Cholesterol esterification and cholesteryl ester accumulation in cultured pigeon and monkey arterial smooth muscle cells

Reduced syndecan-4 expression in arterial smooth muscle cells with enhanced proliferation

Syndecans, a family of cell surface heparan sulfate (HS) containing proteoglycans (PGs), are known regulators of many biological processes including inhibition of smooth muscle cell (SMC) proliferation. Cultured arterial SMCs from atherosclerosis-susceptible White Carneau (WC) pigeons have increased proliferation rates and significant reductions in total cell-surface HS relative to atherosclerosis-resistant Show Racer (SR) SMC. Using a specific syndecan-4 cDNA, 1.5- to 2.0-fold reductions in gene expression were observed in WC SMC compared to SR SMC. Immunolocalization studies demonstrated reduced cell surface syndecan-4 protein in WC cells. Gene induction demonstrated that the reduction in syndecan-4 expression in WC cells was not due to reduced mRNA stability. Studies using cycloheximide to superinduce gene expression indicated transcriptional suppression of syndecan-4 in WC cells. The results suggest that reduced cell surface HS PG in WC arterial SMC can be explained, in part, by reductions in syndecan-4 gene expression. Differential transcriptional regulation of syndecan-4 in WC and SR cells provides a system to explore regulation of the syndecan-4 gene as well as the potential mechanisms by which syndecan-4 can exert a specific antiproliferative effect.

Characterization of alpha 2-macroglobulin receptor low density lipoprotein receptor-related protein (alpha 2 MR/LRP) in White Carneau pigeon peritoneal macrophages: its role in lipoprotein metabolism

White Carneau pigeons develop atherosclerosis naturally, and at an accelerated rate with cholesterol feeding. Macrophages play a central role in the pathogenesis of atherosclerosis in pigeons, as they do in man. The purpose of this study was to determine whether pigeon macrophages express the alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein (alpha 2 MR/LRP) and whether this receptor would recognize beta-VLDL, the major cholesterol-transporting lipoprotein in cholesterol-fed pigeons. The binding of 125I-methylamine-treated alpha 2M (125I-alpha 2 M+) at 4 degrees C was saturable (> 10 nM), specific, Ca2+ dependent, was competed for by the receptor-associated protein (RAP), and had a Kd of binding of 1-5.6 nM, similar to mouse peritoneal macrophages studied simultaneously. At 37 degrees C the bound 125I-alpha 2 M+ was rapidly internalized and degraded in lysosomes. The binding of alpha 2 M+ was not down-regulated with cholesterol loading, as is the LDL receptor on pigeon macrophages. At 4 degrees C there was no competition for binding of 125I-alpha 2 M+ by either pigeon or rabbit beta-VLDL, nor was binding of 125I-pigeon or rabbit beta-VLDL competed for by alpha 2 M+. Stimulation of cholesterol esterification by rabbit or pigeon beta-VLDL was unaffected by RAP, lactoferrin, or alpha 2 M+. Metabolism of 125I-pigeon or rabbit beta-VLDL was not competed by RAP, lactoferrin, or alpha 2 M+ even in the presence of lipoprotein lipase. Pigeon macrophages, and a 500 kDa membrane protein isolated from them, were recognized by several antihuman alpha 2 MR/LRP monoclonal antibodies. The 500 kDa membrane protein also bound 45Ca. These data suggest considerable sequence homology with the human alpha 2 MR/LRP. This is the first study to characterize a functional alpha 2 MR/LRP on peritoneal macrophages from an avian species. There was no evidence, however, that the alpha 2 MR/LRP mediates uptake of beta-VLDL by pigeon macrophages.

Macrophages stimulate cholesteryl ester accumulation in cocultured smooth muscle cells incubated with lipoprotein-proteoglycan complex

