The signal transduction system of the leukotriene D4 receptor

Eicosanoids in inflammation in the blood and the vessel

Polyunsaturated fatty acids (PUFAs) are structural components of membrane phospholipids in cells. PUFAs regulate cellular function through the formation of derived lipid mediators termed eicosanoids. The oxygenation of 20-carbon PUFAs via the oxygenases cyclooxygenases, lipoxygenases, or cytochrome P450, generates a class of classical eicosanoids including prostaglandins, thromboxanes and leukotrienes, and also the more recently identified hydroxy-, hydroperoxy-, epoxy- and oxo-eicosanoids, and the specialized pro-resolving (lipid) mediators. These eicosanoids play a critical role in the regulation of inflammation in the blood and the vessel. While arachidonic acid-derived eicosanoids are extensively studied due to their pro-inflammatory effects and therefore involvement in the pathogenesis of inflammatory diseases such as atherosclerosis, diabetes mellitus, hypertension, and the coronavirus disease 2019; in recent years, several eicosanoids have been reported to attenuate exacerbated inflammatory responses and participate in the resolution of inflammation. This review focused on elucidating the biosynthesis and the mechanistic signaling of eicosanoids in inflammation, as well as the pro-inflammatory and anti-inflammatory effects of these eicosanoids in the blood and the vascular wall.

A review of non-prostanoid, eicosanoid receptors: expression, characterization, regulation, and mechanism of action

Eicosanoid signaling controls a wide range of biological processes from blood pressure homeostasis to inflammation and resolution thereof to the perception of pain and to cell survival itself. Disruption of normal eicosanoid signaling is implicated in numerous disease states. Eicosanoid signaling is facilitated by G-protein-coupled, eicosanoid-specific receptors and the array of associated G-proteins. This review focuses on the expression, characterization, regulation, and mechanism of action of non-prostanoid, eicosanoid receptors.

Leukotriene receptors as potential therapeutic targets

Leukotrienes, a class of arachidonic acid-derived bioactive molecules, are known as mediators of allergic and inflammatory reactions and considered to be important drug targets. Although an inhibitor of leukotriene biosynthesis and antagonists of the cysteinyl leukotriene receptor are clinically used for bronchial asthma and allergic rhinitis, these medications were developed before the molecular identification of leukotriene receptors. Numerous studies using cloned leukotriene receptors and genetically engineered mice have unveiled new pathophysiological roles for leukotrienes. This Review covers the recent findings on leukotriene receptors to revisit them as new drug targets.

Leukotrienes in pulmonary arterial hypertension

Leukotrienes (LTs) are lipid mediators derived from the 5-lipoxygenase (5-LO) pathway of arachidonic acid metabolism and are markers and mediators of pulmonary inflammation. Research over the past two decades has established that LTs modulate inflammation in pulmonary arterial hypertension (PAH). The purpose of this review was to summarize the current knowledge of LTs in the pathophysiology of PAH and to highlight a recent study that advances our understanding of how leukotriene B4 (LTB4) specifically contributes to pulmonary vascular remodeling. The results of these studies suggest that pharmacological inhibition of LT pathways, especially LTB4, has high potential for the treatment of PAH.

Eicosanoids and respiratory viral infection: coordinators of inflammation and potential therapeutic targets

Viruses are frequent causes of respiratory infection, and viral respiratory infections are significant causes of hospitalization, morbidity, and sometimes mortality in a variety of patient populations. Lung inflammation induced by infection with common respiratory pathogens such as influenza and respiratory syncytial virus is accompanied by increased lung production of prostaglandins and leukotrienes, lipid mediators with a wide range of effects on host immune function. Deficiency or pharmacologic inhibition of prostaglandin and leukotriene production often results in a dampened inflammatory response to acute infection with a respiratory virus. These mediators may, therefore, serve as appealing therapeutic targets for disease caused by respiratory viral infection.

Modulation of leukotriene D4 attenuates the development of seizures in mice

The present study has been designed to pharmacologically investigate the effect of Montelukast sodium, a leukotriene D(4) receptor antagonist, and 1,2,3,4, tetrahydroisoquinoline, a leukotriene D(4) synthetic pathway inhibitor, on the pathophysiological progression of seizures using mouse models of kindled epilepsy and status epilepticus induced spontaneous recurrent seizures. Pentylenetetrazole (40 mg kg(-1)) (PTZ) administration every second day for a period of 15 d was used to elicit chemically induced kindled seizure activity in mice. In a separate set of groups, fifty consecutive electroshocks were delivered to mice using corneal electrodes with continuously increasing intensity with an inter-shock interval of 40s. Severity of kindled seizures was assessed in terms of a composite kindled seizure severity score (KSSS). Pilocarpine (100 mg kg(-1)) was injected every twenty minutes until the onset of status epilepticus. A spontaneous recurrent seizure severity score (SRSSS) was recorded as a measure of quantitative assessment of the progressive development of spontaneous recurrent seizures induced after pilocarpine status epilepticus. Sub-acute PTZ administration and electroshock induced the development of severe form of kindled seizures in mice. Severity of kindled seizures was assessed in terms of a composite kindled seizure severity score. Further, pharmacological status epilepticus elicited a progressive evolution of spontaneous recurrent seizures in the animals. However, Montelukast sodium, a leukotriene D(4) receptor antagonist, as well as 1,2,3,4, tetrahydroisoquinoline, a leukotriene D(4) synthetic pathway inhibitor, markedly and dose dependently suppressed the development of kindled seizures as well as pilocarpine induced spontaneous recurrent seizures. Therefore, leukotriene D(4) may be implicated in the pathogenesis of seizures.

Specific leukotriene receptors couple to distinct G proteins to effect stimulation of alveolar macrophage host defense functions

Leukotrienes (LTs) are lipid mediators implicated in asthma and other inflammatory diseases. LTB(4) and LTD(4) also participate in antimicrobial defense by stimulating phagocyte functions via ligation of B leukotriene type 1 (BLT1) receptor and cysteinyl LT type 1 (cysLT1) receptor, respectively. Although both Galpha(i) and Galpha(q) proteins have been shown to be coupled to both BLT1 and cysLT1 receptors in transfected cell systems, there is little known about specific G protein subunit coupling to LT receptors, or to other G protein-coupled receptors, in primary cells. In this study we sought to define the role of specific G proteins in pulmonary alveolar macrophage (AM) innate immune responses to LTB(4) and LTD(4). LTB(4) but not LTD(4) reduced cAMP levels in rat AM by a pertussis toxin (PTX)-sensitive mechanism. Enhancement of FcgammaR-mediated phagocytosis and bacterial killing by LTB(4) was also PTX-sensitive, whereas that induced by LTD(4) was not. LTD(4) and LTB(4) induced Ca(2+) and intracellular inositol monophosphate accumulation, respectively, highlighting the role of Galpha(q) protein in mediating PTX-insensitive LTD(4) enhancement of phagocytosis and microbicidal activity. Studies with liposome-delivered G protein blocking Abs indicated a dependency on specific Galpha(q/11) and Galpha(i3) subunits, but not Galpha(i2) or G(beta)gamma, in LTB(4)-enhanced phagocytosis. The selective importance of Galpha(q/11) protein was also demonstrated in LTD(4)-enhanced phagocytosis. The present investigation identifies differences in specific G protein subunit coupling to LT receptors in antimicrobial responses and highlights the importance of defining the specific G proteins coupled to heptahelical receptors in primary cells, rather than simply using heterologous expression systems.

