Sequential up-regulation of thyroid hormone beta receptor, ornithine transcarbamylase, and carbamyl phosphate synthetase mRNAs in the liver of Rana catesbeiana tadpoles during spontaneous and thyroid hormone-induced metamorphosis

Abstract

During both spontaneous and thyroid hormone (TH)-induced metamorphosis, the Rana catesbeiana tadpole undergoes postembryonic developmental changes in its liver which are necessary for its transition from an ammonotelic larva to a ureotelic adult. Although this transition ultimately results from marked increases in the activities and/or de novo synthesis of the urea cycle enzymes, the precise molecular means by which TH exerts this tissue-specific response are presently unknown. Recent reports, using RNA from whole Xenopus laevis tadpole homogenates and indirect means of measuring TH receptor (TR) mRNAs, suggest a correlation between the up-regulation of TR beta-mRNAs and the general morphological changes occurring during amphibian metamorphosis. To assess whether or not this same relationship exists in a TH-responsive tissue, such as liver, we isolated and characterized a cDNA clone containing the complete nucleotide sequence for a R. catesbeiana urea cycle enzyme, ornithine transcarbamylase (OTC), as well as a genomic clone containing a portion of the hormone-binding domain of a R. catesbeiana TR beta gene. Through use of these homologous sequences and a heterologous cDNA fragment encoding rat carbamyl phosphate synthetase (CPS), we directly determined the relative levels of the TR beta, OTC, and CPS mRNAs in liver from spontaneous and TH-induced tadpoles. Our results establish that TH affects an up-regulation of mRNAs for its own receptor prior to up-regulating CPS and OTC mRNAs. Moreover, results with cultured tadpole liver demonstrate that TH, in the absence of any other hormonal influence, can affect an up-regulation of both the TR beta and OTC mRNAs.