Uptake of [3H]cholesterol from low density lipoprotein by cultured human fibroblasts

Sigma-2 Receptor/TMEM97 and PGRMC-1 Increase the Rate of Internalization of LDL by LDL Receptor through the Formation of a Ternary Complex

CRISPR/Cas gene studies were conducted in HeLa cells where either PGRMC1, TMEM97 or both proteins were removed via gene editing. A series of radioligand binding studies, confocal microscopy studies, and internalization of radiolabeled or fluorescently tagged LDL particles were then conducted in these cells. The results indicate that PGRMC1 knockout (KO) did not reduce the density of binding sites for the sigma-2 receptor (σ2R) radioligands, [¹²⁵I]RHM-4 or [³H]DTG, but a reduction in the receptor affinity of both radioligands was observed. TMEM97 KO resulted in a complete loss of binding of [¹²⁵I]RHM-4 and a significant reduction in binding of [³H]DTG. TMEM97 KO and PGRMC1 KO resulted in an equal reduction in the rate of uptake of fluorescently-tagged or ³H-labeled LDL, and knocking out both proteins did not result in a further rate of reduction of LDL uptake. Confocal microscopy and Proximity Ligation Assay studies indicated a clear co-localization of LDLR, PGRMC1 and TMEM97. These data indicate that the formation of a ternary complex of LDLR-PGRMC1-TMEM97 is necessary for the rapid internalization of LDL by LDLR.

Cellular uptake of lipoproteins and persistent organic compounds--an update and new data

There are a number of interactions related to the transport of lipophilic xenobiotic compounds in the blood stream of mammals. This paper will focus on the interactions between lipoproteins and persistent organic pollutants (POPs) and how these particles are taken up by cells. A number of POPs including the pesticide p,p'-dichlorodiphenyltrichloroethane (DDT), and especially its metabolite p,p'-dichlorodiphenyldichloroethene (DDE), interacts with nuclear hormone receptors causing these to malfunction, which in turn results in a range of deleterious health effects in humans. The aim of the present study was to determine the role of lipoprotein receptors in mouse embryonic fibroblast (MEF) cells in conjunction with uptake of DDT-lipoprotein complexes from supplemented media in vitro. Uptake of DDT by MEF cells was investigated using MEF1 cells carrying the receptors low-density lipoprotein receptor-related protein (LRP) and low-density lipoprotein receptor (LDLR) present and MEF4 cells with no LRP and LDLR expression. Cells were incubated together with the complex of low-density lipoproteins (LDL) and [(14)C]DDT. The receptor function was further evaluated by adding the 40kDa receptor-associated protein (RAP) which blocks receptor activity. The results showed that [(14)C]DDT uptake was decreasing when the LDL concentration was increasing. There was no strong evidence for a receptor-mediated uptake of the [(14)C]DDT-lipoprotein complex. To conclude, DDT travels in the blood stream and can cross cell membranes while being transported as a DDT-lipoprotein complex. The lipoproteins do not need receptors to cross cell membranes since passive diffusion constitutes a major passageway.

Effects of HMG-CoA reductase inhibition on erythrocyte membrane cholesterol and acyl chain composition

The effects of 2 months treatment with simvastatin (40 mg, 20 mg p.o. daily) or placebo on erythrocyte membrane cholesterol content and acyl chain composition have been studied in 36 patients with a clinical history of atherosclerosis enrolled in the Oxford Cholesterol Study. All patients received advice corresponding to a standard phase 1 cholesterol-lowering diet. As expected the mean serum total cholesterol fell substantially (-26.5%, 20 mg simvastatin, P < 0.05; -32.7%, 40 mg simvastatin, P < 0.05) compared to placebo (-6.3%, ns). However, mean erythrocyte cholesterol content did not change significantly in any group (2 months therapy: 20 mg simvastatin, -0.62%; 40 mg simvastatin, +2.2%; placebo, -4.2%). Erythrocyte cholesterol was also unaltered after 5 months of therapy. Erythrocyte osmotic fragility was unchanged in the treatment and placebo groups. In the placebo group dietary advice alone was associated with a significant increase in the linoleic acid content of erythrocytes from 9.4 mole% of total acyl chains to 11.8 mole% (P < 0.05). Treatment with simvastatin was associated with an increase in the arachidonic acid content of the erythrocyte membrane from 12.2 to 15.3 mole% (P < 0.05). Treatment with simvastatin does not alter erythrocyte cholesterol content, but does alter acyl chain distribution. These results suggest that the chemical potential of cholesterol in serum is not markedly altered by HMG-CoA reductase inhibition.

