Interaction of low density lipoproteins with arterial proteoglycans. The role of charge and sialic acid content

Toward reliable low-density lipoprotein ultrastructure prediction in clinical conditions: A small-angle X-ray scattering study on individuals with normal and high triglyceride serum levels

Atherosclerosis is the main killer in the west and therefore a major health challenge today. Total serum cholesterol and lipoprotein concentrations, used as clinical markers, fail to predict the majority of cases, especially between the risk scale extremes, due to the high complexity in lipoprotein structure and composition. In particular, low-density lipoprotein (LDL) plays a key role in atherosclerosis development, with LDL size being a parameter considered for determining the risk for cardiovascular diseases. Determining LDL size and structural parameters is challenging to address experimentally under physiological-like conditions. This article describes the biochemistry and ultrastructure of normolipidemic and hypertriglyceridemic LDL fractions and subfractions using small-angle X-ray scattering. Our results conclude that LDL particles of hypertriglyceridemic compared to healthy individuals 1) have lower LDL core melting temperature, 2) have lower cholesteryl ester ordering in their core, 3) are smaller, rounder and more spherical below melting temperature, and 4) their protein-containing shell is thinner above melting temperature.

Bread enriched with phytosterols with or without curcumin modulates lipoprotein profiles in hypercholesterolaemic individuals. A randomised controlled trial

A novel functional food containing phytosterols and curcumin significantly lowers blood cholesterol concentrations in hypercholesterolaemic individuals.

Effects of altered thyroid states on oxidative stress parameters in rats

BACKGROUND

Thyroid hormones are effective on oxidant-antioxidant balance by leading basal metabolic rate. In this study, the effects of altered thyroid states on low density lipoprotein (LDL) oxidation and oxidative stress parameters were investigated in an experimental animal model.

METHODS

Thirty female Wistar Albino rats were equally divided into 3 groups as follows: control group; hypothyroid group (methimazole (75 mg/100 g was added to diet); hyperthyroid group [L-thyroxine (0.4 mg/100 g was added to diet)]. Oxidized LDL (ox-LDL) levels, thyroid, and lipid parameters were determined in serum. Also lipid peroxidation (LPO), sialic acid (SA) and glutathione levels (GSH), as well as superoxide dismutase (SOD) and catalase (CAT) activities were determined in tissue samples.

RESULTS

A significant increase in lipid parameters was observed in hypothyroid group, whereas these parameters were decreased in hyperthyroid group compared to control group. For ox-LDL levels, a significant increase was observed both in hypothyroid and hyperthyroid groups. In brain, liver and kidney tissues, LPO and SA levels were increased, whereas GSH levels were decreased both in hypothyroid and hyperthyroid groups. The SOD and CAT activities were significantly decreased in hypothyroid group, however, they were increased in hyperthyroid group compared to control group. Both hypothyroid and hyperthyroid conditions modify the oxidant-antioxidant state in serum and tissues.

CONCLUSIONS

Increased SOD and CAT activities in hyperthyroid group suggest that elevated thyroid hormones can reduce oxidative stress by maintaining antioxidant defense and they might have a protective effect on some tissues against oxidants.

Cellular mechanisms of human atherosclerosis: Role of cell-to-cell communications in subendothelial cell functions

The present study was undertaken in order to extend of our earlier work, focusing on the analysis of roles of cell-to-cell communications in the regulation of the subendothelial cell function. In present study, we have found that the expression of connexin43 (Cx43) is dramatically reduced in human atherosclerotic lesions, compared with undiseased intima. In atherosclerotic lesions, the number of so-called 'connexin plaques' was found to be lower in lipid-laden cells than in cells which were free from lipid inclusions. In primary cell culture, subendothelial intimal cells tended to create multicellular structures in the form of clusters. Cluster creation was accompanied by the formation of gap junctions between cells; the degree of gap junctional communication correlated with the density of cells in culture. We found that atherosclerosis-related processes such as DNA synthesis, protein synthesis and accumulation of intracellular cholesterol correlated with the degree of cell-to-cell communication. The relation of DNA and protein synthesis with cell-to-cell communication could be described as "bell-shaped". We further incubated cells, cultured from undiseased subendothelial intima, with various forms of modified LDL causing intracellular cholesterol accumulation. After the incubation of intimal cells with modified LDL, intercellular communication has "dropped" considerably. The findings indicate that intracellular lipid accumulation might be a reason for a decrease of the number of gap junctions. The findings also suggest that the disintegration of cellular network is associated with foam cell formation, the process known as a key event of atherogenesis.

Flaxseed oil intake reduces serum small dense low-density lipoprotein concentrations in Japanese men: a randomized, double blind, crossover study

BACKGROUND

The effects of alpha-linolenic acid (ALA) on cardiovascular risk factors considerably vary between published reports. Therefore, we investigated the effects of 12-week supplementation with flaxseed oil (FO), which is a rich source of ALA, on cardiovascular risk factors such as serum small dense low-density lipoprotein (sd-LDL) concentrations.

METHODS

In a randomized, double blind, crossover study, 15 subjects ingested 10 g of FO or corn oil (CO), containing 5.49 g and 0.09 g of ALA, respectively, once daily with dinner. Blood samples were collected at 0, 4 and 12 weeks, and were used for analysis of serum lipid, lipid-related proteins, serum fatty acids and serum sd-LDL cholesterol. Differences during the test period were identified using a repeated-measures analysis of variance (ANOVA) for within-group effects. Group differences were identified using paired t-test at each blood sampling time point.

RESULTS

ALA and eicosapentaenoic acid concentrations were significantly higher in the FO period at 4 and 12 weeks than in the CO period. No significant differences in docosahexaenoic acid concentrations were observed between two periods, and cholesteryl ester transfer protein and apolipoprotein B concentrations were significantly lower in the FO period than in the CO period at 12 weeks. FO supplementation was associated with a significant decrease in sd-LDL concentrations at 4 and 12 weeks, and CO supplementation had no effect. Moreover, sd-LDL concentrations were significantly lower in the FO period than in the CO period at 4 weeks. Among subjects with triglyceride (TG) concentrations of >100 mg/dl, FO supplementation markedly reduced sd-LDL concentrations at 4 and 12 weeks compared with baseline. Sd-LDL concentrations significantly differed between the periods at both 4 and 12 weeks.

CONCLUSION

This study indicates that the FO, which is a rich source of ALA, leads to lower sd-LDL cholesterol concentrations.

Relationship between carotid artery intima-media thickness and small dense low-density lipoprotein cholesterol concentrations measured by homogenous assay in Japanese subjects

BACKGROUND

Small dense low-density lipoprotein cholesterol (sdLDL-C) concentrations correlate more strongly with coronary heart disease than other LDL-C and large LDL particle concentrations. We investigated the association between carotid artery intima-media thickness (IMT) and sdLDL-C concentrations in Japanese subjects.

METHODS

Carotid artery IMT, blood pressure (BP), fasting plasma sdLDL-C, glucose metabolism, lipid, and C-reactive protein concentrations were measured in 97 native Japanese subjects. Carotid artery IMT was assessed by ultrasonography, and sdLDL-C concentrations were measured by a homogenous assay. Pearson's correlation coefficient analyses and multiple regression analyses were used to examine the relationships between carotid artery IMT values, sdLDL-C values, and other clinical variables.

RESULTS

After multiple regression analysis, including age, sex, body mass index, systolic BP, diastolic BP, fasting plasma glucose, HbA1c, estimated glomerular filtration rate (eGFR), total-C, high-density lipoprotein (HDL)-C, triglyceride, LDL-C, non-HDL-C, large buoyant LDL-C, and sdLDL-C, carotid artery IMT remained significantly associated with age, systolic BP, diastolic BP, and sdLDL-C, whereas sdLDL-C remained significantly associated with age, total-C, HDL-C, triglycerides, and carotid artery IMT.

