Increased urinary excretion of platelet activating factor in mice with lupus nephritis

Platelet activating factor (PAF) is present in urine from humans and experimental animals in normal conditions. Very little is known about changes in PAF urinary excretion under pathologic conditions and no data are available about the origin of PAF in the urine. In the present study we explored the possibility that immunologic renal disease is associated with an increase in PAF urinary excretion using gas chromatography-mass spectrometry technique. To clarify the renal or extrarenal origin of urinary PAF we evaluated whether exogenously administered PAF (1-[1', 2'-3H]alkyl) is filtered through the glomerulus and excreted in the urine. The results show that: 1) urine from mice with lupus nephritis in the early phase of the disease contained amounts of PAF comparable to those excreted in normal mouse urine, 2) PAF levels increased when animals started to develop high grade proteinuria, 3) after intravenous injection of [3H] PAF in nephritic mice, a negligible amount of [3H] ether lipid, corresponding to [3H]1-alkyl -2-acyl-3-phosphocholine (alkyl-2-acyl-GPC), was recovered from the 24 h urine extract.     

Role of spermatozoal platelet-activating factor in fertilization

Platelet-activating factor (PAF), a potent lipid mediator of inflammation, has been shown to play a role in both the implantation and viability of mammalian embryos. We examined whether human and mouse spermatozoa release PAF during in vitro incubation and assessed the effect of exogenous PAF and the PAF receptor antagonist WEB 2086, a thieno-triazolodiazepine, on mouse in vitro fertilization (IVF) rate. PAF biological activity was detected in 11 samples of leukocyte-free, purified human spermatozoa (28 pg PAF/10(6) cells/24 hr) and 5 samples of epididymal mouse spermatozoa (7.8 pg PAF/10(6) cells/3 hr). Exogenous PAF (10(-8) and 10(-6) M) increased (p less than 0.01) the fertilization rate 2- and 3-fold, respectively of mouse oocytes by mouse epididymal spermatozoa. 10(-4) M PAF, however, reduced sperm motility and decreased (p less than 0.05) the fertilization rate. 10(-6) M WEB 2086, decreased IVF to approximately 50% of the control fertilization rate (42% vs. 89%). WEB 2086 treatment also promoted the attachment of supernumerary spermatozoa to both fertilized and unfertilized oocytes. The fertilization rate in the presence of WEB 2086 returned to control levels when zona-pellucida-free oocytes were employed, indicating that WEB 2086 did not interfere with the spermatozoal acrosome reaction. These data suggest that PAF, of spermatozoal origin, may be important in mammalian fertilization.     

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 (TXA,), leukotrienes (LT) or cytokines (TNFx). 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 antagonist might be useful in some specific cardiac disorders in humans.     

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.     

Lipopolysaccharide Cross-Tolerance Delays Platelet-Activating Factor-Induced Sudden Death in Swiss Albino Mice: Involvement of Cyclooxygenase in Cross-Tolerance

Lipopolysaccharide (LPS) signaling through Toll-like receptor-4 (TLR-4) has been implicated in the pathogenesis of many infectious diseases. Some believe that TLR-mediated pathogenicity is due, in part, to the lipid pro-inflammatory mediator platelet-activating factor (PAF), but this has been questioned. To test the direct contribution of PAF in endotoxemia in murine models, we injected PAF intraperitoneally into Swiss albino mice in the presence and absence of LPS. PAF alone (5 μg/mouse) caused death within 15–20 min, but this could be prevented by pretreating mice with PAF-receptor (PAF-R) antagonists or PAF-acetylhydrolase (PAF-AH). A low dose of LPS (5 mg/kg body wt) did not impair PAF-induced death, whereas higher doses (10 or 20 mg/kg body wt) delayed death, probably via LPS cross-tolerance. Cross-tolerance occurred only when PAF was injected simultaneously with LPS or within 30 min of LPS injection. Tolerance does not appear to be due to an abundant soluble mediator. Histologic examination of lungs and liver and measurement of circulating TNF-α and IL-10 levels suggested that the inflammatory response is not diminished during cross-tolerance. Interestingly, aspirin, a non-specific cyclooxygenase (COX) inhibitor, partially blocked PAF-induced sudden death, whereas NS-398, a specific COX-2 inhibitor, completely protected mice from the lethal effects of PAF. Both COX inhibitors (at 20 mg/kg body wt) independently amplified the cross-tolerance exerted by higher dose of LPS, suggesting that COX-derived eicosanoids may be involved in these events. Thus, PAF does not seem to have a protective role in endotoxemia, but its effects are delayed by LPS in a COX-sensitive way. These findings are likely to shed light on basic aspects of the endotoxin cross-tolerance occurring in many disease conditions and may offer new opportunities for clinical intervention.     

