Development of a novel method for determination of acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine acetyltransferase activity and its application to screening for acetyltransferase inhibitors. Inhibition by magnolol and honokiol from Magnoliae cortex

BACE-1 Inhibitors Targeting Alzheimer's Disease

The accumulation of amyloid-β (Aβ) is the main event related to Alzheimer's disease (AD) progression. Over the years, several disease-modulating approaches have been reported, but without clinical success. The amyloid cascade hypothesis evolved and proposed essential targets such as tau protein aggregation and modulation of β-secretase (β-site amyloid precursor protein cleaving enzyme 1 - BACE-1) and γ-secretase proteases. BACE-1 cuts the amyloid precursor protein (APP) to release the C99 fragment, giving rise to several Aβ peptide species during the subsequent γ-secretase cleavage. In this way, BACE-1 has emerged as a clinically validated and attractive target in medicinal chemistry, as it plays a crucial role in the rate of Aβ generation. In this review, we report the main results of candidates in clinical trials such as E2609, MK8931, and AZD-3293, in addition to highlighting the pharmacokinetic and pharmacodynamic-related effects of the inhibitors already reported. The current status of developing new peptidomimetic, non-peptidomimetic, naturally occurring, and other class inhibitors are demonstrated, considering their main limitations and lessons learned. The goal is to provide a broad and complete approach to the subject, exploring new chemical classes and perspectives.

Computational and Experimental Assessments of Magnolol as a Neuroprotective Agent and Utilization of UiO-66(Zr) as Its Drug Delivery System

The phenolic natural product magnolol exhibits neuroprotective properties through β-amyloid toxicity in PC-12 cells and ameliorative effects against cognitive deficits in a TgCRND8 transgenic mice model. Its bioavailability and blood-brain barrier crossing ability have been significantly improved using the metal-organic framework (MOF) UiO-66(Zr) as a drug delivery system (DDS). To investigate the neuroprotective effects of the Zr-based DDS, magnolol and magnolol-loaded-UiO-66(Zr) (Mag@UiO-66(Zr)) were evaluated for inhibitory activity against β-secretase and AlCl₃-induced neurotoxicity. Due to the moderate inhibition observed for magnolol in vitro, in silico binding studies were explored against β-secretase along with 11 enzymes known to affect Alzheimer's disease (AD). Favorable binding energies against CDK2, CKD5, MARK, and phosphodiesterase 3B (PDE3B) and dynamically stable complexes were noted through molecular docking and molecular dynamic simulation experiments, respectively. The magnolol-loaded DDS UiO-66(Zr) also showed enhanced neuroprotective activity against two pathological indices, namely, neutrophil infiltration and apoptotic neurons, in addition to damage reversal compared to magnolol. Thus, MOFs are promising drug delivery platforms for poorly bioavailable drugs.

Inhibitory Effects of Honokiol on the Voltage-Gated Potassium Channels in Freshly Isolated Mouse Dorsal Root Ganglion Neurons

Voltage-gated potassium (K_V) currents, subdivided into rapidly inactivating A-type currents (I _A) and slowly inactivating delayed rectifier currents (I _K), play a fundamental role in modulating pain by controlling neuronal excitability. The effects of Honokiol (Hon), a natural biphenolic compound derived from Magnolia officinalis, on K_V currents were investigated in freshly isolated mouse dorsal root ganglion neurons using the whole-cell patch clamp technique. Results showed that Hon inhibited I _A and I _K in concentration-dependent manner. The IC₅₀ values for block of I _A and I _K were 30.5 and 25.7 µM, respectively. Hon (30 µM) shifted the steady-state activation curves of I _A and I _K to positive potentials by 17.6 and 16.7 mV, whereas inactivation and recovery from the inactivated state of I _A were unaffected. These results suggest that Hon preferentially interacts with the active states of the I _A and I _K channels, and has no effect on the resting state and inactivated state of the I _A channel. Blockade on K⁺ channels by Hon may contribute to its antinociceptive effect, especially anti-inflammatory pain.

