Transcellular metabolism of neutrophil-derived leukotriene A4 by human platelets. A potential cellular source of leukotriene C4

The Leukotriene Receptor Antagonist Montelukast Attenuates Neuroinflammation and Affects Cognition in Transgenic 5xFAD Mice

Alzheimer’s disease (AD) is the most common form of dementia. In particular, neuroinflammation, mediated by microglia cells but also through CD8+ T-cells, actively contributes to disease pathology. Leukotrienes are involved in neuroinflammation and in the pathological hallmarks of AD. In consequence, leukotriene signaling—more specifically, the leukotriene receptors—has been recognized as a potential drug target to ameliorate AD pathology. Here, we analyzed the effects of the leukotriene receptor antagonist montelukast (MTK) on hippocampal gene expression in 5xFAD mice, a commonly used transgenic AD mouse model. We identified glial activation and neuroinflammation as the main pathways modulated by MTK. The treatment increased the number of Tmem119+ microglia and downregulated genes related to AD-associated microglia and to lipid droplet-accumulating microglia, suggesting that the MTK treatment targets and modulates microglia phenotypes in the disease model compared to the vehicle. MTK treatment further reduced infiltration of CD8+T-cells into the brain parenchyma. Finally, MTK treatment resulted in improved cognitive functions. In summary, we provide a proof of concept for MTK to be a potential drug candidate for AD and provide novel modes of action via modulation of microglia and CD8+ T-cells. Of note, 5xFAD females showed a more severe pathology, and in consequence, MTK treatment had a more pronounced effect in the females compared to the males. The effects on neuroinflammation, i.e., microglia and CD8+ T-cells, as well as the effects on cognitive outcome, were dose-dependent, therefore arguing for the use of higher doses of MTK in AD clinical trials compared to the approved asthma dose.

12-Oxo-10-glutathionyl-5,8,14-eicosatrienoic acid (TOG10), a novel glutathione-containing eicosanoid generated via the 12-lipoxygenase pathway in human platelets

Biologically active glutathione (GSH) conjugates of oxygenated fatty acids comprise a group of pro- and anti-inflammatory lipid mediators. While arachidonic acid (AA)-derived conjugates, as the cysteinyl leukotrienes (cys-LTs) and eoxins (EXs) have pro-inflammatory properties, conjugates in tissue regeneration (CTRs) biosynthesized from docosahexaenoic acid (DHA) exhibit pro-resolving activity. Human platelets express abundant amounts of platelet-type 12-lipoxygenase (pt12-LOX) and leukotriene C4 synthase (LTC4S). However, the only two described GSH conjugates formed by platelets are the AA-derived cys-LTs and the recently reported maresin CTRs (MCTRs). While cys-LTs are biosynthesized in a transcellular mechanism via the action of 5-LOX and LTC4S, MCTR1 is formed by 12-LOX and a yet unidentified GSH S-transferase (GST). Here, we present a novel GSH conjugate formed from AA via the 12-LOX pathway in human platelets. The 12-oxo-glutathione adduct, 12-oxo-10-glutathionyl-5,8,14-eicosatrienoic acid (TOG10), was identified by mass spectrometry using positive electrospray ionization. The structural proposal is supported by fragmentation data of the labeled metabolite obtained after incubation of deuterated AA (AA-d8). In platelets as well as in HEK293 cells stably expressing pt12-LOX, TOG10 biosynthesis was inhibited by the 12-LOX inhibitor ML-355 (5 μM), which confirms the involvement of pt12-LOX. Interestingly, TOG10 was formed independently of LTC4S in platelets. This is in accordance with the observation that the conjugate was also generated by AA-stimulated HEK_12-LOX cells in absence of LTC4S. Nevertheless, TOG10 can also be formed by LTC4S as the biosynthesis in HEK_12-LOX_LTC4S cells was reduced by the specific LTC4S inhibitor TK04a. In summary, TOG10 was identified as a new AA-derived GSH conjugate generated in human platelets via the action of pt12-LOX in combination with a GST.

Integral Membrane Enzymes in Eicosanoid Metabolism: Structures, Mechanisms and Inhibitor Design

Eicosanoids are potent lipid mediators involved in central physiological processes such as hemostasis, renal function and parturition. When formed in excess, eicosanoids become critical players in a range of pathological conditions, in particular pain, fever, arthritis, asthma, cardiovascular disease and cancer. Eicosanoids are generated via oxidative metabolism of arachidonic acid along the cyclooxygenase (COX) and lipoxygenase (LOX) pathways. Specific lipid species are formed downstream of COX and LOX by specialized synthases, some of which reside on the nuclear and endoplasmic reticulum, including mPGES-1, FLAP, LTC4 synthase, and MGST2. These integral membrane proteins are members of the family "membrane-associated proteins in eicosanoid and glutathione metabolism" (MAPEG). Here we focus on this enzyme family, which encompasses six human members typically catalyzing glutathione dependent transformations of lipophilic substrates. Enzymes of this family have evolved to combat the topographical challenge and unfavorable energetics of bringing together two chemically different substrates, from cytosol and lipid bilayer, for catalysis within a membrane environment. Thus, structural understanding of these enzymes are of utmost importance to unravel their molecular mechanisms, mode of substrate entry and product release, in order to facilitate novel drug design against severe human diseases.

Lipid mass spectrometry: A path traveled for 50 years

In the middle of the 1960s, I began graduate school and at the same time started on the path of using mass spectrometry to gain insight into various aspects of lipid biochemistry. This was not a straight path but one that went from organic geochemistry, to lunar sample analysis, to a pursuit of the structure of an elusive and very active, lipid mediator slow reacting substance of anaphylaxis (SRS-A). The discovery of the structure of SRS-A opened important questions about phospholipid biochemistry and the arachidonate cycle in cells. I have written this reflection to highlight the various advances in mass spectrometry that occurred during this time that had a great impact on our ability to study lipid biochemistry. I specifically applied these new advances to studies of leukotriene biosynthesis in vivo, leukotriene metabolism, and arachidonate-containing phospholipids that are essential in providing arachidonic acid for the 5-lipoxygenase pathway. Along the way, imaging mass spectrometry was shown to be a powerful tool to probe lipids as they exist in tissue slices. We found this as just one of the ways to use the emerging technology of lipidomics to study human pathophysiology. Our studies of neutral lipids and oxidized phospholipids were especially challenging due to the total number of molecular species that could be found in cells. Many challenges remain in using mass spectrometry for lipid studies, and a few are presented.

Activation of metabolite receptor GPR91 promotes platelet aggregation and transcellular biosynthesis of leukotriene C4

BACKGROUND

Succinate is a Krebs cycle intermediate whose formation is enhanced under metabolic stress, and for which a selective sensor GPR91 has been identified on various cell types including platelets. Platelet-derived eicosanoids play pivotal roles in platelet activation/aggregation, which is key to thrombus formation and progression of atherothrombosis.

OBJECTIVES

This study aims to decipher the molecular mechanism(s) and potential involvement of eicosanoids in succinate enhanced platelet activation/aggregation.

