Structure, function and regulation of Ca2+-sensitive cytosolic phospholipase A2 (cPLA2)

Protein kinase C isoform ε negatively regulates ADP-induced calcium mobilization and thromboxane generation in platelets

OBJECTIVE

Members of the protein kinase C (PKC) family are shown to positively and negatively regulate platelet activation. Although positive regulatory roles are extensively studied, negative regulatory roles of PKCs are poorly understood. We investigated the mechanism and specific isoforms involved in PKC-mediated negative regulation of ADP-induced functional responses.

METHODS AND RESULTS

A pan-PKC inhibitor, GF109203X, potentiated ADP-induced cPLA(2) phosphorylation and thromboxane generation as well as ERK activation and intracellular calcium (Ca(2+)(i)) mobilization, 2 signaling molecules, upstream of cPLA(2) activation. Thus, PKCs inhibit cPLA(2) activation by inhibiting ERK and Ca(2+)(i) mobilization. Because the inhibitor of classic PKC isoforms, GO-6976, did not affect ADP-mediated thromboxane generation, we investigated the role of novel class of PKC isoforms. ADP-induced thromboxane generation, calcium mobilization, and ERK phosphorylation were potentiated in PKCε null murine platelets compared with platelets from wild-type littermates. Interestingly, when thromboxane release is blocked, ADP-induced aggregation in PKCε knockout and wild-type was similar, suggesting that PKCε does not affect ADP-induced aggregation directly. PKCε knockout mice exhibited shorter times to occlusion in an FeCl(3)-induced arterial injury model and shorter bleeding times in tail-bleeding experiments.

CONCLUSIONS

We conclude that PKCε negatively regulates ADP-induced thromboxane generation in platelets and offers protection against thrombosis.

Inhibition of aldose reductase prevents endotoxin-induced inflammation by regulating the arachidonic acid pathway in murine macrophages

The bacterial endotoxin lipopolysaccharide (LPS) is known to induce release of arachidonic acid (AA) and its metabolic products, which play important roles in the inflammatory process. We have shown earlier that LPS-induced signals in macrophages are mediated by aldose reductase (AR). Here we have investigated the role of AR in LPS-induced release of AA metabolites and their modulation using a potent pharmacological inhibitor, fidarestat, and AR siRNA ablation in RAW264.7 macrophages and AR-knockout mouse peritoneal macrophages and heart tissue. Inhibition or genetic ablation of AR prevented the LPS-induced synthesis and release of AA metabolites such as PGE2, TXB, PGI2, and LTBs in macrophages. LPS-induced activation of cPLA2 was also prevented by AR inhibition. Similarly, AR inhibition also prevented the calcium ionophore A23187-induced cPLA2 and LTB4 in macrophages. Further, AR inhibition by fidarestat prevented the expression of AA-metabolizing enzymes such as COX-2 and LOX-5 in RAW264.7 cells and AR-knockout mouse-derived peritoneal macrophages. LPS-induced expression of AA-metabolizing enzymes and their catalyzed metabolic products was significantly lower in peritoneal macrophages and heart tissue from AR-knockout mice. LPS-induced activation of redox-sensitive signaling intermediates such as MAPKs, transcription factor NF-κB, and EGR-1, a transcriptional regulator of mPGES-1, which in collaboration with COX-2 leads to the production of PGE2, was also significantly prevented by AR inhibition. Taken together, our results indicate that AR mediates LPS-induced inflammation by regulating the AA-metabolic pathway and thus provide a novel role for AR inhibition in preventing inflammatory complications such as sepsis.

LDL from obese patients with the metabolic syndrome show increased lipid peroxidation and activate platelets

AIMS/HYPOTHESIS

This study assessed oxidative stress in LDL from obese patients with the metabolic syndrome and compared it with that in LDL from type 2 diabetic patients or control volunteers. It also determined the effect on platelets of LDL from the three groups.

METHODS

The profiles of lipids, fatty acids and fatty acid oxidation products were determined in LDL isolated from plasma of patients with the metabolic syndrome, patients with type 2 diabetes and volunteers (n = 10 per group). The effects of LDL from the participant groups on the platelet arachidonic acid signalling cascade and aggregation were investigated.

RESULTS

Compared with LDL from control volunteers, LDL from obese metabolic syndrome and type 2 diabetic patients had lower cholesteryl ester, higher triacylglycerol and lower ethanolamine plasmalogen levels. Proportions of linoleic acid were decreased in phosphatidylcholine and cholesteryl esters in LDL from both patient groups. Among the markers of lipid peroxidation, oxidation products of linoleic acid (hydroxy-octadecadienoic acids) and malondialdehyde were increased by 59% and twofold, respectively in LDL from metabolic syndrome and type 2 diabetic patients. LDL from metabolic syndrome and type 2 diabetic patients were equally potent in activating the platelet arachidonic acid signalling cascade through increased phosphorylation of p38 mitogen-activated protein kinase and cytosolic phospholipase A(2), and through increased thromboxane B(2) formation. LDL from patients with the metabolic syndrome and type 2 diabetes potentiated platelet aggregation by threefold and 3.5-fold respectively, whereas control LDL had no activating effects on platelets.

CONCLUSIONS/INTERPRETATION

The metabolic syndrome in obese patients, without or with diabetes, is associated with increased oxidative stress in LDL, which triggers platelet activation.

Emodin, a naturally occurring anthraquinone derivative, suppresses IgE-mediated anaphylactic reaction and mast cell activation

The high-affinity receptor for IgE (FcɛRI)-mediated activation of mast cells plays an important role in allergic diseases such as asthma, allergic rhinitis and atopic dermatitis. Emodin, a naturally occurring anthraquinone derivative in oriental herbal medicines, has several beneficial pharmacologic effects, such as anti-cancer and anti-diabetic activities. However, the anti-allergic effect of emodin has not yet been investigated. To assess the anti-allergic activity of emodin, in vivo passive anaphylaxis animal model and in vitro mouse bone marrow-derived mast cells were used to investigate the mechanism of its action on mast cells. Our results showed that emodin inhibited degranulation, generation of eicosanoids (prostaglandin D(2) and leukotriene C(4)), and secretion of cytokines (TNF-α and IL-6) in a dose-dependent manner in IgE/Ag-stimulated mast cells. Biochemical analysis of the FcɛRI-mediated signaling pathways demonstrated that emodin inhibited the phosphorylation of Syk and multiple downstream signaling processes including mobilization of intracellular Ca(2+) and activation of the mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and NF-κB pathways. When administered orally, emodin attenuated the mast cell-dependent passive anaphylactic reaction in IgE-sensitized mice. Thus, emodin inhibits mast cell activation and thereby the anaphylactic reaction through suppression of the receptor-proximal Syk-dependent signaling pathways. Therefore, emodin might provide a basis for development of a novel anti-allergic drug.

Regulation of phosphatidic Acid metabolism by sphingolipids in the central nervous system

This paper explores the way ceramide, sphingosine, ceramide 1-phosphate, and sphingosine 1-phosphate modulate the generation of second lipid messengers from phosphatidic acid in two experimental models of the central nervous system: in vertebrate rod outer segments prepared from dark-adapted retinas as well as in rod outer segments prepared from light-adapted retinas and in rat cerebral cortex synaptosomes under physiological aging conditions. Particular attention is paid to lipid phosphate phosphatase, diacylglycerol lipase, and monoacylglycerol lipase. Based on the findings reported in this paper, it can be concluded that proteins related to phototransduction phenomena are involved in the effects derived from sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide and that age-related changes occur in the metabolism of phosphatidic acid from cerebral cortex synaptosomes in the presence of either sphingosine 1-phosphate/sphingosine or ceramide 1-phosphate/ceramide. The present paper demonstrates, in two different models of central nervous system, how sphingolipids influence phosphatidic acid metabolism under different physiological conditions such as light and aging.

Intrinsic phospholipase A2 activity of adeno-associated virus is involved in endosomal escape of incoming particles

The unique region of the VP1 capsid protein of adeno-associated viruses (AAV) in common with autonomously replicating parvoviruses comprises a secreted phospholipase A2 (sPLA2) homology domain. While the sPLA2 domain of Minute Virus of Mice has recently been shown to mediate endosomal escape by lipolytic pore formation, experimental evidence for a similar function in AAV infection is still lacking. Here, we explored the function of the sPLA2 domain of AAV by making use of the serotype 2 mutant (76)HD/AN. The sPLA2 defect in (76)HD/AN, which severely impairs AAV's infectivity, could be complemented in trans by co-infection with wild-type AAV2. Furthermore, co-infection with endosomolytically active, but not with inactive adenoviral variants partially rescued (76)HD/AN, providing the first evidence for a function of this domain in endosomal escape of incoming AAV particles.

