Phosphorylation-dependent regulation of phospholipase A2 by G-proteins and Ca2+ in HL60 granulocytes

Enhancement of Acetyl-CoA: 1-O-Alkyl-2-lyso-sn-glycero-3-phosphocholine Acetyltransferase Activity by Hydrogen Peroxide

The synthesis of platelet-activating factor (PAF) by human umbilical vein endothelial cell (HUVEC) in response to H2O2 was significantly increased in a concentration-dependent manner. When HUVEC were pretreated with diethyl maleate, which depletes intracellular glutathione, PAF synthesis was enhanced 3-fold upon 5 mM H2O2-treatment. Intracellular redox was involved in regulating PAF synthesis, since the addition of antioxidants such as N-acetylcysteine, pyrrolidinecarbodithioic acid (PDTC), and Trolox reduced PAF production in H2O2-treated HUVEC. The activity of acetyl-CoA: 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase, which is involved in the last step of PAF synthesis, was also activated in H2O2-treated cells. However, exogenous lyso-PAF addition had not effected to acetyltransferase activity. The acetyltransferase activity responded quickly to H2O2-treatment, but the activation was transitory. A tyrosine kinase inhibitor and a calmodulin antagonist blocked acetyltransferase activity in H2O2-stimulated cells, suggesting that tyrosine kinase and calcium/calmodulin-dependent protein kinase are involved in regulating acetyltransferase activity. These observations suggest that H2O2 is one of the modulators of lyso-PAF acetyltransferase activity via a phosphorylation system and platelet-activating factor (PAF) synthesis.

Single Color Fluorescent Indicators of Protein Phosphorylation for Multicolor Imaging of Intracellular Signal Flow Dynamics

Existing monitoring methods for protein phosphorylation involved in intracellular signal transduction in vivo are exclusively based on fluorescence resonance energy transfer, which needs the measurement of the change in fluorescence intensities at two wavelengths. Therefore, it is difficult to monitor protein phosphorylation together with other related signaling processes, such as second messengers and protein translocation. To overcome this problem, we developed novel fluorescent indicators, each containing a differently colored (cyan and green) single fluorophore. The present indicator is a tandem fusion protein containing a kinase substrate domain, a circularly permuted fluorescent protein (cpFP), and a phosphorylation recognition domain. The cpFP is obtained by dividing a green fluorescent protein mutant (GFP) at residue 144-145 and linking the carboxy and amino portions thereof with a peptide linker. The substrate domain used in this study is a peptide sequence that is phosphorylated by insulin receptor. Phosphorylation of the substrate domain induces its interaction with the phosphorylation recognition domain, which causes a conformational change in the cpFP and a change in its fluorescence. The cyan and green indicators exhibited 10% decrease and 15% increase, respectively, in their fluorescence intensities upon phosphorylation. Using this cyan indicator and GFP-tagged mitogen-activated protein kinase (MAPK), we found that insulin-induced protein phosphorylation occurred immediately upon the addition of insulin, whereas nuclear translocation of MAPK occurred 7 min later. By tailoring the substrate domains and the phosphorylation recognition domains in these cyan and green indicators, the present approach should be applicable to the in vivo analysis of a broad range of protein phosphorylation processes, together with other intracellular signaling processes.

Regulation of phospholipase A(2) activity in cockroach (Periplaneta americana) fat body by hypertrehalosemic hormone: evidence for the participation of protein kinase C

Phospholipase A(2) (PLA(2)) associated with the membrane fraction of trophocytes from Periplaneta americana fat body increases by as much as 100% when the cells are incubated with hypertrehalosemic hormone (HTH-II). Activation with HTH-II is approximately halved by inclusion of the PKC inhibitor sphingosine in the incubation medium. Because activation of PLA(2) by HTH-II is blocked by the GDP analogue GDP-beta-S, and the unactivated enzyme is activated by the GTP analogue GTP-gamma-S it is likely that a G protein is involved in activation of the enzyme. Activation of PLA(2) was also achieved by treating the trophocytes with the synthetic diacylglycerol 1-oleoyl-2-acetylglycerol in the presence of thapsigargin. This supports the view that protein kinase C is also involved in the activation process.

fMLP-induced arachidonic acid release in db-cAMP-differentiated HL-60 cells is independent of phosphatidylinositol-4, 5-bisphosphate-specific phospholipase C activation and cytosolic phospholipase A(2) activation

In inflammatory cells, agonist-stimulated arachidonic acid (AA) release is thought to be induced by activation of group IV Ca(2+)-dependent cytosolic phospholipase A(2) (cPLA(2)) through mitogen-activated protein kinase (MAP kinase)- and/or protein kinase C (PKC)-mediated phosphorylation and Ca(2+)-dependent translocation of the enzyme to the membrane. Here we investigated the role of phospholipases in N-formylmethionyl-l-leucyl-l-phenylalanine (fMLP; 1 nM-10 microM)-induced AA release from neutrophil-like db-cAMP-differentiated HL-60 cells. U 73122 (1 microM), an inhibitor of phosphatidyl-inositol-4,5-biphosphate-specific phospholipase C, or the membrane-permeant Ca(2+)-chelator 1, 2-bis¿2-aminophenoxyĕthane-N,N,N',N'-tetraacetic acid (10 microM) abolished fMLP-mediated Ca(2+) signaling, but had no effect on fMLP-induced AA release. The protein kinase C-inhibitor Ro 318220 (5 microM) or the inhibitor of cPLA(2) arachidonyl trifluoromethyl ketone (AACOCF(3); 10-30 microM) did not inhibit fMLP-induced AA release. In contrast, AA release was stimulated by the Ca(2+) ionophore A23187 (10 microM) plus the PKC activator phorbol myristate acetate (PMA) (0.2 microM). This effect was inhibited by either Ro 318220 or AACOCF(3). Accordingly, a translocation of cPLA(2) from the cytosol to the membrane fraction was observed with A23187 + PMA, but not with fMLP. fMLP-mediated AA release therefore appeared to be independent of Ca(2+) signaling and PKC and MAP kinase activation. However, fMLP-mediated AA release was reduced by approximately 45% by Clostridium difficile toxin B (10 ng/ml) or by 1-butanol; both block phospholipase D (PLD) activity. The inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC), D609 (100 microM), decreased fMLP-mediated AA release by approximately 35%. The effect of D609 + 1-butanol on fMLP-induced AA release was additive and of a magnitude similar to that of propranolol (0.2 mM), an inhibitor of phosphatidic acid phosphohydrolase. This suggests that the bulk of AA generated by fMLP stimulation of db-cAMP-differentiated HL-60 cells is independent of the cPLA(2) pathway, but may originate from activation of PC-PLC and PLD.

An SPR-based screening method for agonist selectivity for insulin signaling pathways based on the binding of phosphotyrosine to its specific binding protein

A new screening method was developed that evaluates physiologically relevant chemical selectivity of agonists for insulin-signaling pathways. Phosphorylation (pY939) by an insulin-activated insulin receptor of a target peptide (Y939) derived from an insulin receptor substrate-1 (IRS-1) and its subsequent binding to another downstream target, the SH2 domain of PI-3 kinase (SH2N), were detected by surface plasmon resonance (SPR) spectrometry. This method is based on competitive binding of SH2N to pY939 either in a solution or on the gold surface of the SPR sensor chip. With increasing the concentration of pY939 in solution by the insulin-induced kinase reaction of insulin receptor, SH2N bound to pY939 in solution increases and the one on the sensor chip decreases, thereby causing a decrease in the SPR signal. The amount of thus-detected complex pY939-SH2N was found to depend on added insulin concentrations, confirming that the method utilized part of the sequential transduction mechanism of the insulin-signaling pathways. The kinase activity of insulin receptor-agonist complexes increased in the order of IGF-II < IGF-I < insulin, and neither vanadium ions nor thiazolidine-type medicines for NIDDM, troglitazone and pioglitazone, directly acted on both the kinase reaction of insulin receptor or the binding of pY939 to SH2N. The present approach will thus become a general method for screening agonists for one specific pathway in tyrosine phosphorylation of IRS-1 in insulin signaling, which is regulated by specific protein-protein interaction between a phosphorylated tyrosine in IRS-1 and its corresponding SH2 domain-containing protein such as PI-3 kinase, Grb2-Sos, or SHP2.

