320 Biochimica et Biophysica Acta, 732 (1983) 320-323 Elsevier BBA Report BBA 70100 LIPOSOMES CONTAINING COLLOIDAL GOLD ARE A USEFUL PROBE OF LIPOSOME-CELL INTERACTIONS KEELUNG HONG, DANIEL S. FRIEND a, CHARLES G. GLABE b,. and DEMETRIOS PAPAHADJOPOULOS c Cancer Research Institute and Departments of Pathology ", Anatomy b and Pharmacology ', The University of California, San Francisco, CA 94143 (U.S.A.) (Received April 6th, 1983) Key words: Liposome-cell interaction," Colloidal gold; Endocytosis; Au A method is described for the preparation of liposomes containing colloidal gold as an electron-dense marker to trace liposome-cell interactions. Since gold sols would precipitate at the high concentrations necessary for loading a large proportion of liposomes, gold sois were formed within preformed liposomes which had encapsulated gold chloride. The optimal conditions for encapsulating the marker were ascertained for liposomes prepared by the method of reverse-phase evaporation. Gold sois formed rapidly at ambient temperature and without organic solvent, and produced homogeneous populations of gold granules inside liposomes. Most vesicles contained the marker, allowing us to determine unambiguously the intracellular fate of liposomes and their contents. The in vitro experiments showed that gold-Uposomes were internalized by African green monkey kidney cells in a manner similar to receptor-mediated endocytosis of well-char- acterized ligands. Preliminary in vivo studies also indicated that liposomes were endocytosed by Kupffer cells via the coated vesicle pathway. Liposomes are useful carriers for intracellular de- livery of drugs and macromolecules [1] in vitro, and are the subject of considerable interest as carriers of chemotherapeutic agents in vivo [2,3]. Despite extensive analysis of liposome-cell interac- tion, the mechanisms for cellular incorporation of liposomes and their contents are still uncertain. Radioactive compounds, fluorescent dyes, RNA and DNA [4-9] have been encapsulated success- fully in large liposomes and some of the encapsu- lated material subsequently has been incorporated into cells. At the ultrastructural level, it has been * Present address: Worcester Foundation for Experimental Biology, Shrewsbury, MA 01545, U.S.A. Abbreviation: Hepes, N-2-hydroxyethylpiperazine-N-2- ethanesulfonic acid. difficult to trace unambiguously the path of lipo- some internalization for lack of a suitable marker. Ferritin and horseradish peroxidase have been used to follow liposome fate in vitro, [10-12]. However, the endogenous peroxidase activity of many tis- sues, as well as the natural occurrence of intracell- ular ferritin, complicates the use of such markers. Colloidal gold particles are an attractive histologi- cal marker, owing to their high electron density, uniform size and shape, and versatility [13-15]. However, colloidal gold is unsuitable for encapsu- lation in liposomes, as it precipitates at the high concentrations necessary to load a large fraction of a liposome population. In this report, we describe the preparation of liposomes containing gold gran- ules and demonstrate the utility of gold-containing liposomes as histochemical markers of liposome uptake in cells. 0005-2736/83/$03.00 © 1983 Elsevier Science Publishers B.V.