Foam cells of atherosclerotic lesions originate from both macrophages and smooth muscle cells (SMCs). We explored the mechanism by which SMCs may become lipid laden. Confluent bovine aortic SMCs were cocultured with P388D, macrophages, and the cocultures were incubated for various times with low-density lipoprotein (LDL), acetyl-LDL, or lipoprotein-proteoglycan (PG) complex isolated from human atherosclerotic lesions. Macrophages were then removed from the SMCs and the cholesteryl ester (CE) content of the SMCs was quantitated. Lipoprotein-PG complex but not LDL or acetyl-LDL produced a 6-fold to 9-fold stimulation of CE synthesis and a 4.4-fold increase in cellular CE mass in cocultured SMCs relative to control SMCs. In similar studies with human aortic SMC-macrophage cocultures, macrophages stimulated lipoprotein-PG complex-mediated CE synthesis 7-fold to 13-fold and CE mass 7.8-fold in cocultured SMCs compared with SMCs cultured alone. CE synthesis that was mediated by lipoprotein-PG complex was dose dependent and increased linearly with time. Incubation of lipoprotein-PG complex with SMC-macrophage cocultures but not with SMCs or macrophages alone resulted in aggregation of the complex and stimulation of cholesterol esterification in SMCs by the conditioned media containing the aggregated complex. Cytochalasin D, an inhibitor of phagocytosis, inhibited CE synthesis mediated by lipoprotein-PG complex by 73%, whereas polyinosinic acid, an inhibitor of the scavenger receptor, had no effect. Upregulation or downregulation of apolipoprotein B,E receptors did not affect the lipoprotein-PG complex-mediated CE synthesis by cocultured SMCs. Lipoprotein-PG complex did not stimulate CE synthesis in SMCs cocultured with aortic endothelial cells or macrophages cocultured with SMCs. These results indicate that macrophages can stimulate CE synthesis and accumulation in cocultured SMCs when incubated with lipoprotein-PG complexes isolated from atherosclerotic lesions. This could be a potential mechanism for myocyte foam cell formation.

Possible role of viruses in atherosclerosis

Relative to the role of viruses in the pathogenesis of atherosclerosis a number of questions remain to be answered. Being ubiquitous, are the herpesvirus infections the norm or a diseased state in humans and animals? In all cases studied, including MDV induced atherosclerosis, direct isolation of viruses from arterial tissues has not been successful. Are herpesviruses always latent or dormant in vascular tissues? What is the relationship between provirus and disease development? Are they setting the stage for the pathogenic process triggered by certain environmental factors? Is hypercholesterolemia to trigger the development of atherosclerosis in the presence of proviruses? Is the reactivated and infectious virus the injuring agent that initiates atherogenesis? Or, are the proviral genes activated to transform arterial cells? In the latter case, are these proviral genes equivalent to "proto-atherogenes"? Hopefully, further study on Japanese quail will help clarify many of these questions.

Assay and single dose pharmacokinetics of a novel systemic acyl coenzyme A cholesterol O-acyltransferase inhibitor, RP 73163, in rat plasma using automated solid-phase extraction with high-performance liquid chromatography

RP 73163, (S)-2-[5-(3,5-dimethylpyrazol-1-yl)pent-1-yl]sulphinyl-4,5 diphenylimidazole (I), is a highly potent in vitro and in vivo inhibitor of acyl coenzyme A cholesterol O-acyltransferase (ACAT) (E.C. 2.3.1.26), and as such it has potential therapeutic use as a cholesterol lowering agent in man. A method has been developed for the extraction and assay of I from rat plasma, using a fully automated solid-phase extraction column (ASPEC) technique, coupled to a reversed-phase HPLC system with detection by native fluorescence. The method has been validated over the concentration range 10-500 ng/ml, with demonstrated linearity, precision and accuracy, the mean limit of detection being 6.6 +/- 1.3 ng/ml. Application of the method to the assay of samples following administration of the compound to male and female rats is reported, together with determined pharmacokinetic parameters.

Toxicologic effects of a novel acyl-CoA:cholesterol acyltransferase inhibitor in cynomolgus monkeys

