Pharmacological actions of Y-24180, a new specific antagonist of platelet activating factor (PAF): II. Interactions with PAF and benzodiazepine receptors

Stimulation of Hepatic Apolipoprotein A-I Production by Novel Thieno-Triazolodiazepines: Roles of the Classical Benzodiazepine Receptor, PAF Receptor, and Bromodomain Binding

Expression and secretion of apolipoprotein A-I (apoA-I) by cultured liver cells can be markedly stimulated by triazolodiazepines (TZDs). It has been shown previously that the thieno-TZD Ro 11-1464 increases plasma levels of apoA-I and in vivomacrophage reverse cholesterol transport in mice. However, these effects were only seen at high doses, at which the compound could act on central benzodiazepine (BZD) receptors or platelet activating factor (PAF) receptors, interfering with its potential utility. In this work, we describe 2 new thieno-TZDs MDCO-3770 and MDCO-3783, both derived from Ro 11-1464. These compounds display the same high efficacy on apoA-I production, metabolic stability, and lack of cytotoxicity in cultured hepatocytes as Ro 11-1464, but they do not bind to the central BZD receptor and PAF receptor. The quinazoline RVX-208 was less efficacious in stimulating apoA-I production and displayed signs of cytotoxicity. Certain TZDs stimulating apoA-I production are now known to be inhibitors of bromodomain (BRD) extra-terminal (BET) proteins BRDT, BRD2, BRD3, and BRD4, and this inhibition was inferred as a main molecular mechanism for their effect on apoA-I expression. We show here that the thieno-TZD (+)-JQ1, a potent BET inhibitor, strongly stimulated apoA-I production in Hep-G2 cells, but that its enantiomer (−)-JQ1, which has no BET inhibitor activity, also showed considerable effect on apoA-I production. MDCO-3770 and MDCO-3783 also inhibited BRD3 and BRD4 in vitro, with potency somewhat below that of (+)-JQ1. We conclude that the effect of thieno-TZDs on apoA-I expression is not due to inhibition of the BZD or PAF receptors and is not completely explained by transcriptional repression by BET proteins.

WEB-2086 and WEB-2170 trigger apoptosis in both ATRA-sensitive and -resistant promyelocytic leukemia cells and greatly enhance ATRA differentiation potential

PAF-receptor antagonists WEB-2086 and WEB-2170 (WEBs) have been previously shown to induce differentiation in murine and human leukemia cells. The present study describes the apoptotic-differentiative effect of WEBs in all-trans-retinoic acid (ATRA)-sensitive (NB4) and -resistant (NB4-007-6 and NB4-MR4) acute promyelocytic leukemia (APL) cell lines as well as blasts from patients with t(15;17) APL. NB4 cells exposed to 0.5-1 mM WEBs underwent striking growth arrest and massive apoptosis without appreciable differentiation; IC50 values after 3-day treatment of NB4 were 0.4 and 0.25 mM for WEB-2086 and WEB-2170, respectively. WEBs induced apoptosis also in the two ATRA-resistant NB4-007-6 and NB4-MR4 cell lines and in blasts from patients with t(15;17) APL. Moreover, subapoptotic WEBs acted synergistically with low-dose (0.025-0.05 microM) ATRA; this allowed to increase ATRA differentiation potential up to 40-fold and to improve both number and intensity of NBT-positive NB4 cells at definitely higher levels than with 1 muM ATRA alone. The powerful antiproliferative-apoptotic activities of WEBs in vitro on ATRA-sensitive, ATRA-resistant APL cells and blasts from patients with APL as well as drug capabilities to enhance ATRA differentiation potential suggested that these agents also due to their recognized tolerability in vivo might improve, alone or in combination, clinical treatment of APL.

