Separation and characterization of a phosphatidylinositol kinase activity that co-purifies with the epidermal growth factor receptor

Epidermal growth factor-induced phosphatidylinositol 3-kinase activation and DNA synthesis. Identification of Grb2-associated binder 2 as the major mediator in rat hepatocytes

In previous work we showed that the phosphatidylinositol 3-kinase (PI3-kinase), not the mitogen-activated protein kinase, pathway is necessary and sufficient to account for insulin- and epidermal growth factor (EGF)-induced DNA synthesis in rat hepatocytes. Here, using a dominant-negative p85, we confirmed the key role of EGF-induced PI3-kinase activation and sought to identify the mechanism by which this is effected. Our results show that EGF activates PI3-kinase with a time course similar to that of the association of p85 with three principal phosphotyrosine proteins (i. e. PY180, PY105, and PY52). We demonstrated that each formed a distinct p85-associated complex. PY180 and PY52 each constituted about 10% of EGF-activated PI3-kinase, whereas PY105 was responsible for 80%. PY105 associated with Grb2 and SHP-2, and although it behaved like Gab1, none of the latter was detected in rat liver. We therefore cloned a cDNA from rat liver, which was found to be 95% homologous to the mouse Grb2-associated binder 2 (Gab2) cDNA sequence. Using a specific Gab2 antibody, we demonstrated its expression in and association with p85, SHP-2, and Grb2 upon EGF treatment of rat hepatocytes. Gab2 accounted for most if not all of the PY105 species, since immunoprecipitation of Gab2 with specific antibodies demonstrated parallel immunodepletion of Gab2 and PY105 from the residual supernatants. We also found that the PI3-kinase activity associated with Gab2 was totally abolished by dominant negative p85. Thus, Gab2 appears to be the principal EGF-induced PY protein recruiting and activating PI3-kinase and mitogenesis.

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 4-phosphate synthesis in immunoisolated caveolae-like vesicles and low buoyant density non-caveolar membranes

This study examined phosphatidylinositol 4-phosphate (PtdIns4P) synthesis in caveolae that have been suggested to be discrete signaling microdomains of the plasma membrane and are enriched in the marker protein caveolin. Caveolin-rich light membranes (CLMs) were isolated from A431 cells by detergent-free, discontinuous density-gradient centrifugation method. The CLM fraction was separated from the bulk of the cellular protein and was greatly enriched in PtdIns, PtdIns4P, and phosphatidylinositol 4, 5-bisphosphate (PtdIns(4,5)P2) and an adenosine-sensitive type II PtdIns 4-kinase activity. Preparation of CLMs by an OptiPrep-based cell fractionation procedure confirmed the co-localization of PtdIns 4-kinase and caveolin. Electron microscopy confirmed that an anti-caveolin antiserum immunopurified vesicles from CLMs that were within the size range described for caveolae in other systems. Co-immunoprecipitated PtdIns 4-kinase activity could utilize endogenous PtdIns, present within the caveolae-like vesicles, to produce PtdIns4P. The addition of recombinant phosphatidylinositol transfer protein increased PtdIns 4-kinase activity both in immunoisolated caveolae and CLMs. However, less than 1% of the total cellular PtdIns and PtdIns 4-kinase activity was present in caveolae-like vesicles, indicating that non-caveolar light membrane rafts are the main site for cellular PtdIns4P production.

Characterization of two 85 kd proteins that associate with receptor tyrosine kinases, middle-T/pp60c-src complexes, and PI3-kinase

Affinity-purified bovine brain phosphatidylinositol 3-kinase (PI3-kinase) contains two major proteins of 85 and 110 kd. Amino acid sequence analysis and cDNA cloning reveals two related 85 kd proteins (p85 alpha and p85 beta), which both contain one SH3 and two SH2 regions (src homology regions). When expressed, these 85 kd proteins bind to and are substrates for tyrosine-phosphorylated receptor kinases and the polyoma virus middle-T antigen/pp60c-src complex, but lack PI3-kinase activity. However, an antiserum raised against p85 beta immunoprecipitates PI3-kinase activity. The active PI3-kinase complex containing p85 alpha or p85 beta and the 110 kd protein binds to PDGF but not EGF receptors. p85 alpha and p85 beta may mediate specific PI3-kinase interactions with a subset of tyrosine kinases.

