Interaction of neuronal calcium sensor-1 (NCS-1) with phosphatidylinositol 4-kinase beta stimulates lipid kinase activity and affects membrane trafficking in COS-7 cells

Phosphatidylinositol 4-kinases (PI4K) catalyze the first step in the synthesis of phosphatidylinositol 4,5-bisphosphate, an important lipid regulator of several cellular functions. Here we show that the Ca(2+)-binding protein, neuronal calcium sensor-1 (NCS-1), can physically associate with the type III PI4Kbeta with functional consequences affecting the kinase. Recombinant PI4Kbeta, but not its glutathione S-transferase-fused form, showed enhanced PI kinase activity when incubated with recombinant NCS-1, but only if the latter was myristoylated. Similarly, in vitro translated NCS-1, but not its myristoylation-defective mutant, was found associated with recombinant- or in vitro translated PI4Kbeta in PI4Kbeta-immunoprecipitates. When expressed in COS-7 cells, PI4Kbeta and NCS-1 formed a complex that could be immunoprecipitated with antibodies against either proteins, and PI 4-kinase activity was present in anti-NCS-1 immunoprecipitates. Expressed NCS-1-YFP showed co-localization with endogenous PI4Kbeta primarily in the Golgi, but it was also present in the walls of numerous large perinuclear vesicles. Co-expression of a catalytically inactive PI4Kbeta inhibited the development of this vesicular phenotype. Transfection of PI4Kbeta and NCS-1 had no effect on basal PIP synthesis in permeabilized COS-7 cells, but it increased the wortmannin-sensitive [(32)P]phosphate incorporation into phosphatidylinositol 4-phosphate during Ca(2+)-induced phospholipase C activation. These results together indicate that NCS-1 is able to interact with PI4Kbeta also in mammalian cells and may play a role in the regulation of this enzyme in specific cellular compartments affecting vesicular trafficking.     

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.     

Synergistic activation of a family of phosphoinositide 3-kinase via G-protein coupled and tyrosine kinase-related receptors.

Phosphoinositide 3-kinase (PI 3-kinase) is a key signaling enzyme implicated in a variety of receptor-stimulated cell responses. Stimulation of receptors possessing (or coupling to) protein-tyrosine kinase activates heterodimeric PI 3-kinases, which consist of an 85-kDa regulatory subunit (p85) containing Src-homology 2 (SH2) domains and a 110-kDa catalytic subunit (p110 alpha or p110 beta). Thus, this form of PI 3-kinases could be activated in vitro by a phosphotyrosyl peptide containing a YMXM motif that binds to the SH2 domains of p85. Receptors coupling to alpha beta gamma-trimeric G proteins also stimulate the lipid kinase activity of a novel p110 gamma isoform, which is not associated with p85, and thereby is not activated by tyrosine kinase receptors. The activation of p110 gamma PI 3-kinase appears to be mediated through the beta gamma subunits of the G protein (G beta gamma). In addition, rat liver heterodimeric PI 3-kinases containing the p110 beta catalytic subunit are synergistically activated by the phosphotyrosyl peptide plus G beta gamma. Such enzymatic properties were also observed with a recombinant p110 beta/p85 alpha expressed in COS-7 cells. In contrast, another heterodimeric PI 3-kinase consisting of p110 alpha and p85 in the same rat liver, together with a recombinant p110 alpha/p85 alpha, was not activated by G beta gamma, though their activities were stimulated by the phosphotyrosyl peptide. Synergistic activation of PI 3-kinase by the stimulation of the two major receptor types was indeed observed in intact cells, such as chemotactic peptide (N-formyl-Met-Leu-Phe) plus insulin (or Fc gamma II) receptors in differentiated THP-1 and CHO cells and adenosine (A1) plus insulin receptors in rat adipocytes. Thus, PI 3-kinase isoforms consisting of p110 beta catalytic and SH2-containing (p85 or its related) regulatory subunits appeared to function as a 'cross-talk' enzyme between the two signal transduction pathways mediated through tyrosine kinase and G protein-coupled receptors.     