Involvement of unique mechanisms in the induction of scratching behavior in BALB/c mice by compound 48/80

Compound 48/80 induced scratching behavior in BALB/c mice, and the role of mast cell mediators in this behavior was examined. Mouse scratching behavior was detected and evaluated using a new apparatus, MicroAct. Compound 48/80 increased the incidence of scratching behavior and scratching time in a dose-dependent manner, accompanied by a potent activation of mast cells and a potent increase in vascular permeability. Dibucaine and mu-opioid receptor antagonists inhibited the scratching behavior. Although histamine H(1) receptor antagonists potently inhibited the vascular permeability increase, they did not affect the scratching behavior. Methysergide inhibited the scratching behavior slightly without affecting the vascular permeability increase, whereas cyproheptadine inhibited both. A cyclooxygenase inhibitor, a 5-lipoxygenase-activating protein inhibitor and a PAF receptor antagonist did not affect the scratching behavior. High doses of serotonin induced scratching behavior less frequently than did compound 48/80. Furthermore, mast cell-deficient WBB6F1-W/W(v) mice exhibited frequent scratching behavior after injection of compound 48/80. These results clearly indicate that compound 48/80 can induce scratching behavior in mice independent of mast cell mediators.

Cardiovascular properties of yangambin, a lignan isolated from Brazilian plants

Yangambin was initially selected from a number of lignans isolated from Brazilian plants for its ability to antagonize Platelet-Activating Factor (PAF, 1-O-hexadecyl-2-acetyl- sn-glyceryl-3-phosphorylcholine)-induced biological effects. Subsequently it was shown that, besides its antagonistic properties at PAF receptors, yangambin also prevents the cardiovascular collapse observed during anaphylactic and endotoxic/septic shocks, as well as the vascular and cardiac hyporesponsiveness to catecholamines in endotoxic shock. It is suggested that this naturally occurring compound could be of potential interest in the adjunctive management of the above mentioned pathologies. In the present article, we review the main studies investigating the pharmacological properties of yangambin related to the cardiovascular function.

Acid exposure stimulates the adherence of Streptococcus pneumoniae to cultured human airway epithelial cells: effects on platelet-activating factor receptor expression

To examine the effects of acid exposure on the adherence of Streptococcus pneumoniae to cultured human tracheal epithelial cells, cells were exposed to acid at various pH levels, and various concentrations of S. pneumoniae were added to the culture medium. The number of S. pneumoniae adhering to cultured human tracheal epithelial cells increased after acid exposure. Y-24180, a specific inhibitor of the receptor for the platelet-activating factor (PAF) and PAF itself decreased the number of S. pneumoniae adhering to cultured human tracheal epithelial cells after acid exposure. Acid exposure increased the activation of transcription factor nuclear factor (NF)-kappa B and the expression of protein and messenger RNA (mRNA) of the PAF receptor. The pyrrolidine derivative of dithiocarbamate (PDTC), an inhibitor of NF-kappa B, also decreased the number of S. pneumoniae adhering to the cultured human tracheal epithelial cells after acid exposure. Acid exposure increased the content of interleukin (IL)-1 alpha and IL-1 beta in the culture supernatants, but monoclonal antibodies to IL-1 alpha and IL-1 beta failed to inhibit the increased number of S. pneumoniae adhering to cultured human tracheal epithelial cells after acid exposure. These findings suggest that acid exposure stimulates the adherence of S. pneumoniae to the airway epithelial cells via increases in PAF receptors. Increases in PAF receptor expression may be, in part, mediated via activation of transcription factors and subsequent PAF receptor mRNA expression by acid exposure. Increased adherence of S. pneumoniae may be one of the reasons why pneumonia develops after gastric juice aspiration.

