Inhibition of phospholipase A(2) abrogates intracellular processing of NADPH-oxidase derived reactive oxygen species in human neutrophils

Neutrophil Activated by the Famous and Potent PMA (Phorbol Myristate Acetate)

This review will briefly outline the major signaling pathways in PMA-activated neutrophils. PMA is widely used to understand neutrophil pathways and formation of NETs. PMA activates PKC; however, we highlight some isoforms that contribute to specific functions. PKC α, β and δ contribute to ROS production while PKC βII and PKC ζ are involved in cytoskeleton remodeling. Actin polymerization is important for the chemotaxis of neutrophils and its remodeling is connected to ROS balance. We suggest that, although ROS and production of NETs are usually observed together in PMA-activated neutrophils, there might be a regulatory mechanism balancing both. Interestingly, we suggest that serine proteases might determine the PAD4 action. PAD4 could be responsible for the activation of the NF-κB pathway that leads to IL-1β release, triggering the cleavage of gasdermin D by serine proteases such as elastase, leading to pore formation contributing to release of NETs. On the other hand, when serine proteases are inhibited, NETs are formed by citrullination through the PAD4 pathway. This review puts together results from the last 31 years of research on the effects of PMA on the neutrophil and proposes new insights on their interpretation.

Measurement of Respiratory Burst Products, Released or Retained, During Activation of Professional Phagocytes

Activation of professional phagocytes, potent microbial killers of our innate immune system, is associated with an increased cellular consumption of molecular oxygen (O₂). The O₂ molecules consumed are reduced by electrons delivered by a membrane localized NADPH-oxidase that initially generate one- and two electron reduced superoxide anions (O₂^-) and hydrogen peroxide (H₂O₂), respectively. These oxidants can then be processed into other highly reactive oxygen species (ROS) that can kill microbes, but that may also cause tissue destruction and drive other immune cells into apoptosis. The development of basic techniques to measure and quantify ROS generation by phagocytes is of great importance, and a large number of methods have been used for this purpose. A selection of methods (including chemiluminescence amplified by luminol or isoluminol, absorbance change following reduction of cytochrome c, and fluorescence increase upon oxidation of PHPA) are described in detail in this chapter with special emphasis on how to distinguish between ROS that are released extracellularly, and those that are retained within intracellular organelles. These techniques can be valuable tools in research spanning from basic phagocyte biology to diagnosis of diseases linked to the NADPH-oxidase and more clinically oriented research on innate immune mechanisms and inflammation.

Functional selective ATP receptor signaling controlled by the free fatty acid receptor 2 through a novel allosteric modulation mechanism

A nonactivating allosteric modulator of free fatty acid receptor 2 (FFA2R, also called GPCR 43) turns both propionate (an orthosteric FFA2R agonist) and ATP (an agonist for the purinergic P2Y₂ receptor), into potent activating ligands that trigger an assembly of the superoxide-generating neutrophil NADPH oxidase. The ATP-induced activation requires the participation of FFA2R, and the signaling is biased toward oxidase activation, leaving the ATP-induced rise in intracellular Ca²⁺ unaffected. No NADPH oxidase activity was induced by ATP when propionate replaced the allosteric modulator. Signaling downstream of propionate-activated FFA2Rs was insensitive to Gαq inhibition, but the crosstalk activation involving both FFA2R and P2Y₂R relied on Gαq signaling. The receptor crosstalk, by which allosterically modulated FFA2Rs communicate with P2Y₂Rs and generate NADPH oxidase activating signals downstream of Gαq, represent a novel mechanism by which GPCR activities can be regulated from inside the plasma membrane. Further, the finding that an allosteric FFA2R modulator sensitizes not only the response induced by orthosteric FFA2R agonists, but also the response induced by ATP (P2Y₂R-specific agonist) and formyl peptide receptor-specific agonists, violates the receptor restriction characteristics normally defining the selectivity of allosteric GPCR modulators.-Lind, S., Holdfeldt, A., Mårtensson, J., Sundqvist, M., Björkman, L., Forsman, H., Dahlgren, C. Functional selective ATP receptor signaling controlled by the free fatty acid receptor 2 through a novel allosteric modulation mechanism.

