Glucagon is involved in glucose homeostasis via triggering gluconeogenesis and glycogenolysis. In addition to activation of protein kinase A (PKA), p38α MAPK (p38) is activated by glucagon and promotes glucagon-induced hepatic glucose production. However, underlying mechanisms by which p38 mediates the effect of glucagon are not well elucidated. Previously, we established that glucagon increases the phosphorylation of Foxo1 at Ser273 via PKA to promote hepatic glucose production. In this study, we investigated the potential role of p38 in the action of glucagon by phosphorylation of Foxo1 at Ser273. Using the mouse primary hepatocytes, we showed that activation of p38 by glucagon promoted phosphorylation of Foxo1 at Ser273, which results in both increased stability and nuclear translocation of Foxo1. In vivo kinase assay shows that the active p38 enzyme increased the phosphorylation of Foxo1 at Ser273. Moreover, p38 promoted glucagon-induced PKA activity to increase the phosphorylation of Foxo1 at Ser273. Suppression of p38 by siRNA and SB203580 decreased hepatic glucose production by glucagon in control mouse primary hepatocytes; however, such an effect was diminished in the hepatocytes isolated from the Foxo1-S273D knock-in mice, where Foxo1-Ser273 was replaced by the aspartate mutation (D). In mice, we further showed that injection of p38 inhibitor SB203580 decreased the fasting blood glucose and hepatic glucose production in wild type control mice, while this effect was not observed in Foxo1-S273D mice. These results revealed the novel role of p38 in control of Foxo1 phosphorylation at Ser273 to regulate the action of glucagon in glucose homeostasis.
W. Yang: None. W. Ai: None. X. Li: None. Q. Pan: None. Z. Shen: None. Y. Chen: None. Y. Sun: None. S. Guo: None.
American Diabetes Association (1-15-CD-09 to S.G.); National Institutes of Health (R01DK095118)