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

Inhibitors of cyclo-oxygenase-2 and secretory phospholipase A2 preserve bone architecture following ovariectomy in adult rats

Epidemiological evidence and in vitro data suggest that COX-2 is a key regulator of accelerated remodeling. Accelerated states of osteoblast and osteoclast activity are regulated by prostaglandins in vitro, but experimental evidence for specific roles of cyclooxygenase-2 (COX-2) and secretory phospholipase A2 (sPLA2) in activated states of remodeling in vivo is lacking. The aim of this study was to determine the effect of specific inhibitors of sPLA2-IIa and COX-2 on bone remodeling activated by estrogen deficiency in adult female rats. One hundred and twenty-four adult female Wistar rats were ovariectomized (OVX) or sham-operated. Rats commenced treatment 14 days after surgery with either vehicle, a COX-2 inhibitor (DFU at 0.02 mg/kg/day and 2.0 mg/kg/day) or a sPLA2-group-IIa inhibitor (KH064 at 0.4 mg/kg/day and 4.0 mg/kg/day). Treatment continued daily until rats were sacrificed at 70 days or 98 days post-OVX. The right tibiae were harvested, fixed and embedded in methylmethacrylate for structural histomorphometric bone analysis at the proximal tibial metaphysis. The specific COX-2 or sPLA2 inhibitors prevented ovariectomy-induced (OVX-induced) decreases in trabecular connectivity (P<0.05); suppressed the acceleration of bone resorption; and maintained bone turnover at SHAM levels following OVX in the rat. The sPLA2 inhibitor significantly suppressed increases in osteoclast surface induced by OVX (P<0.05), while the effect of COX-2 inhibition was less marked. These findings demonstrate that inhibitors of COX-2 and sPLA2-IIa can effectively suppress OVX-induced bone loss in the adult rat by conserving trabecular bone mass and architecture through reduced bone remodeling and decreased resorptive activity. Moreover, we report an important role of sPLA2-IIa in osteoclastogenesis that may be independent of the COX-2 metabolic pathway in the OVX rat in vivo.

Activation of phospholipase A2 is associated with generation of placental lipid signals and fetal obesity

CONTEXT

Obesity and diabetes during pregnancy are associated with increased insulin resistance and higher neonatal adiposity. In turn, insulin resistance triggers inflammatory pathways with accumulation of placental cytokines.

OBJECTIVE

To determine placental signals that translate into development of excess adipose tissue, we investigated the role of phospholipases A2 (PLA2) as targets of inflammatory mediators.

SETTING

The study was conducted at Case Western Reserve University, Department of Reproductive Biology.

SUBJECTS

Volunteers gave informed written consent in accordance with the Institutional Review Board guidelines. Placenta and cord blood samples were obtained at the time of elective cesarean section in 15 term pregnancies.

INTERVENTION

Neonatal anthropometric measurements were performed within 48 h of delivery. Placentas were grouped based on neonatal percentage body fat as obese (body fat > or = 16%) and lean control (body fat < or = 8%).

MAIN OUTCOME MEASURES

The primary outcomes were placenta PLA2 expression and fatty acid concentration.

RESULTS

Expression of PLA2G2A and PLA2G5, the main placenta phospholipases, was greater (P < 0.05) in placenta of obese compared with control neonates and was associated with increased 20:3 and 20:5 omega-3 polyunsaturated fatty acids. TNF-alpha and leptin content was increased 3-fold in placenta of obese neonates. TNF-alpha and leptin both induced a time-dependent activation of PLA2G2 and PLA2G5 in placental cells.

CONCLUSION

Accumulation of omega-3 fatty acids through secretory PLA2 activation is associated with high neonatal adiposity. We propose that the generation of placental lipid mediators through TNF-alpha and leptin stimulation represents a key mechanism to favor excess fetal fat accretion.

Comparative analysis of gene expression mechanisms between group IA and IB phospholipase A2 genes from sea snake Laticauda semifasciata

Phospholipase A(2) (PLA(2)) genes expressed in the venom glands of the sea snake, Laticauda semifasciata, were investigated. Both mRNAs, encoding group IA (without a pancreatic loop) and group IB (with pancreatic loop), were detected from venom glands by Northern blot hybridization analysis and RT-PCR. The results of quantitative PCR analysis indicated that the expression amount of group IA genes was around 100-300 times greater than that of group IB genes. Sequence analysis of 5'-upstream regions and a reporter gene assay of the genes (groups IA and IB) previously cloned showed that the functional sequence (411 bp) was inserted in the 5'-flanking region of the group IA PLA(2) genes. It seemed that the contribution of the inserted sequence to the amount of transcribed mRNAs was greater than that of number of genes present in the genome. Comparative analysis of the 5'-flanking sequences from several snake genes encoding toxic PLA(2)s revealed that this sequence was probably inserted into an ancestral gene of PLA(2) with a pancreatic loop. After the duplication of the gene, which contained the inserted sequence, the PLA(2) gene without a pancreatic loop evolved from one of the duplicate genes. This inserted sequence might determine the future of the genes expressed in the venom glands.