A fluorescent indicator for tyrosine phosphorylation-based insulin signaling pathways

A fluorescent indicator for tyrosine phosphorylation-based insulin signaling is described. Upon binding of insulin to cell-surface insulin receptor, the receptor phosphorylates tyrosine residues of insulin receptor substrate 1 (IRS-1) in the cell. A fluorescent indicator was designed by using synthetic phosphopeptide pY939 derived from the tyrosine phosphorylation domain of IRS-1 and its target protein SH2N containing an N-terminal SH2 domain of PI 3-kinase. The SH2N protein and pY939 phosphopeptide were labeled with fluorescein (F-SH2N) and tetramethylrhodamine (T-pY939), respectively. Formation of a F-SH2N-T-pY939 complex (termed a fluorescence resonance energy-transfer (FRET) pair) was evaluated from a change in a fluorescence emission spectrum based on FRET between the two fluorophores. The FRET pair was formed to dissociate in competition with the unlabeled pY939 phosphopeptide, resulting in a decrease in a pY939 phosphopeptide-dependent FRET emission at 580 nm and causing an increase in emission at 520 nm. Tyrosine phosphorylation by the partially purified insulin receptor of substrate peptide Y939 was detected with this formed FRET pair, and resulting changes in fluorescence emission spectra were observed for insulin concentration from about 1.0 x 10(-9) to 1.0 x 10(-6) M. These results indicated that the FRET pair served as a competitive fluorescent indicator for tyrosine phosphorylation-based insulin signaling.

Contribution of endogenously formed arachidonic acid in the presynaptic facilitatory effects of NMDA and carbachol on dopamine release in the mouse striatum

Arachidonic acid stimulated the release of [3H]-dopamine from striatal microdiscs in a concentration-dependent and partially calcium-dependent manner. Inhibitors of cytosolic and membrane-bound phospholipase A2 were used to determine whether endogenously formed arachidonic acid also contributes to the release of [3H]-DA (previously taken up in tissues or endogenously synthesized from [3H]-tyrosine) evoked by N-methyl-d-aspartate (NMDA) and carbachol alone or in combination. In the presence of magnesium, carbachol was found to remove the magnesium block of NMDA receptors and to facilitate the NMDA-evoked release of [3H]-DA from striatal microdiscs and synaptosomes. In addition, in the absence of magnesium, synergistic responses were induced by both agonists on microdiscs but not on synaptosomes. Responses induced by NMDA, carbachol or both agonists on microdiscs were reduced by phospholipase A2 inhibitors, the most striking effects being observed with mepacrine. Mepacrine was also shown to reduce the oxotremorine, but neither the nicotine- nor the potassium-evoked release of [3H]-DA. Tetrodotoxin decreased the release of [3H]-DA evoked by the co-application of NMDA and carbachol on microdiscs, but mepacrine still decreased this tetrodotoxin-resistant response. Similarly, mepacrine still decreased the release of [3H]-DA evoked by NMDA and carbachol on synaptosomes. Altogether, these results indicate that arachidonic acid which is formed in striatal neurons, and to a lesser extent in DA fibres, under stimulation of NMDA and muscarinic receptors, partially contributes to the presynaptic facilitation of DA release evoked by NMDA and carbachol.

Role of Ca2+-ATPase inhibitors in activation of cytosolic phospholipase A2 in human polymorphonuclear neutrophils

In the present study, we investigated the involvement of Ca2+-signaling and protein kinases in the effect of Ca2+-ATPase inhibitors on the activation of cytosolic phospholipase A2 (cPLA2) in human polymorphonuclear neutrophils. We found that activity and mobility on electrophoresis gels of the cPLA2 protein were significantly increased by f-Met-Leu-Phe (fMLP), 12-myristate 13-acetate (PMA) and the Ca2+-ATPase inhibitors, thapsigargin and cyclopiazonic acid. This effect was completely suppressed by staurosporine. Calphostin C partially inhibited the fMLP- and PMA-induced cPLA 2 activation, but had no influence on thapsigargin- and cyclopiazonic acid-treated cells. Thapsigargin and cyclopiazonic acid also showed no effect on protein kinase C activity. However, the thapsigargin- and cyclopiazonic acid-induced cPLA2 activation was completely inhibited by the tyrosine kinase inhibitor, erbstatin, and Ca2+ chelator, EGTA. In addition, the cPLA2 activity was reduced after pretreatment with the mitogen-activated protein kinase kinase inhibitor PD98059. The arachidonic acid release was significantly reduced in cells pretreated with the cPLA2 inhibitor, AACOCF3. Furthermore, we found that the human neutrophil cPLA2 cDNA contain a Ca2+-dependent-lipid binding domain which shares homology to several other enzymes such as protein kinase C and phospholipase C. Our results suggest that tyrosine kinases and the MAP kinase cascade are involved in Ca2+-ATPase inhibitor-induced activation and phosphorylation of cPLA2. Protein kinase C is not required in this event.

Arachidonic acid release from PC12 pheochromocytoma cells is regulated by I1-imidazoline receptors

Rat PC 12 pheochromocytoma cells lack alpha2-adrenergic receptors but express plasma membrane I1-imidazoline receptors. In response to the I1-agonist moxonidine, diglycerides are generated via phosphatidylcholine-selective phospholipase C, and prostaglandin E2 is released. This report characterizes I-receptor-mediated release of arachidonic acid, the precursor to the prostaglandins. PC12 cells were incubated with [3H]arachidonic acid for 24 h and superfused with 0.01% bovine serum albumin in Krebs' physiological buffer at 1 ml/min. Calcium ionophore increased arachidonic acid release only marginally, implying that in PC12 cells arachidonic acid release is not driven by calcium. The I1-agonist moxonidine at concentrations between 10 nM and 1.0 microM rapidly elicited up to two-fold increases in [3H]arachidonic acid release. Guanabenz, a potent alpha2-agonist and I2-ligand, had no effect. The selective I1-antagonist efaroxan blocked the action of moxonidine. The phospholipase A2 inhibitor aristolochic acid had no effect, suggesting that arachidonic acid release may be through an indirect pathway, possibly involving diglycerides. Thus, I1-imidazoline receptors in PC12 cells are coupled to arachidonic acid release through an as yet unknown pathway.

Phospholipase A2: its role in ADP- and thrombin-induced platelet activation mechanisms

ADP and thrombin are two of the most important agonists of platelet aggregation--a cellular response that is critical for maintaining normal hemostasis. However, aberrant platelet aggregation induced by these agonists plays a central role in the pathogenesis of cardiovascular and cerebrovascular diseases. Agonist-induced primary or secondary activation of phospholipases leads to generation of the second messengers that participate in biochemical reactions essential to a number of platelet responses elicited by ADP and thrombin. Phospholipase A2 (PLA2) has been linked to cardiovascular diseases. However, the mechanism(s) of activation of PLA2 in platelets stimulated by ADP and thrombin has remained less well defined and much less appreciated. The purpose of this review is to examine and compare the molecular mechanisms of activation of PLA2 in platelets stimulated by ADP and thrombin.

Growth inhibition of HL-60 cells by extracellular ATP: concentration-dependent involvement of a P2 receptor and adenosine generation

A single addition of ATP (20-1000 microM) to cultures of HL-60 cells resulted here in permanent, Ca(2+)-independent inhibition of cellular proliferation, evident 48 h following treatment. Extracellular ATP (ATPo) was maximally effective at 250 microM giving 90 +/- 1.5% growth inhibition. Up to a concentration of 250 microM ATPo, growth inhibition is solely attributable to ATPo, while at higher ATPo concentrations adenosine generated from ATPo hydrolysis contributes to this effect. The order of potency for growth inhibition was ATP = ADP > AMP > adenosine. Suramin, a P2 receptor antagonist, attenuated growth inhibition by ATP and ADP, indicative of P2 receptor involvement. Equipotency of ATP and ADP excludes the involvement of either an ecto-protein kinase or a P2X7 receptor in growth inhibition. Neither UTP (P2Y2 agonist) nor alpha, beta-methyleneATP (P2X1 agonist) inhibited growth, indicating that such inhibition is mediated by a previously undescribed P2 receptor on HL-60 cells.

Phosphorylation of cytosolic phospholipase A2 by IL-3 is associated with increased free arachidonic acid generation and leukotriene C4 release in human basophils

BACKGROUND

Human basophils secrete leukotriene C4 (LTC4) in response to various stimuli, and a short treatment with IL-3 enhances LTC4 release, although IL-3 alone does not induce LTC4 release. However, the mechanism of this priming effect of IL-3 for LTC4 generation remains unknown in human basophils.

OBJECTIVE

This study was designed to explore the mechanisms by which short treatments with IL-3 enhance stimulated secretion of LTC4, with a focus on the activation of cytosolic phospholipase A2 (cPLA2).

