Ion channel modulators mediated alterations in NO-induced free radical generation and neutrophil membrane potential

Neuroinflammation and Neutrophils: Modulation by Ouabain

Cardiotonic steroids are natural compounds that present many physiological and pharmacological functions. They bind Na+/K+-ATPase (NKA) modifying cellular ion concentration and trigger cell signaling mechanisms without altering ion balance. These steroids are known to modulate some immune responses, including cytokine production, neutrophil migration, and inflammation (peripherally and in the nervous system). Inflammation can occur in response to homeostasis perturbations and is related to the development of many diseases, including immune-mediated diseases and neurodegenerative disorders. Considering the neutrophils role in the general neuroinflammatory response and that these cells can be modulated by cardiac steroids, this work aims to review the possible regulation of neutrophilic neuroinflammation by the cardiac steroid ouabain.

Local anaesthetics upregulate nitric oxide generation in cord blood and adult human neutrophils

Nitric oxide (NO) generation by systemic neonatal neutrophils is not clarified. It is also not known whether local anaesthetics (LAs) transferred to the fetal systemic circulation following maternal epidural blockade may affect this process. In the present study, NO generation was evaluated in neutrophils from cord blood (CB, n = 11) and adult blood (n = 10) following exposure to bupivacaine (0.0005, 0.005, 1 mM), lidocaine (0.002, 0.02, 4 mM) and ropivacaine (0.0007, 0.007, 1.4 mM) using flow cytometry, as well as indirectly by determining nitrite concentrations in cell incubation media. To determine the role of NO synthase (NOS) isoforms in NO generation following exposure to LAs, experiments were repeated in the presence of the NOS inhibitors, N^G-nitro-L-arginine methyl ester and aminoguanidine; in addition, the expression of NOS isoforms was analysed. CB neutrophils produced less NO than adult neutrophils. LAs, especially ropivacaine and lidocaine, stimulated neutrophil NO generation, but in CB neutrophils this effect was negligible at clinically relevant drug concentrations. A mechanism involving NOS activity was responsible for the observed phenomena. In conclusion, LAs are able to upregulate neutrophil NO production, but in neonates this effect is likely to be clinically insignificant.

A Salutary Role of Reactive Oxygen Species in Intercellular Tunnel-Mediated Communication

The reactive oxygen species, generally labeled toxic due to high reactivity without target specificity, are gradually uncovered as signaling molecules involved in a myriad of biological processes. But one important feature of ROS roles in macromolecule movement has not caught attention until recent studies with technique advance and design elegance have shed lights on ROS signaling for intercellular and interorganelle communication. This review begins with the discussions of genetic and chemical studies on the regulation of symplastic dye movement through intercellular tunnels in plants (plasmodesmata), and focuses on the ROS regulatory mechanisms concerning macromolecule movement including small RNA-mediated gene silencing movement and protein shuttling between cells. Given the premise that intercellular tunnels (bridges) in mammalian cells are the key physical structures to sustain intercellular communication, movement of macromolecules and signals is efficiently facilitated by ROS-induced membrane protrusions formation, which is analogously applied to the interorganelle communication in plant cells. Although ROS regulatory differences between plant and mammalian cells exist, the basis for ROS-triggered conduit formation underlies a unifying conservative theme in multicellular organisms. These mechanisms may represent the evolutionary advances that have enabled multicellularity to gain the ability to generate and utilize ROS to govern material exchanges between individual cells in oxygenated environment.

Early events induced by the toxin deoxynivalenol lead to programmed cell death in Nicotiana tabacum cells

Deoxynivalenol (DON) is a mycotoxin affecting animals and plants. This toxin synthesized by Fusarium culmorum and Fusarium graminearum is currently believed to play a decisive role in the fungal phytopathogenesis as a virulence factor. Using cultured cells of Nicotiana tabacum BY2, we showed that DON-induced programmed cell death (PCD) could require transcription and translation processes, in contrast to what was observed in animal cells. DON could induce different cross-linked pathways involving (i) reactive oxygen species (ROS) generation linked, at least partly, to a mitochondrial dysfunction and a transcriptional down-regulation of the alternative oxidase (Aox1) gene and (ii) regulation of ion channel activities participating in cell shrinkage, to achieve PCD.

