Structure and Properties of a Human Non-pancreatic Phospholipase A2

Dioleylphosphatidylglycerol inhibits the expression of type II phospholipase A2 in macrophages

We have recently shown that modified natural pulmonary surfactant Curosurf inhibits the synthesis of type II phospholipase A2 (sPLA2-II) by cultured guinea-pig alveolar macrophages (AM). The goal of the present study was to identify the surfactant components and the mechanisms involved in this process. We show that protein-free artificial surfactant (AS) mimicked the inhibitory effect of Curosurf, suggesting that phospholipid components of surfactant play a role in the inhibition of sPLA2-II expression. Among surfactant phospholipids, dioleylphosphatidylglycerol (DOPG) was the most effective in inhibiting the synthesis of sPLA2-II. By contrast, the concentrations of platelet-activating factor (PAF)-acetylhydrolase and lysophospholipase activities remained unchanged, indicating that inhibition of sPLA2-II synthesis was caused by a specific effect of surfactant. The effect of DOPG on sPLA2-II synthesis was concentration-dependent and was accompanied by a rapid and time-dependent uptake of DOPG by AM whereas dipalmitoylphosphatidylcholine (DPPC) was only marginally taken up. Curosurf, AS, and DOPG inhibited tumor necrosis factor-alpha (TNF-alpha) secretion, a key step in the induction of sPLA2-II synthesis by AM, in contrast to DPPC which had only a marginal effect. We conclude that phospholipid components, especially DOPG, play a major role in the inhibition of sPLA2-II synthesis by surfactant and that this effect can be explained, at least in part, by an impairment of TNF-alpha secretion.

The chemopreventive role of ursodeoxycholic acid in azoxymethane-treated rats: suppressive effects on enhanced group II phospholipase A2 expression in colonic tissue

Great interest has been focused on the chemoprevention of colonic carcinogenesis by oral administration of ursodeoxycholic acid (UDCA) because its administration reportedly reduces the incidence of colon cancer in animal experiments. To elucidate the precise role of UDCA in the chemoprevention of azoxymethane-induced colon carcinogenesis, we examined the expression levels of group II phospholipase A2 in the colonic tissue of UDCA-treated and untreated rats and correlated the levels with the findings of aberrant crypt foci, putative preneoplastic lesions. Twelve weeks after azoxymethane exposure, the total number of aberrant crypt foci in 0.4% UDCA-fed rats and 1% UDCA-fed rats was significantly decreased compared to the untreated animals. The mucosal concentrations of PGE2 and 6-keto PGF1alpha were significantly lower in the UDCA-treated rats than in untreated rats. In correlation with lowering, the enhanced activity, protein mass and mRNA levels of group II phospholipase A2 were significantly attenuated in the UDCA-treated animals. The chemopreventive role of UDCA in colon carcinogenesis may lie in its modulation of the arachidonate metabolism in colonic mucosa.

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.

Extracellular calcium regulates HeLa cell morphology during adhesion to gelatin: role of translocation and phosphorylation of cytosolic phospholipase A2

Attachment of HeLa cells to gelatin induces the release of arachidonic acid (AA), which is essential for cell spreading. HeLa cells spreading in the presence of extracellular Ca2+ released more AA and formed more distinctive lamellipodia and filopodia than cells spreading in the absence of Ca2+. Addition of exogenous AA to cells spreading in the absence of extracellular Ca2+ restored the formation of lamellipodia and filopodia. To investigate the role of cytosolic phospholipase A2 (cPLA2) in regulating the differential release of AA and subsequent formation of lamellipodia and filopodia during HeLa cell adhesion, cPLA2 phosphorylation and translocation from the cytosol to the membrane were evaluated. During HeLa cell attachment and spreading in the presence of Ca2+, all cPLA2 became phosphorylated within 2 min, which is the earliest time cell attachment could be measured. In the absence of extracellular Ca2+, the time for complete cPLA2 phosphorylation was lengthened to <4 min. Maximal translocation of cPLA2 from cytosol to membrane during adhesion of cells to gelatin was similar in the presence or absence of extracellular Ca2+ and remained membrane associated throughout the duration of cell spreading. The amount of total cellular cPLA2 translocated to the membrane in the presence of extracellular Ca2+ went from <20% for unspread cells to >95% for spread cells. In the absence of Ca2+ only 55-65% of the total cPLA2 was translocated to the membrane during cell spreading. The decrease in the amount translocated could account for the comparable decrease in the amount of AA released by cells during spreading without extracellular Ca2+. Although translocation of cPLA2 from cytosol to membrane was Ca2+ dependent, phosphorylation of cPLA2 was attachment dependent and could occur both on the membrane and in the cytosol. To elucidate potential activators of cPLA2, the extracellular signal-related protein kinase 2 (ERK2) and protein kinase C (PKC) were investigated. ERK2 underwent a rapid phosphorylation upon early attachment followed by a dephosphorylation. Both rates were enhanced during cell spreading in the presence of extracellular Ca2+. Treatment of cells with the ERK kinase inhibitor PD98059 completely inhibited the attachment-dependent ERK2 phosphorylation but did not inhibit cell spreading, cPLA2 phosphorylation, translocation, or AA release. Activation of PKC by phorbol ester (12-O-tetradecanoylphorbol-13-acetate) induced and attachment-dependent phosphorylation of both cPLA2 and ERK2 in suspension cells. However, in cells treated with the PKC inhibitor Calphostin C before attachment, ERK2 phosphorylation was inhibited, whereas cPLA2 translocation and phosphorylation remained unaffected. In conclusion, although cPLA2-mediated release of AA during HeLa cell attachment to a gelatin substrate was essential for cell spreading, neither ERK2 nor PKC appeared to be responsible for the attachment-induced cPLA2 phosphorylation and the release of AA.

Action of human group IIa secreted phospholipase A2 on cell membranes. Vesicle but not heparinoid binding determines rate of fatty acid release by exogenously added enzyme

Human group IIa phospholipase A2 (hIIa-PLA2) is a highly basic protein that is secreted from a number of cells during inflammation and may play a role in arachidonate liberation and in destruction of invading bacteria. It has been proposed that rodent group IIa PLA2 is anchored to cell surfaces via attachment to heparan sulfate proteoglycan and that this interaction facilitates lipolysis. hIIa-PLA2 contains 13 lysines, 2 histidines, and 10 arginines that fall into 10 clusters. A panel of 26 hIIa-PLA2 mutants were prepared in which 1-4 basic residues in each cluster were changed to glutamate or aspartate (charge reversal). A detailed analysis of the affinities of these mutants for anionic vesicles and for heparin and heparan sulfate in vitro and of the specific activities of these proteins for hydrolysis of vesicles in vitro and of living cell membranes reveal the following trends: 1) the affinity of hIIa-PLA2 for heparin and heparan sulfate is modulated not by a highly localized site of basic residues but by diffuse sites that partially overlap with the interfacial binding site. In contrast, only those residues on the interfacial binding site of hIIa-PLA2 are involved in binding to membranes; 2) the relative ability of these mutants to hydrolyze cellular phospholipids when enzymes were added exogenously to CHO-K1, NIH-3T3, and RAW 264.7 cells correlates with their relative in vitro affinity for vesicles and not with their affinity for heparin and heparan sulfate. 3) The rates of exogenous hIIa-PLA2-catalyzed fatty acid release from wild type CHO-K1 cells and two mutant lines, one lacking glycosaminoglycan and one lacking heparan sulfate, were similar. Thus basic residues that modulate interfacial binding are important for plasma membrane fatty acid release by exogenously added hIIa-PLA2. Binding of hIIa-PLA2 to cell surface heparan sulfate does not modulate plasma membrane phospholipid hydrolysis by exogenously added hIIa-PLA2.

