Effects of a PAF antagonist, Y-24180, on bronchial hyperresponsiveness in patients with asthma

Forty Years Since the Structural Elucidation of Platelet-Activating Factor (PAF): Historical, Current, and Future Research Perspectives

In the late 1960s, Barbaro and Zvaifler described a substance that caused antigen induced histamine release from rabbit platelets producing antibodies in passive cutaneous anaphylaxis. Henson described a ‘soluble factor’ released from leukocytes that induced vasoactive amine release in platelets. Later observations by Siraganuan and Osler observed the existence of a diluted substance that had the capacity to cause platelet activation. In 1972, the term platelet-activating factor (PAF) was coined by Benveniste, Henson, and Cochrane. The structure of PAF was later elucidated by Demopoulos, Pinckard, and Hanahan in 1979. These studies introduced the research world to PAF, which is now recognised as a potent phospholipid mediator. Since its introduction to the literature, research on PAF has grown due to interest in its vital cell signalling functions and more sinisterly its role as a pro-inflammatory molecule in several chronic diseases including cardiovascular disease and cancer. As it is forty years since the structural elucidation of PAF, the aim of this review is to provide a historical account of the discovery of PAF and to provide a general overview of current and future perspectives on PAF research in physiology and pathophysiology.

A Review on Platelet Activating Factor Inhibitors: Could a New Class of Potent Metal-Based Anti-Inflammatory Drugs Induce Anticancer Properties?

In this minireview, we refer to recent results as far as the Platelet Activating Factor (PAF) inhibitors are concerned. At first, results of organic compounds (natural and synthetic ones and specific and nonspecific) as inhibitors of PAF are reported. Emphasis is given on recent results about a new class of the so-called metal-based inhibitors of PAF. A small library of 30 metal complexes has been thus created; their anti-inflammatory activity has been further evaluated owing to their inhibitory effect against PAF in washed rabbit platelets (WRPs). In addition, emphasis has also been placed on the identification of preliminary structure-activity relationships for the different classes of metal-based inhibitors.

Trans-basement membrane migration of eosinophils induced by LPS-stimulated neutrophils from human peripheral blood in vitro

In the airways of severe asthmatics, an increase of neutrophils and eosinophils is often observed despite high-dose corticosteroid therapy. We previously reported that interleukin-8-stimulated neutrophils induced trans-basement membrane migration (TBM) of eosinophils, suggesting the link between neutrophils and eosinophils. Concentrations of lipopolysaccharide (LPS) in the airway increase in severe asthma. As neutrophils express Toll-like receptor (TLR)4 and can release chemoattractants for eosinophils, we investigated whether LPS-stimulated neutrophils modify eosinophil TBM. Neutrophils and eosinophils were isolated from peripheral blood of healthy volunteers and severe asthmatics. Eosinophil TBM was examined using a modified Boyden's chamber technique. Eosinophils were added to the upper compartment, and neutrophils and LPS were added to the lower compartment. Migrated eosinophils were measured by eosinophil peroxidase assays. LPS-stimulated neutrophils induced eosinophil TBM (about 10-fold increase), although LPS or neutrophils alone did not. A leukotriene B₄ receptor antagonist, a platelet-activating factor receptor antagonist or an anti-TLR4 antibody decreased eosinophil TBM enhanced by LPS-stimulated neutrophils by almost half. Neutrophils from severe asthmatics induced eosinophil TBM and lower concentrations of LPS augmented neutrophil-induced eosinophil TBM. These results suggest that the combination of neutrophils and LPS leads eosinophils to accumulate in the airways, possibly involved the pathogenesis of severe asthma.

Engineered in vitro disease models

The ultimate goal of most biomedical research is to gain greater insight into mechanisms of human disease or to develop new and improved therapies or diagnostics. Although great advances have been made in terms of developing disease models in animals, such as transgenic mice, many of these models fail to faithfully recapitulate the human condition. In addition, it is difficult to identify critical cellular and molecular contributors to disease or to vary them independently in whole-animal models. This challenge has attracted the interest of engineers, who have begun to collaborate with biologists to leverage recent advances in tissue engineering and microfabrication to develop novel in vitro models of disease. As these models are synthetic systems, specific molecular factors and individual cell types, including parenchymal cells, vascular cells, and immune cells, can be varied independently while simultaneously measuring system-level responses in real time. In this article, we provide some examples of these efforts, including engineered models of diseases of the heart, lung, intestine, liver, kidney, cartilage, skin and vascular, endocrine, musculoskeletal, and nervous systems, as well as models of infectious diseases and cancer. We also describe how engineered in vitro models can be combined with human inducible pluripotent stem cells to enable new insights into a broad variety of disease mechanisms, as well as provide a test bed for screening new therapies.

New risk factors for adult-onset incident asthma. A nested case-control study of host antioxidant defense

RATIONALE

Host antioxidant defense, consisting of enzymatic antioxidant activity and nonenzymatic antioxidant micronutrients, is implicated in asthma pathogenesis. Studies of antioxidant defense and adult incident asthma have either used measures of antioxidants estimated from questionnaires or not considered enzymatic aspects of host defense.

OBJECTIVES

We conducted the first study designed and powered to investigate the association of antioxidant defenses on adult incident asthma.