Genetic interactions between a phospholipase A2 and the Rim101 pathway components in S. cerevisiae reveal a role for this pathway in response to changes in membrane composition and shape

Modulating composition and shape of biological membranes is an emerging mode of regulation of cellular processes. We investigated the global effects that such perturbations have on a model eukaryotic cell. Phospholipases A(2) (PLA(2)s), enzymes that cleave one fatty acid molecule from membrane phospholipids, exert their biological activities through affecting both membrane composition and shape. We have conducted a genome-wide analysis of cellular effects of a PLA(2) in the yeast Saccharomyces cerevisiae as a model system. We demonstrate functional genetic and biochemical interactions between PLA(2) activity and the Rim101 signaling pathway in S. cerevisiae. Our results suggest that the composition and/or the shape of the endosomal membrane affect the Rim101 pathway. We describe a genetically and functionally related network, consisting of components of the Rim101 pathway and the prefoldin, retromer and SWR1 complexes, and predict its functional relation to PLA(2) activity in a model eukaryotic cell. This study provides a list of the players involved in the global response to changes in membrane composition and shape in a model eukaryotic cell, and further studies are needed to understand the precise molecular mechanisms connecting them.

TLR4-dependent induction of vascular adhesion molecule-1 in rheumatoid arthritis synovial fibroblasts: Roles of cytosolic phospholipase A(2)alpha/cyclooxygenase-2

Lipopolysaccharide (LPS)/Toll-like receptor 4 (TLR4)-mediated signaling pathways have caught the attention of strategies designed for rheumatoid arthritis (RA). In this study, we identified that cPLA(2)alpha acted as a modulator of LPS-induced VCAM-1 expression and THP-1 (human acute monocytic leukemia cell line) adherence. Treatment of RA synovial fibroblasts (RASFs) with LPS, a TLR4 agonist, promoted the VCAM-1 expression and THP-1 adherence which were decreased by pretreatment with a selective cytosolic phospholipase A(2) (cPLA(2)) inhibitor (AACOCF(3)), implying the involvement of cPLA(2)alpha in these responses. This notion was further confirmed by knockdown of cPLA(2)alpha expression by transfection with cPLA(2)alpha small interfering RNA (siRNA) leading to a decrease in VCAM-1 expression and THP-1 adherence induced by LPS. Subsequently, the LPS-stimulated cPLA(2)alpha phosphorylation was attenuated by pretreatment with a MEK1/2 inhibitor (U0126), suggesting that LPS-stimulated cPLA(2)alpha phosphorylation and activity are mediated through an ERK-dependent mechanism. Moreover, COX-2-derived PGE(2) production appeared to involve in LPS-induced VCAM-1 expression which was attenuated by pretreatment with selective COX-2 inhibitors (NS-398 and celecoxib), transfection with COX-2 siRNA, or PGE(2) receptor antagonists. In addition, pretreatment with ecosapentaenoic acid (EPA), a substrate competitor of arachidonic acid (AA), also blocked LPS-induced VCAM-1 mRNA and protein expression, and THP-1 adherence. Collectively, these results suggest that LPS-induced VCAM-1 expression and adhesion of THP-1 cells are mediated through the TLR4/ERK/cPLA(2)alpha phosphorylation and COX-2 expression/PGE(2) synthesis in RASFs.

Rat brain docosahexaenoic acid metabolism is not altered by a 6-day intracerebral ventricular infusion of bacterial lipopolysaccharide

In a rat model of neuroinflammation, produced by a 6-day intracerebral ventricular infusion of bacterial lipopolysaccharide (LPS), we reported that the brain concentrations of non-esterified brain arachidonic acid (AA, 20:4 n-6) and its eicosanoid products PGE(2) and PGD(2) were increased, as were AA turnover rates in certain brain phospholipids and the activity of AA-selective cytosolic phospholipase A(2) (cPLA(2)). The activity of Ca(2+)-independent iPLA(2), which is thought to be selective for the release of docosahexaenoic acid (DHA, 22:6 n-3) from membrane phospholipid, was unchanged. In the present study, we measured parameters of brain DHA metabolism in comparable artificial cerebrospinal fluid (control) and LPS-infused rats. In contrast to the reported changes in markers of AA metabolism, the brain non-esterified DHA concentration and DHA turnover rates in individual phospholipids were not significantly altered by LPS infusion. The formation rates of AA-CoA and DHA-CoA in a microsomal brain fraction were also unaltered by the LPS infusion. These observations indicate that LPS-treatment upregulates markers of brain AA but not DHA metabolism. All of which are consistent with other evidence that suggest different sets of enzymes regulate AA and DHA recycling within brain phospholipids and that only selective increases in brain AA metabolism occur following a 6-day LPS infusion.

Ceramide 1-phosphate is required for the translocation of group IVA cytosolic phospholipase A2 and prostaglandin synthesis

Little is known about the regulation of eicosanoid synthesis proximal to the activation of cytosolic phospholipase A(2)alpha (cPLA(2)alpha), the initial rate-limiting step. The current view is that cPLA(2)alpha associates with intracellular/phosphatidylcholine-rich membranes strictly via hydrophobic interactions in response to an increase of intracellular calcium. In opposition to this accepted mechanism of two decades, ceramide 1-phosphate (C1P) has been shown to increase the membrane association of cPLA(2)alpha in vitro via a novel site in the cationic beta-groove of the C2 domain (Stahelin, R. V., Subramanian, P., Vora, M., Cho, W., and Chalfant, C. E. (2007) J. Biol. Chem. 282, 20467-204741). In this study we demonstrate that C1P is a proximal and required bioactive lipid for the translocation of cPLA(2)alpha to intracellular membranes in response to inflammatory agonists (e.g. calcium ionophore and ATP). Last, the absolute requirement of the C1P/cPLA(2)alpha interaction was demonstrated for the production of eicosanoids using murine embryonic fibroblasts (cPLA(2)alpha(-/-)) coupled to "rescue" studies. Therefore, this study provides a paradigm shift in how cPLA(2)alpha is activated during inflammation.

In retinal vein occlusion platelet response to thrombin is increased

INTRODUCTION

Retinal vein occlusion is a major cause of ocular morbidity. The precise mechanism leading to thrombosis in retinal vein occlusion has not yet been clearly elucidated. Several risk factors have been identified, including hypertension diabetes, history of cardiovascular disease, hypercholesterolemia, hyperhomocysteinaemia, increased ocular pressure and glaucoma. Although thrombus formation in the vein plays a significant role in the onset of retinal vein occlusion, the relationship between platelet aggregation and retinal vein occlusion remains to be clarified.

MATERIALS AND METHODS

In the present study the platelet response to thrombin in a selected group of retinal vein occlusion patients was investigated. Retinal vein occlusion patients were compared to a group of healthy subjects matched for age, sex, clinical and metabolic characteristics. In resting and activated platelets of both groups of subjects total protein tyrosine phosphorylation, p38MAPK and cytosolic phospholipase A(2) phosphorylation, arachidonic acid release, intracellular calcium levels, thromboxane B(2) and superoxide anion formation were measured.

RESULTS

Results show that platelets of patients were more responsive to thrombin than healthy subjects. In resting or in thrombin stimulated platelets of patients total protein tyrosine phosphorylation, p38MAPK and cytosolic phospholipase A(2) phosphorylation were increased. Also arachidonic acid release, thromboxane B(2) and superoxide anion formation were higher in patients than in healthy subjects. In addition intracellular calcium rise induced by thrombin was increased in patients.

CONCLUSIONS

Altogether data suggest that platelet hyperaggregability inducing thrombus formation might be an important factor in the onset and/or development of retinal vein occlusion.

