Purification and characterization of a phosphatidylinositol kinase from A431 cells

In-vitro Anti-proliferative Effects of Ethanolic Extract of Vanilla Planifolia Leaf Extract against A431 Human Epidermoid Carcinoma Cells

Squamous cell carcinoma is the second largest among skin cancer diseases. The aim of the present study is to reveal the antiproliferative property of vanilla leaf extract against A431 cells. Antiproliferative property was assessed by MTT assay. Apoptotic property was assessed by DNA fragmentation assay. Antiproliferative property of extract was revealed in a dose dependent manner. IC50 of the extract against A431 cells was revealed at 31.2µg/ml. This study revealed the cancer suppression capability of vanilla leaf extract in skin cancer cell lines.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Phosphatidylinositol 4-kinases, phosphatidylinositol 4-phosphate and cancer

This article focuses on the emerging roles for phosphatidylinositol 4-phosphate and the phosphatidylinositol 4-kinases in cancer. Phosphatidylinositol 4-phosphate is a common substrate for both the phosphatidylinositol 3-kinase and phospholipase C pathways, and has been implicated in the membrane targeting of proteins such as Girdin/GIV and OSBP. Alterations to phosphatidylinositol 4-kinase expression levels can modulate MAP kinase and Akt signalling, and are important for chemoresistance, tumour angiogenesis and the suppression of apoptosis and metastases. Recent improvements in high-throughput screening assays, and the discoveries that some anti-viral molecules are isoform selective phosphatidylinositol 4-kinase inhibitors have advanced the drugability of these enzymes.

Synthesis and function of membrane phosphoinositides in budding yeast, Saccharomyces cerevisiae

It is now well appreciated that derivatives of phosphatidylinositol (PtdIns) are key regulators of many cellular processes in eukaryotes. Of particular interest are phosphoinositides (mono- and polyphosphorylated adducts to the inositol ring in PtdIns), which are located at the cytoplasmic face of cellular membranes. Phosphoinositides serve both a structural and a signaling role via their recruitment of proteins that contain phosphoinositide-binding domains. Phosphoinositides also have a role as precursors of several types of second messengers for certain intracellular signaling pathways. Realization of the importance of phosphoinositides has brought increased attention to characterization of the enzymes that regulate their synthesis, interconversion, and turnover. Here we review the current state of our knowledge about the properties and regulation of the ATP-dependent lipid kinases responsible for synthesis of phosphoinositides and also the additional temporal and spatial controls exerted by the phosphatases and a phospholipase that act on phosphoinositides in yeast.

Electroporation loading and photoactivation of caged InsP3: tools to investigate the relation between cellular ATP release in response to intracellular InsP3 elevation

Photolytic liberation of InsP(3) in single cells triggers cell-to-cell propagating calcium changes that are communicated by a gap junctional and a paracrine purinergic pathway involving InsP(3)-triggered ATP release. We investigated the relation between the InsP(3) stimulus and the resulting ATP release in ECV304 cells using UV photolysis of caged compounds and bioluminescent ATP measurements. Careful consideration of all steps, starting from caged InsP(3) loading into the cells by electroporation, up to photoliberation upon UV exposure, allowed to derive a dose-response relation that revealed a first part with a flattening ATP release response in the below 10microM InsP(3) concentration range and a second phase of steeply increasing ATP release in response to above 10microM InsP(3) stimulation. ATP release triggered by below 10microM InsP(3) concentrations attained a level in the order of 30% above baseline ATP release, while the steeply increasing response to high InsP(3) concentrations attained values in the order of 150% above baseline. Our data indicate the involvement of low affinity InsP(3) receptor sites in the pathway leading to triggered ATP release, with activation of these receptors causing the release of 1-2% of the total cellular ATP pool.

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.

