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

Regulation of the Ca(2+)-independent phospholipase A2 in liver mitochondria by changes in the energetic state

The effect of electron transport chain redox status on activity of the mitochondrial Ca(2+)-independent phospholipase A2 (iPLA2) has been examined. When oxidizing NAD-linked substrates, the enzyme is not active unless deenergization occurs. Uncoupler, rotenone, antimycin A, and cyanide are equally effective at upregulating the enzyme, while oligomycin is ineffective. Thenoyltrifluoroacetone causes deenergization and activates the enzyme, but only if succinate is the respiratory substrate. These findings show that the mitochondrial iPLA2 responds to the energetic state overall, rather than to the redox status of individual electron transport chain complexes. With NAD-linked substrates, and using rotenone to deenergize, iPLA2 activation can be reversed by adding succinate to reestablish a membrane potential. For this purpose, ascorbate plus N,N,N'N'-tetramethyl-phenylenediamine can be used instead of succinate and is equally effective. With succinate as substrate, the membrane potential can be reduced in a graded and stable fashion by adding increasing concentrations of malonate, which is a competitive inhibitor of succinate utilization. A partial and stable activation of the iPLA2 accompanies partial deenergization. These findings suggest that in addition to the several functions that have been proposed, the mitochondrial iPLA2 may help to coordinate local capillary blood flow with changing energy demands.

Activation of mitochondrial calcium-independent phospholipase A2γ (iPLA2γ) by divalent cations mediating arachidonate release and production of downstream eicosanoids

Calcium-independent phospholipase A(2)γ (iPLA(2)γ) (PNPLA8) is the predominant phospholipase activity in mammalian mitochondria. However, the chemical mechanisms that regulate its activity are unknown. Here, we utilize iPLA(2)γ gain of function and loss of function genetic models to demonstrate the robust activation of iPLA(2)γ in murine myocardial mitochondria by Ca(2+) or Mg(2+) ions. Calcium ion stimulated the production of 2-arachidonoyl-lysophosphatidylcholine (2-AA-LPC) from 1-palmitoyl-2-[(14)C]arachidonoyl-sn-glycero-3-phosphocholine during incubations with wild-type heart mitochondrial homogenates. Furthermore, incubation of mitochondrial homogenates from transgenic myocardium expressing iPLA(2)γ resulted in 13- and 25-fold increases in the initial rate of radiolabeled 2-AA-LPC and arachidonic acid (AA) production, respectively, in the presence of calcium ion. Mass spectrometric analysis of the products of calcium-activated hydrolysis of endogenous mitochondrial phospholipids in transgenic iPLA(2)γ mitochondria revealed the robust production of AA, 2-AA-LPC, and 2-docosahexaenoyl-LPC that was over 10-fold greater than wild-type mitochondria. The mechanism-based inhibitor (R)-(E)-6-(bromomethylene)-3-(1-naphthalenyl)-2H-tetrahydropyran-2-one (BEL) (iPLA(2)γ selective), but not its enantiomer, (S)-BEL (iPLA(2)β selective) or pyrrolidine (cytosolic PLA(2)α selective), markedly attenuated Ca(2+)-dependent fatty acid release and polyunsaturated LPC production. Moreover, Ca(2+)-induced iPLA(2)γ activation was accompanied by the production of downstream eicosanoid metabolites that were nearly completely ablated by (R)-BEL or by genetic ablation of iPLA(2)γ. Intriguingly, Ca(2+)-induced iPLA(2)γ activation was completely inhibited by long-chain acyl-CoA (IC(50) ∼20 μm) as well as by a nonhydrolyzable acyl-CoA thioether analog. Collectively, these results demonstrate that mitochondrial iPLA(2)γ is activated by divalent cations and inhibited by acyl-CoA modulating the generation of biologically active metabolites that regulate mitochondrial bioenergetic and signaling functions.

