Cytosolic phospholipase A2 is required for macrophage arachidonic acid release by agonists that Do and Do not mobilize calcium. Novel role of mitogen-activated protein kinase pathways in cytosolic phospholipase A2 regulation

Purification and partial characterization of two phospholipases A2 from Bothrops leucurus (white-tailed-jararaca) snake venom

Two proteins with phospholipase A(2) (PLA(2)) activity were purified to homogeneity from Bothrops leucurus (white-tailed-jararaca) snake venom through three chromatographic steps: Conventional gel filtration on Sephacryl S-200, ion-exchange on Q-Sepharose and reverse phase on Vydac C4 HPLC column. The molecular mass for both enzymes was estimated to be approximately 14 kDa by SDS-PAGE. The N-terminal sequences (48 residues) show that one enzyme presents lysine at position 48 and the other an aspartic acid in this position, and therefore they were designated blK-PLA(2) and blD-PLA(2) respectively. blK-PLA(2) presented negligible levels of PLA(2) activity as compared to that of blD-PLA(2). The PLA(2) activity of both enzymes is Ca(2+)-dependent. blD-PLA(2) did not have any effect upon platelet aggregation induced by arachidonic acid, ADP or collagen, but strongly inhibits coagulation and is able to stimulate Ehrlich tumor growth but not angiogenesis.

A novel phospholipase from Trypanosoma brucei

Phospholipase A(1) activities have been detected in most cells where they have been sought and yet their characterization lags far behind that of the phospholipases A(2), C and D. The study presented here details the first cloning and characterization of a cytosolic PLA(1) that exhibits preference for phosphatidylcholine (GPCho) substrates. Trypanosoma brucei phospholipase A(1) (TbPLA(1)) is unique from previously identified eukaryotic PLA(1) because it is evolutionarily related to bacterial secreted PLA(1). A T. brucei ancestor most likely acquired the PLA(1) from a horizontal gene transfer of a PLA(1) from Sodalis glossinidius, a bacterial endosymbiont of tsetse flies. Nano-electrospray ionization tandem mass spectrometry analysis of TbPLA(1) mutants established that the enzyme functions in vivo to synthesize lysoGPCho metabolites containing long-chain mostly polyunsaturated and highly unsaturated fatty acids. Analysis of purified mutated recombinant forms of TbPLA(1) revealed that this enzyme is a serine hydrolase whose catalytic mechanism involves a triad consisting of the amino acid residues Ser-131, His-234 and Asp-183. The TbPLA(1) homozygous null mutants generated here constitute the only PLA(1) double knockouts from any organism.

A regulated adaptor function of p40phox: distinct p67phox membrane targeting by p40phox and by p47phox

In the phagocytic cell, NADPH oxidase (Nox2) system, cytoplasmic regulators (p47(phox), p67(phox), p40(phox), and Rac) translocate and associate with the membrane-spanning flavocytochrome b(558), leading to activation of superoxide production. We examined membrane targeting of phox proteins and explored conformational changes in p40(phox) that regulate its translocation to membranes upon stimulation. GFP-p40(phox) translocates to early endosomes, whereas GFP-p47(phox) translocates to the plasma membrane in response to arachidonic acid. In contrast, GFP-p67(phox) does not translocate to membranes when expressed alone, but it is dependent on p40(phox) and p47(phox) for its translocation to early endosomes or the plasma membrane, respectively. Translocation of GFP-p40(phox) or GFP-p47(phox) to their respective membrane-targeting sites is abolished by mutations in their phox (PX) domains that disrupt their interactions with their cognate phospholipid ligands. Furthermore, GFP-p67(phox) translocation to either membrane is abolished by mutations that disrupt its interaction with p40(phox) or p47(phox). Finally, we detected a head-to-tail (PX-Phox and Bem1 [PB1] domain) intramolecular interaction within p40(phox) in its resting state by deletion mutagenesis, cell localization, and binding experiments, suggesting that its PX domain is inaccessible to interact with phosphatidylinositol 3-phosphate without cell stimulation. Thus, both p40(phox) and p47(phox) function as diverse p67(phox) "carrier proteins" regulated by the unmasking of membrane-targeting domains in distinct mechanisms.

Lipid profiling reveals arachidonate deficiency in RAW264.7 cells: Structural and functional implications

Glycerophospholipids containing arachidonic acid (20:4) serve as the precursors for an array of biologically active lipid mediators, most of which are produced by macrophages. We have applied mass spectrometry-based lipid profiling technology to evaluate the glycerophospholipid structure and composition of two macrophage populations, resident peritoneal macrophages and RAW264.7 cells, with regard to their potential for 20:4-based lipid mediator biosynthesis. Fatty acid analysis indicated that RAW264.7 cells were deficient in 20:4 (10 +/- 1 mol %) compared to peritoneal macrophages (26 +/- 1 mol %). Mass spectrometry of total glycerophospholipids demonstrated a marked difference in the distribution of lipid species, including reduced levels of 20:4-containing lipids, in RAW264.7 cells compared to peritoneal macrophages. Enrichment of RAW264.7 cells with 20:4 increased the fatty acid to 20 +/- 1 mol %. However, the distribution of the incorporated 20:4 remained different from that of peritoneal macrophages. RAW264.7 cells pretreated with granulocyte-macrophage colony stimulating factor followed by lipopolysaccharide and interferon-gamma mobilized similar quantities of 20:4 and produced similar amounts of prostaglandins as peritoneal macrophages treated with LPS alone. LPS treatment resulted in detectable changes in specific 20:4-containing glycerophospholipids in peritoneal cells, but not in RAW264.7 cells. 20:4-enriched RAW264.7 cells lost 88% of the incorporated fatty acid during the LPS incubation without additional prostaglandin synthesis. These results illustrate that large differences in glycerophospholipid composition may exist, even in closely related cell populations, and demonstrate the importance of interpreting the potential for lipid-mediator biosynthesis in the context of overall glycerophospholipid composition.

Sustained activation of proton channels and NADPH oxidase in human eosinophils and murine granulocytes requires PKC but not cPLA2 alpha activity

The prevailing hypothesis that a signalling pathway involving cPLA(2)alpha is required to enhance the gating of the voltage-gated proton channel associated with NADPH oxidase was tested in human eosinophils and murine granulocytes. This hypothesis invokes arachidonic acid (AA) liberated by cPLA(2)alpha as a final activator of proton channels. In human eosinophils studied in the perforated-patch configuration, phorbol myristate acetate (PMA) stimulation elicited NADPH oxidase-generated electron current (I(e)) and enhanced proton channel gating identically in the presence or absence of three specific cPLA(2)alpha inhibitors, Wyeth-1, pyrrolidine-2 and AACOCF(3) (arachidonyl trifluoromethyl ketone). In contrast, PKC inhibitors GFX (GF109203X) or staurosporine prevented the activation of either proton channels or NADPH oxidase. PKC inhibition during the respiratory burst reversed the activation of both molecules, suggesting that ongoing phosphorylation is required. This effect of GFX was inhibited by okadaic acid, implicating phosphatases in proton channel deactivation. Proton channel activation by AA was partially reversed by GFX or staurosporine, indicating that AA effects are due in part to activation of PKC. In granulocytes from mice with the cPLA(2)alpha gene disrupted (knockout mice), PMA or fMetLeuPhe activated NADPH oxidase and proton channels in a manner indistinguishable from the responses of control cells. Thus, cPLA(2)alpha is not essential to activate the proton conductance or for a normal respiratory burst. Instead, phosphorylation of the proton channel or an activating molecule converts the channel to its activated gating mode. The existing paradigm for regulation of the concerted activity of proton channels and NADPH oxidase must be revised.

Impairment of p38 MAPK-mediated cytosolic phospholipase A2 activation in the kidneys is associated with pathogenicity of Candida albicans

In studying the mechanisms underlying the susceptibility of the kidney to candidal infection, we previously reported that the reduced production of cytokines [i.e. tumour necrosis factor-alpha (TNF-alpha)] via platelet-activating factor (PAF)-induced activation of nuclear factor-kappaB (NF-kappaB) renders the organ susceptible to the fungal burden. In this study, we investigated the possibility that pathogenic Candida albicans may evade clearance and perhaps even multiply by inhibiting elements in the signalling pathway that lead to the production of TNF-alpha. The fungal burden of pathogenic C. albicans in the kidneys was 10(4)-10(5)-fold higher than that of a non-pathogenic strain. PAF-induced early activation of NF-kappaB and TNF-alpha mRNA expression were both observed in the kidneys of mice infected with non-pathogenic strains of C. albicans, but not in mice infected with pathogenic strains. Impairment of PAF-mediated early NF-kappaB activation following infection with pathogenic C. albicans was associated with the prevention of activation of the enzyme cytosolic phospholipase A(2) (cPLA(2)) as well as the upstream pathway of cPLA(2), p38 mitogen-activated protein kinase. Collectively, these findings indicate that C. albicans exerts its pathogenicity through impairing the production of anticandidal cytokines by preventing cPLA(2) activity. This novel mechanism provides insight into understanding pathogenic C. albicans and perhaps identifies a target for its treatment.

