Retinoic acid receptor-α regulates synthetic events in human platelets

Essentials Platelets express retinoic acid receptor (RAR)α protein, specifically binding target mRNAs. mRNAs under RARα control include MAP1LC3B2, SLAIN2, and ANGPT1. All-trans retinoic acid (atRA) releases RARα from its target mRNA. RARα expressed in human platelets exerts translational control via direct mRNA binding.

SUMMARY

Background Translational control mechanisms in platelets are incompletely defined. Here, we determined whether the nuclear transcription factor RARα controls protein translational events in human platelets. Methods Isolated human platelets were treated with the pan-RAR agonist all-trans-retinoic acid (atRA). Global and targeted translational events were examined. Results Stimulation of platelets with atRA significantly increased global protein expression. RARα protein bound to a subset of platelet mRNAs, as measured by next-generation RNA-sequencing. In-depth analyses of 5' and 3'-untranslated regions of the RARα-bound mRNAs revealed consensus RARα binding sites in microtubule-associated protein 1 light chain 3 beta 2 (MAP1LC3B2), SLAIN motif-containing protein 2 (SLAIN2) and angiopoietin-1 (ANGPT1) transcripts. When platelets were treated with atRA, binding interactions between RARα protein and mRNA for MAP1LC3B2, SLAIN2 and ANGPT1 were significantly decreased. Consistent with the release of bound RARα protein from MAP1LCB2mRNA, we observed an increase in the synthesis of MAP1LC3B2 protein. Conclusions These findings provide the first evidence that RARα, a nuclear transcriptional factor, regulates synthetic events in anucleate human platelets. They also reveal an additional non-genomic role for RARα in platelets that may have implications for the vitamin A-dependent signaling in humans.

Combined variants in factor VIII and prostaglandin synthase-1 amplify hemorrhage severity across three generations of descendants

Essentials Co-existent damaging variants are likely to cause more severe bleeding and may go undiagnosed. We determined pathogenic variants in a three-generational pedigree with excessive bleeding. Bleeding occurred with concurrent variants in prostaglandin synthase-1 (PTGS-1) and factor VIII. The PTGS-1 variant was associated with functional defects in the arachidonic acid pathway.

SUMMARY

Background Inherited human variants that concurrently cause disorders of primary hemostasis and coagulation are uncommon. Nevertheless, rare cases of co-existent damaging variants are likely to cause more severe bleeding and may go undiagnosed. Objective We prospectively sought to determine pathogenic variants in a three-generational pedigree with excessive bleeding. Patients/methods Platelet number, size and light transmission aggregometry to multiple agonists were evaluated in pedigree members. Transmission electron microscopy determined platelet morphology and granule content. Thromboxane release studies and light transmission aggregometry in the presence or absence of prostaglandin G2 assessed specific functional defects in the arachidonic acid pathway. Whole exome sequencing (WES) and targeted nucleotide sequence analysis identified potentially deleterious variants. Results Pedigree members with excessive bleeding had impaired platelet aggregation with arachidonic acid, epinephrine and low-dose ADP, as well as reduced platelet thromboxane B2 release. Impaired platelet aggregation in response to 2MesADP was rescued with prostaglandin G2 , a prostaglandin intermediate downstream of prostaglandin synthase-1 (PTGS-1) that aids in the production of thromboxane. WES identified a non-synonymous variant in the signal peptide of PTGS-1 (rs3842787; c.50C>T; p.Pro17Leu) that completely co-segregated with disease phenotype. A variant in the F8 gene causing hemophilia A (rs28935203; c.5096A>T; p.Y1699F) was also identified. Individuals with both variants had more severe bleeding manifestations than characteristic of mild hemophilia A alone. Conclusion We provide the first report of co-existing variants in both F8 and PTGS-1 genes in a three-generation pedigree. The PTGS-1 variant was associated with specific functional defects in the arachidonic acid pathway and more severe hemorrhage.

Non-genomic activities of retinoic acid receptor alpha control actin cytoskeletal events in human platelets

Essentials Platelets employ proteins/signaling pathways traditionally thought reserved for nuclear niche. We determined retinoic-acid-receptor alpha (RARα) expression and function in human platelets. RARα/actin-related protein-2/3 complex (Arp2/3) interact via non-genomic signaling in platelets. RARα regulates Arp2/3-mediated actin cytoskeletal dynamics and platelet spreading.

SUMMARY

Background Platelets utilize proteins and pathways classically reserved for the nuclear niche. Methods We determined whether human platelets express retinoic-acid-receptor family members, traditionally thought of as nuclear transcription factors, and deciphered the function of RARα. Results We found that RARα is robustly expressed in human platelets and megakaryocytes and interacts directly with actin-related protein-2/3 complex (Arp2/3) subunit 5 (Arp2/3s5). Arp2/3s5 co-localized with RARα in situ and regulated platelet cytoskeletal processes. The RARα ligand all-trans retinoic acid (atRA) disrupted RARα-Arp2/3 interactions. When isolated human platelets were treated with atRA, rapid cytoskeletal events (e.g. platelet spreading) were inhibited. In addition, when platelets were cultured for 18 h in the presence of atRA, actin-dependent morphological changes (e.g. extended cell body formation) were similarly inhibited. Using in vitro actin branching assays, RARα and Arp2/3-regulated complex actin branch formation was demonstrated. Consistent with inhibition of cytoskeletal processes in platelets, atRA, when added to this branching assay, resulted in dysregulated actin branching. Conclusion Our findings identify a previously unknown mechanism by which RARα regulates Arp2/3-mediated actin cytoskeletal dynamics through a non-genomic signaling pathway. These findings have broad implications in both nucleated and anucleate cells, where actin cytoskeletal events regulate cell morphology, movement and division.

The septic milieu triggers expression of spliced tissue factor mRNA in human platelets

BACKGROUND

Activated platelets have previously-unrecognized mechanisms of post-transcriptional gene expression that may influence hemostasis and inflammation. A novel pathway involves splicing of pre-mRNAs in resting platelets to mature, translatable mRNAs in response to cellular activation.

OBJECTIVES

We asked if bacterial products and host agonists present in the septic milieu induce tissue factor pre-mRNA splicing in platelets from healthy subjects. In parallel, we asked if spliced tissue factor (TF) mRNA is present in platelets from septic patients in a proof-of-principle analysis.

PATIENTS/METHODS

TF pre-mRNA and mRNA expression patterns were characterized in platelets from septic patients and in platelets isolated from healthy subjects activated with bacteria, toxins and inflammatory agonists. Procoagulant activity was also measured.

