Structural requirements for chemotactic activity of leukotriene B4 (LTB4)

Super-Low Dose Lipopolysaccharide Dysregulates Neutrophil Migratory Decision-Making

Neutrophils are the first responders to infection and play a pivotal role in many inflammatory diseases, including sepsis. Recent studies have shown that lipopolysaccharide (LPS), a classical pattern recognition molecule, dynamically programs innate immune responses. In this study, we show that pre-treatment with super-low levels of LPS [1 ng/mL] significantly dysregulate neutrophil migratory phenotypes, including spontaneous migration and altering neutrophil decision-making. To quantify neutrophil migratory decision-making with single-cell resolution, we developed a novel microfluidic competitive chemotaxis-chip (μC³) that exposes cells in a central channel to competing chemoattractant gradients. In this reductionist approach, we use two chemoattractants: a pro-resolution (N-Formyl-Met-Leu-Phe, fMLP) and pro-inflammatory (Leukotriene B₄, LTB₄) chemoattractant to model how a neutrophil makes a decision to move toward an end target chemoattractant (e.g., bacterial infection) vs. an intermediary chemoattractant (e.g., inflammatory signal). We demonstrate that naïve neutrophils migrate toward the primary end target signal in higher percentages than toward the secondary intermediary signal. As expected, we found that training with high dose LPS [100 ng/mL] influences a higher percentage of neutrophils to migrate toward the end target signal, while reducing the percentage of neutrophils that migrate toward the intermediary signal. Surprisingly, super-low dose LPS [1 ng/mL] significantly changes the ratios of migrating cells and an increased percentage of cells migrate toward the intermediary signal. Significantly, there was also an increase in the numbers of spontaneously migrating neutrophils after treatment with super-low dose LPS. These results shed light onto the directional migratory decision-making of neutrophils exposed to inflammatory training signals. Understanding these mechanisms may lead to the development of pro-resolution therapies that correct the neutrophil compass and reduce off-target organ damage.

Neutrophil chemotactic activity of sputum from patients with COPD: role of interleukin 8 and leukotriene B4

STUDY OBJECTIVES

Neutrophilic inflammation is a major feature of COPD. Several factors in bronchial secretions have been identified as chemoattractants for neutrophils. The present study was designed to assess the contribution of interleukin (IL)-8 and leukotriene B(4) (LTB(4)) to neutrophil chemotaxis evoked by sputum obtained from patients with established COPD.

DESIGN

Sputum supernatant of 20 patients with COPD was used as chemoattractant in a 96-well chemotaxis chamber, with subsequent quantification of migrated cells by a luminescence assay. The contribution of IL-8 and LTB(4) to chemotaxis was determined by addition of a neutralizing antibody and a selective receptor antagonist, respectively.

MEASUREMENTS AND RESULTS

COPD sputum caused neutrophil chemotaxis in a concentration-dependent manner, with a maximum response evoked with a 10-fold dilution of the original sample. Pretreatment of sputum or neutrophils with either an anti-IL-8 antibody or the LTB(4) antagonist, SB 201146, led to a concentration-dependent inhibition of sputum-induced neutrophil chemotaxis, with a maximum suppression (mean +/- SEM) of 29.2 +/- 4.9% (p < 0.001) from baseline by 100 ng/mL of anti-IL-8 antibody, and 45.6 +/- 7% (p < 0.02) by 10 micro mol/L of SB 201146. The combination of the anti-IL-8 antibody and SB 201146 inhibited neutrophil chemotaxis, but this was not significantly greater than the effect of SB 201146 or anti-IL-8 alone.

CONCLUSIONS

These data confirm the importance of IL-8 and LTB(4) as chemoattractants for neutrophils in bronchial secretions from patients with COPD, and suggest that specific inhibitors may have therapeutic potential in COPD.

