Jonathan P. Arm

Groups IV, V, and X Phospholipases A2s in Human Neutrophils ROLE IN EICOSANOID PRODUCTION AND GRAM-NEGATIVE BACTERIAL PHOSPHOLIPID HYDROLYSIS

The bacterial tripeptide formyl-Met-Leu-Phe (fMLP) induces the secretion of enzyme(s) with phospholipase A2 (PLA2) activity from human neutrophils. We show that circulating human neutrophils express groups V and X sPLA2 (GV and GX sPLA2) mRNA and contain GV and GX sPLA2 proteins, whereas GIB, GIIA, GIID, GIIE, GIIF, GIII, and GXII sPLA2s are undetectable. GV sPLA2 is a component of both azurophilic and specific granules, whereas GX sPLA2 is confined to azurophilic granules. Exposure to fMLP or opsonized zymosan results in the release of GV but not GX sPLA2 and most, if not all, of the PLA2 activity in the extracellular fluid of fMLP-stimulated neutrophils is due to GV sPLA2. GV sPLA2 does not contribute to fMLP-stimulated leukotriene B4 production but may support the anti-bacterial properties of the neutrophil, because 10–100 ng per ml concentrations of this enzyme lead to Gram-negative bacterial membrane phospholipid hydrolysis in the presence of human serum. By use of a recently described and specific inhibitor of cytosolic PLA2-α (group IV PLA2α), we show that this enzyme produces virtually all of the arachidonic acid used for the biosynthesis of leukotriene B4 in fMLP- and opsonized zymosan-stimulated neutrophils, the major eicosanoid produced by these pro-inflammatory cells.

Effect of dietary fatty acids on inflammatory gene expression in healthy humans

Over the past 100 years, changes in the food supply in Western nations have resulted in alterations in dietary fatty acid consumption, leading to a dramatic increase in the ratio of omega-6 (ω6) to ω3 polyunsaturated fatty acids (PUFA) in circulation and in tissues. Increased ω6/ω3 ratios are hypothesized to increase inflammatory mediator production, leading to higher incidence of inflammatory diseases, and may impact inflammatory gene expression. To determine the effect of reducing the ω6/ω3 ratio on expression of inflammatory pathway genes in mononuclear cells, healthy humans were placed on a controlled diet for 1 week, then given fish oil and borage oil for an additional 4 weeks. Serum and neutrophil fatty acid composition and ex vivo leukotriene B4 production from stimulated neutrophils were measured at the start and end of the supplementation period and after a 2-week washout. RNA was isolated from mononuclear cells and expression of PI3K, Akt, NFκB, and inflammatory cytokines was measured by real-time PCR. A marked increase was seen in serum and neutrophil levels of long-chain ω3 PUFA concomitant with a reduction in the ω6/ω3 PUFA ratio (40%). The ex vivo capacity of stimulated neutrophils to produce leukotriene B4 was decreased by 31%. Expression of PI3Kα and PI3Kγ and the quantity of PI3Kα protein in mononuclear cells was reduced after supplementation, as was the expression of several proinflammatory cytokines. These data reveal that PUFA may exert their clinical effects via their capacity to regulate the expression of signal transduction genes and genes for proinflammatory cytokines.

MOLECULAR CLONING OF THE GENE FOR HUMAN LEUKOTRIENE C4 SYNTHASE : ORGANIZATION, NUCLEOTIDE SEQUENCE, AND CHROMOSOMAL LOCALIZATION TO 5Q35

Leukotriene C (LTC) synthase catalyzes the conjugation of LTA with reduced GSH to form LTC, the parent of the receptor active cysteinyl leukotrienes implicated in the pathobiology of bronchial asthma. Previous cloning of the cDNA for human LTC synthase demonstrated significant homology of its amino acid sequence to that of 5-lipoxygenase activating protein (FLAP) but none to that of the GSH S-transferase superfamily. Genomic cloning from a P1 library now reveals that the gene for LTC synthase contains five exons (ranging from 71 to 257 nucleotides in length) and four introns, which in total span 2.52 kilobase pairs in length. The intron/exon junctions of LTC synthase align identically with those of FLAP; however, the small size of the LTC synthase gene contrasts with the >31-kilobase pair size reported for FLAP. Confirmation of the LTC synthase gene size to ensure that no deletions had occurred during the cloning was obtained by two overlapping polymerase chain reactions from genomic DNA, which provided products of the predicted sizes. Primer extension analysis with poly(A) RNA from culture-derived human eosinophilic granulocytes or the KG-1 myelogenous cell line revealed multiple transcriptional start sites with prominent signals at 66, 69, and 96 base pairs 5′ of the ATG translation start site. The 5′-flanking region revealed a GC-rich promotor sequence consistent with an SP-1 site and consensus sequences for AP-1 and AP-2 enhancer elements, 24, 807, and 877 bp, respectively, 5′ from the first transcription initiation site. Southern blot analysis of a genomic DNA (with full-length cDNA as well as 5′ and 3′ oligonucleotide probes) confirmed the size of the gene and indicated a single copy gene in normal human genomic DNA. Fluorescent in situ hybridization mapped LTC synthase to chromosomal location 5q35, which is in close proximity to the cluster of genes for cytokines and receptors involved in the regulation of cells central to allergic inflammation and implicated in bronchial asthma.