Differential activation of human neutrophil cytosolic phospholipase A2 and secretory phospholipase A2 during priming by 1,2-diacyl- and 1-O-alkyl-2-acylglycerols

We have shown previously that both 1,2-diacylglycerol (AAG) and 1-O-alkyl-2-acylglycerol (EAG) prime neutrophil release of arachidonic acid via uncharacterized phospholipases A2. Therefore, we investigated the actions of EAG and AAG specifically on neutrophil cytosolic (cPLA2) and secretory (sPLA2) phospholipase A2s. We hypothesized that AAG as a protein kinase activator would activate cPLA2 via phosphorylation events. EAG is antagonistic to the AAG activation of PKC, thus it was not expected to act via phosphorylation of cPLA2. Neutrophils were primed with either AAG or EAG and then stimulated with fMLP. When neutrophils were primed with 5-20 microM 1,2-diacylglycerol, a shift was observed in cPLA2 migration on SDS-PAGE gels, consistent with phosphorylation of the protein. This gel shift was not seen after exposure to EAG. AAG also caused a parallel increase in enzymatic activity of cPLA2 that was not seen with EAG. We also investigated whether either diglyceride would cause similar priming or direct secretion of sPLA2. Both AAG and EAG directly caused significant secretion of neutrophil sPLA2. EAG also increased the release of sPLA2 in cells subsequently stimulated with fMLP. Thus, AAG activated cPLA2 and stimulated secretion of sPLA2. In contrast, EAG did not activate cPLA2, but directly activated secretion of sPLA2. We also demonstrated that human synovial fluid sPLA2 increased AA release from resting and fMLP-stimulated neutrophils. Given that diglycerides prime for release of AA, PAF, and LTB4, these current data support the hypothesis that such priming may be mediated by phosphorylation dependent (cPLA2) or phosphorylation independent (e.g. secretion of sPLA2) events.

Expression and characterization of human group V phospholipase A2

Group V phospholipase A2 (GV-PLA2) has been shown to be involved in signal transduction and inflammatory processes in cellular studies, but the physical and biochemical properties of this important enzyme have been unclear. We report the over-expression and characterization of GV-PLA2. The GV-PLA2 cDNA was synthesized from human heart polyA+ mRNA by RT-PCR, and an expression construct containing the GV-PLA2 was established. After expression in Escherichia coli cells, the protein was solubilized and purified to homogeneity in a single step using nickel affinity chromatography. The purified GV-PLA2 protein was folded to form active enzyme. The recombinant GV-PLA2 has an absolute requirement for Ca2+ for enzymatic activity. The optimum pH for this enzyme is pH 8.5 in Tris-HCl buffer with sonicated vesicles as substrate. GV-PLA2 preferentially hydrolyzes phosphatidylethanolamine (PE) vesicles compared to phosphatidylcholine (PC) vesicles. However, hydrolysis of PC and PE is equivalent in mixed vesicles of the phospholipids. The fatty acid preference of GV-PLA2 is linoleoyl>palmitoyl>arachidonyl with a PC head group and sonicated vesicles. 3-(3-Actamide-1-benzyl-2-ethylindolyl-5-oxy)propane phosphonic acid (LY311727), a potent inhibitor of human group IIA PLA2, strongly inhibits GV-PLA2 with an IC50 value of about 36 nM which is comparable to its inhibition of group IIA PLA2.

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

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

Selective Inhibitors of Cytosolic or Secretory Phospholipase A2 Block TNF-Induced Activation of Transcription Factor Nuclear Factor-κB and Expression of ICAM-1

TNF signaling mechanisms involved in activation of transcription factor NF-κB were studied in the human keratinocyte cell line HaCaT. We show that TNF-induced activation of NF-κB was inhibited by the well-known selective inhibitors of cytosolic phospholipase A2 (cPLA2): the trifluoromethyl ketone analogue of arachidonic acid (AACOCF3) and methyl arachidonyl fluorophosphate. The trifluoromethyl ketone analogue of eicosapentaenoic acid (EPACOCF3) also suppressed TNF-induced NF-κB activation and inhibited in vitro cPLA2 enzyme activity with a similar potency as AACOCF3. The arachidonyl methyl ketone analogue (AACOCH3) and the eicosapentanoyl analogue (EPACHOHCF3), which both failed to inhibit cPLA2 enzyme activity in vitro, had no effect on TNF-induced NF-κB activation. TNF-induced NF-κB activation was also strongly reduced in cells stimulated in the presence of the secretory PLA2 (sPLA2) inhibitors 12-epi-scalaradial and LY311727. Addition of excess arachidonic acid suppressed the inhibitory effect of 12-epi-scalaradial and LY311727. Moreover, both methyl arachidonyl fluorophosphate and 12-epi-scalaradial blocked TNF-mediated enhancement of expression of ICAM-1. Activation of NF-κB by IL-1β was markedly less sensitive to both cPLA2 and sPLA2 inhibitors. The results indicate that both cPLA2 and sPLA2 may be involved in the TNF signal transduction pathway leading to nuclear translocation of NF-κB and to NF-κB-activated gene expression in HaCaT cells.

Inhibition of prothrombinase by human secretory phospholipase A2 involves binding to factor Xa

Human group II secretory phospholipase A2 (hsPLA2) exhibits significant anticoagulant activity that does not require its enzymatic activity. We examined which coagulation factor was targeted by hsPLA2 and analyzed which region of the protein may be involved in this inhibition. Prothrombin time coagulation assays indicated that hsPLA2 did not inhibit activated factor V (FVa) activity, whereas activated factor X (FXa) one-stage coagulation assays suggested that FXa was inhibited. The inhibitory effect of hsPLA2 on prothrombinase activity of FXa, FV, phospholipids, and Ca2+ complex was markedly enhanced upon preincubation of hsPLA2 with FXa but not with FV. Prothrombinase activity was also strongly inhibited by hsPLA2 in the absence of PL. High concentrations of FVa in the prothrombinase generation assay reversed the inhibitory effect of hsPLA2. By using isothermal titration calorimetry, we demonstrated that hsPLA2 binds to FXa in solution with a 1:1 stoichiometry and a Kd of 230 nM. By using surface plasmon resonance we determined the rate constants, kon and koff, of the FXa/hsPLA2 interaction and analyzed the Ca2+ effect on these constants. When preincubated with FXa, synthetic peptides comprising residues 51-74 and 51-62 of hsPLA2 inhibited prothrombinase assays, providing evidence that this part of the molecule, which shares similarities with a region of FVa that binds to FXa, is likely involved in the anticoagulant interaction of hsPLA2 with FXa. In conclusion, we propose that residues 51-62 of hsPLA2 bind to FXa at a FVa-binding site and that hsPLA2 decreases the prothrombinase generation by preventing FXa.FVa complex formation.

