Posttranscriptional regulation of rat growth hormone gene expression: increased message stability and nuclear polyadenylation accompany thyroid hormone depletion.

In thyroid hormone-depleted rats, the rate of transcription of the growth hormone (GH) gene in the anterior pituitary gland is lower than the rate in euthyroid controls, and there is a corresponding reduction in the abundance of the GH mRNA. Concomitantly, the poly(A) tail of the GH mRNA increases in length. Examination of nuclear RNA from anterior pituitary glands of control and thyroid hormone-depleted rats revealed no difference in the length of pre-mRNAs containing the first and last introns of the GH gene. However, mature nuclear GH RNA is differentially polyadenylated in euthyroid and hypothyroid animals. We suggest that the extent of polyadenylation of the GH transcript is regulated in the cell nucleus concomitant with or subsequent to the splicing of the pre-mRNA. Experiments with anterior pituitary gland explant cultures demonstrated that the GH mRNA from thyroid hormone-depleted rats is more stable than its euthyroid counterpart and that the poly(A) tail may contribute to the differential stability of free GH ribonucleoproteins.