Acetyl glyceryl ether phosphorylcholine (platelet-activating factor) mediates heightened metabolic activity in macrophages. Studies on PGE, TXB2 and O2- production, spreading, and the influence of calmodulin-inhibitor W-7

Platelet-activating factor and cellular immune responses

Platelet-activating factor is a phospholipid mediator of the allergic reaction that is a potent stimulator of acute inflammatory processes. It is produced by various inflammatory cells upon activation by immune or nonimmune stimuli. As Pierre Braquet and Marek Rola-Pleszcynski review here, evidence is now accumulating that platelet-activating factor is a component in the regulation of cellular immune responses.

Leishmanicidal activity of edelfosine, miltefosine and ilmofosine

The anti-proliferative action of three alkyl-lysophospholipid derivatives, edelfosine (ET-OCH), miltefosine (Hexadecylphosphocholine), and ilmofosine (BM 14.440) has been studied on the promastigotes and amastigotes of Leishmania donovani. The effect of the three drugs has previously been studied, but the action mode was not clearly elucidated. In this study the effect on the intracellular amastigote forms was evaluated by two different methods: the traditional method, counting the amastigotes within the macrophages stained with Giemsa; and by a new method, staining the nuclear macrophages and amastigotes with ethidium bromide and counting the different population by flow cytometry. This new method, based on the flow cytometry, shows an advantage for evaluating the anti-proliferative effects in intracellular parasites. The ED50 were calculated for the drug activity after 72 hr, and for the three alkyl-lysophospholipid derivatives it were in the range of 26.73-33.31 microM against promastigotes and in the range of 16.46-23.16 against amastigotes. Also, studying the effect against macrophages J774A1, the ED50 were in the range of 24.28-26.38 microM. The effect of the alkyl-lysophospholipids in the macromolecular biosynthesis of the Leishmania donovani, was studied comparing the incorporation of labelled analogues ([3H] thymidine, [3H] uridine and [3H] leucine), respectively, in the DNA, RNA, and proteins of the flagellates treated. Miltefosine was the most active of the alkyl-lysophospholipids, especially in the inhibition of the RNA synthesis. The three compounds studied show high in vitro activity against L. donovani promastigotes and amastigotes.

Dual signaling and effector pathways mediate human eosinophil activation by platelet-activating factor

Platelet-activating factor (PAF) induces various cellular functions in eosinophils including chemotaxis, adhesion, superoxide anion (O2-) production, and degranulation. While PAF shares many biological effects with other chemotactic factors such as N-formyl-methionyl-leucyl-phenylalanine, complement fragments, and lipid mediators, PAF is unique in that its action is relatively resistant to pertussis toxin (PTX), and in activating eosinophils more strongly than neutrophils. In this review we consider how PAF might activate human eosinophils in preference to neutrophils, and discuss possible mechanisms of PAF-induced activation of human eosinophils via two distinct signaling and effector pathways. Recently we analyzed O2- production by eosinophils using a sensitive, real-time chemiluminescence method. Our results showed that in human eosinophils PAF activates two distinct signaling and effector pathways coupled to the PAF receptor: one linked to PTX-sensitive G protein(s) and another to PTX-resistant G protein(s), phosphatidylinositol 3-kinase, and cellular adhesion. This activation of two different G proteins by the eosinophil PAF receptor may explain the strong and diverse biological responses of human eosinophils to PAF.

Platelet-activating factor plays a role in the mechanism of major histocompatibility complex in T lymphocytes

In recent studies, using a swine model of single lung transplantation, we demonstrated that IRI alone increased MHC II expression in the host's peripheral T lymphocytes. The inhibition of increased MHC II expression with TCV-309, a specific platelet-activating factor (PAF) antagonist suggested that PAF might play a role in the mechanism of increased MHC II expression. The purpose of the current study was two fold: 1) to investigate the mechanism of PAF-induced increased expression of MHC II in T lymphocytes, 2) to determine whether a specific PAF-antagonist, TCV-309, is capable of inhibiting the increased expression in an in vitro system. This study was subdivided, using four in vitro conditions: 1) purified resting T cells, 2) purified proliferating T cells, 3) PBL treated with PAF, and 4) PBL preincubated with TCV-309 and treated with PAF. The level of MHC II on T cells were measured by two color flow cytometry analysis (swine anti-CD3, MHC II-DR-(beta)antibodies). Both MHC II intensity and the number of CD3+MHC+ T cells did not change in resting purified T cells once treated with PAF, Furthermore, MHC II intensity did not change in purified proliferating T cells treated with PAF. The number of CD3+MHC+ T cells, however, increased significantly (p<0.05) from day 1 to day 4 as compared with pre-treatment value (day 0) for purified proliferating T cells. Treatment of PBL with PAF (10(-7)M) resulted in a significant (p<0.05) increase in MHC II expression from day 2 to day 4 post-treatment. The number of CD3+MHC+ T cells in PBL, however, did not change significantly upon treatment with PAF. The results of this study indicated that PAF did not have a direct effect on increased MHC II expression in resting or proliferating purified T lymphocytes. However, the mechanism of PAF-induced increased expression of MHC II in T cells may be via an indirect pathway involving accessory cells. TCV-309, a specific PAF receptor antagonist, is capable of inhibiting this PAF-induced increased expression of MHC II in T cells.

A platelet-activating factor antagonist (WEB 2170) preserves endothelium-dependent vasodilatation and prevents development of a neo-intima induced by a periarterial collar in rabbit carotid arteries

