Dominant-negative mutant thyroid hormone receptors prevent transcription from Xenopus thyroid hormone receptor beta gene promoter in response to thyroid hormone in Xenopus tadpoles in vivo

We describe a dominant-negative approach in vivo to assess the strong, early upregulation of thyroid hormone receptor beta (TR beta) gene in response to thyroid hormone, characteristic of the onset of natural and thyroid hormone-induced amphibian metamorphosis, 3,3',5-Triiodothyronine (T-3) treatment of organ cultures of premetamorphic Xenopus tadpole tails coinjected in vivo with the wild-type Xenopus TR beta (wt-xTR beta) and three different thyroid responsive element chloramphenicol acetyltransferase (TRE-CAT) reporter constructs, including a direct repeat +4 (DR +4) element in the -200/+87 fragment of the xTR beta promoter, resulted in a 4- to 8-fold enhancement of CAT activity, Two human C-terminal TR beta 1 mutants (Delta-hTR beta 1 and fs-hTR beta 1), an artificial Xenopus C-terminal deletion mutant (mt-xTR beta), and the oncogenic viral homolog v-erbA none of which binds T-3, inhibited this T-3 response of the endogenous wt-xTR in Xenopus XTC-2 cells cotransfected with the -1600/+87 xTR beta promoter-CAT construct, the potency of the dominant-negative effect of these mutant TRs being a function of the strength of their heterodimerization with Xenopus retinoid X receptor gamma, Coinjection of the dominant-negative Xenopus and human mutant TR beta s into Xenopus tadpole tails totally abolished the T-3 responsiveness of the wt-xTR beta with different TREs, including the natural DR +4 TRE of the xTR beta promoter