Induction of type-IIA secretory phospholipase A2 in animal models of acute lung injury

The aim of this study was to evaluate the presence of type-II secretory phospholipase A2 (sPLA2-IIA) in alveolar space and its possible role in the destruction of surfactant in three rat models of acute lung injury. Alveolar instillation of either lipopolysaccaride or live Pseudomonas aeruginosa resulted in a significant increase in lung oedema and in a decrease in static compliance of the respiratory system together with alveolar-neutrophil influx as compared with healthy control rats. The upregulation of messenger ribonucleic acid and sPLA2-IIA by the lung was evident. This was associated with surfactant degradation and a decrease in large:small ratio of surfactant aggregates in bacteria-instilled rats. A negative correlation between compliance and sPLA2-IIA activity in bronchoalveolar lavage fluid was shown. By contrast, during alpha naphthylthiourea-induced injury, neither alveolar-neutrophil influx nor increase in sPLA2-IIA activity was observed. Additional experiments in rats treated with a specific inhibitor of type-II secretory phospholipase A2 activity (3 acetamine-1-benzyl-2 ethylindolyl-5 oxy; propane phosphonic acid (LY311727)) demonstrated no improvement in physiological parameters despite a biochemical effect, suggesting that its activity is only one of the multiple factors involved in the pathophysiology of lung injury.

Mammalian secreted phospholipases A2 and their pathophysiological significance in inflammatory diseases

Phospholipases A2 (PLA2s) represent a growing family of enzymes that catalyze the hydrolysis of phospholipids at the sn-2 position leading to the generation of free fatty acids and lysophospholipids. Mammalian PLA2s are divided into two major classes according to their molecular mass and location: intracellular PLA2 and secreted PLA2 (sPLA2). Type-IIA sPLA2 (sPLA2-IIA), the best studied enzyme of sPLA2, plays a role in the pathogenesis of various inflammatory diseases. Conversely, sPLA2-IIA can exert beneficial action in the context of infectious diseases since recent studies have shown that this enzyme exhibits potent bactericidal effects. Induction of the synthesis of sPLA2-IIA is generally initiated by endotoxin and a limited number of cytokines via paracrine and/or autocrine processes. If the mechanisms involved in the regulation of sPLA2-IIA gene expression have been relatively clarified, little is known on the mechanisms that regulate the expression of other sPLA2. There have been substantial progresses in understanding the transcriptional regulation of sPLA2-IIA expression. Recently, transcription factors including NF-kappaB, PPAR, C/EBP have been identified to play a prominent role in the regulation of sPLA2-IIA gene expression. The activation of these transcription factors is under the control of distinct signaling pathways (PKC, cAMP ...). Accumulating evidences in the literature suggest that cytosolic PLA2 together with two sPLA2 isozymes (sPLA2-IIA and sPLA2-V) are functionally coupled with cyclooxygenase-1 and 2 pathways, respectively, for immediate and delayed PG biosynthesis. This spatio-temporal coupling of cyclooxygenase enzymes with PLA2s may represent a key mechanism in the propagation of inflammatory reaction. Unraveling the mechanisms involved in the regulation of the expression of sPLA2s is important for understanding their pathophysiological roles in inflammatory diseases.

Surfactant protein A suppresses lipopolysaccharide-induced IL-10 production by murine macrophages

Upon LPS exposure, mononuclear phagocytes produce TNF-alpha and IL-10, two cytokines with pro- and anti-inflammatory activities, respectively. We previously described that murine resident alveolar macrophages, which play a central role in the immunosurveillance of the lung alveoli, do not synthesize IL-10 in vivo or in vitro when exposed to LPS. In the present report we demonstrate that during lung inflammation induced by the intranasal administration of LPS, bronchoalveolar cells collected between days 3 and 5 are able to synthesize IL-10 when exposed to LPS. We also show that depletion of resident alveolar macrophages by an intratracheal instillation of liposome-encapsulated clodronate is followed by subsequent replenishment of the airspaces by mononuclear phagocytes. This is accompanied by the transient competence of cells for IL-10 production. The cell capacity to produce IL-10 is evident up to 3 days and then decreases. This led us to hypothesize that the alveolar environment contains a down-regulator of LPS-induced IL-10 synthesis by recently emigrating mononuclear phagocytes. We show that the surfactant protein A, an airspace protein that has known immunomodulatory activities, dramatically inhibits LPS-induced IL-10 formation by bone marrow-derived macrophages. These data show a difference between resident and inflammatory macrophages with respect to IL-10 synthesis. Moreover, this study highlights for the first time the inhibitory role of surfactant protein A in the anti-inflammatory activity of macrophages through inhibition of IL-10 production.

