Isolation and molecular cloning of wortmannin-sensitive bovine type III phosphatidylinositol 4-kinases

Specific interactions among transmembrane 4 superfamily (TM4SF) proteins and phosphoinositide 4-kinase

In earlier work we established that phosphoinositide 4-kinase (PI 4-kinase) may associate with transmembrane 4 superfamily (TM4SF, tetraspanin) proteins, but critical specificity issues were not addressed. Here we demonstrate that at least five different TM4SF proteins (CD9, CD63, CD81, CD151 and A15/TALLA1) can associate with a similar or identical 55 kDa type II PI 4-kinase. These associations were specific, since we found no evidence for other phosphoinositide kinases (e.g. phosphoinositide 3-kinase and phosphoinositide-4-phosphate 5-kinase) associating with TM4SF proteins, and many other TM4SF proteins (including CD82 and CD53) did not associate with PI 4-kinase. CD63-PI 4-kinase complexes were almost entirely intracellular, and thus are distinct from other TM4SF-PI 4-kinase complexes (e.g. involving CD9), which are largely located in the plasma membrane. These results suggest that a specific subset of TM4SF proteins may recruit PI 4-kinase to specific membrane locations, and thereby influence phosphoinositide-dependent signalling.

Molecular mechanisms regulating the retrograde axonal transport of neurotrophins

Neurotrophins are released from target tissues following neural innervation and bind to specific receptors situated on the nerve terminal plasma membrane. The neurotrophin-receptor complex undergoes retrograde axonal transport towards the cell soma, where it signals to the nucleus. This process allows neurotrophins to perform their numerous functions, which include the promotion of neuronal survival and the outgrowth of axons towards certain target tissues. The molecular events controlling each of the components of retrograde axonal transport are beginning to become defined. There is good evidence for the participation of phosphatidylinositol 3-kinase, phosphatidylinositol 4-kinase and the actin cytoskeleton in neurotrophin retrograde axonal transport in vivo. It also appears that the retrograde motor protein dynein mediates the retrograde axonal transport in vivo of neurotrophins such as nerve growth factor. This review discusses the role of the neurotrophin receptors in binding and axonal transport, the endocytic processes required for neurotrophin internalization, the targeting and trafficking of neurotrophins, and the propagation of neurotrophin-induced signals along the axon.

Inhibition of the phosphoinositide 3-kinase pathway induces a senescence-like arrest mediated by p27Kip1

A senescence-like growth arrest is induced in mouse primary embryo fibroblasts by inhibitors of phosphoinositide 3-kinase (PI3K). We observed that senescence-like growth arrest is correlated with an increase in p27(Kip1) but that down-regulation of other cyclin-dependent kinase (CDK) inhibitors, including p15(INK4b), p16(INK4a), p19( INK4d), and p21(Cip1) as well as other negative cell cycle regulators such as p53 and p19(ARF), implies that this senescence-related growth arrest is independent of the activity of p53, p19(ARF), p16(INK4a), and p21(Cip1), which are associated with replicative senescence. The p27(Kip1) binds to the cyclin/CDK2 complexes and causes a decrease in CDK2 kinase activity. We demonstrated that ectopic expression of p27(Kip1) can induce permanent cell cycle arrest and a senescence-like phenotype in wild-type mouse embryo fibroblasts. We also obtained results suggesting that the kinase inhibitors LY294002 and Wortmannin arrest cell growth and induce a senescence-like phenotype, at least partially, through inhibition of PI3K and protein kinase B/Akt, activation of the forkhead protein AFX, and up-regulation of p27(Kip1)expression. In summary, these observations taken together suggest that p27(Kip1) is an important mediator of the permanent cell cycle arrest induced by PI3K inhibitors. Our data suggest that repression of CDK2 activity by p27(Kip1) is required for the PI3K-induced senescence, yet mouse embryo fibroblasts derived from p27(Kip1-/-) mice entered cell cycle arrest after treatment with LY294002. We show that this is due to a compensatory mechanism by which p130 functionally substitutes for the loss of p27(Kip1). This is the first description that p130 may have a role in inhibiting CDK activity during senescence.

