Depletion of human monocyte 85-kDa phospholipase A2 does not alter leukotriene formation

The central role of thromboxane and platelet activating factor receptors in ex vivo regulation of endotoxin-induced monocyte tissue factor activity in human whole blood

Expression of tissue factor (TF) by activated monocytes may initiate thrombotic episodes associated with diseases, such as thrombosis and atherosclerosis. In this study, steps in the regulatory pathways of lipopolysaccharide (LPS)-induced monocyte TF activity and released TNF-α in human whole blood were probed for using an array of inhibitors, comprising specific inhibitors of cytosolic phospholipase A2 (PLA2) (AACOCF3), secretory PLA (SB-203347), protein kinase (PK) (staurosporine), PKC (GF109203; BIM), and serine protease (Pefabloc SC), antagonists of thromboxane prostanoid (TP) receptor (R) (SQ-29548), platelet activating factor (PAF) R (BN-52021), leukotriene B4 R (SC-41930), serotonin R (cyproheptadine), fibronectin/fibrinogen R (RGDS), and finally, creatine phosphate/creatine phosphokinase (CP/CPK) which removes ADP. Whereas when added alone neither of these agents significantly inhibited LPS-induced TF or TNF-α, when presented as a reference cocktail comprising all the agents, TF activity and TNF-α were reduced by 77% and 49%, respectively. By subsequently testing a series of incomplete inhibitory cocktails equal to the reference except for deleted single agents or combinations of two or three active agents, the inhibitory effect of the reference cocktail could be shown to depend on the presence of the protease inhibitor and the thromboxane A2 and PAF antagonists.

Substrate inhibition kinetics in drug metabolism reactions

Inhibition of enzyme activity at high substrate concentrations, so-called "substrate inhibition," is commonly observed and has been recognized in drug metabolism reactions since the last decade. Although the importance of such "atypical" kinetics in vivo remains poorly understood, a substrate with substrate inhibition kinetics has been shown to unconventionally alter the metabolism of other substrates. In recent years, it is becoming increasingly evident that the mechanisms for substrate inhibition are highly complex, which are possibly contributed by multiple (at least two) binding sites within the enzyme protein, the formation of a ternary dead-end enzyme complex, and/or the ligand-induced changes in enzyme conformation. This review primarily discusses the mechanisms for substrate inhibition displayed by the important drug-metabolizing enzymes, such as cytochrome p450s, UDP-glucuronyltransferases, and sulfotransferases. Kinetic modeling of substrate inhibition in the absence or presence of a modifier is another central issue in this review because of its importance in the determination of kinetic parameters and in vitro/in vivo predictions.

A phospholipase A₂ from Bothrops asper snake venom activates neutrophils in culture: expression of cyclooxygenase-2 and PGE₂ biosynthesis

In this study, the production of prostaglandin E₂ (PGE₂) and up-regulation in cyclooxygenase (COX) pathway induced by a phospholipase A₂ (PLA₂), myotoxin-III (MT-III), purified from Bothrops asper snake venom, in isolated neutrophils were investigated. The arachidonic acid (AA) production and the participation of intracellular PLA₂s (cytosolic PLA₂ and Ca(2+)-independent PLA₂) in these events were also evaluated. MT-III induced COX-2, but not COX-1 gene and protein expression in neutrophils and increased PGE₂ levels. Pretreatment of neutrophils with COX-2 and COX-1 inhibitors reduced PGE₂ production induced by MT-III. Arachidonyl trifluoromethyl ketone (AACOCF₃), an intracellular PLA₂ inhibitor, but not bromoenol lactone (BEL), an iPLA₂ inhibitor, suppressed the MT-III-induced AA and PGE₂ release. In conclusion, MT-III directly stimulates neutrophils inducing COX-2 mRNA and protein expression followed by production of PGE₂. COX-2 isoform is preeminent over COX-1 for production of PGE₂ stimulated by MT-III. PGE₂ and AA release by MT-III probably is related to cPLA₂ activation.

Secreted phospholipase A2 group X overexpression in asthma and bronchial hyperresponsiveness

RATIONALE

Secreted phospholipase A(2) enzymes (sPLA(2)s) play key regulatory roles in the biosynthesis of eicosanoids, such as the cysteinyl leukotrienes, but the role of these enzymes in the pathogenesis of asthma is not known.

OBJECTIVES

To establish if sPLA(2)s are overexpressed in the airways of patients with asthma, and to determine if these enzymes may play a role in the generation of eicosanoids in exercise-induced bronchoconstriction.

METHODS

Induced sputum samples were obtained from subjects with asthma with exercise-induced bronchoconstriction and nonasthmatic control subjects at baseline, and on a separate day 30 minutes after exercise challenge. The expression of the PLA(2)s in induced sputum cells and supernatant was determined by quantitative polymerase chain reaction, immunocytochemistry, and Western blot.

MEASUREMENTS AND MAIN RESULTS

The sPLA(2)s expressed at the highest levels in airway cells of subjects with asthma were groups X and XIIA. Group X sPLA(2) (sPLA(2)-X) was differentially overexpressed in asthma and localized to airway epithelial cells and bronchial macrophages. The gene expression, immunostaining in airway epithelial cells and bronchial macrophages, and the level of the extracellular sPLA(2)-X protein in the airways increased in response to exercise challenge in the asthma group, whereas the levels were lower and unchanged after challenge in nonasthmatic control subjects.

CONCLUSIONS

Increased expression of sPLA(2)-X may play a key role in the dysregulated eicosanoid synthesis in asthma.

Differential behavior of sPLA2-V and sPLA2-X in human neutrophils

Neutrophils and differentiated PLB-985 cells contain various types of PLA(2)s including the 85 kDa cytosolic PLA(2) (cPLA(2)), Ca(2+)-independent PLA(2) (iPLA(2)) and secreted PLA(2)s (sPLA(2)s). The present study focuses on the behavior of sPLA(2)s in neutrophils and PLB cells and their relationship to cPLA(2)alpha. The results of the present research show that the two types of sPLA(2) present in neutrophils, sPLA(2)-V and sPLA(2)-X, which are located in the azurophil granules, are differentially affected by physiological stimuli. While sPLA(2)-V is secreted to the extacellular milieu, sPLA(2)-X is detected on the plasma membranes after stimulation. Stimulation of neutrophils with formyl-Met-Leu-Phe (fMLP), opsonized zymosan (OZ) or A23187 resulted in a different kinetics of sPLA(2) secretion as detected by its activity in the neutrophil supernatants. Neutrophil priming by inflammatory cytokines or LPS enhanced sPLA(2) activity detected in the supernatant after stimulation by fMLP. This increased activity was due to increased secretion of sPLA(2)-V to the supernatant and not to release of sPLA(2)-X. sPLA(2) in granulocyte-like PLB cells exhibit identical characteristics to neutrophil sPLA(2), with similar activity and optimal pH of 7.5. Granulocyte-like cPLA(2)alpha-deficient PLB cells serve as a good model to study whether sPLA(2) activity is regulated by cPLA(2)alpha. Secretion and activity of sPLA(2) were found to be similar in granulocyte-like PLB cells expressing or lacking cPLA(2)alpha, indicating that they are not under cPLA(2)alpha regulation.

