Platelet-activating factor (PAF): implications for coronary heart and vascular diseases

Cellular and molecular mechanisms of inflammation and thrombosis

In the last 20 years, the cellular and molecular mechanisms of inflammation and thrombosis have been characterised. These are essentially cell adhesion processes which are regulated by vascular endothelium. Many of the cell adhesion molecules and leucocyte chemoattractants expressed and generated at sites of inflammation have been sequenced and cloned. These inflammatory molecules work together in concert to mediate the adhesion between leucocytes, platelets and vascular endothelium which occurs during the occlusive, thromboembolic, reperfusion and septic complications of atherosclerotic and diabetic vascular diseases. This review aims to summarise our current understanding of the molecular basis of these disorders and the therapeutic implications.

Neutrophil-dependent augmentation of PAF-induced vasoconstriction and albumin flux in coronary arterioles

Platelet-activating factor (PAF) has been implicated in the pathogenesis of ischemic heart disease, reperfusion injury, and inflammatory reactions. Although neutrophils have been shown to primarily mediate PAF-induced microvascular dysfunction, the vasoactive effect of PAF and its neutrophil-dependent mechanism have not been directly and systematically studied in coronary resistance vessels. Therefore, the aim of this study was to examine the effects of PAF on coronary arteriolar function and neutrophil dynamics using an isolated and perfused microvessel preparation. Topical application of PAF to the vessels induced a dose-dependent decrease in the diameter but an increase in the apparent permeability coefficient of albumin. Disruption of the endothelium abolished the vasomotor response to PAF, and perfusion of neutrophils significantly augmented PAF-induced changes in vasomotor tone and permeability. Furthermore, the interaction between neutrophils and the endothelium was studied in the intact perfused coronary arterioles. Under control conditions, there were no adherent neutrophils observed in the vessels at varied intraluminal flow velocities. However, administration of PAF caused neutrophil adhesion to the endothelium of coronary arterioles at low flow velocities. Western blot analysis indicated that PAF upregulated the expression of intercellular adhesion molecule-1 in cultured coronary microvascular endothelial cells. Taken together, the results suggest that 1) PAF induces vasoconstriction and hyperpermeability in coronary arterioles via an endothelium-dependent and neutrophil-mediated mechanism, and 2) PAF is able to stimulate neutrophil adhesion in coronary arterioles under a condition of low flow rate.

Ultrastructural localization of secretory type II phospholipase A2 in atherosclerotic and nonatherosclerotic regions of human arteries

We recently reported on the immunolocalization of type II secretory nonpancreatic phospholipase A2 (snpPLA2) in human atherosclerotic lesions. In the present study, we present data on the distribution and ultrastructural localization of snpPLA2 in adjacent nonatherosclerotic and atherosclerotic regions of human arteries. Electron microscopy (EM) of immunogold labeling techniques with a monoclonal antibody was used to analyze arterial tissue. The human specimens analyzed were obtained from autopsy and surgery cases. The results with EM showed a stronger snpPLA2 immunoreactivity in regions of arteries with atherosclerotic lesions than in regions without lesions from the same individual. snpPLA2 immunoreactivity was stronger in the arterial intima of atherosclerotic than of nonatherosclerotic tissue. EM-immunogold examination revealed that the majority of snpPLA2 was localized along the extracellular matrix, associated with collagen fibers and other extracellular matrix structures. Intracellular snpPLA2 was observed in electron-dense vesicles in intimal cells. snpPLA2 was also found in contact with large, extracellular lipid droplets. These results support the hypothesis that extracellular snpPLA2 is localized at sites where it may hydrolyze phospholipids from lipoproteins and lipid aggregates retained in the extracellular matrix of the arterial wall. This may be a mechanism for in situ release of proinflammatory lipids, free fatty acids, and lysophosphatidylcholine in regions of apolipoprotein B accumulation, which are abundant in atherosclerotic lesions.

Copper-catalyzed oxidation mediates PAF formation in human LDL subspecies. Protective role of PAF:acetylhydrolase in dense LDL