RESULTS AND CONCLUSIONS

Live bacteria, staphylococcal α-toxin and lipopolysaccharide (LPS) induced TF pre-mRNA splicing in platelets isolated from healthy subjects. Toxin-stimulated platelets accelerated plasma clotting, a response that was blocked by a previously-characterized splicing inhibitor and by an anti-tissue factor antibody. Platelets from septic patients expressed spliced TF mRNA, whereas it was absent from unselected and age-matched control subjects. Tissue factor-dependent procoagulant activity was elevated in platelets from a subset of septic patients. Thus, bacterial and host factors induce splicing of TF pre-mRNA, expression of TF mRNA and tissue factor-dependent clotting activity in human platelets. TF mRNA is present in platelets from some septic patients, indicating that it may be a marker of altered platelet phenotype and function in sepsis and that splicing pathways are induced in this syndrome.

Protein synthesis by platelets: historical and new perspectives

In the late 1960s, numerous investigators independently demonstrated that platelets are capable of synthesizing proteins. Studies continued at a steady pace over the next 30 years and into the 21st century. Collectively, these investigations confirmed that platelets synthesize proteins and that the pattern of protein synthesis changes in response to cellular activation. More recent studies have characterized the mechanisms by which platelets synthesize proteins and have shown that protein synthesis alters the phenotype and functions of platelets. Here, we chronologically review our increased understanding of protein synthetic responses in platelets and discuss how the field may evolve over the next decade.

The evolving role of platelets in inflammation

Platelets are small in size and simple in structure. Nevertheless, these anucleate cytoplasts utilize complex molecular systems to regulate a variety of biological functions. Here we review evolutionary paths, traditional roles, and previously unrecognized biological capacities of platelets that interface thrombosis with inflammation and potentially identify new roles in inflammatory diseases.

Altered expression of the mRNA stability factor HuR promotes cyclooxygenase-2 expression in colon cancer cells

Cyclooxygenase-2 (COX-2) expression is normally tightly regulated. However, constitutive overexpression plays a key role in colon carcinogenesis. To understand the molecular nature of enhanced COX-2 expression detected in colon cancer, we examined the ability of the AU-rich element-containing (ARE-containing) 3' untranslated region (3'UTR) of COX-2 mRNA to regulate rapid mRNA decay in human colon cancer cells. In tumor cells displaying enhanced growth and tumorigenicity that is correlated with elevated COX-2, vascular endothelial growth factor (VEGF), and IL-8 protein levels, the corresponding mRNAs were transcribed constitutively and turned over slowly. The observed mRNA stabilization is owing to defective recognition of class II-type AREs present within the COX-2, VEGF, and IL-8 3'UTRs; c-myc mRNA, containing a class I ARE decayed rapidly in the same cells. Correlating with cellular defects in mRNA stability, the RNA-binding of trans-acting cellular factors was altered. In particular, we found that the RNA-stability factor HuR binds to the COX-2 ARE, and overexpression of HuR, as detected in tumors, results in elevated expression of COX-2, VEGF, and IL-8. These findings demonstrate the functional significance rapid mRNA decay plays in controlling gene expression and show that dysregulation of these trans-acting factors can lead to overexpression of COX-2 and other angiogenic proteins, as detected in neoplasia.

Escherichia coli Braun lipoprotein induces a lipopolysaccharide-like endotoxic response from primary human endothelial cells

All bacteria contain proteins in which their amino-terminal cysteine residue is modified with N-acyl S-diacylglycerol functions, and peptides and proteins bearing this modification are immunomodulatory. The major outer membrane lipoprotein of Escherichia coli, the Braun lipoprotein (BLP), is the prototypical triacylated cysteinyl-modified protein. We find it is as active as LPS in stimulating human endothelial cells to an inflammatory phenotype, and a BLP-negative mutant of E. coli was less inflammatory than its parental strain. While the lipid modification was essential, the lipidated protein was more potent than a lipid-modified peptide. BLP associates with CD14, but this interaction, unlike that with LPS, was not required to elicit endothelial cell activation. BLP stimulated endothelial cell E-selectin surface expression, IL-6 secretion, and up-regulation of the same battery of cytokine mRNAs induced by LPS. Quantitative microarray analysis of 4400 genes showed the same 30 genes were induced by BLP and LPS, and that there was near complete concordance in the level of gene induction. We conclude that the lipid modification of at least one abundant Gram-negative protein is essential for endotoxic activity, but that the protein component also influences activity. The equivalent potency of BLP and LPS, and their complete concordance in the nature and extent of endothelial cell activation show that E. coli endotoxic activity is not due to just LPS. The major outer membrane protein of E. coli is a fully active endotoxic agonist for endothelial cells.

Differential regulation of matrix metalloproteinase-9 by monocytes adherent to collagen and platelets

Circulating monocytes adhere to platelets and matrix proteins at sites of vascular injury, where engagement of specific surface tethering molecules mediates outside-in signaling and synthesis of gene products by the leukocytes. Here we demonstrate that interaction of isolated human monocytes with collagen induces matrix metalloproteinase-9 (MMP-9; gelatinase B) synthesis by monocytes, a process that is greatly enhanced in the presence of platelets. MMP-9 is a potent matrix degrading enzyme implicated in atherosclerotic plaque rupture, aneurysm formation, and other vascular syndromes. Synthesis of MMP-9 by monocytes is tightly regulated and synergistically increased following adhesion to collagen and platelets. Adhesion to control matrix proteins alone did not result in MMP-9 protein production and, similarly, adhesion of monocytes to platelets activated with thrombin in suspension was not sufficient to induce MMP-9 synthesis in the absence of monocyte adhesion to collagen. Interruption of intercellular contact between platelets and monocytes dramatically inhibited MMP-9 synthesis. These observations demonstrate that discrete adhesion-dependent signaling pathways govern MMP-9 synthesis by monocytes. The synthesis of MMP-9 by monocytes may be critical in vascular syndromes and other pathological processes that are dependent on dysregulated cell-cell and cell-matrix interactions.

Lysophosphatidylcholine and lyso-PAF display PAF-like activity derived from contaminating phospholipids

Lysophosphatidylcholine is an abundant component of plasma and oxidized LDL that displays several biological activities, some of which may occur through the platelet-activating factor (PAF) receptor. We find that commercial lysophosphatidylcholine, its alkyl homolog (lyso-PAF), and PAF all induce inflammation in a murine model of pleurisy. Hydrolysis of PAF to lyso-PAF by recombinant PAF acetylhydrolase abolished this eosinophilic infiltration, implying that lyso-PAF should not have displayed inflammatory activity. Saponification of lyso-PAF or PAF acetylhydrolase treatment of lyso-PAF or lysophosphatidylcholine abolished activity; neither lysolipid should contain susceptible sn-2 residues, suggesting contaminants account for the bioactivity. Lyso-PAF and to a lesser extent lysophosphatidylcholine stimulated Ca(2+) accumulation in 293 cells stably transfected with the human PAF receptor, and this was inhibited by specific PAF receptor antagonists. Again, treatment of lyso-PAF or lysophosphatidylcholine with recombinant PAF acetylhydrolase, a nonselective phospholipase A(2), or saponification of lyso-PAF destroyed the PAF-like activity, a result incompatible with lyso-PAF or lysophosphatidylcholine being the actual agonist. We conclude that neither lyso-PAF nor lysophosphatidylcholine is a PAF receptor agonist, nor are they inflammatory by themselves. We suggest that PAF or a PAF-like mimetic accounts for inflammatory effects of lysophosphatidylcholine and lyso-PAF.