Effect of verapamil and nifedipine on cholesteryl ester metabolism and low-density lipoprotein oxidation in macrophages

Using mouse macrophage cultures, the effects of verapamil and nifedipine on cholesteryl ester and low-density lipoprotein (LDL) metabolism were studied with special reference to the following parameters: (a) incorporation of [14C]oleate into cholesteryl esters (ChE), (b) contents of total and free cholesterol (FCh), (c) liberation of [14C]oleate from ChE and incorporation of [3H]FCh into ChE, (d) excretion of [3H]Ch from the cells, and (e) LDL oxidation. Verapamil and nifedipine (10-100 microM) were shown to decrease in a dose-dependent manner the incorporation of [14C]oleate into ChE and to increase the concentration of FCh but had no appreciable effect on the concentration of total cholesterol in macrophages cultured in the presence of acetylated LDL. The drugs stimulated the liberation of [14C]oleate from cellular ChE. The pharmacological concentrations (25-75 microM) of verapamil and nifedipine increased the excretion of [3H]FCh from ChE of macrophages in the presence of serum and high-density lipoproteins. The same concentrations of the drugs inhibited both LDL-derived malonyldialdehyde-like products and nitroblue tetrazolium dye reduction in a dose-dependent fashion. The results obtained suggest that verapamil and nifedipine exert their macrophage-mediated antiatherosclerotic effect via reduction of LDL oxidative modification, reduction of intracellular ChE synthesis, stimulation of ChE hydrolysis and cholesterol excretion from the cells.

Lipoprotein interactions with T cells: an update

The role of plasma lipoproteins in atherogenesis is well recognized but the physiological relevance of their immunoregulatory properties is still questioned. Here Karine Traill and colleagues outline the recent advances that have been made towards unravelling the mechanisms of immunoregulation by lipoproteins in vitro and consider whether any of these mechanisms are operative in vivo. In particular they address the possible detrimental effects of high serum lipoprotein levels on immune function and the question of whether hyperlipidemia (or hypercholesterolemia) should be considered a risk factor for diminished immunity, for example in old age.

Cholesterol accumulation in aortic smooth muscle cells exposed to low density lipoproteins. Contribution of free cholesterol transfer

Incubation of cultured arterial smooth muscle cells with large concentrations of low density lipoproteins (LDL) resulted in a net increase in cell cholesterol and cholesteryl ester mass that was dependent on LDL concentration and time of incubation. Use of an inhibitor of acyl-CoA:cholesterol acyl-transferase (ACAT) reduced the accumulation of cholesteryl ester mass by 40% (range 25% to 50%), suggesting that a significant proportion of the cholesteryl ester mass that accumulated from LDL did so without being hydrolyzed and re-esterified. Quiescent arterial smooth muscle cells exposed for 48 hours to 0.5 mg/ml of 125I-LDL accumulated 115 nmol total sterol/mg cell protein. However, these cells took up and degraded only 21 micrograms of 125I-LDL protein, which contains 64 nmol total cholesterol. Hence, only about 60% of the increase in cell-associated cholesterol mass was accounted for by LDL particle uptake and degradation. Further, when cells were incubated with 3H cholesteryl linoleyl ether-labeled LDL, the net increase of total cell cholesterol was 81 nmol/mg cell protein. However, only 49 nmol of total cholesterol was taken up by LDL particle uptake, as calculated from the uptake of the 3H cholesteryl linoleyl ether tracer. It thus appears that about 40% of the accumulated cholesterol mass was derived independent of LDL particle uptake, suggesting the possibility of transfer of free cholesterol from the surface of LDL to the cell surface. The occurrence of cholesterol surface transfer was independently verified by the measurement of the uptake and cellular distribution of LDL-derived free 3H-cholesterol. A substantial fraction of the accumulated cell cholesterol mass (approximately 40%) was derived from surface transfer of LDL free cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)

