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

Phosphatidylcholine-Mediated Aqueous Diffusion of Cellular Cholesterol Down-Regulates the ABCA1 Transporter in Human Skin Fibroblasts

ATP-binding cassette protein A1 (ABCA1) is a cholesterol transporter that contributes to the active transport/removal of excess cellular cholesterol. ABCA1 expression is up-regulated when cells accumulate cholesterol.

AIMS

The purpose of this study was to determine any correlation between extracellular phospholipid levels and ABCA1 expression and function.

METHODOLOGY

Human foreskin fibroblasts were incubated with cholesterol alone or cholesterol and phosphatidylcholine. Total RNA was isolated and subjected to end-point RT-PCR to compare ABCA1 transcript levels. Cell lysates were subjected to Western blot analysis to compare ABCA1 protein levels. Cells were loaded with radiolabeled cholesterol and cellular cholesterol efflux was measured in the presence and absence of apoE, a cholesterol acceptor. ApoE-dependent efflux was calculated as a measure of ABCA1-mediated efflux.

RESULTS

Here we show that incubation of cholesterol-loaded human skin fibroblasts with L-α-phosphatidylcholine (PC) decreases ABCA1 mRNA and protein levels by 93% and 57%, respectively, compared to cells loaded with cholesterol alone. Similarly, PC treatment results in a 25% reduction in ABCG1 mRNA levels compared to cells treated with cholesterol alone, but there is no change in SR-BI transcript levels. Subsequent incubation of phospholipid-treated cells with a cholesterol acceptor such as apoE for 24 hours shows a 65% reduction in ABCA1-mediated cholesterol efflux compared to efflux in cells not treated with PC. During the lipid treatment itself, there is a 2.7-fold greater loss of cholesterol from PC treated cells compared to cells treated with cholesterol alone. Measurement of cholesterol in cellular lipid extracts reveals that cells incubated in the presence of phosphatidylcholine are significantly depleted of cholesterol having only 20% of the cholesterol compared to cells loaded with cholesterol alone.

CONCLUSION

Thus, phosphatidylcholine facilitates removal of cellular cholesterol, thereby negating the cholesterol-dependent induction of ABCA1 message, protein and function.

Liposomal cholesterol delivery activates the macrophage innate immune arm to facilitate intracellular Leishmania donovani killing

Leishmania donovani causes visceral leishmaniasis (VL) by infecting the monocyte/macrophage lineage and residing inside specialized structures known as parasitophorous vacuoles. The protozoan parasite has adopted several means of escaping the host immune response, with one of the major methods being deactivation of host macrophages. Previous reports highlight dampened macrophage signaling, defective antigen presentation due to increased membrane fluidity, and the downregulation of several genes associated with L. donovani infection. We have reported previously that the defective antigen presentation in infected hamsters could be corrected by a single injection of a cholesterol-containing liposome. Here we show that cholesterol in the form of a liposomal formulation can stimulate the innate immune arm and reactivate macrophage function. Augmented levels of reactive oxygen species (ROS) and reactive nitrogen intermediates (RNI), along with proinflammatory cytokines such as tumor necrosis factor alpha (TNF-α) and interleukin 6 (IL-6), corroborate intracellular parasite killing. Cholesterol incorporation kinetics is favored in infected macrophages more than in normal macrophages. Such an enhanced cholesterol uptake is associated with preferential apoptosis of infected macrophages in an endoplasmic reticulum (ER) stress-dependent manner. All these events are coupled with mitogen-activated protein (MAP) kinase activation, while inhibition of such pathways resulted in increased parasite loads. Hence, liposomal cholesterol is a potential facilitator of the macrophage effector function in favor of the host, independently of the T-cell arm.

Adaptation of cholesterol-requiring NS0 mouse myeloma cells to high density growth in a fully defined protein-free and cholesterol-free culture medium

NS0 has been used as a fusion partner for the production of hybridomas and has more recently been engineered to produce recombinant protein. A protein-free culture medium, designated W38 medium, has previously been developed which supported high density growth of rat myeloma and hybridoma cell lines. NS0 cells failed to grow in W38 medium and in a number of protein-free culture media which support the growth of other myeloma cell lines. NS0 cells are derived from the NS-1 cell line, which is known to require exogencus cholesterol. It was found that NS0 cells grew in W38 medium supplemented with phosphatidylcholine, cholesterol, and albumin and that NS0 were auxotrophic for cholesterol. Protein-free growth of NS0 cells was achieved by using β-cyclodextrin to replace albumin as a lipid carrier. The maximal cell density reached in this protein-free medium was in excess of 1.5×10(6) cell ml(-1). The lipid supplements in the medium precipitated after a few days storage at +4°C. In order to overcome this problem a protocol was developed which allowed NS0 cells to be adapted to cholesterol-independent growth in W38 medium. NS0.CF (cholesterol-independent NS0 cells) were cultured continuously in W38 medium for several months. In shake flask culture a cell density of 2.4×10(6) cells ml(-1) was achieved in W38 medium compared with 1.41×10(6) cells ml(-1) in RPMI 1640 medium containing 10% foetal bovine serum. NS0.CF cells readily grew in a 1 litre stirred bioreactor using W38 medium supplemented with Pluronic F68 reaching a density of 3.24×10(6) cells ml(-1). NS0.CF were cloned protein-free by limiting dilution in W38 medium, giving colonies in wells that were seeded at an average density of 0.32 cells per 200 μl. This study has demonstrated for the first time the growth of a cholesterol-requiring mouse myeloma cell line in a completely defined protein-free medium and its subsequent adaptation to cholesterol-independence.

