Regulatory functions of phospholipase A2

Comparison of the activities of wild type and mutant enhancing factor/mouse secretory phospholipase A2 proteins

Enhancing factor (EF) protein, an isoform of secretory phospholipase A2 (PLA2), was purified as a modulator of epidermal growth factor from the small intestine of the Balb/c mouse. It was for the first time that a growth modulatory property of sPLA2 was demonstrated. Deletion mutation analysis of EF cDNA carried out in our laboratory showed that enhancing activity and phospholipase activity are two separate activities that reside in the same molecule. In order to study the specific amino acids involved in each of these activities, two site-directed mutants of EF were made and expressed in vitro. Comparison of enhancing activity as well as phospholipase A2 activity of these mutant proteins with that of wild type protein helped in identification of some of the residues important for both the activities

Control of phospholipase A(2) activity in cockroach (Periplaneta americana) fat body trophocytes by hypertrehalosemic hormone: the role of calcium

Recently, synthetic HTH-I and HTH-II have been shown to increase the formation of free fatty acids in cockroach (Periplaneta americana) fat body. In this study we show that HTH-II increases PLA(2) activity in dispersed trophocytes, thus implying that phospholipid is a potential source of the fatty acids. The increase in HTH-induced PLA(2) activity is triggered by an increase in [Ca(2+)](i) but extracellular Ca(2+) is also required for a maximal Ca(2+) signal: an effect that can be blocked by the introduction of BAPTA into the trophocytes. Treating trophocytes with ryanodine blocks the increase in PLA(2) activity that follows treatment of the cells with HTH-II. This indicates that the Ca(2+) release channels are distinct from those that respond to inositol trisphosphate. Thapsigargin, which releases Ca(2+) to the cytosol from an intracellular store, increases PLA(2) activity. The data show that the enzyme is translocated from the cytosol to the plasma membrane.

Enzymatic properties of human cytosolic phospholipase A(2)gamma

The enzymatic properties of cytosolic phospholipase A(2)gamma (cPLA(2)gamma), an isoform of 85-kDa group IV cPLA(2)alpha (cPLA(2)alpha) were studied in vitro and when the enzyme was expressed in cells. cPLA(2)gamma expressed in Sf9 cells is associated with membrane. Membranes isolated from [(3)H]arachidonic acid-labeled Sf9 cells expressing cPLA(2)gamma, constitutively release [(3)H]arachidonic acid. The membrane-associated activity is inhibited by the group IV PLA(2) inhibitor methylarachidonyl fluorophosphonate, but not effectively by the group VI PLA(2) inhibitor (E)-6-(bromomethylene)-3-(1-naphthalenyl)-2H-tetrahydropyran-2-one. cPLA(2)gamma has higher lysophospholipase activity than PLA(2) activity. Purified His-cPLA(2)gamma does not exhibit phospholipase A(1) activity, but sequentially hydrolyzes fatty acid from the sn-2 and sn-1 positions of phosphatidylcholine. cPLA(2)gamma overexpressed in HEK293 cells is constitutively active in isolated membranes, releasing large amounts of oleic, arachidonic, palmitic, and stearic acids; however, basal fatty acid release from intact cells is not increased. cPLA(2)gamma overexpressed in lung fibroblasts from cPLA(2)alpha-deficient mice is activated by mouse serum resulting in release of arachidonic, oleic, and palmitic acids, whereas overexpression of cPLA(2)alpha results primarily in arachidonic acid release.

Transcriptional regulation of the membrane-associated prostaglandin E2 synthase gene. Essential role of the transcription factor Egr-1

Membrane-associated prostaglandin (PG) E2 synthase (mPGES) is an inducible terminal enzyme in the biosynthetic pathway for prostaglandin E2, which participates in many biological processes. In this study, we investigated the molecular mechanism controlling the inducible expression of mPGES. The mouse mPGES gene consisted of three exons, and its 5'-proximal promoter contained consensus motifs for the binding of several transcription factors. Transgenic expression in mice of the mouse mPGES promoter flanked by a reporter gene resulted in stimulus-dependent induction of the reporter in tissues where mPGES was intrinsically induced. Deletion and site-specific mutation analyses of the 5'-flanking region demonstrated that stimulus-inducible expression of mouse and human mPGES required tandem GC boxes adjacent to the initiation site. The stimulus-induced GC box binding activity was present in nuclear extracts of cells, in which the proximal GC box was essential for binding. An 80-kDa stimulus-inducible nuclear protein that bound to this GC box was identified as the transcription factor Egr-1 (for early growth response-1). These results suggest that Egr-1 is a key transcription factor in regulating the inducible expression of mPGES.

Crystal structure of human group X secreted phospholipase A2. Electrostatically neutral interfacial surface targets zwitterionic membranes

The crystal structure of human group X (hGX) secreted phospholipase A2 (sPLA2) has been solved to a resolution of 1.97 A. As expected the protein fold is similar to previously reported sPLA2 structures. The active site architecture, including the positions of the catalytic residues and the first and second shell water around the Ca2+ cofactor, are highly conserved and remarkably similar to the group IB and group IIA enzymes. Differences are seen in the structures following the (1-12)-N-terminal helix and at the C terminus. These regions are proposed to interact with the substrate membrane surface. The opening to the active site slot is considerably larger in hGX than in human group IIA sPLA2. Furthermore, the electrostatic surface potential of the hGX interfacial-binding surface does not resemble that of the human group IIA sPLA2; the former is highly neutral, whereas the latter is highly cationic. The cationic residues on this face of group IB and IIA enzymes have been implicated in membrane binding and in k(cat*) allostery. In contrast, hGX does not show activation by the anionic charge at the lipid interface when acting on phospholipid vesicles or short-chain phospholipid micelles. Together, the crystal structure and kinetic results of hGX supports the conclusion that it is as active on zwitterionic as on anionic interfaces, and thus it is predicted to target the zwitterionic membrane surfaces of mammalian cells.

Differential effects of arachidonoyl trifluoromethyl ketone on arachidonic acid release and lipid mediator biosynthesis by human neutrophils. Evidence for different arachidonate pools

The goal of this study was to determine the effects of a putative specific cytosolic phospholipase A2 inhibitor, arachidonyl trifluoromethyl ketone (AACOCF3), on arachidonic acid (AA) release and lipid mediator biosynthesis by ionophore-stimulated human neutrophils. Initial studies indicated that AACOCF3 at concentrations 0-10 micro m did not affect AA release from neutrophils. In contrast, AACOCF3 potently inhibited leukotriene B4 formation by ionophore-stimulated neutrophils (IC50 approximately 2.5 micro m). Likewise, AACOCF3 significantly inhibited the biosynthesis of platelet activating factor. In cell-free assay systems, 10 micro m AACOCF3 inhibited 5-lipoxygenase and CoA-independent transacylase activities. [3H]AA labeling studies indicated that the specific activities of cell-associated AA mimicked that of leukotriene B4 and PtdCho/PtdIns, while the specific activities of AA released into the supernatant fluid closely mimicked that of PtdEtn. Taken together, these data argue for the existence of segregated pools of arachidonate in human neutrophils. One pool of AA is linked to lipid mediator biosynthesis while another pool provides free AA that is released from cells. Additionally, the data suggest that AACOCF3 is also an inhibitor of CoA-independent transacylase and 5-lipoxygenase. Thus, caution should be exercised in using AACOCF3 as an inhibitor of cytosolic phospholipase A2 in whole cell assays because of the complexity of AA metabolism.

