Differential activation of phosphatidylinositol deacylation and a pathway via diphosphoinositide in macrophages responding to zymosan and ionophore A23187

Rapid Movement of Palmitoleic Acid from Phosphatidylcholine to Phosphatidylinositol in Activated Human Monocytes

This work describes a novel route for phospholipid fatty acid remodeling involving the monounsaturated fatty acid palmitoleic acid. When administered to human monocytes, palmitoleic acid rapidly incorporates into membrane phospholipids, notably into phosphatidylcholine (PC). In resting cells, palmitoleic acid remains within the phospholipid pools where it was initially incorporated, showing no further movement. However, stimulation of the human monocytes with either receptor-directed (opsonized zymosan) or soluble (calcium ionophore A23187) agonists results in the rapid transfer of palmitoleic acid moieties from PC to phosphatidylinositol (PI). This is due to the activation of a coenzyme A-dependent remodeling route involving two different phospholipase A2 enzymes that act on different substrates to generate free palmitoleic acid and lysoPI acceptors. The stimulated enrichment of specific PI molecular species with palmitoleic acid unveils a hitherto-unrecognized pathway for lipid turnover in human monocytes which may play a role in regulating lipid signaling during innate immune activation.

Dynamics of Docosahexaenoic Acid Utilization by Mouse Peritoneal Macrophages

In this work, the incorporation of docosahexaenoic acid (DHA) in mouse resident peritoneal macrophages and its redistribution within the various phospholipid classes were investigated. Choline glycerophospholipids (PC) behaved as the major initial acceptors of DHA. Prolonged incubation with the fatty acid resulted in the transfer of DHA from PC to ethanolamine glycerophospholipids (PE), reflecting phospholipid remodeling. This process resulted in the cells containing similar amounts of DHA in PC and PE in the resting state. Mass spectrometry-based lipidomic analyses of phospholipid molecular species indicated a marked abundance of DHA in ether phospholipids. Stimulation of the macrophages with yeast-derived zymosan resulted in significant decreases in the levels of all DHA-containing PC and PI species; however, no PE or PS molecular species were found to decrease. In contrast, the levels of an unusual DHA-containing species, namely PI(20:4/22:6), which was barely present in resting cells, were found to markedly increase under zymosan stimulation. The levels of this phospholipid also significantly increased when the calcium-ionophore A23187 or platelet-activating factor were used instead of zymosan to stimulate the macrophages. The study of the route involved in the synthesis of PI(20:4/22:6) suggested that this species is produced through deacylation/reacylation reactions. These results define the increases in PI(20:4/22:6) as a novel lipid metabolic marker of mouse macrophage activation, and provide novel information to understand the regulation of phospholipid fatty acid turnover in activated macrophages.

Compartmentalized regulation of lipid signaling in oxidative stress and inflammation: Plasmalogens, oxidized lipids and ferroptosis as new paradigms of bioactive lipid research

Perturbations in lipid homeostasis combined with conditions favoring oxidative stress constitute a hallmark of the inflammatory response. In this review we focus on the most recent results concerning lipid signaling in various oxidative stress-mediated responses and inflammation. These include phagocytosis and ferroptosis. The best characterized event, common to these responses, is the synthesis of oxygenated metabolites of arachidonic acid and other polyunsaturated fatty acids. Major developments in this area have highlighted the importance of compartmentalization of the enzymes and lipid substrates in shaping the appropriate response. In parallel, other relevant lipid metabolic pathways are also activated and, until recently, there has been a general lack of knowledge on the enzyme regulation and molecular mechanisms operating in these pathways. Specifically, data accumulated in recent years on the regulation and biological significance of plasmalogens and oxidized phospholipids have expanded our knowledge on the involvement of lipid metabolism in the progression of disease and the return to homeostasis. These recent major developments have helped to establish the concept of membrane phospholipids as cellular repositories for the compartmentalized production of bioactive lipids involved in cellular regulation. Importantly, an enzyme classically described as being involved in regulating the homeostatic turnover of phospholipids, namely the group VIA Ca²⁺-independent phospholipase A₂ (iPLA₂β), has taken center stage in oxidative stress and inflammation research owing to its key involvement in regulating metabolic and ferroptotic signals arising from membrane phospholipids. Understanding the role of iPLA₂β in ferroptosis and metabolism not only broadens our knowledge of disease but also opens possible new horizons for this enzyme as a target for therapeutic intervention.

Analytical Methods for Determination of Phytic Acid and Other Inositol Phosphates: A Review

From the early precipitation-based techniques, introduced more than a century ago, to the latest development of enzymatic bio- and nano-sensor applications, the analysis of phytic acid and/or other inositol phosphates has never been a straightforward analytical task. Due to the biomedical importance, such as antinutritional, antioxidant and anticancer effects, several types of methodologies were investigated over the years to develop a reliable determination of these intriguing analytes in many types of biological samples; from various foodstuffs to living cell organisms. The main aim of the present work was to critically overview the development of the most relevant analytical principles, separation and detection methods that have been applied in order to overcome the difficulties with specific chemical properties of inositol phosphates, their interferences, absence of characteristic signal (e.g., absorbance), and strong binding interactions with (multivalent) metals and other biological molecules present in the sample matrix. A systematical and chronological review of the applied methodology and the detection system is given, ranging from the very beginnings of the classical gravimetric and titrimetric analysis, through the potentiometric titrations, chromatographic and electrophoretic separation techniques, to the use of spectroscopic methods and of the recently reported fluorescence and voltammetric bio- and nano-sensors.

Phospholipid Arachidonic Acid Remodeling During Phagocytosis in Mouse Peritoneal Macrophages

Macrophages contain large amounts of arachidonic acid (AA), which distributes differentially across membrane phospholipids. This is largely due to the action of coenzyme A-independent transacylase (CoA-IT), which transfers the AA primarily from diacyl choline-containing phospholipids to ethanolamine-containing phospholipids. In this work we have comparatively analyzed glycerophospholipid changes leading to AA mobilization in mouse peritoneal macrophages responding to either zymosan or serum-opsonized zymosan (OpZ). These two phagocytic stimuli promote the cytosolic phospholipase A₂-dependent mobilization of AA by activating distinct surface receptors. Application of mass spectrometry-based lipid profiling to identify changes in AA-containing phospholipids during macrophage exposure to both stimuli revealed significant decreases in the levels of all major choline phospholipid molecular species and a major phosphatidylinositol species. Importantly, while no changes in ethanolamine phospholipid species were detected on stimulation with zymosan, significant decreases in these species were observed when OpZ was used. Analyses of CoA-IT-mediated AA remodeling revealed that the process occurred faster in the zymosan-stimulated cells compared with OpZ-stimulated cells. Pharmacological inhibition of CoA-IT strongly blunted AA release in response to zymosan but had only a moderate effect on the OpZ-mediated response. These results suggest a hitherto undescribed receptor-dependent role for CoA-independent AA remodeling reactions in modulating the eicosanoid biosynthetic response of macrophages. Our data help define novel targets within the AA remodeling pathway with potential use to control lipid mediator formation.

