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".

In vitro pharmacological evaluation of multitarget agents for thromboxane prostanoid receptor antagonism and COX-2 inhibition.

PURPOSE: Patients with high cardiovascular risk due to ageing and/or comorbidity (diabetes, atherosclerosis) that require effective management of chronic pain may take advantage from new non-steroidal anti-inflammatory drugs (NSAIDs) that at clinical dosages may integrate the anti-inflammatory activity and reduced gastrointestinal side effects of selective cyclooxygenase-2 (COX-2) inhibitor (coxib) with a cardioprotective component involving antagonism of thromboxane A2 prostanoid (TP) receptor. METHODS: New compounds were obtained modulating the structure of the most potent coxib, lumiracoxib, to obtain novel multitarget NSAIDs endowed with balanced coxib and TP receptor antagonist properties. Antagonist activity at TP receptor (pA2) was evaluated for all compounds in human platelets and in an heterologous expression system by measuring prevention of aggregation and Gq-dependent production of intracellular inositol phosphate induced by the stable thromboxane A2 (TXA2) agonist U46619. COX-1 and COX-2 inhibitory activities were assessed in human washed platelets and lympho-monocytes suspension, respectively. COX selectivity was determined from dose-response curves by calculating a ratio (COX-2/COX-1) of IC50 values. RESULTS: The tetrazole derivative 18 and the trifluoromethan sulfonamido-isoster 20 were the more active antagonists at TP receptor, preventing human platelet aggregation and intracellular signalling, with pA2 values statistically higher from that of lumiracoxib. Comparative data regarding COX-2/COX-1 selectivity showed that while compounds 18 and 7 were rather potent and selective COX-2 inhibitor, compound 20 was somehow less potent and selective for COX-2. CONCLUSION: These results indicate that compounds 18 and 20 are two novel combined TP receptor antagonists and COX-2 inhibitors characterized by a fairly balanced COX-2 inhibitor activity and TP receptor antagonism and that they may represent a first optimization of the original structure to improve their multitarget activity.

Autocrine activity of cysteinyl leukotrienes in human vascular endothelial cells: Signaling through the CysLT2 receptor.

We evaluated the autocrine activities of cysteinyl leukotrienes (cysteinyl-LTs) in HUVEC and studied the signaling and the pharmacological profile of the CysLT2 receptor (CysLT2R) expressed by ECs, finally assessing the role of the CysLT2R in permeability alterations in a model of isolated brain. Cysteinyl-LTs and their precursor LTA4 contracted HUVEC and increased permeability to macromolecules, increasing the formation of stress fibers through the phosphorylation of myosin light-chain (MLC) following Rho and PKC activation. Accordingly, in an organ model of cerebral vasculature with an intact intima, neutrophils challenge leaded to significant formation of cysteinyl-LTs and edema. Pretreatment with a selective CysLT2R antagonist prevented cytoskeleton rearrangement and HUVEC contraction, along with edema formation in the brain preparation, while leaving the synthesis of cysteinyl-LTs unaffected. We also demonstrate here that the CysLT1R antagonist zafirlukast, pranlukast, pobilukast and iralukast also possess CysLT2R antagonistic activity, which could help in reconsidering previous data on the role of cysteinyl-LTs in the cardiovascular system. The results obtained are further supporting a potential role for CysLT2R in cardiovascular disease.

Rosuvastatin inhibits human airway smooth muscle cells mitogenic response to eicosanoid contractile agents.