Interaction of low molecular weight group IIA phospholipase A2 with apoptotic human T cells: role of heparan sulfate proteoglycans

Human group IIA phospholipase A2 (hIIA PLA2) is a 14 kDa secreted enzyme associated with inflammatory diseases. A newly discovered property of hIIA PLA2 is the binding affinity for the heparan sulfate proteoglycan (HSPG) glypican-1. In this study, the binding of hIIA PLA2 to apoptotic human T cells was investigated. Little or no exogenous hIIA PLA2 bound to CD3-activated T cells but significant binding was measured on activated T cells induced to undergo apoptosis by anti-CD95. Binding to early apoptotic T cells was greater than to late apoptotic cells. The addition of heparin and the hydrolysis of HSPG by heparinase III only partially inhibited hIIA PLA2 binding to apoptotic cells, suggesting an interaction with both HSPG and other binding protein(s). Two low molecular weight HSPG were coimmunoprecipitated with hIIA PLA2 from apoptotic T cells, but not from living cells. Treatment of CD95-stimulated T cells with hIIA PLA2 resulted in the release of arachidonic acid but not oleic acid from cells and this release was blocked by heparin and heparinase III. Altogether, these results suggest a role for hIIA PLA2 in the release of arachidonic acid from apoptotic cells through interactions with HSPG and its potential implication in the progression of inflammatory diseases.

Biological effects of group IIA secreted phosholipase A(2)

Group IIA secreted phospholipase A(2) (sPLA(2)-IIA) is the most abundant element in human tissues of a large family of low molecular weight phospholipases A(2), which shows properties different from those displayed by the cytosolic phospholipase A(2) involved in the release of arachidonic acid. sPLA(2)-IIA behaves as a ligand for a group of receptors inside the C-type multilectin mannose receptor family and also interacts with heparan sulfate proteoglycans such as glypican, the dermatan/chondroitin sulfate-rich decorin, and the chondroitin sulfate-rich versican, thus being able to internalize to specific compartments within the cell and producing biological responses. This review provides a short summary of the biological actions of sPLA(2)-IIA on intracellular signaling pathways.

Presence of the M-type sPLA(2) receptor on neutrophils and its role in elastase release and adhesion

Secretory phospholipase A(2) (sPLA(2)) produces lipids that stimulate polymorphonuclear neutrophils (PMNs). With the discovery of sPLA(2) receptors (sPLA(2)-R), we hypothesize that sPLA(2) stimulates PMNs through a receptor. Scatchard analysis was used to determine the presence of a sPLA(2) ligand. Lysates were probed with an antibody to the M-type sPLA(2)-R, and the immunoreactivity was localized. PMNs were treated with active and inactive (+EGTA) sPLA(2) (1-100 units of enzyme activity/ml, types IA, IB, and IIA), and elastase release and PMN adhesion were measured. PMNs incubated with inactive, FITC-linked sPLA(2)-IB, but not sPLA(2)-IA, demonstrated the presence of a sPLA(2)-R with saturation at 2.77 fM and a K(d) of 167 pM. sPLA(2)-R immunoreactivity was present at 185 kDa and localized to the membrane. Inactive sPLA(2)-IB activated p38 MAPK, and p38 MAPK inhibition attenuated elastase release. Active sPLA(2)-IA caused elastase release, but inactive type IA did not. sPLA(2)-IB stimulated elastase release independent of activity; inactive sPLA(2)-IIA partially stimulated PMNs. sPLA(2)-IB and sPLA(2)-IIA caused PMN adhesion. We conclude that PMNs contain a membrane M-type sPLA(2)-R that activates p38 MAPK.

Phospholipase A2 receptor: a regulator of biological functions of secretory phospholipase A2

The phospholipase A2 receptor (PLA2R) is a type I transmembrane glycoprotein related to the C-type animal lectin family that includes the mannose receptor. PLA2R regulates a variety of biological responses elicited by specific types of secretory PLA2s (sPLA2s). Group IB sPLA2 (sPLA2-IB) acts as an endogenous PLA2R ligand to induce cell proliferation, cell migration, and lipid mediator production. Analysis of PLA2R-deficient mice has suggested a potential role of the sPLA2-IB/PLA2R pathway in the production of pro-inflammatory cytokines in endotoxic shock. PLA2R is also involved in the clearance of sPLA2s, including group X sPLA2 (sPLA2-X) and a particular type of snake venom sPLA2, and clearance suppresses their potent enzymatic activities. In the circulation, the soluble form of PLA2R is constitutively present as anendogenous inhibitor of sPLA2s. This review will focus on recent findings on the roles of PLA2R in regulating sPLA2 functions and summarize what is known about the otherbinding proteins for mammalian and snake venom sPLA2s.