METHODS

In a nested case-control study, we followed Shanghai women (aged 40-70 years) without prevalent asthma at baseline, over 8 years. Subjects with incident asthma were ascertained prospectively by gold standard testing of symptomatic women and matched to two asymptomatic control subjects.

MEASUREMENTS AND MAIN RESULTS

Baseline urinary F2-isoprostanes, plasma concentrations of antioxidant micronutrients (tocopherols, xanthines, carotenes, and lycopene), and antioxidant enzyme activity (platelet-activating factor acetylhydrolase [PAF-AH] and superoxide dismutase) were measured from samples collected before disease onset. Among 65,372 women, 150 (0.24%) developed asthma. F2-isoprostane levels before asthma onset were not different between cases and control subjects. Doubling of α-tocopherol concentrations and PAF-AH activity was associated with 50 and 37% decreased risk of incident asthma (α-tocopherol: adjusted odds ratio = 0.52; 95% confidence interval, 0.32-0.84; PAF-AH: adjusted odds ratio = 0.63; 95% confidence interval, 0.42-0.93).

CONCLUSIONS

In this prospective study, α-tocopherol, within normal reference ranges, and PAF-AH enzymatic activity were associated with decreased asthma development. These modifiable risk factors may be an effective strategy to test for primary asthma prevention.

Effect of premedications in a murine model of asparaginase hypersensitivity

A murine model was developed that recapitulates key features of clinical hypersensitivity to Escherichia coli asparaginase. Sensitized mice developed high levels of anti-asparaginase IgG antibodies and had immediate hypersensitivity reactions to asparaginase upon challenge. Sensitized mice had complete inhibition of plasma asparaginase activity (P = 4.2 × 10(-13)) and elevated levels of mouse mast cell protease 1 (P = 6.1 × 10(-3)) compared with nonsensitized mice. We investigated the influence of pretreatment with triprolidine, cimetidine, the platelet activating factor (PAF) receptor antagonist CV-6209 [2-(2-acetyl-6-methoxy-3,9-dioxo-4,8-dioxa-2,10-diazaoctacos-1-yl)-1-ethyl-pyridinium chloride], or dexamethasone on the severity of asparaginase-induced allergies. Combining triprolidine and CV-6209 was best for mitigating asparaginase-induced hypersensitivity compared with nonpretreated, sensitized mice (P = 1.2 × 10(-5)). However, pretreatment with oral dexamethasone was the only agent capable of mitigating the severity of the hypersensitivity (P = 0.03) and partially restoring asparaginase activity (P = 8.3 × 10(-4)). To rescue asparaginase activity in sensitized mice without requiring dexamethasone, a 5-fold greater dose of asparaginase was needed to restore enzyme activity to a similar concentration as in nonsensitized mice. Our results suggest a role of histamine and PAF in asparaginase-induced allergies and indicate that mast cell-derived proteases released during asparaginase allergy may be a useful marker of clinical hypersensitivity.

Design, synthesis, and biological evaluation of 3-(1-Aryl-1H-indol-5-yl)propanoic acids as new indole-based cytosolic phospholipase A2α inhibitors

This article describes the design, synthesis, and biological evaluation of new indole-based cytosolic phospholipase A2α (cPLA2α, a group IVA phospholipase A2) inhibitors. A screening-hit compound from our library, (E)-3-{4-[(4-chlorophenyl)thio]-3-nitrophenyl}acrylic acid (5), was used to design a class of 3-(1-aryl-1H-indol-5-yl)propanoic acids as new small molecule inhibitors. The resultant structure-activity relationships studied using the isolated enzyme and by cell-based assays revealed that the 1-(p-O-substituted)phenyl, 3-phenylethyl, and 5-propanoic acid groups on the indole core are essential for good inhibitory activity against cPLA2α. Optimization of the p-substituents on the N1 phenyl group led to the discovery of 56n (ASB14780), which was shown to be a potent inhibitor of cPLA2α via enzyme assay, cell-based assay, and guinea pig and human whole-blood assays. It displayed oral efficacy toward mice tetradecanoyl phorbol acetate-induced ear edema and guinea pig ovalbumin-induced asthma models.

Animal models of asthma: value, limitations and opportunities for alternative approaches

Asthma remains an area of considerable unmet medical need. Few new drugs have made it to the clinic during the past 50 years, with many that perform well in preclinical animal models of asthma, failing in humans owing to lack of safety and efficacy. The failure to translate promising drug candidates from animal models to humans has led to questions about the utility of in vivo studies and to demand for more predictive models and tools based on the latest technologies. Following a workshop with experts from academia and the pharmaceutical industry, we suggest here a disease modelling framework designed to better understand human asthma, and accelerate the development of safe and efficacious new asthma drugs that go beyond symptomatic relief.

