Interleukin-1 beta-induced cytosolic phospholipase A2 activity and protein synthesis is blocked by dexamethasone in rat mesangial cells

Inhibitors of secreted phospholipase A2 suppress the release of PGE2 in renal mesangial cells

The upregulation of PGE2 by mesangial cells has been observed under chronic inflammation condition. In the present work, renal mesangial cells were stimulated to trigger a huge increase of PGE2 synthesis and were treated in the absence or presence of known PLA2 inhibitors. A variety of synthetic inhibitors, mainly developed in our labs, which are known to selectively inhibit each of GIVA cPLA2, GVIA iPLA2, and GIIA/GV sPLA2, were used as tools in this study. Synthetic sPLA2 inhibitors, such as GK115 (an amide derivative based on the non-natural amino acid (R)-γ-norleucine) as well as GK126 and GK241 (2-oxoamides based on the natural (S)-α-amino acid leucine and valine, respectively) presented an interesting effect on the suppression of PGE2 formation.

The ω3-polyunsaturated fatty acid derivatives AVX001 and AVX002 directly inhibit cytosolic phospholipase A(2) and suppress PGE(2) formation in mesangial cells

BACKGROUND AND PURPOSE

ω3-polyunsaturated fatty acids (ω3-PUFAs) are known to exert anti-inflammatory effects in various disease models although their direct targets are only poorly characterized.

EXPERIMENTAL APPROACH

Here we report on two new cPLA(2) inhibitors, the ω3-derivatives AVX001 and AVX002, and their effects on inflammatory PGE(2) production in cultures of renal mesangial cells.

KEY RESULTS

AVX001 and AVX002 dose-dependently inhibited the group IVA cytosolic phospholipase A(2) (cPLA(2) ) in an in vitro activity assay with similar IC(50) values for AVX001 and AVX002, whereas the known cPLA(2) inhibitor AACOCF(3) was less potent and docosahexaenoic acid (DHA) was inactive. In renal mesangial cells, AVX001 and AVX002 suppressed IL-1β-induced PGE(2) synthesis. Mechanistically, this effect occurred by a down-regulation of IL-1β-induced group IIA-sPLA(2) protein expression, mRNA expression and promoter activity. A similar but less potent effect was seen with AACOCF(3) and no effect was seen with DHA. As gene expression of sPLA(2) is known to be regulated by the transcription factor NF-κB, we further investigated NF-κB activation. Both compounds prevented NF-κB activation by blocking degradation of the inhibitor of κB.

CONCLUSIONS AND IMPLICATIONS

These data show for the first time that the novel cPLA(2) inhibitors AVX001 and AVX002 exert an anti-inflammatory effect in cultures of renal mesangial cells and reduce the pro-inflammatory mediator PGE(2) through an inhibitory effect on NF-κB activation. Therefore, these compounds may represent promising novel drugs for the treatment of inflammatory disorders.

Cytokine release from human leukocytes exposed to silorane- and methacrylate-based dental materials

OBJECTIVES

Silorane-based dental monomers contain an epoxy functional group. Less is known about the toxicological and inflammatory potential of silorane-based composites. Therefore we compared the release of 24 cytokines from human leukocytes after incubation with silorane-based Filtek™ Silorane (Silo) and methacrylate-based TetricEvo Flow® (TC).

METHODS

Leukocytes from nine healthy test persons (P) were incubated with Silo or TC for up to 72h. All 24h cytokines were quantified with a magnetic bead assay.

RESULTS

Silo stimulates the leukocytes to higher release of cytokines when compared to TC. 72h after beginning the experiment, leukocytes from P6 incubated with Silo secreted more than an 18-fold amount of interleukin (IL)-6 when compared with leukocytes incubated with TC (771.8 vs 42.1pg/ml). Only leukocytes from P8 incubated with Silo release up to 14.4pg/ml IL-2 after 72h.

SIGNIFICANCE

The significantly higher induction of cytokines with Silo in comparison to TC is test person independent. This indicates a higher sensitization potential for Silo. Because of the cytokine release pattern (especially the release of T-cell dependent IL-2) from leukocytes from P8 after incubation with Silo it is likely that P8 can develop an allergic Type IV sensitization to Silo. Therefore the cytokine release assay is a helpful tool for providing information about possible immunological reactions to dental resins in individual cases as well as for a general risk assessment and comparison between different dental materials.

Interleukin-1beta: a bridge between inflammation and excitotoxicity?

Interleukin-1 (IL-1) is a proinflammatory cytokine released by many cell types that acts in both an autocrine and/or paracrine fashion. While IL-1 is best described as an important mediator of the peripheral immune response during infection and inflammation, increasing evidence implicates IL-1 signaling in the pathogenesis of several neurological disorders. The biochemical pathway(s) by which this cytokine contributes to brain injury remain(s) largely unidentified. Herein, we review the evidence that demonstrates the contribution of IL-1beta to the pathogenesis of both acute and chronic neurological disorders. Further, we highlight data that leads us to propose IL-1beta as the missing mechanistic link between a potential beneficial inflammatory response and detrimental glutamate excitotoxicity.

Paradox of glucocorticoid-induced cytosolic phospholipase A2 group IVA messenger RNA expression involves glucocorticoid receptor binding to the promoter in human amnion fibroblasts

Glucocorticoids (GCs) are well-known anti-inflammatory drugs inhibiting prostaglandin production. Paradoxically, GCs are reported to stimulate cytosolic phosphoplipase A2 group IVA (PLA2G4A) and prostaglandin-endoperoxide synthase 2 (PTGS2) expression in human amnion fibroblasts. This study was designed to examine the molecular mechanisms underlying glucocorticoid-induced PLA2G4A expression in human amnion fibroblasts. Our data showed that cortisol (0.01 approximately 1 microM) increased PLA2G4A mRNA level in a dose-dependent manner in human amnion fibroblasts, which was blocked by glucocorticoid receptor antagonist RU486 (1 microM) as well as by the mRNA transcription inhibitor 5,6-dichlorobenzimidazole riboside (DRB; 75 microM). Concurrently, cortisol (0.01 approximately 1 microM) decreased rather than increased proinflammatory cytokine mRNA levels, including interleukin 1 beta (IL1B), interleukin 6 (IL6), and tumor necrosis factor alpha (TNF), in a dose-dependent manner in human amnion fibroblasts. Chromatin immunoprecipitation assay revealed that glucocorticoid receptor was bound to PLA2G4A promoter in human amnion fibroblasts upon cortisol stimulation. This was confirmed by electrophoretic mobility shift assay showing that nuclear protein extracted from human amnion fibroblasts upon cortisol stimulation could bind the synthesized oligonucleotide sequence corresponding to PLA2G4A promoter region from -95 bp to -65 bp bearing the putative glucocorticoid response element. This binding was super shifted by glucocorticoid receptor antibody. In conclusion, we demonstrated in this study that cortisol increased PLA2G4A mRNA level via GR-dependent ongoing transcription in human amnion fibroblasts by activating the binding of GR to PLA2G4A promoter directly, and this effect appeared unlikely to be secondary to the effect of cortisol on the expression of proinflammatory cytokines in human amnion fibroblasts.

FTY720 suppresses interleukin-1beta-induced secretory phospholipase A2 expression in renal mesangial cells by a transcriptional mechanism

BACKGROUND AND PURPOSE

FTY720 is a potent immunomodulatory prodrug that is converted to its active phosphorylated form by a sphingosine kinase. Here we have studied whether FTY720 mimicked the action of sphingosine-1-phosphate (S1P) and exerted an anti-inflammatory potential in renal mesangial cells.

