L-663,536 (MK-886) (3-[1-(4-chlorobenzyl)-3-t-butyl-thio-5-isopropylindol-2-yl]-2,2 - dimethylpropanoic acid), a novel, orally active leukotriene biosynthesis inhibitor

Modulation of cellular phospholipid fatty acids and leukotriene B4 synthesis in the human intestinal cell (CaCo-2)

The ability of a human colonocyte epithelial cell line (CaCo-2) to synthesise leukotriene B4 was examined. In addition, the effects of stimulation with calcium ionophore, inhibition by a drug which specifically prevents activation of 5-lipoxygenase, and modification of the fatty acid composition of membrane phospholipids on leukotriene B4 synthesis were assessed. Incubation with calcium ionophore (A23187) resulted in a dose and time dependent increase in leukotriene B4 synthesis. After cell phospholipids had been enriched with oleic acid, linoleic acid, and arachidonic acid, leukotriene B4 synthesis was found to be increased 3.2-fold, 5.5-fold, and 6.1-fold above control. Treatment with MK-886 inhibited leukotriene B4 synthesis by 79% to 94% in the various groups. Variations in the polyunsaturated fatty acid content of intestinal epithelial cells could be important in the modulation of cellular responses. We have shown for the first time in this human intestinal epithelial cell its ability to synthesise leukotriene B4. In addition, leukotriene B4 synthesis can be modulated by the fatty acid composition of membrane phospholipids, which can be altered by dietary fatty acids. The synthesis of chemotatic factors, such as leukotriene B4, by the mucosal epithelium may contribute to the recruitment of granulocytes into the colonic mucosa and across the epithelium, giving rise to the crypt abscesses which characterise ulcerative colitis.

Generation of 5-lipoxygenase metabolites following pulmonary reperfusion in isolated rabbit lungs

We characterized the release of arachidonic acid (AA) metabolites in lung effluent following lung ischemia-reperfusion since they may contribute to the pathophysiology of reperfusion lung injury. The left pulmonary artery of rabbits (N = 5) was occluded for 24 hrs with a surgically implanted vascular clip. At 24 hrs, the heart and lungs were removed en bloc and perfused with Ringers-albumin (0.5 gm%) at 60 ml/min while statically inflated with 95% O2-5% CO2. The lipid fraction of the lung effluent was concentrated using the Bligh-Dyer extraction and analyzed by gradient RP-HPLC. Samples obtained in the first minute of reperfusion showed significant increases in LTB4 (+180%), LTC4 (+3600%), 15-HETE (+370%), 5-HPETE (+270%), PGE2 (+140%), 6-keto-PGF1 alpha (+110%) and 12-HHT (+160%) compared to the effluent from the right control lung. The reperfusion-induced increases in LTB4, LTC4, LTD4 and 15-HETE were inhibited greater than or equal to 70% by pretreatment with the 5-LO inhibitors L663,536 or L651,392. The increases in lipid concentrations corresponded to significantly increased pulmonary arterial pressure from a baseline value of 9.5 +/- 0.3 to 29.3 +/- 2.9 (cmH2O) during the first min of reperfusion. The pulmonary arterial pressure remained elevated for at least 20 min of reperfusion. Reperfusion also resulted in PMN uptake (assessed by lung tissue myeloperoxidase content) in the reperfused lung versus control lung (25.0 +/- 2.4 vs. 10.5 +/- 2.5 units). The generation of lipoxygenase metabolites during the initial phase of reperfusion may contribute to post-reperfusion PMN uptake and pulmonary vasoconstriction.

Immune-complex alveolitis in the rat: evidence for platelet activating factor and leukotrienes as mediators of the vascular lesions

In the present study we investigated the involvement of lipid mediators in an experimental model of immune-complex alveolitis induced in rat lungs by intrabronchial instillation of rabbit antibodies to ovalbumin followed by i.v. injection of the antigen. It was found that the reaction did not induce detectable oedema, as measured by the dry:wet weight ratio. A marked influx of neutrophils was observed in the bronchoalveolar lavage fluid, progressing from 6 to 24 h in parallel with the development of haemorrhagic lesions in lung parenchyma. The intensity of these lesions, evaluated by the concentration of extravascular haemoglobin, was not significantly affected by pretreatment of the animals with a cyclo-oxygenase inhibitor (indomethacin), a thromboxane inhibitor (econazole) or a thromboxane antagonist (L-655,240). However, the antagonists of platelet activating factor (PAF), WEB-2086 and BN-52021, and the lipoxygenase inhibitors, nor-dihydroguaiaretic acid and L-663,536, all significantly inhibited the haemorrhagic lesions. A peptide leukotriene antagonist (L-660,711) had no effect. Furthermore, the PAF antagonists inhibited the levels of LTB4, but not of PGE2 and thromboxane, released into the bronchoalveolar space 1 h after induction of the reaction. These results suggest that the haemorrhagic lesions in this model of immune-complex alveolitis are mediated by PAF and leukotrienes, possibly LTB4.

Role of leukotriene B4 in the pathogenesis of hepatic ischemia-reperfusion injury in the rat

A common feature to most models of ischemia-reperfusion injury is the accumulation of polymorphonuclear leukocytes (PMNs) into the post-ischemic tissue during the reperfusion period. Interventions that lead to decreased PMN infiltration protect against tissue injury and therefore a knowledge of the chemotactic mediators leading to PMN accumulation is essential to understanding the pathogenesis of the injury and to the development of successful therapeutic strategies. Leukotriene B4 (LTB4), a metabolite formed via the 5-lipoxygenase pathway from arachidonic acid, is one of the most potent chemotactic mediators known. We have investigated the formation of LTB4 in a well characterized model of hepatic ischemia-reperfusion injury in the rat and made use of a specific leukotriene biosynthesis inhibitor, L663,536, to determine the importance of LTB4 in the pathogenesis of the injury. LTB4 concentrations were measured with a specific and sensitive gas chromatographic-mass spectrometric method previously developed in our laboratory. In liver tissue LTB4 levels were below the detection limit of 20 pg/g before 45 min ischemia and did not increase during the first 6 h of reperfusion. However, at 15 h and 24 h reperfusion LTB4 concentrations had increased to levels 50-fold those in control liver (867 +/- 267 pg/g). The increase of plasma alanine aminotransferase (ALT) activities indicated two phases of injury, an initial phase during the first few hours of reperfusion, and a second more severe injury phase between 6 h and 24 h reperfusion. PMNs accumulated in tissue throughout the reflow period reaching 700 +/- 49 per 50 high power fields (HPF) at 24 h.(ABSTRACT TRUNCATED AT 250 WORDS)

