Platelet-activating factor-mediated endosome formation causes membrane translocation of p67phox and p40phox that requires recruitment and activation of p38 MAPK, Rab5a, and phosphatidylinositol 3-kinase in human neutrophils

Silica nanoparticles induces sperm granuloma formation and blood-epididymal barrier disruption via the p38 MAPK pathway in mice

Previous researches have demonstrated that the silica nanoparticles (SiNPs), which are widely used in all aspects of life, are hazardous to the male reproductive system. However, the cellular and molecular mechanism underlying SiNPs toxicity to the epididymis remain unclear. In this present study, a total of 60 male mice were separated into 4 groups and then treated to SiNPs for 7 consecutive days at a dose of 0, 2.5, 10, and 20 mg/kg body weight. The results showed that SiNPs could alter the histological structure of epididymis and induce sperm granuloma formation, leading to decreased sperm quality and quantity. In addition, the ultrastructure and permeability of blood-epididymal barrier (BEB) were impaired after exposure to SiNPs, and a significant downregulation of integral membrane proteins at the BEB was detected. SiNPs were also found to raise the percentage of macrophages in the epithelium and interstitium of the epididymis, followed by increased expression of pro-inflammatory molecules including TNF α, IL-1β, and IL-6. Meanwhile, SiNPs induced oxidative stress in epididymis, as shown by the markedly elevated generation of reactive oxygen species (ROS) and malondialdehyde (MDA) and upregulated activity of superoxide dismutase (SOD). Further study showed that SiNPs activated the p38 MAPK signaling pathway, which accelerated clathrin-mediated endocytosis of integral membrane proteins and perturb vesicular trafficking. Taken together, exposure to SiNPs could induce sperm granuloma formation and impair the integrity of BEB in mice through activating the p38 MAPK pathway.

Phosphoinositol 3-kinase-driven NET formation involves different isoforms and signaling partners depending on the stimulus

Neutrophil extracellular traps (NETs) serve to immobilize and kill pathogens, but also can contribute to the progression of several inflammatory and auto-immune diseases, as well as cancer. Whence the importance of elucidating the mechanisms underlying NET formation. In this regard, the PI3K signaling pathway has been shown to be crucial; yet little is known about which of its components are involved. Here, we identified the PI3K isoforms and associated signaling partners that are mobilized in response to different classes of physiological NET inducers (inflammatory cytokines, growth factors, chemoattractants). NET generation was assessed by microscopy and signalling molecule activation by immunoblot using phospho-antibodies. Across the various stimuli, PI3Kα and PI3Kγ isoforms clearly contributed to NET induction, while the participation of other isoforms was stimulus-dependent. Some PI3K isoforms were also found to signal through Akt, the canonical downstream effector of PI3K, while others did not. Downstream of PI3K, mTOR and PLCγ2 were used by all stimuli to control NET generation. Conversely, the involvement of other kinases depended on the stimulus - both TNFα and GM-CSF relied on PDK1 and Akt; and both TNFα and fMLP additionally used S6K. We further established that all PI3K isoforms and downstream effectors act belatedly in NET generation, as reported previously for PI3K. Finally, we revisited the PI3K-PDK1-Akt signaling hierarchy in human neutrophils and again found stimulus-dependent differences. Our data uncover unsuspected complexity and redundancy in the signaling machinery controlling NET formation through the all-important PI3K pathway. Conserved signaling molecules represent therapeutic targets for pathologies involving NETs and in this regard, the existence of drugs currently used in the clinic or undergoing clinical trials (which target PI3K isoforms, mTOR or Akt), underscores the translational potential of our findings.

Succinate Activation of SUCNR1 Predisposes Severely Injured Patients to Neutrophil-mediated ARDS

OBJECTIVES

Identify the metabolites that are increased in the plasma of severely injured patients that developed ARDS versus severely injured patients that did not, and assay if these increased metabolites prime pulmonary sequestration of neutrophils (PMNs) and induce pulmonary sequestration in an animal model of ARDS. We hypothesize that metabolic derangement due to advanced shock in critically injured patients leads to the PMNs, which serves as the first event in the ARDS. Summary of Background Data: Intracellular metabolites accumulate in the plasma of severely injured patients.

METHODS

Untargeted metabolomics profiling of 67 critically injured patients was completed to establish a metabolic signature associated with ARDS development. Metabolites that significantly increased were assayed for PMN priming activity in vitro. The metabolites that primed PMNs were tested in a 2-event animal model of ARDS to identify a molecular link between circulating metabolites and clinical risk for ARDS.

RESULTS

After controlling for confounders, 4 metabolites significantly increased: creatine, dehydroascorbate, fumarate, and succinate in trauma patients who developed ARDS ( P < 0.05). Succinate alone primed the PMN oxidase in vitro at physiologically relevant levels. Intravenous succinate-induced PMN sequestration in the lung, a first event, and followed by intravenous lipopolysaccharide, a second event, resulted in ARDS in vivo requiring PMNs. SUCNR1 inhibition abrogated PMN priming, PMN sequestration, and ARDS. Conclusion: Significant increases in plasma succinate post-injury may serve as the first event in ARDS. Targeted inhibition of the SUCNR1 may decrease ARDS development from other disease states to prevent ARDS globally.

Glutamyl-prolyl-tRNA synthetase 1 coordinates early endosomal anti-inflammatory AKT signaling

The AKT signaling pathway plays critical roles in the resolution of inflammation. However, the underlying mechanisms of anti-inflammatory regulation and signal coordination remain unclear. Here, we report that anti-inflammatory AKT signaling is coordinated by glutamyl-prolyl-tRNA synthetase 1 (EPRS1). Upon inflammatory activation, AKT specifically phosphorylates Ser999 of EPRS1 in the cytoplasmic multi-tRNA synthetase complex, inducing release of EPRS1. EPRS1 compartmentalizes AKT to early endosomes via selective binding to the endosomal membrane lipid phosphatidylinositol 3-phosphate and assembles an AKT signaling complex specific for anti-inflammatory activity. These events promote AKT activation-mediated GSK3β phosphorylation, which increase anti-inflammatory cytokine production. EPRS1-deficient macrophages do not assemble the early endosomal complex and consequently exacerbate inflammation, decreasing the survival of EPRS1-deficient mice undergoing septic shock and ulcerative colitis. Collectively, our findings show that the housekeeping protein EPRS1 acts as a mediator of inflammatory homeostasis by coordinating compartment-specific AKT signaling.