CONCLUSIONS

When measured by a homogenous assay, carotid artery IMT may have a closer relationship with sdLDL-C concentrations than other lipid parameters in Japanese subjects. sdLDL-C may be a potentially useful risk marker when assessing carotid artery IMT in Japanese subjects.

Effects of chitosan on plasma lipids and lipoproteins: a 4-month prospective pilot study

Chitosan can favorably modulate plasma lipids, but the available data are not conclusive. We evaluated the effect of chitosan on plasma lipids and lipoproteins in 28 patients with plasma triglyceride levels >150 mg/dL (mean age: 63 ± 12 years), not taking other lipid-lowering agents. All patients received a chitosan derived from fungal mycelium (Xantonet, Bromatech, Italy) at a fixed dose of 125 mg/d in addition to their current medications for 4 months. Polyacrylamide gel electrophoresis was used to measure low-density lipoprotein (LDL) subclasses. After treatment, total cholesterol reduced by 8%, LDL cholesterol by 2%, and triglycerides by 19%, with a concomitant 14% increase in high-density lipoprotein cholesterol. We also found a beneficial effect of chitosan on LDL subclasses, with a significant increase in LDL-2 particles (from 37 ± 8% to 47 ± 8%, P = .0001) and a decrease (although not significant) in atherogenic small, dense LDL. Whether these findings may affect cardiovascular risk remains to be established in future studies.

The interaction of plasma sialylated and desialylated lipoproteins with collagen from the intima and media of uninvolved and atherosclerotic human aorta

We have evaluated the binding of sialylated and desialylated lipoproteins to collagen isolated from the proteoglycan and musculoelastic layers of intima and media of uninvolved human aorta and atherosclerotic lesions. Comparing various collagen preparations from the uninvolved intima-media, the binding of sialylated apoB-containing lipoproteins was best to collagen from the intimal PG-rich layer. Binding of sialylated apoB-containing lipoproteins to collagen from this layer of fatty streak and fibroatheroma was 1.4- and 3.1-fold lower, respectively, in comparison with normal intima. Desialylated VLDL versus sialylated one exhibited a greater binding (1.4- to 3.0-fold) to all the collagen preparations examined. Desialylated IDL and LDL showed a higher binding than sialylated ones when collagen from the intimal layers of fibroatheroma was used. Binding of desialylated HDL to collagen from the intimal PG-rich layer of normal tissue, initial lesion, and fatty streak was 1.2- to 2.0-fold higher compared with sialylated HDL.

Serum amyloid A facilitates the binding of high-density lipoprotein from mice injected with lipopolysaccharide to vascular proteoglycans

OBJECTIVE

Levels of serum amyloid A (SAA), an acute-phase protein carried on high-density lipoprotein (HDL), increase in inflammatory states and are associated with increased risk of cardiovascular disease. HDL colocalizes with vascular proteoglycans in atherosclerotic lesions. However, its major apolipoprotein, apolipoprotein A-I, has no proteoglycan-binding domains. Therefore, we investigated whether SAA, which has proteoglycan-binding domains, plays a role in HDL retention by proteoglycans.

METHODS AND RESULTS

HDL from control mice and mice deficient in both SAA1.1 and SAA2.1 (SAA knockout mice) injected with bacterial lipopolysaccharide (LPS) was studied. SAA mRNA expression in the liver and plasma levels of SAA increased dramatically in C57BL/6 mice after LPS administration, although HDL cholesterol did not change. Fast protein liquid chromatography analysis showed most of the SAA to be in HDL. Mass spectrometric analysis indicated that HDL from LPS-injected control mice had high levels of SAA1.1/2.1 and reduced levels of apolipoprotein A-I. HDL from LPS-injected control mice demonstrated high-affinity binding to biglycan relative to normal mouse HDL. In contrast, HDL from LPS-injected SAA knockout mice showed very little binding to biglycan, consistent with SAA facilitating the binding of HDL to vascular proteoglycans.

CONCLUSION

SAA enrichment of HDL under inflammatory conditions plays an important role in the binding of HDL to vascular proteoglycans.

Should we measure routinely oxidised and atherogenic dense low-density lipoproteins in subjects with type 2 diabetes?

Beyond low-density lipoprotein (LDL)-cholesterol concentrations, in recent years, several clinical studies have shown that both oxidised and small, dense LDL have a strong predictive role for the presence of vascular atherosclerosis. These two lipid parameters seem to have a synergistic impact on cardiovascular risk, with a greater importance in patients at higher-risk, such as those with type-2 diabetes. Increased levels of oxidised and small, dense LDL levels are a feature of diabetic dyslipidaemia, and small, dense LDL have been shown to be a good predictor of future cardiovascular events, at both univariate and multivariate analyses. On the other hand, although the association of oxidised LDL with surrogate markers of atherosclerosis is consistent, the correlation with hard clinical end points seems to be smaller. Yet, measurement of these two lipid parameters has not been widely used in daily practice because of the limited availability of clinical data and methodological problems: lack of availability of easy, cheap and reproducible essays for measurement of oxidised and, particularly, small, dense LDL has reduced their assessment in large clinical end-points trials. However, on the basis of available data, the therapeutic modulation of small, dense LDL is significantly associated with reduced cardiovascular risk, even after adjustment for confounding factors. In conclusion, the routine measurement of oxidised and small, dense LDL in patients with type-2 diabetes cannot be recommended in daily clinical practice so far; yet, their measurement is strongly encouraged to better understand their role on the cardiovascular risk of patients with type-2 diabetes.

Lipoproteins and lipoprotein metabolism in periodontal disease

A growing body of evidence indicates that the incidence of atherosclerosis is increased in subjects with periodontitis - a chronic infection of the oral cavity. This article summarizes the evidence that suggests periodontitis shifts the lipoprotein profile to be more proatherogenic. LDL-C is elevated in periodontitis and most studies indicate that triglyceride levels are also increased. By contrast, antiatherogenic HDL tends to be low in periodontitis. Periodontal therapy tends to shift lipoprotein levels to a healthier profile and also reduces subclinical indices of atherosclerosis. In summary, periodontal disease alters lipoprotein metabolism in ways that could promote atherosclerosis and cardiovascular disease.

Quantitative and qualitative effects of rosuvastatin on LDL-cholesterol: what is the clinical significance?

BACKGROUND

Statins have emerged as the global leader in pharmacologic therapy for dyslipidaemia, and rosuvastatin has demonstrated clinical efficacy as well as safety in several clinical trials and postmarketing analyses.

AIM

The present article reviewed the effects of rosuvastatin on the quantity and the quality of low-density lipoproteins (LDL).

METHODS

We searched for and reviewed all the available evidence in a systematic way. A literature search (by Medline and Scopus) was performed using the following headings: 'LDL-cholesterol', 'LDL size', 'LDL subclasses', 'small dense LDL', 'apolipoprotein B, apo B' and 'rosuvastatin' up to 11 November 2008. The authors also manually reviewed the references of selected articles for any pertinent material.

RESULTS

Rosuvastatin reduces LDL-cholesterol levels to a greater extent than other statins and is able to modulate significantly LDL size and subclasses towards less atherogenic particles as well as the LDL particle number, as indirectly measured by the levels of apo B.

