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

Ca2+ ionophore A23187-stimulated secretion of azurophil granules in human polymorphonuclear leukocytes is largely mediated by endogenously formed leukotriene B4

The mode of action of the new leukotriene synthesis inhibitor BAY X1005 ((R)-2-[4-(quinolin-2-yl-methoxy)phenyl]-2-cyclopentyl acetic acid) and structurally-related quinoline derivatives is reflected by the binding to a high-affinity binding site presumably identical to FLAP (five lipoxygenase activating protein). In addition to FLAP, we have identified a second BAY X1005 (low-affinity) binding site localized in the granule fraction of human PMNL (polymorphonuclear leukocytes). Based on the hypothesis that the corresponding target protein might be involved in the regulation of granule release, the influence of the leukotriene synthesis inhibitors BAY X1005 and MK-886 and the direct 5-LOX (5-lipoxygenase, EC 1.13.11.34) inhibitor A-64077 on the A23187- and fMLP (N-formyl-methionyl-leucyl-phenylalanine)-stimulated release of beta-glucuronidase (as a marker for azurophil granules) and vitamin B12-binding protein (as a marker for specific granules) was investigated. In contrast to MK-886, neither BAY X1005 nor A-64077 significantly affected fMLP-stimulated granule release. This was also true for the A23187-stimulated release of specific granules; however, under the same conditions the A23187-stimulated release of azurophil granules was almost totally inhibited by all three compounds. No obvious relationship between the corresponding IC50 values and the ability of these compounds to compete for BAY X1005 binding at the low-affinity binding site existed. Instead, by extending these studies to additional inhibitors, a correlation between the IC50 values for inhibition of A23187-stimulated (i) beta-glucuronidase release and (ii) LTB4 (leukotriene B4) synthesis was found (r = 0.969, N = 7). This relationship was independent of the mode of action of the compounds, namely direct 5-LOX inhibition or indirect 5-LOX inhibition mediated via binding to FLAP. These results suggest that 5-LOX metabolites may be involved in A23187-stimulated azurophil granule release. Of the two main biologically active 5-LOX metabolites synthesized under these conditions (LTB4 and 5-hydroxyeicosatetraenoic acid), only LTB4 stimulated beta-glucuronidase release to nearly the same extent as A23187. In addition, this metabolite significantly enhanced A23187-stimulated beta-glucuronidase release, but only at A23187 concentrations (> or = 0.25 mumol/L) which by themselves were not sufficient to trigger LTB4 formation. Moreover, the inhibition of A23187-stimulated beta-glucuronidase release by BAY X1005 or A-64077 was totally reversed by the addition of LTB4.(ABSTRACT TRUNCATED AT 400 WORDS)

5-Lipoxygenase: enzyme expression and regulation of activity

5-Lipoxygenase catalyzes the transformation of arachidonic acid to leukotriene A4. This unstable intermediate can be converted to leukotriene B4 by LTA4-hydrolase or to leukotriene C4 by LTC4-synthase. Leukotrienes are involved in host defense reactions and play an important role in inflammatory diseases like asthma, inflammatory bowel disease and arthritis. The capability to release leukotrienes is restricted to a few cell types. Under pathophysiological conditions, leukotrienes are released from granulocytes, mast cells or macrophages. During hematopoiesis the competence of these cells for leukotriene biosynthesis is supposed to be upregulated. In mature cells, 5-lipoxygenase activity is tightly regulated and seems to be under the control of additional cellular components. One cellular component, a membrane-bound peptide termed FLAP, which is necessary for 5-LO activity in intact cells has been recently identified. Inhibitors of FLAP function prevent translocation of 5-lipoxygenase from cytosol to the membrane and inhibit 5-LO activation. Thus, the understanding of the regulatory mechanisms of cellular leukotriene biosynthesis provides new concepts for the development of antiinflammatory drugs. This review focuses on the regulation of gene expression and activity of 5-lipoxygenase.

Reactive oxygen species and not lipoxygenase products are required for mouse B-lymphocyte activation and differentiation

A potential role for lipoxygenase (LO) products and reactive oxygen species (ROS) in mouse B-lymphocyte activation and differentiation was investigated. Previously published investigations with the nonspecific 5-LO (EC 1.13.11.34) and 12-LO (EC 1.13.11.31) inhibitors such as nordihydroguaiaretic acid (NDGA) and 6,7-dihydroxycoumarin (Esculetin), are misleading in that they suggest lymphocyte LO activity is required for activation and differentiation of these cells. In initial support of this concept, we report that NDGA and Esculetin completely inhibited B-lymphocyte activation mediated by either membrane immunoglobulin (mIg), or the lipopolysaccharide (LPS) receptor. NDGA and Esculetin completely inhibited cell enlargement and proliferation, exhibiting half maximal inhibitory concentrations (IC50S) of approximately 1 x 10(-6) M. In contrast, the highly specific 5-LO inhibitors BAY X 1005, MK-886 and Wy 50,295 did not inhibit cell enlargement or proliferation. Moreover, 5,8,11-eicosatriynoic acid (ETI) which inhibits 5- and 12-LO, and 5, 8, 11, 14-eicosatetraynoic acid (ETYA) which inhibits all known LOs did not affect B-lymphocyte proliferation. Interestingly, NDGA and Esculetin are antioxidants, unlike BAY X 1005, MK-886, Wy 50,295, ETI and ETYA. Our hypothesis was that the antioxidant activities of NDGA and Esculetin were reponsible for inhibiting B-lymphocyte activation and proliferation and we speculated that ROS and not LO activity was required for both processes. Additional antioxidants such as butylated hydroxy toluene, o-phenanthroline, thiourea, and alpha-tocopherol (vitamin E), also inhibited B-lymphocyte proliferation induced by either the LPS or mIg receptors. These agents exhibited IC50S of 1 x 10(-8) M, 5 x 10(-10) M, 6 x 10(-3) M and 5 x 10(-5) M, respectively. When resting B-lymphocytes were treated with a source of ROS (1 x 10(-5) M H2O2), cells enlarged in a temperature-sensitive manner, which is similar to LPS-induced enlargement. Both NDGA and Esculetin completely inhibited H2O2-induced enlargement. These results further indicate that ROS are required for B-lymphocyte activation and proliferation. Similar results were obtained for B-lymphocyte differentiation. NDGA and Esculetin completely inhibited the development of plasma cells and displayed IC50S of 5 x 10(-6) M. Conversely, BAY X 1005, MK-886, Wy 50,295, ETI, and ETYA did not block the formation of plasma cells. Therefore, ROS are also crucial for differentiation into plasma cells. These experiments are the first to directly illustrate that intracellular ROS mediate B-lymphocyte activation, proliferation and differentiation and that LO products are not required for these processes.(ABSTRACT TRUNCATED AT 400 WORDS)

Structural requirements for the binding of fatty acids to 5-lipoxygenase-activating protein

5-Lipoxygenase-activating protein is required for cellular leukotriene synthesis and is the target of the leukotriene biosynthesis inhibitors MK-886 (3-[1-(p-chlorophenyl)-5-isopropyl-3-tert-butylthio-1H- indol-2-yl]-2,2-dimethylpropanoic acid) and MK-591 (3-[1-(4-chlorobenzyl)-3-(t-butylthio)-5-(quinolin-2-ylmethoxy)-indol-2-yl] - 2,2-dimethylpropanoic acid). Recent studies demonstrate that 5-lipoxygenase-activating protein binds arachidonic acid and stimulates the utilization of this substrate by 5-lipoxygenase. The present study utilizes a radioligand binding assay to assess the affinity of 5-lipoxygenase-activating protein for arachidonic acid and the specificity of the fatty acid binding site on 5-lipoxygenase-activating protein. Our findings demonstrate that the presence of a free carboxyl group on fatty acids or leukotriene biosynthesis inhibitors which interact with 5-lipoxygenase-activating protein is not required for specific binding to the protein. However, the degree of saturation significantly affects the affinity of fatty acids for 5-lipoxygenase-activating protein.