Reversible covalent inhibition of a phenol sulfotransferase by coenzyme A

Phenol sulfotransferases (SULTs), which normally bind 3'-phosphoadenosine-5'-phosphosulfate as the donor substrate, are inhibited by CoA and its thioesters. Here, we report that inhibition of bovine SULT1A1 by CoA is time-dependent at neutral pH under non-reducing conditions. The rates of inactivation by CoA indicate an initial reversible SULT:CoA complex with a dissociation constant of 5.7 microM and an inactivation rate constant of 0.07 min(-1). Titrations with CoA and prolonged incubations reveal that inactivation of the dimeric enzyme is stoichiometric, consistent with the observation of complete conversion of the protein to a slightly decreased electrophoretic mobility. Both activity and normal electrophoretic migration are restored by 2-mercaptoethanol. Mutagenesis demonstrated that Cys168 is the site of CoA adduction, and a consistent model was constructed that reveals a new SULT molecular dynamic. Cysteine reaction kinetics with Ellman's reagent revealed a PAPS-induced structural change consistent with the model that accounts for binding of CoA.

The role of cytosolic phospholipase A2-alfa in regulation of phagocytic functions

Phospholipase A2(s) (PLA2(s)) are a family of enzymes that is present in a variety of mammalian and nonmammalian sources. Phagocytic cells contain cytosolic PLA2 (cPLA2) as well as several types of secreted PLA2, all of which have the potential to produce proinflammatory lipid mediators. The role of the predominant form of cPLA2 present in neutrophils is cPLA2alpha was studied by many groups. By modulating its expression in a variety of phagocytes it was found that it plays a major role in formation of eicosanoids. In addition, it was reported that cPLA2alpha also regulates the NADPH oxidase activation. The specificity of its effect on the NADPH oxidase is evident by results demonstrating that the differentiation process as well as other phagocytic functions are normal in cPLA2alpha-deficient PLB cell model. The novel dual subcellular localization of cPLA2alpha in different compartments, in the plasma membranes and in the nucleus, provides a molecular mechanism for the participation of cPLA2alpha in different processes (stimulation of NADPH oxidase and formation of eicosanoids) in the same cells.

Knock down of cytosolic phospholipase A2: an antisense oligonucleotide having a nuclear localization binds a C-terminal motif of glyceraldehyde-3-phosphate dehydrogenase

We have previously shown that an antisense, effective in the knock down of cytosolic phospholipase A2 (cPLA2), localizes mainly in the nucleus of human endothelial cells and monocytes and that glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is involved in its nuclear localization. In this study, we clarify how GAPDH participates in the nuclear localization of this antisense oligodeoxynucleotide (ODN) directed against cPLA2 mRNA. A central TAAAT motif providing specificity and high affinity binding was assumed to interact with the enzyme Rossmann fold region on the basis of competition to this site by NAD+. To asses whether the TAAAT motif interacts directly with the enzyme Rossmann fold region, we evaluated the binding to GAPDH of different oligonucleotides and the effect of competitors such as NAD+, NADH, mononucleotides, DNA, polyribonucleic acids and polyanions. We found that the dissociation constant for TAAAT containing oligonucleotides was three--to fivefold higher with respect to oligo not containing this motif. By covalently linking 32P-labeled cPLA2p(N)16 to GAPDH and after executing hydrolysis with hydroxylamine, the labeling was exclusively found in the C-terminal domain (aa 286-334). These results indicate that the antisense oligonucleotide interacts with a site not having a defined function but which can be negatively allosterically regulated when NAD+ or polynucleotides are bound to Rossmann fold.

Cytosolic phospholipase A2 is responsible for prostaglandin E2 and leukotriene B4 formation in phagocyte-like PLB-985 cells: studies of differentiated cPLA2-deficient PLB-985 cells

Our previously established model of cytosolic phospholipase A(2) (cPLA(2))-deficient, differentiated PLB-985 cells (PLB-D cells) was used to determine the physiological role of cPLA(2) in eicosanoid production. Parent PLB-985 (PLB) cells and PLB-D cells were differentiated toward the monocyte or granulocyte lineages using 5 x 10(-)(8) M 1,25 dihydroxyvitamin D(3) or 1.25% dimethyl sulfoxide, respectively. Parent monocyte- or granulocyte-like PLB cells released prostaglandin E(2) (PGE(2)) when stimulated by ionomycin, A23187, opsonized zymosan, phorbol 12-myristate 13-acetate, or formyl-Met-Leu-Phe (fMLP), and monocyte- or granulocyte-like PLB-D cells did not release PGE(2) with any of the agonists. The kinetics of cPLA(2) translocation to nuclear fractions in monocyte-like PLB cells stimulated with fMLP or ionomycin was in correlation with the kinetics of PGE(2) production. Granulocyte-like PLB cells, but not granulocyte-like PLB-D cells, secreted leukotriene B(4) (LTB(4)) after stimulation with ionomycin or A23187. Preincubation of monocyte-like parent PLB cells with 100 ng/ml lipopolysaccharide (LPS) for 16 h enhanced stimulated PGE(2) production, which is in correlation with the increased levels of cPLA(2) detected in these cells. LPS preincubation was less potent in increasing PGE(2) and LTB(4) secretion and did not affect cPLA(2) expression in granulocyte-like PLB cells, which may be a result of their lower levels of surface LPS receptor expression. LPS had no effect on monocyte- or granulocyte-like PLB-D cells. The lack of eicosanoid formation in stimulated, differentiated cPLA(2)-deficient PLB cells indicates that cPLA(2) contributes to stimulated eicosanoid formation in monocyte- and granulocyte-like PLB cells.

Role of monocytes in atherogenesis

This review focuses on the role of monocytes in the early phase of atherogenesis, before foam cell formation. An emerging consensus underscores the importance of the cellular inflammatory system in atherogenesis. Initiation of the process apparently hinges on accumulating low-density lipoproteins (LDL) undergoing oxidation and glycation, providing stimuli for the release of monocyte attracting chemokines and for the upregulation of endothelial adhesive molecules. These conditions favor monocyte transmigration to the intima, where chemically modified, aggregated, or proteoglycan- or antibody-complexed LDL may be endocytotically internalized via scavenger receptors present on the emergent macrophage surface. The differentiating monocytes in concert with T lymphocytes exert a modulating effect on lipoproteins. These events propagate a series of reactions entailing generation of lipid peroxides and expression of chemokines, adhesion molecules, cytokines, and growth factors, thereby sustaining an ongoing inflammatory process leading ultimately to lesion formation. New data emerging from studies using transgenic animals, notably mice, have provided novel insights into many of the cellular interactions and signaling mechanisms involving monocytes/macrophages in the atherogenic processes. A number of these studies, focusing on mechanisms for monocyte activation and the roles of adhesive molecules, chemokines, cytokines and growth factors, are addressed in this review.

Sphingosine 1-phosphate effect on endothelial cell PAF synthesis: Role in cellular migration

Sphingosine 1-phosphate (S1P) and vascular endothelial growth factor (VEGF) are two inflammatory mediators capable of promoting endothelial cell (EC) migration and angiogenesis. As VEGF inflammatory effect is mediated by the synthesis of endothelial platelet-activating factor (PAF) which is also contributing to VEGF chemotactic activity, we wanted to assess if S1P can trigger PAF synthesis in EC and if S1P-induced migration is PAF-dependent. Treatment of bovine aortic EC (BAEC) with S1P (10(-10)-10(-6) M) increased dose- and time-dependently the synthesis of PAF by up to 3.3-fold above the basal level, with a maximal amount of PAF detected at 20 min post-stimulation. This biological response was attenuated by inhibiting p38 mitogen-activated protein kinase (MAPK), cytosolic or secreted phospholipase A(2) (cPLA(2), sPLA(2)) activity, suggesting that p38 MAPK activation by S1P promotes the conversion of membrane phospholipids into PAF through the combined activation of cPLA(2) and sPLA(2). Interestingly, pretreatment of BAEC with extracellular PAF receptor antagonists (BN52021, 10(-5) M and CV3988, 10(-6) M) reduced by up to 42% the cellular migration induced by S1P (10(-6) M). These data demonstrate the capacity of S1P to induce PAF synthesis, which contributes in part to S1P chemotactic activity.