Inhibition by unsaturated fatty acids of type II secretory phospholipase A2 synthesis in guinea-pig alveolar macrophages evidence for the eicosanoid-independent pathway

The effect of arachidonic acid (C20:4) on the production of secretory type II phospholipase A2 (sPLA2-II) by guinea-pig alveolar macrophages was investigated. We show that incubation of these cells with 1-30 microM of arachidonic acid inhibits the synthesis of sPLA2-II in a concentration-dependent manner with an IC50 of approximately 7.5 microM. The inhibition by low concentrations (5 microM) of arachidonic acid was partially reduced by pretreatment of alveolar macrophages with cyclooxygenase or cytochrome P450 inhibitors (aspirin and 1-aminobenzotriazole, respectively), but not by lipoxygenase inhibitor, BW A4C. However, these inhibitors failed to interfere with the effect of high concentrations (30 microM) of arachidonic acid, suggesting that the latter may act on the expression of sPLA2-II, at least in part, independently of eicosanoid generation. Indeed, a similar inhibitory effect on sPLA2-II activity and mRNA expression was observed with other unsaturated fatty acids such as eicosapentaenoic (C20:5) and oleic (C18:1) acids, but not with the saturated fatty acid, palmitic acid (C16:0). In addition, arachidonic acid partially reduced the secretion of tumor necrosis factor alpha, an important intermediate in the induction of sPLA2-II synthesis by guinea-pig alveolar macrophages. However, addition of recombinant tumor necrosis factor alpha failed to reverse the inhibitory effect of arachidonic acid on sPLA2-II expression, suggesting that this process occurs downstream of tumor necrosis factor alpha secretion. We conclude that the expression of sPLA2-II in alveolar macrophages is down-regulated at the transcriptional level by arachidonic acid either directly or via its cyclooxygenase and cytochrome P450-derived metabolites.

Activation of rabbit blood platelets by anandamide through its cleavage into arachidonic acid

Anandamide (ANA), a cannabinoid receptor ligand, stimulated platelet aggregation at concentrations similar to those of arachidonic acid (AA). The aggregating effect of ANA was inhibited by aspirin but not by SR-141716, a cannabinoid receptor antagonist. In addition, HU-210, a cannabinoid receptor agonist, failed to induce platelet activation. Radiolabelling experiments showed that exogenous ANA was cleaved by platelets into AA through a phenylmethylsulfonyl fluoride (PMSF)-sensitive pathway. In agreement, PMSF was shown to abolish the aggregating effect of ANA. In conclusion, ANA is able to induce platelet activation via its cleavage by a PMSF-sensitive amidase activity, leading to the release of AA which in turn activates platelets.

A role for phospholipase A2 in ARDS pathogenesis

Acute respiratory distress syndrome (ARDS) is a life-threatening lung injury that is characterized by arterial hypoxemia and noncardiogenic pulmonary oedema. One feature of ARDS is an alteration of pulmonary surfactant that increases surface tension at the air-liquid interface and results in alveolar collapse and the impairment of gas exchange. Type-II secretory phospholipase A2 (sPLA2-II) plays a major role in the hydrolysis of surfactant phospholipids and its expression is inhibited by surfactant. Here, we discuss the evidence that in pathological situations, such as ARDS, in which surfactant is altered, sPLA2-II production is exacerbated, leading to further surfactant alteration and the establishment of a vicious cycle.

Dioleylphosphatidylglycerol inhibits the expression of type II phospholipase A2 in macrophages

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

Generation of lyso-phospholipids from surfactant in acute lung injury is mediated by type-II phospholipase A2 and inhibited by a direct surfactant protein A-phospholipase A2 protein interaction

Lyso-phospholipids exert a major injurious effect on lung cell membranes during Acute Respiratory Distress Syndrome (ARDS), but the mechanisms leading to their in vivo generation are still unknown. Intratracheal administration of LPS to guinea pigs induced the secretion of type II secretory phospholipase A2 (sPLA2-II) accompanied by a marked increase in fatty acid and lyso-phosphatidylcholine (lyso-PC) levels in the bronchoalveolar lavage fluid (BALF). Administration of LY311727, a specific sPLA2-II inhibitor, reduced by 60% the mass of free fatty acid and lyso-PC content in BALF. Gas chromatography/mass spectrometry analysis revealed that palmitic acid and palmitoyl-2-lyso-PC were the predominant lipid derivatives released in BALF. A similar pattern was observed after the intratracheal administration of recombinant guinea pig (r-GP) sPLA2-II and was accompanied by a 50-60% loss of surfactant phospholipid content, suggesting that surfactant is a major lung target of sPLA2-II. In confirmation, r-GP sPLA2-II was able to hydrolyze surfactant phospholipids in vitro. This hydrolysis was inhibited by surfactant protein A (SP-A) through a direct and selective protein-protein interaction between SP-A and sPLA2-II. Hence, our study reports an in vivo direct causal relationship between sPLA2-II and early surfactant degradation and a new process of regulation for sPLA2-II activity. Anti-sPLA2-II strategy may represent a novel therapeutic approach in lung injury, such as ARDS.

