Phosphorylation of cytosolic group IV phospholipase A(2) is necessary but not sufficient for Arachidonic acid release in P388D(1) macrophages

Phosphorylation of cPLA2α at Ser505 Is Necessary for Its Translocation to PtdInsP2-Enriched Membranes

Group IVA cytosolic phospholipase A₂α (cPLA₂α) is a key enzyme in physiology and pathophysiology because it constitutes a rate-limiting step in the pathway for the generation of pro- and anti-inflammatory eicosanoid lipid mediators. cPLA₂α activity is tightly regulated by multiple factors, including the intracellular Ca²⁺ concentration, phosphorylation reactions, and cellular phosphatidylinositol (4,5) bisphosphate levels (PtdInsP₂). In the present work, we demonstrate that phosphorylation of the enzyme at Ser⁵⁰⁵ is an important step for the translocation of the enzyme to PtdInsP₂–enriched membranes in human cells. Constructs of eGFP-cPLA₂ mutated in Ser⁵⁰⁵ to Ala (S505A) exhibit a delayed translocation in response to elevated intracellular Ca²⁺, and also in response to increases in intracellular PtdInsP₂ levels. Conversely, translocation of a phosphorylation mimic mutant (S505E) is fully observed in response to cellular increases in PtdInsP₂ levels. Collectively, these results suggest that phosphorylation of cPLA₂α at Ser⁵⁰⁵ is necessary for the enzyme to translocate to internal membranes and mobilize arachidonic acid for eicosanoid synthesis.

Inflammation and JNK's Role in Niacin-GPR109A Diminished Flushed Effect in Microglial and Neuronal Cells With Relevance to Schizophrenia

Schizophrenia is a neuropsychiatric illness with no single definitive aetiology, making its treatment difficult. Antipsychotics are not fully effective because they treat psychosis rather than the cognitive or negative symptoms. Antipsychotics fail to alleviate symptoms when patients enter the chronic stage of illness. Topical application of niacin showed diminished skin flush in the majority of patients with schizophrenia compared to the general population who showed flushing. The niacin skin flush test is useful for identifying patients with schizophrenia at their ultra-high-risk stage, and understanding this pathology may introduce an effective treatment. This review aims to understand the pathology behind the diminished skin flush response, while linking it back to neurons and microglia. First, it suggests that there are altered proteins in the GPR109A-COX-prostaglandin pathway, inflammatory imbalance, and kinase signalling pathway, c-Jun N-terminal kinase (JNK), which are associated with diminished flush. Second, genes from the GPR109A-COX-prostaglandin pathway were matched against the 128-loci genome wide association study (GWAS) for schizophrenia using GeneCards, suggesting that G-coupled receptor-109A (GPR109A) may have a genetic mutation, resulting in diminished flush. This review also suggests that there may be increased pro-inflammatory mediators in the GPR109A-COX-prostaglandin pathway, which contributes to the diminished flush pathology. Increased levels of pro-inflammatory markers may induce microglial-activated neuronal death. Lastly, this review explores the role of JNK on pro-inflammatory mediators, proteins in the GPR109A-COX-prostaglandin pathway, microglial activation, and neuronal death. Inhibiting JNK may reverse the changes observed in the diminished flush response, which might make it a good therapeutic target.

Essential Role for Ethanolamine Plasmalogen Hydrolysis in Bacterial Lipopolysaccharide Priming of Macrophages for Enhanced Arachidonic Acid Release

Due to their high content in esterified arachidonic acid (AA), macrophages provide large amounts of eicosanoids during innate immune reactions. Bacterial lipopolysaccharide (LPS) is a poor trigger of AA mobilization in macrophages but does have the capacity to prime these cells for greatly increased AA release upon subsequent stimulation. In this work, we have studied molecular mechanisms underlying this phenomenon. By using mass spectrometry-based lipidomic analyses, we show in this work that LPS-primed zymosan-stimulated macrophages exhibit an elevated consumption of a particular phospholipid species, i.e., the ethanolamine plasmalogens, which results from reduced remodeling of phospholipids via coenzyme A-independent transacylation reactions. Importantly however, LPS-primed macrophages show no changes in their capacity to directly incorporate AA into phospholipids via CoA-dependent acylation reactions. The essential role for ethanolamine plasmalogen hydrolysis in LPS priming is further demonstrated by the use of plasmalogen-deficient cells. These cells, while responding normally to zymosan by releasing quantities of AA similar to those released by cells expressing normal plasmalogen levels under the same conditions, fail to show an LPS-primed response to the same stimulus, thus unambiguously demonstrating a cause–effect relationship between LPS priming and plasmalogen hydrolysis. Collectively, these results suggest a hitherto unrecognized role for ethanolamine plasmalogen hydrolysis and CoA-independent transacylation reactions in modulating the eicosanoid biosynthetic response.

