Nerve growth factor-induced up-regulation of cytosolic phospholipase A2alpha level in rat PC12 cells

Nerve growth factor (NGF) regulates various types of gene transcription in neurons. One of the cytosolic phospholipase A(2)s, cPLA(2)alpha, which preferentially cleaves phospholipids at the sn-2 position to arachidonic acid (AA), is involved in neuronal responses including survival. We investigated the effect of NGF on cPLA(2)alpha expression and its signaling pathways in PC12 cells, which differentiate into neuronal-like cells with neurites by NGF treatment. Treatment with NGF increased cPLA(2)alpha mRNA level after 4h and its protein level 24h after NGF addition. The NGF-induced increase in cPLA(2)alpha mRNA was inhibited by actinomycin D. NGF caused phosphorylation of mitogen-activated protein kinases (MAPKs); sustained phosphorylation of extracellular-regulated kinases (ERK1/2) and transient phosphorylation of p38 MAPK. NGF responses (cPLA(2)alpha mRNA and its protein) were inhibited by selective inhibitors for the ERK1/2 pathway, p38 MAPK and c-Jun NH(2)-terminal kinase. Epidermal growth factor, which transiently activates ERK1/2, did not modify cPLA(2)alpha expression. Although phorbol 12-myristate 13-acetate, an activator of protein kinase C (PKC), alone showed no effect, NGF-induced cPLA(2)alpha mRNA expression decreased due to the inhibition of PKC. These findings suggest that NGF-induced cPLA(2)alpha expression is regulated by gene transcription via the ERK1/2, p38 MAPK and PKC pathways in PC12 cells.

[Profiles of the expression of phospholipase A2 family mRNAs by macrophages infected with Mycobacterium tuberculosis]

Previously, we found that collaboration of reactive nitrogen intermediates (RNI) and free fatty acids (FFA) plays crucial roles in the expression of macrophage (Mphi) antimicrobial activity against Mycobacterium tuberculosis (MTB). Phospholipase A2 (PLA2) families hydrolyze phospholipids and release FFA from their sn-2 position. In the present study, we examined profiles of the mRNA expression of PLA2 families by Mphis stimulated with MTB by using RT-PCR method, and the following results were obtained. First, the expression of type IV cytosolic PLA2 (cPLA2), which is highly specific to arachidnic acid moiety, was significantly up-regulated by MTB stimulation of Mphis. Second, the expression of type IIa secretory PLA2 (sPLA2) was not observed for Mphis with or without MTB stimulation. Third, the profile of mRNA expression of type V sPLA2 was nearly the same as that of type IV cPLA2 for Mphis before and after MTB stimulation. These findings suggest that type IV cPLA2 and type V sPLA2 both play important roles in the FFA-mediated Mphi Cap antimicrobial activity against MTB organisms.

Ceramide-induced enhancement of secretory phospholipase A2 expression via generation of reactive oxygen species in tumor necrosis factor-α-stimulated mesangial cells

Since prostanoids such as prostaglandin E2 play a pivotal role in modulating renal function, we investigated the involvement of ceramide in expression of secretory phospholipase A2 (sPLA2) and cyclooxygenase-2 (COX-2) in tumor necrosis factor-alpha (TNF-alpha)-stimulated mesangial cells. TNF-alpha stimulation increased ceramide generation in parallel with a decrease in sphingomyelin. Pretreatment with exogenous sphingomyelinase (SMase) dose-dependently enhanced TNF-alpha-stimulated increases in COX-2 protein and sPLA) activity. SMase also augmented TNF-alpha-mediated nuclear factor kappaB (NF-kappaB) activation. N-acetylcysteine (NAC), an antioxidant, completely inhibited the SMase-induced increase in sPLA2 activity, whereas NAC inhibited partially the activity stimulated with TNF-alpha alone. Under the conditions, NAC completely inhibited reactive oxygen species (ROS) production induced by SMase followed by TNF-alpha. These results suggest that ceramide elicits up-regulation of NF-kappaB through ROS production, which, in turn, leads to stimulation of COX-2 and sPLA2 expression. Therefore, ceramide may be implicated in the pathogenesis of renal abnormalities.