Free radical-mediated oxidation of cholesterol-rich LDL plays a key role in atherogenesis and involves the formation of oxidized phospholipids with proinflammatory biological activity. We evaluated the production of platelet-activating factor (PAF), a potent inflammatory mediator, in human LDL subspecies on copper-initiated oxidation (4 mumol/L CuCl2, 80 micrograms/mL for hours at 37 degrees C). PAF formation was determined by biological assay of HPLC-purified lipid extracts of copper-oxidized lipoproteins; chemical identity was confirmed by gas chromatographic and mass spectrometric analyses. PAF, characterized as the C16:0 molecular species, was preferentially produced in intermediate LDL (d = 1.029 to 1.039 g/mL) (8.6 +/- 5.7 pmol PAF/3 h per mg LDL protein) and light LDL (d = 1.019 to 1.029 g/mL), but was absent from dense LDL particles (d = 1.050 to 1.063 g/mL). As PAF:acetylhydrolase inactivates PAF and oxidized forms of phosphatidylcholine, we evaluated the relationship of lipoprotein-associated PAF:acetylhydrolase to PAF formation. We confirmed that PAF:acetylhydrolase activity was elevated in native, dense LDL (41.5 +/- 9.5 nmol/min per mg protein) but low in LDL subspecies of light and intermediate density (d 1.020 to 1.039 g/mL) (3.5 +/- 1.6 nmol/min per mg protein) [Tselepis et al, Arterioscler Thromb Vasc Biol. 1995;15:1764-1773]. On copper-mediated oxidation for 3 hours at 37 degrees C, dense LDL particles conserved 20 +/- 14% of their initial enzymatic activity; in contrast, PAF:acetylhydrolase activity was abolished in light and intermediate LDL subspecies. Clearly, the elevated PAF:acetylhydrolase activity of dense LDL efficiently diminishes the potential inflammatory role of endogenously formed PAF; nonetheless, formation of proatherogenic lysophospholipids results. In contrast, LDL particles of the light and intermediate subclasses can accumulate PAF on oxidative modification.

Expression of phospholipase A2 isoforms in human normal and atherosclerotic arterial wall

LDL particles must be modified in the arterial wall to be taken up by macrophages at an excessive rate, leading to foam cell formation. Phospholipase A2 (PLA2) has been shown to modify LDL particles in vitro by degrading its phospholipids, resulting in enhanced uptake by macrophages. Reaction products of PLA2 are lysophospholipids and nonesterified fatty acids (mainly arachidonic acid), which are precursors of potent inflammatory mediators and which have been found in atherosclerotic regions of the arterial wall. To elucidate the expression of PLA2 in normal and diseased arteries, frozen tissue sections of human nonatherosclerotic mesenteric artery and carotid plaques were examined by immunohistochemistry using specific antibodies against secretory PLA2 types I and II and cytosolic PLA2 (85 kd). Secretory PLA2 type I was not detected. High expression of secretory PLA2 type II was found throughout the media in both normal and atherosclerotic artery specimens, in which smooth muscle cells dominated. Cytosolic PLA2 was found exclusively in diseased artery, mainly in the intima in regions with an inflammatory infiltrate consisting of macrophages and smooth muscle cells. Furthermore, both normal and atherosclerotic artery possessed substantial PLA2 activity. It is suggested that secretory PLA2 type II could play an important role in early atherogenesis because it is present in the preatherosclerotic arterial wall, where it may lead to LDL modification, foam cell formation, and activation of immune mechanisms.

Effect of endotoxin and platelet-activating factor on rat vascular permeability: role of vasoactive mediators

The contribution of several vasoactive mediators such as histamine, serotonin, bradykinin, arachidonic acid metabolites and PAF to vascular permeability changes was determined in a rat model of acute endotoxemia. Lipopolysaccharide (10-40 mg/kg, i.v.) from E. coli 0127:B8 (LPS) elicited an increase in Evans blue extravasation in trachea, thymus, seminal vesicle and stomach, whereas other organs remained unaffected. LPS (25 mg/kg)-induced extravasation was not inhibited by intravenous pretreatment with histamine (H1) antagonist mepyramine (5 mg/kg) or bradykinin (B2) antagonist HOE-140 (0.1 mg/kg), whereas other standard drugs selectively inhibited leakage in particular tissues, e.g. the cyclooxygenase inhibitor indomethacin (5 mg/kg) in trachea (78%) and seminal vesicle (64%), the serotonin and H1 antagonists cyproheptadine (2 mg/kg) in trachea (88%) and stomach (56%) and the dual cyclooxygenase/lipoxygenase inhibitor phenidone (10 mg/kg) in seminal vesicle (87%). PAF antagonists lexipafant and UR-12460 (10 mg/kg), but not apafant, potently inhibited extravasation in trachea (59, 84%) and seminal vesicle (81, 78%) and in stomach only UR-12460 (52%), whereas all of them were ineffective in thymus. When extravasation was induced by PAF (4 micrograms/kg) a low dose (0.1 mg/kg) of the three PAF antagonists strongly reduced extravasation in thymus and seminal vesicle, whereas lexipafant and UR-12460 did so in trachea (82, 100%) and only lexipafant in stomach (100%). Mepyramine, cyproheptadine, HOE-140 and indomethacin did not inhibit the effect of PAF, whereas phenidone inhibited it by 58% in trachea. These results suggest that most of the LPS-induced increase in vascular permeability is mediated by secondary vasoactive mediators among which PAF plays a pivotal role, although their relative contribution may vary from tissue to tissue.