Effects of selective protein kinase c isozymes in prostaglandin2alpha-induced Ca2+ signaling and luteinizing hormone-induced progesterone accumulation in the mid-phase bovine corpus luteum

A single-cell approach for measuring the concentration of cytoplasmic calcium ions ([Ca(2+)](i)) and a protein kinase C-epsilon (PKCepsilon)-specific inhibitor were used to investigate the developmental role of PKCepsilon in the prostaglandin F(2alpha)(PGF(2alpha))-induced rise in [Ca(2+)](i) and the induced decline in progesterone accumulation in cultures of cells isolated from the bovine corpus luteum. PGF(2alpha) increased [Ca(2+)](i) in Day 4 large luteal cells (LLCs), but the response was significantly lower than in Day 10 LLCs (4.3 +/- 0.6, n = 116 vs. 21.3 +/- 2.3, n = 110). Similarly, the fold increase in the PGF(2alpha)-induced rise in [Ca(2+)](i) in Day 4 small luteal cells (SLCs) was lower than in Day 10 SLCs (1.6 +/- 0.2, n = 198 vs. 2.7 +/- 0.1, n = 95). A PKCepsilon inhibitor reduced the PGF(2alpha)-elicited calcium responses in both Day 10 LLCs and SLCs to 3.5 +/- 0.3 (n = 217) and 1.3 +/- 0.1 (n = 205), respectively. PGF(2alpha) inhibited LH-stimulated progesterone (P(4)) accumulation only in the incubation medium of Day 10 luteal cells. Both conventional and PKCepsilon-specific inhibitors reversed the ability of PGF(2alpha) to decrease LH-stimulated P(4) accumulation, and the PKCepsilon inhibitor was more effective at this than the conventional PKC inhibitor. In conclusion, the evidence indicates that PKCepsilon, an isozyme expressed in corpora lutea with acquired PGF(2alpha) luteolytic capacity, has a regulatory role in the PGF(2alpha)-induced Ca(2+) signaling in luteal steroidogenic cells, and that this in turn may have consequences (at least in part) on the ability of PGF(2alpha) to inhibit LH-stimulated P(4) synthesis at this developmental stage.

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).

Bronchodilator effect of zafirlukast in subjects with chronic obstructive pulmonary disease

Cysteinyl leukotrienes (LT) are involved in airway inflammation and mucus hypersecretion, characteristically present in asthma and chronic obstructive pulmonary disease (COPD). Zafirlukast is an LT receptor antagonist that improves airway function within 1-3 h after oral administration in subjects with chronic persistent asthma. Through a randomised, double-blind, crossover and placebo-controlled study, we assessed the short-term effects of zafirlukast in patients with severe COPD. We enrolled 23 subjects (seven women) aged 59.4 (1.67) yr [mean (SEM)] with a smoking history of 60.7 (5.2) pack-yr. At screening day the mean FEV(1)was 0.876 (0.72) l; FEV(1) % predicted=35 (3)% and 107 (14) ml increment post-salbutamol. They came two different days, apart from each other at least 72 h. After baseline spirometry, a single oral dose of 40 mg zafirlukast or the corresponding placebo was administered. FVC and FEV(1) was measured every 30 min until 2 hrs. On zafirlukast day, the mean FEV(1) at 90 min [0.813 (0.64) l] and the mean FVC at 90 min [1.76 (0.1) l] were significantly higher than the respective means at placebo day (mean FEV(1)=0.747 (0.55) l; mean FVC=1.63 (0.1) l; p<0.05 Tukey Kramer multiple comparisons test). The maximum mean increase in FEV(1) was 75 (19) ml. A positive correlation was found between absolute response to salbutamol in FEV(1) and the response to zafirlukast (r=0.41; p<0.04). In conclusion, these findings suggest that zafirlukast has a bronchodilator or antibronchoconstrictor effect in COPD patients with severe airflow limitation.

Involvement of prenylated proteins in calcium signaling induced by LTD4 in differentiated U937 cells

We investigated signal transduction pathways for LTD4 in the human promonocytic cell line U937 known, upon differentiation, to express CysLT1 receptors. We confirmed the presence of high-affinity binding sites for 3H-LTD4, which, in functional studies, displayed the features of CysLT1 receptor. In fact, three potent and selective CysLT1 receptor antagonists were able to completely inhibit LTD4-induced response. In turn, cytosolic Ca2+ ([Ca2+]i) increase (EC50 = 3.4 nM +/- 27% CV) was only partially sensitive to pertussis toxin (PTx) as well as to the prenylation inhibitor fluvastatin and to the specific geranylgeranylation and farnesylation inhibitors BAL 9504 and FPT II. Finally, Clostridium sordellii lethal toxin, inhibitor of the Ras family of GTPases, and FTS, a potent methyltransferase inhibitor, were both able to partially inhibit LTD4-induced [Ca2+] increase, suggesting a role for a Ras family member in [Ca2+]i regulation. In conclusion, in dU937 LTD4 signal transduction involves: (a) at least two pathways, one sensitive and one insensitive to PTx; (b) isoprenylated proteins, such as betagamma subunits and, possibly, a small G protein of the Ras family.

Leukotriene D4-induced calcium signaling in human intestinal epithelial cells

LTD4 induces a calcium signal that consists of a mobilization from internal stores regulated by PTX-insensitive G protein, Rho, and, an influx across the plasma membrane regulated by PTX-sensitive Gi protein in human intestinal epithelial cells. The LTD4 induced mobilization of Ca2+ is mediated by a PH domain dependent association between PLC-gamma 1 and G beta gamma subunits. This interaction requires Src kinase activity as well as the association of this kinase with PLC-gamma 1, suggesting a G beta gamma mediated recruitment of proteins to the plasma membrane and formation of a signaling complex which is essential for the downstream Ca2+ signal. We found a cAMP-dependent protein kinase activity upstream of tyrosine kinase(s) and downstream of Gi protein, that is essential for LTD4-induced Ca2+ influx. This model of the LTD4-induced Ca2+ signaling pathways in intestinal epithelial cells is outlined schematically in Fig. 1.

International Union of Pharmacology XXXVII. Nomenclature for leukotriene and lipoxin receptors

The leukotrienes and lipoxins are biologically active metabolites derived from arachidonic acid. Their diverse and potent actions are associated with specific receptors. Recent molecular techniques have established the nucleotide and amino acid sequences and confirmed the evidence that suggested the existence of different G-protein-coupled receptors for these lipid mediators. The nomenclature for these receptors has now been established for the leukotrienes. BLT receptors are activated by leukotriene B(4) and related hydroxyacids and this class of receptors can be subdivided into BLT(1) and BLT(2). The cysteinyl-leukotrienes (LT) activate another group called CysLT receptors, which are referred to as CysLT(1) and CysLT(2). A provisional nomenclature for the lipoxin receptor has also been proposed. LXA(4) and LXB(4) activate the ALX receptor and LXB(4) may also activate another putative receptor. However this latter receptor has not been cloned. The aim of this review is to provide the molecular evidence as well as the properties and significance of the leukotriene and lipoxin receptors, which has lead to the present nomenclature.

Pharmacogenetics of treatment with leukotriene modifiers

PURPOSE OF REVIEW Pharmacogenetics is emerging as a field with great potential to improve both our understanding and treatment options in asthma. This review highlights the importance of pharmacogenetic associations of an important class of asthma therapy, the leukotriene modifiers, and asthma.

