Enhancement of phospholipase A2 activation by phosphatidic acid endogenously formed through phospholipase D action in rat peritoneal mast cell

Phospholipase D1 activity is crucial for cytosolic phospholipase A2 -dependent prostaglandin E2 formation in murine osteoblastic MC3T3-E1 cells

In bone, prostaglandin E2 (PGE2) is highly osteogenic and formed by osteoblasts, a key modulatory event in the regulation of bone cell activity. MC3T3-E1 cells are widely used as an in vitro model of osteoblast function. It is still not clear which pathways contribute to the release of AA in these cells. In this study we have focussed on the contribution of phospholipase D (PLD) enzymes to osteoblastic PGE2 formation after stimulation with endothelin-1 (ET-1). Using specific inhibitors of PLD1 and PLD2 we could show that PGE2 formation was strictly dependent on PLD1 but not PLD2 activity and cytosolic phospholipase A2 (cPLA2) was activated by triggering through PLD1. We have identified diacyl glycerol (DAG) as a possible effector molecule which may serve as a triggering signal for PKC activation and subsequent cPLA2 phosphorylation.

Molecular characterisation of group IVA (cytosolic) phospholipase A2 in murine osteoblastic MC3T3-E1 cells

Formation of the powerful osteogenic prostaglandin E2 by osteoblasts, a key modulatory event in the paracrine and autocrine regulation of bone cell activity, is preceded by release of the precursor arachidonic acid from phospholipid stores. The main routes of arachidonate liberation may involve phospholipase enzymes such as group IVA phospholipase A2 which is believed to be the main effector in many cell system due to its preference for arachidonate-containing lipids. MC3T3-E1 cells are non-transformed osteoblasts and are widely used as an in vitro model of osteoblast function. In these cells there is still no clarity about the main release pathway of arachidonic acid. Besides cytosolic phospholipase A2, phospholipase C and D pathways may play a key role in arachidonate release. Despite the crucial role of osteoblastic prostgalandin synthesis information on the occurrence of involved enzymes at the molecular level is scarse in MC3T3-E1 cells. We have characterised group IVA phospholipase A2 at the mRNA in these cells as a constitutively expressed enzyme which is cytosolic and translocates to the membrane upon endothelin-1 stimulation. Using immunopurification combined with Western blotting and high-resolution mass spectrometry, the enzyme was also identified at the protein level. Using specific gene silencing we were able to show that osteoblastic cytosolic phospholipase A2 is crucially involved in ET-1-induced prostaglandin formation.

Enhanced seed viability and lipid compositional changes during natural ageing by suppressing phospholipase Dα in soybean seed

Changes in phospholipid composition and consequent loss of membrane integrity are correlated with loss of seed viability. Furthermore, phospholipid compositional changes affect the composition of the triacylglycerols (TAG), i.e. the storage lipids. Phospholipase D (PLD) catalyses the hydrolysis of phospholipids to phosphatidic acid, and PLDα is an abundant PLD isoform. Although wild-type (WT) seeds stored for 33 months were non-viable, 30%-50% of PLDα-knockdown (PLD-KD) soybean seeds stored for 33 months germinated. WT and PLD-KD seeds increased in lysophospholipid levels and in TAG fatty acid unsaturation during ageing, but the levels of lysophospholipids increased more in WT than in PLD-KD seeds. The loss of viability of WT seeds was correlated with alterations in ultrastructure, including detachment of the plasma membrane from the cell wall complex and disorganization of oil bodies. The data demonstrate that, during natural ageing, PLDα affects the soybean phospholipid profile and the TAG profile. Suppression of PLD activity in soybean seed has potential for improving seed quality during long-term storage.

Lysophosphatidic acid activates the 70-kDa S6 kinase via the lipoxygenase pathway

Many hormones are known to activate the 70-kDa S6 kinase (p70(S6K)). The signalling pathways mediating p70(S6K) activation are only partially characterized. We investigate, in this report, the mechanisms by which lysophosphatidic acid (LPA) activates p70(S6K). We observed that p70(S6K) activation was conventional, in that it was sensitive to both rapamycin and PI3 kinase inhibition. p70(S6K) activation appeared to be caused by the activation of several phospholipase pathways. LPA was an effective stimulus of phospholipase C induced intracellular calcium mobilization, which appeared to participate in p70(S6K) activation. Similarly, the effect of LPA on p70(S6K) activity was antagonized by butan-1-ol but not butan-2-ol suggesting the involvement of agonist stimulated phospholipase D activity. Further, antagonism of the phospholipase A(2) and lipoxygenase pathways attenuated p70(S6K) activation indicating a novel mechanism of p70(S6K) regulation. We conclude that in Swiss 3T3 cells LPA coordinates activation of several phospholipases to regulate p70(S6K).