Neutrophil and Nanoparticles Delivery to Tumor: Is It Going to Carry That Weight?

The application of cell carriers for transporting nanodrugs to the tumor draws much attention as the alternative to the passive drug delivery. In this concept, the neutrophil (NΦ) is of special interest as this cell is able to uptake nanoparticles (NPs) and cross the vascular barrier in response to tumor signaling. There is a growing body of literature describing NP-NΦ interactions in vitro and in vivo that demonstrates the opportunity of using these cells to improve the efficacy of cancer therapy. However, a number of conceptual and technical issues need to be resolved for translating the technology into clinics. The current review summarizes the recent advances and challenges associated with NP-NΦ interactions, with the special focus on the complex interplay between the NP internalization pathways and the modulation of NΦ activity, and its potential consequences for nanodrug delivery.

Regulation of platelet-activating factor-induced interleukin-8 expression by protein tyrosine phosphatase 1B

Background

Platelet-activating factor (PAF) is a potent lipid mediator whose involvement in the onset and progression of atherosclerosis is mediated by, among others, the modulation of cytokine expression patterns. The presence of multiple potential protein-tyrosine phosphatase (PTP) 1B substrates in PAF receptor signaling pathways brought us to investigate its involvement in PAF-induced cytokine expression in monocyte-derived dendritic cells (Mo-DCs) and to study the pathways involved in this modulation.

Methods

We used in-vitro-matured human dendritic cells and the HEK-293 cell line in our studies. PTP1B inhibition was though siRNAs and a selective inhibitor. Cytokine expression was studied with RT-PCR, luciferase assays and ELISA. Phosphorylation status of kinases and transcription factors was studied with western blotting.

Results

Here, we report that PTP1B was involved in the modulation of cytokine expression in PAF-stimulated Mo-DCs. A study of the down-regulation of PAF-induced IL-8 expression, by PTP1B, showed modulation of PAF-induced transactivation of the IL-8 promoter which was dependent on the presence of the C/EBPß -binding site. Results also suggested that PTP1B decreased PAF-induced IL-8 production by a glycogen synthase kinase (GSK)-3-dependent pathway via activation of the Src family kinases (SFK). These kinases activated an unidentified pathway at early stimulation times and the PI3K/Akt signaling pathway in a later phase. This change in GSK-3 activity decreased the C/EBPß phosphorylation levels of the threonine 235, a residue whose phosphorylation is known to increase C/EBPß transactivation potential, and consequently modified IL-8 expression.

Conclusion

The negative regulation of GSK-3 activity by PTP1B and the consequent decrease in phosphorylation of the C/EBPß transactivation domain could be an important negative feedback loop by which cells control their cytokine production after PAF stimulation.

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0334-6) contains supplementary material, which is available to authorized users.

Disruption of transient receptor potential melastatin 2 decreases elastase release and bacterial clearance in neutrophils

Elastase released by neutrophils is critical for eliminating Gram-negative bacteria. Ca²⁺ influx plays a key role in elastase release and bacterial clearance in neutrophils. Transient receptor potential melastatin 2 (TRPM2) is a Ca²⁺-permeable cation channel highly expressed in neutrophils. Here, we explore the role and possible mechanism of TRPM2 in bacterial clearance in TRPM2 knockout (TRPM2-KO) mice neutrophils. After exposure to Escherichia coli, TRPM2–KO bone marrow neutrophils (BMNs) had increased bacterial burden and decreased elastase release. The same was observed for septic TRPM2-KO mice which also had decreased survival rate. After stimulation with chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP), elastase release was lower in TRPM2-KO BMNs than in wild type (WT) BMNs. Pre-treatment of WT BMNs with p38 MAPK inhibitor reduced fMLP-induced elastase release. Compared with WT BMNs, TRPM2-KO BMNs had decreased p38 MAPK phosphorylation after fMLP stimulation. Removal of extracellular Ca²⁺ reduced fMLP-induced p38 MAPK phosphorylation and elastase release. The concentration of intracellular Ca²⁺ decreased in TRPM2-KO BMNs compared with WT BMNs after fMLP treatment. Hence, TRPM2 plays an important role in bacterial clearance in neutrophils, possibly by regulating elastase release. TRPM2-mediated Ca²⁺ influx regulates elastase release partially via p38 MAPK phosphorylation in neutrophils.

Priming and de-priming of neutrophil responses in vitro and in vivo

The activation status of neutrophils can cycle from basal through primed to fully activated ("green-amber-red"), and at least in vitro, primed cells can spontaneously revert to a near basal phenotype. This broad range of neutrophil responsiveness confers extensive functional flexibility, allowing neutrophils to respond rapidly and appropriately to varied and evolving threats throughout the body. Primed and activated cells display dramatically enhanced bactericidal capacity (including augmented respiratory burst activity, degranulation and longevity), but this enhancement also confers the capacity for significant unintended tissue injury. Neutrophil priming and its consequences have been associated with adverse outcomes in a range of disease states, hence understanding the signalling processes that regulate the transition between basal and primed states (and back again) may offer new opportunities for therapeutic intervention in pathological settings. A wide array of host- and pathogen-derived molecules is able to modulate the functional status of these versatile cells. Reflecting this extensive repertoire of potential mediators, priming can be established by a range of signalling pathways (including mitogen-activated protein kinases, phosphoinositide 3-kinases, phospholipase D and calcium transients) and intracellular processes (including endocytosis, vesicle trafficking and the engagement of adhesion molecules). The signalling pathways engaged, and the exact cellular phenotype that results, vary according to the priming agent(s) to which the neutrophil is exposed and the precise environmental context. Herein we describe the signals that establish priming (in particular for enhanced respiratory burst, degranulation and prolonged lifespan) and describe the recently recognised process of de-priming, correlating in vitro observations with in vivo significance.