Platelet-activating factor antagonists: current status in asthma

Platelet-activating factor (PAF) is a potent lipid-derived mediator of inflammation that is considered to have a potential role in the pathogenesis of asthma. PAF is produced by many cells associated with asthmatic inflammation and has the ability to evoke some of the clinical hallmarks of asthma, such as bronchoconstriction, mucus production and airway hyperresponsiveness (AHR). In addition, PAF has profound chemoattractant properties for eosinophils and neutrophils and it promotes an increase in microvascular permeability and oedema formation within the airways. Nevertheless, the definitive role of PAF in asthma remains elusive. PAF is formed as a result of the action of phospholipase A(2) and acetyltransferase on membrane phospholipids and it is degraded by a PAF-specific acetylhydrolase. The biological effects of PAF are mediated by the activation of specific receptors expressed on effector cell surfaces, although intracellular signalling and paracrine actions have been described. In addition, at least part of the pulmonary effects of PAF could be related to the secondary release of leukotrienes. In the clinical setting, different ways of modifying the activity of PAF have been explored, in particular the inhibitory actions of PAF receptor antagonists. Both natural and synthetic PAF receptor antagonists have shown conflicting results. Although second generation PAF antagonists (apafant, UK-74505, SR-27417A) appear to have a good protective effect against the systemic and pulmonary actions of inhaled PAF, the protective effects of these compounds on allergen-induced responses and AHR are more modest. In the treatment of asthma, PAF receptor antagonists have failed to produce a significant impact in either acute asthma attacks or the maintenance therapy of chronic forms. Other pharmacological interventions of proven efficacy in asthma, such as salbutamol or 5-lipoxygenase antagonists, have shown some anti-PAF effects. Whether the overall negative results with PAF receptor antagonists indicate that extracellular PAF is not a relevant mediator of airway inflammation or that the compounds explored are not capable of blocking the paracrine actions of PAF remains speculative. A PAF synthase inhibitor could be valuable in the elucidation of the role of PAF and it might be a promising and useful complementary therapeutic tool in the future.

Effects of the addition of Beta2-agonist tulobuterol patches to inhaled corticosteroid in patients with asthma

BACKGROUND

Whether the additive effects of the tulobuterol patch (TP), the world's first transdermal beta2-agonist preparation, are useful in asthma patients receiving inhaled corticosteroid (ICS) is unclear. To examine the add-on effects of TP on bronchial hyperresponsiveness and reduction of the percentage of sputum eosinophils, and to compare add-on effects of TP, slow-release theophylline (SRT), and a leukotriene receptor antagonist (LTRA) in patients with asthma receiving ICS.

METHODS

Study 1: We randomly allocated 24 patients with asthma receiving ICS alone in equal numbers to either control treatment (ICS alone at conventional doses) or TP treatment (ICS at conventional doses plus TP at 2mg/day). Following a 2-week observation period, patients received the allocated drug regimens for 4 weeks. Methacholine challenge test and measurement of percentage of eosinophils in hypertonic saline-induced sputum were performed before and after the treatment period. Study 2: We compared add-on effects of TP, SRT, and LTRA in 65 patients with asthma receiving ICS alone, using spirometry and peak expiratory flow (PEF). Participants in these studies had experienced decrease in morning PEF to <80% of the predicted value at least twice a week.

RESULTS

Study 1: In the TP group, improvement of bronchial hyperresponsiveness and decrease in percentage of sputum eosinophils both indicated a statistically significant difference (p < 0.01, and p < 0.05, respectively). These findings were not observed in the control group. Study 2: forced expiratory volume in 1 second (FEV(1)) and PEF markedly increased after treatment with TP compared with treatment with SRT or LTRA.

CONCLUSIONS

These findings suggest that TP can be used as a long-term add-on controller for patients with asthma receiving ICS.

Mouse models of allergic asthma: acute and chronic allergen challenge

Asthma is defined as a chronic inflammatory disease of the airways; however, the underlying physiological and immunological processes are not fully understood. Animal models have been used to elucidate asthma pathophysiology, and to identify and evaluate novel therapeutic targets. Several recent review articles (Epstein, 2004; Lloyd, 2007; Boyce and Austen, 2005; Zosky and Sly, 2007) have discussed the potential value of these models. Allergen challenge models reproduce many features of clinical asthma and have been widely used by investigators; however, the majority involve acute allergen challenge procedures. It is recognised that asthma is a chronic inflammatory disease resulting from continued or intermittent allergen exposure, usually via inhalation, and there has been a recent focus on developing chronic allergen exposure models, predominantly in mice. Here, we review the acute and chronic exposure mouse models, and consider their potential role and impact in the field of asthma research.

Platelet activating factor as a mediator and therapeutic approach in bronchial asthma

Platelet activating factor (PAF) is a potent phospholipid mediator involved in anaphylaxis and chronic inflammatory disorders, including bronchial asthma. PAF is able to act both, directly as a chemotactic factor and indirectly through the release of other inflammatory agents. Apart from its known potent ability to activate platelets, PAF influences other immune and inflammatory cells function involved in asthma, which may be of importance in the pathogenesis of the disease. In addition, PAF administration can mimic some of abnormalities observed in asthma, including bronchoconstriction, bronchial hyper responsiveness, and gas exchange impairment, which may be mediated by leukotrienes acting as secondary mediators of some PAF effects. Therefore, there has been an extensive interest in the role of PAF in human asthma and major efforts have been continued to discover drugs acting thorough inhibition of PAF effects in the disease. Surprisingly, PAF receptor antagonists have not clearly proven their clinical benefits. It may appear that the combined blockage of PAF effects and other mediators involved in asthma is a way to improve clinical efficacy and also an interesting approach to control inflammation in the disease. This review will focus on two main issues: the role of PAF and PAF antagonists in asthma.