Phospholipase A(2)

Considerable progress has been made in characterizing the individual participant enzymes and their relative contributions in the generation of eicosanoids, lipid mediators derived from arachidonic acid, such as prostaglandins and leukotrienes. However, the role of individual phospholipase (PL) A(2) enzymes in providing arachidonic acid to the downstream enzymes for eicosanoid generation in biologic processes has not been fully elucidated. In this review, we will provide an overview of the classification of the families of PLA(2) enzymes, their putative mechanisms of action, and their role(s) in eicosanoid generation and inflammation.

Agonist-specific cross talk between ERKs and p38(mapk) regulates PGI(2) synthesis in endothelium

We have examined the mechanisms regulating prostacyclin (PGI(2)) synthesis after acute exposure of human umbilical vein endothelial cells (HUVEC) to interleukin-1 alpha (IL-1 alpha). IL-1 alpha evoked an early (30 min) release of PGI(2) and [(3)H]arachidonate that was blocked by the cytosolic phospholipase A(2)alpha (cPLA(2)alpha) inhibitor arachidonyl trifluoromethyl ketone. IL-1 alpha-mediated activation of extracellular signal-regulated kinase 1/2 (ERK1/2; p42/p44(mapk)) coincided temporally with phosphorylation of cPLA(2)alpha and with the onset of PGI(2) synthesis. The mitogen-activated protein kinase (MAPK) kinase (MEK) inhibitors, PD-98059 and U-0126, blocked IL-1 alpha-induced ERK activation and partially attenuated cPLA(2)alpha phosphorylation and PGI(2) release, suggesting that ERK-dependent and -independent pathways regulate cPLA(2)alpha phosphorylation. SB-203580 treatment enhanced IL-1 alpha-induced MEK, p42/44(mapk), and cPLA(2)alpha phosphorylation but reduced thrombin-stimulated MEK and p42/44(mapk) activation. IL-1 alpha, but not thrombin, activated Raf-1 as assessed by immune-complex kinase assay, as did SB-203580 alone. These results show that IL-1 alpha causes an acute upregulation of PGI(2) generation in HUVEC, establish a role for the MEK/ERK/cPLA(2)alpha pathway in this early release, and provide evidence for an agonist-specific cross talk between p38(mapk) and p42/44(mapk) that may reflect receptor-specific differences in the signaling elements proximal to MAPK activation.

VEGF stimulation of endothelial cell PAF synthesis is mediated by group V 14 kDa secretory phospholipase A2

1. Vascular endothelial growth factor (VEGF) is a potent inducer of inflammation, and we have shown that this latter effect is mediated through endothelial cell (EC) PAF synthesis. Since the phospholipid remodelling pathway enzymes (CoA-independent transacylase, CoA-IT; phospholipase A2, PLA2; and lyso-PAF acetyltransferase, lyso-PAF-AT) may participate in PAF synthesis, we assessed their contribution to VEGF-induced PAF synthesis in bovine aortic EC (BAEC) and human umbilical vein EC (HUVEC). 2. VEGF enhanced BAEC and HUVEC PAF synthesis by up to 28 and 4 fold above basal levels respectively. 3. A pretreatment with a CoA-IT and lyso-PAF-AT inhibitor (Sanguinarin; 500 nM) blocked VEGF-induced PAF synthesis by 95%, a specific CoA-IT inhibitor (SKF45905; 10 - 50 microM) was without effect, confirming the crucial role of the PLA2 and lyso-PAF-AT. 4. Treatment with secreted PLA2 (sPLA2) inhibitors which have been shown to inhibit both groups IIA and V sPLA2 (SB203347; 10 microM and LY311727; 100 microM) blocked EC PAF synthesis by up to 90%, whereas selective inhibition of group IIA sPLA2 (LY311727; 1 microM) had no significant effect. 5. RT - PCR and Western blot analyses demonstrated the presence of group V sPLA2 whereas group IIA sPLA2 was undetected in EC. 6. Treatment with cytosolic and calcium-independent PLA2 inhibitors (Arachidonyl trifluoromethyl ketone, Bromoenol lactone, Methyl arachydonyl fluorophosphate, up to 50 microM) did not prevent but rather potentiated the VEGF effect on EC PAF synthesis. 7. These results provide evidence that with VEGF activation of EC cells, the group V sPLA2 provides substrate for EC PAF formation.

Functional coupling between secretory and cytosolic phospholipase A2 modulates tumor necrosis factor-alpha- and interleukin-1beta-induced NF-kappa B activation

Tumor necrosis factor (TNF)-alpha and interleukin (IL)-1beta are potent activators of the transcription factor NF-kappaB, induced during inflammatory conditions. We have previously shown that both secretory and cytosolic phospholipase A(2) (PLA(2)) are involved in TNF-alpha- and IL-1beta-induced NF-kappaB activation. In this study, we have addressed the mechanism of PLA(2) involvement with respect to downstream arachidonic acid (AA) metabolites and the functional coupling between PLA(2)s mediating NF-kappaB activation. We show that in addition to inhibitors of secretory and cytosolic PLA(2)s, 5-lipoxygenase inhibitors attenuate TNF-alpha- and IL-1beta-stimulated NF-kappaB activation. Exogenous addition of leukotriene B(4) (LTB(4)) restored NF-kappaB activation reduced by 5-lipoxygenase inhibitors or an LTB(4) receptor antagonist, thus identifying LTB(4) as a mediator in signaling to NF-kappaB. TNF-alpha- and IL-1beta-induced AA release from cellular membranes was accompanied by phosphorylation of cytosolic PLA(2). Inhibitors of secretory PLA(2) and of 5-lipoxygenase/LTB(4) functionality markedly reduced AA release and nearly completely abolished cytosolic PLA(2) phosphorylation. This demonstrates that secretory PLA(2), through 5-lipoxygenase metabolites, is an essential upstream regulator of cytosolic PLA(2) and AA release. Our results therefore suggest the existence of a functional link between secretory and cytosolic PLA(2) in cytokine-activated keratinocytes, providing a molecular explanation for the participation of both secretory and cytosolic PLA(2) in arachidonic acid signaling and NF-kappaB activation in response to proinflammatory cytokines.

Enzymatic aspects of the phenol (aryl) sulfotransferases

The sulfotransferases that are active in the metabolism of xenobiotics represent a large family of enzymes that catalyze the transfer of the sulfuryl group from 3'-phosphoadenosine 5'-phosphosulfate to phenols, to primary and secondary alcohols, to several additional oxygen-containing functional groups, and to amines. Restriction of this review to the catalytic processes of phenol or aryl sulfotransferases does not really narrow the field, because these enzymes have overlapping specificity, not only for specific compounds, but also for multiple functional groups. The presentation aims to provide an overview of the wealth of phenol sulfotransferases that are available for study but concentrates on the enzymology of rat and human enzymes, particularly on the predominant phenol sulfotransferase from rat liver. The kinetics and catalytic mechanism of the rat enzyme is extensively reviewed and is compared with observations from other sulfotransferases.