Cell adhesion regulates gene expression at translational checkpoints in human myeloid leukocytes

Engagement of adhesion molecules on monocytes and other myeloid leukocytes, which are effector cells of the innate immune system, not only tethers the leukocytes in place but also transmits outside-in signals that induce functional changes and alter gene expression. We found that a subset of mRNAs that are induced or amplified by adhesion of human monocytes to P-selectin via its surface ligand, P-selectin glycoprotein 1, have characteristics that suggest specialized translational control. One of these codes for urokinase plasminogen activator receptor (UPAR), a critical surface protease receptor and regulator of cell adhesion and migration. Although UPAR transcripts are induced by adhesion, rapid synthesis of the protein uses constitutive mRNA without a requirement for new transcription and is regulated by mammalian target of rapamycin, demonstrating new biologic roles for the signal-dependent translation pathway controlled by this intracellular kinase. The synthesis of UPAR in monocytic cells is also regulated by eukaryotic translation initiation factor 4E, a second key translational checkpoint, and phosphorylation of eukaryotic translation initiation factor 4E is induced by adhesion of monocytes to P-selectin. Translationally controlled display of UPAR by monocytes confers recognition of the matrix protein, vitronectin. Adhesion-dependent signaling from the plasma membrane to translational checkpoints represents a previously unrecognized mechanism for regulating surface phenotype that may be particularly important for myeloid leukocytes and other cells that are specialized for rapid inflammatory and vascular responses.

Activated platelets mediate inflammatory signaling by regulated interleukin 1beta synthesis

Platelets release preformed mediators and generate eicosanoids that regulate acute hemostasis and inflammation, but these anucleate cytoplasts are not thought to synthesize proteins or cytokines, or to influence inflammatory responses over time. Interrogation of an arrayed cDNA library demonstrated that quiescent platelets contain many messenger RNAs, one of which codes for interleukin 1beta precursor (pro-IL-1beta). Unexpectedly, the mRNA for IL-1beta and many other transcripts are constitutively present in polysomes, providing a mechanism for rapid synthesis. Platelet activation induces rapid and sustained synthesis of pro-IL-1beta protein, a response that is abolished by translational inhibitors. A portion of the IL-1beta is shed in its mature form in membrane microvesicles, and induces adhesiveness of human endothelial cells for neutrophils. Signal-dependent synthesis of an active cytokine over several hours indicates that platelets may have previously unrecognized roles in inflammation and vascular injury. Inhibition of beta3 integrin engagement markedly attenuated the synthesis of IL-1beta, identifying a new link between the coagulation and inflammatory cascades, and suggesting that antithrombotic therapies may also have novel antiinflammatory effects.

Oxidized alkyl phospholipids are specific, high affinity peroxisome proliferator-activated receptor gamma ligands and agonists

Synthetic high affinity peroxisome proliferator-activated receptor (PPAR) agonists are known, but biologic ligands are of low affinity. Oxidized low density lipoprotein (oxLDL) is inflammatory and signals through PPARs. We showed, by phospholipase A(1) digestion, that PPARgamma agonists in oxLDL arise from the small pool of alkyl phosphatidylcholines in LDL. We identified an abundant oxidatively fragmented alkyl phospholipid in oxLDL, hexadecyl azelaoyl phosphatidylcholine (azPC), as a high affinity ligand and agonist for PPARgamma. [(3)H]azPC bound recombinant PPARgamma with an affinity (K(d)((app)) approximately 40 nm) that was equivalent to rosiglitazone (BRL49653), and competition with rosiglitazone showed that binding occurred in the ligand-binding pocket. azPC induced PPRE reporter gene expression, as did rosiglitazone, with a half-maximal effect at 100 nm. Overexpression of PPARalpha or PPARgamma revealed that azPC was a specific PPARgamma agonist. The scavenger receptor CD36 is encoded by a PPRE-responsive gene, and azPC enhanced expression of CD36 in primary human monocytes. We found that anti-CD36 inhibited azPC uptake, and it inhibited PPRE reporter induction. Results with a small molecule phospholipid flippase mimetic suggest azPC acts intracellularly and that cellular azPC accumulation was efficient. Thus, certain alkyl phospholipid oxidation products in oxLDL are specific, high affinity extracellular ligands and agonists for PPARgamma that induce PPAR-responsive genes.

Identification of platelet-activating factor as the inflammatory lipid mediator in CCl4-metabolizing rat liver

Unmitigated oxidative stress is deleterious, as epitomized by CCl4 intoxication. In this well-characterized model of free radical-initiated damage, liver metabolism of CCl4 to CCl3. causes lipid peroxidation, F-ring isoprostane formation, and pathologic leukocyte activation. The nature of the mediator that couples oxidation to the hepatotoxic inflammatory response is uncharacterized. We found that oxidatively modified phosphatidylcholines were present in the livers of CCl4-exposed rats and not in livers from control animals, that CCl4 metabolism generated lipids that activated 293 cells stably transfected with the human platelet-activating factor (PAF) receptor, and that this PAF-like activity was formed as rapidly as isoprostane-containing phosphatidylcholine (iPC) during oxidation. iPC and the PAF-like activity also had similar chromatographic properties. The potential for iPC activation of the PAF receptor has been unexplored, but we conclude that iPC themselves did not activate the PAF receptor, as phospholipase A1 hydrolysis completely destroyed iPC, but none of the PAF-like bioactivity. Oxidatively fragmented phospholipids are potent agonists of the PAF receptor, but mass spectrometry characterized PAF as the major inflammatory component coeluting with iPC. Oxidatively fragmented phospholipids and iPC are markers of free radical generation in CCl4-intoxicated liver, but PAF generation by activated hepatic cells generated the inflammatory agent.

Oxidized LDL contains inflammatory PAF-like phospholipids

Atherosclerosis has an underlying inflammatory component. Oxidation of low-density lipoprotein (LDL) particles to modified forms promotes atherogenesis by supplying cholesterol and through the oxidative generation of agents that activate macrophages, smooth muscle and endothelial cells. A primary target of oxidizing compounds, derived from cigarette smoke, dietary sources, exuberant inflammatory cell responses and normal cellular metabolism among other sources, are the esterified polyunsaturated fatty acids in the phospholipid shell that surrounds the insoluble lipids of the lipoprotein core. One type of phospholipid oxidation product mimics the structure of the potent inflammatory mediator platelet-activating factor (PAF), and these oxidation products activate the PAF receptor found on platelets, monocytes and leukocytes. Production of such PAF mimetics is, in contrast to the physiologic generation of PAF, uncontrolled. PAF mimetics and other phospholipid oxidation products are found in atherosclerotic lesions or even in blood after exposure to cigarette smoke. Here we summarize our data describing the structure, activity and metabolism of the PAF-like lipids found in atherogenic LDL particles.