Plasminogen activator inhibitor-1 in the pathogenesis of asthma

Plasminogen activator inhibitor (PAI)-1 is the main inhibitor of the fibrinolytic system and is known to play an essential role in tissue remodeling. Recent evidence indicates that chronic asthma may lead to tissue remodeling such as subepithelial fibrosis and extracellular matrix (ECM) deposition in the airways. However, the role of PAI-1 in asthma is unknown. Recently the mast cell (MC), which plays a major role in asthma, was found as a novel source of PAI-1, and a large number of MCs expressing PAI-1 are infiltrated in the airways of patients with severe asthma. Furthermore, PAI-1-deficient mice show reduced ECM deposition in the airways of a murine model of chronic asthma by inhibiting MMP-9 activity and fibrinolysis. In a human study, the 4G allele frequency was significantly higher in the asthmatic patients than in the control group. In view of the findings that the 4G allele is associated with elevated plasma PAI-1 level, elevated PAI-1 level in the lung may contribute to the development of asthma. In summary, PAI-1 may play an important role in the pathogenesis of asthma and further studies evaluating the mechanisms of PAI-1 action may lead to the development of a novel therapeutic target for the treatment and prevention of asthma.

Airway hyperresponsiveness in transgenic mice overexpressing platelet activating factor receptor is mediated by an atropine-sensitive pathway

Platelet activating factor (PAF) is a potent mediator potentially involved in the pathogenesis of inflammatory disorders, including bronchial asthma. Recently, transgenic mice overexpressing the PAF receptor (PAFR) gene have been established, and exhibit bronchial hyperresponsiveness, one of the cardinal features of asthma. To elucidate the molecular and pathophysiologic mechanisms underlying PAF-associated bronchial hyperreactivity, we studied airway responsiveness to methacholine (MCh) and serotonin (5-hydroxytryptamine; 5-HT) in PAFR-transgenic mice. In addition, we examined the role of the muscarinic receptor in PAF-induced responses and the binding activities of the muscarinic receptor. The PAFR-transgenic mice exhibited hyperresponsiveness to MCh and PAF; however, no significant differences in 5-HT responsiveness were observed between the control and PAFR-transgenic mice. The administration of atropine significantly blocked PAF-induced responses in PAFR-transgenic mice. There were no differences between the two phenotypes in the binding activities of muscarinic receptor. Morphometric analyses demonstrated that PAFR overexpression did not affect airway structure. These findings suggest that the muscarinic pathway may have a key role in airway hyperresponsiveness associated with PAFR gene overexpression. More generally, PAFR-transgenic mice may provide appropriate models for study of the molecular mechanisms underlying PAF-associated diseases.

PAF acetylhydrolase gene polymorphisms and asthma severity

This review describes the current understanding of the contributions of genetic alterations in platelet-activating factor (PAF) acetylhydrolase to the pathogenesis of asthma. A variety of in vitro and in vivo studies, performed by multiple laboratories, suggest that the lipid substrates of this enzyme, PAF and oxidised derivatives of phosphatidylcholines, play important roles as causative factors in many diseases including asthma. PAF acetylhydrolase inactivates PAF and oxidatively-fragmented lipids thus providing a mechanism to prevent their pro-inflammatory effects. Since it is a most unusual protein, the biochemical, structural and functional characteristics of PAF acetylhydrolase continue to be unravelled. First, the ability of this enzyme to inactivate pro-inflammatory lipid mediators is modulated by its association with lipoproteins and by its susceptibility to oxidative inactivation. Second, mediators of inflammation, such as the substrates for PAF acetylhydrolase, alter expression of the protein at the transcriptional level. Third, naturally-occurring variants of PAF acetylhydrolase have catalytic properties different from those exhibited by the most common form of this protein. Thus, a variety of factors, including genetics, contribute to determine the biological level of lipid substrates known to act as mediators of asthma and other diseases. Here, I summarise key studies that implicate PAF and related molecules as important mediators in the pathogenesis of asthma. Next, I describe clinical findings that are consistent with a role of PAF acetylhydrolase as a modulator of asthma. Third, I focus on the biochemical effects associated with naturally-occurring mutations and polymorphisms in the PAF acetylhydrolase gene and the incidence of these genetic variations in populations of asthmatic subjects. Finally, I present my views on the future of this emerging field and the potential utility of performing additional studies aimed at further characterising the contribution of PAF acetylhydrolase to the pathogenesis of a complex syndrome generally recognised as a multifactorial and heterogeneous disease.

Effect of recombinant human platelet-activating factor-acetylhydrolase on allergen-induced asthmatic responses

Platelet-activating factor (PAF) is a potent lipid mediator associated with key features of asthma such as airway constriction, eosinophil infiltration, edema, and mucus accumulation. Regulation of PAF occurs primarily through degradation to biologically inactive lyso-PAF by cellular and secreted PAF-acetylhydrolase (PAF-AH). We evaluated the effect of human recombinant PAF-AH (rPAF-AH) on the dual phase asthmatic response in atopic subjects with mild asthma. Effects on induced sputum cell counts and differentials, eosinophilic cationic protein (ECP), and tryptase were evaluated. Enrolled subjects demonstrated a positive skin test and a dual asthmatic response to allergen inhalation challenge. Fourteen subjects received rPAF-AH (1 mg/kg) or placebo intravenously in a randomized, double blind, placebo-controlled, two-period crossover study. Treatment with rPAF-AH did not significantly reduce either the early- or late-asthmatic response. Sputum eosinophil cell counts were not affected by treatment, but there was a trend toward a reduction in sputum neutrophils. No significant change in sputum ECP and tryptase was observed between rPAF-AH and placebo. Thus, at the dose studied, the unique anti-PAF agent rPAF-AH demonstrated no significant effect on the allergen-induced dual-phase asthmatic response.