Arachidonyl trifluoromethyl ketone induces lipid body formation in leukocytes

Leukocyte lipid bodies, abundant in cells associated with inflammation, can be induced to form in response to stimuli that include cis -unsaturated, but not saturated, fatty acids. Arachidonyl trifluoromethyl ketone (AACOCF(3)), a non-esterifiable arachidonate analog and an inhibitor of cytosolic phospholipase A(2)enzymes (PLA(2)), dose-dependently (0-20 microM) stimulated neutrophil lipid body formation, but this stimulation was not attributable to PLA(2)inhibition. Palmitoyl trifluoromethyl ketone, also a PLA(2)inhibitor, failed to stimulate lipid body formation, like palmitic acid itself, and did not inhibit stimulated lipid body formation. Moreover, aspirin, indomethacin and ibuprofen, which inhibit cis -unsaturated fatty acid-induced lipid body formation, inhibited AACOCF(3)-induced lipid body formation. Lipid body induction with AACOCF(3)reflected its structural basis as a cis -unsaturated fatty acid analog. These results indicate that cytosolic PLA(2)enzymes are not active in lipid body induction and cis -fatty acid stimulation of lipid body formation does not require esterification of cis -fatty acids into glycerolipids.

The Rossmann fold of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a nuclear docking site for antisense oligonucleotides containing a TAAAT motif

The subcellular localisation of oligodeoxynucleotides (ODN) is a major limitation for their use against nuclear targets. In this study we demonstrate that an antisense ODN directed against cytosolic phospholipase A(2) (cPLA2) mRNA is efficiently taken up and accumulates in the nuclei of endothelial cells (HUVEC), human monocytes and HeLa cells. Gel shift experiments and incubation of cells with oligonucleotide derivatives show that the anti-cPLA2 oligo binds a 37 kDa protein in nuclear extracts. The TAAAT sequence was identified as the major binding motif for the nuclear protein in competition experiments with mutated ODNs. Modification of the AAA triplet resulted in an ODN which failed to localise in the nucleus. Moreover, inserting a TAAAT motif into an ODN localising in the cytosol did not modify its localisation. The 37 kDa protein was purified and identified after peptide sequencing as glyceraldehyde-3-phosphate dehydrogenase (GAPDH). It was shown by confocal microscopy that GAPDH co-localises with anti-cPLA2 ODN in the nucleus and commercial GAPDH effectively binds the oligo. Competition experiments with increasing concentration of NAD(+) co-factor indicate that the GAPDH Rossmann fold is a docking site for antisense oligonucleotides containing a TAAAT motif.

Human calcium-independent phospholipase A2 mediates lymphocyte proliferation

Activation of lymphocytes induces blastogenesis and cell division which is accompanied by membrane lipid metabolism such as increased fatty acid turnover. To date little is known about the enzymatic mechanism(s) regulating this process. Release of fatty acids such as arachidonic acid requires sn-2-deacylation catalyzed by a class of enzymes known as phospholipases A(2) (PLA(2), EC ). Herein, we confirm that human peripheral blood B or T lymphocytes (PBL) do not possess measurable levels of 85-kDa PLA(2) as assessed by Western immunoblot. Low levels of 14-kDa PLA(2) protein and activity were detectable in the particulate fraction of PBL and Jurkat cells. Western immunoblot analysis indicates that PBLs possess the calcium-independent PLA(2) (iPLA(2)) protein. Calcium-independent sn-2-acylhydrolytic activity was measurable in PBL cytosols and could be inhibited by the selective iPLA(2) inhibitor bromoenol lactone. Mitogen activation of PBLs resulted in maintenance of activity levels which remained constant over 72 h suggesting an important role for iPLA(2) in this proliferative process. Indeed, evaluation of iPLA(2) activity in cell cycle-arrested Jurkat T cell fractions revealed the highest iPLA(2) levels occurring at the G(2)/M phase. Addition of the iPLA(2) inhibitors, bromoenol lactone, or arachidonyl trifluoromethyl ketone (AAOCF(3)), inhibited both mitogen-induced PBL as well as Jurkat T cell proliferation. Moreover, specific depletion of iPLA(2) protein by antisense treatment also resulted in marked suppression of cell division. Inhibition of Jurkat cell proliferation was not associated with arrest at a particular phase of the cell cycle nor was it associated with apoptosis as assessed by flow cytometry. These findings provide the first evidence that iPLA(2) plays a key role in the lymphocyte proliferative response.

Respective roles of the 14 kDa and 85 kDa phospholipase A2 enzymes in human monocyte eicosanoid formation

Human monocytes possess both the cytosolic 85 kDa phospholipase (PLA) A2 and a 14 kDa PLA2 and are capable of simultaneously producing prostanoids (PG), leukotrienes (LT) and platelet activating factor (PAF). As the exact roles of the two enzymes in monocyte lipid mediator formation was unclear, both selective PLA2 inhibitors and antisense were used to elucidate their respective roles. Reduction in 85 kDa PLA2 cellular protein levels by initiation site-directed antisense (SK 7111) or exposure to the 85 kDa PLA2 inhibitor, arachidonyl trifluormethyl ketone (AACOCF3), prevented A23187 or zymosan-stimulated monocytes prostanoid formation but not LTC4 or PAF production. This confirmed the important role of the 85 kDa PLA2 in prostanoid formation but indicated a less significant role in LT or PAF biosynthesis. Alternatively, treatment of monocytes with the selective, active-site-directed 14 kDa PLA2 inhibitor, SB 203347, totally inhibited LT and PAF formation, while prostanoid formation was not altered. Addition of 20 uM exogenous arachidonic acid (AA) to monocytes exposed to SB 203347 did not alter A23187-induced LTC4 generation, indicating that SB 203347 had no effect on downstream AA metabolizing enzymes in this setting. Taken together, these results provide evidence that the 14 kDa PLA2 provides substrate for monocyte LT and PAF formation, while the 85 kDa PLA2 plays a more significant role in the formation of PG.

Suppression of acute experimental inflammation by antisense oligonucleotides targeting secretory phospholipase A2 (sPLA2) in vitro and in vivo experiments

In HepG2 cells phosphorothioate modified antisense oligonucleotides against a sequence in the Ca2+ binding domain (AS-Ca2+) of type II sPLA2 mRNA restrained IL-6-induced synthesis of sPLA2 protein, sPLA2 mRNA (northern blot), and abolished IL-6 stimulated PGE2 release. An antisense oligonucleotide corresponding to a sequence in the catalytic domain (AS-Cat) of sPLA2 was less effective. The antisense oligonucleotides did not affect albumin synthesis in HepG2 cells, additionally demonstrating their specificity. The corresponding AS-Ca2+ against a homologous part of the rat sPLA2 mRNA depressed rat carrageenin oedema for 60-70%. Identical suppression was achieved by specific low molecular weight inhibitors of sPLA2. Since cyclo- and 5-lipoxygenase inhibitors exerted similar reductions of carrageenin oedema type II sPLA2 dependent eicosanoid formation seems to be a key cascade in this type of inflammation.