DISCUSSION AND CONCLUSIONS

The recent Justification for the Use of statins in Primary prevention: an Intervention Trial Evaluating Rosuvastatin study provides more evidence about the effectiveness of rosuvastatin therapy in reducing cardiovascular risk, even among persons who would not currently be considered for pharmacotherapy. Further insights on cardiovascular outcomes will be available by the on-going trials included in the GALAXY program that includes subjects with type-2 diabetes, haemodialysis recipients, patients with congestive heart failure and specific ethnic groups, such as African American, Hispanic and South Asian populations.

Hyperelongated biglycan: the surreptitious initiator of atherosclerosis

PURPOSE OF REVIEW

To outline a role for the dermatan sulfate proteoglycan biglycan and specifically its growth factor modified form having elongated glycosaminoglycan chains as being a primary initiator of atherosclerosis.

RECENT FINDINGS

Antiatherosclerotic therapies have mostly targeted epidemiologically identified, experimentally confirmed risk factors. The efficacy of such therapies is less than optimal, and rates of cardiovascular disease remain stubbornly high. A variety of targets have been actively pursued, but as yet no new therapy has emerged that specifically targets the vessel wall. One area concerns the role of proteoglycans in the trapping of atherogenic lipoproteins as an early and initiating step in atherogenesis. On the basis of studies in human coronary arteries, the prime proteoglycan for lipoprotein retention is biglycan. The glycosaminoglycan chains on biglycan are subject to regulation that yields several structural changes, but most prominently elongation of the chains to form 'hyperelongated biglycan'. Multiple animal studies and a recent human disorder study have demonstrated the colocalization of atherogenic lipoproteins with biglycan in atherosclerotic lesions. Moreover, in the human atherosclerosis, the deposition of lipid appears to precede the chronic inflammatory response typical of atherosclerotic lesions.

SUMMARY

The process of biglycan-associated glycosaminoglycan elongation represents a novel potential therapeutic target worthy of full investigation for the prevention of atherosclerosis.

Contribution of macromolecular structure to the retention of low-density lipoprotein at arterial branch points

BACKGROUND

Extracellular deposition of low-density lipoprotein (LDL) in the arterial wall is an essential early step in atherosclerosis. This process preferentially occurs at arterial branch points, reflecting a regional variation in lipoprotein-arterial wall interactions. In this study, we characterized the submicron microstructure of arterial wall collagen and elastin to evaluate its potential role in regional LDL deposition.

METHODS AND RESULTS

With 2-photon microscopy, we used the intrinsic optical properties of collagen and elastin to determine the arterial wall macromolecular microstructure in fresh porcine and murine arteries. This optical approach generated unique nondestructive en face 3-dimensional views of the wall. The collagen/elastin microstructure was found to vary with the topology of the arterial bed. A nearly confluent elastin surface layer was present throughout but was missing at atherosclerosis-susceptible branch points, exposing dense collagen-proteoglycan complexes. In LDL binding studies, this luminal elastin layer limited LDL penetration, whereas its absence at the branches resulted in extensive LDL binding. Furthermore, LDL colocalized with proteoglycans with a sigmoidal dose dependence (inflection point, approximately 130 mg LDL/dL). Ionic strength and competing anions studies were consistent with the initial interaction of LDL with proteoglycans to be electrostatic in nature.

CONCLUSIONS

This optical sectioning approach provided a robust 3-dimensional collagen/elastin microstructure of the arterial wall in fresh samples. At atherosclerosis-susceptible vascular branch points, the absence of a luminal elastin barrier and the presence of a dense collagen/proteoglycan matrix contribute to increased retention of LDL.

Who needs to care about small, dense low-density lipoproteins?

BACKGROUND

Increasing evidence suggest that the 'quality' rather than only the 'quantity' of low-density lipoprotein (LDL) exerts a great influence on the cardiovascular risk. Small, dense LDL seem to be an important predictor of cardiovascular events and progression of coronary artery disease (CAD) and their predominance has been accepted as an emerging cardiovascular risk factor by the National Cholesterol Education Program Adult Treatment Panel III.

DISCUSSION

Some studies showed in past years that small, dense LDL are usually elevated in patients at very high cardiovascular risk, such as those with CAD and type 2 diabetes. More recently elevated levels of these particles have been found in other categories of patients at high cardiovascular risk, such as those with non-coronary forms of atherosclerosis (e.g. with carotid artery disease, aortic abdominal aneurysm and peripheral arterial disease) and metabolic diseases (with polycystic ovary syndrome and growth hormone deficiency); notably, in most of them, the predominance of small, dense LDL characterised their type of dyslipidaemia, alone or in combination with elevated triglycerides and reduced high-density lipoproteins cholesterol concentrations.

CONCLUSIONS

The therapeutical modulation of small, dense LDL have been shown to significantly reduce cardiovascular risk and weight reduction and increased physical activity may constitute first-line therapy. In addition, lipid-lowering drugs are able to favourably alter these particles and fibrates and nicotinic acid seem to be the most effective agents. Promising data are also available with the use of rosuvastatin, the latest statin introduced in the market, and ezetimibe, a cholesterol absorption inhibitor.

The clinical significance of the size of low-density-lipoproteins and the modulation of subclasses by fibrates

BACKGROUND

Beyond total low-density-lipoproteins (LDL) levels, increasing evidence suggests that the 'quality' of LDL exerts a great influence on the cardiovascular risk. Several studies have also shown that the therapeutic modulation of LDL size is of benefit in reducing the risk of cardiovascular events. Hypolipidaemic treatment is able to alter LDL subclass distribution but strong variations have been noticed among different agents. Fibrates have a major impact on triglyceride metabolism and in modulating LDL size and subclasses, but variations exist among the different molecules.

METHODOLOGY

A literature search (by Medline and Scopus) was performed using the following headings: 'small dense LDL', 'LDL size', 'LDL subfractions', 'LDL subclasses', 'LDL distribution' and 'fenofibrate', 'bezafibrate', 'ciprofibrate' and 'gemfibrozil' up to 20 January 2007. The authors also manually reviewed the references of selected articles for any pertinent material.

RESULTS

Analysis of all published studies revealed that treatment with fenofibrate, ciprofibrate, bezafibrate and gemfibrozil is usually beneficial, and fenofibrate may be more efficacious than the other molecules. This is supported by using all the available techniques in subjects with a very wide range of lipid alterations.

CONCLUSION

Among the different agents, fenofibrate has been found to be particularly effective in modulating LDL size and subclasses in patients at higher cardiovascular risk, such as those with type 2 diabetes or the metabolic syndrome.

Small, dense low-density-lipoproteins and the metabolic syndrome

Small, dense low-density-lipoproteins (LDL) are associated with increased risk for cardiovascular diseases and diabetes mellitus and a reduction in LDL size has been reported in patients with coronary and non-coronary forms of atherosclerosis. LDL size has been accepted as an important predictor of cardiovascular events and progression of coronary artery disease as well as an emerging cardiovascular risk factor by the National Cholesterol Education Program Adult Treatment Panel III. Small, dense LDL, with elevated triglyceride levels and low HDL-cholesterol concentrations, constitute the 'atherogenic lipoprotein phenotype (ALP)', a form of atherogenic dyslipidemia that is a feature of type 2 diabetes and the metabolic syndrome. LDL size and subclasses show specific alterations in patients with the metabolic syndrome that probably significantly increase their cardiovascular risk; however, so far it has not been recommended to incorporate LDL size measurements in treatment plans, when hypolipidemic therapies are installed. Patients with type 2 diabetes are at high cardiovascular risk and it is still on debate if the treatment goals may be identical or whether there are distinct groups with different cardiovascular risks and hence with different treatment goals. Measurements beyond traditional lipids, such as measurements on the presence of small, dense LDL in patients with the metabolic syndrome, may help to identify cardiovascular risk subgroups. In addition, it might be possible in the future to individualize hypolipidemic treatments if more than the traditional lipids are taken into account. LDL size measurement may potentially help to assess cardiovascular risk within the metabolic syndrome and adapt the treatment goals thereafter.

