Prelesional events in atherogenesis. Colocalization of apolipoprotein B, unesterified cholesterol and extracellular phospholipid liposomes in the aorta of hyperlipidemic rabbit

Atomic force microscopy study of the complexation of sterols and the glycoalkaloid α-tomatine in Langmuir-Blodgett monolayers

Glycoalkaloids are secondary metabolites produced by plants that aid in their protection from pathogens and pests. They are known to form 1:1 complexes with 3β-hydroxysterols such as cholesterol causing membrane disruption. So far, the visual evidence showcasing the complexes formed between glycoalkaloids and sterols in monolayers has been mainly restricted to some earlier studies using Brewster angle microscopy which were of low resolution showing the formation of floating aggregates of these complexes. This study is aimed at using atomic force microscopy (AFM) for topographic and morphological analysis of the aggregates of these sterol-glycoalkaloid complexes. Langmuir-Blodgett (LB) transfer of mixed monolayers of the glycoalkaloid α-tomatine, sterols, and lipids in varying molar ratios onto mica followed by AFM examination was performed. The AFM method allowed visualization of the aggregation of sterol-glycoalkaloid complexes at nanometer resolution. While aggregation was observed in mixed monolayers of α-tomatine with cholesterol and in mixed monolayers with coprostanol, no sign of complexation was observed for the mixed monolayers of epicholesterol and α-tomatine, confirming their lack of interaction found in prior monolayer studies. Aggregates were observed in transferred monolayers of ternary mixtures of α-tomatine with cholesterol and the phospholipids 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or egg sphingomyelin (egg SM). The formation of aggregates was found to be less prevalent for mixed monolayers of DMPC and cholesterol containing α-tomatine than it was for mixed monolayers containing egg SM and cholesterol with α-tomatine. The observed aggregates were generally elongated structures, of a width ranging from about 40-70 nm.

Transport of Apolipoprotein B-Containing Lipoproteins through Endothelial Cells Is Associated with Apolipoprotein E-Carrying HDL-Like Particle Formation

Passage of apolipoprotein B-containing lipoproteins (apoB-LPs), i.e., triglyceride-rich lipoproteins (TRLs), intermediate-density lipoproteins (IDLs), and low-density lipoproteins (LDLs), through the endothelial monolayer occurs in normal and atherosclerotic arteries. Among these lipoproteins, TRLs and IDLs are apoE-rich apoB-LPs (E/B-LPs). Recycling of TRL-associated apoE has been shown to form apoE-carrying high-density lipoprotein (HDL)-like (HDL_E) particles in many types of cells. The current report studied the formation of HDL_E particles by transcytosis of apoB-LPs through mouse aortic endothelial cells (MAECs). Our data indicated that passage of radiolabeled apoB-LPs, rich or poor in apoE, through the MAEC monolayer is inhibited by filipin and unlabeled competitor lipoproteins, suggesting that MAECs transport apoB-LPs via a caveolae-mediated pathway. The cholesterol and apoE in the cell-untreated E/B-LPs, TRLs, IDLs, and LDLs distributed primarily in the low-density (LD) fractions (d ≤ 1.063). A substantial portion of the cholesterol and apoE that passed through the MAEC monolayer was allotted into the high-density (HD) (d > 1.063) fractions. In contrast, apoB was detectable only in the LD fractions before or after apoB-LPs were incubated with the MAEC monolayer, suggesting that apoB-LPs pass through the MAEC monolayer in the forms of apoB-containing LD particles and apoE-containing HD particles.

A theory for water and macromolecular transport in the pulmonary artery wall with a detailed comparison to the aorta

The pulmonary artery (PA) wall, which has much higher hydraulic conductivity and albumin void space and approximately one-sixth the normal transmural pressure of systemic arteries (e.g, aorta, carotid arteries), is rarely atherosclerotic, except under pulmonary hypertension. This study constructs a detailed, two-dimensional, wall-structure-based filtration and macromolecular transport model for the PA to investigate differences in prelesion transport processes between the disease-susceptible aorta and the relatively resistant PA. The PA and aorta models are similar in wall structure, but very different in parameter values, many of which have been measured (and therefore modified) since the original aorta model of Huang et al. (23). Both PA and aortic model simulations fit experimental data on transwall LDL concentration profiles and on the growth of isolated endothelial (horseradish peroxidase) tracer spots with circulation time very well. They reveal that lipid entering the aorta attains a much higher intima than media concentration but distributes better between these regions in the PA than aorta and that tracer in both regions contributes to observed tracer spots. Solutions show why both the overall transmural water flow and spot growth rates are similar in these vessels despite very different material transport parameters. Since early lipid accumulation occurs in the subendothelial intima and since (matrix binding) reaction kinetics depend on reactant concentrations, the lower intima lipid concentrations in the PA vs. aorta likely lead to slower accumulation of bound lipid in the PA. These findings may be relevant to understanding the different atherosusceptibilities of these vessels.

