A role for phosphatidylinositol 3-kinase in platelet aggregation in response to low, but not high, concentrations of PAF or thrombin

In this study we show that platelet activating factor (PAF) activates PI 3- kinase over a rapid time course that correlates closely with the aggregatio n response. Tyrosine kinases are involved in this response, since there is increased PI 3-kinase activity associated with tyrosine-phosphorylated prot eins. PI 3-kinase inhibitors were used to probe the dependence of PAF-induc ed aggregation on PI 3-kinase. Both wortmannin and LY-294002 inhibited PAF- induced aggregation that correlated with PI 3-kinase inhibition only when u sing lower concentrations of PAF giving reversible aggregation (primary pha se). Similar results were obtained with human platelets using thrombin or t hrombin receptor activating peptide. The same pattern of response was obser ved when activation of GPIIbIIIa was assessed by flow cytometry, i.e., PI 3 -kinase inhibitors blocked integrin activation only when lower concentratio ns of agonist were used. We suggest that PI 3-kinase is important for rever sible (primary) aggregation of platelets in response to PAF or thrombin, pe rhaps by contributing to the 'inside-out' activation of the platelet integr in GPIIbIIIa, only when submaximal concentrations of agonists are used. The lack of effect of PI 3-kinase inhibitors, when high concentrations of agon ist are used, suggests that PI 3-kinase-independent pathways contribute to aggregation under these conditions. (C) 1999 Elsevier Science B.V. All righ ts reserved.