Haloenol pyranones and morpholinones as antineoplastic agents of prostate cancer

Haloenol pyran-2-ones and morpholin-2-ones were synthesized and evaluated as inhibitors of cell growth in two different prostate human cancer cell lines (PC-3 and LNCaP). Analogs derived from L- and D-phenylglycine were found to be the most effective antagonists of LNCaP and PC-3 cell growth. Additional studies reveal that the inhibitors induced G2/M arrest and the (S)-enantiomer of the phenylglycine-based derivatives was a more potent inhibitor of cytosolic iPLA(2)β.

Subcellular localization and role of lipin-1 in human macrophages

The lipins have been described as metabolic enzymes that regulate lipid biosynthesis and also signaling processes by controlling the cellular concentration of bioactive lipids, phosphatidic acid, and diacylgycerol. In the present work we have studied the subcellular localization and role of lipin-1 in human monocyte-derived macrophages. Human macrophages express lipin-1 isoforms α and β. A transfected lipin-1α-enhanced GFP construct associates with membranes of cellular organelles that can be stained with Nile Red. Colocalization experiments with lipid droplet (LD)-specific proteins such as adipophilin/adipose differentiation-related protein/perilipin 2 or TIP47/perilipin 3 show that both proteins colocalize with lipin-1α in the same cellular structures. Reduction of the expression levels of lipin-1 by small interfering RNA technology does not impair triacylglycerol biosynthesis but reduces the size of LDs formed in response to oleic acid. In agreement with these data, peritoneal macrophages from animals that carry a mutation in the Lpin-1 gene (fld animals) also produce less and smaller LDs in response to oleic acid. Mass spectrometry determinations demonstrate that the fatty acid composition of triacylglycerol in isolated LDs from lipin-1-deficient cells differs from that of control cells. Moreover, activation of cytosolic group IVA phospholipase A(2)α, a proinflammatory enzyme that is also involved in LD biogenesis, is also compromised in lipin-1-deficient cells. Collectively, these data suggest that lipin-1 associates with LDs and regulates the activation of cytosolic group IVA phospholipase A(2)α in human monocyte-derived macrophages.

JNK and ceramide kinase govern the biogenesis of lipid droplets through activation of group IVA phospholipase A2

The biogenesis of lipid droplets (LD) induced by serum depends on group IVA phospholipase A(2) (cPLA(2)alpha). This work dissects the pathway leading to cPLA(2)alpha activation and LD biogenesis. Both processes were Ca(2+)-independent, as they took place after pharmacological blockade of Ca(2+) transients elicited by serum or chelation with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester). The single mutation D43N in cPLA(2)alpha, which abrogates its Ca(2+) binding capacity and translocation to membranes, did not affect enzyme activation and formation of LD. In contrast, the mutation S505A did not affect membrane relocation of the enzyme in response to Ca(2+) but prevented its phosphorylation, activation, and the appearance of LD. Expression of specific activators of different mitogen-activated protein kinases showed that phosphorylation of cPLA(2)alpha at Ser-505 is due to JNK. This was confirmed by pharmacological inhibition and expression of a dominant-negative form of the upstream activator MEKK1. LD biogenesis was accompanied by increased synthesis of ceramide 1-phosphate. Overexpression of its synthesizing enzyme ceramide kinase increased phosphorylation of cPLA(2)alpha at Ser-505 and formation of LD, and its down-regulation blocked the phosphorylation of cPLA(2)alpha and LD biogenesis. These results demonstrate that LD biogenesis induced by serum is regulated by JNK and ceramide kinase.

