Endogenous suppression of neutral-active and calcium-dependent phospholipase A2 in human polymorphonuclear leukocytes

Sphingosylphosphorylcholine acts in an anti-inflammatory manner in renal mesangial cells by reducing interleukin-1β-induced prostaglandin E2 formation

Sphingosylphosphorylcholine (SPC) is a bioactive lipid that binds to G protein-coupled-receptors and activates various signaling cascades. Here, we show that in renal mesangial cells, SPC not only activates various protein kinase cascades but also activates Smad proteins, which are classical members of the transforming growth factor-beta (TGFbeta) signaling pathway. Consequently, SPC is able to mimic TGFbeta-mediated cell responses, such as an anti-inflammatory and a profibrotic response. Interleukin-1beta-stimulated prostaglandin E(2) formation is dose-dependently suppressed by SPC, which is paralleled by reduced secretory phospholipase A(2) (sPLA(2)) protein expression and activity. This effect is due to a reduction of sPLA(2) mRNA expression caused by inhibited sPLA(2) promoter activity. Furthermore, SPC upregulates the profibrotic connective tissue growth factor (CTGF) protein and mRNA expression. Blocking TGFbeta signaling by a TGFbeta receptor kinase inhibitor causes an inhibition of SPC-stimulated Smad activation and reverses both the negative effect of SPC on sPLA(2) expression and the positive effect on CTGF expression. In summary, our data show that SPC, by mimicking TGFbeta, leads to a suppression of proinflammatory mediator production and stimulates a profibrotic cell response that is often the end point of an anti-inflammatory reaction. Thus, targeting SPC receptors may represent a novel therapeutic strategy to cope with inflammatory diseases.

FTY720 suppresses interleukin-1beta-induced secretory phospholipase A2 expression in renal mesangial cells by a transcriptional mechanism

BACKGROUND AND PURPOSE

FTY720 is a potent immunomodulatory prodrug that is converted to its active phosphorylated form by a sphingosine kinase. Here we have studied whether FTY720 mimicked the action of sphingosine-1-phosphate (S1P) and exerted an anti-inflammatory potential in renal mesangial cells.

EXPERIMENTAL APPROACH

Prostaglandin E(2) (PGE(2)) was quantified by an enzyme-linked immunosorbent-assay. Secretory phospholipase A(2) (sPLA(2)) protein was detected by Western blot analyses. mRNA expression was determined by Northern blot analysis and sPLA(2)-promoter activity was measured by a luciferase-reporter-gene assay.

KEY RESULTS

Stimulation of cells for 24 h with interleukin-1beta (IL-1beta) is known to trigger increased PGE(2) formation which coincides with an induction of the mRNA for group-IIA-sPLA(2) and protein expression. FTY720 dose-dependently suppressed IL-1beta-induced IIA-sPLA(2) protein secretion and activity in the supernatant. This effect is due to a suppression of cytokine-induced sPLA(2) mRNA expression which results from a reduced promoter activity. As a consequence of suppressed sPLA(2) activity, PGE(2) formation is also reduced by FTY720. Mechanistically, the FTY720-suppressed sPLA(2) expression results from an activation of the TGFbeta/Smad signalling cascade since inhibition of the TGFbeta receptor type I by a specific kinase inhibitor reverses the FTY720-mediated decrease of sPLA(2) protein expression and sPLA(2) promoter activity.

CONCLUSIONS AND IMPLICATIONS

In summary, our data show that FTY720 was able to mimic the anti-inflammatory activity of TGFbeta and blocked cytokine-triggered sPLA(2) expression and subsequent PGE(2) formation. Thus, FTY720 may exert additional in vivo effects besides the well reported immunomodulation and its anti-inflammatory potential should be considered.

Hypoxia increases group IIA phospholipase A(2) expression under inflammatory conditions in rat renal mesangial cells

Hypoxia evokes a common mechanism of oxygen sensing mediated by hypoxia-inducible transcription factors (HIF) in many mammalian cells. This study investigated the effect of hypoxia on group-IIA secretory phospholipase A(2) (sPLA(2)-IIA) expression in renal mesangial cells. Stimulation of cells with IL-1beta under normoxic conditions (21% O(2)) is known to induce expression and secretion of the group sPLA(2)-IIA. This induction is further enhanced by constantly reducing the O(2) concentration to 1% O(2), and is accompanied by increased sPLA(2) activity. To see whether hypoxia potentiates IL-1beta-induced sPLA(2)-IIA gene expression, a 2.67-kb fragment of the rat sPLA(2)-IIA promoter was fused to a luciferase reporter construct and used to transfect mesangial cells. Hypoxia alone is not able to activate the sPLA(2) promoter, whereas it significantly enhances IL-1beta-stimulated promoter activity. A deletion mutant of the promoter that lacks the two putative hypoxia responsive elements (HRE) is devoid of the potentiating effect of hypoxia. Moreover, site-directed mutagenesis of either of the two HRE is sufficient to abolish the potentiating effect of hypoxia. Electrophoretic mobility shift assays show that HIF-2alpha, which is the only HIF subtype expressed in mesangial cells, binds to both HRE in the sPLA(2)-IIA promoter. In summary, the data show that in an inflammatory setting hypoxia is able to potentiate sPLA(2)-IIA expression and activity in renal mesangial cells, and thereby may critically contribute to enhanced formation of inflammatory lipid mediators seen in a diverse range of kidney diseases.

Inhibition of Rho modulates cytokine-induced prostaglandin E2 formation in renal mesangial cells

Stimulation of rat mesangial cells for 24 h with interleukin-1beta (IL- 1beta) plus forskolin (Fk) leads to a marked increase in prostaglandin E2 (PGE2) synthesis. This effect is further enhanced by the small G-protein Rho inhibitor toxin A. A similar increase in PGE2 formation is obtained with Y27632, a Rho-dependent kinase inhibitor, and with lovastatin, a hydroxymethylglutaryl-coenzyme A inhibitor which depletes cells from geranylgeranyl moieties and thus blocks Rho activation. In parallel to the increased PGE2 synthesis, a potentiation of IL-1beta-induced secretory group IIA phospholipases A2 (sPLA2-IIA) protein expression also occurs by Rho inhibition. However, only toxin A triggers an increased sPLA2-IIA activity consistent with the elevated levels of protein expression, whereas Y27632 and lovastatin rather reduced IL-1beta-induced sPLA2-IIA activity. In vitro activity studies reveal that Y27632 and lovastatin can directly block sPLA2-IIA enzyme activity in a concentration-dependent manner. Interestingly, in the absence of IL-1beta/Fk stimulation and the lack of sPLA2-IIA protein expression, all Rho inhibitors exert a small but significant increase in PGE2 formation suggesting that additional PLA2s or downstream enzymes like cyclooxygenases or prostaglandin synthases may be activated by Rho inhibitors. Western blot analyses of toxin A-, Y27632- and lovastatin-stimulated cells reveal that the cytosolic group IV PLA2 (cPLA2) and the cytosolic PGE2 synthase (cPGES), but not the sPLA2-IIA, cyclooxygenase-2 or the microsomal PGE2 synthase (mPGES), are upregulated compared to unstimulated cells. Furthermore, the Rho inhibitors induced arachidonic acid release from intact cells which is blocked by the cPLA2 inhibitor methyl arachidonyl fluorophosphonate (MAFP). In summary, these data show that inhibition of the small G-protein Rho, either by toxin A, lovastatin, or Y27632, exert a dual effect on mesangial cells: (i) in the absence of an inflammatory stimulus it activates the constitutive cPLA2 and cPGE2 synthase and generates low amount of PGE2. (ii) In the presence of inflammatory cytokines it potentiates sPLA2-IIA expression and subsequent PGE2 formation. In addition, we identified lovastatin and Y27632 as direct inhibitors of sPLA2-IIA in a cell-free system.

