Phosphatidylcholine transfer protein regulates size and hepatic uptake of high-density lipoproteins

Phosphatidylcholine transfer protein (PC-TP) is a steroidogenic acute regulatory-related transfer domain protein that is enriched in liver cytosol and binds phosphatidylcholines with high specificity. In tissue culture systems, PC-TP promotes ATP-binding cassette protein A1-mediated efflux of cholesterol and phosphatidylcholine molecules as nascent pre-beta-high-density lipoprotein (HDL) particles. Here, we explored a role for PC-TP in HDL metabolism in vivo utilizing 8-wk-old male Pctp(-/-) and wild-type littermate C57BL/6J mice that were fed for 7 days with either chow or a high-fat/high-cholesterol diet. In chow-fed mice, neither plasma cholesterol concentrations nor the concentrations and compositions of plasma phospholipids were influenced by PC-TP expression. However, in Pctp(-/-) mice, there was an accumulation of small alpha-migrating HDL particles. This occurred without changes in hepatic expression of ATP-binding cassette protein A1 or in proteins that regulate the intravascular metabolism and clearance of HDL particles. In Pctp(-/-) mice fed the high-fat/high-cholesterol diet, HDL particle sizes were normalized, whereas plasma cholesterol and phospholipid concentrations were increased compared with wild-type mice. In the absence of upregulation of hepatic ATP-binding cassette protein A1, reduced HDL uptake from plasma into livers of Pctp(-/-) mice contributed to higher plasma lipid concentrations. These data indicate that PC-TP is not essential for the enrichment of HDL with phosphatidylcholines but that it does modulate particle size and rates of hepatic clearance.     

The effect of estrogens and phytoestrogenic lignans on macrophage uptake of atherogenic lipoproteins

The present study examined the effect of estrogens and compounds with estrogenic activity on the uptake of atherogenic lipoproteins into macrophages, thought to be the initiating step in the development of atherosclerotic lesions. Isolated low density lipoprotein (LDL) and lipoprotein(a) (Lp(a)) were radiolabelled with (3)H-cholesterol linoleate, and incubated with J774 macrophages for 24 hours in the presence of pharmacological doses of estrogens and phytoestrogens. At a concentration of 0.1 microM, the estrogen 17beta-estradiol significantly reduced LDL uptake by macrophages by 14% (p < 0.05), but estrone did not have any effect. At 10 microM, both estrogens significantly reduced macrophage LDL uptake, but the phytoestrogenic-lignans enterodiol and enterolactone had no effect on LDL uptake. Lp(a) uptake into cells was significantly reduced by both estrone and estradiol, and by enterolactone and enterodiol at concentrations of 10 microM (p < 0.01), with enterodiol being most effective. The results of this study suggest that the uptake of these structurally similar lipoproteins is regulated differently. Macrophage Lp(a) uptake appears more phytoestrogen sensitive than does LDL uptake.     

Nanotechnology for synthetic high-density lipoproteins

Atherosclerosis is the disease mechanism responsible for coronary heart disease (CHD), the leading cause of death worldwide. One strategy to combat atherosclerosis is to increase the amount of circulating high-density lipoproteins (HDL), which transport cholesterol from peripheral tissues to the liver for excretion. The process, known as reverse cholesterol transport, is thought to be one of the main reasons for the significant inverse correlation observed between HDL blood levels and the development of CHD. This article highlights the most common strategies for treating atherosclerosis using HDL. We further detail potential treatment opportunities that utilize nanotechnology to increase the amount of HDL in circulation. The synthesis of biomimetic HDL nanostructures that replicate the chemical and physical properties of natural HDL provides novel materials for investigating the structure-function relationships of HDL and for potential new therapeutics to combat CHD.     

Thyrotropin interaction with high-density lipoproteins

Human high-density lipoproteins (HDL), but not other lipoprotein classes, bind bovine thyrotropin (TSH) with moderately high affinity. Binding of 125I-labeled HDL to TSH has been measured in a solid-phase assay; it is saturable and can be displaced by unlabeled HDL but not by other lipoproteins or bovine serum albumin. The interaction of HDL with TSH has been studied by fluorescence spectroscopy: HDL specifically modifies the fluorescence properties of the biologically active dansyl derivative (DNS, (5-dimethyl-aminonaphtalene-1-sulfonyl) chloride) of TSH (DNS-TSH) causing a 12 nm shift to lower wavelength of the emission maximum, a two-fold increase of the quantum yield and a significant increase of fluorescence polarization. The primary site of TSH binding on the HDL particle is likely to be located on its protein moieties, since other lipoprotein classes, which share similar lipids with HDL, do not bind TSH. 125I-labeled apolipoprotein A-I binds TSH in the solid-phase assay and titration of DNS-TSH with apolipoprotein A-I causes perturbations nearly identical to those observed with intact HDL. One HDL particle has at least 12 binding sites for TSH with an association constant, K = 10(7) M-1 whereas one apolipoprotein A-I molecule binds one or two TSH molecules with an association constant slightly lower than that for HDL (K = 10(6) M-1). The lipid moieties of HDL also appears to be perturbed by the interaction with TSH.     