Differential signaling pathways in platelet-activating factor-induced proliferation and interleukin-6 production by rat vascular smooth muscle cells

Vascular smooth muscle cells (SMCs) can be induced to proliferate in response to several cytokines and growth factors, including interleukin (IL)-6. Platelet-activating factor (PAF) also has been shown to induce SMC proliferation. Because PAF can stimulate IL-6 production in monocytes, macrophages, and endothelial cells, our study was undertaken to determine whether PAF could induce IL-6 production by SMCs and to define the underlying signaling pathways. Exposure of rat aortic SMCs to picomolar concentrations of PAF resulted in enhanced production of IL-6. The effect was concentration dependent, selective for the active form of PAF, and mediated by specific PAF receptors. Pretreatment of the cells with Bordatella pertussis toxin (PTX) prevented the effect of PAF, suggesting the involvement of alpha i-type subunits of G proteins in the signal-transduction pathway. PAF-dependent IL-6 production was also prevented by inhibition of tyrosine kinases with genistein or erbstatin. Inhibition of eicosanoid production by blocking either phospholipase A2 or cyclooxygenase also abrogated the effect of PAF on IL-6 production. Moreover, inhibition of Ca2+-calmodulin activity with W7 or blocking of calcium channels with verapamil or nifedipine prevented PAF-mediated enhancement of IL-6 production. Whereas PAF-induced signal-transduction pathways leading to IL-6 production and SMC proliferation were partially common, they appeared to diverge downstream of PLA2 activation: inhibition of cyclooxygenase had no effect on proliferation, whereas augmentation of cyclic adenosine monophosphate (cAMP) levels or activation of protein kinase A inhibited proliferation, in contrast to IL-6 production. Our findings suggest a role for PAF in modulating vascular function by stimulating local production of IL-6 by SMCs and promoting their proliferation. The two effects are, however, associated with partially divergent signaling pathways and may not be causally related.

Dietary flax seed in prevention of hypercholesterolemic atherosclerosis

Oxygen free radicals (OFRs) have been implicated in the development of hypercholesterolemic atherosclerosis. Flax seed is the richest source of omega-3 fatty acid and lignans. omega-3 Fatty acid suppresses the production of interleukin-1 (IL-1), tumor necrosis factor (TNF) and leukotriene B4 (LTB4), and of OFRs by polymorphonuclear leukocytes (PMNLs) and monocytes. Lignans possess anti-platelet activating factor (PAF) activity and are antioxidant. PAF, IL-1, TNF and LTB4 are known to stimulate PMNLs to produce OFRs. Flaxseed would, therefore, reduce the levels of OFRs and hence would prevent the development of hypercholesterolemic atherosclerosis. The effects of dietary flax seed on a high cholesterol diet induced atherosclerosis, lipid profile and OFR-producing activity of PMNLs (PMNL-CL) were investigated in rabbits. The rabbits were divided into 4 groups: group I, control; group II, flax seed diet (7.5 g/kg daily, orally); group III, 1% cholesterol diet; and group IV, same as group III but received flax seed (7.5 g/kg daily, orally). Blood samples were collected before and after 4 and 8 weeks on their respective diets for biochemical measurements and aortae were removed at the end of 8 weeks for estimation of atherosclerotic changes. The high cholesterol diet increased the serum level of total cholesterol (TC) and PMNL-CL without altering the levels of serum triglycerides (TG). These changes were associated with a marked development of atherosclerosis in the aorta. Flax seed reduced the development of aortic atherosclerosis by 46% and reduced the PMNL-CL without significantly lowering the serum cholesterol. Flax seed in normocholesterolemic rabbits increased serum total cholesterol and decreased PMNL-CL without significantly affecting the serum TG. Modest dietary flax seed supplementation is effective in reducing hypercholesterolemic atherosclerosis markedly without lowering serum cholesterol. Its effectiveness against hypercholesterolemic atherosclerosis could be due to suppression of enhanced production of OFRs by PMNLs in hypercholesterolemia. Dietary flax seed supplementation could, therefore, prevent hypercholesterolemia-related heart attack and strokes.

