THYROID-HORMONE DECREASES THE STABILITY AND THE POLY(A) TRACT LENGTH OF RAT THYROTROPIN BETA-SUBUNIT MESSENGER-RNA

Thyroid hormone (T3 and T4) down-regulation of TSH subunit steady state mRNA levels and subunit gene transcription in vitro and in vivo has been well studied. We present evidence here that T3 can also regulate the turnover of TSH subunit mRNA. The apparent half-life of the TSH beta-subunit mRNA was determined by adding actinomycin-D (2-mu-M) to dispersed rat pituitary cultures in hypothyroid medium or medium containing 10(-7) M T3 and analyzing the decline in subunit mRNA levels with time. The half-life of the TSH-beta mRNA from those cultures treated with T3 was shorter than that of the control cultures (9 vs. > 24 h, respectively). A possible mechanism by which TSH beta-subunit mRNA stability is altered is through a change in the size of each mRNA's poly(A) tail. Northern blot analysis of total RNA from the above cultures revealed that T3 treatment reduces the size of the TSH beta-subunit mRNA. To determine if this alteration of mRNA size was due to a loss of a portion of the poly(A) tract and not to alternative splicing of the transcript or use of a secondary transcriptional start site, pooled RNAs were hybridized with oligo(dT) and subsequently digested with RNAse-H to remove the poly(A) tract. RNA blot analysis of these RNAs showed that T3 treatment results in the loss of most of the TSH-beta poly(A) tail. After 8 h in culture, the T3-induced shortening of the poly(A) tail is specific to the TSH beta-subunit, as we saw no such effect on the similarly sized mRNAs (alpha-subunit, LH-beta, or cyclophilin-A). The effect of T3 to shorten the TSH-beta mRNA can be mimicked by cycloheximide (20-mu-g/ml) or actinomycin-D, indicating that a rapidly turning-over protein may be involved in maintaining the size and possibly the stability of the TSH-beta mRNA under euthyroid and hypothyroid conditions. We have also observed that this T3-induced shortening of the TSH-beta mRNA can be produced in vivo within 4 h after a single dose of T3 has been administered to euthyroid animals (20-mu-g/100 g BW, ip). Thus, T3 reduces the apparent half-life of the TSH-beta mRNA and causes a concomitant decrease in its poly(A) tail size. Although a causal relationship has not been established, it is possible that shortening of the poly(A) tract may lead to decreased TSH-beta mRNA stability.