Apoptosis of insulin-secreting cells induced by endoplasmic reticulum stress is amplified by overexpression of group VIA calcium-independent phospholipase A2 (iPLA2 beta) and suppressed by inhibition of iPLA2 beta

The death of insulin-secreting beta-cells that causes type I diabetes mellitus (DM) occurs in part by apoptosis, and apoptosis also contributes to progressive beta-cell dysfunction in type II DM. Recent reports indicate that ER stress-induced apoptosis contributes to beta-cell loss in diabetes. Agents that deplete ER calcium levels induce beta-cell apoptosis by a process that is independent of increases in [Ca(2+)](i). Here we report that the SERCA inhibitor thapsigargin induces apoptosis in INS-1 insulinoma cells and that this is inhibited by a bromoenol lactone (BEL) inhibitor of group VIA calcium-independent phospholipase A(2) (iPLA(2)beta). Overexpression of iPLA(2)beta amplifies thapsigargin-induced apoptosis of INS-1 cells, and this is also suppressed by BEL. The magnitude of thapsigargin-induced INS-1 cell apoptosis correlates with the level of iPLA(2)beta expression in various cell lines, and apoptosis is associated with stimulation of iPLA(2)beta activity, perinuclear accumulation of iPLA(2)beta protein and activity, and caspase-3-catalyzed cleavage of full-length 84 kDa iPLA(2)beta to a 62 kDa product that associates with nuclei. Thapsigargin also induces ceramide accumulation in INS-1 cells, and this response is amplified in cells that overexpress iPLA(2)beta. These findings indicate that iPLA(2)beta participates in ER stress-induced apoptosis, a pathway that promotes beta-cell death in diabetes.

Methylmercury stimulates arachidonic acid release and cytosolic phospholipase A2 expression in primary neuronal cultures

Cytosolic phospholipase A2 (cPLA2) plays an important role in the stimulus-dependent hydrolysis of sn-2 ester bond from membrane phospholipids, releasing arachidonic acid (AA), which along with its metabolites is involved in a number of regulatory functions. The present study examined the effect of methylmercury (MeHg; 0, 2.5, 5.0 microM) on cPLA2 activation in primary hippocampal neurons by assessing the release of 3H-AA. A significant increase in AA release was observed in cultures treated with 5 microM MeHg (10, 30, 60 and 120 min). This effect was due to neuronal cPLA2 activation, since it was completely abolished by arachidonyl trifluoromethyl ketone (AACOCF3), a specific inhibitor of cPLA2. Additional studies confirmed, by means of western blot analysis, that MeHg (5.0 and 10 microM; 16h) potently increases neuronal cPLA2 protein expression. These results suggest that cPLA2-stimulated hydrolysis and release of AA are potential mediators of MeHg-induced neurotoxicity.

Pancreatic islets and insulinoma cells express a novel isoform of group VIA phospholipase A2 (iPLA2 beta) that participates in glucose-stimulated insulin secretion and is not produced by alternate splicing of the iPLA2 beta transcript

Many cells express a group VIA 84 kDa phospholipase A(2) (iPLA(2)beta) that is sensitive to inhibition by a bromoenol lactone (BEL) suicide substrate. Inhibition of iPLA(2)beta in pancreatic islets and insulinoma cells suppresses, and overexpression of iPLA(2)beta in INS-1 insulinoma cells amplifies, glucose-stimulated insulin secretion, suggesting that iPLA(2)beta participates in secretion. Western blotting analyses reveal that glucose-responsive 832/13 INS-1 cells express essentially no 84 kDa iPLA(2)beta-immunoreactive protein but predominantly express a previously unrecognized immunoreactive iPLA(2)beta protein in the 70 kDa region that is not generated by a mechanism of alternate splicing of the iPLA(2)beta transcript. To determine if the 70 kDa-immunoreactive protein is a short isoform of iPLA(2)beta, protein from the 70 kDa region was digested with trypsin and analyzed by mass spectrometry. Such analyses reveal several peptides with masses and amino acid sequences that exactly match iPLA(2)beta tryptic peptides. Peptide sequences identified in the 70 kDa tryptic digest include iPLA(2)beta residues 7-53, suggesting that the N-terminus is preserved. We also report here that the 832/13 INS-1 cells express iPLA(2)beta catalytic activity and that BEL inhibits secretagogue-stimulated insulin secretion from these cells but not the incorporation of arachidonic acid into membrane PC pools of these cells. These observations suggest that the catalytic iPLA(2)beta activity expressed in 832/13 INS-1 cells is attributable to a short isoform of iPLA(2)beta and that this isoform participates in insulin secretory but not in membrane phospholipid remodeling pathways. Further, the finding that pancreatic islets also express predominantly a 70 kDa iPLA(2)beta-immunoreactive protein suggests that a signal transduction role of iPLA(2)beta in the native beta-cell might be attributable to a 70 kDa isoform of iPLA(2)beta.