Prostaglandins and other lipid mediators in Alzheimer's disease

In the central nervous system (CNS), prostaglandin (PG) and other bioactive lipids regulate vital aspects of neural membrane biology, including protein-lipid interactions, trans-membrane and trans-synaptic signaling. However, a series of highly reactive PGs, free fatty acids, lysophospolipids, eicosanoids, platelet-activating factor, and reactive oxygen species (ROS), all generated by enhanced phospholipase A2 (PLA2) activity and arachidonic acid (AA) release, participate in cellular injury, particularly in neurodegeneration. PLA2 activation and PG production are among the earliest initiating events in triggering brain-damage pathways, which can lead to long-term neurologic deficits. Altered membrane-associated PLA2 activities have been correlated with several forms of acute and chronic brain injury, including cerebral trauma, ischemic damage, induced seizures in the brain and epilepsy, schizophrenia, and in particular, Alzheimer's disease (AD). Biochemical mechanisms of PLA2 overactivation and its pathophysiological consequences on CNS structure and function have been extensively studied using animal models and brain cells in culture triggered with PLA2 inducers, PGs, cytokines, and related lipid mediators. Moreover, the expression of both COX-2 and PLA2 appears to be strongly activated during Alzheimer's disease (AD), indicating the importance of inflammatory gene pathways as a response to brain injury. This review addresses some current ideas concerning how brain PLA2 and brain PGs are early and key players in acute neural trauma and in brain-cell damage associated with chronic neurodegenerative diseases such as AD.

Phospholipase A2 enzymes

Phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 position of membrane glycerophospholipids to liberate arachidonic acid (AA), a precursor of eicosanoids including prostaglandins and leukotrienes. The same reaction also produces lysophosholipids, which represent another class of lipid mediators. So far, at least 19 enzymes that possess PLA2 activity have been identified and cloned in mammals. The secretory PLA2 (sPLA2) family, in which 10 isozymes have been identified, consists of low-molecular weight, Ca2+-requiring secretory enzymes that have been implicated in a number of biological processes, such as modification of eicosanoid generation, inflammation, and host defense. The cytosolic PLA2 (cPLA2) family consists of three enzymes, among which cPLA2alpha has been paid much attention by researchers as an essential component of the initiation of AA metabolism. The activation of cPLA2alpha is tightly regulated by Ca2+ and phosphorylation. The Ca2+-independent PLA2 (iPLA2) family contains two enzymes and may play a major role in phospholipid remodeling. The platelet-activating factor (PAF) acetylhydrolase (PAF-AH) family contains four enzymes that exhibit unique substrate specificity toward PAF and/or oxidized phospholipids. Degradation of these bioactive phospholipids by PAF-AHs may lead to the termination of inflammatory reaction and atherosclerosis.

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

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

Deficiency in sPLA(2) does not affect HDL levels or atherosclerosis in mice

Secretory non-pancreatic phospholipase A(2) (sPLA(2)) has been implicated in inflammation and has been found in human atherosclerotic lesions. To test the effect of sPLA(2) deficiency on atherosclerosis, C57BL/Ks mice (apoE(+/+) and PLA(2)(++) were bred with C57BL/6 apoE knockout mice which are sPLA(2)(--) due to a spontaneous mutation. Sibling pairs of mice (apoE(--)/sPLA(2)(++) and apoE(--)/sPLA(2)(--)) on high fat Western diets were dissected at 22 weeks. In vitro enzyme assays confirmed higher serum sPLA(2) activity in the sPLA(2)(++) compared to sPLA(2)(--) for both sexes, while sPLA(2)(--) males had slightly higher serum cholesterol and phospholipids. Analysis of lipoprotein profiles by FPLC showed no effect of sPLA(2) genotype on any measured parameters. Atherosclerosis was quantitated by assaying cholesterol in aortic extracts. Male sPLA(2) trended slightly higher than sPLA(2)(++) with no statistical significance. Female sPLA(2)(++) and sPLA(2)(--) mice showed no significant differences in any of the measured parameters. These results suggest that the endogenous mouse sPLA(2) gene does not significantly affect HDL or atherosclerosis in mice.

Effects of lysophospholipids on the generation of reactive oxygen species by fMLP- and PMA-stimulated human neutrophils

In this study, the effects of exogenous lysophospholipids--lysophosphatidic acid, lysophosphatidylcholine, lysophosphatidylethanolamine and lysophosphatidylserine--on the kinetics of reactive oxygen species (ROS) production by human neutrophils are described. The ROS production by human neutrophils was monitored by luminol-amplified chemiluminescence after cell stimulation with the chemotactic tripeptide, fMLP, or with the phorbol ester, PMA. The interaction of lysophospholipids with the membrane of human neutrophils was additionally tested by mass spectrometry. Lysophosphatidylcholine showed the most pronounced effect on the chemiluminescence pattern, as well as the intensity of the fMLP and PMA-stimulated cells, whereas lysophosphatidic acid showed a slight priming effect when fMLP was used for stimulation. In the case of fMLP-stimulated cells, lysophosphatidylcholine inhibited the first phase and enhanced the second phase of chemiluminescence, whereas the chemiluminescence of PMA-stimulated neutrophils was inhibited in a concentration-dependent manner. We conclude that lysophosphatidylcholine is able to interact with protein kinase C-dependent signalling pathways leading to NADPH oxidase activation.

The VP1 capsid protein of adeno-associated virus type 2 is carrying a phospholipase A2 domain required for virus infectivity

The unique region of the VP1 protein of parvoviruses was proposed to contain a parvoviral phospholipase A2 (pvPLA2) motif. Here, PLA2 activity is shown in the unique region of adeno-associated virus type 2 (AAV-2) VP1 when expressed as an isolated domain in bacteria. Mutations in this region of the capsid protein strongly reduced the infectivity of mutant virions in comparison to wild-type AAV-2. This correlated with effects on the activity of PLA2. The mutations had no influence on capsid assembly, packaging of viral genomes into particles or binding to and entry into HeLa cells. However, a delayed onset and reduced amount of early gene expression, as measured by Rep immunofluorescence, was observed. These results suggest that pvPLA2 activity is required for a step following perinuclear accumulation of virions but prior to early gene expression.

Secretory phospholipase A(2) induces apoptosis via a mechanism involving ceramide generation

Secretory phospholipase A(2) (sPLA(2)) plays important roles in cellular signaling and various biological events. In this study, we examined the biological effects and the potential signaling mechanism of purified sPLA(2) in MV1Lu cells. Three types of snake venom sPLA(2) were purified and their enzymatic activities were characterized by using various lipid substrates prepared from [3H]-myristate-labeled cells and by determining their effects on the induction of arachidonic acid (AA) release. The purified sPLA(2) induced apoptosis in Mv1Lu cells in a dose- and time-dependent manner, and was associated with a rapid increase in the intracellular ceramide level. Similar apoptotic effects were observed in Mv1Lu cells treated with exogenous ceramide analog, C(2)- and C(8)-ceramide. Moreover, treatment of cells with sphingomyelinase (SMase), which reduced the intracellular SM level, enhanced the apoptotic response to sPLA(2)s. sPLA(2)s also displayed an inhibitory effect on bradykinin-induced phospholipase D (PLD) activity, which can be imitated by exogenous ceramide. Our data indicate that sPLA(2) induces cell apoptosis via a mechanism involving increased ceramide generation.

