The human leukocyte platelet-activating factor receptor. cDNA cloning, cell surface expression, and construction of a novel epitope-bearing analog

Platelet-Activating Factor Promotes the Development of Non-Alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) is a multifaceted clinicopathological syndrome characterised by excessive hepatic lipid accumulation that causes steatosis, excluding alcoholic factors. Platelet-activating factor (PAF), a biologically active lipid transmitter, induces platelet activation upon binding to the PAF receptor. Recent studies have found that PAF is associated with gamma-glutamyl transferase, which is an indicator of liver disease. Moreover, PAF can stimulate hepatic lipid synthesis and cause hypertriglyceridaemia. Furthermore, the knockdown of the PAF receptor gene in the animal models of NAFLD helped reduce the inflammatory response, improve glucose homeostasis and delay the development of NAFLD. These findings suggest that PAF is associated with NAFLD development. According to reports, patients with NAFLD or animal models have marked platelet activation abnormalities, mainly manifested as enhanced platelet adhesion and aggregation and altered blood rheology. Pharmacological interventions were accompanied by remission of abnormal platelet activation and significant improvement in liver function and lipids in the animal model of NAFLD. These confirm that platelet activation may accompany a critical importance in NAFLD development and progression. However, how PAFs are involved in the NAFLD signalling pathway needs further investigation. In this paper, we review the relevant literature in recent years and discuss the role played by PAF in NAFLD development. It is important to elucidate the pathogenesis of NAFLD and to find effective interventions for treatment.

Characterization of an anti-FLAG antibody binding protein in V. cholerae

FLAG-tags are commonly used for protein abundance measurements and for identification of protein-protein interactions in living cells. We have observed that the cholera pathogen Vibrio cholerae encodes a FLAG-antibody-reactive protein and identified this protein as an outer membrane porin, Porin4, which contains a sequence very similar to the 3xFLAG epitope tag. We have demonstrated the binding affinity of the conserved peptide sequence (called Porin 4 tag) in Porin4 against monoclonal anti-FLAG M2 antibody. In addition, we created a porin4 deletion mutant, which can be used for background-less FLAG antibody detection experiments.

Markers of anaphylaxis - a systematic review

Anaphylaxis is defined as severe, life-threatening, systemic or general, immediate reaction of hypersensitivity, with repeatable symptoms caused by the dose of stimulus which is well tolerated by healthy persons. The proper diagnosis, immediate treatment and differential diagnosis are crucial for saving patient's life. However, anaphylaxis is relatively frequently misdiagnosed or confused with other clinical entities. Thus, there is a continuous need for identifying detectable markers improving the proper diagnosis of anaphylaxis. Here we presented currently known markers of anaphylaxis and discussed in more detail the most clinically valuable ones: tryptase, platelet activacting factor (PAF), PAF-acethylhydrolase, histamine and its metabolites.

Metabolomics analysis and modeling suggest a lysophosphocholines-PAF receptor interaction in fibromyalgia

Fibromyalgia Syndrome (FMS) is a chronic disease characterized by widespread pain, and difficult to diagnose and treat. We analyzed the plasma metabolic profile of patients with FMS by using a metabolomics approach combining Liquid Chromatography-Quadrupole-Time Of Flight/Mass Spectrometry (LC-Q-TOF/MS) with multivariate statistical analysis, aiming to discriminate patients and controls. LC-Q-TOF/MS analysis of plasma (FMS patients: n = 22 and controls: n = 21) identified many lipid compounds, mainly lysophosphocholines (lysoPCs), phosphocholines and ceramides. Multivariate statistical analysis was performed to identify the discriminating metabolites. A protein docking and molecular dynamic (MD) study was then performed, using the most discriminating lysoPCs, to validate the binding to Platelet Activating Factor (1-alkyl-2-acetyl-sn-glycero-3-phosphocholine, PAF) Receptor (PAFr). Discriminating metabolites between FMS patients and controls were identified as 1-tetradecanoyl-sn-glycero-3-phosphocholine [PC(14∶0/0∶0)] and 1-hexadecanoyl-sn-glycero-3-phosphocholine [PC(16∶0/0∶0)]. MD and docking indicate that the ligands investigated have similar potentialities to activate the PAFr receptor. The application of a metabolomic approach discriminated FMS patients from controls, with an over-representation of PC(14∶0/0∶0) and PC(16∶0/0∶0) compounds in the metabolic profiles. These results and the modeling of metabolite-PAFr interaction, allowed us to hypothesize that lipids oxidative fragmentation might generate lysoPCs in abundance, that in turn will act as PAF-like bioactivators. Overall results suggest disease biomarkers and potential therapeutical targets for FMS.

Traumatic brain injury-associated coagulopathy

Traumatic injury is a common cause of coagulopathy, primarily due to blood loss and hemodilution secondary to fluid resuscitation. Traumatic injury-associated coagulopathy often follows a course of transition from hyper- to hypocoagulable state exemplified in disseminated intravascular coagulation. The incidence of coagulopathy is significantly higher in patients with traumatic brain injury (TBI), especially those with penetrating trauma compared to injury to the trunk and limbs. This occurs despite the fact that patients with isolated TBI bleed less and receive restricted volume load of fluids. TBI-associated coagulopathy is extensively documented to associate with poor clinical outcomes, but its pathophysiology remains poorly understood. Studies in the past have shown that brain tissue is highly enriched in key procoagulant molecules. This review focuses on the biochemical and cellular characteristics of these molecules and pathways that could make brain uniquely procoagulant and prone to coagulopathy. Understanding this unique procoagulant environment will help to identify new therapeutic targets that could reverse a state of coagulopathy with minimal impacts on hemostasis, a critical requirement for neurosurgical treatments of TBI.

The role of platelet-activating factor in the mammalian female reproductive tract

Platelet-activating factor (PAF) is a potent lipid mediator produced by various cell types of mammals and is involved in an inflammatory-like process with increased vascular permeability. Platelet-activating factor exerts its actions through the activation of specific PAF receptors (PAF-R) found in cells and tissues of the female reproductive tract. The aim of this article was summarized briefly in the current research on the role of PAF in female reproductive functions. Platelet-activating factor has been implicated in processes of ovulation, implantation and parturition because of its angiogenic and growth factor properties. This factor is influenced by ovarian steroid hormones in bringing about changes in the uterus and is a candidate molecule for initial embryo-maternal dialogue. Tissue concentrations of PAF are regulated by the equilibrium between biosynthesis and degradation by PAF-acetylhydrolase (PAF-AH). Antagonists of PAF interfere with ovulation and implantation. Platelet-activating factor, its receptor, and PAF-AH activity play an important role in the maintenance of pregnancy.