5-lipoxygenase inhibitors and their anti-inflammatory activities

A wide variety of agents have been reported as 5-LO inhibitors. The majority of the series appear to be lipophilic reducing agents, including phenols, partially saturated aromatics, and compounds containing heteroatom-heteroatom bonds. Many of these are not selective 5-LO inhibitors, but often affect CO and other LOs as well. In vivo systemic activity for many of these has been, in general, disappointing, probably because of poor bioavailability caused by lipophilicity and metabolic instability (oxidation, and conjugation of phenolic compounds). However, topically a number of agents have shown promise for skin inflammation, with Syntex's lonapalene the most advanced of these. Most results published to date appear more disappointing in the allergy/asthma field. More excitingly, a few structural types are selective 5-LO inhibitors which have shown systemic activity in vivo and in the clinic. Abbott's zileuton (136) appears to be one of the leading compounds in this category, along with other hydroxamates such as BW-A4C (129) from Burroughs-Wellcome. Recent selective non-reducing agents such as Wyeth-Ayerst's Wy-50,295 (143) and the similar ICI compounds such as ICI 216800 (145) also hold promise. The enantiospecific effects of (106) and (145) are especially interesting for the design of new inhibitors. If compounds like these validate the hypothesis that inhibition of 5-LO will have a significant anti-inflammatory effect, a redoubling of effort throughout the industry to find second- and third-generation selective agents may be expected. Part of the difficulty in interpreting and comparing the 5-LO literature is the plethora of test methods and activity criteria. As pointed out in the introduction, inhibition of product release from cells, often stimulated with A23187, has commonly been used to demonstrate 5-LO inhibition. However, this type of assay cannot be assumed to be diagnostic for 5-LO inhibition. Only if specificity for 5-LO product generation and (ideally) activity in cell-free enzymes is also shown should mechanistic interpretations be made. Recently, a new class of compounds was found at Merck which inhibited LT biosynthesis without inhibiting 5-LO, but apparently by a novel, specific mechanism. L-655,240 (169) and L-663,536 (MK-886) (170) were both active in human ISN, with IC50 values in the low micromolar range. Both also orally inhibited GPB (< 1 mg/kg). MK-886 was effective in Ascaris-induced asthma in squirrel monkeys, in rat carrageenan pleurisy, in rat Arthus pleurisy, and (topically) in guinea-pig ear oedema induced by A23187.(ABSTRACT TRUNCATED AT 400 WORDS)

Leukotriene D4 participates in colonic transit disturbances induced by intracolonic administration of trinitrobenzene sulfonic acid in rats

The effects of colonic inflammation induced by trinitrobenzene sulfonic acid and influence of previous treatment with specific antagonists of inflammatory mediators (platelet-activating factor, leukotrienes, prostaglandins, and thromboxanes) on colonic transit were examined in conscious rats which were permanently fitted with an intracolonic catheter inserted into the proximal colon. Colonic inflammation was induced by intracolonic administration of trinitrobenzene acid (80 mg/kg) in 50% ethanol. Colonic transit time was evaluated by intracolonic administration of a radiolabeled marker [( 51Cr]sodium chromate) and collection of the feces per hour on a conveyor belt. Excretion of the marker was then plotted vs. time, permitting calculations of the times elapsed to recover 25%, 50%, and 75% of the marker injected (T25, T50, and T75, respectively). In control (saline) animals, excretion of the marker described a regular sigmoid curve with 50% of the marker recovered at 6.92 +/- 0.40 hours after intracolonic administration (T25 = 6.4 +/- 0.43 hours; T75 = 7.49 +/- 0.39 hours). Ethanol (vehicle), 50%, did not modify the profile of marker recovery. On the contrary, single intracolonic administration of trinitrobenzene sulfonic acid/ethanol induced a biphasic response consisting of an early pool of radiolabeled feces (T25 = 4.03 +/- 0.55 hours) with a delayed total one (T50 = 11.74 +/- 0.83 hours; T75 = 13.70 +/- 0.49 hours). Antagonists of the leukotriene pathway, i.e., MK = 886, a lipoxygenase inhibitor, and SKF 104,353 and SR 2640, two different leukotriene D4 receptor antagonists, blocked the effects of trinitrobenzene sulfonic acid on colonic transit time and restored a control profile of radiolabeled marker excretion. In contrast, indomethacin, a cyclooxygenase inhibitor, and SC 19220, a specific prostaglandin E2 receptor antagonist, were inefficient in blocking the effects of trinitrobenzene sulfonic acid on colonic transit time. Specific thromboxane A2 receptor antagonists, KT1-32 and GR 32191B, did not show any improvement in colonic transit after trinitrobenzene sulfonic acid administration. Previous injection of the specific platelet-activating factor receptor antagonists, BN 52021 or BN 50730, was also unable to restore a normal marker excretion profile after administration of trinitrobenzene sulfonic acid. It is concluded that the alterations of colonic transit immediately observed after intracolonic trinitrobenzene sulfonic acid administration are mediated through the release of leukotriene D4. In contrast, platelet-activating factor, prostaglandins, and thromboxanes are not involved in the mediation of these transit disturbances.

Role of the 5-lipooxygenase pathway in obstructive nephropathy

Leukotrienes are products of the 5-lipooxygenase pathway of arachidonic acid metabolism that possess potent inflammatory properties. We examined the potential role of this pathway in the decrease in glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) observed in rats after unilateral release of bilateral ureteral obstruction (BUO) of 24 hours duration. Isolated glomeruli from rats with BUO produced significantly greater amounts of leukotriene B4 (LTB4) than glomeruli from sham-operated rats (SOR; P less than 0.0001). Glomeruli from rats with BUO given MK886, an inhibitor of the 5-lipooxygenase enzyme, or from rats with BUO subjected to both total body irradiation to prevent the leukocyte infiltration of the kidney and also given MK886 prior to obstruction, produced amounts of LTB4 not significantly different from those in glomeruli of SOR. Glomeruli from rats with BUO that had only total body irradiation prior to obstruction produced significantly less LTB4 than glomeruli from untreated BUO rats, but LTB4 production was still significantly greater than in glomeruli from SOR. There were no significant differences in GFR among SOR, SOR given MK886, and SOR subjected to total body irradiation. However, SOR given MK886 had significantly higher ERPF and lower renal vascular resistance (RVR) than SOR not pretreated with the lipooxygenase inhibitor. Rats with BUO given MK886, or subjected to total body irradiation, or both, prior to obstruction had significantly greater GFR and ERPF values and lower RVR than untreated BUO rats. Glomeruli from rats with BUO which were not pretreated had three times the leukocytes of glomeruli from SOR. This leukocyte infiltrate was composed of macrophages (about 55%) and neutrophils (about 45%).(ABSTRACT TRUNCATED AT 250 WORDS)