ATP-mediated killing of intracellular mycobacteria by macrophages is a P2X(7)-dependent process inducing bacterial death by phagosome-lysosome fusion

Mycobacterium tuberculosis survives within host macrophages by actively inhibiting phagosome fusion with lysosomes. Treatment of infected macrophages with ATP induces both cell apoptosis and rapid killing of intracellular mycobacteria. The following studies were undertaken to characterize the effector pathway(s) involved. Macrophages were obtained from p47(phox) and inducible NO synthase gene-disrupted mice (which are unable to produce reactive oxygen and nitrogen radicals, respectively) and P2X(7) gene-disrupted mice. RAW murine macrophages transfected with either the natural resistance-associated macrophage protein gene 1 (Nramp1)-resistant or Nramp1-susceptible gene were also used. The cells were infected with bacille Calmette-Guérin (BCG), and intracellular mycobacterial trafficking was analyzed using confocal and electron microscopy. P2X(7) receptor activation was essential for effective ATP-induced mycobacterial killing, as its bactericidal activity was radically diminished in P2X(7)(-/-) macrophages. ATP-mediated killing of BCG within p47(phox-/-), inducible NO synthase(-/-), and Nramp(s) cells was unaffected, demonstrating that none of these mechanisms have a role in the ATP/P2X(7) effector pathway. Following ATP stimulation, BCG-containing phagosomes rapidly coalesce and fuse with lysosomes. Blocking of macrophage phospholipase D activity with butan-1-ol blocked BCG killing, but not macrophage death. ATP stimulates phagosome-lysosome fusion with concomitant mycobacterial death via P2X(7) receptor activation. Macrophage death and mycobacterial killing induced by the ATP/P2X(7) signaling pathway can be uncoupled, and diverge proximal to phospholipase D activation.

A novel protein kinase target for the lipid second messenger phosphatidic acid

Activation of phospholipase D occurs in response to a wide variety of hormones, growth factors, and other extracellular signals. The initial product of phospholipase D, phosphatidic acid (PA), is thought to serve a signaling function, but the intracellular targets for this lipid second messenger are not clearly identified. The production of PA in human neutrophils is closely correlated with the activation of NADPH oxidase, the enzyme responsible for the respiratory burst. We have developed a cell-free system, in which the activation of NADPH oxidase is induced by the addition of PA. Characterization of this system revealed that a multi-functional cytosolic protein kinase was a target for PA, and that two NADPH oxidase components were substrates for the enzyme. Partial purification of the PA-activated protein kinase separated the enzyme from known protein kinase targets of PA. The partially purified enzyme was selectively activated by PA, compared to other phospholipids, and phosphorylated the oxidase component p47-phox on both serine and tyrosine residues. PA-activated protein kinase activity was present in a variety of hematopoietic cells and cell lines and in rat brain, suggesting it has widespread distribution. We conclude that this protein kinase may be a novel target for the second messenger function of PA.

Muscarinic receptor activation of arachidonate-mediated Ca2+ entry in HEK293 cells is independent of phospholipase C

Receptor-enhanced entry of Ca2+ in non-excitable cells is generally ascribed to a capacitative mechanism in which the activation of the entry pathway is specifically dependent on the emptying of agonist-sensitive intracellular Ca2+ stores. Although such entry can be clearly demonstrated under conditions of maximal or near-maximal stimulation, it is uncertain whether such a mechanism can operate during the oscillatory [Ca2+]i signals that are frequently seen following stimulation with low concentrations of agonists. In this study, we report that the stimulation of human m3 muscarinic receptors stably transfected into HEK293 cells results in the appearance of a novel arachidonate-mediated Ca2+ entry pathway. We show that the generation of arachidonic acid and the activation of this pathway are specifically associated with stimulation at the low agonist concentrations that typically give rise to oscillatory [Ca2+]i signals. At such agonist concentrations, however, the generation of arachidonic acid is independent of the simultaneous activation of the phospholipase C-inositol 1,4,5-trisphosphate pathway. We further show that the arachidonate-mediated Ca2+ entry demonstrates characteristics that distinguish it from the corresponding capacitative pathway in the same cells and therefore is likely to represent an entirely distinct pathway that is specifically responsible for the receptor-enhanced entry of Ca2+ during [Ca2+]i oscillations.