Platelet-activating factor and related lipid mediators

Platelet-activating factor (PAF) is a phospholipid with potent, diverse physiological actions, particularly as a mediator of inflammation. The synthesis, transport, and degradation of PAF are tightly regulated, and the biochemical basis for many of these processes has been elucidated in recent years. Many of the actions of PAF can be mimicked by structurally related phospholipids that are derived from nonenzymatic oxidation, because such compounds can bind to the PAF receptor. This process circumvents much of the biochemical control and presumably is regulated primarily by the rate of degradation, which is catalyzed by PAF acetylhydrolase. The isolation of cDNA clones encoding most of the key proteins involved in regulating PAF has allowed substantial recent progress and will facilitate studies to determine the structural basis for substrate specificity and the precise role of PAF in physiological events.

A novel syndrome of variant leukocyte adhesion deficiency involving defects in adhesion mediated by beta1 and beta2 integrins

Leukocyte adhesion deficiency type I (LAD-1) is a disorder associated with severe and recurrent bacterial infections, impaired extravascular targeting and accumulation of myeloid leukocytes, altered wound healing, and significant morbidity that is caused by absent or greatly diminished surface expression of integrins of the beta2 class. We report clinical features and analysis of functions of cells from a patient with a myelodysplastic syndrome and infectious complications similar to those in the severe form of LAD-1, but whose circulating neutrophils displayed normal levels of beta2 integrins. Analysis of adhesion of these cells to immobilized ligands and to endothelial cells and assays of cell-cell aggregation and chemotaxis demonstrated a profound defect in adhesion mediated by beta2 integrins indicative of a variant form of LAD-1. A novel cell line established from Epstein-Barr virus-transformed lymphoblasts from the subject demonstrated deficient beta2 integrin-dependent adhesive function similar to that of the primary leukocytes. In addition, these cells had markedly impaired beta1 integrin-dependent adhesion. Sequence analysis and electrophoretic mobility of beta1 and beta2 proteins from the cell line demonstrated that the defects were not a result of structural abnormalities in the integrin subunit chains themselves and suggest that the adhesive phenotype of these cells is due to one or more abnormalities of inside-out signaling mechanisms that regulate the activity of integrins of these classes. These features define a unique LAD-1 variant syndrome that may reveal important insights that are generally relevant to inside-out signaling of integrins, a molecular process that is as yet incompletely understood.

Intracellular unesterified arachidonic acid signals apoptosis

Cyclooxygenase-2 (COX-2) is up-regulated in many cancers and is a rate-limiting step in colon carcinogenesis. Nonsteroidal antiinflammatory drugs, which inhibit COX-2, prevent colon cancer and cause apoptosis. The mechanism for this response is not clear, but it might result from an accumulation of the substrate, arachidonic acid, an absence of a prostaglandin product, or diversion of the substrate into another pathway. We found that colon adenocarcinomas overexpress another arachidonic acid-utilizing enzyme, fatty acid-CoA ligase (FACL) 4, in addition to COX-2. Exogenous arachidonic acid caused apoptosis in colon cancer and other cell lines, as did triacsin C, a FACL inhibitor. In addition, indomethacin and sulindac significantly enhanced the apoptosis-inducing effect of triacsin C. These findings suggested that unesterified arachidonic acid in cells is a signal for induction of apoptosis. To test this hypothesis, we engineered cells with inducible overexpression of COX-2 and FACL4 as "sinks" for unesterified arachidonic acid. Activation of the enzymatic sinks blocked apoptosis, and the reduction of cell death was inversely correlated with the cellular level of arachidonic acid. Inhibition of the COX-2 component by nonsteroidal antiinflammatory drugs restored the apoptotic response. Cell death caused by exposure to tumor necrosis factor alpha or to calcium ionophore also was prevented by removal of unesterified arachidonic acid. We conclude that the cellular level of unesterified arachidonic acid is a general mechanism by which apoptosis is regulated and that COX-2 and FACL4 promote carcinogenesis by lowering this level.

Bioactive phospholipid oxidation products

Oxidation of phospholipids results in chain-shortened fragments and oxygenated derivatives of polyunsaturated sn-2 fatty acyl residues, generating a myriad of phospholipid products. Certain oxidation products of phosphatidylcholine bind to and activate the human receptor for PAF, and these PAF-like lipids are potent, selective inflammatory mediators. Formation of PAF-like lipids is nonenzymatic and so their accumulation is unregulated. PAF-like lipids are produced in vivo in response to oxidative stresses and are responsible for attendant acute inflammatory responses. PAF-like lipids almost exclusively contain an ether-linked alkyl residue at the sn-1 position of the phosphatidylcholine backbone and molecular identification of these is facilitated by phospholipase A(1) treatment to remove the bulk of the inactive phospholipids. The identity of biologically active species generated by oxidative fragmentation and oxidation can be elucidated by understanding relevant reactions leading to the formation of PAF-like lipids, and then their structure can be established by tandem mass spectrometry and chemical synthesis.

Induction of cyclooxygenase-2 by human monocytes exposed to group B streptococci

Group B streptococcal (GBS) infections are associated with high morbidity and mortality. The molecular pathways mediating the pathophysiological events in GBS infection are not fully delineated. Cyclooxygenases (COX) are the enzymes that convert arachidonate to active eicosanoids. To identify the effects of GBS on eicosanoid metabolism and regulatory mechanisms, we exposed human monocytes to GBS and found that they secreted prostaglandin E2, prostacyclin, and thromboxane A2. Exposure to GBS caused monocytes to express COX-2 mRNA and protein in both a time- and concentration-dependent manner that correlated with eicosanoid production. COX-1 protein was unchanged. Addition of the anti-inflammatory cytokines interleukin (IL)-4 or IL-10 markedly attenuated GBS-induced COX-2 protein accumulation after GBS exposure, as did inhibition of p38 MAPK. Our experiments are the first to show that exposure of monocytes to a gram-positive bacterium (GBS) results in induction of functional COX-2, suggesting that eicosanoids may play important roles in the pathogenesis of GBS infections.

