Platelet-activating factor acetylhydrolase and transacetylase activities in human plasma low-density lipoprotein

In this study, we demonstrate the presence of a transacetylase activity in human plasma low-density lipoprotein (LDL) that transfers short-chain fatty acids from platelet-activating factor (PAF) and its close ether- and ester-linked analogues to ether/ester-linked lysophospholipids (lyso-PL). We show evidence that both PAF acetylhydrolase (PAF-AH) and transacetylase activities are inhibited to the same extent by serine esterase inhibitors, are resistant to heat treatment, and exhibit identical distributions in lipoprotein classes and in LDL subfractions. Additionally, the competitive inhibition of PAF-AH by lyso-PL, and the evidence that the recombinant PAF-AH also showed a similar transacetylase activity, suggest that PAF-AH is responsible for both activities. Using PAF as a donor molecule and lyso-PAF (1-O-alkyl-sn-glycero-3-phosphocholine) as an acceptor, the transacetylase activity showed typical allosteric kinetics, due to the positive co-operativity of the substrates, with apparent Vmax=19.6+/-3.4 nmol/min per mg of protein, apparent h=2.0+/-0.3 and apparent [S]0.5=9.4+/-2.3 microM at saturation for the concentration of lyso-PAF. The substrate specificity of the donor molecules was decreased by increasing the chain length of the acyl moiety in the sn-2 position of the glycerol. The ether linkage in the sn-1 position of the substrate was 30% more effective than the ester bond; cholesteryl acetate was inactive as an acetyl donor. The two acceptors tested, lyso-PAF and the ester-linked lyso-PC (1-acyl-sn-glycero-3-phosphocholine), showed similar specificity. Addition of exogenous lyso-PAF induced the transient formation of PAF-like aggregating activity predominantely in small dense LDL subfractions upon oxidation. We conclude that PAF-AH possesses both transacetylase and acetylhydrolase activities which remove PAF and its ether-linked analogues from LDL particles upon LDL oxidation. However, in atherogenic small dense LDL-5 particles, the transacetylase activity may acetylate extracellular lyso-PAF into biologically active PAF.

Vitamin E attenuates the injurious effects of bioactive phospholipids on human ciliated epithelium in vitro

Bioactive phospholipids (PL), particularly lysophosphatidylcholine (LPC), are being increasingly implicated in the pathogenesis of various acute and chronic inflammatory disorders, particularly those of the airways, while there is emerging evidence that vitamin E may function as a natural antagonist of these lipid mediators of inflammation. The aims of this study were to document the effects of vitamin E on the inhibition of ciliary beating and damage to structural integrity of human ciliated epithelium induced by the PL, platelet-activating factor (PAF), lyso-PAF and LPC in vitro in relation to the anti-oxidative and membrane-stabilizing properties of the vitamin. Ciliary beat frequency was measured by a phototransistor technique, and damage to structural integrity assessed by a visual-scoring index, while superoxide production by polymorphonuclear leukocytes and membrane-stabilizing potential were measured using lucigenin-enhanced chemiluminescence and haemolytic procedures, respectively. All three PL caused inhibition of ciliary beating and structural damage to human ciliated epithelium by membrane-directed cytotoxic mechanisms, which were potentiated by human polymorphonuclear leukocytes due to induction of oxidant-mediated injury. Both direct and phagocyte-inflicted epithelial injury was attenuated by vitamin E. In haemolytic and chemiluminescence assays, vitamin E neutralized both the membrane-destabilizing and pro-oxidative actions of all three PL, while spectrophotometric analysis of mixtures of vitamin E with PAF, lyso-PAF and LPC revealed alterations in peak intensity, as well as peak shifts, indicative of physicochemical interactions between the vitamin and the PL. Vitamin E status may be a determinant of susceptibility to phospholipid-mediated airway inflammation and damage.

Similar effects of ether phospholipids, PAF and lyso-PAF on the Ca(2+)-ATPase activity of rat brain synaptosomes and leukocyte membranes

The present study is an extension of our previous work with the antineoplastic ether phospholipid ET-18-OCH3 (edelfosine), which was shown to affect the activity of the Ca(2+)-ATPase of rat brain synaptosomes and peritoneal leukocyte membranes. The effect of ET-18-OCH3 was compared with that of the 16-carbon chain analogue ET-16-OCH3 as well as with the structurally related 16- and 18-carbon PAFs (platelet-activating factors) and lyso-PAFs. In addition, the two alkylphosphocholines D-20166 and D-21266 (perifosine) were included in the investigation. The influence of all of the compounds followed the same pattern, i.e., the Ca(2+)-ATPase activity of the synaptosomes was increased over a relatively narrow concentration range (peak at 20-30 microM) and that of the leukocyte membranes was inhibited in a concentration-dependent manner by 10-50 microM concentrations of the drugs. Ether phospholipids with an 18-carbon chain at C-1 were more potent than those with a 16-carbon chain. All of the compounds increased the activity of the synaptosomal ATPase to the same extend (ca. 50%). With the exception of lyso-PAF, all inhibited the enzyme activity of leukocyte membranes by 60-70%, whereas lyso-PAF was less effective (ca. 50% inhibition). The concentration range of activity for PAF and lyso-PAF indicates that their effect on the enzyme activity was caused by receptor-independent mechanisms. The ether phospholipids and alkylphosphocholines are suggested to act by accumulating in the membranes and thereby altering the character of the lipid environment of the enzyme rather than by a direct interaction with the Ca(2+)-ATPase.

Inhibition of human platelet aggregation by amides and ester of salicylic acid with platelet-activating factor analogs

The influence of acetyl salicylic acid (ASA) derivatives with platelet-activating factor (PAF) lipid analogs on PAF-induced human platelet aggregation has been studied. It was found that the ASA amide with an ethanolamine plasmalogen PAF analog (1-0-alk-1'-enyl-2-acetyl-sn-glycero-3-phospho-(N-2'-acetoxybenzoyl)ethanolamine) and the ASA ester with a choline plasmalogen PAF analog (1-0-alk-1'-enyl-2-(2'-acetoxybenzoyl)-sn-glycero-3-phosphocholine) at concentrations of 10-7-10-6 M effectively inhibit PAF-induced aggregation of human platelets. In contrast to these compounds, the ASA amide with an alkyl PAF analog (1-0-alkyl-2-acetyl-sn-glycero-3-phospho-(N-2'-acetoxybenzoyl)ethanolamine) did not inhibit PAF-induced platelet aggregation. As possible mechanisms of action of the studied compounds, the blockade of PAF-receptor and cyclooxygenase inhibition are proposed.

Activation of glycogen synthase kinase 3 beta (GSK-3beta) by platelet activating factor mediates migration and cell death in cerebellar granule neurons

Children with vertically acquired HIV-1 can present with a rapidly progressive encephalopathy and neuronal apoptosis in the first 12-18 months of life. Furthermore, abnormal prenatal platelet activating factor (PAF) signalling may result in lissencephaly, a disorder of neuronal migration. PAF, produced from human immunodeficiency virus type 1 (HIV-1) -infected brain-resident macrophages, induces neuronal apoptosis in cultured cerebellar granule neurons (CGNs) in part by activating glycogen synthase kinase 3 beta (GSK-3beta). However, PAF can also inhibit migration of CGNs that are dispersed and allowed to reaggregate. Therefore, we investigated the biological effects following activation of GSK-3beta by PAF, and whether these effects were dependent on the culture conditions of the CGNs. We show here that activation of neuronal GSK-3beta by PAF is receptor-specific, with similar kinetics of activation in both monolayer cultures of CGNs that have ceased to migrate and reaggregate cultures of CGNs that are actively migrating. However, PAF receptor activation in reaggregated CGNs inhibits neuronal migration and induces approximately half the level of neuronal apoptosis compared with PAF-treated CGN cultures that have ceased to migrate. PAF-mediated inhibition of neuronal migration in reaggregated CGNs or induction of apoptosis in CGNs that have ceased to migrate can be reversed by either PAF receptor antagonists, or the GSK-3beta inhibitors lithium or valproic acid, in a dose-dependent manner. Abnormal PAF signalling that results in GSK-3beta overactivation may represent a common mechanism for pathological defects in neuronal migration in the prenatal period and neuronal apoptosis in the postnatal period.

Effects of platelet-activating factor on cytokine production by human uterine cervical fibroblasts

Platelet-activating factor (PAF), a lipid that acts as a potent proinflammatory mediator, is involved in several reproductive processes including parturition. To investigate the effects of PAF on expression of various cytokines by cultured human uterine cervical fibroblasts obtained at term prior to labour, Northern blot analyses and enzyme-linked immunosorbent assays were performed. C-PAF, a stable analogue of PAF, increased expression of interleukin-6 and -8 mRNA in a dose-dependent manner (10(-10) to 10(-8) mol/l of C-PAF), and the expression peaked within 4 h. The corresponding protein concentrations were increased in culture media. Monocyte chemoattractant protein-1 mRNA showed marked induction by 10(-8) mol/l of C-PAF; this peaked by 4 h and was followed by an increase in the protein concentration. Another cytokine, RANTES (regulated upon activation, normal T cell expressed and secreted) showed marked mRNA induction by 10(-8) mol/l of C-PAF, and continued to increase in a time-dependent manner until 24 h. The protein concentration was correspondingly increased in the medium. The PAF-induced cytokine production was abolished by co-incubation with WEB 2170, a specific PAF receptor antagonist. PAF may stimulate local production of cytokines which may induce migration of leukocytes and accelerate collagenolysis in the uterine cervix, thus contributing to cervical ripening during parturition.

