Hormone-dependent and -independent transcriptional activation by thyroid hormone receptors are mediated by different mechanisms

Transcriptional activation by thyroid hormone (T-3) receptor (T(3)R) generally requires the binding of its high affinity ligand. However, we reported previously that chicken T(3)R alpha (cT(3)Ra) and human T(3)R beta 1 (hT(3)R beta 1) could activate transcription from several promoters containing T(3)R response elements (TREs) in a hormone-independent fashion when expressed in rat anterior pituitary GH(4)C1 cells. In this study we show that rat T(3)R alpha 1 also activates transcription without T-3 in GH4C1 cells and that the oncoprotein v-erbA that is derived from cT(3)Ra but does not bind T-3 is not a constitutive activator in these cells. Increased expression of T(3)R results in transcriptional activation of both native and minimal promoters, and this activation does not appear to require a defined TRE in the promoter. Because hormone-independent activity is not observed in several other cell lines, this activity may involve specific factors present in GH(4)C1 cells. Three mutants with single amino acid changes in a 20-amino acid region of the ligand-binding domain of cT(3)R alpha do not mediate hormone-independent activity. This region is highly conserved within the nuclear receptor family and has been implicated in interactions with other proteins, suggesting participation of other transcription or accessory factors in the hormone-independent activity of T(3)R. Two of these mutants mediate hormone-dependent transcriptional activation similar to wild-type cT(3)R alpha. All three mutants interact in vitro with retinoid X receptor beta similar to wild-type cT(3)R alpha. Our findings suggest that hormone-dependent and hormone-independent transactivation proceed by distinct mechanisms.