Sensitivity of cholecystokinin receptors to membrane cholesterol content

Cholesterol represents a structurally and functionally important component of the eukaryotic cell membrane, where it increases lipid order, affects permeability, and influences the lateral mobility and conformation of membrane proteins. Several G protein-coupled receptors have been shown to be affected by the cholesterol content of the membrane, with functional impact on their ligand binding and signal transduction characteristics. The effects of cholesterol can be mediated directly by specific molecular interactions with the receptor and/or indirectly by altering the physical properties of the membrane. This review focuses on the importance and differential effects of membrane cholesterol on the activity of cholecystokinin (CCK) receptors. The type 1 CCK receptor is quite sensitive to its cholesterol environment, while the type 2 CCK receptor is not. The possible structural basis for this differential impact is explored and the implications of pathological states, such as metabolic syndrome, in which membrane cholesterol may be increased and CCK1R function may be abnormal are discussed. This is believed to have substantial potential importance for the development of drugs targeting the CCK receptor.

Use of cyclodextrins to manipulate plasma membrane cholesterol content: evidence, misconceptions and control strategies

The physiological importance of cholesterol in the cell plasma membrane has attracted increased attention in recent years. Consequently, the use of methods of controlled manipulation of membrane cholesterol content has also increased sharply, especially as a method of studying putative cholesterol-enriched cell membrane domains (rafts). The most common means of modifying the cholesterol content of cell membranes is the incubation of cells or model membranes with cyclodextrins, a family of compounds, which, due to the presence of relatively hydrophobic cavity, can be used to extract cholesterol from cell membranes. However, the mechanism of this activity of cyclodextrins is not completely established. Moreover, under conditions commonly used for cholesterol extraction, cyclodextrins may remove cholesterol from both raft and non-raft domains of the membrane as well as alter the distribution of cholesterol between plasma and intracellular membranes. In addition, other hydrophobic molecules such as phospholipids may also be extracted from the membranes by cyclodextrins. We review the evidence for the specific and non-specific effects of cyclodextrins and what is known about the mechanisms for cyclodextrin-induced cholesterol and phospholipid extraction. Finally, we discuss useful control strategies that may help to verify that the observed effects are due specifically to cyclodextrin-induced changes in cellular cholesterol.

Probing red cell membrane cholesterol movement with cyclodextrin

We probed the kinetics with which cholesterol moves across the human red cell bilayer and exits the membrane using methyl-beta-cyclodextrin as an acceptor. The fractional rate of cholesterol transfer (% s(-1)) was unprecedented, the half-time at 37 degrees C being ~1 s. The kinetics observed under typical conditions were independent of donor concentration and directly proportional to acceptor concentration. The rate of exit of membrane cholesterol fell hyperbolically to zero with increasing dilution. The energy of activation for cholesterol transfer was the same at high and low dilution; namely, 27-28 Kcal/mol. This behavior is not consistent with an exit pathway involving desorption followed by aqueous diffusion to acceptors nor with a simple one-step collision mechanism. Rather, it is that predicted for an activation-collision mechanism in which the reversible partial projection of cholesterol molecules out of the bilayer precedes their collisional capture by cyclodextrin. Because the entire membrane pool was transferred in a single first-order process under all conditions, we infer that the transbilayer diffusion (flip-flop) of cholesterol must have proceeded faster than its exit, i.e., with a half-time of <1 s at 37 degrees C.

Cholesterol enrichment upregulates intercellular adhesion molecule-1 in human vascular endothelial cells

Hypercholesterolemia is a major risk factor for atherosclerosis, but the mechanism by which cholesterol activates the endothelium remains undocumented. The present investigation was undertaken to investigate the role of cholesterol, one of the bioactive moieties of the low-density lipoprotein (LDL) particle, in initiating of intracellular signaling in endothelial cells (ECs) and culminating in increased abundance of the intercellular adhesion molecule-1 (ICAM-1). Cholesterol was delivered to human umbilical vein ECs (HUVECs) via cholesterol-enriched liposomes. In HUVECs, the cellular cholesterol:phospholipid ratio increased after 1 h of exposure to cholesterol. The level of ICAM-1 increased in both mRNA and protein after 24 h of cholesterol exposure. ICAM-1 mRNA half-life was not affected by cholesterol exposure. Promoter studies showed greater than two-fold activation of the ICAM-1 gene expression after cholesterol exposure. Electrophoretic mobility shift assay showed that activator protein-1 (AP-1) activity substantially increased after 2 h of exposure to cholesterol. In contrast, cholesterol did not affect nuclear factor-kappaB (NF-kappaB) activity. Results of trans-reporting assay revealed 2.5-fold increased expression of the AP-1-dependent reporter gene after cholesterol exposure whereas NF-kappaB-dependent expression was not affected. The AP-1/Ets (-891 to -908) site, one of the three AP-1-like sites in the ICAM-1 promoter, was most responsive to cholesterol. These data demonstrate for the first time that cholesterol enrichment phenotypically modulates ECs by transcriptionally upregulating ICAM-1 expression.

Defect of receptor-G protein coupling in human gallbladder with cholesterol stones

Human gallbladders with cholesterol stones (ChS) exhibit an impaired muscle contraction and relaxation and a lower CCK receptor-binding capacity compared with those with pigment stones (PS). This study was designed to determine whether there is an abnormal receptor-G protein coupling in human gallbladders with ChS using (35)S-labeled guanosine 5'-O-(3-thiotriphosphate) ([(35)S]GTPgammaS) binding, (125)I-labeled CCK-8 autoradiography, immunoblotting, and G protein quantitation. CCK and vasoactive intestinal peptide caused significant increases in [(35)S]GTPgammaS binding to Galpha(i-3) and G(s)alpha, respectively. The binding was lower in ChS than in PS (P < 0.01). The reduced [(35)S]GTPgammaS binding in ChS was normalized after the muscles were treated with cholesterol-free liposomes (P < 0.01). Autoradiography and immunoblots showed a decreased optical density (OD) for CCK receptors, an even lower OD value for receptor-G protein coupling, and a higher OD for uncoupled receptors or Galpha(i-3) protein in ChS compared with PS (P < 0.001). G protein quantitation also showed that there were no significant differences in the Galpha(i-3) and G(s)alpha content in ChS and PS. We conclude that, in addition to an impaired CCK receptor-binding capacity, there is a defect in receptor-G protein coupling in muscle cells from gallbladder with ChS. These changes may be normalized after removal of excess cholesterol from the plasma membrane.

