The effect of partial replacements of membrane cholesterol by other steroids on the osmotic fragility and glycerol permeability of erythrocytes

Modulation of human erythrocyte shape and fatty acids by diet

A semi-synthetic diet (Vivonex) was administered via nasogastric tube to three cystic fibrosis patients with pancreatic exocrine deficiency for 14 days to gain weight. Dietary essential fatty acids were provided as safflower oil, which constituted 1.3% of total calories. Plasma and red blood cells were analyzed for the content and composition of lipids at the start of the diet and at days 7 and 14 of the dietary period, and the results were correlated with the morphology of the cells. Feeding Vivonex to the patients led to an essential fatty acid deficiency, which was manifested in a 50% decrease in the linoleic acid content of the phosphatidylcholine of plasma and red blood cells at days 7 and 14 and in a 20% decrease in the linoleic acid content of red cell phosphatidylethanolamine at day 14. There was no significant alteration in the levels or composition of the other phospholipid classes and in the free cholesterol/phospholipid ratio. The decrease in the linoleic acid content of the erythrocytes was accompanied by a dramatic increase in the proportion of cells as echinocytes. We conclude that restricted linoleic acid availability in cystic fibrosis patients causes a change in red blood cell shape either directly by decreasing the linoleoylphosphatidylcholine content of the membrane or indirectly by affecting enzyme activity.

Stimulation of progesterone synthesis, in corpus luteum cells isolated from 4-amino-3,4-d-pyrazolopyrimidine-treated rats, by cholesterol presented in non-lipoprotein form

We have previously shown that the administration of 4-amino-3,4-d-pyrazolopyrimidine (4-APP) to rats reduces progesterone synthesis by cells subsequently isolated from their corpora lutea, and that plasma high density lipoprotein (HDL) can restore this progesterone synthesis to normal. In this paper we demonstrate that a dispersion of phospholipid and cholesterol, but not other sterols, can enhance this 4-APP-disabled progesterone synthesis to the same level as can HDL, thus providing the first direct evidence that cholesterol is the component of HDL upon which rat corpus luteum depends for its ability to synthesize progesterone.

Liposome-cell interactions: in vitro discrimination of uptake mechanism and in vivo targeting strategies to mononuclear phagocytes

The interactions of liposomes with cells have been extensively studied to determine their potential use as vehicles for the delivery of drugs in vivo. Since intravenously administered liposomes are, for the most part, cleared by cells of the reticuloendothelial system (RES), considerable effort has been made to take advantage of this phenomenon rather than view it as an obstacle. Indeed, cells of the RES, in particular macrophages, have been shown to play a vital role in homeostasis and in host defence mechanisms against infection and neoplasia. In this article, we present an overview of liposome-cell interactions, with particular emphasis on the techniques used to monitor the interaction of liposomes with macrophages. Specifically, we discuss methodologies which can be used to differentiate between liposome-cell fusion, adsorption and endocytosis in vitro. In addition, we outline the various strategies that have been employed for both actively and passively targeting liposomes to macrophages in vivo. We also review the rationale and various techniques for designing liposomes for enhanced macrophage uptake, which, in certain cases, results in the selective release of liposome-entrapped compounds in situ.

Biochemical characterization of the cholesterol-dependent growth of the NS-1 mouse myeloma cell line

The biochemical basis for the cholesterol-dependent growth phenotype of the NS-1 myeloma cell line has been investigated. In one series of experiments, the growth response of NS-1 cells to several of the intermediates of cholesterol biosynthesis was studied in serum-free medium. The cholesterol precursors, squalene and lanosterol, were totally ineffective in promoting NS-1 cell growth. In contrast, cholesterol precursors downstream from lanosterol, i.e., desmosterol and 7-dehydrocholesterol, completely replaced cholesterol in supporting NS-1 cell growth. In a second series of experiments, NS-1 cells and NS-1-503 cells (a cholesterol growth-independent variant of NS-1 cells) were labelled with [2-14C]acetate and the distributions of radioactivity between cholesterol and its precursors were determined by thin-layer chromatography using two different solvent systems. The major labelled sterol product (greater than 80%) in NS-1 cells after a 24-h exposure to [2-14C]acetate was lanosterol. In contrast, the major labelled sterol product (greater than 95%) in NS-1-503 cells after a 24-h exposure to [2-14C]acetate was cholesterol. Taken together, these results indicate that NS-1 cells are defective in cholesterol biosynthesis and identify the site of lesion as the demethylation of lanosterol to C-29 sterol intermediates.