Roles of tumor necrosis factor alpha, granulocyte-macrophage colony-stimulating factor, platelet-activating factor, and arachidonic acid metabolites in interleukin-1-induced resistance to infection in neutropenic mice

Treatment with a single low dose (80 to 800 ng) of interleukin-1 (IL-1) 24 h before a lethal bacterial challenge in granulocytopenic and in normal mice enhances nonspecific resistance. The mechanism behind this protection has only partially been elucidated. Since IL-1 induces production of tumor necrosis factor alpha (TNF-alpha), granulocyte-macrophage colony-stimulating factor (GM-CSF), platelet-activating factor (PAF), and arachidonic acid metabolites, we investigated the potential role of these substances in IL-1-induced protection. Low doses of murine TNF-alpha but not of human TNF-alpha enhanced survival, suggesting an effect via the type II TNF receptor rather than the type I TNF receptor, which has little species specificity. In line with this TNF-alpha-induced protection from infection, pretreatment with a low dose of a rat anti-murine TNF-alpha monoclonal antibody tended to inhibit IL-1-induced protection, suggesting a role of TNF-alpha as a mediator of IL-1-induced enhanced resistance to infection. Pretreatment with higher doses of anti-TNF-alpha, however, showed a dose-related protective effect per se, which could be further enhanced by a suboptimal dose of IL-1. A combination of optimal doses of anti-TNF-alpha and IL-1 produced an increase in survival similar to that produced by separate pretreatments. This lack of further enhancement of survival by combined optimal pretreatments suggests a similar mechanism of protection, most likely attenuation of deleterious effects of overproduced proinflammatory cytokines like TNF-alpha during lethal infection. Pretreatment with different doses of GM-CSF before a lethal Pseudomonas aeruginosa challenge in neutropenic mice did not enhance survival. Different doses of WEB 2170, a selective PAF receptor antagonist, of MK-886, a selective inhibitor of leukotriene biosynthesis, or of several cyclooxygenase inhibitors did not reduce the protective effect of IL-1 pretreatment. We conclude that IL-1-induced nonspecific resistance is partially mediated by induction of TNF-alpha and not by GM-CSF, PAF, and arachidonic acid metabolites. The mechanism of action of IL-1 seems to be similar to that of anti-TNF-alpha.

Leukotrienes do not contribute to the pathogenesis of indomethacin-induced ulceration of the gastric antrum in the re-fed rat

The potential involvement of leukotrienes in the pathogenesis of indomethacin-induced ulceration of the rat gastric antrum has been studied. Pretreatment with the leukotriene biosynthesis inhibitor, MK886 (30 mg/kg p.o.), inhibited the increases in blood and antral leukotriene B4 release ex vivo associated with the evolution of antral ulceration. Despite this, however, there was no significant reduction in either the area of antral ulceration, or in the associated blood neutrophilia and neutrophil infiltration into the gastric antrum. Similarly, pretreatment with the leukotriene B4 antagonist, SC41930 (50 mg/kg p.o.) or the peptidyl leukotriene antagonist ICI198,615 (50 mg/kg p.o.) did not inhibit the area of antral ulceration induced by indomethacin. Thus, in contrast to published reports studying fundic ulceration, our results suggest that leukotrienes do not play a major role either in the pathogenesis of indomethacin-induced ulceration of the rat gastric antrum or neutrophil infiltration into the damaged antrum.

Short term effects of indomethacin on rat small intestinal permeability. Role of eicosanoids and platelet activating factor

Short term effects of indomethacin on intestinal permeability were studied on a model of rat isolated vascularly perfused terminal ileum. The objectives of this study were (a) to assess the effects of indomethacin on intestinal permeability and histology; (b) to assess the effects of prostaglandins, leukotrienes, and platelet activating factor (PAF) on the same parameters; (c) to evaluate the role of these inflammation mediators on indomethacin induced permeability modifications. Intravascular administration of 1.25 and 2.5 mM indomethacin induced a significant increase of 51Cr-EDTA transfer rate. Histological analysis showed only mucosal oedema. Pretreatment with 16,16 dimethyl-prostaglandin E2 did not reverse these changes. Intravascular administration of PAF, leukotrienes B4 and D4 provoked a significant rise in 51Cr-EDTA transfer rate and intraluminal protein leakage, with an intense vascocongestion of the mucosal capillaries. These changes were completely prevented by perfusion of the respective specific antagonists (BN52021 for PAF, LY255,583 for leukotriene B4 and MK571 for leukotriene D4). None of these three antagonists, however, or MK886, a selective 5'-lipo-oxygenase inhibitor, could reverse the indomethacin induced permeability changes. Indomethacin induced increased intestinal permeability at these high concentrations does not seem to be a result of changed prostanoid or PAF metabolism. Alternative mechanisms of the initial damage of non-steroid anti-inflammatory drugs should be sought.

Production of inflammatory mediators by human macrophages obtained from ascites

Ascites is a readily available source of human macrophages (M phi), which can be used to study M phi functions in vitro. We characterized the mediators of inflammation produced by human peritoneal M phi (hp-M phi) obtained from patients with portal hypertension and ascites. The production of the cytokines interleukin-1 beta (IL-1 beta), interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) was found to be lipopolysaccharide (LPS) concentration dependent (0-10 micrograms/ml) with a maximal production at 10 micrograms/ml and also dependent on the time of exposure to the stimulus (0-36 h). IL-1 beta, IL-6 and TNF-alpha production after LPS administration reached a plateau at 24 h. In vitro stimulation for 24 h with LPS does not influence the eicosanoid production from endogenous arachidonate. 13 min of exposure of the cells to the calcium ionophore A23187 gives a significant increase in eicosanoid production from both exogenous and endogenous arachidonate. The main eicosanoids produced are the 5-lipoxgenase products LTB4 and 5-hydroxyeicosatetraenoic acid (HETE). The increase in production of the other eicosanoids is not significant. The eicosanoid production depends on the stimulus concentration. The optimal A23187 concentration is 1 microM. Oxygen radical production was measured in the M phi by a flowcytometric method. The fluorescence intensity of phorbol 12-myristate 13-acetate stimulated and dihydro-rhodamine 123 loaded hp-M phi increases significantly after 15 min. We conclude that LPS stimulation of hp-M phi from liver disease results in similar production of IL-1 beta, IL-6 and TNF-alpha, but that the profile of the eicosanoid production of these M phi stimulated with LPS and A23187 differs from M phi of other origin and species.

Effects of loratadine on cytosolic Ca2+ levels and leukotriene release: novel mechanisms of action independent of the anti-histamine activity

Loratadine, a non-sedating anti-histamine drug, displays in vitro potential anti-allergic properties not related to its interaction with the histamine H1 receptor. In a search for the mechanisms of these actions, we have found that loratadine induces an elevation of cytosolic calcium ion, [Ca2+]i, in rat peritoneal macrophages or human platelets. The mechanism of this elevation resides in the ability of loratadine to discharge intracellular Ca2+ stores, similarly to thapsigargin. This in turn brings about the inhibition of [Ca2+]i rise induced by physiological activators (platelet activating factor and ADP), as well as by thapsigargin. One of the active metabolites of loratadine, descarbo-ethoxy-loratadine, and another anti-histamine, namely terfenadine, exhibit the same effects. In addition, loratadine partially inhibits antigen-induced leukotriene release from human bronchi, but is unable to inhibit the concomitant contraction. We conclude that loratadine can interfere with the mechanisms controlling Ca2+ release, thus inhibiting the cell activation elicited by various agonists through [Ca2+]i elevation. This might be the mechanism underlying its anti-allergic actions in vitro. Furthermore, loratadine might represent an interesting tool in the study of Ca2+ homeostasis.

