Functional relationship between cyclic AMP-dependent protein phosphorylation and platelet inhibition.

Exposure of human platelets to prostacyclin (PGI2), iloprost or prostaglandin E1 (PGE1) elicits the cyclic AMP-dependent phosphorylation of proteins of 22, 24, 30, 39, 50, 60 and 250 kDa (P22, P24 etc.). P22 was recently identified as rap 1B, a ras-like protein, and P24 was shown to be the beta-chain of glycoprotein Ib. We found that cyclic AMP-dependent phosphorylation of all proteins except P22 was maximal 1 min after exposure of platelets to PGI2, iloprost or PGE1; maximal phosphorylation of P22 occurred after 45 min of incubation. Inhibition of thrombin-induced platelet activation required only a 30 s incubation with PGI2 or iloprost; at this time phosphorylation of P22 was only slightly increased. Although at maximal concentrations PGI2 was more potent than PGE1 in inhibiting thrombin-induced platelet activation, no difference in the degree and the kinetics of cyclic AMP-dependent protein phosphorylation was found. Platelets that had been preincubated and washed in the presence of PGE1 and later resuspended in the absence of PGE1 responded fully to activation by thrombin despite maximal phosphorylation of P22 and P24. Furthermore, addition of PGI2 to PGE1-washed platelets prevented thrombin-induced platelet activation, but did not evoke further phosphorylation of P22 or P24. Phosphorylation of P39 and P50 correlated better with PGI2-induced inhibition of platelet activation. In experiments in which PGE1-induced inhibition of platelet activation was overcome by the addition of thrombin, no dephosphorylation of proteins phosphorylated by cyclic AMP-dependent kinases was observed. These experiments indicate that: (a) phosphorylation of rap 1B and glycoprotein Ib is not related to platelet inhibition by cyclic AMP; (b) phosphorylation of other proteins such as P39 and P50 probably plays a role in mediating cyclic AMP-dependent platelet inhibition; (c) reactions other than cyclic AMP-dependent protein phosphorylation may participate in platelet inhibition by cyclic AMP.