CCK receptor dysfunction in muscle membranes from human gallbladders with cholesterol stones

Human gallbladders with cholesterol stones exhibit impaired muscle contraction induced by agonists that act on transmembrane receptors, increased membrane cholesterol content, and abnormal cholesterol-to-phospholipid ratio compared with those with pigment stones. The present study was designed to investigate the functions of the CCK receptor of gallbladder muscle membranes by radioreceptor assay and cross-linking. 125I-labeled CCK-8 binding was time-dependent, competitive, and specific. Scatchard analysis showed that the maximum specific binding (Bmax) was significantly decreased in cholesterol compared with pigment stone gallbladders (0.18 +/- 0. 07 vs. 0.38 +/- 0.05 pmol/mg protein, P < 0.05). In contrast, the affinity for CCK was higher in cholesterol than pigment stone gallbladders (0.18 +/- 0.06 vs. 1.2 +/- 0.23 nM). Similar results were observed in binding studies with the CCK-A receptor antagonist [3H]L-364,718. Cross-linking and saturation binding studies also showed significantly less CCK binding in gallbladders with cholesterol stones. These abnormalities were reversible after incubation with cholesterol-free liposomes. The Bmax increased (P < 0.01) and the dissociation constant decreased (P < 0.001) after incubation with cholesterol-free liposomes. In conclusion, human gallbladders with cholesterol stones have impaired CCK receptor binding compared with those with pigment stones. These changes are reversed by removal of the excess membrane cholesterol. These receptor alterations may contribute to the defective contractility of the gallbladder muscle in patients with cholesterol stones.

Excess membrane cholesterol alters human gallbladder muscle contractility and membrane fluidity

BACKGROUND & AIMS

The relationship between muscle contractility, plasma membrane cholesterol, and fluidity was investigated in human gallbladders with gallstones.

METHODS

Isolated gallbladder muscle cells were used to measure contraction. Plasma membranes of gallbladder muscle were purified in a sucrose gradient and measured for cholesterol content and cholesterol/phospholipid mole ratio. Membrane fluidity was determined by using fluorescence polarization and was expressed as the reciprocal of anisotropy.

RESULTS

The maximal contraction induced by cholecystokinin octapeptide was significantly less in gallbladders with cholesterol stones than in those with pigment stones. The membrane cholesterol content and cholesterol/phospholipid mole ratio were significantly higher in gallbladders with cholesterol stones than in those with pigment stones. Membrane anisotropy was also higher than in gallbladders with pigment stones, reflecting lower membrane fluidity in gallbladders with cholesterol stones. After muscle cells from cholesterol stone gallbladders were incubated with cholesterol-free liposomes for 4 hours, cholecystokinin octapeptide-induced contraction, membrane cholesterol content and cholesterol/phospholipid ratio, and membrane fluidity returned to normal levels.

CONCLUSIONS

Gallbladder muscle from patients with cholesterol stones has increased membrane cholesterol/phospholipid mole ratio and decreased membrane fluidity resulting in impaired muscle contractility. These abnormalities are corrected by removing the excess cholesterol from the plasma membranes.

Cholesterol and omega-3 fatty acids inhibit Na, K-ATPase activity in human endothelial cells

We have investigated the effects of cholesterol and omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexenoic acid (DHA) on Na, K-ATPase activity in human endothelial cells (HUVEC). Cultured HUVEC were incubated for 18 h with pure egg phosphatidylcholine (PC), or cholesterol-enriched liposomes (4 mg PC/ml). EPA and DHA alpha-tocopherol-acetate were emulsified with PC and incubated with HUVEC (10 mM). Na, K-ATPase and 5'-nucleotidase activities were determined using the coupled assay method on microsomal fractions obtained from cultured cells using non treated cells as control. Cholesterol enrichment significantly reduced both Na, K-ATPase and 5'-nucleotidase activities by a similar level (- 40%), whereas pure phospholipid liposomes inhibited this activity only by 22%. The dose-response curves of Na, K-ATPase activity were all biphasic assuming the presence of two independent sites exhibiting different affinities for ouabain of nM and microM respectively. The cholesterol induced inhibitory effect was greater for low affinity sites (-54%) as compared to that of the high affinity sites (-24%) whereas omega-3 fatty acids reduced the activity of both sites by 22%. Short term effects of EPA and DHA on Na, K-ATPase activity were determined by incubating microsomal fractions from untreated cells with various concentrations of free fatty acids (from 1 to 200 microM) for 20 min. Both EPA and DHA significantly reduced Na, K-ATPase activity but inhibition by EPA seems to be more effective than DHA. These results suggest that cholesterol and omega-3 fatty acids reduce Na, K-ATPase activity in HUVEC.

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.