Platelet activating factor-induced neuronal apoptosis is initiated independently of its G-protein coupled PAF receptor and is inhibited by the benzoate orsellinic acid

The bioactive lipid mediator platelet activating factor (PAF) is recognized as a key effecter of neuronal apoptosis, yet it is not clear whether its G-protein coupled receptor (PAFR) initiates or prevents PAF neurotoxicity. Using PAFR-/- and congenic wild-type mice, we show that PAF triggers caspase-3/7 activity and neuronal death in PAFR-/- but not PAFR+/+ cerebellar granule neurons. Restoring receptor expression by recombinant adenoviral infection protected cells from PAF challenge. Neuronal death was not mediated by nitric oxide or N-methyl-d-aspartate receptor signaling given that N-nitro-l-arginine methyl ester and MK-801 did not inhibit PAF-induced neuronal loss in PAFR-/- neurons. To intervene in PAFR-independent neurotoxicity, the anti-apoptotic actions of three structurally distinct PAF antagonists were compared to a panel of plant and fungal benzoic acid derivatives. We found that the PAF antagonist BN 52021 but not FR 49175 or CV 3988 inhibited PAFR-independent neurotoxicity. Orsellinic acid, a fungal-derived benzoic acid, blocked PAF-mediated neuronal apoptosis without affecting PAFR-mediated neuroprotection. These findings demonstrate that PAF can transduce apoptotic death in primary neurons independently of its G-protein coupled receptor, that PAFR activation is neuroprotective, and that orsellinic acid effectively attenuates PAFR-independent neuronal apoptosis.

Inverse agonist-induced signaling and down-regulation of the platelet-activating factor receptor

Platelet-activating factor (PAF) is a potent phospholipid mediator involved in several diseases such as allergic asthma, atherosclerosis and psoriasis. The human PAF receptor (PAFR) is a member of the G-protein-coupled receptor family. Following stimulation, PAFR becomes rapidly desensitized; this refractory state is dependent on PAFR phosphorylation, internalization and down-regulation. In this report, we show that the PAFR inverse agonist, WEB2086, can induce phosphorylation and down-regulation of PAFR. Using selective inhibitors, we determined that the agonist, PAF, and WEB2086 could induce phosphorylation of PAFR by PKC. Moreover, dominant-negative (DN) mutant of PKC isoforms beta inhibited WEB2086-stimulated PAFR phosphorylation, whereas PAF-stimulated phosphorylation was inhibited by DN PKCalpha and delta. WEB2086 also induced PAFR down-regulation which could be blocked by PKC inhibitors and by DN PKCbeta. WEB2086-induced down-regulation was dynamin-dependent but arrestin-independent. Unlike PAF, WEB2086-stimulated intracellular trafficking of PAFR was independent of Rab5. Specific inhibitors of lysosomal proteases and of proteasomes were both effective in reducing WEB2086-induced PAFR down-regulation, indicating the importance of receptor targeting to both lysosomes and proteasomes in long-term cell desensitization to WEB2086. These results indicate that although both agonists and inverse agonists induce receptor PAFR down-regulation, this may be accomplished through different signal transduction and trafficking pathways.

Flow cytometric detection of tandem repeat mutations induced by various chemical classes

To facilitate detection of genotoxicity from environmental mutagen exposure, we generated an in vitro enhanced green fluorescence protein (EGFP) reactivation assay that quickly and effectively detects frameshift mutations in tandem repeat sequences (TRS). Two murine cell lines, C3H10T1/2 and mismatch repair deficient MC2a, were stably transfected with EGFP reporter plasmids in which the EGFP constructs contain TRS that put the EGFP sequence out of frame. These included several 2, 3, 4, 5 and 6 bp repeat sequences, a control non-repetitive sequence and a human gene sequence containing a 4 bp repeat motif. Transfected cultures were exposed to five model mutagens and carcinogens: hydrogen peroxide (H(2)O(2)), 12-O-tetradecanoyl-phorbol-13-acetate (TPA), benzo-a-pyrene-diol-epoxide (BPDE), ethyl nitrosourea (ENU), 9-aminoacridine (9AA) and two controls: acetone and ethanol. Frameshift mutations resulted in green fluorescent revertants, as determined by flow cytometry, and were confirmed, for 9AA treatments, by sequencing. All five treatments with model agents induced statistically significant sequence- and exposure-dependent responses in MC2a cells and a negative response with the two negative control treatments, acetone and ethanol. Similar responses were seen in a smaller panel of treatments and plasmids in C3H10T1/2 cells. The mutation frequencies were higher in cells transfected with the plasmids containing TRS than those harbouring the control construct lacking repeats. The highest mutation frequencies were observed with H(2)O(2) and 9AA treatments, yielding up to a 50-fold difference between vehicle and highest concentration treatment. ENU, BPDE, and to a lesser extent TPA treatments, also showed a statistically significant exposure response. Results from these experiments reveal that the assay responds robustly to various classes of mutagenic substances, as well as to rodent carcinogens that are inactive in conventional mutation assays, and that responses are not linked to cytotoxicity. This assay is a promising approach for detecting chemically induced frameshifts within certain DNA sequences of interest, but further characterization and validation are required prior to general use in genotoxicity screening.

Involvement of leukotriene B4 receptor 1 signaling in platelet-activating factor-mediated neutrophil degranulation and chemotaxis

Platelet-activating factor (PAF) is a potent lipid mediator of inflammation that can act on human neutrophils. When neutrophils are stimulated with PAF at concentrations greater than 10 nM, a double peak of intracellular calcium mobilization is observed. The second calcium peak observed in PAF-treated neutrophils has already been suggested to come from the production of endogenous leukotriene B4 (LTB4). Here we demonstrate the involvement of endogenous LTB4 production and subsequent activation of the high affinity LTB4 receptor (BLT1) in this second calcium mobilization peak observed after stimulation with PAF. We also show that the second, but not the first peak, could be desensitized by prior exposure to LTB4. Moreover, when neutrophils were pre-treated with pharmacological inhibitors of LTB4 production or with the specific BLT1 antagonist, U75302, PAF-mediated neutrophil degranulation was inhibited by more than 50%. On the other hand, pre-treating neutrophils with the PAF receptor specific antagonist (WEB2086) did not prevent any LTB4-induced degranulation. Also, when human neutrophils were pre-treated with U75302, PAF-mediated chemotaxis was reduced by more than 60%. These data indicate the involvement of BLT1 signaling in PAF-mediated neutrophil activities.

Inverse agonism: more than reverting constitutively active receptor signaling

Seven-transmembrane receptors constitute one of the major families of proteins encoded by the genome. This type of receptor is one of the most important targets of the pharmaceutical industry, and many of the drugs with significant therapeutic action have been shown to be inverse agonists. Concepts regarding the mechanisms by which ligands activate and inactivate receptors are thought to be far more complex that a simple on-off switch. For both drug design and pharmacology principles, it is important to understand the mechanisms by which these drugs achieve their effects. Recent studies have demonstrated intriguing actions of inverse agonists. They have been shown not only to block constitutive responses of receptors but also to activate and regulate seven-transmembrane receptor signaling and trafficking. The activation of pathways by inverse agonists was shown to occur mainly via G-protein-independent mechanisms. These findings emphasize the importance of inverse agonism as a principle of receptor regulation. In this paper, we will review the evidence supporting inverse agonist promoted signaling and trafficking.