Calcium in the mechanism of ammonia-induced astrocyte swelling

Brain edema, due largely to astrocyte swelling, is an important clinical problem in patients with acute liver failure. While mechanisms underlying astrocyte swelling in this condition are not fully understood, ammonia and associated oxidative/nitrosative stress appear to be involved. Mechanisms responsible for the increase in reactive oxygen/nitrogen species (RONS) and their role in ammonia-induced astrocyte swelling, however, are poorly understood. Recent studies have demonstrated a transient increase in intracellular Ca2+ in cultured astrocytes exposed to ammonia. As Ca2+ is a known inducer of RONS, we investigated potential mechanisms by which Ca2+ may be responsible for the production of RONS and cell swelling in cultured astrocytes after treatment with ammonia. Exposure of cultured astrocytes to ammonia (5 mM) increased the formation of free radicals, including nitric oxide, and such increase was significantly diminished by treatment with the Ca2+ chelator 1,2-bis-(o-aminophenoxy)-ethane-N,N,-N',N'-tetraacetic acid tetraacetoxy-methyl ester (BAPTA). We then examined the activity of Ca2+-dependent enzymes that are known to generate RONS and found that ammonia significantly increased the activities of NADPH oxidase (NOX), constitutive nitric oxide synthase (cNOS), and phospholipase A2 (PLA2) and such increases in activity were significantly diminished by BAPTA. Pre-treatment of cultures with 7-nitroindazole, apocyanin, and quinacrine, respective inhibitors of cNOS, NOX, and PLA2, all significantly diminished RONS production. Additionally, treatment of cultures with BAPTA or with inhibitors of cNOS, NOX, and PLA2 reduced ammonia-induced astrocyte swelling. These studies suggest that the ammonia-induced rise in intracellular Ca2+ activates free radical producing enzymes that ultimately contribute to the mechanism of astrocyte swelling.

Activation of cytosolic phospholipase A2-{alpha} as a novel mechanism regulating endothelial cell cycle progression and angiogenesis

Release of endothelial cells from contact-inhibition and cell cycle re-entry is required for the induction of new blood vessel formation by angiogenesis. Using a combination of chemical inhibition, loss of function, and gain of function approaches, we demonstrate that endothelial cell cycle re-entry, S phase progression, and subsequent angiogenic tubule formation are dependent upon the activity of cytosolic phospholipase A(2)-alpha (cPLA(2)alpha). Inhibition of cPLA(2)alpha activity and small interfering RNA (siRNA)-mediated knockdown of endogenous cPLA(2)alpha reduced endothelial cell proliferation. In the absence of cPLA(2)alpha activity, endothelial cells exhibited retarded progression from G(1) through S phase, displayed reduced cyclin A/cdk2 expression, and generated less arachidonic acid. In quiescent endothelial cells, cPLA(2)alpha is inactivated upon its sequestration at the Golgi apparatus. Upon the stimulation of endothelial cell proliferation, activation of cPLA(2)alpha by release from the Golgi apparatus was critical to the induction of cyclin A expression and efficient cell cycle progression. Consequently, inhibition of cPLA(2)alpha was sufficient to block angiogenic tubule formation in vitro. Furthermore, the siRNA-mediated retardation of endothelial cell cycle re-entry and proliferation was reversed upon overexpression of an siRNA-resistant form of cPLA(2)alpha. Thus, activation of cPLA(2)alpha acts as a novel mechanism for the regulation of endothelial cell cycle re-entry, cell cycle progression, and angiogenesis.

Lysophosphatidic acid signaling during embryo development in sheep: involvement in prostaglandin synthesis

We investigated the lysophosphatidic acid (LPA) pathway during early pregnancy in sheep. LPA was detected in the uteri of early-stage pregnant ewes. Using quantitative RT-PCR, the expression of autotaxin, the LPA-generating enzyme, was found in the endometrium and conceptus. In the latter autotaxin, transcript levels were low on d 12-14 and increased on d 15-16, in parallel with the level of LPA. Autotaxin was localized in the luminal epithelium and superficial glands of the endometrium and in trophectoderm cells of the conceptus. The expression of G protein-coupled receptors for LPA was also examined in the ovine conceptus. LPA receptor LPAR1 and LPAR3 transcripts were expressed during early pregnancy and displayed a peak on d 14, whereas the highest level of protein for both receptors was observed at d 17. LPAR1 was localized in cellular membranes and nuclear compartments of the trophectoderm cells, whereas LPAR3 was revealed only in membranes. LPA activated phosphorylation of the MAPK ERK1/2 in ovine trophectoderm-derived cells. Moreover, the bioactive lipid increased the proliferation of trophectoderm cells in culture, as shown by thymidine and bromodeoxyuridine incorporation. Furthermore, LPA induced changes to the organization of beta-actin and alpha-tubulin, suggesting a role for it in rearrangement of trophectoderm cells cytoskeleton. Because a link had previously been established between prostaglandin and LPA pathways, we analyzed the effect of LPA on prostaglandin synthesis. LPA induced an increase in the release of prostaglandin F2alpha and prostaglandin E2, with no significant modifications to cytosolic phospholipase A2alpha and prostaglandin synthase-2 expression. Taken together, our results suggest a new role for LPA-mediated signaling in the ovine conceptus at the time of implantation.

The phosphoinositide 3-kinase-dependent activation of Btk is required for optimal eicosanoid production and generation of reactive oxygen species in antigen-stimulated mast cells

Activated mast cells are a major source of the eicosanoids PGD(2) and leukotriene C(4) (LTC(4)), which contribute to allergic responses. These eicosanoids are produced following the ERK1/2-dependent activation of cytosolic phospholipase A(2), thus liberating arachidonic acid, which is subsequently metabolized by the actions of 5-lipoxygenase and cyclooxygenase to form LTC(4) and PGD(2), respectively. These pathways also generate reactive oxygen species (ROS), which have been proposed to contribute to FcepsilonRI-mediated signaling in mast cells. In this study, we demonstrate that, in addition to ERK1/2-dependent pathways, ERK1/2-independent pathways also regulate FcepsilonRI-mediated eicosanoid and ROS production in mast cells. A role for the Tec kinase Btk in the ERK1/2-independent regulatory pathway was revealed by the significantly attenuated FcepsilonRI-dependent PGD(2), LTC(4), and ROS production in bone marrow-derived mast cells of Btk(-/-) mice. The FcepsilonRI-dependent activation of Btk and eicosanoid and ROS generation in bone marrow-derived mast cells and human mast cells were similarly blocked by the PI3K inhibitors, Wortmannin and LY294002, indicating that Btk-regulated eicosanoid and ROS production occurs downstream of PI3K. In contrast to ERK1/2, the PI3K/Btk pathway does not regulate cytosolic phospholipase A(2) phosphorylation but rather appears to regulate the generation of ROS, LTC(4), and PGD(2) by contributing to the necessary Ca(2+) signal for the production of these molecules. These data demonstrate that strategies to decrease mast cell production of ROS and eicosanoids would have to target both ERK1/2- and PI3K/Btk-dependent pathways.

Regulation of superoxide production in neutrophils: role of calcium influx

Upon stimulation, activation of NADPH oxidase complexes in neutrophils produces a burst of superoxide anions contributing to oxidative stress and the development of inflammatory process. Store-operated calcium entry (SOCE), whereby the depletion of intracellular stores induces extracellular calcium influx, is known to be a crucial element of NADPH oxidase regulation. However, the mechanistic basis mediating SOCE is still only partially understood, as is the signal-coupling pathway leading to modulation of store-operated channels. This review emphasizes the role of calcium influx in the control of the NADPH oxidase and summarizes the current knowledge of pathways mediating this extracellular calcium entry in neutrophils. Such investigations into the cross-talk between NADPH oxidase and calcium might allow the identification of novel pharmacological targets with clinical use, particularly in inflammatory diseases.

Ischaemic preconditioning and TNF-alpha-mediated preconditioning is associated with a differential cPLA2 translocation pattern in early ischaemia

Both the cytokine tumour necrosis factor-alpha (TNF-alpha) and the enzyme cytosolic phospholipase A2 (cPLA2) have been implicated in ischaemic injury. Apart from the induction of apoptosis, TNF-alpha also mediates cytoprotection when present in low concentrations. However, the relationship between TNF-alpha and cPLA2 activities during cytoprotection is poorly understood. Therefore, we examined the role of cPLA2 in TNF-alpha-mediated (TNF-PC) and ischaemic preconditioning (IPC) in tolerance to ischaemia (SI) in C2C12 myotubes. Significant decreases in cPLA2 phosphorylation in SI compared with the preconditioned groups were observed. This was also mirrored by the p38 mitogen activated protein kinase (MAPK) phosphorylation pattern. These results correlated with fluorescence- and three-dimensional imaging, demonstrating increased translocation of phospho-cPLA2 to the nuclear region in SI compared to TNF-PC and IPC. These data suggest a p38 driven cPLA2 translocation pattern, with a possible role for cPLA2 in deciding cell fate.