METHODS

We used liquid chromatography-mass spectrometry (LC-MS)/MS-based lipid mediator profiling to identify eicosanoids regulated by succinate. We ran light transmittance aggregometry and flow cytometry to assess platelet aggregation, P-selectin expression, and platelet-polymorphonuclear leukocyte (PMN) adherence. Various pharmacological tools were used to assess the contributions of GPR91 signalling and eicosanoids in platelet aggregation.

RESULTS

Succinate and two types of synthetic non-metabolite GPR91 agonists-cis-epoxysuccinate (cES) and Cmpd131-potentiated platelet aggregation, which was partially blocked by a selective GPR91 antagonist XT1. GPR91 activation increased production of 12-hydroxy-eicosatetraenoic acid (12-HETE), thromboxane (TX) A2 , and 12-hydroxy-heptadecatrienoic acid (12-HHT) in human platelets, associated with phosphorylation of cytosolic phospholipase A2 (cPLA2 ), suggesting increased availability of free arachidonic acid. Blocking 12-HETE and TXA2 synthesis, or antagonism of the TXA2 receptor, significantly reduced platelet aggregation enhanced by GPR91 signalling. Moreover, platelet-PMN suspensions challenged with succinate exhibited enhanced transcellular biosynthesis of leukotriene C4 (LTC4 ), a powerful proinflammatory vascular spasmogen.

CONCLUSION

Succinate signals through GPR91 to promote biosynthesis of eicosanoids, which contribute to platelet aggregation/activation and potentially vascular inflammation. Hence, GPR91 may be a suitable target for pharmacological intervention in atherothrombotic conditions.

Platelet lipidome: Dismantling the "Trojan horse" in the bloodstream

The platelet-lipid chapter in the story of atherothrombosis is an old one, recapitulated and revised in many contexts. For decades several stimulating facets have been added to it, both unraveling and increasing the perplexity of platelet-lipid interplay and its pathophysiological consequences. The recent paradigm shift in our perspective has evolved with lipidomic analysis of the intraplatelet compartment and platelet releasate. These investigations have disclosed that platelets are in constant interaction with circulatory lipids, often reflected in their lipid repertoire. In addition, they offer a shielded intracellular space for oxidative lipid metabolism generating "toxic" metabolites that escape degradation by plasma lipases and antioxidant defense, circulate undetected by conventional plasma lipid profile, and deposited at atherosclerotic lesions or thrombus. Lipidomics divulges this silent invader in platelet vehicles, thereby providing potential biomarkers of pathologic manifestations and therapeutic targets to be exploited, which is surmised in this review.

Highly sensitive and specific LC–MS/MS method to determine endogenous leukotriene B4 levels in human plasma

Aim: To develop a high sensitivity and specific analytical method to measure endogenous levels of leukotriene B4 (LTB4) in human plasma. Methodology: LC–MS/MS and ELISA. Results: An LC–MS/MS method was developed with a sensitivity of 1.0 pg/ml, and within and between batch precision of <16% and <13% RSD, respectively. Conclusion: We have developed a sensitive LC–MS/MS method that can detect endogenous LTB4 in human plasma. The LC–MS/MS method displayed correlation with a commercial LTB4 ELISA when analyzing in ex vivo ionophore-stimulated blood samples. For untreated plasma this correlation was lost. Endogenous LTB4 was shown to be unstable in plasma during storage at -20°C and subject to stereoisomer formation. Neither of the assays could quantify endogenous plasma LTB4 in samples stored for long term.

Effects of anticoagulants and storage conditions on clinical oxylipid levels in human plasma

Metabolomics and lipidomics are of fundamental importance to personalized healthcare. Particularly the analysis of bioactive lipids is of relevance to a better understanding of various diseases. Within clinical routines, blood derived samples are widely used for diagnostic and research purposes. Hence, standardized and validated procedures for blood collection and storage are mandatory, in order to guarantee sample integrity and relevant study outcomes. We here investigated different plasma storage conditions and their effect on plasma fatty acid and oxylipid levels. Our data clearly indicate the importance of storage conditions for plasma lipidomic analysis. Storage at very low temperature (-80 °C) and the addition of methanol directly after sampling are the most important measures to avoid ex vivo synthesis of oxylipids. Furthermore, we identified critical analytes being affected under certain storage conditions. Finally, we carried out chiral analysis and found possible residual enzymatic activity to be one of the contributors to the ex vivo formation of oxylipids even at -20 °C.

Importance of the leukotriene B4-BLT1 and LTB4-BLT2 pathways in asthma

For several decades, the leukotriene pathways have been implicated as playing a central role in the pathophysiology of asthma. The presence and elevation of numerous metabolites in the blood, sputum, and bronchoalveolar lavage fluid from asthmatics or experimental animals adds support to this notion. However, targeting of the leukotriene pathways has had, in general, limited success. The single exception in asthma therapy has been targeting of the cysteinyl leukotriene receptor 1, which clinically has proven effective but only in certain clinical situations. Interference with 5-lipoxygenase has had limited success, in part due to adverse drug effects. The importance of the LTB4-BLT1 pathway in asthma pathogenesis has extensive experimental support and findings, albeit limited, from clinical samples. The LTB4-BLT1 pathway was shown to be important as a neutrophil chemoattractant. Despite observations made more than two decades ago, the LTB4-BLT1 pathway has only recently been shown to exhibit important activities on subsets of T lymphocytes, both as a chemoattractant and on lymphocyte activation, as well as on dendritic cells, the major antigen presenting cell in the lung. The role of BLT2 in asthma remains unclear. Targeting of components of the LTB4-BLT1 pathway offers innovative therapeutic opportunities especially in patients with asthma that remain uncontrolled despite intensive corticosteroid treatment.

Twice daily dosing of aspirin improves platelet inhibition in whole blood in patients with type 2 diabetes mellitus and micro- or macrovascular complications

The efficacy of low-dose aspirin in type 2 diabetes mellitus (T2DM) has been questioned. We tested if twice daily dosing of aspirin would be more effective in T2DM, possibly due to increased platelet turnover. A randomised cross-over study compared 75 mg aspirin OD, 75 mg BID and 320 mg OD (≥2 week treatment periods) in 25 patients with T2DM and micro- or macrovascular complications. Platelet responses were examined by impedance aggregometry (WBA) and the IMPACT-R aspirin test in whole blood, light transmittance aggregometry in plateletrich plasma (LTA), and urinary 11-dehydro-thromboxane B2 (TxM). Aspirin 75 mg BID decreased arachidonic acid (AA)-induced WBA compared to 75 mg OD (9.7 ± 4.5 vs. 12.6 ± 3.5 ohm; p=0.003) or to 320 mg OD (11.5 ± 4.2 Ohms; p=0.049). WBA responses to collagen were similarly attenuated by BID or high dosing (by 12–14%; p=0.02 for both). The IMPACT-R showed a better response to 75 mg BID compared to 75 mg OD (p=0.049), but not to 320 mg OD. AA-induced aggregation by LTA was <6.5% on all occasions, with no differences between aspirin dosages. TxM was reduced after 320 mg OD (p=0.002), but not 75 mg BID (p=0.07). Reticulated platelets were highly correlated with mean platelet volume (MPV; r2=0.74, p<0.0001). Both markers for platelet turnover were correlated with AA-induced WBA, but neither identified patients who benefited from BID dosing dependably. In conclusion, twice daily dosing improved laboratory responses to aspirin in high risk T2DM patients. Studies of whether BID dosing of aspirin can improve clinical outcomes in such patients are of interest.