Platelet-activating factor (PAF) in internal jugular venous blood of migraine without aura patients assessed during migraine attacks

The aim of the study was to verify the production of PAF and the activity of PAF acetyl-hydrolase (PAF-AH), the enzyme involved in the catabolism of this phospholipid mediator, in migraine attacks. Their levels were determined during migraine crises in serial samples of internal jugular venous blood taken from five migraine patients without aura, who were admitted to the hospital during the crises. Internal jugular venous blood samples were taken immediately after catheter insertion at 1, 2, and 4 h after attack onset, and within 2 h from its cessation. PAF was purified by high-performance liquid chromatography (HPLC) and determined by radioimmunoassay method. The enzymatic activity of PAF-AH was measured by reverse-phase HPLC, based on the derivatization with 7-diethylaminocoumarin-3-carbonylazide. In the internal jugular venous blood of migraine patients without aura (MO), an increase was observed in PAF levels, which was already evident at the time of catheter insertion (885.6 +/- 82.8) and at the first hour (868.4 +/- 65.24) (ANOVA: P < 0.0001). PAF levels remained elevated through the second (746.8 +/- 82.95), fourth (700.6 +/- 34.93) and sixth hours (644.4 +/- 42.85), and then decreased at the end of the attack, reaching levels significantly lower than those measured at the time of catheter insertion (565.5 +/- 38.34). The activity of PAF-AH showed an opposite trend with higher values at the first hour and significantly lower values at the second and fourth hours from the beginning of the migraine attack (ANOVA: P < 0.02). The increased production of PAF may account for persistent platelet activation during migraine crises, even in the presence of an increased production of nitric oxide (NO) end-products which, on the other hand, should instead intervene in counteracting and limiting platelet activation. Potential sources of PAF production are the endothelial cells from cerebral vessels, stimulated by trigeminal neuropeptides, platelets themselves, and mast cells, as suggested by the neurogenic inflammation model.

Organization of porcine platelet-activating factor receptor gene

Four exons of porcine platelet-activating factor receptor (PAFr) gene expressing transcript 1 and transcript 2 were determined previously. In this study, we cloned and sequenced a new exon, which also initiates transcript 2, and determined the order of 5 exons in the PAFr gene. In addition, two other variants of transcript 2 were found, but no additional variants of transcript 1 were found. Transcript 2 has three variants that were detected in porcine tissues other than in white blood cells.

Synaptic lipid signaling: significance of polyunsaturated fatty acids and platelet-activating factor

Neuronal cellular and intracellular membranes are rich in specialized phospholipids that are reservoirs of lipid messengers released by specific phospholipases and stimulated by neurotransmitters, neurotrophic factors, cytokines, membrane depolarization, ion channel activation, etc. Secretory phospholipases A2 may be both intercellular messengers and generators of lipid messengers. The highly networked nervous system includes cells (e.g., astrocytes, oligodendrocytes, microglial cells, endothelial microvascular cells) that extensively interact with neurons; several lipid messengers participate in these interactions. This review highlights modulation of postsynaptic membrane excitability and long-term synaptic plasticity by cyclooxygenase-2-generated prostaglandin E2, arachidonoyldiacylcylglycerol, and arachidonic acid-containing endocannabinoids. The peroxidation of docosahexaenoic acid (DHA), a critical component of excitable membranes in brain and retina, is promoted by oxidative stress. DHA is also the precursor of enzyme-derived, neuroprotective docosanoids. The phospholipid platelet-activating factor is a retrograde messenger of long-term potentiation, a modulator of glutamate release, and an upregulator of memory formation. Lipid messengers modulate signaling cascades and contribute to cellular differentiation, function, protection, and repair in the nervous system. Lipidomic neurobiology will advance our knowledge of the brain, spinal cord, retina, and peripheral nerve function and diseases that affect them, and new discoveries on networks of signaling in health and disease will likely lead to novel therapeutic interventions.

Trafficking, ubiquitination, and down-regulation of the human platelet-activating factor receptor

Platelet-activating factor (PAF) is a potent phospholipid mediator involved in various disease states such as allergic asthma, atherosclerosis and psoriasis. The human PAF receptor (PAFR) is a member of the G protein-coupled receptor family. Following PAF stimulation, cells become rapidly desensitized; this refractory state can be maintained for hours and is dependent on PAFR phosphorylation, internalization, and down-regulation. In this report, we characterized ligand-induced, long term PAFR desensitization, and pathways leading to its degradation. Some GPCRs are known to be targeted to proteasomes for degradation while others traffic via the early/late endosomes toward lysosomes. Specific inhibitors of lysosomal proteases and inhibitors of the proteasome were effective in reducing the ligand-induced PAFR down-regulation by 40 and 25%, respectively, indicating the importance of receptor targeting to both lysosomes and proteasomes in long term cell desensitization to PAF. The effects of the proteasome and lysosomal protease inhibitors were additive and, together, completely blocked ligand-induced degradation of PAFR. Using dominant-negative Rab5 and 7 and colocalization of the PAFR with the early endosome autoantigen I (EEAI) or transferrin, we confirmed that ligand-induced PAFR down-regulation was Rab5/7-dependent and involved lysosomal degradation. In addition, we also demonstrated that PAFR was ubiquitinated in an agonist-independent manner. However, a dominant negative ubiquitin ligase (NCbl) reduced PAFR ubiquitination and inhibited ligand-induced but not basal receptor degradation. Our results indicate that PAFR degradation can occur via both the proteasome and lysosomal pathways and ligand-stimulated degradation is ubiquitin-dependent.

Platelet-activating factor activates mitogen-activated protein kinases, inhibits proliferation, induces differentiation and suppresses the malignant phenotype of human colon carcinoma cells

Recent studies suggest that the action of platelet-activating factor (PAF), a potent phospholipid modulator of allergic and inflammatory reactions, is diverse and functions as a modulator of a variety of physiological and pathological events in many cell types and tissues. Its role (if any) in modulating the proliferation, transformation and/or differentiation of epithelial colonic cells, however, is not known. In this study, we showed that PAF is biologically active in epithelial-derived human colon carcinoma cells with different phenotypic properties. These cells expressed the PAF receptor. PAF activated three prominent mitogen-activated protein kinase modules (ERK, p38MAPK and Jun N-terminal kinases) in these cells, inhibited proliferation and induced differentiation (measured by the induction of Waf1/p21 and the induction of the differentiation-related marker CEA). The net effect of PAF treatment was the suppression of malignant cell behavior (measured by anchorage-independent growth and cellular invasion). It is concluded that PAF is a modulator of proliferation and differentiation in human epithelial-derived colon carcinoma cells.

Platelet-activating factor receptor

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active phospholipid mediator. Although PAF was named for its potential to induce platelet aggregation, intense investigations have elucidated potent biological actions of PAF in a broad range of cell types and tissues. PAF acts by binding to a unique G-protein-coupled seven transmembrane receptor, and activates multiple intracellular signaling pathways. In the last decade, we have identified the PAF receptor structures, intracellular signaling mechanisms, and genomic organizations. Recently, we found a single nucleotide polymorphism of the human PAF receptor (A224D) with an allele frequency of 7.8% in Japanese. Cells expressing this receptor exhibited the reduced cellular signaling, although the binding parameters remain unchanged. We have established two different types of genetically altered mice, i.e. PAF receptor-overexpressing mouse and PAF receptor-deficient mouse. These mutant mice provide a novel and specific approach for identifying the pathophysiological and physiological functions of PAF in vivo. This review focuses on phenotypes of these mutant mice and summarizes the previous reports regarding PAF and PAF receptor.

