Inhibition of monocyte chemotaxis to C-C chemokines by antisense oligonucleotide for cytosolic phospholipase A2

Role of CCL2/CCR2 axis in the pathogenesis of COVID-19 and possible Treatments: All options on the Table

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is cause of the novel coronavirus disease (COVID-19). In the last two years, SARS-CoV-2 has infected millions of people worldwide with different waves, resulting in the death of many individuals. The evidence disclosed that the host immune responses to SARS-CoV-2 play a pivotal role in COVID-19 pathogenesis and clinical manifestations. In addition to inducing antiviral immune responses, SARS-CoV-2 can also cause dysregulated inflammatory responses characterized by the noticeable release of proinflammatory mediators in COVID-19 patients. Among these proinflammatory mediators, chemokines are considered a subset of cytokines that participate in the chemotaxis process to recruit immune and non-immune cells to the site of inflammation and infection. Researchers have demonstrated that monocyte chemoattractant protein-1 (MCP-1/CCL2) and its receptor (CCR2) are involved in the recruitment of monocytes and infiltration of these cells into the lungs of patients suffering from COVID-19. Moreover, elevated levels of CCL2 have been reported in the bronchoalveolar lavage fluid (BALF) obtained from patients with severe COVID-19, initiating cytokine storm and promoting CD163+ myeloid cells infiltration in the airways and further alveolar damage. Therefore, CCL2/CCR axis plays a key role in the immunopathogenesis of COVID-19 and targeted therapy of involved molecules in this axis can be a potential therapeutic approach for these patients. This review discusses the biology of the CCL2/CCR2 axis as well as the role of this axis in COVID-19 immunopathogenesis, along with therapeutic options aimed at inhibiting CCL2/CCR2 and modulating dysregulated inflammatory responses in patients with severe SARS-CoV-2 infection.

A Metalloproteinase Induces an Inflammatory Response in Preadipocytes with the Activation of COX Signalling Pathways and Participation of Endogenous Phospholipases A2

Matrix metalloproteinases (MMPs) are proteolytic enzymes that have been associated with the pathogenesis of inflammatory diseases and obesity. Adipose tissue in turn is an active endocrine organ capable of secreting a range of proinflammatory mediators with autocrine and paracrine properties, which contribute to the inflammation of adipose tissue and adjacent tissues. However, the potential inflammatory effects of MMPs in adipose tissue cells are still unknown. This study investigates the effects of BmooMPα-I, a single-domain snake venom metalloproteinase (SVMP), in activating an inflammatory response by 3T3-L1 preadipocytes in culture, focusing on prostaglandins (PGs), cytokines, and adipocytokines biosynthesis and mechanisms involved in prostaglandin E2 (PGE2) release. The results show that BmooMPα-I induced the release of PGE2, prostaglandin I2 (PGI2), monocyte chemoattractant protein-1 (MCP-1), and adiponectin by preadipocytes. BmooMPα-I-induced PGE2 biosynthesis was dependent on group-IIA-secreted phospholipase A2 (sPLA2-IIA), cytosolic phospholipase A2-α (cPLA2-α), and cyclooxygenase (COX)-1 and -2 pathways. Moreover, BmooMPα-I upregulated COX-2 protein expression but not microsomal prostaglandin E synthase-1 (mPGES-1) expression. In addition, we demonstrate that the enzymatic activity of BmooMPα-I is essential for the activation of prostanoid synthesis pathways in preadipocytes. These data highlight preadipocytes as important targets for metalloproteinases and provide new insights into the contribution of these enzymes to the inflammation of adipose tissue and tissues adjacent to it.

Definition of a Novel Pathway Centered on Lysophosphatidic Acid To Recruit Monocytes during the Resolution Phase of Tissue Inflammation

Blood-derived monocytes remove apoptotic cells and terminate inflammation in settings as diverse as atherosclerosis and Alzheimer’s disease. They express high levels of the proresolving receptor ALX/FPR2, which is activated by the protein annexin A1 (ANXA1), found in high abundance in inflammatory exudates. Using primary human blood monocytes from healthy donors, we identified ANXA1 as a potent CD14⁺CD16⁻ monocyte chemoattractant, acting via ALX/FPR2. Downstream signaling pathway analysis revealed the p38 MAPK-mediated activation of a calcium independent phospholipase A₂ with resultant synthesis of lysophosphatidic acid (LPA) driving chemotaxis through LPA receptor 2 and actin cytoskeletal mobilization. In vivo experiments confirmed ANXA1 as an independent phospholipase A₂–dependent monocyte recruiter; congruently, monocyte recruitment was significantly impaired during ongoing zymosan-induced inflammation in AnxA1^−/− or alx/fpr2/3^−/− mice. Using a dorsal air-pouch model, passive transfer of apoptotic neutrophils between AnxA1^−/− and wild-type mice identified effete neutrophils as the primary source of soluble ANXA1 in inflammatory resolution. Together, these data elucidate a novel proresolving network centered on ANXA1 and LPA generation and identify previously unappreciated determinants of ANXA1 and ALX/FPR2 signaling in monocytes.

Moving towards a paradigm: common mechanisms of chemotactic signaling in Dictyostelium and mammalian leukocytes

Chemotaxis, or directed migration of cells along a chemical gradient, is a highly coordinated process that involves gradient sensing, motility, and polarity. Most of our understanding of chemotaxis comes from studies of cells undergoing amoeboid-type migration, in particular the social amoeba Dictyostelium discoideum and leukocytes. In these amoeboid cells the molecular events leading to directed migration can be conceptually divided into four interacting networks: receptor/G protein, signal transduction, cytoskeleton, and polarity. The signal transduction network occupies a central position in this scheme as it receives direct input from the receptor/G protein network, as well as feedback from the cytoskeletal and polarity networks. Multiple overlapping modules within the signal transduction network transmit the signals to the actin cytoskeleton network leading to biased pseudopod protrusion in the direction of the gradient. The overall architecture of the networks, as well as the individual signaling modules, is remarkably conserved between Dictyostelium and mammalian leukocytes, and the similarities and differences between the two systems are the subject of this review.

Role of chemokine CCL2 and its receptor CCR2 in neurodegenerative diseases

Chemokines are members of the chemoattractant cytokine family. They play key roles in the trafficking of leukocytes and in the induction of chemotaxis through the activation of G protein-coupled receptor. Considerable interest has been paid to these molecules to elucidate their roles in the unique inflammatory responses elicited in the central nervous system (CNS). Chemokine CCL2 (also known as monocyte chemoattractant protein-1, MCP-1) is one of the vital chemokines that control the migration and infiltration of monocytes/macrophages. CCL2 and its receptor CCR2 have been shown to be induced and involved in various neurodegenerative disorders including Alzheimer's disease, multiple sclerosis, and ischemic brain injury. The present review will focus on the biological and pathophysiological aspects of CCL2 and CCR2 in the CNS and the possible therapeutic approaches for targeting these two proteins to combat neurodegenerative diseases.

