Succinate causes α-SMA production through GPR91 activation in hepatic stellate cells

Succinate acts as an extracellular signaling molecule as well as an intermediate in the citric acid cycle. It binds to and activates its specific G protein-coupled receptor 91 (GPR91). GPR91 is present in hepatic stellate cells (HSCs), but its role in hepatic fibrogenesis remains unclear. Cultured HSCs treated with succinate showed increased protein expression of GPR91 and alpha-smooth muscle actin (alpha-SMA), markers of fibrogenic response. Succinate also increased mRNA expression of alpha-SMA, transforming growth factor 13 (TGF-beta), and collagen type I. Transfection of siRNA against GPR91 abrogated succinate-induced increases in alpha-SMA expression. Malonate, an inhibitor of succinate dehydrogenase (SDH), increased succinate levels in cultured HSCs and increased GPR91 and alpha-SMA expression. Feeding mice a methionine- and choline-deficient (MCD) diet is a widely used technique to create an animal model of nonalcoholic steatohepatitis (NASH). HSCs cultured in MCD media showed significantly decreased SDH activity and increased succinate concentration and GPR91 and alpha-SMA expression. Similarly, palmitate treatment significantly decreased SDH activity and increased GPR91 and alpha-SMA expression. Finally, C57BL6/J mice fed the MCD diet had elevated succinate levels in their plasma. The MCD diet also decreased SDH activity, increased succinate concentration, and increased GPR91 and alpha-SMA expression in isolated HSCs. Collectively, our results show that succinate plays an important role in HSC activation through GPR91 induction, and suggest that succinate and GPR91 may represent new therapeutic targets for modulating hepatic fibrosis. (C) 2015 Elsevier Inc. All rights reserved.