Inhibitory effect of ginsenoside on the mediator release in the guinea pig lung mast cells activated by specific antigen-antibody reactions

We reported that some components of ginsenosides decreased mediator release which was evoked by the activation of mast cells caused by specific antigen-antibody reactions. This study aimed to assess the effects of ginsenoside, Rb1, which belongs to the protopanaxadiol, on the mechanism of mediator release during mast cell activation. Pretreatment of Rb1 (100 microg) significantly decreased histamine and leukotriene in a dose-dependent manner during mast cell activation. The PLD activity during mast cell activation decreased in the pretreatment of Rb1 (300 microg). The amount of DAG produced by PLC activity decreased because of Rb1 pretreatment. The amount of mass DAG decreased due to Rb1 pretreatment during mast cell activation. Rb1 (300 microg) pretreatment strongly inhibited the incorporation of the [3H]methyl moiety into phospholipids. The data suggest that Rb1, purified from Korean Red Ginseng Radix, inhibits an increase of DAG production during mast cell activation caused by antigen-antibody reactions, which is mediated via phosphatidylcholine-PLD and phosphatidylinositol-PLC systems. This is then followed by the inhibition of histamine releases. Furthermore, Rb1 reduces the phosphatidylcholine production by inhibiting the methyl-transferase I and II, and the reduction of phosphatidylcholine production inhibits leukotriene release.

Evidence for role of phospholipase A2 in phosphatidic acid-induced signaling to c-fos serum response element activation

The activity of exogenous phosphatidic acid (PA) to transactivate c-fos serum response element (SRE) was investigated by transient transfection analysis. Incubation of Rat-2 fibroblast cells with exogenous PA caused a stimulation of c-fos SRE-linked luciferase activity in a dose- and time-dependent manner. The SRE stimulation by PA was dramatically reduced by either pre-treatment with mepacrine, an inhibitor of phospholipase A2 (PLA2), or co-transfection with antisense cytosolic phospholipase A2 (cPLA2) oligonucleotide, whereas lysophosphatidic acid (LPA)-induced SRE activation was not affected. Consistent with this specific requirement for PLA2 by PA, the translocation of cPLA2 protein was rapidly induced followed by PA treatment. Together, these results suggest that PLA2, especially cPLA2, plays a critical role in the nuclear signaling cascade of PA in Rat-2 fibroblast cells.

Phospholipase D and phosphatidic acid enhance the hydrolysis of phospholipids in vesicles and in cell membranes by human secreted phospholipase A2

Phosphatidyl-choline (PC) vesicles and normal cell membranes are resistant to hydrolysis by human group II secreted PLA2, an enzyme that can attain high concentrations in extracellular fluids during many inflammatory processes. This highly cationic enzyme (pI>10.5) has a marked preference for anionic phospholipid interfaces, normally present within the cell. Therefore, the ability of one such anionic phospholipid, phosphatidic acid (PA), to enhance the activity of this enzyme has been investigated in detail. Results using model membrane vesicles and a continuous fluorescence assay highlight the ability of low molar proportions of PA to stimulate vesicle hydrolysis and this stimulation with increasing PA was parallelled by enhanced interfacial binding. In contrast, no productive binding of this enzyme could be detected to the surface of pure PC vesicles. The enhancement of hydrolysis in the presence of PA could also be achieved by prior treatment of pure PC vesicles with PLD, an effect that was dependent on the concentration of PLD and the duration of exposure to this enzyme. The fluorescence assay also allowed cell membranes and whole cells to be used as substrates and whereas such membrane presentations were refractory to hydrolysis by the human enzyme, prior treatment with PLD allowed hydrolysis using concentrations of this PLA2 that would be found extracellularly under inflammatory conditions. These results highlight the potential for PA, generated at the surface of the cell membrane, to be hydrolysed by extracellular human sPLA2 with the generation of lysophosphatidic acid and other lipid mediators and provides one possible mechanism whereby this human sPLA2 could become pro-inflammatory.

