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At least 19 recordsLinked to original sources
PubMed1987

Tetranitromethane modification of human high density lipoprotein (HDL3): inactivation of high density lipoprotein binding is not related to cross-linking of phospholipids to apoproteins.

Treatment of human high density lipoprotein (HDL) with tetranitromethane (TNM) inhibits its binding to HDL-specific binding sites of cells and isolated membranes. The mechanism of this inhibition, however, is not known; during treatment of HDL with TNM, in addition to the expected nitration of tyrosine residues, cross-linking of lipids to apoproteins and of apoproteins to one another occurs. In order to determine whether the cross-linking of lipids to apoproteins occurs through the carbon-carbon double bonds in the acyl chains, and to determine whether the cross-linking of phospholipids to apoproteins is a possible mechanism of inhibition of binding, we have prepared a reconstituted HDL3 in which the native phospholipids were replaced with dimyristoyl phosphatidylcholine (DMPC). As a control, a reconstituted HDL3 (C-r-HDL3) was also prepared using the total apoproteins and the total lipid constituents of native HDL3. The reconstituted DMPC-containing HDL3 (DMPC-r-HDL3) was similar to native HDL3 and to C-r-HDL3 in its agarose gel electrophoretic mobility, in its chemical composition, and in its binding to rat liver plasma membranes. When treated with TNM, DMPC-r-HDL3, like the native HDL3 and C-r-HDL3, lost its ability to bind to the HDL binding sites of rat liver plasma membranes, as determined by competitive binding assays with 125I-labeled human HDL3 as the tracer. Nitrated DMPC-r-HDL3 contained only traces of phospholipids covalently linked to apoproteins, whereas 21-26% of the total phospholipids were cross-linked to apoproteins of nitrated C-r-HDL3 and nitrated native HDL3.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1982

Interaction of discoidal complexes of dimyristoyl phosphatidylcholine-cholesterol-apolipoprotein A-I with human plasma high density lipoprotein HDL3.

The interaction of human plasma high density lipoproteins (HDL3) with discoidal complexes of apolipoprotein A-I (apo A-I) and dimyristoyl phosphatidylcholine (DMPC) containing 0, 10, 20 or 30 mol % cholesterol was investigated. Discoidal complexes containing various amounts of cholesterol were prepared by incubating apo A-I and DMPC-cholesterol liposomes for 12 hr at 25 C; the protein-lipid complexes were isolated by gel filtration chromatography on Bio-Gel A15m. Increasing the cholesterol content from 0 to 30 mol % caused a decrease in the fluidity of the discoidal complexes as determined by fluorescence polarization with 1,6-diphenyl-1,3,5-hexatriene; a reduced phase-transition amplitude; a decrease in the ratio of apo A-I to DMPC; and an increase in the width of the discoidal complexes as determined by electron microscopy after negative staining. Incubation of the apo A-I-lipid complexes with HDL3 resulted in a complete breakdown of the discoidal structures and a transfer to DMPC and cholesterol to HDL3. As a result of lipid transfer, there was an increase in the size of HDL3. These in vitro results may be of significance as they relate to the interconversion of HDL subfractions during lipoprotein-lipase-induced lipolysis of triglyceride-rich lipoproteins.

Apolipoprotein A-I↗
PubMed1998

Oxidation of high-density lipoprotein HDL3 leads to exposure of apo-AI and apo-AII epitopes and to formation of aldehyde protein adducts, and influences binding of oxidized low-density lipoprotein to type I and type III collagen in vitro1.

