Foxo1-Ser253 Phosphorylation Regulates Glucose Homeostasis in Mice

The forkhead transcription factor Foxo1 is a key mediator in insulin signaling pathway that controls hepatic glucose production (HGP) and pancreatic beta-cell function. Upon activation of the protein kinase Akt, insulin promotes glycogen synthesis and inhibits gluconeogenesis in the liver, reducing blood glucose. We previously demonstrated human Foxo1-Ser256, an equivalent to mouse Foxo1-Ser253, is a key phosphorylation site for insulin and Akt suppression, promoting Foxo1 nuclear export and suppressing the expression of the target gene for liver gluconeogenesis. Here we investigated the role of Foxo1-Ser253 phosphorylation in control of glucose hemostasis in vivo, by generating Foxo1-S253^A/A knock-in (KI) mice, in which Foxo1-Ser253 replaced by alanine (A mutation) that blocks phosphorylation. Foxo1-S253^A/A mice displayed mild increases in feeding blood glucose and insulin level, but reductions in fasting blood glucose and glucagon concentration, as well as a decrease in the ratio of the number of pancreatic α-cells/β-cells per islet. Foxo1-S253^A/A mice exhibited slight increases in insulin sensitivity but barely changed glucose uptake among tissues, and an enhanced energy expenditure. Further analyses indicate that Foxo1-S253^A/A enhanced Foxo1 ability and promoted the effect of glucagon on HGP. This is the first report demonstrating that Foxo1-Ser253 phosphorylation status itself is sufficient to affect HGP and glucose homeostasis, as well as regulate the synthesis of insulin and glucagon in in vivo.