Lipoprotein aggregation as an essential condition of intracellular lipid accumulation caused by modified low density lipoproteins

Mechanisms modulating foam cell formation in the arterial intima: exploring new therapeutic opportunities in atherosclerosis

In recent years, the role of macrophages as the primary cell type contributing to foam cell formation and atheroma plaque development has been widely acknowledged. However, it has been long recognized that diffuse intimal thickening (DIM), which precedes the formation of early fatty streaks in humans, primarily consists of lipid-loaded smooth muscle cells (SMCs) and their secreted proteoglycans. Recent studies have further supported the notion that SMCs constitute the majority of foam cells in advanced atherosclerotic plaques. Given that SMCs are a major component of the vascular wall, they serve as a significant source of microvesicles and exosomes, which have the potential to regulate the physiology of other vascular cells. Notably, more than half of the foam cells present in atherosclerotic lesions are of SMC origin. In this review, we describe several mechanisms underlying the formation of intimal foam-like cells in atherosclerotic plaques. Based on these mechanisms, we discuss novel therapeutic approaches that have been developed to regulate the generation of intimal foam-like cells. These innovative strategies hold promise for improving the management of atherosclerosis in the near future.

Elevated concentrations of Neu5Ac and Neu5,9Ac2 in human plasma: potential biomarkers of cardiovascular disease

Cardiovascular disease (CVD) is a group of health conditions affecting the heart and vascular system with very high prevalence and mortality rates. The presence of CVD is characterised by high levels of inflammation which have previously been associated with increased plasma concentrations of N-acetyl neuraminic acid (Neu5Ac). While Neu5Ac has been studied in the context of CVD, Neu5,9Ac2 has not, despite being the second most abundant sialic acid in human plasma. A small-scale pilot study of thirty plasma samples from patients with diagnosed CVD, and thirty age and sex-matched healthy controls, was designed to gain insight into sialic acids as biomarkers for CVD and potential future areas of study. Each sample was assayed for Neu5Ac and Neu5,9Ac2 concentrations. Mean Neu5Ac and Neu5,9Ac2 concentrations were significantly elevated in patients with CVD compared to healthy controls (Neu5Ac: P < 0.001; Neu5,9Ac2: P < 0.04). Receiver operator curve (ROC) analysis indicated that both Neu5Ac and Neu5,9Ac2 have reasonable predictive power for the presence of CVD (Neu5Ac AUC: 0.86; Neu5,9Ac2 AUC: 0.71). However, while Neu5Ac had both good sensitivity (0.82) and specificity (0.81), Neu5,9Ac2 had equivalent specificity (0.81) but very poor sensitivity (0.44). A combination marker of Neu5Ac + Neu5,9Ac2 showed improvement over Neu5Ac alone in terms of predictive power (AUC: 0.93), sensitivity (0.87), and specificity (0.90). Comparison to a known inflammatory marker, high sensitivity c-reactive protein (hs-CRP: P-value: NS, ROC:0.50) was carried out, showing that both Neu5Ac and Neu5,9Ac2 outperformed this marker. Further to this, hs-CRP values were combined with the three different sialic acid markers to determine any effect on the AUC values. A slight improvement in AUC was noted for each of the combinations, with Neu5Ac + Neu5,9Ac2 + hs-CRP giving the best AUC of 0.97 overall. Thus, Neu5Ac would appear to offer good potential as a predictive marker for the presence of CVD, which the addition of Neu5,9Ac2 predictive power improves, with further improvement seen by the addition of hs-CRP.

Local Accumulation of Lymphocytes in the Intima of Human Aorta Is Associated with Giant Multinucleated Endothelial Cells: Possible Explanation for Mosaicism of Atherosclerosis

Distribution of different types of atherosclerotic lesions in the arterial wall is not diffuse, but is characterized by mosaicism. The causes of such distribution remain to be established. At the early stages of atherogenesis, low-density lipoprotein (LDL) particles and immune cells penetrate into the intimal layer of the arterial wall through the endothelium. In adult humans, the luminal surface of the arterial wall is a heterogeneous monolayer of cells with varying morphology including typical endothelial cells (ECs) and multinucleated variant endothelial cells (MVECs). We hypothesized that distribution of MVECs in the endothelial monolayer can be related to the distribution pattern of early atherosclerotic lesions. We obtained en face preparations of intact adult (22–59 years old) aortic wall sections that allowed us to study the endothelial monolayer and the subendothelial layer. We compared the distribution of MVECs in the endothelial monolayer with the localization of early atherosclerotic lesions in the subendothelial layer, which were characterized by lipid accumulation and immune cell recruitment. In primary culture, MVECs demonstrated increased phagocytic activity compared to mononuclear ECs. Moreover, we have shown that unaffected aortic intima contained associates formed as a result of aggregation and/or fusion of LDL particles that are non-randomly distributed. This indicated that MVECs may be involved in the accumulation of LDL in the subendothelial layer through increased transcytosis. Interaction of LDL with subendothelial cells of human aorta in primary culture increased their adhesive properties toward circulating immune cells. Study of unaffected aortic intima revealed non-random distribution of leukocytes in the subendothelial layer and increased localization of CD45+ leukocytes in the subendothelial layer adjacent to MVECs. Together, our observations indicate that MVECs may be responsible for the distribution of atherosclerotic lesions in the arterial wall by participating in LDL internalization and immune cell recruitment.

The Pathogenic Role of Foam Cells in Atherogenesis: Do They Represent Novel Therapeutic Targets?

BACKGROUND

Foam cells, mainly derived from monocytes-macrophages, contain lipid droplets essentially composed of cholesterol in their cytoplasm. They infiltrate the intima of arteries, contributing to the formation of atherosclerotic plaques.

PATHOGENESIS

Foam cells damage the arterial cell wall via the release of proinflammatory cytokines, free radicals, and matrix metalloproteinases, enhancing the plaque size up to its rupture.

THERAPY

A correct dietary regimen seems to be the most appropriate therapeutic approach to minimize obesity, which is associated with the formation of foam cells. At the same time, different types of antioxidants have been evaluated to arrest the formation of foam cells, even if the results are still contradictory. In any case, a combination of antioxidants seems to be more efficient in the prevention of atherosclerosis.

Lipid accumulation and novel insight into vascular smooth muscle cells in atherosclerosis

Atherosclerosis is a chronic and progressive process. It is the most important pathological basis of cardiovascular disease and stroke. Vascular smooth muscle cells (VSMCs) are an essential cell type in atherosclerosis. Previous studies have revealed that VSMCs undergo phenotypic transformation in atherosclerosis to participate in the retention of atherogenic lipoproteins as well as the formation of the fibrous cap and the underlying necrotic core in plaques. The emergence of lineage-tracing studies indicates that the function and number of VSMCs in plaques have been greatly underestimated. In addition, recent studies have revealed that VSMCs make up at least 50% of the foam cell population in human and mouse atherosclerotic lesions. Therefore, understanding the formation of lipid-loaded VSMCs and their regulatory mechanisms is critical to elucidate the pathogenesis of atherosclerosis and to explore potential therapeutic targets. Moreover, combination of many complementary technologies such as lineage tracing, single-cell RNA sequencing (scRNA-seq), flow cytometry, and mass cytometry (CyTOF) with immunostaining has been performed to further understand the complex VSMC function. Correct identification of detrimental and beneficial processes may reveal successful therapeutic treatments targeting VSMCs and their derivatives during atherosclerosis. The purpose of this review is to summarize the process of lipid-loaded VSMC formation in atherosclerosis and to describe novel insight into VSMCs gained by using multiple advanced methods.

