In vivo interaction of steroid receptor coactivator (SRC)-1 and the activation function-2 domain of the thyroid hormone receptor (TR) βin TRβ E457A knock-in and SRC-1 knockout mice

The activation function-2 (AF-2) domain of the thyroid hormone (TH) receptor (TR)-β is a THdependent binding site for nuclear coactivators (NCoA), which modulate TH-dependent gene transcription. In contrast, the putative AF-1 domain is a TH-independent region interacting with NCoA. We determined the specificity of the AF-2 domain and NCoA interaction by evaluating thyroid function in mice with combined disruption of the AF-2 domain in TRβ, due to a point mutation (E457A), and deletion of one of the NCoAs, steroid receptor coactivator (SRC)-1. The E457A mutation was chosen because it abolishes NCoA recruitment in vitro while preserving normal TH binding and corepressor interactions resulting in resistance to TH. At baseline, disruption of SRC-1 in the homozygous knock-in (TRβ^E457A/E457A) mice worsened the degree of resistance to TH, resulting in increased serum T ₄ and TSH. During TH deprivation, disruption of AF-2 and SRC-1 resulted in a TSH rise 50% of what was seen when AF-2 alone was removed, suggesting that SRC-1 was interacting outside of the AF-2 domain. Therefore, 1) during TH deprivation, SRC-1 is necessary for activating the hypothalamic-pituitary-thyroid axis; 2) ligand-dependent repression of TSH requires an intact AF-2; and 3) SRC-1 may interact with the another region of the TRβor the TRαto regulate TH action in the pituitary. This report demonstrates the dual interaction of NCoA in vivo: the TH-independent up-regulation possibly through another domain and TH-dependent downregulation through the AF-2 domain.