Magnesium increases insulin-dependent glucose uptake in adipocytes

BACKGROUND: Type 2 diabetes (T2D) is characterized by a decreased insulin sensitivity. Magnesium (Mg(2+)) deficiency is common in people with T2D. However, the molecular consequences of low Mg(2+) levels on insulin sensitivity and glucose handling have not been determined in adipocytes. The aim of this study is to determine the role of Mg(2+) in the insulin-dependent glucose uptake. METHODS: First, the association of low plasma Mg(2+) with markers of insulin resistance was assessed in a cohort of 395 people with T2D. Secondly, the molecular role of Mg(2+) in insulin-dependent glucose uptake was studied by incubating 3T3-L1 adipocytes with 0 or 1 mmol/L Mg(2+) for 24 hours followed by insulin stimulation. Radioactive-glucose labelling, enzymatic assays, immunocytochemistry and live microscopy imaging were used to analyze the insulin receptor phosphoinositide 3-kinases/Akt pathway. Energy metabolism was assessed by the Seahorse Extracellular Flux Analyzer. RESULTS: In people with T2D, plasma Mg(2+) concentration was inversely associated with markers of insulin resistance; i.e., the lower Mg(2+), the more insulin resistant. In Mg(2+)-deficient adipocytes, insulin-dependent glucose uptake was decreased by approximately 50% compared to control Mg(2+)condition. Insulin receptor phosphorylation Tyr1150/1151 and PIP3 mass were not decreased in Mg(2+)-deficient adipocytes. Live imaging microscopy of adipocytes transduced with an Akt sensor (FoxO1-Clover) demonstrated that FoxO1 translocation from the nucleus to the cytosol was reduced, indicting less Akt activation in Mg(2+)-deficient adipocytes. Immunocytochemistry using a Lectin membrane marker and at the membrane located Myc epitope-tagged glucose transporter 4 (GLUT4) demonstrated that GLUT4 translocation was diminished in insulin-stimulated Mg(2+)-deficient adipocytes compared to control conditions. Energy metabolism in Mg(2+) deficient adipocytes was characterized by decreased glycolysis, upon insulin stimulation. CONCLUSIONS: Mg(2+) increases insulin-dependent glucose uptake in adipocytes and suggests that Mg(2+) deficiency may contribute to insulin resistance in people with T2D.