Neuronal calcium sensor-1 and phosphatidylinositol 4-kinase beta regulate IgE receptor-triggered exocytosis in cultured mast cells

We examined the possible occurrence and function of neuronal Ca(2+) sensor 1 (NCS-1/frequenin) in the mast cell line rat basophilic leukemia, RBL-2H3. This protein has been implicated in the control of neurosecretion from dense core granules in neuronal cells as well as in the control of constitutive secretory pathways in both yeast and mammalian cells. We show that RBL-2H3 cells, secretory cells of the immune system, endogenously express the 22-kDa NCS-1 protein as well as an immune-related 50-kDa protein. Both proteins associate in vivo with phosphatidylinositol 4-kinase beta (PI4Kbeta) and colocalize with the enzyme in the Golgi region. We show further that overexpression of NCS-1 in RBL-2H3 cells stimulates the catalytic activity of PI4Kbeta, increases IgE receptor (FcepsilonRI)-triggered hydrolysis of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)), and stimulates FcepsilonRI-triggered, but not Ca(2+) ionophore-triggered, exocytosis. Conversely, expression of a kinase-dead mutant of PI4Kbeta reduces PI4Kbeta activity, decreases FcepsilonRI-stimulated phosphatidylinositol 4,5-bisphosphate hydrolysis, and blocks FcepsilonRI-triggered, but not Ca(2+) ionophore-triggered, exocytosis. Our results indicate that PI(4)P, produced by the Golgi-localized PI4Kbeta, is the rate-limiting factor in the synthesis of the pool of PI(4,5)P(2) that serves as substrate for the generation of lipid-derived second messengers in FcepsilonRI-triggered cells. We conclude that NCS-1 is involved in the control of regulated exocytosis in nonneural cells, where it contributes to stimulus-secretion coupling by interacting with PI4Kbeta and positive regulation of its activity.     

Phosphatidylinositol 4-kinase IIIbeta regulates the transport of ceramide between the endoplasmic reticulum and Golgi

The recently identified ceramide transfer protein, CERT, is responsible for the bulk of ceramide transport from the endoplasmic reticulum (ER) to the Golgi. CERT has a C-terminal START domain for ceramide binding and an N-terminal pleck-strin homology domain that binds phosphatidylinositol 4-phosphate suggesting that phosphatidylinositol (PI) 4-kinases are involved in the regulation of CERT-mediated ceramide transport. In the present study fluorescent analogues were used to follow the ER to Golgi transport of ceramide to determine which of the four mammalian PI 4-kinases are involved in this process. Overexpression of pleckstrin homology domains that bind phosphatidylinositol 4-phosphate strongly inhibited the transport of C5-BODIPY-ceramide to the Golgi. A newly identified PI 3-kinase inhibitor, PIK93 that selectively inhibits the type III PI 4-kinase beta enzyme, and small interfering RNA-mediated down-regulation of the individual PI 4-kinase enzymes, revealed that PI 4-kinase beta has a dominant role in ceramide transport between the ER and Golgi. Accordingly, inhibition of PI 4-kinase III beta either by wortmannin or PIK93 inhibited the conversion of [3H]serine-labeled endogenous ceramide to sphingomyelin. Therefore, PI 4-kinase beta is a key enzyme in the control of spingomyelin synthesis by controlling the flow of ceramide from the ER to the Golgi compartment.     

