Biology and pathophysiology of leukotrienes

The G protein-coupled receptor CysLT1 mediates chemokine-like effects and prolongs survival in chronic lymphocytic leukemia

The G protein-coupled receptor (GPCR) CXCR4 is involved in bone marrow tropism and survival of chronic lymphocytic leukemia (CLL) cells. The function of the GPCRs cysteinyl leukotriene receptor 1 (CysLT1) and CysLT2 remains elusive. Here we demonstrate that in CLL and normal B lymphocytes, CysLT1 mRNA is consistently expressed, in contrast to low CysLT2 levels. Similar to the CXCR4 ligand CXCL12, the cysteinyl leukotriene (cysLT) LTD(4) induces calcium fluxes, actin polymerization, and chemotaxis. These effects are blocked by specific CysLT1 antagonists. Their inhibition by pertussis toxin suggests Giα/o protein involvement. Furthermore, CysLT1 mediates MAP-kinase phosphorylation, which implicates contribution of cysLT to survival. Indeed, CysLT1 antagonists induce apoptosis and reduce viability independent of Gαi/o protein signaling. Considering the production of cysLTs in the bone marrow, our data suggest that CysLT1 induces chemokine-like effects, supports accumulation and survival of CLL cells in the bone marrow and thus represents a potential treatment target.

Ultrafine particles affect the balance of endogenous pro- and anti-inflammatory lipid mediators in the lung: in-vitro and in-vivo studies

Background

Exposure to ultrafine particles exerts diverse harmful effects including aggravation of pulmonary diseases like asthma. Recently we demonstrated in a mouse model for allergic airway inflammation that particle-derived oxidative stress plays a crucial role during augmentation of allergen-induced lung inflammation by ultrafine carbon particle (UfCP) inhalation. The mechanisms how particle inhalation might change the inflammatory balance in the lungs, leading to accelerated inflammatory reactions, remain unclear. Lipid mediators, known to be immediately generated in response to tissue injury, might be strong candidates for priming this particle-triggered change of the inflammatory balance.

Methods

We hypothesize that inhalation of UfCP may disturb the balance of pro- and anti-inflammatory lipid mediators in: i) a model for acute allergic pulmonary inflammation, exposing mice for 24 h before allergen challenge to UfCP inhalation (51.7 nm, 507 μg/m³), and ii) an in-vitro model with primary rat alveolar macrophages (AM) incubated with UfCP (10 μg/1 x 10⁶ cells/ml) for 1 h. Lungs and AM were analysed for pro- and anti-inflammatory lipid mediators, namely leukotriene B₄ (LTB₄), prostaglandin E₂ (PGE₂), 15(S)-hydroxy-eicosatetraenoic acid (15(S)-HETE), lipoxin A₄ (LXA₄) and oxidative stress marker 8-isoprostane by enzyme immunoassays and immunohistochemistry.

Results

In non-sensitized mice UfCP exposure induced a light non-significant increase of all lipid mediators. Similarly but significantly in rat AM all lipid mediators were induced already within 1 h of UfCP stimulation. Also sensitized and challenge mice exposed to filtered air showed a partially significant increase in all lipid mediators. In sensitized and challenged mice UfCP exposure induced highest significant levels of all lipid mediators in the lungs together with the peak of allergic airway inflammation on day 7 after UfCP inhalation. The levels of LTB₄, 8-isoprostane and PGE₂ were significantly increased also one day after UfCP exposure. Immunohistochemistry localized highest concentrations of PGE₂ especially in AM one day after UfCP exposure.

Conclusion

Our results suggest that UfCP exposure affects the balance between pro- and anti-inflammatory lipid mediators. In allergic mice, where the endogenous balance of pro- and anti-inflammatory mediators is already altered, UfCP exposure aggravates the inflammation and the increase in anti-inflammatory, pro-resolving lipid mediators is insufficient to counterbalance the extensive inflammatory response. This may be a contributing mechanism that explains the increased susceptibility of asthmatic patients towards particle exposure.