BACKGROUND: The concept of permanent narrowing of the airways resulting from chronic inflammation and fibrosis is called remodeling and is a common feature of asthma and chronic obstructive pulmonary disease (COPD). The eicosanoid contractile agents thromboxane A(2) (TxA(2)) and cysteinyl-leukotriene D(4) (LTD(4)) are among the recognized mitogens for human airway smooth muscle (ASM) cells. Statins are known to possess anti-inflammatory and immunomodulatory properties that are independent on their cholesterol-lowering effects and may result in clinical lung benefits. Rosuvastatin is the last agent of the lipid-lowering drugs to be introduced and experimental evidence indicates that it possess favorable pleiotropic effects in the cardiovascular and nervous systems. Yet, no data is available in the literature regarding its effects on human airway remodeling. The present study was aimed at examining the effect of rosuvastatin and the involvement of prenylated proteins in the response of human ASM cells to serum, epidermal growth factor (EGF) and eicosanoid contractile mitogens that activate TxA(2) prostanoid and LTD(4) receptors. METHODS: Cell growth was assessed by nuclear incorporation of [(3)H]thymidine in human ASM cells serum-starved and then stimulated for 48 h in MEM plus 0.1% BSA containing mitogens in the absence and presence of modulators of the mevalonate and prenylation pathways. RESULTS: We found that rosuvastatin dose-dependently inhibited serum-, EGF-, the TxA(2) stable analog U46619-, and LTD(4)-induced human ASM cells growth. All these effects were prevented by pretreatment with mevalonate. Addition of the prenylation substrates farnesol and geranylgeraniol reversed the effect of rosuvastatin on EGF and U46619, respectively. Interestingly, only mevalonate showed restoration of cell growth following rosuvastatin treatment in LTD(4) and LTD(4) plus EGF treated cells, suggesting a possible involvement of both farnesylated and geranylgeranylated proteins in the cysteinyl-LT-induced cell growth. CONCLUSIONS: The hydrophilic statin rosuvastatin exerts direct effects on human ASM cells mitogenic response in vitro by inhibiting prenylation of signaling proteins, likely small G proteins. These findings are consistent with previous observed involvement of small GTPase signaling in EGF- and U46619-induced human airway proliferation and corroborate the recent interest in the potential clinical benefits of statins in asthma/COPD.

Eicosanoids and their drugs in cardiovascular diseases: focus on atherosclerosis and stroke.

Eicosanoids are biologically active lipids in both physiologic and pathophysiologic situations. These mediators rapidly generate at sites of inflammation and act through specific receptors that following the generation of a signal transduction cascade, lead to coordinated cellular responses to specific stimuli. Prostanoids, that is, prostaglandins and thromboxane A(2), are active products of the cyclooxygenase pathway, while leukotrienes and lipoxins derive from the lipoxygenase pathway. In addition, a complex family of prostaglandin isomers called isoprostanes is derived as free-radical products of oxidative metabolism. While there is a wide consensus on the importance of the balance between proaggregating (thromboxane A(2)) and antiaggregating (prostacyclin) cyclooxygenase products in cardiovascular homeostasis, an increasing body of evidence suggests a key role also for other eicosanoids generated by lipoxygenases, epoxygenases, and nonenzymatic pathways in cardiovascular diseases. This intricate network of lipid mediators is unique considering that from a single precursor, arachidonic acid, may derive an array of bioproducts that interact within each other synergizing or, more often, behaving as functional antagonists.

Thromboxane A2 is a key regulator of pathogenesis during Trypanosoma cruzi infection.

Chagas' disease is caused by infection with the parasite Trypanosoma cruzi. We report that infected, but not uninfected, human endothelial cells (ECs) released thromboxane A(2) (TXA(2)). Physical chromatography and liquid chromatography-tandem mass spectrometry revealed that TXA(2) is the predominant eicosanoid present in all life stages of T. cruzi. Parasite-derived TXA(2) accounts for up to 90% of the circulating levels of TXA(2) in infected wild-type mice, and perturbs host physiology. Mice in which the gene for the TXA(2) receptor (TP) has been deleted, exhibited higher mortality and more severe cardiac pathology and parasitism (fourfold) than WT mice after infection. Conversely, deletion of the TXA(2) synthase gene had no effect on survival or disease severity. TP expression on somatic cells, but not cells involved in either acquired or innate immunity, was the primary determinant of disease progression. The higher intracellular parasitism observed in TP-null ECs was ablated upon restoration of TP expression. We conclude that the host response to parasite-derived TXA(2) in T. cruzi infection is possibly an important determinant of mortality and parasitism. A deeper understanding of the role of TXA(2) may result in novel therapeutic targets for a disease with limited treatment options.

