Release of 1-O-alkylglyceryl 3-phosphorylcholine, O-deacetyl platelet-activating factor, from leukocytes: chemical ionization mass spectrometry of phospholipids

Forty Years Since the Structural Elucidation of Platelet-Activating Factor (PAF): Historical, Current, and Future Research Perspectives

In the late 1960s, Barbaro and Zvaifler described a substance that caused antigen induced histamine release from rabbit platelets producing antibodies in passive cutaneous anaphylaxis. Henson described a ‘soluble factor’ released from leukocytes that induced vasoactive amine release in platelets. Later observations by Siraganuan and Osler observed the existence of a diluted substance that had the capacity to cause platelet activation. In 1972, the term platelet-activating factor (PAF) was coined by Benveniste, Henson, and Cochrane. The structure of PAF was later elucidated by Demopoulos, Pinckard, and Hanahan in 1979. These studies introduced the research world to PAF, which is now recognised as a potent phospholipid mediator. Since its introduction to the literature, research on PAF has grown due to interest in its vital cell signalling functions and more sinisterly its role as a pro-inflammatory molecule in several chronic diseases including cardiovascular disease and cancer. As it is forty years since the structural elucidation of PAF, the aim of this review is to provide a historical account of the discovery of PAF and to provide a general overview of current and future perspectives on PAF research in physiology and pathophysiology.

Forty five years with membrane phospholipids, phospholipases and lipid mediators: A historical perspective

Phospholipases play a key role in the metabolism of phospholipids and in cell signaling. They are also a very useful tool to explore phospholipid structure and metabolism as well as membrane organization. They are at the center of this review, covering a period starting in 1971 and focused on a number of subjects in which my colleagues and I have been involved. Those include determination of phospholipid asymmetry in the blood platelet membrane, biosynthesis of lysophosphatidic acid, biochemistry of platelet-activating factor, first attempts to define the role of phosphoinositides in cell signaling, and identification of novel digestive (phospho)lipases such as pancreatic lipase-related protein 2 (PLRP2) or phospholipase B. Besides recalling some of our contributions to those various fields, this review makes an appraisal of the impressive and often unexpected evolution of those various aspects of membrane phospholipids and lipid mediators. It is also the occasion to propose some new working hypotheses.

Coupling PAF signaling to dynein regulation: structure of LIS1 in complex with PAF-acetylhydrolase

Mutations in the LIS1 gene cause lissencephaly, a human neuronal migration disorder. LIS1 binds dynein and the dynein-associated proteins Nde1 (formerly known as NudE), Ndel1 (formerly known as NUDEL), and CLIP-170, as well as the catalytic alpha dimers of brain cytosolic platelet activating factor acetylhydrolase (PAF-AH). The mechanism coupling the two diverse regulatory pathways remains unknown. We report the structure of LIS1 in complex with the alpha2/alpha2 PAF-AH homodimer. One LIS1 homodimer binds symmetrically to one alpha2/alpha2 homodimer via the highly conserved top faces of the LIS1 beta propellers. The same surface of LIS1 contains sites of mutations causing lissencephaly and overlaps with a putative dynein binding surface. Ndel1 competes with the alpha2/alpha2 homodimer for LIS1, but the interaction is complex and requires both the N- and C-terminal domains of LIS1. Our data suggest that the LIS1 molecule undergoes major conformational rearrangement when switching from a complex with the acetylhydrolase to the one with Ndel1.

Structural characterization of phosphatidylcholines by atmospheric pressure photoionization mass spectrometry

The potential of atmospheric pressure photoionization was investigated for the structural analysis of phosphatidylcholine lipids (PCs). [M+H]+ ions of high abundance were obtained, along with several fragment ions. Three of these dissociation products corresponded to quite unusual fragmentation pathways but allowed the determination of both the nature and the position on the glycerol backbone (sn-1 or sn-2) of the fatty acyl chains. The loss of a methyl group from the choline head was also observed. These results suggest a complex ionization mechanism in APPI. However, this method proved to be very powerful for the rapid structural analysis of PC species without using MS/MS experiments.

Platelet-activating factor antagonism: a new concept in the management of regional myocardial ischemia-reperfusion injury