Platelet-activating factor (PAF) may be involved in adhesion of leucocytes and migration of cells during vascular remodelling for it is expressed in leucocytes after cytokine priming and is required for cell adhesion. We studied the effects of WEB 2170, a potent PAF antagonist, on the development of an atheroma-like neo-intima induced by a peri-arterial collar in rabbits. Either WEB 2170 (3 mg/kg/day) or vehicle was given by subcutaneous injection once a day for 4 or 9 days, and on day 3 peri-arterial collars were applied to both carotid arteries in all animals. Two or 7 days after implanting the collars vasodilator responses to the endothelium-dependent vasodilator, acetylcholine and the endothelium-independent vasodilator, sodium nitroprusside were studied in isolated artery rings from both groups of rabbits. Neo-intima formation after 7 days (day 10 of treatment) was measured by light microscopy as the ratio of cross-sectional areas of intima and media, and expression of inducible nitric oxide synthase (iNOS) was studied by immunohistochemistry. PAF-induced platelet aggregation ex vivo was inhibited specifically in WEB 2170-treated rabbits. At day 5, acetylcholine-induced vasorelaxation in collared artery rings was markedly impaired as compared to control sections from both vehicle- and WEB 2170-treated rabbits. At day 10, acetylcholine-induced vasorelaxation in collared artery rings from vehicle rabbits was markedly less than in controls, but in WEB 2170-treated rabbits, the acetylcholine response in collared arteries was similar to control sections. Intimal thickening was much reduced in WEB 2170-treated rabbits, ratios of intima/media areas being vehicle: 0.21 +/- 0.02 (n = 5) and WEB 2170: 0.07 +/- 0.01 (n = 7; p < 0.01). Immunofluorescence showed expression of iNOS only in the neo-intima of vehicle-treated, collared arteries, but not in the residual neo-intima of WEB 2170-treated, collared arteries. These results suggest that WEB 2170 is effective in preserving endothelial function, prevents the development of neo-intima and blocks iNOS expression in the neo-intima in this model.

Human monocytes maintained in culture acquire functional responsiveness to platelet-activating factor that is independent of increases in protein tyrosine phosphorylation

1. Acute (day 0) stimulation with platelet-activating factor (PAF) did not elicit superoxide anion (O2-) generation from adherent monocytes. However, by day 2 of culture, PAF induced an increase in O2- generation that was inhibited by pretreatment with the PAF receptor antagonist WEB 2086. 2. The lack of effect of PAF on O2- generation was not due to the absence of receptors, as PAF stimulated an increase in tyrosine phosphorylation and intracellular calcium ([Ca2+]i) on both days 0 and 2 of culture. 3. Pretreatment with the protein tyrosine kinase inhibitor methyl 2,5-dihydroxycinnamate inhibited PAF-induced tyrosine phosphorylation; however, this inhibitor failed to inhibit PAF-induced O2- generation. In contrast, pretreatment with the protein kinase C inhibitor staurosporine had no effect on PAF-induced tyrosine phosphorylation, but did inhibit PAF-induced O2- generation. 4. These results indicate that monocytes maintained in culture acquire a functional response to PAF through a mechanism that appears to be independent of PAF receptor expression, coupling to increases in [Ca2+]i or tyrosine phosphorylation.

Platelet-activating factor and the production of human interferon-gamma

Platelet-activating factor (PAF) was tested for its ability to alter yields of human interferon (IFN) produced from peripheral blood mononuclear cells (PBMC). Using different concentrations of phytohemagglutinin (PHA) we could not demonstrate a consistent effect of PAF at any concentration tested on the yield of IFN-gamma. Similarly, although PAF was associated with a slight enhancement of IFN-gamma yields when PBMC were induced by interleukin-2 (IL-2), the results were not statistically significant. No effect was observed on the accumulation of human IFN-gamma mRNA induced by PHA. Furthermore, PAF did not enhance yields of IFN-gamma when the cells were induced by poly I:poly C. We conclude that although PAF may have a role in sepsis, it is not likely that this is in any way related to its ability to significantly alter the yield of interferons.

Characteristics of vasodilatation induced by acetylcholine and platelet-activating factor in the rat mesenteric arterial bed

We examined the nature of the endothelium-dependent vasodilator effects of acetylcholine and platelet-activating factor (PAF) on the perfused mesenteric arterial bed of the rat. Acetylcholine-induced concentration-dependent vasodilatation of the mesentery was not affected by pretreatment with 10(-4) M NG-monomethyl-L-arginine (L-NMMA), indomethacin, ouabain, or glibenclamide, whereas pretreatment with 10(-5) M oxyhemoglobin, 10(-5) M methylene blue, or 10 mM tetraethylammonium shifted the concentration-response curves to the right. PAF-induced concentration-dependent vasodilatation of the mesentery was inhibited by pretreatment with L-NMMA, oxyhemoglobin, or methylene blue, and slightly but significantly inhibited by tetraethylammonium, whereas indomethacin, glibenclamide, and ouabain had no inhibitory effects. PAF-induced vasodilatation of the mesentery was more sensitive to nitric oxide-cyclic GMP pathway inhibitors (a combined application of L-NMMA, oxyhemoglobin, and methylene blue) than was the vasodilatation induced by acetylcholine. Perfusion of the mesentery preparations with acetylcholine or PAF increased the levels of cyclic GMP in the effluent. These effects were completely inhibited by L-NMMA or oxyhemoglobin. These results suggest that the endothelium-dependent vasodilator effects of PAF are primarily mediated by endothelium-derived nitric oxide (NO) and those of acetylcholine are mediated by both NO and endothelium-derived hyperpolarizing factor (EDHF).

Superoxide production by macrophages stimulated in vivo with synthetic ether lipids

The anticancer activity of synthetic ether lipids may depend in part upon their ability to activate cells of the monocyte/macrophage lineage. In the present study, we have sought to determine whether 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OMe) and related ether lipids enhance superoxide production by mouse peritoneal macrophages. Ether lipids were administered intraperitoneally to C57BL/6 mice 4 d after injection with thioglycollate broth. Elicited peritoneal macrophages were harvested and purified one day later, and superoxide production was detected by measuring the superoxide dismutase inhibitable reduction of cytochrome c. Low levels of superoxide were secreted by macrophages in the absence of phorbol 12-myristate 13-acetate (PMA). When PMA was added in vitro to macrophages from ET-18-OMe-treated mice, these cells secreted 194.2 nmol superoxide/mg protein in comparison to 53.5 nmol superoxide/mg protein for PMA-treated control cells. The in vitro treatment of the macrophages with ET-18-OMe was not effective in stimulating superoxide secretion. Macrophages harvested from mice treated with a series of ether lipids (with and without phosphorus) were examined, and superoxide secretion was found to vary with structure. AM-18-OEt and CP-7 were the most effective compounds, secreting 8.6 and 11.9 times more superoxide, respectively, than PMA-stimulated control cells. Moreover, direct cytotoxicity of the compounds for HL60 human promyelocytic leukemic cells did not necessarily correlate with the ability of each drug to increase macrophage superoxide production.

