Metabolism of native and of lactosylated human low density lipoprotein: evidence for two pathways for catabolism of exogenous proteins in rat hepatocytes.

Human low density lipoprotein (LDL) covalently conjugated with 200-250 residues of lactose per LDL particle (Lac-LDL) was bound and rapidly taken up by the galactose-specific receptor of rat hepatocytes. Uptake of Lac-LDL was associated with inhibition of 3-hydroxy-3-methylglutaryl-CoA reductase and stimulation of cholesterol esterification. Uptake of native human LDL had no significant effects on these enzyme activities even when the rates of LDL uptake equaled those of Lac-LDL. When injected into rats, Lac-LDL was selectively removed by the liver (98% of injected dose). The hepatic subcellular distribution of simultaneously injected native 125I-labeled LDL and 131I-labeled Lac-LDL differed significantly, Lac-LDL was associated with fractions enriched in lysosomal hydrolases whereas native LDL was found predominantly in the supernatant fraction enriched in lactate dehydrogenase. Chloroquine (0.1 mM) markedly suppressed uptake of Lac-LDL by cultured rat hepatocytes (> 80%) but had only a small effect on uptake of native LDL. Leupeptin (0.625 mM) inhibited degradation of Lac-LDL more than it did degradation of native LDL. Colchicine (0.25 microM) dramatically suppressed uptake of Lac-LDL (> 70%) but did not affect native LDL uptake even at concentrations as high as 10 microM. Uptake of human LDL by rat hepatocytes occurs largely by nonspecific mechanisms, including fluid endocytosis, whereas Lac-LDL, as shown here, is taken up by a specific receptor-mediated mechanism. The results show further that native human LDL, representing an example of a protein taken up nonspecifically, is processed intracellularly by a pathway qualitatively distinct from that for Lac-LDL, an example of a protein taken up by a specific mechanism. Lac-LDL may serve as a vehicle for specifically delivering drugs, hormones, or radioactive compounds to hepatocytes for therapeutic or diagnostic purposes.