Interaction of phosphatidylcholine liposomes and plasma lipoproteins with sheep erythrocyte membranes: Preferential transfer of phosphatidylcholine containing unsaturated fatty acids

The interaction of sheep erythrocyte membranes with phosphatidylcholine vesicles (liposomes) or human plasma lipoproteins is described. Isolated sheep red cell membranes were incubated with liposomes containing [¹⁴C]phosphatidylcholine or [^3H]phosphatidylcholine in the presence of EDTA. A time-dependent uptake of phosphatidylcholine into the membranes could be observed. The content of this phospholipid was increased from 2 to 5%. The rate of transfer was dependent on temperature, the amount of phosphatidylcholine present in the incubation mixture and on the fatty acid composition of the liposomal phosphatidylcholine. A possible adsorption of lipid vesicles to the membranes could be monitored by adding cholesteryl [¹⁴C]oleate to the liposomal preparation. As cholesterylesters are not transferred between membranes [1], it was possible to differentiate between transfer of phosphatidylcholine molecules from the liposomes into the membranes and adsorption of liposomes to the membranes. The phosphatidylcholine incorporated in the membranes was isolated, and its fatty acids were analysed by gas chromatography. It could be shown that there was a preferential transfer of phosphatidylcholine molecules containing two unsaturated fatty acids.     

Distribution of negatively charged liposomes on rat liver hepatocytes and non-hepatocytes in cell suspension

The in vitro interactions between negatively charged multilamellar liposomes and purified rat liver parenchymal and non-parenchymal cells were studied. The liposomes were labelled with [¹⁴C]cholesterol and contained [³H]methotrexate. For both cell types the time course of liposomal attachment to the cells slowed down gradually after a rapid initial phase lasting ca 90 min. The rate of attachment at 4 °C was 3–7 times lower than that at 37 °C, and the metabolic inhibitors dinitrophenol and iodoacetic acid caused reduction of 20–30%. Up to 45% of the cell-associated liposomal radioactivity could be detached within 1 h incubation with unlabelled liposomes. Whereas liver parenchymal cell suspension seemed to exhibit similar characteristics in vitro as in vivo, the non-parenchymal cells in vitro showed a 20–50-fold reduction in the rate of liposomal attachment compared to in vivo.     

Lecithin-cholesterol acyltransferase (LCAT) catalyzes transacylation of intact cholesteryl esters. Evidence for the partial reversal of the forward LCAT reaction

Lecithin-cholesterol acyltransferase (LCAT) catalyzes the intravascular synthesis of lipoprotein cholesteryl esters by converting cholesterol and lecithin to cholesteryl ester and lysolecithin. LCAT is unique in that it catalyzes sequential reactions within a single polypeptide sequence, a phospholipase A2 reaction followed by a transacylation reaction. In this report we find that LCAT mediates a partial reverse reaction, the transacylation of lipoprotein cholesteryl oleate, in whole plasma and in a purified, reconstituted system. As a result of the reverse transacylation reaction, a linear accumulation of [3H]cholesterol occurred during incubations of plasma containing high density lipoprotein labeled with [3H]cholesteryl oleate. When high density lipoprotein labeled with cholesteryl [14C]oleate was also included in the incubation the labeled fatty acyl moiety remained in the cholesteryl [14C]oleate pool showing that the formation of labeled cholesterol did not result from hydrolysis of the doubly labeled cholesteryl esters. The rate of release of [3H]cholesterol was only about 10% of the forward rate of esterification of cholesterol using partially purified human LCAT and was approximately 7% in whole monkey plasma. Therefore, net production of cholesterol via the reverse LCAT reaction would not occur. [3H]Cholesterol production from [3H]cholesteryl oleate was almost completely inhibited by a final concentration of 1.4 mM 5,5'-dithiobis(nitrobenzoic acid) during incubation with either purified LCAT or whole plasma. Addition of excess lysolecithin to the incubation system did not result in the formation of [14C]oleate-labeled lecithin, showing that the reverse reaction found here for LCAT was limited to the last step of the reaction. To explain these results we hypothesize that LCAT forms a [14C]oleate enzyme thioester intermediate after its attack on the cholesteryl oleate molecule. Formation of this intermediate allows [3H]cholesterol to be liberated from the enzyme by exchange with unlabeled cholesterol of plasma lipoproteins. The liberated [3H]cholesterol thereby becomes available for reesterification by LCAT as indicated by its appearance as newly synthesized cholesteryl linoleate.     