Reperfusion therapies for treatment of myocardial infarction successfully reduce patient mortality; however, regional myocardial ischemia-reperfusion (RMIR) causes its own expression of cardiovascular dysfunction, including myocardial depression, hemodynamic instability, and dysrhythmias, which have increased patient mortality within the first 24 h after starting reperfusion therapy. Current evidence suggests that the release of oxygen-derived reactive substances and subsequent inflammatory mediators during ischemia-reperfusion contribute toward this injury. Platelet-activating factor (PAF), a mediator released during RMIR, has been emphasized by many investigators as playing a central role in causing RMIR injury. Similar cardiovascular dysfunctions that occur during RMIR, including myocardial depression, hemodynamic instability, and dysrhythmias, occur after administration of PAF and are ameliorated with PAF antagonists. Further, PAF antagonists have been shown to be cardioprotective and improve survival when administered before onset of reperfusion. A variety of phospholipid analogues, naturally derived compounds, and synthetic compounds have been developed that form the different classes of PAF antagonists, each with unique antagonizing properties. Several of these compounds have successfully passed safety and efficacy testing in humans; however, to date, no clinical trials have investigated the protective effects of PAF antagonists against RMIR injury. A current theory in the pathogenesis of RMIR injury considers the ischemic and necrotic portion of the myocardium and regional dysfunction due to tissue necrosis to be solely responsible for global cardiac dysfunction leading to hemodynamic instability and death. Evidence now suggests, however, that the global dysfunction is also due to the effect of inflammatory mediators such as PAF, thromboxanes, leukotrienes, and endothelins that are released during RMIR and are distributed throughout the heart on reperfusion. Antagonizing a central inflammatory mediator such as PAF, as adjunct treatment with currently used reperfusion therapies, improves cardiovascular function and survival in animals and should be introduced into clinical trials to investigate if similar protective effects can be provided in humans.

Treatment of carrageenan induced arthritis by the platelet activating factor antagonist BN 50730

OBJECTIVE

To evaluate the role of platelet activating factor (PAF) in the early stage of arthritis.

METHODS

Arthritis was induced in rabbits by weekly intra-articular injections of carrageenan. A PAF receptor antagonist, BN 50730, was used as a preventive or curative agent.

RESULTS

BN 50730 was able partially to prevent the development of arthritis, and was also active on established arthritis. The joint arthritis scores of BN treated animals were significantly lower than those of the non-treated animals. The blood concentrations of PAF, PAF bound to lipoproteins (lipo-PAF), and its precursor, lyso-PAF, were not correlated with clinical variations.

CONCLUSIONS

The present data demonstrate a therapeutic action of a PAF antagonist in experimental arthritis and suggest a critical role for PAF during the early stage of arthritis.

Correlations between PAF-acether and tumor necrosis factor in rheumatoid arthritis. Influence of parenteral corticosteroids

The aim of this study was to evaluate the presence of PAF-acether (PAF), its specific degrading enzyme acetylhydrolase, and tumor necrosis factor (TNF) concentrations in blood and synovial fluid (SF) from patients with active RA. The variations of the mediators were also evaluated after corticosteroid perfusions in 7 patients. Lipo-PAF (PAF complexed to lipoproteins) was the main form of PAF detected both in blood and in SF, whereas unbound PAF was uncommon. Acetylhydrolase activity was also present in SF, with a strong correlation between serum and SF levels. TNF was detected in most of the samples, and TNF and acetylhydrolase levels were strongly correlated both in blood and in SF. Despite dramatic clinical improvement, corticosteroid treatment was not accompanied by a significant reduction of the concentration of blood mediators, suggesting that these molecules should not be considered as markers of disease activity.

Defective glomerular [3H]lysoPAF metabolism in the autologous phase of rabbit nephrotoxic nephritis

Glomerular infiltration of blood-derived mononuclear cells contributes to the glomerular injury in the autologous phase of nephrotoxic nephritis (NTN). LysoPAF has recently been shown to be chemotactic for human monocytes, thus its accumulation might account for monocyte recruitment. We investigated [3H]lysoPAF metabolism in isolated glomeruli from normal and NTN rabbits studied both in the heterologous and in the autologous phases of the disease. [3H]lysoPAF was converted to [3H]1-O-alkyl-glycerol and [3H]1-O-alkyl-2-acyl-GPC by phospholipase C and acyltransferase, respectively, both in normal and NTN glomeruli. Glomerular metabolism of [3H]lysoPAF was normal during the heterologous phase of NTN. By contrast, in isolated glomeruli from NTN rabbits studied in the autologous phase of the disease, a significantly lower [3H]lysoPAF degradation occurred with respect to normal ones. This defective degradation resulted in a significantly reduced formation of [3H]1-O-alkyl-glycerol. The apparent Km for enzymatic conversion of [3H]lysoPAF to [3H]1-O-alkyl-glycerol, determined at 15 minutes as a function of [3H]lysoPAF concentration, was doubled in glomeruli from rabbits studied in the autologous phase of NTN as compared to normal ones, while Vmax values were similar in the two groups. These results show a defective glomerular lysoPAF degradation in the autologous phase of NTN, likely due to a decreased affinity of phospholipase C to lysoPAF. Altered lysoPAF metabolism results in glomerular accumulation of lysoPAF in the autologous phase of NTN, as shown by significantly higher levels of lysoPAF measured in nephritic glomeruli as compared to normal ones.