Characterization of apolipoprotein M isoforms in low-density lipoprotein

Apo M is a recently discovered human lipoprotein thought to be involved in the metabolism of lipids and lipoprotein particles. Here, a proteomic approach was applied to examine the glycosylation pattern of apo M in human LDL. We treated LDL proteins with N-glycosidase or neuraminidase, studied mobility shifts of Apo M by two-dimensional gel electrophoresis, and different isoforms were then identified with mass spectrometry. This way, we demonstrated the presence of five isoforms of apo M in LDL: three that are both N-glycosylated and sialylated, one that is N-glycosylated but not sialylated, and one that is neither N-glycosylated nor sialylated. As judged from the examination of LDL from 20 healthy human subjects, the three N-glycosylated and sialylated forms are most abundant (80-100% of the total apo M in LDL) whereas the unsialylated and unglycosylated variants constitute at most 20%. Comparative analysis showed that the same five isoforms of apo M are also present in HDL. Further studies aiming at elucidating the role of apo M in health and disease will have to take this polymorphism of apo M proteins into account.

Should we measure routinely the LDL peak particle size?

Low density lipoproteins (LDL) do not show in humans a normal distribution and comprise two different main fractions: large, buoyant (phenotype pattern A) and small, dense (phenotype pattern B) particles, that differ not only in size and density but also in physicochemical composition, metabolic behaviour and atherogenicity. The prevalence of small, dense LDL changes with age (30-35% in adult men, 5-10% in men <20 years and in pre-menopausal women, 15-25% in postmenopausal women) and is genetically influenced, with a heritability ranging from 35% to 45%. Small, dense LDL correlate negatively with plasma HDL levels and positively with plasma triglyceride levels and are associated with the metabolic syndrome and with increased risk for cardiovascular disease and diabetes mellitus. LDL size seems also to be an important predictor of cardiovascular events and progression of coronary artery disease and the predominance of small dense LDL has been accepted as an emerging cardiovascular risk factor by the National Cholesterol Education Program Adult Treatment Panel III. In addition, patients with acute myocardial infarction show an early reduction of LDL size, which persists during hospitalization and seems to precede all other plasma lipoprotein modifications. However, it is still on debate whether to measure the LDL size routinely and in which categories of patients. Since the therapeutic modulation of small, dense LDL particles is of great benefit in reducing the atherosclerotic risk, the LDL size measurement should be extended to patients at high risk of coronary artery disease as much as possible.

Low-density lipoprotein size and cardiovascular prevention

Low-density lipoprotein (LDL) size appears to be an important predictor of cardiovascular events and progression of coronary artery disease, and the predominance of small, dense LDL has been accepted as an emerging cardiovascular risk factor by the National Cholesterol Education Program Adult Treatment Panel III. Yet, other authors have suggested that LDL subclass measurement does not add independent information to that conferred generically by LDL concentration and other standard risk factors. Therefore, the debate continues as to whether to measure LDL particle size for cardiovascular prevention and, if so, in which categories of patients. Since the therapeutic modulation of distinct LDL subspecies is of great benefit in reducing the risk of cardiovascular events, LDL size measurement should be extended as much as possible to patients at high risk of cardiovascular diseases.

Low-density lipoprotein size and cardiovascular risk assessment

A predominance of small, dense low-density lipoproteins (LDL) has been accepted as an emerging cardiovascular risk factor by the National Cholesterol Education Program Adult Treatment Panel III. LDL size seems to be an important predictor of cardiovascular events and progression of coronary heart disease and evidences suggests that both quality (particularly small, dense LDL) and quantity may increase cardiovascular risk. However, other authors have suggested that LDL size measurement does not add information beyond that obtained by measuring LDL concentration, triglyceride levels and HDL concentrations. Therefore, it remains debatable whether to measure LDL particle size in cardiovascular risk assessment and, if so, in which categories of patient. Therapeutic modulation of LDL particle size or number appears beneficial in reducing the risk of cardiovascular events, but no clear causal relationship has been shown, because of confounding factors, including lipid and non-lipid variables. Studies are needed to investigate the clinical significance of LDL size measurements in patients with coronary and non-coronary forms of atherosclerosis; in particular, to test whether LDL size is associated with even higher vascular risk, and whether LDL size modification may contribute to secondary prevention in such patients.

Sialic acid as a protective barrier against neointima development

Arterial sialic acid (SA) has been shown to attenuate the binding of fibrinogen and low-density lipoproteins (LDL) to the vessel wall, presumably protecting against atherosclerosis. This study was aimed to assess the effect of changes in SA content in intimal thickening, an early step in the development of atherosclerosis. New Zealand white rabbits were subjected to bilateral carotid periarterial collaring, followed by in situ-perfusion with neuroaminidase (random artery) and with vehicle (contralateral control artery). The efficiency of SA removal was evaluated in perfusates and arterial homogenates, and arterial tissue samples were obtained 7 and 14 days after the intervention to assess morphological changes. Neuraminidase significantly reduced SA by 16.7%. Arterial desialylation was associated with a significantly increased neointimal formation. Proliferation of smooth muscle cells (SMCs), assessed by incorporation of bromo-2'-deoxyuridine into replicating DNA was also significantly increased in desialylated arteries. In addition, immunohistochemical studies showed a slightly stronger oxidized-LDL (ox-LDL) immunostaining in neointima of desialylated arteries than in control vessels. A mild reduction of SA increases intimal thickening, at least partly due to an enhanced proliferation of SMCs, and may facilitate the accretion of atherogenic lipoproteins, providing evidence for the potential role of SA in the protection against neointimal development.

Low-density lipoprotein size and subclasses are markers of clinically apparent and non-apparent atherosclerosis in type 2 diabetes

The atherogenic lipoprotein phenotype is characterized by an increase in plasma triglycerides, a decrease in high-density lipoprotein (HDL), and the prevalence of small, dense low-density lipoprotein (LDL) particles. The present study investigated the clinical significance of LDL size and subclasses as markers of atherosclerosis in diabetes type 2. Thirty-eight patients with type 2 diabetes, total cholesterol of less than 6.5 mmol/L, and hemoglobin A1c (HbA1c) of less than 9% were studied. Median age was 61 years, mean (+/-SD) body mass index 29 +/- 4.3 kg/m2 , and mean HbA1c 7.1 +/- 0.9 %. Laboratory parameters included plasma lipids and lipoproteins, lipoprotein (a), apolipoprotein (apo) A-I, apo B-100, apo C-III, and high-sensitivity C-reactive protein. Low-density lipoprotein size and subclasses were measured by gradient gel electrophoresis and carotideal intima media thickness (IMT) by duplex ultrasound. By factor analysis, 10 out of 21 risk parameters were selected: age, body mass index, systolic blood pressure, smoking (in pack-years), HbA1c, high-sensitivity C-reactive protein, lipoprotein (a), LDL cholesterol, HDL cholesterol, and LDL particle size. Multivariate analysis of variance of these 10 risk parameters identified LDL particle size as the best risk predictor for the presence of coronary heart disease (P = .002). Smaller LDL particle size was associated with an increase in IMT (P = .03; cut-off >1 mm). Within the different lipid parameters (total cholesterol, LDL cholesterol, HDL cholesterol, triglycerides, apo B, apo A-I, apo C-III, LDL particle size), LDL particle size was most strongly associated with the presence of coronary heart disease (P = .002) and IMT (P = .03). It is concluded that LDL size is the strongest marker for clinically apparent as well as non-apparent atherosclerosis in diabetes type 2.