EXPERIMENTAL APPROACH

Prostaglandin E(2) (PGE(2)) was quantified by an enzyme-linked immunosorbent-assay. Secretory phospholipase A(2) (sPLA(2)) protein was detected by Western blot analyses. mRNA expression was determined by Northern blot analysis and sPLA(2)-promoter activity was measured by a luciferase-reporter-gene assay.

KEY RESULTS

Stimulation of cells for 24 h with interleukin-1beta (IL-1beta) is known to trigger increased PGE(2) formation which coincides with an induction of the mRNA for group-IIA-sPLA(2) and protein expression. FTY720 dose-dependently suppressed IL-1beta-induced IIA-sPLA(2) protein secretion and activity in the supernatant. This effect is due to a suppression of cytokine-induced sPLA(2) mRNA expression which results from a reduced promoter activity. As a consequence of suppressed sPLA(2) activity, PGE(2) formation is also reduced by FTY720. Mechanistically, the FTY720-suppressed sPLA(2) expression results from an activation of the TGFbeta/Smad signalling cascade since inhibition of the TGFbeta receptor type I by a specific kinase inhibitor reverses the FTY720-mediated decrease of sPLA(2) protein expression and sPLA(2) promoter activity.

CONCLUSIONS AND IMPLICATIONS

In summary, our data show that FTY720 was able to mimic the anti-inflammatory activity of TGFbeta and blocked cytokine-triggered sPLA(2) expression and subsequent PGE(2) formation. Thus, FTY720 may exert additional in vivo effects besides the well reported immunomodulation and its anti-inflammatory potential should be considered.

Secretory phospholipase A2 of group IIA: Is it an offensive or a defensive player during atherosclerosis and other inflammatory diseases?

Since its discovery in the serum of patients with severe inflammation and in rheumatoid arthritic fluids, the secretory phospholipase A2 of group IIA (sPLA2-IIA) has been chiefly considered as a proinflammatory enzyme, the result of which has been very intense interest in selective inhibitors of sPLA2-IIA in the hope of developing new and efficient therapies for inflammatory diseases. The recent discovery of the antibacterial properties of sPLA2-IIA, however, has raised the question of whether the upregulation of sPLA2-IIA during inflammation is to be considered uniformly negative and the hindrance of sPLA2-IIA in every instance beneficial. The aim of this review is for this reason, along with the results of various investigations which argue for the proinflammatory and proatherogenic effects of an upregulation of sPLA2-IIA, also to array data alongside which point to a protective function of sPLA2-IIA during inflammation. Thus, it could be shown that sPLA2-IIA, apart from the bactericidal effects, possesses also antithrombotic properties and indeed plays a possible role in the resolution of inflammation and the accelerated clearance of oxidatively modified lipoproteins during inflammation via the liver and adrenals. Based on these multipotent properties the knowledge of the function of sPLA2-IIA during inflammation is a fundamental prerequisite for the development and establishment of new therapeutic strategies to prevent and treat severe inflammatory diseases up to and including sepsis.

Phospholipase A2 inhibitors in development

To date, three isoforms of phospholipase A2 (PLA2) have been identified. Of these, the two Ca2+-dependent isoforms, secretory (sPLA2) and cytosolic phospholipase A2 (cPLA2), are targets for new anti-inflammatory drugs. The catalytic mechanisms and functions of the third isoform, Ca2+-independent cytosolic phospholipase A2 (iPLA2), are unknown at present. sPLA2 and cPLA2 are both implicated in the release of arachidonic acid and prophlogistic lipid mediators. However, recent findings provide evidence that cPLA2 is the dominant isoform in various kinds of inflammation, such as T-cell-mediated experimental arthritis. A triple function of PLA2-derived lipid mediators has been suggested: causing immediate inflammatory signs, involvement in secondary processes, e.g., superoxide free radical (O2) generation, apoptosis, or tumour necrosis factor-alpha (TNF-alpha)-cytotoxicity, and controlling the expression and activation of pivotal proteins implicated in inflammation and cell development, e.g., cytokines, adhesion proteins, proteinases, NF-kappaB, fos/jun/AP-1, c-Myc, or p21ras. In the past, research predominantly focused on the development of sPLA2 inhibitors; however, present techniques enable discrimination of cPLA2, sPLA2, and iPLA2, and specific inhibitors of each of the three isoforms are likely to appear soon. Over the last decade, between 40 and 50 sPLA2 inhibitors have been described; and the list is growing. However, of these, few have the potential for clinical success, and those that do are predominantly active site-directed inhibitors, e.g., BMS-181162, LY311727, ARL-67974, FPL67047, SB-203347, Ro-23-9358, YM-26734, and IS-741. At present, there are no likely clinical candidates emerging from the ranks of cPLA2 and iPLA2 inhibitors in development. Indications for which PLA2 inhibitors are being pursued include, sepsis, acute pancreatitis, inflammatory skin and bowel diseases, asthma, and rheumatoid arthritis. The three main obstacles to the successful development of PLA2 inhibitors include, insufficient oral bioavailability, low affinity for the enzyme corresponding to low in vivo efficacy and insufficient selectivity.

Inhibition of Rho modulates cytokine-induced prostaglandin E2 formation in renal mesangial cells

Stimulation of rat mesangial cells for 24 h with interleukin-1beta (IL- 1beta) plus forskolin (Fk) leads to a marked increase in prostaglandin E2 (PGE2) synthesis. This effect is further enhanced by the small G-protein Rho inhibitor toxin A. A similar increase in PGE2 formation is obtained with Y27632, a Rho-dependent kinase inhibitor, and with lovastatin, a hydroxymethylglutaryl-coenzyme A inhibitor which depletes cells from geranylgeranyl moieties and thus blocks Rho activation. In parallel to the increased PGE2 synthesis, a potentiation of IL-1beta-induced secretory group IIA phospholipases A2 (sPLA2-IIA) protein expression also occurs by Rho inhibition. However, only toxin A triggers an increased sPLA2-IIA activity consistent with the elevated levels of protein expression, whereas Y27632 and lovastatin rather reduced IL-1beta-induced sPLA2-IIA activity. In vitro activity studies reveal that Y27632 and lovastatin can directly block sPLA2-IIA enzyme activity in a concentration-dependent manner. Interestingly, in the absence of IL-1beta/Fk stimulation and the lack of sPLA2-IIA protein expression, all Rho inhibitors exert a small but significant increase in PGE2 formation suggesting that additional PLA2s or downstream enzymes like cyclooxygenases or prostaglandin synthases may be activated by Rho inhibitors. Western blot analyses of toxin A-, Y27632- and lovastatin-stimulated cells reveal that the cytosolic group IV PLA2 (cPLA2) and the cytosolic PGE2 synthase (cPGES), but not the sPLA2-IIA, cyclooxygenase-2 or the microsomal PGE2 synthase (mPGES), are upregulated compared to unstimulated cells. Furthermore, the Rho inhibitors induced arachidonic acid release from intact cells which is blocked by the cPLA2 inhibitor methyl arachidonyl fluorophosphonate (MAFP). In summary, these data show that inhibition of the small G-protein Rho, either by toxin A, lovastatin, or Y27632, exert a dual effect on mesangial cells: (i) in the absence of an inflammatory stimulus it activates the constitutive cPLA2 and cPGE2 synthase and generates low amount of PGE2. (ii) In the presence of inflammatory cytokines it potentiates sPLA2-IIA expression and subsequent PGE2 formation. In addition, we identified lovastatin and Y27632 as direct inhibitors of sPLA2-IIA in a cell-free system.