Differential effect of antiallergic drugs on IgE-mediated cutaneous reaction in passively sensitized mice

We investigated the effect of several antiallergic agents on murine IgE-mediated biphasic cutaneous reaction. Mice were passively sensitized by an intravenous injection of monoclonal anti-dinitrophenol (anti-DNP) IgE antibody. Skin reaction was elicited by an epicutaneous challenge of dinitrofluorobenzene (DNFB) and occurred biphasically with immediate-phase response (IPR) and late-phase response (LPR) at 1 and 24 h, respectively. Classical histamine H1 receptor antagonists and some chemical mediator-release inhibitors, such as diphenhydramine and terfenadine, inhibited IPR, but not LPR. In contrast, leukotriene B(4) (LTB(4)) receptor antagonist (ONO-4057) inhibited LPR only. Antagonists for LTC(4), D(4), E(4) receptor (ONO-1078) and platelet-activating factor (PAF) receptor (Y-24180) significantly inhibited both IPR and LPR, similarly to prednisolone. We recently found that a third-phase inflammatory reaction with marked infiltration of eosinophils (named very-late-phase response; vLPR), which is supposed to be a more important reaction in allergic diseases, was induced, peaking at day 8 following IPR and LPR in this model. The effect of these drugs on the triphasic skin reaction can be scored based on efficacy against IPR / LPR / vLPR; +/+/+ (prednisolone, a PAF receptor antagonist Y-24180, cyclosporin A and FK-506), +/-/- (diphenhydramine), +/+/- (azelastine and LT receptor antagonist ONO-1078), and -/+/+ (an LTB(4) receptor antagonist ONO-4057). Thus the inhibitory effect on vLPR as well as LPR may relate to the inhibition of eosinophil function mediated by LTB(4) and PAF and/or T cells. These findings may provide the basis for a treatment modality using various antiallergic agents in allergic disease.

Inhibition of eosinophil activation in bronchoalveolar lavage fluid from atopic asthmatics by Y-24180, an antagonist to platelet-activating factor

We examined the effects of Y-24180, a potent and long-acting antagonist to platelet-activating factor (PAF) receptor, on the expression of adhesion molecules in peripheral blood and bronchoalveolar lavage fluid (BALF) eosinophils from atopic asthmatics. Y-24180 (20 mg/day) was administered to 4 atopic asthmatics for 8 weeks. The number of eosinophils, the level of eosinophil cationic protein (ECP), the bindings of soluble intercellular adhesion molecule-1 (sICAM-1) and fibronectin (FN), and the expressions of CD11b (alpha chain of Mac-1) and CD49d (alpha chain of VLA-4) on eosinophils were evaluated in peripheral blood (n=4) and BALF (n=3) before and after the administration of Y-24180. The infiltration of eosinophils into the bronchial wall was also examined by taking biopsies. Eosinophil count, sICAM-1 and FN binding to eosinophils in BALF significantly decreased after the administration of Y-24180 (p<0.05). The level of CD11b expression also decreased remarkably after the administration (n=2). In peripheral blood, eosinophil count and ECP level did not change. The binding of sICAM-1 and FN, and expression of CD11b on eosinophils in peripheral blood showed a tendency to decrease after the administration. The level of CD49d expression on eosinophils changed neither in BALF nor in blood. Eosinophil infiltration into the bronchial wall markedly decreased in one out of 3 cases after the administration. These results suggest that Y-24180 inhibits the activation of eosinophils by antagonizing the actions of PAF in atopic asthmatics.

Inhibition of activation of human peripheral blood eosinophils by Y-24180, an antagonist to platelet-activating factor receptor

We examined the effects of Y-24180, a potent and long-acting antagonist to platelet-activating factor (PAF) receptor, on the PAF- or leukotriene B4 (LTB4)-induced activation of eosinophils using human peripheral blood in vitro. As activation markers, CD11b expression level and soluble intercellular adhesion molecule-1 (sICAM-1)-binding activity were analyzed by flow cytometry. Y-24180 significantly inhibited PAF-induced increase in the ratio of strongly positive cells for CD11b expression and sICAM-1 binding at 0.01 microM or more. WEB 2086, another PAF receptor antagonist, also inhibited the increase significantly at 1 microM or more. LTB4-induced increases in the ratio of strongly positive cells for CD11b expression and sICAM-1 binding were inhibited by Y-24180 at 1 microM, but not WEB 2086 up to 10 microM. These results indicate that Y-24180 inhibits the PAF- or LTB4-induced activation of eosinophils in human peripheral blood more potently than WEB 2086.