RECENT FINDINGS

Over the past decade, different leukotriene modifier therapies have emerged and have resulted in improvements in asthma parameters in individuals with this condition. However, there is substantial interindividual variability with respect to the response to this and other asthma therapies. Over the past few years, polymorphisms of two genes in the leukotriene pathway, the gene and the synthase gene, have been identified and have been demonstrated to have pharmacogenetic associations with asthma. However, currently identified genetic determinants explain the response to therapy in only a minority of patients.

SUMMARY

As the field of pharmacogenetics advances, an increasing proportion of individual variation in response to pharmacotherapy will be predictable on the basis of associations with particular genetic polymorphisms or patterns of polymorphisms. The pharmacogenetic association of leukotriene modifiers and asthma is an excellent example of how these associations hold out the promise of being able to individualize pharmacotherapy, by providing specific medications to those most likely to respond while avoiding therapy in those most likely to suffer adverse effects.

Cysteinyl leukotrienes promote human airway smooth muscle migration

Cysteinyl leukotrienes promote airway smooth muscle (ASM) contraction and proliferation. Little is known about their role in ASM migration. We investigated this using cultured human ASMs (between the second and fifth passages) obtained from the large airways of resected nonasthmatic lung. Platelet-derived growth factor-BB (1 ng/ml) promoted significant (3.5-fold) ASM migration of myocytes across collagen-coated 8- micro m polycarbonate membranes in Transwell culture plates. Leukotriene E(4) (10(-7), 10(-8), 10(-9) M) did not demonstrate a chemotactic effect; it did promote chemokinesis. Priming by leukotriene E(4) (10(-7) M) significantly augmented the directional migratory response to platelet-derived growth factor (1.5-fold, p < 0.05). This was blocked by montelukast (10(-6) M), demonstrating the effect to be mediated by the cysteinyl leukotriene receptor. The "priming effect" was also partially attenuated by prostaglandin E(2) (10(-7) M). Whereas both the chemokinetic and the chemotactic "primed" responses were equally attenuated by a p38 mitogen-activated protein kinase inhibitor (SB203580, 25 micro M) and by a Rho-kinase inhibitor (Y27632, 10 micro M), the chemotactic response showed greater inhibition than chemokinesis by a phosphatidylinositol-3 kinase inhibitor (LY294002, 50 micro M). These experiments suggest that cysteinyl leukotrienes play an augmentary role in human ASM migration. The phosphatidylinositol-3 kinase pathway is a key signaling mechanism in the chemotactic migration of ASM cells in response to cysteinyl leukotrienes.

Cysteinyl leukotriene receptors

The cysteinyl leukotrienes, leukotriene C4 (LTC4), leukotriene D4 (LTD4) and leukotriene E4 (LTE4), activate contractile and inflammatory processes via specific interaction with putative seven transmembrane-spanning receptors that couple to G proteins and subsequent intracellular signaling pathways. Pharmacological characterizations identified at least two subtypes of cysteinyl leukotriene (CysLT) receptor based on agonist and antagonist potency for biological responses. The rank potency of agonist activation for the CysLT1 receptor is LTD4 > LTC4 > LTE4 and for the CysLT2 receptor is LTC4 = LTD4 > LTE4. CysLT1 selective receptor antagonists are efficacious in the treatment of asthma. No selective CysLT2 receptor antagonists have been described. Molecular identification of the human and mouse CysLT1 and CysLT2 receptors has confirmed their structure as putative seven transmembrane domain G protein-coupled receptors and largely confirmed the previous pharmacological characterizations. The CysLT1 receptor is most highly expressed in spleen, peripheral blood leukocytes including eosinophils, and lung smooth muscle cells and interstitial lung macrophages. The CysLT2 receptor is most highly expressed in the heart, adrenal medulla, placenta and peripheral blood leukocytes. The molecular identification of the mouse CysLT1 and CysLT2 receptors show similar but not identical profiles to the orthologous human receptors.

Human mast cells express two leukotriene C(4) synthase isoenzymes and the CysLT(1) receptor

Cysteinyl-leukotrienes (cys-LTs) are potent smooth muscle contracting agents, especially in the respiratory tract and microcirculation, and play a key role in inflammatory and allergic diseases. The final step in the biosynthesis of LTC(4), the parent compound of cys-LTs, is catalyzed by a specific GSH transferase termed LTC(4) synthase, which is typically expressed in certain bone marrow-derived cells such as eosinophils and mast cells. Here we report that the human mast cell line HMC-1 as well as human mast cells derived from cord blood (CBMC) express a second enzyme capable of synthesizing leukotriene C(4), i.e., microsomal GSH transferase type 2. Furthermore, these cells abundantly express CysLT(1) receptors that are mostly located at the surface of both types of mast cells, as judged by immunohistochemistry. In addition, stimulation of CBMC with LTC(4) and LTD(4) elicits an immediate and dose-dependent (10(-7)-10(-11) M) mobilization of intracellular Ca(2+), which can be blocked with specific CysLT(1) receptor antagonists. Taken together, our data suggest that human mast cells are equipped with two enzymes that can catalyze the committed step in the biosynthesis of cys-LTs. Moreover, the expression of the cognate receptor CysLT(1) suggests that these lipid mediators may be involved in autocrine signaling pathways regulating mast cell functions.

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

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

Leukotrienes as mediators of asthma

This review describes the aspects of leukotriene (LT) pharmacology and biology that are relevant to their important role in asthma. The biosynthesis and metabolism, including transcellular metabolism, of LTB4 and the cysteinyl-LTs (i.e. LTC4, LTD4 and LTE4) are described, and their transport is briefly outlined. The existence, distribution and pharmacological characterization of the receptors (BLT, CysLT1, CysLT2), as well as the transduction mechanisms triggered, are discussed in detail. We also describe their effects on airway smooth muscle tone, hyperresponsiveness and proliferation, on vascular tone and permeability, on mucus secretion, on neural fibers and inflammatory cell functions. Finally, the evidence supporting their role as asthma mediators is reviewed, including the effects of anti LT drugs (both biosynthesis inhibitors and receptor antagonists) in experimental and clinical asthma.

The epsilon isoform of protein kinase C is involved in regulation of the LTD(4)-induced calcium signal in human intestinal epithelial cells

We investigated the potential roles of specific isoforms of protein kinase C (PKC) in the regulation of leukotriene D(4)-induced Ca(2+) signaling in the intestinal epithelial cell line Int 407. RT-PCR and Western blot analysis revealed that these cells express the PKC isoforms alpha, betaII, delta, epsilon, zeta, and mu, but not betaI, gamma, eta, or theta;. The inflammatory mediator leukotriene D(4) (LTD(4)) caused the TPA-sensitive PKC isoforms alpha, delta, and epsilon, but not betaII, to rapidly translocate to a membrane-enriched fraction. The PKC inhibitor GF109203X at 30 microM but not 2 microM significantly impaired the LTD(4)-induced Ca(2+) signal, indicating that the response involves a novel PKC isoform, such as delta or epsilon, but not alpha. LTD(4)-induced Ca(2+) signaling was significantly suppressed in cells pretreated with TPA for 15 min and was abolished when the pretreatment was prolonged to 2 h. Immunoblot analysis revealed that the reduction in the LTD(4)-induced calcium signal coincided with a reduction in the cellular content of PKCepsilon and, to a limited extent, PKCdelta. LTD(4)-induced Ca(2+) signaling was also markedly suppressed by microinjection of antibodies against PKCepsilon but not PKCdelta. These data suggest that PKCepsilon plays a unique role in regulation of the LTD(4)-dependent Ca(2+) signal in intestinal epithelial cells.

