Phosphatidylinositol-4,5-bisphosphate phosphodiesterase and phosphomonoesterase activities of rat brain. Some properties and possible control mechanisms

A Short Historical Perspective of Methods in Inositol Phosphate Research

The multitudinous inositol phosphate family elicits a wide range of molecular effects that regulate countless biological responses. In this review, I provide a methodological viewpoint of the manner in which key advances in the field of inositol phosphate research were made. I also note some of the considerable challenges that still lie ahead.

PTEN Regulates PI(3,4)P2 Signaling Downstream of Class I PI3K

The PI3K signaling pathway regulates cell growth and movement and is heavily mutated in cancer. Class I PI3Ks synthesize the lipid messenger PI(3,4,5)P₃. PI(3,4,5)P₃ can be dephosphorylated by 3- or 5-phosphatases, the latter producing PI(3,4)P₂. The PTEN tumor suppressor is thought to function primarily as a PI(3,4,5)P₃ 3-phosphatase, limiting activation of this pathway. Here we show that PTEN also functions as a PI(3,4)P₂ 3-phosphatase, both in vitro and in vivo. PTEN is a major PI(3,4)P₂ phosphatase in Mcf10a cytosol, and loss of PTEN and INPP4B, a known PI(3,4)P₂ 4-phosphatase, leads to synergistic accumulation of PI(3,4)P₂, which correlated with increased invadopodia in epidermal growth factor (EGF)-stimulated cells. PTEN deletion increased PI(3,4)P₂ levels in a mouse model of prostate cancer, and it inversely correlated with PI(3,4)P₂ levels across several EGF-stimulated prostate and breast cancer lines. These results point to a role for PI(3,4)P₂ in the phenotype caused by loss-of-function mutations or deletions in PTEN.

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PTEN is a PI(3,4)P₂ 3-phosphatase

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PTEN and INPP4B regulate PI(3,4)P₂ accumulation downstream of class I PI3K

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PTEN regulates PI(3,4)P₂-dependent activation of Akt and formation of invadopodia

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PI(3,4)P₂ signaling may play a role in the tumor suppressor function of PTEN

A short history of inositol lipids

The diverse family of inositol lipids is now known to be central to many aspects of cell biology. The route from the first discovery of inositol to our present day knowledge of inositol lipids spans more than 150 years and is long and complex. This is a brief account of some of the most important stages along that route.

A tale of two inositol trisphosphates

Between spring 1982 and autumn 1984 the physiological role of Ins(1,4,5)P3 as a calcium-mobilizing second messenger was first suggested and then experimentally established. At the same time the unexpected complexity of inositide metabolism began to be exposed by the discovery of Ins(1,3,4)P3. This article recalls my entanglement with these two inositol phosphates.

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.

20 years of Ins(1,4,5)P3, and 40 years before

This year marks the 20th birthday of the discovery of inositol-1,4,5-trisphosphate as a second messenger. The background to this discovery is a complex story that goes back more than 50 years and involves a large cast of characters, both chemical and human.

Kinetic analysis of Arabidopsis phospholipase Ddelta. Substrate preference and mechanism of activation by Ca2+ and phosphatidylinositol 4,5-biphosphate

Phospholipase D (PLD) is a major plant phospholipase family involved in many cellular processes such as signal transduction, membrane remodeling, and lipid degradation. Five classes of PLDs have been identified in Arabidopsis thaliana, and Ca(2+) and polyphosphoinositides have been suggested as key regulators for these enzymes. To investigate the catalysis and regulation mechanism of individual PLDs, surface-dilution kinetics studies were carried out on the newly identified PLDdelta from Arabidopsis. PLDdelta activity was dependent on both bulk concentration and surface concentration of substrate phospholipids in the Triton X-100/phospholipid mixed micelles. V(max), K(s)(A), and K(m)(B) values for PLDdelta toward phosphatidylcholine or phosphatidylethanolamine were determined; phosphatidylethanolamine was the preferred substrate. PLDdelta activity was stimulated greatly by phosphatidylinositol 4,5-bisphosphate (PIP(2)). Maximal activation was observed at a PIP(2) molar ratio around 0.01. Kinetic analysis indicates that PIP(2) activates PLD by promoting substrate binding to the enzyme, without altering the bulk binding of the enzyme to the micelle surface. Ca(2+) is required for PLDdelta activity, and it significantly decreased the interfacial Michaelis constant K(m)(B). This indicates that Ca(2+) activates PLD by promoting the binding of phospholipid substrate to the catalytic site of the enzyme.

Pharmacological modulation of intracellular Ca(2+) channels at the single-channel level

Synaptic signaling, memory formation, neuronal development, and neuronal pathology are strongly influenced by the properties of intracellular Ca2+ channels, ryanodine, and inositol 1, 4, 5 trisphosphate receptors. This review will focus on recently developed and discovered pharmacological tools to modulate these channel proteins at the single-channel level. It will allow the readers of Molecular Neurobiology to evaluate the current knowledge on the pharmacological modulation of intracellular Ca2+ channels and to direct future research efforts effectively using available experimental tools and concepts.

