Phospholipid metabolism in human neutrophils activated by N-formyl-methionyl-leucyl-phenylalanine. Degranulation is not required for release of arachidonic acid: studies with neutrophils and neutrophil-derived cytoplasts

Effect of lithium on superoxide production and intracellular free calcium mobilization in elastin peptide (kappa-elastin) and FMLP stimulated human PMNS. Effect of age

The effect of lithium pretreatment on superoxide anion production and intracellular free calcium levels was investigated in polymorphonuclear leukocytes (PMN) from middle-aged and old individuals after stimulation by elastin peptides or FMLP. K-elastin (KE) significantly stimulated the production of superoxide anion by PMNs from middle-aged subjects, while this stimulation decreased with age and was absent in PMNs of elderly arteriosclerotic patients. Li pretreatment slightly increased this stimulating effect of KE in PMNs from middle-aged subjects and elderly arteriosclerotic patients, while slightly decreased in healthy elderly subjects. Moreover, Li was able to increase superoxide anion production even in the absence of KE, but this effect decreased also in PMNs of healthy and arteriosclerotic elderly patients. FMLP significantly increased superoxide anion production in all age-groups, but this effect was further amplified by Li only in PMNs of middle-aged subjects. In aged individuals Li pretreatment slightly decreased the effect of FMLP and had no effect in arteriosclerotic patients. Ca-mobilization induced by KE was inhibited by Li pretreatement in each age group. This inhibition by Li was much weaker in FMLP-stimulated PMNs. Li pretreatment did however modify the shape of the Ca-transient curves in FMLP stimulated leukocytes suggesting a qualitative modification of ion channel regulation. No such shape change of Ca-transient curves was observed after KE stimulation of Li pretreated PMNs. It appears that the regulation of these two receptors is differently affected by Li treatment.

Comparison of human polymorphonuclear leukocytes from peripheral blood and purulent exudates by high resolution 1H MRS

Human pus samples from various sites, including soft tissue and pleural cavity, as well as sputum from patients with cystic fibrosis have been analyzed by 1H magnetic resonance spectroscopy (MRS) and found to generate high resolution spectra. Assignments have been made for neutral lipid, amino acids, taurine, and lactate. Human peripheral blood polymorphonuclear leukocytes have also been examined and a similar spectrum containing these lipid and metabolite resonances was detected in cells stimulated with lipopolysaccharide. In unstimulated cells, only resonances from taurine and lactate were observed, suggesting that the presence of the lipid and metabolite resonances is an indicator of cellular activation of polymorphonuclear leukocytes. This is the first report of 1H MRS applied to intact human polymorphonuclear leukocytes and extends available documentation as to which types of normal and/or diseased tissue contain MR visible molecules.

The involvement of extracellular calcium in the formation of 5-lipoxygenase metabolites by human polymorphonuclear leukocytes

We have addressed the question why in the presence of a Ca2+ ionophore human polymorphonuclear leukocytes generate leukotrienes in high yields, but in only low amounts after stimulation by receptor agonists like fMLF (fM, formylmethionine), leukotriene B4 or platelet-activating factor (PAF), although a significant release of intracellular calcium can be measured. Using ionomycin we can show that from the two enzymes involved, phospholipase A2 and 5-lipoxygenase, the first requires a threshold level of about 350-400 nM calcium whereas 5-lipoxygenase shows a linear dependence on calcium and saturates at this concentration. Our data indicate that the Ca2+ requirement of phospholipase A2 can only be met by an additional influx of extracellular calcium, whereas 5-lipoxygenase will operate already at levels provided by intracellular stores. Consequently, the complexing of extracellular calcium by EGTA stops phospholipase A2 activity immediately, whereas added arachidonate can be still adequately metabolized by intracellular Ca2+ release triggered by fMLF or PAF. Interestingly, PAF shows a stronger extracellular component in its Ca2+ transient than fMLF, and also generates more 5-lipoxygenase metabolites. However, a clear correlation between the amount of 5-lipoxygenase metabolites and the extracellular Ca2+ signal was lacking, since maximal activity was achieved before the bulk of the extracellular calcium was monitored. Ca2+ influx after PAF stimulation could be blocked after 2 min by EGTA, but a further increase in the formation of 5-lipoxygenase metabolites was observed. In contrast ionomycin-elicited 5-lipoxygenase activity could be stopped at any time shortly after EGTA addition.