Induction of distinct sets of secretory phospholipase A(2) in rodents during inflammation

Although the expression of the prototypic secretory phospholipase A(2) (sPLA(2)), group IIA (sPLA(2)-IIA), is known to be up-regulated during inflammation, it remains uncertain if other sPLA(2) enzymes display similar or distinct profiles of induction under pathological conditions. In this study, we investigated the expression of several sPLA(2)s in rodent inflammation models. In lipopolysaccharide (LPS)-treated mice, the expression of sPLA(2)-V, and to a lesser extent that of sPLA(2)-IID, -IIE, and -IIF, were increased, whereas that of sPLA(2)-X was rather constant, in distinct tissues. 12-O-Tetradecanoylphorbol-13-acetate (TPA)-induced mouse ear edema, in which the expression of sPLA(2)-IID, -IIF and -V was increased, was significantly reduced by YM-26734, a competitive sPLA(2)-IIA inhibitor that turned out to inhibit sPLA(2)-IID, -IIE, -V and -X as well. In contrast, sPLA(2)-IIA was dominant in carageenin-induced pleurisy in rats, where the accumulation of exudate fluids and leukocytes was significantly ameliorated by YM-26734. These results indicate that distinct sPLA(2)s can participate in inflammatory diseases according to tissues, animal species, and types of inflammation.

Purification of recombinant human cPLA2 gamma and identification of C-terminal farnesylation, proteolytic processing, and carboxymethylation by MALDI-TOF-TOF analysis

Cytosolic phospholipase A(2)gamma (cPLA(2)gamma) is a calcium-independent, membrane-associated phospholipase A(2) that possesses a C-terminal prenylation motif (-CCLA) whose covalent structure cannot be deduced from the primary sequence alone. Accordingly, we overexpressed human cPLA(2)gamma containing an N-terminal His tag ((His)(6)cPLA(2)gamma) in Sf9 cells and quantitatively solubilized and purified the enzyme by sequential immobilized metal affinity and Mono Q column chromatographies. The final preparation appeared as a single 61 kDa band after SDS-PAGE/silver-staining, possessed high lysophospholipase activity (50 micromol min(-1) mg(-1)), and was inhibited by, but did not hydrolyze, palmitoyl-CoA. Radiolabeling of recombinant human cPLA(2)gamma with [(3)H]-mevalonolactone in the absence of statins and subsequent cleavage of prenyl groups with Raney nickel revealed that the enzyme is only farnesylated and is not geranylgeranylated. Analysis of CNBr-digested cPLA(2)gamma by matrix-assisted laser desorption/ionization time-of-flight/time-of-flight (MALDI/TOF-TOF) mass spectrometry demonstrated the presence of a farnesyl moiety at Cys-538, cleavage of the Cys(538)-Cys(539) bond, and carboxymethylation of the resultant C-terminal prenylated cysteine. Collectively, these results describe the solubilization and purification of recombinant cPLA(2)gamma to homogeneity and identify cPLA(2)gamma as a farnesylated protein that undergoes at least three sequential posttranslational modifications that likely facilitate its targeting and interactions with its membrane substrates.

Expression of Cytosolic Phospholipase A2α in Murine C12 Cells, a Variant of L929 Cells, Induces Arachidonic Acid Release in Response to Phorbol Myristate Acetate and Ca2+ Ionophores, but Not to Tumor Necrosis Factor-α

Tumor necrosis factor-alpha (TNFalpha)-induced cell death is regulated through the release of arachidonic acid (AA) by group IVA cytosolic phospholipase A2 (cPLA2alpha) in the murine fibroblast cell line L929. However, the signaling pathway by which TNFalpha activates cPLA2alpha remained to be solved. We examined AA release in L929 cells, in a variant of L929 (C12 cells) lacking cPLA2alpha, and in C12 cells transfected with cPLA2alpha expression vectors. In transient and stable clones of C12 cells expressing cPLA2alpha, Ca2+ ionophore A23187 and phorbol myristate acetate (PMA) stimulated AA release within 90 min, although no response to TNFalpha was observed within 6 h. These results suggest that C12 cells may lack the components necessary for TNFalpha-induced AA release, in addition to cPLA2alpha. PMA is known to stimulate AA release via phosphorylation of Ser505 in cPLA2alpha by activating extracellular signal-regulated kinases (ERK1/2). However, PMA-induced AA release from C12 cells expressing mutant cPLA2alpha S505A (mutation of Ser505 to Ala), which is not phosphorylated by ERK1/2, was similar to that from L929 cells and C12 cells expressing wild-type cPLA2alpha. The role of Ser505 phosphorylation in AA release induced by PMA is also discussed.