Fluid flow activates a regulator of translation, p70/p85 S6 kinase, in human endothelial cells

Cellular phenotype is determined not only by genetic transcription but also by subsequent translation of mRNA into protein. Extracellular signals trigger intracellular pathways that distinctly activate translation. The 70/85-kDa S6 kinase (pp70(S6k)) is a central enzyme in the signal-dependent control of translation, but its regulation in endothelial cells is largely unknown. Here we show that fluid flow (in the absence of an exogenous mitogen) as well as humoral agonists activate endothelial pp70(S6k). Rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), and wortmannin, a phosphatidylinositol 3-kinase inhibitor, blocked flow-induced pp70(S6k) activation; FK-506, a rapamycin analog with minimal mTOR inhibitory activity, and PD-98059, an inhibitor of the flow-sensitive mitogen-activated protein kinase pathway, had no effect. Synthesis of Bcl-3, a protein whose translation is controlled by an mTOR-dependent pathway, was induced by flow and inhibited by rapamycin and wortmannin. Transcriptional blockade did not abolish the flow-induced upregulation of Bcl-3. Fluid forces may therefore modify endothelial phenotype by specifically regulating translation of certain mRNA transcripts into protein.

Post-transcriptional control of cyclooxygenase-2 gene expression. The role of the 3'-untranslated region

The cyclooxygenase (COX)-2 enzyme is responsible for increased prostaglandin formation in inflammatory states and is the major target of nonsteroidal anti-inflammatory drugs. Normally COX-2 expression is tightly regulated, however, constitutive overexpression plays a key role in colon carcinogenesis. To understand the mechanisms controlling COX-2 expression, we examined the ability of the 3'-untranslated region of the COX-2 mRNA to regulate post-transcriptional events. When fused to a reporter gene, the 3'-untranslated region mediated rapid mRNA decay (t(1/2) = 30 min), which was comparable to endogenous COX-2 mRNA turnover in serum-induced fibroblasts treated with actinomycin D or dexamethasone. Deletion analysis demonstrated that a conserved 116-nucleotide AU-rich sequence element (ARE) mediated mRNA degradation. In transiently transfected cells, this region inhibited protein synthesis approximately 3-fold. However, this inhibition did not occur through changes in mRNA stability since mRNA half-life and steady-state mRNA levels were unchanged. RNA mobility shift assays demonstrated a complex of cytoplasmic proteins that bound specifically to the ARE, and UV cross-linking studies identified proteins ranging from 90 to 35 kDa. Fractionation of the cytosol showed differential association of ARE-binding proteins to polysomes and S130 fractions. We propose that these factors influence expression at a post-transcriptional step and, if dysregulated, may increase COX-2 protein as detected in colon cancer.

Expression of fatty acid-CoA ligase 4 during development and in brain

Fatty acid utilization is initiated by fatty acid-CoA ligase, which converts free fatty acids into fatty acyl-CoA esters. We have cloned previously the human long-chain fatty acid-CoA ligase 4 (FACL4), which is a central enzyme in controlling the free arachidonic acid level in cells and thereby regulating eicosanoid production. We report here the expression of this gene in tissues, particularly in different parts of the brain. We found that FACL4 encoded a 75 kDa enzyme and that there was a modified translation product expressed in the brain. FACL4 was expressed in early stages of development with a significant amount of FACL4 mRNA detected in an E7 mouse embryo. In addition, FACL4 was highly expressed in both adult and newborn mouse brain especially in the granule cells of the dentate gyrus and the pyramidal cell layer of CA1 in hippocampus, and the granular cell layer and Purkinje cells of the cerebellum.

Inflammatory platelet-activating factor-like phospholipids in oxidized low density lipoproteins are fragmented alkyl phosphatidylcholines

Oxidation of human low density lipoprotein (LDL) generates proinflammatory mediators and underlies early events in atherogenesis. We identified mediators in oxidized LDL that induced an inflammatory reaction in vivo, and activated polymorphonuclear leukocytes and cells ectopically expressing human platelet-activating factor (PAF) receptors. Oxidation of a synthetic phosphatidylcholine showed that an sn-1 ether bond confers an 800-fold increase in potency. This suggests that rare ether-linked phospholipids in LDL are the likely source of PAF-like activity in oxidized LDL. Accordingly, treatment of oxidized LDL with phospholipase A(1) greatly reduced phospholipid mass, but did not decrease its PAF-like activity. Tandem mass spectrometry identified traces of PAF, and more abundant levels of 1-O-hexadecyl-2-(butanoyl or butenoyl)-sn-glycero-3-phosphocholines (C(4)-PAF analogs) in oxidized LDL that comigrated with PAF-like activity. Synthesis showed that either C(4)-PAF was just 10-fold less potent than PAF as a PAF receptor ligand and agonist. Quantitation by gas chromatography-mass spectrometry of pentafluorobenzoyl derivatives shows the C(4)-PAF analogs were 100-fold more abundant in oxidized LDL than PAF. Oxidation of synthetic alkyl arachidonoyl phosphatidylcholine generated these C(4)-PAFs in abundance. These results show that quite minor constituents of the LDL phosphatidylcholine pool are the exclusive precursors for PAF-like bioactivity in oxidized LDL.

A comparison between endovascular and surgical management of basilar artery apex aneurysms

OBJECT

The goal of this retrospective study was to evaluate endovascular treatment by means of Guglielmi detachable coils (GDCs) compared with surgical management for basilar artery (BA) apex aneurysms.

METHODS

Forty-one patients presented with saccular BA apex aneurysms with angiographically definable necks that were judged suitable for either treatment. Of 20 patients who underwent surgery and 21 who underwent GDC embolization, 15 (75%) and 11 (52%), respectively, were treated in the acute phase after subarachnoid hemorrhage (SAH). Twenty-four (92%) of the 26 patients presenting with an SAH had a Hunt and Hess Grade III or better. Fifteen patients with unruptured or ruptured aneurysms more than 14 days post-SAH were treated electively. Patients in the endovascular and surgical treatment groups had aneurysms with comparable dimensions and configurations. Overall, 15 (75%) of the surgical patients and 20 (95%) of the patients in whom GDC embolization was performed had a good outcome (Glasgow Outcome Scale score of 4 or 5). Among those patients treated in the acute stage post-SAH, 11 (73%) of the surgical group and 10 (91%) of the endovascular group did well. Fourteen patients treated electively (93%) had good outcomes. There were two deaths (10%) in the surgical group and none in the endovascular group. Patients treated surgically were hospitalized twice as long and incurred twice the expenses of patients who underwent endovascular treatment (p<0.001).

CONCLUSIONS

Endovascular GDC embolization of select BA apex aneurysms may be a competitive alternative to direct surgical clipping. Long-term follow up is needed to better define the natural history of the endovascularly treated aneurysm and to further evaluate the accuracy of these preliminary results.

Deficiency of platelet-activating factor acetylhydrolase is a severity factor for asthma

Asthma, a family of airway disorders characterized by airway inflammation, has an increasing incidence worldwide. Platelet-activating factor (PAF) may play a role in the pathophysiology of asthma. Its proinflammatory actions are antagonized by PAF acetylhydrolase. A missense mutation (V279F) in the PAF acetylhydrolase gene results in the complete loss of activity, which occurs in 4% of the Japanese population. We asked if PAF acetylhydrolase deficiency correlates with the incidence and severity of asthma in Japan. We found that the prevalence of PAF acetylhydrolase deficiency is higher in Japanese asthmatics than healthy subjects and that the severity of this syndrome is highest in homozygous-deficient subjects. We conclude that the PAF acetylhydrolase gene is a modulating locus for the severity of asthma.

