Prolonged exposure to lipopolysaccharide inhibits macrophage 5-lipoxygenase metabolism via induction of nitric oxide synthesis

Integrated analysis of whole blood oxylipin and cytokine responses after bacterial, viral, and T cell stimulation reveals new immune networks

Oxylipins are major immunomodulating mediators, yet studies of inflammation focus mainly on cytokines. Here, using a standardized whole-blood stimulation system, we characterized the oxylipin-driven inflammatory responses to various stimuli and their relationships with cytokine responses. We performed a pilot study in 25 healthy individuals using 6 different stimuli: 2 bacterial stimuli (LPS and live BCG), 2 viral stimuli (vaccine-grade poly I:C and live H1N1 attenuated influenza), an enterotoxin superantigen and a Null control. All stimuli induced a strong production of oxylipins but most importantly, bacterial, viral, and T cell immune responses show distinct oxylipin signatures. Integration of the oxylipin and cytokine responses for each condition revealed new immune networks improving our understanding of inflammation regulation. Finally, the oxylipin responses and oxylipin-cytokine networks were compared in patients with active tuberculosis or with latent infection. This revealed different responses to BCG but not LPS stimulation highlighting new regulatory pathways for further investigations.

Systemic inhibition of 5-lipoxygenase by MK-886 exacerbates apical periodontitis bone loss in a mouse model

BACKGROUND

To investigate if 5-LO selective inhibitor (MK-886) could be used for systemic treatment of experimentally induced apical periodontitis in a mouse model.

METHODS

Twenty-four C57BL/6 mice were used. After coronal opening, a solution containing Escherichia coli LPS (1.0 µg/µL) was inoculated into the root canals of the lower and upper right first molars (n = 72 teeth). After 30 days apical periodontitis was established, and the animals were treated with MK-886 (5 mg/kg), a 5-LO inhibitor, for 7 and 14 days. The tissues were removed for histopathological and histometric analyses, evaluation of osteoclast number and gene expression for receptor activator of nuclear factor kappa-B (Tnfrsf11a), receptor activator of nuclear factor kappa-B ligand (Tnfsf11), osteoprotegerin (Tnfrsf11b), tartrate-resistant acid phosphatase (Acp5), matrix metalloproteinase-9 (Mmp9), cathepsin K (Ctsk) and calcitonin receptor (Calcr). Statistical data analysis was performed using Kruskal Wallis followed by Dunn's tests (α = 0.05).

RESULTS

Administration of MK-886 for 7 days exerted no effect on apical periodontitis progression compared to LPS inoculation without treatment (p = 0.3549), while treatment for 14 days exacerbated bone loss (p < 0.0001). Administration of MK-886 enhanced osteoclastogenesis signaling and osteoclast formation within 7 days (p = 0.0005), but exerted no effect at 14 days (p > 0.9999). After 7 days of treatment, MK-886 induced mRNA expression for Acp5 (p = 0.0001), Calcr (p = 0.0003), Mmp9 (p = 0.0005) and Ctsk (p = 0.0008), however no effect in those gene expression was observed after 14 days (p > 0.05).

CONCLUSION

Systemic treatment with MK-886 exacerbated LPS-induced apical periodontitis in a mouse model.

Formation of eicosanoids and other oxylipins in human macrophages

In this review it is attempted to summarize current studies about formation of eicosanoids and other oxylipins in different human macrophages. There are several reports on M1 and M2 cells, also other phenotypes have been described. The eicosanoids formed in the largest amounts are the COX products TxB₂ and PGE₂. Thus shortlived bioactive TxA₂ is a dominating product both in M1- and in M2-lineages, one exception seems to be M_{GM-CSF, TGFβ} cells. 5-LOX products are produced in both M1 and M2 macrophages, as well as in not fully polarized cells of both lineages. M_M-CSF as well as M2 macrophages produced LTC₄ more readily compared to M1 lineage cells. In M_{GM-CSF, TGFβ} cells LTB₄ is a major eicosanoid, in line with high expression of LTA₄ hydrolase. Recent reports described increased formation of leukotrienes in macrophages subjected to trained immunity with inflammatory transcriptional reprogramming. Also in macrophages derived from monocytes collected from post-COVID-19 patients. 15-LOX-1 is strongly upregulated in CD206⁺ M2 cells (M2a), differentiated in presence of IL-4. These macrophages also express 15-LOX-2. In incubations with pathogenic E. coli as well as other stimuli 15(S)-HETE and 17(S)-HDHA were major oxylipins formed. Also, the SPM precursor 5,15-diHETE and the SPM RvD5 were produced in considerable amounts, while other SPMs were less abundant. In M2 macrophages incubated with E. coli or S. aureus the cytosolic 15-LOX-1 enzyme accumulated to punctuate structures in a Ca²⁺ dependent manner with a relatively slow time course, leading to formation of mediators from endogenous substrate. Chalcones, flavone-like anti-inflammatory natural products, induced translocation of 15-LOX-1 in M2 cells, with high formation of 15-LOX derived oxylipins.

Deuterated Arachidonic Acid Ameliorates Lipopolysaccharide-Induced Lung Damage in Mice

Arachidonic acid (ARA) is a major component of lipid bilayers as well as the key substrate for the eicosanoid cascades. ARA is readily oxidized, and its non-enzymatic and enzymatic oxidation products induce inflammatory responses in nearly all tissues, including lung tissues. Deuteration at bis-allylic positions substantially decreases the overall rate of ARA oxidation when hydrogen abstraction is an initiating event. To compare the effects of dosing of arachidonic acid (H-ARA) and its bis-allylic hexadeuterated form (D-ARA) on lungs in conventionally healthy mice and in an acute lung injury model, mice were dosed with H-ARA or D-ARA for six weeks through dietary supplementation and then challenged with intranasal lipopolysaccharide (LPS) for subsequent analysis of bronchoalveolar lavage fluid and lung tissue. Dosing on D-ARA resulted in successful incorporation of D-ARA into various tissues. D-ARA significantly reduced LPS-induced adverse effects on alveolar septal thickness and the bronchoalveolar area. Oral deuterated ARA is taken up efficiently and protects against adverse LPS-induced pathology. This suggests novel therapeutic avenues for reducing lung damage during severe infections and other pathological conditions with inflammation in the pulmonary system and other inflammatory diseases.

Mild COVID-19 imprints a long-term inflammatory eicosanoid- and chemokine memory in monocyte-derived macrophages

Monocyte-derived macrophages (MDM) drive the inflammatory response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and they are a major source of eicosanoids in airway inflammation. Here we report that MDM from SARS-CoV-2-infected individuals with mild disease show an inflammatory transcriptional and metabolic imprint that lasts for at least 5 months after SARS-CoV-2 infection. MDM from convalescent SARS-CoV-2-infected individuals showed a downregulation of pro-resolving factors and an increased production of pro-inflammatory eicosanoids, particularly 5-lipoxygenase-derived leukotrienes. Leukotriene synthesis was further enhanced by glucocorticoids and remained elevated at 3–5 months, but had returned to baseline at 12 months post SARS-CoV-2 infection. Stimulation with SARS-CoV-2 spike protein or LPS triggered exaggerated prostanoid-, type I IFN-, and chemokine responses in post COVID-19 MDM. Thus, SARS-CoV-2 infection leaves an inflammatory imprint in the monocyte/ macrophage compartment that drives aberrant macrophage effector functions and eicosanoid metabolism, resulting in long-term immune aberrations in patients recovering from mild COVID-19.

Acinetobacter baumannii LOS Regulate the Expression of Inflammatory Cytokine Genes and Proteins in Human Mast Cells

Herein, we investigated the effect of bacterial lipooligosaccharides (LOS), from Acinetobacter baumannii, on the expression of pro-inflammatory genes that play an essential role in bacterial clearance. LAD2 human mast cells were stimulated with LOS derived from two strains of A. baumannii—ATCC 19606 and MDRA T14. LOS exposure induced the expression of genes for pro-inflammatory mediators, including TNF-α, IL-8, LTC4S, CCL4, and TLR4. The mRNA expression levels of a majority of the pro-inflammatory genes, except TLR4, in A. baumannii-LOS stimulated mast cells were increased. Moreover, co-culture of neutrophils with the supernatant obtained from LOS (ATCC 19606 and MDRA T14)-induced LAD2 cells increased the transmigration of neutrophils, which plays a critical role in the early protection against bacterial infections. The results of the present study suggest that LOS could be involved in the pathogenicity of A. baumannii by inducing inflammatory responses via mast cells and that IL-8 is involved in recruiting neutrophils in response to bacterial invasion.