Bile acids induce cyclooxygenase-2 expression in human pancreatic cancer cell lines

To investigate a possible link between bile acids and the pathogenesis of pancreatic cancer, we determined whether conjugated or unconjugated bile acids induced cyclooxygenase-2 (COX-2) in two human pancreatic cancer cell lines, BxPC-3 and SU 86.86. Bile acids are known promoters of gastric and colon cancer. We demonstrated previously that COX-2, an enzyme that catalyzes the synthesis of prostaglandins, is over-expressed in human pancreatic adenocarcinoma. Both human pancreatic cell lines were treated with conjugated and unconjugated bile acids. COX-2 mRNA and protein were determined. In addition, prostaglandin E2 (PGE2) synthesis was measured. Treatment with conjugated or unconjugated bile acids for 3 h up-regulated COX-2 mRNA. Chenodeoxycholate (CD) or deoxycholate at concentrations ranging from 12.5 to 100 micro M caused a dose-dependent induction of COX-2 protein with a maximal effect at 100 micro M. Induction of COX-2 protein by CD and deoxycholate was detected after treatment for 6 h with maximal induction at 12 h. Taurochenodeoxycholate, a conjugated bile acid, also caused dose-dependent induction of COX-2 but higher concentrations of bile acid (200-1200 micro M) were required. Levels of cyclooxygenase-1 were unaffected by bile acid treatment. Unconjugated and conjugated bile acids caused 7- and 4-fold increases in PGE2 production, respectively. Taken together, these findings suggest a possible role for bile acids in the pathogenesis of pancreatic cancer.

Oxysterol and 9-cis-retinoic acid stimulate the group IIA secretory phospholipase A2 gene in rat smooth-muscle cells

The inflammation that occurs during rheumatoid arthritis or atherosclerosis is characterized by the release of large amounts of sPLA(2) (group IIA secretory phospholipase A(2)). We have shown previously that the sPLA(2) promoter in SMC (smooth-muscle cells) is activated by interleukin-1beta and cAMP-signalling pathways, through the interplay of multiple transcription factors [Antonio, Brouillet, Janvier, Monne, Bereziat, Andreani, and Raymondjean (2002) Biochem. J. 368, 415-424]. In the present study, we have investigated the regulation of sPLA(2) gene expression in rat aortic SMCs by oxysterols. We found that oxysterol ligands that bind to the LXR (liver X receptor), including 25-HC (25-hydroxycholesterol) and 22( R )-HC, cause the accumulation of sPLA(2) mRNA and an increased enzyme activity. Transient transfection experiments demonstrated that the sPLA(2) promoter is synergistically activated by 22( R )-HC in combination with 9- cis -retinoic acid, a ligand for the LXR heterodimeric partner RXR (retinoid X receptor). Promoter activity was also increased in a sterol-responsive fashion when cells were co-transfected with LXRalpha/RXRalpha or LXRbeta/RXRalpha. Mutagenesis studies and gel mobility-shift assays revealed that LXR/RXR heterodimers regulate sPLA(2) transcription directly, by interacting with a degenerated LXRE (LXR response element) at position [-421/-406] of the sPLA(2) promoter. Chromatin immunoprecipitation revealed the in vivo occupancy of LXR on the sPLA(2) promoter. In addition, the orphan nuclear receptor LRH-1 (liver receptor homologue-1) potentiated the sterol-dependent regulation of the sPLA(2) promoter by binding to an identified promoter element (TCAAGGCTG). Finally, we have demonstrated that oxysterols act independent of interleukin-1beta and cAMP pathways to activate the sPLA(2) promoter. In the present study, we have identified a new pathway activating sPLA(2) gene expression in SMCs.

