Uptake and degradation of insulin and α2-macroglobulin-trypsin complex in rat adipocytes. Evidence for different pathways

Abstract

The cell association and degradation of insulin and α 2-macroglobulin-trypsin complex were measured in rat adipocytes with or without various inhibitors in the attempt to clarify whether the two ligands were taken up by the same or by different pathways. Several inhibitors, and particularly those of membrane traffic, lysosomal function and transglutaminase activity, affected the two ligands differently. Thus, chloroquine (100 μM) reduced both the uptake of α 2-macroglobulin · trypsin and its receptor-mediated degradation by about 70%. In contrast, the uptake of insulin was increased 2–3-times and the receptor-mediated degradation was only slightly reduced. Methylamine (10 mM) and ammonium chloride (10 mM) reduced degradation of α 2-macroglobulin · trypsin markedly without affecting that of insulin. Leupeptin (100 μM) increased uptake and reduced degradation of α 2-macroglobulin · trypsin without affecting insulin. Dansylcadaverine (500 μM) almost abolished uptake and degradation of α 2-macroglobulin · trypsin but had little effect on insulin. Moreover, uptake and degradation of α 2-macroglobulin · trypsin was much more sensitive than insulin to the action of metabolic inhibitors such as dinitrophenol and cyanide. The results show that the two ligands are taken up by functionally different systems. In addition, they support the hypothesis that lysosomes play a relatively minor role in the receptor-mediated degradation of insulin.