Cellular regulation of cytosolic group IV phospholipase A2 by phosphatidylinositol bisphosphate levels

Cytosolic group IV phospholipase A2 (cPLA2) is a ubiquitously expressed enzyme with key roles in intracellular signaling. The current paradigm for activation of cPLA2 by stimuli proposes that both an increase in intracellular calcium and mitogen-activated protein kinase-mediated phosphorylation occur together to fully activate the enzyme. Calcium is currently thought to be needed for translocation of the cPLA2 to the membrane via a C2 domain, whereas the role of cPLA2 phosphorylation is less clearly defined. Herein, we report that brief exposure of P388D1 macrophages to UV radiation results in a rapid, cPLA2-mediated arachidonic acid mobilization, without increases in intracellular calcium. Thus, increased Ca2+ availability is a dispensable signal for cPLA2 activation, which suggests the existence of alternative mechanisms for the enzyme to efficiently interact with membranes. Our previous in vitro data suggested the importance of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) in the association of cPLA2 to model membranes and hence in the regulation of cPLA2 activity. Experiments described herein show that PtdInsP2 also serves a similar role in vivo. Moreover, inhibition of PtdInsP2 formation during activation conditions leads to inhibition of the cPLA2-mediated arachidonic acid mobilization. These results suggest that cellular PtdInsP2 levels are involved in the regulation of group IV cPLA2 activation.

Molecular and biochemical properties and physiological roles of plant phospholipase D

Recent advances have thrust the study of plant phospholipase D (PLD) into the molecular era. This review will highlight some of the recent progress made in elucidating the molecular and biochemical nature of plant PLDs as well as their roles in plant physiology.

SH2 domain-mediated activation of phospholipase Cgamma is not required to initiate Ca2+ release at fertilization of mouse eggs

The initiation of Ca2+ release at fertilization of mammalian eggs requires inositol trisphosphate (Miyazaki et al., 1992, Science 257, 251-255), indicating that an enzyme of the phospholipase C family is probably activated. Because Ca2+ release at fertilization in echinoderm eggs is initiated by SH2 domain-mediated activation of phospholipase Cgamma (Carroll et al., 1997, J. Cell Biol. 138, 1303-1311), we examined the possible role of PLCgamma in initiating Ca2+ release at fertilization in mouse eggs. Both PLCgamma isoforms, PLCgamma1 and PLCgamma2, are present in mouse eggs and sperm, and stimulation of these enzymes in the egg by way of an exogenously expressed PDGF receptor causes Ca2+ release. Recombinant SH2 domains of PLCgamma1 and PLCgamma2 inhibit PLCgamma1 and PLCgamma2 activation by the PDGF receptor, completely preventing Ca2+ release in response to PDGF when injected at an approximately 20- to 40-fold excess over the concentrations of endogenous proteins. However, even at an approximately 100- to 400-fold excess over endogenous protein levels, PLCgamma1 and PLCgamma2 SH2 domains do not inhibit Ca2+ release at fertilization. These findings indicate that Ca2+ release at fertilization of mouse eggs does not require SH2-domain-mediated activation of PLCgamma. However, activation of PLCgamma in the egg by an alternative pathway, or introduction of activated PLCgamma from the sperm, may be important.

Substrate selectivities and lipid modulation of plant phospholipase D alpha, -beta, and -gamma

Three classes of phospholipase D (PLD), designated PLD alpha, -beta, and -gamma, have been cloned from plants, but their substrate selectivities have not been established. Using active PLDs expressed from their cDNAs in Escherichia coli, we compared the hydrolytic activities of these three PLDs toward various phospholipids and the influence of substrate composition on their substrate selectivities. When single-class phospholipid vesicles of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylglycerol (PG), phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidylinositol 4,5-bisphosphate (PIP2), N-acylphosphatidylethanolamine (NAPE), and cardiolipin (CL) were examined, PLD alpha hydrolyzed PC, PE, and PG but PLD beta and -gamma showed no activity toward any of these lipids. When PIP2 was included in mixed vesicles with the phospholipids above, PLD alpha showed the same PC-, PE-, and PG-hydrolyzing ability, whereas PLD beta and -gamma were able to hydrolyze both PE and PS. When both PE and PIP2 were included in substrate vesicles, PLD beta and PLD gamma hydrolyzed PC, PG, and NAPE, showing that both PE and PIP2 are required for PC, PG, and NAPE hydrolysis by PLD beta and -gamma. The PE activation of PLD beta and -gamma required lipid vesicles made of mostly PE, suggesting that PE may affect the substrate presentation rather than serve as a cofactor of these PLDs. Under equivalent reaction conditions, PLD beta displayed a similar preference for PC and NAPE, whereas PLD gamma preferred NAPE to PC by nearly three times. None of the three PLDs used PI, CL, or PIP2 as substrates. These results have identified PS- and NAPE-hydrolyzing PLDs and have indicated an important role for lipid composition in regulating the substrate selectivity of PLD beta and -gamma.