Alcohol and high-density lipoproteins.

High-density lipoproteins (HDL) have been shown to be negatively associated with coronary heart disease; some epidemiologic evidence also suggests that alcohol may protect against coronary heart disease, but other evidence shows the opposite. Alcohol ingestion and even alcoholism may be associated with higher serum HDL levels, but the levels tend to return to normal within 2 weeks with abstinence from alcohol. The relation between HDL and alcoholism, however, is complex, since in addition to alcohol itself several other factors have to be considered. Liver disease and cigarette smoking tend to decrease the serum HDL level in alcoholic persons, while certain hormonal and nutritional influences and the concomitant use of other microsomal-enzyme-inducing drugs may lead to increased HDL levels. On balance, while alcohol per se may increase the serum HDL level, alcoholism--particularly alcoholic liver disease--probably negates the HDL-related protection against coronary heart disease.     

Differential uptake and metabolism of free and esterified cholesterol from high-density lipoproteins in the ovary

Rat luteal cells utilize high-density lipoproteins (HDL) as a source of cholesterol for steroid synthesis. Both the free and esterified cholesterol of HDL are utilized by these cells. In this report, we have examined the relative uptake of free and esterified cholesterol of HDL by cultured rat luteal cells. Incubation of the cells with HDL labeled with [3H]cholesterol or [3H]cholesteryl linoleate resulted in 4-6-fold greater uptake of the free cholesterol compared to esterified cholesterol. The increased uptake of free cholesterol correlated with its utilization for progestin synthesis: utilization of HDL-derived free cholesterol was 3-6-fold higher than would be expected from its concentration in HDL. The differential uptake and utilization of free and esterified cholesterol was further examined using egg phosphatidylcholine liposomes containing cholesterol or cholesteryl linoleate as a probe. Liposomes containing free cholesterol were able to deliver cholesterol to luteal cells and support steroid synthesis in the absence of apolipoproteins, and the addition of apolipoprotein A-I (apo A-I) moderately increased the uptake and steroidogenesis. Similar experiments using cholesteryl linoleate/egg phosphatidylcholine liposomes showed that inclusion of apo A-I resulted in a pronounced increase in the uptake of cholesteryl linoleate and progestin synthesis. These experiments suggest that free cholesterol from HDL may be taken up by receptor-dependent and receptor-independent processes, whereas esterified cholesterol uptake requires a receptor-dependent process mediated by apolipoproteins.     

The uptake of the apoprotein and cholesteryl ester of high-density lipoproteins by the perfused rat liver

The uptake of the 125I-labeled apolipoprotein and 3H-labeled cholesteryl ester components of rat apolipoprotein E-deficient HDL by the perfused liver was studied. The uptake of the cholesteryl ester moiety was 4-fold higher than that of apolipoprotein. The concentration-dependent uptake of labeled protein was saturable and competed for by an excess of unlabeled HDL. The uptake of cholesteryl ester was not saturable over the concentration range studied. In the presence of a 50-fold excess of unlabeled HDL, the uptake of both radiolabeled components was decreased by over 75%, indicating that three-quarters of the hepatic uptake of HDL is by a receptor-mediated process. After 15 min of perfusion, 37% of the apolipoprotein radioactivity that was initially bound at 5 min was released into the perfusate as a more dense particle. After 5, 15, 30 and 60 min of perfusion the subcellular distribution of the apolipoprotein and cholesteryl ester components was analyzed by Percoll density gradient centrifugation. Over the 60 min period, there appeared to be transfer of radioactivity from the plasma membrane fraction to the lysosomal fraction. However, the internalization and degradation of cholesteryl ester was more rapid than that of the apolipoprotein. Our findings indicate that there is preferential uptake of HDL cholesteryl ester relative to protein by the liver and that the internalization of these components may occur independently.     

Antioxidant effect of high-density lipoproteins in the peroxidation of low-density lipoproteins

It has been shown that in the solution of low density lipoproteins (LDL) during their incubation at 37 degrees C the turbidity and concentration of malondialdehyde was increased, as compared to that observed at 4 degrees C. Both parameters were slowed down by the addition of high density lipoproteins (HDL) into the medium. The protective effect of HDL depended on the time of incubation and the concentration of HDL added. Delipidated HDL had no effect. Similar action of HDL was established in the experiments where the peroxidation in LDL was induced by the xanthine-xanthine oxidase. The data obtained demonstrate that HDL possess an antioxidant property that may play an important role in their antiatherogenic action.     