Novel aspects of the activation of NADPH oxidase in neutrophils of rheumatic patients on biological therapy

The relationship between inflammation and formation of reactive oxygen species (ROS) is still not completely understood and excessive inflammatory reaction is attributed to increased yet also to reduced ROS formation. To compare ROS formation in severe and low inflammation, neutrophil oxidative burst was analyzed in rheumatic patients before and during therapy with TNFα- or interleukin-6 receptor-neutralizing antibodies. Intracellular and extracellular ROS productions were evaluated on the basis of luminol- and isoluminol-enhanced chemiluminescence in isolated peripheral neutrophils. Disease activity score DAS28 and platelet to lymphocyte ratio were used as markers of arthritis activity and the intensity of systemic inflammation. Biological therapy effectively reduced the intensity of inflammation. Of the twenty-six patients studied eighteen achieved remission or low disease activity. Highly active arthritis persisted only in one patient, though prior to the therapy it was evident in all subjects tested. In patients receiving biological therapy, intracellular chemiluminescence was significantly higher than in patients before this therapy; ROS produced by neutrophils extracellularly were not affected. The increased ROS formation associated with reduced inflammation supports the need to revise the view of the role of ROS in inflammation - from toxic agents promoting inflammation towards a more complex view of ROS as regulators of immune pathways with inflammation-limiting capacity. From this perspective, the interference with neutrophil-derived oxidants may represent a new mechanism involved in the anti-inflammatory activity of biological therapy.

Therapeutic potential of arachidonyl trifluromethyl ketone, a cytosolic phospholipaseA2 IVA specific inhibitor, in cigarette smoke condensate-induced pathological conditions in alveolar type I & II epithelial cells

Cigarette smoke is responsible for multiple disorders and causes almost 10 million annual deaths globally but underlying mechanisms are still underexplored. Continuous exposure of Cigarette smoke condensate (CSC) leads to cytosolic phospholipase A2 (cPLA2) mediated high free radicals where cPLA2s seems to play crucial role in generated various patho-physiological conditions such as chronic inflammation, oxidative stress and cancer. In this view, we assessed the therapeutic potential of arachidonyl trifluromethyl ketone (ATK), a cPLA2 inhibitor, via pharmacological inhibition of most expressible CSC-induced cPLA2 group IVA in type-I and type-II alveolar epithelial cells. The In Vitro inhibitory effect of ATK on CSC-induced PLA2 activity and its cellular role were assessed in terms of cell viability, fluorescein diacetate (FDA) dye uptake assay for membrane integrity, reactive oxygen species (ROS)/reactive nitrogen species (RNS) levels and pro apoptotic as well as anti apoptosis markers via flow cytometry, along with extracellular signal-regulated kinases (ERK) levels using enzyme-linked immunosorbent assay (ELISA). The experimental findings demonstrated that ATK acts as potent inhibitor of cPLA2 activity and shown its effectiveness as therapeutic agent by significantly mimicking CSC-induced levels of free radicals, primary apoptosis, ratio of pro-apoptotic/apoptotic proteins and levels of ERK whereas protected cells from loss of cell viability and membrane integrity. Thus, this study is an important step towards the opening up of avenues for the applicability of the cPLA2 isoform specific inhibitors such as ATK for pre-clinical and clinical studies and could be beneficial during smoking-induced lung pathological conditions.

Quantification of heterotypic granule fusion in human neutrophils by imaging flow cytometry

Human neutrophils are filled with intracellular storage organelles, called granules and secretory vesicles, which differ in their content of soluble matrix proteins and membrane-bound molecules. To date, at least four distinct granule/vesicle subsets have been identified. These organelles may secrete their content extracellularly following mobilization to and fusion with the plasma membrane, but some of them may also fuse with internal membrane-enclosed organelles, typically a plasma membrane-derived phagosome. There are also instances where different granules appear to fuse with one another, a process that would enable mixing of their matrix and membrane components. Such granule fusion enables e.g., myeloperoxidase-processing of intragranular oxygen radicals, a key event in the formation of neutrophil extracellular traps (Björnsdottir et al., 2015) [1]. Described herein are data that show the quantification of such heterotypic granule–granule fusion by the use of imaging flow cytometry, a technique that combines flow cytometry with microscopy. The analysis described is based on immunofluorescent staining of established granule markers (lactoferrin and/or NGAL for one granule subset; the specific granules, and CD63 for another granule subset, the azurophil granules) and calculation of a colocalization score for resting and PMA-stimulated neutrophils.

Neutrophil NET formation is regulated from the inside by myeloperoxidase-processed reactive oxygen species

AIM

Neutrophil extracellular traps (NETs) are mesh-like DNA fibers clad with intracellular proteins that are cast out from neutrophils in response to certain stimuli. The process is thought to depend on reactive oxygen species (ROS) generated by the phagocyte NADPH-oxidase and the ROS-modulating granule enzyme myeloperoxidase (MPO), but when, how, and where these factors contribute is so far uncertain. The neutrophil NADPH-oxidase can be activated at different cellular sites and ROS may be produced and processed by MPO within intracellular granules, even in situations where a phagosome is not formed, e.g., upon stimulation with phorbol myristate acetate (PMA).