Localization of group V phospholipase A2 in caveolin-enriched granules in activated P388D1 macrophage-like cells

In murine P388D1 macrophages, the generation of prostaglandin E2 in response to long term stimulation by lipopolysaccharide involves the action of Group V secreted phospholipase A2 (PLA2), Group IV cytosolic PLA2 (cPLA2), and cyclooxygenase-2 (COX-2). There is an initial activation of cPLA2 that induces expression of Group V PLA2, which in turn induces both the expression of COX-2 and most of the arachidonic acid substrate for COX-2-dependent prostaglandin E2 generation. Because Group V PLA2 is a secreted enzyme, it has been assumed that after cellular stimulation, it must be released to the extracellular medium and re-associates with the outer membrane to release arachidonic acid from phospholipids. In the present study, confocal laser scanning microscopy experiments utilizing both immunofluorescence and green fluorescent protein-labeled Group V PLA2 shows that chronic exposure of the macrophages to lipopolysaccharide results in Group V PLA2 being associated with caveolin-2-containing granules close to the perinuclear region. Heparin, a cell-impermeable complex carbohydrate with high affinity for Group V PLA2, blocks that association, suggesting that the granules are formed by internalization of the Group V sPLA2 previously associated with the outer cellular surface. Localization of Group V PLA2 in perinuclear granules is not observed if the cells are treated with the Group IV PLA2 inhibitor methyl arachidonyl fluorophosphonate, confirming the important role for Group IV PLA2 in the activation process. Cellular staining with antibodies against COX-2 reveals the presence of COX-2-rich granules in close proximity to those containing Group V PLA2. Collectively, these results suggest that encapsulation of Group V PLA2 into granules brings the enzyme to the perinuclear envelope during cell activation where it may be closer to Group IV PLA2 and COX-2 for efficient prostaglandin synthesis.

Induction of astrocytic cytoplasmic phospholipase A2 and neuronal death after intracerebroventricular carrageenan injection, and neuroprotective effects of quinacrine

Glial reaction is often associated with nervous tissue injury, but thus far, few studies have examined whether it can be a cause of neuronal injury. We now study the effect of intracerebroventricular injection of a carrageenan on cytoplasmic phospholipase A(2) (cPLA(2)) expression and neuronal injury in the hippocampus. The enzyme cPLA(2) hydrolyzes neural membrane glycerophospholipids and generates precursors for proinflammatory mediators. An induction of cPLA(2) in astrocytes and death of neurons in the hippocampus were observed following glial reaction induced by intracerebroventricular injections of carrageenan. cPLA(2) levels and neuronal death were modulated by daily intraperitoneal injections of quinacrine, an inhibitor of phospholipase A(2) that can cross the blood brain barrier. These observations support a role for astrocytic cPLA(2) in mediating neuronal death.

Surfactant protein-A and phosphatidylglycerol suppress type IIA phospholipase A2 synthesis via nuclear factor-kappaB

We previously showed that surfactant inhibits the synthesis of type IIA secretory phospholipase A2 (sPLA2-IIA) by alveolar macrophages. These cells have been identified as the main source of this enzyme in an animal model of acute lung injury. The aim of the present study was to identify the surfactant components involved in the inhibition of sPLA2-IIA expression in alveolar macrophages and the signaling pathways that mediate this inhibition. Our results show that various surfactant preparations can inhibit sPLA2-IIA expression in endotoxin-stimulated alveolar macrophages. Both the surfactant protein (SP)-A and the surfactant phospholipid fraction inhibit this expression. The surfactant phospholipid dioleylphosphatidylglycerol (DOPG) abolishes sPLA2-IIA expression, whereas dipalmitoylphosphatidylcholine does not. Chromatographic analysis and confocal microscopy revealed that phosphatidylglycerol was rapidly incorporated and metabolized by alveolar macrophages and that its metabolites accumulate in the cytosol. Nuclear factor-kappaB (NF-kappaB) modulates sPLA2-IIA expression in endotoxin-activated alveolar macrophages, and surfactant preparations, surfactant phospholipid fraction, SP-A, and DOPG indeed suppressed NF-kappaB activation. In summary, our results show that SP-A and DOPG play a role in the surfactant-mediated inhibition of sPLA2-IIA expression in alveolar macrophages and that this inhibition occurs via a downregulation of NF-kappaB activation.

Phospholipase A2 expression in tumours: a target for therapeutic intervention?

Phospholipase A(2) (PLA(2)) enzymes are involved in lipid metabolism and, as such, are central to several cellular processes. The different PLA(2)s identified to date can be classified into three groups: secreted PLA(2) (sPLA(2)), calcium-independent PLA(2) (iPLA(2)) and calcium-dependent cytosolic PLA(2) (cPLA(2)). In addition to their role in cellular signalling, PLA(2)s have been implicated in diverse pathological conditions, including inflammation, tissue repair and cancer. Elevated levels of sPLA(2) and cPLA(2) have been reported in several tumour types. Here, we summarize the current views on the PLA(2)s, and look at their expression, role in human malignancy and potential as targets for anticancer drug development.

