Posttranscriptional Regulation of Thyrotropin β-Subunit Messenger Ribonucleic Acid by Thyroid Hormone in Murine Thyrotrope Tumor Cells: A Conserved Mechanism across Species1

Thyroid hormone (T3) negatively regulates TSH beta-subunit (TSHbeta) messenger RNA (mRNA) gene expression in whole rat pituitary, in part at the level of mRNA stability. However, the regulation of TSHbeta mRNA turnover by T3 in pure populations of thyrotropes and in other species is unknown. To further investigate this, we used murine thyrotropic TtT97 tumor cells. Using primary cultures of TtT97 cells, T3 down-regulated TSHbeta mRNA to approximately 35% of the control level by 8 h. Actinomycin D chase revealed that T3 destabilized TSHbeta mRNA, reducing the half-life from approximately 24 to 7 h, and was accompanied by a decrease in TSHbeta mRNA size. Ribonuclease H analysis revealed that this T3-induced decrease in size was due to a shortening of poly(A) tail from approximately 160 to approximately 30 nucleotides and was specific for TSHbeta mRNA. Cycloheximide mimicked the poly(A) tail effect observed with T3. In the absence of T3, actinomycin D deadenylated TSHbeta mRNA without inducing rapid decay. We conclude that T3 reduces the steady state half-life of TSHbeta mRNA in murine TtT97 thyrotropic tumor cells accompanied by a reduction in poly(A) tail length. However, in the absence of T3, deadenylation alone is not sufficient to induce TSHbeta mRNA decay. Together with the high degree of sequence conservation in the 3'-untranslated region of murine and rat TSHbeta mRNA sequences and the similarities of the T3 effect, these data provide the first evidence for a highly conserved posttranscriptional mechanism operative across species. We propose a model in which T3 coordinately regulates shortening of the poly(A) tail and the activity of a transacting RNA-binding protein and/or an exonuclease to accelerate TSHbeta mRNA turnover.