A novel family of phosphatidylinositol 4-kinases conserved from yeast to humans

Phosphatidylinositolpolyphosphates (PIPs) are centrally involved in many biological processes, ranging from cell growth and organization of the actin cytoskeleton to endo- and exocytosis. Phosphorylation of phosphatidylinositol at the D-4 position, an essential step in the biosynthesis of PIPs, appears to be catalyzed by two biochemically distinct enzymes. However, only one of these two enzymes has been molecularly characterized. We now describe a novel class of phosphatidylinositol 4-kinases that probably corresponds to the missing element in phosphatidylinositol metabolism. These kinases are highly conserved evolutionarily, but unrelated to previously characterized phosphatidylinositol kinases, and thus represent the founding members of a new family. The novel phosphatidylinositol 4-kinases, which are widely expressed in cells, only phosphorylate phosphatidylinositol, are potently inhibited by adenosine, but are insensitive to wortmannin or phenylarsine oxide. Although they lack an obvious transmembrane domain, they are strongly attached to membranes by palmitoylation. Our data suggest that independent pathways for phosphatidylinositol 4-phosphate synthesis emerged during evolution, possibly to allow tight temporal and spatial control over the production of this key signaling molecule.

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.

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.

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.

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

Agonist-sensitive phosphoinositide pools are maintained by recently-identified wortmannin (WT)-sensitive phosphatidylinositol (PI) 4-kinase(s) (Nakanishi, S., Catt, K. J., and Balla, T. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 5317-5321). Two loosely membrane-associated WT-sensitive type III PI 4-kinases were isolated from bovine adrenal cortex as [3H]WT-labeled 110- and 210-kDa proteins. Based on peptide sequences from the smaller enzyme, a 3. 9-kilobase pair (kb) cDNA with an open reading frame encoding a 90-kDa protein was isolated from a bovine brain cDNA library. Expression of this cDNA in COS-7 cells yielded a 110-kDa protein with WT-sensitive PI 4-kinase activity. Northern blot analysis of a human mRNA panel showed a single approximately 3.8-kb transcript. Peptide sequences obtained from the 210-kDa enzyme corresponded to those of a recently described rat 230-kDa PI 4-kinase. A 6.5-kb cDNA containing an open reading frame of 6129 nucleotides that encoded a 230-kDa protein, was isolated from brain cDNA. Northern blot analysis of human mRNA revealed a major 7.5-kb transcript. The molecular cloning of these novel WT-sensitive type III PI 4-kinases will allow detailed analysis of their signaling and other regulatory functions in mammalian cells.

Growth factor-dependent phosphoinositide signalling

A wide variety of messages, in the form of diffusible growth factors, hormones and cytokines, are carried throughout multicellular organisms to coordinate important physiological properties of target cells, such as proliferation, differentiation, migration, apoptosis and metabolism. Most messengers bind to cognate receptors on target cells, which initiate a characteristic cascade of reactions within the cell, ultimately leading to the desired response. The cellular response is defined by the combination of signalling components whose individual activity depends upon the number and type of surface receptors. Consequently the responses of different cell types to one or more stimuli can be quite disparate. A molecular understanding of the signalling pathways employed by each type of receptor therefore underlies the ability to rationalize many cellular functions and to correct disfunctions. As a well studied example of the primary signalling events that take place on the cytoplasmic leaflet of the plasma membrane following receptor activation, we will discuss how the widely expressed receptor for epidermal growth factor (EGF) causes the phosphorylation and hydrolysis of a signalling precursor, the membrane lipid phosphatidylinositol. This paradigm will be used to illustrate certain general principles of signalling, including formation of multienzyme complexes, compartmentation of second messengers and intermediates, and cross-talk between different signalling pathways.

Requirement for phosphatidylinositol transfer protein in epidermal growth factor signaling

Stimulation of phosphatidylinositol-4,5-bisphosphate (PIP2) hydrolysis is a widespread mechanism for receptor-mediated signaling in eukaryotes. Cytosolic phosphatidylinositol transfer protein (PITP) is necessary for guanosine triphosphate (GTP)-dependent hydrolysis of PIP2 by phospholipase C-beta (PLC-beta), but the role of PITP is unclear. Stimulation of phospholipase C-gamma (PLC-gamma) in A431 human epidermoid carcinoma cells treated with epidermal growth factor (EGF) required PITP. Stimulation of PI-4 kinase in cells treated with EGF also required PITP. Coprecipitation studies revealed an EGF-dependent association of PITP with the EGF receptor, with PI-4 kinase, and with PLC-gamma.

