Changes in bronchial responsiveness, circulating leucocytes and ex vivo cytokine production by blood monocytes after PAF inhalation in allergic asthmatics

We investigated the effects of inhaled platelet-activating factor (PAF) on methacholine bronchial responsiveness, circulating leucocyte counts, and ex vivo tumour necrosis factor alpha (TNF alpha) and interleukin-1 (IL-1) production from blood monocytes in eight allergic asthmatics. Bronchial responsiveness was defined as the provocative concentration of methacholine causing a 20% decrease in forced expiratory volume in one second (PC20). Circulating leucocytes were counted by means of an automatic haemocytometer, and cytokines were measured with specific immunoassays. The different variables were measured before and 4, 24, 48, 72 and 168 h after a PAF (225 micrograms), a lyso-PAF (225 micrograms) and a saline bronchial challenge. When compared with lyso-PAF and saline, inhalation of PAF resulted in a significant decrease in PC20 over a period of one week. Two falls in bronchial responsiveness were identified, the first by 4 h and the second beginning 48 h and reaching a maximum by 168 h. The increases in spontaneous TNF alpha and IL-1 production which occurred during the week after both PAF, lyso-PAF and saline, did not differ significantly. Likewise, the changes in circulating neutrophil counts, characterized by a transient rise by 4 h after PAF and lyso-PAF but not saline, followed by a fall by 24 h and a persistent decrease until 168 h, were not significantly different after PAF, lyso-PAF and saline. On the other hand, in comparison with lyso-PAF and saline, inhaled PAF caused a significant protracted augmentation in circulating eosinophil counts, which was maximal by 48 h but did not correlate with the delayed decline in PC20.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of calcium influx by platelet activating factor in Dictyostelium

Platelet activating factor (PAF) induces Ca2+ influx in Dictyostelium discoideum. In this investigation we used this activity to analyze the mechanism of PAF action. We found that PAF activity was confined to the period of spike-shaped oscillations and suggest that the role of PAF is to augment cAMP relay. PAF seems to act only a few times during this time period of two hours, since Ca2+ entry adapted to a subsequent stimulus for about 30 minutes. PAF showed a reduced response in the G protein beta- strain LW14 and was unable to induce Ca2+ influx in the G alpha 2- strains HC85 and JM1. The latter expresses the cAMP receptors cAR1 constitutively, and exhibits cAMP-induced Ca2+ influx, albeit at a reduced level. In order to decide whether the inability of PAF to elicit a Ca2+ response in JM1 cells was due to the lack of differentiation and/or the lack of G alpha 2, we inhibited the IP3-dependent pathway with compound U73122 and found that Ca2+ entry was blocked, whereas a closely related inactive compound, U73343, did not alter the response. In agreement with this, NBD-Cl, an inhibitor of Ca2+ uptake into the IP3-sensitive store in Dictyostelium, also abolished PAF activity. The latter was not inhibited by the plasma membrane antagonists BN-52021 or WEB 2170. Therefore PAF seems to operate intracellularly via the IP3-signalling pathway at or upstream of the IP3-sensitive store.

Synthesis of heterocyclic platelet activating factor analogues

The synthesis of heterocyclic analogues of the platelet activating factor is described. The preparation starts with acylating rac-tetrahydro-1,3-thiazine-4-carboxylic acid ethyl ester, with palmitoyl chloride to form the amide linkage. Following ester reduction, the phosphocholine part is introduced via 2-chloro-2-oxo-1,3,2-dioxaphospholane and subsequent ring opening with trimethylamine under pressure. Furthermore, the related L-thiazolidine analogue is prepared using the same procedure. In addition the sulfinyl and sulfonyl derivatives of this compound are obtained by oxidation with 3-chloro-perbencoic acid. From one sulfinyl intermediate the diastereomeres are separated and their conformations are determinated by 13C-NMR spectroscopy.

The CoA-independent transacylase in PAF biosynthesis: tissue distribution and molecular species selectivity

Microsomal membranes from six different rat tissues (spleen, lung, kidney, brain, testis, and liver) were found to possess CoA-independent transacylase activity that could both acylate lyso-[3H]PAF (1-[3H]hexadecyl-2-lyso-sn-glycero-3-phosphocholine) and then deacylate the 1-[3H]hexadecyl-2-acyl-sn-glycero-3-phosphocholine product via the transacylation of added exogenous 1-alk-1'-enyl-2-lyso-sn-glycero-3-phosphoethanolamine. Platelet-activating factor (1-[3H]hexadecyl-2-acetyl-sn-glycero-3-phosphocholine) was produced when acetyl-CoA was added to the spleen microsomes during generation of lyso-[3H]PAF by the transacylases. More extensive studies with subcellular fractions from spleen revealed that, in addition to microsomes, the transacylase activities were also present in the 15,000 x g membrane fraction but not in the cytosol. Analysis of molecular species of 1-[3H]hexadecyl-2-acyl-sn-glycero-3-phosphocholine before and after addition of 1-alk-1'-enyl-2-lyso-sn-glycero-3-phosphoethanolamine as the acyl acceptor demonstrated a high selectivity for polyunsaturated fatty acids (> 3 double bonds/acyl group) in both the acylation and deacylation processes that occurred in testicular microsomal membranes. The transfer of acyl groups by the transacylase appeared to be equally effective for either arachidonic or docosapentaenoic(n - 6) fatty acids, whereas linoleic and oleic fatty acids were not transferred from 1-[3H]hexadecyl-2-acyl-sn-glycero-3-phosphocholine following the addition of 1-alk-1'-enyl-2-lyso-sn-glycero-3-phosphoethanolamine. Similar experiments with the membrane fraction of undifferentiated HL-60 cells showed that arachidonic acid supplementation of intact cells enhanced both the CoA-independent transacylation of lyso-[3H]PAF and the subsequent deacylation of 1-[3H]hexadecyl-2-acyl-sn-glycero-3-phosphocholine caused by addition of 1-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine. Differentiation of the HL-60 cells into a neutrophil-like form had no effect on the transacylase activity. Our results indicate the PAF-related transacylase is widely distributed among tissues and, although highly selective for polyunsaturated acyl groups, does not discriminate selectively among the polyunsaturates.

Characterization of a behavioral model for peripherally evoked itch suggests platelet-activating factor as a potent pruritogen

The studies described herein characterize animal behavioral models for conjunctival and cutaneous itch. Histamine was used as the reference stimulus for model development because it is firmly established as a pruritogen in both conjunctiva and skin. Itching evokes the desire to scratch in human subjects, so hind limb scratching at the afflicted area was used to identify pruritogenic stimuli. Under optimized environmental conditions, hind limb scratching behavior yielded substantial and highly reproducible responses. The conjunctival itch-scratch response was delineated from pain and foreign body sensations by using appropriate stimuli. Examination of a large and diverse variety of autocoids revealed that only histamine, platelet-activating factor (PAF) and arachidonic acid and its cyclooxygenase metabolite prostaglandin E2 possessed meaningful pruritogenic activity. PAF-induced ocular pruritus did not involve histamine release, according to studies with appropriate antagonists. Thus PAF-induced ocular pruritus was unaffected by the histamine H1-receptor antagonist pyrilamine but was substantially attenuated by the PAF antagonists WEB 2086 and CV-6209 and was virtually abolished by E-6123. Similar itch-scratch behaviors were quantified in hairless guinea pig skin following the application of cowhage or the iontophoretic administration of histamine and PAF. Findings from these newly developed itching models suggest that PAF could be an important mediator of the pruritic sensation by activating a population of nerve endings responsible for encoding the itch sensation.