A novel immunomodulatory effect of ugonin U in human neutrophils via stimulation of phospholipase C

Neutrophils have a crucial role in the immune system and are the first line of defense against pathogenic invaders. Neutrophil activation is required for their defensive function and can be induced by diverse stimuli, through either binding to cell surface receptors or direct intracellular target molecule stimulation. In this study, we found that 4″a,5″,6″,7″,8″,8″a-hexahydro-5,3',4'-trihydroxy-5″,5″,8″a-trimethyl-4H-chromeno [2″,3″:7,6]flavone (ugonin U), a flavonoid isolated from Helminthostachys zeylanica (L) Hook, significantly induced superoxide production and release in a time- and concentration-dependent manner. A series of experiments was performed to dissect the mechanism of ugonin U-induced respiratory burst in human neutrophils. Our results demonstrated that ugonin U induced a slow increase in intracellular Ca(2+), which was necessary for ugonin U-stimulated superoxide release. Use of a formyl peptide receptor (FPR) blocker, G protein inhibitor, and protein tyrosine kinase (PTK) inhibitor proved that FPR, G proteins, and PTKs were not associated with ugonin U-induced respiratory burst. Additionally, immunoblotting results revealed that ugonin U did not affect the phosphorylation of mitogen-activated protein kinases and protein tyrosine. Nevertheless, a phospholipase C (PLC) inhibitor and an inositol triphosphate (IP3) receptor antagonist considerably suppressed ugonin U-stimulated Ca(2+) mobilization and subsequent superoxide release. Ugonin U also induced an increase in intracellular IP3 formation, which could be blocked using a PLC inhibitor. In conclusion, our study reveals that ugonin U represents the first identified natural flavonoid compound to directly stimulate PLC. Moreover, ugonin U induces respiratory burst via the PLC/IP3/Ca(2+) pathway in human neutrophils.

Interaction of inducible nitric oxide synthase with rac2 regulates reactive oxygen and nitrogen species generation in the human neutrophil phagosomes: implication in microbial killing

AIMS

Present study explores importance of inducible nitric oxide synthase (iNOS) and its interaction with Rac2 in reactive oxygen species (ROS)/reactive nitrogen species (RNS) generation, protein-nitration and in microbial killing by neutrophils.

RESULTS

The iNOS transcript and protein were constitutively present in human as well as in mice neutrophils. iNOS protein was found in cytosol, granules containing elastase and gelatinase, and in other subcellular organelles in resting human neutrophils. After phagocytosis of bovine serum albumin (BSA) coated beads, both human and mice neutrophils showed significant elevation in superoxide radicals, nitric oxide (NO), ROS/RNS and consequent BSA nitration. These responses were significantly reduced in presence of iNOS, NADPH oxidase (NOX), myeloperoxidase or Rac inhibitors, as well as in iNOS, Nox2 and Rac2 silenced human or iNOS-knockout mice neutrophils. Complex formed on interaction of iNOS with Rac2 coprecipitated with anti-Rac2, predominantly in cytosol in resting human neutrophils, while iNOS-Rac2 complex translocated to phagosomes after phagocytosis. This was accompanied by generation of superoxide radicals, NO, ROS/RNS and consequent BSA-nitration. Importance of Rac2 in iNOS mediated NO formation and microbial killing was confirmed by pretreatment of mice with Rac inhibitor, NSC23766 that significantly abrogated NO release and microbial killing in vivo.

INNOVATION

Present study highlights previously undefined role of Rac2-iNOS interaction, in translocation of iNOS to phagosomal compartment and consequent NO, superoxide radicals, ROS/RNS generation, BSA nitration and microbial killing.