Lymphatic Dysfunction, Leukotrienes, and Lymphedema

The lymphatic system is essential for the maintenance of tissue fluid homeostasis, gastrointestinal lipid absorption, and immune trafficking. Whereas lymphatic regeneration occurs physiologically in wound healing and tissue repair, pathological lymphangiogenesis has been implicated in a number of chronic diseases such as lymphedema, atherosclerosis, and cancer. Insight into the regulatory mechanisms of lymphangiogenesis and the manner in which uncontrolled inflammation promotes lymphatic dysfunction is urgently needed to guide the development of novel therapeutics: These would be designed to reverse lymphatic dysfunction, either primary or acquired. Recent investigation has demonstrated the mechanistic role of leukotriene B₄ (LTB₄) in the molecular pathogenesis of lymphedema. LTB₄, a product of the innate immune response, is a constituent of the eicosanoid inflammatory mediator family of molecules that promote both physiological and pathological inflammation. Here we provide an overview of lymphatic development, the pathophysiology of lymphedema, and the role of leukotrienes in lymphedema pathogenesis.

Role of the Cysteinyl Leukotrienes in the Pathogenesis and Progression of Cardiovascular Diseases

Cysteinyl leukotrienes (CysLTs) are potent lipid inflammatory mediators synthesized from arachidonic acid, through the 5-lipoxygenase (5-LO) pathway. Owing to their properties, CysLTs play a crucial role in the pathogenesis of inflammation; therefore, CysLT modifiers as synthesis inhibitors or receptor antagonists, central in asthma management, may become a potential target for the treatment of other inflammatory diseases such as the cardiovascular disorders. 5-LO pathway activation and increased expression of its mediators and receptors are found in cardiovascular diseases. Moreover, the cardioprotective effects observed by using CysLT modifiers are promising and contribute to elucidate the link between CysLTs and cardiovascular disease. The aim of this review is to summarize the state of present research about the role of the CysLTs in the pathogenesis and progression of atherosclerosis and myocardial infarction.

Role of platelets in neutrophil extracellular trap (NET) production and tissue injury

In addition to their well-known role as the cellular mediator of thrombosis, numerous studies have identified key roles for platelets during various disease processes. Importantly, platelets play a critical role in the host immune response, directly interacting with, and eliminating pathogens, from the blood stream. In addition to pathogen clearance, platelets also contribute to leukocyte recruitment at sites of infection and inflammation, and modulate leukocyte activity. Platelet interaction with activated neutrophils is a potent inducer of neutrophil extracellular trap (NET). NETs consist of a diffuse, sticky web of extracellular DNA, nuclear and granular proteins, and serve to ensnare and kill pathogens. In addition to catching pathogens, the cytotoxic molecules and proteases on NETs have the potential to inflict significant tissue damage. Additionally, NET components have been suggested to be key activators of infection-induced coagulopathy. These critical roles, at the interface between hemostasis and immunity, highlight the need for balance in the platelet response; too little platelet activity results in bleeding and immune deficit, too much leads to tissue pathogenesis. In this review, we highlight recent advances in our understanding of the role platelets play in inflammation, the link between platelets and NETs and the role platelets play in disease pathogenesis.

Cysteinyl Leukotrienes and Their Receptors: Emerging Therapeutic Targets in Central Nervous System Disorders

Cysteinyl leukotrienes are a group of the inflammatory lipid molecules well known as mediators of inflammatory signaling in the allergic diseases. Although they are traditionally known for their role in allergic asthma, allergic rhinitis, and others, recent advances in the field of biomedical research highlighted the role of these inflammatory mediators in a broader range of diseases such as in the inflammation associated with the central nervous system (CNS) disorders, vascular inflammation (atherosclerotic), and in cancer. Among the CNS diseases, they, along with their synthesis precursor enzyme 5-lipoxygenase and their receptors, have been shown to be associated with brain injury, Multiple sclerosis, Alzheimer's disease, Parkinson's disease, brain ischemia, epilepsy, and others. However, a lot more remains elusive as the research in these areas is emerging and only a little has been discovered. Herein, through this review, we first provided a general up-to-date information on the synthesis pathway and the receptors for the molecules. Next, we summarized the current findings on their role in the brain disorders, with an insight given to the future perspectives.

Development of smart cell-free and cell-based assay systems for investigation of leukotriene C4 synthase activity and evaluation of inhibitors

Cysteinyl leukotrienes (cys-LTs) cause bronchoconstriction in anaphylaxis and asthma. They are formed by 5-lipoxygenase (5-LOX) from arachidonic acid (AA) yielding the unstable leukotriene A₄ (LTA₄) that is subsequently conjugated with glutathione (GSH) by LTC₄ synthase (LTC₄S). Cys-LT receptor antagonists and LTC₄S inhibitors have been developed, but only the former have reached the market. High structural homology to related enzymes and lack of convenient test systems due to instability of added LTA₄ have hampered the development of LTC₄S inhibitors. We present smart cell-free and cell-based assay systems based on in situ-generated LTA₄ that allow studying LTC₄S activity and investigating LTC₄S inhibitors. Co-incubations of microsomes from HEK293 cells expressing LTC₄S with isolated 5-LOX efficiently converted exogenous AA to LTC₄ (~1.3μg/200μg protein). Stimulation of HEK293 cells co-expressing 5-LOX and LTC₄S with Ca²⁺-ionophore A23187 and 20μM AA resulted in strong LTC₄ formation (~250ng/10⁶ cells). MK-886, a well-known 5-LOX activating protein (FLAP) inhibitor that also acts on LTC₄S, consistently inhibited LTC₄ formation in all assay types (IC₅₀=3.1-3.5μM) and we successfully confirmed TK04a as potent LTC₄S inhibitor in these assay systems (IC₅₀=17 and 300nM, respectively). We demonstrated transcellular LTC₄ biosynthesis between neutrophils or 5-LOX-expressing HEK293 cells that produce LTA₄ from AA and HEK293 cells expressing LTC₄S that transform LTA₄ to LTC₄. In conclusion, our assay approaches are advantageous as the substrate LTA₄ is generated in situ and are suitable for studying enzymatic functionality of LTC₄S including site-directed mutations and evaluation of LTC₄S inhibitors.