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.

Treatment of acute and chronic gastrointestinal inflammation

Treating inflammation in the equine gastrointestinal tract remains a challenge. Our most potent anti-inflammatory drugs, COX inhibitors and glucocorticoids, have unwanted effects on the gastrointestinal tract and host defense that often limit their use. Newer strategies targeting specific cells and molecules that regulate a subset of the events occurring during inflammation are rapidly becoming available and should allow clinicians to reduce the detrimental effects of inflammation without inhibiting the beneficial aspects.

Potentiation of tumor necrosis factor alpha-induced secreted phospholipase A2 (sPLA2)-IIA expression in mesangial cells by an autocrine loop involving sPLA2 and peroxisome proliferator-activated receptor alpha activation

In rat mesangial cells, exogenously added secreted phospholipases A2 (sPLA2s) potentiate the expression of pro-inflammatory sPLA2-IIA first induced by cytokines like tumor necrosis factor-alpha (TNFalpha) and interleukin-1 beta. The transcriptional pathway mediating this effect is, however, unknown. Because products of PLA2 activity are endogenous activators of peroxisome proliferator-activated receptor alpha (PPAR alpha, we postulated that sPLA2s mediate their effects on sPLA2-IIA expression via sPLA2 activity and subsequent PPAR alpha activation. This study shows that various sPLA2s, including venom enzymes, human sPLA2-IIA, and wild-type and catalytically inactive H48Q mutant of porcine pancreatic sPLA2-IB, enhance the TNF alpha-induced sPLA2-IIA expression at the mRNA and protein levels. In cells transfected with luciferase sPLA2-IIA promoter constructs, sPLA2s are active only when the promoter contains a functional PPRE-1 site. The effect of exogenous sPLA2s is also blocked by the PPAR alpha inhibitor MK886. Interestingly, the expression of sPLA2-IIA induced by TNF alpha alone is also attenuated by MK886, by the sPLA2-IIA inhibitor LY311727, by heparinase, which prevents the binding of sPLA2-IIA to heparan sulfate proteoglycans, and by the specific cPLA2-alpha inhibitor pyrrolidine-1. Together, these data indicate that sPLA2-IIA released from mesangial cells by TNF alpha stimulates its own expression via an autocrine loop involving cPLA2 and PPAR alpha. This signaling pathway is also used by exogenously added sPLA2s including pancreatic sPLA2-IB and is distinct from that used by TNF alpha.

Phospholipase A2 enzymes

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

Elevated cytosolic phospholipase A2 expression and activity in human neutrophils during sepsis

Sepsis is defined as the systemic inflammatory response to infection. Phospholipase A2 (PLA2) plays an important role in inflammation processes by initiating the production of inflammatory mediators. The role of cytosolic PLA (cPLA2) has not yet been identified in inflammatory and infectious disease clinical settings. The aim of the present research was to determine whether cPLA2 activity has a role during sepsis. Since neutrophil activation has been documented during sepsis, these cells were chosen as a model to evaluate the function of cPLA2 in this clinical setting. cPLA2 was studied at 3 levels: activity, protein expression, and messenger RNA (mRNA). Neutrophils from 32 septic patients with and without bacteremia were examined. cPLA2 activity was measured using labeled phosphatidyl choline vesicles as a substrate, and total PLA2 was determined by the release of labeled arachidonic acid from prelabeled cells. A significant increase in cPLA2activity, protein expression, and total PLA2 activity in neutrophils was detected during sepsis. mRNA levels, detected by reverse transcriptase–polymerase chain reaction, were significantly higher during sepsis, indicating that the increase in the amount of cPLA2 is regulated on the mRNA level. The significant elevation of cPLA2 activity and expression in neutrophils during sepsis suggests that this enzyme plays a major role in neutrophil function in this clinical setting.

Immunopharmacology: anti-inflammatory therapy targeting transcription factors

Immunopharmacology is one of the most dynamic areas in pharmacology encompassing classical immunosuppressive drugs which reveal completely new clues concerning their mode of action as well as novel molecular biology approaches for treating inflammatory and autoimmune diseases, infections and cancer. This article focuses on transcription factors that regulate cell activities involved in immune and inflammatory cell responses and how traditional anti-inflammatory compounds such as glucocorticoids, cyclosporins, tacrolismus and salicylates interfere with the activation cascades triggering the transcription factors. Moreover, promising new initiatives for selective therapeutics including recombinant anti-inflammatory cytokines and proinflammatory cytokine antagonists, and gene therapy will be presented.

Signal transduction pathways involved in the synergistic stimulation of prostaglandin production by interleukin-1beta and tumor necrosis factor alpha in human gingival fibroblasts

Accumulating evidence indicates that prostaglandins play an important role in the pathogenesis of periodontal disease. In this study, the effects and interactions between IL-1beta and TNFalpha on prostaglandin production and its regulation were investigated. The cytokines IL-1beta and TNFalpha stimulated prostaglandin E2 (PGE2) and prostacyclin (PGI2) production in gingival fibroblasts. Simultaneous treatment of the cells with IL-1beta and TNFalpha resulted in a synergistic stimulation of PGE2 and PGI2 formation. IL-1beta and, to a lesser extent, TNFalpha stimulated the release of 3H-arachidonic acid (3H-AA), and simultaneous addition of IL-1beta and TNFalpha further increased the release of 3H-AA from pre-labeled gingival fibroblasts. Furthermore, IL-1beta and, to a lesser extent, TNFalpha induced the expression of cyclooxygenase-2 (COX-2) mRNA. Simultaneous addition of IL-1beta and TNFalpha synergistically enhanced COX-2 mRNA levels, accompanied by a corresponding stimulation of PGE2 synthesis. Neither IL-1beta, TNFalpha, nor the combination of these two cytokines affected COX-1 mRNA levels. PMA, known to activate protein kinase C (PKC), enhanced the stimulatory effect of IL-1beta, TNFalpha, and the combination on COX-2 mRNA levels accompanied by a corresponding increase in PGE2 production. The phospholipase A2 (PLA2) inhibitor, BPB, and the PKC inhibitor, BIS, reduced PGE2 production, whereas dexamethasone, indomethacin, and NS-398 completely abolished PGE2 production induced by IL-1beta, TNFalpha, and the combination. The study indicates that the synergistic stimulation of prostaglandin production by IL-1beta, and TNFalpha is mediated partly at the level of COX-2 and partly at the level of PLA2 and that PKC is involved in the signal transduction of the synergy between the two cytokines. The synergy between IL-1beta and TNFalpha may play an important role in the inflammatory processes in gingival tissue in vivo.

