LIGAND-DEPENDENT CROSS-TALK BETWEEN STEROID AND THYROID-HORMONE RECEPTORS - EVIDENCE FOR COMMON TRANSCRIPTIONAL COACTIVATOR(S)

Steroid and thyroid hormone receptors exhibit striking structural and functional similarity, suggesting that these nuclear receptors may enh ance transcription of target genes by similar mechanisms, To address t his issue, we studied transcriptional interference between progesteron e and thyroid hormone receptors in vivo and in vitro, We observed that transcriptional interference occurred in a ligand-dependent manner be tween progesterone receptor-B (PR-B) and thyroid hormone receptor (TR) alpha or beta in transient transfection experiments. Ligand-occupied TR alpha or TR beta, but not the unliganded receptor, strongly suppres sed transactivation of a progesterone responsive reporter gene by endo genous PRs in human breast carcinoma T47D cells, Ligand dependent inhi bitory cross-talk also occurred between transfected PR-B and TR alpha or TR beta and vice versa in CV1 cells, This phenomenon did not requir e DNA binding by the ''interfering'' receptor but required it to be ho rmone-bound, indicating that a transcriptionally active form of the in terfering receptor is essential for the interfering effect, To analyze further the mechanism of the ligand-dependent cross talk, we reproduc ed transcriptional interference between PR and TR in a cell-free trans cription system, We observed that the addition of triiodothyronine-bou nd recombinant TR beta or a ligand binding domain (LED) peptide (145-4 56) inhibited specifically transcriptional activation of a progesteron e-responsive gene by endogenous PRs in nuclear extracts of T47D cells, while the basal level of transcription from a minimal TATA promoter o r transcription from an adenovirus major-late promoter remained unaffe cted. These results indicated that a transactivation function within t he LED of the interfering receptor TR beta was likely to interact with a mediator protein(s), termed coactivator, that is distinct from basa l transcription factors and is critical for efficient PR-induced trans activation, This concept was reinforced by biochemical evidence that t reatment of T47D extracts with immobilized TR beta LED depleted the ex tract of the coactivator function in a triiodothyronine-dependent mann er and markedly impaired progesterone-induced transactivation of proge sterone response element-linked genes. Deletion of six amino acids (45 1-456) in the extreme COOH terminus of TR beta resulted in a receptor that retained the ability to bind thyroid hormone but failed to inhibi t progesterone-dependent transcription, Interestingly, these six amino acids are present in a region that is highly conserved among various nuclear hormone receptors and contains a ligand-dependent transactivat ion function, AF-2. Based on these results, we propose that a limiting coactivator protein(s) interacts with the AF-2 of PR or TR and mediat es transactivation by the ligand-bound receptor. This regulatory molec ule(s) may therefore serve as a common functional link between the pat hways of hormone-inducible gene activation by various members of the n uclear receptor superfamily.