Signal transduction in the platelet activation induced by IgG anti-streptokinase and anisoylated plasminogen-streptokinase activator complex

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Abstract
Streptokinase (SK) is one of the plasminogen activators currently used in therapeutics. SK antibodies may appear in the blood after thrombolytic therapy with SK or after ß-hemolytic streptococci infection. Such antibodies may both activate platelets and neutralize the ability of SK to convert plasminogen into plasmin. We previously demonstrated that platelet activation induced by the combination of IgG anti-SK and anisoylated plasminogen-SK activator complex (APSAC) is mediated by Fγ7RIIal receptor. However, the mechanism by which IgG anti-SK and APSAC (or SK) transduce an activating signal across the platelet plasma membrane remains unknown. We have demonstrated in the present study that the platelet aggregation induced by the combination of IgG anti-SK and APSAC is accompanied by an increase in inositol phosphate, Ca2+ mobilization and thromboxane (Tx) A2 generation. Neomycin, erbstatin and GF 109203X, which inhibit phospholipase C (PLC), protein tyrosine kinase (PTK) and protein kinase C (PKC) activities, respectively, abolished platelet aggregation induced by IgG anti-SK plus APSAC, indicating the pivotal roles of the PLC, PTK and PKC pathways in this immunological activation. In addition, TxA2 generation is also important since aspirin, a cyclo-oxygenase inhibitor and SQ 29548, a TxA2 receptor antagonist, showed significant inhibition of the platelet response. The contribution of released ADP was confirmed using apyrase, which significantly inhibited IgG anti-SK plus APSAC-induced platelet aggregation. Finally, WEB 2086, a platelet-activating factor (PAF) receptor antagonist, was not effective, indicating that PAF is not involved in this process. APSAC- or SK-induced platelet activation may limit the therapeutic effectiveness of the drug and may contribute to the pathogenesis of early reocclusion. The study of the mechanism leading to APSAC-induced platelet activation could be relevant for a better understanding of the physiopathology of immune complex disorder diseases and thrombolytic treatment failure.