Platelet-activating factor induces leukotriene C4 synthesis by purified human eosinophils

Asthma and the Risk of Rheumatoid Arthritis: An Insight into the Heterogeneity and Phenotypes of Asthma

Asthma is traditionally regarded as a chronic airway disease, and recent literature proves its heterogeneity, based on distinctive clusters or phenotypes of asthma. In defining such asthma clusters, the nature of comorbidity among patients with asthma is poorly understood, by assuming no causal relationship between asthma and other comorbid conditions, including both communicable and noncommunicable diseases. However, emerging evidence suggests that the status of asthma significantly affects the increased susceptibility of the patient to both communicable and noncommunicable diseases. Specifically, the impact of asthma on susceptibility to noncommunicable diseases such as chronic systemic inflammatory diseases (e.g., rheumatoid arthritis), may provide an important insight into asthma as a disease with systemic inflammatory features, a conceptual understanding between asthma and asthma-related comorbidity, and the potential implications on the therapeutic and preventive interventions for patients with asthma. This review discusses the currently under-recognized clinical and immunological phenotypes of asthma; specifically, a higher risk of developing a systemic inflammatory disease such as rheumatoid arthritis and their implications, on the conceptual understanding and management of asthma. Our discussion is divided into three parts: literature summary on the relationship between asthma and the risk of rheumatoid arthritis; potential mechanisms underlying the association; and implications on asthma management and research.

Asthma Status and Risk of Incident Myocardial Infarction: A Population-Based Case-Control Study

BACKGROUND

The role of asthma status and characteristics of asthma in the risk of myocardial infarction (MI) are poorly understood.

OBJECTIVE

We determined whether asthma and its characteristics are associated with risk of MI.

METHODS

The study was designed as a population-based retrospective case-control study, which included all eligible incident MI cases between November 1, 2002, and May 31, 2006, and their matched controls. Asthma was ascertained using predetermined criteria. Active (current) asthma was defined as the occurrence of asthma-related episodes (asthma symptoms, use of asthma medications, unscheduled medical or emergency department visit, or hospitalization for asthma) within 1 year before MI index date.

RESULTS

There were 543 eligible incident MI cases during the study period. Of the 543 MI cases, 81 (15%) had a history of asthma before index date of MI, whereas 52 of 543 controls (10%) had such a history (adjusted odds ratio [OR]: 1.68; 95% CI: 1.06-2.66) adjusting for risk factors for MI and comorbid conditions (excluding chronic obstructive lung disease). Although inactive asthma did not increase the risk of MI, individuals with active asthma had a higher odds of MI, compared with those without asthma (adjusted OR: 3.18; 95% CI: 1.57-6.44) without controlling for chronic obstructive pulmonary disease (COPD). After adjusting for COPD, although asthma overall was no longer statistically significant (adjusted OR: 1.34, 95% CI: 0.84-2.15), active asthma still was associated (adjusted OR: 2.33, 95% CI: 1.12-4.82).

CONCLUSION

Active asthma is an unrecognized risk factor for MI. Further studies are needed to assess the role of asthma control and medications in the risk of MI.

Inhibition of protein kinases A and C demonstrates dual modes of response in human eosinophils stimulated with platelet-activating factor

BACKGROUND

Platelet-activating factor (PAF) is a potent stimulator of human eosinophils involved in the pathogenesis of allergic diseases. However, intracellular signaling mechanisms in eosinophils involving the PAF receptor are incompletely understood.

OBJECTIVE

We sought to determine the roles of protein kinase C (PKC) and cyclic AMP-dependent protein kinase (protein kinase A [PKA]) in signaling pathways of human eosinophils stimulated with PAF.

METHODS

After pretreatment with a PKC inhibitor, bisindolylmaleimide I, or a PKA inhibitor, H89, we investigated PAF-evoked functions, such as CD11b expression, cellular adhesion, superoxide anion generation, and degranulation in human eosinophils.

RESULTS

Preincubation of eosinophils with bisindolylmaleimide I resulted in enhancement of upregulated CD11b expression and adhesion induced by PAF. H89 pretreatment also enhanced PAF-induced cellular adhesion. Superoxide anion generation and degranulation were suppressed by means of inhibition of either PKC or PKA.

CONCLUSION

PKC and PKA negatively regulate PAF-induced CD11b upregulation and cellular adhesion but promote eosinophil effector functions, such as superoxide anion generation and degranulation. PKC and PKA modulate PAF-evoked intracellular signaling of the eosinophil function in distinct ways.

Effect of a specific cysteinyl leukotriene-receptor 1-antagonist (montelukast) on the transmigration of eosinophils across human umbilical vein endothelial cells

BACKGROUND

Leukotrienes have been implicated in the selective infiltration of eosinophils into the bronchial mucosa in asthma.

OBJECTIVE

We studied whether eosinophil transmigration through cultured human umbilical vein endothelial cells (HUVECs) can be blocked by a specific cysteinyl LT1-receptor-antagonist.

METHODS

Unstimulated and stimulated eosinophils from patients with asthma and normal controls were subjected to confluent human umbilical vein endothelial cell (HUVEC) monolayers separating the upper and lower chamber of Transwell culture plates. Unstimulated eosinophils or cells pre-incubated in the presence of the eosinophil activating cytokines GM-CSF or IL-13 were placed in the upper chambers while PAF, a potent chemoattractant factor for eosinophils, was added to the lower chamber. Migration of eosinophils was quantified by a beta-glucuronidase assay.

