Role of platelets and the arachidonic acid pathway in the regulation of neutrophil oxidase activity

The intercellular mechanisms involved in platelet-mediated regulation of neutrophil function remain incompletely understood. This study investigated the role of the arachidonic acid pathway in the modulation of chemoattractant-induced production of oxygen metabolites, measured as luminol-amplified chemiluminescence (CL). We demonstrate that platelets dose-dependently inhibit the CL response in neutrophils stimulated with N-formyl-methionyl-leucyl-phenylalanine (fMLP). Incubation with eicosatetrayonic acid (ETYA), a combined cyclooxygenase and lipooxygenase inhibitor, dramatically decreased the fMLP-induced CL response in neutrophils, an effect that was further enhanced in the presence of platelets. The separate effects of eicosatriyonic acid (ETI ) and indomethacin, specific inhibitors of lipoxygenase and cyclooxygenase, respectively, were significantly lower compared to the action of ETYA. On the contrary, impediment of arachidonic acid release with the phospholipase A 2 inhibitor arachidonyl trifluoromethyl ketone (ATK ) markedly increased the production of oxygen radicals triggered by fMLP. The addition of exogenous arachidonic acid clearly decreased the fMLP-induced CL response in neutrophils, which further strengthens a downregulating effect of arachidonic acid on oxidase activity. This inhibitory action of arachidonic acid, however, was reversed upon co-incubation with platelets. In conclusion, this study suggests that an accumulation of arachidonic acid, following chemotactic peptide stimulation, turns off neutrophil oxidase activity. Furthermore, platelets may support the synthesis of reactive arachidonic acid metabolites, which modulate oxygen radical production in neutrophils.     

The action of extracellular NAD+ in the liver of healthy and tumor-bearing rats: model analysis of the tumor-induced modified response

The chronic inflammatory state induced by cancer is expected to affect the actions of extracellular NAD⁺ in the liver because these are largely mediated by eicosanoids. Under this assumption the present work was planned to investigate the influence of the Walker-256 tumor on the action of extracellular NAD⁺ on metabolism and hemodynamics in the perfused rat liver. The experiments were done with livers from healthy and tumor-bearing rats with measurements of gluconeogenesis from lactate, pyruvate production, oxygen consumption and portal pressure. A model describing the biphasic effects of NAD⁺ was proposed as an auxiliary worktool for interpretation. The Walker-256 tumor modified the responses of metabolism to extracellular NAD⁺ by delaying the peak of maximal responses and by prolonging the inhibitory effects. The transient increase in portal perfusion pressure caused by NAD⁺ was enhanced and delayed. The model was constructed assuming the mediation of a down-regulator (inhibition), an up-regulator (stimulation) and receptor dessensitization. Analysis suggested that the productions of both the down- and up-regulators were substantially increased and delayed in time in the tumor-bearing condition. Since the regulators are probably eicosanoids, this analysis is consistent with the increased capacity of producing these agents in the chronic inflammatory state induced by cancer.     

Role of Ca-independent phospholipase A2 and n−3 polyunsaturated fatty acid docosahexaenoic acid in prostanoid production in brain: perspectives for protection in neuroinflammation

Various diseases of the central nervous system are characterized by induction of inflammatory events, which involve formation of prostaglandins. Production of prostaglandins is regulated by activity of phospholipases A₂ and cyclooxygenases. These enzymes release the prostaglandin precursor, the n−6 polyunsaturated fatty acid, arachidonic acid and oxidize it into prostaglandin H₂. Docosahexaenoic acid, which belongs to the n−3 class of polyunsaturated fatty acids, was shown to reduce production of prostaglandins after in vivo and in vitro administration. Nevertheless, the fact that in brain tissue cellular phospholipids naturally have a uniquely high content of docosahexaenoic acid was ignored so far in studies of prostaglandin formation in brain tissue. We consider the following possibilities: docosahexaenoic acid might attenuate production of prostaglandins by direct inhibition of cyclooxygenases. Such inhibition was found with the isolated enzyme. Another possibility, which has been already shown is reduction of expression of inducible cyclooxygenase-2. Additionally, we propose that docosahexaenoic acid could influence intracellular Ca²⁺ signaling, which results in changes of activity of Ca²⁺-dependent phospholipase A₂, hence reducing the amount of arachidonic acid available for prostaglandin production. Astrocytes, the main type of glial cells in the brain control the release of arachidonic acid, docosahexaenoic acid and the formation of prostaglandins. Our recently obtained data revealed that the release of arachidonic and docosahexaenoic acids in astrocytes is controlled by different isoforms of phospholipase A₂, i.e. Ca²⁺-dependent phospholipase A₂ and Ca²⁺-independent phospholipase A₂, respectively. Moreover, the release of arachidonic and docosahexaenoic acids is differently regulated through Ca²⁺- and cAMP-dependent signal transduction pathways. Based on analysis of the current literature and our own data we put forward the hypothesis that Ca²⁺-independent phospholipase A₂ and docosahexaenoic acid are promising targets for treatment of inflammatory related disorders in brain. We suggest that Ca²⁺-independent phospholipase A₂ and docosahexaenoic acid might be crucially involved in brain-specific regulation of prostaglandins.     