Additional bioactive Lyso-PAF congeners from the sponge Spirastrella abata

A known (1) and four new (2--5) lyso-PAF (platelet activating factor) derivatives were isolated from the sponge Spirastrella abata. Two of them are unprecedented in having a methoxy group at C-2'. The structures have been determined by combined spectroscopic methods. Their inhibitory effect on the biosynthesis of cholesterol and cytotoxicity against human solid tumor cell lines are reported.

Platelet activating factor degradation in tear fluid from guinea pigs with allergic conjunctivitis

The purpose of this study was to investigate the role of platelet-activating factor (PAF) and PAF acetylhydrolase (AH) in conjunctiva. The influence of PAF on conjunctival vascular permeability and the presence of PAF or its metabolites in tears from guinea pigs with allergic conjunctivitis were investigated. We instilled PAF to the eyes of guinea pigs and evaluated vascular permeability. Tear samples were collected from passively sensitized guinea pigs, and the concentration of PAF and its metabolites determined by liquid chromatography-tandem mass spectrometry. Exogenous PAF degradation in tear samples was evaluated with or without diisopropyl fluorophosphate (DFP). Topically applied PAF increased vascular permeability in conjunctiva. In the tear samples from guinea pigs with allergic conjunctivitis, PAF could not be detected. However, 40 +/- 6 ng/ml of lyso-platelet activating factor (lyso-PAF) and 230 +/- 50 ng/ml of 1-alkyl-2-acyl-sn-glycero-3-phosphocholine were detected at 10 min after challenge. Exogenous PAF was rapidly degraded in the tear samples from guinea pigs with allergic conjunctivitis, but not from normal guinea pigs. This PAF degradation was inhibited by DFP. These results suggest that PAF in the tear fluid is quickly hydrolyzed to lyso-PAF by PAF AH, which may be released or activated in allergic conjunctivitis.

The regulation of CoA-independent transacylation reactions in neuronal nuclei by lysophospholipid, free fatty acid, and lysophospholipase: the control of nuclear lyso platelet-activating factor metabolism

CoA-independent transacylase activities generating alkylacylglycerophosphocholine (AAGPC) from alkylglycerophosphocholine (1-alkyl GPC) were considerably enriched in neuronal nuclei isolated from rabbit cerebral cortex. Specific nuclear transacylation activities were 13 times the corresponding microsomal values. Several lysophospholipids, notably 1-acyl glycerophosphocholine (1-acyl GPC), 1-alkenyl GPC and 1-alkenyl GPE (1-alkenyl glycerophosphoethanolamine) inhibited the transacylation of 1-alkyl GPC. The inhibitory effects of 1-acyl GPC were seen in the presence of MAFP (methyl arachidonoylfluorophosphonate) or free oleate, compounds that inhibit neuronal nuclear lysophospholipase. When neuronal nuclei were preincubated with 1-alkyl GPC, the radioactive AAGPC product served as donor in transacylation reactions, to generate 1-alkyl GPC. In these nuclear reactions, 1-palmitoyl GPE and 1-palmitoyl GPC appeared to be poor acceptor substrates, when compared with corresponding 1-alkyl and 1-alkenyl analogues. The presence of free oleate or MAFP in the reactions containing 1-acyl GPC boosted the release of 1-alkyl GPC from AAGPC. These observations are of particular relevance to brain ischemia in which lysophospholipid, free fatty acid, and platelet-activating factor (PAF) levels rise dramatically. PAF can be made by the nuclear acetylation of 1-alkyl GPC, which is formed by nuclear transacylation mechanisms. Yet transacylase also removes 1-alkyl GPC, and thus this enzyme activity can regulate 1-alkyl GPC availability. Our observations indicate that lysophospholipids promote the formation of 1-alkyl GPC from nuclear AAGPC via transacylation, while free fatty acid likely prolongs the lifetime of 1-acyl lysophospholipids substrates by lysophospholipase inhibition. Similarly, once 1-alkyl GPC is formed, other lysophospholipids effectively compete with this 1-alkyl analogue and reduce its conversion back to AAGPC by transacylation. Free oleate, in this case, sustains 1-acyl lysophospholipid inhibitors of 1-alkyl GPC transacylation. Thus the cycle of transacylation may favour 1-alkyl GPC formation during ischemia, increasing levels of 1-alkyl GPC for nuclear acetylation reactions and PAF formation. The nuclear generation of PAF is of considerable importance as PAF can play regulatory roles in transcription events associated with inflammation.

Regulation of the biosynthesis of acyl analogs of platelet-activating factor by purinergic agonist in endothlial cells

We have previously shown that platelet-activating factor (PAF)-dependent transacetylase (TA) contains three catalytic activities, namely PAF: lysophospholipid TA (TAL), PAF: sphingosine TA (TAs) and PAF acetylhydrolase. It serves as a modifier of PAF actions by producing different lipid signal molecules. The TAL activity is involved in the biosynthesis of acyl analogs of PAF (acyl-PAF, 1-acyl-2-acetyl-sn-glycero-3-phosphocholine, acylacetyl-GPC) in agonist-stimulated endothelial cells. In the present investigation, we have studied the mechanism(s) by which the TA activity is regulated in ATP-treated endothelial cells. We have demonstrated that ATP, and thiol-modifying agents with ATP, specifically regulate only the TAL part of the TA activities.

Evidence for involvement of the epidermal platelet-activating factor receptor in ultraviolet-B-radiation-induced interleukin-8 production

Ultraviolet B radiation has been shown to generate cutaneous inflammation in part through inducing oxidative stress and cytokine production in human keratinocytes. Amongst the proinflammatory cytokines synthesized in response to ultraviolet B radiation is the potent chemoattractant interleukin-8. Though the lipid mediator platelet-activating factor (PAF) is synthesized in response to oxidative stress, and keratinocytes express PAF receptors linked to cytokine biosynthesis, it is not known whether PAF is involved in ultraviolet-B-induced epidermal cell cytokine production. These studies examined the role of the PAF system in ultraviolet-B-induced epidermal cell interleukin-8 biosynthesis using a novel model system created by retroviral-mediated transduction of the PAF-receptor-negative human epidermal cell line KB with the human PAF receptor. Treatment of PAF-receptor-expressing KB cells with the metabolically stable PAF receptor agonist carbamoyl-PAF resulted in increased interleukin-8 mRNA and protein, indicating that activation of the epidermal PAF receptor was linked to interleukin-8 production. Ultraviolet B irradiation of PAF-receptor-expressing KB cells resulted in significant increases in both interleukin-8 mRNA and protein in comparison to ultraviolet-B-treated control KB cells. Pretreatment with PAF receptor antagonists inhibited both carbamoyl-PAF-induced and ultraviolet-B-induced interleukin-8 production in the PAF-receptor-positive cells, but not in control KB cells. Similarly, treatment of the PAF-receptor-expressing primary cultures of human keratinocytes or the human epidermal cell line A-431 with carbamoyl-PAF or ultraviolet B radiation resulted in interleukin-8 production that was partially inhibited by PAF receptor antagonists. These studies suggest that the epidermal PAF receptor may be a pharmacologic target for ultraviolet B radiation in skin and thus may act to augment ultraviolet-B-mediated production of cytokines such as interleukin-8.

Two-dimensional analysis of phospholipids by capillary liquid chromatography/electrospray ionization mass spectrometry

A phospholipid mixture extracted from cultured cells was directly analyzed by capillary (Cap) liquid chromatography (LC)/electrospray ionization (ESI) mass spectrometry (MS). Using a quadrupole mass spectrometer, we analyzed positive molecular ions, negative molecular ions, positive fragment ions and negative fragment ions under four different functions. In the analysis of the elution patterns of the phospholipids, a two-dimensional map, in which the first dimension is elution time and the second dimension is mass, proved useful. Consequently, four different maps can be obtained by each of four different functions. Among them, from negative fragment ions at high cone voltage in the negative ion mode, ions that originated from acyl fatty acid and phosphorylcholine, phosphorylethanolamine and cyclic inositol phosphate can be detected at specific elution times. The map from positive fragment ions at high cone voltage in the positive ion mode indicated ions such as diradylglycerol and derivatives of 1-alkyl or 1-alkenyl cyclic phosphatidic acid from phosphatidylethanolamine (PE), and phosphorylcholine from choline-containing phospholipids. The map produced from positive molecular ions indicated choline-containing phospholipids such as phosphatidylcholine, sphingomyelin, lysophosphatidylcholine and PE. The map of negative molecular ions effectively indicated acidic phospholipids such as phosphatidylinositol. We were able to obtain more than 500 molecular species of phospholipids by this method within a few hours immediately after extraction from culture cells using a mixture of chloroform and methanol (2:1). In this context, we concluded that the combination of Cap-LC and ESIMS seems to be very effective in the analysis of phospholipid classes and their molecular species.

Neuronal cytoskeletal alterations evoked by a platelet-activating factor (PAF) analogue

Platelet-activating factor (PAF), a phospholipid signaling molecule found in brain, modulates several neural functions and is implicated in the human developmental brain disorder Miller-Dieker Lissencephaly (MDL). Exposure to PAF, and a non-hydrolyzable analogue, methyl carbamyl PAF (mc-PAF), produces the following rapid, reversible effects upon cultured hippocampal neurites: growth cone collapse, neurite retraction, and neurite varicosity formation. In this study, the cytoskeletal alterations that mediate these shape changes were investigated by comparing the effects of mc-PAF with other cytoskeletal-altering drugs, through the fluorescent labeling of cytoskeletal proteins and mitochondria, and by electron microscopy. Results indicate that rearrangements of microtubules (MTs), F-actin, and mitochondria underlie the neurite shape changes produced by mc-PAF. Evidence for MT alteration was obtained by comparing the effects of mc-PAF with nocodozole and taxol. Exposure to nocodazole, a MT-depolymerizing agent, produced growth cone collapse and neurite varicosity formation similar to mc-PAF, whereas pre-incubation of neurites in taxol, a MT-stabilizing drug, was effective in blocking mc-PAF-induced neurite effects. Immunofluorescent labeling and EM revealed MT splaying and unbundling within neurite varicosities following mc-PAF treatment. Immunofluorescent labeling also revealed that F-actin shifted from concentration in the growth cone to a diffuse distribution along the neurite shaft following mc-PAF exposure. Fluorescent labeling and EM also revealed retrograde movement and morphological alterations of mitochondria following mc-PAF exposure, resulting in mitochondrial aggregates within neurite varicosities. These cytoskeletal rearrangements may provide insights into the mechanisms by which PAF influences neuronal activity, and could have important implications for the impairment of neuronal motility observed in MDL.

