Melatonin influences insulin secretion primarily via MT 1 receptors in rat insulinoma cells (INS‐1) and mouse pancreatic islets

  Several studies have revealed that melatonin affects the insulin secretion via MT 1 and MT 2 receptor isoforms. Owing to the lack of selective MT 1 receptor antagonists, we used RNA interference technology to generate an MT 1 knockdown in a clonal β‐cell line to evaluate whether melatonin modulates insulin secretion specifically via the MT 1 receptor. Incubation experiments were carried out, and the insulin concentration in supernatants was measured using a radioimmunoassay. Furthermore, the intracellular cAMP was determined using an enzyme‐linked immunosorbent assay. Real‐time RT‐PCR indicated that MT 1 knockdown resulted in a significant increase in the rIns1 mRNA and a significantly elevated basal insulin secretion of INS‐1 cells. Incubation with melatonin decreased the amount of glucagon‐like peptide 1 or inhibited the glucagon‐stimulated insulin release of INS‐1 cells, while, in MT 1 ‐knockdown cells, no melatonin‐induced reduction in insulin secretion could be found. No decrease in 3‐isobutyl‐1‐methylxanthine‐stimulated intracellular cAMP in rMT 1 ‐knockdown cells was detectable after treatment with melatonin either, and immunocytochemistry proved that MT 1 knockdown abolished phosphorylation of cAMP‐response‐element‐binding protein. In contrast to the INS‐1 cells, preincubation with melatonin did not sensitize the insulin secretion of rMT 1 ‐knockdown cells. We also monitored insulin secretion from isolated islets of wild‐type and melatonin‐receptor knockout mice ex vivo. In islets of wild‐type mice, melatonin treatment resulted in a decrease in insulin release, whereas melatonin treatment of islets from MT 1 knockout and MT 1/2 double‐knockout mice did not show a significant effect. The data indicate that melatonin inhibits insulin secretion, primarily via the MT 1 receptor in rat INS‐1 cells and isolated mouse islets.