Identification of a murine cysteinyl leukotriene receptor by expression in Xenopus laevis oocytes

Leukotriene Receptor Antagonists and Antiallergy Drugs

As one of the candidates of the therapeutic strategy for asthma in addition to inhaled corticosteroids (ICS), leukotriene receptor antagonists (LTRAs) are known to be useful for long-term management of asthma patients complicated by allergic rhinitis (AR) or exercise-induced asthma (EIA). Currently available LTRAs are pranlukast hydrate, zafirlukast, and montelukast. These LTRAs have a bronchodilator action and inhibit airway inflammation, resulting in a significant improvement of asthma symptoms, respiratory function, inhalation frequency of as-needed inhaled β2-agonist, airway inflammation, airway hyperresponsiveness, dosage of ICSs, asthma exacerbations, and patients' QOL. Although cys-LTs are deeply associated with the pathogenesis of asthma, LTRAs alone are less effective compared with ICS. However, the effects of LTRAs in combination with ICS are the same as those of LABAs in combination with ICS in steroid-naïve asthmatic patients. Concerning antiallergy drugs other than LTRAs, some mediator-release suppressants, H1 histamine receptor antagonists (H1RAs), thromboxane A2 (TXA2) inhibitors/antagonists, and Th2 cytokine inhibitor had been used mainly in Japan until the late 1990s. However, the use of these agents rapidly decreased after ICS/long acting beta agonist (LABA) combination was introduced and recommended for the management of asthma in the early 2000s. The effectiveness of other antiallergic agents on asthma management seems to be quite limited, and the safety of oral antiallergic agents has not been demonstrated in fetuses during pregnancy. Further effectiveness studies are needed to determine the true value of these orally administered agents in combination with ICS as an anti-asthma treatment.

Cutting edge: Leukotriene C4 activates mouse platelets in plasma exclusively through the type 2 cysteinyl leukotriene receptor

Leukotriene C4 (LTC4) and its extracellular metabolites, LTD4 and LTE4, mediate airway inflammation. They signal through three specific receptors (type 1 cys-LT receptor [CysLT1R], CysLT2R, and GPR99) with overlapping ligand preferences. In this article, we demonstrate that LTC4, but not LTD4 or LTE4, activates mouse platelets exclusively through CysLT2R. Platelets expressed CysLT1R and CysLT2R proteins. LTC4 induced surface expression of CD62P by wild-type mouse platelets in platelet-rich plasma (PRP) and caused their secretion of thromboxane A2 and CXCL4. LTC4 was fully active on PRP from mice lacking either CysLT1R or GPR99, but completely inactive on PRP from CysLT2R-null (Cysltr2(-/-)) mice. LTC4/CysLT2R signaling required an autocrine ADP-mediated response through P2Y12 receptors. LTC4 potentiated airway inflammation in a platelet- and CysLT2R-dependent manner. Thus, CysLT2R on platelets recognizes LTC4 with unexpected selectivity. Nascent LTC4 may activate platelets at a synapse with granulocytes before it is converted to LTD4, promoting mediator generation and the formation of leukocyte-platelet complexes that facilitate inflammation.

Cysteinyl leukotriene receptors, old and new; implications for asthma

The cysteinyl leukotrienes (cys-LTs) are three structurally similar, but functionally distinct lipid mediators of inflammation. The parent cys-LT, LTC(4) , is synthesized by and released from mast cells, eosinophils, basophils, and macrophages, and is converted to the potent constrictor LTD(4) and the stable metabolite, LTE(4) . While only two cys-LT-selective receptors (CysLTRs) have been identified, cloned, and characterized, studies dating back three decades predicted the existence of at least three functional CysLTRs, each with a characteristic physiological function in airways and other tissues. The recent demonstration that mice lacking both known CysLTRs exhibit full (and in some instances, augmented) physiological responses to cys-LTs verifies the existence of unidentified CysLTRs. Moreover, the ability to manipulate receptor expression in both whole animal and cellular systems reveals that the functions of CysLTRs are controlled at multiple levels, including receptor-receptor interactions. Finally, studies in transgenic mice have uncovered a potentially major role for cys-LTs in controlling the induction of Th(2) responses to common allergens. This review focuses on these recent findings and their potential clinical implications.