Signal-activated phospholipase regulation of leukocyte chemotaxis

Signal-activated phospholipases are a recent focus of the rapidly growing field of lipid signaling. The extent of their impact on the pathways regulating diverse cell functions is beginning to be appreciated. A critical step in inflammation is the attraction of leukocytes to injured or diseased tissue. Chemotaxis of leukocytes, a requisite process for monocyte and neutrophil extravasation from the blood into tissues, is a critical step for initiating and maintaining inflammation in both acute and chronic settings. Recent studies have identified new important and required roles for two signal-activated phospholipases A2 (PLA2) in regulating chemotaxis. The two intracellular phospholipases, cPLA2alpha (Group IVA) and iPLA2beta (Group VIA), act in parallel to provide distinct lipid mediators at different intracellular sites that are both required for leukocytes to migrate toward the chemokine monocyte chemoattractant protein-1. This review will summarize the separate roles of these phospholipases as well as what is currently known about the influence of two other classes of intracellular signal-activated phospholipases, phospholipase C and phospholipase D, in regulating chemotaxis in eukaryotic cells, but particularly in human monocytes. The contributions of these phospholipases to chemotaxis both in vitro and in vivo will be highlighted.

How chemokines invite leukocytes to dance

A prominent activity of the chemokine system is the regulation of leukocyte trafficking. Here we summarize recent findings on the initial steps in chemokine receptor-induced signal transduction in leukocytes. In particular, we discuss the potential influences of the formation of oligomers of ligand and receptor and of coupling between chemokine signals and regulators of the cytoskeleton, such as small GTPases.

Arachidonic acid as a retrograde signal controlling growth and dynamics of retinotectal arbors

In the developing visual system, correlated presynaptic activity between neighboring retinal ganglion cells (RGC) stabilizes retinotopic synapses via a postsynaptic NMDAR (N-methyl-D-aspartate receptor)-dependent mechanism. Blocking NMDARs makes individual axonal arbors larger, which underlies an unsharpened map, and also increases branch turnover, as if a stabilizing factor from the postsynaptic partner is no longer released. Arachidonic acid (AA), a candidate retrograde stabilizing factor, is released by cytoplasmic phospholipase A2 (cPLA2) after Ca(2+) entry through activated NMDARs, and can activate presynaptic protein kinase C to phosphorylate various substrates such as GAP43 to regulate cytoskeletal dynamics. To test the role of cPLA2 in the retinotectal system of developing zebrafish, we first used PED6, a fluorescent reporter of cPLA2 activity, to show that 1-3 min of strobe flashes activated tectal cPLA2 by an NMDAR-dependent mechanism. Second, we imaged the dynamic growth of retinal arbors during both local inhibition of tectal cPLA2 by a pharmacological inhibitor, arachidonic tri-fluoromethylketone, and its suppression by antisense oligonucleotides (both injected intraventricularly). Both methods produced larger arbors and faster branch dynamics as occurs with blocking NMDARs. In contrast, intraocular suppression of retinal cPLA2 with large doses of antisense oligos produced none of the effects of tectal cPLA2 inhibition. Finally, if AA is the retrograde messenger, the application of exogenous AA to the tectum should reverse the increased branch turnover caused by blocking either NMDARs or cPLA2. In both cases, intraventricular injection of AA stabilized the overall branch dynamics, bringing rates down below the normal values. The results suggest that AA generated postsynaptically by cPLA2 downstream of Ca(2+) entry through NMDARs acts as a retrograde signal to regulate the dynamic growth of retinal arbors.

Inhibitors of brain phospholipase A2 activity: their neuropharmacological effects and therapeutic importance for the treatment of neurologic disorders

The phospholipase A(2) family includes secretory phospholipase A(2), cytosolic phospholipase A(2), plasmalogen-selective phospholipase A(2), and calcium-independent phospholipase A(2). It is generally thought that the release of arachidonic acid by cytosolic phospholipase A(2) is the rate-limiting step in the generation of eicosanoids and platelet activating factor. These lipid mediators play critical roles in the initiation and modulation of inflammation and oxidative stress. Neurological disorders, such as ischemia, spinal cord injury, Alzheimer's disease, multiple sclerosis, prion diseases, and epilepsy are characterized by inflammatory reactions, oxidative stress, altered phospholipid metabolism, accumulation of lipid peroxides, and increased phospholipase A(2) activity. Increased activities of phospholipases A(2) and generation of lipid mediators may be involved in oxidative stress and neuroinflammation associated with the above neurological disorders. Several phospholipase A(2) inhibitors have been recently discovered and used for the treatment of ischemia and other neurological diseases in cell culture and animal models. At this time very little is known about in vivo neurochemical effects, mechanism of action, or toxicity of phospholipase A(2) inhibitors in human or animal models of neurological disorders. In kainic acid-mediated neurotoxicity, the activities of phospholipase A(2) isoforms and their immunoreactivities are markedly increased and phospholipase A(2) inhibitors, quinacrine and chloroquine, arachidonyl trifluoromethyl ketone, bromoenol lactone, cytidine 5-diphosphoamines, and vitamin E, not only inhibit phospholipase A(2) activity and immunoreactivity but also prevent neurodegeneration, suggesting that phospholipase A(2) is involved in the neurodegenerative process. This also suggests that phospholipase A(2) inhibitors can be used as neuroprotectants and anti-inflammatory agents against neurodegenerative processes in neurodegenerative diseases.

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.

CCL3/Macrophage inflammatory protein-1α induces fever and increases prostaglandin E2 in cerebrospinal fluid of rats: Effect of antipyretic drugs

The aim of this study was to investigate whether the increase in body temperature caused by intracerebroventricular (i.c.v.) injection of recombinant mouse CCL3/MIP1alpha [C-C (two adjacent conserved cysteines) ligand 3/macrophage inflammatory protein-1alpha] constitutes solely a hyperthermic response or a true integrated fever. Additionally, we examined the effects of systemic administration of different antipyretic drugs including the glucocorticoid dexamethasone, on cerebrospinal fluid (CSF) concentration of prostaglandin (PG) E2 and on febrile response induced by CCL3/MIP1alpha. I.c.v. administration of CCL3/MIP1alpha evokes an integrated fever accompanied by a reduction in tail skin temperature and an increase in PGE2 concentration in the CSF. Dexamethasone and indomethacin markedly reduced the fever and the elevation of CSF PGE2 concentration induced by lipopolysaccharide (LPS) whereas both response evoked by i.c.v. CCL3/MIP1alpha were insensitive to this steroid. Indomethacin only blocked the PGE2 increase in the CSF whereas ibuprofen and celecoxib each blocked the fever and the elevation of CSF PGE2. In this study, we have demonstrated for the first time that CCL3/MIP1alpha evokes an integrated febrile response accompanied by an increase of PGE2 levels in the CSF. These events are dissociated, especially in animals treated with indomethacin. If PGE2 does not participate in the febrile response evoked by CCL3/MIP1alpha, the inhibition of this response by celecoxib and ibuprofen indicates additional mechanisms to the well-known inhibition of COX enzymes by these drugs. Such mechanisms do not seem to depend on cytokine synthesis and subsequent COX-2 induction.