Synthesis of cysteinyl leukotrienes in human endothelial cells: subcellular localization and autocrine signaling through the CysLT2 receptor.

The purpose of this study was to characterize enzyme, receptor, and signaling involved in the synthesis and the activity of cysteinyl leukotrienes (cys-LTs) in human umbilical vein endothelial cells (HUVECs). We used primary cultures of HUVECs and evaluated the formation of cys-LTs by RP-HPLC. Suicide inactivation and subcellular localization of the enzyme responsible for the conversion of leukotriene (LT) A(4) into LTC(4) were studied by repeated incubations with LTA(4) and immunogold electron microscopy. The CysLT(2) receptor in HUVECs was characterized by equilibrium binding studies, Western blot analysis, and immunohistochemistry. Concentration-response curves in HUVECs and in transfected COS-7 cells were used to characterize a novel specific CysLT(2) receptor antagonist (pA(2) of 8.33 and 6.79 against CysLT(2) and CysLT(1) receptors, respectively). The results obtained provide evidence that the mGST-II synthesizing LTC(4) in HUVECs is pharmacologically distinguishable from the LTC(4)-synthase (IC(50) of MK886 <5 µM for LTC(4)-synthase and >30 µM for mGST-II), is not suicide-inactivated and is strategically located on endothelial transport vesicles. The CysLT(2) receptor is responsible for the increase in intracellular Ca(2+) following exposure of HUVECs to cys-LTs and is coupled to a pertussis toxin-insensitive G(q) protein. The synthesis of cys-LTs from LTA(4) by endothelial cells is directly associated with the activation of the CysLT(2) receptor (EC(50) 0.64 µM) in a typical autocrine fashion.

Light on the structure of thromboxane A2receptor heterodimers.

The structure-based design of a mutant form of the thromboxane A(2) prostanoid receptor (TP) was instrumental in characterizing the structural determinants of the hetero-dimerization process of this G protein coupled receptor (GPCR). The results suggest that the hetero-dimeric complexes between the TPα and ß isoforms are characterized by contacts between hydrophobic residues in helix 1 from both monomers. Functional characterization confirms that TPα-TPß hetero-dimerization serves to regulate TPα function through agonist-induced internalization, with important implications in cardiovascular homeostasis. The integrated approach employed in this study can be adopted to gain structural and functional insights into the dimerization/oligomerization process of all GPCRs for which the structural model of the monomer can be achieved at reasonable atomic resolution.

Superactive mutants of thromboxane prostanoid receptor: functional and computational analysis of an active form alternative to constitutively active mutants.

In class A GPCRs the E/DRY motif is critical for receptor activation and function. According to experimental and computational data, R3.50 forms a double salt bridge with the adjacent E/D3.49 and E/D6.30 in helix 6, constraining the receptor in an inactive state. The disruption of this network of interactions facilitates conformational transitions that generate a signal or constitutive activity. Here we demonstrate that non-conservative substitution of either E129((3.49)) or E240((6.30)) of thromboxane prostanoid receptor (TP) resulted in mutants characterized by agonist-induced more efficient signaling properties, regardless of the G protein coupling. Results of computational modeling suggested a more effective interaction between G(q) and the agonist-bound forms of the TP mutants, compared to the wild type. Yet, none of the mutants examined revealed any increase in basal activity, precluding their classification as constitutively active mutants. Here, we propose that these alternative active conformations might be identified as superactive mutants or SAM.

A role for inflammatory mediators in heterologous desensitization of CysLT1 receptor in human monocytes.

