ARDS in patients with thermal injury.

OBJECTIVE: To determine the time to onset of the adult respiratory distress syndrome (ARDS) in patients with thermal injury requiring mechanical ventilation. Secondarily, to consider the burn-related risk factors, demographics, incidence, and mortality for ARDS in this population. DESIGN: Retrospective chart review; ARDS defined according to the American-European Consensus Conference and the Lung Injury Severity Score definitions. SETTING: Regional, tertiary referral, adult burn unit in a university teaching hospital. PATIENTS AND PARTICIPANTS: Patients with thermal injury requiring mechanical ventilation, admitted between 1 January 1991 and 28 February 1995. INTERVENTIONS: None. MEASUREMENTS AND RESULTS: Of 469 consecutive admissions, 126 (26.9%) received intubation and mechanical ventilation. ARDS was defined according to the American-European Consensus and Lung Injury Severity Score (score > 2.5) definitions. The mean time to onset of ARDS from admission to the burn unit was 6.9 +/- 5.2 and 8.2 +/- 10.7 days when defined by the American-European Consensus and Lung Injury Severity Score definitions respectively (p = 0.41). Of the intubated patients, 53.6 and 45.2% developed ARDS according to the American-European Consensus and Lung Injury Severity Score definitions, respectively (p = 0.19). Using multivariate logistic analysis, only age proved to be an independent risk factor for the development of ARDS (p = 0.03), although there was a trend toward an increased incidence of inhalation injury in patients with ARDS. Mortality was not significantly greater (41.8 vs 32.2%) in those with ARDS compared to those without (p = 0.27). CONCLUSIONS: According to the American-European Consensus Conference and the Lung Injury Severity Score definitions, ARDS is common in the adult burn population and has a delayed onset compared to most critical care populations. We found age to be a major predisposing factor for ARDS.

Unilamellar liposomes modulate secretion of tumor necrosis factor by lipopolysaccharide-stimulated macrophages.

Liposomal encapsulation of antimicrobial agents has been used to improve drug delivery, particularly against intracellular pathogens. The effect of unilamellar liposomes on macrophage activation in response to Escherichia coli lipopolysaccharide was examined. Liposomes caused a dose- and time-dependent inhibition of tumor necrosis factor release by lipopolysaccharide-treated cells. The accumulation of tumor necrosis factor mRNA transcripts was unaffected, suggesting a posttranscriptional mechanism for this effect. However, induction of macrophage procoagulant activity was unaffected by liposomes, indicating a selective rather than a global inhibition. These data suggest that liposomes used for drug delivery may modulate the host response to infection.

Synthesis of the dioleoyl derivative of iododeoxyuridine and its incorporation into reconstituted high density lipoprotein particles.

We investigated the potential use of reconstituted HDL particles (NeoHDL) as a carrier for lipophilic (pro)drugs. The antiviral drug iododeoxyuridine (IDU) was used as model compound. [3H]-IDU was derivatized with two oleoyl residues to dioleoyl[3H]iododeoxyuridine ([3H]IDU-Ol2), and the lipophilic prodrug was incorporated into NeoHDL by cosonication of [3H]IDU-Ol2 with lipids and HDL apoproteins. NeoHDL particles with the same density, size, and electrophoretic mobility as native HDL were obtained, which contained 7.3 +/- 0.8% (w/w) [3H]IDU-Ol2 (about 30 molecules of prodrug per particle). NeoHDL-associated [3H]IDU-Ol2 was stable during 2 h of incubation with human plasma; the prodrug was not appreciably hydrolyzed, nor exchanged with LDL. After intravenous injection of [3H]-IDU-Ol2-loaded 125I-NeoHDL into rats, [3H]IDU-Ol2 disappeared more rapidly from the circulation than the 125I-apoproteins (78.0 +/- 8.0% vs 30.1 +/- 4.5% of the dose cleared from plasma in 60 min, respectively). The hepatic association of the prodrug was higher than that of the apoproteins (21.6 +/- 0.5 vs 5.2 +/- 1.0% of the dose at 10 min after injection, respectively). As selective clearance and uptake of lipid esters is also observed with native HDL, this suggests that, in vivo, prodrug-loaded NeoHDL may be subject to physiological HDL-specific processing. Lactosylated [3H]IDU-Ol2-loaded 125I-NeoHDL, which contains galactose residues that can be recognized by galactose receptors on parenchymal liver cells, was rapidly cleared from plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholesteryl esters from oxidized low-density lipoproteins are in vivo rapidly hydrolyzed in rat Kupffer cells and transported to liver parenchymal cells and bile.