Subcellular distribution of fatty acids, phospholipids and phospholipase A2 in human neutrophils

We have examined the putative relationship between the presence of neutral pH optimum phospholipase A2 activities and the phospholipid and fatty acid composition in the distinct fractions obtained after subcellular fractionation in a continuous sucrose gradient of human neutrophils. Neutral phospholipase A2 activity was found mainly in the plasma membrane, the azurophilic granules and an unidentified fraction which was a little denser than that containing specific granules and endoplasmic reticulum. These differently located phospholipase A2 activities resulted in activation upon calcium ionophore A23187-cell treatment, but the phospholipase A2 activity found in the unidentified region showed the highest degree of activation after cell treatment with A23187 and hydrolyzed preferentially ethanolamine-containing glycerophospholipids. The subcellular distribution of inositol-containing phosphoglycerides was bimodal, with a main peak at the plasma membrane and another peak at the azurophilic granule region. Choline- and ethanolamine-containing glycerophospholipids showed a broad distribution throughout the gradient, with preponderance in the denser part of the gradient, where the intracellular organelle phospholipase A2 activities were located. Interestingly, ethanolamine-containing glycerophospholipids were shown to be enriched in fractions containing the phospholipase A2 which hydrolyzes preferentially this phospholipid class. Most of the arachidonic acid present in the postnuclear fraction of human neutrophils was found in the plasma membrane and resulted to be the only fatty acid released during A23187 stimulation. Thus, plasma membrane phospholipase A2 seems to be involved mainly in the release of arachidonic acid in A23187-stimulated cells. These findings indicate that differences in substrate specificity found in vitro among the phospholipase A2 activities located in distinct subcellular particulates might be a reflection of the distinct phospholipid and fatty acid composition in these particulates.

Lipoxin A4 and lipoxin B4 stimulate the release but not the oxygenation of arachidonic acid in human neutrophils: dissociation between lipid remodeling and adhesion

The profiles of actions of lipoxin A4 (LXA4) and lipoxin B4 (LXB4), two lipoxygenase-derived eicosanoids, were examined with human neutrophils. At nanomolar concentrations, LXA4 and LXB4 each stimulated the release of [1-14C]arachidonic acid from esterified sources in neutrophils. Lipoxin-induced release of [1-14C]arachidonic acid was both dose- and time-dependent and was comparable to that induced by the chemotactic peptide f-met-leu-phe. Time-course studies revealed that lipoxin A4 and lipoxin B4 each induced a biphasic release of [1-14C]arachidonic acid, which was evident within seconds (5-15 sec) in its initial phase and minutes (greater than 30 sec) in the second phase. In contrast, the all-trans isomers of LXA4 and LXB4 did not provoke [1-14C]AA release. Lipoxin-induced release of arachidonic acid was inhibited by prior treatment of the cells with pertussis toxin but not by its beta-oligomers, suggesting the involvement of guaninine nucleotide-binding regulatory proteins in this event. Dual radiolabeling of neutrophil phospholipid classes with [1-14C]arachidonic acid and [3H]palmitic acid showed that phosphatidylcholine was a major source of lipoxin-induced release of [1-14C]arachidonic acid. They also demonstrated that lipoxins rapidly stimulate both formation of phosphatidic acid as well as phospholipid remodeling. Although both LXA4 and LXB4 (10(-8)-10(-6) M) stimulated the release of [1-14C]arachidonic acid, neither compound evoked its oxygenation by either the 5- or 15-lipoxygenase pathways (including the formation of LTB4, 20-COOH-LTB4, 5-HETE, or 15-HETE). LXA4 and LXB4 (10(-7) M) each stimulated the elevation of cytosolic Ca2+ as monitored with Fura 2-loaded cells, albeit to a lesser extent than equimolar concentrations of FMLP. Neither lipoxin altered the binding of [3H]LTB4 to its receptor on neutrophils. In addition, they did not stimulate aggregation or induce adhesion of neutrophils to human endothelial cells. Results indicate that both LXA4 and LXB4 stimulate the rapid remodeling of neutrophil phospholipids to release arachidonic acid without provoking either aggregation or the formation of lipoxygenase-derived products within a similar temporal and dose range. Together they indicate that LXA4 and LXB4 display selective actions with human neutrophils and suggest that these eicosanoids possess unique profiles of action which may regulate neutrophil function during inflammation.

