The Anticoagulant Effect of the Human Secretory Phospholipase A₂ on Blood Plasma and on a Cell‐Free System is Due to a Phospholipid‐Independent Mechanism of Action Involving the Inhibition of Factor Va

Abstract

Blood platelets play a central role in haemostasis by leading to plug formation and by increasing the efficiency of blood coagulation. We have previously shown that blood platelets contain a group I1 secretory phospholipase A, (sPLA, grII) which is released into the extracellular medium upon activation but is unable to stimulate blood platelets. We presently reported an investigation of the putative involvement of the human sPLA, grII (hsPLA, grII) in the coagulation process, both in the absence and in the presence of activated platelets. We show that this enzyme prolongs the recalcification time of blood plasma even in the presence of activated platelets.The positive action of blood platelets on coagulation is correlated, at least in part, with the appearence at the cellular surface of procoagulant phospholipids which constitute a potential target for hsPLA, grII. We therefore investigated the involvement of its enzymatic activity in the anticoagulant effect of this enzyme. We observed that the replacement of CaCl, by SrC1, to initiate the coagulation cascade did not suppress, but rather increased, the inhibitory action of hsPLA, grII. Moreover, hsPLA, grII hydrolyzed only a minor proportion of platelet phospholipids, and it did not affect plasma phospholipids. Taken together, these observations strongly suggest that the major action of hsPLA, grII on blood coagulation does not involve the hydrolysis of phospholipids, in contrast with the strong anticoagulant effect of the group I1 venom phospholipase A, from Crotalus durrissus terrificus.We next studied which step of the coagulation cascade was affected by hsPLA, grII. Using purified coagulation factors, we demonstrated that hsPLA, grII strongly inhibited the prothrombinase activity. This inhibitory effect was independent of the presence of phospholipids but required factor Va, leading to the hypothesis that hsPLA, grII inhibited this factor. Further, the anticoagulant effect of hsPLA, grII was observed on normal and factor-X-deficient plasma, but not on factor-V-deficient plasma.In conclusion, the anticoagulant action of hsPLA, grII is based on a nonenzymatic mechanism of action involving the inhibition of factor Va.

Cell‐derived microparticles in synovial fluid from inflamed arthritic joints support coagulation exclusively via a factor VII–dependent mechanism

“…Actually, this procoagulant nature was not anticipated, because previous studies showed that sPLA 2 , which is present in high concentrations in synovial fluid, inhibits coagulation (23,24,37,38). In line with such an effect of sPLA 2 was the lower exposure of PS by synovial microparticles, as reflected by the reduced binding of annexin V, most likely due to the breakdown of PS to lyso-PS.…”

Distinct Arachidonate-releasing Functions of Mammalian Secreted Phospholipase A2s in Human Embryonic Kidney 293 and Rat Mastocytoma RBL-2H3 Cells through Heparan Sulfate Shuttling and External Plasma Membrane Mechanisms

“…To date, nine genes coding for structurally related and enzymatically active sPLA 2 s have been identified in mammals (groups IB, IIA, IIC, IID, IIE, IIF, III, V, and X) (6 -10). Understanding the physiological functions of this diverse set of sPLA 2 s is now a complex and challenging area of research in the eicosanoid field, and the possibility that some of these enzymes are involved in processes unrelated to eicosanoid generation should be considered (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21).…”

“…Group IIA sPLA 2 (sPLA 2 -IIA), known as inflammatory PLA 2 , is expressed in a variety of tissues and hematopoietic cells, and its expression is markedly induced following challenge with proinflammatory stimuli (1,(23)(24)(25)(26). This sPLA 2 is thought to play a role in inflammation (1), host defense against bacteria (13)(14)(15), tumor suppression (16), exocytosis (17,18), blood coagulation (19), and atherosclerosis (20,21). Group IIC sPLA 2 (sPLA 2 -IIC) is expressed in rodent testes, but only a pseudogene for this enzyme has been found in the human genome (5,27).…”

Inhibition of Prothrombinase by Human Secretory Phospholipase A2 Involves Binding to Factor Xa

“…The prothrombinase complex, composed of FVa, FXa, phospholipids (PL), and Ca 2ϩ , plays a central role in the coagulation cascade (17,21). hsPLA 2 exerts a moderate anticoagulant effect on plasma (22) and inhibits prothrombinase activity (23,24). The anticoagulant effect of hsPLA 2 is phospholipid-independent and not observed when FV-deficient plasma is used for coagulation assays (24).…”