Potentiation of tumor necrosis factor alpha-induced secreted phospholipase A2 (sPLA2)-IIA expression in mesangial cells by an autocrine loop involving sPLA2 and peroxisome proliferator-activated receptor alpha activation

In rat mesangial cells, exogenously added secreted phospholipases A2 (sPLA2s) potentiate the expression of pro-inflammatory sPLA2-IIA first induced by cytokines like tumor necrosis factor-alpha (TNFalpha) and interleukin-1 beta. The transcriptional pathway mediating this effect is, however, unknown. Because products of PLA2 activity are endogenous activators of peroxisome proliferator-activated receptor alpha (PPAR alpha, we postulated that sPLA2s mediate their effects on sPLA2-IIA expression via sPLA2 activity and subsequent PPAR alpha activation. This study shows that various sPLA2s, including venom enzymes, human sPLA2-IIA, and wild-type and catalytically inactive H48Q mutant of porcine pancreatic sPLA2-IB, enhance the TNF alpha-induced sPLA2-IIA expression at the mRNA and protein levels. In cells transfected with luciferase sPLA2-IIA promoter constructs, sPLA2s are active only when the promoter contains a functional PPRE-1 site. The effect of exogenous sPLA2s is also blocked by the PPAR alpha inhibitor MK886. Interestingly, the expression of sPLA2-IIA induced by TNF alpha alone is also attenuated by MK886, by the sPLA2-IIA inhibitor LY311727, by heparinase, which prevents the binding of sPLA2-IIA to heparan sulfate proteoglycans, and by the specific cPLA2-alpha inhibitor pyrrolidine-1. Together, these data indicate that sPLA2-IIA released from mesangial cells by TNF alpha stimulates its own expression via an autocrine loop involving cPLA2 and PPAR alpha. This signaling pathway is also used by exogenously added sPLA2s including pancreatic sPLA2-IB and is distinct from that used by TNF alpha.

High-performance liquid chromatographic assay with ultraviolet spectrometric detection for the evaluation of inhibitors of secretory phospholipase A(2)

A non-radioactive spectrometric assay for the evaluation of inhibitors of pancreatic group IB and non-pancreatic group IIA secretory phospholipase A(2) (sPLA(2)) is described. Mixed-micelles consisting of 1 mM of a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol and 6 mM of sodium deoxycholate were used as substrate. The enzyme activity was determined directly without any sample clean-up by measuring the sPLA(2)-mediated oleic acid release with reversed-phase HPLC and UV-detection at 200 nm. The known sPLA(2) inhibitors MJ33 and AR-C 67047MI were analyzed in this assay for their inhibitory potency. While MJ33 revealed only a very weak inhibition of group IB and IIA sPLA(2) at the highest test concentration (33 microM), AR-C 67047MI proved to be a potent inhibitor of both enzymes with IC(50)-values of 0.36 and 0.14 microM, respectively.

Biological effects of secretory phospholipase A(2) group IIA on lipoproteins and in atherogenesis

Secretory phospholipase A(2) group IIA(sPLA(2) IIA) can be produced and secreted by various cell types either constitutionally or as an acute-phase reactant upon stimulation by proinflammatory cytokines. The enzyme prefers phosphatidylethanolamine and phosphatidylserine as substrates. One important biological function may be the hydrolytic destruction of bacterial membranes. It has been demonstrated, however, that sPLA(2) can also hydrolyse the phospholipid monolayers of high density lipoprotein (HDL) and low density lipoprotein (LDL) in vitro. Secretory phospholipase A(2)-modified LDL show increased affinity to glycosaminoglycans and proteoglycans, a tendency to aggregate, and an enhanced ability to deliver cholesterol to cells. Incubation of cultured macrophages with PLA(2)-treated LDL and HDL is associated with increased intracellular lipid accumulation, resulting in the formation of foam cells. Elevated sPLA(2)(IIA) activity in blood serum leads to an increased clearance of serum cholesterol. Secretory phospholipase A(2)(IIA) can also be detected in the intima, adventitia and media of the atherosclerotic wall not only in developed lesions but also in very early stages of atherosclerosis. The presence of DNA of Chlamydia pneumoniae, herpes simplex virus, and cytomegalovirus was found to be associated with sPLA(2)(IIA) expression and other signs of local inflammation. Thus, sPLA(2)(IIA) appears to be one important link between the lipid and the inflammation hypothesis of atherosclerosis.

Characterization of cockroach (Periplaneta americana) fat body phospholipase A(2) activity

A phospholipase has been identified in the fat body of the American cockroach, Periplaneta americana, which removes fatty acid from the sn-2 acyl position of an artificial substrate. The enzyme has been characterized using a crude preparation obtained by low-speed centrifugation of the homogenized tissue. With 1-hexadecanoyl-2-(1-pyrenedecanoyl)-sn-glycero-3-phosphocholine as the substrate, the K(m) has been estimated to be 1.17 microM and the v(max) 113.5 pmol/min/mg protein. The phospholipase has a pH optimum close to 7 and shows maximal activity at 50 degrees C. Activity of the phospholipase has been determined in cytosolic and plasma membrane fractions. The specific activity of the latter fraction is approximately twice that of the cytosol. The enzyme in both fractions is Ca(2+)-independent. Arch.

Lipopolysaccharide-induced release of arachidonic acid and prostaglandins in liver macrophages: regulation by Group IV cytosolic phospholipase A2, but not by Group V and Group IIA secretory phospholipase A2

Lipopolysaccharide (LPS) induces a delayed release (lag phase of 2-4 h) of arachidonic acid (AA) and prostaglandin (PG) D2 in rat liver macrophages. Group IV cytosolic phospholipase A2 (cPLA2) becomes phosphorylated within minutes after the addition of LPS. The phosphorylated form of cPLA2 shows an enhanced in vitro activity. The Ca2+ dependence of cPLA2 activity is not affected by phosphorylation of the enzyme. In addition, LPS induces an enhanced expression of cPLA2 mRNA (after 2-4 h) and an enhanced expression of cPLA2 protein (after 8 h). The cellular cPLA2 activity is enhanced about twofold 24 h after LPS treatment. Liver macrophages constitutively express mRNAs encoding Groups V and IIA secretory PLA2 (sPLA2). LPS has no effect on the levels of Groups V and IIA sPLA2 mRNA expression. Despite mRNA expression, Groups V and IIA sPLA2 protein and sPLA2 activity are not detectable in unstimulated or LPS-stimulated liver macrophages. Collectively, these and earlier [Mediators Inflammation 8 (1999) 295.] results suggest that in liver macrophages the LPS-induced delayed release of AA and prostanoids is mediated by phosphorylation and an enhanced expression of cPLA2, a de novo expression of cyclooxygenase (COX)-2, but not by the actions of Group V or Group IIA sPLA2.

Potentiation of TNF-alpha-stimulated group IIA phospholipase A(2) expression by peroxisome proliferator-activated receptor alpha activators in rat mesangial cells

Natural activators of peroxisome proliferator-activated receptors (PPAR) are lipid metabolites, including those produced by phospholipases A(2) (PLA(2)). In glomerular mesangial cells, the secreted group IIA PLA(2) (sPLA(2)-IIA), which is thought to be a crucial factor in pathologic processes in the kidney, may provide free fatty acids and eicosanoids directly or indirectly, by activating a cytosolic PLA(2). The scope of this study was to investigate whether synthetic PPAR(alpha) activators have an effect on sPLA(2)-IIA mRNA expression in rat mesangial cells, thus constituting a feedback modulation of sPLA(2)-IIA transcription. In the presence of tumor necrosis factor-alpha (TNF-alpha), the PPAR(alpha) agonists WY14643 and LY171883 as well as the lipid-lowering compound clofibrate potentiated expression, secretion, and activity of group IIA sPLA(2) in mesangial cells. MK886, known as a noncompetitive inhibitor of PPAR(alpha), completely abolished the potentiation of sPLA(2)-IIA secretion and activity by WY14643, thus indicating that the effect of WY14643 is specifically mediated by PPAR(alpha). When cells were transfected with different constructs of the rat sPLA(2)-IIA promoter fused to a luciferase reporter gene, a stimulation with TNF-alpha in the presence of the PPAR(alpha) activators caused an enhanced promoter activity compared with that induced by TNF-alpha alone. Site-directed mutagenesis of a putative PPRE site in the sPLA(2)-IIA promoter abolished the potentiating effect of PPAR(alpha) agonists, thus strongly indicating its contribution to the enhanced promoter activity. In summary, this study shows that the rat sPLA(2)-IIA promoter is sensitive to PPAR(alpha) agonists, which act synergistically with cytokines, resulting in an enhanced expression of sPLA(2)-IIA in rat mesangial cells.