[14C]acetate incorporation by cultured normal, familial hypercholesterolemia and Down's syndrome fibroblasts

Fibroblasts from patients with homozygous familial hypercholesterolemia (FH), a disease characterized by accelerated atherogenesis, are known to lack functional low-density-lipoprotein receptors, which ultimately results in increased cholesterol biosynthesis in the cultured cells. [14C]Acetate incorporation in these cells was compared to that of normal fibroblasts and to fibroblasts from patients with Down's syndrome, a disease in which atherosclerosis is rare. Total [14C]acetate incorporation did not differ significantly between normal and Down's fibroblasts, nor did its partitioning into the hexane-extractable and aqueous fractions of the cell hydrolysates. [14C]Acetate incorporation was much greater in FH cells in both the aqueous and hexane-extractable fractions. Preincubation in fetal bovine serum increased acetate incorporation only by FH cells, while 50 micrograms low-density lipoprotein/ml medium depressed acetate incorporation in all three groups. A C27 sterol, identified by gas chromatography-mass spectrometry as a probable isomer of cholesterol, was present in small amounts in FH fibroblasts, but was not detectable in the normal or Down's cells. The absolute amounts of [14C]acetate incorporated into the non-sterol lipids were greater in the FH fibroblasts, indicating that these cells may have to synthesize, in addition to cholesterol, other required cellular lipids which are delivered to the normal cells by low-density lipoproteins.

[3H]cholesterol transfer from microemulsion particles of different sizes to human fibroblasts

A new technique for preparing microemulsion particles of well-defined sizes and compositions is presented. Utilization of these microemulsions is advocated as lipoprotein models in studies of lipid transport and metabolism, rather than the currently used phospholipid-cholesterol vesicles. The emulsion particles consisted of egg phosphatidylcholine and cholesterol as surface lipids and cholesteryl oleate as core lipid. They were prepared by a combined injection and sonication technique and size-separated by a two-step procedure of gel filtration chromatography and density gradient centrifugation. By varying the ratios of core and surface material, particles covering a size range of 20-200 nm in diameter could be produced. The adequacy of these microemulsions as lipoprotein models was tested by studying the transfer of [3H]cholesterol and [14C]cholesterol oleate from the particles to cultured human fibroblasts. Up to a particle size of 100 nm, there was a slight increase of [3H]cholesterol transfer. The transfer of [14C]cholesteryl oleate was very slow, yet measurable. Studies of the exchangeability of cholesterol between the microemulsion core and surface phases indicated that all cholesterol can be transferred from microemulsions to cultured cells as a single pool.

Uptake of persistent environmental chemicals by cultured human cells

Uptake of the persistent environmental chemicals 2,2',4,4',5,5'-hexachlorobiphenyl and 1,1,1-trichloro-2,2-di-(4-chlorophenyl)ethane (the insecticide DDT) by Chang liver cells, an established human cell line, has been investigated. Monolayer cells were incubated with culture medium to which the lipophilic model compounds had been added. The time course of uptake of either compound was biphasic, reaching equilibrium after about 5 hr of incubation. The ratio of DDT:hexachlorobiphenyl uptake was dependent on the presence of serum proteins. Increasing concentrations of serum proteins in the culture medium progressively inhibited uptake. Efflux from the cells was not entirely reversible: 10-20% of the chemicals were not released. Uptake was a linear function of the external concentration of the compounds. Absorptive binding to the outer cell plasma membrane could be determined by removing bound chemicals with fetal calf serum ("back exchange"). With this method, temperature-dependent translocation through the cell plasma membrane could directly be demonstrated. The effect of low temperature as well as the influence of metabolic inhibitors point out the contribution of energy-driven uptake pathways. Demonstration of LDL receptor-like binding protein on Chang liver cells facilitated estimation of the role of receptor-mediated uptake. This route of uptake proved to be of minor importance only, as was transport of the protein-bound chemicals via fluid pinocytosis. The results demonstrate that cellular endocytosis of plasma membrane-bound chemicals constitutes a major uptake pathway for lipophilic chemicals.