Cysteinyl-leukotrienes (cysteinyl-LT) are rapidly generated at sites of inflammation and, in addition to their role in asthma, rhinitis, and other immune disorders, are increasingly regarded as significant inflammatory factors in cancer, gastrointestinal, cardiovascular diseases. We recently demonstrated that in monocyte/macrophage-like U937 cells, extracellular nucleotides heterologously desensitize CysLT(1) receptor (CysLT(1)R)-induced Ca(2+) transients. Given that monocytes express a number of other inflammatory and chemoattractant receptors, this study was aimed at characterizing transregulation between these different stimuli. We demonstrate that in U937 cells and in primary human monocytes, a series of inflammatory mediators activating G(i)-coupled receptor (FPR1, BLT(1)) desensitize CysLT(1)R-induced Ca(2+) response unidirectionally through activation of PKC. Conversely, PAF-R, exclusively coupled to G(q), cross-desensitizes CysLT(1)R without the apparent involvement of any kinase. Interestingly, G(s)-coupled receptors (beta(2)AR, H(1/2)R, EP(2/4)R) are also able to desensitize CysLT(1)R response through activation of PKA. Heterologous desensitization seems to affect mostly the G(i)-mediated signaling of the CysLT(1)R. The hierarchy of desensitization among agonists may be important for leukocyte signal processing at the site of inflammation. Considering that monocytes/macrophages are likely to be the major source of cysteinyl-LT in many immunological and inflammatory processes, shedding light on how their receptors are regulated will certainly help to better understand the role of these cells in orchestrating this complex network of integrated signals.

Montelukast Use Decreases Cardiovascular Events in Asthmatics.

Cysteinyl leukotrienes are proinflammatory mediators with a clinically established role in asthma and a human genetic and preclinical role in cardiovascular pathology. Given that cardiovascular disease has a critical inflammatory component, the aim of this work was to conduct an observational study to verify whether the use of a cysteinyl leukotriene antagonist, namely, montelukast, may protect asthmatic patients from a major cardiovascular event and, therefore, represent an innovative adjunct therapy to target an inflammatory component in cardiovascular disease. We performed an observational retrospective 3-year study on eight hundred adult asthmatic patients 18 years or older in Albania, equally distributed into two cohorts, exposed or nonexposed to montelukast usage, matched by age and gender according to information reported in the data collection. Patients with a previous history of myocardial infarction or ischemic stroke were excluded. In summary, 37 (4.6%) of the asthmatic patients, 32 nonexposed, and five exposed to montelukast suffered a major cardiovascular event during the 3-year observation period. All the cardiovascular events, in either group, occurred among patients with an increased cardiovascular risk. Our analyses demonstrate that, independent from gender, exposure to montelukast remained a significant protective factor for incident ischemic events (78% or 76% risk reduction depending on type of analysis). The event-free Kaplan-Meier survival curves confirmed the lower cardiovascular event incidence in patients exposed to montelukast. Our data suggest that there is a potential preventative role of montelukast for incident cardiac ischemic events in the older asthmatic population, indicating a comorbidity benefit of montelukast usage in asthmatics by targeting cysteinyl leukotriene-driven cardiac disease inflammation.

Leukotriene modifiers in the treatment of cardiovascular diseases.

Cysteinyl-leukotrienes (Cys-LTs) and LTB4 are potent proinflammatory mediators derived from arachidonic acid through the 5-lipoxygenase (5-LO) pathway, which exerts important pharmacological effects through their interaction with specific receptors: Cys-LT receptors (CysLT1 and CysLT2) and LTB4 receptors (BLT1 and BLT2). Published evidence justifies a broader role for LT receptor antagonists (LTRAs), in particular, montelukast, in the treatment of bronchial asthma, allergic rhinitis, and recently, in cardiocerebrovascular disease. The actions of Cys-LTs on the cardiovascular (CV) system are well-documented and include a broad array of activities with promising therapeutic targets in animal models exploring the use of selective 5-LO (or 5-LO-activating protein) inhibitors or dual LO-cycloxygenase-blocking agents in experimentally induced acute myocardial infarction. The picture that emerges from studies with LTRAs is more controversial at the moment, and some findings suggest a role for Cys-LTs in the extension of ischemic damage and in cardiac dysfunction during reperfusion; others do not. The aim of this short review is to summarize the state of present research about LT modifier treatment in CV disease.

