Mechanism and role of insulin receptor endocytosis

Like many other cell surface receptors for nutrients and polypeptide hormones, the insulin receptor undergoes a complex endocytotic itinerary. Upon insulin binding, the receptor is activated as a tyrosine-specific protein kinase and autophosphorylates. This autophosphorylation is necessary for the receptor to internalize. After endocytosis, the ligand (insulin) and its receptor are dissociated. Most of the insulin is degraded, whereas the receptors are largely recycled to the cell surface. The signals in the receptor that control and specify its endocytotic pathway are beginning to be understood. Through the techniques of in vitro mutagenesis, noninternalizing receptors have been engineered and their structural and functional properties have been analyzed. For example, the immediate submembranous domain of the insulin receptor has been found to contain sequences (Gly-Pro-Leu-Tyr and, to a lesser extent, Asn-Pro-Gln-Tyr) that are necessary for normal endocytosis. Receptors deleted or mutated in these sequences retain tyrosine kinase activity but fail to undergo endocytosis. Unlike the better understood low density lipoprotein and transferrin receptors, however, these sequences are not sufficient for endocytosis. An insulin receptor with only these sequences exposed in the cytoplasm does not internalize. Tyrosine kinase activity is thought to be needed to lead to autophosphorylation and a conformational change that exposes the otherwise buried endocytosis sequences in the normally dimerized insulin receptor. Non-internalizing mutants of the insulin receptor have been used to examine the role of endocytosis in insulin action. It was found that an endocytosis-defective receptor could induce a short-term metabolic action of insulin (glycogen synthetase stimulation) as well as longer-term mitogenic effects of insulin. Furthermore, insulin action deactivated after the hormone was removed from the noninternalizing receptors. Apparently, endocytosis is not necessary for insulin action, but probably is important for removing the insulin from the cell so the target cell for insulin responds in a time-limited fashion to the hormone.