Role of co-activators and co-repressors in the mechanism of steroid/thyroid receptor action.

Steroid/thyroid hormone receptors are ligand-dependent transcription factors that regulate diverse aspects of growth, development, and homeostasis by binding as homodimers or heterodimers to their cognate DNA response elements to modulate transcription of target genes. Transactivation by steroid/ thyroid hormone receptors involves a conserved AF-2 domain located in the distal carboxy-terminus of the receptors. The existence of co-factors, termed co-activators or adapters, was first suggested by transcriptional squelching between progesterone receptors and estrogen receptors. Co-repressors were also postulated to contribute to the silencing function of unliganded thyroid hormone receptor (TR). The yeast two-hybrid system and Far-Western blotting have been used to identify several proteins that interact with members of the steroid/thyroid hormone receptor superfamily in a ligand-sensitive manner. Our laboratory cloned the first functional co-activator, termed steroid receptor co-activator-one (SRC-1), that appears to be a general co-activator for all steroid receptors tested and enhances transactivation of steroid hormone-dependent target genes. Subsequently, many more putative co-activators have been reported, including the SRC-1 related proteins, TIF2 and GRIP1, and other putative and unrelated co-activators such as ARA70, Trip1, RIP140, and TIF1. In addition, another co-activator, CREB-binding protein (CBP), has been shown to enhance steroid receptor-dependent target gene transcription. CBP and SRC-1 interact and synergistically enhance transcriptional activation by the ER and PR. Therefore, a ternary complex-consisting of CBP, SRC-1, and liganded steroid receptors-may form to increase the rate of hormone-responsive gene transcription. Similarly, co-repressors, such as SMRT and N-CoR, for TR and retinoic acid receptors (RAR) have been identified. The unliganded TR and RAR have been shown to inhibit basal promoter activity; this silencing of target gene transcription by unliganded receptors is mediated by these co-repressors. Collectively, available evidence supports the following model of steroid-responsive gene transcription. Upon binding of agonist the receptor changes its conformation in the ligand-binding domain that enables recruitment of co-activators, which allows the receptor to interact with the basal transcriptional machinery more efficiently and to activate transcription. In contrast, binding of antagonists induces a different conformational change in the receptor. Although some antagonist-bound receptor can dimerize and bind to its cognate DNA element, it fails to dislodge the associated co-repressors, which results in a nonproductive interaction with the basal transcriptional machinery. Similarly, the TR and RAR associate with co-repressors in the absence of ligand, thereby resulting in a negative interaction with the transcriptional machinery that silences target gene expression. In the case of mixed agonist/antagonists, such as 4-hydroxytamoxifen, activation of gene transcription may depend on the relative ratio of co-activators and co-repressors in the cell or cell-specific factors that determine the relative agonistic or antagonistic potential of different compounds. These co-activators and co-repressors appear to act as an accelerator and/or a brake that modulates transcriptional regulation of hormone-responsive target gene expression. Thus, the recent discovery of co-activators and co-repressors expands our knowledge of the mechanisms of steroid receptor action.