Characterization of effect of N-formyl-methionyl-leucyl-phenylalanine on leukotriene synthesis in human polymorphonuclear leukocytes

Toll-like receptor ligands induce polymorphonuclear leukocyte migration: key roles for leukotriene B4 and platelet-activating factor

Activation of toll-like receptors (TLRs) and polymorphonuclear leukocyte (PMN) accumulation at infection sites are critical events of host defense. The involvement of leukotriene (LT) B(4) and platelet-activating factor (PAF) in TLR ligand-induced activation of inflammatory cell functions is essentially unknown. Using an in vitro model of human PMN migration through human endothelial cell monolayers, we demonstrate that prototypic ligands of TLR1/2, 2/6, 3, 4, 5, and 7/8 promote PMN migration, an effect markedly inhibited by 3 LTB(4) receptor antagonists (70-80% inhibition at 100 nM compared to vehicle-treated cells), 3 PAF receptor antagonists (20-50% inhibition at 10 nM), 3 LT biosynthesis inhibitors (75-85% inhibition at 100 nM), and 1 cytosolic phospholipase A(2)alpha (cPLA(2)alpha) inhibitor (90% inhibition at 1 microM). Accordingly, selected TLR ligands caused Ser-505-phosphorylation of cPLA(2)alpha and measurable LTB(4) and PAF biosynthesis in the transmigration assay. As negative controls, interleukin-8- and formyl-methionyl-leucyl-phenylalanine-elicited migration in vitro was not inhibited either by an LTB(4) receptor antagonist or by the cPLA(2)alpha inhibitor. Finally, LTB(4) and PAF receptor antagonists inhibited (up to approximately 65% at optimal doses) TLR ligand-induced PMN infiltration in the mouse air-pouch model. These studies unravel the critical involvement of de novo LTB(4) and PAF biosynthesis in PMN migration elicited by TLR ligands.

The Toll‐like receptor 7/8‐ligand resiquimod (R‐848) primes human neutrophils for leukotriene B4, prostaglandin E2and platelet‐activating factor biosynthesis

Toll-like receptors (TLR) recognize pathogen-associated molecular patterns and play important roles in the innate immune system. While single-stranded viral RNA is the natural ligand of TLR7/TLR8, the imidazoquinoline resiquimod (R-848) is recognized as a potent synthetic agonist of TLR7/TLR8. We investigated the effects of TLR7/8 activation on lipid mediator production in polymorphonuclear leukocytes exposed to R-848. Although R-848 had minimal effects by itself, it strongly enhanced leukotriene B4 formation on subsequent stimulation by fMLP, platelet-activating factor, and the ionophore A23187. R-848 acted via TLR8 but not TLR7 as shown by the lack of effect of the TLR7-specific ligand imiquimod. Priming with R-848 also resulted in enhanced arachidonic acid release and platelet-activating factor formation following fMLP stimulation, as well as enhanced prostaglandin E2 synthesis following the addition of arachidonic acid. Western blot analysis demonstrated that R-848 induced the phosphorylation of the cytosolic phospholipase A2alpha, promoted 5-lipoxygenase translocation and potently stimulated the expression of the type 2 cyclooxygenase. Bafilomycin A1, an inhibitor of endosomal acidification, efficiently inhibited all R-848-induced effects. These studies demonstrate that TLR8 signaling strongly promotes inflammatory lipid mediator biosynthesis and provide novel insights on innate immune response to viral infections.

Inhibition of platelet-activating factor biosynthesis by adenosine and histamine in human neutrophils: involvement of cPLA2alpha and reversal by lyso-PAF

Leukotrienes (LT) and platelet-activating factor (PAF) are important lipid mediators of inflammation. We and others reported previously that autacoids such as adenosine, histamine, prostaglandin E2, and beta-adrenergic agents inhibit LT biosynthesis in activated human polymorphonuclear leukocytes (PMN). In this study, we demonstrate that CGS-21680 (a selective agonist of the adenosine A2A receptor) and histamine also potently inhibit PAF biosynthesis in agonist [formyl Met-Leu-Phe (fMLP)]- and thapsigargin-activated human PMN. The observed inhibitions of PAF biosynthesis were reversed effectively by exogenous 1-O-alkyl-lyso-sn-glyceryl-3-phosphocholine (lyso-PAF), suggesting that these effects of CGS-21680 and histamine implicate the blockade of cytosolic phospholipase A2alpha (cPLA2alpha) activity and lyso-PAF release and that the acetyl-coenzyme A/lyso-PAF acetyl transferase is not inhibited by the autacoids. Accordingly, the cPLA2alpha inhibitor pyrrophenone completely blocked PAF formation, and lyso-PAF similarly prevented this effect of pyrrophenone. The inhibitory effects of CGS-21680 and histamine on PAF biosynthesis were prevented by the protein kinase A inhibitor H-89, supporting roles for the Gs -coupled receptors A2A and H2, respectively, and cyclic adenosine monophosphate in the inhibitory mechanism. The fMLP-induced phosphorylations of p38 and extracellular signal-regulated kinase 1/2 were not altered significantly by the CGS-21680, indicating that inhibition of these kinases is not involved in the inhibitory effect of the adenosine A2A receptor ligand on LT and PAF biosynthesis. These data further emphasize the multiple and potent inhibitory effects of adenosine and histamine on leukocyte functions, in particular, on the biosynthesis of two classes of important lipid mediators and their putative regulatory roles in immune processes in health and diseases.