Lipoxin and resolvin biosynthesis is dependent on 5-lipoxygenase activating protein

Resolution of acute inflammation is an active process coordinated by proresolving lipid mediators (SPMs) such as lipoxins (LXs) and resolvins (Rvs), which are formed by the concerted action of 2 lipoxygenases (LOs). Because the exact molecular mechanisms of SPM biosynthesis are not completely understood, we aimed to investigate LX and D-type Rv formation in human leukocytes and HEK293T cells overexpressing leukotriene (LT) pathway enzymes. Activity assays in precursor (15-hydroxyeicosatetraenoic acids, 17-HDoHE)-treated granulocytes [polymorphonuclear leukocytes (PMNLs)] showed a strict dependence of LXA4/RvD1 biosynthesis on cell integrity, and incubation with recombinant human 5-LO did not lead to LX or Rv formation. Pharmacologic inhibition of 5-LO activating protein (FLAP) by MK-886 inhibited LXA4/RvD1 biosynthesis in precursor-treated PMNLs (drug concentration causing 50% inhibition ∼ 0.3/0.2 µM), as did knockdown of the enzyme in MM6 cells, and precursor-treated HEK293T overexpressing 5-LO produced high amounts of LXA4 only in the presence of FLAP. In addition, inhibition of cytosolic phospholipase A2α (cPLA2α) interfered with LXA4/RvD1 formation from exogenous precursors in PMNLs. Furthermore, inhibition of the LT synthases LTA4 hydrolase and LTC4 synthase in PMNL/platelet coincubations augmented LXA4 levels. These findings show that several enzymes known to be involved in the biosynthesis of proinflammatory LTs, such as FLAP and cPLA2α, also contribute to LX and Rv formation.

Platelet activation markers overexpressed specifically in patients with aspirin-exacerbated respiratory disease

BACKGROUND

Aspirin-exacerbated respiratory disease (AERD) is characterized by respiratory reactions on ingestion of COX-1 inhibitors and cysteinyl leukotriene overproduction. The hypersensitivity reaction is induced by low doses of aspirin that inhibit COX-1 in platelets.

OBJECTIVE

We sought to explore the role of platelets in the pathogenesis of AERD in patients under stable conditions and during an aspirin challenge test.

METHODS

Stable patients with AERD (n = 30), aspirin-tolerant asthma (ATA; n = 21), or idiopathic chronic eosinophilic pneumonia (n = 10) were enrolled. Platelet activation was estimated based on expression levels of P-selectin (CD62P), CD63, CD69, and GPIIb/IIIa (PAC-1) in peripheral platelets; percentages of circulating platelet-adherent leukocytes; and plasma levels of soluble P-selectin (sP-selectin) and soluble CD40 ligand (sCD40L).

RESULTS

In the stable condition, expression of all surface markers on platelets, the percentage of platelet-adherent eosinophils, and the plasma levels of sP-selectin and sCD40L were significantly higher in patients with AERD compared with those in patients with ATA. P-selectin and CD63 expression on platelets and plasma sP-selectin and sCD40L levels were positively correlated with the percentage of platelet-adherent eosinophils. Among these markers, P-selectin expression and plasma sP-selectin levels positively correlated with urinary concentrations of leukotriene E4. Additionally, plasma sP-selectin and sCD40L levels were negatively correlated with lung function. In contrast, platelet activation markers in patients with AERD did not change during the aspirin challenge test.

CONCLUSION

Peripheral platelets were activated more in patients with stable AERD compared with those in patients with stable ATA, patients with idiopathic chronic eosinophilic pneumonia, and control subjects. Platelet activation was involved in cysteinyl leukotriene overproduction and persistent airflow limitations in patients with AERD.

Platelets and allergic inflammation

Irrefutable clinical evidence demonstrates the activation of platelets in allergic diseases, including asthma, allergic rhinitis, and eczema. Indeed, experimental models of allergic disease have now shown that platelets play a fundamental role in the tissue recruitment of leucocytes following exposure to allergens. Furthermore, the extravascular presence of platelets in lungs of patients with asthma, and in animal models of allergic lung inflammation suggests that platelets may also contribute directly to allergic inflammation, through alterations in lung function, or by modulating processes involved in airway wall remodelling. Despite significant platelet activation in patients with allergic diseases, it is of note that these patients have been described as having a mild haemostastic defect, rather than an increased incidence of thrombosis. This suggests a dichotomy exists in platelet activation during inflammation compared to haemostasis, and that hitherto undiscovered platelet activation pathways might be exploited to create novel anti-inflammatory therapies without affecting the critical function of platelets in haemostasis.

Transcellular biosynthesis of eicosanoid lipid mediators

The synthesis of oxygenated eicosanoids is the result of the coordinated action of several enzymatic activities, from phospholipase A2 that releases the polyunsaturated fatty acids from membrane phospholipids, to primary oxidative enzymes, such as cyclooxygenases and lipoxygenases, to isomerases, synthases and hydrolases that carry out the final synthesis of the biologically active metabolites. Cells possessing the entire enzymatic machinery have been studied as sources of bioactive eicosanoids, but early on evidence proved that biosynthetic intermediates, albeit unstable, could move from one cell type to another. The biosynthesis of bioactive compounds could therefore be the result of a coordinated effort by multiple cell types that has been named transcellular biosynthesis of the eicosanoids. In several cases cells not capable of carrying out the complete biosynthetic process, due to the lack of key enzymes, have been shown to efficiently contribute to the final production of prostaglandins, leukotrienes and lipoxins. We will review in vitro studies, complex functional models, and in vivo evidences of the transcellular biosynthesis of eicosanoids and the biological relevance of the metabolites resulting from this unique biosynthetic pathway. This article is part of a Special Issue entitled "Oxygenated metabolism of PUFA: analysis and biological relevance".

Prostaglandin E2 resistance in granulocytes from patients with aspirin-exacerbated respiratory disease

BACKGROUND

Aspirin-exacerbated respiratory disease (AERD) is an inflammatory condition of the respiratory tract and is characterized by overproduction of leukotrienes (LT) and large numbers of circulating granulocyte-platelet complexes. LT production can be suppressed by prostaglandin E(2) (PGE(2)) and the cyclic AMP-dependent protein kinase A (PKA).

OBJECTIVE

To determine if PGE(2)-dependent control of LT production by granulocytes is dysregulated in AERD.

METHODS

Granulocytes from well-characterized patients with and without AERD were activated ex vivo and subjected to a range of functional and biochemical analyses.

RESULTS

Granulocytes from subjects with AERD generated more LTB4 and cysteinyl LTs than did granulocytes from controls with aspirin-tolerant asthma and controls without asthma. When compared with controls, granulocytes from subjects with AERD had comparable levels of EP(2) protein expression and PGE(2)-mediated cAMP accumulation, yet were resistant to PGE(2)-mediated suppression of LT generation. Percentages of platelet-adherent neutrophils correlated positively with LTB4 generation and inversely with responsiveness to PGE(2)-mediated suppression of LTB(4). The PKA inhibitor H89 potentiated LTB4 generation by control granulocytes but was inactive in granulocytes from individuals with AERD and had no effect on platelet P-selectin induction. Both tonic PKA activity and levels of PKA catalytic gamma subunit protein were significantly lower in granulocytes from individuals with AERD relative to those from controls.

