Activation of phospholipase A2 and beta-thromboglobulin release in human platelets: comparative effects of thrombin and fluoroaluminate stimulation.
Several reports have suggested that the activity of platelet phospholipase A2 is modulated by GTP-binding protein(s) whose nature and properties need to be defined. Fluoroaluminate is known to activate G-proteins and this leads to a number of cellular responses including the activation of phospholipases. This paper demonstrates that human platelets, prelabelled with [3H]arachidonic acid, produce free arachidonic acid when stimulated with fluoroaluminate and this effect is time- and dose-dependent. The production of arachidonic acid is not inhibited by neomycin, a PI-cycle inhibitor, but is completely abolished by mepacrine, an inhibitor of both phospholipase A2 and C. At low concentration of fluoroaluminate (10 mM NaF) phospholipase A2 but not phospholipase C is activated. In addition, fluoroaluminate treatment releases beta-thromboglobulin (beta-TG) and this effect is not inhibited by acetylsalicylic acid. Under identical conditions both neomycin and mepacrine suppress the release of arachidonic acid and beta-TG induced by thrombin. Sodium nitroprusside, which increases cGMP levels in platelets, inhibits arachidonic acid liberation and beta-TG release in thrombin-stimulated platelets but has no effect in fluoroaluminate-treated platelets; cGMP was reported to suppress phospholipase C activation. These results are consistent with the hypothesis that, in thrombin-stimulated platelets, the liberation of arachidonic acid and beta-TG are strictly dependent on the activation of phospholipase C. We have also provided evidence for the existence of a phospholipase A2 activated by a G-protein which is independent from the degradation of phosphoinositides and, contrary to phospholipase C, it is not down regulated by cGMP.
Stasi
M
M
Istituto di Biochimica e Chimica Medica, Università di Perugia, Italy.
Gresele
P
P
Porcellati
S
S
Quero
E
E
Nenci
G G
GG
Goracci
G
G
D000535:Aluminum / Q000494:pharmacology*; D016718:Arachidonic Acid / Q000378:metabolism; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D006152:Cyclic GMP / Q000378:metabolism; D004789:Enzyme Activation; D005461:Fluorine / Q000494:pharmacology*; D019204:GTP-Binding Proteins / Q000378:metabolism; D006801:Humans; D009355:Neomycin / Q000494:pharmacology; D009599:Nitroprusside / Q000494:pharmacology; D010741:Phospholipases A / Q000037:antagonists & inhibitors / Q000378:metabolism*; D054467:Phospholipases A2; D011796:Quinacrine / Q000494:pharmacology; D013917:Thrombin / Q000494:pharmacology*; D010738:Type C Phospholipases / Q000378:metabolism; D001620:beta-Thromboglobulin / Q000378:metabolism*
D001620:beta-Thromboglobulin; D009599:Nitroprusside; C053629:fluoroaluminum; D016718:Arachidonic Acid; D005461:Fluorine; D000535:Aluminum; D010741:Phospholipases A; D054467:Phospholipases A2; D010738:Type C Phospholipases; D013917:Thrombin; D019204:GTP-Binding Proteins; D011796:Quinacrine; D006152:Cyclic GMP; D009355:Neomycin