Presence of a novel form of phosphatidylinositol 4-kinase in rat liver

Rat liver microsomes contain two distinct forms of PtdIns 4-kinase which were resolved by heparin-Sepharose chromatography. One enzyme was identified as the type II PtdIns kinase previously isolated from exocytotic vesicles. The other enzyme, however, was a novel PtdIns 4-kinase isoform with properties differing from any other PtdIns kinase so far characterized. Both kinases were recognized by a monoclonal antibody specific for type II PtdIns 4-kinase, but the novel enzyme was considerably less sensitive to inhibition by adenosine and Ca2+ than type II enzymes, and in addition was specifically inhibited by submillimolar concentrations of dithioerythritol. The presence of a novel PtdIns 4-kinase isoform in rat liver raises the question of whether this enzyme is unique for this organ or whether it has a more widespread distribution but so far has avoided detection.

TOR1 and TOR2 are structurally and functionally similar but not identical phosphatidylinositol kinase homologues in yeast

The Saccharomyces cerevisiae genes TOR1 and TOR2 were originally identified by mutations that confer resistance to the immunosuppressant rapamycin. TOR2 was previously shown to encode an essential 282-kDa phosphatidylinositol kinase (PI kinase) homologue. The TOR1 gene product is also a large (281 kDa) PI kinase homologue, with 67% identity to TOR2. TOR1 is not essential, but a TOR1 TOR2 double disruption uniquely confers a cell cycle (G1) arrest as does exposure to rapamycin; disruption of TOR2 alone is lethal but does not cause a cell cycle arrest. TOR1-TOR2 and TOR2-TOR1 hybrids indicate that carboxy-terminal domains of TOR1 and TOR2 containing a lipid kinase sequence motif are interchangeable and therefore functionally equivalent; the other portions of TOR1 and TOR2 are not interchangeable. The TOR1-1 and TOR2-1 mutations, which confer rapamycin resistance, alter the same potential protein kinase C site in the respective protein's lipid kinase domain. Thus, TOR1 and TOR2 are likely similar but not identical, rapamycin-sensitive PI kinases possibly regulated by phosphorylation. TOR1 and TOR2 may be components of a novel signal transduction pathway controlling progression through G1.

Phosphatidylinositol 4-kinase: gene structure and requirement for yeast cell viability

Phosphatidylinositol (PtdIns) 4-kinase catalyzes the first step in the biosynthesis of PtdIns-4,5-bisphosphate (PtdIns[4,5]P2). Hydrolysis of PtdIns[4,5]P2 in response to extracellular stimuli is thought to initiate intracellular signaling cascades that modulate cell proliferation and differentiation. The PIK1 gene encoding a PtdIns 4-kinase from the yeast Saccharomyces cerevisiae was isolated by polymerase chain reaction (PCR) with oligonucleotides based on the sequence of peptides derived from the purified enzyme. The sequence of the PIK1 gene product bears similarities to that of PtdIns 3-kinases from mammals (p110) and yeast (Vps34p). Expression of PIK1 from a multicopy plasmid elevated PtdIns 4-kinase activity and enhanced the response to mating pheromone. A pik1 null mutant was inviable, indicating that PtdIns4P and presumably PtdIns[4,5]P2 are indispensable phospholipids.

Phosphatidylcholine enhances the activity of rat liver type II phosphatidylinositol-kinase

A PtdIns 4-kinase was purified extensively from rat liver exocytotic vesicles. The enzyme had a low Km for ATP, was inhibited by adenosine, and had an apparent molecular mass of 54 kDa, indicating it to be a type II PtdIns-kinase. The activity of the purified enzyme was enhanced several-fold by PtdCho, and to some extent by other phospholipids with basic polar head groups, and was inhibited by PtdSer. Kinetic analyses, presenting the substrate in mixed micelles of Triton X-100, PtdIns and PtdCho, showed that the effect of PtdCho was both to increase Vmax and to decrease the apparent Km for micellar PtdIns.

Activation of phosphatidylinositol 3-kinase and phosphatidylinositol 4-kinase during rat parotid acinar cell proliferation