Henk van den Bosch: chemist and biochemist

Henk van den Bosch is a native of The Netherlands and recently retired from his position as Professor at Utrecht University. This article summarizes the many scientific achievements of Dr. van den Bosch. He enjoys an international reputation for his research on phospholipases A, cardiolipin biosynthesis in eukaryotes, lysophospholipases, phosphatidylcholine biosynthesis for lung surfactant, plasmalogen biosynthesis in peroxisomes, diagnosis of peroxisomal disorders and most recently his work on alkyl-dihydroxyacetone phosphate synthase. During his research career Henk van den Bosch published approximately 280 articles and presented 110 invited lectures.

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.

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

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

Distribution of group II phospholipase A2 protein and mRNA in rat tissues

Group II phospholipase A2 (PLA2) is an acute-phase protein and an important component of the host defense against bacteria. In this study we investigated the distribution of PLA2 protein by immunohistochemistry and the distribution of mRNA of PLA2 by Northern blotting and in situ hybridization in rat tissues. PLA2 protein was localized in the Paneth cells of the intestinal mucosa, chondrocytes and the matrix of cartilage, and megakaryocytes in the spleen. By Northern blotting, mRNA of PLA2 was found in the gastrointestinal tract, lung, heart, and spleen. By in situ hybridization, PLA2 mRNA was localized in the Paneth cells of the small intestinal mucosa but in no other cell types. Our results show specific distribution of PLA2 in a limited number of cell types in rat tissues. The reagents developed in this study (the anti-rat PLA2 antibody and probes for Northern blotting and in situ hybridization of mRNA of rat PLA2) will provide useful tools for future studies concerning the role of PLA2 in various experimental disease models.

Modulation of cytokine-induced expression of secretory phospholipase A2-type IIA by protein kinase C in rat renal mesangial cells

Renal mesangial cells express the 14 kDa secretory phospholipase A2-type IIA (sPLA2-IIA) in response to interleukin-1beta (IL-1beta). In order to understand the regulation of cytokine-induced sPLA2-IIA induction in more detail, we investigated whether phorbol ester-activated protein kinase C (PKC) has an influence on the IL-1beta-induced expression of sPLA2-IIA. We found that treatment of mesangial cells with the biologically active phorbol 12-myristate 13-acetate (PMA) and phorbol 12,13-dibutyrate inhibited IL-1beta induction of sPLA2-IIA mRNA, protein, and activity, whereas the inactive compound 4alpha-phorbol 12,13-didecanoate was without effect. An 8-hr pretreatment with PMA, which led to down-regulation of PKC-alpha and -delta isoenzymes, still inhibited sPLA2-IIA induction. Only after down-regulation of PKC-epsilon isoenzyme by 24-hr preincubation with PMA were we able to reconstitute the IL-1beta-induced sPLA2-IIA expression. Thrombin as a physiological activator of PKC in mesangial cells exerted similar effects as PMA and inhibited sPLA2-IIA expression. The selective PKC inhibitor calphostin C potentiated IL-1beta induction of sPLA2-IIA mRNA levels and partially reconstituted the thrombin-induced inhibition of sPLA2-IIA mRNA and activity. These data show that IL-1beta induction of sPLA2-IIA can be modulated by PKC and that the epsilon-isoenzyme of the PKC family is the most likely candidate mediating the suppression of cytokine-induced sPLA2-IIA expression in mesangial cells.

Secretory group IIA phospholipase A(2) generates anti-apoptotic survival signals in kidney fibroblasts