The changes in the immunological properties of apolipoprotein AI (apo-AI) and AII (apo-AII) during the oxidation of the high-density lipoprotein HDL3 and its influence on the binding of heavily oxidized low-density lipoprotein (LDL) to type I and III collagen were investigated. Oxidation of HDL3 or Eu3+-labelled HDL3 was performed with CuSO4, varying the time of oxidation. Oxidation of HDL3 resulted in an increase in lipid hydroperoxides and enhanced the negative charge of this lipoprotein. Immunological studies with a solid-phase sandwich immunoassay revealed a strong increase in binding of Eu3+-labelled HDL3 to polyclonal antibodies against apo-AI and apo-AII within the first 4 h of oxidation. Neo-epitopes were also formed by interaction of the apolipoproteins with degradation products from the lipid peroxidation of polyunsaturated fatty acids, as evidenced by an immunoreaction of oxidized Eu3+-labelled HDL3 with antibodies to 4-hydroxynonenal (4-HNE)- and malondialdehyde (MDA)-protein adducts. Western blot analysis of oxidized HDL3 samples showed, as well as apo-AI and apo-AII bands, larger aggregated apolipoproteins, occurring after 0.5-2.5 h of oxidation. These aggregates were recognized by antibodies to apo-AI and apo-AII as well as by antibodies to 4-HNE- and MDA-protein adducts. Furthermore the original apo-AI monomers and apo-AII dimers decreased during the oxidation. The ability of native and oxidized HDL3 to prevent the binding of Eu3+-labelled 24 h-oxidized LDL to collagen on microtitration plates was estimated. Interestingly, 2 h-oxidized HDL3 competed most with the binding of 24 h-oxidized LDL on collagen type I and type III, followed by native HDL3. However, 24 h-oxidized HDL3 was a weaker competitor. Thus oxidative modification of HDL3 strongly alters the immunological properties of this lipoprotein and its binding affinity for collagen.

Aldehydes↗
PubMed1998

High-density lipoprotein (HDL3)-associated alpha-tocopherol is taken up by HepG2 cells via the selective uptake pathway and resecreted with endogenously synthesized apo-lipoprotein B-rich lipoprotein particles.

alpha-Tocopherol (alphaTocH) is transported in association with lipoproteins in the aqueous milieu of the plasma. Although up to 50% of circulating alphaTocH is transported by high-density lipoproteins (HDLs), little is known about the mechanisms of uptake of HDL-associated alphaTocH. During the current study, human apolipoprotein (apo)E-free HDL subclass 3 (HDL3) labelled with [14C]alphaTocH was used to investigate uptake mechanisms of HDL3-associated alphaTocH by a permanent hepatoblastoma cell line (HepG2). HDL3-associated alphaTocH was taken up independently of HDL3 holoparticles in excess of apoA-I comparable with the non-endocytotic delivery of cholesteryl esters to cells termed the 'selective' cholesteryl ester uptake pathway. Experiments with unlabelled HDL3 demonstrated net mass transfer of alphaTocH to HepG2 cells. Time-dependent studies with [14C]alphaTocH-labelled HDL3 revealed tracer uptake in 80-fold excess of apoA-I and in 4-fold excess of cholesteryl linoleate. In addition to HLDs, low-density lipoprotein (LDL)-associated alphaTocH was also taken up in excess of holoparticles, although to a lesser extent. These findings were confirmed with unlabelled lipoprotein preparations, in which HDL3 displayed a 2- to 3-fold higher alphaTocH donor efficiency than LDLs (lipoproteins adjusted for equal amounts of alphaTocH). An important factor affecting particle-independent uptake of alphaTocH was the cellular cholesterol content (a 2-fold increase in cellular cholesterol levels resulted in a 2.3-fold decrease in uptake). Pulse-chase studies demonstrated that some of the HDL3-associated alphaTocH taken up independently of holoparticle uptake was resecreted along with a newly synthesized apoB-containing lipoprotein fraction.

Acrylamide↗
PubMed1982

Human high density lipoprotein (HDL3) binding to rat liver plasma membranes.

The binding of human 125I-labeled HDL3 to purified rat liver plasma membranes was studied. 125I-labeled HDL3 bound to the membranes with a dissociation constant of 10.5 micrograms protein/ml and a maximum binding of 3.45 micrograms protein/mg membrane protein. The 125I-labeled HDL3-binding activity was primarily associated with the plasma membrane fraction of the rat liver membranes. The amount of 125I-labeled HDL3 bound to the membranes was dependent on the temperature of incubation. The binding of 125I-labeled HDL3 to the rat liver plasma membranes was competitively inhibited by unlabeled human HDL3, rat HDL, HDL from nephrotic rats enriched in apolipoprotein A-I and phosphatidylcholine complexes of human apolipoprotein A-I, but not by human or rat LDL, free human apolipoprotein A-I or phosphatidylcholine vesicles. Human 125I-labeled apolipoprotein A-I complexed with egg phosphatidylcholine bound to rat liver plasma membranes with high affinity and saturability, and the binding constants were similar to those of human 125I-labeled HDL3. The 125I-labeled HDL3-binding activity of the membranes was not sensitive to pronase or phospholipase A2; however, prior treatment of the membranes with phospholipase A2 followed by pronase digestion resulted in loss of the binding activity. Heating the membranes at 100 degrees C for 30 min also resulted in an almost complete loss of the 125I-labeled HDL3-binding activity.