Phosphorylation of STAT-3 in response to basic fibroblast growth factor occurs through a mechanism involving platelet-activating factor, JAK-2, and Src in human umbilical vein endothelial cells. Evidence for a dual kinase mechanism

Platelet-activating factor (PAF) is a potent proinflammatory phospholipid with multiple pathological and physiological effects. We have shown that basic fibroblast growth factor (bFGF) supplementation induces rapid proliferation of human umbilical vein endothelial cells (HUVEC), which is reduced upon removal of bFGF or by bFGF immunoneutralization. The PAF receptor antagonist LAU-8080 inhibited bFGF-stimulated HUVEC proliferation, indicating the involvement of PAF in the bFGF-mediated signaling of HUVEC. Although FGF receptor phosphorylation was not affected by LAU-8080, the bFGF-mediated prolonged phosphorylation, and activation of Erk-1 and -2 were attenuated. Phosphorylation of STAT-3 was observed in the presence of PAF or bFGF, which was attenuated by PAFR antagonists. PAF-induced STAT-3 phosphorylation observed in HUVEC pretreated with either Src inhibitor PP1 or JAK-2 inhibitor AG-490 indicated (i) immediate (1 min) phosphorylation of STAT-3 is dependent on Src, (ii) JAK-2-dependent STAT-3 phosphorylation occurs after the delayed (30 min) PAF exposure, and (iii) prolonged (60 min) STAT-3 phosphorylation may be either through Src and/or JAK-2. Attenuation of the STAT-3 phosphorylation by the PAFR antagonists indicated signaling through the PAF receptor. Taken together, these findings suggest the production of PAF is important for bFGF-mediated signaling and that a dual kinase mechanism is involved in the PAF-mediated signal transduction cascade.

A pivotal role of cytosolic phospholipase A(2) in bleomycin-induced pulmonary fibrosis

Pulmonary fibrosis is an interstitial disorder of the lung parenchyma whose mechanism is poorly understood. Potential mechanisms include the infiltration of inflammatory cells to the lungs and the generation of pro-inflammatory mediators. In particular, idiopathic pulmonary fibrosis is a progressive and fatal form of the disorder characterized by alveolar inflammation, fibroblast proliferation and collagen deposition. Here, we investigated the role of cytosolic phospholipase A(2) (cPLA(2)) in pulmonary fibrosis using cPLA(2)-null mutant mice, as cPLA(2) is a key enzyme in the generation of pro-inflammatory eicosanoids. Disruption of the gene encoding cPLA(2) (Pla2g4a) attenuated IPF and inflammation induced by bleomycin administration. Bleomycin-induced overproduction of thromboxanes and leukotrienes in lung was significantly reduced in cPLA(2)-null mice. Our data suggest that cPLA(2) has an important role in the pathogenesis of pulmonary fibrosis. The inhibition of cPLA(2)-initiated pathways might provide a novel therapeutic approach to pulmonary fibrosis, for which no pharmaceutical agents are currently available.

Receptor-dependent metabolism of platelet-activating factor in murine macrophages

Degradation of platelet-activating factor (PAF) was examined by incubating PAF with macrophages from PAF receptor-deficient mice. The degradation rate was halved as compared with wild-type mice. The reduction of the rate was comparable with the presence of a PAF antagonist WEB 2086 in wild-type cells. PAF was internalized rapidly (t(12) approximately 1 min) into wild-type macrophages. The PAF internalization was inhibited by the treatment of 0.45 m sucrose but was not affected by phorbol 12-myristate 13-acetate, suggesting that PAF internalizes into macrophages with its receptor in a clathrin-dependent manner. Internalized PAF was degraded into lyso-PAF with a half-life of 20 min. Treatment of concanavalin A inhibited the conversion of PAF into lyso-PAF, suggesting that uptake of PAF enhances PAF degradation. Lyso-PAF was subsequently metabolized into 1-alkyl-2-acyl-phosphatidylcholine. In addition, release of PAF acetylhydrolase from macrophages was enhanced when wild-type macrophages were stimulated with PAF but not from macrophages of PAF receptor-deficient mice. Thus, the PAF stimulation of macrophages leads to its degradation through both intracellular and extracellular mechanisms.

Agonist-induced internalization of the platelet-activating factor receptor is dependent on arrestins but independent of G-protein activation. Role of the C terminus and the (D/N)PXXY motif

As with most G-protein-coupled receptors, repeated agonist stimulation of the platelet-activating factor receptor (PAFR) results in its desensitization, sequestration, and internalization. In this report, we show that agonist-induced PAFR internalization is independent of G-protein activation but is dependent on arrestins and involves the interaction of arrestins with a limited region of the PAFR C terminus. In cotransfected COS-7 cells, both arrestin-2 and arrestin-3 could be coimmunoprecipitated with PAFR, and agonist stimulation of PAFR induced the translocation of both arrestin-2 and arrestin-3. Furthermore, coexpression of arrestin-2 with PAFR potentiated receptor internalization, whereas agonist-induced PAFR internalization was inhibited by a dominant negative mutant of arrestin-2. The coexpression of a minigene encoding the C-terminal segment of the receptor abolished PAF-induced arrestin translocation and inhibited PAFR internalization. Using C terminus deletion mutants, we determined that the association of arrestin-2 with the receptor was dependent on the region between threonine 305 and valine 330 because arrestin-2 could be immunoprecipitated with the mutant PAFRstop330 but not PAFRstop305. Consistently, stop330 could mediate agonist-induced arrestin-2 translocation, whereas stop305 could not. Two other deletion mutants with slightly longer regions of the C terminus, PAFRstop311 and PAFRstop317, also failed to induce arrestin-2 translocation. Finally, the PAFR mutant Y293A, containing a single substitution in the putative internalization motif DPXXY in the seventh transmembrane domain (which we had shown to be able to internalize but not to couple to G-proteins) could efficiently induce arrestin translocation. Taken together, our results indicate that ligand-induced PAFR internalization is dependent on arrestins, that PAFR can associate with both arrestin-2 and -3, and that their translocation involves interaction with the region of residues 318-330 in the PAFR C terminus but is independent of G-protein activation.

Airway hyperresponsiveness in transgenic mice overexpressing platelet activating factor receptor is mediated by an atropine-sensitive pathway

Platelet activating factor (PAF) is a potent mediator potentially involved in the pathogenesis of inflammatory disorders, including bronchial asthma. Recently, transgenic mice overexpressing the PAF receptor (PAFR) gene have been established, and exhibit bronchial hyperresponsiveness, one of the cardinal features of asthma. To elucidate the molecular and pathophysiologic mechanisms underlying PAF-associated bronchial hyperreactivity, we studied airway responsiveness to methacholine (MCh) and serotonin (5-hydroxytryptamine; 5-HT) in PAFR-transgenic mice. In addition, we examined the role of the muscarinic receptor in PAF-induced responses and the binding activities of the muscarinic receptor. The PAFR-transgenic mice exhibited hyperresponsiveness to MCh and PAF; however, no significant differences in 5-HT responsiveness were observed between the control and PAFR-transgenic mice. The administration of atropine significantly blocked PAF-induced responses in PAFR-transgenic mice. There were no differences between the two phenotypes in the binding activities of muscarinic receptor. Morphometric analyses demonstrated that PAFR overexpression did not affect airway structure. These findings suggest that the muscarinic pathway may have a key role in airway hyperresponsiveness associated with PAFR gene overexpression. More generally, PAFR-transgenic mice may provide appropriate models for study of the molecular mechanisms underlying PAF-associated diseases.