Identification of genes deregulated during serum-free medium adaptation of a Burkitt's lymphoma cell line

OBJECTIVE

Serum is usually added to growth media when mammalian cells are cultured in vitro to supply the cells with growth factors, hormones, nutrients and trace elements. Defined proteins and metal ions, such as insulin, growth factors, transferrin and sodium selenite, are sometimes also included and can in some cases substitute serum components. How adaptation to serum free media influences cells has not been studied in detail.

MATERIALS AND METHODS

We have adapted the Burkitt's lymphoma line Ramos to a serum-free medium that supports long-term survival and studied gene expression changes that occurred during the adaptation process.

RESULTS AND CONCLUSIONS

The adaptation process was characterized by initial cell population growth arrest, and after that extensive cell death, followed by proliferation and long-term survival of clonal cultures. Proliferation and cell cycle progression of the serum-free cultures closely mimicked that of serum-dependent cells. Affymetrix micro-array technology was used to identify gene expression alterations that had occurred during the adaptation. Most changes were subtle, but frequently the genes with altered expression were involved in basal cellular functions such as cell division, cell cycle regulation, apoptosis and cell signalling. Some alterations were restored when the cells were transferred back to serum-containing medium, indicating that expression of these genes was controlled by components in serum. Others were not, and may represent changes that were selected during the adaptation process. Among these were, for example, several genes within the Wnt signalling pathway.

Anionic lipids activate group IVA cytosolic phospholipase A2 via distinct and separate mechanisms

Previously, ceramide-1-phosphate (C1P) and phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] were demonstrated to be potent and specific activators of group IVA cytosolic phospholipase A2 (cPLA2alpha). In this study, we hypothesized that these anionic lipids functionally activated the enzyme by distinctly different mechanisms. Indeed, surface plasmon resonance and surface dilution kinetics demonstrated that C1P was a more potent effector than PI(4,5)P2 in decreasing the dissociation constant of the cPLA2alpha-phosphatidylcholine (PC) interaction and increasing the residence time of the enzyme on the vesicles/micelles. PI(4,5)P2, in contrast to C1P, decreased the Michaelis-Menten constant, increasing the catalytic efficiency of the enzyme. Furthermore, PI(4,5)P2 activated cPLA2alpha with a stoichiometry of 1:1 versus C1P at 2.4:1. Lastly, PI(4,5)P2, but not C1P, increased the penetration ability of cPLA2alpha into PC-rich membranes. Therefore, this study demonstrates two distinct mechanisms for the activation of cPLA2alpha by anionic lipids. First, C1P activates cPLA2alpha by increasing the residence time of the enzyme on membranes. Second, PI(4,5)P2 activates the enzyme by increasing catalytic efficiency through increased membrane penetration.

The Confluence-dependent Interaction of Cytosolic Phospholipase A2-α with Annexin A1 Regulates Endothelial Cell Prostaglandin E2 Generation

The regulated generation of prostaglandins from endothelial cells is critical to vascular function. Here we identify a novel mechanism for the regulation of endothelial cell prostaglandin generation. Cytosolic phospholipase A(2)-alpha (cPLA(2)alpha) cleaves phospholipids in a Ca(2+)-dependent manner to yield free arachidonic acid and lysophospholipid. Arachidonic acid is then converted into prostaglandins by the action of cyclooxygenase enzymes and downstream synthases. By previously undefined mechanisms, nonconfluent endothelial cells generate greater levels of prostaglandins than confluent cells. Here we demonstrate that Ca(2+)-independent association of cPLA(2)alpha with the Golgi apparatus of confluent endothelial cells correlates with decreased prostaglandin synthesis. Golgi association blocks arachidonic acid release and prevents functional coupling between cPLA(2)alpha and COX-mediated prostaglandin synthesis. When inactivated at the Golgi apparatus of confluent endothelial cells, cPLA(2)alpha is associated with the phospholipid-binding protein annexin A1. Furthermore, the siRNA-mediated knockdown of endogenous annexin A1 significantly reverses the inhibitory effect of confluence on endothelial cell prostaglandin generation. Thus the confluence-dependent interaction of cPLA(2)alpha and annexin A1 at the Golgi acts as a novel molecular switch controlling cPLA(2)alpha activity and endothelial cell prostaglandin generation.

Dietary palatinose and oleic acid ameliorate disorders of glucose and lipid metabolism in Zucker fatty rats

Excessive dietary intake of carbohydrates and fats has been linked to the development of obesity. However, the mechanism by which these dietary factors interact to bring about metabolic changes has not been elucidated. We examined the combined effects of different types of dietary carbohydrates and fats on the etiology of obesity and its complications in the Zucker fatty (fa/fa) rat, a model of obesity. Specifically, these rats were fed an isocaloric diet containing various combinations of carbohydrates [palatinose (P), an insulin-sparing sucrose analogue, and sucrose (S)] and fatty acids [oleic acid (O) and linoleic acid (L)]. After 8 wk, palatinose feeding (PO and PL) led to significant reductions in visceral fat mass, adipocyte cell size, hyperglycemia, and hyperlipidemia compared with sucrose feeding (SO and SL); pancreatic islet hypertrophy was also prevented by palatinose feeding. Linoleic-acid-fed rats (PL and SL) exhibited reduced insulin-immunoreactive staining of the pancreatic islets, enhanced macrophage infiltration in adipose tissue, and an elevated plasma tumor necrosis factor-alpha concentration when compared with oleic-acid-fed rats (PO and SO). Furthermore, sucrose and linoleic acid synergistically increased the expression of genes involved in hepatic gluconeogenesis and lipogenesis [sterol regulatory-element binding protein (SREBP)-1c and SREBP-2]. In conclusion, a diet containing palatinose and oleic acid may prevent diet-induced metabolic abnormalities. The combination of palatinose and oleic acid holds promise for a new approach to preventing and treating obesity and its complications.

Ceramide-1-phosphate Binds Group IVA Cytosolic Phospholipase a2 via a Novel Site in the C2 Domain

Previously, ceramide-1-phosphate (C1P) was demonstrated to be a potent and specific activator of group IV cytosolic phospholipase A(2)alpha (cPLA(2)alpha) via interaction with the C2 domain. In this study, we hypothesized that the specific interaction site for C1P was localized to the cationic beta-groove (Arg(57), Lys(58), Arg(59)) of the C2 domain of cPLA(2)alpha. In this regard, mutants of this region of cPLA(2)alpha were generated (R57A/K58A/R59A, R57A/R59A, K58A/R59A, R57A/K58A, R57A, K58A, and R59A) and examined for C1P affinity by surface plasmon resonance. The triple mutants (R57A/K58A/R59A), the double mutants (R57A/R59A, K58A/R59A, and R57A/K58A), and the single mutant (R59A) demonstrated significantly reduced affinity for C1P-containing vesicles as compared with wild-type cPLA(2)alpha. Examining these mutants for enzymatic activity demonstrated that these five mutants of cPLA(2)alpha also showed a significant reduction in the ability of C1P to: 1) increase the V(max) of the reaction; and 2) significantly decrease the dissociation constant (K (A)(s)) of the reaction as compared with the wild-type enzyme. The mutational effect was specific for C1P as all of the cationic mutants of cPLA(2)alpha demonstrated normal basal activity as well as normal affinities for phosphatidylcholine and phosphatidylinositol-4,5-bisphosphate as compared with wild-type cPLA(2)alpha. This study, for the first time, demonstrates a novel C1P interaction site mapped to the cationic beta-groove of the C2 domain of cPLA(2)alpha.