The role of platelet-activating factor and peptidoleukotrienes in the vascular changes of rat passive anaphylaxis

1. The role of platelet-activating factor (PAF) and peptidoleukotrienes as putative mediators of some of the vascular changes triggered by antigen was investigated in rats passively sensitized with monoclonal anti-DNP (2,4-dinitrophenyl) IgE. 2. Lethal anaphylaxis with respiratory distress, systemic hypotension, detachment of the intestinal mucosa, leukopenia and extravasation of protein-rich plasma was observed after antigen challenge of rats sensitized with partially purified monoclonal IgE at concentrations of 15 mg protein kg-1. 3. Analysis of the peritoneal fluid obtained after i.v. challenge with DNP-BSA (bovine serum albumin) showed the presence of significant amounts of PAF (101 +/- 8 pg/rat), whereas this mediator was undetectable in control animals. Leukotriene D4 was the predominant peptidoleukotriene that could be recovered after antigen challenge, and showed an extremely high concentration (92 +2- 15 ng/rat) as compared to PAF levels. 4. Extravasation of protein-rich plasma was observed shortly after challenge and reached a maximum at 30 min. Treatment of animals with i.v. PCA 4248 (1-2 mg kg-1) and WEB 2086 (1 mg kg-1), two chemically unrelated compounds which are antagonists of the PAF-receptor, produced a significant reduction of the extravasation of protein-rich plasma. 5. The same degree of protection could be afforded by MK-886, an inhibitor of leukotriene biosynthesis. Combined treatment with WEB 2086 and MK-886 provided greater inhibition of protein-rich plasma extravasation than either compound alone. PCA 4248 was also found to inhibit in a dose-dependent manner the systemic hypotension observed upon DNP-BSA challenge.6. These data indicate that the lipid mediators PAF and peptidoleukotrienes are major effectors of the vascular disturbances observed in rat passive IgE-mediated anaphylaxis.

The pathobiology of bronchial asthma

Early studies of patients dying from status asthmaticus revealed marked inflammation of the bronchial tree. Subsequent histological studies of the airways and examination of bronchoalveolar lavage fluid of subjects with mild asthma have confirmed the presence of airway inflammation in life. There is epithelial edema and desquamation, subepithelial deposition of collagen and fibronectin, and an inflammatory cell infiltrate in the mucosa. There are increased numbers of activated eosinophils, CD25-positive T lymphocytes, and immature macrophages with the phenotypic characteristics of blood monocytes. An increased expression of HLA class II is present on epithelium, macrophages, and other infiltrating cells. The severity of clinical asthma correlates with several measurements of the severity of the inflammatory response, suggesting a crucial role for airway inflammation in the pathophysiology of the disease. There is considerable interest and research into the mechanisms underlying the pathogenesis and maintenance of the inflammatory response in asthma. The development and maintenance of the inflammatory response in asthma is likely to be a consequence of a complicated interaction between various cells and the mediators they generate. The characterization of an ever-increasing number of cytokines is of particular interest. Interleukin-3, interleukin-5, and granulocyte-macrophage colony-stimulating factor are hematopoietic growth factors that increase the survival of eosinophils in culture and enhance certain eosinophil functions, such as mediator generation and toxicity. Alveolar macrophages derived from asthmatic subjects produce twofold to threefold more GM-CSF than do those from normal control subjects. Using in situ hybridization, the presence of IL-5 mRNA has been demonstrated in bronchial biopsies from asthmatic subjects. Thus IL-3, IL-5, and GM-CSF influence eosinophil function and survival, and may be generated by T lymphocytes and/or alveolar macrophages within the airways in asthma. In addition to these three cytokines, IL-4 and interferon-gamma may be crucial to the regulation of IgE biosynthesis. TNF-alpha and IL-1 are potentially important in the up-regulation of endothelial adhesion molecules. An important step in the recruitment of leukocytes to an inflammatory focus is margination to the vascular endothelium. Our understanding of the molecular events involved in migration of leukocytes to an inflammatory focus has been advanced by the discovery and characterization of a variety of cell adhesion molecules. The potential role of ELAM-1 and ICAM-1 in allergic inflammation is suggested by their up-regulation on vascular endothelium in association with late cutaneous responses to allergen and by their role in certain primate models of asthma.(ABSTRACT TRUNCATED AT 400 WORDS)

Cellular oxidative modification of low density lipoprotein does not require lipoxygenases

The oxidative modification of low density lipoprotein (LDL) may play an important role in the pathogenesis of atherosclerosis. LDL can be oxidatively modified in vitro by endothelial cells, mouse peritoneal macrophages, or copper ions. Studies using lipoxygenase inhibitors have suggested that lipoxygenase(s) is required for the cellular modification of LDL [Rankin, S. M., Parthasarathy, S. & Steinberg, D. (1991) J. Lipid Res. 32, 449-456]. We have reexamined the effect of lipoxygenase inhibitors on cellular modification and found that (i) inhibitors specific for 5-lipoxygenase do not block LDL modification; (ii) inhibitors that block lipoxygenase by donating one electron to the enzyme (reductive inactivation) prevent LDL modification by cells and also modification mediated by copper ions, implying that they act as general antioxidants; (iii) the lipoxygenase inhibitor 5,8,11,14-eicosatetraynoic acid blocks 15-lipoxygenase activity in intact macrophages at concentrations 100 times less than those required to block LDL modification by macrophages; and (iv) 5,8,11,14-eicosatetraynoic acid is cytotoxic at concentrations about twice those required to prevent modification. Furthermore, macrophages and the RECB4 line of endothelial cells modify LDL with similar efficiencies despite dramatic differences in 15-lipoxygenase activity. Thus we conclude that neither 5-lipoxygenase nor 15-lipoxygenase is required for modification of LDL by cultured cells.