Ceruloplasmin and oxidized LDL colocalize in atherosclerotic lesions of hamster

Epidemiological studies show that the risk for cardiovascular diseases increases with increasing levels of free-copper in plasma. It is known that intact ceruloplasmin (CP), the major protein transporter of copper in human plasma, oxidizes low density lipoproteins (LDL) in vitro. Our aim was to study the interaction between LDL and CP in vitro and in vivo, in an animal model of diet-induced atherosclerosis. In order to visualize the pathway of LDL into the arterial wall, human native LDL was labeled with fluorescent DiI and injected into male, Golden Syrian hyperlipemic hamsters. In vitro results demonstrated that slightly degraded CP has a significant oxidation potential against LDL at neutral pH. In vivo, after 24 hours circulation, LDL-DiI was taken up by the enlarged intima and fatty streaks of the arterial wall. Immunohistochemical localization of oxidized LDL and CP revealed their presence in the same areas of the arteries that take up LDL-DiI. Co-localization of LDL and CP in the enlarged intima of pro-atherosclerotic areas might explain the possible copper-induced oxidation process that might occur after native LDL is taken-up from the blood, transcytosed through the endothelium and accumulated in focalized deposits.

Mechanopathobiology of Atherogenesis: A Review

Cardiovascular disease is the number one cause of mortality in the United States. Atherosclerosis, the primary etiology of cardiovascular disease is hypothesized to be a time-dependent response to arterial injury. Although risk factors for atherosclerosis are systemic in nature, certain arteries (e.g., coronary arteries) are more susceptible to plaque formation than others. The heterogeneous distribution of atherosclerosis in the vasculature is thought to be related to biomechanical factors. A review of the relevant pathological features of atherogenesis and how physiologically-consistent mechanical stimuli can impact those processes supports this notion. However, specific investigations geared toward finding the mechanistic link between mechanical stimuli and early atherogenic processes are required to differentiate those stimuli that facilitate and those that inhibit atherogenesis. Such knowledge is required for intelligent direction in the search for potential targets for clinical intervention.

HMG-CoA reductase inhibitors (statins) decrease MMP-3 and MMP-9 concentrations in abdominal aortic aneurysms

BACKGROUND

An imbalance in matrix metalloproteinases (MMPs) and their inhibitors (TIMPs) are implicated in AAA formation. 3-Hydroxy-3-methylglutaryl coenzyme-A reductase inhibitors (statins) are known to reduce MMP levels. The aim of this study was to investigate the in vivo effect of statins on MMP levels in AAA.

METHODS

Infra-renal aortic biopsies were obtained from the anterior sac of 63 patients undergoing asymptomatic repair. Seventeen patients were taking a statin pre-operatively, while 46 were not. The concentrations of MMP-1, -2, -3, -8, -9, -13, TIMP-1 and TIMP-2 were quantified using ELISA.

RESULTS

There was no difference in the concentration of MMP-1, -2, -8, -13, TIMP-1 or -2 in patients taking versus not taking a statin pre-operatively. In contrast levels of MMP-9 and MMP-3 were significantly lower in patients taking a statin.

CONCLUSIONS

These data demonstrate that statins decrease MMP-9 and MMP-3 levels and represent a potential pharmacotherapy in established AAA.

Calcification of elastic fibers in human atherosclerotic plaque

The present study was undertaken to systematically investigate whether calcification of elastic fibers occurs in human atherosclerotic plaques. Fourteen carotid artery segments obtained by endarterectomy were examined by a combination of electron microscopy and cytochemistry. The analysis demonstrated that calcification of elastic fibers occurred in all 14 specimens. Two distinct types of calcification of elastic fibers were identified. In type I calcification, elastin itself was observed to undergo calcification and no visible structural alterations preceded the calcification. In type II of calcification, structural alteration of elastin preceded calcification of elastic fibers and included vacuolization of elastin accompanied by the accumulation of neutral lipids and unesterified cholesterol within altered elastic fibers. In type II calcification, calcified deposits were found to form in an association with unesterified cholesterol. Type II calcification was widespread throughout the plaque matrix while type I calcification occurred only in the deep portions of plaques.

The smooth muscle cell membrane during atherogenesis: a potential target for amlodipine in atheroprotection

BACKGROUND

Atherosclerotic disease has been present in the human population apparently from the beginning of time. However, it has only been in the 20th century that improvements in the control of infectious diseases have allowed the average life span to increase to the point where atherosclerosis has been able to affect the general population. By the middle of the 20th century, atherosclerosis had reached epidemic levels, and it is currently pandemic and increasing worldwide. Despite its growing significance to health care, we still know relatively little about the cellular basis for plaque genesis in the vessel wall. Current thinking holds that atherosclerosis is caused by an unchecked chronic inflammatory process involving the cells of the arterial wall and their interaction with LDL and various inflammatory cells. Considerable evidence suggests that the principal insults underlying atherogenesis are serum dyslipidemias and oxidative stress mediated primarily by oxidized LDL. However, just how these insults alter the cell biology of vascular cells and lead to the atherosclerotic phenotype is still under intense investigation. Moreover, recent clinical trials have provided evidence that certain classes of drugs, including newer calcium channel blockers (CCBs), can remodel the arterial smooth muscle cell (SMC) membrane and inhibit the progression of atherosclerotic disease.