Anti-phosphotyrosine immunoprecipitation of phosphatidylinositol 3' kinase activity in different cell types after exposure to epidermal growth factor

In previous studies from this laboratory, it was shown that mouse epidermal growth factor (mEGF) or insulin increased the labeling of phosphaditylinositol-3,4-bisphosphate (PI-3,4-P2) in MA-10 cells prelabeled with different radioactive precursors (Pignataro, O.P., and Ascoli, M. (1990) J. Biol. Chem. 265, 1718-1723 and Mol. Endocrinol. (1990) 4, 758-765). In order to further characterize this phenomenon we sought to determine if we could use anti-phosphotyrosine antibodies to immunoprecipitate a phosphatidylinositol (PI) kinase activity from MA-10 cells treated with mEGF or insulin. Our data indicate that this is indeed the case, and that the PI kinase precipitated is a PI-3' kinase. A second cell type, A431 cells, in which we were unable to detect an increase in PI-3,4-P2 labeling when stimulated with mEGF or insulin, was also studied. It was found that, as in MA-10 cells, A431 cells also contain an immunoprecipitable PI-3' kinase activity that is increased in response to mEGF or insulin.

Reduced tyrosine phosphorylation and nonresponsiveness to EGF-mediated cytotoxicity in EGF receptor-hyperproducing UCVA-1 cells

UCVA-1 cells, derived from human pancreas adenocarcinoma, have a high number of epidermal growth factor (EGF) receptors (1.0 x 10(6) per cell) but their growth is not inhibited by EGF, unlike other EGF receptor-hyperproducing tumour cells. In UCVA-1 cells EGF activates neither the phosphatidylinositol turnover nor protein kinase C. EGF, however, enhances the phosphorylation of EGF receptors at specific tyrosine residues, indicating that the EGF receptor kinase is active and subject to autophosphorylation. Downmodulation of EGF receptors by 12-O-tetradecanoylphorbol 13-acetate (TPA) is also observed. Using an anti-phosphotyrosine antibody several phosphoproteins, including EGF receptors, were immunoprecipitated from UCVA-1 cell lysates, whereas more than 20 phosphoproteins were detected in other EGF receptor-hyperproducing tumour cells (NA), indicating that tyrosine-phosphorylation of endogenous substrates by EGF receptor kinase is significantly reduced in UCVA-1 cells. Thus, non-responsiveness of UCVA-1 cells to EGF is correlated with the reduced tyrosine phosphorylation.

Legionella micdadei protein kinase catalyzes phosphorylation of tubulin and phosphatidylinositol

Legionella micdadei, a pathogen which enters into host phagocyte phagolysosomal structures, contains at least two protein kinases. We have purified to homogeneity the predominant, nucleotide-independent protein kinase and examined its ability to catalyze the transfer of phosphate from ATP to acceptors in human neutrophils. The L. micdadei protein kinase catalyzed the phosphorylation of proteins of 11.5, 14, 19, 23, 28, 34, and 38 kilodaltons (kDa) present in a Triton X-100 extract of neutrophil membranes and of 11.5, 13.5, 25, and 38 kDa in the neutrophil cytosol. Tubulin was a good substrate for the L. micdadei protein kinase in vitro. The bacterial kinase also catalyzed the phosphorylation of phosphatidylinositol (PI) at about half the rate at which histones were phosphorylated; phosphatidylinositol-4-phosphate (PIP) was not phosphorylated by the kinase. The PI kinase activity of the L. micdadei enzyme was optimum at pH 7.0, and the divalent cation requirement was satisfied best by Mg2+ and Ca2+. The maximum rate of PI phosphorylation was obtained with 0.6 mM PI; in the presence of MgCl2 (10 mM), the Km for PI was 0.9 mM and the Km for ATP was 1.5 mM. The detergents octyl-beta-D-glucoside (10 to 20 mM) and Triton X-100 (0.5%) stimulated kinase activity twofold when PI was the phosphate acceptor; however, only octyl glucoside stimulated histone kinase activity. Various membrane phospholipids inhibited PI kinase activity. The most potent phospholipid inhibitor was the product of the PI kinase reaction, PIP, which at a 0.6 mM concentration inhibited both PI and tubulin phosphorylation by 80%. The inhibition of kinase activity by PIP when histone served as the acceptor was noncompetitive in character. The L. micdadei kinase also phosphorylated PI in intact. (3H)inositol-labeled neutrophils. The PI kinase and histone kinase activities of teh L. micdadei kinase copurified and cofucused (pI, 5.8) when subjected to isoelectric focusing, suggesting that the two enzymatic activities reside in a single protein.