Inhibition of leukotriene biosynthesis in the rat peritoneal cavity

In the search for a model of leukotriene (LT) production to provide a method to determine in vivo 5-lipoxygenase (5-LO) inhibitory activity by various compounds, a passive anaphylactic reaction in the rat peritoneal cavity was examined, refined and characterized. The reaction, produced by passive sensitization with an i.p. injection of rabbit anti-bovine serum albumin (anti-BSA) followed by an i.p. injection of BSA, resulted in the biosynthesis of large amounts of sulfidopeptide LTs measurable by immunoassay or by reversed phase high performance liquid chromatography. The oral activity of several 5-LO inhibitors has been examined using this model. An example of these is zileuton (Abbott-64077), a potent 5-lipoxygenase inhibitor now under clinical evaluation. Zileuton inhibited sulfidopeptide LT biosynthesis in the rat peritoneal cavity in a dose-dependent manner (ED50 = 3 mg/kg). WY-49,232, MK-866, BW A4C and phenidone also produced good activity with ED50 values of 6, 8, 11 and 17 mg/kg, respectively. This modified rat peritoneal anaphylaxis model appears to be a valuable tool for establishing in vivo activity of 5-LO inhibitors.

Interaction of vasoactive substances released by platelet-activating factor in the rat perfused heart

1. The coronary vascular effects of platelet-activating factor (PAF) have been intensively studied and it has been proposed that they are mediated by the release of vasoactive substances. In this study, a cascade perfusion model using two rat perfused hearts was developed to investigate the properties of PAF-released vasoactive substances and the interplay of these substances. The properties of the vasoactive substances after an injection of PAF (100 pmol) in the rat perfused heart were examined by collecting the effluent from the first heart for the perfusion of a second (recipient) heart. The presence of vasoconstrictor substances in the effluent was characterized by an increase in the perfusion pressure of the recipient heart. 2. Previous exposure of the recipient heart of PAF (100 pmol) abolished the response of the heart to subsequent administration of PAF, but did not affect the response of the recipient heart to the effluent. This suggested that the coronary vasoconstrictor response of the recipient heart was not due to the presence of PAF in the effluent but to other vasoactive substances. 3. Pretreatment of the recipient heart with the leukotriene receptor antagonist, L-649,923 (5 microM), partially reduced the vasoconstrictor effect of the effluent. Pretreatment of the first heart with indomethacin (2.8 microM) also partially reduced the vasoconstrictor effect of the effluent. The combination of indomethacin pretreatment of the first heart and L-649,923 pretreatment of the recipient heart completely abolished the vasoconstrictor effect of the effluent suggesting that both prostaglandins and leukotrienes are involved in the vasoconstrictor effect of the effluent. 4. Pretreatment of both hearts with L-649,923 or the first heart with the leukotriene synthesis inhibitor (MK-886, 10 microM) completely abolished the vasoconstrictor effect of the effluent. This suggested that the indomethacin sensitive vasoconstrictor component of the effluent might be regulated by leukotrienes in the first heart. However, infusion of leukotrienes (LTB4, LTC4 and LTD4) to the first heart did not reproduce this vasoconstrictor component of the effluent in the recipient heart.5. In conclusion, our study demonstrated through the use of a leukotriene receptor antagonist, a leukotriene synthesis inhibitor and a cyclo-oxygenase inhibitor that the vasoconstrictor effect of the effluent of the rat perfused heart after an injection of PAF is mediated by leukotrienes and prostaglandins. The ability of leukotriene receptor blockade and inhibition of leukotriene synthesis to mimic the effect of indomethacin indicates that the production and/or release of cyclo-oxygenase products in the effluent by PAF can be modulated by leukotrienes. The inability of exogenously applied leukotrienes to modulate the production and/or the release of cyclo-oxygenase products in the effluent suggests that the PAF-induced production of prostaglandins may be mediated by intracellular leukotrienes or at sites not accessible to exogenously applied leukotrienes.

Lack of evidence for a role for the lipoxygenase pathway in increases in cytosolic calcium evoked by ADP and arachidonic acid in human platelets

We have investigated the possibility that metabolism of arachidonic acid by the lipoxygenase pathway contributes to ADP-evoked rises in [Ca2+], in human platelets. 30 microM BW A4C did not affect ADP-evoked Ca2+ signals, but inhibited 12-lipoxygenase activity in platelet homogenates. Another lipoxygenase inhibitor, MK 866 was similarly without effect on ADP-evoked Ca2+ signals. ADP was found to liberate little arachidonic acid, and formation of the lipoxygenase product 12-HETE was not detectable. The rise in [Ca2+]i evoked by arachidonic acid was completely inhibited by the cyclooxygenase inhibitors aspirin or indomethacin. These results indicate that lipoxygenase products do not play an essential role in mediating rises in [Ca2+]i evoked by ADP, or by arachidonic acid.

Role of leukotrienes in leukocyte adhesion following systemic administration of oxidatively modified human low density lipoprotein in hamsters

In vitro studies indicate that oxidatively modified low density lipoprotein (oxLDL) promotes leukocyte adhesion to the vascular endothelium, a constant feature of early atherogenesis. Using intravital fluorescence microscopy in the dorsal skinfold chamber model in awake Syrian golden hamsters, we studied whether (a) oxLDL elicits leukocyte/endothelium interaction in vivo, and whether (b) leukotrienes play a mediator role in this event. Leukocyte/endothelium interaction was assessed in the time course after intravenous injection of native human LDL (4 mg/kg body wt) and of oxLDL (7.5 microM Cu++, 6 h, 37 degrees C) into control hamsters and into hamsters, pretreated with the selective leukotriene biosynthesis inhibitor MK-886 (20 mumol/kg, i.v.). While no effect was seen after injection of native LDL, oxLDL elicited an immediate induction of leukocyte adhesion to the endothelium of arterioles and postcapillary venules. Total and differential leukocyte counts suggest that all leukocyte subsets were likewise affected by oxLDL with no specific preference for monocytes. Stimulation of leukocyte adhesion was entirely prevented in inhibitor-treated animals, suggesting the important mediator role of leukotrienes in oxLDL-induced leukocyte/endothelium interaction.

Leukotrienes as mediators in ischemia-reperfusion injury in a microcirculation model in the hamster

Leukotriene (LT)B4 promotes leukocyte chemotaxis and adhesion to the endothelium of postcapillary venules. The cysteinyl leukotrienes, LTC4, LTD4, and LTE4, elicit macromolecular leakage from this vessel segment. Both leukocyte adhesion to the endothelium and macromolecular leakage from postcapillary venules hallmark the microcirculatory failure after ischemia-reperfusion, suggesting a role of leukotrienes as mediators of ischemia-reperfusion injury. Using the dorsal skinfold chamber model for intravital fluorescence microscopy of the microcirculation in striated muscle in awake hamsters and sequential RP-HPLC and RIA for leukotrienes, we demonstrate in this study that (a) the leukotrienes (LT)B4 and LTD4 elicit leukocyte/endothelium interaction and macromolecular leakage from postcapillary venules, respectively, that (b) leukotrienes accumulate in the tissue after ischemia and reperfusion, and that (c) selective inhibition of leukotriene biosynthesis (by MK-886) prevents both postischemic leukotriene accumulation and the microcirculatory changes after ischemia-reperfusion, while blocking of LTD4/E4 receptors (by MK-571) inhibits postischemic macromolecular leakage. These results demonstrate a key role of leukotrienes in ischemia-reperfusion injury in striated muscle in vivo.