METHODS

This review summarizes our current thinking on atherogenesis in the arterial SMC and considers recent developments regarding alterations in the SMC membrane during the very early period of atherogenesis. We also discuss how certain CCBs might operate to produce atheroprotection.

RESULTS

The SMC membrane becomes enriched in unesterified cholesterol soon after the development of serum hypercholesterolemia. With excess membrane cholesterol, the membrane becomes thicker and develops distinct cholesterol domains. These alterations in the membrane increase the permeability of SMC to calcium and induce a variety of alterations in SMC function that contribute to cellular atherogenic processes during plaque genesis. Amlodipine, a third-generation CCB, markedly inhibits the progression of lesions. The explanation of this novel action may lie in the effects of this drug on various potential cellular targets.

CONCLUSIONS

Evidence is accumulating that excess membrane cholesterol may contribute to the cellular defects responsible for the transformation of the SMC to the atherosclerotic phenotype. Amlodipine, which has membrane-remodeling properties, is emerging as an important atheroprotective drug.

Accumulation of co-localised unesterified cholesterol and neutral lipids within vacuolised elastin fibres in athero-prone areas of the human aorta

To investigate whether there are alterations of elastin fibres in the arterial intima at the pre-atherosclerotic stage, grossly normal areas of human thoracic aorta were taken soon after death from 13 healthy trauma victims whose ages ranged from 16 to 40 years. Two areas were compared: atherosclerosis-prone (AP) areas localised to the dorsal aspect of the aorta along the rows of intercostal branch origins, and atherosclerosis-resistant (AR) areas from the ventral aorta. Electron microscopic analysis combined with cytochemical staining was applied. Unesterified cholesterol was identified using the filipin-staining technique while neutral lipids were visualised by the OTO-technique. Intimal features were studied by combining the filipin-staining and the OTO-technique. Electron microscopical examination showed that in both AR and AP areas, some elastin fibres in the intima were vacuolised. Unesterified cholesterol was found to be predominantly localised in the musculoelastic layer, in particular, inside the vacuolised elastin fibres. This localisation was seen in all 13 AP areas studied in contrast to the AR areas where it was observed in only four of 13 aortas studied (P < 0.0005, chi2-test). Accumulation of neutral lipids inside vacuolised elastin fibres was found in five out of 13 AP areas but was not observed in any of the AR areas (P=0.01, chi2). A combination of the filipin-staining and OTO-techniques showed that some deposits of neutral lipids and unesterified cholesterol within vacuolised elastin fibres were independently located from each other, but more frequently, neutral lipids were co-located with unesterified cholesterol. The present observations indicate a difference between AP and AR intimal areas which, in particular, relates to the structure of elastin fibres in the musculoelastic layer. The observations suggest that alterations of the extracellular matrix are involved in the trapping and retention of cholesterol and neutral lipids within the intima at an early stage in the development of atherosclerotic lesions.

Complement C6 deficiency protects against diet-induced atherosclerosis in rabbits

Low-density lipoprotein (LDL) can be transformed to an atherogenic moiety by nonoxidative, enzymatic degradation. Enzymatically degraded LDL induces macrophage foam cell formation, provokes release of cytokines, and also activates complement. To determine whether complement activation may contribute to atherogenesis, 6 pairs of homozygous C6-deficient rabbits and their non-C6-deficient heterozygous siblings were fed a cholesterol-rich diet for 14 weeks. Cholesterol levels and plasma lipoprotein profiles of the animals in the C6-competent and C6-deficient groups did not significantly differ, and the high density lipoprotein and LDL cholesterol ratios at the end of the experiment were 0.07+/-0.01 and 0.08+/-0.01 (SEM), respectively. However, differences in atherosclerotic plaque formation were discernible macroscopically, with extensive aortic lesions being visible in all C6-competent animals and absent in all C6-deficient animals. Aortas were sectioned from thorax to abdomen, and 10 sections were stained from each aorta. Quantification of atherosclerotic lesions and lumen stenosis with the use of computer-based morphometry documented a dramatic protective effect of C6 deficiency on the development of diet-induced atherosclerosis. We conclude that the terminal complement sequence is centrally involved in atherosclerotic lesion progression.