Nucleotide sequence of phospholipase A(2) gene expressed in snake pancreas reveals the molecular evolution of toxic phospholipase A(2) genes

We have cloned two phospholipase A(2) (PLA(2)) DNA complementary to RNA that contained nucleotide sequences encoding pancreas loop by the reverse transcription-polymerase chain reaction cloning procedure using messenger RNA isolated from Laticauda semifasciata pancreas. Additionally, a gene clone encoding PLA(2) with the pancreatic loop sequence was isolated from a L. semifasciata genomic library. Subsequent sequence analysis revealed that PLA(2) clones encoding group IB" PLA(2). Comparative analysis of group IA and IB" PLA(2) genes revealed that the exon-intron organization is conserved in the genes of both groups. The invaded sequences in the second intron were very similar to those of the L. semifasciata group IA gene. This observation suggested that the integration of the invaded sequences occurred before the divergence of groups IA and IB" during the evolution of PLA(2) gene. The comparative analysis revealed that the arising of group IA PLA(2) occurred by the deletion and substitution of nucleotide sequences in exon III region during the process of accelerated evolution.

Axonemal dynein expression in human fetal tracheal epithelium

Ciliogenesis in human fetal airway epithelium occurs from 11 to 24 gestational weeks. Using genetic and antigenic markers specific for human axonemal dynein heavy chain 9, we characterized temporal aspects of axonemal dynein expression associated with large airway epithelial ciliogenesis during human fetal development. Late in the first trimester, an undifferentiated columnar epithelium is characteristic of the large airways, and immunocytochemical studies exhibited focal localization of axonemal dynein antigen on luminal epithelial cell borders at sites consistent with emergent ciliary beds. From 12 to 22 wk, immunocytochemical labeling of new ciliary beds was prominent, and localization within the cytoplasm of epithelial cells suggested avid synthesis of axonemal dynein in advance of ciliogenic events. Quantitative RT-PCR of tracheal RNA and in situ hybridization studies compared favorably with immunocytochemical findings with the earliest expression of axonemal dynein at 9-10 wk gestation. These studies have documented that axonemal dynein is expressed early in human fetal life during airway epithelial maturation and well before histological or ultrastructural evidence of ciliogenesis is apparent.

Enhancement of mast cell survival: a novel function of some secretory phospholipase A(2) isotypes

This study tested the hypothesis that certain secretory phospholipase A(2) (sPLA(2)) isotypes act in a cytokine-like fashion through cell surface receptors to influence mast cell survival. Initial experiments revealed that sPLA(2) activity and sPLA(2) receptor expression are increased, and mast cells lost their capacity to maintain membrane asymmetry upon cytokine depletion. Groups IB and III, but not group IIA PLA(2), prevented the loss of membrane asymmetry. Similarly, group IB prevented nucleosomal DNA fragmentation in mast cells. Providing putative products of sPLA(2) hydrolysis to cytokine-depleted mast cells did not influence survival. Furthermore, catalytic inactivation of sPLA(2) did not alter its capacity to prevent apoptosis. Inhibition of protein synthesis using cycloheximide or actinomycin reversed the antiapoptotic effect of sPLA(2). Additionally, both wild-type and catalytically inactive group IB PLA(2) induced IL-3 synthesis in mast cells. However, adding IL-3-neutralizing Ab did not change Annexin V(FITC) binding and only partially inhibited thymidine incorporation in sPLA(2)-supplemented mast cells. In contrast, IL-3-neutralizing Ab inhibited both Annexin V(FITC) binding and thymidine incorporation in mast cells maintained with IL-3. sPLA(2) enhanced phosphoinositide 3'-kinase activity, and a specific inhibitor of phosphoinositide 3'-kinase reversed the antiapoptotic effects of sPLA(2). Likewise, sPLA(2) increased the degradation of I-kappaBalpha, and specific inhibitors of nuclear factor kappa activation (NF-kappaB) reversed the antiapoptotic effects of sPLA(2). Together, these experiments reveal that certain isotypes of sPLA(2) enhance the survival of mast cells in a cytokine-like fashion by activating antiapoptotic signaling pathways independent of IL-3 and probably via sPLA(2) receptors rather than sPLA(2) catalytic products.

sPLA(2) cooperates with cPLA(2)alpha to regulate prostacyclin synthesis in human endothelial cells

The first step in prostacyclin (PGI(2)) synthesis involves the generation of arachidonic acid (AA) from membrane phospholipids mediated by the 85 kDa cytosolic phospholipase A(2) (cPLA(2)alpha). The current study examined the effects of secretory PLA(2)s (sPLA(2)s) on PGI(2) production by human umbilical vein endothelial cells (HUVEC). We demonstrate that exposure of HUVEC to sPLA(2) dose- and time-dependently enhances AA release and PGI(2) generation. sPLA(2)-stimulated AA mobilisation was blocked by AACOCF(3), an inhibitor of cPLA(2)alpha, suggesting cross-talk between the two classes of PLA(2). sPLA(2) induced the phosphorylation of cPLA(2)alpha and enhanced the phosphorylation states of p42/44(mapk), p38(mapk), and JNK, concomitant with elevated AA and PGI(2) release. The MEK inhibitor PD98059 attenuated sPLA(2)-stimulated cPLA(2)alpha phosphorylation and PGI(2) release. These data show that sPLA(2) cooperates with cPLA(2)alpha in a MAPK-dependent manner to regulate PGI(2) generation and suggests that cross-talk between sPLA(2) and cPLA(2)alpha is a physiologically important mechanism for enhancing prostanoid production in endothelial cells.

