Dual regulation of platelet protein kinase B

Protein kinase B (PKB) is a serine/threonine kinase that is activated by gr owth hormones and implicated in prevention of apoptosis, glycogen metabolis m, and glucose uptake. A key enzyme in PKB activation is phosphatidylinosit ide 3-kinase (PI-3K), which triggers the dual phosphorylation of PKB by pho sphatidylinositol-dependent kinases (PDKs). Here we report that the major P KB subtype in platelets is PKB alpha, which is activated by phosphorylation of Thr(308) and Ser(473) and has a constitutively phosphorylated Thr(450) that does not contribute to PKB activation. alpha-Thrombin and thrombopoiet in activate PKB alpha via PI-3K and trigger the concurrent phosphorylation of Thr(308) (via PDK1) and Ser473 (via a not vet identified PDK2). In addit ion, alpha-thrombin activates a PI-3K-independent pathway involving phospho lipase C beta and calcium-dependent protein kinase C subtypes (PKC alpha/be ta). This route is specific for phosphorylation of Ser(473) and can be init iated by direct PRC activation with phorbol ester or purified active PKC ca talytic fragment in platelet lysate. Different degrees of Ser(473) and Thr( 308) phosphorylation correlate with different degrees of enzyme activity. T hese data reveal a PI-3K-independent PKB activation in which PKC alpha/beta regulates the phosphorylation of Ser(473) in PKB alpha. The independent co ntrol of the two phosphorylation sites may contribute to fine regulation of PKB alpha activity.