Lii-Nao countertransport and Li leak in erythrocytes are differentially affected by membrane enrichment with cholesteryl hemisuccinate

Enrichment of erythrocytes with cholesteryl hemisuccinate caused a marked reduction in Li leak but did not change kinetic and thermodynamic properties of Lii-Nao countertransport of either normotensive persons or patients with essential hypertension. As cholesteryl hemisuccinate was shown to affect the membrane similarly to cholesterol, it is likely that the unique thermodynamic properties of erythrocyte Lii-Nao countertransport in essential hypertension are not caused by changes in cholesterol.

The effect of dietary lipids on the trout erythrocyte membrane

Rainbow trout were fed either a commercial diet or semi-purified diets containing 8% by weight of either cod liver oil, grape seed oil or hydrogenated coconut oil. Important changes in the fatty acid composition of erythrocyte membrane phospholipids were induced by these dietary fats. No changes were seen in the cholesterol/phospholipid ratio and in the cellular ATP level. Shape changes (crenation of cell margin and shrinkage) were only seen in trout fed hydrogenated coconut oil in connection with an accumulation of high amounts of (n-9) fatty acids including 20:3 (n-9) phospholipids. The compositional changes affect the stability of the erythrocyte membrane. An increased rate of osmotic hemolysis appeared to be associated with an increased unsaturated fatty acid content of the membranes.

Oxygenated cholesterols synergistically immobilize acyl chains and enhance protein helical structure in human erythrocyte membranes

Fourier transform infrared spectroscopy revealed that insertion of 20 alpha-hydroxycholesterol into human erythrocyte membranes (10% of total membrane sterol) immobilized the lipid acyl chains to a degree equivalent to enriching total membrane cholesterol by 50% (Rooney, M.W., Lange, Y. and Kauffman, J.W. (1984) J. Biol. Chem. 259, 8281-8285). Raman spectroscopy showed that the amount of acyl chain rotamers was not significantly altered by the presence of 20 alpha-hydroxycholesterol, indicating that acyl chain immobilization was limited to an inhibition of lateral motion. The presence of 20 alpha-hydroxycholesterol may synergistically enhance the acyl-chain-immobilizing behavior of membrane cholesterol. In addition, protein helical structure was not altered by 20 alpha-hydroxycholesterol. The insertion of 7 alpha-hydroxycholesterol into erythrocyte membranes resulted in an increase in protein helical structure which was comparable to that observed for erythrocyte membranes enriched with pure cholesterol by 50%. However, both acyl chain mobility and conformation were unchanged. These results suggest a synergistic behavior between oxysterols and cholesterol in modifying erythrocyte membrane packing.

Relation of red cell membrane properties to invasion by Plasmodium falciparum

The effects of changes in red cell membrane properties on invasion by Plasmodium falciparum have been studied by varying the cholesterol content and the intracellular concentration of polyamines. Increased cholesterol content is known to cause large reductions in the internal fluidity of the phospholipid bilayer and a change in its preferred direction of bending, but does not cause changes in gross mechanical rigidity. Polyamines, on the other hand, are thought to increase the cohesion of the membrane cytoskeleton and impede translational diffusion of transmembrane particles, as well as increase the mechanical rigidity of the membrane. Cells with membranes augmented by 50% in cholesterol show no reduction in their susceptibility to parasitic invasion, whereas an increase in cytosolic polyamine (especially spermine) concentration leads to strong inhibition of invasion. In neither case is the development of the intracellular parasite affected. We conclude that it is the macroscopic, rather than the microscopic rheoelastic properties of the membrane that influence the invasion process. Depletion of membrane cholesterol leads to a substantial reduction in parasitaemia; it is suggested that this is linked to the reduced phosphorus incorporation into spectrin in these cells. Polyamines may exert a significant effect at physiological concentrations and the possibility must be considered that the elevated polyamine levels found in red cells in sickle cell disease may account for the protection against P. falciparum.