Molecular characterization of murine pancreatic phospholipase A(2)

The pancreatic secretory phospholipase A(2) (sPLA(2)IB) is considered to be a digestive enzyme, although it has several important receptor-mediated functions. In this study, using the newly isolated murine sPLA(2)IB cDNA clone as a probe, we demonstrate that in addition to the pancreas, the sPLA(2)IB mRNA was expressed in extrapancreatic organs such as the liver, spleen, duodenum, colon, and lungs. We also demonstrate that sPLA(2)IB mRNA expression was detectable from the 17(th) day of gestation in the developing mouse fetus, coinciding with the time of completion of differentiation of the pancreas. Furthermore, the mRNA expression pattern of sPLA(2)IB was distinct from those of sPLA(2)IIA and cPLA(2) in various tissues examined. The murine sPLA(2)IB gene structure is well conserved, consistent with findings in other mammalian species, and this gene mapped to the region of mouse chromosome 5F1-G1.1. Taken together, our results suggest that sPLA(2)IB plays important roles both in the pancreas and in extrapancreatic tissues and that in the mouse, its expression is developmentally regulated.

[Functional analysis of phospholipase A2 receptor by gene knockout studies]

Phospholipase A2 receptor (PLA2R) is a type I transmembrane glycoprotein related to the C-type animal lectin family and mediates a variety of biological responses elicited by group IB secretory phospholipase A2 (sPLA2-IB). In the present study, we have shown the evidence that a novel type of sPLA2, sPLA2-X, also acts as one of the high-affinity ligands for mouse PLA2R. We then generated PLA2R-deficient mice and found that the knockout mice exhibited the resistance to an endotoxic shock with reduced plasma levels of proinflammatory cytokines, such as TNF-alpha and IL-1 beta. In situ hybridization analysis revealed that the expression of PLA2R transcript was markedly enhanced in type II alveolar epithelial cells and a subset of splenic lymphocytes in accordance with the elevated expression of sPLA2-IB and TNF-alpha mRNAs during endotoxic shock. In addition, the elevated expression level of TNF-alpha transcript was significantly reduced by the deficiency of PLA2R, suggesting that PLA2R plays a role in the regulation of TNF-alpha expression in these cell types. We then synthesized a specific sPLA2 inhibitor, indoxam, which blocked the binding of sPLA2-IB and X to PLA2R. Indoxam was found to suppress the elevation of the plasma level of TNF-alpha and prolonged the survival of endotoxin-challenged mice. The inhibitory effects of indoxam were abolished by the deficiency of PLA2R, demonstrating the involvement of PLA2R in the progression of endotoxic shock. We also detected and characterized a soluble form of PLA2R protein in the plasma of mouse with anti-PLA2R antibody, and showed its potential role as an endogenous sPLA2 inhibitor. Taken together, a series of studies with PLA2R-knockout mice have elucidated a critical role of PLA2R in the regulation of the development of endotoxic shock.

Leucocyte recruitment induced by type II phospholipases A(2) into the rat pleural cavity

Bothropstoxin-I (BthTX-I) and bothropstoxin-II (BthTX-II) are Lys-49 and Asp-49 phospholipases A(2) (PLA(2)s), respectively, isolated from Bothrops jararacussu venom. Piratoxin-I (PrTX-I) is a Lys-49 PLA(2) isolated from Bothrops pirajai venom. In this study, the ability of BthTX-I, BthTX-II and PrTX-I to recruit leucocytes into the rat pleural cavity and potential mechanisms underlying this effect were investigated. Intrapleural injection of either BthTX-I or PrTX-I (10-100 microg/cavity each) caused a significant leucocyte infiltration at 12 h after injection. The maximal cell migration was observed with the dose of 30 microg/cavity (14.9+/-15.5 and 17.6+/-1. 6x10(6) cells/cavity, respectively). Leucocyte counts consisted mainly of mononuclear cells, but significant amounts of neutrophils and eosinophils were also observed. Intrapleural injection of BthTX-II (10-100 microg/cavity) caused a marked leucocyte infiltration at 6 and 12 h after injection. The maximal response was observed with the dose of 100 microg/cavity (57.3+/-3.4x10(6) cells/cavity, 6 h). The leucocyte counts were mainly composed of neutrophils and mononuclear cells. The treatment of either BthTX-I (30 microg/cavity, 12 h) or BthTX-II (30 microg/cavity, 6 h) with the PLA(2) inhibitor p-bromophenacyl bromide (p-BPB) had no effect on the total and differential leucocyte counts induced by these proteins. The same treatment partially reduced the PrTX-I-induced pleural leucocyte infiltration. In rats depleted of the histamine and 5-hydroxytryptamine (5-HT) stores by chronic treatment with compound 48/80, the total leucocyte counts in response to BthTX-I, BthTX-II and PrTX-I was not significantly affected compared to control animals. In addition, BthTX-I, BthTX-II and PrTX-I (100 microg/ml each) significantly degranulated pleural mast cells in vitro leading to the release of [(14)C]5-hydroxytryptamine ([(14)C]5-HT). p-BPB and heparin (50 IU/ml) significantly reduced the [(14)C]5-HT release induced by these PLA(2)s. Our results demonstrate that BthTX-I, BthTX-II and PrTX-I recruit leucocyte into the pleural cavity of the rat by mechanisms unrelated to enzymatic activity and pleural mast cell degranulation.