Comparison of the coding sequence of the platelet-activating factor receptor gene in three species

The actions of platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) are mediated through the PAF receptor (PAFr), which is a member of G-protein coupled superfamily of receptors. Our laboratory has data showing PAF has a role(s) in reproduction in domestic animals. Porcine, bovine and caprine PAFr genes cloned in BAC vectors were sequenced. Each PAFr coding sequence (cds) in these three species is 1029 nucleotides long and contains no intervening sequences. The deduced amino acid sequences (AAS) appear to contain seven putative transmembrane domains with an extracellular N-terminus in each species. There is a common glycosylation site at the fourth asparagine residue of N-terminus. In the tail of each deduced amino acid sequence, five to six serines and five threonine residues could act as phosphorylation sites, which play an important role in rapid receptor desensitization. The degree of homology of the three species is from 89 to 96% in nucleotide sequences (NtS), and 87-96% in identities (I) and 94-97% in positives (P) in amino acid sequences (AAS). The degree of homology with human, guinea pig, mouse and rat is 84-87, 82-88 and 83-88% in NtS, 77-84 (I) or 85-90 (P), 77-84 (I) or 85-90 (P) and 75-83 (I) or 87-90% (P) in AAS for caprine, bovine and pig, respectively. Southern blotting results suggested that the PAFr gene exists as a single copy in the genome of pig.

Oxidized low density lipoproteins downregulate LPS-induced platelet-activating factor receptor expression in human monocyte-derived macrophages: implications for LPS-induced nuclear factor-kappaB binding activity

Monocytes/macrophages play a key role in atherogenesis due to their inflammatory properties including formation of lipid mediators such as platelet-activating-factor (PAF). We investigated the effect of oxidized low-density lipoprotein (oxLDL) on lipopolysaccharide (LPS)-induced PAF receptor (PAF-R) expression in human macrophages and the implication of the nuclear factor (NF)-kappaB in this regulation. LPS-treatment (1 microg.mL(-1)) of macrophages increased PAF binding and PAF-R mRNA expression by 56% (P < 0.05) and twofold (P < 0.01), respectively. In contrast, highly oxidized low-density lipoprotein [ox24hLDL; 100 microg.mL(-1); thiobarbituric acid reacting substances: 31 +/- 3 nmol equiv. malondialdehyde (MDA).mg protein LDL-1] diminished PAF-R expression (-69%; P < 0.05) and mRNA level (- 45%; P < 0.01). LPS pretreatment induced the activated form of p65 in the nuclear compartment of macrophages (detected by Western blotting) and NF-kappaB binding activity (by electrophoretic mobility shift assay). Treatment of macrophages with ox24hLDL suppressed the LPS-induced binding of NF-kappaB to DNA. In addition, treatment of macrophages with lysophosphatidylcholine (2 and 10 microM), a major component of oxLDL, inhibited the LPS-induced NF-kappaB binding to DNA and reduced PAF binding by 30 and 70%, respectively. In conclusion, oxLDL may downregulate PAF-R expression in human macrophages by inhibiting LPS-induced NF-kappaB binding to DNA.

Platelet-activating factor (PAF)-like activity, localization of PAF receptor (PAF-R) and PAF-acetylhydrolase (PAF-AH) activity in bovine endometrium at different stages of the estrous cycle and early pregnancy

PAF-like activity in the endometrium increased from days 2-4 to day 12 and day 20 in both cyclic and pregnant cows. There was an increase in platelet aggregation induced by PAF-like activity in the endometrium of pregnant animals on day 20 as compared to cyclic animals at the same point in time. Two major bands of PAF-R protein at 67 kDa and 97 kDa were detected by Western blot analysis. PAF-R was localized mainly in luminal and glandular epithelium of the endometrium, but the staining was markedly increased in the endometrium of pregnant cows on day 20 compared to cyclic animals on the same day. The purified PAF-AH from the endometrium is similar to in plasma. In cyclic cattle, no changes in PAF-AH activity of endometrium were observed, whereas a decrease in enzyme activity occurred in pregnant cows on day 20 as compared to cyclic animals on the same day. We suggest that the bovine endometrium produces PAF-like activity, expresses the PAF-R and possesses a PAF-AH activity which varies during pregnancy.

Fluorometric detection of platelet activating factor receptor in cultured oviductal epithelial and stromal cells and endometrial stromal cells from bovine at different stages of the oestrous cycle and early pregnancy

During the oestrous cycle and early pregnancy, the oviduct and uterus undergo a variety of morphological and physiological modifications in which the platelet activating factor receptor (PAF-R) plays an important role. PAF-R levels were quantified in bovine oviductal epithelial and stromal cells and endometrial stromal cells at days 2 to 4, 12, and 20 of the estrous cycle and during early pregnancy. Cells were grown in vitro and their intracellular PAF-R concentration was measured by flow cytometry using a polyclonal anti-PAF-R antibody system. A significant increase (P < 0.05) in the portion of PAF-R-positive oviductal epithelial and stromal cells was detected in both non-pregnant and pregnant cattle on days 2 to 4 in comparison to day 12 and 20. In endometrial stromal cells derived from day 20 pregnant bovine, a significant increase (P < 0.05) in PAF-R staining was observed in comparison to the day 20 non-pregnant and days 2 to 4 or 12 pregnant and non-pregnant animals. The PAF-R was detected in oviductal cells by using immunoblotting and immuno-gold postembedding method. Positive binding of the anti-PAF-R antibody was found on the cell membrane and in the cytoplasm. We concluded that the increased PAF-R concentration measured in cultured oviductal epithelial and stromal cells of cyclic and pregnant heifers on days 2 to 4 was hormonally regulated. The increased PAF-R in endometrial stromal cells on day 20 of pregnant heifers was a pregnancy-specific effect and may mediate a local increase in endometrial vascular permeability known to precede the implantation.

Acid exposure stimulates the adherence of Streptococcus pneumoniae to cultured human airway epithelial cells: effects on platelet-activating factor receptor expression

To examine the effects of acid exposure on the adherence of Streptococcus pneumoniae to cultured human tracheal epithelial cells, cells were exposed to acid at various pH levels, and various concentrations of S. pneumoniae were added to the culture medium. The number of S. pneumoniae adhering to cultured human tracheal epithelial cells increased after acid exposure. Y-24180, a specific inhibitor of the receptor for the platelet-activating factor (PAF) and PAF itself decreased the number of S. pneumoniae adhering to cultured human tracheal epithelial cells after acid exposure. Acid exposure increased the activation of transcription factor nuclear factor (NF)-kappa B and the expression of protein and messenger RNA (mRNA) of the PAF receptor. The pyrrolidine derivative of dithiocarbamate (PDTC), an inhibitor of NF-kappa B, also decreased the number of S. pneumoniae adhering to the cultured human tracheal epithelial cells after acid exposure. Acid exposure increased the content of interleukin (IL)-1 alpha and IL-1 beta in the culture supernatants, but monoclonal antibodies to IL-1 alpha and IL-1 beta failed to inhibit the increased number of S. pneumoniae adhering to cultured human tracheal epithelial cells after acid exposure. These findings suggest that acid exposure stimulates the adherence of S. pneumoniae to the airway epithelial cells via increases in PAF receptors. Increases in PAF receptor expression may be, in part, mediated via activation of transcription factors and subsequent PAF receptor mRNA expression by acid exposure. Increased adherence of S. pneumoniae may be one of the reasons why pneumonia develops after gastric juice aspiration.