Spectrophotometric assay for serum platelet-activating factor acetylhydrolase activity

We developed a spectrophotometric assay for serum platelet-activating factor acetylhydrolase (PAF-AH, EC 3.1.1.47.) activity using a platelet-activating factor (PAF) analogue with a 4-nitrophenyl group as substrate. PAF-AH hydrolyzes the sn-2 position of the substrate ¿1-myristoyl-2-(p-nitrophenylsuccinyl)phosphatidylcholine, producing p-nitrophenyl succinate. This liberation was spectrophotometrically monitored and the activity determined from the change in absorption. The assay does not require radioisotopes and is applicable to an automatic analyzer. Utilizing this assay with an automatic analyzer, it is possible to measure the activities of thousands of samples in a few hours with excellent precision (CV 0.5%, n=30) and high correlation (r=0.979, n=100) with the results of a conventional radioisotopic assay. The assay should be particularly useful for clinical diagnostics.

Cloning and expression of rat neutral sphingomyelinase: enzymological characterization and identification of essential histidine residues

Using cross-species sequence homology, we cloned a cDNA for rat neutral sphingomyelinase (nSMase) composed of 422 amino acids that shares 87.6 and 79.0% identity with the mouse and human forms respectively. The rat nSMase expressed in Escherichia coli catalyzed sphingomyelin hydrolysis at neutral pH in a Mg(2+)-dependent manner, and required Triton X-100, dithiothreitol, and KCl for its full activity. The cloned rat enzyme shares conserved sequences with nSMases from both eukaryotes and prokaryotes. Introduction of single mutations into either of the histidine residues at positions 136 and 272, putative active sites, entirely abolished the activity, supporting a common mechanism for the nSMase family independent of the species. However, mutation in histidine 151, conserved only in eukaryotes, also abolished the activity, suggesting eukaryote-specific control of nSMase linked to this histidine 151. This enzyme also catalyzed the hydrolysis of lyso-platelet activating factor to yield 1-alkylglycerol at a rate that is slightly lower than that with sphingomyelin.

Bioactive amide of prostaglandin E1 and ethanolamine plasmalogen analog of platelet-activating factor inhibits several pathways of human platelet aggregation

The influence of an amide of prostaglandin E1 and ethanolamine plasmalogen platelet-activating factor analog 1-O-alk-1;-enyl-2-acetyl-sn-glycero-3-phospho-(N-11alpha, 15alpha-dioxy-9-keto-13-prostenoyl)ethanolamine (PGE1-PPAF) on platelet-activating factor (PAF)-, ADP-, and thrombin-induced human platelet aggregation has been studied. It was found that PGE1-PPAF inhibits the PAF-, ADP-, and thrombin-induced platelet aggregation in platelet-rich plasma. 1-O-alk-1;-enyl-2-acetyl-sn-glycero-3-phosphoethanolamine inhibited PAF-induced aggregation up to 50% but had no influence on platelet aggregation induced by ADP or thrombin. The ethanolamine plasmalogen analog of PAF 1-O-alk-1;-enyl-2-acetyl-sn-glycero-3-phospho-(N-palmitoyl)ethanolami ne, having a palmitoyl residue instead of PGE1, did not inhibit platelet aggregation induced by PAF, ADP, or thrombin. We propose that inhibition of human platelet aggregation by PGE1-PPAF is mediated by its action on platelet PAF-receptors and the adenylate cyclase system.

Mechanisms of Helicobacter pylori-induced rat gastric mucosal microcirculatory disturbances in vivo

The exact mechanisms by which Helicobacter pylori infection results in gastric mucosal injury are unclear. However, it has been demonstrated that surface protein extracts of the bacterium can induce a number of disturbances within the rat gastric mucosal microcirculation, including platelet aggregation and macromolecular leakage (MML) of labeled albumin. This study aimed to determine the mechanisms involved in inducing these events using the technique of fluorescent in vivo microscopy. Male Wistar rats were pretreated with either ketotifen, a mast cell stabilizer (1 mg/kg), pyrilamine, an H1-receptor antagonist (30 mg/kg), hexanolamine-PAF, a PAF-receptor antagonist (10 microg/kg), L-arginine, the nitric oxide precursor (300 mg/kg) or vehicle, saline. Then 0.5 ml of H. pylori extract was administered to the exteriorized gastric mucosa of the anesthetized rat. Alterations in fluorescein-labeled albumin leak, vessel diameters, and acridine red-labeled leukocyte and platelet activity were determined over a 2-hr period. Saline pretreated animals demonstrated significant MML with a peak at 5 min (11%, P<0.02). This was prevented with ketotifen and pyrilamine, but not with hexanolamine-PAF (17.5%, P<0.05) and L-arginine (13%, P<0.05). Significant numbers of platelet emboli and thrombi were observed within mucosal capillaries and postcapillary venules with vehicle pretreatment; this was prevented with hexanolamine-PAF and L-arginine, but not with ketotifen and pyrilamine. In conclusion, these studies demonstrate that more than one mediator is involved in inducing the rat gastric mucosal microcirculatory disturbances associated with H. pylori administration. Mast cells and histamine are linked to MML, with PAF, probably not derived from mast cells, involved in platelet activation.

Structure-dependent effects of glucose-containing analogs of platelet activating factor (PAF) on membrane integrity

Synthetic choline-containing phospholipids comprise a new class of compounds with antineoplastic properties. We have investigated the effect of recently synthesized glucose-containing analogs of lysophosphatidylcholine (glyceroglucophospholipid, Glc-PC) and of lysoplatelet activating factor (Glc-PAF) and its C16, C14 and C12 derivatives (ET-16, ET-14, and ET-12) on proliferation of immortalized human keratinocyte (HaCaT) cells. The data were compared to the ability of the compounds to intercalate into phosphatidylserine liposomes and to form lesions in planar bilayer membranes. A correlation between bioactivity and membrane activity was found. The number of molecules that intercalated into phosphatidylserine liposomes depended on the chemical structure of the compounds and was in the order Glc-PAF approximately ET-16 approximately ET-14 > Glc-PC > ET-12. All compounds induced membrane lesions, and the lesion forming activity was in the same order. Similar activity rankings were found for the release of lactate dehydrogenase from HaCaT cells as a measure of lytic activity and for the influence on cell number as a measure of proliferation. In the latter test, however, proliferation was already inhibited at non-toxic concentrations. From these findings, it may be concluded that the intercalation of the compounds at toxic concentrations leads to the formation of membrane lesions and finally results in membrane rupture leading to cell death.

Platelet activating factor acetylhydrolase activity in lamb lungs is up-regulated in the immediate newborn period

We recently showed that platelet activating factor (PAF) is an important modulator of pulmonary vasomotor tone in the fetus, with a significant decrease in circulating PAF levels in the immediate newborn period. In this study, we have determined PAF catabolism by PAF acetylhydrolase (PAF-Ah) in lungs of near-term fetal and newborn 2- to 16-h (<1 day) and 6- to 12-day-old lambs. The rate of PAF catabolism by lung homogenate protein from the three groups of lamb lungs was studied at 37 degrees C in 30 mM Tris buffer, pH 7.5, containing 0.01% BSA. Each lung homogenate protein was incubated for 10 min with 50 microM [(3)H]acetyl-PAF at pO(2) <50 Torr (hypoxia) and approximately 100 Torr (normoxia). PAF-Ah activity was quantified as amount of lyso-PAF produced. PAF-Ah activity (means +/- SEM, nmol lyso-PAF/min/mg protein) in fetal lung homogenate was 1.19 +/- 0.14 and 2.46 +/- 0.05 during hypoxia and normoxia, respectively. The corresponding values for the newborns were newborn <1 day, 1.65 +/- 0.26 and 2.95 +/- 0.07 and newborn 6-12 days, 1.25 +/- 0.10 and 2.84 +/- 0.05. In all groups, PAF-Ah activity was higher in normoxia than in hypoxia. During normoxia, PAF-Ah activity in newborn <1 day was significantly higher than the activity in fetus, but similar to the activity in newborn 6- to 12-day-old lamb lungs. These data show a significant up-regulation of PAF-Ah activity in lungs in the immediate newborn period. PAF-Ah gene expression measured by RT-PCR showed a significant up-regulation of the PAF-Ah gene in lungs of lambs <1 day old, suggesting a transcriptional regulation of the PAF-Ah gene in the immediate newborn period. These results suggest that up-regulation of PAF-Ah activity after birth with oxygenation will result in a decrease in circulating PAF levels, thereby facilitating the fall in pulmonary vascular resistance in the immediate newborn period.