Mechanism of lipoprotein retention by the extracellular matrix

PURPOSE OF REVIEW

Considerable evidence suggests that the subendothelial retention of atherogenic lipoproteins is a key early step in atherogenesis. In humans and experimental animals, elevated levels of plasma lipoproteins are associated with increased atherosclerosis, and lipoproteins with higher affinity for arterial proteoglycans are more atherogenic. Here we discuss the molecular mechanisms underlying lipoprotein retention in the arterial wall and how this interaction can be modulated.

RECENT FINDINGS

Functional proteoglycan binding sites in lipoproteins containing apolipoprotein B have been identified and shown to have atherogenic potential in vivo. In addition to apolipoprotein B, novel bridging molecules, those that can interact with both proteoglycans and lipoproteins, have been identified that mediate the retention of atherogenic particles in the vessel wall. The interaction between lipoproteins and proteoglycans can be enhanced by the modification of lipoproteins in the circulation and in the arterial wall, by alterations in the subendothelium, and by changes in proteoglycan synthesis that result in a more atherogenic profile. The retention of atherogenic lipoproteins is a potential target for therapies to reverse atherosclerosis, and in-vitro studies have identified compounds that decrease the affinity of proteoglycans for lipoproteins.

SUMMARY

Considerable progress has been made in understanding the association between lipoproteins and cardiovascular disease. This review highlights the importance of the interaction between lipoproteins and the arterial matrix.

Insights into the pathogenesis of vein graft disease: lessons from intravascular ultrasound

The success of coronary artery bypass grafting (CABG) is limited by poor long-term graft patency. Saphenous vein is used in the vast majority of CABG operations, although 15% are occluded at one year with as many as 50% occluded at 10 years due to progressive graft atherosclerosis. Intravascular ultrasound (IVUS) has greatly increased our understanding of this process. IVUS studies have shown that early wall thickening and adaptive remodeling of vein grafts occurs within the first few weeks post implantation, with these changes stabilising in angiographically normal vein grafts after six months. Early changes predispose to later atherosclerosis with occlusive plaque detectable in vein grafts within the first year. Both expansive and constrictive remodelling is present in diseased vein grafts, where the latter contributes significantly to occlusive disease. These findings correlate closely with experimental and clinicopathological studies and help define the windows for prevention, intervention or plaque stabilisation strategies. IVUS is also the natural tool for evaluating the effectiveness of pharmacological and other treatments that may prevent or slow the progression of vein graft disease in clinical trials.

Model study of interactions of high-molecular dextran sulfate with lipid monolayers and foam films

The interaction of high-molecular dextran sulfate (DS-5000) with dimyristoylphosphatidylcholine (DMPC) monolayers and foam films (FF) at the air-water interface in the presence of Ca2+ and Na+ ions was studied. DS-5000 was added in monolayer films (MF) and in FF as monomer molecules and in liposomal form. When added in liposomal form in FF, DS-5000 decreased the stability of DMPC common black films (CBF), and no formation of Newton black films (NBF) was observed. However, when included as monomer molecules in FF, DS-5000 caused film thinning, and drastically decreased the expansion rate of the black spots and transition of thick films to NBF, thus avoiding formation of CBF. The above effects were observed in both gel and liquid-crystalline phase states of DMPC in the presence of Ca2+ ions only, and not in the presence of Na+ ions. We postulate that the interaction of DMPC with DS-5000 in the plane of FF is mediated by Ca2+ bridges and results in dehydration of the DMPC polar heads. The interaction between DMPC and DS-5000 in monolayers resulted in slower adsorption and spreading of DMPC molecules at the interface, lower monolayer surface pressure, and penetration of DS-5000 molecules to DMPC monolayers when surface lipid density was higher than 50 A2 per DMPC molecule. The applicability of the FF model for studying the interactions of phospholipids with polysaccharides at interfaces surrounded by bulk solution, and for modeling such interactions in biological systems, e.g. LDL adhesion to the arterial walls, aggregation and fusion of liposomes, etc., is discussed.

Plasma clearance of human low-density lipoprotein in human apolipoprotein B transgenic mice is related to particle diameter

To test for intrinsic differences in metabolic properties of low-density lipoprotein (LDL) as a function of particle size, we examined the kinetic behavior of 6 human LDL fractions ranging in size from 251 to 265 A injected intravenously into human apolipoprotein (apo) B transgenic mice. A multicompartmental model was formulated and fitted to the data by standard nonlinear regression using the Simulation, Analysis and Modeling (SAAM II) program. Smaller sized LDL particles (251 to 257 A) demonstrated a significantly slower fractional catabolic rate (FCR) (0.050 +/- 0.045 h(-1)) compared with particles of larger size (262 to 265 A) (0.134 +/- -0.015 h(-1), P <.03), and there was a significant correlation between FCR and the peak LDL diameter of the injected fractions (R(2) =.71, P <.034). The sum of the equilibration parameters, k(2,1) and k(1,2), for smaller LDL (0.255 h(-1) and 0.105 h(-1), respectively) was significantly smaller than that for larger LDL (0.277 h(-1) and 0.248 h(-1), respectively; P <.01), indicative of slower intravascular-extravascular exchange for smaller LDL. Therefore in this mouse model, smaller LDL particles are cleared more slowly from plasma than larger LDL and are exchanged more slowly with the extravascular space. This might be due to compositional or structural features of smaller LDL that lead to retarded clearance.

Intimal retention of cholesterol derived from apolipoprotein B100- and apolipoprotein B48-containing lipoproteins in carotid arteries of Watanabe heritable hyperlipidemic rabbits

OBJECTIVE

The arterial retention of apolipoprotein (apo) B100- and apoB48-containing lipoproteins was simultaneously determined in a rabbit model of human hypercholesterolemia using 3D confocal microscopy.

METHODS AND RESULTS

Lipoproteins containing apoB100 (LDL) and apoB48 (chylomicron remnants) were differentially conjugated with fluorophores and simultaneously perfused at equivalent concentrations under physiological conditions in situ through carotid vessels of Watanabe heritable hyperlipidemic rabbits and compared with controls. Retention of lipoproteins was defined as the amount remaining after an extensive washout phase. LDL and chylomicron remnants were both retained, primarily within the subendothelial space. Without a concomitant increase in exposure to lipoproteins, we found a marked increase in the retention of cholesterol within the intima of Watanabe heritable hyperlipidemic rabbits compared with controls, specifically because of increased entrapment of apoB48-containing lipoproteins.

CONCLUSIONS

Collectively, our data suggest that hypercholesterolemia induced as a consequence of LDL receptor deficiency differentially influences retention of LDL and of chylomicron remnants.

Metabolic origins and clinical significance of LDL heterogeneity

LDLs in humans comprise multiple distinct subspecies that differ in their metabolic behavior and pathologic roles. Metabolic turnover studies suggest that this heterogeneity results from multiple pathways, including catabolism of different VLDL and IDL precursors, metabolic remodeling, and direct production. A common lipoprotein profile designated atherogenic lipoprotein phenotype is characterized by a predominance of small dense LDL particles. Multiple features of this phenotype, including increased levels of triglyceride rich lipoprotein remnants and IDLs, reduced levels of HDL and an association with insulin resistance, contribute to increased risk for coronary heart disease compared with individuals with a predominance of larger LDL. Increased atherogenic potential of small dense LDL is suggested by greater propensity for transport into the subendothelial space, increased binding to arterial proteoglycans, and susceptibility to oxidative modification. Large LDL particles also can be associated with increased coronary disease risk, particularly in the setting of normal or low triglyceride levels. Like small LDL, large LDL exhibits reduced LDL receptor affinity compared with intermediate sized LDL. Future delineation of the determinants of heterogeneity of LDL and other apoB-containing lipoproteins may contribute to improved identification and management of patients at high risk for atherosclerotic disease.

