Thyroid hormone direct repeat 4 response element is a positive regulatory element for the human TR2 orphan receptor, a member of steroid receptor superfamily

We demonstrate that TR2 orphan receptor (TR2) may induce transactivation activities via an AGGTCA-like-direct-repeat-4 consensus thyroid hormone response element (DR4-TRE) system. TR2 showed a slightly greater binding affinity than thyroid hormone receptor alpha 1 (TR alpha 1)/retinoid X receptor alpha (RXR alpha) heterodimer with Kds 0.5 nM and 2.3 nM, respectively. These receptors, TR2 and TR alpha 1/RXR alpha heterodimer, competed with each other on binding to limited amounts of DR4-TRE. TR2 canceled the suppression effect of unliganded-TR alpha 1 on CAT reporter activity in a dose-dependent fashion. Estrogen receptor (ER) and 2P2 (a mutated TR2 with P box sequence of androgen receptor) failed not only to bind to DR4-TRE but also to recover this inhibitory effect of unliganded TR alpha 1. However, when T-3 was supplemented, estradiol-ER competed for a full CAT activity while TR2 showed an additive effect on the transcriptional activation. These results indicate that DNA binding is essential for TR2 to take action and fully functional liganded TR alpha 1 may rely on common factors shared with ER but not TR2.