Promotion-resistant JB6 mouse epidermal cells exhibit defects in phosphatidylethanolamine synthesis and phorbol ester-induced phosphatidylcholine hydrolysis

The tumour-promotion-sensitive (P+) and -resistant (P-) variants of mouse JB6 epidermis-derived cells have often been used to study the requirements for the tumour-promoting effect of PMA. As part of an effort to identify the defect(s) in JB6 P- cells that might prevent the promoting effect of PMA, stimulation of phospholipase D (PLD)-mediated hydrolysis of phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) by PMA as well as the rate of phospholipid synthesis were compared in three P+ variants, two P- variants and a transformed variant of the JB6 cell line. PMA (5-100 nM) had significantly less stimulatory effect on PtdCho hydrolysis in P- cells than in P+ or transformed JB6 cells. The effects of PMA on PtdEtn hydrolysis in the P+ and P- cell lines were similar, whereas in transformed cells PMA had slightly less effect. Each JB6 cell line was found to contain similar amounts of PtdCho. In contrast, P- cells contained significantly less PtdEtn and a correspondingly higher level of ethanolamine phosphate compared with P+ and transformed cells. P- cells also secreted ethanolamine phosphate into the medium; this process was greatly enhanced by PMA. In the two P- variants the synthesis of PtdEtn from [14C]ethanolamine was reduced to various extents, whereas the rate of PtdCho synthesis was comparable in each JB6 cell line. The synthesis of PtdCho, but not PtdEtn, was greatly stimulated by PMA in both the P+ and P- clones. The results indicate that decreased synthesis/level of PtdEtn and suboptimal functioning of a PtdCho-specific PLD are common characteristics of the P- JB6 cells examined so far. The observed alterations in phospholipid metabolism may play a role in the resistance of P- cells to the tumour-promoting action of PMA.

Inhibition of phospholipase C-delta 1 catalytic activity by sphingomyelin

We measured the ability of sphingomyelin (SPM) to inhibit phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] hydrolysis catalyzed by human phospholipase C-delta 1 (PLC-delta 1) in model membranes and detergent phospholipid mixed micelles. SPM strongly inhibited PLC-delta 1 catalytic activity measured in large unilamellar vesicles (LUVs) composed of egg phosphatidylcholine (PC), PI(4,5)P2, and SPM from brain or egg. At 37 or 45 degrees C, the rate of PI(4,5)P2 hydrolysis in PC/SPM/PI(4,5)P2 vesicles (15:80:5 mol:mol) was less than 25% of that observed in PC/PI(4,5)P2 vesicles (95:5). By contrast, catalysis was only weakly inhibited by equivalent concentrations of the SPM analog, 3-deoxy-2-O-stearoyl-SPM, which lacks hydrogen bond-donating groups at the C-3 and C-2 positions of the sphingolipid backbone. Inhibition by SPM was not observed in detergent/phospholipid mixed micelles. The binding affinity of PLC-delta 1 for vesicles containing PC and PI(4,5)P2 was slightly diminished by inclusion of SPM in the lipid mixture, but not enough to account for the decreased rate of catalysis. We could find no evidence of specific binding of the enzyme to SPM, which argues against a simple negative allosteric mechanism. To understand the cause of inhibition, the effects of SPM and 3-deoxy-2-O-stearoyl-SPM on the bulk properties of the substrate bilayers were examined. Increasing the mole fraction of SPM altered the fluorescence emission spectra of two sets of head group probes, 6-lauronyl(N,N-dimethylamino)naphthalene and N-[5-(dimethylamino)naphthalene-1-sulfonyl]-1,2-dihexadecanoyl-sn- glycero-3-phosphoethanolamine, that are sensitive to water content at the membrane/solution interface. Results obtained with both probes suggested a reduction in hydration with increasing SPM content. Vesicles containing 3-deoxy-2-O-stearoyl-SPM produced intermediate changes. Our results are most consistent with a model in which SPM inhibits PLC by increasing interlipid hydrogen bonding and by decreasing membrane hydration; both factors raise the energy barrier for activation of PLC-delta 1 at the membrane/protein microinterface.

betagamma-Transducin stimulates hydrolysis and synthesis of phosphatidylinositol 4,5-bisphosphate in bovine rod outer segment membranes

T betagamma was shown to stimulate the hydrolysis and synthesis of PtdInsP2 in dark-adapted bovine retinal rod outer segments. In contrast, T alphaGDP blocked the effect of betagamma-transducin. It was also demonstrated that T betagamma was a stimulator of 32P incorporation into PtdInsP2 in ROS. These findings explain the modulating actions of GTP and light on PtdInsP2 hydrolysis and synthesis in ROS. The possible existence of cross-talk between the cGMP and phosphoinositide cascades in retinal rods was discussed.

Role of glucose metabolism and phosphoinositide hydrolysis in glucose-induced sensitization/desensitization of insulin secretion from mouse pancreatic islets

The role of glucose metabolism and phosphoinositide hydrolysis in glucose-induced sensitization/desensitization of insulin secretion was studied. A change in glucose concentration from 5.5 to 16.7 mM during 22-24 h of pre-exposure of mouse islets in TCM 199 culture medium (0.26 mM Ca2+) led to sensitization of glucose-induced insulin secretion. This change in islet responsiveness to glucose was not mediated by increases in glucose utilization ([5-3H]glucose conversion to 3H2O) and glucose oxidation ([U-14C]glucose oxidation to 14CO2). Glucose-induced sensitization of insulin secretion was associated with an increase in glucose-induced phosphoinositide hydrolysis, leading to a significant increase in inositol 1-monophosphate formation, but not in inositol 1,4-bisphosphate or in inositol 1,4,5-trisphosphate plus inositol 1,3,4-trisphosphate formation. Diacylglycerol, which may arise from both phosphoinositide hydrolysis and de novo from glucose metabolism, was, on the other hand, not increased during acute exposure to glucose and not changed after pre-exposure to glucose. At 16.7 mM glucose in TCM 199 medium, a change in Ca2+ concentration from 0.26 to 1.26 mM led to a reduction in glucose-induced insulin secretion. This Ca(2+)-dependent desensitization of insulin secretion in the presence of glucose was associated with a decrease in glucose-induced phosphoinositide hydrolysis, but not with a change in glucose metabolism or diacylglycerol accumulation. In conclusion, it is suggested that glucose-induced sensitization/desensitization of insulin secretion may involve changes in phosphoinositide hydrolysis, but may occur independently of concomitant changes in glucose metabolism or diacylglycerol accumulation.