Inhibitors of actin polymerisation stimulate arachidonic acid release and 5-lipoxygenase activation by upregulation of Ca2+ mobilisation in polymorphonuclear leukocytes involving Src family kinases

Here, we show that actin polymerisation inhibitors such as latrunculin B (LB), and to a minor extent also cytochalasin D (Cyt D), enhance the release of arachidonic acid (AA) as well as nuclear translocation of 5-lipoxygenase (5-LO) and 5-LO product synthesis in human polymorphonuclear leukocytes (PMNL), challenged with thapsigargin (TG) or N-formyl-methionyl-leucyl-phenylalanine. The concentration-dependent effects of LB (EC50 approximately 200 nM) declined with prolonged preincubation (>3 min) prior TG and were barely detectable when PMNL were stimulated with Ca2+-ionophores. Investigation of the stimulatory mechanisms revealed that LB (or Cyt D) elicits Ca2+ mobilisation and potentiates stimulus-induced elevation of intracellular Ca2+, regardless of the nature of the stimulus. LB caused rapid but only moderate activation of p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK)2. The selective Src family kinase inhibitors PP2 and SU6656 blocked LB- or Cyt D-mediated Ca2+ mobilisation and suppressed the upregulatory effects on AA release and 5-LO product synthesis, without affecting AA metabolism evoked by ionophore alone. We conclude that in PMNL, inhibitors of actin polymerisation cause enhancement of intracellular Ca2+ levels through Src family kinase signaling, thereby facilitating stimulus-induced release of AA and 5-LO product formation.

Cysteinyl-leukotrienes and the liver

Leukotrienes are potent biological mediators implicated in an increasing number of disease processes. This review outlines the basic biology of leukotrienes and discusses recent developments in our understanding of the specific role of cysteinyl-leukotrienes (cLTs) in cholestasis, hepatic inflammation, portal hypertension, and the pathogenesis of the hepatorenal syndrome (HRS).

Role of arachidonic acid and its metabolites in the priming of NADPH oxidase in human polymorphonuclear leukocytes by peritoneal dialysis effluent

Peritoneal dialysis effluent (PDE) contains a low-molecular-weight solute that will activate and prime the NADPH oxidase of human neutrophils via a phospholipase A2 (PLA2)-dependent mechanism. Since the products of PLA2 are known to activate and prime the oxidase we have investigated their role in the dialysis effluent-mediated activation and priming of human neutrophils. NADPH oxidase activity of PDE-primed and -unprimed neutrophils was measured by lucigenin-enhanced chemiluminescence in the presence of known inhibitors of the arachidonic acid cascade. Incubation of neutrophils with the nonselective PLA2 inhibitor quinacrine (0 to 100 microM) reduced oxidase activity in both primed and unprimed cells. Furthermore, primed cells were more sensitive to the action of quinacrine than were unprimed cells. We were unable to determine the relative roles of secretory PLA2 (sPLA2) and cytosolic PLA2 (cPLA2) since the selective sPLA2 inhibitor scalaradial (0 to 100 microM) inhibited oxidase activity in both groups of cells by similar degrees, while the specific cPLA2 inhibitor AACO-CF3 (0 to 50 microM) failed to affect activity in either group. Inhibition of platelet-activating factor (PAF), cycloxygenase, and 5-lipoxygenase-activating protein by hexanolamino-PAF (0 to 25 microM), flurbiprofen (0 to 25 microM), and MK886 (0 to 5 microM), respectively, had no effect upon oxidase activity. However, the direct inhibition of 5-lipoxygenase by caffeic acid or lipoxin A4 resulted in a similar concentration-dependent attenuation of oxidase activity in both primed and unprimed cells. Leukotriene B4 (LTB4) release from primed neutrophils was comparable to that from unprimed cells with the exception of phorbol myristate acetate-stimulated cells, which released fivefold more LTB4 than control. Taken together, these results suggest that it is arachidonic acid per se, and not its metabolites, that is important in priming of the neutrophil NADPH oxidase by dialysis effluent.

Leukotriene B4 stimulates the release of arachidonate in human neutrophils via the action of cytosolic phospholipase A2

Leukotriene B4 (LTB4) is a potent lipid mediator of inflammation and is involved in the receptor-mediated activation of a number of leukocyte responses including degranulation, superoxide formation, and chemotaxis. In the present research, stimulation of unprimed polymorphonuclear leukocytes (neutrophils) with LTB4 results in the transient release of arachidonate as measured by mass. This release of arachidonate was maximal at an LTB4 concentration of 50-75 nM and peaked at 45 s after stimulation with LTB4. The transient nature of this release can be attributed, in part, to a fast (< 60 s) metabolism of the added LTB4. Moreover, the inhibition of the reacylation of the released arachidonate with thimerosal results in greater than 4-times as much arachidonate released. Thus, a rapid reacylation of the released arachidonate also contributes to the transient nature of its measured release. Multiple additions of LTB4, which would be expected to more closely resemble the situation in vivo where the cell may come into contact with an environment where LTB4 is in near constant supply, yielded a more sustained release of arachidonate. No release of [3H]arachidonate was observed when using [3H]arachidonate-labeled cells. This indicates that the release of arachidonate as measured by mass is most probably the result of hydrolysis of arachidonate-containing phosphatidylethanolamine within the cell since the radiolabeled arachidonate is almost exclusively incorporated into phosphatidylcholine and phosphatidylinositol pools under the non-equilibrium radiolabeling conditions used. Consistent with the role of cytosolic phospholipase A2 (cPLA2) in the release of arachidonate, potent inhibition of the LTB4-stimulated release was observed with methylarachidonylfluorophosphonate, an inhibitor of cPLA2 (IC50 of 1 microM). The bromoenol lactone of the calcium-independent phosphospholipase A2. failed to affect LTB4-stimulated release of arachidonate in these cells.

Neutrophil release of arachidonic acid, oxidants, and proteinases: causally related or independent

This investigation examined the concept that arachidonic acid (AA) serves as a second messenger in stimulation of the respiratory burst and degranulation of polymorphonuclear neutrophils (PMN). The main support for this idea is from observations that reagent AA, added to cell suspensions, stimulates the respiratory burst and degranulation and these events are blocked by PLA2 inhibitors. We verified that exogenously-added AA stimulated release of O2-, myeloperoxidase (MPO), and lysozyme (LZ), but this required amounts of AA which approximated the critical micellar concentration. This suggested that such administration of AA might act as an extracellular agonist, similar to particulate stimuli, rather than acting as a second messenger as might occur following mobilization of AA from cellular membranes. To investigate the role of fatty acids released by hydrolysis of cellular phospholipids, exogenously-added group I, II or III PLA2's were used to mobilize fatty acids from cellular membranes. Mole quantities of cell-associated free fatty acids were measured by negative ion chemical ionization gas chromatography/mass spectrometry. AA mobilization in response to exogenous PLA2 was dose- (0.1 to 10 U/ml PLA2) and time-dependent (peak at 1 to 2 min with a reduction by 4 min). Resting neutrophils contained < 10 pmol free AA/10(7) PMN; the receptor-mediated agonist N-formyl-methionyl-leucyl-phenylalanine (fMLP) alone did not increase these values. Exogenously-added PLA2 generated large quantities of free AA in control and fMLP-treated cells (462 +/- 122 and 2097 +/- 176 pmol/10(7) PMN, respectively); however, this did not induce O2-, nor did it augment the level of O2- stimulated by fMLP. Also, PLA2 caused no degranulation and did not alter degranulation induced by fMLP. PLA2 also did not alter O2- or degranulation responses in primed PMN. The data indicate that mobilization of AA from cellular phospholipids neither stimulates nor modulates the respiratory burst or degranulation of PMN.

