Thyroid hormone regulation of a transcriptional coactivator in Xenopus laevis: implication for a role in postembryonic tissue remodeling.

Thyroid hormone (TH) affects biological processes by regulating gene transcription through TH receptors (TRs). In the presence of TH, TR activates target gene transcription by recruiting one or more transcription coactivators belonging to diverse groups. Here, we demonstrate that during TH-dependent anuran metamorphosis, one such coactivator gene, the Xenopus laevis homolog of human Trip7, is up-regulated by TH. Kinetic studies suggest that Xenopus Trip7 is most likely induced indirectly by TH in a tissue-dependent manner. In the intestine, which undergoes extensive remodeling as the animal changes from being herbivorous to carnivorous, Trip7 is expressed at high levels during but not before or after metamorphosis. It is also up-regulated in other growing or remodeling tissues such as the brain and limb but not in degenerating tadpole tail skin. By using frog oocyte as a model, we show that Trip7 influences basal transcription in a chromatin structure-dependent manner but enhances the function of liganded TR regardless of the chromatin structure of the target promoter. In vitro studies indicate that Trip7 interacts directly with TR. These results suggest that during Xenopus metamorphosis, TH up-regulates, albeit indirectly, Trip7 to enhance TR function during larval-to-adult tissue transformation. Copyright 2002 Wiley-Liss, Inc.