In vitro interaction of Zajdela ascites hepatoma cells with lipid vesicles.

We studied the in vitro interaction between Zajdela ascites hepatoma cells and small unilamellar vesicles, consisting of 14C-labeled phosphatidylacholine, cholesterol, and phosphatidylserine (molar ratio 5 : 4 : 1), containing high intravesicular concentrations of carboxyfluorescein or fluorescein isothiocyanate tagged dextran. The entrapped markers were found to be associated with the cells to a lesser degree than the vesicle membrane marker. This discrepancy, which is slightly less pronounced for fluorescein isothiocyanate tagged dextran than for carboxyfluorescein, increases with incubation time and decreases with increasing vesicle lipid concentration in the incubation mixture. Vesicle-plasma membrane exchange of the vesicle lipid marker could not entirely explain the observed discrepancy. It is tentatively concluded that the gap mainly arises from a selective loss of entrapped dyes from vesicles actually interacting with the cell surface. Both spectrofluorimetry and fluorescence microscopic observations, as well as the relative insensitivity of vesicle uptake towards the presence of metabolic inhibitors, exclude a major contribution of endocytosis as a vesicle uptake route. We therefore conclude that vesicles are primarily internalized by a vesicle-cell fusion-like process. The observed discrepancy in uptake between entrapped materials and vesicle lipid is discussed in terms of a two-site vesicle-cell surface interaction model.

Thin-layer chromatography with agarose gels. A quick, simple method for evaluating liposome size.

Thin-layer gels can be made with agarose and used to assess within a few minutes the efficiency with which multilamellar vesicles are converted to small unilamellar ones by sonication. A fluroescent lipid marker or vesicle-encapsulated solute permits continuous monitoring of the chromatography. Advantages over agarose gel column chromatography include speed of analysis, small sample size, the possibility of running multiple samples and simultaneously, and direct accessibility to fluorescence microscopy. This approach should also be useful in the study of liposome-lipoprotein interactions and in affinity chromatography of liposomes.