Momordica charantia Suppresses Inflammation and Glycolysis in Lipopolysaccharide-Activated RAW264.7 Macrophages

Macrophage activation is a key event that triggers inflammatory response. The activation is accompanied by metabolic shift such as upregulated glucose metabolism. There are accumulating evidences showing the anti-inflammatory activity of Momordica charantia. However, the effects of M. charantia on inflammatory response and glucose metabolism in activated macrophages have not been fully established. The present study aimed to examine the effect of M. charantia in modulating lipopolysaccharide (LPS)-induced inflammation and perturbed glucose metabolism in RAW264.7 murine macrophages. The results showed that LPS-induced NF-κB (p65) nuclear translocation was inhibited by M. charantia treatment. In addition, M. charantia was found to reduce the expression of inflammatory genes including IL6, TNF-α, IL1β, COX2, iNOS, and IL10 in LPS-treated macrophages. Furthermore, the data showed that M. charantia reduced the expression of GLUT1 and HK2 genes and lactate production (-28%), resulting in suppression of glycolysis. Notably, its effect on GLUT1 gene expression was found to be independent of LPS-induced inflammation. A further experiment also indicated that the bioactivities of M. charantia may be attributed to its key bioactive compound, charantin. Taken together, the study provided supporting evidences showing the potential of M. charantia for the treatment of inflammatory disorders.

Metabolic reprograming of LPS-stimulated human lung macrophages involves tryptophan metabolism and the aspartate-arginosuccinate shunt

Lung macrophages (LM) are in the first line of defense against inhaled pathogens and can undergo phenotypic polarization to the proinflammatory M1 after stimulation with Toll-like receptor agonists. The objective of the present work was to characterize the metabolic alterations occurring during the experimental M1 LM polarization. Human LM were obtained from resected lungs and cultured for 24 hrs in medium alone or with 10 ng.mL-1 lipopolysaccharide. Cells and culture supernatants were subjected to extraction for metabolomic analysis with high-resolution LC-MS (HILIC and reverse phase -RP- chromatography in both negative and positive ionization modes) and GC-MS. The data were analyzed with R and the Worklow4Metabolomics and MetaboAnalyst online infrastructures. A total of 8,741 and 4,356 features were detected in the intracellular and extracellular content, respectively, after the filtering steps. Pathway analysis showed involvement of arachidonic acid metabolism, tryptophan metabolism and Krebs cycle in the response of LM to LPS, which was confirmed by the specific quantitation of selected compounds. This refined analysis highlighted a regulation of the kynurenin pathway as well as the serotonin biosynthesis pathway, and an involvement of aspartate-arginosuccinate shunt in the malate production. Macrophages M1 polarization is accompanied by changes in the cell metabolome, with the differential expression of metabolites involved in the promotion and regulation of inflammation and antimicrobial activity. The analysis of this macrophage immunometabolome may be of interest for the understanding of the pathophysiology of lung inflammatory disesases.

Long-term stimulation of toll-like receptor-2 and -4 upregulates 5-LO and 15-LO-2 expression thereby inducing a lipid mediator shift in human monocyte-derived macrophages

Macrophage polarization switches during the course of inflammation along with the lipid mediators released. We investigated the lipid mediator formation in human monocyte-derived macrophages during in vitro differentiation and pathogen stimulation. For this, peripheral blood monocytes were differentiated into M1 (CSF-2/IFNγ) or M2 (CSF-1/IL-4) macrophages followed by stimulation with the toll-like receptor (TLR) ligands zymosan (TLR-2), Poly(I:C) (TLR-3) or bacterial lipopolysaccharides (TLR-4) mimicking fungal, viral and bacterial infection, respectively. Expression of enzymes involved in lipid mediator formation such as 5- and 15-lipoxygenases (LO), the 5-LO activating protein and cyclooxygenase-2 (COX-2) was monitored on mRNA and protein level and lipid mediator formation was assessed. In addition, cytokine release was measured. In vitro differentiation of human peripheral blood monocytes to M1 and M2 macrophages considerably attenuated 5-LO activity. Furthermore, while TLR-2 and -4 stimulation of M1 macrophages primarily triggered pro-inflammatory cytokines and lipid mediators, persistent stimulation (16 h) of human M2 macrophages induced a coordinated upregulation of 5- and 15-LO-2 expression. This was accompanied by a marked increase in IL-10 and monohydroxylated 15-LO products in the conditioned media of the cells. After additional stimulation with Ca²⁺ ionophore combined with supplementation of arachidonic, eicosapentaenoic and docosahexaenoic acid these cells also released small amounts of SPM such as lipoxins and resolvins. From this we conclude that activation of TLR-2 or -4 triggers the biosynthesis of pro-inflammatory 5-LO and COX-2 derived lipid mediators in human monocyte-derived M1 macrophages while persistent stimulation of M2 macrophages induces a shift towards pro-resolving 15-LO derived oxylipins.

The Potential of Telomeric G-quadruplexes Containing Modified Oligoguanosine Overhangs in Activation of Bacterial Phagocytosis and Leukotriene Synthesis in Human Neutrophils

Human neutrophils are the first line of defense against bacterial and viral infections. They eliminate pathogens through phagocytosis, which activate the 5-lipoxygenase (5-LOX) pathway resulting in synthesis of leukotrienes. Using HPLC analysis, flow cytometry, and other biochemical methods, we studied the effect of synthetic oligodeoxyribonucleotides (ODNs) able to fold into G-quadruplex structures on the main functions of neutrophils. Designed ODNs contained four human telomere TTAGGG repeats (G4) including those with phosphorothioate oligoguanosines attached to the end(s) of G-quadruplex core. Just modified analogues of G4 was shown to more actively than parent ODN penetrate into cells, improve phagocytosis of Salmonella typhimurium bacteria, affect 5-LOX activation, the cytosol calcium ion level, and the oxidative status of neutrophils. As evident from CD and UV spectroscopy data, the presence of oligoguanosines flanking G4 sequence leads to dramatic changes in G-quadruplex topology. While G4 folds into a single antiparallel structure, two main folded forms have been identified in solutions of modified ODNs: antiparallel and dominant, more stable parallel. Thus, both the secondary structure of ODNs and their ability to penetrate into the cytoplasm of cells are important for the activation of neutrophil cellular effects. Our results offer new clues for understanding the role of G-quadruplex ligands in regulation of integral cellular processes and for creating the antimicrobial agents of a new generation.

Looking for a partner: ceruloplasmin in protein-protein interactions

Ceruloplasmin (CP) is a mammalian blood plasma ferroxidase. More than 95% of the copper found in plasma is carried by this protein, which is a member of the multicopper oxidase family. Proteins from this group are able to oxidize substrates through the transfer of four electrons to oxygen. The essential role of CP in iron metabolism in humans is particularly evident in the case of loss-of-function mutations in the CP gene resulting in a neurodegenerative syndrome known as aceruloplasminaemia. However, the functions of CP are not limited to the oxidation of ferrous iron to ferric iron, which allows loading of the ferric iron into transferrin and prevents the deleterious reactions of Fenton chemistry. In recent years, a number of novel CP functions have been reported, and many of these functions depend on the ability of CP to form stable complexes with a number of proteins.

Resolving Lipid Mediators Maresin 1 and Resolvin D2 Prevent Atheroprogression in Mice

RATIONALE

Atheroprogression is a consequence of nonresolved inflammation, and currently a comprehensive overview of the mechanisms preventing resolution is missing. However, in acute inflammation, resolution is known to be orchestrated by a switch from inflammatory to resolving lipid mediators. Therefore, we hypothesized that lesional lipid mediator imbalance favors atheroprogression.

OBJECTIVE

To understand the lipid mediator balance during atheroprogression and to establish an interventional strategy based on the delivery of resolving lipid mediators.

