Lipid vesicles as carriers for introducing materials into cultured cells: influence of vesicle lipid composition on mechanism(s) of vesicle incorporation into cells.

The mechanisms involved in the uptake of uni- and multi-lamellar lipid vesicles by BALB/c mouse 3T3 cells have been investigated. Vesicles are incorporated into cells both by endocytosis and by a nonendocytotic mechanism which we propose involves fusion of vesicles with the plasma membrane. The nonendocytotic pathway predominates in the uptake of negatively charged vesicles composed of phospholipids that are "fluid" (phosphatidylserine/phosphatidylcholine) at 37 degrees. Neutral fluid vesicles (phosphatidylcholine) and negatively charged vesicles prepared from "solid" phospholipids (phosphatidylserine/distearylphosphatidylcholine/dipalmitoylphosphatidylcholine) are instead incorporated largely by endocytosis. Uptake of the latter classes of vesicle is reduced (80-90% inhibition) by inhibitors of cellular energy metabolism and by cytochalasin B.