Prevention by metformin of alterations induced by chronic exposure to high glucose in human islet beta cells is associated with preserved ATP/ADP ratio

AimWe have explored whether the insulin secretory defects induced by glucotoxicity in human pancreatic islets could be prevented by metformin and investigated some of the possible mechanisms involved.MethodsHuman pancreatic islets and INS-1E cells were cultured for 24 h with or without high glucose (16.7 mM) concentration in the presence or absence of therapeutical concentration of metformin and then glucose-stimulated insulin release, adenine nucleotide levels and mitochondrial complex I and II activities were measured. Islet ultrastructure was analyzed by electron microscopy.ResultsCompared to control islets, human islets cultured with high glucose showed a reduced glucose-stimulated insulin secretion that was associated with lower ATP levels and a lower ATP/ADP ratio. These functional and biochemical defects were significantly prevented by the presence of metformin in the culture medium, that was also able to significantly inhibit the activity of mitochondrial complex I especially in beta cells exposed to high glucose. Ultrastructural observations showed that mitochondrial volume density was significantly increased in high glucose cultured islets. The critical involvement of mitochondria was further supported by the observation of remarkably swollen organelles with dispersed matrix and fragmented cristae. Metformin was able to efficiently prevent the appearance of all these ultrastructural alterations in human islets exposed to high glucose.ConclusionsOur results show that the functional, biochemical and ultrastructural abnormalities observed in human islet cells exposed to glucotoxic condition can be significantly prevented by metformin, further highlighting a direct beneficial effect of this drug on the insulin secreting human pancreatic beta cells.