Novel Mechanism of Foxo1 Phosphorylation in Glucagon Signaling in Control of Glucose Homeostasis

Abstract

Dysregulation of hepatic glucose production (HGP) serves as a major underlying mechanism for the pathogenesis of type 2 diabetes. The pancreatic hormone glucagon increases and insulin suppresses HGP, controlling blood glucose homeostasis. The forkhead transcription factor Foxo1 promotes HGP through increasing expression of genes encoding the rate-limiting enzymes responsible for gluconeogenesis. We previously established that insulin suppresses Foxo1 by Akt-mediated phosphorylation of Foxo1 at Ser in human hepatocytes. In this study, we found a novel Foxo1 regulatory mechanism by glucagon, which promotes Foxo1 nuclear translocation and stability via cAMP- and protein kinase A-dependent phosphorylation of Foxo1 at Ser Replacing Foxo1-S276 with alanine (A) or aspartate (D) to block or mimic phosphorylation, respectively, markedly regulates Foxo1 stability and nuclear localization in human hepatocytes. To establish in vivo function of Foxo1-Ser phosphorylation in glucose metabolism, we generated Foxo1-S273A and Foxo1-S273D knock-in (KI) mice. The KI mice displayed impaired blood glucose homeostasis, as well as the basal and glucagon-mediated HGP in hepatocytes. Thus, Foxo1-Ser is a new target site identified in the control of Foxo1 bioactivity and associated metabolic diseases.

Selective inhibition of CBP/p300 HAT by A-485 results in suppression of lipogenesis and hepatic gluconeogenesis

“…FOXO1 is traditionally regarded as an important component of insulin signaling cascades in suppressing gluconeogenesis 44,45 . A recent study demonstrates a novel regulatory mechanism of FOXO1 by glucagon, which promotes FOXO1 stability and nuclear retention via cAMP/PKA-dependent phosphorylation of FOXO1 at Ser276 18 . In this study, A-485 blocked cAMP-increased abundance of FOXO1 protein in nucleus and enhanced proteasome-mediated FOXO1 ubiquitination in primary hepatocytes.…”

“…Nuclear and cytoplasmic protein was extracted from primary mouse hepatocytes by means of commercial protocol and reagents (Thermo scientific) as previously described 18 .…”

A Review of FoxO1-Regulated Metabolic Diseases and Related Drug Discoveries

“…In the liver, insulin activates the PI3K/PKB signaling pathway and results in FOXO1 protein phosphorylation and degradation [32]. FOXO1 TF binds and promotes transcription of glucose 6-phosphatase (G6PC) and phosphoenolpyruvate carboxykinase (PEPCK), which are key enzymes stimulating gluconeogenesis and glycogenolysis [33,34]. When FOXO1 is suppressed, transcription of G6PC and PEPCK subsequently decreases, which consequently inhibits the rates of glucose production in the liver.…”

Effects of isotretinoin on glucose metabolism in patients with acne: A systematic review and meta‐analysis

“…In addition to the influence of dyslipidemia on insulin sensitivity, isotretinoin also plays a key role in the regulation of glucose homeostasis and insulin resistance via FoxO1. FoxO1 regulates most critical metabolic steps of hepatic gluconeogenesis as well as pancreatic β‐cell function . In the current study, eight of the nine included studies showed increased insulin resistance after isotretinoin treatment, but the increase was only significant in one study.…”