Effect of eotaxin and platelet-activating factor on airway inflammation and hyperresponsiveness in guinea pigs in vivo

Although eotaxin causes selective infiltration of eosinophils into the lung, its role in airway hyperresponsiveness remains unclear. We studied the effects of local administration of eotaxin on airway inflammation and hyperresponsiveness in guinea pigs in vivo. Airway responsiveness to inhaled histamine and differential cell counts in bronchoalveolar lavage fluid (BALF) were evaluated 12 h, 24 h, 3 d, and 7 d after intratracheal instillation of eotaxin. Significant eosinophilia in BALF was observed between 6 h and 7 d after eotaxin administration. Histologically, eosinophil accumulation was observed in the airways but not in the alveoli. In contrast, eotaxin did not affect airway responsiveness between 12 h and 7 d after its administration. We then studied the effects on airway responsiveness of subthreshold doses of interleukin 5, leukotriene D(4) (LTD(4)), and platelet-activating factor (PAF) combined with eotaxin. Neither interleukin 5 nor LTD(4) affected airway responsiveness. After eotaxin treatment, PAF significantly enhanced airway responsiveness without further increases in eosinophil counts. Eotaxin plus PAF significantly increased in eosinophil peroxidase activity in BALF compared with control and with eotaxin alone. These data indicate that eotaxin alone causes eosinophil accumulation in the airways but not hyperresponsiveness, and that additional factors such as PAF are needed to activate eosinophils for the development of airway hyperresponsiveness.

Mechanisms of eosinophil-associated inflammation

Increasing evidence supports a critical role for the eosinophil in disease. Here, the mechanisms underlying eosinophil-associated inflammation are reviewed including eosinophil constituents, eosinophil maturation and release from the bone marrow, and eosinophil tissue recruitment and activation. Eosinophil effector functions in bronchial asthma are summarized with particular attention to pulmonary M2 muscarinic receptors and bronchial hyperreactivity. Recent findings supporting roles for IL-5, the eosinophil, and its ribonucleases in viral immunity are presented. Overall, this information supports an expanded view of eosinophil participation in health and disease.

Recombinant human platelet-activating factor-acetylhydrolase inhibits airway inflammation and hyperreactivity in mouse asthma model

Numerous in vitro and in vivo studies in both animal models and human asthmatics have implicated platelet-activating factor (PAF) as an important inflammatory mediator in asthma. In a murine asthma model, we examined the anti-inflammatory activities of recombinant human PAF-acetylhydrolase (rPAF-AH), which converts PAF to biologically inactive lyso-PAF. In this model, mice sensitized to OVA by i.p. and intranasal (i.n.) routes are challenged with the allergen by i.n. administration. The OVA challenge elicits an eosinophil infiltration into the lungs with widespread mucus occlusion of the airways and results in bronchial hyperreactivity. The administration of rPAF-AH had a marked effect on late-phase pulmonary inflammation, which included a significant reduction in airway eosinophil infiltration, mucus hypersecretion, and airway hyperreactivity in response to methacholine challenge. These studies demonstrate that elevating plasma levels of PAF-AH through the administration of rPAF-AH is effective in blocking the late-phase pulmonary inflammation that occurs in this murine allergen-challenge asthma model. These results suggest that rPAF-AH may have therapeutic effects in patients with allergic airway inflammation.

Mast-cell responses in the development of asthma

Many cells participate in the pathogenesis of asthmatic inflammation. The mast cell is localized at the interface of the internal and external environment within the lung where it may respond to allergens and other exogenous stimuli. The activation of mast cells leads to the release of mediators that contribute to the early phase of asthmatic inflammation. Mast-cell-derived products may also contribute to the late-phase asthmatic response. This review summarizes the developmental biologic features of the mast cell, its receptor-mediated activation, and its range of preformed, newly synthesized, and induced mediators that contribute to asthmatic inflammation.

Platelet-activating factor (PAF) receptor and genetically engineered PAF receptor mutant mice

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active phospholipid mediator. Although PAF was initially recognized for its potential to induce platelet aggregation and secretion, intense investigations have elucidated potent biological actions of PAF in a broad range of cell types and tissues, many of which also produce the molecule. PAF acts by binding to a unique G-protein-coupled seven transmembrane receptor. PAF receptor is linked to intracellular signal transduction pathways, including turnover of phosphatidylinositol, elevation in intracellular calcium concentration, and activation of kinases, resulting in versatile bioactions. On the basis of numerous pharmacological reports, PAF is thought to have many pathophysiological and physiological functions. Recently advanced molecular technics enable us not only to clone PAF receptor cDNAs and genes, but also generate PAF receptor mutant animals, i.e., PAF receptor-overexpressing mouse and PAF receptor-deficient mouse. These mutant mice gave us a novel and specific approach for identifying the pathophysiological and physiological functions of PAF. This review also describes the phenotypes of these mutant mice and discusses them by referring to previously reported pharmacological and genetical data.