OBJECTIVES

We investigated the subcellular location of ROS production and processing by MPO in the context of PMA-induced NET formation.

RESULTS

Complete neutralization of extracellular ROS was not sufficient to block NET formation triggered by PMA, indicating that intragranular ROS are critical for NETosis. Employing a set of novel MPO-inhibitors, inhibition of NET formation correlated with inhibition of intragranular MPO activity. Also, extracellular addition of MPO was not sufficient to rescue NET formation in completely MPO-deficient neutrophils and specific neutralization by luminol of MPO-processed ROS within intracellular granules led to a complete block of PMA-triggered NET formation.

CONCLUSION

We show for the first time that inhibition of intragranular MPO activity, or neutralization of intragranular MPO-processed ROS by luminol effectively block NET formation. Our data demonstrate that ROS must be formed and processed by MPO in order to trigger NET formation, and that these events have to occur within intracellular granules.

Discrete roles of intracellular phospholipases A2 in human neutrophil cytotoxicity

BACKGROUND

The role of calcium-independent phospholipase A2 (iPLA2), a component of the three major PLA2 families, in acute/chronic inflammatory processes remains elusive. Previous investigations have documented iPLA2-mediated respiratory burst of neutrophils (PMNs); however, the causative isoform of iPLA2 is unidentified. We also demonstrated that the iPLA2γ-specific inhibitor attenuates trauma/hemorrhagic shock-induced lung injury. Therefore, iPLA2γ may be implicated in acute inflammation. In addition, arachidonic acid (AA), which is primarily produced by cytosolic PLA2 (cPLA2), is known to display PMN cytotoxicity, although the relationship between AA and the cytotoxic function is still being debated on. We therefore hypothesized that iPLA2γ regulates PMN cytotoxicity via AA-independent signaling pathways. The study aim was to distinguish the role of intracellular phospholipases A2, iPLA2, and cPLA2, in human PMN cytotoxicity and explore the possibility of the presence of signaling molecule(s) other than AA.

METHODS

Isolated human PMNs were incubated with the PLA2 inhibitor selective for iPLA2β, iPLA2γ, or cPLA2 and then activated with formyl-methionyl-leucyl-phenylalanine (fMLP) or phorbol 12-myristate 13-acetate (PMA). Superoxide production was assayed according to the superoxide dismutase-inhibitable cytochrome c reduction method, and the degree of elastase release was measured using a p-nitroanilide-conjugated elastase-specific substrate. In addition, chemotaxis toward platelet activating factor/fMLP was determined with a modified Boyden chamber system.

RESULTS

The iPLA2γ-specific inhibitor reduced the fMLP/PMA-stimulated superoxide generation by 90% and 30%, respectively; in addition, the inhibitor completely blocked the fMLP/PMA-activated elastase release. However, the cPLA2-specific inhibitor did not abrogate these effects to any degree at all concentrations. Likewise, the inhibitor for iPLA2γ, but not iPLA2β or cPLA2, completely inhibited the platelet activating factor/fMLP-induced chemotaxis.

CONCLUSION

iPLA2 is involved in extracellular reactive oxygen species production, elastase release, and chemotaxis in response to well-defined stimuli. In addition, the ineffectiveness of the cPLA2 inhibitor suggests that AA may not be relevant to these cytotoxic functions.

Effects of coarse chalk dust particles (2.5-10 μm) on respiratory burst and oxidative stress in alveolar macrophages

The main aim of the present study was to examine in vitro responses of rat alveolar macrophages (AMs) exposed to coarse chalk dust particles (particulate matter in the size range 2.5-10 μm, PM(coarse)) by respiratory burst and oxidative stress. Chalk PM(coarse)-induced respiratory burst in AMs was measured by using a luminol-dependent chemiluminescence (CL) method. Also, the cell viability; lactate dehydrogenase (LDH) release; levels of cellular superoxide dismutase (SOD), catalase (CAT), glutathione (GSH), malondialdehyde (MDA), and acid phosphatase (ACP); plasma membrane ATPase; and extracellular nitric oxide (NO) level were determined 4 h following the treatment with the different dosages of chalk PM(coarse). The results showed that chalk PM(coarse) initiated the respiratory burst of AMs as indicated by strong CL, which was inhibited by diphenyleneiodonium chloride and L-N-nitro-L-arginine methyl ester hydrochloride. It suggested that chalk PM(coarse) induced the production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in AMs. This hypothesis was confirmed by the fact that chalk PM(coarse) resulted in a significant decrease of intracellular SOD, GSH, ACP, and ATPase levels and a notable increase of intracellular CAT, MDA content, and extracellular NO level, consequently leading to a decrease of the cell viability and a increase of LDH release. It was concluded that AMs exposed to chalk PM(coarse) can suffer from cytotoxicity which may be mediated by generation of excessive ROS/RNS. Graphical Abstract The possible mechanism of coarse chalk particles-induced adverse effects in AMs.