Ion channels involved in cell volume regulation: effects on migration, proliferation, and programmed cell death in non adherent EAT cells and adherent ELA cells

This mini review outlines studies of cell volume regulation in two closely related mammalian cell lines: nonadherent Ehrlich ascites tumour cells (EATC) and adherent Ehrlich Lettre ascites (ELA) cells. Focus is on the regulatory volume decrease (RVD) that occurs after cell swelling, the volume regulatory ion channels involved, and the mechanisms (cellular signalling pathways) that regulate these channels. Finally, I shall also briefly review current investigations in these two cell lines that focuses on how changes in cell volume can regulate cell functions such as cell migration, proliferation, and programmed cell death.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Swelling-activated ion channels: functional regulation in cell-swelling, proliferation and apoptosis

Cell volume regulation is one of the most fundamental homeostatic mechanisms and essential for normal cellular function. At the same time, however, many physiological mechanisms are associated with regulatory changes in cell size meaning that the set point for cell volume regulation is under physiological control. Thus, cell volume is under a tight and dynamic control and abnormal cell volume regulation will ultimately lead to severe cellular dysfunction, including alterations in cell proliferation and cell death. This review describes the different swelling-activated ion channels that participate as key players in the maintenance of normal steady-state cell volume, with particular emphasis on the intracellular signalling pathways responsible for their regulation during hypotonic stress, cell proliferation and apoptosis.

Role of the Cys18–Cys274 disulfide bond and of the third extracellular loop in the constitutive activation and internalization of angiotensin II type 1 receptor

An insertion of residues in the third extracellular loop and a disulfide bond linking this loop to the N-terminal domain were identified in a structural model of a G-protein coupled receptor specific to angiotensin II (AT1 receptor), built in homology to the seven-transmembrane-helix bundle of rhodopsin. Both the insertion and the disulfide bond were located close to an extracellular locus, flanked by the second extracellular loop (EC-2), the third extracellular loop (EC-3) and the N-terminal domain of the receptor; they contained residues identified by mutagenesis studies to bind the angiotensin II N-terminal segment (residues D1 and R2). It was postulated that the insertion and the disulfide bond, also found in other receptors such as those for bradykinin, endothelin, purine and other ligands, might play a role in regulating the function of the AT1 receptor. This possibility was investigated by assaying AT1 forms devoid of the insertion and with mutations to Ser on both positions of Cys residues forming the disulfide bond. Binding and activation experiments showed that abolition of this bond led to constitutive activation, decay of agonist binding and receptor activation levels. Furthermore, the receptors thus mutated were translocated to cytosolic environments including those in the nucleus. The receptor form with full deletion of the EC-3 loop residue insertion, displayed a wild type receptor behavior.

Targeted Gene Disruption Reveals the Role of the Cysteinyl Leukotriene 2 Receptor in Increased Vascular Permeability and in Bleomycin-induced Pulmonary Fibrosis in Mice

The cysteinyl leukotrienes (cys-LTs) mediate both acute and chronic inflammatory responses in mice, as demonstrated by the attenuation of the IgE/antigen-mediated increase in microvascular permeability and of bleomycin-induced pulmonary fibrosis, respectively, in a strain with targeted disruption of leukotriene C(4) synthase to prevent cys-LT synthesis. Our earlier finding that the acute, but not the chronic, injury was attenuated in a strain with targeted disruption of the cysteinyl leukotriene 1 (CysLT(1)) receptor suggested that the chronic injury might be mediated through the CysLT(2) receptor. Thus, we generated CysLT(2) receptor-deficient mice by targeted gene disruption. These mice developed normally and were fertile. The increased vascular permeability associated with IgE-dependent passive cutaneous anaphylaxis was significantly reduced in CysLT(2) receptor-null mice as compared with wild-type mice, whereas plasma protein extravasation in response to zymosan A-induced peritoneal inflammation was not altered. Alveolar septal thickening after intratracheal injection of bleomycin, characterized by interstitial infiltration with macrophages and fibroblasts and the accumulation of collagen fibers, was significantly reduced in CysLT(2) receptor-null mice as compared with the wild-type mice. The amounts of cys-LTs in bronchoalveolar lavage fluid after bleomycin injection were similar in the CysLT(2) receptor-null mice and the wild-type mice. Thus, in response to a particular pathobiologic event the CysLT(2) receptor can mediate an increase in vascular permeability in some tissues or promote chronic pulmonary inflammation with fibrosis.