Groups IV, V, and X phospholipases A2s in human neutrophils: role in eicosanoid production and gram-negative bacterial phospholipid hydrolysis

The bacterial tripeptide formyl-Met-Leu-Phe (fMLP) induces the secretion of enzyme(s) with phospholipase A(2) (PLA(2)) activity from human neutrophils. We show that circulating human neutrophils express groups V and X sPLA(2) (GV and GX sPLA(2)) mRNA and contain GV and GX sPLA(2) proteins, whereas GIB, GIIA, GIID, GIIE, GIIF, GIII, and GXII sPLA(2)s are undetectable. GV sPLA(2) is a component of both azurophilic and specific granules, whereas GX sPLA(2) is confined to azurophilic granules. Exposure to fMLP or opsonized zymosan results in the release of GV but not GX sPLA(2) and most, if not all, of the PLA(2) activity in the extracellular fluid of fMLP-stimulated neutrophils is due to GV sPLA(2). GV sPLA(2) does not contribute to fMLP-stimulated leukotriene B(4) production but may support the anti-bacterial properties of the neutrophil, because 10-100 ng per ml concentrations of this enzyme lead to Gram-negative bacterial membrane phospholipid hydrolysis in the presence of human serum. By use of a recently described and specific inhibitor of cytosolic PLA(2)-alpha (group IV PLA(2)alpha), we show that this enzyme produces virtually all of the arachidonic acid used for the biosynthesis of leukotriene B(4) in fMLP- and opsonized zymosan-stimulated neutrophils, the major eicosanoid produced by these pro-inflammatory cells.

Potentiation of cytokine induction of group IIA phospholipase A(2) in rat mesangial cells by ATP and adenosine via the A2A adenosine receptor

1. In rat mesangial cells extracellular nucleotides were found to increase arachidonic acid release by a cytosolic phospholipase A(2) through the P2Y(2) purinergic receptor. 2. In this study we investigated the effects of ATP and UTP on interleukin-1ss (IL-1ss)-induced mRNA expression and activity of group IIA phospholipase A(2) (sPLA(2)-IIA) in rat mesangial cells. 3. Treatment of cells for 24 h with extracellular ATP potentiated IL-1ss-stimulated sPLA(2)-IIA induction, whereas UTP had no effect. 4. We obtained the following evidence that the P2Y(2) receptor is not involved in the potentiation of sPLA(2)-IIA induction: (i) ATP-gamma-S had no enhancing effect; (ii) suramin, a P(2) receptor antagonist, did not inhibit ATP-mediated potentiation; (iii) inhibition of degradation of extracellular nucleotides by the 5'-ectonucleotidase inhibitor AOPCP did not enhance sPLA(2)-IIA induction and (iv) adenosine deaminase treatment completely abolished the ATP-mediated potentiation of sPLA(2)-IIA induction. 5. In contrast, treatment of mesangial cells with adenosine or the A2A receptor agonist CGS 21680 mimicked the effects of ATP in enhancing IL-1ss-stimulated sPLA(2)-IIA induction, whereas the specific A2A receptor antagonist ZM 241385 completely abolished the potentiating effect of ATP or adenosine. 6. The protein kinase A inhibitor Rp-8-Br-cyclic AMPS dose-dependently inhibited the enhancing effect of ATP or adenosine indicating the participation of an adenosine receptor-mediated cyclic AMP-dependent signalling pathway. 7. These data indicate that ATP mediates proinflammatory long-term effects in rat mesangial cells via its degradation product adenosine through the A2A receptor resulting in potentiation of sPLA(2)-IIA induction.

Group-II phospholipase A(2) enhances oxidized low density lipoprotein-induced macrophage growth through enhancement of GM-CSF release

Inflammatory process plays an important role in the development and progression of atherosclerotic lesions. Recently, group-II phospholipase A(2) (PLA(2)), an inflammatory mediator, was reported to exist in human atherosclerotic lesions and to enhance the development of murine atherosclerotic lesions. Oxidized low density lipoprotein (Ox-LDL) stimulates the growth of several types of macrophages in vitro. Since proliferation of macrophages occurs in atherosclerotic lesions, it is possible to assume that the Ox-LDL-induced macrophage proliferation might be involved in the progression of atherosclerosis. In this study, the role of group-II PLA(2) in the Ox-LDL-induced macrophage growth was investigated using thioglycollate-elicited mouse peritoneal macrophages. Thioglycollate-elicited macrophages significantly expressed group-II PLA(2) and released it into the culture medium. The Ox-LDL-induced thymidine incorporation into thioglycollate-elicited macrophages was three times higher than that into resident macrophages, whereas under the same conditions, granulocyte/macrophage colony-stimulating factor (GM-CSF) equally induced thymidine incorporation into both types of macrophages. Moreover, the Ox-LDL-induced GM-CSF release from thioglycollate-elicited macrophages was significantly higher than that from resident macrophages. In addition, the Ox-LDL-induced thymidine incorporation into macrophages obtained from human group-II PLA(2) transgenic mice and the GM-CSF release from these cells were significantly higher than those from their negative littermates, and the Ox-LDL-induced thymidine incorporation into human group-II PLA(2) transgenic macrophages was significantly inhibited by a polyclonal anti-human group-II PLA(2) antibody. These results suggest that the expression of group-II PLA(2) in thioglycollate-elicited macrophages may play an enhancing role in the Ox-LDL-induced macrophage growth through the enhancement of the GM-CSF release.

Inhibition of NFkappaB-mediated pro-inflammatory gene expression in rat mesangial cells by the enolized 1,3-dioxane-4, 6-dione-5-carboxamide, CGP-43182

1. CGP-43182 has been described as a potent inhibitor of group IIA secreted phospholipase A(2) (group IIA sPLA(2)) activity in vitro. In rat mesangial cells, inhibition of group IIA sPLA(2) activity by CGP-43182 results in a 70% reduction of cytokine-stimulated prostaglandin E(2) biosynthesis, suggesting that group IIA sPLA(2) participates in arachidonic acid release and eicosanoid formation. Under these conditions the cytosolic phospholipase A(2) is not affected. 2. In mesangial cells, in addition to inhibition of catalytic activity, the membrane-permeant CGP-43182 completely blocked interleukin 1beta (IL1beta)-stimulated group IIA sPLA(2) gene expression. 3. A further action of CGP-43182 was a complete inhibition of cyclo-oxygenase-2 gene expression, resulting in a drastic reduction of prostaglandin formation in mesangial cells. 4. Moreover, CGP-43182 completely blocked IL1beta-induced gene expression of the inducible nitric oxide synthase, leading to an inhibition of cytokine-stimulated nitric oxide formation. 5. In contrast, the stimulatory effect of the cell-permeant cyclic AMP-analogue, dibutyryl-cAMP, on the induction of these enzymes was not inhibited by CGP-43182. These data indicate that CGP-43182 interferes with IL1beta- but not cyclic AMP-activated transcriptional regulation. 6. By studying components of the upstream transcription machinery, we observed an inhibition of NFkappaB activation by CGP-43182 in IL1beta-treated cells. Moreover, we observed that CGP-43182 prevented the phosphorylation and proteolytic degradation of the endogenous NFkappaB inhibitor, IkappaB, a process necessary for NFkappaB activation. 7. From our data, we propose that CGP-43182 is a potent anti-inflammatory drug useful for preventing the consequences of a concerted action of cytokine-stimulated pro-inflammatory genes mediated by NFkappaB.