The metabolism of arachidonic and eicosapentaenoic acids in human neutrophils stimulated by A23187 and FMLP

A23187 stimulates the metabolism of endogenous as well as exogenous arachidonic acid (AA) and eicosapentaenolc acid (EPA) to their corresponding leukotrienes in human neutrophils. In contrast, conflicting results have been obtained concerning the effect of FMLP on the metabolism of these fatty acids. In the present study we compared the effect of A23187 and FMLP on the release and metabolism of these fatty acids in neutrophils. Stimulation of neutrophils with A23187, but not with FMLP, resulted in detectable levels of AA in the presence or absence of BW755C (a dual inhibitor of cyclooxygenase and lipoxygenase). The absolute amount of nonesterified AA in the extracts of neutrophils exposed to the agonist A23187 in the presence of BW755C was 20% higher than that obtained in the absence of BW755C, indicating that only a small fraction of the released AA was converted to lipoxygenase products. Furthermore, significant quantities of AA and EPA metabolites were detected only after treatment of neutrophils with A23187, but not with FMLP. Both A23187 and FMLP stimulated the conversion of exogenous EPA to 5-lipoxygenase products, with A23187 being somewhat more effective. In addition, significant differences were noted on the effect of EPA and DHA on the conversion of AA to its metabolites in A23187-stimulated neutrophils. Our results provide strong evidence that the amounts of eicosanoid precursors mobilized in response to FMLP are extremely small, if any, and this appears to be the likely explanation for the lack of eicosanoid detection by HPLC in FMLP-stimulated neutrophils.

Phospholipase A2-mediated release of arachidonic acid in stimulated guinea pig alveolar macrophages: interaction with lipid mediators and cyclic AMP

The stimulation of cultured guinea pig alveolar macrophages by the chemotactic peptide N-formyl-L-methionyl-L-leucyl-L-phenylalanine, or by the phospholipid inflammatory mediator platelet activating factor (PAF) induced an increase in arachidonic acid release and its cyclooxygenase products. This release, which was mimicked by the association of threshold concentrations of the calcium ionophore A 23187 and of the protein kinase C activator tetradecanoyl phorbol acetate arose mainly from diacyl- and alkyl-acyl-phosphatidylcholine and phosphatidylinositol. Using [1-14C]arachidonic acid-labeled membranes as an endogenous substrate as well as dioleoyl-phosphatidyl [14C]ethanolamine as an exogenous substrate, we showed that phospholipase A2 activity of stimulated macrophages increases upon stimulation. Treatment of macrophages by prostaglandin E2 decreased the arachidonic acid release elicited by the chemotactic peptide and PAF. Furthermore, prostaglandin E2 increased and PAF decreased the cellular content in cyclic AMP. From these results we suggest that an initial stimulation of alveolar macrophages by a bacterial signal initiates the sequential activation of a phospholipase C and of phospholipase A2, leading to the release of PAF and eicosanoids. These mediators may in turn modulate the cell response by increasing or decreasing cyclic AMP, Ca2+, or diacyglycerol macrophage content.