Cysteinyl-leukotriene receptors and cellular signals.

Cysteinyl-leukotrienes (cysteinyl-LTs) exert a range of proinflammatory effects, such as constriction of airways and vascular smooth muscle, increase of endothelial cell permeability leading to plasma exudation and edema, and enhanced mucus secretion. They have proved to be important mediators in asthma, allergic rhinitis, and other inflammatory conditions, including cardiovascular diseases, cancer, atopic dermatitis, and urticaria. The classification into subtypes of the cysteinyl-LT receptors (CysLTRs) was based initially on binding and functional data, obtained using the natural agonists and a wide range of antagonists. CysLTRs have proved remarkably resistant to cloning. However, in 1999 and 2000, the CysLT1R and CysLT2R were successfully cloned and both shown to be members of the G-protein coupled receptors (GPCRs) superfamily. Molecular cloning has confirmed most of the previous pharmacological characterization and identified distinct expression patterns only partially overlapping. Recombinant CysLTRs couple to the Gq/11 pathway that modulates inositol phospholipids hydrolysis and calcium mobilization, whereas in native systems, they often activate a pertussis toxin-insensitive Gi/o-protein, or are coupled promiscuously to both G-proteins. Interestingly, recent data provide evidence for the existence of an additional receptor subtype that seems to respond to both cysteinyl-LTs and uracil nucleosides, and of an intracellular pool of CysLTRs that may have roles different from those of plasma membrane receptors. Finally, a cross-talk between the cysteinyl-LT and the purine systems is being delineated. This review will summarize recent data derived from studies on the molecular and cellular pharmacology of CysLTRs.

The DRY motif and the four corners of the cubic ternary complex model.

Recent structural data on GPCRs using a variety of spectroscopic approaches suggest that GPCRs adopt a dynamic conformational landscape, with ligands stabilizing subsets of these states to activate one or more downstream signaling effectors. A key outstanding question posed by this emerging dynamic structural model of GPCRs is what states, active, inactive, or intermediate are captured by the numerous crystal structures of GPCRs complexed with a variety of agonists, partial agonists, and antagonists. In the early nineties the discovery of inverse agonists and constitutive activity led to the idea that the active receptor state (R⁎) is an intrinsic property of the receptor itself rather than of the RG complex, eventually leading to the formulation of the cubic ternary complex model (CTC). Here, by a careful analysis of a series of data obtained with a number of mutants of the highly conserved E/DRY motif, we show evidences for the existence of all the receptor states theorized by the CTC, four 'uncoupled (R, R⁎ and HR and HR⁎), and, consequently four 'coupled' (RG, R⁎G, HRG and HR⁎G). The E/DRY motif located at the cytosolic end of transmembrane helix III of Class A GPCRs has been widely studied and analyzed because it forms a network of interactions believed to lock receptors in the inactive conformation (R), and, thus, to play a key role in receptor activation. Our conclusions are supported by recent crystal and NMR spectra, as well as by results obtained with two prototypical GPCRs using a new FRET technology that de-couples G protein binding to the receptor from signal transduction. Thus, despite its complexity and limitations, we propose that the CTC is a useful framework to reconcile pharmacological, biochemical and structural data.

Impaired thromboxane receptor dimerization reduces signaling efficiency: A potential mechanism for reduced platelet function in vivo.