Effects of Y-24180, a receptor antagonist to platelet-activating factor, on allergic cutaneous eosinophilia in mice

We examined the effects of Y-24180, a potent and long-acting antagonist to platelet-activating factor (PAF), on allergic cutaneous eosinophilia and cytokine production in the skin of mice. Mice sensitized actively with ovalbumin (OA) were challenged by an intradermal injection of OA solution. The number of inflammatory cells, including eosinophils and eosinophil peroxidase (EPO) activity reflecting eosinophil infiltration into the tissue increased in OA-challenged skin 12 hr after the challenge. The levels of interleukin-4 (IL-4) and IL-5 also increased significantly in the challenged skin 12 hr and 3-24 hr, respectively, but that of interferon-gamma (IFN-gamma) did not change. Then, we evaluated the effects of Y-24180, ketotifen, suplatast and prednisolone on the increase in EPO activity, IL-4 and IL-5. These drugs were orally administered once a day for 5 days beginning 4 days before the challenge. Y-24180 (10 mg/kg) and prednisolone (5 mg/kg) significantly suppressed these parameters. Suplatast did not affect EPO activity, but significantly decreased the levels of IL-4 and IL-5. Ketotifen had no effect on them. These results indicate that the inhibition of IL-4, IL-5 and PAF are required to suppress the cutaneous eosinophilia and Y-24180 contributes to the treatment of allergic cutaneous eosinophilia.

Inhibition of leukotriene B4-induced increase in intracellular calcium ion level of human peripheral blood polymorphonuclear leukocytes by Y-24180, an antagonist of platelet-activating factor receptor

Y-24180 ((+/-)-4-(2-chlorophenyl)-2-[2-(4-isobutylphenyl)ethyl]-6,9-dim eth yl-6H-thieno[3,2-f] [1,2,4]triazolo[4,3-a] [1,4] diazepine), an antagonist of platelet-activating factor (PAF) receptor, has already been reported to inhibit leukotriene B4 (LTB4)-induced activation of polymorphonuclear leukocytes. In this article, to clarify the mechanism of inhibition of LTB4-induced activation by Y-24180, we examined the effect of Y-24180 on LTB4-induced increase in intracellular calcium ion ([Ca2+]i) level of human polymorphonuclear leukocytes at a single cell level using a laser scanning confocal microscope. Preincubation with Y-24180 significantly inhibited the increase in [Ca2+]i level at 0.3-3 microM, while WEB 2086 (4-[3-[4-(2-chlorophenyl)-9-methyl-6H-thieno[3,2-f][1,2,4]triazolo[4,3-a ][1,4]diazepin-2-yl]propionyl]morpholine), another PAF receptor antagonist, did not show the inhibitory effect at 3-30 microM, indicating that the difference in potency between these compounds was more than 100-fold. The LTB4-induced increase in [Ca2+]i level was suppressed by microinjection of anti-PAF antibody, but not control antibody, into leukocytes. Microinjection of Y-24180 at 0.003-0.03 microM or WEB 2086 at 0.03-0.3 microM into leukocytes also inhibited the LTB4-induced increase. The microinjection of WEB 2086 at the 10-fold greater concentrations than those of Y-24180 inhibited to the almost equal level to that obtained by Y-24180. In addition, microinjection of PAF into leukocytes induced the increase in [Ca2+]i level. Preincubation with Y-24180 at 1 or 3 microM significantly attenuated the PAF microinjection-induced increase, but WEB 2086 showed little effect at 3-30 microM. These results indicate that Y-24180 inhibits LTB4-induced activation of leukocytes by suppressing the increase in [Ca2+]i level and the inhibitory effect is mediated by antagonistic action against intracellular PAF-induced up-regulation of [Ca2+]i level. The difference in inhibitory activity for the increase in [Ca2+]i level between Y-24180 and WEB 2086 when these are added in the culture medium may depend upon their aptitude for the transmembrane influx.