Down-regulation by prostaglandins of type-II phospholipase A2 expression in guinea-pig alveolar macrophages: a possible involvement of cAMP

We have demonstrated previously that isolated guinea-pig alveolar macrophages (AM) synthesize type-II phospholipase A2 (PLA2-II) through a tumour necrosis factor-alpha (TNF-alpha)-dependent process. This synthesis is enhanced by lipopolysaccharide (LPS) and accompanied by a release of prostaglandin E2 (PGE2) into the medium. Because agents elevating intracellular cAMP, such as PGE2, have been shown to stimulate PLA2-II expression in various cell types, we investigated the modulation of PLA2-II synthesis by cAMP in AM. Surprisingly, incubation of AM with PGE2, dibutyryl-cAMP, cholera toxin or rolipram (an inhibitor of specific cAMP-phosphodiesterase) inhibited both basal and LPS-stimulated PLA2-II expression. The inhibitory effect of PGE2 was observed at concentrations similar to those released by AM. Moreover, treatment of AM with either aspirin or neutralizing PGE2 monoclonal antibody stimulated PLA2-II synthesis. These effects were closely correlated with the ability of these agents to modulate TNF-alpha release, which was decreased by dibutyryl-cAMP and exogenous PGE2, whereas neutralizing PGE2 antibody markedly increased this release. Hence, in contrast to other cell systems, we report that: (i) agents elevating intracellular cAMP levels down-regulate both basal and LPS-induced PLA2-II synthesis, (ii) prostaglandins exert a negative feedback effect on this synthesis, probably through an elevation of intracellular cAMP levels, and (iii) inhibition of TNF-alpha release may account, at least in part, for the down-regulation of PLA2-II expression by endogenously produced prostaglandins and cAMP-elevating agents.

Inhibition by pulmonary surfactant Curosurf of secretory phospholipase A2 expression in guinea-pig alveolar macrophages

Replacement therapy with exogenous surfactant has been proven successful in animal models of acute respiratory distress syndrome (ARDS). Here, we investigated the effect of seminatural surfactant Curosurf on the expression of secretory phospholipase A2 (sPLA2) in guinea-pig alveolar macrophages (AM). The latter produced an sPLA2 activity whose level was markedly reduced when culture medium was supplemented with Curosurf. This effect was concentration-dependent and was accompanied by a decrease in sPLA2 mRNA levels. By contrast, when AM were first cultured for 20 hr and then incubated with Curosurf, no significant change was observed in their sPLA2 activity. Finally, f-Met-Leu-Phe (FMLP)-induced thromboxane B2 release from AM was not altered by Curosurf, indicating that the inhibition of sPLA2 expression cannot be attributed to a nonspecific membraneous effect of Curosurf. These findings show that pulmonary surfactant modulates the expression of sPLA2 in AM and suggest that this effect may account for the clinical efficacy of surfactant replacement therapy in ARDS.

Endotoxin induces expression of type II phospholipase A2 in macrophages during acute lung injury in guinea pigs: involvement of TNF-alpha in lipopolysaccharide-induced type II phospholipase A2 synthesis

Elevated levels of secretory type II phospholipase A2 (sPLA2-II) have been associated with a poor clinical outcome in the acute respiratory distress syndrome. This study identifies the cell source(s) and the mechanisms of sPLA2-II synthesis in the guinea pig model of acute respiratory distress syndrome induced by intratracheal injection of LPS. Administration of LPS led to an increase in lung membrane-associated calcium-dependent sPLA2 activity, which was abrogated by LY311727, a selective inhibitor of sPLA2-II. No sPLA2 activity was detected in the vascular compartment of the lung. LPS administration induced a parallel accumulation of sPLA2-II mRNA in lung tissues. In situ hybridization showed that sPLA2-II transcripts were synthesized in interstitial and alveolar macrophages (AM). Incubation of AM with LPS enhanced the expression of sPLA2-II mRNA, leading to stimulation of sPLA2-II synthesis and secretion. This increase was prevented by the addition of anti-TNF-alpha and anti-p55 TNF receptor Abs. Furthermore, the addition to AM of cellfree bronchoalveolar fluid collected from LPS-treated guinea pigs increased sPLA2-II expression, which was abrogated by anti-TNF-alpha Ab. These findings demonstrate that 1) macrophages are in vivo the major cell source of sPLA2-II in LPS-induced acute lung injury; 2) in contrast to that in other cell systems, regulation of LPS-induced sPLA2-II synthesis in AM is TNF-alpha dependent; and 3) production of TNF-alpha in the air-lung interface is an important step for sPLA2-II synthesis in macrophages.