Development of a high-performance liquid chromatographic-mass spectrometric technique, with an ionspray interface, for the determination of platelet-activating factor (PAF) and lyso-PAF in biological samples

An HPLC-mass spectrometric technique with an ionspray interface was developed for the determination of platelet-activating factor (PAF) and PAF-related compounds in biological samples. HPLC separations were performed using a reversed-phase column. The mass spectra showed intense [M + H]+ ions. Collision-induced dissociation of protonated molecular ions gave characteristic daughter ions corresponding to the phosphorylcholine group. By selective-ion monitoring, a detection limit of 0.3 ng was obtained for all molecules; by multiple reaction monitoring, the same sensitivity was achieved for PAF whereas for lyso-PAF the limit was 3 ng. Finally, PAF was comparatively determined by bioassay and HPLC-MS after extraction from the cell pellets and the supernatants of human polymorphonuclear neutrophils unstimulated or stimulated with opsonized zymosan. The good correlation observed between these techniques indicated the reliability of HPLC-MS for biochemical studies on PAF and PAF-related molecules.

Outer-membrane porins from gram-negative bacteria stimulate platelet-activating-factor biosynthesis by cultured human endothelial cells

Porins are a family of hydrophobic proteins located in the outer membrane of the cell wall in Gram-negative bacteria. The effect of porins on the biosynthesis of platelet-activating factor (PAF) by cultured human umbilical-cord-vein-derived endothelial cells (HUVEC) was investigated. The results demonstrate that porins were able to induce a dose-dependent synthesis of PAF in HUVEC. PAF, synthesized after stimulation with porins, was mainly cell associated and the synthesis peaked at 15 min, decreasing rapidly thereafter. Experiments with radiolabeled precursors demonstrated that PAF, a 1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine, was synthesized via the remodeling pathway involving the acetylation of 1-O-alkyl-2-lyso-sn-glyceryl-3-phosphorylcholine (2-lysoPAF) generated from 1-O-alkyl-2-acyl-sn-glyceryl-3-phosphorylcholine by phospholipase-A2 activity. The activation of phospholipase A2 in HUVEC stimulated by porins was detected by observing the mobilization of [14C]arachidonic acid. In addition, the activity of acetyl-CoA:1-alkyl-sn-glycero-3-phosphorylcholine 2-O-acetyltransferase was transiently increased in porin-stimulated HUVEC and, after incubation with [3H]CoASAc or [3H]acetate, the [3H]acetyl group was incorporated into newly synthesized PAF. Porins, by forming transmembrane channels, induced a sustained influx of extracellular 45Ca2+ into the cytosol. The activation of PAF synthesis by porins depended on this influx rather than on intracellular calcium mobilization, since PAF synthesis did not occur in the absence of extracellular Ca2+.

Platelet-activating factor in bronchoalveolar lavage from patients with sarcoidosis

Platelet-activating factor (PAF), a lipid mediator of inflammation and anaphylaxis, may play a role in several physiopathologic alterations of the lung. A lipid compound with physicochemical and biologic characteristics similar to synthetic PAF was extracted and purified from bronchoalveolar lavage (BAL) fluid of 15 of 34 patients with sarcoidosis. PAF was quantitated by a bioassay on washed rabbit platelets. The specificity of platelet aggregation was assessed by using two different PAF receptor antagonists. The incidence of detectable amounts of PAF in BAL fluid of sarcoid patients was statistically significant (chi 2 = 4.064, p = 0.044) when compared with the 14 normal control subjects. The results demonstrated an increased production of PAF in the lower respiratory tract of patients with sarcoidosis. The presence of PAF in BAL fluid, however, did not correlate with radiologic stage, intensity of alveolitis, gallium scanning positivity, angiotensin-converting enzyme serum level, or lung function tests. Therefore, a direct relationship between presence of PAF in BAL fluid and activity of lung disease in patients with sarcoidosis was not directly established.

Presence of paf-acether in rheumatic diseases

Paf-acether (paf) is a naturally occurring phospholipid involved in inflammatory processes. The presence of paf, its precursor lyso paf, and lipo-paf has been determined in blood and synovial fluid from 13 patients with rheumatoid arthritis (RA), 11 with spondylarthropathies, eight with other inflammatory rheumatisms, 13 with chondrocalcinosis, 15 with osteoarthritis, and also in blood from nine healthy subjects. Paf and lipo-paf were measured by rabbit platelet aggregation after isolation by high performance liquid chromatography, whereas lyso paf was first chemically acetylated to give paf. Lipo-paf in blood was higher in patients than in controls; lipo-paf concentrations in blood and in synovial fluid were significantly higher in rheumatoid arthritis than in osteoarthritis and chondrocalcinosis. By contrast, paf and lyso paf reached their lower values in rheumatoid arthritis. The amounts of lipid mediators were not correlated with biological parameters of inflammation. Lipo-paf, which is considered as a storage form of paf, may be the important form of paf in active inflammatory rheumatism.