RESULTS

The assumption that eosinophils express CysLT1 (cysteinyl-leukotriene 1)-receptors was based on our demonstration of mRNA-expression for the CysLT-1-receptor by polymerase chain reaction on purified eosinophils. The chemotactic response to PAF was significantly reduced when eosinophils were pre-incubated with montelukast for 15 min. When eosinophils were pre-incubated with GM-CSF and/or IL-13, the migratory response to PAF was also significantly reduced by montelukast.

CONCLUSION

From these data we conclude that the specific cysteinyl LT1-receptor antagonist montelukast can inhibit PAF-induced eosinophil transmigration through cultured HUVEC monolayers.

Endogenous Platelet-Activating Factor Is Critically Involved in Effector Functions of Eosinophils Stimulated with IL-5 or IgG

Eosinophil activation and subsequent release of inflammatory mediators are implicated in the pathophysiology of allergic diseases. Eosinophils are activated by various classes of secretagogues, such as cytokines (e.g., IL-5), lipid mediators (e.g., platelet-activating factor (PAF)), and Ig (e.g., immobilized IgG). However, do these agonists act directly on eosinophils or indirectly through the generation of intermediate active metabolites? We now report that endogenous PAF produced by activated eosinophils plays a critical role in eosinophil functions. Human eosinophils produced superoxide when stimulated with immobilized IgG, soluble IL-5, or PAF. Pretreating eosinophils with pertussis toxin abolished their responses to these stimuli, suggesting involvement of a metabolite(s) that acts on G proteins. Indeed, PAF was detected in supernatants from eosinophils stimulated with IgG or IL-5. Furthermore, structurally distinct PAF antagonists, including CV6209, hexanolamine PAF, and Y-24180 (israpafant), inhibited IgG- or IL-5-induced superoxide production and degranulation. Previous reports indicated that exogenous PAF stimulates eosinophil eicosanoid production through formation of lipid bodies. We found in this study that IgG or IL-5 also induces lipid body formation and subsequent leukotriene C4 production mediated by endogenous PAF. Finally, inhibition of cytosolic phospholipase A2, one of the key enzymes involved in PAF synthesis, attenuated both PAF production and effector functions of eosinophils. These findings suggest that endogenous PAF plays important roles in eosinophil functional responses to various exogenous stimuli, such as cytokines and Igs. Therefore, inhibition of PAF synthesis or action may be beneficial for the treatment of eosinophilic inflammation.

Inhibition by the anti-mitotic drug doxorubicin of platelet-activating-factor-induced late eosinophil accumulation in rats

Platelet-activating factor (PAF) has been shown, in the rat model of pleural inflammation, to induce the generation of an intermediate proteic factor able to cause eosinophil proliferation in vitro. This study was undertaken to investigate the effect of the anti-mitotic compound doxorubicin on PAF-induced eosinophilia in rats, in order to evaluate the contribution of local cell proliferation to this phenomenon. The late eosinophil infiltration caused by another chemoattractant leukotriene B4 was used for comparison. We observed that local treatment with doxorubicin (20 and 40 microg/cavity), given 6 h after PAF (1 microg/cavity), suppressed the eosinophil accumulation within 24 h, whilst only the higher dose was effective when the drug was given 12 h post-PAF. An effect on chemotaxis was ruled out, since local doxorubicin (40 microg/cavity) failed to modify the eosinophil migration noted 24 h after leukotriene B4 (0.5 microg/cavity) and the neutrophil/eosinophil infiltration noted at 6 h after PAF injection. Transfer of the pleural fluids collected 6 h after PAF from donors to recipient rats caused significant eosinophil accumulation in the recipient rats, an effect which was inhibited by the co-administration of doxorubicin (40 microg/cavity). No inhibitory effect was noted when the drug was given 6 h after the pleural fluids were transferred. We also found no change in the number of blood or bone marrow eosinophils after PAF stimulation. We conclude that doxorubicin selectively impaired the late eosinophil accumulation triggered by PAF in the pleural cavity of rats, clearly indicating that local cell proliferation seems to contribute to the development of this inflammatory response.

Inhibition of eosinophil chemotaxis by chronic blockade of nitric oxide biosynthesis