CONCLUSIONS

Impaired granulocyte PKA function in AERD may lead to dysregulated control of 5-lipoxygenase activity by PGE(2), whereas adherent platelets lead to increased production of LTs, which contributes to the features of persistent respiratory tract inflammation and LT overproduction.

Platelet lipidomics: modern day perspective on lipid discovery and characterization in platelets

Lipids are diverse families of biomolecules that perform essential structural and signaling roles in platelets. Their formation and metabolism are tightly controlled by enzymes and signal transduction pathways, and their dysregulation leads to significant defects in platelet function and disease. Platelet activation is associated with significant changes to membrane lipids, and formation of diverse bioactive lipids plays essential roles in hemostasis. In recent years, new generation mass spectrometry analysis of lipids (termed lipidomics) has begun to alter our understanding of how these molecules participate in key cellular processes. Although the application of lipidomics to platelet biology is still in its infancy, seminal earlier studies have shaped our knowledge of how lipids regulate key aspects of platelet biology, including aggregation, shape change, coagulation, and degranulation, as well as how lipids generated by platelets influence other cells, such as leukocytes and the vascular wall, and thus how they regulate hemostasis, vascular integrity, and inflammation, as well as contribute to pathologies, including arterial/deep vein thrombosis and atherosclerosis. This review will provide a brief historical perspective on the characterization of lipids in platelets, then an overview of the new generation lipidomic approaches, their recent application to platelet biology, and future perspectives for research in this area. The major platelet-regulatory lipid families, their formation, metabolism, and their role in health and disease, will be summarized.

The interrelationship between leukotriene B4 and leukotriene-A4-hydrolase in collagen/adjuvant-induced arthritis in rats

This study aimed to check the involvement of lipid mediator leukotriene (LT) B4 and the activity of LTA4 hydrolase (LTA4H) in the development of arthritis induced in rats by collagen and adjuvant (CIA). High-performance liquid chromatography (HPLC) and enzyme immunoassay (EIA) were used for measurements of LTB4 and LTA4H in plasma, synovial fluid (SF), soluble (SO), and solubilized membrane-bound fraction (MB) from synovial tissue (ST) and peripheral blood mononuclear cells (PBMCs) of CIA-arthritic and CIA-resistant. EIA process is simple, clean, and rapid and offered advantages over HPLC, showing that in SF and MB-PBMCs of CIA-arthritic and CIA-resistant, and in MB-ST of CIA-resistant, LTB4 and LTA4H were altered in parallel and were positively related. In the plasma and SO-ST and SO-PBMCs of CIA-arthritic and CIA-resistant, and in MB-ST of CIA-arthritic, this pattern was not found. The primordial role played by LTA4H in the biosynthesis of LTB4 was confirmed together with the existence of alternative steps that regulate LTB4 without participation of LTA4H. The involvement of compartmentalized and coupled changes of LTB4 and LTA4H in the resistance and development of arthritis in CIA model was demonstrated for the first time.

Prostaglandin E2 deficiency causes a phenotype of aspirin sensitivity that depends on platelets and cysteinyl leukotrienes

Aspirin-exacerbated respiratory disease (AERD) is characterized by asthma, tissue eosinophilia, overproduction of cysteinyl leukotrienes (cysLTs), and respiratory reactions to nonselective cyclooxygenase (COX) inhibitors. Ex vivo studies suggest that functional abnormalities of the COX-2/microsomal prostaglandin (PG)E2 synthase-1 system may underlie AERD. We demonstrate that microsomal PGE2 synthase-1 null mice develop a remarkably AERD-like phenotype in a model of eosinophilic pulmonary inflammation. Lysine aspirin (Lys-ASA)-challenged PGE2 synthase-1 null mice exhibit sustained increases in airway resistance, along with lung mast cell (MC) activation and cysLT overproduction. A stable PGE2 analog and a selective E prostanoid (EP)2 receptor agonist blocked the responses to Lys-ASA by ∼90%; EP3 and EP4 agonists were also active. The increases in airway resistance and MC products were blocked by antagonists of the type 1 cysLT receptor or 5-lipoxygenase, implying that bronchoconstriction and MC activation were both cysLT dependent. Lys-ASA-induced cysLT generation and MC activation depended on platelet-adherent granulocytes and T-prostanoid (TP) receptors. Thus, lesions that impair the inducible generation of PGE2 remove control of platelet/granulocyte interactions and TP-receptor-dependent cysLT production, permitting MC activation in response to COX-1 inhibition. The findings suggest applications of antiplatelet drugs or TP receptor antagonists for the treatment of AERD.

Neutrophil and platelet complexes and their relevance to neutrophil recruitment and activation

The manifestation of platelet 'satallitism' around neutrophils in whole blood is a long acknowledged phenomenon [1]. Circulating platelet-neutrophil complexes (PNC) occur in a diverse range of inflammatory disorders and infections that affect numerous organs of the body. Animal models have revealed that the formation of PNC is required for the recruitment of neutrophils to inflamed tissue, since platelets 'prime' neutrophils for efficient adhesion to vascular endothelium via the up-regulation of integrins and enhanced responsiveness to chemokines (Fig. 1). Perhaps surprisingly, the surface contact between platelets and neutrophils additionally enhances other neutrophil functions, such as chemotaxis that is required for migration into tissues, trans-cellular production of eicosanoids, phagocytosis and trapping of pathogens, increased respiratory burst leading to the production of reactive oxygen species (ROS), and modulation of neutrophil apoptosis (Fig. 1). Platelet P-selectin appears to have a particular role in enhancing the majority of these activities, and the influence of platelet P-selectin is not therefore confined to the initial rolling events in the process of neutrophil extravasation.

Pathogenesis of aspirin-exacerbated respiratory disease and reactions

Physiologic and pharmacologic studies support the hypothesis that aspirin-exacerbated respiratory disease (AERD) involves fundamental dysregulation in the production of and end-organ responsiveness to both antiinflammatory eicosanoids (prostaglandin E2) and proinflammatory effectors (cysteinyl leukotrienes). The acquired nature of AERD implies a disturbance in a potential epigenetic control mechanism of the relevant mediator systems, which may be a result of incompletely clarified environmental factors (eg, viral or bacterial infections, inhaled pollutants).

Platelet-leukocyte crosstalk: Linking proinflammatory responses to procoagulant state

Platelet activation is known to be associated with the release of a vast array of chemokines and proinflammatory lipids which induce pleiotropic effects on a wide variety of tissues and cells, including leukocytes. During thrombosis, the recruitment of leukocytes to activated platelets is considered an important step which not only links thrombosis to inflammatory responses but may also enhance procoagulant state. This phenomenon is highly regulated and influenced by precise mutual interactions between the cells at site of vascular injury and thrombi formation. Platelet-leukocyte interaction involves a variety of mediators including adhesion molecules, chemokines and chemoattractant molecules, shed proteins, various proinflammatory lipids and other materials. The current review addresses the detailed mechanisms underlying platelet-leukocyte crosstalk. This includes their adhesive interactions, transcellular metabolisms, induced tissue factor activity and neutrophil extracellular traps formation as well as the impacts of these phenomena in modulation of the proinflammatory and procoagulant functions in a reciprocal manner that enhances the physiological responses.