Identification and characterization of the autophosphorylation sites of phosphoinositide 3-kinase isoforms beta and gamma

Class I phosphoinositide 3-kinases (PI3Ks) are bifunctional enzymes possessing lipid kinase activity and the capacity to phosphorylate their catalytic and/or regulatory subunits. In this study, in vitro autophosphorylation of the G protein-sensitive p85-coupled class I(A) PI3K beta and p101-coupled class I(B) PI3K gamma was examined. Autophosphorylation sites of both PI3K isoforms were mapped to C-terminal serine residues of the catalytic p110 subunit (i.e. serine 1070 of p110 beta and serine 1101 of p110 gamma). Like other class I(A) PI3K isoforms, autophosphorylation of p110 beta resulted in down-regulated PI3K beta lipid kinase activity. However, no inhibitory effect of p110 gamma autophosphorylation on PI3K gamma lipid kinase activity was observed. Moreover, PI3K beta and PI3K gamma differed in the regulation of their autophosphorylation. Whereas p110 beta autophosphorylation was stimulated neither by G beta gamma complexes nor by a phosphotyrosyl peptide derived from the platelet-derived growth factor receptor, autophosphorylation of p110 gamma was significantly enhanced by G beta gamma in a time- and concentration-dependent manner. In summary, we show that autophosphorylation of both PI3K beta and PI3K gamma occurs in a C-terminal region of the catalytic p110 subunit but differs in its regulation and possible functional consequences, suggesting distinct roles of autophosphorylation of PI3K beta and PI3K gamma.     

Purification and characterization of a soluble phosphatidylinositol 4-kinase from the yeast Saccharomyces cerevisiae.

A phosphatidylinositol (PI) 4-kinase was purified 25,000-fold from the cytosolic fraction of extracts from the yeast Saccharomyces cerevisiae. The purification consisted of an ammonium sulfate fractionation followed by chromatography on sulfonated-agarose (S-Sepharose), phosphocellulose, threonine-agarose, and quaternary amino (Mono Q), and sulfonated (Mono S) beads. Major contaminants in the purification, Hsc82 and Hsp82 (yeast homologs of the mammalian heat shock protein Hsp90), were eliminated by using a combination of molecular genetics (to construct a null mutation in HSC82), altered growth conditions (to minimize expression from the inducible HSP82 gene), and high ionic strength fractionation conditions (to remove the residual Hsp82). The purified enzyme had an apparent subunit molecular weight of 125,000, much larger than any other well characterized PI-4-kinase reported previously. Like mammalian PI-4-kinases, the yeast enzyme specifically phosphorylated PI on position 4 of the inositol ring and was stimulated by Triton X-100. However, activity was not inhibited by adenosine, a potent inhibitor of certain (type II) mammalian PI-4-kinases. The enzyme displayed typical Michaelis-Menten kinetics with apparent Km values of 100 microM for ATP and 50 microM for PI. To date, this yeast enzyme is the first soluble PI-4-kinase purified from any source.     

Multiple roles for phosphatidylinositol 4-kinase in biosynthetic transport in polarized Madin-Darby canine kidney cells.

Phosphatidylinositols (PI) play important roles in regulating numerous cellular processes including cytoskeletal organization and membrane trafficking. The control of PI metabolism by phosphatidylinositol kinases has been the subject of extensive investigation; however, little is known about how phosphatidylinositol kinases regulate traffic in polarized epithelial cells. Because phosphatidylinositol 4-kinase (PI4K)-mediated phosphatidylinositol 4-phosphate (PI(4)P) production has been suggested to regulate biosynthetic traffic in yeast and mammalian cells, we have examined the role of PI4Kbeta in protein delivery in polarized MDCK cells, at different levels of the biosynthetic pathway. Expression of wild type PI4Kbeta had no effect on the rate of transport of influenza hemagglutinin (HA) through the Golgi complex, but inhibited the rate of trans-Golgi network (TGN)-to-cell surface delivery of this protein. By contrast, expression of dominant-negative, kinase-dead PI4Kbeta (PI4Kbeta(D656A)) inhibited intra-Golgi transport but stimulated TGN-to-cell surface delivery of HA. Moreover, expression of PI4Kbeta(D656A) significantly increased the solubility in cold Triton X-100 of HA staged in the TGN, suggesting that altered association of HA with lipid rafts may be responsible for the enhanced transport rate. Both wild type and kinase-dead PI4Kbeta inhibited basolateral delivery of vesicular stomatitis virus G protein, suggesting an effector function for PI4Kbeta in the regulation of basolateral traffic. Thus, by contrast with the observed requirement for PI4Kbeta activity and PI(4)P for efficient transport in yeast, our data suggest that changes in PI(4)P levels can stimulate and inhibit Golgi to cell surface delivery in mammalian cells.     

