Molecular cloning and characterization of the platelet-activating factor receptor gene expressed in the human heart

Debris-stimulated tumor growth: a Pandora's box?

Current cancer therapies aim at eradicating cancer cells from the body. However, killing cells generates cell “debris” which can promote tumor progression. Thus, therapy can be a double-edged sword. Specifically, injury and debris generated by cancer therapies, including chemotherapy, radiation, and surgery, may offset their benefit by promoting the secretion of pro-tumorigenic factors (e.g., eicosanoid-driven cytokines) that stimulate regrowth and metastasis of surviving cells. The debris produced by cytotoxic cancer therapy can also contribute to a tumor microenvironment that promotes tumor progression and recurrence. Although not well understood, several molecular mechanisms have been implicated in debris-stimulated tumor growth that we review here, such as the involvement of extracellular vesicles, exosomal miR-194-5p, Bax, Bak, Smac, HMGB1, cytokines, and caspase-3. We discuss the cases of pancreatic and other cancer types where debris promotes postoperative tumor recurrence and metastasis, thus offering a new opportunity to prevent cancer progression intrinsically linked to treatment by stimulating resolution of tumor-promoting debris.

Platelet activating factor receptor antagonists improve the efficacy of experimental chemo- and radiotherapy

Platelet activating factor is a lipid mediator of inflammation, and in recent decades, it has emerged as an important factor in tumor outcomes. Platelet activating factor acts by specific binding to its receptor, which is present in both tumor cells and cells that infiltrate tumors. Pro-tumorigenic effects of platelet activating factor receptor in tumors includes promotion of tumor cell proliferation, production of survival signals, migration of vascular cells and formation of new vessels and stimulation of dendritic cells and macrophages suppressor phenotype. In experimental models, blocking of platelet activating factor receptor reduced tumor growth and increased animal survival. During chemotherapy and radiotherapy, tumor cells that survive treatment undergo accelerated proliferation, a phenomenon known as tumor cell repopulation. Work from our group and others showed that these treatments induce overproduction of platelet activating factor-like molecules and increase expression of its receptor in tumor cells. In this scenario, antagonists of platelet activating factor markedly reduced tumor repopulation. Here, we note that combining chemo- and radiotherapy with platelet activating factor antagonists could be a promising strategy for cancer treatment.

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.

Platelet activating factor: the good and the bad in the ischemic/reperfused heart

The present review is focused on the dual role played by platelet-activating factor (PAF) in ischemia and reperfusion (I/R) injury of the heart. Although the involvement of PAF in the pathogenesis of myocardial reperfusion injury is well established, in the last few years it has emerged that very low concentrations of PAF exert cardioprotective effects, comparable to that afforded by ischemic preconditioning (IP). PAF is a potent phosphoglyceride involved in different pathophysiological conditions affecting the cardiovascular system, including the development of myocardial I/R injury. PAF is released from the I/R myocardium in concentrations (1-10 nmol/L) high enough to negatively modulate coronary circulation as well as electrical and contractile activities. PAF may act either directly, via generation of secondary mediators, or through the activation of inflammatory cells like platelets and polymorphonuclear neutrophils, which exacerbate postischemic myocardial injury. The effects of PAF are mediated through specific receptors (PAFRs) that belong to the superfamily of G protein-coupled receptors. Since cardiomyocytes not only produce PAF but also possess PAFRs, it is likely that PAF acts as an autocrine/paracrine mediator. Although the negative effects exerted by high concentrations of PAF are well established, several recent findings from our and other laboratories have demonstrated that very low concentrations (pmol/L) of PAF infused before ischemia induce cardioprotective effects similar to those afforded by IP, and that endogenous PAF production participates in the induction of IP itself. The IP-like action exerted by low concentrations of PAF is due to the activation/phosphorylation of kinases included in the reperfusion injury salvage kinase (RISK) pathway, such as protein kinase C, Akt/PkB and nitric oxide synthase. Together with the activation of mitochondrial K(ATP) channels, these events may allow prevention of mitochondrial permeability transition pores opening at reperfusion. Moreover, the nitric oxide-dependent S-nitrosylation of L-type Ca(2+) channels induced by PAF reduces intracellular Ca(2+) overload.

Significance of lipid mediators in corneal injury and repair

Corneal injury induces an inflammatory reaction and damages the sensory nerves that exert trophic influences in the corneal epithelium. Alterations in normal healing disrupt the integrity and function of the tissue with undesirable consequences, ranging from dry eye and loss of transparency to ulceration and perforation. Lipids play important roles in this complex process. Whereas lipid mediators such as platelet activating factor (PAF) and cyclooxygenease-2 metabolites contribute to tissue damage and neovascularization, other mediators, such as the lipoxygenase (LOX) derivatives from arachidonic acid, 12- and 15-hydroxy/hydroperoxyeicosatetraenoic acids, and lipoxin A4, act as second messengers for epidermal growth factor to promote proliferation and repair. Stimulation of the cornea with pigment epithelial derived factor in the presence of docosahexaenoic acid gives rise to the synthesis of neuroprotectin D1, a derivative of LOX activity, and increases regeneration of corneal nerves. More knowledge about the role that lipids play in corneal wound healing can provide insight into the development of new therapeutic approaches for treating corneal injuries. PAF antagonists, lipoxins, and neuroprotectins can be effective therapeutic tools for maintaining the integrity of the cornea.