Characterization of recombinant phosphatidylinositol 4-kinase beta reveals auto- and heterophosphorylation of the enzyme

Phosphatidylinositol (PI) 4-kinases catalyze the synthesis of PI 4-phosphate, an important intermediate for the synthesis of membrane polyphosphoinositides, regulators of multiple cellular functions. Two mammalian PI 4-kinases have been cloned, a 230-kDa enzyme (alpha-form) and a 110-kDa (beta-form), both of which are inhibited by >0.1 microm concentrations of the PI 3-kinase inhibitor, wortmannin (WT). In the present study, we created a glutathione S-transferase-PI4Kbeta fusion protein for expression in Escherichia coli. The purified protein was biologically active and phosphorylated PI in its 4-position with WT sensitivity and kinetic parameters that were identical to those of purified bovine brain PI4Kbeta. In addition to its lipid kinase activity, the enzyme exhibited autophosphorylation that was enhanced by Mn(2+) ions and inhibited by WT and another PI 3-kinase inhibitor, LY 294002. The recombinant protein was unable to transphosphorylate, but its isolated C-terminal catalytic domain still displayed autophosphorylation, suggesting that the autophosphorylation site resides within the C-terminal catalytic domain of the protein and is held in position by intramolecular interactions. Autophosphorylation inhibited subsequent lipid kinase activity, which was reversed upon dephosphorylation, by protein phosphatases, PP1 and PP2A(1), suggesting that it may represent a regulatory mechanism for the enzyme. Phosphorylation of endogenous or overexpressed PI4Kbeta was also observed in COS-7 cells; however, the in vivo phosphorylation of the expressed protein was only partially inhibited by WT and also occurred in a catalytically inactive form of the enzyme, indicating the presence of additional phosphorylation site(s). Successful bacterial expression of PI4Kbeta should aid research on the structure-function relationships of this protein as well as of other, structurally related enzymes.

Localization of two distinct type III phosphatidylinositol 4-kinase enzyme mRNAs in the rat

Inositol lipid kinases generate polyphosphoinositides, important regulators of several cellular functions. We have recently cloned two distinct phosphatidylinositol (PI) 4-kinase enzymes, the 210-kDa PI4KIIIalpha and the 110-kDa PI4KIIIbeta, from bovine tissues. In the present study, the distribution of mRNAs encoding these two enzymes was analyzed by in situ hybridization histochemistry in the rat. PI4KIIIalpha was found predominantly expressed in the brain, with low expression in peripheral tissues. PI4KIIIbeta was more uniformly expressed being also present in various peripheral tissues. Within the brain, PI4KIIIbeta showed highest expression in the gray matter, especially in neurons of the olfactory bulb and the hippocampus, but also gave a signal in the white matter indicating its presence in glia. PI4KIIIalpha was highly expressed in neurons, but lacked a signal in the white matter and the choroid plexus. Both enzymes showed expression in the pigment layer and nuclear layers as well as in the ganglion cells of the retina. In a 17-day-old rat fetus, PI4KIIIbeta was found to be more widely distributed and PI4KIIIalpha was primarily expressed in neurons. These results indicate that PI4KIIIbeta is more widely expressed than PI4KIIIalpha, and that the two enzymes are probably coexpressed in many neurons. Such expression pattern and the conservation of these two proteins during evolution suggest their nonredundant functions in mammalian cells.

Metabolism and function of 3-D-phosphorylated phosphoinositides in C5a-stimulated eosinophils

Eosinophils play a central role in the pathogenesis of parasitic infections, atopic diseases, and bullous dermatoses. To understand the regulative function of phosphatidylinositol 3-kinases in cell responses of eosinophils, phospholipid metabolism and production of reactive oxygen metabolites were followed after stimulation with C5a. Measurements of phosphatidylinositol lipids and analysis of deacylated products of separated lipid extracts showed fast and transient formation of phosphatidylinositol 3,4,5-trisphosphate (PIP(3)). Cell studies in the presence of the tyrosine kinase blocker genistein indicated that C5a-stimulated PIP(3) formation occurred independently of tyrosine kinase activity. To analyze the function of PI4,5P(2)-3-kinase in eosinophils, the influence of wortmannin and LY294002 on production of reactive oxygen metabolites was studied. Both compounds inhibited with similar concentration dependency C5a-induced formation of PIP(3) and production of reactive oxygen metabolites. In summary, these data showed for the first time the involvement of PI4,5P(2)-3-kinase in the production of reactive oxygen metabolites in eosinophils.