A Penicillium sp. F33 metabolite and its synthetic derivatives inhibit acetyl-CoA:1-O-alkyl-sn-glycero-3-phosphocholine acetyltransferase (a key enzyme in platelet-activating factor biosynthesis) and carrageenan-induced paw edema in mice

Acetyl-CoA:1-O-alkyl-sn-glycero-3-phosphocholine (lyso-PAF) acetyltransferase is a key enzyme in the biosynthesis of 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (PAF) in inflammatory cells. Substances which inhibit this enzyme are of therapeutic interest. In this study, we screened for new inhibitors of lyso-PAF acetyltransferase with anti-inflammatory effects. In a metabolite from Penicillium sp. F33, we isolated an acetyltransferase inhibitor identified as dihydrofumigatin (2-methoxy-1,3,4-trihydroxy-5-methylbenzene) from high resolution mass spectrometer and NMR data. Dihydrofumigatin had strong acetyltransferase inhibitory activity, but was not stable in aqueous solution. Thus, we chemically synthesized its oxidized form fumigatin (3-hydroxy-2-methoxy-5-methyl-1,4-benzoquinone) and derivatives thereof, and evaluated their inhibitory effects. Strong inhibitory activity was observed for saturated fatty acid esters of fumigatin; the order of inhibition was 3-decanoyloxy-2-methoxy-5-methyl-1,4-benzoquinone (termed FUD-7, IC₅₀ = 3 μM)>2-methoxy-5-methyl-3-tetradecanoyloxy-1,4-benzoquinone (termed FUD-8, IC₅₀ = 20 μM)>3-hexanoyloxy-2-methoxy-5-methyl-1,4-benzoquinone (IC₅₀ = 139 μM). Interestingly, these compounds also significantly suppressed the gene expression of lyso-PAF acetyltransferase/LPCAT2 in mouse bone marrow-derived macrophages stimulated by lipopolysaccharide (LPS). We further evaluated the effect of these substances on anti-inflammatory activity in vivo using the carrageenan-induced mouse paw edema test. FUD-7 and FUD-8 at 2.5 mg/kg showed significant, 47.9-51.7%, inhibition stronger than that of prednisolone at 10 mg/kg (41.9%). These results suggest that FUD-7 and FUD-8 are potent inhibitors with anti-inflammatory activity.

Platelet-activating factor (PAF)-antagonists of natural origin

Presently herbal medicines are being used by about 80% of the world population for primary health care as they stood the test of time for their safety, efficacy, cultural acceptability and lesser side effects. The discovery of platelet activating factor antagonists (PAF antagonists) during these decades are going on with different framework, but the researchers led their efficiency in studying in vitro test models. Since it is assumed that PAF play a central role in etiology of many diseases in humans such as asthma, neuronal damage, migraine, cardiac diseases, inflammatory, headache etc. Present days instinctively occurring PAF antagonist exists as a specific grade of therapeutic agents for the humans against these and different diseases either laid hold of immunological or non-immunological types. Ginkgolide, cedrol and many other natural PAF antagonists such as andrographolide, α-bulnesene, cinchonine, piperine, kadsurenone, different Piper species' natural products and marine origin plants extracts or even crude drugs having PAF antagonist properties are being used currently against different inflammatory pathologies. This review is an attempt to summarize the data on PAF and action of natural PAF antagonists on it, which were evaluated by in vivo and in vitro assays.

Cardiovascular protection of magnolol: cell-type specificity and dose-related effects

Magnolia officinalis has been widely used in traditional Chinese medicine. Magnolol, an active component isolated from Magnolia officinalis, is known to be a cardiovascular protector since 1994. The multiplex mechanisms of magnolol on cardiovascular protection depends on cell types and dosages, and will be reviewed and discussed in this article. Magnolol under low and moderate dosage possesses the ability to protect heart from ischemic/reperfusion injury, reduces atherosclerotic change, protects endothelial cell against apoptosis and inhibits neutrophil-endothelial adhesion. The moderate to high concentration of magnolol mainly acts on smooth muscle cells and platelets. Magnolol induces apoptosis in vascular smooth muscle cells at moderate concentration and inhibits proliferation at moderate and high concentration. High concentration of magnolol also abrogates platelet activation, aggregation and thrombus formation. Magnolol also serves as an smooth muscle relaxant only upon the high concentration. Oral intake of magnolol to reach the therapeutic level for cardiovascular protection is applicable, thus makes magnolol an agent of great potential for preventing cardiovascular diseases in high-risk patients.