Involvement of cytosolic phospholipase A2 and secretory phospholipase A2 in arachidonic acid release from human neutrophils

The purpose of this study was to define the role of secretory phospholipase A2 (sPLA2), calcium-independent PLA2, and cytosolic PLA2 (cPLA2) in arachidonic acid (AA) release from fMLP-stimulated human neutrophils. While fMLP induced the release of extracellular sPLA2 activity and AA, 70% of sPLA2 activity remained associated with the cell. Treatment with the cell-impermeable sPLA2 inhibitors DTT or LY311-727, or the anti-sPLA2 Ab 3F10 all inactivated extracellular sPLA2 activity, but had minimal effect on neutrophil AA mass release. In contrast, coincubation of streptolysin-O toxin-permeabilized neutrophils with DTT, LY311-727, or 3F10 all decreased [3H8]AA release from [3H8]AA-labeled, fMLP-stimulated cells. Exposure to fMLP resulted in a decrease in the electrophoretic mobility of cPLA2, a finding consistent with cPLA2 phosphorylation, and stimulated the translocation of cPLA2 from cytosolic to microsomal and nuclear compartments. The role of cPLA2 was further evaluated with the cPLA2 inhibitor methyl arachidonyl fluorophosphonate, which attenuated cPLA2 activity in vitro and decreased fMLP-stimulated AA mass release by intact neutrophils, but had no effect on neutrophil sPLA2 activity. Inhibition of calcium-independent PLA2 with haloenol lactone suicide substrate had no effect on neutrophil cPLA2 activity or AA mass release. These results indicate a role for cPLA2 and an intracellular or cell-associated sPLA2 in the release of AA from fMLP-stimulated human neutrophils.

Modulation of cytokine-induced expression of secretory phospholipase A2-type IIA by protein kinase C in rat renal mesangial cells

Renal mesangial cells express the 14 kDa secretory phospholipase A2-type IIA (sPLA2-IIA) in response to interleukin-1beta (IL-1beta). In order to understand the regulation of cytokine-induced sPLA2-IIA induction in more detail, we investigated whether phorbol ester-activated protein kinase C (PKC) has an influence on the IL-1beta-induced expression of sPLA2-IIA. We found that treatment of mesangial cells with the biologically active phorbol 12-myristate 13-acetate (PMA) and phorbol 12,13-dibutyrate inhibited IL-1beta induction of sPLA2-IIA mRNA, protein, and activity, whereas the inactive compound 4alpha-phorbol 12,13-didecanoate was without effect. An 8-hr pretreatment with PMA, which led to down-regulation of PKC-alpha and -delta isoenzymes, still inhibited sPLA2-IIA induction. Only after down-regulation of PKC-epsilon isoenzyme by 24-hr preincubation with PMA were we able to reconstitute the IL-1beta-induced sPLA2-IIA expression. Thrombin as a physiological activator of PKC in mesangial cells exerted similar effects as PMA and inhibited sPLA2-IIA expression. The selective PKC inhibitor calphostin C potentiated IL-1beta induction of sPLA2-IIA mRNA levels and partially reconstituted the thrombin-induced inhibition of sPLA2-IIA mRNA and activity. These data show that IL-1beta induction of sPLA2-IIA can be modulated by PKC and that the epsilon-isoenzyme of the PKC family is the most likely candidate mediating the suppression of cytokine-induced sPLA2-IIA expression in mesangial cells.

Role of p38 in the priming of human neutrophils by peritoneal dialysis effluent

Peritoneal dialysis effluent (PDE) contains a low-molecular-weight substance that is able to prime human neutrophils for the release of arachidonic acid and superoxide anion. Conventional priming agents, such as tumor necrosis factor alpha (TNF-alpha), are known to signal via mitogen-activated protein (MAP) kinases; at least one possible substrate for MAP kinases is cytosolic phospholipase A(2) (cPLA(2)). Phosphorylation of this enzyme results in arachidonic acid release, and this fatty acid is a potent primer and activator of the human neutrophil NADPH oxidase. Because of the striking similarities between the priming of neutrophils with agents such as TNF-alpha and PDE, we have investigated the signalling pathways evoked by PDE and explored the possibility that cPLA(2) is a target for activated MAP kinases. Our results show that PDE treatment of human neutrophils results in the phosphorylation of the p38 kinase rather than the p42 and p44 kinases. Phosphorylation of p38 is transient with maximal activity being observed 1 min after exposure to PDE. We were unable to demonstrate that activation of p38 resulted in phosphorylation of cPLA(2); furthermore, translocation of this enzyme to a membrane-containing fraction was not enhanced in PDE-treated neutrophils. Taken together, these data suggest that, in a manner similar to that of TNF-alpha, PDE primes human neutrophils by the activation of the p38 kinase. However, unlike the cytokine, the activation of this protein does not result in phosphorylation or activation of cPLA(2).

Cross-talk between group IIA-phospholipase A2 and inducible NO-synthase in rat renal mesangial cells

Features of glomerulonephritis are expression of the inducible form of NO synthase (iNOS) as well as expression of the secretory group IIA-phospholipase A2 (sPLA2) in mesangial cells. Interleukin 1beta (IL-1beta) induces both enzymes with a similar time course resulting in an increase in nitrite production and sPLA2-IIA activity. In this study we investigated the relationship between the formation of NO and sPLA2-IIA induction in rat renal mesangial cells. Incubation of mesangial cells with the NO-donor, spermine-NONOate, for 24 h induced sPLA2-IIA mRNA expression and activity, whereas S-nitroso glutathione alone had only a small stimulatory effect. Stimulation of cells with IL-1beta caused a marked increase in sPLA2-IIA mRNA and activity that were potentiated 3 fold by both NO donors. Coincubation of cells with IL-1beta and the NOS inhibitor, L-N(G) monomethylarginine (L-NMMA), caused a dose-dependent inhibition of cytokine-induced sPLA2-IIA mRNA expression and activity. sPLA2-IIA activity was not stimulated by 8-bromo-cyclic GMP indicating that NO-induced sPLA2-IIA induction is independent of cyclic GMP-mediated signal transduction. These data show that NO contributes to the expression by cytokines of sPLA2-IIA and establishes a novel type of interaction between iNOS and sPLA2-IIA in mesangial cells. This cross-talk between inflammatory mediators may help to promote and sustain an inflammatory state in the kidney.

Phospholipase A2: its usefulness in laboratory diagnostics

Phospholipase A2 (PLA2) is an enzyme that catalyzes the hydrolysis of membrane phospholipids. This article reviews the source and structure of PLA2, the involvement of the enzyme in various biological and pathological phenomena, and the usefulness of PLA2 assays in laboratory diagnostics. Of particular importance is the role of PLA2 in the cellular production of mediators of inflammatory response to various stimuli. Assays for PLA2 activity and mass concentration are discussed, and the results of enzyme determinations in plasma from patients with different pathological conditions are presented. The determination of activity and mass concentration in plasma is particularly useful in the diagnosis and prognosis of pancreatitis, multiple organ failure, septic shock, and rheumatoid arthritis. A very important result is the demonstration that PLA2 is an acute phase protein, like CRP. Indeed, there is a close correlation between PLA2 mass concentration and CRP levels in several pathological conditions. Although the determination of C-reactive protein is much easier to perform and is routinely carried out in most clinical laboratories, the assessment of PLA2 activity or mass concentration has to be considered as a reliable approach to obtain a deeper understanding of some pathological conditions and may offer additional information concerning the prognosis of several disorders.

Study on the activation of phospholipases A2 by purinergic agonists in rat submandibular ductal cells

Extracellular ATP and benzoyl-ATP (Bz-ATP) increased the release of [3H]arachidonic acid ([3H]AA) from prelabeled rat submandibular gland (RSMG) ductal cells respectively two- and threefold. Both agonists also increased the release of [3H]AA from acini but at a lower level (+50% and +100% respectively). Carbachol had no significant effect on either cellular population. In ductal cells phorbol myristate acetate, an activator of protein kinase C, slightly increased the basal release of [3H]AA but did not affect the release of [3H]AA in response to ATP. Staurosporine, an inhibitor of protein kinases, inhibited the response to the purines. The removal of calcium from the extracellular medium decreased the response to ATP and Bz-ATP. Only barium could partly substitute for calcium to restore the purinergic response. Zinc inhibited the release of [3H]AA. Permeabilization of the cells with streptolysin O (SLO) activated the calcium-independent phospholipase A2 activity (iPLA2). The iPLA2, not the calcium-dependent PLA2 (cPLA2), released [3H]oleic acid ([3H]OA) from RSMG ductal cells. It is concluded that RSMG ducts have a higher PLA2 activity when compared to acini. This activity is accounted for by iPLA2 and cPLA2. Both enzymes are activated by P2X agonists by a staurosporine-sensitive mechanism. Cells permeabilized with SLO or membranes from Escherichia coli as a substrate are not good models to study the regulation of these enzymes. In intact RSMG ductal cells the two activities can be distinguished by rather specific inhibitors, by different ionic conditions and also by the fatty acid used to label the cells.