Tendon biomechanics and mechanobiology--a minireview of basic concepts and recent advancements

Due to their unique hierarchical structure and composition, tendons possess characteristic biomechanical properties, including high mechanical strength and viscoelasticity, which enable them to carry and transmit mechanical loads (muscular forces) effectively. Tendons are also mechanoresponsive by adaptively changing their structure and function in response to altered mechanical loading conditions. In general, mechanical loading at physiological levels is beneficial to tendons, but excessive loading or disuse of tendons is detrimental. This mechanoadaptability is due to the cells present in tendons. Tendon fibroblasts (tenocytes) are the dominant tendon cells responsible for tendon homeostasis and repair. Tendon stem cells (TSCs), which were recently discovered, also play a vital role in tendon maintenance and repair by virtue of their ability to self-renew and differentiate into tenocytes. TSCs may also be responsible for chronic tendon injury, or tendinopathy, by undergoing aberrant differentiation into nontenocytes in response to excessive mechanical loading. Thus, it is necessary to devise optimal rehabilitation protocols to enhance tendon healing while reducing scar tissue formation and tendon adhesions. Moreover, along with scaffolds that can mimic tendon matrix environments and platelet-rich plasma, which serves as a source of growth factors, TSCs may be the optimal cell type for enhancing repair of injured tendons.

Oxymetazoline inhibits and resolves inflammatory reactions in human neutrophils

The nasal decongestant oxymetazoline (OMZ) exhibits anti-oxidative and anti-inflammatory properties (I. Beck-Speier et al., J Pharmacol Exp Ther. 2006;316:842-851). In a follow up study, we hypothesized that OMZ generates pro-resolving lipoxins being paralleled by production of immune-modulating prostaglandin E(2) (PGE(2)) and anti-inflammatory 15(S)-hydroxy-eicosatetraenoic acid [15(S)-HETE] and depletion of pro-inflammatory leukotriene B(4) (LTB(4)). Human neutrophils (PMN) were chosen as the cellular system. The effect of OMZ on these parameters as well as on respiratory burst activity and oxidative stress marker 8-isprostane was analyzed in unstimulated and co-stimulated PMN by ultrafine carbon particles (UCP) or opsonized zymosan (OZ), respectively. In unstimulated cells, OMZ induced formation of PGE(2), 15(S)-HETE, and LXA(4). The levels of LTB(4) and 8-isoprostane were not affected, whereas respiratory burst activity was drastically inhibited. In UCP- and OZ-stimulated control cells, all parameters were elevated. Here, OMZ maintained the increased levels of PGE(2), 15(S)-HETE, and LXA(4), but substantially suppressed levels of LTB(4) and 8-isoprostane and inhibited the respiratory burst activity. These findings suggest a switch from the pro-inflammatory eicosanoid class LTB(4) to the pro-resolving LXA(4). Since LXA(4) is most relevant in returning inflamed tissue to homeostasis, OMZ is postulated to terminate rhinitis-related inflammation, thus contributing to shortening of disease duration.

The CysLT1 ligand leukotriene D4 supports alpha4beta1- and alpha5beta1-mediated adhesion and proliferation of CD34+ hematopoietic progenitor cells

Cytokines and chemokines control hematopoietic stem and progenitor cell (HPC) proliferation and trafficking. However, the role of nonpeptide mediators in the bone marrow microenvironment has remained elusive. Particularly CysLT(1), a G protein-coupled receptor recognizing inflammatory mediators of the cysteinyl leukotriene family, is highly expressed in HPCs. We therefore analyzed the effects of its ligands on human CD34(+) HPCs. The most potent CysLT(1) ligand, LTD(4), rapidly and significantly up-regulated alpha(4)beta(1) and alpha(5)beta(1) integrin-dependent adhesion of both primitive and committed HPC. LTD(4)-triggered adhesion was inhibited by specific CysLT(1) antagonists. The effects of other CysLT(1) ligands were weak (LTC(4)) or absent (LTE(4)). In serum-free liquid cultures supplemented with various hematopoietic cytokines including IL-3, only LTD(4) significantly augmented the expansion of HPCs in a dose-dependent manner comparable to that of peptide growth factors. LTC(4) and LTE(4) were less effective. In CD34(+) cell lines and primary HPCs, LTD(4) induced phosphorylation of p44/42 ERK/MAPK and focal adhesion kinase-related tyrosine kinase Pyk2, which is linked to integrin activation. Bone marrow stromal cells produced biologically significant amounts of cysteinyl leukotrienes only when hematopoietic cells were absent, suggesting a regulatory feedback mechanism in the hematopoietic microenvironment. In contrast to antagonists of the homing-related G protein-coupled receptor CXCR4, administration of a CysLT(1) antagonist failed to induce human CD34(+) HPC mobilization in vivo. Our results suggest that cysteinyl leukotriene may contribute to HPC retention and proliferation only when cysteinyl leukotriene levels are increased either systemically during inflammation or locally during marrow aplasia.