Activation of phospholipase D by interleukin-8 in human neutrophils

Interleukin 8 (IL-8), a member of the C-X-C branch of the chemokine superfamily, stimulated the breakdown of 1-O-[3H]alkyl-2-acyl-sn-glycero-3-phosphocholine ([3H]EAPC) and the formation of 1-O-[3H]alkyl-2-acyl-phosphatidic acid ([3H]-EAPA) in human polymorphonuclear leukocytes (PMN) in the presence of cytochalasin B. In addition, the mass of diradyl-PA was increased with similar kinetics. In the presence of ethanol, 1-O-[3H]alkyl-2-acyl-phosphatidylethanol ([3H]EAPEt) was formed at the expense of [3H]EAPA formation, indicating the activation of phospholipase D by the cytokine. The effect was time- and concentration-dependent, reaching a plateau at 30 seconds with the maximally activating concentration of 120 nmol/L IL-8. Preincubation of cells with 1 microgram/mL Bordetella pertussis toxin inhibited the breakdown of [3H]EAPC and [3H]EAPA formation, indicating a role for a pertussis toxin-sensitive guanosine triphosphate-binding protein. Formation of phosphatidic acid (PA) correlated with activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, the oxidative burst enzyme, with both events occurring in the same concentration range. Inhibition of PA formation, by the presence of ethanol, also inhibited the oxidative burst stimulation by IL-8. Pretreatment of PMN with 10 nmol/L platelet-activating factor potentiated both [3H]EAPA accumulation and activation of NADPD oxidase by IL-8. Collectively, these data show that IL-8 stimulates the metabolism of choline-containing phosphoglycerides in human PMN and support a role for PA in the signaling mechanisms used by IL-8 to stimulate PMN function.

Activation of phospholipase D by interleukin-8 in human neutrophils

Interleukin 8 (IL-8), a member of the C-X-C branch of the chemokine superfamily, stimulated the breakdown of 1-O-[3H]alkyl-2-acyl-sn-glycero-3-phosphocholine ([3H]EAPC) and the formation of 1-O-[3H]alkyl-2-acyl-phosphatidic acid ([3H]-EAPA) in human polymorphonuclear leukocytes (PMN) in the presence of cytochalasin B. In addition, the mass of diradyl-PA was increased with similar kinetics. In the presence of ethanol, 1-O-[3H]alkyl-2-acyl-phosphatidylethanol ([3H]EAPEt) was formed at the expense of [3H]EAPA formation, indicating the activation of phospholipase D by the cytokine. The effect was time- and concentration-dependent, reaching a plateau at 30 seconds with the maximally activating concentration of 120 nmol/L IL-8. Preincubation of cells with 1 microgram/mL Bordetella pertussis toxin inhibited the breakdown of [3H]EAPC and [3H]EAPA formation, indicating a role for a pertussis toxin-sensitive guanosine triphosphate-binding protein. Formation of phosphatidic acid (PA) correlated with activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, the oxidative burst enzyme, with both events occurring in the same concentration range. Inhibition of PA formation, by the presence of ethanol, also inhibited the oxidative burst stimulation by IL-8. Pretreatment of PMN with 10 nmol/L platelet-activating factor potentiated both [3H]EAPA accumulation and activation of NADPD oxidase by IL-8. Collectively, these data show that IL-8 stimulates the metabolism of choline-containing phosphoglycerides in human PMN and support a role for PA in the signaling mechanisms used by IL-8 to stimulate PMN function.

Production and metabolism of platelet-activating factor in the normal and ischemic fetal rat brain

Production and metabolism of platelet-activating factor (PAF) in the fetal rat brain under normal and under ischemic stress conditions were examined. Endogenous PAF levels, determined by a bioassay using PAF-stimulated platelet release of [3H]serotonin, averaged 2.32 +/- 2.14 pg/mg in control brains and was reduced to 1.10 +/- 1.06 pg/mg after 20 min of maternal-fetal blood flow occlusion. [3H]PAF administered intracranially into the fetuses in utero was removed in a biphasic, time-dependent manner: a rapid component with an estimated elimination rate constant of 0.067 min-1 and t1/2 of 10 min and a slower component with an elimination rate of 0.017 min-1 and t1/2 of 41 min. In fetal brains subjected to ischemia a delayed elimination of [3H]PAF was noticed in the slow component (t1/2 = 59 min), indicating a possible difference between the clearance of exogenous and endogenous PAF. The disappearance of [3H]PAF was accompanied by an increase in the radioactivity associated with lyso-PAF that reached a plateau after 2.5 min, possibly indicating the degradation of the fast component. A steady increase in the alkyl-acyl-glycerophosphorylcholine radioactivity commenced after 5 min and continued up to 30 min. The endogenous production of PAF and the rapid degradation due to maternal-fetal blood flow occlusion indicate an additional target for therapeutic intervention in the pathology of intrauterine ischemia. Addition of the calcium ionophore A23187 stimulated in vitro formation of PAF and lyso-PAF from [3H]-choline-labeled fetal brain phospholipids, suggesting that intracellular calcium may play a major stimulatory role in PAF production.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of heterocyclic platelet activating factor analogues

The synthesis of heterocyclic analogues of the platelet activating factor is described. The preparation starts with acylating rac-tetrahydro-1,3-thiazine-4-carboxylic acid ethyl ester, with palmitoyl chloride to form the amide linkage. Following ester reduction, the phosphocholine part is introduced via 2-chloro-2-oxo-1,3,2-dioxaphospholane and subsequent ring opening with trimethylamine under pressure. Furthermore, the related L-thiazolidine analogue is prepared using the same procedure. In addition the sulfinyl and sulfonyl derivatives of this compound are obtained by oxidation with 3-chloro-perbencoic acid. From one sulfinyl intermediate the diastereomeres are separated and their conformations are determinated by 13C-NMR spectroscopy.

Coenzyme A-independent transacylation in amnion-derived (WISH) cells

Total membranes or microsomal fractions prepared from the amnion-derived WISH cell line posses a coenzyme A-independent transacylase activity. The transacylase utilizes 1-alkenyl- and 1-alkyl-2-lysoglycerophospholipids as preferred acceptors. Marginal transacylation was observed with 1-acyl-2-lysoglycerophospholipids. The reaction occurred in the presence of ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid and was not affected by phospholipase A2 inhibitors. Both 1-acyl- or 1-alkyl-glycerophosphocholines containing an arachidonoyl residue in the sn-2 position were effective as donors, while 2-oleoyl- or 2-palmitoyl-glycerophosphocholines were ineffective. The presence of the transacylase, the specificity of the substrates, and the stability of the 1-alkenyl bond provide a biochemical model that may explain the increased proportion of a highly enriched arachidonate-containing phosphatidyl-ethanolamine-plasmalogens fraction that is found in amnion at term.

Role of platelet activating factor in the intestinal epithelial secretory and Chinese hamster ovary cell cytoskeletal responses to cholera toxin

With the recent heightened concern about cholera around the world come new questions about the mechanism by which cholera toxin causes diarrhea. Peterson and Ochoa have suggested that prostaglandin synthesis is key to both the intestinal epithelial secretory and the CHO cell responses to cholera toxin [Peterson, J. W. and Ochoa, G. (1989) Science 245, 857-859]. Because platelet activating factor (PAF) can be a potent stimulus for prostaglandin synthesis, we examined its role in the intestinal and tissue culture effects of cholera toxin. We report that the specific PAF receptor antagonists BN 52021 and SR 27417 inhibit the effects of cholera toxin on intestinal secretion in rabbit ileal loops in vivo and on the cytoskeleton of Chinese hamster ovary cells in vitro. We also show that PAF itself can cause net fluid secretion in the rabbit model and that PAF potentiates the effects of cholera toxin on intestinal secretion. Finally, we demonstrate that cholera toxin stimulates significant PAF production (2.6-fold) in isolated T-84 intestinal epithelial cells. We conclude that cholera toxin stimulates PAF production and that PAF is involved in both the secretory and cytoskeletal responses to cholera toxin. These findings further support the involvement of additional mediators of cholera toxin effects other than mucosal cell cyclic AMP and help explain the effects of cholera toxin on prostaglandin synthesis.

Platelet-activating factor acetylhydrolase (PAF-AH) in human kidney

1. PAF-AH activity in human kidney (cortex and medulla) has been demonstrated and shares the following properties. 2. Does not require the presence of Ca2+ and appears to be different from phospholipase A2. 3. The pH optimum shows a peak at 7-7.4. 4. It is stable for 4 days at -30 degrees C. 5. It is mainly distributed in the microsomal fraction. 6. The apparent Km values of the enzymes of cortex and medulla are 0.553 and 0.207 microM, respectively and distinct from serum PAF-AH (1.439 microM). 7. The apparent molecular weight values are 60,000 and 25,000 for medulla and cortex, respectively and distinct from serum PAF-AH (94,000).