The role of platelet-activating factor in musculocutaneous flap reperfusion injury

Platelet-activating factor is an extremely potent lipid-inflammatory mediator implicated in the pathophysiologic mechanism of reperfusion injury in a variety of organs. The purpose of this study, employing a porcine latissimus dorsi flap model, was to (1) examine the expression of platelet-activating factor and (2) evaluate the possible benefit and mechanism of action of platelet-activating factor antagonism in musculocutaneous flap reperfusion injury. Experiment 1: In 6 pigs, bilateral flaps underwent 8 hours of arterial ischemia followed by 12 hours of reperfusion. Biopsies were collected sequentially and analyzed immunohistochemically for platelet-activating factor expression. Different processing techniques, however, were unable to detect specific tissue expression of platelet-activating factor. Experiment 2: In 11 pigs, bilateral flaps underwent 8 hours of arterial ischemia followed by 20 hours of reperfusion. A lipophilic platelet-activating factor receptor antagonist (L-659,989) was administered as a single dose to treated flaps by a local intraarterial route prior to reperfusion. This treatment augmented the survival of both muscle (48.3 versus 19.7 percent) and skin (49.8 versus 42.0 percent) components of the flaps in a statistically significant fashion (p = 0.001). Experiment 3: In 3 pigs, a radiolabeled structural analogue of L-659,989 (14C-L-680,573) was administered to flaps in a fashion similar to experiment 2. After 8 hours of ischemia, sequential full-thickness flap biopsies were collected over the initial 6 hours of reperfusion. The radio-labeled platelet-activating factor receptor antagonist was found to be highly concentrated within treated flaps, with gradual decay over the initial 6 hours of reperfusion. Experiment 4: Thirty minutes prior to completion of 8 hours of arterial ischemia, autologous neutrophils labeled with indium-111 were reintroduced into the systemic-circulation of 5 pigs. Prior to reperfusion, treated flaps received L-659,989 as in experiment 2. Over the initial 4 hours of reperfusion, the flaps were imaged in situ by a gamma camera at 3-minute intervals. The platelet-activating factor receptor antagonist was found to significantly attenuate the accumulation of radioactivity within treated flaps.

CONCLUSION

Platelet-activating factor expression within musculocutaneous flaps subjected to ischemia and reperfusion was non directly demonstrated in this study. Still, we have shown that (1) the specific platelet-activating factor receptor antagonist L-659,989 is beneficial to the survival of both muscle and skin flap components, (2) a single, prereperfusion local dose of this lipophilic drug remains concentrated within the flap during the early inflammatory phase of reperfusion, and (3) during reperfusion, platelet-activating factor antagonism is able to directly or indirectly diminish the accumulation of acute inflammatory cells in musculocutaneous flaps.

Expression of the PAF receptor in human monocyte-derived macrophages is downregulated by oxidized LDL: relevance to the inflammatory phase of atherogenesis

Human monocyte-derived macrophages play a major role in the initiation and progression of atherosclerotic lesions as a result of the production of a wide spectrum of proinflammatory and prothrombotic factors. Among such factors is a potent inflammatory phospholipid, platelet-activating factor (PAF), which is produced after macrophage activation. Because the cells involved in PAF biosynthesis are typically targets for the bioactions of PAF via specific cell surface receptors, we evaluated the expression of the PAF receptor in human monocyte-derived macrophages. Oxidized LDL (oxLDL) exerts multiple cellular effects that enhance lesion progression; we therefore investigated the potential modulation of expression of the macrophage PAF receptor by oxLDL. [3H]PAF bound to adherent human macrophages with a K(d) of 2.1 nmol/L and a B(max) of 19 fmol/10(6) cells; approximately 5300 binding sites per cell were detected. OxLDL (100 microg protein per milliliter) induced a twofold decrease in cellular PAF binding after 3 hours at 37 degrees C. Analysis of macrophage mRNA by reverse transcription-polymerase chain reaction (RT-PCR) revealed two forms corresponding to the PAF receptor, of which the leukocyte type (type 1 promoter) predominated. Expression of PAF receptor mRNA, evaluated by quantitative RT-PCR using an actin or a GAPDH mimic, was progressively reduced (up to 70%) by oxLDL up to 6 hours and remained low for at least 24 hours. Such downregulation was reversible after incubation of the cells for 24 hours in oxLDL-free medium. Addition of forskolin (3 micromol/L) or dibutyryl cAMP (1 mmol/L) to macrophage cultures reproduced the oxLDL-mediated inhibition of PAF receptor expression; carbamyl PAF reduced PAF binding and PAF mRNA to a similar degree (approximately 50%). These data demonstrate that atherogenic oxLDL downregulates the expression of both cellular PAF receptors and PAF receptor mRNA in macrophages, consistent with both a diminished bioresponse to PAF and decreased cell motility. Such diminished bioresponse to a powerful antacoid reflects the suppression of an acute inflammatory reaction, thereby leading to chronic, low-level inflammation, such as that characteristic of fatty streaks and more advanced atherosclerotic plaques.

