Specific binding of the Akt-1 protein kinase to phosphatidylinositol 3,4,5-trisphosphate without subsequent activation

Signaling by phosphoinositide-3,4,5-trisphosphate through proteins containing pleckstrin and Sec7 homology domains

Signal transmission by many cell surface receptors results in the activation of phosphoinositide (PI) 3-kinases that phosphorylate the 3' position of polyphosphoinositides. From a screen for mouse proteins that bind phosphoinositides, the protein GRP1was identified. GRP1 binds phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4, 5)P3] through a pleckstrin homology (PH) domain and displays a region of high sequence similarity to the yeast Sec7 protein. The PH domain of the closely related protein cytohesin-1, which, through its Sec7 homology domain, regulates integrin beta2 and catalyzes guanine nucleotide exchange of the small guanine nucleotide-binding protein ARF1, was also found to specifically bind PtdIns(3,4,5)P3. GRP1 and cytohesin-1 appear to connect receptor-activated PI 3-kinase signaling pathways with proteins that mediate biological responses such as cell adhesion and membrane trafficking.

High affinity binding of inositol phosphates and phosphoinositides to the pleckstrin homology domain of RAC/protein kinase B and their influence on kinase activity

The influence of inositol phosphates and phosphoinositides on the alpha isoform of the RAC-protein kinase B (RAC/PKB) was studied using purified wild type and mutant kinase preparations and a recombinant pleckstrin homology (PH) domain. Binding of inositol phosphates and phosphoinositides to the PH domain was measured as the quenching of intrinsic tryptophan fluorescence. Inositol phosphates and D3-phosphorylated phosphoinositides bound with affinities of 1-10 microM and 0.5 microM, respectively. Similar values were obtained using RAC/PKB expressed and purified from baculovirus-infected Sf9 cells in the fluorescence assay. The influence of synthetic dioctanoyl derivatives of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate on the activity of RAC/PKB purified from transfected COS-1 cells was studied. Phosphatidylinositol 3,4,5-trisphosphate was found to inhibit the RAC/PKB kinase activity with half-maximal inhibition at 2.5 microM. In contrast, phosphatidylinositol 3, 4-bisphosphate stimulated kinase activity (half-maximal stimulation at 2.5 microM). A mutant RAC/PKB protein lacking the PH domain was not affected by D3-phosphorylated phosphoinositides. These results demonstrate that the PH domain of RAC/PKB binds inositol phosphates and phosphoinositides with high affinity, and suggest that the products of the phosphatidylinositide 3-kinase can act as both a membrane anchor and modulator of RAC/PKB activity. The data also provide further evidence for a link between phosphatidylinositide 3-kinase and RAC/PKB regulation.

Regulation of protein kinase B and glycogen synthase kinase-3 by insulin and beta-adrenergic agonists in rat epididymal fat cells. Activation of protein kinase B by wortmannin-sensitive and -insensitive mechanisms

Previous studies using L6 myotubes have suggested that glycogen synthase kinase-3 (GSK-3) is phosphorylated and inactivated in response to insulin by protein kinase B (PKB, also known as Akt or RAC) (Cross, D. A. E., Alessi, D. R., Cohen, P., Andjelkovic, M., and Hemmings, B. A. (1995) Nature 378, 785-789). In the present study, marked increases in the activity of PKB have been shown to occur in insulin-treated rat epididymal fat cells with a time course compatible with the observed decrease in GSK-3 activity. Isoproterenol, acting primarily through beta3-adrenoreceptors, was found to decrease GSK-3 activity to a similar extent (approximately 50%) to insulin. However, unlike the effect of insulin, the inhibition of GSK by isoproterenol was not found to be sensitive to inhibition by the phosphatidylinositol 3'-kinase inhibitors, wortmannin or LY 294002. The change in GSK-3 activity brought about by isoproterenol could not be mimicked by the addition of permeant cyclic AMP analogues or forskolin to the cells, although at the concentrations used, these agents were able to stimulate lipolysis. Isoproterenol, but again not the cyclic AMP analogues, was found to increase the activity of PKB, although to a lesser extent than insulin. While wortmannin abolished the stimulation of PKB activity by insulin, it was without effect on the activation seen in response to isoproterenol. The activation of PKB by isoproterenol was not accompanied by any detectable change in the electrophoretic mobility of the protein on SDS-polyacrylamide gel electrophoresis. It would therefore appear that distinct mechanisms exist for the stimulation of PKB by insulin and isoproterenol in rat fat cells.

