Mechanism of the association of HDL3 with endothelial cells, smooth muscle cells, and fibroblasts. Evidence against the role of specific ligand and receptor proteins

Endocytosis of lipoproteins

During their metabolism, all lipoproteins undergo endocytosis, either to be degraded intracellularly, for example in hepatocytes or macrophages, or to be re-secreted, for example in the course of transcytosis by endothelial cells. Moreover, there are several examples of internalized lipoproteins sequestered intracellularly, possibly to exert intracellular functions, for example the cytolysis of trypanosoma. Endocytosis and the subsequent intracellular itinerary of lipoproteins hence are key areas for understanding the regulation of plasma lipid levels as well as the biological functions of lipoproteins. Indeed, the identification of the low-density lipoprotein (LDL)-receptor and the unraveling of its transcriptional regulation led to the elucidation of familial hypercholesterolemia as well as to the development of statins, the most successful therapeutics for lowering of cholesterol levels and risk of atherosclerotic cardiovascular diseases. Novel limiting factors of intracellular trafficking of LDL and the LDL receptor continue to be discovered and to provide drug targets such as PCSK9. Surprisingly, the receptors mediating endocytosis of high-density lipoproteins or lipoprotein(a) are still a matter of controversy or even new discovery. Finally, the receptors and mechanisms, which mediate the uptake of lipoproteins into non-degrading intracellular itineraries for re-secretion (transcytosis, retroendocytosis), storage, or execution of intracellular functions, are largely unknown.

HDL, ABC transporters, and cholesterol efflux: implications for the treatment of atherosclerosis

High-density lipoprotein (HDL) has been identified as a potential target in the treatment of atherosclerotic vascular disease. The failure of torcetrapib, an inhibitor of cholesteryl ester transfer protein (CETP) that markedly increased HDL levels in a clinical trial, has called into doubt the efficacy of HDL elevation. Recent analysis suggests that failure may have been caused by off-target toxicity and that HDL is functional and promotes regression of atherosclerosis. New studies highlight the central importance of the ATP-binding cassette (ABC) transporters ABCA1 and ABCG1 in reducing macrophage foam cell formation, inflammation, and atherosclerosis. A variety of approaches to increasing HDL may eventually be successful in treating atherosclerosis.

Secretory phospholipase A2 of group IIA: Is it an offensive or a defensive player during atherosclerosis and other inflammatory diseases?

Since its discovery in the serum of patients with severe inflammation and in rheumatoid arthritic fluids, the secretory phospholipase A2 of group IIA (sPLA2-IIA) has been chiefly considered as a proinflammatory enzyme, the result of which has been very intense interest in selective inhibitors of sPLA2-IIA in the hope of developing new and efficient therapies for inflammatory diseases. The recent discovery of the antibacterial properties of sPLA2-IIA, however, has raised the question of whether the upregulation of sPLA2-IIA during inflammation is to be considered uniformly negative and the hindrance of sPLA2-IIA in every instance beneficial. The aim of this review is for this reason, along with the results of various investigations which argue for the proinflammatory and proatherogenic effects of an upregulation of sPLA2-IIA, also to array data alongside which point to a protective function of sPLA2-IIA during inflammation. Thus, it could be shown that sPLA2-IIA, apart from the bactericidal effects, possesses also antithrombotic properties and indeed plays a possible role in the resolution of inflammation and the accelerated clearance of oxidatively modified lipoproteins during inflammation via the liver and adrenals. Based on these multipotent properties the knowledge of the function of sPLA2-IIA during inflammation is a fundamental prerequisite for the development and establishment of new therapeutic strategies to prevent and treat severe inflammatory diseases up to and including sepsis.

ATP-binding cassette transporters G1 and G4 mediate cellular cholesterol efflux to high-density lipoproteins

The mechanisms responsible for the inverse relationship between plasma high-density lipoprotein (HDL) levels and atherosclerotic cardiovascular disease are poorly understood. The ATP-binding cassette transporter A1 (ABCA1) mediates efflux of cellular cholesterol to lipid-poor apolipoproteins but not to HDL particles that constitute the bulk of plasma HDL. We show that two ABC transporters of unknown function, ABCG1 and ABCG4, mediate isotopic and net mass efflux of cellular cholesterol to HDL. In transfected 293 cells, ABCG1 and ABCG4 stimulate cholesterol efflux to both smaller (HDL-3) and larger (HDL-2) subclasses but not to lipid-poor apoA-I. Treatment of macrophages with an liver X receptor activator results in up-regulation of ABCG1 and increases cholesterol efflux to HDL. RNA interference reduced the expression of ABCG1 in liver X receptor-activated macrophages and caused a parallel decrease in cholesterol efflux to HDL. These studies indicate that ABCG1 and ABCG4 promote cholesterol efflux from cells to HDL. ABCG1 is highly expressed in macrophages and probably mediates cholesterol efflux from macrophage foam cells to the major HDL fractions, providing a mechanism to explain the relationship between HDL levels and atherosclerosis risk.

High-density lipoprotein increases intracellular calcium levels by releasing calcium from internal stores in human endothelial cells

Elevated levels of high-density lipoproteins (HDL) appear to delay or prevent the development of atherosclerosis. The intracellular signaling mechanisms activated by HDL in vascular cells are currently under active investigation. In this study the effects of HDL on endothelial intracellular Ca levels (EC Ca(i)) are investigated. We show that HDL, like low density lipoproteins (LDL), increases EC Ca(i) in a dose-dependent fashion by releasing Ca from internal stores. Neither apolipoprotein A-I (apo A-I) nor apolipoprotein A-II (apo A-II) was responsible for the increase in EC Ca(i). HDL appeared to release Ca from the same internal stores as did LDL, since preincubation of EC with LDL prevented subsequent responses to HDL but not to the vasodilator ATP. In addition, preincubation of EC with pertussis toxin, an inhibitor of specific G proteins, as well as U73122, an inhibitor of phospholipase C, prevented a rise in EC Ca(i) in response to HDL. These findings suggest that HDL, like LDL, can modulate EC Ca(i) and that this occurs via a pertussis toxin-sensitive G protein-mediated pathway which involves phospholipase C.

