Identification and target-size analysis of the leukotriene D4 receptor in the human THP-1 cell line

Neutralization of leukotriene C4 and D4 activity by monoclonal and single-chain antibodies

BACKGROUND

Cysteinyl leukotrienes (LTs) are key mediators in inflammation. To explore the structure of the antigen-recognition site of a monoclonal antibody against LTC4 (mAbLTC), we previously isolated full-length cDNAs for heavy and light chains of the antibody and prepared a single-chain antibody comprising variable regions of these two chains (scFvLTC).

METHODS

We examined whether mAbLTC and scFvLTC neutralized the biological activities of LTC4 and LTD4 by competing their binding to their receptors.

RESULTS

mAbLTC and scFvLTC inhibited their binding of LTC4 or LTD4 to CysLT1 receptor (CysLT1R) and CysLT2 receptor (CysLT2R) overexpressed in Chinese hamster ovary cells. The induction by LTD4 of monocyte chemoattractant protein-1 and interleukin-8 mRNAs in human monocytic leukemia THP-1 cells expressing CysLT1R was dose-dependently suppressed not only by mAbLTC but also by scFvLTC. LTC4- and LTD4-induced aggregation of mouse platelets expressing CysLT2R was dose-dependently suppressed by either mAbLTC or scFvLTC. Administration of mAbLTC reduced pulmonary eosinophil infiltration and goblet cell hyperplasia observed in a murine model of asthma. Furthermore, mAbLTC bound to CysLT2R antagonists but not to CysLT1R antagonists.

CONCLUSIONS

These results indicate that mAbLTC and scFvLTC neutralize the biological activities of LTs by competing their binding to CysLT1R and CysLT2R. Furthermore, the binding of cysteinyl LT receptor antagonists to mAbLTC suggests the structural resemblance of the LT-recognition site of the antibody to that of these receptors.

GENERAL SIGNIFICANCE

mAbLTC can be used in the treatment of inflammatory diseases such as asthma.

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

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

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.

Levels of cysteinyl leukotriene receptor mRNA in human peripheral leucocytes: significantly higher expression of cysteinyl leukotriene receptor 2 mRNA in eosinophils

BACKGROUND

Cysteinyl leukotrienes (CysLTs) have been implicated as important contributors in the pathophysiology of asthma and their biological effects are mediated by at least two distinct G-protein-coupled receptors. cDNA sequences of cysteinyl leukotriene receptor 1 (CysLTR1) and cysteinyl leukotriene receptor 2 (CysLTR2) have recently been elucidated.

OBJECTIVES

Our aim is to explore gene expression and the comparative expression of CysLTR1 mRNA and CysLTR2 mRNA in human peripheral blood leucocytes.

METHODS

Gene expression of CysLTR1 and CysLTR2 mRNAs in human peripheral blood eosinophils, neutrophils, monocytes and T lymphocytes has been measured by competitive reverse transcription-polymerase chain reactions using RNA or DNA competitors.

RESULTS

(a) When cellular levels of CysLTR1 mRNA were normalized to those of G3PDH mRNA, the relative concentration of CysLTR1 mRNA in eosinophils (43.8 +/- 37.2, n = 29) was significantly higher than that in neutrophils (18.7 +/- 23.3, n = 11), monocytes (0.93 +/- 1.1, n = 10) and T lymphocytes (3.4 +/- 2.4, n = 11). (b) When measured using each DNA competitor, mRNAs for both types of CysLTR coexisted in each type of leucocyte. The ratio of CysLTR1 mRNA to CysLTR2 mRNA was significantly lower in eosinophils (0.65 +/- 0.42, n = 12) than in neutrophils (6.9 +/- 4.9, n = 12), monocytes (1.8 +/- 0.9, n = 10) and T lymphocytes (4.5 +/- 5.7, n = 10). (c) Human umbilical vein endothelial cells expressed CysLTR2 mRNA, but not CysLTR1 mRNA.

CONCLUSION

These studies reveal that CysLTR1 mRNA and, in particular, CysLTR2 mRNA are abundantly expressed at high levels in eosinophils, raising the possibility that CysLTR2 may have an important physiological role in eosinophils and a CysLTR2 antagonist may be a good target for preventing signal transduction by CysLTs in eosinophils.

Molecular cloning and functional characterization of murine cysteinyl-leukotriene 1 (CysLT(1)) receptors

We sought to clone and characterize the murine cysteinyl-leukotriene D(4) receptor (mCysLT(1)R) to complement our studies with leukotriene-deficient mice. A cDNA, cloned from trachea mRNA by reverse transcriptase-polymerase chain reaction, has two potential initiator ATG codons that would encode for polypeptides of 352 and 339 amino acids, respectively. These two potential forms, predicted to be seven transmembrane-spanning domain proteins, have 87% amino acid identity with the human CysLT(1) receptor (hCysLT(1)R). Membrane fractions of Cos-7 cells transiently expressing the short mCysLT(1)R demonstrated high affinity and specific binding for leukotriene D(4) (LTD(4), K(d) = 0.25 +/- 0.04 nM). In competition binding experiments, LTD(4) was the most potent competitor (K(i) = 0.8 +/- 0.2 nM) followed by LTE(4) and LTC(4) (K(i) = 86.6 +/- 24.5 and 100.1 +/- 17.1 nM, respectively) and LTB(4) (K(i) > 1.5 microM). Binding of LTD(4) was competitively inhibited by the specific CysLT(1) receptor antagonists MK-571 [(+)-3-(((3-(2-(7-chloro-2-quinolinyl)ethenyl)phenyl) ((3-(dimethylamino)-3-oxopropyl)thio)methyl)thio)propanoic acid], pranlukast (Onon), and zafirlukast (Accolate), while the CysLT(1)/CysLT(2) receptor antagonist BAY-u9773 [6(R)-(4'-carboxyphenylthio)-5(S)-hydroxy-7(E),9(E),11(Z),14(Z)-eicosatetrenoic acid] was 1000 times less potent than LTD(4). In transiently transfected HEK293-T cells expressing either the long or short form of mCysLT(1)R, LTD(4) induced an increase of intracellular calcium. In Xenopus laevis melanophores transiently expressing either isoform, LTD(4) induced the dispersion of pigment granules, consistent with the activation by LTD(4) of a G(alphaq) (calcium) pathway. Functional elucidation of mCysLT(1)R properties as described here will enable further experiments to clarify the selective role of LTD(4) in murine models of inflammation and asthma.

