Abstract
AMP-activated protein kinase (AMPK) is an important effector of metformin action on glucose uptake in skeletal muscle cells. We recently reported that metformin improved insulin receptor substrate-1 (IRS-1)-associated insulin signaling by downregulating sterol regulatory element-binding protein-1c (SREBP-1c) expression. In this study, we investigated whether AMPK activation and SREBP-1c inhibition contribute to the beneficial effects of metformin on IRS-1-associated insulin signaling in L6 myotubes. L6 muscle cells were incubated with palmitic acid (PA) to induce insulin resistance and then treated with metformin and/or the AMPK inhibitor, compound C. AMPK, SREBP-1c, IRS-1 and Akt protein expression levels were determined by western blot analysis. The effects of metformin on SREBP-1c gene transcription were determined by a luciferase reporter assay. Glucose uptake was evaluated using the 2-NBDG method. In the PA-treated L6 cells, metformin treatment enhanced AMPK phosphorylation, reduced SREBP-1c expression and increased IRS-1 and Akt protein expression, whereas treatment with compound C blocked the effects of metformin on SREBP-1c expression and the IRS-1 and Akt levels. Moreover, metformin suppressed SREBP-1c promoter activity and promoted glucose uptake through AMPK. The results from this study demonstrate that metformin ameliorates PA-induced insulin resistance through the activation of AMPK and the suppression of SREBP-1c in skeletal muscle cells.