Characterization of human monocytic cell line, U937, in taking up acetylated low-density lipoprotein and cholesteryl ester accumulation. A flow cytometric and HPLC study

Endothelial progenitor cells: their potential in the placental vasculature and related complications

Endothelial progenitor cells (EPCs) have received significant attention in recent times. A role for EPCs has been suggested in a range of pathologies and some recent studies of EPCs in pregnancy have been published. This review provides a guide to the confusing field of EPCs. Attention is paid to their phenotyping, as although elementary this remains a highly debated topic. The current understanding of different subtypes and physiological role of EPCs in the placenta, fetus and adult are also considered. An overview is given as to role of EPC's in the pathophysiology of different disease states and the possible therapeutic and diagnostic applications expected from EPC-related research in obstetrics.

Effects of adrenergic agents on rat peritoneal macrophages activated in vitro by acetylated low-density lipoprotein

Macrophages interact with modified lipoproteins and alter their functional status. In this study, the effects of the adrenergic agents adrenaline, isoproterenol, and dobutamine on macrophages activated with acetylated low-density lipoprotein were tested. The aim of this investigation was to determine whether adrenergic agents influence biologically significant functions of these cells in an in vitro model of macrophage-lipoprotein acute interaction. Rat peritoneal macrophages were incubated with acetylated low-density lipoprotein for 16 h, with or without adrenergic agents. Hydrogen peroxide and nitric oxide production and acid phosphatase activities in the supernatant and cell lysate were assayed. Adrenaline and isoproterenol inhibited the production of hydrogen peroxide, stimulated the production of nitric oxide, and increased the extracellular activity of acid phosphatase in the lipoprotein-activated cells. Dobutamine increased the extracellular, but decreased the intracellular acid phosphatase activity. Adrenaline and dobutamine also produced changes in the cell size and nuclear morphology of the macrophages. Macrophages are activated in vitro by acetylated low-density lipoprotein, and their functions and morphology are modified under the influence of adrenergic agents. Certain effects could be attributed to changes in cyclic AMP levels.

Atorvastatin and Simvastatin Decrease the Uptake of Acetylated Low-Density Lipoprotein by Human Monocytes

Several lines of evidence indicate that HMG-CoA reductase inhibitors, calcium channel blockers, and angiotensin-converting enzyme inhibitors exert anti-atherogenic effects in vitro and in vivo. In the present study, we determined the effect of members of the above classes of drugs on the uptake of modified low-density lipoprotein (LDL) by U937 cells (human monocytic cell line), a key event in the progression of atherosclerosis. U937 cells were treated with drugs and subsequently the uptake of fluorescent acetylated LDL was assessed by flow cytometry. The HMG-CoA reductase inhibitors, atorvastatin and simvastatin (1-30 mumol/l), but not calcium channel blockers or angiotensin-converting enzyme inhibitors, inhibited the uptake of modified LDL by monocytes. Therefore, atorvastatin and simvastatin may slow down the progression of atherosclerosis by inhibiting the formation of foam cells.

Lipid microspheres incorporated by U937 cells via their specific receptors

Lipid microspheres (LM), currently in clinical use as drug carriers, mainly consist of soybean oil as a core and lecithin as a surfactant. The purpose of our study wass to determine whether or not LM incorporation is receptor-mediated. U937 cells resuspended in a serum-free medium abundantly took up unmodulated LM. A binding study showed that U937 cells had a single binding site for LM (410 sites/cell at 24°C; 100 sites/cell at 4°C). Inhibition assays revealed that lecithin liposome, lysophosphatidylcholines, activated α2-macroglobulin, and HDL did not affect the binding of LM to U937 cells. VLDL strongly, and LDL and AcLDL moderately, inhibited the binding of LM to U937 cells. Ligand blotting analysis revealed that unmodulated LM in an apoprotein-free buffer directly bound to a 40 kDa protein in the cell membrane fraction. These results suggest that LM that is not modulated by any protein is incorporated by specific cells via receptor-mediated processes.Key words: lipid emulsion, drug delivery system, monocyte, free fatty acids.

