Differential effects of leupeptin, monensin and colchicine on ligand degradation mediated by the two asialoglycoprotein receptor pathways in isolated rat hepatocytes

We have shown that degradation of asialo-orosomucoid (ASOR) in isolated rat hepatocytes occurs by two different intracellular pathways [Clarke, Oka and Weigel (1987) J. Biol. Chem. 262, 17384-17392] mediated by two subpopulations of cell surface galactosyl (Gal) receptors, designated State 1 or State 2 receptors. In the present study, several inhibitors were tested for their effects on ligand degradation by the State 1 or State 2 pathway. Leupeptin, monensin and chloroquine completely inhibited degradation of ¹²⁵I-labelled ASOR in both pathways. Dose-response studies showed, however, that the State 2 pathway was more sensitive to leupeptin or monensin than the State 1 pathway. No differences were observed with chloroquine. For example, the onset of the inhibition in the State 2 and State 1 pathways occurred at about 0.05 and 0.3 μM-leupeptin respectively, a 6-fold difference. At 3.5 μM-monensin, ¹²⁵I-ASOR degradation in the State 2 pathway was completely blocked, whereas degradation in the State 1 pathway was essentially unaffected. Colchicine was observed to give the largest differential sensitivity between the two pathways. The State 2 degradation pathway was about 30-fold more sensitive to colchicine than the State 1 pathway. Lumicolchicine had no affect. The onset of inhibition of the rate of ¹²⁵I-ASOR degradation in the State 2 and State 1 pathways occurred at ~ 0.1 and 3.0 μM-colchicine respectively. At very high concentrations (> 0.1 mM), the State 1 pathway could be completely inhibited. We conclude that intracellular ¹²⁵I-ASOR processing or delivery to degradative compartments in both the State 1 and State 2 Gal receptor pathways requires low pH. Ligand delivery to the degradative compartment does not require microtubules in the State 1 pathway, consistent with the very rapid onset of degradation in this pathway. The State 2 degradation pathway does require microtubules.