Cellular Plasmalogen Content Does Not Influence Arachidonic Acid Levels or Distribution in Macrophages: A Role for Cytosolic Phospholipase A2γ in Phospholipid Remodeling

Availability of free arachidonic acid (AA) constitutes a rate limiting factor for cellular eicosanoid synthesis. AA distributes differentially across membrane phospholipids, which is largely due to the action of coenzyme A-independent transacylase (CoA-IT), an enzyme that moves the fatty acid primarily from diacyl phospholipid species to ether-containing species, particularly the ethanolamine plasmalogens. In this work, we examined the dependence of AA remodeling on plasmalogen content using the murine macrophage cell line RAW264.7 and its plasmalogen-deficient variants RAW.12 and RAW.108. All three strains remodeled AA between phospholipids with similar magnitude and kinetics, thus demonstrating that cellular plasmalogen content does not influence the process. Cell stimulation with yeast-derived zymosan also had no effect on AA remodeling, but incubating the cells in AA-rich media markedly slowed down the process. Further, knockdown of cytosolic-group IVC phospholipase A₂γ (cPLA₂γ) by RNA silencing significantly reduced AA remodeling, while inhibition of other major phospholipase A₂ forms such as cytosolic phospholipase A₂α, calcium-independent phospholipase A₂β, or secreted phospholipase A₂ had no effect. These results uncover new regulatory features of CoA-IT-mediated transacylation reactions in cellular AA homeostasis and suggest a hitherto unrecognized role for cPLA₂γ in maintaining membrane phospholipid composition via regulation of AA remodeling.

Cytosolic group IVA and calcium-independent group VIA phospholipase A2s act on distinct phospholipid pools in zymosan-stimulated mouse peritoneal macrophages

Phospholipase A2s generate lipid mediators that constitute an important component of the integrated response of macrophages to stimuli of the innate immune response. Because these cells contain multiple phospholipase A2 forms, the challenge is to elucidate the roles that each of these forms plays in regulating normal cellular processes and in disease pathogenesis. A major issue is to precisely determine the phospholipid substrates that these enzymes use for generating lipid mediators. There is compelling evidence that group IVA cytosolic phospholipase A2 (cPLA2α) targets arachidonic acid-containing phospholipids but the role of the other cytosolic enzyme present in macrophages, the Ca(2+)-independent group VIA phospholipase A2 (iPLA2β) has not been clearly defined. We applied mass spectrometry-based lipid profiling to study the substrate specificities of these two enzymes during inflammatory activation of macrophages with zymosan. Using selective inhibitors, we find that, contrary to cPLA2α, iPLA2β spares arachidonate-containing phospholipids and hydrolyzes only those that do not contain arachidonate. Analyses of the lysophospholipids generated during activation reveal that one of the major species produced, palmitoyl-glycerophosphocholine, is generated by iPLA2β, with minimal or no involvement of cPLA2α. The other major species produced, stearoyl-glycerophosphocholine, is generated primarily by cPLA2α. Collectively, these findings suggest that cPLA2α and iPLA2β act on different phospholipids during zymosan stimulation of macrophages and that iPLA2β shows a hitherto unrecognized preference for choline phospholipids containing palmitic acid at the sn-1 position that could be exploited for the design of selective inhibitors of this enzyme with therapeutic potential.

GPR55 regulates cannabinoid 2 receptor-mediated responses in human neutrophils

The directional migration of neutrophils towards inflammatory mediators, such as chemokines and cannabinoids, occurs via the activation of seven transmembrane G protein coupled receptors (7TM/GPCRs) and is a highly organized process. A crucial role for controlling neutrophil migration has been ascribed to the cannabinoid CB(2) receptor (CB(2)R), but additional modulatory sites distinct from CB(2)R have recently been suggested to impact CB(2)R-mediated effector functions in neutrophils. Here, we provide evidence that the recently de-orphanized 7TM/GPCR GPR55 potently modulates CB(2)R-mediated responses. We show that GPR55 is expressed in human blood neutrophils and its activation augments the migratory response towards the CB(2)R agonist 2-arachidonoylglycerol (2-AG), while inhibiting neutrophil degranulation and reactive oxygen species (ROS) production. Using HEK293 and HL60 cell lines, along with primary neutrophils, we show that GPR55 and CB(2)R interfere with each other's signaling pathways at the level of small GTPases, such as Rac2 and Cdc42. This ultimately leads to cellular polarization and efficient migration as well as abrogation of degranulation and ROS formation in neutrophils. Therefore, GPR55 limits the tissue-injuring inflammatory responses mediated by CB(2)R, while it synergizes with CB(2)R in recruiting neutrophils to sites of inflammation.

The glycerophospho metabolome and its influence on amino acid homeostasis revealed by brain metabolomics of GDE1(-/-) mice

GDE1 is a mammalian glycerophosphodiesterase (GDE) implicated by in vitro studies in the regulation of glycerophophoinositol (GroPIns) and possibly other glycerophospho (GroP) metabolites. Here, we show using untargeted metabolomics that GroPIns is profoundly (>20-fold) elevated in brain tissue from GDE1(-/-) mice. Furthermore, two additional GroP metabolites not previously identified in eukaryotic cells, glycerophosphoserine (GroPSer) and glycerophosphoglycerate (GroPGate), were also highly elevated in GDE1(-/-) brains. Enzyme assays with synthetic GroP metabolites confirmed that GroPSer and GroPGate are direct substrates of GDE1. Interestingly, our metabolomic profiles also revealed that serine (both L-and D-) levels were significantly reduced in brains of GDE1(-/-) mice. These findings designate GroPSer as a previously unappreciated reservoir for free serine in the nervous system and suggest that GDE1, through recycling serine from GroPSer, may impact D-serine-dependent neural signaling processes in vivo.

Phospholipase-dependent signalling during the AvrRpm1- and AvrRpt2-induced disease resistance responses in Arabidopsis thaliana

Bacterial pathogens deliver type III effector proteins into plant cells during infection. On susceptible host plants, type III effectors contribute to virulence, but on resistant hosts they betray the pathogen to the plant's immune system and are functionally termed avirulence (Avr) proteins. Recognition induces a complex suite of cellular and molecular events comprising the plant's inducible defence response. As recognition of type III effector proteins occurs inside host cells, defence responses can be elicited by in planta expression of bacterial type III effectors. We demonstrate that recognition of either of two type III effectors, AvrRpm1 or AvrRpt2 from Pseudomonas syringae, induced biphasic accumulation of phosphatidic acid (PA). The first wave of PA accumulation correlated with disappearance of monophosphatidylinosotol (PIP) and is thus tentatively attributed to activation of a PIP specific phospholipase C (PLC) in concert with diacylglycerol kinase (DAGK) activity. Subsequent activation of phospholipase D (PLD) produced large amounts of PA from structural phospholipids. This later wave of PA accumulation was several orders of magnitude higher than the PLC-dependent first wave. Inhibition of phospholipases blocked the response, and feeding PA directly to leaf tissue caused cell death and defence-gene activation. Inhibitor studies ordered these events relative to other known signalling events during the plant defence response. Influx of extracellular Ca(2+) occurred downstream of PIP-degradation, but upstream of PLD activation. Production of reactive oxygen species occurred downstream of the phospholipases. The data presented indicate that PA is a positive regulator of RPM1- or RPS2-mediated disease resistance signalling, and that the biphasic PA production may be a conserved feature of signalling induced by the coiled-coil nucleotide binding domain leucine-rich repeat class of resistance proteins.

