Contrasting patterns of expression of thyroid hormone and retinoid X receptor genes during hormonal manipulation of Xenopus tadpole tail regression in culture.

The precocious induction of amphibian metamorphosis is an ideal system for analyzing the developmental action of TH, while the hormonal activation of tadpole tail regression offers the further advantage of studying programmed cell death. One of the striking features of thyroid hormone (TH)-induced tail regression (as with morphogenetic responses of all tadpole tissues) in Xenopus is the rapid autoinduction of TRbeta gene, but it is not known how TH would affect the expression of the genes encoding TR's heterodimeric partner, retinoid X receptor (RXR). Here we first show that the synthetic glucocorticoid dexamethasone (Dex) potentiates and prolactin (PRL) suppresses, 3,3',5-triiodothyronine (T3)-induced regression of pre-metamorphic Xenopus tadpole tails in organ culture. T3 strongly upregulated (11-35-fold) the concentration of Xenopus TRbeta (xTRbeta) mRNA in these cultures while downregulating by 50% that of Xenopus RXRgamma (xRXRgamma) mRNA in the same samples of tail RNA. DEX and PRL enhanced or diminished the T3-regulated expression of these two transcripts, respectively, which parallels their other effects in whole tadpoles or cultured tails. The contrasting effects of the three hormones on the steady-state levels of xTRbeta and XRXRgamma mRNAs were time- and dose-dependent. T3 and DEX also strongly upregulated the transcription of xTRbeta gene transfected into Xenopus XTC-2 cells but PRL failed to prevent this autoinduction. The actions of these three hormones involved in amphibian metamorphosis, as judged by the expression of xTRbeta and xRXRgamma genes, reveal a new facet of hormonal interplay underlying their developmental actions.