Human low-density lipoprotein was labeled in its cholesteryl ester moiety with [3H]cholesteryl oleate or [3H]cholesteryl oleoyl ether and oxidized by exposure to 10 mumol/L of cupric sulfate. The in vivo metabolism of cholesteryl esters of oxidized low-density lipoprotein was determined after injection into rats. When oxidized low-density lipoprotein was labeled with [3H]cholesteryl oleoyl ether, a nonhydrolyzable analog of cholesteryl oleate, Kupffer cells contributed to 55.1% +/- 4.1% of the total liver uptake 10 min after injection. When [3H]cholesteryl oleate-labeled oxidized low-density lipoprotein was injected, the radiolabeled cholesterol esters were nearly completely hydrolyzed within 1 hr of injection. Within this time, the Kupffer cell-associated radioactivity declined to 32% of the maximal uptake value. In serum, the highest specific resecreted [3H]cholesteryl (esters) were associated with the serum high-density lipoprotein fraction, suggesting a role for high-density lipoprotein as an in vivo cholesterol acceptor. The kinetics of biliary secretion were studied in rats equipped with catheters in the bile duct, duodenum and heart. One hour after injection of [3H]cholesteryl oleate-labeled oxidized low-density lipoprotein, 4.15% +/- 0.67% of the injected dose was secreted in the bile, mainly as bile acids. Six hours after injection, this value was 19.2% +/- 1.2%. These values are three times higher than those for injected [3H]cholesteryl oleate-labeled acetylated low-density lipoprotein, which is initially mainly taken up by liver endothelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Visualization of the uptake and processing of oxidized low-density lipoproteins in human and rat liver.

The interaction of oxidized human low-density lipoproteins with human and rat liver was analyzed by light and electron microscopy. At the light microscopic level oxidized low-density lipoprotein was visualized by the fluorescent dye 1,1' dioctadecyl 3,3,3',3' tetramethyl indocarbocyanine perchlorate, whereas at the electron microscopic level, an indirect immunolabeling procedure was used that detected the apoprotein B of the oxidized low-density lipoprotein. In rats, oxidized low-density lipoprotein was administered intravenously, and uptake by human liver was studied by perfusion of tissue blocks. Both in human and in rat liver, fluorescently labeled oxidized low-density lipoprotein was mainly found to become concentrated in Kupffer cells and, to a lesser extent, in endothelial cells. In both species the cell association of fluorescently labeled oxidized low-density lipoprotein could be inhibited by preadministration of polyinosinic acid, indicating a scavenger receptor-mediated process. At the electron microscopic level, oxidized low-density lipoprotein was found to bind mainly to areas of the plasma membrane of the Kupffer cells without clathrin coating, although binding to coated regions was also noticed. Internalization of the ligand occurred through coated vesicle formation and through membrane folding of interacting lamellipodia and wormlike structures. No indication for phagocytosis of aggregated oxidized low-density lipoprotein particles was noticed. After internalization, the immunoreactive oxidized low-density lipoprotein was detected in relatively electron-lucent endosomes and, subsequently, in lysosomes. Endothelial cells internalized oxidized low-density lipoprotein solely through coated pits, after which the particles were transferred through endosomes into lysosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of lipoprotein-like lipid particles for drug targeting: neo-high density lipoproteins.