Platelet-activating factor induces mediator release by human basophils primed with IL-3, granulocyte-macrophage colony-stimulating factor, or IL-5.

The phospholipid platelet-activating factor (PAF) is a potent cell-derived bioactive molecule thought to be involved in diverse inflammatory processes. It has been shown that PAF can activate different leukocyte types and platelets, particularly in synergy with other agonists. In this study we examined the effect of PAF upon the release of histamine and leukotriene (LT) C4 by basophils when added alone and in combination with different agonists and cytokines. PAF by itself did neither induce histamine release nor the generation of LTC4 by basophils. However, basophils primed by the hematopoietic growth factors (hGF) IL-3, granulocyte-macrophage (GM)-CSF, or IL-5 (10 ng/ml) released preformed and de novo synthesized mediators in response to PAF at 10 to 100 nM concentrations. The extent of mediator release by hGF primed basophils in response to PAF was similar to that induced by an optimal concentration of monoclonal anti-IgE. Thus, similar to NAP-1/IL-8 and C3a, PAF efficiently stimulates mediator release in hGF-primed basophils only. However, PAF was clearly a more potent trigger of LTC4 formation in IL-3-primed cells than NAP-1/IL-8 or C3a. When PAF was used as a second trigger, the priming effect of IL-5 was less than that of IL-3 or GM-CSF, whereas the response for other IgE-independent agonists (i.e., C5a or FMLP) was augmented equally by all three hGF. IL-1 beta-pretreated basophils released minimal amounts of histamine in response to PAF. Neither TNF-alpha nor PAF, nor the combination thereof, was able to induce basophil mediator release. The efficiency of the different cytokines to prime for PAF responsiveness was strikingly similar to their capacity to enhance anti-IgE-induced mediator release. Similar to other IgE-independent agonists, the kinetic of mediator release in response to PAF was very rapid. PAF pretreatment of basophils did not enhance mediator release in response to diverse agonists, such as C5a and FMLP, in contrast to the capacity of PAF to augment the response of other leukocyte types to appropriate stimuli. Thus, depending on the presence of IL-3, GM-CSF, or IL-5, PAF is a potent basophil agonist capable of inducing histamine release as well as de novo synthesis of LTC4.     

Evidence for the autocrine induction of capacitation of mammalian spermatozoa

Mammalian spermatozoa require a maturational event after ejaculation that allows them to acquire the capacity for fertilization. This process, known as capacitation, occurs spontaneously in simple defined medium implicating a potential role of autocrine induction. This study shows that the ether phospholipid 1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphocholine (PAF) meets the criteria for an autocrine mediator of capacitation. Sperm released PAF after their dilution into capacitation medium and expressed a receptor for PAF on their membranes. PAF stimulated changes in the motility of sperm and enhanced fertilization in vitro. These actions were inhibited by a PAF receptor antagonist (UR-12519) and by extracellular recombinant PAF:acetylhydrolase (an enzyme that degrades PAF to a biologically inert form). Seminal plasma contained an acid-labile PAF:acetylhydrolase, whereas capacitation was inhibited by an acid-labile factor within seminal plasma, implicating this factor as a potential decapacitation factor within seminal plasma. Sperm from a PAF receptor knock-out mouse strain failed to express the receptor and displayed a significantly (p < 0.01) reduced rate of capacitation, as assessed by the spontaneous onset of the acrosome reaction in vitro. When used for in vitro fertilization, sperm from PAF receptor knock-out mice gave a significantly lower rate of fertilization (21.5%) than did wild-type sperm (66.7%). The study shows for the first time the operation of an autocrine loop that induces capacitation in sperm in vitro and shows that this loop acts in concert with other mediators of capacitation to promote efficient fertilization.     