Areca nut extracts-activated secretion of leukotriene B4, and phosphorylation of p38 mitogen-activated protein kinase and elevated intracellular calcium concentrations in human polymorphonuclear leukocytes

BACKGROUND AND OBJECTIVE

Polymorphonuclear leukocytes are the major source of leukotriene B4, which is synthesized via the 5-lipoxygenase pathway. Activation of the 5-lipoxygenase pathway is regulated by intracellular calcium and the phosphorylation of p38 mitogen-activated protein kinase (MAPK). The impact of areca nut extracts on the biosynthesis of leukotriene B4 by human polymorphonuclear leukocytes was evaluated, and some of the possible mechanisms underlying the responses were examined.

MATERIAL AND METHODS

Polymorphonuclear leukocytes were treated with various concentrations of areca nut extracts. The concentrations of leukotriene B4 released into the supernatants were evaluated using enzyme immunoassay. The phosphorylation of p38 MAPK was monitored using immunoblotting, and the cytosolic calcium kinetics were assessed fluorometrically using Fura-2.

RESULTS

Exposure of polymorphonuclear leukocytes to areca nut extracts led to a dose-dependent increase in the production of leukotriene B4, with levels peaking at 30 min and decreasing thereafter. Areca nut extracts enhanced the phosphorylation of p38 MAPK, an enzyme known to activate 5-lipoxygenase. Incubation with areca nut extracts also resulted in a rapid elevation of intracellular calcium concentrations in polymorphonuclear leukocytes. The induction of leukotriene B4 by areca nut extracts was suppressed with the p38 MAPK inhibitor, SB203580, or with the intracellular calcium chelator, BAPTA-AM.

CONCLUSION

The interaction of areca nut extracts with polymorphonuclear leukocytes activated the arachidonic acid metabolic cascade. Incubation of polymorphonuclear leukocytes with areca nut extracts resulted in the activation of intracellular events, such as phosphorylation of p38 MAPK and Ca2+ mobilization, involved in the release of pro-inflammatory lipid mediators. The results of this study emphasize the potential importance of polymorphonuclear leukocytes as a source of leukotriene B4, which may modulate the inflammatory response in areca chewers.

Cysteinyl leukotrienes: multi-functional mediators in allergic rhinitis

Cysteinyl leukotrienes (CysLTs) are a family of inflammatory lipid mediators synthesized from arachidonic acid by a variety of cells, including mast cells, eosinophils, basophils, and macrophages. This article reviews the data for the role of CysLTs as multi-functional mediators in allergic rhinitis (AR). We review the evidence that: (1) CysLTs are released from inflammatory cells that participate in AR, (2) receptors for CysLTs are located in nasal tissue, (3) CysLTs are increased in patients with AR and are released following allergen exposure, (4) administration of CysLTs reproduces the symptoms of AR, (5) CysLTs play roles in the maturation, as well as tissue recruitment, of inflammatory cells, and (6) a complex inter-regulation between CysLTs and a variety of other inflammatory mediators exists.