We have recently shown that beta-adrenergic agonist, isoproterenol-induced parotid acinar cell proliferation is in part mediated by elevated levels of surface galactosyltransferase which undergoes interaction with the EGF-R. The receptor subsequently undergoes autophosphorylation on the tyrosine residues in a manner similar to its 'receptor-ligand' interaction (Purushotham et al. (1992) Biochem. J. 284, 767-776). In this study, we provide evidence for phosphatidylinositol 3-kinase and 4-kinase as cytoplasmic signalling proteins involved in both the isoproterenol and EGF-stimulated signal transduction upon in vitro and in-vivo stimulation of parotid acinar cells. Total cell lysate activity for the PtdIns 4-kinase was 2- and 3-fold higher than unstimulated control cells, while the PtdIns 3-kinase was 1.4- and 2.8-fold higher following stimulation by isoproterenol or EGF, respectively. Increases of 6- and 2-fold in phosphatidylinositol 3-kinase were observed in anti-phosphotyrosine-antibody-immunoprecipitated cell lysates upon in-vitro growth stimulation with isoproterenol or EGF, respectively. There was an increase in tyrosine phosphorylation of the holoenzyme and association of the p85 subunit of phosphatidylinositol 3-kinase with EGF-R in response to both isoproterenol and EGF treatments. This corresponded with the mobilization of p85 from the cytoplasm to the plasma membrane upon growth stimulation. These results further implicate the phosphoinositide metabolites in the second messenger signalling pathways of isoproterenol-induced rat parotid cell proliferation. The parallel utilization of EGF indicate that the post-transductional mechanisms of isoproterenol-induced acinar cell proliferation are similar to the growth-factor-mediated activation of intracellular signalling pathways for cell growth.

Affinity labelling of the active site of brain phosphatidylinositol 4-kinase with 5'-fluorosulphonylbenzoyl-adenosine

5'-p-Fluorosulphonylbenzoyl-adenosine (FSO2BzAdo), an affinity labelling analogue of ATP, was used to label the active site of sheep brain phosphatidylinositol 4-kinase (PtdIns 4-kinase). The incubation of PtdIns 4-kinase with concentrations of FSO2BzAdo as low as 50 microM resulted in considerate inactivation of the enzyme. (e.g. 55% less after 60 min with 50 microM FSO2BzAdo). The kinetics of inactivation of PtdIns 4-kinase by FSO2BzAdo suggest a two-step mechanism, in which a rapid reversible binding of FSO2BzAdo to the enzyme is followed by a covalent sulphonation step. The first-order rate constant (k2) for the inactivation of PtdIns 4-kinase was calculated to be 0.063 min-1, and the steady-state constant of inactivation (Ki) to be 200 microM. Preincubation of the enzyme with either ATP plus Mg2+, or PtdIns alone, prior to addition of FSO2BzAdo reduced the degree of inactivation of the enzyme; suggesting that FSO2BzAdo binds within the active site PtdIns 4-kinase. Moreover, since ATP plus Mg2+ provided the greatest protection against inactivation, it is concluded that the main site of labelling of PtdIns 4-kinase by FSO2BzAdo is within the ATP-binding site of the enzyme. Results obtained from chemical modification experiments, which employed pyridoxal 5'-phosphate and tetranitromethane, are consistent with a catalytically-essential lysine being present within the ATP-binding site of PtdIns 4-kinase. Therefore, it is hypothesised that the inactivation of PtdIns 4-kinase by FSO2BzAdo may be due to the labelling of this lysine residue.

Two distinct membrane-bound phosphatidylinositol-4-phosphate phosphatases in bovine brain

Solubilized phosphatidylinositol-4-phosphate 4-phosphatase from bovine brain resolved into two peaks of activity by ion exchange chromatography. Both exhibited substantial detergent binding characteristic of integral membrane proteins, and both appear specific for phosphatidylinositol-4-phosphate, but their pH optima differ: the earlier eluting fraction (peak 1) is optimally active between pH 5.5 and 6, whereas the later eluting fraction (peak 2) is most active around pH 8.5. Detergent inhibition studies suggest that peak 2, but not peak 1, interacts with phosphatidylinositol-4-phosphate in the context of a single mixed micelle. Further characterization of these activities should help shed light on the biological function of polyphosphoinositide phosphatases.

