Regulation of protein kinase B/Akt activity and Ser473 phosphorylation by protein kinase Cα in endothelial cells

Protein kinase Balpha (PKBalpha/Akt-1) is a key mediator of multiple signaling pathways involved in angiogenesis, cell proliferation and apoptosis among others. The unphosphorylated form of Akt-1 is virtually inactive and its full activation requires two phosphatidylinositol-3,4,5-triphosphate-dependent phosphorylation events, Thr-(308) by 3-phosphoinositide-dependent kinase-1 (PDK1) and Ser(473) by an undefined kinase that has been termed PDK2. Recent studies have suggested that the Ser(473) kinase is a plasma membrane raft-associated kinase. In this study we show that protein kinase Calpha (PKCalpha) translocates to the membrane rafts in response to insulin growth factor-1 (IGF-1) stimulation. Overexpression of PKCalpha increases Ser(473) phosphorylation and Akt-1 activity, while inhibition of its activity or expression decreases IGF-1-dependent activation of Akt-1. Furthermore, in vitro, in the presence of phospholipids and calcium, PKCalpha directly phosphorylates Akt-1 at the Ser(473) site. We conclude, therefore, that PKCalpha regulates Akt-1 activity via Ser(473) phosphorylation and may function as PDK2 in endothelial cells. (C) 2004 Elsevier Inc. All rights reserved.