Regulation of Akt signaling byO-GlcNAc in euglycemia

Abstract

The hexosamine biosynthesis pathway (HBP) regulates the posttranslational modification of nuclear and cytoplasmic protein by O-linked N-acetylglucosamine ( O-GlcNAc). Numerous studies have demonstrated that, in hyperglycemic conditions, excessive glucose flux through this pathway contributes to the development of insulin resistance. The role of the HBP in euglycemia, however, remains largely unknown. Here we investigated the effect of O-GlcNAc on hepatic Akt signaling at physiological concentrations of glucose. In HepG2 cells cultured in 5 mM glucose, removal of O-GlcNAc by adenoviral-mediated overexpression of O-GlcNAcase increased Akt activity and phosphorylation. We also observed that Akt was recognized by succinylated wheat germ agglutinin (sWGA), which specifically binds O-GlcNAc. Overexpression of O-GlcNAcase in HepG2 cells reduced the levels of Akt in sWGA precipitates. The increased Akt activity was accompanied by increased phosphorylation of Akt substrates and reduced mRNA for glucose-6-phosphatase and phospho enolpyruvate carboxykinase (PEPCK). The increased Akt activity was not a result of activation of its upstream activator phosphoinositide 3-kinase (PI 3-kinase). Further demonstrating Akt regulation by O-GlcNAc, we found that overexpression of O-GlcNAcase in the livers of euglycemic mice also significantly increased Akt activity, resulting in increased phosphorylation of downstream targets and decreased mRNA for glucose-6-phosphatase. Together, these data suggest that O-GlcNAc regulates Akt signaling in hepatic models under euglycemic conditions.

Conditional Knock-out Reveals a Requirement for O-Linked N-Acetylglucosaminase (O-GlcNAcase) in Metabolic Homeostasis

“…7108 JOURNAL OF BIOLOGICAL CHEMISTRY VOLUME 290 • NUMBER 11 • MARCH 13, 2015 phosphatidylinositol 3-kinase inhibition (62), altered insulin secretion (63), and the reduction of AKT activity (64). The results of this study suggest that GSK3␤ may play a key role in lipid and glucose metabolism.…”

Lipopolysaccharide (LPS)-stimulated iNOS Induction Is Increased by Glucosamine under Normal Glucose Conditions but Is Inhibited by Glucosamine under High Glucose Conditions in Macrophage Cells

“…Inflammation is basically triggered by nutrient surpluses, and inflammatory responses and metabolic disorders share common signaling molecules and pathways. A large body of emerging evidence suggests that nutrient signaling through the HBP serves an important role in cellular adaptation to fluctuations in nutrient availability even at physiological glucose concentrations (36). Our previous study (37) discussed the distinct role of the GlcN, in which metabolic processes are allowed to adapt to nutritional states of normal and fuel excess: GlcN impaired the insulin response when fed to normal diet-fed mice yet it alleviated insulin resistance in high fat diet-fed mice.…”

Dynamic O-GlcNAcylation and its roles in the cellular stress response and homeostasis

“…It is also important to note that Akt itself is dynamically modified by O-GlcNAc at Ser473 (Gandy et al 2006;Kang et al 2008). While hyper-O-GlcNAcylation of Akt has deleterious effects in hepatic cell and tissue models of euglycemia (Soesanto et al 2008) and in pancreatic β-cells under hyperglycemic conditions (Kang et al 2008), it is unclear if Akt is glycosylated in models of tissue injury.…”