Intestinal sugar absorption is regulated by phosphorylation and turnover of protein kinase C betaII mediated by phosphatidylinositol 3-kinase- and mammalian target of rapamycin-dependent pathways

Stimulation of intestinal fructose absorption by phorbol 12-myristate 13-acetate (PMA) results from rapid insertion of GLUT2 into the brush-border membrane and correlates with protein kinase C (PKC) betaII activation. We have therefore investigated the role of phosphatidylinositol 3 (PI3)-kinase and mammalian target of rapamycin in the regulation of fructose absorption by PKC betaII phosphorylation. In isolated jejunal loops, stimulation of fructose absorption by PMA was inhibited by preperfusion with wortmannin or rapamycin, which blocked GLUT2 activation and insertion into the brush-border membrane. Antibodies to the last 18 and last 10 residues of the C-terminal region of PKC betaII recognized several species differentially in Western blots. Extensive cleavage of native enzyme (80/78 kDa) to a catalytic domain product of 49 kDa occurred. PMA and sugars provoked turnover and degradation of PKC betaII by dephosphorylation to a 42-kDa species, which was converted to polyubiquitylated species detected at 180 and 250+ kDa. PMA increased the level of the PKC betaII 49-kDa species, which correlates with the GLUT2 level; wortmannin and rapamycin blocked these effects of PMA. Rapamycin and wortmannin inhibited PKC betaII turnover. PI3-kinase, PDK-1, and protein kinase B were present in the brush-border membrane, where their levels were increased by PMA and blocked by the inhibitors. We conclude that GLUT2-mediated fructose absorption is regulated through PI3-kinase and mammalian target of rapamycin-dependent pathways, which control phosphorylation of PKC betaII and its substrate-induced turnover and ubiquitin-dependent degradation. These findings suggest possible mechanisms for short term control of intestinal sugar absorption by insulin and amino acids.     

Insulin treatment stimulates the tyrosine phosphorylation of the alpha-type 85-kDa subunit of phosphatidylinositol 3-kinase in vivo.

After adding insulin to cells overexpressing the insulin receptor, the activity of phosphatidylinositol (PI) 3-kinase in the anti-phosphotyrosine immunoprecipitates was rapidly and greatly increased. This enzyme may therefore be a substrate for the insulin receptor tyrosine kinase and may be one of the mediators of insulin signal transduction. However, it is unclear whether or not activated tyrosine kinase of the insulin receptor directly phosphorylates PI 3-kinase at tyrosine residue(s) and whether insulin stimulates the specific activity of PI 3-kinase. We reported previously that the 85-kDa subunit of purified PI 3-kinase was phosphorylated at tyrosine residue(s) by the insulin receptor in vitro. To examine the tyrosine phosphorylation of PI 3-kinase and change of its activity by insulin treatment in vivo, we used a specific antibody to the 85-kDa subunit of PI 3-kinase. The activity of PI 3-kinase in immunoprecipitates with the antibody against the p85 subunit of PI 3-kinase was increased about 3-fold by insulin treatment of cells overexpressing insulin receptors. Insulin treatment also stimulated the tyrosine, serine, and threonine phosphorylation of the alpha-type 85-kDa subunit of PI 3-kinase in vivo. Phosphatase treatment of the immunoprecipitates abolished the increase in PI 3-kinase activity. The phosphorylation(s) of the kinase itself, tyrosine phosphorylation(s) of associated protein(s), or the complex formation of the phosphorylated PI 3-kinase with associated proteins may increase the activity of PI 3-kinase.     

