Properties of aqueous dispersions of phospholipid and cholesterol

Protection of boar spermatozoa from cold shock damage by 2-hydroxypropyl-beta-cyclodextrin

This study examined whether 2-hydroxypropyl-beta-cyclodextrin (HBCD) could play a role in protecting spermatozoa from cold shock, as judged by motility parameters, intact acrosomes, and membrane integrity. Motility parameters were assessed by a computer-assisted sperm motility analysis (CASA) system, and the acrosome and membrane integrity were evaluated by fluorescent staining with FlTC-labeled peanut agglutinin and SYBR-14 plus Propidium Iodide, respectively. The addition of HBCD to the BF5 extender significantly increased the percentages of spermatozoa with intact acrosomes and increased membrane integrity after cold shock. The motility, progressive motility, and progressive velocity of the cold-shocked spermatozoa in the presence of HBCD were significantly higher than in the absence of HBCD. In contrast, further supplement of HBCD with cholesterol-3-sulfate (a cholesterol analogue) resulted in a decrease in all the aforementioned criteria, suggesting that the ability of HBCD to protect spermatozoa from cold shock injury is blocked by saturating the cholesterol binding sites of HBCD. It is therefore concluded that HBCD protects spermatozoa against cold shock injury, possibly due to its ability to remove membrane cholesterol.

Effects of membrane incorporation of short-chain phospholipids on sodium pump function in human erthrocytes

Erythrocyte membrane incorporation of exogenous short-chain diacyl phosphatidylcholines (PC) has been quantified by gas chromatography of fatty acid methyl esters of extracted membrane lipids after incubation of cells with sonicated aqueous suspensions of PC. The PCs studied included didecanoyl PC (C10-PC), dilauroyl PC (C12-PC), dimyristoyl PC (C14-PC) and dipalmitoyl PC (C16-PC). PC incorporation of 10-15 mol% was achieved by incubation at 37 degrees C for 0.5-24 h. Control cells incubated in saline alone showed a progressive reduction in endogenous polyunsaturated acyl chain content. Incubation with C10-PC and C16-PC was associated with reductions in membrane cholesterol. Experiments were performed with mixtures of PC and cholesterol in order to minimise this effect. Short-chain PC incorporation was associated with increases in intracellular Na+ and reduced intracellular K+ concentrations. Sodium pump activity was measured as the ouabain-sensitive rate of 86Rb+ influx and was significantly reduced by all PCs tested; mean reductions were 13-30%. These results confirm that the sodium pump in situ is sensitive to lipid acyl chain composition.

Enhancement of the turtle olfactory responses to fatty acids by treatment of olfactory epithelium with phosphatidylserine

The turtle olfactory epithelium was treated with suspensions of various lipids and their effects on the olfactory responses were examined by measuring the olfactory bulbar responses. The phosphatidylserine (PS)-treatment greatly lowered the threshold for n-valeric acid and enhanced its responses at all concentrations examined. The responses to isovaleric acid and n-butyric acid were also greatly enhanced by the PS-treatment. The responses to ten other odorants examined were a little enhanced or unchanged by the PS-treatment. The enhanced responses to the fatty acids returned to the original level about 10 h after the treatment. It was confirmed that PS was incorporated into olfactory epithelium by incubating the epithelium with PS-suspension containing [14C]PS. The treatment of the epithelium with phosphatidic acid or cardiolipin unchanged or suppressed the responses to odorants including the fatty acids. The present results suggest that lipids as well as proteins in the receptor membranes play an important role in odor reception.

Decreased erythrocyte cholesterol/phospholipid ratio in untreated patients with essential hypertension

The erythrocyte cholesterol/phospholipid ratio was determined in eight patients with untreated essential hypertension and compared with that of eight age-matched control subjects. The ratio was significantly lower in patients (Wilcoxon's paired rank test; P less than 0.01), and a correlation existed between the ratio and serum cholesterol concentration in patients (r = 0.63) but not in controls (r = 0.02). A reduction in the cholesterol/phospholipid ratio may play a direct role in destabilizing the plasma membrane, which will in turn result in an increase in membrane permeability in essential hypertension.