The effect of chronic N omega-nitro-L-arginine methyl ester (L-NAME) treatment on the in vivo eosinophil migration induced by bradykinin, platelet-activating factor (PAF), lipopolysaccharide and carrageenin has been investigated in the rat using the pleurisy model. The in vitro (microchemotaxis chamber) eosinophil migration induced by N-formyl-methionyl-leucyl-phenylalanine (fMLP), PAF and zymosan-activated serum was also evaluated in the rat. The eosinophils were obtained from the peritoneal cavity of male Wistar rats and isolated on a discontinuous metrizamide gradient. Chronic inhibition of nitric oxide biosynthesis was achieved by adding L-NAME to the drinking water to give an intake of approximately 75 mumol/rat/day for 4 weeks. Rats treated chronically with L-NAME developed a significant level of hypertension (163 +/- 4.8 mmHg; P < 0.01) compared with animals which received either the same dose of the inactive enantiomer D-NAME (124 +/- 3.2 mmHg) or tap water alone (119 +/- 1.6 mmHg). The intrapleural injection of bradykinin (50 micrograms), PAF (1 microgram), lipopolysaccharide (0.25 microgram) and carrageenin (125 micrograms) into untreated rats in vivo induced a significant level of eosinophil migration by 24 h post-injection. This migration was markedly reduced in L-NAME-treated rats. Eosinophils obtained from untreated rats showed a significant level of migration in vitro in response to fMLP (5 X 10(-8) M), PAF (10(-8) M) and zymosan-activated serum (27 microliters). In contrast, the migration induced by these chemotactic agents was markedly reduced in cells isolated from animals treated chronically with L-NAME. L-Arginine (5.5 mM), but not D-arginine (5.5 mM), restored the ability of eosinophils from L-NAME-treated animals to migrate in response to fMLP. Our results indicate that nitric oxide plays a major role in the in vivo and ex vivo migration of eosinophils.

Priming effect of platelet activating factor on leukotriene C4 from stimulated eosinophils of asthmatic patients

BACKGROUND

Eosinophils from asthmatic patients are known to release greater amounts of leukotrienes than normal eosinophils when stimulated by the calcium ionophore A23187. The effect of platelet activating factor (PAF) in priming eosinophils was investigated.

METHODS

Eosinophils were obtained from 18 asthmatic patients and 18 healthy donors. Cells separated by the Percoll gradients were incubated with PAF (C-18) for 30 minutes and then stimulated with the calcium ionophore A23187 (2.5 microM) for 15 minutes. The amount of leukotriene C4 (LTC4) in supernatants was measured using a combination of high pressure liquid chromatography and radioimmunoassay.

RESULTS

The mean (SD) amount of LTC4 released by eosinophils from asthmatic patients upon stimulation with the calcium ionophore A23187 alone was 27.9 (9.9) ng/10(6) cells (n = 6). The amount of LTC4 released following stimulation with the calcium ionophore A23187 after pretreatment with PAF (1, 5, and 10 microM) was 57.2 (8.9), 75.1 (14.3), and 52.6 (10.7) ng/10(6) cells (n = 6), respectively. Trace amounts of LTC4 (0.9 (0.02) ng/10(6) cells, n = 6) were detected in the supernatant of the cells after stimulation by PAF alone (5 microM). The amount of LTC4 released upon stimulation by calcium ionophore A23187 alone in eosinophils from healthy donors was 10.3 (3.7) ng/10(6) cells (n = 4). The amounts of LTC4 released upon stimulation with calcium ionophore A23187 after pretreatment with PAF at concentrations of 1, 5, and 10 microM were 11.9 (3.5), 17.8 (5.6), and 12.7 (5.1) ng/10(6) cells (n = 4), respectively. Trace amounts of LTC4 (0.6 (0.02) ng/10(6) cells, n = 4) were detected in the supernatant of the cells upon stimulation with PAF alone (5 microM). The amounts of LTC4 released upon stimulation with calcium ionophore A23187 after pretreatment with lyso-PAF at concentrations of 1, 5, and 10 microM (n = 4 or 6) were 30.8 (5.2), 22.9 (5.1), and 27.3 (4.3) ng/10(6) cells (n = 6) from the eosinophils of asthmatic patients and 13.7 (3.3), 15.2 (4.9), and 14.7 (3.8) ng/10(6) cells (n = 4) from the eosinophils of healthy donors.

CONCLUSIONS

The results indicated that PAF enhanced LTC4 formation by eosinophils obtained from asthmatic patients stimulated with the calcium ionophore A23187, but not those obtained from normal subjects.

Platelet-activating factor and leukotrienes in acute otitis media, secretory otitis media, and chronic otitis media on the acute excerbation

The present study was carried out for the purpose of measuring platelet activating factor (PAF) and leukotrienes (LTs) in middle ear fluid (MEF) or in otorrhea of children with acute otitis media (AOM), with secretory otitis media (SOM), and with chronic otitis media (COM) on the acute exacerbation. PAF, LTC4, and LTD4+LTE4 concentrations were measured by radioimmunoassay in purulent MEFs obtained from 15 ears of 15 children with AOM, in mucoid or gluey MEFs from 16 ears of 15 children with SOM, and in purulent otorrhea from 9 ears of 9 children with COM on the acute exacerbation. PAF concentrations were 25.4 +/- 9.0 ng/mg total phospholipid (TPL) in AOM, 4.9 +/- 1.9 ng/mg TPL in SOM, 20.7 +/- 15.7 ng/mg TPL in COM, demonstrating significant differences between AOM and SOM (p < 0.05) and between COM and SOM (p < 0.01). LTC4 concentrations were 92.9 +/- 73.8 pg/mg total protein (TP) in AOM, 52.0 +/- 42.5 pg/mg TP in SOM, and 28.5 +/- 11.2 pg/mg TP in COM. LTD4+LTE4 concentrations were 326.5 +/- 177.0 pg/mg TP in AOM, 288.2 +/- 144.6 pg/mg TP in SOM, and 94.0 +/- 58.1 pg/mg TP in COM, demonstrating significant differences between AOM and COM and between SOM and COM (p < 0.01). The results obtained indicated that PAF was involved in the onset of AOM and COM on the acute exacerbation, and that LTs played an important role in SOM.