Platelet-activating factor and oxidized LDL induce immune activation by a common mechanism

Platelet activating factor (PAF) is a phospholipid with proinflammatory and thrombogenic properties, which has been implicated in inflammatory disorders including vasculitis and asthma. PAF-like compounds are present in oxidized LDL (oxLDL), which has been detected in the atherosclerotic lesion, where it may activate monocytes, macrophages, and T cells. OxLDL may therefore both initiate and perpetuate inflammatory reactions in the artery wall. Herein we demonstrate that PAF has the capacity to induce enhanced interferon gamma (IFN-gamma) secretion in peripheral blood mononuclear leukocytes (PBMCs), as does oxLDL. Both oxLDL- and PAF-induced IFN-gamma secretions were inhibited by a specific PAF-receptor antagonist, WEB 2170. PAF-like lipids in oxLDL could thus be responsible for oxLDL-induced activation of immune-competent cells. The effects of PAF and oxLDL were inhibited by antibodies to major histocompatibility complex class II and thus depend on accessory cells like monocytes. Both PAF and oxLDL induced tumor necrosis factor-alpha (TNF-alpha) synthesis in peripheral blood. PAF-mediated TNF-alpha production was inhibited by WEB 2170, whereas oxLDL-induced TNF-alpha was only partially inhibited. These findings indicate that both PAF and oxLDL have the capacity to induce TNF-alpha, which may increase atherogenesis due to its pleiotropic proinflammatory effects. Our findings suggest that the PAF receptor plays an important role in the inflammatory component of atherosclerosis.

Localization of nonpancreatic secretory phospholipase A2 in normal and atherosclerotic arteries. Activity of the isolated enzyme on low-density lipoproteins

Secretory nonpancreatic type II phospholipase A2 (snpPLA2) hydrolyzes fatty acids at the sn-2 position in phospholipids releasing free fatty acids (FFAs) and lysophospholipids. These products may act as intracellular second messengers or can be further metabolized into proinflammatory lipid mediators. The presence of snpPLA2 in extracellular fluids and serum during inflammation has suggested a role of the enzyme in this process. However, the presence of snpPLA2 in a variety of normal tissues suggests that snpPLA2 may also have physiological functions. Atherosclerosis appears to have an inflammatory component. Here we report on the snpPLA2 localization in normal and atherosclerotic lesions and on the properties of the isolated enzyme. A strong snpPLA2 immunoreactivity was observed in the arterial media that was colocalized with alpha-actin-positive vascular smooth muscle cells (SMCs) in both normal and atherosclerotic vessels. In aortic atherosclerotic lesions, snpPLA2 was observed colocalized with CD68-positive macrophages and HHF-35-positive SMCs and extracellularly in the lipid core. snpPLA2 was isolated from human normal arteries and from aorta with lesions. The enzyme was isolated by acid extraction of normal arterial tissues followed by immunoaffinity chromatography. The purified snpPLA2 had an expected molecular weight of 14 kD by polyacrylamide gel electrophoresis and appeared as a single band in immunoblotting. The enzymatic activity was followed by measuring release of fatty acids from phospholipid liposomes or LDL as substrates. The enzymatic activity was inhibited with two specific inhibitors for human snpPLA2: (1) monoclonal antibody 187 and (2) LY311727, a synthetic selective inhibitor. The mRNA for snpPLA2 was detected with reverse transcriptase polymerase chain reaction. These results indicate that snpPLA2 is present in human arteries and that it is able to hydrolyze phospholipids in LDL. The results support the hypothesis that snpPLA2 can release proinflammatory lipids at places of LDL deposition in the arterial wall.

PAF-acetylhydrolase activity and PAF levels in pancreas and plasma of well-fed, diabetic and fasted rat

PAF-AH activity was determined in pancreas homogenates. The enzyme activity was moderately stable upon storage at -20 degrees C. PAF and lyso-PAF were identified in rat pancreas and their concentrations were determined. PAF levels and PAF-AH activity were compared in the pancreatic tissue and plasma of three different groups of animals: well-fed, STZ-induced diabetic and fasted rats. The concentration of PAF in the pancreas of fasted rats was ten fold lower as compared with that of the well-fed or the diabetic animals. The last two groups had similar pancreatic PAF concentration. PAF levels in the plasma of fasted rats were seven fold lower than those of well-fed or diabetic rats, which were found to be similar. The enzyme PAF-AH had the highest activity in the pancreas of well-fed rats. On the contrary, the enzyme seems to be more active in the plasma of fasted as compared with diabetic and well-fed animals.

Pefabloc, 4-[2-aminoethyl]benzenesulfonyl fluoride, is a new, potent nontoxic and irreversible inhibitor of PAF-degrading acetylhydrolase

We report here that 4-[2-aminoethyl]benzenesulfonyl fluoride (Pefabloc SC, Pefabloc), a new irreversible serine proteinase inhibitor, efficiently inhibits both human and rat platelet activating factor (PAF)-degrading acetylhydrolase (acetylhydrolase). Indeed, low concentrations of Pefabloc (0.1 mM) rapidly and totally inactivate both human plasma-, VLDL-, IDL-, LDL- and HDL-associated acetylhydrolase, and in addition, acetylhydrolase synthesized and released by human adherent monocytes in culture, as well as rat brain cytosolic acetylhydrolase. By contrast, Pefabloc only minimally inhibited the phospholipase A2 (PLA2) activity from Naja naja and from porcine pancreas. In addition, Pefabloc is relatively nontoxic, stable and convenient to use. Henceforth, Pefabloc may replace both DFP and PMSF and therefore constitutes a useful and valuable tool in future studies of acetylhydrolase.