METHODS

The phosphorylation state of cPLA2 in human basophils was examined by its shift in electrophoretic mobility as detected by Western blotting. Free arachidonic acid (AA) and LTC4 were measured by gas chromatography-negative ion chemical ionization mass spectrometry and LTC4-specific RIA, respectively.

RESULT

Human basophils expressed cPLA2. IL-3, as well as the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate, caused a shift in the electrophoretic mobility of cPLA2, which indicated phosphorylation of cPLA2 and therefore its activation. Ionomycin at a concentration of 0.1 microg/mL was used to induce a modest elevation of cytosolic calcium response ([Ca2+]I), no apparent cPLA2 phosphorylation, and little free AA and LTC4 generation. Pretreatment with IL-3 (1 to 10 ng/mL) markedly enhanced ionomycin (0.1 microg/mL)-mediated AA and LTC4 generation. The concentration dependence of cPLA2 phosphorylation by IL-3 and its effects on free AA and LTC4 generation were similar. The selective PKC inhibitors, bis-indolylmaleimide II and Ro-31-8220 inhibited the phorbol 12-myristate 13-acetate-mediated cPLA2 electrophoretic mobility shift, but not the IL-3-mediated shift, suggesting that the IL-3 effect is PKC independent. Both the anaphylatoxin split product of the C component C5 (C5a) and f-Met-Leu-Phe induced PKC-independent cPLA2 phosphorylation with a similar time course most notable for the absence of observable changes in cPLA2 phosphorylation before 30 seconds. These results suggested an explanation for the absence of free AA generation by C5a. When [Ca2+]I was elevated in response to C5a, there was no phosphorylation of cPLA2, and by the time cPLA2 became phosphorylated, [Ca2+]I had returned to resting levels. Treatment with IL-3 preconditioned the cPLA2 by causing its phosphorylation so that the transient [Ca2+]I response, which followed stimulation by C5a, could induce the generation of free AA and LTC4.

CONCLUSION

Taken together, these results suggest that the effect of IL-3 for free AA generation and LTC4 release might be due to induction of cPLA2 phosphorylation. The studies demonstrated a need for synchronous cPLA2 phosphorylation and elevations in [Ca2+]I.

Contribution of cyclooxygenase-1 and cyclooxygenase-2 to prostanoid formation by human enterocytes stimulated by calcium ionophore and inflammatory agents

The stimulation of intestinal epithelial cell cyclooxygenase (COX) enzymes with inflammatory agents and the inhibition of COX-1 and COX-2 enzymes has the potential to increase understanding of the role of these enzymes in intestinal inflammation. The aim of this study was to determine the contributions of COX-1 and -2 to the production of specific prostanoids by unstimulated and stimulated intestinal epithelial cells. Cultured enterocytes were stimulated with lipopolysaccharide (LPS), interleukin-1 (IL-1)beta (IL-1 beta), and calcium ionophore (Ca Ion), with and without COX inhibitors. Valerylsalicylic acid (VSA) was employed as the COX-1 inhibitor, and SC-58125 and NS398 were used as the COX-2 inhibitors. Prostanoids were quantitated by Elisa assay. Western immunoblotting demonstrated the presence of constitutive COX-1 and inducible COX-2 enzyme. Unstimulated prostanoid formation was not decreased by the COX-1 inhibitor. All of the stimulants evaluated increased prostaglandin E2 (PGE2) production. Only Ca Ion stimulated prostaglandin D2 (PGD2) production while IL-1 beta, and Ca Ion, but not LPS, increased prostaglandin F2 alpha (PGF2 alpha) formation. Ca Ion-stimulated prostanoid formation was uniformly inhibited by COX-2, but not COX-1, inhibitors. IL-1 beta-stimulated PGE2 and PGE2 alpha formation was significantly decreased by both COX-1 and COX-2 inhibitors. VSA, in a dose-dependent manner, significantly decreased IL-1 beta-stimulated PGE2 and PGF2 alpha production. Unstimulated prostanoid formation was not dependent on constitutive COX-1 activity. The stimulation of intestinal epithelial cells by Ca Ion seemed to uniformly produce prostanoids through COX-2 activity. There was no uniform COX-1 or COX-2 pathway for PGE and PGF2 alpha formation stimulated by the inflammatory agents, suggesting that employing either a COX-1 or COX-2 inhibitor therapeutically will have varying effects on intestinal epithelial cells dependent on the prostanoid species and the inflammatory stimulus involved.

Pharmacological comparison of UTP- and thapsigargin-induced arachidonic acid release in mouse RAW 264.7 macrophages

1. Although stimulation of mouse RAW 264.7 macrophages by UTP elicits a rapid increase in intracellular free Ca2+ ([Ca2+]i), phosphoinositide (PI) turnover, and arachidonic acid (AA) release, the causal relationship between these signalling pathways is still unclear. In the present study, we investigated the involvement of phosphoinositide-dependent phospholipase C (PI-PLC) activation, Ca2+ increase and protein kinase activation in UTP-induced AA release. The effects of stimulating RAW 264.7 cells with thapsigargin, which cannot activate the inositol phosphate (IP) cascade, but results in the release of sequestered Ca2+ and an influx of extracellular Ca2+, was compared with the effects of UTP stimulation to elucidate the multiple regulatory pathways for cPLA2 activation. 2. In RAW 264.7 cells UTP (100 microM) and thapsigargin (1 microM) caused 2 and 1.2 fold increases, respectively, in [3H]-AA release. The release of [3H]-AA following treatment with UTP and thapsigargin were non-additive, totally abolished in the Ca2+-free buffer, BAPTA (30 microM)-containing buffer or in the presence of the cPLA2 inhibitor MAFP (50 microM), and inhibited by pretreatment of cells with pertussis toxin (100 ng ml(-1)) or 4-bromophenacyl bromide (100 microM). By contrast, aristolochic acid (an inhibitor of sPLA2) had no effect on UTP and thapsigargin responses. 3. U73122 (10 microM) and neomycin (3 mM), inhibitors of PI-PLC, inhibited UTP-induced IP formation (88% and 83% inhibition, respectively) and AA release (76% and 58%, respectively), accompanied by a decrease in the [Ca2+]i rise. 4. Wortmannin attenuated the IP response of UTP in a concentration-dependent manner (over the range 10 nM-3 microM), and reduced the UTP-induced AA release in parallel. RHC 80267 (30 microM), a specific diacylglycerol lipase inhibitor, had no effect on UTP-induced AA release. 5. Short-term treatment with PMA (1 microM) inhibited the UTP-stimulated accumulation of IP and increase in [Ca2+]i, but had no effect on the release of AA. In contrast, the AA release caused by thapsigargin was increased by PMA. 6. The role of PKC in UTP- and thapsigargin-mediated AA release was shown by the blockade of these effects by staurosporine (1 microM), Ro 31-8220 (10 microM), Go 6976 (1 microM) and the down-regulation of PKC. 7. Following treatment of cells with SK&F 96365 (30 microM), thapsigargin-, but not UTP-, induced Ca2+ influx, and the accompanying AA release, were down-regulated. 8. Neither PD 98059 (100 microM), MEK a inhibitor, nor genistein (100 microM), a tyrosine kinase inhibitor, had any effect on the AA responses induced by UTP and thapsigargin. 9. We conclude that UTP-induced cPLA2 activity depends on the activation of PI-PLC and the sustained elevation of intracellular Ca2+, which is essential for the activation of cPLA2 by UTP and thapsigargin. The [Ca2+]i-dependent AA release that follows treatment with both stimuli was potentiated by the activity of protein kinase C (PKC). A pertussis toxin-sensitive pathway downstream of the increase in [Ca2+]i was also shown to be involved in AA release.

