Purification and partial characterization of extracellular liposomes isolated from the hyperlipidemic rabbit aorta

Ultramorphological analysis of plaque advancement and cholesterol crystal formation in Ldlr knockout mouse atherosclerosis

BACKGOUND AND AIMS

The low-density lipoprotein receptor-deficient (Ldlr^-/-) mouse has been utilized by cardiovascular researchers for more than two decades to study atherosclerosis. However, there has not yet been a systematic effort to document the ultrastructural changes that accompany the progression of atherosclerotic plaque in this model.

METHODS

Employing several different staining and microscopic techniques, including immunohistochemistry, as well as electron and polarized microscopy, we analyzed atherosclerotic lesion development in Ldlr^-/- mice fed an atherogenic diet over time.

RESULTS

Lipid-like deposits occurred in the subendothelial space after only one week of atherogenic diet. At two weeks, cholesterol crystals (CC) formed and increased thereafter. Lipid, CC, vascular smooth muscles cells, and collagen progressively increased over time, while after 4 weeks, relative macrophage content decreased. Accelerated accumulation of plate- and needle-shaped CC accompanied plaque core necrosis. Lastly, CC were surrounded by cholesterol microdomains, which co-localized with CC through all stages of atherosclerosis, indicating that the cholesterol microdomains may be a source of CC.

CONCLUSIONS

Here, we have documented, for the first time in a comprehensive way, atherosclerotic plaque morphology and composition from early to advanced stages in the Ldlr^-/- mouse, one of the most commonly used animal models utilized in atherosclerosis research.

Quantification of sterol-specific response in human macrophages using automated imaged-based analysis

BACKGROUND

The transformation of normal macrophage cells into lipid-laden foam cells is an important step in the progression of atherosclerosis. One major contributor to foam cell formation in vivo is the intracellular accumulation of cholesterol.

METHODS

Here, we report the effects of various combinations of low-density lipoprotein, sterols, lipids and other factors on human macrophages, using an automated image analysis program to quantitatively compare single cell properties, such as cell size and lipid content, in different conditions.

RESULTS

We observed that the addition of cholesterol caused an increase in average cell lipid content across a range of conditions. All of the sterol-lipid mixtures examined were capable of inducing increases in average cell lipid content, with variations in the distribution of the response, in cytotoxicity and in how the sterol-lipid combination interacted with other activating factors. For example, cholesterol and lipopolysaccharide acted synergistically to increase cell lipid content while also increasing cell survival compared with the addition of lipopolysaccharide alone. Additionally, ergosterol and cholesteryl hemisuccinate caused similar increases in lipid content but also exhibited considerably greater cytotoxicity than cholesterol.

CONCLUSIONS

The use of automated image analysis enables us to assess not only changes in average cell size and content, but also to rapidly and automatically compare population distributions based on simple fluorescence images. Our observations add to increasing understanding of the complex and multifactorial nature of foam-cell formation and provide a novel approach to assessing the heterogeneity of macrophage response to a variety of factors.

Controlling liposomal drug release with low frequency ultrasound: mechanism and feasibility

The ability of low-frequency ultrasound (LFUS) to release encapsulated drugs from sterically stabilized liposomes in a controlled manner was demonstrated. Three liposomal formulations having identical lipid bilayer compositions and a similar size ( approximately 100 nm) but differing in their encapsulated drugs and methods of drug loading have been tested. Two of the drugs, doxorubicin and methylpredinisolone hemisuccinate, were remote loaded by transmembrane gradients (ammonium sulfate and calcium acetate, respectively). The third drug, cisplatin, was loaded passively into the liposomes. For all three formulations, a short exposure to LFUS (<3 min) released nearly 80% of the drug. The magnitude of drug release was a function of LFUS amplitude and actual exposure time, irrespective of whether irradiation was pulsed or continuous. Furthermore, no change in liposome size distribution or in the chemical properties of the lipids or of the released drugs occurred due to exposure to LFUS. Based on our results, we propose that the mechanism of release is a transient introduction of porelike defects in the liposome membrane, which occurs only during exposure to LFUS, after which the membrane reseals. This explains the observed uptake of the membrane-impermeable fluorophore pyranine from the extraliposomal medium during exposure to LFUS. The implications of these findings for clinical applications of controlled drug release from liposomes are discussed.