Mechanisms that account for the selective release of arachidonic acid from intact cells by secretory phospholipase A2

The current study examined mechanisms that account for the selective release of arachidonic acid (AA) from cells by secretory phospholipase A2 (sPLA2). Initial studies demonstrated that low concentrations of group I and group III PLA2 isotypes and an sPLA2-enriched extract from bone marrow-derived mast cells (BMMC) selectively released AA from mast cells. Much higher concentrations of group II PLA2 were required to release comparable quantities of AA. Group I PLA2 also selectively released AA from another mast cell line (CFTL-15) and a monocytic cell line (THP-1). In contrast, high concentrations of group I PLA2 were required to release fatty acids from a promyelocytic cell line (HL-60) and this release was not selective for AA. Binding studies revealed that cell types (BMMC, CFTL-15 and THP-1) which selectively released AA also had the capacity to specifically bind group I PLA2. However, group II PLA2, which did not selectively release AA from cells, also did not specifically bind to these same cell types. Additional studies revealed that sPLA2 binding to the mast cell receptor was attenuated after stimulation with antigen or ionophore A23187. Reverse transcriptase-polymerase chain reaction analyses indicated the presence of mRNA for the sPLA2 receptor in BMMC, CFTL-15 and THP-1 and the absence of this mRNA in HL-60. Final studies demonstrated that p-aminophenyl-alpha-D-mannopyranoside BSA, a known ligand of the sPLA2 receptor, also selectively released AA from mast cells but not from HL-60 cells. These experiments indicated that receptor occupancy alone (without PLA2 activity) is sufficient to induce the release of AA from mast cells. Together, these data reveal that specific isotypes of sPLA2 have the capacity to selectively release AA from certain cells by their capacity to bind to sPLA2 receptors on the cell surface.

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.

Effects of nimesulide on constitutive and inducible prostanoid biosynthesis in human beings

BACKGROUND

The aim of this study was to test the hypothesis that nimesulide, a nonsteroidal antiinflammatory drug, or its principal metabolite 4-hydroxynimesulide, is a selective inhibitor of prostaglandin H synthase-2 in human beings.

METHODS

Heparinized whole blood samples obtained from healthy subjects were incubated with lipopolysaccharide (10 micrograms/ml) for 24 hours at 37 degrees C and prostaglandin E2 was measured in plasma as an index of monocyte prostaglandin H synthase-2 activity. The production of thromboxane B2 in whole blood allowed to clot at 37 degrees C for 60 minutes was assessed as an index of platelet prostaglandin H synthase-1 activity. We also measured the urinary excretion of 11-dehydrothromboxane B2, prostaglandin E2, 6-ketoprostaglandin F1 alpha, and thromboxane B2 as in vivo indexes of cyclooxygenase activity. All prostanoids were measured by previously validated radioimmunoassay techniques.

RESULTS

In the whole blood assays in vitro, nimesulide was twentyfold more potent than 4-hydroxynimesulide toward the two isozymes and both compounds displayed a twentyfold preference for prostaglandin H synthase-2 versus prostaglandin H synthase-1. The administration of a single oral dose of 100 mg nimesulide to six healthy subjects significantly (p < 0.01) reduced monocyte prostaglandin H synthase-2 and prostaglandin H synthase-1 activity ex vivo by more than 90% and 50%, respectively, up to 6 hours. At 24 hours, prostaglandin H synthase-2 but not prostaglandin H synthase-1 activity was significantly reduced by 49% (p < 0.05). Nimesulide significantly (p < 0.05) reduced the urinary excretion of 11-dehydrothromboxane B2 and 6-ketoprostaglandin F1 alpha by approximately 30% and 25%, respectively, while not affecting that of prostaglandin E2 and thromboxane B2.

CONCLUSIONS

Nimesulide is a potent inhibitor of human monocyte prostaglandin H synthase-2. However, despite a twentyfold selectivity ratio, therapeutic plasma levels of nimesulide are sufficiently high to cause detectable inhibition of platelet prostaglandin H synthase-1.

Secretory phospholipase A2 is the principal bactericide for staphylococci and other gram-positive bacteria in human tears

We examined human tears for molecules that killed gram-positive bacteria. The principal mediator of bactericidal activity against staphylococci proved to be a calcium-dependent enzyme, secretory phospholipase A2. Whereas the concentration of secretory phospholipase A2 in the normal tear film exceeded 30 microg/ml, only 1.1 ng (<0.1 nM) of the enzyme per ml sufficed to kill Listeria monocytogenes and 15 to 80 ng/ml killed Staphylococcus aureus. Despite its efficacy against gram-positive bacteria, secretory phospholipase A2 lacked bactericidal activity against gram-negative organisms (Escherichia coli, Salmonella typhimurium, and Pseudomonas aeruginosa) when tested in the ionic environment of tears. Given the presence of secretory phospholipase A2 in tears, intestinal secretions, and leukocytes, this enzyme may play a substantial role in innate mucosal and systemic bactericidal defenses against gram-positive bacteria.

Synergistic Effect of Type II Phospholipase A2 and Platelet-Activating Factor on Mac-1 Surface Expression and Exocytosis of Gelatinase Granules in Human Neutrophils: Evidence for the 5-Lipoxygenase-Dependent Mechanism

Stimulation of human neutrophils with inflammatory mediators such as TNF-α or platelet-activating factor (PAF) induces translocation of adhesion molecule Mac-1 (CD11b/CD18) from secretory vesicles to the plasma membrane. Type II phospholipase A2 (PLA2-II) also induces translocation of Mac-1 from secretory vesicles. However, there are more Mac-1 molecules in gelatinase granules and specific granules than in secretory vesicles. Therefore, different combinations of PLA2-II and other mediators were examined for their ability to induce gelatinase granules and specific granules to induce Mac-1 surface expression. The combination of PLA2-II and PAF synergistically increased Mac-1 surface expression, and the effect was greater than the combinations of PLA2-II with TNF-α, IL-8, or FMLP. Additionally, the combination of PLA2-II and PAF induced exocytosis of both secretory vesicles and gelatinase granules, which did not occur with either PLA2-II alone or PAF alone. The induction was accompanied by marked production of leukotriene B4. AA861, an inhibitor of 5-lipoxygenase, did not inhibit exocytosis of secretory vesicles but did inhibit exocytosis of gelatinase granules and decrease Mac-1 surface expression. It was also found that Ca2+ influx is essential for 5-lipoxygenase activation, because Ni2+, which blocks the influx of extracellular Ca2+, inhibited the production of leukotriene B4. These results suggest that stimulation by the combination of PLA2-II and PAF, unlike stimulation by each mediator alone, causes exocytosis of gelatinase granules via the 5-lipoxygenase pathway, resulting in a synergistic increase in neutrophil Mac-1 surface expression during inflammatory processes.