Phosphorylation and calcium influx are not sufficient for the activation of cytosolic phospholipase A2 in U937 cells: requirement for a Gi alpha-type G-protein

Differentiation with dibutyryl cyclic AMP (dBcAMP) of the human, premonocytic U937 cell line toward a monocyte/granulocyte-like cell results in the cell acquiring an ability to release arachidonate upon stimulation. In contrast, the calcium ionophore ionomycin was able to stimulate phospholipase C, as measured by inositol 1,4,5-trisphosphate formation, to equal extents in both undifferentiated and dBcAMP-differentiated U937 cells. The role and regulation of cytosolic phospholipase A2 (cPLA2) in the production of arachidonate in these cells when either the chemotactic peptide fMLP or ionomycin are used as stimulus were investigated. The ionomycin- and fMLP-stimulated release of arachidonate were sensitive to the cPLA2 inhibitor arachidonyl trifluoromethylketone (IC50 values of 32 and 18 microM, respectively), but were not inhibited by E-6-(bromomethylene)-tetrahydro-3-(1-naphthalenyl)-2 H-pyran-2-one, a bromoenol lactone inhibitor of the calcium-independent phospholipase A2. These results, coupled with the inhibition of ionomycin-induced arachidonate production by electroporation of differentiated cells to introduce an anti-cPLA2, demonstrate that the cPLA2 is the enzyme responsible for arachidonate release in differentiated cells. SDS-PAGE and immunoblot analysis of differentiated cells showed the cells to contain both phosphorylated and unphosphorylated forms of cPLA2 (ratio of about 2: 3). Surprisingly, undifferentiated cells contain 30% more enzyme than differentiated cells and contain a higher percentage (approximately 75%) of the phosphorylated in the absence of stimulation. The inability of undifferentiated cells to produce arachidonate is not due to insufficient intracellular calcium concentrations since ionomycin induces large (820-940 nM) influxes of intracellular calcium in both differentiated and undifferentiated cells. This demonstrates that phosphorylation of cPLA2 andan influx of intracellular calcium are not sufficient to activate the enzyme to produce arachidonate. Instead, activation of a pertussis toxin-sensitive Gi alpha-type G-protein is required as evidenced by the production of arachidonate in undifferentiated cells stimulated with mastoparan, an activator of Gi alpha subunits, in combination with ionomycin. This activation of a Gi alpha-type G-protein is independent of modulations of adenylyl cyclase activity since cellular cAMP levels were not modulated upon treatment with mastoparan and ionomycin.

Inhibition of cytosolic phospholipase A2 by annexin V in differentiated permeabilized HL-60 cells. Evidence of crucial importance of domain I type II Ca2+-binding site in the mechanism of inhibition

Annexin V belongs to a family of proteins that interact with phospholipids in a Ca2+-dependent manner. This protein has been demonstrated to have anti-phospholipase A2 activity. However, this effect has never yet been reported with the 85-kDa cytosolic PLA2 (cPLA2). We studied, in a model of differentiated and streptolysin O-permeabilized HL-60 cells, the effect of annexin V on cPLA2 activity after stimulation by calcium, GTPgammaS (guanosine 5'-O-(3-thiotriphosphate)), formyl-Met-Leu-Phe, or phorbol 12-myristate 13-acetate. Both recombinant and human placental purified annexin V inhibit cPLA2 activity whatever the stimulus used. The decrease of arachidonic acid release is of 40 and 50%, respectively, at [Ca2+] of 3 and 10 microM. The mechanism of inhibition was also analyzed. cPLA2 requires calcium and protein kinase C (PKC) or mitogen-activated protein kinase phosphorylation for its activation. As annexin V was shown to be an endogenous inhibitor of PKC, PKC-stimulated cPLA2 activity was analyzed. Using GF109203x, a specific PKC inhibitor, we demonstrated that this pathway is of minor importance in our model. cPLA2 inhibition by annexin V is not linked to PKC inhibition. To test the hypothesis of phospholipid depletion, mutants of annexin V were constructed using mutagenesis directed to Ca2+ site. We demonstrate that the Ca2+ site located in domain I is necessary for the inhibitory effect of annexin V on cPLA2 activity. The site in domain IV is also involved but with less efficiency. In contrast, mutations in site II and III do not modify this effect. Moreover, annexin V mutated on all sites does not inhibit cPLA2. Thus, we propose a predominant role of module (I/IV) in the biological action of annexin V, which, in physiological conditions, may control cPLA2 activity by depletion of the phospholipid substrate.

Volume-sensitive chloride current in pigmented ciliary epithelial cells: role of phospholipases

The whole cell recording technique was used to examine an outwardly rectifying chloride current activated by hypotonic shock in bovine pigmented ciliary epithelial (PCE) cells. Removal of internal and external Ca2+ did not affect the activation of these currents, but they were abolished by the phospholipase C inhibitor neomycin. The current was blocked by 5-nitro-2-(3-phenylpropylamino)benzoic acid, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, and 4,4'-disothiocyanostilbene-2,2'-disulfonic acid (DIDS) in a voltage-dependent manner, but tamoxifen, dideoxyforskolin, and quinidine did not affect it. This blocking profile differs from that of the volume-sensitive chloride channel in neighboring nonpigmented ciliary epithelial cells (Wu, J., J. J. Zhang, H. Koppel, and T. J. C. Jacob, J. Physiol, Lond. 491: 743-755, 1996), and this difference implies that the volume responses of the two cell types are mediated by different chloride channels (Jacob, T. J. C., and J. J. Zhang. J. Physiol. Lond. In press). Intracellular administration of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) to PCE cells induced a transient, time-independent, outwardly rectifying chloride current that closely resembled the current activated by hypotonic shock. DIDS produced a voltage-dependent block of the GTP gamma S-activated current similar to the block of the hypotonically activated current. Intracellular neomycin completely prevented activation of this current as did incubation of the cells in calphostin C. and inhibitor of protein kinase C (PKC). Removal of Ca2+ did not affect activation of the current by GTP gamma S but extended the duration of the response. Inhibition of phospholipase A2 (PLA2) with p-bromophenacyl bromide prevented the activation of the hypotonically induced current and also inhibited the current once activated by hypotonic solution. The findings imply that the hypotonic response in PCE cells is mediated by both phospholipase C (PLC) and PLA2. Both phospholipases generate arachidonic acid, and, in addition, the PLC pathway regulates the PLA2 pathway via a PKC-dependent phosphorylation of PLA2.

Pyrimidinoceptor-mediated activation of phospholipase C and phospholipase A2 in RAW 264.7 macrophages

1. As well as the presence of P2Z purinoceptors previously found in macrophages, we identified pyrimidinoceptors in RAW 264.7 cells, which activate phospholipase C (PLC) and phospholipase A2 (PLA2). 2. The relative potency of agonists to stimulate inositol phosphate (IP) formation and arachidonic acid (AA) release was UTP = UDP > > ATP, ATP gamma S, 2MeSATP. For both signalling pathways, the EC50 values for UTP and UDP (3 microM) were significantly lower than that for ATP and all other analogues tested (> 100 microM). 3. UTP and UDP displayed no additivity in terms of IP formation and AA release at maximally effective concentrations. 4. UTP-, but not ATP-, evoked AA release was 60% inhibited by pertussis toxin (PTX), while stimulation of IP formation by both agonists was unaffected. Short-term treatment with phorbol 12-myristate 13-acetate (PMA) led to a dose-dependent inhibition of IP responses to UTP and UDP, but failed to affect the AA responses. Removal of extracellular Ca2+ inhibited the PI response to UTP, but abolished its AA response. 5. ATP-induction of these two transmembrane signal pathways was decreased in high Mg(2+)-containing medium but potentiated by the removal of extracellular Mg2+. 6. Suramin and reactive blue displayed equal potency to inhibit the IP responses of UTP and ATP. 7. Both UTP and UDP (0.1-100 microM) induced a sustained increase in [Ca2+]i which lasted for more than 10 min. 8. Taken together, these results indicate that in mouse RAW 264.7 macrophages, pyrimidinoceptors with specificity for UTP and UDP mediate the activation of PLC and cytosolic (c) PLA2. The activation of PLC is via a PTX-insensitive G protein, whereas that of cPLA2 is via a PTX-sensitive G protein-dependent pathway. The sustained Ca2+ influx caused by UTP contributes to the activation of cPLA2. RAW 264.7 cells also possess P2z purinoceptors which mediate ATP(4-)-induced PLC and PLA2 activation.

Comparison of tBuBHQ with chemotactic peptide and phorbol ester in O2- production in HL-60 cells

The effect of 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tBuBHQ), a Ca2+ pump inhibitor, on superoxide anion (O2-) production was examined with a special reference to Ca2+ in HL-60 cells differentiated by dibutyryl cAMP, and compared with the effect of N-formyl-Met-Leu-Phe (fMLP) and phorbol 12-myristate 13-acetate (PMA). tBuBHQ caused O2- production and Ca2+ mobilization, but not phosphoinositide hydrolysis. fMLP caused O2- production, Ca2+ mobilization and phosphoinositide hydrolysis. PMA caused O2- production without affecting Ca2+ mobilization and phosphoinositide hydrolysis. EGTA and O,O'-bis(2-aminophenyl)ethyleneglycol- N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester (BAPTA/AM), an intracellular Ca2+ chelator, inhibited O2- production induced by fMLP, but not by tBuBHQ. Thapsigargin, another Ca2+ pump inhibitor, had a weak ability to produce O2-. fMLP, but not tBuBHQ, caused BAPTA/AM-sensitive activation of phospholipase A2 and D. tBuBHQ caused O2- production by interacting with phosphatidylcholine in a cell-free system. The results suggest that tBuBHQ causes O2- production independent of Ca2+, and Ca2+ might be a cofactor in the activation of phospholipase A2 and D upstream in fMLP-induced O2- production.

