Identification of Transcripts Initiated from an Internal Promoter in the c-erbAα Locus That Encode Inhibitors of Retinoic Acid Receptor-α and Triiodothyronine Receptor Activities

The thyroid hormone receptor-coding locus, c-erbAα, generates several mRNAs originating from a single primary transcript that undergoes alternative splicing. We have identified for the first time two new transcripts, called TRΔα1 and TRΔα2[ mRNA for isoform α1 and α2 of the T3 receptor (TR), respectively], whose transcription is initiated from an internal promoter located within intron 7 of the c-erbAα gene. These two new transcripts exhibit tissue-specific patterns of expression in the mouse. These two patterns are in sharp contrast with the expression patterns of the full-length transcripts generated from the c-erbAα locus. TRΔα1 and TRΔα2 mRNAs encode N-terminally truncated isoforms of T3Rα1 and T3Rα2, respectively. The protein product of TRΔα1 antagonizes the transcriptional activation elicited by T3 and retinoic acid. This protein inhibits the ligand-induced activating functions of T3Rα1 and 9-cis-retinoic acid receptor-α but does not affect the retinoic acid-dependent activating function of retinoic acid receptor-α. We predict that these truncated proteins may work as down-regulators of transcriptional activity of nuclear hormone receptors in vivo.