Coordinate regulation of TLR-mediated arachidonic acid mobilization in macrophages by group IVA and group V phospholipase A2s

Macrophages can be activated through TLRs for a variety of innate immune responses. In contrast with the wealth of data existing on TLR-dependent gene expression and resultant cytokine production, very little is known on the mechanisms governing TLR-mediated arachidonic acid (AA) mobilization and subsequent eicosanoid production. We have previously reported the involvement of both cytosolic group IVA phospholipase A(2) (cPLA(2)) and secreted group V phospholipase A(2) (sPLA(2)-V) in regulating the AA mobilization response of macrophages exposed to bacterial LPS, a TLR4 agonist. In the present study, we have used multiple TLR agonists to define the role of various PLA(2)s in macrophage AA release via TLRs. Activation of P388D(1) and RAW2647.1 macrophage-like cells via TLR1/2, TLR2, TLR3, TLR4, TLR6/2, and TLR7, but not TLR5 or TLR9, resulted in AA mobilization that appears to involve the activation of both cPLA(2) and sPLA(2) but not of calcium-independent phospholipase A(2). Furthermore, inhibition of sPLA(2)-V by RNA interference or by two cell-permeable compounds, namely scalaradial and manoalide, resulted in a marked reduction of the phosphorylation of ERK1/2 and cPLA(2) via TLR1/2, TLR2, TLR3, and TLR4, leading to attenuated AA mobilization. Collectively, the results suggest a model whereby sPLA(2)-V contributes to the macrophage AA mobilization response via various TLRs by amplifying cPLA(2) activation through the ERK1/2 phosphorylation cascade.

TLR-4 and sustained calcium agonists synergistically produce eicosanoids independent of protein synthesis in RAW264.7 cells

Arachidonic acid is released by phospholipase A(2) and converted into hundreds of distinct bioactive mediators by a variety of cyclooxygenases (COX), lipoxygenases (LO), and cytochrome P450s. Because of the size and diversity of the eicosanoid class of signaling molecules produced, a thorough and systematic investigation of these biological processes requires the simultaneous quantitation of a large number of eicosanoids in a single analysis. We have developed a robust liquid chromatography/tandem mass spectrometry method that can identify and quantitate over 60 different eicosanoids in a single analysis, and we applied it to agonist-stimulated RAW264.7 murine macrophages. Fifteen different eicosanoids produced through COX and 5-LO were detected either intracellularly or in the media following stimulation with 16 different agonists of Toll-like receptors (TLR), G protein-coupled receptors, and purinergic receptors. No significant differences in the COX metabolite profiles were detected using the different agonists; however, we determined that only agonists creating a sustained Ca(2+) influx were capable of activating the 5-LO pathway in these cells. Synergy between Ca(2+) and TLR pathways was detected and discovered to be independent of NF-kappaB-induced protein synthesis. This demonstrates that TLR induction of protein synthesis and priming for enhanced phospholipase A(2)-mediated eicosanoid production work through two distinct pathways.

Polychlorinated biphenyls induce arachidonic acid release in human platelets in a tamoxifen sensitive manner via activation of group IVA cytosolic phospholipase A2-alpha

Polychlorinated biphenyls (PCBs) are stable compounds commonly found in nature as environmental pollutants. PCBs can affect the endocrine function of hormones such as steroid-hormones. Also, PCBs are known to be inducers of arachidonic acid release in various cells. We report, here, the effects of PCBs on eicosanoid formation, arachidonic acid release and cytosolic phospholipase A2-alpha (cPLA2-alpha) activation in human platelets. Ortho-substituted PCBs induced a time and dose-dependent release of arachidonic acid and the concomitant formation of 12(S)-hydroxy-5,8-cis-10-trans-14-cis-eicosatetraenoic acid (12-HETE) and 12(S)-hydroxy-5-cis-8,10-trans-heptadecatrienoic acid (12-HHT) in human platelets. The release of arachidonic acid and the formation of 12-HETE was completely blocked by the cPLA2-alpha inhibitors AACOCF3 or pyrrolidine-1. PCB-treatment of platelets demonstrated that the cPLA2-alpha protein as well as PLA2 activity translocated to the membrane fraction, independent of a rise in intracellular Ca2+. Furthermore, electrophoretic gel mobility shift analysis of cPLA2-alpha on SDS-PAGE demonstrated a PCB-dependent phosphorylation of cPLA2-alpha. The effects of 17beta-estradiol and two structurally unrelated anti-estrogens, nafoxidin and tamoxifen on PCB-induced arachidonic acid release in platelets were also investigated. Both nafoxidin and tamoxifen inhibited PCB-induced arachidonic acid release as well as 12-HETE and 12-HHT formation. Interestingly, platelets incubated with PCBs did not aggregate despite the fact that robust release of arachidonic acid was observed. In summary, these results demonstrate that certain PCBs induce activation of cPLA2-alpha independent of a rise in intracellular calcium and a robust release of arachidonic acid release with resulting eicosanoid formation in human platelets.