A role for oxygen radicals in rat monocytic leukemia cell differentiation under stimulation with platelet-activating factor

Combined stimulation, by superoxide ions generated by the xanthine-xanthine oxidase reaction, and platelet-activating factor (PAF), induced cell differentiation of rat monocytic leukemia cells (c-WRT-LR) to macrophage-like mature cells. Monitoring of cytochrome c reduction revealed that PAF stimulation induced the release of superoxide ions from c-WRT-LR. To further investigate the effect of superoxide ions in the autocrine or paracrine mechanism in cell differentiation, molecular species of the oxygen radicals under PAF stimulation were examined using the EPR spin trap, 5,5'-dimethyl-1-pyrroline N-oxide (DMPO). PAF and/or phorbol myristate acetate caused the formation of EPR spectra, a combination of DMPO/.OOH and DMPO/.OH. Since both spectra were diminished in the presence of superoxide dismutase, it was concluded that DMPO/.OH was derived from superoxide ions. Mannitol and catalase suppressed cell differentiation induced by combined stimulation with PAF and oxygen radicals generated by the xanthine-xanthine oxidase reaction. Taken together, these results suggest that hydroxyl radicals generated by Fenton reaction from H2O2 may be involved in the mechanism of cell differentiation in rat monocytic leukemia cells.

Cyclic-AMP level and eicosanoid release from alveolar macrophages are differentially affected by high and low dose of platelet activating factor

Antigen challenged alveolar macrophages (ac-AM) showed much higher basal prostaglandin E2 (PGE2) release (4,4-fold) and cAMP content (2,4-fold) than naive alveolar macrophages (AM). In naive AM 1 fM platelet activating factor (PAF) enhanced PGE2 release from 115 to 157 ng/5 x 10(6) cells but was inactive at 1 nM or 1 microM. In ac-AC 1 fM PAF enhanced PGE2 release from 510 to 670 ng/5 x 10(6) cells and inhibited leukotriene B4 (LTB4) release (from 6.0 to 4.8 ng/5 x 10(6) cells). At a 10(6)-fold higher concentration PAF inhibited PGE2 release (from 510 to 400 ng/5 x 10(6) cells) and stimulated LTB4 release (from 6.0 to 8.2 ng/5 x 10(6) cells). PAF-induced increase or decrease in PGE2 release was paralleled by changes in cellular cAMP (+35 and -17%, respectively). The specific PAF-antagonist BN 52021 completely reversed all PAF-induced effects while indomethacin inhibited only PAF-induced increase in PGE2 release and cAMP leaving LTB4 release unaffected. Similarly, the lipoxygenase inhibitor AA-861 inhibited PAF-induced rise in LTB4 release leaving the enhancement in PGE2 release and cAMP content unaffected. Present data show that PAF dose-dependently affects eicosanoid production and cAMP level in alveolar macrophages.

Platelet-activating factor modulates endotoxin-induced macrophage procoagulant activity by a protein kinase C-dependent mechanism

Macrophage procoagulant activity is an important mediator of extravascular fibrin deposition at sites of infection and appears to contribute to the pathogenesis of several infectious disease processes. Previous studies have shown that the inflammatory mediator platelet-activating factor was able to prime macrophages for induction of procoagulant activity by bacterial lipopolysaccharide. The present studies were designed to examine the mechanism of this priming effect. Platelet-activating factor (100 nM) primed macrophages for procoagulant activity generation in response to endotoxin at concentrations as low as 100 ng/ml and also following exposure to Escherichia coli, Bacteroides fragilis, and Staphylococcus aureus. The priming effect occurred following a pretreatment with platelet-activating factor for as short as 1 min, suggesting a rapid activation event. Two different doses of the calcium ionophore ionomycin were used to mimic the peak and sustained effects of platelet-activating factor on cytoplasmic calcium levels (1 microM and 100 nM, respectively). Neither dose was able to mimic the priming effect. However, extracellular calcium was necessary for induction of procoagulant activity and the priming effect. By contrast, the protein kinase C agonist phorbol myristate acetate reproduced the priming phenomenon observed for platelet-activating factor. In further support of the concept that protein kinase C activation mediated the effect of platelet-activating factor, the specific protein kinase C inhibitor staurosporine reversed the ability of platelet-activating factor to augment induction of macrophage procoagulant activity by endotoxin. These data suggest mechanisms by which inflammatory mediators within the microenvironment of infection might modulate the host response to bacterial pathogens.

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.

Biological response of guinea pig peritoneal macrophages to platelet-activating factor

We investigated the effects of platelet-activating factor (PAF) on guinea pig peritoneal macrophages. Specific and high-affinity binding sites for PAF were detected on guinea pig peritoneal macrophages. Scatchard analysis of PAF binding revealed high affinity binding sites (7.9 x 10(4)/cell) with a dissociation constant of 2.3 x 10(-10) M. When treated with 10(-9)-10(-5) M PAF, guinea pig peritoneal macrophages released hydrogen peroxide into the medium in a time-dependent manner. The release reaction upon stimulation with 10(-5) M PAF reached a plateau within 30 min and the extent of release was twice as high as that when stimulated by N-formyl-L-methionyl-leucyl-L-phenylalanine (fMLP; 2 microM)-treated cells. Neither lysoPAF nor the PAF enantiomer was effective. PAF-induced H2O2 release was inhibited specifically by PAF antagonists, suggesting that PAF activated macrophages through binding to specific sites. Lysosomal enzyme (N-acetyl-beta-D-glucosaminidase) was released from guinea pig peritoneal macrophages upon treatment with 10(-5) M PAF for 60 min. Guinea pig peritoneal macrophages were treated with PAF for 8 hr and the conditioned medium was examined for cytokines. The medium exhibited cytocidal activity against mouse fibroblast L929 cells [tumor necrosis factor (TNF) activity], and this activity was comparable to that detected after treatment of cells with the bacterial lipopolysaccharide (LPS). Furthermore, the same conditioned medium also showed colony-stimulating factor (CSF) activity. Generation of these cytokines was stereospecific. Our findings suggest that PAF is a unique macrophage activator that potentiates both respiratory burst/lysosomal enzyme release (early-phase response) and monokine production/glucose consumption (late-phase response).