Measurement of respiratory burst products, released or retained, during activation of professional phagocytes

Activation of professional phagocytes, potent microbial killers of our innate immune system, is associated with an increase in cellular consumption of molecular oxygen (O2). The consumed O2 is utilized by an NADPH-oxidase to generate highly reactive oxygen species (ROS) by a one electron reduction, initially generating superoxide anion (O2 (-)) that then dismutates to hydrogen peroxide (H2O2). The ROS are strongly bactericidal molecules but may also cause tissue destruction, and are capable of driving immune competent cells of both the innate and the adaptive immune systems into apoptosis. The development of basic techniques to measure/quantify ROS generation by phagocytes during activation of the respiratory burst is of great importance, and a large number of methods have been used for this purpose. A selection of methods, including chemiluminescence amplified by luminol or isoluminol, the absorbance change following reduction of cytochrome c, and the fluorescence increase upon oxidation of PHPA, are described in detail in this chapter with special emphasis on how to distinguish between ROS that are released extracellularly, and those that are retained within intracellular organelles. These techniques can be valuable tools in research spanning from basic phagocyte biology to more clinically oriented research on innate immune mechanisms and inflammation.

Study of possible mechanisms involved in the inhibitory effects of coumarin derivatives on neutrophil activity

To specify the site of action of the synthetic coumarin derivatives 7-hydroxy-3-(4'-hydroxyphenyl) coumarin (HHC) and 7-hydroxy-3-(4'-hydroxyphenyl) dihydrocoumarin (HHDC), we evaluated their effects on extra- and intracellular reactive oxygen species (ROS) formation in phorbol-myristate-13-acetate (PMA) stimulated human neutrophils. We studied also the effects of HHC and HHDC on possible molecular mechanisms which participate in the activation of NADPH oxidase, that is, on PKC activity, on phosphorylation of some PKC isoforms (α, βII, and δ), and on phosphorylation of the NADPH oxidase subunit p40(phox). Without affecting cytotoxicity, both coumarines tested were effective inhibitors/scavengers of ROS produced by neutrophils on extracellular level. HHC markedly diminished oxidant production and also, intracellularly, decreased PKC activity and partly phosphorylation of PKCα, βII. On the other hand, we did not observe any effect of coumarin derivatives on phosphorylation of PKC δ and on phosphorylation of the NADPH oxidase subunit p40(phox), which were suggested to be involved in the PMA-dependent intracellular activation process. In agreement with our previous findings, we assume that the different molecular structures of HHC and HHDC with their different physicochemical and free radical scavenging characteristics are responsible for their diverse effects on the parameters tested.

The natural stilbenoid piceatannol decreases activity and accelerates apoptosis of human neutrophils: involvement of protein kinase C

Neutrophils are able to release cytotoxic substances and inflammatory mediators, which, along with their delayed apoptosis, have a potential to maintain permanent inflammation. Therefore, treatment of diseases associated with chronic inflammation should be focused on neutrophils; formation of reactive oxygen species and apoptosis of these cells represent two promising targets for pharmacological intervention. Piceatannol, a naturally occurring stilbenoid, has the ability to reduce the toxic action of neutrophils. This substance decreased the amount of oxidants produced by neutrophils both extra- and intracellularly. Radicals formed within neutrophils (fulfilling a regulatory role) were reduced to a lesser extent than extracellular oxidants, potentially dangerous for host tissues. Moreover, piceatannol did not affect the phosphorylation of p40^phox—a component of NADPH oxidase, responsible for the assembly of functional oxidase in intracellular (granular) membranes. The stilbenoid tested elevated the percentage of early apoptotic neutrophils, inhibited the activity of protein kinase C (PKC)—the main regulatory enzyme in neutrophils, and reduced phosphorylation of PKC isoforms α, βII, and δ on their catalytic region. The results indicated that piceatannol may be useful as a complementary medicine in states associated with persisting neutrophil activation and with oxidative damage of tissues.