METHODS AND RESULTS

Aortic lipid mediator profiling of aortas from Apoe^-/- mice fed a high-fat diet for 4 weeks, 8 weeks, or 4 months revealed an expansion of inflammatory lipid mediators, Leukotriene B4 and Prostaglandin E2, and a concomitant decrease of resolving lipid mediators, Resolvin D2 (RvD2) and Maresin 1 (MaR1), during advanced atherosclerosis. Functionally, aortic Leukotriene B4 and Prostaglandin E2 levels correlated with traits of plaque instability, whereas RvD2 and MaR1 levels correlated with the signs of plaque stability. In a therapeutic context, repetitive RvD2 and MaR1 delivery prevented atheroprogression as characterized by halted expansion of the necrotic core and accumulation of macrophages along with increased fibrous cap thickness and smooth muscle cell numbers. Mechanistically, RvD2 and MaR1 induced a shift in macrophage profile toward a reparative phenotype, which secondarily stimulated collagen synthesis in smooth muscle cells.

CONCLUSIONS

We present evidence for the imbalance between inflammatory and resolving lipid mediators during atheroprogression. Delivery of RvD2 and MaR1 successfully prevented atheroprogression, suggesting that resolving lipid mediators potentially represent an innovative strategy to resolve arterial inflammation.

Atherosclerosis - A matter of unresolved inflammation

Atherosclerosis is commonly looked upon as a chronic inflammatory disease of the arterial wall arising from an unbalanced lipid metabolism and a maladaptive inflammatory response. However, atherosclerosis is not merely an inflammation of the vessel wall. In fact, the cardinal signs of unstable atherosclerotic lesions are primarily characteristics of failed resolution of a chronic inflammation. In contrast to acute inflammatory events which are typically self-limiting, atherosclerosis is an unresolved inflammatory condition, lacking the switch from the pro-inflammatory to the pro-resolving phase, the latter characterized by termination of inflammatory cell recruitment, removal of inflammatory cells from the site of inflammation by apoptosis and dead cell clearance, reprogramming of macrophages toward an anti-inflammatory, regenerative phenotype, and finally egress of effector cells and tissue regeneration. Here we present an overview on mechanisms of failed resolution contributing to atheroprogression and deliver a summary of novel therapeutic strategies to restore resolution in inflamed arteries.

Phospholipase A2 regulates eicosanoid class switching during inflammasome activation

Initiation and resolution of inflammation are considered to be tightly connected processes. Lipoxins (LX) are proresolution lipid mediators that inhibit phlogistic neutrophil recruitment and promote wound-healing macrophage recruitment in humans via potent and specific signaling through the LXA4 receptor (ALX). One model of lipoxin biosynthesis involves sequential metabolism of arachidonic acid by two cell types expressing a combined transcellular metabolon. It is currently unclear how lipoxins are efficiently formed from precursors or if they are directly generated after receptor-mediated inflammatory commitment. Here, we provide evidence for a pathway by which lipoxins are generated in macrophages as a consequence of sequential activation of toll-like receptor 4 (TLR4), a receptor for endotoxin, and P2X7, a purinergic receptor for extracellular ATP. Initial activation of TLR4 results in accumulation of the cyclooxygenase-2-derived lipoxin precursor 15-hydroxyeicosatetraenoic acid (15-HETE) in esterified form within membrane phospholipids, which can be enhanced by aspirin (ASA) treatment. Subsequent activation of P2X7 results in efficient hydrolysis of 15-HETE from membrane phospholipids by group IVA cytosolic phospholipase A2, and its conversion to bioactive lipoxins by 5-lipoxygenase. Our results demonstrate how a single immune cell can store a proresolving lipid precursor and then release it for bioactive maturation and secretion, conceptually similar to the production and inflammasome-dependent maturation of the proinflammatory IL-1 family cytokines. These findings provide evidence for receptor-specific and combinatorial control of pro- and anti-inflammatory eicosanoid biosynthesis, and potential avenues to modulate inflammatory indices without inhibiting downstream eicosanoid pathways.

Interleukin-1β inhibits synthesis of 5-lipooxygenase in lipopolysaccharide-stimulated equine whole blood

Interleukin-1β (IL-1β) is a pro-inflammatory cytokine. It induces the synthesis of prostaglandin E2 (PGE2) catalyzed by cyclooxygenase (COX) and microsomal prostaglandin E synthase (m-PGES). Besides its pro-inflammatory properties, PGE2 also exhibits anti-inflammatory properties by inhibiting synthesis of 5-lipooxygenase (5-LO) products which are in themselves, pro-inflammatory mediators. Thus, inhibition of 5-LO products is beneficial in regulating immune-responses and pro-inflammatory processes. To investigate the hypothesis that IL-1β is responsible for the increase in the synthesis of PGE2 and in the reduction of 5-LO products, equine whole blood (EWB) was treated with lipopolysaccharide (LPS). In vitro treatment of EWB with LPS resulted in increased expression of IL-1β while expression of 5-LO was suppressed. Quantification of eicosanoids using liquid-chromatography-mass spectrometry/multiple reaction monitoring (LC-MS/MRM) showed increased concentrations of prostaglandins and decreased 5-LO products in LPS-treated EWB. Pretreatment of EWB with IL-1β followed by calcium ionophore A23187 (CI) reduced synthesis of 5-LO products. However, pretreatment of EWB with COX-2 inhibitor (NS-398) or m-PGES-1 inhibitor (CAY 10526) and IL-1β followed with CI resulted in a significant (p<0.0001) increase in 5-LO products. Pretreatment of EWB with phospholipase C inhibitor (U73122) followed with LPS reduced PGE2 production but increased 5-LO products. The result of this study indicated that increased PGE2 production led to reduction in 5-LO products in LPS-treated EWB via IL-1β. However, other pathways, cytokines and mediators may be involved in inhibiting 5-LO products but the present study did not include those other potential pathways. Inhibition of 5-LO products by PGE2 in EWB may regulate the initiation and pathogenesis of inflammatory responses in the horse.

GM-CSF priming drives bone marrow-derived macrophages to a pro-inflammatory pattern and downmodulates PGE2 in response to TLR2 ligands

In response to pathogen recognition by Toll-like receptors (TLRs) on their cell surface, macrophages release lipid mediators and cytokines that are widely distributed throughout the body and play essential roles in host responses. Granulocyte macrophage colony-stimulating factor (GM-CSF) is important for the immune response during infections to improve the clearance of microorganisms. In this study, we examined the release of mediators in response to TLR2 ligands by bone marrow-derived macrophages (BMDMs) primed with GM-CSF. We demonstrated that when stimulated with TLR2 ligands, non-primed BMDMs preferentially produced PGE₂ in greater amounts than LTB₄. However, GM-CSF priming shifted the release of lipid mediators by BMDMs, resulting in a significant decrease of PGE₂ production in response to the same stimuli. The decrease of PGE₂ production from primed BMDMs was accompanied by a decrease in PGE-synthase mRNA expression and an increase in TNF-α and nitric oxide (NO) production. Moreover, some GM-CSF effects were potentiated by the addition of IFN-γ. Using a variety of TLR2 ligands, we established that PGE₂ release by GM-CSF-primed BMDMs was dependent on TLR2 co-receptors (TLR1, TLR6), CD14, MyD88 and the nuclear translocation of NFκB but was not dependent on peroxisome proliferator-activated receptor-γ (PPAR-γ) activation. Indeed, GM-CSF priming enhanced TLR2, TLR4 and MyD88 mRNA expression and phospho-IκBα formation. These findings demonstrate that GM-CSF drives BMDMs to present a profile relevant to the host during infections.