Lipid signaling in neural plasticity, brain repair, and neuroprotection

The extensive networking of the cells of the nervous system results in large cell membrane surface areas. We now know that neuronal membranes contain phospholipid pools that are the reservoirs for the synthesis of specific lipid messengers on neuronal stimulation or injury. These messengers in turn participate in signaling cascades that can either promote neuronal injury or neuroprotection. Prostaglandins are synthesized as a result of cyclooxygenase activity. In the first step of the arachidonic acid cascade, the short-lived precursor, prostaglandin H2, is synthesized. Additional steps in the cascade result in the synthesis of an array of prostaglandins, which participate in numerous physiological and neurological processes. Our laboratory recently reported that the membrane polyunsaturated fatty acid, docosahexaenoic acid, is the precursor of oxygenation products now known as the docosanoids, some of which are powerful counter-proinflammatory mediators. The mediator 10,17S-docosatriene (neuroprotectin D1, NPD1) counteracts leukocyte infiltration, NF-kappa activation, and proinflammatory gene expression in brain ischemia-reperfusion and is an apoptostatic mediator, potently counteracting oxidative stress-triggered apoptotic DNA damage in retinal pigment epithelial cells. NPD1 also upregulates the anti-apoptotic proteins Bcl-2 and Bcl-xL and decreases pro-apoptotic Bax and Bad expression. Another biologically active messenger derived from membrane phospholipids in response to synaptic activity is platelet-activating factor (PAF). The tight regulation of the balance between synthesis (via phospholipases) and degradation (via acetylhydrolases) of PAF modulates the functions of this lipid messenger. Under pathological conditions, this balance is tipped, and PAF becomes a proinflammatory mediator and neurotoxic agent. The newly discovered docosahexaenoic acid signaling pathways, as well as other lipid messengers related to synaptic activation, may lead to the clarification of clinical issues relevant to stroke, age-related macular degeneration, spinal cord injury, Alzheimer's disease, and other diseases that include neuroinflammatory components.

Platelet-activating factor induces cardioprotection in isolated rat heart akin to ischemic preconditioning: role of phosphoinositide 3-kinase and protein kinase C activation

Ischemic preconditioning (IP) is a cardioprotective mechanism against myocellular death and cardiac dysfunction resulting from reperfusion of the ischemic heart. At present, the precise list of mediators involved in IP and the pathways of their mechanisms of action are not completely known. The aim of the present study was to investigate the role of platelet-activating factor (PAF), a phospholipid mediator that is known to be released by the ischemic-reperfused heart, as a possible endogenous agent involved in IP. Experiments were performed on Langendorff-perfused rat hearts undergoing 30 min of ischemia followed by 2 h of reperfusion. Treatment with a low concentration of PAF (2 × 10−11 M) before ischemia reduced the extension of infarct size and improved the recovery of left ventricular developed pressure during reperfusion. The cardioprotective effect of PAF was comparable to that observed in hearts in which IP was induced by three brief (3 min) periods of ischemia separated by 5-min reperfusion intervals. The PAF receptor antagonist WEB-2170 (1 × 10−9 M) abrogated the cardioprotective effect induced by both PAF and IP. The protein kinase C (PKC) inhibitor chelerythrine (5 × 10−6 M) or the phosphoinositide 3-kinase (PI3K) inhibitor LY-294002 (5 × 10−5 M) also reduced the cardioprotective effect of PAF. Western blot analysis revealed that following IP treatment or PAF infusion, the phosphorylation of PKC-ε and Akt (the downstream target of PI3K) was higher than that in control hearts. The present data indicate that exogenous applications of low quantities of PAF induce a cardioprotective effect through PI3K and PKC activation, similar to that afforded by IP. Moreover, the study suggests that endogenous release of PAF, induced by brief periods of ischemia and reperfusion, may participate to the triggering of the IP of the heart.

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Left regional cardiac perfusion in vitro with platelet-activating factor, norepinephrine and K+ reveals that ischaemic arrhythmias are caused by independent effects of endogenous "mediators" facilitated by interactions, and moderated by paradoxical antagonism