Mechanisms of the coronary vascular effects of platelet-activating factor in the rat perfused heart

1. In a previous study it was demonstrated that bolus injections of platelet-activating factor (PAF) in the rat perfused heart resulted in coronary vasodilatation, vasoconstriction or the combination of both, depending on the amount of PAF that was injected. In the present study, the mechanisms of these coronary vascular effects of PAF in the rat perfused heart were investigated. 2. Pretreatment of the rat perfused heart with the PAF antagonists FR-900452 or BN-52021 did not affect the vasodilator effect of PAF but eliminated the vasoconstrictor effect of PAF. FR-900452 had no effect on the vasoconstrictor response to leukotriene C4 (LTC4) or LTD4. 3. The cyclo-oxygenase inhibitor, indomethacin, did not modify the coronary vascular effects of PAF. However L-649,923 (a leukotriene antagonist) and MK-886 (a leukotriene synthesis inhibitor) eliminated both the vasodilator and vasoconstrictor effects of PAF. 4. When leukotrienes were administered by bolus injection in the rat perfused heart, LTB4 produced vasodilatation while LTC4 and LTD4 produced vasoconstriction. L-649,923 blocked both the vasodilator and vasoconstrictor effects of the leukotrienes tested. 5. The results suggest that lipoxygenase products are responsible for both the vasodilator and vasoconstrictor actions of PAF in the coronary vasculature of the rat perfused heart while the cyclo-oxygenase products do not play a significant role. The ineffectiveness of PAF antagonists in blocking the vasodilatation produced by PAF is compatible with the concept that there may be multiple PAF receptors.

Tumour necrosis factor production in a rat airpouch model of inflammation: role of eicosanoids

TNF is a potent cytokine which can induce many of the pathological changes associated with inflammatory disease. In vitro studies have demonstrated that 5-lipoxygenase products promote the production of TNF by activated macrophages, suggesting that 5-lipoxygenase inhibitors may have therapeutic utility for the treatment of inflammatory conditions. A rat airpouch model of inflammation has been used to investigate the relationship between eicosanoid generation and TNF production in vivo. Injection of zymosan into the airpouch caused a time-dependent stimulation of TNF production which preceded leukotriene generation by at least 30 minutes. Injection of LPS into the airpouch also stimulated TNF production but not leukotriene generation. The selective 5-lipoxygenase inhibitors, ICI207968, A64077 and BWA4C, and the 5-lipoxygenase translocation inhibitor MK886, decreased leukotriene generation but enhanced TNF production. Taken together, these results do not support a role for 5-lipoxygenase products in the regulation of TNF production in vivo.

Absence of modulation of monokine production via endogenous cyclooxygenase or 5-lipoxygenase metabolites: MK-886 (3-[1-(4-chlorobenzyl)-3-t-butyl-thio-5-isopropylindol-2-yl]-2,2- dimethylpropanoic acid), indomethacin, or arachidonate fail to alter immunoreactive interleukin-1 beta, or TNF-alpha production by human monocytes in vitro

Human peripheral blood monocytes exposed to MK-886 (3-[1-(4-chlorobenzyl)-3-t-butyl-thio-5-isopropylindol-2-yl]-2,2- dimethylpropanoic acid) at doses which abolish formation of 5-lipoxygenase metabolites showed unaltered interleukin-1 beta (IL-1 beta) or tumor necrosis factor-alpha (TNF-alpha) levels in response to phorbol ester, concanavalin A, serum-treated zymosan, or lipopolysaccharide. Indomethacin (10 microM), alone or in combination with MK-886, also failed to modulate monokine production in response to any stimulus. Exogenous arachidonate (3-30 microM) which augmented the formation of PGE2 and LTB4 in the absence of stimulation, also had no effect on monokine production. LPS-induced IL-1 and TNF production occurred despite stimulation of PGE2 synthesis. The results make a role for endogenous prostaglandins and leukotrienes in the regulation of monocyte IL-1 beta and TNF-alpha production unlikely. These data also indicate that MK-886, a novel inhibitor of 5-lipoxygenase product formation, is a potentially useful leukotriene inhibitor which does not affect monokine production.

ATP-dependent leukotriene export from mastocytoma cells

The biosynthesis of leukotrienes (LT) C4 and B4 is followed by an export of these mediators into the extracellular space. This transport was characterized using plasma membrane vesicles prepared from mastocytoma cells and identified as an ATP-dependent primary active process. The apparent Km-values were 110 nM for LTC4 and 48 microM for ATP. The transport rate was highest for LTC4, whereas LTD4, LTE4, and N-acetyl-LTE4 were transported with relative rates of 31, 12 and 8%, respectively, at a concentration of 10 nM. LTB4 transport was also dependent on ATP. LTC4 transport was inhibited by LTD4 receptor antagonists (IC50 = 1.0 microM for MK-571 and 1.3 microM for LY245769) and by the inhibitor of leukotriene biosynthesis MK-886 (IC50 = 1.8 microM). The ATP-dependent export carrier for leukotrienes in leukotriene-synthesizing cells represents a novel member of the family of ATP-dependent exit pumps.

Inhibition of leukotriene biosynthesis and polymorphonuclear leukocyte functions by orally active quinolylmethoxyphenylamines

The N-substituted quinolylmethoxyphenylamines, ETH603, ETH615 and ETH647, inhibited the formation of LTB4 in rat peritoneal leukocytes, human peripheral polymorphonuclear leukocytes and canine whole blood. In rat and human cells, the compounds also inhibited the formation of 5-HETE and stimulated the synthesis of 15-HETE. In rat leukocytes, the compounds were 15-30 times more potent inhibitors of LTB4 synthesis than nordihydroguaiaretic acid, but in canine whole blood they were significantly less potent, possibly due to protein binding. However, after oral administration of the compounds to dogs a long-lasting inhibition of LTB4 production in peripheral blood was observed at serum concentrations much lower than those required in vitro. Furthermore, the compounds inhibited the LTB4-directed chemotaxis and the phagocytosis of C. albicans blastospores by canine polymorphonuclear leukocytes both in vitro and following oral administration. The calcium ionophore A23187-induced release of LTB4 in the peritoneal cavity of rats was also inhibited by systemic administration of the compounds. We therefore conclude that these novel quinolines are orally active 5-lipoxygenase inhibitors which may accumulate in inflammatory cells in vivo, leading to potent inhibition of leukotriene biosynthesis and cell function.