Mitochondrion as a Selective Target for the Treatment of Atherosclerosis: Role of Mitochondrial DNA Mutations and Defective Mitophagy in the Pathogenesis of Atherosclerosis and Chronic Inflammation

BACKGROUND

Atherosclerosis is a chronic inflammatory condition that affects different arteries in the human body and often leads to severe neurological complications, such as stroke and its sequelae. Affected blood vessels develop atherosclerotic lesions in the form of focal thickening of the intimal layer, so called atherosclerotic plaques.

OBJECTIVES

Despite the high priority of atherosclerosis research for global health and the numerous preclinical and clinical studies conducted, currently, there is no effective pharmacological treatment that directly impacts atherosclerotic plaques. Many knowledge gaps exist in our understanding of the mechanisms of plaque formation. In this review, we discuss the role of mitochondria in different cell types involved in atherogenesis and provide information about mtDNA mutations associated with the disease.

RESULTS

Mitochondria of blood and arterial wall cells appear to be one of the important factors in disease initiation and development. Significant experimental evidence connects oxidative stress associated with mitochondrial dysfunction and vascular disease. Moreover, mitochondrial DNA (mtDNA) deletions and mutations are being considered as potential disease markers. Further study of mtDNA damage and associated dysfunction may open new perspectives for atherosclerosis treatment.

CONCLUSION

Mitochondria can be considered as important disease-modifying factors in several chronic pathologies. Deletions and mutations of mtDNA may be used as potential disease markers. Mitochondria-targeting antioxidant therapies appear to be promising for the development of treatment of atherosclerosis and other diseases associated with oxidative stress and chronic inflammation.

Do Mitochondrial DNA Mutations Play a Key Role in the Chronification of Sterile Inflammation? Special Focus on Atherosclerosis

BACKGROUND

The aim of the elucidation of mechanisms implicated in the chronification of inflammation is to shed light on the pathogenesis of disorders that are responsible for the majority of the incidences of diseases and deaths, and also causes of ageing. Atherosclerosis is an example of the most significant inflammatory pathology. The inflammatory response of innate immunity is implicated in the development of atherosclerosis arising locally or focally. Modified low-density lipoprotein (LDL) was regarded as the trigger for this response. No atherosclerotic changes in the arterial wall occur due to the quick decrease in inflammation rate. Nonetheless, the atherosclerotic lesion formation can be a result of the chronification of local inflammation, which, in turn, is caused by alteration of the response of innate immunity.

OBJECTIVE

In this review, we discussed potential mechanisms of the altered response of the immunity in atherosclerosis with a particular emphasis on mitochondrial dysfunctions.

CONCLUSION

A few mitochondrial dysfunctions can be caused by the mitochondrial DNA (mtDNA) mutations. Moreover, mtDNA mutations were found to affect the development of defective mitophagy. Modern investigations have demonstrated the controlling mitophagy function in response to the immune system. Therefore, we hypothesized that impaired mitophagy, as a consequence of mutations in mtDNA, can raise a disturbed innate immunity response, resulting in the chronification of inflammation in atherosclerosis.

Mutations of mtDNA in some Vascular and Metabolic Diseases

BACKGROUND

The present review article considers some chronic diseases of vascular and metabolic genesis, the causes of which may be mitochondrial dysfunction. Very often, in the long course of the disease, complications may occur, leading to myocardial infarction or ischemic stroke and, as a result, death. In particular, a large percentage of human deaths nowadays belongs to cardiovascular diseases, such as coronary heart disease (CHD), arterial hypertension, cardiomyopathies, and type 2 diabetes mellitus.

OBJECTIVE

The aim of the present review was the analysis of literature sources, devoted to an investigation of a link of mitochondrial DNA mutations with chronic diseases of vascular and metabolic genesis.

RESULTS

The analysis of literature indicates the association of the mitochondrial genome mutations with coronary heart disease, type 2 diabetes mellitus, hypertension, and various types of cardiomyopathies.

CONCLUSION

The detected mutations can be used to analyze the predisposition to chronic diseases of vascular and metabolic genesis. They can also be used to create molecular-cell models necessary to evaluate the effectiveness of drugs developed for the treatment of these pathologies. MtDNA mutations associated with the absence of diseases of vascular and metabolic genesis could be potential candidates for gene therapy of the said diseases.

Cellular Mechanisms of Human Atherogenesis: Focus on Chronification of Inflammation and Mitochondrial Mutations

Atherosclerosis is one of the most common diseases of the cardiovascular system that leads to the development of life-threatening conditions, such as heart attack and stroke. Arthrosclerosis affects various arteries in the human body, but is especially dangerous in the arteries alimenting heart and brain, aorta, and arteries of the lower limbs. By its pathophysiology, atherosclerosis is an inflammatory disease. During the pathological process, lesions of arterial intima in the form of focal thickening are observed, which form atherosclerotic plaques as the disease progresses further. Given the significance of atherosclerosis for the global health, the search for novel effective therapies is highly prioritized. However, despite the constant progress, our understanding of the mechanisms of atherogenesis is still incomplete. One of the remaining puzzles in atherosclerosis development is the focal distribution of atherosclerotic lesions in the arterial wall. It implies the existence of certain mosaicism within the tissue, with some areas more susceptible to disease development than others, which may prove to be important for novel therapy development. There are many hypotheses explaining this phenomenon, for example, the influence of viruses, and the spread in the endothelium of the vessel multinucleated giant endothelial cells. We suggest the local variations of the mitochondrial genome as a possible explanation of this mosaicism. In this review, we discuss the role of genetic variations in the nuclear and mitochondrial genomes that influence the development of atherosclerosis. Changes in the mitochondrial and nuclear genome have been identified as independent factors for the development of the disease, as well as potential diagnostic markers.

Signaling Pathways Potentially Responsible for Foam Cell Formation: Cholesterol Accumulation or Inflammatory Response-What is First?