Thyroid hormone efflux from monolayer cultures of human fibroblasts and hepatocytes. Effect of lipoproteins and other thyroxine transport proteins

We have previously shown that human skin fibroblasts exposed to preformed low density lipoprotein (LDL)-thyroxine (T4) complexes internalize more T4 than they do when exposed to T4 alone. The system is set to function when the LDL receptor is up-regulated by reducing the intracellular concentration of cholesterol, and the LDL concentration outside the cell is in the range of the kDa of the receptor. High density lipoproteins (HDL), albumin (HSA), transthyretin (TTR), and thyroxine-binding globulin (TBG) interfere with, rather than facilitate, T4 entry. Of the three classes of lipoproteins (VLDL, LDL, and HDL), HDL is the major carrier of thyroid hormones. While LDL delivers cholesterol (and T4) to cells, HDL is the scavenger of cholesterol. We thus hypothesized that HDL could also facilitate thyroid hormone exit from cells. This hypothesis was tested on two human cell lines: skin fibroblasts and hepatocytes (Hep G2), using physiological concentrations of HDL or, as control, physiological concentrations of LDL, HSA, TTR, and TBG or buffer. Because cell surface receptors for HDL are regulated by intracellular cholesterol in a manner opposite to that of LDL receptors, we evaluated the effect of HDL (and other proteins) in three states: normal, high, and low intracellular cholesterol content (i.e. normal, high, and low expression of HDL receptors). In both cell lines and with either T4 or T3, we found that: 1) HDL as well as the other proteins tested increased the efflux and augmented both the initial rate of exit and the equilibrium value. 2) The efflux did not saturate over a wide range of protein concentrations. 3) The effect of HDL, LDL, and the other proteins on the fractional efflux rate of thyroid hormones remained the same irrespective of the intracellular cholesterol content (and, therefore, irrespective of the expression of either LDL or HDL receptors). 4) HSA, TTR, and TBG were, on a mass basis, equipotent and more efficient than lipoproteins. However, the effect of lipoproteins--whose Ka for T4 is comparable to that of HSA--was disproportionately high. On a molar basis, LDL (about 80% of the weight being accounted for by lipids) was more effective than HDL2 (about 60% lipids) and HDL2 was more effective than HDL3 (about 40% lipids), suggesting that the disproportionate effect of lipoproteins was due to transfer of the lypophylic thyroid hormones to the lipid moiety of lipoproteins. 5. A mixture of HDL and LDL gave the same efflux rate as a mixture of HSA, TTR, and TBG. The data indicate that the efflux of T4 and T3 from cells is rapid and appears not to be mediated by a particular lipoprotein. The disproportionately large effect of lipoproteins, which are low affinity thyroid hormone carriers, compared with nonlipoprotein carriers, and the greater effect of LDL compared with HDL, might indicate that the lipoproteins establish a nonspecific physical contact with the plasma membrane and that their hydrophobic nature favors the release of the similarly hydrophobic thyroid hormones.

Receptor binding of an apolipoprotein E-rich subfraction of high density lipoprotein to rat and human brain membranes

During nerve cell degeneration, cholesterol released from the degrading cells is conserved through the formation of a cholesterol-apolipoprotein (apo) E complex which is subsequently taken up by regenerating nerve cells. The aim of the present project was to identify the physiologically relevant lipoprotein receptor for this lipoprotein complex which has remained elusive. HDL was separated into apo E-rich and apo E-poor subfractions and labelled with [14C]-sucrose. Labelled apo E-rich HDL bound to rat brain membranes in a time- and ligand concentration-dependent manner and was a saturable process. Essentially no binding occurred with [14C]-apo E-poor HDL or with free apo E. Binding was partially inhibited by low density lipoprotein (LDL) and by alpha 2-macroglobulin. These results provide new evidence that native apoE-rich HDL particles resembling those present in the brain bind to rat brain membranes and that the binding may be due, at least in part, to the LDL receptor and to the LDL-receptor related protein. Evidence was also provided for the presence of a receptor which binds [14C]-sucrose human apoE-rich HDL in human brain. Characterisation of the receptor which mediates the uptake of cholesterol from HDL-like complexes by brain cells is important in understanding the role of apoE in the central nervous system and of the alterations which occur in disorders such as Alzheimer's disease.

Alterations in the physiochemical characteristics of low and high density lipoproteins after lipolysis with phospholipase A2. A spin-label study

Human low and high density lipoproteins (LDL and HDL, respectively) were treated with porcine pancreatic phospholipase A2 (PLA2) in the presence of albumin resulting in hydrolysis of 40-84% of the lipoproteins phospholipids. The resulting PLA2-treated LDL and HDL and concurrent control lipoproteins incubated without PLA2 were reisolated by ultracentrifugation and labelled with 5-doxyl- and 16-doxyl-stearic acid, and with a spin-labelled analogue of maleimide. Analysis of ESR spectra showed that phospholipid hydrolysis both of LDL and HDL resulted in an increase in order, micro-viscosity and polarity of lipid regions in the surface monolayer of the particles. In the temperature range from 3 degrees C to 50-60 degrees C, Arrhenius plots of a spectral parameter of LDL and HDL labelled with 5-doxyl-stearate exhibited alterations which suggest an increase in free cholesterol content near the surface of the lipoproteins after PLA2-treatment. ESR spectra of the maleimide analogue bound covalently to the protein moiety of the lipoproteins have demonstrated that, following phospholipid hydrolysis, the conformation of the apoproteins became more condensed, with more masked domains. The possible implications of the revealed alterations for enhanced delivery of LDL and HDL cholesterol to cells after phospholipolysis of the lipoproteins are discussed.