Reverse cholesterol transport in the rat following a short-term intravenous infusion of fat emulsion

The effect on cholesterol transport of an intravenous infusion of a fat emulsion (10%) Intralipid or 10% Lipovenös) in vivo was investigated in the rat. Intralipid (1.85 ml/hr/kg body weight in rts for 3 hr) caused a reduction (P < 0.05) in free cholesterol in the aorta (by 25%), in plasma high-density lipoproteins (64%) and in erythrocytes (11%) with a concomitant enrichment of liver free cholesterol (16%), suggesting an enhanced reverse cholesterol transport in this species. Lipovenös under the same conditions gave similar results. Our data support our previous in vivo study in man indicating that infusion of a fat emulsion is able even to remove cholesterol from the arterial wall and thereby possibly be considered as an antiatherosclerotic agent.

[3H]ouabain binding to cultured endothelial cells: effect of cholesterol enrichment

Binding experiments were performed with [3H]ouabain on cultured human umbilical vein endothelial cells (huvEC). Saturation studies yielded a binding capacity (Bmax) of 820 +/- 81 fmole/mg pr.(n = 4) and dissociation constant (KD) of 11.7 +/- 2.1nM (n = 4) in K(+)-free buffer for specific [3H] ouabain binding on these cells. External K+ inhibited this binding in a dose-dependent manner. The mean value of Bmax is equivalent to about 4 x 10(5) sites per cell, comparable with that of smooth muscle cell. These data demonstrated the presence of specific [3H]ouabain binding linked to Na+/K+ pump, consistent with the observations of ouabain-sensitive 86Rb uptake in huvEC. Effect of cholesterol enrichment was also studied. Incubation in media supplemented with cholesterol-phospholipid liposomes of molar ratio of 2:1 for 18 hours reduced the Bmax by 31% (P < 0.05) without significantly changed the value of KD. This reduction of [3H]ouabain binding appeared to be specific for cholesterol since liposome made with pure phospholipid did not alter binding. Recent findings indicate that cholesterol-enrichment and plasma lipoproteins enhance vascular contractile response, our results suggest that the cholesterol-enrichment of endothelial cells may also indirectly affect the vascular response via disturbing the function of Na+/K+ pump.

Acute exposure to cholesterol increases arterial nitroprusside- and endothelium-mediated relaxation

The effect of cholesterol enrichment on arterial relaxation was studied by evaluating sodium nitroprusside (SNP)- and endothelium-mediated relaxation of isolated rabbit carotid artery. Arterial segments were perfused in vitro (4 h) with cholesterol-rich liposomes consisting of free cholesterol (FC) and phospholipid (PL) in a 2:1 molar ratio. Ring segments from arteries exposed to cholesterol-rich liposomes exhibited a 60% increase (P < 0.01) in FC content without affecting PL content. Cholesterol-enrichment was associated with a twofold increase (r = 0.92, P < 0.05) in acetylcholine- and A23187-induced endothelium-mediated relaxation. Bioassay of endothelium-derived relaxing factor(s) (EDRF) after cholesterol exposure indicated that EDRF half-life and/or release increased (P < 0.05) threefold. A trend (P = 0.07) toward increased smooth muscle cell sensitivity to EDRF after cholesterol enrichment was also observed. Cholesterol enrichment increased (P < 0.05) sensitivity to SNP 12-fold, and this difference was further augmented (P < 0.01) twofold with endothelium removal. Cholesterol enrichment had no effect on relaxation to N2,2'-O-dibutyrylguanosine 3',5'-cyclic monophosphate. These data indicate that acute cholesterol enrichment increases EDRF activity from arterial endothelium and increases smooth muscle responses to both EDRF and SNP.

The effect of cholesterol on the accumulation of intracellular calcium

Cholesterol/egg phosphatidylcholine (PC) liposomes (1:1 or 4:1, M/M), in which the absolute amount of PC was adjusted to be the same, were incubated with cultured bovine arterial smooth muscle cells for up to 8 h at 37 degrees C. The effect of increased cellular cholesterol on the accumulation of intracellular calcium in these cells was studied. The results indicate that the intracellular calcium content, measured by Fura-2/AM, was increased 2.3-fold by incubation with 4:1, cholesterol/PC liposomes. Kinetic analysis using 45Ca2+ indicated that the increased calcium influx was due to increase of pool size, not from a change of rate constant. (Ca2+ + Mg2+)-ATPase activity was decreased by 4:1, cholesterol/PC liposomes. The molar ratio of cholesterol/phospholipids in the cell membranes was directly proportional to that in liposomes. No change in phospholipid composition was noted. We suggest that the accumulation of intracellular calcium was a composite result due to the altering effect of inserted cholesterol on surface area, and to direct interactions between cholesterol and the proteins of the Ca2+ channel and (Ca2+ + Mg2+)-ATPase.

Modulation of membrane cholesterol levels: effects on endothelial cell function

The endothelial cell lining of blood vessels is now recognized as an active interface between blood and the underlying tissue. Modulation of cholesterol levels in several cell types has resulted in altered cell function. We have removed cholesterol from the endothelial cell membrane and have observed corresponding alterations in endothelial cell function. Following depletion of cholesterol from the endothelial cells, polymorphonuclear leukocyte adhesion to the cells was decreased. Angiotensin-converting enzyme activity of the endothelial cells was increased following removal of cholesterol from the endothelial cell membranes. The results of fluorescence polarization measurements suggest that these changes may be partially explained by altered membrane order.

Cholesterol movement between the plasma membrane and the cholesteryl ester droplets of cultured Leydig tumour cells

The present studies characterize the turnover of plasma membrane cholesterol in MA-10 Leydig tumour cells. Plasma membrane cholesterol of MA-10 cells was slowly internalized and converted into cholesteryl ester. Low-density lipoprotein (LDL) stimulated, in a dose- and time-dependent fashion, plasma membrane cholesterol conversion into intracellular esters. Stimulation of membrane internalization was not simply the consequence of accelerated uptake of membrane with LDL, since binding and internalization of epidermal growth factor and transferrin had no effect on turnover of plasma membrane cholesterol. The protein of LDL is unimportant as well, since delipidated LDL had no effect on membrane turnover. The action of LDL on cholesterol turnover was explained entirely by its contribution to cholesteryl ester stores. The degree of plasma membrane cholesterol internalization and esterification was directly proportional to the size of cellular ester stores.