Oncogenic Mutations of p110α Isoform of PI 3-Kinase Upregulate Its Protein Kinase Activity

In addition to lipid kinase activity, the class-I PI 3-kinases also function as protein kinases targeting regulatory autophosphorylation sites and exogenous substrates. The latter include a recently identified regulatory phosphorylation of the GM-CSF/IL-3 βc receptor contributing to survival of acute myeloid leukaemia cells. Previous studies suggested differences in the protein kinase activity of the 4 isoforms of class-I PI 3-kinase so we compared the ability of all class-I PI 3-kinases and 2 common oncogenic mutants to autophosphorylate, and to phosphorylate an intracellular fragment of the GM-CSF/IL-3 βc receptor (βic). We find p110α, p110β and p110γ all phosphorylate βic but p110δ is much less effective. The two most common oncogenic mutants of p110α, H1047R and E545K have stronger protein kinase activity than wildtype p110α, both in terms of autophosphorylation and towards βic. Importantly, the lipid kinase activity of the oncogenic mutants is still inhibited by autophosphorylation to a similar extent as wildtype p110α. Previous evidence indicates the protein kinase activity of p110α is Mn²⁺ dependent, casting doubt over its role in vivo. However, we show that the oncogenic mutants of p110α plus p110β and p110γ all display significant activity in the presence of Mg²⁺. Furthermore we demonstrate that some small molecule inhibitors of p110α lipid kinase activity (PIK-75 and A66) are equally effective against the protein kinase activity, but other inhibitors (e.g. wortmannin and TGX221) show different patterns of inhibition against the lipid and protein kinases activities. These findings have implications for the function of PI 3-kinase, especially in tumours carrying p110α mutations.     

Regulation of phosphoinositide 3-kinase by its intrinsic serine kinase activity in vivo

One potentially important mechanism for regulating class Ia phosphoinositide 3-kinase (PI 3-kinase) activity is autophosphorylation of the p85 alpha adapter subunit on Ser608 by the intrinsic protein kinase activity of the p110 catalytic subunit, as this downregulates the lipid kinase activity in vitro. Here we investigate whether this phosphorylation can occur in vivo. We find that p110 alpha phosphorylates p85 alpha Ser608 in vivo with significant stoichiometry. However, p110 beta is far less efficient at phosphorylating p85 alpha Ser608, identifying a potential difference in the mechanisms by which these two isoforms are regulated. The p85 alpha Ser608 phosphorylation was increased by treatment with insulin, platelet-derived growth factor, and the phosphatase inhibitor okadaic acid. The functional effects of this phosphorylation are highlighted by mutation of Ser608, which results in reduced lipid kinase activity and reduced association of the p110 alpha catalytic subunit with p85 alpha. The importance of this phosphorylation was further highlighted by the finding that autophosphorylation on Ser608 was impaired, while lipid kinase activity was increased, in a p85 alpha mutant recently discovered in human tumors. These results provide the first evidence that phosphorylation of Ser608 plays a role as a shutoff switch in growth factor signaling and contributes to the differences in functional properties of different PI 3-kinase isoforms in vivo.     

Analysis of the catalytic domain of phosphatidylinositol 4-kinase type II

Phosphatidylinositol (PtdIns) 4-kinases catalyze the conversion of PtdIns to PtdIns 4-phosphate, the major precursor of phosphoinositides that regulates a vast array of cellular processes. Based on enzymatic differences, two classes of PtdIns 4-kinase have been distinguished termed Types II and III. Type III kinases, which belong to the phosphatidylinositol (PI) 3/4-kinase family, have been extensively characterized. In contrast, little is known about the Type II enzymes (PI4KIIs), which have been cloned and sequenced very recently. PI4KIIs bear essentially no sequence similarity to other protein or lipid kinases; hence, they represent a novel and distinct branch of the kinase superfamily. Here we define the minimal catalytic domain of a rat PI4KII isoform, PI4KIIalpha, and identify conserved amino acid residues required for catalysis. We further show that the catalytic domain by itself determines targeting of the kinase to membrane rafts. To verify that the PI4KII family extends beyond mammalian sources, we expressed and characterized Drosophila PI4KII and its catalytic domain. Depletion of PI4KII from Drosophila cells resulted in a severe reduction of PtdIns 4-kinase activity, suggesting the in vivo importance of this enzyme.     