Remodeling of neutrophil phospholipids with 15(S)-hydroxyeicosatetraenoic acid inhibits leukotriene B4-induced neutrophil migration across endothelium

5-Lipoxygenase products, such as leukotrienes, are important stimuli for leukocyte-mediated tissue injury in acute inflammation. 15-Hydroxyeicosatetraenoic acid (15-HETE) is an eicosanoid generated by a variety of cell types via the actions of 15-lipoxygenases and, in addition, cyclooxygenases and epoxygenases. 15-HETE levels are frequently elevated at sites of inflammation, and extracellular 15(S)-HETE is esterified rapidly into neutrophil (PMN) phospholipids in vitro to levels that are comparable with arachidonic acid. We present evidence that remodeling of PMN phospholipids with 15(S)-HETE stereoselectively inhibits PMN migration across endothelium in response to leukotriene B4 (LTB4) and other chemoattractants. Esterified 15(S)-HETE causes a striking reduction in the affinity of LTB4 cell-surface receptors for their ligand and inhibition of LTB4-triggered stimulus-response coupling. As a result of these actions, esterified 15(S)-HETE attenuates the cytoskeletal rearrangements and CD11/CD18-mediated adhesive events that subserve directed locomotion of PMN across endothelium. These observations indicate that products of the 5-lipoxygenase and 15-lipoxygenase pathways can exert counterbalancing influences on PMN trafficking across endothelium. They suggest that 15(S)-HETE may be a potent endogenous inhibitor of PMN-endothelial interactions in vivo and serve to limit or reverse acute inflammation.

Comparative evaluation of arachidonic acid (AA)- and tetradecanoylphorbol acetate (TPA)-induced dermal inflammation

The effects of topical application of arachidonic acid (AA) or phorbol ester, tetradecanoylphorbol 13-acetate (TPA), on edema response, vascular permeability, MPO, NAG, and generation of eicosanoids were studied in two murine models of cutaneous inflammation. AA produced a short-lived edema response with a rapid onset that was associated with marked increases in levels of prostaglandins (PGE2, 6-keto-PGF1 alpha, PGF2 alpha), thromboxane B2 (TxB2) and leukotriene B4 (LTB4), with smaller increases in levels of LTC4. TPA produced a longer-lasting edema that was associated with marked influx of neutrophils and predominant formation of LTB4 along with significant changes in levels of TxB2. Circulating T lymphocytes have no apparent role in the acute inflammatory responses induced by either agent. Arachidonic acid-induced vascular permeability preceded the edema response and neutrophil influx, whereas TPA-induced vascular permeability paralleled the edema response and influx of neutrophils. Mast cells appear to be important in the complete expression of inflammatory response, i.e., edema, cellular influx, and vascular permeability induced by either AA or TPA, as these responses were blunted in mast cell-deficient mice. Inhibitors of CO or 5-LO attenuated inflammatory responses in both models. The LTB4 receptor antagonist, SC-41930, inhibited the inflammatory response to TPA but had little effect on that initiated by AA. This suggests that LTB4 is an important mediator in the phorbol ester-induced inflammatory response, whereas peptidoleukotrienes and prostaglandins regulate vascular permeability responses in the arachidonate model.

Formation of sulphidopeptide-leukotrienes by cell-cell interaction causes coronary vasoconstriction in isolated, cell-perfused heart of rabbit

1. We have studied the transcellular biosynthesis of bioactive leukotrienes (LTs), generated upon blood cell-vascular wall interactions and their functional consequences, in the spontaneously beating, cell-perfused, heart of the rabbit. Rabbit isolated hearts were perfused under recirculating conditions (50 ml) with 5 x 10(6) cells of unpurified (buffy coat) or purified human neutrophils (PMNL), and challenged with 0.5 microM A23187 for 30 min. Coronary perfusion pressure (CPP), heart rate (HR), left ventricular end-diastolic pressure (LVEDP) and left ventricular pressure (LVP) were monitored continuously. Leukotriene formation was measured by specific enzyme-immunoassay and confirmed by reversed phase h.p.l.c. and u.v. spectral analysis. 2. Basal CPP values averaged 44 +/- 1.4 mmHg; A23187 triggered a marked increase in CPP both in the presence of buffy coat cells (+100% above basal) and PMNL (+270% above basal); the latter change in CPP was accompanied by a rise in LVEDP (+138% above basal). 3. The increase in CPP was preceded by a statistically significant rise in iLTC4-D4 concentration in the circulating buffer. Pretreatment with two structurally unrelated LTD4 receptor antagonists, LY171883 and SKF104353 (10 microM), fully prevented the increase in CPP and LVEDP. A similar protection was also observed when the rabbit heart was perfused with PMNL that had been pretreated with MK886 (1 microM), a potent inhibitor of leukotriene biosynthesis. 4. The increased coronary tone was accompanied by a marked release of lactate dehydrogenase (LDH), a marker of ischaemic damage; pretreatment of the heart with the LTD4 receptor antagonists as well as of the PMNL with MK886 resulted in a complete suppression of LDH activity release. 5. Positive identification of LTC4-D4 in the perfusates was obtained and a significant correlation observed between the CPP values and iLTC4-D4 concentrations.6. This study suggests that challenge of PMNL present within the coronary vasculature, causes a LTD4-dependent coronary vasoconstriction, favoured by an efficient uptake of PMNL-derived LTA4 by endothelial cells. The activation of the 5-lipoxygenase pathway in the context of tight interactions between blood cells and coronary vasculature, is suggested to have an important outcome in the alterations of coronary flow and cardiac contractility.

Partial agonist effect of the platelet-activating factor receptor antagonists, WEB 2086 and WEB 2170, in the rat perfused heart

1. WEB 2086 and WEB 2170 are potent platelet-activating factor (PAF) receptor antagonists and have been used widely as pharmacological tools to investigate the actions of PAF in a variety of biological systems. 2. Low concentrations of WEB 2086 and WEB 2170 blocked the vasoconstrictor action of PAF in the rat perfused heart. In this study, we observed that moderate concentrations of WEB 2086 and WEB 2170 increased the perfusion pressure in rat isolated hearts under constant flow perfusion. The vasoconstrictor actions of WEB 2086 and WEB 2170 were not observed with a structurally different PAF receptor antagonist, FR-900452. 3. To determine whether this vasoconstrictor action of WEB 2086 involved non-specific effects or was via the activation of PAF receptors, hearts were pretreated with 1000 pmol PAF or 50 microM FR-900452. These pretreatments attenuated the vasoconstrictor action of 1 microM WEB 2086, suggesting that the action of WEB 2086 may be mediated via PAF receptors. Pretreatment with the leukotriene receptor antagonist (L-649,923, 5 microM) and the leukotriene synthesis inhibitor (MK-886, 10 microM) that are known to block the vasoconstrictor action of PAF receptor activation also attenuated the vasoconstrictor action of WEB 2086. Pretreatment with PAF or MK-886 attenuated the vasoconstrictor action of 0.5 microM WEB 2170. 4. When PAF receptors were activated by PAF in the perfused heart, significant amounts of leukotriene C4 and leukotriene C4/D4/E4 were detected in the coronary effluent. However, no significant amount of these leukotrienes was detected in the coronary effluent when hearts were perfused with 1 microM WEB 2086 or 0.5 microM WEB 2170. 5. In summary, our results indicate that WEB 2086 and WEB 2170 possess partial agonist effects in the rat perfused heart where they produced vasoconstriction via the activation of PAF receptor. This action could be attenuated by PAF pretreatment or a PAF receptor antagonist. The vasoconstrictor action of WEB 2086 and WEB 2170 involved the production of leukotrienes. But unlike the vasoconstrictor action of PAF, no significant amount of leukotrienes was detected in the effluent suggesting that the vasoconstrictor action of WEB 2086 and WEB 2170 may be explained on the basis of intracellularly or locally produced leukotrienes.

