mTORC2 Protein-mediated Protein Kinase B (Akt) Serine 473 Phosphorylation Is Not Required for Akt1 Activity in Human Platelets

Protein kinase B (PKB, Akt) is a Ser/Thr kinase, involved in the regulation of cell survival, proliferation and metabolism, and is activated by dual phosphorylation on Thr308 in the activation loop and Ser473 in the hydrophobic motif. It plays a contributory role to platelet function, although little is known about its regulation. In this study, we investigated the role of the mammalian target of rapamycin complex (mTORC)-2 in Akt regulation using the recently identified small-molecule ATP competitive mTOR inhibitors PP242 and Torin1. Both PP242 and Torin1 blocked thrombin and IGF-1-mediated Akt Ser473 phosphorylation with an IC50 between 1 and 5 nM, whereas the mTORC1 inhibitor rapamycin had no effect. Interestingly, PP242 and Torin1 had no effect on Akt Thr308 phosphorylation, Akt1 activity and phosphorylation of the Akt substrate GSK-3β, indicating that Ser473 phosphorylation is not necessary for Thr308 phosphorylation and maximal Akt1 activity. In contrast, Akt2 activity was significantly reduced, concurrent with weak inhibition of PRAS40 phosphorylation, in the presence of PP242 and Torin1. Other signalling pathways, including PLC/PKC and the MAPK pathway, were unaffected by PP242 and Torin1. Together, these results demonstrate that mTORC2 is the kinase that phosphorylates Akt Ser473 in human platelets, but that this phosphorylation is dispensable for Thr308 phosphorylation and Akt1 activity.