Neuromedin U suppresses insulin secretion by triggering mitochondrial dysfunction and endoplasmic reticulum stress in pancreatic β-cells.

Neuromedin U (NMU), a highly conserved peptide in mammals, is involved in a wide variety of physiological processes. NMU, which is synthesized in β-cells and co-localizes with insulin, directly acts on β-cells via NMU receptor 1 (NMUR1) to suppress glucose-stimulated insulin secretion (GSIS). The mechanism underlying this insulinostatic effect has yet to be elucidated. We observed that NMU caused mitochondrial dysfunction by impairing mitochondrial biogenesis, respiration, and mitochondrial Ca2+ uptake in β-cell-derived MIN6-K8 cells. NMU administration induced the endoplasmic reticulum (ER) stress, as reflected by the activation of ER stress signaling pathways involving ATF6, XBP-1s, and PERK-ATF4-CHOP. Nmu knockdown in MIN6-K8 cells increased the number of insulin granules and improved mitochondrial biogenesis and function. NMU was upregulated in both the islets of db/db mice and palmitate-treated MIN6-K8 cells. Our results highlight the crucial role of NMU in the maintenance of β-cell function and glucose metabolism through regulation of mitochondria dysfunction and ER stress. In pathological stages that develop into diabetes, upregulation of NMU could suppress the insulin secretion by inducing mitochondrial dysfunction and ER stress, which may contribute to subsequent β-cell dysfunction.