Platelet activating factor (PAF-acether): total synthesis of 1-O-octadecyl 2-O-acetyl sn-glycero-3-phosphoryl choline

Synthesis of ether lipids: natural compounds and analogues

Ether lipids are compounds present in many living organisms including humans that feature an ether bond linkage at the sn-1 position of the glycerol. This class of lipids features singular structural roles and biological functions. Alkyl ether lipids and alkenyl ether lipids (also identified as plasmalogens) correspond to the two sub-classes of naturally occurring ether lipids. In 1979 the discovery of the structure of the platelet-activating factor (PAF) that belongs to the alkyl ether class of lipids increased the interest in these bioactive lipids and further promoted the synthesis of non-natural ether lipids that was initiated in the late 60's with the development of edelfosine (an anticancer drug). More recently, ohmline, a glyco glycero ether lipid that modulates selectively SK3 ion channels and reduces in vivo the occurrence of bone metastases, and other glyco glycero ether also identified as GAEL (glycosylated antitumor ether lipids) that exhibit promising anticancer properties renew the interest in this class of compounds. Indeed, ether lipid represent a new and promising class of compounds featuring the capacity to modulate selectively the activity of some membrane proteins or, for other compounds, feature antiproliferative properties via an original mechanism of action. The increasing interest in studying ether lipids for fundamental and applied researches invited to review the methodologies developed to prepare ether lipids. In this review we focus on the synthetic method used for the preparation of alkyl ether lipids either naturally occurring ether lipids (e.g., PAF) or synthetic derivatives that were developed to study their biological properties. The synthesis of neutral or charged ether lipids are reported with the aim to assemble in this review the most frequently used methodologies to prepare this specific class of compounds.

Chemo-enzymatic synthesis of rac 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine and its analogues

The synthesis of rac 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholines (6a-c), blood platelet activating ether lipid analogues has been achieved in a four-step sequence from epichlorohydrin (1). Etherification of epichlorohydrin with different alcohols namely tetradecyl (2a), hexadecyl (2b) and octadecyl (2c) alcohols gave glycidyl ethers (3a-c) with 78-80% yields. The second step involved opening of the epoxide by acetic anhydride to give acetylated products (4a-c, 76-78% yield), which were subsequently hydrolyzed selectively, a key step of the method employing a 1,3 specific lipase to obtain rac 1-O-alkyl-2- acetylglycerol (5a-c) with 45-50% yields. The hydrolyzed products (5a-c) were phosphorylated to obtain rac 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholines (6a-c) in 80-85% yields.

Asthma: a simple concept but in reality a complex disease

BACKGROUND

Asthma is a disorder of the conducting airways that contract too easily and too much to cause variable airflow obstruction with symptoms of wheeze, cough, chest tightness and shortness of breath. Based on this knowledge, initial treatments were directed to dilating the contracted airways with anticholinergic and adrenergic drugs. The recognition that allergic-type inflammation underlay the hyperresponsive airways in asthma led to the introduction of anti-inflammatory drugs such as sodium cromoglicate and corticosteroids. Over the 2 decades that followed, these drugs have been progressively improved by increasing their therapeutic index and duration of action.

METHODS

A review of the recent literature indicates that since the 1980s, the explosive increase in knowledge of the cell and mediator mechanisms of asthma has only led to modest improvements in therapy including the introduction of leukotriene modifiers and a blocking monoclonal antibody against IgE. Indeed, biologics targeting allergic cytokines and effector cells have on the whole proven disappointing despite initial promise being shown in animal models.

RESULTS

Part of the difficulty lies in the oversimplified concept that asthma is only driven by allergic processes when in reality there are many environmental causes and triggers and the view that it is a homogeneous disorder only varying in severity.

CONCLUSIONS

The more recent views that asthma is a complex disorder made up of different subtypes with differing causes, treatment responses and natural histories creates a new opportunity for stratified medicine in which therapies acting upstream selectively target specific disease subtypes identified by specific diagnostic biomarkers.

