Amino acid composition and NH2-terminal amino acid sequence of human phospholipase A2 purified from rheumatoid synovial fluid

Secreted Phospholipases A2: Drivers of Inflammation and Cancer

Secreted phospholipase 2 (sPLA2) is the largest family of phospholipase A2 (PLA2) enzymes with 11 mammalian isoforms. Each sPLA2 exhibits different localizations and specific properties, being involved in a very wide spectrum of biological processes. The enzymatic activity of sPLA2 has been well described; however, recent findings have shown that they could regulate different signaling pathways by acting directly as ligands. Arachidonic acid (AA) and its derivatives are produced by sPLA2 in collaboration with other molecules in the extracellular space, making important impacts on the cellular environment, being especially relevant in the contexts of immunity and cancer. For these reasons, this review focuses on sPLA2 functions in processes such as the promotion of EMT, angiogenesis, and immunomodulation in the context of tumor initiation and progression. Finally, we will also describe how this knowledge has been applied in the search for new sPLA2 inhibitory compounds that can be used for cancer treatment.

Inflammatory Effects of Bothrops Phospholipases A2: Mechanisms Involved in Biosynthesis of Lipid Mediators and Lipid Accumulation

Phospholipases A₂s (PLA₂s) constitute one of the major protein groups present in the venoms of viperid and crotalid snakes. Snake venom PLA₂s (svPLA₂s) exhibit a remarkable functional diversity, as they have been described to induce a myriad of toxic effects. Local inflammation is an important characteristic of snakebite envenomation inflicted by viperid and crotalid species and diverse svPLA₂s have been studied for their proinflammatory properties. Moreover, based on their molecular, structural, and functional properties, the viperid svPLA₂s are classified into the group IIA secreted PLA₂s, which encompasses mammalian inflammatory sPLA₂s. Thus, research on svPLA₂s has attained paramount importance for better understanding the role of this class of enzymes in snake envenomation and the participation of GIIA sPLA₂s in pathophysiological conditions and for the development of new therapeutic agents. In this review, we highlight studies that have identified the inflammatory activities of svPLA₂s, in particular, those from Bothrops genus snakes, which are major medically important snakes in Latin America, and we describe recent advances in our collective understanding of the mechanisms underlying their inflammatory effects. We also discuss studies that dissect the action of these venom enzymes in inflammatory cells focusing on molecular mechanisms and signaling pathways involved in the biosynthesis of lipid mediators and lipid accumulation in immunocompetent cells.

Human Group IIA Phospholipase A2-Three Decades on from Its Discovery

Phospholipase A₂ (PLA₂) enzymes were first recognized as an enzyme activity class in 1961. The secreted (sPLA₂) enzymes were the first of the five major classes of human PLA₂s to be identified and now number nine catalytically-active structurally homologous proteins. The best-studied of these, group IIA sPLA₂, has a clear role in the physiological response to infection and minor injury and acts as an amplifier of pathological inflammation. The enzyme has been a target for anti-inflammatory drug development in multiple disorders where chronic inflammation is a driver of pathology since its cloning in 1989. Despite intensive effort, no clinically approved medicines targeting the enzyme activity have yet been developed. This review catalogues the major discoveries in the human group IIA sPLA₂ field, focusing on features of enzyme function that may explain this lack of success and discusses future research that may assist in realizing the potential benefit of targeting this enzyme. Functionally-selective inhibitors together with isoform-selective inhibitors are necessary to limit the apparent toxicity of previous drugs. There is also a need to define the relevance of the catalytic function of hGIIA to human inflammatory pathology relative to its recently-discovered catalysis-independent function.

Mann T, Bastard K, F. Scott K, Church WB. Structural and Functional Aspects of Targeting the Secreted Human Group IIA Phospholipase A2

Human group IIA secretory phospholipase A₂ (hGIIA) promotes the proliferation of cancer cells, making it a compelling therapeutic target, but it is also significant in other inflammatory conditions. Consequently, suitable inhibitors of hGIIA have always been sought. The activation of phospholipases A₂ and the catalysis of glycerophospholipid substrates generally leads to the release of fatty acids such as arachidonic acid (AA) and lysophospholipid, which are then converted to mediator compounds, including prostaglandins, leukotrienes, and the platelet-activating factor. However, this ability of hGIIA to provide AA is not a complete explanation of its biological role in inflammation, as it has now been shown that it also exerts proinflammatory effects by a catalysis-independent mechanism. This mechanism is likely to be highly dependent on key specific molecular interactions, and the full mechanistic descriptions of this remain elusive. The current candidates for the protein partners that may mediate this catalysis-independent mechanism are also introduced in this review. A key discovery has been that selective inhibition of the catalysis-independent activity of hGIIA is achieved with cyclised derivatives of a pentapeptide, FLSYK, derived from the primary sequence of hGIIA. The effects of hGIIA on cell function appear to vary depending on the pathology studied, and so its mechanism of action is complex and context-dependent. This review is comprehensive and covers the most recent developments in the understanding of the many facets of hGIIA function and inhibition and the insight they provide into their clinical application for disease treatment. A cyclic analogue of FLSYK, c2, the most potent analogue known, has now been taken into clinical trials targeting advanced prostate cancer.

Molecular dynamics simulations reveal structural insights into inhibitor binding modes and functionality in human Group IIA phospholipase A2

Human Group IIA phospholipase A₂ (hGIIA) promotes inflammation in immune-mediated pathologies by regulating the arachidonic acid pathway through both catalysis-dependent and -independent mechanisms. The hGIIA crystal structure, both alone and inhibitor-bound, together with structures of closely related snake-venom-derived secreted phospholipase enzymes has been well described. However, differentiation of biological and nonbiological contacts and the relevance of structures determined from snake venom enzymes to human enzymes are not clear. We employed molecular dynamics (MD) and docking approaches to understand the binding of inhibitors that selectively or nonselectively block the catalysis-independent mechanism of hGIIA. Our results indicate that hGIIA behaves as a monomer in the solution environment rather than a dimer arrangement that is in the asymmetric unit of some crystal structures. The binding mode of a nonselective inhibitor, KH064, was validated by a combination of the experimental electron density and MD simulations. The binding mode of the selective pentapeptide inhibitor FLSYK to hGIIA was stipulated to be different to that of the snake venom phospholipases A₂ of Daboia russelli pulchella (svPLA₂ ). Our data suggest that the application of MD approaches to crystal structure data is beneficial in evaluating the robustness of conclusions drawn based on crystal structure data alone. Proteins 2017; 85:827-842. © 2016 Wiley Periodicals, Inc.

