Objective: TR4 is a nuclear receptor without clear pathophysiological roles. We investigated the roles of hepatic TR4 in the regulation of lipogenesis and insulin sensitivity in vivo and in vitro.
Research design and methods: TR4 activity and phosphorylation assays were carried out using hepatocytes and various TR4 wild-type and mutant constructs. Liver tissues from TR4 knockout, C57BL/6, and db/db mice were examined to investigate TR4 target gene stearoyl-CoA desaturase (SCD) 1 regulation.
Results: TR4 transactivation is inhibited via phosphorylation by metformin-induced AMP-activated protein kinase (AMPK) at the amino acid serine 351, which results in the suppression of SCD1 gene expression. Additional mechanistic dissection finds TR4-transactivated SCD1 promoter activity via direct binding to the TR4-responsive element located at -243 to -255 on the promoter region. The pathophysiological consequences of the metformin→AMPK→TR4→SCD1 pathway are examined via TR4 knockout mice and primary hepatocytes with either knockdown or overexpression of TR4. The results show that the suppression of SCD1 via loss of TR4 resulted in reduced fat mass and increased insulin sensitivity with increased β-oxidation and decreased lipogenic gene expression.
Conclusions: The pathway from metformin→AMPK→TR4→SCD1→insulin sensitivity suggests that TR4 may function as an important modulator to control lipid metabolism, which sheds light on the use of small molecules to modulate TR4 activity as a new alternative approach to battle the metabolic syndrome.