The pulmonary pharmacology of [4-methoxy-N1-(4-trans-nitrooxycyclohexyl)-N3-(3-pyridinylmethyl)-1,3-benzenedicarboxamide] (2NTX-99), an anti-atherotrombotic compound with therapeutic potential in pathological conditions that target lung vasculature

The pharmacological activity of 2NTX-99 ([4-methoxy-N1-(4-trans-nitrooxycyclohexyl)-N3-(3-pyridinylmethyl)-1,3-benzenedicarboxamide]) was investigated in vitro in the intact, rat pulmonary vasculature and in guinea pig airways. Rat lungs were perfused at constant flow and changes in vascular tone recorded. Challenge with the TXA₂ analogue 9,11-dideoxy-9α11α-methanoepoxy ProstaglandinF₂ (U46619, 0.5 μM) increased vessel tone (32.48±1.5 vs 13.13±0.56 mmHg; n=12). 2NTX-99 (0.1-100 μM; n=5), caused a concentration-dependent relaxation, prevented by 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, 10 μM, n=4), an inhibitor of soluble guanylate cyclase. Acetylcholine (0.1-10 μM; n=3) and a reference NO-donor, isosorbide-5-mononitrate (5-100 μM; n=4), were ineffective. Intraluminal perfusion of washed human platelets (2 × 10⁸ cells/ml) increased intravascular pressure after challenge with arachidonic acid (AA, 2 μM; n=5), an increase abolished by acetylsalicylic acid and significantly reduced by 2NTX-99 (40 μM; n=5). TXB₂ in the lung perfusate was detected after platelet activation, 2NTX-99 inhibited TXA₂ synthesis (6.45±0.6 and 1.10±0.2 ng/ml, respectively). 2NTX-99 did not alter central or peripheral airway responsiveness to Histamine (0.001-300 μM; n=6), U46619 (0.001-3 μM, n=3) or LTD₄ (1 pM-1 μM; n=6). 2NTX-99 vasodilates the pulmonary vasculature via the release of nitric oxide (NO) and reduces intraluminal, AA-induced, TXA₂ formation. The combined activity of 2NTX-99 as an NO-donor and a TXA₂-synthesis inhibitor provides strong support for its potential therapeutic use in pathologies of the pulmonary vascular bed (e.g. pulmonary hypertension).

Involvement of red blood cells in the regulation of leukotriene synthesis in polymorphonuclear leucocytes upon interaction with Salmonella Typhimurium

Leukotriene (LT) B4 is the primary eicosanoid product of polymorphonuclear leucocytes (PMNLs). We studied LT synthesis in PMNLs upon interaction with Salmonella enterica serovar Typhimurium. Human PMNLs exposed to Salmonella produced LTs; mostly LTB4 and ω-hydroxy-LTB4. Opsonization with normal serum increased the capacity of S. Typhimurium to induce LT synthesis in PMNLs. Addition of red blood cells (RBCs) alone did not activate LT synthesis in PMNLs but did further increase the Salmonella-induced release of LTs. Priming of PMNLs with lipopolysaccharide before the addition of bacteria potentiated LT synthesis in these cells. The effect was more pronounced in the presence of RBCs. We found that RBCs diminished the effect of exogenously added NO donors on LT synthesis in PMNLs. We conclude that RBCs mediate the activation of LT synthesis in PMNLs exposed to Salmonella bacteria at least in part by regulating the intercellular exchange and metabolism of NO.

Distinct role of nitric oxide and peroxynitrite in mediating oligodendrocyte toxicity in culture and in experimental autoimmune encephalomyelitis

Nitric oxide has been implicated in the pathogenesis of multiple sclerosis. However, it is still unclear whether nitric oxide plays a protective role or is deleterious. We have previously shown that peroxynitrite, a reaction product of nitric oxide and superoxide, is toxic to mature oligodendrocytes (OLs). The toxicity is mediated by intracellular zinc release, phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), activation of 12-lipoxygenase (12-LOX) and the formation of reactive oxygen species (ROS). In this study, we found that the donors of nitric oxide, dipropylenetriamine NONOate (DPT NONOate) and diethylenetriamine NONOate (DETA NONOate), protected OLs from peroxynitrite or zinc-induced toxicity. The protective mechanisms appear to be attributable to their inhibition of peroxynitrite- or zinc-induced ERK1/2 phosphorylation and 12-LOX activation. In cultures of mature OLs exposed to lipopolysaccharide (LPS), induction of inducible nitric oxide synthase (iNOS) generated nitric oxide and rendered OLs resistant to peroxynitrite-induced toxicity. The protection was eliminated when 1400W, a specific inhibitor of iNOS, was co-applied with LPS. Using MOG35-55-induced experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis, we found that nitrotyrosine immunoreactivity, an indicator of peroxynitrite formation, was increased in the spinal cord white matter, which correlated with the loss of mature OLs. Targeted gene deletion of the NADPH oxidase component gp91phox reduced clinical scores, the formation of nitrotyrosine and the loss of mature OLs. These results suggest that blocking the formation specifically of peroxynitrite, rather than nitric oxide, may be a protective strategy against oxidative stress induced toxicity to OLs.

Modulation of neutrophil apoptosis by murine pulmonary microvascular endothelial cell inducible nitric oxide synthase

Neutrophils contribute significantly to ALI (acute lung injury) through adhesion to pulmonary microvascular endothelial cells (PMEC), trans-PMEC migration and alveolar infiltration. Trans-PMEC migration delays expression of neutrophil apoptosis, which promotes intra-alveolar neutrophil survival and neutrophil mediated ALI. We assessed the role of neutrophil vs PMEC inducible nitric oxide (NO) synthase (iNOS) in modulating neutrophil apoptosis. Apoptosis of wild-type vs iNOS-/- neutrophils was quantified by microscopy and FACS annexin-V binding. In a murine model of ALI, neutrophils isolated by BAL(broncho-alveolar lavage) from iNOS-/- mice had increased expression of apoptosis after 24h culture ex vivo than wild-type neutrophils (15.2±3.3 vs 3.0±0.4%, mean±sd, p<0.01). Apoptosis rates of isolated bone marrow iNOS+/+ vs iNOS-/- neutrophils were similar under basal and LPS/IFN-γ stimulation, and following LPS/IFN-γ-stimulated trans-PMEC migration. Apoptosis of both iNOS+/+ and iNOS-/- neutrophils was inhibited by trans-PMEC migration only across iNOS+/+ PMEC (1.6±0.3 and 1.5±0.3%, respectively; p<0.05 for each vs non-migrated neutrophils) but not across iNOS-/- PMEC (4.3±1 and 3.1±0.6%, respectively). PMEC iNOS-dependent inhibition of neutrophil apoptosis was independent of changes in neutrophil caspase-3 activity. We conclude that PMEC iNOS, but not neutrophil iNOS, has an important inhibitory effect on neutrophil apoptosis during trans-PMEC neutrophil migration, which is independent of caspase-3 activity. Further studies will define the mechanism of PMEC iNOS-dependent inhibition of neutrophil apoptosis and assess the potential relevance of this phenomenon in human neutrophils and ALI.

Signal transduction pathways linking the activation of alveolar macrophages with the recruitment of neutrophils to lungs in chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is a major and increasing global health problem. It is predicted by the World Health Organization to become the third most common cause of death and the fifth most common cause of disability in the world by 2020. COPD is a complex inflammatory disease involving several types of inflammatory cells and multiple inflammatory mediators. Although abnormal numbers of inflammatory cells such as macrophages, dendritic cells, neutrophils, and T lymphocytes have been documented in COPD, the relationship between these cell types and the sequence of their appearance and persistence is largely unknown. Alveolar macrophages have been identified as one of the major cell types that plays a key role in orchestrating the inflammatory events associated with the pathophysiology of COPD. One of the major functions of macrophages is the secretion of chemotactic factors and this function is markedly increased on exposure to cigarette smoke (CS). This enhanced release of chemoattractants results in increased lung neutrophil infiltration, which is thought to be a key event in the development of COPD. The molecular basis for this amplified inflammatory response is not very clear, but it could be due to an alteration in signal transduction pathways within the macrophage. Based on existing literature, an attempt has been made to create a comprehensive review of the signal transduction pathways that link the activation of macrophages with the increased recruitment of neutrophils into the airways. Some of the major stimuli that activate macrophages and cause them to secrete chemotactic factors have been identified as CS, wood smoke, ozone, bacterial endotoxin, and proinflammatory cytokines such as interleukin (IL)-1beta and tumor necrosis factor (TNF)-alpha. These stimuli seem to activate mainly redox-sensitive transcription factors such as nuclear factor (NF)-kappa B and activator protein (AP)-1, both of which play a major role in the synthesis and secretion of chemotactic factors such as IL-8 and leukotriene B(4) (LTB(4)). The pathways involved in the synthesis and secretion of other factors such as macrophage chemotactic protein-1 (MCP-1) and growth-related oncogene-alpha (Gro-alpha) have also been reviewed.

Corticosteroid suppression of lipoxin A4 and leukotriene B4 from alveolar macrophages in severe asthma

BACKGROUND

An imbalance in the generation of pro-inflammatory leukotrienes, and counter-regulatory lipoxins is present in severe asthma. We measured leukotriene B4 (LTB4), and lipoxin A4 (LXA4) production by alveolar macrophages (AMs) and studied the impact of corticosteroids.