Phagocytic activation induces formation of platelet-activating factor in human monocyte-derived macrophages and in macrophage-derived foam cells. Relevance to the inflammatory reaction in atherogenesis

Monocyte-derived macrophages and macrophage-derived foam cells in arterial tissue may undergo phagocytic activation and thereby contribute to an inflammatory reaction. We have investigated the effect of phagocytic activation on the formation of platelet-activating factor (1-0-alkyl-2-acetyl-sn-glycero-3-phosphocholine, PAF-acether, PAF), a proinflammatory phospholipid, in human monocyte-derived macrophages (macrophages) and in cholesterol-loaded macrophage foam cells (foam cells). Adherent human monocyte-derived macrophages were transformed into foam cells upon incubation with acetylated low-density lipoproteins (Ac-LDL). Such foam cells characteristically displayed a markedly increased content of cholesteryl esters compared with macrophages (4.3 +/- 1.3 microgram/microgram DNA and 0.2 +/- 0.3 microgram/microgram DNA, n = 5, respectively). After phagocytic stimulation with serum-opsonized zymosan (OPZ), both macrophages and foam cells synthesized PAF transiently with maximal production (0.5-1.1 pmol PAF/microgram DNA, n = 5, corresponding to 4.0-8.8 pmol PAF/10(6) cells, as assessed by bioassay) occurring approximately 15 min after stimulation. A major fraction of the synthesized PAF remained cell-associated; such PAF was composed mainly of the hexadecyl (16:0 PAF, approximately 75%) and the octadecenyl (18:1 PAF) species and of trace amounts of octadecyl (18:0 PAF), as assessed by reverse-phase liquid chromatography. Addition of exogenous 16:0 lyso-PAF alone triggered PAF formation (0.9-1.7 pmol PAF/microgram DNA, after 15 min of cellular stimulation); simultaneous cellular stimulation with OPZ and 16:0 lyso-PAF increased PAF formation in an additive manner. Acetyltransferase, the enzyme which acetylates the precursor lyso-PAF and transforms it into PAF, displayed elevated activity both in macrophages and in foam cells, attaining 83-240 pmol PAF formed per min per mg DNA (n = 4); such elevated activity was not increased by OPZ-stimulation. The activity of acetylhydrolase, the PAF-degrading enzyme, was similar in macrophages and in foam cells, and varied between 120 pmol and 320 pmol PAF degraded per min per mg DNA (n = 5). Cell-associated acetylhydrolase activity was increased significantly by 40+/-15 % (P < 0.003, n = 5) after 15 - 30 min of activation with OPZ compared with non-stimulated cells and may account for the rapid decrease in cellular PAF content observed approximately 30 min after stimulation. These studies have established that metabolism of PAF in foam cells closely resembles that in macrophages, and thus PAF metabolism is largely independent of cellular cholesterol content. Moreover our data are consistent with the hypothesis that both macrophages and macrophage-derived foam cells upon phagocytic-activation constitute a significant transient source of PAF at inflammatory sites in the arterial intima where this phospholipidic mediator may exert potent proatherogenic and prothrombotic effects.

Inhibition of PAF-acether effects on isolated guinea pig hearts by zinc ions (Zn2+)

PAF-acether is a phospholipid synthesized by most animal tissues and exerting a strong decrease on the heart's contractile force and coronary flow. PAF-acether (10(-9) and 10(-10)M) was administered to isolated guinea pig hearts perfused via the Langendorff apparatus with Chenoweth solution. Zinc (1.5 microM) is known to benefit heart function thus, Zn2+ (1.5, 7.5, and 30 microM) was added in the perfusing solution before or after PAF-acether administration. Contractile force, coronary flow, and heart rate were recorded by means of a Narco MK-IV Physiograph throughout all modes of perfusion. Calcium inhibitor (Verapamil 10(-10)M) and Pb+2 Co2+ (1.5 x 10(-6)M) were used subsequently in the perfusing solutions in order to elucidate some of the Zn and PAF interactions observed. All hearts were analyzed for their Zn and Ca content by means of an Atomic Absorption Spectrophotometry (AAS). Our data suggest that low concentrations of zinc (1.5 microM) can strongly inhibit PAF-induced decrease of contractile force and coronary flow. Zinc-inhibiting effects on PAF's negative inotropic action (myocytic level) is not exerted through Zn-Ca antagonism. Nevertheless, a Zn-Ca antagonism in the arteriolar level cannot be excluded. Zinc inhibits PAF selectively only if it is administered before PAF injection and this strongly suggests a receptor interaction between the metal and the phospholipid at the heart level.