Enzymes and receptors of prostaglandin pathways with arachidonic acid-derived versus eicosapentaenoic acid-derived substrates and products

Dietary fish oil containing omega 3 highly unsaturated fatty acids has cardioprotective and anti-inflammatory effects. Prostaglandins (PGs) and thromboxanes are produced in vivo both from the omega 6 fatty acid arachidonic acid (AA) and the omega 3 fatty acid eicosapentaenoic acid (EPA). Certain beneficial effects of fish oil may result from altered PG metabolism resulting from increases in the EPA/AA ratios of precursor phospholipids. Here we report in vitro specificities of prostanoid enzymes and receptors toward EPA-derived, 3-series versus AA-derived, 2-series prostanoid substrates and products. The largest difference was seen with PG endoperoxide H synthase (PGHS)-1. Under optimal conditions purified PGHS-1 oxygenates EPA with only 10% of the efficiency of AA, and EPA significantly inhibits AA oxygenation by PGHS-1. Two- to 3-fold higher activities or potencies with 2-series versus 3-series compounds were observed with PGHS-2, PGD synthases, microsomal PGE synthase-1 and EP1, EP2, EP3, and FP receptors. Our most surprising observation was that AA oxygenation by PGHS-2 is only modestly inhibited by EPA (i.e. PGHS-2 exhibits a marked preference for AA when EPA and AA are tested together). Also unexpectedly, TxA(3) is about equipotent to TxA(2) at the TP alpha receptor. Our biochemical data predict that increasing phospholipid EPA/AA ratios in cells would dampen prostanoid signaling with the largest effects being on PGHS-1 pathways involving PGD, PGE, and PGF. Production of 2-series prostanoids from AA by PGHS-2 would be expected to decrease in proportion to the compensatory decrease in the AA content of phospholipids that would result from increased incorporation of omega 3 fatty acids such as EPA.

Chronic lithium administration attenuates up-regulated brain arachidonic acid metabolism in a rat model of neuroinflammation

Neuroinflammation, caused by a 6-day intracerebroventricular infusion of lipopolysaccharide (LPS) in rats, is associated with the up-regulation of brain arachidonic acid (AA) metabolism markers. Because chronic LiCl down-regulates markers of brain AA metabolism, we hypothesized that it would attenuate increments of these markers in LPS-infused rats. Incorporation coefficients k* of AA from plasma into brain, and other brain AA metabolic markers, were measured in rats that had been fed a LiCl or control diet for 6 weeks, and subjected in the last 6 days on the diet to intracerebroventricular infusion of artificial CSF or of LPS. In rats on the control diet, LPS compared with CSF infusion increased k* significantly in 28 regions, whereas the LiCl diet prevented k* increments in 18 of these regions. LiCl in CSF infused rats increased k* in 14 regions, largely belonging to auditory and visual systems. Brain cytoplasmic phospholipase A(2) activity, and prostaglandin E(2) and thromboxane B(2) concentrations, were increased significantly by LPS infusion in rats fed the control but not the LiCl diet. Chronic LiCl administration attenuates LPS-induced up-regulation of a number of brain AA metabolism markers. To the extent that this up-regulation has neuropathological consequences, lithium might be considered for treating human brain diseases accompanied by neuroinflammation.

Reprint of “Oxidative alterations of cyclooxygenase during atherogenesis” [Prostag. Oth. Lipid. M. 80 (2006) 1–14]

Nitric oxide (*NO) and eicosanoids are critical mediators of physiological and pathophysiological processes. They include inflammation and atherosclerosis. *NO production and eicosanoid synthesis become disrupted during atherosclerosis and thus, it is important to understand the mechanisms that may contribute to this outcome. We, and others, have shown that nitrogen oxide (NOx) species modulate cyclooxygenase (COX; also known as prostaglandin H2 synthase) activity and alter eicosanoid production. We have determined that peroxynitrite (ONOO-) has multiple effects on COX activity. ONOO- can provide the peroxide tone necessary for COX activation, such that simultaneous exposure of COX to its arachidonic acid substrate and ONOO- results in increased eicosanoid production. Alternatively, in the absence of arachidonic acid, ONOO- can modify COX through nitration of an essential tyrosine residue (Tyr385) such that it is incapable of catalysis. In this regard, we have shown that COX nitration occurs in human atherosclerotic tissue and in aortic lesions from ApoE-/- mice kept on a high fat diet. Additionally, we have demonstrated that Tyr nitration in ApoE-/- mice is dependent on the inducible form of NO synthase (iNOS). Under conditions where ONOO- persists and arachidonic acid is not immediately available, the cell may try to correct the situation by responding to ONOO- and releasing arachidonic acid via a signaling pathway to favor COX activation. Other post-translational modifications of COX by NOx species include S-nitrosation of cysteine (Cys) residues (which may have an activating effect) and Cys oxidation. The central focus of this review will include a discussion of how NOx species alter COX activity at the molecular level and how these modifications may contribute to altered eicosanoid output during atherosclerosis and lesion development.

Group VIA Calcium-independent Phospholipase A2 Mediates Endothelial Cell S Phase Progression

Arachidonic acid and its metabolites have been previously implicated in the regulation of endothelial cell proliferation. Arachidonic acid may be liberated from cellular phospholipids by the action of group VIA calcium-independent phospholipase A2 (iPLA2-VIA). Consequently, we tested the hypothesis that iPLA2-VIA activity is linked to the regulation of endothelial cell proliferation. Inhibition of iPLA2 activity by bromoenol lactone (BEL) was sufficient to entirely block endothelial cell growth. BEL dose-dependently inhibited endothelial cell DNA synthesis in a manner that was reversed upon the exogenous addition of arachidonic acid. DNA synthesis was inhibited by the S-isomer and not by the R-isomer of BEL, demonstrating that endothelial cell proliferation is mediated specifically by iPLA2-VIA. iPLA2-VIA activity was critical to the progression of endothelial cells through S phase and is required for the expression of the cyclin A/cdk2 complex. Thus, inhibition of iPLA2-VIA blocks S phase progression and results in exit from the cell cycle. Inhibition of iPLA2-VIA-mediated endothelial cell proliferation is sufficient to block angiogenic tubule formation in co-culture assays. Consequently, iPLA2-VIA is a novel regulator of endothelial cell S phase progression, cell cycle residence, and angiogenesis.

Oxidative alterations of cyclooxygenase during atherogenesis

Nitric oxide (*NO) and eicosanoids are critical mediators of physiological and pathophysiological processes. They include inflammation and atherosclerosis. *NO production and eicosanoid synthesis become disrupted during atherosclerosis and thus, it is important to understand the mechanisms that may contribute to this outcome. We, and others, have shown that nitrogen oxide (NO(x)) species modulate cyclooxygenase (COX; also known as prostaglandin H(2) synthase) activity and alter eicosanoid production. We have determined that peroxynitrite (ONOO(-)) has multiple effects on COX activity. ONOO(-) can provide the peroxide tone necessary for COX activation, such that simultaneous exposure of COX to its arachidonic acid substrate and ONOO(-) results in increased eicosanoid production. Alternatively, in the absence of arachidonic acid, ONOO(-) can modify COX through nitration of an essential tyrosine residue (Tyr385) such that it is incapable of catalysis. In this regard, we have shown that COX nitration occurs in human atherosclerotic tissue and in aortic lesions from ApoE(-/-) mice kept on a high fat diet. Additionally, we have demonstrated that Tyr nitration in ApoE(-/-) mice is dependent on the inducible form of NO synthase (iNOS). Under conditions where ONOO(-) persists and arachidonic acid is not immediately available, the cell may try to correct the situation by responding to ONOO(-) and releasing arachidonic acid via a signaling pathway to favor COX activation. Other post-translational modifications of COX by NO(x) species include S-nitrosation of cysteine (Cys) residues (which may have an activating effect) and Cys oxidation. The central focus of this review will include a discussion of how NO(x) species alter COX activity at the molecular level and how these modifications may contribute to altered eicosanoid output during atherosclerosis and lesion development.