Regulation of the cellular expression of secretory and cytosolic phospholipases A2, and cyclooxygenase-2 by peptide growth factors

Secretory group II (sPLA2) and cytosolic (cPLA2) phospholipases A2 and cyclooxygenase-2 (Cox-2) play a pivotal role in release of proinflammatory eicosanoids. Excessive activity of sPLA2 per se can also propagate inflammation. Endogenous control of the above enzymes has not been completely elucidated. We investigated the combined impact of promoting cytokines and inhibitory peptide growth factors on the expression of mRNA of the above enzymes, on protein content and extracellular release of sPLA2 and on PGE2 production in osteoblasts (FRCO). The synthesis and release of sPLA2 were enhanced by about 20-fold by 0.5 ng/ml IL-1beta or by 50 ng/ml of TNFalpha. Coaddition of both cytokines resulted in synergistic 150-fold increase in the release of sPLA2 implying the existence of two paths of induction. IL-1beta and TNFalpha markedly enhanced the transcription of sPLA2 mRNA. Kinetic study showed that IL-1/TNF initiated sPLA2 release after 12 h, reaching maximum at 48 h. IL-1alpha was a weak stimulator of sPLA2 release, whereas IL-6, IL-8, IGF, IFN-gamma, growth hormone, insulin and GM-CSF were not stimulatory. Peptide growth hormones TGFbeta, PDGF-BB, EGF and bFGF markedly inhibited the extracellular release of sPLA2. TGFbeta and PDGF-BB significantly reduced the level of sPLA2 mRNA, thus acting upon transcription whereas EGF and bFGF were not inhibitory, acting rather upon the translational or posttranslational steps. IL-1/TNF and growth factors had no significant effect on cPLA2 mRNA expression. Cox-2 mRNA expression was markedly enhanced by IL-1/TNF and suppressed by all growth factors tested. Cytokines enhanced the extracellular release of PGE2 and further enhancement was induced by growth factors with the exception of TGFbeta. Cycloheximide abolished completely the release of sPLA2 and markedly reduced the release of PGE2 from cytokine-stimulated FRCO, regardless of whether growth factors were present or not. NS-398, a specific inhibitor of Cox-2 abolished almost completely the release of PGE2 from cytokine-stimulated cells, regardless of the presence of growth factors. Thus, different signalling mechanisms are involved in the impact of growth factors on mRNA expression of sPLA2, cPLA2 and Cox-2. The differences between the impact on FRCO sPLA2 and that reported in other cells, imply that endogenous control of arachidonic acid cascade is cell-specific.

Multiple controls in inflammation. Extracellular and intracellular phospholipase A2, inducible and constitutive cyclooxygenase, and inducible nitric oxide synthase

Inflammation occurs as a defensive response to invasion of the host by foreign material, often of microbial nature. This response is normally a localized protective response that at the microscopic level involves a complex series of events including dilatation of arterioles, venules, and capillaries with increased vascular permeability, exudation of fluids including plasma proteins, and leukocyte migration into the inflammatory area. Since disease characterized by inflammation is an important cause of morbidity and mortality in humans, the processes involved in the host defense in inflammation have been and continue to be the object of several experimental studies. The role of several mediators such as histamine, serotonin, bradykinin, prostaglandins, and, more recently, cytokines and nitric oxide has been evaluated, and a contribution for each one of these mediators has been proposed. With the development of powerful molecular biology tools, it has become possible to study enzymes involved in this complex phenomenon by measuring the expression or evaluating the signaling pathways following a specific stimulus. These techniques have generated a proliferation of studies on the role of several enzymes and cytokines in inflammation. Most of these studies have been conducted in vitro on cell lines, and not many of the results have been confirmed by in vivo studies. This commentary does not pretend to analyze all of the studies and their possible in congruences, but endeavors to provoke in the reader a critical review of dogmas and current beliefs that most of the time are built on unilateral interpretation of the data.

Differential routes of Ca2+ influx in Swiss 3T3 fibroblasts in response to receptor stimulation

Ca2+ influx into cells in response to stimulation of various receptors was studied with Swiss 3T3 fibroblasts. The mechanisms involved were found to be so diverse that they were classified into four groups, Type I to IV. Type-I influx occurred, via pertussis toxin-susceptible G-proteins, immediately after internal Ca2+ mobilization by bradykinin, thrombin, endothelin, vasopressin or angiotensin II. Type-II influx induced by bombesin differed from Type I in its insusceptibility to pertussis toxin treatment. Ca2+ influx induced by prostaglandin E1, referred to as Type-III influx, was unique in that phospholipase C was apparently not activated without extracellular Ca2+, strongly suggesting that the Ca2+ influx preceded and was responsible for InsP3 generation and internal Ca2+ mobilization. More Ca2+ entered the cells more slowly via the Type-IV route opened by platelet-derived and other growth factors. These types of Ca2+ influx could be differentiated by their different susceptibilities to protein kinase C maximally activated by 1 h of exposure of cells to PMA, which inhibited phospholipase Cbeta coupled to receptors involved in Type-I and -II influx but did not inhibit growth-factor-receptor-coupled phospholipase Cgamma. Type-I and -II Ca2+ influxes, together with store-operated influx induced by thapsigargin, were not directly inhibited by exposure of cells to PMA, but Type-III and -IV influxes were completely inhibited. In addition, stimulation of receptors involved in Type-I and -IV Ca2+ influx, but not Type-II and -III influx, led to phospholipase A2 activation in the presence of extracellular Ca2+. Inhibition of Type-I and -IV Ca2+ influxes by their respective inhibitors, diltiazem and nifedipine, resulted in abolition of phospholipase A2 activation induced by the respective receptor agonists, in agreement with the notion that Ca2+ influx via these routes is responsible for receptor-mediated phospholipase A2 activation.

Increased renal and vascular cytosolic phospholipase A2 activity in rats with cirrhosis and ascites

Indirect evidence suggests that the renal and vascular production of prostaglandins is increased in cirrhosis with ascites. However, the activity of the enzymes regulating the prostaglandin pathway has not been investigated in cirrhosis. The aim of the current study was to determine the activity of phospholipase A2 (PLA2), the key enzyme in the regulation of prostaglandin synthesis, in kidney and vascular tissue obtained from rats with carbon tetrachloride-induced cirrhosis and ascites (n = 9) and control rats (n = 6). PLA2 activity was assayed in vitro using [14C]arachidonyl-phosphatidylcholine (PC) and [14C]arachidonyl-phosphatidylethanolamine (PE) as substrates in the presence of Ca2+. Kidneys from cirrhotic rats had significantly higher PLA2 activity compared with control rats, with both PC and PE (35 +/- 5 and 40 +/- 6 vs. 21 +/- 2 and 26 +/- 3 pmol/mg/min, respectively; P < .05 for both). PLA2 activity was increased in the renal cortex as well as in the renal medulla. Fractionation of the kidney extracts by Mono-Q anion-exchange chromatography showed that the elution position of PLA2 activity corresponded to the cytosolic PLA2 isoform (cPLA2). Increased amounts of cPLA2 protein were found in kidney extracts immunoblotted with an anti-cPLA2 antibody However, reverse-transcriptase polymerase chain reaction (RT-PCR) analysis did not detect any difference in cPLA2 mRNA. PLA2 activity was also higher in aortic tissue from cirrhotic rats than in controls (PC 38 +/- 5 vs. 26 +/- 1 and PE 66 +/- 8 vs. 41 +/- 3 pmol/mg/min; P < .05 for both). Incubation of renal and aortic extracts from cirrhotic rats with anti-cPLA2 antibody reduced PLA2 activity by 64% and 88%, respectively. In conclusion, PLA2 activity is increased in kidneys and vascular tissue from cirrhotic rats with ascites. This can be accounted for by an induction of cPLA2, which would mediate, at least in part, the increased renal and vascular production of prostaglandins in cirrhosis.