Comparison of weal and flare responses to platelet activating factor (PAF) and histamine, and the ultrastructural effects of PAF in the skin of atopic and normal subjects

Weal volume and flare area responses to intradermal platelet activating factor (PAF) and histamine, both in the presence and absence of the protein carrier, human serum albumin (HSA), were measured in the skin of normal subjects, atopics without dermatitis, and in the normal-appearing skin of patients with atopic dermatitis. The ultrastructural effects of intradermal PAF were also studied in the same groups of subjects. Weal and flare dose-responses to PAF were enhanced by the presence of human serum albumin, HSA itself producing a significantly greater mean weal response in each group than lyso-PAF (P < 0.01). However, comparison of normal subjects, atopics without dermatitis and patients with atopic dermatitis, showed the differences in dose-response effects of PAF and histamine on weal volume and flare area to be nonsignificant. The cellular response to PAF in normal subjects and atopics consisted predominantly of neutrophil extravasation; eosinophils were noted in the normal-appearing skin of atopics and patients with atopic dermatitis 30 min and 4 h after injection. At an ultrastructural level PAF induced opening of endothelial gaps and extravasation of neutrophils in each group.

The effect of intravenous administration of WEB 2086 on PAF-induced platelet aggregation in healthy Friesian calves

The in vivo ability of the specific PAF-antagonist WEB 2086, a thienotriazolodiazepine, to inhibit platelet-activating factor (PAF) in cattle was investigated by in vitro determination of platelet aggregation curves. WEB 2086 was infused intravenously into a group of 5 healthy male Friesian calves in a dose of 3 mg/kg over 1 min. The resultant inhibition peaked between 30 min and 1 h after administration of WEB 2086. The inhibition was significantly reduced after 3 h and became non-significant after 6 h, but maximal pre-treatment aggregation had not been restored by 24 h after the injection of WEB 2086. These results confirm previous results obtained in vitro and suggest that WEB 2086 is a potent antagonist of PAF activity in calves. They also suggest that further clinical studies with WEB 2086 in cattle are desirable.

Inhibition of PAF-induced platelet aggregation by WEB 2086 'in-vitro', an antagonist to the receptor for platelet-activating factor, in bovine

The sensitivity of bovine platelet aggregation in response to PAF stimulation and the ability of WEB 2086 (a thieno-triazolodiazepine) to inhibit response to PAF-induced platelet aggregation were investigated in the blood from five healthy male Belgian Blue calves. The recorded response to PAF showed a plateau which was dependent on the PAF concentration. Platelet aggregation induced by PAF consists of two mechanisms: reversible and irreversible aggregations which are accompanied by the release of platelet granule contents. Reversible aggregation occurred above (2 . 10(-9) mol/l) PAF, and irreversible aggregation occurred above (2 . 10(-7) mol/l) PAF. Addition of WEB 2086 to bovine platelets in vitro induced a rightward shift in the dose-response curve to PAF. WEB 2086 inhibited PAF-induced aggregation in a competitive reversible manner (pA2 = 7.61). The results of our study show that PAF induces platelet aggregation in platelet-rich plasma (PRP) and that addition of WEB 2086 to bovine platelets in vitro inhibits PAF-induced Platelet Aggregation.

Interdigitation of bilayers from ether lipid analogs: (R)-PAF, (R)-Lyso-PAF and the antineoplastic (R)-ET-18-OMe

The ether lipid analogs (R)-PAF, (R)-ET-18-OMe and (R)-Lyso-PAF can form bilayers. Using differential scanning calorimetry (DSC) and small angle X-ray diffraction, we have investigated bilayers formed entirely of these molecules. Small angle X-ray diffraction experiments strongly indicated that these bilayers are interdigitated at 98% relative humidity in the gel phase. The slight differences in the structures of the ether lipids affect their thermal behavior and packing properties. Of the three, (R)-Lyso-PAF shows the highest phase transition temperature, broadest phase transition peak and smallest enthalpy change. These characteristics may be attributed to hydrogen bonding of the -OH group at sn-2 position of its glycerol backbone to water or to other (R)-Lyso-PAF molecules. The interdigitated structure shows that these bilayers are similar to lipid bilayers and suggests that these molecules could easily insert themselves into membranes to reach their respective active sites or to perturb the membranes. The packing and thermal characteristics of (R)-Lyso-PAF may help explain its biological inactivity.

On the synthesis of platelet-activating factor via acetylation of 1-alkyl-sn-glycero-3-phosphocholine. Formation of structural isomer of PAF in the presence of bases

It has been shown that platelet-activating factor (PAF) specimens prepared via acetylation of 1-alkyl-sn-glycero-3-phosphocholine (lyso-PF) with acetic anhydride are heterogeneous. The contaminated compound was isolated and identified to be the structural isomer of PAF, 1-alkyl-3-acetyl-sn-glycero-2-phosphocholine (iso-PAF). It appeared, that iso-PAF is formed when performing the reaction in the presence of organic bases,but not under acid catalysis. The mechanism of iso-PAF formation is discussed.