Antibodies against type II phospholipase A2 prevent renal injury due to ischemia and reperfusion in rats

This study was performed to determine the involvement of type II phospholipase A2 (PLA2-II) in renal injury caused by ischemia and reperfusion. Ischemia and reperfusion significantly elevated levels of blood urea nitrogen and serum creatinine in rats. These increases were significantly reduced by i.v. administration of rabbit IgG F(ab')2 fragments against rat PLA2-II. Increased levels of acid-stable PLA2 activity in the kidney were caused by ischemia and reperfusion, and were suppressed by administration of anti-PLA2-II F(ab')2. Increased levels of myeloperoxidase activity, a marker of neutrophil infiltration, in the kidney were also reduced after anti-PLA2-II F(ab')2 treatment. These results suggest that PLA2-II plays a pivotal role in pathogenesis of ischemia and reperfusion injury through induction of neutrophil infiltration.

PX-52, A novel inhibitor of 14 kDa secretory and 85 kDa cytosolic phospholipases A2

Previously we reported that PGBx, a prostaglandin oligomer with anti-inflammatory activity, inhibited 14 kDa phospholipase A2 (PLA2) activity and blocked arachidonic acid mobilization in prelabeled human neutrophils (Biochim. Biophys. Acta 1006:272-277, 278-286, 1989) This study describes a new inhibitor of phospholipase A2, PX-52, that also blocks agonist induced arachidonic acid mobilization in prelabeled cells. PX-52, a fatty acid polymer, inhibited hydrolysis of 14C-oleate labeled E.coli by a variety of 14 kDa PLA2s including human PMN, sperm, synovial fluid and disc, as well as porcine pancreas, N. naja, and bee venom in a dose-dependent manner with IC50s ranging from 1.0-3.7 uM. Inhibition of activity was comparable at different Ca2+ concentrations, but was relieved by increasing substrate concentration or by methylation of PX-52. Hydrolysis of [14C]-arachidonyl phosphatidylcholine by 85 kDa, cytosolic PLA2 from U937 cells was similarly inhibited by PX-52, the IC50 = 5 uM. Arachidonic acid mobilization induced by A23187 in prelabeled human PMNs was blocked by PX-52; IC50 = 10-15 uM while concentrations of up to 80 uM oleate had no effect. These results demonstrate that PX-52 inhibits the in vitro activity of secretory and cytosolic PLA2s and agonist-induced arachidonic acid release from human cells. Given its ability to block the arachidonic acid cascade, PX-52 may be useful in the control of inflammation.

Platelet-derived growth factor and fibroblast growth factor differentially regulate interleukin 1beta- and cAMP-induced group II phospholipase A2 expression in rat renal mesangial cells

Expression of group II phospholipase A2 (PLA2; EC 3.1.1.4) in rat renal mesangial cells is triggered in response to two principal classes of activating signals. These two groups of activators comprise inflammatory cytokines such as interleukin 1beta (IL-1beta) or tumor necrosis factor alpha and agents that elevate cellular levels of cyclic AMP (cAMP) such as forskolin, an activator of adenylate cyclase. Treatment of mesangial cells with IL-1beta or forskolin for 24 h induces group II PLA2 activity secreted into cell culture supernatants by about 15-fold and 11-fold, respectively. Platelet-derived growth factor (PDGF)-BB potently inhibits secretion of IL-1beta- and forskolin-induced group II PLA2 activity. By Western and Northern blot analyses, we demonstrate that this is due to a reduction of PLA2 protein levels and the corresponding PLA2 mRNA steady-state levels. Basic fibroblast growth factor (bFGF) virtually does not inhibit IL-1beta-stimulated group II PLA2 activity, but markedly inhibits forskolin-induced expression of group II PLA2 activity. These effects are caused by changes in the corresponding PLA2 protein and PLA2 mRNA steady-state levels. Inhibition of protein kinase C (PKC) by the potent and selective PKC inhibitor calphostin C converted the inhibitory action of PDGF into a bFGF-type of response thus suggesting that PKC is a major effector in PDGF-induced inhibition of IL-1beta-stimulated group II sPLA2 expression. In summary, our data suggest that PDGF and bFGF differentially modulate in a stimulus-specific manner the expression of group II PLA2 in mesangial cells.

Quantitation of human tissue and immune cell type II 14 kDa phospholipase A2 by enzyme immunoassay

The metabolism of arachidonic acid into inflammatory mediators (e.g. prostaglandin, leukotrienes) is dependent upon the rate-limiting enzyme phospholipase A(2). Localization and quantification of type II 14 kDa phospholipase A(2) (PLA(2)) in cells or tissue preparations has historically been accomplished through activity measurements, a process that can provide variable results due to interference by exogenous substances with hydrolysis assessment. Others have reported on the use of sandwich enzyme immunoassays (EIA) to measure 14 kDa PLA(2) by mass in serum and exudate fluids, e.g. synovial fluid. Herein, we report the utilization of a human recombinant type II 14 kDa PLA(2) sandwich EIA to directly measure cell or tissue-residing 14 kDa PLA(2). It is known that type II 14 kDa PLA(2) resists acid treatment, and this technique was applied to cell fractions which liberated the enzyme from cellular membrane components prior to quantitation by EIA. Two human immune cell populations were assessed and shown to contain measurable levels of 14 kDa PLA(2). Neutrophil or monocyte cytosolic fractions contained no measurable levels whereas the respective 100 000g particulate fractions contained 2.6+/-0.8 pg (neutrophil) and 2.1+/-0.6 pg (monocyte) 14 kDa PLA(2)/mug protein. Human placenta cytosolic fractions contained no measurable levels while 100 000g particulate contained approximately 25 ng 14 kDa PLA(2)/mg protein. This EIA, in conjunction with acid extraction, provides an easy and reproducible assay to identify and quantify this enzyme in cells and whole tissues, expanding our ability to study the relationship of this enzyme to inflammatory processes.

Suppression by cyclosporin A of interleukin 1 beta-induced expression of group II phospholipase A2 in rat renal mesangial cells

1. We investigated whether cyclosporin A, a potent immunosuppressive drug, affects group II phospholipase A2. (PLA2; EC 3.1.1.4) induction in rat renal mesangial cells. 2. Previously we showed that the expression of group II PLA2 in rat renal mesangial cells is triggered by exposure of the cells to inflammatory cytokines such as interleukin 1 beta (IL-1 beta) or tumour necrosis factor alpha and agents that elevate cellular levels of cyclic AMP. Treatment of mesangial cells with IL-1 beta for 24 h induced PLA2 activity secreted into cell culture supernatants by about 16 fold. Incubation of mesangial cells with cyclosporin A inhibited IL-1 beta-induced PLA2 section in a dose-dependent fashion, with an IC50 value of 4.3 microM. Cyclosporin A did not directly inhibit enzymatic activity of PLA2. 3. Immunoprecipitation of radioactively labelled PLA2 protein from mesangial cell supernatants revealed that the inhibition of PLA2 activity is due to a suppression of PLA2 protein levels. This effect was preceded by a reduction of PLA2 mRNA steady state levels, as demonstrated by Northern blot analyses of total cellular RNA isolated from stimulated mesangial cells. 4. In order to evaluate whether cyclosporin A would affect the transcriptional activity of the PLA2 gene, we performed nuclear run on transcription experiments and provided evidence that the transcription rate of the PLA2 gene is reduced by cyclosporin A. 5. Previously we found that the nuclear transcription factor kappa B (NF kappa B) is an essential component of the IL-1 beta-dependent upregulation of PLA2 gene transcription. By electrophoretic mobility shift analysis, we demonstrated that cyclosporin A diminishes the formation of NF kappa B DNA-binding complexes, thus suggesting that this transcription factor is a target for cyclosporin A-mediated repression of PLA2 gene transcription. 6. The data presented in this study strongly suggest that the cellular mechanism involved in the IL1 beta-dependent transcriptional upregulation of the PLA2 gene in mesangial cells is a target for the action of cyclosporin A.