Platelet-activating factor and group I metabotropic glutamate receptors interact for full development and maintenance of long-term potentiation in the rat medial vestibular nuclei

In rat brainstem slices, we investigated the interaction between platelet-activating factor and group I metabotropic glutamate receptors in mediating long-term potentiation within the medial vestibular nuclei. We analysed the N1 field potential wave evoked in the ventral portion of the medial vestibular nuclei by primary vestibular afferent stimulation. The group I metabotropic glutamate receptor antagonist, (R,S)-1-aminoindan-1,5-dicarboxylic acid, prevented long-term potentiation induced by a platelet-activating factor analogue [1-O-hexadecyl-2-O-(methylcarbamyl)-sn-glycero-3-phosphocholine], as well as the full development of potentiation, induced by high-frequency stimulation under the blocking agent for synaptosomal platelet-activating factor receptors (ginkolide B), at drug washout. However, potentiation directly induced by the group I glutamate metabotropic receptor agonist, (R,S)-3,5-dihydroxyphenylglycine, was reduced by ginkolide B. These findings suggest that platelet-activating factor, whether exogenous or released following potentiation induction, exerts its effect through presynaptic group I metabotropic glutamate receptors, mediating the increase of glutamate release. In addition, we found that this mechanism, which led to full potentiation through presynaptic group I metabotropic glutamate receptor activation, was inactivated soon after application of potentiation-inducing stimulus. In fact, the long-lasting block of the platelet-activating factor and metabotropic glutamate receptors prevented the full potentiation development and the induced potentiation progressively declined to null. Moreover, ginkolide B, given when high-frequency-dependent potentiation was established, only reduced it within 5 min after potentiation induction. We conclude that to fully develop vestibular long-term potentiation requires presynaptic events. Platelet-activating factor, released after the activation of postsynaptic mechanisms which induce potentiation, is necessary for coupling postsynaptic and presynaptic phenomena, through the activation of group I metabotropic glutamate receptors, and its action lasts only for a short period. If this coupling does not occur, a full and long-lasting potentiation cannot develop.

Lyso-PAF analogues and lysophosphatidylcholines from the marine sponge Spirastrella abata as inhibitors of cholesterol biosynthesis

A series of phospholipids, including previously undescribed compounds 4-7, were isolated by a bioactivity-guided fractionation from the marine sponge Spirastrella abata as inhibitors of cholesterol biosynthesis in human liver cells. These compounds were identified as lyso-PAF analogues (1-5) and lysophosphatidylcholines (6, 7) based on NMR and MS analyses. Compounds 1-7 specifically blocked the conversion of lanosterol into cholesterol in the Chang liver cell.

Occurrence of lysophosphatidic acid and its alkyl ether-linked analog in rat brain and comparison of their biological activities toward cultured neural cells

Rat brain was found to contain substantial amounts of potent bioactive lipids lysophosphatidic acid (acyl LPA) (3.73 nmol/g tissue) and lysoplasmanic acid (alkyl LPA) (0.44 nmol/g tissue). The presence of alkyl LPA was confirmed by mild alkaline hydrolysis analysis and by gas chromatography/mass spectrometry analysis of the trimethylsilyl derivative. This is the first clear evidence of the occurrence of an alkyl LPA in nature. The predominant molecular species of acyl LPA are 18:1-, 18:0- and 16:0-containing species (46. 9, 22.5 and 18.8%, respectively). A significant amount of a 20:4-containing species (7.2%) was also detected in the acyl LPA fraction. We also confirmed that rat brain alkyl LPA consists of 16:0-, 18:0- and 18:1-containing species. Noticeably, either acyl or alkyl LPA is capable of stimulating neuroblastomaxglioma hybrid NG108-15 cells to elicit a Ca(2+) transient, the potencies being almost the same. Both acyl and alkyl LPAs also induce cell rounding upon addition to the cells. These results suggest that acyl and alkyl LPAs play important physiological roles as intercellular signaling molecules as well as the roles as metabolic intermediates in the nervous system.

Intravitreally injected platelet activating factor induces retinitis in experimental animals

PURPOSE

Platelet activating factor is a lipid which has been strongly implicated in anterior uveitis. In order to investigate further the role of platelet activating factor in intraocular inflammation, we have characterized the histological changes associated with the intravitreal injection of platelet activating factor, PAF analogs, or lyso-PAF in laboratory rabbits and rats.

METHODS

Initial studies utilized a PAF analog (rac 1-0-octadecyl 2-0-ethyl glycero phosphoryl choline or ethoxy PAF), because this compound is relatively resistant to degradation by hydrolase, the major degradative enzyme for PAF. Doses ranging from 1 ug to 5 mg and time points from 6 hours to 7 days after injection were studied.

RESULTS

In either rats or rabbits, 100 ug of ethoxy PAF consistently induced a marked uveitis with the predominance of inflammation focused in the retina and choroid. Retinitis was also induced in rabbits by either 1 mg PAF injected intravitreally or a similar dose of the PAF precursor/metabolite, lyso PAF. Retinal inflammation was not induced by an inactive lipid, 1,1-0,0-dihexadecyl-rac-glycero-3-phosphocholine, although this compound resulted in mild vitreous inflammation. The histological changes induced by PAF could be readily distinguished from the predominantly anterior inflammation induced by intravitreal injections of substances such an interleukin-1 or endotoxin.

CONCLUSIONS

Recent studies indicating that PAF antagonists inhibit a variety of retinal toxicities and our own observations suggest that PAF could be a major mediator of retinal inflammation.

Subcellular localization of enzyme activities involved in the metabolism of platelet-activating factor in rainbow trout leukocytes

The subcellular distribution of an alkyllyso-GPC: acetyl-CoA acetyltransferase (EC 2.3.1.67) and transacylase, two important enzyme activities involved in the remodeling pathway for the biosynthesis of platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine, PAF) have been examined in leukocytes isolated from the pronephros of the rainbow trout, Oncorhynchus mykiss. Contrary to mammalian systems, in which the acetyltransferase is localized to intracellular membranes, the subcellular distribution of an acetyltransferase activity in rainbow trout leukocytes was localized to the plasma membrane. Analysis of the acetyltransferase products by thin-layer chromatography (TLC) and high performance liquid chromatography (HPLC) confirmed synthesis of two subclasses of PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine and 1-acyl-2-acetyl-sn-glycero-3-phosphocholine. The transacylase activity in this study was detected in membrane fractions in two domains of the intermediate density region which also contained the NADH dehydrogenase activity, a marker enzyme for the endoplasmic reticulum. Acylation of lysoPAF (1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine) exhibited approximately 95% specificity for omega-3 fatty acids. Acylation patterns were not significantly different in either domain of the endoplasmic reticulum. A model is proposed herein for the metabolism of PAF in rainbow trout leukocytes.

Substrate specificity of lysophospholipase D which produces bioactive lysophosphatidic acids in rat plasma

Previously we reported that lysophospholipase D in rat plasma hydrolyzes endogenous unsaturated lysophosphatidylcholines (LPCs) preferentially to saturated LPCs to lysophosphatidic acids with growth factor-like and hormone-like activities. In this study, we examined the possibility that association of LPCs with different proteins in rat plasma has an effect on the preference of lysophospholipase D for unsaturated LPCs. Large portions of various LPCs were found to be recovered in the lipoprotein-poor bottom fraction. Furthermore, the percentages of LPCs associated with albumin isolated from rat plasma were shown not to be consistent with their percentage conversions to lysophosphatidic acids by lysophospholipase D on incubation of rat plasma at 37 degrees C. These results indicate that distinct distributions of LPCs in the plasma protein fractions are not critical factors for the substrate specificity of lysophospholipase D. Experiments with Nagase analbuminemic rats suggested that albumin-LPC complexes are not necessarily required for the hydrolysis by lysophospholipase D; lipoprotein-associate LPCs appeared to be good substrates for the phospholipase. We found that both saturated and unsaturated LPCs are present mainly as 1-acyl isomers in rat plasma. This result indicates that the preference of lysophospholipase D for unsaturated LPCs is not attributable to a difference in position of the acyl group attached to the glycerol backbone of LPC. In addition, lysophospholipase D was also found to attack choline phospholipids with a long chain group and a short chain alkyl group, although their percentage hydrolyses were low. Taken altogether, these results suggest that lysophospholipase D shows higher affinities for free forms of unsaturated acyl type LPCs equilibrated with albumin-bound and lipoprotein-associated forms, than for free forms of saturated acyl type LPCs and analogs of platelet-activating factor.

Platelet activating factor is elevated in cerebral spinal fluid and plasma of patients with relapsing-remitting multiple sclerosis

Platelet-activating factor (PAF) is a phospholipid mediator of inflammation with a wide range of biological activities, including the alteration of barrier function of endothelium. A biological assay combined with high pressure liquid chromatography-tandem mass spectrometry showed that plasma and cerebral spinal fluid (CSF) PAF levels in 20 patients with relapsing/remitting or secondary progressive multiple sclerosis (MS) studied by magnetic resonance imaging (MRI) were significantly higher than in healthy controls (plasma: 3.29+/-4.52 vs. 0.48+/-0.36 ng/ml, p < 0.002; CSF: 4.95+/-6.22 ng/ml vs. 0.01+/-0.04 ng/ml, p < 0.0001). Values were also significantly higher in relapsing/remitting than in secondary progressive (plasma: 5.10+/-4.97 vs. 0.52+/-0.85 ng/ml, p < 0.005; CSF: 8.59+/-6.39 vs. 0.55+/-0.68 ng/ml, p < 0.002). It was also found that both plasma (R2: 0.65) and CSF (R2:0.72) levels were correlated with the MRI number of gadolinium enhancing lesions, which are markers of blood-brain barrier (BBB) injury, whereas their peaks were not correlated with the MRI number of white matter lesions, nor with the expanded disability status score (EDSS) according to Kurtze [Kurtze, J.F., 1983. Rating neurological impairment in multiple sclerosis: an expanded disability scale (EDSS). Neurology 33, 1444-1452]. Both plasma and CSF in patients with relapsing/remitting MS and marked gadolinium enhancement contained the two major molecular species of PAF: 1-0-hexadecyl- (C16:O) and 1-0-octadecyl-sn-glycero-3-phosphocholine (C18:O). The ratio of the two molecular species was different in the two biological fluids, being PAF C18:0 more abundant in CSF and PAF C16:0 in plasma, indicating a different cellular origin of PAF or different enzymatic processing. These findings suggest that PAF is a significant mediator of BBB injury in the early stages of MS, rather than a marker of its progression and severity.

