ENDOCYTOSED LDL AND BETA-VLDL FOLLOW DIFFERENT INTRACELLULAR PATHWAYSIN RAT-LIVER

The intracellular transport of [I-125]tyramin cellobiose low-density l ipoprotein ([(ITC)-I-125]LDL) and [(ITC)-I-131]beta-very-low-density l ipoprotein ([(ITC)-I-131]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 [(ITC)-I-131]beta-VLDL appeared at 1.08 g/ml partly coincid ing with the distribution of the cation independent (alpha)mannose-6-p hosphate receptor, whereas [(ITC)-I-125]LDL was found at 1.13 mg/ml, w here the degradation of [(ITC)-I-125]LDL started. [(ITC)-I-131]beta-VL DL, on the other hand, was transferred to denser vesicles, banding at 1.16 g/ml, and degradation started in these organelles, similar to tha t observed with asialoorosomucoid (ASOR) that was used as a control li gand. Since degradation products coincided with beta-N-acetylglucosami nidase we assume that these organelles are secondary lysosomes. [(ITC) -I-125]LDL was subsequently also transferred to these dense secondary lysosomes, and the distribution of degraded [(ITC)-I-125]LDL was there fore bimodal until [(ITC)-I-125]LDL was completely cleared from the ci rculation. Furthermore our results show that the different intracellul ar pathways observed are not due to uptake in different liver cell typ es, since the bimodal distribution of [(ITC)-I-125]LDL was also eviden t in purified liver parenchymal cells. The data suggest that LDL and b eta-VLDL follow different endosomal pathways in the rat hepatocytes an d that both pathways meet in a common final lysosome. The data also su pport the notion that LDL and beta-VLDL are taken up through different endocytic receptors. However, following estradiol treatment, both lig ands seem to follow a common pathway. In this case the density distrib utions of the two ligands coincide and resemble the pathway of LDL obs erved in control animals. This may be due to a pronounced up-regulatio n of LDL receptors following estradiol treatment, and beta-VLDL may un der these conditions be taken up via the LDL receptor.