Topical glucocorticoids and the skin--mechanisms of action: an update

The topical glucocorticoids (GCs) represent the treatment of choice for many types of inflammatory dermatoses. Despite the extensive use of this class of drugs as first line therapy the mechanism of their action is uncertain. It is clear that the multiplicity of actions of the topical GCs is an important facet of their scope in the treatment of dermal disorders. The aim of this update is to review past and current theories regarding how these agents might work. Current understanding of the molecular mechanisms of GC action has advanced significantly over the past decade with the realisation that multiple systems are responsible for transduction of GC effects at a molecular level. The two primary modes of action are via interaction directly with DNA or indirectly through modulation of specific transcription factors: the endpoint in both cases being modulation of specific protein synthesis. Both of these mechanisms will be discussed. In particular this review will concentrate on the possibility that a GC-inducible protein, termed lipocortin 1, may have a significant role to play in the anti-inflammatory actions of these drugs. Additionally it has become apparent that several inflammatory enzymes induced in inflammation are sites of inhibitory action of the GCs, and the possibility that this occurs in the skin will be discussed paying particular attention to the inducible phospholipase A2, nitric oxide synthase and cyclooxygenase systems.

Influence of interleukin 1alpha on superoxide anion, platelet activating factor release and phospholipase A2 activity of naive and sensitized guinea-pig alveolar macrophages

1. We studied the effect exerted by hr-interleukin-1alpha (IL-1alpha) on responsiveness of alveolar macrophages (AM) from naive and sensitized guinea-pigs, through O2.- production (by ferricytochrome C reduction), platelet-activating factor (PAF) release (by platelet aggregation), prostaglandin E2 (PGE2) release (by a radioimmunoassay), and cytosolic phospholipase A2 (cPLA2) activity (by hydrolysis of radioactive substrate). 2. In naive guinea-pig AM, 0.06 nM hr-IL-1alpha pretreatment decreased by 65% O2.- release stimulated with 10 nM fMLP. In contrast, O2.- production was not affected in sensitized guinea-pig AM. 3. O2.- release elicited by fMLP stimulation in both cell groups was affected by PLA2 inhibitors (10 microM bromophenacyl bromide, BPB or 10 microM methylprednisolone, MP). In contrast, 10 microM arachidonyl trifluoromethyl ketone (AACOCF3), a cPLA2 inhibitor, was ineffective. 4. In naive AM, PAF release was elicited by hr-IL-1alpha pretreatment and by separate fMLP-stimulation, but when the stimulus was added to hr-IL-1alpha-pretreated cells inhibition of PAF release was observed. In sensitized AM, PAF release was lower than that found in naive guinea-pig AM in both hr-IL-1alpha-pretreated and fMLP-stimulated cells. 5. PGE2 release was unaffected by hr-IL-1alpha pretreatment and it was decreased by fMLP in both naive and sensitized AMs. The latter released less PGE2 than naive cells in basal conditions and after fMLP treatment. 6. Sensitized AM showed a greater cPLA2 activity in all experimental conditions in comparison to naive cells. cPLA2 activity assayed in the cytosolic fraction was found to be enhanced by hr-IL-1alpha pretreatment and by fMLP stimulation in naive but not in sensitized AM. However, when the stimulus was added to hr-IL-1alpha-pretreated cells we observed a decrease in cPLA2 activity in the cytosol and an increase in the membranes, thus suggesting a translocation of enzymatic activity. 7. In conclusion, hr-IL-1alpha can modulate the responsiveness of AM from naive and sensitized guinea-pigs, as suggested by changes found in the release of PAF and O2.- and in cPLA2 activity; therefore, sensitization itself may affect cellular responsiveness.

Cross-talk between secretory phospholipase A2 and cytosolic phospholipase A2 in rat renal mesangial cells

Incubation of rat glomerular mesangial cells with potent proinflammatory cytokines like interleukin 1beta, (IL- 1beta) triggers the expression of a non-pancreatic secretory phospholipase A2 (sPLA2) and increases the formation of prostaglandin E2. We show here that sPLA2 acts in an autocrine fashion on mesangial cells and induces a rapid activation of protein kinase C (PKC) isoenzymes delta and epsilon and of p42 mitogen-activated protein kinase (MAPK), two putative activators of cytosolic phospholipase A2 (cPLA2). sPLA2 also activates Raf-1 kinase in mesangial cells which integrates the signals coming from PKC for further processing along the MAPK cascade. Subsequently a phosphorylation and activation of cPLA2 is observed, thus arguing for a cross-talk between the two classes of PLA2. Pretreatment of cells with either the highly specific PKC inhibitor Ro-318220 or the highly specific MAPK kinase (MEK) inhibitor PD 98059 completely blocked the sPLA2-induced cPLA2 activation, indicating that both kinases are essential for the cross-talk between the two types of PLA2. The effect of sPLA2 is mimicked by lysophosphatidylcholine (LPC), a reaction product of sPLA2 activity. LPC stimulates PKC-epsilon, Raf-1 kinase and MAPK activation as well as cPLA2 activation with a subsequent increase in arachidonic acid release from mesangial cells. These data suggest that sPLA2 by cleaving membrane phospholipids and generating LPC and other lysophospholipids activates cPLA2 via the PKC/Raf-1/MAPK signalling pathway. Hence a network of interactions between different PLA2s is operative in mesangial cells and may contribute to the progression of glomerular inflammatory processes.

Synthesis, biological evaluation, and structure-activity relationships of 3-acylindole-2-carboxylic acids as inhibitors of the cytosolic phospholipase A2

3-Acylindole-2-carboxylic acid derivatives were prepared and evaluated for their ability to inhibit the cytosolic phospholipase A2 of intact bovine platelets. To define the structural requirements for enzyme inhibition, the carboxylic acid group, the acyl residue, and the moiety in position 1 were systematically modified. Furthermore, different substituents were introduced into the phenyl part of the indole. Replacement of the carboxylic acid group in position 2 of the indole with an acetic or propionic acid substituent led to a decrease of inhibitory potency. Enzyme inhibition was optimal when the acyl residue in position 3 had a length of 12 or more carbons. Conformational restriction of the acyl residue did not influence activity. Introduction of alkyl chains at position 1 of the indole with 8 or more carbons resulted in a loss of activity. However, replacing the omega-methyl group of such compounds with a carboxylic acid moiety was found to increase inhibitory potency significantly. Among the tested indole derivatives, 1-[2-(4-carboxyphenoxy)ethyl]-3-dodecanoylindole-2-carboxyli c acid (29b) had the highest potency. With an IC50 of 0.5 microM it was about 20-fold more active than the standard cPLA2 inhibitor arachidonyl trifluoromethyl ketone (IC50: 11 microM).

Possible participation of cyclooxygenase-2 in the recurrence of allergic inflammation in rats

In the recurrence of allergic inflammation in a rat air pouch model, pouch fluid volume, prostaglandin E2 concentration in the pouch fluid, leukocyte infiltration into the pouch fluid, and granulation tissue weight were markedly increased by the antigen challenge. To clarify the role of cyclooxygenase-2 in the recurrence of allergic inflammation, the time-course of changes in protein levels of cyclooxygenase-1 and cyclooxygenase-2 in the granulation tissue and in the infiltrated leukocytes was examined by Western blot analysis. It was shown that cyclooxygenase-1 levels in the granulation tissue and in the infiltrated leukocytes were not changed by the antigen challenge, but cyclooxygenase-2 levels were increased. Furthermore, treatment with the selective cyclooxygenase-2 inhibitor, NS-398 ([N-2(cyclohexyloxy-4-nitrophenyl]-methanesulfonamide), suppressed the recurrence of allergic inflammation as did the non-selective cyclooxygenase-1/cyclooxygenase-2 inhibitor, indomethacin. The steroidal anti-inflammatory drug, dexamethasone, inhibited the induction of cyclooxygenase-2, and suppressed the allergic inflammation. These findings strongly suggested that cyclooxygenase-2 induced by the antigen challenge plays a role in the recurrence of inflammation induced by the allergic mechanism.

