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Platelet Activating Factor (PAF-Acether) Promotes an Early Degradation of Phosphatidylinositol-4,Sbiphosphate in Rabbit Platelets

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Platelet Activating Factor (PAF-Acether) Promotes an Early Degradation of Phosphatidylinositol-4,Sbiphosphate in Rabbit Platelets View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Volume 153, number 2 FEBS LETTERS March 1983 Platelet activating factor (PAF-acether) promotes an early degradation of phosphatidylinositol-4,Sbiphosphate in rabbit platelets GCrard Mauco, Hugues Chap and Louis Douste-Blazy* INSERM Unite 101, Biochimie des Lipides, HGpital Purpan, 31059 Toulouse, France Received 14 January 1983 32P-Labelled washed rabbit platelets were incubated with 0.6 nM platelet activating factor (PAF-acether), giving a full aggregation and release response within 30-60 s. The major phospholipid changes observed under these conditions were: (1) An increased labelling of phosphatidic acid (PA) within 10 s and of phosphatidylinositol (MPI) at 30 s, reflecting the activation of the MPI cycle via the cytosolic phospholipase C; (2) an enhancement of phosphatidylinositol-4-phosphate (DPI) and phosphatidyl- inositol-4,5-bisphosphate (TPI) labelling at later incubation times; (3) an early degradation of TPI with a counterbalancing formation of DPI. The latter changes suggest a receptor-mediated stimulation of TPI- phosphomonoesterase, the role of which in the mechanism of platelet activation is discussed. Platelets Platelet activating factor @oly)Phosphoinositides phosphatidic acid Phosphomonoesterase 1. INTRODUCTION cells involve the polyphosphoinositides (poly(P1)): phosphatidylinositol-4-phosphate (DPI) and phos- Phospholipid metabolism in stimulated platelets phatidylinositol-4,Sbisphosphate (TPI). An in- has received much attention. One of the earliest creased labelling of poly(P1) was revealed in changes subsequent to platelet stimulation by platelets stimulated by various agonists specific agonists like ADP, thrombin, collagen or [3,4,13,18-201. In [21] thrombin promoted a net platelet activating factor (PAF-acether) seems to synthesis of platelet DPI and TPI, which occurs as be the activation of a phosphatidylinositol (MPI) a late event and parallels the release of lysosomal specific phospholipase C [ 1,2] leading to the pro- enzymes. In [22] a specific degradation of TPI was duction of diglycerides, phosphatidic acid (PA), observed in the first seconds of platelet activation lyso PA and arachidonate [1,3-131. This so-called by thrombin: TPI hydrolysis was not dependent on MPI effect was described in numerous tissues calcium mobilization - it was suggested to par- under specific conditions of stimulation [ 14- 171. ticipate in the release of membrane-bound calcium Other metabolic changes observed in stimulated ~231. This study was undertaken to investigate the Abbreviations: PAF-acether, platelet-activating factor mechanism of action of PAF-acether, a powerful or 1-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine; PA, phospholipid mediator able to promote platelet ag- phosphatidic acid; MPI, phosphatidylinositol; DPI, gregation and secretion [24-271: besides pro- phosphatidylinositol-4-phosphate; TPI, phosphatidyl- moting an early acitvation of MPI-specific phos- inositol-4,5bisphosphate; poly (PI), polyphosphoinosi- pholipase C and a secondary phosphorylation of tides DPI and TPI, PAF-acether also induces an early * To whom correspondence should be adressed degradation of TPI. Published by Elsevier Biomedical Press 00145793/83/0000-0000/.S3.00 0 Federation of European Biochemical Societies 361 Volume 153, number 2 FEBS LETTERS March 1983 2. MATERIALS AND METHODS were separated on oxalate-impregnated silica gel plates, using the bidimensional system in [3]. After HP32PO:-, di-sodium salt, carrier-free, was ob- autoradiography, the radioactive spots were tained from the Commissariat a I’Energie Atom- scraped off and submitted to liquid scintillation ique (Saclay). 5-Hydroxy [G-3H]-tryptamine, counting. creatinine sulfate (lo-20 Ci/mmol) was from NEN (Boston, MA). I-Hexadecyl (rat) glycero- 2.4. Determination of the release reaction phosphorylcholine was purchased from Medmark [ 3H]Serotonin was detected for radioactivity in (Heidelberg). Liquid scintillation counting was supernatants obtained by centrifugation (1500 x g, performed using Picofluor 30 (Packard, Palo Alto 5 min, +4”C) after platelet fixation by formalin CA) and an LS 3000 spectrophotometer (Kontron, [ 121. N-Acetyl-P-D-glucosaminidase was measured Basel). in supernatants obtained after addition of 20 mM EDTA according to [31]. 2.1. Preparation of washed rabbit blood platelets Blood was withdrawn by intracardiac puncture 3. RESULTS of male New Zealand rabbits using 8 mM EDTA (pH 7.4) as an anticoagulant. The platelets were 3.1. Labelling pattern of platelet phospholipids prepared essentially as in [28]. Labelling was per- Fig. 1 shows an autoradiogram of phospholipids formed by incubating either platelet-rich plasma extracted from 32P-prelabelled platelets. As with [3H]serotonin (100 nCi/ml, 30 min, 20°C) or described in [3-4,6,13,18], only TPI, DPI, MPI the first washing fluid (5 x lo8 cells/ml) with and PA incorporated significant radioactivity [32P]phosphate (50,&i/ml, 90 min, 20°C). Final (table 1). Another unidentified compound X suspension (5 x lo8 cells/ml) was in Tyrode buffer represented 6% of the total radioactivity. Special (pH 7.4) containing 1 mM MgCL2, 1 mM CaC12, care was taken to identify lyso MPI. A pure stan- 5.5 mM glucose and 3.5 g/l of bovine serum dard was clearly separated from DPI (fig. 1). albumin. 