Honokiol and magnolol as multifunctional antioxidative molecules for dermatologic disorders

Chinese herbs have been and still are widely used as important remedies in Oriental medicine. Over the recent years, a variety of biologically active constituents have been isolated from these sources and confirmed to have multifunctional activity in experimental studies. Honokiol is a small-molecule polyphenol isolated from the genus Magnolia. It is accompanied by other related polyphenols, including magnolol, with which it shares certain biological properties. Recently, honokiol and magnolol have been found to have anti-oxidative, anti-inflammatory, anti-tumor, and anti-microbial properties in preclinical models, without appreciable toxicity. These findings have increased interest in bringing honokiol and magnolol to the clinic as novel therapeutic agents in dermatology. In this review, the findings concerning the major mechanisms of action of honokiol and magnolol are described. Knowledge of the multiple activities of honokiol and magnolol can assist with the development of honokiol and magnolol derivatives and the design of clinical trials that will maximize the potential benefit of honokiol and magnolol in the patient setting for dermatologic disorders.

Honokiol, a multifunctional antiangiogenic and antitumor agent

Honokiol is a small-molecule polyphenol isolated from the genus Magnolia. It is accompanied by other related polyphenols, including magnolol, with which it shares certain biologic properties. Recently, honokiol has been found to have antiangiogenic, antiinflammatory, and antitumor properties in preclinical models, without appreciable toxicity. These findings have increased interest in bringing honokiol to the clinic as a novel chemotherapeutic agent. In addition, mechanistic studies have tried to find the mechanism(s) of action of honokiol, for two major reasons. First, knowledge of the mechanisms of action may assist development of novel synthetic analogues. Second, mechanistic actions of honokiol may lead to rational combinations with conventional chemotherapy or radiation for enhanced response to systemic cancers. In this review, we describe the findings that honokiol has two major mechanisms of action. First, it blocks signaling in tumors with defective p53 function and activated ras by directly blocking the activation of phospholipase D by activated ras. Second, honokiol induces cyclophilin D, thus potentiating the mitochondrial permeability transition pore, and causing death in cells with wild-type p53. Knowledge of the dual activities of honokiol can assist with the development of honokiol derivatives and the design of clinical trials that will maximize the potential benefit of honokiol in the patient setting.

Protective effect of the ethanol extract of Magnolia officinalis and 4-O-methylhonokiol on scopolamine-induced memory impairment and the inhibition of acetylcholinesterase activity

Magnolol, honokiol, and obovatol are well-known bioactive constituents of the bark of Magnolia officinalis and have been used as traditional Chinese medicines for the treatment of neurosis, anxiety, and stroke. We recently isolated novel active compound (named 4-O-methylhonokiol) from the ethanol extract of Magnolia officinalis. The present study aimed to test two different doses of ethanol extracts of Magnolia officinalis (5 and 10 mg/kg/mouse, p.o., 1 week) and 4-O-methylhonokiol (0.75 and 1.5 mg/kg/mouse, p.o., 1 week) administered for 7 days on memory impairment induced by scopolamine (1 mg/kg body weight i.p.) in mice. Memory and learning were evaluated using the Morris water maze and the step-down avoidance test. Both the ethanol extract of Magnolia officinalis and 4-O-methylhonokiol prevented memory impairment induced by scopolamine in a dose-dependent manner. The ethanol extract of Magnolia officinalis and 4-O-methylhonokiol also dose-dependently attenuated the scopolamine-induced increase of acetylcholinesterase (AChE) activity in the cortex and hippocampus of mice, and inhibited AChE activity in vitro with IC₅₀ (12 nM). This study, therefore, suggests that the ethanol extract of Magnolia officinalis and its major ingredient, 4-O-methylhonokiol, may be useful for the prevention of the development or progression of AD.