Compartmentalisation and characteristics of a Ca2+-dependent phospholipase A2 in human colon mucosa

The biochemical properties of the phospholipase A2 (PLA2) found in the 100,000 x g centrifugate cytosol or particulate fractions of human colonic mucosa have been investigated using both deoxycholate-solubilized and Escherichia coli (E. coli) phospholipids as substrates. PLA2 activity was present in both subcellular fractions and the profiles of biochemical activites were similar. Activity in the particulate fraction was approximately twofold greater than the cytosol fraction when expressed on the basis of protein concentration. The PLA2 is Ca2+ dependent and using EGTA-regulated buffers cytosolic or particulate fraction activity was similar at both 10 microm or 10 mm Ca2+ concentrations. Using deoxycholate-phospholipid micelles as substrate a small but statistically significant twofold preference for glycero-phosphatidylcholine bearing sn-2-arachidonate compared with sn-2-oleate was seen, but this preference was not noted using arachidonate or oleate labelled E. coli membranes. Dithiothreitol (10 mM) reduced colon mucosal cytosol PLA2 activity significantly by 63.5 +/- 1.90% in cytosol and by 30.54 +/- 1.27% in microsomes using micelles as substrate or by 84.3 +/- 2.30% in cytosol and by 69.33 +/- 11.30% in microsomes using oleate-labelled E. coli as substrates. Warming at 57 degrees C reduced activity significantly by 35.0 +/- 5.80% in microsomes and by 40.0 +/- 7.08% in cytosol. Acid treatment increased PLA2 activity to 148 +/- 16.3% in microsomes and 145 +/- 18.6% in cytosol. When mucosal preparations were subjected to heparin-Sepharose chromatography, it bound tightly and eluted in the same position on a salt gradient as authentic human group II PLA2. Further purification by gel-permeation chromatography gave activity in the 14 kDa region of the elution profile. These features have many of the characteristics expected of a 14 kDa isoform of PLA2 but exhibit activity at concentrations of Ca2+ that are relevant in the intracellular environment and may participate in cellular lipid metabolism.

Liver cirrhosis induces renal and liver phospholipase A2 activity in rats

Maintenance of renal function in liver cirrhosis requires increased synthesis of arachidonic acid derived prostaglandin metabolites. Arachidonate metabolites have been reported to be involved in modulation of liver damage. The purpose of the present study was to establish whether the first enzyme of the prostaglandin cascade synthesis, the phospholipase A2(PLA2) is altered in liver cirrhosis induced by bile duct excision. The mRNA of PLA2(group I and II) and annexin-I a presumptive inhibitor of PLA2 enzyme was measured by PCR using glyceraldehyde 3-phosphate dehydrogenase (GAPDH) as an internal standard. The mean mRNA ratio of group II PLA2/GAPDH was increased in liver tissue by 126% (P < 0.001) and in kidney tissue by 263% (P < 0.006) following induction of liver cirrhosis. The increase in group II PLA2 mRNA in cirrhotic animals was reflected by an increase in PLA2 protein and enzyme activity in both liver and kidney tissues. Since the mRNA of group I PLA2 was not detectable and Group IV PLA2 activity measured in liver and kidney tissue samples was very low and not changed following induction of cirrhosis, it is likely that the major PLA2 activity measured in liver and kidney corresponds to group II PLA2 enzyme. The mean mRNA ratio of annexin-I/GAPDH was increased in liver tissue by 115% (P < 0.05) but unchanged in kidney tissue following induction of cirrhosis. The protein content of annexin-I and -V were not affected by bile duct excision in liver and kidney tissue indicating that upregulation of group II PLA2 activity was not due to downregulation of annexin-I or -V. Group II PLA2 activity of glomerular mesangial cells stimulated by interleukin-1 beta was enhanced by bile juice and various bile salts. In conclusion, activity of group II PLA2 is upregulated partly due to enhanced transcription and translation in cirrhosis and is furthermore augmented by elevated levels of bile salts.

Tetranactin inhibits interleukin 1 beta and cAMP induction of group II phospholipase A2 in rat renal mesangial cells

Renal mesangial cells express secretory phospholipase A2 in response to two principal classes of activating signals that may interact in a synergistic fashion. These two groups of activators comprise inflammatory cytokines, such as interleukin 1 beta, and agents that elevate cellular levels of cAMP. Treatment of mesangial cells with tetranactin, a cyclic antibiotic produced by Streptomyces aureus with a molecular structure similar to cyclosporin A inhibits interleukin 1 beta- and cAMP-dependent group II phospholipase A2 secretion in a dose-dependent manner with IC50 values of 43 and 33 nM, respectively. However, tetranactin does not directly inhibit group II phospholipase A2 activity. Western blot analyses of mesangial cell supernatants reveal that the inhibition of phospholipase A2 activity is due to suppression of phospholipase A2 protein synthesis. This effect is preceded by the reduction of phospholipase A2 mRNA steady-state levels as shown by Northern blot analyses of total cellular RNA isolated from stimulated mesangial cells. Thus, tetranactin is a potent inhibitor of group II phospholipase A2 expression in cytokine- and cAMP-stimulated mesangial cells and represents a new class of group II phospholipase A2 inhibitors with IC50 values in the low nanomolar range. This compound may be useful in the therapy of diseases associated with increased group II phospholipase A2 secretion.

Effect of degree of unsaturation in dietary fatty acids on arachidonic acid mobilization by peritoneal macrophages

Cells from rats fed with a tripalmitin diet showed a depletion of phospholipid arachidonate and n-3 fatty acids such as eicosapentaenoic and docosahexaenoic acids (EPA and DHA). In rats fed fish oil diet, a significant reduction in arachidonic acid (AA) content was observed whereas EPA and DHA were incorporated into membranes lipids. These changes in lipid composition of membranes did not affect cellular adherence, phagocytic capability, or [3H]AA incorporation. However, both tripalmitin and fish oil diets induced a decrease in [3H]AA mobilization stimulated by 4 beta-phorbol-12-myristate 13-acetate, A23187, or opsonized-zymosan in rat peritoneal macrophages. These results demonstrate that the antiinflammatory effects of essential fatty acids deficiency or n-3 enrichment diets may be associated with a decreased AA mobilization in resident rat peritoneal macrophages treated with proinflammatory agents.

Biosynthesis and biological activities of lantibiotics with unique post-translational modifications

Lantibiotics are biologically active peptides which contain the thioether amino acid lanthionine as well as several other modified amino acids. They can be broadly divided into two groups on the basis of their structures: type-A lantibiotics are elongated, amphiphilic peptides, while type-B lantibiotics are compact and globular. In the last decade there has been a marked increase in research interest in these peptides due both to the novel biosynthetic mechanisms by which they are produced, as well as to their potential applications. Lantibiotics are synthesised on the ribosome as a prepeptide which undergoes several post-translational modification events, including dehydration of specific hydroxyl amino acids to form dehydroamino acids, addition of neighbouring sulfhydryl groups to form thioethers and, in specific cases, other modifications such as introduction of D-alanine residues from L-serine, formation of lysinoalanine bridges, formation of novel N-terminal blocking groups and oxidative decarboxylation of a C-terminal cysteine. The genetic elements responsible for these specific modification reactions encode unique enzymes with hitherto unknown reaction mechanisms. Production of these peptides also requires accessory proteins including processing proteases, translocators of the ATP-binding cassette transporter family, regulatory proteins and dedicated producer self-protection mechanisms. While the principle biological activity of most type-B lantibiotics appears to be directed at the inhibition of enzyme functions, the type-A lantibiotics kill bacterial cells by forming pores in the cytoplasmic membrane.

Pyrrolidine dithiocarbamate differentially affects cytokine- and cAMP-induced expression of group II phospholipase A2 in rat renal mesangial cells

Renal mesangial cells express group II phospholipase A2 in response to two principal classes of activating signals that may interact in a synergistic fashion. These two groups of activators comprise inflammatory cytokines such as interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF alpha) and agents that elevate cellular levels of cAMP such as forskolin, an activator of adenylate cyclase. Using pyrrolidine dithiocarbamate (PDTC), a potent inhibitor of nuclear factor NF kappa B, we determined its role in cytokine--and cAMP--triggered group II PLA2 expression. Micromolar amounts of PDTC suppress the IL-1 beta- and TNF alpha-dependent, but not the forskolin-stimulated group II PLA2 activity in mesangial cells. Furthermore, PDTC inhibited the increase of group II PLA2 mRNA steady state levels in response to IL-1 beta and TNF alpha, while only marginally affecting forskolin-induced PLA2 mRNA levels. Our data suggest that NF kappa B activation is an essential component of the cytokine signalling pathway responsible for group II PLA2 gene regulation and that cAMP triggers a separate signalling cascade not involving NF kappa B. These observations may provide a basis to study the underlying mechanisms involved in the regulation of group II PLA2 gene expression.

