Targeting of Liposomes to Cells Bearing Nerve Growth Factor Receptors Mediated by Biotinylated Nerve Growth Factor

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Abstract
We have used biologically active derivatives of β‐nerve growth factor (NGF), modified by biotinylation via carboxyl groups, to target the specific binding of liposomes to cultured rat and human tumor cells bearing NGF receptors. Liposomes, to be used for targeting, were prepared by conjugating streptavidin to phospholipid amino groups on liposomes prepared by reverse‐phase evaporation. Approximately 2,000 streptavidin molecules were incorporated per liposome. Addition of biotinylated NGF, but not of unmodified NGF, could mediate the subsequent binding of radiola‐beled streptavidin‐liposomes to rat pheochromocytoma PC 12 cells in suspension at 4°C. In contrast, incubation with biotinylated NGF did not mediate the binding of hemoglobin‐conjugated liposomes. Under optimal incubation conditions, approximately 570 streptavidin‐liposomes were specifically bound per cell. Biotinylated NGF was also used to obtain specific binding of streptavidin‐liposomes containing encapsulated fluorescein isothiocyanate‐labeled dextran to PC12 cells or human melanoma HS294 cells. When HS294 cells were incubated at 37°C following targeted liposome binding at 4°C, the cell‐associated fluorescence appeared to become internalized, displaying a perinu‐clear pattern of fluorescence similar to that observed when lysosomes were stained with acridine orange. Trypsin treatment abolished cell‐associated fluorescence when cells were held at 4°C but did not alter the fluorescence pattern in cells following incubation at 37°C. When liposomes containing carboxyfluorescein, a dye capable of diffusing out of acidic compartments, were targeted to HS294 cells, subsequent incubation at 37°C resulted in diffuse cytoplasmic fluorescence, suggesting that internalized liposomes encounter ly‐sosomal or prelysosomal organelles.