Neuroprotective effect of graded postischemic reoxygenation in spinal cord ischemia in the rabbit

Early ischemia/reperfusion-induced changes of four phospholipid compounds bound to the inner cell membrane leaflet, i.e., phosphatidic acid, inositol phospholipids, serine phospholipids, and ethanolamine plasmalogens, were studied in a model of spinal cord ischemia in the rabbit during normoxic and graded postischemic reoxygenation. Light and electron microscopic analysis after normoxic reoxygenation disclosed neuronal membrane argyrophilia of the interneuronal pool located in lamina VII of L4-L6 segments. The number of small neurons (10-25 microm in diameter) affected by somatodendritic argyrophilia was greatly reduced, and concomitantly the ultrastructure of the endoplasmic reticulum, mitochondria, and Golgi complexes remained almost undamaged when graded postischemic reoxygenation had been applied. A statistically significant increase of phosphatidylserine and ethanolamine plasmalogen levels, and a decrease of phosphatidic acid, were detected after a short-lasting graded postischemic reoxygenation. The formation of thiobarbituric acid-reactive substances was significantly reduced during 60 min of graded postischemic reoxygenation and remained close to control or ischemic levels. The present data indicate that graded postischemic reoxygenation, which is considered to be neuroprotective, can prevent neuronal argyrophilia and the development of reperfusion-induced alterations of organelles. Moreover, reoxygenation can positively modify ischemia-induced changes of some membrane-bound phospholipids.

Phosphatidic acid activation of protein kinase C in LA-N-1 neuroblastoma cells

Phosphatidic acid (PA), a hydrolytic product of phospholipase D activity, stimulated cytosolic protein kinase C (PKC) activity when LA-N-1 neuroblastoma cells in culture were treated with PA, without translocating the enzyme to the membrane. Treatment of cells with 12-O-tetradecanoylphorbol-13-acetate (TPA) translocated and activated PKC in a dogmatic manner. Partially purified PKC activity derived from LA-N-1 neuroblastoma cells was stimulated by PA alone or in the presence of phosphatidylserine or TPA, without affecting [3H]phorbol dibutyrate binding, indicating that the site of action of PA was different from the phorbol ester or diacylglycerol binding site. These results suggest an unorthodox pattern of PKC stimulation mediated by PA which appears to be yet another mode of PA signal transduction.

Stimulation of protein and DNA synthesis in mouse C2C12 satellite cells: evidence for phospholipase D-dependent and -independent pathways

In C2C12 myoblasts, 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulated a phospholipase D (PLD) to degrade phosphatidylcholine (PC) as measured by the release of choline and an increase in the formation of phosphatidic acid (PA) (or phosphatidylbutanol [PtdBuOH] in the presence of 0.5% butanol). Exogenous PLD also stimulated choline release, PA and PtdBuOH formation. The protein kinase C (PKC) inhibitor, Ro-31-8220, and PKC downregulation significantly inhibited the effects of TPA but Ro-31-8220 had no effect on PLD action. Neither basic Fibroblast Growth Factor (bFGF) or Epidermal Growth Factor (EGF) increased PLD activity. All agonists stimulated protein synthesis during both a 90 min and a 6 hr incubation and increased RNA accretion after 6 hr. The response at 90 min was not inhibited by the transcription inhibitor, actinomycin D. Ro-31-8220 and PKC downregulation significantly inhibited all the effects of TPA. In contrast, Ro-31-8220 significantly inhibited the increase in RNA accretion elicited by PLD but had no effect on the ability of agonists other than TPA to enhance protein synthesis. All agonists also stimulated thymidine incorporation into DNA. The effects of EGF, bFGF, and PLD were rapid and transient whereas that of TPA was delayed and sustained. Ro-31-8220 and PKC downregulation significantly inhibited the response due to TPA. Furthermore, Ro-31-8220 also significantly inhibited the effects elicited by EGF and PLD but not that induced by bFGF. In differentiated myotubes, TPA and PLD, but not bFGF or EGF, again stimulated choline release and PtdBuOH formation. However, all agents failed to stimulate protein synthesis and RNA accretion. The data demonstrate the presence in C2C12 myoblasts, but not differentiated myotubes, of both a PLD-dependent and PLD-independent pathway(s) leading to the stimulation of protein synthesis, RNA accretion, and DNA synthesis.

