Fibroblast growth factor (FGF) 21 is an endocrine growth factor mainly secreted by the liver in response to a ketogenic diet and alcohol consumption. FGF21 signaling requires co-receptor β-klotho (KLB) co-acting with FGF receptors, which has pleiotropic metabolic effects, including induced hepatic fatty acid oxidation and ketogenesis, in human and animal models of obesity. We examined the hepatocyte-specific enhancer/promoter of FGF21 expression plasmids in high-fat diet-fed mice for 12 weeks. Hydrodynamic injection for FGF21 delivery every 6 weeks sustained high circulating levels of FGF21, resulting in marked reductions in body weight, epididymal fat mass, insulin resistance, and liver steatosis. FGF21-induced lipolysis in the adipose tissue enabled the liver to be flooded with fat-derived FFAs. The hepatic expression of Glut2 and Bdh1 was upregulated, whereas that of gluconeogenesis-related genes, G6p and Pepck, and lipogenesis-related genes, Srebp-1 and Srebp-2, was significantly suppressed. FGF21 induced the phosphorylation of AMPK at Thr172 and Raptor at ser792 and suppressed that of mTOR at ser2448, which downregulated mTORC1 signaling and reduced IRS-1 phosphorylation at ser1101. Finally, in the skeletal muscle, FGF21 increased Glut4 and Mct2, a membrane protein that acts as a carrier for ketone bodies. Enzymes for ketone body catabolism (Scot) and citrate cycle (Cs, Idh3a), and a marker of regenerating muscle (myogenin) were also upregulated via increased KLB expression. Thus, FGF21-induced lipolysis was continuously induced by a high-fat diet and fat-derived FFAs might cause liver damage. Hepatic fatty acid oxidation and ketone body synthesis may act as hepatic FFAs' disposal mechanisms and contribute to improved liver steatosis. Liver-derived ketone bodies might be used for energy in the skeletal muscle. The potential FGF21-related crosstalk between the liver and extraliver organs is a promising strategy to prevent and treat metabolic syndrome-related nonalcoholic steatohepatitis.