Intracellular Ca2+ concentration and H2O2 production in mouse peritoneal macrophages are stimulated by platelet activating factor

The intracellular calcium concentration ([Ca2+]i) in mouse peritoneal macrophages cultured on a coverglass was measured in the superfusion system using fura-2 as a fluorescent calcium probe and platelet activating factor (PAF) as a stimulant. In the presence of extracellular Ca2+, 10(-7) M PAF sharply increased [Ca2+]i from a basal level of 90 nM to 340 nM. Thereafter, the [Ca2+]i level gradually decreased in two phases, in an initial rapid phase and a subsequent slow phase of decrease. The calcium response was dependent on the PAF concentration. In the absence of extracellular Ca2+, a single sharp peak was observed, suggesting two different modes of Ca2+ movement, one from intracellular stores and the other from the extracellular medium. A simple, sensitive fluorometric assay system was developed for measuring H2O2 in the superfusate of macrophages after stimulation by use of immobilized peroxidase and 3-(p-hydroxyphenyl)propionic acid as a fluorogenic substrate. With this system, as little as 2 pmol of H2O2 could be measured. PAF (1 microM) increased H2O2 production in peritoneal macrophages in the presence of extracellular Ca2+, but H2O2 production was not observed in the absence of extracellular Ca2+.

Activation mechanisms of platelet-activating factor in U937 cells: possible involvement of protein kinase C

We have previously demonstrated that platelet-activating factor (PAF) binds specifically on cell membranes isolated from U937 cells. We now describe biological evidence showing that the effect of PAF on U937 cells is a receptor-mediated event. myo-[3H]Inositol-labeled U937 cells were used to investigate the possible role of phosphoinositide metabolism in these cells after binding of PAF. Formation of inositol phosphates (IP1, IP2, and IP3) in response to PAF was increased two- to threefold more than in vehicle control in U937 cells. The effect of PAF on endogenous protein phosphorylation was also studied by using 32PO4-labeled cells. PAF stimulates the phosphorylation of a 45-kDa protein in a time-dependent and dose-related fashion. Since the phospholipase C-generated diglyceride is an important activator of protein kinase C, the phosphorylated 45-kDa protein could be the substrate of protein kinase C. In this regard, we were able to demonstrate that phorbol ester enhances the phosphorylation of the same 45-kDa protein band. In addition, sphingosine, a protein kinase C inhibitor, inhibits the phosphorylation of the same 45-kDa protein band. Down-regulation of the protein kinase C also inhibits the 45-kDa protein phosphorylation. These results suggest that protein kinase C is involved in the PAF-U937 cell interaction.

Cholesterol modulates PAF-stimulated Ca2(+)-mobilization in monocytic U937 cells

We investigated the effect of cellular cholesterol content on platelet activating factor (PAF)-stimulated Ca2+ mobilization in the human monocytic cell line U937. When cholesterol auxotroph U937 cells were depleted of cellular cholesterol by a 48-h incubation in delipidated medium, a 40% reduction in the PAF (100 nM)-stimulated increase in cytosolic Ca2+ concentration was seen. Ca2+ mobilization following stimulation with LTD4 (10 nM) or ATP (10 microM) was not affected. Addition of LDL (100 micrograms/ml, 24 h) to the delipidated medium completely recovered cellular cholesterol content and PAF-induced Ca2+ mobilization. These two LDL effects had very similar time- and dose-dependences. Partial recoveries of PAF-induced Ca2+ mobilization, seen after addition of pure cholesterol dissolved in ethanol (30 micrograms/ml, 24 h) or acetyl-LDL (100 micrograms/ml, 24 h), were associated with partial recoveries of cellular cholesterol content. Our results indicate that cellular cholesterol influences PAF-stimulated events in monocytic cells.

Primed stimulation of isolated perfused rabbit lung by endotoxin and platelet activating factor induces enhanced production of thromboxane and lung injury

Bacterial sepsis often precedes the development of the adult respiratory distress syndrome (ARDS) and bacterial endotoxin (LPS) produces a syndrome similar to ARDS when infused into experimental animals. We determined in isolated, buffer-perfused rabbit lungs, free of plasma and circulating blood cells that LPS synergized with platelet activating factor (PAF) to injure the lung. In lungs perfused for 2 h with LPS-free buffer (less than 100 pg/ml), stimulation with 1, 10, or 100 nM PAF produced transient pulmonary hypertension and minimal edema. Lungs perfused for 2 h with buffer containing 100 ng/ml of Escherichia coli 0111:B4 LPS had slight elevation of pulmonary artery pressure (PAP) and did not develop edema. In contrast, lungs exposed to 100 ng/ml of LPS for 2 h had marked increases in PAP and developed significant edema when stimulated with PAF. LPS treatment increased capillary filtration coefficient, suggesting that capillary leak contributed to pulmonary edema. LPS-primed, PAF-stimulated lungs had enhanced production of thromboxane B2 (TXB) and 6-keto-prostaglandin F1 alpha (6KPF). Indomethacin completely inhibited PAF-stimulated production of TXB and 6KPF in control and LPS-primed preparations, did not inhibit the rise in PAP produced by PAF in control lungs, but blocked the exaggerated rise in PAP and edema seen in LPS-primed, PAF-stimulated lungs. The thromboxane synthetase inhibitor dazoxiben, and the thromboxane receptor antagonist, SQ 29,548, similarly inhibited LPS-primed pulmonary hypertension and edema after PAF-stimulation. These studies indicate that LPS primes the lung for enhanced injury in response to the physiologic mediator PAF by amplifying the synthesis and release of thromboxane in lung tissue.

Binding of platelet activating factor by isolated membranes from U937 cells

Platelet activating factor (PAF), 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (1-O-alkyl-2-acetyl-GPC) has been known to have biological effect on cells. The mechanisms of the effect of the potent phospholipid on cells has not been established. We have used 1-O-[3H]alkyl-2-acetyl-GPC [( 3H]PAF) to study the interaction on the isolated membranes of U937 cells. The binding process was time, protein concentration, temperature dependent and reversible. The binding of [3H]PAF to the U937 cell membranes was slightly inhibited by the addition of PAF analogue, 3-O-Hexadecyl-2-acetyl-sn-glycerol-1-phosphorylcholine. U937 cell membranes showed high affinity binding sites for PAF with equilibrium dissociation constant (Kd) of 5 x 10(-9) M. The displacement of bound [3H]PAF with 500-fold excess of nonlabeled PAF was not altered suggesting that the bound [3H]PAF was not degraded during the binding. Binding of [3H]PAF on U937 cell membranes was inhibited by PAF antagonist, 59227RP. The kinetic of the inhibition by PAF antagonist is competitive suggesting that PAF and PAF antagonist bind at the same site.