Purification and characterization of human erythrocyte phosphatidylinositol 4-kinase. Phosphatidylinositol 4-kinase and phosphatidylinositol 3-monophosphate 4-kinase are distinct enzymes

PtdIns 4-kinase has been purified 83,000-fold from human erythrocyte membranes. The major protein detected by SDS/PAGE is of molecular mass 56 kDa, and enzymic activity can be renatured from this band of the gel. The characteristics of this enzyme are similar to other type II PtdIns kinases previously described: PtdIns presented in Triton X-100 micelles is preferred as a substrate over PtdIns vesicles, the enzyme possesses a relatively low Km for ATP (20 microM), and adenosine is an effective inhibitor. A monoclonal antibody raised against bovine brain type II PtdIns 4-kinase is an effective inhibitor of the purified enzyme. PtdIns(4,5)P2 inhibits by approx. 50% when added in equimolar amounts with PtdIns; PtdIns4P has little effect on activity. A PtdIns3P 4-kinase activity has also been detected in erythrocyte lysates. Approximately two-thirds of this activity is in the cytosolic fraction and one-third in the membrane fraction. No PtdIns3P 4-kinase activity could be detected in the purified type II PtdIns 4-kinase preparation, nor could this activity be detected in a bovine brain type III PtdIns 4-kinase preparation. The monoclonal antibody that inhibits the type II PtdIns 4-kinase does not affect the PtdIns3P 4-kinase activity in the membrane fraction. The cytosolic PtdIns3P 4-kinase can be efficiently recovered from a 60%-satd.-(NH4)2SO4 precipitate that is virtually free of PtdIns 4-kinase activity. We conclude that PtdIns3P 4-kinase is a new enzyme distinct from previously characterized PtdIns 4-kinases, and that this enzyme prefers PtdIns3P over PtdIns as a substrate.

Cross-talk between cAMP and formylmet-leu-phe in human neutrophils: phosphorylation of a 52,000 molecular weight protein

The mechanism of inhibition of neutrophil phagocytic functions by cAMP-elevating agents has not yet been clarified. In the present work, the effects of adenylate cyclase agonists on protein phosphorylation in the formylmethionyl-leucyl-phenylalanine (fMLP)-stimulated human neutrophils were studied. Before stimulation, 32Pi-labelled cells were incubated with adenosine deaminase to remove the endogenously produced adenosine, an adenylate cyclase agonist itself. A protein of about 52,000 molecular weight was rapidly and transiently phosphorylated when neutrophils were stimulated with fMLP in the presence of isoproterenol, prostaglandin E1, histamine or 2-chloroadenosine. This phosphorylation was blocked by the antagonists of the receptors for the above-listed agents. No phosphorylation of the 52,000 molecular weight protein could be observed if either fMLP or the cAMP-elevating agent were applied alone. A calcium ionophore A23187 and dibutyryl-cAMP could replace fMLP and a cAMP-elevating agent, respectively. Phosphorylation of the 52,000 molecular weight protein was also demonstrated in cell lysates in the presence of cAMP, and in membrane preparations in the presence of the catalytic subunit of cAMP-dependent protein kinase. These data suggest that phosphorylation of the 52,000 molecular weight protein in intact cells is dependent on the cross-talk between the fMLP- and the cAMP-signalling pathways, and may thus be involved in the cAMP-regulatory mechanism.

Release of carrot plasma membrane-associated phosphatidylinositol kinase by phospholipase A2 and activation by a 70 kDa protein

Plasma membranes were isolated from carrot (Daucus carota L.) cells grown in suspension culture and treated with phospholipase A2 from snake or bee venom for 10 min. As a result of this treatment, phosphatidylinositol kinase activity was recovered in the soluble fraction. There was no detectable diacylglycerol kinase or phosphatidylinositol monophosphate kinase activity released from the membranes after the phospholipase A2 treatment. Treating the plasma membranes with phospholipase C or D did not release PI kinase activity. The phospholipase A2-released PI kinase was activated over 2-fold by a heat stable, soluble 70 kDa protein. The partially purified 70 kDa activator increases the Vmax but does not affect the Km of the phospholipase A2-released PI kinase.

Platelet membrane phosphatidylinositol kinase activity. Triton X-100 effects provide evidence for intramicellar reaction

Phosphatidylinositol (PI) kinase activity of platelet membranes was solubilized and partially purified by anion-exchange chromatography to measure the initial enzymatic rates. Kinetic studies were performed in the presence of Triton X-100 to obtain mixed micelles. The partially purified enzyme exhibited a Michaelian behaviour towards ATP, with a Km of 58 microM. The enzymatic rates were dependent upon Triton concentrations. Upon increasing its concentration, this detergent exhibited an activating effect followed by an inhibitory one. The optimal micellar Triton concentration was proportionnal to the PI concentration used in the assay. Conversely, the behaviour of the enzyme towards PI was dependent upon the Triton concentration. However, when PI concentration was expressed as its surfacic concentration within the micelles, the activity became independent of the detergent concentration, and a Km value of 0.09 mol/mol was estimated. Therefore, in vitro phosphorylation of phosphatidylinositol by PI kinase is rate-limited by an intramicellar reaction, and provides a study model for the in vivo reaction.