PAF-receptor. III. Conformational and electronic properties of PAF-like agonists and antagonists

In order to compare electronic and conformational properties of PAF-agonists and PAF-antagonists, 14 analogues structurally related to PAF were studied. A common conformation of the glycerol backbone was present in all agonists and all constrained or flexible antagonists. The distinction between agonists and antagonists appears to be casted on position-2 where the folded conformation of the substituent for agonists should be the most probable. In position-3 the gauche conformation can be adopted by all the analysed compounds. The electrostatic potential well at -30 kcal/mol stretches to the carbonyl group in position-2 in the folded conformation of the agonists. On the contrary, in constrained antagonists, a second negative zone appears around the carbamate group. Given the modelling results, the triethylammonium PAF analogue considered in literature as a weak agonist, was resynthesized and proved to be more potent than previously reported. These experimental results confirm our hypothesis in terms of a common conformation of agonist and antagonist PAF-like molecules.

Lyso platelet activating factor (LysoPAF) and its enantiomer. Total synthesis and carbon-13 NMR spectroscopy

Described is a reaction sequence for the total synthesis of lyso platelet activating factor (lysoPAF; 1-O-alkyl-sn-glycero-3-phosphocholine) and its enantiomer. The procedure is versatile and yields optically pure isomers of defined chain length. The synthesis is equally suited for the preparation of lysoPAF analogues and its enantiomers with unsaturation in the long aliphatic chain. First, rac-1(3)-O-alkylglycerol is prepared by alkylation of rac-isopropylideneglycerol with alkyl methanesulfonate followed by acid-catalyzed removal of the ketal group. The primary hydroxy group of alkylglycerol is then protected by tritylation, the secondary hydroxy group is acylated, and the protective trityl group is removed under mild acidic conditions with boric acid on silicic acid, essentially without acyl migration. Condensation of the diradylglycerol with bromoethyl dichlorophosphate in diethyl ether, hydrolysis of the resulting chloride, and nucleophilic displacement of the bromine with trimethylamine gives rac-1-O-alkyl-2-acylglycero-3-phosphocholine in good overall yield. The racemic alkylacylglycerophosphocholine is finally treated with snake venom phospholipase A2 (Ophiophagus hannah) which affords 1-O-alkyl-sn-glycero-3-phosphocholine (lysoPAF) of natural configuration in optically pure form. The "unnatural" 3-O-alkyl-2-O-acyl-sn-glycero-1-phosphocholine enantiomer, which is not susceptible to phospholipase A2 cleavage, gives 3-O-alkyl-sn-glycero-1-phosphocholine upon deacylation with methanolic sodium hydroxide. Homogeneity and structure of the intermediates and final products were ascertained by carbon-13 nuclear magnetic resonance spectroscopy on monomeric solutions.

A facile synthesis of an aldehydic analog of platelet activating factor and its use in the production of specific antibodies

The multistep synthesis of a platelet activating factor (PAF) analog having a reactive aldehyde group at the omega-end of the sn-1 position is described. A novel ozonolysis of a double bond was employed to generate the aldehyde group in high yield under mild conditions. The aldehyde group was generated at the last step of the synthesis to avoid any reactions of protection and deprotection. The natural chiral center at the sn-2 position was introduced at the first step so that no steric resolution of the final product was needed. This analog of PAF was conjugated to thyroglobulin via reductive amination and then used to immunize rabbits for production of specific antibodies. The purified antibodies bind stereospecifically to tritiated PAF and crossreact minimally with lyso-PAF, plasmalogens and other phospholipids. The solid-phase radioimmunoassay thus developed detects as low as 20 pg of PAF per assay tube and should be applicable to the quantitation of PAF in biological systems.

An asymmetric synthesis of (R) and (S)-1-alkoxy-2,3-propanediols including precursors to platelet activating factor

The titanium-assisted nucleophilic opening of glycidol with primary aliphatic alcohols gives 1-alkoxy-2,3-propanediols. The titanium alkoxide used in the reaction should be the alkoxide of the alcohol used for the reaction. When optically active (S)-glycidol is used in the reaction, (S)-1-alkoxy-2,3-propanediols are obtained without loss of optical activity. When the reaction is carried out at 70-75 degrees C without solvent, the 1-alkoxy-2,3-propanediols are obtained in yields of 45-59%. The regioisomeric 2-alkoxy-1,3-propanediols are found to the extent of 4-6% in the reaction. The optical purity of glycidol can be measured from the high field (500 MHz) nuclear magnetic resonance spectrum of the Mosher ester.

