Visualization of the uptake and processing of oxidized low-density lipoproteins in human and rat liver

Abstract

The interaction of oxidized human low-density lipoproteins with human and rat liver was analyzed by light and electron microscopy. At the light microscopic level oxidized low-density lipoprotein was visualized by the fluorescent dye 1,1′ dioctadecyl 3,3,3′,3′ tetramethyl indocarbocyanine perchlorate, whereas at the electron microscopic level, an indirect immunolabeling procedure was used that detected the apoprotein B of the oxidized low-density lipoprotein. In rats, oxidized low-density lipoprotein was administered intravenously, and uptake by human liver was studied by perfusion of tissue blocks. Both in human and in rat liver, fluorescently labeled oxidized low-density lipoprotein was mainly found to become concentrated in Kupffer cells and, to a lesser extent, in endothelial cells. In both species the cell association of fluorescently labeled oxidized low-density lipoprotein could be inhibited by preadministration of polyinosinic acid, indicating a scavenger receptor-mediated process. At the electron microscopic level, oxidized low-density lipoprotein was found to bind mainly to areas of the plasma membrane of the Kupffer cells without clathrin coating, although binding to coated regions was also noticed. Internalization of the ligand occurred through coated vesicle formation and through membrane folding of interacting lamellipodia and wormlike structures. No indication for phagocytosis of aggregated oxidized low-density lipoprotein particles was noticed. After internalization, the immunoreactive oxidized low-density lipoprotein was detected in relatively electron-lucent endosomes and, subsequently, in lysosomes. Endothelial cells internalized oxidized lowdensity lipoprotein solely through coated pits, after which the particles were transferred through endosomes into lysosomes. The endosomes often contained tubular extensions, which were devoid of immunolabel. In human Kupffer and endothelial liver cells, essentially the same organelles were demonstrated to be involved in the internalization and processing of oxidized low-density lipoprotein as in the rat. Our morphological results confirm earlier biochemical data on the relative involvement of the various liver cell types in the uptake of oxidized low-density lipoprotein in rats, and the relevance of these data for the human situation is indicated. The uptake process, coupled to oxidized low-density lipoprotein recognition by Kupffer cells, as presently analyzed, indicates that both rat and human Kupffer cells are equipped with a similar removal system to protect the body against the occurrence of the atherogenic oxidized low-density lipoprotein particles in the blood. (HEPATOLOGY 1993;18:537–545.)