HORMONE-DEPENDENT AND HORMONE-INDEPENDENT TRANSCRIPTIONAL ACTIVATION BY THYROID-HORMONE RECEPTORS ARE MEDIATED BY DIFFERENT MECHANISMS

Transcriptional activation by thyroid hormone (T-3) receptor (T(3)R) g enerally 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 pro moters containing T(3)R response elements (TREs) in a hormone-independ ent 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 transcriptio n without T-3 in GH4C1 cells and that the oncoprotein v-erbA that is d erived from cT(3)Ra but does not bind T-3 is not a constitutive activa tor in these cells. Increased expression of T(3)R results in transcrip tional activation of both native and minimal promoters, and this activ ation does not appear to require a defined TRE in the promoter. Becaus e hormone-independent activity is not observed in several other cell l ines, this activity may involve specific factors present in GH(4)C1 ce lls. Three mutants with single amino acid changes in a 20-amino acid r egion of the ligand-binding domain of cT(3)R alpha do not mediate horm one-independent activity. This region is highly conserved within the n uclear receptor family and has been implicated in interactions with ot her proteins, suggesting participation of other transcription or acces sory factors in the hormone-independent activity of T(3)R. Two of thes e mutants mediate hormone-dependent transcriptional activation similar to wild-type cT(3)R alpha. All three mutants interact in vitro with r etinoid X receptor beta similar to wild-type cT(3)R alpha. Our finding s suggest that hormone-dependent and hormone-independent transactivati on proceed by distinct mechanisms.