Lipids as targets for novel anti-inflammatory therapies

Lipids serve important functions as membrane constituents and also as energy storing molecules. Besides these functions certain lipid species have now been recognized as signalling molecules that regulate a multitude of cellular responses including cell growth and death, and also inflammatory reactions. Bioactive lipids are generated by hydrolysis from membrane lipids mainly by phospholipases giving rise to fatty acids and lysophospholipids that either directly exert their function or are further converted to active mediators. This review will summarize the present knowledge about bioactive lipids that either promote or attenuate inflammatory reactions. These lipids include polyunsaturated fatty acids (PUFA), eicosanoids including the epoxyeicosatrienoic acids (EET), peroxisome proliferation activating receptor (PPAR) activators, cannabinoids and the sphingolipids ceramide, sphingosine 1-phosphate and sphingosylphosphorylcholine.     

Influence of polyunsaturated fatty acids and their metabolites on stem cell biology

Proinflammatory cytokines and essential fatty acids (EFAs) and their metabolites are altered in coronary heart disease, stroke, diabetes mellitus, hypertension, cancer, depression, schizophrenia, Alzheimer's disease, and collagen vascular diseases, indicating that these diseases not only are low-grade systemic inflammatory conditions but also have defects in the metabolism of EFAs. EFAs and their metabolites such as eicosanoids, lipoxins, resolvins, protectins, maresins, and nitrolipids are biologically active molecules that regulate gene expression and enzyme activity, modulate inflammation, the immune response, and gluconeogenesis by direct and indirect pathways, function directly as agonists of a number of G-protein-coupled receptors, and thus regulate several cellular processes. EFAs and their metabolites activate phosphatidylinositol 3-kinase/murine thymoma viral oncogene homolog 1 (Akt) and p44/42 mitogen-activated protein kinases and stimulate gluconeogenesis and cell proliferation by Ca²⁺, phospholipase C/protein kinase, events that are also necessary for stem cell proliferation. Stem cells are pluripotent and expected to be of benefit in the management of many clinical conditions. Therefore, I propose that the beneficial actions of EFAs and their metabolites seen in coronary heart disease, stroke, diabetes mellitus, hypertension, atherosclerosis, cancer, depression, schizophrenia, Alzheimer's disease, and collagen vascular diseases could be ascribed to their ability to enhance the proliferation and differentiation of embryonic stem cells in addition to their capacity to suppress inflammation.     

Endogenous cannabinoids may regulate chronic inflammation in aspirin-exacerbated respiratory disease

Aspirin-exacerbated respiratory disease (AERD) is characterized by the triad of chronic rhinosinusitis with nasal polyposis, adult-onset asthma and non-IgE mediated reactions to aspirin and other cyclooxygenase-1 (COX-1) inhibitors. Patients with AERD are dependent on COX-1 activity to maintain production of prostaglandin (PG) species, such as PGE2, which maintain physiologic levels of inflammation and limit the production of pro-inflammatory cysteinyl leukotrienes. The endogenous cannabinoid system is a family of immunomodulatory lipids and their innate g-protein coupled receptors that are closely related to arachidonic acid and may modulate inflammation via several pathways, including the direct production of metabolically active prostaglandin glycerol-esters. A recent pilot study has identified the significant up-regulation of the peripherally expressed, type-2 cannabinoid receptor (CB2) in AERD nasal polyps versus control tissues from patients with either allergic fungal rhinosinusitis or no history of chronic sinonasal inflammation. These early findings suggest the involvement of increased endogenous cannabinoid activity in prostaglandin deficient states such as AERD. Future study is needed to explore the significance of these findings, with specific investigation of the impact of CB2 activation on markers of airway inflammation, as well as the potential to measure CB2 expression as a screening biomarker for the evaluation of unrecognized disease.     

Inhibition of the synthesis of eicosanoid-like substances in a human oral cancer cell line by interferon-γ and eicosapentaenoic Acid

The objectives were to examine the production of eicosanoids in a Chinese human oral cancer cell line (OEC-M1) and to test the effects of interferon-γ (IFN-γ), eicosapentaenoic acid (EPA) and enzyme inhibitors on this biosynthesis. The eicosanoids were identified by reverse phase-high performance liquid chromatography. Two predominant peaks appeared in the chromatograms. One compound (P-1) was identified by ultraviolet absorption at a λ_max of 278 nm with shoulders at 272 and 284 nm. The other compound (P-2) was identified by ultraviolet absorption at a λ_max of 284 nm with shoulders at 278 and 290 nm. The production of P-1 was significantly inhibited by the addition of IFN-γ (200 and 400 U/ml), and EPA (10 to 40 μM). It was only partially inhibited (p<0.05) by indomethacin (INDO) (0.5 and 1μM), nordihydroguaiaretic acid (NDGA) (30 and 60μM/ml), and eicosa-5,8,11,14-tetraynoic acid (ETYA) (20–60μM). It was almost completely inhibited by indomethacin (2 and 3μM), and dexamethasone (0.6 and 6μM). The production of P-2 was almost completely inhibited by IFN-γ (200 and 400 U/ml), and partially inhibited (p<0.05) by EPA (10 and 20μM), NDGA (30 and 60μM), ETYA (20 and 40μM), dexamethasone (0.6 and 6μM). The production of both peaks was significantly reduced by excluding arachidonic acid (AA), and almost completely inhibited by heating at 100°C for 10 min during incubation. These results demonstrate that two eicosanoid-like compounds are synthesized by the OEC-M1 cell line and that their production can be modulated by IFN-γ, EPA, indomethacin, NDGA, ETYA, and dexamethasone.