Phosphatidylinositol 4-kinase associated with spinach plasma membranes. Isolation and characterization of two distinct forms

Highly purified plasma membranes from spinach (Spinacia oleracea L.) leaves contained phosphatidylinositol (PtdIns) kinase activity that was firmly associated with the membrane. The enzyme was solubilized by detergent treatment (2% [w/v] Triton X-100) and purified by heparin-Sepharose and Q-Sepharose chromatography. Two enzymically active fractions, QI and QII, both exhibiting PtdIns 4-kinase activity, were resolved and purified 100- to 300-fold over the plasma membrane. QI and QII shared similar high apparent K(m) values for ATP (approximately 0.45 mM) and PtdIns (approximately 0.2 mM) and were insensitive to inhibition by adenosine. While Mg(2+) was the preferred divalent cation, Mn(2+) could partly substitute in the reaction catalyzed by the QII enzyme but not in that catalyzed by QI. Mn(2+) acted synergistically with suboptimal Mg(2+) concentrations to activate not only the QII enzyme, but also to some extent QI. Both enzymes were inhibited by millimolar concentrations of Ca(2+) and micromolar concentrations of wortmannin. The apparent molecular mass for QI was 120 kD, which was determined by SDS-PAGE and western blotting using an antibody against a peptide unique for lipid kinases and the binding of (3)H-wortmannin, and for QII 65 kD as determined by immunodetection and renaturation of PtdIns kinase activity in the 65-kD region of polyacrylamide gels.

Two compartments for insulin-stimulated exocytosis in 3T3-L1 adipocytes defined by endogenous ACRP30 and GLUT4

Insulin stimulates adipose cells both to secrete proteins and to translocate the GLUT4 glucose transporter from an intracellular compartment to the plasma membrane. We demonstrate that whereas insulin stimulation of 3T3-L1 adipocytes has no effect on secretion of the alpha3 chain of type VI collagen, secretion of the protein hormone adipocyte complement related protein of 30 kD (ACRP30) is markedly enhanced. Like GLUT4, regulated exocytosis of ACRP30 appears to require phosphatidylinositol-3-kinase activity, since insulin-stimulated ACRP30 secretion is blocked by pharmacologic inhibitors of this enzyme. Thus, 3T3-L1 adipocytes possess a regulated secretory compartment containing ACRP30. Whether GLUT4 recycles to such a compartment has been controversial. We present deconvolution immunofluorescence microscopy data demonstrating that the subcellular distributions of ACRP30 and GLUT4 are distinct and nonoverlapping; in contrast, those of GLUT4 and the transferrin receptor overlap. Together with supporting evidence that GLUT4 does not recycle to a secretory compartment via the trans-Golgi network, we conclude that there are at least two compartments that undergo insulin-stimulated exocytosis in 3T3-L1 adipocytes: one for ACRP30 secretion and one for GLUT4 translocation.

Coupled inositide phosphorylation and phospholipase D activation initiates clathrin-coat assembly on lysosomes

Adaptors appear to control clathrin-coat assembly by determining the site of lattice polymerization but the nucleating events that target soluble adaptors to an appropriate membrane are poorly understood. Using an in vitro model system that allows AP-2-containing clathrin coats to assemble on lysosomes, we show that adaptor recruitment and coat initiation requires phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) synthesis. PtdIns(4,5)P2 is generated on lysosomes by the sequential action of a lysosome-associated type II phosphatidylinositol 4-kinase and a soluble type I phosphatidylinositol 4-phosphate 5-kinase. Phosphatidic acid, which potently stimulates type I phosphatidylinositol 4-phosphate 5-kinase activity, is generated on the bilayer by a phospholipase D1-like enzyme located on the lysosomal surface. Quenching phosphatidic acid function with primary alcohols prevents the synthesis of PtdIns(4, 5)P2 and blocks coat assembly. Generating phosphatidic acid directly on lysosomes with exogenous bacterial phospholipase D in the absence of ATP still drives adaptor recruitment and limited coat assembly, indicating that PtdIns(4,5)P2 functions, at least in part, to activate the PtdIns(4,5)P2-dependent phospholipase D1. These results provide the first direct evidence for the involvement of anionic phospholipids in clathrin-coat assembly on membranes and define the enzymes responsible for the production of these important lipid mediators.