Supernatants and lipids from stored red blood cells activate pulmonary microvascular endothelium through the BLT2 receptor and protein kinase C activation

BACKGROUND

Although transfusion is a lifesaving intervention, it may be associated with significant morbidity in injured patients. We hypothesize that stored red blood cells (RBCs) induce proinflammatory activation of human pulmonary microvascular endothelial cells (HMVECs) resulting in neutrophil (PMN) adhesion and predisposition to acute lung injury (ALI).

STUDY DESIGN AND METHODS

Ten units of RBCs were collected; 50% (by weight) were leukoreduced (LR-RBCs) and the remainder was unmodified and stored in additive solution-5 (AS-5). An additional 10 units of RBCs were collected, leukoreduced, and stored in AS-3. HMVECs were incubated with [10%-40%]_FINAL of the supernatants on Day (D)1 to D42 of storage, lipid extracts, and purified lipids. Endothelial surface expression of intercellular adhesion molecule-1 (ICAM-1), interleukin (IL)-8 release, and PMN adhesion to HMVECs were measured. HMVEC signaling via the BLT2 receptor was evaluated. Supernatants and lipids were also employed as the first event in a two-event model of ALI.

RESULTS

The supernatants [10%-40%]_FINAL from D21 LR-RBCs and D42 RBCs and LR-RBCs and the lipids from D42 stored in AS-5 induced increased ICAM-1 surface expression on endothelium, IL-8 release, and PMN adhesion. In addition, the supernatants [20%-40%]_FINAL from D21 and D42 RBCs in AS-5 also increased endothelial surface expression of ICAM-1. D42 supernatants and lipids also caused coprecipitation of β-arrestin-1 with BLT2, protein kinase C (PKC)β_I , and PKCδ and served as the first event in a two-event rodent model of ALI.

CONCLUSION

Lipids that accumulate during RBC storage activate endothelium and predispose to ALI, which may explain some of the adverse events associated with the transfusion of critically injured patients.

Endocytosis is required for exocytosis and priming of respiratory burst activity in human neutrophils

OBJECTIVE AND DESIGN

Neutrophil generation of reactive oxygen species (ROS) is enhanced by exposure to pro-inflammatory agents in a process termed priming. Priming is depending on exocytosis of neutrophil granules and p47^phox phosphorylation-dependent translocation of cytosolic NADPH oxidase components. Clathrin-mediated endocytosis was recently reported to be necessary for priming, but the mechanism linking endocytosis to priming was not identified. The present study examined the hypothesis that endocytosis regulates neutrophil priming by controlling granule exocytosis.

MATERIALS AND METHODS

Clathrin-mediated endocytosis by isolated human neutrophils was inhibited by chlorpromazine, monodansylcadaverine, and sucrose. Exocytosis of granule subsets was measured as release of granule components by ELISA or chemiluminescence. ROS generation was measured as extracellular release of superoxide as reduction of ferrocytochrome c. p38 MAPK activation and p47^phox phosphorylation were measured by immunoblot analysis. Statistical analysis was performed using a one-way ANOVA with the Tukey-Kramer multiple-comparison test.

RESULTS

Inhibition of endocytosis prevented priming of superoxide release by TNFα and inhibited TNFα stimulation and priming of exocytosis of all four granule subsets. Inhibition of endocytosis did not reduce TNFα-stimulated p38 MAPK activation or p47^phox phosphorylation. Inhibition of NADPH oxidase activity blocked TNFα stimulation of secretory vesicle and gelatinase granule exocytosis.

CONCLUSIONS

Endocytosis is linked to priming of respiratory burst activity through ROS-mediated control of granule exocytosis.

Anti-inflammatory signaling actions of electrophilic nitro-arachidonic acid in vascular cells and astrocytes

Nitrated derivatives of unsaturated fatty acids (nitro-fatty acids) are being formed and detected in human plasma, cell membranes and tissue, triggering signaling cascades via covalent and reversible post-translational modifications of nucleophilic amino acids in transcriptional regulatory proteins. Arachidonic acid (AA) represents a precursor of potent signaling molecules, i.e., prostaglandins and thromboxanes through enzymatic and non-enzymatic oxidative pathways. Arachidonic acid can be nitrated by reactive nitrogen species leading to the formation of nitro-arachidonic acid (NO₂-AA). A critical issue is the influence of NO₂-AA on prostaglandin endoperoxide H synthases, modulating inflammatory processes through redirection of AA metabolism and signaling. In this prospective article, we describe the key chemical and biochemical actions of NO₂-AA in vascular and astrocytes. This includes the ability of NO₂-AA to mediate unique redox signaling anti-inflammatory actions along with its therapeutic potential.

LysoPCs induce Hck- and PKCδ-mediated activation of PKCγ causing p47phox phosphorylation and membrane translocation in neutrophils

Lysophosphatidylcholines (lysoPCs) are effective polymorphonuclear neutrophil (PMN) priming agents implicated in transfusion-related acute lung injury (TRALI). LysoPCs cause ligation of the G2A receptor, cytosolic Ca²⁺ flux, and activation of Hck. We hypothesize that lysoPCs induce Hck-dependent activation of protein kinase C (PKC), resulting in phosphorylation and membrane translocation of 47 kDa phagocyte oxidase protein (p47^phox). PMNs, human or murine, were primed with lysoPCs and were smeared onto slides and examined by digital microscopy or separated into subcellular fractions or whole-cell lysates. Proteins were immunoprecipitated or separated by polyacrylamide gel electrophoresis and immunoblotted for proteins of interest. Wild-type (WT) and PKCγ knockout (KO) mice were used in a 2-event model of TRALI. LysoPCs induced Hck coprecipitation with PKCδ and PKCγ and the PKCδ:PKCγ complex also had a fluorescence resonance energy transfer (FRET)⁺ interaction with lipid rafts and Wiskott-Aldrich syndrome protein family verprolin-homologous protein 2 (WAVE2). PKCγ then coprecipitated with p47^phox Immunoblotting, immunoprecipitation (IP), specific inhibitors, intracellular depletion of PKC isoforms, and PMNs from PKCγ KO mice demonstrated that Hck elicited activation/Tyr phosphorylation (Tyr311 and Tyr525) of PKCδ, which became Thr phosphorylated (Thr507). Activated PKCδ then caused activation of PKCγ, both by Tyr phosphorylation (Τyr514) and Ser phosphorylation, which induced phosphorylation and membrane translocation of p47^phox In PKCγ KO PMNs, lysoPCs induced Hck translocation but did not evidence a FRET⁺ interaction between PKCδ and PKCγ nor prime PMNs. In WT mice, lysoPCs served as the second event in a 2-event in vivo model of TRALI but did not induce TRALI in PKCγ KO mice. We conclude that lysoPCs prime PMNs through Hck-dependent activation of PKCδ, which stimulates PKCγ, resulting in translocation of phosphorylated p47^phox.