A23187 causes release of inositol phosphates from cultured rat Kupffer cells

The Ca2+ ionophore A23187 is routinely used to illustrate the extracellular Ca2+-dependence of a variety of cellular reactions. We found that A23187-induced hydrolysis of phosphoinositides to various inositol phosphates in rat Kupffer cells was accompanied by their release from the cells. The synthesis and release of inositol phosphates was A23187 concentration-dependent (0.5-10 microM), and was apparent at the lowest concentration tested. A23187-induced release of inositol phosphates increased time-dependently, was apparent at 5 s of stimulation and maximal at 20 min. The effects of A23187 were reversed by EGTA. The integrity of the cells was not affected by A23187 treatment as indicated by their exclusion of trypan blue and the lack of release of lactate dehydrogenase. We propose that such effects should be considered while evaluating the Ca2+-dependence of biological processes based on the actions of A23187.

High-performance thin-layer chromatography method for inositol phosphate analysis

A simple and inexpensive high-performance thin-layer chromatography (HPTLC) method for the analysis of inositol mono- to hexakisphosphates on cellulose precoated plates is described. Plates were developed in 1-propanol-25% ammonia solution-water (5:4:1) and substance quantities as low as 100-200 pmol were detected by molybdate staining. Chromatographic mobilities of nucleotides and phosphorylated carbohydrates were also characterized. Charcoal treatment was employed to separate nucleotides from inositol phosphates with similar R(F) values prior to HPTLC analysis. Practical application of the HPTLC system is demonstrated by analysis of grain extracts from wild type and low-phytate mutant barley as well as phytate degradation products resulting from barley phytase activity.

Role of intracellular second messengers and reactive oxygen species in the pathophysiology of V. cholera O139 treated rabbit ileum

Vibrio cholerae O139 has pandemic potential and it produces copious amounts of fluid secretion. The levels of various second messengers (intracellular Ca2+, cAMP, IP3, PKC) were measured to determine the cause of fluid secretion produced by this strain of V. cholerae. There was a significant increase in the levels of these second messengers in V. cholerae O139 treated ileum as compared to control ileum (enterocytes). Levels of these second messengers were also assessed in V. cholerae 569B induced fluid secretion in rabbit ileum and it was found that the levels were raised more in V. cholerae O139 treated ileum than in V. cholerae 569B treated rabbit ileum. The intestinal damage was assessed by measuring changes in the extent of lipid peroxidation of the enterocytes. Intracellular second messengers are known to raise the extent of lipid peroxidation. In V. cholerae O139 treated loops calcium ionophore A23187 enhanced the extent of lipid peroxidation whereas l-verapamil could only marginally decrease the lipid peroxidation. Dantrolene and H7 significantly decreased the extent of lipid peroxidation of enterocytes in V. cholerae O139 treated rabbit ileum. However, PMA could not enhance further the extent of lipid peroxidation in V. cholerae O139 treated rabbit ileum. So intracellular calcium and protein kinase C appear to be involved in intestinal damage caused by V. cholerae O139. Reactive oxygen species are responsible for causing tissue damage and the extent of oxidative damage depends on the balance between the pro-oxidants and the anti-oxidants. So the changes in the enterocytes' antioxidant level during V. cholerae O139 mediated intestinal infection was estimated. There was a significant decrease in the enterocyte level of the antioxidant enzymes SOD, catalase, glutathione peroxidase, glutathione reductase, glutathione transferase and glucose-6-phosphate dehydrogenase in V. cholerae O139 mediated intestinal infection. So a significant decrease in the levels of antioxidant defenses and a significant increase in the levels of second messengers appear to be important in mediating V. cholerae O139 induced lipid peroxidation which contributes to the changes in membrane permeability and thus to fluid secretion.

Antimalarial drugs inhibit phospholipase A2 activation and induction of interleukin 1beta and tumor necrosis factor alpha in macrophages: implications for their mode of action in rheumatoid arthritis

1. The effects of antimalarial drugs on the intracellular signaling leading to activation of the phospholipase C and phospholipase A2 pathways and the induction of proinflammatory cytokines have been studied in mouse macrophages. 2. Both chloroquine and quinacrine, and to a lesser extent hydroxychloroquine, inhibited arachidonate release and eicosanoid formation induced by phorbol diester. This inhibition was due to that of the activation of the arachidonate-mobilizing phospholipase A2. 3. All three antimalarials potently inhibited arachidonate release induced by zymosan. They also inhibited the zymosan-induced formation of inositol phosphates, which hints that an inhibitory effect at the phospholipase C level might explain the inhibition of the response to zymosan. 4. Quinacrine, and to a lesser extent chloroquine, has an inhibitory effect on the lipopolysaccharide- or zymosan-induced expression of interleukin 1beta and tumor necrosis factor alpha, both at the mRNA and protein levels. This, in particular, has important implications for the mode of action of these compounds in rheumatoid arthritis.

The mechanisms of action of disease-modifying antirheumatic drugs: a review with emphasis on macrophage signal transduction and the induction of proinflammatory cytokines

1. Rheumatoid arthritis (RA) is probably the most common source of treatable disability. A major problem in modern rheumatology is that the mechanism(s) of action of the currently used disease-modifying antirheumatic drugs (DMARDs) remain unclear. Many of these drugs entered rheumatology mainly through clinical intuition and have been used for decades. 2. The former T-cell-centered paradigm of rheumatoid inflammation has given way to a model of inflammation highlighting the macrophage and its proinflammatory cytokines. In particular, tumor necrosis factor alpha (TNF-alpha) has gained prominence as a central proinflammatory mediator in RA, and antibodies against TNF-alpha have been successfully used in patients with RA. 3. This review will summarize the recent advances in determining the mechanisms of action of the currently used DMARDs, with particular emphasis on their effects on the induction of TNF-alpha and interleukin 1 (IL-1) in mononuclear phagocytes. Although some DMARDs, such as auranofin, antimalarials and tenidap, act as inhibitors of the induction of these cytokines in monocytes or macrophages or both, other drugs, such as methotrexate, D-penicillamine and aurothiomalate, do not seem to affect either TNF-alpha or IL-1. 4. The drugs' effects on proinflammatory cytokine induction are correlated to those on other macrophage responses.