Endotoxin induces expression of type II phospholipase A2 in macrophages during acute lung injury in guinea pigs: involvement of TNF-alpha in lipopolysaccharide-induced type II phospholipase A2 synthesis

Elevated levels of secretory type II phospholipase A2 (sPLA2-II) have been associated with a poor clinical outcome in the acute respiratory distress syndrome. This study identifies the cell source(s) and the mechanisms of sPLA2-II synthesis in the guinea pig model of acute respiratory distress syndrome induced by intratracheal injection of LPS. Administration of LPS led to an increase in lung membrane-associated calcium-dependent sPLA2 activity, which was abrogated by LY311727, a selective inhibitor of sPLA2-II. No sPLA2 activity was detected in the vascular compartment of the lung. LPS administration induced a parallel accumulation of sPLA2-II mRNA in lung tissues. In situ hybridization showed that sPLA2-II transcripts were synthesized in interstitial and alveolar macrophages (AM). Incubation of AM with LPS enhanced the expression of sPLA2-II mRNA, leading to stimulation of sPLA2-II synthesis and secretion. This increase was prevented by the addition of anti-TNF-alpha and anti-p55 TNF receptor Abs. Furthermore, the addition to AM of cellfree bronchoalveolar fluid collected from LPS-treated guinea pigs increased sPLA2-II expression, which was abrogated by anti-TNF-alpha Ab. These findings demonstrate that 1) macrophages are in vivo the major cell source of sPLA2-II in LPS-induced acute lung injury; 2) in contrast to that in other cell systems, regulation of LPS-induced sPLA2-II synthesis in AM is TNF-alpha dependent; and 3) production of TNF-alpha in the air-lung interface is an important step for sPLA2-II synthesis in macrophages.

Expression of the type-II phospholipase A2 in alveolar macrophages. Down-regulation by an inflammatory signal

We have shown previously that guinea pig alveolar macrophages (AM) synthesize a secretory phospholipase A2 (PLA2) during in vitro incubation. Here, we report the molecular cloning of this enzyme and show that it has structural features closely related to all known mammalian type-II PLA2. The mRNA and PLA2 activity were undetectable in freshly collected AM, but their levels increased dramatically to reach maximal values after 16 h of culture. Thereafter, the PLA2 activity remained constant with a parallel secretion in the medium, in contrast to mRNA level which returned to near basal values after 32 h. Incubation of AM for 16 h with the inflammatory secretagogue peptide f-Met-Leu-Phe (fMLP) markedly reduced the PLA2 activity and mRNA levels. This inhibition was prevented by preexposure of AM to pertussis toxin, an inhibitor of G-protein. In contrast, when AM were first cultured for 16 h and then incubated with fMLP, no significant change was observed in their PLA2 activity. In conditions where the type-II PLA2 was completely abrogated by fMLP, the latter did not alter the lipopolysaccharide-induced accumulation of tumor necrosis factor alpha mRNA or the release of arachidonic acid induced by the subsequent addition of the calcium ionophore A23187. These studies show that the inflammatory peptide fMLP down-regulates the expression of the type-II PLA2 by AM through a process mediated by G-protein. A possible negative control of the type-II PLA2 expression during AM activation is suggested.

Platelet secretory phospholipase A2 fails to induce rabbit platelet activation and to release arachidonic acid in contrast with venom phospholipases A2

The ability of platelet secretory phospholipase A2 (sPLA2) to induce platelet activation was investigated. sPLA2 (group II) contained in an activated platelet supernatant, as well as high concentrations of purified recombinant platelet sPLA2, failed to induce platelet activation. Furthermore, sPLA2 did not modify platelet activation induced by various agonists. The possible relationship between the failure of this enzyme to induce platelet activation and its origin (mammalian) or its structural group (group II) was then investigated, using pancreatic PLA2s (group I) and venom PLA2s from groups I, II and III. All venom PLA2s induced platelet activation that was accompanied by the liberation of arachidonic acid and was abolished by aspirin. In contrast, as observed for platelet sPLA2, enzymes from hog or bovine pancreas were unable to induce platelet activation even when used at high concentrations. Interestingly, PLA2 able to induce platelet activation efficiently hydrolyse phosphatidylcholine, while those inactive on platelets did not. Taken together, these results suggest that the catalytic activity of added PLA2 is necessary but not sufficient to induce platelet activation. Moreover, the ability of PLA2 to induce platelet activation is not related to its structural group (I, II, III) but rather to its origin (venom vs. mammalian) and capacity to hydrolyse phosphatidylcholine, the major phospholipid of the outer leaflet of the plasma membrane.