Bacterial exploitation of phosphorylcholine mimicry suppresses inflammation to promote airway infection

Regulation of neutrophil activity is critical for immune evasion among extracellular pathogens, yet the mechanisms by which many bacteria disrupt phagocyte function remain unclear. Here, we have shown that the respiratory pathogen Streptococcus pneumoniae disables neutrophils by exploiting molecular mimicry to degrade platelet-activating factor (PAF), a host-derived inflammatory phospholipid. Using mass spectrometry and murine upper airway infection models, we demonstrated that phosphorylcholine (ChoP) moieties that are shared by PAF and the bacterial cell wall allow S. pneumoniae to leverage a ChoP-remodeling enzyme (Pce) to remove PAF from the airway. S. pneumoniae-mediated PAF deprivation impaired viability, activation, and bactericidal capacity among responding neutrophils. In the absence of Pce, neutrophils rapidly cleared S. pneumoniae from the airway and impeded invasive disease and transmission between mice. Abrogation of PAF signaling rendered Pce dispensable for S. pneumoniae persistence, reinforcing that this enzyme deprives neutrophils of essential PAF-mediated stimulation. Accordingly, exogenous activation of neutrophils overwhelmed Pce-mediated phagocyte disruption. Haemophilus influenzae also uses an enzyme, GlpQ, to hydrolyze ChoP and subvert PAF function, suggesting that mimicry-driven immune evasion is a common paradigm among respiratory pathogens. These results identify a mechanism by which shared molecular structures enable microbial enzymes to subvert host lipid signaling, suppress inflammation, and ensure bacterial persistence at the mucosa.

Clathrin complexes with the inhibitor kappa B kinase signalosome: imaging the interactome

Many receptors involved with innate immunity activate the inhibitor kappa B kinase signalosome (IKK). The active complex appears to be assembled from the two kinase units, IKKα and IKKβ with the regulatory protein NEMO. Because we previously found that RNA silencing of clathrin heavy chains (CHC), in transformed human lung pneumocytes (A549), decreased TNFα‐induced signaling and phosphorylation of inhibitor kappa B (IκB), we hypothesized that CHC forms cytoplasmic complexes with members of the IKK signalosome. Widely available antibodies were used to immunoprecipitate IKKα and NEMO interactomes. Analysis of the affinity interactomes by mass spectrometry detected clathrin with both baits with high confidence. Using the same antibodies for indirect digital immunofluorescence microscopy and FRET, the CHC–IKK complexes were visualized together with NEMO or HSP90. The natural variability of protein amounts in unsynchronized A549 cells was used to obtain statistical correlation for several complexes, at natural levels and without invasive labeling. Analyses of voxel numbers indicated that: (i) CHC–IKK complexes are not part of the IKK signalosome itself but, likely, precursors of IKK–NEMO complexes. (ii) CHC–IKKβ complexes may arise from IKKβ–HSP90 complexes.

Clathrin forms complexes with IKKa, IKKb, and NEMO, but apparently not the canonical signalosome. These complexes are identified, for the first time, by affinity proteomics and triple FRET without altering molecular structure.

MicroRNA-451 down-regulates neutrophil chemotaxis via p38 MAPK

OBJECTIVE

MicroRNAs (miRNAs) are endogenous small noncoding RNAs that regulate the activity of target messenger RNAs (mRNAs) and cellular processes. MicroRNA-451 (miR-451) is one of the miRNAs that is conserved perfectly among vertebrates, and it regulates cell proliferation, invasion, and apoptosis in tumors. However, the role of miR-451 in autoimmune arthritis is unknown. This study was undertaken to identify the role of miR-451 in autoimmune arthritis.

METHODS

We compared the expression of miR-451 in neutrophils from patients with rheumatoid arthritis (RA) and healthy controls. We also evaluated the role of miR-451 in neutrophil chemotaxis in vivo and in vitro using murine neutrophils. The regulation of p38 MAPK by miR-451 was assessed. Double-stranded miR-451 was administered to SKG mice, the arthritis score was determined, and histologic examination was performed.

RESULTS

MicroRNA-451 expression in neutrophils isolated from patients with RA was lower than that in healthy controls. Systemic administration of miR-451 significantly disrupted the infiltration of neutrophils in an air-pouch model of local inflammation without affecting apoptosis of neutrophils. Overexpression of miR-451 significantly suppressed the migration of neutrophils to fMLP. We identified CPNE3 and Rab5a as direct targets of miR-451. Overexpression of miR-451 suppressed the phosphorylation of p38 MAPK via 14-3-3ζ, a known target of miR-451, and Rab5a. In SKG mice, miR-451 treatment reduced the severity of arthritis and the number of infiltrating cells.

CONCLUSION

These results suggest that miR-451 suppresses neutrophil chemotaxis via p38 MAPK and is a potential target in the treatment of RA.

Platelet-activating factor antagonists enhance intracellular degradation of amyloid-β42 in neurons via regulation of cholesterol ester hydrolases

INTRODUCTION

The progressive dementia that is characteristic of Alzheimer's disease is associated with the accumulation of amyloid-beta (Aβ) peptides in extracellular plaques and within neurons. Aβ peptides are targeted to cholesterol-rich membrane micro-domains called lipid rafts. Observations that many raft proteins undertake recycling pathways that avoid the lysosomes suggest that the accumulation of Aβ in neurons may be related to Aβ targeting lipid rafts. Here we tested the hypothesis that the degradation of Aβ by neurons could be increased by drugs affecting raft formation.