Mammalian group IIA phospholipase A(2) (PLA(2)) is believed to play important roles in inflammation, cell injury, and tumor resistance. However, the cellular site of action has not been clearly defined as it has long been recognized that group IIA PLA(2) is both a secretory and mitochondrial protein. The purpose of this study was to determine the subcellular target of the group IIA PLA(2) and its role in apoptosis stimulated by growth factor withdrawal. Cloning of the rat liver group IIA PLA(2) demonstrated a typical secretory signal and no alternative splicing of the primary transcript. When a sequence including the signal peptide and first 8 residues in the mature enzyme or the entire PLA(2) (including the signal peptide) was fused to enhanced green fluorescent protein, the fusion protein was directed to the secretory pathway rather than mitochondria in baby hamster kidney (BHK) cells. To examine the role of group IIA PLA(2) in cell injury, wild type (wt) rat group IIA PLA(2) and a mutant group IIA PLA(2) containing a His-47 --> Gln mutation (at the catalytic center) were transfected into BHK cells and cells stably expressing these constructs were isolated. After deprivation of growth factors, both normal BHK cells and BHK cells expressing mutant PLA(2) underwent massive apoptosis, while BHK cells expressing wt PLA(2) showed considerable resistance to growth factor withdrawal-induced apoptosis. The secretory PLA(2) inhibitors 12-epi-scalaradial and aristolochic acid abrogated resistance to apoptosis in the wt PLA(2) expressing cells. These two inhibitors did not induce cell death in the presence of fetal bovine serum, suggesting that they induce cell death by blocking PLA(2) generated survival signals. This study demonstrates that group IIA PLA(2) generates anti-apoptotic survival signals in BHK cells targeting the secretory pathway, and suggests that high levels of group IIA PLA(2) accumulated at inflammatory sites may not only regulate inflammation, but also may protect cells from unnecessary death induced by pro-inflammatory agents.

Cross-talk between group IIA-phospholipase A2 and inducible NO-synthase in rat renal mesangial cells

Features of glomerulonephritis are expression of the inducible form of NO synthase (iNOS) as well as expression of the secretory group IIA-phospholipase A2 (sPLA2) in mesangial cells. Interleukin 1beta (IL-1beta) induces both enzymes with a similar time course resulting in an increase in nitrite production and sPLA2-IIA activity. In this study we investigated the relationship between the formation of NO and sPLA2-IIA induction in rat renal mesangial cells. Incubation of mesangial cells with the NO-donor, spermine-NONOate, for 24 h induced sPLA2-IIA mRNA expression and activity, whereas S-nitroso glutathione alone had only a small stimulatory effect. Stimulation of cells with IL-1beta caused a marked increase in sPLA2-IIA mRNA and activity that were potentiated 3 fold by both NO donors. Coincubation of cells with IL-1beta and the NOS inhibitor, L-N(G) monomethylarginine (L-NMMA), caused a dose-dependent inhibition of cytokine-induced sPLA2-IIA mRNA expression and activity. sPLA2-IIA activity was not stimulated by 8-bromo-cyclic GMP indicating that NO-induced sPLA2-IIA induction is independent of cyclic GMP-mediated signal transduction. These data show that NO contributes to the expression by cytokines of sPLA2-IIA and establishes a novel type of interaction between iNOS and sPLA2-IIA in mesangial cells. This cross-talk between inflammatory mediators may help to promote and sustain an inflammatory state in the kidney.

Genes encoding multiple forms of phospholipase A2 are expressed in rat brain

The polymerase chain reaction was utilized to determine which of six cloned phospholipase A2 (PLA2) messenger RNAs (mRNAs), encoding four distinct low molecular weight forms of secretory PLA2 (PLA2-IB, PLA2-IIA, PLA2-IIC and PLA2-V), a calcium-dependent high molecular weight PLA2 (PLA2-IV, cPLA2) and a calcium-independent high molecular weight PLA2 (PLA2-VI, iPLA2), were expressed in different regions of rat brain and in a number of peripheral tissues. Pancreatic PLA2-IB mRNA was not expressed in the brain. PLA2-IIA, PLA2-IV and PLA2-VI mRNAs appeared to be ubiquitously expressed in brain, with relatively similar levels detected in all regions. PLA2-IIC mRNA was expressed in all brain regions but not in any of the peripheral tissues studied. PLA2-V mRNA was found at low levels in most areas of the brain, but at very high levels in the hippocampus. These results indicate that mammalian brain has the potential to express multiple isoforms of PLA2 which could be important given the potential role of these enzyme activities in ischaemic damage and in the regulation of synaptic plasticity.