Nitric oxide donors induce apoptosis in glomerular mesangial cells, epithelial cells and endothelial cells

Renal mesangial cells exposed to inflammatory cytokines produce high concentrations of nitric oxide (NO) which may exert cytotoxic actions. We report here that glomerular mesangial cells, endothelial cells and epithelial cells in culture are themselves targets for NO and undergo apoptotic cell death upon exposure to high concentrations of NO. NO generated from different NO-releasing compounds as well as NO-saturated solution induce apoptosis in all three cell types as demonstrated by internucleosomal DNA fragmentation, an enrichment of cytosolic DNA/histone complexes, an increasing number of cellular 3'-OH-fragmented DNA ends and typical nuclear chromatin condensation. Induction of apoptosis was found to be dependent on protein synthesis and is preceded by expression of the tumour suppressor gene product p53 in mesangial cells. Induction of inducible NO synthase in mesangial cells by interleukin-1 beta leads to excessive formation of NO by the cells as measured by nitrite production. However, there was no evidence for apoptotic changes in mesangial cells triggered by endogenously produced NO. Co-cultures of glomerular endothelial or epithelial cells with interleukin-1 beta-activated mesangial cells expressing inducible NO synthase do not show apoptotic alterations in endothelial or epithelial cells. Moreover, preincubation of mesangial cells with interleukin-1 beta protects the cells from apoptosis induced by subsequent addition of exogenous NO thus suggesting that interleukin-1 beta not only triggers the expression of inducible NO synthase and massive NO formation but simultaneously stimulates a protecting principle in the cells. In summary, these results suggest that exogenous NO can induce apoptosis in all three types of intrinsic glomerular cells. However, whether endogenously produced NO can fulfil this function critically depends on a balance between a yet to be defined protective mechanism and inducible NO synthase expression in mesangial cells in response to interleukin-1 beta and eventually other inflammatory cytokines.

3-(3,5-Dimethyl-4-octadecanoylpyrrol-2-yl)propionic acids as inhibitors of 85 kDa cytosolic phospholipase A2

3-(1,4-Diacylpyrrol-2-yl)propionic acids were designed as inhibitors of cytosolic phospholipase A2. Enzyme inhibition was assayed by evaluation of calcium ionophore A23187-induced arachidonic acid release from bovine platelets. While the synthesized bisacyl compound 3-[3,5-dimethyl-4-octadecanoyl-1-(3-phenylpropionyl)pyrrol-2-yl] propionic acid was inactive at 33 microM, the related monoacylated 3-(3,5-dimethyl-4-octadecanoylpyrrol-2-yl)-propionic acid and 3-(1,3,5-trimethyl-4-octadecanoylpyrrol-2-yl)-propionic acid proved to be inhibitors of cytosolic phospholipase A2(IC50: 24 microM and 13 microM, respectively).

Induction of both cytosolic phospholipase A2 and prostaglandin H synthase-2 by interleukin-1 beta in WISH cells in inhibited by dexamethasone

In previous studies we have shown that IL-1 beta induced both PGE2 release and total cellular cPLA2 activity and cPLA2 protein synthesis in human amnion-derived WISH cells. In this study, the effect of IL-1 beta on cPLA2 and PGHS-2 mRNA expression was investigated. Using RT-PCR, we found that IL-1 beta (0.1 ng/ml) coordinately induced both cPLA2 and PGHS-2 mRNA expression within 2 hours. The synthetic glucocorticoid dexamethasone (10(-10)-10(-6)M) inhibited IL-1 beta-induced cPLA2 and PGHS-2 mRNA expression activity and protein synthesis and PGE2 release in a concentration dependent manner. In the absence of IL-1 beta, dexamethasone alone (10(-6)M) inhibited basal cPLA2 activity, mRNA expression and protein synthesis. In addition, cycloheximide (5 micrograms/ml) apparently superinduced, but actinomycin D (2 micrograms/ml) inhibited IL-1 beta-induced cPLA2 and PGHS-2 mRNA expression suggesting that both are immediate early genes and a transcriptional mechanism is involved in the induction of both cPLA2 and PGHS-2 mRNA by IL-1 beta.

Interleukin-1 enhances pancreatic islet arachidonic acid 12-lipoxygenase product generation by increasing substrate availability through a nitric oxide-dependent mechanism

Interleukin-1 (IL-1) impairs insulin secretion from pancreatic islets and may contribute to the pathogenesis of insulin-dependent diabetes mellitus. IL-1 increases islet expression of nitric oxide (NO) synthase, and the resultant overproduction of NO participates in inhibition of insulin secretion because NO synthase inhibitors, e.g. NG-monomethyl-arginine (NMMA), prevent this inhibition. While exploring effects of IL-1 on islet arachidonic acid metabolism, we found that IL-1 increases islet production of the 12-lipoxygenase product 12-hydroxyeicosatetraenoic acid 12-(HETE). This effect requires NO production and is prevented by NMMA. Exploration of the mechanism of this effect indicates that it involves increased availability of the substrate arachidonic acid rather than enhanced expression of 12-lipoxygenase. Evidence supporting this conclusion includes the facts that IL-1 does not increase islet 12-lipoxygenase protein or mRNA levels and does not enhance islet conversion of exogenous arachidonate to 12-HETE. Mass spectrometric stereochemical analyses nonetheless indicate that 12-HETE produced by IL-1-treated islets consists only of the S-enantiomer and thus arises from enzyme action. IL-1 does enhance release of nonesterified arachidonate from islets, as measured by isotope dilution mass spectrometry, and this effect is suppressed by NMMA and mimicked by the NO-releasing compound 3-morpholinosydnonimine. Although IL-1 increases neither islet phospholipase A2 (PLA2) activities nor mRNA levels for cytosolic or secretory PLA2, a suicide substrate which inhibits an islet Ca(2+)-independent PLA2 prevents enhancement of islet arachidonate release by IL-1. IL-1 also impairs esterification of [3H8]arachidonate into islet phospholipids, and this effect is prevented by NMMA and mimicked by the mitochondrial ATP-synthase inhibitor oligomycin. Experiments with exogenous substrates indicate that NMMA does not inhibit and that the NO-releasing compound does not activate islet 12-lipoxygenase or PLA2 activities. These results indicate that a novel action of NO is to increase levels of nonesterified arachidonic acid in islets.

