Role of phosphoinositide 3-kinase β in platelet aggregation and thromboxane A2 generation mediated by Gi signalling pathways

PI3Ks (phosphoinositide 3-kinases) play a critical role in platelet functional responses. PI3Ks are activated upon P2Y(12) receptor stimulation and generate pro-aggregatory signals. P2Y(12) receptor has been shown to play a key role in the platelet aggregation and thromboxane A(2) generation caused by co-stimulation with G(q) or G(z), or super-stimulation of G(i) pathways. In the present study, we evaluated the role of specific PI3K isoforms alpha, beta, gamma and delta in platelet aggregation, thromboxane A(2) generation and ERK (extracellular-signal-regulated kinase) activation. Our results show that loss of the PI3K signal impaired the ability of ADP to induce platelet aggregation, ERK phosphorylation and thromboxane A(2) generation. We also show that G(q) plus G(i)- or G(i) plus G(z)-mediated platelet aggregation, ERK phosphorylation and thromboxane A(2) generation in human platelets was inhibited by TGX-221, a PI3K beta-selective inhibitor, but not by PIK75 (a PI3K alpha inhibitor), AS252424 (a PI3K gamma inhibitor) or IC87114 (a PI3K delta inhibitor). TGX-221 also showed a similar inhibitory effect on the G(i) plus G(z)-mediated platelet responses in platelets from P2Y(1)(-/-) mice. Finally, 2MeSADP (2-methyl-thio-ADP)-induced Akt phosphorylation was significantly inhibited in the presence of TGX-221, suggesting a critical role for PI3K beta in G(i)-mediated signalling. Taken together, our results demonstrate that PI3K beta plays an important role in ADP-induced platelet aggregation. Moreover, PI3K beta mediates ADP-induced thromboxane A(2) generation by regulating ERK phosphorylation.