High-density lipoprotein and low-density lipoprotein-mediated signal transduction in cultured human skin fibroblasts

The signalling mechanisms whereby high-density lipoproteins (HDL) and low-density lipoproteins (LDL) affect a number of cellular functions in fibroblasts are unclear. This study has analyzed the influence of HDL3 and LDL on the phosphatidylinositol specific phospholipase C pathway in human skin fibroblasts. Exposure of myo-[2-3H]-inositol prelabelled fibroblasts to HDL3 or LDL elicited major increases in IP1 and minor increases in IP2 and IP3 within 30 s. In fura-2 loaded suspended fibroblasts, HDL3 and LDL increased intracellular Ca2+ concentrations ([Ca2+]i) with comparable rapid, transient kinetics. The dose-profiles for HDL3- and LDL-induced increases in [Ca2+]i were also comparable, with half-maximally and maximally effective concentrations being approximately 15 micrograms/mL and approximately 50 micrograms/mL, respectively. HDL3- and LDL-induced increases in [Ca2+]i were diminished by approximately 60% (vs. control fibroblasts) in thapsigargin-pretreated fibroblasts, indicating that release of Ca2+ from intracellular pools is the major contributor toward lipoprotein-induced increases in [Ca2+]i. Pertussis toxin-pretreatment of cells completely abolished lipoprotein induced Ca(2+)-transient, indicating the involvement of a guanine nucleotide-binding protein in the signalling process. In [3H]-palmitic acid-prelabelled fibroblasts, both HDL3 and LDL were observed to stimulate production of DAG. Activation of protein kinase C (PKC) was analysed by determining the cytosol-to-membrane translocation of both enzymatic activity and immunoreactivity of specific PKC isoforms (alpha, delta, epsilon, and zeta). Stimulation with HDL3 and LDL evoked a rapid (within 2.5 min) translocation of PKC activity, with PKC alpha and PKC epsilon being the isoforms translocated. It is concluded that HDL3 and LDL acutely stimulate a phosphoinositide-specific phospholipase C pathway in human skin fibroblasts. However, the specific cell membrane events mediating this signal transduction remain to be further elucidated.

High-density lipoprotein stimulates endothelial cell movement by a mechanism distinct from basic fibroblast growth factor

Endothelial cell (EC) migration is a regulatory event in the formation and repair of blood vessels. Although serum contains substantial promigratory activity, the responsible components and especially the role of lipoproteins have not been determined. We examined the effect of plasma high-density lipoprotein (HDL) on the movement of ECs in vitro. Confluent cultures of bovine aortic ECs in serum-free medium were "wounded," and migration was measured after 24 hours. HDL stimulated migration in a concentration-dependent manner with a half-maximal response at 25 to 40 micrograms cholesterol per milliliter and a maximal twofold stimulation at approximately 150 micrograms cholesterol per milliliter. HDL-stimulated migration was not due to cell proliferation, since migration was increased in the presence of hydroxyurea at a concentration that blocked proliferation. At optimal concentrations, HDL was at least as stimulatory as basic fibroblast growth factor (FGF). However, the activity of HDL was not due to contamination by basic FGF, since antibodies to basic FGF did not block HDL-stimulated movement and since the maximum promigratory activities of basic FGF and HDL were additive. These results indicate that HDL and basic FGF may use distinct signaling pathways to initiate EC movement. This possibility was confirmed by results showing that pertussis toxin suppressed basic FGF-stimulated but not HDL-stimulated EC motility and that inhibitors of phospholipase A2, aristolochic acid and ONO-RS-082, also blocked the promigratory activity of basic FGF but had no effect on the activity of HDL.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipoproteins inhibit the secretion of tissue plasminogen activator from human endothelial cells

We studied the effect of lipoprotein(a) [Lp(a)], low-density lipoprotein (LDL), and high-density lipoprotein (HDL) on tissue plasminogen activator (TPA) secretion from human endothelial cells. At 1 mumol/L, Lp(a) inhibited constitutive TPA secretion by 50% and phorbol myristate acetate- and histamine-enhanced TPA secretion by 40%. LDL and HDL also depressed TPA secretion by 45% and 35% (constitutive) and 40% to 60% (stimulated). TPA mRNA levels were also examined and found to change in parallel with antigen secretion. In contrast to TPA, plasminogen activator inhibitor type-1 secretion and mRNA levels were not affected by any of the three lipoproteins. These results suggest that the interaction of lipoproteins with certain cell-surface binding sites may interfere with the proper production and/or secretion of TPA.

Binding of HDL to basolateral membranes of the renal cortex. Evidence for two components in the HDL-membrane association

The binding of porcine 125I-HDL to purified basolateral membrane fractions isolated from pig kidney cortex displays two categories of sites, one with high affinity ((Kd = (3.0 +/- 0.7) x 10(-9) M) and low capacity (Bmax = 52 +/- 32 ng/mg proteins) another with low affinity (Kd = (5.3 +/- 0.7) x 10(-8) M) but a higher capacity (Bmax = 795 +/- 115 ng/mg proteins). Binding was competitively inhibited to the same extent by unlabeled HDL from swine, human or rat, demonstrating an absence of species specificity. Porcine LDL partially competed for binding even in the presence of 30 mM EDTA which prevents apo B/E specific binding. Membrane proteins solubilized with CHAPS were analyzed by electrophoresis followed by ligand blotting using porcine 125I-HDL and 125I-apoAI-HDL to show that HDL bound to two proteins of respective molecular masses 120 +/- 2 and 95 +/- 9 kDa. 125I-apoAI associated mostly with the 95 kDa protein. A 100-fold excess of unlabeled HDL greatly decreased binding to the 95 kDa protein but less to the 120 kDa protein. We conclude that part of HDL binding occurs through the lipid moiety, while another is the result of a specific interaction between apoAI and a membrane protein of 95 kDa.