Membrane raft association of CD47 is necessary for actin polymerization and protein kinase C theta translocation in its synergistic activation of T cells

CD47 is a ubiquitously expressed membrane protein with an extracellular Ig domain and a multiple membrane-spanning domain that can synergize with antigen to induce interleukin (IL)-2 secretion by T lymphocytes. Ligation of CD47 induced actin polymerization and increased protein kinase Ctheta (PKCtheta) association with the cytoskeleton independent of antigen receptor ligation, but ligation of mutant forms of the molecule missing either the Ig domain or the multiple membrane-spanning domain did not. Simultaneous ligation of CD47 and CD3 led to additive effects on F-actin and synergistic effects on PKCtheta cytoskeletal association. Disruption of membrane rafts by removal of cholesterol with cyclodextrin blocked CD47-induced actin polymerization, and mutant forms of CD47 that localized poorly to rafts failed to effect cytoskeletal rearrangement. However, raft association alone was not sufficient, because a raft-localized CD47 Ig domain bound to the membrane by a glycan phosphoinositol anchor was unable to induce actin polymerization. A mutant form of CD47 without its Ig domain that did not induce actin polymerization or localize to rafts still enhanced T cell receptor (TCR)-dependent tyrosine phosphorylation of PLCgamma and associated Ca(2+) signaling but did not augment IL-2 secretion. Thus, CD47 synergy with TCR to increase [Ca(2+)](i) is independent of actin and rafts but is insufficient to explain CD47 cooperation with TCR in IL-2 synthesis. Full synergy with TCR requires CD47 localization to membrane rafts where ligation leads to TCR-independent signals causing actin polymerization and PKCtheta translocation.

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.

Role of platelet-activating factor in cardiovascular pathophysiology

Platelet-activating factor (PAF) is a phospholipid mediator that belongs to a family of biologically active, structurally related alkyl phosphoglycerides. PAF acts via a specific receptor that is coupled with a G protein, which activates a phosphatidylinositol-specific phospholipase C. In this review we focus on the aspects that are more relevant for the cell biology of the cardiovascular system. The in vitro studies provided evidence for a role of PAF both as intercellular and intracellular messenger involved in cell-to-cell communication. In the cardiovascular system, PAF may have a role in embryogenesis because it stimulates endothelial cell migration and angiogenesis and may affect cardiac function because it exhibits mechanical and electrophysiological actions on cardiomyocytes. Moreover, PAF may contribute to modulation of blood pressure mainly by affecting the renal vascular circulation. In pathological conditions, PAF has been involved in the hypotension and cardiac dysfunctions occurring in various cardiovascular stress situations such as cardiac anaphylaxis and hemorrhagic, traumatic, and septic shock syndromes. In addition, experimental studies indicate that PAF has a critical role in the development of myocardial ischemia-reperfusion injury. Indeed, PAF cooperates in the recruitment of leukocytes in inflamed tissue by promoting adhesion to the endothelium and extravascular transmigration of leukocytes. The finding that human heart can produce PAF, expresses PAF receptor, and is sensitive to the negative inotropic action of PAF suggests that this mediator may have a role also in human cardiovascular pathophysiology.

Deficient homologous desensitization of formyl peptide receptors stably expressed in undifferentiated HL-60 cells

The ability of formyl peptide receptors (FPRs) stably expressed in undifferentiated HL-60 cells to undergo ligand-induced desensitization was compared with their ability in normal and vector-transfected HL-60 cells following granulocyte differentiation with DMSO. fMet-Leu-Phe failed to induce uncoupling of FPRs from G-proteins in FPR-transfected cells, whereas uncoupling was induced in differentiated HL-60 cells and differentiated vector-transfected HL-60 cells, as determined by ligand-stimulated guanosine 5'-(gamma-thio)triphosphate (GTPgammaS) binding and GTPgammaS inhibition of fMet-Leu-Phe binding to isolated membranes. Immunoprecipitation of Galpha(i2) from solubilized, azidoanalide (AA-gammaGTP) photolabeled membranes showed that receptors in desensitized FPR-transfected HL-60 cells remained coupled to Galpha(i2), whereas desensitized receptors in differentiated HL-60 cell membranes were uncoupled from Galpha(i2). As determined by immunoblotting, Galpha(i2) expression was similar in undifferentiated and differentiated HL-60 cells and FPR-transfected cells. Ligand-stimulated receptor internalization and desensitization of calcium redistribution were similar in all three groups of cells. Immunoblotting also indicated that G-protein-coupled receptor kinases (GRKs) 2 and 4 were present in undifferentiated FPR-transfected HL-60 cells at 50% of the level seen in differentiated HL-60 cells. However, differentiation did not increase GRK2 or GRK4 expression, indicating that differences in GRK expression do not explain deficient desensitization. The data indicated that undifferentiated HL-60 cells are unable to induce homologous desensitization of FPRs.

Acute lung injury by sepsis and acid aspiration: a key role for cytosolic phospholipase A2

Adult respiratory distress syndrome (ARDS) is characterized by acute lung injury with a high mortality rate and yet its mechanism is poorly understood. Sepsis syndrome and acid aspiration are the most frequent causes of ARDS, leading to increased lung permeability, enhanced polymorphonuclear neutrophil (PMN) sequestration and respiratory failure. Using a murine model of acute lung injury induced by septic syndrome or acid aspiration, we investigated the role of cytosolic phospholipase A2 (cPLA2) in ARDS. We found that disruption of the gene encoding cPLA2 significantly reduced pulmonary edema, PMN sequestration and deterioration of gas exchange caused by lipopolysaccharide and zymosan administration. Acute lung injury induced by acid aspiration was similarly reduced in mice with a disrupted cpla2 gene. Our observations suggest that cPLA2 is a mediator of acute lung injury induced by sepsis syndrome or acid aspiration. Thus, the inhibition of cPLA2-initiated pathways may provide a therapeutic approach to acute lung injury, for which no pharmaceutical agents are currently effective.

Interferon-γ–induced membrane PAF-receptor expression confers tumor cell susceptibility to NK perforin-dependent lysis

Perforin is known to display a membranolytic activity on tumor cells. Nevertheless, perforin release during natural killer (NK)–cell activation is not sufficient to induce membrane target-cell damage. On the basis of the ability of perforin to interact with phospholipids containing a choline phosphate headgroup, we identify the platelet-activating factor (PAF) and its membrane receptor as crucial components in tumor cell killing activity of human resting NK cells. We demonstrate for the first time that upon activation, naive NK cells release the choline phosphate–containing lysolipid PAF, which binds to perforin and acts as an agonist on perforin-induced membrane damage. PAF is known to incorporate cell membranes using a specific receptor. Here we show that interferon-γ (IFN–γ) secreted from activated NK cells ends in PAF-receptor expression on perforin-sensitive K562 cells but not on perforin-resistant Daudi cells. In order to prove the capacity of PAF to interact simultaneously with its membrane PAF receptor and with perforin, we successfully co-purified the 3 components in the presence of bridging PAF molecules. The functional activity of this complex was further examined. The aim was to determine whether membrane PAF-receptor expression on tumor cells, driven to express this receptor, could render them sensitive to the perforin lytic pathway. The results confirmed that transfection of the PAF-receptor complementary DNA into major histocompatibility complex class I and Fas-receptor negative tumor cells restored susceptibility to naive NK cells and perforin attack. Failure of IFN-γ to induce membrane PAF receptor constitutes the first described mechanism for tumor cells to resist the perforin lytic pathway.