Calcium and protein kinase C play a significant role in response to Shigella toxin in rabbit ileum both in vivo and in vitro

The role of second messengers in Shigella toxin (STx) induced fluid secretion in rabbit ileum was evaluated. In vivo and in vitro studies were carried out in presence or absence of following modulators: Ca2+ ionophore A23187 (15 microM), l-verapamil (200 microM), phorbol-12-myristate-13-acetate (PMA, 200 ng), 1-(5-isoquinolinyl-sulphonyl)-2-methyl-piperazine (H-7, 15 microg) and indomethacin (20 microM). In in vivo studies, the fluid accumulation into rabbit ileal loops in response to STx was measured in presence or absence of these modulators. In in vitro studies, unidirectional fluxes of Na+ and Cl- were carried out in presence or absence of these modulators. The addition of Ca2+ ionophore A23187 along with STx further increases the amount of fluid already induced by STx. Whereas the presence of l-verapamil along with STx did not decrease the amount of fluid induced by STx. In vitro findings were in consonance with the in vivo studies. A significant increase in inositol triphosphate (IP3) levels was observed in enterocytes isolated from STx treated rabbit ileum. The addition of PMA into rabbit ileal loops in presence of STx mimicked the effect of STx while the presence of H-7 reversed the secretion caused by STx to absorption. Similar results were obtained while determining unidirectional fluxes of Na+ and Cl- in presence of PMA and also with H-7. A significant increase in PKC levels was observed in the membrane fraction of enterocytes isolated from STx treated rabbit ileum as compared to control. Further a marked decrease in PKC levels was observed in the presence of H-7 in membrane fraction of enterocytes isolated from STx treated rabbit ileum. The addition of indomethacin into rabbit ileal loops reversed the secretion (caused by STx) to absorption. In vitro findings were in consonance with in vivo studies. Besides, there was a significant increase in PG-E levels in enterocytes isolated from STx treated rabbit ileum as compared to control. These findings suggested that STx induced enteritis involves the role of PKC, intracellular calcium stores and prostaglandins. The extracellular calcium pool probably does not play a significant role in this process.

Positive charge at position 549 is essential for phosphatidylinositol 4,5-bisphosphate-hydrolyzing but not phosphatidylinositol-hydrolyzing activities of human phospholipase C delta1

Point mutagenesis, phosphatidylinositol (PI), and phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis assays and equilibrium centrifugation PIP2 assays were used to study the functional roles of four highly conserved arginine residues in the Y region of human phospholipase C delta1 (PLCdelta1) (Arg-527, -549, -556, -701). Most of the mutant enzymes were either partially defective or fully active in their abilities to catalyze the hydrolysis of PI or PIP2. However, upon substitution of Arg-549 by glycine or histidine, the mutant enzyme was defective in its ability to catalyze the hydrolysis of PIP2, but it is still able to hydrolyze PI. Replacing Arg-549 with lysine had little effect on the level of PI and PIP2 hydrolytic activities of the mutant enzyme. The residual PIP2 hydrolyzing activity of R549H is highly dependent on pH. R549H showed 5-10% of the PIP2-hydrolyzing activity of the native enzyme between pH 5 and 7 and nondetectable PIP2-hydrolyzing activity at pH 8. The PIP2-hydrolyzing activity of R549G was not detectable at all pH values. Kinetic analysis of PLCdelta1-catalyzed PIP2 hydrolysis revealed that the micellar dissociation constant Ks and interfacial Michaelis constant Km were similar in the native, R549K, and R549H enzymes; but the specific activity at the saturated substrate mole fraction and infinite level of substrate (Vmax) of the R549H mutant were reduced by a factor of 15. PIP2 competitively inhibits the native enzyme to hydrolyze PI at both pH 7 and 8. However, PIP2 inhibits R549H only at pH 7.0 and does not inhibit R549G at either pH. Taken together, these results suggest that positive charge at position 549 of PLCdelta1 protein is essential for the enzyme to recognize and catalyze the hydrolysis of PIP2 but not PI.

Involvement of secretory phospholipase A2 activity in the zymosan rat air pouch model of inflammation

1. In the zymosan rat air pouch model of inflammation we have assessed the time dependence of phospholipase A2 (PLA2) accumulation in the inflammatory exudates as well as cell migration, myeloperoxidase activity, prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) levels. 2. A significant increase in PLA2 activity was detected in 1,200 g supernatants of exudates 8 h after injection of zymosan into rat air pouch. This event coincided with peaks in cell accumulation (mainly neutrophils) and myeloperoxidase activity in exudates and was preceded by a rise in eicosanoid levels. 3. This enzyme (without further purification) behaved as a secretory type II PLA2 with an optimum pH at 7-8 units, lack of selectivity for arachidonate release and dependence on mM calcium concentrations for maximal activity. 4. The PLA2 inhibitors manoalide and scalaradial inhibited this enzyme activity in vitro in a concentration-dependent manner. Scalaradial also inhibited zymosan stimulated myeloperoxidase release in vitro. 5. Injection of the marine PLA2 inhibitor scalaradial together with zymosan into the pouch at doses of 0.5, 1 and 5 mumol per pouch resulted in a dose-dependent inhibition of PLA2 activity in exudates collected 8 h later. Myeloperoxidase levels and cell migration were also decreased, while eicosanoid levels were not modified. 6. Colchicine administration to rats prevented infiltration and decreased PLA2 levels in the 8 h zymosan-injected air pouch. 7. These results indicate that during inflammatory response to zymosan in the rat air pouch a secretory PLA2 activity is released into the exudates. The source of this activity is mainly the neutrophil which migrates into the pouch. 8. Scalaradial exerts anti-inflammatory effects in the zymosan air pouch.

Quantitative chromatographic analysis of inositol phospholipids and related compounds

The metabolism of phospholipids and the mobilization of second messengers such as inositol-1,4,5-trisphosphate, 1,2-diacylglycerol (DAG) and arachidonic acid (AA) from phospholipids is commonly studied by radiolabelling phospholipids with [3H]myo-inositol or [32P]ATP and measuring the incorporation of radioactivity in different phospholipids or their hydrolysis products. However, for the radiolabelling method to accurately reflect changes in the compound's mass, it is essential that the tissue is labelled to isotopic equilibrium which is difficult to achieve. To circumvent the disadvantages of the radiolabelling method, several analytical procedures have been developed for the mass analysis of phospholipids and inositolphosphates (IPs). Quantitation of the mass or the radiolabelling of phospholipids is a complex multi-step procedure that involves quantitative isolation of phospholipids, fractionation of individual phospholipids and either determination of radioactivity in each component or the measurement of their mass. Phospholipids, DAG and AA are extracted from tissue sample with organic solvents such as chloroform-methanol (2:1) containing HCl or formic acid. The extract is separated by TLC, cartridge-column chromatography or HPLC on a reversed-phase column. Phospholipids are quantitated by measuring inorganic phosphate, absorption at 200 nm or mass spectrometry. Inositol phosphates are extracted with perchloric acid or trichloroacetic acid and separated by ion-exchange cartridge-column or HPLC with an ion-exchange column. IPs are quantitated by measuring inorganic phosphate or by using enzymatic reaction, metal-dye coupling, NMR or mass spectrometry.