Induction of secretory phospholipase A2 confirms the systemic inflammatory nature of adjuvant arthritis

Adjuvant arthritis (AA) is an accepted model of inflammatory arthritis. Until now, however, there is little information about inflammatory mediators, specifically in relation to the arachidonic acid cascade in AA. Our objective was to study the expression of secretory (sPLA2) and cytosolic (cPLA2) phospholipases A2 in various organs during the course of AA. AA was induced in Lewis rats which were sacrificed at days 0, 7, 14, 21, 28 and 42. Expression of sPLA2 mRNA and protein and mRNA of cPLA2 in paws, regional lymph nodes, spleen, liver, lungs and aorta was investigated. Serum sPLA2 activity increased from 15213 +/- 1131 to a maximum of 32,455 +/- 4109 nmol/30' on day 21. Maximal increase in sPLA2 mRNA in paws, lung and aorta was observed on day 14, and in the lymph nodes and spleen on day 28. In the liver, trace levels were found with no corresponding protein expression. In paws, lung, aorta and lymph nodes maximum increase in sPLA2 protein was noted on day 14 whereas the spleen showed constant sPLA2 protein level during AA. cPLA2 mRNA detected in all organs, did not significantly change during the course of AA, with the exception of regional lymph nodes where the message increased between 14 and 28 day. Induction of mRNA and protein of sPLA2 in several organs is an evidence that AA is a systemic inflammatory process. The parallelity of the sPLA2 expression to the severity of inflammatory process, implies that sPLA2 may play pathogenic role in AA. Lack of enhancement of cPLA2 mRNA may mean that this enzyme is either not induced in AA, or it increases earlier in the course of the inflammatory process.

Platelet-derived growth factor and fibroblast growth factor differentially regulate interleukin 1beta- and cAMP-induced group II phospholipase A2 expression in rat renal mesangial cells

Expression of group II phospholipase A2 (PLA2; EC 3.1.1.4) in rat renal mesangial cells is triggered in response to two principal classes of activating signals. These two groups of activators comprise inflammatory cytokines such as interleukin 1beta (IL-1beta) or tumor necrosis factor alpha and agents that elevate cellular levels of cyclic AMP (cAMP) such as forskolin, an activator of adenylate cyclase. Treatment of mesangial cells with IL-1beta or forskolin for 24 h induces group II PLA2 activity secreted into cell culture supernatants by about 15-fold and 11-fold, respectively. Platelet-derived growth factor (PDGF)-BB potently inhibits secretion of IL-1beta- and forskolin-induced group II PLA2 activity. By Western and Northern blot analyses, we demonstrate that this is due to a reduction of PLA2 protein levels and the corresponding PLA2 mRNA steady-state levels. Basic fibroblast growth factor (bFGF) virtually does not inhibit IL-1beta-stimulated group II PLA2 activity, but markedly inhibits forskolin-induced expression of group II PLA2 activity. These effects are caused by changes in the corresponding PLA2 protein and PLA2 mRNA steady-state levels. Inhibition of protein kinase C (PKC) by the potent and selective PKC inhibitor calphostin C converted the inhibitory action of PDGF into a bFGF-type of response thus suggesting that PKC is a major effector in PDGF-induced inhibition of IL-1beta-stimulated group II sPLA2 expression. In summary, our data suggest that PDGF and bFGF differentially modulate in a stimulus-specific manner the expression of group II PLA2 in mesangial cells.