Lymph node localization of non-specific antibody-coated liposomes

Subcutaneously injected small unilamellar liposomes are drained into the lymphatics and localized in the regional lymph nodes, and thus they can be used for the detection of metastatic spread in breast cancer patients and for delivery of drugs to diseased lymph nodes (1-8). An aqueous phase marker, [125I]-polyvinylpyrrolidone, and a lipid phase marker, [3H]-cholesterol, were used to study the lymph node localization of IgG-coated liposomes injected subcutaneously into mouse and rat footpads. The results show that human immunoglobulin G (IgG) coated liposomes are rapidly removed from the site of injection and are localized in the regional lymph nodes to a greater extent than control liposomes (i.e. liposomes without IgG). Free IgG was found to inhibit the uptake of IgG-coated liposomes by the lymph nodes. The localization of IgG-coated liposomes in the regional lymph nodes is influenced by charge of the liposomes. The results presented here suggest that antibody-coated liposomes may provide a more efficient way of delivering therapeutic agents to the lymph nodes in the treatment of diseases such as breast cancer with lymph node involvement. Similarly, monoclonal antibody-coated liposomes containing lymphoscintigraphic material may improve the detection of lymph node metastases.

Cholesterol oxidation derivatives and arterial endothelial damage

Three groups of New Zealand male while rabbits were given either 2.5 mg/kg of 25-hydroxycholesterol, cholestane-3 beta, 5 alpha, 6 beta-triol or vehicle only, intravenously. 24 h after treatment, the luminal surfaces of aortae of rabbits receiving 25-hydroxycholesterol were examined by scanning electron microscopy (SEM) and showed numerous balloon-like protrusions and crater-like defects as well as circulating, formed elements adhering on the luminal surface. The luminal surface of aortae of rabbits given cholestane-3 beta, 5 alpha, 6 beta-triol had similar but more frequent lesions when compared with those of the 25-hydroxycholesterol group. Microthrombi were occasionally found. The aortae of the control group had significantly fewer lesions. Transmission electron-microscopic studies showed intracytoplasmic vacuoles and diffuse subendothelial edema in the aortae of the two groups receiving the oxidation derivatives of cholesterol. The balloon-like protrusions and crater-like defects observed by SEM appeared to represent the initial sterol-induced endothelial cell injury. Repeated episodes of arterial injury followed by thrombus formation could eventually lead to atherosclerosis.

The cholesterol content of the human erythrocyte influences calcium influx through the channel

In order to study the influence of the cholesterol content on the calcium entry channel, the human red blood cell was used as a model system. The cholesterol to lecithin ratio (C/L ratio) of the membrane was modified experimentally by incubating the cells (15h, 25 degrees) with liposomes of defined C/L ratios. Subsequently, net 45Calcium-influx into the cell was measured by inhibiting the Ca-ejecting ATPase with vanadate. Additionally, the use of nitrendipine, a potent calcium channel inhibitor, during incubation allowed the determination of Ca-influx through the calcium channel. A positive correlation between the 45Ca++-influx and the molar C/L ratio of the membrane was found over a wide C/L range. A molar C/L ratio of 1.4 in the membrane increased calcium influx by 150 % compared to controls (molar C/L ratio = 0.8, calcium influx rate = 100 %), while a molar C/L ratio at less than 0.75 decreased calcium influx by 50 %. We conclude, that the cholesterol content of the membrane greatly influences the calcium channel and thus plays a pivotal role for the availability of calcium as a second messenger. These findings may provide a link between high plasma cholesterol and the development of atherosclerosis as well as enhanced platelet aggregability.

Role of cholesterol in fusion of Semliki Forest virus with membranes

The low pH-triggered membrane fusion activity of Semliki Forest virus is dependent on the presence of cholesterol in the target membrane. When liposomes containing phospholipids and cholesterol analogs were used, fusion activity was observed with steroids which did not have a planar nucleus or an isooctyl side chain at C-17, but fusion activity was not observed when analogs which lacked the 3 beta-OH group were used. Binding of virus to liposomes at low pH was similarly, but not totally, dependent on the presence of a 3 beta-OH sterol.