A novel, rapid, and highly sensitive mass assay for phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) and its application to measure insulin-stimulated PtdIns(3,4,5)P3 production in rat skeletal muscle in vivo

The pivotal role of phosphatidylinositol 3-kinase (PI 3-kinase) in signal transduction has been well established in recent years. Receptor-regulated forms of PI 3-kinase are thought to phosphorylate phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) at the 3-position of the inositol ring to give the putative lipid second messenger, phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4, 5)P3). Cellular levels of PtdIns(3,4,5)P3 are currently measured by time-consuming procedures involving radiolabeling with high levels of 32PO4, extraction, and multiple chromatography steps. To avoid these lengthy and hazardous procedures, many laboratories prefer to assay PI 3-kinase activity in cell extracts and/or appropriate immunoprecipitates. Such approaches are not readily applied to measurements of PtdIns(3,4,5)P3 in extracts of animal tissues. Moreover, they can be misleading since the association of PI 3-kinases in molecular complexes is not necessarily correlated with the enzyme's activity state. Direct measurements of PtdIns(3,4,5)P3 would also be desirable since its concentration may be subject to additional control mechanisms such as activation or inhibition of the phosphatases responsible for PtdIns(3,4,5)P3 metabolism. We now report a simple, reproducible isotope dilution assay which detects PtdIns(3,4,5)P3 at subpicomole sensitivity, suitable for measurements of both basal and stimulated levels of PtdIns(3,4,5)P3 obtained from samples containing approximately 1 mg of cellular protein. Total lipid extracts, containing PtdIns(3,4,5)P3, are first subjected to alkaline hydrolysis which results in the release of the polar head group Ins(1,3,4,5)P4. The latter is measured by its ability to displace [32P]Ins(1,3,4,5)P4 from a highly specific binding protein present in cerebellar membrane preparations. We show that this assay solely detects PtdIns(3,4,5)P3 and does not suffer from interference by other compounds generated after alkaline hydrolysis of total cellular lipids. Measurements on a wide range of cells, including rat-1 fibroblasts, 1321N1 astrocytoma cells, HEK 293 cells, and rat adipocytes, show wortmannin-sensitive increased levels of PtdIns(3,4,5)P3 upon stimulation with appropriate agonists. The enhanced utility of this procedure is further demonstrated by measurements of PtdIns(3,4,5)P3 levels in tissue derived from whole animals. Specifically, we show that stimulation with insulin increases PtdIns(3,4,5)P3 levels in rat skeletal muscle in vivo with a time course which parallels the activation of protein kinase B in the same samples.

A specific product of phosphatidylinositol 3-kinase directly activates the protein kinase Akt through its pleckstrin homology domain