1-(2,3,4-trimethoxyphenyl)-3-(3-(2-chloroquinolinyl))-2-propen-1-one, a chalcone derivative with analgesic, anti-inflammatory and immunomodulatory properties

OBJECTIVE AND DESIGN

The synthetic chalcone derivative 1-(2,3,4-trimethoxyphenyl)-3-(3-(2-chloroquinolinyl))-2-propen-1-one (TQ) was evaluated for its immunomodulatory and anti-inflammatory efficacy in vitro and in vivo.

MATERIAL AND SUBJECTS

Human neutrophils and lymphocytes from healthy volunteers and RAW 264.7 murine macrophages. Swiss mice and Lewis rats were randomly divided into groups of six animals.

TREATMENT

TQ was orally administered in all in vivo assays (10-30 mg/kg).

METHODS

Elastase, superoxide and LTB(4) release were assayed in human neutrophils, NO/PGE(2) production and NF-kappaB activation in RAW 264.7, and (3)H thymidine incorporation in human lymphocytes. Zymosan-stimulated air pouches, DNFB-DTH, PBQ-induced writhings and formalin-induced pain were assayed in mice. Adjuvant-induced arthritis was tested in rats. Dunnett's t-test was employed for statistical analysis.

RESULTS

Human T-cell proliferation, neutrophil functions and NO/PGE(2) production in murine macrophages were inhibited by TQ (IC(50) in the microM range), which showed anti-inflammatory, immunomodulatory and analgesic effects.

CONCLUSIONS

Our findings indicate the potential interest of TQ in the modulation of some immune and inflammatory responses probably by NF-kappaB inhibition.

[Study on the expression level of Annexin I, cPLA(2) and PCNA in cleft-palate mice induced by dexamethasone]

OBJECTIVE

To test the changing expression level of Annexin I, cPLA(2) and PCNA in the palatine process of cleft-palate mice A/J and C57B/6j induced by dexamethasone. To discuss the developing mechanism of cleft-palate and the corresponding preventive methods.

METHOD

s Pregnant mice A/J and C57BL/6j were randomly divided into normal group as blank control, group with deformity induced by dexamethasone, group given VitB(12) as antagonist to deforming factor, group given only VitB(12). The relative quantity of Annexin I, Cpla2 and PCNA were detected by immunoblotting.

RESULTS

Among mice A/J, with the development of palatine process, Annexin I's expression level was increased in the normal group, and other groups showed the similar changes. Annexin I's expression level was significantly higher in group DEX and group DEX + VitB(12) than in normal group and VitB(12) group of, while there was no significant difference between normal group and VitB(12) group. But the changes of cPLA(2) and PCNA expression level was in an opposite direction, with development it decreased in the normal mice's palatine process. In mice C57B/6j there was no significant difference between normal group and group DEX on the measured quantity of Annexin I, cPLA(2) and

CONCLUSIONS

Annexin I and cPLA(2) introduce glucocorticoid to induce cleft-palate. VitB(12) can not inhibit DEX's enhancing effect on the expression level of Annexin I, but it can antagonize DEX's inhibiting effect on expression level of cPLA(2), which is probably one of the mechanisms why VitB(12) antagonize glucocorticoid's deforming effect.

Neuronal damage by secretory phospholipase A2: modulation by cytosolic phospholipase A2, platelet-activating factor, and cyclooxygenase-2 in neuronal cells in culture

Activation of cytosolic phospholipase A(2) (cPLA(2)) is an early event in brain injury, which leads to the formation and accumulation of bioactive lipids: platelet-activating factor (PAF), free arachidonic acid, and eicosanoids. A cross-talk between secretory PLA(2) (sPLA(2)) and cPLA(2) in neural signal transduction has previously been suggested (J Biol Chem 271:32722; 1996). Here we show, using neuronal cell cultures, an up-regulation of cPLA(2) expression and an inhibition by the selective cPLA(2) inhibitor AACOCF3 after exposure to neurotoxic concentrations of sPLA(2)-OS2. Pretreatment of neuronal cultures with recombinant PAF acetylhydrolase (rPAF-AH) or the presynaptic PAF receptor antagonist, BN52021, partially blocked neuronal cell death induced by sPLA(2)-OS2. Furthermore, selective COX-2 inhibitors ameliorated sPLA(2)-OS2-induced neurotoxicity. We conclude that sPLA(2)-OS2 activates a neuronal signaling cascade that includes activation of cPLA(2), arachidonic acid release, PAF production, and induction of COX-2.