Nasal eosinophilia induced by PAF-acether is accompanied by the release of eosinophil cationic protein

It has been demonstrated that platelet-activating factor (PAF)-acether can induce nasal neutrophilia and eosinophilia, with a different degree of responsiveness in atopic and in nonatopic subjects. The aim of this study was to evaluate whether PAF can also induce the release of secondary mediators in the human nose. Ten patients with allergic rhinitis and 10 normal subjects underwent nasal challenge with PAF (500 nmol), lyso-PAF (500 nmol) and saline solution. Nasal lavages were performed before and after challenge to evaluate changes in nasal cytology and release of histamine, immunoreactive leukotriene (iLT) C4 and eosinophil cationic protein (ECP). PAF caused neutrophilia and eosinophilia, which appeared earlier in atopic than in nonatopic subjects (30 min vs 1 h), and peaked 3 h after challenge in both groups. Lyso-PAF caused mild neutrophilia, which appeared 3 h after challenge in both groups; an increase in eosinophil counts was observed 3 h after challenge in atopic subjects, but not in nonatopic subjects. PAF insufflation caused a significant release of ECP in nasal lavage fluids 30 min and 3 h after challenge in atopic subjects, and 3 h after challenge in nonatopic subjects. ECP levels in the nasal lavages collected 30 min and 3 h after challenge with PAF were higher in atopic than in nonatopic subjects. Eosinophil counts correlated with ECP levels in the nasal lavages collected 30 min after PAF challenge in atopic subjects. Nasal challenge with lyso-PAF did not provoke any release of ECP. No significant increase of histamine and iLTC4 levels in nasal lavages was found after challenge with either PAF or lyso-PAF.(ABSTRACT TRUNCATED AT 250 WORDS)

Platelet activating factor (PAF) and lyso-PAF in psoriasis

Previous studies have shown that scale from lesional psoriatic skin contains substantial amounts of platelet activating factor (PAF). In this study, PAF and its immediate precursor, lyso-PAF, were measured in exudates from abrasions on lesional and uninvolved psoriatic skin, and from skin of healthy subjects. The mean amounts of PAF recovered from lesional and uninvolved psoriatic skin (n = 13) and from healthy skin (n = 14) were not significantly different (range 0.05-2.14 pmol/sample). Mean recoveries of lyso-PAF from lesional psoriatic skin (n = 9) and skin of healthy subjects (n = 13) were also similar (9.5 +/- 1.9 and 11.0 +/- 1.9 pmol/sample, respectively), but significantly less lyso-PAF was found in exudates from the uninvolved psoriatic skin (n = 9; 3.1 +/- 0.4 pmol/sample; P < 0.01 relative to both lesional psoriasis and healthy skin). The finding of reduced lyso-PAF in uninvolved psoriatic skin was unexpected because increased phospholipase-A2 activity is associated with psoriasis. These results do not support the hypothesis that extracellular PAF contributes significantly to the inflammation associated with psoriasis.

Accumulation of platelet-activating factor acetylhydrolase in the peritoneal cavity of guinea pig after endotoxin shock

We examined the production of PAF, a mediator of shock, and LysoPAF, an inactive metabolite of PAF, in the guinea pig peritoneal cavity after i.p. administration of Escherichia coli LPS. Within 1 h of LPS administration, the level of PAF in the peritoneal fluid increased from 4.9 to 37.2 pmol/animal and decreased to the control value thereafter. In contrast, the level of lysoPAF gradually rose from 63.5 to 268 pmol/animal for up to 6 h. The activity of acetylhydrolase, which converts PAF to lysoPAF, in the peritoneal cavity increased in parallel with the increase in the lysoPAF level. The enzyme was distinguishable from phospholipase A2, because p-bromophenacylbromide (p-BPB), Ca2+, and ethylenediaminetetraacetic acid (EDTA) did not affect its enzymatic activity. In addition, this acetylhydrolase revealed similar biochemical properties to that detected in plasma. Both acetylhydrolases were resistant to trypsin treatment and had the same apparent molecular weight, as shown by gel-filtration column chromatography. These results suggest that the acetylhydrolase, which accumulates in the peritoneal cavity, infiltrates from the plasma in response to LPS, and then participates in the exclusion of PAF during endotoxin shock.

Agonist-induced down-regulation of platelet-activating factor receptor gene expression in U937 cells

Prolonged exposure (8-24 h) of human promonocytic U937 cells to 100 nM 1-O-hexadecyl-2-N-methylcarbamyl-sn-glycero-3-phosphocholine (carbarmyl-PAF), a non-metabolizable analogue of platelet-activating factor (PAF), reduced the numbers of PAF receptors by 50-75%, as determined by the radioligand-binding assay. To clarify whether the down-regulation of receptor numbers is due to decreased expression level of the PAF-receptor gene, the effect of carbamyl-PAF on the steady-state level of PAF-receptor mRNA was examined by a highly sensitive reverse-transcriptase PCR method. A 50% decline in the level of PAF-receptor mRNA was observed in U937 cells pretreated with 100 nM carbamyl-PAF for 24 h. The effect of carbamyl-PAF was dose-dependent, with an EC50 value around 10 nM. PAF-receptor antagonist, SRI-63675, was able to attenuate the effect of carbamyl-PAF. Furthermore lysoPAF, at 1 uM, was unable to induce a significant decrease in PAF-receptor mRNA after incubation for 24 h, indicating that the effect of carbamyl-PAF was specific. The half-life of the PAF-receptor mRNA measured in the presence of actinomycin D was unaffected by carbamyl-PAF treatment. In contrast, nuclear run-off experiments demonstrated that the transcription rate of the PAF-receptor gene in carbamyl-PAF-treated cells was about 65% of that in control cells. These results suggest that the PAF receptor in U937 cells is subject to down-regulation by agonist, at least partly, at the transcriptional level.

Excretion of platelet activating factor-acetylhydrolase and phospholipase A2 into nasal fluids after allergenic challenge: possible role in the regulation of platelet activating factor release

Platelet activating factor (PAF), a proinflammatory mediator synthesized through a phospholipase A2 (PLA2)-dependent reaction, is hydrolyzed into its inactive metabolite, lyso-PAF, by a specific acetylhydrolase. Previous studies have shown that allergen challenge of patients with allergic rhinitis leads to an increase of the concentrations of lyso-PAF in nasal lavage fluid (NLF), whereas PAF is detected only marginally. PAF-hydrolyzing enzymes are expected to be released on allergenic challenge, to account for the reduced concentrations of PAF in NLF. Here, we show that allergen challenge of patients with allergic rhinitis induces an increase of acetylhydrolase-like activity in NLF, which peaks within 10 minutes and returns to basal values 1 hour later. Acetylhydrolase hydrolyzed exogenous PAF with a complete loss of its ability to induce platelet aggregation. Allergen challenge also led to a parallel release of a PLA2 in nasal fluids. This enzyme preferentially hydrolyzes negatively charged phospholipids (phosphatidic acid monomethyl ester and phosphatidylgylcerol) versus phosphatidylcholine. More interestingly, the hydrolysis of phosphatidic acid monomethyl ester and phosphatidylglycerol by NLF was completely abolished by the addition of ethylenediaminetetraacetic acid which had no effect on the hydrolysis of PAF, indicating that the PLA2 secreted in nasal fluids is not involved in the degradation of PAF. Finally, our results show that allergen-induced increase in the concentrations of lyso-PAF and prostaglandin D2 occurred with a kinetic similar to that of tosyl-L-arginine-methyl-ester esterase, suggesting that mast cells are implicated in this process. Although no direct relationship was demonstrated between the absence of PAF and the increase of acetylhydrolase levels in NLF, we suggest a potential role for this enzyme in the inactivation of PAF if the latter is released in the nasal lumen.