METHODS

AMs obtained by fiberoptic bronchoscopy from 14 non-asthmatics, 12 non-severe and 11 severe asthmatics were stimulated with lipopolysaccharide (LPS,10 microg/ml) with or without dexamethasone (10(-6)M). LTB4 and LXA4 were measured by enzyme immunoassay.

RESULTS

LXA4 biosynthesis was decreased from severe asthma AMs compared to non-severe (p < 0.05) and normal subjects (p < 0.001). LXA4 induced by LPS was highest in normal subjects and lowest in severe asthmatics (p < 0.01). Basal levels of LTB4 were decreased in severe asthmatics compared to normal subjects (p < 0.05), but not to non-severe asthma. LPS-induced LTB4 was increased in severe asthma compared to non-severe asthma (p < 0.05). Dexamethasone inhibited LPS-induced LTB4 and LXA4, with lesser suppression of LTB4 in severe asthma patients (p < 0.05). There was a significant correlation between LPS-induced LXA4 and FEV1 (% predicted) (r(s) = 0.60; p < 0.01).

CONCLUSIONS

Decreased LXA4 and increased LTB4 generation plus impaired corticosteroid sensitivity of LPS-induced LTB4 but not of LXA4 support a role for AMs in establishing a pro-inflammatory balance in severe asthma.

INDUCTION OF INDUCIBLE NITRIC OXIDE SYNTHASE BY LIPOPOLYSACCHARIDE/INTERFERON GAMMA AND SEPSIS DOWN-REGULATES 5-LIPOXYGENASE METABOLISM IN MURINE ALVEOLAR MACROPHAGES

Pretreatment with lipopolysaccharide (LPS) suppresses rat alveolar macrophage leukotriene synthesis in a nitric oxide (NO)-dependent mechanism. The authors examined the effect of NO on alveolar macrophage leukotriene synthesis following in vitro and in vivo models of sepsis. Treatment of alveolar macrophages from inducible NO synthase (iNOS) wild-type but not knock-out mice with LPS inhibited leukotriene synthesis. iNOS was induced early in alveolar macrophages from cecal ligation and puncture rats and mice compared to sham animals with associated reduced leukotriene synthesis. iNOS knock-out mice were protected from the decrease in alveolar macrophage 5-lipoxygenase metabolism. iNOS regulates alveolar macrophage 5-lipoxygenase metabolism following endotoxin exposure.

Specific mediator inhibition by the NO donors SNP and NCX 2057 in the peripheral lung: implications for allergen-induced bronchoconstriction

Background

The aim of this study was to examine potential therapeutic effect of the two NO donors NCX 2057 (3-(4-hydroxy-3-methoxyphenyl)-2-propenoic acid) 4-(nitrooxy)butyl ester) and SNP (sodium nitroprusside) on the early allergic airway response in the peripheral lung.

Methods

The experiments were performed in guinea pig lung parenchyma (GPLP) derived from ovalbumin (OVA) sensitized guinea pigs. The effects of NCX 2057 and SNP were evaluated by contractile responses and mediator release during OVA challenge. The generation of nitrite and nitrate was assessed by chemiluminescence. Statistical analysis was evaluated by ANOVA.

Results

Cumulatively increasing concentrations of OVA (1–10,000 ng/ml) induced concentration-dependent contractions of the GPLP that were reduced by NCX 2057 (100 μM, p < 0.001) and SNP (100 μM, p < 0.05). Antigen-induced eicosanoid release was decreased by NCX 2057 (100 μM, p < 0.001) but not by SNP (100 μM), whereas the release of histamine was reduced by SNP (100 μM, p < 0.001) but not by NCX 2057 (100 μM). In addition, NCX 2057 (0.1–100 μM), but not SNP (0.1–100 μM), relaxed leukotriene D₄ (10 nM) precontracted GPLP (p < 0.01). The guanylyl cyclase inhibitor ODQ had no effect on the NCX 2057 mediated relaxation. SNP released significantly less nitrite than NCX 2057.

Conclusion

Although both SNP and NCX 2057 reduced the release of pro-inflammatory mediators, their profiles were distinctly different. Furthermore, NCX 2057 also induced smooth muscle dilation in the GPLP. The findings point to specific anti-inflammatory effects of different NO donors in the peripheral lung tissue.

Convergence of nitric oxide and lipid signaling: anti-inflammatory nitro-fatty acids

The signaling mediators nitric oxide ( NO) and oxidized lipids, once viewed to transduce metabolic and inflammatory information via discrete and independent pathways, are now appreciated as interdependent regulators of immune response and metabolic homeostasis. The interactions between these two classes of mediators result in reciprocal control of mediator synthesis that is strongly influenced by the local chemical environment. The relationship between the two pathways extends beyond coregulation of NO and eicosanoid formation to converge via the nitration of unsaturated fatty acids to yield nitro derivatives (NO(2)-FA). These pluripotent signaling molecules are generated in vivo as an adaptive response to oxidative inflammatory conditions and manifest predominantly anti-inflammatory signaling reactions. These actions of NO(2)-FA are diverse, with these species serving as a potential chemical reserve of NO, reacting with cellular nucleophiles to posttranslationally modify protein structure, function, and localization. In this regard these species act as potent endogenous ligands for peroxisome proliferator-activated receptor gamma. Functional consequences of these signaling mechanisms have been shown in multiple model systems, including the inhibition of platelet and neutrophil functions, induction of heme oxygenase-1, inhibition of LPS-induced cytokine release in monocytes, increased insulin sensitivity and glucose uptake in adipocytes, and relaxation of preconstricted rat aortic segments. These observations have propelled further in vitro and in vivo studies of mechanisms of NO(2)-FA signaling and metabolism, highlighting the therapeutic potential of this class of molecules as anti-inflammatory drug candidates.

Differential modulation of leukotriene B4 synthesis and degradation in human bronchoalveolar lavage cells by lipopolysaccharide and tobacco smoke

Leukotrienes have been implicated to play a prominent inductive role in carcinogenesis. We previously reported that bronchoalveolar lavage (BAL) cells from smokers manifested higher levels of leukotriene B4 (LTB4) production than ex-smokers. This study aims to elucidate the underlying mechanism(s). BAL cells from current and former smokers were exposed to lipopolysaccharide (LPS) for up to 7 days. LPS induced the release of LTB4 from BAL cells and down-regulated 5-lipoxygenase (5-LOX) mRNA expression in a dose-dependent manner, followed by a decrease in 5-LOX protein production and normalization of LTB4 levels. Exogenous LTB4 inhibited LPS-induced 5-LOX activity and accentuated the down-regulation of 5-LOX mRNA, whereas suppression of 5-LOX abrogated the LPS-induced changes, suggesting a negative feedback mechanism. LPS concomitantly induced expression and activity of the LTB4 metabolizing enzyme LTB4 omega-hydroxylase (LTB4OH) in ex-smokers' BAL cells, but not in smokers' BAL cells. In vitro smoke exposure of ex-smokers' BAL cells also abrogated the LPS-induced up-regulation of LTB4OH mRNA expression. Furthermore, ex-smokers' BAL cells expressed significantly higher LTB4OH mRNA levels than smokers' BAL cells. Such differential modulation of LTB4 synthesis and degradation by LPS in the setting of tobacco smoke exposure suggests that mechanisms responsible for sustained elevation of LTB4 levels in the lung microenvironment may contribute to the pathogenesis of tobacco-related respiratory diseases such as lung cancer. By regulating the balance of LTB4 in the lung, LTB4OH may function as a suppressor of lung carcinogenesis.