Production of paf-acether by human epidermal cells

The production of the inflammatory mediator paf-acether (paf) from human epidermal cells was investigated in vitro. Human epidermal cells, freshly isolated from normal skin or in culture, were incubated in Tyrode's buffer containing 0.25% lipid-free bovine serum albumin in the presence of 2 microM calcium ionophore A23187, at 37 degrees C, for 1 to 60 min. Paf production slightly began at the first min of stimulation, was significant after 10 min, reached a maximum at 20 min (251 +/- 25 pg/l X 10(6) cells, mean +/- 1 SD), and decreased thereafter. About 50% of the paf amount produced by epidermal cells was recovered in supernatants. Addition of the non-acetylated paf precursor 1-O-octadecyl-sn-glycero-3-phosphocholine, i.e., lyso-paf, at 0.1 microM to epidermal cells during A23187-stimulation did not alter this production. In contrast, addition of acetyl-coenzyme A at 0.1 mM enhanced paf production by 5 times. The material produced by epidermal cells was identical to synthetic paf because: 1) the aggregation of aspirin-treated and ADP-insensitive washed rabbit platelets it induced was inhibited by BN 52021, an antagonist of the paf putative receptor; 2) the factor was inactivated by phospholipase A2 but was insensitive to lipase from Rhizopus arrhizus; 3) it exhibited the same retention time as synthetic paf during standard and reverse-phase (RP) high-pressure liquid chromatography (HPLC) elution. The paf precursors, i.e., lyso-paf and 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine, were also detected in epidermal cells, stimulated with A23187 or not. As determined by RP-HPLC analysis and confirmed by gas chromatography analysis, these precursors and the paf produced by epidermal cells exhibited more than 90% of a hexadecyl chain at the sn-1 position of the molecule. The present results demonstrate the synthesis and release of paf by normal human epidermal cells. Paf production within the epidermis might account for the development of cutaneous inflammation and the pathogenesis of many skin disorders.

Biosynthesis of paf-acether. Activators of protein kinase C stimulate cultured mast cell acetyltransferase without stimulating paf-acether synthesis

Antigen stimulation of cultured bone-marrow-derived mast cells sensitized with specific monoclonal IgE induced cell degranulation and paf-acether (paf; 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) biosynthesis via the deacylation/acetylation (remodelling) pathway. Phorbol myristate acetate (PMA; 20-100 ng/ml) triggered only acetyltransferase activation, without concomitant lyso-paf (1-O-alkyl-sn-glycero-3-phosphocholine) and paf formation. A low concentration of PMA (5 ng/ml) potentiated antigen-induced degranulation, acetyltransferase activation and paf formation by about 30% but did not change the level of lyso-paf formation. Stimulation of mast cells with antigen increased intracellular Ca2+ from 61 to 269 nM, whereas no modification of Ca2+ influx was observed when cells were pretreated with PMA (5 ng/ml) before antigen challenge. Gas chromatography coupled to electron capture detection revealed that the composition of paf formed by cells stimulated by antigen alone was similar to that of paf formed by PMA-primed antigen-stimulated cells; 84 +/- 8% and 79 +/- 2% (means +/- S.E.M., n = 3) of molecules respectively bore the C16:0 alkyl chain moiety, with the remainder bearing essentially C18:0 molecules. Overnight treatment of mast cells with PMA (200 ng/ml) caused disappearance of protein kinase C (PKC) from both cytosol and membranes. When such cells were stimulated further with antigen, they failed to degranulate, and acetyltransferase activation, paf production and lyso-paf production were decreased by 33 +/- 11%, 57 +/- 4% and 96 +/- 3% respectively (n = 3 or 5). The PKC inhibitors chlorpromazine and staurosporine inhibited to a significant extent both cell degranulation and all steps leading to paf biosynthesis. Our data suggest that PKC-dependent mechanisms are operational during cell degranulation and contribute only in part to paf biosynthesis. The PKC-dependent signal directly generated by PMA or diacylglycerol is not sufficient to trigger the full cell response, which is obtained only through receptor-operated antigen challenge.

Platelet activating factor is produced during infectious peritonitis in CAPD patients

Peritonitis, a frequent complication of continuous ambulatory peritoneal dialysis (CAPD), is a model of inflammation which provides the opportunity to recover the exudate fluid. To date, various endogenous mediators (histamine, bradykinin, activated complement factors, prostanoids) have been implicated in the mediation of peritoneal inflammation and increased peritoneal permeability. In the present study, a lipid compound with physicochemical and biological characteristics similar to platelet activating factor (PAF) (1-0-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine) was extracted in significant amounts from the dialysate of eight out of nine peritonitis episodes in seven CAPD patients (Group A; 6771.4 +/- 3025.9 pM, mean +/- SEM at the first exchange during peritonitis). The amounts of PAF recovered in the first exchange dialysate from patients of Group A were linearly correlated with the loss of albumin (y = -3157.64 + 91.4x; r = 0.7394; N = 9; P less than 0.03) and number of leukocytes (y = 902.45 + 1.52x; r = 0.7576 N = 9; P less than 0.02). PAF was not detectable in the dialysate fluid from patients of Group A after recovery. Twelve patients on CAPD who had no past or present history of peritonitis (Group B) were used as controls; no PAF (9 patients) or only minimal amounts (3 patients: 7.0 pM; 23.0 pM; 70.0 pM) of this mediator were detected. This is the first direct demonstration of the local generation of PAF in a septic inflammatory reaction involving the peritoneal serosa in man. PAF produced by various cell types (neutrophils, peritoneal macrophages, endothelial cells) during peritoneal inflammation may contribute to the increased permeability of the peritoneal vascular bed.