Elevated prostaglandin E2 level via cPLA2--COX-2--mPGES-1 pathway involved in bladder carcinogenesis induced by terephthalic acid-calculi in Wistar rats

To investigate the prostaglandin E2 (PGE2) biosynthetic mechanism in bladder carcinogenesis, we established Wistar rat model of bladder papilloma and transitional cell carcinoma (TCC) induced by 5% terephthalic acid (TPA) treatment. Then, the mRNA level of cytosolic phospholipase A2 (cPLA2), cyclooxygenases (COX)-1 and -2, membrane-bound PGE2 synthases (mPGES)-1 and -2 was detected using reverse transcription polymerase chain reaction (RT-PCR). Immunoblotting was applied to detect the expression of COX-2 protein. Proliferating cell nuclear antigen (PCNA) was determined by immunohistochemistry. In addition, the level of PGE2 was measured by radioimmunoassay (RIA). Bladder papilloma (100%, 8/8) was examined in rats after 24-week treatment, and bladder TCC (80%, 16/20) was found after 48-week treatment. Histopathological changes were not found in control group rats. The incidence of bladder papilloma and TCC in test group was significantly higher than that in control group (P<0.01). The mRNA levels of cPLA2, COX-2 and mPGES-1 in the bladder papilloma and TCC were significantly higher than those in normal bladder (P<0.01), while the mRNA levels of COX-1 and mPGES-2 in TCC were unchanged compared with normal bladder. Bladder TCC exhibited a substantial expression of COX-2 protein. On the contrary, normal bladder tissue barely expresses COX-2 protein. PCNA labeling index (LI) and the level of PGE2 in bladder papilloma are much higher than those in normal bladder (P<0.01), but lower than those in bladder TCC (P<0.05). In conclusion, increasing PGE2 level via cPLA2--COX-2--mPGES-1 pathway may play an important role in rat bladder carcinogenesis. PGE2 may be a biomarker for the development of bladder TCC. cPLA2 and mPGES-1 may be targets for development of novel chemoprevention strategies for bladder TCC.

Targeting cytosolic phospholipase A2 by arachidonyl trifluoromethyl ketone prevents chronic inflammation in mice

Cytosolic phospholipase A(2) (cPLA(2)) plays a pivotal role in inflammation by catalyzing the release of arachidonic acid, a substrate for lipoxygenase and cyclooxygenase enzymes, from membrane phospholipids. In the present study we examined the role of cPLA(2) in inflammatory responses through the use of a specific inhibitor of the enzyme, cPLA(2), arachidonyl trifluoromethyl ketone (AACOCF3). Interestingly, we observed that AACOCF3 is an inhibitor of chronic but not acute inflammatory responses. Specifically, AACOCF3 inhibited phorbol 12-myristate 13-acetate (PMA)-induced chronic ear edema in mice. Additionally, oral treatment of ovalbumin-sensitized/ovalbumin-challenged BALB/c mice with 20 mg/kg AACOCF3 prevented the development of airway hyper-responsiveness in a model of asthma. Furthermore, AACOCF3 decreased cellular recruitment in the airway lumen and airway inflammation after the ovalbumin challenge. Taken together, these results suggest that a potent and specific chemical inhibitor of cPLA(2) may be useful for the treatment of chronic inflammatory diseases including rheumatoid arthritis, inflammatory bowel disease, psoriasis, and asthma.

Attenuation of the pulmonary inflammatory response following butylated hydroxytoluene treatment of cytosolic phospholipase A2 null mice

Administration of butylated hydroxytoluene (BHT) to mice causes lung damage characterized by the death of alveolar type I pneumocytes and the proliferation and subsequent differentiation of type II cells to replace them. Herein, we demonstrate this injury elicits an inflammatory response marked by chemokine secretion, alveolar macrophage recruitment, and elevated expression of enzymes in the eicosanoid pathway. Cytosolic phospholipase A(2) (cPLA(2)) catalyzes release of arachidonic acid from membrane phospholipids to initiate the synthesis of prostaglandins and other inflammatory mediators. A role for cPLA(2) in this response was examined by determining cPLA(2) expression and enzymatic activity in distal respiratory epithelia and macrophages and by assessing the consequences of cPLA(2) genetic ablation. BHT-induced lung inflammation, particularly monocyte infiltration, was depressed in cPLA(2) null mice. Monocyte chemotactic protein-1 (MCP-1) content in bronchoalveolar lavage fluid increases after BHT treatment but before monocyte influx, suggesting a causative role. Bronchiolar Clara cells isolated from cPLA(2) null mice secrete less MCP-1 than Clara cells from wild-type mice, consistent with the hypothesis that cPLA(2) is required to secrete sufficient MCP-1 to induce an inflammatory monocytic response.

Activation of p38 MAPKinase/cPLA2 pathway in homocysteine-treated platelets

Hyperhomocysteinemia is considered a risk factor in arterial and venous thrombosis. The mechanism by which homocysteine (HCy) supports atherothrombosis is still unknown and may be multifactorial. Earlier in vitro studies demonstrated that HCy induced arachidonic acid release and increased thromboxane B2 (TXB2) formation. In this work, we found that HCy stimulated the rapid and sustained phosphorylation of platelet p38 mitogen-activated protein kinase (p38 MAPK). The effect was time- and dose-dependent. The HCy effect on p38 MAPK phosphorylation was prevented by N-acetyl-L-cysteine and iloprost and was partially inhibited by nordihydroguaiaretic acid. Moreover, the incubation of platelets with HCy led to the phosphorylation of cytosolic phospholipase A2 (cPLA2). In addition HCy promoted cPLA2 activation, assessed as arachidonic acid release. The cPLA2 phosphorylation and activation were both impaired by the inhibition of p38 MAPK through SB203580. This effect was not complete, reaching at the most the 50% of the total. In FURA 2-loaded platelets, HCy induced a dose-dependent intracellular calcium rise suggesting that the calcium elevation promoted by HCy could participate in the cPLA2 activation, leading to arachidonic acid release and TXB2 formation. In conclusion, our data provide insight into the mechanisms of platelet activation induced by HCy, suggesting that the p38 MAPK/cPLA2 pathway could play a relevant role in platelet hyperactivity described in hyperhomocysteinemia.

Oxidant-mediated activation of cytosolic phospholipase a(2) in pulmonary endothelium: role of protein kinase C alpha and a pertussis toxin-sensitive protein

The authors have previously demonstrated that the oxidant t-buOOH stimulates phospholipase A(2) (PLA(2)) activity in bovine pulmonary artery endothelial cells (S. Chakraborti et al. American Journal of Physiology, 257, L430-L437, 1989). Herein, the authors sought to investigate the mechanism by which t-buOOH stimulates PLA(2) activity and the role of protein kinase C (PKC) in this scenario. Treatment of bovine pulmonary artery endothelial cells with t-buOOH stimulated an aprotinin-sensitive protease activity, PKC activity, and PLA(2) activity in the cell membrane. Pretreatment with intracellular Ca(2+) chelator (BAPTA-AM), PKCalpha inhibitor (Go6976), cPLA(2) inhibitor (AACOCF(3)), and pertussis toxin prevented t-buOOH-stimulated PLA(2) activity. Immunoblot studies with aprotinin, cPLA(2), PKCalpha, and Gialpha antibodies revealed their presence in the endothelial membrane. Immunoblot studies of the cell membrane isolated from t-buOOH-stimulated cells with cPLA(2) and PKCalpha antibodies elicited an apparent increase in their immunoreactive protein profiles along with an additional 47-kDa immunoreactive fragment in the membrane. t-buOOH caused Gialpha phosphorylation in the membrane and pretreatment with Go6976 prevented the phosphorylation. Overall, these results suggest that t-buOOH stimulates an aprotinin-sensitive protease activity that proteolytically activates PKCalpha and that subsequently phosphorylates a pertussis toxin-sensitive protein, resulting in the stimulation of cPLA(2) activity in the cell membrane.

Cellular and molecular pathways of ischemic neuronal death

Three routes have been identified triggering neuronal death under physiological and pathological conditions. Excess activation of ionotropic glutamate receptors cause influx and accumulation of Ca2+ and Na+ that result in rapid swelling and subsequent neuronal death within a few hours. The second route is caused by oxidative stress due to accumulation of reactive oxygen and nitrogen species. Apoptosis or programmed cell death that often occurs during developmental process has been coined as additional route to pathological neuronal death in the mature nervous system. Evidence is being accumulated that excitotoxicity, oxidative stress, and apoptosis propagate through distinctive and mutually exclusive signal transduction pathway and contribute to neuronal loss following hypoxic-ischemic brain injury. Thus, the therapeutic intervention of hypoxic-ischemic neuronal injury should be aimed to prevent excitotoxicity, oxidative stress, and apoptosis in a concerted way.