Inhibition of cytosolic phospholipase A(2) attenuates activation of mitogen-activated protein kinases in human monocytic cells

Eicosanoids and platelet-activating factor generated upon activation of cytosolic phospholipase A(2) enhance activity of transcription factors and synthesis of proinflammatory cytokines. Here, we show that selective inhibitors and antisense oligonucleotides against this enzyme suppressed expression of the interleukin-1beta gene at the level of transcription and promoter activation in human monocytic cell lines. This inhibitory effect was due to failure of activation of mitogen-activated protein kinases (MAPK) through phosphorylation by upstream mitogen-activated protein kinase kinases (MKK). Consequently, phosphorylation and degradation of inhibitor-kappaBalpha (I-kappaBalpha) and subsequent cytoplasmic mobilization, DNA-binding and the transactivating potential of nuclear factor-kappaB (NF-kB), nuclear factor-interleukin-6 (NF-IL6), activation protein-1 (AP-1) and signal-transducer-and-activator-of-transcription-1 (STAT-1) were impaired. It is concluded, that lipid mediators promote activation of MAPKs, which in turn lead to phosphorylation and liberation of active transcription factors. Since inhibition of cytosolic phospholipase A(2) ameliorates inflammation in vivo, this potency may reside in interference with the MAPK pathway.

Identification of a cellular protein that functionally interacts with the C2 domain of cytosolic phospholipase A(2)alpha

Cytosolic phospholipase A(2) (cPLA(2)) alpha plays critical roles in lipid mediator synthesis. We performed far-Western analysis and identified a 60-kDa protein (P60) that interacted with cPLA(2)alpha in a Ca(2+)-dependent manner. Peptide microsequencing revealed that purified P60 was identical to vimentin, a major component of the intermediate filament. The interaction occurred between the C2 domain of cPLA(2)alpha and the head domain of vimentin. Immunofluorescence microscopic analysis demonstrated that cPLA(2)alpha and vimentin colocalized around the perinuclear area in cPLA(2)alpha-overexpressing human embryonic kidney 293 cells following A23187 stimulation. Forcible expression of vimentin in vimentin-deficient SW13 cells augmented A23187-induced arachidonate release. Moreover, overexpression of the vimentin head domain in rat fibroblastic 3Y1 cells exerted a dominant inhibitory effect on arachidonate metabolism, significantly reducing A23187-induced arachidonate release and attendant prostanoid generation. These results suggest that vimentin is an adaptor for cPLA(2)alpha to function properly during the eicosanoid-biosynthetic process.

Cyclooxygenases-1 and 2 couple to cytosolic but not group IIA phospholipase A2 in COS-1 cells

Phospholipases A2 (PLA2) and cyclooxygenases (COX) are important enzymes responsible for production of potent lipid mediators, including prostaglandins (PG) and thromboxane A2. We investigated coupling between PLA2 and COX isoforms by using transient transfection in COS-1 cells. Untransfected cells, incubated with or without phorbol ester + the Ca2+ ionophore ionomycin, generated trivial amounts of PGE2. In cells co-transfected with cytosolic PLA2 (cPLA2) and COX-1 or COX-2, phorbol ester + ionomycin markedly stimulated PGE2 production. There was no preferential coupling of cPLA2 to either of the COX isoforms. In contrast, group IIA secretory PLA2 (sPLA2) co-transfected with COX-1 or COX-2 did not lead to an increase in PGE2 production, despite high levels of sPLA2 enzymatic activity. Transfection of cPLA2 did not affect basal free arachidonic acid (AA) levels. Phorbol ester + ionomycin stimulated release of AA in cPLA2-transfected COS-1 cells, but not in untransfected cells, whereas sPLA2 transfection (without stimulation) led to high basal free AA. Thus, AA released by cPLA2 is accessible to both COX isoforms for metabolism to PG, whereas AA released by sPLA2 is not metabolized by COX.

A continuous assay for the spectrophotometric analysis of sulfotransferases using aryl sulfotransferase IV

We have developed a continuous spectrophotometric coupled-enzyme assay for sulfotransferase activity. This assay is based on the regeneration of 3'-phosphoadenosine-5'-phosphosulfate (PAPS) from the desulfated 3'-phosphoadenosine-5'-phosphate (PAP) by a recombinant aryl sulfotransferase using p-nitrophenyl sulfate as the sulfate donor and visible spectrophotometric indicator of enzyme turnover. Here recombinant rat aryl sulfotransferase IV (AST-IV) is expressed, resolved to the pure beta-form during purification, and utilized for the regeneration. The activity of betaAST-IV to catalyze the synthesis of PAPS from PAP and p-nitrophenyl sulfate is demonstrated via capillary zone electrophoresis, and the kinetics of this reverse-physiological reaction are calculated. betaAST-IV is then applied to the coupled enzyme system, where the steady-state activity of the commercially available Nod factor sulfotransferase is verified with an enzyme concentration study and substrate-specificity assays of N-chitoses. The potential applications of this assay include rapid kinetic determinations for carbohydrate and protein sulfotransferases, high-throughput screening of potential sulfotransferase substrates and inhibitors, and biomedical screening of blood samples and other tissues for specific sulfotransferase enzyme activity and substrate concentration.

Cytosolic Phospholipase A2 Activation Is Essential for β1 and β2 Integrin-Dependent Adhesion of Human Eosinophils

We examined the role of cytosolic phospholipase A2 (cPLA2) during human eosinophil adherence to ICAM-1- or VCAM-1-coated plates. IL-5-stimulated eosinophils adhered to ICAM-1 through the β2 integrin CD11b/CD18, while nonstimulated eosinophils did not. By contrast, nonstimulated eosinophils adhered to VCAM-1 through the β1-integrin VLA-4/CD29. Both IL-5-induced adhesion to ICAM-1 and spontaneous adhesion to VCAM-1 corresponded temporally to cPLA2 phosphorylation, which accompanied enhanced catalytic activity of cPLA2. The structurally unrelated cPLA2 inhibitors, arachidonyl trifluoromethylketone and surfactin, significantly inhibited eosinophil adhesion to ICAM-1 and VCAM-1 in a concentration-dependent manner. Inhibition of secretory PLA2, 5-lipoxygenase, or cyclooxygenase did not affect eosinophil adhesion. Addition of arachidonic acid to eosinophils after cPLA2 inhibition with arachidonyl trifluoromethylketone or surfactin did not reverse the blockade of adhesion to ICAM-1 or VCAM-1. However, CV-6209, a receptor-specific antagonist of platelet-activating factor, inhibited all integrin-mediated adhesion. The activated conformation of CD11b as identified by the mAb, CBRM1/5, as well as quantitative surface CD11b expression were up-regulated after IL-5 stimulation. However, cPLA2 inhibition neither prevented CBRM1/5 expression nor blocked surface Mac-1 up-regulation caused by IL-5. Our data suggest that cPLA2 activation and its catalytic product platelet-activating factor play an essential role in regulating β1 and β2 integrin-dependent adhesion of eosinophils. This blockade occurs even in the presence of up-regulated eosinophil surface integrin.

Inhibition and binding studies of coenzyme A and bovine phenol sulfotransferase

Phenol sulfotransferases (PSTs, EC 2.8.2.1) catalyze sulfonyl group transfer from 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to the hydroxyl oxygen of aromatic acceptor substrates. The structural overlap between PAPS and coenzyme A (CoA) suggested a possible role of this common acyl carrier in modulating PST activity. To test this hypothesis, purified recombinant bovine PST was examined by kinetic and affinity chromatographic approaches. After demonstrating PST enzyme inhibition by CoA, systematic variation of CoA and PAPS concentrations indicated simple competitive inhibition with K(i) = 1. 3 microM. PST bound to CoA-agarose, attached via the pantetheinyl thiol group, was eluted with PAP but not by 2-naphthol. This observation was consistent with the pattern of inhibition. Additional members of the sulfotransferase superfamily, as well as acylated CoAs, should be further investigated.