Alkyl analogs of diacylglycerol as activators of protein kinase C

Diacylglycerols which activate protein kinase C have the 1,2-sn configuration. Short-chain saturated fatty acids or long-chain unsaturated fatty acids are required for supporting the potency of these lipids. Using alkyl analogs such as 1-O-decyl-2-O-decanoylglycerol, 1-O-decanoyl-2-O-decylglycerol, 1,2-O-didecylglycerol, 1-O-hexadecyl-2-O-acetylglycerol and 1-O-decyl-2-O-acetylglycerol, we showed that the ether bond was consistently associated with a loss of activity to varying extents. The results suggest that the ester bond in the 1-position is a major determinant of diacylglycerol-mediated protein kinase activation.

In vitro and in vivo effects of treatment by platelet-activating factor on N-formyl-met-leu-phe-mediated responses of polymorphonuclear leucocytes

Two chemoattractants, the peptide N-formyl-met-leu-phe (FMLP), and the ether phospholipid, platelet activating factor (PAF), each stimulate a variety of in vitro responses in polymorphonuclear leucocytes (PMN). Because often more than one inflammatory mediator is active during inflammation, we determined the effect on PMN of sequential stimulation with these two agents. Before FMLP stimulation, human PMN were exposed to PAF, at concentrations which gave little or no response when administered alone. PAF enhanced FMLP-elicited superoxide release in a dose-dependent fashion. Likewise, release of granular lysozyme from the cells was increased in PAF treated cells. Similar treatment with other phospholipids, including the lyso derivation of PAF, failed to produced these effects. Incubation with nordihydroguaiaretic acid, an inhibitor of arachidonic acid metabolism, had little effect on the enhancement of lysozyme release by PAF. To determine if enhancing effects by PAF might occur also in vivo, we studied rabbits receiving PAF and/or FMLP intravenously. When rabbits received 0.01 micrograms PAF (a dose which does not elicit the sustained neutropenia observed with higher doses of PAF) followed by 0.05 micrograms FMLP the absolute granulocyte count (AGC) dropped at 1 min (46 +/- 11% of original value), and continued to fall (24 +/- 12% at 10 min). Controls, treated with the suspending fluid for PAF, and then 0.05 micrograms FMLP, had a similar 1 min AGC value, but at 10 min AGC returned to 65 +/- 6.1% (P less than 0.001 for comparison of 10 min values). Thus PAF pretreatment enhanced FMLP-elicited granulocytopenia in vivo. Study of in vitro human PMN aggregation revealed that, at certain relative concentrations of PAF and FMLP, aggregation was enhanced. These studies show that both in vitro and in vivo responses of FMLP-stimulated PMN may be exaggerated by pre-exposure to PAF.

Biochemical characterization of acetyl-CoA:1-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase in rat spleen microsomes

Acetyl-CoA:1-alkyl-2-lyso-sn-glycero-3-phosphocholine (lyso-PAF) ultrasonic disruption in the presence of 25% glycerol from rat spleen microsomes. About 26% of the enzymatic activity was recovered in the 225,000 X g supernatant by this treatment, although the specific activity was slightly decreased compared with the original microsomes. The solubilized enzyme was remarkably susceptible to various kinds of metal ions. Sulfhydryl reagents such as p-chloromercuribenzoate and N-ethyl-maleimide significantly inhibited the enzyme reaction, suggesting that the enzyme is an SH enzyme. Based on the sedimentation pattern in sucrose density centrifugation, the isoelectric point, the kinetic characteristics and the sensitivity to tryptic digestion of microsomes, it appears that acetyl-CoA:lyso-PAF acetyltransferase does not differ from the acetyltransferase responsible for the transfer of acetate from acetyl-CoA to 1-acyl-2-lyso-sn-glycero-3-phosphocholine.