Phosphoinositide kinases and the synthesis of polyphosphoinositides in higher plant cells

Phosphoinositides are a family of inositol-containing phospholipids which are present in all eukaryotic cells. Although in most cells these lipids, with the exception of phosphatidylinositol, constitute only a very minor proportion of total cellular lipids, they have received immense attention by researchers in the past 15-20 years. This is due to the discovery that these lipids, rather than just having structural functions, play key roles in a wide range of important cellular processes. Much less is known about the plant phosphoinositides than about their mammalian counterparts. However, it has been established that a functional phosphoinositide system exists in plant cells and it is becoming increasingly clear that inositol-containing lipids are likely to play many important roles throughout the life of a plant. It is not our intention to give an exhaustive overview of all aspects of the field, but rather we focus on the phosphoinositide kinases responsible for the synthesis of all phosphorylated forms of phosphatidylinositol. Also, we mention some of the aspects of current phosphoinositide research which, in our opinion, are most likely to provide a suitable starting point for further research into the role of phosphoinositides in plants.

Pathways for phosphoinositide synthesis

In eukaryotic cells, phosphatidylinositol can be phosphorylated on the inositol ring by a series of kinases to produce at least seven distinct phosphoinositides. These lipids have been implicated in a variety of cellular processes, including calcium regulation, actin rearrangement, vesicle trafficking, cell survival and mitogenesis. The phosphorylated lipids can act as precursors of second messengers or act directly to recruit specific signaling proteins to the membrane. A number of the kinases responsible for producing these lipids have been purified and their cDNA clones have been isolated. The most well characterized of these enzymes are the phosphoinositide 3-kinases. However, progress has also been made in the characterization of phosphatidylinositol 4-kinases and phosphatidylinositol-4-phosphate 5-kinases. In addition, new pathways involving phosphatidylinositol-5-phosphate 4-kinases, phosphatidylinositol-3-phosphate 5-kinases and phosphatidylinositol-3-phosphate 4-kinases have recently been described. The various enzymes and pathways involved in the synthesis of cellular phosphoinositides will be discussed.

Functional expression and characterisation of a new human phosphatidylinositol 4-kinase PI4K230

By constructing DNA probes we have identified and cloned a human PtdIns 4-kinase, PI4K230, corresponding to a mRNA of 7.0 kb. The cDNA encodes a protein of 2044 amino acids. The C-terminal part of ca. 260 amino acids represents the catalytic domain which is highly conserved in all recently cloned PtdIns 4-kinases. N-terminal motifs indicate multiple heterologous protein interactions. Human PtdIns 4-kinase PI4K230 expressed in vitro exhibits a specific activity of 58 micromol mg-1min-1. The enzyme expressed in Sf9 cells is essentially not inhibited by adenosine, it shows a high Km for ATP of about 300 microM and it is half-maximally inactivated by approximately 200 nM wortmannin. These data classify this enzyme as type 3 PtdIns 4-kinase. Antibodies raised against the N-terminal part moderately activate and those raised against the C-terminal catalytic domain inhibit the enzymatic activity. The coexistence of two different type 3 PtdIns 4-kinases, PI4K92 and PI4K230, in several human tissues, including brain, suggests that these enzymes are involved in distinct basic cellular functions.