Accumulation of lipid-laden (foam) cells in the arterial wall is known to be the earliest step in the pathogenesis of atherosclerosis. There is almost no doubt that atherogenic modified low-density lipoproteins (LDL) are the main sources of accumulating lipids in foam cells. Atherogenic modified LDL are taken up by arterial cells, such as macrophages, pericytes, and smooth muscle cells in an unregulated manner bypassing the LDL receptor. The present study was conducted to reveal possible common mechanisms in the interaction of macrophages with associates of modified LDL and non-lipid latex particles of a similar size. To determine regulatory pathways that are potentially responsible for cholesterol accumulation in human macrophages after the exposure to naturally occurring atherogenic or artificially modified LDL, we used transcriptome analysis. Previous studies of our group demonstrated that any type of LDL modification facilitates the self-association of lipoprotein particles. The size of such self-associates hinders their interaction with a specific LDL receptor. As a result, self-associates are taken up by nonspecific phagocytosis bypassing the LDL receptor. That is why we used latex beads as a stimulator of macrophage phagocytotic activity. We revealed at least 12 signaling pathways that were regulated by the interaction of macrophages with the multiple-modified atherogenic naturally occurring LDL and with latex beads in a similar manner. Therefore, modified LDL was shown to stimulate phagocytosis through the upregulation of certain genes. We have identified at least three genes (F2RL1, EIF2AK3, and IL15) encoding inflammatory molecules and associated with signaling pathways that were upregulated in response to the interaction of modified LDL with macrophages. Knockdown of two of these genes, EIF2AK3 and IL15, completely suppressed cholesterol accumulation in macrophages. Correspondingly, the upregulation of EIF2AK3 and IL15 promoted cholesterol accumulation. These data confirmed our hypothesis of the following chain of events in atherosclerosis: LDL particles undergo atherogenic modification; this is accompanied by the formation of self-associates; large LDL associates stimulate phagocytosis; as a result of phagocytosis stimulation, pro-inflammatory molecules are secreted; these molecules cause or at least contribute to the accumulation of intracellular cholesterol. This chain of events may explain the relationship between cholesterol accumulation and inflammation. The primary sequence of events in this chain is related to inflammatory response rather than cholesterol accumulation.

Signaling Pathways and Key Genes Involved in Regulation of foam Cell Formation in Atherosclerosis

Atherosclerosis is associated with acute cardiovascular conditions, such as ischemic heart disease, myocardial infarction, and stroke, and is the leading cause of morbidity and mortality worldwide. Our understanding of atherosclerosis and the processes triggering its initiation is constantly improving, and, during the last few decades, many pathological processes related to this disease have been investigated in detail. For example, atherosclerosis has been considered to be a chronic inflammation triggered by the injury of the arterial wall. However, recent works showed that atherogenesis is a more complex process involving not only the immune system, but also resident cells of the vessel wall, genetic factors, altered hemodynamics, and changes in lipid metabolism. In this review, we focus on foam cells that are crucial for atherosclerosis lesion formation. It has been demonstrated that the formation of foam cells is induced by modified low-density lipoprotein (LDL). The beneficial effects of the majority of therapeutic strategies with generalized action, such as the use of anti-inflammatory drugs or antioxidants, were not confirmed by clinical studies. However, the experimental therapies targeting certain stages of atherosclerosis, among which are lipid accumulation, were shown to be more effective. This emphasizes the relevance of future detailed investigation of atherogenesis and the importance of new therapies development.

Role of Phagocytosis in the Pro-Inflammatory Response in LDL-Induced Foam Cell Formation; a Transcriptome Analysis

Excessive accumulation of lipid inclusions in the arterial wall cells (foam cell formation) caused by modified low-density lipoprotein (LDL) is the earliest and most noticeable manifestation of atherosclerosis. The mechanisms of foam cell formation are not fully understood and can involve altered lipid uptake, impaired lipid metabolism, or both. Recently, we have identified the top 10 master regulators that were involved in the accumulation of cholesterol in cultured macrophages induced by the incubation with modified LDL. It was found that most of the identified master regulators were related to the regulation of the inflammatory immune response, but not to lipid metabolism. A possible explanation for this unexpected result is a stimulation of the phagocytic activity of macrophages by modified LDL particle associates that have a relatively large size. In the current study, we investigated gene regulation in macrophages using transcriptome analysis to test the hypothesis that the primary event occurring upon the interaction of modified LDL and macrophages is the stimulation of phagocytosis, which subsequently triggers the pro-inflammatory immune response. We identified genes that were up- or downregulated following the exposure of cultured cells to modified LDL or latex beads (inert phagocytosis stimulators). Most of the identified master regulators were involved in the innate immune response, and some of them were encoding major pro-inflammatory proteins. The obtained results indicated that pro-inflammatory response to phagocytosis stimulation precedes the accumulation of intracellular lipids and possibly contributes to the formation of foam cells. In this way, the currently recognized hypothesis that the accumulation of lipids triggers the pro-inflammatory response was not confirmed. Comparative analysis of master regulators revealed similarities in the genetic regulation of the interaction of macrophages with naturally occurring LDL and desialylated LDL. Oxidized and desialylated LDL affected a different spectrum of genes than naturally occurring LDL. These observations suggest that desialylation is the most important modification of LDL occurring in vivo. Thus, modified LDL caused the gene regulation characteristic of the stimulation of phagocytosis. Additionally, the knock-down effect of five master regulators, such as IL15, EIF2AK3, F2RL1, TSPYL2, and ANXA1, on intracellular lipid accumulation was tested. We knocked down these genes in primary macrophages derived from human monocytes. The addition of atherogenic naturally occurring LDL caused a significant accumulation of cholesterol in the control cells. The knock-down of the EIF2AK3 and IL15 genes completely prevented cholesterol accumulation in cultured macrophages. The knock-down of the ANXA1 gene caused a further decrease in cholesterol content in cultured macrophages. At the same time, knock-down of F2RL1 and TSPYL2 did not cause an effect. The results obtained allowed us to explain in which way the inflammatory response and the accumulation of cholesterol are related confirming our hypothesis of atherogenesis development based on the following viewpoints: LDL particles undergo atherogenic modifications that, in turn, accompanied by the formation of self-associates; large LDL associates stimulate phagocytosis; as a result of phagocytosis stimulation, pro-inflammatory molecules are secreted; these molecules cause or at least contribute to the accumulation of intracellular cholesterol. Therefore, it became obvious that the primary event in this sequence is not the accumulation of cholesterol but an inflammatory response.

The Atherogenic Role of Circulating Modified Lipids in Atherosclerosis

Lipid accumulation in the arterial wall is a crucial event in the development of atherosclerotic lesions. Circulating low-density lipoprotein (LDL) is the major source of lipids that accumulate in the atherosclerotic plaques. It was discovered that not all LDL is atherogenic. In the blood plasma of atherosclerotic patients, LDL particles are the subject of multiple enzymatic and non-enzymatic modifications that determine their atherogenicity. Desialylation is the primary and the most important atherogenic LDL modification followed by a cascade of other modifications that also increase blood atherogenicity. The enzyme trans-sialidase is responsible for the desialylation of LDL, therefore, its activity plays an important role in atherosclerosis development. Moreover, circulating modified LDL is associated with immune complexes that also have a strong atherogenic potential. Moreover, it was shown that antibodies to modified LDL are also atherogenic. The properties of modified LDL were described, and the strong evidence indicating that it is capable of inducing intracellular accumulation of lipids was presented. The accumulated evidence indicated that the molecular properties of modified LDL, including LDL-containing immune complexes can serve as the prognostic/diagnostic biomarkers and molecular targets for the development of anti-atherosclerotic drugs.