A phospholipase C with a high specificity for platelet-activating factor in rabbit liver light mitochondria

The light mitochondrial fraction from rabbit liver was found to catalyze the hydrolysis of platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) by the phospholipase C reaction to form 1-O-alkyl-2-acetyl-glycerol and phosphocholine. The highest specific phospholipase C activity occurred in the liver and kidney. A subcellular survey showed that the enzyme was of lysosomal origin. The enzyme was solubilized with 2% Triton X-100 from rabbit liver light mitochondria and purified ca. 600- to 700-fold with a 17% yield using procedures that included hydroxyapatite, Sepharose 4B and isoelectric focusing column chromatography followed by fast protein liquid chromatography. The enzyme consists of two forms having a pl of 4.7 and 5.8. Each form was purified to a homogeneous state as judged by sodium dodecyl sulfate-polyacrylamide disc gel electrophoresis. The enzyme migrated to positions corresponding to apparent molecular weights of 33,000 and 75,000, respectively. The purified enzymes of pl 4.7 and 5.8 had pH optima of 8.2 and 8.5 and apparent Km values of 55.6 and 45.5 microM for PAF, respectively. Furthermore, their phospholipase C activity was significantly inhibited by the addition of 1 mM EDTA. EDTA-inactivated enzyme, however, recovered completely upon addition of Ca2+ to the original level. p-Chloromercuribenzoate markedly inhibited enzyme activity, suggesting that phospholipase C is a -SH enzyme. The physiological role of the enzyme should be evaluated, considering its specificity for a highly potent, biologically active ether-phospholipid.

Inhibition and reversal of endotoxin-, aggregated IgG- and paf-induced hypotension in the rat by SRI 63-072, a paf receptor antagonist

Platelet activating factor (paf) given intravenously produces systemic hypotension in the rat. Similar effects can be induced using endotoxin or heat-aggregated IgG challenges, which are thought to involve endogenous paf release. Extending this concept, we have examined the ability of the paf antagonist SRI 63-072 to inhibit or reverse systemic hypotension induced with paf, heat-aggregated IgG or endotoxin 0111-B4 in rats. At 100 ng kg-1 paf, there occurred a 38.6 +/- 5.1% decrease in carotid mean arterial pressure (MAP) followed by a 3.2 +/- 0.7 min recovery period (RP) to return to normal pressure values. The ED50 of SRI 63-072 was 0.16 mg kg-1 i.v. (MAP) and 0.25 mg kg-1 (RP) when given 1-5 min before the paf challenge. Endotoxin (15 mg kg-1 i.v.) produced a hypotensive response (54 +/- 8% decrease in MAP) and a corresponding 80% decrease in mesenteric artery blood flow. When given 2-8 min after endotoxin, 1.0 mg kg-1 i.v. SRI 63-072 totally restored blood pressure and artery blood flow. SRI 63-072 similarly reversed heat-aggregated IgG (10 mg kg-1) induced reduction of MAP, with an ED50 of 0.05 mg kg-1 i.v. The observations that SRI 63-072 can inhibit or reverse systemic vascular effects produced from paf and other provocators of endogenous paf release strongly implicates paf as a common final mediator of hypotension and shock. As SRI 63-072 is a competitive receptor antagonist, the hypotensive effects of these provocators appear to be mediated by vascular receptors for paf.

Vascular responses of platelet-activating factor in the Cebus apella primate and inhibitory profiles of antagonists SRI 63-072 and SRI 63-119

We have evaluated several effects of intravenous administration of synthetic platelet-activating factor (PAF) in the non-human primate Cebus apella. Parameters measured were hemoconcentration (monitored by changes in hematocrit), thrombocytopenia (platelet counts), leukopenia (loss of buffy coat), bronchoconstriction (increased airway resistance to fixed airway ventilation), thromboxane A2 production (radioimmunoassay to thromboxane B2) and in vitro aggregation responses of platelets in platelet-rich plasma. Cebus platelets were refractory to PAF-induced aggregation (up to 50 microM) and there was no evidence of thrombocytopenia, elevated thromboxane B2 levels, loss of buffy coat or bronchoconstriction following systemic PAF injection. Animals exhibited reproducible but varying sensitivities to PAF-induced hemoconcentration, where 3.5-30 micrograms/kg PAF (6.6-57 nmol/kg) was required to produce 28-32% increased hematocrit range for the colony. Hemoconcentration induced by PAF in baboons and rhesus occurred at similar doses, suggesting comparable sensitivity. Prior administration of PAF receptor antagonists SRI 63-072 or SRI 63-119 at 3 mg/kg inhibited cebus hemoconcentration responses to 3.5 micrograms/kg PAF by 96% and 100%, respectively. The ED50 values were 0.95 and 0.60 mg/kg, respectively. These results suggest that the cebus exhibits a reproducible hemoconcentration effect to PAF and that these vascular responses can be inhibited by a PAF receptor antagonist.

