The Molecular Mechanism of Farnesoid X Receptor Alleviating Glucose Intolerance in Turbot (Scophthalmus maximus)

To explore the molecular targets for regulating glucose metabolism in carnivorous fish, the turbot (<i>Scophthalmus maximus</i>) was selected as the research object to study. Farnesoid X receptor (FXR; NR1H4), as a ligand-activated transcription factor, plays an important role in glucose metabolism in mammals. However, the mechanisms controlling glucose metabolism mediated by FXR in fish are not understood. It was first found that the protein levels of FXR and its target gene, small heterodimer partner (SHP), were significantly decreased in the high-glucose group (50 mM, HG) compared with those in the normal glucose group (15 mM, CON) in primary hepatocytes of turbot. By further exploring the function of FXR in turbot, the full length of FXR in turbot was cloned, and its nuclear localization function was characterized by subcellular localization. The results revealed that the FXR had the highest expression in the liver, and its capability to activate SHP expression through heterodimer formation with retinoid X receptor (RXR) was proved, which proved RXR could bind to 15 binding sites of FXR by forming hydrogen bonds. Activation of FXR in both the CON and HG groups significantly increased the expression of glucokinase (<i>gk</i>) and pyruvate kinase (<i>pk</i>), while it decreased the expression of cytosolic phosphoenolpyruvate carboxykinase (<i>cpepck</i>), mitochondrial phosphoenolpyruvate carboxykinase (<i>mpepck</i>), glucose-6-phosphatase 1 (<i>g6pase1</i>) and glucose-6-phosphatase 2 (<i>g6pase2</i>), and caused no significant different in glycogen synthetase (<i>gs</i>). ELISA experiments further demonstrated that under the condition of high glucose with activated FXR, it could significantly decrease the activity of PEPCK and G6PASE in hepatocytes. In a dual-luciferase reporter assay, the FXR could significantly inhibit the activity of G6PASE2 and cPEPCK promoters by binding to the binding site 'ATGACCT'. Knockdown of SHP after activation of FXR reduced the inhibitory effect on gluconeogenesis. In summary, FXR can bind to the <i>mpepck</i> and <i>g6pase2</i> promoters to inhibit their expression, thereby directly inhibiting the gluconeogenesis pathway. FXR can also indirectly inhibit the gluconeogenesis pathway by activating <i>shp</i>. These findings suggest the possibility of FXR as a molecular target to regulate glucose homeostasis in turbot.