A Novel Mechanism by Foxo1 Phosphorylation Mediates Glucagon Signaling in Control of Glucose Homeostasis

Abstract

Glucagon increases hepatic glucose production (HGP) to maintain blood glucose homeostasis. The forkhead transcription factor Foxo1 promotes HGP through increasing expression of genes encoding the rate-limiting metabolic enzymes for gluconeogenesis. We previously established that insulin suppresses Foxo1 by Akt-mediated phosphorylation at Ser256 in human hepatocytes. Here we demonstrated glucagon promotes Foxo1 nuclear translocation and stability via protein kinase A (PKA)-dependent phosphorylation of Foxo1 at Ser276, serving as a novel mechanism of glucagon action in control of glucose homeostasis. In vitro protein kinase assay showed a direct phosphorylation of Foxo1 at Ser276 by PKA. Replacing Foxo1-Ser276 with alanine (S276A) or aspartate (S276D) reduced or increased Foxo1 stability in human hepatocytes, respectively. To establish the in vivo function of Foxo1-Ser276 phosphorylation in glucose metabolism, we generated Foxo1-S273A and Foxo1-S273D knock-in mice, revealing the novel mechanism by which glucagon, via PKA-dependent phosphorylation of Foxo1 at Ser276, promotes Foxo1 nuclear localization, stability, and HGP. Thus, Foxo1-Ser276 is a potential target site for the control of Foxo1 bioactivity and associated metabolic diseases. Disclosure Y. Wu: None. H. Yan: None. Q. Pan: None. H. Zheng: None. S.A. Dahanayaka: None. Y. Sun: None. A. Zhou: None. L. Zhang: None. S. Guo: None.