Influence of some prostaglandins and prostaglandin analogues on PAF-induced shock in mice.

Prostaglandins and Prostaglandin-analogues were investigated for their ability to protect mice from platelet-activating factor (PAF) induced shock. 75% mortality in female NMRI mice was induced by i.v. injection of 75 micrograms/kg PAF. Nileprost and PGE1, the most potent substances, produced a dose dependent protection against PAF. Iloprost and PGI2 were less effective. PGE2, nalador, flunoprost and U 46619 were neither protective nor deleterious. The strong difference in the effectiveness between the two prostaglandins of the E-series and the poor effect of PGI2 and the PGI2 analogue is remarkable. Flunoprost and U 46619 that increased the TXB2 synthesis or release in two experimental models did not enhance the PAF mortality; TXA2 seems to be only a secondary mediator of the acute PAF-induced death.     

Cytochrome P-450-dependent metabolism in alveolar type II epithelial cells: modulation by platelet-activating factor

Platelet-activating factor (PAF), an ether lipid mediator released from activated pulmonary phagocytes, was evaluated for its ability to affect cytochrome P-450-dependent activities in isolated rat alveolar type II cells. The data indicate that at non-toxic doses, PAF caused an increase in beta-naphthoflavone (BNF) inducible/alpha-naphthoflavone (ANF) sensitive ethoxyphenoxazone deethylase (EtOPx'ase) activity. At high concentrations of PAF, inhibition of both EtOPx'ase and metyrapone (MP) sensitive benzyloxyphenoxazone debenzylase (BzOPx'ase) activities and aggregation of type II cells were observed. The PAF analogs, lyso-PAF and enantio-PAF, exhibited actions similar to those observed with PAF. PAF-induced enhancement of EtOPx'ase activity required the presence of intact cells, whereas at high PAF concentrations decreased enzyme activities were observed in both intact cell and sonicated cell preparations. The data thus suggest that xenobiotic metabolism in alveolar type II cells can be modified by an inflammatory mediator, such as PAF, produced by alveolar phagocytes.     

The platelet activating factor (PAF) signaling cascade in systemic inflammatory responses

The platelet-activating factor (PAF) signaling cascade evolved as a component of the repertoire of innate host defenses, but is also an effector pathway in inflammatory and thrombotic diseases. This review focuses on the PAF signaling cascade in systemic inflammatory responses and, specifically, explores its activities in experimental and clinical sepsis and anaphylaxis in the context of the basic biochemistry and biology of signaling via this lipid mediator system.     

Platelet-activating factor: an endogenous mediator for bowel necrosis in endotoxemia

We have developed a model of isochemic bowel necrosis in the rat by injecting platelet-activating factor (PAF) or PAF in combination with bacterial endotoxin. PAF causes profound hypotension, and it has been suggested that it is released during endotoxin shock. Because ischemic bowel necrosis is often associated with shock or infection, it is possible that PAF is the endogenous mediator that causes shock and bowel necrosis during sepsis. In this study, we have demonstrated that: 1) normal intestine contained a small amount of PAF; 2) necrotic lesions of the intestine could be induced by endotoxin injection; 3) PAF production in the bowel is markedly increased in animals treated with endotoxin; 4) pretreatment of the animal with PAF antagonists prevent endotoxin-induced necrosis; 5) isolated, buffer-perfused small intestine produced a small quantity of PAF in response to endotoxin injection. Therefore, we conclude that PAF is a likely endogenous mediator in endotoxemia, which causes bowel necrosis and shock.     

Elevated plasma phospholipase A2 and platelet-activating factor acetylhydrolase activity in colorectal cancer

This clinical study reports that blood levels of the pro-inflammatory mediator platelet-activating factor (PAF) did not change in colorectal cancer patients. In contrast, plasma levels of two enzymatic activities, one implicated in PAF production (i.e. phospholipase A2) and one in PAF degradation (i.e. PAF acetylhydrolase activity) were significantly elevated.     

Urinary platelet-activating factor excretion is elevated in non-insulin dependent diabetes mellitus.