The possibility was explored of synthesizing, from commercially available lipids, high density lipoprotein (HDL)-like particles (neo-HDL) with the same physico-chemical and biological properties as native HDL. A preparation method involving egg yolk phosphatidylcholine, cholesterol, and apoproteins from HDL led to the formation of particles with a composition, size, electrophoretic mobility, and density similar to those of discoidal HDL. In vitro experiments with isolated parenchymal liver cells showed that unlabeled HDL and neo-HDL competed for the same high affinity binding sites as did radiolabeled neo-HDL, whereas an excess of unlabeled low density lipoprotein was ineffective. In vivo experiments with radio-labeled neo-HDL indicated that neo-HDL showed a slow decay upon injection into rats, whereas the liver uptake did not exceed > 10% of the injected dose. The small additional liver uptake of radioactivity from neo-HDL, compared with HDL, was due to enhanced uptake by endothelial and Kupffer cells. Lactosylation of neo-HDL led to a markedly increased decay rate and a rapid uptake by rat liver (80% in 10 min). Parenchymal cells accounted for > 90% of the total liver uptake of radiolabeled lactosylated neo-HDL. Because the liver uptake of lactosylated 125I-neo-HDL could be blocked by preinjection of N-acetylgalactosamine, we conclude that the asialoglycoprotein receptor, which is specifically localized on parenchymal liver cells, is responsible for the avid liver uptake. With a fibroblast cell line transfected with the human asialoglycoprotein receptor, it was found that lactosylated neo-HDL binds with high affinity (Kd, 40 nM), in a galactose-specific way. It can be concluded that, with commercially available lipid components, HDL-like particles (neo-HDL) with virtually the same characteristics as found for native apolipoprotein E-free HDL can be reconstituted. Lactosylated neo-HDL, which is rapidly taken up by galactose-specific receptors on parenchymal liver cells, might be used to transport antiviral drugs specifically to parenchymal liver cells.

Cholesterol esters selectively delivered in vivo by high-density-lipoprotein subclass LpA-I to rat liver are processed faster into bile acids than are LpA-I/A-II-derived cholesterol esters.

High-density lipoprotein (HDL) subclass LpA-I has been reported to promote cholesterol efflux from mouse adipose cells in vitro, whereas subclass LpA-I/A-II has no effect. To investigate whether the apolipoprotein composition of HDL plays a role in the selective delivery of cholesterol esters to the liver in vivo, we labelled HDL in its cholesterol ester moiety and separated [3H]cholesterol oleate-labelled HDL into subclasses LpA-I and LpA-I/A-II by immuno-affinity chromatography. Serum decay and liver association of LpA-I and LpA-I/A-II were compared for the apoprotein and cholesterol ester moieties. Both LpA-I and LpA-I/A-II selectively delivered cholesterol esters to the liver with similar kinetics. The kinetics of biliary secretion of processed cholesterol esters, initially associated with LpA-I or LpA-I/A-II, were studied in rats equipped with permanent catheters in bile, duodenum and heart. For both LpA-I and LpA-I/A-II, liver association was coupled to bile acid synthesis, with an increase in secretion rate during the night. During the first night period, the biliary secretion of LpA-I-derived radio-activity was significantly greater than for LpA-I/A-II. The data indicate that with both LpA-I and LpA-I/A-II selective delivery of cholesterol esters from HDL to the liver occurs, but that cholesterol esters delivered by LpA-I are more efficiently coupled to bile acid synthesis.

Selective uptake of cholesteryl esters from apolipoprotein-E-free high-density lipoproteins by rat parenchymal cells in vivo is efficiently coupled to bile acid synthesis.

[3H]Cholesteryl ester-labelled human high-density lipoprotein (HDL) was injected into rats and its decay, intrahepatic cellular distribution and the kinetics of biliary secretion were determined. At 10 min after injection the hepatic uptake of cholesteryl esters from HDL was 3-fold higher as compared with the apolipoprotein. Selective uptake was exerted only by parenchymal cells (5.6-fold more cholesteryl esters than apolipoprotein) and not by liver endothelial or Kupffer cells. The kinetics of biliary secretion of processed cholesteryl esters initially associated with HDL or low-density lipoprotein (LDL) were compared in unrestrained rats, equipped with permanent catheters in bile duct, duodenum and heart. At 72 h after injection of [3H]cholesteryl oleate-labelled HDL, 51.0 +/- 2.5% of the injected dose was recovered as bile acids, which is about twice as high as the secretion of biliary radioactivity after injection of [3H]cholesteryl oleate-labelled LDL. Oestradiol treatment stimulated only liver uptake of LDL cholesteryl esters, and resulted in a 2-fold higher liver uptake than with HDL. However, the rate of radioactive bile acid formation from [3H]cholesteryl oleate-labelled HDL was still more rapid than for LDL. It is concluded that the selective uptake pathway for cholesteryl esters from HDL in parenchymal cells is more efficiently coupled to the formation of bile acids than is the cholesteryl ester uptake from LDL. This efficient coupling may facilitate the role of HDL in reverse cholesterol transport.

