Dual regulation of platelet protein kinase B

Abstract

Protein kinase B (PKB) is a serine/threonine kinase that is activated by growth hormones and implicated in prevention of apoptosis, glycogen metabolism, and glucose uptake. A key enzyme in PKB activation is phosphatidylinositide 3-kinase (PI-3K), which triggers the dual phosphorylation of PKB by phosphatidylinositol-dependent kinases (PDKs). Here we report that the major PKB subtype in platelets is PKBα, which is activated by phosphorylation of Thr308 and Ser473 and has a constitutively phosphorylated Thr450 that does not contribute to PKB activation, α-Thrombin and thrombopoietin activate PKBα via PI-3K and trigger the concurrent phosphorylation of Thr308 (via PDK1) and Ser473 (via a not yet identified PDK2). In addition, α-thrombin activates a PI-3K-independent pathway involving phospholipase Cβ and calcium-dependent protein kinase C subtypes (PKCα/β). This route is specific for phosphorylation of Ser473 and can be initiated by direct PKC activation with phorbol ester or purified active PKC catalytic fragment in platelet lysate. Different degrees of Ser473 and Thr308 phosphorylation correlate with different degrees of enzyme activity. These data reveal a PI-3K-independent PKB activation in which PKCα/β regulates the phosphorylation of Ser473 in PKBα. The independent control of the two phosphorylation sites may contribute to fine regulation of PKBα activity.