Thromboxane A2 is a potent mediator of inflammation and platelet aggregation exerting its effects through the activation of a G protein-coupled receptor (GPCR), termed TP. Although the existence of dimers/oligomers in Class A GPCRs is widely accepted, their functional significance still remains controversial. Recently, we have shown that TPα and TPß homo-/hetero-dimers interact through an interface of residues in transmembrane domain 1 (TM1) whose disruption impairs dimer formation. Here, biochemical and pharmacological characterization of this dimer deficient mutant (DDM) in living cells indicates a significant impairment in its response to agonists. Interestingly, two single loss-of-function TPα variants, namely W29C and N42S recently identified in two heterozygous patients affected by bleeding disorders, match some of the residues mutated in our DDM. These two naturally occurring variants display a reduced potency to TP agonists and are characterized by impaired dimer formation in transfected HEK-293T cells. These findings provide proofs that lack of homo-dimer formation is a crucial process for reduced TPα function in vivo, and might represent one molecular mechanism through which platelet TPα receptor dysfunction affects the patient(s) carrying these mutations.

CysLT1 receptor-induced human airway smooth muscle cells proliferation requires ROS generation, EGF receptor transactivation and ERK1/2 phosphorylation.

BACKGROUND: Cysteine-containing leukotrienes (cysteinyl-LTs) are pivotal inflammatory mediators that play important roles in the pathophysiology of asthma, allergic rhinitis, and other inflammatory conditions. In particular, cysteinyl-LTs exert a variety of effects with relevance to the aetiology of asthma such as smooth muscle contraction, eosinophil recruitment, increased microvascular permeability, enhanced mucus secretion and decreased mucus transport and, finally, airway smooth muscle cells (ASMC) proliferation. We used human ASMC (HASMC) to identify the signal transduction pathway(s) of the leukotriene D4 (LTD4)-induced DNA synthesis. METHODS: Proliferation of primary HASMC was measured by [3H]thymidine incorporation. Phosphorylation of EGF receptor (EGF-R) and ERK1/2 was assessed with a polyclonal anti-EGF-R or anti-phosphoERKl/2 monoclonal antibody. A Ras pull-down assay kit was used to evaluate Ras activation. The production of reactive oxygen species (ROS) was estimated by measuring dichlorodihydrofluorescein (DCF) oxidation. RESULTS: We demonstrate that in HASMC LTD4-stimulated thymidine incorporation and potentiation of EGF-induced mitogenic signaling mostly depends upon EGF-R transactivation through the stimulation of CysLT1-R. Accordingly, we found that LTD4 stimulation was able to trigger the increase of Ras-GTP and, in turn, to activate ERK1/2. We show here that EGF-R transactivation was sensitive to pertussis toxin (PTX) and phosphoinositide 3-kinase (PI3K) inhibitors and that it occurred independently from Src activity, despite the observation of a strong impairment of LTD4-induced DNA synthesis following Src inhibition. More interestingly, CysLT1-R stimulation increased the production of ROS and N-acetylcysteine (NAC) abolished LTD4-induced EGF-R phosphorylation and thymidine incorporation. CONCLUSION: Collectively, our data demonstrate that in HASMC LTD4 stimulation of a Gi/o coupled CysLT1-R triggers the transactivation of the EGF-R through the intervention of PI3K and ROS. While PI3K and ROS involvement is an early event, the activation of Src occurs downstream of EGF-R activation and is followed by the classical Ras-ERK1/2 signaling pathway to control G1 progression and cell proliferation.

Cysteinyl Leukotrienes Pathway Genes, Atopic Asthma and Drug Response: From Population Isolates to Large Genome-Wide Association Studies.