Visualization of the interaction of native and modified lipoproteins with parenchymal, endothelial and Kupffer cells from human liver.

The interaction of low density lipoprotein, acetylated low density lipoprotein and apolipoprotein E-free high density lipoprotein with parenchymal, endothelial and Kupffer cells of human liver was visualized. For this purpose, the fluorescent phospholipid analog 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate was used to label the lipoproteins. The involvement of both parenchymal and nonparenchymal cells in the uptake of 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-labeled low density lipoprotein and acetylated low density lipoprotein was studied using in vitro perfusion of human liver tissue blocks. In addition, primary hepatocyte cultures were used to visualize the interaction with 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-labeled apolipoprotein E-free high density lipoprotein and (modified) low density lipoprotein. 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-low density lipoprotein showed a time-dependent and concentration-dependent interaction with both hepatocytes and Kupffer cells, although the intensity of the interaction with parenchymal cells varied strongly among the liver donors. Uptake of 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-low density lipoprotein by both cell types was strongly inhibited by the presence of excess unlabeled low density lipoprotein in the (perfusion) medium. Methylation and hydroxyacetaldehyde treatment of low density lipoprotein prevented the uptake of low density lipoprotein. This indicated that the uptake of low density lipoprotein by Kupffer and parenchymal cells was mediated by the low density lipoprotein receptor. 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-acetylated low density lipoprotein was mainly taken up in situ by liver endothelial cells and by a minor population of Kupffer cells. Polyinosinic acid, a known inhibitor of the scavenger receptor, prevented the uptake by liver endothelial cells. Therefore human liver endothelial cells express active scavenger receptors on their surface. Apolipoprotein E-free 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate-high density lipoprotein was found to be associated with the membrane of cultured liver parenchymal cells but was not taken up intracellularly, indicating a cholesterol exchange process occurring extracellularly at the plasma membrane. The cellular localization of lipoprotein receptors and uptake of the various classes of lipoproteins are comparable with the situation in rats.

Characterization in vitro of interaction of human apolipoprotein E-free high density lipoprotein with human hepatocytes.

Characterization of the interaction of iodinated apolipoprotein (apo) E-free high density lipoprotein (HDL) with cultured human hepatocytes provides evidence for a saturable, Ca2(+)-independent, high affinity binding site with an apparent km value of 20 micrograms/ml of apolipoprotein. Nitrated HDL and low density lipoprotein (LDL) did not compete for the binding of HDL, in contrast to very low density lipoprotein (VLDL). It is suggested that VLDL competition is exerted by the presence of apo Cs. Degradation of HDL was relatively low and in some cases not detectable. In cases where degradation was found, inhibitors of the lysosomal pathway of protein degradation had no effect, while LDL degradation was inhibited more than 80%. In the presence of 10 microM of monensin, the cell-association of HDL was unaffected, but the degradation was inhibited by 30%. Under similar conditions, LDL association was inhibited by 40% and LDL degradation, by 90%. Incubation of human hepatocytes with fluorescently labeled HDL (Dil-HDL) revealed (in contrast to Dil-LDL) mainly strong membrane-bound fluorescence and hardly any labeling of small intracellular vesicles. It is concluded that human hepatocytes possess a specific high affinity site for human HDL with recognition properties similar to those described earlier on rat hepatocytes. No evidence that the binding of HDL is actively coupled to uptake and lysosomal degradation could be obtained, indicating that binding of LDL and HDL to human hepatocytes is coupled differently to intracellular pathways.

Nitrosylated high density lipoprotein is recognized by a scavenger receptor in rat liver.