[An alternative hypothesis of the pathogenesis of atherosclerosis]

Why LDL entrapped in the subendothelium should trigger events leading to chronic inflammation and to arterial wall injury is a major enigma of modern medicine. Oxidation of LDL in vitro renders the molecule potentially atherogenic, and the concept that oxidation is the major single event underlying the transformation of LDL to a proinflammatory molecule dominates the world literature. Here, an alternative hypothesis on the pathogenesis of atherosclerosis will be presented. We have found that non-oxidative, enzymatic modification of LDL with ubiquitous enzymes (protease + cholesterol esterase + neuraminidase) also transforms the molecule to an atherogenic moiety. Enzymatically altered LDL (E-LDL) shares major properties in common with lipoproteins that have been isolated from atherosclerotic lesions. It activates complement via the alternative pathway and is recognized by a scavenger receptor on human macrophages, thus inducing foam cell formation. Uptake of E-LDL is accompanied by potent induction of MCP-1 synthesis and secretion. In contrast, E-LDL does not stimulate IL-1 or TNF-production and is only a weak inducer of IL-6. Monoclonal antibodies were produced that recognize neoepitopes on E-LDL, but that do not react with native or oxidized LDL. With the use of these antibodies, extensive deposition of E-LDL in very early atherosclerotic lesions was demonstrated. Activated complement components colocalized with E-LDL, corroborating the concept that subendothelially deposited LDL is enzymatically transformed to a complement activator at the earliest stages in lesion development. The pathogenetic relevance of unhalted complement activation in atherogenesis was demonstrated with the use of C6-deficient rabbits. It was found that C6-deficiency markedly protected against development of diet-induced atherosclerosis in the experimental animals. In sum, our hypothesis departs from the mainstream of atherosclerosis research and derives from the recognition that extracellular exposition of free cholesterol in LDL-particles by itself confers pro-inflammatory properties onto the lipoprotein molecule. We believe that the degrading enzymes are ubiquitously present in the extracellular matrix, so the only requirement for atherogenesis to occur is the deposition of large amounts of LDL. Oxidative processes or infections probably play only minor roles, and reduction of LDL plasma levels will predictably represent the single most important prophylactic measure against development and progression of atherosclerosis.

Immunohistochemical demonstration of enzymatically modified human LDL and its colocalization with the terminal complement complex in the early atherosclerotic lesion

Treatment of low density lipoprotein (LDL) with degrading enzymes transforms the molecule to a moiety that is micromorphologically indistinguishable from lipoproteinaceous particles that are present in atherosclerotic plaques, and enzymatically modified LDL (E-LDL), but not oxidized LDL (ox-LDL), spontaneously activates the alternative complement pathway, as do lesion lipoprotein derivatives. Furthermore, because E-LDL is a potent inducer of macrophage foam cell formation, we propose that enzymatic degradation may be the key process that renders LDL atherogenic. In this article, we report the production of two murine monoclonal antibodies recognizing cryptic epitopes in human apolipoprotein B that become exposed after enzymatic attack on LDL. One antibody reacted with LDL after single treatment with trypsin, whereas recognition by the second antibody required combined treatment of LDL with trypsin and cholesterol esterase. In ELISAs, both antibodies reacted with E-LDL produced in vitro and with lesion complement activator derived from human atherosclerotic plaques, but they were unreactive with native LDL or ox-LDL. The antibodies stained E-LDL, but not native LDL or ox-LDL, that had been artificially injected into arterial vessel walls. With the use of these antibodies, we have demonstrated that early human atherosclerotic coronary lesions obtained at autopsy as well as lesions examined in freshly explanted hearts always contain extensive extracellular deposits of E-LDL. Terminal complement complexes, detected with a monoclonal antibody specific for a C5b-9 neoepitope, colocalized with E-LDL within the intima, which is compatible with the proposal that subendothelially deposited LDL is enzymatically transformed to a complement activator at the earliest stages in lesion development.

Modified low density lipoprotein enhances the secretion of bile salt-stimulated cholesterol esterase by human monocyte-macrophages. species-specific difference in macrophage cholesteryl ester hydrolase

Reverse transcriptase-polymerase chain reaction was used to study the biosynthesis of two different cholesteryl ester hydrolases by human and mouse macrophages. Oligonucleotide primers for bile salt-stimulated cholesterol esterase yielded positive reactions with RNA isolated from human peripheral blood monocytes, monocyte-derived macrophages, the human monocytic THP-1 cells, and phorbol ester-induced THP-1 macrophages. In contrast, oligonucleotide primers for hormone-sensitive lipase yielded positive reactions only with RNA isolated from non-differentiated human THP-1 monocytic cells and peripheral blood monocytes, but not those obtained from differentiated THP-1 macrophages or monocyte-derived macrophages. Thus, while human monocytes were capable of synthesizing both enzymes, human macrophages synthesized only bile salt-stimulated cholesterol esterase and not the hormone-sensitive lipase. The synthesis of bile salt-stimulated cholesterol esterase by human macrophages was confirmed by detection of bile salt-stimulated cholesteryl ester hydrolytic activity in conditioned media of differentiated THP-1 cells and human peripheral blood monocyte-derived macrophages. Moreover, incubating human macrophages with oxidized low density lipoprotein (LDL) or acetylated LDL increased bile salt-stimulated cholesterol esterase activity in the conditioned media of these cells. These results with human macrophages were contrasted with results of studies with mouse macrophages, which showed the presence of hormone-sensitive lipase mRNA but not the bile salt-stimulated cholesterol esterase mRNA. Taken together, these results demonstrated species-specific differences in expression of cholesteryl ester hydrolytic enzymes in macrophages. The expression of bile salt-stimulated cholesterol esterase by human macrophages, in a process inducible by modified LDL, suggests a role of this protein in atherogenesis.