Nitroprusside stimulates contractility and the synthesis of 14C-acetylated PAF-like substances in estrogen primed-mouse uterine horns

We investigated the effect of sodium nitroprusside (SNP), a donor of nitric oxide, on the formation of platelet-activating factor (PAF) and uterine contractility in mouse uterine horns from mice treated with estrogen. Because the major pathway of PAF synthesis is the remodeling pathway in uterine tissue, we evaluated the incorporation of 14C-acetate into PAF-like molecules. Our results showed that SNP (100-300 mumol/L) caused a transient increase in the synthesis of PAF, which remained cell-associated. The addition of SNP (100-300 mumol/L) to a mouse uterine horn in an isolated organ bath preparation evoked a transient increase in contractility, which was inhibited by hemoglobin (2 micrograms/mL), a nitric oxide scavenger, but not by methylene blue (10 mumol/L), a guanylate cyclase inhibitor. The pharmacological characteristics of the contractions evoked by SNP resembled those evoked after mast cell activation, in that they were blocked by ritodrine (a beta 2 adrenergic agonist, 0.1 mumol/L); indomethacin (a cyclooxygenase inhibitor, 10 mumol/L); ketotifen (a mast cell stabilizer, 1.0 mumol/L); cromolyn sodium (a mast cell stabilizer, 100 mumol/L); pyrilamine (an H1 antagonist, 10 mumol/L); and ketanserine (5HT2 antagonist, 0.1 mumol/L). These data demonstrate that nitric oxide generated from SNP stimulated the synthesis of PAF and evoked contractility in uterine horns from mice treated with estrogen. This result suggests the possibility that these tissue conditions might be favorable for the generation of peroxynitrites, possible mediators of both effects. It is also shown that the contractility evoked by the addition of SNP was not due to production of PAF, because its antagonist, WEB 2086 (10-30 mumol/L, a concentration that blocked contractions evoked by PAF 1 nmol/L), had no effect on the SNP-evoked contractions.

Effect of platelet activation on coronary collateral blood flow

BACKGROUND

The platelet products thromboxane A2 and serotonin have been shown to cause constriction of well-developed coronary collateral vessels. This study was performed to determine whether intravascular platelet activation produced with platelet activating factor (PAF) can cause a decrease in coronary collateral blood flow.

METHODS AND RESULTS

Collateral vessel growth was induced by embolization of a hollow stainless steel plug into the left anterior descending coronary artery (LAD) of adult dogs. The animals were returned to the laboratory 3 to 6 weeks later for surgical instrumentation and measurement of collateral blood flow. Collateral flow was assessed by measuring retrograde blood flow from the cannulated collateral-dependent artery. PAF (10 nmol) was injected into the left main coronary artery to allow products of platelet activation to reach collateral vessels arising from the left coronary system. PAF caused a vasoconstrictor response, which became maximal 3 minutes after injection and resulted in a 40.3+/-7.4% decrease in retrograde blood flow (32.1+/-2.1 to 19.6+/-3.2 mL/min; P<0.05). By 15 minutes after the PAF injection, both retrograde blood flow and transcollateral resistance had returned to normal. After pretreatment with the thromboxane A2 receptor antagonist SQ30, 741, the vasoconstrictor response to PAF was abolished and, in contrast to the decrease in retrograde blood flow from PAF alone, a weak vasodilator effect was unmasked.

CONCLUSIONS

PAF caused a decrease in coronary collateral blood flow. This vasoconstrictor response required the participation of thromboxane A2.

Identification and pharmacological characterization of platelet-activating factor and related 1-palmitoyl species in human inflammatory blistering diseases

Through its pro-inflammatory effects on leukocytes, endothelial cells, and keratinocytes, the lipid mediator platelet-activating factor (PAF) has been implicated in cutaneous inflammation. Although the 1-alkyl PAF species has been considered historically the most abundant and important ligand for the PAF receptor (PAF-R), other putative ligands for this receptor have been described including 1-acyl analogs of sn-2 acetyl glycerophosphocholines. Previous bioassays have demonstrated a PAF-like activity in lesions of the autoimmune blistering disease bullous pemphigoid. To assess the actual sn-2 acetyl glycerophosphocholine species that result in this PAF agonistic activity, we measured PAF and related sn-2 acetyl GPCs in fresh blister fluid samples from bullous pemphigoid and noninflammatory (suction-induced) bullae by mass spectrometry. We report the presence of 1-hexadecyl as well as the 1-acyl PAF analog 1-palmitoyl-2-acetyl glycerophosphocholine (PAPC) in inflammatory blister fluid samples. Because PAPC is the most abundant sn-2 acetyl glycerophosphocholine species found in all samples examined, the pharmacological effects of this species with respect to the PAF-R were determined using a model system created by transduction of a PAF-R-negative epidermoid cell line with the PAF-R. Radioligand binding and intracellular calcium mobilization studies indicated that PAPC is approximately 100x less potent than PAF. Though a weak agonist, PAPC could induce PAF biosynthesis and PAF-R desensitization. Finally, intradermal injections of PAF and PAPC into the ventral ears of rats demonstrated that PAPC was 100x less potent in vivo. These studies suggest possible involvement of PAF and related species in inflammatory bullous diseases.

Platelet-activating factor modulates cardiorespiratory responses in the conscious rat

Platelet-activating factor receptor (PAFR) activation is associated with increases in neuronal excitability. We hypothesized that PAF may play a role in cardiorespiratory control. Ventilatory responses to microinjection of a long-acting PAF analog (mc-PAF, 1 microg in 1 microl) within the dorsocaudal brain stem were measured in unrestrained adult rats. mc-PAF elicited significant minute ventilation (VE) enhancements that were primarily due to tidal volume increases and were accompanied by respiratory alkalosis, heart rate increase, and reduction of arterial blood pressure. Such cardiovascular and respiratory effects did not occur after administration of either vehicle or the inactive analog lyso-PAF. The effect was blocked when animals were coadministered the presynaptic PAFR antagonist BN-52021 or recombinant PAF acetyl hydrolase. To determine the relative contribution of PAF to hypercapnic and hypoxic ventilation, microinjections were performed in additional animals with either vehicle (CO, 1 microl) or with 5 microg in 1 microl of BN-52021. Hypercapnic challenges with 5% CO2 were unaffected by BN-52021. In contrast, although 10% O2 breathing increased VE from 120.4 +/- 7.5 to 204.6 +/- 11.4 ml/min in CO, after BN-52021, VE increased only from 118.7 +/- 6.9 to 137.3 +/- 8. 9 ml/min (CO vs. BN-52021, P < 0.001). We conclude that PAFR activation in the dorsocaudal brain stem exerts significant cardioventilatory effects during normoxia and appears to play an important modulatory role in the VE response to hypoxia in conscious rats.

Dose-dependent agonist and antagonist effects of the platelet-activating factor analogue 1-palmitoyl-2-acetoyl-sn-glycero-3-phosphocholine on B lymphocytes

BACKGROUND

Platelet activating-factor (PAF), an ether-linked phospholipid, is a potent activator of B lymphocyte cell lines. The related ester-linked phospholipid, 1-palmitoyl-2-acetoyl-sn-glycero-3-phosphocholine (PAGPC), is synthesized by tissues important in B-cell development.

OBJECTIVES

We examined whether PAGPC was capable of influencing immunoglobulin synthesis in B lymphocytes and compared its action with that of PAF. We also examined the interaction of the two mediators as agonists or competitive antagonists.

METHODS

Ramos, an IgM-secreting immature B-cell line that expresses PAF receptor, was used in these experiments. The effect of PAF, PAGPC, or both mediators together on IgM secretion and anti-IgM-mediated apoptosis was measured.

RESULTS

Both PAF and PAGPC stimulated IgM production in Ramos cells in a dose-dependent fashion, with PAGPC being approximately three logs less potent than PAF. The effect of both mediators was inhibited by specific PAF receptor antagonists. Preincubation with suboptimal concentrations of PAGPC inhibited the ability of PAF to increase IgM secretion by Ramos cells. Additionally, preincubation with low concentrations of PAGPC prevented PAF from rescuing Ramos cells from apoptosis induced by cross-linking the B-cell receptor with anti-IgM antibodies. PAGPC caused PAF receptor desensitization because displacement of bound PAGPC with high concentrations of bovine serum albumin did not reverse its PAF antagonist effect.

CONCLUSIONS

PAF and PAGPC are biologically active phospholipids, but PAF is approximately 1000 times more potent. At high concentrations, PAGPC acts similarly to PAF, whereas at lower concentrations, PAGPC acts as a functional PAF antagonist. Because it is secreted at sites of inflammation and allergic reactions, PAGPC may be an endogenous regulator of the effects of PAF.

