Glucagon, secreted from the α-cells of pancreatic islets, plays an important, yet underappreciated role in glucose homeostasis. As the major counter-regulatory hormone to insulin, it helps maintain normal blood glucose levels and prevent life-threatening hypoglycemia. However, while the regulation of glucagon has been shown to be perturbed in subjects with diabetes leading to inadequately high levels and worsening of the hyperglycemia, the molecular mechanisms involved in this abnormal regulation are still unknown. We therefore first investigated the possibility that microRNAs might be involved in the regulation of glucagon expression. Indeed, analysis of the glucagon 3’UTR revealed two potential miR-320a binding sites in human and one potential site in mouse and rat, respectively. In addition, we performed luciferase reporter assays and found that miR-320a directly binds to the 3’ UTR of the human glucagon gene. To further determine the role of miR-320a in the regulation of glucagon gene expression, we performed gain and loss of function studies with miR-320a in αTC1-6 cells. Glucagon gene expression was reduced in response to miR-320a overexpression, whereas inhibition of miR-320a upregulated glucagon gene expression. Interestingly, consistent with its role in the regulation of glucagon, miR-320a is more highly expressed in α-cells than beta cells and its expression was decreased by high glucose. Moreover, this was associated with an increase in glucagon gene expression in mouse αTC 1-6 cells as well as human islets. Finally, these findings are not only consistent with the results in response to high glucose, but also demonstrate the pathophysiological relevance of this phenomenon. Taken together, our data suggest that reduced miR-320a expression may contribute to the increase in glucagon gene expression in type 2 diabetes. These findings indicate for the first time that glucagon gene expression is under the control of a microRNA and provide novel insight into the abnormal regulation of glucagon in diabetes.
S. Jo: None. G. Xu: None. G. Jing: None. A. Shalev: None.