METHODS

Primary neurons were incubated with soluble Aβ preparations. The amounts of Aβ42 in neurons or specific cellular compartments were measured by enzyme-linked immunosorbent assay. The effects of drugs on the degradation of Aβ42 were studied.

RESULTS

Aβ42 was targeted to detergent-resistant, low-density membranes (lipid rafts), trafficked via a pathway that avoided the lysosomes, and was slowly degraded by neurons (half-life was greater than 5 days). The metabolism of Aβ42 was sensitive to pharmacological manipulation. In neurons treated with the cholesterol synthesis inhibitor squalestatin, less Aβ42 was found within rafts, greater amounts of Aβ42 were found in lysosomes, and the half-life of Aβ42 was reduced to less than 24 hours. Treatment with phospholipase A2 inhibitors or platelet-activating factor (PAF) antagonists had the same effects on Aβ42 metabolism in neurons as squalestatin. PAF receptors were concentrated in the endoplasmic reticulum (ER) along with enzymes that constitute the cholesterol ester cycle. The addition of PAF to ER membranes triggered activation of cholesterol ester hydrolases and the release of cholesterol from stores of cholesterol esters. An inhibitor of cholesterol ester hydrolases (diethylumbelliferyl phosphate) also increased the degradation of Aβ42 in neurons.

CONCLUSIONS

We conclude that the targeting of Aβ42 to rafts in normal cells is a factor that affects its degradation. Critically, pharmacological manipulation of neurons can significantly increase Aβ42 degradation. These results are consistent with the hypothesis that the Aβ-induced production of PAF controls a cholesterol-sensitive pathway that affects the cellular localization and hence the fate of Aβ42 in neurons.

Expression of p38 MAPK in Acute Lung Injury Induced by LPS in Mice

Objective To investigate the expression and roles of p38 mitogen-activated protein kinase (p38 MAPK) in LPS-induced acute lung injury (ALI) in mice. Methods The ALI mice models were set up by intraperineal injection of lipopolysaccharide (LPS). The expressions of p38 MAPK in lung tissues were detected by immunohistochemistry and Western-blot. Results The positive expressions of p38 MAPK distribute mainly in infiltrative inflammatory cells, epithelial cells and endothelial cells. And the level of expression of phosphated p38 MAPK in ALI group were higher obviously than that in the control group, and it reached a peak after two hours. Conclusion p38 MAPK signaling pathway was triggered by ALI induced by endotoxin.

The subcellular localization of the receptor for platelet-activating factor in neutrophils affects signaling and activation characteristics

The localization in neutrophils, of the receptor for platelet-activating factor (PAFR), has been determined using subcellular fractionation and a receptor mobilization protocol. We show that the PAFR is expressed primarily in the plasma membrane. Although activation of neutrophils by PAF induces responses typical also of agonists that bind the formyl peptide receptors (FPR), known to be stored in mobilizable organelles, some quantitative as well as qualitative differences were observed when neutrophils were activated through these receptors. PAF is equipotent to fMLF (high affinity agonist for FPR1) to cleave off L-selectin and to induce granule/vesicle secretion but is more potent than fMLF to induce a rise in intracellular Ca²⁺. Similar to fMLF, PAF induced also a robust release of reactive oxygen species, but with higher EC₅₀ value and was less sensitive to a PI3K inhibitor compared to the fMLF response. Despite the lack of a granule localized storage pool of receptors, the PAF-induced superoxide production could be primed; receptor mobilization was, thus, not required for priming of the PAF response. The desensitized PAFR could not be reactivated, suggesting that distinct signaling pathways are utilized for termination of the responses triggered through FPR1 and PAFR.

Nitroarachidonic acid prevents NADPH oxidase assembly and superoxide radical production in activated macrophages

Nitration of arachidonic acid (AA) to nitroarachidonic acid (AANO2) leads to anti-inflammatory intracellular activities during macrophage activation. However, less is known about the capacity of AANO2 to regulate the production of reactive oxygen species under proinflammatory conditions. One of the immediate responses upon macrophage activation involves the production of superoxide radical (O2(•-)) due to the NADPH-dependent univalent reduction of oxygen to O2(•-) by the phagocytic NADPH oxidase isoform (NOX2), the activity of NOX2 being the main source of O2(•-) in monocytes/macrophages. Because the NOX2 and AA pathways are connected, we propose that AANO2 can modulate macrophage activation by inhibiting O2(•-) formation by NOX2. When macrophages were activated in the presence of AANO2, a significant inhibition of NOX2 activity was observed as evaluated by cytochrome c reduction, luminol chemiluminescence, Amplex red fluorescence, and flow cytometry; this process also occurs under physiological mimic conditions within the phagosomes. AANO2 decreased O2(•-) production in a dose- (IC50=4.1±1.8 μM AANO2) and time-dependent manner. The observed inhibition was not due to a decreased phosphorylation of the cytosolic subunits (e.g., p40(phox) and p47(phox)), as analyzed by immunoprecipitation and Western blot. However, a reduction in the migration to the membrane of p47(phox) was obtained, suggesting that the protective actions involve the prevention of the correct assembly of the active enzyme in the membrane. Finally, the observed in vitro effects were confirmed in an in vivo inflammatory model, in which subcutaneous injection of AANO2 was able to decrease NOX2 activity in macrophages from thioglycolate-treated mice.

β-Arrestins in the immune system

β-Arrestins regulate G protein-coupled receptors through receptor desensitization while also acting as signaling scaffolds to facilitate numerous effector pathways. Recent studies have provided evidence that β-arrestins play a key role in inflammatory responses. Here, we summarize these advances on the roles of β-arrestins in immune regulation and inflammatory responses under physiological and pathological conditions, with an emphasis on translational implications of β-arrestins on human diseases.