Net transport of cholesterol from cells of the human EA.hy 926 endothelial cell line to high density lipoproteins

EA.hy 926 cells, a human endothelial cell line, show characteristics of differentiated endothelial cells. The cells express saturable binding of apo E-free 125I-high density lipoprotein3 (HDL3). Bmax increased from 71 to 226 ng HDL3 bound/mg cell protein after cholesterol loading of the confluent endothelial cells with cationized low density lipoprotein (LDL). The affinity did not change after cholesterol enrichment (Kd was 37 micrograms HDL3 protein/ml for control cells and 31 micrograms/ml for loaded cells). Incubation of cholesterol-loaded EA.hy 926 cells with native HDL and LDL had different effects on cellular cholesterol levels. Incubation with HDL decreased both esterified and unesterified cellular cholesterol, but LDL did not change total cellular cholesterol. However, LDL tended to increase cellular cholesteryl esters, with a concomitant decrease of unesterified cellular cholesterol. Incubation of endothelial cells with both HDL and LDL also resulted in decreased total cellular cholesterol levels. These data show that cationized LDL-loaded human endothelial EA.hy 926 cells can be used to study the net transport of cellular cholesterol to HDL, the first step in reverse cholesterol transport.

Cholesterol is poorly available for free apolipoprotein-mediated cellular lipid efflux from smooth muscle cells

To study the mechanism for resistance of smooth muscle cells (SMC) to cholesterol efflux caused by lipid-free apolipoproteins [Komaba, A., et al. (1992) J. Biol. Chem. 267, 17560-17566], the efflux of phospholipids and cholesterol was induced from mouse peritoneal macrophages (MP) and rat aortic SMC by phospholipid/triglyceride microemulsion, by human plasma high- and low-density lipoproteins (HDLs and LDLs), and by lipid-free human apolipoprotein (apo) A-I. The efflux of both lipids by the lipid microemulsion showed essentially the same kinetic profile for these two types of cells except that the rate of phospholipid efflux was 5-6 times slower by weight than cholesterol in both cases. The same ratio of cholesterol to phospholipid was also found in the efflux to LDLs. Lipid-free apoA-I mediated cellular cholesterol efflux, but the rate was much slower from SMC than from MP. However, the rate of apoA-I-mediated phospholipid efflux was similar between these two cells generating HDL-like particles, resulting in a high phospholipid:cholesterol ratio, (4-5):1 by weight, in the lipid efflux from SMC, in contrast with (0.8-1):1 in the lipid efflux from MP. When standardized for the cellular free cholesterol, the Vmax of cholesterol efflux induced by lipid-free apoA-I was 10 times slower from SMC than from MP, but only by at most 2-fold slower when lipid microemulsion was the acceptor. Thus, free cholesterol of SMC is less available than that of MP for free apolipoprotein-mediated generation of HDLs with cellular lipids.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective association of lipoprotein cholesteryl esters with liver plasma membranes

High-density lipoprotein (HDL) cholesteryl esters are taken up by hepatocytes without parallel uptake of HDL apolipoproteins. This selective uptake of HDL cholesteryl esters is mediated by a non-endocytotic mechanism. Recently, selective uptake of cholesteryl esters also from low-density lipoprotein (LDL) was demonstrated. In this study, the role of the plasma membrane in selective uptake by the liver was investigated. Plasma membranes were prepared from rat liver or from human Hep G2 hepatoma cells. Human HDL3 (d = 1.125-1.21 g/ml) was either radioiodinated or labeled with [3H]cholesteryl oleate. Human low-density lipoprotein (d = 1.019-1.05 g/ml) was labeled in its protein and in its lipid moiety as well. Labeled lipoproteins, unlabeled lipoproteins and membranes were incubated. After separation by ultracentrifugation, apparent lipoprotein particle association with membranes was determined. Plasma membranes from rat liver and Hep G2 cells bound 125I-HDL3, indicating specific HDL3 particle binding. With both types of membrane, apparent HDL3 particle association according to [3H]cholesteryl oleate-labeled HDL3 was in significant excess on that due to 125I-HDL3. This indicates selective, i.e., particle binding independent, association of cholesteryl esters with the membrane. Excess unlabeled HDL3 competed for selective association, indicating a specific process. Selective association of HDL3 cholesteryl esters was concentration-, time-, temperature-dependent; however, parameters differed from HDL3 particle binding. HDL3 was modified by nitration; this modification inhibited HDL3 particle binding in contrast to unchanged selective association. These results suggested distinct membrane sites for HDL3 particle binding and selective cholesteryl ester association. Regulation of selective association was investigated. Hep G2 cells were cholesterol-loaded or cholesterol-depleted. Cellular cholesterol-loading down-regulated selective association of HDL3 cholesteryl esters with isolated membranes prepared from these cells. In parallel, selective uptake of HDL3 cholesteryl esters by Hep G2 cells was down-regulated in cholesterol-loaded cells. This parallel regulation of selective association with membranes and selective uptake by cells suggests a functional relationship. LDL, radiolabeled in its protein and in its lipid moiety, was incubated with liver plasma membranes. Besides LDL holo-particle receptor binding, also LDL cholesteryl esters were selectively associated with membranes. These data showed that selective association with membranes is not restricted to HDL but can occur from LDL as well. It is concluded that HDL3 as well as LDL cholesteryl esters can selectively be associated with hepatic plasma membranes, i.e., independent from particle binding. Results suggest an important role of the plasma membrane in the mechanism of selective cholesteryl ester uptake by the liver.