Localization and functional interrelationships among cytosolic Group IV, secreted Group V, and Ca2+-independent Group VI phospholipase A2s in P388D1 macrophages using GFP/RFP constructs

P388D(1) cells exposed to bacterial lipopolysaccharide (LPS) mobilize arachidonic acid (AA) for prostaglandin synthesis in two temporally distinct pathways. The "immediate pathway" is triggered within minutes by receptor agonists such as platelet-activating factor (PAF) but only if the cells have previously been primed with LPS for 1 h. The "delayed pathway" occurs in response to LPS alone over the course of several hours. We have now investigated the subcellular localization of both the Group IV cytosolic phospholipase A(2) (cPLA(2)) and the Group V secreted PLA(2) (sPLA(2)) during these two temporally distinct routes of AA release. We have prepared cells overexpressing fusion proteins of sPLA(2)-GFP and cPLA(2)-RFP. In the resting cells, cPLA(2)-RFP was uniformly located throughout the cytoplasm, and short-term treatment with LPS did not induce translocation to perinuclear and/or Golgi membranes. However, such a translocation occurred almost immediately after the addition of PAF to the cells. Long-term exposure of the cells to LPS led to the translocation of cPLA(2)-RFP to intracellular membranes after 3 h, and correlates with a significant release of AA in a cPLA(2)-dependent manner. At the same time period that the delayed association of cPLA(2) with perinuclear membranes is detected, an intense fluorescence arising from the sPLA(2)-GFP was found around the nucleus in the sPLA(2)-GFP stably transfected cells. In parallel with these changes, significant AA release was detected from the sPLA(2)-GFP transfectants in a cPLA(2)-dependent manner, which may reflect cross-talk between sPLA(2) and cPLA(2). The subcellular localization of the Group VIA Ca(2+)-independent PLA(2) (iPLA(2)) was also investigated. Cells overexpressing iPLA(2)-GFP showed no fluorescence changes under any activation condition. However, the iPLA(2)-GFP-expressing cells showed relatively high basal AA release, confirming a role for iPLA(2) in basal deacylation reactions. These new data illustrate the subcellular localization changes that accompany the distinct roles that each of the three kinds of PLA(2) present in P388D(1) macrophages play in AA mobilization.

Activation of phospholipase A2 and MAP kinases by oxidized low-density lipoproteins in immortalized GP8.39 endothelial cells