Liposome-Mediated transfer of bacterial RNA into carrot protoplasts

The uptake of liposome-encapsulated E. coli [³H]RNA by carrot (Daucus carota L.) protoplasts was examined. [³H]RNA extracted from protoplasts that had been incubated with [³H]RNA-containing, large, unilamellar lipid vesicles (liposomes) obtained by ether infusion, and examined by sucrose gradient centrifugation and formamide-polyacrylamide gel electrophoresis, appeared substantially degraded, with a total elimination of 23S RNA and a partial loss of 16S RNA. In contrast, no breakdown of the [³H]RNA was apparent in the liposomes after sequestration, even in the presence of externally added ribonuclease, or in the unfused liposomes remaining after incubation of protoplasts with liposomes. Thus, the degradation of the [³H]RNA extracted from the protoplasts must have occurred within the protoplasts and represents evidence for liposome-mediated RNA uptake. Naked RNA added to the protoplast culture was found to be totally degraded after incubation with the protoplasts. The uptake of liposome-sequestered RNA by protoplasts was demonstrated to be a function both of the lipid composition of the liposomal membrane and of the temperature of incubation of the liposomeprotoplast mixture. Furthermore, the mode of this uptake (fusion versus endocytosis) could be manipulated by adjusting the cholesterol content of the liposomal membrane. The implications of the ability to insert RNA into protoplasts without degradation by extracellular nucleases are discussed.     

Acyl-coenzyme A: cholesterol acyltransferase. Transfer of cholesterol to its substrate pool and modulation of activity

The preincubation at 37 degrees C of rat liver microsomal fraction, followed by re-isolation of the treated vesicles, results in a time-dependent increase in the activity of acyl-CoA: cholesterol acyltransferase. The presence of cholesterol-phospholipid (1:1, mol/mol) liposomes results in higher rate of increase in activity and under these conditions the rate of increase is liposomal cholesterol concentration-dependent. The preincubation of the microsomal fraction in the presence of [3H]cholesterol-phospholipid liposomes results in transfer of [3H]cholesterol to the re-isolated microsomal vesicles and this transfer follows first-order kinetics in respect to the donor concentration. These preincubations result also in a time-dependent and liposomal cholesterol concentration-dependent increase in the incorporation of [3H]cholesterol into the cholesteryl oleate produced on assay of cholesterol acyltransferase activity. From specific radioactivity data of the cholesteryl esters synthesised on assay of cholesterol acyltransferase in treated microsomal preparations, the rate of liposomal [3H]cholesterol equilibration with the cholesterol acyltransferase substrate pool can be calculated. The half-time of this transfer decreased with the concentration of liposomal cholesterol present during the preincubation. The activation energy for the transfer of liposomal cholesterol to the cholesterol acyltransferase substrate pool was 87.9 kJ/mol and was independent of the concentration of liposomal cholesterol. The activation energy for the rate of increase of total cholesteryl oleate was similar to this value for low concentrations of liposomal cholesterol and progressively decreased with increasing concentrations of liposomal cholesterol. The data suggest that under the present conditions, the time-dependent and temperature-dependent increase in cholesterol acyltransferase activity is due to the transfer of non-esterified cholesterol from other microsomal and/or liposomal vesicles to the vesicles that contain the enzyme and therefore to increased availability of substrate.     

