Molecular and cellular mechanisms governing the ligand-specific and non-specific steps of insulin receptor internalization.

The surface events leading to insulin-induced internalization of its specific receptor can be subdivided in three major steps: the first step consists in the surface redistribution of the receptor from the villous to the non-villous region of the cell surface, it is ligand-specific, depends on kinase activation and phosphorylation of tyrosines 1146, 1150 and 1151, and consists in the relief of a constraint immobilizing the receptor on microvilli; the second step is characterized by the shift of the insulin-receptor complex in the plane of the membrane allowing it to get access to the nonvillous domain of the cell surface where internalization gates (clathrin-coated pits) are located; this stage is controlled, at least in part, by the transmembrane domain of the molecule and its flanking amino acids; the third step corresponds to the segregation of the insulin-receptor complex in clathrin-coated pits, this step is relatively non-specific and is governed by well defined signal sequences present in the juxtamembrane domain of the cytoplasmic segment of the b-subunit. These surface events are then automatically followed by the entry of the insulin receptor inside the cells through the formation of clathrin-coated vesicles, in its subsequent association with endosomes which acidic pH allows insulin dissociation from its receptor and the sorting of the receptor and the hormone in different directions: insulin is targetted to lysosomes to be degraded while the receptor is recycled back to the cell surface to be reused. This complex process does not seem to be involved in the transmission of the biological signal of the hormone. Nevertheless, it is initiated and controlled by insulin and results in the intracellular degradation of insulin and in the modulation of the number of surface insulin receptors. Thus, even if it does not directly participate in insulin signaling, insulin receptor internalization plays a crucial role in the control of insulin action.