Expression of the platelet-activating factor receptor results in enhanced ultraviolet B radiation-induced apoptosis in a human epidermal cell line

Recent studies have demonstrated that ultraviolet B radiation (UVB) damages human keratinocytes in part by inducing oxidative stress and cytokine production. Severe UVB damage to the keratinocyte can also result in apoptosis or programmed cell death. Although the lipid mediator platelet-activating factor (PAF) is synthesized in response to epidermal cell damage and epidermal cells express PAF receptors, it is not known whether PAF is involved in UVB-induced epidermal cell apoptosis. These studies examined the role of the PAF system in UVB-induced epidermal cell apoptosis using a novel model system created by retroviral-mediated transduction of the PAF receptor-negative human epidermal cell line KB with the human PAF receptor (PAF-R). Expression of the PAF-R in KB cells did not affect base-line growth or apoptosis, yet resulted in a decrease in the lag time between treatment of the cells and the induction of apoptosis following irradiation with 400 J/m2 UVB. This effect was inhibited by pretreatment with the PAF-R antagonists WEB 2086 and A-85783, confirming involvement of the PAF-R in this process. At lower doses (100-200 J/m2) of UVB, only KB cells that expressed the PAF-R became apoptotic. Treatment of PAF-R-expressing KB clones with the metabolically stable PAF-R agonist 1-hexadexyl-2-N-methylcarbamoyl-3-glycerophosphocholine (CPAF) alone did not induce apoptosis but augmented the degree of apoptosis observed if CPAF was used in combination with lower doses (200 J/m2) of UVB irradiation. Interestingly, UVB irradiation was found to stimulate PAF synthesis only in PAF-R-expressing KB cell clones. The antioxidants N-acetyl cysteine, 1,1,3,3-tetramethyl-2-thiourea, and vitamin E inhibited both UVB-induced PAF biosynthesis as well as the augmentation of UVB-induced apoptosis in PAF-R-expressing KB clones, suggesting the possibility that UVB stimulates the production of oxidized lipid species with PAF-R agonistic activity in this model system. Thus, these studies indicate that a component of UVB-induced epidermal cell cytotoxicity can be modulated by PAF-R activation through the production of PAF and PAF-like species.

Stimulation of primed neutrophils by soluble immune complexes: priming leads to enhanced intracellular Ca2+ elevations, activation of phospholipase D, and activation of the NADPH oxidase

Soluble immune complexes activate a rapid burst of reactive oxidant secretion from neutrophils that have previously been primed with GM-CSF. Binding of these complexes to the cell surface of unprimed neutrophils results in the generation of intracellular Ca2+ transients, but the NADPH oxidase fails to become activated. No phospholipase D activity was observed following the addition of soluble immune complexes to unprimed cells. Upon priming with GM-CSF, the intracellular Ca2+ signal generated following soluble complex binding was greatly extended and phospholipase D was activated: there was also increased phosphorylation of proteins on tyrosine residues and the NADPH oxidase was activated. When Ca2+ influx was prevented, this phospholipase D activity was not observed. This primed oxidase activity was completely inhibited by erbstatin. Treatment of unprimed neutrophils with pervanadate (to inhibit protein tyrosine phosphatases) mimicked the effects of priming in that pervanadate-treated neutrophils secreted reactive oxidants in response to soluble immune complexes. The data indicate that during priming a new signaling pathway is activated that involves Ca2+ influx, phosphorylation on tyrosine residues, phospholipase D activity, and NADPH oxidase activation.

MgATP has different inhibitory effects on the use of 1-acyl-lysophosphatidylcholine and lyso platelet-activating factor acceptors by neuronal nuclear acetyltransferase activities

The inhibitory effects of MgATP on neuronal nuclear acetyltransferase activities were studied using lyso platelet-activating factor (lyso-PAF, 1-alkyl-sn-glycero-3-phosphocholine) and lysophosphatidylcholine (lyso-PC, 1-acyl-sn-glycero-3-phosphocholine). The nuclear (N1) acetylation of lyso-PC was more profoundly inhibited by MgATP. MgATP did not alter the apparent Km for acetyl-CoA in either acetylation reaction. The inhibitory effects of MgATP were not seen for other nucleotides or MgAMP-PCP. Kinase inhibitors such as staurosporine (1 microM), chelerythrine, and R59022 (diglyceride kinase inhibitor I) did not block the MgATP inhibition of either acetylation. However, the addition of phospholipids to the assays indicated a selective inhibitory effect for PIP (25-50 microM) in the nuclear acetylation of lyso-PAF. When N1 was incubated with [gamma-33P]ATP, phosphatidic acid and PIP were the principal radioactive lipid products. While the extent of MgATP inhibition of lyso-PAF acetylation was similar at different concentrations of lyso-PAF, increasing lyso-PC concentrations greatly decreased the MgATP inhibition seen in lyso-PC acetylations. Nuclear envelopes prepared in the presence of PMSF, and fraction N1 exposed to PMSF, did not show the inhibitory effect of MgATP on lyso-PC acetylation. PMSF (an inhibitor of certain phospholipase and lysophospholipase activities) did not reduce the MgATP inhibition of lyso-PAF acetylation. Arachidonoyl trifluoromethylketone, an inhibitor of cytosolic phospholipases A2 and of lysophospholipase activity associated with cPLA2, also blocked the inhibitory effect of MgATP on lyso-PC acetylation. Using radioactive lyso-PC substrate, fraction N1 produced labeled free fatty acid and phosphatidylcholine. In the presence of acetyl-CoA, the production of radioactive phosphatidylcholine increased almost 6-fold when MgATP was also included in these incubations. In the presence of MgATP and acetyl-CoA, PMSF reduced the levels of radioactive free fatty acid and phosphatidylcholine derived from lyso-PC, while Triacsin C, an inhibitor of acyl CoA synthetase, decreased phosphatidylcholine labeling. These findings suggest that MgATP inhibition of lyso-PC acetylation results from a loss of lyso-PC substrate that is largely mediated by nuclear lysophospholipase, acyl-CoA synthetase and lyso-PC acylation. Thus the neuronal nuclear production of Acyl PAF may be regulated by paths that compete for the lyso-PC substrate. In contrast, the acetylation of lyso-PAF is inhibited by PIP, a product of nuclear PI kinase reactions.

Metabolic fate of platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) and lyso-PAF (1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine) in FRTL5 cells

The metabolism of platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) and lyso-PAF (1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine) was investigated in FRTL5 cells, a normal rat thyroid cell line. FRTL5 cells incorporated [3H]PAF and deacetylated this compound to the corresponding [3H]lyso-PAF which was not accumulated or secreted but converted mainly to alkyl-acyl-phosphocholine indicating that this acylation process was particularly active in these cells. Among metabolic products of both [3H]PAF and [3H]lyso-PAF were alkylglycerol as well as its mono- and diacyl derivatives. [3H]alkylglycerol could be the intermediate compound for the production of [3H]alkyl- and [3H]alkenyl-phosphoethanolamine (plasmalogen) which were also metabolic products. FRTL5 cells were able to convert lyso-PAF to PAF especially when they were stimulated by ionophore A23187 in the presence of [3H]Iyso-PAF and phenylmethylsulfonyl fluoride. The amount of PAF increased for the first 30 min and declined thereafter. PAF resting levels were found low in the same cells. Furthermore, PAF-acetylhydrolase activity was determined in cell homogenates. The presence of metabolic products such as alkyl-phosphatidylcholine, alkyl- and alkenyl-phosphatidylethanolamine and alkyl-glycerol, as well as, its mono- and diacyl derivatives, indicates that FRTL5 cells and probably other thyroid cells, are very active in metabolizing PAF and lyso-PAF and suggests the co-operation of the corresponding metabolic pathways in these cells.

Role of platelet-activating factor in long-term potentiation of the rat medial vestibular nuclei

In rat brain stem slices, we investigated the role of platelet activating factor (PAF) in long-term potentiation (LTP) induced in the ventral part of the medial vestibular nuclei (MVN) by high-frequency stimulation (HFS) of the primary vestibular afferent. The synaptosomal PAF receptor antagonist, BN-52021 was administered before and after HFS. BN-52021 did not modify the vestibular potentials under basal conditions, but it reduced the magnitude of potentiation induced by HFS, which completely developed after the drug wash-out. The same effect was obtained by using CV-62091, a more potent PAF antagonist at microsomal binding sites, but with concentrations higher than those of BN-52021. By contrast both BN-52021 and CV-6209 had no effect on the potentiation once induced. This demonstrates that PAF is involved in the induction but not in the maintenance of vestibular long-term effect through activation of synaptosomal PAF receptors. In addition, we analyzed the effect of the PAF analogue, 1-O-hexadecyl-2-O- (methylcarbamyl)-sn-glycero-3-phosphocoline (MC-PAF) and the inactive PAF metabolite, 1-O-hexadecyl-sn-glycero-3-phosphocoline (Lyso-PAF) on vestibular responses. Our results show that MC-PAF, but not Lyso-PAF induced potentiation. This potentiation was prevented by D,L-2-amino 5-phosphonopentanoic acid, suggesting an involvement of N-methyl-D-aspartate receptors. Furthermore, under BN-52021 and CV-6209, the MC-PAF potentiation was reduced or abolished. The dose-effect curve of MC-PAF showed a shift to the right greater under BN-52021 than under CV-6209, confirming the main dependence of MC-PAF potentiation on the activation of synaptosomal PAF receptors. Our results suggest that PAF can be released in the MVN after the activation of postsynaptic mechanisms triggering LTP, and it may act as a retrograde messenger which activates the presynaptic mechanisms facilitating synaptic plasticity.