Inhibition of histamine and eicosanoid release from dispersed human lung cells in vitro by quinotolast

We have examined the effects of a new anti-allergic drug, quinotolast [sodium 5-(4-oxo-1-phenoxy-4H-quinolizine-3-carboxamido) yetrazolate monohydrate], in inhibiting the release of histamine and the generation of leukotriene (LT) C4 and prostaglandin (PG) D2 from dispersed human lung cells and compared this with those of its active metabolite in the rat, hydroxy quinotolast, and reference drugs, tranilast and sodium cromoglycate (SCG). Quinotolast in the concentration range of 1-100 micrograms/ml inhibited histamine and LTC4 release in a concentration-dependent manner. The inhibitory effect of quinotolast on histamine release from dispersed lung cells was largely independent of the preincubation period, no tachyphylaxis being observed. Hydroxy quinotolast and tranilast showed a weak inhibition of histamine release only when the drugs were added to the cells simultaneously with anti-IgE challenge. Quinotolast, 100 micrograms/ml, and SCG, 1 mM, significantly inhibited PGD2 and LTC4 release. Quinotolast inhibited PGD2 release by 100% and LTC4 release by 54%, whereas SCG inhibited PDG2 release by 33% and LTC4 release by 100%. No cross-tachyphylaxis between quinotolast and SCG was observed. The results demonstrated that quinotolast showed a significant inhibition of inflammatory mediators from human dispersed lung cells, suggesting that quinotolast is a good candidate for a clinical anti-allergic drug.

Pharmacologic heterogeneity of human lung and colon cells: effect of terfenadine and cetirizine

H1-blockers may have antiallergic properties which cause the blocking of eicosanoid release, and the effect of these drugs may differ according to the phenotype of mast cells. This study examined the ability of terfenadine and cetirizine to inhibit the release of arachidonic acid-derived mediators from human lung and colon cells. Dispersed cells were challenged with anti-IgE in the presence or absence of 10 microM of terfenadine or cetirizine, and the release of prostaglandin (PG)D2 and leukotriene (LT)C4/D4 was assessed by enzyme immunoassay (EIA). Terfenadine caused significant inhibition of both PGD2 and LTC4/D4 (49 +/- 9 and 29 +/- 19%, respectively) from human lung cells but had a less marked effect on PGD2 release from human colon cells (21 +/- 9% for PGD2 and 18 +/- 9% for LTC4/D4). In contrast, although cetirizine caused significant inhibition of both mediators measured in lung cells (38 +/- 16% for PGD2 and 34 +/- 19% for LTC4), it did not cause any significant inhibition of either mediator from human colon cells. These findings suggest that H1-antagonists may have additional properties, and the differential effects of cetirizine on lung and colon tissue may indicate differences in mast cell phenotype.

Platelet-activating factor: receptors and signal transduction

During the past two decades, studies describing the chemistry and biology of PAF have been extensive. This potent phosphoacylglycerol exhibits a wide variety of physiological and pathophysiological effects in various cells and tissues. PAF acts, through specific receptors and a variety of signal transduction systems, to elicit diverse biochemical responses. Several important future directions can be enumerated for the characterization of PAF receptors and their attendant signalling mechanisms. The recent cloning and sequence analysis of the gene for the PAF receptor will allow a number of important experimental approaches for characterizing the structure and analysing the function of the various domains of the receptor. Using molecular genetic and immunological technologies, questions relating to whether there is receptor heterogeneity, the precise mechanism(s) for the regulation of the PAF receptor, and the molecular details of the signalling mechanisms in which the PAF receptor is involved can be explored. Another area of major significance is the examination of the relationship between the signalling response(s) evoked by PAF binding to its receptor and signalling mechanisms activated by a myriad of other mediators, cytokines and growth factors. A very exciting recent development in which PAF receptors undoubtedly play a role is in the regulation of the function of various cellular adhesion molecules. Finally, there remain many incompletely characterized physiological and pathophysiological situations in which PAF and its receptor play a crucial signalling role. Our laboratory has been active in the elucidation of several tissue responses in which PAF exhibits major autocoid signalling responses, e.g. hepatic injury and inflammation, acute and chronic pancreatitis, and cerebral stimulation and/or trauma. As new experimental strategies are developed for characterizing the fine structure of the molecular mechanisms involved in tissue injury and inflammation, the essential role of PAF as a primary signalling molecule will be affirmed. Doubtless the next 20 years of experimental activity will be even more interesting and productive than the past two decades.

Modulation of eicosanoid and histamine release from human dispersed lung cells by terfenadine

Terfenadine is an H1-blocker that may have antiallergic properties. A study was carried out to examine the ability of terfenadine to inhibit the release of histamine and arachidonic-acid-derived mediators from human lung cells. Cells were dispersed from fresh human lung tissue obtained from four accident victims whose hearts were donated for transplantation and four lung cancer resections. Cells were dispersed by enzymatic digestion with type XIV protease and chymopapain, and this resulted in a cell population containing approximately 5% mast cells. The remaining cells were mainly macrophages. The cells were challenged with anti-IgE at a 1/1000 dilution. Cells were challenged without terfenadine and after a preincubation of 0.1, 1, and 10 mumol terfenadine. The release of PGD2 and LTC4/D4 was assessed with an EIA. Histamine was assayed by RIA with a monoclonal antibody against acylated histamine. A release of both eicosanoids and histamine was observed in all experiments. An inhibition of eicosanoids was observed at both 1 and 10 mumol terfenadine (median percentage of inhibition of PGD2: 38.00 +/- 15.65 and 56.00 +/- 13.12; median percentage of inhibition of LTC4/D4: 37.5 +/- 19.80 and 52.5 +/- 26.8). On the other hand, histamine release was not blocked by terfenadine. Terfenadine inhibits, in a dose-dependent manner, the release of eicosanoids after challenge of dispersed lung cells by anti-IgE, and this effect may have some clinical relevance.