On the interaction of the liposomal membrane with blood components

Liposome-encapsulated hemoglobin (LEH) has been shown to be a viable candidate as a blood replacement. However, few data have been presented as to how LEH interacts with normal blood components. Liposomes were prepared from egg lecithin, cholesterol, and dicetyl phosphate or phosphatidic acid, and mixed with fresh blood plasma or whole blood. Erythrocyte osmotic fragility, prothrombin time (extrinsic coagulation efficiency), activated partial thromboplastin time (intrinsic coagulation efficiency), plasma clot stability in urea (fibrin stabilizing factor), and clot retraction (platelet activation) were measured. Although liposomes were found to bind extensively to erythrocytes, all tests indicated that the liposomes had no significant adverse effects, provided that normal levels of plasma Ca++ were maintained. The ability of liposomes to absorb Ca++ from the plasma was related directly to the amount of dicetyl phosphate or phosphatidic acid present and thus, presumably, to the presence of negatively charged species in the membrane. The mechanics of deformation of the LEH membrane were investigated by encapsulating Hemoglobin S in liposomes. Liposomes containing Hemoglobin S were found to sickle when deoxygenated, but not liposomes containing normal hemoglobin. Shape analysis of sickled liposomes yielded a deforming stress of 10(6) dynes/cm2, about 50 times greater than the reported limit for shear elasticity of the erythrocyte membrane.

Modulation of Na-H antiporter activity in human lymphoblasts by altered membrane cholesterol

The effects of changes in membrane cholesterol on Na-H antiporter activity in culture human lymphoblasts are described. Lymphoblast cholesterol alteration was achieved with liposomes of phosphatidylcholine (cholesterol depletion) or phosphatidylcholine plus cholesterol (cholesterol enrichment). Lymphoblast intracellular pH (pHi) was examined by fluorimetry using cells loaded with the pH-sensitive dye 2',7'-bis(2-carboxyethyl)5(6)-carboxyfluorescein, and the Na-dependent proton efflux rate at a pHi of 6.0 was taken as the maximum velocity of the Na-H antiporter. Lymphoblast membrane cholesterol depletion activated the Na-H antiporter, and enrichment of membrane cholesterol caused inhibition of the antiporter activity. This study demonstrates that in situ modification of membrane cholesterol can modulate the activity of the Na-H antiporter.

Platelet secreted lipoprotein-like particle is taken up by the macrophage scavenger receptor and enhances cellular cholesterol accumulation

Enhanced macrophage cholesterol accumulation is associated with foam cell formation in the atherosclerotic lesion. Since platelet activation plays an important role in atherogenesis, we questioned whether products released from activated platelets could affect macrophage cholesterol metabolism. The addition of platelet-conditioned medium (PCM, obtained from collagen activated platelets) to a J-774 macrophage cell line, enhanced cellular cholesteryl ester content by 32%. The cholesterol esterification rate was also increased by 29%. Pre-loading the macrophages with cholesterol by incubation with acetyl-LDL, resulted in a further elevation of 48% in PCM-mediated cholesterol esterification. Possible mechanisms for the enhanced cholesterol esterification by J-774 macrophages following incubation with PCM include increased cholesterol influx and/or decreased cholesterol efflux (These cells were recently shown not to synthesize cholesterol). However, both increased uptake of PCM cholesterol by the macrophages as well as increased cellular cholesterol efflux (by 22%) were noted. The enhancement of cholesterol esterification by PCM was competitively inhibited by fucoidin and polyinosinic acid, implicating PCM binding to the scavenger receptor. This was further evidenced by the observations that apolipoprotein E which reduces cellular uptake via the scavenger receptor but not via the LDL receptor, also inhibited the effect of PCM, whereas IgG C-7, the LDL receptor antibody, did not alter the effect of PCM. Lysosomal involvement in the cellular processing of PCM was observed since PCM activity was inhibited by the lysosomal inhibitor, chloroquine. Partial purification of PCM by gel filtration revealed that the cholesterol component was associated with both phospholipids and proteins in a lipoprotein-like particle. Delipidation of PCM resulted in its inactivation but both heat treatment and tryptic digestion of PCM, revealed that the protein (and not only the cholesterol) component was also essential for the effect of PCM on cellular cholesterol esterification. Furthermore, PCM prepared from platelets of a patient with Gray Platelet Syndrome that lack platelet alfa granules (which contain platelet specific proteins), failed to enhance cholesterol esterification. These results demonstrate that lipoprotein-like particles released during platelet activation can interact with the macrophage scavenger receptor thus leading to enhanced cellular cholesterol accumulation.