Transient activation of the acetyltransferase necessary for paf-acether biosynthesis in thrombin-activated platelets

Thrombin-activated platelets formed paf-acether (1-0-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine), a molecule susceptible to play a role in haemostasis and thrombosis, and its deacetylated analogue, lyso paf-acether, a biologically inactive molecule. We presently show the presence in human and rabbit platelet lysates of an acetyltransferase which transfers the acetyl moiety of acetyl-coenzyme A (acetyl-CoA) onto synthetic lyso paf-acether, yielding the fully active paf-acether molecule. Under our optimal standard conditions, 0.36 +/- 0.23 nmol paf-acether/10 min/mg proteins was formed by the acetyltransferase from resting human platelets. Upon thrombin stimulation, the acetyltransferase activity doubled within 30 s, reaching a maximum at 2 min (1.17 +/- 0.31 nmol paf-acether/10 min/mg proteins) and decreased progressively. Similar results were obtained using rabbit platelets. In addition we demonstrated that the activation and deactivation of the acetyltransferase correlated with the kinetics of paf-acether formation by thrombin-activated rabbit platelets. It is hypothesized that this enzyme may represent one of the regulating mechanism of paf-acether formation by platelets.

Evidence for distinct systemic extravasation effects of platelet activating factor, leukotrienes B4, C4, D4 and histamine in the guinea pig

The relative potencies of platelet-activating factor (PAF), leukotrienes B4 (LTB4), C4 (LTC4), D4 (LTD4) and histamine to induce hemoconcentration (HC) were evaluated in the guinea pig. The maximal hematocrit increase (MHI) from PAF, LTD4 and histamine occurred 5-7 min after i.v. injection, whereas the MHI of LTC4 occurred 13-15 min after injection. LTB4 (2.97-5.95 nmol kg-1) did not produce HC. The magnitude of PAF-induced MHI was 2-fold that of LTC4 or LTD4, regardless of the dose of leukotrienes used. The doses (nmol kg-1) needed to produce 30% HC were: 0.14-PAF, 0.71-LTD4 and 3.37-LTC4 and 2,400 histamine. The HC effects of LTD4 were markedly reduced by prior administration of FPL-55712. However, neither LTC4 or LTD4 HC effects were significantly reduced by prior i.v. injection of CV-3988 (3.4 mg kg-1), a competitive receptor antagonist to PAF which is 98% effective in abolishing HC response to 0.14 nmol kg-1 PAF. Diphenhydramine abolished histamine-induced hemoconcentration but was without effect on PAF or LTD4. These results suggest that the responses of PAF, leukotrienes and histamine differ in their potency and may involve separate vascular recognition sites related to acute increases in vascular permeability.

A role for PAF-acether (platelet-activating factor) in platelet-dependent vascular diseases?

Platelets-isolated or in conjunction with leukocytes-interact with vessel walls in many experimental and human diseases. Several mediators are held responsible for platelet activation and interaction with leukocytes, among which PAF-acether (platelet-activating factor) is a prime candidate. This phospholipid mediator is released by most inflammatory cells, including neutrophils, by isolated organs such as kidney and heart, is a potent platelet and neutrophil agonist, and exerts major vasoactive properties. Its biosynthesis involves a two-step enzymatic process yielding the active molecule from the membrane alkyl-ether choline-containing phospholipids. The first step implicates a phospholipase A2 that hydrolyzes a long-chain fatty acid (which can be arachidonic acid) from membrane phospholipids, leaving the intermediate compound lyso PAF-acether, a PAF-acether precursor that is acetylated by an acetyltransferase in a second step. It can also result from deacetylation of PAF-acether by an acetylhydrolase. PAF-acether release might explain the intervention of platelets in diseases such as glomerulonephritis and allergic vasculitis, in which the involvement of neutrophils and platelets is frequently noted. The end result of these complex sets of cell-to-cell interactions is the release of most known inflammatory mediators, influencing vascular permeability, cell infiltration, and smooth muscle contraction. Nevertheless, direct evidence for the implication of these rather well-defined cellular and molecular interactions in human pathologic states remains to be obtained.