Expression of low-density lipoprotein receptors in peripheral blood and tonsil B lymphocytes

B lymphocytes, purified from peripheral leucocytes from young normolipaemic humans, expressed and internalized low-density lipoprotein receptors (LDLR). The expression was assessed by a monoclonal anti-LDLR. The internalization of LDL was assessed by LDL labelled with 125I (125I-LDL) and 1,1'-dioctadecyl-3,3,3',3' tetramethyl-indocarboxycyanine perchlorate (LDL-DiI). The expression of LDLR, assessed by anti-LDLR, was: 38 +/- 8% (n = 5) for fresh purified cells, 60 +/- 10% (n = 12) for non-stimulated cells, 79 +/- 5% (n = 10) for IL-2 (100 U/ml)-stimulated cells and 95 +/- 5% (n = 8) for pokeweed mitogen (PWM) (1:200 dilution)-stimulated cells. The optimal concentrations of agonist were 100 U/ml of IL-2, and 1:200 dilution of PWM. IL-2 and PWM increased the internalization of LDL-DiI by 1.5-fold. The internalization of LDL-DiI was maximal at 60 microg of protein/ml (48 +/- 8%). Scatchard analysis revealed a Kd of 3.2 +/- 0.22 x 10(-8) M and 2180 +/- 190 binding sites in non-stimulated cells, a Kd of 7.73 +/- 0.36 x 10(-9) M and 12,500 +/- 430 binding sites for IL-2 (100 U/ml)-stimulated cells, and a Kd of 7.2 +/- 0.43 x 10(-9) M and 13,250 +/- 450 binding sites for PWM (1:200 dilution)-stimulated cells. Lineweaver-Burk analysis of LDL binding (LDL-DiI) revealed that the apparent Kd for non-stimulated cells was 1.3 +/- 0.11 x 10(-8) M, and 9.2 +/- 0.2 x 10(-9) M and 7.5 +/- 0.25 x 10(-9) M for IL-2- and PWM-stimulated cells, respectively. B lymphocytes from tonsils also showed a high expression of LDLR assessed with anti-LDLR (70 +/- 6%). The high expression of LDLR and the avid internalization of LDL suggest that LDL may be important for B cell physiological responses.

Uremic serum enhances scavenger receptor expression and activity in the human monocytic cell line U937

The macrophage scavenger receptor (SR) plays a leading role in atherogenesis, but little is known about the relevance of SR to atherosclerosis in uremia. In this study, the impact of uremic serum on SR expression and activity was examined in the human monocytic cell line U937. The cells were cultured with serum from ten healthy subjects, ten hemodialysis (HD) and ten continuous ambulatory peritoneal dialysis (CAPD) patients. SR mRNA expression was examined using reverse transcriptase-polymerase chain reaction followed by Southern blot. SR protein amount was evaluated by ligand blot. SR activity was analyzed by cellular uptake of fluorescently labeled acetylated low-density lipoprotein using flow cytometry. Uremic serum dose-dependently enhanced SR activity primarily by increasing the amount of receptor protein. Heat-inactivated uremic serum had a stimulatory effect, but ultrafiltrate of uremic serum, which included molecules with a molecular weight less than ten kDa, had no effect. The serum levels of macrophage-colony stimulating factor (M-CSF), an activator of SR, were fourfold higher in uremia and significantly correlated with SR activity in cells treated with uremic serum. Pre-treatment of uremic serum with a neutralizing antibody to M-CSF abolished the effect of uremic serum on SR activity. In conclusion, uremic serum contains a factor(s) that enhances SR expression and activity in U937 cells. Elevated M-CSF in uremic serum could be responsible for this enhancement.

Oxidative degradation of cholesteryl esters in low-density lipoproteins: analysis by liquid chromatography-light scattering and protection by a new synthetic antioxidant, S20478