Antioxidative and hepatoprotective effects of magnolol on acetaminophen-induced liver damage in rats

Acute liver failure (ALF), an often fatal condition characterized by massive hepatocyte necrosis, is frequently caused by drug poisoning, particularly with acetaminophen (N-acetyl-p-aminophenol/APAP). Hepatocyte necrosis is consecutive to glutathione (GSH) depletion and mitochondrial damage caused by reactive oxygen species (ROS) overproduction. Magnolol, one major phenolic constituent of Magnolia officinalis, have been known to exhibit potent antioxidative activity. In this study, the anti-hepatotoxic activity of magnolol on APAP-induced toxicity in the Sprague-Dawley rat liver was examined. After evaluating the changes of several biochemical parameters in serum, the levels of aspartate aminotransferase (AST), alanine aminotransferase (ALT), and lactate dehydrogenase (LDH) were elevated by APAP (500 mg/kg) intraperitoneal administration (8 and 24 h) and reduced by treatment with magnolol (0.5 h after APAP administration; 0.01, 0.1, and 1 mug/kg). Histological changes around the hepatic central vein, lipid peroxidation (thiobarbituric acid-reactive substance/TBARS), and GSH depletion in liver tissue induced by APAP were also recovered by magnolol treatment. The data show that oxidative stress followed by lipid peroxidation may play a very important role in the pathogenesis of APAP-induced hepatic injury; treatment with lipid-soluble antioxidant, magnolol, exerts anti-hepatotoxic activity. Our study points out the potential interest of magnolol in the treatment of toxic ALF.

Application of a TCA-precipitation method for the determination of 1-alkyl-sn-glycero-3-phosphate: Acetyl-CoA acetyltransferase in human renal tissue

The activity of 1-alkyl-sn-glycero-3-phosphate:Acetyl-CoA acetyltransferase, which catalyses the first step of the de novo biosynthesis of PAF, was determined and characterised in cortical and medullary human renal tissues. A novel thin-layer chromatographic system as well as a trichloroacetic acid precipitation method, were utilised in order to determine the enzyme's activity. The acetyltransferase activity was associated with the membranous fractions of the renal tissue, it showed an optimum pH of 8.4 and it had a bell-shaped dependence on BSA concentration. One or more disulphide bonds were necessary for the action of acetyltransferase while the enzyme seemed to be independent from divalent cations. Two assay products were extracted from the incubation mixture namely alkylacetylphosphatidic acid, produced by the acetylating action of the acetyltransferase on alkyllyso-phosphatidic acid and alkylacetyl-glycerol, which is produced by the action of a phosphohydrolase on alkylacetylphosphatidic acid. The presence of NaF in the assay mixture resulted to a decreased degradation of alkylacetylphosphatidic acid, as well as to an increased overall product formation. Cortical and medullary acetyltransferases share similar biochemical properties and there is no statistical difference between the two activities.

Tea polyphenols inhibit acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine acetyltransferase (a key enzyme in platelet-activating factor biosynthesis) and platelet-activating factor-induced platelet aggregation

BACKGROUND

Tea extracts have antiallergic and anti-inflammatory actions in rats and mice. However the mechanism through which tea polyphenols act in vivo are still incompletely understood. We found inhibitory effects of black tea extracts on an fMLP-induced aggregating response in a rabbit platelet-polymorphonuclear leukocyte (PMN) system.

METHOD

To elucidate whether 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (PAF) production in PMNs and/or PAF-stimulated platelet activation were inhibited, the effects of tea polyphenols were investigated on the enzyme activity of acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine acetyltransferase (EC 2.3.1.67), PAF biosynthesis in A23187-activated rabbit PMNs, and rabbit platelet aggregation. By comparing the inhibitory effects of 31 galloyl esters and gallic acid, the structure-inhibitory activity relationship was characterized.

RESULTS

Theaflavin and its galloyl esters and pentagalloyl glucose were found to be potent inhibitors of the acetyltransferase (IC(50) = 28-58 microM) and the PAF biosynthesis as well as (-)-epicatechin-3-O-gallate (IC(50) = 72 +/- 13 microM) and (-)-epigallocatechin-3-O-gallate (IC(50) = 46 +/- 6 microM). On the other hand, flavan-3-ols without galloyl group at C-3 and gallic acid did not show significant enzyme inhibition. In addition, theaflavin and its galloyl esters (IC(50) = 32-77 microM) and geranyl gallate, farnesyl gallate and geranylgeranyl gallate (IC(50) = 6.4-7.6 microM) were found to be potent inhibitors of PAF- and TPA-induced rabbit platelet aggregation but not A23187-induced aggregation.

CONCLUSIONS

Theaflavin and its galloyl esters in black tea extract, and isoprenyl gallates were potent inhibitors of PAF synthesis and platelet aggregation and these activities may be relevant to the claimed therapeutic effects of tea extracts.

