Thyroid hormone receptor mediates transcriptional activation and repression of different promoters in vitro

The thyroid hormone receptor (TR) has the dual ability to activate or repress transcription of specific genes. A cell-free transcription system was used to study the effects of TR on transcription by positively (TREpMLP) and negatively (TSH alpha) regulated promoters. Receptor-deficient HeLa cell extracts were complemented with baculovirus-produced TR. TR stimulated transcription from the TREpMLP promoter by 3-fold, and trans-activation did not require hormone. Transcriptional stimulation by TR required the presence of the TRE sequence and was diminished by the addition of competitor TRE binding sites. Baculovirus-produced TR repressed transcription in vitro from the TSH alpha promoter by 30-50%, also in a hormone-independent manner. Transcription from a control adenovirus 2 major late promoter was unaffected by added TR. Receptor-specific antisera and competition with TRE binding sites impaired TR-mediated repression of the TSH alpha promoter. Unlike transcriptional stimulation, which was optimal when TR and HeLa extracts were added concomitantly, transcriptional repression by the TR was most effective when the receptor was preincubated with the alpha-promoter, suggesting that receptor binding to the promoter may block access of other proteins to cause transcriptional repression. These results indicate that baculovirus-expressed TR mediates transcriptional activation and repression in a promoter-specific manner in vitro. This system provides a valuable model for examining transcriptional control by the TR.