Imaging and manipulation of high-density lipoproteins.

The atomic force microscope (AFM) has been used to image a variety of biological systems, but has rarely been applied to soluble protein-lipid complexes. One of the primary physiological protein-lipid complexes is the high-density lipoproteins (HDL), responsible for the transport of cholesterol from the peripheral tissues and other lipoproteins to the liver. We have used the AFM to directly image discoidal reconstituted HDL (rHDL) particles for the first time. The height of these particles is consistent with a phospholipid bilayer structure, but careful high resolution measurements of particle diameters has indicated that they fuse when adsorbed to mica. Furthermore, it has been demonstrated that the AFM can be used to initiate this bilayer fusion in a controlled manner, allowing the fabrication of stabilized, nanometer scale, phospholipid bilayer "domains."     

Cholesterol transport between cells and high-density lipoproteins

Various types of studies in humans and animals suggest strongly that HDL is anti-atherogenic. The anti-atherogenic potential of HDL is thought to be due to its participation in reverse cholesterol transport, the process by which cholesterol is removed from non-hepatic cells and transported to the liver for elimination from the body. Extensive studies in cell culture systems have demonstrated that HDL is an important mediator of sterol transport between cells and the plasma compartment. The topic of this review is the mechanisms that account for sterol movement between HDL and cells. The most prominent and easily measured aspect of sterol movement between HDL and cells is the rapid bidirectional transfer of cholesterol between the lipoprotein and the plasma membrane. This movement occurs by unmediated diffusion, and in most situations its rate in each direction is limited by the rate of desorption of sterol molecules from the donor surface into the adjacent water phase. The net transfer of sterol mass out of cells occurs when there is either a relative enrichment of sterol within the plasma membrane or a depletion of sterol in HDL. Recent studies suggest that certain minor subfractions of HDL (with pre-beta mobility on agarose gel electrophoresis and containing apoprotein A-I but no apo A-II) are unusually efficient at promoting efflux of cell sterol. To what extent efflux to these HDL fractions is balanced by influx from the lipoprotein has not yet been established clearly. The prevention and reversal of atherosclerosis require the mobilization of cholesterol from internal (non-plasma membrane) cellular locations. To some extent, this may involve the retroendocytosis of HDL. However, most mobilization probably involves the transport of internal sterol to the plasma membrane, followed by desorption to extracellular HDL. Several laboratories are investigating the transport of sterol from intracellular locations to the plasma membrane. Studies on biosynthetic sterol (probably originating mostly in the smooth endoplasmic reticulum) suggest that there is rapid transport to the plasma membrane in lipid-rich vesicles. Important features of this transport are that it bypasses the Golgi apparatus and may be positively regulated by the specific binding of HDL to the plasma membrane.(ABSTRACT TRUNCATED AT 400 WORDS)     

Metabolism of high-density lipoproteins in rainbow trout.

Trout high-density lipoproteins have been labelled with residualizing tracers for the lipid and protein moieties ([3H]cholesteryloleyl ether and 125I-tyramine-cellobiose, respectively). Plasma kinetics and tissue site of catabolism were determined for both tracers. The lipid tracer was cleared about twice as fast from the blood as the protein tracer (half lifes were 63.5 and 125.3 h, respectively). This selective removal of lipid from the lipoprotein was mainly accomplished by the higher liver uptake of the cholesteryl ether. The main catabolic site for HDL protein was kidney tissue. This data established the existence of differential HDL catabolism in a lower vertebrate, in which HDL is the dominant plasma lipoprotein. In addition, the findings confirm the importance of fish kidney as a major site of endocytosis of macromolecules, of both exogenous and endogenous origin.     

Myeloperoxidase-mediated oxidation of high-density lipoproteins: fingerprints of newly recognized potential proatherogenic lipoproteins

Substantial evidence supports the notion that oxidative processes participate in the pathogenesis of atherosclerotic heart disease. Major evidence for myeloperoxidase (MPO) as enzymatic catalyst for oxidative modification of lipoproteins in the artery wall has been suggested in numerous studies performed with low-density lipoprotein. In contrast to low-density lipoprotein, plasma levels of high-density lipoprotein (HDL)-cholesterol and apoAI, the major apolipoprotein of HDL, inversely correlate with the risk of developing coronary artery disease. These antiatherosclerotic effects are attributed mainly to HDL's capacity to transport excess cholesterol from arterial wall cells to the liver during 'reverse cholesterol transport'. There is now strong evidence that HDL is a selective in vivo target for MPO-catalyzed oxidation impairing the cardioprotective and antiinflammatory capacity of this antiatherogenic lipoprotein. MPO is enzymatically active in human lesion material and was found to be associated with HDL extracted from human atheroma. MPO-catalyzed oxidation products are highly enriched in circulating HDL from individuals with cardiovascular disease where MPO concentrations are also increased. The oxidative potential of MPO involves an array of intermediate-generated reactive oxygen and reactive nitrogen species and the ability of MPO to generate chlorinating oxidants-in particular hypochlorous acid/hypochlorite-under physiological conditions is a unique and defining activity for this enzyme. All these MPO-generated reactive products may affect structure and function of HDL as well as the activity of HDL-associated enzymes involved in conversion and remodeling of the lipoprotein particle, and represent clinically useful markers for atherosclerosis.     