PAF-degrading acetylhydrolase is preferentially associated with dense LDL and VHDL-1 in human plasma. Catalytic characteristics and relation to the monocyte-derived enzyme

In human plasma, platelet activating factor (PAF)-degrading acetylhydrolase (acetylhydrolase) is principally transported in association with LDLs and HDLs; this enzyme hydrolyzes PAF and short-chain forms of oxidized phosphatidylcholine, transforming them into lyso-PAF and lysophosphatidylcholine, respectively. We have examined the distribution, catalytic characteristics, and transfer of acetylhydrolase activity among plasma lipoprotein subspecies separated by isopycnic density gradient ultracentrifugation; the possibility that the plasma enzyme may be partially derived from adherent monocytes has also been evaluated. In normolipidemic subjects with Lp(a) levels < 0.1 mg/mL, acetylhydrolase was associated preferentially with small, dense LDL particles (LDL-5; d = 1.050 to 1.063 g/mL) and with the very-high-density lipoprotein-1 subfraction (VHDL-1; d = 1.156 to 1.179 g/mL), representing 23.9 +/- 1.7% and 20.6 +/- 3.2%, respectively, of total plasma activity. The apparent Km values for PAF of the enzyme associated with such lipoproteins were 89.7 +/- 23.4 and 34.8 +/- 4.5 mumol/L for LDL-5 and VHDL-1, respectively: indeed, the Km value for LDL-5 was some 10-fold higher than that of the light LDL-1, LDL-2, and LDL-3 subspecies, whereas the Km of VHDL-1 was some twofold greater than those of the HDL-2 and HDL-3 subspecies. Furthermore, when expressed on the basis of unit plasma volume, the Vmax of the acetylhydrolase associated with LDL-5 was some 150-fold greater than that in LDL-1 (d = 1.019 to 1.023 g/mL). No significant differences in the pH dependence of enzyme activity or in sensitivity to protease inactivation, sulfydryl reagents, the serine protease inhibitor Pefabloc, or the PAF antagonist CV 3988 could be detected between apo B-containing and apo A-I-containing lipoprotein particle subspecies. Incubation of LDL-1 (Km = 8.4 +/- 2.6 mumol/L) and LDL-2 (d = 1.023 to 1.029 g/mL; Km = 8.4 +/- 3.3 mumol/L) subspecies with LDL-5, in which acetylhydrolase had been inactivated by pretreatment with Pefabloc, demonstrated preferential transfer of acetylhydrolase to LDL-5. Acetylhydrolase transferred to LDL-5 from the light LDL subspecies exhibited a Km of 9.4 +/- 2.2 mumol/L, a value characteristic of the particle donors. Finally, acetylhydrolase (Km = 23.4 +/- 7.6 mumol/L) released by adherent human monocytes in culture was found to bind preferentially to small, dense LDL subspecies upon incubation of Pefabloc-inactivated plasma with monocyte supernatant.(ABSTRACT TRUNCATED AT 400 WORDS)

PAF effects on eicosanoid release in neonatal rat cardiomyocytes

The aim of this study was to find out whether, in neonatal rat cardiomyocytes, platelet-activating factor (PAF) can stimulate eicosanoid formation. For this purpose neonatal cardiomyocytes were incubated for 60 min at 37 degrees C in HANKS buffer with PAF (10-1000 nM), and the eicosanoids thromboxane A2 (TXA2) and prostacyclin (PGI2) were assessed in the supernatant as TXB2 and 6-keto-PGF1 alpha, respectively, by an enzyme immunoassay. PAF caused concentration-dependent release of PGI2; TXA2, however, was significantly released only at the highest concentration of PAF (1000 nM). Acetylsalicylic acid (556 microM) and the PAF antagonist WEB 2086 (10 microM) significantly attenuated PAF-induced eicosanoid formation. We conclude that in neonatal rat cardiomyocytes PAF can induce eicosanoid formation and this effect is brought about by activation of a specific PAF receptor.

