Insulin-stimulated protein kinase B phosphorylation on Ser-473 is independent of its activity and occurs through a staurosporine-insensitive kinase

Inhibition of protein kinase B/Akt. implications for cancer therapy

Protein kinase B (PKB, also called Akt) is an important regulator of cell proliferation and survival. Amplification of genes encoding PKB isoforms has been found in several types of human cancers. In addition, mutations in the phosphatase and tensin homolog deleted on chromosome ten (PTEN), one of the most frequently mutated tumor suppressor genes, results in elevated PKB activity. PKB has a wide range of cellular targets, and the oncogenicity of PKB arises from activation of both proliferative and anti-apoptotic signaling. Furthermore, PKB contributes to tumor progression by promoting cell invasiveness and angiogenesis. These observations establish PKB as an attractive target for cancer therapy. A cellular inhibitor of PKB, termed carboxyl-terminal modulator protein, reverts the phenotype of viral akt-transformed cells, suggesting that a specific PKB inhibitor will be useful in the treatment of tumors with elevated PKB activity. Since inhibition of PKB activity induces apoptosis in a range of mammalian cells, a PKB inhibitor may be effective, in combination with other anticancer drugs, for the treatment of tumors with other mutations.

Differential regulation of Akt kinase isoforms by the members of the TCL1 oncogene family

The members of the TCL1 proto-oncogene family (TCL1, MTCP1, and TCL1b) bind to Akt1, increasing its phosphorylation status and kinase activity. This is thought to be secondary to the formation of TCL1-Akt oligomers within which Akt is preferentially phosphorylated. Here we show that, in contrast to Akt1 and Akt2, which bind to all members of the TCL1 family, Akt3 specifically interacts with TCL1 but not with MTCP1 or TCL1b. This association is functional, as the presence of TCL1 but not MTCP1 or TCL1b increased Akt3 kinase activity in in vitro kinase assays. Functional specificity is determined by the Akt pleckstrin homology domain as chimeric Akt1, where Akt1 PH domain was replaced by that of Akt3 was no longer able to interact with MTCP1 or TCL1b and its kinase activity was solely enhanced by TCL1. Moreover, we show that, in TCL1-overexpressing SUPT-11 T-cell leukemia and P3HR-1 Burkitt's lymphoma cell lines, TCL1 interacts with endogenous Akt1, Akt2, and Akt3. TCL1 enhanced hetero-oligomerization of Akt1 with Akt3 and as a consequence facilitated transphosphorylation of Akt molecules, which may contribute to Akt activation and TCL1-induced leukemogenesis in vivo.

Dual signaling via protein kinase C and phosphatidylinositol 3'-kinase/Akt contributes to bradykinin B2 receptor-induced cardioprotection in guinea pig hearts

We investigated the role of protein kinase C (PKC) and phosphatidylinositol 3;-kinase (PI3-K) in the signaling mechanism of cardioprotection afforded by bradykinin (BK). Coronary-perfused guinea pig ventricular muscles were subjected to 20-min no-flow ischemia and 60-min reperfusion. Pretreatment for 5 min with BK (1 microm) significantly improved the recovery of developed tension measured after 60 min of reperfusion (86.8+/-2.6%v 34.8+/-4.1% in control). Prior treatment with B2 receptor antagonist HOE 140 completely abolished the protective effect of BK (37.0+/-7.6%). The protection was reduced by either PKC inhibitor chelerythrine (CH, 58.9+/-2.2%) or PI3-K inhibitor wortmannin (WM, 59.4+/-2.5%); however, the recovery of contractility was intermediate between the BK and control groups. Nevertheless, pretreatment with CH and WM together completely eliminated the protective effect of BK (38.9+/-4.2%). The mitochondrial ATP-sensitive K+ (mitoK(ATP)) channel blocker 5-hydroxydecanoate (5HD) significantly but partially inhibited the effect of BK (59.0+/-2.2%). Pretreatment with 5HD and CH together could not generate further inhibition (61.1+/-3.3%), while pretreatment with 5HD and WM together totally eliminated the protection (34.9+/-2.9%). We conclude that BK B2 receptors can precondition guinea pig hearts via the dual activation of PKC and PI3-K. The mitoK(ATP) channels act as downstream targets of PKC, whereas PI3-K is not associated with mitoK(ATP) channels.

PKB/AKT: functional insights from genetic models

Since its discovery 10 years ago, the potential functions of protein kinase B (PKB)/AKT have been catalogued with increasing efficiency. The physiological relevance of some of the proposed mechanisms by which PKB/AKT mediates many of its effects has been questioned, and recent work using new reagents and approaches has revealed some cracks in our understanding of this important molecule, and also hinted that these effects may involve other players.