Bacterial lipopolysaccharides prime human neutrophils for enhanced production of leukotriene B4

Neutrophils can be "primed" for an enhanced respiratory burst by lipopolysaccharide (LPS) in concentrations measurable in patients with septic shock. Leukotriene B4 (LTB4) is the primary eicosanoid product of neutrophils and is felt to be a mediator of host defense and inflammation. We investigated the in vitro effects of LPS on neutrophil production of LTB4 and the omega-oxidation metabolites of LTB4. Incubation of neutrophils with LPS in concentrations ranging from 0.01 to 100 ng/ml did not result in production of LTB4 or metabolites in the absence of a second stimulus. Priming neutrophils with LPS and then stimulating with opsonized zymosan, phorbol-myristate-acetate or a low concentration of the calcium ionophore A23187 resulted in enhanced production of LTB4. LPS priming of neutrophils occurred in a concentration dependent manner. LPS did not result in LTB4 production in response to the chemoattractant peptide FMLP. LPS priming of neutrophils had no effect on cytosolic calcium concentrations of resting or zymosan-stimulated cells. These results suggest that LPS might effect host defense and tissue injury by potentiating the effect of other stimulants on neutrophil production of LTB4. This LPS induced enhancement may represent an important pathogenetic pathway in patients with gram negative sepsis.

Biosynthesis of 20,20,20-trifluoroleukotriene B4 from 20,20,20-trifluoroarachidonic acid: a metabolically stable analog of leukotriene B4 and its application to a study of stimulation of leukotriene B4 synthesis by immunoglobulin G1,2

Leukotriene B4 is rapidly metabolized through omega-oxidation, preventing its detection when it is produced under certain biological conditions. To investigate leukotriene B4 production in various physiological conditions, analogs of arachidonic acid which are converted to metabolically stable analogs of leukotriene B4 would be useful. We have synthesized 20,20,20-trifluoroarachidonic acid by the cis-selective Wittig reaction of the C12-C20 fragment with phosphonium salt. 20,20,20-trifluoroarachidonic acid was transformed into 20,20,20-trifluoroleukotriene B4 when incubated with human neutrophils in the presence of the calcium ionophore A23187. The product was identified by uv absorption spectrophotometry, gas chromatography-mass spectrometry, and coelution on high-performance liquid chromatography with 20,20,20-trifluoroleukotriene B4, which was enantioselectively synthesized by the reaction of the fluorine-containing C11-C20 fragment with the C1-C10 phosphonate. The fluorinated leukotriene B4 demonstrated as much chemotactic activity on human neutrophils as natural leukotriene B4 and was metabolically stable when incubated with human neutrophils, probably by blocking omega-oxidation. Also, enzymes catalyzing the transformation of arachidonic acid (AA) into leukotriene B4 did not discriminate the fluorinated precursors from the natural, nonfluorinated AA, thus 20-F3-AA is a valid analog of AA to be used in the study of AA metabolism. When 50 microM of the fluorinated acid was incubated with neutrophils stimulated with heat-aggregated human immunoglobulin G, a significant amount of fluorinated leukotriene B4 (4.3 ng/10(6) cells/40 min, at most) was formed in a dose-dependent manner while little leukotriene B4 was detected with incubation with 50 microM arachidonic acid, probably due to omega-oxidation of the product, leukotriene B4. 20,20,20-Trifluoroarachidonic acid appears to be a useful tool for studying the capacity of leukotriene B4 synthesis in various biological systems while long-lasting 20,20,20-trifluoroleukotriene B4 would serve as an excellent analog of leukotriene B4 in pharmacological studies to understand functions of leukotrienes B4.

Stimulation of arachidonic acid release by guanine nucleotide in saponin-permeabilized neutrophils: evidence for involvement of GTP-binding protein in phospholipase A2 activation

Addition of a guanine nucleotide analog, guanosine 5'-O-(thiotriphosphate) (GTP gamma S)(1-100 microM) induced release of [3H]arachidonic acid from [3H]arachidonate-prelabeled rabbit neutrophils permeabilized with saponin. The chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP)-induced arachidonate release was enhanced by GTP gamma S, Ca2+, or their combination. Ca2+ alone (up to 100 microM) did not effectively stimulate lipid turnover. However, the combination of fMLP plus GTP gamma S elicited greater than additional effects in the presence of resting level of free Ca2+. The addition of 100 microM of GTP gamma S reduced the Ca2+ requirement for arachidonic acid liberation induced by fMLP. Pretreatment of neutrophils with pertussis toxin resulted in the abolition of arachidonate release and diacylglycerol formation. Neomycin (1 mM) caused no significant reduction of arachidonate release. In contrast, about 40% of GTP gamma S-induced arachidonate release was inhibited by a diacylglycerol lipase inhibitor, RHC 80267 (30 microM). These observations indicate that liberation of arachidonic acid is mediated by phospholipase A2 and also by phospholipase C/diacylglycerol lipase pathways. Fluoride, which bypasses the receptor and directly activates G proteins, induced arachidonic acid release and diacylglycerol formation. The fluoride-induced arachidonate release also appeared to be mediated by these two pathways. The loss of [3H]arachidonate was seen in phosphatidylinositol, phosphatidylcholine, and phosphatidylethanolamine. These data indicate that a G protein is involved between the binding of fMLP to its receptor and activation of phospholipase A2, and also that the arachidonic acid release is mediated by both phospholipase A2 and phospholipase C/diacylglycerol lipase.