GTP-binding proteins are involved in the modulated activity of human neutrophils treated with the Panton-Valentine leukocidin from Staphylococcus aureus

Significant amounts of leukotriene B4 (LTB4) are generated by human polymorphonuclear neutrophils (PMNs) after incubation with the Panton-Valentine leukocidin (Luk-PV) from Staphylococcus aureus V8 strains. We showed that GTP-binding proteins (G proteins) are involved in the Luk-PV-activated signal transduction of PMNs. ADP-ribosylation of heterotrimeric G proteins by cholera and pertussis toxins decreased the Luk-PV-induced LTB4-generation. In contrast, ADP-ribosylation of the low-molecular-weight G proteins rho and rac by Clostridium botulinum exoenzyme C3 increased the Luk-PV-induced LTB4 synthesis. The subsequent stimulation of Luk-PV-treated PMNs by either calcium ionophore A23187, sodium fluoride, or formylmethionyl-leucyl-phenylalanine was significantly inhibited. This decrease was paralleled by a loss of G-protein functions, including GTPase activity and GTP-binding capacity. An increase of G-protein functions was obtained with low amounts of Luk-PV. In addition to the modulated G-protein functions, ADP-ribosylation of 24-, 40-, and 45-kDa proteins by Luk-PV was detected. As shown in control experiments, the ADP-ribosylated 24-kDa proteins were not substrates for C. botulinum exoenzyme C3. Introduction of ras p21 into digitonin-permeabilized PMNs was without effect on subsequent Luk-PV stimulation. In addition, the translocation of ras p21, ras GAP, and 5-lipoxygenase into the membrane of Luk-PV-treated PMNs, as well as the expression of chemotactic membrane receptors for LTB4 and formylmethionyl leucyl phenylalanine, was significantly diminished.

Staphylococcus aureus toxic shock syndrome toxin 1 and Streptococcus pyogenes erythrogenic toxin A modulate inflammatory mediator release from human neutrophils

We studied the influence of staphylococcal toxic shock syndrome toxin 1 and streptococcal erythrogenic (pyrogenic) toxin A (ETA) on intact and digitonin-permeabilized human polymorphonuclear granulocytes (PMNs). As was shown by reversed-phase high-performance liquid chromatography analysis, toxic shock syndrome toxin 1 or ETA alone, in the absence of any additional stimulus, did not induce the generation of the chemoattractant leukotriene B4 (LTB4) from PMNs in a wide range of concentrations. In addition, pretreatment of intact PMNs with either toxin potentiated formyl-methionyl-leucyl-phenylalanine (fMLP)- and washed Staphylococcus aureus cell-induced generation of LTB4 in a time- and dose-dependent manner. This increase included LTB4 as well as its inactive omega-oxidated compounds. Further studies revealed evidence that toxin exposure was accompanied by enhanced cellular receptor expression for fMLP as well as for LTB4. The intrinsic GTPase activity of membrane fractions was modulated by both toxins. Short-term incubation with ETA increased the GTPase activity of PMNs up to 141%. Inhibitory effects were obtained when GTP-binding protein functions were stimulated with sodium fluoride (NaF). In addition, specific binding of Gpp(NH)p to GTP-binding protein was inhibited by both toxins during the first 10 min of incubation and was restored at later times of incubation. Our data therefore suggest that both toxins significantly affect the signal transduction pathways of human PMNs, which results in immunomodulatory functions.

Inhibition of human neutrophil responses by alpha-cyano-3,4-dihydroxythiocinnamamide; a protein-tyrosine kinase inhibitor

1. Activation of neutrophils results in increased tyrosine phosphorylation of several proteins that may have important roles in receptor/effector coupling. In this study, the effect of a protein tyrosine kinase inhibitor on receptor-mediated neutrophil activation by platelet-activating factor (PAF), leukotriene, B4 (LTB4) and N-formylmethionylleucylphenylalanine (FMLP) is investigated. 2. alpha-Cyano-3,4-dihydroxythiocinnamamide dose-dependently inhibited intracellular calcium release and superoxide generation from human neutrophils activated by 1 microM LTB4, PAF, and FMLP. 3. In the presence of cytochalasin B, FMLP stimulated elastase release from neutrophils was also inhibited to unstimulated levels by 5 min pretreatment with alpha-cyano-3,4-dihydroxythiocinnamamide. 4. The inhibitory action of alpha-cyano-3,4-dihydroxythiocinnamamide was found to be at or upstream of phospholipase C activation, blocking both phosphatidylinositol hydrolysis and protein kinase C activation. alpha-Cyano-3,4-dihydroxythiocinnamamide did not affect agonist receptor binding sites or receptor affinity in neutrophils. 5. Immunoblot analysis demonstrated the tyrosine phosphorylation of proteins of 41, 56, 66, and 104 kDa in neutrophils treated with agonists. Treatment of neutrophils with alpha-cyano-3,4-dihydroxythiocinnamamide prior to stimulation with chemoattractants reduced tyrosine phosphorylation of the above phosphoproteins. 6. These results indicate that alpha-cyano-3,4-dihydroxythiocinnamamide might be a useful agent in characterizing the essential proteins and biochemical pathways that regulate neutrophil activation.