Redundant and segregated functions of granule-associated heparin-binding group II subfamily of secretory phospholipases A2 in the regulation of degranulation and prostaglandin D2 synthesis in mast cells

We herein demonstrate that mast cells express all known members of the group II subfamily of secretory phospholipase A2 (sPLA2) isozymes, and those having heparin affinity markedly enhance the exocytotic response. Rat mastocytoma RBL-2H3 cells transfected with heparin-binding (sPLA2-IIA, -V, and -IID), but not heparin-nonbinding (sPLA2-IIC), enzymes released more granule-associated markers (beta-hexosaminidase and histamine) than mock- or cytosolic PLA2alpha (cPLA2alpha)-transfected cells after stimulation with IgE and Ag. Site-directed mutagenesis of sPLA2-IIA and -V revealed that both the catalytic and heparin-binding domains are essential for this function. Confocal laser and electron microscopic analyses revealed that sPLA2-IIA, which was stored in secretory granules in unstimulated cells, accumulated on the membranous sites where fusion between the plasma membrane and granule membranes occurred in activated cells. These results suggest that the heparin-binding sPLA2s bind to the perigranular membranes through their heparin-binding domain, and lysophospholipids produced in situ by their enzymatic action may facilitate the ongoing membrane fusion. In contrast to the redundant role of sPLA2-IIA, -IID, and -V in the regulation of degranulation, only sPLA2-V had the ability to markedly augment IgE/Ag-stimulated immediate PGD2 production, which reached a level comparable to that elicited by cPLA2alpha. The latter observation reveals an unexplored functional segregation among the three related isozymes expressed in the same cell population.

Mouse group X secretory phospholipase A2 induces a potent release of arachidonic acid from spleen cells and acts as a ligand for the phospholipase A2 receptor

Group X secretory phospholipase A2 (sPLA2-X) has recently been shown to possess a powerful potency for releasing arachidonic acid from cell membrane phospholipids. Here, we report the purification of mouse pro- and mature forms of sPLA2-X, as well as its expression and biological functions. Purified pro-sPLA2-X was found to possess a propeptide of 11 amino acid residues attached at the NH2-terminals of the mature protein, and showed as little as 8% of the PLA2 activity of the mature form. Limited proteolysis of pro-sPLA2-X with trypsin resulted in the appearance of the mature form with a concomitant increase in PLA2 activity, suggesting a requirement of proteolytic removal of the propeptide for the optimal activity. The expression of sPLA2-X mRNA was detected in various tissues including the lung, thymus, and spleen, and immunohistochemical analysis revealed its expression in splenic macrophages. In the spleen cells, mature sPLA2-X elicited a prompt release of arachidonic acid with significant production of prostaglandin E2 more efficiently than group IB and IIA sPLA2s. In addition, sPLA2-X was identified as a high-affinity ligand for both native and recombinant form of mouse PLA2 receptor (PLA2R). However, there was no significant difference in the sPLA2-X-induced arachidonic acid release responses in the spleen cells between wild-type and PLA2R-deficient mice. These findings strongly suggest that sPLA2-X possesses two distinct biological functions in mice: it elicits a marked release of arachidonic acid from membrane phospholipids leading to the production of lipid mediators based on its enzymatic potency, and it acts as a natural ligand for the PLA2R that has been shown to play a critical role in the production of inflammatory cytokines during endotoxic shock.

Secretory phospholipase A2 receptor-mediated activation of cytosolic phospholipase A2 in murine bone marrow-derived mast cells

The current study examined the signal transduction steps involved in the selective release of arachidonic acid (AA) induced by the addition of secretory phospholipase A2 (sPLA2) isotypes to bone marrow-derived mast cells (BMMC). Overexpression of sPLA2 receptors caused a marked increase in AA and PGD2 release after stimulation of BMMC, implicating sPLA2 receptors in this process. The hypothesis that the release of AA by sPLA2 involved activation of cytosolic PLA2 (cPLA2) was next tested. Addition of group IB PLA2 to BMMC caused a transient increase in cPLA2 activity and translocation of this activity to membrane fractions. Western analyses revealed that these changes in cPLA2 were accompanied by a time-dependent gel shift of cPLA2 induced by phosphorylation of cPLA2 at various sites. A noncatalytic ligand of the sPLA2 receptor, p-amino-phenyl-alpha-D-mannopyranoside BSA, also induced an increase in cPLA2 activity in BMMC. sPLA2 receptor ligands induced the phosphorylation of p44/p42 mitogen-activated protein kinase. Additionally, an inhibitor of p44/p42 mitogen-activated protein kinase (PD98059) significantly inhibited sPLA2-induced cPLA2 activation and AA release. sPLA2 receptor ligands also increased Ras activation while an inhibitor of tyrosine phosphorylation (herbimycin) inhibited the increase in cPLA2 activation and AA release. Addition of partially purified sPLA2 from BMMC enhanced cPLA2 activity and AA release. Similarly, overexpression of mouse groups IIA or V PLA2 in BMMC induced an increase in AA release. These data suggest that sPLA2 mediate the selective release of AA by binding to cell surface receptors and then inducing signal transduction events that lead to cPLA2 activation.