In immortalized rat brain endothelial cells (GP8.39), we have previously shown that oxidized LDL (oxLDL), after 24-h treatment, stimulates arachidonic acid release and phosphatidylcholine hydrolysis by activation of cytosolic phospholipase A(2) (cPLA(2)). A putative role for MAPKs in this process has emerged. Here, we studied the contribution of Ca(2+)-independent phospholipase A(2) (iPLA(2)), and the role of the MAP kinase family as well as both cPLA(2) and iPLA(2) mRNA expression by RT-PCR in oxLDL toxicity to GP8.39 cells in vitro. The activation of extracellular signal-regulated kinases ERK1/2, p38 and c-Jun NH(2)-terminal kinase (JNK) was assessed with Western blotting and kinase activity assays. iPLA(2) activity, which was found as a membrane-associated enzyme, was more stimulated by oxLDL compared with native LDL. The phosphorylation of ERK1/2, p38 and JNKs was also significantly enhanced in a dose-dependent manner. PD98059, an ERK inhibitor, SB203580, a p38 inhibitor, and SP600125, an JNK inhibitor, abolished the stimulation of all three members of the MAPK family by oxLDL. Confocal microscopy analysis and subcellular fractionation confirmed either an increase in phosphorylated form of ERKs, p38 and JNKs, or their nuclear translocation upon activation. A strong inhibition of MAPK activation was also observed when endothelial cells were treated with GF109203X, a PKC inhibitor, indicating the important role of both PKC and all three MAPKs in mediating the maximal oxLDL response. Finally, compared with samples untreated or treated with native LDL, treatment with oxLDL (100 muM hydroperoxides) for 24 h significantly increased the levels of constitutively expressed iPLA(2) protein (by 5.1-fold) and mRNA (by 3.1-fold), as well as cPLA(2) protein (by 4.4-fold) and mRNA (by 1.5-fold). Together, these data link the stimulation of PKC-ERK-p38-JNK pathways and PLA(2) activity by oxLDL to the prooxidant mechanism of the lipoprotein complex, which may initially stimulate the endothelial cell reaction against noxious stimuli as well as metabolic repair, such as during inflammation and atherosclerosis.

Molecular characterization of the lipopolysaccharide/platelet activating factor- and zymosan-induced pathways leading to prostaglandin production in P388D1 macrophages

P388D1 cells release free arachidonic acid (AA) and prostaglandin E2 (PGE2) upon stimulation with platelet-activating factor (PAF) and zymosan. The response to PAF is dependent on priming of the cells with bacterial lipopolysaccharide (LPS). In the LPS/PAF pathway, both AA and PGE2 release are dependent on transcription and translation, whereas in the zymosan pathway the release of these compounds appears to be largely independent of these processes. Using quantitative real-time PCR, we analyzed the expression of mRNAs that encode proteins potentially responsible for the dependency of the LPS/PAF pathway on gene expression. These include all the phospholipases A2 (PLA2) that we detected in P388D1 cells, cyclooxygenases (COX), COX-1 and COX-2, the membrane-associated prostaglandin E synthase-1 (mPGES-1), the lipocalin-type prostaglandin D2 synthase (PGDS), hematopoietic PGDS and the subunit G(alpha i2) of heterotrimeric G-proteins. None of the mRNAs encoding PLA2s, PGDSs, or G(alpha i2) are substantially altered during LPS priming. However, cyclooxygenase-2 is up-regulated during LPS priming and after stimulation of the cells with zymosan. A modest but significant increase of mPGES-1 mRNA was also detected upon stimulation with zymosan. Thus, the dependency of the LPS/PAF-induced PGE2 production on gene expression can be attributed to the production of cyclooxygenase-2. The dependency of AA release on gene expression is not due to altered expression of any of the PLA2s. We suggest that an accessory regulatory protein affecting the release of AA must be responsible. Using HPLC we separated lipids that are secreted upon stimulation with LPS/PAF and zymosan and found that in both pathways PGD2 is the dominant prostaglandin produced and also detected PGE2, PGF(2alpha) and AA besides several unidentified compounds.

Negative feedback between secretory and cytosolic phospholipase A2 and their opposing roles in ovalbumin-induced bronchoconstriction in rats

Phospholipase A2 (PLA2) hydrolyzes cell membrane phospholipids (PL) to produce arachidonic acid and lyso-PL. The PLA2 enzymes include the secretory (sPLA2) and cytosolic (cPLA2) isoforms, which are assumed to act synergistically in production of eicosanoids that are involved in inflammatory processes. However, growing evidence raises the possibility that in airways and asthma-related inflammatory cells (eosinophils, basophils), the production of the bronchoconstrictor cysteinyl leukotrienes (CysLT) is linked exclusively to sPLA2, whereas the bronchodilator prostaglandin PGE2 is produced by cPLA2. It has been further reported that the capacity of airway epithelial cells to produce CysLT is inversely proportional to PGE2 production. This seems to suggest that sPLA2 and cPLA2 play opposing roles in asthma pathophysiology and the possibility of a negative feedback between the two isoenzymes. To test this hypothesis, we examined the effect of a cell-impermeable extracellular sPLA2 inhibitor on bronchoconstriction and PLA2 expression in rats with ovalbumin (OVA)-induced asthma. It was found that OVA-induced bronchoconstriction was associated with elevation of lung sPLA2 expression and CysLT production, concomitantly with suppression of cPLA2 expression and PGE2 production. These were reversed by treatment with the sPLA2 inhibitor, resulting in amelioration of bronchoconstriction and reduced CysLT production and sPLA2 expression, concomitantly with enhanced PGE2 production and cPLA2 expression. This study demonstrates, for the first time in vivo, a negative feedback between sPLA2 and cPLA2 and assigns opposing roles for these enzymes in asthma pathophysiology: sPLA2 activation induces production of the bronchoconstrictor CysLT and suppresses cPLA2 expression and the subsequent production of the bronchodilator PGE2.

