Chronic insulin exposure with a decrease of insulin receptors results in diminished insulin sensitivity of glucagon gene transcription in pancreatic alpha cells

Objectives: Insulin resistance of peripheral tissues like liver, muscle and fat and beta cell dysfunction contribute to the pathogenesis of diabetes mellitus type 2. Considering that insulin inhibits glucagon secretion and gene transcription, hyperglucagonemia and hyperinsulinemia observed in diabetes suggest that insulin resistance also affects the glucagon-producing pancreatic alpha-cells. However, the molecular mechanisms of insulin resistance at the level of the pancreatic islet alpha-cells are unknown. In the present study the effect of chronic insulin treatment on the regulation of glucagon gene transcription in the pancreatic islet alpha cell-line and on different steps of insulin signaling was investigated.

Methods: Transient transfection assays and immunoblot analysis in the pancreatic alpha cell line InR1G9.

Results: Insulin treatment for 48h abolished the insulin-induced inhibition of a luciferase reporter gene under the control of the rat glucagon promoter from -350 to +58 bp. Chronic insulin treatment reduced the phosphorylation of protein kinase B on Ser-473, indicating an upstream defect in insulin signaling. In addition, the phosphorylation on Tyr-612 of the insulin receptor substrate 1 was decreased by chronic insulin treatment. Moreover, chronic insulin treatment caused an insulin receptor down-regulation in a reversible and time-dependent process which was attributed to enhanced lysosomal degradation.

Conclusion: Our results suggest that chronic insulin exposure of alpha cells overrides insulin-induced inhibition of glucagon gene transcription, most likely by reducing the number of insulin receptors on the alpha cells. Thus, hyperinsulinemia might result in insulin resistance at the level of the pancreatic alpha cells, thereby leading to hyperglucagonemia and further increase in hepatic glucose production.