Lipid Vesicle-Cell Interactions: Analysis of a Model for Transfer of Contents from Adsorbed Vesicles to Cells

Abstract

The association of dioleoylphosphatidylcholine vesicles with P388 murine tumor cells was monitored using fluorescent lipid probes incorporated into the vesicle membrane. As shown by fluorescence microscopy and by fluorescence photobleaching recovery measurements, most of the vesicles were stably adsorbed on the cell surface with little incorporation of vesicle lipid into the cell membrane. After 10 minutes of incubation only about 15% as much solute (³H-sucrose) remained cell-associated, as would have been expected on the basis of adsorbed lipid. These data indicate that the major component of interaction between these vesicles and cells is stable adsorption to the cell surface with considerable leakage of vesicle contents (cf., Szoka et al., Biochim. Biophys. Acta. (1979, 1980]). However, data in the literature indicate effects of vesicle-encapsulated solutes on physiological processes in the cytoplasm of non-endocytosing cells. Moreover, our data [Weinstein et al., Science (Wash. D.C.) 1977.] show fluorescent staining of the cytoplasm of non-endocytosing cells incubated with vesicle-encapsulated carbox-yfluorescein. We confirmed this observation using carbox-yfluorescein purified according to Ralston et al., Biochim. Biophys. Acta. [1981]. Our findings indicate that small amounts of solute are transferred from vesicles into cells. It has been suggested that this transfer could occur by leakage at the cell surface and entry into the cell before diffusion out of the unstirred layer. We here analyze such a model and show the following: such an “unstirred layer” mechanism could not increase solute incorporation beyond that for an equivalent bathing concentration of free solute unless the diffusion coefficient for the vesicles were larger than that for the free solute. Such is certainly not the case. We conclude that the small amount of vesicle-mediated entry occurs either by a low level of fusion or by some other unknown mechanism.