Acetyl-CoA:1-O-alkyl-sn-glycero-3-phosphocholine acetyltransferase (lyso-PAF AT) activity in cortical and medullary human renal tissue

Platelet-activating factor (PAF) is one of the most potent inflammatory mediators. It is biosynthesized by either the de novo biosynthesis of glyceryl ether lipids or by remodeling of membrane phospholipids. PAF is synthesized and catabolized by various renal cells and tissues and exerts a wide range of biological activities on renal tissue suggesting a potential role during renal injury. The aim of this study was to identify whether cortex and medulla of human kidney contain the acetyl-CoA:1-O-alkyl-sn-glycero-3-phosphocholine acetyltransferase (lyso-PAF AT) activity which catalyses the last step of the remodeling biosynthetic route of PAF and is activated in inflammatory conditions. Cortex and medulla were obtained from nephrectomized patients with adenocarcinoma and the enzymatic activity was determined by a trichloroacetic acid precipitation method. Lyso-PAF AT activity was detected in both cortex and medulla and distributed among the membrane subcellular fractions. No statistical differences between the specific activity of cortical and medullary lyso-PAF AT was found. Both cortical and medullary microsomal lyso-PAF ATs share similar biochemical properties indicating common cellular sources.

Magnolol reduces myocardial ischemia/reperfusion injury via neutrophil inhibition in rats

The accumulation of oxygen-free radicals and activation of neutrophils are strongly implicated as important pathophysiological mechanisms mediating myocardial ischemia/reperfusion injury. It has been proven that various antioxidants have cardioprotective effects. Magnolol, an active component extracted from the Chinese medicinal herb Magnolia officinalis, possesses potent antioxidant and free radical scavenging activities. In this study, the cardioprotective activity of magnolol was evaluated in an open-chest anesthetized rat model of myocardial ischemia/reperfusion injury. The results demonstrated that pretreatment with magnolol (0.2 and 0.5 microg/kg, i.v. bolus) at 10 min before 45 min of left coronary artery occlusion, significantly suppressed the incidence of ventricular fibrillation and mortality when compared with the control group. Magnolol (0.2 and 0.5 microg/kg) also significantly reduced the total duration of ventricular tachycardia and ventricular fibrillation. After 1 h of reperfusion, pretreatment with magnolol (0.2 and 0.5 microg/kg) caused a significant reduction in infarct size. In addition, magnolol (0.2 microg/kg) significantly reduced superoxide anion production and myeloperoxidase activity, an index of neutrophil infiltration in the ischemic myocardium. In addition, pretreatment with magnolol (0.2 and 0.5 microg/kg) suppressed ventricular arrhythmias elicited by reperfusion following 5 min of ischemia. In vitro studies of magnolol (5, 20 and 50 microM) significantly suppressed N-formylmethionyl-leucyl-phenylalanine (fMLP; 25 nM)-activated human neutrophil migration in a concentration-dependent manner. It is concluded that magnolol suppresses ischemia- and reperfusion-induced ventricular arrhythmias and reduces the size of the infarct resulting from ischemia/reperfusion injury. This pronounced cardioprotective activity of magnolol may be mediated by its antioxidant activity and by its capacity for neutrophil inhibition in myocardial ischemia/reperfusion.

Honokiol and magnolol increased hippocampal acetylcholine release in freely-moving rats

Honokiol and magnolol, phenolic compounds isolated from the stem bark of Magnolia officinalis, have been demonstrated to increase choline acetyltransferase activity, inhibit acetylcholinesterase, promote potassium-induced acetylcholine release and exhibit neurotrophic function in in vitro studies. The objective of the present study was to determine the effect of these compounds on hippocampal acetylcholine release in conscious, freely-moving rats. 10(-4) M-10(-6) M of honokiol or magnolol was perfused into rat hippocampus via a dialysis probe. The results showed that at 10(-4) M concentration, honokiol and magnolol markedly increased extracellular acetylcholine release to 165.5+/-5.78% and 237.83+/-9.47% of the basal level, respectively. However, lower concentrations of either compounds failed to elicit significant acetylcholine release. This result suggests that a high dose of honokiol or magnolol may enhance in vivo hippocampal acetylcholine release.