PD 132301-2, an acyl-CoA: cholesterol acyltransferase (ACAT) inhibitor, was administered orally to cynomolgus monkeys for 2 wk at doses of 25, 50, 100, and 200 mg/kg to assess potential subacute toxicity. Sporadic episodes of soft feces and diarrhea increased in incidence from 100 to 200 mg/kg. Histopathologic alterations in adrenocortical cells of treated monkeys consisted of a dose-related decrease in cytoplasmic fine vacuolation and an increase in cytoplasmic eosinophilia most conspicuous in the zona fasciculata and reticularis. At 50, 100, and 200 mg/kg, a narrow discontinuous zone of cytotoxic cortical cell degeneration occurred in the outer zona fasciculata. Decreased fine vacuolation of cortical cells correlated ultrastructurally with reduced size and number of intracellular lipid vacuoles and biochemically with a dose-related decrease in adrenal total cholesterol (from 56 to 13% of control) and cholesteryl ester (from 51 to 3% of control) concentrations. Other ultrastructural changes noted in zona fasciculata cortical cells at all doses were an apparent increase in both smooth endoplasmic reticulum and variably sized autophagic vacuoles. Ovarian corpora lutea in some females at all doses had increased coarse vacuolation of luteal cells, foci of cellular degeneration, increased numbers of cholesterol clefts, and slight infiltrates of mononuclear cells. Sebaceous glands were atrophic in all treated monkeys due largely to a reduction in size and number of differentiated foam cells. Sebaceous gland reserve cells were hypertrophic and hyperplastic. Toxicity data from this study indicated that PD 132301-2 at 25-200 mg/kg targeted cholesterol-rich cells of the adrenals, ovaries, and skin adnexa.

Influence of vascular smooth muscle heterogeneity on angiotensin converting enzyme activity in chicken embryonic aorta and in endothelial cells in culture

The smooth muscle of the abdominal region of the chicken aorta derives from locally recruited mesenchyme (mesenchymal smooth muscle), whereas that of the thoracic region derives from the neural crest (ectomesenchymal smooth muscle). We hypothesized that this smooth muscle heterogeneity might affect important enzymatic functions of the vessel wall. Therefore, we measured angiotensin converting enzyme (ACE) activity in homogenates of chicken thoracic and abdominal aorta at different embryonic stages (days 10, 14, and 18 of gestation). ACE activity increased in both regions over the time of gestation (p less than 0.001 in both cases); the increase was steeper and ACE activity was higher in thoracic than in abdominal segments (p less than 0.001). Km values were similar (approximately 7 microM) at all times and between the two segments, whereas changes in Vmax values closely paralleled those in enzyme activity, indicating gestation-dependent increases in the amount of enzyme. Neural crest ablation at an early developmental stage resulted in an increase of ACE activity in thoracic homogenates (p less than 0.001), predictably leaving that in abdominal homogenates unaffected. Bovine pulmonary artery endothelial cell monolayers exposed to media conditioned with cultured mesenchymal or ectomesenchymal smooth muscle cells exhibited elevated ACE activity (46% and 83%, respectively, relative to control medium, with p less than 0.01 in both cases; p less than 0.05 between the two media). Increases in endothelial cell ACE activity corresponded to proportional increases in ACE protein determined by enzyme-linked immunosorbent assay (r = 0.99) and were interpreted as indicative of enhanced enzyme synthesis subsequent to exposure of endothelial cells to smooth muscle-conditioned media.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term treatment of neonatal aortic smooth muscle cells with beta VLDL induces cholesterol accumulation

A model for smooth muscle derived foam cells was developed by treating smooth muscle cells isolated from the aortae of neonatal rabbits with beta VLDL for up to 1 month. Hyperlipidemic beta VLDL isolated from cholesterol fed rabbits induced proliferation of the cells that were maintained in lipid deficient serum. In addition, the lipoprotein fraction stimulated [14C]oleic acid incorporation into [14C]cholesteryl ester, even in cultures that had been chronically exposed to the lipoprotein. The accumulation of cholesterol was evaluated and small amounts of cholesteryl ester were demonstrated in cultures treated for 3 days with beta VLDL. However, continued exposure to the lipoprotein resulted in larger elevations in total cholesterol, approximately 65% of which was in the esterified form in cultures treated with 100 micrograms beta VLDL/ml for 24 days. When cholesterol levels were examined as a function of time, it was determined that both total cholesterol and cholesteryl ester levels increased. Approximately 2-3 weeks after lipoprotein was introduced to the culture, maximum levels were attained. Triglyceride levels were also measured and found to increase more than two-fold in cultures that had been incubated in the presence of beta VLDL for 24 days, when compared to cultures incubated in its absence. Examination of the cultures by electron microscopy revealed intracytoplasmic lipid droplets in beta VLDL treated cells. These results suggest that beta VLDL treatment of neonatal aortic smooth muscle cells provides an ideal model in which to study the lipid laden smooth muscle cells that characterize the atherosclerotic plaque.

