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 a nd thyroid hormone-induced amphibian metamorphosis, 3,3',5-Triiodothyr onine (T-3) treatment of organ cultures of premetamorphic Xenopus tadp ole tails coinjected in vivo with the wild-type Xenopus TR beta (wt-xT R beta) and three different thyroid responsive element chloramphenicol acetyltransferase (TRE-CAT) reporter constructs, including a direct r epeat +4 (DR +4) element in the -200/+87 fragment of the xTR beta prom oter, resulted in a 4- to 8-fold enhancement of CAT activity, Two huma n C-terminal TR beta 1 mutants (Delta-hTR beta 1 and fs-hTR beta 1), a n artificial Xenopus C-terminal deletion mutant (mt-xTR beta), and the oncogenic viral homolog v-erbA none of which binds T-3, inhibited thi s T-3 response of the endogenous wt-xTR in Xenopus XTC-2 cells cotrans fected 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 re ceptor gamma, Coinjection of the dominant-negative Xenopus and human m utant TR beta s into Xenopus tadpole tails totally abolished the T-3 r esponsiveness of the wt-xTR beta with different TREs, including the na tural DR +4 TRE of the xTR beta promoter.