Biosynthesis of paf-acether. Paf-acether but not leukotriene C4 production is impaired in cultured macrophages

After adherence for 24 or 48 h mouse peritoneal macrophages, upon a zymosan challenge, synthesized 114 +/- 55 and 82 +/- 31 pmol of paf-acether (paf)/mg of protein respectively, as compared with 513 +/- 195 pmol of paf/mg of protein in 2 h-adherent macrophages (means +/- S.D., n = 10). By contrast, 24 h- and 48 h-adherent macrophages exposed to zymosan produced more leukotriene C4 (2.7 +/- 1.1 and 1.4 +/- 0.2 nmol/mg of protein respectively, n = 5) than did 2 h-adherent macrophages (0.5 +/- 0.2 nmol/mg of protein, n = 5). Paf production was not altered when 2 h- and 24 h-adherent cells were cultured and/or stimulated in the presence of 5 microM-indomethacin, 10 microM-nordihydroguaiaretic acid or 100 microM-BW755C as compared with untreated cells. These results indirectly exclude the regulation of paf production by arachidonic acid metabolites. We investigated the efficiency of the enzymic steps which govern paf synthesis. We showed that the anabolic process was not impaired since (1) the amounts of alkylacylglycerophosphocholine and lyso-paf were similar in 2 h-, 24 h- and 48 h-adherent macrophages; (2) adding synthetic lyso-paf or acetyl-CoA to intact cells did not increase paf production in zymosan-stimulated 24 h- and 48 h-adherent macrophages; (3) the basal level of acetyltransferase was comparable in 2 h-, 24 h- and 48 h-adherent macrophages and in all cases was increased by 2-3 times upon zymosan challenge. We also showed that impaired paf production in 24 h- and 48 h-cultured macrophages was not due to the nature of the stimulus used to induce its synthesis.

Synthesis of trideuterated O-alkyl platelet activating factor and lyso derivatives

Racemic heavy isotope analogs of 1-O-alkyl-sn-glycero-3-phosphocholine (lysoPAF) and 1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholine (PAF) were prepared for use as internal standards to facilitate quantitative studies based on mass spectrometry. Starting from pentadecane-1,15-diol and rac-glycerol-1,2-acetonide, a convergent synthesis of 1-O-[16'-2H3]hexadecyl and 1-O-[18'-2H3]octadecyl rac-glycero-3-phosphocholine and their acetyl derivatives is described. Three deuterium atoms were introduced at the terminal position of the 1-O-alkyl group by displacement of the p-toluensulfonyl group from 1-O-alkyl-15'-p-toluensulfonate and 1-O-alkyl-17'-p-toluensulfonate with [2H3]-methylmagnesium iodide. The 1-O-alkyl-17'-p-toluensulfonate was obtained by reaction of the 1-O-alkyl-15'-p-toluensulfonate with allylmagnesium bromide, followed by reductive ozonolysis and treatment with p-toluene-sulfonyl chloride. The hydroxyl group at C-2 was protected by a benzyl group and removed at a late stage in the synthesis. This provided the corresponding lyso-derivatives or allowed preparation of racemic PAF by subsequent acetylation of the free hydroxy group. The phosphocholine moiety was introduced at glycerol C-3 by reaction with bromoethyldichlorophosphate and trimethylamine. The synthetic compounds were analyzed by FAB/MS and GC/NICIMS. They were shown to contain less than 0.6% protium impurity.

Evidence of PAF-acether metabolic pathway activation in antigen challenge of upper respiratory airways

Lyso-PAF-acether and PAF-acether (formerly platelet-activating factor) were detected in nasal secretions from patients with hay fever who underwent local antigen challenge. Lyso-PAF release was observed in 12 of 13 patients, with a maximum (p less than 0.001) 5 min after stimulation and a progressive decrease during the first hour. PAF was detected in the 5-min postchallenge nasal washings from two of 13 subjects. After HPLC, this mediator was found in four of seven postchallenge nasal washings submitted to this procedure, with a peak 5 min and 10 min after provocation. Histamine analysis revealed a significant (p less than 0.001) but time-limited (5 min) release in nasal secretion. The pattern of immunoreactive leukotriene C4 showed a maximal peak (p less than 0.01) 5 min after allergen provocation, with raised levels for 20 min. Nasal stimulation with nebulized saline solution or grass pollens in healthy subjects and in patients suffering from allergic rhinitis caused by Dermatophagoides pteronyssinus was followed by no local mediator release. These data indicate that, in addition to histamine and peptide-leukotrienes, lyso-PAF and PAF are released in nasal secretions after local antigen stimulation in patients with hay fever, with a preponderance of lyso-PAF response. On the basis of these results, it is conceivable that these ether-phospholipids may be involved in allergic inflammation of human nasal airways.

