Stimulation by lipoxygenase products of superoxide anion production in FMLP-treated neutrophils

Esterified eicosanoids are acutely generated by 5-lipoxygenase in primary human neutrophils and in human and murine infection

5-Lipoxygenase (5-LOX) plays key roles in infection and allergic responses. Herein, four 5-LOX-derived lipids comprising 5-hydroxyeicosatetraenoic acid (HETE) attached to phospholipids (PLs), either phosphatidylethanolamine (PE) or phosphatidylcholine (18:0p/5-HETE-PE, 18:1p/5-HETE-PE, 16:0p/5-HETE-PE, and 16:0a/5-HETE-PC), were identified in primary human neutrophils. They formed within 2 minutes in response to serum-opsonized Staphylococcus epidermidis or f-methionine-leucine-phenylalanine, with priming by lipopolysaccharide, granulocyte macrophage colony-stimulating factor, or cytochalasin D. Levels generated were similar to free 5-HETE (0.37 ± 0.14 ng vs 0.55 ± 0.18 ng/10(6) cells, esterified vs free 5-HETE, respectively). They remained cell associated, localizing to nuclear and extranuclear membrane, and were formed by fast esterification of newly synthesized free 5-HETE. Generation also required Ca(2+), phospholipase C, cytosolic and secretory phospholipase A(2), 5-LOX activating protein, and mitogen-activated protein kinase/extracellular signal-regulated kinase kinase 1. 5-HETE-PLs were detected in murine S epidermidis peritonitis, paralleling neutrophil influx, and in effluent from Gram-positive human bacterial peritonitis. Formation of neutrophil extracellular traps was significantly enhanced by 5-LOX inhibition but attenuated by HETE-PE, whereas 5-HETE-PE enhanced superoxide and interleukin-8 generation. Thus, new molecular species of oxidized PL formed by human neutrophils during bacterial infection are identified and characterized.

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.

5-Hydroxyeicosanoids selectively stimulate the human neutrophil 15-lipoxygenase to use endogenous substrate

When human neutrophils, prelabeled with [3H]arachidonic acid, were incubated with 5S,15S-dihydroxyeicosatetraenoic acid (5,15-diHETE), a dose-dependent increase in the 15-lipoxygenase product [3H]-15-HETE was observed relative to untreated cells. Typically, a fivefold increase in [3H]-15-HETE formation was obtained upon exposure of these cells to 3 muM 5,15-diHETE. There was no appreciable enhancement of the 5-lipoxygenase metabolite [3H]-5-HETE. Product identities were confirmed by comparing retention times on straight- and reversed-phase HPLC with authentic standards, and RIA. Other 5-hydroxyeicosanoids, such as 5-HETE, 5-HETE methyl ester, and leukotriene B4(5S,12R-diHETE), were equally effective in stimulating the formation of [3H]-15-HETE, but exogenously added lipoxin A4, lipoxin B4, 15-HETE, and 12-HETE were much less potent, whereas stearic acid was ineffective. The diHETEs also showed a greater selectivity in activating the 15-lipoxygenase relative to the 5-lipoxygenase. A likely source of substrate for the 15- and 5-lipoxygenases is a pool of cell-associated but noncovalently bound arachidonic acid. In [3H]arachidonic acid-prelabeled neutrophils, the amount of free [3H]arachidonic acid ranged between 50 and 700 fmol/10(7) cells, whereas unlabeled neutrophils contained 100-2,200 pmol/10(7) cells of nonesterified arachidonic acid. The exogenously added hydroxyeicosanoids induce a 0.5-3% conversion of this substrate pool to product. These findings indicate that the 15-lipoxygenase in human neutrophils is a cryptic enzyme that needs to be stimulated in order to metabolize endogenous substrate. It is possible that 5-hydroxyeicosanoids may mimic an as yet unidentified physiological activator of the 15-lipoxygenase.

Three new and bioactive icosanoids from the temperate red marine alga Farlowia mollis

Three new dihydroxyicosanoids, 12(R),13(R)-dihydroxyicosa-5(Z),8(Z),10(E),14(Z)-tetraenoic acid, 12(R),13(R)-dihydroxyicosa-5(Z),8(Z),10(E),14(Z),17(Z)-pentaeno ic acid and 10(R*),11(R*)-dihydroxyoctadeca-6(Z),8(E),12(Z)-trienoic acid, have been isolated from a previously unstudied temperate red marine alga, Farlowia mollis (Cryptonemiales, Rhodophyta). The structures of these new metabolites have been deduced from detailed nuclear magnetic resonance and mass spectrometry analyses on stabilized diacetate-methyl esters and stereochemistry deduced by 1H NMR couplings and CD analysis of a dibenzoate derivative. Collectively, these new natural products modulate fMLP-induced superoxide anion generation in human neutrophils, inhibit the conversion of arachidonic acid to lipoxygenase products by human neutrophils, and inhibit the functioning of the dog kidney Na+/K+ ATPase.