Leukotriene D4-induced activation of smooth-muscle cells from human bronchi is partly Ca2+-independent

Cysteine-containing leukotrienes (cysteinyl-LTs) are potent bronchoconstrictors and play a key role in asthma. We found that histamine and LTD4 markedly constrict strips of human bronchi (HB) with similar efficacy. However, in human airway smooth-muscle (HASM) cells, LTD4, at variance with histamine, elicited only a small, transient change in intracellular calcium ion concentration. HASM cells express both Ca2+-dependent and -independent isoforms of protein kinase C (PKC) (i.e., PKC-alpha and PKC-alpha ). Western blot analysis showed that PKC-alpha is activated by histamine and, to a lesser extent, by LTD4, whereas only LTD4 translocates PKC-alpha. This translocation was specifically inhibited by the LTD4 antagonist pobilukast. Phorbol-dibutyrate ester (PDBu) (a PKC activator) contracted HB strips to the same extent in the presence as in the absence of extra- and intracellular Ca2+. In the absence of Ca2+, LTD4 contracted HB strips to the same extent as did PDBu, suggesting the involvement of a Ca2+-independent PKC in LTD4-mediated signal transduction. PDBu-induced desensitization and the PKC inhibitor H7 abolished the slow and sustained LTD4-triggered contraction of HB strips in the absence of Ca2+, although H7 did not greatly affect the response in the presence of the ion. Thus, in human airways, we identified a novel LTD4 transduction mechanism linked to bronchial smooth-muscle contraction, which is partly independent of Ca2+ and involves the activation of PKC-alpha.

Lipoxin A4 antagonizes the mitogenic effects of leukotriene D4 in human renal mesangial cells. Differential activation of MAP kinases through distinct receptors

The lipoxygenase-derived eicosanoids leukotrienes and lipoxins are well defined regulators of hemeodynamics and leukocyte recruitment in inflammatory conditions. Here, we describe a novel bioaction of lipoxin A(4) (LXA(4)), namely inhibition of leukotriene D(4) (LTD(4))-induced human renal mesangial cell proliferation, and investigate the signal transduction mechanisms involved. LXA(4) blocked LTD(4)-stimulated phosphatidylinositol 3-kinase (PI 3-kinase) activity in parallel to inhibition of LTD(4)-induced mesangial cell proliferation. Screening of a human mesangial cell cDNA library revealed expression of the recently described cys-leukotriene(1)/LTD(4) receptor. LTD(4)-induced mesangial cell proliferation required both extracellular-related signal regulated kinase (erk) and PI 3-kinase activation and may involve platelet-derived growth factor receptor transactivation. LTD(4)-stimulated the MAP kinases erk and p38 via a pertussis toxin (PTX)-sensitive pathway dependent on PI 3-kinase and protein kinase C activation. On screening a cDNA library, mesangial cells were found to express the previously described LXA(4) receptor. In contrast to LTD(4), LXA(4) showed differential activation of erk and p38. LXA(4) activation of erk was insensitive to PTX and PI 3-kinase inhibition, whereas LXA(4) activation of p38 was sensitive to PTX and could be blocked by the LTD(4) receptor antagonist SKF 104353. These data suggest that LXA(4) stimulation of the MAP kinase superfamily involves two distinct receptors: one shared with LTD(4) and coupled to a PTX-sensitive G protein (G(i)) and a second coupled via an alternative G protein, such as G(q) or G(12), to erk activation. These data expand on the spectrum of LXA(4) bioactions within an inflammatory milieu.

Influence of the epithelium on acetylcholine release from parasympathetic nerves of the rat trachea

1. The present study was undertaken to investigate the influence of the airway epithelium on the release of acetylcholine (ACh) from parasympathetic nerves of the rat trachea. Epithelium-intact and epithelium-denuded preparations of rat trachea were incubated with [3H]-choline to incorporate [3H]-ACh into the cholinergic transmitter stores. Release of radiolabelled transmitter ACh was evoked by electrical field stimulation (60 s trains of 1 ms pulses, 5 Hz, 15 V). 2. Field stimulation both of epithelium-intact and epithelium-denuded radiolabelled tracheal preparations evoked an increase in the efflux of radioactivity; however, the mean stimulation-induced (S-I) efflux from epithelium-denuded preparations (2932 +/- 190 d.p.m., n = 9) was approximately 60% of that from epithelium-intact preparations (4802 +/- 820 d.p.m., n = 11). We have shown previously that, in epithelium-intact (but not epithelium-denuded) tracheal preparations, a substantial proportion of the S-I efflux is resistant to tetrodotoxin (1 microM) and to the removal of extracellular Ca2+, indicating that much of the S-I efflux is not caused by exocytotic release of neuronal [3H]-ACh. In epithelium-denuded tracheal preparations, superfused individually, phosphorylcholine (1 and 100 microM) did not alter S-I efflux. In epithelium-intact tracheal preparations, both in the absence and in the presence of atropine (1 microM), neither N(G)-nitro-L-arginine (100 microM), superoxide dismutase (100 units ml(-1)), indomethacin (10 microM), capsaicin (30 microM) nor alpha-chymotrypsin (1 unit ml(-1)) altered S-I efflux. 3. Experiments were also performed using two tracheal preparations superfused in series. When unlabelled epithelium-intact preparations were present in the upper chamber (superfused first), the S-I efflux from radiolabelled epithelium-denuded preparations in the lower chamber (superfused second) did not differ significantly from radiolabelled epithelium-denuded preparations superfused individually. Moreover, there was no significant difference in the S-I efflux from radiolabelled epithelium-denuded preparations in the lower chamber between experiments in which the upper chamber contained epithelium-intact or epithelium-denuded preparations. 4. Field stimulation of epithelium-intact tracheal preparations in the upper chamber with 90, 120 and 300-s periods (trains of 1 ms pulses, 5 Hz, 15 V) did not significantly alter the S-I efflux from radiolabelled epithelium-denuded tracheal preparations in the lower chamber. 5. When introduced into the upper (unlabelled epithelium-intact) and subsequently allowed to superfuse the lower (radiolabelled epithelium-denuded) tracheal preparations, the stable cholinomimetic carbachol (3 microM) markedly reduced the S-I efflux whereas ACh (0.1 and 1 microM) had no significant effect. However, in the presence of the anti-cholinesterase neostigmine (1 microM), ACh (1 microM) significantly reduced S-I efflux, indicating that ACh is subject to rapid hydrolysis by cholinesterase enzymes. When atropine (10 microM) was only exposed to radiolabelled epithelium-denuded preparations in the lower chamber, the inhibitory effects of ACh (1 microM) and carbachol (3 microM) on S-I efflux were prevented. 6. In conclusion, the findings of the present study do not support the notion that the airway epithelium exerts an inhibitory influence on ACh release from parasympathetic nerves of the rat trachea. Alternatively, if epithelium-dependent modulation of cholinergic transmission does occur in the rat trachea, then the mechanism does not appear to involve phosphorylcholine, nitric oxide, superoxide radicals, cyclo-oxygenase products of arachadonic acid, capsaicin-sensitive neuropeptides or vasoactive intestinal peptide. Moreover, the inhibitory effect of carbachol and ACh on transmitter ACh release in the rat trachea appears to be due solely to activation of prejunctional inhibitory muscarinic cholinoceptors on parasympathetic nerves and does not involve the liberation of a putative epithelium-derived inhibitory factor.