Tumor necrosis factor release by murine macrophages stimulated by the cytotoxic ether lipid 1-O-hexadecyl-2-O-methyl-SN-glycero-3-phosphorylcholine (ET-18-O-OCH3)

The cytotoxic ether lipid 1-O-hexadecyl-2-O-methyl-SN-glycero-3-phosphorylcholine (ET-18-O-OCH3) is a structural analog of the mediator of inflammation platelet-activating factor (PAF). Recent studies demonstrated that ET-18-O-OCH3 activates human monocytes selectively at non-cytotoxic concentrations. The current studies determined the capacity of ET-18-O-OCH3 to stimulate release of TNF alpha by murine peritoneal macrophages. Macrophage receptors for ET-18-OCH3 and PAF were also assessed. ET-18-O-OCH3 and PAF stimulated TNF alpha release by resident BALB/c macrophages in the presence of LPS but not in the absence of this co-factor. In contrast, both ET-18-O_OCH3 and PAF stimulated TNF alpha release by thioglycollate-elicited macrophages in the absence of LPS although release was greater in the presence of this co-stimulus. Optimal stimulation of TNF alpha release occurred at 10(-14)-10(-11) M ET-18-O-OCH3 and PAF. Elicited macrophages and splenic macrophages from C57Bl/6 mice, unlike those from BALB/c mice, did not respond to 10(-15)-10(-8) M ET-18-O-OCH3 or PAF without or with LPS. Scatchard analysis of [3H]PAF binding to elicited BALB/c macrophages revealed the existence of high affinity receptors for PAF. In contrast, there was no evidence for receptors for ET-18-O-OCH3. ET-18-O-OCH3 did not compete with PAF for binding; macrophage activation by ET-18-O-OCH3 was not stereospecific; and, binding studies using [3H]ET-18-O-OCH3 did not reveal saturable binding characteristic of binding to specific receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Nasal allergen challenge generates 1-0-hexadecyl-2-lyso-sn-glycero-3-phosphocholine

We studied antigen-induced platelet activating factor and the 1-0-hexadecyl-2-lyso-sn-glycero-3-phosphocholine (lyso-PAF) in nasal lavage fluids (NLF) by combined gas chromatography/mass spectrometric analysis (GC/MS). During the early allergic reaction, there was a dramatic increase in the levels of lyso-PAF that peaked at 15 min (2.6 +/- 5.2 ng/ml, mean +/- SEM, n = 6). Increasing doses of antigen produced a dose-dependent increase in the levels of lyso-PAF that peaked at the highest dose. Levels of lyso-PAF correlated strongly with those of N-alpha-tosyl-L-arginine methyl ester (TAME)-esterase activity (rs = 0.82, p = 0.0001) and histamine (rs = 0.57, p = 0.002). There was a no significant increase in the quantity of lyso-PAF found in NLF from allergic individuals challenged with diluent or nonallergic individuals challenged with antigen. In subjects showing a late phase reaction, as indicated by symptoms and histamine release, we detected lyso-PAF along with TAME-esterase activity and histamine during the late phase reaction. In contrast to lyso-PAF, PAF levels were near or below the detection limit of the assay in NLF and remained unchanged after antigen challenge. We also investigated the potential pathways for lyso-PAF generation from 2-acetylated phospholipids. We found that the time required for deacetylation of 50% of [3H]PAF (t1/2) to lyso-PAF was 50 min in baseline secretions and 10 and 22 min in NLF obtained 10 min and 24 h after antigen challenge, respectively. These data suggested that catabolic pathways were present in NLF.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium influx in platelet activating factor-induced atrial natriuretic peptide release in rat cardiomyocytes

Atrial natriuretic peptide (ANP) is released from the myocardium after the activation of protein kinase C and/or ischemia, events that are associated with an increase in platelet activating factor (PAF) production in this tissue. In this study we demonstrate that PAF, but not lyso-PAF, induces a concentration-dependent increase in ANP secretion in spontaneously beating neonatal rat cardiomyocytes, a response associated with increases in cellular adenosine 3',5'-cyclic monophosphate (cAMP) formation, calcium influx, and the mobilization of calcium from intracellular stores. cAMP formation and calcium influx appear to play major roles in PAF-induced ANP secretion in this system, insofar as PAF-induced ANP release was substantially reduced in the presence of the (R)-p-diastereoisomer of adenosine 3',5'-cyclic monophosphorothioate (10 microM), whereas both PAF-induced calcium influx and ANP secretion were abolished in the presence of the calcium channel antagonist nifedipine (0.1 microM). Consistent with these results, N6-2'-O-dibutyryl cAMP (DBcAMP, 10 microM) and/or forskolin (0.1 microM) simultaneously increased cAMP production, calcium influx, and ANP release in these cells, with both DBcAMP- and forskolin-induced ANP secretion being fully abolished in the presence of 0.1 microM nifedipine. Taken together, these results suggest that PAF, DBcAMP, and forskolin promote ANP secretion in spontaneously beating cardiomyocytes via the activation of a cAMP-dependent, nifedipine-sensitive myocardial calcium channel and that calcium influx is a major requirement for cAMP-induced ANP secretion in this system.

Platelet-activating factor inhibits (Na+,K+) ATPase activity in rat brain

In the present study, experiments were conducted to determine the effect of platelet-activating factor (PAF) on (Na+,K+)-ATPase in rat cerebral cortex. PAF, but not lysoPAF, inhibited (Na+,K+)ATPase activity, in a dose- and time-dependent manner, 10(-7) to 10(6) M being the most effective dose. These effects were abolished in the presence of PCA-4248, a PAF antagonist, indicating that the PAF effect may be mediated by its specific membrane receptors. Omission of external calcium caused an increase in the basal activity and abolished the PAF effect on (Na+,K+)ATPase. The present study demonstrates that PAF inhibits (Na+,K+)ATPase activity in the cerebral cortex and suggests that PAF released during certain pathological conditions, such as ischemia, may act on ATPase. This could be one possible mechanism of PAF action that needs further attention.

Vitamin E enhances the acylation of 1-O-alkyl-sn-glycero-3-phosphocholine in human endothelial cells

1-O-Alkyl-2-acyl-sn-glycero-3-phosphocholine (alkylacyl-GPC) is the precursor of platelet-activating factor. It is formed via the CoA-independent transacylase reaction, which transfers the polyenoyl acyl group from the sn-2 position of a diacyl phospholipid to the sn-2 position of 1-O-alkyl-sn-glycero-3-phosphocholine (alkyl-GPC). We have reported previously that vitamin E alters phospholipid turnover in the endothelial cells by increasing arachidonic acid release and prostacyclin synthesis. In the present study, the role of vitamin E in the formation of alkylacyl-GPC was investigated. Incubation of endothelial cells with vitamin E resulted in an increase in the formation of [3H]alkylacyl-GPC from [3H]alkyl-GPC. The effect of vitamin E was dose-dependent at concentrations below 23 microM. However, vitamin E did not have a direct effect on the transacylase activity. When endothelial cells were incubated with vitamin E, the CoA-independent transacylase activity in the cell homogenate was found to be enhanced. Kinetic analysis of the transacylase activity in the pre-incubated cells showed that the enhancement of enzyme activity was at the enzyme-substrate level. When endothelial cells were incubated with vitamin E analogues (Trolox, tocol and tocopherol acetate), only limited enhancement of the transacylation process was detected. It is clear that vitamin E enhanced the synthesis of alkylacyl-GPC from alkyl-GPC in a very specific manner by an indirect stimulation of the CoA-independent transacylase activity. The regulation by vitamin E of the formation of alkylacyl-GPC may mediate the transfer of arachidonate from the diacyl phospholipid pool into the ether-linked phospholipid pool.

Nasal neutrophilia and eosinophilia induced by challenge with platelet activating factor

BACKGROUND

It has been demonstrated that in vitro platelet activating factor-acether (1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphorylcholine; PAF) has the capacity to attract eosinophils and neutrophils. We investigated whether the same applies when human nasal airways are stimulated with PAF.

METHODS

Symptom scores and cytologic changes in nasal lavage fluids were evaluated in 10 atopic and 10 nonatopic subjects after nasal challenge with PAF, its precursor and metabolite, 1-O-hexadecyl-sn-glycero-3-phosphorylcholine (lyso-PAF), or saline solution.