Suppression by cyclosporin A of interleukin 1 beta-induced expression of group II phospholipase A2 in rat renal mesangial cells

1. We investigated whether cyclosporin A, a potent immunosuppressive drug, affects group II phospholipase A2. (PLA2; EC 3.1.1.4) induction in rat renal mesangial cells. 2. Previously we showed that the expression of group II PLA2 in rat renal mesangial cells is triggered by exposure of the cells to inflammatory cytokines such as interleukin 1 beta (IL-1 beta) or tumour necrosis factor alpha and agents that elevate cellular levels of cyclic AMP. Treatment of mesangial cells with IL-1 beta for 24 h induced PLA2 activity secreted into cell culture supernatants by about 16 fold. Incubation of mesangial cells with cyclosporin A inhibited IL-1 beta-induced PLA2 section in a dose-dependent fashion, with an IC50 value of 4.3 microM. Cyclosporin A did not directly inhibit enzymatic activity of PLA2. 3. Immunoprecipitation of radioactively labelled PLA2 protein from mesangial cell supernatants revealed that the inhibition of PLA2 activity is due to a suppression of PLA2 protein levels. This effect was preceded by a reduction of PLA2 mRNA steady state levels, as demonstrated by Northern blot analyses of total cellular RNA isolated from stimulated mesangial cells. 4. In order to evaluate whether cyclosporin A would affect the transcriptional activity of the PLA2 gene, we performed nuclear run on transcription experiments and provided evidence that the transcription rate of the PLA2 gene is reduced by cyclosporin A. 5. Previously we found that the nuclear transcription factor kappa B (NF kappa B) is an essential component of the IL-1 beta-dependent upregulation of PLA2 gene transcription. By electrophoretic mobility shift analysis, we demonstrated that cyclosporin A diminishes the formation of NF kappa B DNA-binding complexes, thus suggesting that this transcription factor is a target for cyclosporin A-mediated repression of PLA2 gene transcription. 6. The data presented in this study strongly suggest that the cellular mechanism involved in the IL1 beta-dependent transcriptional upregulation of the PLA2 gene in mesangial cells is a target for the action of cyclosporin A.

Half-life of interleukin-1 beta-induced group II phospholipase A2 in rat mesangial cells

Group II phospholipase A2 (sPLA2) has been implicated as an important agent involved in a number of inflammatory processes. Potent pro-inflammatory cytokines, such as interleukin-1 beta (IL-1 beta) and tumor necrosis factor (TNF) have been found to induce sPLA2 synthesis and release from many cell types among which mesangial cells. Although considerable research has been devoted to unravelling the mechanisms underlying the induction of sPLA2 not much is known about the time scale at which the cytokine elicited signals for sPLA2 induction persist in target cells. In this study we addressed that question by using rat renal mesangial cells as a model target cell. We found that after removal of IL-1 beta from the culture medium, the induced-sPLA2 synthesis continues at gradually decreasing rates for approximately 8 h. This is accompanied by a decrease in sPLA2 mRNA levels. Furthermore, with pulse-chase experiments we investigated the half-life of sPLA2 disappearance from the cells. This disappearance was found to be biphasic. A rapidly disappearing pool, constituting approx. 74% of the total, exhibited a half-life of 1.6 +/- 0.2 h. The remaining pool of the induced enzyme was much more stable and its level remained constant for at least 24 h. Analysis of the appearance of newly synthesized enzyme in the culture medium indicated this process to be completed in an hour.

Purification and characterization of nuclear alkaline phospholipase A2 in rat ascites hepatoma cells

The alkaline phospholipase A2 (PLA2) was purified from nuclei of rat ascites hepatoma cells (AH7974) by column chromatography with a Sephacryl S-300 column and an immunoadsorbent using anti-group II PLA2 monoclonal antibody. From these two columns, the alkaline PLA2 was eluted in parallel with a 17-kDa protein which is reactive to another anti-group II PLA2 polyclonal antibody. Approximately 80% of nuclear PLA2 was inhibited by this antibody. The alkaline PLA2 was found in association with the chromatin fraction among subnuclear fractions. By an immunocytochemical staining, the nuclei of AH7974 were stained more strongly than other parts of cells with anti-group II PLA2 antiserum.

