Platelet-activating factor and related ether lipid mediators. Biological activities, metabolism, and regulation

Characterization of acetyl-CoA: lyso-PAF acetyltransferase of human mesangial cells

Platelet activating factor (PAF) is a potent inflammatory mediator produced by various renal cells and it is implicated in renal pathology. The aim of this study is the characterization of remodeling lyso-PAF acetyltransferase, which is activated under inflammatory conditions, in human mesangial cell. Total membranes of mesangial cells were isolated and enzymatic activity and kinetic parameters were determined by trichloroacetic acid precipitation method. The effect of BSA, divalent cations, EDTA, and various chemicals on the activity of lyso-PAF acetyltransferase was also studied. Various detergents were also tested for the solubilization of the enzyme and only glycerol did not affect its activity. Partial purification of solubilized enzyme preparations of human kidney tissue and mesangial cells was performed on anion exchange column chromatography and native-PAGE electrophoresis and two active fractions were detected.

Synthesis and biological evaluation of novel steroid-modified ether phospholipids

Platelet activating factor is one of the most potent inflammatory ether phospholipid mediators known and structurally modified analogues are of considerable interest as potential therapeutic preparations. Inspired by the proposed structure for a novel endogenous hydroxy-PAF analogue isolated recently from gingival crevicular fluid, we designed and prepared two novel steroid-modified ether phospholipids. These two novel compounds exhibit marked chemical and biological similarities to their endogenous prototype and they antagonize it being less active in inducing washed platelet aggregation through PAF receptors.

Efficient synthesis of phospholipids from glycidyl phosphates

New efficient routes to enantiopure phospholipids, starting from (S)-glycidol, are described. Lysophosphatidic acids and phosphatidic acids were obtained in good overall yields from (S)-glycidol, in only three and four steps, respectively. Moreover, the strategy can also be used to produce phosphatidylcholines in three steps. Using dialkylphosphoramidites, (S)-glycidol was phosphorylated to give (R)-1-O-glycidyl dialkyl phosphates. Regiospecific epoxide opening, using hexadecanol or cesium palmitate, followed by phosphate deprotection, provided lysophosphatidic acids. 2-O-Esterification prior to phosphate deprotection provided 1,2-O-diacyl and 1-O-alkyl-2-O-acyl phosphatidic acids. Phosphorylation of (S)-glycidol using phosphorus oxychloride followed by in situ treatment with choline tosylate produced (R)-glycidyl phosphocholine. Subsequent nucleophilic opening of the epoxide using cesium palmitate produced 1-O-palmitoyl-sn-glycero-3-phosphocholine, which has been used in syntheses of phosphatidylcholines.

Oxidative response and membrane modification of diabetic platelets challenged with PAF

Alterations in the functional activities of platelets (PLT) in type I diabetes have been widely observed. These changes play a key role in the development of cardiovascular complications in diabetes. Various functional activities of PLT are the result of the interaction of numerous stimuli with PLT plasma membrane. This study was designed to evaluate the oxidative response and membrane modifications of diabetic PLT stimulated by platelet activating factor (PAF). The oxidative response was assessed by employing luminol- and lucigenin-amplified chemiluminescence. Luminol-amplified chemiluminescence is sensitive to the release of hydrogen peroxide whereas lucigenin-amplified chemiluminescence is sensitive to the production of superoxide anion. Membrane fluidity and polarity were studied using fluorescence spectroscopy. Membrane fluidity was investigated by measuring steady-state fluorescence anisotropy of 1-[4-trimethylammonium-phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH) and membrane polarity was studied by measuring the steady-state fluorescence emission and excitation spectra of 2-dimethylamino[6-lauroyl]-naphthalene (Laurdan). The diabetic group consisted of 20 type I diabetic children with good metabolic control. Our results show a significant decrease in the luminol- and lucigenin-amplified chemiluminescence of PAF stimulated PLT in the diabetic group with respect to controls. These data indicate a decrement in the release of reactive oxygen species by diabetic PLT. We observed a significant increase in steady-state fluorescence anisotropy of diabetic PLT membrane that reflects a decrease in membrane fluidity. Laurdan showed a blue shift of the fluorescence emission and excitation spectra in diabetic PLT with respect to the control group, indicating a decrease in membrane polarity. The addition of PAF to PLT induced a red shift of Laurdan spectra in both groups, indicating an increase in membrane polarity. Our study [table: see text] demonstrates an altered oxidative response to PAF stimulation of diabetic PLT, probably due to altered generation or handling of reactive oxygen species, and alterations in the physico-chemical properties of the plasma membrane which could influence various functional activities of PLT.