Platelet-activating factor contributes to acute lung leak in rats given interleukin-1 intratracheally

Lung lavage fluid of patients with acute lung injury (ALI) has increased levels of interleukin-1 (IL-1) and neutrophils, but their relationship to the lung leak that characterizes these patients is unclear. To address this concern, we investigated the role of the neutrophil agonist platelet-activating factor [1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (PAF)] in the development of the acute neutrophil-dependent lung leak that is induced by giving IL-1 intratracheally to rats. We found that PAF acetyltransferase and PAF activities increased in lungs of rats given IL-1 intratracheally compared with lungs of sham-treated rats given saline intratracheally. The participation of PAF in the development of lung leak and lung neutrophil accumulation after IL-1 administration was suggested when treatment with WEB-2086, a commonly used PAF-receptor antagonist, decreased lung leak, lung myeloperoxidase activity, and lung lavage fluid neutrophil increases in rats given IL-1 intratracheally. Additionally, neutrophils recovered from the lung lavage fluid of rats given IL-1 intratracheally reduced more nitro blue tetrazolium (NBT) in vitro than neutrophils recovered from control rats or rats that had been given WEB-2086 and then IL-1. Histological examination indicated that the endothelial cell-neutrophil interfaces of cerium chloride-stained lung sections of rats given IL-1 contained increased cerium perhydroxide (the reaction product of cerium chloride with hydrogen peroxide) compared with lungs of control rats or rats treated with WEB-2086 and then given IL-1 intratracheally. These in vivo findings were supported by parallel findings showing that WEB-2086 treatment decreased neutrophil adhesion to IL-1-treated cultured endothelial cells in vitro. We concluded that PAF contributes to neutrophil recruitment and neutrophil activation in lungs of rats given IL-1 intratracheally.

Magnolol induces cytosolic-free Ca2+ elevation in rat neutrophils primarily via inositol trisphosphate signalling pathway

In the present study, we describe the role of inositol trisphosphate in the signalling pathway that leads to the elevation of cytosolic-free Ca2+ in rat neutrophils stimulated with magnolol, a compound isolated from the cortex of Magnolia officinalis. Magnolol increased [Ca2+]i, by stimulating Ca2+ release from internal stores and Ca2+ influx across the plasma membrane, in a concentration-dependent manner. Ni2+ and [6-[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H -pyrrole-2,5-dione (U73122), but not pertussis toxin, inhibited the magnolol-induced Ca2+ influx. Measurement of cellular levels of inositol trisphosphate showed a clear increase upon exposure to magnolol. U73122 but not ryanodine suppressed the Ca2+ release from internal stores caused by magnolol. Pretreatment of cells with formyl-Met-Leu-Phe (fMLP) or cyclopiazonic acid greatly reduced the [Ca2+]i changes caused by the subsequent addition of magnolol. Collectively, these findings suggest that a pertussis toxin-insensitive inositol trisphosphate signalling pathway is involved in the magnolol-induced [Ca2+]i elevation in rat neutrophils.

The sensitivity of bovine corneal epithelial lyso-PAF acetyltransferase to cyclooxygenase and lipoxygenase inhibitors is independent of arachidonate metabolites

Lyso-platelet-activating factor (PAF) acetyltransferase is a critical regulatory step in PAF synthesis. PAF accumulates in the cornea in response to injury and is a potent inflammatory mediator that stimulates corneal cyclooxygenase but not lipoxygenase reactions. 12(S)-hydroxyeicosatetraenoic (HETE) acid, a major lipoxygenase product of mammalian corneas, is also generated during injury. In the bovine corneal epithelium, both PAF acetyltransferase and 12-lipoxygenase are microsomal enzymes. A potential interaction between these two lipid mediators was, therefore, examined. PAF acetyltransferase activity was assayed by determining radioactivity in the trichloroacetic acid-precipitated complex of [3H]PAF bound to albumin, formed after incubation of corneal epithelial microsomes with lyso-PAF and [3H]acetyl CoA. Lipoxygenase metabolism by bovine corneal epithelial microsomes was also studied. While 12(S)-HETE did not activate lyso-PAF acetyltransferase, PAF synthesis was decreased when microsomes were treated with lipoxygenase inhibitors. The IC50 values for nordihydroguaiaretic acid (NDGA), and baicalein were 75 and 105 microM, respectively. The IC50 value for CDC, a more potent inhibitor of platelet 12-lipoxygenase, was greater than 200 microM. The results indicate that concentrations suppressing lyso-PAF acetyltransferase activity exceed those required to inhibit lipoxygenases from bovine corneal epithelia. Similarly, concentrations of aspirin and indomethacin, cyclooxygenase inhibitors that decrease PAF formation, were greater than those reported to block prostaglandin generation. A number of other compounds, some structurally similar to the lipoxygenase inhibitors that suppress lyso-PAF acetyltransferase, and others unrelated chemically but known as anti-oxidant and cationic-chelators, also inhibited lyso-PAF acetyltransferase. This suggests that lyso-PAF acetyltransferase activity is inhibited by mechanisms independent of lipoxygenase and cyclooxygenase metabolites.

