Gerrit L. Scherphof

Disintegration of phosphatidylcholine liposomes in plasma as a result of interaction with high-density lipoproteins.

1. During in vitro incubation of liposomes or unilamellar vesicles prepared from egg-yolk or rat-liver phosphatidylcholine with human, monkey or rat plasma the phospholipid becomes associated with a high molecular weight protein-containing component. 2. The phosphatidylcholine . protein complex thus formed co-chromatographs with high-density lipoprotein on Ultrogel AcA34 and has the same immunoelectrophoretic properties as this lipoprotein. 3. Release of the phosphatidylcholine from liposomes was also observed when liposomes were incubated with pure monkey high-density lipoproteins. Under those conditions some transfer of protein from the lipoprotein to the liposomes was observed as well. 4. The observed release of phospholipid from the liposomes is a one-way process, as the specific radioactivity of liposome-associated phosphatidylcholine remained constant during incubation with plasma. 5. It is concluded that either the lipoprotein particle takes up additional phospholipid or that a new complex is formed from protein constituents of the lipoprotein and the liposomal phosphatidylcholine. 6. Massive release of entrapped 125I-labeled albumin from the liposome during incubation with plasma suggests that the observed release of phosphatidylcholine from the liposomes has a highly destructive influence on the liposomal structure. 7. Our results are discussed with special reference to the use of liposomes as intravenous carriers of drugs and enzymes.

POSITIONAL SPECIFICITY OF SATURATED AND UNSATURATED FATTY ACIDS IN PHOSPHATIDIC ACID FROM RAT LIVER

Abstract 1. 1. The relative incorporation of a number of radioactive fatty acids into the different glycerolipids of rat liver microsomes has been investigated. 2. 2. Studies on the distribution of the radioactivity incorporated into phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid showed that in all three lipids, the majority of the saturated fatty acids were at the 1-position while the polyunsaturated fatty acids were largely confined to the 2-position. 3. 3. The phosphatidic acid fraction of rat liver was isolated in a pure form, and its fatty acid distribution agreed with the results of the incorporation studies. 4. 4. The incorporation of radioactive fatty acids into different molecular species of phosphatidic acid, separated as dimethyl phosphatidates was investigated.

Transfer of [14C]phosphatidylcholine between liposomes and human plasma high density lipoprotein Partial purification of a transfer-stimulating plasma factor using a rapid transfer assay

A simple method was developed for the rapid determination of [14C]phosphatidylcholine transfer from small unilamellar liposomes to human plasma HDL, based on the selective precipitation of liposomes by heparin and MnCl2. The assay was utilized to monitor the progress in the partial purification of a phospholipid transfer factor from human plasma. The purification procedure included ultracentrifugation at d = 1.25 g/ml, hydrophobic chromatography on phenyl-Sepharose, affinity chromatography on heparin-Sepharose and gel filtration. The partially purified protein(s) catalyzed the net transfer of phospholipid from small unilamellar phosphatidylcholine liposomes to isolated HDL. The transfer of [14C]phosphatidylcholine from liposomes consisting of phosphatidylcholine/phosphatidylserine/cholesterol (molar ratio, 4:1:5) to HDL was stimulated without affecting the permeability barrier of the liposomal membranes and is, therefore, taken to represent exchange with HDL phospholipid rather than net transfer.

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 was 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.

TRANSFER AND EXCHANGE OF PHOSPHOLIPID BETWEEN SMALL UNILAMELLAR LIPOSOMES AND RAT PLASMA HIGH-DENSITY LIPOPROTEINS - DEPENDENCE ON CHOLESTEROL CONTENT AND PHOSPHOLIPID-COMPOSITION

Abstract We investigated the ability of small unilamellar liposomes of various lipid compositions to maintain their integrity in the presence of rat plasma or plasma fractions. Liposomal damage was determined in terms of release of an entrapped water-soluble marker, carboxyfluorescein, and, simultaneously, loss of liposomal 14C-labeled phospholipid. Complete retentions of carboxyfluorescein during a 30-min incubation with plasma could be attained with liposomes containing at least 40 mol% cholesterol. By substituting sphingomyelin for phosphatidylcholine a considerable prolongation of solute retention was achieved. Sphingomyelin/cholesterol (molar ratio 3 : 2) liposomes retained nearly all of the entrapped dye during a 20-h incubation with plasma, while phosphatidylcholine/cholesterol liposomes lost as much as 40%. Incorporation of cholesterol also reduces the transfer of 14C-labeled phospholipid from liposomes to plasma HDL. While transfer of phosphatidylcholine was partially reduced, transfer of sphingomyelin was completely inhibited under the same conditions. Isolated HDL was unable to bring about the extent of carboxyfluorescein release and phospholipid transfer which was observed with whole plasma, but required the addition of lipoprotein-free plasma. The subfraction of HDL that is rich in apolipoprotein E (HDL1) plays at most a minor role in liposome-plasma interactions. Native HDL phospholipids were biosynthetically labeled with 32P and isolated HDL was incubated with 14C-labeled liposomes in presence of lipoprotein-free plasma. In this way we demonstrated that, whereas phosphatidylcholine transfer from cholesterol-poor liposomes is mainly an unidirectional process leading to excessive liposome degradation, phosphatidylcholine transfer from cholesterol-rich liposomes involves an exchange process and is attended by almost complete retention of entrapped solute.