Platelet activating factor amplifies human neutrophil adherence to bovine endothelial cells: evidence for a lipoxygenase dependent mechanism

Platelet activating factor (PAF) is a potent lipid mediator that induces the release of leukotrienes and prostaglandins from various cells and tissues. We examined the capacity of PAF alone and in combination with soluble stimuli to enhance eicosanoid synthesis and adherence of human neutrophils. Neutrophils were preincubated with PAF and washed before exposure to the soluble stimuli F-Met-Leu-Phe (FMLP), calcium ionophore A23187, and phorbol myristate acetate. Preincubation of neutrophils with 1 microM PAF enhanced the release of both LTB4 and LTC4 in response to each of the three agonists, in contrast with the unprimed neutrophils. Priming was specific for PAF since lyso-PAF was inactive. Priming concentrations of PAF also augmented the adherence of neutrophils to endothelium in the presence of the soluble agonists A23187, phorbol myristate acetate, and FMLP. The priming effect of PAF on eicosanoid release and neutrophil adherence was shown to have similar time- and dose-dependent effects. Further, the priming effects of PAF on adherence could be reversed by preincubation of neutrophils with the lipoxygenase inhibitors nordihydroguiaretic acid and 5,8,11,14-ETYA but not by preincubation with the cyclooxygenase inhibitor indomethacin. These data demonstrate that PAF amplifies neutrophil adherence to endothelium through a lipoxygenase dependent mechanism.

Cytokines, platelet activating factor and eosinophils in asthma

Understanding of the pathogenesis of asthma has increased considerably during the past few years. These advances were possible through scientific progress in three areas which contribute to this complex and multifaceted disease: (a) the much clearer understanding of eosinophil function; (b) the defining of lipid mediators in tissue inflammation and bronchial obstruction; and (c) the growing knowledge about the biological action of a new class of protein hormones, collectively called cytokines. In line with this, evidence has accumulated of how these components may interact with each other in providing the basis of inflammatory processes in asthma. Hence it seems appropriate to review the potential implications of this new information for the pathogenesis and therapy of this disease.

Platelet-activating factor stimulates a rapid accumulation of inositol (1,4,5)trisphosphate in guinea pig eosinophils: relationship to calcium mobilization and degranulation

The effect of platelet-activating factor (PAF) on inositol (1,4,5)trisphosphate (Ins[1,4,5]P3) mass, calcium mobilization, and the release of granule enzymes was studied on guinea pig peritoneal eosinophils (EOSs). PAF evoked a concentration-dependent accumulation of Ins(1,4,5)P3 with a drug concentration that elicits 50% of the maximum attainable response (EC50) of 10 nmol/L; the production of this second messenger was maximal at 1 mumol/L of PAF. Kinetic analysis of PAF (1 mumol/L)-induced Ins(1,4,5)P3 accumulation demonstrated it to be transient with a 3.8-fold increase over resting levels observed at 5 seconds. Thereafter, the level of Ins(1,4,5)P3 declined, returning to vehicle-treated levels 60 seconds after PAF challenge. Lyso-PAF, the inactive precursor and metabolite of PAF, was inactive at all concentrations examined. PAF also induced a rapid, concentration-dependent (EC50, 12 nmol/L) rise in the cytosolic-free calcium concentration ([Ca++]i) in fura 2-AM-loaded EOSs that was transient, peaking after the maximum increase in Ins(1,4,5)P3 mass was observed. A highly significant positive correlation was found between the peak increase in Ins(1,4,5)P3 and the peak rise in [Ca++]i. Functionally, PAF evoked a concentration-dependent release of granule constituents from both the small (arylsulfatase B; EC50, 3 nmol/L) and specific (EOS peroxidase; EC50, 2.7 nmol/L) granules that lagged, temporally, behind both Ins(1,4,5)P3 accumulation and the rise in [Ca++]i. Both the biochemical and functional effects of PAF examined in this study were antagonized by WEB 2086 (300 nmol/L), a selective PAF receptor-blocking drug. It is concluded that stimulus (PAF)-response coupling in guinea pig peritoneal EOSs may involve the receptor-mediated formation of Ins(1,4,5)P3 and subsequent release of intracellularly stored Ca++. This sequence of events may link PAF receptor activation to Ca(++)-dependent cellular responses, such as degranulation.