RESULTS

Nasal obstruction was reported by all the atopic subjects and seven of the 10 nonatopic subjects after nasal challenge with PAF; other symptoms such as rhinorrhea, itching, and sneezing were generally mild. PAF induced neutrophilia, which appeared after 30 minutes in atopic subjects and after 1 hour in nonatopic subjects, and peaked at 3 hours in both. Less neutrophilia was found 3 hours after stimulation with lyso-PAF in both groups of subjects. PAF also induced eosinophilia, which appeared after 30 minutes in atopic subjects and only after 3 hours in nonatopic subjects. An increase in eosinophil counts was observed 3 hours after lyso-PAF stimulation in atopic but not in nonatopic subjects.

CONCLUSION

PAF can attract neutrophils and eosinophils into human nasal airways; however, the recruitment of inflammatory cells is more rapid in atopic than in nonatopic subjects, suggesting a different degree of responsiveness to PAF challenge in the two groups of subjects.

Plasma degradation of platelet-activating factor in severely ill patients with clinical sepsis

OBJECTIVE

To study the plasma degradation of platelet-activating factor in severely ill patients with clinical sepsis.

DESIGN

A prospective, nonrandomized control study.

SETTING

Intensive care unit in a university hospital.

PATIENTS

Thirteen critically ill male patients with clinical sepsis, due to medical or surgical illness, and ten normal male volunteers were studied. Measurements were repeated in seven patients who survived.

MEASUREMENTS AND MAIN RESULTS

The plasma activity of acetylhydrolase, the lipoprotein-associated enzyme that hydrolyses platelet-activating factor to its biologically inactive lyso-derivative was determined using an optimized enzyme assay. The plasma half-life of platelet-activating factor was also measured, along with phospholipase A2 activity, lyso-platelet-activating factor, and serum lipid concentrations. Patients results were compared with those results of normal controls and followed once in survivors. Acetylhydrolase activity in the patient group was significantly lower than in normal subjects (median 34, interquartile range 17 to 54 nmol/min/mL vs. median 60, interquartile range 56 to 80 nmol/min/mL; p < .002), while overall, the plasma half-life of platelet-activating factor did not differ significantly between the groups. However, the half-life of platelet-activating factor in six patients who died (median 3.3, range 3.3 to 4.3 mins) was significantly greater than in either survivors (median 2.1, range 1.4 to 2.9 mins; p < .001) or the normal group (median 2.5, range 2.2 to 2.8 mins; p < .001). Consistent with theoretical prediction, a significant linear relationship existed between platelet-activating factor half-life and the reciprocal of acetylhydrolase activity in the patient group (p < .05). Plasma phospholipase A2 activity was markedly increased in the patient group, while plasma lyso-platelet-activating factor and serum lipid concentrations were severely decreased.

CONCLUSIONS

Depression of acetylhydrolase activity was consistent with the concentration of lipids with which it is associated. Platelet-activating factor half-life was relatively well preserved because of the nature of its relationship with enzyme activity. The half-life was prolonged in those patients with the worst outcome and the breakdown in plasma degradation of platelet-activating factor could have contributed to pathophysiology in these subjects.

Human endothelial cells are targets for platelet-activating factor (PAF). Activation of alpha and beta protein kinase C isozymes in endothelial cells stimulated by PAF

We evaluated the role of the protein kinase C (PKC) and its isozymes in the activation of human endothelial cells (EC) stimulated by platelet-activating factor (PAF). Exposure of confluent EC to PAF resulted in a rapid and concentration-dependent redistribution of PKC from cytosol to plasma-membrane, rearrangement of cytoskeleton (i.e. decrease in F-actin content and redistribution of vinculin), and finally increase in the transendothelial flux of 125I-albumin. Stimulation of EC with oleylacetylglycerol or phorbol 12-myristate 13-acetate induced the modification of the cytoskeletal structures and the increase of 125I-albumin clearance. Inhibitors of PKC prevented the effects induced by PAF on the cytoskeleton and on the barrier function of the EC monolayer. Confluent EC expressed only alpha, beta, and epsilon PKC isoforms. Biochemical and immunochemical analysis showed that the time course of the PKC isozymes translocation from cytosol to the membrane fraction of EC stimulated by PAF was different: beta isoform was redistributed more quickly than alpha isoform. PAF did not induce translocation of PKC epsilon. These results suggest that activation of PKC alpha and beta is an important signal transduction pathway by which PAF activates endothelial monolayer and modify its function of barrier to macromolecules.

1 Acyl-2 acetyl-sn-glycero-3 phosphocholine decreases the susceptibility of low-density lipoprotein to oxidative modification by copper ions, monocytes or endothelial cells

The effects of platelet-activating factor (PAF) and its analogue, 1 acyl-2 acetyl-sn-glycero-3 phosphocholine (1 acyl-2 acetyl-GPC), were investigated on the oxidative modification of low-density lipoprotein (LDL) by copper ions, U937 monocyte-like cells or endothelial cells, by determination of the lipid peroxidation end products (TBARS) content and measurement of the electrophoretic mobility of the particle. 1 Acyl-2 acetyl-GPC, in the concentration range 1-5 micrograms/ml, inhibited LDL oxidation in a dose-dependent manner in the three systems, whereas PAF had no effect. The protective effect of 1 acyl-2 acetyl-GPC was markedly more important when oxidative modification was performed with endothelial cells, leading to total inhibition at 5 micrograms/ml. At the same concentration, the TBARS production was inhibited by 60% and 20% with monocytes and copper ions, respectively. The degradation by J774 macrophage-like cells of LDL modified by copper ions, U937 monocyte-like cells or endothelial cells was also inhibited when modification was performed in the presence of 1 acyl-2 acetyl-GPC. Furthermore, preincubation of the LDL particle with 1 acyl-2 acetyl-GPC before modification protected the lipoprotein against oxidation, whereas preincubation of the cultured cells with the phospholipid had no effect. Thus 1 acyl-2 acetyl-GPC decreases the susceptibility of the LDL particle to oxidative modification, possibly by intercalation within the lipid phase of the particle. Since LDL oxidation is believed to play an important role in the initiation and progression of atherosclerosis, this inhibitory effect of 1 acyl-2 acetyl-GPC might be of importance in view of the fact that this phospholipid is produced concomitantly with PAF in some inflammatory cells.

Comparison of 1-O-alkyl-, 1-O-alk-1'-enyl-, and 1-O-acyl-2-acetyl-sn-glycero-3-phosphoethanolamines and -3-phosphocholines as agonists of the platelet-activating factor family

Four naturally occurring platelet-activating factor (PAF) analogs, 1-alk-1'-enyl-2-acetyl-sn-glycero-3-phosphocholine, 1-hexadecanoyl-2-acetyl-sn-glycero-3-phosphocholine, 1-octadecanoyl-2-acetyl-sn-glycero-3-phosphocholine, and 1-alkyl-2-acetyl-sn-glycero-3-phosphoethanolamine, stimulated human neutrophils (PMN) to mobilize Ca2+, degranulate, and produce superoxide anion. They were, respectively, 5-, 300-, 500-, and 4000-fold weaker than PAF in each assay; inhibited PMN-binding of [3H]PAF at concentrations paralleling their biological potencies; and showed sensitivity to the inhibitory effects of PAF antagonists. PAF and the analogs, moreover, desensitized PMN responses to each other but not to leukotriene B4 and actually increased (or primed) PMN responses to N-formyl-MET-LEU-PHE. Finally, 5-hydroxyeicosatetraenoate-enhanced PMN responses to PAF and the analogs without enhancing the actions of other stimuli. It stereospecifically raised each analog's potency by as much as 100-fold and converted a fifth natural analog, 1-alk-1'-enyl-2-acetyl-sn-glycero-3-phosphoethanolamine from inactive to a weak stimulator of PMN. PAF and its analogs thus represent a structurally diverse family of cell-derived phospholipids which can activate, prime, and desensitize neutrophils by using a common, apparently PAF receptor-dependent mechanism.