Glycosylation of human plasma lipoproteins reveals a high level of diversity, which directly impacts their functional properties

AIMS

Human plasma lipoproteins are known to contain various glycan structures whose composition and functional importance are starting to be recognized. We assessed N-glycosylation of human plasma HDL and LDL and the role of their glycomes in cellular cholesterol metabolism.

METHODS

N-glycomic profiles of native and neuraminidase-treated HDL and LDL were obtained using HILIC-UHPLC-FLD. Relative abundance of the individual chromatographic peaks was quantitatively expressed as a percentage of total integrated area and N-glycan structures present in each peak were elucidated by MALDI-TOF MS. The capacity of HDL to mediate cellular efflux of cholesterol and the capacity of LDL to induce cellular accumulation of cholesteryl esters were evaluated in THP-1 cells.

RESULTS

HILIC-UHPLC-FLD analysis of HDL and LDL N-glycans released by PNGase F resulted in 22 and 18 distinct chromatographic peaks, respectively. The majority of N-glycans present in HDL (~70%) and LDL (~60%) were sialylated with one or two sialic acid residues. The most abundant N-glycan structure in both HDL and LDL was a complex type biantennary N-glycan with one sialic acid (A2G2S1). Relative abundances of several N-glycan structures were dramatically altered by the neuraminidase treatment, which selectively removed sialic acid residues. Native HDL displayed significantly greater efficacy in removing cellular cholesterol from THP-1 cells as compared to desialylated HDL (p < 0.05). Cellular accumulation of cholesteryl esters in THP-1 cells was significantly higher after incubations with desialylated LDL particles as compared to native LDL (p < 0.05).

CONCLUSIONS

N-glycome of human plasma lipoproteins reveals a high level of diversity, which directly impacts functional properties of the lipoproteins.

Angiotensin II induces the aggregation of native and oxidized low-density lipoprotein

Modifications of low-density lipoprotein (LDL), such as oxidation and aggregation, and angiotensin (Ang) peptides are involved in the pathogenesis of atherosclerosis. Here, we investigated the relationship between one of the Ang peptides, AngII, and two LDL modifications, oxidation and aggregation. Using polyacrylamide gel electrophoresis and aggregation assays, we noted that AngII markedly induced the aggregation of LDL and oxidized LDL (Ox-LDL), and bound to both the aggregated and non-aggregated forms. In contrast, a peptide (AngIII) formed by deletion of N-terminal Asp of AngII induced the aggregation of Ox-LDL but not LDL. From tyrosine fluorescence measurements, we noted that AngII interacted with two major lipid components in LDL and Ox-LDL, phosphatidylcholine (PC) and oxidized PC, while AngIII interacted with oxidized PC, but not with PC and lysophosphatidylcholine. Moreover, results from thiobarbituric acid-reactive substances assay proved that AngII did not induce oxidation of LDL. These results suggest that AngII can be involved in the pathogenesis of atherosclerosis by binding to LDL and Ox-LDL-especially to the major lipid components, PC and oxidized PC-followed by inducing the aggregation of LDL and Ox-LDL and that the N-terminal Asp of AngII is important for the binding and aggregation specificity of LDL and Ox-LDL.

The P2Y2 receptor mediates uptake of matrix-retained and aggregated low density lipoprotein in primary vascular smooth muscle cells

BACKGROUND AND AIMS

The internalization of aggregated low-density lipoproteins (agLDL) mediated by low-density lipoprotein receptor related protein (LRP1) may involve the actin cytoskeleton in ways that differ from the endocytosis of soluble LDL by the LDL receptor (LDLR). This study aims to define novel mechanisms of agLDL uptake through modulation of the actin cytoskeleton, to identify molecular targets involved in foam cell formation in vascular smooth muscle cells (VSMCs). The critical observation that formed the basis for these studies is that under pathophysiological conditions, nucleotide release from blood-derived and vascular cells activates SMC P2Y2 receptors (P2Y2Rs) leading to rearrangement of the actin cytoskeleton and cell motility. Therefore, we tested the hypothesis that P2Y2R activation mediates agLDL uptake by VSMCs.

METHODS

Primary VSMCs were isolated from aortas of wild type (WT) C57BL/6 and.P2Y2R-/- mice to investigate whether P2Y2R activation modulates LRP1 expression. Cells were transiently transfected with cDNA encoding a hemagglutinin-tagged (HA-tagged) WT P2Y2R, or a mutant P2Y2R that unlike the WT P2Y2R does not bind the cytoskeletal actin-binding protein filamin-A (FLN-A).

RESULTS

P2Y2R activation significantly increased agLDL uptake, and LRP1 mRNA expression decreased in P2Y2R-/- VSMCs versus WT. SMCs, expressing P2Y2R defective in FLN-A binding, exhibit 3-fold lower LDLR expression levels than SMCs expressing WT P2Y2R, while cells transfected with WT P2Y2R show greater agLDL uptake in both WT and P2Y2R-/- VSMCs versus cells transfected with the mutant P2Y2R.

CONCLUSIONS

Together, these results show that both LRP1 and LDLR expression and agLDL uptake are regulated by P2Y2R in VSMCs, and that agLDL uptake due to P2Y2R activation is dependent upon cytoskeletal reorganization mediated by P2Y2R binding to FLN-A.

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.

Aggregation and fusion of low-density lipoproteins in vivo and in vitro

Low-density lipoproteins (LDLs, also known as 'bad cholesterol') are the major carriers of circulating cholesterol and the main causative risk factor of atherosclerosis. Plasma LDLs are 20- to 25-nm nanoparticles containing a core of cholesterol esters surrounded by a phospholipid monolayer and a single copy of apolipoprotein B (550 kDa). An early sign of atherosclerosis is the accumulation of LDL-derived lipid droplets in the arterial wall. According to the widely accepted 'response-to-retention hypothesis', LDL binding to the extracellular matrix proteoglycans in the arterial intima induces hydrolytic and oxidative modifications that promote LDL aggregation and fusion. This enhances LDL uptake by the arterial macrophages and triggers a cascade of pathogenic responses that culminate in the development of atherosclerotic lesions. Hence, LDL aggregation, fusion, and lipid droplet formation are important early steps in atherogenesis. In vitro, a variety of enzymatic and nonenzymatic modifications of LDL can induce these reactions and thereby provide useful models for their detailed analysis. Here, we summarize current knowledge of the in vivo and in vitro modifications of LDLs leading to their aggregation, fusion, and lipid droplet formation; outline the techniques used to study these reactions; and propose a molecular mechanism that underlies these pro-atherogenic processes. Such knowledge is essential in identifying endogenous and exogenous factors that can promote or prevent LDL aggregation and fusion in vivo and to help establish new potential therapeutic targets to decelerate or even block these pathogenic reactions.