Cellular actions of inositol phosphates and other natural calcium and magnesium chelators

Naturally occurring chelators of Ca2+ and Mg2+ have largely been unrecognized due to their low binding affinities. They include carbohydrate and cyclitol phosphates, nucleotides and nucleic acids. The calciotrophic inositol phosphates Ins(1,4,5)P3 and Ins(1,3,4,5)P4 form chelates within the range of Ca2+ concentrations found in biological systems. As well as being a likely source of experimental artifact where these compounds have been investigated at unphysiological cation concentrations, chelation may have important physiological roles. The autoregulation of Ca2+ entry into the cell cytosol is one, whereas the coupling of chelation with enzyme or receptor interactions offers a general mechanism for divalent cation control of diverse biological processes. Inositol monophosphate 1-phosphatase and inositol polyphosphate 1-phosphatase are two related enzymes which may conform to this mechanism. If so, it would provide a possible explanation for their sensitivity to divalent cations and for their non-competitive inhibition by lithium ion.

Simultaneous measurements of cytosolic pH and calcium interactions in bovine lactotrophs using optical probes and four-wavelength quantitative video microscopy

The properties of pH and calcium homeostasis have been investigated in single bovine lactotrophs by the use of the fluorescent indicators 2',7'-bis(carboxyethyl)-carboxyfluorescein (BCECF) and fura-2 respectively. A method of simultaneous recording from both dyes loaded in the same cell was used. Despite slight crosstalk between the two dyes, physiologically relevant information about the interrelationship between pH and calcium homeostasis was obtained. Three types of interactions were recorded. First, an increase in calcium due to the discharge of intracellular stores by thyrotropin-releasing hormone resulted in no change in cytosolic pH. Secondly, alkalinization by the addition of a weak base, NH4Cl, induced a large transient (around 1000 nM) and a small (a few tens of nanomoles per liter) sustained increase in cytosolic calcium. The former is partly due to release from agonist-sensitive stores. Thirdly, upon the removal of NH4Cl the cytoplasm became acidic, which induced a release of calcium from intracellular stores in some cells. In addition we demonstrate that the recovery from acid load is sensitive to extracellular Na+, suggesting the presence of Na+/H+ exchange mechanisms in bovine lactotrophs. Interestingly we have also found that, at rest, removal of Na+ from the bathing medium results in a decrease in resting [Ca2+]i, paralleled by a reduction in pHi. This suggests a role for Na+/H+ exchange in determining resting [Ca2+]i.

Cooperative effects of ethanol and protein kinase C activators on phospholipase-D-mediated hydrolysis of phosphatidylethanolamine in NIH 3T3 fibroblasts

In a previous study, ethanol was shown to enhance the stimulatory effect of phorbol 12-myristate 13-acetate (PMA), a prominent activator of protein kinase C (PKC), on phospholipase-D (PLD)-mediated hydrolysis of phosphatidylethanolamine (PtdEtn) in NIH 3T3 fibroblasts (Kiss et al. (1991) Eur. J. Biochem. 197, 785-790). Here, the mechanism and possible significance of ethanol-stimulated PtdEtn hydrolysis was further studied. In [14C]ethanolamine-labeled NIH 3T3 fibroblasts, 10 mM ethanol enhanced PMA-induced hydrolysis of PtdEtn 1.5-2.0-fold during a 2.5-15-min incubation period. Other alcohols, including glycerol, methanol, and 1-propanol, also enhanced PMA-induced PtdEtn hydrolysis. Of the other PLD activators tested, ethanol potentiated the PKC-dependent stimulatory effect of bombesin but failed to alter the apparently PKC-independent stimulatory effect of serum. Pretreatment of [14C]ethanolamine-labeled fibroblasts with 200 mM ethanol for 20 min resulted in increased (approx. 2-fold) hydrolysis of [14C]PtdEtn in isolated membranes. In membranes from ethanol-treated, but not from untreated, cells, PMA further enhanced (approx. 1.5-fold) the production of [14C]ethanolamine. Ethanol exerted none of the above stimulatory effects on phosphatidylcholine hydrolysis. These results suggest that the specific stimulatory action of ethanol on PLD-mediated PtdEtn hydrolysis can occur in vivo and may involve increased binding of a regulatory PKC-isoform to membranes.