Intracerebroventricular injection of phospholipases A2 inhibitors modulates allodynia after facial carrageenan injection in mice

The present study was carried out, using inhibitors to secretory phospholipase A2 (sPLA2, 12-epi-scalaradial), cytosolic phospholipase A2 (cPLA2, AACOCF3), or calcium-independent phospholipase A2 (iPLA2, bromoenol lactone), to compare possible contributions of central nervous PLA2 isoforms to the development of allodynia after facial carrageenan injection in mice. C57BL/6J (B6) mice showed increased responses to facial stimulation using a von Frey hair (1 g force), at 8 h, 1 day, and 3 days after facial carrageenan injection. On the other hand, BALB/c mice did not show increased responses at any of the time points. In both B6 and BALB/c mice, intracerebroventricular injection of inhibitors to each of the three PLA2 isoforms significantly reduced responses to von Frey hair stimulation at 8 h and 1 day after facial carrageenan injection, but at 3 days after injection, only the sPLA2 inhibitor had an effect. Since BALB/c mice did not show increased responses after facial carrageenan injection, the reduction in responses actually indicates that there is loss of normal sensitivity to von Frey hair stimulation after intracerebroventricular injection of each of these inhibitors, in this strain of mice. The effects of PLA2 inhibitors are unlikely to be due simply to inhibition of arachidonic acid generation, since intracerebroventricular injection of arachidonic acid also had an anti-nociceptive effect. The above results support an important role of central nervous PLA2s in neurotransmission and pain transmission.

Amyloid β(1–42) and its β(25–35) fragment induce activation and membrane translocation of cytosolic phospholipase A2 in bovine retina capillary pericytes

We investigated changes in cytosolic phospholipase A(2) (cPLA(2)) and calcium-independent PLA(2) (iPLA(2)) activities in bovine retina capillary pericytes after stimulation with 50 microM amyloid-beta (Abeta) (1-42) and its (25-35) fragment, over 24 h (mild, sublethal model of cell damage). In the presence of Abeta peptides, we found that cPLA(2) activity was increased and translocated from the cytosolic fraction to the membrane system, particularly in the nuclear region. Reversed-sequence Abeta(35-25) peptide did not stimulate or induce cPLA(2) translocation. Exposure to both Abeta peptides had no significant effect on cPLA(2) protein content as tested by Western immunoblot analysis. The addition of Abetas to quiescent pericytes was followed by phosphorylation of cPLA(2) and arachidonic acid release. Treatment with inhibitors (AACOCF(3), staurosporine and cycloheximide) resulted in a sharp decrease in basal and stimulated cPLA(2) activity. Inactivating effects of bromoenol lactone (BEL), inhibitor of iPLA(2), demonstrated that the stimulation of total PLA(2) activity by Abetas was mediated by both PLA(2) enzymes. Taken together with our previous observations that both Abeta peptides may induce hydrolysis of phosphatidylcholine, the present results provide evidence that this process is cooperatively mediated by cPLA(2) activation/translocation and iPLA(2) activation. The effect is very likely triggered by a mild prooxidant mechanism which was not able to divert the cell to degeneration. The data confirm the hypothesis that pericytes could be a target of potential vascular damage and reactivity during processes involving amyloid accumulation.