Maximal epidermal growth-factor-induced cytosolic phospholipase A2 activation in vivo requires phosphorylation followed by an increased intracellular calcium concentration

The 85 kDa cytosolic phospholipase A2 (cPLA2) preferentially catalyses the hydrolysis of arachidonic acid from the sn-2 position of phospholipids. cPLA2 can be activated by extracellular stimuli such as thrombin, platelet-derived growth factor and epidermal growth factor (EGF): A full activation of cPLA2 requires an increase of intracellular Ca2+ concentration and phosphorylation on Ser-505 by mitogen-activated protein (MAP) kinase. Because EGF can provoke an increase in intracellular [Ca2+] ([Ca2+]i) and activation of MAP kinase, we investigated the role of these pathways in EGF-induced activation of cPLA2. Characterization of two cell lines expressing different numbers of EGF receptors (HERc13 and HER14) revealed that both were activating MAP kinase in response to EGF, but only HER14 responded with an increase in [Ca2+]i. In this study we used both cell lines as a tool to clarify the role of each pathway in cPLA2 activation. We show that EGF stimulates cPLA2 activity in both cell lines in vitro as measured in cytosolic fractions, but only in HER14 in vivo as measured by 3H release from cells prelabelled with [3H]arachidonic acid. This latter activation can be restored in HERc13 cells by the addition of the ionophore A23187. Interestingly, this effect is only observed when EGF stimulation precedes A23187 addition. The phosphorylation of MAP kinase, however, was identical under identical conditions. We conclude that a maximal cPLA2 activation by EGF requires both, and in this order: MAP kinase activation followed by a rise in [Ca2+]i concentration.

Calcium-mediated translocation of cytosolic phospholipase A2 to the nuclear envelope and endoplasmic reticulum

Cytosolic phospholipase A2 (cPLA2) is activated by a wide variety of stimuli to release arachidonic acid, the precursor of the potent inflammatory mediators prostaglandin and leukotriene. Specifically, cPLA2 releases arachidonic acid in response to agents that increase intracellular Ca2+. In vitro data have suggested that these agents induce a translocation of cPLA2 from the cytosol to the cell membrane, where its substrate is localized. Here, we use immunofluorescence to visualize the translocation of cPLA2 to distinct cellular membranes. In Chinese hamster ovary cells that stably overexpress cPLA2, this enzyme translocates to the nuclear envelope upon stimulation with the calcium ionophore A23187. The pattern of staining observed in the cytoplasm suggests that cPLA2 also translocates to the endoplasmic reticulum. We find no evidence for cPLA2 localization to the plasma membrane. Translocation of cPLA2 is dependent on the calcium-dependent phospholipid binding domain, as a calcium-dependent phospholipid binding deletion mutant of cPLA2 (delta CII) fails to translocate in response to Ca2+. In contrast, cPLA2 mutated at Ser-505, the site of mitogen-activated protein kinase phosphorylation, translocates normally. This observation, combined with the observed phosphorylation of delta CII, establishes that translocation and phosphorylation function independently to regulate cPLA2. The effect of these mutations on cPLA2 translocation was confirmed by subcellular fractionation. Each of these mutations abolished the ability of cPLA2 to release arachidonic acid, establishing that cPLA2-mediated arachidonic acid release is strongly dependent on both phosphorylation and translocation. These data help to clarify the mechanisms by which cPLA2 is regulated in intact cells and establish the nuclear envelope and endoplasmic reticulum as primary sites for the liberation of arachidonic acid in the cell.

Cytosolic phospholipase A2

To summarize the regulation of cPLA2, we have proposed a model for the activation of cPLA2 based both on our previous studies (Clark et al., 1991; Lin et al., 1993) and the work of many others (Fig. 5). In this model, cPLA2 is tightly regulated by multiple pathways, including those that control Ca2+ concentration, phosphorylation states and cPLA2 protein levels, to exert both rapid and prolonged effects on cellular processes, such as inflammation. cPLA2 is rapidly activated by increased intracellular Ca2+ concentration and phosphorylation by MAP kinase. When cells are stimulated with a ligand for a receptor, such as ATP or PDGF, PLC is activated via either a G protein-dependent or -independent process, leading to the production of diacylglycerol (DAG) and inositol triphosphate (IP3). The rise in these intracellular messengers cause the activation of PKC and mobilization of intracellular Ca2+. Alternatively, the increase in intracellular Ca2+ can result from a Ca2+ influx. Increased Ca2+ acts through the CaLB domain to cause translocation of cPLA2 from the cytosol to the membrane where its substrate, phospholipid, is localized. This step is essential for the activation of cPLA2 and may account for the partial activation of cPLA2 in the absence of phosphorylation. MAP kinase activation can occur through both PKC-dependent and -independent mechanisms (Cobb et al., 1991; Posada and Cooper, 1992; Qiu and Leslie, 1994). In many cases, this pathway is also G protein-dependent. Activated MAP kinase phosphorylates cPLA2 at Ser-505, causing increased enzymatic activity of cPLA2, which is realized only upon translocation of cPLA2 to the membrane. Therefore, full activation of cPLA2 requires both increased cytosolic Ca2+ and cPLA2 phosphorylation at Ser-505. In a more delayed response, cPLA2 activity in the cells can be controlled by changes in its expression levels, such as in response to inflammatory cytokines and certain growth factors. Thus the expression level of cPLA2 is regulated by both transcriptional and post-transcriptional mechanisms.

Epidermal growth factor (EGF) induces serine phosphorylation-dependent activation and calcium-dependent translocation of the cytosolic phospholipase A2

Phospholipase A2 (PLA2) is a key enzyme in the release of arachidonic acid and subsequent production of eicosanoids, which play an important role in a variety of biological processes, including mitogenic signalling by epidermal growth factor (EGF). In a previous study [Spaargaren, M. et al. (1992) Biochem J. 287, 37-43] we identified the EGF-activated PLA2 as being similar to the recently cloned high-molecular-mass cytosolic phospholipase A2 (cPLA2). In the present study we demonstrate a rapid transient EGF-induced activation of this cPLA2 and an EGF-induced increase in phosphorylation of the cPLA2. The EGF-induced activation of cPLA2 is reversed upon phosphatase treatment showing phosphorylation-dependent activation of the cPLA2. No direct association of the cPLA2 to the EGF receptor was detected under conditions where such an association with phospholipase C-gamma was demonstrated. Phosphoamino acid analysis of this cPLA2 showed that EGF induced an increase in serine phosphorylation exclusively, no tyrosine phosphorylation being observed. EGF treatment of the cells resulted in a Ca(2+)-dependent translocation of the cPLA2 from the cytosol to the membrane fraction. This is due to an EGF-induced [Ca2+]i rise which is dependent on the influx of extracellular Ca2+ via voltage-independent Ca2+ channels. It is shown that the Ca(2+)-dependent association of cPLA2 to membranes does not require accessory membrane molecules.

Detection and purification of two 14 kDa phospholipase A2 isoforms in rat kidney: their role in eicosanoid synthesis

Phospholipase A2 (PLA2) activity in the soluble fraction of rat kidney yielded three peaks on DEAE cellulose column chromatography. From these three, we purified two PLA2 isoforms to near-homogeneity. Both had a molecular weight of approx. 14,000 on SDS-PAGE, and immunochemical and enzymological studies indicated that one is a 14 kDa type I PLA2 and the other a 14 kDa type II PLA2. RNA blot analysis confirmed that rat kidney contains both types of PLA2 and that administration of lipopolysaccharides and mercury chloride into rats increased type II PLA2 mRNA levels in kidney. When cultured rat mesangial cells were incubated with purified type I or type II PLA2 in combination with the calcium ionophore A23187 at suboptimal condition, augmentation of prostaglandin E2 production was observed. Type I and type II forms of PLA2 may play a role in arachidonate metabolism in rat kidney.

