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

Abstract

Stimulation of horse platelets with platelet-activating factor (PAF) induces a rapid degradation of phosphatidylinositol 4,5-bisphosphate [Ptdlns(4,5)P2j. Addition of 0.1 JM PAF for 5 sec to platelets prelabeled with 32P induces a 50% loss of [32P]Ptdlns(4,5)P2. 32P-Labeled phosphatidylinositol 4-monophosphate (PtdIns4P) and [32P]phosphatidylinositol (PtdIns) also are decreased, albeit at a slower rate. Loss Of 32P radioactivity correlates with a net loss of fatty acids from both polyphosphoinositides. Stimulation of platelets with PAF also produces formation of [32P]phosphatidic acid and [32P]lysophosphatidylinositol. The initial disappearance of inositol lipids is subsequently followed by resynthesis, as evidenced by increased incorporation of 32p into Ptdlns(4,5)P2, PtdIns4P, and PtdIns. The resynthesis of the inositides increases with time and is proportional to the concentration of PAF. Prostacyclin (1 ,uM) inhibits (i) the formation of phosphatidic acid and lysophosphatidylinositol and (ii) the resynthesis of polyphosphoinositides induced by 0.03 ,uM PAF without affecting the initial loss of Ptdlns(4,5)P2. The loss of inositol lipids appears to be a primary event of platelet activation. The initial loss of polyphosphoinositides might be linked to the initiation of cellular activation by mobilizing membrane-bound Ca2+, whereas the subsequent formation of these lipids might be involved in mechanisms to prevent overstimulation of the cell.Platelet-activating factor (PAF), a phospholipid mediator of anaphylaxis (1), is released from leucocytes after immunologic and nonimmunologic stimulation (2). The structure of this compound has been elucidated as 1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine (3, 4). PAF induces hypotension (5, 6), neutropenia, and thrombocytopenia in vivo (7,8); in vitro, PAF promotes chemotaxis and degranulation in neutrophils (9-11) and induces shape change, release of granule content, and aggregation in platelets (12)(13)(14)(15)(16).The biochemical mechanisms by which PAF elicits physiological responses are not fully understood. In platelets, PAF has been shown to stimulate the phosphatidylinositol (PtdIns) cycle (16, 17), release of arachidonic acid from membrane phospholipids (16,18), and the phosphorylation of specific proteins (10). These biochemical responses are induced also by other platelet stimuli such as ADP, thrombin, or ionophore A23187 (19)(20)(21) and are thought to be the consequence of the release of Ca2" from a cellular pool inaccessible to chelating agents (20,22,23 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact. platelets (25). Because PtdIns(4,5)P2 might provide Ca2+-binding sites in the plasma membrane (26-28), it has been suggested that the rapid loss of this phospholipid might be a mechanism by which the initial release of Ca2+ could be achieved (25). We now show that PAF also causes a ra...

Purification and characterization of the specific binding protein for platelet activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) from human platelets.

“…The intracellular events subsequent to PAF binding during platelet activation are not well understood. One possibility involves the liberation of arachidonic acid from membrane phospholipids by activation of phospholipases and their subsequent formation into endoperoxides and thromboxane A2 (Lapetina 1982;Billah and Lapetina 1983;Samuelsson, 1983). However, no significant inhibition of PAF-induced platelet aggregation was found among several cyclooxygenase inhibitors, such as indomethacin (Chingard et al 1979).…”

Gonadotropin-Releasing Hormone-Stimulated Phosphoinositide Hydrolysis in the Anterior Pituitary

“…To date there is no obvious example of a tissue in which cAMP potentiates signal-in duced phosphoinositide turnover [27], Conversely, there are reports on an inhibitory effect of cAMP on stimulated phosphoinositide breakdown [6,16,36]. Cyclic GMP in volvement in GnRH-induced gonadotropin release has also been invoked [25,31].…”

Platelet activation.

“…184 In contrast, DAG, OAG, or PMA activates protein kinase C, leading to phosphorylation of the 40 K protein and secretion, with little phosphorylation of myosin light chain. 183185 The present consensus is that both the increase in cytoplasmic Ca 2+ and production of DAG by PIP 2 189 or PAF-acether, 190 although it inhibits the formation of PA and resynthesis of PIP 2 . 190 This suggests that resynthesis of PIP 2 (i.e., turnover) is important, a conclusion reached from studies in other tissues.…”