Purification and reconstitution of phosphatidylinositol 4-kinase from human erythrocytes

A membrane-bound phosphatidylinositol 4-kinase (PtdIns kinase) has been purified to apparent homogeneity from human erythrocytes. Enzyme activity was solubilized from urea-KCl-stripped, inside-out membrane vesicles by 3% Triton X-100. Purification to apparent homogeneity was accomplished by cation-exchange chromatography on phosphocellulose, followed by heparin-acrylamide chromatography. This resulted in a nearly 3900-fold purification of PtdIns kinase activity to a specific activity of 44 nmol min-1 mg-1. The purified enzyme has an Mr of 59,000 on silver-stained SDS-PAGE; however, many preparations also contain 54 kDa and 50 kDa proteins which are related to the 59 kDa protein and have PtdIns kinase activity. Kinetic analysis of the PtdIns kinase indicate apparent Km values of 40 and 35 microM for phosphatidylinositol and ATP, respectively. The purified enzyme has been reconstituted into phospholipid liposomes and shown to phosphorylate phosphatidylinositol.

Purification and chemical modification of a phosphatidylinositol kinase from sheep brain

A membrane-bound phosphatidylinositol (PtdIns) kinase has been purified approximately 9500-fold to apparent homogeneity from sheep brains. The purification procedure involves: solubilisation of the membrane fraction with Triton X-100, ammonium sulphate fractionation and a number of ion-exchange and gel-filtration chromatography steps. The purified enzyme exhibited a final specific activity of 1149 nmol.min-1.mg-1. The molecular mass of the enzyme was estimated to be 55 kDa by SDS/PAGE and 150 +/- 10 kDa by HPLC gel filtration in the presence of Triton X-100. Kinetic measurements have shown that the apparent Km value of PtdIns kinase for the utilisation of PtdIns is 22 microM and for ATP 67 microM. Mg2+ was the most effective divalent cation activator of PtdIns kinase, with maximal enzymatic activity reached at a concentration of 10 mM Mg2+. In addition to adenosine and ADP, the 2'(3')-O-(2,4,6-trinitrophenyl) derivative of ATP was found to be a strong competitive inhibitor of the enzyme, with a Ki of 32 microM. Enzymatic activity was found to be stimulated by Triton X-100 but inhibited by deoxycholate.

Purification and characterization of phosphatidylinositol 4-kinase from human erythrocyte membranes

Two species of PtdIns 4-kinase with molecular masses of 50 kDa and 45 kDa were detected in human erythrocyte membranes using SDS/PAGE. These enzymes were purified to near homogeneity and found to display very similar enzymatic characteristics. The purification scheme consisted of solubilization from erythrocyte membranes in the presence of Triton X-100, followed by Cibacron-blue-Sephadex, phosphocellulose and Mono Q anion-exchange chromatography. The final step in the purification protocol was preparative SDS/PAGE, followed by electroelution and renaturation of the enzyme. This procedure afforded an about 4000-fold purification of the enzyme from erythrocyte membranes. Characterization of the [32P]PtdInsP products formed by the purified PtdIns kinases indicated that these enzymes specifically phosphorylated the D-4 position of the inositol ring. The Km values of both PtdIns 4-kinase species for PtdIns and ATP were found to be 0.2 mM and 0.1 mM, respectively. The enzymes are both activated by Mg2+, and inhibited by Ca2+ and by adenosine. The potential importance of these effectors for the regulation of PtdIns phosphorylation in cells is discussed.

A monoclonal antibody distinguishes two types of phosphatidylinositol 4-kinase

A monoclonal antibody has been developed against the type II PtdIns 4-kinase from bovine brain. This antibody, 4C5G, causes greater than 90% inhibition of the type II PtdIns 4-kinase from bovine brain, rat brain and human erythrocytes. However, it fails to inhibit type III PtdIns 4-kinase from bovine brain or PtdIns 3-kinase from rat liver. These results suggest that type II and type III PtdIns 4-kinases are distinct gene products, and that 4C5G will be useful in studying the function of the type II PtdIns 4-kinase.