Targeting of lactosylceramide-containing liposomes to hepatocytes in vivo

Incorporation of 8 mol% lactosylceramide in small unilamellar vesicles consisting of cholesterol, dimyristoylphosphatidylcholine and phosphatidylserine in a molar ratio of 5:4:1 and containing [3H]inulin as an aqueous-space marker resulted in a 3-fold decreased half-life of the vesicles in blood and a corresponding increase in liver uptake after intracardial injection into rats. The increase in liver uptake was mostly accounted for by an enhanced uptake in the parenchymal cells, while the uptake by the non-parenchymal cells was only slightly increased. The uptake of both the control and the glycolipid-containing vesicles by the non-parenchymal cell fraction could be attributed completely to the Kupffer cells; no radioactivity was found in the endothelial cells. The effect of lactosylceramide on liver uptake and blood disappearance of the liposomes was effectively counteracted by desialylated fetuin, injected shortly before the liposome dose. This observation supports the notion that a galactose-specific receptor is involved in the liver uptake of lactosylceramide liposomes.

Action of phospholipases A2 on phosphatidylcholine bilayers. Effects of the phase transition, bilayer curvature and structural defects.

We examined the action of porcine pancreatic and bee-venom phospholipase A2 towards bilayers of phosphatidylcholine as a function of several physical characteristics of the lipid-water interface. 1. Unsonicated liposomes of dimyristoyl phosphatidylcholine are degraded by both phospholipases in the temperature region of the phase transition only (cf. Op den Kamp et al. (1974) Biochim. Biophys. Acta 345, 253--256 and Op den Kamp et al. (1975) Biochim. Biophys. Acta 406, 169--177). With sonicates the temperature range in which hydrolysis occurs is much wider. This discrepancy between liposomes and sonicates cannot be ascribed entirely to differences in available substrate surface. 2. Below the phase-transition temperature the phospholipases degrade dimyristoyl phosphatidylcholine single-bilayer vesicles with a strongly curved surface much more effectively than larger single-bilayer vesicles with a relatively low degree of curvature. 3. Vesicles composed of egg phosphatidylcholine can be degraded by pancreatic phospholipase A2 at 37 degrees C, provided that the substrate bilayer is strongly curved. The bee-venom enzyme shows a similar, but less pronounced, preference for small substrate vesicles. 4. In a limited temperature region just above the transition temperature of the substrate the action of both phospholipases initially proceeds with a gradually increasing velocity. This stimulation is presumably due to an increase of the transition temperature, effectuated by the products of the phospholipase action. 5. Structural defects in the substrate bilayer, introduced by sonication below the phase-transition temperature (cf. Lawaczeck et al. (1976) Biochim. Biophys. Acta 443, 313--330) facilitate the action of both phospholipases. The results lead to the general conclusion that structural irregularities in the packing of the substrate molecules facilitate the action of phospholipases A2 on phosphatidylcholine bilayers. Within the phase transition and with bilayers containing structural defects these irregularities represent boundaries between separate lipid domains. The stimulatory effect of strong bilayer curvature can be ascribed to an overall perturbation of the lipid packing as well as to a change in the phase-transition temperature.

Selective transfer of a lipophilic prodrug of 5-fluorodeoxyuridine from immunoliposomes to colon cancer cells.

