Stepwise autophosphorylation regulates biased agonism of the insulin receptor

Abstract

Insulin is a key regulator of energy metabolism in peripheral tissues but also functions as a growth factor. Insulin binding to the insulin receptor (IR) leads to autophosphorylation of intracellular tyrosine residues, which simultaneously initiates a multitude of signals and functions. In contrast, some artificial (non-insulin) ligands for the IR result in biased agonism, selectively activating the PI3K/AKT pathway and metabolic effects without activating mitogen-activated protein kinase (MAPK) pathway and mitogenic effects. However, the precise mechanism of biased agonism at the receptor level remains unclear. The biased agonist IR-A48 aptamer selectively induces mono-phosphorylation of Tyr1150 residue (m-pY1150) in the kinase domain of IR. Hence, we hypothesized that IR autophosphorylation is a stepwise process in which formation of m-pY1150 represents an intermediate step. To explore this idea, we used hybrid receptors in which insulin can bind only one of the two binding sites of the dimeric receptor. Asymmetric insulin binding selectively induced symmetric m-pY1150 in both kinase domains. Moreover, the juxtamembrane domain, which interacts with the kinase domain, restricted the full activation of IR, and symmetric m-pY1150 played a crucial role in the rearrangement of intracellular domains to release this restriction. Our findings demonstrate that the symmetry of insulin binding to a dimeric receptor determines the stepwise autophosphorylation of IR. Furthermore, considering that the degree of ligand symmetry on a receptor depends mainly on ligand concentration, our results suggest that IR may play metabolic-biased roles in peripheral tissues dependent on local insulin concentrations.