Interactions of PC/Chol and PS/Chol liposomes with human cells in vitro

Interactions between phosphatidylcholine (PC) or phosphatidylserine (PS) li posomes and human umbilical vein endothelial cells (HUVEC) or human promyel ocytic leukemia cells (HL60) were investigated. Pyramine encapsulating or r hodamine incorporating small unilamellar liposomes with mean diameters arou nd 80 nm (demonstrated to retain encapsulated material and to be nontoxic u nder experimental conditions) were used. Liposome uptake by both types of c ells increased when increasing amounts of vesicles were co-incubated. For b oth lipid compositions, the interaction with HUVEC was very fast (associati on reached a plateau within 5 min) and so was the release of internalized v esicles (90% within 10 min at 37 degreesC). The reduced association values at 4 degreesC and the punctuate fluorescence observed in the cell cytoplasm after interaction, were indicative of whole liposome internalization. This internalization was clathrin-independent, since it was not inhibited by so dium azide and deoxyglucose, Pre-treatment of HUVEC with filipin or NEM res ulted in modification of the interaction, something that could be due to al terations in the biochemical characteristics of HUVEC membranes that inhibi t vesicular processes. In HL-60 cells, a slower association and raster rele ase of PC/Chol liposomes was demonstrated, while association of both liposo mes with these cells was energy-and temperature-independent. Nevertheless, morphological studies revealed differences in the interactions: A bright fl uorescent rim observed after interaction with PC/Chol liposomes, suggests t hat these liposomes were adsorbed on the surface of HL60 cells, while the u niform cytoplasmic fluorescence observed after incubation with PS/Chol lipo somes was indicative of fusion as the interaction mechanism.