COX-2, inflammatory secreted PLA2, and cytoplasmic PLA2 protein expression in small bowel adenocarcinomas compared with colorectal adenocarcinomas

Cyclooxygenase-2 (COX-2), human synovial inflammatory secreted phospholipase A2 (sPLA2) and cytoplasmic phospholipase A2 (cPLA2) are involved in eicosanoid production and also seem to participate in colorectal tumorigenesis. As there are no data regarding these enzymes in human small bowel tumors, we wanted to determine whether they were involved in human small bowel tumorigenesis, and whether their expression was different in small bowel compared to colorectal adenocarcinomas, as suggested by animal studies. We studied their protein expression by immunohistochemistry in 25 small bowel adenocarcinomas and compared it to 48 colorectal adenocarcinomas. Seventy-six percent of the small bowel and 88% of the colorectal adenocarcinomas had a moderate or strong COX-2 expression. Sixty-eight percent of the small bowel and 67% of the colorectal adenocarcinomas had a moderate or strong sPLA2 expression. Forty-eight percent of the small bowel and 35% of the colorectal adenocarcinomas had a moderate or strong cPLA2 expression. In conclusion, the increased expression of COX-2, sPLA2, and sometimes cPLA2 in both small bowel and colorectal adenocarcinomas is in accordance with the likely eicosanoid involvement in tumor development. The same pattern of protein expression found in both types of adenocarcinoma contradicts experimental results in mice. Moreover, our results strengthen the similarities between these two types of human cancer.

Macrophage-expressed group IIA secretory phospholipase A2 increases atherosclerotic lesion formation in LDL receptor-deficient mice

OBJECTIVE

Transgenic mice expressing human group IIA secretory phospholipase A(2) (group IIA sPLA(2)) spontaneously develop atherosclerotic lesions. The mechanism for this proatherogenic effect is likely multifactorial, because HDL-cholesterol is significantly lower and LDL/VLDL cholesterol is slightly higher in transgenic mice compared with nontransgenic littermates. In the present study, we show for the first time that elicited peritoneal macrophages from transgenic mice express human group IIA sPLA(2). This study tested whether macrophage-expressed sPLA(2) contributes to atherogenesis.

METHODS AND RESULTS

Bone marrow cells from either sPLA(2) transgenic mice or control C57BL/6 mice were transplanted into LDL receptor-deficient mice. After hematopoietic engraftment, animals were fed a diet enriched with saturated fat and cholesterol for 12 weeks. Despite a lack of effect on serum lipoprotein concentrations, the presence of bone marrow-derived cells expressing human group IIA sPLA(2) resulted in a significant increase in the extent of atherosclerosis in the aortic arch (12.8+/-1.4% versus 7.4+/-0.9%; P<0.005) and aortic sinus (0.3+/-0.03 mm(2) versus 0.2+/-0.04 mm(2); P<0.05).

CONCLUSIONS

Group IIA sPLA(2) can contribute to atherosclerotic lesion development through a mechanism that is independent of systemic lipoprotein metabolism.

Mechanisms of enhanced macrophage-mediated prostaglandin E2 production and its suppressive role in Th1 activation in Th2-dominant BALB/c mice

PGE(2) has been known to suppress Th1 responses. We studied the difference in strains of mice in PGE(2) production by macrophages and its relation to Th1 activation. Macrophages from BALB/c mice produced greater amounts of PGE(2) than those from any other strains of mice, including C57BL/6, after LPS stimulation. In accordance with the amount of PGE(2) produced, macrophage-derived IL-12 and T cell-derived IFN-gamma production were more strongly suppressed in BALB/c macrophages than in C57BL/6 macrophages. When macrophages were treated with indomethacin or EP4 antagonist, Th1 cytokines were more markedly increased in cells from BALB/c mice than in those from C57BL/6 mice. Although cyclooxygenase-2 was expressed similarly after LPS stimulation in these mouse strains, the release of arachidonic acid and the expression of type V secretory phospholipase A(2) mRNA were greater in BALB/c macrophages. However, exogenous addition of arachidonic acid did not reverse the lower production of PGE(2) by C57BL/6 macrophages. The expression of microsomal PGE synthase, a final enzyme of PGE(2) synthesis, was also greater in BALB/c macrophages. These results indicate that the greater production of PGE(2) by macrophages, which is regulated by secretory phospholipase A(2) and microsomal PGE synthase but not by cyclooxygenase-2, is related to the suppression of Th1 cytokine production in BALB/c mice.