Evidence for a platelet-activating factor receptor on human lymphoblastoid B cells: activation of the phosphatidylinositol cycle and induction of calcium mobilization

In this report we demonstrate evidence which strongly suggests that a receptor for platelet-activating factor (PAF) exists on a lymphoblastoid B cell line, LA350. PAF ranging in concentration from 10(-6)-10(-9)M initiated the incorporation of 32P into phosphatidic acid (PA) and phosphatidylinositol (PI) with no change in phosphatidylethanolamine (PE) and phosphatidylcholine (PC) over baseline. Lyso-PAF, the inactive precursor, at 10(-7)M had no effect on membrane phospholipid metabolism. In addition, PAF from 10(-6)-10(-8)M when added to Fura-2 containing B cells induced a rapid and significant rise of calcium within the cell, with lyso-PAF having no effect. These data suggest that PAF binds to a receptor on B cells and induces the hydrolysis of PI and a subsequent increase of intracellular calcium.

Interaction between lipids and bovine brain calmodulin: lysophosphatidylcholine-induced conformation change

We have monitored the interaction of several lipids with the bovine brain calmodulin(CaM) and analyzed the effect of lysophosphatidylcholine(lyso-PC, 2-50 micrograms/ml) on conformation of CaM and the interaction between CaM and CaM-binding protein(CaMBP), using a fluorescence signal of 1-(dimethylamino)naphthalene-5-sulfonate-labeled CaM(DNS-CaM). Lyso-PC(egg, 20 micrograms/ml), among various natural lipids including phosphatidylserine(PS), phosphatidylinositol(PI), phosphatidylethanolamine (PE) and their lyso forms, greatly and dose-dependently enhanced the intensity of DNS fluorescence of DNS-CaM in the presence (100 microM CaCl2) and absence (1 mM EGTA) of Ca2+. Apparent dissociation constants calculated from the fluorometric titrations of binding of lyso-PC to DNS-CaM were 0.6 and 3.7 micrograms/ml in the presence and absence of Ca2+, respectively. Lyso-PC remarkably prevented both trypsin-induced quenching of the fluorescence of DNS-CaM and tryptic digestion of native CaM in the absence of Ca2+. Enhancement of DNS fluorescence of DNS-CaM by CaMBP was observed only in the presence of Ca2+ and lyso-PC could further increase the fluorescence intensity of the complex. These all results suggest that lyso-PC can modulate the interaction between CaM and CaMBP as a result of its direct effect on conformation of CaM.

Antagonism of platelet activating factor-induced chemiluminescence in guinea-pig peritoneal macrophages in differing states of activation

1. The effects of the platelet activating factor (Paf) antagonists alprazolam, BN 52021, kadsurenone, L 652,731 and SRI 63119 have been studied on Paf-induced chemiluminescence (CL) of guinea-pig, C. parvum-activated peritoneal macrophages in vitro. 2. All antagonists produced a shift to the right in the dose-response curve to Paf (0.001-10 mumol l-1). Schild plots for BN 52021, L 652,731, kadsurenone and SRI 63119 were linear, but only for BN 52021 and kadsurenone did the mean slope not differ significantly from unity. Mean pA2 values for BN 52021 and kadsurenone were 6.60 +/- 0.05 and 6.41 +/- 0.14 (mean + s.e.mean) respectively. Calculation of IC50 values for all antagonists (at 0.1 mumol l-1 Paf) gave an order of potency: L 652731 greater than kadsurenone greater than or equal to BN 52021 greater than alprazolam greater than SRI 63119. 3. When individual pA2 values for BN 52021 and kadsurenone were plotted against the maximal CL response to Paf of cell suspensions in the absence of antagonist (reflecting the degree of activation of the macrophages by the C. parvum), it was found that the affinity of both antagonists for macrophage Paf receptors remained relatively constant irrespective of the activation state of the cells. 4. We conclude that activation of guinea-pig peritoneal macrophages does not account for the increased affinity for macrophage Paf receptors previously observed for kadsurenone. Kadsurenone and BN 52021 presumably bind to a site on Paf receptors which is not affected by the activation process, while alprazolam and SRI 63119 are non-specific antagonists. The reason for the difference between the competitive nature of kadsurenone and its structural analogue L 652,731 is unclear.

Platelet-activating factor induces the production of leukotrienes by human monocytes

The aim of this study was to evaluate the role of platelet-activating factor (PAF) as a stimulator of leukotriene production by human monocytes. The production of leukotrienes was time- and concentration-dependent. Release of leukotrienes was half-maximal after 2 min and reached a maximum after 10 min. At a concentration of 10(-8) M, PAF induced the production of 0.14 +/- 0.01 ng LTB4/10(6) cells (mean +/- S.E., n = 8). At concentrations of 10(-6) M, PAF induced the production of 1.0 +/- 0.04 ng LTB4 and 0.22 +/- 0.03 ng peptidoleukotrienes (mean +/- S.E., n = 16). There was no metabolism of LTB4 as judged from stability of [3H]LTB4 added to the incubations. LTC4 was slowly metabolized by human monocytes to LTD4 and LTE4. The two specific PAF-receptor antagonists BN 52021 and WEB 2086 in concentrations of 10(-4) and 10(-6) M, respectively, inhibited the PAF (10(-6) M) stimulated LTB4 production completely. In this study, we demonstrate that nanomolar concentrations of PAF can stimulate the production of LTB4 and peptidoleukotrienes in human monocytes by a receptor-mediated mechanism.