Effect of alcohol on lipids and lipoproteins in relation to atherosclerosis

Several studies indicate that light-to-moderate alcohol consumption is associated with a low prevalence of coronary heart disease. An increase in high-density lipoprotein (HDL) cholesterol is associated with alcohol intake and appears to account for approximately half of alcohol's cardioprotective effect. In addition to changes in the concentration and composition of lipoproteins, alcohol consumption may alter the activities of plasma proteins and enzymes involved in lipoprotein metabolism: cholesteryl ester transfer protein, phospholipid transfer protein, lecithin:cholesterol acyltransferase, lipoprotein lipase, hepatic lipase, paraoxonase-1 and phospholipases. Alcohol intake also results in modifications of lipoprotein particles: low sialic acid content in apolipoprotein components of lipoprotein particles (e.g., HDL apo E and apo J) and acetaldehyde modification of apolipoproteins. In addition, "abnormal" lipids, phosphatidylethanol, and fatty acid ethyl esters formed in the presence of ethanol are associated with lipoproteins in plasma. The effects of lipoproteins on the vascular wall cells (endothelial cells, smooth muscle cells, and monocyte/macrophages) may be modulated by ethanol and the alterations further enhanced by modified lipids. The present review discusses the effects of alcohol on lipoproteins in cholesterol transport, as well as the novel effects of lipoproteins on vascular wall cells.

Myeloperoxidase and hypochlorite, but not copper ions, oxidize heparin-bound LDL particles and release them from heparin

A key factor in atherosclerosis is the retention of low density lipoprotein (LDL) in the extracellular matrix of the arterial intima, where it binds to the negatively charged glycosaminoglycan chains of proteoglycans. Oxidation may lead to modification of the lysine residues of apolipoprotein B-100 of LDL, which normally mediate the binding of LDL to glycosaminoglycans. Here, we studied whether various modes of oxidation can release LDL from heparin, a glycosaminoglycan with a strong negative charge, in vitro. We found that copper ions were unable to oxidize heparin-bound LDL particles because of their redox inactivation by the glycosaminoglycans. In contrast, myeloperoxidase and hypochlorite, a product of myeloperoxidase, were able to oxidize heparin-bound LDL, and this oxidation led to the release of the oxidized particles from heparin. When the released LDL particles were compared with the residual heparin-bound LDL particles, the released particles were more electronegative and contained more modified lysine residues than did the particles that remained bound. Because human atherosclerotic lesions contain catalytically active myeloperoxidase and (lipo)proteins modified by hypochlorite, the results suggest that myeloperoxidase-secreting monocytes/macrophages in the arterial intima can oxidize and extract LDL from the extracellular matrix with ensuing uptake by the macrophages of the oxidized and released LDL, with eventual formation of foam cells.

Characterization of human apolipoprotein B100 oligosaccharides in LDL subfractions derived from normal and hyperlipidemic plasma: deficiency of alpha-N-acetylneuraminyllactosyl-ceramide in light and small dense LDL particles

The carbohydrate composition of apolipoprotein (apo) B100, particularly its degree of sialylation, may contribute to the atherogenic properties of low-density lipoprotein (LDL). We analyzed LDL apoB100 glycans derived from normolipidemic, hypercholesterolemic, and hypertriglyceridemic diabetic subjects. Using exoglycosidase carbohydrate sequencing and matrix-assisted laser desorption/ionization mass spectrometry to analyze fluorescently labeled oligosaccharides, we report evidence for several carbohydrates not previously identified on apoB100, including truncated complex biantennary N-glycans and hybrid N-glycans. The distribution and diversity of the apoB100 glycans isolated from all individuals was highly conserved. The N-glycan composition of apoB100 derived from five LDL subpopulations (LDL1, d = 1.018-1.023; LDL2, d = 1.023-1.030; LDL3, d = 1.030-1.040; LDL4, d = 1.040-1.051; LDL5, d = 1.051-1.065 g/ml) did not vary in normolipidemic or hypercholesterolemic subjects. Furthermore, we found no evidence for "desialylated" apoB100 glycans in any of the samples analyzed. Analysis of the most abundant LDL ganglioside, alpha-N-acetylneuraminyllactosyl-ceramide, revealed a deficiency in small dense LDL and in the most buoyant subpopulation. These data provide a novel explanation for the apparent deficiency of sialic acid in small dense LDL and indicate that the global apoB100 N-glycan composition is invariable in the patient groups studied.

The content of lipoperoxidation products in normal and atherosclerotic human aorta

To evaluate the role of lipid oxidation in atherogenesis the levels of lipid- and protein-bound products of peroxidation in normal and atherosclerotic areas of human aorta were investigated. The level of fluorescent (360/430 nm) lipid products was measured in chloroform-methanol extracts of aortic tissue. Normal intima, initial lesions and fatty streaks had a similar content of fluorescent substances. On the other hand, high level of fluorescent products was found in atherosclerotic plaques. Cholesterol covalently bound to proteins, which serve as a marker of lipoperoxidation, was measured by high performance liquid chromatography after mild alkaline hydrolysis of delipidated tissue protein samples. The levels of protein-bound cholesterol in initial lesions and fatty streaks were close to its content in uninvolved intima (59 +/- 18 and 92 +/- 18 vs. 70 +/- 13 nmol/g protein). The content of covalently bound cholesterol in atherosclerotic plaques was dramatically higher (90-fold) than in the normal tissue. In addition to protein-bound cholesterol, considerable amount of lipofuscin was revealed in the cells of atherosclerotic plaques, but not in the cells of normal intima, initial lesions or fatty streaks. Thus, the contents of all investigated lipid- and protein-bound products of lipoperoxidation in earlier atherosclerotic lesions were similar to their levels in normal tissue. It can be due to a low rate of oxidized product formation and/or high rate of its degradation in or elimination from the vessel wall.

Measurement of asialylated LDL in the blood of patients with coronary artery disease by antibody-lectin sandwich assay

Elevated concentrations of asialylated LDL (asialo-LDL) have been reported in patients with coronary artery disease (CAD). This may stimulate lipid accumulation in arterial intima cells and promote atherosclerosis. To investigate asialo-LDL as a potential risk-factor for coronary atherogenesis, we developed an antibody-lectin sandwich assay to measure levels in serum from CAD patients and age-matched control subjects. LDL was captured with an anti-apolipoprotein (apo) B antibody and asialylated oligosaccharides measured using the biotinylated D-galactose (D-gal) binding lectin, Ricinus communis agglutinin 120 (RCA120), and a streptavidin-alkaline phosphatase conjugate. For the control and atherosclerotic subjects, median [interquartile range (IQR)] values for total concentrations of asialo-LDL were 240 mg,L (180-310 mg/L) and 220 mg/L (186-390 mg/L), respectively (P = 0.82). When expressed as a percentage of serum apo B-100, median (IQR) values were 18% (16-23%) and 19% (15-29%), respectively (P = 0.78). These results suggest asialo-LDL has little value as a risk factor for coronary atherosclerosis.

