Neomycin does not interfere with the inositol phospholipid metabolism, but blocks binding of alpha-thrombin to intact human platelets

Convulxin-induced platelet aggregation is accompanied by a powerful activation of the phospholipase C pathway

Platelet aggregation and stimulation of phosphoinositide-specific phospholipase C (PLC) by thrombin and by convulxin (Cvx), a non-enzymic snake venom glycoprotein, were compared. Cvx-stimulated production of inositol phosphates by washed platelets was independent of the cyclo-oxygenase pathway, formation of platelet-activating factor and ADP release, but prostacyclin (prostaglandin I2), a stimulator of cyclic AMP formation, suppressed its effects on platelet and PLC activation. Kinetic analysis showed that inositol 1,4,5-trisphosphate formation reached its maximal value 15 s after platelet stimulation with Cvx and persisted for at least 5 min. Neomycin sulphate (10 mM), which complexes phosphatidylinositol 4-phosphate and phosphatidyl-inositol 4,5-bisphosphate, decreased the production of inositol phosphates, partially prevented platelet aggregation induced by a high concentration of Cvx (10 nM) and abolished both platelet aggregation and inositol phosphate formation induced by thrombin (2 units/ml) and by a stable prostaglandin H2 analogue, U46619 (1 microM). In contrast with neomycin sulphate, Na2SO4 had no significant effect against all agonists tested. It is concluded that platelet activation by Cvx is partially mediated by PLC and involves other mechanisms as well.

Phosphoinositide metabolism in a polyoma-BK-virus-transformed pancreatic islet cell line: evidence for constitutively activated phospholipase C

We have characterized the phosphoinositide metabolism in a polyoma-BK-virus-transformed rat pancreatic islet cell line which has highly malignant characteristics, expresses viral T-antigen and has lost insulin-secreting capacity. After incorporation with [3H]inositol to isotopic equilibrium, all inositol metabolites were analyzed. When compared with normal pancreatic islets, increased levels of inositol 1,4,5-trisphosphate (Ins-1,4,5-P3), inositol 1,3,4-trisphosphates and inositol tetrakisphosphate (Ins-P4), and decreased levels of phosphatidylinositol monophosphate (PIP) and phosphatidylinositol bisphosphate (PIP2) were found. The Ins-1,4,5-P3/PIP2 ratio increased, whereas the PIP2/PIP ratio was not altered after the transformation. In the pancreatic islet cell line there was a stable accumulation of inositol phosphates at 3.3 mM glucose. Glucose, KCl, cholecystokinin (CCK) and carbachol with and without LiCl were all without effect on the accumulation of inositol phosphates. Somatostatin inhibited the accumulation of inositol phosphates but a Ca(2+)-free/EDTA solution did not. Preincubation with cholera toxin or pertussis toxin inhibited the accumulation of inositol phosphates at 3.3 mM glucose except for Ins-P4, whereas no effect was observed on the phosphoinositides. NaF stimulated the accumulation of inositol phosphates, with a concomitant decrease in the phosphoinositides, whereas neomycin was without effect on the inositol phosphates. In normal pancreatic islets, pertussis toxin inhibited the CCK-induced increase in Ins-1,4,5-P3, whereas no effect was seen at 3.3 mM glucose. Finally, pertussis toxin inhibited the CCK-induced increase in the Ins-1,4,5-P3/PIP2 ratio in normal pancreatic islets. The same inhibition was also found in the pancreatic islet cell line at 3.3 mM glucose. We conclude that in the transformed pancreatic islet cell line the phosphoinositide hydrolysis is constitutively activated at the level of phospholipase C, with a substantial loss of regulatory control.

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.