THE JOURNEY OF THE INSULIN-RECEPTOR INTO THE CELL - FROM CELLULAR BIOLOGY TO PATHOPHYSIOLOGY

The data that we have reviewed indicate that insulin binds to a specific cell-surface receptor. The complex then becomes involved in a series of steps which lead the insulin-receptor complex to be internalized and rapidly delivered to endosomes. From this sorting station, the hormone is targeted to lysosomes to be degraded while the receptor is recycled back to the cell surface. This sequence of events presents two degrees of ligand specificity: (a) The first step is ligand-dependent and requires insulin-induced receptor phosphorylation of specific tyrosine residues. It consists in the surface redistribution of the receptor from microvilli where it preferentially localizes in its unoccupied form. (b) The second step is more general and consists in the association with clathrincoated pits which represents the internalization gate common to many receptors. This sequence of events participates in the regulation of the biological action of the hormone and can thus be implicated in the pathophysiology of diabetes mellitus and various extreme insulin resistance syndromes, including type A extreme insulin resistance, leprechaunism, and Rabson-Mendehall syndrome. Alterations of the internalization process can result either from intrinsic abnormalities of the receptor or from more general alteration of the plasma membrane or of the cell metabolism. Type I diabetes is an example of the latter possibility, since general impairment of endocytosis could contribute to extracellular matrix accumulation and to an increase in blood cholesterol. Thus, better characterization of the molecular and cellular biology of the insulin receptor and of its journey inside the cell definitely leads to better understanding of disease states, including diabetes. © 1993 Springer-Verlag.