The role of Ca2+ in regulating the catabolism of PAF-acether (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) in rabbit platelets

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

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

Presence and metabolism of lyso platelet-activating factor in human bone marrow

Lyso platelet-activating factor (PAF) is the precursor of PAF, an inflammatory phospholipid molecule present in human bone marrow. The present study shows that in healthy volunteers lyso PAF concentrations are significantly lower (P = 0.0001, Mann-Whitney U-test) in bone marrow plasma (594 +/- 67 ng/ml, n = 47) than in blood plasma (1448 +/- 99 ng/ml, n = 31). Marrow plasma lyso PAF concentrations are similar in patients with lymphoid and nonlymphoid malignancies as compared with controls. Freshly isolated mononuclear marrow cells and cultures of marrow stromal cells contain lyso PAF. Experiments with [3H]lyso PAF indicate that human mononuclear bone marrow cells and marrow stromal cells actively acylate lyso PAF into a 1-alkyl analogue of phosphatidylcholine. Results of this investigation indicate: (1) that lyso PAF is present in human marrow cells and plasma; and (2) that marrow cells and stromal cells metabolize it, thus suggesting their role in the regulation of lyso PAF amounts in human bone marrow.

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.

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.

Interactions of prostanoids with the platelet activating factors

The platelet-activating factor (PAF) induced a marked increase of the thromboxane (TX) B2-formation in the incubation medium of isolated myocardium and tissue from other organs. The content of the 6-oxo-prostaglandin (PG)F1 alpha, the inactive metabolite of PGI2, remained uninfluenced or showed a small decrease. PAF, given in a concentration of 2.10(-9) mol/l or a single dose of 100 ng, significantly reduced the contraction force and the coronary flow of isolated guinea-pig hearts. This effect was connected with a high efflux of TXA2. The PAF-antagonist, WEB 2086, nearly abolished the cardiac effects of PAF, and iloprost or a pretreatment with indomethacin markedly reduced the PAF-influence on the heart. The TXA2-antagonist BM 13177 was ineffective. The results indicate a close interaction between the myocardial PAF-effect and the TXA2-formation of the heart tissue, but gave no suggestion for a mediation of the PAF-effect by TXA2. The PAF-antagonistic action of WEB 2086, iloprost and indomethacin could be of some interest in the therapy of cardiovasculatory diseases.

Catabolism of platelet-activating factor by human colonic mucosa. Calcium dependence of the catabolizing enzymes

The catabolism of platelet-activating factor (PAF) and lyso PAF by a supernatant fraction of human colon mucosa homogenates has been studied in vitro. PAF is initially catabolized to lyso PAF by mucosal enzymes via removal of its acetyl group. Incubates in Ca(2+)-free Tris with EDTA showed that the acetyl hydrolase was Ca2+ independent. Addition of the hydrolase inhibitor, phenyl methyl sulphonyl fluoride, significantly reduced the catabolism of PAF. Lyso PAF was further catabolized in at least two ways. An acyl group was incorporated into the sn-2 position of lyso PAF to give 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine (alkyl acyl GPC); this step was Ca2+ independent as shown by omitting Ca2+ and adding EDTA to the incubate. Formation of alkyl acyl GPC was confirmed by HPLC. Alternatively, choline was removed from the head group of lyso PAF by a calcium-dependent lyso phospholipase D. Under the experimental conditions utilized a neutral lipid product was formed but significant amounts of the intermediate lysophosphatidic acid could not be detected. A substance with a chromatographic mobility of Rf = 0.8 on TLC plates having an intact phosphorylcholine head group was also formed but has not yet been identified. It is concluded that the human colon mucosa contains enzymes that actively catabolize pro-inflammatory PAF and lyso PAF.