Various putative drug targets for suppression of ischaemia-induced ventricular fibrillation (VF) have been proposed, but therapeutic success in the suppression of sudden cardiac death (SCD) has been disappointing. Platelet-activating factor (PAF) is a known component of the ischaemic milieu. We examined its arrhythmogenic activity, and its interaction with two other putative mediators, norepinephrine and K(+), using an ischaemia-free in vitro heart bioassay, and a specific PAF antagonist (BN-50739). PAF (0.1-100 nmol) was administered selectively to the left coronary bed of rat isolated hearts using a specially designed catheter. In some hearts, PAF was administered to the left coronary bed during concomitant regional perfusion with norepinephrine and/or K(+). In separate studies, PAF accumulation in the perfused cardiac tissue was evaluated using (3)H-PAF. PAF evoked ventricular arrhythmias concentration-dependently (P<0.05). It also widened QT interval and reduced coronary flow selectively in the PAF-exposed left coronary bed (both P<0.05). Two exposures of hearts to PAF were necessary to evoke the QT and rhythm effects. The PAF-induced arrhythmias and coronary vasoconstriction were partially suppressed by the PAF antagonist BN-50739 (10 microm), although BN-50739 itself widened QT interval. K(+) (8 and 15 mm) unexpectedly antagonised the arrhythmogenic effects of PAF without itself eliciting arrhythmias (P<0.05). Norepinephrine (0.1 microm) had little or no effect on the actions of PAF, while failing to evoke arrhythmias itself. Nevertheless, the combination of 15 mm K(+) and 0.1 microm norepinephrine evoked arrhythmias of a severity similar to arrhythmias evoked by PAF alone, without adding to or diminishing the arrhythmogenic effects of PAF. (3)H-PAF accumulated in the cardiac tissue, with 43+/-5% still present 5 min after bolus administration, accounting for the need for two exposures of the heart to PAF for evocation of arrhythmias. Thus, PAF, by activating specific receptors in the ventricle, can be expected to contribute to arrhythmogenesis during ischaemia. However, its interaction with other components of the ischaemic milieu is complex, and selective block of its actions (or its accumulation) in the ischaemic milieu is alone unlikely to reduce VF/SCD.

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.

Activation of ERK1/2 by platelet-activating factor receptor is independent of receptor internalisation and G-protein activation

Platelet-activating factor (PAF) is a potent pro-inflammatory phospholipid mediator involved in a broad range of physiological and pathophysiological processes. The receptor of PAF (PAFR) is a heptahelical G-protein-coupled receptor. We have shown previously that upon agonist stimulation, PAFR internalised through clathrin-coated vesicles in an arrestin-dependent, but G-protein-coupling-independent manner. In the current report, we demonstrate that PAF stimulates Erk1/2 phosphorylation and: (1). dominant negative mutants of arrestins and dynamin do not influence Erk1/2 activation, (2). hypertonic conditions do not decrease the extent of Erk1/2 phosphorylation, (3). internalisation-deficient and/or G-protein-coupling-deficient mutants of PAFR activate Erk1/2 as efficiently as the wild-type PAFR, and (4). inhibition of epidermal growth factor receptor (EGFR) does not block Erk1/2 activation. Taken together, our results suggest that PAFR-mediated activation of mitogen-activated protein kinases Erk1/2 does not require receptor endocytosis, receptor tyrosine kinase transactivation or G-protein activation. In addition, our studies reveal that PAFR-mediated signals of G-protein activation, receptor internalisation and MAPK activation are differentially regulated by receptor structure and/or conformation.

The role of platelet-activating factor in the corneal response to injury

Platelet-activating factor (PAF) is a potent bioactive lipid that is generated in the cornea after injury and whose actions are mediated through specific receptors. Studies from our laboratory have shown that PAF interactions with its receptor activate several transmembrane signals involved in inflammation, wound healing, and apoptosis. The wide variety of responses to PAF implicate this lipid as a central player in many responses of the cornea after a pathologic stimulus. An exciting facet of PAF is that it induces the expression of specific genes involved in the remodeling of components of the extracellular matrix, such as some metalloproteinases, urokinase plasminogen activator, and selective inhibitors of metalloproteinases. These enzymes, when overexpressed, could lead to corneal ulceration. Continuous exposure to PAF during prolonged inflammation produces increase keratocyte apoptosis and inhibition of epithelial adhesion to the basement membrane. As a consequence, there is a marked delay in wound healing, which is not countered by the actions of growth factors. In this review, we present data mainly from our laboratory showing actions of PAF in corneal epithelium in vivo and in vitro in corneal models of injury as well as in cells in culture. We also discuss the signal-transduction mechanisms involved in the different actions of PAF. A therapeutic role for PAF antagonists in blocking the effects of PAF is guaranteed in the future.

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.

Cardiovascular properties of yangambin, a lignan isolated from Brazilian plants

Yangambin was initially selected from a number of lignans isolated from Brazilian plants for its ability to antagonize Platelet-Activating Factor (PAF, 1-O-hexadecyl-2-acetyl- sn-glyceryl-3-phosphorylcholine)-induced biological effects. Subsequently it was shown that, besides its antagonistic properties at PAF receptors, yangambin also prevents the cardiovascular collapse observed during anaphylactic and endotoxic/septic shocks, as well as the vascular and cardiac hyporesponsiveness to catecholamines in endotoxic shock. It is suggested that this naturally occurring compound could be of potential interest in the adjunctive management of the above mentioned pathologies. In the present article, we review the main studies investigating the pharmacological properties of yangambin related to the cardiovascular function.

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.

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.

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.

