(1999) J. Biol. Chem. 274, 27529 -27535). In the present investigation, we define a second pathway contributing to CREB-dependent up-regulation of Bcl-2 expression as a novel anti-apoptotic function of Akt signaling. To examine the role of Akt on Bcl-2 expression, a series of transient transfections using a luciferase reporter gene driven by the promoter region of Bcl-2 containing a CRE were carried out. Pharmacological inhibition of phosphatidylinositol (PI) 3-kinase, the upstream kinase of Akt, with LY294002 led to a 45% decrease in Bcl-2 promoter activity. The reporter activity was enhanced 2.3-fold by overexpression of active p110 subunit of PI 3-kinase and inhibited 44% by the dominant negative p85 subunit of PI 3-kinase. Cotransfection with 3-phosphoinositide-dependent kinase (PDK1), which is required for the full activation of Akt, resulted in enhanced luciferase activity. Insulin-like growth factor-I-mediated induction of Bcl-2 promoter activity was decreased significantly (p < 0.01) by the dominant negative forms of p85 subunit of PI 3-kinase, PDK1, and Akt. These data indicate that regulation of Bcl-2 expression by IGF-I involves a signaling cascade mediated by PI 3-kinase/PDK1/Akt/CREB. Furthermore, we measured the Bcl-2 mRNA in PC12 cells overexpressing Akt by real-time quantitative reverse transcriptionpolymerase chain reaction using the TaqMan TM fluorogenic probe system. We observed a 2.1-fold increase in Bcl-2 mRNA levels in the Akt cell line compared with control PC12 cells, supporting the observation that enhanced CREB activity by Akt signaling leads to increased Bcl-2 promoter activity and cell survival.The serine threonine kinase Akt/protein kinase B is an important mediator of metabolic as well as survival responses to insulin and growth factors (1). Akt is activated by translocation to plasma membrane when the PI 3-kinase-generated 3-phosphoinositides bind to its pleckstrin homology domain (2). For its full activation it needs to be further phosphorylated by 3-phosphoinositide-dependent kinase1 (PDK1) 1 at Thr-308 and by PDK2 at Ser-473. The metabolic actions of insulin mediated by Akt include stimulation of GLUT4 translocation and activation of glycogen synthase and the glycolytic enzyme 6-phosphofructose-

Activation of protein kinase B (PKB) by growth factors and hormones has been demonstrated to proceed via phosphatidylinositol 3-kinase (PI3-kinase).In this report, we show that PKB can also be activated by PKA (cyclic AMP [cAMP]-dependent protein kinase) through a PI3-kinase-independent pathway. Although this activation required phosphorylation of PKB, PKB is not likely to be a physiological substrate of PKA since a mutation in the sole PKA consensus phosphorylation site of PKB did not abolish PKA-induced activation of PKB. In addition, mechanistically, this activation was different from that of growth factors since it did not require phosphorylation of the S473 residue, which is essential for full PKB activation induced by insulin. These data were supported by the fact that mutation of residue S473 of PKB to alanine did not prevent it from being activated by forskolin. Moreover, phosphopeptide maps of overexpressed PKB from COS cells showed differences between insulin-and forskolin-stimulated cells that pointed to distinct activation mechanisms of PKB depending on whether insulin or cAMP was used. We looked at events downstream of PKB and found that PKA activation of PKB led to the phosphorylation and inhibition of glycogen synthase kinase-3 (GSK-3) activity, a known in vivo substrate of PKB. Overexpression of a dominant negative PKB led to the loss of inhibition of GSK-3 in both insulin-and forskolin-treated cells, demonstrating that PKB was responsible for this inhibition in both cases. Finally, we show by confocal microscopy that forskolin, similar to insulin, was able to induce translocation of PKB to the plasma membrane. This process was inhibited by high concentrations of wortmannin (300 nM), suggesting that forskolin-induced PKB movement may require phospholipids, which are probably not generated by class I or class III PI3-kinase. However, high concentrations of wortmannin did not abolish PKB activation, which demonstrates that translocation per se is not important for PKA-induced PKB activation.