Phosphatidylinositol (PI) 3-kinase is a cytoplasmic signaling molecule that is recruited to activated growth factor receptors after growth factor stimulation of cells. Activation of PI 3-kinase results in increased intracellular levels of 3' phosphorylated inositol phospholipids and the induction of signaling responses, including the activation of the protein kinase Akt, which is also known as RAC-PK or PKB. We tested the possibility that the phospholipid products of PI 3-kinase directly mediate the activation of Akt. We have previously described a constitutively active PI 3-kinase, p110, which can stimulate Akt activity. We used purified p110 protein to generate a series of 3' phosphorylated inositol phospholipids and tested whether any of these lipids could activate Akt in vitro. Phospholipid vesicles containing PI3,4 bisphosphate (P2) specifically activated Akt in vitro. By contrast, the presence of phospholipid vesicles containing PI3P or PI3,4,5P3 failed to increase the kinase activity of Akt. Akt could also be activated by synthetic dipalmitoylated PI3,4P2 or after enzymatic conversion of PI3,4,5P3 into PI3,4P2 with the signaling inositol polyphosphate 5' phosphatase SIP. We show that PI3,4P2-mediated activation is dependent on a functional pleckstrin homology domain in Akt, since a point mutation in the pleckstrin homology domain abrogated the response to PI3,4P2. Our findings show that a phospholipid product of PI 3-kinase can directly stimulate an enzyme known to be an important mediator of PI 3-kinase signaling.

R-Ras can activate the phosphoinositide 3-kinase but not the MAP kinase arm of the Ras effector pathways

BACKGROUND

The small GTPase R-Ras displays a less potent transforming activity than the closely related Ras oncogene products. Although R-Ras has been reported to interact with c-Raf1 and Ral-GDS in vitro, the pathways by which it exerts its effects on cellular proliferation are not known.

RESULTS

Both Ras and R-Ras interact with phosphoinositide (PI) 3-kinase in vitro, and induce elevation of the levels of PI 3-kinase lipid products in intact cells. Unlike Ras, R-Ras does not activate Raf or mitogen-activated protein (MAP) kinase in cells. In co-transfection assays, the serine/threonine protein kinase PKB (or Akt) is effectively stimulated by R-Ras, Ras, mutants of Ras that activate PI 3-kinase but not other effectors, and activated forms of PI 3-kinase. Ras and R-Ras stimulate PKB/Akt through a non-autocrine mechanism that involves PI 3-kinase. The constitutive activation of PI 3-kinase alone is sufficient to activate PKB/Akt, but not the MAP kinase ERK or the stress-activated protein kinase, Jun N-terminal kinase. Transformation assays in fibroblasts suggest that PKB/Akt and Raf are part of distinct oncogenic signalling pathways.

CONCLUSIONS

Both the Raf-MAP kinase and PI 3-kinase-PKB/Akt pathways are activated by Ras, but only the PI 3-kinase-PKB/Akt pathway is activated by R-Ras. PI 3-kinase, and downstream targets such as PKB/Akt, are likely to be essential mediators of transformation induced by R-Ras. PI 3-kinase, as well as Raf, is thus implicated also in Ras transformation.

Mechanism of activation of protein kinase B by insulin and IGF-1

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).

Distinct specificity in the recognition of phosphoinositides by the pleckstrin homology domains of dynamin and Bruton's tyrosine kinase

Pleckstrin homology (PH) domains may act as membrane localization modules through specific interactions with phosphoinositide phospholipids. These interactions could represent responses to second messengers, with scope for regulation by soluble inositol polyphosphates. A biosensor-based assay was used here to probe interactions between PH domains and unilamellar liposomes containing different phospholipids and to demonstrate specificity for distinct phosphoinositides. The dynamin PH domain specifically interacted with liposomes containing phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] and, more weakly, with liposomes containing phosphatidylinositol-4-phosphate [PI(4)P]. This correlates with phosphoinositide activation of the dynamin GTPase. The functional GTPase of a dynamin mutant lacking the PH domain, however, cannot be activated by PI(4,5)P2. The phosphoinositide-PH domain interaction can be abolished selectively by point mutations in the putative binding pocket predicted by molecular modelling and NMR spectroscopy. In contrast, the Bruton's tyrosine kinase (Btk)PH domain specifically bound liposomes containing phosphatidylinositol-3,4,5-trisphosphate [PI(3,4,5)P3]: an interaction requiring Arg28, a residue found to be mutated in some X-linked agammaglobulinaemia patients. A rational explanation for these different specificities is proposed through modelling of candidate binding pockets and is supported by NMR spectroscopy.