Leukotriene D(4) triggers an association between gbetagamma subunits and phospholipase C-gamma1 in intestinal epithelial cells

The proinflammatory mediator leukotriene D(4) (LTD(4)) binds to the seven-transmembrane receptor CYSLT(1). Although this leukotriene plays an important biological role, its intracellular signaling pathways are only partly known. In previous experiments, we found that LTD(4) induced tyrosine phosphorylation and translocation of phospholipase (PLC)-gamma1 to a plasma membrane fraction in a human epithelial cell line (Int 407). In the present study, we further examined these signaling events and found that LTD(4) induced a rapid interaction between Gbetagamma subunits and PLC-gamma1; results obtained with GST fusion proteins of PLC-gamma1 suggest that this interaction is mediated via the pleckstrin homology domain of PLC-gamma1. Moreover, LTD(4) induced an increased association of c-Src with PLC-gamma1, and the selective Src family tyrosine kinase inhibitor PP1 blocked both LTD(4)-induced tyrosine phosphorylation of PLC-gamma1 and the association of PLC-gamma1 with Gbetagamma subunits. The relevance of these observations in intracellular calcium signaling was investigated by microinjecting cells with anti-Gbeta, anti-PLC-gamma1, or anti-c-Src antibodies and by pretreatment with PP1. LTD(4)-induced calcium mobilization was blocked by each of the indicated antibodies (but not isotype-matched control antibodies) and by PP1. Our data suggest that Gbetagamma subunits can, directly or indirectly, serve as membrane-bound partners for PLC-gamma1 and c-Src and that each of these proteins is essential for LTD(4)-induced downstream PLC-gamma1 signaling.

Asthma therapy with agents preventing leukotriene synthesis or action

Elucidation of the biochemistry of leukotriene production and the pharmacology of its actions has led to the development of a number of therapeutic agents shown to be of value in the treatment of asthma. These agents either prevent the synthesis of the leukotrienes, by preventing the action of the 5-lipoxygenase-activating protein or the catalytic action of the 5-lipoxygenase, or by inhibiting the action of leukotrienes at the CysLT1 receptor. Numerous clinical trials in exercise-induced asthma, allergen-induced asthma, aspirin-induced asthma, and spontaneously occurring asthmatic episodes have indicated that these agents are safe and effective asthma treatments.

The effect of montelukast (MK-0476), a cysteinyl leukotriene receptor antagonist, on allergen-induced airway responses and sputum cell counts in asthma

BACKGROUND

Cysteinyl leukotrienes are capable of inducing chemotaxis of eosinophils in vitro and within the airways of animals and humans in vivo.

OBJECTIVE

We hypothesized that montelukast (MK-0476), a potent cysLT1 receptor antagonist, would protect against allergen-induced early (EAR) and late (LAR) asthmatic responses by virtue of anti-inflammatory properties. Hence, we studied the effect of pretreatment with oral montelukast on allergen-induced airway responses. As an exploratory endpoint, changes in inflammatory cell differentials and eosinophil cationic protein (ECP) were evaluated in hypertonic saline-induced sputum.

METHODS

Twelve asthmatic men (20-34 years, FEV1 79-109% predicted, histamine PC20FEV1 <4 mg/mL) with dual responses to inhaled house dust mite extract participated in a two-period, double-blind, placebo-controlled, crossover study. Three oral doses of montelukast (10 mg) or matching placebo were administered 36 and 12 h before, and 12 h post-allergen. The airway response to allergen was measured by FEV1, and the EAR and LAR were expressed as the corresponding areas under the time-response curves (AUC0-3 h and AUC3-8h, respectively). During each study period, sputum was induced with 4.5% NaCl 24 h before and 24 h after a standardized allergen challenge. Processed whole sputum cytospins were stained with Giemsa, and cell counts expressed as percentage nonsquamous cells. ECP was measured by FEIA in sputum supernatants.

RESULTS

All subjects completed the study. The changes in baseline FEV1 were not significantly different between the two pretreatments (P = 0.183). Montelukast significantly inhibited the EAR and LAR, reducing the AUC0-3h by 75.4% (P<0.001) and the AUC3-8h by 56.9% (P = 0.003) as compared with placebo. Sputa of nine subjects could be included in the analysis (<80% squamous cells). Allergen challenge significantly increased sputum eosinophils after placebo (mean change +/- SD: 4.8 +/- 5.8%, P = 0.038), with a similar trend after montelukast (mean change +/- SD: 4.1 +/- 5.4%; P = 0.056). The allergen-induced changes in sputum eosinophils and ECP, however, were not significantly different between the two pretreatments (P = 0.652 and P = 0.506, respectively).

CONCLUSION

We conclude that oral montelukast protects against allergen-induced early and late airway responses in asthma. However, using the present dosing and sample size, this protection was not accompanied with changes in sputum eosinophil percentage or activity, which may require more prolonged pretreatment with cysLT1 receptor antagonists.

Leukotrienes as mediators of airway obstruction

The cysteinyl leukotrienes are potent mediators of airway narrowing derived from the lipoxygenation of arachidonic acid and the adduction of glutathione to this eicosanoid backbone. In lower animals and humans, the cysteinyl leukotrienes are among the most potent airway contractile substances ever identified. Furthermore, these moieties can be recovered from the urine during induced or spontaneous asthma attacks. Most important, inhibition of the synthesis of the leukotrienes or prevention of their action at the CysLT1 receptor is associated with an improvement in the airway dysfunction that occurs in both induced and spontaneous asthma. These data indicate that the cysteinyl leukotrienes have a clinically significant role in the airway obstruction that characterizes asthma.

Leukotriene D4 induces a rapid increase in cAMP in the human epithelial cell line, Int 407: a potential role for this signal in the regulation of calcium influx through the plasma membrane

Although the LTD4-induced Ca2+ influx in human epithelial cells has been shown to be regulated by a pertussis toxin-sensitive heterotrimeric G-protein, most likely a G alpha i3 protein [Adolfsson J.L.P., Ohd J.F., Sjölander A. Leukotriene D4-induced activation and translocation of the G-protein alpha i3-subunit in human epithelial cells. Biochem Biophys Res Commun 1996; 226: 413-419], the signalling pathway further downstream is still unclear. In the present study, we investigated the possible involvement of cAMP and protein kinase A activity in the LTD4-induced Ca2+ influx in the epithelial cell line Int 407. Stimulation with LTD4, but not with the calcium ionophore ionomycin, triggered a rapid increase (peak at 7 s) in the cellular cAMP level, an effect that was totally abolished by pertussis toxin. Furthermore, the LTD4-induced Ca2+ signal was reduced by 60% when cells that had been pre-incubated with the protein kinase A inhibitor Rp-cAMPS (50 microM for 30 min) were stimulated in a calcium containing medium. In contrast, Rp-cAMPS had no apparent effect on the LTD4-induced Ca2+ signal when the cells were stimulated in a calcium-depleted medium. The immediate LTD4-induced protein tyrosine phosphorylation (15 s), previously shown to be necessary for the subsequent Ca2+ influx, was abolished not only by pretreatment with pertussis toxin but also by exposure to Rp-cAMPS. Furthermore, direct activation of the cellular adenylyl cyclase activity by treatment with forskolin alone induced a prompt Ca2+ signal in the presence, but not in the absence, of extracellular Ca2+, identical results were obtained with the cell permeable cAMP analogue 8-bromo-cAMP. In addition, forskolin induced protein tyrosine phosphorylation similar to that seen with LTD4. These results suggest that protein kinase A activity participates in the regulation of the LTD4-induced Ca2+ influx at a site that is downstream of the activation of the pertussis toxin-sensitive G-protein but upstream of a LTD4-stimulated tyrosine kinase(s).