Airway eosinophils from actively sensitized guinea pigs exhibit enhanced superoxide anion release in response to antigen challenge

Antigen challenge of actively sensitized guinea pigs produces airway eosinophilia, airway hyperreactivity, and late-phase bronchoconstriction. The recruited eosinophils are thought to be important cells in the development of the airway hyperreactivity and the late-phase bronchoconstriction. However, the functional abilities of these eosinophils have not been determined in response to antigen challenge. The purpose of this study was to describe the characteristics of superoxide anion release from airway eosinophils obtained 24 h after ovalbumin challenge of actively sensitized guinea pigs. Eosinophils were collected by bronchoalveolar lavage. The total bronchoalveolar lavage eosinophil count was 17- to 27-fold greater in sensitized, ovalbumin-challenged guinea pigs (9.30 +/- 0.11 x 10(6)/guinea pig) than in unsensitized guinea pigs (0.35 +/- 0.07 x 10(6)/guinea pig) or sensitized, saline-challenged guinea pigs (0.56 x 10(6)/guinea pig; n = 2). The increase in eosinophils was due to increased lavage leukocyte count and increased eosinophil differential. Eosinophils were isolated on a Percoll-plasma discontinuous gradient. Two populations of eosinophils were collected, one at the 1.093 g/ml gradient step and one at the 1.107 g/ml gradient step. Unstimulated or phorbol myristate acetate (PMA)-stimulated superoxide anion release was measured by the reduction of ferricytochrome c. Unstimulated superoxide anion release from both eosinophil populations of challenged guinea pigs (4.50 +/- 2.37 and 4.07 +/- 1.48 nmol from 1.093 and 1.107 g/ml eosinophils, respectively) was 6- to 7-fold greater than superoxide anion release from eosinophils of control guinea pigs (0.74 +/- 0.43 and 0.56 +/- 025 nmol from 1.093 and 1.107 g/ml eosinophils, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Eosinophil-eicosanoid interactions: inhibition of eosinophil chemotaxis in vivo by a LTD4-receptor antagonist

Chemotaxis of guinea-pig eosinophils in vivo has been studied in the guinea-pig conjunctiva. Guinea-pig eosinophils were labelled with 111In oxine and injected i.v. into recipient animals. Circulating radioactivity remained constant over a period of 4 h but dropped transiently in response to a bolus injection (1 nmol) of platelet activating factor (PAF), suggesting that the 111In-labelled eosinophils had retained their responsiveness to PAF. Leukotriene D4 (LTD4, 1 nmol/eye) and PAF (10 nmol/eye), but not histamine (5 nmol/eye), induced a significant 2.5-fold increase in conjunctival radioactivity, a measure of eosinophil chemotaxis in vivo, after 17 h. Antigen challenge (250 micrograms/eye) in ovalbumin-sensitized animals produced larger responses (8-fold increases) at the same time point. A specific LTD4 receptor antagonist MK-571 (10 micrograms/eye) completely inhibited in vivo chemotactic responses to LTD4, and partially inhibited (54%) the responses to ovalbumin (observations subsequently confirmed by histological studies). As eosinophils may play an important role in allergic diseases, the results with MK-571 indicate that selective and potent LTD4 receptor antagonists may provide a novel therapy for allergic conjunctivitis, rhinitis and asthma.

Lecithins enhance leukotriene production from white cells

36 x 10(7) WBC were isolated from 120 ml heparinized venous blood by 5% dextran T-500 sedimentation. 20 mg egg lecithin and 20 mg dipalmitoyl lecithin were respectively pretreated in 2 ml 0.15 M Tris buffer by vibration and sonication. WBC were incubated with the pretreated lecithins for 20 min. Leukotrienes (LTs) were identified by HPLC and bioassay, and quantified with an RIA Kit. Crude incubation medium of both lecithin groups caused guinea pig ileum contractions which were antagonized with FPL55712. Incubation media were partially purified with Bond elut C18. Purified samples of both lecithin groups showed LTC4 and LTD4 peaks on HPLC. LTC4 production (pg/10(7) WBC, M +/- SD) was 194.5 +/- 61.7 (n = 5) in control group, 348.9 +/- 95.4 (n = 6) in dipalmitoyl lecithin group, 543.8 +/- 105.6 (n = 6) in egg lecithin group and 105.62 +/- 63.2 (n = 6) in AA-861 + dipalmitoyl lecithin group. LTC4 production of both lecithin groups was significantly higher than that of control group (P less than 0.01 in dipalmitoyl lecithin group and P less than 0.001 in egg lecithin group). Both egg lecithin and dipalmitoyl lecithin enhanced LT production from WBC. LT production was suppressed in the presence of AA-861. The mechanism of the enhancement in LT production is unclear, but these lecithins are apparently not substrates because dipalmitoyl lecithin contains no arachidonic acid.