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

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

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

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

The antibacterial properties of secreted phospholipases A2: a major physiological role for the group IIA enzyme that depends on the very high pI of the enzyme to allow penetration of the bacterial cell wall

The antibacterial properties of human group IIA secreted phospholipase A(2) against Gram-positive bacteria as a result of membrane hydrolysis have been reported. Using Micrococcus luteus as a model system, we demonstrate the very high specificity of this human enzyme for such hydrolysis compared with the group IB, IIE, IIF, V, and X human secreted phospholipase A(2)s. A unique feature of the group IIA enzyme is its very high pI due to a large excess of cationic residues on the enzyme surface. The importance of this global positive charge in bacterial cell membrane hydrolysis and bacterial killing has been examined using charge reversal mutagenesis. The global positive charge on the enzyme surface allows penetration through the bacterial cell wall, thus allowing access of this enzyme to the cell membrane. Reduced bacterial killing was associated with the loss of positive charge and reduced cell membrane hydrolysis. All mutants were highly effective in hydrolyzing the bacterial membrane of cells in which the cell wall was permeabilized with lysozyme. These same overall characteristics were also seen with suspensions of Staphylococcus aureus and Listeria innocua, where cell membrane hydrolysis and antibacterial activity of human group IIA enzyme was also lost as a result of charge reversal mutagenesis.

Identification of the phospholipase A(2) isoforms that contribute to arachidonic acid release in hypoxic endothelial cells: limits of phospholipase A(2) inhibitors

Changes in endothelium functions during ischemia are thought to be of importance in numerous pathological conditions, with, for instance, an increase in the release of inflammatory mediators like prostaglandins. Here, we showed that hypoxia increases phospholipase A(2) (PLA(2)) activity in human umbilical vein endothelial cells. Both basal PLA(2) activity and PG synthesis are sensitive to BEL and AACOCF3, respectively, inhibitors of calcium-independent PLA(2) (iPLA(2)) and cytosolic PLA(2) (cPLA(2)), while OPC, an inhibitor of soluble PLA(2) (sPLA(2)) only inhibited the hypoxia-induced AA release and PGF(2alpha) synthesis. Hypoxia does not alter expression of iPLA(2), sPLA(2) and cPLA(2) and cycloheximide did not inhibit PLA(2) activation, indicating that hypoxia-induced increase in PLA(2) activity is due to activation rather than induction. However, mRNA levels for sPLA(2) displayed a 2-fold increase after 2 hr incubation under hypoxia. BAPTA, an intracellular calcium chelator, partially inhibited the AA release in normoxia and in hypoxia. Direct assays of specific PLA(2) activity showed an increase in sPLA(2) activity but not in cPLA(2) activity after 2hr hypoxia. Taken together, these results indicate that the hypoxia-induced increase in PLA(2) activity is mostly due to the activation of sPLA(2).

Functional site of endogenous phospholipase A2 inhibitor from python serum

The functional site of 'phospholipase A2 inhibitor from python' (PIP) was predicted based on the hypothesis of proline brackets. Using different sources of secretory phospholipase A2 (sPLA2s) as enzyme, and [3H]arachidonate-labelled Escherichia coli as substrate, short synthetic peptides representing the proposed site were examined for their secretory phospholipase A2 (sPLA2) inhibitory activity. A decapeptide P-PB.III proved to be the most potent of the tested peptides in inhibiting sPLA2 enzymatic activity in vitro, and exhibited striking anti-inflammatory effects in vivo in a mouse paw oedema model. P-PB.III inhibited the enzymatic activity of class I, II and III PLA2s, including that of human synovial fluid from arthritis patients. When tested by ELISA, biotinylated P-PB.III interacted positively with various PLA2s, suggesting that the specific region of PIP corresponding to P-PB.III, is likely to be involved in the PLA2-PLI interaction. The effect of P-PB.III on the peritoneal inflammatory response after surgical trauma in rats was also examined. P-PB.III effectively reduced the extent of postsurgical peritoneal adhesions as compared to controls. sPLA2 levels at seventh postoperative day in the peritoneal tissue of P-PB.III-treated rats were also significantly reduced (P < 0.05) in comparison to those of the untreated controls. The present results shed additional insight on the essential structural elements for PLA2 binding, and may be useful as a basis for the design of novel therapeutic agents.

Human neutrophil migration in vitro induced by secretory phospholipases A2: a role for cell surface glycosaminoglycans

The purpose of this study was to examine the ability of type I- (porcine pancreas and Naja mocambique mocambique venom), type II- (bothropstoxin-I, bothropstoxin-II, and piratoxin-I), and type III- (Apis mellifera venom) secretory phospholipases A2 (sPLA2s) to induce human neutrophil chemotaxis, and the role of the cell surface proteoglycans, leukotriene B4 (LTB4), and platelet-activating factor (PAF), in mediating this migration. The neutrophil chemotaxis assays were performed by using a 48-well microchemotaxis chamber. Piratoxin-I, bothropstoxin-I, N. m. mocambique venom PLA2 (10-1000 microg/mL each), bothropstoxin-II (30-1000 microg/mL), porcine pancreas PLA2 (0.3-30 microg/mL), and A. mellifera venom PLA2 (30-300 microg/mL) caused concentration-dependent neutrophil chemotaxis. Heparin (10-300 U/mL) concentration-dependently inhibited the neutrophil migration induced by piratoxin-I, bothropstoxin-II, and N. m. mocambique and A. mellifera venom PLA2s (100 microg/mL each), but failed to affect the migration induced by porcine pancreas PLA2. Heparan sulfate (300 and 1000 microg/mL) inhibited neutrophil migration induced by piratoxin-I, whereas dermatan sulfate and chondroitin sulfate (30-1000 microg/mL each) had no effect. Heparitinase I and heparinase (300 mU/mL each) inhibited by 41.5 and 47%, respectively, piratoxin-I-induced chemotaxis, whereas heparitinase II and chondroitinase AC failed to affect the chemotaxis. The PAF receptor antagonist WEB 2086 (3-[4-(2-chlorophenyl)-9-methyl-6H-thienol-[3,2-f] -triazolo-[4,3-a] -diazepine-2-yl]-1-(4-morpholynil)-1-propionate) (0.1-10 microM) and the LTB4 synthesis inhibitor AA-861 [2-(12-hydroxydodeca-5,10-diynyl)-3,5,6-trimethyl-1,4-benzoquinone] (0.1-10 microM) significantly inhibited the piratoxin-I-induced chemotaxis. Piratoxin-I (30-300 microg/mL) caused a concentration-dependent release of LTB4. Our results suggest that neutrophil migration in response to sPLA2s is independent of PLA activity, and involves an interaction of sPLA2s with cell surface heparin/heparan binding sites triggering the release of LTB4 and PAF.

Clinical implications of prostaglandin inhibition in the small bowel

NSAID-induced intestinal toxicity is more common than previously recognized and may have clinically significant sequelae, especially in elderly arthritic patients. Increased awareness of the potential intestinal complications associated with prostaglandin inhibition is required for early recognition and appropriate management. An increase in the level of suspicion by physicians may lead to earlier diagnosis and subsequent discontinuation of the offending NSAID; this is important in that discontinuation of the offending agent may be preferable to multiple endoscopic radiologic and surgical procedures in the patient with obscure blood loss and anemia. Appropriate diagnosis in selected patients may prevent the increased morbidity and mortality associated with small intestinal surgery. The emergence of selective COX-2 inhibitors likely will bring this issue to the forefront because it will become increasingly important to determine the effects of these agents on the small intestine and colon, in addition to their effects on the gastroduodenal mucosa. The new generation of selective COX-2 inhibitors may offer a potential therapeutic advantage over the nonselective NSAIDs with respect to their intestinal toxicity. Well-designed safety trials that have intestinal injury as a predefined end point will provide important information as to the overall gastrointestinal safety of these compounds. These agents must be evaluated with respect to their overall safety profile and not just by their gastrointestinal safety. Nevertheless, these agents are continuing to provide new directions for exciting basic and clinical scientific investigation.