Excretion of platelet activating factor-acetylhydrolase and phospholipase A2 into nasal fluids after allergenic challenge: possible role in the regulation of platelet activating factor release

Platelet activating factor (PAF), a proinflammatory mediator synthesized through a phospholipase A2 (PLA2)-dependent reaction, is hydrolyzed into its inactive metabolite, lyso-PAF, by a specific acetylhydrolase. Previous studies have shown that allergen challenge of patients with allergic rhinitis leads to an increase of the concentrations of lyso-PAF in nasal lavage fluid (NLF), whereas PAF is detected only marginally. PAF-hydrolyzing enzymes are expected to be released on allergenic challenge, to account for the reduced concentrations of PAF in NLF. Here, we show that allergen challenge of patients with allergic rhinitis induces an increase of acetylhydrolase-like activity in NLF, which peaks within 10 minutes and returns to basal values 1 hour later. Acetylhydrolase hydrolyzed exogenous PAF with a complete loss of its ability to induce platelet aggregation. Allergen challenge also led to a parallel release of a PLA2 in nasal fluids. This enzyme preferentially hydrolyzes negatively charged phospholipids (phosphatidic acid monomethyl ester and phosphatidylgylcerol) versus phosphatidylcholine. More interestingly, the hydrolysis of phosphatidic acid monomethyl ester and phosphatidylglycerol by NLF was completely abolished by the addition of ethylenediaminetetraacetic acid which had no effect on the hydrolysis of PAF, indicating that the PLA2 secreted in nasal fluids is not involved in the degradation of PAF. Finally, our results show that allergen-induced increase in the concentrations of lyso-PAF and prostaglandin D2 occurred with a kinetic similar to that of tosyl-L-arginine-methyl-ester esterase, suggesting that mast cells are implicated in this process. Although no direct relationship was demonstrated between the absence of PAF and the increase of acetylhydrolase levels in NLF, we suggest a potential role for this enzyme in the inactivation of PAF if the latter is released in the nasal lumen.

Increased synthesis and secretion of a 14-kDa phospholipase A2 by guinea pig alveolar macrophages. Dissociation from arachidonic acid liberation and modulation by dexamethasone

The occurrence of a 14-kDa secretory phospholipase A2 (PLA2) in guinea pig alveolar macrophages (AM) and its relationship with the release of arachidonic acid (AA) were investigated. Freshly collected AM showed no detectable PLA2 activity as measured by the in vitro hydrolysis of phosphatidic acid. However, the PLA2 activity increased progressively when AM were maintained in culture to reach a level 60- to 100-fold greater than basal values within 20 h, with a parallel secretion into the incubation medium. By contrast, the activities of other phospholipid-hydrolyzing enzymes (platelet-activating factor acetylhydrolase and lysophospholipase) were modified only marginally. Both intra- and extracellular increases of PLA2 activity were abrogated with actinomycin D or cycloheximide. The enhanced PLA2 activity preferentially hydrolyzed negatively charged phospholipids in the order phosphatidic acid > phosphatidylglycerol > phosphatidylethanolamine > phosphatidylcholine, had an optimum pH of 7.5, and required a millimolar Ca2+ concentration for optimal activity and an apparent molecular mass of 14 kDa. Taken together, these results suggest that cultured AM elaborate an enzyme similar to the group II PLA2. On the other hand, our results show that AM hydrolyzed exogenous 2-arachidonoyl phosphatidylcholine and released AA and metabolites on FMLP stimulation. However, in contrast to the increase observed in the activity of the 14-kDa PLA2, the enzymatic activity involved in the hydrolysis of 2-arachidonoyl phosphatidylcholine and AA release remained constant with the culture duration of AM. Finally, dexamethasone markedly inhibited the increase of PLA2 activity, but only marginally inhibited the release of AA and metabolites from FMLP-stimulated AM. We conclude that guinea pig AM elaborate a 14-kDa PLA2 similar to the group II PLA2 through RNA- and protein synthesis-dependent processes. This elaboration appears to be induced by the adhesion of AM and is clearly dissociated from the liberation of AA.