Approach to reduction of blood atherogenicity

We have earlier found that blood sera of patients with coronary heart disease (CHD) increase lipid levels in cells cultured from subendothelial intima of human aorta. We have also revealed that the ability of blood sera to raise intracellular cholesterol; that is, their atherogenicity is caused by at least modified low density lipoprotein (LDL) circulating in the blood of patients and autoantibodies to modified LDL. In the present work we have demonstrated significant impact of nonlipid factor(s) to blood atherogenicity. We have developed an approach to removal of nonlipid atherogenicity factor(s) from blood serum based on the use of immobilized LDL. This approach was used for extracorporeal perfusion of patient's blood through the column with immobilized LDL. Pilot clinical study confirmed the efficacy of this approach for prevention of coronary atherosclerosis progression.

Comparison of hydrophobic properties of thoracic duct lymph chylomicrons from rats given different fats or oils by gavage

Lipoprotein aggregation is generated by hydrophobic nature of lipoproteins that is known to be one of the causes of atherosclerosis. Low density lipoproteins (LDL) has been extensively studied in this respect but not chylomicrons. There is strong evidence that post-prandial triacylglycerol-rich lipoproteins are atherogenic. Because biophysical properties of lipoproteins are largely determined by their lipid compositions, hydrophobic nature of thoracic lymph duct chylomicrons obtained from rats given different fats or oils by gavage was investigated by vortexing-induced aggregation and hydrophobic interaction chromatography. Contrary to LDL, vortexing did not cause aggregation in chylomicrons. Vortexing of fish oil and butter chylomicrons resulted in more prominent reduction in absorbances compared with chylomicrons from other sources that might indicate less micelle stability. Hydrophobic interaction chromatography of fish oil, palm oil and olive oil chylomicrons yielded three fractions, whereas that of sunflower, margarine and butter chylomicrons gave rise to two fractions. These results suggest that surface hydrophobicity of chylomicrons might be heterogenous. Our results also demonstrate that fish oil chylomicrons have less hydrophobicity and lower stability against vortexing compared with chylomicrons from other sources. Considering beneficial effects of fish oil in cardiovascular health, less hydrophobicity together with lower stability might provide an additional atherogeneicity index for lipoproteins.

Lipid loading of human vascular smooth muscle cells induces changes in tropoelastin protein levels and physical structure

Aggregated low-density lipoprotein (agLDL), one of the main LDL modifications in the arterial intima, contributes to massive intracellular cholesteryl ester (CE) accumulation in human vascular smooth muscle cells (VSMC), which are major producers of elastin in the vascular wall. Our aim was to analyze the levels, physical structure, and molecular mobility of tropoelastin produced by agLDL-loaded human VSMC (agLDL-VSMC) versus that produced by control VSMC. Western blot analysis demonstrated that agLDL reduced VSMC-tropoelastin protein levels by increasing its degradation rate. Moreover, our results demonstrated increased levels of precursor and mature forms of cathepsin S in agLDL-VSMC. Fourier transform infrared analysis revealed modifications in the secondary structures of tropoelastin produced by lipid-loaded VSMCs. Thermal and dielectric analyses showed that agLDL-VSMC tropoelastin has decreased glass transition temperatures and distinct chain dynamics that, in addition to a loss of thermal stability, lead to strong changes in its mechanical properties. In conclusion, agLDL lipid loading of human vascular cells leads to an increase in cathepsin S production concomitantly with a decrease in cellular tropoelastin protein levels and dramatic changes in secreted tropoelastin physical structure. Therefore, VSMC-lipid loading likely determines alterations in the mechanical properties of the vascular wall and plays a crucial role in elastin loss during atherosclerosis.

Pluronic block copolymers inhibit low density lipoprotein self-association

Little is known about exogenous inhibitors of low-density lipoprotein (LDL) aggregation. The search for nontoxic and bioavailable inhibitors of LDL aggregation is of interest, especially considering that the suppression of the aggregation of LDL might represent a therapeutic approach. We hypothesized that amphiphilic copolymers of propylene oxide and ethylene oxide, the so-called Pluronic block copolymers, can be used to influence the aggregation of LDL. In this work we used Pluronic® P85, L61 and F68. A comparative study of the effects of Pluronic block copolymers with various hydrophilic-lipophilic properties on the aggregation process of LDL showed that Pluronic copolymers with strong hydrophobic properties (P85 and L61) at concentrations close to or greater than the respective critical concentration of micelle formation inhibited the aggregation process of LDL; however, the "hydrophilic" Pluronic F68 had no effect on the aggregation of LDL at any concentration. Thus, the study demonstrated for the first time that Pluronic® block copolymers inhibit LDL self-association. The possibility of modulating the aggregation of LDL by various Pluronic copolymers can be regarded as a prerequisite in the creation of new types of anti-atherosclerotic drugs.

Deglycosylation of apo B-containing lipoproteins increase their ability to aggregate and to promote intracellular cholesterol accumulation in vitro

Sub-fractions of all apo B-100 containing lipoproteins (low density lipoproteins, very low density lipoproteins and intermediate density lipoproteins) with reduced contents of sialic acid were found in vivo in human blood. These lipoproteins were inclined to spontaneously form aggregates and were able to stimulate accumulation of cholesterol in cells cultured from human aortic intima. In vitro treatment of apo B-containing lipoproteins with 2,6- and 2,3-specific sialidases, alpha-mannosidase, endoglycosidases F1 or F2 or peptide-N-glycanase F also stimulated aggregation and increased the ability of these particles to potentiate cholesterol accumulation in cells of the intact human aortic intima. So, deglycosylation of various apo B-containing lipoproteins possibly occurs in the blood, decreases their resistance to aggregation and increases the ability of these particles to stimulate accumulation of cholesterol in human aortic intima cells, thereby increasing their atherogenic potential.

Oxidation-induced aggregation of LDL increases their uptake by smooth muscle cells from human aorta

Oxidative modification of human blood LDL induced by Cu2+, NaOCl, or 2,2-azobis-(2-aminopropane hydrochloride) was followed by their partial aggregation. Separation of oxidized LDL into aggregates and nonaggregated particles showed that they are characterized by a similar degree of oxidative modification. In contrast to nonaggregated particles, LDL aggregates in the same concentration significantly increased cholesterol content in smooth muscle cells from the intact (no involoved in atherosclerosis) human aortic intima. Our results indicate that atherogenicity of LDL oxidized by various factors is mainly associated with the formation of aggregates, but does not depend on the degree of oxidative modification.

Sialic acid: a novel marker of cardiovascular disease?