A plant 126-kDa phosphatidylinositol 4-kinase with a novel repeat structure. Cloning and functional expression in baculovirus-infected insect cells

Phosphatidylinositol metabolism plays a central role in signaling pathways in animals and is also believed to be of importance in signal transduction in higher plants. We report here the molecular cloning of a cDNA encoding a previously unidentified 126-kDa phosphatidylinositol (PI) 4-kinase (AtPI4Kbeta) from the higher plant Arabidopsis thaliana. The novel protein possesses the conserved domains present in animal and yeast PI 4-kinases, namely a lipid kinase unique domain and a catalytic domain. An additional domain, approximately 300 amino acids long, containing a high percentage (46%) of charged amino acids is specific to this plant enzyme. Recombinant AtPI4Kbeta expressed in baculovirus-infected insect (Spodoptera frugiperda) cells phosphorylated phosphatidylinositol exclusively at the D4 position of the inositol ring. Recombinant protein was maximally activated by 0.6% Triton X-100 but was inhibited by adenosine with an IC50 of approximately 200 microM. Wortmannin at a concentration of 10 microM inhibited AtPI4Kbeta activity by approximately 90%. AtPI4Kbeta transcript levels were similar in all tissues analyzed. Light or treatment with hormones or salts did not change AtPI4Kbeta transcript levels to a great extent, indicating constitutive expression of the AtPI4Kbeta gene.

Phosphoinositide lipids as signaling molecules: common themes for signal transduction, cytoskeletal regulation, and membrane trafficking

Signaling roles for phosphoinositides that involve their regulated hydrolysis to generate second messengers have been well characterized. Recent work has revealed additional signaling roles for phosphoinositides that do not involve their hydrolysis. PtdIns 3-P, PtdIns 3,4,5-P3, and PtdIns 4,5-P2 function as site-specific signals on membranes that recruit and/or activate proteins for the assembly of spatially localized functional complexes. A large number of phosphoinositide-binding proteins have been identified as the potential effectors for phosphoinositide signals. Common themes of localized signal generation and the spatially localized recruitment of effector proteins appear to underlie mechanisms employed in signal transduction, cytoskeletal, and membrane trafficking events.

Phosphoinositide kinases

Phosphatidylinositol, a component of eukaryotic cell membranes, is unique among phospholipids in that its head group can be phosphorylated at multiple free hydroxyls. Several phosphorylated derivatives of phosphatidylinositol, collectively termed phosphoinositides, have been identified in eukaryotic cells from yeast to mammals. Phosphoinositides are involved in the regulation of diverse cellular processes, including proliferation, survival, cytoskeletal organization, vesicle trafficking, glucose transport, and platelet function. The enzymes that phosphorylate phosphatidylinositol and its derivatives are termed phosphoinositide kinases. Recent advances have challenged previous hypotheses about the substrate selectivity of different phosphoinositide kinase families. Here we re-examine the pathways of phosphoinositide synthesis and the enzymes involved.

Phosphatidylinositol 3,4,5-trisphosphate-dependent stimulation of phospholipase C-gamma2 is an early key event in FcgammaRIIA-mediated activation of human platelets

Platelets express a single class of Fcgamma receptor (FcgammaRIIA), which is involved in heparin-associated thrombocytopenia and possibly in inflammation. FcgammaRIIA cross-linking induces platelet secretion and aggregation, together with a number of cellular events such as tyrosine phosphorylation, activation of phospholipase C-gamma2 (PLC-gamma2), and calcium signaling. Here, we show that in response to FcgammaRIIA cross-linking, phosphatidylinositol (3,4, 5)-trisphosphate (PtdIns(3,4,5)P3) is rapidly produced, whereas phosphatidylinositol (3,4)-bisphosphate accumulates more slowly, demonstrating a marked activation of phosphoinositide 3-kinase (PI 3-kinase). Inhibition of PI 3-kinase by wortmannin or LY294002 abolished platelet secretion and aggregation, as well as phospholipase C (PLC) activation, indicating a role of this lipid kinase in the early phase of platelet activation. Inhibition of PLCgamma2 was not related to its tyrosine phosphorylation state, since wortmannin actually suppressed its dephosphorylation, which requires platelet aggregation and integrin alphaIIb/beta3 engagement. In contrast, the stable association of PLCgamma2 to the membrane/cytoskeleton interface observed at early stage of platelet activation was fully abolished upon inhibition of PI 3-kinase. In addition, PLCgamma2 was able to preferentially interact in vitro with PtdIns(3,4,5)P3. Finally, exogenous PtdIns(3,4,5)P3 restored PLC activation in permeabilized platelets treated with wortmannin. We propose that PI 3-kinase and its product PtdIns(3,4,5)P3 play a key role in the activation and adequate location of PLCgamma2 induced by FcgammaRIIA cross-linking.