Inhibition of eosinophil activation in bronchoalveolar lavage fluid from atopic asthmatics by Y-24180, an antagonist to platelet-activating factor

We examined the effects of Y-24180, a potent and long-acting antagonist to platelet-activating factor (PAF) receptor, on the expression of adhesion molecules in peripheral blood and bronchoalveolar lavage fluid (BALF) eosinophils from atopic asthmatics. Y-24180 (20 mg/day) was administered to 4 atopic asthmatics for 8 weeks. The number of eosinophils, the level of eosinophil cationic protein (ECP), the bindings of soluble intercellular adhesion molecule-1 (sICAM-1) and fibronectin (FN), and the expressions of CD11b (alpha chain of Mac-1) and CD49d (alpha chain of VLA-4) on eosinophils were evaluated in peripheral blood (n=4) and BALF (n=3) before and after the administration of Y-24180. The infiltration of eosinophils into the bronchial wall was also examined by taking biopsies. Eosinophil count, sICAM-1 and FN binding to eosinophils in BALF significantly decreased after the administration of Y-24180 (p<0.05). The level of CD11b expression also decreased remarkably after the administration (n=2). In peripheral blood, eosinophil count and ECP level did not change. The binding of sICAM-1 and FN, and expression of CD11b on eosinophils in peripheral blood showed a tendency to decrease after the administration. The level of CD49d expression on eosinophils changed neither in BALF nor in blood. Eosinophil infiltration into the bronchial wall markedly decreased in one out of 3 cases after the administration. These results suggest that Y-24180 inhibits the activation of eosinophils by antagonizing the actions of PAF in atopic asthmatics.

Inhibition of activation of human peripheral blood eosinophils by Y-24180, an antagonist to platelet-activating factor receptor

We examined the effects of Y-24180, a potent and long-acting antagonist to platelet-activating factor (PAF) receptor, on the PAF- or leukotriene B4 (LTB4)-induced activation of eosinophils using human peripheral blood in vitro. As activation markers, CD11b expression level and soluble intercellular adhesion molecule-1 (sICAM-1)-binding activity were analyzed by flow cytometry. Y-24180 significantly inhibited PAF-induced increase in the ratio of strongly positive cells for CD11b expression and sICAM-1 binding at 0.01 microM or more. WEB 2086, another PAF receptor antagonist, also inhibited the increase significantly at 1 microM or more. LTB4-induced increases in the ratio of strongly positive cells for CD11b expression and sICAM-1 binding were inhibited by Y-24180 at 1 microM, but not WEB 2086 up to 10 microM. These results indicate that Y-24180 inhibits the PAF- or LTB4-induced activation of eosinophils in human peripheral blood more potently than WEB 2086.

Deficiency of platelet-activating factor acetylhydrolase is a severity factor for asthma

Asthma, a family of airway disorders characterized by airway inflammation, has an increasing incidence worldwide. Platelet-activating factor (PAF) may play a role in the pathophysiology of asthma. Its proinflammatory actions are antagonized by PAF acetylhydrolase. A missense mutation (V279F) in the PAF acetylhydrolase gene results in the complete loss of activity, which occurs in 4% of the Japanese population. We asked if PAF acetylhydrolase deficiency correlates with the incidence and severity of asthma in Japan. We found that the prevalence of PAF acetylhydrolase deficiency is higher in Japanese asthmatics than healthy subjects and that the severity of this syndrome is highest in homozygous-deficient subjects. We conclude that the PAF acetylhydrolase gene is a modulating locus for the severity of asthma.

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.

Plasma platelet-activating factor acetylhydrolase deficiency in Japanese patients with asthma

Platelet-activating factor (PAF), a phospholipid with a wide range of proinflammatory actions, is immediately degraded and inactivated in vivo by PAF acetylhydrolase (PAF-AH). Surprisingly, 4% of the Japanese population lacks the extracellular isoform of this enzyme, plasma PAF-AH, due to a genetic missense (V279F) mutation. We studied the association of this mutation with asthma prevalence and phenotypes in the Japanese adult population. The allele frequency of V279F mutation was 18.6% in 279 patients with asthma (28.7% heterozygotes and 4.3% homozygotes) and 21.7% in 217 healthy subjects (32.3% heterozygotes and 5.5% homozygotes). V279F mutant allele prevalence was consistent regardless of asthma type (16.3% in atopic [n = 156] and 21.6% in nonatopic [n = 123]), or the severity of disease (21.7% in patients with mild [n = 97], 17.5% in those with moderate [n = 131], and 15.8% in those with severe [n = 51] asthma). Plasma PAF-AH activity was inversely proportional to the number of mutant alleles, and did not correlate with asthma prevalence, type, or severity. We concluded that plasma PAF-AH deficiency due to V279F mutation is not essential to the pathophysiology of asthma in the Japanese adult population.

Effect of azelastine on platelet-activating factor-induced microvascular leakage in rat airways

To determine the effect of the antiallergic drug azelastine on airway mucosal inflammation, we studied airway microvascular permeability in response to platelet-activating factor (PAF) in pathogen-free rats. Vascular permeability and neutrophil accumulation were assessed by the percent area occupied by Monastral blue-labeled blood vessels and by myeloperoxidase-containing granulocytes, respectively, in whole mounts of the trachea and main bronchus. Intravenous PAF caused dose-dependent increases in the area density of Monastral blue-labeled vessels and neutrophil influx, and the former effect was inhibited by depletion of circulating neutrophils by cyclophosphamide or treatment with the neutrophil elastase inhibitor ONO-5046. Pretreatment with azelastine inhibited PAF-induced vascular leakage without affecting neutrophil accumulation. This inhibitory effect of azelastine was not seen in neutropenic rats and ONO-5046-treated rats. PAF increased neutrophil elastase contents in bronchoalveolar lavage fluid, an effect that was inhibited by azelastine. Therefore, azelastine attenuates PAF-induced airway mucosal microvascular leakage, probably involving inhibition of the release of neutrophil elastase from activated neutrophils.