Cytosolic phospholipase A2 alpha-deficient mice are resistant to experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE), a Th1-mediated inflammatory disease of the central nervous system (CNS), is a model of human multiple sclerosis. Cytosolic phospholipase A2alpha (cPLA2alpha), which initiates production of prostaglandins, leukotrienes, and platelet-activating factor, is present in EAE lesions. Using myelin oligodendrocyte glycoprotein (MOG) immunization, as well as an adoptive transfer model, we showed that cPLA2alpha-/- mice are resistant to EAE. Histologic examination of the CNS from MOG-immunized mice revealed extensive inflammatory lesions in the cPLA2alpha+/- mice, whereas the lesions in cPLA2alpha-/- mice were reduced greatly or completely absent. MOG-specific T cells generated from WT mice induced less severe EAE in cPLA2alpha-/- mice compared with cPLA2alpha+/- mice, which indicates that cPLA2alpha plays a role in the effector phase of EAE. Additionally, MOG-specific T cells from cPLA2alpha-/- mice, transferred into WT mice, induced EAE with delayed onset and lower severity compared with EAE that was induced by control cells; this indicates that cPLA2alpha also plays a role in the induction phase of EAE. MOG-specific T cells from cPLA2alpha-/- mice were deficient in production of Th1-type cytokines. Consistent with this deficiency, in vivo administration of IL-12 rendered cPLA2alpha-/- mice susceptible to EAE. Our data indicate that cPLA2alpha plays an important role in EAE development and facilitates differentiation of T cells toward the Th1 phenotype.

Phospholipase A2 inhibitors in development

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

Knock down of cytosolic phospholipase A2: an antisense oligonucleotide having a nuclear localization binds a C-terminal motif of glyceraldehyde-3-phosphate dehydrogenase

We have previously shown that an antisense, effective in the knock down of cytosolic phospholipase A2 (cPLA2), localizes mainly in the nucleus of human endothelial cells and monocytes and that glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is involved in its nuclear localization. In this study, we clarify how GAPDH participates in the nuclear localization of this antisense oligodeoxynucleotide (ODN) directed against cPLA2 mRNA. A central TAAAT motif providing specificity and high affinity binding was assumed to interact with the enzyme Rossmann fold region on the basis of competition to this site by NAD+. To asses whether the TAAAT motif interacts directly with the enzyme Rossmann fold region, we evaluated the binding to GAPDH of different oligonucleotides and the effect of competitors such as NAD+, NADH, mononucleotides, DNA, polyribonucleic acids and polyanions. We found that the dissociation constant for TAAAT containing oligonucleotides was three--to fivefold higher with respect to oligo not containing this motif. By covalently linking 32P-labeled cPLA2p(N)16 to GAPDH and after executing hydrolysis with hydroxylamine, the labeling was exclusively found in the C-terminal domain (aa 286-334). These results indicate that the antisense oligonucleotide interacts with a site not having a defined function but which can be negatively allosterically regulated when NAD+ or polynucleotides are bound to Rossmann fold.

B-Myb-Dependent Regulation of c-Myc Expression by Cytosolic Phospholipase A2

Cytosolic phospholipase A(2) (cPLA(2)) cleaves membrane phospholipids to release arachidonic acid, initiating lipoxygenase and cyclooxygenase pathways. Mice lacking a gene for cPLA(2) suggested important roles of the protein in allergic responses, fertility, and neural cell death. Here we show that cPLA(2) negatively regulates c-Myc expression in a B-Myb-dependent manner. Overexpression of cPLA(2) protein but not a mutant cPLA(2) protein that lacks in vitro binding ability with B-Myb inhibits B-Myb-dependent c-myc gene expression. The inhibition was associated with physical interaction of B-Myb protein with cPLA(2) both in the cytoplasm and the nucleus. Binding site analysis demonstrated that both the N and C termini of cPLA(2) interact with B-Myb. Macrophage colony stimulating factor (MCSF) stimulated cPLA(2) redistribution into the nucleus and also association with B-Myb in human monocytes. Importantly, macrophages from mice with a disrupted cPLA(2) gene demonstrated significantly increased levels of c-Myc protein in the nucleus compared with cells from the wild-type mice, whereas B-Myb levels were similar in the cells from the cPLA(2)(+/+) and cPLA(2)(-/-) mice. Moreover, an introduction of cPLA(2) into cPLA(2)(-/-) mouse macrophages resulted in decreased c-Myc protein levels, and an inhibition of cPLA(2) expression by small interfering RNAs or antisense RNA increased the c-myc transcription in macrophage colony stimulating factor-activated human monocytes. These findings provide new insights into the function of cPLA(2) in B-Myb-dependent gene expression.

Cytosolic phospholipase A2 plays a key role in the pathogenesis of multiple sclerosis-like disease

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS) that results in motor and sensory deficits. Although MS and its animal model, experimental autoimmune encephalomyelitis (EAE), are thought to be T cell-mediated diseases, the mechanisms underlying the lesions in the CNS are not fully understood. We propose that a strong candidate as a central mediator in evoking the complex pathological changes seen in MS and EAE is the enzyme cytosolic phospholipase A2 (cPLA2). One of the metabolic products of this enzyme is pro-inflammatory, while the other induces myelin breakdown, demyelination, and chemokine/cytokine expression. We provide evidence that cPLA2 is highly expressed in EAE lesions and show that blocking this enzyme leads to a remarkable reduction in the onset and progression of EAE.

A novel laminin-induced LPA autocrine loop in the migration of ovarian cancer cells

We have reported previously that levels of lysophosphatidic acid (LPA) are elevated in the blood and ascites from patients with ovarian cancer. LPA stimulates proliferation of ovarian cancer cells and has been proposed as an autocrine growth factor. Here, we show that a novel autocrine loop of LPA promotes the migration of ovarian cancer cells, which is a critical step of tumor metastasis. We report that laminin, but not other extracellular matrix proteins, induces LPA production in ovarian cancer cells. A neutralizing antibody against beta1 integrin and a calcium-independent phospholipase A2-specific inhibitor, HELSS, block both LPA production and the haptotactic activity of laminin. Exogenously added LPA restores the migratory ability of HEY ovarian cancer cells to laminin. These data suggest that laminin-induced cell migration is mediated by LPA. We further show that a specific receptor for LPA, LPA3, is required for mediating the chemotactic activity of LPA. In addition, we show that cytosolic PLA2 is required for cell migration and its activation is phosphatidylinositol-3 kinase-dependent. These findings have revealed a new mechanism of crosstalk between a beta1 integrin receptor and a G protein-coupled receptor.