Isolation of the active form of RAC-protein kinase (PKB/Akt) from transfected COS-7 cells treated with heat shock stress and effects of phosphatidylinositol 3,4,5-trisphosphate and phosphatidylinositol 4,5-bisphosphate on its enzyme activity

RAC-protein kinase (PKB/Akt) has been shown to be activated by growth factor stimulation as a downstream target of phosphatidylinositol 3-kinase and also by heat shock through a pathway independent of phosphatidylinositol 3-kinase. RAC-protein kinase was purified by antibody affinity chromatography from COS-7 cells transfected with the epitope-tagged expression plasmid. The protein kinase activity of RAC-protein kinase purified from heat-treated cells was 9-fold higher than the enzyme isolated from untreated control cells. Phosphatidylinositol 3,4,5-trisphosphate did not enhance the activity of RAC-protein kinase purified from either heat-treated cells or control cells, whereas phosphatidylinositol 4,5-bisphosphate suppressed the enzyme isolated from heat-treated cells. These results indicate that RAC-protein kinase may interact with phosphoinositides, however, it could not be activated by simple association with the product of phosphatidylinositol 3-kinase reaction.

Akt, a pleckstrin homology domain containing kinase, is activated primarily by phosphorylation

Akt is a serine/threonine kinase that is stimulated by receptor tyrosine kinases and contains a pleckstrin homology domain. One model proposed to explain this activation suggests that receptor tyrosine kinases stimulate a phosphatidylinositol 3-kinase whose lipid products directly activate Akt kinase by interacting with its pleckstrin homology domain. In the present study, we show, in three cell types, that Akt does not require its pleckstrin homology domain to respond to either insulin or platelet-derived growth factor. Moreover, attachment of the src myristoylation signal to target Akt, without its pleckstrin homology domain, to the membrane constitutively activates Akt by causing an increase in its basal level of phosphorylation. This constitutively active form of Akt can also activate p70(S6K), indicating that the pleckstrin homology domain is not necessary for downstream interactions. Fusion of the inter src homology 2 domain from the p85 regulatory subunit of the phosphatidylinositol 3-kinase to Akt also constitutively activated Akt and induced an association with the lipid kinase. Phosphorylation of this fusion protein still critically contributes toward its increased activity. The sum of these results indicates that the primary mechanism of Akt activation is via protein phosphorylation.

Thrombin receptors modulate insulin-stimulated phosphatidylinositol 3,4,5-trisphosphate accumulation in 1321N1 astrocytoma cells

Thrombin and insulin receptor signaling via phosphoinositide (PI)-specific phospholipase C (PLC) and PI 3-kinase was studied in [3H]inositol-labelled 1321N1 cells. Thrombin stimulated a dramatic, transient activation of PLC which is probably mediated via receptors of the 'tethered-ligand' type, since it was both reproduced by, and abolished following, pretreatment of cells with a synthetic peptide (SFLLRN) corresponding to the ligand domain of the human thrombin receptor. However, neither thrombin nor SFLLRN stimulated PI 3-kinase. By contrast, insulin did not influence [3H]InsP3 concentration but stimulated accumulation of [3H]PtdIns(3,4,5)P3 and [3H]PtdIns(3,4)P2, the relative steady-state concentrations of which may indicate degradation of [3H]PtdIns(3,4,5)P3 by 5- and 3-phosphatases. The independent coupling of thrombin and insulin receptors to PLC and PI 3-kinase respectively in 1321N1 cells allowed interactions between these systems to be examined. Thus insulin-stimulated [3H]PtdIns(3,4,5)P3 accumulation was attenuated on co-stimulation of the thrombin receptor, whereas concentrations of [3H]PtdIns(3,4)P2 were transiently enhanced but then reduced. These results indicate that thrombin receptors in 1321N1 cells do not activate PI 3-kinase, but can modulate signalling by this enzyme.