Evidence that atypical vasopressin V(2) receptor in inner medulla of kidney is V(1B) receptor

Vasopressin V(2) receptors at high-density and V(1B) receptors are candidates for the V(2)-like receptor, which evokes an increase in [Ca(2+)](i) when stimulated by the vasopressin V(2) receptor agonist 1-desamino-8-D-arginine vasopressin (dDAVP) in kidney inner medullary collecting duct. We compared the pharmacological characteristics of vasopressin V(2) and V(1B) receptors in Chinese hamster ovary (CHO) cells to those of vasopressin V(2)-like receptors in rat inner medullary collecting duct cells. The vasopressin V(1B) receptor-selective agonist [deamino-Cys(1), D-3-(Pyridyl)-Ala(2), Arg(8)]vasopressin (D3PVP) did not stimulate the [Ca(2+)](i) increase in high-density vasopressin V(2) receptor-expressing CHO cells, but did in inner medullary collecting duct cells. Moreover, the vasopressin V(1A)/V(2) receptor dual antagonist 4'-[(2-methyl-1,4,5,6-tetrahydroimidazo[4,5-d][1] benzazepin-6-yl)carbonyl] 2-phenylbenzanilide (YM087), which has no effect on vasopressin V(1B) receptors, did not block the [Ca(2+)](i) increase in inner medullary collecting duct cells when stimulated by dDAVP and D3PVP. On reverse transcription-polymerase chain reaction (RT-PCR) analysis of kidney, vasopressin V(1B) receptor mRNA was detected only in the medulla. We propose that the true nature of the vasopressin V(2)-like receptor in the inner medullary collecting duct is the vasopressin V(1B) receptor, rather than the vasopressin V(2) receptor expressed at high-density.

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.

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.

Human intestinal epithelial cells express receptors for platelet-activating factor

The intestinal epithelium produces and responds to cytokines and lipid mediators that play a key role in the induction and regulation of mucosal inflammation. The lipid mediator platelet-activating factor (PAF) can be produced and degraded by the human intestinal epithelium and is known to mediate a range of proinflammatory and other biological effects in the intestinal mucosa. In the studies herein, we assessed whether or not human intestinal epithelial cells express cell surface or intracellular PAF receptors (PAF-R), whether expression of these receptors can be regulated, and whether human intestinal epithelial cells respond to PAF. Several human colon epithelial cell lines (HT-29, Caco-2, T84, HCT-8, HCA-7, I407, and LS-174T) were shown by RT-PCR to constitutively express mRNA for PAF-R. In addition, PAF-R expression was demonstrated by immunoblot analysis and PAF-R was shown to be constitutively expressed on the cell surface of several of these cell lines, as assessed by flow cytometry. PAF-R expression by human colon epithelial cells was upregulated by stimulation with retinoic acid but not by stimulation with PAF, proinflammatory agonists (tumor necrosis factor-alpha, interleukin-1, interferon-gamma), or transforming growth factor-alpha. PAF-R on intestinal epithelial cells were functional, as PAF stimulation of the cells increased tyrosine phosphorylation of several cellular proteins, including proteins of 75 and 125 kDa, and this response was blocked by a PAF-R antagonist. Consistent with the findings using cell lines, PAF-R were also constitutively expressed by normal human colon and small intestinal epithelium in vivo, as shown by immunohistology. The constitutive and regulated expression of functional PAF-R by human intestinal epithelium suggests PAF produced by the intestinal epithelial cells or cells underlying the epithelium has autocrine or paracrine effects on intestinal epithelial cells.

Association of a G994-->T missense mutation in the plasma platelet-activating factor acetylhydrolase gene with genetic susceptibility to nonfamilial dilated cardiomyopathy in Japanese

BACKGROUND

Although several genes or genetic loci that are responsible for or confer susceptibility to familial dilated cardiomyopathy (DCM) have been identified, genetic defects that underlie nonfamilial DCM remain to be characterized. Mice lacking manganese superoxide dismutase exhibit DCM, suggesting that impairment of the defense mechanisms against oxidative stress is an important susceptibility factor for DCM. Plasma platelet-activating factor (PAF) acetylhydrolase also acts as a key defense against oxidative stress by hydrolyzing PAF and oxidized phospholipids. Thus, abnormalities in the activity of this enzyme may result in predisposition to myocardial damage.

METHODS AND RESULTS

The possible association of a G994 (M allele)-->T (m allele) missense mutation in the plasma PAF acetylhydrolase gene with genetic susceptibility to nonfamilial DCM has now been investigated in 122 Japanese individuals with this condition and 226 healthy control subjects. PAF acetylhydrolase activity in plasma was significantly associated with plasma PAF acetylhydrolase genotype in both DCM patients and healthy control subjects. The frequency of the mutant m allele was significantly higher in DCM patients than in control subjects. Left ventricular mass (LVM) and the LVM index in DCM patients with Mm or mm genotypes were significantly greater than those in patients with the MM genotype.

CONCLUSIONS

The G994-->T mutation in the plasma PAF acetylhydrolase gene is associated with nonfamilial DCM in Japanese subjects. Although the mutation is unlikely to be a causative factor, it may contribute to genetic susceptibility to or progression of nonfamilial DCM.