Contribution of type II PLA2 to prostaglandin formation: a study using a type II PLA2 specific inhibitor SB 203347

Arachidonic acid (AA) mobilization by phospholipase A2 (PLA2) and subsequent prostaglandin synthesis is considered to be a pivotal event in inflammation. The purpose of this study was to assess the efficacy of a Type II PLA2 specific inhibitor, SB 203347, in reducing prostaglandin production in Type II PLA2-transfected Chinese hamster ovary (CHO) cells and in human placenta. In both experimental models utilised, Type II PLA2 represents the principal isozyme contributing to total PLA2 enzymatic activity. PLA2 enzymatic activity released into cell culture media and placental explant media was quantified by radiolabelled substrate assay [14C-phosphatidylethanolamine (PE)]. Immunoreactive prostaglandin F2alpha (PGF2alpha) concentrations were determined by radioimmunoassay. SB 203347 (at 0.1-10 microM final concentration) inhibited PLA2 enzymatic activity released by Zn++ -activated CHO cells by up to 60% (P<0.0001). The concentration of PGF2alpha present in culture media was concomitantly reduced by up to 90% (P<0.0001). Similar results were observed for human placental explants. Treatment of human placental explants with SB 203347 (1 microM final concentration) significantly reduced PLA2 enzymatic activity recovered in media after 24 h incubation (P<0.0001; n = 10). Incubation media PGF2alpha concentrations were also reduced by 60% (P<0.00001). The addition of endogenous arachidonic acid (30 microM final concentration) significantly attenuated SB 203347-inhibition of PGF2alpha release (P<0.01). The data obtained in this study are consistent with the hypothesis that Type II PLA2 contributes to the liberation of arachidonic acid for prostanoid formation in human placenta and in cells that abundantly express this isozyme.

A 38 kDa nuclear protein is involved in the retention of an antisense oligonucleotide directed against cytosolic phospholipase A2

Recent studies suggest that antisense phosphorothioate oligonucleotides (APO) are useful tools not only to impair gene expression, but also to modify the splicing of pre-mRNA, as the classical view that they act by suppressing the translation of mature mRNA has been challenged by several examples showing their nuclear site of action. In this work we show that an APO directed against cytosolic phospholipase A2 (cPLA2) mRNA localises in the nucleus and interacts with a specific nuclear protein.

Inhibitors of intracellular phospholipase A2 activity: their neurochemical effects and therapeutical importance for neurological disorders

Intracellular phospholipases A2 (PLA2) are a diverse group of enzymes with a growing number of members. These enzymes hydrolyze membrane phospholipids into fatty acid and lysophospholipids. These lipid products may serve as intracellular second messengers or can be further metabolized to potent inflammatory mediators, such as eicosanoids and platelet-activating factors. Several inhibitors of nonneural intracellular PLA2 have been recently discovered. However, nothing is known about their neurochemical effects, mechanism of action or toxicity in human or animal models of neurological disorders. Elevated intracellular PLA2 activities, found in neurological disorders strongly associated with inflammation and oxidative stress (ischemia, spinal cord injury, and Alzheimer's disease), can be treated with specific, potent and nontoxic inhibitors of PLA2 that can cross blood-brain barrier without harm. Currently, potent intracellular PLA2 inhibitors are not available for clinical use in human or animal models of neurological disorders, but studies on this interesting topic are beginning to emerge. The use of nonspecific intracellular PLA2 inhibitors (quinacrine, heparin, gangliosides, vitamin E) in animal model studies of neurological disorders in vivo has provided some useful information on tolerance, toxicity, and effectiveness of these compounds.

Interleukin-6 production by activated human monocytic cells is enhanced by MK-571, a specific inhibitor of the multi-drug resistance protein-1

1. The intracellular transport of leukotriene C4 (LTC4) in hematopoietic cells such as human monocytes is controlled by an ATP dependent carrier encoded by the multidrug resistance protein1 (MRPI) gene whose function can be blocked by the compound MK-571. Since LTs play a major role in control of cytokine expression in monocytes, we questioned whether blocking of the MRPI mediated function by MK-571 might affect cytokine production. 2. MK-571 strongly enhanced IL-6 expression at mRNA and protein level in lipopolysaccharide (LPS) and interleukin-1 (IL-1) stimulated human monocytes giving rise to 2.0+/-0.4 (x+/-s.d.) and 5.7+/-3.5 fold induction of IL-6 protein secretion. The increase in IL-6 secretion was accompanied by an enhanced phosphorylation of p38 but not of c-Jun-N terminal kinase. 3. The involvement of the kinase signalling pathways was further analysed by using SB203580 and PD98059, specific inhibitors of the p38 and ERK1/2 signalling route. MK-571 mediated upregulation of IL-6 in the presence of IL-1 was partially attenuated by SB203580 and PD98059. Electrophoretic mobility shift assays demonstrated that MK-571 did not affect the IL-1 induced DNA binding activity of Activator Protein-1 and Nuclear Factor-kappaB but rather enhanced the transactivational activity of an IL-6 promoter construct. Finally it was shown that the MK-571 mediated effects on IL-6 secretion could not be inhibited by the LT synthesis inhibitor SB203347 or by the anti-oxidant pyrrolidine dithiocarbamate (PDTC). 4. These results indicate that the membrane transporter MRP1 is involved in the regulation of IL-6 expression in activated human peripheral blood monocytes.

Cytosolic phospholipase A2 is essential for both the immediate and the delayed phases of eicosanoid generation in mouse bone marrow-derived mast cells

We have used mice in which the gene for cytosolic phospholipase A2 (cPLA2) has been disrupted to demonstrate the absolute requirement for cPLA2 in both the immediate and the delayed phases of eicosanoid generation by bone marrow-derived mast cells. For the immediate phase, quantitative analysis of the products of the 5-lipoxygenase pathway showed that gene disruption of cPLA2 prevented the provision of arachidonic acid substrate for biosynthesis of proximal intermediates. By analogy, we conclude that arachidonic acid substrate was also not available to prostaglandin endoperoxide synthase 1 in the immediate phase of prostaglandin (PG) D2 generation. These defects occurred with two distinct stimuli, stem cell factor and IgE/antigen, which were, however, sufficient for signal transduction defined by exocytosis of beta-hexosaminidase. Whereas cPLA2 is essential for immediate eicosanoid generation by providing arachidonic acid, its role in delayed-phase PGD2 generation is more complex and involves the activation-dependent induction of prostaglandin endoperoxide synthase 2 and the supply of arachidonic acid for metabolism to PGD2.

Asthma and leukotrienes: antileukotrienes as novel anti-asthmatic drugs

Antileukotriene drugs inhibit the formation or action of leukotrienes, which are potent lipid mediators generated from arachidonic acid in lung tissue and inflammatory cells. The leukotrienes were discovered in basic studies of arachidonic acid metabolism in leucocytes 20 years ago and were found to display a number of biological activities which may contribute to airway obstruction. Clinical studies with antileukotriene drugs have indeed demonstrated that leukotrienes are significant mediators of airway obstruction evoked by many common trigger factors in asthma. Moreover, treatment trials have established that this new class of drugs has beneficial anti-asthmatic properties, and several antileukotrienes have recently been introduced as new therapy of asthma. This communication presents an overview of the biosynthesis of leukotrienes, their biological effects and clinical effects of antileukotrienes in the treatment of asthama.