Effects of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet activating factor) on cardiac function in perfused guinea-pig heart

The direct cardiac action of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (PAF) was studied in isolated perfused guinea-pig heart preparations. PAF produced a fall in left ventricular pressure, decreases in the rate of rise of the left ventricular pressure (dp/dt) and coronary flow, but had no effect on heart rate. These results indicate that PAF is a cardiodepressant with inotropic selectivity and this effect on heart is blocked by CV-3988, a specific PAF antagonist.

Hypotensive mechanism of acetyl glyceryl ether phosphorylcholine (AGEPC) in dogs. Effects on hemodynamics and humoral factors

One-O-hexadecyl-2-O-acetyl-sn-glycero-3-phosphorylcholine (AGEPC) was intravenously administered to anesthetized dogs to study the effects on hemodynamics and several endocrine factors. The effect of AGEPC on local blood flow was also studied by direct intra-arterial injection. Following intravenous injection, blood pressure and cardiac output decreased significantly (p less than 0.001). Changes in total peripheral resistance (TPR) and heart rate were biphasic. TPR increased significantly (p less than 0.01) after an initial slight reduction. Heart rate decreased significantly (p less than 0.01) with only a transient slight elevation. Femoral blood flow was increased (p less than 0.001) by intraarterial injection and decreased (p less than 0.05) by intravenous administration. Plasma norepinephrine (p less than 0.001), epinephrine (p less than 0.01), thromboxane B2 (p less than 0.001), 6-0-PGF1 alpha (p less than 0.01), aldosterone (p less than 0.001) and cortisol (p less than 0.001) were elevated, but plasma renin activity did not change. These results suggest that the hypotensive mechanism of AGEPC is due to both cardiosuppression and vasodilation. AGEPC increased plasma catecholamines, thromboxane A2, PGI2, aldosterone and cortisol which, in turn, may modify hemodynamics.

Synthesis and characterization of a radioiodinated, photoreactive and physiologically active analogue of platelet activating factor

The multistep synthesis of a photoreactive, radioactive and aggregating analogue of platelet-activating factor (PAF)-acether is described. The photoreactive and radioactive moiety was added at the last step; the specific radioactivity was higher than 1000 Ci/mmol. The concentration of this new analogue which causes 50% of aggregation of platelets were of the same order of magnitude as for synthetic snPAF-acether, so as for two other analogues having a bulky group at the omega end of the fatty ether chain. The photoreactivity was proved by the covalent binding of the analogue to protein (BSA) after 10-min irradiation times at 300 nm. The binding was largely prevented by prior (not by later) addition of a high concentration of lyso phosphatidyl choline.

Accumulation of inflammatory cells in response to intracutaneous platelet activating factor (Paf-acether) in man

Platelet activating factor (Paf-acether, AGEPC) is a family of ether-linked phospholipids known to be released from a range of inflammatory cell types. In vitro and in experimental animals, it seems to be a mediator of inflammation, and intradermal injection of Paf-acether in man elicits a biphasic inflammatory response, reminiscent of the dual response to allergen in sensitized individuals. In the present study, cutaneous histology was assessed in sequential skin biopsies from six normal volunteers after intradermal injection of 200 or 800 pmol Paf-acether. Paf-acether (200 pmol) induced intravascular accumulation of neutrophils, accompanied by a perivascular mixed cellular infiltrate which was composed predominantly of neutrophils at 4 and 12 hours, and lymphocytes and histiocytes at 24 hours. Control injections of lyso-Paf and normal saline induced no noteworthy histological changes. Paf-acether (800 pmol) resulted in vessel destruction, gross endothelial swelling and a perivascular infiltrate of mononuclear cells and neutrophils, accompanied by occasional evidence of leucocytoclasis. By virtue of its ability to induce inflammatory cell accumulation in human skin, Paf-acether should be considered as a potential mediator of inflammatory disorders such as psoriasis.