3.2. Time course of phospholipid changes during 2.2. Synthesis of I-hexadecyl-2-acetyl (rat)- platelet stimulation by PAF-acether glycerophosphorylcholine (PAF-acether) Various changes occurred during platelet ag- PAF-acether was obtained according to [25] and gregation induced by PAF-acether (fig. 2) PA purified by thin-layer chromatography on silica gel radioactivity was increased by 60% after 10 s [29]. It was stored in ethanol at -20°C until use. stimulation and levelled off at 30 s, whereas an enhanced labelling of MPI appeared secondarily. 2.3. Stimulation of the platelets and analytical A biphasic response was observed for poly (PI): procedures after 10 s, TPI was significantly reduced by 12%, PAF-acether was added under a minimal volume whereas DPI displayed a concomitant increase. At of ethanol (12J/ml) to prewarmed platelets suspension (1 min, 37’C) and incubation was per- Table 1 formed at 37°C under stirring. The same volume Radioactive labelling of phospholipid classes in rabbit of ethanol was added to the controls. In alle the ex- platelets periments considered, a full aggregation response was obtained with 6 x lo-” M PAF-acether. Phospholipids Percentages 54.9 + 1.5 Phospholipid analysis: TPI DPI 24.3 + 5.5 Incubation- was blocked by adding 3 vol. PI 9.1 +3 CHC13/CH30H (l/2, v/v) and EDTA (5 mM, PA 5.1 + 1 final cont.). Lipid extraction was performed as in X 6.1 k 1 [30]. The procedure was repeated twice and the Results are mean + SD of the percentage of the label in organic layers were washed by 4 vol. each phospholipid class vs the whole platelet lipid CH3OH/1.2 N HCl (l/l, v/v) [30]. Phospholipids radioactivity 362 Volume 153, number 2 FEBS LETTERS March 1983 OTPI 0 0 0 10 30 so Fig. 1. Chromatogram of phospholipids extracted from INCUEWION TIME (S) pre-labelled platelets. For extraction conditions see section 2. (1) Solvent 1: CHC&/CHsOHN N ammonia Fig. 2. Effect of PAF-acether on phospholipid meta- (9/4/Z, by vol.); (2) Solvent 2: CHCl~/CH3OH/CH3 bolism of pre-labelled rabbit platelets. Results are COOH/HzO (60/30/12/S, by ~01.); (---> non- expressed as the ratio of the radioactivity of a given radioactive iodine-stained lipids; (-) radioactive phospholipid vs the radioactivity of the same phospholipids detected by autoradiography; (0) origin; phospholipid at time zero. Mean f SD of 6 (10 s) or LPI, lysophosphatidylinositol (pure standard added to 10 (30 s and 60 s) independent experiments: assays the lipid extract); SPH, sphingomyeline; PE, phosphati- (e---+); controls (O---O); p < 0.05, according to dylethanolamine; LPE, lysophosphatidylethanolamine; Student’s f-test (*); p < 0.02, (+*); p < 0.01, (or**); PS, phosphatidylserine; PC, phosphatidylcholine; PA, non-significant (a). phosphatidic acid; NL. neutral lipids; FFA, free fatty acids; X, unkown; for other abbreviations, see the footnote. 4. DISCUSSION later incubation times, radioactivity of TPI and DPI was enhanced over controls, the difference be- This study reports 3 modifications of inositol- ing significant for DPI at 60 s. phospholipid metabolism occurring in rabbit In contrast, phospholipid X did not display any platelets stimulated with PAF-acether. One is the significant modification during the whole incuba- activation of the MPI-specific phospholipase C, as tion time. Also, no radioactive spot was detected at evidenced by the production of PA, followed by an the position of lyso MPI. increased labelling of MPI confirming 19,131. As Fig. 3 gives for comparison the time course of 3 discussed in [9], PA produced in the first seconds platelet responses. At the [PAF-acether] used, of stimulation might play a central role in the serotonin release appeared as the earliest event and mechanism of platelet activation. This would fit reached 30% of total serotonin at 10 s, when ag- with the observation that lyso PA are able to pro- gregation and N-acetyl-#7-D-glucosaminidase mote platelet aggregation 132,331. release were still hardly detectable. However, the As to the increased labelling of DPI and TPI two secretory responses were completed within happening at later times, similar results were 30 s, whereas a full aggregation was obtained at’ reported in [ 131 by measuring the incorporation of 60 s. exogenouso-[32P]phosphate into platelet phospho- 363 Volume 153. number 2 FEBS LETTERS March 1983 this counterbalancing increase of DPI is not stoichiometric to the decrease of TPI since relative changes were taken in account, whereas TPI radio- activity is twice higher than DPI radioactivity. According to [22,23], activation of TPI phos- / phomonoesterase might be linked to occupancy of the thrombin receptor and could be involved in the release of membrane-bound calcium, leading to further activation of various calcium-dependent events.
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