Effect of cholesterol on the uptake and intracellular degradation of liposomes by liver and spleen; a combined biochemical and gamma-ray perturbed angular correlation study

We investigated the effect of cholesterol on the uptake and intracellular degradation of liposomes by rat liver and spleen macrophages. Multilamellar vesicles (MLV) consisting of distearoylphosphatidylcholine/phosphatidylserine (molar ratio 9:1) or distearoylphosphatidylcholine/cholesterol/phosphatidylserine (molar ratio 4:5:1) were labeled with [3H]cholesteryl hexadecyl ether and/or cholesteryl [14C]oleate. After i.v. injection the cholesterol-containing liposomes were eliminated less rapidly from the bloodstream and taken up to a lesser extent by the liver (macrophages) than the cholesterol-free liposomes. Assessment of the 3H/14C ratios in liver and spleen cells revealed that the cholesterol-containing liposomes are substantially more resistant towards intracellular degradation than the cholesterol-free liposomes. These results could be confirmed by measuring the release of 111In from liposomes after uptake by liver and spleen by means of gamma-ray perturbed angular correlation spectroscopy. Experiments with cultured Kupffer cells in monolayer also revealed that incorporation of cholesterol results in a decrease of the uptake and an increase of the intracellular stability of cholesteryl [14C]oleate-labeled liposomes. Finally, incubation of both types of liposomes with lysosomal fractions prepared from rat liver demonstrated a difference in susceptibility to lysosomal degradation: the cholesterol-free vesicles were much more sensitive to lysosomal esterase than the cholesterol-containing liposomes. These results may be relevant to the application of liposomes as a drug carrier system to liver and spleen (macrophages).     

Uptake and processing of liposomal phospholipids by Kupffer cells in vitro

We investigated the intracellular metabolic fate of [Me-14C]choline-labeled phosphatidylcholines and sphingomyelin taken up by rat Kupffer cells in maintenance culture during interaction with large unilamellar liposomes composed of cholesterol, labeled choline-phospholipid and phosphatidylserine (molar ration 5:4:1). With both labeled compounds only small proportions of water-soluble radioactivity were found to accumulate in the cells and in the culture medium, suggesting limited phospholipid degradation. However, after a lag period of 30 min progressively increasing proportions of cell-associated liposomal phospholipid were found to be converted to cellular phospholipid, nearly all of which was phosphatidylcholine. This conversion as well as the limited release of water-soluble label from the cells was inhibited by the lysosomotropic agents ammonium chloride and chloroquine. With [Me-14C]choline-labeled lysophosphatidylcholine, label was found to become cell-associated far in excess of an encapsulated liposomal label, [3H]inulin. Without a lag period virtually all of this was rapidly converted to phosphatidylcholine, a process which was not inhibited by the lysosomotropic agents. It is concluded that Kupffer cells, after endocytosis of liposomes, degrade the liposomal phospholipids effectively but reutilize the choline moiety for de novo synthesis of cellular phosphatidylcholine.     

Intrahepatic uptake and processing of intravenously injected small unilamellar phospholipid vesicles in rats

Small unilamellar vesicles consisting of sphingomyelin, cholesterol and phosphatidylserine in a molar ratio of 4:5:1 containing [³H]inulin as a marker of the aqueous space or [Me-¹⁴C]choline-labeled sphingomyelin as a marker of the lipid phase were injected intravenously into rats. After separation of the non-parenchymal cells into a Kupffer cell fraction and an endothelial cell fraction by elutriation centrifugation analysis of the radioactivity contents demonstrated that Kupffer cells were actively involved in the uptake of the vesicles whereas endothelial cells did not contribute at all. Uptake by total parenchymal cells was also substantial but, on a per cell base, significantly lower than that by the Kupffer cells. By comparising the fate of the [³H]inulin label and the [¹⁴C]sphingomyelin label it was concluded that release of liposomal lipid degradation products especially occurred from Kupffer cells rather than from parenchymal cells. In both cell types, however, substantial proportions of the ¹⁴C-label accumulated in the phosphatidylcholine fraction, indicating intracellular degradation of sphingomyelin and subsequent phosphatidylcholine synthesis. Treatment of the animals with the lysosomotropic agent chloroquine prior to liposome injection effectively blocked the conversion of the choline-labeled sphingomyelin into phosphatidylcholine in both cell types. This observation indicates that uptake of the vesicles occurred by way of an endocytic mechanism.     