Effects of membrane lipid composition on the kinetics of cholesterol exchange between lipoproteins and different species of red blood cells

To better understand the effects of plasma membrane structure on the kinetics of cellular cholesterol efflux to extracellular lipoprotein particles, the influence of plasma membrane sphingomyelin (SM) on the kinetics of cholesterol exchange was examined in both a model membrane system comprised of egg SM/egg phosphatidylcholine (PC) unilamellar vesicles and in various types of mammalian red blood cells (RBC) containing differing levels of SM. The kinetics and mechanism of the bidirectional flux of unesterified cholesterol (FC) between RBC and lipoproteins were established by using human RBC (labeled with [14C]FC) incubated with varying concentrations of human [3H]FC high-density lipoprotein (HDL3) or [3H]FC low-density lipoprotein (LDL). A maximal rate constant for FC efflux was obtained when the lipoprotein FC was in excess (6-fold and 15-fold, for HDL3 and LDL, respectively) of RBC FC; under this condition, the rate-limiting step is desorption of cholesterol molecules from the RBC membrane into the extracellular aqueous phase. At 37 degrees C, the halftime (t1/2) for efflux was 4.6 +/- 0.6 h for HDL3 and 6.2 +/- 0.2 h for LDL; FC efflux exhibited first-order kinetics and the RBC FC comprised a single kinetic pool. To investigate the effect of different membrane SM/PC ratios on the rate of FC desorption from the plasma membrane, the kinetics of cholesterol efflux from bovine RBC (5:1, w/w ratio of SM/PC), human RBC (1:1 ratio), rabbit RBC (0.6:1 ratio) and rat RBC (0.3:1 ratio) were compared. With excess HDL3 present, bovine, rabbit, and rat RBC exhibited t1/2 of 5.5 +/- 0.8, 4.0 +/- 0.2, and 3.7 +/- 0.6 h, respectively, for cholesterol efflux. Changing the ratio from 0.3:1 to 3:1 in egg SM/egg PC small unilamellar vesicles increased the t1/2 for cholesterol efflux at 45 degrees C from 1.1 to 6.9 h. The results described in this paper suggest that increasing membrane SM content raises the t1/2 for cholesterol exchange in both the RBC plasma membrane and in simple mixed SM/PC bilayers. However, the influence of SM is less in the natural plasma membrane, perhaps because of modulating factors such as membrane proteins and the presence of a complex phospholipid mixture.

Hepatic uptake and antihepatotoxic properties of vitamin E and liposomes in the mouse

Intravenous administration of soybean phosphatidylcholine liposomes containing different amounts of tocopherol acetate leads to a dose and time dependent increase of mouse liver tocopherol content, which was not observed when the preparation was given orally. When benzo[a]pyrene pretreated mice intoxicated with 400 mg/kg AAP were pretreated 2 h before with 1 g/kg phosphatidylcholine liposomes containing 4 mg/kg vitamin E acetate, these animals were protected against liver damage. Vitamin E alone or liposomes lacking vitamin E showed no protection. In an inflammatory liver disease model, i.e. fulminant hepatitis induced by intraperitoneal administration of 700 mg/kg galactosamine and 1 microgram/kg lipopolysaccharide phosphatidylcholine liposomes protected at a dose of 1 g/kg i.v. In this case, however, the protection was not due to the presence of vitamin E. These findings demonstrate the usefulness of phosphatidylcholine for liver protection and show that the protective spectrum is improved when they contain vitamin E. The data suggest that phosphatidylcholine is an excellent carrier for delivery of vitamin E to the liver.

Intralipid infusion abolishes ability of human serum to cholesterol-load cultured macrophages