Leukotriene B4 at low dosage negates the catabolic effect of prostaglandin E2 in human patellar tendon fibroblasts

Tendinopathy often involves inflammation and matrix degeneration. The inflammatory mediators such as prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) are implicated in the development of tendinopathy. Therefore, the purpose of this study was to determine the effect of PGE2 and LTB4 on the proliferation of human patellar tendon fibroblasts (HPTFs), the gene expression of collagen type I, MMP-1 and MMP-3, as well as the protein secretion of these gene products by the cells. The results showed that LTB4 at low doses (0.1 and 1 nM) significantly increased cell proliferation compared to controls and LTB4 at 0.1 nM negated the PGE2-induced decrease in cell proliferation. In addition, PGE2 at 100 ng/ml significantly increased the expression of MMP-1 and MMP-3 at both mRNA and protein levels. These stimulatory effects were significantly diminished by co-treatment with LTB4 at 0.1 nM. Finally, neither PGE2 nor LTB4 treatment affected collagen type I gene expression. These results suggest that low levels of LTB4 counterbalance the negative effects mediated by PGE2 on tendon fibroblast proliferation and MMP production, which may lead to matrix degradation. Thus, our findings suggest that although LTB4 is generally thought to be pathogenic, low levels of LTB4 are actually beneficial in maintaining tendon tissue homeostasis.

Development of a high-capacity homogeneous fluorescent assay for the measurement of leukotriene B4

Current immunoassays for the measurement of leukotriene B(4) (LTB(4)) typically utilize an enzyme-linked immunosorbent assay (ELISA) format that requires multiple incubations and washing steps and often expensive immunoassay kits. We have developed a bead-based, mix and read, indirect fluorescence-linked immunosorbent assay utilizing fluorometric microvolume assay technology (FMAT). The assay employs a monoclonal anti-LTB(4) antibody-coated onto goat antimouse antibody coupled polystyrene beads and an AlexaFluor-647-coupled LTB(4) ligand. Because the FMAT measurement is made only in the portion of the well volume containing the settled beads coated with AF647-LTB(4), the free label in the solution is not measured. Similarly, substances present in plasma that interfere with other immunoassays are largely ignored. The assay is robust (Z=0.8; S/N=250) and can be measured in the presence of relatively high concentrations of dimethyl sulfoxide or serum. It is inexpensive (<0.10 dollars/assay) and amenable to robotics and has a sensitivity comparable to that of the most sensitive ELISA assays; the concentration of LTB(4) giving 50% inhibition (IC(50)) was ca. 55pg/ml. Cross-reactivity in the FMAT assay was comparable to that of the ELISA assay with significant cross-reactivity found only with 20-hydroxy LTB(4) and 12-epi LTB(4). Measurements of LTB(4) determined by FMAT were equivalent to those measured by standard ELISA in samples of ionophore-stimulated human neutrophils or whole blood.

Oxymetazoline Inhibits Proinflammatory Reactions: Effect on Arachidonic Acid-Derived Metabolites

The nasal decongestant oxymetazoline effectively reduces rhinitis symptoms. We hypothesized that oxymetazoline affects arachidonic acid-derived metabolites concerning inflammatory and oxidative stress-dependent reactions. The ability of oxymetazoline to model pro- and anti-inflammatory and oxidative stress responses was evaluated in cell-free systems, including 5-lipoxygenase (5-LO) as proinflammatory, 15-lipoxygenase (15-LO) as anti-inflammatory enzymes, and oxidation of methionine by agglomerates of ultrafine carbon particles (UCPs), indicating oxidative stress. In a cellular approach using canine alveolar macrophages (AMs), the impact of oxymetazoline on phospholipase A(2) (PLA(2)) activity, respiratory burst and synthesis of prostaglandin E(2) (PGE(2)), 15(S)-hydroxy-eicosatetraenoic acid (15-HETE), leukotriene B(4) (LTB(4)), and 8-isoprostane was measured in the absence and presence of UCP or opsonized zymosan as particulate stimulants. In cell-free systems, oxymetazoline (0.4-1 mM) inhibited 5-LO but not 15-LO activity and did not alter UCP-induced oxidation of methionine. In AMs, oxymetazoline induced PLA(2) activity and 15-HETE at 1 mM, enhanced PGE(2) at 0.1 mM, strongly inhibited LTB(4) and respiratory burst at 0.4/0.1 mM (p < 0.05), but did not affect 8-isoprostane formation. In contrast, oxymetazoline did not alter UCP-induced PLA(2) activity and PGE(2) and 15-HETE formation in AMs but inhibited UCP-induced LTB(4) formation and respiratory burst at 0.1 mM and 8-isoprostane formation at 0.001 mM (p < 0.05). In opsonized zymosan-stimulated AMs, oxymetazoline inhibited LTB(4) formation and respiratory burst at 0.1 mM (p < 0.05). In conclusion, in canine AMs, oxymetazoline suppressed proinflammatory reactions including 5-LO activity, LTB(4) formation, and respiratory burst and prevented particle-induced oxidative stress, whereas PLA(2) activity and synthesis of immune-modulating PGE(2) and 15-HETE were not affected.

