Glycogen synthase kinase-3 (GSK3) is implicated in the regulation of several physiological processes, including the control of glycogen and protein synthesis by insulin, modulation of the transcription factors AP-1 and CREB, the specification of cell fate in Drosophila and dorsoventral patterning in Xenopus embryos. GSK3 is inhibited by serine phosphorylation in response to insulin or growth factors and in vitro by either MAP kinase-activated protein (MAPKAP) kinase-1 (also known as p90rsk) or p70 ribosomal S6 kinase (p70S6k). Here we show, however, that agents which prevent the activation of both MAPKAP kinase-1 and p70S6k by insulin in vivo do not block the phosphorylation and inhibition of GSK3. Another insulin-stimulated protein kinase inactivates GSK3 under these conditions, and we demonstrate that it is the product of the proto-oncogene protein kinase B (PKB, also known as Akt/RAC). Like the inhibition of GSK3 (refs 10, 14), the activation of PKB is prevented by inhibitors of phosphatidylinositol (PI) 3-kinase.

Insulin activated endogenous protein kinase B alpha (also known as RAC/Akt kinase) activity 12‐fold in L6 myotubes, while after transfection into 293 cells PKBalpha was activated 20‐ and 50‐fold in response to insulin and IGF‐1 respectively. In both cells, the activation of PKBalpha was accompanied by its phosphorylation at Thr308 and Ser473 and, like activation, phosphorylation of both of these residues was prevented by the phosphatidylinositol 3‐kinase inhibitor wortmannin. Thr308 and/or Ser473 were mutated to Ala or Asp and activities of mutant PKBalpha molecules were analysed after transfection into 293 cells. The activity of wild‐type and mutant PKBalpha was also measured in vitro after stoichiometric phosphorylation of Ser473 by MAPKAP kinase‐2. These experiments demonstrated that activation of PKBalpha by insulin or insulin‐like growth factor‐1 (IGF‐1) results from phosphorylation of both Thr308 and Ser473, that phosphorylation of both residues is critical to generate a high level of PKBalpha activity and that the phosphorylation of Thr308 in vivo is not dependent on phosphorylation of Ser473 or vice versa. We propose a model whereby PKBalpha becomes phosphorylated and activated in insulin/IGF‐1‐stimulated cells by an upstream kinase(s).

Since their discovery, protein tyrosine phosphatases have been speculated to play a role in tumor suppression because of their ability to antagonize the growthpromoting protein tyrosine kinases. Recently, a tumor suppressor from human chromosome 10q23, called PTEN or MMAC1, has been identified that shares homology with the protein tyrosine phosphatase family. Germ-line mutations in PTEN give rise to several related neoplastic disorders, including Cowden disease. A key step in understanding the function of PTEN as a tumor suppressor is to identify its physiological substrates. Here we report that a missense mutation in PTEN, PTEN-G129E, which is observed in two Cowden disease kindreds, specifically ablates the ability of PTEN to recognize inositol phospholipids as a substrate, suggesting that loss of the lipid phosphatase activity is responsible for the etiology of the disease. Furthermore, expression of wild-type or substrate-trapping forms of PTEN in HEK293 cells altered the levels of the phospholipid products of phosphatidylinositol 3-kinase and ectopic expression of the phosphatase in PTEN-deficient tumor cell lines resulted in the inhibition of protein kinase (PK) B͞Akt and regulation of cell survival.