Sera of patients suffering from inflammatory diseases contain group IIA but not group V phospholipase A(2)

During recent years, the high phospholipase A(2) (PLA(2)) concentrations at sites of inflammation and in circulation in several life-threatening diseases, such as sepsis, multi-organ dysfunction and acute respiratory distress syndrome, has generally been ascribed to the non-pancreatic group IIA PLA(2). Recently the family of secreted low molecular mass PLA(2) enzymes has rapidly expanded. In some cases, a newly described enzyme appeared to be cross-reactive with antibodies against the group IIA enzyme. For this reason, reports describing the expression of group IIA PLA(2) during inflammatory conditions need to be reevaluated. Here we describe the identification of the PLA(2) activity in sera of acute chest syndrome patients and in sera of trauma victims. In both cases, the PLA(2) activity was identified as group IIA. This classification was based upon cross-reactivity with monoclonal antibodies against group IIA PLA(2) which do not recognize the recombinant human group V enzyme. Moreover, purification of the enzymatic activity from the two sera followed by N-terminal amino acid sequence analyses revealed only the presence of group IIA enzyme.

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.

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.

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.

Purification of a novel phospholipase A2 from bovine seminal plasma

Phospholipases A2 are enzymes believed to play important roles in numerous physiological systems including sperm cell maturation. Relatively little work has, however, been devoted to study these enzymes in seminal plasma. We therefore undertook the purification and characterization of this enzyme from bovine seminal plasma. After a 330-fold purification, an activity corresponding to a protein of 100 kDa was identified by gel filtration. SDS-polyacrylamide gel electrophoresis analysis of the purified fraction revealed the presence of a 60-kDa band that comigrated with the activity during ion-exchange and gel filtration chromatography as well as polyacrylamide gel electrophoresis. The enzyme possessed a pH optimum around pH 6.5 and was calcium-dependent. Using isoelectric focusing, its isoelectric point was determined to be 5.6 +/- 0.07. The enzymatic activity was resistant to p-bromophenacyl bromide, but was sensitive to gossypol and dithiothreitol. The enzyme was 2 orders of magnitude more active toward micelles formed with deoxycholate than with Triton X-100. Slight differences in the specificity toward head groups and/or sn-2-side chains were found in both assay systems. The enzyme was acid-labile and did not display affinity for heparin. It would therefore appear that the phospholipase A2 form isolated from bovine seminal plasma is of a novel type.

Aspirin inhibits expression of the interleukin-1beta-inducible group II phospholipase A2

Nonsteroidal anti-inflammatory drugs (NSAIDs) clearly inhibit the synthesis and release of prostaglandins. However, these actions are not sufficient to explain all the anti-inflammatory effects of these drugs. Recently, it has been shown that aspirin and sodium salicylate inhibit the activation of the transcription factor NF-kappaB. Group II phospholipase A2 (sPLA2) is expressed in rat glomerular mesangial cells upon exposure to the inflammatory cytokine interleukin-1beta (IL-1beta) and this induction is attenuated by the NF-kappaB inhibitor pyrrolidine dithiocarbamate (PDTC). We now report that aspirin inhibits the IL-1beta-induced sPLA2 activity in rat mesangial cells in a dose-dependent manner. The IC50 value of aspirin for sPLA2 inhibition was 6.5 mM. This decrease in sPLA2 activity was not due to direct inhibition of enzymatic activity but rather to the fact that aspirin inhibits the expression of IL-1beta-induced sPLA2 protein and mRNA. Furthermore, by electrophoretic mobility shift analysis we demonstrate reduced DNA binding of the nuclear factor kappaB, an essential component of the IL-1beta-dependent upregulation of sPLA2 gene transcription, after treatment of the cells with aspirin. The study described in this report indicates that the inhibition of sPLA2 expression as induced by pro-inflammatory cytokines potentially represents an additional mechanism of action for aspirin.

Regulation of arachidonic acid, eicosanoid, and phospholipase A2 levels in murine mast cells by recombinant stem cell factor

The current study evaluates the capacity of recombinant rat stem cell factor (rrSCF) to regulate enzymes that control AA release and eicosanoid generation in mouse bone marrow-derived mast cells (BMMCs). Initial studies indicated that rrSCF provided for 24 h inhibited the release of AA into supernatant fluids of antigen- and ionophore A23187-stimulated BMMCs. Agonist-induced increases in cellular levels of AA were also inhibited, albeit to a lesser degree by rrSCF. To determine the inhibitory mechanism, several steps (e.g., mobilization of cytosolic calcium, release of BMMC granules, and regulation of phospholipase A2 [PLA2] activity) that could influence AA release were measured in rrSCF-treated cells. rrSCF did not alter the capacity of BMMCs to mobilize cytosolic calcium or release histamine in response to antigen and ionophore. BMMCs released large amounts of PLA2 with characteristics of the group II family in response to antigen and ionophore A23187. rrSCF treatment of BMMCs reduced the secretion of this PLA2 activity by BMMCs. Partial purification of acid-extractable PLA2 from rrSCF-treated and untreated BMMCs suggested that rrSCF decreased the quantity of acid-stable PLA2 within the cells. In contrast to group II PLA2, the quantity of cPLA2 (as determined by Western blot analysis) increased in response to rrSCF. To assess the ramifications of rrSCF-induced reductions in AA and group II PLA2, eicosanoid formation was measured in antigen- and ionophore-stimulated BMMCs, rrSCF-inhibited (100 ng/ml, 24 h) prostaglandin D2 (PGD2), thromboxane B2, and leukotriene B4 by 48.4 +/- 7.7%, 61.1 +/- 10.0% AND 38.1 +/- 3.6%, respectively, in antigen-stimulated cells. Similar patterns of inhibition were observed in ionophore-stimulated BMMCs. The addition of a group I PLA2 or exogenous AA to BMMCs reversed the inhibition of eicosanoid generation induced by rrSCF. Together, these data indicate that rrSCF differentially regulates group II and cytosolic PLA2 activities in BMMCs. The resultant reductions in eicosanoid generation suggest that group II PLA2 provides a portion of AA that is used for eicosanoid biosynthesis by BMMCs.

