A key role of adenosine diphosphate in the irreversible platelet aggregation induced by the PAR1-activating peptide through the late activation of phosphoinositide 3-kinase

Although adenosine diphosphate (ADP), per se, is a weak platelet agonist, i ts role as a crucial cofactor in human blood platelet functions has now bee n clearly demonstrated in vitro and in vivo. The molecular basis of the ADP -induced platelet activation is starting to be understood since the discove ry that 2 separate P2 purinergic receptors may be involved simultaneously i n the activation process. However, little is known about how ADP plays its role as a cofactor in platelet activation and which signaling pathway initi ated by a specific agonist can be modulated by the released ADP To investig ate these points, we took advantage of a model of platelet activation throu gh the thrombin receptor PAR1 in which both ADP scavengers and phosphoinosi tide 3-kinase (PI 3-kinase) inhibitors have been shown to transform the cla ssical irreversible aggregation into a reversible one, We have observed tha t, among the different PI 3-kinase products, the accumulation of phosphatid ylinositol 3,4-bisphosphate [PtdIns(3,4)P-2] was dramatically and specifica lly attenuated when ADP was removed by apyrase treatment, A comparison betw een the effects of PI 3-kinase inhibitors and apyrase strongly suggest that the late, ADP-dependent, PtdIns(3,4)P-2 accumulation is necessary for PAR1 -induced irreversible aggregation. Using selective antagonists, we found th at the effect of ADP was due to the ADP receptor coupled to inhibition of a denylyl cyclase, Finally, we found that both ADP and PI B-kinase play an im portant role in PAR1-dependent reorganization of the cytoskeleton through a control of myosin heavy chain translocation and the stable association of signaling complexes with the actin cytoskeleton. (C) 1999 by The American S ociety of Hematology.