Inhibition by platelet-activating factor of Src- and hepatocyte growth factor-dependent invasiveness of intestinal and kidney epithelial cells. Phosphatidylinositol 3'-kinase is a critical mediator of tumor invasion

This study was designed to characterize platelet-activating factor receptor (PAF-R) expression and function in normal and cancerous human colonic epithelial cells. PAF-R gene transcripts were analyzed by reverse transcription-polymerase chain reaction and Southern blot, using three sets of primers corresponding either to the coding region of the human PAF-R sequence (polymerase chain reaction product: 682 base pairs (bp)) or to the leukocyte- and tissue-type transcripts of 166 and 252 bp, respectively. An elongated splice variant was identified in the 5'-untranslated region of the tissue-type PAF-R transcript (334 bp) in colonic epithelial crypts and tumors. In human colonic PCmsrc cells transformed by c-src oncogene, the hepatocyte growth factor (HGF)-dependent invasiveness of collagen gels was abolished by 0.1 microM PAF and restored by the PAF-R antagonists WEB2086 and SR27417. PAF blocked HGF-induced tyrosine phosphorylation of p125 focal adhesion kinase. The phosphatidylinositol 3'-kinase (PI3'-K) inhibitors wortmannin and LY294002 totally blocked the HGF-induced invasion. Similar effects were observed in ts-srcMDCK kidney epithelial cells transformed by a v-Src temperature-sensitive mutant: (i) PAF and wortmannin exerted additive inhibitory effects on Src-induced invasion and (ii) activated and dominant negative forms of p110alpha PI3'-K, respectively, amplified and abrogated the Src- and HGF-dependent invasiveness of parental and ts-srcMDCK cells. We also provided the first evidence for the contribution of rapamycin-insensitive, pertussis toxin-dependent G-protein pathways to the integration of the signals emerging from activated Met and PAF receptors. These results indicate that PI3'-K is a critical transducer of invasiveness and strongly suggest that PAF exerts a negative control on invasion by inhibiting this signaling pathway. A possible beneficial role of PAF analogs on tumor invasion is therefore proposed.

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.

Impaired anaphylactic responses with intact sensitivity to endotoxin in mice lacking a platelet-activating factor receptor

Platelet-activating factor (PAF) is a potent phospholipid mediator with diverse biological activities in addition to its well-known ability to stimulate platelet aggregation. Pharmacologic studies had suggested a role for PAF in pregnancy, neuronal cell migration, anaphylaxis, and endotoxic shock. Here we show that disruption of the PAF receptor gene in mice caused a marked reduction in systemic anaphylactic symptoms. Unexpectedly, however, the PAF receptor-deficient mice developed normally, were fertile, and remained sensitive to bacterial endotoxin. These mutant mice clearly show that PAF plays a dominant role in eliciting anaphylaxis, but that it is not essential for reproduction, brain development, or endotoxic shock.

Roles of plasma platelet-activating factor acetylhydrolase in allergic, inflammatory, and atherosclerotic diseases

Platelet-activating factor (PAF) mediates a variety of physiologic and pathologic events by activating platelets, neutrophils, monocytes, macrophages, and smooth muscle cells. A strongly oxidizing environment induces fragmentation of the polyunsaturated fatty acids of membrane phospholipids, and the resulting oxidized phospholipids are structurally similar to PAF and mimic its biologic actions. The effects of PAF and oxidized phospholipids are abolished by hydrolysis of the sn-2 residue, a reaction catalyzed by PAF acetylhydrolase. Plasma and intracellular forms of PAF acetylhydrolase have been purified and characterized. The plasma form binds with high affinity to lipoproteins in plasma. Furthermore, changes in the activity of this enzyme are associated with various human diseases and animal models of human pathology, suggesting that it may play important roles in their pathogenesis. Studies that have defined the properties of this enzyme and its roles in physiologic and pathologic processes are reviewed. Such studies have provided insight into the functions of PAF and oxidized phospholipids as well as into the etiology of allergic, inflammatory, and atherosclerotic diseases.

Airway responsiveness in transgenic mice overexpressing platelet-activating factor receptor. Roles of thromboxanes and leukotrienes

Platelet-activating factor (PAF) is a potent proinflammatory compound potentially involved in the pathogenesis of inflammatory disorders, including bronchial asthma. To elucidate the pathophysiologic roles of PAF in bronchial asthma, we studied airway responsiveness in transgenic mice overexpressing PAF receptor. In the transgenic mice, PAF-induced airway smooth muscle contraction was demonstrated by physiologic and morphometric analyses, whereas there was no significant response in the littermate control group. The PAF-elicited bronchoconstriction in the transgenic mice was significantly reduced not only by a PAF receptor antagonist (WEB-2086) but also by a thromboxane synthesis inhibitor (indomethacin or ozagrel), an inhibitor of 5-lipoxygenase-activating protein (MK-886), or a cysteinyl leukotriene (LT) antagonist (pranlukast). LTB4 receptor antagonist (ONO-4057), however, had no effect on the PAF-induced responses. The transgenic mice showed a bronchial hyperreactivity to methacholine challenge, which was also inhibited by pretreatment with either thromboxane synthesis inhibitor or cysteinyl LT antagonist. These observations suggest that both thromboxane A2 and cysteinyl LTs (LTC4, LTD4, and LTE4) are involved in the bronchial responses to PAF or cholinergic stimulus in mice. The transgenic mice overexpressing PAF receptor may provide an appropriate model to study various PAF-related lung diseases, including bronchial asthma.