Effects of tenidap on Ca(2+)- and protein kinase C-mediated protein phosphorylation, activation of the arachidonate-mobilizing phospholipase A2 and subsequent eicosanoid formation in macrophages

Tenidap is a novel antirheumatic drug which combines non-steroidal antiinflammatory drug-like cyclooxygenase inhibition with cytokine modulating qualities in rheumatoid arthritis. We show herein that tenidap (5-20 microM) inhibited protein kinase C-mediated signalling leading to release of arachidonate in mouse macrophages by interfering with the up-regulation of the 85 kDa arachidonate-mobilizing phospholipase A2, although it did not inhibit this enzyme directly. The Ca(2+)-mediated activation of arachidonate mobilization was inhibited only at higher concentrations (20-40 microM). Studies of protein phosphorylation indicated that tenidap in itself was capable of inducing the phosphorylation of several protein bands through interaction with intracellular protein kinases and/or phosphatases. Importantly, tenidap inhibited both arachidonate release and the increase in intracellular protein phosphorylation when the cells were stimulated with zymosan. We propose that the main inhibitory influence of tenidap on the macrophage signalling investigated here is exerted at some level between protein kinase C and the 85 kDa phospholipase A2 and quite possibly also at the receptor-linked activation of phospholipase C.

Involvement of intracellular calcium stores in Giardia lamblia induced diarrhoea in mice

The transmucosal fluxes of Na+ and Cl- were studied in Giardia lamblia-infected mice in the presence or absence of dantrolene (1-(5(p-nitrophenyl)furfurilidene-amino) hydantoin sodium hydrate). There was net secretion of Na+ and Cl- in infected animals, while in control animals there was net absorption of these ions. The addition of dantrolene resulted in significant net increase in absorption of Na+ and Cl- in control and experimental groups. Further, mouse intestinal epithelial cells were labelled with [32P]Pi and then treated with G. lamblia trophozoites and their excretory secretory products separately. The optimum time for inositol triphosphate formation was 15 min in control enterocytes as well as in treated enterocytes. A plateau was formed at higher concentrations. Since raised inositol triphosphate levels mobilize Ca2+ from intracellular stores and dantrolene traps Ca2+ within intracellular calcium stores, the present study thus suggests that intracellular calcium stores are involved in G. lamblia-induced diarrhoea in mice.

Stimulation of inositide degradation in clumping Stigmatella aurantiaca

Numerous external signals which activate inositol phospholipid hydrolysis in eukaryotes are known; probably all of these signals are transduced by G proteins. So far, neither signal-transducing G protein nor receptor-regulated phospholipase C has been found in prokaryotes. However, a group of bacteria, the myxobacteria, displays cellular and tissue-like differentiation; therefore, it appeared that a search for the various activities involved in a signal-activated phosphatidylinositol cycle might be rewarding. Here, we report that in Stigmatella aurantiaca, under conditions which promote clumping, inositol phospholipid synthesis and degradation were stimulated with the resulting formation of inositol phosphate and inositol bisphosphate. The turnover was Ca2+ dependent and was increased by fluoride ions. Membrane preparations from these cells showed a phospholipase C activity which increased with the stage of incubation and which was stimulated by GTP gamma S, suggesting G protein dependency. To what extent this system in a prokaryotic cell shares properties of the phosphatidylinositol cycle in eukaryotes remains unexamined.

Endothelin-1-induced phosphoinositide hydrolysis and contraction in isolated rabbit detrusor and urethral smooth muscle

1. Endothelin-1 (ET-1) caused a concentration-dependent increase in the formation of inositol phosphates (IPs) in isolated rabbit detrusor and urethral smooth muscle preparations prelabelled with myo-[3H]inositol. 2. The increase in accumulation of IPs was slow in onset in both detrusor and urethra, with no significant accumulation demonstrable during the first 30 min. The increase in IPs accumulation found after exposure of detrusor tissue to ET-1 (10(-7) M) for 2 hr (250 +/- 38%, n = 7) was not significantly different from that found in the urethra (279 +/- 40%, n = 6), when expressed as per cent of corresponding control values. 3. Pretreatment with nifedipine (10(-6) M) did not reduce IPs formation. In contrast, no increase in IPs formation was demonstrated in Ca(2+)-free medium. 4. ET-1 (10(-11)-10(-7) M) produced concentration-dependent, slowly developing contractions in both detrusor and urethral preparations. Pretreatment with H-7 (3 x 10(-5) M) for 30 min before ET-1 application resulted in a non-parallel shift of the ET-1 concentration-response curve with significant reductions in maximal responses in both tissues. 5. ET-1-induced contractions in urethral preparations were markedly inhibited by Ni2+ (3 x 10(-4) M), whereas the effect of Ni2+ in the detrusor was less pronounced. 6. The results suggest that ET-1 stimulates phosphoinositide hydrolysis in the rabbit detrusor and urethra. Both IPs formation and contractile activation evoked by ET-1 are dependent on extracellular Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Phospholipase C activation by endothelin-1 and noradrenaline in isolated penile erectile tissue from rabbit

The effects of endothelin-1 and noradrenaline on phospholipase C activity in the rabbit isolated corpus cavernosum were investigated by measuring the accumulation of inositol phosphates. Both endothelin-1 and noradrenaline caused a time- and concentration-dependent increase in the accumulation of 3H-inositol phosphates in preparations prelabelled with 3H-myo-inositol. The reaction was slow in onset with no significant accumulation of 3H-inositol phosphates, including inositol trisphosphate, demonstrable during the first 15 minutes. At 60 minutes, the mean increases in 3H-inositol inositol phosphates induced by 3 x 10(-7) M endothelin-1 and 10(-3) M noradrenaline amounted to 341 and 530% of time-matched controls, respectively. However, when given at concentrations having the same contractile amplitude on rabbit corpus cavernosum, there was no difference in the amounts of 3H-inositol phosphates generated by endothelin-1 and noradrenaline. Prazosin (10(-6) M) significantly inhibited the stimulatory effect of noradrenaline on phosphoinositide hydrolysis. Pretreatment with 10(-6) M nimodipine did not reduce the increases in 3H-inositol phosphates induced by 3 x 10(-7) M endothelin-1 and 10(-3)M noradrenaline. Also in Ca(2+)-free medium, both agonists had significant stimulatory effects on phosphoinositide turnover, although under this condition, the responses were greatly reduced. The results suggest that exogenous endothelin-1 and noradrenaline activate phospholipase C in corpus cavernosum, and that this mechanism is partly independent of extracellular Ca2+. Considering the slow onset of action, phospholipase C activation is probably not directly involved in rapid contractile events, but may be of importance in the long-term regulation of penile smooth muscle tone.