Preparation and characterization of mast cell cytoplasts

A new model system for studying biochemical reactions in mast cell plasma membranes was developed. Particles termed cytoplasts consisting of organelle-depleted cytoplasm surrounded by an intact plasma membrane were formed from cytochalasin B-treated mast cells ultracentrifuged through a discontinuous Ficoll gradient. Two cytoplasts were formed per mast cell and 95% were recovered. Mast cell cytoplasts had a mean diameter of 3.2 microns with a median volume of 38 microns 3. Enzyme marker studies indicated that subcellular recoveries in the mast cell cytoplast were: plasma membrane = 16%, cytoplasm = 39%, nucleus = 1.1%, granule = 0.5%. Analysis of IgE receptors indicated that mast cell cytoplasts retained the normal asymmetric orientation of the plasma membrane. Mast cell cytoplasts synthesized ATP, incorporated labeled fatty acids into complex lipids and retained fluorescein after deacylation of diacetylfluorescein. The quantity of cAMP (adenosine 3':5'-cyclic monophosphate) maintained in mast cell cytoplasts was 0.0304 pmol/10(6) original mast cells. Cytoplasts offer the opportunity to study plasma membrane and cytoplasmic biochemical events that occur during stimulation in a relatively physiologic environment.

The leukotriene B4 paradox: neutrophils can, but will not, respond to ligand-receptor interactions by forming leukotriene B4 or its omega-metabolites

Leukotriene B4 (5S,12R-dihydroxy-6,14-cis,8,10-trans-eicosatetraenoic acid, LTB4) is released from neutrophils exposed to calcium ionophores. To determine whether LTB4 might be produced by ligand-receptor interactions at the plasmalemma, we treated human neutrophils with serum-treated zymosan (STZ), heat-aggregated IgG and fMet-Leu-Phe (fMLP), agonists at the C3b, Fc and fMLP receptors respectively. STZ (10 mg/ml) provoked the formation of barely detectable amounts of LTB4 (0.74 ng/10(7) cells); no omega-oxidized metabolites of LTB4 were found. Adding 10 microM-arachidonate did not significantly increase production of LTB4 or its metabolites. Addition of 50 microM-arachidonate (an amount which activates protein kinase C) before STZ caused a 40-fold increase in the quantity of LTB4 and its omega-oxidation products. Neither phorbol myristate acetate (PMA, 200 ng/ml) nor linoleic acid (50 microM), also activators of protein kinase C, augmented generation of LTB4 by cells stimulated with STZ. Neither fMLP (10(-6) M) nor aggregated IgG (0.3 mg/ml) induced LTB4 formation (less than 0.01 ng/10(7) cells). Moreover, cells exposed to STZ, fMLP, or IgG did not form all-trans-LTB4 or 5-hydroxyeicosatetraenoic acid; their failure to make LTB4 was therefore due to inactivity of neutrophil 5-lipoxygenase. However, adding 50 microM-arachidonate to neutrophil suspensions before fMLP or IgG triggered LTB4 production, the majority of which was metabolized to its omega-oxidized products (fMLP, 20.2 ng/10(7) cells; IgG, 17.1 ng/10(7) cells). The data show that neutrophils exposed to agonists at defined cell-surface receptors produce significant quantities of LTB4 only when treated with non-physiological concentrations of arachidonate.