Differences in lipid composition and fluidity of cardiac sarcolemma prepared from newborn and adult rabbits

As the lipid composition of sarcolemma isolated from the rabbit heart changes during ontogeny, the object of this study is to determine if concomitant changes in sarcolemmal fluidity occur. Highly purified sarcolemmal fractions of newborn and adult myocardium were prepared and lipids of those fractions were analyzed. The fluorescence anisotropy of sarcolemmal fractions of both groups was also measured on a fluorometer using 1,6-diphenylhexatriene (DPH) as a probe. Total phospholipid, cholesterol, total fatty acid, and saturated fatty acid contents in the sarcolemmal fraction of the newborn were significantly higher than those of the adult. Total fluorescence intensity at saturating levels of DPH was significantly higher in the sarcolemmal fraction isolated from the newborn compared to the adult, but anisotropy was not significantly different between the newborn and the adult. An Arrhenius plot demonstrated phase transition points at nearly 23 degrees C in both myocardium, but an apparent phase transition at breaking point was not observed in the newborn sarcolemmal fraction. The slight difference in the value of the fusion activation energy above and below the phase transition point was observed between the newborn and the adult. Thus, it is apparent that the changes of sarcolemmal fluidity with age do occur but are smaller than expected, based on the difference in the sarcolemmal lipid composition.

The effect of the sterol oxygen function on the interaction with phospholipids

The effect of cholesteryl ethers (namely cholesteryl methyl ether, cholesteryl ethyl ether, cholesteryl n-propyl ether, cholesteryl isopropyl ether, cholesteryl butyl ether, cholesteryl methoxymethyl ether, cholesteryl (2'-hydroxy)-3-ethyl ether) and cholesteryl ester (namely cholesteryl acetate) is tested on the interaction with phosphatidylcholines in liquid-crystalline and crystalline state. The interfacial properties of sterols are tested at the air-water interface. The cholesteryl ethers show a reduced interfacial stability with increasing hydrophobicity of the ether-linked moiety. The interaction between the sterol derivatives and phospholipids in mixed monolayers is indicated by measuring the deviation from the simple addivity rule (condensing effect). An interaction is found only for cholesteryl (2'-hydroxy)-3-ethyl ether, cholesteryl methyl ether and cholesteryl ethyl ether. These sterols also reduce the glucose permeability of liposomal membranes in this order. In this respect cholesteryl (2'-hydroxy)-3-ethyl ether is as effective as cholesterol. Cholesteryl methyl ether and cholesteryl ethyl ether show 62 and 33 percent of the effect observed with cholesterol. The effect of the sterol derivatives on the gel-to-liquid-crystalline phase transition of dipalmitoylphosphatidylcholine is measured by differential scanning calorimetry. Cholesteryl methyl ether, cholesteryl ethyl ether, and cholesteryl (2'-hydroxy)-3-ethyl ether reduce the energy content of the phase transition nearly as effective as cholesterol, cholesteryl n-propyl ether has only a small effect. Although cholesteryl acetate, and cholesteryl methoxymethyl ether have no condensing or permeability-reducing effect, they have a considerable effect on the gel-to-liquid-crystalline phase transition. Cholesteryl isopropyl ether and cholesteryl butyl ether have no effect. It is concluded that a free 3 beta-hydroxy group is not a prerequisite to observe a sterol-like effect in membranes. However, the interfacial stability and the orientation of the sterol and oxygen moiety at the sterol 3-position are important.

The solubility of cholesterol and its exchange between membranes

It has been proposed that exchange between membrane cholesterol pools occurs by desorption of molecules into the aqueous environment rather than by formation of a transitory collision complex between the membranes. The rate of exchange is likely to be determined by the rate of dissociation of cholesterol from the membrane bilayer and by the concentration of cholesterol monomers or aggregates of cholesterol molecules in solution. The aim of this study was to measure the effects of agents known to increase cholesterol exchange rates on cholesterol solubility, critical micellar concentration and on the activation energy of exchange. A comparison was also made with regard to these parameters, of the exchange of cholesterol to that of 4-cholesten-3-one, another steroid which exchanges more rapidly than cholesterol. Acetone and dimethylsulphoxide increased cholesterol exchange between liposomes and erythrocytes, but only modestly increased the apparent solubility of cholesterol in saline and had no effect on the activation energy of the exchange process. However, acetone and dimethylsulphoxide increased the critical micellar concentration of the cholesterol 3-fold, although tetraethylammonium iodide, which had a smaller effect on exchange, did not. 4-Cholesten-3-one had a lower solubility and critical micellar concentration than that of cholesterol, but had the same activation energy for exchange. It is concluded that the apparent solubility of steroid aggregates are unlikely to determine the rate of exchange, but that agents which substantially increase exchange also increase the critical micellar concentration. The low critical micellar concentration of cholestenone suggests that the actual monomer concentration in an exchange system is low and that the rate of dissociation of the molecules from the liposomes must determine the exchange rate. This is not reflected in the activation energy measurements since these are a composite of all the elements of the exchange process.