Purification and characterization of a low-molecular-weight phospholipase A2 from developing seeds of elm

Phospholipase A2 (PLA2) was purified about 180,000 times compared with the starting soluble-protein extract from developing elm (Ulmus glabra) seeds. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis the purified fraction showed a single protein band with a mobility that corresponded to 15 kD, from which activity could be recovered. When analyzed by matrix-assisted laser-desorption ionization-time-of-flight mass spectrometry, the enzyme had a deduced mass of 13,900 D. A 53-amino acid-long N-terminal sequence was determined and aligned with other sequences, giving 62% identity to the deduced amino acid sequence of some rice (Oryza sativa) expressed sequence tag clones. The purified enzyme had an alkaline pH optimum and required Ca2+ for activity. It was unusually stable with regard to heat, acidity, and organic solvents but was sensitive to disulfide bond-reducing agents. The enzyme is a true PLA2, neither hydrolyzing the sn-1 position of phosphatidylcholine nor having any activity toward lysophosphatidylcholine or diacylglycerol. The biochemical data and amino acid sequence alignments indicate that the enzyme is related to the well-characterized family of animal secretory PLA2s and, to our knowledge, is the first plant enzyme of this type to be described.

Ultrastructural localization of secretory type II phospholipase A2 in atherosclerotic and nonatherosclerotic regions of human arteries

We recently reported on the immunolocalization of type II secretory nonpancreatic phospholipase A2 (snpPLA2) in human atherosclerotic lesions. In the present study, we present data on the distribution and ultrastructural localization of snpPLA2 in adjacent nonatherosclerotic and atherosclerotic regions of human arteries. Electron microscopy (EM) of immunogold labeling techniques with a monoclonal antibody was used to analyze arterial tissue. The human specimens analyzed were obtained from autopsy and surgery cases. The results with EM showed a stronger snpPLA2 immunoreactivity in regions of arteries with atherosclerotic lesions than in regions without lesions from the same individual. snpPLA2 immunoreactivity was stronger in the arterial intima of atherosclerotic than of nonatherosclerotic tissue. EM-immunogold examination revealed that the majority of snpPLA2 was localized along the extracellular matrix, associated with collagen fibers and other extracellular matrix structures. Intracellular snpPLA2 was observed in electron-dense vesicles in intimal cells. snpPLA2 was also found in contact with large, extracellular lipid droplets. These results support the hypothesis that extracellular snpPLA2 is localized at sites where it may hydrolyze phospholipids from lipoproteins and lipid aggregates retained in the extracellular matrix of the arterial wall. This may be a mechanism for in situ release of proinflammatory lipids, free fatty acids, and lysophosphatidylcholine in regions of apolipoprotein B accumulation, which are abundant in atherosclerotic lesions.

Type IB secretory phospholipase A2 is contained in insulin secretory granules of pancreatic islet beta-cells and is co-secreted with insulin from glucose-stimulated islets

Stimulation of pancreatic islets with d-glucose induces insulin secretion from secretory granules contained within the islet beta-cells. Accumulating evidence suggests that secretory phospholipases A2 (sPLA2) may play a role in the distal events of secretory processes in many different cell types. Since intact pancreatic islets have been reported to contain sPLA2, it was of interest to determine the cellular and subcellular localization of the sPLA2 enzymes in pancreatic islets. Our findings indicate that rat pancreatic islets express mRNA for both types IB and IIA sPLA2 enzymes and mRNA for an sPLA2 membrane receptor. Immunoblotting analyses with antibodies directed against type IB sPLA2 or against type IIA sPLA2 indicate that the type IB isoform is much more abundant than the type IIA isoform in islets. Studies with purified populations of islet beta-cells prepared from dispersed islet cells by fluorescence-activated cell sorting indicate that both sPLA2 activity and type IB sPLA2 immunoreactive protein are substantially more abundant in beta-cells than in non-beta-cells. Subcellular fractionation studies indicate that sPLA2 activity and type IB sPLA2 immunoreactive protein are contained in insulin secretory granules. Stimulation of intact islets with insulin secretagogues results in the co-secretion of insulin and of sPLA2 activity and type IB sPLA2 immunoreactive protein into the incubation medium. These findings raise the possibility that type IB sPLA2 participates in the secretory process of pancreatic islet beta-cells.

Cellular source of human platelet secretory phospholipase A2

Platelets are one source of the group II extracellular form of phospholipase A2 (sPLA2) which is involved in the amplification of local and systemic inflammation. Although sPLA2 protein has been described in human platelets, its presence in human megakaryocytes has not been yet established. We demonstrated in this study that the human erythroleukaemia (HEL) cell line, which has megakaryoblastic features, constitutively expresses sPLA2. Using an anti-rhsPLA2 monoclonal antibody (mAb BA11) and dot-blot detection, we showed that HEL cells and platelets release sPLA2 into incubation medium upon stimulation by thrombin. Similar results were obtained for sPLA2 activity detected by a spectrofluorescence assay. Enzymatic activity was abolished by mAb BA11 and by protamine. In both cell types, although released, the major part of sPLA2 remained in the cell pellet, and was probably adsorbed at non-specific membrane sites. Double labelling experiments using mAb BA11 and an anti-GPIIb antiserum revealed the presence of sPLA2 in human bone-marrow megakaryocytes. The use of reverse transcription-polymerase chain reaction conjugated with hybridization analysis demonstrated the presence of mRNA encoding for sPLA2 in platelets and HEL cells. Expression of sPLA2 in platelets and megakaryocytes at both transcriptional and post-translational levels strongly argues in favour of a megakaryocytic origin of platelet sPLA2 and rules out a role for endocytosis of the enzyme from plasma by circulating platelets.

Secretory and cytosolic phospholipases A2 are activated during TNF priming of human neutrophils

Cytokines alter neutrophil (PMN) function during inflammation, and Tumor Necrosis Factor (TNF) in vitro primes PMN such that receptor-mediated stimulation causes markedly enhanced release of arachidonic acid. We hypothesized that two Ca(2+)-dependent PLA2's in PMN might be activated during priming of the cell, thus affecting arachidonate release. A low molecular weight, secretory PLA2 was identified by enzymatic activity in the cell free supernates of primed or stimulated PMN, and in PMN disrupted by nitrogen cavitation. The enzymatic activity was calcium-dependent, acid stable, destroyed by dithiothreitol, and blocked by anti-sPLA2 antibodies. TNF caused secretion of sPLA2 and also caused an increase in cell-associated sPLA2 enzymatic activity. Activation and release were maximal with fMLP stimulation of TNF-primed PMN. Neutrophils also contained a cytosolic PLA2 (cPLA2) characterized by enzymatic activity which was calcium dependent, enhanced by dithiothreitol, and blocked by anti-cPLA2 antibody. TNF caused a doubling of cPLA2 enzymatic activity which was associated with phosphorylation of the enzyme as judged by a migration shift on Western blots. Thus, TNF priming of human PMN caused marked increase in fMLP stimulated AA release in parallel to enhanced activity of two different PLA2's.