A monoclonal antibody against the rat colon carcinoma CC531 was covalently coupled to liposomes containing a dipalmitoylated derivative of the anticancer drug FUdR as a prodrug in their bilayers. We investigated the in vitro interaction of these liposomes with CC531 target cells and the mechanism by which they deliver the active drug FUdR intracellularly to the cells by monitoring the fate of the liposomal bilayer markers cholesterol-[(14)C]oleate and [(3)H]cholesteryloleylether as well as the (3)H-labeled prodrug and colloidal gold as an encapsulated liposome marker. After binding of the immunoliposomes to the cell surface, only limited amounts were internalized as demonstrated by a low level of hydrolysis of liposomal cholesterol ester and by morphological studies employing colloidal gold-labeled immunoliposomes. By contrast, already within 24 h immunoliposome-incorporated FUdR-dP was hydrolyzed virtually completely to the parent drug FUdR intracellularly. This process was inhibited by a variety of endocytosis inhibitors, indicating that the prodrug enters and is processed by the cells by a mechanism involving an endocytic process, resulting in intracellular FUdR concentrations up to 3000-fold higher than those in the medium. Immunoliposomes containing poly(ethyleneglycol) (PEG) chains on their surface, with the antibody coupled either directly to the bilayer or at the distal end of the PEG chains were able to deliver the prodrug into the tumor cells at the same rate as immunoliposomes without PEG. Based on these observations, we tentatively conclude that during the interaction of the immunoliposomes with the tumor cells the lipophilic prodrug FUdR-dP is selectively transferred to the cell surface and subsequently internalized by constitutive endocytic or pinocytic invaginations of the plasma membrane, thus ultimately delivering the prodrug to a lysosomal compartment where hydrolysis and release of parent drug takes place. This concept allows for an efficient delivery of a liposome-associated drug without the need for the liposome as such to be internalized by the cells.

Processing of different liposome markers after in vitro uptake of immunoglobulin-coated liposomes by rat liver macrophages

We compared the metabolic fate of [3H]cholesteryl[14C]oleate, [3H]cholesteryl hexadecylether, 125I-labeled bovine serum albumin and [3H]inulin as constituents of large immunoglobulin-coupled unilamellar lipid vesicles following their internalization by rat liver macrophages (Kupffer cells) in monolayer culture. Under serum-free conditions, the cholesteryl oleate that is taken up is hydrolyzed, for the greater part, within 2 h. This occurs in the lysosomal compartment as judged by the inhibitory effect of the lysosomotropic agents monensin and chloroquin. After hydrolysis, the cholesterol moiety is accommodated in the cellular pool of free cholesterol and the oleate is reutilized for the synthesis mainly of phospholipids and, to a lesser extent of triacylglycerols. During incubation in plasma, however, substantial proportions of both the cholesterol and the oleate are shed from the cells, predominantly in the unesterified form. When the liposomes are labeled with the cholesteryl ester analog [3H]cholesteryl hexadecylether only a very small fraction of the label is released from the cells, even in the presence of plasma. Similar to the label remaining associated with the cells, the released label is identified in that case as unchanged cholesteryl ether. The liposomal aqueous phase marker 125I-labeled bovine serum albumin is also readily degraded intralysosomally and the radioactive label is rapidly released from the cells in a trichloroacetic acid-soluble form. Also, as much as 20% of the aqueous phase marker [3H]inulin that becomes cell-associated during a 2-h incubation with inulin-containing liposomes, is released from the cells during a subsequent 4-h incubation period in medium or rat plasma. The usefulness of the various liposomal labels as parameters of liposome uptake and intracellular processing is discussed.

Intrahepatic distribution of small unilamellar liposomes as a function of liposomal lipid composition

We investigated the intrahepatic distribution of small unilamellar liposomes injected intravenously into rats at a dose of 0.10 mmol of lipid per kg body weight. Sonicated liposomes consisting of cholesterol/sphingomyelin (1:1), (A); cholesterol/egg phosphatidylcholine (1:1), (B); cholesterol/sphingomyelin/phosphatidylserine (5:4:1), (C) or cholesterol/egg-phosphatidylcholine/phosphatidylserine (5:4:1), (D) were labeled by encapsulation of [3H]inulin. The observed differences in rate of blood elimination and hepatic accumulation (A much less than B approximately equal to C less than D) confirmed earlier observations and reflected the rates of uptake of the four liposome formulations by isolated liver macrophages in monolayer culture. Fractionation of the liver into a parenchymal and a non-parenchymal cell fraction revealed that 80-90% of the slowly clearing type-A liposomes were taken up by the parenchymal cells while of the more rapidly eliminated type-B liposomes even more than 95% was associated with the parenchymal cells. Incorporation of phosphatidylserine into the sphingomyelin-based liposomes caused a significant increase in hepatocyte uptake but a much more substantial increase in non-parenchymal cell uptake, resulting in a major shift of the intrahepatic distribution towards the non-parenchymal cell fraction. For the phosphatidylcholine-based liposomes incorporation of phosphatidylserine did not increase the already high uptake by the parenchymal cells while uptake by the non-parenchymal cells was only moderately elevated; this resulted in only a small shift in distribution towards the non-parenchymal cells. The phosphatidylserine-induced increase in liposome uptake by non-parenchymal liver cells was paralleled by an increase in uptake by the spleen. Fractionation of the non-parenchymal liver cells in a Kupffer cell fraction and an endothelial cell fraction showed that even for the slowly eliminated liposomes of type A endothelial cells do not participate to a measurable extent in the elimination process, thus excluding involvement of fluid-phase pinocytosis in the uptake process.