Platelet activating factor and tracheobronchial respiratory glycoconjugate release in feline and human explants: involvement of the lipoxygenase pathway

It has been suggested that platelet activating factor (PAF) may participate in many aspects of bronchial asthma, including stimulation of mucus secretion. Feline tracheal and human bronchial explant production of respiratory glycoconjugates (RGC) in response to platelet activating factor (PAF) was investigated, in order to differentiate the actions of this putative mediator on mucus secretion. PAF caused a dose-dependent increase in RGC release in concentrations ranging from 100-0.5 microM during a 1-2 hours incubation with either feline or human explants, and the effect was inhibited by the PAF receptor antagonists Ro 19-3704. Several lines of evidence suggest that PAF enhances RGC release indirectly through stimulation of the production of lipoxygenase metabolites of arachidonic acid. 1) Incubation of 10 microM PAF together with arachidonic acid (100 micrograms/ml) enhances PAF's stimulatory effect on RGC release in cats. 2) The cyclooxygenase inhibitor ibuprofen (65 and 420 microM) either failed to effect or slightly enhanced PAF induced RGC release in both species. 3) The combined cyclooxygenase and lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA) as well as the putatively specific 5-lipoxygenase inhibitor L-651,392 (both at 50 microM) inhibited the response to PAF in both species. 4) The putative LTD4 receptor antagonists (L-660,711, 100 microM) slightly reduced the PAF secretory response in human bronchi. We conclude that PAF causes specific receptor mediated RGC release. This response is indirectly mediated through the generation of lipoxygenase metabolite formation including 5-lipoxygenase pathway metabolites.

The role of platelet activating factor in allergic respiratory disease

1. Platelet activating factor (PAF) is an ether-linked phospholipid capable of eliciting many of the factors of the allergic response including bronchoconstriction, mucosal oedema, eosinophil infiltration and bronchial hyperresponsiveness. 2. A wide range of selective PAF receptor antagonists have now been described which have been reported to reduce many aspects of allergen-induced inflammatory responses in experimental animals. 3. Early clinical studies have indicated that some of these drugs are also effective PAF antagonists in man, but no controlled studies have been reported using these compounds in patients with allergic asthma. 4. The gingkolide mixture BN 52063 has recently been reported to inhibit allergen-induced cutaneous inflammation in man; a response which has certain pathological similarities to allergen-induced late-onset airways obstruction in the lung. 5. Therefore, drugs antagonizing the actions of PAF may well lead to a better understanding of and may be a novel therapeutic approach for allergic respiratory disorders.

The generation and metabolism of leukotrienes in the ionophore-stimulated blood of normal and asthmatic subjects

The generation and metabolism of leukotrienes (LTs) B4, C4, D4, and E4 were studied in vitro in the A23187-stimulated whole blood of normal (N) and atopic asthmatic (AA) human subjects. Using a combination of reversed-phase high performance liquid chromatography and radioimmunoassay, we have demonstrated that the blood cells of atopic asthmatic patients have an enhanced ability to release LTB4 and LTC4 when compared to those of normal subjects. The release of LTB4 and LTC4 in response to ionophore is dose- and time-dependent. Half-maximal doses of ionophore caused the generation of high, sustained levels of LTB4, which are significantly higher in the AA blood than in N blood. Incubations of 3H-LTB4 in ionophore-stimulated N and AA blood revealed a slow metabolism to 20-OH-LTB4 and 20-COOH-LTB4. LTC4 is generated in smaller amounts than LTB4, with an early peak after 10 min which is significantly higher (p less than 0.01) in the AA blood compared to the N blood. Subsequent metabolism of LTC4 elicits significantly greater amounts of LTD4, and consistently higher levels of LTE4, in the AA blood. Parallel incubations of 3H-LTC4 in ionophore-stimulated N and AA blood demonstrated rapid metabolism of LTC4 by the glutathione detoxification pathway. The elevated production of LTB4 and LTC4 in AA blood was not accounted for by differences in leukocyte sub-type counts in the two groups, nor by differences in their rates of catabolism. The novel, selective 5-lipoxygenase inhibitor BW A4C [N-(3-phenoxycinnamyl) acetohydroxamic acid] caused dose-dependent inhibition of LTB4 and LTC4 generation and was equipotent in N and AA blood.

Platelet-activating factor induces the production of leukotrienes by human monocytes

The aim of this study was to evaluate the role of platelet-activating factor (PAF) as a stimulator of leukotriene production by human monocytes. The production of leukotrienes was time- and concentration-dependent. Release of leukotrienes was half-maximal after 2 min and reached a maximum after 10 min. At a concentration of 10(-8) M, PAF induced the production of 0.14 +/- 0.01 ng LTB4/10(6) cells (mean +/- S.E., n = 8). At concentrations of 10(-6) M, PAF induced the production of 1.0 +/- 0.04 ng LTB4 and 0.22 +/- 0.03 ng peptidoleukotrienes (mean +/- S.E., n = 16). There was no metabolism of LTB4 as judged from stability of [3H]LTB4 added to the incubations. LTC4 was slowly metabolized by human monocytes to LTD4 and LTE4. The two specific PAF-receptor antagonists BN 52021 and WEB 2086 in concentrations of 10(-4) and 10(-6) M, respectively, inhibited the PAF (10(-6) M) stimulated LTB4 production completely. In this study, we demonstrate that nanomolar concentrations of PAF can stimulate the production of LTB4 and peptidoleukotrienes in human monocytes by a receptor-mediated mechanism.