A phosphatidylinositol 4-kinase pleckstrin homology domain that binds phosphatidylinositol 4-monophosphate

Pleckstrin homology (PH) domains are found in many proteins involved in signal transduction, including the family of large molecular mass phosphatidylinositol (PI) 4-kinases. Although the exact function of these newly discovered domains is unknown, it is recognized that they may influence enzyme regulation by binding different ligands. In this study, the recombinant PI 4-kinase PH domain was explored for its ability to bind to different phospholipids. First, we isolated partial cDNAs of the >7-kilobase transcripts of PI 4-kinases from carrot (DcPI4Kalpha) and Arabidopsis (AtPI4Kalpha). The deduced primary sequences were 41% identical and 68% similar to rat and human PI 4-kinases and contained the telltale lipid kinase unique domain, PH domain, and catalytic domain. Antibodies raised against the expressed lipid kinase unique, PH, and catalytic domains identified a polypeptide of 205 kDa in Arabidopsis microsomes and an F-actin-enriched fraction from carrot cells. The 205-kDa immunoaffinity-purified Arabidopsis protein had PI 4-kinase activity. We have used the expressed PH domain to characterize lipid binding properties. The recombinant PH domain selectively bound to phosphatidylinositol 4-monophosphate (PI-4-P), phosphatidylinositol 4,5-bisphosphate (PI-4,5-P2), and phosphatidic acid and did not bind to the 3-phosphoinositides. The PH domain had the highest affinity for PI-4-P, the product of the reaction. Consideration is given to the potential impact that this has on cytoskeletal organization and the PI signaling pathway in cells that have a high PI-4-P/PI-4,5-P2 ratio.

Rac regulates integrin-mediated spreading and increased adhesion of T lymphocytes

Leukocyte adhesion to the extracellular matrix (ECM) is tightly controlled and is vital for the immune response. Circulating lymphocytes leave the bloodstream and adhere to ECM components at sites of inflammation and lymphoid tissues. Mechanisms for regulating T-lymphocyte-ECM adhesion include (i) an alteration in the affinity of cell surface integrin receptors for their extracellular ligands and (ii) an alteration of events following postreceptor occupancy (e.g., cell spreading). Whereas H-Ras and R-Ras were previously shown to affect T-cell adhesion by altering the affinity state of the integrin receptors, no signaling molecule has been identified for the second mechanism. In this study, we demonstrated that expression of an activated mutant of Rac triggered dramatic spreading of T cells and their increased adhesion on immobilized fibronectin in an integrin-dependent manner. This effect was not mimicked by expression of activated mutant forms of Rho, Cdc42, H-Ras, or ARF6, indicating the unique role of Rac in this event. The Rac-induced spreading was accompanied by specific cytoskeletal rearrangements. Also, a clustering of integrins at sites of cell adhesion and at the peripheral edges of spread cells was observed. We demonstrate that expression of RacV12 did not alter the level of expression of cell surface integrins or the affinity state of the integrin receptors. Moreover, our results indicate that Rac plays a role in the regulation of T-cell adhesion by a mechanism involving cell spreading, rather than by altering the level of expression or the affinity of the integrin receptors. Furthermore, we show that the Rac-mediated signaling pathway leading to spreading of T lymphocytes did not require activation of c-Jun kinase, serum response factor, or pp70(S6 kinase) but appeared to involve a phospholipid kinase.