Zymosan-induced glycerylprostaglandin and prostaglandin synthesis in resident peritoneal macrophages: roles of cyclo-oxygenase-1 and -2

COX [cyclo-oxygenase; PG (prostaglandin) G/H synthase] oxygenates AA (arachidonic acid) and 2-AG (2-arachidonylglycerol) to endoperoxides that are converted into PGs and PG-Gs (glycerylprostaglandins) respectively. In vitro, 2-AG is a selective substrate for COX-2, but in zymosan-stimulated peritoneal macrophages, PG-G synthesis is not sensitive to selective COX-2 inhibition. This suggests that COX-1 oxygenates 2-AG, so studies were carried out to identify enzymes involved in zymosan-dependent PG-G and PG synthesis. When macrophages from COX-1-/- or COX-2-/- mice were treated with zymosan, 20-25% and 10-15% of the PG and PG-G synthesis observed in wild-type cells respectively was COX-2 dependent. When exogenous AA and 2-AG were supplied to COX-2-/- macrophages, PG and PG-G synthesis was reduced as compared with wild-type cells. In contrast, when exogenous substrates were provided to COX-1-/- macrophages, PG-G but not PG synthesis was reduced. Product synthesis also was evaluated in macrophages from cPLA(2alpha) (cytosolic phospholipase A2alpha)-/- mice, in which zymosan-induced PG synthesis was markedly reduced, and PG-G synthesis was increased approx. 2-fold. These studies confirm that peritoneal macrophages synthesize PG-Gs in response to zymosan, but that this process is primarily COX-1-dependent, as is the synthesis of PGs. They also indicate that the 2-AG and AA used for PG-G and PG synthesis respectively are derived from independent pathways.

The role of cytosolic phospholipase A2-alfa in regulation of phagocytic functions

Phospholipase A2(s) (PLA2(s)) are a family of enzymes that is present in a variety of mammalian and nonmammalian sources. Phagocytic cells contain cytosolic PLA2 (cPLA2) as well as several types of secreted PLA2, all of which have the potential to produce proinflammatory lipid mediators. The role of the predominant form of cPLA2 present in neutrophils is cPLA2alpha was studied by many groups. By modulating its expression in a variety of phagocytes it was found that it plays a major role in formation of eicosanoids. In addition, it was reported that cPLA2alpha also regulates the NADPH oxidase activation. The specificity of its effect on the NADPH oxidase is evident by results demonstrating that the differentiation process as well as other phagocytic functions are normal in cPLA2alpha-deficient PLB cell model. The novel dual subcellular localization of cPLA2alpha in different compartments, in the plasma membranes and in the nucleus, provides a molecular mechanism for the participation of cPLA2alpha in different processes (stimulation of NADPH oxidase and formation of eicosanoids) in the same cells.

The phospholipase A2 superfamily and its group numbering system

The superfamily of phospholipase A(2) (PLA(2)) enzymes currently consists of 15 Groups and many subgroups and includes five distinct types of enzymes, namely the secreted PLA(2)s (sPLA(2)), the cytosolic PLA(2)s (cPLA(2)), the Ca(2+) independent PLA(2)s (iPLA(2)), the platelet-activating factor acetylhydrolases (PAF-AH), and the lysosomal PLA(2)s. In 1994, we established the systematic Group numbering system for these enzymes. Since then, the PLA(2) superfamily has grown continuously and over the intervening years has required several updates of this Group numbering system. Since our last update, a number of new PLA(2)s have been discovered and are now included. Additionally, tools for the investigation of PLA(2)s and approaches for distinguishing between the different Groups are described.

Group V sPLA2: classical and novel functions

Group V sPLA(2) is unique among the family of secretory sPLA(2) enzymes in being able to bind to cell membranes through both interfacial-binding and through binding to proteoglycan. The function of group V sPLA(2) as an enzyme and its cross-talk with cPLA(2)alpha in initiating eicosanoid generation is well documented. Evidence, though, is emerging on the ability of this molecule to act as a regulator of several intracellular and extracellular pathways independently of its ability to provide arachidonic acid for eicosanoid generation, acting within the cell or as a secreted enzyme. In this article we will provide an overview of the properties of the enzyme and how they relate to our current understanding of its function.

PKCalpha regulates phosphorylation and enzymatic activity of cPLA2 in vitro and in activated human monocytes

Phospholipases A(2) (PLA(2)) are potent regulators of the inflammatory response. We have observed that Group IV cPLA(2) activity is required for the production of superoxide anion (O(2)(-)) in human monocytes [Li Q., Cathcart M.K. J. Biol. Chem. 272 (4) (1997) 2404-2411.]. We have previously identified PKCalpha as a kinase pathway required for monocyte O(2)(-) production [Li Q., Cathcart M.K. J. Biol. Chem. 269 (26) (1994) 17508-17515.]. We therefore investigated the potential interaction between PKCalpha and cPLA(2) by evaluating the requirement for specific PKC isoenzymes in the process of activating cPLA(2) enzymatic activity and protein phosphorylation upon monocyte activation. We first showed that general PKC inhibitors and antisense oligodeoxyribonucleotides (ODN) to the cPKC group of PKC enzymes inhibited cPLA(2) activity. To distinguish between PKCalpha and PKCbeta isoenzymes in regulating cPLA(2) protein phosphorylation and enzymatic activity, we employed our previously characterized PKCalpha or PKCbeta isoenzyme-specific antisense ODN [Li Q., Subbulakshmi V., Fields A.P., Murray, N.R., Cathcart M.K., J. Biol. Chem. 274 (6) (1999) 3764-3771]. Suppression of PKCalpha expression, but not PKCbeta expression, inhibited cPLA(2) protein phosphorylation and enzymatic activity. Additional studies ruled out a contribution by Erk1/2 to cPLA(2) phosphorylation and activation. We also found that cPLA(2) co-immunoprecipitated with PKCalpha and vice versa. In vitro studies demonstrated that PKCalpha could directly phosphorylate cPLA(2).and enhance enzymatic activity. Finally, we showed that addition of arachidonic acid restored the production of O(2)(-) in monocytes defective in either PKCalpha or cPLA(2) expression. Taken together, our data suggest that PKCalpha, but not PKCbeta, is the predominant cPKC isoenzyme required for cPLA(2) protein phosphorylation and maximal induction of cPLA(2) enzymatic activity upon activation of human monocytes. Our data also support the concept that the requirements for PKCalpha and cPLA(2) in O(2)(-) generation are solely due to their seminal role in generating arachidonic acid.

Intracellular actions of group IIA secreted phospholipase A2 and group IVA cytosolic phospholipase A2 contribute to arachidonic acid release and prostaglandin production in rat gastric mucosal cells and transfected human embryonic kidney cells

Gastric epithelial cells liberate prostaglandin E(2) in response to cytokines as part of the process of healing of gastric lesions. Treatment of the rat gastric epithelial cell line RGM1 with transforming growth factor-alpha and interleukin-1beta leads to synergistic release of arachidonate and production of prostaglandin E(2). Results with highly specific and potent phospholipase A(2) inhibitors and with small interfering RNA show that cytosolic phospholipase A(2)-alpha and group IIA secreted phospholipase A(2) contribute to arachidonate release from cytokine-stimulated RGM1 cells. In the late phase of arachidonate release, group IIA secreted phospholipase A(2) is induced (detected at the mRNA and protein levels), and the action of cytosolic phospholipase A(2)-alpha is required for this induction. Results with RGM1 cells and group IIA secreted phospholipase A(2)-transfected HEK293 cells show that the group IIA phospholipase acts prior to externalization from the cells. RGM1 cells also express group XIIA secreted phospholipase A(2), but this enzyme is not regulated by cytokines nor does it contribute to arachidonate release. The other eight secreted phospholipases A(2) were not detected in RGM1 cells at the mRNA level. These results clearly show that cytosolic and group IIA secreted phospholipases A(2) work together to liberate arachidonate from RGM1 cell phospholipids in response to cytokines.

