Effect of Metformin on Mitochondrial Pathways in Human Skeletal Muscle Cells

Metformin (Met) is the most commonly prescribed antidiabetic agent worldwide. Although it has been marketed in the U.S. for over two decades, the exact molecular mechanisms of Met remain controversial. In addition to suppression of hepatic gluconeogenesis, increasing evidence suggests Met induces extrahepatic tissue-specific effects, such as in skeletal muscle (SKM), where the mitochondrion is believed to be the primary cellular target. We investigated the effects of Met on mitochondrial pathways in human SKM cells isolated by percutaneous needle biopsy from lean control, obese nondiabetic, and type 2 diabetic (T2D) volunteers. Mitochondrial function was assessed by Seahorse XF24 analyzer in SKM cells treated with or without Met for 24 hours. Glycolytic rate increased in Met-treated SKM cells from lean (P<0.01), obese (P<0.0001), and T2D subjects (P<0.01) compared to untreated cells. In contrast, basal respiration and ATP production were dramatically reduced in SKM cells treated with Met (P<0.0001 for all phenotypes). Evaluation of the insulin signaling pathway revealed increased Akt phosphorylation at Ser⁴⁷³ (P<0.05) and Thr³⁰⁸ (P<0.01) in cells pre-treated with Met compared to insulin stimulation alone, indicating an additive effect of Met on insulin action. As expected, muscle cells incubated with Met showed enhanced AMPK phosphorylation at Thr¹⁷² (P<0.001). Recent studies have identified a direct link between AMPK activation and mitochondrial dynamics pathways. We therefore assessed changes in markers of mitochondrial fission and fusion and found that Met treatment significantly induced phosphorylation of mitochondrial fission factor (MFF; P<0.01) in human SKM cells. Our data suggest Met may mediate improvements in glycemic control by activating AMPK to directly regulate the mitochondrial fission machinery and alter cellular bioenergetics. Functional morphometric and biochemical studies are ongoing to validate mechanisms by which Met exerts its antihyperglycemic effects in SKM.