Time-dependent effects of endogenous hyperglucagonemia on glucose homeostasis and hepatic glucagon action

Elevation of glucagon levels and increase in alpha cell proliferation is associated with states of hyperglycemia in diabetes. A better understanding of the molecular mechanisms governing glucagon secretion could have major implications for understanding abnormal responses to hypoglycemia in patients with diabetes and provide novel avenues for diabetes management. Using mice with inducible induction of Rheb1 in alpha cells (alpha RhebTg mice), we showed that short-term activation of mTORC1 signaling is sufficient to induce hyperglucagonemia through increased glucagon secretion. Hyperglucagonemia in alpha RhebTg mice was also associated with an increase in alpha cell size and mass expansion. This model allowed us to identify the effects of chronic and short-term hyperglucagonemia on glucose homeostasis by regulating glucagon signaling in the liver. Short-term hyperglucagonemia impaired glucose tolerance, which was reversible over time. Liver glucagon resistance in alpha RhebTg mice was associated with reduced expression of the glucagon receptor and genes involved in gluconeogenesis, amino acid metabolism, and urea production. However, only genes regulating gluconeogenesis returned to baseline upon improvement of glycemia. Overall, these studies demonstrate that hyperglucagonemia exerts a biphasic response on glucose metabolism: Short-term hyperglucagonemia lead to glucose intolerance, whereas chronic exposure to glucagon reduced hepatic glucagon action and improved glucose tolerance

Automated summary

Elevation of glucagon levels and increase in alpha cell proliferation are associated with hyperglycemia in diabetes. Understanding the molecular mechanisms of glucagon secretion could have significant implications for the management of diabetes.