CONCLUSIONS

Altogether results obtained demonstrate the role of iNOS in NO and ROS/RNS generation, after phagocytosis of coated latex beads by human polymorphonuclear neutrophils. These studies imply functional importance of iNOS and its interaction with Rac2 in pathogen killing by the neutrophils.

Endothelial calcium-activated potassium channels as therapeutic targets to enhance availability of nitric oxide

The vascular endothelium plays a critical role in vascular health by controlling arterial diameter, regulating local cell growth, and protecting blood vessels from the deleterious consequences of platelet aggregation and activation of inflammatory responses. Circulating chemical mediators and physical forces act directly on the endothelium to release diffusible relaxing factors, such as nitric oxide (NO), and to elicit hyperpolarization of the endothelial cell membrane potential, which can spread to the surrounding smooth muscle cells via gap junctions. Endothelial hyperpolarization, mediated by activation of calcium-activated potassium (K(Ca)) channels, has generally been regarded as a distinct pathway for smooth muscle relaxation. However, recent evidence supports a role for endothelial K(Ca) channels in production of endothelium-derived NO, and indicates that pharmacological activation of these channels can enhance NO-mediated responses. In this review we summarize the current data on the functional role of endothelial K(Ca) channels in regulating NO-mediated changes in arterial diameter and NO production, and explore the tempting possibility that these channels may represent a novel avenue for therapeutic intervention in conditions associated with reduced NO availability such as hypertension, hypercholesterolemia, smoking, and diabetes mellitus.

Neutrophil extracellular traps contain mitochondrial as well as nuclear DNA and exhibit inflammatory potential

Neutrophils expel extracellular traps (NETs) to entrap and exterminate the invaded micro-organisms. Acute/chronic inflammatory disorders are often observed with aberrantly enhanced NETs formation and high nitric oxide (NO) availability. Recent study from this laboratory demonstrated release of NETs from human neutrophils following treatment with SNP or SNAP. This study is an extension of our previous finding to explore the extracellular bacterial killing, source of DNA in the expelled NETs, their ability to induce proinflammatory cytokines release from platelets/THP-1 cells, and assessment of NO-mediated free radical formation by using a consistent NO donor, DETA-NONOate. NO-mediated NETs exhibited extracellular bacterial killing as determined by colony forming units. NO-mediated NETs formation was due to the activation of NADPH oxidase and myeloperoxidase. NO- or PMA-mediated NETs were positive for both nuclear and mitochondrial DNA as well as proteolytic enzymes. Incubation of NETs with human platelets enhanced the release of IL-1β and IL-8, while with THP-1 cells, release of IL-1β, IL-8, and TNFα was observed. This study demonstrates that NO by augmenting enzymatic free radical generation release NETs to promote extracellular bacterial killing. These NETs were made up of mitochondrial and nuclear DNA and potentiated release of proinflammatory cytokines.

Nitric oxide-mediated augmentation of neutrophil reactive oxygen and nitrogen species formation: Critical use of probes

Previous reports from this laboratory and others demonstrated NO-mediated biphasic modulation of NADPH oxidase and attenuation of neutrophil reactive oxygen species generation, whereas recently we reported augmentation in DCF fluorescence following NO treatment. These discrepancies seem to be due to utilization of different probes/methods to assess effect of NO on reactive oxygen and nitrogen species (ROS/RNS, reactive species) generation. This study aims to look into this and evaluate NO-mediated enzymatic reactive species formation by using multiple probes, human neutrophils/HL60 cells and various interventions. Addition of NO donor, SNP or SNAP (100 nM-1 mM) to PMNs suspension, exhibited a concentration- and time-dependent augmentation in DCF fluorescence, but reduced DHE fluorescence. Collective generation of reactive species was confirmed by enhanced DMPO-nitrone adduct, dityrosine and rhodamine-123 and quenching of scopoletin. NO also enhanced bacterial killing, without altering phagocytosis. Addition of NO to HL-60 cells lacking functional NADPH oxidase enhanced reactive species formation, indicating importance of other enzyme(s) too. NO-dependent ROS/RNS generation was substantially reduced by NADPH oxidase inhibitor (DPI), MPO inhibitor (ABAH), or NOS inhibitor (7-NI). However, 7-NI reduced MPO activity, warranting reappraisal of those reports, which implied NOS in reactive species formation. The results obtained demonstrated NO-mediated reactive species augmentation in human PMNs. Furthermore, superoxide scavenging by NO seems to be the key process in the decrease of DHE fluorescence and suggest usefulness of DCF as the most appropriate probe to measure the NO-mediated modulation of reactive oxygen species in particular in various pathological situations.

