Metformin attenuates albumin-induced alterations in renal tubular cells in vitro

Proteinuria (albuminuria) plays a crucial role in the etiology of chronic kidney disease (CKD) via alteration of multiple signaling pathways and cellular process in renal cells. The objectives of this study are to investigate the effects of activation of the energy-sensing molecule AMP-activated kinase (AMPK) in renal cells using metformin on endoplasmic reticulum (ER) stress, AKT, mTOR, epithelial-to-mesenchymal transition (EMT), autophagy, and apoptosis that are thought to mediate renal cell injury during proteinuria, and to dissect the AMPK- and non-AMPK mediated effects of metformin using an in vitro model of albumin-induced renal cell injury. Rat renal proximal tubular (NRK-52E) cells were exposed to 10 and 15mg/ml of albumin for 72h in the presence of 1mM Metformin and/or 0.5 mu M compound C, and assessed for alterations in the aforementioned pathways. Metformin treatment restored AMPK phosphorylation and augmented autophagy in renal cells exposed to albumin. In addition, metformin treatment attenuated the albumin-induced phosphorylation of AKT and the downstream targets of mTOR, and prevented albumin-mediated inductions of EMT marker (-SMA), pro-apoptotic ER stress marker CHOP, and apoptotic caspases -12 and -3 in renal cells. Blockade of metformin-induced AMPK activation with compound C blunted the ER defense response and autophagy but had no effect on the markers of EMT and apoptosis in our model. Our studies suggest that metformin protects renal cells against proteinuric cytotoxicity via suppression of AKT and mTOR activation, inhibition of EMT and apoptosis, and augmentation of autophagy and ER defense response through AMPK-independent and AMPK-dependent mechanisms, respectively.