Receptor-Mediated Endocytosis in Liver Endothelial Cells. Evidence of Lysosomal Heterogeneity

Abstract

Hepatic lysosomal heterogeneity is partly due to differences between lysosomes in the different types of liver cells. It has been shown, for instance, that the composition and contents of various lysosomal enzyme activities are distinctly different in the parenchymal cells (PC) and the nonparenchymal liver cells (Berg and Boman 1973). The biochemical and morphological differences are also reflected in the finding that lysosomes of PC, Kupffer cells (KC), and endothelial cells (EC) show different density distributions after isopycnic centrifugation in sucrose and Nycodenz gradients (Kindberg et al 1990). The density distribution of lysosomes of individual cells can be followed after injecting intravenously 1251-tyramine-cellobiose-labelled ligands that are selectively taken up by receptor-mediated endocytosis in only one cell type (Berg et al 1985). Following degradation the labelled degradation products are trapped in the lysosomes of the cells in which uptake took place and may therefore serve as markers for these organelles. Lysosomal heterogeneity may also be seen in the individual cells. By means of subcellular fractionation techniques combined with the “trap-label” method it has been possible to observe subpopulations of lysosomes in PC and EC that may be involved in degradation of substrates brought into the lysosomes by heterophagy and autophagy.