Inositol-lipid-specific phospholipase C isoenzymes and their differential regulation by receptors

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The long-term combined stimulatory effects of ethanol and phorbol ester on phosphatidylethanolamine hydrolysis are mediated by a phospholipase C and prevented by overexpressed alpha-protein kinase C in fibroblasts

The protein kinase C (PKC) activator 12-O-tetradecanoylphorbol 13-acetate (TPA) has been shown to potentiate the stimulatory effect of ethanol on the hydrolysis of phosphatidylethanolamine (PtdEtn) in NIH 3T3 fibroblasts. Following an initial 20-min period, the main product of PtdEtn degradation in cells treated with TPA plus ethanol was ethanolamine phosphate. Here, we have examined the regulatory role of PKC and the possible catalytic role of phospholipase C in the formation of ethanolamine phosphate. TPA, bryostatin, and bombesin, direct or indirect activators of PKC, had similar potentiating effects on ethanol-induced formation of [14C]ethanolamine phosphate from [14C]PtdEtn in [14C]ethanolamine-prelabelled NIH 3T3 fibroblasts. At lower concentrations of ethanol (40-80 mM), significant stimulation of ethanolamine phosphate formation required longer treatments (2 h or longer). The combined effects of TPA (100 nM) and ethanol (50-200 mM) on ethanolamine phosphate formation were not inhibited by the PKC inhibitors staurosporine or 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7). In contrast, these inhibitors significantly inhibited TPA-induced formation of ethanolamine, catalyzed by a phospholipase-D-type enzyme. In membranes isolated from TPA+ethanol-treated cells, enhanced formation of ethanolamine phosphate was maintained for at least 20 min. Down-regulation of PKC by prolonged (24-h) treatment of NIH 3T3 fibroblasts by 300 nM TPA enhanced, while overexpression of alpha-PKC in Balb/c fibroblasts diminished, the stimulatory effect of ethanol on the formation of ethanolamine phosphate. Finally, addition of the protein phosphatase inhibitor okadaic acid (2 microM) to fibroblasts inhibited TPA+ethanol-induced formation of ethanolamine phosphate. These results suggest that alpha-PKC-mediated protein phosphorylation may negatively regulate PtdEtn hydrolysis and that the potentiating effect of TPA may result, at least partly, from increased degradation of this PKC isoform.

Activity and distribution of phosphoinositidase C in rat sciatic nerve

The hydrolysis of phosphatidylinositol-4,5-bisphosphate (PIP2) by rat sciatic nerve cytosolic phosphoinositidase C [phosphoinositide-specific phospholipase C (PIC)] was studied at neutral pH and at ionic concentrations that approximate intracellular conditions. The principal water-soluble product formed was shown to be inositol trisphosphate by anion exchange chromatography. The maximum hydrolysis rate (2.5 nmol/min/mg protein) was achieved at less than 100 nM Ca2+. Hydrolysis was markedly increased to 15 nmol/min/mg protein by inclusion of K+ in the reaction mixture. In the presence of 200 mM K+, the optimum Ca2+ was increased to approximately 600 nM. Higher Ca2+ concentrations progressively inhibited PIP2 hydrolysis. Mg2+ also inhibited the reaction, but the presence of equimolar amounts of ATP and Mg2+ had no effect. Appreciable degradation of phosphatidylinositol-4-phosphate (PIP) also occurred in the nanomolar Ca2+ range, whereas breakdown of phosphatidylinositol (PI) required millimolar Ca2+. The presence of PIP but not PI inhibited PIP2 hydrolysis. Upon subcellular fractionation of nerve, more than 50% of recovered PIC activity was in the cytosol and about 20% was located in a myelin-enriched fraction. Using PIP2 as substrate, PIC activities in nerves from normal and streptozotocin-induced diabetic animals were not different. However, the myelin-associated enzyme from diabetic animals was more labile to freezing and thawing.

Inositol lipids in cellular signalling mechanisms

At the opening of the 1980s, two camps vigorously contested whether receptor-stimulated inositol lipid hydrolysis was a transmembrane signalling reaction that brought about an elevation of cytosolic [Ca2+] or simply a frequent, but unexplained, response of many stimulated cells to a stimulated elevation of cytosolic [Ca2+]. Since 1984, this discussion has been replaced by intensive work that is well on the way to providing a detailed description of the complex set of signalling pathways initiated by phosphatidylinositol 4,5-bisphosphate hydrolysis to form the second messengers inositol 1,4,5-trisphosphate and 1,2-diacylglycerol. In addition, it has been realized that cells closely regulate their levels both of a novel family of 3-phosphorylated inositol lipids and of a large number of water-soluble inositol polyphosphates; the definition of the functions of these molecules will be a job for the 1990s.

Effects of muscarinic agonists and depolarizing agents on inositol monophosphate accumulation in the rabbit vagus nerve

The effects of muscarinic agonists and depolarizing agents on inositol phospholipid hydrolysis in the rabbit vagus nerve were assessed by the measurement of [3H]inositol monophosphate production in nerves that had been preincubated with [3H]inositol. After 1 h of drug action, carbachol, oxotremorine, and arecoline increased the inositol monophosphate accumulation, though the maximal increase induced by these agonists differed. Addition of the muscarinic antagonists atropine or pirenzepine shifted the carbachol dose-response curves to the right, without decreasing the carbachol maximal stimulatory effects. The KB for pirenzepine was 35 nM, which is characteristic of muscarinic high-affinity binding sites coupled to phosphoinositide turnover and often associated with the M1 receptor subtype. On the other hand, agents known to depolarize or to increase the intracellular Ca2+ concentration, e.g., elevated extracellular K+, ouabain, Ca2+, and the Ca2+ ionophore A23187, also increased inositol monophosphate accumulation. These effects were not mediated by the release of acetylcholine, as suggested by the fact that they could not be potentiated by the addition of physostigmine nor inhibited by the addition of atropine. The Ca(2+)-channel antagonist Cd2+, also known to inhibit the Na+/Ca2+ exchanger, was able to block the effects of K+ and ouabain, but did not alter those of carbachol. These results suggest that depolarizing agents increase inositol monophosphate accumulation in part through elevation of the intracellular Ca2+ concentration and that muscarinic receptors coupled to phosphoinositide turnover are present along the trunk of the rabbit vagus nerve.