Pharmacological modulation of the inflammatory actions of platelets

Patients with inflammatory diseases often exhibit a change in platelet function, with these alterations being clearly distinct from the well-characterized role of platelets in haemostasis and thrombosis. It has recently been revealed that platelets can behave as innate inflammatory cells in immune responses with roles in leukocyte recruitment, migration into tissues, release of cytotoxic mediators, and in tissue remodelling following injury.Platelets exhibit a wide range of receptors for mediators involved in the inflammatory pathway and the immune response (Fig. 1). These include purinergic receptors, selectins, integrins, toll-like receptors, immunoglobulins, and chemokine receptors, but the precise role platelets play in the inflammatory process is still under investigation. Nevertheless, given that many of these receptors are distinct from those involved in thrombosis and haemostasis, this raises the real possibility of targeting these receptors to regulate inflammatory diseases without compromising haemostasis.

Enhanced formation of 5-oxo-6,8,11,14-eicosatetraenoic acid by cancer cells in response to oxidative stress, docosahexaenoic acid and neutrophil-derived 5-hydroxy-6,8,11,14-eicosatetraenoic acid

The 5-lipoxygenase (5-LO) product 5-oxo-6,8,11,14-eicosatetraenoic acid (5-oxo-ETE), which is a potent chemoattractant for myeloid cells, is known to promote the survival of prostate cancer cells. In the present study, we found that PC3 prostate cancer cells and cell lines derived from breast (MCF7) and lung (A-427) cancers contain 5-hydroxyeicosanoid dehydrogenase (5-HEDH) activity and have the ability to synthesize 5-oxo-ETE from its precursor 5S-hydroxy-6,8,11,14-eicosatetraenoic acid (5-HETE) when added as an exogenous substrate. H(2)O(2) strongly stimulated the synthesis of 5-oxo-ETE and induced dramatic increases in the levels of both glutathione disulfide and NADP(+). The effects of H(2)O(2) on 5-oxo-ETE and NADP(+) were blocked by N-ethylmaleimide (NEM), indicating that this effect was mediated by the glutathione reductase-dependent generation of NADP(+), the cofactor required by 5-HEDH. 5-Oxo-ETE synthesis was also stimulated by agents that have cytotoxic effects on tumor cells, including 4,7,10,13,16,19-docosahexaenoic acid, tamoxifen and MK-886. Because PC3 cells have only modest 5-LO activity compared with inflammatory cells, we investigated their ability to contribute to the transcellular biosynthesis of 5-oxo-ETE from neutrophil-derived 5-HETE. Stimulation of neutrophils with arachidonic acid and calcium ionophore in the presence of PC3 cells led to a large and selective increase in 5-oxo-ETE synthesis compared with controls in which PC3 cell 5-oxo-ETE synthesis was selectively blocked by pretreatment with NEM. The ability of prostate tumor cells to synthesize 5-oxo-ETE may contribute to tumor cell proliferation as well as the influx of inflammatory cells, which may further induce cell proliferation through the release of cytokines. 5-Oxo-ETE may be an attractive target in cancer therapy.

ALOX5 gene variants affect eicosanoid production and response to fish oil supplementation

The objective of this study was to determine whether 5-lipoxygenase (ALOX5) gene variants associated with cardiovascular disease affect eicosanoid production by monocytes. The study was a randomized, double-masked, parallel intervention trial with fish oil (5.0 g of fish oil daily, containing 2.0 g of eicosapentaenoic acid [EPA] and 1.0 g of docosahexaenoic acid [DHA]) or placebo oil (5.0 g of corn/soy mixture). A total of 116 subjects (68% female, 20-59 years old) of African American ancestry enrolled, and 98 subjects completed the study. Neither ALOX5 protein nor arachidonic acid-derived LTB4, LTD4, and LTE4 varied by genotype, but 5-hydroxyeicosatetraenoate (5-HETE), 6-trans-LTB4, 5-oxo-ETE, 15-HETE, and 5,15-diHETE levels were higher in subjects homozygous for the ALOX5 promoter allele containing five Sp1 element tandem repeats ("55" genotype) than in subjects with one deletion (d) (three or four repeats) and one common ("d5" genotype) allele or with two deletion ("dd") alleles. The EPA-derived metabolites 5-HEPE and 15-HEPE and the DHA-derived metabolite 17-HDoHE had similar associations with genotype and increased with supplementation; 5-HEPE and 15-HEPE increased, and 5-oxo-ETE decreased to a greater degree in the 55 than in the other genotypes. This differential eicosanoid response is consistent with the previously observed interaction of these variants with dietary intake of omega-3 fatty acids in predicting cardiovascular disease risk.

Claude H. Organ, Jr. memorial lecture: splanchnic hypoperfusion provokes acute lung injury via a 5-lipoxygenase-dependent mechanism

Postinjury multiple organ failure (MOF) is the net result of a dysfunctional immune response to injury characterized by a hyperactive innate system and a suppressed adaptive system. Acute lung injury (ALI) is the first clinical manifestation of organ failure, followed by renal and hepatic dysfunction. Circulatory shock is integral in the early pathogenesis of MOF, and the gut has been invoked as the motor of MOF. Mesenteric lymph is recognized as the mechanistic link between splanchnic ischemia/reperfusion and distant organ dysfunction, but the specific mediators remain to be defined. Current evidence suggests the lipid fraction of postshock mesenteric lymph is central in the etiology of ALI. Specifically, our recent work suggests that intestinal phospholipase A2 generated arachidonic acid and its subsequent 5-lipoxygenase products are essential in the pathogenesis of ALI. Proteins conveyed via postshock mesenteric lymph also may have an important role. Elucidating these mediators and the timing of their participation in pulmonary inflammation is critical in translating our current knowledge to new therapeutic strategies at the bedside.

Sulfido-peptide leukotrienes in coronary heart disease - relationship with disease instability and myocardial ischaemia

BACKGROUND

Urinary excretion of leukotriene (LT) E(4) is an index of LTC(4) biosynthesis and platelet-neutrophil interactions, which may occur in coronary heart disease and contribute to myocardial ischaemia. Enhanced LTC(4) biosynthesis may be a consequence of myocardial ischaemia or be linked to its pathogenetic substrate.