Intralipid is widely used for intravenous alimentation and contains triglyceride-emulsion particles and phospholipid liposomes. After infusion, triglyceride-emulsion particles resemble chylomicron remnants and thus may be atherogenic. On the other hand, intravenous infusion of phospholipid liposomes produces regression of experimental atherosclerosis and abolishes the ability of hypercholesterolemic rabbit plasma to cholesterol-load cultured macrophage foam cells. To determine the net effect of intralipid infusion on cellular cholesterol balance, J-774 macrophages were incubated for 18 hours with human serum obtained before, during, and after a 6-hour infusion of 10% Intralipid. Compared to serum-free medium, pre-infusion serum increased cellular unesterified cholesterol by 76% and cholesteryl ester by 78%. In contrast, serum obtained after the 6-hour infusion reduced cellular unesterified cholesterol by 23% and cholesteryl ester by 15%. Serum obtained 18 hours after the end of the infusion still showed impaired cholesterol-loading ability. Mouse peritoneal macrophages incubated with these serum samples behaved similarly. Compared to pre-infusion serum, postinfusion serum inhibited cellular uptake of 125I-low density lipoprotein and 125I-very low density lipoprotein by 50% and 80%, respectively, and also enhanced the efflux of cellular cholesterol by 46%. We conclude that the ability of human serum to cause cholesterol accumulation in cultured macrophages is abolished by an infusion of Intralipid. This effect is mediated by a reduction in cholesterol uptake by the cells and by an increase in cell cholesterol efflux. If similar events occur in the arterial wall, Intralipid infusion might inhibit foam cell formation in vivo.

Increase of erythrocyte resistance to hemolysis and modification of membrane lipids induced by hemodialysis

Mechanical fragility, deformability and lipid composition of erythrocyte membranes were studied in 22 uremic dialysed patients before and after dialysis in comparison to controls. While deformability was not affected by a dialysis session, osmotic and saponin resistances to hemolysis were significantly increased after dialysis (p less than 0.001). The lipid composition of erythrocyte membranes was also altered during dialysis, with a notable increase in cholesterol and a different phospholipid distribution, i.e. phosphatidylcholine and sphingomyelin significantly decreased (p less than 0.001). Likewise, the fatty acid distribution showed large variations with an increase in polyunsaturated fatty acid. The acute effect of dialysis on the membrane lipid composition seems to be linked to erythrocyte alterations. Its significance is discussed in relation to the asymmetric transverse distribution of phospholipids previously described in human erythrocyte.

Low density lipoprotein receptor-independent hepatic uptake of a synthetic, cholesterol-scavenging lipoprotein: implications for the treatment of receptor-deficient atherosclerosis

The metabolism of infused 111In-labeled phospholipid liposomes was examined in Watanabe heritable hyperlipidemic (WHHL) rabbits, which lack low density lipoprotein (LDL) receptors, and in normal control rabbits. The half-times (t1/2) for clearance of 111In and excess phospholipid from plasma were 20.8 +/- 0.9 hr and 20.3 +/- 4.6 hr in WHHL and 20.0 +/- 0.8 hr and 19.6 +/- 2.2 hr in the normal rabbits (means +/- SEM; n = 4). By 6 hr postinfusion, the plasma concentration of unesterified cholesterol increased by 2.2 +/- 0.23 mmol/liter in WHHL and 2.1 +/- 0.04 mmol/liter in normal rabbits, presumably reflecting mobilization of tissue stores. Disappearance of excess plasma cholesterol was greater than 90% complete in both groups of rabbits by 70 hr postinfusion. By quantitative gamma camera imaging, hepatic trapping of 111In-labeled liposomes over time was indistinguishable between the two groups. At autopsy, the liver was the major organ of clearance, acquiring 22.0% +/- 1.7% (WHHL) and 16.8% +/- 1.0% (normal of total 111In. Aortic uptake of 111In was less than 0.02%. Thus, mobilization of cholesterol and hepatic uptake of phospholipid liposomes do not require LDL receptors. Because phospholipid infusions produce rapid substantial regression of atherosclerosis in genetically normal animals, our results suggest that phospholipid liposomes or triglyceride phospholipid emulsions (e.g., Intralipid) might reduce atherosclerosis in WHHL rabbits and in humans with familial hypercholesterolemia.

Studies on liposome-encapsulated heparin

In order to prolong the anticoagulant activity of heparin in vivo, attempts were made to encapsulate heparin into liposomes. Liposome-encapsulated heparin (lipo-heparin) prepared was large multilamellar vesicles (0.5-4.0 micron in diameter). The activity of lipo-heparin was 1.6-5.2 X 10(3) U/g lipid with recovery rate ranged between 0.4 to 1.3% and stable in saline at 4 degrees C for at least two weeks. When intravenously administered into rats, the anticoagulant activity of lipo-heparin was significantly prolonged (approximately three times), as compared with that of untreated heparin. Furthermore, the activity of lipo-heparin could be neutralized by protamine sulfate. From these observations, it was concluded that liposome-encapsulation of heparin results in the prolonged anticoagulant effect in vivo and lipo-heparin may be applicable for clinical use, after further studies on side effects of liposomes are completed.