Molecular evolution of snake toxins: is the functional diversity of snake toxins associated with a mechanism of accelerated evolution?

Recent studies revealed that animal toxins with unrelated biological functions often possess a similar architecture. To tentatively understand the evolutionary mechanisms that may govern this principle of functional prodigality associated with a structural economy, two complementary approaches were considered. One of them consisted of investigating the rates of mutations that occur in cDNAs and/or genes that encode a variety of toxins with the same fold. This approach was largely adopted with phospholipases A2 from Viperidae and to a lesser extent with three-fingered toxins from Elapidae and Hydrophiidae. Another approach consisted of investigating how a given fold can accommodate distinct functional topographies. Thus, a number of topologies by which three-fingered toxins exert distinct functions were investigated either by making chemical modifications and/or mutational analyses or by studying the three-dimensional structure of toxin-target complexes. This review shows that, although the two approaches are different, they commonly indicate that most if not all the surface of a snake toxin fold undergoes natural engineering, which may be associated with an accelerated rate of evolution. The biochemical process by which this phenomenon occurs remains unknown.

Secretory phospholipase A2 activates the cascade of mitogen-activated protein kinases and cytosolic phospholipase A2 in the human astrocytoma cell line 1321N1

The biological effects of type IIA 14-kDa phospholipase A2 (sPLA2) on 1321N1 astrocytoma cells were studied. sPLA2 induced a release of [3H]arachidonic acid ([3H]AA) similar to that elicited by lysophosphatidic acid (LPA), a messenger acting via a G-protein-coupled receptor and a product of sPLA2 on lipid microvesicles. In contrast, no release of [1-14C]oleate could be detected in cells labeled with this fatty acid. As these findings pointed to a selective mechanism of [3H]AA release, it was hypothesized that sPLA2 could act by a signaling mechanism involving the activation of cytosolic PLA2 (cPLA2), i.e. the type of PLA2 involved in the release of [3H]AA elicited by agonists. In keeping with this view, stimulation of 1321N1 cells with sPLA2 elicited the decrease in electrophoretic mobility that is characteristic of the phosphorylation of cPLA2, as well as activation of p42 mitogen-activated protein (MAP) kinase, c-Jun kinase, and p38 MAP kinase. Incubation with sPLA2 of quiescent 1321N1 cells elicited a mitogenic response as judged from an increased incorporation of [3H]thymidine. Attempts to correlate the effect of extracellular PLA2 with the generation of LPA were negative. Incubation with pertussis toxin prior to the addition of either sPLA2 or LPA only showed abrogation of the response to LPA, thus suggesting the involvement of pertussis-sensitive Gi-proteins in the case of LPA. Treatments with inhibitors of the catalytic effect of sPLA2 such as p-bromophenacyl bromide and dithiothreitol did not prevent the effect on cPLA2 activation. In contrast, preincubation of 1321N1 cells with the antagonist of the sPLA2 receptor p-aminophenyl-alpha-D-mannopyranoside-bovine serum albumin, blocked cPLA2 activation with a EC50 similar to that described for the inhibition of binding of sPLA2 to its receptor. Moreover, treatment of 1321N1 cells with the MAP kinase kinase inhibitor PD-98059 inhibited the activation of both cPLA2 and p42 MAP kinase produced by sPLA2. In summary, these data indicate the existence in astrocytoma cells of a signaling pathway triggered by engagement of a sPLA2-binding structure, that produces the release of [3H]AA by activating the MAP kinase cascade and cPLA2, and leads to a mitogenic response after longer periods of incubation.

Essential requirement of cytosolic phospholipase A2 for activation of the phagocyte NADPH oxidase

Arachidonic acid (AA) can trigger activation of the phagocyte NADPH oxidase in a cell-free assay. However, a role for AA in activation of the oxidase in intact cells has not been established, nor has the AA generating enzyme critical to this process been identified. The human myeloid cell line PLB-985 was transfected to express p85 cytosolic phospholipase A2 (cPLA2) antisense mRNA and stable clones were selected that lack detectable cPLA2. cPLA2-deficient PLB-985 cells differentiate similarly to control PLB-985 cells in response to retinoic acid or 1,25-dihydroxyvitamin D3, indicating that cPLA2 is not involved in the differentiation process. Neither cPLA2 nor stimulated [3H]AA release were detectable in differentiated cPLA2-deficient PLB-985 cells, demonstrating that cPLA2 is the major type of PLA2 activated in phagocytic-like cells. Despite the normal synthesis of NADPH oxidase subunits during differentiation of cPLA2-deficient PLB-985 cells, these cells fail to activate NADPH oxidase in response to a variety of soluble and particulate stimuli, but the addition of exogenous AA fully restores oxidase activity. This establishes an essential requirement of cPLA2-generated AA for activation of phagocyte NADPH oxidase.

The three-dimensional structures of two toxins from snake venom throw light on the anticoagulant and neurotoxic sites of phospholipase A2

The three-dimensional structures of the class II anticoagulant phospholipase A2 (PLA2) toxin RVV-VD from the venom of Russell's viper, Vipera russelli russelli, and the class I neurotoxic PLA2 Notechis II-5 from the, Australian tiger snake, Notechis scutatus scutatus, were determined to 2.2 A and 3.0 A resolution, respectively. Both enzymes are monomeric and consist of 121 and 119 residues, respectively. A comparison of ten class I/II PLA2 structures showed, among other differences, that the beta-sheet of these enzymes (residues 76-83) is about 90 degrees less twisted in class I than in class II PLA2s. This, along with the insertion of some residues in the region 57-59 in class I enzymes (the elapid loop), could be the main reason for the significant difference in the anticoagulant and (presynaptic) neurotoxic properties between the two classes of PLA2. It seems apparent from sequence and structural comparisons that the toxic site of PLA2 responsible for the strong anticoagulancy of these toxins consists of a negatively charged part, Glu53, together with a positively charged ridge of lysine residues free for intermolecular interactions. These lysines differ between the two classes of PLA2.

Secretory phospholipases and membrane polishing

Although secretory phospholipase A2 (PLA2) isozymes have been identified in human gestational tissues, their role in homeostasis and pathophysiology during pregnancy has yet to be clearly established. The aims of this brief commentary are: (1) to review recent data concerning the expression of secretory PLA2 isozymes in human gestational tissues; and (2) to present a case for their involvement in regulating the expression of glycerophospholipids in the exoplasmic monolayer of the cell membrane. Three secretory PLA2 isozymes and a secretory PLA2 cell-surface receptor have been identified in human term gestational tissues. In addition to their potential role in the formation of glycerophospholipid-derived metabolites (such as prostaglandins), these isozymes may function to regulate the expression of aminophospholipids on the cell surface. The exposure of aminophospholipids on the cell surface dramatically affects many aspects of cell function. Secreted PLA2 isozymes that display a substrate preference for the negatively charged aminophospholipids (e.g. phosphatidylserine or phosphatidylethanolamine) in the exoplasmic membrane may affect cell function and reactivity via a process of 'membrane polishing', that is, the preferentially removal of aminophospholipids from the exoplasmic leaflet of the cell membranes. By this process, secreted PLA2 isozymes may limit unsolicited cell-surface binding of exogenous proteins, membrane fusion events and recognition by cellular surveillance systems.