Viral chemokine receptors and chemokines in human cytomegalovirus trafficking and interaction with the immune system. CMV chemokine receptors

The ubiquitous, opportunistic pathogen human cytomegalovirus (CMV) encodes several proteins homologous to those of the host organism. Four different CMV genes encode chemokine receptor-like peptides. These genes, UL33, UL78, US27, and US28, are expressed at various stages of infection in vitro. Their functions remain largely unknown. To date, chemokine binding and signalling has only been demonstrated for the US28 gene product. Putative ligands for the other CMV-encoded chemokine receptors are discussed on basis of phylogenetic analysis. The potential roles of these receptors in virus trafficking, persistence, and immune evasion are summarized. Similarly, modulation of expression of the host chemokines IL-8, MCP-1a and RANTES in relation to viral dissemination and persistence is reviewed.

Calcium-independent phospholipase A2 through arachidonic acid mobilization is involved in Caco-2 cell growth

Several studies indicate that phospholipase A(2) (PLA(2)) expression and/or activation account for the high levels of arachidonic acid (AA) detected in cancer and, together with the elevated expression of cyclooxygenase-2, lead to cell proliferation and tumor formation. Using Caco-2 cells, a human colorectal carcinoma cell, we studied the role of high-molecular-weight PLA(2)s, cytosolic PLA(2) (cPLA(2)), and calcium-independent PLA(2) (iPLA(2)) in the AA cascade and in cell growth. Treatment with an antisense oligonucleotide against cPLA(2)alpha decreased [(3)H]AA release induced by ionophore A23187 or by a phorbol ester but did not affect the release of [(3)H]AA, [(3)H]thymidine incorporation, or Caco-2 growth induced by fetal calf serum (FCS). However, these parameters were significantly modified by iPLA(2) inhibitors and by an antisense oligonucleotide against iPLA(2)beta. Our results show that iPLA(2) was involved in AA release and the subsequent prostaglandin production induced by serum. Moreover, these data indicate that iPLA(2) may be involved in the signaling pathways involved in the control of Caco-2 proliferation.

Cytosolic phospholipase A2 (cPLA2) regulation of human monocyte NADPH oxidase activity. cPLA2 affects translocation but not phosphorylation of p67(phox) and p47(phox)

The NADPH oxidase of human monocytes is activated upon exposure to opsonized zymosan and a variety of other stimuli to catalyze the formation of superoxide anion. Assembly of the NADPH oxidase complex is believed to be a highly regulated process, and molecular mechanisms responsible for this regulation have yet to be fully elucidated. We have previously reported that cytosolic phospholipase A(2) (cPLA(2)) expression and activity are essential for superoxide anion production in activated human monocytes. In this study, we investigated the mechanisms involved in cPLA(2) regulation of NADPH oxidase activation by evaluating the effects of cPLA(2) on translocation and phosphorylation of p67(phox) and p47(phox). We report that translocation and phosphorylation of p67(phox), as well as p47(phox), occur upon activation of human monocytes and that decreased cPLA(2) protein expression, mediated by antisense oligodeoxyribonucleotides (AS-ODN) specific for cPLA(2) mRNA, blocked the stimulation-induced translocation of p47(phox) and p67(phox) from the cytosol to the membrane fraction. Inhibition of translocation of both p47(phox) and p67(phox) by cPLA(2) AS-ODN was above 85%. Arachidonic acid (AA), a product of cPLA(2) enzymatic activity, completely restored translocation of both of these oxidase components in the AS-ODN-treated, cPLA(2)-deficient human monocytes. These results represent the first report that cPLA(2) activity or AA is required for p67(phox) and p47(phox) translocation in human monocytes. Although cPLA(2) was required for translocation of p47(phox) and p67(phox), it did not influence phosphorylation of these components. These results suggest that one mechanism of cPLA(2) regulation of NADPH oxidase activity is to control the arachidonate-sensitive assembly of the complete oxidase complex through modulating the translocation of both p47(phox) and p67(phox). These studies provide insight into the mechanisms by which activation signals are transduced to allow the induction of superoxide anion production in human monocytes.

Calcium-independent phospholipase A(2) is required for human monocyte chemotaxis to monocyte chemoattractant protein 1

Monocyte chemoattractant protein 1 (MCP-1) has an important influence on monocyte migration into sites of inflammation. Our understanding of the signal transduction pathways involved in the response of monocytes to MCP-1 is quite limited yet potentially significant for understanding and manipulating the inflammatory response. Prior studies have demonstrated a crucial regulatory role for cytosolic phospholipase A(2) (cPLA(2)) in monocyte chemotaxis to MCP-1. In these studies we investigated the role for another PLA(2), calcium-independent PLA(2) (iPLA(2)) in comparison to cPLA(2). Pharmacological inhibitors of PLA(2) were found to substantially inhibit chemotaxis. Using antisense oligodeoxyribonucleotide treatment we found that iPLA(2) expression is required for monocyte migration to MCP-1. Complete blocking of the chemotactic response was observed with inhibition of either iPLA(2) or cPLA(2) expression by their respective antisense oligodeoxyribonucleotide. In reconstitution experiments, lysophosphatidic acid completely restored MCP-1-stimulated migration in iPLA(2)-deficient monocytes, whereas lysophosphatidic acid was without effect in restoring migration in cPLA(2)-deficient monocytes. To the contrary, arachidonic acid fully restored migration of cPLA(2)-deficient monocytes while having no effect on the iPLA(2)-deficient monocytes. Additional studies revealed that neither enzyme appears to be upstream of the other indicating that iPLA(2) and cPLA(2) represent parallel regulatory pathways. These data demonstrate novel and distinct roles for these two phospholipases in this critical step in inflammation.

Role of Ca2+-independent phospholipase A2 on arachidonic acid release induced by reactive oxygen species

Previous studies have shown that reactive oxygen species (ROS) enhance arachidonic acid (AA) release and the subsequent AA metabolism in macrophages. The purpose of this study was determined the implication of phospholipases A2 (PLA2s) in these events. Our results show that oxidative stress induced by exogenous adding of hydrogen peroxide or superoxide anion in macrophage RAW 264.7 and mouse peritoneal macrophage cultures caused a marked enhancement of calcium-independent PLA2 (iPLA2) activity,whereas the increment of secreted PLA2 (sPLA2) and calcium-dependent cytosolic PLA2 (cPLA2) activities were slight. This increase of iPLA2 activity by ROS was rapid and dose-dependent. ROS also induced a significant [3H] arachidonic acid (AA) release. The iPLA2 selective inhibitor, bromoenol lactone, almost completely suppressed the mobilization of [3H]AA induced by ROS whereas antisense oligonucleotide against cPLA2 did not have any appreciable effect. Thus, our data show that iPLA2 activity is involved in the mechanism by which ROS increases the availability of free AA in macrophages RAW 264.7. Moreover, the protein kinase C (PKC) inhibitor, calphostin C, and calcium chelators had no effect on the [3H]AA release induced by ROS, suggesting this is a regulatory role of iPLA2.