Experimental colitis in animal models

Colitis may be induced in animals by oral administration of sulfated polysaccharides (carrageenan, amylopectin sulfate, dextran sulfate), chemical irritation by rectal instillation of diluted acetic acid, delayed hypersensitivity reaction after sensitization to DNCB or after one single administration of TNBS, and Arthus reaction induced by intravenous injection of immune complexes after chemical irritation of the colon, and by chemoattractant peptides such as FMLP. It appears that all models of colon inflammation in the rat, mouse, or rabbit produce increased amounts of eicosanoids similar to that found in human colitis. Thus, animal studies provide useful information on the origin, regulation, and function of inflammatory mediators. However, with the possible exception of the cotton-top tamarin, no animal model of induced or spontaneous inflammation of the colon is analogous to human ulcerative colitis in etiology, course of disease activity, or histology (114). The observation that two different immune-mediated models gave similar results suggests that the colitis is not a specific response to delayed-type hypersensitivity or immune-complex-mediated reactions but rather an unspecific, stereotype response (125). The original disturbance may not determine the nature of the lesions ultimately produced but may instead serve as an initiator of a final common immunologic pathway.

Protein kinase C impairs the coupling of the GTP-binding protein to LTB4 receptor in neutrophil

In the present study, the mechanism of LTB4 receptor down regulation by protein kinase C (PKC) has been investigated using porcine neutrophil membranes. Pretreatment of intact porcine neutrophils with 12-O-tetradecanoylphorbol-13-acetate (TPA) for 2 min prior to the preparation of plasma membrane, demonstrated a reduced binding sites (Bmax) for LTB4 without altering the receptor affinity (Kd). This effect of TPA on LTB4 receptor binding was found to be due to the activation of PKC as membrane treated with purified PKC (type III) produced the same effect. When membranes from neutrophils pretreated with TPA were exposed to non-hydrolyzable GTP analog, GTP-gamma S, or GMP-PNP, no further decrease in receptor Kd was observed, while the Bmax was reduced to the level observed in TPA treated samples. Treatment of isolated neutrophil membranes with purified PKC reduced the Bmax and blocked the effect of GTP analogs on the receptor affinity. These results suggest that, PKC interrupts the receptor binding to G-protein.

Studies on the activation of human neutrophil 5-lipoxygenase induced by natural agonists and Ca2+ ionophore A23187

By using exogenous substrates, activation of human neutrophil 5-lipoxygenase can be investigated independently of the release of endogenous arachidonic acid. We have developed a sensitive assay to measure 5-LO activation which takes advantage of the 5-LO-mediated conversion of 15S-hydroperoxy-5,8,11,13(Z,Z,Z,E)-eicosatetraenoic acid (15-HpETE) into 5S,15S-dihydroxy-6,8,11,13(E,Z,Z,E)-eicosatetraenoic acid (5,15-DiHETE). When resting neutrophils were incubated with low micromolar concentrations of 15-HpETE, a minor dose- and time-dependent formation of 5,15-DiHETE was observed. In contrast, co-addition of 15-HpETE with Ca2+ ionophore A23187 or with the neutrophil agonists platelet-activating factor (PAF), fMetLeuPhe or complement component C5a resulted in a sizeable concentration-dependent synthesis of 5,15-DiHETE, while lyso-PAF and phorbol myristate acetate were without effect on 5,15-DiHETE formation from 15-HpETE. This stimulation of 5,15-DiHETE synthesis by A23187 or by natural agonists was effectively inhibited by MK-886, a compound that has recently been reported to inhibit the A23187-induced translocation of 5-LO to membrane structures. Furthermore, natural-agonist-induced activation of the 5-LO-mediated transformation of 15-HpETE was inhibited by pertussis toxin, indicating the involvement of a GTP-binding protein in the 5-LO activation process.

Regulation of leukotriene B4 generation from human polymorphonuclear granulocytes after stimulation with formyl-methionyl-leucyl phenylalanine: effects of pertussis and cholera toxins

The effects of holotoxins and toxin subunits from Bordetella pertussis and Vibrio cholerae strains on intact and digitonin-permeabilized human polymorphonuclear neutrophils were studied. Our data clearly demonstrate that formyl-methionyl-leucyl-phenylalanine (fMLP)-induced generation of chemotactic active leukotriene B4 was inhibited by both holotoxins as well as by their isolated enzymatic A protomers. In contrast, the respective binding components (B oligomers) did not affect leukotriene formation. Priming of digitonin-permeabilized neutrophils with either guanylylimidodiphosphate or inositol trisphosphate increased subsequent stimulation with fMLP. In contrast, diacylglycerol decreased fMLP-induced leukotriene B4 formation, but inositol trisphosphate and diacylglycerol had no effect on inhibition mediated by the toxins. In addition, pertussis and cholera toxins reduced the specific binding of [3H]fMLP. Scatchard plot analysis revealed that the observed decrease of peptide binding was due to a reduced number of receptor sites. The fMLP-stimulated [3H]guanylylimidodiphosphate binding and GTPase activity used as parameters for the activation of G proteins were decreased in parallel. These results suggest altered chemotactic receptor numbers and G-protein functions responsible for the toxin-dependent suppression of fMLP-mediated response for neutrophils.

Thrombin-activated platelets promote leukotriene B4 synthesis in polymorphonuclear leucocytes stimulated by physiological agonists