Activation of cytosolic phospholipase A2 in permeabilized human neutrophils

Neutrophils (PMN) contain two types of phospholipase A2 (PLA2), a 14 kDa 'secretory' Type II PLA2 (sPLA2) and an 85 kDa 'cytosolic' PLA2 (cPLA2), that differ in a number of key characteristics: (1) cPLA2 prefers arachidonate (AA) as a substrate but hydrolyzes all phospholipids; sPLA2 is not AA specific but prefers ethanolamine containing phosphoacylglycerols. (2) cPLA2 is active at nM calcium (Ca2+) concentrations; sPLA2 requires microM Ca2+ levels. (3) cPLA2 activity is regulated by phosphorylation; sPLA2 lacks phosphorylation sites. (4) cPLA2 is insensitive to reduction; sPLA2 is inactivated by agents that reduce disulfide bonds. We utilized PMN permeabilized with Staphylococcus aureus alpha-toxin to determine whether one or both forms of PLA2 were activated in porated cells under conditions designed to differentiate between the two enzymes. PMN were labeled with [3H]AA to measure release from phosphatidylcholine and phosphatidylinositol; gas chromatography-mass spectrometry was utilized to determine total AA release (mainly from phosphatidylethanolamine) and to assess oleate and linoleate mass. A combination of 500 nM Ca2+, a guanine nucleotide, and stimulation with n-formyl-met-leu-phe (FMLP) were necessary to induce maximal AA release in permeabilized PMN measured by either method; AA was preferentially released. [3H]AA and AA mass release occurred in parallel over time. A hydrolyzable form of ATP was necessary for maximum AA release and staurosporin inhibited PLA2 activation. Dithiothreitol treatment had little affect on [3H]AA release and metabolism but inhibited AA mass release. Assay of cell supernatants after cofactor addition did not detect sPLA2 activity and the cytosolic buffer utilized did not support activity of recombinant sPLA2. These results strongly suggested that cPLA2 was the enzyme activated in the permeabilized cell model and this is the first report which unambiguously demonstrates AA release in response to activation of a specific type of PLA2 in PMN.

Involvement of an arachidonic-acid-dependent pathway in the interferon-beta-mediated expression of C202 gene in Ehrlich-ascites-tumor cells

We have investigated the signal transduction mechanism of the expression of the C202 gene mediated by interferon beta (IFN-beta) in the murine Ehrlich's ascites tumor cell line. We have shown that treatment of cells with IFN-beta transiently enhances within minutes the release of free arachidonic acid through membrane phospholipase activity. Furthermore, prior treatment with either p-bromophenacyl bromide, an antagonist of both cytosolic and secretory phospholipase A2, or neomycin, which blocks phospholipase C activity, significantly decreased the activation of the murine IFN-beta-inducible gene, C202. Moreover, an increase of the expression of the C202 gene was observed after blocking of both the cyclooxygenase and lipoxygenase pathways. This suggests that further metabolism of arachidonic acid to epoxides via epoxygenase-catalysed pathways may be a mechanism by which second messengers for IFN-beta-mediated effects on C202 gene expression are generated. Taken together, these results indicate that lipids as second messengers may be important mediators in the IFN-beta-based activation of C202 gene expression.

Alterations in the activity of phospholipases A2 in postmortem white matter from patients with multiple sclerosis

Activities toward arachidonyl-labelled phospholipase A2 substrates were assayed in fractions of white matter and cerebral cortex from control subjects and in fractions of demyelinated plaque, normal-appearing white matter and cerebral cortex from subjects who died with multiple sclerosis. Membranous activity at pH 8.6 in the presence of Ca2+, characteristic of 14 kDa "secretory" phospholipase A2, in either multiple sclerosis white matter or cortex did not differ from controls, whereas membranous activity at pH 4.5 in the absence of added Ca2+, characteristic of lysosomal enzymes was increased over controls in both plaque and normal-appearing white matter but not cerebral cortex. Activity in the cytosol fraction, at pH 8.6 in the presence of Ca2+ and glycerol characteristic of the "cytosolic" 85 kDa enzyme was decreased by greater than 50% in both white matter and cortex samples from multiple sclerosis subjects. Immuno-precipitation and -blotting confirmed that the deficient activity was largely attributable to the 85 kDa enzyme although the enzyme protein was not similarly reduced.

Cytosolic phospholipase A2 is phosphorylated in collagen- and thrombin-stimulated human platelets independent of protein kinase C and mitogen-activated protein kinase

Human platelets pretreated with indomethacin release arachidonic acid predominantly through the activity of cytosolic phospholipase A2 (cPLA2), an 85-kDa protein. This enzyme is regulated by an increase in intracellular Ca2+, a necessary condition of for arachidonic acid liberation, and by phosphorylation. Phosphorylation of cPLA2 enhanced the Ca(2+)-induced arachidonic acid release in platelets stimulated by the ionophore A23187 and phorbol ester (phorbol 12,13-dibutyrate (PDBu)). In thrombin-stimulated platelets, however, phosphorylation appeared not to be necessary for arachidonic acid release since the latter response was not impaired in the presence of staurosporine, which inhibited phosphorylation. Collagen, thrombin, and PDBu induced phosphorylation of platelet cPLA2 as well as activation of mitogen-activated protein kinase (MAPK; p42mapk and p44mapk). cPLA2 activation was not dependent on protein kinase C (PKC) in thrombin- and collagen-stimulated platelets, as preincubation with the PKC inhibitor Ro 31-8220 neither interfered with cPLA2 phosphorylation nor reduced arachidonic acid release. However, collagen- and thrombin-induced activation of MAPK was inhibited by Ro 31-8220, indicating that PKC is necessary for MAPK stimulation in platelets. Although MAPK may underlie phosphorylation of cPLA2 in PDBu-activated human platelets, our results provide evidence for PKC- and MAPK-independent phosphorylation of cPLA2 in platelets stimulated by the physiological activators collagen and thrombin.

Evidence for the involvement of 5-lipoxygenase products in the regulation of the expression of the proenkephalin gene in cultured astroglial cells

Cultured astroglial cells secrete eicosanoids which are produced by the cyclooxygenase and lipoxygenases. These cells also transcribe the proenkephalin gene. In the present study, it was investigated whether agents which inhibit the metabolism of arachidonic acid affect the basal and stimulated expression of the gene. Tetradecanoyl phorbol acetate (TPA; 1-1000 nmol/l) increases the concentration of proenkephalin mRNA in these cells by activating protein kinase C. The enhancement in proenkephalin mRNA caused by TPA (10 nmol/l) was not affected by the cyclooxygenase inhibitor indomethacin (5 mumol/l). However, nordihydroguaiaretic acid, which blocks cyclooxygenase and lipoxygenases, potentiated the effect of TPA on proenkephalin mRNA, when used at concentrations of 0.5-50 mumol/l. Two selective inhibitors of 5-lipoxygenase, i.e. MK886 (5 mumol/l) and BAY X1005 (1 mumol/l), also enhanced the effect of TPA (10 nmol/l) without affecting the basal expression of the gene. When added to the incubation medium, leukotriene E4 (10-1000 nmol/l) diminished in a dose-dependent manner the basal and TPA-induced expression of the proenkephalin gene. It is concluded that in astroglial cells derived from cortex of new-born rats products of 5-lipoxygenase can diminish the action of protein kinase C on the proenkephalin gene.