Chemokine signaling and functional responses: the role of receptor dimerization and TK pathway activation

A broad array of biological responses, including cell polarization, movement, immune and inflammatory responses, and prevention of HIV-1 infection, are triggered by the chemokines, a family of structurally related chemoattractant proteins that bind to specific seven-transmembrane receptors linked to G proteins. Here we discuss one of the early signaling pathways activated by chemokines, the JAK/STAT pathway. Through this pathway, and possibly in conjunction with other signaling pathways, the chemokines promote changes in cellular morphology, collectively known as polarization, required for chemotactic responses. The polarized cell expresses the chemokine receptors at the leading cell edge, to which they are conveyed by rafts, a cholesterol-enriched membrane fraction fundamental to the lateral organization of the plasma membrane. Finally, the mechanisms through which the chemokines promote their effect are discussed in the context of the prevention of HIV-1 infection.

The Rossmann fold of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a nuclear docking site for antisense oligonucleotides containing a TAAAT motif

The subcellular localisation of oligodeoxynucleotides (ODN) is a major limitation for their use against nuclear targets. In this study we demonstrate that an antisense ODN directed against cytosolic phospholipase A(2) (cPLA2) mRNA is efficiently taken up and accumulates in the nuclei of endothelial cells (HUVEC), human monocytes and HeLa cells. Gel shift experiments and incubation of cells with oligonucleotide derivatives show that the anti-cPLA2 oligo binds a 37 kDa protein in nuclear extracts. The TAAAT sequence was identified as the major binding motif for the nuclear protein in competition experiments with mutated ODNs. Modification of the AAA triplet resulted in an ODN which failed to localise in the nucleus. Moreover, inserting a TAAAT motif into an ODN localising in the cytosol did not modify its localisation. The 37 kDa protein was purified and identified after peptide sequencing as glyceraldehyde-3-phosphate dehydrogenase (GAPDH). It was shown by confocal microscopy that GAPDH co-localises with anti-cPLA2 ODN in the nucleus and commercial GAPDH effectively binds the oligo. Competition experiments with increasing concentration of NAD(+) co-factor indicate that the GAPDH Rossmann fold is a docking site for antisense oligonucleotides containing a TAAAT motif.

Lysophosphatidylcholine inhibits endothelial cell migration and proliferation via inhibition of the extracellular signal-regulated kinase pathway

Lysophosphatidylcholine (lysoPC), a major lipid component of oxidized low density lipoprotein, inhibits endothelial cell (EC) migration and proliferation, which are critical processes during angiogenesis and the repair of injured vessels. However, the mechanism(s) of lysoPC-induced inhibition of EC migration and proliferation has not been clarified. In this report, we demonstrate the critical role of extracellular signal-regulated kinase (ERK) in growth factor-stimulated EC migration and proliferation as well as their inhibition by lysoPC. EC migration and proliferation stimulated by basic fibroblast growth factor (FGF-2) were blocked by inhibition of ERK activity by both the specific mitogen-activated protein kinase kinase (MEK) 1 inhibitor PD98059 and the overexpression of a dominant-negative mutant of MEK1. Conversely, overexpression of a constitutively active mutant of MEK1 increased EC migration and proliferation, which were comparable to those of ECs stimulated with FGF-2. LysoPC inhibited FGF-2-induced ERK activation via prevention of Ras activation without inhibiting tyrosine phosphorylation of phospholipase C-gamma. Taken together, our data demonstrate that ERK activity is required for FGF-2-induced EC migration and proliferation and suggest that inhibition of the Ras/ERK pathway by lysoPC contributes to the reduced EC migration and proliferation.

Identification of surface residues of the monocyte chemotactic protein 1 that affect signaling through the receptor CCR2

The CC chemokine, monocyte chemotactic protein, 1 (MCP-1) functions as a major chemoattractant for T-cells and monocytes by interacting with the seven-transmembrane G protein-coupled receptor CCR2. To identify which residues of MCP-1 contribute to signaling though CCR2, we mutated all the surface-exposed residues to alanine and other amino acids and made some selective large changes at the amino terminus. We then characterized the impact of these mutations on three postreceptor pathways involving inhibition of cAMP synthesis, stimulation of cytosolic calcium influx, and chemotaxis. The results highlight several important features of the signaling process and the correlation between binding and signaling: The amino terminus of MCP-1 is essential as truncation of residues 2-8 ([1+9-76]hMCP-1) results in a protein that cannot stimulate chemotaxis. However, the exact peptide sequence may be unimportant as individual alanine mutations or simultaneous replacement of residues 3-6 with alanine had little effect. Y13 is also important and must be a large nonpolar residue for chemotaxis to occur. Interestingly, both Y13 and [1+9-76]hMCP-1 are high-affinity binders and thus affinity of these mutants is not correlated with ability to promote chemotaxis. For the other surface residues there is a strong correlation between binding affinity and agonist potency in all three signaling pathways. Perhaps the most interesting observation is that although Y13A and [1+9-76]hMCP are antagonists of chemotaxis, they are agonists of pathways involving inhibition of cAMP synthesis and, in the case of Y13A, calcium influx. These results demonstrate that these two well-known signaling events are not sufficient to drive chemotaxis. Furthermore, it suggests that specific molecular features of MCP-1 induce different conformations in CCR2 that are coupled to separate postreceptor pathways. Therefore, by judicious design of antagonists, it should be possible to trap CCR2 in conformational states that are unable to stimulate all of the pathways required for chemotaxis.

Phospholipase A2 enzymes in eicosanoid generation

Phospholipase A2 (PLA2) enzymes cleave esterified fatty acids from membrane glycerophospholipids. The 20-carbon polyunsaturated fatty acid, arachidonic acid, is used as substrate by intermediate enzymes for the generation of eicosanoids, including leukotrienes and prostanoid products. An expanding number of PLA2 enzymes has now been identified that may participate in arachidonic acid release and thus serve a rate-limiting role in eicosanoid biosynthesis. Cellular PLA2 function for various members is regulated by constitutive or elicited expression, as well as by posttranslational events such as phosphorylation. In addition, the function of some cellular PLA2 enzymes is regulated by a requirement for calcium or by localization to a particular subcellular compartment. Finally, some PLA2 enzymes are secreted from the cell where they may directly interact with plasma membrane or transmembrane receptors to function as autocrine or paracrine mediators. Evaluating the roles of a number of these functionally similar PLA2 enzymes in the biosynthesis of leukotrienes and other eicosanoids is the focus of this review.