Increased phosphatidylinositol kinase activity in psoriatic epidermis

Phosphatidylinositol (PI) kinase is activated by growth factors, such as epidermal growth factor (EGF), and is thought to be involved in cellular proliferation. Psoriasis is a hyperproliferative epidermal disease in which EGF receptor expression is altered and phospholipase C activity is increased. Considering the potential importance of growth factor stimulated phosphoinositide metabolism in the genesis of abnormal growth, we measured PI kinase activity in epidermal keratome biopsies from normal skin and the lesional and nonlesional skin of psoriatic patients. The PI kinase activity in 10 psoriatic involved plaques was increased 6.7-fold (Vmax = 67.1 +/- 23.9 pmol formed/min/mg protein +/- SE) when compared with 11 normal epidermal biopsies (Vmax = 10.0 +/- 1.3 pmol/min/mg protein, p less than 0.025). Similar results were noted when enzyme activity was standardized using DNA content. The apparent Km of PI kinase for ATP in involved psoriatic biopsies (0.45 +/- 0.14 mM) was also significantly (p less than 0.025) increased compared with normals (0.11 +/- 0.02 mM). The PI kinase activity in 11 biopsies of nonlesional psoriatic epidermis was not statistically different from normal epidermis. Both psoriatic and normal PI kinases required Mg++ and were inhibited by Ca++. The polyamine, spermine, a known activator of PI kinase in other tissues, stimulated normal but not psoriatic epidermal PI kinase. Both normal and psoriatic PI kinase activities had an apparent mol wt of 85,000. Increased synthesis of phosphoinositides by PI kinase in psoriatic tissue may provide more substrate for phospholipase C; a key enzyme in growth factor-mediated signal transduction.

Role of phosphatidylinositol kinase in PDGF receptor signal transduction

The molecules with which the platelet-derived growth factor (PDGF) receptor interacts to elicit the biochemical reactions responsible for cell proliferation have not been identified. Antisera directed against specific PDGF receptor peptides coprecipitated a phosphatidylinositol (PI) kinase and the PDGF receptor. Immunoprecipitates from PDGF-stimulated cells contained 10 to 50 times as much PI kinase as those from unstimulated cells. Mutation of the PDGF receptor by deletion of its kinase insert region resulted in a receptor markedly less effective than the wild type in eliciting cell proliferation and defective in PDGF-stimulated PI kinase, but still capable of PDGF-induced receptor autophosphorylation and phosphoinositide hydrolysis. These data show that the PDGF receptor is physically associated with a PDGF-sensitive PI kinase that is distinct from tyrosine kinase and is not required for PDGF-induced PI hydrolysis. The finding that the mutant PDGF receptor missing the kinase insert domain elicited known early biochemical responses to PDGF, but did not associate with or regulate PI kinase, suggests a novel role for the receptor-associated PI kinase in the transmission of mitogenic signals.

Nerve growth factor potentiates the hormone-stimulated intracellular accumulation of inositol phosphates and Ca2+ in rat PC12 pheochromocytoma cells: comparison with the effect of epidermal growth factor

The effects of nerve growth factor (NGF) and epidermal growth factor (EGF) on the intracellular accumulation of inositol phosphates and on cytosolic free Ca2+ concentrations were studied in rat PC12 pheochromocytoma cells. Both NGF and EGF potentiate in these cells the increase in the accumulation of inositol phosphates that is elicited by bradykinin and carbachol. A corresponding potentiation was also found for the agonist-induced increase of cytosolic Ca2+ concentrations. The effect of NGF, but not that of EGF, is abolished when the cells are preincubated with 5'-deoxy-5'-methylthioadenosine, an inhibitor of S-adenosylhomocysteine hydrolase. These results suggest that an increased response to hormones, which act via phosphoinositide-derived second messengers, may be important in the mechanism of action of NGF and EGF.

Separation of the protein-tyrosine kinase and phosphatidylinositol kinase activities of the human placental insulin receptor