5-Lipoxygenase reaction products modulate alveolar macrophage phagocytosis of Klebsiella pneumoniae

The leukotrienes are potent lipid mediators of inflammation formed by the 5-lipoxygenase-catalyzed oxidation of arachidonic acid. Although the effects of leukotrienes on neutrophil chemotaxis and activation have been established, their role in modulating innate host defense mechanisms is poorly understood. In a previous study (M. Bailie, T. Standiford, L. Laichalk, M. Coffey, R. Strieter, and M. Peters-Golden, J. Immunol. 157:5221-5224, 1996), we used 5-lipoxygenase knockout mice to establish a critical role for endogenous leukotrienes in pulmonary clearance and alveolar macrophage phagocytosis of Klebsiella pneumoniae. In the present study, we investigated the role of specific endogenous leukotrienes in phagocytosis of K. pneumoniae and explored the possibility that exogenous leukotrienes could restore phagocytosis in alveolar macrophages with endogenous leukotriene synthesis inhibition and enhance this process in leukotriene-competent cells. Rat alveolar macrophages produced leukotriene B4 (LTB4), LTC4, and 5-hydoxyeicosatetraenoic acid (5-HETE) during the process of phagocytosis, and the inhibition of endogenous leukotriene synthesis with zileuton and MK-886 dramatically attenuated phagocytosis. We also observed a reduction in phagocytosis when we treated alveolar macrophages with antagonists to the plasma membrane receptors for either LTB4, cysteinyl-leukotrienes, or both. In leukotriene-competent cells, LTC4 augmented phagocytosis to the greatest extent, followed by 5-HETE and LTB4. These 5-lipoxygenase reaction products demonstrated similar relative abilities to reconstitute phagocytosis in zileuton-treated rat alveolar macrophages and in alveolar macrophages from 5-lipoxygenase knockout mice. We conclude that endogenous synthesis of all major 5-lipoxygenase reaction products plays an essential role in phagocytosis. The restorative and pharmacologic effects of LTC4, LTB4, and 5-HETE may provide a basis for their exogenous administration as an adjunctive treatment for patients with gram-negative bacterial pneumonia.

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.

Leukotriene D4 activates mitogen-activated protein kinase through a protein kinase Calpha-Raf-1-dependent pathway in human monocytic leukemia THP-1 cells

Leukotriene D4 (LTD4) is a major lipid mediator involved in inflammatory and allergic disorders including bronchial asthma. Despite its potent biological activity, little is known about the receptor and intracellular signaling pathways. Here we analyzed the signal transduction mechanisms through LTD4 receptors using human monocytic leukemia THP-1 cells. When these cells were stimulated with LTD4, intracellular calcium concentration was increased and mitogen-activated protein kinase (MAP kinase) was activated severalfold. This activation was inhibited by staurosporine or GF109203X treatment or abolished by protein kinase C depletion. Cytosolic protein kinase Calpha was translocated to the membrane, and Raf-1 was activated by LTD4 treatment in a similar time course. LTD4-induced Raf-1 activation was diminished by protein kinase C depletion in the cells. A chemotactic response of THP-1 cells toward LTD4 was observed which was inhibited by pertussis toxin (PTX) pretreatment. Thus, LTD4 has at least two distinct signaling pathways in THP-1 cells, a PTX-insensitive mitogen-activated protein kinase activation through protein kinase Calpha and Raf-1 and a PTX-sensitive chemotactic response. This cellular signaling can explain in part the versatile activities of LTD4 in macrophages under inflammatory and allergic conditions.

Effects of synthetic peptido-leukotrienes on bone resorption in vitro

Peptido-leukotrienes are short-lived organic molecules known to have potent biological effects as mediators of inflammation, hypersensitivity and respiratory disorders. However, little is known concerning their effects on bone cells. We have shown previously that stromal cells isolated from a human giant cell tumor secrete 5-HETE (5-hydroxyeicosatetraenoic acid) and the peptido-leukotrienes, also known as the cysteinyl leukotrienes LTC4, LTD4, and LTE4. These eicosanoids were shown to stimulate the multinucleated giant cells obtained from these tumors to form resorption lacunae on sperm whale dentine. Here, we show that the peptido-leukotrienes also stimulate isolated avian osteoclast-like cells to form resorption lacunae and to increase their content of tartrate-resistant acid phosphatase. LTD4 increased 45Ca release from murine calvarial bone organ cultures, but not from fetal rat long bone cultures. Isolated avian osteoclast-like cells were chosen to perform receptor binding studies, as this population is the most homogeneous source of osteoclasts available. After the precursors had fused to form multinucleated cells, receptor binding assays were performed. Scatchard analysis of saturation binding data showed a single class of binding sites, with a dissociation constant (Kd) of 0.53 nM and a receptor density of 5,200 receptors per cell. Competition binding studies showed receptor specificity using a specific LTD4 receptor antagonist ZM 198,615. These data show that the peptido-leukotrienes activate highly enriched populations of isolated avian osteoclast-like cells, and also that specific LTD4 receptors are present in this cell population.

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.

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.