Role of TLR-4 in liver macrophage and endothelial cell responsiveness during acute endotoxemia

Liver macrophages and endothelial cells have been implicated in hepatotoxicity induced by endotoxin (ETX). In these studies, we analyzed the role of toll-like receptor 4 (TLR-4) in the response of these cells to acute endotoxemia. Treatment of control C3H/HeOuJ mice with ETX (3 mg/kg, i.p.) resulted in increased numbers of activated macrophages in the liver. This was associated with morphological changes in the cells and a rapid (within 3 h) induction of nitric oxide synthase-2, cyclooxygenase-2, microsomal PGE synthase-1, interleukin-1 beta and tumor necrosis factor alpha gene expression. In endothelial cells, acute endotoxemia led to increased expression of these genes, as well as 5-lipoxygenase. In contrast, liver sinusoidal cells from C3H/HeJ TLR-4 mutant mice were relatively unresponsive to ETX. Treatment of C3H/HeOuJ, but not C3H/HeJ mice with ETX, resulted in activation of transcription factors AP-1 and NF-kappaB in liver sinusoidal cells, which was evident within 3 h. Whereas in macrophages, transcription factor activation was transient, in endothelial cells, it persisted for 24 h. In C3H/HeOuJ mice treated with ETX, activation of p38 MAP kinase was also evident in macrophages and endothelial cells, and JNK kinase in macrophages. In contrast, reduced protein kinase B (AKT) was noted in macrophages. In C3H/HeJ mice, ETX administration also led to activation of p38 MAP kinase in macrophages with no effects on JNK, p44/42 MAP kinase or AKT. These studies demonstrate that liver macrophages and endothelial cells are highly responsive to acute endotoxemia. Moreover, this activity is largely dependent on TLR-4.

TLR-4 and sustained calcium agonists synergistically produce eicosanoids independent of protein synthesis in RAW264.7 cells

Arachidonic acid is released by phospholipase A(2) and converted into hundreds of distinct bioactive mediators by a variety of cyclooxygenases (COX), lipoxygenases (LO), and cytochrome P450s. Because of the size and diversity of the eicosanoid class of signaling molecules produced, a thorough and systematic investigation of these biological processes requires the simultaneous quantitation of a large number of eicosanoids in a single analysis. We have developed a robust liquid chromatography/tandem mass spectrometry method that can identify and quantitate over 60 different eicosanoids in a single analysis, and we applied it to agonist-stimulated RAW264.7 murine macrophages. Fifteen different eicosanoids produced through COX and 5-LO were detected either intracellularly or in the media following stimulation with 16 different agonists of Toll-like receptors (TLR), G protein-coupled receptors, and purinergic receptors. No significant differences in the COX metabolite profiles were detected using the different agonists; however, we determined that only agonists creating a sustained Ca(2+) influx were capable of activating the 5-LO pathway in these cells. Synergy between Ca(2+) and TLR pathways was detected and discovered to be independent of NF-kappaB-induced protein synthesis. This demonstrates that TLR induction of protein synthesis and priming for enhanced phospholipase A(2)-mediated eicosanoid production work through two distinct pathways.

TNF-alpha downregulates the leukotriene C4 synthase gene in mononuclear phagocytes

We studied the effect of tumor necrosis factor (TNF)-alpha exposure on cysteinyl leukotriene (LT) synthesis by cells of monocyte/macrophage lineage. TNF-alpha conditioning of monocytic THP-1 cells and primary human monocytes resulted in a decreased capacity for LTC(4) release. TNF-alpha exposure (for 16-24 h) decreased LTC(4) synthase mRNA in THP-1 cells, primary mouse bone marrow-derived macrophages, and eosinophilic AML14.3D10 cells. TNF-alpha downregulated LTC(4) synthase mRNA in THP-1 cells in a dose- and time-dependent manner, with downregulation observed as early as 4 h. The effect of TNF-alpha on LTC(4) synthase mRNA expression was mediated via the MEK/ERK pathway, but not via cyclooxygenase or nitric oxide synthase pathways. Conditioning of actinomycin D-treated cells with TNF-alpha did not accelerate degradation of LTC(4) synthase mRNA. TNF-alpha produced sustained activation of p50 and p65, which were previously reported by our group to decrease LTC(4) synthase promoter activity. In transiently transfected THP-1 cells, TNF-alpha decreased promoter activity via an element located within the first 620 bp of the promoter. We conclude that TNF-alpha exposure downregulates the synthetic capacity for cysteinyl LT release and LTC(4) synthase gene expression in monocytes/macrophages via a transcriptional mechanism.

Inhibition of nitric-oxide synthase enhances antigen-induced contractions and increases release of cysteinyl-leukotrienes in guinea pig lung parenchyma: nitric oxide as a protective factor

Nitric oxide (NO) in exhaled air is a biomarker of airway inflammation. However, the role of NO in the peripheral lung is not known. The aim of this study was to determine the role of endogenous NO in antigen-induced contractions of ovalbumin (OVA)-sensitized guinea pig lung parenchyma (GPLP). The contraction in this in vitro model of the peripheral lung closely resembles the corresponding response in human airways. Cumulatively increasing concentrations (10-10,000 microg/l) of OVA induced concentration-dependent contractions of the GPLP that were enhanced by the NO synthase (NOS) inhibitors N(omega)-nitro-L-arginine (L-NOARG; 100 microM), N(omega)-monomethyl-L-arginine (100 microM), N(omega)-nitro-L-arginine methyl ester (100 microM), and N-(3-(aminomethyl)benzyl)acetamidine (1400W; 1 microM). The enhancement induced by L-NOARG was reversed by coadministration with the 5-lipoxygenase inhibitor (R)-2-[4-(quinolin-2-yl-methoxy)phenyl]-2-cyclopentyl acetic acid (BAY x1005; 3 microM), whereas coadministration of L-NOARG with the cyclooxygenase inhibitor indomethacin (10 microM) did not change the effect of L-NOARG alone. L-NOARG (100 microM) did not affect the cumulative concentration-response relations for either leukotriene (LT) D4 (0.1-100 nM) or histamine (1-30 microM). The NO donor NONOate (0.001-100 microM) was ineffective in GPLP but potently relaxed precontracted guinea pig pulmonary artery. Furthermore, L-NOARG enhanced the release of LTE4 and decreased the release of prostaglandin E2 induced by OVA. In conclusion, endogenous NO exerts an inhibitory effect on antigen-induced contractions in the peripheral lung. The action of NO apparently involves inhibition of the release of mediators rather than direct relaxation of airway smooth muscle. The findings support the belief that endogenous NO has a protective anti-inflammatory effect in the airways.

Lipopolysaccharide induces 5-lipoxygenase-activating protein gene expression in THP-1 cells via a NF-κB and C/EBP-mediated mechanism

We examined induced expression of the 5-lipoxygenase-activating protein (FLAP), which is critical for leukotriene synthesis in mononuclear phagocytes. Prolonged exposure to the bacterial component, lipopolysaccharide (LPS), increased FLAP gene transcription, mRNA expression, and protein expression in the human monocyte-like THP-1 cell line. Activation and inhibition of the NF-κB pathway modulated LPS induction of FLAP gene expression. An NF-κB-mediated mechanism of action was supported by overexpression of dominant-negative IκBα and p50/p65 proteins. EMSA/supershift and DNase I footprint analyses revealed that p50 binds to an NF-κB site located in the proximal FLAP promoter, while chromatin immunoprecipitation assays demonstrated that LPS induced binding of p50 but not of p65. Moreover, EMSA/supershift analyses demonstrated that LPS induced time-dependent binding of THP-1 nuclear extracts (containing p50) to this promoter region. Mutation of the NF-κB site decreased basal promoter activity and abolished the p50- and p65-associated induction. EMSA/supershift analyses also demonstrated that LPS induced binding of THP-1 nuclear extracts [containing CCAAT/enhancer binding protein (C/EBP)-α, -δ, and -ε] to a C/EBP site located adjacent to the NF-κB site in the FLAP promoter. We conclude that LPS enhances FLAP gene expression via both NF-κB- and C/EBP-mediated transcriptional mechanisms in mononuclear phagocytes.

Modulation of pulmonary leukotriene B4 production by cyclooxygenase-2 inhibitors and lipopolysaccharide

PURPOSE

Emerging data continue to link carcinogenesis to inflammatory events involving the eicosanoid metabolic pathways. We therefore evaluated the effects of cyclooxygenase (COX)-2 inhibition on leukotriene (LT) B(4) synthesis in the lungs of active smokers, as part of a pilot lung cancer chemoprevention study with celecoxib (Celebrex), an oral COX-2 inhibitor.

EXPERIMENTAL DESIGN

Bronchoalveolar lavage was performed before celecoxib treatment and after 1 month of celecoxib treatment to recover alveolar macrophages (AMs) and lining fluid for study. After harvest, AMs were immediately stimulated in vitro with the calcium ionophore A23187. AMs obtained from smokers before treatment and from ex-smoker control subjects were also cultured overnight with SC58236, a selective COX-2 inhibitor, with or without lipopolysaccharide stimulation.