Interferon-γ–induced membrane PAF-receptor expression confers tumor cell susceptibility to NK perforin-dependent lysis

Perforin is known to display a membranolytic activity on tumor cells. Nevertheless, perforin release during natural killer (NK)–cell activation is not sufficient to induce membrane target-cell damage. On the basis of the ability of perforin to interact with phospholipids containing a choline phosphate headgroup, we identify the platelet-activating factor (PAF) and its membrane receptor as crucial components in tumor cell killing activity of human resting NK cells. We demonstrate for the first time that upon activation, naive NK cells release the choline phosphate–containing lysolipid PAF, which binds to perforin and acts as an agonist on perforin-induced membrane damage. PAF is known to incorporate cell membranes using a specific receptor. Here we show that interferon-γ (IFN–γ) secreted from activated NK cells ends in PAF-receptor expression on perforin-sensitive K562 cells but not on perforin-resistant Daudi cells. In order to prove the capacity of PAF to interact simultaneously with its membrane PAF receptor and with perforin, we successfully co-purified the 3 components in the presence of bridging PAF molecules. The functional activity of this complex was further examined. The aim was to determine whether membrane PAF-receptor expression on tumor cells, driven to express this receptor, could render them sensitive to the perforin lytic pathway. The results confirmed that transfection of the PAF-receptor complementary DNA into major histocompatibility complex class I and Fas-receptor negative tumor cells restored susceptibility to naive NK cells and perforin attack. Failure of IFN-γ to induce membrane PAF receptor constitutes the first described mechanism for tumor cells to resist the perforin lytic pathway.

Platelet-activating factor receptors in lamb lungs are downregulated immediately after birth

Platelet-activating factor (PAF) is a phospholipid with diverse biological functions mediated by a G protein-coupled receptor. We determined PAF receptor binding in lung membranes of four groups of perinatal lambs. Membrane protein (100 microg/ml) was incubated for 60 min at 30 degrees C with 0.5-24 nM of acetyl-[(3)H]PAF in 30 mM Tris buffer, pH 7.2, containing 0.25% BSA, 10 mM MgCl(2), and 125 mM choline chloride. PAF bound to membrane was isolated and quantified by scintillation spectrometry, followed with Scatchard analysis for receptor density (B(max)). The B(max) (means +/- SE, fmol/mg protein) were 445.8 +/- 12.3, 244.2 +/- 3.3, 250.6 +/- 3.6, and 419. 9 +/- 8.6 for the fetal, 90-min-old, <1-day-old, and 6- to 12-day-old lambs, respectively. The B(max) for the 90-min-old and <1-day-old lambs were not different but were significantly lower than those of either the term fetal or 6- to 12-day-old lambs. These data show a significant decrease in PAF binding to its receptor and in PAF B(max) in lung membranes of immediate newborn lambs. The dissociation constants (K(D), nM) were 7.7 +/- 0.52, 11.5 +/- 0.34, 6.9 +/- 0.48, and 5.0 +/- 0.53 for fetal, 90-min-old, <1-day-old, and 6- to 12-day-old newborn lamb lungs, respectively. The K(D) of the 90-min-old lamb was the highest of all. PAF receptor gene measured by RT-PCR showed a significant downregulation of PAF receptor gene mRNA in lungs of lambs <1 day old, suggesting a transcriptional regulation of PAF receptor gene expression in the immediate newborn period. We speculate that decreased PAF receptor binding immediately after birth will facilitate the fall in pulmonary vascular resistance in the immediate newborn period.

Platelet-activating factor (PAF) receptor and genetically engineered PAF receptor mutant mice

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active phospholipid mediator. Although PAF was initially recognized for its potential to induce platelet aggregation and secretion, intense investigations have elucidated potent biological actions of PAF in a broad range of cell types and tissues, many of which also produce the molecule. PAF acts by binding to a unique G-protein-coupled seven transmembrane receptor. PAF receptor is linked to intracellular signal transduction pathways, including turnover of phosphatidylinositol, elevation in intracellular calcium concentration, and activation of kinases, resulting in versatile bioactions. On the basis of numerous pharmacological reports, PAF is thought to have many pathophysiological and physiological functions. Recently advanced molecular technics enable us not only to clone PAF receptor cDNAs and genes, but also generate PAF receptor mutant animals, i.e., PAF receptor-overexpressing mouse and PAF receptor-deficient mouse. These mutant mice gave us a novel and specific approach for identifying the pathophysiological and physiological functions of PAF. This review also describes the phenotypes of these mutant mice and discusses them by referring to previously reported pharmacological and genetical data.

Platelet-activating factor mediates acid-induced lung injury in genetically engineered mice

Adult respiratory distress syndrome (ARDS) is an acute lung injury of high mortality rate, and the molecular mechanisms underlying it are poorly understood. Acid aspiration-induced lung injury is one of the most common causes of ARDS, characterized by an increase in lung permeability, enhanced polymorphonuclear neutrophil (PMN) sequestration, and respiratory failure. Here, we investigated the role of platelet-activating factor (PAF) and the PAF receptor (PAFR) gene in a murine model of acid aspiration-induced lung injury. Overexpression of the PAFR gene in transgenic mice enhanced lung injury, pulmonary edema, and deterioration of gas exchange caused by HCl aspiration. Conversely, mice carrying a targeted disruption of the PAFR gene experienced significantly less acid-induced injury, edema, and respiratory failure. Nevertheless, the efficiency of PMN sequestration in response to acid aspiration was unaffected by differences in PAFR expression level. The current observations suggest that PAF is involved in the pathogenesis of acute lung injury caused by acid aspiration. Thus, inhibition of this pathway might provide a novel therapeutic approach to acute lung injury, for which no specific pharmaceutical agents are currently available.

In vivo regulation of plasma platelet-activating factor acetylhydrolase during the acute phase response

Plasma platelet-activating factor acetylhydrolase (PAF-AH) hydrolyzes PAF and oxidized phospholipids and is associated with lipoproteins in the circulation. Endotoxin [lipopolysaccharide (LPS)], a potent inducer of the acute phase response (APR), produces marked changes in several proteins that play important roles in lipoprotein metabolism. We now demonstrate that LPS produces a 2.5- to 3-fold increase in plasma PAF-AH activity in Syrian hamsters. The plasma PAF-AH activity is found in the high-density lipoprotein (HDL) fraction and is increased threefold with LPS treatment despite a decrease in plasma HDL levels, indicating that plasma PAF-AH activity is increased per HDL particle. LPS markedly increased PAF-AH mRNA levels in liver, spleen, lung, and small intestine. The maximal increase in plasma PAF-AH activity and mRNA expression in liver and spleen is seen 24 h after LPS treatment. Both tumor necrosis factor and interleukin-1 modestly increased plasma PAF-AH activity and mRNA levels in liver and spleen, suggesting that they may partly mediate the effect of LPS on PAF-AH. Surgical removal of spleen had no effect on basal or LPS-induced plasma PAF-AH activity, suggesting that spleen per se may not contribute to plasma PAF-AH activity. Finally, LPS, turpentine and zymosan increased plasma PAF-AH activity in mice and/or rats, indicating that multiple APR inducers upregulate plasma PAF-AH and this effect is consistent across different rodent species. Taken together, our results indicate that plasma PAF-AH activity and mRNA expression is markedly upregulated during the host response to infection and inflammation. An increase in plasma PAF-AH may enhance the degradation of PAF as well as alter the structure and function of HDL during infection and inflammation.