Oxidative stress and lipid mediators induced in alveolar macrophages by ultrafine particles

In ambient aerosols, ultrafine particles (UFP) and their agglomerates are considered to be major factors contributing to adverse health effects. Reactivity of agglomerated UFP of elemental carbon (EC), Printex 90, Printex G, and diesel exhaust particles (DEP) was evaluated by the capacity of particles to oxidize methionine in a cell-free in vitro system for determination of their innate oxidative potential and by alveolar macrophages (AMs) to determine production of arachidonic acid (AA), including formation of prostaglandin E2 (PGE2), leukotriene B4 (LTB4), reactive oxygen species (ROS), and oxidative stress marker 8-isoprostane. EC exhibiting high oxidative potential induced generation of AA, PGE2, LTB4, and 8-isoprostane in canine and human AMs. Printex 90, Printex G, and DEP, showing low oxidative capacity, still induced formation of AA and PGE2, but not that of LTB4 or 8-isoprostane. Aging of EC lowered oxidative potential while still inducing production of AA and PGE2 but not that of LTB4 and 8-isoprostane. Cellular ROS production was stimulated by all particles independent of oxidative potential. Particle-induced formation of AA metabolites and ROS was dependent on mitogen-activated protein kinase kinase 1 activation of cytosolic phospholipase A2 (cPLA2) as shown by inhibitor studies. In conclusion, cPLA2, PGE2, and ROS formation was activated by all particle types, whereas LTB4 production and 8-isoprostane were strongly dependent on particles' oxidative potential. Physical and chemical parameters of particle surface correlated with oxidative potential and stimulation of AM PGE2 and 8-isoprostane production.

Sulfur-related air pollutants induce the generation of platelet-activating factor, 5-lipoxygenase- and cyclooxygenase-products in canine alveolar macrophages via activation of phospholipases A2

Recent studies have shown that long-term in vivo exposure of dogs to neutral sulfur(IV)/sulfite aerosols induces mild inflammatory reactions, whereas the combination of neutral sulfite with acidic sulfur(VI)/sulfate aerosols evokes less pronounced effects. To understand underlying mechanisms, we studied in vitro the role of lipid mediators in the responses of alveolar macrophages (AMs) to sulfur-related compounds under neutral (pH 7) or moderate acidic (pH 6) conditions. Canine AMs incubated with sulfite at pH 7 released threefold higher amounts of platelet-activating factor than control (P < 0.005). Generation of arachidonic acid, leukotriene B4, 5-hydroxy-eicosatetraenoic acid, prostaglandin E2, thromboxane B2 and 12-hydroxyheptadecatrienoic acid increased twofold (P < 0.0005). However, these metabolites remained unchanged following incubation of AMs with sulfite at pH 6 or with sulfate at pH 7 or pH 6. Mediator release by sulfite-treated AMs at pH 7 stimulated respiratory burst activity of neutrophils. Inhibition of MAPK pathway by PD 98059, of cytosolic (cPLA2) and secretory phospholipases A2 by AACOCF3 and thioetheramide-PC, respectively, reduced sulfite-induced eicosanoid formation in AMs. Sulfite activated cPLA2 activity twofold at pH 7. This mechanism of sulfite-stimulated responses in phospholipid metabolism predicts that chronic exposure to sulfur(IV)/sulfite is associated with a considerable health risk.