Increased synthesis and secretion of a 14-kDa phospholipase A2 by guinea pig alveolar macrophages. Dissociation from arachidonic acid liberation and modulation by dexamethasone

The occurrence of a 14-kDa secretory phospholipase A2 (PLA2) in guinea pig alveolar macrophages (AM) and its relationship with the release of arachidonic acid (AA) were investigated. Freshly collected AM showed no detectable PLA2 activity as measured by the in vitro hydrolysis of phosphatidic acid. However, the PLA2 activity increased progressively when AM were maintained in culture to reach a level 60- to 100-fold greater than basal values within 20 h, with a parallel secretion into the incubation medium. By contrast, the activities of other phospholipid-hydrolyzing enzymes (platelet-activating factor acetylhydrolase and lysophospholipase) were modified only marginally. Both intra- and extracellular increases of PLA2 activity were abrogated with actinomycin D or cycloheximide. The enhanced PLA2 activity preferentially hydrolyzed negatively charged phospholipids in the order phosphatidic acid > phosphatidylglycerol > phosphatidylethanolamine > phosphatidylcholine, had an optimum pH of 7.5, and required a millimolar Ca2+ concentration for optimal activity and an apparent molecular mass of 14 kDa. Taken together, these results suggest that cultured AM elaborate an enzyme similar to the group II PLA2. On the other hand, our results show that AM hydrolyzed exogenous 2-arachidonoyl phosphatidylcholine and released AA and metabolites on FMLP stimulation. However, in contrast to the increase observed in the activity of the 14-kDa PLA2, the enzymatic activity involved in the hydrolysis of 2-arachidonoyl phosphatidylcholine and AA release remained constant with the culture duration of AM. Finally, dexamethasone markedly inhibited the increase of PLA2 activity, but only marginally inhibited the release of AA and metabolites from FMLP-stimulated AM. We conclude that guinea pig AM elaborate a 14-kDa PLA2 similar to the group II PLA2 through RNA- and protein synthesis-dependent processes. This elaboration appears to be induced by the adhesion of AM and is clearly dissociated from the liberation of AA.

Expression of annexin I, II, V, and VI by rat osteoblasts in primary culture: stimulation of annexin I expression by dexamethasone

To determine whether rat osteoblasts synthesize proteins of the annexin family and to evaluate the extent to which glucocorticoids modulate the expression of annexins by these cells, osteoblasts were grown in primary cultures in the absence or presence of dexamethasone, and the expression of annexins was evaluated by immunoblotting using polyclonal antibodies against human annexins. Four different annexins (I, II, V, and VI) were found to be expressed by rat osteoblasts. The expression of annexin I, but not the other annexins studied, was increased in osteoblasts cultured in the presence of dexamethasone (173 +/- 33% increase comparing untreated cells and cells treated for 10 days with 5 x 10(-7) M dexamethasone). Increased expression of annexin I was observed after the third day of exposure to dexamethasone and rose thereafter until day 10; annexin I expression increased with dexamethasone concentrations above 10(-10) M throughout the range of concentrations studied. The increase in annexin I protein was associated with an increase in annexin I mRNA and was completely blocked by the concomitant addition of the glucocorticoid receptor antagonist RU 38486. The increase in annexin I content following dexamethasone treatment was associated with an increase in alkaline phosphatase activity and PTH-induced cAMP stimulation, whereas phospholipase A2 activity in the culture medium was reduced to undetectable levels. The finding that four annexins are expressed in rat osteoblasts in primary culture raises the possibility that these proteins could play an important role in bone formation by virtue of their ability to bind calcium and phospholipids, serve as Ca2+ channels, interact with cytoskeletal elements, and/or regulate phospholipase A2 activity. In addition, the dexamethasone-induced increase in annexin I may represent a mechanism by which glucocorticoids modify osteoblast function.

Inhibition of phospholipase A2 activity in guinea pig eosinophils by human recombinant IL-1 beta

The effect of human rIL-1 beta on the release of arachidonic acid (AA) and on the phospholipase A2 (PLA2) activity in guinea pig eosinophils was investigated. Stimulation of [3H]AA-labeled eosinophils with the ionophore A23187 resulted in a time and concentration-dependent release of AA in parallel to hydrolysis of endogenous phosphatidylcholine (PC). Both events were abrogated by the chelation of intracellular free calcium, but not by its depletion from the medium, suggesting that the ionophore-induced AA release involves a PLA2 activity dependent on the mobilization of intracellular calcium. Addition of human rIL-1 beta (0.01 to 100 ng/ml) to eosinophils for 15 min had no effect on the release of AA induced by the ionophore. However, prolonged incubation with human rIL-1 beta (30 to 180 min) inhibited in a concentration- and time-dependent manner the release of AA and the hydrolysis of phosphatidylcholine in ionophore-stimulated eosinophils. Our results also showed that eosinophil homogenates contain a calcium-dependent PLA2 whose activity was markedly reduced when eosinophils were pretreated with human rIL-1 beta. The inhibition was time and concentration dependent and was observed in the presence of calcium and phospholipid excess. Finally, studies with Fura-2-loaded eosinophils showed that the ionophore A23187 stimulated an increase in intracellular calcium concentration that was not altered by pretreating the eosinophils with human rIL-1 beta. These results suggest that human rIL-1 beta inhibits the release of AA by eosinophils via the inhibition of a PLA2 activity and through a calcium-independent mechanism. Inhibition by human rIL-1 beta required a prolonged incubation (30 to 180 min) and was observed after its removal from the medium, suggesting that human rIL-1 beta did not interact directly with the PLA2 itself, but with a metabolic process involved with the regulation of its activity in eosinophils.

