Endocytosis of insulin receptors is not required for activation or deactivation of the hormone response

To examine the role of endocytosis in insulin action, hormone responsiveness was studied in transfected Rat 1 fibroblasts stably expressing a noninternalizing insulin receptor. The latter receptor (hIR delta ex16) was engineered by deleting the immediately submembranous 22 amino acids encoded by the 16th exon of the human insulin receptor and has previously been shown not to internalize despite having normal insulin-stimulated tyrosine kinase activity. It is shown in the present study that hIR delta ex16 receptors do mediate insulin action. Insulin dose-response curves both for activation of glycogen synthetase and for mitogenic stimulation demonstrate greater insulin sensitivity in hIR delta ex16 cells compared with untransfected Rat 1 cells. In addition, increases in the absolute levels of glycogen synthetase activity are seen in the hIR delta ex16 cells. Species-specific agonistic antibodies to the insulin receptor also stimulate hIR delta ex16 cells, confirming the activity of the mutant receptor. The non-internalizing receptors are rapidly dephosphorylated after removal of insulin, and the activation of glycogen synthetase decays no more slowly in hIR delta ex16 cells than in cells expressing wild-type receptors. The results demonstrate that receptor endocytosis is not necessary for activation or deactivation of the insulin response.