Platelet-activating factor and related acetylated lipids as potent biologically active cellular mediators

Platelet-activating factor (PAF or 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is the most potent lipid mediator yet discovered. It is known to stimulate a wide span of biological responses ranging from aggregation and degranulation of platelets and neutrophils to a variety of cellular effects involving the stimulation of chemotaxis; chemokinesis; superoxide formation; protein phosphorylation; activation of protein kinase C, arachidonic acid, and phosphoinositide metabolites; glycogenolysis; and tumor necrosis factor production. Obviously, with such a diversity of biological activities, it is not surprising that PAF has been considered to be a key component in numerous diseases related to hypersensitivity and inflammatory responses. Evidence has also been presented for the role of PAF in physiological processes, particularly those involving reproduction and fetal development. Furthermore, because of its potent hypotensive action, PAF has been implicated as a contributing factor in blood pressure regulation. PAF is produced by two independent enzymatic pathways. The remodeling route involves the structural modification of a membrane lipid (1-alkyl-2-acyl-sn-glycero-3-phosphocholine) by replacement of the acyl moiety with an acetate group. An alternate route is the de novo synthesis of PAF from an O-alkyl analogue of a lysophosphatidic acid that requires a reaction sequence of acetylation, dephosphorylation, and phosphocholine addition steps. Hypersensitivity and other pathophysiological reactions are thought to be caused by activation of the remodeling pathway, whereas the de novo route is believed to be the source of endogenous levels of PAF required for physiological functions. Inactivation of PAF occurs when the acetate group is hydrolyzed by an acetylhydrolase that is present in both extra- and intracellular compartments, although the catalytic activity of the two forms of acetylhydrolase are identical, some of their properties differ. The control of PAF metabolism is very complex, but acetylhydrolase, Ca2+, phosphorylation/dephosphorylation of enzymes, and fatty acids (especially polyunsaturates) appear to be important regulatory factors. Specific PAF receptors have clearly been demonstrated on several different types of cells, and although the mechanism of PAF actions is poorly understood, it appears that the PAF/receptor-induced responses are closely associated with the signal transduction process; both G proteins and adenyl cyclase appear to be involved. Because significant quantities of PAF are often retained within certain cells, the possibility of PAF serving as an intracellular mediator has also been proposed.

Platelet-activating factor (PAF) and its relation to prostaglandins, leukotrienes and other aspects of arachidonate metabolism

This article summarizes some of the previously reported findings regarding a lipid mediator known as platelet-activating factor (PAF), and briefly describes its effects on cells and tissues. The effects of PAF have also been considered in relation to certain products of arachidonate metabolism released in response to PAF.

Possible origins of PAF-acether and lyso-PAF-acether in rat lung alveoli secondary to hypoxia

After 4 h hypoxia, platelet activating factor (PAF-acether or 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) and its deacetylated derivative, lyso-PAF-acether, accumulate in rat lung surfactant, the latter in a 1000-fold excess (Prévost, M.C., Cariven, C., Simon, M.F., Chap, H. and Douste-Blazy, L. (1984) Biochem. Biophys. Res. Commun. 119, 58-63). In order to determine the origin of these two phospholipids, rat lung alveolar lavages and rat lung macrophages were examined for phospholipid composition before and after 4 h of hypoxic treatment. Our data indicate an activation of phospholipase A2 in both compartments, as detected by the accumulation of lysophosphatidylcholine. The main effect was observed in lung surfactant, where phosphatidylcholine hydrolysis attained 13%. This change was concomitant with the activation of a calcium-independent phospholipase A2 present in lung alveolar lavages, which might be responsible for the accumulation of some lyso-PAF-acether, alkylacylcholine glycerophospholipids being present in low but significant amounts in lung surfactant. However, the main source of PAF and lyso-PAF-acether appears to be alveolar macrophages, which secreted significant amounts of the two phospholipids upon in vitro hypoxic treatment, although the participation of other cells, such as type II pneumocytes, cannot be excluded. The relative amounts of the two compounds might be regulated by both an intracellular and an extracellular acetylhydrolase, the two enzymes being distinct proteins on the basis of their different isoelectric points.