Prostaglandin biology

PGs are important mediators of normal physiology, response to injury, and pathologic processes. Dissecting these biochemical and molecular pathways allows development of therapeutic agents that can be [figure: see text] applied to specific clinical situations, while preserving PGs that play a role in normal physiology.

p38 MAPK regulates group IIa phospholipase A2 expression in interleukin-1beta -stimulated rat neonatal cardiomyocytes

Group IIa phospholipase A(2) (GIIa PLA(2)) is released by some cells in response to interleukin-1beta. The purpose of this study was to determine whether interleukin-1beta would stimulate the synthesis and release of GIIa PLA(2) from cardiomyocytes, and to define the role of p38 MAPK and cytosolic PLA(2) in the regulation of this process. Whereas GIIa PLA(2) mRNA was not identified in untreated cells, exposure to interleukin-1beta resulted in the sustained expression of GIIa PLA(2) mRNA. Interleukin-1beta also stimulated a progressive increase in cellular and extracellular GIIa PLA(2) protein levels and increased extracellular PLA(2) activity 70-fold. In addition, interleukin-1beta stimulated the p38 MAPK-dependent activation of the downstream MAPK-activated protein kinase, MAPKAP-K2. Treatment with the p38 MAPK inhibitor, SB202190, decreased interleukin-1beta stimulated MAPKAP-K2 activity, GIIa PLA(2) mRNA expression, GIIa PLA(2) protein synthesis, and the release of extracellular PLA(2) activity. Infection with an adenovirus encoding a constitutively active form of MKK6, MKK6(Glu), which selectively phosphorylates p38 MAPK, induced cellular GIIa PLA(2) protein synthesis and the release of GIIa PLA(2) and increased extracellular PLA(2) activity 3-fold. In contrast, infection with an adenovirus encoding a phosphorylation-resistant MKK6, MKK6(A), did not result in GIIa PLA(2) protein synthesis or release by unstimulated cardiomyocytes. In addition, infection with an adenovirus encoding MKK6(A) abrogated GIIa PLA(2) protein synthesis and release by interleukin-1beta-stimulated cells. These results provide direct evidence that p38 MAPK activation was necessary for interleukin-1beta-induced synthesis and release of GIIa PLA(2) by cardiomyocytes.

Assessment of preferential Th1 or Th2 induction by low-molecular-weight compounds using a reverse transcription-polymerase chain reaction method: comparison of two mouse strains, C57BL/6 and BALB/c

The aim of this study was to examine whether the RT-PCR method for various Th1/Th2 cytokines is appropriate for determination of response to allergens using C57BL/6 and Balb/c mice, which are known to preferentially demonstrate Th1 and Th2 responses, respectively. To this end, both strains of mice were sensitized by skin painting with the contact allergen dinitrochlorobenzene (DNCB) or the respiratory allergen trimellitic anhydride (TMA). We used the sensitizing protocol adopted by Kimber and coworkers (Toxicology 103, 63-73, 1995). At various time points after the last application, the levels of mRNA expression for Th1-type cytokines IFN-gamma, IL-18, and IL-12p40, as well as receptor IL-18R, and the Th2-type cytokine IL-4 and the receptor ST2L, in lymph nodes were measured. The results suggest that differential expression of IL-12p40 and IL-4 mRNA after 24 h allows clear discrimination between DNCB and TMA in C57BL/6 mice, more obviously than in Balb/c mice. Furthermore, to examine this method, C57BL/6 mice were exposed to OXA, DNFB, and TNCB (Th1-predominant allergens) or PA, TDI, and MDI (Th2-predominant allergens). Elevation of IL-12p40 expression was significant with the Th1 inducers, while the level of IL-4 was higher with Th2-predominant allergens. The results of the present study demonstrate, for the first time, that differential expression of IL-12p40 and IL-4 in C57BL/6 mice may be useful as a parameter for assessing influence of contact and respiratory allergens.

Human and rat alveolar macrophages express multiple EDG receptors

Endothelial differentiation gene (EDG) receptors are a new family of eight G protein-coupled receptors for the lysophospholipids lysophosphatitic acid and sphingosine-1-phosphate. In the present experiments, the expression of EDG receptors in rat and human alveolar macrophages was studied by reverse transcription-polymerase chain reaction (RT-PCR). In alveolar macrophages of both species, mRNA for multiple EDG receptors could be detected, but the pattern of expression was different in both species. In human alveolar macrophages, mRNA for EDG1, EDG2, EDG4, EDG7 receptors and, to a lesser extent, for the EDG7 receptor was detected, whereas in rat macrophages, mRNA for EDG2, EDG5 receptors and, to a lesser extent, for the EDG6 receptor was found. In functional experiments, it was observed that lysophosphatitic acid and sphingosine-1-phosphate can stimulate O(2)(-) generation in rat and human alveolar macrophages suggesting that lysophosphatitic acid and sphingosine-1-phosphate possibly acting via EDG receptors may play a role in controlling the activation of macrophages.

Lipopolysaccharide-induced increase of prostaglandin E(2) is mediated by inducible nitric oxide synthase activation of the constitutive cyclooxygenase and induction of membrane-associated prostaglandin E synthase

NO produced by the inducible NO synthase (NOS2) and prostanoids generated by the cyclooxygenase (COX) isoforms and terminal prostanoid synthases are major components of the host innate immune and inflammatory response. Evidence exists that pharmacological manipulation of one pathway could result in cross-modulation of the other, but the sense, amplitude, and relevance of these interactions are controversial, especially in vivo. Administration of 6 mg/kg LPS to rats i.p. resulted 6 h later in induction of NOS2 and the membrane-associated PGE synthase (mPGES) expression, and decreased constitutive COX (COX-1) expression. Low level inducible COX (COX-2) mRNA with absent COX-2 protein expression was observed. The NOS2 inhibitor aminoguanidine (50 and 100 mg/kg i.p.) dose dependently decreased both NO and prostanoid production. The LPS-induced increase in PGE(2) concentration was mediated by NOS2-derived NO-dependent activation of COX-1 pathway and by induction of mPGES. Despite absent COX-2 protein, SC-236, a putative COX-2-specific inhibitor, decreased mPGES RNA expression and PGE(2) concentration. Ketoprofen, a nonspecific COX inhibitor, and SC-236 had no effect on the NOS2 pathway. Our results suggest that in a model of systemic inflammation characterized by the absence of COX-2 protein expression, NOS2-derived NO activates COX-1 pathway, and inhibitors of COX isoforms have no effect on NOS2 or NOS3 (endothelial NOS) pathways. These results could explain, at least in part, the deleterious effects of NOS2 inhibitors in some experimental and clinical settings, and could imply that there is a major conceptual limitation to the use of NOS2 inhibitors during systemic inflammation.

Retinoic acid and lipopolysaccharide act synergistically to increase prostanoid concentrations in rats in vivo

Vitamin A and its active metabolite retinoic acid (RA) modulate host-pathogen interactions by interfering with the host immune and inflammatory response including prostaglandin (PG) biosynthesis. The effects of RA on phospholipase A(2) (PLA(2)) and cyclooxygenase (COX) isoforms in vitro are controversial, and few in vivo studies exist. We investigated the in vivo effects of RA on PG biosynthesis in the presence or absence of lipopolysaccharide (LPS) in rats. RA alone [10 mg/(kg. d) for 5 d] increased plasma and liver PG concentrations by increasing COX-1 protein expression (twofold that of control rats). RA acted synergistically with LPS to increase plasma (400-fold) and liver (15-fold) concentrations of prostaglandin E(2) (PGE(2)) and significantly, but to a lesser extent, other PG compared with RA rats, in the absence of major differences in PLA(2) expression or activity or COX-1 and COX-2 mRNA or protein expression. The RA + LPS-mediated increase in PGE(2) was significantly attenuated (97%) by aminoguanidine (AG), a relatively specific inhibitor of the inducible nitric oxide synthase (NOS2), consistent with the previously reported synergistic effect of RA and LPS on NOS2 expression and activity. In addition, RA and LPS induced the expression of the microsomal isoform of PGE synthase (mPGES). In conclusion, in vivo, RA and LPS increased PG and especially PGE(2) concentrations. The PGE(2) increase was associated with NOS2-mediated activation of COX and induction of mPGES. These results contribute to the characterization of the effects of vitamin A on the host inflammatory response.