Differential regulation of elicited-peritoneal macrophage 14 kDa and 85 kDa phospholipase A2(s) by transforming growth factor-beta

Elicited guinea pig macrophages collected from inflammatory peritoneal exudate release soluble 14 kDa phospholipase A2 (PLA2) and prostaglandin E2 (PGE2) into surrounding media during culture (Marshall et al. (1994) J. Lipid Med. 10, 295-313). The effect of transformation growth factor beta 1 (TGF beta), an immunoregulatory growth factor, was examined in this system. Exposure of cultured macrophages to TGF beta reduced both the activity and protein levels of 14 kDa PLA2 measured in conditioned media. This inhibition occurred within the first 6-8 h, was prevalent through 72 h of exposure and was dependent on TGF beta concentration. The reduction, however, never reached more than 40-60%. Evaluation of the cellular PLA2 activity confirmed the existence of an immunologically-related 14 kDa PLA2 (ELISA) in the particulate fraction and an 85 kDa PLA2 (Western analysis) in the cytosol. Exposure to TGF beta halved the particulate activity and protein levels of 14 kDa PLA2 which was consistent with the reduction in the secreted form. Alternatively, TGF beta induced an increase in cytosolic 85 kDa PLA2 (activity and protein) which was not apparent until 12 h and significant at 20-24 h of exposure. This demonstrates that TGF beta differentially regulates the production of these two enzymes. Despite this, neither PGE2 synthesis nor the up-regulated cyclooxygenase -II were altered by TGF beta treatment suggesting that maximal prostanoid synthesis had been reached.

Dexamethasone down-regulates the 85 kDa phospholipase A2 in mouse macrophages and suppresses its activation

We have studied the effects of dexamethasone (dex) (i) on the level of the arachidonate-mobilizing phospholipase A2 (PLA2-85) in macrophages, (ii) on the stimulus-induced activation of this enzyme, and (iii) on the stimulus-induced release of arachidonate. Treatment of macrophages with 10 nM dex led to progressive reduction of PLA2-85 down to approx. 35% of control levels in 20 h in the absence of stimuli. This was accompanied by a partial inhibition of calcium-ionophore-induced arachidonate release. In contrast, the ability of zymosan or phorbol ester to cause both persistent activation of PLA2-85 and arachidonate release was greatly reduced or abolished. However, the protein phosphatase inhibitor okadaic acid, previously shown to cause enhanced phosphorylation and persistent activation of PLA2-85, was still able to exert this effect on the dex-suppressed PLA2-85. This suggests that the effect of okadaic acid was exerted at, or downstream of, the dex-sensitive step(s). Treatment with dex also led to inhibition of the characteristic changes in phosphoprotein labelling induced by phorbol ester or zymosan. However, phorbol-dibutyrate-binding isoforms of protein kinase C were not severely down-regulated. Thus dex was found to down-regulate PLA2-85 and, in addition, to affect one or more component(s) in the signal chain that normally leads to its activation. However, okadaic acid retained the ability to cause activation of PLA2-85.

Recent insights into the structure, function and biology of cPLA2

The 85-kDa cytosolic PLA2 (cPLA2) is present in most cells and tissues and its structural and functional properties have been described. Different agonists, growth factors and cytokines activate cPLA2 to hydrolyze cellular phospholipids thereby providing the precursor substrates for the biosynthesis of eicosanoids and platelet-activating factor (PAF), the well-known mediators of inflammatory and allergic reactions. Recent studies discussed here suggest that cPLA2 is a receptor-regulated enzyme involved in the inflammatory response. Therefore, inhibitors of cPLA2 may be useful as therapeutic agents in the treatment of inflammatory diseases.

Phosphorylation and activation of Ca(2+)-sensitive cytosolic phospholipase A2 in MCII mast cells mediated by high-affinity Fc receptor for IgE

In the present study we examined the activation of Ca(2+)-sensitive cytosolic phospholipase A2 (cPLA2) after aggregation of cell-surface high-affinity Fc receptors for IgE (Fc epsilon RI) on mast cells. MCII mast cells (a factor-dependent bone-marrow-derived murine mast cell line) produce significant amounts of leukotriene C4 (LTC4) (70 ng/10(6) cells) on cross-linking of Fc epsilon RI. Using enzymic and immunochemical analysis we found that cPLA2 is the predominant form of this enzyme in MCII mast cells (0.2 micrograms/mg of total protein) and other forms (i.e. secretory PLA2 or Ca2+ independent cytosolic PLA2) could not be detected. Therefore MCII mast cells represent an excellent cellular model for the study of the biochemical mechanism(s) responsible for Fc epsilon RI-induced activation of cPLA2 and the involvement of cPLA2 in Fc epsilon RI-mediated production of LTC4. After activation of Fc epsilon RI by cross-linking, cPLA2 in MCII mast cells exhibited a decreased electrophoretic mobility and its enzyme activity was increased 3-fold. Treatment with phosphatase reversed both the altered electrophoretic mobility and the enhanced enzyme activity demonstrating that they were the result of Fc epsilon RI-induced phosphorylation. On cross-linking of Fc epsilon RI, cPLA2 was phosphorylated within 30 s and appeared to be an early substrate for Fc epsilon RI-activated protein kinases in MCII mast cells. Tyrosine phosphorylation may be a critical component in this process, as genistein, an inhibitor of protein tyrosine kinases, blocked the activation of cPLA2. Using anti-phosphotyrosine antibodies we observed that the activating phosphorylation was not on tyrosine residues of cPLA2, indicating that tyrosine kinases participate upstream in the signalling cascade that couples Fc epsilon RI to cPLA2. We conclude that in MCII mast cells cPLA2 is activated by kinase-dependent mechanisms and may be responsible for Fc epsilon RI-induced mobilization of arachidonic acid for the generation of LTC4.

Endothelin-1 stimulates cytosolic phospholipase A2 activity and gene expression in rat glomerular mesangial cells

Endothelin-1 (ET-1) stimulates vascular smooth muscle and mesangial cells to release prostaglandin E2 (PGE2), which can attenuate the vasoconstrictor and mitogenic effects of this peptide. Phospholipase A2 (PLA2)-mediated release of arachidonic acid from the sn-2 position of membrane phospholipids is thought to be one of the rate-limiting steps in prostaglandin (PG) synthesis. We evaluated the role of ET-1 to regulate gene expression, protein synthesis and enzymatic activity of cytosolic PLA2 (cPLA2), an intracellular form of the PLA2 enzyme family, in cultured rat mesangial cells using both acute and chronic incubation protocols. Acute ET-1-induced stimulation of cPLA2 activity was maximal after 10 minutes (181.1 +/- 6.84% of control), persisted for 40 minutes and did not require new protein synthesis. Heparin, a potent inhibitor of intracellular Ca2+ increase as well as mitogen-activated protein (MAP) kinase activation and cell proliferation, did not affect the rapid cPLA2 stimulation by ET-1. Chronic incubation of glomerular mesangial cells with ET-1 (1 to 24 hr) led to time- and dose-dependent increases in cPLA2 mRNA expression which was maximal after six hours, persisted up to 24 hours and which was accompanied by both cPLA2 protein formation, as assessed by Western analysis, as well as by stimulation of enzymatic activity. Inhibition of protein synthesis by cycloheximide increased basal cPLA2 mRNA accumulation in quiescent mesangial cells, and the combination of ET-1 and cycloheximide resulted in a greater induction of cPLA2 gene expression when compared to ET-1 alone. Actinomycin D treatment blocked the effect of ET-1 on cPLA2 mRNA accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)