A Defective Protein Kinase C Anchoring System Underlying Age-Associated Impairment in TNF-α Production in Rat Macrophages

The ability of macrophages to secrete cytokines is important in host responses to infections inflammatory stimuli, both of which are altered with aging. In this study, age-associated changes in the release of TNF-α from LPS-stimulated rat alveolar macrophages were determined and correlated with a decrease in the level of RACK1, the anchoring protein involved in protein kinase C translocation and activation. Macrophages from aged rats produced ∼50% less TNF-α than those from young rats. This effect was observed independently from the concentration of LPS used and the time considered. The decrease observed was associated with a defective PKC translocation, due to a reduction in the expression of RACK1, whereas no differences were detected in the expression of LPS receptor (CD14) or total PKC isoforms (α and βΙΙ) in old and young rats. Use of RACK1 antisense oligonucleotide reduced the ability of young macrophages to respond to LPS, further supporting the idea that a deficit in RACK1 contributes to the functional impairment in aged macrophages and that age-induced macrophage immunodeficiencies are associated with alteration in signal transduction pathways.

Inhibitors of intracellular phospholipase A2 activity: their neurochemical effects and therapeutical importance for neurological disorders

Intracellular phospholipases A2 (PLA2) are a diverse group of enzymes with a growing number of members. These enzymes hydrolyze membrane phospholipids into fatty acid and lysophospholipids. These lipid products may serve as intracellular second messengers or can be further metabolized to potent inflammatory mediators, such as eicosanoids and platelet-activating factors. Several inhibitors of nonneural intracellular PLA2 have been recently discovered. However, nothing is known about their neurochemical effects, mechanism of action or toxicity in human or animal models of neurological disorders. Elevated intracellular PLA2 activities, found in neurological disorders strongly associated with inflammation and oxidative stress (ischemia, spinal cord injury, and Alzheimer's disease), can be treated with specific, potent and nontoxic inhibitors of PLA2 that can cross blood-brain barrier without harm. Currently, potent intracellular PLA2 inhibitors are not available for clinical use in human or animal models of neurological disorders, but studies on this interesting topic are beginning to emerge. The use of nonspecific intracellular PLA2 inhibitors (quinacrine, heparin, gangliosides, vitamin E) in animal model studies of neurological disorders in vivo has provided some useful information on tolerance, toxicity, and effectiveness of these compounds.

Regulation and inhibition of phospholipase A2

In recent years, there has been great interest in the study of phospholipid metabolism in intact cell systems. Such an interest arises mainly from the discovery that cellular membrane phospholipids serve not only in structural roles, but are also reservoirs of preformed second messenger molecules with key roles in cellular signaling. These second messenger molecules are generated by agonist-induced activation and secretion of intracellular and extracellular phospholipases, respectively, i.e. enzymes that cleave ester bonds within phospholipids. Prominent members of the large collection of signal-activated phospholipases are the phospholipase A2s. These enzymes hydrolyze the sn-2 ester bond of phospholipids, releasing a free fatty acid and a lysophospholipid, both of which may alter cell function. In addition to its role in cellular signaling, phospholipase A2 has recently been recognized to be involved in a wide number of pathophysiological situations, ranging from systemic and acute inflammatory conditions to cancer. A growing number of pharmacologic inhibitors will help define the role of particular phospholipase A2s in signaling cascades.

Characterization of the Signal Transduction Pathway Activated in Human Monocytes and Dendritic Cells by MPIF-1, a Specific Ligand for CC Chemokine Receptor 1

The receptor specificity and signal transduction pathway has been identified and characterized for a truncated form of myeloid progenitor inhibitory factor-1 (MPIF-124–99). MPIF-1 binds specifically to sites, in particular CCR1, shared with macrophage inflammatory protein-1α (MIP-1α) on the surface of human monocytes and dendritic cells, as inferred by its ability to compete for [125I]MIP-1α, but not for [125I]MIP-1β or [125I]monocyte chemotactic protein-1(MCP-1) binding to intact cells. Based on calcium flux, MPIF-1 is an agonist on CCR1-transfected HEK-293 cells, monocytes, and dendritic cells, but not on CCR5-, CCR8-, or CX3CR1-transfected cells. The inhibitory effect of guanosine 5′-O-(3-thio-triphosphate) (GTP-γS) or pertussis toxin pretreatment on MPIF-1 binding and calcium mobilization, respectively, indicates the involvement of G proteins in the interaction of MPIF-1 and its receptor(s). The increase in intracellular free calcium concentration following MPIF-1 treatment is mainly due to the influx of calcium from an extracellular pool. However, a portion of the intracellular free calcium concentration is derived from a phospholipase C inhibitor-sensitive intracellular pool. MPIF-1 induces a rapid dose-dependent release of [3H]arachidonic acid from monocytes that is dependent on extracellular calcium and is blocked by phospholipase A2 (PLA2) inhibitors. Furthermore, PLA2 activation is shown to be necessary for filamentous actin formation in monocytes. Thus, the MPIF-1 signal transduction pathway appears to include binding to CCR1; transduction by G proteins; effector function by phospholipase C, protein kinase C, calcium flux, and PLA2; and cytoskeletal remodeling.

Extracellular calcium regulates HeLa cell morphology during adhesion to gelatin: role of translocation and phosphorylation of cytosolic phospholipase A2

Attachment of HeLa cells to gelatin induces the release of arachidonic acid (AA), which is essential for cell spreading. HeLa cells spreading in the presence of extracellular Ca2+ released more AA and formed more distinctive lamellipodia and filopodia than cells spreading in the absence of Ca2+. Addition of exogenous AA to cells spreading in the absence of extracellular Ca2+ restored the formation of lamellipodia and filopodia. To investigate the role of cytosolic phospholipase A2 (cPLA2) in regulating the differential release of AA and subsequent formation of lamellipodia and filopodia during HeLa cell adhesion, cPLA2 phosphorylation and translocation from the cytosol to the membrane were evaluated. During HeLa cell attachment and spreading in the presence of Ca2+, all cPLA2 became phosphorylated within 2 min, which is the earliest time cell attachment could be measured. In the absence of extracellular Ca2+, the time for complete cPLA2 phosphorylation was lengthened to <4 min. Maximal translocation of cPLA2 from cytosol to membrane during adhesion of cells to gelatin was similar in the presence or absence of extracellular Ca2+ and remained membrane associated throughout the duration of cell spreading. The amount of total cellular cPLA2 translocated to the membrane in the presence of extracellular Ca2+ went from <20% for unspread cells to >95% for spread cells. In the absence of Ca2+ only 55-65% of the total cPLA2 was translocated to the membrane during cell spreading. The decrease in the amount translocated could account for the comparable decrease in the amount of AA released by cells during spreading without extracellular Ca2+. Although translocation of cPLA2 from cytosol to membrane was Ca2+ dependent, phosphorylation of cPLA2 was attachment dependent and could occur both on the membrane and in the cytosol. To elucidate potential activators of cPLA2, the extracellular signal-related protein kinase 2 (ERK2) and protein kinase C (PKC) were investigated. ERK2 underwent a rapid phosphorylation upon early attachment followed by a dephosphorylation. Both rates were enhanced during cell spreading in the presence of extracellular Ca2+. Treatment of cells with the ERK kinase inhibitor PD98059 completely inhibited the attachment-dependent ERK2 phosphorylation but did not inhibit cell spreading, cPLA2 phosphorylation, translocation, or AA release. Activation of PKC by phorbol ester (12-O-tetradecanoylphorbol-13-acetate) induced and attachment-dependent phosphorylation of both cPLA2 and ERK2 in suspension cells. However, in cells treated with the PKC inhibitor Calphostin C before attachment, ERK2 phosphorylation was inhibited, whereas cPLA2 translocation and phosphorylation remained unaffected. In conclusion, although cPLA2-mediated release of AA during HeLa cell attachment to a gelatin substrate was essential for cell spreading, neither ERK2 nor PKC appeared to be responsible for the attachment-induced cPLA2 phosphorylation and the release of AA.