Structure-based analysis of GPCR function: conformational adaptation of both agonist and receptor upon leukotriene B4 binding to recombinant BLT1

We produced the human leukotriene B(4) (LTB(4)) receptor BLT1, a G-protein-coupled receptor, in Escherichia coli with yields that are sufficient for the first structural characterization of this receptor in solution. Overexpression was achieved through codon optimization and the search for optimal refolding conditions of BLT1 recovered from inclusion bodies. The detergent-solubilized receptor displays a 3D-fold compatible with a seven transmembrane (TM) domain with ca 50% alpha-helix and an essential disulfide bridge (circular dichroism evidence); it binds LTB(4) with K(a)=7.8(+/-0.2)x10(8)M(-1) and a stoichiometric ratio of 0.98(+/-0.02). Antagonistic effects were investigated using a synthetic molecule that shares common structural features with LTB(4). We report evidence that both partners, LTB(4) and BLT1, undergo a rearrangement of their respective conformations upon complex formation: (i) a departure from planarity of the LTB(4) conjugated triene moiety; (ii) a change in the environment of Trp234 (TM-VI helix) and in the exposure of the cytoplasmic region of this transmembrane helix.

Nonpeptide mediators in the hematopoietic microenvironment

Migration of hematopoietic stem and progenitor cells (HPCs) is controlled by chemotactic factors released in the hematopoietic microenvironment. In particular, the chemokine SDF-1, which activates the G protein-coupled receptor (GPR) CXCR4, plays an important role in progenitor cell mobilization and homing. However, we provide evidence that ligands of other GPRs similarly act on CD34(+) hematopoietic progenitors. These ligands comprise non-peptide mediators, including the cysteinyl-leukotriene receptor CysLT1, and stimulate migration and integrin-dependent adhesion of HPCs. Moreover, continuous activation of a GPR by a specific ligand upregulates the responsiveness of other GPRs to their corresponding ligands. These findings suggest that HPC migration may not depend on a single chemokine receptor (e.g., CXCR4). Rather, mobilization and homing of HPCs involve several GPRs, which interact with each other as well as with adhesion molecules. Pharmacological activation and inhibition of the GPR may allow HPC mobilization and homing to be modulated.

Transcription and replication of a natural hepatitis B virus nucleocapsid promoter variant is regulated in vivo by peroxisome proliferators

A hepatitis B virus (HBV) transgenic mouse containing a naturally occurring mutation in the nucleocapsid promoter (A1764T plus G1766A) that inhibits the retinoid X receptor alpha (RXRalpha) plus peroxisome proliferator-activated receptor alpha (PPARalpha) heterodimer from binding to the proximal nuclear hormone receptor recognition sequence has been generated. Viral transcription and replication occur in the liver and kidney. The nucleocapsid promoter mutation does not prevent peroxisome proliferators from increasing viral transcription and replication in the liver of these variant HBV transgenic mice. This suggests that peroxisome proliferators may enhance viral transcription directly in a PPARalpha-dependent manner through the nuclear hormone receptor recognition site in the enhancer 1 region of the HBV genome. Hepatocyte nuclear factor 4 (HNF4) binding to the proximal nuclear hormone receptor recognition sequence in the nucleocapsid promoter appears to limit RNA synthesis from the precore transcription initiation site. Consequently, the variant HBV transgenic mice transcribe very little precore RNA and secrete extremely low levels of hepatitis B e antigen (HBeAg) compared with the wild-type HBV transgenic mice. This is consistent with the suggestion that viruses expressing HBeAg are preferentially eliminated in infected individuals when they seroconvert from HBeAg positive to anti-HBe antibody-positive status, leaving escape HBV variants that have reduced HBeAg expression.

