Metformin: a potential risk for Mg2+ transport disorders and hypomagnesemia in type 2 diabetes

Abstract

Background/Aims: Metformin is the first-line treatment in T2D. Previously and recently, it has been shown that metformin treatment is associated with lower serum Mg2+ levels in T2D patients. To confirm this association and to investigate how metformin affects Mg2+ homeostasis, human, in vitro and in vivo studies were performed. Methods: A cohort of T2D patients, diabetic mice housed in metabolic cages, mouse DCT15 kidney cells, human Caco-2 colon cells and TRPM6-transfected HEK293 cells were included in this study. Plasma and urinary Mg2+ concentrations were measured using a spectrophotometric assay. In the human study, variables were correlated to plasma Mg2+ levels and to FEMg by using Pearson correlation analyses. After excluding confounding variables, all parameters correlating (p < 0.1) with plasma Mg2+ or with FEMg were included in a stepwise backward regression model. Whole-cell patch clamp recordings and cell surface biotinylation assays were performed to study short-term (1 h) metformin treatment in TRPM6-transfected HEK293 cells. Long-term (24 h) treatment of mouse DCT15 kidney cells and human Caco-2 colon cells with metformin was applied to measure the effects on endogenous TRPM6 mRNA expression and on Mg2+ uptake. mRNA expression of magnesiotropic genes was determined using RT-qPCR. To assess Mg2+ absorption, 25Mg2+ uptake measurements were performed using inductively coupled plasma mass spectrometry. Results: In the cohort of T2D subjects, the mean plasma Mg2+ concentration and the mean FEMg were 0.74 ± 0.10 mmol/L and 4 ± 3% respectively. 30.6% of patients suffered from hypomagnesemia (blood [Mg2+] < 0.7 mmol/L) and 77% showed renal Mg2+ loss (FEMg > 2%). In the patients with T2D, the use of metformin (n = 251, 62%) was associated with lower plasma Mg2+ concentrations (r = −0.268, p < 0.001) and a lower FEMg (r = −0.278, p < 0.001). Short-term effects of metformin significantly increased TRPM6 activity and its cell surface trafficking. In contrast, long-term effects significantly decreased TRPM6 mRNA expression and Mg2+ uptake in the colon and DCT. Db/db mice had significantly lower serum Mg2+ levels than db/m mice. However, metformin-treatment of mice did not result in effects on the serum Mg2+ concentration, urinary Mg2+ excretion and gene expression levels. Conclusion: Indeed, metformin therapy is strongly associated with reduced serum Mg2+ levels in patients with T2D. The prolonged use of metformin may lower serum Mg2+ concentrations through downregulation of TRPM6 gene expression, which in turn, could lead to modulation of Mg2+ (re)absorption in the intestine and kidney. Therefore, metformin-treated patients with type T2D are at risk for hypomagnesemia.