Abstract
Liposome uptake by HepG2 human hepatoma cells was investigated in comparison with the uptake by J774 murine macrophage-like cells. HepG2 cells accumulated liposomes (egg yolk phosphatidylcholine (EPC)/Chol; 75/25, diameter 0.2 μm) at 37°C comparably to J774 macrophage-like cells. Confocal microscopic observations revealed that J774 cells internalized EPC/Chol liposomes efficiently but HepG2 cells kept most or the liposomes bound on their plasma membrane surfaces. Poly(ethylene glycol) (PEG)-coated liposomes (0.2 μm) containing poly(ethylene glycol) cholesteryl ether (PEG-Chol) avoided cellular uptake at 37°C by either cell line. In both cell lines, binding of PEG-coated liposomes was lower than that of EPC/Chol liposomes when incubation was carried out at 4°C. To analyze the binding process at 37°C, surface-bound liposomes were removed from the cells by pronase treatment. A reduction of the amount of bound-liposomes on cell surfaces was observed in the case of PEG-coated liposomes. Therefore, PEG/coating reduces direct binding of liposomes to the cell surfaces. The presence of apolipoprotein E (apoE) increased the uptake of EPC/Chol liposomes via its receptor in both cell lines. In contrast, cellular uptake of PEG-coated liposomes was not enhanced by treatment with apoE. Therefore, while apoE-mediated liposome uptake occurs in the case of EPC/Chol liposomes, it does not occur for PEAG-coated liposomes; PEG-coating also inhibits protein-mediated binding to the cells. These results further imply that elusion from liver clearance 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 go through the fenestrates of the endothelial lining.