In order to assess the presence of specific recognition sites for high density lipoprotein (HDL) in vivo, HDL was nitrosylated with tetranitromethane and the decay and liver uptake were compared with that of native HDL. The association of intravenously injected nitrosylated HDL (TNM-HDL) with liver was greatly increased as compared to native HDL. Using a cold cell isolation method, it became evident that the liver endothelial cells were responsible for the increased uptake of the modified HDL. The involvement of the endothelial cells in the uptake of TNM-HDL from the circulation could also be demonstrated morphologically by using the fluorescent dye dioctadecyl-tetramethyl-indocarbocyanine perchlorate (Dil) to label HDL. In vitro competition studies with isolated liver endothelial cells indicated that unlabeled modified HDL and acetylated LDL displaced iodine-labeled TNM-HDL, while no competition was seen with LDL and a slight displacement was seen with unlabeled native HDL. Nonlipoprotein competitors of the scavenger receptor such as fucoidin and polyinosinic acid blocked the interaction of TNM-HDL with the liver endothelial cells. Also the degradation of TNM-HDL was blocked by low concentrations of chloroquine. It can be concluded that a scavenger receptor on liver endothelial cells is involved in the clearance of tetranitromethane-modified HDL, which excludes the possibility of using TNM-HDL in vivo to assess the non-receptor-dependent uptake of HDL. The use of nitrosylated HDL in vitro as a low affinity control is limited to cell types that do not possess scavenger receptors, because cell types with scavenger receptors will recognize and internalize TNM-HDL by a high affinity scavenger pathway.

Interaction in vivo and in vitro of apolipoprotein E-free high-density lipoprotein with parenchymal, endothelial and Kupffer cells from rat liver.

The interaction of apolipoprotein (apo) E-free high-density lipoprotein (HDL) with parenchymal, endothelial and Kupffer cells from liver was characterized. At 10 min after injection of radiolabelled HDL into rats, 1.0 +/- 0.1% of the radioactivity was associated with the liver. Subfractionation of the liver into parenchymal, endothelial and Kupffer cells, by a low-temperature cell-isolation procedure, indicated that 77.8 +/- 2.4% of the total liver-associated radioactivity was recovered with parenchymal cells, 10.8 +/- 0.8% with endothelial cells and 11.3 +/- 1.7% with Kupffer cells. It can be concluded that inside the liver a substantial part of HDL becomes associated with endothelial and Kupffer cells in addition to parenchymal cells. With freshly isolated parenchymal, endothelial and Kupffer cells the binding properties for apo E-free HDL were determined. For parenchymal, endothelial and Kupffer cells, evidence was obtained for a saturable, specific, high-affinity binding site with Kd and Bmax. values respectively in the ranges 10-20 micrograms of HDL/ml and 25-50 ng of HDL/mg of cell protein. In all three cell types nitrosylated HDL and low-density lipoproteins did not compete for the binding of native HDL, indicating that lipids and apo B are not involved in specific apo E-free HDL binding. Very-low-density lipoproteins (VLDL), however, did compete for HDL binding. The competition of VLDL with apo E-free HDL could not be explained by label exchange or by transfer of radioactive lipids or apolipoproteins between HDL and VLDL, and it is therefore suggested that competition is exerted by the presence of apo Cs in VLDL. The results presented here provide evidence for a high-affinity recognition site for HDL on parenchymal, liver endothelial and Kupffer cells, with identical recognition properties on the three cell types. HDL is expected to deliver cholesterol from peripheral cells, including endothelial and Kupffer cells, to the liver hepatocytes, where cholesterol can be converted into bile acids and thereby irreversibly removed from the circulation. The observed identical recognition properties of the HDL high-affinity site on liver parenchymal, endothelial and Kupffer cells suggest that one receptor may mediate both cholesterol efflux and cholesterol influx, and that the regulation of this bidirectional cholesterol (ester) flux lies beyond the initial binding of HDL to the receptor.

Stimulation of the LDL receptor activity in the human hepatoma cell line Hep G2 by high-density serum fractions.

