The interaction of prostaglandins with serum low-density lipoproteins

New fluorescent cholesterol analogs as membrane probes

New fluorescent cholesterol analogs, (22E, 20R)-3beta-hydroxy-23-(9-anthryl)-24-norchola-5,22-die ne (R-AV-Ch), and the 20S-isomer (S-AV-Ch) were synthesized, their spectral and membrane properties were characterized. The probes bear a 9-anthrylvinyl (AV) group instead of C22-C27 segment of the cholesterol alkyl chain. Computer simulations show that both of the probes have bulkier tail regions than cholesterol and predict some perturbation in the packing of membranes, particularly for R-AV-Ch. In monolayer experiments, the force-area behavior of the probes was compared with that of cholesterol, pure and in mixtures with palmitoyloleoyl phosphatidylcholine (POPC) and N-stearoyl sphingomyelin (SSM). The results show that pure R-AV-Ch occupies 35-40% more cross-sectional area than cholesterol at surface pressures below film collapse (0-22 mN/m); whereas S-AV-Ch occupies nearly the same molecular area as cholesterol. Isotherms of POPC or SSM mixed with 0.1 mol fraction of either probe are similar to isotherms of the corresponding mixtures of POPC or SSM with cholesterol. The probes show typical AV absorption (lambda 386, 368, 350 and 256 nm) and fluorescence (lambda 412-435 nm) spectra. Steady-state anisotropies of R-AV-Ch and S-AV-Ch in isotropic medium or liquid-crystalline bilayers are higher than the values obtained for other AV probes reflecting hindered intramolecular mobility of the fluorophore and decreased overall rotational rate of the rigid cholesterol derivatives. This suggestion is confirmed by time-resolved fluorescence experiments which show also, in accordance with monolayer data, that S-AV-Ch is better accommodated in POPC-cholesterol bilayers than R-AV-Ch. Model and natural membranes can be labeled by either injecting the probes via a water-soluble organic solvent or by co-lyophilizing probe and phospholipid prior to vesicle production. Detergent-solubilization studies involving 'raft' lipids showed that S-AV-Ch almost identically mimicked the behavior of cholesterol and that of R-AV-Ch was only slightly inferior. Overall, the data suggest that the AV-labeled cholesterol analogs mimic cholesterol behavior in membrane systems and will be useful in related studies.

Modified LDL decreases the binding of prostaglandin E2, I2, and E1 onto monocytes in patients with peripheral vascular disease

Recent data suggest that various eicosanoids including prostaglandins play an important regulatory role in the development of atherosclerotic lesions. Peripheral blood monocytes have been implemented in early atherogenesis because they express receptors specific for modified LDL. In this study we investigated the binding of tritium prostaglandins E2 (3H-PGE2), E1 (3H-PGE1) and I2 (3H-PGI2) onto intact peripheral monocytes isolated from 20 patients (32-71 years) with manifested ischemic peripheral vascular disease stage II according to Fontaine and compared the results with those obtained in 16 healthy volunteers (21-68 years). In control subjects, Scatchard analyses of the binding data indicated a single class of high-affinity binding sites for 3H-PGE2 (maximal binding capacity [Bmax] = 11,400 +/- 3200 sites/cell; dissociation constant [Kd] = 1.3 +/- 0.5 nmol/L) and two classes of binding sites for 3H-PGE1 (Bmax1 = 11,200 +/- 4900 sites/cell, Kd1 = 1.5 +/- 0.5 nmol/L; Bmax2 = 47,800 +/- 6100 sites/cell, Kd2 = 12.8 +/- 5.9 nmol/L) as well as for 3H-PGI2 (Bmax1 = 10,100 +/- 3700 sites/cell, Kd1 = 1.7 +/- 0.7 nmol/L; Bmax2 = 81,200 +/- 5200 sites/cell, Kd2 = 14.2 +/- 6.5 nmol/L). In the patients, an absence of the higher-affinity binding class and significantly (P < .01) fewer lower-affinity binding sites were found for each ligand (PGE2: Bmax = 6600 +/- 3600 sites/cell, Kd = 12.1 +/- 3.2 nmol/L; PGI2: Bmax = 6400 +/- 3100 sites/cell, Kd = 22.1 +/- 8.3; PGE1: Bmax = 5300 +/- 1700 sites/ cell, Kd = 20.5 +/- 7.0 nmol/L). After incubation of monocytes with modified LDL (oxidized LDL or acetylated LDL), the binding of prostaglandins was significantly (P < .01 to P < .001) decreased, whereas native VLDL, LDL, and HDL did not interfere with prostaglandin binding. Prostaglandin-induced adenosine 3'-5' cyclic monophosphate (cAMP) formation by monocytes was significantly (P < .01) lower in patients (the concentrations causing 50% elevation of basal cAMP formation [ED50] were 3.8 +/- 2.4 nmol/L for PGE2, 6.3 +/- 3.5 nmol/L for PGE1, and 5.6 +/- 4.1 nmol/L for PGI2) than in the control subjects (ED50 was 1.6 +/- 1.2 nmol/L for PGE2, 4.8 +/- 2.5 nmol/L for PGE1, and 3.1 +/- 1.4 nmol/L for PGI2). After preincubation with modified LDL, the PG-induced cAMP production by monocytes was remarkably decreased in both patients and control subjects (P < .05). Our results suggest a direct effect of modified LDL on PGE2, PGE1, and PGI2 binding onto monocytes by reducing the number of cell surface-expressed receptors available. Modified LDL also reduces the sensitivity of monocytes to prostaglandins, which results in decreased cAMP production. The complex interactions between prostaglandins and lipoproteins may play an important role during atherogenesis.