Role of cytosolic phospholipase A2 in the enhancement of α2-adrenoceptor-mediated vasoconstriction by the thromboxane-mimetic U46619 in the porcine isolated ear artery: Comparison with vasopressin-enhanced responses

Pre-contraction with the thromboxane-mimetic U46619 enhances the subsequent alpha(2)-adrenoceptor-mediated vasoconstriction in the porcine ear artery through an enhanced activation of ERK-MAP kinase. In this study we determined the role of cPLA(2) in this enhanced response, and determined whether vasopressin is also able to enhance alpha(2)-adrenoceptor-mediated vasoconstriction through the same pathway. The cPLA(2) inhibitors AACOCF3 (50 microM) and MAFP (50 microM) both inhibited the U46619-enhanced alpha(2)-adrenoceptor response, but had no effect on the direct alpha(2)-adrenoceptor response. AACOCF3 also inhibited the enhanced ERK activation associated with the enhanced alpha(2)-adrenoceptor-mediated vasoconstriction. Pre-contraction with arachidonic acid mimicked the effect of U46619 by enhancing the contractile response to the alpha(2)-adrenoceptor agonist UK14304 (1 microM) and enhancing the alpha(2)-adrenoceptor-mediated ERK activation. Pre-contraction with vasopressin also enhanced the contractile response to UK14304, but neither PD98059 (50 microM) nor AACOCF3 (50 microM) had any effect this vasopressin-enhanced response, indicating that neither the ERK pathway, nor cPLA(2) are involved in vasopressin-enhanced responses. The alpha(2)-adrenceptor-stimulated activation of ERK was also unaffected by pre-contraction with vasopressin. On the other hand, inhibition of PKCzeta inhibited the enhanced alpha(2)-adrenoceptor contraction after pre-contraction with both U46619 and vasopressin. This study demonstrates that alpha(2)-adrenoceptor-mediated vasoconstriction can be enhanced through two different pathways-one dependent upon the enhanced activation of ERK-MAP kinase through activation of cPLA(2), and the other through a different, ERK/cPLA(2)-independent pathway.

Positive cooperativity between the thrombin and bradykinin B2 receptors enhances arachidonic acid release

Bradykinin (BK) or kallikreins activate B2 receptors (R) that couple Galpha(i) and Galpha(q) proteins to release arachidonic acid (AA) and elevate intracellular Ca2+ concentration ([Ca2+]i). Thrombin cleaves the protease-activated-receptor-1 (PAR1) that couples Galpha(i), Galpha(q), and Galpha(12/13) proteins. In Chinese hamster ovary cells stably transfected with human B2R, thrombin liberated little AA, but it significantly potentiated AA release by B2R agonists. We explored mechanisms of cooperativity between constitutively expressed PAR1 and B2R. We also examined human endothelial cells expressing both Rs constitutively. The PAR1 agonist hexapeptide (TRAP) was as effective as thrombin. Inhibitors of components of Galpha(i), Galpha(q), and Galpha(12/13) signaling pathways, and a protein kinase C (PKC)-alpha inhibitor, Gö-6976, blocked potentiation, while phorbol, an activator, enhanced it. Several inhibitors, including a RhoA kinase inhibitor, a [Ca2+]i antagonist, and an inositol-(1,3,4)-trisphosphate R antagonist, reduced mobilization of [Ca2+]i by thrombin and blocked potentiation of AA release by B2R agonists. Because either a nonselective inhibitor (isotetrandrine) of phospholipase A2 (PLA2) or a Ca2+-dependent PLA2 inhibitor abolished potentiation of AA release by thrombin, while a Ca2+-independent PLA2 inhibitor did not, we concluded that the mechanism involves Ca2+-dependent PLA2 activation. Both thrombin and TRAP modified activation and phosphorylation of the B2R induced by BK. In lower concentrations they enhanced it, while higher concentrations inhibited phosphorylation and diminished B2R activation. Protection of the NH2-terminal Ser1-Phe2 bond of TRAP by an aminopeptidase inhibitor made this peptide much more active than the unprotected agonist. Thus PAR1 activation enhances AA release by B2R agonists through signal transduction pathway.

Toll-like receptor 4 signaling regulates cytosolic phospholipase A2 activation and lipid generation in lipopolysaccharide-stimulated macrophages

Inflammatory lipid mediators such as prostaglandins and leukotrienes play crucial roles in the pathogenesis of bacterial lipopolysaccharide (LPS)-induced inflammation. Cytosolic phospholipase A(2) (cPLA(2)) is a key enzyme in the generation of pro-inflammatory lipid mediators. Here, we found that Toll-like receptor 4 (TLR4) is essential for LPS-induced cPLA(2) activation and lipid release. Inhibition of TLR4 protein expression by TLR4 small interfering RNA or neutralization of TLR4 by the specific antibody against TLR4/MD2 blocked cPLA(2) phosphorylation and cPLA(2)-hydrolyzed arachidonic acid release. Furthermore, activation of the TLR4 signaling pathway by LPS regulated cPLA(2) activation and lipid release. cPLA(2) phosphorylation and cPLA(2)-hydrolyzed lipid release were significantly impaired when TLR4 adaptor protein, either MyD88 or TRIF, was knocked down in LPS-stimulated macrophages. Similarly, LPS-induced arachidonate release was inhibited in cells transfected with a dominant-negative MyD88 or TRIF construct. Subsequently, cPLA(2) activation could be suppressed by inhibition of the TLR4 adaptor protein-directed p38 and ERK MAPK pathways. These findings suggest that, in LPS-induced inflammation, the TLR4-mediated MyD88- and TRIF-dependent MAPK pathways result in cPLA(2) activation and production of pro-inflammatory lipid mediators.

Cytosolic phospholipase A2-alpha mediates endothelial cell proliferation and is inactivated by association with the Golgi apparatus

Arachidonic acid and its metabolites are implicated in regulating endothelial cell proliferation. Cytosolic phospholipase A2-alpha (cPLA2alpha) is responsible for receptor-mediated arachidonic acid evolution. We tested the hypothesis that cPLA2alpha activity is linked to endothelial cell proliferation. The specific cPLA2alpha inhibitor, pyrrolidine-1, inhibited umbilical vein endothelial cell (HUVEC) proliferation in a dose-dependent manner. Exogenous arachidonic acid addition reversed this inhibitory effect. Inhibition of sPLA2 did not affect HUVEC proliferation. The levels of cPLA2alpha did not differ between subconfluent and confluent cultures of cells. However, using fluorescence microscopy we observed a novel, confluence-dependent redistribution of cPLA2alpha to the distal Golgi apparatus in HUVECs. Association of cPLA2alpha with the Golgi was linked to the proliferative status of HUVECs. When associated with the Golgi apparatus, cPLA2alpha activity was seen to be 87% inhibited. Relocation of cPLA2alpha to the cytoplasm and nucleus, and cPLA2alpha enzyme activity were required for cell cycle entry upon mechanical wounding of confluent monolayers. Thus, cPLA2alpha activity and function in controlling endothelial cell proliferation is regulated by reversible association with the Golgi apparatus.

Proteolytic activation of protein kinase Calpha by peroxynitrite in stimulating cytosolic phospholipase A2 in pulmonary endothelium: involvement of a pertussis toxin sensitive protein

We sought to determine the roles of PKCalpha and G(i)alpha in regulating cPLA(2) activity in bovine pulmonary artery endothelial cell membrane under peroxynitrite (ONOO(-)) stimulation. Treatment of bovine pulmonary artery endothelial cells with ONOO(-) markedly stimulates the cell membrane associated protease activity, protein kinase C (PKC) activity, phospholipase A(2) (PLA(2)) activity, and arachidonic acid (AA) release from the cells. ONOO(-) significantly increases (Ca(2+))(i) in the cells, and pretreatment with the intracellular Ca(2+) chelator BAPTA-AM prevents the increase in (Ca(2+))(i), protease activity, PKC activity, and cPLA(2) activity in the cell membrane and AA release from the cells. Pretreatment of the cells with arachidonyl trifluoromethyl ketone (AACOCF(3)) (a cPLA(2) inhibitor) prevents ONOO(-)-stimulated cPLA(2) activity and AA release without producing a significant alteration of the protease activity. Pretreatment with vitamin E and aprotinin prevents ONOO(-)-induced increase in the protease activity, PKC activity, and cPLA(2) activity in the cell membrane and AA release from the cells. Pretreatment with the PKC inhibitor calphostin C prevents ONOO(-)-caused increase in PKC activity and cPLA(2) activity in the cell membrane and AA release from the cells. An immunoblot study of the cell membrane isolated from the ONOO(-)-treated cells with polyclonal PKCalpha antibody elicited an increase in the 80 kDa immunoreactive protein band along with an additional 47 kDa immunoreactive fragment. An immunoblot study with anti-nitrotyrosine antibody revealed that ONOO(-) induces nitration of tyrosine residues in PKCalpha. Pretreatment of the cells with aprotinin abolished the 47 kDa immunoreactive fragment in the immunoblot. An immunoblot study of the endothelial cell membrane with polyclonal cPLA(2) antibody revealed that treatment of the cells with ONOO(-) markedly increases the cPLA(2) immunoreactive protein profile in the membrane. Pretreatment of the endothelial cells with Go6976, a PKCalpha inhibitor, prevents the increase in PKC activity and cPLA(2) activity in the cell membrane under ONOO(-)-triggered condition. It, therefore, appears from the present study that treatment of the cells with ONOO(-) causes an increase in the protease activity, and that plays an important role in activating PKCalpha, which subsequently stimulates cPLA(2) activity in the cell membrane and AA release from the cells. An immunoblot assay with polyclonal G(i)alpha antibody elicited an immunoreactive band having a molecular mass of 41 kDa. Pretreatment of the cells with pertussis toxin markedly inhibits ONOO(-)-induced increase in cPLA(2) activity and AA release without significantly altering (Ca(2+))(i), protease activity, and PKC activity in the cell membrane. Treatment of the cells with ONOO(-) causes phosphorylation of G(i)alpha in the cell membrane, and pretreatment with Go6976 prevents its phosphorylation. We suggest the existence of a pertusssis toxin sensitive G protein-mediated mechanism for activation of cPLA(2) by ONOO(-) in bovine pulmonary artery endothelial cell membrane, which is regulated by PKCalpha-dependent phosphorylation and sensitive to aprotinin for its inhibition.