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."

The biosynthetic incorporation of diacetylenic fatty acids into the biomembranes of Acholeplasma laidlawii A cells and polymerisation of the biomembranes by irradiation with ultraviolet light

Acholeplasma laidlawii A has been grown in media containing synthetic, long chain C20- and C23-fatty acids possessing a diacetylene group in their acyl chains. Growth on the C23 diacetylenic acid was poor but was good on the C20 acid. Biosynthetic incorporation of the fatty acids occurs; as much as 90% of the membrane lipid fatty acyl chains consisting of the C20-diacetylenic fatty acid, the remainder being shorter chain, saturated fatty acids. The thermal phase transition of this biomembrane has been studied and a differential scanning calorimetry heating curve shows the presence of an endotherm corresponding to a membrane lipid phase transition occurring at about 26 degrees C. The lipid class composition of membranes containing the C20-diacetylene lipids was examined and found to be similar to membranes from cells grown on oleic acid-containing medium. (The ratio of monoglucosyl- to diglucosyldiacylglycerols was the same but the ratio of glycolipid to phosphatidylglycerol was higher in the cells grown with diacetylene fatty acids). Upon irradiation with ultraviolet light the cells and isolated biomembranes become coloured, either red or yellow depending upon their thermal history. The colour change indicates that extensive cross-linking of the lipids of the biomembranes of A. laidlawii has occurred and that a conjugated polymeric structure has been formed. Analysis of the extracted lipids from the biomembranes by GLC indicates that extensive cross-linking of the lipid chains within the biomembrane of a natural cell system has been achieved. The monoglucosyldiacylglycerols cross-link more readily that do the phosphatidylglycerol lipids. The effect of such lipid cross-linking or polymerisation on the activity at 35 degrees C of an intrinsic membrane-bound enzyme, NADH oxidase, and ribonuclease, an extrinsic membrane-bound enzyme, was studied. The NADH oxidase activity decreased rapidly upon cross-linking of the lipid environment whereas ribonuclease activity was unaffected. The potential for future studies of polymerised model and natural biomembranes is discussed.

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.

Studies on the mechanisms of the epileptiform activity induced by U186661. I Gross alteration of the lipids of synaptosomes and myelin

U18666A, an inhibitor of desmosterol reductase (a terminal enzyme in cholesterol synthesis), has been found to produce chronic epileptiform activity in laboratory animals. Since desmosterol might substitute for cholesterol in neuronal membranes without detriment, the present study was undertaken to examine the possibility that this drug-induced epilepsy was related to changes in other brain lipids. Chronic treatment of rat with U18666A, beginning at one day of age, resulted in pronounced decreases in the concentration of phospholipids and increases in gangliosides of brain microsomal, synaptosomal, and crude myelin fractions. Since total sterol levels were not changed, the ratio of sterols to phospholipids also increased. If drug treatment was stopped at 4 weeks of age, brain lipids of all subcellular fractions examined returned to normal levels by 8 weeks, and no epileptiform activity was detected. However, following 8 weeks of continuous treatment, epileptiform activity was present, and the changes in brain lipids were focused in the myelin fraction. Phospholipid levels and the sterol:phospholipid ratio of microsomes and synaptosomes, in contrast to myelin, were near normal; however, gangliosides were still clearly elevated in all fractions. A reported ability to induce epileptiform activity in rats by treatment with antiserum to brain gangliosides could indicate a special significance of the altered myelin and synaptic gangliosides to the U18666A-induced epilepsy. We suggest that some epileptiform conditions could be directly related to alterations in the lipid composition of critical neuronal structures.