Inhibition of phospholipase A2 activity in guinea pig eosinophils by human recombinant IL-1 beta

The effect of human rIL-1 beta on the release of arachidonic acid (AA) and on the phospholipase A2 (PLA2) activity in guinea pig eosinophils was investigated. Stimulation of [3H]AA-labeled eosinophils with the ionophore A23187 resulted in a time and concentration-dependent release of AA in parallel to hydrolysis of endogenous phosphatidylcholine (PC). Both events were abrogated by the chelation of intracellular free calcium, but not by its depletion from the medium, suggesting that the ionophore-induced AA release involves a PLA2 activity dependent on the mobilization of intracellular calcium. Addition of human rIL-1 beta (0.01 to 100 ng/ml) to eosinophils for 15 min had no effect on the release of AA induced by the ionophore. However, prolonged incubation with human rIL-1 beta (30 to 180 min) inhibited in a concentration- and time-dependent manner the release of AA and the hydrolysis of phosphatidylcholine in ionophore-stimulated eosinophils. Our results also showed that eosinophil homogenates contain a calcium-dependent PLA2 whose activity was markedly reduced when eosinophils were pretreated with human rIL-1 beta. The inhibition was time and concentration dependent and was observed in the presence of calcium and phospholipid excess. Finally, studies with Fura-2-loaded eosinophils showed that the ionophore A23187 stimulated an increase in intracellular calcium concentration that was not altered by pretreating the eosinophils with human rIL-1 beta. These results suggest that human rIL-1 beta inhibits the release of AA by eosinophils via the inhibition of a PLA2 activity and through a calcium-independent mechanism. Inhibition by human rIL-1 beta required a prolonged incubation (30 to 180 min) and was observed after its removal from the medium, suggesting that human rIL-1 beta did not interact directly with the PLA2 itself, but with a metabolic process involved with the regulation of its activity in eosinophils.

The metabolism of 1-acyl-PAF in rabbit platelets and its possible interaction with PAF

The metabolism of 1-acyl-2-acetyl-sn-glycero-3-phosphocholine (1-acyl-PAF), a naturally occurring analogue of platelet activating factor (PAF), was investigated in rabbit platelets. Our studies showed that 1-acyl-[3H]PAF (1-palmitoyl-2-acetyl-sn-glycero-3-phospho[N-methyl-3H]-choline) was converted by platelets into phosphatidyl-[3H]choline [( 3H]PC) in a time-dependent fashion. The formation of [3H]PC occurred at a rate similar to that observed when lyso-[3H]PC (palmitoyl-sn-glycero-3-phospho[N-methyl-3H]choline) was used as substrate. In addition, a time-dependent increase in the level of water-soluble radioactivity was observed during the incubation of platelets with either 1-acyl-[3H]PAF or lyso-[3H]PC. This increase was parallel to the formation of [3H]PC and was not observed in the presence of [14C]PAF (1-octadecyl-2-acetyl-sn-glycero-3-phospho[N methyl-14C]-choline). Analysis by thin-layer chromatography showed that the soluble radioactivity was mainly associated with glycerophosphocholine (GPC). On the other hand, the preincubation of platelets with phenylmethylsulfonyl fluoride, an inhibitor of the acetylhydrolase, reduced the hydrolysis of 1-acyl-[3H]PAF to [3H]GPC with a concomitant accumulation of radioactivity in 1-acyl-PAF. These findings suggest that 1-acyl-PAF is converted into PC through deacetylation-reacylation with lysoPC as an obligatory intermediate. The findings also indicate that the lysoPC resulting from 1-acyl-PAF is either reacylated to phosphatidylcholine (PC) or hydrolyzed to GPC by lysophospholipase. Finally, we showed that the stimulation of platelets with PAF led to a time- and concentration-dependent increase in the conversion of 1-acyl-[3H]PAF to [3H]PC.(ABSTRACT TRUNCATED AT 250 WORDS)