Characterization of serum platelet-activating factor (PAF) acetylhydrolase. Correlation between deficiency of serum PAF acetylhydrolase and respiratory symptoms in asthmatic children

Platelet-activating factor (PAF) acetylhydrolase has been recognized as an enzyme that inactivates PAF. We developed a convenient and reproducible method for determining human serum PAF acetylhydrolase activity. The assay was based on measurement of [14C]acetate produced from 1-O-alkyl-2-[14C]-acetyl-sn-glycero-3-phosphocholine upon precipitation of the complex of radioactive substrate and albumin with TCA. The apparent Km value of PAF acetylhydrolase (near the physiological concentration of serum protein) was 1.5 X 10(-4) M PAF. 32 subjects with serum PAF acetylhydrolase deficiency were found among 816 healthy Japanese adults. The low PAF acetylhydrolase activity in the deficient serum might not be due to the presence of enzyme inhibitor. Both the sensitivity to PAF and the metabolism of PAF in platelets from PAF acetylhydrolase-deficient subjects were almost the same as those of normal subjects. Deficiency in serum PAF acetylhydrolase appeared to be transmitted by autosomal recessive heredity among five Japanese families. Among healthy adults, healthy children, and asthmatic children, who were grouped into five classes on the basis of respiratory symptoms (remission, wheezy, mild, moderate, and severe groups), the probability of PAF acetylhydrolase deficiency was significantly higher in groups with severe symptoms (moderate and severe) (P less than 0.01). These results suggest that deficiency of serum PAF acetylhydrolase might be one of the factors leading to severe respiratory symptoms in asthmatic children.

Actions of platelet activating factor (PAF) homologues and their combinations on neutrophil chemokinesis and cutaneous inflammatory responses in man

The inflammatory actions of synthetic C16:0 and C18:0 platelet activating factor (PAF) homologues, both alone and in combination, have been compared in an in vitro human neutrophil chemokinesis assay and by intradermal injection in human skin. In the chemokinesis assay, the maximum distance moved by neutrophils in the presence of C18:0 PAF was significantly greater than that seen with the C16:0 compound. A mixture of C16:0 and C18:0 PAFs in a ratio of 1:9 appeared to be more active than in a ratio of 3:1. Intradermal injection of the C16:0 and C18:0 PAF homologues induced dose-dependent increases in weal volume and flare area responses which were not significantly different. Combination of these phospholipids in a ratio of 3:1 or 1:9 of C16:0:C18:0 did not significantly alter the dose response curves. Thus, changes in the chain length of the alkyl substituent of synthetic PAF homologues and combination of these homologues, in ratios found in vivo or formed by leukocytes in vitro, did not alter the cutaneous inflammatory responses to PAF in man. The C18:0 homologue was, however, more active as a human neutrophil chemoattractant in vitro.

Preformed PAF-acether and lyso PAF-acether are bound to blood lipoproteins

PAF-acether (PAF) is a newly formed mediator not normally present in circulating blood. A compound exhibiting all of its biological characteristics but coeluting with phosphatidylcholine (PC) in high-pressure liquid chromatography (HPLC) was unveiled ('peak X') in normal human plasma. A second HPLC run of peak X HPLC fractions revealed the presence of PAF itself with concomitant disappearance of peak X. Beside PAF, immunoreactive apolipoproteins A-I and E were found in peak X. Also lipoproteins (Ls) purified using either ultracentrifugation or immunoaffinity chromatography yielded peak X and, in a second HPLC run, authentic PAF. L-free plasma was devoid of peak X. Finally, after preincubation with plasma, labeled PAF was found associated with Ls. Thus in human blood preformed PAF is bound in high amounts to Ls, a result of interest given the role of Ls and platelets in vascular diseases and the present knowledge on PAF biosynthesis.

Characterization and quantification of PAF-acether (platelet-activating factor) as a heptafluorobutyrate derivative of 1-O-alkyl-2-acetyl-sn-glycerol by capillary column gas chromatography with electron-capture detection

A gas chromatographic method with a glass capillary column and electron-capture detection is proposed for the characterization and quantification of 1-O-hexadecyl-2-O-acetyl-sn-glycero-3-phosphocholine (PAF C16) using the corresponding 1-O-hexadecyl-2-O-acetyl-sn-glycero-3-heptafluorobutyrate derivative and, as an internal standard, 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphocholine. The reproducibility was approximately 5% and amounts as low as 20 pg could be measured. The method was specific and allowed the quantification of PAF C16 in supernatants from stimulated human polymorphonuclear neutrophils.