Endothelin-1-induced contractions of isolated pig detrusor and vesical arterial smooth muscle: calcium dependence and phosphoinositide hydrolysis

1. In isolated pig detrusor and vesical arterial smooth muscle preparations, endothelin-1 (ET-1) caused concentration-dependent contractions. Nifedipine (10(-6) M) did not significantly affect the action of ET-1 in the vessels, but almost abolished its effect in the detrusor. Incubation for 30 min in Ca(2+)-free solution markedly reduced the ET-1-induced contractions in both detrusor and vesical arteries. 2. The protein kinase C inhibitor H-7 (3 x 10(-5) M), reduced the response to ET-1 in detrusor muscle as well as in vessels, and abolished the contractions evoked by ET-1 in Ca(2+)-free solution. 3. ET-1 caused an increase in the accumulation of inositol phosphates (IPs) in preparations prelabelled with myo-[3H]inositol. After exposure to ET-1 (10(-7) M) for 60 min, an approx. 4-fold increase in IPs levels were demonstrated, compared to untreated controls, in both detrusor and vessel preparations. Pretreatment with nifedipine (10(-6) M) did not reduce IPs formation. In contrast, no increase in IPs formation was demonstrated in Ca(2+)-free medium. 4. The increase in accumulation of IPs was slow in onset in both detrusor and vesical arteries, with no significant accumulation demonstrable during the first 30 min. Time-course studies of tension development for ET-1 revealed that maximum tension was reached before significant levels of IPs could be detected.

Production of inositol phosphates and reactive oxygen metabolites in quartz-dust-stimulated human polymorphonuclear leukocytes

The present paper explores phosphoinositide turnover in quartz-stimulated human polymorphonuclear leukocytes. Separation of inositol phosphates was carried out with a new ion-pair, reverse-phase high performance liquid chromatographic method applying a gentle tetrabutyl ammonium phosphate buffer gradient. The method separates inositol monophosphates, inositol 1,4-bisphosphate, inositol trisphosphates and inositol 1,3,4,5-tetrakisphosphate. Reactive oxygen metabolites, indices for leukocyte activation, were measured with a luminometric assay. Quartz increased the production of reactive oxygen metabolites, preceded by facilitated inositol phosphate turnover. This finding provides evidence that inositol phosphate second messengers may be involved in quartz-induced leukocyte activation and subsequent production of reactive oxygen metabolites.

Protein-kinase-C-independent activation of arachidonate release and prostaglandin E2 formation in macrophages interacting with certain bacteria

Certain bacterial species, of which we selected Fusobacterium nucleatum, Gardnerella vaginalis, Peptostreptococcus anaerobius and Propionibacterium acnes, were found to induce release of arachidonic acid in a dose- and time-dependent manner in mouse macrophages. The release of arachidonic acid showed a characteristic lag period of approximately 10 min and was accompanied by selective transformation into prostaglandin E2. Bacteria killed by various methods caused a similar response, indicating that bacterial surface structures rather than secreted products were involved. Down-regulation of protein kinase C by treatment of macrophages with 4 beta-phorbol 12-myristate 13-acetate hardly affected the response to bacteria at all, except for a partial inhibition in the case of P. acnes. Furthermore, the generation of prostaglandin E2 was synergistically enhanced when macrophages were exposed to both bacteria and phorbol ester. It is also unlikely that bacterial activation was mediated exclusively via a rise in cytosolic [Ca2+], since bacteria stimulated the release of arachidonic acid also when [Ca2+] was clamped at various levels and since the response to bacteria was enhanced in an additive to synergistic manner when combined with calcium ionophore. Changes in protein phosphorylation in macrophages exposed to F. nucleatum (Gram-negative) were virtually identical to those seen with bacterial lipopolysaccharide, while P. anaerobius (Gram-positive) induced enhanced labeling of a single detectable phosphoprotein. In both cases, the changes in protein phosphorylation showed a time lag of 4-8 min and occurred independently of protein kinase C, consistent with a possible role in signal transduction. These results demonstrate that certain bacteria cause activation of arachidonic acid release and prostaglandin E2 formation in mouse macrophages; that the response is independent of protein kinase C and that it is not wholly mediated via a rise in cytosolic [Ca2+].

Ca(2+)-independent phospholipase A2 activity associated with secretory granular membranes in rat parotid gland

Phospholipase A2 activity was detected in a secretory granular fraction (SG) purified by Percoll gradient centrifugation from rat parotid gland using [3H]phosphatidylcholine (PC) as a substrate. High activity of this enzyme was observed at neutral pH. The enzyme was activated by Triton X-100 and did not require Ca2+ for its activity. In the absence of Ca2+, its apparent Km for exogenous PC was 28 microM while it was slightly increased by adding 5 mM CaCl2 (73 microM). Furthermore, the enzyme was located essentially in a granular membrane fraction separated from granular lysate. The deacylation activities were also detected in other subcellular fractions, which showed a different detergent-susceptibility or pH-dependency from that in SG. These results suggest that secretory granules have membrane-bound phospholipase A2 which has properties different from that found in other organelles.

Colony stimulating factor-1 stimulates diacylglycerol generation in murine bone marrow-derived macrophages, but not in resident peritoneal macrophages

Colony stimulating factor-1 (CSF-1) stimulates DNA synthesis in murine bone marrow-derived macrophages (BMM); however, unlike BMM, murine resident peritoneal macrophages (RPM) undergo a poor proliferative response. It has previously been shown that phosphatidylinositol-4,5-bisphosphate hydrolysis is not associated with CSF-1 action in BMM. In this report we demonstrate that, despite a lack of inositol trisphosphate generation, CSF-1 transiently elevated both [3H]myristoyl- and [3H]arachidonyl-diacylglycerol (DAG) in BMM in a dose-dependent fashion. CSF-1 failed, however, to stimulate an increase in either species of DAG in RPM. Thus, DAG could be a second messenger for the proliferative action of CSF-1 in macrophages. Other mitogenic agents, 12-0-tetradecanoyl phorbol 13-acetate (TPA) and exogenous phospholipase C, also increased BMM levels of [3H]myristoyl- and [3H]arachidonyl-DAG. The nonmitogenic agents, lipopolysaccharide (LPS), tumor necrosis factor-alpha (TNF-alpha) and zymosan, had different effects on the generation of either species of DAG in BMM. LPS failed to elevate either form, TNF-alpha increased only [3H]arachidonyl-DAG, while zymosan stimulated levels of both species of DAG. It therefore appears that increased diacylglycerol generation may be necessary, but perhaps not sufficient, for macrophage proliferation.