Tetranactin inhibits interleukin 1 beta and cAMP induction of group II phospholipase A2 in rat renal mesangial cells

Renal mesangial cells express secretory phospholipase A2 in response to two principal classes of activating signals that may interact in a synergistic fashion. These two groups of activators comprise inflammatory cytokines, such as interleukin 1 beta, and agents that elevate cellular levels of cAMP. Treatment of mesangial cells with tetranactin, a cyclic antibiotic produced by Streptomyces aureus with a molecular structure similar to cyclosporin A inhibits interleukin 1 beta- and cAMP-dependent group II phospholipase A2 secretion in a dose-dependent manner with IC50 values of 43 and 33 nM, respectively. However, tetranactin does not directly inhibit group II phospholipase A2 activity. Western blot analyses of mesangial cell supernatants reveal that the inhibition of phospholipase A2 activity is due to suppression of phospholipase A2 protein synthesis. This effect is preceded by the reduction of phospholipase A2 mRNA steady-state levels as shown by Northern blot analyses of total cellular RNA isolated from stimulated mesangial cells. Thus, tetranactin is a potent inhibitor of group II phospholipase A2 expression in cytokine- and cAMP-stimulated mesangial cells and represents a new class of group II phospholipase A2 inhibitors with IC50 values in the low nanomolar range. This compound may be useful in the therapy of diseases associated with increased group II phospholipase A2 secretion.

Effects of dexamethasone and transforming growth factor-beta 2 on group II phospholipase A2 mRNA and activity levels in interleukin 1 beta- and forskolin-stimulated mesangial cells

The expression of 14 kDa group II phospholipase A2 [also referred to as secretory PLA2 (sPLA2)] is induced in rat glomerular mesangial cells by exposure to inflammatory cytokines and forskolin, a cAMP elevating agent. Previously we have shown that dexamethasone and transforming growth factor-beta 2 (TGF-beta 2) suppress sPLA2 protein synthesis and enzyme activity induced by cytokines and forskolin. The regulation of sPLA2 by pro-inflammatory cytokines suggests that the enzyme may play a role in glomerular inflammatory reactions. In order to understand the regulation of sPLA, in more detail, we investigated whether dexamethasone and TGF-beta 2 also suppress sPLA, mRNA after its induction by either interleukin-1 beta (IL-1 beta) or forskolin. We found that IL-1 beta-induced sPLA2 mRNA in rat mesangial cells is not down-regulated by pretreatment of the cells with dexamethasone, even at a concentration of 10 microM, which dramatically decreases sPLA2 protein levels and activity. Metabolic labelling experiments indicated that the decreased sPLA2 levels under these conditions can be explained by inhibition of the rate of sPLA2 synthesis from the elevated mRNA levels. In contrast, the forskolin-induced elevation of sPLA, mRNA is inhibited by dexamethasone in a concentration-dependent manner. Likewise, TGF-beta 2 inhibits the elevation of sPLA, mRNAs induced by either IL-1 beta or forskolin. The decrease in sPLA2 mRNA caused by TGF-beta 2 corresponds with the decrease in sPLA2 enzyme levels and activity. These data suggest that cytokine- and forskolin-induced sPLA2, expression is tightly controlled via both transcriptional and post-transcriptional mechanisms. Furthermore, we show that pretreatment of mesangial cells with epidermal growth factor prior to stimulation with IL-1 beta or forskolin had no suppressing effect on sPLA2 levels or enzyme activity, as has been reported previously for osteoblasts.

A natural disruption of the secretory group II phospholipase A2 gene in inbred mouse strains