Platelet-activating factor antagonism: a new concept in the management of regional myocardial ischemia-reperfusion injury

Reperfusion therapies for treatment of myocardial infarction successfully reduce patient mortality; however, regional myocardial ischemia-reperfusion (RMIR) causes its own expression of cardiovascular dysfunction, including myocardial depression, hemodynamic instability, and dysrhythmias, which have increased patient mortality within the first 24 h after starting reperfusion therapy. Current evidence suggests that the release of oxygen-derived reactive substances and subsequent inflammatory mediators during ischemia-reperfusion contribute toward this injury. Platelet-activating factor (PAF), a mediator released during RMIR, has been emphasized by many investigators as playing a central role in causing RMIR injury. Similar cardiovascular dysfunctions that occur during RMIR, including myocardial depression, hemodynamic instability, and dysrhythmias, occur after administration of PAF and are ameliorated with PAF antagonists. Further, PAF antagonists have been shown to be cardioprotective and improve survival when administered before onset of reperfusion. A variety of phospholipid analogues, naturally derived compounds, and synthetic compounds have been developed that form the different classes of PAF antagonists, each with unique antagonizing properties. Several of these compounds have successfully passed safety and efficacy testing in humans; however, to date, no clinical trials have investigated the protective effects of PAF antagonists against RMIR injury. A current theory in the pathogenesis of RMIR injury considers the ischemic and necrotic portion of the myocardium and regional dysfunction due to tissue necrosis to be solely responsible for global cardiac dysfunction leading to hemodynamic instability and death. Evidence now suggests, however, that the global dysfunction is also due to the effect of inflammatory mediators such as PAF, thromboxanes, leukotrienes, and endothelins that are released during RMIR and are distributed throughout the heart on reperfusion. Antagonizing a central inflammatory mediator such as PAF, as adjunct treatment with currently used reperfusion therapies, improves cardiovascular function and survival in animals and should be introduced into clinical trials to investigate if similar protective effects can be provided in humans.

Cloning and characterization of a murine platelet-activating factor receptor gene

A murine gene encoding a platelet-activating factor receptor (PAFR) was cloned. The gene was mapped to a distal region of chromosome 4 both by fluorescence in situ hybridization and by molecular linkage analysis. Northern blot analysis showed a high expression of the PAFR message in peritoneal macrophages. When C3H/HeN macrophages were treated with bacterial lipopolysaccharide (LPS) or synthetic lipid A, the PAFR gene expression was induced. Bacterial LPS, but not lipid A, induced the level of PAFR mRNA in LPS-unresponsive C3H/HeJ macrophages. These induction patterns were parallel to those of tumor necrosis factor-alpha mRNA. Thus, the PAFR in macrophages plays important roles in LPS-induced pathologies.

Structural and functional requirements for agonist-induced internalization of the human platelet-activating factor receptor

The receptor for platelet-activating factor (PAF) is a member of the G-protein-coupled receptor family. To study the structural elements and mechanisms involved in the internalization of human PAF receptor (hPAFR), we used the following mutants: a truncated mutant in the C-terminal tail of the receptor (Cys317 --> Stop) and mutations in the (D/N)P(X)2,3Y motif (Asp289 --> Asn,Ala and Tyr293 --> Phe,Ala). Chinese hamster ovary cells expressing the Cys317 --> Stop mutant exhibited a marked reduction in their capacity to internalize PAF, suggesting the existence of determinants important for endocytosis in the last 26 amino acids of the cytoplasmic tail. Substitution of Asp289 to alanine abolished both internalization and G-protein coupling, whereas substitution of Tyr293 to alanine abolished coupling but not internalization. Inhibition or activation of protein kinase C did not significantly affect the internalization process. Receptor sequestration and ligand uptake was, at least in part, blocked by concanavalin A and blockers of endocytosis mediated by clathrin-coated pits. Our data suggest that the internalization of a G-protein-coupled receptor and coupling to a G-protein can be two independent events. Moreover, the C terminus tail of hPAFR, but not the putative internalization motifs, may be involved in the internalization of hPAFR.

Alanine exchanges of polar amino acids in the transmembrane domains of a platelet-activating factor receptor generate both constitutively active and inactive mutants

To determine ligand-binding sites of a platelet-activating factor (PAF) receptor, alanine-scanning mutagenesis was carried out. All 23 polar amino acids in the putative 7-transmembrane (TM) domains of a guinea pig PAF receptor were individually replaced with alanine. The ligand-binding properties of mutant receptors were determined after transient expression in COS-7 cells. Mutants in TM II (N58A, D63A), TM III (N100A, T101A, S104A) and TM VII (D289A) displayed higher PAF-binding affinities than seen with the wild-type receptor. In contrast, mutants in TM V (H188A), TM VI (H248A, H249A, Q252A), and TM VII (Q276A, T278A) showed lower affinities. Representative mutants were then stably expressed in Chinese hamster ovary cells to observe PAF-induced cellular signals (arachidonate release, phosphatidylinositol hydrolysis, adenylyl cyclase inhibition). An N100A mutant with the highest affinity was constitutively active and was responsive to lyso-PAF, an inactive derivative of PAF. One nanomolar PAF induced no signals in low affinity mutants, an EC50 value for the wild-type receptor. Three histidines (His-188, His-248, His-249) might form a binding pocket for the phosphate group of PAF, since zinc effectively inhibited ligand binding. Based on these results, a three-dimensional molecular model of PAF and its receptor was generated using bacteriorhodopsin as a reference protein.