CGP 47969A: a novel inhibitor of the synthesis of inflammatory cytokines

CGP 47969A is a novel piperazine derivative that inhibits the synthesis of inflammatory cytokines, such as interleukin-1 alpha (IL-1), IL-1 beta and tumor necrosis factor alpha (TNF), in human monocytes stimulated with lipopolysaccharide (LPS), zymosan or IL-1 itself. IC50 values are in the range of 0.3-5 mumol/l. CGP 47969A does not inhibit total protein or RNA synthesis indicating selectivity for cytokine inhibition. CGP 47969A exerts its inhibitory effect at a post-transcriptional level, most probably by reducing translational efficiency of IL-beta mRNA, as steady-state levels of IL-1 beta mRNA are not inhibited while the primary translation product, the 31 kD IL-1 beta precursor molecule, is dose-dependently inhibited by CGP 47969A. The compound is devoid of cyclooxygenase and phospholipase A2 inhibitory activity but efficiently inhibits the generation of PGE2 and LTC4 in zymosan-stimulated mouse macrophages with an IC50 of 1.2 and 0.6 mumol/l, respectively. Antagonism of IL-1 and/or TNF is thought to have a beneficial effect on the course of inflammatory diseases. CGP 47969A may therefore represent a mechanistically new approach to the treatment of such diseases.

Phospholipases A2 in ras-transformed and immortalized human mammary epithelial cells

Phospholipase A2 (PLA2) activities of non-tumorigenic and tumorigenic human mammary epithelial cells, 184B5 cells (immortalized cell line from a reduction mammoplasty) and B5KTu cells (cells from a tumor induced by ras-transformed 184B5 cells), are characterized, with emphasis on lipid biomediator-related phospholipases A2. Phospholipases A2 associated with regulation of arachidonic acid metabolism include the high molecular mass cytosolic PLA2 (cPLA2) and group II PLA2. The major PLA2 activity in the mammary epithelial cells has the characteristics of cPLA2; this activity is higher in the B5KTu cells. In contrast, the 184B5 and B5KTu cells have similar levels of a Ca(2+)-independent, cytosolic PLA2 activity and low levels of a particulate fraction PLA2 activity, which does not have the properties of group II PLA2. Thus, cPLA2 activity is selectively elevated in the tumorigenic human mammary epithelial cells and this may result in increased generation of lipid biomediators such as arachidonic acid metabolites.

Separation of agonist-stimulated arachidonate mobilization from subsequent leukotriene B4 synthesis in human neutrophils: different effects of oleoylacetylglycerol and phorbol myristate acetate as priming agents

Preincubation of human neutrophils with phorbol esters or soluble diglycerides enhances subsequent f-Met-Leu-Phe (fMLP)-stimulated arachidonate mobilization and leukotriene B4 (LTB4) synthesis. We have recently reported that 1,3-dioctanoylglycerol (1,3-diC8) is equipotent with 1,2-sn-dioctanoylglycerol (1,2-diC8) as priming agent, thus suggesting that the priming effects of diacylglycerols are protein kinase C (PKC) independent (Rosenthal et al., 1993, Biochim. Biophys. Acta 1177:79-86). In order to further investigate this question, the present study has directly compared the effects of oleoylacetylglycerol (OAG) and the PKC activator, phorbol 12-myristate 13-acetate (PMA), on agonist-stimulated lipid metabolism. The results indicate that both OAG and PMA dose dependently enhance f-Met-Leu-Phe (fMLP)-stimulated release of [3H]arachidonate. Optimal concentrations of OAG (5 microns) and PMA (10 nM) are equipotent in increasing fMLP-stimulated arachidonate mobilization as quantitated either with total radioactivity or by mass measurements of free arachidonate. By contrast OAG is sixfold more effective than PMA in enhancing synthesis of 5-lipoxygenase (5-LO) metabolites by mass and two to threefold more effective than PMA in enhancing synthesis of [3H]eicosanoids. Furthermore, OAG, but not PMA, enhances fMLP-stimulated synthesis of platelet-activating factor. By contrast, PMA directly stimulates [3H]arachidonate mobilization, while OAG (20 microM) does not; despite these differences, the combined effects of PMA + OAG on subsequent agonist-stimulated arachidonate release are not greater than those of PMA alone. In cells challenged with subthreshold concentrations (< 0.1 microM) of the calcium ionophore A23187, both OAG and PMA stimulate [3H]arachidonate release but not [3H]LTB4 synthesis. These findings suggest that OAG does not directly activate 5-LO, but instead couples arachidonate mobilization to leukotriene synthesis in a PKC-independent manner.

Determination of the catalytic activity of phospholipase A2: E. coli-based assay compared to a photometric micelle assay

Phospholipase A2 activity in human sera was determined of the basis of the E. coli assay and compared to a photometric micelle assay. The E. coli assay is based on the hydrolysis of phospholipids from [1-14C]oleic acid-labelled E. coli biomembranes. In the photometric assay the phospholipase A2 acts on mixed phospholipid micelles. The amount of fatty acid produced is quantitated in a subsequent photometric assay by coupling in the reaction to the coenzyme A metabolism. The E. coli membranes are essentially resistant to other lipases in human sera, i.e. lipoprotein lipases, hepatic triacylglycerolipase or pancreatic lipase and thus a very specific substrate for the phospholipase A2 of human serum. The photometric assay, though, is susceptible to other lipases in human serum. The ratio of [1-14C]oleic acid to released total fatty acids served as the basis for the calculation of the true enzymatic activity. The assay closely correlated with the photometric assay based on mixed micelles in the higher ranges of phospholipase A2 activity, but not in the normal range. The sensitivity is higher by at least two powers of 10. The human serum phospholipase A2 strongly preferred E. coli membranes as substrate to the mixed micelles containing phosphatidylcholine/phosphatidylethanolamine. In conclusion, the modified phospholipase A2 assay based on E. coli membranes is a sensitive, specific, reliable, and convenient method for the measurement of phospholipase A2 activity in human sera. The photometric assay suffers from low sensitivity but has the advantage of practicability in a normal routine laboratory, including the amenability to automation.

Cytokine-stimulated secretion of group II phospholipase A2 by rat mesangial cells. Its contribution to arachidonic acid release and prostaglandin synthesis by cultured rat glomerular cells

Potent pro-inflammatory cytokines, such as interleukin 1 (IL-1) or tumor necrosis factor (TNF) alpha have been found to increase group II phospholipase A2 (PLA2) synthesis and secretion by mesangial cells. In all cases 85-90% of the enzyme is secreted from the cells and a parallel increase in prostaglandin (PG)E2 synthesis is observed. We report here that co-incubation with a monoclonal antibody that specifically binds and neutralizes rat group II PLA2 attenuates IL-1 beta and TNF alpha-stimulated PGE2 production by 45% and 52%, respectively. CGP43182, a specific inhibitor of group II PLA2, potently blocks mesangial cell group II PLA2 in vitro with a half-maximal inhibitory concentration (IC50) of 1.5 microM, while only slightly affecting mesangial cell high molecular weight PLA2. CGP 43182 markedly attenuates IL-1 beta- and TNF alpha-stimulated PGE2 synthesis in intact mesangial cells with IC50's of 1.3 and 1.0 microM, respectively. PLA2 secreted from cytokine-stimulated mesangial cells was purified to homogeneity. Addition of the purified enzyme to unstimulated mesangial cells causes a marked release of arachidonic acid and a subsequent increased synthesis of PGE2. Moreover, addition of purified PLA2 to a cloned rat glomerular epithelial cell line and cultured bovine glomerular endothelial cells augmented both arachidonic acid release and PGE2 synthesis, with the endothelial cells being especially sensitive. Thus, cytokine-triggered synthesis and secretion of group II PLA2 by mesangial cells contributes, at least in part, to the observed synthesis of PGE2 that occurs in parallel to the enzyme secretion. Furthermore, extracellular PLA2 secreted by mesangial cells is able to stimulate arachidonic acid release and PGE2 synthesis by the adjacent endothelial and epithelial cells. These data suggest that expression and secretion of group II PLA2 triggered by pro-inflammatory cytokines may crucially participate in the pathogenesis of inflammatory processes within the glomerulus.