Fast atom bombardment and tandem mass spectrometry of phosphatidylserine and phosphatidylcholine

Fast atom bombardment (FAB) desorption of phosphatidylserine and various phosphatidylcholines produces a limited number of very informative negative ions. Especially significant is the formation of (M-H)- ions for phosphatidylserine, a compound which does not yield informative high mass ions by other ionization methods. Phosphatidylcholines do not yield (M-H)- ions but instead produce three characteristic high mass ions, (M-CH+3)-, [M-HN(CH3)+3]- and [M-HN(CH3)+3-C2H2]-. Both classes of lipids also yield anions attributed to the carboxylate components of these complex lipids. FAB desorption in combination with collisional activation allows for characterization of fragmentation and determination of structural features. Collisional activation of the carboxylate anion fragments from the complex lipids is especially informative. Structural characterization of the fatty acid chain can be achieved as the released saturated carboxylate anions undergo a highly specific 1,4-elimination of H2, which results in the losses of the elements of CH4, C2H6, C3H8 . . . in a fashion entirely consistent with the chemistry of carboxylate anions desorbed from free fatty acids. These CnH2n + 2 losses begin at the alkyl terminus and progress along the entire alkyl chain. Modified fatty acids undergo a similar fragmentation; however, the modification affects the series of CnH2n + 2 losses in a manner which permits determining the type of modification and its location on the fatty acid chain.

Binding and metabolism of platelet-activating factor by human neutrophils

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

Platelet-activating factor and the release of a platelet-derived coronary artery vasodilator substance in the canine

Platelet-activating factor (acetyl-glyceryl-ether-phosphorylcholine; 1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphorylcholine), which is released by stimulated neutrophils and platelets, possesses the ability to alter vascular tone and permeability and to activate various formed blood elements. We have characterized the hemodynamic effects of intracoronary injections of platelet-activating factor and the influences of pharmacological blockade and platelet depletion on its activity. Intracoronary injections of platelet-activating factor produced maximum increases in left circumflex coronary artery blood flow of 55 +/- 8, 52 +/- 8, and 52 +/- 7 ml/min at 0.5, 1.0, and 2.0 nM, respectively. Only modest changes in systemic arterial blood pressure and regional developed isometric contractile force were associated with the intracoronary artery administration of platelet-activating factor over the range of doses studied. The increase in left circumflex coronary artery blood flow in response to platelet-activating factor was attenuated (44%), but not prevented, by pretreatment with diphenhydramine, (4 mg/kg, iv), and was not affected by pretreatment with aspirin (20 mg/kg, iv) or the systemic administration of the serotonin receptor antagonist, methysergide. The coronary vasodilator response to platelet-activating factor was reduced significantly by the induction of thrombocytopenia (95 +/- 3% platelet depletion) through the administration of sheep-derived canine platelet antiserum. The intracoronary artery injection of platelet-rich plasma activated with platelet-activating factor into thrombocytopenic dogs produced a significantly greater increase in coronary artery blood flow than the injection of either non-activated platelet-rich plasma or platelet-depleted plasma to which platelet-activating factor was added. Similar changes in coronary artery blood flow could be obtained with the intracoronary artery injection of cell-free supernates from washed platelets activated with platelet-activating factor. The observed results suggest that circulating platelets, when exposed to platelet-activating factor, can release a coronary dilator substance, and that the coronary artery dilation is not prevented by pharmacological receptor antagonists for histamine, serotonin, or inhibitors of cyclooxygenase.

Evidence that biosynthesis of platelet-activating factor (paf-acether) by human neutrophils occurs in an intracellular membrane

Human polymorphonuclear leukocytes (PMN) were incubated in the absence or presence of the calcium ionophore A23187 (6 microM) for 10 min at 37 degrees C. They were then lysed by nitrogen cavitation and fractionated using Percoll gradients. Three major fractions of increasing density corresponding to plasma membrane, intracellular membranes and secretory granules were detected using [3H]concanavalin A, NADH-dehydrogenase and beta-D-glucuronidase as respective markers. In both cases, the acetyltransferase activity responsible for biosynthesis of paf-acether (platelet-activating factor of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) was detected in the intermediary fraction, the enzyme activity being increased 3-4-fold in stimulated cells. From the comparison with the distribution of various markers, it is concluded that in human PMN the final step of paf-acether assembly occurs in an intracellular membrane, possibly the endoplasmic reticulum.

Binding kinetics of PAF-acether (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) to intact human platelets

The binding of [3H]PAF-acether (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) to intact human gel-filtered platelets was measured at 22 degrees C. Specific binding reached saturation within 15 min at high doses of [3H]PAF-acether (0.5-0.9 nM), whereas about 90 min were required when low doses (0.02-0.5 nM) were used. Above 1 nM, [3H]PAF-acether non-specific binding increased progressively, which together with the demonstration of a 3H-labelled metabolite suggested uptake and metabolism of [3H]PAF-acether. Equilibrium analysis revealed one class of specific receptors with a Ka of 18.86 +/- 4.82 X 10(9) M-1 and 242 +/- 64 binding sites per platelet. Non-equilibrium binding revealed a similar Ka (16.87 X 10(9) M-1). Specific binding became irreversible after prolonged incubation, a process that was enhanced at increasing concentrations of [3H]PAF-acether. Platelets made desensitized to PAF-acether by prior incubation with unlabelled PAF-acether failed to bind a second dose of PAF-acether (3H-labelled), suggesting that desensitization resulted from loss of available binding sites. Under the conditions of the binding studies, PAF-acether induced exposure of the fibrinogen receptor, aggregation (in a stirred suspension) and alterations in (poly)-phosphatidylinositides. These results suggest that PAF-acether initiates platelet responses via receptor-mediated processes.

