Action of Metformin on Beta Cell Lipotoxicity

Abstract

Introduction: Chronic exposure to elevated levels of free fatty acids contributes to pancreatic beta cell dysfunction. Although it is well known that metformin induces cellular energy depletion and a concomitant activation of adenosine monophosphate-activated protein kinase (AMPK) through inhibition of respiratory chain, previous studies have shown inconsistent results with regard to action of metformin on pancreatic beta cells. Methods: To examine the effects of metformin on pancreatic beta cells under lipotoxic stress, I measured viability and glucose-stimulated insulin secretion (GSIS) in NIT-1 cells and isolated mouse islets exposed to palmitate and various concentrations of metformin. To determine the dependence on AMPK, I treated AMPK activator and AMPK antagonist in parallel with metformin and measured levels of AMPK phosphorylation. As markers for cellular metabolism, cellular adenosine diphosphate and triphosphate levels were measured and autofluorescence imaging of the pyridine nucleotides was obtained. I measured messenger RNA levels of endoplasmic reticulum (ER) stress markers, glucose-stimulated calcium influx, intracellular reactive oxygen species (ROS) levels and caspase-3 activity as markers for lipotoxicity. Results: I find that metformin has protective effects on palmitate-induced beta cell dysfunction. Metformin at concentrations lower than 0.5 mM inhibits palmitate-induced elevations in expression levels of ER stress markers, intracellular ROS level, and caspase-3 activity in a AMPK-independent manner, whereas metformin at the higher concentrations depletes cellular ATP levels, restores calcium influx reduced by palmitate and improves lipotoxic beta cell dysfunction in a AMPK-dependent manner. Cytosolic redox state is increased by metformin at concentrations lower than 0.5 mM at which AMPK activation does not occur. Conclusions: This study suggests that metformins action on beta cell lipotoxicity is implemented by different molecular pathways depending on its concentration. Metformin at usual therapeutic dose is supposed to alleviate lipotoxic beta cell dysfunction through inhibition of oxidative stress and ER stress.