Regional differences in rat brain lipids during global ischemia

BACKGROUND AND PURPOSE

Membrane lipid degradation plays an important role in the pathogenesis of ischemic brain damage, but there is little information on changes in cerebrosides, sulfatides, and sphingomyelin. We studied regional changes in the quantities of these lipids during complete global brain ischemia in rats.

METHODS

Nitrous oxide-anesthetized rats were subjected to ischemia by a high-pressure neck cuff and arterial hypotension for 0 (control), 3, 10, or 30 minutes (n = 5 at each time). Brain temperature was allowed to fall spontaneously during ischemia, and the brain was frozen in situ with liquid N2 without recirculation. The frontal cortex, hippocampus, and basal ganglia were dissected at -15 degrees C. The lipids were separated by column and high-performance thin-layer chromatography and quantified by charring and densitometry.

RESULTS

Total lipid content was higher (p less than 0.01) in the hippocampus (72.6 +/- 2.8 mg/g wet wt, mean +/- SD) than in the frontal cortex and basal ganglia (57.7 +/- 2.1 and 62.6 +/- 1.5 mg/g wet wt, respectively). Ischemic changes occurred only in the frontal cortex, where total lipid content fell (p less than 0.01) by 11% after 30 minutes of ischemia because sulfatide and cerebroside contents fell by 44% and 38%, respectively.

CONCLUSIONS

Despite a marked accumulation of free fatty acids during complete global brain ischemia in rats, the only detectable changes in brain lipids were in the amounts of cerebrosides and sulfatides in the frontal cortex.

A specific transduction mechanism for the glutamate action on phosphoinositide metabolism via the quisqualate metabotropic receptor in rat brain synaptoneurosomes: II. Calcium dependency, cadmium inhibition

In this article, we demonstrate that an increase in intracellular Ca2+ concentration may represent a specific common step(s) in the mechanism(s) of action of glutamate (Glu) and depolarizing agents on formation of inositol phosphates (IPs) in 8-day-old rat forebrain synaptoneurosomes. In fact, A23187, a Ca2+ ionophore, induces a dose-dependent accumulation of IPs, which is not additive with that evoked by Glu and K+ but is slightly synergistic with that induced by carbachol. In addition, Glu and K+ augment the intracellular Ca2+ concentration in synaptoneurosome preparations as measured by the fura-2 assay. The absence of external Ca2+ decreases basal and Glu-, and K(+)-stimulated formation of IPs. Cd2+ (100 microM) fully inhibits both Glu- and K(+)-evoked formation of IPs without affecting the carbachol-elicited response of IPs. Zn2+ inhibits Glu- and K(+)-stimulated accumulation of IPs (IC50 approximately 0.4 mM) but with a lower affinity than Cd2+ (IC50 approximately 0.035 mM). The organic Ca2+ channel blockers verapamil (10 microM), nifedipine (10 microM), omega-conotoxin (2 microM), and amiloride (10 microM) as well as the inorganic blockers Co2+ (100 microM) and La3+ (100 microM) block neither Glu- nor K(+)-evoked formation of IPs, a result suggesting that the opening of the L-, T-, N-, or P-type Ca2+ channels does not participate in these responses. All these data suggest that an increase in intracellular Ca2+ concentration resulting from an influx of Ca2+, sensitive to Cd2+ but not to other classical Ca2+ antagonists, may play a key role in the transduction mechanism activated by Glu or depolarizing agents.

Characteristics of phosphoinositide-specific phospholipase C activity from mouse pancreatic islets

In pancreatic islets the bulk of phosphoinositide-specific phospholipase C (PI-PLC) activity was cytosolic. The soluble enzyme was activated by submicromolar concentrations of Ca2+, independent of calmodulin. It was unaffected by glucose and a series of glycolytic intermediates, including glyceraldehyde 3-phosphate. These observations lend support to the hypothesis that glucose-stimulated inositol triphosphate production in islets may be secondary to and provoked by glucose-mediated Ca2+ influx. All four pyridine nucleotides stimulated PI-PLC. Phosphatidylinositol hydrolysis was also stimulated by dioleine and arachidonic acid, and by the polyamines, putrescine and spermine. Phosphatidylinositol hydrolysis was inhibited by chlorpromazine, tetracaine, ATP, 5'-AMP, inorganic pyrophosphate and by phosphatidylinositol 4,5-bisphosphate, phosphatidylcholine and phosphatidylserine--but not affected by phosphatidylethanolamine. The cyclic nucleotides, cAMP and cGMP had no effect on the enzyme, and GTP-gamma-S did not activate the enzyme event at very low Ca2+ concentrations. The diglyceride lipase inhibitor, RHC 80267, and the cyclooxygenase inhibitor, indomethacin, had no effect on PI-PLC activity.