Cytosolic phospholipase A2

To summarize the regulation of cPLA2, we have proposed a model for the activation of cPLA2 based both on our previous studies (Clark et al., 1991; Lin et al., 1993) and the work of many others (Fig. 5). In this model, cPLA2 is tightly regulated by multiple pathways, including those that control Ca2+ concentration, phosphorylation states and cPLA2 protein levels, to exert both rapid and prolonged effects on cellular processes, such as inflammation. cPLA2 is rapidly activated by increased intracellular Ca2+ concentration and phosphorylation by MAP kinase. When cells are stimulated with a ligand for a receptor, such as ATP or PDGF, PLC is activated via either a G protein-dependent or -independent process, leading to the production of diacylglycerol (DAG) and inositol triphosphate (IP3). The rise in these intracellular messengers cause the activation of PKC and mobilization of intracellular Ca2+. Alternatively, the increase in intracellular Ca2+ can result from a Ca2+ influx. Increased Ca2+ acts through the CaLB domain to cause translocation of cPLA2 from the cytosol to the membrane where its substrate, phospholipid, is localized. This step is essential for the activation of cPLA2 and may account for the partial activation of cPLA2 in the absence of phosphorylation. MAP kinase activation can occur through both PKC-dependent and -independent mechanisms (Cobb et al., 1991; Posada and Cooper, 1992; Qiu and Leslie, 1994). In many cases, this pathway is also G protein-dependent. Activated MAP kinase phosphorylates cPLA2 at Ser-505, causing increased enzymatic activity of cPLA2, which is realized only upon translocation of cPLA2 to the membrane. Therefore, full activation of cPLA2 requires both increased cytosolic Ca2+ and cPLA2 phosphorylation at Ser-505. In a more delayed response, cPLA2 activity in the cells can be controlled by changes in its expression levels, such as in response to inflammatory cytokines and certain growth factors. Thus the expression level of cPLA2 is regulated by both transcriptional and post-transcriptional mechanisms.

Evidence for the activation of the signal-responsive phospholipase A2 by exogenous hydrogen peroxide

The intracellular events that lead to arachidonic acid release from bovine endothelial cells in culture treated with hydrogen peroxide were characterized. The hydrogen peroxide-stimulated release of arachidonic acid was time- and dose-dependent, with maximal release achieved at 15 minutes after the addition of 100 microM hydrogen peroxide. Hydrogen peroxide-stimulated release of arachidonic acid was blocked with the phospholipase A2 inhibitor quinacrine. Treatment of the cells with hydrogen peroxide did not result in liberation of oleic acid, indicating that hydrogen peroxide exercised its effect on an arachidonate-specific phospholipase. Pretreatment of the cells with antioxidants, transition metal chelators, and hydroxyl radical scavengers did not affect the hydrogen peroxide-stimulated arachidonic acid release, indicating that the response to hydrogen peroxide is not oxygen radical-mediated. The response to hydrogen peroxide does not appear to be calcium-dependent, due to the following two observations: (a) No increase in intracellular calcium was seen upon exposure of the FURA2-loaded cells to hydrogen peroxide at concentrations sufficient to release arachidonic acid, and (b) no change in the release response was detected in cells loaded with the intracellular calcium chelator BAPTA. Significant inhibition of arachidonic acid release was seen when the cells were pretreated with inhibitors of protein kinase C, but not with inhibitors of tyrosine kinase. The results of these studies indicate that hydrogen peroxide-stimulated arachidonic acid release is mediated by a specific signal-responsive phospholipase A2, and that this process is not mediated via the actions of either lipid peroxidation or calcium but, rather, that a stimulation of intracellular kinase activity is necessary for this response.

Platelet-activating factor preferentially stimulates the phospholipase A2/cyclooxygenase cascade in the rabbit cornea

Platelet-activating factor (PAF) is formed in the cornea after injury as well as by infiltrating inflammatory cells. We have studied the effects of PAF on the release and metabolism of arachidonic acid (AA) in the rabbit cornea. Corneal lipids were labeled in vivo by injecting [3H]AA and subsequently incubated in vitro with 100 nM PAF in the presence or absence of 10 microM BN50727, a PAF antagonist. The AA and eicosanoids released by incubated corneas were analyzed by high-performance liquid chromatography (HPLC). Tissue lipids were examined by mono- and bidimensional thin-layer chromatography (TLC). Within 5 min, PAF stimulated AA release to 76% above control levels. BN50727 inhibited the AA release elicited by PAF at all time points studied. The decreased content of [3H]AA in phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylinositol (PI) following PAF exposure and the lack of stimulation by PAF on the release of [3H] linoleic acid suggest that the cytosolic phospholipase A2 was activated. PAF also stimulated depletion of AA from the inositol lipids, phosphatidylinositol-4-phosphate (PIP) and phosphatidylinositol-4,5-biphosphate (PIP2) and increased content of [3H]AA into diacylglycerol (DAG) and phosphatidic acid (PA). This reaction indicates that PAF could also mediate activation of other phospholipases in the cornea. In addition, PAF preferentially stimulated the cyclooxygenase pathway. The PAF antagonist BN50727 mainly suppressed the PAF-stimulated release of PGE2. The antagonist did not inhibit lipoxygenase activity even after 30 min of PAF stimulation. These results suggest that PAF activate a phospholipase A2/cyclooxygenase pathway in the cornea via a PAF-receptor mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple enzymatic activities of the human cytosolic 85-kDa phospholipase A2: hydrolytic reactions and acyl transfer to glycerol

The recombinant human 85-kDa cytosolic phospholipase A2 (cPLA2), when assayed in the presence of glycerol, catalyzes the transfer of acyl chains of radiolabeled phosphatidylcholine and para-substituted phenyl esters of fatty acids to glycerol, in addition to hydrolyzing these substrates. The product of the transacylation reaction is monoacylglycerol (MAG), and the acyl chain is predominantly esterified (> or = 95%) to a primary hydroxyl group of glycerol (sn-1/3); the stereochemistry is not known. Increasing concentrations of glycerol accelerate enzyme turnover both by providing an additional mechanistic pathway for the enzyme-substrate complex to form products and by increasing the intrinsic hydrolytic and transacylation activities of the enzyme. Significant enzymatic hydrolysis of sn-1/3-arachidonylmonoacylglycerol was measured, while sn-1/3-alpha-linolenoyl- and sn-2-arachidonylmonoacylglycerols were not detectably hydrolyzed. 1,3-Propanediol also serves as an acyl acceptor for the enzyme. cPLA2 hydrolyzes analog of lysophosphatidylcholine that lacks the sn-2 hydroxyl group. The enzyme will hydrolyze sn-1-acyl chains of rac-1-(arachidonyl, alpha-linolenoyl, palmitoyl)-2-O-hexadecyl-glycero-3-phosphocholine lipids and transfer the acyl chain to glycerol. Thus, cPLA2 has phospholipase A1 activity but only if an ether linkage rather than an ester linkage is present at the sn-2 position, and it is shown that the sn-1 acyl chains of both enantiomers of phosphatidylcholine are hydrolyzed. Phenyl [14C]-alpha-linolenate and five para-substituted phenyl esters of [3H]-alpha-linolenic acid with pKa values ranging from 7.2 to 10.2 for the phenol leaving groups were incorporated into 1,2-ditetradecyl-sn-glycero-3-phosphomethanol/Triton X-100 mixed micelles as substrates for the transacylation/hydrolysis reactions of the enzyme. Average product ratios, which are defined as the amount of monoacylglycerol formed to phenyl ester hydrolyzed, were 2.1 +/- 0.1 (n = 5) for the para-substituted phenyl esters and 2.0 +/- 0.3 (n = 7) for phenyl alpha-linolenate. The similarity of the ratios, despite the range of pKa values for the leaving groups, is consistent with the formation of a common enzyme intermediate that partitions to give either fatty acid or MAG. That intermediate may be a covalent acyl enzyme. Finally, the acyl chain specificity of cPLA2 was investigated to better understand the preference of the enzyme for phospholipids with sn-2-arachidonyl chains.

The guanine-nucleotide-binding protein subunit G alpha i2 is involved in calcium activation of phospholipase A2. Effects of the dominant negative G alpha i2 mutant, [G203T]G alpha i2, on activation of phospholipase A2 in Chinese hamster ovary cells

The mechanisms of activation of cytoplasmic phospholipase A2 (cPLA2) are complex and incompletely defined. In Chinese hamster ovary (CHO) cells, receptor stimulation of cPLA2 is due to the interaction of pathways involving the alpha subunits of at least two guanine-nucleotide-binding (G) proteins, G alpha i2 and G alpha q. Activation of cPLA2 is inhibited by pertussis toxin and G alpha i2 mutants. In addition, activation of phospholipase C via G alpha q results in increased intracellular calcium ([Ca2+]i) and activation of protein kinase C, both of which interact with and activate cPLA2. The present study was undertaken to analyze the mechanism of interaction of G alpha i2 with the phospholipase-C-stimulated pathway in the activation of cPLA2. We addressed this question using a dominant negative G alpha i2 mutant, [G203T]G alpha i2, in which Gly203 is mutated to Thr. [G203T]G alpha i2 inhibits ATP receptor activation of cPLA2. The effect of [G203T]G alpha i2 was specific to G alpha i2-activated pathways, as shown by its lack of effect on other purinergic receptor stimulated pathways: ATP stimulation of [Ca2+]i or mitogen-activated protein kinase phosphorylation is unaltered by [G203T]G alpha i2. We addressed the possibility that the activation of cPLA2 by Ca2+ and/or protein kinase C is dependent on G alpha i2. Activation of cPLA2 by the Ca2+ ionophore, ionomycin, was inhibited by 61 +/- 9% (n = 5) in [G203T]G alpha i2-expressing cells; however the ionomycin-induced [Ca2+]i rise was unaffected by [G203T]G alpha i2. Thus, [G203T]G alpha i2. specifically inhibits Ca2+ activation of cPLA2. In contrast, activation of cPLA2 via protein kinase C by phorbol 12-myristate 13-acetate was unaffected by [G203T]G alpha i2. Our results demonstrate that Ca2+ but not phorbol ester activation of cPLA2 in CHO cells is G alpha i2-dependent. The possibility is discussed that G alpha i2 is downstream of Ca2+ but upstream of protein kinase C activation of cPLA2.

