Analysis of TR4 orphan nuclear receptor knockout mouse: Roles of TR4 in glucose and lipid metabolism.

摘要

Testicular Receptor 4 (TR4, NR2C2), an orphan member of the nuclear receptor superfamily, is known to be involved in postnatal growth, neural development, spermatogenesis, and many signal pathways. However, the physiological roles of TR4 in glucose, lipid metabolism, and insulin sensitivity remain unclear. Our studies revealed novel roles of TR4 in glucose and lipid metabolism.;Hepatic gluconeogenesis is robustly activated in response to fasting, in which phosphoenolpyruvate carboxykinase (PEPCK) is the important gene regulating the de novo glucose production process. We found hypoglycemia in mice lacking TR4 (TR4-/-) with reduced PEPCK gene expression during fasting. Overexpression of TR4 induced PEPCK gene transcription and led to increased hepatic glucose production. Furthermore, mechanistic dissection suggested TR4 binds to the PEPCK gene promoter and induces its expression via a transcriptional regulation. In response to fasting, the hepatic expression of TR4 and the association of TR4 with PEPCK promoter were induced by the cAMP/PKA axis and C/EBPs leading to increased PEPCK expression and elevated gluconeogenesis.;In addition to the up-regulated TR4 expression, we investigated other mechanisms regulating TR4 biological activity. Here we showed that, in the absence of specific ligand(s), the activity of TR4 could be modulated by metformin via AMP-activated protein kinase (AMPK)-mediated phosphorylation. This demonstrated that TR4 is a downstream target of metformin/AMPK signaling pathway and raised the possibility that small molecule(s) can modulate TR4 activity through AMPK.;Stearoyl-CoA desaturase 1 (SCD1) is a critical regulator in the biosynthesis of monounsaturated fatty acid. Previous studies suggest that both SREBP-1c-dependent and -independent regulation could control SCD1 gene expression. However, the mechanisms of SREBP-1c-independent regulation on SCD1 remain unclear. We demonstrated that SCD1 was induced by TR4 in a SREBP-1c-independent manner. Deficiency of SCD1 in TR4-/- mice resulted in less fat mass and increased insulin sensitivity with altered beta-oxidation and lipogenesis. Knockdown of TR4 via RNAi in wild-type mouse primary hepatocytes resulted in reduction of SCD1. Together, our results showed that modulation of TR4 via genetic knockout, RNAi knockdown, or phosphonyl inactivation, all affect SCD1 transcription, demonstrating a novel SREBP-1c-independent pathway to modulate SCD1 gene expression.