Uptake and degradation of insulin and alpha 2-macroglobulin-trypsin complex in rat adipocytes. Evidence for different pathways.

The cell association and degradation of insulin and alpha 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 microM) reduced both the uptake of alpha 2-macroglobulin X 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 alpha 2-macroglobulin X trypsin markedly without affecting that of insulin. Leupeptin (100 microM) increased uptake and reduced degradation of alpha 2-macroglobulin X trypsin without affecting insulin. Dansylcadaverine (500 microM) almost abolished uptake and degradation of alpha 2-macroglobulin X trypsin but had little effect on insulin. Moreover, uptake and degradation of alpha 2-macroglobulin X 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.