Human arterial proteoglycans increase the rate of proteolytic fusion of low density lipoprotein particles

Low density lipoprotein (LDL) particles can undergo fusion in the arterial intima, where they are bound to proteoglycans. Here we studied the effect of human arterial proteoglycans on proteolytic fusion of LDL in vitro. For this purpose, an assay was devised based on fluorescence resonance energy transfer that allowed continuous monitoring of fusion of proteoglycan-bound LDL particles. We found that addition of human arterial proteoglycans markedly increased the rate of proteolytic fusion of LDL. The glycosaminoglycans isolated from the proteoglycans also increased the rate of fusion, demonstrating that this effect was produced by the negatively charged sulfated polysaccharides in the proteoglycans. Furthermore, heparin, chondroitin 6-sulfate, and dextran sulfate, three commercially available sulfated polysaccharides, also increased the rate of LDL fusion, with heparin and chondroitin 6-sulfate being as effective as and dextran sulfate more effective than human proteoglycans. The ability of the sulfated polysaccharides to increase the rate of proteolytic fusion of LDL depended critically on their ability to form insoluble complexes with LDL, which, in turn, resulted in an increased rate of LDL proteolysis and, in consequence, in an increased rate of LDL fusion. The results reveal a novel mechanism regulating LDL fusion and point to the potentially important role of arterial proteoglycans in the generation of LDL-derived lipid droplets in the arterial intima during atherogenesis.

A novel technique for mapping the lipid composition of atherosclerotic fatty streaks by en face fluorescence microscopy

We introduce here a new fluorescence microscopy technique for en face analysis of the atherosclerotic fatty streaks (FS). This technique is semiquantitative and has the sensitivity and resolution to map lipids to individual cells in FS less than 100 microns in diameter. New Zealand White rabbits were fed an atherogenic diet for up to 26 weeks. Aortas were fixed in formalin and stained en bloc with the fluorescent dyes Nile red and filipin. Fluorescent staining was validated by correlating microfluorimetric and biochemical measurements of the lipid content in FS. To determine the cell types associated with the different staining patterns, FS were also evaluated by transmission electron microscopy (TEM) and immunohistochemistry (IH). Correlation of microfluorimetry, TEM, IH, and biochemical data indicated that regions rich in non-esterified cholesterol stained with filipin and fluoresced blue owing to accumulations of lipid vesicles and/or cholesterol crystals. Regions rich in neutral and polar lipids stained with Nile red and fluoresced yellow or orange, respectively, owing to accumulations of lipids in both macrophages and smooth muscle cells (SMC). Digital overlays of the filipin and Nile red images revealed that larger lesions (> 0.5 mm diameter) had a "nested" distribution of lipids, with a blue (filipin) fringe surrounding an orange (Nile red) fringe surrounding a yellow (Nile red) center.

On the pathogenesis of atherosclerosis: enzymatic transformation of human low density lipoprotein to an atherogenic moiety

Combined treatment with trypsin, cholesterol esterase, and neuraminidase transforms LDL, but not HDL or VLDL, to particles with properties akin to those of lipid extracted from atherosclerotic lesions. Single or double enzyme modifications, or treatment with phospholipase C, or simple vortexing are ineffective. Triple enzyme treatment disrupts the ordered and uniform structure of LDL particles, and gives rise to the formation of inhomogeneous lipid droplets 10-200 nm in diameter with a pronounced net negative charge, but lacking significant amounts of oxidized lipid. Enzymatically modified LDL (E-LDL), but not oxidatively modified LDL (ox-LDL), is endowed with potent complement-activating capacity. As previously found for lipid isolated from atherosclerotic lesions, complement activation occurs to completion via the alternative pathway and is independent of antibody. E-LDL is rapidly taken up by human macrophages to an extent exceeding the uptake of acetylated LDL (ac-LDL) or oxidatively modified LDL. After 16 h, cholesteryl oleate ester formation induced by E-LDL (50 micrograms/ml cholesterol) was in the range of 6-10 nmol/mg protein compared with 3-6 nmol/mg induced by an equivalent amount of acetylated LDL. At this concentration, E-LDL was essentially devoid of direct cytotoxic effects. Competition experiments indicated that uptake of E-LDL was mediated in part by ox-LDL receptor(s). Thus, approximately 90% of 125I-ox-LDL degradation was inhibited by a 2-fold excess of unlabeled E-LDL. Uptake of 125I-LDL was not inhibited by E-LDL. We hypothesize that extracellular enzymatic modification may represent an important step linking subendothelial deposition of LDL to the initiation of atherosclerosis.