A pharmacophore for high affinity PAF antagonists. II. Hydrophobicity study using the molecular lipophilicity potential

Platelet-activating factor (PAF) is a powerful phospholipid-derived autacoid involved in many physiopathological mechanisms. Many PAF antagonists have been synthesized and evaluated as therapeutic candidates. In a previous report, we have described an electronic pharmacophore of PAF antagonists using the molecular electrostatic potential. In the present study, a molecular lipophilicity potential is used to compare the hydrophobic properties of 49 "heterocyclic sp2 nitrogen' highly potent PAF antagonists, belonging to six structurally different series (nine hetrazepines, five pyrrolo[1,2-c]thiazoles, 14 carboxamides, nine dihydropyridines, nine pyridinyl-thiazolidines and three imidazo[4,5-c]pyridines). Their common features consist of three hydrophilic (HYD2, HY14(3)B and HYD3) and two lipophilic zones (LIP3 and LIP4), defining the lipophilic pharmacophore of the antagonists. This pharmacophore is also characterized by several zone-to-zone distances: HYD3-HYD2 = 1.3 +/- 1.0 A, HY3B-HYD2 = 7.8 +/- 1.1, HYD3-HY3B = 5.1 +/- 1.1 A, LIP4-LIP3 = 5.4 +/- 1.1 A, LIP3-HYD2 = 11.3 +/- 1.6 A, LIP3-HY3B = 5.9 +/- 1.0 A, LIP3-HYD3 = 4.3 +/- 0.9 A, LIP4-HYD2 = 14.7 +/- 1.6 A, LIP4-HY3B = 8.1 +/- 1.2 A and LIP4-HYD3 = 3.9 +/- 1.1 A. These results represent a new step in the determination of a global pharmacophore for PAF antagonists.

Effect of Y-24180, a platelet-activating factor-receptor antagonist, on the antigen-induced airway microvascular leakage in guinea pigs

The inhibitory effect of Y-24180 ((+/-)-4-(2-chlorophenyl)-2-[2-(4-isobutylphenyl)ethyl]-6, 9-dimethyl-6H-thieno[3,2-f][1,2,4]triazolo[4,3-a][1,4]diazepine) on platelet-activating factor (PAF)- or antigen-induced airway microvascular leakage was studied in guinea pigs by oral administration. The tissue content of extravasated Evans blue dye was used as an index of plasma exudation in the trachea, main bronchi, central intrapulmonary airways and peripheral intrapulmonary airways. In all of these tissues, Y-24180 potently inhibited the leakage induced by PAF. The ED50 value of Y-24180 determined in each of the tissues was approximately 0.02 mg/kg, demonstrating that the inhibitory potency of Y-24180 is 4-6 times that of WEB 2086, another PAF antagonist. Even at a dose of 10 mg/kg, however, Y-24180 showed no inhibitory effect on the leakage induced by leukotriene (LT) D4, histamine or bradykinin. In the antigen-induced model of guinea pigs sensitized with aerosolized ovalbumin, Y-24180 (0.1-10 mg/kg) and WEB 2086 (1-100 mg/kg) potently inhibited the microvascular leakage in all of the examined airway tissues except for the trachea. At 1-100 mg/kg, however, both ONO-1078, an LT-receptor antagonist, and OKY-046, a thromboxane A2 synthetase inhibitor, prevented partially but not significantly the antigen-induced leakage. These results provide evidence that endogenous PAF partially mediates the antigen-induced airway microvascular leakage in guinea pigs.