Microplate chromatography assay for acetyl-CoA: lysoplatelet-activating factor acetyltransferase

Acetyl-CoA:lysoplatelet-activating factor (1-O-alkyl-sn-glycero-3-phosphocholine) acetyltransferase (lysoPAF-AT) (EC 2.3.1.67) is a key enzyme in the biosynthesis of platelet-activating factor (PAF) and has been shown to be activated by various extracellular stimuli. A novel method to determine the enzyme activity is described here, which enables 96 simultaneous assays in a standard 96-well microplate format. The assay is based on the quantification of the incorporation of [3H]acetyl-CoA into PAF in the presence of lysoPAF. The radioactive products are separated from the substrate with a 96-well-formatted chromatography device using a Multiscreen plate (Millipore) prefilled with octyl-silica gel. As little as 1 mg octyl-silica gel was sufficient for the efficient recovery of the radioactive product, resulting in the very low background and thus high sensitivity. The enzyme activity could be measured directly with whole cell lysates from various cells cultured in 96-well microplate scale. This tailor-made microplate chromatography separation step is readily applicable for other kinds of enzyme assays.

Inhibition of lysoPAF acetyltransferase activity by flavonoids

OBJECTIVE AND DESIGN

Several kinds of flavonoids, widely distributed natural products of the vegetable kingdom which possess anti-inflammatory activity, were examined for inhibitory effects on the acetyl-CoA: 1-alkyl-2-lyso-sn-glycero-3-phosphocholine (lysoPAF) acetyltransferase activity.

METHODS

Acetyl-CoA:lysoPAF acetyltransferase activity was determined using homogenates of a rat mucosal-type mastocytoma cell line, RBL-2H3 as an enzyme source. The production of platelet-activating factor (PAF) in rat peripheral white blood cells stimulated with the calcium ionophore A23187 was studied.

RESULTS

Of the flavonoids tested, luteolin and quercetin exhibited significant inhibitory effects (IC50, 45 microM and 80 microM, respectively), whereas other structurally-related flavonoids failed to affect the lysoPAF acetyltransferase activity. Luteolin did not suppress the activity of choline acetyltransferase, suggesting that the inhibition observed here was specific. Luteolin also inhibited the production of PAF in rat peripheral white blood cells.

CONCLUSIONS

These results indicate that luteolin could become a leading compound for developing a novel type of anti-inflammatory, anti-allergic drugs that target lysoPAF acetyltransferase.

High-density lipoprotein inhibits the synthesis of platelet-activating factor in human vascular endothelial cells

The regulation of platelet-activating factor (PAF) synthesis by serum lipoproteins was investigated in human umbilical vein endothelial cells. High-density lipoprotein (HDL) inhibited PAF synthesis in agonist (thrombin, histamine, and A23187)-stimulated endothelial cells, that was determined by incorporation of [3H]acetate into PAF and by bioassay. The inhibition by HDL was increased in a concentration-dependent manner, but was reversed as the concentration of thrombin increased. HDL did not affect the time course of PAF production. HDL lipids suppressed the PAF production to a lesser extent than HDL. The reduction of PAF accumulation in HDL, did not result from degradation of PAF but inhibition of PAF synthesis, which was mainly mediated via the blockade of acetyl-CoA:1-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase activation. HDL did not prevent the release of [3H]arachidonic acid in thrombin-stimulated endothelial cells. The binding of 125I-HDL to endothelial cells and its uptake were not enhanced by thrombin stimulation. These results demonstrate that HDL may inhibit the activation of acetyltransferase by thrombin at the cell surface. This observation may explain a part of mechanism of HDL action.