Co-expression of mCysLT1 receptors and IK channels in Xenopus laevis oocytes elicits LTD4-stimulated IK current, independent of an increase in [Ca2+]i

Addition of LTD4 (10 nM) to Xenopus laevis oocytes expressing the mCysLT1 receptor together with hBK or hIK channels resulted in the activation of both channels secondary to an LTD4-induced increase in [Ca2+]i. In addition, the hIK channel is activated by low concentrations of LTD4 (<0.1 nM), which did not result in any increase in [Ca2+]i. Even though activation of hIK by low concentrations of LTD4 was independent of an increase in [Ca2+]i, a certain "permissive" level of [Ca2+]i was required for its activation, since buffering of intracellular Ca2+ by EGTA completely abolished the response to LTD4. Neither hTBAK1 nor hTASK2 was activated following stimulations with LTD4 (0.1 and 100 nM).

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.

Targeted gene disruption reveals the role of cysteinyl leukotriene 1 receptor in the enhanced vascular permeability of mice undergoing acute inflammatory responses

The cysteinyl leukotrienes (cysLTs), leukotriene (LT) C(4), LTD(4), and LTE(4), are proinflammatory lipid mediators generated in the mouse by hematopoietic cells such as macrophages and mast cells. There are two mouse receptors for the cysLTs, CysLT(1) receptor (CysLT(1)R) and CysLT(2)R, which are 38% homologous and are located on mouse chromosomes X and 14, respectively. To clarify the different roles of the CysLT(1)R and CysLT(2)R in inflammatory responses in vivo, we generated CysLT(1)R-deficient mice by targeted gene disruption. These mice developed normally and were fertile. In an intracellular calcium mobilization assay with fura-2 acetoxymethyl ester, peritoneal macrophages from wild-type littermates, which express both CysLT(1)R and CysLT(2)R, responded substantially to 1 x 10(-6) m LTD(4) and slightly to 1 x 10(-6) m LTC(4), whereas the macrophages from CysLT(1)R-deficient mice did not respond to either LTD(4) or LTC(4). Plasma protein extravasation, but not neutrophil infiltration, was significantly reduced in CysLT(1)R-deficient mice subjected to zymosan A-induced peritoneal inflammation. Plasma protein extravasation was also significantly diminished in CysLT(1)R-deficient mice undergoing IgE-mediated passive cutaneous anaphylaxis as compared with the wild-type mice. Thus, the cysLTs generated in vivo by either monocytes/macrophages or mast cells utilize CysLT(1)R for the response of the microvasculature in acute inflammation.

Intracellular signalling involved in activation of the volume-sensitive K+ current in Ehrlich ascites tumour cells

The cell swelling-activated K+ channel in Ehrlich ascites tumour cells has a conductance of 5 pS estimated from noise analysis of the volume-sensitive whole-cell K+ current (I(K,vol)). I(K,vol) exhibits Goldman-Hodgkin-Katz type behaviour and is insensitive to clotrimazole, apamin and charybdotoxin (ChTX), but inhibited by clofilium. Its small conductance, lack of intrinsic voltage-dependence and peculiar pharmacological profile are similar to properties described for the two-pore domain background K+ TASK channels. Neither Ca2+ nor ATP work as initiators in the activation of I(K,vol). In contrast, several investigations in Ehrlich cells suggest an important role for leukotriene D4 (LTD4) in the activation of I(K,vol). Under isotonic conditions, LTD4 activates Ca2+-dependent, ChTX-sensitive K+ channels as well as Ca2+-independent. ChTX-insensitive K+ channels. The LTD4-activated, ChTX-insensitive K+ current exhibits a current-voltage relation, pharmacological profile and single channel conductance similar to that of I(K,vol), indicating that LTD4 is the signalling molecule responsible for activation of the volume-sensitive K+ channels in Ehrlich cells. Hypotonic swelling of Ehrlich cells results in translocation of the 85-kDa cytosolic (c) PLA2alpha to the nucleus where it is activated. This activation leads to an increase in arachidonic acid release followed by an increased release of leukotrienes, and is essential in cell swelling-induced activation of I(K,vol) and of the organic osmolyte channels.