Nonoxidative modifications of lipoproteins in atherogenesis

The key initiating event in atherosclerosis is the retention of plasma lipoproteins in the subendothelial matrix. Subsequently, a series of biological responses to this retained material leads to specific molecular and cellular processes that promote lesion formation. There is considerable evidence that many of these biological responses, notably macrophage cholesteryl ester loading (foam cell formation), require subendothelial modification of the retained lipoproteins. Oxidation of lipoproteins is one such modification that likely occurs in vivo and promotes certain atherogenic events, but oxidation cannot explain all aspects of atherogenesis, including certain elements of macrophage foam cell formation. For this reason, there has been renewed interest in other modifications of lipoproteins that may be important in atherogenesis. This review addresses five such lipoprotein modifications, namely aggregation, glycation, immune complex formation, proteoglycan complex formation, and conversion to cholesterol-rich liposomes. The focus is on the evidence that these modifications occur in atherosclerotic lesions and on the potential role of these modified lipoproteins in atherogenesis, with an emphasis on macrophage foam cell formation.

Apolipoprotein B stimulates formation of monocyte-macrophage surface-connected compartments and mediates uptake of low density lipoprotein-derived liposomes into these compartments

Much of the cholesterol that accumulates in atherosclerotic plaques is found within monocyte-macrophages transforming these cells into "foam cells." Native low density lipoprotein (LDL) does not cause foam cell formation. Treatment of LDL with cholesterol esterase converts LDL into cholesterol-rich liposomes having >90% cholesterol in unesterified form. Similar cholesterol-rich liposomes are found in early developing atherosclerotic plaques surrounding foam cells. We now show that cholesterol-rich liposomes produced from cholesterol esterase-treated LDL can cause human monocyte-macrophage foam cell formation inducing a 3-5-fold increase in macrophage cholesterol content of which >60% is esterified. Although cytochalasin D inhibited LDL liposome-induced macrophage cholesteryl ester accumulation, LDL liposomes did not enter macrophages by phagocytosis. Rather, the LDL liposomes induced and entered surface-connected compartments within the macrophages, a unique endocytic pathway in these cells that we call patocytosis. LDL liposome apoB rather than LDL liposome lipid mediated LDL liposome uptake by macrophages. This was shown by the findings that: 1) protease treatment of the LDL liposomes prevented macrophage cholesterol accumulation; 2) liposomes prepared from LDL lipid extracts did not cause macrophage cholesterol accumulation; and 3) purified apoB induced and accumulated within macrophage surface-connected compartments. Although apoB mediated the macrophage uptake of LDL liposomes, this uptake did not occur through LDL, LDL receptor-related protein, or scavenger receptors. Also, LDL liposome uptake was not sensitive to treatment of macrophages with trypsin or heparinase. Cholesterol esterase-mediated transformation of LDL into cholesterol-rich liposomes is an LDL modification that: 1) stimulates uptake of LDL cholesterol by apoB-dependent endocytosis into surface-connected compartments, and 2) causes human monocyte-macrophage foam cell formation.

Chromatographic approaches to liposomes, proteoliposomes and biomembrane vesicles

Size-exclusion chromatography has been used for fractionation of liposomes, proteoliposomes and biomembrane vesicles of up to approximately 500 nm in size and for separation of these entities from smaller components. Liposome sizes, encapsulation stability, and solute affinities for membrane proteins have been determined. Counter-current distribution in aqueous two-phase systems has widened the range of applications to larger structures. Immobilized biomembrane vesicles and (proteo)liposomes provide stationary phases for chromatographic analysis of specific or nonspecific membrane-solute interactions.

Is oxidized low-density lipoprotein present in vivo?