Cholesteryl esters in the hydrophobic core of low-density lipoprotein (LDL) particles constitute a major molecular target during copper-mediated oxidation. To facilitate the rapid analysis and quantitation of the oxidative degradation of LDL cholesteryl esters, we describe a new approach based on light scattering detection following separation by HPLC. We have applied this approach to the evaluation of the protective capacity of a new synthetic antioxidant, S20478, during oxidation of LDL in the presence of copper ions. HPLC separation of cholesterol and the four major molecular species of cholesteryl esters (C16:0, C18:1, C18:2 and C20:4) of LDL was achieved in a single run of 20 min with high sensitivity (50 ng) and low background. Time course studies of the oxidative modification of LDL (ratio LDL protein: copper, 100 micrograms/mL: 1 microM) revealed that the content of unsaturated cholesteryl esters (C20:4 and C18:2) decreased (-30% and -15%, respectively) within 90 min of copper-mediated oxidation, while only minor degradation (up to 15%) of monounsaturated (C18:1) and saturated (C16:0) esters occurred. At 24 hours of oxidation, only traces (< 5%) of the C20:4 and C18:2 esters were detectable; whereas 52% of the C18:1 ester remained (P < 0.01). Of the saturated esters, only minor proportions (35% or less) underwent oxidative modification. In addition, some 81% of free cholesterol was conserved as the native sterol. The synthetic antioxidant, S20478 (50 microM) was capable of inhibiting the initiation and the propagation of copper-mediated LDL oxidation as determined by the time- and dose-dependent inhibition of the formation of conjugated dienes and thiobarbituric acid-reactive substances, as well as the conservation of the net electrical charge of LDL; indeed S20478 conserved cholesteryl esters in their native form up to 24 hours. However, after prolonged exposure to copper ions (48 hours), only 47% of the unsaturated esters remained (C18:2, P < 0.05). Nonetheless, S20478 (10 microM) was more efficient in inhibiting copper-mediated LDL oxidation as compared to probucol at the same concentration. These findings suggest that S20478 may be of potential interest in a new antioxidant approach to therapeutic stabilisation and regression of atherosclerotic plaques. Moreover, this method should prove useful in the assessment of the integrity of native LDL, and provides a new chemical marker of the degree of LDL oxidation.

Enhanced scavenger receptor expression in monocyte-macrophages in dialysis patients

The macrophage scavenger receptor (SR), which has two isoforms named type I and II, plays a leading role in the atherosclerotic process. To elucidate the mechanism of atherosclerosis in dialysis patients, SR expression was studied in monocyte-macrophages from thirteen dialysis patients and eight healthy controls. SR mRNA expression was examined in four hemodialysis patients and four controls matched for age and sex. Monocytes were allowed to differentiate into macrophages in in vitro cultures for seven days and SR type I and II mRNA expression were analyzed with the reverse transcription-polymerase chain reaction and quantitated using radiation densities of Southern blots. Only SR type I expression increased during differentiation and was accelerated by one or two days and enhanced after five days in patients, as compared to controls. To detect SR protein expression, uptake of fluorescently labeled acetylated low-density lipoprotein (Ac-LDL) was evaluated by flow cytometry. The mean fluorescence intensity of labeled cells was significantly higher in hemodialysis and continuous ambulatory peritoneal dialysis patients than in controls, although the number of SR-positive cells remained constant. In conclusion, SR expression is enhanced in macrophages from dialysis patients, probably by up-regulation of type I, which may contribute to the development of atherosclerosis in these patients.

Desialylated LDL uptake in human and mouse macrophages can be mediated by a lectin receptor

We have compared the uptake of desialylated low density lipoprotein (LDL) with other modified forms of LDL in mouse peritoneal macrophages and PMA-activated human U937 monocytes. Neuraminidase-treated LDL (NT-LDL) caused significant cholesterol ester accumulation in both cell types, although the efficiency relative to loading with acetylated LDL (AcLDL) was markedly different, suggesting a very different complement of receptors in the cells. We therefore determined the effect of PMA-activation on lipoprotein receptor expression in U937 cells and found that while scavenger receptor concentration was elevated after PMA-activation, there was no significant change in the expression of the LDL receptor. Receptor specificity of NT-LDL uptake was examined by competition experiments using the degradation assay. This showed that 125I-labelled NT-LDL uptake in U937 cells could largely be accounted for by the persistent expression of the LDL receptor in these cells. In contrast, in mouse peritoneal macrophages where LDL receptor expression is very low, 125I-labelled NT-LDL degradation was also effectively competed by asialofetuin. Surprisingly, 125I-labelled NT-LDL degradation was also effectively competed by AcLDL. Measurement of sialic acid content of AcLDL showed that approximately 14% of the LDL sialic acid, equivalent to 2 to 3 residues per particle, was lost during acetylation of LDL with acetic anhydride. Thus competition between 125I-labelled NT-LDL and AcLDL could be due to lectin receptor binding rather than competition for scavenger receptor binding.