Role of the macrophage galactose lectin in the uptake of desialylated LDL

Desialylated low density lipoprotein (LDL) is rapidly taken up and accumulated by both peripheral blood monocytes and cells isolated from human arterial intima consisting predominantly of smooth muscle cells. It is shown that thioglycollate (TG)-elicited mouse macrophages and mouse peritoneal macrophages stimulated with lipopolysaccharide (LPS) show increased expression of a membrane-bound, galactose-specific lectin that could be responsible for this uptake. In LPS-stimulated macrophages accumulation of desialylated LDL is increased ca. 2.6-fold. Accumulation of acetylated LDL in the same cells is reduced, suggesting that the galactose-specific lectin might be responsible for the uptake of desialylated LDL. Transfection of cells with the mouse macrophage Gal/GalNAc-specific lectin (MMGL) increased their capacity to take up asialofetuin (ASF) and, to a smaller extent, desialylated LDL. The uptake of desialylated LDL was small, most likely due to the high k(d) of MMGL for biantennary oligosaccharides as found on LDL, and low concentration of LDL achieved in tissue culture experiments. The data suggest that the expression of galactose-specific lectins can be elevated under inflammatory conditions, and that these receptors could contribute to foam cell formation under conditions of high desialylated LDL concentration, as might be found in arterial intima.

Significance of acidic sugar chains of apolipoprotein B-100 in cellular metabolism of low-density lipoproteins

We have elucidated the carbohydrate structures of the N-linked sugar chains of human and rabbit apolipoprotein B-100 (apo B-100), which is similar in composition to oligosaccharides (Arch Biochem Biophys 1989;273:197-205, Arteriosclerosis 1990; 10:386-93). We have also shown the negative correlation of the ratio of acidic sugar chains of apo B-100 to the serum cholesterol levels in Watanabe heritable hyperlipidemic rabbits (Atherosclerosis 1992;93:229-35). The acidity of sugar chains is determined by the existence of sialic acid residues at the terminal of oligosaccharides. In the present study we investigated N-linked sugar chains of apo B-100 from patients with coronary artery disease (CAD) who had moderate hypercholesterolemia (less than 400 mg/dL). There was no difference in the structure of their oligosaccharides and the ratio of acidic sugar chains of apo B-100 from CAD patients as compared with that from healthy individuals reported previously. To clarify the role of sialic acid residues in apo B-100 for lipoprotein metabolism, we studied cellular uptake of low-density lipoproteins (LDLs) treated with sialidase (desialylated LDL). Desialylated LDLs were taken up and degraded to a 2-fold greater degree than control LDL by human monocyte-derived macrophages and stimulated cholesterol esterification in these cells. These results indicate that sialic acid residues of apo B- 100 play an important role in cellular uptake and degradation of LDL.

Electronegative LDL from normolipemic subjects induces IL-8 and monocyte chemotactic protein secretion by human endothelial cells

The presence in plasma of an electronegative LDL subfraction [LDL(-)] cytotoxic for endothelial cells (ECs) has been reported. We studied the effect of LDL(-) on the release by ECs of molecules implicated in leukocyte recruitment [interleukin-8 (IL-8) and monocyte chemotactic protein-1 (MCP-1)] and in the plasminogen activator inhibitor-1 (PAI-1). LDL(-), isolated by anion-exchange chromatography, differed from nonelectronegative LDL [LDL(+)] in its higher triglyceride, nonesterified fatty acid, apoprotein E and apoprotein C-III, and sialic acid contents. No evidence of extensive oxidation was found in LDL(-); its antioxidant and thiobarbituric acid-reactive substances contents were similar to those of LDL(+). However, conjugated dienes were increased in LDL(-), which suggests that mild oxidation might affect these particles. LDL(-) increased, in a concentration-dependent manner, the release of IL-8 and MCP-1 by ECs and was a stronger inductor of both chemokines than oxidized LDL (oxLDL) or LDL(+). PAI-1 release increased slightly in ECs incubated with both LDL(-) and oxLDL but not with LDL(+). However, no cytotoxic effects of LDL(-) were observed on ECs. Actinomycin D inhibited the release of IL-8 and MCP-1 induced by LDL(-) and oxLDL by up to 80%, indicating that their production is mediated by protein synthesis. Incubation of ECs with N:-acetyl cysteine inhibited production of IL-8 and MCP-1 induced by LDL(-) and oxLDL by >50%. The free radical scavenger butylated hydroxytoluene slightly inhibited the effect of oxLDL but did not modify the effect of LDL(-). An antagonist (BN-50730) of the platelet-activating factor receptor inhibited production of both chemokines by LDL(-) and oxLDL in a concentration-dependent manner. Our results indicate that LDL(-) shows proinflammatory activity on ECs and may contribute to early atherosclerotic events.

Interaction of native and modified low-density lipoproteins with extracellular matrix

Lipoprotein-matrix interactions play an important role in arterial disease. Extracellular matrix proteoglycans bind and retain specific positively charged domains on apolipoproteins B- and E-containing lipoproteins during atherogenesis. Retained lipoproteins can undergo several modifications, which may alter their interaction with extracellular matrix molecules. Growth factors, cytokines and oxidized low density lipoproteins influence proteoglycan structure, rendering them more likely to bind and retain lipoproteins during atherogenesis. Lipoproteins, native and modified, also can modulate the expression of several of the matrix degrading enzymes present in vascular tissue, thereby influencing plaque stability. Thus, the interaction of atherogenic lipoproteins with arterial wall matrix molecules can influence the genesis and progression of atherosclerosis and its complications.

Pomegranate juice consumption reduces oxidative stress, atherogenic modifications to LDL, and platelet aggregation: studies in humans and in atherosclerotic apolipoprotein E-deficient mice

BACKGROUND

Dietary supplementation with nutrients rich in antioxidants is associated with inhibition of atherogenic modifications to LDL, macrophage foam cell formation, and atherosclerosis. Pomegranates are a source of polyphenols and other antioxidants.

OBJECTIVE

We analyzed, in healthy male volunteers and in atherosclerotic apolipoprotein E-deficient (E(0)) mice, the effect of pomegranate juice consumption on lipoprotein oxidation, aggregation, and retention; macrophage atherogenicity; platelet aggregation; and atherosclerosis.

DESIGN

Potent antioxidative effects of pomegranate juice against lipid peroxidation in whole plasma and in isolated lipoproteins (HDL and LDL) were assessed in humans and in E(0) mice after pomegranate juice consumption for </=2 and 14 wk, respectively.

RESULTS

In humans, pomegranate juice consumption decreased LDL susceptibility to aggregation and retention and increased the activity of serum paraoxonase (an HDL-associated esterase that can protect against lipid peroxidation) by 20%. In E(0) mice, oxidation of LDL by peritoneal macrophages was reduced by up to 90% after pomegranate juice consumption and this effect was associated with reduced cellular lipid peroxidation and superoxide release. The uptake of oxidized LDL and native LDL by mouse peritoneal macrophages obtained after pomegranate juice administration was reduced by 20%. Finally, pomegranate juice supplementation of E(0) mice reduced the size of their atherosclerotic lesions by 44% and also the number of foam cells compared with control E(0) mice supplemented with water.

CONCLUSION

Pomegranate juice had potent antiatherogenic effects in healthy humans and in atherosclerotic mice that may be attributable to its antioxidative properties.