Reduction by inhibitors of mono(ADP-ribosyl)transferase of chemotaxis in human neutrophil leucocytes by inhibition of the assembly of filamentous actin

1. Chemotaxis of human neutrophils is mediated by numerous agents [e.g. N-formyl-methionyl-leucyl-phenylalanine (FMLP) and platelet activating factor (PAF)] whose receptors are coupled to phospholipase C. However, the subsequent transduction pathway mediating cell movement remains obscure. We now propose involvement of mono(ADP-ribosyl)transferase activity in receptor-dependent chemotaxis. 2. Human neutrophils were isolated from whole blood and measurements were made of FMLP or PAF-dependent actin polymerization and chemotaxis. The activity of cell surface Arg-specific mono(ADP-ribosyl)transferase was also measured. Each of these activities was inhibited by vitamin K3 and similar IC50 values obtained (4.67 +/- 1.46 microM, 2.0 +/- 0.1 microM and 4.7 +/- 0.1 microM respectively). 3. There were similar close correlations between inhibition of (a) enzyme activity and (b) actin polymerization or chemotaxis by other known inhibitors of mono(ADP-ribosyl)transferase, namely vitamin K1, novobiocin, nicotinamide and the efficient pseudosubstrate, diethylamino(benzylidineamino)guanidine (DEA-BAG). 4. Intracellular Ca2+ was measured by laser scanning confocal microscopy with two fluorescent dyes (Fluo-3 and Fura-Red). Exposure of human neutrophils to FMLP or PAF was followed by transient increases in intracellular Ca2+ concentration, but the inhibitors of mono(ADP-ribosyl)transferase listed above had no effect on the magnitude of the response. 5. A panel of selective inhibitors of protein kinase C, tyrosine kinase, protein kinases A and G or phosphatases 1 and 2A showed no consistent inhibition of FMLP-dependent polymerization of actin. 6. We conclude that eukaryotic Arg-specific mono(ADP-ribosyl)transferase activity may be implicated in the transduction pathway mediating chemotaxis of human neutrophils, with involvement in the assembly of actin-containing cytoskeletal microfilaments.

Important contribution of the methylene part of LTB4 toward binding affinity to the LTB4 receptors and rise in intracellular-free calcium concentration

In order to examine a role of the C(16)-C(20) methylene part of leukotriene B4 (LTB4) toward the activation of leukocytes, we synthesized the LTB4-analogues in which the length of the C(16)-C(20) part of LTB4 is varied systematically while the two hydroxyl groups at C(5) and C(12) positions and the 6(Z), 8(E), 10(E) conjugated triene unit remained untouched. We examined their binding affinity to the LTB4 receptors present in the rat polymorphonuclear leukocytes (PMNLs) and their ability to raise intracellular-free calcium concentration ([Ca2+]i) in the rat PMNLs loaded with fura-2. As the length of the chain of LTB4 was increased or decreased one by one, the binding affinity to the LTB4 receptors diminished, and the analogues of more than three carbon atoms shorter chain were of about three log order less activity than LTB4. The biological potency as assessed in [Ca2+]i rises pararelled that of the binding affinity to the PMNL membrane. These results indicate that the C(16)-C(20) part of LTB4 plays important role for the activity. In a similar way we prepared the LTB4-analogues of a different chain length between C(2)-C(4) of LTB4 and tested their biological activity. We found that the C(2)-C(4) part of LTB4 also affects the activity.

Calcium mobilization and right-angle light scatter responses to 12-oxo-derivatives of arachidonic acid in neutrophils: evidence for the involvement of the leukotriene B4 receptor