Effects of chlorpromazine on phosphatidylinositol turnover following thrombin stimulation of human platelets

Thrombin stimulation of human platelets is known to result in phosphatidylinositol turnover (PI response), the activation of protein kinase C (C-kinase), and the release of arachidonic acid (AA). The authors studied the effects of chlorpromazine (CPZ) on these responses. At a concentration of 100 microM, CPZ inhibited the phosphorylation of 40,000-dalton protein through C-kinase activation. CPZ failed to inhibit initial transient synthesis of 1,2-diacylglycerol (1,2-DAG) through the PI response, although it slowed the concurrent decreasing process. CPZ inhibited promotion of compensatory synthesis of phosphatidylinositol 4,5-bisphosphate (PIP2), and also inhibited the synthesis of phosphatidic acid (PA). These results suggest that phosphatidylinositol 4-monophosphate kinase and diacylglycerol kinase (DAG-kinase) may be inhibited by CPZ. CPZ also intensified the accumulation of inositol phosphates caused by the PI response, indicating possible inhibition of the phosphatases that metabolize these phosphates. Thus, CPZ partially inhibited the PI response. However, it appears likely that the inhibition of C-kinase activity by CPZ was not due to inhibition of 1,2-DAG production nor to direct inhibition of phospholipase C. CPZ also inhibited AA release. This action might be partially a result of the inhibitory effect of CPZ on PA production.

Pathway of phosphatidylinositol(3,4,5)-trisphosphate synthesis in activated neutrophils

Neutrophils activated by the formyl peptide f-Met-Leu-Phe transiently accumulate a small subset of highly polar inositol lipids. A similar family of lipids also appear in many other cells in response to a range of growth factors and activated oncogenes, and are presumed to be the direct or indirect products of 3-phosphatidylinositol kinase. The structures of these lipids are shown to be phosphatidylinositol 3-phosphate, phosphatidylinositol-(3,4)bisphosphate and phosphatidylinositol-(3,4,5)trisphosphate, and we present evidence that in intact neutrophils a phosphatidyl-inositol-(4,5)bisphosphate-3-kinase seems to be the focal point through which agonists stimulate the formation of 3-phosphorylated inositol lipids.

Polyphosphoinositol lipids in Chlamydomonas eugametos gametes

In Chlamydomonas eugametos gametes, phosphatidylinositol 4-phosphate (PtdInsP) and phosphatidylinositol 4,5-bisphosphate (PtdInsP2) comprised 0.4 and 0.3% of the whole-cell phospholipids. They were concentrated in the plasma membrane around the cell body and were present in low concentrations in the flagellar membrane. When gametes were fed (32)PO 4 (-) , the label was rapidly incorporated into PtdInsP and PtdInsP2 and only slowly incorporated into structural lipids such as phosphatidylethanolamine and phosphatidylglycerol. Similarly, when a pulse of (32)PO 4 (-) was chased with PO 4 (-) , the label was rapidly lost from the polyphosphoinositol lipids but not from the structural lipids. The major fatty acids in the polyphosphoinositides were C-22 carbon polyenoic acids (70%). The significance of these results in relationship to intracellular signalling via inositol phosphates and Ca(2+) is discussed.

Assay of a phosphatidylinositol bisphosphate phospholipase C activity in postmortem human brain

The activity of a phospholipase C which hydrolyses exogenous phosphatidylinositol-4,5-bisphosphate [( 3H]PtdIns(4,5)P2) in membranes prepared from frozen postmortem human brain and rat brain was investigated. Enzyme characteristics were essentially similar in membranes prepared from frozen postmortem brain and fresh or frozen rat brain. The [3H]PtdIns(4,5)P2 solubilization and assay procedure employed resulted in an efficient availability of the substrate for the enzyme. The non-hydrolysable guanosine triphosphate analogue guanosine 5'-[beta gamma-imido]diphosphate (Gpp[NH]p) stimulated hydrolysis rapidly with a half maximum activity of approximately 25 microM. This stimulation was not specific for guanine nucleotides as ATP, imidodiphosphate and pyrophosphate also caused enzyme activation. However these activation effects could be distinguished by the polyanion spermine. The non-hydrolysable guanine dinucleotide analogue guanosine 5'-[beta-thio]diphosphate acted as a partial agonist thereby inhibiting the stimulatory effect of Gpp[NH]p. Gpp[NH]p-stimulated enzyme activity showed a maximum response in the presence of 1 mM deoxycholate and displayed a pH optima in the range 7.0-7.5. PtdIns(4,5)P2 hydrolysis was observed in the absence of added calcium, but hydrolytic cleavage was inhibited in the presence of divalent ion chelators. Magnesium inhibited PtdIns(4,5)P2 hydrolysis in a concentration-dependent manner. Elucidation of these aspects of the phosphatidylinositol cycle in normal human postmortem brain will permit comparative studies in CNS disease states.

Cholinergic inositol phosphate formation in striatal neurons is mediated by distinct mechanisms

In murine striatal neurons devoid of functional synapses (6 days in vitro) the cholinergic agonists carbachol and arecoline evoked dose-dependent inositol phosphate (InsP) responses with mean log EC50s of -4.1 +/- 0.5 and -4.48 +/- 0.1, respectively. Carbachol (1 mM) and arecoline (1 mM) responses were insensitive to tetrodotoxin, a voltage-sensitive Na+ channel blocker, and were blocked by pirenzepine with relatively low affinity (logIC50 = -5.9 +/- 0.3 for the carbachol response and logIC50 = -5.8 +/- 0.3 for the arecoline response). After synaptogenesis (13 days in vitro) the maximal carbachol effect doubled whereas the arecoline response remained unchanged. This additional effect was sensitive to tetrodotoxin and the voltage-dependent Ca2+ channel blocker, omega-conotoxin. The tetrodotoxin-sensitive carbachol response was blocked by lower concentrations of pirenzepine than the tetrodotoxin-insensitive carbachol response. More than 75% of the InsP response evoked by low concentrations of muscarine (1 and 10 microM) was sensitive to tetrodotoxin whereas only 38% of the InsP response stimulated by 1 mM of muscarine could be blocked by tetrodotoxin. These results suggest that there are at least two different mechanisms (depending on the stage of development), activated most probably by two different muscarinic receptors responsible for the carbachol-induced InsP formation in striatal neurons.