The inflammatory mediator leukotriene D4 triggers a rapid reorganisation of the actin cytoskeleton in human intestinal epithelial cells

Epithelial cells play an important role in maintaining the intestinal mucosa barrier, a barrier that is impaired in several inflammatory conditions. The mechanisms behind this impairment are not known, but it can be presumed that structural alterations of the epithelial cells are involved. In support of this notion, we here show the inflammatory mediator leukotriene D4 (LTD4) triggered first a rapid (10 s) increase and immediately thereafter (30 s) a sustained decrease in the cellular filamentous actin (F-actin) level in intestinal epithelial cells. The initial LTD4-induced increase in F-actin content was effectively blocked by preincubating the cells with either pertussis toxin or the tyrosine kinase inhibitor genistein. A possible involvement of the tyrosine kinase-dependent phosphatidylinositol-3-kinase (PI-3-kinase) in the polymerisation of actin was supported by the observations that LTD4 induced a translocation to a membrane fraction of PI-3-kinase and by the findings that wortmannin, a PI-3-kinase inhibitor, totally abolished both this translocation of PI-3-kinase as well as the initial LTD4-induced polymerisation of actin. In addition, pertussis toxin and genistein, both known to interfere with the LTD4-induced calcium signal, completely or partially reversed the actin-depolymerising effect of LTD4. The calcium ionophore ionomycin (30s) induced actin depolymerisation to the same extent as LTD4 (30 s) did, but lacked the initial effect on actin polymerisation. In cells loaded with the calcium chelator MAPT, LTD4 induced a normal actin polymerisation response but the subsequent depolymerisation was completely inhibited. Similar results were obtained when the cells were preincubated with the protein kinase A inhibitor Rp-cAMPS, which has been shown to impair the LTD4-induced calcium signal in these epithelial cells. The present results show that the inflammatory mediator LTD4 triggers a reorganisation of the actin network in intestinal epithelial cells that is likely to contribute to the impairment of the intestinal barrier function.

Leukotriene D4-induced signalling events in human epithelial cells: G alpha i3 activation and translocation

Our model of LTD4-induced signal transduction in epithelial cells is summarised in Figure 2. Extending what is already known about LTD4 signalling in epithelial cells, we identified the Gi3-protein as the crucial PTX sensitive G-protein and found that it is translocated to what might be a cytoskeletal fraction. This finding suggests a subtle response to LTD4, mediated via the bifurcation at the alpha/beta gamma junction. Although little is known about the role of epithelial cells in inflammation, it has been shown that such cells produce the potent chemoattractant LTB4 and the proinflammatory 5-HETE in response to intracellular accumulation of Ca2+ 24. The target protein(s) and the effect(s) of the translocation of the activated G alpha i3-proteins, as well as the possible role of the beta/gamma-subunits of Gi3, remain to be elucidated.

Identification and characterization of two cysteinyl-leukotriene high affinity binding sites with receptor characteristics in human lung parenchyma

We report the characterization of two distinct binding sites with receptor characteristics for leukotriene (LT)D4 and LTC4 in membranes from human lung parenchyma. The use of S-decyl-glutathione allowed us to characterize a previously unidentified high affinity binding site for LTC4. Computerized analysis of binding data revealed that each leukotriene interacts with two distinct classes of binding sites (Kd = 0.015 and 105 nM for LTC4 and 0.023 and 230 nM for LTD4) and that despite cross-reactivity, the two high affinity sites are different entities. LTD4 binding sites displayed features of G protein-coupled receptors, whereas LTC4 binding sites did not show any significant modulation by guanosine-5'-(beta, gamma-imido)triphosphate or stimulation of GTPase activity. The antagonists ICI 198,615 and SKF 104353 were unselective for the high and low affinity states of LTD4 receptor, whereas only SKF 104353 was able to recognize the two [3H]LTC4 binding sites although with different affinities. These data indicate that in human lung parenchyma, LTD4 and LTC4 recognize two different binding sites; these binding sites are different entities; and for LTD4, the two binding sites represent the interconvertible affinity states of a G protein-coupled receptor, whereas for LTC4, the high affinity site is likely to be a specific LTC4 receptor.

Pharmacological characterization of the cysteinyl-leukotriene antagonists CGP 45715A (iralukast) and CGP 57698 in human airways in vitro

1. Cysteinyl-leukotrienes (cysteinyl-LTs) are important mediators in the pathogenesis of asthma. They cause bronchoconstriction, mucus hypersecretion, increase in microvascular permeability, plasma extravasation and eosinophil recruitment. 2. We investigated the pharmacological profile of the cysteinyl-LT antagonists CGP 45715A (iralukast), a structural analogue of LTD4 and CGP 57698, a quinoline type antagonist, in human airways in vitro, by performing binding studies on human lung parenchyma membranes and functional studies on human isolated bronchial strips. 3. Competition curves vs [3H]-LTD4 on human lung parenchyma membranes demonstrated that: (a) both antagonists were able to compete for the two sites labelled by [3H]-LTD4; (b) as in all the G-protein coupled receptors, iralukast and CGP 57698 did not discriminate between the high and the low affinity states of the CysLT receptor labelled by LTD4 (Ki1=Ki2= 16.6 nM+/-36% CV and Ki1= Ki2 = 5.7 nM+/-19% CV, respectively); (c) iralukast, but not CGP 57698, displayed a slow binding kinetic, because preincubation (15 min) increased its antagonist potency. 4. In functional studies: (a) iralukast and CGP 57698 antagonized LTD4-induced contraction of human bronchi, with pA2 values of 7.77+/-4.3% CV and 8.51+/-1.6% CV, respectively, and slopes not significantly different from unity; (b) the maximal LTD4 response in the presence of CGP 57698 was actually increased, thus clearly deviating from apparent simple competition. 5. Both antagonists significantly inhibited antigen-induced contraction of human isolated bronchial strips in a concentration-dependent manner, lowering the upper plateau of the anti-IgE curves. 6. In conclusion, the results of the present in vitro investigation indicate that iralukast and CGP 57698 are potent antagonists of LTD4 in human airways, with affinities in the nanomolar range, similar to those obtained for ICI 204,219 and ONO 1078, two of the most clinically advanced CysLT receptor antagonists. Thus, these compounds might be useful drugs for the therapy of asthma and other allergic diseases.

[Potential importance of antileukotrienes in the treatment of asthma and other inflammatory diseases: apropos of a new pharmacological class]

OBJECTIVES

Among the mediators involved in the asthma bronchoconstriction and inflammation mechanisms, there is now substantial evidence that the sulfidopeptide leukotrienes (LTs) are important. Antagonists of their receptors and inhibitors of their synthesis have been developed.

IMPORTANT POINTS

Antagonists of LTs, as well as inhibitors of their synthesis, reduce the LTs actions: bronchoconstriction, bronchial hyperresponsiveness, hypersecretion and inflammation. They produce an acute bronchodilating effect in mild asthma, reduce the hyperresponsiveness responses due to allergens, aspirin and cold and dry air, and also cutaneous and gastrointestinal reactions. Oral administrations tested during 4 or 6 weeks diminish the use of the beta-agonists, decrease the asthma symptom scores and other inflammatory signs.