The biological activities of two carbonyl compounds derived from arachidonic acid, (5Z,8Z,10E,14Z)-12-keto-5,8,10,14-eicosatetraeno ic acid (12-OxoETE) and (5Z,8Z,10E)-12-oxo-5,8,10-dodecatrienoic acid (12-OxoDTrE) were investigated. The ability of these compounds to induce a mobilization of calcium and to trigger a right-angle scatter response in isolated peripheral blood human neutrophils was determined. The two compounds induced a rapid and dose-dependent increase in the concentration of cytoplasmic free calcium; these effects were clearly detectable at concentrations greater than or equal to 10(-8) M. Pre-exposure of neutrophils to leukotriene B4 completely abolished the calcium mobilization induced by 12-OxoDTre and 12-OxoETE, while pre-exposure of the cells to the carbonyl compounds only slightly reduced the response to subsequent stimulation of neutrophils by leukotriene B4. The carbonyl compounds also induced a decrease in right-angle light scatter and these effects were abolished by pretreatment of neutrophils with leukotriene B4. These data demonstrate that 12-OxoETE and 12-OxoDTrE show significant agonist activities towards human neutrophils and strongly suggest that their mechanisms of action involve the leukotriene B4 binding sites or a common activation sequence.

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.

Characterization of the soluble leukotriene B4 receptor from sheep lung membranes

Leukotriene B4 (LTB4) is an arachidonate metabolite which elicits a variety of pro-inflammatory responses by activation of a guanine-nucleotide-binding protein-coupled membrane receptor. As a prelude to receptor isolation and purification, we have established assay methods for LTB4 receptor solubilization and characterization from sheep lung membranes. [3H]LTB4 binding to the soluble receptor was saturable, specific, protein-concentration- and time-dependent and reversible. Binding of [3H]LTB4 was enhanced by divalent cations and inhibited by sodium ions in a manner analogous to its binding to the human leukocyte membrane receptor. Saturation binding yielded a dissociation constant (Kd) of 0.50 +/- 0.05 nM and a receptor density (Bmax) of 330 +/- 90 fmol/mg of protein for [3H]LTB4 binding to detergent-solubilized receptor. In competition experiments, the rank order of binding affinity was LTB4 greater than 20-OH-LTB4 greater than trans-homo-LTB4 greater than 6-trans-LTB4 greater than U-75302. Gel-filtration chromatography showed that the LTB4 receptor protein in the detergent micellar state has a molecular mass in the range 800-1000 kDa. These results demonstrate that the physiologically and pharmacologically important LTB4 receptor may be readily solubilized from sheep lung membranes without alteration in binding specificity and characteristics, suggesting that sheep lung membranes represent a rich source with which to pursue receptor isolation and purification.

Acute antiinflammatory and gastric effects of the seleno-organic compound ebselen

Ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)-one), a seleno-organic compound with glutathione peroxidase-like activity in vitro, was compared with indomethacin, BW 755C, and levamisole as an inhibitor of carrageenan- and CVF (cobra venom factor)-induced paw oedema in the rat. The antiinflammatory potency of ebselen against CVF-induced oedema (ED50 = 56 mg/kg p.o.) was similar to that of BW 755C, while indomethacin was weakly active in this model, and levamisole exerted stronger activity. In the carrageenan model, ebselen exhibited weak inhibitory potency, like BW 755C, while indomethacin markedly inhibited this inflammatory response, and levamisole was inactive. Unlike cyclooxygenase inhibitors, ebselen produced almost no gastric irritation in rats up to 316 mg/kg p.o. Moreover, ebselen inhibited significantly diclofenac-induced gastric intolerance at 31.6 and 316 mg/kg p.o. Thus, ebselen represents a new tool for antiinflammatory therapy.