Bactericidal properties of murine intestinal phospholipase A2

We purified a molecule from the murine small intestine that killed both Escherichia coli and Listeria monocytogenes, and identified it as intestinal phospholipase A2 (iPLA2) by NH2-terminal sequencing and enzymatic measurements. The ability of iPLA2 to kill. L. monocytogenes was greatly enhanced by 5 mM calcium, inhibited by EGTA and abolished after reduction and alkylation, suggesting that enzymatic activity was required for iPLA2-mediated bactericidal activity. A mouse-avirulent phoP mutant, S. typhimurium 7953S, was 3.5-fold more susceptible to iPLA2 than its isogenic virulent parent, S. typhimurium 14028S (estimated minimal bactericidal concentrations 12.7 +/- 0.5 micrograms/ml vs. 43.9 +/- 4.5 micrograms/ml P < 0.001). Overall, these findings identify iPLA2 as part of the antimicrobial arsenal that equips Paneth cells to protect the small intestinal crypts from microbial invasion. Because iPLA2 is identical to Type 2 phospholipase A2 molecules found in other sites, including spleen, platelets and inflammatory exudate cells, this enzyme may also contribute to antibacterial defenses elsewhere in the body.

Dissociation between the phospholipases C and A2 activities in stimulated platelets and their involvement in the arachidonic acid liberation

In previous work we have demonstrated that platelets depleted from secretory phospholipase A2 (sPLA2) produced similar amounts of thromboxane (Tx)B2 as control platelets upon stimulation by thrombin. However, since depletion of sPLA2 was not total, this sole finding only suggested the non-involvement of sPLA2 in arachidonic acid release. In the present study we provide further evidence for the non-involvement of sPLA2 in arachidonic acid liberation during platelet activation. Thus, rabbit platelets exposed to thrombin secreted sPLA2, released free arachidonic acid and formed TxB2 and inositol phosphates. In contrast, U46619, a stable prostaglandin (PG)H2 analogue, activates phospholipase C (PLC) and induces release of sPLA2 without TXB2 generation nor arachidonic acid liberation. At each concentration tested of both agonists, stimulation of sPLA2 activity paralleled the production of inositol phosphates. These data suggest that sPLA2 is dependent on phosphoinositide hydrolysis and on the release reaction and that it is not involved in the liberation of arachidonic acid from stimulated platelets. In addition, a dissociation was observed between sPLA2 and the enzyme involved in the arachidonic acid mobilization, suggesting that the liberation of this fatty acid from membrane phospholipids was mediated by cytosolic phospholipase A2 (cPLA2). Finally, PLC does not play a major role in arachidonic acid liberation, since U46619, which induced the breakdown of inositol phospholipids, failed to release arachidonic acid. In confirmation, neomycin, which inhibits PLC activity, failed to inhibit ATP, sPLA2 and arachidonic acid release upon stimulation of platelets by fluoroaluminate. These data demonstrate that sPLA2 is not involved in the arachidonic acid release by stimulated platelets and indicate that the activations of PLC, sPLA2 and cPLA2 are independent events.

Synovial-type (group II) phospholipase A2 human seminal plasma

Phospholipase A2 (PLA2) was analysed in seminal plasma of fertile, subfertile, and vasectomized men as well as in prostatic secretion and tissue. Immunological cross-reactivity was observed between synovial-type PLA2 antiserum and the enzyme present in seminal plasma. There was a highly significant correlation between the concentration of the synovial-type PLA2, as measured by a time-resolved fluoroimmunoassay and the catalytic activity of the PLA2. The results show that the PLA2 content in human seminal plasma is very high (approximately 1000 times of that present in blood plasma) and that the enzyme belongs to the synovial-type group II phospholipase A2. The results also indicate that the enzyme is secreted by the prostate.

Cloning of the cDNA coding for 14 kDa group II phospholipase A2 from rat liver

The amino acid sequence of rat liver phospholipase A2 was partially elucidated using peptide fragments generated by enzymatic or chemical cleavage. Based on this sequence information, two oligonucleotide probes were constructed which were applied in a polymerase chain reaction on cDNA generated from rat liver total RNA. This resulted in cloning of the cDNA corresponding to the coding region of the mature phospholipase A2. The deduced amino acid sequence showed the enzyme belongs to the group II phospholipases, and is almost completely identical to rat platelet and spleen membrane-associated phospholipase A2. However, in the cDNA isolated one codon was different as compared to the platelet and spleen enzymes, resulting in the substitution of Ala94 by Arg94 in the liver enzyme. In Northern blot analyses the mRNA for rat group II phospholipase A2 could not be detected in rat liver, neither in total RNA nor in poly(A)+ RNA. However, a polymerase chain reaction using total RNA originating from freshly isolated hepatocytes resulted in the amplification of the described phospholipase A2 cDNA. This indicates that group II PLA2 mRNA is present in these cells, but presumably at very low abundance. The observed increase in rat group II phospholipase A2 secretion in rat mesangial cells upon stimulation with interleukin-1 beta (Pfeilschifter et al. (1989), Biochem. Biophys. Res. Commun. 159, 385-394) was shown to be accompanied by an increased transcription of the rat group II phospholipase A2 gene, indicating interleukin exerts its effect via increased phospholipase A2 mRNA synthesis. Based on Northern blot analyses of stimulated rat mesangial cells, the size of the mRNA for rat group II phospholipase A2 was determined to be 0.9 kb.