Lipid transport pathways in mammalian cells

A major deficit in our understanding of membrane biogenesis in eukaryotes is the definition of mechanisms by which the lipid constituents of cell membranes are transported from their sites of intracellular synthesis to the multiplicity of membranes that constitute a typical cell. A variety of approaches have been used to examine the transport of lipids to different organelles. In many cases the development of new methods has been necessary to study the problem. These methods include cytological examination of cells labeled with fluorescent lipid analogs, improved methods of subcellular fractionation, in situ enzymology that demonstrates lipid translocation by changes in lipid structure, and cell-free reconstitution with isolated organelles. Several general patterns of lipid transport have emerged but there does not appear to be unifying mechanism by which lipids move among different organelles. Significant evidence now exists for vesicular and metabolic energy-dependent mechanisms as well as mechanisms that are clearly independent of cellular ATP content.

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)

Fusion of lipid vesicles with ascites tumor cells and their lipid-depleted variants. Studies with radioactive- and fluorescent-labeled vesicles

Cultured ascites tumor cells and their lipid-depleted variants, which contained 35-40% less membrane phospholipid and cholesterol, were used for fusion experiments with unilamellar lipid vesicles which were between 300 and 600 nm in diameter. Vesicle-cell interaction was followed by tracer studies using vesicles double-labeled in the lipid moiety, by vesicle-encapsulated [3H] dextran, and by measurements of energy transfer between N-(10-[1-pyrene]decanoyl)sphingomyelin-labeled vesicles and alpha-parinaric acid-labeled cells in the presence of poly(ethylene glycol) (PEG) as fusogen. The reaction rates measured with the radiolabeled vesicles were found to follow patterns similar to those obtained with the resonance energy transfer assay. This latter method revealed a vesicle-cell membrane fusion reaction, which was substantiated by radiolabeling the internal cellular compartment after treatment of the cells with [3H]dextran-encapsulated vesicles as shown by electron microscopic autoradiography on semi-thin sections. Endocytosis as a reaction mechanism can be excluded, since no energy transfer was observed at 25 degrees C in the absence of PEG. Investigations of vesicle bilayer order and fluidity on vesicle-cell interaction revealed optimal reactivity, with intermediate fluidity corresponding to cholesterol/phospholipid ratios between 0.7 and 1.0 and fluorescence depolarization (P) values of 0.18 and 0.21. Lipid depletion decreased the reaction velocity between cells and vesicles by about 20%, exhibiting V values of 33.2 mumol/min, as compared to the control of 41.4 mumol/min determined for 10(7) cells. The affinity constants for vesicle lipid were affected only slightly with Km values of 0.195 mM (0.210 mM). The activation energies for the reaction were calculated to give values of EA = 22.44 kJ/mol for the control and of EA = 20.4 kJ/mol for the modified cells. These data indicate that the decrease in membrane lipid content apparently has no major influence on the extent of the interaction.

Effect of surface curvature on the rate of cholesterol transfer between lipid vesicles

The effect of surface curvature on the spontaneous movement of cholesterol between membranes was investigated by measuring the rates of cholesterol transfer from donor vesicles of various sizes to a common acceptor vesicle. Donor vesicles of size in the range 40-240 nm were prepared by extruding multilamellar dispersions through polycarbonate filters of different pore sizes under pressure. The smallest donor vesicle and the acceptor vesicles were obtained by the normal sonication procedures. The rate of cholesterol transfer, as measured by the movement of [3H]cholesterol, decreases with increasing size of the donor vesicle in an almost linear fashion. The extrapolation of the results gave a half-time (t1/2) of 16-20 h of the desorption of cholesterol from a planar bilayer, and this can be considered as a reference value for most cellular membranes which are characterized by very low curvatures. Our earlier studies have shown that the t1/2 for cholesterol efflux is influenced by the presence of gangliosides and phosphatidylethanolamine, and the asymmetric distribution of these lipids in the plasma membrane could partially account for the large difference in the rates of cholesterol movement from the two sides of the plasma membrane. The small differences in rates arising from asymmetric distribution will be magnified by the longer t1/2 obtained here for membranes of low curvatures, so that the large difference in rates might be a coupled effect of lipid asymmetry and low curvature of the plasma membrane. This, in turn, may have a role in maintaining the large differences in cholesterol/phospholipid molar ratios observed between plasma membrane and intracellular membranes.

Cholesterol transfer between lipid vesicles. Effect of phospholipids and gangliosides

The effect of lipid composition on the rate of cholesterol movement between cellular membranes is investigated using lipid vesicles. The separation of donor and acceptor vesicles required for rate measurement is achieved by differential centrifugation so that the lipid effect can be quantified in the absence of a charged lipid generally used for ion-exchange-based separation. The rate of cholesterol transfer from small unilamellar vesicles (SUVs) containing 50 mol% cholesterol to a common large unilamellar vesicle (LUV) acceptor containing 20 mol% cholesterol decreases with increasing mol% of sphingomyelin in the SUVs, while phosphatidylethanolamine and phosphatidylserine have no appreciable effect at physiologically relevant levels. There is a large decrease in rate when phosphatidylethanolamine constitutes 50 mol% of donor phospholipids. Interestingly, gangliosides which have the same hydrocarbon moiety as sphingomyelin exert an opposite effect. The effect of spingomyelin seems to be mediated by its ability to decrease the fluidity of the lipid matrix, while that of gangliosides may arise from a weakening of phosphatidylcholine-cholesterol interactions or from a more favourable (less polar) microenvironment for the desorption of cholesterol provided by the head-group interactions involving sugar residues. If the effect of asymmetric transbilayer distribution of lipids is taken into consideration, the observed composition-dependent rate changes could partly account for the large difference in the rates of cholesterol desorption from the inner and outer layers of plasma membrane. Such rate differences may be responsible for an unequal steady-state distribution of cholesterol among various cellular membranes and lipoproteins.