Beta-chemokines MCP-1 and RANTES are selectively increased in cerebrospinal fluid of patients with human immunodeficiency virus-associated dementia

Human immunodeficiency virus-associated dementia (HAD) is associated with increased numbers of activated central nervous system (CNS) macrophages. Chemokines, which regulate infiltration of macrophages, were measured in the cerebrospinal fluid (CSF) of human immunodeficiency virus (HIV)-negative and HIV-positive individuals with and without neurological disease. Monocyte chemotactic protein (MCP)-1 and RANTES (but not MCP-3), macrophage inflammatory protein (MIP)-1alpha, MIP-1beta, or interleukin-8 (IL-8) was higher in HAD. MCP-1 correlated with CSF viral load and severity of dementia, and it increased over time in patients who developed dementia.

Chemokines and chemokine receptors during activation and deactivation of monocytes and dendritic cells and in amplification of Th1 versus Th2 responses

Chemokines are a superfamily of small cytokines which are generally chemotactic for leukocytes. The general features of chemokines and their receptors are reviewed. Recent evidence indicates that receptor expression dictates the spectrum of action of chemokines, as shown recently for Th1 and Th2 cells. Chemokines represent amplification loops of polarized Th1 and Th2 responses. Receptor expression is tightly regulated during differentiation, activation, and deactivation of mononuclear phagocytes and dendritic cells. Thus, regulation of receptor expression is crucial as a set point of the chemokine system.

A mild and efficient solid-support synthesis of novel oligonucleotide conjugates

Conjugates of oligodeoxyribonucleotide phosphorothioate (ODN-PS) with folic acid, retinoic acid, arachidonic acid, and methoxypoly(ethylene glycol)propionic acid have been synthesized. The procedure involved the initial solid-phase preparation of 5'-amino-functionalized ODN-PS using N-pent-4-enoyl-derived (PNT) nucleoside phosphoramidites followed by conjugation of the oligonucleotide either to the ligand acids, using 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide as a coupling reagent, or to their corresponding succinimidyl derivatives. Subsequent exposure of the support to aqueous ammonium hydroxide (28%, 2 h, 55 degrees C) resulted in the release of the fully deprotected ODN conjugates, which were purified by reversed-phase HPLC or by preparative polyacrylamide gel electrophoresis. The identity of the oligonucleotide conjugates was confirmed by MALDI-TOF mass spectral analysis.

Inhibition of HIV type 1 BaL replication by MIP-1alpha, MIP-1beta, and RANTES in macrophages

The beta-chemokines RANTES, MIP-1alpha, and MIP-1beta have been shown to inhibit the infection of T cells by macrophage-tropic HIV-1 strains by blocking env-driven HIV-1 fusion through competition for the chemokine receptors or receptor downregulation. This study was aimed at testing whether beta-chemokines also inhibit the productive infection of monocyte-derived macrophages (MDMs) by a monocytotropic HIV-1 strain, by using virus yield assays. The action of the beta-chemokines MIP-1alpha, MIP-1beta, and RANTES was captured with that of the alpha-chemokine interleukin 8 (IL-8) and of interferon alpha (IFN-alpha), which is a well-known broad-range inhibitor of viral replication. While IL-8 did not inhibit HIV-1 BaL replication in MDMs, the beta-chemokines were dose-dependently inhibitory. RANTES was the most effective, reaching at 300 ng/ml a protection similar to that obtained with IFN-alpha at 1000 IU/ml, and was even more inhibitory when added to MDMs after virus attachment. In contrast to IFN-alpha, the antiviral activity of beta-chemokines was restricted to HIV, because another virus was not inhibited. As compared with untreated MDMs, full-length proviral DNA at day 1 postinfection was inhibited in MDMs treated with RANTES either before or after the absorption phase, and even more so in IFN-treated MDMs, whereas in IL-8-treated MDMs no inhibition was observed. Our results indicate that in MDMs both RANTES and IFN affect early steps of HIV-1 BaL replication, preceding the completion of viral DNA synthesis.

Essential requirement of cytosolic phospholipase A2 for activation of the phagocyte NADPH oxidase

Arachidonic acid (AA) can trigger activation of the phagocyte NADPH oxidase in a cell-free assay. However, a role for AA in activation of the oxidase in intact cells has not been established, nor has the AA generating enzyme critical to this process been identified. The human myeloid cell line PLB-985 was transfected to express p85 cytosolic phospholipase A2 (cPLA2) antisense mRNA and stable clones were selected that lack detectable cPLA2. cPLA2-deficient PLB-985 cells differentiate similarly to control PLB-985 cells in response to retinoic acid or 1,25-dihydroxyvitamin D3, indicating that cPLA2 is not involved in the differentiation process. Neither cPLA2 nor stimulated [3H]AA release were detectable in differentiated cPLA2-deficient PLB-985 cells, demonstrating that cPLA2 is the major type of PLA2 activated in phagocytic-like cells. Despite the normal synthesis of NADPH oxidase subunits during differentiation of cPLA2-deficient PLB-985 cells, these cells fail to activate NADPH oxidase in response to a variety of soluble and particulate stimuli, but the addition of exogenous AA fully restores oxidase activity. This establishes an essential requirement of cPLA2-generated AA for activation of phagocyte NADPH oxidase.

Antisense inhibition of group VI Ca2+-independent phospholipase A2 blocks phospholipid fatty acid remodeling in murine P388D1 macrophages

A major issue in lipid signaling relates to the role of particular phospholipase A2 isoforms in mediating receptor-triggered responses. This has been difficult to study because of the lack of isoform-specific inhibitors. Based on the use of the Group VI Ca2+-independent phospholipase A2 (iPLA2) inhibitor bromoenol lactone (BEL), we previously suggested a role for the iPLA2 in mediating phospholipid fatty acid turnover (Balsinde, J., Bianco, I. D., Ackermann, E. J., Conde-Frieboes, K., and Dennis, E. A. (1995) Proc. Natl. Acad. Sci. U. S. A. 92: 8527-8531). We have now further evaluated the role of the iPLA2 in phospholipid remodeling by using antisense RNA technology. We show herein that inhibition of iPLA2 expression by a specific antisense oligonucleotide decreases both the steady-state levels of lysophosphatidylcholine and the capacity of the cell to incorporate arachidonic acid into membrane phospholipids. These effects correlate with a decrease in both iPLA2 activity and protein in the antisense-treated cells. Collectively these data provide further evidence that the iPLA2 plays a major role in regulating phospholipid fatty acyl turnover in P388D1 macrophages. In stark contrast, experiments with activated cells confirmed that the iPLA2 does not play a significant role in receptor-coupled arachidonate mobilization in these cells, as manifested by the lack of an effect of the iPLA2 antisense oligonucleotide on PAF-stimulated arachidonate release.