Immunohistochemical study of phospholipase A2 in bovine prostate

Approximately 1,100-fold purified phospholipase A2 (PLA2) from bovine prostate was injected into rabbit to prepare polyclonal antibodies. Antibodies produced showed specific immunoprecipitation only with the purified enzyme, as well as with homogenate of bovine prostatic tissue. By Western blot analysis or by immunodiffusion test, no cross-reactivity with PLA2 purified from human seminal plasma, bovine pancreas, Crotalus adamanteus venom, or with partially purified PLA2 from bovine seminal vesicle fluid or Cowper's glands was observed. Using the indirect peroxidase technique, PLA2 was localized in the cytoplasm of bovine prostatic epithelial cells. By immunogold microscopy, this enzyme was directly visualized inside the lysosomes, as well as in the endoplasmic reticulum of the glandular epithelial cells. Enzyme activity was localized in two principal subcellular sites: the mitochondria and lysosome-enriched fraction, and in the microsomal fraction.

Recombinant human secretory phospholipase A2: purification and characterization of the enzyme for active site studies

A secreted form of phospholipase A2 (PLA2) is thought to play an important role in inflammatory diseases. To characterize this enzyme the cDNA encoding a low molecular weight PLA2 was cloned from a human placental cDNA library. The cDNA encoding the human PLA2 was subcloned into an expression vector and subsequently transfected into Chinese hamster ovary (CHO) cells. A stable CHO cell clone, secreting ca 1 mg/L of recombinant PLA2 into the medium, was scaled up in culture to 180 L. The recombinant enzyme was purified from the cell supernatant to apparent homogeneity by a novel procedure combining adsorption to poly(vinylidene difluoride) membranes, ion exchange chromatography and size exclusion chromatography. The final recovery of PLA2 activity was 58%. A direct comparison between the purified recombinant human PLA2 and PLA2 purified from human synovial fluid, including molecular weight, antigenicity, ionic dependence, substrate specificity and sensitivity to known PLA2 inhibitors, indicated that the two enzymes exhibit identical biochemical properties. These results show that the recombinant PLA2 can be efficiently expressed and purified in sufficient quantities to characterize the enzyme active site, to aid in the rational development of PLA2 inhibitors as potential anti-inflammatory drugs, and to investigate further the role of PLA2 in inflammatory disease.

Demonstration of similar calcium dependencies by mammalian high and low molecular mass phospholipase A2

The in vitro Ca2+ dependencies of arachidonyl (AA)-selective high molecular mass phospholipase A2 (HMM, 85 kDa-PLA2) and human low molecular mass (LMM-Type II, 14 kDa)-PLA2 were compared. When the LMM-PLA2 and HMM-PLA2 enzymes were examined for hydrolysis against [3H]AA Escherichia coli in an ethyleneglycol-bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA)-free buffer system, neither enzyme demonstrated activity below 10 microM free Ca2+. Beyond 11 microM Ca2+ both enzyme activities increased steadily exhibiting 50% of maximal activity at 0.1 and 1.0 mM, respectively. Using EGTA-regulated free Ca2+ buffers, both enzymes responded in a biphasic manner, achieving 50% of the maximum response by 0.5 microM Ca2+, stabilizing up to 0.1 mM, then further increasing with exposure to millimolar Ca2+ concentrations. Replacement of [3H]AA-labeled phosphatidylethanolamine vesicles for [3H]AA E. coli or using Tris-HCl buffer instead of HEPES buffer did not alter these findings significantly. The presence of EGTA had a pronounced concentration-dependent effect on the activity of both the HMM- and LMM-PLA2 enzymes but only in the range of 0 to 100 microM free Ca2+. EGTA (EC50 approximately 200 microM) reduced the concentration of Ca2+ required by PLA2 to achieve 50% of maximal acylhydrolysis. In contrast, the Type I bovine pancreatic PLA2 required millimolar Ca2+ concentrations to elicit 50% of the maximal response in both EGTA-free or EGTA-containing systems, which is concordant with its extracellular role as a digestive enzyme. These data suggest that the LMM-Type II PLA2 and HMM-PLA2 are both activated at submicromolar, intracellularly relevant, Ca2+ concentrations and therefore have the ability to contribute to cellular lipid metabolism.

Purification of recombinant human secretory phospholipase A2 (group II) produced in long-term immobilized cell culture

Recombinant human secretory phospholipase A2 (Group II) was expressed in long-term culture of immobilized Chinese hamster ovary cells utilizing a continuous-perfusion airlift bioreactor. The bioreactor was continuously perfused with cell-culture medium supplemented with 5% fetal calf serum at an average flow rate of 5 liters/day for 30 days. Recombinant phospholipase A2, at concentrations ranging from 100 to 500 micrograms/liter, was purified to apparent homogeneity by an efficient two-step procedure involving a silica-based cation-exchange resin and hydrophobic interaction chromatography (greater than 65% recovery of phospholipase A2). The purified recombinant protein has an apparent molecular weight of 16 kDa, identical to that of purified human placental or synovial fluid phospholipase A2, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Application of the purified protein onto several different gel filtration columns resulted in elution of the protein at molecular weights corresponding to 3.1-4.7 kDa, suggesting an interaction of the protein with the column resins. However, analytical ultracentrifugation experiments revealed that the protein behaves as a monomer (13.8-14.2 kDa) over a protein concentration range of approximately 10 micrograms/ml to 5 mg/ml. With autoclaved Escherichia coli membranes as substrate, the recombinant protein has catalytic properties (pH optimum, effects of bovine serum albumin, sodium chloride concentration, and requirement for calcium) similar to those of the protein purified from human placenta.