Proteinuria is currently considered a very sensitive predictor of diabetic nephropathy, but 20-25% of all diabetic patients with negative Albustix reaction excrete higher than normal (< 20 mg/24 h) amounts of albumin in their urine. It is our hypothesis that platelet-activating factor (PAF), a potent glycerophospholipid that acts as a chemical mediator for a wide spectrum of biological activities, including increased vascular permeability, may be produced in significant amounts during periods preceding microalbuminuria. In this study, we compared urinary PAF excretion in Mexican-American subjects who were diagnosed with non-insulin dependent diabetes mellitus (NIDDM) with their healthy control counterparts. The age of the NIDDM subjects (45.9 +/- 2.1 years) was not significantly different from the healthy control group, which was 39.4 +/- 2.7 years (P < 0.0672). The NIDDM subjects (body mass index, 29.9 +/- 1.1 compared to 26.1 +/- 0.9 kg/m2 in healthy controls) were characterized by significantly increased (P < 0.05) fasting plasma glucose (192 +/- 11 vs. 97 +/- 4 mg/dl in healthy controls), fasting insulin (20.9 +/- 2.4 vs. 12.3 +/- 1.6 microU/ml), fasting C-peptide (2.93 +/- 1.26 vs. 1.48 +/- 0.51 ng/ml), and hemoglobin A1c (10.3 +/- 0.7 vs. 5.6 +/- 0.3%), respectively. The urine output for the NIDDM and control subjects were 1942 +/- 191 ml/24 h and 1032 +/- 94 ml/24 h, respectively, and urinary albumin excretion (UAE) rates were estimated to be 38 +/- 7 micrograms/min and 11 +/- 1 micrograms/min, respectively. The NIDDM subjects produced significantly increased levels of urinary PAF (2606.3 +/- 513.1 ng/24 h compared with 77.9 +/- 14.1 ng/24 h in controls (or 1706.3 +/- 420.8 ng/ml compared with 85.4 +/- 17.8 pg/ml of urine, in NIDDM and control subjects, respectively). We found that urinary PAF excretion was significantly correlated with microalbumin excretion (r = 0.7) especially at UAE rates greater than 30 mg/day and more importantly, some NIDDM patients with negative Albustix reaction (i.e. normal UAE) produced significantly more PAF, suggesting that PAF excretion may precede microalbuminuria and that subtle injury to the kidneys are present in NIDDM long before overt albuminuria ensues, urinary PAF measurements could potentially therefore serve as a sensitive indicator of renal injury in diabetes mellitus. These results lend further credence to our hypothesis that PAF may be the biochemical compound linking the various members of the insulin resistance syndrome.     

Synthesis of platelet activating factor by tissues from the rainbow trout, Salmo gairdneri

In mammals, platelet activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a lipid mediator with biological activity at concentrations in the subnanomolar range. Although PAF is known to have many activities in mammals, little is known about its synthesis and importance in other vertebrate groups. We demonstrate here the synthesis of PAF from [3H]acetate by slices of trout gill, kidney, liver and spleen. PAF synthesis was stimulated by the calcium ionophore A23187 and was time-dependent. The radiolabeled PAF produced was characterized by TLC, HPLC, derivatization and by saponification and phospholipase A2 hydrolysis. These findings suggest that PAF may be an important mediator in fish.     

Rapid Production of Platelet-activating Factor Is Induced by Protein Kinase Cα-mediated Phosphorylation of Lysophosphatidylcholine Acyltransferase 2 Protein

Platelet-activating factor (PAF), a potent proinflammatory lipid mediator, is synthesized rapidly in response to extracellular stimuli by the activation of acetyl-CoA:lyso-PAF acetyltransferase (lyso-PAFAT). We have reported previously that lyso-PAFAT activity is enhanced in three distinct ways in mouse macrophages: rapid activation (30 s) after PAF stimulation and minutes to hours after LPS stimulation. Lysophosphatidylcholine acyltransferase 2 (LPCAT2) was later identified as a Ca²⁺-dependent lyso-PAFAT. However, the mechanism of rapid lyso-PAFAT activation within 30 s has not been elucidated. Here we show a new signaling pathway for rapid biosynthesis of PAF that is mediated by phosphorylation of LPCAT2 at Ser-34. Stimulation by either PAF or ATP resulted in PKCα-mediated phosphorylation of LPCAT2 to enhance lyso-PAFAT activity and rapid PAF production. Biochemical analyses showed that the phosphorylation of Ser-34 resulted in augmentation of V_max with minimal K_m change. Our results offer an answer for the previously unknown mechanism of rapid PAF production.     