Platelet-activating factor and cardiac diseases: therapeutic potential for PAF inhibitors

Platelet-activating factor (PAF) is a potent phospholipid mediator released from inflammatory cells in response to diverse immunologic and non-immunologic stimuli. Animal studies have implicated PAF as a major mediator involved in coronary artery constriction, modulation of myocardial contractility and the generation of arrhythmias which may bear on cardiac disorders such as ischemia, infarction and sudden cardiac death. PAF effects are induced by direct actions of PAF on cardiac tissue to modify chronotropic and inotropic activity, or indirectly via the release of eicosanoids such as thromboxane A2 (TXA2), leukotrienes (LT) or cytokines (TNF alpha). The development of selective, high affinity PAF receptor antagonists has permitted investigations on the role of PAF in experimental animal models of cardiac injury. In vivo and in vitro studies strongly suggest that PAF receptor antagonists might convey therapeutic benefits in ischemic conditions and certain arrhythmias. In addition, PAF antagonists might have a cardiac allograft-preservation effect. Although clinical studies with PAF receptor antagonists in patients with cardiac diseases have not yet been reported, the experimental results to date suggest that PAF receptor antagonists might be useful in some specific cardiac disorders in humans.

Signal transduction and gene expression in the eye: a contemporary view of the pro-inflammatory, anti-inflammatory and modulatory roles of prostaglandins and other bioactive lipids

Eye tissues respond to physiological and pathophysiological stimuli by the activation of phospholipases and the consequent release from membrane phospholipids of biologically active metabolites. These rapid events have profound effects on long-term ocular physiology. Activation of phospholipase A2 is the first step in the synthesis of two important classes of lipid second messengers, the eicosanoids and platelet-activating factor (PAF). PAF accumulates in the cornea in response to injury. It has been shown to stimulate metalloproteinase gene expression in the corneal epithelium, and is, thus, implicated in the extracellular matrix remodeling that accompanies wound healing and ulceration. PAF antagonists confer protection in animal models of acute and chronic anterior segment inflammation, and block the PAF-enhanced glutamate release from retina. The latter effect suggests a role for PAF in glaucomatous neuronal damage. The eicosanoids, in particular the prostaglandins, have long been implicated in the pathophysiology of ocular inflammation and there is pharmacological evidence for their role in the regulation of intraocular pressure. The induction by PAF of the inducible prostaglandin synthase in neurons and in the corneal epithelium provides a link between the actions of these two lipid second messengers. There may be thresholds of lipid second messenger concentrations which govern their activities as physiological, defensive, or harmful.

Identification of transmembrane domain residues determinant in the structure-function relationship of the human platelet-activating factor receptor by site-directed mutagenesis

Platelet-activating factor (PAF) is a potent phospholipid mediator that produces a wide range of biological responses. The PAF receptor is a member of the seven-transmembrane GTP-binding regulatory protein-coupled receptor superfamily. This receptor binds PAF with high affinity and couples to multiple signaling pathways, leading to physiological responses that can be inhibited by various structurally distinct PAF antagonists. We have used site-directed mutagenesis and functional expression studies to examine the role of the Phe97 and Phe98 residues located in the third transmembrane helix and Asn285 and Asp289 of the seventh transmembrane helix in ligand binding and activation of the human PAF receptor in transiently transfected COS-7 cells. The double mutant FFGG (Phe97 and Phe98 mutated into Gly residues) showed a 3-4-fold decrease in affinity for PAF, but not for the specific antagonist WEB2086, when compared with the wild-type (WT) receptor. The FFGG mutant receptor, however, displayed normal agonist activation, suggesting that these two adjacent Phe residues maintain the native PAF receptor conformation rather than interacting with the ligand. On the other hand, substitution of Ala for Asp289 increased the receptor affinity for PAF but abolished PAF-dependent inositol phosphate accumulation; it did not affect WEB2086 binding. Substitution of Asn for Asp289, however, resulted in a mutant receptor with normal binding and activation characteristics. When Asn285 was mutated to Ala, the resulting receptor was undistinguishable from the WT receptor. Surprisingly, substitution of Ile for Asn285 led to a loss of ligand binding despite normal cell surface expression levels of this mutant, as verified by flow cytometric analysis. Our data suggest that residues 285 and 289 are determinant in the structure and activation of the PAF receptor but not in direct ligand binding, as had been recently proposed in a PAF receptor molecular model.