Human group II phospholipase A2. Characterization of monoclonal antibodies and immunochemical quantitation of the protein in synovial fluid

Murine monoclonal and rabbit polyclonal antibodies were generated against human group II phospholipase A2 (PLA2) in order to study the role of this enzyme in inflammatory disease, the source of its synthesis, and the interaction of PLA2 with its substrate. Monoclonal antibody PLA187 exhibits potent inhibitory activity toward human PLA2 using autoclaved E. coli membranes as the substrate. Three other monoclonal antibodies (PLA184, PLA185, and PLA186) also inhibit enzyme activity, but with about 50-fold less potency. Based on the results of double-antibody competition experiments and enzyme inhibition profiles, PLA184 and PLA185 appear to recognize the same epitope. Monoclonal antibody PLA186 recognizes an epitope which is spatially distinct from that recognized by PLA184/185. The results also suggest that the epitope recognized by PLA187 may overlap with both epitopes recognized by PLA186 and PLA184/185. A double-antibody sandwich ELISA was developed using a combination of PLA185 and rabbit polyclonal antibody against PLA2. The ELISA provides a sensitive and quantitative method for monitoring specifically group II PLA2 in various biological sources, independent of factors which may affect enzyme activity. We have utilized this assay to quantitate PLA2 levels in synovial fluid from the joints of individuals with rheumatoid arthritis as well as from non-arthritic joints. Our results indicate that elevated levels of group II PLA2 in synovial fluid are not necessarily associated with arthritis.

Structures of free and inhibited human secretory phospholipase A2 from inflammatory exudate

Phospholipase A2 (PLA2) participates in a wide range of cellular processes including inflammation and transmembrane signaling. A human nonpancreatic secretory PLA2 (hnps-PLA2) has been identified that is found in high concentrations in the synovial fluid of patients with rheumatoid arthritis and in the plasma of patients with septic shock. This enzyme is secreted from certain cell types in response to the proinflammatory cytokines, tumor necrosis factor or interleukin-1. The crystal structures of the calcium-bound form of this enzyme have been determined at physiological pH both in the presence [2.1 angstrom (A) resolution] and absence (2.2 A resolution) of a transition-state analogue. Although the critical features that suggest the chemistry of catalysis are identical to those inferred from the crystal structures of other extracellular PLA2s, the shape of the hydrophobic channel of hnps-PLA2 is uniquely modulated by substrate binding.

Circulating phospholipase A2 activity associated with sepsis and septic shock is indistinguishable from that associated with rheumatoid arthritis

Elevation of circulating phospholipase A2 (PLA2) activity is associated with sepsis and septic shock. Elevated levels of PLA2 activity also are seen in association with chronic inflammatory disorders such as rheumatoid arthritis. The relationship between these phospholipases is unclear. We have developed a highly specific enzyme-linked immunosorbent assay (ELISA) capable of measuring human synovial PLA2 in plasma, using monoclonal antibodies raised to recombinant synovial PLA2. This ELISA has been used to quantitate circulating PLA2 levels in patients clinically diagnosed with sepsis. These elevated levels positively correlated with the elevation seen in plasma PLA2 enzyme activity. The antibodies also have been used to purify immunoreactive PLA2 from plasma of patients with sepsis, thus enabling characterization of the purified protein by amino-terminal sequence analysis. We conclude from this study that the increase in PLA2 activity seen in association with sepsis and septic shock results from a dramatic elevation in levels of a circulating PLA2 enzyme. This inflammatory PLA2 is indistinguishable, both immunologically and chemically, from that associated with rheumatoid arthritis. Therapeutic agents directed towards inhibition of this inflammatory PLA2 enzyme may have utility in the treatment of both chronic and acute inflammatory disease.

Detection of 14-kDa group II phospholipase A2 in human seminal plasma

About 90% of phospholipase A2 activity detected in human seminal plasma reacted with monoclonal antibodies raised against human synovial fluid phospholipase A2. The crude seminal plasma yielded a pure immuno-cross-reactive phospholipase A2 preparation in a single purification step using immuno-affinity chromatography. The amino acid sequence of the N-terminal 20 residues of this seminal enzyme was determined and found to be identical with that of human synovial phospholipase A2. Thus, it is suggested that human seminal plasma contains phospholipase A2, belonging to the 14-kDa group II enzyme family, as the major isoenzyme.

Intracellular localization of group II phospholipase A2 in rat vascular smooth muscle cells and its possible relationship to eicosanoid formation

We investigated the localization of group II phospholipase A2 (PLA2-II) in rat vascular smooth muscle cells (VSMCs) by applying immunofluorescence and immunoelectron microscopy with its polyclonal antibody. In unstimulated cells, no immunolabelling was detected in the cells. On the other hand, in the cells stimulated with tumor necrosis factor (TNF) and/or forskolin (FK), intense fluorescence was detected in the cytoplasm. The immunoperoxidase reactions were detected in the cisternae of rough endoplasmic reticulum (rER), trans-cisternae of Golgi apparatus, and small vesicles beneath the plasma membrane. Western blot analysis showed VSMCs secrete PLA2-II after stimulation. Secreted PLA2-II was associated with the plasma membrane and extracellular matrix. Colchicine inhibited PLA2-II synthesis and its secretion to the extracellular space. These observations indicate that in VSMCs PLA2-II is synthesized at rER. transported to Golgi apparatus, discharged into extracellular space via the small vesicles, and microtubules may concern with its process. Furthermore, in VSMCs treated with TNF or TNF + FK, prostaglandin E2 formation was also increased. Actinomycin D and cycloheximide inhibited the potentiation of the prostaglandin E2 formation induced by TNF or TNF + FK, indicating that both RNA and protein synthesis are required for the potentiation. These results suggest an involvement of PLA2-II in the prostaglandin formation.