Transition of affinity states for leukotriene B4 receptors in sheep lung membranes

Leukotriene B4 (LTB4) is a pro-inflammatory arachidonate metabolite. We have characterized the LTB4 receptors in sheep lung membranes and have assessed the contribution of the guanine-nucleotide-binding (G) protein in the regulation of receptor affinity states. Saturation isotherms have demonstrated a single class of LTB4 receptor with a Kd of 0.18 +/- 0.03 nM and a density (Bmax.) of 410 +/- 84 fmol/mg of protein in sheep lung membranes. The effect of the G-protein on receptor affinity was assessed in the presence of non-hydrolysable GTP analogues (e.g. GTP[S]) and in membranes following alkali treatment (pH 12.1) to remove the G-protein. Saturation isotherms produced either in the presence of GTP[S] (Kd.GTP[S] = 0.51 +/- 0.02 nM) or with alkali-treated membranes (Kd.alk. = 0.52 +/- 0.02 nM) demonstrated a 3-fold shift in receptor affinity for [3H]LTB4 binding. In competition experiments, the rank order of affinity of LTB4 analogues was LTB4 greater than 20-OH-LTB4 greater than trans-homo-LTB4 greater than 6-trans-LTB4 greater than 20-COOH-LTB4, using either untreated or alkali-treated membranes, both in the presence and absence of GTP[S]. These findings demonstrate that, in sheep lung membranes, there is only one class of LTB4 receptor. Removal of the G-protein or uncoupling of the receptor from the G-protein shifted the agonist-binding affinity of the receptor by 3-4-fold, without affecting the specificity of the LTB4 receptor in either the high- or the low-affinity state.

Platelet-activating factor induces the production of leukotrienes by human monocytes

The aim of this study was to evaluate the role of platelet-activating factor (PAF) as a stimulator of leukotriene production by human monocytes. The production of leukotrienes was time- and concentration-dependent. Release of leukotrienes was half-maximal after 2 min and reached a maximum after 10 min. At a concentration of 10(-8) M, PAF induced the production of 0.14 +/- 0.01 ng LTB4/10(6) cells (mean +/- S.E., n = 8). At concentrations of 10(-6) M, PAF induced the production of 1.0 +/- 0.04 ng LTB4 and 0.22 +/- 0.03 ng peptidoleukotrienes (mean +/- S.E., n = 16). There was no metabolism of LTB4 as judged from stability of [3H]LTB4 added to the incubations. LTC4 was slowly metabolized by human monocytes to LTD4 and LTE4. The two specific PAF-receptor antagonists BN 52021 and WEB 2086 in concentrations of 10(-4) and 10(-6) M, respectively, inhibited the PAF (10(-6) M) stimulated LTB4 production completely. In this study, we demonstrate that nanomolar concentrations of PAF can stimulate the production of LTB4 and peptidoleukotrienes in human monocytes by a receptor-mediated mechanism.

Leukotrienes in experimental spinal cord injury

Leukotrienes are a group of noncyclized fatty acid eicosanoids which are formed from the breakdown of arachidonic acid. They are potent mediators of inflammation and may contribute to secondary injury in the central nervous system. All mammalian tissue including cerebral cortex is capable of synthesizing these; however, clear documentation of leukotriene formation in the spinal cord is lacking. We subjected 55 rabbits to weight drops of 200, 300, and 400 gm/cm, respectively, on an exposed spinal cord. The traumatized spinal cord was removed after periods of 15, 30, and 45 minutes and 1, 2, 4, 8, and 24 hours. Radioimmunoassay for leukotriene B-4 was then performed on the specimens. Significant (p less than 0.05) elevation was noted in the 200 and 300 gm/cm groups with peak levels occurring in the first 4 hours. The 400 gm/cm group showed significant depression of values below control levels from 2 to 24 hours. We conclude that in sublethal central nervous system injury leukotrienes are produced in significant amounts and may contribute to secondary spinal cord injury.

Failure of leukotriene B4 to translocate calcium across phosphatidylcholine membranes

It has been reported that leukotriene B4 can translocate calcium across model membranes (Serhan et. al., (1982) J. Biol. Chem., 257: 4746). Such ionophoretic behavior could account for its biological effects. We have examined the effect of chromatographically pure leukotriene B4 on Ca2+ permeability when added exogenously at 3 microM to phosphatidylcholine liposomes and when incorporated at 5 mole % in the lipid mixture used to prepare liposomes. No effect was observed with either procedure. An oxidized preparation of leukotriene B4 stimulated calcium permeability, however, suggesting that oxidation may account for the previously reported ionophoretic behavior of leukotriene B4.