Mast cell-mediated inhibition of reverse cholesterol transport

Net cholesterol efflux from cholesterol-loaded macrophages, i.e., foam cells, was induced by incubating the foam cells with high density lipoprotein3 (HDL3). However, when the incubation system included rat serosal mast cells stimulated to trigger exocytosis of their cytoplasmic secretory granules, the ability of HDL3 to induce cholesterol efflux was largely lost. This loss was found to be due to the proteolytic action of chymase, the neutral protease of the granules, which degraded the apolipoproteins of HDL3, so rendering them unable to mediate cholesterol efflux from the foam cells. The observation defines a novel cell-dependent mechanism that blocks the initial steps of reverse cholesterol transport and suggests a role for mast cell chymase in cellular accumulation of cholesterol, an early stage in atherogenesis.

Binding of 111In-labeled HDL to platelets from normolipemic volunteers and patients with heterozygous familial hypercholesterolemia

High density lipoproteins (HDLs; d = 1.063 - 1.21 g/ml) were isolated by ultracentrifugation and radiolabeled with 111In. The in vitro binding onto platelets from healthy volunteers (n = 15) and patients (n = 36) with heterozygous familial hypercholesterolemia (FH) was investigated. Binding was saturable and indicated high-affinity binding sites, which bound 1,882 +/- 361 ng protein of 111In-HDL/10(9) platelets (dissociation constant [Kd] = 7 +/- 3 micrograms protein/ml) in healthy volunteers and significantly (p less than 0.01) lower amounts in the FH patients (1,012 +/- 439 ng protein of 111In-HDL/10(9) platelets [Kd = 12 +/- 4 micrograms protein/ml]; p less than 0.01). The capacity to displace one half of the bound ligand (IC50) amounted to 14 +/- 3 micrograms protein/ml in healthy volunteers and 22 +/- 9 micrograms protein/ml in FH patients (p less than 0.001). Treatment with lipid-lowering drugs (gemfibrozil, alone or in combination with cholestyramine) in 10 patients resulted in an increased HDL binding capacity: before treatment, 1,280 +/- 883; after 2 months of treatment, 2,052 +/- 873 (p less than 0.05); and after 6 months of treatment, 2,127 +/- 812 ng protein/10(9) platelets (p less than 0.01). There was a significant (p less than 0.001) correlation between 111In-HDL binding data and plasmatic lipid and lipoprotein values. Furthermore, those FH patients with the additional risk factors of smoking (p less than 0.05) and hypertension (p less than 0.01) showed significantly lower 111In-HDL binding onto platelets. The findings indicate specific 111In-HDL binding sites for human platelets, which may be decreased in patients with heterozygous FH. Upregulation of HDL binding sites during lipid-lowering medication therapy supports the hypothesis that high-affinity HDL binding is involved in hyperlipemic disorders and is possibly related to the reactivity of platelets.

Probucol increases the selective uptake of HDL cholesterol esters by Hep G2 human hepatoma cells

A previous study in rats showed that even though probucol substantially lowers high density lipoprotein (HDL) levels, near-normal mass transport of HDL cholesterol esters (CE) to the liver is maintained by the induction of "selective" (direct) uptake of HDL CE. The present study describes a parallel result in cultured Hep G2 human hepatoma cells. Cells were preincubated in the presence or absence of probucol before measuring the uptake of doubly labeled HDL3 in the absence of probucol. Preincubation with probucol decreased the uptake of HDL3 particles (iodine-125-labeled N-methyltyramine cellobiose-apolipoprotein [125I-NMTC-apo] A-I uptake) but increased the uptake of [3H]cholesteryl oleyl ether in excess of 125I-NMTC-apo A-I (i.e., selective uptake) in a dose-dependent fashion. The reversibly cell-associated pool of CE tracer, a precursor for selective uptake, enlarged on probucol treatment, but the increase was not in proportion to the increase in selective uptake. HDL3 particle uptake decreased on probucol treatment. The decrease was evident after less than 20 minutes of probucol exposure and was maximal after 6 hours; in contrast, HDL3 CE selective uptake increased only after greater than 13 hours and had not reached a plateau after 20 hours. Thus, effects on particle uptake and selective uptake were dissociated in time.

Aortic features in Tangier disease and pathogenetic considerations--Part I. Fatty dots and streaks

Morphological studies of aortic fatty dots and streaks, and the adjacent normally appearing intima, in a 5 3/4-year-old boy who died of pneumonia, showed several hitherto unreported features. In lesions, lipid vacuoles and/or other cytoplasmic "inclusions" (ultrastructurally considered to present complex forms of lipids) were present on occasion in the endothelium but consistently involved the (intimal) smooth muscle cells (SMC). Similar changes were present in the adjacent intima, but were here less prominent and "tapered off" distally. A moderate number of macrophages also contained cytoplasmic lipids but such cells entirely free of lipid inclusions were observed, too. Most surprisingly, dilated and many cisternae of rough-surfaced endoplasmic reticulum (ER) in the SMC of lesions were associated spatially with cytoplasmic droplets and other forms of lipids. The results of these studies question the generally accepted central role of macrophages as being primarily involved in the pathogenesis of tissue changes in Tangier disease. It is possible that in view of the absence or paucity of high-density lipoproteins (HDL) and alterations of (their) apo A-I and apo A-II (as well as of other lipids), the arterial SMC may be in some way involved in the metabolism of the above substances in this disorder. Support of this tentative (and highly speculative) assumption must await further work utilizing tissues and cells other than those containing macrophages and other derivatives of reticulo-endothelial system.