Increased anion channel activity is an unavoidable event in ozone-induced programmed cell death

BACKGROUND

Ozone is a major secondary air pollutant often reaching high concentrations in urban areas under strong daylight, high temperature and stagnant high-pressure systems. Ozone in the troposphere is a pollutant that is harmful to the plant.

PRINCIPAL FINDINGS

By exposing cells to a strong pulse of ozonized air, an acute cell death was observed in suspension cells of Arabidopsis thaliana used as a model. We demonstrated that O(3) treatment induced the activation of a plasma membrane anion channel that is an early prerequisite of O(3)-induced cell death in A. thaliana. Our data further suggest interplay of anion channel activation with well known plant responses to O(3), Ca(2+) influx and NADPH-oxidase generated reactive oxygen species (ROS) in mediating the oxidative cell death. This interplay might be fuelled by several mechanisms in addition to the direct ROS generation by O(3); namely, H(2)O(2) generation by salicylic and abscisic acids. Anion channel activation was also shown to promote the accumulation of transcripts encoding vacuolar processing enzymes, a family of proteases previously reported to contribute to the disruption of vacuole integrity observed during programmed cell death.

SIGNIFICANCE

Collectively, our data indicate that anion efflux is an early key component of morphological and biochemical events leading to O(3)-induced programmed cell death. Because ion channels and more specifically anion channels assume a crucial position in cells, an understanding about the underlying role(s) for ion channels in the signalling pathway leading to programmed cell death is a subject that warrants future investigation.

Functional and molecular characterization of NOS isoforms in rat neutrophil precursor cells

Previous studies from this laboratory have demonstrated importance of neutrophil-derived nitric oxide (NO) in free radical generation, characterized nitric oxide synthase (NOS) isoforms, and have reported subcellular distribution of NOS in rat peripheral neutrophils. Maximum number of neutrophils are added per day to the circulation from bone marrow, thus neutrophils might add substantial amount of NO in the bone marrow. NO generating ability and NOS isoforms characteristics in bone marrow neutrophil precursor cells is, however, still unexplored. This study was, therefore, undertaken to investigate NO generation ability and the molecular/biochemical characteristics of NOS isoforms in neutrophil precursor cells. The neutrophil precursors were separated on Percoll density gradient and characterized by Giemsa staining, CD markers, and by their size and granularity at various stages of maturation as Bands 1, 2, and 3. Mature neutrophils were efficient in free radical generation and phagocytosis, whereas immature cells had more mitochondria and myeloperoxidase. Amount of NO augmented from immature to mature neutrophils as assessed by fluorescent probe DAF-2DA and Griess reagent. Measurement of NOS enzyme activity further confirmed the functional status of NOS in these cells. NOS isoforms were differentially expressed during neutrophil maturation as confirmed by enzyme activity, Western blotting, flowcytometry, and RT-PCR. Expression of nNOS was predominantly stable in all the stages of neutrophil maturation. iNOS expression was, however, consistently augmented during maturation, whereas eNOS expression was downregulated with neutrophil maturation. Furthermore, all NOS isoforms proteins were distributed in cytosol as well as nucleus as assessed by confocal microscopy. This study for the first time report biochemical and molecular characteristics of NOS isoforms in rat neutrophil precursor cells.