Design and Synthesis of 1-Indol-1-yl-propan-2-ones as Inhibitors of Human Cytosolic Phospholipase A2α

The synthesis and structure-activity relationship study of a series of 1-indol-1-yl-3-phenoxypropan-2-one inhibitors of cytosolic phospholipase A(2)alpha (cPLA(2)alpha) are described. The compounds were evaluated in a vesicle assay with isolated cPLA(2)alpha and in cellular assays with intact human platelets. Systematic variation led to 3-methylhydrogen 1-[3-(4-decyloxyphenoxy)-2-oxopropyl]indole-3,5-dicarboxylate (57), which revealed the highest activity against the isolated enzyme. With an IC(50) value of 4.3 nM in this assay, it is one of the most potent in vitro cPLA(2)alpha inhibitors known today.

Group V secretory phospholipase A2 translocates to the phagosome after zymosan stimulation of mouse peritoneal macrophages and regulates phagocytosis

We have previously reported that group V secretory phospholipase A2 (sPLA2) amplifies the action of cytosolic phospholipase A2(cPLA2) alpha in regulating eicosanoid biosynthesis by mouse peritoneal macrophages stimulated with zymosan (Satake, Y., Diaz, B. L., Balestrieri, B., Lam, B. K., Kanaoka, Y., Grusby, M. J., and Arm, J. P. (2004) J. Biol. Chem. 279, 16488-16494). To further understand the role of group V sPLA2, we studied its localization in resting mouse peritoneal macrophages before and after stimulation with zymosan and the effect of deletion of the gene encoding group V sPLA2 on phagocytosis of zymosan. We report that group V sPLA2 is present in the Golgi apparatus and recycling endosome in the juxtanuclear region of resting peritoneal macrophages. Upon ingestion of zymosan by mouse peritoneal macrophages, group V sPLA2 is recruited to the phagosome. There it co-localizes with cPLA2alpha, 5-lipoxygenase, 5-lipoxygenase-activating protein, and leukotriene C4 synthase. Using immunostaining for the cysteinyl leukotrienes in carbodiimide-fixed cells, we show, for the first time, that the phagosome is a site of cysteinyl leukotriene formation. Furthermore, peritoneal macrophages from group V sPLA2-null mice demonstrated a >50% attenuation in phagocytosis of zymosan particles, which was restored by adenoviral expression of group V sPLA2 but IIA not group sPLA2. These data demonstrate that group V sPLA2 contributes to the innate immune response both through regulation of eicosanoid generation in response to a phagocytic stimulus and also as a component of the phagocytic machinery.

Estrogen induces phospholipase A2 activation through ERK1/2 to mobilize intracellular calcium in MCF-7 cells

The principal secreted estrogen, 17beta-estradiol rapidly activates signaling cascades that regulate important physiological processes including ion transport across membranes, cytosolic pH and cell proliferation. These effects have been extensively studied in the MCF-7 estrogen-responsive human breast carcinoma cell line. Here, we demonstrate that a physiological concentration of 17beta-estradiol caused a rapid, synchronous and transient increase in intracellular calcium concentration in a confluent monolayer of MCF-7 cells 2-3 min after treatment. This response was abolished when cells were pre-incubated with the phospholipase A(2) (PLA(2)) inhibitor quinacrine or with the cyclooxygenase inhibitor indomethacin. The translocation of GFP-cPLA(2)alpha to perinuclear membranes occurred 1-2 min after 17beta-estradiol treatment; this translocation was concurrent with the transient phosphorylation of cPLA(2)alpha at serine residue 505. The phosphorylation and translocation of cPLA(2) were sensitive to inhibition of the extracellular signal regulated kinase (ERK) signaling cascade and occurred simultaneously with a transient activation of ERK. The phosphorylation of cPLA(2) could be stimulated by membrane impermeable 17beta-estradiol conjugated to bovine serum albumen and was blocked by an antagonist of the classical estrogen receptor. Here we show, for the first time, that PLA(2) and the eicosanoid biosynthetic pathway are involved in the 17beta-estradiol induced rapid calcium responses of breast cancer cells.

Overexpression of Cytosolic Group IVA Phospholipase A2 Protects Cells from Ca2+-dependent Death

The calcium ionophore ionomycin induces apoptosis-like events in the human embryonic kidney cell line at early times. Plasma membrane blebbing, mitochondrial depolarization, externalization of phosphatidylserine, and nuclear permeability changes can all be observed within 15 min of treatment. However, there is no activation of caspases or chromatin condensation. Expression of a fusion protein containing the enhanced green fluorescent protein (EGFP) and human cytosolic Group IVA phospholipase A(2)alpha (EGFP-cPLA(2)alpha) in these cells prevents ionomycin-induced phosphatidylserine externalization and death. Cells expressing the cPLA(2)alpha mutant D43N, which does not bind calcium, retain their susceptibility to ionomycin-induced cell death. Both nonexpressing and EGFP-D43N-cPLA(2)alpha-expressing human embryonic kidney cells can be spared from ionomycin-induced cell death by pretreating them with exogenous arachidonic acid. Moreover, during calcium overload, mitochondrial depolarization is significantly lower in the EGFP-cPLA(2)alpha-expressing cells than in cells expressing normal amounts of cPLA(2)alpha. These results suggest that early cell death events promoted by an overload of calcium can be prevented by the presence of high levels of arachidonic acid.

Regulation of cytosolic phospholipase A2 activation and cyclooxygenase 2 expression in macrophages by the beta-glucan receptor

Phagocytosis of non-opsonized microorganisms by macrophages initiates innate immune responses for host defense against infection. Cytosolic phospholipase A(2) is activated during phagocytosis, releasing arachidonic acid for production of eicosanoids, which initiate acute inflammation. Our objective was to identify pattern recognition receptors that stimulate arachidonic acid release and cyclooxygenase 2 (COX2) expression in macrophages by pathogenic yeast and yeast cell walls. Zymosan- and Candida albicans-stimulated arachidonic acid release from resident mouse peritoneal macrophages was blocked by soluble glucan phosphate. In RAW264.7 cells arachidonic acid release, COX2 expression, and prostaglandin production were enhanced by overexpressing the beta-glucan receptor, dectin-1, but not dectin-1 lacking the cytoplasmic tail. Pure particulate (1, 3)-beta-D-glucan stimulated arachidonic acid release and COX2 expression, which were augmented in a Toll-like receptor 2 (TLR2)-dependent manner by macrophage-activating lipopeptide-2. However, arachidonic acid release and leukotriene C(4) production stimulated by zymosan and C. albicans were TLR2-independent, whereas COX2 expression and prostaglandin production were partially blunted in TLR2(-/-) macrophages. Inhibition of Syk tyrosine kinase blocked arachidonic acid release and COX2 expression in response to zymosan, C. albicans, and particulate (1, 3)-beta-D-glucan. The results suggest that cytosolic phospholipase A(2) activation triggered by the beta-glucan component of yeast is dependent on the immunoreceptor tyrosine-based activation motif-like domain of dectin-1 and activation of Syk kinase, whereas both TLR2 and Syk kinase regulate COX2 expression.

Impaired NADPH oxidase activity in Rac2-deficient murine neutrophils does not result from defective translocation of p47phox and p67phox and can be rescued by exogenous arachidonic acid

Rac2 is a hematopoietic-specific Rho-GTPase that plays a stimulus-specific role in regulating reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activation and other functional responses in neutrophils. In this study, rac2-/- neutrophils were shown to have significantly decreased NADPH oxidase activity and actin remodeling in response to exogenous arachidonic acid (AA), as previously observed for phorbol 12-myristate 13-acetate (PMA) or formyl-Met-Leu-Phe (fMLP) as agonists. PMA-, fMLP-, or AA-induced translocation of p47phox and p67phox to the plasma membrane was not impaired in rac2-/- neutrophils. Combined stimulation of rac2-/- neutrophils with exogenous AA and PMA had a synergistic effect on NADPH oxidase activity, and superoxide production increased to a level that was at least as high as wild-type cells and had no effect on fMLP-elicited enzyme activity. Membrane translocation of p47phox and p67phox as well as Rac1 activation was not increased further by combined PMA and AA stimulation. Inhibitor studies were consistent with important roles for phorbol ester-activated protein kinase C (PKC) isoforms and an atypical isoform, PKCzeta, in superoxide production by wild-type and rac2-/- neutrophils stimulated with AA and PMA. In addition, PMA-stimulated release of AA and cytoplasmic phospholipase A2 expression in rac2-/- neutrophils were similar to wild-type, suggesting that deficient AA production by PMA-stimulated rac2-/- neutrophils does not explain the effect of exogenous AA on oxidase activity. Although not required for translocation of p47phox and p67phox, Rac2 is necessary for optimal activity of the assembled oxidase complex, an effect that can be replaced by exogenous AA, which may act directly or via an exogenous AA-induced mediator.

