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.


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.

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