Biosynthesis of acyl-specific glycerophospholipids in mammalian tissues. Postulation of new pathways

A review of the literature concerning the synthesis of acyl-specific glycerophospholipids indicates that the known biosynthetic sequences cannot satisfactorily explain these specific acylations . New de novo and salvage pathways are proposed to account for the acyl composition of highly unsaturated and saturated glycerophospholipids. In these hypothetical pathways, de novo synthesized glycerophosphodiesters are postulated to be key intermediates to establish the specific acyl composition of the resulting glycero-phospholipids, and to be integrated with the known cytidine pathways. A re-interpretation of the experimental literature in terms of these postulated pathways is provided, with some methods to test these proposed sequences.

Dissociation between inhibition of phospholipid methylation and production of PAF-acether by rabbit platelets

Platelet-activating-factor (PAF-acether, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine) is formed by and released from rabbit platelets stimulated with thrombin, with the ionophore A23187, with collagen and with the platelet-stimulating glycoprotein convulxin. We here show that 3-deazaadenosine (C3ado) and L-homocysteine (HCy), two inhibitors of platelet methylation, reduced the formation of PAF-acether and of its deacetylated product lyso-PAF-acether by rabbit platelets challenged with thrombin, under conditions where the accompanying aggregation was not significantly modified. In contrast, platelet aggregation induced by convulxin was completely and irreversibly blocked when C3ado and HCy were associated. Aggregation by thrombin was not affected by the methylation inhibitors even when ADP was scavenged and thromboxane formation was suppressed. Our results indicate that phospholipid methylation, thrombin-induced platelet aggregation and formation of PAF-acether can be dissociated. The formation of PAF-acether by rabbit platelets can be blocked by mechanisms other than inhibition of phospholipase A2, since the latter is not affected by C3ado and/or HCy.

Effect of structural analogues of PAF-acether on platelet desensitization

The 1-O-alkyl-2-O-acetyl-sn-glyceryl-3-phosphorylcholine (PAF-acether) aggregates rabbit platelets and desensitizes them to a second challenge with the same agonist but not to arachidonic acid. The desensitizing activities of 14 analogues of PAF-acether were explored with particular attention to the dose-response dependency of the desensitization process. PAF-acether was 500-fold more active than its 1-O-acyl analogues. The 2-lyso PAF-acether was inactive and the PAF-acether enantiomer 2000 times less effective than the natural isomer, thus confirming the importance of the presence and steric position of the 2-acetate group. The desensitizing activities of the 2-propionyl and the 2-butyryl analogues were close to that of PAF-acether. Substituting an ether to an ester bond at the 2-position indicated that the number of carbon of the 2-substituant seems more determinant than the nature of the linkage for the desensitizing process. Indeed, the 2-ethoxy and the 2-methoxy analogues were 87 and 5000 times less active than PAF-acether respectively. The presence of methyl groups on the nitrogen base is also critical to desensitize platelets. The desensitizing potency of the tested phospholipids was always identical to their aggregating efficiency. It is suggested that these compounds activate cells through a common mechanism.

Platelet-activating factor stimulates metabolism of phosphoinositides in horse platelets: possible relationship to Ca2+ mobilization during stimulation

Stimulation of horse platelets with platelet-activating factor (PAF) induces a rapid degradation of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]. Addition of 0.1 microM PAF for 5 sec to platelets prelabeled with 32P induces a 50% loss of [32P]PtdIns(4,5)P2. 32P-Labeled phosphatidylinositol 4-monophosphate (PtdIns4P) and [32P]phosphatidylinositol (PtdIns) also are decreased, albeit at a slower rate. Loss of 32P radioactivity correlates with a net loss of fatty acids from both polyphosphoinositides. Stimulation of platelets with PAF also produces formation of [32P]phosphatidic acid and [32P]lysophosphatidylinositol. The initial disappearance of inositol lipids is subsequently followed by resynthesis, as evidenced by increased incorporation of 32P into PtdIns(4,5)P2, PtdIns4P, and PtdIns. The resynthesis of the inositides increases with time and is proportional to the concentration of PAF. Prostacyclin (1 microM) inhibits (i) the formation of phosphatidic acid and lysophosphatidylinositol and (ii) the resynthesis of polyphosphoinositides induced by 0.03 microM PAF without affecting the initial loss of PtdIns(4,5)P2. The loss of inositol lipids appears to be a primary event of platelet activation. The initial loss of polyphosphoinositides might be linked to the initiation of cellular activation by mobilizing membrane-bound Ca2+, whereas the subsequent formation of these lipids might be involved in mechanisms to prevent overstimulation of the cell.