Lysosomal lipid accumulation in vascular smooth muscle cells

Using an inverted culture technique, the accumulation of lipid within vascular smooth muscle cells incubated with lipid droplets was studied. Initially, lipid was found exclusively within cytoplasmic inclusions but, as accumulation continued, lysosomes became the predominant site of lipid storage. After 3 hr of incubation, 84% of lipid was within lysosomes. This lysosomal lipid accumulation produced a tripling of the average size of lysosomes and resulted in lysosomes with complex, multilobed shapes. In contrast, although the number of cytoplasmic inclusions increased with lipid loading, individual inclusions maintained a spherical shape and a consistent diameter of 1-1.3 microns. Concomitant with changes in cellular lipid storage, incubation with lipid droplets induced development of an anastomosing network of acid phosphatase-containing tubules which were spatially related to sites of lysosomal lipid accumulation. Thus lipid accumulation produced ultrastructural alterations in a number of metabolic compartments. Similar alterations in the intracellular compartmentalization of acquired lipid have been demonstrated in foam cells during atherogenesis and have been hypothesized to have profound effects on lipid metabolism and disease progression.

Lysosomal hydrolysis of lipids in a cell culture model of smooth muscle foam cells

Rabbit aortic smooth muscle cells take up lipid droplets when they are presented using an inverted culture technique. These droplets were localized in secondary lysosomes as demonstrated by staining for acid phosphatase. Initially, 69% of the cell volume was occupied by lipid, and 94% of the lipid was in lysosomes. After a 24-hr clearance period, the cell volume occupied by lipid decreased to 53%, although there was no change in the fraction of cell lipid that was in lysosomes. To confirm that hydrolysis of droplet lipid was occurring in lysosomes, cultures were exposed to medium containing Sandoz 58-035, an inhibitor of acyl CoA:cholesterol acyl transferase, for 24 hr in the presence and absence of chloroquine, ammonium chloride, or methylamine. Although the hydrolysis of cholesteryl oleate was sensitive to these lysosomotropic agents, the hydrolysis of triolein was not. Using reconstituted LDL containing cholesteryl oleate and triolein, we demonstrated that the hydrolyses of cholesteryl oleate and triolein were equally sensitive to the lysosomotropic agents when the cells were not loaded with droplet lipid. However, in cells loaded with lipid, hydrolysis of LDL cholesteryl ester was sensitive to the lysosomotropic agents but hydrolysis of triolein was not. We therefore conclude that both droplet lipids were hydrolyzed in lysosomes, and we attribute the failure of the lysosomotropic agents to inhibit fully the hydrolysis of droplet triolein to the presence of a large mass of free fatty acids in the lysosome that maintains a sufficiently low pH to sustain the triglyceridase activity, but not the cholesteryl esterase activity, of the lysosomal acid lipase.

Insolubilization of low density lipoprotein induces cholesterol accumulation in cultured subendothelial cells of human aorta

Primary cultures of typical and modified smooth muscle cells isolated from the intima of human aorta were used to study the mechanism whereby low density lipoprotein (LDL) induces accumulation of intracellular cholesterol. Incubation of intimal cells with native LDL obtained from human plasma did not lead to deposition of total cholesterol. LDL added to the cultures simultaneously with hyaluronic acid, heparin, chondroitin sulfate, fibronectin, and mouse monoclonal antibody against LDL also failed to alter the cellular cholesterol. On the other hand, 24-h incubation of the cells with LDL in the presence of dextran sulfate, gelatin, particles of aortic elastin, particles of collagenase-resistant aortic matrix, goat polyclonal antibodies against LDL or latex beads caused a significant (1.5-7-fold) increase in total cholesterol. The compounds which stimulated cholesterol deposition are able to form precipitating complexes with LDL. On the contrary, the agents which failed to induce cholesterol accumulation were unable to insolubilize LDL. A direct correlation (r = 0.927) was found between the cholesterol content of the insoluble complex and the increment of cholesterol in the cultured cells. To find out whether LDL plays a specific role in the deposition of intracellular cholesterol, very low density lipoproteins and high density lipoproteins were used. These lipoproteins stimulated the accumulation of intracellular cholesterol in the presence of agents capable of forming insoluble associates with them. Our data suggest that insolubilization of lipoproteins is a key event in the LDL-mediated accumulation of intracellular cholesterol induced by various agents.

