Poly(ethylene glycol) derivative of cholesterol reduces binding step of liposome uptake by murine macrophage-like cell line J774 and human hepatoma cell line HepG2

Liposome uptake by HepG2 human hepatoma cells was investigated in compariso n with the uptake by J774 murine macrophage-like cells. HepG2 cells accumul ated liposomes (egg yolk phosphatidylcholine (EPC)/Chol; 75/25, diameter 0. 2 mu m) at 37 degrees C comparably to J774 macrophage-like cells. Confocal microscopic observations revealed that J774 cells internalized EPC/Chol lip osomes efficiently but HepG2 cells kept most of the liposomes bound on thei r plasma membrane surfaces. Poly(ethylene glycol) (PEG)-coated liposomes (0 .2 mu m) containing poly(ethylene glycol) cholesteryl ether (PEG-Chol) avoi ded cellular uptake at 37 OC by either cell line. In both cell lines, bindi ng of PEG-coated liposomes was lower than that of EPC/Chol liposomes when i ncubation was carried out at 4 degrees C. To analyze the binding process at 37 degrees C, surface-bound liposomes were removed from the cells by prona se treatment. A reduction of the amount of bound-liposomes on cell surfaces was observed in the case of PEG-coated liposomes. Therefore, PEG-coating r educes direct binding of liposomes to the cell surfaces. The presence of ap olipoprotein E (apoE) increased the uptake of EPC/Chol liposomes via its re ceptor in both cell lines. In contrast, cellular uptake of PEG-coated lipos omes was not enhanced by treatment with apoE. Therefore, while apoE-mediate d liposome uptake occurs in the case of EPC/Chol liposomes, it does not occ ur for PEG-coated liposomes; PEG-coating also inhibits protein-mediated bin ding to the cells. These results further imply that elusion from liver clea rance of PEG-coated liposomes is not only due to the reduction of uptake by Kupffer cells but also by hepatocytes when liposomes are small enough to g o through the fenestrates of the endothelial lining.