Platelet-activating factor increases leukotriene B4 release in stimulated alveolar macrophages from asthmatic patients

This study was designed to examine further the role of platelet-activating factor (PAF) in asthma, comparing leukotriene B4 (LTB4) release, 5-lipoxygenase activity and intracellular calcium levels ([Ca2+]i) in macrophages. LTB4 and other lipoxygenase metabolites in macrophages in bronchoalveolar lavage fluids obtained from 23 asthmatic patients and 20 control subjects were measured by reverse-phase high-performance liquid chromatography. [Ca2+]i was monitored using the fluorescent probe fura-2. The basal LTB4 release of resting macrophages was not different between groups (0.02+/-0.01 versus 0.05+/-0.02 ng x 10(-6) cells). When stimulated with calcium ionophore A23187 (2.5 microM), however, macrophages from asthmatic patients released more LTB4 than cells from control subjects (30.2+/-3.4 versus 13.7+/-2.1 ng x 10(-6) cells). Although PAF alone did not alter LTB4 release, it enhanced the response to subsequent A23187 stimulation. This effect was noted following short treatment (i.e., 5 min) at concentrations of > or =1.0 microM PAF, with the maximal effect noted after treatment with 5.0 microM PAF + 2.5 microM A23187 (105.1+/-6.7 versus 15.3+/-2.6 ng x 10(-6) cells). Treatment of macrophages with PAF also increased 5-lipoxygenase activity and [Ca2+]i more in cytosols from asthmatic patients than in cytosols from control subjects. These findings support a role of intracellular calcium in the activation of 5-lipoxygenase which, in turn, augments the release of leukotriene B4. Because levels of platelet-activating factor may be increased in the lung during asthma and can increase the subsequent release of a chemotactic mediator leukotriene B4, from macrophages, these findings suggest that platelet-activating factor may prime the constitutive cells of the lung to augment inflammatory effects important in the pathogenesis of asthma.

Substrate specificities of neuronal nuclear acetyltransferases involved in the synthesis of platelet-activating factor: differences in the use of 1-alkyl and 1-acyl lysophospholipid acceptors

The selectivity of alkylglycerophosphate (AGP) acetyltransferase and lyso-platelet-activating factor (lyso-PAF) acetyltransferase was studied in neuronal nuclei isolated from cerebral cortices of 15-day-old rabbits. Specifically, 1-alkyl and 1-acyl analogues were compared as acceptors in these acetylation reactions. A number of observations supported one nuclear activity in the acetylation of AGP and lyso-PA. Lyso-PA was a competitive substrate for AGP, Km values for AGP and lyso-PA were similar, as were acetylation rates measured at individual AGP or lyso-PA concentrations, and the acetylation of both substrates was unaffected by preincubations with protein phosphatase 1 (PP-1). In contrast, there were a number of differences seen in the acetylation of lyso-PAF and lyso-PC. The kinetics for lyso-PC acetylation (as a function of lyso-PC concentration) were not hyperbolic, and lyso-PC was not a competitive substrate for the acetylation of lyso-PAF. Unlike acetylation rates with lyso-PAF, lyso-PC acetylation was not reduced by preincubations with PP-1, and was less susceptible to inhibition particularly at high levels of free fatty acid. In addition, rates of acetylation of lyso-PC were selectively increased by the presence of lyso-PA. When neuronal nuclear envelope fractions (NE) were prepared from N1, the specific acetylation activity with lyso-PAF was significantly lower in NE, while the activities for lyso-PC were comparable in NE and the parent N1 fraction. The results with the acetylation of lyso-PC and lyso-PAF suggest that the lyso-PC acetyltransferase may be in a uniquely sequestered state within the neuronal nucleus. This could explain the smaller inhibition of lyso-PC acetylation by free fatty acid, the maintenance of lyso-PC acetylation during PP-1 preincubations, the non-hyperbolic response to lyso-PC concentrations and the selective preservation of lyso-PC acetylation during NE isolation. This protected status could result from a more internal location for this acetyltransferase within the membranes of the nuclear envelope, or possibly an association of the enzyme with the nuclear matrix that is disrupted with the exposure of N1 to lyso-PA.

Oxatomide inhibits synthesis and release of platelet-activating factor in human neutrophils

Effect of oxatomide on release and production of platelet-activating factor (PAF) in neutrophils obtained from asthmatic and non-asthmatic patients was investigated. Neutrophils were preincubated with or without oxatomide and stimulated with N-formyl-L-methionyl-L-leucyl-L-phenylalanine (f-MLP, 10 microM) for 15 min. PAF activity was detected by aggregation of washed guinea pig platelets. PAF activity released from asthmatic neutrophils without preincubation of oxatomide was 7.97[0.22] (mean[SEM], ng/10(7) cells) in supernatants and 33.4[0.26] in cell pellets. After preincubation with 10(-8), 10(-6), and 10(-4) M of oxatomide, PAF activity reduced to 6.77[0.37] (mean[SEM], ng/10(7) cells), 3.99[0.25], and 0.96[0.05] (n = 15) in the supernatants, and 22.4[0.31], 16.7[0.22], and 6.35[0.11] (n = 15) in the cell pellets, respectively. PAF activity in non-asthmatic neutrophils without preincubation of oxatomide was 6.35[0.12] (mean[SEM], ng/10(7) cells) in supernatants and 27.9[0.25] in cell pellets. After preincubation with 10(-8), 10(-6), and 10(-4) M of oxatomide, PAF activity reduced to 5.02[0.16] (mean [SEM], ng/10(7) cells), 3.96[0.11], and 0.94[0.03] (n = 10) in the supernatants, and 28.4[0.69], 13.78[0.17], and 2.88[0.27] (n = 10) in the cell pellets, respectively. Our results showed that preincubation with oxatomide caused a dose-dependent inhibition of intra- and extracellular PAF activity from asthmatic and non-asthmatic neutrophils in the same manner.

The bioregulatory role of platelet-activating factor in intracellular processes and cell-cell interactions

The role of platelet-activating factor (PAF, a phospholipid compound) in regulation of cell functions and cell-cell interactions is reviewed. The biological effects of PAF on platelets, neutrophils, basophils, eosinophiles, lymphocytes, and endothelial cells are described. Mechanisms of cell activation by PAF are discussed. Interactions of PAF with other biological regulators (prostaglandins, leukotrienes, NO, tumor necrosis factor, and interleukins) are considered.

Production, metabolism and effect of platelet-activating factor on the growth of the human K562 erythroid cell line

The human immature K562 erythroid cell line was studied for its capacity to produce and to metabolize the phospholipid molecule platelet-activating factor (PAF). K562 cells produced PAF under calcium ionophore stimulation. Lyso PAF and acetyl-CoA (the acetate donor molecule for the acetylation of lyso PAF into PAF) had no effect on the amounts of PAF produced by ionophore-stimulated cells. The metabolism of PAF and lyso PAF by K562 cells was compared to that of freshly-isolated human bone marrow erythroblasts and blood erythrocytes. K562 cells rapidly metabolized [3H]PAF and [3H]lyso PAF with 1-alkyl analogue of phosphatidylcholine as the major metabolic product. In contrast, blood erythrocytes did not. PAF acetylhydrolase activity levels in K562 cells and bone marrow erythroblasts were similar and higher than in blood erythrocytes. PAF (1-100 nM) stimulated [3H]thymidine incorporation in K562 cells grown in low serum concentration, a non-metabolizable PAF agonist being more potent than PAF to stimulate thymidine incorporation. PAF receptor mRNA was detected in K562 cells by polymerase chain reaction on reverse transcripts. The present study demonstrates that K562 cells produce and metabolize PAF and underlines the putative role of erythroid precursors in the modulation of bone marrow PAF concentrations. The effect of PAF on the growth of K562 cells might be mediated through PAF receptors suggesting a potential role of PAF on the proliferation and functions of human erythroid marrow precursors.

Copper-catalyzed oxidation mediates PAF formation in human LDL subspecies. Protective role of PAF:acetylhydrolase in dense LDL

Free radical-mediated oxidation of cholesterol-rich LDL plays a key role in atherogenesis and involves the formation of oxidized phospholipids with proinflammatory biological activity. We evaluated the production of platelet-activating factor (PAF), a potent inflammatory mediator, in human LDL subspecies on copper-initiated oxidation (4 mumol/L CuCl2, 80 micrograms/mL for hours at 37 degrees C). PAF formation was determined by biological assay of HPLC-purified lipid extracts of copper-oxidized lipoproteins; chemical identity was confirmed by gas chromatographic and mass spectrometric analyses. PAF, characterized as the C16:0 molecular species, was preferentially produced in intermediate LDL (d = 1.029 to 1.039 g/mL) (8.6 +/- 5.7 pmol PAF/3 h per mg LDL protein) and light LDL (d = 1.019 to 1.029 g/mL), but was absent from dense LDL particles (d = 1.050 to 1.063 g/mL). As PAF:acetylhydrolase inactivates PAF and oxidized forms of phosphatidylcholine, we evaluated the relationship of lipoprotein-associated PAF:acetylhydrolase to PAF formation. We confirmed that PAF:acetylhydrolase activity was elevated in native, dense LDL (41.5 +/- 9.5 nmol/min per mg protein) but low in LDL subspecies of light and intermediate density (d 1.020 to 1.039 g/mL) (3.5 +/- 1.6 nmol/min per mg protein) [Tselepis et al, Arterioscler Thromb Vasc Biol. 1995;15:1764-1773]. On copper-mediated oxidation for 3 hours at 37 degrees C, dense LDL particles conserved 20 +/- 14% of their initial enzymatic activity; in contrast, PAF:acetylhydrolase activity was abolished in light and intermediate LDL subspecies. Clearly, the elevated PAF:acetylhydrolase activity of dense LDL efficiently diminishes the potential inflammatory role of endogenously formed PAF; nonetheless, formation of proatherogenic lysophospholipids results. In contrast, LDL particles of the light and intermediate subclasses can accumulate PAF on oxidative modification.

