Conclusion
Akt phosphorylation at Ser473 by ginsenoside Rg1 is critical for its gluconeogenesis-lowering effect, suggesting a potential for pharmaceutical intervention in response to fasting hormone glucagon.
Methods
The effects of Rg1 were investigated in high-fat-diet (HFD)-fed mice and glucagon-challenged C57BL/6J mice. Glucose metabolism was evaluated by oral glucose tolerance test and pyruvate tolerance test. The role of Rg1 on the regulation of Akt-FoxO1 interaction was performed using immunofluorescence, immunoprecipitation, siRNA silencing, pharmacological inhibitor and active-site mutant in primary hepatocytes or HepG2 cells.
Results
Abnormally elevated fasting glucagon levels were observed in HFD-fed mice, contributing significantly to increased fasting plasma glucose levels. Inappropriate fasting glucagon secretion inactivated Akt and promoted hepatic glucose production via upregulation of FoxO1 activity. Rg1 preserved glucagon-impaired Akt activation partly by binding to Akt at Ser473 site. Rg1 also promoted Akt binding to FoxO1 and inactivated FoxO1 by phosphorylation. Consequently, Rg1 decreased the hepatic glucose production through a decrease in transcription of phosphoenolpyruvate carboxykinase (PEPCK) and glucose 6-phosphatase (G6Pase). Both siRNA silencing of Akt and Akt inhibitor triciribine attenuated the effects of Rg1 in response to fasting hormone glucagon.