Platelet-activating factor causes ventilation-perfusion mismatch in humans

We hypothesized that platelet-activating factor (PAF), a potent inflammatory mediator, could induce gas exchange abnormalities in normal humans. To this end, the effect of aerosolized PAF (2 mg/ml solution; 24 micrograms) on ventilation-perfusion (VA/Q) relationships, hemodynamics, and resistance of the respiratory system was studied in 14 healthy, nonatopic, and nonsmoking individuals (23 +/- 1 [SEM]yr) before and at 2, 4, 6, 8, 15, and 45 min after inhalation, and compared to that of inhaled lyso-PAF in 10 other healthy individuals (24 +/- 2 yr). PAF induced, compared to lyso-PAF, immediate leukopenia (P < 0.001) followed by a rebound leukocytosis (P < 0.002), increased minute ventilation (P < 0.05) and resistance of the respiratory system (P < 0.01), and decreased systemic arterial pressure (P < 0.05). Similarly, compared to lyso-PAF, PaO2 showed a trend to fall (by 12.2 +/- 4.3 mmHg, mean +/- SEM maximum change from baseline), and arterial-alveolar O2 gradient increased (by 16.7 +/- 4.3 mmHg) (P < 0.02) after PAF, because of VA/Q mismatch: the dispersion of pulmonary blood flow and that of ventilation increased by 0.45 +/- 0.1 (P < 0.01) and 0.29 +/- 0.1 (P < 0.04), respectively. We conclude that in normal subjects, inhaled PAF results in considerable immediate VA/Q inequality and gas exchange impairment. These results reinforce the notion that PAF may play a major role as a mediator of inflammation in the human lung.

Differences in the acyl composition of the platelet-activating factor (PAF) precursor and other choline phosphoglycerides of the rabbit retinal rod outer segments and neural retina

Choline phosphoglycerides comprise almost half of vertebrate retinal phospholipids. This lipid pool contains the precursor of the potent lipid mediator, platelet-activating factor. The acyl composition and distribution of the different subclasses of the choline phosphoglycerides (alkylacyl-[or the precursor of platelet-activating factor], alkenylacyl-[or choline plasmalogen] and diacyl-glycero-3-phosphocholine) were studied in intact rabbit retina, neural retina and rod outer segments. Choline phosphoglycerides were isolated by high performance liquid chromatography and derivatized by acetylation after phospholipase C treatment. The derivatives were purified by high performance liquid chromatography and subjected to methanolysis. Fatty acids were analyzed by capillary gas liquid chromatography. In the intact retina and in the neural retina, the alkylacyl-glycero-3-phosphocholine and alkenylacyl-glycero-3-phosphocholine comprise 1.2% and 1.5%, respectively, of the total choline phosphoglycerides, whereas the rod outer segments contain twice the proportion of the precursor of platelet-activating factor and no detectable plasmalogens. On a mole percent basis, arachidonic acid was highest in the neural retinal alkenylacyl-glycero-3-phosphocholine (27%), 18% in the alkylacyl-glycero-3-phosphocholine and only 5% in the diacyl-glycero-3-phosphocholine. However, alkylacyl-glycero-3-phosphocholine from rod outer segments was enriched in docosapentaenoic acid (18%) while arachidonic acid was in the 3-4% range. Our results suggest that, in the neural retina, alkyl-arachidonoyl-glycero-3-phosphocholine is a source of both platelet-activating factor and of arachidonic acid which may be a substrate for both prostaglandins and lipoxygenase metabolites during an inflammatory episode and may contribute to the retinal pathology.

Anti-edema action of formoterol in rat trachea does not depend on capsaicin-sensitive sensory nerves

The beta 2-adrenergic agonist formoterol has been shown to inhibit plasma extravasation in the respiratory mucosa associated with neurogenic inflammation as well as that caused by histamine or bradykinin. It is unknown whether these effects of formoterol are mediated through an action of sensory nerves or through a direct effect on the leaky blood vessels. In the present study we sought to determine whether capsaicin-sensitive sensory nerves are essential for the anti-edema effect of formoterol in the rat trachea. Substance P (5 micrograms/kg), PAF (hexadecyl-PAF, 5 micrograms/kg), or bradykinin (10 mg/kg) was injected intravenously to increase vascular permeability. The amount of plasma extravasation was measured with two tracers, Evans blue dye and Monastral blue pigment. The effectiveness of formoterol's anti-edema action was assessed in two groups of rats. One was pretreated with capsaicin to eliminate tachykinin-containing sensory nerves and another, the control group, was not pretreated. We found that in control rats formoterol inhibited to a similar extent the extravasation of Evans blue and Monastral blue caused by all three mediators. The highest intravenous dose of formoterol (10 micrograms/kg) reduced substance P-induced extravasation of Monastral blue by 59%, reduced PAF-induced extravasation by 74%, and reduced bradykinin-induced extravasation by 58%. Pretreatment of rats with a dose of capsaicin that eliminated at least 94% of the substance P-immunoreactive nerve fibers did not significantly reduce the effectiveness of formoterol against any of the mediators.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of a new de-N-acetyl-lysoglycosphingolipid on chemically-induced inflammatory bowel disease: possible mechanism of action

A new, orally active de-N-acetylated lysoglycosphingolipid (WILD20) was evaluated as antiinflammatory agent using a model of chemically-induced inflammatory bowel disease (IBD) in the rat to mimic human ulcerative colitis and Chron's disease. IBD was induced by hapten trinitrobenzenesulphonic acid (TNB). WILD20, orally administered as preventive or curative, was demonstrated to be efficacious at daily dosages of 0.1-1 mg/kg for 4-5 days. Damage scores, body weight, spleen weight, colonic tissular levels of LTB4, myeloperoxidase (MPO) and malondialdehyde (MDA) are influenced and brought into parameters of normality. Histological observation demonstrated quicker healing, better repair, reduced inflammation, and poor eosinophil degranulation. The mechanisms underlying WILD20 antiinflammatory effects were investigated: whereas WILD20 fails to show a direct effect on PKC, it reduces PKC translocation to the membrane; cellular PLA2 was consequently greatly reduced through this mechanism and thought to be responsible for WILD20 efficacy towards chemically-induced IBD.

Studies on the subcellular distribution of 1-O-alkyl-2-acetyl-sn-glycero phosphocholine (PAF) and on the enzymic activities involved in its biosynthesis within the ciliate Tetrahymena pyriformis

The ciliated protozoan Tetrahymena pyriformis contains platelet-activating factor (PAF) as a physiological minor lipid. Its subcellular localization was found as follows: 13.7% in the pellicles, 24.9% in mitochondria, 56.5% in microsomes and 7.1% in the cytosol. Succinate dehydrogenase was used as marker enzyme. PAF remains cell-associated unless bovine serum albumin is included in the extracellular medium. In this case 15% of total PAF, portion comparable to that found in the pellicles, is released. Investigation of the principal enzymic activities involved in PAF formation showed that PAF-acetyltransferase (2.3.167) is totally absent from the protozoan. This means that the 'remodelling' pathway occurring in pro-inflammatory cells does not contribute in PAF formation in our system. A dithiothreitol (DTT)-insensitive CDPcholine phosphocholinetransferase activity involved in PAF biosynthesis is shown for the first time to be responsible for PAF production in T. pyriformis. It uses exogenous alkyl-acetyl-glycerol as substrate and is saturated over substrate concentration 250 microM. It can also use endogenous lipids as substrate. It is distributed mainly in mitochondria and microsomes, much less is found in the pellicles and it is totally absent from the cytosol. Its insensitivity to DTT, its selectivity to alkyl-acetyl-G and its different distribution compared to the enzymic activity involved in PC formation (EC 2.7.8.2) suggest that a different enzyme, specific for PAF formation (EC2.7.8.16) via the de novo pathway exists in the protozoan.

[Formation of a structural isomer of platelet activating factor during acetylation of 1-alkyl-sn-glycero-3-phosphocholine]

In studying acetylation of 1-alkyl-sn-glycero-3-phosphocholine (lyso PAF) with acetic anhydride, a key step of the platelet activating factor (PAF) synthesis, we have found that in the presence of triethylamine or 4-dimethylaminopyridine some 1-alkyl-3-acetyl-sn-glycero-2-phosphocholine as an admixture to PAF formed, whereas the acid catalysed reaction resulted in isomerically pure PAF. The mechanism of the reaction leading to the PAF structural isomer is discussed.

