac2(-/-) mice generated by gene targeting, we previously reported that PMA-induced superoxide production was reduced by about 4-fold, which was partially corrected in TNF-alpha-primed BM neutrophils and in peritoneal exudate neutrophils. We investigated receptor-mediated activation of the NADPH oxidase in the current study, finding that superoxide production in rac2(-/-) BM and peritoneal exudate neutrophils was normal in response to opsonized zymosan, reduced to 22% of wild type in response to IgG-coated SRBC, and almost absent in response to fMLP. In wild-type murine BM neutrophils, phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), p38 mitogen-activated protein kinase, c-Jun N-terminal kinase, and Akt was induced by PMA or fMLP, which was decreased in rac2(-/-) neutrophils for ERK1/2 and p38. Activation of p38 by either opsonized zymosan or IgG-coated SRBC was similar in wild-type and rac2(-/-) cells. Inhibition of ERK1/2 or p38 activation using either PD98059 or SB203580, respectively, had only a modest effect on fMLP-elicited superoxide production and no effect on the PMA-induced response. These data provide genetic evidence supporting an important role for Rac2 in regulating neutrophil NADPH oxidase activation downstream of chemoattractant and Fcgamma receptors. The effect of Rac2 deficiency on superoxide production is probably exerted through multiple pathways, including those independent of mitogen-activated protein kinase activation.

Role of reactive oxygen species and phosphatidylinositol 3-kinase in cardiomyocyte differentiation of embryonic stem cells

Cardiotypic development in embryonic stem cell-derived embryoid bodies may be regulated by reactive oxygen species (ROS). ROS were generated by a NADPH oxidase-like enzyme which was transiently expressed during the time course of embryoid body development. Incubation with either H(2)O(2) or menadione enhanced cardiomyogenesis, whereas the radical scavengers trolox, pyrrolidinedithiocarbamate and N-acetylcysteine exerted inhibitory effects. The phosphatidylinositol 3-kinase (PI-3-kinase) inhibitors LY294002 and wortmannin abolished cardiac commitment and downregulated ROS in embryoid bodies. Coadministration of LY294002 with prooxidants resumed cardiomyocyte differentiation, indicating a role for PI-3-kinase in the regulation of the intracellular redox state.

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.