Movement of plasma-membrane sterols to the endoplasmic reticulum in cultured cells

The spontaneous turnover of plasma-membrane sterols, as measured by their transfer to the endoplasmic reticulum, was measured in quiescent cultured human skin fibroblasts and monkey arterial smooth-muscle cells. The plasma-membrane sterol pool was pulse-labelled with trace amounts of either [3H]desmosterol or [3H]cholesterol. We then measured the enzymic conversion of [3H]desmosterol into [3H]cholesterol and of [3H]cholesterol into [3H]cholesteryl esters in intact cells. Depending on the probe used, markedly different transfer or conversion rates were found in these cells. In quiescent human skin fibroblasts, incubated in a serum-free medium, about 1.1% of the plasma-membrane [3H]desmosterol was converted into [3H]cholesterol/h, whereas in monkey arterial smooth-muscle cells the corresponding rate was 0.4%. Under similar experimental conditions, these cells esterified less than 0.02% (fibroblasts) and 0.12% (smooth-muscle cells) of the plasma-membrane [3H]cholesterol/h. The movement of sterols from the plasma membrane to the endoplasmic reticulum, as measured by the conversion of [3H]desmosterol into [3H]cholesterol was not blocked by colchicine, but was markedly enhanced by 3% (w/v) dimethyl sulphoxide. In all, these results indicate that plasma-membrane sterols of cultured cells are continuously transferred to the interior of the cell at a rate substantially higher than previously appreciated. This turnover of plasma-membrane sterol molecules took place even when there was no mass transfer of sterols into the cells.

The transfer of lipids from protein-free lipoprotein models to human fibroblasts in culture

Lipid microemulsions were prepared by sonication of mixtures of cholesteryl ester, triacylglycerol, phosphatidylcholine and cholesterol in aqueous dispersions and were purified by gel filtration. The resulting emulsion particles were characterized by differential scanning calorimetry, electron microscopy and analytical gel filtration and were shown to have the size and general organization of low-density lipoprotein. The lipid microemulsions were used as protein-free plasma lipoprotein models for studies of the receptor-independent transfer of lipids to human fibroblasts in culture. The transfer rate of [3H]cholesterol increased with the donor concentration and with the molar ratio between cholesterol and phosphatidylcholine in the donor particles. A maximal transfer value of 1 nmol per mg protein per h was obtained for cholesterol/phosphatidylcholine 1:1 particles. There was a profound temperature effect on the cholesterol transfer. The effect of altering the core lipid of the emulsion particles on the [3H]cholesterol transfer rate was small giving a somewhat higher rate with cholesteryl oleate and cholesteryl stearate than with cholesteryl linoleate. Addition of trioleoylglycerol to the cholesteryl ester core had no effect on the transfer rate. The transfer rate of palmitoyl[14C]oleoylphosphatidylcholine was found to be about 1/5 of that obtained for [3H]cholesterol. About 50% of the cell-associated [14C]cholesteryl oleate was found in the trypsin-releasable pool, while 25% was internalized by the cells at a rate of 0.06 nmol X mg-1 X h-1. Trioleoylglycerol was internalized at the same rate as the cholesteryl ester. Our data suggest that the lipoprotein lipid composition may play a role in the receptor-independent cellular uptake of cholesterol.

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)

Intracellular processing of exogenously derived non-lipoprotein [3H]cholesterol in normal and mutant human skin fibroblasts deficient in acid sterol ester hydrolase

By studying the incorporation and esterification of non-lipoprotein, free [3H]cholesterol in normal and acid sterol ester hydrolase-deficient human fibroblasts, it was examined whether the esterification reaction of the lysosomal acid sterol ester hydrolase contributed to the formation of cellular [3H]cholesteryl esters. Both the normal and the acid sterol ester hydrolase-deficient cells incorporated exogenous, vesicle-derived free [3H]cholesterol linearly as a function of time. Also, the rate of [3H]cholesteryl ester formation was almost the same in normal and mutant fibroblasts, indicating that the apparent esterification activity of the acid sterol ester hydrolase in normal fibroblasts did not contribute to the formation of [3H]cholesteryl esters in intact cells. To examine whether the incorporated [3H]cholesterol was transported into the endoplasmic reticulum and esterified by the acyl-CoA: cholesterol acyltransferase, the rate of [3H]cholesteryl ester formation was measured in the presence or absence of the acyl-CoA: cholesterol acyltransferase-inhibitor 58-035 (Sandoz Inc.). Results showed that the formation of [3H]cholesteryl esters was reduced markedly when cells were co-incubated with the acyltransferase inhibitor. Maximal inhibition (i.e., 75%) was obtained at an inhibitor concentration of 1 microgram/ml. Since the inhibitor 58-035 is very specific for acyl-CoA: cholesterol acyltransferase, this finding clearly shows that exogenous, exchangeable [3H]cholesterol can reach and mix with the intracellular substrate pool of the enzyme.

