Negative and Positive Transcriptional Regulation by Thyroid Hormone Receptor Isoforms

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Abstract
Multiple forms of human thyroid hormone (T3) receptor have been identified, including true receptors that bind T3 (.alpha.1 and .beta.) and a splicing variant (.alpha.2) that does not bind T3. The .alpha.1- and .beta.-receptors activate transcription through interactions with positive thyroid response elements (TREs). The .alpha.2 variant is unable to activate transcription and has been reported to inhibit .alpha.1 or .beta. 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 .alpha.1-, .alpha.2-, and .beta.-receptors were each coexpressed in JEG-3 cells with either TreTKCAT (CAT = chloramphenicol acetyltransferase), a reporter gene that contains a positive TRE, or TSH.alpha.CAT, a negatively regulated reporter gene. The .alpha.1 and .beta. isoforms stimulated transcription in a T3-dependent manner, whereas the .alpha.2 variant was inactive. When coexpressed with .alpha.1- or .beta.-receptors, .alpha.2 inhibited regulation of positive TREs, but the effects of .alpha.2 were modest and only occurred when relatively high doses of receptor were transfected. The .alpha.2-receptor variant did not affect negative regulation by .alpha.1- or .beta.-receptors. Thus, in both positive and negative regulation, thyroid hormone receptor isoforms that bind T3 (.alpha.1,.beta.) are functional, whereas the .alpha.2 isoform, which does not bind T3, is not functional. The inhibitory properties of .alpha.2 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 .alpha.2 interacts with a limiting nuclear factor(s) that is required for receptor function or transcriptional stimulation.