Regulation of leukotriene and platelet-activating factor synthesis in human alveolar macrophages

It has been suggested that phospholipase A2 (PLA2) contributes to the regulation of leukotriene (LT) and platelet-activating factor (PAF) synthesis by controlling the release of their precursors, arachidonic acid (AA) and lysophosphatidylcholine (lysoPC), from membrane phospholipids. In rat alveolar macrophages (AMs), PLA2 appears to have a major role in LT synthesis but a more limited role in PAF synthesis. The present study was designed to define the role of PLA2 in LT and PAF synthesis in human AMs and determine whether differences exist between AMs obtained from normal subjects and those from patients with asthma. In the normal subjects, the calcium ionophore A23187 (Cal) increased AM PAF synthesis (percent incorporation of tritiated acetate) by 135% (p < 0.01) and LTB4 synthesis 88-fold (p < 0.001). Phorbol myristate acetate (PMA) had little effect alone, but it had a synergistic effect with Cal, increasing PAF synthesis by 466% and LTB4 synthesis to 229-fold above the control values (p < 0.001 for both). Ro 25-4331, a combined cytosolic (c) and secretory (s) PLA2 inhibitor, had little effect on the Cal-stimulated PAF synthesis, but it completely blocked the effect of PMA. It also blocked the Cal- and Cal+PMA-stimulated LTB4 synthesis. AACOCF3, a cPLA2 inhibitor, had no effect on either Cal or Cal+PMA-stimulated PAF synthesis. It reduced LTB4 synthesis, but it did so less effectively than Ro 25-4331. CoA-independent transacylase (CoAI-TA) activity in the AMs increased after stimulation and exposure to Ro 25-4331. SK&F 45905, a CoAI-TA inhibitor, reduced stimulated PAF synthesis by 30% to 40%. Patients with asthma had similar results except that cPLA2 had a greater role in stimulated LTB4 synthesis. These data indicate that PLA2 plays a direct role in human AM LT synthesis; both the cytosolic and secretory forms contribute to LT synthesis; PLA2 appears to have a more limited role in PAF synthesis, although it mediates the synergistic effect of PMA, probably via sPLA2; and CoAI-TA contributes to PAF synthesis during PLA2 inhibition. With the exception of the greater role for cPLA2 in stimulated LTB4 synthesis in the patients with asthma, the contributions of PLA2 and CoAI-TA to AM LT and PAF synthesis appear to be similar in normal subjects and patients with asthma.

Cytosolic 85-kDa phospholipase A2-mediated release of arachidonic acid is critical for proliferation of vascular smooth muscle cells

Recent evidence suggests that arachidonic acid (AA) may be involved in regulating cellular proliferation. The predominant mechanism of AA release from cellular phospholipids is via phospholipase A2 (PLA2) hydrolysis. The purpose of this study was to examine the roles of the distinct 14-kDa and 85-kDa PLA2 enzymes in human coronary artery vascular smooth muscle cell (hCAVSMC) proliferation. Cultured hCAVSMCs proliferate in the presence of growth medium with a typical doubling time of 30-40 h, grow at a slower proliferative rate upon reaching confluency (day 8), and eventually undergo contact inhibition of growth (day 10). Neither Type II 14-kDa PLA2 activity nor mass changed over a 10-day culture period. In contrast, 85-kDa PLA2 protein activity and mRNA decreased as time in culture progressed. This reduction in 85-kDa PLA2 correlated with reductions in DNA synthesis and suggested a possible association between 85-kDa PLA2 and proliferation. To directly evaluate the role of the 85-kDa PLA2 in proliferation we examined the effects of an 85-kDa PLA2 inhibitor (AACOCF3) and 85-kDa PLA2 antisense oligonucleotides on proliferation. Both reagents dose dependently inhibited proliferation, whereas a 14-kDa PLA2 inhibitor (SB203347), a calcium-independent PLA2 inhibitor (HELSS), an 85-kDa sense oligonucleotide, and a nonrelevant scrambled control oligonucleotide had no effect. The mechanism by which 85-kDa PLA2 influences cellular proliferation remains unclear. Inhibition of 85-kDa PLA2 activity produced neither phase-specific cell cycle arrest nor apoptosis (fluorescence-activated cell sorter analysis). Addition of AA (20 mu M) attenuated the effects of both AACOCF3 and 85-kDa antisense oligonucleotides implicating AA as a key mediator in cellular proliferation. However, although prostaglandin E2 (PGE2) was present in the culture medium, it peaked early (day 3) in culture, and indomethacin had no effect on cellular proliferation indicating that hCAVSMC proliferation was not mediated through PGE2. These data provide the first direct evidence that PLA2 is involved in control of VSMC proliferation and indicate that 85-kDa PLA2-mediated liberation of AA is critical for cellular proliferation.

Group II phospholipase A2 in nasal fluid, mucosa and paranasal sinuses

The aim of this study was to determine the type of phospholipase A2 (PLA2) in nasal fluid and to demonstrate its cellular origin. The concentration of group II PLA2 was high (591.5 micrograms/l) in nasal fluid compared with serum level (10.8 micrograms/l) and the fluid of paranasal sinuses (10.6 micrograms/l). Methacholine stimulated nasal fluid contained only small amounts (19.1 micrograms/l) of group II PLA2 when the flow of tear fluid through the nasolacrimal duct was obstructed. Occasional glands secreting group II PLA2 were found in nasal and paranasal mucosa by immunohistochemistry. Lysozyme was found in the majority of mucosal glands. It was concluded that nasal and paranasal mucosal glands contain group II PLA2. In nasal fluid, however, PLA2 is mainly derived from tear fluid.