Extracellular phospholipase A2 detected at inflamed sites in rats does not originate from platelets

Extracellular phospholipase A2 activity has been detected in caseinate-induced peritoneal fluid in rats. We studied the source of this extracellular phospholipase A2 in 'platelet-poor' rats which had been pretreated with an intravenous injection of rabbit anti-rat platelet serum. In these rats, the increase in the peritoneal fluid extracellular phospholipase A2 level at the inflamed sites was almost identical to that observed in control rats, although platelet numbers in peripheral blood were decreased markedly. This observation suggests that extracellular phospholipase A2 in the peritoneal cavity is not derived mainly from platelets.

Myotoxin II from Bothrops asper (Terciopelo) venom is a lysine-49 phospholipase A2

A basic, dimeric myotoxic protein, myotoxin II, purified from Bothrops asper venom has a similar molecular weight and is immunologically cross-reactive with antibodies raised to previously isolated B. asper phospholipases A2, except that it shows only 0.1% of the phospholipase activity against L-alpha-phosphatidylcholine in the presence of Triton X-100. Its 121 amino acid sequence, determined by automated Edman degradation, clearly identifies it as a Lys-49 phospholipase A2. Key amino acid differences between myotoxin II and phospholipase active proteins in the Ca2(+)-binding loop region, include Lys for Asp-49, Asn for Tyr-28, and Leu for Gly-32. The latter substitution has not previously been seen in Lys-49 proteins. Other substitutions near the amino terminus (Leu for Phe-5 and Gln for several different amino acids at position 11) may prove useful for identifying other Lys-49 proteins in viperid and crotalid venoms. Myotoxin II shows greater sequence identity with other Lys-49 proteins from different snake venoms (Agkistrodon piscivorus piscivorus, Bothrops atrox, and Trimeresurus flavoviridis) than with another phospholipase A2 active Asp-49 molecule isolated from the same B. asper venom. This work demonstrates that phospholipase activity per se, is not required in phospholipase molecules for either myotoxicity or edema inducing activities.

Cloning and nucleotide sequence of a cDNA encoding ammodytoxin A, the most toxic phospholipase A2 from the venom of long-nosed viper (Vipera ammodytes)

A venom gland cDNA library was constructed in pUC9 and screened with a mixed oligonucleotide probe deduced from the unique Glu-4 to Ile-9 region of ammodytoxins. Twenty-one strongly positive clones were found by hybridization of about 5000 bacterial colonies, nine of them with the inserts encoding ammodytoxin A. The cDNA for ammodytoxin A encodes a 122 amino acid residue mature protein, preceded by a 16 residue signal peptide. Its complete nucleotide sequence shows 99% similarity to those of ammodytoxins B and C.

A simple and sensitive method for determining transcription initiation site: identification of two transcription initiation sites in rat group II phospholipase A2 gene

We developed a simple and sensitive method for assigning transcriptional initiation sites, and applied it to characterize the transcriptional unit of rat group II phospholipase A2 (PLA2) gene. Our method involves the primer extension reaction followed by detection of its products by hybridization. Using this method, we were able to map two transcriptional initiation sites on the nucleotide sequence of the core promoter region of PLA2 gene with one-base resolution without any difficulties.

Purification and characterization of a soluble phospholipase A2 from guinea pig lung

Guinea pig lung cytosolic phospholipase A2 was purified to near homogeneity by chromatography on a phosphocellulose column, followed by Q-Sepharose, S-Sepharose, gel filtration chromatography and reverse-phase HPLC. The purified enzyme exhibited an apparent molecular weight of 16,700 by SDS-polyacrylamide gel electrophoresis. Active enzyme eluted from the gel at an apparent molecular weight of 16,700. The purified enzyme exhibited a pH optimum of 9.0 and was calcium-dependent. Guinea pig lung phospholipase A2 hydrolyzed phosphatidylcholine and phosphatidylethanolamine equally well. Substrates containing unsaturated fatty acids in the sn-2 position were hydrolyzed preferentially to those containing saturated fatty acids. Anionic detergents stimulated enzyme activity while nonionic detergents inhibited the enzyme. Disulfide reducing agents dithiothreitol, glutathione and 2-mercaptoethanol modestly stimulated enzyme activity. The sulfhydryl aklylating agent n-ethylmaleimide had no effect on enzyme activity and only high concentrations of p-hydroxymercuribenzoic acid inhibited enzyme activity. The histidine modifying agent, bromophenacyl bromide did not inhibit guinea pig lung phospholipase A2 under conditions in which Crotalus adamanteus phospholipase A2 was inhibited 80%. Manoalide inhibited guinea pig lung phospholipase A2 in a concentration-dependent manner (IC50 = 2 microM). Antibodies prepared against porcine pancreatic phospholipase A2 specifically immunoprecipitated guinea pig lung phospholipase A2 suggesting that the major phospholipase A2 in guinea pig lung cytosol is immunologically related to pancreatic phospholipase A2 in agreement with the biochemical properties of the enzyme.

Enhanced expression of group II phospholipase A2 gene in the tissues of endotoxin shock rats and its suppression by glucocorticoid

We studied the regulation of group II phospholipase A2 (PLA2-II) gene in vivo, using endotoxin shock rat as a model for systemic inflammation. Administration of endotoxin into rats increased PLA2 activity in the plasma, as described by Vadas and Hay, using endotoxin-challenged rabbit. Specific absorption of this activity by anti-PLA2-II antibody indicated that the released PLA2 was PLA2-II. The levels of PLA2-II mRNA were elevated in the aorta, spleen, lung, and thymus but not in the liver and kidney. The tissues with high PLA2-II mRNA contents released a greater amount of PLA2-II than the tissues of control rats. These results suggest that in endotoxin shock rats, PLA2-II is synthesized de novo in the above tissues and released into circulation. Furthermore, our present study demonstrates that glucocorticoid suppresses the enhanced expression of the PLA2-II gene in the tissues of endotoxin shock rats.