Genetic variants associated with asthma pathogenesis and altered response to drug therapy are discussed. Many studies implicate polymorphisms in genes encoding the enzymes responsible for leukotriene synthesis and intracellular signaling through activation of seven transmembrane domain receptors, such as the cysteinyl leukotriene 1 (CYSLTR1) and 2 (CYSLTR2) receptors. The leukotrienes are polyunsaturated lipoxygenated eicosatetraenoic acids that exhibit a wide range of pharmacological and physiological actions. Of the three enzymes involved in the formation of the leukotrienes, arachidonate 5 lipoxygenase 5 (ALOX5), leukotriene C4 synthase (LTC4S), and leukotriene hydrolase (LTA4H) are all polymorphic. These polymorphisms often result in variable production of the CysLTs (LTC4, LTD4, and LTE4) and LTB4. Variable number tandem repeat sequences located in the Sp1-binding motif within the promotor region of the ALOX5 gene are associated with leukotriene burden and bronchoconstriction independent of asthma risk. A 444A > C SNP polymorphism in the LTC4S gene, encoding an enzyme required for the formation of a glutathione adduct at the C-6 position of the arachidonic acid backbone, is associated with severe asthma and altered response to the CYSLTR1 receptor antagonist zafirlukast. Genetic variability in the CysLT pathway may contribute additively or synergistically to altered drug responses. The 601 A > G variant of the CYSLTR2 gene, encoding the Met201Val CYSLTR2 receptor variant, is associated with atopic asthma in the general European population, where it is present at a frequency of ∼2.6%. The variant was originally found in the founder population of Tristan da Cunha, a remote island in the South Atlantic, in which the prevalence of atopy is approximately 45% and the prevalence of asthma is 36%. In vitro work showed that the atopy-associated Met201Val variant was inactivating with respect to ligand binding, Ca2+ flux and inositol phosphate generation. In addition, the CYSLTR1 gene, located at Xq13-21.1, has been associated with atopic asthma. The activating Gly300Ser CYSLTR1 variant is discussed. In addition to genetic loci, risk for asthma may be influenced by environmental factors such as smoking. The contribution of CysLT pathway gene sequence variants to atopic asthma is discussed in the context of other genes and environmental influences known to influence asthma.

CysLT1 leukotriene receptor antagonists inhibit the effects of nucleotides acting at P2Y receptors.

Montelukast and pranlukast are orally active leukotriene receptor antagonists selective for the CysLT1 receptor. Conversely, the hP2Y(1,2,4,6,11,12,13,14) receptors represent a large family of GPCRs responding to either adenine or uracil nucleotides, or to sugar-nucleotides. Montelukast and pranlukast were found to inhibit nucleotide-induced calcium mobilization in a human monocyte-macrophage like cell line, DMSO-differentiated U937 (dU937). Montelukast and pranlukast inhibited the effects of UTP with IC50 values of 7.7 and 4.3 microM, respectively, and inhibited the effects of UDP with IC50 values of 4.5 and 1.6 microM, respectively, in an insurmountable manner. Furthermore, ligand binding studies using [3H]LTD4 excluded the possibility of orthosteric nucleotide binding to the CysLT1 receptor. dU937 cells were shown to express P2Y2, P2Y4, P2Y6, P2Y11, P2Y13 and P2Y14 receptors. Therefore, these antagonists were studied functionally in a heterologous expression system for the human P2Y receptors. In 1321N1 astrocytoma cells stably expressing human P2Y(1,2,4,6) receptors, CysLT1 antagonists inhibited both the P2Y agonist-induced activation of phospholipase C and intracellular Ca2+ mobilization. IC50 values at P2Y1 and P2Y6 receptors were <1 microM. In control astrocytoma cells expressing an endogenous M3 muscarinic receptor, 10 microM montelukast had no effect on the carbachol-induced rise in intracellular Ca2+. These data demonstrated that CysLT1 receptor antagonists interact functionally with signaling pathways of P2Y receptors, and this should foster the study of possible implications for the clinical use of these compounds in asthma or in other inflammatory conditions.

Pharmacological differences among CysLT(1) receptor antagonists with respect to LTC(4) and LTD(4) in human lung parenchyma.