On immunoprecipitation using a specific antiphosphotyrosine antibody, phosphatidylinositol kinase (EC 2.7.1.67) activity was separated from the protein-tyrosine kinase (EC 2.7.1.112) activity of the wheat germ agglutinin (WGA) -purified insulin receptor from human placenta. This clearly indicates that protein-tyrosine kinase and phosphatidylinositol kinase activity do not reside on the same polypeptide chain as previously has been suggested. Quantitatively, the fraction of phosphatidylinositol kinase that was bound to WGA sepharose and eluted together with the insulin receptor amounted to 2% of the Triton X-100 soluble phosphatidylinositol kinase. The apparent Km values of the bound and unbound phosphatidylinositol kinase with respect to PI and ATP were very similar (0.4 and 0.3 mmol/l and 8 and 7 mumol/l, respectively) suggesting that the WGA-bound phosphatidylinositol kinase is not a different enzyme, but rather represents a small portion of the bulk Triton X-100-soluble phosphatidylinositol kinase that is bound to the lectin tightly associated with the insulin receptor. The synthetic polymer (Glu80Tyr20)n, a model substrate of the insulin receptor tyrosine kinase, at 0.5 mmol/l, inhibited phosphatidylinositol kinase of WGA-purified insulin receptor by 70-90%. This inhibition was not overcome by increasing the concentrations of ATP or PI as one would expect if a functional interrelationship of the protein-tyrosine kinase and the phosphatidylinositol kinase would exist.

Purification and characterization of a phosphatidylinositol kinase from A431 cells

A phosphatidylinositol kinase from A431 cells has been purified to near homogeneity. Purification was achieved through the use of a combination of chromatography steps including affinity elution of the enzyme from a heparin-agarose column with PI. Characterization of the [32P]PIP formed by the purified PI kinase indicates that the enzyme phosphorylates the inositol on the 4-position and is therefore a phosphatidylinositol 4-kinase. The enzyme has a subunit weight of 55,000 as estimated by SDS gel electrophoresis and appears to be active as a monomer. Studies of the hydrodynamic properties of the enzyme indicate that the PI kinase binds substantial amounts of Triton X-100 and is actually present in detergent-containing solutions as a complex with a molecular weight of approximately 120,000. The Km of the enzyme for PI is 16 microM and for ATP is 74 microM. The enzyme is inhibited by adenosine with an IC50 of 100 microM. These properties are essentially identical with those of the membrane-bound PI kinase in A431 cells which is stimulated by EGF. The data therefore suggest that the EGF-stimulated PI kinase is a 55,000-Da monomer.

Epidermal growth factor-induced increases in inositol trisphosphates, inositol tetrakisphosphates, and cytosolic Ca2+ in a human hepatocellular carcinoma-derived cell line

A human hepatocellular carcinoma-derived cell line, PLC/PRF/5, was examined for its ability to respond to epidermal growth factor (EGF) exposure with increased phosphatidylinositol 4,5-bisphosphate hydrolysis. Upon addition of EGF (25 ng/ml), a rapid (10-15 s) but transient increase in Ins(1,4,5)P3 levels and large, prolonged (2 min) increases in Ins(1,3,4,5)P4 and Ins(1,3,4)P3 levels were detected. Increases in cytosolic Ca2+ were observed after a 10 to 20 s lag, reaching peak value at 1 min, and remaining elevated for 10 min. The initial burst of cytosolic Ca2+ occurred in the absence of extracellular Ca2+ and probably reflects mobilization of intracellular Ca2+ stores. In cells pretreated with EGTA, the sustained component of the Ca2+ response was not observed. Comparison of the inositol phosphate and Ca2+ responses of PLC/PRF/5 cells to responses reported in other cell types indicates that this cell line is a good model for EGF action in liver.

Partial purification and characterization of phosphatidylinositol kinase from bovine brain

Purification of Phosphatidylinositol (PI) kinase was attempted from bovine brain. A seven step purification protocol increased the specific activity 100 x but attempts at further purification were unsuccessful. Labeling of the partially purified PI kinase with the ATP analog fluorosulfonylbenzoyl adenosine reproducibly identified three bands on polyacrylamide gel electrophoresis of 76 K, 45 K, and 29 K, one of which likely represents PI kinase. Kinetic studies showed a Km of 17 microM for ATP, 0.02 mg/ml for PI and a Vm of 1830 pmol/min/mg protein for ATP and 820 pmol/min/mg protein for PI.