ADAM-9 (MDC-9/meltrin-gamma), a member of the a disintegrin and metalloproteinase family, regulates myeloma-cell-induced interleukin-6 production in osteoblasts by direct interaction with the alpha(v)beta5 integrin

ADAM-9, a member of the a disintegrin and metalloproteinase family, contains both metalloproteinase and disintegrin domains. Myeloma cell lines express ADAM-9; however, its function and role in the pathophysiology of multiple myeloma is unknown. The aim of this study was to establish whether primary myeloma cells express ADAM-9, whether ADAM-9 regulates IL-6 production in human osteoblasts (hOBs), whether ADAM-9 interacts with specific integrin heterodimers, and the identity of downstream signaling pathways. Primary myeloma cells demonstrated increased expression of ADAM-9 (P < .01). ADAM-9 promoted a 5-fold increase in IL-6, but not IL-1beta mRNA, and a dose- and time-dependent increase in IL-6 production by hOBs (P < .01). IL-6 induction was inhibited by an antibody to the alpha(v)beta5 integrin (P < .01) but not by antibodies to other integrin heterodimers. ADAM-9 was shown to bind directly to the alpha(v)beta5 integrin on hOBs. Antibodies to ADAM-9 and alpha(v)beta5 integrin inhibited myeloma cell-induced IL-6 production by hOBs (P < .01). Furthermore, inhibitors of p38 MAPK and cPLA2, but not NF-kappaB and JAK2, signaling pathways inhibited ADAM-9-induced IL-6 production by hOBs (P < .01). These data demonstrate that ADAM-9, expressed by myeloma cells, stimulates IL-6 production in hOBs by binding the alpha(v)beta5 integrin. This may have important consequences for the growth and survival of myeloma cells in bone.

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.

Roles of Rac and p38 kinase in the activation of cytosolic phospholipase A2 in response to PMA

The roles of Rac and p38 kinase in the activation of cPLA2 (cytosolic PLA2) in Rat-2 fibroblasts were investigated. In the present study, we found that PMA activates cPLA2 by a Rac-p38 kinase-dependent pathway. Consistent with this, Rac, if activated, was shown to stimulate cPLA2 in a p38 kinase-dependent manner. In another experiment to understand the signalling mechanism by which the Rac-p38 kinase cascade mediates cPLA2 activation in response to PMA, we observed that PMA-induced cPLA2 translocation to the perinuclear region is completely inhibited by the expression of Rac1N17 or treatment with SB203580 (inhibitor of p38 kinase), suggesting that Rac-p38 kinase cascade acts in this instance by mediating the translocation of cPLA2. The mediatory role of p38 kinase in cPLA2 activation was further demonstrated after a treatment with anisomycin, a very effective activator of p38 kinase. Consistent with the mediatory role of p38 kinase in stimulating cPLA2, anisomycin induced the translocation and activation of cPLA2 in a p38 kinase-dependent manner.

Phosphatidylinositol 4,5-bisphosphate anchors cytosolic group IVA phospholipase A2 to perinuclear membranes and decreases its calcium requirement for translocation in live cells

The eicosanoids are centrally involved in the onset and resolution of inflammatory processes. A key enzyme in eicosanoid biosynthesis during inflammation is group IVA phospholipase A2 (also known as cytosolic phospholipase A2alpha, cPLA2alpha). This enzyme is responsible for generating free arachidonic acid from membrane phospholipids. cPLA2alpha translocates to perinuclear membranes shortly after cell activation, in a process that is governed by the increased availability of intracellular Ca2+. However, cPLA2alpha also catalyzes membrane phospholipid hydrolysis in response to agonists that do not mobilize intracellular Ca2+. How cPLA2alpha interacts with membranes under these conditions is a major, still unresolved issue. Here, we report that phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] promotes translocation of cPLA2alpha to perinuclear membranes of intact cells in a manner that is independent of rises in the intracellular Ca2+ concentration. PtdIns(4,5)P2 anchors the enzyme to perinuclear membranes and allows for a proper interaction with its phospholipid substrate to release arachidonic acid.

PGE2 release is independent of upregulation of Group V phospholipase A2 during long-term stimulation of P388D1 cells with LPS

P388D1 cells release arachidonic acid (AA) and produce prostaglandin E2 (PGE2) upon long-term stimulation with lipopolysaccharide (LPS). The cytosolic Group IVA (GIVA) phospholipase A2 (PLA2) has been implicated in this pathway. LPS stimulation also results in increased expression and secretion of a secretory PLA2, specifically GV PLA2. To test whether GV PLA2 contributes to PGE2 production and whether GIVA PLA2 activation increases the expression of GV PLA2, we utilized the specific GIVA PLA2 inhibitor pyrrophenone and second generation antisense oligonucleotides (AS-ONs) designed to specifically inhibit expression and activity of GV PLA2. Treatment of P388D1 cells with antisense caused a marked decrease in basal GV PLA2 mRNA and prevented the LPS-induced increase in GV PLA2 mRNA. LPS-stimulated cells release active GV PLA2 into the medium, which is inhibited to background levels by antisense treatment. However, LPS-induced PGE2 release by antisense-treated cells and by control cells are not significantly different. Collectively, the results suggest that the upregulation of GV PLA2 during long-term LPS stimulation is not required for PGE2 production by P388D1 cells. Experiments employing pyrrophenone suggested that GIVA PLA2 is the dominant player involved in AA release, but it appears not to be involved in the regulation of LPS-induced expression of GV PLA2 or cyclooxygenase-2.

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.

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.

Intact actin filaments are required for cytosolic phospholipase A2 translocation but not for its activation by norepinephrine in vascular smooth muscle cells

Cytosolic phospholipase A(2) (cPLA(2)) is activated and translocated to the nuclear envelope by various vasoactive agents, including norepinephrine (NE), and releases arachidonic acid (AA) from tissue phospholipids. We previously demonstrated that NE-induced cPLA(2) translocation to the nuclear envelope is mediated via its phosphorylation by calcium/calmodulin-dependent kinase-II in rabbit vascular smooth muscle cells (VSMCs). Cytoskeletal structures actin and microtubule filaments have been implicated in the trafficking of proteins to various cellular sites. This study was conducted to investigate the contribution of actin and microtubule filaments to cPLA(2) translocation to the nuclear envelope and its activation by NE in rabbit VSMCs. NE (10 microM) caused cPLA(2) translocation to the nuclear envelope as determined by immunofluorescence. Cytochalasin D (CD; 0.5 microM) and latrunculin A (LA; 0.5 microM) that disrupted actin filaments, blocked cPLA(2) translocation elicited by NE. On the other hand, disruption of microtubule filaments by 10 microM colchicine did not block NE-induced cPLA(2) translocation to the nuclear envelope. CD and LA did not inhibit NE-induced increase in cytosolic calcium and cPLA(2) activity, determined from the hydrolysis of l-1-[(14)C]arachidonyl phosphatidylcholine and release of AA. Coimmunoprecipitation studies showed an association of actin with cPLA(2), which was not altered by CD or LA. Far-Western analysis showed that cPLA(2) interacts directly with actin. Our data suggest that NE-induced cPLA(2) translocation to the nuclear envelope requires an intact actin but not microtubule filaments and that cPLA(2) phosphorylation and activation and AA release are independent of its translocation to the nuclear envelope in rabbit VSMCs.

RAW264.7 cells lack prostaglandin-dependent autoregulation of tumor necrosis factor-alpha secretion

Studies of the response of RAW264.7 cells (RAW) to lipopolysaccharide (LPS) were carried out to determine why these cells do not demonstrate the prostaglandin (PG)-dependent autocrine regulation of tumor necrosis factor-alpha (TNF-alpha) secretion observed in primary resident peritoneal macrophages (RPMs). The major cyclooxygenase (COX) product of LPS-stimulated RAW was PGD2, with lesser amounts of PGE2. LPS-treated RAW produced PGs more slowly and reached their maximal PG synthetic rate later than did LPS-treated RPMs, as a result of lower constitutive COX-1 expression and a slower rate of COX-2 induction. Cytosolic phospholipase A2 and levels of free arachidonic acid were similar in RAW and RPMs. In contrast to RPMs, LPS-treated RAW produced high quantities of TNF-alpha, which were not altered in the presence of COX inhibitors. This failure of endogenous PGs to suppress TNF-alpha secretion was explained by the absence of the prostaglandin D2 receptor and the low levels of PGE2 produced during the first 2 h of the LPS response. These studies demonstrate that autocrine regulation of TNF-alpha secretion in response to LPS is greatly facilitated by a COX-1-mediated rapid accumulation of PGs as well by a correspondence between the PGs produced and the receptors expressed by the cells.