Insulin-like growth factor-I (IGF-I

Activation of protein kinase B (PKB) by growth factors has been demonstrated to proceed via phosphatidylinositol 3-kinase (PI3-kinase). Here, we show that agents which raise intracellular cAMP can also stimulate PKB. However, this effect is not sensitive to wortmannin, indicating that it is PI3-kinase independent. This activation does not appear to result from direct phosphorylation by protein kinase A (PKA) since GST-PKB is not an effective PKA substrate. In addition, the activation pathway of PKB by cAMP seems to be linked to that of growth factors, albeit downstream of PI3-kinase. Evidence for this is that a constitutive active PKB, T308D, S473D, containing activating mutations in the serine and threonine residues which are phosphorylated subsequent to PI3-kinase activation, cannot be further stimulated by cAMP elevations. Hence, these data suggest that, in addition to growth factors, cAMP can also lead to activation of PKB. This cAMP stimulatory action appears to require phosphorylation of T308 and S473, and hence would indicate that cAMP modulates the phosphorylation event of these PKB regulatory sites.

In this report we investigated the function of phosphoinositide-dependent protein kinase 1 (PDK1) in protein kinase B (PKB) activation and translocation to the cell surface. Wild-type and PDK1 mutants were transfected into HeLa cells, and their subcellular localization was analyzed. PDK1 was found to translocate to the plasma membrane in response to insulin, and this process did not require a functional catalytic activity, since a catalytically inactive kinase mutant (Kd) of PDK1 was capable of translocating. The PDK1 presence at the cell surface was shown to be linked to phospholipids and therefore to serum-dependent phosphatidylinositol 3-kinase activity. Using confocal microscopy in HeLa cells we found that PDK1 colocalizes with PKB at the plasma membrane. Further, after cotransfection of PKB and a PDK1 mutant (Mut) unable to translocate to the plasma membrane, PKB was prevented from moving to the cell periphery after insulin stimulation. In response to insulin, a PKB mutant with its PH domain deleted (⌬PH-PKB) retained the ability to translocate to the plasma membrane when coexpressed with PDK1. Finally, we found that ⌬PH-PKB was highly active independent of insulin stimulation when cotransfected with PDK1 mutants defective in their PH domain. These findings suggest that PDK1 brings PKB to the plasma membrane upon exposure of cells to insulin and that the PH domain of PDK1 acts as a negative regulator of its enzyme activity.Protein kinase B␣ (PKB␣), also known as c-Akt or RAC (related to protein kinase A and C), is composed of a Nterminal pleckstrin homology (PH) domain followed by a kinase catalytic domain which shares homology with the A and C protein kinases. Two other isoforms of PKB (termed PKB␤ and PKB␥) have been identified and are expressed in ovarian, pancreatic, and breast cancer cells (10,11). This serine/threonine kinase is rapidly activated in response to stimulation of tyrosine kinase receptors such as those for platelet-derived growth factor (PDGF), insulin, basic fibroblast growth factor, and epidermal growth factor (9,15,18). PKB stimulation by insulin or growth factors is thought to be dependent on phosphatidylinositol 3Ј-kinase (PI 3-kinase) for the following reasons: (i) it is sensitive to pharmacological inhibitors of PI 3-kinase (18), (ii) dominant-negative PI 3-kinase mutants block PKB activation (24), and (iii) constitutively active PI 3-kinase mutants stimulate PKB (9).A model has been proposed to explain activation of PKB in response to hormones and growth factors (2). According to this model, stimulation of cell surface receptors leads to an increase in the level of phosphatidylinositol-3,4,5-trisphosphate (PtdIns-3,4,5-P 3 ) and PtdIns-3,4-P 2 via PI 3-kinase. Although it was initially reported that phospholipids could directly activate PKB by interacting with its PH domain (16), more recently it has been shown that this process most likely fulfills other and/or additional functions. Indeed, it may facilitate PKB localization to the plasma membrane. This view stems from the observ...