Selective inhibitors of a PAF biosynthetic enzyme lysophosphatidylcholine acyltransferase 2

Platelet-activating factor (PAF) is a potent pro-inflammatory phospholipid mediator. In response to extracellular stimuli, PAF is rapidly biosynthesized by lyso-PAF acetyltransferase (lyso-PAFAT). Previously, we identified two types of lyso-PAFATs: lysophosphatidylcholine acyltransferase (LPCAT)1, mostly expressed in the lungs where it produces PAF and dipalmitoyl-phosphatidylcholine essential for respiration, and LPCAT2, which biosynthesizes PAF and phosphatidylcholine (PC) in the inflammatory cells. Under inflammatory conditions, LPCAT2, but not LPCAT1, is activated and upregulated to produce PAF. Thus, it is important to develop inhibitors specific for LPCAT2 in order to ameliorate PAF-related inflammatory diseases. Here, we report the first identification of LPCAT2-specific inhibitors, N-phenylmaleimide derivatives, selected from a 174,000-compound library using fluorescence-based high-throughput screening followed by the evaluation of the effects on LPCAT1 and LPCAT2 activities, cell viability, and cellular PAF production. Selected compounds competed with acetyl-CoA for the inhibition of LPCAT2 lyso-PAFAT activity and suppressed PAF biosynthesis in mouse peritoneal macrophages stimulated with a calcium ionophore. These compounds had low inhibitory effects on LPCAT1 activity, indicating that adverse effects on respiratory functions may be avoided. The identified compounds and their derivatives will contribute to the development of novel drugs for PAF-related diseases and facilitate the analysis of LPCAT2 functions in phospholipid metabolism in vivo.     

Involvement of platelet activating factor in immediate heart response to lipopolysaccharide.

Although lipopolysaccharide (LPS) is recognized to induce a biphasic cardiovascular response its mechanism is not fully elucidated. In this study we analysed the involvement of PAF, TXA(2) and cysteinyl leukotrienes (cysLTs) in the acute cardiovascular effects of LPS in the isolated rat heart as well as in delayed phase of LPS response using a surrogate cellular model of the induction of NOS-2 by LPS in mouse macrophages. Perfusion of rat hearts with LPS resulted, in an immediate fall in heart contractility and coronary flow by 2.5 +/- 0.59 ml x min(-1) and 560 +/- 81 mmHg x sec(-1), respectively. This response was fully blocked by platelet activating factor (PAF) antagonist - WEB 2170 and partially inhibited, by inhibitor of cyclooxygenase (indomethacin) or by inhibitor of thromboxane synthase (camonagrel). The inhibition of leukotriene synthesis (BAY x1005) or cysLTs receptors (BAY x7195) was without effect. Administration of stable PAF analog (methylcarbamyl-PAF - MC-PAF) alone, mimicked heart response to LPS. In cultured mouse macrophages, MC-PAF did not induce NOS-2 expression and when given with LPS it slightly potentiated NOS-2 induction by LPS. However, in presence of WEB 2170 NOS-2 induction by LPS was inhibited in a dose-dependent manner. Inhibition of cyclooxygenase and leukotriene pathways had no effect on NOS-2 induced by LPS. These results indicate that PAF and TXA(2) but not cysLTs mediate the instant heart response induced by LPS, while PAF alone mediates a delayed NOS-2 induction by LPS. Accordingly, PAF may constitute the mediator that links acute and delayed phases of LPS-induced cardiovascular response.     

