Receptor-mediated degradation and internalization of insulin in the adenocarcinoma cell line HT-29 from human colon

Abstract

In the adenocarcinoma cell line HT-29 receptor-bound insulin is substrate for a proteolytic process leading to the release of about half of the cell-associated [ 125I]monoiodoinsulin in the form of [ 125I]iodide and [ 125I]monoiodotyrosine. Classical lysosomal inhibitors (NH + 4 methylamine, leupeptin) did not inhibit this proteolysis. Inhibitors of membrane traffic (chloroquine and monensin) and of metabolism (CN −) inhibited the fractional receptor-mediated degradation. The former led to an increased cell-associated 125I activity whereas the latter reduced the uptake. Sulphydryl reagents inhibited the receptor-mediated degradation. The data are not compatible with a quantitatively major role of lysosomes in the receptormediated insulin degradation. However, since the process requires energy it is suggested that the receptormediated degradation takes place in vesicles other than secondary lysosomes. The responsible enzyme(s) may belong to the thiol group of proteases. Both insulin and the insulin receptor are internalized as a consequence of incubation of HT-29 cells with insulin.