Role of lipoxygenase metabolites in platelet-activating factor- and antigen-induced bronchial hyperresponsiveness and eosinophil infiltration

Pharmacological modulation of the inflammatory actions of platelets

Patients with inflammatory diseases often exhibit a change in platelet function, with these alterations being clearly distinct from the well-characterized role of platelets in haemostasis and thrombosis. It has recently been revealed that platelets can behave as innate inflammatory cells in immune responses with roles in leukocyte recruitment, migration into tissues, release of cytotoxic mediators, and in tissue remodelling following injury.Platelets exhibit a wide range of receptors for mediators involved in the inflammatory pathway and the immune response (Fig. 1). These include purinergic receptors, selectins, integrins, toll-like receptors, immunoglobulins, and chemokine receptors, but the precise role platelets play in the inflammatory process is still under investigation. Nevertheless, given that many of these receptors are distinct from those involved in thrombosis and haemostasis, this raises the real possibility of targeting these receptors to regulate inflammatory diseases without compromising haemostasis.

Alternative splicing of the G protein-coupled receptor superfamily in human airway smooth muscle diversifies the complement of receptors

G protein-coupled receptors (GPCRs) are the largest signaling family in the genome, serve an expansive array of functions, and are targets for approximately 50% of current therapeutics. In many tissues, such as airway smooth muscle (ASM), complex, unexpected, or paradoxical responses to agonists/antagonists occur without known mechanisms. We hypothesized that ASM express many more GPCRs than predicted, and that these undergo substantial alternative splicing, creating a highly diversified receptor milieu. Transcript arrays were designed detecting 434 GPCRs and their predicted splice variants. In this cell type, 353 GPCRs were detected (including 111 orphans), with expression levels varying by approximately 900-fold. Receptors used for treating airway disease were expressed lower than others with similar signaling properties, indicating potentially more effective targets. A disproportionate number of Class-A peptide-group receptors, and those coupling to G(q)/(11) or G(s) (vs. G(i)), was found. Importantly, 192 GPCRs had, on average, five different expressed receptor isoforms because of splicing events, including alternative splice donors and acceptors, novel introns, intron retentions, exon(s) skips, and novel exons, with the latter two events being most prevalent. The consequences of splicing were further investigated with the leukotriene B4 receptor, known for its aberrant responsiveness in lung. We found transcript expression of three variants because of alternative donor and acceptor splice sites, representing in-frame deletions of 38 and 100 aa, with protein expression of all three isoforms. Thus, alternative splicing, subject to conditional, temporal, and cell-type regulation, is a major mechanism that diversifies the GPCR superfamily, creating local recepteromes with specialized environments.

5-Oxo-6,8,11,14-eicosatetraenoic Acid Stimulates the Release of the Eosinophil Survival Factor Granulocyte/Macrophage Colony-stimulating Factor from Monocytes

Allergic diseases such as asthma are characterized by tissue eosinophilia induced by the combined effects of chemoattractants and cytokines. Lipid mediators are a major class of endogenous chemoattractants, among which 5-oxo-6,8,11,14-eicosatetraenoic acid (5-oxo-ETE) is the most potent for human eosinophils. In this study, we investigated the effects of 5-oxo-ETE on eosinophil survival by flow cytometry. We found that this compound could promote eosinophil survival in the presence of small numbers of contaminating monocytes, but not in their absence. The conditioned medium from monocytes treated for 24 h with 5-oxo-ETE also strongly promoted eosinophil survival, whereas the medium from vehicle-treated monocytes had no effect. An antibody against the granulocyte/macrophage colony-stimulating factor (GM-CSF) completely blocked the response of eosinophils to the conditioned medium from 5-oxo-ETE-treated monocytes, whereas an antibody against interleukin-5 had no effect. Furthermore, 5-oxo-ETE stimulated the release of GM-CSF from cultured monocytes in amounts compatible with eosinophil survival activity, with a maximal effect being observed after 24 h. This effect was concentration-dependent and could be observed at concentrations in the picomolar range. 5-Oxo-ETE and leukotriene B(4) had similar effects on GM-CSF release at low concentrations, but 5-oxo-ETE induced a much stronger response at concentrations of 10 nm or higher. This is the first report that 5-oxo-ETE can induce the release of any cytokine, suggesting that it could be an important mediator in allergic and other inflammatory diseases due both to its chemoattractant properties and to its potent effects on the synthesis of the survival factor GM-CSF.

Protective effect of silymarin in antigen challenge- and histamine-induced bronchoconstriction in in vivo guinea-pigs

The effects of silymarin on bronchoconstriction induced by antigen challenge and on post-antigen challenge hyperresponsiveness to substance P were evaluated in sensitized guinea-pigs. Silymarin significantly decreased the bronchoconstriction due to antigen administration in the early phase of the response. In contrast, the dose-response curve for substance P recorded 1 h after antigen challenge was not modified by pretreatment with silymarin. The influence of the flavonoid on hyperresponsiveness to histamine in propranolol- and PAF (platelet-activating factor)-treated animals was also assessed. Silymarin did not affect hyperresponsiveness to histamine induced by either propranolol or PAF although it had inhibitory activity on the bronchial contractile response to the autacoid. These results suggest that silymarin has a protective effect in the early phase of allergic asthma, an effect, which may be related to a negative influence of the flavonoid on bronchial responsiveness to histamine.