Regulation of alveolar macrophage p40phox: hierarchy of activating kinases and their inhibition by PGE2

PGE(2), produced in the lung during infection with microbes such as Klebsiella pneumoniae, inhibits alveolar macrophage (AM) antimicrobial functions by preventing H(2)O(2) production by NADPH oxidase (NADPHox). Activation of the NADPHox complex is poorly understood in AMs, although in neutrophils it is known to be mediated by kinases including PI3K/Akt, protein kinase C (PKC) δ, p21-activated protein kinase (PAK), casein kinase 2 (CK2), and MAPKs. The p40phox cytosolic subunit of NADPHox has been recently recognized to function as a carrier protein for other subunits and a positive regulator of oxidase activation, a role previously considered unique to another subunit, p47phox. The regulation of p40phox remains poorly understood, and the effect of PGE(2) on its activation is completely undefined. We addressed these issues in rat AMs activated with IgG-opsonized K. pneumoniae. The kinetics of kinase activation and the consequences of kinase inhibition and silencing revealed a critical role for a PKCδ-PAK-class I PI3K/Akt1 cascade in the regulation of p40phox activation upon bacterial challenge in AMs; PKCα, ERK, and CK2 were not involved. PGE(2) inhibited the activation of p40phox, and its effects were mediated by protein kinase A type II, were independent of interactions with anchoring proteins, and were directed at the distal class I PI3K/Akt1 activation step. Defining the kinases that control AM p40phox activation and that are the targets for inhibition by PGE(2) provides new insights into immunoregulation in the infected lung.

CD13 regulates dendritic cell cross-presentation and T cell responses by inhibiting receptor-mediated antigen uptake

Dendritic cell (DC) Ag cross-presentation is generally associated with immune responses to tumors and viral Ags, and enhancement of this process is a focus of tumor vaccine design. In this study, we found that the myeloid cell surface peptidase CD13 is highly and specifically expressed on the subset of DCs responsible for cross-presentation, the CD8(+) murine splenic DCs. In vivo studies indicated that lack of CD13 significantly enhanced T cell responses to soluble OVA Ag, although development, maturation, and Ag processing and presentation of DCs are normal in CD13KO mice. In vitro studies showed that CD13 regulates receptor-mediated, dynamin-dependent endocytosis of Ags such as OVA and transferrin but not fluid-phase or phagocytic Ag uptake. CD13 and Ag are cointernalized in DCs, but CD13 did not coimmunoprecipitate with Ag receptors, suggesting that CD13 does not control internalization of specific receptors but regulates endocytosis at a more universal level. Mechanistically, we found that phosphorylation of the endocytic regulators p38MAPK and Akt was dysregulated in CD13KO DCs, and blocking of these kinases perturbed CD13-dependent endocytic uptake. Therefore, CD13 is a novel endocytic regulator that may be exploited to enhance Ag uptake and T cell activation to improve the efficacy of tumor-targeted vaccines.

Endotoxin priming of neutrophils requires endocytosis and NADPH oxidase-dependent endosomal reactive oxygen species

NADPH oxidase 2 (Nox2)-generated reactive oxygen species (ROS) are critical for neutrophil (polymorphonuclear leukocyte (PMN)) microbicidal function. Nox2 also plays a role in intracellular signaling, but the site of oxidase assembly is unknown. It has been proposed to occur on secondary granules. We previously demonstrated that intracellular NADPH oxidase-derived ROS production is required for endotoxin priming. We hypothesized that endotoxin drives Nox2 assembly on endosomes. Endotoxin induced ROS generation within an endosomal compartment as quantified by flow cytometry (dihydrorhodamine 123 and Oxyburst Green). Inhibition of endocytosis by the dynamin-II inhibitor Dynasore blocked endocytosis of dextran, intracellular generation of ROS, and priming of PMN by endotoxin. Confocal microscopy demonstrated a ROS-containing endosomal compartment that co-labeled with gp91(phox), p40(phox), p67(phox), and Rab5, but not with the secondary granule marker CD66b. To further characterize this compartment, PMNs were fractionated by nitrogen cavitation and differential centrifugation, followed by free flow electrophoresis. Specific subfractions made superoxide in the presence of NADPH by cell-free assay (cytochrome c). Subfraction content of membrane and cytosolic subunits of Nox2 correlated with ROS production. Following priming, there was a shift in the light membrane subfractions where ROS production was highest. CD66b was not mobilized from the secondary granule compartment. These data demonstrate a novel, nonphagosomal intracellular site for Nox2 assembly. This compartment is endocytic in origin and is required for PMN priming by endotoxin.

Lysophosphatidylcholines activate G2A inducing G(αi)₋₁-/G(αq/)₁₁- Ca²(+) flux, G(βγ)-Hck activation and clathrin/β-arrestin-1/GRK6 recruitment in PMNs

Lyso-PCs (lysophosphatidylcholines) are a mixture of lipids that accumulate during storage of cellular blood components, have been implicated in TRALI (transfusion-related acute lung injury) and directly affect the physiology of neutrophils [PMNs (polymorphonuclear leucocytes)]. Because the G2A receptor, expressed on PMNs, has been reported to recognize lyso-PCs, we hypothesize that lyso-PC activation of G2A causes the increases in cytosolic Ca²(+) via release of G(α) and G(βγ) subunits, kinase activation, and the recruitment of clathrin, β-arrestin-1 and GRK6 (G-protein receptor kinase 6) to G2A for signal transduction. PMNs were isolated by standard techniques, primed with lyso-PCs for 5-180 s, and lysed for Western blot analysis, immunoprecipitation or subcellular fractionation, or fixed and smeared on to slides for digital microscopy. The results demonstrated that lyso-PCs cause rapid activation of the G2A receptor through S-phosphorylation and internalization resulting in G(αi)₋₁ and G(αq/)₁₁ release leading to increases in cytosolic Ca²(+), which was inhibited by an antibody to G2A or intracellular neutralization of these subunits. Lyso-PCs also caused the release of the G(βγ) subunit which demonstrated a physical interaction (FRET+) with activated Hck (haemopoietic cell kinase; Tyr⁴¹¹). Moreover, G2A recruited clathrin, β-arrestin-1 and GRK6: clathrin is important for signal transduction, GRK6 for receptor de-sensitization, and β-arrestin-1 both propagates and terminates signals. We conclude that lyso-PC activation of G2A caused release of G(αi)₋₁, G(αq/)₁₁ and G(βγ), resulting in cytosolic Ca²(+) flux, Hck activation, and recruitment of clathrin, β-arrestin-1 and GRK6.