PTH and phospholipase A2 in the aging process of intestinal cells

In this study we analyzed, for the first time, alterations in phospholipase A2 (PLA2) activity and response to parathyroid hormone (PTH) in rat enterocytes with aging. We found that PTH, rapidly stimulate arachidonic acid (AA) release in rat duodenal cells (+1- to 2-fold), an effect that is greatly potentiated by aging (+4-fold). We also found that hormone-induced AA release in young animals is Ca2+-dependent via cPLA2, while AA released by PTH in cells from aged rats is due to the activation of cPLA2 and the Ca2+-independent PLA2 (iPLA2). In enterocytes from 3 months old rats, PTH induced, in a time and dose-dependent fashion, the phosphorylation of cPLA2 on serine 505, with a maximum at 10 min (+7-fold). Basal levels of cPLA2 serine-phosphorylation were higher in old enterocytes, affecting the hormone response which was greatly diminished (+2-fold at 10 min). cPLA2 phosphorylation impairment in old animals was not related to changes of cPLA2 protein expression and did not explain the substantial increase on PTH-induced AA release with aging, further suggesting the involvement of a different PLA2 isoform. Intracellular Ca2+ chelation (BAPTA-AM, 5 microM) suppressed the serine phosphorylation of cPLA2 in both, young and aged rats, demonstrating that intracellular Ca2+ is required for full activation of cPLA2 in enterocytes stimulated with PTH. Hormone effect on cPLA2 was suppressed to a great extent by the MAP kinases ERK 1 and ERK2 inhibitor, PD 98059 (20 microM), the cAMP antagonist, Rp-cAMP, and the PKC inhibitor Ro31820 both, in young and aged animals. Enterocytes exposure to PTH also resulted in phospho-cPLA2 translocation from cytosol to nuclei and membrane fractions, where phospholipase substrates reside. Hormone-induced enzyme translocation is also modified by aging where, in contrast to young animals, part of phospho-cPLA2 remained cytosolic. Collectively, these data suggest that PTH activates in duodenal cells, a Ca2+-dependent cytosolic PLA2 and attendant AA release and that this activation requires prior stimulation of intracellular ERK1/2, PKA, and PKC. cPLA2 is the major enzyme responsible for AA release in young enterocytes while cPLA2 and the Ca2+-independent iPLA2, potentiate PTH-induced AA release in aged cells. Impairment of PTH activation of PLA2 isoforms upon aging may result in abnormal hormone regulation of membrane fluidity and permeability and thereby affecting intestinal cell membrane function.

Effect of EGF on H2O2-induced inhibition of ?-MG uptake in renal proximal tubule cells: Involvement of MAPK and AA release

Both oxidative stress and epidermal growth factor (EGF) contribute to the initiation and progression of renal proximal tubular dysfunction under pathophysiologic conditions. Thus, this study was performed (1) to examine both the individual, and the combined effects of H2O2 and EGF on alpha-methyl-D-glucopyranoside uptake (alpha-MG uptake) in the primary cultured renal proximal tubule cells (PTCs), and (2) to elucidate the involvement of p44/42 mitogen activated protein kinase (MAPK) and phospholipase A2 in mediating these actions. Both H2O2 and EGF inhibited alpha-MG uptake individually, while the combination of H2O2 and EGF further potentiated the inhibitory effect on alpha-MG uptake, which was elicited by each agent. H2O2 not only caused a rapid increase in the phosphorylation of p44/42 MAPK, but also promoted the translocation of cytosolic phospholipase A2 (cPLA2) from the cytosolic to particulate fraction, and stimulated cellular [3H]-arachidonic acid (AA) release. EGF similarly activates phosphorylation of p44/42 MAPK and stimulates [3H]-AA release. When PTCs were exposed to 100 microM H2O2 and 50 ng/ml EGF simultaneously, a further increase in the phosphorylation of p44/42 MAPK, of [3H]-AA release, and of prostaglandin E2 (PGE2) production was elicited as compared with the effects of each individual agonist alone. Moreover, the additive phosphorylation of p44/42 MAPK, [3H]-AA release, and PGE2 production by H2O2 and EGF was almost completely inhibited by the p44/42 MAPK inhibitor, PD 98059. In conclusion, these results are consistent with the hypothesis that under conditions of oxidative stress, the H2O2-induced inhibition of alpha-MG uptake in the renal proximal tubule is mediated through a modulation of the EGF signaling pathway, promoting further phosphorylation of p44/42 MAPK, activation of PLA2.

HPLC assay with UV spectrometric detection for the evaluation of inhibitors of cytosolic phospholipase A2

A non-radioactive spectrometric assay for the evaluation of inhibitors of cytosolic phospholipase A(2) (cPLA(2)) is described. The enzyme was isolated from human platelets applying anion exchange chromatography. Sonicated covesicles consisting of 0.2mM 1-stearoyl-2-arachidonoyl-sn-glycero-3-phosphocholine and 0.1mM 1,2-dioleoyl-sn-glycerol were used as enzyme substrate. The cPLA(2) activity was determined by measuring the arachidonic acid released by the enzyme with reversed-phase HPLC and UV detection at 200 nm after cleaning up the samples by solid phase extraction. Two known cPLA(2) inhibitors were used to validate the test assay.

Cytosolic phospholipase A2-alpha is necessary for platelet-activating factor biosynthesis, efficient neutrophil-mediated bacterial killing, and the innate immune response to pulmonary infection: cPLA2-alpha does not regulate neutrophil NADPH oxidase activity

The role of a cytosolic phospholipase A(2)-alpha (cPLA(2)-alpha) in neutrophil arachidonic acid release, platelet-activating factor (PAF) biosynthesis, NADPH oxidase activation, and bacterial killing in vitro, and the innate immune response to bacterial infection in vivo was examined. cPLA(2)-alpha activity was blocked with the specific cPLA(2)-alpha inhibitor, Pyrrolidine-1 (human cells), or by cPLA(2) -alpha gene disruption (mice). cPLA(2)-alpha inhibition or gene disruption led to complete suppression of neutrophil arachidonate release and PAF biosynthesis but had no effect on neutrophil NADPH oxidase activation, FcgammaII/III or CD11b surface expression, primary or secondary granule secretion, or phagocytosis of Escherichia coli in vitro. In contrast, cPLA(2)-alpha inhibition or gene disruption diminished neutrophil-mediated E. coli killing in vitro, which was partially rescued by exogenous arachidonic acid or PAF but not leukotriene B(4). Following intratracheal inoculation with live E. coli in vivo, pulmonary PAF biosynthesis, inflammatory cell infiltration, and clearance of E. coli were attenuated in cPLA(2)-alpha(-/-) mice compared with wild type littermates. These studies identify a novel role for cPLA(2)-alpha in the regulation of neutrophil-mediated bacterial killing and the innate immune response to bacterial infection.

Rac and protein kinase C-delta regulate ERKs and cytosolic phospholipase A2 in FcepsilonRI signaling to cysteinyl leukotriene synthesis in mast cells

Although cysteinyl leukotrienes (cysLTs) are known to be principal inflammatory lipid mediators released from IgE-stimulated mast cells, the signaling mechanisms involved in the synthesis of cysLTs remain largely unknown. In the present study, therefore, we investigated the signaling pathway by which IgE induces cysLTs synthesis after binding to its high affinity receptor (FcepsilonRI) in RBL-2H3 mast cells. We found that IgE-induced cysLT synthesis is completely abolished in RBL-2H3(Rac-N17) cells, a stable cell line expressing Rac(N17), a dominant negative Rac1 mutant; conversely, synthesis was enhanced in cells expressing Rac(V12), a constitutively active Rac1 mutant, suggesting that Rac1 is a key mediator of IgE signaling to cysLT synthesis. Further analysis aimed at identifying mediators downstream of Rac1 revealed that pretreating cells with a protein kinase C-delta (PKC-delta) inhibitor or infection with an adenoviral vector harboring a dominant negative PKC-delta mutant significantly attenuates IgE-induced ERKs phosphorylation, cytosolic phospholipase A(2) phosphorylation/translocation, and cysLT synthesis. In addition, the expression of Rac(N17) blocked PKC-delta translocation and impaired the phosphorylation of ERKs and cytosolic phospholipase A(2) otherwise elicited by IgE stimulation. Taken together these results suggest that PKC-delta also plays a critical mediatory role in the IgE signaling pathway leading to cysLT synthesis, acting downstream of Rac1. Finally, the physiological significance of PKC-delta in the IgE signaling pathway was demonstrated in an Ag (OVA)-challenged in vivo mouse model, in which induced levels of cysLTs and airway responsiveness in lung airways were significantly diminished by prior i.p. injection of a PKC-delta inhibitor.