Hydrogen peroxide activation of cytosolic phospholipase A2 in vascular smooth muscle cells

We have reported previously that hydrogen peroxide induces arachidonic acid release from prelabeled vascular smooth muscle cells. Here, we studied the effect of hydrogen peroxide on the phosphorylation of cytosolic phospholipase A2 in these cells. Hydrogen peroxide induced a rapid, time-dependent increase in the phosphorylation of cytosolic phospholipase A2. Hydrogen peroxide also increased arachidonic acid release from prelabeled cells in a time-dependent manner similar to that of phosphorylation of cytosolic phospholipase A2. Protein kinase C depletion significantly inhibited the hydrogen peroxide-stimulated cytosolic phospholipase A2 phosphorylation and arachidonic acid release. Hydrogen peroxide caused a time-dependent increase in mitogen activated protein kinase activity. Taken together, these findings suggest that cytosolic phospholipase A2 may, at least in part, contribute to arachidonic acid release induced by hydrogen peroxide and this effect appears to be mediated by protein kinase C and mitogen activated protein kinase.

N-ethyl maleimide stimulates arachidonic acid release through activation of the signal-responsive phospholipase A2 in endothelial cells

Treatment of bovine endothelial cells with the alkylator N-ethyl maleimide results in arachidonic acid mobilization. N-ethyl maleimide-stimulated arachidonic acid release was dose and time dependent and maximum release was achieved after 10-15 min with 50 microM N-ethyl maleimide, N-ethyl maleimide-stimulated arachidonic acid release could be prevented by pretreating the cells with the phospholipase A2 inhibitor quinacrine. Based on the finding that N-ethyl maleimide was not able to release oleic acid from oleic acid-preloaded cells, it was clear that the effect of N-ethyl maleimide was limited to an arachidonic acid-specific phospholipase. The effect of N-ethyl maleimide does not appear to be dependent on calcium, as shown by the observation that N-ethyl maleimide was not able to increase intracellular calcium concentration in FURA2-loaded cells. Pretreatment of the cells with staurosporine totally inhibited N-ethyl maleimide-stimulated arachidonic acid liberation. The tyrosine kinase inhibitor genistein was also able to significantly inhibit arachidonic acid release. It is concluded from the results obtained in this study that N-ethyl maleimide stimulates arachidonic acid release by stimulating the activity of a specific, signal-responsive phospholipase A2. Furthermore this activation is not mediated by intracellular calcium fluxes but by a stimulation of intracellular kinase activity which eventually leads to the activation of this signal-responsive phospholipase A2.

The role of protein kinase C in arachidonic acid release and prostaglandin E production from CHO cells transfected with EGF receptors

Arachidonic acid release and prostaglandin production are stimulated by both phorbol esters and growth factors in various cell types. Whereas phorbol esters activate and transmit a signal via protein kinase C, this pathway is not necessarily involved in growth factor signal transduction. We investigated the involvement of protein kinase C in the pathway of arachidonic acid metabolism by CHO cells transfected with full-length EGF receptor (CHOwt). Two isoforms of protein kinase C were identified in CHOwt cells, alpha and zeta. On downregulation, the parallel loss of phorbol ester-stimulated arachidonic acid release and the alpha-isoform suggests a possible involvement of this isoform in phospholipase A2 activation in these cells. In addition, we propose that the zeta-isoform may be separately involved in prostaglandin production as residual phorbol ester-stimulation of PGE production occurs in downregulated cells where PKC zeta is the sole remaining isoform. EGF stimulation of arachidonic acid release, as a measure of phospholipase A2 activation, and subsequent prostaglandin production are unaffected by inhibition of protein kinase C in CHOwt cells. Indeed one such inhibitor, staurosporine, augmented the EGF effect. These results suggest that PKC is not required for EGF activation of phospholipase A2 in these cells.

Multiple mechanisms of arachidonic acid release in Chinese hamster ovary cells transfected with cDNA of substance P receptor

We investigated the release of [3H]arachidonic acid ([3H]AA) and its relationship to the formation of [3H]inositol trisphosphate ([3H]IP3) elicited by substance P (SP) in prelabeled Chinese hamster ovary cells stably expressing the SP receptor. Activation of the SP receptor resulted in a concentration- and time-dependent stimulation of [3H]AA release. Half-maximal release was obtained at 10(-9) M, comparable to that for [3H]IP3 formation reported previously, and the maximal release effected by 0.1 microM SP was 8 to 10-fold above the basal value. Both the [3H]AA release and the [3H]-IP3 accumulation stimulated in the cells by 0.1 microM SP were concentration-dependently blocked with the specific SP receptor antagonist CP-96,345, with IC50 values of 2.5 and 0.4 microM, respectively. The time course of [3H]AA release showed a biphasic pattern: an initial rapid release essentially independent of Ca2+, followed by a sustained release markedly suppressed by removal of extracellular Ca2+ or chelation of intracellular Ca2+ with 1,2-bis(2-aminophenoxyethane)-N,N,N',N'-tetraacetic acid (BAPTA). While pretreatment with pertussis toxin (200 ng/mL, 6 hr) did not block [3H]IP3 formation, it did reduce [3H]AA release by 50% at 1 and 10 min after SP stimulation. Treatment of the cells with a phorbol ester, a protein kinase C activator, augmented the SP-stimulated [H]AA release, and sphingosine, a protein kinase C inhibitor, reversed the phorbol ester-potentiated [3H]AA release, but not the release stimulated by SP alone, suggesting a synergistic effect of protein kinase C on SP-stimulated AA release. These results demonstrate that SP, acting at the SP receptor, stimulates [3H]AA release via mechanisms that are (1) mediated by a pertussis toxin-sensitive G-protein, (2) dependent on extracellular Ca2+, and (3) enhanced by activation of protein kinase C.

A role for reactive oxygen species in zymosan and beta-glucan induced protein tyrosine phosphorylation and phospholipase A2 activation in murine macrophages

Previously we have shown that reactive oxygen species (ROS) formation induced by phorbol ester in association with vanadate is essential for protein tyrosine phosphorylation and phospholipase A2 (PLA2) activation. Here we show that the interaction of beta-glucan particles (glucanp) or zymosan with complement receptor type 3 (CR3) leads, when associated with vanadate, to a cascade of reactions culminating in PLA2 activation. Vanadate + zymosan (or glucanp) markedly enhance protein tyrosine phosphorylation in bone marrow derived macrophages (BMMs), whereas neither of the agents alone has any effect. The enhancement was due to both sustained activation of protein tyrosine kinase (PTK) and inactivation of protein tyrosine phosphatase (PTP) as assessed in lysates of treated cells. Zymosan elevates membranal PKC, an effect that is potentiated by vanadate. Activation of both PTK and PKC leads to the activation of NADPH oxidase and to ROS formation. The formed ROS together with vanadate are potent inactivators of PTP leading to amplification of tyrosine phosphorylation and myelin basic protein kinase (MBP-K) activation. The activation of the cascade of protein kinases eventually leads to activation of PLA2. All the activation steps, i.e., activation of PTK, NADPH oxidase, MBP-K,PLA2 and the inactivation of PTP are sensitive to the NADPH oxidase inhibitor diphenyleneiodonium (DPI), to antioxidants and to PKC inhibitors. Thus, ROS formation (in the presence of vanadate) is critical for protein phosphorylation processes constituting the regulatory pathway of PLA2 activation by ligand-CR3 interaction.