Beta-VLDL metabolism by pigeon macrophages. Evidence for two binding sites with different potentials promoting cholesterol accumulation

Previous studies from our laboratory (J Lipid Res 1988;29:643-656) have shown that thioglycolate-elicited peritoneal macrophages from White Carneau and Show Racer pigeons, like mammalian macrophages, have on their surfaces specific receptors for acetylated low density lipoprotein (acLDL) and beta-migrating very low density lipoproteins (beta-VLDL). The binding kinetics of beta-VLDL were complex, however, suggesting more than one binding site. The purpose of the present study was to further characterize these beta-VLDL binding sites. Scatchard analysis of 125I-beta-VLDL binding curves indicated at least two classes of binding sites. The first binds pigeon beta-VLDL and LDL with high affinity (Kd approximately 7 micrograms/ml), is down-regulated by cholesterol loading, requires calcium, and is destroyed by the proteolytic enzyme, pronase. This pigeon beta-VLDL receptor is specific for pigeon beta-VLDL and LDL and does not recognize HDL, acLDL, methyl LDL, cynomolgus monkey LDL, or rabbit beta-VLDL. Like the mammalian macrophage beta-VLDL receptor, the "pigeon beta-VLDL receptor" has many of the characteristics of an LDL receptor. The second class of binding sites is relatively nonspecific, recognizing both pigeon and rabbit beta-VLDL, LDL, acLDL, methyl LDL, and HDL. Binding to this site is not altered by incubation of macrophages with pronase or by cholesterol loading. This binding site has low affinity for beta-VLDL (Kd approximately 100 micrograms/ml), but high capacity. We have called this the "lipoprotein binding site," a term used by others to describe similar lipoprotein binding characteristics on a variety of cells. Not only does binding to this site promote the internalization and degradation of lipoproteins, but it may also facilitate the independent uptake of cholesterol. This conclusion is based on the observation that more cholesterol accumulates in cells incubated with rabbit beta-VLDL, which binds only to the lipoprotein binding site, than can be accounted for by beta-VLDL uptake and degradation. Since the lipoprotein binding site recognizes a variety of normal, as well as abnormal, lipoproteins, it would not require the generation of abnormal lipoprotein products, as must occur with the scavenger receptor, to promote the accumulation of cholesteryl esters in macrophages of atherosclerotic lesions. This, coupled with the fact that the lipoprotein binding site is not down-regulated by cholesterol loading, suggests that it could provide an alternative mechanism to the scavenger receptor pathway for the formation of foam cells.

beta-VLDL and acetylated-LDL binding to pigeon monocyte macrophages

Blood-derived monocytes are an important source of foam cells in atherosclerotic lesions of White Carneau pigeons. Based upon studies with cultured blood monocytes (monocyte macrophages) and peritoneal macrophages from a variety of mammalian species, it has been proposed that these cells become loaded with cholesteryl esters through the uptake of lipoproteins including beta-migrating very low density lipoproteins (beta-VLDL) and low density lipoproteins that have been chemically modified in a manner analogous to experimental acetylation (Ac-LDL). The purpose of this study was to determine whether similar mechanisms functioned in pigeon monocyte macrophages. Radioiodinated pigeon beta-VLDL and Ac-LDL were incubated with White Carneau pigeon monocyte macrophages that had been maintained in culture for 7 days. Scatchard analysis of the specific binding data revealed the presence of specific and saturable receptors for both beta-VLDL and Ac-LDL. beta-VLDL receptors had both low and high affinity binding components, whereas Ac-LDL receptors displayed a single class of high affinity binding sites. beta-VLDL binding remained relatively constant from 3 to 10 days in culture while Ac-LDL binding increased with time in culture. Competition studies demonstrated a high degree of binding specificity for 125I-Ac-LDL, but less for 125I-beta-VLDL. Binding of 125I-beta-VLDL was not competed for by Ac-LDL, but was by beta-VLDL and by low density lipoproteins from both normal and hypercholesterolemic pigeons. Following binding of beta-VLDL and Ac-LDL, the lipoproteins were rapidly internalized and degraded. Although the majority of degradation was secondary to internalization by the monocyte macrophages, approx. 5% of the degradation resulted from enzymatic activity in the culture medium, presumably due to secretion of proteolytic enzymes by the cells. As measured by esterification of [14C]oleate to cholesterol, it was shown that the cholesterol liberated from the degradation of both beta-VLDL and Ac-LDL stimulated cholesteryl ester synthesis in pigeon monocyte macrophages. These studies confirm the existence of specific beta-VLDL and Ac-LDL receptors on the surface of pigeon monocyte macrophages which facilitate both internalization of the lipoproteins and subsequent stimulation of cholesteryl ester synthesis. This is the first demonstration of beta-VLDL and Ac-LDL receptors on monocyte macrophages from an avian species, and the findings support the potential role for the receptor-mediated uptake of a variety of abnormal lipoproteins in the formation of monocyte-derived foam cells in the arterial wall of White Carneau pigeons during the development of atherosclerosis.