Effect of the 5-lipoxygenase inhibitor piriprost on superoxide production by human neutrophils

The effect of 6,9-deepoxy-6,9-(phenylimino)-delta 6,8-prostaglandin I1 (Piriprost) on the oxidative response was studied in human neutrophils stimulated by N-formyl-methionyl-leucyl-phenylalanine (fMLP), phorbol 12-myristate, 13-acetate (PMA) or opsonized zymosan. Piriprost inhibited the stimulatory effect of fMLP on superoxide anion (O2-) generation, at concentrations higher than those which depress leukotriene B4 (LTB4) formation. This inhibition was overcome by increasing the concentration of fMLP. Neither exogenous LTB4 nor indomethacin were able to reverse the inhibitory effect of piriprost on fMLP action. In contrast, piriprost did not inhibit the stimulation of O2- production induced by PMA or zymosan. Piriprost behaves thus as a specific and apparently competitive antagonist of fMLP: this action does not seem to involve lipoxygenase inhibition and might be exerted at the level of the fMLP receptor or its associated mechanisms of transduction.

Role of the 15-lipoxygenase in the immune system

Recent data from our laboratory, as well as supporting evidence from other investigators, strongly suggest that the PMN 15-LO exists in a cryptic state. Several stimuli, including HETEs, can convert the inactive 15-LO to an active species that can metabolize AA to a variety of products. Many of these metabolites have been reported to modulate various components of the immune response.

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.

In vitro and in vivo effects of treatment by platelet-activating factor on N-formyl-met-leu-phe-mediated responses of polymorphonuclear leucocytes

Two chemoattractants, the peptide N-formyl-met-leu-phe (FMLP), and the ether phospholipid, platelet activating factor (PAF), each stimulate a variety of in vitro responses in polymorphonuclear leucocytes (PMN). Because often more than one inflammatory mediator is active during inflammation, we determined the effect on PMN of sequential stimulation with these two agents. Before FMLP stimulation, human PMN were exposed to PAF, at concentrations which gave little or no response when administered alone. PAF enhanced FMLP-elicited superoxide release in a dose-dependent fashion. Likewise, release of granular lysozyme from the cells was increased in PAF treated cells. Similar treatment with other phospholipids, including the lyso derivation of PAF, failed to produced these effects. Incubation with nordihydroguaiaretic acid, an inhibitor of arachidonic acid metabolism, had little effect on the enhancement of lysozyme release by PAF. To determine if enhancing effects by PAF might occur also in vivo, we studied rabbits receiving PAF and/or FMLP intravenously. When rabbits received 0.01 micrograms PAF (a dose which does not elicit the sustained neutropenia observed with higher doses of PAF) followed by 0.05 micrograms FMLP the absolute granulocyte count (AGC) dropped at 1 min (46 +/- 11% of original value), and continued to fall (24 +/- 12% at 10 min). Controls, treated with the suspending fluid for PAF, and then 0.05 micrograms FMLP, had a similar 1 min AGC value, but at 10 min AGC returned to 65 +/- 6.1% (P less than 0.001 for comparison of 10 min values). Thus PAF pretreatment enhanced FMLP-elicited granulocytopenia in vivo. Study of in vitro human PMN aggregation revealed that, at certain relative concentrations of PAF and FMLP, aggregation was enhanced. These studies show that both in vitro and in vivo responses of FMLP-stimulated PMN may be exaggerated by pre-exposure to PAF.

Human monocyte and polymorphonuclear leukocyte chemotactic and chemokinetic responses to leukotriene B4 and FMLP

Comparable investigations of the chemotactic and chemokinetic responses of purified monocytes (MO) and polymorphonuclear leukocytes (PMN) to leukotriene B4 (LTB4) and N-formyl-methionyl-leucyl-phenylalanine (FMLP) were made in this study. Using a sensitive and objective 51Cr-chemotactic assay, it was shown that both MO and PMN showed a bell-shaped response to LTB4 and FMLP, with a maximum response at 10(-8)M for both drugs. For PMN, the maximal response elicited by LTB4 was similar in magnitude to that produced by FMLP, whereas the MO chemotaxis induced by 10(-8)M FMLP was significantly higher than the response evoked by 10(-8)M LTB4. For both cell types, LTB4 at low concentrations (less than 10(-9)M) gave rise to higher chemotactic responses than FMLP. Chemokinesis was differentiated from chemotaxis, using a checkerboard system. At concentrations less than 10(-9)M the LTB4-evoked contribution of chemokinesis to the total migrational response was significantly higher than the chemokinetic contribution of FMLP. Preincubation with LTB4 produced only homologous chemotactic deactivation to subsequent LTB4 stimulation, whereas preincubation with FMLP resulted in diminished secondary response to both FMLP and LTB4. The degree of deactivation was dependent upon the dose of attractant used, with a LTB4 concentration of 10(-7)M leading to about 40% and 25% deactivation of PMN and MO, respectively. Preincubation with 10(-7)M FMLP led to about 50% and 32% suppression of the subsequent chemotactic response of PMN and MO, respectively.