High-density lipoprotein stimulates endothelial cell movement by a mechanism distinct from basic fibroblast growth factor

Endothelial cell (EC) migration is a regulatory event in the formation and repair of blood vessels. Although serum contains substantial promigratory activity, the responsible components and especially the role of lipoproteins have not been determined. We examined the effect of plasma high-density lipoprotein (HDL) on the movement of ECs in vitro. Confluent cultures of bovine aortic ECs in serum-free medium were "wounded," and migration was measured after 24 hours. HDL stimulated migration in a concentration-dependent manner with a half-maximal response at 25 to 40 micrograms cholesterol per milliliter and a maximal twofold stimulation at approximately 150 micrograms cholesterol per milliliter. HDL-stimulated migration was not due to cell proliferation, since migration was increased in the presence of hydroxyurea at a concentration that blocked proliferation. At optimal concentrations, HDL was at least as stimulatory as basic fibroblast growth factor (FGF). However, the activity of HDL was not due to contamination by basic FGF, since antibodies to basic FGF did not block HDL-stimulated movement and since the maximum promigratory activities of basic FGF and HDL were additive. These results indicate that HDL and basic FGF may use distinct signaling pathways to initiate EC movement. This possibility was confirmed by results showing that pertussis toxin suppressed basic FGF-stimulated but not HDL-stimulated EC motility and that inhibitors of phospholipase A2, aristolochic acid and ONO-RS-082, also blocked the promigratory activity of basic FGF but had no effect on the activity of HDL.(ABSTRACT TRUNCATED AT 250 WORDS)

Complete discrimination of docosahexaenoate from arachidonate by 85 kDa cytosolic phospholipase A2 during the hydrolysis of diacyl- and alkenylacylglycerophosphoethanolamine

In our previous report (Shikano, M., Masuzawa, Y. and Yazawa, K. (1993) J. Immunol. 150, 3525-3533), we described that the enrichment of docosahexaenoic acid (DHA, 22:6(n - 3)) reduces both arachidonic acid (AA, 20:4(n - 6)) release and platelet-activating factor (PAF) synthesis in human eosinophilic leukemia cells, Eol-1. Since no DHA release was observed in response to Ca-ionophore stimulation, we presumed that the phospholipase A2 (PLA2) responsible for AA release and PAF synthesis can not hydrolyze the DHA moiety of phospholipids. In the present paper, we examined whether DHA-containing diacyl- and alkenylacylglycerophosphoethanolamine (DHA-diacylGPE and DHA-alkenylacyGPE) are susceptible to the action of AA-preferential 85 kDa cytosolic phospholipase A2 (cPLA2) from rabbit platelets in comparison with AA and eicosapentaenoic acid (EPA, 20:5(n - 3)) derivatives. When diacylGPE was used as a substrate, DHA release was almost negligible under the assay condition that allowed AA and EPA to be liberated at the rates of 4.3 mumol/min per mg protein and 2.5 mumol/min per mg protein, respectively. On the other hand, 14 kDa type II PLA2 hydrolyzed DHA-diacylGPE as well as AA-diacylGPE and EPA-diacylGPE. When DHA-diacylGPE and AA-diacylGPE were mixed at equimolar concentrations, DHA release by cPLA2 was not observed and AA release was reduced to 32% in the case without DHA-diacylGPE. This indicated that DHA-diacylGPE is a poor substrate but possesses the inhibitory activity for cPLA2. cPLA2 does not clearly discriminate between AA-alkenylacylGPE and AA-diacylGPE. As in the case using diacylGPE as a substrate, DHA-alkenylacylGPE was completely discriminated from AA-alkenylacylGPE by cPLA2. The roles of DHA and cPLA2 in the synthesis of lipid mediators will be discussed in relation to the new aspects of the substrate specificity of cPLA2 provided here.

Autocrine enhancement of leukotriene synthesis by endogenous leukotriene B4 and platelet-activating factor in human neutrophils

1. Platelet-activating factor (PAF) and leukotriene B4 (LTB4), two potent lipid mediators synthesized by activated neutrophils, are known to stimulate several neutrophil functional responses. In this study, we have determined that endogenous LTB4 and PAF exert autocrine effects on LT synthesis, as well as the underlying mechanism involved. 2. Pretreatment of neutrophils with either pertussis toxin (PT), or with receptor antagonists for LTB4 and PAF, resulted in an inhibition of LT synthesis induced by calcium ionophore, A23187. This inhibition was most marked at submaximal (100-300 nM) A23187 concentrations, whilst it was least at ionophore concentrations which induce maximal LT synthesis (1-3 microM). Thus newly-synthesized PAF and LTB4 can enhance LT synthesis induced by A23187 under conditions where the LT-generating system is not fully activated. 3. In recombinant human (rh) granulocyte-macrophage colony-stimulating factor (GM-CSF)-primed neutrophils, LT synthesis in response to chemoattractants (fMet-Leu-Phe or rhC5a) was also significantly inhibited by the LTB4 receptor antagonist, and to a lesser extent by PAF receptor antagonists. 4. Further investigation revealed that LTB4 and/or PAF exert their effects on LT synthesis via an effect on arachidonic acid (AA) availability, as opposed to 5-lipoxygenase (5-LO) activation. Indeed, the receptor antagonists, as well as PT, inhibited LT synthesis and AA release to a similar extent, whereas 5-LO activation (assessed with an exogenous 5-LO substrate) was virtually unaffected under the same conditions. Accordingly, we showed that addition of exogenous LTB4 could enhance AA availability in response to chemoattractant challenge in rhGM-CSF-primed cells, without significantly affecting the 5-LO activation status. Our data show that newly-generated PAF and LTB4 have the ability to positively feedback on LT synthesis by acting at the level of the phospholipase A2/re-esterification component of the LT biosynthetic pathway in neutrophils. Such autocrine affects are likely to represent an important amplification step of LT synthesis, and may as such contribute to the rapid onset, as well as to the evolution, of inflammatory responses.

Calcium-dependent release of arachidonic acid in response to purinergic receptor activation in airway epithelium

The effect of purinergic receptor agonists on arachidonic acid release was investigated in [3H]arachidonic acid-prelabeled human airway epithelial cells. Exposure of bronchial epithelial BEAS39 cells to extracellular ATP resulted in a marked release of unesterified [3H]arachidonic acid with maximal effect observed within 60-90 s. [3H]diacylglycerol and [3H]phosphatidic acid accumulated in parallel with [3H]arachidonic acid. ATP-stimulated [3H]arachidonic acid release with a K0.5 of 9 +/- 2 microM and UTP was equipotent; no effect was observed with P2Y- or P2X-purinergic receptor agonists or with adenosine. Similar results were obtained with primary cultures of normal human nasal epithelium, CF/T43 and HBE1 airway epithelial cell lines derived from a cystic fibrosis patient and from a normal donor, respectively, and HT-29 human colon carcinoma cells. ATP stimulated inositol phosphate formation in BEAS39 cells with a concentration dependence identical to that for [3H]arachidonic acid release. The effect of ATP on both [3H]arachidonic acid release and inositol phosphate formation was equally inhibited by pertussis toxin. The Ca2+ ionophore A-23187 mimicked the effects of ATP or UTP on arachidonic acid release, and a marked inhibitory effect was observed with thapsigargin. The protein kinase C inhibitor staurosporine partially inhibited ATP-stimulated [3H]arachidonic acid release. These data are consistent with the hypothesis that phospholipase A2 activation is secondary to P2U-purinergic receptor stimulation of D-myoinositol 1,4,5-trisphosphate production and calcium mobilization from intracellular stores.

Ethanol inhibits zymosan-stimulated eicosanoid production in mouse peritoneal macrophages

Resident peritoneal macrophages synthesized and released eicosanoids when challenged by zymosan, a phagocytosable particle. Incubation of these cells with ethanol resulted in dose-dependent inhibition of arachidonic acid release and eicosanoid generation in response to zymosan. Ethanol affected the extent but not the ratio of eicosanoids released. When assayed in a cell-free system, endogenous phospholipase A2 activity was neither affected by the presence of ethanol in the incubation medium nor by preincubation of the cells with ethanol. Ethanol also inhibited arachidonic acid release in response to phorbol myristate acetate, a compound that, like zymosan, triggered a pertussis-toxin-sensitive response. When cells that had been previously treated with pertussis toxin were used, no further inhibitory effect of ethanol was seen in response to both zymosan and phorbol myristate acetate. On the other hand, ethanol had no effect on arachidonic acid release stimulated by ionophore A23187 or lipopolysaccharide, two compounds that triggered a pertussis-toxin-insensitive response. Moreover, ethanol was able to nearly abolish arachidonic acid release in response to fluoroaluminate, a direct activator of G-proteins. Altogether, the results of this study suggest that ethanol inhibits zymosan-stimulated eicosanoid production by interacting with a G-protein--or a G-protein-mediated process--that is critically involved in arachidonic acid mobilization.