Liposome-cell interactions. Studies of lipid transfer using isotopically asymmetric vesicles.

Phospholipid transfer between small unilamellar vesicles and Chinese hamster fibroblasts, a process in which cellular uptake of vesicle lipids without concomitant uptake of vesicle contents occurs, was studied. Isotopically asymmetric vesicles were formed by incubation of 3H-labeled dioleoyl phosphatidylcholine or 3H-labeled dioleoyl phosphatidylethanolamine-containing vesicles with appropriate phospholipid exchange proteins and unlabeled acceptor membranes. Isolated vesicles were then incubated with cell suspensions at 2°C and cellular uptake of radioactive lipid was determined. Uptake of exogenous lipids was found to be directly proportional to the amount of radioactivity present in the outer (but not inner) leaflet of the vesicles, thus demonstrating that only the outer monolayer lipids become cell-associated during vesicle-cell lipid transfer. Immediately following incubation of cells with 3H-labeled phosphatidylethanolamine-containing vesicles at 2”C, more than 80% of the exogenously supplied lipid was converted to trinitrophenyl phosphatidylethanolamine when the cells were reacted with trinitrobenzenesulfonic acid under nonpenetrating conditions. This result demonstrates that vesicle lipids are introduced preferentially into the outer leaflet of the cell plasma membrane by the lipid transfer process. Spectrophotometric determination of the total amount of phosphatidylethanolamine accessible to trinitrobenzenesulfonic acid labeling before and after incubation of cells with vesicles revealed that lipid transfer is not accompanied by a net change in cell surface phosphatidylethanolamine, suggesting that the transfer of lipids between vesicles and cells actually represents a one-for-one exchange. This exchange process is phospholipid species-specific and sensitive to pretreatment of cells with trypsin.