Protective effect of BN 52021, a specific antagonist of platelet-activating factor (PAF-acether) against diet-induced cholesteryl ester deposition in rabbit aorta

Platelet-activating factor (PAF-acether), a phospholipid mediator involved in inflammatory reactions, has been reported to induce endovascular surface lesions. We investigated the possible involvement of PAF-acether in the mechanism of arterial cholesterol deposition. Rabbits fed a normal or hypercholesterolic diet were treated orally for 1 month with BN 52021 (20 mg/kg per day), a specific PAF-acether antagonist, and killed at the end of treatment. Cholesterol feeding resulted in a marked (50-fold) increase in plasma cholesterol. However, the drug had no significant effect on the diet-induced hypercholesterolemia. Free and esterified cholesterol were markedly increased (635%) in the aorta of animals receiving the atherogenic diet. This accumulation was reduced by 36% upon simultaneous administration of BN 52021 (P less than 0.02, n = 15). This decrease essentially affected the esterified cholesterol content. Conversely, BN 52021 showed no effect on the cellular cholesterol esterification, since liver acyl-CoA: cholesterol acyltransferase activity remained unchanged. This study indicates that BN 52021 is effective in reducing cholesterol accumulation in rabbit atherosclerotic aorta, without changing the plasma cholesterol levels.

Biphasic increase in intracellular calcium induced by platelet-activating factor in macrophages

In single mouse macrophages stimulated by platelet-activating factor (PAF), the intracellular calcium concentration (Cai) monitored with fura-2 at room temperature presents a biphasic increase, including a transient and a more sustained component. After pulse administration of PAF, the first phase lasts for a few seconds and reaches a peak value of 0.5-1 microM Ca2+ at high PAF concentration. The amplitude of this peak is independent of extracellular Ca2+ concentration, suggesting that the initial Ca2+ transient is due to the release of Ca2+ from intracellular stores. The second phase of the response lasts for several minutes; its maximum amplitude is reached 1-2 min after the brief initial PAF stimulation. This phase, suppressed in zero external Ca2+ and increased in 10 mM Ca2+, is probably due to influx of Ca2+ through the plasma membrane. This secondary Ca2+ increase is blocked by 10-50 microM lanthanum. At low PAF concentration, the initial Ca2+ transient is not followed by a second phase, showing that the initial rises of Ca2+ and of its activator (presumably inositol trisphosphate) are not sufficient to trigger the second phase of Ca2+ increase.

Augmentation of DNA synthesis in guinea pig bone marrow cells by platelet-activating factor (PAF)

When guinea pig bone marrow cells were incubated in the presence of 10(-7) to 10(-5) M platelet-activating factor (PAF) for 24 to 72 hours, [3H]thymidine incorporation of cells was time-dependently augmented. The enantiomer of PAF and lysoPAF, a major metabolite of PAF, did not show significant enhancement. A non-metabolizable potent PAF agonist, 1-O-octadecyl-2-O-(N,N-dimethylcarbamoyl)-sn-glycero-3-phospholine, enhanced the [3H]thymidine incorporation at 10(-10) to 10(-8) M. This augmentation of DNA synthesis in bone marrow cells was abolished by specific PAF antagonists, CV-6209 or FR-900452. When the conditioned medium of PAF-stimulated bone marrow cells was added to another culture of bone marrow cells, the augmentation of DNA synthesis was also observed. These results suggest that PAF may affect the proliferation of one or some classes of guinea pig bone marrow cells through release of soluble factor(s).

A novel bioaction of PAF: induction of microbicidal activity in guinea pig bone marrow cells

When guinea pig bone marrow cells were incubated in the presence of 10(-8) to 10(-6) M platelet activating factor (PAF) for 24 to 72 hr, microbicidal activity against Candida parapsilosis of cells was augmented. This augmentation was inhibited by PAF-specific antagonists, CV6209 or FR900452. PAF-specific binding sites with a high affinity were found on guinea pig bone marrow cells. Carrageenan or 2-chloroadenosine, reagents known to be preferentially cytotoxic to macrophages, abolished the microbicidal activity of PAF-treated bone marrow cells. Macrophages prepared from the peritoneal cavity, however, acquired no appreciable microbicidal action by treatment with PAF. These observations suggest that PAF may affect a class of guinea pig bone marrow cells through binding to receptors specific to PAF, resulting in activation and/or induction of differentiation of monocyte-macrophage lineage cells.

Effects of platelet activating factor (PAF) on human citrated whole blood

Effects of PAF on citrated whole blood (C-WB) from 38 healthy donors have been studied by impedance aggregometry and by morphologic examination of blood cells using scanning and transmission electron microscopy. In the aggregometer, the C-WB samples showed distinct differences in PAF sensitivity. C-WB specimens from high responders (= 15 donors) displayed a dose-related response to PAF stimulation but those from low responders (= 23 donors) did not indicate an impedance alteration even after the addition of high PAF doses (greater than or equal to 10(-6) mol/l). Morphologic studies revealed shape-changed platelets and primary aggregates in all C-WB samples, whereas secondary aggregates occurred only in C-WB specimens from high responders. Monocytes and neutrophil PMNs showed typical morphologic alterations which were observed in PAF-stimulated C-WB samples from all donors. Both cell types appeared polarized in shape and exhibited large vacuoles in the cytoplasm after PAF activation. In addition, monocytes came into close contact with shape-changed platelets as well as primary and secondary aggregates, whereas PMNs had no special relationship to single or aggregated platelets. In summary, our study indicates that PAF acts on different cell types in C-WB including platelets, monocytes and PMNs. The sensitivity of platelets against PAF stimulation appears to vary between different donors and in certain cases seems to be limited to the formation of primary aggregates.

Biochemical and functional responses stimulated by platelet-activating factor in murine peritoneal macrophages

Platelet-activating factor (PAF) is a potent stimulant of leukocytes, including macrophages. To analyze the mechanisms of its effects upon macrophages, we determined whether macrophages bear specific surface receptors for PAF. By competitive radioactive binding assays, we determined two classes of specific receptors to be present on purified membranes derived from murine peritoneal macrophages (one having a Kd of approximately 1 X 10(-10) M and one a Kd of approximately 2 X 10(-9) M). When the macrophages were incubated with PAF, rapid formation of several inositol phosphates including inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate were observed. PAF also elevated intracellular levels of calcium to 290 +/- 27% of basal levels which were 82.7 +/- 12 nM. Increases in calcium were observed first in submembranous areas of the macrophages. PAF also led to increases of 1,2-diacylglycerol of approximately 200 pmol/10(7) cells. A characteristic pattern of enhanced protein phosphorylation, similar to that initiated by both phorbol 12,13-myristate and lipopolysaccharide, was observed and involved enhanced phosphorylation of proteins of 28, 33, 67, and 103 kD. The half-maximal dose of PAF for initiating all the above effects was approximately 5 X 10(-9) M. PAF also initiated significant chemotaxis of the cells; the half-maximal dose for this effect was approximately 1 X 10(-11) M. Taken together, these observations suggest that murine mononuclear phagocytes bear specific membrane receptors for PAF and that addition of PAF leads to generation of break-down products of polyphosphoinositides, subsequent changes in intracellular calcium and protein phosphorylation, and chemotaxis.