Cholesteryl ester cycle in cultured hepatoma cells

The existence of a cholesteryl ester cycle in cultured Fu5AH hepatoma cells was documented and factors affecting the rate of turnover of the cholesteryl ester cycle in this cell line were explored. The influence of the physical state of the lipid inclusion in which the cholesteryl esters are stored could be addressed in this cell line because these cells can be induced to store cholesteryl esters in anisotropic (liquid-crystalline) cytoplasmic inclusions by exposure to free cholesterol-rich phospholipid dispersions or in isotropic (liquid) inclusions by addition of oleic acid to the phospholipid dispersions. To examine the relative rates of turnover of the cholesteryl ester cycle in the cells with the two types of inclusions, the fraction of cholesteryl linolenate, a cholesteryl ester present in low amounts in these inclusions, was examined after cells were exposed to medium containing linolenate. After 12 h, cells with anisotropic inclusions contained 17.5% cholesteryl linolenate and cells with isotropic inclusions contained 29.8% cholesteryl linolenate, suggesting an approximately 2-fold difference in turnover of the cholesteryl ester pool. To determine whether this difference was due to a differential rate of cholesteryl ester hydrolysis, the acyl CoA: cholesterol acyl transferase arm of the cholesteryl ester cycle was blocked using a specific inhibitor, Sandoz 58-035. In the presence of this compound, cholesteryl ester was hydrolysed twice as fast in cells with isotropic inclusions as compared to that in cells with anisotropic inclusions. The difference in rate of turnover of the cholesteryl ester cycle was shown to be related to the rate of hydrolysis of cholesteryl ester which, in turn, is related to the physical state of the stored cholesteryl ester.

Development of the smooth muscle foam cell: uptake of macrophage lipid inclusions

A possible mechanism for the formation of smooth muscle foam cells in the atherosclerotic lesion was explored. Cultured macrophages (J774 cell line) were induced to form cytoplasmic cholesteryl ester inclusions by exposure to acetylated low density lipoprotein in the presence of cholesterol-rich phospholipid dispersions. The macrophages were disrupted by brief sonication, and the inclusions were isolated by flotation. When these inclusions were placed in direct contact with cultured smooth muscle cells, cellular uptake of the inclusions in a time- and dose-dependent manner was observed. Light and electron microscopy indicated the presence of lipid inclusions throughout the cytoplasm of the cells. Uptake of inclusion lipid by the smooth muscle cells was inhibited by several metabolic inhibitors, indicating that the process is dependent on metabolic activity. A modest but significant hydrolysis of the cholesteryl ester was observed, showing that the stored cholesteryl esters are metabolically available.

Metabolism of low density lipoproteins by pigeon skin fibroblasts and aortic smooth muscle cells. Comparison of cells from atherosclerosis-susceptible and atherosclerosis-resistant pigeons

Aortic smooth muscle cells from atherosclerosis-susceptible White Carneau (WC) pigeons lack a functional low density lipoprotein (LDL) receptor pathway. The purpose of the present study was to determine if atherosclerosis-resistant Show Racer pigeons (SR) shared this lack of an LDL receptor pathway and if LDL from normal and hypercholesterolemic pigeons were metabolized similarly. The amount of LDL bound, internalized, and degraded by skin fibroblasts, embryo fibroblasts, and aortic smooth muscle cells from WC and SR pigeons were similar and averaged from 2% to 25% of that seen with monkey smooth muscle cells incubated with the same LDL. LDL uptake by pigeon cells was due largely to nonspecific processes, while specific uptake predominated in monkey cells. A similar lack of specific uptake was obtained with LDL from normal and hypercholesterolemic pigeons. Sterol synthesis and HMG-CoA reductase activity were 10- to 35-fold higher in pigeon cells than in monkey cells incubated in serum-containing medium. LDL had little effect on cholesterol esterification and cholesteryl ester accumulation in pigeon cells. These results indicate that despite major changes in the size and composition of LDL from hypercholesterolemic pigeons, this LDL, like normal pigeon and monkey LDL, was not metabolized by specific uptake processes by pigeon cells. Cells from both WC and SR pigeons lack a functional LDL receptor pathway.

