Dissociation between secretion and protein phosphorylation in agonist-stimulated platelets; action of PCA-4230, a new antithrombotic drug

Selective translocation of protein kinase c isozymes by PAF in rabbit platelets

The action of platelet activating factor (PAF) on subcellular distribution and activity of protein kinase C (PKC) isoforms in rabbit platelets was analyzed. The results showed an increase of PKC alpha in membrane fraction, concomitantly with a decrease in cytosolic fraction after 5 min PAF treatment, indicating that a translocation of PKC alpha occurred. In addition, PKC zeta was redistributed in a "reverse" form, from the membrane to cytosolic fraction after PAF treatment. PAF induced an increase of PKC alpha activity, whereas a decrease rather than increase in PKC zeta was observed by using immunoprecipitation assays. In addition, some results indicated that PI3 kinase activation was not involved in PAF-induced PKC zeta translocation as occur in several cells and with other agonists. These actions were time- and concentration-dependent, and were inhibited by the treatment with a PAF antagonist. No translocation was observed when the platelets were incubated with lysoPAF, a PAF related compound. The redistribution of PKC isoforms take place through the activation of high specificity PAF binding sites. The pretreatment of the rabbit platelets with staurosporine, a putative inhibitor of PKC, completely blocked the PAF-evoked aggregation without affecting to PAF-evoked shape change and serotonin release. All together, these data could suggest that the specific translocation of PKC isoforms play an important role in the activation of rabbit platelets.

Involvement of cyclic GMP in the mode of action of a new antithrombotic agent PCA-4230; inhibition of the platelet cyclic GMP dependent phosphodiesterase

The effect of PCA-4230, a new dihydropyridine derivative with a potent antithrombotic activity, on cyclic nucleotide phosphodiesterase in platelets was studied. PCA-4230 inhibited (54%) cyclic GMP hydrolytic activity of a platelet cytosolic fraction, whereas it did not affect that of cyclic AMP. Results suggested that PCA-4230 inhibited a cyclic GMP-dependent phosphodiesterase, known as cGB PDE or type V, on a purified enzyme from rabbit platelets by a non-competitive-uncompetitive type inhibition. In addition, PCA-4230 potentiated the increase in both cyclic GMP and cyclic AMP levels evoked by sodium nitroprusside. Furthermore, PCA-4230 and forskolin caused a synergistic effect in cyclic AMP, and also potentiated the phosphorylation of 50 kDa and 22 kDa proteins, reported as substrates of cyclic GMP- and cyclic AMP-dependent protein kinases that are related to the inhibition of platelet functions. Finally, PCA-4230 also potentiated the forskolin- and sodium nitroprusside-inhibited serotonin release evoked by thrombin, probably related to the increased cyclic nucleotide level.

Regulation of PAF-induced platelet responses by cyclic nucleotides

The existence of cross-talk mechanisms between the cyclic nucleotide system and other transduction systems involved in PAF-activated platelets is described in this study. A protein of 125 kDa, identified as pp 125(FAK), is tyrosine phosphorylated by PAF in a time- and concentration-dependent manner. The presence of a cAMP- or a cGMP-elevating agent, used alone or in combination, together with PAF diminished tyrosine phosphorylation. The sensitivity to cAMP shown by PAF-induced ppl25 phosphorylation on tyrosine residues was similar to PAF-induced phosphorylation of a 47-kDa protein (pp47) on serine and threonine. In contrast, the latter was not affected in the presence of a cGMP-elevating agent, although it was able to enhance synergistically the inhibitory effect of forskolin. Data reported herein also show that pp47 phosphorylation and serotonin secretion are not closely correlated. Accordingly, sodium nitroprusside (SNP) did not have any effect on phosphorylation of pp47, but it was able to inhibit serotonin secretion when added alone, and it showed a synergistic inhibitory action with forskolin.