Binding of leukotriene B4 and its analogs to human polymorphonuclear leukocyte membrane receptors

LTB4-induced proinflammatory responses in PMN including chemotaxis, chemokinesis, aggregation and degranulation are thought to be initiated through the binding of LTB4 to membrane receptors. To explore further the nature of this binding, we have established a receptor binding assay to investigate the structural specificity requirements for agonist binding. Human PMN plasma membrane was enriched by homogenization and discontinuous sucrose density gradient purification. [3H]-LTB4 binding to the purified membrane was dependent on the concentration of membrane protein and the time of incubation. At 20 degrees C, binding of [3H]-LTB4 to the membrane receptor was rapid, required 8 to 10 min to reach a steady-state and remained stable for up to 50 min. Equilibrium saturation binding studies showed that [3H]-LTB4 bound to high affinity (dissociation constant, Kd = 1.5 nM), and low capacity (density, Bmax = 40 pmol/mg protein) receptor sites. Competition binding studies showed that LTB4, LTB4-epimers, 20-OH-LTB4, 2-nor-LTB4, 6-trans-epi-LTB4 and 6-trans-LTB4, in decreasing order of affinity, bound to the [3H]-LTB4 receptors. The mean binding affinities (Ki) of these analogs were 2, 34, 58, 80, 1075 and 1275 nM, respectively. Thus, optimal binding to the receptors requires stereospecific 5(S), 12(R) hydroxyl groups, a cis-double bond at C-6, and a full length eicosanoid backbone. The binding affinity and rank-order potency of these analogs correlated with their intrinsic agonistic activities in inducing PMN chemotaxis. These studies have demonstrated the existence of high affinity, stereoselective and specific receptors for LTB4 in human PMN plasma membrane.

Analogs of leukotriene B4: effects of modification of the hydroxyl groups on leukocyte aggregation and binding to leukocyte leukotriene B4 receptors

The syntheses and agonist and binding activities of 5(S)-hydroxy- 6(Z), 8(E), 10(E), 14(Z)-eicosatetraenoic acid (12-deoxy LTB4), 5(S), 12(S)-dihydroxy-6(Z), 8(E), 10(E), 14(Z)-eicosatetraenoic acid (12-epi LTB4), 12(R)-hydroxy-6(Z), 8(E), 10(E), 14(Z)-eicosatetraenoic acid (5-deoxy LTB4), 5(R), 12(S)-dihydroxy-6(Z), 8(E), 10(E), 14(Z)-eicosatetraenoic acid (5-epi LTB4), 6(Z), 8(E), 10(E), 14(Z)-eicosatetraenoic acid (5, 12-deoxy LTB4) are described. These leukotriene B4 analogs were all able to aggregate rat leukocytes and compete with [3H]-leukotriene B4 for binding to rat and human leukocyte leukotriene B4 receptors with varying efficacy. The analog in which the 12-hydroxyl group was removed was severely reduced both in agonist action (aggregation) and binding. The epimeric 12-hydroxyl analog demonstrated better agonist and binding properties than the analog without a hydroxyl at this position. In contrast, in the case of the 5-hydroxyl the epimeric hydroxyl analog had greatly reduced agonist and binding activities while the 5-deoxy analog demonstrated potency only several fold less than leukotriene B4 itself. The dideoxy leukotriene B4 analog was more than a thousand fold less active than leukotriene B4 as an agonist and in binding to the leukotriene B4 receptor. These results show that binding to the leukocyte leukotriene B4 receptor requires a hydroxyl group at the 12 position in either stereochemical orientation but that the presence of a hydroxyl at the 5 position is less important. However, the epimeric C5 leukotriene B4 analog clearly interacts unfavourably with the binding site of the leukotriene B4 receptor.