Dual antagonists of platelet activating factor and histamine. 3. Synthesis, biological activity and conformational implications of substituted N-acyl-bis-arylcycloheptapiperazines

A series of N-acyl-4-(5,6-dihydro-11H-benzo[5,6]cyclohepta[1,2-b]pyridin- 11-ylidene)piperazines is described that are dual antagonists of PAF and histamine. The structural requirements for activity in this series parallel those of their previously reported piperidinylidene counterparts. Whereas their global minimum energy conformations are different for both series of compounds, computer assisted molecular modeling suggests that a common bioactive conformation is possible.

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.

Lipid mediators in inflammatory disorders

During the past few decades, intensive collaborative research in the fields of chronic and acute inflammatory disorders has resulted in a better understanding of the pathophysiology and diagnosis of these diseases. Modern therapeutic approaches are still not satisfactory and shock, sepsis and multiple organ failure remain the great challenge in intensive care medicine. However, the treatment of inflammatory diseases like rheumatoid arthritis, ulcerative colitis or psoriasis also represents an unresolved problem. Many factors contribute to the complex course of inflammatory reactions. Microbiological, immunological and toxic agents can initiate the inflammatory response by activating a variety of humoral and cellular mediators. In the early phase of inflammation, excessive amounts of interleukins and lipid-mediators are released and play a crucial role in the pathogenesis of organ dysfunction. Arachidonic acid (AA), the mother substance of the pro-inflammatory eicosanoids, is released from membrane phospholipids in the course of inflammatory activation and is metabolised to prostaglandins and leukotrienes. Various strategies have been evaluated to control the excessive production of lipid mediators on different levels of biochemical pathways, such as inhibition of phospholipase A2, the trigger enzyme for release of AA, blockade of cyclooxygenase and lipoxygenase pathways and the development of receptor antagonists against platelet activating factor and leukotrienes. Some of these agents exert protective effects in different inflammatory disorders such as septic organ failure, rheumatoid arthritis or asthma, whereas others fail to do so. Encouraging results have been obtained by dietary supplementation with long chain omega-3 fatty acids like eicosapentaenoic acid (EPA). In states of inflammation, EPA is released to compete with AA for enzymatic metabolism inducing the production of less inflammatory and chemotactic derivatives.

Activation of platelets in bronchial asthma

STUDY OBJECTIVES

To investigate whether platelets are activated in asthmatics with increased release of preformed mediators and to investigate the influence of oral administration of theophylline on them.

DESIGN

Comparison of the intracellular free calcium concentration ([Ca2+]i) in platelets as an indicator of platelet activation, CD62P expression on platelets, and the chemokine regulated upon activation in normal T cells expressed and presumably secreted (RANTES) level in platelet-rich buffer supernatants between asthmatics and normal subjects.

SETTING

The respiratory outpatient clinics, Hiroshima University, Japan.

PARTICIPANTS

Twenty-five normal volunteers, 19 asthmatics taking no oral drugs associated with asthma treatment (group A), and 18 asthmatics taking oral theophylline (group B).

MEASUREMENTS AND RESULTS

While the resting [Ca2+]is in platelets were similar among the three groups, the [Ca2+]is in group A were significantly higher than those in normal subjects (p<0.05) and group B (p<0.01) after thrombin or 9,11-epithia-11,12-methano-thromboxane A2 (STA2) stimulation in the absence of external Ca2+. The CD62P expression level and RANTES level in group A after STA2 stimulation were significantly higher than those in normal subjects and group B (p<0.05).

CONCLUSIONS

We conclude that agonist-mediated activation of platelets is augmented in asthmatics resulting in enhanced release of chemokine such as RANTES, which could be suppressed by oral administration of theophylline.

Airway responsiveness in transgenic mice overexpressing platelet-activating factor receptor. Roles of thromboxanes and leukotrienes

Platelet-activating factor (PAF) is a potent proinflammatory compound potentially involved in the pathogenesis of inflammatory disorders, including bronchial asthma. To elucidate the pathophysiologic roles of PAF in bronchial asthma, we studied airway responsiveness in transgenic mice overexpressing PAF receptor. In the transgenic mice, PAF-induced airway smooth muscle contraction was demonstrated by physiologic and morphometric analyses, whereas there was no significant response in the littermate control group. The PAF-elicited bronchoconstriction in the transgenic mice was significantly reduced not only by a PAF receptor antagonist (WEB-2086) but also by a thromboxane synthesis inhibitor (indomethacin or ozagrel), an inhibitor of 5-lipoxygenase-activating protein (MK-886), or a cysteinyl leukotriene (LT) antagonist (pranlukast). LTB4 receptor antagonist (ONO-4057), however, had no effect on the PAF-induced responses. The transgenic mice showed a bronchial hyperreactivity to methacholine challenge, which was also inhibited by pretreatment with either thromboxane synthesis inhibitor or cysteinyl LT antagonist. These observations suggest that both thromboxane A2 and cysteinyl LTs (LTC4, LTD4, and LTE4) are involved in the bronchial responses to PAF or cholinergic stimulus in mice. The transgenic mice overexpressing PAF receptor may provide an appropriate model to study various PAF-related lung diseases, including bronchial asthma.