Peroxisome proliferators enhance cyclooxygenase-2 expression in epithelial cells

The formation of prostaglandins requires the catalytic activity of cyclooxygenase (COX) which converts arachidonic acid to the prostaglandin endoperoxide PGH2, from which all other prostaglandins are formed. COX-2 is the highly inducible isozyme of COX which is responsible for much of the prostaglandin production in inflammation and is a key factor in colon carcinogenesis. Because COX-2 activity can be rate-limiting in prostaglandin formation, COX-2 expression must be regulated tightly. Numerous factors, including mitogens, tumor promoters, and cytokines have been found to stimulate the transcription of COX-2. We show that fatty acids, prostaglandins, and non-steroidal anti-inflammatory drugs, compounds that are substrates, products, and inhibitors, respectively, of COX enzymatic activity, also increase its expression. These compounds are members of a heterogeneous group of compounds known as peroxisome proliferators, and the prototypical peroxisome proliferator, WY-14, 643, also enhanced COX-2 expression. We demonstrate that these compounds increase COX-2 transcription, and we identify a region of the COX-2 promoter containing a peroxisome proliferator response element that is responsible for the enhancement of COX-2 expression seen with these compounds.

Integrin-dependent control of translation: engagement of integrin alphaIIbbeta3 regulates synthesis of proteins in activated human platelets

Integrins are widely expressed plasma membrane adhesion molecules that tether cells to matrix proteins and to one another in cell-cell interactions. Integrins also transmit outside-in signals that regulate functional responses of cells, and are known to influence gene expression by regulating transcription. In previous studies we found that platelets, which are naturally occurring anucleate cytoplasts, translate preformed mRNA transcripts when they are activated by outside-in signals. Using strategies that interrupt engagement of integrin alphaIIbbeta3 by fibrinogen and platelets deficient in this integrin, we found that alphaIIbbeta3 regulates the synthesis of B cell lymphoma 3 (Bcl-3) when platelet aggregation is induced by thrombin. We also found that synthesis of Bcl-3, which occurs via a specialized translation control pathway regulated by mammalian target of rapamycin (mTOR), is induced when platelets adhere to immobilized fibrinogen in the absence of thrombin and when integrin alphaIIbbeta3 is engaged by a conformation-altering antibody against integrin alphaIIbbeta3. Thus, outside-in signals delivered by integrin alphaIIbbeta3 are required for translation of Bcl-3 in thrombin-stimulated aggregated platelets and are sufficient to induce translation of this marker protein in the absence of thrombin. Engagement of integrin alpha2beta1 by collagen also triggered synthesis of Bcl-3. Thus, control of translation may be a general mechanism by which surface adhesion molecules regulate gene expression.

The cloning and characterization of a novel human diacylglycerol kinase, DGKiota

Diacylglycerol (DAG) plays a central role in both the synthesis of complex lipids and in intracellular signaling; diacylglycerol kinase (DGK) catalyzes the phosphorylation of DAG, which yields phosphatidic acid. A family of DGKs has been identified in multicellular organisms over the past few years, but the physiological function(s) of this diversity is not clear. One clue has come from the Drosophila DGK2, rdgA, since mutations in this gene cause retinal degeneration. We isolated a novel DGK, which we designated DGKiota, from human retina and brain libraries. DGKiota contains two cysteine-rich repeats, a region similar to the phosphorylation site domain of myristoylated alanine-rich C kinase substrate, a conserved catalytic domain, and four ankyrin repeats at its C terminus. By primary structure, it is most similar to human DGKzeta and Drosophila rdgA. An >12-kilobase mRNA for DGKiota was detected only in brain and retina among the tissues examined. In cells transfected with the DGKiota cDNA, we detected an approximately 130-kDa protein by immunoassay, and activity assays demonstrated that it encodes a functional DAG kinase. The protein was found to be in both the cytoplasm and nucleus with the localization controlled by PKC isoforms alpha and gamma. The gene encoding DGKiota was localized to human chromosome 7q32.3-33, which is known to be a locus for an inherited form of retinitis pigmentosa. These results have defined a novel isoform of DAG kinase, which may have important cellular functions in the retina and brain.

Engagement of ICAM-3 activates polymorphonuclear leukocytes: aggregation without degranulation or beta 2 integrin recruitment

ICAM-3 is a preferred counterreceptor for the leukocyte alpha(L)beta2 integrin. It activates T cells through outside-in signaling, but polymorphonuclear leukocytes (PMN) are reported to be refractory to ICAM-3 stimulation. We found that engagement of ICAM-3 by a mAb (CAL3.10), which binds in the region where alpha(L)beta2 integrin binds, activates PMN homotypic aggregation and adhesion to surfaces. These functional changes were due to ICAM-3 outside-in signaling because aggregation and adhesion were beta2 integrin-dependent, tyrosine kinase and protein kinase C activities were activated, and there was a reorganization of the cytoskeleton. This reorganization and kinase activity was required for ICAM-3-, but not FMLP-, induced aggregation. This is not an Fc-mediated event as an appropriate anti-ICAM-3 F(ab')2 fragment still induced aggregation. Another anti-ICAM-3 Ab (HP2/19), which activates T cells, did not activate PMN. Strikingly, anti-ICAM-3 did not induce degranulation or cause an increase in surface beta2 integrin expression, so adhesion and aggregation were due solely to the activation of the constitutively expressed beta2 integrins. Aggregation in response to ICAM-3, but not FMLP, was compromised at lower cell densities, showing that beta2 integrin recruitment enhances aggregation under suboptimal conditions. We conclude that engagement of ICAM-3 stimulates PMN as well as T cells, but that the appropriate epitope varies between these two cells. ICAM-3 outside-in signaling reorganizes the cytoskeleton without causing degranulation, induces serine and tyrosine kinase activation, and activates existing surface beta2 integrins to a proadhesive state.

Protein kinase C regulates the nuclear localization of diacylglycerol kinase-zeta

Diacylglycerol kinases (DGKs) terminate signalling from diacylglycerol by converting it to phosphatidic acid. Diacylglycerol regulates cell growth and differentiation, and its transient accumulation in the nucleus may be particularly important in this regulation. Here we show that a fraction of DGK-zeta is found in the nucleus, where it regulates the amount of nuclear diacylglycerol. Reducing nuclear diacylglycerol levels by conditional expression of DGK-zeta attenuates cell growth. The nuclear-localization signal of DGK-zeta is located in a region that is homologous to the phosphorylation-site domain of the MARCKS protein. This is, to our knowledge, the first evidence that this domain, which is a major target for protein kinase C, can localize a protein to the nucleus. Two isoforms of protein kinase C, but not others, regulate the localization of DGK-zeta. Our results define a cycle in which diacylglycerol activates protein kinase C, which then regulates the metabolism of diacylglycerol by alternating the intracellular location of DGK-zeta. This may be a general mechanism to control mitogenic signals that depend on nuclear diacylglycerol.