Purification and Characterization of a Soluble Phosphatidylinositol 4-Kinase from Carrot Suspension Culture Cells

Previously we reported the presence of a soluble phosphatidylinositol 4-kinase (PI 4-Kinase) in carrot (Daucus carota L.) suspension culture cells (C.M. Okpodu, W. Gross, W.F. Boss [1990] Plant Physiol 93: S-63). We have purified the enzyme over 1000-fold using Q-Sepharose ion exchange, hydroxylapatite, and G-100 gel filtration column chromatography. The Mr of the enzyme was estimated to be 83,000 by gel filtration. PI 4-kinase activity was recovered after renaturation of the 80-kD region of polyacrylamide gels, and an 80-kD peptide cross-reacted with antibodies to the yeast 55-kD membrane-associated PI 4-kinase on western blots. The isolated lipid kinase phosphorylated PI but not lysophosphatidylinositol or phosphatidylinositol monophosphate. Maximal PI kinase activity occurred when the substrate was added as Triton X-100/PI mixed micelles at pH 8. The enzyme required divalent cations. At low concentrations (1-5 mM), Mn2+ was more effective than Mg2+ in increasing enzyme activity; however, maximal activity occurred at 25 to 40 mM Mg2+. Calcium from 0.01 [mu]M to 1 mM had no effect on the enzyme activity. The Km of the enzyme for ATP was estimated to be between 400 and 463 [mu]M. The enzyme was inhibited by adenosine (100 [mu]M); however, ADP (up to 100 [mu]M) had no effect on the activity. The biochemical characteristics of the carrot soluble PI 4-kinase are compared with the previously reported PI 4-kinases from animals and yeast.     

Anionic phospholipids are involved in membrane targeting of PI 3-kinase gamma

Phosphoinositide 3-kinases (PI 3-kinases) have critical roles in diverse cellular signaling processes and in protein trafficking. In contrast to the class I PI 3-kinases alpha, beta, and delta which bind via src homology 2 (SH2) domains of adaptor proteins to tyrosine kinase receptors, the mechanism of recruitment of the PI 3-kinase gamma to membranes is unknown. We report in vitro experiments using immobilized proteins and small unilamellar vesicles which suggest an involvement of anionic phospholipids in membrane association of PI 3-kinase gamma. Furthermore we provide evidence that the enzyme displays beside the catalytic center a phospholipid binding domain which is essential for enzyme activity.     

Activation of host cell phosphatidylinositol 3-kinases by Trypanosoma cruzi infection

Trypanosoma cruzi, the causative agent of Chagas' disease in humans, is an intracellular protozoan parasite with the ability to invade a wide variety of mammalian cells by a unique and remarkable process in cell biology that is poorly understood. Here we present evidence suggesting a role for the host phosphatidylinositol (PI) 3-kinases during T. cruzi invasion. The PI 3-kinase inhibitor wortmannin marked inhibited T. cruzi infection when macrophages were pretreated for 20 min at 37 degrees C before inoculation. Infection of macrophages with T. cruzi markedly stimulated the formation of the lipid products of the phosphatidylinositol (PI) 3-kinases, PI 3-phospate, PI 3,4-biphosphate, and PI 3,4,5-triphosphate, but not PI 4-phosphate or PI 4,5-biphosphate. This activation was inhibited by wortmannin. Infection with T. cruzi also stimulated a marked increase in the in vitro lipid kinase activities that are present in the immunoprecipitates of anti-p85 subunit of class I PI 3-kinase and anti-phosphotyrosine. In addition, T. cruzi invasion also activated lipid kinase activity found in immunoprecipitates of class II and class III PI 3-kinases. These data demonstrate that T. cruzi invasion into macrophages strongly activates separated PI 3-kinase isoforms. Furthermore, the inhibition of the class I and class III PI 3-kinase activities abolishes the parasite entry into macrophages. These findings suggest a prominent role for the host PI 3-kinase activities during the T. cruzi infection process.     