Effects of a potent platelet-activating factor antagonist, SR27417A, on allergen-induced asthmatic responses

Platelet-activating factor (PAF) is a lipid-derived mediator that has been implicated in the pathophysiology of airway inflammation in asthma. Its actions include chemotaxis and activation of inflammatory cells, particularly eosinophils. Inhaled PAF causes bronchoconstriction and increased airway responsiveness in human subjects. However, PAF antagonists have so far failed to show benefits in allergen challenge or in the treatment of chronic asthma. SR27417A is a novel PAF antagonist with increased potency compared with previously tested compounds. Twelve asthmatic subjects received treatment with either SR27417A or placebo for 1 wk in a double-blind crossover study. After treatment each subject underwent allergen challenge. Effects were assessed in terms of early and late asthmatic responses and allergen-induced effects on airway responsiveness. Baseline lung function and airway responsiveness were also examined. Treatment with SR27417A significantly attenuated the late asthmatic response (AUC LAR4-10h: 107 +/- 24 after placebo, 79 +/- 17 after SR27417A, p < 0.05; mean maximal percent fall in FEV1 LAR: 29 +/- 6% after placebo, 23.5 +/- 5.4% after SR27417A, p < 0.05). There were no effects on early asthmatic responses, allergen-induced airway responsiveness, or baseline lung measurements. SR27417A is the most potent PAF antagonist to date, and it has a modest inhibitory effect on the late asthmatic response. This suggests that PAF has a small role in allergic inflammation.

Apafant (a PAF receptor antagonist) suppresses the early and late airway responses in guinea pigs: a comparison with antiasthmatic drugs

We studied the effects of apafant (WEB 2086 BS), a specific and potent platelet activating factor (PAF) receptor antagonist, on the early and late airway responses in conscious and actively sensitized guinea pigs. An increase in airway resistance (Rs) was seen 1 min after the inhaled antigen challenge (early airway response), followed by another increase in Rs which peaked between 4 and 8 h after the provocation (late airway response). Oral administration of apafant as well as theophylline inhibited both early and late airway responses. Ozagrel, an inhibitor of thromboxane A2 synthetase, salbutamol, a beta2-adrenoceptor agonist, and dexamethasone significantly inhibited either the early or the late airway response only. Disodium cromoglycate inhibited neither the early nor the late airway response. The results showed that apafant inhibited both the early and late airway responses in sensitized guinea pigs and its effect was comparable or superior to that of anti-asthmatic drugs used clinically.

Effect of Y-24180, a platelet-activating factor-receptor antagonist, on the antigen-induced airway microvascular leakage in guinea pigs

The inhibitory effect of Y-24180 ((+/-)-4-(2-chlorophenyl)-2-[2-(4-isobutylphenyl)ethyl]-6, 9-dimethyl-6H-thieno[3,2-f][1,2,4]triazolo[4,3-a][1,4]diazepine) on platelet-activating factor (PAF)- or antigen-induced airway microvascular leakage was studied in guinea pigs by oral administration. The tissue content of extravasated Evans blue dye was used as an index of plasma exudation in the trachea, main bronchi, central intrapulmonary airways and peripheral intrapulmonary airways. In all of these tissues, Y-24180 potently inhibited the leakage induced by PAF. The ED50 value of Y-24180 determined in each of the tissues was approximately 0.02 mg/kg, demonstrating that the inhibitory potency of Y-24180 is 4-6 times that of WEB 2086, another PAF antagonist. Even at a dose of 10 mg/kg, however, Y-24180 showed no inhibitory effect on the leakage induced by leukotriene (LT) D4, histamine or bradykinin. In the antigen-induced model of guinea pigs sensitized with aerosolized ovalbumin, Y-24180 (0.1-10 mg/kg) and WEB 2086 (1-100 mg/kg) potently inhibited the microvascular leakage in all of the examined airway tissues except for the trachea. At 1-100 mg/kg, however, both ONO-1078, an LT-receptor antagonist, and OKY-046, a thromboxane A2 synthetase inhibitor, prevented partially but not significantly the antigen-induced leakage. These results provide evidence that endogenous PAF partially mediates the antigen-induced airway microvascular leakage in guinea pigs.

A study of the effect of a platelet activating factor (PAF) receptor antagonist on antigen challenge of horses with chronic obstructive pulmonary disease

Antigen challenge involving exposure to straw and mouldy hay for 7 h produced lung function changes and neutrophil recruitment to the lungs in horses with chronic obstructive pulmonary disease (COPD). During the challenge, an increase in radiolabelled neutrophils in the lungs occurred, together with increased respiratory rate and pleural pressure. The role of platelet activating factor (PAF) in antigen-induced neutrophil accumulation, and increased pleural pressure and respiratory rate was investigated by administering the PAF receptor antagonist WEB 2086 to asymptomatic COPD horses prior to antigen challenge. WEB 2086 (3 mg/kg i.v.) did not affect antigen-induced changes in either neutrophil accumulation or respiratory function. These results suggest that PAF may not be an important mediator of the response to antigen in equine COPD.