Peroxidation of liposomes in the presence of human erythrocytes and induction of membrane damage of erythrocytes by peroxidized liposomes

Hemolysis (Kobayashi, T., Takahashi, K., Yamada, A., Nojima, S. and Inoue, K. (1983) J. Biochem. 93, 675-680) and shedding of acetylcholinesterase-enriched membrane vesicles (diameter 150-200 nm) were observed when human erythrocytes were incubated with liposomes of phosphatidylcholine which contained polyunsaturated fatty acyl chains. These events occurring on erythrocyte membrane were inhibited by radical scavengers or incorporation of alpha-tocopherol into liposomes, suggesting that lipid peroxidation is involved in the process leading to membrane vesiculation and hemolysis. The idea was supported by findings that generation of chemiluminescence, formation of thiobarbituric acid reactive substance, accumulation of conjugated diene compounds in liposomes and decrease of polyunsaturated fatty acids in liposomes occurred concomitantly during incubation. Hemolysis was also suppressed by the addition of extra liposomes, insensitive to peroxidation, or of serum albumin even after the completion of peroxidation of liposomes. These results suggest that peroxidized lipids, responsible for vesiculation and hemolysis, may be formed first in liposomes and then gradually transferred to erythrocyte membranes. The accumulation of these lipids peroxides may eventually cause membrane vesiculation followed by hemolysis.

Protein-mediated exchange of synthetic phosphatidylcholines into synaptosomal membranes

A phosphatidylcholine (PC) exchange protein from bovine liver was used to exchange endogenous synaptosomal membrane PC's with PC's of defined fatty-acid composition from phospholipid vesicles. Up to 50% of the total synaptosomal PC could be exchanged during a 3 h incubation with PC's which were in the liquid-crystalline state at the temperature of incubation (dimyristoyl-, dioleoyl- and dielaidoyl-PC). The biphasic kinetics of the exchange of 14C-labeled 1-palmitoyl-2-oleoyl-PC into isolated synaptic plasma membrane vesicles indicated that the half-time for transbilayer equilibrium of PC in these membranes was about 10 h. Hence, the observed 50% exchange of total synaptosomal PC probably represented nearly complete exchange of PC in the outer face of the synaptosomal plasma membrane. This extensive exchange was accomplished without apparent loss of synaptosomal function, including membrane potential and high-affinity uptake of choline and gamma-aminobutyric acid. PC's in the gel state (dipalmitoyl- and distearoyl-PC) could not be exchanged extensively into the synaptosomal membranes. However, from within gel-state distearoyl-PC liposomes, a trace amount of fluid 1-palmitoyl-2-oleoyl-PC (Tm less than 10 degrees C) could be preferentially exchanged into the synaptosomes at 32 degrees C with little transfer of the saturated PC.

Role of cholesterol in the capping of surface immunoglobulin receptors on murine lymphocytes

Previously, we have shown that the capping of surface immunoglobulins on murine lymphocytes can be affected by modulating the lipid environment of the surface membrane with free fatty acids. In the present study, murine lymphocytes were depleted of cholesterol by incubation with phospholipid vesicles. As the cellular cholesterol:phospholipid ratio decreased, the capping of the surface immunoglobulin was seen to decrease. This inhibition of capping could not be reversed by calcium and is not accompanied by changes in either the cytoskeletal element alpha-actinin or cellular ATP levels. Incubation of the cholesterol-depleted cells with cholesterol-containing phospholipid vesicles raised both the cholesterol:phospholipid ratio and capping levels to values close to those of untreated control cells. Remarkably, stearic acid, a saturated fatty acid, could also restore the capping levels in the cholesterol-depleted cells. On the basis of the present data and measurements of the fluorescence polarization of the probe diphenyl hexatriene, we propose a model in which the protein(s) involved in capping is located in a gel-like lipid domain, and that removal of cholesterol makes this domain less gel-like and inhibits capping. Restoration of the gel-like nature of this domain by the addition of either cholesterol or stearic acid enables the protein(s) to function normally.