Changes in plasma phospholipids in the presence and absence of erythrocytes 31P-NMR time-course studies

Whole human blood and plasma were incubated at 37 degrees C and 31P-NMR spectra were acquired from aliquots of plasma dissolved in sodium cholate. The narrow resonances of phospholipids were well resolved, allowing identification and accurate quantification of the various phospholipid classes. During the course of plasma incubation, the rate of increase in 2-lysphosphatidylcholine (2-LysoPtdCho) corresponded closely to the rate of decrease in phosphatidylcholine. However, little or no change was observed in the sphingomyelin concentrations. The rate of cholesteryl ester formation in plasma was also determined using HPLC and was observed to be similar to the rate of 2-LysoPtdCho increase in plasma; this is consistent with the rate of acyl transfer from phosphatidylcholine to free cholesterol catalysed by lecithin:cholesterol acyltransferase. However, during the course of whole blood incubation, the rate of 2-LysoPtdCho increase in plasma was significantly lower than the rate of 2-LysoPtdCho production during incubation of plasma alone (31 +/- 4 micromol x 1(-1) x h(-1) and 80 +/- 8 micromol x 1(-1) x h(-1), respectively). The difference between the rates can be attributed to the action of enzymes present in blood cells including those in the erythrocytes that catalyse acylation or hydrolysis of 2-LysoPtdCho.

Study of LDL and acetylated LDL endocytosis by mononuclear cells in HIV infection

Activated lymphocytes have a high level of low density lipoprotein (LDL) uptake as compared to resting lymphocytes, whereas scavenger receptors for acetylated LDL (Ac-LDL) are expressed on limited number of immune cells, i.e., monocytes/macrophages. The endocytosis of LDL and Ac-LDL by mononuclear cells was studied during in vitro and in vivo HIV infection, in order to use LDL and Ac-LDL as carriers of antiviral and/or immunomodulatory drugs towards lymphocytes and monocytes. The uptake of LDL and Ac-LDL was analyzed by cytofluorimetry. LDL endocytosis in PHA/IL2-activated lymphocytes was higher than in resting lymphocytes. In vitro HIV infection of PHA/IL2-activated lymphocytes did not alter the high LDL endocytosis in lymphocytes. CD4+ and CD8+ cells. In a group of 12 symptomatic patients there was no alteration of LDL endocytosis in lymphocytes, CD4 and CD8 lymphocytes. In another group of 23 individuals, the Ac-LDL endocytosis mediated by CD14+ monocytes was unaltered in asymptomatic patients (n = 6) and in some symptomatic patients (n = 6, CD14+ cells > 100/mm3). On the contrary, in other symptomatic patients (n = 11, CD14+ cells < 100/mm3), the number of Ac-LDL+ CD14+ cells decreased, whereas their efficiency of Ac-LDL endocytosis increased as compared to those of other HIV+ patients. In conclusion, the use of lipoproteins as carriers to increase the drug delivery to CD4+ lymphocytes and to CD14+ monocytes can be envisaged, since: (i) the LDL endocytosis was not impaired in CD4 lymphocytes of HIV+ patients, and (ii) the Ac-LDL uptake by monocytes was altered only in some patients of stage IV.

MacMARCKS mutation blocks macrophage phagocytosis of zymosan

A major protein kinase C substrate, MacMARCKS (F52, MPR), was examined for its role in phagocytosis. In macrophage-phagocytosing zymosan particles, MacMARCKS was concentrated around nascent phagosomes as detected by immunofluorescent microscopy. The effector domain of MacMARCKS contains the phosphorylation sites, a calmodulin binding site, as well as a putative actin binding site. Stable J774 macrophage cell lines constitutively expressing effector domain deletion mutants of MacMARCKS were generated. When given zymosan particles, these transfectants showed approximately a 90% reduction in their phagocytic capacity. The receptor-mediated endocytosis of acetylated low density lipoproteins, however, was not affected by the mutant. These results strongly suggest the involvement of MacMARCKS in macrophage phagocytosis.