Platelet-activating factor and serum components from oestrous mice co-operate to mimic the activity of 'early pregnancy factor' in the rosette inhibition assay

When male mouse spleen cells were incubated with a combination of platelet activating factor (PAF, 1-0-alkyl-2-acetyl-sn-glycero-3-phosphocholine) and sera from female mice in oestrus, the cells displayed a markedly increased rosette inhibition titre (RIT) when subsequently tested in the rosette inhibition assay. Neither PAF nor oestrous mouse sera alone could induce this effect, the combined action was required. Lyso-PAF could not substitute for the PAF, nor could male mouse sera nor the sera from females in dioestrus or metoestrus substitute for the oestrous mouse serum requirement. Pro-oestrous mouse sera could replace oestrous mouse sera but were less effective in their dose-responses. Studies on the mechanism of action of the PAF and oestrous mouse serum components suggested that the PAF stimulated the production and release of soluble factors (termed S2 factors) which by themselves could induce increased RIT values when applied to fresh spleen cells. The PAF-stimulated cell populations were rendered refractory to the action of these S2 factors and did not display increased RIT values, unless oestrous mouse serum was added. This serum acted to reverse the refractory state, allowing the S2 factors to exert their effect, and so cells treated with PAF and oestrous mouse serum displayed increased RIT values.     

A single enzyme catalyzes both platelet-activating factor production and membrane biogenesis of inflammatory cells. Cloning and characterization of acetyl-CoA:LYSO-PAF acetyltransferase

Platelet-activating factor (PAF) is a potent proinflammatory lipid mediator eliciting a variety of cellular functions. Lipid mediators, including PAF are produced from membrane phospholipids by enzymatic cascades. Although a G protein-coupled PAF receptor and degradation enzymes have been cloned and characterized, the PAF biosynthetic enzyme, aceyl-CoA:lyso-PAF acetyltransferase, has not been identified. Here, we cloned lyso-PAF acetyltransferase, which is critical in stimulus-dependent formation of PAF. The enzyme is a 60-kDa microsomal protein with three putative membrane-spanning domains. The enzyme was induced by bacterial endotoxin (lipopolysaccharide), which was suppressed by dexamethasone treatment. Surprisingly, the enzyme catalyzed not only biosynthesis of PAF from lyso-PAF but also incorporation of arachidonoyl-CoA to produce PAF precursor membrane glycerophospholipids (lysophosphatidylcholine acyltransferase activity). Under resting conditions, the enzyme prefers arachidonoyl-CoA and contributes to membrane biogenesis. Upon acute inflammatory stimulation with lipopolysaccharide, the activated enzyme utilizes acetyl-CoA more efficiently and produces PAF. Thus, our findings provide a novel concept that a single enzyme catalyzes membrane biogenesis of inflammatory cells while producing a prophlogistic mediator in response to external stimuli.     

Differential expression of platelet-activating factor acetylhydrolase in macrophages and monocyte-derived dendritic cells

Although macrophages (Mphi) and monocyte-derived dendritic cells (MDDC) come from a common precursor, they are distinct cell types. This report compares the two cell types with respect to the metabolism of platelet-activating factor (PAF), a biologically active lipid mediator. These experiments were prompted by our studies of localized juvenile periodontitis, a disease associated with high IgG2 production and a propensity of monocytes to differentiate into MDDC. As the IgG2 Ab response is dependent on PAF, and MDDC selectively induce IgG2 production, we predicted that PAF levels would be higher in MDDC than in Mphi. To test this hypothesis, human MDDC were prepared by treating adherent monocytes with IL-4 and GM-CSF, and Mphi were produced by culture in M-CSF. Both Mphi and MDDC synthesized PAF; however, MDDC accumulated significantly more of this lipid. We considered the possibility that PAF accumulation in MDDC might result from reduced turnover due to lower levels of PAF acetylhydrolase (PAFAH), the enzyme that catabolizes PAF. Although PAFAH increased when monocytes differentiated into either cell type, MDDC contained significantly less PAFAH than did Mphi and secreted almost no PAFAH activity. The reduced levels of PAFAH in MDDC could be attributed to lower levels of expression of the enzyme in MDDC and allowed these cells to produce PGE(2) in response to exogenous PAF. In contrast, Mphi did not respond in this manner. Together, these data indicate that PAF metabolism may impinge on regulation of the immune response by regulating the accessory activity of MDDC.