Biochemical signal transduction of mechanical strain in osteoblast-like cells

The responses to mechanical loading of two types of osteoblast-like cells and skin fibroblasts were investigated using two new devices for applying defined and homogeneous strains to cells. The results indicate that only periostal (bone surface) osteoblasts are sensitive to strains within the physiological range and that a specific strain mechanism is responsible. Osteoblasts derived from the haversian system and skin fibroblasts do not respond except at higher, unphysiological strains. The mechanism is located in the cytoskeleton and activates the membrane phospholipase C within milliseconds and may react to distension of a strain sensitive protein. Activation of phospholipase C can account for only some of the observed responses of bone to mechanical loading such as stimulation of cell division, increase in collagen and collagenase production. Application of over 10,000 mu strains results in a de-differentiation of the osteoblasts and a change in cell morphology to become fibroblast-like.

Tumor necrosis factor-alpha stimulates phosphatidylinositol breakdown by phospholipase C to coordinately increase the levels of diacylglycerol, free arachidonic acid and prostaglandins in an osteoblast (MC3T3-E1) cell line

The effects of (human recombinant) tumor necrosis factor-alpha on phosphatidylinositol breakdown, release of 1,2-diacylglycerols, mobilization of arachidonate from diacylglycerol and prostaglandin synthesis were examined in a model osteoblast cell line (MC3T3-E1). Tumor necrosis factor-alpha (10 nM) caused a specific (30%) decrease in the mass of phosphatidylinositol (and no other phospholipids) within 30 min of exposure. Tumor necrosis factor-alpha doubled the rate of incorporation of [32P]orthophosphoric acid into phosphatidylinositol, indicating that the turnover of inositol phosphate was enhanced, and increased the content of diacylglycerol in parallel with phosphatidylinositol breakdown. The cytokine (10-50 nM; 4 h) also promoted a specific release of 24-34% of the [3H]arachidonate from prelabeled phosphatidylinositol, a release of 80% of the 3H-fatty acid from the diacylglycerol pool, and a 30-fold increase in the synthesis of prostaglandin E2. The tumor necrosis factor-alpha induced liberation of [3H]arachidonate from diacylglycerol, cellular arachidonate release and the synthesis of prostaglandin E2 were each blocked by an inhibitor of diacylglycerol lipase, the compound RHC 80267 (30 microM). Therefore, we conclude that, in the MC3T3-E1 cell line, tumor necrosis factor-alpha activates a phosphatidylinositol-specific phospholipase C (phosphatidylinositol inositolphosphohydrolase; EC 3.1.4.3) to release diacylglycerol, and increases the metabolism of diacylglycerol to liberate arachidonate for prostaglandin synthesis.

Membrane-associated phosphoinositidase C activity in Dictyostelium discoideum

Membrane-associated phosphoinositidase C activity has been identified in Dictyostelium discoideum using phosphatidylinositol 4,5-bisphosphate as exogenous substrate. Maximal activity was observed with 0.4 mM phosphatidylinositol 4,5-bisphosphate at pH 7.0. The enzyme was stimulated by micromolar concentrations of free calcium with maximal activity at 100 microM.

The ratio of macrophage prostaglandin and leukotriene synthesis is determined by the intracellular free calcium level

The induction of eicosanoid synthesis in various cell types by different physiological stimuli is dependent on an increase in the intracellular calcium level and stimulation of the protein kinase C (PKC). In a model system this can be mimicked by using calcium ionophores and direct PKC activators. In mouse peritoneal macrophages calcium ionophores induced the formation of prostaglandin E2 (PGE2) and leukotriene C4 (LTC4). A synergistic enhancement of both eicosanoids could be achieved by simultaneous addition of the calcium ionophore A23187 together with a suboptimal dose of the direct protein kinase C activator 12-O-tetradecanoylphorbol 13-acetate (TPA). Low concentrations of the ionophore, resulting in only marginally increased intracellular calcium levels, led to a more than additive prostaglandin E2 production in combination with TPA. Higher concentrations of A23187 together with TPA favoured LTC4 synthesis, whereas PGE2 levels at the same time were even diminished. This observed shift from prostaglandin to leukotriene formation was amplified by simultaneous addition of indomethacin. Manganese as inhibitor of the A23187-induced calcium influx decreased PGE2 synthesis. On the other hand, in the presence of manganese LTC4 production was also inhibited at high concentrations of A23187 but elevated in the absence or at low doses of A23187. Our data provide evidence that in macrophages the ratio of cyclooxygenase and lipoxygenase products caused by mediators, acting via the phospholipase C or D/PKC signal transduction pathway, is regulated by the extent of the intracellular calcium increase.

A glycerol ether induces mobilization and 12-lipoxygenation of arachidonic acid in macrophages. Synergistic effect on mobilization and induction of leukotriene C formation by activators of protein kinase C

A glycerol triether, 1,2-isopropylidene 3-0-decanyl-sn-glycerol, was found to induce mobilization of arachidonic acid from ethanolamine phosphoglycerides and phosphatidylinositol in mouse peritoneal macrophages. This effect showed structural specificity, occurred without activation of protein kinase C and resulted in formation and release of predominantly 12-hydroxy-eicosatetraenoic acid. Activators of kinase C (4-beta-phorbol 12-myristate 13-acetate and 1,2-dioctanoyl-sn-glycerol) instead specifically enhance prostaglandin E2 formation. When macrophages were exposed to both a kinase C activator and the glycerol triether, the mobilization of arachidonic acid was synergistically enhanced and formation of leukotriene C was induced.

Retinoic acid rapidly decreases phosphatidylinositol turnover during neuroblastoma cell differentiation

Phosphatidylinositol (PI) turnover has recently been implicated in the regulation of cell proliferation and transformation. We have investigated its role in differentiation using LAN-1 cells, a human neuroblastoma cell line that can be induced to differentiate along the neuronal pathway by retinoic acid (RA). We have found that treatment of LAN-1 cells with RA is followed by a rapid decrease of inositol phospholipid metabolism, using myo-[1,2-3H]inositol or [1(3)-3H]glycerol. No changes were observed in both [3H]inositol and [3H]glycerol uptake within 24 h of RA treatment. Decreased incorporation of the metabolic precursor into PI 4-monophosphate and PI 4,5-bisphosphate occurred within 1 h of RA treatment. No changes were seen in the specific radioactivity of the precursor pools up to 1 h of treatment with RA. Analysis of labeled PI metabolites from prelabeled cells indicated a rapid decrease of inositol 1,4,5-trisphosphate and 1,2-diacylglycerol content within 1 min of induction of LAN-1 cell differentiation. These findings constitute the earliest reported events in neuroblastoma cell differentiation.