1. The addition of 2 x 10(8) human platelets to 8 x 10(6) polymorphonuclear leucocytes (PMNL) incubated in presence of 2.5 u ml-1 thrombin and 0.1 microM N-formyl-Met-Leu-Phe (FMLP) (or C5a or PAF) led to enhancement of leukotriene B4 (LTB4) synthesis by the PMNL (measured by h.p.l.c. as 20-hydroxy- and 20-carboxy-LTB4) from 4 +/- 1 pmol (in absence of platelets) to 26 +/- 4 pmol (mean +/- s.e.mean, n = 9). Platelets and thrombin were both essential for the enhancement of LTB4 synthesis. 2. Platelets also caused enhancement of LTB4 synthesis from (30 +/- 12 to 134 +/- 25 pmol, n = 6) when PMNL pretreated with granulocyte-macrophage colony-stimulating factor were used in similar experiments. 3. Enhancement of LTB4 synthesis was also observed (from 5 +/- 1.5 to 26.5 +/- 5 pmol, n = 9) when the supernatants of thrombin-activated platelet suspensions were added to FMLP-stimulated PMNL. 4. Supernatants of platelet suspensions activated by thrombin in presence of cyclo-oxygenase and 12-lipoxygenase inhibitors led to greater enhancement (from 5 +/- 3 to 153.5 +/- 27.5 pmol, n = 3) of LTB4 synthesis by FMLP-stimulated PMNL, suggesting that arachidonic acid itself, rather than its metabolites was responsible for the effects of platelets. 5. Addition of arachidonic acid to FMLP-stimulated PMNL at a concentration comparable to that measured in thrombin-activated platelet supernatants (0.2 +/- 0.025 microM, n = 6) mimicked the effect of platelets or platelet supernatants on LTB4 synthesis in FMLP-activated PMNL. 6. The present data indicate that under conditions of cell activation by physiological agonists, platelets can significantly increase the formation of the proinflammatory compound LTB4 in PMNL by providing arachidonic acid. These data lend support to the concept that platelet-PMNL interactions could modulate the inflammatory process.

Biosynthesis and biological activity of leukotriene B4

The leukotrienes are a family of biologically active molecules derived from arachidonic acid. While prostaglandins and thromboxanes are products of the cyclooxygenase pathway of arachidonic acid metabolism, the leukotrienes are formed by arachidonate 5-lipoxygenase, an enzyme present in phagocytes, mast cells, and basophils. Inflammatory stimuli, such as chemotactic peptides, platelet-activating factor, phagocytic particles, and immunological stimuli, which activate phagocytes and mast cells, stimulate leukotriene synthesis. Leukotriene B4, a dihydroxy derivative of arachidonic acid, has a unique stimulatory activity on important functional responses of phagocytes; leukotriene B4 exerts chemotactic and chemokinetic activity towards phagocytes in vitro and in vivo, and it is a putative mediator of inflammation.

Studies on the mechanism of platelet-activating factor production in GM-CSF primed neutrophils: involvement of protein synthesis and phospholipase A2 activation

GM-CSF regulates the growth of hemopoietic progenitor cells, enhances the responsiveness of mature PMN and primes these cells for synthesis of leukotrienes and PAF in response to secondary stimuli. The biochemical requirements for PAF production in GM-CSF primed PMN was examined using different metabolic inhibitors. GM-CSF stimulates uridine incorporation into RNA and inhibitors for RNA and protein synthesis decrease PAF synthesis in our model. This suggests a role for gene expression and de novo synthesis of proteins in the action of GM-CSF. Different PLA2 inhibitors, including a 9 amino-acid peptide derived from a conserved region of the calpactin superfamily, decrease PAF production, indicating that in GM-CSF primed PMN the chemotactic peptide fMLP triggers lipid mediator synthesis by activating PLA2.

The monocyte-derived neutrophil activating peptide (NAP/interleukin 8) stimulates human neutrophil arachidonate-5-lipoxygenase, but not the release of cellular arachidonate

LPS and mitogen-stimulated mononuclear cells secrete a cytokine, which is able to activate the PMNL-arachidonate-5-lipoxygenase. This cytokine has been proven to be identical with the recently characterized novel neutrophil-activating peptide NAP/IL-8. NAP/IL-8 is able to activate human PMNL for release of LTB4, omega-oxidized LTB4, and 5-HETE in the presence of exogenous AA. Half-maximal concentration of NAP/IL-8 for release of LTB4 has been found to be near 4 x 10(-8) mol/liter. Time course studies revealed rapid activation of PMNL, with maximal release of LTB4 within the first 10 min with a decline up to 40 min. High amounts of omega-oxidized LTB4 were detected up to that time. Significant amounts of AA-5-LO-products can be detected only when PMNL were stimulated with NAP/IL-8 in the presence of exogenous AA. The concentration of AA necessary for half-maximal LTB4 release has been found to be 3 x 10(-6) mol/liter. In the presence of 8 x 10(-9) mol/liter [3H]AA, NAP/IL-8 (10(-9) to 10(-7) mol/liter) did not induce the production of LTB4, omega-oxidized LTB4, or 5-HETE. In addition, PMNL prelabeled with [3H]AA did not release either [3H]AA or 5-lipoxygenase metabolites when stimulated with NAP/IL-8 (10(-9) to 10(-7) mol/liter), indicating that NAP/IL-8 apparently does not activate cellular phospholipases/diacylglycerol-lipases. Apart from FMLP, C5a, and PAF NAP/IL-8 is the fourth clearly characterized neutrophil chemotaxin able to activate the PMNL-5-lipoxygenase. The detection of large amounts of NAP/IL-8, arachidonic acid, as well as LTB4-like material, in lesional material of patients with psoriasis points towards a possibly important role of NAP/IL-8 in amplifying inflammatory processes by induction of LTB4-production.

Activation of the human neutrophil 5-lipoxygenase by exogenous arachidonic acid: involvement of pertussis toxin-sensitive guanine nucleotide-binding proteins

1. The mechanism by which incubation of human peripheral blood neutrophils with exogenous arachidonic acid leads to 5-lipoxygenase product synthesis was investigated. 2. Incubation of neutrophils with arachidonic acid caused a concentration- and time-dependent synthesis of leukotriene B4, its omega-oxidation products, and 5-hydroxyeicosatetraenoic acid. 3. The threshold concentration of arachidonic acid required for this effect was equal to, or greater than 3.3 microM and the synthesis increased with up to 33 microM arachidonic acid, the highest concentration used. Synthesis induced by arachidonic acid increased with time for up to 15 min and the major products detected were the omega-oxidation products of leukotriene B4. 4. Pre-incubation of neutrophils with pertussis toxin inhibited the synthesis of 5-lipoxygenase products induced by arachidonic acid by 75% or more, but had no effect on either arachidonic acid-induced synthesis of the 15-lipoxygenase product, 15-hydroxyeicosatetraenoic acid, or activation of the 5-lipoxygenase induced by the calcium ionophore A23187. 5. Pre-incubation of neutrophils with granulocyte-macrophage colony-stimulating factor lead to enhanced leukotriene synthesis in response to arachidonic acid. 6. These results imply that exogenous arachidonic acid is not only used as a substrate, but also activates the 5-lipoxygenase. Possible mechanisms of action are discussed.