Cholesterol content of red blood cells and low-density lipoproteins in hypertriglyceridemia

The red blood cells and the low-density lipoproteins in hypertriglyceridemia have a lower ratio of unesterified cholesterol to phospholipid than normal. The low-density lipoproteins are also smaller and more dense in hypertriglyceridemia, and contain only 45% of the normal unesterified cholesterol mass. The phase behavior of the lipids shows that normal red cells and low-density lipoproteins are close to saturation with cholesterol, whereas in hypertriglyceridemia less cholesterol is present. Because newly secreted triacylglycerol-rich lipoproteins are poor in cholesterol, their excess production and transport in hypertriglyceridemia may prevent maintenance of the normal cholesterol content of blood cells and low-density lipoproteins. Partitioning of cholesterol into triacylglycerol-rich lipoproteins is able to account for significant fluxes of unesterified cholesterol in the plasma compartment.

In vitro effects of vitamin D3 on the phospholipids of isolated renal brush border membranes

A model system is described in which cholecalciferol (vitamin D3) is incorporated into phosphatidylcholine liposomes and then the liposomes are incubated in vitro with isolated renal brush border membrane vesicles. The incubation results in an alteration of the phospholipid composition, the fluidity, and the transport properties of the membrane. The findings provide evidence consistent with the hypothesis that vitamin D3 and metabolites modify membrane structure and function by "liponomic regulation."

Further studies on the charge-related alterations of methotrexate transport in Ehrlich ascites tumor cells by ionic liposomes: correlation with liposome-cell association

Interaction of positively (phosphatidylcholine/stearylamine 5:1) or negatively (phosphatidylcholine/stearic acid 5:1) charged liposomes with Ehrlich ascites tumor cells for 1-5 min increases or decreases, respectively, the bidirectional fluxes of the folic acid analog, methotrexate. These effects on influx and efflux appear to be symmetrical since the liposomes do not change the intracellular level of methotrexate at the steady state. Influx kinetics show that these alterations result from an increase or decrease in the Vmax with no change in the Kinm. These effects appear to be specific for the methotrexate-tetrahydrofolate carrier system since the transport of other compounds which utilize this carrier, aminopterin, 5-methyltetrahydrofolate, and 5-formyltetrahydrofolate, is affected similarly to methotrexate, whereas, the transport of folic acid, a compound similar in structure and charge but not significantly transported by this carrier is unaffected by liposomes. Once cells are exposed to charged liposomes, the effects on methotrexate transport cannot be reversed by washing the cells free of the extracellular liposomes. If, however, cells are exposed to liposomes of one charge, washed and then exposed to liposomes of the opposite charge, methotrexate influx is reversed to control rates. The effects of charged liposomes on methotrexate influx were not abolished by treating the cells with neuraminidase, metabolic inhibitors or lowering the temperature to 4 degrees C. Studies on the uptake of [14C] liposomes show that these effects are not proportional to the total amount of lipid associated with the cell but result from an initial rapid liposome-cell association that is not dependent on temperature or energy metabolism nor related to cell surface charge.

Role of phospholipids in the DDT-induced efflux of potassium in human erythrocytes

Incubation of human erythrocytes for 1-2 h at 37 degrees C in a suspension of dipalmitoylphosphatidylcholine (DDPC) liposomes results in a phospholipid enrichment of erythrocyte membranes by 45-55% and a depletion of cholesterol by 19-24%. The enrichment by DPPC was time and concentration dependent. By contrast, dioleoylphosphatidylcholine (DOPC) liposomes were less effective in enriching the membranes with phospholipid and in depleting the membranes of cholesterol. Concomitantly, the DDT-induced efflux of K+ was reduced in the case of DPPC-enriched erythrocytes but enhanced in DOPC-enriched erythrocytes. These results suggest that DDT partitions more readily into the unsaturated than the saturated phospholipids of the erythrocyte membrane. It is concluded that the extent to which DDT affects the flux of K+ across the membrane is dependent on the fluidity of the lipid phase. We also report here a rapid method for cholesterol depletion of red blood cells in comparison to previously reported methods.

Effect of sterol structure on the transfer of sterols and phospholipids from liposomes to erythrocytes in vitro

Human erythrocytes were incubated for 24 h with liposomes containing [3H]phosphatidylcholine, [14C]cholesterol and one of several other sterols. Of the other sterols, 3-hydroxycholest-3-en-2-one did not appear to be taken up by the cells, sterophenol was taken up at about the same rate as cholesterol, and cholesta-4,6-dien-3-one and 7-oxocholesterol were taken up much more rapidly than cholesterol. Each component of the liposomes was incorporated into the cells independently of the others and the rate of incorporation of the test sterol had little, if any, effect on the rate of incorporation of phospholipid or cholesterol. The results support the proposal that sterol exchange is mediated via the pool of monomers present in the medium rather than by a collision mechanism.

