Interaction of phospholipid vesicles with cultured mammalial cells. I. Characteristics of uptake.

The interaction of monolayer cultures of Chinese hamster V79 cells with artificially generated, unilamellar lipid vesicles (approximately 500 A diameter) was examined. Vesicles prepared from a variety of natural and synthetic radiolabeled phosphatidyl cholines (lecithins) were incubated with V79 cells bathed in a simple balanced salt solution. After incubation, the cells were analyzed for exogenous lipid incorporation. Large quantities (approximately 10(8) molecules/cell/h) of lecithin became cell associated without affecting cell viability. The effects of pH, charged lipids, and the influence of the vesicle lipid phase transition on the uptake process were examined. Glutaraldehyde fixation of cells before vesicle treatment, or incubation in the presence of metabolic inhibitors, failed to reduce the lecithin uptake by more than 25-50%, suggesting that the lipid uptake is largely energy independent. Cells in sparse culture took up about ten times more lipid than dense cultures. Prolonged incubation (greater than 15 h) of sparse cell cultures with lecithin vesicles resulted in significant cell death while no deleterious effect was found in dense cultures, or with 1:1 lecithin/cholesterol vesicles. When vesicle-treated cells were homogenized and fractionated, about 20-30% of the exogenous lipid was found in the plasma membrane fraction, with the remainder being distributed into intracellular fractions. Electron microscope radioautography further demonstrated that most of the internalized lipid was present in the cytoplasm, with little in the nucleus. These results are discussed in terms of possible modification of cell behavior by lipid vesicle treatment.