The uptake of glycerol, α-aminoisobutyric acid and 2-deoxy-D-glucose by spermatozoa of a line of chickens selected for fertility of frozen-thawed semen and a control line

The uptake of glycerol, alpha-aminoisobutyric acid and 2-deoxy-D-glucose by fowl spermatozoa was each measured in two trials using seventh generation males of a line selected for duration of fertility of frozen-thawed semen and those of a randomly selected control line. The selected line had significantly (P< 0.01) higher fertility of frozen-thawed and fresh semen than the control line. The association between glycerol level in spermatozoa before freezing and the fertility of frozen-thawed semen was examined. Significantly greater amounts (CPM/10(9) cells) of glycerol, alpha-aminoisobutyric acid and 2-deoxy-D-glucose were taken up by the spermatozoa of the selected line than those of the control line at 15, 30 and 60 min of incubation. Rank correlations between the fertility of frozen-thawed semen and glycerol levels were generally positive but low in magnitudes.

Studies on (Na+ + K+)-activated ATPase. XLIX. Content and role of cholesterol and other neutral lipids in highly purified rabbit kidney enzyme preparation

(1) The neutral lipids and the free and bound fatty acids of a highly purified (Na+ + K+)-ATPase preparation from rabbit kidney outer medulla have been analysed. (2) On a dry weight basis, the total lipid content is nearly the same as the total protein content, and consists for 66% of phospholipids and for 34% of neutral lipids and free fatty acids. In the latter category cholesterol is the main component (71%). (3) On a molar basis the enzyme preparation contains 382 mol phospholipids, 67 mol free fatty acids, 9, 16 and 12 mol mono-, di- and triacylglycerols, 249 and 19 mol free and esterified cholesterol per mol enzyme. (4) The fatty acid composition of each lipid and of the free fatty acid fraction, present in the enzyme preparation, is reported. (5) All cholesterol and part of the phospholipids can be removed by hexane extraction, leaving 66% of the (Na+ + K+)-ATPase activity. Oxidation of all cholesterol to cholest-4-en-3-one by cholesterol oxidase leaves 85% of the (Na+ + K+)-ATPase activity. These results indicate that cholesterol is not essential for (Na+ + K+)-ATPase activity.

Altered phospholipid composition affects endocytosis in cultured LM fibroblasts

The phospholipid polar head group composition of LM fibroblast membranes was altered by growing the cells in a chemically defined, serum-free medium containing choline, N,N'-dimethylethanolamine, N-monomethylethanolamine, or ethanolamine. The cells incorporated these bases into their membrane phospholipid such that 29-40% of the total plasma membrane phospholipids contained these polar head groups. Alteration of the phospholipid composition correlated with a depression of polystyrene bead phagocytosis by 36, 55 and 85% when the cells had been supplemented with N,N'-dimethylethanolamine, N-monoethylethanolamine, or ethanolamine, respectively. Pinocytotic uptake of horseradish peroxidase was depressed 44, 39, and 32%, respectively. The phagosomal membrane phospholipid composition qualitatively resembled that of the primary plasma membrane from which it was derived. However, enrichment of phosphatidylcholine, and other quantitative differences were noted in the phagosomal membranes as compared to the parent primary plasma membrane. Approx. 50% of the phagosomal membrane's phosphatidylethanolamine was accessible to the chemical labelling reagent trinitrobenzenesulfonate at 4 degrees C. The asymmetric distribution of phosphatidylethanolamine across the phagosomal membrane did not appear to be altered by base analogues except in the case of phagosomes from cells supplemented with ethanolamine. The data were consistent with a nonrandom site for endocytosis with regard to phospholipid composition.

Exchange of lipids between spermatozoa and seminal plasma in normal and pathological human semen

Normal and pathological semen were studied with regard to cholesterol and phospholipid content of sperm cells and seminal plasma. Spermatozoa from pathologic semen have similar concentrations of phospholipid-phosphorous and significantly higher cholesterol concentration than spermatozoa from normal semen. However, only oligoasthenospermic spermatozoa showed a significantly higher cholesterol/phospholipid ratio. Azoospermic seminal plasma showed the lowest values of both cholesterol and phospholipids, but the ratio of cholesterol to phospholipids was equal to that in normal spermatozoa. No significant difference was found in the cholesterol concentration of seminal plasma from oligoasthenospermic, asthenospermic, and normospermic subjects and only asthenospermic plasma showed a significantly lower concentration of this compound. Cholesterol and phospholipid exchange between sperm cells and seminal plasma was shown by the striking correlation between the lipid composition of seminal plasma with that of sperm cells.