Basic research has provided strong evidence that oxidation of LDL plays an important role in the pathogenesis of atherosclerosis. Several mechanisms have been identified which can lead to LDL oxidation in vivo. Clinical and epidemiological studies have provided circumstantial evidence that oxidized LDL, as measured by serum autoantibody levels, may be associated with the progression of atherosclerotic vascular disease. This review discusses recent findings regarding the presence of oxidized LDL (ox-LDL) in vivo and the significance of ox-LDL autoantibody measurements as a tool to predict cardiovascular diseases in various patient populations.

Phospholipid transfer protein can transform reconstituted discoidal HDL into vesicular structures

The present study investigated the effect of phospholipid transfer protein (PLTP) on transformation of discoidal HDL (d-HDL) to vesicular structures by using primarily KBr density gradient centrifugation, non-denaturing gradient gel electrophoresis, and electron microscopy. The incubation of reconstituted d-HDL preparations containing apo-AI with PLTP resulted in the formation of vesicular structures differing in hydrated densities and sizes. The extents of transformation were dependent upon PLTP concentrations and incubation times. Substantial transformations occurred, even with plasma concentrations of PLTP, within 4 h of incubation at 37 degrees C. After 8 h of incubation, almost 80% of d-HDL was converted to vesicular structures with a hydrated density of 1.07 g ml-1. The d-HDL-vesicle transformation appeared to be triggered by the PLTP-mediated displacement of apo-AI. This apo-AI displacement might have led to the fusion of transiently produced apo-AI deficient particles, producing thermodynamically stable vesicular structures. The cross-linking of apo-AI in d-HDL almost completely prevented d-HDL-vesicle transformation. The addition of free apo-AI to the PLTP/d-HDL incubation mixtures also greatly reduced the transformation. The conversion of smaller vesicles of density 1.07 g ml-1 to larger vesicles of density 1.05 g ml-1 also seemed to have been affected by PLTP-mediated apo-AI displacement. We described the possible implications of the transformation of d-HDL into vesicular structures in lipid and lipoprotein transport processes under physiological and pathological conditions.

Selective increase in cholesterol at atherosclerosis-susceptible aortic sites after short-term cholesterol feeding

In rabbits, the aortic arch and branch sites of the descending thoracic and abdominal aortas are susceptible to atherosclerosis. This study investigated the hypothesis that the reported focal increase in LDL concentration and mean residence time at susceptible aortic sites after feeding cholesterol for 4 to 8 days precede atherosclerotic change as indicated by increased aortic cholesterol concentration. Cholesterol concentrations for all aortic sites of normal rabbits were similar (approximately equal to 2.8 mumol/g). No change in aortic cholesterol concentration could be detected after feeding cholesterol for 8 days. However, after feeding cholesterol for 12 and 16 days, cholesterol concentrations for abdominal branch sites were increased compared with abdominal branch sites of normal rabbits (4.47 +/- 0.50, n = 8, and 4.85 +/- 0.33, n = 11, mumol/g, respectively, versus 2.87 +/- 0.27, n = 12, mumol/g; P < .025 and P < .005, respectively). In contrast, the cholesterol concentration of atherosclerosis-resistant nonbranch abdominal aorta was unchanged after feeding cholesterol for 16 days and was much less than that of the branch sites (2.72 +/- 0.12 versus 4.85 +/- 0.33, mumol/g, n = 11; P < .001). Cholesterol concentrations for other susceptible sites were also increased after feeding cholesterol for 12 and 16 days. Cholesterol concentrations for susceptible sites were linearly related to a combined measure of duration and extent of hypercholesterolemia (P < .001 to P < .0001), whereas no such relationship could be detected for resistant sites. Most (59% to 93%) of the cholesterol accumulating in susceptible aortic sites after feeding cholesterol for 12 and 16 days was nonesterified, suggesting that the increased cholesterol concentration did not reflect development of foam cells or the insudation of plasma lipoproteins. This study suggests that the reported focal increases in LDL concentration and mean residence time at susceptible aortic sites during cholesterol feeding precede atherosclerosis.