Atypical lambda/iota PKC conveys 5-lipoxygenase/leukotriene B4-mediated cross-talk between phospholipase A2s regulating NF-kappa B activation in response to tumor necrosis factor-alpha and interleukin-1beta

The transcription factor nuclear factor kappaB (NF-kappaB) plays crucial roles in a wide variety of biological functions such as inflammation, stress, and immune responses. We have shown previously that secretory nonpancreatic (snp) and cytosolic (c) phospholipase A(2) (PLA(2)) regulate NF-kappaB activation in response to tumor necrosis factor (TNF)-alpha or interleukin (IL)-1beta activation and that a functional coupling mediated by the 5-lipoxygenase (5-LO) metabolite leukotriene B(4) (LTB(4)) exists between snpPLA(2) and cPLA(2) in human keratinocytes. In this study, we have further investigated the mechanisms of PLA(2)-modulated NF-kappaB activation with respect to specific kinases involved in TNF-alpha/IL-1beta-stimulated cPLA(2) phosphorylation and NF-kappaB activation. The protein kinase C (PKC) inhibitors RO 31-8220, Gö 6976, and a pseudosubstrate peptide inhibitor of atypical PKCs attenuated arachidonic acid release, cPLA(2) phosphorylation, and NF-kappaB activation induced by TNF-alpha or IL-1beta, thus indicating atypical PKCs in cPLA(2) regulation and transcription factor activation. Transfection of a kinase-inactive mutant of lambda/iotaPKC in NIH-3T3 fibroblasts completely abolished TNF-alpha/IL-1beta-stimulated cellular arachidonic acid release and cPLA(2) activation assayed in vitro, confirming the role of lambda/iotaPKC in cPLA(2) regulation. Furthermore, lambda/iotaPKC and cPLA(2) phosphorylation was attenuated by phosphatidyinositol 3-kinase (PI3-kinase) inhibitors, which also reduced NF-kappaB activation in response to TNF-alpha and IL-1beta, indicating a role for PI3-kinase in these processes in human keratinocytes. TNF-alpha- and IL-1beta-induced phosphorylation of lambda/iotaPKC was attenuated by inhibitors toward snpPLA(2) and 5-LO and by an LTB(4) receptor antagonist, suggesting lambda/iotaPKC as a downstream effector of snpPLA(2) and 5-LO/LTB(4) the LTB(4) receptor. Hence, lambda/iotaPKC regulates snpPLA(2)/LTB(4)-mediated cPLA(2) activation, cellular arachidonic acid release, and NF-kappaB activation induced by TNF-alpha and IL-1beta. In addition, our results demonstrate that PI3-kinase and lambda/iotaPKC are involved in cytokine-induced cPLA(2) and NF-kappaB activation, thus identifying lambda/iotaPKC as a novel regulator of cPLA(2).

The molecular biology of the group VIA Ca2+-independent phospholipase A2

The group VIA PLA2 is a member of the PLA2 superfamily. This enzyme, which is cytosolic and Ca2+-independent, has been designated iPLA2beta to distinguish it from another recently cloned Ca2+-independent PLA2. Features of iPLA2beta molecular structure offer some insight into possible cellular functions of the enzyme. At least two catalytically active iPLA2beta isoforms and additionalsplicing variants are derived from a single gene that consists of at least 17 exons located on human chromosome 22q13.1. Potential tumor suppressor genes also reside at or near this locus. Structural analyses reveal that iPLA2beta contains unique structural features that include a serine lipase consensus motif (GXSXG), a putative ATP-binding domain, an ankyrin-repeat domain, a caspase-3 cleavage motif DVTD138Y/N, a bipartite nuclear localization signal sequence, and a proline-rich region in the human long isoform. iPLA2beta is widely expressed among mammalian tissues, with highest expression in testis and brain. iPLA2beta prefers to hydrolyze fatty acid at the sn-2 fatty acid substituent but also exhibits phospholipase A1, lysophospholipase, PAF acetylhydrolase, and transacylase activities. iPLA2beta may participate in signaling, apoptosis, membrane phospholipid remodeling, membrane homeostasis, arachidonate release, and exocytotic membrane fusion. Structural features and the existence of multiple splicing variants of iPLA2beta suggest that iPLA2beta may be subject to complex regulatory mechanisms that differ among cell types. Further study of its regulation and interaction with other proteins may yield insight into how its structural features are related to its function.

Cytosolic phospholipase A(2) activity during the ongoing cell cycle

Cytosolic phospholipase A(2) (cPLA(2)) is of special interest because it selectively releases arachidonic acid from membrane phospholipids. Arachidonic acid has been implicated to play an important role in various cellular responses. Recently arachidonic acid release and prostaglandin synthesis have been shown to be cell cycle dependent and therefore the activity of cPLA(2) during the ongoing cell cycle was investigated, using the mitotic shake off method for cell synchronisation. cPLA(2) activity was high in mitotic cells and decreased rapidly in the early G1 phase. A strong increase in activity was measured following the G1/S transition in both neuroblastoma and Chinese hamster ovary cells. The changes in activity were not due to a difference in cPLA(2) expression but due to phosphorylation of cPLA(2). Phosphorylation of cPLA(2) occurs through MAPK since the use of a specific MAPK kinase inhibitor and serum depletion of synchronised cells inhibited cPLA(2) activity.

Effect of shigatoxin-1 on arachidonic acid release by human glomerular epithelial cells

BACKGROUND

Altered arachidonic acid (AA) metabolism has been implicated in the pathogenesis of renal injury in the hemolytic uremic syndrome (HUS). However, there is very little information of the effect of shigatoxin (Stx; the putative mediator of renal damage in HUS) on AA release or metabolism by renal cells. Since recent studies have demonstrated that glomerular epithelial cells (GECs) may be important early targets of Stx, the current study was undertaken to examine the effects of Stx on AA release and metabolism by GECs.

METHODS

Cultured human GECs were exposed to Stx1 +/- lipopolysaccharide (LPS) for 4 to 48 hours followed by determination of (3)H-arachidonate release, thromboxane A(2) (TxA(2)) and prostacyclin (PGI(2)) production, cyclooxygenase (COX) activity, and Western and Northern analyses for phospholipase A(2) (PLA(2)) and COX protein and mRNA levels, respectively.

RESULTS

Stx1 increased arachidonate release by GECs. LPS alone had no such effect, but increased arachidonate release in response to Stx1. Stx1-stimulated arachidonate release correlated with elevations in cPLA(2) and sPLA(2) protein and cPLA(2) mRNA levels. Stx1 also increased both TxA(2) and PGI(2) production by GECs; LPS alone did not alter eicosanoid production, but augmented Stx1 effects. Both Stx1 and LPS stimulated COX activity; however, these effects were not additive. Although there was an accompanying elevation of COX-1 and COX-2 mRNA, Stx1 decreased and LPS did not change COX1 and COX2 protein levels.

CONCLUSIONS

Stx1 alone or in conjunction with LPS increases arachidonate release and eicosanoid production by human GECs; this effect correlates with increased PLA(2) protein and mRNA levels. To our knowledge, this is the first study identifying the mechanisms of Stx1-stimulated AA release. These results raise the possibility that arachidonate release and metabolism by GECs, and conceivably other renal cell types, are involved in renal injury in HUS.