Mechanisms and consequences of cellular cholesterol exchange and transfer

It is apparent from consideration of the reactions involved in cellular cholesterol homeostasis that passive transfer of unesterified cholesterol molecules plays a role in cholesterol transport in vivo. Studies in model systems have established that free cholesterol molecules can transfer between membranes by diffusion through the intervening aqueous layer. Desorption of free cholesterol molecules from the donor lipid-water interface is rate-limiting for the overall transfer process and the rate of this step is influenced by interactions of free cholesterol molecules with neighboring phospholipid molecules. The influence of phospholipid unsaturation and sphingomyelin content on the rate of free cholesterol exchange are known in pure phospholipid bilayers and similar effects probably occur in cell membranes. The rate of free cholesterol clearance from cells is determined by the structure of the plasma membrane. It follows that the physical state of free cholesterol in the plasma membrane is important for the kinetics of cholesterol clearance and cell cholesterol homeostasis, as well as the structure of the plasma membrane. Bidirectional flux of free cholesterol between cells and lipoproteins occurs and rate constants characteristic of influx and efflux can be measured. The direction of any net transfer of free cholesterol is determined by the relative free cholesterol/phospholipid molar ratios of the donor and acceptor particles. Cholesterol diffuses down its gradient of chemical potential generally partitioning to the phospholipid-rich particle. Such a surface transfer process can lead to delivery of cholesterol to cells. This mechanism operates independently of any lipoprotein internalization by receptor-mediated endocytosis. The influence of enzymes such as lecithin-cholesterol acyltransferase and hepatic lipase on the direction of net transfer of free cholesterol between lipoproteins and cells can be understood in terms of their effects on the pool sizes and the rate constants for influx and efflux. Excess accumulation of free cholesterol in cells stimulates the rate of cholesteryl ester formation and induces deposition of cholesteryl ester inclusions in the cytoplasm similar to the situation in the 'foam' cells of atherosclerotic plaque. Clearance of cellular cholesteryl ester requires initial hydrolysis to free cholesterol followed by efflux of this free cholesterol. The rate of clearance of cholesteryl ester from cytoplasmic droplets is influenced by the physical state of the cholesteryl ester; liquid-crystalline cholesteryl ester is removed more slowly than cholesteryl ester in a liquid state.(ABSTRACT TRUNCATED AT 400 WORDS)

Relationships between lymphocyte cholesterol homeostasis and LDL-cholesterol

The homeostasis of cholesterol was studied in lymphocytes freshly isolated from the blood and cultured with or without low-density lipoprotein (LDL). The content of cholesterol decreased in the lymphocytes cultured without LDL, whereas LDL substituted for cellular cholesterol losses, in spite of almost suppressed LDL-receptor and lymphocyte cholesterol synthesis. Free cholesterol was taken up from LDL mainly via cholesterol exchange and, in contrast to esterified cellular cholesterol, rapidly excreted into the medium. In vitro stimulation of lymphocyte cholesterol synthesis was correlated to the ratio of esterified to free LDL-cholesterol in the blood from which the lymphocytes had been isolated. This result probably reflects the different rates of influx and efflux of esterified or free cholesterol between plasma lipoproteins and lymphocytes. These effects should be taken into account if LDL-cholesterol is determined in plasma for the evaluation of an individual's atherosclerotic risk.