Selective inhibition of cytosolic phospholipase A2 in activated human monocytes. Regulation of superoxide anion production and low density lipoprotein oxidation

Our previous studies have shown that monocyte activation and release of O-2 are required for monocyte-mediated low density lipoprotein (LDL) lipid oxidation. We have also found that intracellular Ca2+ levels and protein kinase C activity are requisite participants in this potentially pathogenic process. In these studies, we further investigated the mechanisms involved in the oxidation of LDL lipids by activated human monocytes, particularly the potential contributions of the cytosolic phospholipase A2 (cPLA2) signaling pathway. The most well-studied cPLA2, has a molecular mass of 85 kDa and has been reported to be regulated by both Ca2+ and phosphorylation. We found that cPLA2 protein levels and cPLA2 enzymatic activity were induced upon activation of human monocytes by opsonized zymosan. Pharmacologic inhibition of cPLA2 activity by AACOCF3, which has been reported to be a specific inhibitor of cPLA2 as compared with sPLA2, caused a dose-dependent inhibition of cPLA2 enzymatic activity and LDL lipid oxidation by activated human monocytes, whereas sPLA2 activity was not affected. To corroborate these findings, we used specific antisense oligonucleotides to inhibit cPLA2. We observed that treatment with antisense oligonucleotides caused suppression of both cPLA2 protein expression and enzymatic activity as well as monocyte-mediated LDL lipid oxidation. Furthermore, antisense oligonucleotide treatment caused a substantial inhibition of O-2 production by activated human monocytes. In parallel experimental groups, cPLA2 sense oligonucleotides did not affect cPLA2 protein expression, cPLA2 enzymatic activity, O-2 production, or monocyte-mediated LDL lipid oxidation. These studies support the proposal that cPLA2 activity is required for activated monocytes to oxidize LDL lipids.

The role of chemokines in inflammation

Chemokines, together with adhesion molecules, cytokines, and proteases, are essential for the directional migration of leukocytes during normal and inflammatory processes. Interleukin-8 and monocyte chemotactic protein-1 are the best-characterized members of the C-X-C and C-C chemokine subfamilies, respectively. However, more than 20 human chemokines have been identified but are only partially characterized at the biological level. Chemokines are involved in chemotaxis of monocytes, lymphocytes, neutrophils, eosinophils, basophils, natural killer cells, dendritic cells, and endothelial cells. This review describes the chemokine subfamilies, the chemokine producer and target cells, their receptors, signal transduction mechanisms, and the role of chemokines during physiological and pathological conditions. More and more evidence points to a role for chemokines in chemotaxis-related phenomena, such as the expression of adhesion molecules, the secretion of proteinases, inhibition of apoptosis, hematopoiesis, and angiogenesis. Chemokines are also involved in diseases such as cancer (tumor regression and tumor metastasis), autoimmune diseases, and bacterial or viral infection.

The activity of cytosolic phospholipase A2 is required for the lysis of adenovirus-infected cells by tumor necrosis factor

Most cell types are resistant to apoptosis induced by tumor necrosis factor (TNF) unless the cells are treated with a sensitizing agent. Inhibitors of transcription or translation act as sensitizing agents, as do adenoviruses lacking one or more resistance genes. We have reported recently that the activity of cytosolic phospholipase A2 (cPLA2) is necessary for the TNF-induced lysis of cells that are sensitized by inhibitors of transcription or translation (C. Voelkel-Johnson, T. E. Thorne, and S. M. Laster, J. Immunol. 156:201-207, 1996). In this report we have asked whether the lysis of cells infected by the adenovirus dl758 (which lacks the E3 14.7-kDa resistance gene product) also involves the activity of cPLA2. We report that a phosphorothioate-modified antisense oligonucleotide specific for cPLA2, but not the control oligonucleotide, inhibited the TNF-induced release of both [3H]arachidonic acid and 51Cr from infected cells. Arachidonyltrifluoromethyl ketone (AA COCF3), an inhibitor of cPLA2, also inhibited the release of 51Cr, and we found that the release of [3H]arachidonic acid was highly selective and was preferred over the release of [3H]palmitic acid. Taken together, these results suggest strongly that cPLA2 is indeed the phospholipase responsible for the release of [3H]arachidonic acid during the lysis of infected cells and that its activity is necessary for cell death. Finally, since arachidonic acid serves as the substrate for the synthesis of inflammatory lipids, our results suggest a possible link between the TNF-induced lysis of infected cells and inflammation. The E3 14.7-kDa resistance protein may, therefore, play two roles: preventing TNF-induced cell death and, as our results show, preventing the TNF-induced release of arachidonic acid.

Type II secretory phospholipase A2 associated with cell surfaces via C-terminal heparin-binding lysine residues augments stimulus-initiated delayed prostaglandin generation

Type II secretory phospholipase A2 (sPLA2) has been shown to be induced by a variety of proinflammatory stimuli and, therefore, has been implicated in the inflammatory process. In order to determine whether association of sPLA2 with cell surfaces via heparan sulfate proteoglycan is important for its effects on cellular functions, we have identified the critical domain in sPLA2 for heparin and cell surface binding and examined its role in cellular prostaglandin (PG) biosynthesis. Replacement of several conserved Lys residues in the C-terminal region of mouse and rat sPLA2s by Glu resulted in a marked reduction of their capacities to bind to heparin and mammalian cell surfaces without affecting their enzymatic activities toward dispersed phospholipid as a substrate. CHO cells stably transfected with wild-type sPLA2 released about twice as much arachidonic acid (AA) during culture for 10 h with fetal calf serum and interleukin-1beta than cells transfected with vector alone, whereas the ability to enhance AA release was impaired in sPLA2 mutants incapable of binding to cell surfaces. AA released by wild-type sPLA2-transfected CHO cells was metabolized to prostaglandin E2 via prostaglandin endoperoxide H synthase (PGHS)-2 after IL-1beta stimulation, revealing a particular functional linkage of sPLA2 to PGHS-2. In contrast, A23187-initiated immediate AA release over 30 min was not affected by sPLA2 overexpression. Taken together, these results suggest that sPLA2 expressed endogenously and anchored on cell surfaces via its C-terminal heparin-binding domain is involved in the PGHS-2-dependent delayed PG biosynthesis initiated by growth factors and cytokines during long term culture.