Role of autocrine stimulation for the effects of cyclic AMP on protein and lipid phosphorylation in platelets activated by particles

Abstract

We have compared responses in platelets stimulated with the particulate materials, Intralipid (liposome-suspension) and a potential contrast medium IEEC (1P-(ethyloxycarbonyloxy)-ethyl-5-acetyl-amino-3-(N-methyl-acetylamino)-2,4,6-triiodo-benzenecarboxylate coated with human serum albumin), with and without forskolin and inhibitors of autocrine stimulation (IAS: an ADP-removing system of creatine phosphate/creatine phosphokinase; RGDS to prevent fibrinogen/fibronectin binding to GPIIb/IIIa ; SQ 29.548 as a TXA P receptor antagonist; cyproheptadine as a serotonin receptor antagonist; BN 52021 as a platelet-activating factor receptor antagonist). The pattern of tyrosine-phosphorylated proteins, phosphorylation of initial lipids and phosphorylation of pleckstrin (P47) were used as markers for early signal transducing responses, while secretion of ADP+ATP and L L-N-acetyl-glycosaminidase were used as final responses. Intralipid showed no platelet activation except for some weak tyrosine protein phosphorylation that was inhibited by elevated cAMP. IEEC induced strong platelet activation that was partly inhibited by increased levels of cAMP and IAS. The inhibition of elevated cAMP seemed to be due to removal of the G protein-mediated activation from secreted autocrine stimulators either by IAS or forskolin. The remaining activity is a pure effect from IEEC which is not affected by elevated cAMP.z 1998 Federation of European Biochemical Societies.

Insertion of poly (ethylene glycol)-lipid reduces the liposome-encapsulated hemoglobin-induced thrombocytopenic reaction

“…Unfortunately, foreign particles and substances that are recognized as foreign surfaces lead to platelet activation followed by their hepatic sequestration. This phenomenon has been observed in clinically used contrast agents [8,19] and may result in a prolonged clotting time…”

“…Apparently, the particle-induced platelet activation is different from the activation induced by soluble activators, such as thrombin [8] . Thrombininduced platelet activation does not cause protein tyrosine phosphorylation and is dependent on intracellular cyclic adenosine monophosphate (cAMP i ).…”

“…Complement activation may also occur due to the presence of anti-phospholipid antibodies in circulation [7] . The molecular basis of this activation has not been yet established, but it appears that intracellular protein phosphorylation is one of the initial and mandatory steps in the process that is partially dependent on cyclic adenosine monophosphate [8] . A recent article has also shown that anionic charge on the surface of the liposome plays a key role in activation of both classical and alternative complement pathway [9] .…”

Phospholipase D activity in human platelets is inhibited by protein kinase A, involving inhibition of phospholipase D1 translocation

“…In order to investigate whether forskolin could act as a PLD activator in the same manner as PGE 1 , platelets were pre-incubated with 1 mM of forskolin [28,29] alone and also for 5 min before the addition of thrombin. Forskolin alone gave a minor PLD activity that could not be distinguished from the basal activity (not shown) and inhibited the effect of thrombin (Figure 3) by 50%.…”