METHODS AND RESULTS

Overnight urine collections were obtained from 17 patients with chronic stable angina, three patients with Prinzmetal's angina, 16 patients with non ST-elevation acute coronary syndromes (NSTE-ACS) and six patients with acute ST-elevation myocardial infarction (STEMI). LTE(4) excretion was measured by enzyme immunoassay after HPLC separation. Compared with healthy controls (51.1 +/- 21.3 pg mg(-1) creatinine, mean +/- SD, n = 11) and with non-coronary cardiac controls (36.6 +/- 9.8 pg mg(-1) creatinine, n = 9), LTE(4) excretion was unchanged in stable angina (40.5 +/- 25.8 pg mg(-1) creatinine), but significantly (P < 0.01) increased in NSTE-ACS (122.7 +/- 137.2 pg mg(-1) creatinine) and STEMI (213.4 +/- 172.4 pg mg(-1) creatinine). In these patients, LTE(4) excretion rapidly dropped after day 1, consistent with effective coronary reperfusion. In patients with NSTE-ACS, the increase in LTE(4) excretion was entirely restricted to patients with recent (< 48 h) spontaneous anginal episodes. Myocardial ischaemia elicited by a positive exercise stress test was not accompanied by any detectable increase in LTE(4) excretion, while a significant (P < 0.01) increase was detected after a single-vessel percutaneous coronary interventions (PCI) procedure (n = 10), as compared with diagnostic angiography (n = 9).

CONCLUSIONS

In coronary heart disease, increased LTC(4) biosynthesis is restricted to ACS and not linked to myocardial ischaemia per se, but likely to the occurrence of plaque disruption.

Leukotriene pathway genetics and pharmacogenetics in allergy

Leukotrienes (LT) are biologically active lipid mediators known to be involved in allergic inflammation. Leukotrienes have been shown to mediate diverse features of allergic conditions including inflammatory cell chemotaxis/activation and smooth muscle contraction. Cysteinyl leukotrienes (LTC(4), LTD(4) and, LTE(4)) and the dihydroxy leukotriene LTB(4) are generated by a series of enzymes/proteins constituting the LT synthetic pathway or 5-lipoxygenase (5-LO) pathway. Their function is mediated by interacting with multiple receptors. Leukotriene receptor antagonists (LTRA) and LT synthesis inhibitors (LTSI) have shown clinical efficacy in asthma and more recently in allergic rhinitis. Despite growing knowledge of leukotriene biology, the molecular regulation of these inflammatory mediators remains to be fully understood. Genes encoding enzymes of the 5-LO pathway (i.e. ALOX5, LTC4S and LTA4H) and encoding for LT receptors (CYSLTR1/2 and LTB4R1/2) provide excellent candidates for disease susceptibility and severity; however, their role remains unclear. Preliminary data also suggest that 5-LO pathway/receptor gene polymorphism can predict patient responses to LTSI and LTRA; however, the exact mechanisms require elucidation. The aim of this review was to summarize the recent advances in the knowledge of these important mediators, focusing on genetic and pharmacogenetic aspects in the context of allergic phenotypes.

Transcellular biosynthesis of cysteinyl leukotrienes in vivo during mouse peritoneal inflammation

Leukotrienes (LTs) are lipid mediators of inflammation formed by enzymatic oxidation of arachidonic acid. One intriguing aspect of LT production is transcellular biosynthesis: cells expressing 5-lipoxygenase (5LO) form LTA(4) and transfer it to cells expressing LTA(4) hydrolase (LTA(4)H) or LTC(4) synthase (LTC(4)S) to produce LTB(4) or LTC(4). This process has been demonstrated in vivo for LTB(4), but not for cysteinyl LTs (cysLTs). We examined transcellular cysLT synthesis during zymosan-induced peritonitis, using bone marrow transplants with transgenic mice deficient in key enzymes of LT synthesis and analyzing all eicosanoids by liquid chromatography/tandem mass spectrometry. WT mice time-dependently produced LTB(4) and cysLTs (LTC(4), LTD(4), and LTE(4)). 5LO(-/-) mice were incapable of producing LTs. WT bone marrow cells restored this biosynthetic ability, but 5LO(-/-) bone marrow did not rescue LT synthesis in irradiated WT mice, demonstrating that bone marrow-derived cells are the ultimate source of all LTs in this model. Total levels of 5LO-derived products were comparable in LTA(4)H(-/-) and WT mice, but were reduced in LTC(4)S(-/-) animals. No differences in prostaglandin production were observed between these transgenic or chimeric mice. Bone marrow cells from LTA(4)H(-/-) or LTC(4)S(-/-) mice injected into 5LO(-/-) mice restored the ability to synthesize LTB(4) and cysLTs, providing unequivocal evidence of efficient transcellular biosynthesis of cysLTs. These results highlight the potential relevance of transcellular exchange of LTA(4) for the synthesis of LTs mediating biological activities during inflammatory events in vivo.

Determination of leukotriene A(4) stabilization by S100A8/A9 proteins using mass spectrometry

Leukotriene A(4) (LTA(4)) is the precursor for the formation of bioactive leukotrienes, but is highly susceptible to nonenzymatic hydrolysis. Although it is chemically reactive, LTA(4) participates in the process of transcellular metabolism, which requires the transfer of LTA(4) from one cell to another for the production of additional leukotrienes. Due to the susceptibility of LTA(4) to hydrolysis, various methods have been used to measure the half-life of LTA(4) in the presence of different proteins in efforts to understand how it is transported between cells. In this work, a new liquid chromatography mass spectrometry technique was developed to improve upon these previous assays that analyzed LTA(4) directly. The new technique derivatizes LTA(4) to stable compounds for analysis and removes the potential for sample decomposition between analytical runs. This assay was used in measuring the capabilities of the S100A8/A9 protein complex isolated from human neutrophils to stabilize LTA(4). It was determined that the S100A8/A9 protein complex protects LTA(4) from hydrolysis in a Ca(2+) dependent manner and increases LTA(4) half-life to in excess of 35 and 5 min at 4 degrees C and 37 degrees C, respectively.

A novel clade of cysteinyl leukotriene scavengers in soft ticks

Inflammation is an important vertebrate defense mechanism against ecto-parasites for which ticks have evolved numerous mechanisms of modulation. AM-33 and TSGP4, related lipocalins from the soft ticks Argas monolakensis and Ornithodoros savignyi bind cysteinyl leukotrienes with high affinity as measured by isothermal titration calorimetry. This was confirmed in a smooth muscle bioassay that measured contraction of guinea pig ileum induced by leukotriene C4 where both proteins inhibited contraction effectively. Conservation of this function in two diverse soft tick genera suggests that scavenging of cysteinyl leukotrienes evolved in the ancestral soft tick lineage. In addition soft ticks also evolved mechanisms that target other mediators of inflammation that include scavenging of histamine, serotonin, leukotriene B4, thromboxane A2, ATP and inhibition of the complement cascade. Inhibitors of blood-coagulation and platelet aggregation were also present in the ancestral soft tick lineage. Because histamine and cysteinyl leukotrienes are mainly produced by mast cells and basophils, and these cells are important in the mediation of tick rejection reactions, these findings indicate an ancient antagonistic relationship between ticks and the immune system. As such, modulation of the hemostatic system as well as inflammation was important adaptive responses in the evolution of a blood-feeding lifestyle in soft ticks.