Increased receptor-mediated phospholipase D activation and Ca2+ mobilization in peritoneal polymorphonuclear leukocytes from streptozotocin-induced diabetic rats

Phospholipase D activation was investigated in peritoneal polymorphonuclear leukocytes from streptozotocin-induced diabetic rats. Stimulation of the cells with formyl-Met-Leu-Phe resulted in a time- and dose-dependent increase in phosphatidylethanol in the presence of ethanol, and this lipid formation in cells prepared from diabetic rats was enhanced as compared to that in the case of nondiabetic rats. Furthermore, the increase in the intracellular Ca2+ concentration was also enhanced in the stimulated cells from diabetic rats. Under the present conditions, N-acetyl-beta-glucosaminidase release and superoxide generation, which are known to be dependent on phospholipase D activation, were higher in the cells from diabetic rats than those in the cells from control rats. However, there was no difference in the dissociation constant and the number of binding sites for formyl-Met-Leu-Phe between the cells from diabetic and control rats. Phosphatidylethanol formation, N-acetyl-beta-glucosaminidase release and superoxide generation in response to ionomycin or 4 beta-phorbol 12-myristate 13-acetate were not enhanced in diabetic rat cells, as compared with those in control rat cells. These results suggest that receptor-mediated phospholipase D activation and Ca2+ mobilization are enhanced in diabetic rat polymorphonuclear leukocytes, which might be due to acceleration of receptor-mediated signaling.

Arachidonic acid liberation induced by phosphatidic acid endogenously generated from membrane phospholipids in rabbit platelets

The action of phosphatidic acid generated from membrane phospholipids on phospholipase A2 activation in rabbit platelets was investigated. When [3H]arachidonic acid-labelled platelets were treated with phorbol 12-myristate 13-acetate (PMA) and the membranes isolated from the cells incubated at 37 degrees C with 50 microM CaCl2 and 50 microM guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), both phosphatidic acid production and arachidonic acid liberation increased in PMA- and GTP gamma S-concentration-dependent manners. Ethanol dose-dependently inhibited these responses, accompanied by the formation of phosphatidylethanol. Since propranolol, an inhibitor of phosphatidic acid phosphohydrolase, had no influence on the production of phosphatidic acid, the arachidonic acid liberated does not appear to be derived from diacylglycerol which may be produced from phosphatidic acid through the action of this enzyme. In another approach, treatment of [3H]arachidonic acid-labelled membranes with phospholipase D from Streptomyces chromofuscus induced arachidonic acid liberation as well as phosphatidic acid formation in time- and dose-dependent manners. The former response was suppressed by p-bromophenacyl bromide, a phospholipase A2 inhibitor. These results suggest that phosphatidic acid derived from membrane phospholipids potentiates phospholipase A2 activation and contributes to the amplification of platelet activation.

Contribution of phospholipases A2 and D to arachidonic acid liberation and prostaglandin D2 formation with increase in intracellular Ca2+ concentration in rat peritoneal mast cells

The contribution of phospholipases A2 (PLA2) and D (PLD) activation to arachidonic acid liberation and prostaglandin D2 (PGD2) formation was studied in stimulated rat peritoneal mast cells. Stimulation of the cells with ionomycin induced time-dependent and Ca(2+)-concentration-dependent increase in arachidonic acid liberation and PGD2 formation, and the Ca(2+)-dependent increase was especially remarkable at extracellular Ca2+ concentration higher than 200 microM. Staurosporine did not induce any effect on the arachidonic acid liberation, indicating that protein kinase C is not involved in the liberation. Addition of ethanol to the cells decreased the ionomycin-stimulated arachidonic acid liberation to 40% of the control, while it decreased the PGD2 formation almost completely, with the increase in phosphatidylethanol formation. Propranolol, a phosphatidate phosphohydrolase inhibitor, caused similar effects. p-Bromophenacyl bromide, a PLA2 inhibitor, inhibited partially the arachidonic acid liberation. The inhibition of the liberation by combination of p-bromophenacyl bromide and ethanol was additive and reached approximately 90%. Under the conditions used p-bromophenacyl bromide did not influence significantly the PLD activity assessed by the phosphatidylethanol formation. Histamine release was decreased by ethanol treatment to 35% of the control. These results suggest that more than half of the total arachidonic acid liberation is mediated by the sequential pathway of PLD/phosphatidate phosphohydrolase/diacylglycerol lipase and more than half of histamine release is also dependent on PLD activation, while the PGD2 formation is fully mediated by the pathway. PLA2 also contributes to arachidonic acid liberation but to a lower extent.