The global burden posed by cardiovascular disease (CVD), due to a rising incidence of known risk factors, underlines an urgent need to identify other potential risk factors. Sialic acid (SA), an abundant terminal monosaccharide of glycoconjugates, is a possible risk factor for CVD. Although large-scale epidemiological surveys have shown that serum total sialic acid (TSA) is positively associated with mortality from coronary artery disease (CAD) and stroke, studies investigating the correlation between serum TSA and the severity of atherosclerosis are conflicting. Clinical and epidemiological studies indicate that serum TSA is a marker of a sustained inflammatory response in CVD, rather than causal in nature. Data also indicates ethnic variation in baseline TSA. This article reviews current methods for determining serum TSA and evidence supporting serum TSA as a risk factor for CVD. Potential mechanisms for this role are examined. The use of serum TSA as a marker of atherosclerotic disease is evaluated.

Phospholipid Hydrolysis with Phospholipases A2 and C Impairs Apolipoprotein B-100 Conformation on the Surface of Low Density Lipoproteins by Reducing Their Association Resistance

Modification of apolipoprotein B-100 conformation on the surface of LDL isolated from human blood was demonstrated by enzyme immunoassay with a panel of monoclonal antibodies to this protein. The study by the light transmission fluctuation method showed that incubation of LDL with phospholipases A2 or C led to association of LDL particles. This lipolytic modification seems to impair LDL surface properties inducing association of these particles, which can play an important role in lipid accumulation in the vascular wall and at early stages promote the development of atherosclerosis.

Lipoprotein aggregation protects human monocyte-derived macrophages from OxLDL-induced cytotoxicity

Oxidative modifications render low density lipoprotein cytotoxic and enhance its propensity to aggregate and fuse into particles similar to those found in atherosclerotic lesions. We showed previously that aggregation of oxidized LDL (OxLDL) promotes the transformation of human macrophages into lipid-laden foam cells (Asmis, R., and J. Jelk. 2000. Large variations in human foam cell formation in individuals. A fully autologous in vitro assay based on the quantitative analysis of cellular neutral lipids. Atherosclerosis. 148: 243-253). Here, we tested the hypothesis that aggregation of OxLDL enhances its clearance by human macrophages and thus may protect macrophages from OxLDL-induced cytotoxicity. We found that increased aggregation of OxLDL correlated with decreased macrophage injury. Using 3H-labeled and Alexa546-labeled OxLDL, we found that aggregation enhanced OxLDL uptake and increased cholesteryl ester accumulation but did not alter free cholesterol levels in macrophages. Acetylated LDL was a potent competitor of aggregated oxidized LDL (AggOxLDL) uptake, suggesting that scavenger receptor A plays an important role in the clearance of AggOxLDL. Inhibitors of actin polymerization, cytochalasin B, cytochalasin D, and latrunculin A, also prevented AggOxLDL uptake and restored OxLDL-induced cytotoxicity. This suggests that OxLDL-induced macrophage injury does not require OxLDL uptake and may occur on the cell surface. Our data demonstrate that aggregation of cytotoxic OxLDL enhances its clearance by macrophages without damage to the cells, thus allowing macrophages to avoid OxLDL-induced cell injury.

Resistance of native and circulating modified low-density lipoproteins in human blood to association

The resistance of native and circulating modified low-density lipoproteins from human blood to spontaneous and polyethylene glycol-induced association was studied by recording light transmission fluctuations. Circulating modified low-density lipoproteins were less resistant to association than native low-density lipoproteins. Polyethylene glycol-induced association of low-density lipoproteins was irreversible. Our results suggest that atherogenic activity of circulating modified low-density lipoproteins is associated with their increased predisposition to irreversible association.

Low ionic strength promotes association of circulating modified LDL human blood

The resistance to association of circulating multiply-modified low-density lipoproteins (LDL) isolated from human blood and characterized by a decreased content of sialic acids in comparison with native LDL was studied by analysing light transmission fluctuations. LDL association was stimulated by decreasing environmental ionic strength. It is established that circulating modified LDL are less resistant to association than native LDL. Association of LDL in a medium with low ionic strength was irreversible. Probably, increased capacity to irreversible association determines the atherogenic properties of circulating modified LDL subfraction.

Retention of aggregated LDL by cultured human coronary artery endothelial cells

Aggregated LDL (AgLDL) accumulates within the subendothelial space of developing atherosclerotic lesions. We were interested to learn whether endothelial cells can interact with AgLDL. Incubation of endothelial cells with AgLDL resulted in apparent cholesterol retention. Microscopic examination revealed that cholesterol retention resulted mainly from endothelial cell surface attachment of AgLDL. Little AgLDL entered endothelial cells consistent with the small amount of endothelial cell degradation of AgLDL. Although endothelial cell retention of AgLDL was inhibited by LDL, AgLDL retention was not blocked by lactoferrin, C7 anti-LDL receptor monoclonal antibody, or receptor-associated protein, suggesting that LDL receptor family members did not mediate this retention. Surface retention of AgLDL depended on microtubule function and could be regulated by the protein kinase C activator, PMA. Treatment of endothelial cells with PMA either before or during, but not after incubation with AgLDL, inhibited retention of AgLDL. Our findings show that endothelial cells can retain AgLDL but internalize and metabolize little of this AgLDL. Thus, it is unlikely that endothelial cells can transport AgLDL out of atherosclerotic lesions, but it is likely that retention of AgLDL affects endothelial function.

Hydrophobic nature of rat lymph chylomicrons

BACKGROUND

A typical molecular structure of a lipoprotein is composed of hydrophobic lipids at the core and hydrophilic apolipoprotein side chains and lipid head groups at the surface. Some of the hydrophobic characteristics of rat lymph chylomicrons were investigated.

METHODS

Thoracic duct was cannulated and lymph was collected overnight. Chylomicrons (>100 nm) were isolated by ultracentrifugation at 4 x 10(6)xg min. Since particle aggregation is a characteristic of hydrophobic nature of lipoproteins, as an index of aggregation, the turbidity generated by vortexing and storage of chylomicrons was measured spectrophotometrically at 680 nm. We also assessed the ability of chylomicrons to interact with five different hydrophobic interaction chromatography (HIC) media.

RESULTS

Neither shaking nor prolonged storage at 4 degrees C produced an increase in the optical density of chylomicron solution indicating no aggregation took place. Typical elution profiles of chylomicrons through octyl, phenyl (high substance) and butyl sepharose columns showed two peaks. Peak I material emerged with 4 mol/l NaCl in a position corresponding to the void volume and peak II material eluted with water. Phenyl sepharose (high performance) media exhibited the maximum binding strength towards chylomicrons among the five different media. In the case of phenyl sepharose (low substance) column, an additional material was eluted with 3 mol/l NaCl between peaks I and II. These results indicate the heterogeneity of chylomicron surface hydrophobicity.

CONCLUSION

Since particle aggregation is a characteristics of hydrophobicity of lipoproteins and believed to be an underlying cause of atherosclerosis, fractionation of lipoproteins by hydrophobic interaction chromatography may introduce a new approach into the assessment of lipoprotein atherogeneicity.