Aspirin induces non-enzymatic formation of platelet-activating factor from lyso platelet-activating factor

Substantial amounts of platelet-activating factor (PAF) were formed when lysoPAF was mixed with aspirin (e.g. 0.04% of added lysoPAF (200 nmol) was converted to PAF when mixed with aspirin (2 mumol) for 1 h). Non-enzymatic formation of PAF from aspirin and lysoPAF also occurs in the aqueous solution or in organic solvents in time-dependent and dose-dependent manners. Possible meanings of the non-enzymatic formation of PAF are discussed.

The potential for platelet-activating factor synthesis in brain: properties of cholinephosphotransferase and 1-alkyl-sn-glycero-3-phosphate acetyltransferase in microsomal fractions of immature rabbit cerebral cortex

The synthesis of platelet-activating factor (PAF) was studied in microsomal fractions of cerebral cortices of 15-day-old rabbits. These included: a total microsomal fraction P3, rough and smooth microsomes, R and S, and microsomal fraction P derived from isolated nerve cell bodies. Cholinephosphotransferase (CPT) generating PAF from alkylacetylglycerol had the highest specific activities in fractions R and P (24 and 6 times the homogenate values, based on membrane phospholipid content). This CPT activity differed from that which synthesized phosphatidylcholine as the latter was sensitive to dithiothreitol inhibition and was more readily inhibited by Triton X-100. As the CPT activity for PAF synthesis relies on the production of alkylacetylglycerol we studied the acetyltransferase which forms 1-alkyl-2-acetyl-sn-glycero-3-phosphate (AAGP). This enzyme had the highest specific activity in fraction R, followed by fractions P3 and P. There was evidence that the acetyltransferase was more active in a phosphorylated form. NaF maximized the recovery of AAGP products in the assays. The pH optimum for acetylation was in a range of 8.0-9.0. Lyso PAF did not inhibit the formation of AAGP and the rates of formation of PAF by acetylation were less than 5% of values for AAGP synthesis. During AAGP formation there was no evidence for subsequent alkylacetylglycerol formation in the absence of NaF, but a small formation of radioactive PAF could be demonstrated from AAGP under the CPT assay conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Agonistic activity and charge driven interaction potentiality of PAF derivatives

Taking into account the contribution of the C2 substituent to the interaction ability of PAF derivatives, an investigation has been undertaken on: i) the role of the presence of two unitary charges of opposite sign in the molecule; ii) the role of the spacing of the same charged moieties. For this purpose, the theoretical analysis of model compounds for newly synthesized and already known compounds has been performed by means of computer-aided modelling, molecular mechanics and semiempirical quantum chemical computational methods. The combination of the results with the available pharmacological data allows a discussion about the stereoelectronic features required for agonist interaction at the PAF receptor.

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.

Platelet-activating factor induces collagenase expression in corneal epithelial cells

Platelet-activating factor (PAF), a potent lipid mediator involved in inflammatory and immune responses, accumulates rapidly in response to injury in a variety of tissues, including the corneal epithelium. However, the precise role of this compound in the cascade of events following insult has not been defined. Here we examined the effect of PAF on gene expression in the epithelial cells of rabbit corneas in organ culture. We found that incubation with 100 nM methylcarbamoyl PAF, a nonhydrolyzable analog of PAF, produced rapid transient 2.8- and 3.5-fold increases in the expression of c-fos and c-jun, respectively, at 1 hr, followed by increased expression of the collagenase type I gene beginning at 3 hr and peaking at 14-fold by 8 hr. Addition of the protein-synthesis-inhibitor cycloheximide superinduced c-fos and c-jun, strongly potentiating the PAF effect, but inhibited the induction of collagenase type I expression, suggesting the existence of a transcriptional factor linking the two events. BN-50730, a selective antagonist of intracellular PAF-binding sites, blocked the expression of the immediate-early genes as well as the increase in collagenase type I mRNA. Our results suggest that one of the functions of PAF may be to enhance the breakdown of the extracellular matrix as a part of the remodeling process during corneal wound healing after injury. Pathologically, a PAF-induced overproduction of collagenase may be a factor in the development of corneal ulcers, as well as other pathophysiological conditions such as cartilage destruction in arthritis. If so, inhibitors of this signal-transduction pathway may be useful as tools for further investigation and, eventually, as therapeutic agents to treat such disorders.

Biosynthesis of platelet-activating factor in cultured mast cells. Involvement of the CoA-independent transacylase demonstrated by analysis of the molecular species of platelet-activating factor

We have recently demonstrated that arachidonate [20:4(5,8,11,14)] was primarily linked to the hexadecyl (16:0) and octadecenyl (18:1) species of alkylacyl derivatives of glycerolphosphocholine (GroPCho). Consistent with the involvement of arachidonate-specific CoA-independent transacylase in the synthesis of platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-GroPCho), 16:0 and 18:1 PAF species were formed upon antigen stimulation [Joly, F., Breton, M., Wolf, C., Ninio, E. & Colard, O. (1992) Biochim. Biophys. Acta 1125, 305-312]. In the present work, addition of lyso-PAF to mast cells resulted in PAF production. We analyzed the PAF species formed in the presence of a defined lyso-PAF molecular species in order to differentiate between either direct acetylation or involvement of the membrane precursor. The 18:1 lyso-PAF was more effective than the 16:0 in producing PAF which was composed of 95% 18:1 PAF, the balance being 16:0, indicating that part of the acetylated lyso-PAF originated from the cellular pool of alkyl-arachidonyl-GroPCho in resting cells. Consistent with alkyl-arachidonyl-GroPCho species content and acetyltransferase specificity, similar amounts of 16:0 and 18:1 PAF species were formed when mast cells were stimulated with antigen. Supplemented with 16:0 or 18:1 lyso-PAF, antigen-stimulated mast cells responded by 230% and 125% increase in PAF synthesis, respectively. As expected, the amount of the PAF species corresponding to the added lyso-PAF was increased. More interestingly, addition of 16:0 lyso-PAF almost doubled the amount of 18:1 PAF content as compared to antigen alone, thus indicating that the lyso-PAF formed via the CoA-independent transacylase was significantly used for PAF synthesis, despite a large excess of exogenous lyso-PAF. The CoA-independent transacylase, measured using [3H]lyso-PAF as a substrate in sonicates from antigen-stimulated cells, was decreased concurrently with PAF formation. In conclusion, we show that when lyso-PAF is added to mast cells, a direct acetylation may occur. However, PAF is preferentially synthesized through a mechanism involving the CoA-independent transacylase reaction.

Effects of platelet activating factor on human sperm function in vitro

The direct effects of platelet activating factor (PAF) and the specific PAF receptor antagonist, CV-3988, on the fertilizing ability of human spermatozoa were investigated. PAF (10(-7)-10(-11) M) increased the human sperm penetration rates in a sperm penetration assay at all doses > 10(-11) M. In contrast, treatment of the spermatozoa with 10(-5) CV-3988 caused a significant decrease in human sperm penetration of zona-free hamster oocytes and adversely affected sperm motility after 24 h of incubation. This suppression was reversed by the addition of PAF. The acrosome reaction was also enhanced by PAF treatment of spermatozoa but this effect was not observed in calcium-free medium. While 10(-5) M CV-3988 decreased the acrosome reaction, the inhibition was also reversed by the addition of PAF. These results suggest that PAF may have a direct role in the fertilizing capacity of human spermatozoa. These findings also suggest that PAF may have a clinical application in an in-vitro fertilization programme.