Essential Ca(2+)-independent role of the group IVA cytosolic phospholipase A(2) C2 domain for interfacial activity

The cytosolic Group IVA phospholipase A2 (GIVAPLA2) translocates to intracellular membranes to catalyze the release of lysophospholipids and arachidonic acid. GIVAPLA2 translocation and subsequent activity is regulated by its Ca2+-dependent phospholipid binding C2 domain. Phosphatidylinositol 4,5-bisphosphate (PI-4,5-P2) also binds with high affinity and specificity to GIVAPLA2, facilitating membrane binding and activity. Herein, we demonstrate that GIVAPLA2 possessed full activity in the absence of Ca2+ when PI-4,5-P2 or phosphatidylinositol 3,4,5-trisphosphate were present. A point mutant, D43N, that is unable to bind Ca2+ also had full activity in the presence of PI-4,5-P2. However, when GIVAPLA2 was expressed without its Ca2+-binding C2 domain (DeltaC2), there was no interfacial activity. GIVAPLA2 and DeltaC2 both had activity on monomeric lysophospholipids. DeltaC2, but not the C2 domain alone, binds to phosphoinositides (PIPns) in the same manner as the full-length GIVAPLA2, confirming the location of the PIPn binding site as the GIVAPLA2 catalytic domain. Moreover, proposed PIPn-binding residues in the catalytic domain (Lys488, Lys541, Lys543, and Lys544) were confirmed to be essential for PI-4,5-P2-dependent activity increases. Exploiting the effects of PI-4,5-P2, we have discovered that the C2 domain plays a critical role in the interfacial activity of GIVAPLA2 above and beyond its Ca2+-dependent phospholipid binding.

Secretory phospholipase A2 activity in the normal and kainate injected rat brain, and inhibition by a peptide derived from python serum

The present study aimed to elucidate sPLA(2) activity in the normal and kainate-lesioned hippocampus using selective inhibitors of sPLA(2). In normal rats the highest levels of sPLA(2) were observed in the hippocampus, pons, and medulla, followed by the cerebral neocortex and caudate nucleus. After intracerebroventricular kainate injections an increase in total PLA(2) activity was observed in the rat hippocampus. Using a selective sPLA(2) inhibitor 12-epi-scalaradial, sPLA(2) activity was found to be significantly increased by 2.5-fold on the side of the intracerebroventricular injection compared to the contralateral side. A peptide P-NT.II, derived from the amino acid sequence of "PLA(2)-inhibitory protein," discovered in the serum of the reticulated python, also showed potent sPLA(2) inhibitory activity in homogenates from the kainate-injected hippocampus. These results show that there is a high level of sPLA(2) activity in the normal hippocampus, pons, and medulla oblongata, and that the level increases further in the hippocampus after kainate-induced excitotoxic injury. The increased PLA(2) activity was inhibited by P-NT.II, indicating a potential use of this peptide as a PLA(2) inhibitory agent in the brain.

Hyperosmotic stress induces phosphorylation of cytosolic phospholipase A(2) in HaCaT cells by an epidermal growth factor receptor-mediated process

Cytosolic phospholipase A(2) (cPLA(2)) is an enzyme involved in the formation of proinflammatory mediators by catalyzing the release of arachidonic acid, thereby mediating eicosanoid biosynthesis. Using HaCaT keratinocytes as a model system, we present experimental evidence that in these cells, cPLA(2) is constitutively phosphorylated and that the degree of phosphorylation dramatically increases in cells under hyperosmotic stress induced by sorbitol. In parallel, a rapid release of arachidonic acid followed by prostaglandin E(2) formation was detected. Elucidating the mechanism of cPLA(2) upregulation, we observed that it is mediated via epidermal growth factor receptor (EGFR) activation, since tyrphostin AG1478, a selective inhibitor of EGFR tyrosine kinase, completely inhibited cPLA(2) phosphorylation. Furthermore, addition of PD98059, which is an inhibitor of MEK1 activation, but not of SB203580, which is an inhibitor of p38 stress kinase, inhibited cPLA(2) phosphorylation, indicating that the ras-raf-MEK cascade is the major signalling pathway involved in cPLA(2) phosphorylation. In addition, depletion of the cells from intracellular calcium does not prevent sorbitol-elicited cPLA(2) phosphorylation, suggesting that this process is independent of the presence of calcium. Together, our results demonstrate that hyperosmotic stress phosphorylates cPLA(2) in human keratinocytes by an EGFR-mediated process.

