Posttranscriptional regulation of thyrotropin β-subunit messenger ribonucleic acid by thyroid hormone in murine thyrotrope tumor cells:: A conserved mechanism across species

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