Immunohistochemical detection of macrophage-derived foam cells and macrophage colony-stimulating factor in pulmonary atherogenesis of cholesterol-fed rabbits

In order to investigate the role of monocyte/macrophages and their relationship to the expression of macrophage colony-stimulating factor (MCSF) in pulmonary atherosclerosis, lungs were excised from rabbits that had been fed for 60 and 90 days on a diet containing 0.5% cholesterol. In the lungs, fatty streaks and elevated foam cell lesions predominated in the large or medium-sized elastic pulmonary arteries, while massive accumulation of foam cells in the intima of muscular arteries produced marked luminal narrowing and nearly complete occlusion. In these lesions, most of the foam cells were reactive with RbM2, a monoclonal antibody (mAb) against rabbit macrophages, while smooth muscle cell-derived foam cells were detected by mAb against smooth muscle actin in the deeper area of elevated foam cell lesions of elastic arteries. Ultrastructural observation confirmed the presence of monocytes in the intima, their differentiation into macrophages, and their transformation into foam cells in the atherosclerotic lesions. Immunohistochemical expression of MCSF was demonstrated in the endothelial cells, smooth muscle cells and foam cells. A minor macrophage-derived foam cell population was demonstrated to possess a proliferative capacity. These data suggest that MCSF is involved in the differentiation of monocytes into macrophages, their transformation into foam cells, and their proliferation during pulmonary atherogenesis.

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.

The pathogenesis of atherosclerosis: a perspective for the 1990s

Atherosclerosis, the principal cause of heart attack, stroke and gangrene of the extremities, is responsible for 50% of all mortality in the USA, Europe and Japan. The lesions result from an excessive, inflammatory-fibroproliferative response to various forms of insult to the endothelium and smooth muscle of the artery wall. A large number of growth factors, cytokines and vasoregulatory molecules participate in this process. Our ability to control the expression of genes encoding these molecules and to target specific cell types provides opportunities to develop new diagnostic and therapeutic agents to induce the regression of the lesions and, possibly, to prevent their formation.

Monoclonal antibodies against LDL further enhance macrophage uptake of LDL aggregates

Self-aggregates of low density lipoprotein (LDL) are taken up and degraded more rapidly by macrophages than is native LDL. That enhanced uptake is attributable in part to phagocytosis via the LDL receptor pathway. However, arterial macrophages appear to express little LDL receptor activity. The present studies demonstrate an alternative mechanism by which LDL aggregates could contribute to foam cell formation. This could occur by the formation of large immune complexes that are taken up by macrophages via the Fc receptor. When immune complexes were formed with native, soluble LDL and MB47, a monoclonal antibody specific to the apoprotein B domain recognized by the LDL receptor, the subsequent uptake and degradation of the LDL by macrophages were inhibited 50-80% compared with native LDL alone. In contrast, when aggregated LDL was bound to MB47 at a similar molar ratio, the subsequent degradation of the insoluble immune complexes was two- to fivefold greater than that of aggregated LDL alone. The enhanced uptake was abolished when Fab or F(ab')2 fragments of MB47 were substituted for the intact antibody, indicating that the increased uptake was via the Fc receptor pathway. Furthermore, the uptake of the immune complexes of aggregated LDL was reduced by competition for the Fc receptor with heat-aggregated immunoglobulin. There was also an increase in the rate of cellular cholesterol esterification and an increase in macrophage cholesteryl ester mass. Since aggregates of LDL as well as autoantibodies against modified LDL have been demonstrated in atherosclerotic lesions, it is possible that immune complexes of aggregates of modified LDL may be generated in the intima.(ABSTRACT TRUNCATED AT 250 WORDS)

Liposome interactions with hydroxyapatite crystals: a possible mechanism in the calcification of atherosclerotic plaques

Some stages in the calcification of atherosclerotic plaques may involve associations between lipids and hydroxyapatite (HA) by surface interactions. Liposomes, artificial membranous lipid vesicles, have been used in this study as model structures for biological calcification processes. Liposome (containing cholesterol and phosphatidylcholine in most cases) suspensions were prepared by sonication, after which HA seed crystals were added to the suspensions and stirred at 37 degrees C. Aliquots of the liposome suspensions were analyzed for particle size distribution and by transmission electron microscopy and electron diffraction. The results showed that HA induced aggregation of liposomes and modifications of the microscopic shapes of the liposomes in the aggregates. These data can be explained by the electron diffraction pattern where superimposition of liposome reflection and crystal reflection exists and may suggest organic-inorganic interaction. The potential of HA crystals to induce formation of liposome aggregates may be seen as a step in atherosclerotic plaques calcification.

Ultrastructure appearance of atherosclerosis in human and experimentally-induced animal models

We describe here the basic structure of the aorta, the changes with aging and ultrastructural appearance of atherosclerosis of human and animal models. The architecture of the aortic wall is highly organized, for adaptation to changes of blood pressure. The main cells composing the vessel are endothelial cells and smooth muscle cells. They maintain the integrity and homeostasis of the aorta along with the extracellular matrix of collagen fibrils, elastic fibers and glycosaminoglycans. The structural changes with aging and atherogenesis are a compensative or degenerative phenomenon caused by many factors. Three major cells are the endothelial cell, smooth muscle cell and monocyte-derived macrophages (as well as platelets) all of which are involved in atherogenesis. Foam cells in atheromatous lesions are derived from macrophages and smooth muscle cells. Recently, the molecular biological nature and function of these cells and their derived-factors have been thoroughly investigated in cell culture and in experimental animal models caused by a mechanical injury of the endothelium or by a dietary induced hypercholesterolemia. However, the mechanism of the endothelial injury in vivo as well as formation of atheromatous cores of human atherosclerosis is not exactly understood. Some structural and functional changes inherent to the arterial wall during aging may play an important role in initiation or progression of human atherosclerosis.