A pharmacophore for high affinity PAF antagonists. I. Electronic model using molecular electrostatic potential

PAF is a powerful phospholipid-derived autacoid involved in many physio-pathological mechanisms. Many PAF antagonists have been synthesized and assayed for therapeutic purposes. In this study, molecular electrostatic potential is used to compare the electronic properties of 48 'heterocyclic sp2 nitrogen' highly potent PAF antagonists, belonging to six series (nine hetrazepines, five pyrrolo[1,2-c]thiazoles, 14 carboxamides, nine dihydropyridines, nine pyridinylthiazolidines and two imidazo[4,5-c]pyridines). Their common features consist of three main electronegative zones (A, B1 and B2) describing the electronic pharmacophore of these ligands. The high affinity of these PAF antagonists seems to be related to this electronegative system A-B(x), which is characterized by three distances A-B1 (9.3 +/- 1.0 A), A-B2 (13.4 +/- 0.7 A) and B1-B2 (4.9 +/- 0.9 A). Moreover, B1 and B2 may surround a common anchorage point in the binding site of the receptor.

Inhibition of Mac-1-dependent leukocyte adhesion by Y-24180, an anti-inflammatory agent, in mice

Y-24180 (4-(2-chlorophenyl)-2-[2-(4-isobutylphenyl)ethyl]-6,9-dimethyl-6H-thieno[3,2-f][1,2,4]triazolo[4,3-a][1,4]-diazepine), an antagonist of platelet-activating factor (PAF), has been already clarified to suppress the expression of an adhesion molecule, Mac-1, on human neutrophils in the previous in vitro study. In the present paper, we examined the effect of Y-24180 on in vivo Mac-1 expression on mouse neutrophils using a lipopolysaccharide-induced leukocyte reduction model in which Mac-1-dependent infiltration of neutrophils was involved. Prophylactic oral administration of Y-24180 inhibited the induction of Mac-1-strongly positive neutrophils by intraperitoneal injection of lipopolysaccharide and prevented the reduction of leukocyte number. In contrast, WEB 2086 (3-[4-(2-chlorophenyl)-9-methyl-6H-thieno[3,2-f][1,2,4]triazolo[4,3-a][1,4]diazepine-2-yl]-1-(4-morpholinyl)-1-propanone), another PAF antagonist, showed little effect. PAF injection failed to induce the Mac-1-strongly positive neutrophils in peripheral blood and the reduction of circulating leukocytes, indicating that PAF was not concerned with the lipopolysaccharide-induced up-regulation of Mac-1 expression and leukocyte reduction. Y-24180 inhibited the leukocyte infiltration also in the thioglycollate medium-induced peritonitis, which was mediated by Mac-1-dependent leukocyte adhesion. These results indicate that Y-24180 inhibits the leukocyte infiltration into the inflamed sites by suppressing Mac-1 expression on leukocytes in vivo and can contribute to the improvement of inflammatory diseases in which the Mac-1-dependent leukocyte adhesion is involved.

The neuroprotective properties of the Ginkgo biloba leaf: a review of the possible relationship to platelet-activating factor (PAF)

Ginkgo biloba (Ginkgoaceae) is an ancient Chinese tree which has been cultivated and held sacred for its health-promoting properties. There is substantial experimental evidence to support the view that Ginkgo biloba extracts have neuroprotective properties under conditions such as hypoxia/ischemia, seizure activity and peripheral nerve damage. Research on the biochemical effects of Ginkgo biloba extracts is still at a very early stage. One of the components of Ginkgo biloba, ginkgolide B, is a potent platelet-activating factor (PAF) antagonist. Although the terpene fraction of Ginkgo biloba, which contains the ginkgolides, may contribute to the neuroprotective properties of the Ginkgo biloba leaf, it is also likely that the flavonoid fraction, containing free radical scavengers, is important in this respect. Taken together, the evidence suggests that Ginkgo biloba extracts are worthy of further investigation as potential neuroprotectant agents.