Animals↗
PubMed1985

Specific binding of high density lipoprotein (HDL3) is not related to sterol synthesis in rat intestinal mucosa.

There is good evidence that high density lipoprotein (HDL) is involved in the flux of cholesterol into the cells of some organs and out of the cells of other tissues. Because we have previously found that HDL is bound specifically by mucosal cells of the small intestine, we have examined the possibility that this was associated with regulation of cholesterol flux. We have, therefore, compared the specific binding of 125I-labeled HDL3 with cholesterol synthesis in mucosal cells obtained from rats that had been treated to alter intestinal cholesterol metabolism. The rate of sterol synthesis measured in tissue slices, by the incorporation of [3H]water into sterols, was altered up to fivefold by treatment with cholestyramine (to induce bile salt loss), by surformer treatment (to reduce absorption of cholesterol), and by biliary diversion. Yet the capacity of mucosal cells to bind, internalize, and degrade 125I-labeled HDL3 was unchanged. Cholesterol feeding influenced neither the interaction of 125I-labeled HDL3 with cells nor the rate of sterol synthesis. Furthermore, the interactions of 125I-labeled HDL3 with mucosal cells isolated from the proximal and distal halves of the intestine or between the upper and lower villus cells were similar, despite differences in sterol synthesis. These data suggest that, in rat intestine, the specific binding of HDL is not related to sterol synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1985

Modification of human high density lipoprotein (HDL3) with tetranitromethane and the effect on its binding to isolated rat liver plasma membranes.

Apolipoprotein E-free high density lipoproteins (HDL) bind to various cells and cell membrane preparations, with properties typical of ligand-receptor interactions. In order to further characterize the binding sites and to investigate the functional role of binding, a chemically modified HDL without the specific binding properties would be highly desirable. We have reacted human HDL3 with tetranitromethane, a relatively specific nitrating reagent for tyrosine residues, in 50 mM Tris HCL buffer, pH 8.0, and at a reagent concentration 10 times the molar excess of tyrosine residues. The resulting nitrated HDL3 completely lost its ability to bind to high affinity saturable binding sites of rat liver plasma membranes, as determined by competitive binding with 125I-labeled HDL3, and also by direct binding assays using 125I-labeled nitrated HDL3. Although nitrated HDL3 did not bind to the high affinity saturable binding sites, it bound to the membranes, but the binding was not saturable, and was not competed for by unlabeled nitrated HDL3. On agarose gel electrophoresis, pH 8.6, the nitrated HDL3 moved ahead of the control HDL3, indicating an increase in negative charges in the molecule. No difference in size was noted in the nitrated HDL3 when analyzed either by negative stain electron microscopy or by gel filtration chromatography. Spectroscopic analysis of the nitrated HDL3 at pH 8.0 revealed a prominent absorption with maximum at around 360 nm, but none in the region expected for nitrotyrosine residues. At pH 10.0, however, the nitrated HDL3 showed an absorption band with a maximum at around 440 nm, possibly related to nitrotyrosine residues. Nitrotyrosine was detected in the nitrated HDL3 on amino acid analysis. Comparison of the amino acid analysis of the nitrated HDL3 and control HDL3 showed no difference in composition of any of the amino acids except tyrosine; tyrosine content was reduced more than 90% in the nitrated HDL3. SDS-polyacrylamide gel electrophoresis analysis of apoproteins of nitrated HDL3 revealed changes in apolipoprotein profile. Bands corresponding to the apolipoproteins of the starting HDL3 almost disappeared and a series of new bands appeared at the high molecular weight region of the gel, indicating extensive cross-linking of apolipoproteins during the reaction. In addition, a substantial amount of phospholipids and cholesteryl esters, but not unesterified cholesterol, was found covalently linked, possibly through the unsaturated centers of the fatty acid chains, to apolipoproteins.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1976

Rat high density lipoprotein subfraction (HDL3) uptake and catabolism by isolated rat liver parenchymal cells.