Desaggregation of PAF-acether-aggregated platelets by verapamil and TMB-8 with reversal of phosphorylation of 40K and 20K proteins

Verapamil at a concentration of 10(-4) M inhibited aggregation and release of [3H]5HT induced by platelet activating factor (PAF-acether, PAF) in rabbit platelet-rich plasma and washed labelled platelets. When added to platelets previously aggregated by PAF-acether verapamil caused them to desaggregate at doses as low as 2 X 10(-6) M. The desaggregated platelets were refractory to further additions of similar doses of PAF-acether but could further be aggregated by A23187. Simultaneous to full aggregation PAF-acether caused phosphorylation of 40K and 20K proteins in particular. Addition of verapamil at the concentration of 2 X 10(-6) M to platelets already aggregated by PAF-acether resulted in dephosphorylation of 40K protein and reduction of phosphorylation of 20K protein to the level of control parallel to desaggregation. TMB-8 (10(-3) M) also caused desaggregation and reversal of phosphorylation of 40K and 20K proteins. When A23187 was added to verapamil desaggregated platelets, 40K and 20K proteins were rephosphorylated. The extracellular calcium antagonists EGTA or La3+, when added to PAF-acether aggregated platelets, did not abolish the phosphorylation of 40K and 20K proteins. The experiments suggest that inhibition of intracellular calcium-dependent reactions is involved in the desaggregatory action of verapamil.

Formation of prostaglandins, leukotrienes and paf-acether by macrophages

Prostaglandins (PG) and leukotrienes (LT)--arachidonic acid-dependent metabolites--and paf-acether (platelet-activating factor)--an ether phospholipid--are potent mediators of allergic and inflammatory reactions. Their structures, chemical synthesis and biosynthetic pathways have been recently described. These mediators are produced by various cells with proinflammatory activities including the macrophages upon interaction with a specific secretagogue stimulus (phagocytosis of zymosan particles, immune-complexes); in IgE-dependent hypersensitivity reactions; upon interaction with one of these mediators. Formation of these mediators by macrophages depends upon their local environment. Qualitative and/or quantitative variations in their synthesis are observed depending on the tissue they are derived from (alveole or peritoneum) and on the type of inflammation (immunologic specific or not). Their potent biological activities (increase of vascular permeability, smooth muscle contraction, cardiac and vascular effects and/or chemotactism) suggest a role for these mediators in various pathologies.

Cutaneous and pulmonary histopathological responses to platelet activating factor (Paf-acether) in the guinea-pig

The effect of synthetic Paf-acether has been studied in guinea-pig skin, following intradermal injection, and in guinea-pig lung, following intravenous administration. Histopathological responses to Paf-acether were assessed by both light microscopy and electron microscopy. In addition, plasma protein extravasation and platelet accumulation were quantitatively assessed using radiolabelling techniques. Intradermal injection of Paf-acether, but not lyso-Paf, elicited acute increased vascular permeability, accompanied by intravascular accumulation of platelets and neutrophils. There was evidence, 2-8 h after intradermal injection of Paf-acether, of perivascular infiltration with neutrophils. At 24 h there was a mixed cellular infiltrate comprising mononuclear cells in addition to neutrophils. Following systemic administration of Paf-acether, aggregates of platelets in close association with neutrophils were evident within the pulmonary vasculature. Intravenous injection of Paf-acether, but not lyso-Paf, caused intrathoracic accumulation of radiolabelled platelets. These results suggest that Paf-acether has properties consistent with those of a mediator of inflammation.

Paf-acether generates chemiluminescence in human neutrophils in the absence of cytochalasin B

In the absence of cytochalasin B, synthetic Paf-acether (0.1-10 microM) induced oxygen radical production in polymorphonuclear neutrophils as measured by the luminol-dependent chemiluminescence ( LDCL ) test. This effect was observed after a lag period of 10 s and was maximal between 5 and 15 min. In the presence of cytochalasin B, the kinetics were shortened, but the lag period was not modified and the same concentrations of the agonist had to be used to induce LDCL . None of the structural analogs tested (2-lyso Paf-acether, Paf-acether enantiomer, 1 ester analog of Paf-acether, lyso-phosphatidylcholine) were active, irrespective of the presence of cytochalasin B. Paf-acether (10 microM) shortened the kinetics of opsonized zymosan (10 micrograms/ml)-induced LDCL and enhanced it by 550% and 250% at 5 min and 10 min respectively, without affecting the peak value. Similar results were obtained using non-opsonized zymosan (100 micrograms/ml). Lower concentrations of Paf-acether (0.1 microM) were also able to increase oxygen radical production induced by low doses of zymosan and opsonized zymosan. The triggering and enhancing effects of Paf-acether on oxygen radical production by resting and stimulated polymorphonuclear neutrophils support the role of Paf-acether in inflammation.