Role of platelet-activating factor and thromboxane A2 in radical production during ischemia and reperfusion of the rat brain

Oxygen radicals produced by activated neutrophils have been involved in brain injury during ischemia-reperfusion. Platelet-activating factor (PAF) is a candidate as one of the mediators of neutrophil activation during cerebral ischemia-reperfusion. Recent evidence indicates that PAF-induced neutrophil activation is mediated by thromboxane A2 (TXA2). To study the role of PAF and TXA2 in radical production during cerebral ischemia-reperfusion, we evaluated the effects of a PAF antagonist, Y-24180, and a TXA2 antagonist, S-1452, on radical formation in rats with 1 h middle cerebral artery (MCA) occlusion. In the present study, we employed a new electron spin resonance (ESR) method coupled with brain microdialysis. The method uses the endogenous ascorbyl radical (AR) concentration as a marker of oxygen radicals and requires no spin-trapping agents. In the vehicle controls, extracellular AR from the ischemic brain cortex decreased during MCA occlusion. Following reperfusion, AR significantly increased at 30 mm and 1 h, returned to near the basal levels at 2 h, and increased again at 24 h after reperfusion. In the rats treated with S-1452 or Y-24180, AR decreased during MCA occlusion to the same extent as in the vehicle control. However, pretreatment with Y-24180 or S-1452 significantly attenuated the increase in extracellular AR after reperfusion, while it exerted no effect on the changes in extracellular ascorbate or tissue pO2 throughout the experimental period. In conclusion, PAF and TXA2 might contribute to cerebral ischemia-reperfusion injury by increasing the generation of oxygen radicals.

The specific binding of the platelet-activating factor (PAF) receptor antagonist WEB 2086 and the benzodiazepine flunitrazepam to rat hepatocytes

Thieno-triazolodiazepines WEB 2086 and BN 50739 have been described as the potent PAF receptor antagonists. Binding of radiolabeled [3H]WEB 2086 has been widely employed to characterize PAF receptors in different cells. In a search for a PAF receptor in isolated rat hepatocytes, we discovered that the binding of [3H]WEB to rat hepatocytes was highly specific but had a relatively low affinity with a Kd of 113 nM and Bmax of 0.65 pmol/10(6) cells in freshly isolated cell suspension and Kd of 1.65 muM and Bmax of 2.0 pmol/plate in cultured hepatocytes. No consistent specific binding of [3H]PAF itself was found in the same cell preparations. The binding of [3H]flunitrazepam in the presence of the peripheral type of benzodiazepine receptor antagonist Ro 5-4864 was saturated and exhibited a K(i) of 3.8 nM and Bmax of 3.5 pmol/plate. The central type of benzodiazepine receptor antagonist clonazepam was competed for the [3H]flunitrazepam binding, however with a much lower affinity. Various antagonists inhibited the binding of [3H]WEB 2086 with a rank order BN 50739>>Ro 5-4864 > or = clonazepam. Interestingly, bicuculline, specific antagonist of GABA(A) recognition sites, also significantly reduced the binding of [3H]WEB 2086. The binding of [3H]flunitrazepam was inhibited with a rank potency BN 50739>>WEB 2086. Taken together, these findings suggest that the specific binding of PAF receptor antagonists WEB 2086 and BN 50739 in rat hepatocytes does not involve PAF receptors and occurs via peripheral benzodiazepine and, possibly GABA(A) receptor sites.

Inhibition of leukocyte adhesion to endothelial cells of human umbilical vein by Y-24180, an antagonist of platelet-activating factor

Effect of Y-24180 (4-(2-chlorophenyl)-2-[2-(4-isobutylphenyl)ethyl]-6,9-dimethyl-6H- thieno [3,2-f][1, 2, 4]triazolo[4,3-a][1,4]diazepine), an antagonist of platelet-activating factor (PAF), on adhesion of leukocytes to endothelial cells of human umbilical vein was examined. Y-24180 inhibited the adhesion of guinea-pig peritoneal exudate cells activated by leukotriene B4 to endothelial cells in a concentration-dependent manner. WEB2086, another PAF antagonist, did not show the inhibitory effect. Inhibition by Y-24180 was not influenced by addition of PAF. Y-24180 had no effect on the expression of intercellular adhesion molecule-1 or endothelial-leukocyte adhesion molecule-1 on endothelial cells activated with interleukin-1 beta, while it suppressed the expression of CD18 antigen on human peripheral blood polymorphonuclear cells activated with leukotrien B4, inhibiting their adhesion to endothelial cells activated with interleukin-1 beta. In oxazolone-induced ear edema in mice, which involves delayed-type hypersensitivity with infiltration by inflammatory cells, Y-24180 dose-dependently inhibited the increase in ear weights, but WEB2086 did not. These results indicate that Y-24180 inhibits not only the actions of PAF but also the adhesion of leukocytes by suppressing the expression of CD18 antigen, inducing the inhibition of infiltration by inflammatory cells.