Metabolism of atherogenic lipoproteins by smooth muscle cells of different phenotype in culture

Smooth muscle cells of the rabbit aorta, when grown in vitro, express three distinguishable forms of phenotype (contractile, reversible synthetic, and irreversible synthetic). We compared the interactions of these three smooth muscle phenotypes with rabbit very low density lipoprotein (VLDL), low density lipoprotein (LDL), and very low density lipoprotein from cholesterol-fed rabbits (beta-VLDL). beta-VLDL showed saturable. high-affinity binding characteristics with each phenotype predominantly through the B/E receptor. The irreversible synthetic cells displayed the greatest binding capacity and the contractile cells, the least. Binding and degradation of normal VLDL was less than that of beta-VLDL and higher than that of LDL. Only the irreversible synthetic cells showed substantial (about threefold) cholesteryl ester formation and cholesterol accumulation, and then only when incubated with beta-VLDL. Substantial stainable lipid, shown chemically to include triglyceride, cholesterol and cholesteryl ester, was also observed only when irreversible synthetic cells were exposed to beta-VLDL. The high capacity of irreversible synthetic-state, smooth muscle cells to bind and accumulate beta-VLDL in contrast to the relative immunity of contractile cells may be relevant to the genesis of atherosclerosis in the rabbit and possibly also in humans.

Role of acid lipase in cholesteryl ester accumulation during atherogenesis. Correlation of enzyme activity with acid lipase-containing macrophages in rabbit and human lesions

Purified acid lipase was previously shown to hydrolyze the artificial substrate, alpha-naphthyl palmitate, as well as triglycerides and cholesteryl esters and to form cholesteryl esters. To determine to what extent these activities are associated with acid lipase-containing cells in atherosclerotic plaques, we examined rabbit aortas at different stages of experimental lesion induction and human atherosclerotic arteries. Assays of cholesteryl ester formation, and alpha-naphthyl palmitate and cholesteryl ester hydrolysis were performed on homogenates of lesions and the hydrolysis of the artificial fatty acid ester was used as a histochemical marker to identify acid lipase positive foam cells in sections of the same lesions. The volume of lesions occupied by cells stained for acid lipase correlated strongly with the enzyme activities of the arterial homogenates. These results suggest that acid lipase-containing cells may mediate the accumulation of cholesteryl ester during atherogenesis. Since acid lipase activity marks macrophages, these methods may be useful for relating macrophage distribution and function to lesion progression, regression, and complication.

Lipid accumulation in arterial smooth muscle cells in culture. Morphological and biochemical changes caused by low density lipoproteins and chloroquine

Cultured smooth muscle cells from pig aortas were incubated with low density lipoproteins (LDL) and chloroquine for up to 5 days, as an in vitro model for lipid accumulation in atherosclerosis. Cells incubated with LDL alone had a normal morphology, except that some cells contained large lipid droplets. The activities of acid phosphatase, catalase and malate dehydrogenase were increased in homogenates prepared from these cells. Cells incubated with chloroquine alone developed large autophagic vacuoles. The activities of the three acid hydrolases, acid phosphatase, N-acetyl-beta-glucosaminidase and beta-glucuronidase, were decreased, as was the proteolytic activity of the cell homogenates at acid pH toward 125I-labelled LDL. There was, however, a transient increase in the activity of malate dehydrogenase. Chloroquine by itself was toxic to the cells, but LDL protected against this toxic effect. Cells incubated with LDL and chloroquine together developed both autophagic vacuoles and large lipid droplets. The cholesteryl ester content of the cells was increased many-fold and the non-esterified cholesterol content was increased to a lesser extent. The above four acid hydrolase activities were decreased, as was the activity of catalase, whereas the activities of lactate dehydrogenase and malate dehydrogenase were increased.