Differential expression of platelet-activating factor acetylhydrolase in macrophages and monocyte-derived dendritic cells

Although macrophages (Mphi) and monocyte-derived dendritic cells (MDDC) come from a common precursor, they are distinct cell types. This report compares the two cell types with respect to the metabolism of platelet-activating factor (PAF), a biologically active lipid mediator. These experiments were prompted by our studies of localized juvenile periodontitis, a disease associated with high IgG2 production and a propensity of monocytes to differentiate into MDDC. As the IgG2 Ab response is dependent on PAF, and MDDC selectively induce IgG2 production, we predicted that PAF levels would be higher in MDDC than in Mphi. To test this hypothesis, human MDDC were prepared by treating adherent monocytes with IL-4 and GM-CSF, and Mphi were produced by culture in M-CSF. Both Mphi and MDDC synthesized PAF; however, MDDC accumulated significantly more of this lipid. We considered the possibility that PAF accumulation in MDDC might result from reduced turnover due to lower levels of PAF acetylhydrolase (PAFAH), the enzyme that catabolizes PAF. Although PAFAH increased when monocytes differentiated into either cell type, MDDC contained significantly less PAFAH than did Mphi and secreted almost no PAFAH activity. The reduced levels of PAFAH in MDDC could be attributed to lower levels of expression of the enzyme in MDDC and allowed these cells to produce PGE(2) in response to exogenous PAF. In contrast, Mphi did not respond in this manner. Together, these data indicate that PAF metabolism may impinge on regulation of the immune response by regulating the accessory activity of MDDC.

Prostaglandins inhibit 5-lipoxygenase-activating protein expression and leukotriene B4 production from dendritic cells via an IL-10-dependent mechanism

PGs produced from arachidonic acid by the action of cyclooxygenase enzymes play a pivotal role in the regulation of both inflammatory and immune responses. Because leukotriene B4 (LTB4), a product of 5-lipoxygenase (5-LO) pathway, can exert numerous immunoregulatory and proinflammatory activities, we examined the effects of PGs on LTB4 release from dendritic cells (DC) and from peritoneal macrophages. In concentration-dependent manner, PGE1 and PGE2 inhibited the production of LTB4 from DC, but not from peritoneal macrophage, with an IC50 of 0.04 microM. The same effect was observed with MK-886, a 5-LO-activating protein (FLAP)-specific inhibitor. The decreased release of LTB4 was associated with an enhanced level of IL-10. Furthermore, the inhibition of LTB4 synthesis by PGs was significantly reversed by anti-IL-10, suggesting the involvement of an IL-10-dependent mechanism. Hence, we examined the effects of exogenous IL-10 on the 5-LO pathway. We demonstrate that IL-10 suppresses the production of LTB4 from DC by inhibiting FLAP protein expression without any effect on 5-LO and cytosolic phospholipase A2. Taken together, our results suggest links between DC cyclooxygenase and 5-LO pathways during the inflammatory response, and FLAP is a key target for the PG-induced IL-10-suppressive effects.

Bimodal regulatory effect of melittin and phospholipase A2-activating protein on human type II secretory phospholipase A2

Melittin and phospholipase A2-activating protein (PLAP) are known as efficient activators of secretory phospholipase A2(sPLA2) types I, II, and III when phospholipid liposomes are used as substrate. The present study demonstrates that both peptides can either inhibit or activate sPLA2 depending on the peptide/phospholipid ratio when erythrocyte membranes serve as a biologically relevant substrate. Low concentrations of melittin and PLAP were observed to inhibit sPLA2-triggered release of fatty acids from erythrocyte membranes. The inhibition was reversed at melittin concentrations above 1 microM. PLAP-induced inhibition of sPLA2 persisted steadily throughout the used concentration range (0-150 nM). The two peptides induced a dose-dependent activation of sPLA2 at low concentrations, followed by inhibition when model membranes were used as substrate. This opposite modulatory effect on biological membranes and model membranes is discussed with respect to different mechanisms the interaction of the regulatory peptides with the enzyme molecules and the substrate vesicles.