The cloning and developmental regulation of murine diacylglycerol kinase zeta

Diacylglycerol kinases (DGKs) regulate the key signaling intermediates diacylglycerol (DAG) and phosphatidic acid (PA). We isolated cDNA clones of mouse diacylglycerol kinase zeta (mDGKzeta) and found that it shares 88% identity at the nucleic acid level and 95.5% identity at the amino acid level with human DGKzeta (hDGKzeta). Murine DGKzeta protein rose gradually during embryonic development, and was abundant in newborn and adult brains. By RNA whole-mount in situ hybridization, mDGKzeta was shown to be expressed in spinal ganglia and limb buds at low level in E11.5 embryos and at higher level in E12.5 embryos. In E13.5 embryos, DGKzeta mRNA was highly expressed in vibrissa follicles, in spinal ganglia, and in the interdigital regions of the developing limbs. Northern blotting showed that DGKzeta expression was limited to specific anatomical regions of the brain. Thus, the expression of DGKzeta is regulated temporally and spatially during mammalian development and correlates with the development of sensory neurons and regions undergoing apoptosis.

Human endothelial cells regulate polymorphonuclear leukocyte degranulation

Neutrophil degranulation is an important event in inflammatory responses. We examined the regulation of neutrophil (PMN) degranulation by resting and activated human endothelial cells. Whereas PMNs adherent to endothelial cells that were stimulated to express P-selectin and platelet-activating factor did not release the specific granule marker lactoferrin or the primary granule enzyme, elastase, PMNs adherent to endothelial cells stimulated with interleukin-1 (IL-1) or tumor necrosis factor secreted both. PMN degranulation was dependent on the time of incubation of endothelial cells with the cytokine, its concentration, and the time of incubation of the PMNs with endothelial cells. Degranulation of PMNs and their adhesion to stimulated endothelial cells are correlated events, but they could be dissociated by blocking the tethering molecules used by the endothelial cells and neutrophils under these conditions. This suggested that paracrine signaling molecules that induce PMN degranulation are produced by cytokine-stimulated endothelial cells. We found that endothelial cells stimulated with IL-1 release newly synthesized degranulating factors that require transcription and translation. IL-8 was synthesized, released, and signaled granular secretion by PMNs. However, experiments with blocking antibodies indicated the presence of an additional degranulating factor not accounted for by IL-8. These experiments demonstrate that human endothelial cells regulate degranulation of neutrophils by generating signaling factors that are expressed differentially depending on the endothelial agonist and other features. Active modification of neutrophil granular secretion by endothelial cells can influence physiologic acute inflammatory responses but may also contribute to pathologic vascular and tissue damage.

Bacterial lipopolysaccharide induces endothelial cells to synthesize a degranulating factor for neutrophils

Enzymes and other factors secreted by degranulating neutrophils (polymorphonuclear leukocytes, PMNs) mediate endothelial injury, thrombosis, and vascular remodeling. In bacteremia and sepsis syndrome and their consequent complications (including acute respiratory distress syndrome and systemic ischemia-reperfusion resulting from septic shock), neutrophil degranulation is an important mechanism of injury. In related studies, we found that human endothelial cells regulate neutrophil degranulation and that inflammatory cytokines induce synthesis of degranulating factors by human endothelial cells. Here we show that lipopolysaccharides (LPS) from gram-negative bacteria were the most potent agonists for release of degranulating activity by endothelial cells when compared to several cytokines and stimulatory factors. LPS also induced the release of degranulating signals for PMNs from a human endothelial cell line, EA.hy 926. Interleukin 8 (IL-8) is synthesized by endothelial and EA.hy 926 cells in response to LPS and induces neutrophil degranulation. However, complementary strategies using receptor desensitization, translation of messenger RNA by Xenopus laevis oocytes, and purification and analysis of factors from conditioned supernatants demonstrated that degranulating factors distinct from IL8 are generated in response to LPS. The characteristics of a partially purified degranulating factor isolated from conditioned supernatants distinguished it from known chemokines and other factors that induce PMN degranulation and are generated by endothelial cells in response to LPS. Thus, cultured human endothelial cells and endothelial cell lines synthesize several unique signaling molecules that can trigger neutrophil granular secretion. If produced in vivo in response to LPS or other pathologic agonists, these degranulating signals may activate PMNs in combination or in sequence, initiating or propagating vascular damage.

Coengagement of ICAM-3 and Fc receptors induces chemokine secretion and spreading by myeloid leukocytes

ICAM-3 is expressed at high levels on myeloid leukocytes, but its function on these cells is unknown. We tested the hypothesis that it transduces outside-in proinflammatory signals using immobilized mAbs to engage ICAM-3 on freshly isolated human monocytes and neutrophils. Two immobilized Abs that recognize epitopes in the extracellular domain 1 of ICAM-3, which is critical for recognition by the alphaL/beta2 integrin, potently induced secretion of MIP-1alpha, IL-8, and MCP-1 by monocytes and triggered IL-8 secretion by neutrophils. These chemokines are products of immediate-early genes that are induced when myeloid cells are activated. Chemokine secretion induced by "triggering" Abs was greater than that induced by isotype-matched immobilized Abs against ICAM-1, ICAM-2, PECAM-1, control Igs, or immobilized control proteins. Coengagement of ICAM-3 and Fc receptors (FcgammaRI or FcgammaRII) was required for maximal chemokine secretion by monocytes. Microscopy documented that there is also dramatic spreading of monocytes when surface ICAM-3 is engaged by immobilized Abs. Spreading was induced by Fab and F(ab')2 fragments of triggering anti-ICAM-3 mAb, demonstrating direct outside-in signaling, but was not required for chemokine secretion. These experiments indicate that ICAM-3 may transmit outside-in signals when it is engaged by beta2 integrins during myeloid cell-cell interactions in inflammatory lesions. Binding of Fc receptors by Ig in the local environment can amplify the responses.