The effect of platelet-activating factor on the responsiveness of the human nasal airway

1. The effects of inhaled platelet-activating factor (PAF) on responsiveness of the human nasal airway were examined in normal subjects by measuring nasal airway resistance in response to histamine and bradykinin at 2, 6, 24, 48 h and 7 d after PAF administration. Eosinophil cationic protein (ECP) in nasal secretions was also measured. 2. Intranasal aerosol administration of PAF, 30 or 60 micrograms per nostril to normal human subjects induced an increased responsiveness to inhaled histamine, 50 to 400 micrograms and bradykinin, 100 micrograms per nostril at 2, 6 and 24 h following PAF treatment. However the effect was not apparent at 48 h or 7 days after PAF administration. 3. Intranasal administration of lyso-PAF, 60 micrograms by aerosol did not increase the reactivity of the nasal airway in response to histamine, 200 micrograms. 4. There was no difference in the time course of the PAF-induced hyperresponsiveness to histamine or bradykinin. 5. PAF-induced nasal hyperresponsiveness at 2 and 6 h was associated with increases in the ECP concentration of the nasal lavage fluid. 6. Vitamin E pretreatment of subjects resulted in the attenuation of the PAF-induced hyperresponsiveness to histamine, and a decrease in ECP levels of the nasal lavage fluid. 7. The results suggest that in the human nasal airway, PAF induces a non-specific hyperresponsiveness which is accompanied by eosinophil activation in the nasal cavity. Free radical production induced by PAF may contribute to the hyperresponsiveness and the activation of eosinophils.

Hormonal regulation of platelet-activating factor-acetyltransferase activity in rat tissues

Platelet-activating factor (PAF) plays a very important role in a number of physiological or pathological conditions. Recently, we and others have suggested that the PAF concentration in various tissues may be regulated by the enzyme that inactivates PAF, PAF acetylhydrolase (PAF-AH). Furthermore, it has been shown that the plasma isozyme of PAF-AH is regulated by various steroid hormone. Lyso-PAF:acetyl-CoA acetyltransferase (PAF-AT) is a key enzyme in PAF biosynthesis by the remodeling pathway. In the present study, the hormonal effects on the tissue levels of PAF-AT activity were investigated in various organs of the rat. The microsomal fraction was prepared from the spleen, lung, uterus, and liver of female rats which had been pretreated with dexamethasone, 17 alpha-ethynylestradiol, medroxyprogesterone-acetate, or vehicle and the PAF-AT activity was assayed. The rats treated with dexamethasone demonstrated significantly lower PAF-AT specific activity in the liver (P < 0.001) and spleen (P < 0.01), compared to the control group. A dose-dependent decrease of PAF-AT specific activity was demonstrated following dexamethasone treatment. Medroxyprogesterone injection also caused a significant decrease of PAF-AT activity in the liver (P < 0.01). No change of PAF-AT activity in any tissue was observed in the 17 alpha-ethynylestradiol-injected animals. Plasma PAF-AH activity was increased significantly by the administration of dexamethasone (P < 0.001) or medroxyprogesterone (P < 0.001), and decreased by 17 alpha-ethynylestradiol (P < 0.001), as previously described. The observation that dexamethasone or medroxyprogesterone treatment decreases the activity of PAF-AT and increases the plasma PAF-AH activity provides a mechanism for the tight control of PAF in tissues.

Gastric secretion of platelet activating factor and precursors in healthy humans: effect of pentagastrin

The release of platelet activating factor (PAF-ACETHER or PAF) and its precursors in the gastric lumen was assessed in 13 normal subjects in basal condition and after stimulation by gastrin. Acid, pepsin, and sialic acid outputs were determined under the same conditions. Gastric juice was collected using a nasogastric tube after overnight fast in basal condition for 60 minutes, then under pentagastrin infusion (6 micrograms/kg/hr for 60 minutes). Platelet activating factor was detected at low concentration in 4/13 subjects under basal condition (mean (SEM) 1.2 (0.6) pg/hr) while high concentrations of lyso platelet activating factor (6.1 (1.8) microgram/hr) and of alkyl-acyl-glycerophosphocholine (AAGPC) (11.5 (3) micrograms/hr) were found in 13 and 11 subjects, respectively. Platelet activating factor was not detected during pentagastrin infusion, while lyso platelet activating factor and alkyl-acyl-glycerophosphocholine were detected in 13 and in 12 subjects, respectively. Compared with the basal condition these platelet activating factor precursors increased significantly (p < 0.001) going up to fivefold baseline (31.8 (6.8) micrograms/hr and 53 (9.3) micrograms/hr respectively) in response to pentagastrin. There was a positive correlation between platelet activating factor precursors and acid or pepsin output but not between platelet activating factor precursors and sialic acid. As sialic acid may be considered an index of mucus glycoprotein degradation, it seems that gastrin stimulation of gastric epithelial cells results in a concomittant secretion of platelet activating factor precursors, acid, and pepsin irrespective of mucus glycoprotein degradation.

A comparative study of the effects of 1-acyl-2-acetyl-sn-glycero-3-phosphocholine and platelet activating factor on histamine and leukotriene C4 release from human leukocytes

BACKGROUND

IgE-mediated stimulation of human basophils and lung mast cells causes the synthesis of larger amounts of 1-acyl-2-acetyl-sn-glycero-3-phosphocholine (1-acyl-2-acetyl-GPC) than 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet activating factor [PAF]).

METHODS

To study the biologic activity of 1-acyl-2-acetyl-GPC, we compared its effects and those of PAF on histamine and leukotriene C4 (LTC4) release from human mixed leukocytes that contained basophils.

RESULTS

1-Acyl-2-acetyl-GPC (0.1 to 10 mumol/L) failed to release significant amounts of histamine (> or = 10%) in most donors tested (20 of 24), whereas PAF (0.01 to 1 mumol/L) was active in 58%. 1-Acyl-2-acetyl-GPC (0.1 to 10 mumol/L) was a stimulus for LTC4 release (132 +/- 30 ng/micrograms of histamine) with a potency of about 1000 times less than PAF. The kinetics of 1-acyl-2-acetyl-GPC-activated LTC4 release were similar to those of PAF (half-life approximately equal to 2 minutes). The specific PAF receptor antagonist, WEB 2086 (10 nmol/L to 10 mumol/L), inhibited both 1-acyl-2-acetyl-GPC- and PAF-mediated LTC4 release with the same potency (inhibitory concentration of 50% approximately equal to 1.5 mumol/L). Brief (2-minute) cell preincubation with 1-acyl-2-acetyl-GPC in the absence of extracellular Ca2+ induced a decrease in the subsequent Ca2+ dependent activation of PAF. Similarly, 1-acyl-2-acetyl-GPC (0.1 to 10 mumol/L) caused a concentration-dependent inhibition of PAF-activated histamine secretion (inhibitory concentration of 50% approximately equal to 0.2 mumol/L).

CONCLUSIONS

Our data suggest that 1-acyl-2-acetyl-GPC may represent, under certain circumstances, a modulator of human basophil mediator release via mechanisms shared with PAF.

Distribution of lysophospholipids and metabolism of platelet-activating factor in human follicular and peritoneal fluids

The distribution of lysophosphatidylcholine, lyso-platelet-activating factor and platelet-activating factor (PAF) was studied in human plasma and in follicular and peritoneal fluid. In plasma, peritoneal and follicular fluids, 51%, 87% and 89%, respectively, of the total lipids were found in the protein fraction (the density > 1.21 fraction). Two forms of lysophospholipids were identified in this fraction: one of high affinity and one of low affinity for albumin. The metabolism of PAF in human follicular fluid, peritoneal fluid and plasma was also investigated. PAF-acetylhydrolase activity was found in both peritoneal and follicular fluids which induced a time-dependent hydrolysis of [3H]PAF. The half-life of PAF was estimated to be 7-12 min in plasma, 15-25 min in peritoneal fluid and approximately 2 h in follicular fluid. PAF-acetylhydrolase activity in embryo culture media supplemented with 10% serum was markedly inhibited by addition of commercial serum albumin. When 25 g albumin l-1 was added, 22% of [3H]PAF was hydrolysed h-1 compared with 72% in media without albumin. The concentrations of lysophosphatidylcholine measured in plasma, in follicular and peritoneal fluids were 252, 286 and 53 mumol l-1, respectively. The distribution of these lysophospholipids and the metabolism of PAF in the female genital tract fluids reported in the present study provide evidence for the involvement of these biologically active lipid mediators in a variety of reproductive processes including sperm-egg interactions and embryonic development.