Phosphatidylserine-specific phospholipase A1 stimulates histamine release from rat peritoneal mast cells through production of 2-acyl-1-lysophosphatidylserine

Lysophosphatidylserine (1-acyl-2-lyso-PS) has been shown to stimulate histamine release from rat peritoneal mast cells (RPMC) triggered by FcepsilonRI (high affinity receptor for IgE) cross-linking, although the precise mechanism of lyso-PS production has been obscure. In the present study we show that phosphatidylserine-specific phospholipase A(1), PS-PLA(1), stimulates histamine release from RPMC through production of 2-acyl-1-lyso-PS in the presence of FcepsilonRI cross-linker. The potency of 2-acyl-1-lyso-PS was almost equal to that of 1-acyl-2-lyso-PS. A catalytically inactive PS-PLA(1), in which an active serine residue (Ser(166)) was replaced with an alanine residue did not show such activity. sPLA(2)-IIA, another secretory PLA(2) that is capable of producing lyso-PS in vitro, was also a poor histamine inducer against RPMC. PS-PLA(1) significantly stimulated histamine release from crude RPMC, indicating that lyso-PS is mainly derived from cells other than mast cells. In agreement with this phenomenon, the enzyme stimulated the histamine release more efficiently when RPMC were mixed with apoptotic Jurkat cells. Under these conditions, lyso-PS with unsaturated fatty acid was released from the apoptotic cells treated with PS-PLA(1). Finally, heparin, which has affinity for PS-PLA(1), completely blocked the stimulatory effect of the enzyme. In conclusion, PS-PLA(1) may bind to heparan sulfate proteoglycan, efficiently hydrolyze PS appearing on plasma membranes of apoptotic cells, and stimulate mast cell activation mediated by 2-acyl-1-lyso-PS.

A pyrrolidine-based specific inhibitor of cytosolic phospholipase A(2)alpha blocks arachidonic acid release in a variety of mammalian cells

We analyzed a recently reported (K. Seno, T. Okuno, K. Nishi, Y. Murakami, F. Watanabe, T. Matsuur, M. Wada, Y. Fujii, M. Yamada, T. Ogawa, T. Okada, H. Hashizume, M. Kii, S.-H. Hara, S. Hagishita, S. Nakamoto, J. Med. Chem. 43 (2000)) pyrrolidine-based inhibitor, pyrrolidine-1, against the human group IV cytosolic phospholipase A(2) alpha-isoform (cPLA(2)alpha). Pyrrolidine-1 inhibits cPLA(2)alpha by 50% when present at approx. 0.002 mole fraction in the interface in a number of in vitro assays. It is much less potent on the cPLA(2)gamma isoform, calcium-independent group VI PLA(2) and groups IIA, X, and V secreted PLA(2)s. Pyrrolidine-1 blocked all of the arachidonic acid released in Ca(2+) ionophore-stimulated CHO cells stably transfected with cPLA(2)alpha, in zymosan- and okadaic acid-stimulated mouse peritoneal macrophages, and in ATP- and Ca(2+) ionophore-stimulated MDCK cells.

A peptide derived from bee venom-secreted phospholipase A2 inhibits replication of T-cell tropic HIV-1 strains via interaction with the CXCR4 chemokine receptor

We have previously shown that secreted phospholipases A2 (sPLA2) from bee and snake venoms have potent anti-human immunodeficiency virus (HIV) activity. These sPLA2s block HIV-1 entry into host cells through a mechanism linked to sPLA2 binding to cells. In this study, 12 synthetic peptides derived from bee venom sPLA2 (bvPLA2) have been tested for inhibition of HIV-1 infection. The p3bv peptide (amino acids 21 to 35 of bvPLA2) was found to inhibit the replication of T-lymphotropic (T-tropic) HIV-1 isolates (ID(50) = 2 microM) but was without effect on monocytotropic (M-tropic) HIV-1 isolates. p3bv was also found capable of preventing the cell-cell fusion process mediated by T-tropic HIV-1 envelope. Finally, p3bv can inhibit the binding of radiolabeled stromal cell-derived factor (SDF)-1alpha, the natural ligand of CXCR4, and the binding of 12G5, an anti-CXCR4 monoclonal antibody. Taken together, these results indicate that p3bv blocks the replication of T-tropic HIV-1 strains by interacting with CXCR4. Its mechanism of action however appears distinct from that of bvPLA2 because the latter inhibits replication of both T-tropic and M-tropic isolates and does not compete with SDF-1alpha and 12G5 binding to CXCR4.

A viral phospholipase A2 is required for parvovirus infectivity

Sequence analysis revealed phospholipase A2 (PLA2) motifs in capsid proteins of parvoviruses. Although PLA2 activity is not known to exist in viruses, putative PLA2s from divergent parvoviruses, human B19, porcine parvovirus, and insect GmDNV (densovirus from Galleria mellonella), can emulate catalytic properties of secreted PLA2. Mutations of critical amino acids strongly reduce both PLA2 activity and, proportionally, viral infectivity, but cell surface attachment, entry, and endocytosis by PLA2-deficient virions are not affected. PLA2 activity is critical for efficient transfer of the viral genome from late endosomes/lysosomes to the nucleus to initiate replication. These findings offer the prospect of developing PLA2 inhibitors as a new class of antiviral drugs against parvovirus infections and associated diseases.

Mechanical stress and nitric oxide influence leukotriene production in cartilage

Nitric oxide (NO) and leukotrienes regulate a variety of processes in joint tissues and are frequently elevated in arthritis. Mechanical stress can induce biochemical and functional changes in cartilage that may influence mediator production. To investigate the relationship between mechanical stress and the production of leukotriene B(4) (LTB(4)) and NO, explants of porcine articular cartilage were subjected to mechanical compression for 1 h followed by 23 h recovery in the presence or absence of the NOS2 inhibitor 1400W. Dynamic compression significantly increased LTB(4) and LOX protein production in the presence of 1400W. The induced LTB(4) was functional as evidenced by its ability to promote chemotaxis of RBL-2H3 cells expressing the LTB(4) receptor. Increased LOX protein but not LTB(4) occurred in response to compression alone. These findings provide a direct link between mechanical stress and inflammation in cartilage and may have implications in the pathogenesis and treatment of arthritis.

Contrasting effects of cPLA2 on epithelial Na+ transport

Activity of the epithelial Na+ channel (ENaC) is the limiting step for discretionary Na+ reabsorption in the cortical collecting duct. Xenopus laevis kidney A6 cells were used to investigate the effects of cytosolic phospholipase A2 (cPLA2) activity on Na+ transport. Application of aristolochic acid, a cPLA2 inhibitor, to the apical membrane of monolayers produced a decrease in apical [3H]arachidonic acid (AA) release and led to an approximate twofold increase in transepithelial Na+ current. Increased current was abolished by the nonmetabolized AA analog 5,8,11,14-eicosatetraynoic acid (ETYA), suggesting that AA, rather than one of its metabolic products, affected current. In single channel studies, ETYA produced a decrease in ENaC open probability. This suggests that cPLA2 is tonically active in A6 cells and that the end effect of liberated AA at the apical membrane is to reduce Na+ transport via actions on ENaC. In contrast, aristolochic acid applied basolaterally inhibited current, and the effect was not reversed by ETYA. Basolateral application of the cyclooxygenase inhibitor ibuprofen also inhibited current. Both effects were reversed by prostaglandin E2 (PGE2). This suggests that cPLA2 activity and free AA, which is metabolized to PGE2, are necessary to support transport. This study supports the fine-tuning of Na+ transport and reabsorption through the regulation of free AA and AA metabolism.