Nedocromil sodium inhibits the A23187- and opsonized zymosan-induced leukotriene formation by human eosinophils but not by human neutrophils

1. Inflammatory cells such as eosinophils and neutrophils are thought to contribute actively to the pathogenesis of asthma by the release of bronchoconstrictor mediators including leukotrienes. Previous studies have revealed the almost exclusive synthesis of leukotriene C4 (LTC4) by human eosinophils and of leukotriene B4 (LTB4), 20-OH-LTB4 and the non-enzymatically formed LTB4-isomers by neutrophils when stimulated in vitro with the calcium ionophore A23187 or opsonized zymosan (OZ). In this study we have investigated whether nedocromil sodium, a new anti-asthma drug, was capable of inhibiting A23187- and OZ-induced leukotriene formation by these cells. 2. Nedocromil sodium inhibited A23187- and OZ-induced LTC4 formation by eosinophils in a concentration-dependent manner (mean IC30 for A23187: 5.6 X 10(-5) M; mean IC30 for OZ: 6.3 X 10(-5) M), whereas it did not inhibit A23187- and OZ-induced LTB4 formation by neutrophils. 3. Extension of the preincubation time of the cells with the drug did not alter the observed inhibitory capacity. The optimal preincubation time was 5 min. 4. The in vitro inhibition of LTC4 formation by eosinophils by nedocromil sodium may be a valuable property of this drug in the treatment of asthma.

Characterization of platelet-activating factor-induced elevation of cytosolic free calcium concentration in eosinophils

In order to evaluate the role of calcium in the activation processes in eosinophils induced by platelet-activating factor (PAF), we investigated the changes in free cytoplasmatic Ca2+ concentration using fura-2. PAF causes a rapid and transitory rise of the intracellular free calcium ion concentration [( Ca2+]i) in purified guinea pig eosinophils of approx. 1000 nM above a basal level of 120.7 +/- 36.5 nM (n = 10). The effect was dose-related with a maximum rise at 1000 nM PAF and an EC50 of 17.4 nM and specifically inhibited by the PAF antagonist WEB 2086 with an IC50 of 95.5 nM. WEB 2086 did not affect either the leukotriene B4- or the fMet-Leu-Phe-induced elevation of [Ca2+]i. The response to PAF was dependent on external Ca2+ as it was significantly inhibited by EGTA (85.6 +/- 5.4%) and Ni2+ (95.8 +/- 2.1%) but not by the dihydropyridine antagonist nimodipine. We conclude that Ca2+ entry via receptor-operated Ca2+ channels may be involved in PAF-induced degranulation of eosinophils.

Leukotriene C4 formation by purified human eosinophils can be induced by arachidonic acid in the absence of calcium-ionophore A23187

Addition of arachidonic acid (50 microM) to purified human eosinophils leads to the formation of considerable amounts of LTC4 [11.3 +/- 1.3) x 10(6) molecules/cell, mean +/- SEM, n = 10), 15-HETE [412 +/- 142) x 10(6) molecules/cell, mean +/- SEM, n = 3) and 15-series leukotrienes [35 +/- 15) x 10(6) molecules/cell, mean +/- SEM, n = 3). The ratio of the amounts of LTC4 and 15-lipoxygenase products was found to be strongly dependent on the arachidonic acid concentration, being relatively large at low arachidonic acid concentrations and very small at high arachidonic acid concentrations. Platelet activating factor (1 microM) was able to enhance significantly the production of LTC4 but not that of 15-lipoxygenase products. As arachidonic acid was found to be capable of inducing a fast, transient rise in the cytosolic free Ca2+ concentration, this explains, at least partly, its ability to induce the Ca2+-dependent formation of LTC4.

Inhibitory effects of nedocromil sodium on the in vitro induced migration and leukotriene formation of human granulocytes

Inflammatory cells, such as neutrophils and eosinophils, are thought to actively contribute to the pathogenesis of asthma since they infiltrate into the lung tissue and may be activated locally to release bronchoconstrictor mediators. In this study we provide evidence that nedocromil sodium is capable of effectively inhibiting the platelet-activating factor (PAF) and zymosan-activated serum (ZAS)-induced chemotaxis of polymorphonuclear granulocytes (PMN) [IC50 approximately 1 nmol/L and 0.1 mumol/L respectively]. The same inhibitory potency was obtained with sodium cromoglycate. Thus, nedocromil sodium may effectively inhibit the mobilisation of inflammatory cells in the lung. Furthermore, nedocromil sodium is capable of inhibiting the formation of the bronchoconstrictor mediator leukotriene-C4 (LTC4) by eosinophils in a concentration-dependent way [IC30 for A23187: 5.6 10(-5) mol/L; IC30 for opsonised zymosan (OZ): 6.3 10(-5) mol/L], whereas this drug is not capable of inhibiting leukotriene-B4 (LTB4) formation by neutrophils. These findings indicate that nedocromil sodium inhibits the release of bronchoconstrictor mediators not only from mast cells but also from eosinophils.

Arachidonic acid can induce leukotriene C4 formation by purified human eosinophils in the absence of other stimuli

Stimulation of purified human eosinophils with 50 microM arachidonic acid leads to the production of leukotriene C4, 15-hydroxy-eicosatetraenoic acid and 15-series leukotrienes. The ratio of the amounts of leukotriene C4 and 15-lipoxygenase products was found to be strongly dependent on the arachidonic acid concentration, being relatively large at low arachidonic acid concentrations and very small at high arachidonic acid concentrations. In the presence of 1 microM platelet-activating factor a significant elevation of leukotriene C4 formation is observed, whereas the formation of 15-lipoxygenase products remains unaltered. As arachidonic acid was found to be capable of inducing a fast, transient rise in the cytosolic free Ca2+ concentration, this explains at least partly its ability to induce the Ca2+-dependent formation of leukotriene C4.