Cytosolic phospholipase A2 in rat decidual cells: evidence for its role in decidualization

We investigated the existence and possible role of cytosolic phospholipase A2 (cPLA2) in rat decidualized uteri. PLA2 activity in the cytosol of a decidualized uterine horn, induced by intraluminal oil infusion, was significantly higher than that in contralateral intact horn. The activity was almost completely depressed by cPLA2 inhibitors including arachidonyl trifluoromethyl ketone (ATK). The immunoreactive signals for cPLA2 were intense in decidua and glandular epithelial cells. In vivo administration of ATK (0.1-100 microg) caused a dose-dependent inhibition of decidualization. These results show the presence of cPLA2 and its probable implication in decidualization in rat uterus.

Genetic determinants of 5-lipoxygenase transcription

BACKGROUND

5 Lipoxygenase (5-LO) is a critical enzyme in the production of the leukotrienes. We have identified a series of mutations in the 5-LO gene that modify gene transcription. These mutations consist of addition of an Sp-1 binding motif (-GGGCGG-) or deletion of one or two Sp-1 binding motifs in the 5-LO core promoter.

METHODS

Mutant forms of the 5-LO core promoter were placed in a chloramphenicol acetyl transferase (CAT) reporter construct using either HeLa or SL-2 cells.

RESULTS

In HeLa cells all of the mutant forms are less effective in driving CAT reporter activity than the wild-type promoter. In SL-2 cells the construct containing the addition mutation was more effective in driving CAT reporter activity, while the constructs containing the deletion mutations were less effective.

CONCLUSIONS

These data indicate that naturally occurring mutations in the 5-LO core promoter modify gene transcription in vitro.

Functional coupling between various phospholipase A2s and cyclooxygenases in immediate and delayed prostanoid biosynthetic pathways

Several distinct phospholipase A2s (PLA2s) and two cyclooxygenases (COXs) were transfected, alone or in combination, into human embryonic kidney 293 cells, and their functional coupling during immediate and delayed prostaglandin (PG)-biosynthetic responses was reconstituted. Signaling PLA2s, i.e. cytosolic PLA2 (cPLA2) (type IV) and two secretory PLA2s (sPLA2), types IIA (sPLA2-IIA) and V (sPLA2-V), promoted arachidonic acid (AA) release from their respective transfectants after stimulation with calcium ionophore or, when bradykinin receptor was cotransfected, with bradykinin, which evoked the immediate response, and interleukin-1 plus serum, which induced the delayed response. Experiments on cells transfected with either COX alone revealed subtle differences between the PG-biosynthetic properties of the two isozymes in that COX-1 and COX-2 were favored over the other in the presence of high and low exogenous AA concentrations, respectively. Moreover, COX-2, but not COX-1, could turn on endogenous AA release, which was inhibited by a cPLA2 inhibitor. When PLA2 and COX were coexpressed, AA released by cPLA2, sPLA2-IIA and sPLA2-V was converted to PGE2 by both COX-1 and COX-2 during the immediate response and predominantly by COX-2 during the delayed response. Ca2+-independent PLA2 (iPLA2) (type VI), which plays a crucial role in phospholipid remodeling, failed to couple with COX-2 during the delayed response, whereas it was linked to ionophore-induced immediate PGE2 generation via COX-1 in marked preference to COX-2. Finally, coculture of PLA2 and COX transfectants revealed that extracellular sPLA2s-IIA and -V, but neither intracellular cPLA2 nor iPLA2, augmented PGE2 generation by neighboring COX-expressing cells, implying that the heparin-binding sPLA2s play a particular role as paracrine amplifiers of the PG-biosynthetic response signal from one cell to another.

Modulation of acute and chronic inflammatory processes by cacospongionolide B, a novel inhibitor of human synovial phospholipase A2

1. Cacospongionolide B is a novel marine metabolite isolated from the sponge Fasciospongia cavernosa. In in vitro studies, this compound inhibited phospholipase A2 (PLA2), showing selectivity for secretory PLA2 (sPLA2) versus cytosolic PLA2 (cPLA2), and its potency on the human synovial enzyme (group II) was similar to that of manoalide. 2. This activity was confirmed in vivo in the 8 h zymosan-injected rat air pouch, on the secretory enzyme accumulating in the pouch exudate. Cacospongionolide B, that is bioavailable when is given orally, reduced the elevated levels of sPLA2 present in paw homogenates of rats with adjuvant arthritis. 3. This marine metabolite showed topical anti-inflammatory activity on the mouse ear oedema induced by 12-O-tetradecanoylphorbol acetate (TPA) and decreased carrageenin paw oedema in mice after oral administration of 5, 10 or 20 mg kg(-1). 4. In the mouse air pouch injected with zymosan, cacospongionolide B administered into the pouch, induced a dose-dependent reduction in the levels of eicosanoids and tumour necrosis factor alpha (TNFalpha) in the exudates 4 h after the stimulus. It also had a weak effect on cell migration. 5. The inflammatory response of adjuvant arthritis was reduced by cacospongionolide B, which did not significantly affect eicosanoid levels in serum, paw or stomach homogenates and did not induce toxic effects. 6 Cacospongionolide B is a new inhibitor of sPLA2 in vitro and in vivo, with anti-inflammatory properties in acute and chronic inflammation. This marine metabolite was active after oral administration and able to modify TNFalpha levels, and may offer an interesting approach in the search for new anti-inflammatory agents.

Mechanisms of the priming effect of lipopolysaccharides on the biosynthesis of leukotriene B4 in chemotactic peptide-stimulated human neutrophils

The goal of this study was to explain the priming effect of lipopolysaccharides (LPS) in human polymorphonuclear leukocytes on leukotriene B4 (LTB4) biosynthesis after stimulation with the receptor-mediated agonist formyl-methionyl-leucyl-phenylalanine (fMLP). This priming effect for LTB4 biosynthesis was maximal after a 30 min preincubation with LPS but was lost when incubations were extended to 90 min or longer. Priming with LPS resulted in an enhanced maximal activation of 5-lipoxygenase (5- to15-fold above unprimed cells) as well as a prolonged activation of the enzyme after stimulation with fMLP compared to that measured in unprimed cells. The activation of 5-lipoxygenase was associated with its translocation to the nuclear fraction of the cell after stimulation of LPS-primed cells but not of unprimed cells. Priming of cells with LPS also resulted in an enhanced capacity (fivefold increase) for arachidonic acid (AA) release after stimulation with fMLP compared to unprimed cells as measured by mass spectrometry. This release of AA was very efficiently blocked in a dose-dependent manner by the 85 kDa cytosolic phospholipase A2 (PLA2) inhibitor MAFP (IC50=10nM) but not by the 14 kDa secretory PLA2 inhibitor SB 203347 (up to 5 microM), indicating that the 85 kDa cPLA2 is the PLA2 responsible for AA release in response to receptor-mediated agonists. In accord with inhibitor studies, the LPS-mediated phosphorylation of cPLA2 followed the same kinetics as the priming for AA release, and a measurable fMLP-induced translocation of cPLA2 was observed only in primed cells. As with AA release and LTB4 biosynthesis, both the phosphorylation and capacity to translocate cPLA2 were reversed when the preincubation period with LPS was extended to 120 min. These results explain some of the cellular events responsible for the potentiation and subsequent decline of functional responses of human polymorphonuclear leukocytes recruited to inflammatory foci.

