IRS1IRS2

IRS1 and IRS2, often discussed together as IRS1/IRS2, are cytoplasmic adaptor proteins that function downstream of insulin and insulin-like growth factor 1 (IGF-1) receptors. When insulin binds its receptor, the receptor's tyrosine kinase activity phosphorylates IRS1 and IRS2 on specific tyrosine residues, creating docking sites for SH2-domain–containing signaling proteins and initiating multiple signaling cascades.

Structurally, IRS proteins feature an N-terminal pleckstrin homology (PH) domain and a phosphotyrosine binding (PTB) domain that help recruit them to activated receptors, followed by long regions rich in tyrosine and serine phosphorylation sites. Tyrosine phosphorylation of IRS1/IRS2 recruits the p85 regulatory subunit of PI3K and other effectors, enabling activation of the PI3K-Akt pathway as well as the mitogen-activated protein kinase (MAPK) cascade. These pathways coordinate metabolic actions such as glucose uptake and growth-related signals.

Physiologically, IRS1 is broadly expressed and primarily mediates metabolic actions of insulin, including glucose uptake in muscle and adipose tissue. IRS2 is more prominent in liver and pancreatic beta cells and contributes to hepatic metabolism and beta-cell survival and function. Mouse models show that IRS1 deficiency causes insulin resistance with modest hyperglycemia, while IRS2 deficiency leads to beta-cell failure and more severe diabetes-like phenotypes; together, they account for much of insulin signaling’s complexity.

Regulation of IRS1/IRS2 involves serine phosphorylation and feedback inhibition by mTOR/S6K and inflammatory kinases such as JNK, which can dampen signaling. Dysregulation of IRS signaling is linked to insulin resistance, obesity, and type 2 diabetes. Variants in IRS1 have associations with obesity and insulin resistance, while IRS2 variants relate to beta-cell dysfunction and diabetes susceptibility. In research, IRS1/IRS2 signaling is central to understanding insulin action, metabolic control, and related diseases, with interactions extending to PI3K, Grb2, SHP2, and downstream metabolic and growth pathways.