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We have investigated the role of subcellular localization in the regulation of protein kinase B (PKB) activation. The myristoylation/palmitylation motif from the Lck tyrosine kinase was attached to the N terminus of protein kinase B to alter its subcellular location. Myristoylated/palmitylated (m/p)-PKB␣ was associated with the plasma membrane of transfected cells, whereas the wild-type kinase was mostly cytosolic. The activity of m/p-PKB␣ was 60-fold higher compared with the unstimulated wild-type enzyme, and could not be stimulated further by growth factors or phosphatase inhibitors. In vivo 32 P labeling and mutagenesis demonstrated that m/p-PKB␣ activity was due to phosphorylation on ). Three mammalian isoforms of PKB have been identified so far, termed PKB␣, -␤, and -␥ (7-9). 2 All three isoforms contain a pleckstrin homology (PH) domain at the N terminus (10), followed by a catalytic domain related to protein kinases A and C, and a C-terminal regulatory region. PKB␣ was found to mediate insulin-and insulin-like growth factor (IGF-1)-induced cellular responses, such as the inhibition of glycogen synthase kinase-3 (11), the stimulation of glucose uptake (12), and the promotion of cell survival by inhibiting apoptosis (Ref. 13; reviewed in Refs. 14 and 15). PKB␣ is the cellular homologue of the oncogene product v-Akt encoded by the AKT8 retrovirus, which induces thymic lymphomas in mice (16). Cloning of v-akt revealed that it was created by fusion of viral Gag sequences to the N terminus of mouse PKB␣, which adds an N-terminal myristoylation signal to the oncoprotein and could account for its transforming ability (2, 17). Overexpression of PKB␣ or -␤ is associated with some human ovarian, pancreatic, and breast carcinomas (8, 18 -20).PKB␣ is activated by a variety of growth factors and phosphatase inhibitors (5, 6, 21) through a phosphorylation mechanism (21-23). The activation of PKB␣ by insulin or IGF-1 is mediated by phosphorylation of Thr 308 in the catalytic domain and Ser 473 at the C terminus (22). The phosphorylation of both sites is blocked by pretreatment of the cells with the PI3-K inhibitor wortmannin. Substitution of both regulatory sites by aspartic acid residues to mimic phosphorylation by the introduction of a negative charge, produces a constitutively active enzyme (22). This work predicted the existence of an upstream kinase(s) that phosphorylate(s) these sites, and recently a protein kinase activity was identified and purified capable of phosphorylating Thr 308 in the presence of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P 3 ) or phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P 2 ) (Refs. 24 and 25; reviewed in Ref. 26). The enzyme has therefore been termed 3-phosphoinositide-dependent protein kinase-1 (PDK1).The PH domain of PKB has been reported to play a role in the activation process (6), but PKB activation can also occur in its absence, depending on the agonist and the type of deletion mutants used (21,23,27). The PH domain of PKB binds PtdIns(3,4,5)P 3 and PtdIns(3,4)P...

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Serum-and glucocorticoid-inducible kinases (SGKs) form a novel family of serine/threonine kinases that are activated in response to a variety of extracellular stimuli. SGKs are related to Akt (also called PKB), a serine/threonine kinase that plays a crucial role in promoting cell survival. Like Akt, SGKs are activated by the phosphoinositide-3 kinase (PI3K) and translocate to the nucleus upon growth factor stimulation. However the physiological substrates and cellular functions of SGKs remained to be identified. We hypothesized that SGKs regulate cellular functions in concert with Akt by phosphorylating common targets within the nucleus. The best-characterized nuclear substrates of Akt are transcription factors of the Forkhead family. Akt phosphorylates Forkhead transcription factors such as FKHRL1, leading to FKHRL1's exit from the nucleus and the consequent shutoff of FKHRL1 target genes. We show here that SGK1, like Akt, promotes cell survival and that it does so in part by phosphorylating and inactivating FKHRL1. However, SGK and Akt display differences with respect to the efficacy with which they phosphorylate the three regulatory sites on FKHRL1. While both kinases can phosphorylate Thr-32, SGK displays a marked preference for Ser-315 whereas Akt favors Ser-253. These findings suggest that SGK and Akt may coordinately regulate the function of FKHRL1 by phosphorylating this transcription factor at distinct sites. The efficient phosphorylation of these three sites on FKHRL1 by SGK and Akt appears to be critical to the ability of growth factors to suppress FKHRL1-dependent transcription, thereby preventing FKHRL1 from inducing cell cycle arrest and apoptosis. These findings indicate that SGK acts in concert with Akt to propagate the effects of PI3K activation within the nucleus and to mediate the biological outputs of PI3K signaling, including cell survival and cell cycle progression.Serum-and glucocorticoid-induced kinases (SGKs) belong to a new family of serine/threonine kinases that are regulated at both the transcriptional and posttranslational levels by external stimuli. The mRNA encoding SGK1, the best-studied member of the SGK family, is rapidly induced in response to a variety of stimuli, including growth factors (51, 52), steroid and peptide hormones (3, 51, 52), cytokines (15, 50), changes in cell volume (49), and brain injury (24).The SGK gene is conserved from yeast to human, and the SGK protein is expressed in a variety of tissues and cell lines in mammals (10, 51, 52). Although it has been proposed that SGK may play a role in cell cycle progression (8) or sodium homeostasis control (4, 12), the cellular functions of SGK are largely uncharacterized, and to date no in vivo SGK substrates have been identified.Within the protein kinase superfamily, SGK is closely related to Akt (also called PKB), another serine/threonine kinase that is activated in response to growth and survival factors and plays a critical role in promoting cell survival (16,20). Several recent reports have shown that growth and s...

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