Role of oxygen radicals and arachidonic acid metabolites in the reverse passive Arthus reaction and carrageenin paw oedema in the rat

1. The role of arachidonic acid metabolites and oxygen radicals in carrageenin-induced rat paw oedema and dermal reverse passive Arthus reaction (RPA) have been investigated. 2. Indomethacin (10 mg kg-1, p.o.) inhibited carrageenin paw oedema when administered 30 min before, but not 2 h after carrageenin. BWB70C (10 mg kg-1, p.o.), a selective inhibitor of 5-lipoxygenase, had no effect whether administered before or after carrageenin. Administration of both indomethacin and BWB70C had no greater anti-inflammatory effect than indomethacin alone. 3. BW755C (20 mg kg-1, p.o.), which inhibits the cyclo-oxygenase and lipoxygenase pathways of arachidonic acid metabolism, or superoxide dismutase-polyethylene glycol conjugate (SOD-PEG, 3000 u, i.v.) inhibited carrageenin paw oedema whether administered either 30 min before, or 2 h after carrageenin. 4. Pretreatment with dexamethasone (0.1 mg kg-1) or colchicine (2 mg kg-1), likewise suppressed carrageenin paw oedema. 5. BW755C (25-100 mg kg-1, p.o.) dose-dependently reduced plasma leakage in the RPA, whereas indomethacin (5 mg kg-1, p.o.) or BWB70C either alone or in combination, did not. 6. SOD-PEG (300-3000 u, i.v.) dose-dependently inhibited plasma leakage in the RPA. In addition, the iron chelator and peroxyl radical scavenger, desferrioxamine (200 mg kg-1, s.c.) also inhibited plasma leakage. 7. Pretreatment with dexamethasone (0.1 mg kg-1) or colchicine (1 mg kg-1) reduced the plasma leakage in RPA, whereas MK-886 (10 mg kg-1) had no effect. 8. These results indicate an important role for oxygen radicals but not arachidonic acid metabolites in the maintenance of carrageenin paw oedema and the plasma leakage in RPA. Furthermore, the results suggest that the anti-inflammatory actions of BW755C can be dissociated from its effects on arachidonic acid metabolism and are attributed to its anti-oxidant activity.

Epidermal growth factor-induced actin remodeling is regulated by 5-lipoxygenase and cyclooxygenase products

In a number of cell types, epidermal growth factor (EGF) evokes dramatic morphological changes, cortical actin polymerization, and stress fiber breakdown. The molecular processes by which increased EGF receptor tyrosine kinase activity results in actin reorganization and morphological changes are unresolved. Recently, we demonstrated that arachidonic acid metabolites function in EGF signal transduction. We now report that in A431 cells, HeLa cells, and rat-1 fibroblasts, the EGF-induced cortical actin polymerization is produced by lipoxygenase metabolism, whereas in these cells stress fiber breakdown is mediated by cyclooxygenase metabolites. Also, the EGF-provoked rounding up in A431 cells is dependent on arachidonic acid metabolism. We conclude that leukotrienes and prostaglandins act in concert, as second messengers, to produce morphological effects and actin reorganization, providing a novel mechanism for directing growth factor-induced cytoskeletal changes.

5-lipoxygenase-activating protein stimulates the utilization of arachidonic acid by 5-lipoxygenase

5-Lipoxygenase (5-LO) and its activating protein (FLAP) are both required for cellular leukotriene (LT) synthesis, with 5-LO catalyzing both the synthesis of (5S)-5-hydroperoxy-6,8,11,14-eicosatetraenoic acid (5-HPETE) from arachidonic acid and the subsequent synthesis of LTA4 from 5-HPETE. We have previously expressed both human 5-LO and human FLAP to high levels in Spodoptera frugiperda (Sf9) insect cells, using recombinant baculoviruses. To study the mechanism by which FLAP activates 5-LO, we compared cellular 5-LO activity in Sf9 cells expressing this enzyme to that in Sf9 cells coexpressing FLAP and 5-LO. In this system, FLAP stimulates the utilization of arachidonic acid by 5-LO as a substrate, and increases the efficiency with which 5-LO converts 5-HPETE to LTA4. LT synthesis in cells coexpressing FLAP and 5-LO is inhibited by 3-[1-(p-chlorophenyl)-5-isopropyl-3-tert-butylthio-1H-indol-2-yl]-2,2- dimethyl-propanoic acid (MK-886), an LT biosynthesis inhibitor which specifically binds to FLAP. These studies in Sf9 cells, together with our recent demonstration that FLAP specifically binds arachidonic acid, suggests that FLAP activates 5-LO by acting as an arachidonic acid transfer protein.

Lipoxin A4 modulates transmigration of human neutrophils across intestinal epithelial monolayers

Neutrophil (PMN) migration across intestinal epithelial barriers, such as occurs in many disease states, results in modifications in epithelial barrier. Here, we investigated the impact of lipoxin A4 (LXA4), an eicosanoid with counterregulatory inflammatory roles, on PMN migration across cultured monolayers of the human intestinal epithelial cell line T84. Transepithelial migration of PMN was assessed in the apical-to-basolateral direction and in the basolateral-to-apical direction. In the apical-to-basolateral direction, preexposure of PMN to LXA4 (10 nM, 15 min) stimulated an 87 +/- 5% increase in transepithelial migration of PMN as determined by a PMN myeloperoxidase assay. The LXA4-elicited effect on transmigration was present throughout the 2-h assay period and was not secondary to LXA4 effects on epithelial monolayer integrity as judged by measurement of transepithelial resistance. PMN migration in the basolateral-to-apical direction was modulated by LXA4 with a comparable time- and concentration-dependence to that in the apical-to-basolateral direction. However, qualitative differences in how LXA4 modulates transmigration in the two opposing directions were observed. In the basolateral-to-apical direction, preexposure of PMN to LXA4 (10 nM, 15 min) diminished PMN transepithelial migration by 33 +/- 4%. Structure-function studies revealed that LXA4 and 11-trans-LXA4 (50% of LXA4 effect), but not LXB4, inhibited basolateral-to-apical PMN transmigration. The action of LXA4 was not sensitive to inhibitors of cyclooxygenase or specific leukotriene biosynthesis, but was sensitive to staurosporine, a protein kinase C inhibitor. These results suggest that migration of PMN across epithelia in the physiological direction may be qualitatively different following PMN exposure to eicosanoids. We propose that such retention of PMN at this specific anatomic location may serve an important role in mucosal defense.