Endotoxin transduces Ca2+ signaling via platelet-activating factor receptor

Lipopolysaccharide (LPS) is a pathogenic substance causing severe multiple organ failures and high mortality. Although several LPS binding proteins have been identified, the molecular mechanism underlying the LPS signaling pathway still remains obscure. We have found that the LPS-induced Ca2+ increase in platelets and platelet aggregation is blocked by selective platelet-activating factor (PAF) receptor antagonists, thus suggesting a cross-talk between LPS and the PAF receptor. Next, we confirmed this hypothesis using the cloned PAF receptors [(1991) Nature 349, 342-346; (1991) J. Biol. Chem. 266, 20400-20405] expressed in Xenopus oocytes and Chinese hamster ovary (CHO) cells. In both systems, cells responded to LPS only when PAF receptors were expressed, and specific PAF binding was successfully displaced and reversibly dissociated by LPS. PAF receptor activation by LPS may represent a novel important pathway in the pathogenesis of circulatory collapse and systemic thrombosis caused by endotoxin.

Suppression of the Arthus reaction by Y-24180, a potent and specific antagonist of platelet-activating factor

A novel and potent antagonist of platelet-activating factor (PAF), Y-24180 (4-(2-chlorophenyl)-2-[2-(4-isobutylphenyl)ethyl]-6,9-dimethyl-6H- thieno[3,2-f][1,2,4]triazolo[4,3-a][1,4] diazepine) was investigated for the effects on the skin reactions induced by chemical mediators and the Arthus reactions. In the rat dorsal skin, Y-24180 (0.1-10 mg/kg, p.o.) inhibited increase in vascular permeability by the intradermal PAF injection in a dose dependent manner and the inhibitory activity was 60 times more potent than that of WEB 2086. While even at doses as large as 10 mg/kg, p.o., it had no effect on vascular permeability in the rat skin induced by histamine, serotonin, bradykinin and leukotriene D4. On a reversed passive Arthus reaction in rat dorsal skin, Y-24180 (0.1-1 mg/kg, p.o.) markedly inhibited vascular permeability in a dose dependent manner and the inhibitory activity was 15 times more potent than that of WEB 2086. Y-24180 also inhibited the Arthus dermal reaction in rabbits (0.03-0.3 mg/kg, p.o.) and guinea pigs (0.1-1 mg/kg, p.o.). In addition, Y-24180 (0.1-10 mg/kg, p.o.) significantly reduced the exudate volume and the number of infiltrated inflammatory cells in the reversed passive Arthus pleural reaction in rats. Furthermore, in rat passive Arthus pancreatitis, Y-24180 (0.3-10 mg/kg, p.o.) significantly inhibited the dye extravasation from the pancreas. These results provide strong evidence that endogenous PAF plays an important role as a mediator in the type III allergic inflammation.

Activation of K+ current in macrophages by platelet activating factor

Puff application of platelet activating factor (10(-8) M) onto peritoneal macrophages from thioglycollate-stimulated mice induced an outward current at a holding potential of -63 mV. The current was suppressed by an antagonist Y-24180 but not by CV-3988. Charybdotoxin (10(-6) M) suppressed the current. Reversal potentials were dependent on external K+ concentrations. The current was not suppressed in Ca(2+)-free EGTA-containing solution but was completely abolished in BAPTA-AM containing solution. These results suggest that platelet activating factor activates a Ca(2+)-dependent K+ channel.