Modulation of cell-substrate adhesion by arachidonic acid: lipoxygenase regulates cell spreading and ERK1/2-inducible cyclooxygenase regulates cell migration in NIH-3T3 fibroblasts

Adhesion of cells to an extracellular matrix is characterized by several discrete morphological and functional stages beginning with cell-substrate attachment, followed by cell spreading, migration, and immobilization. We find that although arachidonic acid release is rate-limiting in the overall process of adhesion, its oxidation by lipoxygenase and cyclooxygenases regulates, respectively, the cell spreading and cell migration stages. During the adhesion of NIH-3T3 cells to fibronectin, two functionally and kinetically distinct phases of arachidonic acid release take place. An initial transient arachidonate release occurs during cell attachment to fibronectin, and is sufficient to signal the cell spreading stage after its oxidation by 5-lipoxygenase to leukotrienes. A later sustained arachidonate release occurs during and after spreading, and signals the subsequent migration stage through its oxidation to prostaglandins by newly synthesized cyclooxygenase-2. In signaling migration, constitutively expressed cyclooxygenase-1 appears to contribute approximately 25% of prostaglandins synthesized compared with the inducible cyclooxygenase-2. Both the second sustained arachidonate release, and cyclooxygenase-2 protein induction and synthesis, appear to be regulated by the mitogen-activated protein kinase extracellular signal-regulated kinase (ERK)1/2. The initial cell attachment-induced transient arachidonic acid release that signals spreading through lipoxygenase oxidation is not sensitive to ERK1/2 inhibition by PD98059, whereas PD98059 produces both a reduction in the larger second arachidonate release and a blockade of induced cyclooxygenase-2 protein expression with concomitant reduction of prostaglandin synthesis. The second arachidonate release, and cyclooxygenase-2 expression and activity, both appear to be required for cell migration but not for the preceding stages of attachment and spreading. These data suggest a bifurcation in the arachidonic acid adhesion-signaling pathway, wherein lipoxygenase oxidation generates leukotriene metabolites regulating the spreading stage of cell adhesion, whereas ERK 1/2-induced cyclooxygenase synthesis results in oxidation of a later release, generating prostaglandin metabolites regulating the later migration stage.

Microsomal prostaglandin E synthase is regulated by proinflammatory cytokines and glucocorticoids in primary rheumatoid synovial cells

The selective induction of PGE(2) synthesis in inflammation suggests that a PGE synthase may be linked to an inducible pathway for PG synthesis. We examined the expression of the recently cloned inducible microsomal PGE synthase (mPGES) in synoviocytes from patients with rheumatoid arthritis, its modulation by cytokines and dexamethasone, and its linkage to the inducible cyclooxygenase-2. Northern blot analysis showed that IL-1beta or TNF-alpha treatment induces mPGES mRNA from very low levels at baseline to maximum levels at 24 h. IL-1beta-induced mPGES mRNA was inhibited by dexamethasone in a dose-dependent fashion. Western blot analysis demonstrated that mPGES protein was induced by IL-1beta, and maximum expression was sustained for up to 72 h. There was a coordinated up-regulation of cyclooxygenase-2 protein, although peak expression was earlier. Differential Western blot analysis of the microsomal and the cytosolic fractions revealed that the induced expression of mPGES protein was limited to the microsomal fraction. The detected mPGES protein was catalytically functional as indicated by a 3-fold increase of PGES activity in synoviocytes following treatment with IL-1beta; this increased synthase activity was limited to the microsomal fraction. In summary, these data demonstrate an induction of mPGES in rheumatoid synoviocytes by proinflammatory cytokines. This novel pathway may be a target for therapeutic intervention for patients with arthritis.

CTP:phosphocholine cytidylyltransferase inhibition by ceramide via PKC-alpha, p38 MAPK, cPLA2, and 5-lipoxygenase

In a companion paper (Vivekananda J, Smith D, and King RJ. Am J Physiol Lung Cell Mol Physiol 281: L98-L107, 2001), we demonstrated that tumor necrosis factor (TNF)-alpha inhibited the activity of CTP:phosphocholine cytidylyltransferase (CT), the rate-limiting enzyme in the de novo synthesis of phosphatidylcholine (PC), and that its actions were likely exerted through a metabolite of sphingomyelin. In this paper, we explore the signaling pathway employed by TNF-alpha using C2 ceramide as a cell-penetrating sphingolipid representative of the metabolites induced by TNF-alpha. We found that in H441 cells, as reported in other cell types, cytosolic phospholipase A2 (cPLA2) is activated by TNF-alpha. We also observed that the inhibiting action of C2 ceramide on CT requires protein kinase C-alpha, p38 mitogen-activated protein kinase, and cPLA2. The actions of C2 ceramide on CT activity can be duplicated by adding 2 microM lysoPC to these cells. Furthermore, we found that the effects of C2 ceramide are dependent on 5-lipoxygenase but that cyclooxygenase II is unimportant. We hypothesize that CT activity is inhibited by the lysoPC generated as a consequence of the activation of cPLA2 by protein kinase C-alpha and p38 mitogen-activated protein kinase. The other product of the activation of cPLA2, arachidonic acid, is a substrate for the synthesis of leukotrienes, which raise intracellular Ca2+ levels and complete the activation of cPLA2.

Physical properties of erythrocyte ghosts that determine susceptibility to secretory phospholipase A2

Artificial membranes may be resistant or susceptible to catalytic attack by secretory phospholipase A(2) (sPLA(2)) depending on the physical properties of the membrane. Living cells are normally resistant but become susceptible during trauma, apoptosis, and/or a significant elevation of intracellular calcium. Intact erythrocytes and ghosts were studied to determine whether the principles learned from artificial systems apply to biological membranes. Membrane properties such as phospholipid and/or protein composition, morphology, and microscopic characteristics (e.g. fluidity) were manipulated by preparing ghosts under different experimental conditions such as in the presence or absence of divalent cations with or without ATP. The properties of each membrane preparation were assessed by biochemical and physical means (fluorescence spectroscopy and electron and two-photon microscopy using the membrane probes bis-pyrene and laurdan) and compared with sPLA(2) activity. The properties that appeared most relevant were the degree of phosphatidylserine exposure on the outer face of the membrane and changes to the membrane physical state detected by bis-pyrene and laurdan. Specifically, vulnerability to hydrolysis by sPLA(2) was associated with an increase in bilayer order apparently reflective of expansion of membrane regions of diminished fluidity. These results argue that the general principles identified from studies with artificial membranes apply to biological systems.

Differential effects of calcium-dependent and calcium-independent phospholipase A(2) inhibitors on kainate-induced neuronal injury in rat hippocampal slices

Brain tissue contains multiple forms of intracellular phospholipase A(2) (PLA(2)) activity that differ from each other in many ways including their response to specific inhibitors. The systemic administration of kainic acid to rats produces a marked increase in cPLA(2) activity in neurons and astrocytes. This is associated with increased lipid peroxidation as evidenced by accumulation of 4-hydroxynonenal (4-HNE) modified proteins. The present study describes the effect of specific inhibitors of Ca(2+)-dependent or Ca(2+)-independent PLA(2) on kainite-induced excitotoxic injury in rat hippocampal slices. Specific inhibitors of Ca(2+)-dependent PLA(2) prevented the decrease of a neuronal marker, GluR1, and increase in cPLA(2) and 4-HNE immunoreactivities in slices treated with kainate. This shows that cPLA(2) plays an important role in kainite-induced neurotoxicity and that cPLA(2) inhibitors can be used to protect hippocampal slices from damage induced by kainate.