Role of apolipoproteins in cholesterol efflux from macrophages to lipid microemulsion: proposal of a putative model for the pre-beta high-density lipoprotein pathway

Lipid microemulsion of phospholipid and triglyceride with the size of low-density lipoprotein was capable of removing cholesterol from cholesterol-loaded mouse peritoneal macrophages, resulting in reduction of intracellularly accumulated cholesteryl ester. Apolipoproteins (apo) A-I, A-II, C-III, and E bound to the surface of the microemulsion did not modulate the interaction of the microemulsion with the cells in terms of the cholesterol efflux. The cholesterol removal by the microemulsion was enhanced by some 30% only when apoA-I, -A-II, and -E were present in excess to provide their free forms in the medium, but apoC-III did not show such an effect even by its excess amount. The kinetics including the results with apoC-III were consistent with a model that the apparent enhancement was due to generation of pre-beta high-density lipoprotein (HDL)-like particles upon the interaction of free apolipoproteins with macrophages [Hara, H., & Yokoyama, S. (1991) J. Biol. Chem. 266, 3080-3086]. However, pre beta-HDL-like particle was not detected after 6- and 24-h incubation in the medium where cholesterol efflux to the emulsion was maximally enhanced by the apolipoproteins, and cholesterol and phospholipids removed from the cells were all found with the microemulsions. It was also shown separately that the lipids in pre beta-HDL-like particles generated by apoA-I and macrophages were rapidly, within the order of minutes, transferred to the apo-lipoprotein-covered microemulsions when they were incubated together. Thus, the data were consistent with a model that the free form of certain apolipoproteins, such as apoA-I, -A-II, and -E but not apoC-III, generates pre beta-HDL-like particles with cellular lipids in situ and these particles act as mediators for cholesterol transfer from the cells to other lipoproteins.

Cholesterol efflux from macrophages mediated by high-density lipoprotein subfractions, which differ principally in apolipoprotein A-I and apolipoprotein A-II ratios

High-density lipoprotein (HDL) was fractionated by preparative isoelectric focussing into six distinct subpopulations. The major difference between the subfractions was in the molar ratio of apolipoprotein A-I to apolipoprotein A-II, ranging from 2.1 to 0.5. The least acidic particles had little apolipoprotein A-II, were larger and contained the most lipid. The efflux capacity of the HDL subfractions was tested with mouse peritoneal macrophages and a mouse macrophage cell line (P388D1), either fed with acetylated low-density lipoprotein or free cholesterol. All the HDL subfractions were equally able to efflux cholesterol. The efflux was concentration dependant and linear for the first 6 h. The HDL subfractions bound with high affinity (Kd = 6.7-7.9 micrograms/ml) at 4 degrees C to the cell surface of P388D1 cells (211,000-359,000 sites/cell). Ligand blotting showed that all the HDL subfractions bound to membrane polypeptides at 60, 100, and 210 kDa. These HDL binding proteins may represent HDL receptors. In summary HDL particles, which differed principally in ratio of apolipoprotein A-I to apolipoprotein A-II behaved in a similar manner for both cholesterol efflux and cell surface binding.

High density lipoprotein interaction with human placenta: biochemical and ultrastructural characterization of binding to microvillous receptor and lack of internalization

Specific receptor and internalization process for low density lipoprotein (LDL) and modified LDL (acetyl-LDL) have been well characterized in placental microvilli and in trophoblastic cells in culture. The aim of this study was to investigate high density lipoprotein (HDL3) binding and its eventual subsequent internalization in both these purified placental preparations. Isolated term placental microvilli were used for binding of [125I]HDL3 (devoid of apoprotein E). HDL3 were conjugated to colloidal gold for ultrastructural visualization of binding and internalization in syncytiotrophoblast in culture. Saturable binding of HDL3 was identified. Scatchard analysis revealed a Kd value of 24.2 +/- 8.0 micrograms HDL3 protein/ml and a maximum binding capacity at 4 degrees C of 128.2 +/- 54.5 micrograms HDL3 protein/mg of membrane protein. These sites have broad specificity: both LDL and acetyl-LDL were able to partially inhibit the HDL3 binding. Ultrastructural study confirms that gold-HDL3 bind specifically to syncytiotrophoblast membrane. However, after incubation at 37 degrees C, an internalization process similar to those described for gold-LDL and gold-acetyl-LDL was not observed for gold-HDL3. These results demonstrate specific HDL3 binding without internalization. The physiological significance of an HDL3 membranous interaction and the placental steroidogenesis remains to be established.

Cholesterol stimulation of HDL binding to human endothelial cells EAhy 926 and skin fibroblasts: evidence for a mechanism independent of cellular metabolism

The properties of the HDL binding site on the permanent human cell line EAhy 926 were studied. This cell line presents with highly differentiated functions of vascular endothelium. EAhy 926 cells possess HDL3 saturable binding sites with a Kd of about 20 micrograms/ml, which were up-regulated by cholesterol and were pronase- and EDTA-insensitive. Furthermore, HDL3 promoted cholesterol efflux from EAhy 926 cells in a dose-dependent manner. Thus, the HDL-binding site in EAhy 926 cells is similar to that present in fibroblasts, smooth muscle cells and endothelial cells. Up-regulation of HDL binding by cholesterol did not require de novo synthesis of HDL 'receptor' protein, as shown by the lack of effect of cycloheximide and alpha-amanitin and also occurred in fixed, non-living cells. Similar results were obtained using human skin fibroblasts. From these data we conclude that: (a) EAhy 926 cells are a good model for studying the HDL interaction with endothelial cells; (b) a mechanism independent of cellular metabolism is involved in the cholesterol-mediated up-regulation of HDL binding sites in EAhy 926 cells and human skin fibroblasts.

High density lipoprotein apolipoproteins mediate removal of sterol from intracellular pools but not from plasma membranes of cholesterol-loaded fibroblasts

Cultured cells possess high-affinity binding sites (receptors) for high density lipoprotein (HDL) that appear to mediate removal of excess intracellular cholesterol from cells. To examine the role of intact HDL apoproteins in receptor-mediated cholesterol removal, HDL3 apoproteins were digested with the proteolytic enzymes trypsin and pronase, and the residual particles were used in sterol efflux experiments. Protease treatment abolished the interaction of HDL3 with the 110-kd cell membrane protein postulated to represent the HDL receptor molecule, indicating that this interaction is mediated by HDL apoproteins rather than lipids. Compared with native HDL3 protease-modified HDL3 had a markedly reduced ability to selectively remove sterol from intracellular pools, even though modified particles promoted greater cholesterol efflux from the plasma membrane than did native particles. These results indicate that whereas sterol efflux from plasma membranes is mediated by HDL lipids, removal of excess intracellular sterol from cells is mediated by HDL apoproteins. These findings are consistent with the hypothesis that receptor binding of HDL apoproteins stimulates translocation of excess intracellular sterol to the cell surface where it becomes accessible for removal by HDL or other lipid-rich acceptor particles.

