Marked alteration of proteoglycan metabolism in cholesterol-enriched human arterial smooth muscle cells

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.

The extracellular matrix on atherogenesis and diabetes-associated vascular disease

Atherosclerosis is remarkably increased in type 2 diabetes suggesting that mechanisms causing arterial lesion are enhanced by the metabolic disturbances of insulin resistance (IR) and diabetes. Several lines of research suggest that processes taking place in the arterial intima extracellular matrix may be part of a shared pathogenic mechanism. The intima extracellular matrix is where atherogenesis takes place. This layer contains fibrilar macromolecules like collagens, proteoglycans (PGs), hyaluronate, and extracellular multi-domain proteins. Specific interaction of lysine, arginine-rich segments of the apoB-100 lipoproteins, LDL, IDL and Lp (a), with the negatively charged glycosaminoglycans (GAGs) of PGs cause retention of the lipoproteins, one of the initiation process of atherogenesis. Such interactions cause structural modifications of the lipid and protein moieties of the lipoproteins that appear to increase their susceptibility to proteases, phospholipases and free radical-mediated processes. The association of apoB-lipoproteins, specially small and dense LDL, with intima PGs increases their uptake by macrophages and human arterial smooth muscle cells (HASMC) leading to 'foam cell' formation. In vitro, elevated levels of non-esterified fatty acids (NEFA) alter the matrix of endothelial cells basement membrane making them more permeable to macromolecules. NEFA cause changes in the expression of genes controlling the PGs composition of the PGs secreted by HASMC causing formation of a matrix with high affinity for LDL. These results lead us to speculate that an important component of the dyslipidemia of IR and type 2 diabetes, chronic high NEFA, may contribute to cellular alterations that cause changes of the arterial intima extracellular matrix. Such changes may increase the atherogenicity of the retention of apoB lipoproteins in the intima and contribute to the systemic alteration of the arterial wall frequently observed in IR and type 2 diabetes.

Lipid mediators that modulate the extracellular matrix structure and function in vascular cells

Treatment with moderate levels of albumin-bound, nonesterified fatty acids (NEFA) induce important alterations of the structure and functionality of proteoglycans secreted by endothelial cells and arterial smooth muscle cells. In endothelial cell monolayers, the reduction on relative amount and sulfation of heparan sulfate proteoglycans is associated with an increased permeability to albumin. In smooth muscle cells, NEFA-albumin complex increased the expression of the genes for the core proteins of the proteoglycans syndecan, decorin and perlecan. This effect appears mediated by peroxisome proliferator-activated receptor gamma (PPARg). The matrix produced by the cells treated with NEFA-albumin had a higher affinity with low-density lipoproteins (LDLs). We speculate about the possibility that under dyslipidemias associated with increased exposure of vascular cells to NEFA, like in type 2 diabetes, similar alterations may contribute to associated macrovascular and microvascular complications.

Effect of hypoxia and hypoxia/reoxygenation on proteoglycan metabolism by vascular smooth muscle cells

Hypoxia and hypoxia/reoxygenation are known to affect vascular smooth muscle cell physiology. In this study, we first investigated proteoglycan synthesis by human aortic smooth muscle cells exposed to normoxia, hypoxia, or hypoxia/reoxygenation. We then compared the newly synthesized proteoglycans from normoxic and hypoxic-reoxygenation cultures for their ability to bind low density lipoprotein (LDL). Confluent smooth muscle cells under normoxia, hypoxia, or hypoxia/reoxygenation were pulsed with [35S]sulfate, and secreted and cell-associated proteoglycans were analyzed. Secreted proteoglycans in cultures exposed to hypoxia (4 h)/reoxygenation (19 h) increased 28% over those of cells continuously exposed to normoxia. Cell-associated proteoglycans did not differ significantly between the two groups. In contrast, hypoxia (4 h) followed by a 30-min reoxygenation produced a 37% decrease in newly synthesized proteoglycans. Hypoxia alone also resulted in a 24% decrease in secreted proteoglycans and a 20% decrease in cell-associated proteoglycans. Proteoglycans newly synthesized by smooth muscle cells exposed to normoxia and hypoxia/reoxygenation did not differ in their charge densities and molecular size but did differ in glycosaminoglycan composition. Exposure of smooth muscle cells to hypoxia/reoxygenation produced a 60% increase in a proteoglycan subfraction that bound LDL with very high affinity. The incorporation of [3H]leucine into total cellular protein decreased significantly following exposure of smooth muscle cells to hypoxia as well as hypoxia/reoxygenation. These results indicate that hypoxia and hypoxia/reoxygenation cause major alterations in proteoglycan metabolism by vascular smooth muscle cells.