Early endosomal antigen 1 (EEA1) is an obligate scaffold for angiotensin II-induced, PKC-alpha-dependent Akt activation in endosomes

Akt/protein kinase B (PKB) activation/phosphorylation by angiotensin II (Ang II) is a critical signaling event in hypertrophy of vascular smooth muscle cells (VSMCs). Conventional wisdom asserts that Akt activation occurs mainly in plasma membrane domains. Recent evidence that Akt activation may take place within intracellular compartments challenges this dogma. The spatial identity and mechanistic features of these putative signaling domains have not been defined. Using cell fractionation and fluorescence methods, we demonstrate that the early endosomal antigen-1 (EEA1)-positive endosomes are a major site of Ang II-induced Akt activation. Akt moves to and is activated in EEA1 endosomes. The expression of EEA1 is required for phosphorylation of Akt at both Thr-308 and Ser-473 as well as for phosphorylation of its downstream targets mTOR and S6 kinase, but not for Erk1/2 activation. Both Akt and phosphorylated Akt (p-Akt) interact with EEA1. We also found that PKC-α is required for organizing Ang II-induced, EEA1-dependent Akt phosphorylation in VSMC early endosomes. EEA1 expression enables PKC-α phosphorylation, which in turn regulates Akt upstream signaling kinases, PDK1 and p38 MAPK. Our results indicate that PKC-α is a necessary regulator of EEA1-dependent Akt signaling in early endosomes. Finally, EEA1 down-regulation or expression of a dominant negative mutant of PKC-α blunts Ang II-induced leucine incorporation in VSMCs. Thus, EEA1 serves a novel function as an obligate scaffold for Ang II-induced Akt activation in early endosomes.

Inhibition of phospholipase A2 increased the removal of the prion derived peptide PrP82-146 from cultured neurons

The prion diseases are characterised by the formation of the disease-associated isoform of the prion protein (PrP(Sc)) and the production of disease-related peptides. The prion derived peptide PrP82-146 bound readily to cortical neurons and was found within detergent resistant membranes that are commonly called lipid rafts. It was not found within lysosomes and the slow degradation of PrP82-146 resulted in a half-life of approximately 5 days. In cortical neurons pre-treated with phospholipase A(2) (PLA(2)) inhibitors (AACOCF(3) or MAFP) less PrP82-146 entered lipid rafts, more PrP82-146 was found within lysosomes and the half-life of PrP82-146 was reduced to 24 h. Similarly, pre-treatment of neurons with platelet-activating factor (PAF) receptor antagonists (Hexa-PAF and ginkgolide B) increased the entry of PrP82-146 into lysosomes and reduced its half-life. Furthermore, the addition of PAF reversed the effects of PLA(2) inhibitors on PrP82-146 trafficking. PAF controlled the amount of cholesterol in cell membranes and the effects of PAF receptor antagonists on the trafficking of PrP82-146 were reversed by the addition of cholesterol. We conclude that activation of PLA(2) and the production of PAF control a cholesterol-sensitive pathway that affects the cellular localisation and hence the fate of PrP82-146 in neurons.

GnRH signaling, the gonadotrope and endocrine control of fertility

Mammalian reproductive cycles are controlled by an intricate interplay between the hypothalamus, pituitary and gonads. Central to the function of this axis is the ability of the pituitary gonadotrope to appropriately respond to stimulation by gonadotropin-releasing hormone (GnRH). This review focuses on the role of cell signaling and in particular, mitogen-activated protein kinase (MAPK) activities regulated by GnRH that are necessary for normal fertility. Recently, new mouse models making use of conditional gene deletion have shed new light on the relationships between GnRH signaling and fertility in both male and female mice. Within the reproductive axis, GnRH signaling is initiated through discrete membrane compartments in which the receptor resides leading to the activation of the extracellular signal-regulated kinases (ERKs 1/2). As defined by gonadotrope-derived cellular models, the ERKs appear to play a central role in the regulation of a cohort of immediate early genes that regulate the expression of late genes that, in part, define the differentiated character of the gonadotrope. Recent data would suggest that in vivo, conditional, pituitary-specific disruption of ERK signaling by GnRH leads to a gender-specific perturbation of fertility. Double ERK knockout in the anterior pituitary leads to female infertility due to LH biosynthesis deficiency and a failure in ovulation. In contrast, male mice are modestly LH deficient; however, this does not have an appreciable impact on fertility.

Tumor necrosis factor-alpha causes release of cytosolic interleukin-18 from human neutrophils

Neutrophils (PMNs) are a vital part of host defense and are the principal leukocyte in innate immunity. Interleukin (IL)-18 is a proinflammatory cytokine with roles in both innate and adaptive immunity. We hypothesize that PMNs contain preformed IL-18, which is released in response to specific inflammatory stimuli. Isolated PMNs were stimulated with a battery of chemoattractants (5 min to 24 h), and IL-18 release was measured. PMNs were also separated into subcellular fractions and immunoblotted with antibodies against IL-18 or were fixed and probed with antibodies to IL-18 as well as to the contents of granules, intracellular organelles, and filamentous actin (F-actin), incubated with fluorescent secondary antibodies, and examined by digital microscopy. Quiescent PMNs contained IL-18 in the cytoplasm, associated with F-actin, as determined by positive fluorescence resonance energy transfer (FRET+). In turn, TNF-alpha stimulation disrupted the association of IL-18 with F-actin, induced a FRET+ interaction of IL-18 with lipid rafts, and elicited IL-18 release. Manipulation of F-actin status confirmed the relationship between IL-18 and F-actin in resting PMNs. Consequently, incubation with monomeric IL-18 binding protein inhibited TNF-alpha-mediated priming of the PMN oxidase. We conclude that human PMNs contain IL-18 associated with F-actin in the cytoplasm and TNF-alpha stimulation causes dissociation of IL-18 from F-actin, association with lipid rafts, and extracellular release. Extracellular IL-18 participates in TNF-alpha priming of the PMN oxidase as demonstrated by inhibition with the IL-18 binding protein.