Familial hypercholesterolaemia: effect of low density lipoproteins on esterification of cholesterol in lymphocytes from homozygous and heterozygous subjects

The effect of low density lipoproteins on esterification of cholesterol was studied in lymphocytes from patients with familial hypercholesterolaemia; results were compared with those obtained using cells from normal individuals. Freshly isolated lymphocytes were maintained in lipoprotein-deficient medium for 48 h and the rate of formation of [3H] cholesteryl oleate from [3H] oleate was then determined in the presence or absence of low density lipoproteins. In the absence of low density lipoproteins, incorporation of [3H] oleate was higher in heterozygote and homozygote cells than in normal lymphocytes. Incorporation in the presence of low density lipoproteins was increased relative to that measured in their absence for all of the subjects studied; heterozygotes and homozygotes showed marked changes in some cases but not in others.

Uptake of 2,3,7,8-tetrachlorodibenzo-p-dioxin from plasma lipoproteins by cultured human fibroblasts

Tritiated 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) added to human plasma in vitro associated with the plasma lipoproteins. The effects of plasma and lipoproteins on cellular uptake of dioxin were studied using normal human skin fibroblasts and mutant fibroblasts from a patient with homozygous familial hypercholesterolemia. The latter cells lack the normal cell membrane receptor for low density lipoprotein (LDL). The time- and temperature-dependent cellular uptake of [3H]dioxin was greatest from LDL, intermediate from high density lipoprotein (HDL) and least from serum. A significantly greater uptake from LDL by the normal cells compared to the mutant cells indicated the involvement of the LDL receptor-mediated pathway. Concentration-dependent studies indicated that the cellular uptake at 37 degrees C of [3H]dioxin varied linearly with dioxin concentration at constant LDL concentration. Thin-layer chromatography (TLC) showed that conversion to more polar compounds may have occurred after 24-h incubation with cells. [3H]Dioxin could be removed from cells efficiently by incubation with 20% serum greater than HDL greater than LDL. Since the vehicle of delivery may influence subsequent location and metabolism of this compound in cells, it is concluded that the physiologic vehicles (either serum- or LDL-associated dioxin), rather than organic solvents, should be used in experiments with cultured cells or perfused organs.

Cholesterol synthesis in cultured human peripheral lymphocytes. Influence of LDL, HDL, cholesterol/phosphatidylcholine liposomes and complete serum

Lipoprotein-deficient milieu, freshly isolated human peripheral blood lymphocytes lose about 50% of their membrane cholesterol into the medium within 8 h. The cholesterol loss is counter-regulated by de novo synthesis commencing after a lag phase of 8-12 h, and reaching a steady state within 24 h at a diminished membrane cholesterol level. About 50 micrograms free cholesterol/ml, offered in the form of low-density lipoproteins (LDL) and cholesterol/phosphatidylcholine liposomes, suppressed cholesterol synthesis to about 20% of that controls (lipoprotein-deficient culture). By contrast, pure phosphatidylcholine liposomes enhanced cholesterol synthesis to about 150% of control values. High-density lipoproteins (HDL) exerted a slightly suppressive effect on cholesterol synthesis only at high concentrations (greater than 100 micrograms HDL cholesterol/ml). HDL added to cultures containing fixed concentrations of LDL led to a dose-dependent neutralization of LDL suppression of cholesterol synthesis. Culture medium containing complete serum caused a suppression of cholesterol synthesis to about 50% of the control. The lesser reduction in cholesterol synthesis caused by complete serum compared with LDL or cholesterol/phosphatidylcholine liposomes can be explained by the presence of HDL in the former. Our results support the view that the cholesterol requirement of blood lymphocytes in their lipid-rich milieu is met by cholesterol neosynthesis as well as an exchange mechanism with surrounding lipoproteins. In our system, the cholesterol neosynthesis appears to be controlled by the ratio of LDL to HDL in the surrounding medium.

