Endocytosed LDL and beta-VLDL follow different intracellular pathways in rat liver

The intracellular transport of [125I]tyramine cellobiose low-density lipoprotein ([125ITC]LDL) and [131ITC]beta-very-low-density lipoprotein ([131ITC]beta-VLDL) in rat liver was studied by means of centrifugation in sucrose and Nycodenz gradients. At time-points up to 45 min after intravenous injection, the two ligands were found in endosomes with distinctly different buoyant densities. In the Nycodenz gradients [131ITC]beta-VLDL appeared at 1.08 g/ml partly coinciding with the distribution of the cation independent (alpha)mannose-6-phosphate receptor, whereas [125ITC]LDL was found at 1.13 mg/ml, where the degradation of [125ITC]LDL started. [131ITC]beta-VLDL, on the other hand, was transferred to denser vesicles, banding at 1.16 g/ml, and degradation started in these organelles, similar to that observed with asialoorosomucoid (ASOR) that was used as a control ligand. Since degradation products coincided with beta-N-acetylglucosaminidase we assume that these organelles are secondary lysosomes. [125ITC]LDL was subsequently also transferred to these dense secondary lysosomes, and the distribution of degraded [125ITC]LDL was therefore bimodal until [125ITC]LDL was completely cleared from the circulation. Furthermore our results show that the different intracellular pathways observed are not due to uptake in different liver cell types, since the bimodal distribution of [125ITC]LDL was also evident in purified liver parenchymal cells. The data suggest that LDL and beta-VLDL follow different endosomal pathways in the rat hepatocytes and that both pathways meet in a common final lysosome. The data also support the notion that LDL and beta-VLDL are taken up through different endocytic receptors. However, following estradiol treatment, both ligands seem to follow a common pathway. In this case the density distributions of the two ligands coincide and resemble the pathway of LDL observed in control animals. This may be due to a pronounced up-regulation of LDL receptors following estradiol treatment, and beta-VLDL may under these conditions be taken up via the LDL receptor.