Direct inhibition of the signaling functions of the mammalian target of rapamycin by the phosphoinositide 3-kinase inhibitors, wortmannin and LY294002.

The immunosuppressant, rapamycin, inhibits cell growth by interfering with the function of a novel kinase, termed mammalian target of rapamycin (mTOR). The putative catalytic domain of mTOR is similar to those of mammalian and yeast phosphatidylinositol (PI) 3-kinases. This study demonstrates that mTOR is a component of a cytokine-triggered protein kinase cascade leading to the phosphorylation of the eukaryotic initiation factor-4E (eIF-4E) binding protein, PHAS-1, in activated T lymphocytes. This event promotes G1 phase progression by stimulating eIF-4E-dependent translation initiation. A mutant YAC-1 T lymphoma cell line, which was selected for resistance to the growth-inhibitory action of rapamycin, was correspondingly resistant to the suppressive effect of this drug on PHAS-1 phosphorylation. In contrast, the PI 3-kinase inhibitor, wortmannin, reduced the phosphorylation of PHAS-1 in both rapamycin-sensitive and -resistant T cells. At similar drug concentrations (0.1-1 microM), wortmannin irreversibly inhibited the serine-specific autokinase activity of mTOR. The autokinase activity of mTOR was also sensitive to the structurally distinct PI 3-kinase inhibitor, LY294002, at concentrations (1-30 microM) nearly identical to those required for inhibition of the lipid kinase activity of the mammalian p85-p110 heterodimer. These studies indicate that the signaling functions of mTOR, and potentially those of other high molecular weight PI 3-kinase homologs, are directly affected by cellular treatment with wortmannin or LY294002.     

Role of protein phosphatase 2A in calcium-dependent chloride secretion by human colonic epithelial cells

EGF inhibits carbachol-induced chloride secretion by regulating a basolateral potassium channel via phosphatidylinositol 3-kinase (PI 3-kinase) and PKC activation. Although both EGF and carbachol cause tyrosine phosphorylation of p85 of PI 3-kinase, only EGF activates the enzyme. Serine phosphorylation of p85 is thought to suppress the lipid kinase of PI 3-kinase. Our present study examined whether the differential effects of carbachol and EGF on PI 3-kinase activity correspond to varying phosphorylation of p85, and the mechanisms and consequences. T₈₄ colonic epithelial cells were treated with either EGF or carbachol. Cell lysates were immunoprecipitated with p85 antibody and blotted with either phosphotyrosine or phosphoserine antibodies. Protein phosphatase (PP) 1 and 2A activities were also measured. Both tyrosine and serine residues of p85 were phosphorylated by carbachol, whereas EGF induced only tyrosine phosphorylation. Moreover, EGF abolished carbachol-induced serine phosphorylation of p85 and activated PP2A without affecting PP1. Carbachol did not affect either phosphatase. Calyculin A or okadaic acid pretreatment reversed the inhibitory action of EGF on carbachol-induced chloride secretion and restored serine phosphorylation of p85. Although carbachol recruits p85, it phosphorylates both serine and tyrosine residues so that the lipid kinase of PI 3-kinase is inhibited. EGF results in p85 tyrosine phosphorylation as well as dephosphorylation of serine residues via the activation of PP2A. This explains the differential induction of PI 3-kinase enzyme activity in response to EGF and/or carbachol and has functional implications. Our data provide further insights into negative signals that regulate chloride secretion and into the molecular basis of signaling diversification in the intestinal epithelium. epithelial secretion; PI 3-kinase; EGF