Leukotriene formation by human polymorphonuclear leukocytes from endogenous arachidonate. Physiological triggers and modulation by prostanoids

Human polymorphonuclear leukocytes (PMN) were isolated from freshly drawn venous blood by Dextran sedimentation and discontinuous Percoll gradient centrifugation. The effects of several putative triggers of the leukotriene formation such as C5a, PAF, FMLP, C3a, PMA, LTC4, LTD4, LTB4 or arachidonate were studied by RP-HPLC analysis. 280 nM C5a, 100 nM FMLP, 1 microM PAF or 20 microM arachidonate induced a marginal formation of 1.5-18 ng of LTB4 plus LTB4 metabolites/2 x 10(7) PMN. 560 nM C3a, 100 nM PMA, 1 microM LTC4, 1 microM LTD4 and 1 microM LTB4 each failed to induce any formation of 5-lipoxygenase products. Pretreatment of the cells with 40 microM ethylmercurithiosalicylate (merthiolate) enhanced the leukotriene formation by 100 nM FMLP about 40-fold, by 280 nM C3a about 120-fold and by 1 microM PAF about 14-fold. Merthiolate itself induced no leukotriene formation from human PMN and reduced the leukotriene formation by 20 microM arachidonate. The FMLP/merthiolate-induced activation of the PMN was concentration-dependent in respect to both FMLP and merthiolate. 1 microM LTC4, 1 microM LTD4 or 1 microM LTB4 also failed to trigger any LTB4 formation of merthiolate-treated PMN. 560 nM C3a or 100 nM PMA in combination with 40 microM merthiolate induced a slight formation of 28 ng and 10 ng of LTB4 plus LTB4 metabolites, respectively. The FMLP/merthiolate-induced leukotriene formation was modulated by prostanoids. PGE2, PGE1, PGD2 and 6-keto-PGE1 each evoked a concentration-dependent inhibition of the leukotriene formation with IC50 values of 0.07 microM, 0.18 microM, 0.27 microM and 6 microM respectively. In addition, significant inhibitory effects by PGI2, Iloprost (a carbacyclin analogue of prostacyclin), PGF2a or 6-keto-PGF1a were achieved; the corresponding IC50 values, however, amounted to 19-59 microM. Thus these compounds were about 500-fold less potent in comparison with PGE2 in inhibiting LTB4 formation by human PMN.

Function and stimulus-specific effects of phorbol 12-myristate 13-acetate on human polymorphonuclear neutrophils: autoregulatory role for protein kinase C in signal transduction

Exposure of polymorphonuclear neutrophils (PMNs) to phorbol 12-myristate 13-acetate (PMA) resulted in a concentration-dependent (1-10 ng/ml) inhibition of granule exocytosis induced with the receptor-specific ligands, N-formyl-methionyl-leucyl-phenylalanine (FMLP), pepstatin A, 5(S),12(R)-dihydroxy-6,14-cis-8,10-trans-eicosatetraenoic acid (LTB4), and acetyl-sn-glyceryl-3-phosphorylcholine (AGEPC). PMA exerted a marginal inhibitory effect on calcium ionophore A23187-induced PMN degranulation, and the PMA analog, 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD), was inactive. However, PMA potentiated AGEPC, pepstatin A, FMLP, LTB4, and A23187-stimulated superoxide anion (O2-) production. The mobilization of intracellular sequestered calcium (Ca2+) by the receptor-specific ligands, as reflected by a rise in the cytosolic-free Ca2+ concentration ([Ca2+]i) in PMNs loaded with the Ca2+-sensitive dye, Fura-2, was suppressed by PMA. A protein kinase C (PKC) inhibitor, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) reversed the PMA-mediated inhibition of PMN degranulation and intracellular CA2+ mobilization. However, another, but less potent PKC inhibitor, N-(2-guanidino-ethyl)-5-isoquinolinesulfonamide (HA1004), had no effect on the inhibition of PMN activation by PMA.

Characterization of the priming effects of human granulocyte-macrophage colony-stimulating factor on human neutrophil leukotriene synthesis

Human granulocyte-macrophage colony-stimulating factor (GM-CSF) is an in vitro and in vivo stimulator of human bone marrow myelomonocytic precursor cells and mature granulocyte and macrophage effector cells. We have compared the effect of GM-CSF on the synthesis of 5-lipoxygenase products induced by the chemotactic peptide fMet-Leu-Phe and the calcium ionophore A23187 in human neutrophils. Although GM-CSF alone did not stimulate detectable synthesis of products of the 5-lipoxygenase pathway, pre-incubation of neutrophils with 200 pM GM-CSF for 1 hour at 23 degrees C enhanced synthesis of leukotriene B4, its all-trans isomers and omega-oxidation products, and 5-hydroxyeicosatetraenoic acid in response to both the calcium ionophore A23187 (1.5 microM), and the chemotactic peptide fMet-Leu-Phe (0.1 microM). This priming effect of GM-CSF was maximal after a 60 min incubation at 23 degrees C, or after a 30 min preincubation at 37 degrees C. The effect of GM-CSF was maximal using a concentration of 1 nM. Enhancement of the leukotriene synthesis stimulated by A23187 was only observed when the cells were stimulated by the ionophore for periods of 3 minutes or less. In contrast, the enhancing effect of GM-CSF was still apparent when cells were exposed to fMet-Leu-Phe for as long as 15 minutes. Furthermore, the enhancing effect of GM-CSF was ablated when neutrophils were stimulated with A23187 and exogenous arachidonic acid. However, co-addition of exogenous arachidonic acid with fMet-Leu-Phe did not entirely mask the effect of GM-CSF. Possible mechanisms of action of GM-CSF are discussed.