Failure of the intracellular itinerary of beta very low density lipoproteins to augment cholesterol esterification in macrophages from Watanabe heritable hyperlipidemic rabbits

beta very low density lipoproteins (beta-VLDL) interact with mouse peritoneal macrophages via specific receptors leading to pronounced stimulation of cholesterol esterification. The present study has defined an alternative pathway for the processing of beta-VLDL in alveolar macrophages from Watanabe heritable hyperlipidemic (WHHL) rabbits. Macrophages from either New Zealand (NZ) or WHHL rabbits degraded 125I-beta-VLDL to an equivalent extent. Degradation was competed to a similar extent in both cell types by either excess unlabeled beta-VLDL or low density lipoprotein, indicative of a specific receptor involvement. Accumulation of intracellular degradation products of beta-VLDL labeled with the residualizing label, dilactitol-125I-tyramine, was similar in both cell types demonstrating that degradation was not due to secreted proteolytic enzymes. beta-VLDL promoted the incorporation of [3H]oleate into cholesteryl-[3H]oleate and increased the cellular mass of cholesterol in NZ macrophages. In contrast, beta-VLDL did not augment cholesteryl-[3H]oleate deposition in WHHL macrophages. This lack of cholesterol esterification occurred despite equivalent acyl-CoA:cholesterol acyltransferase activity in microsomal fractions of both cell types, and similar augmentations in cholesteryl-[3H]oleate during incubation with phospholipase C-treated LDL. Incubation of WHHL macrophages with beta-VLDL increased cellular cholesterol mass, although the response was attenuated compared to NZ cells. To determine whether these disparities in cholesterol esterification were related to the catabolic fate of beta-VLDL-derived cholesterol esters, [3H]cholesteryl oleate was exchanged into the core of beta-VLDL and incubated with macrophages in medium containing [14C]oleate. NZ macrophages accumulated both [3H]cholesterol and [3H]cholesteryl-[14C]oleate after 5 h, indicating hydrolysis and re-esterification of cholesterol esters. In contrast, WHHL macrophages only accumulated [3H]cholesterol esters, suggesting uptake of cholesterol esters without subsequent hydrolysis. These data demonstrate that WHHL macrophages possess a pathway for the intracellular processing of beta-VLDL that permits internalization of the particle without stimulation of cholesterol esterification.     

Reversible accumulation of cholesteryl esters in macrophages incubated with acetylated lipoproteins

Mouse peritoneal macrophages accumulate large amounts of cholesteryl ester when incubated with human low-density lipoprotein that has been modified by chemical acetylation (acetyl-LDL). This accumulation is related to a high-affinity cell surface binding site that mediates the uptake of acetyl-LDL by adsorptive endocytosis and its delivery to lysosomes. The current studies demonstrate that the cholesteryl ester accumulation can be considered in terms of a two-compartment model: (a) the incoming cholesteryl esters of acetyl-LDL are hydrolyzed in lysosomes, and (b) the resultant free cholesterol is re-esterified in the cytosol where the newly formed esters are stored as lipid droplets. The following biochemical and morphologic evidence supports the hydrolysis-re-esterification mechanism: (a) Incubation of macrophages with acetyl-LDL markedly increased the rate of cholesteryl ester synthesis from [14C]oleate, and this was accompanied by an increase in the acyl-CoA:cholesteryl acyltransferase activity of cell-free extracts. (b) When macrophages were incubated with reconstituted acetyl-LDL in which the endogenous cholesterol was replaced with [3H]-cholesteryl linoleate, the [3H]cholesteryl linoleate was hydrolyzed, and at least one-half of the resultant [3H]cholesterol was re-esterified to form [3H]cholesteryl oleate, which accumulated within the cell. The lysosomal enzyme inhibitor chloroquine inhibited the hydrolysis of the [3H]cholesteryl linoleate, thus preventing the formation of [3H]cholesteryl oleate and leading to the accumulation of unhydrolyzed [3H]cholesteryl linoleate within the cells. (c) In the electron microscope, macrophages incubated with acetyl-LDL had numerous cytoplasmic lipid droplets that were not surrounded by a limiting membrane. The time course of droplet accumulation was similar to the time course of cholesteryl ester accumulation as measured biochemically. (d) When acetyl-LDL was removed from the incubation medium, biochemical and morphological studies showed that cytoplasmic cholesteryl esters were rapidly hydrolyzed and that the resultant free cholesterol was excreted from the cell.     