Lipid accumulation in rabbit aortic intima 2 hours after bolus infusion of low density lipoprotein. A deep-etch and immunolocalization study of ultrarapidly frozen tissue

The intima from aortas of normal New Zealand White rabbits was studied 2 hours after infusion of 320 mg human low density lipoprotein (LDL), resulting in a plasma concentration of at least five times and maximally 20 times the values found in normal rabbit serum. The following techniques were used: 1) ultrarapid freezing without chemical fixation, followed by freeze-etching; 2) immunofluorescence microscopy; and 3) postembedding immunogold-labeling electron microscopy. In the latter two methods MB47, a murine monoclonal antibody against human apolipoprotein B, was used as the primary antibody. Control rabbits were infused with the same volume of buffer only. Rotary-shadowed replicas of samples from the LDL-injected rabbits showed the deposition of lipidlike particles in the subendothelial-intimal space that were the size of the injected LDL (23 nm). In focal areas of the intima, groups of 23-nm-sized lipidlike particles and larger lipidlike structures were found enmeshed in the extracellular matrix. Control replicas were essentially free of lipid deposition. Immunofluorescence microscopy of frozen aortic cross sections showed an overall increase in apolipoprotein B in the intima of the LDL-injected rabbits. The presence of apolipoprotein B in the intima was also confirmed by immunoelectron microscopy. These in vivo results show that clustering of LDL-sized particles occurs in the intima within 2 hours of excessive LDL uptake. It also demonstrates the interaction of these LDL-sized particles with the filaments of the extracellular matrix. The clustering of the LDL-sized particles supports the possibility that LDL self-aggregation may occur in vivo and that components of the extracellular matrix are involved in this process.

Isolation and characterization of a complement-activating lipid extracted from human atherosclerotic lesions

The major characteristics of human atherosclerotic lesions are similar to those of a chronic inflammatory reaction, namely fibrosis, mesenchymal cell proliferation, the presence of resident macrophages, and cell necrosis. Atherosclerosis exhibits in addition the feature of lipid (mainly cholesterol) accumulation. The results of the present report demonstrate that a specific cholesterol-containing lipid particle present in human atherosclerotic lesions activates the complement system to completion. Thus, lipid could represent a stimulatory factor for the inflammatory reaction, whose underlying mechanistic basis may be, at least in part, complement activation. The complement-activating lipid was purified from saline extracts of aortic atherosclerotic lesions by sucrose density gradient centrifugation followed by molecular sieve chromatography on Sepharose 2B. It contained little protein other than albumin, was 100-500 nm in size, exhibited an unesterified to total cholesterol ratio of 0.58 and an unesterified cholesterol to phospholipid ratio of 1.2. The lipid, termed lesion lipid complement (LCA), activated the alternative pathway of complement in a dose-dependent manner. Lesion-extracted low density lipoprotein (LDL) obtained during the purification procedure failed to activate complement. Specific generation of C3a desArg and C5b-9 by LCA indicated C3/C5 convertase formation with activation proceeding to completion. Biochemical and electron microscopic evaluations revealed that much of the C5b-9 present in atherosclerotic lesions is membraneous, rather than fluid phase SC5b-9. The observations reported herein establish a link between lipid insudation and inflammation in atherosclerotic lesions via the mechanism of complement activation.

Atherogenesis during low level hypercholesterolemia in the nonhuman primate. I. Fatty streak formation

Although a large body of data is available concerning atherogenesis in animals maintained at high levels of hypercholesterolemia, little data are available for animals maintained at lower levels of hypercholesterolemia for longer periods of time, closer to those observed in humans. The chronologic sequence of cellular events and interactions that occur during the formation of the lesions of atherosclerosis was investigated during relatively low level hypercholesterolemia (200 to 400 mg/dl) in a series of nonhuman primates (Macaca nemestrina). The arterial tree of each animal was examined by light microscopy and scanning and transmission electron microscopy. Immunohistochemical staining with monoclonal antibodies specific for smooth muscle cells, monocyte-macrophages, and T-lymphocytes was performed to analyze the cellular composition of the lesions. After 6 months of low level hypercholesterolemia, the surface of the aorta contained large numbers of adherent leukocytes, many of which were in the process of entering the artery. This resulted in irregularly shaped nodular elevations, or fatty streaks, preferentially located at branch sites and bifurcations. The fatty streaks consisted of intimal accumulations of numerous lipid-laden macrophages together with relatively small numbers of T-lymphocytes. With lesion progression, the thickness of the fatty streaks increased, and their surfaces became irregular and frequently showed disruptions of covering endothelial cells resulting in exposure of subendothelial macrophages. Platelet microthrombi were observed over some of the exposed macrophages at some branches or bifurcations in every animal studied. These observations made during the early phases of atherosclerosis lesion formation are virtually identical to those described in our previous reports in high level hypercholesterolemic nonhuman primates (600 to 1000 mg/dl) with the exception that the changes occurred more slowly in the lower levels of hypercholesterolemia.