Nitric oxide and arachidonic acid modulation of calcium currents in postganglionic neurones of avian cultured ciliary ganglia

1. A study has been made of the modulation of high-voltage activated transient and sustained calcium currents in cultured neurones of avian ciliary ganglia by nitric oxide (NO) and arachidonic acid. 2. Sodium nitroprusside (100 microM) reduced the transient calcium current (ICa) on average by 31% and the sustained ICa by 32% during a test depolarization to +20 mV from a holding potential of -100 mV. This reduction was maintained for at least 30 min following a single application of sodium nitroprusside. 3. L-Arginine (270 microM) reduced the transient ICa on average by 28% and the sustained ICa by 22% and these effects were prevented by the presence of the NO-synthase competitive blocker NG-nitro-L-arginine methylester (L-NAME; 100 microM) in the bathing solution. 4. Arachidonic acid (50 microM) reduced the transient ICa on average by 28% and the sustained ICa by 33%. When added together, arachidonic acid (50 microM) and L-arginine (270 microM) produced the same effects as arachidonic acid alone. 5. Blocking the conversion of arachidonic acid to prostaglandins by addition of indomethacin (20 microM) to the bathing solution did not prevent the depression of either the transient or the sustained calcium current during application of arachidonic acid (50 microM). The effects of arachidonic acid were also not occluded by L-NAME (100 microM) when present in the bathing solution. 6. Inhibiting the biosynthesis of leukotrienes by applying L-663,536 (MK-886; 3 microM) to the bathing solution prevented the depression of both components of ICa during application of arachidonic acid (50 microM). 7. These results indicate that endogenous NO and arachidonic acid pathways are present in parasympathetic ciliary neurones, and that both act to depress high-voltage, gated, calcium channel activity.

WY-50,295 tromethamine, a novel, orally active 5-lipoxygenase inhibitor: biochemical characterization and antiallergic activity

WY-50,295 tromethamine demonstrates significant 5-lipoxygenase inhibitory activity with IC50 values ranging from 0.055 microM in rat peritoneal exudate cells, to 0.16 microM in mouse macrophages, 1.2 microM in human peripheral neutrophils and 8.1 microM in rat blood leukocytes. This activity appeared selective for 5-lipoxygenase as concentrations up to 10 microM in rat peritoneal exudate cells, and 1 microM in mouse macrophages did not effect prostaglandin generation. In non-cellular enzyme assays, WY-50,295 tromethamine displayed inhibitory activity against a soluble 5-lipoxygenase from guinea pig peritoneal exudate cells (IC50 = 5.7 microM), while it was essentially inactive against 12-lipoxygenase, 15-lipoxygenase, or prostaglandin H synthetase at concentrations up to 500 microM, or against human phospholipase A2 at concentrations up to 50 microM. In purified human blood neutrophils the inhibitory activity was reversible but did not appear dependent upon substrate concentration. IN contrast, in the guinea pig cell-free 5-lipoxygenase assay changing the arachidonic acid substrate concentration from 5 to 500 microM produced a concentration-dependent reduction in inhibitory activity. WY-50,295 tromethamine inhibited the release of peptidoleukotrienes from fragmented guinea pig lung with an IC50 of 0.63 microM. When administered p.o. with a 4 h pretreatment time, WY-50,295 tromethamine inhibited ex vivo leukotriene B4 production in rat blood leukocytes with an ED50 of 19.6 mg/kg. Against an ovalbumin-induced leukotriene dependent bronchoconstriction in anesthetized sensitized guinea pigs, WY-50,295 tromethamine inhibited the ovalbumin-induced bronchoconstriction with an i.v. ED50 of 2.5 mg/kg (5 min pretreatment) and a p.o. ED50 of 7.3 mg/kg (4 h pretreatment). Significant activity was also evident with an 18 h pretreatment. Thus WY-50,295 tromethamine is an potent and selective 5-lipoxygenase inhibitor in a number of in vitro systems. Additionally the compound is orally efficacious and has a long duration of action in an allergic bronchoconstriction model. This data suggests that WY-50,295 tromethamine may have utility in the treatment of asthma and other leukotriene-dependent pathologies.

Anti-leukotriene effects of WY-50,295 tromethamine in isolated guinea pig pulmonary tissues

The abilities of WY-50,295 tromethamine, a 5-lipoxygenase inhibitor, to inhibit antigen-induced leukotriene (LT) release from guinea pig lung fragments, and to prevent LTD4 or antigen-induced contraction of isolated guinea pig tracheal muscle were compared with the activities of zileuton and MK-886 (two selective 5-lipoxygenase inhibitors), and LY171883 (a LTD4 receptor antagonist). In fragmented guinea pig lung, WY-50,295 tromethamine inhibited antigen-induced LT release with an IC50 of 0.63 microM, and was 4.6- and 5.2-fold more potent than zileuton and MK-886, respectively. WY-50,295 tromethamine differed from these 5-lipoxygenase inhibitors, however, in that WY-50,295 tromethamine competitively antagonized LTD4-induced tracheal contractions (pA2 = 6.06) at concentrations that inhibited LT release. LY171883 was an effective LTD4 receptor antagonist (pA2 = 6.96), that only inhibited antigen-induced LT release at higher concentrations (IC50 = 7.9 microM). WY-50,295 tromethamine almost completely inhibited antigen-induced leukotriene-dependent tracheal contractions, whereas high concentrations of zileuton, MK-886, or LY171883 produced only partial inhibition. This partial inhibition was likely to result from 'breakthrough' 5-lipoxygenase activity, because combinations of zileuton plus MK-886 or zileuton plus LY171883, were more effective than zileuton, MK-886, or LY171883 alone. The greater efficacy of WY-50,295 tromethamine in the antigen-challenged guinea pig trachea is likely to result from its combined abilities to prevent LT biosynthesis and block LTD4 receptors.

Oral leukotriene inhibitor (MK-886) blocks allergen-induced airway responses

To elucidate the role of leukotrienes (LT) in allergic asthma in humans the effect of MK-886, an LT biosynthesis inhibitor, was evaluated on antigen-induced early (EAR) and late (LAR) asthmatic reactions and bronchial responsiveness to histamine. Eight atopic men participated in a two-part, double-blind, placebo-controlled, crossover trial. MK-886 was administered in two oral doses of 500 mg and 250 mg, 1 h before and 2 h after allergen inhalation, respectively. Biochemical effects of MK-886 were evaluated by the inhibition of urinary LTE4 excretion and calcium ionophore-stimulated LTB4 biosynthesis in whole blood ex vivo. MK-886 significantly inhibited the EAR by 58.4% (AUC0-3 h) and the LAR by 43.6% (AUC3-7 h) when compared with placebo (p < 0.01). There was no difference in PC20 histamine 30 h post allergen challenge between MK-886 and placebo (0.33 and 0.27 doubling doses, p > 0.1). MK-886 inhibited calcium ionophore-stimulated LTB4 production in whole blood (54.2 +/- 25.6%) for up to 6 h post allergen challenge. LTE4 excretion in urine was inhibited by 51.5% during the EAR by as much as 80% during the LAR. This indicates that LT play a role in allergen-induced asthmatic reactions in humans in vivo and that LT synthesis inhibitors such as MK-886 should be further explored for the treatment of asthma.

The role of 5-lipoxygenase products in preclinical models of asthma

BACKGROUND

The action of 5-lipoxygenase on arachidonic acid generates potent inflammatory mediators that may contribute to the pathophysiology of asthma.

METHODS

Using the potent and selective 5-lipoxygenase inhibitor BI-L-239, we have examined the role of 5-lipoxygenase products in three animal models of asthma.