Chemotaxis and transendothelial migration of CD34(+) hematopoietic progenitor cells induced by the inflammatory mediator leukotriene D4 are mediated by the 7-transmembrane receptor CysLT1

Recent studies suggest that bone marrow (BM)-derived chemotactic mediators such as chemokines play key roles in hematopoietic stem cell trafficking. Lipid mediators, particularly leukotrienes, are involved in leukocyte chemotaxis during inflammation but have also been detected in the normal BM. Therefore, the effects of leukotrienes on hematopoietic progenitor cells were analyzed. Cysteinyl leukotrienes, particularly leukotriene D4 (LTD4), induced strong intracellular calcium fluxes and actin polymerization in mobilized and BM CD34(+) progenitors. Chemotaxis and in vitro transendothelial migration of CD34(+) and more primitive CD34(+)/CD38(-) cells were 2-fold increased by LTD4 at an optimum concentration of 25 to 50 nM. Accordingly, CD34(+) cells expressed the 7-transmembrane LTD4 receptor CysLT1 by reverse transcriptase-polymerase chain reaction and Western blot. Effects of LTD4 were suppressed by the CysLT1 receptor antagonist MK-571 and reduced by pertussis toxin. In contrast, LTB4 induced strong responses only in mature granulocytes. LTD4-induced calcium fluxes were also observed in granulocytes but were not reduced by MK-571, suggesting that these effects were mediated by other receptors (eg, CysLT2) rather than by CysLT1. In addition, expression of 5-lipoxygenase, the key enzyme of leukotriene biosynthesis, was detected in both hematopoietic progenitor cells and mature leukocytes. The study concludes that the functionally active LTD4 receptor CysLT1 is preferentially expressed in immature hematopoietic progenitor cells. LTD4 released in the BM might regulate progenitor cell trafficking and could also act as an autocrine mediator of hematopoiesis. This would be a first physiologic effect of cysteinyl leukotrienes apart from the many known pathophysiologic actions related to allergy and inflammation. (Blood. 2001;97:3433-3440)

Transendothelial migration of hematopoietic progenitor cells. Role of chemotactic factors

There is increasing evidence that hematopoietic stem cell mobilization and homing is regulated not only by adhesion molecules and cytokines, but also by chemotactic factors that support transendothelial migration across the bone marrow sinusoidal endothelium. Many receptors for chemotactic mediators belong to the family of G protein-coupled seven-transmembrane receptors (7-TMR). Signaling via G proteins, particularly Gi proteins, results in a chemotactic response of the cells towards a gradient of the corresponding ligand. Recent studies have provided evidence for expression of several 7-TMR on immature hematopoietic progenitor cells, which potentially mediate chemotactic effects: chemokine receptors (e.g., CXCR4, receptor for stromal cell-derived factor-1), receptors for lipid mediators (e.g., the cysteinyl leukotriene receptor cysLT1 and the peripheral cannabinoid receptor cb2), and receptors for neuroendocrine hormones (e.g., the somatostatin receptor sst2). From these studies it can be concluded that migration of hematopoietic progenitor and stem cells is controlled by a variety of chemotactic factors rather than by a single chemokine (e.g., SDF-1). Trafficking of immature hematopoietic cells may require combined and interactive regulatory functions of these mediators.

Nasal secretion in ragweed-sensitized dogs: effect of leukotriene synthesis inhibition

Allergic rhinitis is an inflammatory disease associated with local leukotriene release during periods of symptoms. Zileuton, a leukotriene synthesis inhibitor, is known to inhibit the release of leukotriene B4. Because we previously showed that leukotriene B4 is a potent mediator of neutrophil-dependent nasal secretion, we investigated whether Zileuton inhibited allergen-induced nasal secretion. Using a newly developed method for isolating and superfusing a nasal segment, we examined the effect of Zileuton on nasal secretion and neutrophil recruitment in ragweed-sensitized dogs. Instillation of ragweed into the nasal segment caused time-dependent increases in the volume of airway fluid and the recruitment of neutrophils. Zileuton prevented ragweed-induced neutrophil recruitment and nasal secretion. These results indicate that leukotrienes are important mediators of allergy-induced nasal secretion in dogs. Future clinical studies in allergic patients will determine whether there is a therapeutic role for leukotriene synthesis inhibitors in modulating neutrophil recruitment and hypersecretion in the nose.