Cell-specific expression of group X and group V secretory phospholipases A(2) in human lung airway epithelial cells

Secretory phospholipase A(2) (sPLA(2)) enzymes contribute to inflammatory injury in human lungs by several mechanisms, including eicosanoid production and hydrolytic damage to surfactant phospholipids. Several distinct sPLA(2) genes have been described in human tissue but little is known regarding their presence, localization, or function(s) within lungs. We hypothesized that sPLA(2)s would have cell-specific distributions within lung. We used reverse transcriptase/polymerase chain reaction to identify sPLA(2) messenger RNAs (mRNAs) in adult human lung tissue. Resulting complementary DNA (cDNA) sequences indicated that total lung extracts contained mRNA for Groups IB, IIA, V, and X sPLA(2). An epithelial cell line, BEAS cells, expressed only Groups IIA, V, and X. We used these cDNAs to clone these enzymes, especially the recently described Group X and Group V enzymes. Digoxigenin-labeled complementary RNA probes were used to determine localization of each sPLA(2) by in situ hybridization of human lung. Hybridization was strongly positive for Group X and Group V in airway epithelial cells, which failed to hybridize Group IB or IIA probes. Although four known mammalian sPLA(2) isotypes were expressed in lung, only Group X and Group V sPLA(2) mRNAs appear uniquely expressed in airway epithelium, suggesting they could provide a mechanism of pulmonary surfactant hydrolysis during lung injury.

Functional crosstalk between phospholipase D(2) and signaling phospholipase A(2)/cyclooxygenase-2-mediated prostaglandin biosynthetic pathways

We performed reconstitution analyses of functional interaction between phospholipase A(2) (PLA(2)) and phospholipase D (PLD) enzymes. Cotransfection of HEK293 cells with cytosolic (cPLA(2)) or type IIA secretory (sPLA(2)-IIA) PLA(2) and PLD(2), but not PLD(1), led to marked augmentation of stimulus-induced arachidonate release. Interleukin-1-stimulated arachidonate release was accompanied by prostaglandin E(2) production via cyclooxygenase-2, the expression of which was augmented by PLD(2). Conversely, activation of PLD(2), not PLD(1), was facilitated by cPLA(2) or sPLA(2)-IIA. Thus, our results revealed functional crosstalk between signaling PLA(2)s and PLD(2) in the regulation of various cellular responses in which these enzymes have been implicated.

Secretory phospholipases A2 induce beta-glucuronidase release and IL-6 production from human lung macrophages

Secretory phospholipases A2 (sPLA2s) are a group of extracellular enzymes that release fatty acids at the sn-2 position of phospholipids. Group IIA sPLA2 has been detected in inflammatory fluids, and its plasma level is increased in inflammatory diseases. To investigate a potential mechanism of sPLA2-induced inflammation we studied the effect of group IA (from cobra venom) and group IIA (human synovial) sPLA2s on human macrophages. Both sPLA2s induced a concentration- and Ca2+-dependent, noncytotoxic release of beta-glucuronidase (16.2 +/- 2.4% and 13.1 +/- 1.5% of the total content with groups IA and IIA, respectively). Both sPLA2s also increased the rate of secretion of IL-6 and enhanced the expression of IL-6 mRNA. Preincubation of macrophages with inhibitors of the hydrolytic activity of sPLA2 or cytosolic PLA2 did not influence the release of beta-glucuronidase. Incubation of macrophages with p-aminophenyl-mannopyranoside-BSA (mp-BSA), a ligand of the mannose receptor, also resulted in beta-glucuronidase release. However, while preincubation of macrophages with mp-BSA had no effect on beta-glucuronidase release induced by group IIA sPLA2, it enhanced that induced by group IA sPLA2. A blocking Ab anti-mannose receptor inhibited both mp-BSA- and group IIA-induced beta-glucuronidase release. Taken together, these data indicate that group IA and IIA sPLA2s activate macrophages with a mechanism independent from their enzymatic activities and probably related to the activation of the mannose receptor or sPLA2-specific receptors. The secretion of enzymes and cytokines induced by sPLA2s from human macrophages may play an important role in inflammation and tissue damage associated with the release of sPLA2s.