Metabolism of cholesteryl ester lipid droplets in a J774 macrophage foam cell model

J774 macrophages rapidly incorporated [3H]cholesteryl oleate droplets by a non-saturable phagocytic process. In less than 2 h, foam cell morphology was acquired. The extent of loading obtained after 2 h was a linear function of the mass of cholesteryl oleate provided to the cells. The cholesteryl oleate incorporated was hydrolyzed in the cells at a linear rate over 24 h and the fractional hydrolysis was constant over a wide range of cellular esterified cholesterol contents. The rate of hydrolysis was influenced by the physical state of the cholesteryl ester; cholesteryl oleate in isotropic droplets was hydrolyzed 2-3-fold more rapidly than cholesteryl oleate in anisotropic droplets. The hydrolysis of both types of droplets was inhibited by lysosomotropic agents, indicating that hydrolysis occurred in the lysosomes. Only a small fraction (less than 10% after 24 h) of the free [3H]cholesterol generated in the lysosomes was esterified by ACAT resulting in a doubling of the cell free cholesterol content. Electron microscopy of cells treated with digitonin revealed the accumulation of free cholesterol in lipid-laden lysosomes. ACAT was active as endogenous free [14C]cholesterol was esterified in a linear manner over 24 h and was responsive to the presence of lysosomally-derived cholesterol, as the extent of esterification of the endogenous pool was directly proportional to the mass of [3H]cholesterol generated in the lysosomes.     

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 μg/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.     

Interaction of liposomes with human leukocytes in whole blood

The uptake of multilamellar liposomes into human leukocytes in whole blood in vitro was evaluated on the basis of the cellular association of liposomal markers (3H-labelled cholesterol, lipid phase; [14C]inulin, aqueous phase). The entry of liposomes into human blood leukocytes was linear for 60 min and was mediated by a saturable mechanism displaying affinity constants of 0.28 +/- 0.17 and 0.16 +/- 0.05 mM liposomal lipid (means +/- S.E.) for liposomal lipid and aqueous phase markers, respectively. Amicon filtration analysis of incubation mixtures containing blood and liposomes (phosphatidylcholine:dicetyl phosphate:cholesterol, 70:20:10) showed that 34% of [14C]inulin was lost (neither liposome-associated nor cell-associated) after 60 min. By preincorporating sphingomyelin (35 mol%) into multilamellar liposomes, the leakage of the model aqueous phase marker inulin was reduced to 8% after 60 min, thus enhancing the drug carrier potential of liposomes in blood. As a consequence of their interaction with liposomes, the polymorphonuclear leukocytes in whole blood decreased in apparent buoyant density, while maintaining their viability. These results indicate that blood leukocytes in their natural milieu of whole blood are capable of interacting with, and taking up multilamellar liposomes.     