Stimulation by epidermal growth factor of inositol phosphate production in plasma membranes from A431 cells

Plasma membranes were isolated from A431 cells previously labelled with myo-[3H]inositol during exponential growth, using a rapid procedure on Percoll gradients. They displayed a significant phospholipase (PLC) activity against phosphoinositides, which was stimulated by guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), epidermal growth factor (EGF) and fetal calf serum (FCS) (24%, 11% and 97% over controls, respectively). The effect of EGF was not significantly increased by GTP gamma S. Upon addition of cytosol, EGF promoted an almost 100% stimulation of inositol 1,4,5-trisphosphate and inositol bisphosphate generation, which displayed an absolute requirement for GTP gamma S. This dose-dependent effect of cytosol was linear until 60 micrograms/ml of cytosolic protein and decreased afterwards; it was abolished by heat treatment and trypsin hydrolysis, and it was not reproduced by an identical amount of bovine serum albumin. The same biphasic stimulation was observed with phosphotyrosyl proteins immunopurified from cytosol of A431 cells previously stimulated by EGF. Since phosphotyrosyl proteins displayed PLC activity, our data suggest that soluble protein substrates of EGF receptor tyrosine kinase, including PLC, could be involved in the regulation of phosphoinositide hydrolysis in response to EGF. Using phosphatidyl[3H]inositol 4,5-bisphosphate (PIP2) dispersed with unlabelled phosphatidylethanolamine and phosphatidylserine as an exogenous substrate, no stimulation of PLC activity by EGF could be detected, either with membranes or with membranes plus cytosol. It is concluded that EGF might stimulate hydrolysis of phosphoinositides by PLC through complex interactions between plasma membrane and cytosolic factors which still remain to be identified.

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.

Participation of both intracellular free Ca2+ and protein kinase C in tonic vasoconstriction induced by prostaglandin F2 alpha

The roles of intracellular free Ca2+ and protein kinase C in the tonic contraction induced by prostaglandin were studied. Prostaglandin F2 alpha induced tonic contraction of rat thoracic aorta in both control and Ca2(+)-free solution. Close correlations were observed between the contractile response of aortic strips and the changes in intracellular free Ca2+ concentration in vascular smooth muscle cells assessed with the fluorescent Ca2+ indicator fura 2, both in control and Ca2(+)-free solutions. Prostaglandin F2 alpha also enhanced the production of inositol 1,4,5-trisphosphate in vascular smooth muscle cells before the rise of the intracellular free Ca2+ concentration. Moreover, 1-(5-isoquinoline-sulfonyl)-2-methylpiperazine, an inhibitor of protein kinase C, inhibited the tonic contractions induced by PGF2 alpha and 12-O-tetradecanoyl phorbol-13-acetate, a direct activator of protein kinase C, at similar concentrations. These results suggest that both intracellular free Ca2+ and protein kinase C participate in prostaglandin F2 alpha-induced tonic contraction.

Phosphatidylinositol-4,5-bisphosphate phospholipase C in bovine rod outer segments

Preparations of rod outer segments from cattle retinas contained soluble and particulate phospholipase C activities which hydrolyzed phosphatidylinositol 4,5-bisphosphate (PIP2) and the other phosphoinositides. Ca2+ was required for PIP2 hydrolysis, but high (greater than 300 microM) concentrations were inhibitory. Mg2+ and spermine at low concentrations stimulated the particulate activity but inhibited the soluble. Mn2+ inhibited both. High (greater than 100 microM) concentrations of the nonhydrolyzable GTP analogue guanylyl beta,gamma-methylenediphosphonate inhibited PIP2 hydrolysis by both the soluble and particulate activities, but guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), fluoride, and cholera and pertussis toxins were without effect. Overall phospholipase C activity in ROS was unaffected by light. Evidence was found for multiple forms of the enzyme, requiring isolation and separate characterization before ruling out regulation by light or G-protein.

The actin-binding protein profilin binds to PIP2 and inhibits its hydrolysis by phospholipase C

Profilin is generally thought to regulate actin polymerization, but the observation that acidic phospholipids dissociate the complex of profilin and actin raised the possibility that profilin might also regulate lipid metabolism. Profilin isolated from platelets binds with high affinity to small clusters of phosphatidylinositol 4,5-bisphosphate (PIP2) molecules in micelles and also in bilayers with other phospholipids. The molar ratio of the complex of profilin with PIP2 is 1:7 in micelles of pure PIP2 and 1:5 in bilayers composed largely of other phospholipids. Profilin competes efficiently with platelet cytosolic phosphoinositide-specific phospholipase C for interaction with the PIP2 substrate and thereby inhibits PIP2 hydrolysis by this enzyme. The cellular concentrations and binding characteristics of these molecules are consistent with profilin being a negative regulator of the phosphoinositide signaling pathway in addition to its established function as an inhibitor of actin polymerization.