Effect of fluid mechanical stresses and plasma constituents on aggregation of LDL

LDL aggregates when exposed to even moderate fluid mechanical stresses in the laboratory, yet its half-life in the circulation is 2-3 days, implying that little aggregation occurs. LDL may be protected from aggregation in vivo by components of plasma, or by a qualitative difference in flows. Previous studies have shown that HDL and albumin inhibit the aggregation induced by vortexing. Using a more reproducible method of inducing aggregation and assessing aggregation both spectrophotometrically and by sedimentation techniques, we showed that at physiological concentrations, albumin is the more effective inhibitor, and that aggregation is substantially but not completely inhibited in plasma. Heat denatured and fatty-acid-stripped albumin were more effective inhibitors than normal albumin, supporting the idea that hydrophobic interactions are involved. Aggregation of LDL in a model reproducing several aspects of flow in the circulation was 200-fold slower, but was still inhibited by HDL and albumin, suggesting similar mechanisms are involved. Within the sensitivity of our technique, LDL aggregation did not occur in plasma exposed to these flows. Thus, as a result of the characteristics of blood flow and the inhibitory effects of plasma components, particularly albumin, LDL aggregation is unlikely to occur within the circulation.

Macrophage-expressed group IIA secretory phospholipase A2 increases atherosclerotic lesion formation in LDL receptor-deficient mice

OBJECTIVE

Transgenic mice expressing human group IIA secretory phospholipase A(2) (group IIA sPLA(2)) spontaneously develop atherosclerotic lesions. The mechanism for this proatherogenic effect is likely multifactorial, because HDL-cholesterol is significantly lower and LDL/VLDL cholesterol is slightly higher in transgenic mice compared with nontransgenic littermates. In the present study, we show for the first time that elicited peritoneal macrophages from transgenic mice express human group IIA sPLA(2). This study tested whether macrophage-expressed sPLA(2) contributes to atherogenesis.

METHODS AND RESULTS

Bone marrow cells from either sPLA(2) transgenic mice or control C57BL/6 mice were transplanted into LDL receptor-deficient mice. After hematopoietic engraftment, animals were fed a diet enriched with saturated fat and cholesterol for 12 weeks. Despite a lack of effect on serum lipoprotein concentrations, the presence of bone marrow-derived cells expressing human group IIA sPLA(2) resulted in a significant increase in the extent of atherosclerosis in the aortic arch (12.8+/-1.4% versus 7.4+/-0.9%; P<0.005) and aortic sinus (0.3+/-0.03 mm(2) versus 0.2+/-0.04 mm(2); P<0.05).

CONCLUSIONS

Group IIA sPLA(2) can contribute to atherosclerotic lesion development through a mechanism that is independent of systemic lipoprotein metabolism.

Association between serum total and lipid-bound sialic acid concentration and the severity of coronary atherosclerosis

Serum total sialic acid has recently been shown to be a cardiovascular risk factor. Increased levels of this substance are associated with higher cardiovascular mortality and with cerebrovascular disease. It has also been shown that serum concentrations of total and lipid-associated sialic acid are significantly increased in hypertriglyceridemia. On the other hand, several circulating lipoproteins have been suggested to be related to the severity of coronary atherosclerosis, but contradictory results have been reported in the possible relationship between the concentrations of sialic acid and the severity of coronary lesions in patients with coronary heart disease. The purpose of this study was to investigate the possible relationship between serum total sialic acid concentration, recently shown to be a cardiovascular risk factor, and serum lipid-bound sialic acid concentration and the severity of coronary lesions. The study comprised 90 subjects, divided into three subgroups according to angiography results: 30 patients with no vessel disease, 30 patients with single-vessel disease, and 30 patients with double/triple-vessel disease. Serum total sialic acid determination was carried out with the thiobarbituric acid method of Warren; lipid-associated sialic acid was assayed with the method of Katopodis. Mean serum total sialic acid levels in patients with single-vessel disease (P <.05) and patients with double/triple-vessel disease (P <.001) were found to be significantly increased compared with that in patients with no vessel disease, whereas mean serum lipid-bound sialic acid levels were found to be significantly different between patients with double- or triple-vessel disease and patients with no vessel disease (P <.001). We also noted a significant difference between the levels of serum total sialic acid (P <.001) and lipid-bound sialic acid (P <.001) in patients with single-vessel disease and patients with double/triple-vessel disease. We found a significant correlation only between serum lipid-bound sialic acid and coronary angiographic score in patients with double/triple-vessel disease (r = 0.425, P <.05). Although the concentration of serum total sialic acid is increased proportionally with the number of diseased coronary arteries, only the concentration of serum lipid-bound sialic acid is related to the severity of coronary atherosclerosis, especially in patients with double/triple-vessel disease.

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.

Enhancement of macrophage survival and DNA synthesis by oxidized-low-density-lipoprotein (LDL)-derived lipids and by aggregates of lightly oxidized LDL

Human atherosclerotic plaque contains a partially characterized range of normal and oxidized lipids formed mainly from free and esterified cholesterol and phospholipids, some of which can be located in macrophage-derived "foam" cells. Oxidation of low-density lipoprotein (LDL) is often considered as an important event leading to subsequent foam-cell development, which may also include enhanced cell survival and/or proliferation. The active component(s) in oxidized LDL (ox.LDL) causing macrophage proliferation is debated. We report here that the lipid component of ox.LDL can promote macrophage survival and DNA synthesis, the latter response showing a synergistic effect in the presence of low concentrations of macrophage colony-stimulating factor. 7-Ketocholesterol showed some stimulation of macrophage DNA synthesis whereas hypochlorite-oxidized (i.e. apolipoprotein B-oxidized) LDL did not. Plaque-derived lipids could enhance macrophage survival. It has not been proven that LDL in lesions is oxidized sufficiently to be the dominant source of sterols in vivo or to be able to induce macrophage growth in vitro or in vivo; it has been suggested that aggregation of modified LDL in vivo is an important step in the deposition of intracellular lipid. We found that aggregation of lightly oxidized LDL potentiated dramatically its ability to stimulate macrophage DNA synthesis, indicating that extensive oxidation of LDL is not required for this response in vitro and perhaps in vivo.