LTB(4)-induced nasal gland serous cell secretion mediated by neutrophil elastase

Local allergen challenge causes nasal hypersecretion and also causes local leukotriene (LT) release, including LTB(4). Because LTB(4) causes leukocyte recruitment, and because neutrophil elastase is a potent secretagogue, we examined the hypothesis that LTB(4) causes nasal hypersecretion via neutrophil elastase. We developed a method for isolating and superfusing a nasal segment in dogs. Instillation of LTB(4) into the nasal segment caused a time-dependent increase in the volume of airway fluid, in lysozyme secretion, and in the recruitment of neutrophils. ICI 200,355, a selective inhibitor of neutrophil elastase, prevented LTB(4)-induced nasal secretion and lysozyme secretion, but it had no effect on neutrophil recruitment. We conclude that LTB(4) causes potent nasal secretion via release of elastase, and therefore LTB(4) may play a major role in allergic nasal hypersecretion.

Leukotriene A4 hydrolase and leukotriene C4 synthase activities in human chondrocytes: transcellular biosynthesis of Leukotrienes during granulocyte-chondrocyte interaction

OBJECTIVE

To investigate the cooperation of chondrocytes and polymorphonuclear cells (PMN) in the biosynthesis of leukotrienes (LT).

METHODS

PMN, resting and interleukin-1beta-stimulated cultured human chondrocytes, and mixtures of both cell types were incubated with A23187 and/or 14C-arachidonic acid (14C-AA). To explore the presence of LTC4 synthase and LTA4 hydrolase, the chondrocytes were incubated with authentic LTA4. Eicosanoids were analyzed using high performance liquid chromatography techniques.

RESULTS

Chondrocytes formed only prostaglandin E2 and minor amounts of 15-HETE and 11-HETE, the production of all of which was inhibited by 1 microM indomethacin. Incubation of PMN and chondrocytes produced more LTC4 from endogenous and exogenous AA, and more LTB4 from endogenous AA, than incubation of PMN alone, which was consistent with the presence of LTC4 synthase and LTA4 hydrolase activities in chondrocytes. Chondrocytes also slightly increased the level of PMN production of all 5-lipoxygenase (5-LO)-derived products from endogenous AA.

CONCLUSION

Human chondrocytes form eicosanoids from AA only by the cyclooxygenase pathway. Chondrocytes cooperate in the transcellular biosynthesis of LT since they possess LTA4 hydrolase and LTC4 synthase activities and increase metabolism by the 5-LO pathway in PMN.

Pharmacological inhibition of leukotriene actions

Leukotrienes represent a group of lipid mediators that play a very important role in a wide variety of pathological conditions. The presence of leukotrienes in inflammatory sites has been extensively documented, and accordingly research efforts have been directed towards the development of drugs that interfere with the formation or effects of leukotrienes. Although clinical application of such drugs has been disappointing in the past, recent discoveries of more potent and selective drugs seem to be promising. This review attempts to highlight some of these exciting developments.

MK-886, a leukotriene biosynthesis inhibitor, induces antiproliferative effects and apoptosis in HL-60 cells

MK-886, a specific inhibitor of 5-lipoxygenase inhibited DNA replication in leukemic HL-60 cells in a dose-dependent manner. Addition of exogenous leukotriene B4 reversed this effect, whereas addition of leukotriene B4 failed to modulate a prostaglandin D2-induced inhibition of DNA replication. The reversal of MK-886-induced inhibition was not observed with leukotriene C4. These results suggest that the effect of MK-886 is mediated by inhibition of leukotriene B4 biosynthesis. Moreover, MK-886 not only impaired DNA replication in HL-60 cells but also decreased cell proliferation and induced apoptotic cell death. Our results suggest a crucial role of leukotriene B4 in the regulation of cell proliferation and cell survival in HL-60 cells.