Complement-independent Ab-induced peroxide lysis of platelets requires 12-lipoxygenase and a platelet NADPH oxidase pathway

Antiplatelet GPIIIa49-66 Ab of HIV-related thrombocytopenic patients induces thrombocytopenia and platelet fragmentation by the generation of peroxide and other reactive oxygen species (ROS). Here we report the presence of a functional platelet NADPH oxidase pathway that requires activation by the platelet 12-lipoxygenase (12-LO) pathway to fragment platelets. A new Ab-mediated mechanism is described in which the platelet 12-LO product, 12(S)-HETE activates the NADPH oxidase pathway to generate ROS.

Inhibition of the arachidonic acid cascade by norathyriol via blockade of cyclooxygenase and lipoxygenase activity in neutrophils

Recent studies have suggested that dual inhibitors of cyclooxygenase (COX) and lipoxygenase (LO) may be more beneficial in the treatment of inflammatory diseases in which platelet-leukocyte interaction dominates the underlying inflammatory process, than inhibitors of COX or LO alone. In this study, we examined oxygenated xanthones, shown previously to inhibit platelet and neutrophil activation, with respect to the potency of COX inhibition. 1,3,6,7-Tetrahydroxyxanthone (norathyriol) was the most potent. Norathyriol suppressed thromboxane B(2) (TXB(2)) and leukotriene B(4) (LTB(4)) formation in calcium ionophore (A23187)- and formyl-methionyl-leucyl-phenylalanine (fMLP)-stimulated rat neutrophils. Norathyriol was 3-4 times more active against LTB(4) formation than against TXB(2) formation (IC(50) about 2.8 vs. 10 microM, respectively). Norathyriol also inhibited prostaglandin D(2) (PGD(2)) formation in A23187-stimulated rat mast cells (IC(50) 3.0+/-1.2 microM) and in arachidonic acid (AA)-activated mast cell lysate. Norathyriol was a more effective inhibitor of 5-LO activity than of COX, as shown also by analyses of enzyme activities in a cell-free system, of COX and 5-LO metabolic capacity in neutrophils and of ex vivo TXB(2) and LTB(4) formation in A23187-stimulated neutrophils. Moreover, norathyriol inhibited COX-2 and 12-LO with IC(50) values (19.6+/-1.5 and 1.2+/-0.1 microM, respectively) similar to those required for the inhibition of COX-1 and 5-LO (16.2+/-1.5 and 1.8+/-0.4 microM, respectively). Inhibition of 15-LO by norathyriol was slightly less active. Norathyriol had no effect on A23187-induced AA release from neutrophils and did not affect phospholipase A(2) (PLA(2)) activity in a cell-free system. These results indicate that norathyriol inhibits the formation of PGs and LTs in neutrophils probably through direct blockade of COX and 5-LO activities. Norathyriol, a single molecule with multiple targets, might provide a potential therapeutic benefit in the treatment of inflammatory diseases.

Cytosolic Phospholipase A2 Translocates to Forming Phagosomes during Phagocytosis of Zymosan in Macrophages

Resident tissue macrophages mediate early innate immune responses to microbial infection. Cytosolic phospholipase A(2)alpha (cPLA(2)alpha) is activated in macrophages during phagocytosis of non-opsonized yeast (zymosan) triggering arachidonic acid release and eicosanoid production. cPLA(2)alpha translocates from cytosol to membrane in response to intracellular calcium concentration ([Ca(2+)](i)) increases. Enhanced green fluorescent protein (EGFP)-cPLA(2)alpha translocated to forming phagosomes, surrounding the zymosan particle by 5 min and completely overlapping with early endosome (Rab5) and plasma membrane (F4/80) markers but only partially overlapping with resident endoplasmic reticulum proteins (GRP78 and cyclooxygenase 2). EGFP-cPLA(2)alpha also localized to membrane ruffles during phagocytosis. Zymosan induced an initial high amplitude calcium transient that preceded particle uptake followed by a low amplitude sustained calcium increase. Both phases were required for optimal phagocytosis. Extracellular calcium chelation prevented only the sustained phase but allowed a limited number of phagocytic events, which were accompanied by translocation of cPLA(2)alpha to the phagosome although [Ca(2+)](i) remained at resting levels. The results demonstrate that cPLA(2)alpha targets the phagosome membrane, which may serve as a source of arachidonic acid for eicosanoid production.

AHNAK-mediated activation of phospholipase C-gamma1 through protein kinase C

We have recently shown that phospholipase C-gamma (PLC-gamma) is activated by the central repeated units (CRUs) of the AHNAK protein in the presence of arachidonic acid. Here we demonstrate that four central repeated units (4 CRUs) of AHNAK act as a scaffolding motif networking PLC-gamma and PKC-alpha. Specifically, 4 CRUs of AHNAK bind and activate PKC-alpha, which in turn stimulates the release of arachidonic acid near where PLC-gamma1 is localized. Moreover, 4 CRUs of AHNAK interacted with PLC-gamma and the concerted action of 4 CRUs with arachidonic acid stimulated PLC-gamma activity. Stimulation of NIH3T3 cells expressing 4 CRUs of AHNAK with phorbol 12-myristate 13-acetate resulted in the increased generation of total inositol phosphates (IP(T)) and mobilization of the intracellular calcium. Phorbol 12-myristate 13-acetate-dependent generation of IP(T) was completely blocked in NIH3T3 cells depleted of PLC-gamma1 by RNA interference. Furthermore, bradykinin, which normally stimulated the PLC-beta isozyme resulting in the generation of a monophasic IP(T) within 30 s in NIH3T3 cells, led to a biphasic pattern for generation of IP(T) in NIH3T3 cells expressing 4 CRUs of AHNAK. The secondary activation of PLC is likely because of the scaffolding activity of AHNAK, which is consistent with the role of 4 CRUs as a molecular linker between PLC-gamma and PKC-alpha.

Arachidonic acid release from mammalian cells transfected with human groups IIA and X secreted phospholipase A(2) occurs predominantly during the secretory process and with the involvement of cytosolic phospholipase A(2)-alpha

Stable expression of human groups IIA and X secreted phospholipases A(2) (hGIIA and hGX) in CHO-K1 and HEK293 cells leads to serum- and interleukin-1beta-promoted arachidonate release. Using mutant CHO-K1 cell lines, it is shown that this arachidonate release does not require heparan sulfate proteoglycan- or glycosylphosphatidylinositol-anchored proteins. It is shown that the potent secreted phospholipase A(2) inhibitor Me-Indoxam is cell-impermeable. By use of Me-Indoxam and the cell-impermeable, secreted phospholipase A(2) trapping agent heparin, it is shown that hGIIA liberates free arachidonate prior to secretion from the cell. With hGX-transfected CHO-K1 cells, arachidonate release occurs before and after enzyme secretion, whereas all of the arachidonate release from HEK293 cells occurs prior to enzyme secretion. Immunocytochemical studies by confocal laser and electron microscopies show localization of hGIIA to the cell surface and Golgi compartment. Additional results show that the interleukin-1beta-dependent release of arachidonate is promoted by secreted phospholipase A(2) expression and is completely dependent on cytosolic (group IVA) phospholipase A(2). These results along with additional data resolve the paradox that efficient arachidonic acid release occurs with hGIIA-transfected cells, and yet exogenously added hGIIA is poorly able to liberate arachidonic acid from mammalian cells.

