The N-terminal region (A/B) of rat thyroid hormone receptors alpha 1, beta 1, but not beta 2 contains a strong thyroid hormone-dependent transactivation function.

In this study we have investigated the role of the N-terminal region of thyroid hormone receptors (TRs) in thyroid hormone (TH)-dependent transactivation of a thymidine kinase promoter containing TH response elements composed either of a direct repeat or an inverted palindrome. Comparison of rat TR beta 1 with TR beta 2 provides an excellent model since they share identical sequences except for their N termini. Our results show that TR beta 2 is an inefficient TH-dependent transcriptional activator. The degree of transactivation corresponds to that observed for the mutant TR delta N beta 1/2, which contains only those sequences common to TR beta 1 and TR beta 2. Thus, TH-dependent activation appears to be associated with two separate domains. The more important region, however, is embedded in the N-terminal domain. Furthermore, the transactivating property of TR alpha 1 was also localized to the N-terminal domain between amino acids 19 and 30. Using a coimmunoprecipitation assay, we show that the differential interaction of the N terminus of TR beta 1 and TR beta 2 with transcription factor IIB correlates with the TR beta 1 activation function. Hence, our results underscore the importance of the N-terminal region of TRs in TH-dependent transactivation and suggest that a transactivating signal is transmitted to the general transcriptional machinery via a direct interaction of the receptor N-terminal region with transcription factor IIB.

Estrogen and thyroid hormone interaction on regulation of gene expression.

Estrogen receptor (ER) and thyroid hormone receptors (TRs) are ligand-dependent nuclear transcription factors that can bind to an identical half-site, AGGTCA, of their cognate hormone response elements. By in vitro transfection analysis in CV-1 cells, we show that estrogen induction of chloramphenicol acetyltransferase (CAT) activity in a construct containing a CAT reporter gene under the control of a minimal thymidine kinase (tk) promoter and a copy of the consensus ER response element was attenuated by cotransfection of TR alpha 1 plus triiodothyronine treatment. This inhibitory effect of TR was ligand-dependent and isoform-specific. Neither TR beta 1 nor TR beta 2 cotransfection inhibited estrogen-induced CAT activity, although both TR alpha and TR beta can bind to a consensus ER response element. Furthermore, cotransfection of a mutated TR alpha 1 that lacks binding to the AGGTCA sequence also inhibited the estrogen effect. Thus, the repression of estrogen action by liganded TR alpha 1 may involve protein-protein interactions although competition of ER and TR at the DNA level cannot be excluded. A similar inhibitory effect of liganded TR alpha 1 on estrogen induction of CAT activity was observed in a construct containing the preproenkephalin (PPE) promoter. A study in hypophysectomized female rats demonstrated that the estrogen-induced increase in PPE mRNA levels in the ventromedial hypothalamus was diminished by coadministration of triiodothyronine. These results suggest that ER and TR may interact to modulate estrogen-sensitive gene expression, such as for PPE, in the hypothalamus.