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.

Effects of prior sterol depletion on neurite outgrowth in neuroblastoma cells

The effect of decreasing cellular sterol content on neurite outgrowth in C1300 (Neuro 2A) neuroblastoma cells in serum-free medium has been studied. Sterol-depleted, undifferentiated neuroblastoma cells were obtained by growing cells for 24 h in medium containing lipoprotein-poor serum and 25-hydroxycholesterol (25-OHC). Under these conditions the activity of 3-hydroxy-3-methyl-glutaryl-CoA reductase and the incorporation of [14C] acetate into sterols were almost completely suppressed, and the sterol/phospholipid ratio of the cells declined to 60% of that in cultures grown without 25-OHC. The sterol-depleted cells were viable and exhibited rats of DNA, RNA, protein and fatty acid synthesis comparable to those measured in control cultures. Sterol depletion had no detectable effect on the number of cells that were able to undergo morphological differentiation within 3 h after removal of serum from the medium. However, by 24 h most of the sterol-depleted cells had retracted their neurites. The observation that addition of low-density lipoprotein was able to restore neurite outgrowth in cultures treated with 25-OHC indicates that the inability of sterol-depleted cells to maintain their neurites is related specifically to the decline in the sterol content rather than to a general cytotoxic effect of 25-OHC. Our findings suggest that incorporation of cholesterol into the cell membrane is important for long-term maintenance and elongation of neuroblastoma neurites, but that the initial morphological change (e.e., within 3 h after removal of serum) is apparently a separate and distinct event, not dependent on the availability of cholesterol.

Structural and functional lability induced by diethyl ether on the sarcoplasmic reticulum membrane

Structural and functional changes occurring in sarcoplasmic reticulum vesicles following exposure to low concentrations (5--7%, v/v) of diethyl ether in aqueous media, were studied by electron microscopy and by kinetic measurements of Ca2+ transport and ATPase activity. Electron microscopy of thin sectioned and freeze-fractured sarcoplasmic reticulum vesicles provided detailed resolution of Ca-ATPase amphiphilic molecules displaying 'lollipop' portions on the outer surface of the vesicle, and non-polar moieties penetrating the membrane's hydrophobic interior. This asymmetric disposition of ATPase molecules was disrupted in vesicles exposed to ether and then centrifuged and/or resuspended in aqueous media. Such vesicles had a tendency to undergo fragmentation, and the distribution of ATPase molecules was markedly altered. The continuous fuzzy layer of lollipops became discontinuous, and the intramembranous particles became randomly distributed over both the concave and the convex freeze-fracture membrane faces. Functionally, the vesicles lost their ability to accumulate calcium in the presence of ATP, although high rates of ATPase activity were maintained. Vesicles which were simply exposed to ether, without being subjected to centrifugation and/or homogenization, did not appear altered ultrastructurally, and retained their ability to accumulate calcium. In fact, the enzyme turnover and the maximal levels of calcium uptake were increased. It is concluded that diethyl ether interferes with lipid-lipid and protein-lipid interactions in the sarcoplasmic reticulum vesicle membrane, thereby facilitating molecular motions which may be a limiting factor in the transport mechanism. On the other hand, these weakened interactions permit structural denaturation and loss of the ability to maintain a transmembrane Ca2+ gradient when the vesicles are subjected to mechanical perturbations which are harmless in the absence of ether.

Use of fluorescent probes in the study of phospholipid--sterol bilayers

1. The transfer of excitation energy between the fluorescent probes 1,6-diphenylhexa-1,3,5-triene and 12-(9-anthroyl)stearic acid and the cholesterol analogue cholesta-4,6-dien-3-one in phosphatidylcholine liposomes has been investigated. 2. The results indicate that probes and steroid are randomly distributed in the bilayer at steroid concentrations up to 35 mol%. 3. The degree of polarization of diphenylhexatriene fluorescence increases with increasing cholesterol content. Other sterols, differing in structure in the region of the polar group or in the side chain at position-17, produce similar but not identical effects. 4. the results are consistent with the proposal that diphenylhexatriene gives a general picture of the state of the bilayer and that there is no segregation of sterols in liquid-crystalline phosphatidylcholine bilayers.

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.

