Leukotriene receptors

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.

Leukotriene D(4)-induced Rho-mediated actin reorganization in human bronchial smooth muscle cells

We investigated the role of cysteinyl leukotriene (CysLT) receptors on leukotriene D(4)-induced actin reorganization and the signaling pathways of the response in human bronchial smooth muscle cells. The effects of leukotriene D(4) on actin reorganization in human bronchial smooth muscle cells were evaluated by dual-fluorescence labeling of filamentous (F) and monomeric (G) actin with fluorescein isothiocyanate (FITC)-labeled phalloidin and Texas Red-labeled DNase I, respectively. Leukotriene D(4) (100 nM) induced actin reorganization in the presence and absence of extracellular Ca(2+). The CysLT type 1 (CysLT(1)) receptor antagonist ONO 1078 (4-oxo-8(-)[p-(4-phenylbutyloxy) benzoylamino]-2-(tetrazol-5-yl)-4H-1-benzopyran hemihydrate) inhibited leukotriene D(4)-induced actin reorganization. Pretreatment with pertussis toxin, C3 exoenzyme, or tyrosine kinase inhibitors significantly reduced leukotriene D(4)-induced actin reorganization. However, phosphatidylinositol-3-kinase and protein kinase C inhibitors had little effect on these responses. These results suggest that leukotriene D(4)-induced actin reorganization in human bronchial smooth muscle cells is extremely dependent on the CysLT(1) receptor coupled with pertussis toxin-sensitive G protein, Rho GTPases and tyrosine phosphorylation pathways.

Leukotriene (LT)-receptor antagonist is more effective in asthmatic patients with a low baseline ratio of urinary LTE4 to 2,3-dinor-6-keto-prostaglandin (PG)F1alpha

BACKGROUND

To test the hypothesis that urinary levels of arachidonic acid metabolites may be a predicting factor of the effects of pranlukast, a selective leukotriene (LT) antagonist, on chronic adult asthma, we investigated the relationship between its clinical efficacy and urinary eicosanoid levels.

METHODS

An open, multicenter trial was conducted involving 38 stable moderate and severe asthmatic patients (mean percent predicted FEV1 was 71%). All patients received pranlukast (225 mg twice daily) for 4 weeks after a 2-week run-in period. Urinary levels of LTE4, 11-dehydro-thromboxane (TX) B2, 2,3-dinor-6-keto-prostaglandin (PG) F1alpha, and creatinine were measured in 3-h urine collected on day 1 of the treatment. The responder was defined by an improvement of asthma symptom scores and peak expiratory flow rate (PEFR).

RESULTS

One patient was excluded because of an adverse effect, nausea. Thirteen out of 37 subjects were responders and 24 were nonresponders. There were no significant differences in patients' backgrounds and urinary arachidonate levels between the two groups. The urinary LTE4 to 2,3-dinor-6-keto-PGF1alpha ratio in the responder was significantly lower (P=0.01) than that in the nonresponder. In all patients, a significant inverse correlation was revealed between the baseline urinary LTE4/2,3-dinor-6-keto-PGF1alpha ratio and the improvement of PEFR in the morning (r=-0.43, P=0.007).

CONCLUSIONS

These data suggested that the urinary ratio of LTE4 to 2,3-dinor-6-keto-PGF1alpha might be one of the predictive markers of the clinical efficacy of this LT-receptor antagonist in asthmatic subjects.

Pharmacology of leukotriene receptor antagonists. More than inhibitors of bronchoconstriction

The cysteinyl leukotrienes (LTs) are chemical mediators that are thought to contribute to the pathophysiologic condition of asthma and other inflammatory diseases. The biological effects of the cysteinyl LTs in the lung are pleiotropic, including both bronchoconstrictor and a growing list of nonbronchoconstrictor activities that extend to inflammatory cell recruitment, vascular leakage, mucus production, neuronal dysfunction, and airways remodeling. This spectrum of effects of cysteinyl LTs is consistent with an expanded view of asthma that extends beyond simply bronchoconstriction and inflammation. Consequently, the clinical efficacy of cysteinyl LT receptor antagonists (LTRAs) in asthma may be related to antagonism of more than cysteinyl LT-induced bronchoconstriction. The relationship of antagonism of the multiple effects of cysteinyl LTs by cysteinyl LTRAs to their utility in the therapy of asthma is addressed, and the preclinical and clinical pharmacology of cysteinyl LTRAs is reviewed.