Lyso-PAF:acetyl-CoA acetyltransferase and CDP-choline cholinephosphotransferase activities in the rabbit endometrium

Endometrial platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine) levels change significantly during the pre-implantation period in the rabbit uterus, but under in vitro culture conditions, constitutive PAF biosynthesis by isolated endometrial tissues is not easily demonstrable. Rapid metabolism of PAF relative to its synthesis may account for this disparity because we have recently shown that in stromal cells there is a significant build-up of lyso-PAF suggesting that lyso-PAF-acetyl-CoA acetyltransferase might be a limiting factor. In the glandular epithelial cells however, the lyso-PAF build-up was replaced by a significant accumulation of a neutral lipid which was tentatively identified as 1-O-hexadecyl-2-acetylglycerol. It was hypothesized that, during endometrial growth and development, this lipid might serve as the substrate for the alkylacetylglycerol CDP-choline cholinephosphotransferase enzyme for PAF synthesis via the de novo pathway. We have therefore examined the activities of lyso-PAF:acetyl-CoA acetyltransferase and the CDP-cholinephosphotransferase enzymes. Microsomal preparations containing lyso-PAF:acetyl-CoA acetyltransferase activity catalyzed the incorporation of [3H]acetyl-CoA lyso-PAF into two distinct lipid products. One co-migrated with authentic PAF and the other with 1-O-hexadecyl-2-acetylglycerol, the latter being formed subsequent to PAF formation. The alkylacetylglycerol CDP-choline cholinephosphotransferase enzyme, which would potentially utilize the alkylacetylglycerol synthesized via the remodeling pathway, was also demonstrable. Unlike the species present in other tissues however, it was found to be sensitive to the presence of 10 mM DTT. The diacylglycerol CDP-choline cholinephosphotransferase species was also demonstrable and supported the synthesis of both PAF and phosphatidylcholine, in the absence of DTT, when only the synthesis of phosphatidylcholine was expected. It is hypothesized that the rabbit endometrium possesses active enzymes which may catalyze PAF synthesis via both the de novo and 'remodeling' pathways.

Platelet-activating factor is a messenger in the electroconvulsive shock-induced transcriptional activation of c-fos and zif-268 in hippocampus

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine), undetectable in resting neural tissue, accumulates in brain during seizures. A hetrazepine, BN-50730, is shown here to displace [3H]PAF-specific binding from microsomal, but not from synaptosomal membranes, indicating selectivity for a high affinity intracellular binding site. Rats pretreated with BN-50730 by intraperitoneal or intracerebroventricular injection exhibited an inhibition of the electroconvulsive shock (ECS)-induced expression of c-fos and zif-268 in hippocampus. A much more pronounced, dose-dependent inhibition of ECS-induced zif-268 mRNA in hippocampus by intracerebroventricular injection of BN-50730 was observed. It is concluded that, in the hippocampus, PAF is a mediator of the expression of zif-268 and, to a lesser extent, c-fos through an intracellular specific binding site. Thus, PAF may be a messenger in signal regulated zinc-finger transcription factors, and in other immediate-early genes involved in long-term synaptic plasticity changes.

Classical and novel forms of multidrug resistance and the physiological functions of P-glycoproteins in mammals

In this paper, we review recent work on multidrug resistance (MDR) in Amsterdam. We have generated mice homozygous for a disruption of one of their P-glycoprotein (Pgp) genes. The mutations do not interfere with viability or fertility, showing that these Pgps have no indispensable role in early development or metabolism. Mice homozygous for a disruption of their mdr2 gene, however, develop liver disease and this appears to be due to their complete inability to secrete phospholipids into bile. This suggests that the mdr2 Pgp (and, by inference, its human MDR3 homologue) is essential for translocating phospholipids through the hepatocyte canalicular membrane in which this Pgp is located. These and other results show the importance of the genetic approach for studying drug metabolism. MDR is not only caused by increased activity of Pgps. When the human non-small cell lung carcinoma cell line SW-1573 is selected in vitro for low level doxorubicin resistance, the resistant variants are nearly always multidrug resistant, but this is not due to increased Pgp activity. Only when resistance is pushed to higher levels does activation of the MDR1 Pgp gene occur. This suggests that clinically relevant levels of drug resistance in some cells may be caused predominantly by non-Pgp-mediated drug resistance mechanisms. The protein responsible for MDR in the SW-1573 cells has not yet been identified and experiments are in progress to find the gene encoding it.

Interaction between PAF and human platelet membranes: a fluorescence study

The interaction between PAF and human platelet membranes was investigated by measuring the steadystate fluorescence anisotropy and fluorescence decay of 1 (4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) incorporated in platelet plasma membranes. PAF induced a time-limited and significant increase of the lipid order in the exterior part of the membrane and a decrease in membrane heterogeneity. These changes were blocked in the presence of the PAF antagonists, L-659,989 and 1-O-hexadecyl-2-acetyl-sn-glycero-3-phospho(N,N,N-trimethyl)hexanolamine.H(2)O. These results indicate that the observed changes in the physico-chemical properties of the membrane are attributed to the PAF-receptor interaction and signal transduction.