Macrophage released proteoglycans are involved in cell-mediated aggregation of LDL

Aggregated low density lipoprotein (LDL) is taken up by macrophages at enhanced rate, leading to macrophage cholesterol accumulation and foam cell formation. Since macrophages were shown to mediate self aggregation of modified forms of LDL, we sought to study the effect of macrophages on the susceptibility of native LDL to aggregation. Incubation of LDL (100 microg of protein/ml) with J-774A.1 macrophage-like cell line for 18 h at 37 degrees C, led to a 114 and 56% enhanced susceptibility of LDL to aggregation by vortexing and by Bacillus cereus SMase respectively. Macrophage conditioned media (MCMs) that were obtained from J-774A.1 cells also enhanced the susceptibility of LDL to aggregation by vortexing and SMase by 134 and 75% respectively, suggesting the involvement of macrophage secretory products in the enhanced aggregation of LDL. As proteoglycans were shown to be involved in lipoprotein aggregation, we analyzed the possible involvement of macrophage-released proteoglycans in LDL aggregation. Incubation of LDL (100 microg protein/ml) with 25 microg of proteoglycans that were isolated from MCM led to a dose-dependent enhanced susceptibility of LDL to aggregation by vortexing or by SMase by up to 62 and 77% respectively. The stimulatory effect of the MCMs on LDL aggregation was markedly reduced upon MCMs treatment with the glycosaminoglycan hydrolyzing enzyme chondroitinase ABC, chondroitinase AC, but not heparinase. On the contrary, incubation of LDL (100 microg of protein/ml) with increasing concentrations (up to 50 microg/ml) of chondroitin sulfate, or heparan sulfate enhanced the susceptibility of LDL to aggregation by up to 98 or by only 18% respectively, in comparison with non-treated LDL. Since macrophages under atherogenic conditions (cholesterol-loading, cellular lipid peroxidation and activation) demonstrate enhanced secretion of proteoglycans, we finally studied the effect of J-774A.1 macrophages on the susceptibility of native LDL to aggregation under the above atherogenic conditions. Incubation of LDL with cholesterol-loaded macrophages led to a 62% enhanced susceptibility of LDL to undergo aggregation by vortexing, in comparison with LDL that was incubated with non-loaded cells. Macrophage activation with phorbol myristate acetate (5 microM of PMA) also significantly increased cell-mediated aggregation of LDL by 50%, in comparison with non-activated cells. Lipid peroxidized macrophages obtained by cell treatment with either FeSO4 (50 microM), or angiotensin II (10(-7) M) enhanced the susceptibility of LDL to aggregation by 22 or by 39% respectively. These results suggest that under atherogenic conditions, macrophages release proteoglycans, and mainly chondroitin sulfate, which can contribute to cell-mediated formation of aggregated LDL, a potent inducer of macrophage foam cells which are the hallmark of early atherogenesis.

Human chondroitin 6-sulfotransferase: cloning, gene structure, and chromosomal localization

Chondroitin 6-sulfotransferase (C6ST) is the key enzyme in the biosynthesis of chondroitin 6-sulfate, a glycosaminoglycan implicated in chondrogenesis, neoplasia, atherosclerosis, and other processes. C6ST catalyzes the transfer of sulfate from 3'-phosphoadenosine 5'-phosphosulfate to carbon 6 of the N-acetylgalactosamine residues of chondroitin. Based on the previously published avian sequence, we searched the database of expressed sequence tags (dbEST) and obtained partial-length cDNAs that we completed by 5'-RACE using human chondrosarcoma and endothelial-cell RNA as template. Stable transfection of our full-length expression construct into CHO-K1 cells resulted in marked increases in C6ST and keratan sulfate sulfotransferase (KSST) enzymatic activities in cell homogenates. The predicted 411 amino acid sequence of human C6ST contains an N-terminal hydrophobic domain consistent with membrane insertion, four potential sites for N-linked glycosylation, several consensus sequences for protein phosphorylation, and one RGD sequence. The human and chick C6ST cDNA share 51% nucleotide identity, 40% amino acyl identity, and 75% amino acyl conservation. The human C6ST gene structure has been elucidated and exhibits an intron-less coding region, and the gene has been mapped to human chromosome 11 by radiation hybrid panel mapping.

Age-related changes in populations of aortic glycosaminoglycans: species with low affinity for plasma low-density lipoproteins, and not species with high affinity, are preferentially affected

Glycosaminoglycans were extracted from the intima and media layers of normal human thoracic aortas from donors of different ages. The arterial segments were devoid of macroscopically visible lesions obtained from patients who had no clinically evident cardiovascular disease. Total glycosaminoglycan content increases during the first 40 years of life. Changes in the content of hyaluronic acid and heparan sulfate are less noticeable. The content of chondroitin sulfate (mainly the 6-isomer) increases, whereas dermatan sulfate remains constant. Plasma LDL-affinity chromatography of dermatan sulfate+chondroitin 4/6-sulfate fractions allowed the separation of LDL high- and low-affinity glycosaminoglycan species. Remarkably, only glycosaminoglycan species with low affinity for plasma LDL increase with age in the disease-free areas of human thoracic aortas studied. These results suggest that age-related changes in glycosaminoglycan composition of the arterial wall do not contribute to increased deposition of plasma LDL. However, the alternative explanation that individuals with arterial glycosaminoglycans that avidly bind LDL would develop early and severe cardiovascular disease and would thus be excluded from our analysis cannot be ruled out.