PERSPECTIVES AND PROJECTS

More studies for longer periods, double blind trials and comparisons with classical treatments will be necessary to define the real place of LTs antagonists in the treatment of asthma. So their efficacy has to be confirmed as well as their good tolerance profile (particularly for hepatic functions).

CONCLUSION

Antagonists of receptors and synthesis inhibitors of LTs have known a recent and important development. They constitute a new therapeutic class: further studies are needed to better define the place of these new drugs in the treatment of asthma and other inflammatory diseases.

Leukotriene D4-induced mobilization of intracellular Ca2+ in epithelial cells is critically dependent on activation of the small GTP-binding protein Rho

We have previously shown that the leukotriene D4 (LTD4)-induced mobilization of intracellular Ca2+ in epithelial cells is mediated by a G-protein that is distinctly different from the pertussis toxin-sensitive G-protein that regulates the subsequent influx of Ca2+. In the present study, we attempted to gain further knowledge about the mechanisms involved in the LTD4-induced mobilization of intracellular Ca2+ in epithelial cells by investigating the effects of compactin, an inhibitor of the isoprenylation pathway, on this signalling event. In cells preincubated with 10 microM compactin for 48 h, the LTD4-induced mobilization of intracellular Ca2+ was reduced by 75% in comparison with control cells. This reduction was reversed by co-administration of mevalonate (1 mM). The effect of compactin occurred regardless of whether or not Ca2+ was present in the extracellular medium, suggesting that isoprenylation must occur before Ca2+ is released from intracellular stores. In accordance with this, we also found that both the LTD4-induced formation of inositol 1,4,5-trisphosphate and the LTD4-induced phosphorylation of phospholipase C gamma 1 (PLC gamma 1) on tyrosine residues were significantly reduced in compactin-pretreated cells. These results open up the possibility that the activation of PLC gamma 1 is related to a molecule that is sensitive to impaired activity of the isoprenylation pathway, such as a small monomeric G-protein. This idea was supported by the observation that Clostridium botulinum C3 exoenzyme-induced inhibition of Rho proteins abolished the LTD4-induced intracellular mobilization of Ca2+. A regulatory role of Rho proteins in the LTD4-induced activation of PLC gamma 1 is unlikely to be indirectly mediated via an effect on the cytoskeleton, since cytochalasin D had no major effect on the LTD4-induced mobilization of Ca2+. Although the mechanism of interaction remains to be elucidated, the present findings indicate an important role of an isoprenylated protein such as Rho in the LTD4-induced Ca2+ signal.

Role of leukotriene C4 in follicle-stimulating hormone (FSH) secretion in female rat pituitary

Leukotriene C4, at doses of 0.01 and 0.1 nmol/l added to superfused cells in pulse of 4-min duration, evoked follicle-stimulating hormone (FSH) release up to 12- to 26-fold of basal secretion. Higher and lower concentrations of leukotriene C4 were not able to induce FSH secretion. Gonadotropin-releasing hormone (GnRH)-induced FSH release was reduced by 38-57% by the leukotriene receptor antagonist FPL 55712 (10 mumol/l). Moreover, we have shown that FSH release occurs parallel to leukotriene C4 synthesis in rat anterior pituitary cells. Mellitin (100 nmol/l), an activator of phospholipase A2, induced FSH and radioactivity secretion in rat anterior pituitary cells previously preincubated for 24 h with [3H]arachidonic acid (AA).

Biochemical aspects of chronic pain and its relationship to treatment

This review presents an overview of the neurotransmitters and neuromodulators involved in acute and chronic pain. Although there is little evidence that the neuronal pathways differ in the two types of pain, it is clear that different transmitters or receptor types are involved in hyperalgesia and chronic pain. While most attention has been focussed on spinal processes, it is apparent that some types of chronic pain have both a peripheral and a supraspinal component. The presently available drugs are probably adequate for acute pain, but the treatment of chronic pain may need to be tailored to the individual patient.

Evidence for the involvement of 5-lipoxygenase products in the regulation of the expression of the proenkephalin gene in cultured astroglial cells

Cultured astroglial cells secrete eicosanoids which are produced by the cyclooxygenase and lipoxygenases. These cells also transcribe the proenkephalin gene. In the present study, it was investigated whether agents which inhibit the metabolism of arachidonic acid affect the basal and stimulated expression of the gene. Tetradecanoyl phorbol acetate (TPA; 1-1000 nmol/l) increases the concentration of proenkephalin mRNA in these cells by activating protein kinase C. The enhancement in proenkephalin mRNA caused by TPA (10 nmol/l) was not affected by the cyclooxygenase inhibitor indomethacin (5 mumol/l). However, nordihydroguaiaretic acid, which blocks cyclooxygenase and lipoxygenases, potentiated the effect of TPA on proenkephalin mRNA, when used at concentrations of 0.5-50 mumol/l. Two selective inhibitors of 5-lipoxygenase, i.e. MK886 (5 mumol/l) and BAY X1005 (1 mumol/l), also enhanced the effect of TPA (10 nmol/l) without affecting the basal expression of the gene. When added to the incubation medium, leukotriene E4 (10-1000 nmol/l) diminished in a dose-dependent manner the basal and TPA-induced expression of the proenkephalin gene. It is concluded that in astroglial cells derived from cortex of new-born rats products of 5-lipoxygenase can diminish the action of protein kinase C on the proenkephalin gene.

Leukotriene receptors

The current challenge in research on leukotriene receptors is to clone these molecules. Traditional protein purification approaches have not been successful in providing sequence information. Solubilization of cys-LT1 has been achieved but results in the dissociation of G-proteins and the loss of high affinity binding (Mong et al., 1986b; Mong and Sarau, 1990), while cys-LT2 activity cannot be monitored by other than functional assays and there have not been any purification attempts. Partial purification of B-LT has been reported but has not been continued to homogeneity (Sherman et al., 1992; Votta et al., 1990; Miki et al., 1990). Nor have attempts to clone these receptors through either homology screening or expression cloning been successful. The cloning of the prostanoid receptors, described in detail elsewhere in this volume, has shown that these receptors belong to a distinct family within the G-protein-coupled receptor superfamily. It is probable, therefore, that the leukotriene receptors will also belong to a separate group within this superfamily since phylogenic comparisons have shown that receptors displaying high affinity for structurally related ligands exist as discrete families. Recently, a human cDNA encoding an orphan FMLP-related receptor cloned from HL60 cells of myeloid lineage was identified as the receptor for another eicosanoid, lipoxin A (Fiore et al., 1994). FMLP has a similar profile of biological actions to LTB4. Moreover, LTD4 showed a high degree of cross-reactivity with this receptor with an affinity only 20-fold less that of lipoxin A, although LTB4 was inactive. It remains to be determined whether the leukotriene receptors will fall into this class of receptors. The cloning of the leukotriene receptors will allow identification of the different receptor types and subtypes and potentially splice variants. Evaluation of currently developed antagonists at these receptor types could also open the way for novel therapies for inflammatory conditions.