RESULTS

In vitro BI-L-239 inhibited 5-lipoxygenase product generation from human lung mast cells, alveolar macrophages, and peripheral blood leukocytes with a concentration that would provide 50% inhibition values of 28 to 340 nmol/L. A 36-fold selectivity for immunoreactive leukotriene C4 versus immunoreactive prostaglandin D2 inhibition was demonstrated in mast cells. In anesthetized cynomolgus monkeys, inhaled BI-L-239 provided dose-dependent inhibition of the inhaled Ascaris-induced immunoreactive leukotriene C4 release (maximum, 73%; bronchoalveolar lavage [BAL], 20 minutes), late-phase bronchoconstriction (maximum, 41%; +6 to 8 hours), and neutrophil infiltration (maximum, 63%; BAL, +8 hours). In conscious sheep, inhaled BI-L-239 provided dose-dependent inhibition of the inhaled Ascaris-induced late-phase bronchoconstriction (maximum, 66%; +6 to 8 hours) and increase in airway responsiveness (maximum, 82%; carbachol, +24 hours). The acute bronchoconstriction was shortened, and neutrophil infiltration diminished (maximum, 61%; BAL, +8 hours) in this model. Finally in conscious actively sensitized guinea pigs pretreated with pyrilamine and indomethacin, inhaled BI-L-239 attenuated acute bronchoconstriction (maximum, 80%; +5 to 15 minutes), leukocyte infiltration (58%; BAL, +3 days) and increase in airway responsiveness (100%; methacholine, +3 days) induced by three alternate-day ovalbumin inhalations.

CONCLUSIONS

In conclusion, results in these three animal models indicate that 5-lipoxygenase products may be major contributors to the bronchoconstriction (especially late phase), leukocyte infiltration, and airway hyperresponsiveness that characterize asthma.

Leukotriene D4 is a mediator of proteinuria and glomerular hemodynamic abnormalities in passive Heymann nephritis

We assessed the role of leukotrienes (LTs) in Munich-Wistar rats with passive Heymann nephritis (PHN), an animal model of human membranous nephropathy. 10 d after injection of anti-Fx1A antibody, urinary protein excretion rate (Upr) in PHN was significantly higher than that of control. Micropuncture studies demonstrated reduced single nephron plasma flow and glomerular filtration rates, increased transcapillary hydraulic pressure difference, pre- and postglomerular resistances, and decreased ultrafiltration coefficient in PHN rats. Glomerular LTB4 generation from PHN rats was increased. Administration of the 5-LO activating protein inhibitor MK886 for 10 d markedly blunted proteinuria and normalized glomerular hemodynamic abnormalities in PHN rats. An LTD4 receptor antagonist SK&F 104353 led to an immediate reduction in Upr and to reversal of glomerular hemodynamic impairment. Ia(+) cells/glomerulus were increased in PHN rats. In x-irradiated PHN rats, which developed glomerular macrophage depletion, augmented glomerular LT synthesis was abolished. Thus, in the autologous phase of PHN, LTD4 mediates glomerular hemodynamic abnormalities and a hemodynamic component of the accompanying proteinuria. The synthesis of LTD4 likely occurs directly from macrophages or from macrophage-derived LTA4, through LTC4 synthase in glomerular cells.

5-lipoxygenase-activating protein is an arachidonate binding protein

5-Lipoxygenase-activating protein (FLAP) is an 18-kDa integral membrane protein which is essential for cellular leukotriene (LT) synthesis, and is the target of LT biosynthesis inhibitors. However, the mechanism by which FLAP activates 5-LO has not been determined. We have expressed high levels of human FLAP in Spodoptera frugiperda (Sf9) insect cells infected with recombinant baculovirus, and used this system to demonstrate that FLAP specifically binds [125I]L-739,059, a novel photoaffinity analog of arachidonic acid. This binding is inhibited by both arachidonic acid and MK-886, an LT biosynthesis inhibitor which specifically interacts with FLAP. These studies suggest that FLAP may activate 5-LO by specifically binding arachidonic acid and transferring this substrate to the enzyme.

Effect of 5-lipoxygenase inhibitors on in situ LTB4 biosynthesis following calcium ionophore stimulation in the rat pleural cavity

The intrapleural injection of carrageenan in the rat induces exudate formation, cellular influx and leukotriene generation in the pleural cavity. We have demonstrated that the inflammatory response (exudate volume, and LTB4 levels) is increased in situ by the intrapleural administration of calcium ionophore A 23187 (100 nmol/rat) at 4, 16, 24, 48, and 72 h after the injection of carrageenan and that the A 23187-induced increase is dose-dependent. The oral administration of A-64077 and MK-886, two 5-lipoxygenase inhibitors (5-LOIs), at 10 mg/kg causes marked decreases in LTB4 release at the above-mentioned time intervals. However, A 23187-induced augmented exudate formation is not affected by the treatment with 5-LOIs. The results suggest that the use of 5-LOIs to inhibit LTB4 biosynthesis may be beneficial in various LTB4-dependent pathological conditions.

In vitro pharmacology of BAY X1005, a new inhibitor of leukotriene synthesis

BAY X1005, (R)-2-[4-(quinolin-2-yl-methoxy)phenyl]-2-cyclopentyl acetic acid, is an enantioselective inhibitor of leukotriene biosynthesis. It effectively inhibits the synthesis of LTB4 in A23187-stimulated leukocytes from rats, mice and humans (IC50 0.026, 0.039 and 0.22 mumol/l, respectively) as well as the formation of LTC4 (IC50 0.021 mumol/l) in mouse peritoneal macrophages stimulated with opsonized zymosan. The compound is, however, less active in inhibiting LTB4 synthesis in human whole blood (IC50 17.0 and 11.6 mumol/l, as measured by RIA or HPLC, respectively). BAY X1005 exhibits a high enantioselectivity in human whole blood (31 times over the (S)-enantiomer). BAY X1005 is shown to be a selective inhibitor of the formation of 5-lipoxygenase-derived metabolites in vitro, without effects on other routes of arachidonic acid metabolism such as 12-lipoxygenase in human whole blood and cyclooxygenase in both mouse macrophages and human whole blood. BAY X1005 is devoid of any antioxidant activity (methemoglobin induction and xanthine-xanthine oxidase assay), without effects on granule release and with only weak effects on reactive oxygen species generation in human PMNL.

Evidence that endogenous generation of leukotrienes does not regulate proliferation of malignant hemopoietic cell lines

The proliferation of malignant hemopoietic cell lines is inhibited by antagonists of 5-lipoxygenase, suggesting that the endogenous generation of leukotrienes via the action of this enzyme may play some role in the proliferation of these cells (Snyder D. S., Castro R. & Desforges J. F. (1989), Expl Hemat. 17, 6). Here we have confirmed that the lipoxygenase inhibitors piriprost, nordihydroguiaretic acid and BW755C decreased DNA synthesis and proliferation of leukemic cell lines. However, the concentrations of these drugs required for half-maximal inhibition of proliferation were significantly greater than their IC50 values for 5-lipoxygenase inhibition. We therefore studied the actions of two novel, potent lipoxygenase inhibitors, BWA4C and MK886, on proliferation (as measured by estimating the number of viable, trypan blue-excluding cells) and DNA synthesis (measured by the incorporation of radiolabeled thymidine) in the leukemia cell lines HL60, K562 and Jurkat. Neither parameter was affected by concentrations of these drugs which were shown in parallel studies to substantially inhibit leukotriene generation in whole blood. The data show that endogenous leukotriene generation does not play a significant role in the regulation of proliferation of these leukemic cell lines and suggest that conclusions about leukotriene involvement in the control of cellular metabolic pathways based on the use of lipoxygenase inhibitors should be re-assessed.

Do infiltrating neutrophils contribute to the pathogenesis of indomethacin induced ulceration of the rat gastric antrum?

The potential involvement of neutrophils in the pathogenesis of indomethacin induced ulceration of the gastric antrum in the re-fed rat was studied. Indomethacin was associated with a time dependent increase in the extent and severity of ulceration, blood neutrophilia, neutrophil infiltration into the gastric antrum, and calcium ionophore induced immunoreactive leukotriene B4 (LTB4) release from the antrum ex vivo. Neutrophil infiltration into the antrum was detectable 1 hour after dosing with indomethacin, at which time damage was apparent microscopically but not macroscopically. Thus, cell infiltration may contribute to the development, if not the initiation, of ulceration. Consistent with this suggestion, oral dexamethasone (5 mg/kg) significantly attenuated indomethacin induced ulceration, the associated neutrophil infiltration, and calcium ionophore induced immunoreactive leukotriene B4 release from the gastric antrum and whole blood ex vivo, although the blood neutrophilia was unaffected. These results suggest that indomethacin induced ulceration of the rat gastric antrum may have a dependence on neutrophil infiltration for its pathogenesis.