We have previously reported, by means of equilibrium binding studies, the existence of two distinct binding sites with receptor characteristics for LTC(4) and LTD(4) in human lung parenchyma (HLP) membranes using S-decyl-glutathione (S-decyl-GSH) to inhibit LTC(4) binding to a number of non-receptor sites. Recently, we have been able to avoid the use of S-decyl-GSH in kinetic experiments and to characterize a distinctive pharmacological profile for the LTC(4) high affinity binding sites which do not correlates with the ability of both LTD(4) and LTC(4) to contract isolated HLP strips through the CysLT(1) receptor. Here, we report that the most advanced CysLT(1) receptor antagonists, some of which are already in clinical use, displayed a different behavior toward LTC(4) and LTD(4) in HLP. Equilibrium and kinetic binding studies demonstrated the following rank order of potency for (3)H-LTD(4) receptor (CysLT(1)): zafirlukast = montelukast > LM-1507 = LM-1484 = pranlukast. In addition, LM-1507, LM-1484, pranlukast and montelukast but not zafirlukast are able to interact also with the high affinity site for (3)H-LTC(4) (LM-1507 = LM-1484 > pranlukast; montelukast not detectable in the presence of S-decyl-GSH). In this respect, the behavior of the LM antagonists closely resembles that of pranlukast although LM-1507 and LM-1484 display a higher affinity for (3)H-LTC(4) sites. Montelukast has an intermediate behavior, inasmuch as its interaction with (3)H-LTC(4) sites can be revealed only in kinetic studies, while zafirlukast is totally unable to inhibit (3)H-LTC(4) binding. It might be, therefore, most relevant for a complete understanding of the clinical efficacy, besides their nominal potency, of the most advanced CysLT(1) receptor antagonists to consider their pharmacological differences with respect not only to LTD(4)/LTE(4), but also to LTC(4).

CysLT1 signal transduction in differentiated U937 cells involves the activation of the small GTP-binding protein Ras.

We investigated the signal transduction pathway(s) of leukotriene D(4) (LTD(4)) in the human promonocytic U937 cells, a cell line known to constitutively express CysLT(1) receptors. Herein, we demonstrate that LTD(4) specifically acts on a CysLT(1) receptor to dose-dependently increase (three to five-fold over basal) RasGTP through a G(i/o) protein. In fact, while cytosolic Ca(2+) ([Ca(2+)](i)) increase was only partially sensitive to pertussis toxin (PTx), Ras activation was almost completely inhibited by the same toxin. Furthermore, the phospholipase C (PLC) inhibitor U73122 completely inhibited both [Ca(2+)](i) and RasGTP increase, suggesting that in these cells PLC is the point of convergence for both PTx insensitive and sensitive pathways leading to [Ca(2+)](i) release and Ras activation. Indeed, chelating intracellular Ca(2+) strongly (>70%) prevented LTD(4)-induced Ras activation, indicating that this ion plays an essential role for CysLT(1)-induced downstream signaling in differentiated U937 (dU937) cells. In addition, while Src did not appear to be substantially involved in CysLT(1)-induced signaling, genistein was able to partially inhibit LTD(4)-induced [Ca(2+)](i) transient ( approximately 34%) and almost completely prevented Ras activation (>90%), suggesting a potential role for other Ca(2+)-dependent tyrosine kinases in LTD(4)-induced signaling. Finally, agonist-induced CysLT(1) stimulation was followed by a specific extracellular regulated kinase (ERK) 1/2 phosphorylation, an event with a pharmacological profile similar to that of Ras activation, partially ( approximately 40%) sensitive to Clostridium sordellii lethal toxin and totally blocked by PTx. In conclusion, LTD(4)-induced CysLT(1) receptor activation in dU937 cells leads to Ras activation and ERK phosphorylation mostly through a PTx-sensitive G(i/o) protein, PLC, and Ca(2+)-dependent tyrosine kinase(s).