The synovial fluid or group II secretory phospholipase A2 (sPLA2) has been implicated as an important agent involved in a number of inflammatory processes. In an attempt to determine the role of sPLA2 in inflammation, we set out to generate sPLA2-deficient mice. During this investigation, we observed that in a number of inbred mouse strains, the sPLA2 gene was already disrupted by a frameshift mutation in exon 3. This mutation, a T insertion at position 166 from the ATG of the cDNA, terminates out of frame in exon 4, resulting in the disruption of the calcium binding domain in exon 3 and loss of both activity domains coded by exons 4 and 5. The mouse strains C57BL/6, 129/Sv, and B10.RIII were found to be homozygous for the defective sPLA2 gene, whereas outbred CD-1:SW mice had variable genotype at this locus. BALB/c, C3H/HE, DBA/1, DBA/2, NZB/BIN, and MRL lpr/lpr mice had a normal sPLA2 genotype. The sPLA2 mRNA was expressed at very high levels in the BALB/c mouse small intestine, whereas in the small intestine of the sPLA2 mutant mouse strains, sPLA2 mRNA was undetectable. In addition, PLA2 activity in acid extracts of the small intestine were approximately 40 times higher in BALB/c than in the mutant mice. Transcription of the mutant sPLA2 gene resulted in multiple transcripts due to exon skipping. None of the resulting mutant mRNAs encoded an active product. The identification of this mutation should not only help define the physiological role of sPLA2 but also has important implications in mouse inflammatory models developed by targeted mutagenesis.

Growth hormone signaling leading to CYP2C12 gene expression in rat hepatocytes involves phospholipase A2

The expression of CYP2C12 is liver-specific and regulated at the transcriptional level by growth hormone (GH). In attempts to elucidate the nature of signaling molecules mediating the GH regulation of this gene in rat hepatocytes, a role for phospholipase A2 (PLA2) as a transducer of GH-induced levels of P4502C12 mRNA was investigated. GH was shown to induce tyrosyl-phosphorylation of p42 and p44 microtubule-associated protein (MAP) kinases and to reduce the electrophoretic mobility of a 100-kDa protein, immunologically related to cPLA2. These events were observed in parallel with GH-stimulated release of [3H]arachidonic acid ([3H]AA) from cellular phospholipids of rat hepatocytes labeled with [3H]AA. These rapid effects of GH action, as well as the GH-induced expression of CYP2C12, were inhibited in cells treated with the tyrosine kinase inhibitor herbimycin A. Similarly, when the GH-induced liberation of [3H]AA was blocked by the PLA2 inhibitor mepacrine or the Ca2+ channel blocker verapamil, GH-induced accumulation of P4502C12 mRNA was absent. These results suggest a correlation between PLA2 activity and GH regulation of the CYP2C12 gene. The inhibitory effect of mepacrine on GH induction of P4502C12 mRNA was reversed by AA addition, further supporting a role for eicosanoids in the regulation of CYP2C12. Finally, inhibitors of P450-mediated AA metabolism, SKF-525A and ketoconazole as well as eicosatetraynoic acid, blocked the GH-mediated induction of P4502C12 mRNA, whereas more specific inhibitors of cyclooxygenase or lipoxygenase metabolism did not. Based on these results, we suggest that GH signaling in rat hepatocytes, leading to increased expression of CYP2C12, involves PLA2 activation and subsequent P450-catalyzed formation of an active AA metabolite.

Pyrrolidine dithiocarbamate differentially affects cytokine- and cAMP-induced expression of group II phospholipase A2 in rat renal mesangial cells

Renal mesangial cells express group II phospholipase A2 in response to two principal classes of activating signals that may interact in a synergistic fashion. These two groups of activators comprise inflammatory cytokines such as interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF alpha) and agents that elevate cellular levels of cAMP such as forskolin, an activator of adenylate cyclase. Using pyrrolidine dithiocarbamate (PDTC), a potent inhibitor of nuclear factor NF kappa B, we determined its role in cytokine--and cAMP--triggered group II PLA2 expression. Micromolar amounts of PDTC suppress the IL-1 beta- and TNF alpha-dependent, but not the forskolin-stimulated group II PLA2 activity in mesangial cells. Furthermore, PDTC inhibited the increase of group II PLA2 mRNA steady state levels in response to IL-1 beta and TNF alpha, while only marginally affecting forskolin-induced PLA2 mRNA levels. Our data suggest that NF kappa B activation is an essential component of the cytokine signalling pathway responsible for group II PLA2 gene regulation and that cAMP triggers a separate signalling cascade not involving NF kappa B. These observations may provide a basis to study the underlying mechanisms involved in the regulation of group II PLA2 gene expression.