Cholesterol movement between human skin fibroblasts and phosphatidylcholine vesicles

Cholesterol readily exchanges between human skin fibroblasts and unilamellar phospholipid vesicles. Only a fraction of the exchangeable cholesterol and only 10-15% of the total cellular free cholesterol is available for net movement or depletion to cholesterol-free phosphatidylcholine vesicles. [14C]Cholesterol introduced into the fibroblast plasma membrane by exchange from lipid vesicles does not readily equilibrate with fibroblast cholesterol labelled endogenously from [3H]mevalonic acid. While endogenously-synthesized [3H]cholesterol readily becomes incorporated into a pool of esterified cholesterol, little, if any, of the [14C]cholesterol introduced into the fibroblast membrane by exchange from lipid vesicles becomes available for esterification. We interpret these findings as suggesting that: (1) net cholesterol movement from fibroblasts to an acceptor membrane is limited to a small percentage of the plasma membrane cholesterol, and (2) separate pools of cholesterol exist in human skin fibroblasts, one associated with the plasma membrane and the second associated with intracellular membranes, and equilibration of cholesterol between the two pools is a very limited process.

Effects of enrichment of phosphatidylcholine liposomes with cholesterol or alpha-tocopherol on the response of lymphocytes to phytohemagglutinin

Studies were performed to determine the ability of alpha-tocopherol and cholesterol to influence the effect of phosphatidylcholine (PC) liposomes on the blastogenic response of bovine peripheral blood lymphocytes (BPBL) to phytohemagglutinin (PHA). BPBL were cultured with liposomes having a molar ratio of cholesterol to PC (C/P) ranging from 0 to 2.0, a molar ratio of alpha-tocopherol to PC (E/P) of 1.0 and a molar ratio of cholesterol + alpha-tocopherol to PC [(C + E)/P] of 2.0 and 4.0 PC liposomes significantly suppressed BPBL blastogenic response to PHA. Cholesterol-rich (C/P greater than or equal to 1.0) liposomes, alpha-tocopherol-rich (E/P = 1.0) liposomes and liposomes rich in cholesterol and alpha-tocopherol [(C + E)/P greater than or equal to 2.0] were able to completely reverse PC liposome suppression of BPBL. There was no molar ratio [C/P, E/P or (C + E)/P] that was able to enhance the blastogenic response of BPBL above the response obtained with PHA alone. The results suggest that the augmentation of PC liposomes rich in cholesterol, alpha-tocopherol and cholesterol with alpha-tocopherol (C/P and E/P greater than or equal to 1.0) was equally capable of restoring normal responses in BPBL but did not enhance or suppress the response to PHA.

The effects of membrane lipid order and cholesterol on the internal and external cationic sites of the Na+-K+ pump in erythrocytes

cholesterol depletion alters the apparent affinity of the internal cationic sites and the maximal translocation rate but not the affinity of the external cationic sites of the Na+-K+ pump in human erythrocytes. To test whether these effects were mediated by a direct cholesterol-internal site interaction or by a change in membrane lipid order, the effects of five fluidizing amphiphiles (chlorpromazine, imipramine, benzyl alcohol, sodium oleate and sodium benzenesulphonate) on the kinetic parameters of the Na+-K+ pump were determined. The cholesterol removal and all the agents used induced dose-response decreases in membrane lipid order as measured by fluorescence polarization or ESR. Positive and neutral amphiphiles mimicked the effects of cholesterol removal on the affinity of the internal sites of the pump and to a lesser extent on the maximal translocation rate. Anionic amphiphiles had no effect on internal sites, probably because they distributed preferentially within the outer leaflet on the membrane. These results indicate that cholesterol controls the affinity of the internal sites of the Na+-K+ pump by altering the membrane lipid order. In contrast, neither cholesterol depletion nor the agents used altered the affinity of the external sites of the Na+-K+ pump. This difference in sensitivity to membrane lipids order suggests that internal and external cationic sites, although borne by the same protein, are in different lipid environments.