Cross-talk between secretory phospholipase A2 and cytosolic phospholipase A2 in rat renal mesangial cells

Incubation of rat glomerular mesangial cells with potent proinflammatory cytokines like interleukin 1beta, (IL- 1beta) triggers the expression of a non-pancreatic secretory phospholipase A2 (sPLA2) and increases the formation of prostaglandin E2. We show here that sPLA2 acts in an autocrine fashion on mesangial cells and induces a rapid activation of protein kinase C (PKC) isoenzymes delta and epsilon and of p42 mitogen-activated protein kinase (MAPK), two putative activators of cytosolic phospholipase A2 (cPLA2). sPLA2 also activates Raf-1 kinase in mesangial cells which integrates the signals coming from PKC for further processing along the MAPK cascade. Subsequently a phosphorylation and activation of cPLA2 is observed, thus arguing for a cross-talk between the two classes of PLA2. Pretreatment of cells with either the highly specific PKC inhibitor Ro-318220 or the highly specific MAPK kinase (MEK) inhibitor PD 98059 completely blocked the sPLA2-induced cPLA2 activation, indicating that both kinases are essential for the cross-talk between the two types of PLA2. The effect of sPLA2 is mimicked by lysophosphatidylcholine (LPC), a reaction product of sPLA2 activity. LPC stimulates PKC-epsilon, Raf-1 kinase and MAPK activation as well as cPLA2 activation with a subsequent increase in arachidonic acid release from mesangial cells. These data suggest that sPLA2 by cleaving membrane phospholipids and generating LPC and other lysophospholipids activates cPLA2 via the PKC/Raf-1/MAPK signalling pathway. Hence a network of interactions between different PLA2s is operative in mesangial cells and may contribute to the progression of glomerular inflammatory processes.

Expression of phospholipase A2 isoforms in human normal and atherosclerotic arterial wall

LDL particles must be modified in the arterial wall to be taken up by macrophages at an excessive rate, leading to foam cell formation. Phospholipase A2 (PLA2) has been shown to modify LDL particles in vitro by degrading its phospholipids, resulting in enhanced uptake by macrophages. Reaction products of PLA2 are lysophospholipids and nonesterified fatty acids (mainly arachidonic acid), which are precursors of potent inflammatory mediators and which have been found in atherosclerotic regions of the arterial wall. To elucidate the expression of PLA2 in normal and diseased arteries, frozen tissue sections of human nonatherosclerotic mesenteric artery and carotid plaques were examined by immunohistochemistry using specific antibodies against secretory PLA2 types I and II and cytosolic PLA2 (85 kd). Secretory PLA2 type I was not detected. High expression of secretory PLA2 type II was found throughout the media in both normal and atherosclerotic artery specimens, in which smooth muscle cells dominated. Cytosolic PLA2 was found exclusively in diseased artery, mainly in the intima in regions with an inflammatory infiltrate consisting of macrophages and smooth muscle cells. Furthermore, both normal and atherosclerotic artery possessed substantial PLA2 activity. It is suggested that secretory PLA2 type II could play an important role in early atherogenesis because it is present in the preatherosclerotic arterial wall, where it may lead to LDL modification, foam cell formation, and activation of immune mechanisms.

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.

Comparison of alkylacylglycerol vs. diacylglycerol as activators of mitogen-activated protein kinase and cytosolic phospholipase A2 in human neutrophil priming

In human neutrophils, the choline-containing phosphoglycerides contain almost equal amounts of alkylacyl- and diacyl-linked subclasses. In contrast to phosphatidylinositol hydrolysis which yields diacylglycerol, hydrolysis of choline-containing phosphoglycerides by phospholipase D coupled with phosphohydrolase yields both alkylacyl- and diacylglycerol. While diacylglycerol activates protein kinase C, alkylacylglycerol does not, and its role is unclear. Yet previous studies have shown that exogenous alkylacyl- and diacylglycerols can prime for the release of radiolabeled arachidonic acid (AA) in intact neutrophils stimulated by formyl-methionyl-leucyl-phenylalanine. We have now examined the effects of both diacylglycerol (1-oleoyl-2-acetylglycerol; OAG) and alkylacylglycerol (1-O-hexadecyl-2-acetylglycerol; EAG) on the activation of mitogen-activated protein (MAP) kinase and the 85-kDa cytosolic phospholipase A2 (cPLA2) in human neutrophils. We observed that while OAG could effectively activate p42 and p44 MAP kinases along with cPLA2 in a time- and concentration-dependent manner, EAG could not. A novel p40 MAP kinase isoform is also present and activated in response to OAG treatment; the behavior of this MAP kinase isoform is discussed. The activation of cPLA2 and MAP kinase by 20 microM OAG could be inhibited by pretreatment with 1 microM GF-109203X, a selective inhibitor of protein kinase C. Although only OAG activated cPLA2, both OAG and EAG primed for the release of AA mass as determined by gas chromatography/mass spectrometry. The priming of AA release by OAG may be explained by the phosphorylation of cPLA2 through the activation of protein kinase C linked to MAP kinase. However, priming by EAG appears to involve a separate mechanism that is dependent on a different PLA2. Our results support a role for phospholipase D-derived products modulating the activation of cPLA2, further supporting the idea of cross-talk among various phospholipases.

Expression of human group II phospholipase A2 in transgenic mice

Group II phospholipase A2 (PLA2) has been proposed to play an important role in inflammation and defense against bacterial infection. We investigated tissues of transgenic mice expressing the human group II PLA2 gene by immunohistochemistry using rabbit anti-human group II PLA2 antibodies, and by in situ hybridization by probing with human group II PLA2 mRNA anti-sense (test) and sense (control) riboprobes. By immunohistochemistry, human group II PLA2 was found in various mouse tissues and cell types including hepatocytes, proximal tubule cells of the kidney, epithelial cells of the renal pelvis, urinary bladder and ureter, granulosa cells of Graafian follicles, aortic intima and media, cartilage, epiphyseal bone, bronchial epithelial cells, and connective tissue cells in the dermis. By in situ hybridization, group II PLA2 mRNA was localized in hepatocytes, epidermal cells, dermal cells, connective tissue fibroblasts, epithelial and smooth muscle cells of the urinary bladder, and cells of Bowman's capsule. These results show that human group II PLA2 is expressed in large amounts in hepatocytes and many extrahepatic tissues of the transgenic mice. These animals provide a useful new tool for studies on the metabolism, in vivo effects, and physiological and pathological roles of phospholipase A2.

Total chemical synthesis of enzymatically active human type II secretory phospholipase A2

Human group II secretory phospholipase A2 (sPLA2) is an enzyme found in the alpha granules of platelets and at inflammatory sites. Although its physiological function is unclear, sPLA2 can inhibit blood coagulation reactions independent of its lipolytic action. To study the molecular basis of PLA2 activities, we developed a total chemical synthesis of sPLA2 by chemical ligation of large unprotected peptides. The synthetic segments PLA2-(1-58)-alphaCOSCH2COOH and PLA2-(59-124) were prepared by stepwise solid-phase peptide synthesis and ligated to yield a peptide bond between Gly58 and Cys59. The 124-residue polypeptide product (mass: 13,920 +/- 2 Da) was folded to yield one major product (mass: 13,905 +/- 1 Da), the loss of 15 +/- 3 Da reflecting the formation of seven disulfide bonds. Circular dichroism studies of synthetic sPLA2 showed alpha-helix, beta-structure, and random coil contents consistent with those found in the crystal structure of sPLA2. Synthetic sPLA2 had kcat and Km values identical to those of recombinant sPLA2 for hydrolysis of 1,2-bis(heptanoylthio)-phosphatidylcholine. Synthetic sPLA2, like recombinant sPLA2, inhibited thrombin generation from prothrombinase complex (factors Xa, V, II, Ca2+, and phospholipids). In the absence of phospholipids, both synthetic and recombinant sPLA2 inhibited by 70% prothrombin activation by factors Xa, Va, and Ca2+. Thus, synthetic sPLA2 is a phospholipid-independent anticoagulant like recombinant or natural sPLA2. This study demonstrates that chemical synthesis of sPLA2 yields a fully active native-like enzyme and offers a straightforward tool to provide sPLA2 analogs for structure-activity studies of anticoagulant, lipolytic, or inflammatory activities.