Purification and characterization of phospholipase A2 from rat stomach

Phospholipase A2, which is localized in the mucosal part of the corpus of rat stomach (Hirohara et al. (1987) Biochim. Biophys. Acta 919, 231-238), was purified 990-fold from the supernatant of a tissue homogenate by heat treatment at acidic pH, ammonium sulfate fractionation, ion-exchange chromatography, gel-filtration and reverse-phase high-performance liquid chromatography (reverse-phase HPLC). The purified enzyme gave a single protein band on sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis with a molecular mass of approx. 17 kDa. The enzyme had a pH optimum of 8.0 and hydrolyzed the 2-arachidonoyl residue of phosphatidylcholine preferentially to the 2-oleoyl residue, the Vmax and Km values for the two being 227 and 29 mumol/min per mg protein and 0.037 and 0.019 mM, respectively. The activity was calcium-dependent and was markedly increased by SDS and dimethyl sulfoxide (DMSO). The enzyme showed typical product inhibition. Free unsaturated fatty acids (oleic, arachidonic and docosahexaenoic acids), which are supposedly the main enzymatic products in vivo, inhibited the activity. Arachidonic acid caused noncompetitive inhibition and its concentration for its maximal inhibition (50% inhibition) was 5 x 10(-5) M. Lysophosphatidylcholine, free saturated fatty acids (palmitic and stearic acids) and arachidonic acid metabolites (leukotrienes and prostaglandins) had no effect on the activity.

Structure of genomic DNA for rat platelet phospholipase A2

A genomic DNA for rat platelet phospholipase A2 was isolated by screening a rat genomic library with oligonucleotide probes based on its published amino acid sequence. The rat platelet phospholipase A2 gene had a total length of about 2.5 kb and contained five exons and four introns. The intron-exon structure of the rate gene was similar to that of human non-pancreatic phospholipase A2.

Characterization of extracellular phospholipase A2 (PLA2) activity in fluid and peritoneal cells from casein-treated rats

Extracellular phospholipase A2 activity (PLA2) found in the fluid and cells of the peritoneal cavity of rats injected with casein is described. PLA2 activities from both the fluid and cells require Ca2+ and have pH optima of 7. Acid-extraction increased PLA2 activity in the polymorphonuclear leukocyte (PMN) homogenates 20-fold but not the PLA2 activity in the extracellular fluid. Acid extraction also increased the sensitivity of the PLA2 activities to standard inhibitors. Since the PLA2 activities described in this model have characteristics similar to other inflammatory PLA2s, including human synovial fluid PLA2, casein stimulation should prove useful for testing potential inhibitors.

Structure of gene coding for rat group II phospholipase A2

The gene coding for rat group II phospholipase A2 was isolated from a rat genomic library by using the cDNA for rat platelet phospholipase A2 as a hybridization probe. The rat group II phospholipase A2 gene spanned about 3.5 kilobase pairs and consisted of five exons. Southern blot analysis revealed that a single copy of this gene exists in the rat haploid genome. A TATA-like sequence and two AP-2 binding site-like loci were found upstream from the tentatively identified transcription initiation site.

Phospholipases: old enzymes with new meaning

Phospholipases are enzymes that hydrolyze specific portions of phospholipid molecules. Their role in the digestion of exogenous phospholipids and as the active principle in snake and bee venoms has long been appreciated. Interest has increased in phospholipases recently because of new data implicating them in the inflammatory response. The ability of phospholipases to hydrolyze bacterial phospholipids has also received considerable attention. These new data have brought pertinence to studies of the physicochemical nature of potential substrates that greatly influence enzyme activity. Interest in the regulation of enzyme activity, both by physiological and pharmacological means, has increased as the importance of the phospholipases in response to various stimuli has become better appreciated. Finally, considerable interest has focused on the role of the phospholipases in response to hormones in a variety of cell systems. Data pertinent to all of these areas of interest will be discussed in this review with a view toward stimulating those with an interest in gastrointestinal physiology to apply them to their own areas of research in the gastrointestinal tract or liver.

Proteinaceous inhibitors of phospholipase A2 purified from inflammatory sites in rats

We have purified two phospholipase A2 inhibitory proteins (37 and 33 kDa) from peritoneal fluid of dexamethasone-treated rats. The extracellular phospholipase A2 found in inflammatory sites differed from the exocrine phospholipase A2 in susceptibility to these endogenous inhibitors; both proteins inhibited the activity of the extracellular phospholipase A2 purified from sites of inflammation but did not affect appreciably the activity of either porcine pancreatic or Naja naja venom phospholipase A2. The amino acid sequence of the NH2-terminal portion of the purified proteins did not resemble that of lipocortins so far reported, but it was almost identical to that of parts of human or mouse complement component C3. These findings may indicate that degraded products of C3 are involved in the regulation of activity of a class of mammalian phospholipase A2.

Group II phospholipase A2 mRNA synthesis is stimulated by two distinct mechanisms in rat vascular smooth muscle cells

Two potent inflammatory mediators, interleukin 1 (IL-1) and tumor necrosis factor (TNF) as well as lipopolysaccharide (LPS) increased group II phospholipase A2 (PLA2) mRNA levels, which resulted in enhanced secretion of the PLA2 enzyme from rat smooth muscle cells. cAMP-elevating agents also stimulated the release of PLA2 and increased the mRNA, but IL-1, TNF and LPS did not affect cAMP levels. Furthermore, the effects of TNF and cAMP-elevating agents were not additive but synergistic. Therefore, we concluded that the level of rat group II PLA2 mRNA is controlled at least by two distinct mechanisms, one involves cAMP and the other is mediated by TNF, IL-1 and LPS. This study also suggests important roles of group II PLA2 in pathogenesis of vascular inflammation.