Gamma-thrombin-induced phospholipase A2 activation in rabbit platelets: comparison with alpha-thrombin

gamma-Thrombin stimulated release of [3H]arachidonic acid ([3H]AA) accompanied by a significant production of PAF and lyso-PAF by rabbit platelets. These responses, which reflect PLA2 activation, were observed after a prolonged lag and to a lower extent when compared to those induced by alpha-thrombin which evoked a much higher elevation in intracellular calcium. This elevation together with [3H]AA release were markedly reduced by EDTA. However, addition of ionophore A23187 enhanced the release of [3H]AA by gamma-thrombin to the levels similar to those of alpha-thrombin. We conclude that gamma-thrombin is able to activate PLA2 and suggest that calcium influx may be a limiting factor for this activation.

Selective inhibition of adrenaline-induced human platelet aggregation by the structurally related Paf antagonist Ro 19-3704

1. Two non-lipid antagonists of platelet-activating factor acether (Paf), BN 52021 and WEB 2086, at concentrations which completely blocked Paf-induced platelet aggregation, failed to interfere with aggregation by adrenaline. In contrast, Ro 19-3704, a structurally related antagonist of Paf, inhibited concentration-dependently aggregation induced by adrenaline or by the simultaneous addition of submaximal concentrations of adrenaline and Paf. Reversal of aggregation was obtained when Ro 19-3704 was added to the platelet suspension after adrenaline. 2. Ro 19-3704 was selective for Paf and adrenaline since it failed to interfere with platelet aggregation induced by arachidonic acid or ADP. CV-3988, an antagonist of Paf structurally similar to Ro 19-3704, also inhibited adrenaline-induced aggregation. However, a morpholine analogue (MA) of Paf, which has no anti-Paf activity, failed to interfere with the aggregation induced by adrenaline. This suggests that the effect of Ro 19-3704 and CV-3988 on adrenaline is not simply due to their lipid structure. 3. Experiments on plasma membrane preparations showed that Ro 19-3704 inhibited [3H]-yohimbine binding with an inhibition constant (Ki) of 7 +/- 3 microM. In contrast, BN 52021 and MA did not interfere with [3H]-yohimbine binding. Equilibrium binding experiments showed that Ro 19-3704 increased the apparent KD of [3H]-yohimbine binding from 2.02 +/- 0.15 to 7.3 +/- 0.4 nM. The Paf antagonist Ro 19-3704 interacts specifically with the alpha 2-adrenoceptor and may thus prevent the early steps involved in the mechanism of adrenaline-induced platelet activation.

The role of Ca2+ in regulating the catabolism of PAF-acether (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) in rabbit platelets

In the present study we have investigated the effect of changes in the concentration of cytosolic free Ca2+ ([Ca2+]i) on the deacetylation-reacylation of PAF-acether (alkylacetylglycerophosphocholine, alkylacetyl-GPC) by rabbit platelets. Washed platelets were incubated with alkyl[3H]acetyl-GPC ([3H]acetyl-PAF) or [3H]alkylacetyl-GPC ([3H]alkyl-PAF) and [Ca2+]i was subsequently elevated by the addition of the ionophore A23187 or thrombin. The catabolism of PAF-acether was studied by measuring the release of [3H]acetate or the formation of [3H]alkylacyl-GPC. The ionophore inhibited the release of [3H]acetate and the formation of [3H]alkylacyl-GPC with no accumulation of lyso-[3H]PAF, indicating that the deacetylation of PAF-acether was blocked. The effect of ionophore on the deacetylation of PAF-acether was parallel with the increase of [Ca2+]i and could be reversed by the addition of EGTA. In contrast with the prolonged inhibition evoked by ionophore, thrombin, which induced a transient elevation of [Ca2+]i, merely delayed the deacetylation of PAF-acether. Since intact platelets failed to convert exogenous lyso-PAF, the effect of Ca2+ on its acylation was investigated by using platelet homogenates. These experiments showed that the acylation of lyso-PAF was inhibited by the exogenously added Ca2+, with a maximum effect at 1 mM. When the formation of endogenous lyso-PAF from the labelled pool of alkylacyl-GPC was examined, a prolonged increase in the concentration of lyso-PAF with a parallel and equally prolonged decrease in the cellular level of alkylacyl-GPC were observed after the addition of ionophore to intact platelets. The addition of EGTA reversed the effect of ionophore, thus permitting reacylation of lyso-PAF. In contrast, only a transient change in the level of lyso-PAF and alkylacyl-GPC was evoked by the addition of thrombin. Therefore we conclude that the inhibitory effect of Ca2+ on the deacetylation-reacylation of PAF-acether may have an important role in the regulation of its biosynthesis.