Apelin protects against liver X receptor-mediated steatosis through AMPK and PPARα in human and mouse hepatocytes

Non-alcoholic fatty liver disease is the most commonly occurring chronic liver disease, and hepatic steatosis, a condition defined as extensive lipid accumulation in hepatocytes, is associated with liver dysfunction and metabolic diseases, such as, obesity and type II diabetes. Apelin is an adipokine that acts on a G protein-coupled receptor named APJ, and has been established to play pivotal roles in various physiological conditions. However, the function of apelin in hepatocytes has not been fully investigated. In order to assess the functional roles of apelin and APJ in hepatocytes, we used an in vitro model of liver X receptor (LXR)-mediated hepatocellular steatosis. In Hep3B human hepatoma cells, T0901317 (a specific LXR activator) induced lipid accumulation and this was inhibited by apelin. T0901317 also induced the expression of SREBP-lc, a key transcription factor for lipogenesis. Apelin not only inhibited SREBP-lc induction at the mRNA and protein levels but also induced lipolytic PPAR alpha expression. Furthermore, these protective effects of apelin were inhibited by apelin F13A (a specific APJ antagonist). Furthermore, silencing of APJ by siRNA transfection also inhibited the actions of apelin. Specific inhibitors of cellular signaling components showed inhibition of lipid accumulation by apelin was mediated through G(i/o) proteins, AMPK, and SREBP-1c suppression during the early stage and through AMPK, ERKs, and PPAR alpha induction during the late stage. In addition, the protective effect of apelin was confirmed in mouse primary hepatocytes. Our observations suggest apelin-APJ signaling in hepatocytes functions to protect against lipid accumulation in liver through two signaling pathways, that is, via AMPK activation and PPAR alpha induction.