Role of macrophage glycosaminoglycans in the cellular catabolism of oxidized LDL by macrophages

Macrophage binding sites for oxidized LDL (Ox-LDL) include class A scavenger receptors (SR-As), the CD-36 molecule, and an additional but hitherto unidentified binding site. Because cell-surface glycosaminoglycans (GAGs) were previously shown to be involved in the cellular uptake of native LDL and lipoprotein(a), several strategies to assess the participation of heparan sulfate (HS) and chondroitin sulfate (CS) in macrophage catabolism of Ox-LDL were used. First, incubation of J-774 A.1 macrophage-like cells with either heparinase or chondroitinase, or with both enzymes together, reduced the binding, uptake, and degradation of 125I-Ox-LDL by 20% to 45%, in comparison with control nontreated cells, while catabolism of 125I-labeled acetylated LDL (Ac-LDL) and native LDL were unaffected. Second, the proteoglycan (PG) cellular content was increased by cell enrichment with exogenous GAGs or by using human monocyte-derived macrophages from two patients with Sanfilippo mucopolysaccharidosis, which are characterized by cellular HS accumulation. In these macrophages, cellular uptake of 125I-Ox-LDL increased, while catabolism of 125I-Ac-LDL and native LDL were unaffected. Experiments using conditioned media from control, heparinase-digested, or chondroitinase-digested macrophages indicated that neither secreted GAGs nor released digestion products played any role in Ox-LDL catabolism. To evaluate potential interactions between cell-surface GAGs and known receptors for Ox-LDL, we used excess unlabeled Ac-LDL to block SR-As or anti-CD-36 antibodies to block CD-36, and then examined the catabolism of 125I-Ox-LDL by GAG-enriched or -depleted macrophages. Both excess unlabeled Ac-LDL and anti-CD-36 antibodies reduced 125I-Ox-LDL catabolism, but only excess unlabeled Ac-LDL completely abolished the increase in 125I-Ox-LDL catabolism on GAG enrichment of the cells, indicating a cooperation between exogenous GAGs and cell-surface SR-As in the catabolism of OX-LDL. Moreover, the addition of GAGases to macrophages that were preincubated with anti-CD-36 antibodies and excess Ac-LDL further reduced macrophage degradation of Ox-LDL in comparison with cells that were pretreated only with anti-CD-36 antibodies and Ac-LDL, indicating a more complex role for endogenous GAGs. Overall, these studies demonstrate a substantial contribution of macrophage-associated GAGs in the catabolism of Ox-LDL, which is mediated in part by a cooperation between GAGs and cell-surface SR-As.

Changes in distribution of glycosaminoglycans during the progression of cholesterol induced atherosclerosis in Japanese quail

The temporal and spatial distribution and relative concentration of the proteoglycan glycosaminoglycan component were studied during the progression of atherosclerosis in the systemic arteries of Japanese quail selected for cholesterol induced atherosclerosis (CIA). The CIA quail were placed on either control or 0.5% added cholesterol diets at 3 months of age. The major systemic arteries (dorsal aorta, right and left brachiocephalic) were collected at 1- or 2-week intervals over the 10-week period of cholesterol feeding. In the cholesterol fed quail, alcian blue staining of the dorsal aorta showed elevations of glycosaminoglycans in regions of the artery with atherosclerotic plaque, beginning at the 6-week time point. By biochemical analysis, increases in glycosaminoglycan relative concentration was detected at the 10-week time point. In addition to the change in glycosaminoglycan relative concentration and distribution, the cholesterol fed animals also formed foam cells characteristic of atherosclerotic plaques. Therefore, the conclusion reached was that the CIA line of Japanese quail is a valid animal model for the study of alterations in proteoglycan metabolism in atherosclerotic plaques induced by hypercholesterolemia.

The proteoglycan decorin links low density lipoproteins with collagen type I

Decorin is a small dermatan sulfate-rich proteoglycan which binds to collagen type I in vitro and in vivo. In atherosclerotic lesions the contents of low density lipoprotein (LDL), decorin, and collagen type I are increased, and ultrastructural studies have suggested an association between LDL and collagen in the lesions. To study interactions between LDL, decorin, and collagen type I, we used solid phase systems in which LDL was coupled to a Sepharose column, or in which LDL, decorin, or collagen type I was attached to microtiter wells. The interaction between LDL and decorin in the fluid phase was evaluated using a gel mobility shift assay. We found that LDL binds to decorin by ionic interactions. After treatment with chondroitinase ABC, decorin did not bind to LDL, showing that the glycosaminoglycan side chain of decorin is essential for LDL binding. Acetylated and cyclohexanedione-treated LDL did not bind to decorin, demonstrating that both lysine and arginine residues of apoB-100 are necessary for the interaction. When collagen type I was attached to the microtiter plates, only insignificant amounts of LDL bound to the collagen. However, if decorin was first allowed to bind to the collagen, binding of LDL to the decorin-collagen complexes was over 10-fold higher than to collagen alone. Thus, decorin can link LDL with collagen type I in vitro, which suggests a novel mechanism for retention of LDL in collagen-rich areas of atherosclerotic lesions.