Lipid vesicle-mediated alterations of membrane cholesterol levels: effects on Na+ and K+ currents in squid axon

We demonstrate that cholesterol can exchange from sonicated lipid vesicles to a perfused squid axon membrane and that vesicles with varying cholesterol/phospholipid (C/P) mole ratios can be used to achieve either net loading or net depletion of axon membrane cholesterol. Two types of evidence were obtained which show that net loading or depletion of cholesterol was achieved: (i) changes in the cholesterol/phospholipid (C/P) mole ratios of axons, and (ii) visualization of cholesterol depleted from the preparation by cholesterol-free vesicles by thin-layer chromatography. The C/P mole ratios indicate that cholesterol levels in the preparation were increased or decreased by 30-40%. Increasing or decreasing membrane cholesterol levels were ineffective in altering the Na+ or K+ occurrents in voltage-clamped axons. In addition, we determined that cholesterol "flip-flop" across the axonal membrane occurred with a t 1/2 of 7.3 to 15.3 min.

Mechanism of cholesterol exchange between phospholipid vesicles

The kinetics of cholesterol exchange between two populations of small unilamellar vesicles has been investigated. There is no change in the initial rate of this exchange process over a 100-fold change in the acceptor vesicle concentration at a constant donor concentration. These results are not consistent with a collision-dependent exchange mechanism. In support of transfer via the aqueous phase, the inclusion of a negatively charged lipid into the vesicles did not affect the exchange rate. Evidence for a water-soluble pool of cholesterol that had partitioned ut of the vesicle was obtained. Finally, cholesterol exchange was observed when donor and acceptor membranes were separated by a barrier through which neither could pass. These data together support our contention that the exchange of cholesterol between these vesicles involves a water-soluble intermediate.

Interaction of lipids with the plasma membrane of sperm cells. I. The antifertilization action of cholesterol

The inhibitory action of cholesterol-containing suspensions on fertilizing capacity in uterine-capacitated rabbit sperm cells showed a direct dependence on the concentration of sterol. Dispersion with synthetic phosphatidylcholine as a nonsonicated suspension or as liposomes did not alter this antifertilization effect. Esterification of the sterol, however, caused a complete loss of inhibitory activity. Cholesterol inhibited induction of the acrosome reaction among epididymal rat spermatozoa incubated under chemically defined conditions. Other agents with a negative effect on the acrosome reaction were seminal plasma membrane vesicles and palmitic acid. Egg lecithin-liposomes and bovine serum albumin, especially after being delipidated, facilitated it. These results corroborate the viewpoint that changes in the lipid bilayer of sperm plasma membrane significantly influence fertilizing capacity among mammalian spermatozoa.

Plasma membrane cholesterol regulates human lymphocyte cytotoxic function

Cell cholesterol is believed to be confined mainly to the plasma membrane. Treatment here of human peripheral blood lymphocytes with cholesterol-free and cholesterol-containing liposomes to effect, respectively, decreases or increases in cholesterol content measureable by chemical analysis, markedly altered effector functions of the cells. Depletion of cholesterol evoked inhibition of spontaneous and phytohemagglutinin-dependent lymphocyte cytotoxicity against allogeneic target cells. Opposite effects resulted from cholesterol enrichment, with PHA-dependent and antibody-dependent cytotoxicities increasing significantly. Treatment, instead, with the known inhibitor of cholesterol biosynthesis, 25-hydroxycholesterol, had suppressive effects like those resulting from lowering the cholesterol level physically by liposome treatment. Our data suggest that the plasma membrane cholesterol content of different categories of lymphocytes in man is both essential and regulatory for their cytotoxic function.

Membrane composition modulates thiopental partitioning in bilayers and biomembranes

The nonspecific interaction of thiopental with erythrocyte ghosts, synaptic membranes, microsomes and mitochondria has been measured at 25 degrees C and pH 6.6. In cholesterol-depleted erythrocyte ghosts the partition coefficient decreases with increasing cholesterol content. In sonicated liposomes made from egg lecithin and cholesterol the partition coefficient also decreases with increasing cholesterol content. The dependence of the partition coefficient on cholesterol content in the biological membranes, on average, parallels that in the lipid bilayers. The partition coefficient in lipid bilayers made from lipids extracted from erythrocyte ghosts was comparable to that in the corresponding egg lecithin/cholesterol bilayer. The partition coefficients of all the biomembranes are consistently lower than those in the corresponding egg lecithin/cholesterol bilayer, the free energy of transfer between biomembrane and corresponding bilayer being -1 kcal/mol.