Immune complexes of LDL induce atherogenic responses in human monocytic cells

The ability of immune complexes of LDL or acetylated LDL (acLDL), together with antibodies to LDL, to induce a proatherogenic phenotype in human monocytic cells has been explored. Treatment of THP-1 monocytic cells or peripheral human monocytes with LDL immune complexes containing intact anti-LDL markedly enhanced the ability of these cells to subsequently bind and take up LDL, whereas aggregated LDL or LDL immune complexes prepared with F(ab')2 fragments of anti-LDL had no significant effect. Activation of THP-1 cells with intact LDL immune complexes also stimulated mRNA expression for the scavenger receptor. Additionally, activation of THP-1 cells with insoluble immune complexes of LDL or LDL stimulated generation of reactive oxygen intermediates that, in turn, could oxidize exogenous LDL. These results indicate that the binding of lipoprotein immune complexes to Fc receptors on monocytic cells activates a series of responses that could accelerate the initiation or progression of atherosclerosis.

A chemotactic S100 peptide enhances scavenger receptor and Mac-1 expression and cholesteryl ester accumulation in murine peritoneal macrophages in vivo

In the early development of atherosclerotic plaque, monocytes are recruited to the arterial intima where they accumulate lipid and become foam cells. The recently described murine chemotactic S100 protein, CP-10, may have an important role in this process. Intraperitoneal injection of CP-10(42-55) (chemotactic hinge region peptide) into mice caused a sustained leukocyte recruitment with a sixfold increase in monocyte numbers over 24 h. CP-10(42-55)--elicited monocyte/macrophages accumulated significantly increased cholesteryl esters in response to acetylated LDL, both in vivo and in vitro and this was associated with a twofold increase in scavenger receptor expression. By contrast, thioglycollate- and macrophage colony-stimulating factor-elicited macrophages expressed levels of scavenger receptor similar to those on resident macrophages and did not exhibit enhanced acetylated LDL loading in vitro. The leukocyte integrin Mac-1 (CD11b/CD18) and its beta subunit (CD18), but neither lymphocyte function-associated antigen-1 nor very late activation antigen-4, were upregulated on monocyte/macrophages elicited by CP-10(42-55), thioglycollate, and macrophage colony-stimulating factor. Cholesteryl ester accumulation in vitro was significantly enhanced by adhesion, which appeared to involve macrophage activation via ligation of Mac-1. The initial events of monocyte recruitment and adhesion to the vessel wall may be important in macrophage foam cell development, and CP-10 or related S100 proteins may contribute to the early inflammatory events of atherogenesis by stimulating these events.

Activated CD4+ T cells preferentially take up lipid microspheres, but resting cells do not

Lipid microspheres (LM) used as drug carriers increase the effectiveness and reduce the toxicity of incorporated drugs. The present study is designed to determine whether or not activated T lymphocytes, which were the cells chosen first from the 'inflammatory cells', can take up LM in vitro. LM were labelled with a fluorescent probe, DiI (DiI-LM), to examine the kinetics. Flow cytometric analysis demonstrated that in freshly isolated peripheral blood mononuclear cells (PBMC), monocytes principally took up DiI-LM, while lymphocytes and granulocytes did not. When PBMC were stimulated with immobilized anti-CD3 MoAb and IL-2, cells expressing CD3, CD4, CD8 and CD16 incorporated DiI-LM. Purified CD4+ T cells, obtained by positive panning selection, were stimulated with this system. They were CD25, CD71, LFA-1-positive, and also showed an ability to take up DiI-LM, which resting cells did not. The findings were confirmed by flow cytometry and quantitative analysis of DiI. Confocal micrographs showed fluorescent granules from the probe in the cytoplasm of stimulated CD4+ T cells after incubation with DiI-LM. These results suggest that immunomodulatory agents incorporated into LM might selectively regulate the function of CD4+ or CD8+ T cells when these are activated.