Cytochemical localization of beta-lipoproteins and their components in successive stages of hyperlipidemic atherogenesis of rabbit aorta

We have investigated cytochemically the correlative distribution of lipoprotein (LP) components in successive lesional stages of plaque formation in hyperlipoproteinemic atherogenesis in the rabbit. Apoprotein B (apo B) was detected by an immunoperoxidase procedure, unesterified cholesterol (UC) by filipin and tomatine and phospholipid lamellae of the extracellular liposomes (EL) as they appear in standard EM. The changes were evaluated in relation to the state of endothelial cells and their transport pathways, and the reaction of the cellular and extracellular components of the intima. Each lesional stage has a relatively characteristic pattern distribution of the LP components. In fatty streaks with no endothelial denudation, apo B reaction product occurs mostly in non-particulate form associated with UC-rich EL; this suggests that transcytosed LP upon partial degradation and interaction with the extracellular components, reassemble as polymorphic EL-UC-apo B complexes. Serofibrinous insudates, although commonly devoid of EL and apo B, may contain UC presumably transported by a carrier other than LP. In advanced fibrolipidic lesions with open endothelial junctions and deendothelialized areas, a bulky intramural insudation of plasma results in the presence of large amounts of apparently little modified LP. This may represent what several investigators have isolated as 'aortic LP', which may be insudated rather than transported plasma LP.

Transport pathways of beta-VLDL by aortic endothelium of normal and hypercholesterolemic rabbits

The uptake and transport of beta-VLDL by the aortic endothelium was investigated in normal and hyperlipidemic rabbits fed a cholesterol-enriched diet for 1 week to 5 months. Weekly (in the first month) or every other week afterwards, animals were given one of the following probes: (a) [125I]-beta-VLDL injected in vivo and after 24 h the whole aorta or its intima and media were separately collected and examined by spectrometry and autoradiography; (b) [125I]-beta-VLDL coupled to the fluorescent probe 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate perfused in situ for 1-2 h and aorta examined by radioassay and fluorescence microscopy; (c) beta-VLDL-gold complex perfused in situ for 10-15 min and aortic fragments examined by electron microscopy. In addition, cryosections of aortic wall were processed for the immunocytochemical detection of apolipoprotein B and apolipoprotein E. The results showed that both in normal and hyperlipidemic rabbits, the aortic endothelium transports plasma beta-VLDL by a dual pathway: (i) endocytosis involving coated pits and vesicles, endosomes, multivesicular bodies and lysosomes, and (ii) transcytosis, the predominant process, carried out by plasmalemmal vesicles. Both processes, and especially transcytosis, are markedly increased in hyperlipidemia leading to progressive accumulation of beta-VLDL or/and its components in the subendothelial extracellular matrix. In prelesional stages of atherogenesis, beta-VLDL-gold complexes or deposits of apo B and apo E were detected in close association with extracellular liposomes. With the appearance of intimal macrophage-derived foam cells, the immunoperoxidase reaction product, revealing the presence of the two apolipoproteins, could also be seen in intracellular lipid inclusions.

Development of intracellular lipid deposits in the lipid-laden cells of atherosclerotic lesions. A cytochemical and ultrastructural study

In atherosclerotic lesions of rabbits fed a cholesterol-rich diet, the lipid deposits of foam cells derived from monocytes, smooth muscle and endothelial cells were studied by physical, cytochemical and ultrastructural methods. Beginning with the third week of diet, the lipid material that could be visualized at the light microscope level by Oil red O and Nile red staining was progressively accumulated in the intimal cells of the atherosclerotic lesions. In the early stages of foam cell formation, the deposits occurred especially as intracytoplasmic non-membrane bound lipid inclusions (lipid droplets). In polarizing microscopy these appeared as a mixture of iso-, and anisotropic material. The latter were birefringent and showed an axial symmetry with a black cross image, suggesting that the lipids were in a liquid crystalline state. In chemically-fixed specimens, the content of lipid inclusions was preserved in various degrees. In freeze-fractured preparations they displayed a layered onion-like arrangement with smooth cleavage faces surrounding an amorphous core. Upon incubation with filipin, that specifically binds to 3 beta-hydroxysterols, the peripheral layers of the inclusions were labeled, revealing the existence of unesterified cholesterol. In the advanced stages of foam cell formation, lipids were additionally accumulated in the lysosomal compartment as polymorphic multilamellar structures concentrically arranged, with cleavage faces devoid of intralamellar particles. The presence of acid phosphatase showed that these features were modified lysosomes and were tentatively named lysosomal lipid bodies. In the latest stages examined cholesterol crystals developed within lysosomal lipid bodies usually enclosed in multilamellar structures. This lipid coat may represent the place of crystal formation and presumably acts as barrier for the turnover of the crystalline cholesterol, thus impeding plaque regression.