Influence of leukotrienes B4 and C4, 12-hydroxyeicosatetraenoic acid, and erythro epimers of hepoxilin B3 on the plasticity of cholinoreceptors of neurons of the common snail

The influences of a number of acyclic eicosanoids which are formed from arachidonic acid under the influence of 5- and 12-lipoxygenases on the plasticity of cholinoreceptors of neurons of the common snail, which was assessed on the basis of the degree of frequency depression of the amplitude of the inward current induced by local application of acetylcholine to the soma, were investigated in identified RPa3 and LPa3 neurons of the common snail using the method of bielectrode recording of the potential on the membrane. The plasticity of the cholinoreceptors was enhanced by leukotrienes B4 and C4, and was not altered by 12(S)-hydroxy-(5Z, 8Z, 10E, 14Z)-eicosatetraenoic acid, or by the threo and erythro epimers of hepoxilin B3. An inference is drawn regarding the regulation of the plasticity of cholinoreceptors by leukotrienes B4 and C4, which are formed under the influence of 5-lipoxygenase. One of these pathways includes inositol-1,4,5-triphosphate-dependent mobilization of deposited Ca2+.

Regulation of neuron cholinoreceptor plasticity of Helix lucorum by second messengers and opioids

The rhythmical local ionophoretic applications of acetylcholine (ACh) to the somatic membrane of Helix lucorum identified neurons evokes the reversible depression of the ACh-induced response which shows cholinoreceptor (ChR) desensitization. ChR desensitization is regulated not by one but by several known second messengers and G-proteins. The endogenous opioids perform the excitation or inhibitory tonic control of the membrane potential in some neurons constantly activating the ionotropic opiate receptors. The direction of neuron ChR desensitization modulation by opioids depends on the type of the activated modulatory opiate receptors (mu or kappa) on the neuron membrane. Second messengers are involved in intracellular mechanism of modulation of the ChR desensitization by opiate kappa-agonist bremazocine.

The regulation of leukotriene D4-induced calcium influx in human epithelial cells involves protein tyrosine phosphorylation

Leukotriene D4 (LTD4) has been found to induce calcium signalling in the intestinal epithelial cell line Int 407, and this action involves the activation of both different GTP-binding proteins (G-proteins) and phospholipase C of the gamma-subtype (PLC-gamma). With this knowledge as the incentive, we investigated the possible regulatory role of protein tyrosine kinase activities in the calcium signalling system of the LTD4 receptor. The tyrosine kinase inhibitors genistein and herbimycin. A both reduced the LTD4-induced calcium signal by 70% when Int 407 cells were stimulated in the presence of extracellular calcium, but had no effect on the signal when the cells were stimulated in a calcium-free medium. In accordance with these findings, pretreatment with a tyrosine kinase inhibitor also blocked thapsigargin-induced cellular influx of calcium. These inhibitors had no effect on the intracellular mobilisation of calcium, which was supported by the findings that LTD4 was able to induce an increase in the tyrosine phosphorylation of PLC-gamma even when one of the tyrosine kinase inhibitors was present. Of possible interest regarding the effect of genistein on LTD4-induced calcium influx is that two major tyrosine phosphorylated protein bands were detected in immunoprecipitates obtained with PLC-gamma antibodies from LTD4-stimulated cells. These proteins, which associate with PLC-gamma, have estimated molecular weights of 84 and 97 kD. Preincubation with genistein completely abolished the LTD4-induced increase in tyrosine phosphorylation of the major 97 kD band, whereas the 84 kD protein band, like the PLC-gamma band, still exhibited an increased phosphorylation of tyrosine residues in response to LTD4. Neither this effect nor any of the other effects of genistein were induced when cells were preincubated with daidzein, an inactive analogue of genistein. The present results suggest that LTD4-induced calcium signalling in epithelial cells involves not only tyrosine phosphorylation of PLC-gamma, but also a tyrosine kinase-dependent step which occurs downstream of PLC-gamma activation and is directly implicated in the regulation of agonist-mediated calcium influx.

Membrane mechanisms and intracellular signalling in cell volume regulation

Recent work on selected aspects of the cellular and molecular physiology of cell volume regulation is reviewed. First, the physiological significance of the regulation of cell volume is discussed. Membrane transporters involved in cell volume regulation are reviewed, including volume-sensitive K+ and Cl- channels, K+, Cl- and Na+, K+, 2Cl- cotransporters, and the Na+, H+, Cl-, HCO3-, and K+, H+ exchangers. The role of amino acids, particularly taurine, as cellular osmolytes is discussed. Possible mechanisms by which cells sense their volumes, along with the sensors of these signals, are discussed. The signals are mechanical changes in the membrane and changes in macromolecular crowding. Sensors of these signals include stretch-activated channels, the cytoskeleton, and specific membrane or cytoplasmic enzymes. Mechanisms for transduction of the signal from sensors to transporters are reviewed. These include the Ca(2+)-calmodulin system, phospholipases, polyphosphoinositide metabolism, eicosanoid metabolism, and protein kinases and phosphatases. A detailed model is presented for the swelling-initiated signal transduction pathway in Ehrlich ascites tumor cells. Finally, the coordinated control of volume-regulatory transport processes and changes in the expression of organic osmolyte transporters with long-term adaptation to osmotic stress are reviewed briefly.

Leukotriene D4-induced signal transduction

In a human epithelial cell line LTD4 induces a calcium signal that is dependent on both intracellular mobilization and influx of calcium. This calcium signal is generated via the activation of dual G protein pathways. Whereas the intracellular mobilization of calcium is regulated by a pertussis toxin-insensitive G protein, the subsequent influx of calcium is regulated by a pertussis toxin-sensitive G protein. Furthermore, a LTD4-induced cellular elevation of cAMP also participates in the regulation of this calcium signal. The increase in cAMP is directly related to the LTD4-induced influx of calcium, perhaps by an activation of protein kinase A and a subsequent phosphorylation of a plasma membrane channel. This model of the LTD4-induced signaling pathway in epithelial cells is outlined in Figure 2.

Two-phase increment of Ca2+ uptake, intracellular Ca2+ concentration, and histamine release following antigen stimulation in mouse bone marrow-derived mast cells (BMMC)

The relationship between the influx of Ca2+ into cells or cytosolic Ca2+ concentration ([Ca2+]i) and the histamine release following antigen stimulation in mouse bone marrow-derived mast cells (BMMC) was examined, and the results were compared with those from human lung mast cells (HLMC) and rat peritoneal mast cells (RPMC) in some experiments. Anaphylactic histamine release from BMMC as well as HLMC, but not that from RPMC, was dependent on the extracellular Ca2+. When BMMC were challenged by antigen following radioactive 45Ca2+ addition, two phases of 45Ca2+ influx into the cells were observed. The first phase, which was initiated and completed within 30 sec and 2 min, respectively, after antigen treatment, appeared to be related to anaphylactic histamine release. The second influx began 30 sec subsequent to the first one and lasted for at least 2 min, and this occurred after the completion of the histamine release; So far, it is not known how this second influx participates in the intracellular event(s). On the other hand, only one sustained elevation of [Ca2+]i occurred that reached its maximum within 2 min after antigen stimulation. Following stimulation of BMMC with antigen in the absence of Ca2+, Ca2+ addition 1 to 5 min later time-dependently enhanced the histamine release, although the release was deteriorated by further extension of Ca2+ addition. In contrast, the releasability of HLMC was rapidly decreased. These results indicate that extracellular Ca2+ not only is prerequisite for anaphylactic histamine release from BMMC, but also may modulate the release and participate in some intracellular event(s) which has yet to be focused upon.