A phospholipase A2 kinetic and binding assay using phospholipid-coated hydrophobic beads

A novel kinetic and membrane-binding assay for phospholipase A2 (PLA2) has been developed utilizing phospholipid-coated hydrophobic styrene-divinylbenzene beads (5.2 +/- 0.3 microm diameter). Phospholipids formed a stable monolayer film on styrene-divinylbenzene beads with average surface packing density of (1.3 +/- 0.2) x 10(-2) molecule/A2. Secretory PLA2 readily hydrolyzed 1-palmitoyl-2-[3H]-oleoyl-sn-glycero-3-phosphoglycerol coated on styrene-divinylbenzene beads which could be easily monitored by measuring the radioactivity of fatty acid released to solution in the presence of bovine serum albumin. For human cytosolic PLA2 with high specificity for sn-2 arachidonyl group, styrene-divinylbenzene beads coated with 1-stearoyl-2-[14C]-arachidonyl-sn-glycero-3-phosphocholine and dioleoylglycerol (7:3, mol/mol) were used as substrate. PLA2 activity was linearly proportional to the enzyme concentration in the range from 1 to 150 nM for human class II secretory PLA2 and from 1 to 20 nM for cytosolic PLA2; the specific activity was 1.6 and 1.7 micromol/min/mg, respectively. Finally, styrene-divinylbenzene beads coated with polymerized 1,2-bis[12-(lipoyloxy) dodecanoyl]-sn-glycero-3-phosphoglycerol were used to measure the membrane binding affinity of PLA2, which in conjunction with kinetic data provides important insights into how PLA2 interacts with membranes.

Cloning, chromosomal mapping, and expression of a novel human secretory phospholipase A2

Secretory phospholipases A2 (sPLA2s) represent a rapidly expanding family of structurally related enzymes found in mammals as well as in insect and snake venoms. In this report, a cDNA coding for a novel sPLA2 has been isolated from human fetal lung, and its gene has been mapped to chromosome 16p13.1-p12. The mature sPLA2 protein has a molecular mass of 13.6 kDa, is acidic (pI 5.3), and made up of 123 amino acids. Key structural features of the sPLA2 include: (i) a long prepropeptide ending with an arginine doublet, (ii) 16 cysteines located at positions that are characteristic of both group I and group II sPLA2s, (iii) a C-terminal extension typical of group II sPLA2s, (iv) and the absence of elapid and pancreatic loops that are characteristic of group I sPLA2s. Based on these structural properties, this sPLA2 appears as a first member of a new group of sPLA2s, called group X. A 1.5-kilobase transcript coding for the human group X (hGX) sPLA2 was found in spleen, thymus, and peripheral blood leukocytes, while a less abundant 0.8-kilobase transcript was detected in the pancreas, lung, and colon. When the hGX sPLA2 cDNA was expressed in COS cells, sPLA2 activity preferentially accumulated in the culture medium, indicating that hGX sPLA2 is an actively secreted enzyme. It is maximally active at physiological pH and with 10 mM Ca2+. hGX sPLA2 prefers phosphatidylethanolamine and phosphatidylcholine liposomes to those of phosphatidylserine.

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.

Activation of MAP kinase cascade induced by human pancreatic phospholipase A2 in a human pancreatic cancer cell line

We have found that the growth of human pancreatic cancer cells MIAPaCa-2, induced by human pancreatic phospholipase A2 group I (hPLA2-I), is mediated via its specific receptor but not via its catalytic property. The present study showed that the activation of mitogen-activated protein kinase (MAPK) cascade in MIAPaCa-2 cells is induced by hPLA2-I: this digestive enzyme induced phosphorylation of MEK1/2, p44/42 MAPK and ATF-2, and the phosphorylation in the MAPK cascade was inhibited after the cells were pre-incubated with a selective inhibitor of MEK, PD98059. In addition, this inhibitor dose-dependently blocked the hPLA2-I-induced MIAPaCa-2 proliferation, suggesting that activation of the MAPK cascade is essential for the hPLA2-I-induced MIAPaCa-2 proliferation.

Group II phospholipase A2 in serum after knee surgery and intramedullary nailing of tibial shaft fracture

This prospective study investigates the effect of injury and surgery of cartilage and bone on serum group II phospholipase A2 (PLA2-II) and C-reactive protein (CRP) levels. Serum concentrations of PLA2-II and CRP were measured before and after the operation in nine patients with closed tibial shaft fractures treated by nailing, 11 patients with fractures of lateral tibial plateau treated by bone grafting, and 19 patients with ruptured anterior cruciate ligament treated by reconstruction. The postoperative PLA2-II and CRP values were statistically significantly higher than the pre-operative values in the tibial plateau fracture and ligament rupture groups, whereas the increase in the PLA2-II values in the tibial fracture group was not statistically significant. The highest values of both parameters were found on the second postoperative day. The changes in the PLA2-II and CRP values were parallel in the lateral condyle fracture and in anterior cruciate ligament rupture groups. PLA2-II behaves as an acute phase reactant in the serum of patients undergoing acute and elective knee surgery.

Morelloflavone, a novel biflavonoid inhibitor of human secretory phospholipase A2 with anti-inflammatory activity

The flavanonylflavone morelloflavone inhibited secretory phospholipase A2 (PLA2) in vitro, with a high potency on the human recombinant synovial and bee venom enzymes (IC50 = 0.9 and 0.6 microM, respectively). The inhibition was apparently irreversible. In contrast, the compound was inactive on cytosolic PLA2 activity from human monocytes. Morelloflavone scavenged reactive oxygen species generated by human neutrophils (IC50 = 2.7 and 1.8 microM for luminol and lucigenin, respectively) but did not modify cellular responses such as degranulation or eicosanoid release. This biflavonoid exerted anti-inflammatory effects in animal models, with a potent inhibition of 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced ear inflammation in mice after topical administration. In this test, morelloflavone was found to decrease oedema and myeloperoxidase levels in ear homogenates ID50 = 58.5 and 74.3 micrograms/ear, respectively). In contrast, this biflavonoid failed to modify arachidonic acid-induced ear inflammation or eicosanoid levels in ear homogenates. A significant anti-inflammatory effect was also observed in the mouse paw carrageenan edema after oral administration, with the highest inhibition at 3 hr after induction of inflammation. Morelloflavone is an inhibitor of secretory PLA2 with selectivity for groups II and III enzymes and may be a pharmacological tool. In addition, it shows anti-inflammatory activity apparently not related to the synthesis of eicosanoids, but likely dependent on other mechanisms such as scavenging of reactive oxygen species.