The interaction of low density lipoprotein, acetylated low density lipoprotein and apolipoprotein E—free high density lipoprotein with parenchymal, endothelial and Kupffer cells of human liver was visualized. For this purpose, the fluorescent phospholipid analog 1,1′-dioctadecyl-3,3,3′,3′-tetramethyl indocarbocyanine perchlorate was used to label the lipoproteins. The involvement of both parenchymal and nonparenchymal cells in the uptake of 1,1′-dioctadecyl-3,3,3′,3′-tetramethyl indocarbocyanine perchlorate—labeled low density lipoprotein and acetylated low density lipoprotein was studied using in vitro perfusion of human liver tissue blocks. In addition, primary hepatocyte cultures were used to visualize the interaction with 1,1′-dioctadecyl-3,3,3′,3′-tetramethyl indocarbocyanine perchlorate-labeled apolipoprotein E—free high density lipoprotein and (modified) low density lipoprotein.
1,1′-dioctadecyl-3,3,3′,3′-tetramethyl indocarbocyanine perchlorate-low density lipoprotein showed a time-dependent and concentration-dependent interaction with both hepatocytes and Kupffer cells, although the intensity of the interaction with parenchymal cells varied strongly among the liver donors. Uptake of 1,1′-dioctadecyl-3,3,3′,3′-tetramethyl indocarbocyanine perchlorate-low density lipoprotein by both cell types was strongly inhibited by the presence of excess unlabeled low density lipoprotein in the (perfusion) medium. Methylation and hydroxyac-etaldehyde treatment of low density lipoprotein prevented the uptake of low density lipoprotein. This indicated that the uptake of low density lipoprotein by Kupffer and parenchymal cells was mediated by the low density lipoprotein receptor.
1,1′-dioctadecyl-3,3,3′,3′-tetramethyl indocarbocyanine perchlorate-acetylated low density lipoprotein was mainly taken up in situ by liver endothelial cells and by a minor population of Kupffer cells. Polyinosinic acid, a known inhibitor of the scavenger receptor, prevented the uptake by liver endothelial cells. Therefore human liver endothelial cells express active scavenger receptors on their surface.
Apolipoprotein E—free 1,1′-dioctadecyl-3,3,3′,3′-tetramethyl indocarbocyanine perchlorate-high density lipoprotein was found to be associated with the membrane of cultured liver parenchymal cells but was not taken up intracellularly, indicating a cholesterol exchange process occurring extracellularly at the plasma membrane. The cellular localization of lipoprotein receptors and uptake of the various classes of lipoproteins are comparable with the situation in rats. (HEPATOLOGY 1991;13:79–90.)