Ceramide 1-Phosphate Acts as a Positive Allosteric Activator of Group IVA Cytosolic Phospholipase A2α and Enhances the Interaction of the Enzyme with Phosphatidylcholine

Previous findings from our laboratory have demonstrated that cPLA(2)alpha is directly activated by the emerging bioactive sphingolipid, ceramide 1-phosphate (C-1-P) (1). In this study, a Triton X-100/phosphatidylcholine (PC) mixed micelle assay was utilized to determine the kinetics and specificity of this lipid-enzyme interaction. Using this assay, the addition of C-1-P induced a dramatic increase in the activity of cPLA(2)alpha (>15-fold) with a K(a) of 2.4 mol % C-1-P/Triton X-100 micelle. This activation was highly specific as the addition of other lipids had insignificant effects on cPLA(2)alpha activity. Studies using surface-dilution kinetics revealed that C-1-P had no effect on the Michaelis-Menten constant, K(m)(B), but decreased the dissociation constant (K (A)(s)) value by 87%. Thus, C-1-P not only increases the membrane affinity of cPLA(2)alpha but also may act as an allosteric activator of the enzyme. Surface plasmon resonance analysis of the C-1-P/cPLA(2)alpha interaction verified a decrease in the dissociation constant, demonstrating that cPLA(2)alpha bound PC vesicles containing C-1-P with increased affinity (5-fold) compared with PC vesicles alone. The effect on the dissociation rate of cPLA(2)alpha was also found to be lipid-specific with the exception of phosphatidylinositol 4,5-bisphosphate, which caused a modest increase in vesicle affinity (2-fold). Lastly, the binding site for C-1-P was determined to be within the C2-domain of cPLA(2)alpha, unlike phosphatidylinositol 4,5-bisphosphate. These data demonstrate a novel interaction site for C-1-P and suggest that C-1-P may function to recruit cPLA(2)alpha to intracellular membranes as well as allosterically activate the membrane-associated enzyme.

Cyclooxygenases, peroxide tone and the allure of fish oil

Skepticism about the health benefits of fish oil is largely the result of our incomplete understanding of the biochemistry of omega3 essential fatty acids. Recent work has confirmed the roles of omega3 fatty acids in gene transcription and signal transduction, and has given insight into the effects of eicosapentaenoic acid (EPA) and the EPA/arachidonic acid (AA) ratio on prostanoid (PG) metabolism and function. One pronounced effect of fish-oil-induced increases in EPA/AA ratios is decreased PG formation from AA via cyclooxygenase-1, because EPA inhibits this isoform. In addition, cells lacking endogenous alkyl-peroxide-generating systems and thus having a low 'peroxide tone' cannot oxygenate EPA via cyclooxygenase-1. Platelets, however, which are equipped with a lipoxygenase that can produce an abundance of hydroperoxide from AA, can form small amounts of thromboxane A3 from EPA via cyclooxygenase-1. A second major consequence of elevated EPA/AA ratios is significantly increased production of 3-series PGs, including PGE3, via cyclooxygenase-2. There are four PGE receptor subtypes and at least one of these types--not yet identified--has a significantly different response to PGE3 than to PGE2; this difference may underlie the ability of omega3 fatty acids to mitigate inflammation and tumorigenesis.

Role of an aprotinin-sensitive protease in protein kinase Calpha-mediated activation of cytosolic phospholipase A2 by calcium ionophore (A23187) in pulmonary endothelium

Treatment of bovine pulmonary artery endothelial cells with the calcium ionophore, A23187, stimulates the cell membrane associated protease activity, phospholipase A2 (PLA2) activity, and arachidonic acid (AA) release from the cells. Pretreatment of the cells with arachidonyl-trifluomethylketone (AACOCF3), a cPLA2 inhibitor, but not bromoenollactone (BEL), a iPLA2 inhibitor, prevents A23187 stimulated PLA2 activity and AA release without producing an appreciable alteration of the protease activity. Pretreatment of the cells with aprotinin, an ambient protease inhibitor, prevents the increase in the protease activity and cPLA2 activity in the membrane and AA release from the cells caused by both low and high doses of A23187, and also inhibits protein kinase C (PKC) activity caused by high doses of A23187. Immunoblot study of the endothelial cell membrane isolated from A23187 (10 microM)-treated cells with polyclonal PKCalpha antibody elicited an increase in the 80 kDa immunoreactive protein band along with an additional 47 kDa immunoreactive fragment. Pretreatment of the cells with aprotinin abolished the 47 kDa immunoreactive fragment in the immunoblot. Immunoblot study of the endothelial membrane with polyclonal cPLA2 antibody revealed that treatment of the cells with A23187 dose-dependently increases cPLA2 immunoreactive protein profile in the membrane. It therefore appears from the present study that treatment of the cells with a low dose of A23187 (1 microM) causes a small increase in an aprotinin-sensitive protease activity and that stimulates cPLA2 activity in the cell membrane without an involvement of PKC. By contrast, treatment of the cells with a high dose of 10 microM of A23187 causes optimum increase in the protease activity and that plays an important role in activating PKCalpha, which subsequently stimulates cPLA2 activity in the cell membrane. Although pretreatment of the cells with pertussis toxin caused ADP ribosylation of a 41 kDa protein in the cell membrane, it did not inhibit the cPLA2 activity and AA release caused by both low and high doses of A23187.

Regulatory mechanism and physiological role of cytosolic phospholipase A2

Cytosolic phospholipase A2alpha (cPLA2alpha) preferentially hydrolyzes phospholipids containing arachidonic acid and plays a key role in the biosynthesis of eicosanoids. This review discusses the essential features of cPLA2alpha regulation and addresses new insights into the functional properties of this enzyme. Full activation of the enzyme requires Ca2+ binding to an N-terminal C2 domain and phosphorylation on serine residues. Ca2+ binding induces translocation of cPLA2alpha from the cytosol to the perinuclear membranes. Serine phosphorylation is mediated by mitogen-activated protein kinases (MAPKs), Ca2+/calmodulin-dependent protein kinase II, and MAPK-interacting kinase Mnk1. Interaction with proteins and lipids, which include vimentin, annexins, NADPH oxidase, phosphatidylcholine, phosphatidylinositol 4,5-bisphosphate (PIP2), and ceramide-1-phosphate, can also modulate the activity of cPLA2alpha. Recent evidence has established the physiological and pathological roles of cPLA2alpha using cPLA2alpha knockout mice. This enzyme has been implicated in fertility, striated muscle growth, renal concentration, postischemic brain injury, arthritis, inflammatory bone resorption, intestinal polyposis, pulmonary fibrosis, acute respiratory distress syndrome, and autoimmune encephalomyelitis. Now novel three paralogs, cPLA2beta, cPLA2gamma, and cPLA2delta, have been identified in humans. cPLA2gamma is distinct from others in that it is farnesylated and lacks the C2 domain. Biological roles for these new enzymes have not yet been defined.