Platelet-activating factor in the modulation of excitatory amino acid neurotransmitter release and of gene expression

Selective phospholipids of synaptic membranes are reservoirs for lipid second messengers. 1-Alkyl-2 arachidonoyl glycero-3-phosphocholine is hydrolyzed by phospholipase A2 (PLA2) into two products: lyso-PAF, which is transacetylated to yield platelet-activating factor (PAF), and free arachidonic acid (20:4), which can undergo oxidative metabolism to eicosanoids. Alternative pathways of PAF synthesis, such as CoA-independent transacylase and the de novo route of synthesis, remain to be explored and compared to the PLA2-dependent route. At low concentrations, PAF is a retrograde messenger of LTP in CA1 hippocampal neurons, and is also a memory enhancer in inhibitor avoidance tasks. PAF enhances excitatory amino acid release in synaptic pairs from primary hippocampal cultures by a presynaptic mechanism. Ischemia and convulsions activate synaptic PLA2. Thus, increased concentrations of PAF promote massive glutamate exocytosis, glutamate receptor activation, and elevated intracellular calcium levels in target cells. As a result, calcium-sensitive cascades are affected. PAF thus had dual roles as a lipid mediator: under physiological conditions it modulates neurotransmitter release, but at high concentrations it becomes neurotoxic. Through an intracellular high affinity binding site, PAF activates the expression of immediate-early genes. Some of these genes encode transcription factors (e.g. zif-268, c-fos), and others encode enzymes (COX-2 or inducible prostaglandin synthase). PAF also activates the expression of metalloproteinases which participate in the remodeling of the extracellular matrix. These effects have been studied in cells in culture as well as in the brain. A PAF antagonist specific for the intracellular binding site inhibits COX-2 expression elicited by a single electroconvulsive shock or vasogenic edema. COX-1, the constitutive prostaglandin synthase, is not induced and is unaffected by the antagonist. Most of the cerebral induction occurs in the hippocampus and results from transcriptional activation. PAF mediated gene expression may be involved in neural plasticity as well as in pathophysiological conditions in which the neural tissue activates repair-injury pathways.

Mutations of two adjacent amino acids generate inactive and constitutively active forms of the human platelet-activating factor receptor

We have mutated two residues, Ala230 and Leu231, in the C-terminal portion of the third intracellular loop of the human platelet-activating factor (PAF) receptor into Glu230 and Arg231, respectively. The Leu231 --> Arg231 substitution led to two major modifications: 1) increased constitutive activity of the PAF receptor resulting in agonist-independent production of inositol phosphates and 2) increased affinity of the receptor for binding PAF (agonist) but not WEB2086 (antagonist). The L231R mutant was able to adopt at least two conformations: (i) a higher affinity state than the corresponding state of the wild-type receptor (WT), dependent on G protein coupling, and (ii) a low affinity state, higher than the one for the uncoupled WT receptor. The Ala230 --> Glu230 substitution also resulted in two major modifications: 1) unresponsiveness in terms of phosphatidylinositol hydrolysis in response to PAF and 2) a marked decrease in affinity of the receptor for binding the agonist but not the antagonist. Competition binding studies of transient receptor expression in COS-7 cells and the inability of guanosine 5 -O-(3-thiotriphosphate) to modulate the decrease in affinity of a stable A230E mutant in Chinese hamster ovary cells suggest an inherent low affinity conformation for this mutant. Alternatively, mutation of Ala230 to Gln230 suggested that the residue 230 has a fundamental effect on receptor affinity and its charge is determinant in G protein coupling of the PAF receptor. In this report, we show that substitution of two immediately adjacent residues of the PAF receptor, Ala230 and Leu231, surprisingly leads to an inactive and a constitutively active phenotype, respectively. These results further support the concept of constitutively active G protein-coupled receptors as adopting ''active'' state conformations similar to those induced by agonist binding to WT receptors.

A murine platelet-activating factor receptor gene: cloning, chromosomal localization and up-regulation of expression by lipopolysaccharide in peritoneal resident macrophages

A murine gene encoding a platelet-activating factor receptor (PAFR) was cloned. The gene was mapped to a region of the D2.2 band of chromosome 4 both by fluorescence in situ hybridization and by molecular linkage analysis. Northern blot analysis showed a high expression of the PAFR message in peritoneal macrophages. When C3H/HeN macrophages were treated with bacterial lipopolysaccharide (LPS) or synthetic lipid A, the PAFR gene expression was induced. Bacterial LPS, but not lipid A, induced the level of PAFR mRNA in LPS unresponsive C3H/HeJ macrophages. These induction patterns were parallel to those of tumor necrosis factor-alpha mRNA. Thus the PAFR in macrophages is important in LPS-induced pathologies.

Tissue-specific response of the human platelet-activating factor receptor gene to retinoic acid and thyroid hormone by alternative promoter usage

We have studied the effects of retinoic acid (RA) and thyroid hormone (3,3',5-triiodothyronine; T3) on platelet-activating factor receptor (PAFR) gene expression in intact rats and the ability of two human PAFR gene promoters (PAFR promoters 1 and 2) to generate two transcripts (PAFR transcripts 1 and 2). Northern blotting showed that RA and T3 regulated PAFR gene expression only in rat tissues that express PAFR transcript 2. Functional analysis of the human PAFR promoter 2 revealed that responsiveness to RA and T3 was conferred through a 24-bp element [PAFR-hormone response element (HRE) located from -67 to -44 bp of the transcription start site, whereas PAFR promoter 1 did not respond to these hormones. The PAFR-HRE is composed of three direct repeated TGACCT-like hexamer motifs with 2-and 4-bp spaces, and the two upstream and two downstream motifs were identified as response elements for RA and T3. Thus, the PAF-PAFR pathway is regulated by the PAFR level altered by a tissue-specific response to RA and T3 through the PAFR-HRE of the PAFR promoter 2.