Uptake and utilization of lipoprotein cholesteryl esters by rat granulosa cells

Earlier studies have shown that rat granulosa cells grown in serum-free medium are exquisitely responsive to exogenously provided lipoprotein cholesterol. In this study we compare the amount of cholesterol (cholesteryl ester) actually delivered from various homologous and heterologous cholesterol-rich lipoproteins and examine the intracellular pathways used in the delivery system. Granulosa cells were incubated for 5 or 24 h with 125I-labeled human (h) HDL3, rat (r) HDL or hLDL equipped with non-releasable apoprotein and cholesteryl ether tags which accumulate within cells, even after degradation. We show that all the tested lipoproteins were similarly efficient in cholesteryl ester delivery; i.e., based on cholesterol: protein ratios of the starting ligands, each delivered approximately the same cholesteryl ester mass and evoked a similar progestin response. However, each lipoprotein was processed quite differently by the granulosa cells: hHDL3-cholesteryl ester was taken up almost exclusively by an non-endocytic pathway, hLDL-cholesteryl ester almost exclusively by an endocytic pathway and rHDL-cholesteryl ester by both pathways. In general, there was no correlation between the total amount of lipoprotein bound or apoprotein internalized and/or degraded by the cells with the amount of cholesteryl ester received or the level of the progestin response. Hormone stimulation upregulated the preferred pathway for each lipoprotein.

Studies on the interaction between apolipoprotein A-II-enriched HDL3 and cultured bovine aortic endothelial (BAE) cells

The specific binding of high-density lipoproteins (HDL) to a number of cell and membrane types has been reported. The aim of this study was to investigate the ligand specificity of HDL binding sites on bovine aortic endothelial (BAE) cells and in particular to investigate the role of apo A-II in the interaction. In order to do this we prepared AII-HDL3 particles by incubating HDL3 with apo HDL. These particles were enriched in apo A-II, contained virtually no apo A-I, and were similar to HDL3 in terms of size and lipid composition. As these particles resemble the native HDL3 structure we believe they are probably a more suitable model for investigation of ligand specificity than artificial recombinants. AII-HDL3 particles were shown to bind to cells with similar affinity and capacity as HDL3. Further experiments indicated that HDL3 and AII-HDL3 competed with each other for binding and displayed similar affinities for a common binding site(s). The results suggest that apo A-II, as well as apo A-I, play an important role in the process of HDL recognition by putative HDL receptors on endothelial cells.

Behaviour of phospholipase-modified HDL towards cultured hepatocytes. I. Enhanced transfers of HDL sterols and apoproteins

Human HDL subfractions (HDL2, HDL3, or HDL separated by heparin affinity chromatography) were labelled either on their apolipoprotein moiety with 125I or on their sterols: unesterified [14C]cholesterol and [3H]cholesteryl linoleyl ether, a non-hydrolysable analog of esterified cholesterol. HDL subfractions were then treated with or without phospholipase A2 from Crotalus adamanteus in presence of albumin leading to a 72-82% phosphatidylcholine degradation. Control and treated HDL were reisolated and then addressed to cultured rat hepatocytes. (A) During incubations, unesterified [14C]cholesterol from HDL3 readily appeared in hepatocytes. The specific uptake of HDL esterified cholesterol calculated from [3H]cholesteryl ether was 2-4-times less important. Uptake of HDL cholesterol tended to saturate at 150-200 micrograms/ml HDL protein. A prior phospholipase treatment of HDL3 stimulated by 2-5-fold the uptake of [3H]cholesteryl ether, whereas the transfer of free [14C]cholesterol was minimally increased. The uptake of 3H/14C-labelled sterols from HDL2 was 2-3-times higher than from HDL3. (B) Parallel experiments were conducted with 125I-labelled HDL subfractions. At 37 degrees C, the specific uptake and degradation of HDL3 125I-apolipoprotein were about 2-fold enhanced following treatment of HDL3 with phospholipase A2. Uptakes of apolipoprotein and of esterified cholesterol were compared, indicating a preferential delivery of the sterol over apoprotein (X5). The dissociation was still more pronounced with phospholipase-treated HDL3. Competition experiments showed that 12-times more unlabelled HDL3 were required to half reduce the uptake of HDL3 [3H]cholesteryl ether than to impede similarly the HDL 125I-apolipoprotein recovered in cells. Uptake of 125I-labelled apolipoprotein from HDL2 was quantitatively comparable to that from HDL3. (C) Binding of 125I-HDL subfractions was followed at 4 degrees C. A specific binding was observed for HDL2 and HDL3, although kinetic parameters were quite different (KD of 9 and 25 micrograms/ml, respectively). Following phospholipolysis, both the specific and non-specific contributions to total binding were increased. Hence, hepatocytes take up more 125I-labelled apolipoprotein and 3H/14C-labelled sterols from lipolysed HDL than from unmodified particles. This is associated to changes in the binding characteristics.