Role of PKC and MAPK in cytosolic PLA2 phosphorylation and arachadonic acid release in primary murine astrocytes

Although Group IV cytosolic phospholipase A2 (cPLA2) in astrocytes has been implicated in a number of neurodegenerative diseases, mechanisms leading to its activation and release of arachidonic acid (AA) have not been clearly elucidated. In primary murine astrocytes, phorbol myristate acetate (PMA) and ATP stimulated phosphorylation of ERK1/2 and cPLA2 as well as evoked AA release. However, complete inhibition of phospho-ERK by U0126, an inhibitor of mitogen-activated protein kinase kinase (MEK), did not completely inhibit PMA-stimulated cPLA2 and AA release. Epidermal growth factor (EGF) also stimulated phosphorylation of ERK1/2 and cPLA2[largely through a protein kinase C (PKC)-independent pathway], but EGF did not evoke AA release. These results suggest that phosphorylation of cPLA2 due to phospho-ERK is not sufficient to evoke AA release. However, complete inhibition of ATP-induced cPLA2 phosphorylation and AA release was observed when astrocytes were treated with GF109203x, a general PKC inhibitor, together with U0126, indicating the important role for both PKC and ERK in mediating the ATP-induced AA response. There is evidence that PMA and ATP stimulated AA release through different PKC isoforms in astrocytes. In agreement with the sensitivity of PMA-induced responses to PKC down-regulation, prolonged treatment with PMA resulted in down-regulation of PKCalpha and epsilon in these cells. Furthermore, PMA but not ATP stimulated rapid translocation of PKCalpha from cytosol to membranes. Together, our results provided evidence for an important role of PKC in mediating cPLA2 phosphorylation and AA release in astrocytes through both ERK1/2-dependent and ERK1/2-independent pathways.

Inhibition of phospholipase A(2) as a therapeutic target

The hydrolysis of cell membrane phospholipids by phospholipase A(2) (PLA(2)) leads to the production of numerous lipid mediators of diverse pathological conditions, mainly inflammatory diseases. These include lysophospholipids and their derivatives, and arachidonic acid and its derivatives (the eicosanoids). Both these groups of mediators are produced predominantly by the secretory PLA(2)s (sPLA(2)s) which hydrolyze the phospholipids of the cell surface membrane. Protection of cell membrane from these 'inflammatory enzymes' can therefore be used for the treatment of inflammatory processes. A prototype of cell-impermeable PLA(2) inhibitors, which protect the cell membrane from different sPLA(2)s without affecting vital phospholipid metabolism, is presented and discussed in the present review.

Cellular regulation of cytosolic group IV phospholipase A2 by phosphatidylinositol bisphosphate levels

Cytosolic group IV phospholipase A2 (cPLA2) is a ubiquitously expressed enzyme with key roles in intracellular signaling. The current paradigm for activation of cPLA2 by stimuli proposes that both an increase in intracellular calcium and mitogen-activated protein kinase-mediated phosphorylation occur together to fully activate the enzyme. Calcium is currently thought to be needed for translocation of the cPLA2 to the membrane via a C2 domain, whereas the role of cPLA2 phosphorylation is less clearly defined. Herein, we report that brief exposure of P388D1 macrophages to UV radiation results in a rapid, cPLA2-mediated arachidonic acid mobilization, without increases in intracellular calcium. Thus, increased Ca2+ availability is a dispensable signal for cPLA2 activation, which suggests the existence of alternative mechanisms for the enzyme to efficiently interact with membranes. Our previous in vitro data suggested the importance of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) in the association of cPLA2 to model membranes and hence in the regulation of cPLA2 activity. Experiments described herein show that PtdInsP2 also serves a similar role in vivo. Moreover, inhibition of PtdInsP2 formation during activation conditions leads to inhibition of the cPLA2-mediated arachidonic acid mobilization. These results suggest that cellular PtdInsP2 levels are involved in the regulation of group IV cPLA2 activation.