Intravascular metabolism of lipoprotein cholesteryl esters in African green monkeys: differential fate of doubly labeled cholesteryl oleate

High density lipoproteins (HDL), doubly labeled with [3H]cholesteryl oleate and cholesteryl [14C]oleate, were reinjected to study HDL cholesteryl ester metabolism in African green monkeys. The transfer of labeled HDL cholesteryl ester to low density lipoprotein (LDL) was rapid and equilibration of the [3H]cholesteryl oleate and cholesteryl [14C]oleate specific activities in LDL and HDL occurred within 90 min after reinjection. The apparent rates of disappearance from the circulation of the two moieties of the cholesteryl ester were different. In the same four animals, the residence time for the turnover of plasma [3H]cholesterol averaged 6.1 days while the residence time for the removal of cholesteryl [14C]oleate from plasma was approximately 2.1 days. These results suggest that for some lipoprotein cholesteryl esters removed from plasma, the cholesterol moiety subsequently reappeared in plasma. The difference between the rate of decay of the 14C-labeled fatty acid moiety, which represents all of the cholesteryl ester removed from plasma (0.48 pools/day) and the decay of the 3H-labeled cholesterol moiety, which represents the sum of cholesteryl ester removal and cholesterol reappearance (0.16 pools/day), is the fraction of the cholesteryl ester pool recycled per day (0.32 pools/day or 22.5 mg/kg per day). In other words, approximately 68% of the cholesterol moiety that was removed from plasma as cholesteryl oleate reappeared in the plasma cholesterol pool. These studies support the concept that an efficient reutilization cycle for plasma cholesterol occurs, i.e., the cholesteryl ester molecule can exit and the cholesterol moiety can re-enter plasma without effective equilibration of the cholesterol moiety with extravascular cholesterol pools.     

Lipolytic cleavage of dolichyl oleate catalyzed by calf brain membranes

Calf brain membranes catalyze the lipolytic cleavage of dolichyl [¹⁴C]oleate added as an aqueous dispersion in Triton X-100. The enzymatic release of [¹⁴C]oleate from the dolichyl ester is not affected by divalent cations or EDTA, but the lipase activity is inhibited by iodoacetamide and pHMB. The amount of [¹⁴C]oleate released is dependent on the time of incubation, the amount of membrane protein added and the concentration of the radiolabeled lipid substrate. Dolichyl ester hydrolase activity exhibits a pH optimum of 7.5, distinguishing this lipase activity from cholesteryl ester hydrolase (5.0–5.5) and triolein hydrolase (5.0) activity associated with the same membrane preparations. The enzymatic hydrolysis of dolichyl [¹⁴C]oleate is also partially inhibited by oleate and free dolichol, possibly by end-product inhibition.     

The involvement of parenchymal,Kupffer and endothelial liver cells in the hepatic uptake of intravenously injected liposomes effects of lanthanum and gadolinium salts

¹²⁵I-labeled albumin or poly(vinyl pyrrolidone) encapsulated in intermediate size multilamellar or unilamellar liposomes with 30–40% of cholesterol were injected intravenously into rats. In other experiments liposomes containing phosphatidyl[Me-¹⁴C]choline were injected. 1 h after injection parenchymal or non-parenchymal cells were isolated. Non-parenchymal cells were separated by elutriation centrifugation into a Kupffer cell fraction and an endothelial cell fraction. From the measurements of radioactivities in the various cell fractions it was concluded that the liposomes are almost exclusively taken up by the Kupffer cells. Endothelial cells did not contribute at all and hepatocytes only to a very low extent to total hepatic uptake of the ¹²⁵I-labels. Of the ¹⁴C-label, which orginates from the phosphatidylcholine moiety of the liposomes, much larger proportions were recovered in the hepatocytes. A time-dependence study suggested that besides the involvement of phosphatidylcholine exchange between liposomes and high density lipoprotein, a process of intercellular transfer of lipid label from Kupffer cells to the hepatocytes may be involved in this phenomenon. Lanthanum or gadolinium salts, which effectively block Kupffer cell activity, failed to accomplish an increase in the fraction of liposomal material recovered in the parenchymal cells. This is compatible with the notion that liposomes of the type used in these experiments have no, or at most very limited, access to the liver parenchyma following their intravenous administration to rats.     