Glycosphingolipid accumulation in the aortic wall is another feature of human atherosclerosis

High accumulation of lipids is a typical feature of an atherosclerotic lesion. We have previously identified the chemical structure of the major glycosphingolipids (GSLs) of human aorta; however, quantification of the absolute concentration of GSLs was not carried out. In the present study, for the first time we have performed a quantitative comparative analysis of GSL composition in the media and two sublayers of the intima taken from normal regions, fatty streaks, and atherosclerotic plaques of the human aorta. The intimal tissue containing fatty streaks and atherosclerotic plaques accumulated GSLs, predominantly glucosylceramide (GlcCer), lactosylceramide (LacCer), and ganglioside GM3. GSL levels in plaques were highest: GlcCer was 18- and 8-fold, LacCer was 8- and 7-fold, and GM3 was 2.5- and 12-fold higher than in musculoelastic and elastic-hyperplastic intimal layers of normal regions, respectively. We did not observe a significant increase in other GSLs. An increase in the content of gangliosides GD3 and GD1a was detected in the media underlying atherosclerotic lesions. On the basis of an analysis of the ratio of GlcCer, LacCer, and GM3 accumulated in the tissue and cells of the elastic-hyperplastic layer of intima, we have concluded that the accumulation of the above-mentioned GSLs occurs mainly in the extracellular space of the intima. In this study, we have also demonstrated that extracellular lipid liposomes, which appear in the early stages of atherogenesis, are one locus of GSL accumulation in the extracellular space of the intima.(ABSTRACT TRUNCATED AT 250 WORDS)

Irreversibly glycated albumin alters the physico-chemical characteristics of low density lipoproteins of normal and diabetic subjects

In diabetic plasma, glycated albumin and glycated LDL coexist with augmented levels of peroxides, conditions frequently associated with the development of accelerated atherosclerosis. The direct interaction between irreversibly glycated albumin, LDL and oxidation have not been explored yet. We tried to elucidate whether irreversibly glycated albumin (AGE-Alb) induces changes in the chemistry and morphology of LDL particle, and if AGE-Alb has the ability to scavenge free radicals, as reported for native albumin. LDL isolated from normal (nLDL) or diabetic human subjects (dLDL) was incubated in vitro with AGE-Alb in conditions of autoxidation (37 degrees C, 24-48 h in the absence of oxidation inhibitors) or of Cu2+ induced-oxidation. The results showed that, especially in the latter condition, AGE-Alb induced marked physico-chemical modifications of both nLDL and dLDL without significant changes in the level of peroxides. Incubation with AGE-Alb decreased the cholesteryl esters/unesterified cholesterol ratio of nLDL by 30% and of dLDL by approximately 50%. Concomitantly, in oxidative conditions a marked increase (approximately 3-fold) in the lysophosphatidylcholine/phosphatidylcholine ratio of dLDL was detected. Apolipoprotein B integrity as well as the morphology of the lipoprotein particles were drastically affected. To a lesser extent, these modifications occurred also in the presence of inhibitors of oxidation at 37 degrees C, but not at 4 degrees C. The above described effects were constantly more pronounced in the case of dLDL. These results indicated that in the absence of other plasma or vascular tissue components (e.g., endothelial cells, extracellular matrix) AGE-Alb by itself induces alterations in the chemistry and morphology of LDL, especially of glycated LDL, modifications that may account for the occurrence of accelerated atherogenesis in diabetes.