Mechanisms of the influence of magnolol on eicosanoid metabolism in neutrophils

We have demonstrated that magnolol suppressed thromboxane B2 (TXB2) and leukotriene B4 (LTB4) formation in A23187-stimulated rat neutrophils. Maximum inhibition was obtained with about 10 microM magnolol. Magnolol was more effective in the inhibition of cyclooxygenase (COX) activity than in the inhibition of 5-lipoxygenase (5-LO) activity as assessed by means of enzyme activity determination in vitro and COX and 5-LO metabolic capacity analyses in vivo. Magnolol alone stimulated cytosolic phospholipase A2 (cPLA2) phosphorylation and the translocation of 5-LO and cPLA2 to the membrane, and evoked arachidonic acid (AA) release. Recruitment of both 5-LO and cPLA2 to the membranes was suppressed by EGTA. Arachidonyl trifluoromethyl ketone (AACOCF3), a PLA2 inhibitor, bromoenol lactone (BEL), a Ca2+-independent PLA2 (iPLA2) inhibitor, and EGTA suppressed the magnolol-induced AA release. However, none of the follows affected magnolol-induced AA-release: 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole (SB203580), a p38 mitogen-activated protein kinase (MAPK) inhibitor, 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene (U0126), a MAPK kinase (MEK) inhibitor, or 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide (GF109203X), a protein kinase C (PKC) inhibitor. In addition, magnolol at 30 microM did not stimulate the p38 MAPK and extracellular signal-regulated kinase 2 (ERK2) enzyme activities. These results indicated that magnolol inhibits the formation of prostaglandins and leukotrienes in A23187-stimulated rat neutrophils, probably through a direct blockade of COX and 5-LO activities. The stimulatory effects of magnolol at high concentration on the membrane association of 5-LO and cPLA2 are attributable to the elevation of [Ca2+]i, and on the AA release is likely via activation of cPLA2 and iPLA2.

Role of group V phospholipase A2 in zymosan-induced eicosanoid generation and vascular permeability revealed by targeted gene disruption

Conclusions regarding the contribution of low molecular weight secretory phospholipase A2 (sPLA2) enzymes in eicosanoid generation have relied on data obtained from transfected cells or the use of inhibitors that fail to discriminate between individual members of the large family of mammalian sPLA2 enzymes. To elucidate the role of group V sPLA2, we used targeted gene disruption to generate mice lacking this enzyme. Zymosan-induced generation of leukotriene C4 and prostaglandin E2 was attenuated approximately 50% in peritoneal macrophages from group V sPLA2-null mice compared with macrophages from wild-type littermates. Furthermore, the early phase of plasma exudation in response to intraperitoneal injection of zymosan and the accompanying in vivo generation of cysteinyl leukotrienes were markedly attenuated in group V sPLA2-null mice compared with wild-type controls. These data provide clear evidence of a role for group V sPLA2 in regulating eicosanoid generation in response to an acute innate stimulus of the immune response both in vitro and in vivo, suggesting a role for this enzyme in innate immunity.

Regulation of arachidonic acid availability for eicosanoid production

Mammalian cells have developed specific pathways for the incorporation, remodeling, and release of arachidonic acid. Acyltransferase and transacylase pathways function to regulate the levels of esterified arachidonic acid in specific phospholipid pools. There are several distinct, differentially regulated phospholipases A2in cells that mediate agonist-induced release of arachidonic acid. These pathways are important in controlling cellular levels of free arachidonic acid. Both arachidonic acid and its oxygenated metabolites are potent bioactive mediators that regulate a myriad of physiological and pathophysiological processes.Key words: phospholipase A2, arachidonic acid, eicosanoid, phospholipid.

A requirement for calcium-independent phospholipase A2 in thrombin-induced arachidonic acid release and growth in vascular smooth muscle cells

Thrombin is a potent mitogen for vascular smooth muscle cells (VSMC). To understand its mitogenic signaling events, we have studied the role of calcium-independent phospholipase A2 (iPLA2). Without affecting its levels, thrombin increased iPLA2 activity in a time-dependent manner in VSMC. Thrombin also induced arachidonic acid release and DNA synthesis by about 2-fold as compared with control. Down-regulation of iPLA2 activity by its specific inhibitor, bromoenol lactone, or its expression by antisense oligonucleotides, significantly reduced thrombin-induced arachidonic acid release and DNA synthesis in VSMC. To learn the mechanism of thrombin-stimulated iPLA2 activity, we next tested the role of p38 MAPK. Thrombin stimulated p38 MAPK phosphorylation and activity in a time-dependent manner in VSMC. Inhibition of p38 MAPK activity by SB203580 and SB202190 resulted in decreased iPLA2 activity, arachidonic acid release, and DNA synthesis induced by thrombin in VSMC. Together, these results for the first time demonstrate that iPLA2 plays a role in thrombin-induced arachidonic acid release and growth in VSMC and that these responses are mediated by p38 MAPK.

Effects of hypoxia on monocyte inflammatory mediator production: Dissociation between changes in cyclooxygenase-2 expression and eicosanoid synthesis

Blood-derived monocytes are found at sites of inflammation as well as in solid tumors and atherosclerotic arteries. They are an abundant source of inflammatory eicosanoids such as prostaglandin E2 (PGE2) and thromboxane A2, which are formed via arachidonic acid (AA) metabolism by cyclooxygenase-1/2 (COX-1/2). In vitro studies of inflammatory mediator production are conducted invariably in room air, which does not reflect the oxygen tensions found in monocyte-containing lesions, which are frequently hypoxic. In this work we examined the effects of hypoxia at levels reported in these lesions, on monocyte COX-2 expression, the related events that lead to eicosanoid synthesis, and relationships with tumor necrosis factor (TNF)-alpha synthesis. In fresh human monocytes exposed to hypoxia (1% O2), there was an increase in COX-2 protein compared with cells in normoxia, and this was attributable to increased transcription and mRNA stability. However, the synthesis of PGE2 and thromboxane A2 was reduced in hypoxia and did not reflect the increased level of COX-2. Monocytes prelabeled with [3H]AA followed by lipopolysaccharide stimulation in the presence of hypoxia showed a reduced release of AA compared with cells in normoxia. In addition, hypoxia resulted in decreased phosphorylation of the p44/42 mitogen-activated protein kinase and of cytosolic phospholipase A2. Hypoxia also increased TNF-alpha synthesis, which appeared to play a role in COX-2 expression, and the observed increase TNF-alpha synthesis appeared to result from reduced PGE2 synthesis. Overall, the results suggest the existence of an autocrine loop of regulation between monocyte eicosanoid and TNF-alpha production, which is dysregulated in hypoxia and establishes hypoxia as being an important environmental determinant of inflammatory mediator production.

Unique targeting of cytosolic phospholipase A2 to plasma membranes mediated by the NADPH oxidase in phagocytes

Cytosolic phospholipase A2 (cPLA2)-generated arachidonic acid (AA) has been shown to be an essential requirement for the activation of NADPH oxidase, in addition to its being the major enzyme involved in the formation of eicosanoid at the nuclear membranes. The mechanism by which cPLA2 regulates NADPH oxidase activity is not known, particularly since the NADPH oxidase complex is localized in the plasma membranes of stimulated cells. The present study is the first to demonstrate that upon stimulation cPLA2 is transiently recruited to the plasma membranes by a functional NADPH oxidase in neutrophils and in granulocyte-like PLB-985 cells. Coimmunoprecipitation experiments and double labeling immunofluorescence analysis demonstrated the unique colocalization of cPLA2 and the NADPH oxidase in plasma membranes of stimulated cells, in correlation with the kinetic burst of superoxide production. A specific affinity in vitro binding was detected between GST-p47phox or GST-p67phox and cPLA2 in lysates of stimulated cells. The association between these two enzymes provides the molecular basis for AA released by cPLA2 to activate the assembled NADPH oxidase. The ability of cPLA2 to regulate two different functions in the same cells (superoxide generation and eicosanoid production) is achieved by a novel dual subcellular localization of cPLA2 to different targets.

Phospholipase A(2)

Considerable progress has been made in characterizing the individual participant enzymes and their relative contributions in the generation of eicosanoids, lipid mediators derived from arachidonic acid, such as prostaglandins and leukotrienes. However, the role of individual phospholipase (PL) A(2) enzymes in providing arachidonic acid to the downstream enzymes for eicosanoid generation in biologic processes has not been fully elucidated. In this review, we will provide an overview of the classification of the families of PLA(2) enzymes, their putative mechanisms of action, and their role(s) in eicosanoid generation and inflammation.

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.

