Stimulation of mTORC2 by integrin alpha IIb beta 3 is required for PI3K beta-dependent activation of Akt but is dispensable for platelet spreading on fibrinogen

摘要

Phosphatidylinositol 3 kinase (PI3K) is a major player in platelet activation and regulates thrombus formation and stabilization. The beta isoform of PI3K is implicated in integrin alpha IIb beta 3 outside-in signaling, is required for the phosphorylation of Akt, and controls efficient platelet spreading upon adhesion to fibrinogen. In this study we found that during integrin alpha IIb beta 3 outside-in signaling PI3K beta-dependent phosphorylation of Akt on Serine473 is mediated by the mammalian target of rapamycin complex 2 (mTORC2). The activity of mTORC2 is stimulated upon platelet adhesion to fibrinogen, as documented by increased autophosphorylation. However, mTORC2 activation downstream of integrin alpha IIb beta 3 is PI3K beta-independent. Inhibition of mTORC2, but not mTORC1, also prevents Akt phosphorylation of Threonine308 and affects Akt activity, resulting in the inhibition of GSK3 alpha/beta phosphorylation. Nevertheless, mTORC2 or Akt inhibition does not alter PI3K beta-dependent platelet spreading on fibrinogen. The activation of the small GTPase Rap1b downstream of integrin alpha IIb beta 3 is regulated by PI3K beta but is not affected upon inhibition of either mTORC2 or Akt. Altogether, these results demonstrate for the first time the activation of mTORC2 and its involvement in Akt phosphorylation and stimulation during integrin alpha IIb beta 3 outside-in signaling. Moreover, the results demonstrate that the mTORC2/Akt pathway is dispensable for PI3K beta-regulated platelet spreading on fibrinogen.