Differential responses to ligands of overexpressed thyroid hormone and retinoid X receptors in a Xenopus cell line and in vivo.

In an attempt to explain the contrasting patterns of expression of Xenopus thyroid hormone (xTR) and retinoid X (xRXR) receptor genes and to extend our understanding of the role of heterodimerization of these receptors during amphibian metamorphosis, we have investigated the response to their respective ligands of cells in which xTR and xRXR were overexpressed. Results obtained with two separate approaches are now described. In the first, 3,3'5-triiodothyronine (t3) was found to strongly upregulate xTR beta mRNA in XTC-2 cells, but not of xTR alpha or xRXR alpha mRNAs, while xRXR gamma transcripts could not be detected. 9-cis-retinoic acid (9-cis-RA) did not substantially influence the expression of any of these four receptor genes. When transcription from three different thyroid response elements (TREs) (a palindromic TREpal, an inverted repeat +6 [F2] and a direct repeat +4[DR+4] as present in the promoter of xTR beta gene) was measured in XTC-2 cells in which xTR beta and xRXR alpha were overexpressed, only T3 upregulated transcription while 9-cis-RA, alone or together with T3, was ineffective. 9-cis-RA however enhanced transcription from an RXR responsive element (RXR-RE). THe second approach involved overexpression of xTR beta and xRXR alpha in premetamorphic Xenopus tadpole tail muscle followed by measuring the response of the tails to T3 in organ culture. After validating the microinjection/culture procedure histochemically, we found that T3 enhanced transcription from the xTR beta DR +4 TRE in tails in which xTR beta was overexpressed but the overexpression of xRXR alpha failed to modify this response. It is concluded that in both XTC cells and tadpole tails, overexpressed xRXR fails to modify the enhanced transcriptional response of endogenous and overexpressed xTR beta to T3 and that exogenous 9-cis-RA is ineffective.