Phosphatidylinositol hydrolysis by human plasma phospholipase D

A phospholipase D activity able to hydrolyze phosphatidylinositol has previously been described in the cytosol of human neutrophils. The experiments reported here demonstrate that this phosphatidylinositol-hydrolyzing phospholipase D activity is also present in human plasma. This activity was assessed by free inositol release from phosphatidylinositol substrate, by phosphatidate formation and by phosphatidylethanol formation through its capacity of catalyzing a transphosphatidylation reaction. This plasma enzyme activity shows an optimum pH of 8.0 and is inhibited by EGTA.

Pyruvate dehydrogenase activation by insulin in human circulating lymphocytes and the possible pathway involved

1. The incubation of human fresh circulating lymphocytes with insulin leads to modifications in the behaviour of the pyruvate dehydrogenase complex (PDH) when the contact medium is supplemented with 50 microM Ca2+ and Mg2+. 2. To investigate the mechanism involved in the PDH responsiveness to insulin in circulating lymphocytes and the role of Ca2+ and Mg2+ in this process, the PDH activity was assayed in lymphocytes combined with insulin and/or a number of substances whose mechanism of action is partially known. 3. Of these some have been seen to mimick insulin effects on PDH, whereas other were tested for the first time in this study.

Methods for the analysis of inositol phosphates

Interest in the inositol phospholipids was stimulated by the simultaneous discoveries that the products of hydrolysis of these lipids could serve as messengers to activate to synergistic signaling pathways in hormonally responsive cells, namely, inositol 1,4,5-trisphosphate which causes the release of Ca2+ from intracellular stores and diacylglycerol which promotes the activation of protein kinase C. At the same time, Berridge and co-workers introduced relatively simple approaches to study the inositol phospholipid cycle. These included the use of [3H]inositol to label the inositol metabolites, all of which are confined to this cycle, and of Li+ to decrease the rate of degradation of the inositol phosphates. Water-soluble inositol phosphates and chloroform-soluble inositol phospholipids could then be separated by solvent partition and the inositol phosphates further separated by use of an anion-exchange resin. However, the subsequent application of high-performance liquid chromatography as a separation technique indicated the existence of many isomers of the inositol phosphates formed by different pathways of dephosphorylation and phosphorylation. Mapping of these metabolic pathways may be substantially complete, but novel pathways may still be discovered. We review both old and new methods of analysis of the inositol phosphates for the measurement of mass and radioactivity. Although the complexity of the cycle sometimes demands the use of sophisticated methods of separation and rigorous identification, older and inexpensive methods may still be useful for some purposes.

Activation and proliferation signals in murine macrophages: relationships among c-fos and c-myc expression, phosphoinositide hydrolysis, superoxide formation, and DNA synthesis

Murine bone marrow-derived macrophages (BMM) undergo DNA synthesis in response to growth factors such as colony stimulating factor-1 (CSF-1) and granulocyte-macrophage CSF (GM-CSF). These macrophages can also be "activated," but without subsequent DNA synthesis, by a number of other agents, including lipopolysaccharide (LPS), concanavalin A, zymosan, formyl-methionyl-leucyl-phenylalanine (FMLP), and the Ca2+ ionophore, A23187. When BMM are treated with a range of stimuli, there is some, although not perfect, correlation between transient elevations in both c-myc mRNA and c-fos mRNA levels and increases in DNA synthesis. However, enhanced DNA synthesis and oncogene expression are readily dissociated from rises in inositol phosphates and, by implication, phospholipase C-mediated hydrolysis of phosphatidyl inositol 4,5-bisphosphate. Superoxide formation in BMM can also be dissociated from the other responses and does not necessarily depend on protein kinase C activation.

Kinetic analysis of A23187-mediated polyphosphoinositide breakdown in rat cortical synaptosomes suggests that inositol bisphosphate does not arise primarily by degradation of inositol trisphosphate

The kinetics of polyphosphoinositide breakdown and inositol phosphate formation have been studied in rat cortical synaptosomes labelled in vitro with myo-[2-3H]inositol. Intrasynaptosomal Ca2+ concentrations have been varied by the use of Ca-EGTA buffers or by adding the ionophore A23187 in the presence and absence of 1 mM Ca2+. The former studies have revealed that, at very low (20 nM) intrasynaptosomal free Ca2+ levels, inositol bisphosphate, but not inositol monophosphate levels are reduced. Addition of A23187 in the absence of added Ca2+ gives rise to greatly enhanced inositol bisphosphate accumulation, which is further enhanced if 1 mM Ca2+ is present in the extrasynaptosomal medium. At all time points examined (down to 2 s after adding ionophore), the ratio of inositol trisphosphate/inositol bisphosphate accumulation does not exceed 0.2, and calculations based on inositol bis- and trisphosphate breakdown rates in synaptosomal lysates suggest that only a minority of the inositol bisphosphate arises from degradation of inositol trisphosphate. Addition of ionophore in the presence (but not in the absence) of 1 mM Ca2+ leads to rapid breakdown of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) and ATP and slower breakdown of phosphatidylinositol 4-phosphate (PtdInsP). The rates of loss of PtdinsP2 and ATP are very highly correlated, suggesting that polyphosphoinositide resynthesis may be limited by ATP availability at high Ca2+ levels. Analysis of 32P-labelled synaptosomes also reveals that A23187 produces Ca2+-dependent losses of PtdInsP2, PtdInsP, ATP, and GTP radioactivity and a marked increase in the radioactivity of a compound distinct from nucleotides or any of the lipid breakdown products tested.(ABSTRACT TRUNCATED AT 250 WORDS)

Phospholipase A2-mediated release of arachidonic acid in stimulated guinea pig alveolar macrophages: interaction with lipid mediators and cyclic AMP

The stimulation of cultured guinea pig alveolar macrophages by the chemotactic peptide N-formyl-L-methionyl-L-leucyl-L-phenylalanine, or by the phospholipid inflammatory mediator platelet activating factor (PAF) induced an increase in arachidonic acid release and its cyclooxygenase products. This release, which was mimicked by the association of threshold concentrations of the calcium ionophore A 23187 and of the protein kinase C activator tetradecanoyl phorbol acetate arose mainly from diacyl- and alkyl-acyl-phosphatidylcholine and phosphatidylinositol. Using [1-14C]arachidonic acid-labeled membranes as an endogenous substrate as well as dioleoyl-phosphatidyl [14C]ethanolamine as an exogenous substrate, we showed that phospholipase A2 activity of stimulated macrophages increases upon stimulation. Treatment of macrophages by prostaglandin E2 decreased the arachidonic acid release elicited by the chemotactic peptide and PAF. Furthermore, prostaglandin E2 increased and PAF decreased the cellular content in cyclic AMP. From these results we suggest that an initial stimulation of alveolar macrophages by a bacterial signal initiates the sequential activation of a phospholipase C and of phospholipase A2, leading to the release of PAF and eicosanoids. These mediators may in turn modulate the cell response by increasing or decreasing cyclic AMP, Ca2+, or diacyglycerol macrophage content.