Rat liver parenchymal cell binding, uptake, and proteolytic degradation of rat 125I-labeled high density lipoprotein (HDL) subfraction, HDL3 (1.10 less than d less than 1.210 g/ml), in which apo-A-I is the major polypeptide, were investigated. Structural and metabolic integrity of the isolated cells was verified by trypan blue exclusion, low lactic dehydrogenase leakage, expected morphology, and gluconeogenesis from lactate and pyruvate. 125I-labeled HDL3 was incubated with 10 X 10(6) cells at 37 degrees and 4 degrees in albumin and Krebs-Henseleit bicarbonate buffer, pH 7.4. Binding and uptake were determined by radioactivity in washed cells. Proteolytic degradation was determined by trichloroacetic acid-soluble radioactivity in the incubation medium. At 37 degrees, maximum HDL3 binding (Bmax) and uptake occurred at 30 min with a Bmax of 31 ng/mg dry weight of cells. The apparent dissociation constant of the HDL3 receptor system (Kd) was 60 X 10(-8) M, based on Mr = 28,000 of apo-A-I, the predominant rat HDL3 protein. Proteolytic degradation showed a 15-min lag and then constant proteolysis. After 2 hours 5.8% of incubated 125I-labeled HDL3 was degraded. Sixty per cent of cell radioactivity at 37 degrees was trypsin-releasable. At 37 degrees, 125I-labeled HDL3 was incubated with cells in the presence of varying concentrations of native (cold) HDL3, very low density lipoproteins, and low density lipoproteins. Incubation with native HDL3 resulted in greatest inhibition of 125I-labeled HDL3 binding, uptake, and proteolytic degradation. When 125I-labeled HDL3 was preincubated with increasing amounts of HDL3 antiserum, binding and uptake by cells were decreased to complete inhibition. Cell binding, uptake, and proteolytic degradation of 125I-labeled HDL3 were markedly diminished at 4 degrees. Less than 1 mM chloroquine enhanced 125I-labeled HDL3 proteolysis but at 5 mM or greater, chloroquine inhibited proteolysis with 125I-labeled HDL3 accumulation in cells. L-[U-14C]Lysine-labeled HDL3 was bound, taken up, and degraded by cells as effectively as 125I-labeled HDL3. These data suggest that liver cell binding, uptake, and proteolytic degradation of rat HDL3 are actively performed and linked in the sequence:binding, then uptake, and finally proteolytic degradation. Furthermore, there may be a specific HDL3 (lipoprotein A) receptor of recognition site(s) on the plasma membrane. Finally, our data further support our previous reports of the important role of liver lysosomes in proteolytic degradation of HDL3.

Animals↗
PubMed1999

Uptake and fate of class B scavenger receptor ligands in HepG2 cells.

Class B scavenger receptors (SR-Bs) interact with native, acetylated and oxidized low-density lipoprotein (LDL, AcLDL and OxLDL), high-density lipoprotein (HDL3) and maleylated BSA (M-BSA). The aim of this study was to analyze the catabolism of CD36- and LIMPII-analogous-1 (CLA-1), the human orthologue for the scavenger receptor class B type I (SR-BI), and CD36 ligands in HepG2 (human hepatoma) cells. Saturation binding experiments revealed moderate-affinity binding sites for all the SR-B ligands tested with dissociation constants ranging from 20 to 30 microg.mL-1. Competition binding studies at 4 degrees C showed that HDL and modified and native LDL share common binding site(s), as OxLDL competed for the binding of 125I-LDL and 125I-HDL3 and vice versa, and that only M-BSA and LDL may have distinct binding sites. Degradation/association ratios for SR-B ligands show that LDL is very efficiently degraded, while M-BSA and HDL3 are poorly degraded. The modified LDL degradation/association ratio is equivalent to 60% of the LDL degradation ratio, but is three times higher than that of HDL3. All lipoproteins were good cholesteryl ester (CE) donors to HepG2 cells, as a 3.6-4.7-fold CE-selective uptake ([3H]CE association/125I-protein association) was measured. M-BSA efficiently competed for the CE-selective uptake of LDL-, OxLDL-, AcLDL- and HDL3-CE. All other lipoproteins tested were also good competitors with some minor variations. Hydrolysis of [3H]CE-lipoproteins in the presence of chloroquine demonstrated that modified and native LDL-CE were mainly hydrolyzed in lysosomes, whereas HDL3-CE was hydrolyzed in both lysosomal and extralysosomal compartments. Inhibition of the selective uptake of CE from HDL and native modified LDL by SR-B ligands clearly suggests that CLA-1 and/or CD36 are involved at least partially in this process in HepG2 cells.