Small-angle X-ray scattering of human serum high-density lipoproteins

Small-angle x-ray scattering (SAXRS) studies of the human serum high-density lipoprotein HDL₂ indicate a symmetrical particle with a radius of gyration Rg = 46 Å. The positions and intensities of subsidiary maxima in the scattering curves are not consistent with those of a uniformly electron dense sphere. Scattering curves calculated for spheres with a step-model radial electron density distribution, show good agreement with the experimental scattering curve for HDL₂ only for specific values of the step function used. The dimensions obtained for the electron-deficient core and electron-rich shell model are quantitatively consistent with a predominantly surface location for the HDL₂ protein and phospholipid head groups, the more hydrocarbon species being located in the interior of the particle.     

Effects of high-density lipoproteins on thrombin-induced platelet aggregation

Using new-developed method of aggregates radius measurement in suppression of washed human platelets, it has been shown, that HDL, in concentration of 190 ug/ml and in higher ones, totally inhibited aggregation, induced by 0.075 U/ml thrombin. For the same effect on aggregation, induced by 0.225 U/ml thrombin, HDL in concentration of 1320 ug/ml were needed.     

A plasma membrane pool of cholesteryl esters that may mediate the selective uptake of cholesteryl esters from high-density lipoproteins

Selective uptake of high-density lipoprotein (HDL) cholesteryl esters without parallel uptake of HDL particles occurs by a nonendocytotic pathway that requires no specific apolipoprotein and results in the net delivery of cholesteryl esters to cells. Here we examine a reversibly cell-associated pool of cholesteryl ester tracer and its relationship to selective uptake. A fraction of cholesteryl ester tracer selectively taken up from HDL by rat primary or mouse Y1-BS1 adrenocortical cells was chased from the cells by subsequent incubation with unlabeled HDL. This pool of cholesteryl ester tracer was distinct from that irreversibly internalized, and in excess of that accounted for by dissociation of labeled HDL particles bound to the cell surface. In response to various metabolic effectors, cholesteryl ester tracer in this reversibly cell-associated pool of Y1-BS1 cells correlated linearly with irreversible selective uptake. Both reversibly and irreversibly cell-associated pools of cholesteryl ester tracer displayed similar saturation kinetics for uptake from HDL, and both pools correlated inversely with cell-free cholesterol levels. Cholesteryl ester tracer in the reversible pool was shown to serve as a precursor for irreversible selective uptake. A pool with properties similar to the reversibly cell-associated pool was identified in plasma membrane fractions; enough tracer was incorporated into this pool to account for the reversibly cell-associated pool of intact cells. The data suggest that a pool of cholesteryl esters in the plasma membrane is involved in selective uptake at a step prior to irreversible internalization.     

Effect of lipoproteins on macrophage superoxide generation

Superoxide production by macrophages and leukocytes may have an important role in atherogenesis. Whether lipoproteins modulate the superoxide production of these cells is not clear. Therefore, the effect of lipoproteins on the production of superoxide by rat peritoneal macrophages was tested. VLDL and LDL inhibited digitonin-stimulated superoxide production in a dose-dependent manner. Maximum inhibition was observed at 10 μg ml^?1 of VLDL protein and 50 μg ml^?1 of LDL protein respectively. In contrast, HDL (40 μg protein ml^?1) enhanced digitoninstimulated superoxide production (by 47 per cent). Macrophage superoxide production induced by arachidonic acid was enhanced by both VLDL (130 per cent) and HDL (84 per cent), whereas LDL had no effect. The lipoproteins had no effect on macrophage superoxide stimulated by other agonists such as phorbol myristate 13-acetate, sodium fluoride or the calcium ionophore, A23187. The effect of lipoproteins was also tested on human polymorphonuclear leukocyte superoxide generation, stimulated by digitonin and PMA. Ten μg of VLDL, 50 μg of LDL and 50 μg of HDL proteins ml^?1, inhibited digitonin-induced superoxide production by 50, 100 and 33 per cent respectively. Lipoproteins had no effect on PMA stimulated superoxide generation by human polymorphonuclear leukocytes. The stimulatory and inhibitory effects of lipoproteins on macrophage and neutrophil superoxide generation could be important in the understanding of oxidation-mediated development of atherosclerosis.