Effect of surgery on neutrophil functions, superoxide and leukotriene production

Perioperative changes of neutrophil function were investigated in 28 patients who underwent major surgery, particularly focusing on the potential capacity for superoxide and leukotriene production, which seem to be important in host defence. The superoxide-producing capacity of neutrophils, which was examined using N-formyl-peptide or phorbol myristate acetate as an attractant, significantly decreased postoperatively to 55 and 69 per cent of the pre-operative values, respectively. The leukotriene-producing capacity of neutrophils, which was stimulated with calcium ionophore A23187 in the presence of arachidonic acid, significantly changed postoperatively. The leukotriene B4 (LTB4) production increased together with an increment of production of 6-trans 6-trans LTB4, however, increased postoperatively to 1.2 times the (LTC4) production decreased postoperatively. The total production of leukotriene A4 (LTA4) metabolites consisting of LTB4, LTC4, and 6-trans LTB4, however, increased postoperatively to 1.2 times the pre-operative values. This indicates that neutrophils in the postoperative period have a higher capacity for LTA4 production but a lower capacity for superoxide production than those in the pre-operative period.

Inhibition of phospholipase A2 activity of guinea-pig alveolar macrophages by lipocortin-like proteins purified from mice lung

Guinea-pig alveolar macrophages were harvested by bronchoalveolar lavage and purified by differential adhesion. They were labeled with 14C-Arachidonic acid and then exposed to platelet-activating factor or to the calcium ionophore A23187. The activity of cellular phospholipase A2 was considered as the release of free 14C-Arachidonic acid in the cell supernatant. The pretreatment of guinea-pig alveolar macrophages with two lipocortin-like proteins (36 kDa and 40 kDa) purified from mice lung induced a significant inhibition of their phospholipase A2 activity upon platelet-activating factor and calcium ionophore stimulation. These results indicate that lipocortin-like proteins can modulate the phospholipase A2 activity of isolated cells in vitro.

Chemotactic peptides. Mechanisms, functions, and possible role in inflammatory bowel disease

An important component of host defenses is the ability of inflammatory cells to detect and respond to minute concentrations of chemoattractant substances. Chemotactic peptides elaborated by both bacteria and leukocytes are the focus of this review. These peptides induce directed migration of inflammatory cells towards their targets, and stimulate biological functions including degranulation, release of oxygen radicals, phagocytosis, and eicosanoid production. Among the released eicosanoids, leukotriene B4 potentiates the leukocyte response. As with other chemotactic factors, these functions are regulated partially through differential coupling to high- and low-affinity receptors and via calcium as the second messenger. Some chemotactic peptides are elaborated by normal colonic luminal bacteria. Recent evidence demonstrates that these peptides can produce mucosal inflammation in vivo. A possible mechanism for this effect involves abnormal colonic permeability in susceptible individuals that allows bacterial chemotactic peptides access to the mucosa where they may induce inflammation. Remaining questions include the mechanism by which the mucosal barrier is breached and the role of leukotrienes in the potentiation of colonic inflammation.

Rat neutrophil function, and leukotriene generation in essential fatty acid deficiency

Since the essential fatty acid linoleic acid is the precursor of arachidonic acid and thus of leukotrine B4 (LTB4), essential fatty acid deficiency (EFAD) may result in decreased synthesis of this stimulator of neutrophil granulocyte functions. Peritoneal and blood neutrophils from rats fed a diet with only 0.3% of energy requirements as linoleic acid and exhibiting biochemical evidence of EFAD showed substantial functional impairments compared to neutrophils from rats maintained on a diet with 3% of the energy requirement as linoleic acid. Oxidative burst activation (assessed by chemiluminescence), chemotaxis and aggregation were impaired upon stimulation with formylpeptides or the ionophore A23187. In contrast, these functions were intact on stimulation with exogenous LTB4. Chemiluminescence was slightly but not significantly enhanced in EFAD rat neutrophils compared to controls when stimulated with phorbol myristate acetate (PMA). There were no differences between EFAD and control peritoneal neutrophils in the number of f-met-leu-phe (fMLP) receptors, or in their affinity for the ligand, assessed with fML(3H)P. The fraction of responding cells also were similar, assessed with dichlorofluorescein diacetate fluorescence. Moreover, the endogeneous LTB4 production in response to A23187 or fMLP was decreased by 57.7% and 63.5%, respectively, in EFAD peritoneal neutrophils. Thus, EFAD was associated with reductions of LTB4 production and neutrophil responsiveness to A23187 and formylpeptides but not to LTB4 or PMA, which supports the hypothesis that endogeneous LTB4 may contribute to the activation of neutrophil functions involved in inflammation and host defense.

Studies on the effects of disease-modifying antirheumatic drugs on lipoxygenase product synthesis in human neutrophils in vitro

The disease-modifying antirheumatic drugs sodium aurothiomalate, D-penicillamine and chloroquine phosphate were tested on leukotriene (LT) synthesis in human blood polymorphonuclear leukocytes stimulated with either the ionophore A23187, zymosan or f-Met-Leu-Phe. Lipoxygenase products were analyzed by reversed-phase high-performance liquid chromatography. The study demonstrated that the drugs can either inhibit or enhance 5-lipoxygenase product synthesis in human leukocytes, depending on the stimulus used to activate the cells and the drug concentration. The data also suggested that increased substrate availability accounted for the stimulatory effects of the drugs on leukotriene synthesis.

Arachidonic acid and 15(S)-hydroxy-5,8,11-cis-13-trans-eicosatetraenoic acid modulate human polymorphonuclear neutrophil activation by monocyte derived neutrophil activating factor

Exposure of human polymorphonuclear neutrophils (PMN) to human monocyte derived neutrophil activating factor(s) (NAF) resulted in a concentration-dependent extracellular release of granule constituents. NAF also induced the generation of 5(S),12(R)-dihydroxy-6,14-cis-8,10-trans-eicosatetraenoic acid [Leukotriene B4 (LTB4)] by PMNs which was enhanced in the presence of exogenous arachidonic acid (AA). In contrast to its enhancing effect on LTB4 production, AA inhibited NAF-stimulated PMN degranulation. 15(S)-hydroxy-5,8,11-cis-13-trans-eicosatetraenoic acid (15-HETE), a product of the 15-lipoxy-genation of AA in PMNS, caused a concentration-dependent suppression of degranulation and LTB4 generation by PMNs in contact with NAF. 15-HETE also inhibited the rise in cytosolic-free calcium [( Ca2+]i) observed in NAF activated PMNs. These data suggest that AA and a 15-lipoxygenase product modulate the NAF-associated activation pathway in human PMNs.