Membrane-permeant esters of phosphatidylinositol 3,4,5-trisphosphate

Phosphoinositide 3-OH kinases and their products, D-3 phosphorylated phosphoinositides, are increasingly recognized as crucial elements in many signaling cascades. A reliable means to introduce these lipids into intact cells would be of great value for showing the physiological roles of this pathway and for testing the specificity of pharmacological inhibitors of the kinases. We have stereospecifically synthesized di-C8-PIP3/AM and di-C12-PIP3/AM, the heptakis(acetoxymethyl) esters of dioctanoyl- and dilauroylphosphatidylinositol 3,4,5-trisphosphate, in 14 steps from myo-inositol. The ability of these uncharged lipophilic derivatives to deliver phosphatidylinositol 3,4,5-trisphosphate across cell membranes was demonstrated on 3T3-L1 adipocytes and T84 colon carcinoma monolayers. Insulin stimulation of hexose uptake into adipocytes was inhibited by the kinase inhibitor wortmannin and was largely restored by di-C8-PIP3/AM, which had no effect in the absence of insulin. Thus phosphatidylinositol 3,4,5-trisphosphate or a metabolite was necessary but not sufficient for stimulation of hexose transport. In T84 epithelial monolayers, di-C12-PIP3/AM mimicked epidermal growth factor in inhibiting chloride secretion and potassium efflux, suggesting that phosphatidylinositol 3,4, 5-trisphosphate was sufficient to modulate these fluxes and mediate epidermal growth factor's action.

Wortmannin, an inhibitor of phosphatidyl-inositol 3-kinase, induces oocyte maturation through a MPF-MAPK-dependent pathway

Wortmannin has been shown to be a non-competitive and irreversible inhibitor of PI3 kinase. For this reason, it has attracted considerable interest and it has been used, as a selective inhibitor of the PI3 kinase, for the study of signal transduction pathways in different systems including Xenopus oocytes. We show here that wortmannin itself is able to induce meiotic maturation at doses slightly higher that those required for complete inhibition of PI3 kinase. This effect was shown to be independent of the ability to inhibit PI3K since another unrelated PI3K inhibitor, LY294002, was unable to induce oocyte maturation at inhibitory concentrations for PI3 kinase. The mechanism for wortmannin-induced maturation involves the activation of maturation promoting factor (MPF) and MAP kinase activities in a time course that preceded the appearance of germinal vesicle breakdown. Thus, the pathway activated by wortmannin directly or indirectly affects other protein or proteins, besides PI3 kinase, responsible for its activity. This new target is placed independently or downstream of the PI3 kinase inhibition and upstream of protein synthesis. Moreover, the inhibition of either MPF or cAMP phosphodiesterase blocks wortmannin-induced maturation. We conclude that wortmannin may be a valuable tool for the study of the pathway leading to mitotic maturation of oocytes, but cannot be used as a specific PI3 kinase inhibitor.

Dexamethasone stimulates the expression of GLUT1 and GLUT4 proteins via different signalling pathways in L6 skeletal muscle cells

It was recently demonstrated that dexamethasone treatment of L6 skeletal muscle cells resulted in an elevation of GLUT1 protein. However, the level of GLUT4 protein under these conditions was not examined. In addition, the signalling mechanism(s) leading to dexamethasone-induced expression of GLUT1 protein was not investigated. In the present study we investigated the effect of dexamethasone on the expression of GLUT1 and GLUT4 proteins in differentiated L6 muscle cells and the signalling mechanism(s) via which dexamethasone may act. Dexamethasone (300 nM) treatment for 24 h elevated GLUT1 and GLUT4 proteins by 68% and 94%, respectively, above control levels. These increases were due to de novo synthesis as shown by metabolic labelling with [35S]methionine. Incubation of cells with 100 nM wortmannin or 30 ng/ml rapamycin prevented the dexamethasone-stimulated elevation of GLUT1 protein. In contrast, neither of these inhibitors affected the elevation of GLUT4 protein by dexamethasone. Furthermore, dexamethasone down-regulated insulin receptor substrate-1 protein content by 42% and insulin-induced tyrosine phosphorylation of insulin receptor substrate-1 by 28%. The p70 ribosomal S6 kinase was not activated by dexamethasone and instead, dexamethasone attenuated the stimulation of this enzyme activity by insulin. These results suggest that dexamethasone induces the expression of GLUT1 and GLUT4 protein by independent signalling mechanisms with a concomitant depression of intracellular signalling by insulin.