Tyrosyl and phosphatidylinositol kinases of human erythrocyte membranes

The tyrosyl kinase and phosphatidylinositol (PI) kinase activities of human red cells have been partially purified and characterized. Although the PI kinase required detergent for solubilization, the major tyrosyl kinase of the red cell could be extracted by high salt. A very small residual activity remained associated with the membranes, however, that was solubilized with the PI kinase and copurified through an ammonium sulfate precipitation and diethylaminoethyl (DEAE) ion-exchange step gradient elution. However, the two activities were found to differ with respect to their apparent KmS for ATP and Mg2+; they showed different half-lives for temperature inactivation, possessed different relative activities in the presence of Mn2+ and Ca2+, and were separable by elution from a DEAE-Trisacryl ion exchange column using a linear NaCl gradient. The kinetic parameters of the membrane-associated tyrosyl kinase differed from those of the salt-extracted enzyme. PI kinase was not activated by pretreatment with the tyrosyl kinase p68v-ros or by addition of the phosphotyrosyl phosphatase inhibitor, vanadate, to intact membranes, and was not competitively inhibited by the tyrosyl kinase substrate poly(Glu4, Tyr). We conclude that the human red cell phosphatidylinositol and tyrosyl kinases are distinct and separate activities, and that at least two separable tyrosyl kinases are present in human erythrocytes.

Mitogen-stimulated release of inositol phosphates in human fibroblasts

Mitogenic stimulation of quiescent human fibroblasts (HSWP) with a growth factor mixture (consisting of epidermal growth factor (EGF), insulin, bradykinin, and vasopressin) rapidly induces an increase in Na influx via a Ca-mediated activation of an amiloride-sensitive Na/H exchanger. Inositol phosphates (specifically inositol-1',4',5'-phosphate) have been implicated in mediating the mobilization of intracellular Ca stores in other cell types and we have now completed a detailed analysis of the mitogen-induced release of inositol phosphates in HSWP cells. Stimulation of inositol trisphosphate release is rapid (within 5 s) and reaches a maximum level (416-485% basal) within 10-15 s after the addition of growth factor mixture. Inositol bisphosphate and inositol monophosphate reach maximum levels by 30 s (1257% basal) and 60 s (291% basal), respectively. Levels of all three compounds then decay toward basal levels but remain elevated (150-350% of basal levels) after 10 min of incubation with mitogens. The effects of different combinations of these growth factors and of the bee venom peptide, melittin, have also been determined. We have also found that 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate, which prevents the mitogen-induced rise in intracellular calcium activity and activation of Na influx, does not alter the mitogen-stimulated accumulation of inositol trisphosphate. In addition, the calcium ionophore A23187, which increases cytosolic Ca activity and induces a Na influx, does not stimulate the release of inositol trisphosphate. Assays performed in the presence of lithium, which inhibits inositol phosphate monophosphatase, promotes the prolonged and enhanced accumulation of inositol monophosphate. Treatment with the phospholipase inhibitor mepacrine or pretreatment with dexamethasone reduces the amount of inositol phosphates released upon mitogenic stimulation. Hence mitogenic stimulation of HSWP cells leads to the rapid stimulation of inositol phosphate release via a calcium-independent mechanism and suggests inositol trisphosphate as a candidate to mediate the release of intracellular calcium stores which is involved in the processes responsible for the activation of the Na/H exchanger.

A p60v-src-related tyrosine kinase in the acetylcholine receptor-rich membranes of Narke japonica: association and dissociation of phosphatidylinositol kinase activity

We have isolated a tyrosine-specific protein kinase from the acetylcholine receptor (AChR)-rich membranes of the electric ray Narke japonica. The enzyme is immunologically related to p60v-src, the product of the transforming gene of Rous sarcoma virus. A substantial phosphatidylinositol (PI) kinase activity was associated with this enzyme when it was purified through tyrosine-agarose affinity chromatography used previously for the purification of p60v-src. However, by subsequent chromatography on casein-agarose, most of the associated PI kinase activity was separated from the tyrosine kinase activity. The results suggest that the tyrosine-specific protein kinase in the AChR-rich membranes of N. japonica has no intrinsic PI kinase activity.

Phosphoinositide phosphorylation precedes growth in rat mammary tumors

DMBA-induced rat mammary tumors were used to study the possible association of phosphoinositide phosphorylation to tumor growth. These membranous enzymatic activities were measured during various stages of tumor growth induced by pharmacological manipulation of plasma prolactin level. An increase in phosphorylation of both phosphatidyl inositol and phosphatidyl inositol 4-phosphate preceded the growth induced by prolactin concomitantly with an increase in tyrosine phosphorylation. Good correlation (r = 0.87) existed between the tyrosine kinase activity and phosphatidyl inositol kinase activity of 21 individual tumors taken from animals at different stages of hormonal manipulation. Phosphoinositide phosphorylation was inhibited by quercetin and was not affected by cAMP, similar to tyrosine kinase. Phosphorylation of angiotensin II by tyrosine kinase was inhibited by 0.2 mg/ml phosphatidyl inositol 4 phosphate or phosphatidyl inositol 4,5-bisphosphate.