Antagonism of the platelet activating factor-induced rise of the intracellular calcium ion concentration of U937 cells

1. U937 cells are a continuous line of human cells of committed monocytic origin which serve as a useful model for studying human monocytic function. The present study investigated the effect of platelet-activating factor (Paf) on intracellular free calcium ion concentration ([Ca2+]i) in U937 cells using the calcium fluorescent probe fura-2. 2. The naturally-occurring stereoisomer (R)-Paf (0.01-300 nM) and the stable, less hydrolysable racemic Paf analogue PR1501 (10 nM-3 microM) produced dose-related and rapid elevations of 100-1200 nM [Ca2+]i above a basal value of 135 +/- 9 nM (n = 22). 3. The unnatural stereoisomer (S)-Paf and the natural stereoisomer lyso-(R)-Paf had no effect on basal [Ca2+]i at 30 microM, approximately 100,000 times the concentration found to be the threshold concentration to elicit a response to (R)-Paf. 4. Leukotriene B4 (LTB4) also induced increases in [Ca2+]i in the concentration range 28.5 nM-2.85 microM but the responses were smaller and of shorter duration than those induced by Paf. 5. Five compounds, WEB 2086, Ro 19-3704, L-652,731, BN 52021, and CV 3988, inhibited suboptimal Paf (10 nM)-induced increase in [Ca2+]i with IC50s of 48 +/- 2, 118 +/- 33, 318 +/- 131, 340 +/- 205 and 2320 +/- 183 nM respectively. All five compounds have previously been reported as specific Paf receptor antagonists, at least with respect to platelets. 6. The above compounds at 10 microM had no effect upon the increased [Ca2+]i induced by either LTB4 or the calcium ionophore, ionomycin. 7. These results suggest that U937 cells respond to Paf at least with respect to elevated [Ca2+]i as measured by fura-2 and that these cells may well possess a Paf receptor as suggested by the action of specific antagonists and the stereoselectivity observed with Paf.

Effect of azelastine on the intracellular Ca2+ mobilization in guinea pig peritoneal macrophages

Azelastine, an orally effective anti-allergic agent, has been demonstrated to inhibit the release of histamine and leukotrienes. This suggests that azelastine might alter the mobilization of intracellular Ca2+. We have examined the effect of azelastine on the change in intracellular free calcium concentration ([Ca2+])i) in guinea pig peritoneal macrophages induced by platelet activating factor (PAF-acether) or N-formylmethionyl-leucyl-phenylalanine (FMLP) using a fluorescent Ca2+ indicator, fura2. PAF-acether raised [Ca2+]i from 89 +/- 4 to 243 +/- 26 nM (n = 15) within 20 s after addition of PAF-acether in the presence of 2 mM EGTA. This indicates that the stimulation of macrophages by PAF-acether induced intracellular mobilization of Ca2+, and pretreatment with azelastine reduced the PAF-acether-induced increase in [Ca2+]i in a dose-dependent manner (IC50 = 16 microM). Azelastine also inhibited the FMLP-induced increase in [Ca2+]i. Furthermore, PAF-acether and FMLP both caused the release of prostaglandin E2 from macrophages, and pretreatment with azelastine reduced the PGE2 release dose dependently (IC50 = 10 microM). These results suggest that azelastine inhibits the release of Ca2+ from intracellular storage sites induced by PAF-acether or FMLP, and that this effect possibly causes reduction in the release of PGE2 from the cells.

Platelet-activating factor-induced hydrolysis of phosphatidylinositol 4,5-bisphosphate stimulates the production of reactive oxygen intermediates in macrophages

To investigate the relationship between inositol lipid hydrolysis and reactive oxygen-intermediate (ROI) production in macrophages we have examined the effect of platelet-activating factor (PAF) on normal bone marrow-derived macrophages. Addition of PAF to macrophages prelabelled with [3H]inositol caused a marked and rapid increase in [3H]inositol trisphosphate levels. Similarly when PAF was added to [3H]-glycerol prelabelled macrophages there was a rapid increase in 1,2-diacyl[3H]glycerol levels. These events preceded any increase in the rate of PAF-stimulated ROI production by a discernible period of several seconds. Increasing concentrations of PAF led to a markedly similar increase in both ROI production and [3H]inositol lipid hydrolysis suggesting that inositol lipid hydrolysis may lead to the generation of ROI in macrophages. Further evidence that this is the case came from experiments in which pretreatment of macrophages with phorbol esters was shown to inhibit both PAF-stimulated [3H]inositol phosphate production and ROI production to a markedly similar degree. Similarly pertussis toxin inhibited both PAF-stimulated ROI production and [3H]inositol phosphate production. Phorbol esters were shown to activate ROI production in normal bone marrow-derived macrophages whereas the Ca2+ ionophore, A23187, did not. These experiments suggest that PAF stimulates a pertussis toxin-sensitive activation of inositol lipid hydrolysis leading to the formation of inositol trisphosphate and diacylglycerol. The diacylglycerol formed can then activate protein kinase C leading to the stimulation of ROI production in normal bone marrow-derived macrophages.

GTP regulation of platelet-activating factor binding to human neutrophil membranes

Radiolabeled ligand binding studies showed that specific receptors for platelet-activating factor are present in human neutrophil membranes. GTP at 10(-7) to 10(-3) M decreased the specific binding of platelet-activating factor to neutrophil membranes in a dose-dependent manner. Inhibition of platelet-activating factor binding was also induced by other guanine nucleotides but not by adenine nucleotides. Our results suggest that platelet-activating factor receptor in human neutrophil membranes may be coupled to a guanine nucleotide binding protein.