Receptor-mediated and protein kinase-dependent growth enhancement of primary human fibroblasts by platelet activating factor

Chronic inflammation is a recognized risk factor for human cancer, but the causal mechanisms are poorly understood. We previously demonstrated that platelet activating factor (PAF) can induce alterations in the in vitro growth properties of primary rat fibroblasts. In the study reported here, exposure of primary human skin fibroblasts to PAF for 1 h in serum-free medium was shown to cause sustained proliferation over 50 d in medium containing low serum and anchorage-independent growth in soft agarose. Both properties could be inhibited by pretreatment with a PAF receptor antagonist, CV3988 (10 microM); a tyrosine-kinase inhibitor, genistein (1 microgram/mL); or a protein kinase C (PKC) inhibitor, staurosporine (50 nM) but not with a cyclooxygenase inhibitor, indomethacin (200 nM-20 microM). PAF had no effect on doubling time, saturation density, or cell viability under normal monolayer growth conditions in complete medium. Treatment with lyso-PAF, an inactive metabolite of PAF, had no effect in either of the assays. Control and PAF-induced cell proliferation in low-serum medium was inhibited by PAF receptor antagonists present during the extended growth period. The presence of PAF receptor mRNA in human skin fibroblasts was demonstrated by reverse transcriptase-polymerase chain reaction. The presence of a functional receptor was indicated by an early (2 min) transient increase in PKC activity and an increase in fos mRNA after PAF treatment. PAF-induced PKC activity was blocked by pretreatment with either staurosporine (50 nM) or CV3988 (1 microM). These results suggest that PAF is a mitogenic factor that contributes to the known increase in risk of malignancy associated with chronic inflammatory conditions.

Possible mechanisms for the differential effects of high linoleate safflower oil and high alpha-linolenate perilla oil diets on platelet-activating factor production by rat polymorphonuclear leukocytes

As compared with high dietary linoleate safflower oil, high dietary alpha-linolenate perilla oil decreased platelet-activating factor (PAF) production by nearly half in calcium ionophore (CaI)-stimulated rat polymorphonuclear leukocytes (PMN). In the CaI-stimulated PMN from the perilla oil group, the accumulated amount of arachidonate (AA) plus eicosapentaenoate (EPA) was 30% less and that of lyso-PAF was 50% less, indicating that the decreased availability of lyso-PAF is a factor contributing to the relatively low PAF production. Consistently, eicosatetraynoic acid (ETYA), a dual inhibitor of cyclooxygenase and lipoxygenase, increased free fatty acids (FFA) and decreased PAF production possibly by decreasing the availability of lyso-PAF. Although, leukotrienes (LTs) have been proposed to stimulate PAF production synergistically, a potent LTB4 receptor antagonist, ONO-4057, decreased the formation of free fatty acids and LTB4, but stimulated PAF production somewhat, indicating that LTB4 may not stimulate PAF production in PMN. Lysophospholipid-induced transacylase (CoA-independent transacylase) activity in PMN homogenates was 25-30% lower in the perilla oil group but no significant differences were observed in the lyso-PAF acetyltransferase and PAF acetylhydrolase activities between the two dietary groups. Thus, decreased transacylase activity is another factor associated with the relatively low PAF production in the perilla oil group.

PAF analogues capable of inhibiting PAF acetylhydrolase activity suppress migration of isolated rat cerebellar granule cells

Intracellular platelet-activating factor (PAF) acetylhydrolase in the bovine brain is a heterotrimeric enzyme composed of alpha1, alpha2 and beta subunits. The trimeric enzyme may be involved in neural cell migration, since the human homolog of the non-catalytic beta subunit is a product of the LIS-1 gene which is a causative gene for Miller-Dieker syndrome. Miller-Dieker syndrome is a form of lissencephaly that is characterized by widespread agyria of the brain and defects of neuronal cell migration. In the present study, we attempted to determine whether the catalytic activity of either the alpha1 or alpha2 subunit is required for the regulation of granule cell migration. Granule cells prepared from rat cerebellum at postnatal day 0 express all three subunit proteins (alpha1, alpha2 and beta) as determined by western blotting. Granule cell migration, which was observed in vitro on a layer coated with laminin, was effectively blocked by PAF analogs which showed PAF receptor-antagonistic activity (CV-6209 and CV-3988) and PAF receptor-agonistic activity (carbamoyl PAF). These PAF analogs also inhibited the activity of bovine brain PAF acetylhydrolase. Cell migration was restored when the inhibitors were removed by washing the treated cells with buffer, indicating that the inhibitory effect of PAF analogs is reversible. Structurally-unrelated PAF antagonists (SM-12502, TCV-309 and YM-264), none of which showed any appreciable inhibitory activity against PAF acetylhydrolase, did not block granule cell migration under the same conditions. It is suggested that the catalytic activity of PAF acetylhydrolase may play a crucial role in neural cell migration.

Platelet activating factor compromises airway epithelial defense functions

BACKGROUND

The mechanism of disruption of the epithelial defense function observed in asthmatic airways is considered to be largely the result of mediators involved in allergic responses. Platelet activating factor (PAF) might be a key mediator involved in this mechanism.

OBJECTIVE

This study was designed to determine whether PAF is capable of compromising the epithelial defense functions, such as tight junctional barriers and the mucociliary system.

METHODS

A total of 120 healthy rabbits were used. Twenty of them were used as normal controls. Eighty rabbits were treated with inhalation of 10 ml of PAF (200 microg/ml), and 20 animals were used for the examination of epithelial defense functions of the trachea at 1, 10, 20, and 30 days after inhalation of PAF. Epithelial defense functions of the trachea were evaluated by ciliary activity, mucociliary transport velocity, epithelial permeability to fluorescein isothiocyanate dextrans (70,000 d), and electron microscopy of the epithelial structure.

RESULTS

PAF inhalation induced a significant decrease in ciliary activity and mucociliary transport velocity, which persisted for up to 20 days. PAF inhalation also caused a significant 7.4-fold increase in epithelial permeability to fluorescein isothiocyanate dextrans at I and 10 days. This increased epithelial permeability returned to the normal level 20 days after PAF inhalation. However, electron microscopy demonstrated no apparent evidence of epithelial shedding.

CONCLUSIONS

PAF-induced prolonged dysfunction of both the epithelial junctional barrier and the mucociliary system may allow enhanced entry of allergen molecules, as well as bronchoactive agonists to the airway parenchyma and may also significantly contribute to an increased airway responsiveness.

[Effect of structural analogs of platelet activating factor on the intracellular signal transduction in murine peritoneal neutrophils and macrophages of P388D1 line]

Direct and modulate effects of platelet activating factor (PAF), its structural analogues and ATP on primary and second processes at peritoneal neutrophils and P388D1 cells activation has been studied. The effect of compounds was evaluated on changes in Ca2+ transport and generation of reactive oxygen species. It was shown, that the synthetic analogues of MS series interact with PAF receptor, mobilize Ca2+ from thapsigargin-dependent intracellular stores and inhibit Ca2+ response on PAF in both types of cells. Unlike PAF the analogues do not induce the formations of reactive oxygen species in neutrophils and inhibit the PMA-induced respiratory burst. The activation of pyrinoreceptor of P388D1 cells by exogenous ATP does not inhibit PAF induced Ca2+ rise in cytoplasm, though partly releases Ca2+ from the same store.

Platelet-activating factor in plasma of patients with sickle cell disease in steady state

The role of platelet-activating factor (PAF) in the pathogenesis of microvascular vaso-occlusion in sickle cell disease (SCD) is not known. In order to assess a role for PAF in vaso-occlusion in patients with SCD in steady state conditions, we measured plasma PAF level and plasma PAF acetylhydrolase activity as indices of PAF metabolism in vivo. We also studied PAF synthesis, from (3H)-acetate, by purified platelets stimulated with A23187. PAF was extracted from plasma of ten patients with SCD in steady state and from age-matched controls. PAF, purified by thin-layer chromatography, was quantitated by radioimmunoassay. PAF level (mean +/- SEM, pg/ml) in plasma of controls was 393 +/- 65, which was significantly lower than the 797 +/- 62 measured in plasma of patients with SCD. There was no difference in acetylhydrolase activity between the two groups. PAF synthesis (mean +/- SEM, nmol/10(6) cells) by platelets of controls without exogenous lyso-PAF was 1.69 +/- 0.24, higher than the 0.59 +/- 0.038 synthesized by platelets of patients with SCD. Incubation of platelets with 1.0 micromol/L lyso-PAF increased PAF synthesis by controls to 8.93 +/- 1.76, still higher than the 4.59 +/- 0.98 synthesized by platelets of patients with SCD. Our data show that patients with SCD are susceptible to a higher circulating levels of PAF in vivo during steady-state conditions. We speculate that higher levels of PAF may be a contributing factor to the persistent stress and inflammatory state of the microcirculation of patients with SCD.

Specific binding sites for 1-O-alkyl-sn-glyceryl-3-phosphorylcholine on intact human blood neutrophils

Infection, inflammation and allergy are characterized by an infiltration of neutrophils or eosinophils in the tissue and are associated with an increased level of lysophospholipids. In this study it is shown that labeled 1-O-alkyl-sn-glyceryl-3-phosphorylcholine (lyso-paf) binds to intact human blood neutrophils. Unlabeled lyso-paf(500 nM) inhibits the binding of [3H]lyso-paf to neutrophils in the presence of fatty acid-free serum albumin (0.25%). A linear Scatchard plot analysis of the specific [3H]lyso-paf binding to neutrophils revealed a KD value of 9.2 nM with 4,100 lyso-paf binding sites per neutrophil at 4 degrees C. Lyso-paf increased the specific binding of labeled platelet-activating factor ([3H]paf) to neutrophils at 20 degrees C. The increased specific binding of labeled paf to neutrophils could only be demonstrated when sterile cell preparation methods were used. Intact human blood eosinophils did not express specific lyso-paf binding sites. These results suggest that the lyso-paf binding sites on neutrophils have an up-regulatory function for paf which might be involved in neutrophil-mediated disorders.