Binding, Competitive↗
PubMed1989

Use of a dual-precipitation procedure for measuring high-density lipoprotein 3 (HDL3) in normolipidemic serum.

We compared results by a dual-precipitation method (J Lipid Res 1982;23:1206-23) for measuring high-density lipoprotein 3 (HDL3) cholesterol with those by ultracentrifugation at d 1.125, using 56 fresh and 105 frozen-stored serum samples. For both methods, HDL2-cholesterol was calculated as the difference between total HDL-cholesterol and HDL3-cholesterol. In general, for pooled serum samples, agreement was closest with ultracentrifugation when we used a dextran sulfate concentration of 5.0 mg/L to precipitate the HDL2-rich fraction, although the optimal concentration varied from 3.0 to 6.8 mg/L for different pools. For individual samples, the values for HDL3 by dual precipitation averaged 12.8% lower than by ultracentrifugation. The coefficients of correlation between the two methods were HDL3, r = 0.70; and HDL2, r = 0.92. The dual-precipitation method reflected the expected sex-related differences in HDL2-cholesterol concentration and inverse relationship with triglyceride concentration.

Adult↗
PubMed1982

Plasma high density lipoproteins HDL2, HDL3 and postheparin plasma lipases in relation to parameters of physical fitness.

A number of studies has shown that the plasma levels of high density lipoprotein (HDL) are increased by regular aerobic exercise. The plasma HDL, particularly HDL2, is regulated by the activity of 2 endothelial lipases, viz. lipoprotein lipase (LPL) and hepatic lipase (HL), which both can be assayed in postheparin plasma. In the present study the plasma levels of HDL2 and HDL3 cholesterol and the postheparin plasma lipase activities were related to parameters of physical fitness obtained from a pulse conducted maximal bicycle ergometer test. There was a significant positive correlation between HDL2 cholesterol and physical fitness (r = 0.52, P less than 0.01). On the other hand, the postheparin plasma hepatic lipase activity showed a significant negative correlation to physical fitness (r = -0.57, P less than 0.01). The HDL2 cholesterol was inversely correlated with the HL activity (r = 0.57, P less than 0.001). Application of partial correlation analysis to the data showed that the relationship between HDL2 cholesterol and fitness disappeared by keeping the HL activity constant whereas the correlation between HDL2 and HL was not influenced by fitness. The relation of HDL2 to fitness was independent in body fat and basal plasma insulin level; in addition the relationship between HL and fitness was not accounted for by body fatness. No relationship was found between physical fitness and LPL activity or between HDL3 and fitness. The results support the hypothesis that hepatic endothelial lipase has a role in the regulation of plasma HDL2 cholesterol and that the activity of this enzyme decreases upon increase of physical fitness.

Adult↗
PubMed1990

Effects of endogenous and exogenous cholesterol on the ultrastructure and steroid secretion of undifferentiated rat adrenocortical cells in primary culture.

