Human group II 14 kDa phospholipase A2 activates human platelets

• Published in: Biochemical journal Vol. 327 ( Pt 1); no. 1; pp. 259 - 265
• Main Authors: Polgár, J, Kramer, R M, Um, S L, Jakubowski, J A, Clemetson, K J
• Format: Journal Article
• Language: English
• Published: England 01-10-1997
• Subjects: Arachidonic Acid - pharmacology; Blood Platelets - drug effects; Blood Platelets - physiology; Calcium - metabolism; Calcium - pharmacology; Electrophoresis, Polyacrylamide Gel; Enzyme Inhibitors - pharmacology; Humans; Indoles - pharmacology; Indomethacin - pharmacology; Lysophosphatidylcholines - pharmacology; Magnesium - pharmacology; Phosphatidylinositol Diacylglycerol-Lyase; Phosphoinositide Phospholipase C; Phospholipases A - antagonists & inhibitors; Phospholipases A - pharmacology; Phospholipases A2; Phosphorylation; Phosphotyrosine - metabolism; Platelet Activation; Platelet Aggregation - drug effects; Platelet Aggregation - physiology; Polysaccharide-Lyases - metabolism; Recombinant Proteins - pharmacology; Thromboxane A2 - metabolism; Type C Phospholipases - pharmacology
• ISSN: 0264-6021; 1470-8728
• DOI: 10.1042/bj3270259

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.