LDL receptor-related protein mediates uptake of aggregated LDL in human vascular smooth muscle cells

Foam cell formation is a key event in the onset and progression of atherosclerotic lesions. We have previously reported that internalization of aggregated low density lipoproteins (agLDLs) by vascular smooth muscle cells (VSMCs) produces cholesteryl ester (CE) accumulation in these cells. The aim of this study was to analyze whether the low density lipoprotein receptor-related protein (LRP) mediates the uptake of agLDL by VSMCs. First, immunocytochemistry and fluorescence microscopic analysis with the use of anti-LRP antibodies indicated that there was a high expression of LRP in VSMCs. Confocal microscopic analysis with the use of agLDLs labeled with fluorochrome 1,1'-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine and anti-LRP antibodies showed the colocalization of agLDL and LRP. The second approach was to analyze the effect of LRP ligands on agLDL internalization; lactoferrin strongly inhibited CE accumulation from agLDLs (85.0+/-5.7% at 25 microg/mL) by impairing agLDL binding. Coincubation of agLDL with anti-LRP antibodies decreased in a dose-dependent manner agLDL-derived CE accumulation (from 20% at 12.5 microg/mL to 80% at 50 microg/mL). The third approach was to evaluate whether antisense LRP oligodeoxynucleotides were able to block agLDL internalization. Treatment of VSMCs with 5 micromol/L antisense LRP oligodeoxynucleotides reduced agLDL-derived CE accumulation by 84+/-2%. In conclusion, these results from immunologic, biochemical, and molecular interventions demonstrate that LRP mediates the binding and internalization of agLDL in human VSMCs. Because LRP is highly expressed in VSMCs and the uptake of 1 LDL aggregate amounts to the deposition of several hundreds of LDL particles, the uptake of agLDL through LRP could have a crucial role for lipid deposition in VSMCs.

Macrophage-released proteoglycans enhance LDL aggregation: studies in aorta from apolipoprotein E-deficient mice

Aggregated low-density lipoprotein (LDL) was shown to be present in the atherosclerotic lesion, but the mechanism responsible for its formation in vivo is not known yet. To find out whether LDL aggregation occurs in the arterial wall during atherogenesis, LDLs were extracted from the aortas of apolipoprotein E-deficient (E(0)) mice during their aging (and the development of atherosclerosis), and were analyzed for their aggregation states, in comparison to LDLs isolated from aortas of control mice. LDL isolated from aortas of E(0) mice was already aggregated at 1 month of age and its aggregation state substantially increased with age, with 3-fold elevation at 6 months of age compared to younger, 1-month-old, mice. Only minimal aggregation could be detected in LDL derived from control mice. Electron microscopy examination revealed that LDL particles from aortas of the E(0) mice were heterogeneous in their size, ranging between 20 and 300 nm. The mouse aortic LDL contained proteoglycans (PGs) and their content increased with the age of the mice, with about 2-fold higher levels than those found in LDLs derived from aortas of control mice. Macrophage-released PGs were previously demonstrated to enhance LDL aggregation in vitro. However, their involvement in LDL aggregation in vivo has not been studied yet. Thus, we next studied the effect of arterial macrophage-released PGs on the susceptibility of plasma LDL to aggregation by Bacillus cereus sphingomyelinase (SMase). Foam cell macrophages were isolated from aortas of the atherosclerotic E(0) mice at 6 months of age and were found to be loaded with cholesterol and to contain oxidized lipids. To analyze the effect of macrophage-released PGs on LDL aggregation, PGs were prelabeled by cell incubation with [35S]sulfate, followed by incubation of macrophage-released PGs with E(0) mouse plasma LDL (200 microg protein/ml) for 1 h at 37 degrees C. [35S]Sulfated PGs were found to be LDL-associated and the susceptibility of PG-associated LDL to aggregation by SMase was increased by up to 45% in comparison to control LDL. Similar results demonstrating the involvement of PGs in LDL aggregation were obtained upon incubation of LDL with increasing concentrations of PGs that were isolated from the entire aorta of E(o) mice (rather than the isolated macrophages). The stimulatory effect of macrophage-released PGs on LDL aggregation was markedly reduced when the PGs were pretreated with the glycosaminoglycan-hydrolyzing enzymes, chondroitinase ABC or chondroitinase AC, and to a much lesser extent with heparinase. We thus conclude that macrophage-released chondroitin sulfate PG can contribute to the formation of atherogenic aggregated LDL in the arterial wall.

Large variations in human foam cell formation in individuals: a fully autologous in vitro assay based on the quantitative analysis of cellular neutral lipids

The transformation of monocyte-derived macrophages into lipid-laden foam cells constitutes a characteristic and crucial event in the development of the earliest atherosclerotic lesions. We investigated whether the propensity to form foam cells varies among individuals. We developed a fully autologous foam cell assay based on a recently developed novel culture technique for human monocyte-derived macrophages (Wintergerst ES, Jelk J, Asmis, R. Differential expression of CD14, CD36 and the LDL receptor on human monocyte-derived macrophages. A novel cell culture system to study macrophage differentiation and heterogeneity, Histochem. Cell Biol. 1998;110:231-241). Thin layer chromatography and laser densitometry were used to determine cholesterol, triglyceride and cholesteryl ester levels in human macrophages. Aggregated LDL obtained by vortexing was found to be a reproducible stimulus of foam cell formation in human macrophages. In our hands, Cu(2+)-oxidized LDL also induced cholesteryl ester accumulation, but only when vortexed. We found that foam cell formation in an individual varied by less than 25% over a 10-month period. In contrast, we observed a sevenfold difference in foam cell formation among eight male volunteers. The transfer of foam cells into culture medium with freshly thawed autologous serum resulted in a 75% regression within 1 week, independent of the amount of cellular cholesteryl esters accumulated. Foam cell formation correlated neither to serum nor to cellular cholesterol and triglyceride levels. The propensity to form foam cells could therefore represent a novel indicator of individual risk of atherogenesis.

Unique cellular events occurring during the initial interaction of macrophages with matrix-retained or methylated aggregated low density lipoprotein (LDL). Prolonged cell-surface contact during which ldl-cholesteryl ester hydrolysis exceeds ldl protein degradation

A critical event in atherogenesis is the interaction of arterial wall macrophages with subendothelial lipoproteins. Although most studies have investigated this interaction by incubating cultured macrophages with monomeric lipoproteins dissolved in media, arterial wall macrophages encounter lipoproteins that are mostly bound to subendothelial extracellular matrix, and these lipoproteins are often aggregated or fused. Herein, we utilize a specialized cell-culture system to study the initial interaction of macrophages with aggregated low density lipoprotein (LDL) bound to extracellular matrix. The aggregated LDL remains extracellular for a relatively prolonged period of time and becomes lodged in invaginations in the surface of the macrophages. As expected, the degradation of the protein moiety of the LDL was very slow. Remarkably, however, hydrolysis of the cholesteryl ester (CE) moiety of the LDL was 3-7-fold higher than that of the protein moiety, in stark contrast to the situation with receptor-mediated endocytosis of acetyl-LDL. Similar results were obtained using another experimental system in which the degradation of aggregated LDL protein was delayed by LDL methylation rather than by retention on matrix. Additional experiments indicated the following properties of this interaction: (a) LDL-CE hydrolysis is catalyzed by lysosomal acid lipase; (b) neither scavenger receptors nor the LDL receptor appear necessary for the excess LDL-CE hydrolysis; and (c) LDL-CE hydrolysis in this system is resistant to cellular potassium depletion, which further distinguishes this process from receptor-mediated endocytosis. In summary, experimental systems specifically designed to mimic the in vivo interaction of arterial wall macrophages with subendothelial lipoproteins have demonstrated an initial period of prolonged cell-surface contact in which CE hydrolysis exceeds protein degradation.