The present study was undertaken to define the effects of lipoprotein-derived cholesterol and endogenous, de novo synthesized cholesterol on the ultrastructure and function of undifferentiated rat adrenocortical cells [lipoprotein (HDL3 and LDL) receptor-negative, zona glomerulosa-like adrenocortical cells] in primary culture. For this purpose human plasma high density lipoprotein (HDL3) or low density lipoprotein (LDL) was added to culture medium devoid of cholesterol. Steroid secretion remained at the low basal level even after addition of lipoproteins, and the amount of intracellular lipid droplets did not increase. When mevinolin (0.96 microgram/ml), an inhibitor of cholesterol synthesis, was added to the culture medium, a low secretion of corticosterone was measured both in serum-free and serum-containing media. Ultrastructurally, lipid droplets disappeared after treatment with mevinolin in both media used. At this concentration of mevinolin cell proliferation was similar to that in the controls, but at higher concentrations (4.8 or 9.6 micrograms/ml) proliferation was inhibited to 42% and 26% in serum-free medium, and 20% and 12% in serum-supplemented medium, respectively. This study demonstrates that cell proliferation and synthesis of corticosterone by undifferentiated rat adrenocortical cells is identical in the absence or presence of exogenous lipoprotein cholesterol. Inhibition of de novo cholesterol synthesis by mevinolin over a period of 7 days does not inhibit corticosterone secretion or proliferation of cells but decreases the amount of intracellular lipid droplets, thus suggesting utilization of intracellular cholesterol esters. However, higher concentrations of mevinolin inhibit proliferation of cells both in serum-free and serum-containing media.

Animals↗
PubMed1989

High density lipoprotein3 binding and biological action: high affinity binding is not necessary for stimulation of placental lactogen release from trophoblast cells.

High density lipoprotein (HDL3) binds with high affinity to many types of cells, but controversy exists concerning the nature and biological significance of the binding. We have recently demonstrated that HDL and apoproteins (apo)-AI, -AII, and -CI stimulate a specific and dose-dependent increase in placental lactogen (hPL) release from human trophoblast cells. To examine the possible relationship between HDL3 binding and stimulation of hPL release, we have characterized the binding of [125I]HDL3 to an enriched fraction of hPL-producing trophoblast cells. Binding studies were performed on trophoblast cells isolated by isopycnic centrifugation of collagenase/hyaluronidase-dispersed placental tissue and apo-E free-HDL3 (density, 1.125-1.215 g/ml). Scatchard analysis of binding studies performed at 37 C for 2 h revealed two classes of binding sites: 1) high affinity binding sites with a Kd of 9.7 +/- 2.2 micrograms/ml (1.3 x 10(-7) M) and 9.8 +/- 3.2 x 10(5) binding sites/trophoblast cell, and 2) low affinity binding sites with a Kd of 172.8 +/- 64.8 micrograms/ml (2.3 x 10(-6) M) and an estimated 3.2 x 10(6) sites/cell. As has been found in hepatocytes and other cells, the number of HDL3-binding sites per trophoblast cell (but not the binding affinity) decreased at lower incubation temperatures. In addition, HDL3 binding to trophoblasts cells did not require calcium and was not affected by prior treatment of the cells with pronase or trypsin. HDL3-binding sites on trophoblast cells, however, were not specific for HDL3. Low density lipoprotein (density, 1.063-1.055 g/ml), which does not stimulate hPL release, was nearly as potent on a molar basis as HDL3 in binding to the high and low affinity binding sites on trophoblast cells. Furthermore, nitrated HDL3, which does not compete for high affinity binding to trophoblast cells, stimulated hPL release. Although the characteristics of HDL3 binding to trophoblast cells are similar to those of other cells, these results strongly suggest that the binding of HDL3 to high affinity binding sites is not essential for HDL-mediated hPL release.

Binding Sites↗
PubMed1987

Influence of prostaglandins on the lipid transfer between human high density and low density lipoproteins.

Prostaglandin (PG) E1 was demonstrated to stimulate the transfer of phosphatidylcholine and cholesterol esters from human high density lipoproteins (HDL3) to low density lipoproteins (LDL). The enhancement effect of PGE1 on the interlipoprotein lipid transfer was seen at low PG concentrations under conditions of spontaneous exchange as well as in the presence of lipoprotein-depleted plasma, or partly purified plasma lipid exchange protein. PGE2 and PGF2 alpha showed no significant influence on the interlipoprotein lipid transfer. Evidence is presented suggesting that the PGE1-induced stimulation of interlipoprotein lipid exchange results in enhancement of LCAT-catalyzed cholesterol esterification in plasma. It is proposed that the effect of PGE1 is due to the previously described PGE1-induced reorganization of the HDL surface [(1984) FEBS Lett. 173, 291-293] and that PG-lipoprotein interaction may be a factor regulating cholesterol homeostasis.

Alprostadil↗