Effect of endogenously produced leukotrienes and thromboxane on renal vascular resistance in rabbit hydronephrosis

Ureteral obstruction in rabbits is characterized by mononuclear cell invasion of the renal cortex and proliferative fibrosis that is associated with exaggerated prostaglandin synthesis in response to vasoactive and inflammatory cell agonists. In this investigation, we studied the effects of the chemotactic peptide N-formylmethionyl-leucyl-phenylalanine (fMLP) and bradykinin (BK) on eicosanoid synthesis and renal vascular resistance in the ex vivo perfused hydronephrotic kidney (HNK). Administration of fMLP resulted in the dose-dependent synthesis of leukotrienes, thromboxane A2 (TXA2), prostaglandin E2 (PGE2), and prostacyclin (PGI2). Peptidoleukotriene synthesis was monitored by specific radioimmunoassay and by guinea pig ileum bioassay and it was then validated by inhibition of the ileal contractile activity with the peptidoleukotriene receptor antagonist FPL-55712. The leukotrienes produced were identified as LTB4, LTC4, LTD4, and LTE4 by comigration with authentic standards on reverse phase high-performance liquid chromatography (RP-HPLC) and by ultraviolet spectroscopy. BK administration stimulated the synthesis of TXA2, PGE2, and PGI2 but not the synthesis of leukotrienes, in contrast to the results with fMLP, suggesting the involvement of different cell types. Administration of fMLP to the HNK also resulted in a renal vasoconstriction that was partially inhibited by FPL-55712 and that was completely inhibited by the thromboxane synthase inhibitor OKY-1581. Consistent with this result, exogenous administration of LTC4 resulted in the synthesis of TXA2 and in a renal vasoconstriction that was inhibited by either FPL-55712 or OKY-1581.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of leukotriene generation in polymorphonuclear leukocytes by staphylococcal alpha-toxin

The effects of staphylococcal alpha-toxin on arachidonic acid metabolism in rabbit polymorphonuclear leukocytes (PMNs) were investigated and compared with those of the ionophore A23187 and the chemotactic tripeptide formylmethionyl-leucyl-phenylalanine (fMLP). Sublytic amounts of alpha-toxin stimulated the release of leukotriene B4 (LTB4) in PMNs in a dose-dependent manner. The toxin was several times more potent than fMLP but was not as effective as the ionophore. Preincubation of the toxin with neutralizing antibodies abolished the effect. Extracellular calcium was strictly required for eliciting LTB4 generation. Verapamil, a calcium channel blocker, inhibited fMLP-mediated LTB4 generation but had no effect on alpha-toxin- or A23187-exposed PMNs. Agents such as trifluoperazine and N-6(aminohexyl)-5-chloro-1-naphthalene sulfonamid that interfered with calmodulin activity, however, inhibited LTB4 generation in all cases. One minute after the addition of alpha-toxin, PMNs exhibited a severalfold enhancement in passive permeability to 45Ca2+. In addition, these cells became permeable to sucrose but not to inulin or dextran. The influx pattern was consistent with the previous observation that alpha-toxin creates discrete transmembrane channels in erythrocytes with an effective internal diameter of 2 to 3 nm. The results suggest that alpha-toxin triggers the arachidonic acid pathway in PMNs by facilitating calcium influx into the cells, possibly via transmembrane toxin pores that serve as calcium gates. Generation of arachidonic acid metabolites in PMNs by sublytic amounts of alpha-toxin may represent an important cellular reaction that generally occurs during infections with Staphylococcus aureus.

Anthralin (1,8-dihydroxyanthrone) is a potent inhibitor of leukotriene production and LTB4-omega oxidation by human neutrophils

The effect of anthralin and its oxidation products danthrone and anthralin-dimer on the production of 5-lipoxygenase products (5-HETE, leukotriene B4, omega-oxidized LTB4) by Ca-ionophore A 23187-stimulated human neutrophils has been studied in vitro. Anthralin exhibited dose-dependent inhibitory activity showing 50% inhibition at 7 microM with 10(7) neutrophils. Inhibitory effects strongly depended upon cell densities and maximal inhibition occurred at low cell concentrations, whereas inhibitory rates of anthralin were low at high cell densities. Inhibition of leukotriene production persisted after washing of anthralin-treated neutrophils. Also, with increasing amounts of arachidonic acid as substrate only slight changes of inhibitory activity were detected, indicating a noncompetitive way of action. In addition to the inhibition of leukotriene-production, the formation of omega-OH-LTB4 from LTB4 as well as omega-COOH-LTB4 from omega-OH-LTB4 was inhibited with IC50 (half maximum inhibition concentration) near 4.4 microM and 2.2 microM, respectively. In contrast to anthralin, both metabolites--danthrone as well as anthralin-dimer--did not show any effect on leukotriene production and omega-oxidation even at high concentrations (up to 70 microM and 44 microM, respectively).

Requirement of free arachidonic acid for leukotriene B4 biosynthesis by 12-hydroperoxyeicosatetraenoic acid-stimulated neutrophils

Stimulation of human neutrophils with 12-hydroperoxyeicosatetraenoic acid (12-HPETE) led to formation of 5S, 12S-dihydroxyeicosatetraenoic acid (DiHETE), but leukotriene B4 (LTB4) or 5-hydroxyeicosatetraenoic acid (5-HETE) was not detectable by reversed-phase high-performance liquid chromatography analysis. N-formylmethionylleucylphenylalanine (FMLP) induced the additional synthesis of small amounts of LTB4 in 12-HPETE-stimulated neutrophils. The addition of arachidonic acid greatly increased the synthesis of LTB4 and 5-HETE by neutrophils incubated with 12-HPETE. In experiments using [1-14C]arachidonate-labeled neutrophils, little radioactivity was released by 12-HPETE alone or by 12-HPETE plus FMLP, while several radiolabeled compounds, including LTB4 and 5-HETE, were released by A23187. These findings demonstrate that LTB4 biosynthesis by 12-HPETE-stimulated neutrophils requires free arachidonic acid which may be endogenous or exogenous.