RESULTS

Treatment with oral celecoxib only modestly increased LTB(4) levels in bronchoalveolar lavage, without increasing the mRNA transcription of 5-lipoxygenase (5-LOX) or 5-LOX-activating protein in AMs, whereas the acute calcium ionophore-stimulated LTB(4) production from smokers' AMs was markedly increased by 10.6-fold. In addition, smokers' AMs were twice as responsive in producing LTB(4) when exposed to lipopolysaccharide compared with ex-smokers' AMs. Concomitant COX-2 inhibition with SC58236, however, did not significantly impact these changes, whereas the 5-LOX inhibitor Zileuton blocked the generation of LTB(4) in a dose-responsive manner. Finally, cycloheximide increased the production of LTB(4) under all conditions, suggesting a shunting phenomenon and/or the presence of pathway inhibitors.

CONCLUSIONS

Our findings suggest that whereas oral celecoxib is capable of modulating LTB(4) production in the lung microenvironment, under physiologic conditions, this effect is probably not functionally significant.

Leukotrienes: underappreciated mediators of innate immune responses

Leukotrienes are bronchoconstrictor and vasoactive lipid mediators that are targets in the treatment of asthma. Although they are increasingly recognized to exert broad proinflammatory effects, their role in innate immune responses is less well appreciated. These molecules are indeed synthesized by resident and recruited leukocytes during infection. Acting via cell surface G protein-coupled receptors and subsequent intracellular signaling events, they enhance leukocyte accumulation, phagocyte capacity for microbial ingestion and killing, and generation of other proinflammatory mediators. Interestingly, a variety of acquired states of immunodeficiency, such as HIV infection and malnutrition, are characterized by a relative deficiency of leukotriene synthesis. The data reviewed herein point to leukotrienes as underappreciated yet highly relevant mediators of innate immunity.

Lysosomal phospholipase A2 is selectively expressed in alveolar macrophages

Lung surfactant is the surface-active agent comprised of phospholipids and proteins that lines pulmonary alveoli. Surfactant stabilizes the alveolar volume by reducing surface tension. Previously, we identified a lysosomal phospholipase A2, termed LPLA2, with specificity toward phosphatidylcholine and phosphatidylethanolamine. The phospholipase is localized to lysosomes, is calcium-independent, has an acidic pH optimum, and transacylates ceramide. Here, we demonstrate that LPLA2 is selectively expressed in alveolar macrophages but not in peritoneal macrophages, peripheral blood monocytes, or other tissues. Other macrophage-associated phospholipase A2s do not show a comparable distribution. LPLA2 is of high specific activity and recognizes disaturated phosphatidylcholine as a substrate. The lysosomal phospholipase A2 activity is six times lower in alveolar macrophages from mice with a targeted deletion of the granulocyte macrophage colony-stimulating factor (GM-CSF), a model of impaired surfactant catabolism, compared with those from wild-type mice. However, LPLA2 activity and protein levels are measured in GM-CSF null mice in which GM-CSF is expressed as a transgene under the control of the surfactant protein C promoter. Thus LPLA2 may be a major enzyme of pulmonary surfactant phospholipid degradation by alveolar macrophages and may be deficient in disorders of surfactant metabolism.

5-Lipoxygenase Pathway, Dendritic Cells, and Adaptive Immunity

5-lipoxygenase (5-LO) pathway is the major source of potent proinflammatory leukotrienes (LTs) issued from the metabolism of arachidonic acid (AA), and best known for their roles in the pathogenesis of asthma. These lipid mediators are mainly released from myeloid cells and may act as physiological autocrine and paracrine signalling molecules, and play a central role in regulating the interaction between innate and adaptive immunity. The biological actions of LTs including their immunoregulatory and proinflammatory effects are mediated through extracellular specific G-protein-coupled receptors. Despite their role in inflammatory cells, such as neutrophils and macrophages, LTs may have important effects on dendritic cells (DC)-mediated adaptive immunity. Several lines of evidence show that DC not only are important source of LTs, but also become targets of their actions by producing other lipid mediators and proinflammatory molecules. This review focuses on advances in 5-LO pathway biology, the production of LTs from DC and their role on various cells of immune system and in adaptive immunity.

Human group V phospholipase A2 induces group IVA phospholipase A2-independent cysteinyl leukotriene synthesis in human eosinophils

We previously reported that exogenously added human group V phospholipase A2 (hVPLA2) could elicit leukotriene B4 biosynthesis in human neutrophils through the activation of group IVA phospholipase A2 (cPLA2) (Kim, Y. J., Kim, K. P., Han, S. K., Munoz, N. M., Zhu, X., Sano, H., Leff, A. R., and Cho, W. (2002) J. Biol. Chem. 277, 36479-36488). In this study, we determined the functional significance and mechanism of the exogenous hVPLA2-induced arachidonic acid (AA) release and leukotriene C4 (LTC4) synthesis in isolated human peripheral blood eosinophils. As low a concentration as 10 nm exogenous hVPLA2 was able to elicit the significant release of AA and LTC4 from unstimulated eosinophils, which depended on its ability to act on phosphatidylcholine membranes. hVPLA2 also augmented the release of AA and LTC4 from eosinophils activated with formyl-Met-Leu-Phe + cytochalasin B. A cellular fluorescent PLA2 assay showed that hVPLA2 had a lipolytic action first on the outer plasma membrane and then on the perinuclear region. hVPLA2 also caused the translocation of 5-lipoxygenase from the cytosol to the nuclear membrane and a 2-fold increase in 5-lipoxygenase activity. However, hVPLA2 induced neither the increase in intracellular calcium concentration nor cPLA2 phosphorylation; consequently, cPLA2 activity was not affected by hVPLA2. Pharmacological inhibition of cPLA2 and the hVPLA2-induced activation of eosinophils derived from the cPLA2-deficient mouse corroborated that hVPLA2 mediates the release of AA and leukotriene in a cPLA2-independent manner. As such, this study represents a unique example in which a secretory phospholipase induces the eicosanoid formation in inflammatory cells, completely independent of cPLA2 activation.

5-lipoxygenase and FLAP

The initial steps in the biosynthesis of leukotrienes from arachidonic acid are carried out by the enzyme 5-lipoxygenase (5-LO). In intact cells, the helper protein 5-LO activating protein (FLAP) is necessary for efficient enzyme utilization of endogenous substrate. The last decade has witnessed remarkable progress in our understanding of these two proteins. Here we review the molecular and cellular aspects of the expression, function, and regulation of 5-LO and FLAP.

A leukotriene receptor antagonist modulates iNOS in the lung and in a leukotriene-free cell model

Nitric oxide (NO), an important cell signaling molecule, is considered a marker of inflammatory response and is elevated in asthmatics. This study investigated the effects of montelukast (a leukotriene receptor antagonist) on iNOS expression and activity in a Brown Norway (BN) rat allergic inflammation model and in L2 lung epithelial cells. Allergic inflammation was induced by ovalbumin (OVA) injection in BN rats followed by treatment with either montelukast or dexamethasone (DX). Allergen inhalation was performed, and post-allergen Penh was measured 5 min after the challenge. Cysteinyl leukotriene levels were measured in bronchoalveolar lavage (BAL) fluid and lung iNOS expression and activity determined. These parameters were also measured in cytokine stimulated L2 lung epithelial cells. iNOS expression was significantly higher in OVA challenged rats compared to the naive, DX, and montelukast treated groups, as confirmed by immunohistochemistry and Western blot analysis. However, no significant differences in NOS activity were found. Cysteinyl leukotriene measured in BAL was significantly higher in all OVA challenged rats compared to naive controls. Incubation of L2 cells with a mixture of interferon gamma (IFNgamma), lipopolysaccharide (LPS), and tumor necrosis factor (TNFalpha) resulted in high levels of nitrite formation resulting from iNOS induction. Treatment of cytokine stimulated cells with DX or montelukast significantly decreased iNOS expression and activity. No detectable cysteinyl leukotrienes were found in the supernatant fluid of L2 cells. This study confirms the ability of montelukast to modulate iNOS function and raises the possibility that changes in iNOS expression and activity may occur via pathways independent of cysteinyl leukotrienes.