Binding and metabolism of platelet-activating factor by human neutrophils

Human polymorphonuclear neutrophils rapidly incorporated radiolabeled platelet-activating factor, 1-O-[hexadecyl-9, 10-3H2]-2-acetyl-sn-glycero-3-phosphocholine ([3H]PAF), and then metabolized it into its sn-2-fatty acyl derivative. Fractionation of radiolabel-pretreated cells over Percoll gradients revealed that virtually all of the intact [3H]PAF was located in nongranule membranes that were enriched with alkaline phosphatase and cell surface glycoproteins. While still membrane associated, the ligand was rapidly converted to its acyl derivative and then more slowly transferred to specific granules and, to a lesser extent, azurophilic granules. In contrast, neutrophils did not metabolize [3H]PAF at 4 degrees C but rather gradually accumulated it in their alkaline phosphatase-enriched membrane subfractions. These same subfractions contained receptors for the ligand, as determined by their capacity to bind [3H]PAF specifically. Binding was readily saturated, partially reversible, and fit a two receptor model; dissociation constant (Kd) values for high and low affinity sites were 0.2 and 500 nM, respectively. Receptors with similar affinities were detected in whole cells. Furthermore, the potencies of several structural analogues in inhibiting binding of [3H]PAF to membranes correlated closely with their respective potencies in stimulating degranulation responses. Finally, quantitative studies suggested all or most of the cell's receptors were membrane associated. We conclude that PAF rapidly enters cellular membranes to bind with specific receptors that trigger function. The intramembranous ligand is also deacetylated, acylated, and then transferred to granules. This metabolism may be sufficiently rapid to limit ligand-receptor binding and distort quantitative analyses of receptors.

Platelet activating factor raises intracellular calcium ion concentration in macrophages

Peritoneal cells from thioglycollate-stimulated mice were allowed to adhere to coverglasses for 2 h to give a dense monolayer of adherent cells greater than 95% of which were macrophages. After incubation with the tetra-acetoxymethyl ester of quin2, coverglasses were rinsed with Ca2+-free saline, oriented at a 45 degree angle in square cuvettes containing a magnetically driven stir bar, and analyzed for changes in quin2 fluorescence in a spectrofluorimeter. Such fluorescence, taken as an indication of intracellular calcium ion concentration ([Ca2+]i), increased as exogenous calcium ion concentration ([Ca2+]o) was raised to 1 mM. At [Ca2+]o approximately equal to 10 microM, [Ca2+]i = 72 +/- 14 nM (n = 26); at [Ca2+]o = 1 mM, [Ca2+]i = 140-220 nM, levels not increased by N, N, N', N'-tetrakis (2-pyridylmethyl) ethylenediamine, a membrane-permeant chelator of heavy metals than can quench quin2. Addition of mouse alpha + beta fibroblast interferon, lipopolysaccharide, thrombin, collagen, vasopressin, ADP, compound 48/80, or U46619 did not change [Ca2+]i. However, addition of platelet activating factor (PAF) (2-20 ng/ml) raised [Ca2+]i by 480 nM within 1 min if [Ca2+]o = 1 mM. In the presence of 5 mM EGTA, PAF raised [Ca2+]i by 25 nM. This suggests that PAF causes influx of exogenous Ca2+, as well as releasing some Ca2+ from intracellular stores. Consistent with these results, when PAF was added to 1 mM Ca2+ in the presence of 100 microM Cd2+ or Mn2+ to block Ca2+ influx, [Ca2+]i increased by only intermediate amounts; at the times of such dampened peak response, [Ca2+]i could be raised within 1 min to normal PAF-stimulated levels by chelation of the exogenous heavy metals with diethylenetriaminepentaacetic acid. Normal PAF responses were observed in the presence of indomethacin. The lowest dose of PAF observed to raise [Ca2+]i was 0.1 ng/ml. Response of [Ca2+]i to 2-20 ng/ml PAF was transient, and second applications had no effect. The PAF response also was seen in cell suspensions. These results suggest that an increase in [Ca2+]i may be an early event in PAF activation of macrophages.

Pharmacological analysis of guinea-pig macrophage chemiluminescence responses to platelet activating factor and opsonized zymosan

The luminol-dependent chemiluminescence (CL) response in vitro of guinea-pig C. parvum-activated peritoneal macrophages to platelet activating factor (PAF) has been compared with that to opsonized zymosan (OpZ). The response to PAF (5 X 10(-6) mol/l.) reached a peak within 1 min, that to OpZ (0.17 mg/ml) within 10-20 min. Peak responses to both stimuli were dose-dependently inhibited in a similar manner by p-hydroxymercuribenzoate (10(-5) - 10(-3) mol/l), sodium benzoate (10(-5) - 10(-3) mol/l.) and quinacrine (10(-6) - 10(-3) mol/l.). In contrast, the xanthine oxidase inhibitor allopurinol (IC50 vs OpZ, 220 mumol/l.; vs PAF greater than 1000 mumol/l.), the methylation-inhibiting combination homocysteine + 3-deazaadenosine (IC50 vs OpZ, 22 mumol/l.; vs PAF greater than 100 mumol/l.), the phospholipase A2 inhibitor and alkylating agent p-bromophenacylbromide (pBPB; IC50 vs OpZ, 2.6 mumol/l.; vs PAF 15 mumol/l.) and the beta-adrenoceptor agonist isoprenaline (IC50 vs OpZ, 0.1 mumol/l.; PAF greater than 10 mumol/l.) all exerted differential inhibitory effects on the CL responses to the two stimuli, though colour quenching by adrenochrome cannot be ruled out in the differential effect of isoprenaline. In screening studies, carried out with CL responses measured 2 or 5 min after PAF and OpZ, respectively, verapamil (less than or equal to 10(-4) mol/l.), trifluoperazine (less than or equal to 10(5) mol/l.) EDTA (less than or equal to 10(6) mol/l.), mannitol (less than or equal to 10(-2) mol/l.), metyrapone (less than or equal to 10(-5) mol/l.), SQ 22536 (less than or equal to 10 micrograms/ml.), iso-butyl methylxanthine (less than or equal to 10(-5) mol/l.).(ABSTRACT TRUNCATED AT 250 WORDS)