Mode of action of the new selective leukotriene synthesis inhibitor BAY X 1005 ((R)-2-[4-(quinolin-2-yl-methoxy)phenyl]-2-cyclopentyl acetic acid) and structurally related compounds

BAY X 1005 ((R)-2-[4-(quinolin-2-yl-methoxy)phenyl]-2-cyclopentyl acetic acid) has been demonstrated to be a potent inhibitor of leukotriene B4 (LTB4) and 5-hydroxyeicosatetraenoic acid (5-HETE) synthesis in various in vitro systems. Using mainly human polymorphonuclear leukocytes (PMNL) this study elucidates the mechanism of inhibition of 5-lipoxygenase (5-LOX, EC 1.13.11.34)-derived arachidonic acid metabolites by BAY X 1005. At concentrations of BAY X 1005 which almost totally inhibited the formation of 5-LOX-derived metabolites, both arachidonic acid release and platelet-activating factor synthesis were only modestly affected. This suggests that the inhibitory effect of BAY X 1005 is not due to a limitation of substrate availability for 5-LOX. Compared to the inhibition of leukotriene synthesis in intact human PMNL about 800-fold higher concentrations of BAY X 1005 were required to inhibit leukotriene formation in a cell-free system suggesting that the inhibitory effect of BAY X 1005 cannot be explained by a direct effect on 5-LOX. In an attempt to identify possible target proteins of BAY X 1005, [14C]BAY X 1005 was used in binding studies under equilibrium conditions. The quantitative analysis of specific binding in intact human PMNL revealed two binding sites for BAY X 1005. Upon subcellular fractionation of these cells the BAY X 1005 high affinity binding site was localized in the microsomal fraction whereas the low affinity binding site was localized in the granule fraction. The Kd for BAY X 1005 binding to the high affinity binding site (0.165 mumol/L) was almost identical to the IC50 value for inhibition of LTB4 synthesis (0.22 mumol/L). Furthermore, the IC50 values for competition of BAY X 1005 binding at the high affinity binding site were almost identical to the IC50 values for inhibition of LTB4 synthesis in the case of BAY X 1005, 12 other structurally related quinoline derivatives and the reference compounds REV-5901, WY-50,295 and MK-886, but not in the case of the direct 5-LOX inhibitors A-64077 and AA-861. The analysis of BAY X 1005 binding in rat PMNL also revealed two binding sites. Whereas the low affinity binding site in rat PMNL exhibited a Kd similar to the human, the rat high affinity binding site showed a 5.5-fold higher affinity for BAY X 1005 compared to the human. This correlates well with the 8.5-fold higher sensitivity of rat versus human PMNL concerning inhibition of LTB4 synthesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Platelet-activating factor and interleukin 1 are involved in colonic dysmotility in experimental colitis in rats

BACKGROUND

Intracolonic administration of trinitrobenzene sulfonic acid (TNBS) to rats produces chronic colitis associated with an increased release of eicosanoids, platelet-activating factor (PAF), and interleukins.

METHODS

Motor effects of TNBS on proximal colon were evaluated electromyographically in rats. Mediator involvement was investigated using eicosanoids and PAF antagonists.

RESULTS

The colonic myoelectrical activity was 59 +/- 17 spike bursts per hour lasting 6.9 +/- 1.3 seconds. Two to eight days after TNBS treatment, spike-burst duration was significantly (P < 0.05) higher, with a maximal 1.5-4-fold enhancement at day 3. These alterations were significantly (P < 0.05) reduced by daily treatment with MK-886, a 5-lipoxygenase inhibitor (10 mg/kg, orally), whereas indomethacin (1 mg/kg per day, intramuscularly) was ineffective. At day 3, RP55778, a PAF antagonist (45, 60 mg/kg, intraperitoneally), and rIRAP, an interleukin 1 antagonist (0.3 mg/kg, intraperitoneally) but not KT1-32, a thromboxane A2 antagonist (30, 60 mg/kg orally), nor SKF104,353, a leukotriene D4 antagonist (2, 4 mg/kg, orally), significantly (P < 0.05) reduced the TNB-induced motor effects.

CONCLUSION

TNBS-induced colitis in rats involves a delayed long-lasting dysmotility involving PAF, interleukin 1, and some leukotrienes but not leukotriene D4, thromboxane A2, or other cyclo-oxygenase products.

Are leukotrienes or PAF involved in hyperbaric oxygen toxicity?

Several very selective leukotriene inhibitors, and a PAF inhibitor, suitable for in vivo use, have been tested for their effects on hyperbaric oxygen toxicity. The leukotriene D4 inhibitor, L660771, and the 5-lipoxygenase pathway inhibitor L663536, failed to affect convulsions or lung damage induced by hyperbaric oxygen (pressure range 515-615 kPa) in either rats or mice. The specific PAF antagonist L659989 showed marginal protection against hyperoxic convulsions and did not alter pulmonary damage. The specific LTB4 antagonist SC-41930 was very effective in inhibiting hyperbaric oxygen-induced convulsions in both rats and mice. SC-41930 also very significantly protected rats against pulmonary oxygen toxicity, but had only marginally significant effects on pulmonary protection in mice.

The effect of inhibition of leukotriene B4 release on lipopolysaccharide-induced production of neutrophil attractant/activation protein-1 (interleukin-8) by human alveolar macrophages

Whereas we observed previously that concentrations of the lipoxygenase inhibitor nordihydroguaiaretic acid that inhibited leukotriene B4 release from lipopolysaccharide-stimulated human alveolar macrophages in vitro also inhibited subsequent interleukin-8 release, we hypothesized that leukotriene B4 release was required for the release of interleukin-8. Alveolar macrophages from normal nonsmoking volunteers were adhered to plastic and incubated with varying concentrations (25-250nM) of the 5-lipoxygenase activating protein inhibitor MK-886 prior to stimulation with lipopolysaccharide. MK-886 inhibited leukotriene B4 release in a concentration-dependent manner. The concentration of MK-886 that inhibited release by 50% was 53.3 +/- 23.1nM (mean +/- SD), n = 4. Interleukin-8 concentrations in 24hr supernatants were not inhibited by incubation of the cells with any concentration of MK-886, including those that inhibited leukotriene B4 release by > 95%. Thus, MK-886 is an effective inhibitor of human alveolar macrophage release of leukotriene B4, and the release of leukotriene B4 is not a prerequisite for alveolar macrophage release of interleukin-8.