Dynamic lipid heterogeneity and receptor events

Receptor occupation by specific ligands induces changes in the dynamic domain organization of surrounding lipids. Such changes were observed by measuring changes in the fluorescence parameters of fluorescent-labelled lipids incorporated into plasma membranes of intact cells, membrane vesicles or lipoprotein particles in response to specific binding of a broad range of biologically active agents, including drugs, prostaglandins, neuropeptides, antibodies and viruses. The high sensitivity of the fluorescence response allowed us to register changes in lipid heterogeneity induced in a multitude of discrete targets by transient weak binding of a single rapidly translocating molecule. To explain these observations a non-equilibrium model of ligand-receptor interaction based on low relaxation phenomena in heterogeneous lipid matrixes is proposed.

Anthrylvinyl-labeled phospholipids as fluorescent membrane probes. The action of melittin on multilipid systems

The interaction of melittin with multicomponent lipid mixtures composed of phosphatidylcholine, sphingomyelin and phosphatidylserine or phosphatidylglycerol was investigated by measuring the intrinsic fluorescence of the peptide, steady state fluorescence anisotropy of, and Trp-fluorescence energy transfer to fluorescent analogs of the same phospholipids bearing the anthrylvinyl fluorophore in one of the aliphatic chains at various distances from the polar head group. Based on the finding that at high lipid/peptide ratio the peptide induces unequal changes in the fluorescence parameters of phospholipid probes differing structurally only in their polar head groups, it is concluded that melittin induces lipid demixing in its nearest environment. Comparison of the fluorescence energy transfer from Trp to different lipid probes indicates that the depth of penetration of melittin into the bilayer depends on the polar head group composition of the phospholipid matrix and that certain segments of the melittin chain display a specific affinity for a given lipid head group.

Effect of prostaglandin E1 on low density lipoprotein apo-B-receptor binding in human, rat and swine liver in vitro

The effect of PGE1 on low density lipoprotein (LDL) apo-B-receptor binding was examined in human, rat and swine liver. Autologous LDL (for humans and swines) and homologous LDL (for rats) were isolated by ultracentrifugation and labelled with 123I using Iodogen followed by purification with dialysis. LDL-concentrations of 0.1-6 micrograms protein/ml were used for direct binding assays investigating the specific binding of labelled LDL in presence of increasing PGE1-concentrations (100 pM to 100 microM). In separate experiments the effect of PGE1 on displacement of specifically bound 123I-LDL by unlabelled ones was studied. The binding capacities estimated by Scatchard analysis were similar for human and rat liver LDL-apo-B-receptor binding, however, swine liver exhibited a significantly (p less than 0.001) lower binding capacity for 123I-LDL. PGE1 significantly (p less than 0.01-0.001) increased the amount of 123I-LDL specifically bound to the liver apo-B-receptors and the binding affinity in all liver preparations of the 3 species in a dose-dependent manner. PGE1 also significantly increased competition of unlabelled LDL for 123I-LDL bound to its specific apo-B-receptors in a dose-dependent manner (p less than 0.01-0.001) with an ED50 of 123 +/- 64 nM for human liver, 901 +/- 102 nM for rat liver obtained during anaesthesia, 74 +/- 23 nM for rat liver obtained after decapitation and 941 +/- 121 nM for swine liver. In human liver iloprost (ED50 = 876 +/- 53 nM) and PGI2 (ED50 = 52 +/- 12 microM) were less effective than PGE1, PGE2 had no effect on LDL-induced competition. It is concluded that PGE1 renders LDL more sensitive for apo-B-receptor binding suggesting a potential hypolipidemic action of PGE1.

The interaction of prostaglandin E1 with serum lipoproteins. Possible role in cholesterol homeostasis

Prostaglandin (PG) E1 significantly stimulates the rate of cholesterol esterification in plasma. This effect could be attributed to an enhancement by PGE1 of the interlipoprotein transfer of phosphatidylcholine and cholesteryl esters, i.e., the substrate and product of lecithin-cholesterol acyltransferase (LCAT). The enhancement effect appears to be due to a rearrangement of the lipoprotein surface induced by specific interaction of PGE1 with some apolipoproteins, although the binding capacity of serum lipoproteins for PGE1 was found to be rather weak. To explain these findings, an hypothetical non-equilibrium model was put forward. The purpose of the present article is to summarize available data on the PGE1-lipoprotein interaction.