Human group II 14 kDa phospholipase A2 activates human platelets.
Recombinant human group II phospholipase A2 (sPLA2) added to human platelets in the low microg/ml range induced platelet activation, as demonstrated by measurement of platelet aggregation, thromboxane A2 generation and influx of intracellular free Ca2+ concentration and by detection of time-dependent tyrosine phosphorylation of platelet proteins. The presence of Ca2+ at low millimolar concentrations is a prerequisite for the activation of platelets by sPLA2. Mg2+ cannot replace Ca2+. Mg2+, given in addition to the necessary Ca2+, inhibits sPLA2-induced platelet activation. Pre-exposure to sPLA2 completely blocked the aggregating effect of a second dose of sPLA2. Albumin or indomethacin inhibited sPLA2-induced aggregation, similarly to the inhibition of arachidonic acid-induced aggregation. Platelets pre-treated with heparitinase or phosphatidylinositol-specific phospholipase C lost their ability to aggregate in response to sPLA2, although they still responded to other agonists. This suggests that a glycophosphatidylinositol-anchored platelet-membrane heparan sulphate proteoglycan is the binding site for sPLA2 on platelets. Previous reports have stated that sPLA2 is unable to activate platelets. The inhibitory effect of albumin and Mg2+, frequently used in aggregation studies, and the fact that isolated platelets lose their responsiveness to sPLA2 relatively quickly, may explain why the platelet-activating effects of sPLA2 have not been reported earlier.
Polgár
J
J
Theodor Kocher Institute, University of Berne, Berne 3012, Switzerland.
Kramer
R M
RM
Um
S L
SL
Jakubowski
J A
JA
Clemetson
K J
KJ
D016718:Arachidonic Acid / Q000494:pharmacology; D001792:Blood Platelets / Q000187:drug effects / Q000502:physiology; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D004591:Electrophoresis, Polyacrylamide Gel; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D007211:Indoles / Q000494:pharmacology; D007213:Indomethacin / Q000494:pharmacology; D008244:Lysophosphatidylcholines / Q000494:pharmacology; D008274:Magnesium / Q000494:pharmacology; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D054801:Phosphoinositide Phospholipase C; D010741:Phospholipases A / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D054467:Phospholipases A2; D010766:Phosphorylation; D019000:Phosphotyrosine / Q000378:metabolism; D015539:Platelet Activation; D010974:Platelet Aggregation / Q000187:drug effects / Q000502:physiology; D011133:Polysaccharide-Lyases / Q000378:metabolism; D011994:Recombinant Proteins / Q000494:pharmacology; D013928:Thromboxane A2 / Q000378:metabolism; D010738:Type C Phospholipases / Q000494:pharmacology
D004791:Enzyme Inhibitors; D007211:Indoles; C095553:LY 311727; D008244:Lysophosphatidylcholines; D011994:Recombinant Proteins; D019000:Phosphotyrosine; D016718:Arachidonic Acid; D013928:Thromboxane A2; D010741:Phospholipases A; D054467:Phospholipases A2; D010738:Type C Phospholipases; D054801:Phosphoinositide Phospholipase C; D011133:Polysaccharide-Lyases; C021349:heparitinsulfate lyase; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D008274:Magnesium; D002118:Calcium; D007213:Indomethacin