Beta-VLDL metabolism by pigeon macrophages. Evidence for two binding sites with different potentials promoting cholesterol accumulation

Previous studies from our laboratory (J Lipid Res 1988;29:643-656) have shown that thioglycolate-elicited peritoneal macrophages from White Carneau and Show Racer pigeons, like mammalian macrophages, have on their surfaces specific receptors for acetylated low density lipoprotein (acLDL) and beta-migrating very low density lipoproteins (beta-VLDL). The binding kinetics of beta-VLDL were complex, however, suggesting more than one binding site. The purpose of the present study was to further characterize these beta-VLDL binding sites. Scatchard analysis of 125I-beta-VLDL binding curves indicated at least two classes of binding sites. The first binds pigeon beta-VLDL and LDL with high affinity (Kd approximately 7 micrograms/ml), is down-regulated by cholesterol loading, requires calcium, and is destroyed by the proteolytic enzyme, pronase. This pigeon beta-VLDL receptor is specific for pigeon beta-VLDL and LDL and does not recognize HDL, acLDL, methyl LDL, cynomolgus monkey LDL, or rabbit beta-VLDL. Like the mammalian macrophage beta-VLDL receptor, the "pigeon beta-VLDL receptor" has many of the characteristics of an LDL receptor. The second class of binding sites is relatively nonspecific, recognizing both pigeon and rabbit beta-VLDL, LDL, acLDL, methyl LDL, and HDL. Binding to this site is not altered by incubation of macrophages with pronase or by cholesterol loading. This binding site has low affinity for beta-VLDL (Kd approximately 100 micrograms/ml), but high capacity. We have called this the "lipoprotein binding site," a term used by others to describe similar lipoprotein binding characteristics on a variety of cells. Not only does binding to this site promote the internalization and degradation of lipoproteins, but it may also facilitate the independent uptake of cholesterol. This conclusion is based on the observation that more cholesterol accumulates in cells incubated with rabbit beta-VLDL, which binds only to the lipoprotein binding site, than can be accounted for by beta-VLDL uptake and degradation. Since the lipoprotein binding site recognizes a variety of normal, as well as abnormal, lipoproteins, it would not require the generation of abnormal lipoprotein products, as must occur with the scavenger receptor, to promote the accumulation of cholesteryl esters in macrophages of atherosclerotic lesions. This, coupled with the fact that the lipoprotein binding site is not down-regulated by cholesterol loading, suggests that it could provide an alternative mechanism to the scavenger receptor pathway for the formation of foam cells.

The low density lipoprotein receptor

The study of familial hypercholesterolemia at the molecular level has led to its advancement from a clinical syndrome to a fascinating experimental system. FH was first described 50 years ago by Carl Müller who concluded that the disease produces high plasma cholesterol levels and myocardial infarctions in young people, and is transmitted as an autosomal dominant trait determined by a single gene. The existence of two forms of FH, namely heterozygous and homozygous, was recognized by Khachadurian and Fredrickson and Levy much later. The value of FH as an experimental model system lies in the availability of homozygotes, because mutant genes can be studied without interference from the normal gene. The first and most important breakthrough was the realization that the defect underlying FH could be studied in cultured skin fibroblasts. Rapidly, the LDL receptor pathway was conceptualized and its dysfunction in cells from FH homozygotes was demonstrates. Isolation of the normal LDL receptor protein and studies on the biosynthesis and structure of abnormal receptors in mutant cell lines provided essential groundwork for elucidation of defects at the DNA level. The power of the experimental system, FH, became nowhere more obvious than in work that correlated structural information at the protein level with the elucidation of defined defects in the LDL receptor gene. In addition to revealing important structure-function relationships in the LDL receptor polypeptide and delineating mutational events, studies of FH have established several more general concepts. First, the tight coupling of LDL binding to its internalization suggested that endocytosis was not a non-specific process as suggested from early observations. The key finding was that LDL receptors clustered in coated pits, structures that had been described by Roth and Porter 10 years earlier. These investigators had demonstrated, in electron microscopic studies on the uptake of yolk proteins by mosquito oocytes, that coated pits pinch off from the cell surface and form coated vesicles that transport extracellular fluid into the cell. Studies on the LDL receptor system showed directly that receptor clustering in coated pits is the essential event in this kind of endocytosis, and thus established receptor-mediated endocytosis as a distinct mechanism for the transport of macromolecules across the plasma membrane. Subsequently, many additional systems of receptor-mediated endocytosis have been defined, and variations of the overall pathway have been described.(ABSTRACT TRUNCATED AT 400 WORDS)

Serum high-density lipoprotein particles of alcohol-fed rats are deficient in apolipoprotein E

Effects of chronic ethanol consumption on serum lipoproteins have been studied in the rat. The serum levels of triglycerides, cholesterol, phospholipids and apolipoproteins AI and AIV increased significantly after 1 week of ethanol feeding, and they remained elevated up to 7 weeks of alcohol drinking. By contrast, serum total apolipoprotein E decreased or, sometimes, did not change. Very-low-density lipoprotein cholesterol, triglycerides and very-low-density lipoprotein apolipoprotein E of the alcohol-fed rats increased in parallel and were about 2- to 2.5-fold over the controls. Whereas high-density lipoprotein cholesterol, phospholipids, apolipoprotein AI and AIV increased 1.2-fold by chronic alcohol feeding, the level of high-density lipoprotein apolipoprotein E decreased to 70% of that of the control rats. The rates of secretion of apolipoprotein AI, E and AIV into the culture medium by hepatocytes isolated from ethanol-fed rats were 1.8-, 1.3- and 1.1-fold higher than those from control rats. These data indicate that (i) chronic ethanol feeding increases very-low-density lipoprotein and high-density lipoprotein in the rat; (ii) serum high-density lipoprotein particles of the ethanol-fed rats are deficient in apolipoprotein E, and (iii) chronic ethanol feeding increases hepatic secretion of apolipoprotein AI, E and AIV. Since the steady-state serum level of apolipoprotein E decreases or remains unchanged in the presence of increased hepatic apolipoprotein E secretion, this imbalance suggests that alcohol feeding either accelerates the rate of degradation of serum apolipoprotein E or suppresses apolipoprotein E synthesis by nonhepatic tissues.