Studies of liposome interactions with rat thymocytes

1. The consequences of incubating liposomes with rat thymocytes have been studied using liposomes of dipalmitolyphosphatidylcholine and cholesterol or dipalmitoylphosphatidylcholine only. 2. Dipalmitoylphosphatidylcholine-cholesterol liposomes do not bind to the cells can be removed by washing. An increase in cellular cholesterol is observed. However dipalmitoylphosphatidylcholine liposomes bind rapidly to the cells and cannot be removed by repeated washing. Cholesterol is removed from the cells. 3. There are small changes in intracellular cations in the cholesterol-enriched cells, but no transport studies have been made. Cells depleted of cholesterol lose K+ with little change in intracellular Na+. Na+ influx is increased. The majority of this increase appears to be ouabain-sensitive, indicating in pump-mediated Na+ influx. K+ influx is reduced. 4. The significance of these results is discussed.

Hemolysis of rat erythrocytes by replacement of the natural phosphatidylcholine by various phosphatidylcholines

Vesicles of a variety of types of phosphatidylcholine have been shown to be suitable donors of phosphatidylcholine to intact rat erythrocytes in the presence of a specific phosphatidylcholine exchange protein. Coincident with the progression of phosphatidylcholine exchange is the onset of hemolysis, occurring at degrees of exchange characteristic for the type of phosphatidylcholine employed. During exchange with the dimyristoyl, dipalmitoyl and distearoyl species, hemolysis starts when 27%, 25% and 22% of the native phosphatidylcholine is replaced by these disaturated species, respectively. In the case of dielaidoylglycerophosphocholine and 1-stearoyl-2-oleoylglycerophosphocholine hemolysis starts after the introduction of 30% and 28%, respectively. In contrast, the replacement of native erythrocyte phosphatidylcholine with egg phosphatidylcholine, and the dioleoyl species up to levels of 60% does not result in rapid hemolysis. Despite such functional consequences, overall contents of the individual phospholipids and cholesterol are normal. Accompanying the biochemical events are morphologic changes, including echinocyte an spherocyte formation. The structure-function implications and possible mechanisms of hemolysis are discussed.

Dual effect of membrane cholesterol on simple and mediated transport processes in human erythrocytes

The influence of cholesterol on simple and facilitated transport processes across the membrane of intact human erythrocytes was studied after graded depletion or enrichment of membrane cholesterol by incubation of the cells in phospholipid or phospholipid/cholesterol suspensions. 1. The carrier-mediated transfer of L-lactate and of L-arabinose proved to be enhanced in this effect. In contrast, the self-exchange of SO4(2-), mediated by the inorganic anion-exchange system, and the simple diffusion of erythritol via the lipid phase of the membrane are inhibited by cholesterol. 2. Reversibility of these two opposite effects of cholesterol was demonstrated by measurements on cells depleted again after cholesterol enrichment and enriched again after previous depletion. 3. Certain phospholipids used for preparing the lipid dispersions that are required for cholesterol variation have effects on permeability of their own, due, for example, to traces of contaminants. A discrimination of such artifacts due, for example, to traces of contaminants. A discrimination of such artifacts from the effects of cholesterol is only possible by demonstrating reversibility. 4. The opposite effects of cholesterol on various facilitated transfer processes, which have a correlation in the opposite effects of other modifications of the membrane lipid phase (Deuticke, B., Grunze, M. and Haest, C.W.M. (1979) Alfred Benzon Symposium 14, Munksgaard, Copenhagen, in the press), are indicative of different types of lipid-protein interaction in the erythrocyte membrane.

The influence of the fatty acid composition of Acholeplasma laidlawii membranes on the growth inhibitory activity of narasin, a polyether ionophorous antibiotic

Narasin, apolyether ionophorous antibiotic capable of acting as a transmembrane carrier of cations, has a growth inhibitory effect on Acholeplasma laidlawii, permitting only 20% survival when present as 0.1 micrograms/ml in an undefined growth nutrient or fatty acid-deficient nutrient supplemented only with palmitic acid. When A. laidlawii is propagated in fatty acid-deficient nutrient supplemented with linoleic acid, however, the organisms become 40 times more sensitive to the growth inhibitory effect of this antibiotic. The actual fatty acid composition of the membranes would indicate that a higher degree of unsaturation enhances ionophore activity.