Prolonged lipopolysaccharide inhibits leukotriene synthesis in peritoneal macrophages: mediation by nitric oxide and prostaglandins

Resident rat peritoneal macrophages synthesize a variety of prostanoids and leukotrienes from arachidonic acid. Overnight treatment with lipopolysaccharide (LPS) induces the synthesis of cyclooxygenase-2 (COX-2) and an altered prostanoid profile that emphasizes the preferential conversion of arachidonic acid to prostacyclin and prostaglandin E2. In these studies, we report that exposure to LPS also caused a strong suppression of 5-lipoxygenase but not 12-lipoxygenase activity, indicated by the inhibition of synthesis of both leukotriene B4 and 5-hydroxyeicosatetraenoic acid (5-HETE), but not of 12-HETE. Inhibition of 5-lipoxygenase activity by LPS was both time- and dose-dependent. Treatment of macrophages with prostaglandin E2 partially inhibited leukotriene synthesis, and cyclooxygenase inhibitors partially blocked the inhibition of leukotriene generation in LPS-treated cells. In addition to COX-2, nitric oxide synthase (NOS) was also induced by LPS. Treatment of macrophages with an NO donor mimicked the ability of LPS to significantly reduce leukotriene B4 synthesis. Inhibition of NOS activity in LPS-treated cells blunted the suppression of leukotriene synthesis. Inhibition of both inducible NOS and COX completely eliminated leukotriene suppression. Finally, macrophages exposed to prolonged LPS demonstrated impaired killing of Klebsiella pneumoniae and the combination of NOS and COX inhibitors restored killing to the control level. These results indicate that prolonged exposure to LPS severely inhibits leukotriene production via the combined action of COX and NOS products. The shift in mediator profile, to one that minimizes leukotrienes and emphasizes prostacyclin, prostaglandin E2 and NO, provides a signal that reduces leukocyte function, as indicated by impaired killing of Gram-negative bacteria.

Lipopolysaccharide down-regulates the leukotriene C4 synthase gene in the monocyte-like cell line, THP-1

We studied the effects of LPS on cysteinyl leukotriene (LT) synthesis and LTC(4) synthase expression in mononuclear phagocytes. Conditioning of the monocyte-like cell line, THP-1, with LPS for 7 days resulted in significantly decreased ionophore-stimulated LTC(4) release. The putative LPS receptor, Toll-like receptor 4, was expressed in THP-1 cells. LPS down-regulated LTC(4) synthase mRNA in THP-1 cells in a dose- and time-dependent manner, with down-regulation observed as early as 4 h. Conditioning of actinomycin D-treated cells with LPS resulted in no change in the rate of LTC(4) synthase mRNA decay. LPS treatment of THP-1 cells, transiently transfected with a LTC(4) synthase promoter (1.35 kb)-reporter construct, decreased promoter activity. Neutralization of TNF-alpha and inhibition of mitogen-activated protein kinase kinase/extracellular signal-regulated kinase did not inhibit the effect of LPS. Treatment of cells with a Toll-like receptor 4-blocking Ab and an inhibitor of NF-kappaB activation resulted in inhibition of the LPS effect, while activation of NF-kappaB and p50/p65 overexpression down-regulated the LTC(4) synthase gene. LPS down-regulates cysteinyl LT release and LTC(4) synthase gene expression in mononuclear phagocytes by an NF-kappaB-mediated mechanism.

Cysteinyl leukotriene interactions with other mediators and with glucocorticosteroids during airway inflammation

Unexpected aspects of the antiasthmatic efficacy of leukotriene modifiers and glucocorticosteroids have been observed. For both classes, the observed effects may be partially explainable on the basis of underrecognized interactions involving leukotrienes. This review examines the interactions between leukotrienes and other mediators of asthma. It details the effects of glucocorticosteroids on leukotriene synthesis and on leukocyte populations in asthmatic airways. Unexpected controller effects of the leukotriene modifiers may reflect the fact that leukotrienes and other mediators of asthma, such as T(H)2 cytokines, positively influence each other's generation. The ability of the leukotriene modifiers to disrupt such extensive interactions means that other relevant mediators are targeted indirectly by leukotriene blockade. Among asthma therapies, the glucocorticosteroids have numerous anti-inflammatory activities, but their effects may be unpredictable. Many processes involved in inflammation appear to escape modulation by glucocorticosteroids, including leukotriene synthesis, and leukotriene generation is among them. Understanding whether glucocorticosteroids reduce cysteinyl leukotriene levels in the airway is important in determining the clinical value of combining glucocorticosteroid therapy with leukotriene modifier therapy.

Effects of non-steroidal anti-inflammatory drugs on cyclo-oxygenase and lipoxygenase activity in whole blood from aspirin-sensitive asthmatics vs healthy donors

1. Cyclo-oxygenase (COX) and lipoxygenase (LO) share a common substrate, arachidonic acid. Aspirin and related drugs inhibit COX activity. In a subset of patients with asthma aspirin induces clinical symptoms associated with increased levels of certain LO products, a phenomenon known as aspirin-sensitive asthma. The pharmacological pathways regulating such responses are not known. 2. Here COX-1 and LO activity were measured respectively by the formation of thromboxane B(2) (TXB(2)) or leukotrienes (LT) C(4), D(4) and E(4) in whole blood stimulated with A23187. COX-2 activity was measured by the formation of prostaglandin E(2) (PGE(2)) in blood stimulated with lipopolysaccharide (LPS) for 18 h. 3. No differences in the levels of COX-1, COX-2 or LO products or the potency of drugs were found in blood from aspirin sensitive vs aspirin tolerant patients. Aspirin, indomethacin and nimesulide inhibited COX-1 activity, without altering LO activity. Indomethacin, nimesulide and the COX-2 selective inhibitor DFP [5,5-dimethyl-3-(2-isopropoxy)-4-(4-methanesulfonylphenyl)-2(5H)-furanone] inhibited COX-2 activity. NO-aspirin, like aspirin inhibited COX-1 activity in blood from both groups. However, NO-aspirin also reduced LO activity in the blood from both patient groups. Sodium salicylate was an ineffective inhibitor of COX-1, COX-2 or LO activity in blood from both aspirin-sensitive and tolerant patients. 4. Thus, when COX activity in the blood of aspirin-sensitive asthmatics is blocked there is no associated increase in LO products. Moreover, NO-aspirin, unlike other NSAIDs tested, inhibited LO activity in the blood from both aspirin sensitive and aspirin tolerant individuals. This suggests that NO-aspirin may be better tolerated than aspirin by aspirin-sensitive asthmatics.

Interaction between nitric oxide, reactive oxygen intermediates, and peroxynitrite in the regulation of 5-lipoxygenase metabolism

We have shown that overnight lipopolysaccharide (LPS) suppresses alveolar macrophage (AM) leukotriene (LT) synthesis mediated in part by induction of inducible nitric oxide synthase (iNOS) and NO production. Here we examined the possibility that reactive oxygen intermediates (ROI) generated by LPS pretreatment contribute to the suppression of 5-lipoxygenase (5-LO) metabolism. Pretreatment of AM with xanthine/xanthine oxidase, which generates high concentrations of ROI, resulted in suppression of LT synthetic capacity. Since NO and ROI reactive species are known to react and form peroxynitrite (ONOO(-)), we examined the effect of ONOO(-) on 5-LO metabolism. Exogenous ONOO(-) caused a dose-dependent suppression of recombinant 5-LO cell-free activity. ONOO(-) also suppressed LT synthesis in intact AM, which was reversed by the ONOO(-) scavenger tetrakis(4-benzoic acid)porphyrin. ONOO(-) treatment also resulted in dose-dependent nitrotyrosination and S-nitrosylation of the recombinant 5-LO enzyme. Since the direct 5-LO inhibitor zileuton prevents the LPS-induced suppression of LT synthesis, we examined if 5-LO itself was the source of ROI. Zileuton reduced ROI generation in LPS-treated cells. These studies identify an important role for ROI and ONOO(-) in the suppression of 5-LO metabolism by LPS.

Macrophage defences against respiratory tract infections

Pulmonary macrophages with a key role in defence against respiratory infection are a heterogeneous family of cells with phagocytic, antigen processing and immunomodulatory functions. Macrophages are important in both innate and acquired immunity in the respiratory tract, and have a role in lung defence against viruses, bacteria, mycobacteria and fungi. Interactions of pathogens with lung macrophages is strongly influenced by soluble immune components including complement, collectins and immunoglobulins. Macrophage function can be modulated by cytokines, environmental exposures, recent and chronic infection including HIV infection, drug therapy and gene transfer.