The regulation of the LDL receptor activity in the human hepatoma cell line Hep G2 was studied. In Hep G2 cells, in contrast with fibroblasts, the LDL receptor activity was increased 2.5-fold upon increasing the concentration of normal whole serum in the culture medium from 20 to 100% by volume. Incubation of the Hep G2 cells with physiological concentrations of LDL (up to 700 micrograms/ml) instead of incubation under serum-free conditions resulted in a maximum 2-fold decrease in LDL receptor activity (10-fold decrease in fibroblasts). Incubation with physiological concentrations of HDL with a density of between 1.16 and 1.20 g/ml (heavy HDL) resulted in an approximately 7-fold increase in LDL receptor activity (1.5-fold increase in fibroblasts). This increased LDL receptor activity is due to an increase in the number of LDL receptors. Furthermore, simultaneous incubation of Hep G2 cells with LDL and heavy HDL (both 200 micrograms/ml) resulted in a 3-fold stimulation of the LDL receptor activity as compared with incubation in serum-free medium. 3-Hydroxy-3-methylglutaryl-CoA reductase activity was also stimulated after incubation of Hep G2 with heavy HDL (up to 3-fold). The increased LDL receptor activity in Hep G2 cells after incubation with heavy HDL was independent of the action of lecithin:cholesterol acyltransferase during that incubation. However, previous modification of heavy HDL by lecithin:cholesterol acyltransferase resulted in an enhanced ability of heavy HDL to stimulate the LDL receptor activity. Our results indicate that in Hep G2 cells the heavy HDL-mediated stimulation of the LDL receptor activity overrules the LDL-mediated down-regulation and raises the suggestion that in man the presence of heavy HDL and the action of lecithin:cholesterol acyltransferase in plasma may be of importance in receptor-mediated catabolism of LDL by the liver.

High-affinity uptake and degradation of acetylated low density lipoprotein by confluent human vascular endothelial cells.

Confluent human vascular endothelial cells take up and degrade acetylated low density lipoproteins (Ac-LDL) via a high-affinity binding process, comparable to that for native LDL. The degradation of 125I-labelled Ac-LDL by endothelial cells was inhibited by the addition of unlabelled Ac-LDL but not by the addition of unlabelled LDL. Similarly, the degradation of 125I-labelled LDL could be inhibited by unlabelled LDL but not by unlabelled Ac-LDL. Unlabelled apolipoprotein E-free HDL did not compete with the degradation of either 125I-labelled LDL or Ac-LDL. Electron microscopical studies, using gold-labelled LDL and gold-labelled Ac-LDL, showed that at 4 degrees C both LDL and Ac-LDL bind to indented regions of the endothelial cell plasma membrane. At 37 degrees C both LDL and Ac-LDL were taken up and associated with lysosomes. Although morphologically identical, we conclude that the binding of LDL and Ac-LDL to human endothelial cells proceeds via 2 different high-affinity receptors. The uptake and degradation of Ac-LDL by human endothelial cells was about 25 and 15%, respectively, of that of native LDL. The uptake and degradation of either LDL or Ac-LDL did not lead to a massive increase in cellular cholesterol content.

Effects of compactin, mevalonate and low-density lipoprotein on 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity and low-density-lipoprotein-receptor activity in the human hepatoma cell line Hep G2.

Compactin, an inhibitor of HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) reductase, decreased cholesterol synthesis in intact Hep G2 cells. However, after the inhibitor was washed away, the HMG-CoA-reductase activity determined in the cell homogenate was found to be increased. Also the high-affinity association of LDL (low-density lipoprotein) to Hep G2 cells was elevated after incubation with compactin. Lipoprotein-depleted serum, present in the incubation medium, potentiated the compactin effect compared with incubation in the presence of human serum albumin. Addition of either mevalonate or LDL prevented the compactin-induced rise in activities of both HMG-CoA reductase and LDL receptor in a comparable manner. It is concluded that in this human hepatoma cell line, as in non-transformed cells, both endogenous mevalonate or mevalonate-derived products and exogenous cholesterol are able to modulate the HMG-CoA reductase activity as well as the LDL-receptor activity.

Characterisation of the binding of apolipoprotein E-free high density lipoprotein to cultured human endothelial cells.

[125I]-labelled apolipoprotein E-free high density lipoprotein (apo E-free HDL) binds to cultured human endothelial cells with high affinity. Competitive binding experiments showed that complexes of egg phosphatidyl choline with respectively apo A-1, A-2 and E, and phosphatidyl choline vesicles alone, competed efficiently with [125I]-apo E-free HDL for binding, suggesting that the binding of HDL to the high affinity receptor is not mediated by recognition of one specific apolipoprotein. Analyses of the respective incubation media of the competitive binding experiments by density gradient ultracentrifugation showed that the [125I]-label of [125I]-HDL redistributes to the competitors used. This implies that the usual competitive binding experiments may not be used in order to investigate which HDL component is involved in the high affinity binding of HDL to the plasma membrane.