The Role of Nuclear Receptor Corepressors NCoR1 and SMRT on Physiologic Function in the Adult Mouse

Thyroid hormone (TH) plays an essential role in maintaining homeostasis and regulating metabolism in all organ systems beginning with embryogenesis and continuing throughout life. TH action is mediated by the thyroid hormone receptor (TR), which is a nuclear receptor, and it’s coregulators. The nuclear receptor corepressor 1 (NCoR1) and the silencing mediator of retinoid and thyroid hormone receptors (SMRT) are two critical corepressors of the TR that inhibit gene transcription in the absence of TH. Repression is mediated by complexing with histone deacetylase 3 (HDAC3), which is stabilized by NCoR1 and SMRT. NCoR1 and SMRT are critical for maintaining metabolic homeostasis and act to mediate energy expenditure, insulin sensitivity, and body weight. We sought to elucidate the roles of NCoR1 and SMRT in maintaining global physiologic function in the adult mouse. In order to study the post-natal role of these corepressors, we used a tamoxifen-inducible Cre recombinase (UBC-Cre-ERT2) to knock-out (KO) NCoR1, SMRT, or NCoR1 and SMRT together in adult mice because global deletion of either corepressor during embryogenesis is lethal. Mice were injected with tamoxifen at 8 weeks of age to KO either NCoR1 (NCoR1-KO; NKO), SMRT (SMRT-KO; SKO), or both NCoR1 and SMRT (double KO; DKO) and metabolic parameters were analyzed. While postnatal deletion of either NCoR1 or SMRT did not impact mortality, KO of both NCoR1 and SMRT resulted in a rapidly lethal phenotype heralded by weight loss, hypoglycemia and hypothermia. Metabolic phenotyping confirmed a loss of body mass and in particular fat mass in addition to a reduction in energy expenditure and increase in fecal caloric density. Further analysis showed the rapid development of hepatosteatosis and disturbances in lipid metabolism with a profound increase in beta-oxidation. We also found a reduction in HDAC3 protein levels in the DKO mice but no rapidly lethal phenotype in HDAC3 KO mice. Overall, we show that NCoR1 and SMRT together are critical for life as their deletion results in a rapidly lethal phenotype. While NCoR1 and SMRT are required to stabilize the corepressor complex, including HDAC3, HDAC3 KO resulted in a distinct and separate phenotype.