FLAP: a novel drug target for inhibiting the synthesis of leukotrienes

FLAP (5-lipoxygenase-activating protein) is a novel 18 kDa membrane protein that together with 5-lipoxygenase is required for cellular leukotriene biosynthesis. Tony Ford-Hutchinson describes a novel class of leukotriene biosynthesis inhibitor, exemplified by MK0886, that can bind to FLAP with high affinity, prevent the activation of 5-lipoxygenase by preventing the translocation of the enzyme from the cytosol to the membrane and inhibit leukotriene production. The term 'translocation inhibitors' may be used for inhibitors such as MK0886 and may represent a new class of drug.

Comparison of several new 5-lipoxygenase inhibitors in a rat Arthus pleurisy model

The 5-lipoxygenase inhibitors WY-50,295 tromethamine, A-64,077, L-663,536 and ICI-207,968 were compared in a reverse passive Arthus reaction-induced pleurisy model of eicosanoid biosynthesis in the rat. When a 1 h pretreatment schedule was employed, all four inhibitors equivalently blocked leukotriene B4 (LTB4) production with ED50 values of 2.0-2.9 mg/kg p.o. Conversely, WY-50,295 tromethamine (225 mg/kg p.o.) and L-663,536 (100 mg/kg p.o.) did not significantly alter thromboxane B2 (TxB2) levels, whereas A-64,077 (50 mg/kg p.o.) and ICI-207,968 (100 mg/kg p.o.) significantly reduced TxB2 by 50 and 72%, respectively. When 3 and 18 h pretreatment schedules were employed, WY-50,295 tromethamine demonstrated a longer duration of action than the other 5-lipoxygenase inhibitors with ED50 values of 1.7 and 6.3 mg/kg p.o., respectively. At doses of 50 and 100 mg/kg p.o., all drugs tested significantly inhibited inflammatory cell influx by 15-27%, albeit in a non-dose-related manner. However, only A-64,077 significantly lowered fluid extravasation by 35%, presumably due to inhibition of cyclooxygenase product formation. These results demonstrate that in this rat reverse passive Arthus pleurisy model, WY-50,295 tromethamine potently and selectively inhibits 5-lipoxygenase in vivo, and possesses a longer duration of action than the other 5-lipoxygenase inhibitors employed.

Inhibition of leukotriene B4 biosynthesis by disulfiram and A-64077 during carrageenan-induced pleurisy in the rat

1. The effect of disulfiram and A-64077 on leukotriene B4 biosynthesis was investigated using human polymorphonuclear leukocyte preparations and an in vivo rat pleurisy assay. 2. Disulfiram inhibited the calcium ionophore-induced release of LTB4 by human leukocytes in vitro with an IC50 of 4.6 +/- 0.3 microM, a value similar to that observed with the 5-lipoxygenase inhibitor A-64077 (IC50 = 1.2 +/- 0.3 microM). These inhibitors were at least 100-fold more potent than diethyldithiocarbamate, the primary metabolite of disulfiram. 3. In a rat pleurisy model, the administration of A-64077 (p.o., 2 hr pretreatment) caused a marked decrease in LTB4 levels measureable after ionophore stimulation at doses of 3 and 10 mg kg (67 and 96% inhibition, respectively). Disulfiram was about a 100-fold less potent, inhibiting LTB4 release by 65% at 300 mg kg (p.o., 6 hr pretreatment). 4. In contrast to A-64077, the inhibitory effect of disulfiram on LTB4 production by isolated leukocytes from the pleural cavity was reduced by the addition of the cell-free pleural exudate, suggesting that protein binding or conversion of disulfiram to inactive species contributes to diminish the potency of the drug. 5. The results indicate that disulfiram, after oral administration in rats, causes an inhibition of leukotriene biosynthesis in the pleural cavity and further illustrate the limited specificity of this drug as an inhibitor of aldehyde dehydrogenase at doses generally used to inhibit this enzyme in vivo.

Differential protective activities of site specific lipoxygenase inhibitors in endotoxic shock and production of tumor necrosis factor

Lipoxygenase inhibitors have been shown to exert beneficial effects in experimental models of endotoxin shock. In the present study it was found that lipoxygenase inhibitors prevented LPS, but not tumor necrosis factor alpha (TNF alpha)-evoked leukopenia in mice. These inhibitors protected against endotoxin lethality but not against TNF alpha induced lethality. When the protective potency of the specific 5-lipoxygenase inhibitors (MK 886, CGS 81585) was tested in endotoxin-induced leukopenia and shock, they were found to be ineffective. Site specificity of the inhibitors was assessed by comparison of their effects on the formation of LTC4 and the conversion of linoleic acid to 13-hydroxyoctadecadienoic acid (13-HODD) by macrophages. The 5-lipoxygenase inhibitors interfered with LTC4 formation in macrophages, however, they did not affect endotoxin-induced TNF alpha formation, neither in cell cultures nor in mice. The inhibitory strength of other, less specific lipoxygenase blockers to suppress TNF alpha formation correlated quantitatively with their ability to interfere with 13-HODD synthesis. From these findings it is concluded that lipoxygenase inhibitors interfere with endotoxic effects because they block TNF alpha formation. Since 5-lipoxygenase inhibitors neither prevented the formation of TNF alpha nor endotoxin leukopenia and lethality, it is suggested that a lipoxygenase product distinct from the leukotrienes is involved in TNF alpha synthesis. Based on the fact that a tight correlation exists between inhibition of TNF alpha synthesis and 13-HODD formation, activation of 15-lipoxygenase might be important for TNF alpha formation.