Stimulation of purified phosphatidylinositol 4-kinase by cobra venom cardiotoxin

Cobra venom cardiotoxin was found to stimulate the phosphatidylinositol kinase activity present in A431 cell membranes and in detergent extracts of these membranes. Incubation of highly purified phosphatidylinositol 4-kinase from this source with cardiotoxin resulted in a 2- to 3-fold stimulation of phosphatidylinositol kinase activity. The activation of the purified phosphatidylinositol 4-kinase by cardiotoxin was time- and dose-dependent and appeared to be associated with a decrease in the Km apparent of the enzyme for phosphatidylinositol with no change in the Vmax apparent of the enzyme. The data suggest that the phosphatidylinositol 4-kinase is activated by direct interaction of the enzyme with cobra venom cardiotoxin.

Activation of a membrane-associated phosphatidylinositol kinase through tyrosine-protein phosphorylation by naphthoquinones and orthovanadate

We have previously reported that several naphthoquinones stimulated tyrosine-specific protein phosphorylation in isolated rat liver membranes. Our more recent study demonstrated a similar effect by orthovanadate, which concomitantly stimulated phosphorylation of protein-tyrosine and phosphatidylinositol (Ptd-Ins). Results presented here show a simultaneous increase in PtdIns phosphorylation along with stimulation of tyrosine-protein phosphorylation by naphthoquinones. This PtdIns kinase resembles the type I PtdIns kinase in that it was insensitive to adenosine inhibition. The product, nevertheless, comigrated with a PtdIns-4-phosphate standard in TLC using three different solvent systems. Stimulation of PtdIns phosphorylation by vanadate or naphthoquinones could be achieved in the following preparations: intact rat liver membranes, Triton X-100-solubilized membranes, solubilized membranes partially purified by Sephacryl chromatography, solubilized membranes purified by wheat germ agglutinin chromatography. The naphthoquinone or vanadate-activated PtdIns kinase activity could be isolated by antiphosphotyrosine antibody-agarose affinity chromatography. The relative potencies of a series of ring-substituted naphthoquinones in the stimulation of tyrosine-protein phosphorylation, PtdIns kinase activity, dithiothreitol-dependent oxygen consumption, and cytochrome c reduction were highly correlated. We conclude that oxidant(s) produced by redox cycling of naphthoquinones stimulated an adenosine-insensitive PtdIns kinase through tyrosine phosphorylation of the enzyme.

myo-inositol metabolites as cellular signals

The discovery of the second-messenger functions of inositol 1,4,5-trisphosphate and diacylglycerol, the products of hormone-stimulated inositol phospholipid hydrolysis, marked a turning point in studies of hormone function. This review focuses on the myo-inositol moiety which is involved in an increasingly complex network of metabolic interconversions, myo-Inositol metabolites identified in eukaryotic cells include at least six glycerophospholipid isomers and some 25 distinct inositol phosphates which differ in the number and distribution of phosphate groups around the inositol ring. This apparent complexity can be simplified by assigning groups of myo-inositol metabolites to distinct functional compartments. For example, the phosphatidylinositol 4-kinase pathway functions to generate inositol phospholipids that are substrates for hormone-sensitive forms of inositol-phospholipid phospholipase C, whilst the newly discovered phosphatidylinositol 3-kinase pathway generates lipids that are resistant to such enzymes and may function directly as novel mitogenic signals. Inositol phosphate metabolism functions to terminate the second-messenger activity of inositol 1,4,5-trisphosphate, to recycle the latter's myo-inositol moiety and, perhaps, to generate additional signal molecules such as inositol 1,3,4,5-tetrakisphosphate, inositol pentakisphosphate and inositol hexakisphosphate. In addition to providing a more complete picture of the pathways of myo-inositol metabolism, recent studies have made rapid progress in understanding the molecular basis underlying hormonal stimulation of inositol-phospholipid-specific phospholipase C and inositol 1,4,5-trisphosphate-mediated Ca2+ mobilisation.

