Thyroid Hormone-induced Changes In Gene Expression In The Bullfrog, Rana Catesbeiana, Tadpole Liver

During spontaneous and 3,5,3{dollar}\\sp\\prime{dollar}-triiodothyronine (T{dollar}\\sb3{dollar})-induced metamorphosis, the Rana catesbeiana tadpole undergoes an obligatory transition from ammonotelism to ureotelism. Although this ultimately results from marked increases in the activities and/or de novo synthesis of urea cycle enzymes in the liver, the precise molecular means governing this tissue-specific response are unknown. T{dollar}\\sb3{dollar} acts primarily on gene transcription by interacting with specific nuclear receptors (TRs). Recent reports suggest a strong correlation between TR{dollar}\\beta{dollar} mRNA up-regulation and gross metamorphic changes. To address whether or not this same relationship exists within a T{dollar}\\sb3{dollar}-responsive tissue, cDNA sequences encoding three Rana catesbeiana urea cycle enzymes, carbamyl phosphate synthetase I (CPS I), ornithine transcarbamylase (OTC) and arginase, and a partial TR{dollar}\\beta{dollar} genomic clone were isolated and characterized. Hybridization and RT-PCR analyses of RNA from control and T{dollar}\\sb3{dollar}-treated tadpole liver, using these sequences and a Rana catesbeiana TR{dollar}\\alpha{dollar} cDNA probe, reveal that T{dollar}\\sb3{dollar} induces an accumulation of mRNAs for its own receptors and arginase prior to up-regulating mRNAs encoding CPS I and OTC.;The onset of T{dollar}\\sb3{dollar}-induced metamorphosis is inhibited by low temperature. T{dollar}\\sb3{dollar}-injected tadpoles maintained at 5{dollar}\\sp\\circ{dollar}C do not show overt morphological changes nor does their liver accumulate CPS I, OTC, or TR{dollar}\\alpha{dollar} mRNAs, whereas, in the same time period, TR{dollar}\\beta{dollar} and arginase mRNAs increase. Shifting T{dollar}\\sb3{dollar}-treated tadpoles from 5{dollar}\\sp\\circ{dollar}C to 25{dollar}\\sp\\circ{dollar}C results in an accelerated metamorphosis and an up-regulation of liver mRNAs similar to that seen in tadpoles maintained at 25{dollar}\\sp\\circ{dollar}C. These results suggest that the mechanism(s) regulating the T{dollar}\\sb3{dollar}-induced expression of the TR{dollar}\\alpha{dollar}, TR{dollar}\\beta{dollar} and arginase genes differs from CPS I and OTC genes.;Experimentally shifting tadpoles from 5{dollar}\\sp\\circ{dollar}C to 25{dollar}\\sp\\circ{dollar}C imposes a thermal stress (heat shock). To assess the effects of this thermal shift, a hsp30 gene from Rana catesbeiana was isolated and characterized. Dot-blot hybridization and RT-PCR analyses using the aforementioned sequences disclose that: (1) heat shock does not affect and/or can stabilize T{dollar}\\sb3{dollar}-induced CPS I mRNA and protein levels and, (2) hsp30 and hsp30-like genes are differentially-expressed and influenced by T{dollar}\\sb3{dollar} during heat shock in various tissues and may play a role during amphibian metamorphosis.