Transcellular biosynthesis of cysteinyl leukotrienes in rat neuronal and glial cells

Leukotrienes are mediators of inflammation that belong to a family of lipids derived from arachidonic acid by the action of 5-lipoxygenase. Leukotrienes have been detected in the central nervous system in association with different pathological events, but little is known about their biosynthesis or function in the brain. When rat neurons and glial cells in primary culture were stimulated with the calcium ionophore, no significant biosynthesis of leukotrienes was detected using liquid chromatography/mass spectrometry (LC/MS) techniques. However, when exogenous LTA(4) was added to these cultured cells, both neurons and glia were able to synthesize LTC(4). Activated neutrophils are known to supply LTA(4) to other cells for transcellular biosynthesis of cysteinyl-leukotrienes. Since neutrophils can infiltrate brain tissue after stroke or traumatic brain injury, we examined whether neutrophils play a similar role in the central nervous system. When peripheral blood neutrophils were co-cultured with rat neurons, glia cells, and then stimulated with calcium ionophore, a robust production of LTC(4), LTD(4), and LTE(4) was observed, revealing that neurons and glia can participate in the transcellular mechanism of leukotriene biosynthesis. The formation of LTC(4) through this mechanism may be relevant in the genesis and progression of the inflammatory response as a result of brain injury.

ATP-dependent transport of leukotrienes B4 and C4 by the multidrug resistance protein ABCC4 (MRP4)

The proinflammatory mediators leukotriene (LT) B(4) and LTC(4) must be transported out of cells before they can interact with LT receptors. Previously, we identified the multidrug resistance protein ABCC1 (MRP1) as an efflux pump for LTC(4). However, the molecular basis for the efflux of LTB(4) was unknown. Here, we demonstrate that human ABCC4 mediates the ATP-dependent efflux of LTB(4) in the presence of reduced glutathione (GSH), whereby the latter can be replaced by S-methyl GSH. Transport studies were performed with inside-out membrane vesicles from V79 fibroblasts and Sf9 insect cells that contained recombinant ABCC4, with vesicles from human platelets and myelomonocytic U937 cells, which were rich in endogenous ABCC4, but ABCC1 was below detectability. Moreover, human polymorphonuclear leukocytes contained ABCC4. K(m) values for LTB(4) were 5.2 muM with vesicles from fibroblasts and 5.6 muM with vesicles from platelets. ABCC4, with its broad substrate specificity, also functioned as an ATP-dependent efflux pump for LTC(4) with a K(m) of 0.13 muM in vesicles from fibroblasts and 0.32 muM in vesicles from platelets. However, GSH was not required for the transport of this glutathionylated leukotriene. The transport of LTC(4) by ABCC4 explains its release from platelets during transcellular synthesis. ATP-dependent transport of LTB(4) and LTC(4) by ABCC4 was inhibited by several organic anions, including S-decyl GSH, sulindac sulfide, and by the LTD(4) receptor antagonists montelukast and 3-(((3-(2-(7-chloro-2-quinolinyl)ethenyl)phenyl)-((3-dimethyl-amino-3-oxopropyl)-thio)-methyl)thio)propanoic acid (MK571). Thus, as an efflux pump for the proinflammatory mediators LTB(4) and LTC(4), ABCC4 may represent a novel target for anti-inflammatory therapies.

Cysteinyl-leukotrienes and their receptors in asthma and other inflammatory diseases: critical update and emerging trends

Cysteinyl-leukotrienes (cysteinyl-LTs), that is, LTC4, LTD4, and LTE4, trigger contractile and inflammatory responses through the specific interaction with G protein-coupled receptors (GPCRs) belonging to the purine receptor cluster of the rhodopsin family, and identified as CysLT receptors (CysLTRs). Cysteinyl-LTs have a clear role in pathophysiological conditions such as asthma and allergic rhinitis (AR), and have been implicated in other inflammatory conditions including cardiovascular diseases, cancer, atopic dermatitis, and urticaria. Molecular cloning of human CysLT1R and CysLT2R subtypes has confirmed most of the previous pharmacological characterization and identified distinct expression patterns only partially overlapping. Interestingly, recent data provide evidence for the immunomodulation of CysLTR expression, the existence of additional receptor subtypes, and of an intracellular pool of CysLTRs that may have roles different from those of plasma membrane receptors. Furthermore, genetic variants have been identified for the CysLTRs that may interact to confer risk for atopy. Finally, a crosstalk between the cysteinyl-LT and the purine systems is being delineated. This review will summarize and attempt to integrate recent data derived from studies on the molecular pharmacology and pharmacogenetics of CysLTRs, and will consider the therapeutic opportunities arising from the new roles suggested for cysteinyl-LTs and their receptors.

Biosynthesis and metabolism of leukotrienes

Leukotrienes are metabolites of arachidonic acid derived from the action of 5-LO (5-lipoxygenase). The immediate product of 5-LO is LTA4 (leukotriene A4), which is enzymatically converted into either LTB4 (leukotriene B4) by LTA4 hydrolase or LTC4 (leukotriene C4) by LTC4 synthase. The regulation of leukotriene production occurs at various levels, including expression of 5-LO, translocation of 5-LO to the perinuclear region and phosphorylation to either enhance or inhibit the activity of 5-LO. Several other proteins, including cPLA2α (cytosolic phospholipase A2α) and FLAP (5-LO-activating protein) also assemble at the perinuclear region before production of LTA4. LTC4 synthase is an integral membrane protein that is present at the nuclear envelope; however, LTA4 hydrolase remains cytosolic. Biologically active LTB4 is metabolized by ω-oxidation carried out by specific cytochrome P450s (CYP4F) followed by β-oxidation from the ω-carboxy position and after CoA ester formation. Other specific pathways of leukotriene metabolism include the 12-hydroxydehydrogenase/15-oxo-prostaglandin-13-reductase that forms a series of conjugated diene metabolites that have been observed to be excreted into human urine. Metabolism of LTC4 occurs by sequential peptide cleavage reactions involving a γ-glutamyl transpeptidase that forms LTD4 (leukotriene D4) and a membrane-bound dipeptidase that converts LTD4 into LTE4 (leukotriene E4) before ω-oxidation. These metabolic transformations of the primary leukotrienes are critical for termination of their biological activity, and defects in expression of participating enzymes may be involved in specific genetic disease.

The role of antileukotriene drugs in management of rhinitis and rhinosinusitis

Antileukotriene drugs have been studied for more than 15 years. In this review we examine the role of leukotrienes in rhinitis and rhinosinusitis, and explore the clinical literature supporting the use of anti-leukotriene agents in these diseases. Although these medications clearly are efficacious in rhinitis, it is unclear where in the armamentarium they should be used. The evidence for use in sinusitis has not been well studied except in sinusitis-associated aspirin-exacerbated respiratory disease. In this circumstance there is information that allows use of antileukotriene agents to be considered efficacious. We provide our rationale for use and await future clinical studies to answer this important question.