Group V phospholipase A(2)-mediated oleic acid mobilization in lipopolysaccharide-stimulated P388D(1) macrophages

P388D(1) macrophages prelabeled with [(3)H]arachidonic acid (AA) respond to bacterial lipopolysaccharide (LPS) by mobilizing AA in a process that takes several hours and is mediated by the concerted actions of the group IV cytosolic phospholipase A(2) and the group V secretory phospholipase A(2) (sPLA(2)). Here we show that when the LPS-activated cells are prelabeled with [(3)H]oleic acid (OA), they also mobilize and release OA to the extracellular medium. The time and concentration dependence of the LPS effect on OA release fully resemble those of the AA release. Experiments in which both AA and OA release are measured simultaneously indicate that AA is released 3 times more efficiently than OA. Importantly, LPS-stimulated OA release is strongly inhibited by the selective sPLA(2) inhibitors 3-(3-acetamide-1-benzyl-2-ethylindolyl-5-oxy)propane sulfonic acid and carboxymethylcellulose-linked phosphatidylethanolamine. The addition of exogenous recombinant sPLA(2) to the cells also triggers OA release. These data implicate a functionally active sPLA(2) as being essential for the cells to release OA upon stimulation with LPS. OA release is also inhibited by methyl arachidonyl fluorophosphonate but not by bromoenol lactone, indicating that the group IV cytosolic phospholipase A(2) is also involved in the process. Together, these data reveal that OA release occurs during stimulation of the P388D(1) macrophages by LPS and that the regulatory features of the OA release are strikingly similar to those previously found for the AA release.

Role of protein phosphorylation in activation of phospholipase A2 by the polychlorinated biphenyl mixture Aroclor 1242

Polychlorinated biphenyls (PCBs) activate neutrophils to induce degranulation and undergo superoxide production through a mechanism that involves stimulation of phospholipase A(2) (PLA(2)). Since the biochemical processes leading to the PCB-induced activation of this enzyme are unknown, the objective of this study was to determine whether protein phosphorylation has a role in this mechanism. Isolated rat neutrophils were labeled with [(3)H]-arachidonic acid ([(3)H]-AA), and activation of PLA(2) was determined from release of radioactivity into the medium. Exposure to the PCB mixture Aroclor 1242 induced release of [(3)H]-AA, and pretreatment with bromoenol lactone (BEL), an inhibitor of calcium-independent PLA(2), diminished release by 80%. Genistein, an inhibitor of tyrosine kinases, caused a small but significant decrease in Aroclor 1242-stimulated release of [(3)H]-AA. Daidzein, a genistein analog with no activity to inhibit tyrosine kinases, had no effect on [(3)H]-AA release. An inhibitor of p38 mitogen-activated protein kinase (MAPK), SB203580, did not affect Aroclor 1242-induced PLA(2) activity at concentrations selective for p38 MAPK; however, PD 98059, which inhibits MAPK kinase (MEK), decreased [(3)H]-AA release to about the same extent as genistein. Treatment of neutrophils with Aroclor 1242 induced phosphorylation of p44 MAPK, and this phosphorylation was unaffected by BEL but was inhibited by PD 98059. Staurosporine, a nonselective inhibitor of protein kinase C (PKC), inhibited PCB-induced release of [(3)H]-AA. Ro 32-0432, a selective inhibitor of PKC(alpha) and PKC(beta1), produced the greatest degree of inhibition (40%) among the tested protein kinase inhibitors. These results suggest that tyrosine kinases, PKC, and the MEK/MAPK pathway are involved in a fraction of Aroclor 1242-induced activation of PLA(2).

Purified group X secretory phospholipase A(2) induced prominent release of arachidonic acid from human myeloid leukemia cells

Group X secretory phospholipase A(2) (sPLA(2)-X) possesses several structural features characteristic of both group IB and IIA sPLA(2)s (sPLA(2)-IB and -IIA) and is postulated to be involved in inflammatory responses owing to its restricted expression in the spleen and thymus. Here, we report the purification of human recombinant COOH-terminal His-tagged sPLA(2)-X, the preparation of its antibody, and the purification of native sPLA(2)-X. The affinity-purified sPLA(2)-X protein migrated as various molecular species of 13-18 kDa on SDS-polyacrylamide gels, and N-glycosidase F treatment caused shifts to the 13- and 14-kDa bands. NH(2)-terminal amino acid sequencing analysis revealed that the 13-kDa form is a putative mature sPLA(2)-X and the 14-kDa protein possesses a propeptide of 11 amino acid residues attached at the NH(2) termini of the mature protein. Separation with reverse-phase high performance liquid chromatography revealed that N-linked carbohydrates are not required for the enzymatic activity and pro-sPLA(2)-X has a relatively weak potency compared with the mature protein. The mature sPLA(2)-X induced the release of arachidonic acid from phosphatidylcholine more efficiently than other human sPLA(2) groups (IB, IIA, IID, and V) and elicited a prompt and marked release of arachidonic acid from human monocytic THP-1 cells compared with sPLA(2)-IB and -IIA with concomitant production of prostaglandin E(2). A prominent release of arachidonic acid was also observed in sPLA(2)-X-treated human U937 and HL60 cells. Immunohistochemical analysis of human lung preparations revealed its expression in alveolar epithelial cells. These results indicate that human sPLA(2)-X is a unique N-glycosylated sPLA(2) that releases arachidonic acid from human myeloid leukemia cells more efficiently than sPLA(2)-IB and -IIA.