Aspirin-sensitive versus non-aspirin-sensitive nasal polyp patients: analysis of leukotrienes/Fas and Fas-ligand expression

OBJECTIVE

The goal of the present study was to identify levels of leukotrienes released by leukocytes and nasal polyps from aspirin-sensitive patients (ASPs) and non-aspirin-sensitive patients (NASPs) after exposure to various concentrations of aspirin. Eosinophil expressions of apoptosis regulators were also compared.

STUDY DESIGN AND SETTING

Leukotriene release from nasal polyps and leukocytes of ASPs and NASPs was measured after exposure to differing concentrations of aspirin. Eosinophils were analyzed for Fas and Fas ligand receptors using flow cytometry.

RESULTS

NASPs showed increased leukotriene release from nasal polyps on increased aspirin exposure. Polyps from ASPs did not show any change. ASPs showed greater leukotriene release from blood on increasing aspirin exposure compared with NASPs. Flow cytometric analysis showed eosinophils in the peripheral blood of ASPs have decreased Fas expression compared with those of NASPs.

CONCLUSIONS

This study showed differences in leukotriene release and eosinophil apoptosis between ASPs and NASPs.

SIGNIFICANCE

These findings suggest a role of leukotrienes and eosinophils in aspirin sensitivity mechanisms in blood.

Eotaxin and RANTES enhance 5-oxo-6,8,11,14-eicosatetraenoic acid-induced eosinophil chemotaxis

BACKGROUND

The 5-lipoxygenase product 5-oxo-6,8,11,14-eicosatetraenoic acid (5-oxo-ETE) is a potent activator of human eosinophils and, among lipid mediators, is the most active chemoattractant for these cells. Studies have demonstrated the importance of 5-lipoxygenase products in allergen-induced pulmonary eosinophilia. Because CC chemokines such as eotaxin and RANTES also play critical roles in this phenomenon, it would seem likely that members of both classes of mediators contribute to this response.

OBJECTIVE

The study was designed to directly compare the effects of 5-oxo-ETE on eosinophils with those of eotaxin and RANTES and to determine whether these chemokines could enhance the chemotactic response to 5-oxo-ETE.

METHODS

Eosinophil chemotaxis was measured with microchemotaxis chambers. CD11b, L-selectin, and actin polymerization were measured by flow cytometry. Calcium mobilization was measured by fluorescence.

RESULTS

5-Oxo-ETE stimulated eosinophil chemotaxis with a potency between those of eotaxin and RANTES and a maximal response about 50% higher than that of eotaxin. Threshold concentrations of eotaxin and RANTES increased the chemotactic potency of 5-oxo-ETE by more than 4-fold. 5-Oxo-ETE and eotaxin were approximately equipotent in mobilizing cytosolic calcium in eosinophils. Eotaxin was more potent in inducing CD11b expression and actin polymerization, but the maximal responses to 5-oxo-ETE were about 50% higher. 5-Oxo-ETE strongly induced L-selectin shedding, whereas eotaxin elicited only a weak and variable response.

CONCLUSION

5-Oxo-ETE is a strong activator of human eosinophils with a chemotactic potency comparable to those of eotaxin and RANTES, both of wwhich enhance 5-oxo-ETE-induced chemotaxis. 5-Oxo-ETE and CC chemokines may combine to induce pulmonary eosinophilia in asthma.

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.

Hyperresponsiveness in the human nasal airway: new targets for the treatment of allergic airway disease

Allergic rhinitis is a condition which affects over 15% of the population in the United Kingdom. The pathological process involves two stages: nasal inflammation, and the development of nasal airway hyperresponsiveness (AHR) to allergen and a number of other stimuli. This results in the amplification of any subsequent allergic reaction, contributing to the chronic allergic state. A number of different hypotheses have been proposed to explain the underlying mechanism of AHR, including a role for eosinophil-derived proteins, free radicals and neuropeptides. While there may be a number of independent pathways which can result in AHR, evidence obtained from both animal models and in vivo experiments in humans indicate that some mediators may interact with one another, resulting in AHR. Further research into these interactions may open new avenues for the pharmacological treatment of chronic allergic rhinitis, and possibly other allergic airway diseases.

5-oxo-ETE induces pulmonary eosinophilia in an integrin-dependent manner in Brown Norway rats

We have shown previously that the 5-lipoxygenase product 5-oxo-6,8, 11,14-eicosatetraenoic acid (5-oxo-ETE) is a highly potent eosinophil chemoattractant in vitro. To determine whether this substance can induce pulmonary eosinophil infiltration in vivo, it was administered to Brown Norway rats by tracheal insufflation. Eosinophils were then counted in lung sections that had been immunostained with an antibody to eosinophil major basic protein. 5-Oxo-ETE induced a dramatic increase in the numbers of eosinophils (ED50, 2.5 microg) around the walls of the airways, which reached maximal levels (five times control levels) between 15 and 24 h after administration, and then declined. LTB4 also induced pulmonary eosinophil infiltration with a similar ED50 but appeared to be somewhat less effective. In contrast, LTD4 and LTE4 were inactive. 5-Oxo-ETE-induced eosinophilia was unaffected by the LTB4 and PAF antagonists LY255283 and WEB 2170, respectively. However, it was inhibited by approximately 75% by monoclonal antibodies to CD49d (VLA-4) or CD11a (LFA-1) but was not significantly affected by an antibody to CD11b (Mac-1). In conclusion, 5-oxo-ETE induces pulmonary eosinophilia in Brown Norway rats, raising the possibility that it may be a physiological mediator of inflammation in asthma.