Partial purification and characterization of phosphatidylinositol kinases from human platelets

Most of human platelet phosphatidylinositol (PI) kinase activity (approx. 80%) was associated with the membrane fraction and its majority was released by the extraction with Triton X-100 after KCl treatment. Two major activity peaks (mPIK-I and mPIK-III) were obtained by Mono Q column chromatography. They were distinct from each other with regard to Mr (76,000 and 80,000 as determined by gel-filtration chromatography), apparent Km values for ATP, effect of arachidonic acid and phosphatidylserine and detergent requirement. Triton X-100 inhibited the activity of mPIK-I but rather weakly enhanced the mPIK-III activity, and sodium cholate remarkably inhibited both mPIK-I and mPIK-III activities. Their products were identified to be phosphatidylinositol 4-phosphate. On the other hand, about 20% of PI kinase activity was recovered from the cytosolic fraction and two activity peaks (cPIK-I and cPIK-II) were resolved on Mono Q column chromatography. There were no significant differences in biochemical properties between cPIK-I and cPIK-II. Both of them had Mr approx. 550,000 as determined by gel-filtration chromatography and were activated by sodium cholate to a greater extent than by Triton X-100. The results suggest that the major PI kinases (mPIK-I and mPIK-III) are PI 4-kinase and mPIK-I is distinct from PI 4-kinases in other sources especially with regard to the effect of Triton X-100.

Purification and characterization of bovine brain type I phosphatidylinositol kinase

Investigation into the phosphatidylinositol kinase activities in bovine brain has revealed the presence of a type I PtdIns kinase activity. This classification is based upon potent inhibition by neutral detergent and the production of a phosphatidylinositol phosphate that can be distinguished from phosphatidyl-inositol-4-phosphate [PtdIns(4)P] by thin-layer chromatography. The enzyme has been substantially purified and the activity is associated with an 85-kDa polypeptide on SDS/polyacrylamide gel electrophoresis. Analysis of the product confirms the identification of the enzyme as a type I PtdIns kinase. The purified kinase has been characterized with respect to substrate dependence (Mg2+, ATP, PtdIns), substrate presentation (pure lipid versus mixed micelle) and specificity [PtdIns versus PtdIns(4)P and phosphatidylinositol 4,5-bisphosphate].

Effect of genistein, a tyrosine kinase inhibitor, on U46619-induced phosphoinositide phosphorylation in human platelets

Recent evidence suggests that the agonist-induced formation of phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) via PI and PIP kinases may play an important role in transmembrane signalling. In the present work, the effect of genistein, a specific inhibitor of protein-tyrosine kinase, on phosphoinositide phosphorylation was studied in human platelets stimulated with the endoperoxide analogue, U46619. At 100 microM concentration, genistein, but not the related compounds, flavone and biochanin A, which possess only weak anti-protein-tyrosine kinase activity, significantly inhibited the U46619-induced accumulation of [3H]PIP (by 71%) and [3H]PIP2. These data suggest that phosphoinositide phosphorylation may be regulated, in part, by tyrosine phosphorylation in U46619-stimulated platelets.

Enzymatic synthesis of pyrene-labeled polyphosphoinositides and their behavior in organic solvents and phosphatidylcholine bilayers

A method is reported for the synthesis of pyrene-labeled analogues of phosphatidylinositol 4-phosphate (Pyr-PIP) and phosphatidylinositol 4,5-biphosphate (Pyr-PIP2) from sn-2-(pyrenyl-decanoyl)phosphatidylinositol (Pyr-PI) using partially purified PI and PIP kinase preparations. Phosphorylation of Pyr-PI and Pyr-PIP was extensive (more than 50%) provided that the ATP concentration was high and that stabilizing agents such as sucrose and polyethylene glycol were present in the incubation medium. Pyr-PIP and Pyr-PIP2 were isolated by chromatography on immobilized neomycin. The identity of the products was established by thin-layer chromatography, UV-absorption spectroscopy, and spectrofluorometry. The pyrene excimer/monomer fluorescence technique revealed that, in contrast to Pyr-PI, Pyr-PIP and Pyr-PIP2 formed clusters in organic solvents. By use of the same technique for model membranes, it was shown that in phosphatidylcholine bilayers the collision frequency of the three fluorescent phosphoinositides decreased in the order PI greater than PIP greater than PIP2. Addition of Ca2+ at concentrations above 0.1 mM increased the collision frequency of Pyr-PIP2 and, to a much lesser extent, Pyr-PIP; Ca2+ had no effect on Pyr-PI.