Phospholipase A2 enzymes in eicosanoid generation

Phospholipase A2 (PLA2) enzymes cleave esterified fatty acids from membrane glycerophospholipids. The 20-carbon polyunsaturated fatty acid, arachidonic acid, is used as substrate by intermediate enzymes for the generation of eicosanoids, including leukotrienes and prostanoid products. An expanding number of PLA2 enzymes has now been identified that may participate in arachidonic acid release and thus serve a rate-limiting role in eicosanoid biosynthesis. Cellular PLA2 function for various members is regulated by constitutive or elicited expression, as well as by posttranslational events such as phosphorylation. In addition, the function of some cellular PLA2 enzymes is regulated by a requirement for calcium or by localization to a particular subcellular compartment. Finally, some PLA2 enzymes are secreted from the cell where they may directly interact with plasma membrane or transmembrane receptors to function as autocrine or paracrine mediators. Evaluating the roles of a number of these functionally similar PLA2 enzymes in the biosynthesis of leukotrienes and other eicosanoids is the focus of this review.

Low molecular weight group IIA and group V phospholipase A(2) enzymes have different intracellular locations in mouse bone marrow-derived mast cells

The subcellular location of the enzymes of eicosanoid biosynthesis is critical for their co-ordinate action in the generation of leukotrienes and prostaglandins. This activity is thought to occur predominantly at a perinuclear location. Whereas the subcellular locations of cytosolic phospholipase (PL) A(2) and each of the pathway enzymes of eicosanoid generation have been defined, the distribution of the low molecular weight species of PLA(2) has remained elusive because of the lack of antibodies that distinguish among homologous family members. We have prepared affinity-purified rabbit antipeptide IgG antibodies that distinguish mouse group IIA PLA(2) and group V PLA(2). Immunofluorescence staining and immunogold electron microscopy reveal different subcellular locations for the enzymes. Group IIA(2) PLA(2) is present in the secretory granules of mouse bone marrow-derived mast cells, consistent with its putative role in facilitating secretory granule exocytosis and its consequent extracellular action. In contrast, group V PLA(2) is associated with various membranous organelles including the Golgi apparatus, nuclear envelope, and plasma membrane. The perinuclear location of group V PLA(2) is consistent with a putative interaction with translocated cytosolic PLA(2) in supplying arachidonic acid for generation of eicosanoid products, while the location in Golgi cisternae may also reflect its action as a secreted enzyme. The spatial segregation of group IIA PLA(2) and group V PLA(2) implies that these enzymes are not functionally redundant.

Regulation of type V phospholipase A2 expression and function by proinflammatory stimuli

Types IIA and V secretory phospholipase A2 (sPLA2) are structurally related to each other and their genes are tightly linked to the same chromosome locus. An emerging body of evidence suggests that sPLA2-IIA plays an augmentative role in long-term prostaglandin (PG) generation in cells activated by proinflammatory stimuli; however, the mechanism underlying the functional regulation of sPLA2-V remains largely unknown. Here we show that sPLA2-V is more widely expressed than sPLA2-IIA in the mouse, in which its expression is elevated by proinflammatory stimuli such as lipopolysaccharide. In contrast, proinflammatory stimuli induced sPLA2-IIA in marked preference to sPLA2-V in the rat. Cotransfection of sPLA2-V with cyclooxygenase (COX)-2, but not with COX-1, into human embryonic kidney 293 cells dramatically increased the interleukin-1-dependent PGE2 generation occurring over a 24 h of culture period. Rat mastocytoma RBL-2H3 cells overexpressing sPLA2-V exhibited increased IgE-dependent PGD2 generation and accelerated beta-hexosaminidase exocytosis. These results suggest that sPLA2-V acts as a regulator of inflammation-associated cellular responses. This possible compensation of sPLA2-V for sPLA2-IIA in many, if not all, tissues may also explain why some mouse strains with natural disruption of the sPLA2-IIA gene exhibit few abnormalities during their life-spans.

Suppression of murine endotoxic shock by sPLA2 inhibitor, indoxam, through group IIA sPLA2-independent mechanisms

Endotoxic shock is a systemic inflammatory process, involving a variety of proinflammatory mediators. Two types of secretory phospholipase A2 (sPLA2) have been implicated in this process. Group IB sPLA2 (PLA2-IB) binds to the PLA2 receptor (PLA2R), and PLA2R-deficient mice exhibit resistance to endotoxin-induced lethality with reduced plasma levels of proinflammatory cytokines, such as TNF-alpha. Group IIA sPLA2 (PLA2-IIA) is found in many tissues and cell types, and local and systemic levels are elevated under numerous inflammatory conditions including sepsis. In this study, we investigated the effect of a specific sPLA2 inhibitor, indoxam, on murine endotoxic shock. Indoxam suppressed the elevation of plasma TNF-alpha with a similar potency in PLA2-IIA-expressing and PLA2-IIA-deficient mice after LPS challenge. In PLA2-IIA-deficient mice, indoxam also suppressed the elevation of plasma IL-1beta, IL-6 and NO, and prolonged survival after LPS challenge. Indoxam was found to block the PLA2-IB binding to murine PLA2R with a high potency (Ki=30 nM). The inhibitory effects of indoxam on the LPS-induced elevation of plasma TNF-alpha levels could not be observed in mice deficient in PLA2R. These findings suggest that indoxam blocks the production of proinflammatory cytokines during endotoxemia through PLA2-IIA-independent mechanisms, possibly via blockade of the PLA2R function.