Glycation of plasma low density lipoproteins increases interaction with arterial proteoglycans

The increased susceptibility to atherosclerosis of diabetic individuals, may result from diabetes-associated modification in plasma low density lipoproteins (LDL) which enhance their interaction with arterial extracellular matrix proteoglycans. Using a nonhuman primate model for human diabetes, studies were conducted to examine diabetes-induced changes in LDL. Plasma LDL were isolated from control (n = 4) and streptozotocin-induced diabetic (n = 3) cynomolgus macaques by differential ultracentrifugation. An in vitro binding assay was used to measure LDL interaction with arterial proteoglycans. Significantly more diabetic LDL bound to proteoglycans than control LDL (12.9+/-0.7 microg LDL cholesterol/microg proteoglycan versus 8.9+/-0.5 microg LDL cholesterol/microg proteoglycan (mean +/- S.E.M.), P < 0.005). Glycation of LDL, determined by fructosamine content, was significantly enhanced in diabetic versus control animals (37+/-3.1 versus 20+/-1.5 micromol/l (mean +/- S.E.M.) P < 0.005). The correlation coefficient between fructosamine content of LDL and its binding to arterial proteoglycans was 0.95. No LDL compositional variables other than glycation correlated with proteoglycan binding. Removal of the glycated portion of LDL from diabetic animals returned LDL proteoglycan binding to normal. These data demonstrate that the diabetes induced glycation of LDL increases its proteoglycan binding properties: thus, a critical mechanism in atherosclerosis, enhanced LDL interaction with arterial proteoglycans, may be accelerated by the diabetic state.

Surface Plasmon Resonance Measurement of the Binding of Low-Density Lipoprotein at a Heparin Surface

Low-density lipoprotein (LDL) adsorption to heparin-like surface is of wide clinical interest since such a mechanism might be responsible for cholesterol accumulation in the arterial wall. By modifying the surface plasmon resonance sensor surface with heparin and albumin (BSA) LDL adsorption to this surface was investigated and characterized. Heparin was seen to be a potentially useful ligand for LDL detection and analysis in a clinical context. It was found that N-acetylheparin had a lower affinity for LDL than heparin and that the binding strength of LDL to N-acetylheparin was reduced. Assuming a random distribution of heparin on the surface, it was calculated from the data obtained that a maximum of 4.0 x 10(9) heparin or 4.8 x 10(9) N-acetylheparin "rods" can be found on a millimeter square and that one LDL molecule (380 nm(2)) covers on average only 1.5 heparin molecules or 1.8 N-acetylheparin molecules, yet maximum LDL binding cannot be increased beyond a surface coverage of 7.5 or 5.8% for heparin and N-acetyl heparin, respectively. This could lead to the suggestion that the glycosaminoglycan-LDL atheroclerosis mechanism would involve only 1-2 heparin molecules in "binding" each LDL, but 20-30 molecules are required to attract it to the surface in the first place. Copyright 1999 Academic Press.

Investigation of phospholipid area compression induced by calcium-mediated dextran sulfate interaction

The association of anionic polyelectrolytes such as dextran sulfate (DS) to zwitterionic phospholipid surfaces via Ca(2+) bridges results in a perturbation of lipid packing at physiologically relevant Ca(2+) concentrations. Lipid area compression was investigated in 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) multilamellar bilayer dispersions by (2)H-NMR and in monolayer studies. Binding of DS to DMPC surfaces via Ca(2+) results in denser lipid packing, as indicated by higher lipid chain order. DMPC order parameters are homogeneously increased throughout the lipid bilayer. Higher order translates into more extended hydrocarbon chains and decreased average lipid area per molecule. Area compression is reported as a function of DS concentration and molecular weight. Altering the NaCl and Ca(2+) concentrations modified electrostatic interactions between DS and phospholipid. A maximal area reduction of DeltaA = 2.7 A(2) per DMPC molecule is observed. The lipid main-phase transition temperature increases upon formation of DMPC/Ca(2+)/DS-complexes. Lipid area compression after addition of DS and Ca(2+) to the subphase was also observed in monolayer experiments. A decrease in surface tension of up to 3.5 mN/m at constant molecular area was observed. DS binds to the lipid headgroups by formation of Ca(2+) bridges without penetrating the hydrophobic region. We suggest that area compression is the result of an attractive electrostatic interaction between neighboring lipid molecules induced by high local Ca(2+) concentration due to the presence of DS. X-ray diffraction experiments demonstrate that DS binding to apposing bilayers reduces bilayer separation. We speculate that DS binding alters the phase state of low-density lipoproteins that associate with polyelectrolytes of the arterial connective tissue in the early stages of arteriosclerosis.

Sialic acid-containing components of lipoproteins influence lipoprotein-proteoglycan interactions

Sialic acid is a negatively charged sugar associated with the protein and lipid portions of lipoproteins. Sialic acid has been hypothesised to play an anti-atherogenic role in lipoprotein metabolism through the electrostatic inhibition of lipoprotein interactions with chondroitin-6-sulphate-rich arterial proteoglycans (APG). We conducted a series of studies using native and modified lipoproteins (VLDL1 Sf 60-400, VLDL2 Sf 20-60, IDL1 Sf 16-20, IDL2 Sf 12-16, LDL(A) Sf 8-12, and LDL(B) Sf0-8) that vary in their sialic acid content to examine the relationship between lipoprotein sialic acid content and its interaction with APG. Lipoprotein sialic acid was greatest in VLDL1 and decreased progressively with particle density until the IDL2 fraction (VLDL1 > VLDL2 > IDL1 > IDL2 = LDL(A) = LDL(B)). The pattern of reactivity of each fraction with APG was different from the pattern observed for lipoprotein sialic acid content (IDL2 > LDL(A) > LDL(B) > IDL1 > VLDL2 > VLDL1). Levels of sialic acid were lower in subjects with CHD as compared to control subjects but the presence of CHD had no effect on lipoprotein-APG complex formation when sex and plasma triglyceride levels were taken into account. There was also no significant relationship between the lipoprotein sialic acid content and the reactivity with APG within each lipoprotein fraction. Treatment of hypertriglyceridaemic subjects with ciprofibrate decreased lipoprotein-APG complex formation in all lipoprotein fractions. This was associated with a decrease in the total sialic acid content of apo B100-containing lipoproteins suggesting that the total sialic acid content of apo B100-containing lipoproteins has no influence on lipoprotein-APG complex formation. We next conducted in vitro experiments to manipulate LDL sialic acid content. Enzymatic removal of sialic acid from LDL with neuraminidase resulted in an increase in LDL-APG complex formation. This was accompanied by an increase in the exposure of free amino groups on LDL possibly due to disruption of interactions between free amino groups and sialic acid-containing components on LDL. Increasing LDL sialic acid content through incubation with ganglioside resulted in a decrease in lipoprotein-APG complex formation without any changes in the exposure of free amino groups on LDL. We conclude that total sialic acid content of lipoproteins is not a major determinant of their binding to APG. However, specific sialic acid-containing components on lipoproteins can affect their interaction with APG.

Ultrastructure of early lipid accumulation in ApoE-deficient mice

Apolipoprotein (apo) E-deficient mice develop severe hypercholesterolemia and have lesions that progress from fatty streaks to fibrous plaques distributed in lesion-prone areas throughout the aorta. Lesions develop in apoE-deficient mice on a regular chow diet and will occur faster on a diet higher in cholesterol. Examination of the aortas from these mice on a chow diet by high-resolution, freeze-etch electron microscopy demonstrated lipid retention in the intima by 3 weeks of age. Lipid was retained in the matrix as individual particles between 33 and 48 nm in diameter, aligned along the collagen fibrils and in aggregates consisting of lipid particles with average diameters of 33 and 68 nm. Larger particles seemed to have formed from fusion of smaller particles. Lipid retention was more widespread in 5- and 9-week-old mice. Monocyte attachment to endothelial cells was observed by electron microscopy at 5 weeks of age. The appearance of the intimal lipid was similar to that previously described in rabbit models and suggests that lipid interaction with matrix filaments and subsequent aggregation of lipid particles are critical first steps in the process of foam cell formation.