Evidence for the existence of non-esterified cholesterol carried by albumin in rat serum

Isolation of non-esterified [14C]cholesterol bound to albumin from rat serum, 8 days after i.p. injection of [14C]cholesterol, was achieved by affinity chromatography, using Cibacron blue F3GA bound to Sepharose 4B and by Sephadex G-150 column chromatography. Both methods permit isolation of large quantities of cholesterol-loaded albumin, free of globulins and lipoproteins. The isolated albumin-cholesterol fraction was estimated to be 4.6 mg/100 ml serum, which represents approx. the 24% of the non-esterified cholesterol present in the rat serum. Albumin-cholesterol, cholesterol glucoside, cholesterol hemisuccinate and hydroxylated derivatives of cholesterol produced a biphasic curve of changes in synaptosomal plasma membranes (SPM)-bound (Na+ + K+)-stimulated ATPase activity. Low concentrations of the ligand progressively increased the enzyme activity, while increasing the ligand concentration above that which maximally stimulated the enzyme activity, produced a progressive inhibition. Lipoproteins did not have any effect on the enzyme activity. The fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene-labeled SPM, increased in albumin-cholesterol derivatives-treated SPM, which is consistent with a general decrease in membrane bilayer fluidity. The results provide evidence that the 'albumin-cholesterol' fraction of the serum may directly affect the cell membrane-bound enzyme activity.

Role of apolipoproteins in cellular cholesterol efflux

The effects of serum apolipoproteins, particle size and concentration on the effectiveness of phosphatidylcholine (PC)-containing acceptor particles in causing release of cholesterol from cells growing in culture have been investigated. The acceptor particles were prepared by detergent-dialysis procedures and were either egg PC small unilamellar vesicles (SUV) or discoidal complexes of egg PC with apoproteins from human high-density lipoprotein (HDL). Gel filtration chromatography was employed to isolate particles of defined composition and size. The half-times (t 1/2) for the unidirectional efflux of cholesterol from cells prelabeled with [3H]cholesterol were measured as a function of acceptor PC concentration in the extracellular medium. HDL apolipoprotein-egg PC discoidal complexes at 100 micrograms PC/ml gave the following t 1/2 values when incubated with rat Fu5AH hepatoma, human HepG2 hepatoma, human GM3468 skin fibroblast, L-cell and mouse J774 macrophage-tumor cells: 11 +/- 2, 22 +/- 5, 84 +/- 18, 17 +/- 2 and 32 +/- 6 h, respectively. Equivalent experiments using purified apolipoprotein A-I or the total apolipoprotein C fraction to form the egg PC complexes showed that the t 1/2 values for the hepatoma cells were unaltered. However, with the fibroblasts, L-cells and J774 macrophages, the apolipoprotein C complexes gave significantly longer t 1/2 than complexes of egg PC with either apolipoprotein A-I or HDL apolipoprotein which gave the same t 1/2. An analysis based on the theory of fast coagulation of colloid particles to describe collisions between desorbed cholesterol molecules and acceptor particles predicts that the dependence of t 1/2 for cholesterol efflux from a given cell to different acceptors should be normalized when the extracellular level of acceptors is expressed in terms of the product of the radius of the particle times the number concentration of acceptor particles. The decrease in t 1/2 for cholesterol efflux from fibroblasts when the egg PC acceptor was changed from an SUV to an apolipoprotein HDL discoidal complex is consistent with the above concepts. The primary effect of the apolipoproteins in promoting cellular cholesterol efflux seems to be the solubilization of PC so that the PC is present in the extracellular medium as many small particles.

The synthesis and esterification of cholesterol by hepatocytes and H35 hepatoma cells are independent of the level of nonspecific lipid transfer protein

The level of the nonspecific lipid transfer protein (i.e., sterol carrier protein 2) is 16-fold lower in the Reuber H35 hepatoma cells as compared to the hepatocytes in culture (49 and 810 ng of protein per mg of 105000 X g supernatant protein, respectively). In order to establish whether there is a relationship between the level of nonspecific transfer protein and intracellular cholesterol metabolism, we have determined the biosynthesis and esterification of cholesterol in these hepatoma cells and hepatocytes. Both types of cells incorporated [3H]mevalonate into cholesterol and cholesterol ester. Incubation of both cell types with [3H]cholesterol in the medium resulted in a time-dependent uptake and subsequent conversion into cholesterol ester. In both instances, the amount of 3H label incorporated into cholesterol per mg of cellular protein was about 2-fold higher for the hepatoma cells. The kinetics of esterification of endogenously synthesized cholesterol were similar for both hepatoma cells and hepatocytes. Esterification of cholesterol derived from the medium proceeded 2-times faster in the hepatoma cells than in the hepatocytes. From the kinetics of cholesterol esterification we conclude that cells do not discriminate between cholesterol synthesized de novo and cholesterol derived from the medium. In addition, the proposition that the nonspecific lipid transfer protein is involved in cholesterol synthesis and esterification is not substantiated by this study.

Liposomes of dipalmitoylphosphatidylcholine associate with natural surfactant

Unilamellar liposomes of an average diameter of 0.05 micron formed by sonication of dipalmitoylphosphatidylcholine associate in vitro with the large aggregate forms of natural surfactant. The liposomal-surfactant aggregates are stable and previously associated liposomes are not released from the aggregates by the addition of more liposomes. Radiolabeled liposomes, surfactant, and preformed liposomal-surfactant aggregates were injected at a dose of 8-10 mg lipid (about 2-times the endogenous surfactant pool size) into the airways of 3-day-old rabbits. Following airway injection, labeled phosphatidylcholine from the liposomal-surfactant aggregates were recovered in approximately equal amounts by alveolar wash and in the residual lung tissue fractions. This recovery pattern and the clearance kinetics were equivalent for 48 h after airway injection to those measured with radiolabeled surfactant alone. In contrast, following the injection of liposomes alone, labeled phosphatidylcholine from the liposomes was recovered primarily by alveolar wash at 3 and 24 h. The overall clearance of the liposomal-derived phosphatidylcholine from the lung was more rapid than was the clearance of the phosphatidylcholine from the surfactant or liposome-surfactant complexes. Liposomes can interact with surfactant in vitro, and the liposomes associated with the surfactant aggregate have a metabolic fate in vivo similar to surfactant and different from liposomes alone.