Purification and characterization of a phospholipase A2 from human osteoarthritic synovial fluid

Phospholipase A2 (PLA2) from human osteoarthritic synovial fluid was purified to homogeneity in three steps. The NH2-terminal amino acid sequence and biochemical characteristics of the enzyme were identical to the Peak A PLA2 activity of rheumatoid synovial fluid (1). The enzyme exhibited an apparent mass of 14,000, an absolute Ca+(+)-dependence, an alkaline pH optimum, and was inhibited by sodium deoxycholate (DOC), NaCl and 0.5 M Tris-HCl. The enzyme strongly preferred phosphatidylethanolamine (PE) as substrate over phosphatidylcholine (PC) or phosphatidylinositol (PI), and hydrolyzed PE containing arachidonic acid or linoleic acid in the sn-2 position at similar rates. Heparin bound to the enzyme but did not inhibit catalytic activity. In addition, the human enzyme was not inhibited by the acidic 'chaperone' subunit of crotoxin despite considerable sequence similarity with the basic PLA2 subunit of the neurotoxin. The enzyme was capable of hydrolyzing E. coli membrane phospholipids in the presence of the neutrophil bactericidal/permeability increasing protein (BPI). This finding, coupled to the reported pro-inflammatory activity and presence of the enzyme in inflammatory cells, supports the hypothesis that it may be a component of the host defense mechanism which can, under certain conditions, contribute to the pathogenesis of inflammatory disease.

Phospholipases, enzymes that share a substrate class

Considerable work has gone into the study of PLs since the first suggestions of their existence nearly a century ago. This work has intensified enormously since the mid-1970s when their role in signal-coupling mechanisms and in pathophysiology was recognized. While much has been done to understand this diverse group of enzymes at the molecular and mechanistic levels, the discovery of new PLs has far outstripped our capacity to study them in sufficient detail to appreciate what makes each unique while perhaps having some common mechanisms of action and regulation. One would almost plead: No new PLs - Let us study those at hand! That is not the case in our field and the discovery of new PLs will continue. It is important, however, that an understanding be gained of these enzymes at the molecular level, how they interact with their substrates, and how regulatory factors can target the function of PLs in situ.

Extracellular phospholipase A2 activity in two in vivo models of inflammation

Two "in vivo" models of inflammation have been used to investigate the role of phospholipases A2 (PLA2) in inflammation. These models are casein-induced peritonitis in the rat and zymosan-induced peritonitis in the mouse. The extracellular PLA2 activities from peritoneal lavage fluid in these two models are similar: they are calcium dependent and have broad neutral pH optima. However, the relationship between extracellular PLA2 activity and cell influx in these models are not identical. In zymosan peritonitis, PLA2 activity preceded peak cell influx, reaching a maximum within 15 min after zymosan injection, while cell influx peaked by 8 hr. In casein-induced peritonitis, the PLA2 activity peaked at 24 hr, while cell influx continued through 48 hr. The origins of the PLA2 activities in both models remain unclear; one potential source is the plasma. Understanding the role of extracellular PLA2 activity in "in vivo" models, and investigating specific inhibitors in these models may aid in our understanding of the role of extracellular PLA2 in diseases such as rheumatoid arthritis, endotoxin shock and pancreatitis.

Localization and evolution of two human phospholipase A2 genes and two related genetic elements

Mammals are now known to contain at least two distinct classes of phospholipases A2, the progenitors of which can be seen in the venoms of snakes. Mammalian "Type I" PLA2, synthesized primarily by the pancreas, is also present in smaller amounts in other tissues including lung, spleen, and kidney. Recently, a mammalian "Type II" PLA2 has been sequenced, and shown to occur in platelets, synovial cells and fluid, cells of inflammatory peritoneal exudate, liver, intestine, kidney, and placenta. This form, referred to here as Type IIA PLA2, could play a key role in arachidonate release in both normal and pathologic inflammation. The genes encoding both forms have also been recently cloned. Here, the sites of synthesis and respective roles of the two known enzymes are discussed, along with an analysis of the evolutionary conservation of Type IIA PLA2 gene sequence. In addition, two related genetic elements containing sequences homologous to a portion of Type II PLA2 are described, which map to the same chromosome as the Type IIA PLA2 gene (chromosome 1). Either or both of these could also encode a portion of additional mammalian PLA2s.

In vivo release and clearance of rat platelet phospholipase A2

Intravenous injection of ADP into rats produced a rapid increase of plasma phospholipase A2 activity with a concomitant decrease in platelet count. Phospholipase A2 activity in plasma reached a peak within 1 min and thereafter declined sharply. This phospholipase A2 activity was reactive with a monoclonal antibody specific for rat platelet-derived phospholipase A2. These findings, together with the fact that ADP is a stimulant specific for platelets, suggest that the phospholipase A2 observed may be released into plasma from activated platelets in vivo. When platelet-derived phospholipase A2 was purified, labeled with 125I, and injected intravenously into rats, the radioactivity in the plasma decreased rapidly: the half-life of the enzyme in the blood stream was less than 30 s. Addition of either heparin or anti-phospholipase A2 monoclonal antibody directed against the domain of the enzyme responsible for binding to heparin retarded the clearance of the enzyme, suggesting that phospholipase A2 might be adsorbed onto heparan sulfate proteoglycans on the endothelial cell surface. Examination of the distribution of radioactivity in vivo revealed that most of the enzyme was rapidly taken up by the liver and degraded to acid-soluble materials.

The primary structure of a membrane-associated phospholipase A2 from human spleen

The complete primary structure of membrane-associated phospholipase A2 purified from a human splenic membrane fraction was determined by sequence analysis of the peptides generated by lysyl endopeptidase and Staphylococcus aureus V8 protease cleavage. The enzyme consists of 124 amino acid residues corresponding to a molecular weight of 13,904. The primary structure reveals the characteristics of Group II phospholipases A2 and a large ratio of basic amino acid residues to acidic ones, that ratio being 3.4 : 1.