Inhibitors of TRP channels reveal stimulus-dependent differential activation of Ca2+ influx pathways in human neutrophil granulocytes

A pharmacological characterization of Ca(2+) influx pathways in neutrophil granulocytes is problematic because of the lack of specific inhibitors. The activation of transient receptor potential cation channel, subfamily M, member 2 (TRPM2) channels by intracellular adenosine diphosphate ribose (ADPR), well characterized in neutrophils, is reportedly inhibited by 8-bromo-ADPR (8Br-ADPR). TRPM2 is blocked by N-(p-amylcinnamoyl)anthranilic acid (ACA) interfering with the pore, but ACA is as well effective on other transient receptor potential channels, especially transient receptor potential canonical (TRPC) channels. We wished to analyze whether ACA and 8Br-ADPR were suitable probes to demonstrate that different Ca(2+) entry pathways are activated in human neutrophil granulocytes by the receptor-dependent stimuli N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP) and platelet-activating factor (PAF) and the receptor-independent thapsigargin. Ca(2+)-influx-related increases in [Ca(2+)](i) were calculated by comparing aliquots of fluo-3-loaded neutrophils in the presence and absence of extracellular Ca(2+). Moreover, Mn(2+) quenching was used in fura-2-loaded cells. We compared 8Br-ADPR with ACA. 8Br-ADPR was exclusively effective when Ca(2+) influx (or Mn(2+) quenching) was induced by fMLP; it did not affect influx when PAF or thapsigargin was the stimulus. ACA inhibited Ca(2+) influx significantly more strongly when this was induced by PAF than by fMLP. Moreover, it reduced thapsigargin-induced Ca(2+) influx. The contribution of TRPM2 to Ca(2+) influx in neutrophils strongly depends on the stimulus; it is sizeable in the case of fMLP and minimal in the case of PAF. PAF induces Ca(2+) entry pathways different from TRPM2; the inhibition by ACA suggests the contribution of channels of the TRPC family.

Amantadine inhibits platelet-activating factor induced clathrin-mediated endocytosis in human neutrophils

Receptor signaling is integral for adhesion, emigration, phagocytosis, and reactive oxygen species production in polymorphonuclear neutrophils (PMNs). Priming is an important part of PMN emigration, but it can also lead to PMN-mediated organ injury in the host. Platelet-activating factor (PAF) primes PMNs through activation of a specific G protein-coupled receptor. We hypothesize that PAF priming of PMNs requires clathrin-mediated endocytosis (CME) of the PAF receptor (PAFr), and, therefore, amantadine, known to inhibit CME, significantly antagonizes PAF signaling. PMNs were isolated by standard techniques to >98% purity and tested for viability. Amantadine (1 mM) significantly inhibited the PAF-mediated changes in the cellular distribution of clathrin and the physical colocalization [fluorescence resonance energy transfer positive (FRET+)] of early endosome antigen-1 and Rab5a, known components of CME and similar to hypertonic saline, a known inhibitor of CME. Furthermore, amantadine had no effect on the PAF-induced cytosolic calcium flux; however, phosphorylation of p38 MAPK was significantly decreased. Amantadine inhibited PAF-mediated changes in PMN physiology, including priming of the NADPH oxidase and shape change with lesser inhibition of increases in CD11b surface expression and elastase release. Furthermore, rimantadine, an amantadine analog, was a more potent inhibitor of PAF priming of the N-formyl-methionyl-leucyl-phenylalanine-activated oxidase. PAF priming of PMNs requires clathrin-mediated endocytosis that is inhibited when PMNs are pretreated with either amantadine or rimantadine. Thus, amantadine and rimantadine have the potential to ameliorate PMN-mediated tissue damage in humans.

Redox signaling across cell membranes

Generation of reactive oxygen species (ROS) by plasma membrane-localized NADPH oxidase (Nox 2) is a major mechanism of cell signaling associated with activation of the enzyme by a variety of agonists. With activation, the integral membrane flavocytochrome of Nox 2 transfers an electron from intracellular NADPH to extracellular O(2), generating superoxide anion (O(2)(*-)). The latter dismutes to H(2)O(2) which can diffuse through aquaporin channels in the plasma membrane to elicit an intracellular signaling response. O(2)(*-) also can initiate intracellular signaling by penetration of the cell membrane through anion channels (Cl(-) channel-3, ClC-3). Endosomes containing Nox2 and ClC-3 (called signaling endosomes) are composed of internalized plasma membrane and generate O(2)(*-) in the endosomal lumen to initiate signaling at intracellular sites. Thus, cellular signaling by Nox2 is dependent on the transmembrane flux of ROS. The role of this pathway has only recently been described and will require additional investigation to appreciate its physiological significance fully.

β-arrestins attenuate p38-mediated endosome to Golgi transport

Shiga toxin (Stx) is after endocytosis transported via early endosomes to the Golgi apparatus and endoplasmic reticulum. It is then translocated to the cytosol where it exerts its toxic effect. We recently reported that p38 is required for endosome to Golgi transport of Stx. In the present study, we investigated whether β-arrestins are effectors of this pathway. β-arrestin knockdown led to enhanced Stx transport. A similar phenotype was achieved upon p38 activation. We demonstrate that p38 and β-arrestin act on the same pathway. β-arrestin colocalized with internalized Stx and, interestingly, was recruited to endosomes upon p38 activation. After Stx treatment, p38 and β-arrestin formed a transient complex. From these data we propose that β-arrestin negatively regulates Stx transport via an interaction with activated p38 and attenuation of its signalling. Interestingly, also mannose 6-phosphate receptor transport was regulated by p38 and β-arrestin. β-arrestins therefore seem to regulate an endosome to Golgi pathway used by multiple cargo proteins.