Leukotriene receptors in HL-60 cells differentiated into eosinophils, monocytes and neutrophils

The expression of leukotriene B4 (LTB4) and leukotriene D4 (LTD4) receptors was determined, by binding assay, in HL-60 cells differentiated into the monocyte/macrophage, neutrophil, and eosinophil lineages. Monocyte/ macrophage- and neutrophil-differentiated cells developed specific LTB4 receptors with high affinities (Kd = 1.27 nM and 2.65 nM, respectively) and low affinities (Kd = 26.41 nM and 55.63 nM, respectively). These receptors were functional and specific as indicated by the ability of LTB4 to elicit an increase in intracellular calcium concentration antagonised by specific antagonists. Eosinophil-differentiated cells developed mainly LTD4 receptors (Kd = 41.91 nM), and stimulation with LTD4 induced an increase in intracellular calcium that was antagonised by a specific LTD4 antagonist. These results show, for the first time, that eosinophil-differentiated HL-60 cells express specific functional LTD4 receptors. These cells could be used for the study of the actions of peptidoleukotrienes on eosinophils, and for studies on the molecular mechanisms regulating LTD4 receptor expression.

Cardiovascular effects of N-methyl leukotriene C4, a nonmetabolizable leukotriene C4 analogue, and the antagonism of leukotriene-induced hypotension by Ro 23-3544, in the American bullfrog, Rana catesbeiana

Although some leukotriene antagonists have been reported to block leukotriene (LT) C4 responses in vivo, it is difficult to determine whether those antagonists block the effect of LTC4 directly or act via blocking the action of LTD4, as LTC4 is metabolized to LTD4 rapidly in vivo. In this study, the dose-response curves of N-methyl LTC4 (NMLTC4), the nonmetabolizable LTC4 analogue, and the peptidoleukotrienes (LTC4, LTD4, and LTE4) were obtained in the absence and presence of the leukotriene antagonist Ro 23-3544 in cannulated frogs. The more potent effect of NMLTC4 suggests that receptors that preferentially bind LTC4 exist in frog vascular smooth muscle and the previously reported LTC4 effect is a combination of LTC4 and its less potent metabolite LTD4. The NMLTC4- and LTC4-induced hypotensive effects were antagonized by Ro 23-3544. Ro 23-3544 also antagonized the effects induced by high doses of LTD4 and LTE4. Ro 23-3544 had no effect on duration of response and did not affect heart rate responses to LTC4 at low dose of the antagonist. The data suggest that receptors that preferentially bind LTC4 in bullfrog vascular smooth muscle regulate the hypotensive effect and that they can be antagonized by Ro 23-3544.

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.

Characterization of leukotriene C4 binding sites in the brain of the American bullfrog, Rana catesbeiana

In this study, specific binding sites for [3H]-LTC4 on membrane preparations from American bullfrog (Rana catesbeiana) brain were characterized. Binding assays were done in the presence of serine (5mM) borate (10 mM) for 30 min at 23 degrees C. Under these conditions, no metabolism of LTC4 to LTD4 occurred. Specific binding of [3H]-LTC4 reached steady state within 10 min, remained constant for 60 min, and was reversible with the addition of 1,000-fold excess unlabelled LTC4. Scatchard analysis of the binding data indicated a single class of binding sites with an estimated Kd of 89.83 nM and Bmax of 43.79 pmol/mg protein. Competition binding studies demonstrated that LTD4 and LTE4 were ineffective in displacing [3H]-LTC4 from its binding site. The Ki for LTC4 was 51 nM. S-decylglutathione, glutathione and hematin had Ki values of 44, 312,602, and 25,576 nM, respectively. The mammalian cysteinyl leukotriene antagonist L-660,711 inhibited specific binding of [3H]-LTC4, with a Ki of 87,149 nM. Guanosine-5'-0-3-thiotriphosphate (GTP gamma S) did not affect specific binding of [3H]-LTC4 indicating that, like mammalian LTC4 receptors, a Gi protein is not involved in the transduction mechanism. The LTC4 binding site in bullfrog brain demonstrates both similarities and differences from its mammalian counterpart.