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.

Regulation by thyrotropin of acyl-CoA: cholesterol acyltransferase in cultured thyroid cells

ACAT activity measured in microsomes of thyroid cells cultured for 4 days in the presence of TSH (1 mU/ml) was two or three times lower than that of the control cells cultured for the same period in the absence of TSH. The pool of cellular cholesterol which served as the ACAT substrate, was not exchangeable with exogenous cholesterol provided in the form of liposomes. However, the incubation of microsomes with liposomes made it possible to increase the cholesterol content in the microsomes by 35%, and this resulted in the activation of ACAT activity. Nevertheless, maximum activity measured after activation in the microsomes of the control cells remained higher than that of the microsomes of cells cultured in the presence of TSH. These findings would suggest that TSH acted by diminishing the cellular content in ACAT enzyme, as well as modifying the distribution of cholesterol in the intracellular membranes.

Role of the plasma membrane in the mechanism of cholesterol efflux from cells

In order to investigate the role of the plasma membrane in determining the kinetics of removal of cholesterol from cells, the efflux of [3H]cholesterol from intact cells and plasma membrane vesicles has been compared. The release of cholesterol from cultures of Fu5AH rat hepatoma and WIRL-3C rat liver cells to complexes of egg phosphatidylcholine (1 mg/ml) and human high-density apolipoprotein is first order with respect to concentration of cholesterol in the cells, with half-times (t 1/2) for at least one-third of the cell cholesterol of 3.2 +/- 0.6 and 14.3 +/- 1.5 h, respectively. Plasma membrane vesicles (0.5-5.0 micron diameter) were produced from both cell lines by incubating the cells with 50 mM formaldehyde and 2 mM dithiothreitol for 90 min. The efflux of cholesterol from the isolated vesicles follows the same kinetics as the intact, parent cells: the t 1/2 values for plasma membrane vesicles of Fu5AH and WIRL cells are 3.9 +/- 0.5 and 11.2 +/- 0.7 h, respectively. These t 1/2 values reflect the rate-limiting step in the cholesterol efflux process, which is the desorption of cholesterol molecules from the plasma membrane into the extracellular aqueous phase. The fact that intact cells and isolated plasma membranes release cholesterol at the same rates indicates that variations in the plasma membrane structure account for differences in the kinetics of cholesterol release from different cell types. In order to investigate the role of plasma membrane lipids, the kinetics of cholesterol desorption from small unilamellar vesicles prepared from the total lipid isolated from plasma membrane vesicles of Fu5AH and WIRL cells were measured. Half-times of cholesterol release from plasma membrane lipid vesicles of Fu5AH and WIRL cells were the same, with values of 3.1 +/- 0.1 and 2.9 +/- 0.2 h, respectively. Since bilayers formed from isolated plasma membrane lipids do not reproduce the kinetics of cholesterol efflux observed with the intact plasma membranes, it is likely that the local domain structure, as influenced by membrane proteins, is responsible for the differences in t 1/2 values for cholesterol efflux from these cell lines.

Serum albumin enhances the uptake of [3H]cholesterol from phosphatidylcholine vesicles by cultured human fibroblasts

Cultured human lung fibroblasts, incubated with cholesterol/phosphatidylcholine vesicles (cholesterol: phosphatidylcholine molar ratio 1:1) incorporated vesicle [3H]-cholesterol linearly for at least 48 h by an exchange process without gaining sterol mass. The incorporation of [3H]cholesterol by the cells was markedly enhanced in the presence of purified bovine serum albumin. A fraction of the incorporated vesicle [3H]cholesterol was esterified by the cells.

Intracellular transport and esterification of exchangeable cholesterol in cultured human lung fibroblasts

We have studied the intracellular fate of exchangeable cholesterol in a model system with lipid vesicles (cholesterol/phospholipid mole ratio 1:1) and cultured human lung fibroblasts. Exchangeable [3H]cholesterol in lipid vesicles was readily incorporated into cellular plasma membranes and transported to intracellular esterification sites. The formation of [3H]cholesteryl esters was not affected by cytoskeleton-disrupting drugs. A reduction of cellular pinocytosis by 75% did not reduce the formation of tracer-labelled esters, suggesting that membrane flow due to the energy-dependent pinocytosis is no major contributor to the intracellular transport of molecular cholesterol between plasma membranes and esterification sites. The formation of [3H]cholesteryl esters was not significantly affected by energy poisons (NaF and KCN) but was inhibited (to 50%) by chloroquine at 50 microM. This may indicate that membrane-derived cholesterol passes through the lysosomal compartment on its way to intracellular esterification sites.

Interactions in the hydrogen belts of membranes: cholesterol leaving phosphatidylcholine bilayers

Cholesterol transfer from sonicated liposomes of phosphatidylcholine containing 10 or 30 mole percent cholesterol was measured with erythrocytes as acceptor. The activation energies of the (rate-limiting) bilayer-cholesterol dissociation were determined. In parallel experiments, phosphatidylcholine was replaced by an analog lacking the carbonyl oxygens, diether-phosphatidylcholine. The activation energies for dissociation of cholesterol from this phospholipid were three Cal/mole smaller than those for cholesterol-phosphatidylcholine dissociation, at both concentrations of cholesterol. These results demonstrate the involvement of the carbonyl oxygen in cholesterol-phospholipid bonding and support the hypothesis of lipid-lipid hydrogen bonding in the hydrogen belts of membranes.