On the expression of cytosolic calcium-independent phospholipase A2 (88kDa) in immature and mature myeloid cells and its role in leukotriene synthesis in human granulocytes

The human calcium-independent phospholipase A2 (iPLA2; 88 kDa) has recently been cloned (Larsson, P.K.A., Claesson, H.-E. and Kennedy, B.P. (1998) J. Biol. Chem. 272, 207-214). Here we demonstrate the expression of the human iPLA2 mRNA and its splice variants in blood progenitor cells, immature leukemic cells and mature granulocytes. Chromatographical resolvable iPLA2 activity was found in the cytosolic fraction of granulocytes and the activity was inhibited by the iPLA2 inhibitor bromoenol lactone. This drug also inhibited leukotriene synthesis in human granulocytes, induced by low concentration of calcium ionophore A23187 (0.10-0.15 microM) or opsonized zymosan. These results suggest that iPLA2 is involved in the regulation of the pool of arachidonic acid destined for leukotriene synthesis in human granulocytes.

U937 cells deprived of endogenous annexin 1 demonstrate an increased PLA2 activity

Annexin 1 (An 1), a phospholipid and calcium binding protein, is strongly expressed in differentiated U 937 cells. In attempting to correlate the expression of An 1 with phospholipase A2 (PLA2) activity, U 937 cells were stably transfected both with a Sense and Antisense cDNA for An 1. PLA2 activity was measured by Flow cytometry analysis utilizing the bis-Bodipy-C11-PC fluorescent probe. U 937 cells stably transfected with the sense or antisense vectors were differentiated for 24 h with phorbol 12-myristate 13-acetate (PMA, 6 ng ml(-1)). Both in undifferentiated and differentiated cells, the Antisense clone (36.4 AS) showed consistently higher PLA2 activity than the control Sense clone (15 S). Since the fluorescent probe measures the total PLA2 activity, we used two different stimuli, PMA: (100 ng ml(-1)) or lipopolysaccharide (LPS, 10 ng ml(-1)), and two different inhibitors, to discriminate the PLA2 involved (namely arachidonyl trifluoromethyl ketone or AACOCF3, which is specific for the cytosolic PLA2, and SB 203347 specific for the secretory PLA2). In the Antisense clone the inhibitory effect of AACOCF was stronger [68%, P<0.025] than in the Sense, which may reflect the lower endogenous level of An 1 present in the cells. On the contrary, the inhibitory effect of SB 203347 [60% of inhibition] was identical in both clones. Since cPLA2 activity is correlated with its phosphorylation, Western and shift blot analysis were performed. They did not show any significative difference between the phosphorylated and non phosphorylated form of the enzyme in both the differentiated or not, Sense and Antisense clones. Furthermore the tyrosine phosphorylation analysis of An 1 showed that less than 10% of An 1 was phosphorylated irrespective of PMA presence or absence. From the pattern of inhibition observed, we propose that the endogenous unphosphorylated form of An 1 may act intracellularly to block the activity of a cytosolic PLA2.

The functions of five distinct mammalian phospholipase A2S in regulating arachidonic acid release. Type IIa and type V secretory phospholipase A2S are functionally redundant and act in concert with cytosolic phospholipase A2

We examined the relative contributions of five distinct mammalian phospholipase A2 (PLA2) enzymes (cytosolic PLA2 (cPLA2; type IV), secretory PLA2s (sPLA2s; types IIA, V, and IIC), and Ca2+-independent PLA2 (iPLA2; type VI)) to arachidonic acid (AA) metabolism by overexpressing them in human embryonic kidney 293 fibroblasts and Chinese hamster ovary cells. Analyses using these transfectants revealed that cPLA2 was a prerequisite for both the calcium ionophore-stimulated immediate and the interleukin (IL)-1- and serum-induced delayed phases of AA release. Type IIA sPLA2 (sPLA2-IIA) mediated delayed AA release and, when expressed in larger amounts, also participated in immediate AA release. sPLA2-V, but not sPLA2-IIC, behaved in a manner similar to sPLA2-IIA. Both sPLA2s-IIA and -V, but not sPLA2-IIC, were heparin-binding PLA2s that exhibited significant affinity for cell-surface proteoglycans, and site-directed mutations in residues responsible for their membrane association or catalytic activity markedly reduced their ability to release AA from activated cells. Pharmacological studies using selective inhibitors as well as co-expression experiments supported the proposal that cPLA2 is crucial for these sPLA2s to act properly. The AA-releasing effects of these sPLA2s were independent of the expression of the M-type sPLA2 receptor. Both cPLA2, sPLA2s-IIA, and -V were able to supply AA to downstream cyclooxygenase-2 for IL-1-induced prostaglandin E2 biosynthesis. iPLA2 increased the spontaneous release of fatty acids, and this was further augmented by serum but not by IL-1. Finally, iPLA2-derived AA was not metabolized to prostaglandin E2. These observations provide evidence for the functional cross-talk or segregation of distinct PLA2s in mammalian cells in regulating AA metabolism and phospholipid turnover.

The role of platelet activating factor and other lipid mediators in inflammatory angiogenesis

Chronic inflammatory diseases are often accompanied by intense angiogenesis. A model of inflammatory angiogenesis is the murine air pouch granuloma which has a hyperangiogenic component. Proinflammatory lipid mediator generation is also a hallmark of chronic inflammation and the role of endogenous production of these mediators in angiogenesis is not known. The 14 kDa phospholipase A2 (PLA2) deacylates phospholipid, liberating arachidonic acid, which is used for leukotriene production, and lysophospholipid, which can drive the production of platelet-activating factor (PAF). Therefore, SB 203347, an inhibitor of the 14 kDa PLA2, zileuton, an inhibitor of 5-lipoxygenase, and Ro 24-4736 a PAF receptor antagonist were evaluated for their effects in the murine air pouch granuloma. SB 203347 reduced both LTB4 and PAF, but not PGD2 levels measured in the day 6 granuloma. This correlated with a significant reduction in angiogenesis. Zileuton reduced LTB4 levels as expected, but did not significantly inhibit angiogenesis, whereas Ro 24-4736 potently reduced angiogenesis. These data support the hypothesis that PAF, and to a lesser extent leukotrienes contribute to the angiogenic phenotype in chronic inflammation.

Pasteurella haemolytica leukotoxin-induced increase in phospholipase A2 activity in bovine neutrophils

Exposure of bovine neutrophils to Pasteurella haemolytica leukotoxin (LKT) stimulates the production of leukotriene B4 (LTB4), which is believed to be an important chemotactic agent in the development of acute fibrinopurulent pneumonic infection in cattle. The involvement of phospholipase A2 (PLA2) in LKT-induced synthesis of LTB4 was studied by using bovine neutrophils labeled with 3H-arachidonate ([3H]AA). Incubation of isolated neutrophils with [3H]AA resulted in incorporation of radioactivity in the PLA2 substrates phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine. Exposure of radiolabeled neutrophils to LKT caused concentration- and time-dependent release of radioactivity and redistribution of radioactivity in neutrophil membranes consistent with utilization of phosphoglyceride substrate and release of free fatty acid and eicosanoid products. These LKT-induced effects could be inhibited by pretreatment with arachidonyl trifluoromethyl ketone, an inhibitor of type IV cytoplasmic PLA2, and were dependent on extracellular calcium. These results support the conclusion that LKT-induced synthesis of LTB4 involves a calcium-mediated increase in PLA2 activity.