The effect of non-receptor-mediated uptake of cholesterol on intracellular cholesterol metabolism in human skin fibroblasts

Unilamellar lipid vesicles of various cholesterol:phosphatidylcholine molar ratios were used to alter, via passive exchange at the plasma membrane, the cellular free cholesterol content of cultured human skin fibroblasts which had been preincubated in lipoprotein-deficient serum. The effects of these net surface transfers of cholesterol on cellular cholesterol biosynthesis, cholesterol esterification and low density lipoprotein (LDL) binding were determined and were compared with the effects of cholesterol delivered to the cell interior via the receptor-mediated endocytosis of LDL. Both LDL and cholesterol-rich lipid vesicles increased cell cholesterol within 6 h. Cells exposed to LDL also showed, within 6 h, decreased cholesterol synthesis, decreased LDL binding and increased cholesterol esterification. Cells incubated with the cholesterol-rich vesicles showed similar changes but these were delayed and did not occur until 24 h. Fibroblasts incubated with cholesterol-free phosphatidylcholine vesicles had decreased cell cholesterol, increased cholesterol synthesis, increased LDL binding, and decreased esterification, but only after 24 h of incubation. These results suggest that passive net transfers of cholesterol occurring at the cell surface can with time modulate intracellular cholesterol metabolism. These findings are consistent with the idea that the movement of cholesterol from the cell surface to the cell interior is a limited and relatively slow process.

Chemical composition and physical state of lipid deposits in atherosclerosis

The composition, morphology, and physical properties of lipids in atherosclerotic lesions from human aortas were studied in order to elucidate the factors for the accumulation of cholesterol and its esters in the vessel wall. Lesions were classified histologically into 3 groups: fatty streak, fibrous plaque, and advanced plaque. The relative lipid composition of the lesions was plotted on the phase diagram of the 3 major lipids: cholesterol, cholesteryl ester, and phospholipid. Early fatty streaks had compositions within the 2-phase zone with a cholesterol-phospholipid liquid crystalline phase and a cholesteryl ester oily phase. Advanced fatty streaks and fibro-fatty plaques fell within the 3-phase zone with excess free cholesterol. Advanced plaques also had an average lipid composition within the 3-phase zone, but with a larger excess of free cholesterol. From the lipid-chemical point of view there is a continuous progression from early fatty streaks through advanced fatty streaks and fibro-fatty plaques to advanced plaques. In fatty streaks the cholesteryl esters accumulate in the form of isotropic and anisotropic droplets. The latter are in the smectic liquid crystalline state with the molecules arranged in layers and have surfaces that are spherical and smooth. Fibrous and advanced plaques showed beside droplets also amorphous lipids and cholesterol monohydrate crystals. Some of the amorphous lipids were solid up to about 45 degrees C and exhibited a smectic phase at cooling, indicating cholesteryl esters as the major component. The transition temperatures of high-melting cholesteryl esters, e.g. palmitate, are depressed by low-melting ones. Most of the triglycerides are present in the cholesteryl ester droplets and abolish the cholesteric liquid crystalline phase.

Physicochemical transfer of [3H]cholesterol from plasma lipoproteins to cultured human fibroblasts