Cholesterol effects on nonelectrolyte membrane transport in human erythrocytes: NMR magnetization transfer studies

The cholesterol content of human erythrocytes was altered by incubating them with sonicated dispersions of cholesterol/phosphatidylcholine at 37 degrees C. 31P NMR saturation transfer experiments were used to measure the rate constant for efflux of dimethyl methylphosphonate (DMMP) from the cells, and thereby gain an estimate of the permeability coefficient. It was shown that up to 39% depletion of membrane cholesterol (cholesterol/phospholipid molar ratio of 0.46) increased the efflux rate constant and permeability coefficient of DMMP 1.55- and 1.86-fold, respectively. Enrichment of the membranes with cholesterol by 45% (cholesterol/phospholipid molar ratio of 1.57) on the other hand, decreased the efflux rate constant and permeability coefficient 1.63- and 1.79-fold, respectively. It was concluded that DMMP may be used as a probe molecule to study the functional consequences of changes in the lipid composition of erythrocytes in diseases that are associated with disorders of lipid metabolism.

Fluorescent probe-labeled lipid microsphere uptake by human endothelial cells: a flow cytometric study

Lipid microspheres have been used as carriers of drugs such as prostaglandin E1 (lipo-PGE1) and corticosteroid (liposteroid). Lipo-PGE1 is used for the treatment of chronic arterial occlusive diseases because its activity is far greater than that of free PGE1 in vivo. To verify the fact that the drug carriers, lipid microspheres, are preferentially taken up by endothelial cells, we labeled lipid microspheres with a fluorescent probe, DiI (DiI-LM), and observed them in some in vitro models. Stoichiometric fluorescence was obtainable, and the fluorescence was stable between pH 3.3 and pH 8.9. Human umbilical vein endothelial cells and cells of a human endothelial cell line, ECV304, showed increased uptake of DiI-LM, 81% and 61%, respectively. In contrast, uptakes were less than 7% in human skin fibroblasts, 3T3 cells, and human neutrophils. Prominent perinuclear fluorescence was also observed in endothelial cells by fluorescence microscopy. DiI-LM and flow cytometric analysis will be useful for studies to elucidate the precise mechanism of the selective accumulation of lipid microspheres by cells in blood vessel walls.

Close similarity between cultured human omental mesothelial cells and endothelial cells in cytochemical markers and plasminogen activator production

The mesothelial cells obtained from human omental adipose tissue showed a typical cobblestone monolayer and reacted strongly with keratin, but did not have Von Willebrand factor. Ultrastructurally these cells revealed the existence of desmosome-like cell junctions as well as intracellular canaliculi, tubular structures surrounded by microvilli, and tonofilament-like filaments. The mesothelial cells grew much faster in the medium containing epidermal growth factor, actively took up acetylated-low density lipoprotein into their cytoplasm, and released angiotensin-converting enzyme. They also released urokinase-type plasminogen activator, but only half as much as do human umbilical vein endothelial cells; release of tissue-type plasminogen activator was not observed. Inasmuch as the mesothelial cells also released plasminogen activator inhibitor-1, as do human umbilical vein endothelial cells, we could not detect u-PA activity in culture medium. u-Pa may play a role in the protection against adhesion among visceral organs. These observations indicate that cultured human mesothelial cells have characteristics closely related to those found in human endothelial cells.

The labeling of lipoproteins for studies of cellular binding with a fluorescent lipophilic dye

N,N-dipentadecylaminostyrylpyridinium iodide is a dye that is approximately 100-fold more intensely fluorescent in a lipid than aqueous environment. This observation suggests its potential as a fluorescence stain for lipoproteins. This work reports the staining of LDL with this dye for use in studies of cellular binding. The staining procedure is simple, resulting in stable attachment of the dye as determined by transfer experiments, physical properties essentially identical to native LDL as demonstrated by virtually identical electrophoretic mobility, and consistent results in studies of cellular binding using flow cytometry. Increased signal to noise ratio over other dyes used for lipoprotein staining including the widely used Dil (3,3'-dioctadecylindocarbocyanine iodide) allows determinations of greater sensitivity and precision to be made. This is demonstrated by the flow cytometric determination of the 4 degrees C binding curve of LDL with freshly isolated human peripheral blood lymphocytes (i.e., cells not LDL receptor upregulated). Mediation of binding by the LDL receptor is demonstrated by correspondence between the LDL receptor dissociation constant derived from this work and literature values; increased specific binding in lymphocytes cultured in lipoprotein-deficient media to up-regulate the LDL receptor; and decreased specific binding in lymphocytes cultured in the presence of 25-hydroxy cholesterol for 48 h to suppress the LDL receptor.