Internalization of the TXA2 receptor alpha and beta isoforms. Role of the differentially spliced cooh terminus in agonist-promoted receptor internalization

Thromboxane A2 (TXA2) potently stimulates platelet aggregation and smooth muscle constriction and is thought to play a role in myocardial infarction, atherosclerosis, and bronchial asthma. The TXA2 receptor (TXA2R) is a member of the G protein-coupled receptor family and is found as two alternatively spliced isoforms, alpha (343 residues) and beta (407 residues), which share the first 328 residues. In the present report, we demonstrate by enzyme-linked immunosorbent assay and immunofluorescence microscopy that the TXA2Rbeta, but not the TXA2Ralpha, undergoes agonist-induced internalization when expressed in HEK293 cells as well as several other cell types. Various dominant negative mutants were used to demonstrate that the internalization of the TXA2Rbeta is dynamin-, GRK-, and arrestin-dependent in HEK293 cells, suggesting the involvement of receptor phosphorylation and clathrin-coated pits in this process. Interestingly, the agonist-stimulated internalization of both the alpha and beta isoforms, but not of a mutant truncated after residue 328, can be promoted by overexpression of arrestin-3, identifying the C-tails of both receptors as necessary in arrestin-3 interaction. Simultaneous mutation of two dileucine motifs in the C-tail of TXA2Rbeta did not affect agonist-promoted internalization. Analysis of various C-tail deletion mutants revealed that a region between residues 355 and 366 of the TXA2Rbeta is essential for agonist-promoted internalization. These data demonstrate that alternative splicing of the TXA2R plays a critical role in regulating arrestin binding and subsequent receptor internalization.

Heterogeneous platelet-activating factor (PAF) receptors and calcium increase in platelets and macrophages

We used the increase in cytosolic Ca2+ levels, [Ca2+]i, as a way to characterize PAF (platelet-activating factor, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) receptors in human platelets and rat and human macrophages. [Ca2+] was measured by means of the fluorescent probe fura-2/acetoxymethylester. PAF recognized heterogeneous receptors in human macrophages only (curve slope <1). The PAF antagonist SCH 37370 (1-acetyl-4(8-chloro-5,6-dihydro-11H-benzo[5.6]cyclohepta[1,2-b]pyridine -11-ylidine)piperidine) abolished [Ca2+]i elevation in human platelets, while in rat and human macrophages the maximal inhibition was 76% and 85%, respectively. On the contrary, the antagonist WEB 2086 (3-[4-(2-chlorophenyl)-9-methyl-6Hthieno[3,2-f] [1,2,4]triazolo-[4,3-a] [1,4]-diazepin-2-yl]-1-(4-morpholiny)-1-propanon, apafant) totally inhibited the effect of PAF in both platelets and macrophages. The WEB 2086 concentration-response curves had a slope <1 in the three cell types, indicating interaction with heterogeneous receptors. Accordingly, 3H-WEB 2086 bound to two different classes of sites. Both phases of [Ca2+]i elevation (influx or release) were equally affected by the antagonists. These data support the notions that: 1) PAF receptors are heterogeneous; 2) the two antagonists have a different selectivity toward the receptor subtypes: WEB 2086 recognizes two different receptors both in platelets and in macrophages, while SCH 37370 does not discriminate between receptor subtypes in platelets, and only interacts with one subtype in macrophages; and 3) both SCH 37370 and WEB 2086 display different potencies in rat and human macrophages.

Isolation and characterization of PDE9A, a novel human cGMP-specific phosphodiesterase

We have cloned and characterized the first human isozyme in a new family of cyclic nucleotide phosphodiesterases, PDE9A. By sequence homology in the catalytic domain, PDE9A is almost equidistant from all eight known mammalian PDE families but is most similar to PDE8A (34% amino acid identity) and least like PDE5A (28% amino acid identity). We report the cloning of human cDNA encoding a full-length protein of 593 amino acids, including a 261-amino acid region located near the C terminus that is homologous to the approximately 270-amino acid catalytic domain of other PDEs. PDE9A is expressed in all eight tissues examined as a approximately 2. 0-kilobase mRNA, with highest levels in spleen, small intestine, and brain. The full-length PDE9A was expressed in baculovirus fused to an N-terminal 9-amino acid FLAG tag. Kinetic analysis of the baculovirus-expressed enzyme shows it to be a very high affinity cGMP-specific PDE with a Km of 170 nM for cGMP and 230 microM for cAMP. The Km for cGMP makes PDE9A one of the highest affinity PDEs known. The Vmax for cGMP (4.9 nmol/min/microg recombinant enzyme) is about twice as fast as that of PDE4 for cAMP. The enzyme is about twice as active in vitro in 1-10 mM Mn2+ than in the same concentration of Mg2+ or Ca2+. PDE9A is insensitive (up to 100 microM) to a variety of PDE inhibitors including rolipram, vinpocetine, SKF-94120, dipyridamole, and 3-isobutyl-1-methyl-xanthine but is inhibited (IC50 = 35 microM) by zaprinast, a PDE5 inhibitor. PDE9A lacks a region homologous to the allosteric cGMP-binding regulatory regions found in the cGMP-binding PDEs: PDE2, PDE5, and PDE6.

Molecular cloning and expression of human cGMP-binding cGMP-specific phosphodiesterase (PDE5)

A human PDE5 cDNA has been isolated which contains an open reading frame encoding an 875 amino acid, 100,012 Da polypeptide, the expression of which yields a protein of the predicted size and is capable of hydrolyzing cGMP. The deduced amino acid sequence is very similar (95%) to that of bovine PDE5, and comprises a conserved cGMP-binding domain and catalytic domain. Northern analysis reveals a major and minor transcript of approximately 9 kb and approximately 8 kb respectively, thus indicating the existence of at least two splice variants, the major form being readily detected in bladder, colon, lung, pancreas, placenta, prostate, small intestine, and stomach.

In situ expression of platelet-activating factor (PAF)-receptor gene in rat skin and effects of PAF on proliferation and differentiation of cultured human keratinocytes

Platelet-activating factor (PAF) is a potent lipid mediator that exhibits versatile biologic activities in many diverse systems by binding to a specific cell-surface receptor (PAFR). Although the production of PAF in cultured keratinocytes and fibroblasts has been reported, physiologic roles of this mediator in skin remain unclear. In this study, we examined in situ expression of PAFR gene in rat skin and the effects of PAF on the proliferation and differentiation of cultured human keratinocytes. In rat epidermis, PAFR mRNA expression was found from the basal cells to the granular cells, and strong signals were seen in the stratum spinosum. In cultured human keratinocytes, a 3.8 kb PAFR mRNA expression was demonstrated by northern blotting, and two distinct type transcripts driven by different promoters were detected by reverse transcriptase polymerase chain reaction analysis. Addition of PAF (30-100 nM) to cultured keratinocytes during a growth phase inhibited the proliferation. This effect was receptor dependent, because the inhibition was completely blocked by a PAFR antagonist, WEB 2086 (100 nM). On the other hand, whereas PAF (30-100 nM) alone did not affect the cornified envelope formation during the process of keratinocyte differentiation, WEB 2086 (30-300 nM) accelerated it in a concentration-dependent manner. Addition of PAF (100 nM) reversed the effect of WEB 2086, suggesting that WEB 2086 induced cornification by inhibiting PAF endogeneously produced by keratinocytes in an autocrine manner. Thus, we propose that PAF is an intrinsic regulator of keratinocyte during proliferation and differentiation.