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) α 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 cPLA 2 α, 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.

Cytosolic Phospholipase A2 Participates with TNF-α in the Induction of Apoptosis of Human Macrophages Infected with Mycobacterium tuberculosis H37Ra

Macrophage (MΦ) apoptosis, an important innate microbial defense mechanism induced by Mycobacterium tuberculosis (Mtb) H37Ra, depends on the induction of TNF-α synthesis. When protein synthesis is blocked, both infection with Mtb and addition of TNF-α are required to induce caspase 9 activation, caspase 3 activation and apoptosis. In this study, we show that the second protein synthesis-independent signal involves activation of group IV cytosolic phospholipase A2 (cPLA2). Apoptosis of Mtb-infected MΦ and concomitant arachidonic acid release are abrogated by group IV cPLA2 inhibitors (methyl arachidonyl fluorophosphate and methyl trifluoromethyl ketone), but not by inhibitors of group VI Ca2+-independent (iPLA2 ; bromoenol lactone) or of secretory low molecular mass PLA2. In MΦ homogenates, the predominant PLA2 activity showed the same inhibitor sensitivity pattern and preferred arachidonic acid over palmitic acid in substrates, also indicating the presence of one or more group IV cPLA2 enzymes. In concordance with these findings, MΦ lysates contained transcripts and protein for group IV cPLA2-α and cPLA2-γ. Importantly, group IV cPLA2 inhibitors significantly reduced MΦ antimycobacterial activity and addition of arachidonic acid, the major product of group IV cPLA2, to infected MΦ treated with cPLA2 inhibitors completely restored the antimycobacterial activity. Importantly, addition of arachidonic acid alone to infected MΦ significantly reduced the mycobacterial burden. These findings indicate that Mtb induces MΦ apoptosis by independent signaling through at least two pathways, TNF-α and cPLA2, which are both also critical for antimycobacterial defense of the MΦ .

A Novel Group of Phospholipase A2s Preferentially Expressed in Type 2 Helper T Cells

We report a novel phospholipase A2 (PLA2), group XII (GXII) PLA2, distinct from other cysteine-rich groups with a catalytic histidine motif, by its 20-kDa size and distribution of the 14 cysteine residues within the protein. Alternative spliced forms with distinct subcellular localization, designated GXII-1 and GXII-2, were identified by reverse transcription-polymerase chain reaction. Importantly, GXII PLA2s, in particular GXII-2 PLA2, and group V PLA2, but not group X PLA2, were selectively expressed in murine type 2 helper T (Th2) clones and in vitro differentiated mouse CD4 Th2 cells as compared with type 1 helper T clones and cells. Stimulation with anti-CD3 appreciably up-regulated expression of GXII PLA2s and group V PLA2 by steady state analysis of the Th2 cells as compared with type 1 helper T cells. These results suggest that group XII and group V PLA2s might participate in helper T cell immune response through release of immediate second signals and generation of downstream eicosanoids.

Human Mast Cell-Derived Gelatinase B (Matrix Metalloproteinase-9) Is Regulated by Inflammatory Cytokines: Role in Cell Migration

Mast cells are key effectors in the pathogenesis of inflammatory and tissue destructive diseases such as rheumatoid arthritis (RA). These cells contain specialized secretory granules loaded with bioactive molecules including cytokines, growth factors, and proteases that are released upon activation. This study investigated the regulation of matrix metalloproteinase MMP-9 (gelatinase B) in human mast cells by cytokines that are known to be involved in the pathogenesis of RA. Immunohistochemical staining of synovial tissue showed abundant expression of MMP-9 by synovial tissue mast cells in patients with RA but not in normal controls. The expression, activity, and production of MMP-9 in mast cells was confirmed by RT-PCR, zymography, and Western blotting using cord blood-derived human mast cells (CB-HMC). Treatment of CB-HMC with TNF-α significantly increased the expression of MMP-9 mRNA and up-regulated the activity of MMP-9 in a time- and dose-dependent manner. By contrast, IFN-γ inhibited MMP-9 mRNA and protein expression. The cytokine-mediated regulation of MMP-9 was also apparent in the human mast cell line (HMC-1) and in mouse bone marrow-derived mast cells. Furthermore, TNF-α significantly increased the invasiveness of CB-HMC across Matrigel-coated membranes while the addition of IFN-γ, rTIMP-1, or pharmacological MMP inhibitors significantly reduced this process. These observations suggest that MMP-9 is not a stored product in mast cells but these cells are capable of producing this enzyme under inflammatory conditions that may facilitate the migration of mast cell progenitors to sites of inflammation and may also contribute to local tissue damage.