Effect of BI-L-239, A-64077 and MK-886 on leukotriene B4 synthesis by chopped guinea pig lung and on antigen-induced tracheal contraction in vitro

The 5-lipoxygenase (5-LO) inhibitors BI-L-239 and A-64077 were compared with the 5-LO translocation inhibitor MK-886 for the ability to inhibit leukotriene B4 (LTB4) biosynthesis by chopped (1 mm3) guinea pig lung. LTB4 synthesis by ovalbumin-sensitized chopped lung tissue was determined after stimulation with either calcium ionophore (A23187) or antigen. With A23187 stimulation, MK-886 was more potent (IC50 = 0.39 +/- 0.23 microM, mean +/- SEM, p < 0.01) than BI-L-239 (IC50 = 2.48 +/- 0.46 microM) or A-64077 (IC50 = 4.68 +/- 0.70 microM) and BI-L-239 was more potent than A64077 (p < 0.02). Thus, the order of potency was MK-886 > BI-L-239 > A-64077 for inhibition of calcium ionophore-induced LTB4 generation. There was no significant differences in potency of the compounds in chopped lung stimulated with antigen: IC50 for LTB4 synthesis by A-64077 = 3.31 +/- 1.70 microM, for BI-L-239 = 9.06 +/- 4.94 microM, and for MK-886 = 13.33 +/- 7.91 microM. The ability of these compounds to inhibit contraction of tracheal tissue from actively sensitized guinea pigs in response to antigen was also determined in the presence of indomethacin (15 micrograms/ml), mepyramine, and atropine (5 micrograms each/ml). Both 5-LO inhibitors inhibited antigen-induced contraction, with IC50 values for BI-L-239 and A-64077 of 1.58 and 4.35 microM respectively. MK-886 was ineffective at inhibiting antigen-induced tracheal contraction in vitro at concentrations up to 30 microM. In summary, these compounds inhibit antigen-induced and A23187-induced leukotriene biosynthesis in guinea pig tissue. These 5-LO inhibitors were similarly effective at inhibiting antigen-induced tracheal contraction where MK-886 was ineffective.

Leukotriene B4 mediates shear rate-dependent leukocyte adhesion in mesenteric venules

Previous studies have demonstrated that low shear rates promote leukocyte adherence to microvascular endothelium in postcapillary venules. The objective of this study was to determine whether an accumulation of inflammatory mediators such as platelet activating factor and leukotriene B4 is responsible for shear rate-dependent leukocyte-endothelial cell adhesion. Postcapillary venules (25-39 microns in diameter) in cat mesentery were studied by intravital microscopy. Venular wall shear rate was varied over a wide range by graded occlusion of the mesenteric artery. Red blood cell velocity, vessel diameter, leukocyte rolling velocity, and the numbers of rolling and adherent leukocytes were measured at each shear rate. In one series of experiments, shear rate-dependent leukocyte adherence was monitored at different superfusion rates (1.0 and 2.5 ml/min). At the lower superfusion rate, the number of adherent leukocytes was significantly higher at any given shear rate when compared with results obtained at the higher superfusion rate. This suggests that reduced washout of inflammatory mediators contributes to shear rate-dependent leukocyte adhesion. Pretreatment with different platelet activating factor receptor antagonists (WEB 2086 or WEB 2170) had no effect on the number of adherent leukocytes normally observed at lower shear rates, suggesting that platelet activating factor does not play a major role in this process. However, shear rate-dependent leukocyte adhesion was largely prevented by pretreatment with either a leukotriene B4 receptor antagonist (SC-41930) or a leukotriene synthesis inhibitor (L663,536). The results of this study indicate that a reduced washout of leukotriene B4 is responsible for the enhanced leukocyte adherence that occurs at low venular wall shear rates.

Relationship between histamine, lipoxygenase and cyclooxygenase products in antigen-induced contraction in guinea-pig tracheal tube preparations

We have used a tracheal tube preparation to study antigen-induced contraction in sensitized guinea pig airways. Treatment with both the cyclooxygenase inhibitor indomethacin and the lipoxygenase inhibitor MK-886 (L-663,536) affected this contraction in preparations with intact epithelium. Indomethacin potentiated and MK-886 inhibited part of the contraction. Leukotriene release from tracheal tubes was measured after antigen challenge, and was found to be significant in preparations with an intact epithelium. When the epithelium was removed, the histamine receptor antagonist mepyramine reduced antigen-induced contraction by 90%. Our results show that when the epithelium is absent, histamine is the most important mediator in the contraction. With the epithelium left intact, the contraction is more complex: both the cyclooxygenase and lipoxygenase pathways are involved, and our findings indicate that eicosanoid production is associated with the airway epithelium.

Nonessential role of leukotrienes as mediators of acute gastric mucosal injury induced by aspirin in rats

The present study was designed to determine the role of leukotrienes in aspirin-induced acute gastric mucosal injury in rats. We examined the effects of aspirin, indomethacin, and sodium salicylate on gastric mucosal injury, and on eicosanoid synthesis and content. Aspirin, indomethacin, and acidified salicylate caused significant mucosal injury, while salicylate at pH 7 did not induce significant injury. Aspirin and indomethacin significantly reduced mucosal prostaglandin synthesis and content. No significant changes in mucosal leukotriene C4 synthesis and content were observed. There were no correlations between changes in mucosal leukotriene B4 synthesis and the extent of mucosal injury. We also evaluated the effects of MK-571 (a leukotriene D4 receptor antagonist) and MK-886 (a leukotriene biosynthesis inhibitor) on aspirin-induced gastric mucosal injury. Neither MK-571 nor MK-886 could reduce the mucosal lesions induced by aspirin. These findings suggest that leukotrienes are not involved in aspirin-induced acute gastric mucosal injury in rats.

Excessive production of reactive oxygen metabolites by inflamed colon: analysis by chemiluminescence probe

Reactive oxygen metabolites (ROMs) are involved in inflammatory diseases and are postulated to contribute to tissue injury in colitis. To determine whether excessive ROMs are generated by inflamed colonic mucosa and to identify possible sources and type of ROMs, mucosal ROMs were estimated in rats and humans using a chemiluminescence probe. Colitis was induced in rats by intracolonic injection of acetic acid or intraperitoneal injection of mitomycin C. Intact, inflamed colon in rats produced more ultraweak chemiluminescence than normal colon. Inflamed mucosal scrapings from both rat models produced significantly more luminol-enhanced chemiluminescence. Addition of catalase, an H2O2 scavenger, or azide, a myeloperoxidase inhibitor, into the media significantly decreased chemiluminescence from inflamed mucosal scrapings. Indomethacin, an antioxidant cyclo-oxygenase inhibitor, also decreased chemiluminescence, but MK-866, a 5-lipoxygenase inhibitor, had no effect. Colonic biopsy specimens obtained during colonoscopy from patients with ulcerative colitis also produced more catalase-inhibitable chemiluminescence than normal colonic mucosa. These data indicate that excessive ROMs are produced by inflamed colonic mucosa in both humans and rats, which may contribute to tissue injury.

Modulation of leukocyte adhesion in rat mesenteric venules by aspirin and salicylate

Erythrocyte velocity, vessel diameter, leukocyte rolling velocity, and number of adherent and emigrated leukocytes were measured in postcapillary venules both before and during superfusion of rat mesentery with either aspirin or sodium salicylate. In some experiments, animals were treated with either a leukotriene (LT)-synthesis inhibitor (L-663,536), an LTD4 antagonist (MK-571), an LTB4 antagonist (SC-41930), misoprostol, or prostaglandin (PG) I2, then the aspirin protocol was repeated. Superfusion of aspirin but not sodium salicylate resulted in increased leukocyte adherence and a reduced leukocyte rolling velocity but did not affect leukocyte emigration. Aspirin-induced leukocyte adhesion was effectively prevented by the LT-synthesis inhibitor and LTB4 antagonist but not by the LTD4 antagonist. Misoprostol and PGI2 also prevented the aspirin-induced adhesion responses. Superfusion of the mesentery with either platelet-activating factor (PAF) or LTB4 enhanced leukocyte adherence and emigration while reducing leukocyte rolling velocity. Sodium salicylate prevented all of the adhesion responses elicited by LTB4. Although salicylate did not affect the PAF-induced leukocyte adherence and rolling responses, it completely prevented the increased leukocyte emigration. These results indicate that aspirin promotes, whereas sodium salicylate inhibits, leukocyte-endothelial cell adhesive interactions at therapeutically relevant concentrations.