Background: Chronic liver disease is becoming a major cause of morbidity and mortality worldwide. During liver injury, hepatic stellate cells (HSCs) trans-differentiate into activated myofibroblasts, which produce extracellular matrix. Succinate and succinate receptor (G-protein coupled receptor91, GPR91) signaling pathway has now emerged as a regulator of metabolic signaling. A previous study showed that succinate and its specific receptor, GPR91, are involved in the activation of HSCs and the overexpression of cc-smooth muscle actin (alpha-SMA). Metformin, a well-known anti-diabetic drug, inhibits hepatic gluconeogenesis in the liver. Many studies have shown that metformin not only prevented, but also reversed, steatosis and inflammation in a nonalcoholic steatohepatitis (NASH) animal model. However, the role of metformin in HSC activation and succinate-GPR91 signaling has not been clarified. Methods: The immortalized human HSCs, LX-2 cells, were used for the in vitro study. For the in vivo study, male C57BL/J6 mice were randomly divided into 3 groups and were fed with a methionine-choline-deficient diet (MCD diet group) as a nonalcoholic steatohepatitis (NASH) mouse model with or without 0.1% metformin for 12 weeks, or were fed a control methionine-choline-sufficient diet (MCS diet group). Results: In our study, metformin and 5-aminoimidazole-4-carboxamide 1-beta-D ribofuranoside (AICAR), which is an analog of adenosine monophosphate, were shown to suppress alpha-SMA expression via enhanced phosphorylation of AMP-activated protein kinase (AMPK) and inhibition of succinate-GPR91 signaling in activated LX-2 cells induced by palmitate- or succinate. Metformin and AICAR also reduced succinate concentration in the cell lysates when LX-2 cells were treated with palmitate. Moreover, metformin and AICAR reduced interleukin-6 and, transforming growth factor-beta 1 production in succinate-treated LX-2 cells. Both metformin and AICAR inhibited succinate-stimulated HSC proliferation and cell migration. Mice fed a MCD diet demonstrated increased steatohepatitis and liver fibrosis compared to that of mice fed control diet. Metformin ameliorated steatohepatitis, liver fibrosis, inflammatory cytokine production and decreased alpha-SMA and GPR91 expression in the livers of the MCD diet-fed mice. Conclusion: This study shows that metformin can attenuate activation of HSCs by activating the AMPK pathway and inhibiting the succinate-GPR91 pathway. Metformin has therapeutic potential for treating steatohepatitis and liver fibrosis. (C) 2017 Elsevier Inc. All rights reserved.