Intimal thickenings of human aorta contain modified reassembled lipoproteins

The aim of this study was to determine whether in human aortas early minute changes such as minimal intimal thickenings (MIT), developed in areas known to have a predilection to atherosclerosis, contain modified reassembled lipoproteins (MRLp) such as extracellular liposomes (EL) and lipid droplets (LD). These features have been previously detected in the aortic lesion-prone areas of rabbits and hamsters fed a fat-rich diet. Tissue samples of the aortic arch and thoracic aorta from 12 young subjects who died in accidents were selectively collected from grossly normal regions. By light microscopy, some of these regions were found to contain MIT. The normal areas and the MIT were separately examined by electron microscopy or subjected to fractionation and partial biochemical characterization. The MIT (approximately 25-100 microns thick) were constituted by a pronounced proliferation of extracellular matrix, especially elastin and microfibrils, with interspersed lipid deposits appearing as EL and LD. Commonly, MIT did not contain smooth muscle cells, macrophages, foam cells or cytolytic debris. Such components were only occasionally found in specimens excised from the vicinity of fatty streaks. Saline extracts of MIT or grossly normal aortic regions were subjected to a four-step purification procedure consisting of gel filtration, affinity chromatography on anti-apo B and anti-albumin Sepharose, followed by density gradient ultracentrifugation. The entire procedure was monitored by negative staining, lipid assays, SDS PAGE and immunoblotting. From the initial MRLp mixture, two fractions were obtained: fraction 1 containing multilamellar EL and LD, and fraction 2 composed mostly of unilamellar EL. As compared with serum LDL, the cholesteryl ester/unesterified cholesterol ratio was 4-6-fold lower in fraction 1 and 15-19-fold lower in fraction 2. On SDS-PAGE the fraction 2 displayed a single protein band of 66 kDa, immunochemically identified as albumin. The MRLp isolated from human aortas with minimal intimal thickenings appeared to be similar to those purified from the prelesional stage aorta of hyperlipidemic rabbits and hamsters.

Liposome-like particles isolated from human atherosclerotic plaques are structurally and compositionally similar to surface remnants of triglyceride-rich lipoproteins

Recent studies have demonstrated the presence of unesterified cholesterol-rich, liposome-like vesicles in the extracellular space of atherosclerotic lesions in humans and animals. Liposome-like vesicles accumulate in the subendothelial space in rabbits within 2 weeks of initiation of cholesterol feeding, well before foam cells appear. These observations suggest that extracellular liposome-like vesicles may play a pivotal role in atherogenesis. The origin of these particles is unknown. We report a combination of in vivo and in vitro experiments that suggest a novel origin for these liposome-like vesicles. We demonstrate that the liposome-like particles isolated from postmortem human atherosclerotic plaques are rich in intact apolipoprotein (apo) A-I, C apolipoproteins, and sphingomyelin. We show that the in vivo derived particles are virtually identical, structurally and compositionally, to liposome-like lipolytic surface remnants of triglyceride (TG)-rich lipoproteins produced during in vitro lipolysis of hypertriglyceridemic serum. In vitro lipolysis of isolated very-low-density lipoprotein has shown that the lipolytic surface remnants remain attached to the core remnants in the absence of high-density lipoprotein (HDL), dissociate to form liposome-like vesicles in the presence of low levels of HDL, and are assimilated into HDL to form larger HDL particles in the presence of excess HDL. Thus, the in vitro produced, liposome-like particles represent a complex of lipolytic surface remnants of TG-rich lipoproteins and apo A-I derived from HDL. Two possible origins have been suggested for the extracellular liposome-like vesicles in atherosclerotic plaques: (1) modified, aggregated, and/or degraded LDL particles entrapped in an intimal matrix and (2) intracellular lipid products of arterial wall cells. Neither possibility directly explains the presence of A-I and C apolipoproteins and excess sphingomyelin that we observe. We propose as an alternate explanation that the in vivo liposome-like particles are lipolytic surface remnants of TG-rich lipoproteins. We further suggest that these remnants are produced in the intimal space by undefined processes and/or are transcytosed into the intima from the plasma compartment as a product of normal lipolysis gone awry. We conjecture that one role of HDL may be to assimilate the highly atherogenic liposome-like particles in a (1) "mop-up" fashion to remove them from the artery wall and/or (2) preventive fashion in the plasma compartment to prevent their transcytosis into the artery wall. The suggestion that elevated concentrations of surface remnants act as a "sink" for apo A-I can also account for the well-established but poorly understood link between hypertriglyceridemia and low HDL.