Signal-dependent translation of a regulatory protein, Bcl-3, in activated human platelets

Circulating human platelets lack nuclei, cannot synthesize mRNA, and are considered incapable of regulated protein synthesis. We found that thrombin-activated, but not resting, platelets synthesize Bcl-3, a member of the IkappaB-alpha family of regulatory proteins. The time- and concentration-dependent generation of Bcl-3 in platelets signaled by thrombin was blocked by translational inhibitors, by rapamycin, and by inhibitors of phosphatidylinositol-3-kinase, indicating that it occurs via a specialized translational control pathway that involves phosphorylation of the inhibitory protein 4E-BP1. After its synthesis in activated platelets Bcl-3 binds to the SH3 domain of Fyn (p59(fyn)), a Src-related tyrosine kinase. This, along with its expression in anucleate cells, suggests that Bcl-3 has previously unrecognized functions aside from modulation of transcription. We also demonstrate that platelets synthesize and secrete numerous proteins besides Bcl-3 after they adhere to fibrinogen, which mediates adhesion and outside-in signaling of these cells by engagement of alphaIIb/beta3 integrin. Taken together, these data demonstrate that regulated synthesis of proteins is a signal-dependent activation response of human platelets.

Cloning, expression, and chromosomal localization of human long-chain fatty acid-CoA ligase 4 (FACL4)

Long-chain fatty acid-CoA ligase (also called fatty acid acyl-CoA synthetase) plays an essential role in lipid biosynthesis and fatty acid degradation. We report herein the cDNA cloning of the human long-chain fatty acid-CoA ligase 4 from a brain library. The cDNA encodes a functional long-chain fatty acid-CoA ligase that shows preference for arachidonic acid as substrate. We also studied the tissue distribution of gene expression by Northern hybridization. Human placenta, brain, testis, ovary, spleen, and adrenal cortex have the highest levels of expression of the long-chain fatty acid-CoA ligase 4, whereas the GI system has the lowest. Finally, this gene was localized to chromosome Xq23 in human by FISH analysis.

A juxtacrine mechanism for neutrophil adhesion on platelets involves platelet-activating factor and a selectin-dependent activation process

The aim of this study was to identify the molecular mechanisms involved in neutrophil adhesion to immobilized platelets with particular focus on the possible existence of a juxtacrine system for neutrophil-platelet interactions. Platelets were immobilized onto collagen (type I)-coated coverslips that were placed in a flow chamber and neutrophils were perfused across these confluent monolayers at a shear stress of 1 to 4 dynes/cm2. Neutrophils rolled, and a significant proportion (25% to 50%) adhered to platelet monolayers. P-selectin was expressed in very large quantities on the surface of platelets and mediated all of the rolling, whereas the beta2-integrin mediated firm adhesion. An activation mechanism for adhesion was necessary inasmuch as fixed neutrophils continued to roll on immobilized platelets, but did not adhere. Platelets adherent to collagen produced significant levels of platelet-activating factor (PAF). Accordingly, the firm adhesion of neutrophils to platelets was significantly inhibited by a PAF receptor antagonist (WEB 2086). Treatment of only the platelets with acetylhydrolase, which converts membrane-associated PAF to lyso-PAF, prevented 60% of the adhesion. These data suggest that PAF, on the surface of platelets, mediated a significant portion of the adhesive interaction. Addition of some selectin-binding carbohydrates (fucoidan or soluble SLEx analogs but not dextran sulfate) to the platelets caused rolling neutrophils to immediately adhere, an event that was not observed on histamine or thrombin-treated endothelium or P-selectin transfectants. These data support the view that a juxtacrine activation process exists on immobilized platelets for neutrophils. This process can be greatly enhanced on platelets and may involve a signaling mechanism through P-selectin.

Expression of plasma platelet-activating factor acetylhydrolase is transcriptionally regulated by mediators of inflammation

Platelet-activating factor (PAF) is a potent phospholipid with diverse physiological and pathological actions, and it is inactivated by PAF acetylhydrolase. In this study, we analyzed the tissue distribution of the plasma PAF acetylhydrolase mRNA in humans. We isolated a 3.5-kilobase fragment containing the 5' genomic sequence of the plasma PAF acetylhydrolase gene and further characterized the promoter activity. We determined the transcriptional initiation site by primer extension. We then prepared constructs containing various lengths of 5' genomic fragments fused to a luciferase reporter gene and transfected these constructs into COS-7 cells. We found that there is more than one region in the 1.3-kilobase 5' genomic sequence conferring promoter activity and that a very short 5'-flanking region (72 base pairs) is sufficient for more than 65% of the basal activity. In parallel, we examined the regulation of expression of the PAF acetylhydrolase gene. We found that interferon-gamma (IFNgamma) and lipopolysaccharide (LPS) significantly inhibited synthesis of PAF acetylhydrolase, whereas other cytokines, including IFNalpha, interleukin (IL) 1alpha, IL4, IL6, tumor necrosis factor-alpha, granulocyte/macrophage colony-stimulating factor, and macrophage colony-stimulating factor, had a smaller or no effect in human monocyte-derived macrophages. Furthermore, transfection of the promoter/reporter construct into macrophage RAW264.7 cells revealed that IFNgamma and LPS decreased the promoter activity by 35% and 50%, respectively, whereas PAF stimulated it by 52% via its receptor. The promoter activity was much lower in monocytic U937 cells compared with the basal level in COS-7 cells, while the activities in P388D1 and RAW264.7 macrophagic cells were considerably higher than the basal level in COS-7 cells. There are multiple regions in the PAF acetylhydrolase promoter that contain responsive elements for signal transducer and activators of transcription-related proteins, and also for myeloid-specific transcription factors. Our data indicate that the opposite of mRNA expression in monocytes versus macrophages is due to inhibition of the promoter activity in the former and activation in the latter cells.

Inflammatory agonists induce cyclooxygenase type 2 expression by human neutrophils

The synthesis of prostanoids is regulated by cyclooxygenases (prostaglandin H synthases), which catalyze the conversion of arachidonic acid to PGH2. Cyclooxygenases are the target of aspirin and other nonsteroidal anti-inflammatory agents. In this study, we found that human polymorphonuclear leukocytes (PMNs) express the inducible isoform of cyclooxygenase, COX-2, when stimulated by LPS whereas the protein was not detectable in freshly isolated human PMNs. We also found by immunohistochemical analysis that COX-2 is expressed in PMNs in inflamed human tissues. COX-2 was induced in a time- and concentration-dependent fashion when isolated human PMNs were exposed to LPS; COX-2 was also induced, or its expression was increased, by TNF-alpha, IL-1, and IL-8. Expression of COX-2 in stimulated PMNs was paralleled by secretion of PGE2. The release of PGE2 was blocked by a selective nonsteroidal inhibitor of COX-2, indicating that the enzyme is responsible for the prostanoids produced, and was inhibited by dexamethasone. The time course of LPS-induced COX-2 expression and other features were different in freshly isolated PMNs, monocytes, and macrophages, indicating that COX-2 expression is differentially regulated in myeloid cells of different lineages and degrees of maturation. Consistent with this, IL-4 and IL-10, which suppressed LPS-induced COX-2 expression in monocytes, had little effect on this response by PMNs. These experiments demonstrate that PMNs express COX-2 when appropriately stimulated. Thus, they may actively influence the eicosanoid composition of the acute inflammatory milieu.