Role of eicosanoids in PAF-induced increases of the vascular permeability in rat airways

1. Platelet activating factor (PAF; 1.0 and 5.0 micrograms kg-1) injected in the tail vein of unanaesthetized rats dose-dependently increased the vascular permeability of the trachea, upper and lower bronchi (up to 400%) as measured by the extravasation of Evans blue dye. The permeability of the parenchyma was not affected by PAF treatment. 2. Pretreatment of the animals with an intravenous injection of the PAF antagonist BN-52021 (10 mg kg-1) abolished almost totally the vascular permeability changes elicited by PAF injection (5.0 micrograms kg-1). 3. Pretreatment of the animals with intravenous injections of inhibitors of thromboxane formation, indomethacin (10 mg kg-1) and compound OKY-046 (10 mg kg-1), and thromboxane antagonist, compound L-655,240 (5 mg kg-1), partially reduced PAF effects in the airways (from 28 to 69%). The thromboxane mimic U-44069 (5.0 micrograms kg-1) did not modify the vascular permeability of rat airways. The effect of a low dose of PAF (0.1 microgram kg-1) on the vascular permeability of the trachea and bronchi (but not of the parenchyma) was potentiated by compound U-44069 (5.0 micrograms kg-1) or noradrenaline (400 ng kg-1) whereas the effect of a high dose of PAF (5.0 micrograms kg-1) was not affected. 4. Neither the peptidoleukotriene antagonist MK-571 (10 mg kg-1) nor the 5-lipoxygenase inhibitor, L-663,536 (10 mg kg-1) given before the injection of PAF (5.0 micrograms kg-1) affected the protein extravasation in rat lung tissues. 5. These data suggest that the effect of PAF on rat vascular permeability is partly modulated by thromboxane formation although thromboxanes have no direct effect on the permeability. Thromboxane may act via a vasoconstriction that increases hydrostatic pressure and potentiates the extravasation elicited by PAF effect on endothelial cells. 6. Leukotrienes do not appear to be involved in the changes of rat airway permeability induced by PAF.

Effects of racemic, S- and R-indobufen on cyclooxygenase and lipoxygenase activities in human whole blood

Racemic indobufen inhibits human platelet aggregation by reducing thromboxane (TX) A2 biosynthesis. In order to ascertain which of the two optical isomers is responsible for its pharmacological activity, we compared the effects of racemic (SR +/-), S(+) enantiomer and R(-) enantiomer indobufen on cyclooxygenase and 5-lipoxygenase activities by assessing the biosynthesis of TXB2, prostaglandin (PG) E2 and leukotriene (LT) B4 in human whole blood stimulated with the Ca2+ ionophore A23187. Racemic indobufen caused a dose-dependent inhibition of TXB2 and PGE2 production (IC50: 0.53 +/- 0.06 and 0.34 +/- 0.02 micrograms/ml, respectively; mean +/- S.D., n = 4). S-Indobufen was approximately 2-fold more potent than the racemate in inhibiting the synthesis of cyclooxygenase products. R-Indobufen affected the same enzyme but only at considerably higher concentrations (IC50: 53 +/- 8 micrograms/ml, n = 3). Serum LTB4 concentrations were significantly reduced only at indobufen concentrations greater than 50 micrograms/ml. In conclusion, indobufen is a selective inhibitor of the cyclooxygenase activity of platelet PGG/H synthase in a concentration range corresponding to the therapeutic plasma levels in man. This inhibitory effect is largely due to the S isomer of the drug.

Role of leukotriene B4 in granulocyte infiltration into the postischemic feline intestine

Several studies have demonstrated that granulocytes accumulate in the intestinal mucosa following ischemia/reperfusion. It has been suggested that leukotriene B4 may be released during ischemia/reperfusion and consequently may promote granulocyte infiltration into the mucosa. The objectives of this study were to determine whether (a) leukotriene B4 is produced in the gut mucosa during ischemia and reperfusion, and (b) inhibition of leukotriene B4 attenuates granulocyte infiltration into the postischemic intestinal mucosa. Isolated segments of cat intestine were subjected to 3 hours of ischemia and 1 hour of reperfusion. Mucosal samples were obtained during baseline, ischemia at 3 hours and reperfusion at 1 hour. Leukotriene B4 production was determined by radioimmunoassay. Tissue-associated myeloperoxidase activity was used to quantitate granulocyte accumulation in the mucosal samples. In untreated animals, mucosal leukotriene B4 concentration was higher at reperfusion compared with baseline levels. The reperfusion-induced increase in mucosal leukotriene B4 was entirely prevented by pretreatment with either nordihydroguaiaretic acid (Sigma Chemical Co., St. Louis, MO) or L663,536 (Merck-Frosst, Montreal, Quebec, Canada), two potent lipoxygenase inhibitors. Both lipoxygenase inhibitors, as well as leukotriene B4 antagonist (SC-41930) significantly attenuated the reperfusion-induced infiltration of granulocytes. These results indicate that leukotriene B4 plays an important role in mediating the granulocyte accumulation elicited by reperfusion of the ischemic bowel.

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.

Leukotrienes do not regulate interleukin 1 production by activated macrophages

The objective of this work was to investigate the role of leukotrienes in the production of IL-1 by activated human peripheral blood monocytes and mouse peritoneal macrophages. Using overnight adherent macrophages, stimulation with lipopolysaccharide or zymosan caused a time-dependent increase in IL-1 production. LTC4 was detected and preceded IL-1 production only in zymosan-treated macrophages. Lipopolysaccharide did not stimulate macrophages to produce LTC4. Zymosan-stimulated LTC4 production was inhibited by the lipoxygenase inhibitors, ICI207968 (3.20 microM), nordihydroguaiaretic acid (0.22 microM), phenidone (4.60 microM), REV5901 (0.20 microM), and the Merck 5-lipoxygenase "translocation inhibitor" MK886 (0.02 microM) with IC50 values as shown in parenthesis. However, none of these inhibitors reduced IL-1 production at concentrations which completely inhibited leukotriene synthesis. Taken together, these results do not support a role for leukotrienes in the production of IL-1 by zymosan-activated macrophages.

Leukotriene B4 potentiates colonic ulceration in the rat

The ability of various leukotrienes, platelet-activating factor and N-formyl-methyl-leucyl-phenylalanine to augment colonic damage induced by 30% ethanol was investigated in the rat. Each of the mediators was tested at a dose of 2 nmol, administered intracolonically in the ethanol vehicle. When colonic damage was assessed 72 hr later, only leukotriene B4 significantly augmented damage compared to the controls. The incidence of ulcers increased from 35% in the control group to 90% in the group receiving leukotriene B4. Leukotriene B4 administration also resulted in significant increases in colonic myeloperoxidase activity and colonic leukotriene B4 synthesis. To assess the possible contribution of infiltrating neutrophils to the increase in colonic leukotriene B4 synthesis that accompanies colonic inflammation, colitis was induced in normal and neutropenic rats by intracolonic administration of trinitrobenzene sulfonic acid. Neutropenia was achieved by treatment with an antineutrophil serum. In the neutropenic animals killed 4 hr after induction of colitis significant changes in leukotriene B4 synthesis were not observed, whereas a fourfold increase was observed in the controls. From these studies we conclude the following: (1) leukotriene B4, at a dose of 2 nmol, can significantly potentiate the colonic ulceration induced by 30% ethanol; (2) this action of leukotriene B4 is not shared by the same dose of the other inflammatory mediators tested; and (3) infiltrating neutrophils are the major source of colonic leukotriene B4 synthesis in a rat model of colitis.

