TY - JOUR T1 - The Orphan Nuclear Receptor, DAX-1, Functions as a Potent Co-repressor of the Constitutive Androstane Receptor (CAR, NR1I3) JF - Molecular Pharmacology JO - Mol Pharmacol DO - 10.1124/mol.112.080721 SP - mol.112.080721 AU - Elizabeth M Laurenzana AU - Tao Chen AU - Malavika Kannuswamy AU - Brian E Sell AU - Stephen C Strom AU - Yong Li AU - Curtis J Omiecinski Y1 - 2012/01/01 UR - http://molpharm.aspetjournals.org/content/early/2012/08/15/mol.112.080721.abstract N2 - Regulation of gene transcription is controlled in part by nuclear receptors (NR) that function coordinately with coregulator proteins. The human constitutive androstane receptor (CAR; NR1I3) is primarily expressed in liver and regulates the expression of genes involved in xenobiotic metabolism as well as hormone, energy and lipid homeostasis. In this report, DAX-1, a NR family member with corepressor properties, was identified as a potent CAR regulator. Results of transaction and mutational studies demonstrated that both DAX-1's downstream 'LXXLL' and its 'PCFQVLP' motifs were critical contributors to DAX-1's co-repression activities, although two other 'LXXM/LL' motifs located nearer the N-terminus had no impact on the CAR functional interaction. Deletion of DAX-1’s carboxy-terminal transcription silencing domain (TSD) restored CAR1 transactivation activity in reporter assays to ~90% of control, demonstrating its critical function in mediating the CAR repression activities. Further, results obtained from mammalian two-hybrid experiments assessing various domain configurations of the respective receptors showed that full length DAX-1 inhibited CAR-SRC1 interaction by ~50%, whereas the same interaction was restored to 90% of control when the DAX-1 TSD was deleted. Direct interaction between CAR and DAX-1 was demonstrated with both alpha-screen and co-immunoprecipitation experiments, and this interaction was enhanced in the presence of the CAR activator, CITCO. Results obtained in primary human hepatocytes further demonstrated DAX-1 inhibition of CAR-mediated CITCO induction of the CYP2B6 target gene. The results of this investigation identify DAX-1 as a novel and potent CAR corepressor and suggest that DAX-1 functions as a coordinate hepatic regulator of CAR's biological function. ER -