Biliary lipid secretion in the rat. The uncoupling of biliary cholesterol and phospholipid secretion from bile acid secretion by sulfated glycolithocholic acid

Glycolithocholic acid and its sulfated derivative are major metabolites of the secondary bile acid lithocholic acid in man. Both compounds are known to induce cholestasis in experimental animals. We compared the effects of these endogenous hepatotoxins on bile production and biliary lipid composition in rats with chronic biliary drainage. The compounds were administered enterally at relatively low rates (5-50% of the rats' endogenous bile acid secretion in these experiments) to simulate enterohepatic circulation. Both compounds were substantially secreted into bile (more than 90% of dose); sulfated glycolithocholic acid unchanged and glycolithocholic acid after hepatic hydroxylation predominantly in the form of glyco-beta-muricholic acid (cf. Kuipers et al. (1986) Am. J. Physiol. 251, G189-G194). Neither glycolithocholic acid nor its sulfated derivative affected the biliary excretion of endogenous bile acids or bile flow in these experiments. In spite of this, phospholipid and cholesterol secretion were significantly reduced by sulfated glycolithocholic acid but were not altered by glycolithocholic acid. Phospholipid and cholesterol secretion rapidly decreased to 25 and 50% of their initial values, respectively, at biliary output rates of sulfated glycolithocholic acid up to 2 mumol/h, and did not further decrease when this output was increased to 6 mumol/h. Small unilamellar liposomes consisting of cholesterol, [Me-14C]choline-labeled phosphatidylcholine, phosphatidylserine and [3H]cholesteryl oleate in a 5:4:1:0.1 molar ratio were employed to label intrahepatic lipid pools. Administration of sulfated glycolithocholic acid slightly reduced bile acid synthesis from [3H]cholesteryl oleate, but significantly reduced the biliary secretion of [14C]phospholipid. Glycolithocholic acid did not affect the hepatic processing of liposomal lipids. It is concluded that sulfated glycolithocholic acid at low doses causes the uncoupling of biliary lipid secretion from that of bile acids, which might represent in initiating event in sulfated glycolithocholic acid hepatotoxicity.     

Differential distribution of liposome-entrapped [3H]methotrexate and labelled lipids after intravenous injection in a primate

Positive liposomes consisting of phosphatidylcholine, cholesterol and stearylamine and negatively charged liposomes consisting of phosphatidylcholine, cholesterol and phosphatidylserine, were double labelled with either ³H-labelled dipalmitoyl phosphatidylcholine and [¹⁴C]cholesterol or with [¹⁴C]cholesterol and [³H]methotrexate entrapped in the aqueous phase. The plasma levels and urinary excretion of radioactivity from sonicated and non-sonicated liposomes were then compared with the levels of radioactivity from free [³H]methotrexate during a 4 h experimental period after an initial intravenous injection in cynomolgous monkeys. Tissue uptake at the completion of the 4 h experimental period was also measured.It was found that plasma radioactivity from [³H]methotrexate and [¹⁴C]cholesterol in sonicated positive liposomes was cleared more slowly than from comparable non-sonicated liposomes, and considerably slower than from free [³H]methotrexate. Radioactivity from sonicated negative liposomes was cleared more rapidly than from positive sonicated liposomes. Positive liposomes captured considerably more [³H]methotrexate than negative liposomes and showed very low permeability to [³H]methotrexate in in vitro studies, even in the presence of high concentrations of serum.[¹⁴C]Cholesterol radioactivity was cleared more rapidly from plasma than ³H-radioactivity from liposome-entrapped [³H]methotrexate for double-labelled sonicated liposomes and generally showed greater uptake into tissues and red blood cells. ³H-labelled dipalmitoyl phosphatidylcholine in sonicated positive liposomes was cleared faster than [¹⁴C]cholesterol during the first 3 h. The more rapid disappearance of [¹⁴C]cholesterol from the plasma was complemented by greater uptake into a number of tissues, and positive non-sonicated liposomes were taken up to a greater extent by the spleen than equivalent sonicated liposomes.Renal excretion of ³H from liposome-entrapped [³H]methotrexate was considerably less than that of ³H from free [³H]methotrexate. There was insignificant excretion, however, of ¹⁴C from cholesterol in the urine.Entrapment in liposomes completely prevented the otherwise considerable breakdown of free methotrexate to ³H-containing products in plasma and partially prevented its breakdown in tissues.These studies indicate marked differences in the distribution of liposomes in vivo due to surface charge and size, and some degree of exchange of the lipid components of the liposome bilayer independent of the distribution of the entrapped species. They also show that entrapment in liposomes can reduce metabolic degradation of a drug, maintain high plasma levels and reduce its renal excretion.     