Inhibitory effects of MK-886 on arachidonic acid metabolism in human phagocytes

1. We have investigated the inhibitory activity of compound MK-886 (formerly L-663,536), an indole derivative, on 5-lipoxygenase product synthesis in various human phagocytes stimulated with either the ionophore A23187, in the presence and absence of exogenous arachidonic acid, or platelet-activating factor (PAF). The lipoxygenase products were analysed by reversed-phase h.p.l.c. 2. MK-886 inhibited the formation of 5-hydroxy-eicosatetraenoic acid (5-HETE), leukotriene B4 (LTB4), its omega-oxidation products and 6-trans-isomers with an IC50 value of 10-14 nM in A23187-stimulated neutrophils. In the same system, nordihydroguaiaretic acid (NDGA), AA-861 and L-655,240 showed IC50 values of 250-510, 110-420 nM and 1.7-3.9 microM, respectively. 3. MK-886 inhibited 5-lipoxygenase product synthesis in A23187-stimulated blood eosinophils and monocytes, and in neutrophils primed with granulocyte-macrophage colony-stimulating factor and stimulated with PAF with IC50 values of 1-13 nM. 4. The inhibitory activity of MK-886 was not reversed by addition of 10 microM arachidonic acid to A23187-stimulated neutrophils. 5. Compound MK-886 had no effect on 15-lipoxygenase product synthesis in blood eosinophils and neutrophils up to a concentration of 1 microM. 6. At 100 nM compound MK-886 had no significant effects on calcium ion mobilization, superoxide anion production and actin polymerization in neutrophils. 7. In conclusion, MK-886 is a very potent and specific inhibitor of both LTB4 and LTC4 synthesis in various types of human phagocytes.

Identification and isolation of a membrane protein necessary for leukotriene production

Several inflammatory diseases, including asthma, arthritis and psoriasis are associated with the production of leukotrienes by neutrophils, mast cells and macrophages. The initial enzymatic step in the formation of leukotrienes is the oxidation of arachidonic acid by 5-lipoxygenase (5-LO) to leukotriene A4. Osteosarcoma cells transfected with 5-LO express active enzyme in broken cell preparations, but no leukotriene metabolites are produced by these cells when stimulated with the calcium ionophore A23187, indicating that an additional component is necessary for cellular 5-LO activity. A new class of indole leukotriene inhibitor has been described that inhibits the formation of cellular leukotrienes but has no direct inhibitory effect on soluble 5-LO activity. We have now used these potent agents to identify and isolate a novel membrane protein of relative molecular mass 18,000 which is necessary for cellular leukotriene synthesis.

Requirement of a 5-lipoxygenase-activating protein for leukotriene synthesis

Leukotrienes, the biologically active metabolites of arachidonic acid, have been implicated in a variety of inflammatory responses, including asthma, arthritis and psoriasis. Recently a compound, MK-886, has been described that blocks the synthesis of leukotrienes in intact activated leukocytes, but has little or no effect on enzymes involved in leukotriene synthesis, including 5-lipoxygenase, in cell-free systems. A membrane protein with a high affinity for MK-886 and possibly representing the cellular target for MK-886 has been isolated from rat and human leukocytes. Here, we report the isolation of a complementary DNA clone encoding the MK-886-binding protein. We also demonstrate that the expression of both the MK-886-binding protein and 5-lipoxygenase is necessary for leukotriene synthesis in intact cells. Because the MK-886-binding protein seems to play a part in activating this enzyme in cells, it is termed the five-lipoxygenase activating protein (FLAP).

Functional consequences of phospholipase A2 activation in human monocytes

Human monocytes release arachidonic acid upon stimulation with a variety of soluble or particulate agents. These include: phorbol esters (i.e., 12-O-tetradecanoate phorbol-13-acetate, TPA), calcium ionophores (ionomycin), serum-treated zymosan (STZ) concanavalin A (Con A), and, to a minor degree, lipopolysaccharides (LPS). Protein Kinase C activation or increased intracellular Ca2+ are common features of the actions of most, if not all, of these stimuli. Prevention of PKC activation by the use of staurosporine or chelation of extracellular calcium by EGTA selectively impaired AA release, indicating that PLA2 may be regulated by either pathway concurrently. The generation of inositol phosphates and diacylglycerol by the action of phospholipase C, notably upon interaction with opsonized particles during phagocytosis, apparently constitutes the physiological correlate of stimulation via these agents. Release of arachidonic acid by the action of PLA2 or other phospholipid hydrolyzing enzymes leads directly to the formation of cyclooxygenase products. In the presence of markedly elevated calcium concentrations, 5-lipoxygenase (LO) is activated as well, leading to the formation and release of leukotrienes. Agents which stimulate AA release also initiate other monocyte functions, including generation of reactive oxygen intermediates and lymphokine release. This observation makes it tempting to implicate PLA2 activation in many aspects of monocyte physiology. However, no correlation with PLA2 activation and either superoxide or lymphokine release was found when multiple stimuli, including TPA, ionomycin, serum-treated zymosan, concanavalin A, or LPS, were compared simultaneously. Instead, our results indicate that PLA2 activation is regulated by the same mechanisms, including PKC activation and increased Ca2+, as are other enzymes which determine expression of monocyte function. Phospholipase A2 (PLA2) hydrolyzes fatty acid from the sn-2 position of a wide variety of phospholipids. Substrates for this (these) enzyme(s) include species which contain a variety of polar head groups (choline, serine, ethanolamine, etc.) and some phospholipids with either linkages in sn-1. In many cell types, including human monocytes, phospholipase A2 commonly acts on substrates containing arachidonic acid (AA). The liberation of free arachidonate is a first step in the metabolism of prostaglandins, hydroxyeicosatetraeinoic acids, (HETE'S), and leukotrienes (Lt's). Monocytes and macrophages have been shown to be rich sources of arachidonate and its metabolites. Some biologic properties of monocytes, notably their role as immunomodulating cells, have been attributed to eicosanoid production and release. Accordingly, much of the interest regarding PLA2 in human monocytes centers on this aspect of their function.(ABSTRACT TRUNCATED AT 400 WORDS)