Metformin induces renal medullary interstitial cell apoptosis in type 2 diabetic mice

Objective: Metformin is a first-line antidiabetic drug for type 2 diabetes (T2D) with a relatively good safety profile. Metformin activates AMP-activated protein kinase (AMPK), which is crucial in maintaining renal medullary function, with inappropriate AMPK activation facilitating renal medullary interstitial cells (RMICs) apoptosis under hypertonic challenge. The present study was to determine the effects of metformin on RMIC survival in both normal and T2D mice. Methods: Mice (C57BL/6, db/m, and db/db) were treated with 450mg/kg metformin for 7 days and subjected to 24-h water restriction (=dehydration) before being killed. Cell apoptosis in the renal medulla was determined by the terminal deoxyribonucleotidyl transferase-mediated dUTP-digoxigenin nick end-labeling (TUNEL) assay. Cultured RMIC were treated with 10mmol/L metformin in the presence or absence of hypertonic stress. Cell viability was determined and the underlying mechanisms were investigated. Results: Metformin induced significant apoptosis of RMIC in dehydrated normal mice and both hydrated and dehydrated T2D mice. Hypertonicity increased ATP production and inhibited AMPK phosphorylation in RMIC, which was attenuated by metformin. Metformin augmented hypertonicity-induced apoptosis of RMIC, suppressed the nuclear factor-κB/cyclo-oxygenase-2 pathway, reduced reactive oxygen species production and inhibited transcriptional activation of tonicity-responsive enhancer binding protein (TonEBP) and its downstream osmoprotective gene expression. Conclusions: Metformin treatment is associated with increased RMIC apoptosis in both normally hydrated and dehydrated T2D mice. The results confirm AMPK as a critical factor involved in the maintenance of RMIC viability in T2D and raise safety concerns for metformin and other AMPK-activating antidiabetic drugs in dehydrated diabetic patients.