Effects of PAF on cardiac function and eicosanoid release in the isolated perfused rat heart: comparison between normotensive and spontaneously hypertensive rats

The aim of this study was (a) in isolated perfused rat heart to characterize the effects of platelet-activating factor (PAF) on coronary flow, ventricular contractility, and eicosanoid release and (b) to determine whether PAF effects are altered in hearts from spontaneously hypertensive rats (SHR). PAF (10(-10)-10(-7) mol) dose-dependently decreased coronary flow and ventricular contractility; concomitantly, coronary effluent concentrations of thromboxane (TX)B2 and prostaglandin F2 alpha (PGF2 alpha) were elevated but not those of prostacyclin. The PAF receptor antagonist WEB 2086 (10(-7)-10(-5) mol/l) concentration-dependently antagonized these PAF effects. In addition; the cyclo-oxygenase inhibitor indomethacin (5 x 10(-5) mol/l) prevented PAF (10(-9)-10(-7) mol) induced eicosanoid release; in the presence of indomethacin PAF caused coronary constriction and ventricular depression only at the highest dose (10(-7) mol) but had no effect at 10(-9) or 10(-8) mol. Moreover, the TXA2 antagonist SQ29,548 (10(-6) mol/l) completely inhibited 10(-8) mol PAF induced ventricular depression but did not effect coronary constriction. In SHR PAF (10(-9)-10(-7) mol) evoked decreases in coronary flow and ventricular contractility did not differ from those in normotensive Wistar-Kyoto rats while PAF induced TXA2 and PGF2 alpha release was markedly enhanced. In addition, decreases in coronary flow and ventricular contractility induced by the TXA2 agonist U 46619 (10(-7) mol/l) were markedly depressed in SHR. We conclude that in isolated perfused rat heart PAF causes coronary constriction and depression of ventricular function mainly indirectly through released TXA2 and/or PGF2 alpha. Moreover, the fact that in SHR the PAF effects on coronary flow and ventricular function are not altered despite markedly enhanced TXA2 and PGF2 alpha release supports the view that in the SHR the receptors mediating TXA2 and/or PGF2 alpha effects are desensitized.

Inhibition of PAF-induced human platelet aggregation by antithrombotic nipecotomides

Nipecotamides (piperidine-3-carboxamides) are potent inhibitors of platelet aggregation induced by a variety of agonists in vitro and in vivo. The inhibitory effects of six structural types of nipecotamides on human platelet aggregation induced by platelet-activating factor (PAF) in vitro, are studied. Evaluation of 15 racemates and stereoisomers of two nipecotamides showed that bis-nipecotoyl alkanes were more active than their mono congeners. Mono- and bis-nipecotoyl decanes were more potent than the corresponding hexanes. Lipophilicity was found to play a significant role in the antiplatelet activity of these compounds. The stereoselectivity in the PAF-antagonist potential of nipecotamides was less pronounced than that resulting from their action on ADP- or collagen-induced aggregation. Oxidation of the two benzylic carbon atoms of alpha, alpha'-bis[3-(N,N-diethylcarbamoyl)piperidino]-p-xylene.2HBr (A-1) to form 1,4-bis[3-N,N-diethylcarbamoyl) piperidino]benzenedicarboxamide (A-40K), which has a second set of carbonyl oxygens but lacks basic N atoms, resulted in a remarkable loss of ADP-antagonist potency while retaining PAF-antagonist activity. It is suggested that in addition to their membrane effects, nipecotamides act at other sites, including the PAF receptor. Double reciprocal plots of PAF binding to gel-filtered platelets (GFP) in the presence and absence of a typical nipecotamide (A-1C) were indicative of competitive inhibition (Ki = 19.28 microM). Scatchard analysis of 3H-PAF binding to GFP suggested the presence of high, intermediate (I) and low affinity binding sites, of which the I site gave a KD/app of 0.332 nM with an estimated 564 sites/platelet. Key interactions of nipecotamides with the PAF receptor appear to be the following (i) electrostatic interactions of the two amide oxygens with a primary set of electropositive areas spaced at 5-7 A, (ii) in the case of appropriate compounds, electrostatic interactions of the two amide oxygens spaced at 10-12 A with corresponding secondary receptor sites carrying positive electrostatic potential, (iii) a hydrophobic moiety fitting into a hydrophobic pocket in the receptor, and (iv) the cationic piperidine N+ (at pH 7.4) interacting with a counterion, probably aspartic acid.

Enhancement of the antineoplastic effect of anticarcinogens on benzo[a]pyrene-treated Wistar rats, in relation to their number and biological activity

Naturally occurring anticarcinogens, such as vitamins C and E, and the microelement selenium were found to inhibit the induction of benzo[a]pyrene-induced malignant tumors in Wistar rats to various extends. The antineoplastic effect of the tested anticarcinogens is gradually increased according to the number of inhibitors selected. To date the maximum action against malignancy is manifested by use of the above three inhibitors. In the group of rats receiving vitamins C, E and selenium, the prolongation of life induced by adding more than one anticarcinogen to the treatment regime reached, and in some cases surpassed, the normal life expectancy of the rats. It is expected that by adding even more anticarcinogens, the antineoplastic potency (Ap) of the inhibitors will be further improved. These results encouraged us to conduct a clinical trial in terminal human cancer cases, in conjunction with the usual treatments of surgery or chemotherapy and irradiation.