The transfer of free cholesterol from [3H]cholesterol-labelled plasma lipoproteins to cultured human lung fibroblasts was studied in a serum-free medium. The uptake of [3H]cholesterol depended upon time of incubation, concentration of lipoprotein in the medium, and temperature. Modified (reduced and methylated) low-density lipoprotein (LDL), which did not enter the cells by the receptor pathway, gave a somewhat lower transfer rate than unmodified LDL, but if the transfer values for native LDL were corrected for the receptor-mediated uptake of cholesterol the difference was eliminated. The initial rates of transfer of [3H]cholesterol from LDL and high-density lipoprotein (HDL) were of the same order of magnitude (0.67 +/- 0.05 and 0.75 +/- 0.06 nmol of cholesterol/h per mg of cell protein, respectively) while that from very-low-density lipoprotein (VLDL) was much lower (0.23 +/- 0.02 nmol of cholesterol/h per mg) (means +/- S.D., n = 5). The activation energy for transfer of cholesterol from reduced, methylated LDL to fibroblasts was determined to be 57.5 kJ/mol. If albumin was added to the incubation medium the transfer of [3H]cholesterol was enhanced, while that of [14C]dipalmitoyl phosphatidylcholine was decreased compared with the protein-free system. The results demonstrate that, in spite of its low water solubility, free cholesterol can move from lipoproteins to cellular membranes, probably by aqueous diffusion. We propose that physicochemical transfer of free cholesterol may be a significant mechanism for net uptake of the sterol into the artery during atherogenesis.

Uptake and esterification of exogenous cholesterol by low-density-lipoprotein-receptor-negative human fibroblasts in culture

The incorporation and metabolism of both vesicle- and LDL (low-density lipoprotein)- derived [3H]cholesterol by LDL-receptor-negative fibroblasts were studied. Independent of the cholesterol source, free [3H]cholesterol was readily incorporated into the cells and was available for esterification. 7-Oxocholesterol stimulated both [3H]cholesterol incorporation, by increasing the exchange rate, and the subsequent esterification of it irrespective of the source of exogenous [3H]cholesterol. The 7-oxocholesterol-stimulated esterification of exogenously derived LDL free [3H]cholesterol was progesterone-sensitive and energy-requiring.

Imipramine associations with plasma components and its uptake by cultured human cells

It has been proposed that in vivo variability in response to certain hydrophobic chemicals or drugs, such as imipramine, may be due in part to the varying plasma lipid levels in patients. The distribution of [3H]imipramine into the lipoproteins of human plasma was therefore studied. Differential density centrifugation of plasma containing [3H]imipramine resulted in flotation of very low density, low density and high density lipoproteins (VLDL, LDL, HDL) and approximately one-third of the total 3H radioactivity. Twelve percent of the radioactivity was present in the sedimented fraction which included most of the plasma proteins. There appeared to be little specific binding of [3H]imipramine to VLDL or LDL, as shown by ultracentrifugation, dialysis and column chromatography. [3H]Imipramine was readily incorporated into cultured human fibroblasts;o no differences were observed in cellular uptake whether it was added to the medium in plasma, LDL or HDL. Also, no differences in uptake of [3H]imipramine by LDL-receptor positive and receptor negative cells were noted. These experiments indicate that LDL is not a major vehicle for the transport of this drug and that both the bound and free fractions are available for cellular uptake.

Transfer of [3H]cholesterol between lipid vesicles and rat arterial smooth muscle cells in vitro

Unesterified [3H]cholesterol is rapidly transferred between cholesterol-phosphatidylcholine vesicles and rat arterial smooth muscle cells in vitro. Exchange rate is influenced by the vesicle/cell ratio in a saturable way. The maximal transfer of cholesterol, which is 3.76 micrograms per mg cell protein during 4 h, is achieved with a vesicle/cell ratio of 3.4 X 10(7). Bovine serum albumin enhances the exchange by a factor of 4.5 compared to a protein-free system. The activation energy for the process is + 38.5 kJ X mol-1 with vesicles of 1:1 mole ratio of cholesterol to phosphatidylcholine (C/P). A fraction of the incorporated free [3H]cholesterol is esterified within 4 h with donor vesicles of over 1:1 C/P. When cells were incubated with vesicles of low C/P mole ratio (1:2) a fraction of the incorporated free [3H]cholesterol was esterified within 16 h. Our results are compatible with the aqueous diffusion mechanism of cholesterol exchange. Furthermore, we suggest that, in rat smooth muscle cells, the cell membrane cholesterol pool is not metabolically isolated from internal cholesterol pools, at least as judged by the ability of the cells to esterify incorporated free cholesterol.