Glucocorticoid deficiency increases phospholipase A2 activity in rats

An important mechanism for the antiinflammatory effect of pharmacological doses of glucocorticoids is the inhibition of arachidonic acid release from phospholipids by phospholipase A2 (PLA2). As a corollary, one might predict that low endogenous concentrations of glucocorticoids favor inflammatory disease states. Indeed, clinical and experimental observations revealed an association between glucocorticoid deficiency and disease states caused by immunological and/or inflammatory mechanisms. The purpose of the present investigation was to study the regulation of PLA2 mRNA, protein, and enzyme activity in adrenalectomized (ADX) rats where glucocorticoid concentrations were below physiological levels. The mRNA of group I and II PLA2 were measured by PCR. Group II PLA2 mRNA was increased by 126 +/- 9% in lung tissue of ADX rats, whereas group I PLA2 was increased only by 27 +/- 1.5%. The increase in group II mRNA in ADX rats was reflected by a corresponding increase of group II PLA2 protein (70-100%) in lung, spleen, liver, and kidney. This increase was reversed by the administration of exogenous corticosterone. After ADX, the percentage increase in total PLA2 activity was higher than that of mRNA or PLA2 protein, suggesting that the activity of the enzyme was modulated by inhibitors or activators. The concentration of lipocortin-I, an inhibitor of PLA2 enzyme was strongly correlated with the activity of PLA2 in the tissues (lung, spleen, liver, and kidney). In all these tissues, the concentrations of lipocortin-I declined after ADX. Thus upregulation of PLA2 enzyme and downregulation of lipocortin-I might account for the enhanced inflammatory response in hypoglucocorticoid states.

Time-course of phospholipase A2, eicosanoid release and cellular accumulation in rat immunological air pouch inflammation

Phospholipase A2 activity in the rat air pouch cavity was determined after induction of a reverse passive Arthus reaction. Time-course of phospholipase A2 activity appeared to correlate with increased prostaglandin E2 levels in inflammatory exudate and with the influx of mononuclear inflammatory cells. Local administration of anti-inflammatory drugs such as dexamethasone, indomethacin, or a PLA2 inhibitor such as p-bromophenacyl bromide significantly inhibited exudate volume, cellular influx, granuloma formation, exudate PGE2 levels and PLA2 activity, to varying degrees. Dexamethasone treatment significantly reduced all parameters determined, whereas p-bromophenacyl bromide had a significant inhibitory effect on PLA2 activity and PGE2 release, and indomethacin only restored PGE2 levels. These results show that PLA2 is neither the only nor the most important factor involved in the development of subchronic inflammation.

1,3-Dioctanoylglycerol modulates arachidonate mobilization in human neutrophils and its inhibition by PGBx: evidence of a protein-kinase-C-independent role for diacylglycerols in signal transduction

Preincubation of human neutrophils with 1-oleoyl-2-acetylglycerol (OAG) enhances subsequent f-Met-Leu-Phe (fMLP)-stimulated arachidonate mobilization. We have recently demonstrated that preincubation of neutrophils with OAG also reverses inhibition of A23187 stimulated [3H]arachidonate mobilization by the phospholipase A2 inhibitors, PGBx and aristolochic acid. The present study has compared the effects of 1,2-sn-dioctanoylglycerol (1,2-diC8) and 1,3-dioctanoylglycerol (1,3-diC8) on these cellular events. Dose-dependent priming (ED50 < 2.5 microM) of fMLP-stimulated [3H]arachidonate mobilization is obtained with both 1,2-diC8 and 1,3-diC8. Both diC8s also enhance fMLP-stimulated synthesis of leukotriene B4, 5-hydroxyeicosatetraenoic acid and platelet-activating factor, and generation of superoxide. Furthermore, both 1,2-diC8 and 1,3-diC8 reverse the effects of PGBx on A23187-stimulated [3H]arachidonate mobilization and platelet-activating factor synthesis. By contrast, higher concentrations (5-10 microM) of 1,2-diC8, but not 1,3-diC8, directly stimulate both [3H] arachidonate mobilization and superoxide generation. Since 1,3-diC8 does not activate protein kinase C (PKC), these results suggest that PKC is involved in direct activation of neutrophils by diacylglycerols but not in priming. Furthermore, reversal of the inhibitory effects of PGBx by diacylglycerols also appears to involve a PKC-independent mechanism.

Influence of arachidonic acid on indices of phospholipase A2 activity in the human neutrophil

The present studies were conducted to understand better the regulation of phospholipase A2 (PLA2)-dependent mobilization of lipid mediators by arachidonic acid (C20:4). After stimulation of human neutrophils, g.l.c./m.s. analysis of non-esterified fatty acids indicated that the quantity of C20:4 increased as a function of time after stimulation, from undetectable quantities to > 800 pmol/10(7) cells. In contrast with C20:4, the quantities of other free fatty acids such as oleic and linoleic were high in resting cells and did not change after stimulation. Some 15% of the C20:4 released from cellular lipids remained cell-associated. To examine the effect of C20:4 on its own release, neutrophils were exposed to [2H8]C20:4, to differentiate it by g.l.c./m.s. from naturally occurring C20:4. In A23187-stimulated neutrophils, low concentrations (5-10 microM) of [2H8]C20:4 added just before A23187 increased the quantity of C20:4 produced by the cell, whereas higher concentrations (30-50 microM) decreased the quantity of C20:4 released from phospholipids. As other measures of PLA2 activity, the effects of C20:4 on production of platelet-activity factor (PAF) and leukotriene B4 (LTB4) were assessed. C20:4 treatment just before stimulation of neutrophils blocked PAF and LTB4 production in a concentration-dependent manner (IC50 10-20 microM). The effect of C20:4 was not blocked by the cyclo-oxygenase inhibitor naproxine (10 microM), nor could it be mimicked by 1 microM LTB4, 5-hydroxyeicosa-6,8,11,14-tetraenoic acid (5HETE), 5-hydroperoxyeicosa-6,8,11,14-tetraenoic acid (5HPETE) or 15-hydroxyeicosa-5,8,11,13-tetraenoic acid (15HETE). The 5-lipoxygenase (5LO) inhibitor zileuton induced a concentration-dependent decrease in PAF, with a maximal effect of a 50% decrease at 10-50 microM. The decrease in PAF by the 5LO inhibitor could not be circumvented by addition of 1 microM 5HETE, 5HPETE and LTB4, and may be attributed to the capacity of zileuton to increase the quantity of C20:4 in A23187-treated neutrophils. The inhibitory effect of C20:4 (20-40 microM) on PAF production could be antagonized by the protein kinase C inhibitor staurosporine (30 nM), but not by inhibitors of protein kinase A, tyrosine kinase or calmodulin kinase II. Taken together, these data demonstrate that C20:4 is selectively released from membrane phospholipids of A23187-stimulated neutrophils, and this C20:4 may play an important role in regulating the mobilization of C20:4 by altering PLA2 activity.

Differential inhibition of human secretory and cytosolic phospholipase A2

The roles and relative contributions of secretory and cytosolic phospholipases A2 in physiology and pathology are not precisely known. In a search for differential inhibitors of these enzymes, which could serve as tools to clarify this issue, we evaluated the potencies of reference compounds and three series of new compounds, viz. substrate analogues, 1,2-amino alcohols and enolized beta-tricarbonyl derivatives, as inhibitors of secretory phospholipase A2 from human polymorphonuclear leukocytes (sPLA2) and of cytosolic phospholipase A2 from human U937 cells (cPLA2). With few exceptions, the compounds selected are potent inhibitors of sPLA2 with IC50 values (concentration inhibiting 50%) in the low micromolar range. Inhibition of cPLA2 was only observed with some phosphate-free substrate analogues, with 1,2-amino alcohols and two of seven reference compounds. These results suggest that inhibition of secretory and of cytosolic phospholipases A2 are independent effects. Several inhibitors could be identified with a marked selectivity for sPLA2.