In vitro formation of oxidatively-modified and reassembled human low-density lipoproteins: antioxidant effect of albumin

In early atherogenesis, excess plasma lipoproteins accumulate into the arterial lesion-prone areas as modified and reassembled lipoproteins (MRLp) appearing mostly as lipid droplets and vesicles. In the present study we produced such MRLp, in a cell-free system, devoid of any component of extracellular matrix, by subjecting in vitro human low-density lipoproteins (LDL) to autoxidation or copper-induced oxidation, for up to 96 h. As visualized by negative staining electron microscopy, a large number of lipoprotein particles (Lp) were progressively transformed into aggregates (ALp), fused particles (FLp) and vesicles (VLp). These modifications were paralleled by peroxidation of the samples as revealed by chemical analysis of each MRLp fraction isolated by a three-step purification procedure. LDL peroxidation in the above conditions was inhibited by the presence of albumin as assessed by TBARS and lipid analysis, and by the lack of MRLp formation. This protective effect was independent of albumin source (bovine, human, rabbit) and occurs at an albumin/LDL ratio of 1 when Cu2+ was present, and at a ratio of 0.25 in autoxidative conditions. The results show that: (i) in vitro LDL autoxidation or copper-induced peroxidation in a cell-free system can generate modified and reassembled lipoproteins similar to those detected in vivo in the arterial intima at the inception of atherogenesis; (ii) Lp particles appear to be sequentially transformed in self-aggregates, droplets and vesicles; (iii) serum albumin can completely prevent these LDL alterations.

Scavenger receptor-independent stimulation of cholesterol esterification in macrophages by low density lipoprotein extracted from human aortic intima

There is a growing body of evidence that suggests that modification of low density lipoprotein (LDL) in the artery wall may contribute to atherogenesis. A number of physiologically plausible modifications have been studied in vitro, including oxidation, aggregation, formation of complexes with glycosaminoglycans, and generation of LDL-immune complexes. Several studies of the properties of LDL extracted from the aortic intima have been published, but these indicate disagreement about both the nature and the extent of modification of LDL in the artery wall. The objectives of the present study were to determine the nature and extent of modification of LDL extracted from both normal and diseased human aortic intimas and to correlate this with the rate of LDL uptake in cultured cells. Analyses were performed on LDLs isolated from aortic intimas obtained at autopsy or at the time of organ harvest from 33 subjects. LDL from normal intima showed no clear evidence of oxidation but had slightly increased electrophoretic mobility compared with native plasma LDL, whereas LDL from plaques or fatty streaks exhibited variable but usually modest signs of oxidative change. Aortic LDL was more rapidly degraded by cultured macrophages than was plasma LDL and resulted in a greater stimulation of cholesterol esterification. The degree of stimulation of cholesterol esterification was correlated with the extent of modification of LDL as reflected by the degree of apolipoprotein B fragmentation. However, in all aortic LDLs the extent of oxidative change, as assessed by electrophoretic mobility or other physical parameters, was less than that required for scavenger receptor-mediated uptake. In all cases where sufficient amounts of LDL were recovered to permit degradation experiments, the uptake of aortic LDL was nonsaturable and could not be inhibited by polyinosinic acid or acetylated LDL. Chromatography on Sepharose CL-4B showed that most LDLs isolated from plaque contained a fraction that eluted in the void volume, and the size of this void peak correlated well with the stimulation of cholesterol esterification. Electron microscopy showed that the high-molecular-weight fraction contained several different types of aggregates. Some appeared to be clusters of LDL-size particles, but large vesicular structures with numerous adherent LDL particles as well as lipid droplets were also identified. These results indicate that the accelerated uptake by macrophages of LDL isolated from the arterial intima can largely be attributed to phagocytosis of LDL-containing aggregates.(ABSTRACT TRUNCATED AT 400 WORDS)