The role of apolipoproteins of HDL in the selective uptake of cholesteryl linoleyl ether by cultured rat and bovine adrenal cells

Rat adrenal cells in culture were used to study the uptake of cholesteryl linoleyl ether [( 3H]cholesteryl linoleyl ether), a nonhydrolyzable analog of cholesteryl ester. When [3H]cholesteryl linoleyl ether was added in the form of liposomes, its uptake was enhanced by adrenocorticotropin (ACTH) and by addition of milk lipoprotein lipase and interfered by heparin. When the adrenal cells were incubated with homologous [3H]cholesteryl linoleyl ether-HDL, ACTH treatment also resulted in an increase in [3H]cholesteryl linoleyl ether uptake. The uptake of [3H]cholesteryl linoleyl ether was in excess of the uptake and metabolism of 125I-labeled HDL protein and was not sensitive to heparin. Unlabeled HDL or delipidated HDL reduced very markedly the uptake of [3H]cholesteryl linoleyl ether, while addition of phosphatidylcholine liposomes had little effect. Attempts were made to deplete and enrich the adrenal cells in cholesterol and, while depletion resulted in a decrease in [3H]cholesteryl linoleyl ether-HDL uptake, enrichment of cells with cholesterol had no effect. Among the individual apolipoproteins tested, apolipoprotein A-I and the C apolipoproteins reduced [3H]cholesteryl linoleyl ether uptake, while apolipoprotein E was not effective. Since the labeled ligand studied was a lipid, these effects could not be due to an exchange of apolipoproteins, but indicated competition for binding sites. Preferential uptake of human [3H]cholesteryl linoleyl ether-HDL3 by bovine adrenal cells was found when compared to the uptake and metabolism of 125I-labeled HDL. The present results suggest that the preferential uptake of HDL cholesteryl ester (as studied with [3H]cholesteryl linoleyl ether) requires an interaction between the apolipoproteins of HDL and cell surface components.     

Effect of apolipoprotein E-free high density lipoproteins on cholesterol metabolism in cultured pig hepatocytes

We studied cholesterol synthesis from [14C]acetate, cholesterol esterification from [14C]oleate, and cellular cholesterol and cholesteryl ester levels after incubating cells with apoE-free high density lipoproteins (HDL) or low density lipoproteins (LDL). LDL suppressed synthesis by up to 60%, stimulated esterification by up to 280%, and increased cell cholesteryl ester content about 4-fold. Esterification increased within 2 h, but synthesis was not suppressed until after 6 h. ApoE-free HDL suppressed esterification by about 50% within 2 h. Cholesterol synthesis was changed very little within 6 h, unless esterification was maximally suppressed; synthesis was then stimulated about 4-fold. HDL lowered cellular unesterified cholesterol by 13-20% within 2 h and promoted the removal of newly synthesized cholesterol and cholesteryl esters. These changes were transient; by 24 h, both esterification and cellular unesterified cholesterol returned to control levels, and cholesteryl esters increased 2-3-fold. HDL core lipid was taken up selectively from 125I-labeled [3H]cholesteryl ester- and ether-labeled HDL. LDL core lipid uptake was proportional to LDL apoprotein uptake. The findings suggest that 1) the cells respond initially to HDL or LDL with changes in esterification, and 2) HDL mediates both the removal of free cholesterol from the cell and the delivery of HDL cholesteryl esters to the cell.