Differential regulation of lipopolysaccharide-induced monocyte matrix metalloproteinase (MMP)-1 and MMP-9 by p38 and extracellular signal-regulated kinase 1/2 mitogen-activated protein kinases

Signal transduction events in monocyte matrix metalloproteinase (MMP) production have been shown to include a PGE(2)-cAMP-dependent step. To determine earlier pathway components, we examined the role of mitogen-activated protein kinases (MAPKs) in the regulation of monocyte MMP-1 and MMP-9, two major MMPs induced by LPS. Stimulation with LPS resulted in the activation of the extracellular signal-regulated kinase 1 and 2 (ERK1/2) and mitogen-activated kinase p38. The p38-specific inhibitor SB203580 suppressed p38 activity and MMP-1 mRNA and protein, but increased ERK activity and MMP-9 mRNA and protein. In contrast, the MAPK kinase 1/2-specific inhibitor PD98059 inhibited MMP-1 and MMP-9. However, both MAPK inhibitors decreased the production of cyclooxygenase-2 and PGE(2), but only the inhibition of MMP-1 by SB203580 was reversed by PGE(2) or dibutyryl cAMP. Examination of the effect of these MAPK inhibitors on the promoters of MMP-1 and MMP-9 revealed that PD98059 inhibited the binding of transcription factors to all of the MMP promoter-specific complementary oligonucleotides tested. However, SB203580 only inhibited the binding of MMP-1-specific CREB and SP 1 oligonucleotides, which was reversed by PGE(2). Additionally, SB203580 enhanced transcription factor binding to the oligonucleotides complementary to a NF-kappaB site in the promoter of MMP-9. Thus, LPS induction of MMP-1 production by monocytes is regulated by both ERK1/2 and p38, whereas MMP-9 stimulation occurred mainly through the ERK1/2 pathway. Moreover, p38 regulates MMP-1 mainly through a PGE(2)-dependent pathway, whereas ERK1/2-mediated MMP-1 and MMP-9 production involves the activation of additional MMP promoter sites through a PGE(2)-independent mechanism.

Cytosolic phospholipase A2 is an effector of Jak/STAT signaling and is involved in platelet-derived growth factor BB-induced growth in vascular smooth muscle cells

Platelet-derived growth factor-BB (PDGF-BB) is a potent mitogen and chemoattractant for vascular smooth muscle cells (VSMC). To understand its mitogenic and chemotactic signaling events, we studied the role of cytosolic phospholipase A(2) (cPLA(2)) and the Jak/STAT pathway. PDGF-BB induced the expression and activity of cPLA(2) in a time-dependent manner in VSMC. Arachidonyl trifluoromethyl ketone, a potent and specific inhibitor of cPLA(2), significantly reduced PDGF-BB-induced arachidonic acid release and DNA synthesis. PDGF-BB stimulated tyrosine phosphorylation of Jak-2 in a time-dependent manner. In addition, PDGF-BB activated STAT-3 as determined by its tyrosine phosphorylation, DNA-binding activity, and reporter gene expression, and these responses were suppressed by AG490, a selective inhibitor of Jak-2. AG490 and a dominant-negative mutant of STAT-3 also attenuated PDGF-BB-induced expression of cPLA(2,) arachidonic acid release, and DNA synthesis in VSMC. Together, these results suggest that induction of expression of cPLA(2) and arachidonic acid release are involved in VSMC growth in response to PDGF-BB and that these events are mediated by Jak-2-dependent STAT-3 activation.

Inhibition of the NF-kappaB signaling pathway mediates the anti-inflammatory effects of petrosaspongiolide M

Petrosaspongiolide M (PT) is a potent secretory phospholipase A(2) inhibitor and anti-inflammatory agent. This marine metabolite reduced the production of nitrite, prostaglandin E(2), and tumor necrosis factor-alpha in the mouse air pouch injected with zymosan. These effects were also observed in mouse peritoneal macrophages stimulated with zymosan. Inhibition of these inflammatory mediators was related to reductions in inducible nitric oxide synthase, cyclo-oxygenase-2, and tumor necrosis factor-alpha expression. Since nuclear factor-kappaB (NF-kappaB) appears to play a central role in the transcriptional regulation of these proteins by macrophages, we investigated the effects of PT on this transcription factor. We found that PT was a potent inhibitor of the NF-kappaB pathway since at 1 microM it strongly decreased NF-kappaB-DNA binding in response to zymosan, in mouse peritoneal macrophages. Our study also indicated that PT could interfere with a key step in NF-kappaB activation, the phosphorylation of IkappaBalpha, resulting in inhibition of IkappaBalpha degradation. The control of a wide range of mediators by PT suggests a potentially wide therapeutic spectrum for this marine metabolite in inflammatory conditions.

Erk1/2- and p38 MAP kinase-dependent phosphorylation and activation of cPLA2 by m3 and m2 receptors

This study examined the upstream signaling pathways initiated by muscarinic m2 and m3 receptors that mediate sustained ERK1/2- and p38 MAP kinase-dependent phosphorylation and activation of the 85-kDa cytosolic phospholipase (cPL)A(2) in smooth muscle. The pathway initiated by m2 receptors involved sequential activation of Gbetagamma(i3), phosphatidylinositol (PI)3-kinase, Cdc42, and Rac1, p21-activated kinase (PAK1), p38 mitogen-activated protein (MAP) kinase, and cPLA(2), and phosphorylation of cPLA(2) at Ser(505). cPLA(2) activity was inhibited to the same extent (61 +/- 5 to 72 +/- 4%) by the m2 antagonist methoctramine, Gbeta antibody, pertussis toxin, the PI3-kinase inhibitor LY 294002, PAK1 antibody, the p38 MAP kinase inhibitor SB-203580, and a Cdc42/Rac1 GEF (Vav2) antibody and by coexpression of dominant-negative Cdc42 and Rac1 mutants. The pathway initiated by m3 receptors involved sequential activation of Galpha(q), PLC-beta1, PKC, ERK1/2, and cPLA(2), and phosphorylation of cPLA(2) at Ser(505). cPLA(2) activity was inhibited to the same extent (35 +/- 3 to 41 +/- 5%) by the m3 antagonist 4-diphenylacetoxy-N-methylpiperdine (4-DAMP), the phosphoinositide hydrolysis inhibitor U-73122, the PKC inhibitor bisindolylmaleimide, and the ERK1/2 inhibitor PD 98059. cPLA(2) activity was not affected in cells coexpressing dominant-negative RhoA and PLC-delta1 mutants, implying that PKC was not derived from phosphatidylcholine hydrolysis. The effects of ERK1/2 and p38 MAP kinase on cPLA(2) activity were additive and accounted fully for activation and phosphorylation of cPLA(2).

CaM kinase IIalpha mediates norepinephrine-induced translocation of cytosolic phospholipase A2 to the nuclear envelope

Several growth factors, hormones and neurotransmitters, including norepinephrine, increase cellular calcium levels, promoting the translocation of cytosolic phospholipase A(2) to the nuclear envelope. This study was conducted to investigate the contributions of the calcium-binding protein calmodulin and of calcium-calmodulin-dependent protein kinase II to cytosolic phospholipase A(2) translocation to the nuclear envelope elicited by norepinephrine in rabbit aortic smooth-muscle cells. Norepinephrine caused cytosolic phospholipase A(2) accumulation around the nuclear envelope as determined from its immunofluorescence; cytosolic phospholipase A(2) translocation was blocked by inhibitors of calmodulin and calcium-calmodulin-dependent protein kinase II or calcium-calmodulin-dependent protein kinase IIalpha antisense oligonucleotide. Calmodulin and calcium-calmodulin-dependent protein kinase II inhibitors did not prevent cytosolic calcium increase but attenuated cytosolic phospholipase A(2) phosphorylation caused by norepinephrine or ionomycin. In vascular smooth-muscle cells reversibly permeabilized with beta-escin and treated with alkaline phosphatase, norepinephrine failed to cause cytosolic phospholipase A(2) phosphorylation and translocation to the nuclear envelope; these effects of norepinephrine were minimized by the phosphatase inhibitor okadaic acid. Recombinant cytosolic phospholipase A(2) phosphorylated by purified calcium-calmodulin-dependent protein kinase II, but not unphosphorylated or dephosphorylated cytosolic phospholipase A(2), introduced into permeabilized vascular smooth-muscle cells in the absence of calcium accumulated around the nuclear envelope. These data suggest that norepinephrine-induced translocation of cytosolic phospholipase A(2) to the nuclear envelope is mediated by its phosphorylation by calcium-calmodulin-dependent protein kinase II and that calcium alone is insufficient for cytosolic phospholipase A(2) translocation to the nuclear envelope in rabbit vascular smooth-muscle cells.