Interleukin-1beta-induced type IIA secreted phospholipase A2 gene expression and extracellular activity in rat vascular endothelial cells

Two phospholipase A2 (PLA2) isoforms, secretory and cytosolic, have been implicated in inflammation. Secretory type IIA PLA2 (sPLA2-IIA), which hydrolyzes fatty acids bound at the sn-2 position of glycerophospholipids, has been detected universally in a variety of mammalian tissues and cells. The expression of the sPLA2-IIA gene and its extracellular activity were shown to be regulated by different factors such as hypoxia, cytokines and phorbol esters. In the present study, we examined the effects of interleukin-1beta (IL-1beta) on the expression of the 14kDa sPLA2-IIA, determined using reverse transcription polymerase chain reaction and radiometric Escherichia coli enzyme assay in primary cultures of rat endothelial cells and in two different rat endothelial cell lines (SVAREC and RBE4). These experiments revealed that IL-1beta induces sPLA2-IIa gene expression and secretion of the enzyme in endothelial cells in a dose- and time-dependent manner. The cAMP-elevator forskolin did not augment the cytokine-induced elevation of sPLA2-IIa enzyme activity but significantly increased the IL-1beta-stimulated sPLA2-IIa mRNA contents in endothelial cells.

Regulation of synoviocyte phospholipase A2 and cyclooxygenase 2 by macrophage migration inhibitory factor

OBJECTIVE

Macrophage migration inhibitory factor (MIF) is a proinflammatory cytokine with known actions in macrophage and T cell activation. MIF also has the unique capacity to reverse the inhibitory effects of glucocorticoids on these cells. We have recently demonstrated MIF expression in human rheumatoid arthritis (RA) synovium and cultured fibroblast-like synoviocytes (FLS), as well as the ability of FLS-derived MIF to induce monocyte release of tumor necrosis factor alpha. We investigated the effects of MIF on aspects of RA FLS activation, including the induction of phospholipase A2 (PLA2) and cyclooxygenase (COX).

METHODS

PLA2 activity was measured by 3H-arachidonic acid released from treated FLS supernatants. COX activity was measured by prostaglandin E2 enzyme-linked immunosorbent assay. Cytosolic PLA2 (cPLA2) and COX-2 messenger RNA (mRNA) were determined using semiquantitative reverse transcriptase-polymerase chain reaction.

RESULTS

Constitutive PLA2 activity was detected in RA FLS. Recombinant human MIF up-regulated PLA2 activity (P < 0.01) and cPLA2 mRNA expression, but had no effect on secretory PLA2. Recombinant human MIF up-regulated COX activity (P < 0.05) and COX-2 mRNA, but had no observable effect on COX-1. Interleukin-1beta (IL-1beta) significantly up-regulated PLA2 activity (P < 0.005) and cPLA2 mRNA expression while anti-MIF monoclonal antibody (mAb) significantly inhibited this IL-1beta-induced PLA2 activity (P < 0.02). Anti-MIF mAb significantly reduced IL-1beta-induced COX activity (P < 0.05) and COX-2 mRNA expression.

CONCLUSION

MIF exerts a proinflammatory effect on key aspects of RA FLS activation. That anti-MIF mAb inhibited IL-1beta up-regulation of FLS indicates an additional cofactor role for MIF in IL-1beta-induced FLS activation. These data suggest that MIF antagonism has important therapeutic potential in RA.

Zafirlukast: an update of its pharmacology and therapeutic efficacy in asthma

Zafirlukast is a selective and competitive orally administered inhibitor of the cysteinyl leukotrienes LTC4, LTD4 and LTE4. The drug is indicated for the prophylaxis and treatment of chronic asthma, and has been developed in response to mounting evidence indicating the importance of the cysteinyl leukotrienes in the pathogenesis of this disorder. The efficacy of zafirlukast 20 mg twice daily has been shown in double-blind placebo-controlled studies of up to 13 weeks' duration in patients aged > or = 12 years. Zafirlukast was consistently superior to placebo in improving objective measures of lung function and subjective measures such as symptom scores and use of as-required bronchodilator therapy. This dosage is also as effective when added to low-dosage inhaled corticosteroid therapy as doubling of corticosteroid dosages. Recent studies indicate superior efficacy over zafirlukast of twice-daily inhaled fluticasone propionate 88 microg or salmeterol 42 microg, although zafirlukast was nevertheless associated with clinical improvement. Data also show zafirlukast 40 mg to be of similar efficacy to pranlukast 225 mg (both twice daily). Overall, preliminary pharmacoeconomic data suggest that healthcare costs are reduced by zafirlukast therapy, although superior cost effectiveness has been reported with inhaled fluticasone propionate. and further studies are needed. Data are available to show improvements in patient-rated quality of life, and preference for and high rates of compliance with zafirlukast. In clinical trials, zafirlukast has shown an adverse event profile similar to that of placebo. Isolated reports of hepatic dysfunction in a small number of individuals receiving the drug have been received, and recommendations for monitoring of patients are in place. Although no causal relationship has been established between zafirlukast and Churg-Strauss Syndrome, patients undergoing corticosteroid dosage reductions require careful surveillance.

CONCLUSIONS

zafirlukast is an effective and well tolerated agent for preventive monotherapy in mild to moderate persistent asthma. Emerging data indicate benefit of the drug when added to low-dosage inhaled corticosteroids and show that it may be a viable alternative to inhaled adjunctive treatments and increased corticosteroid dosages in some patients. Although inhaled fluticasone propionate and salmeterol have been associated with greater clinical improvement than zafirlukast in clinical studies, compliance considerations and the confirmed clinical efficacy relative to placebo of the drug denote zafirlukast as an effective alternative in treatment programmes based on individualised therapy. As experience with zafirlukast accumulates, it is expected that the drug will be positioned more definitively in national and international treatment guidelines.

Anti-CD3 and concanavalin A-induced human T cell proliferation is associated with an increased rate of arachidonate-phospholipid remodeling. Lack of involvement of group IV and group VI phospholipase A2 in remodeling and increased susceptibility of proliferating T cells to CoA-independent transacyclase inhibitor-induced apoptosis

In this study arachidonate-phospholipid remodeling was investigated in resting and proliferating human T lymphocytes. Lymphocytes induced to proliferate with either the mitogen concanavalin A or with anti-CD3 (OKT3) in combination with interleukin 2 (OKT3/IL-2) showed a greatly accelerated rate of [3H]arachidonate-phospholipid remodeling compared with resting lymphocytes or with lymphocytes stimulated with OKT3 or IL-2 alone. The concanavalin A-stimulated cells showed a 2-fold increase in the specific activity of CoA-independent transacylase compared with unstimulated cells, indicating that this enzyme is inducible. Stimulation with OKT3 resulted in greatly increased quantities of the group VI calcium-independent phospholipase A2 but not of the quantity of group IV cytosolic phospholipase A2. However, group IV phospholipase A2 became phosphorylated in OKT3-stimulated cells, as determined by decreased electrophoretic mobility. Incubation of cells with the group VI phospholipase A2 inhibitor, bromoenol lactone, or the dual group IV/group VI phospholipase A2 inhibitor, methyl arachidonyl fluorophosphonate, did not block arachidonate-phospholipid remodeling resting or proliferating T cells, suggesting that these phospholipases A2 were not involved in arachidonate-phospholipid remodeling. The incubation of nonproliferating human lymphocytes with inhibitors of CoA-independent transacylase had little impact on cell survival. In contrast, OKT3/IL-2-stimulated T lymphocytes were very sensitive to apoptosis induced by CoA-independent transacylase inhibitors. Altogether these results indicate that increased arachidonate-phospholipid remodeling is associated with T cell proliferation and that CoA-independent transacylase may be a novel therapeutic target for proliferative disorders.