NEGATIVE AND POSITIVE TRANSCRIPTIONAL REGULATION BY THYROID-HORMONE RECEPTOR ISOFORMS

Multiple forms of human thyroid hormone (T3) receptor have been identified, including true receptors that bind T3 (α1 and β) and a splicing variant (α2) that does not bind T3. The α1- and β-receptors activate transcription through interactions with positive thyroid response elements (TREs). The α2 variant is unable to activate transcription and has been reported to inhibit α1, or β stimulation of positive TREs, a property referred to as dominant negative activity. In this report we have performed studies to assess the functional properties of different members of the thyroid receptor family with regard to both positive and negative transcriptional regulation. The α1-, α2-, and β-receptors were each coexpressed in JEG-3 cells with either TreTKCAT (CAT = chloramphenicol acetyltransferase), a reporter gene that contains a positive TRE, or TSHαCAT, a negatively regulated reporter gene. The α1, and β isoforms stimulated transcription of TreTKCAT and inhibited TSHαCAT transcription in a T3-dependent manner, whereas the α2 variant was inactive. When coexpressed with at-or β-receptors, α2 inhibited regulation of positive TREs, but the effects of α2 were modest and only occurred when relatively high doses of receptor were transfected. The α2-receptor variant did not affect negative regulation by α1- or β-receptors. Thus, in both positive and negative regulation, thyroid hormone receptor isoforms that bind T3 (α1,β) are functional, whereas the α2isoform, which does not bind T3, is not functional. The inhibitory properties of α2 are dependent on total receptor dose for positive regulation and are not evident in negative regulation. These data are consistent with a mechanism of inhibition in which α2 interacts with a limiting nuclear factor(s) that is required for receptor function or transcriptional stimulation. © 1990 by The Endocrine Society.