The insulin receptor substrate proteins IRS1 and IRS2 are key targets of the insulin receptor tyrosine kinase and are required for hormonal control of metabolism. Tissues from insulinresistant and diabetic humans exhibit defects in IRS-dependent signalling, implicating their dysregulation in the initiation and progression of metabolic disease. However, IRS1 and IRS2 are regulated through a complex mechanism involving phosphorylation of >50 serine/threonine residues (S/T) within their long, unstructured tail regions. In cultured cells, insulin-stimulated kinases (including atypical PKC, AKT, SIK2, mTOR, S6K1, ERK1/2 and ROCK1) mediate feedback (autologous) S/T phosphorylation of IRS, with both positive and negative effects on insulin sensitivity. Additionally, insulin-independent (heterologous) kinases can phosphorylate IRS1/2 under basal conditions (AMPK, GSK3) or in response to sympathetic activation and lipid/ inflammatory mediators, which are present at elevated levels in metabolic disease (GRK2, novel and conventional PKCs, JNK, IKKβ, mPLK). An emerging view is that the positive/negative regulation of IRS by autologous pathways is subverted/co-opted in disease by increased basal and other temporally inappropriate S/T phosphorylation. Compensatory hyperinsulinaemia may contribute strongly to this dysregulation. Here, we examine the links between altered patterns of IRS S/T phosphorylation and the emergence of insulin resistance and diabetes.

Summary The Forkhead transcription factor Foxo1 regulates expression of genes involved in stress resistance and metabolism. To assess the contribution of Foxo1 to metabolic dysregulation during hepatic insulin resistance, we disrupted Foxo1 expression in the liver of mice lacking hepatic Irs1 and Irs2 (DKO-mice). DKO-mice were small and developed diabetes; analysis of the DKO-liver transcriptome identified perturbed expression of growth and metabolic genes, including increased Ppargc1a and Igfbp1, and decreased glucokinase, Srebp1c, Ghr and Igf1. Liver-specific deletion of Foxo1 in DKO-mice resulted in significant normalization of the DKO-liver transcriptome and partial restoration of the response to fasting and feeding, near normal blood glucose and insulin concentrations, and normalization of body size. These results demonstrate that constitutively active Foxo1 significantly contributes to hyperglycemia during severe hepatic insulin resistance, and that the Irs1/2→PI-3K→Akt→Foxo1 branch of insulin signaling is largely responsible for hepatic insulin-regulated glucose homeostasis and somatic growth.