Activation of human eosinophils through leukocyte immunoglobulin-like receptor 7

Eosinophils are implicated prominently in allergic diseases and the host response to parasitic infections. Eosinophils may be activated in vitro by diverse classes of agonists such as immunoglobulins, lipid mediators, and cytokines. The leukocyte Ig-like receptors (LIRs) comprise a family of inhibitory and activating cell-surface receptors. Inhibitory LIRs down-regulate cellular responses through cytoplasmic immunoreceptor tyrosine-based inhibitory motifs. There are limited data on the action of the activating LIRs, which are thought to signal through the Fc receptor γ chain, which contains an immunoreceptor tyrosine-based activation motif. We now demonstrate the expression of LIR1 (inhibitory), LIR2 (inhibitory), LIR3 (inhibitory), and LIR7 (activating) on eosinophils from 4, 4, 12, and 11, respectively, of 12 healthy donors. Cross-linking of LIR7 with plate-bound antibody elicited the dose- and time-dependent release of eosinophil-derived neurotoxin and leukotriene C4. Eosinophils activated with antibodies to LIR7 embedded in gel-phase EliCell preparations showed leukotriene C4 generation at the nuclear envelope and the release of IL-12 but not IL-4 by vesicular transport. Thus, LIR7 is an activating receptor for eosinophils that elicited the release of cytotoxic granule proteins, de novo lipid mediator generation, and cytokine release through vesicular transport.

A novel anti-inflammatory role for secretory phospholipase A2 in immune complex-mediated arthritis

Phospholipase A2 (PLA2) catalyses the release of arachidonic acid for generation of lipid mediators of inflammation and is crucial in diverse inflammatory processes. The functions of the secretory PLA2 enzymes (sPLA2), numbering nine members in humans, are poorly understood, though they have been shown to participate in lipid mediator generation and the associated inflammation. To further understand the roles of sPLA2 in disease, we quantified the expression of these enzymes in the synovial fluid in rheumatoid arthritis and used gene‐deleted mice to examine their contribution in a mouse model of autoimmune erosive inflammatory arthritis. Contrary to expectation, we find that the group V sPLA2 isoform plays a novel anti‐inflammatory role that opposes the pro‐inflammatory activity of group IIA sPLA2. Mechanistically, group V sPLA2 counter‐regulation includes promotion of immune complex clearance by regulating cysteinyl leukotriene synthesis. These observations identify a novel anti‐inflammatory function for a PLA2 and identify group V sPLA2 as a potential biotherapeutic for treatment of immune‐complex‐mediated inflammation.

Deletion of Secretory Group V Phospholipase A2 Attenuates Cell Migration and Airway Hyperresponsiveness in Immunosensitized Mice

We investigated the role of group V phospholipase A2 (gVPLA2) in OVA-induced inflammatory cell migration and airway hyperresponsiveness (AHR) in C57BL/6 mice. Repeated allergen challenge induced biosynthesis of gVPLA2 in airways. By aerosol, gVPLA2 caused dose-related increase in airway resistance in saline-treated mice; in allergic mice, gVPLA2 caused persistent airway narrowing. Neither group IIa phospholipase A2, a close homolog of gVPLA2, nor W31A, an inactive gVPLA2 mutant with reduced activity, caused airway narrowing in immune-sensitized mice. Pretreatment with MCL-3G1, a blocking Ab against gVPLA2, before OVA challenge blocked fully gVPLA2-induced cell migration and airway narrowing as marked by reduction of migrating leukocytes in bronchoalveolar lavage fluid and decreased airway resistance. We also assessed whether nonspecific AHR caused by methacholine challenge was elicited by gVPLA2 secreted from resident airway cells of immune-sensitized mice. MCL-3G1 also blocked methacholine-induced airway bronchoconstriction in allergic mice. Blockade of bronchoconstriction by MCL-3G1 was replicated in allergic pla2g5−/− mice, which lack the gene encoding gVPLA2. Bronchoconstriction caused by gVPLA2 in pla2g4−/− mice was comparable to that in pla2g4+/+ mice. Our data demonstrate that gVPLA2 is a critical messenger enzyme in the development of AHR and regulation of cell migration during immunosensitization by a pathway that is independent of group IVa phospholipase A2.