Hormonal regulation of the growth hormone gene. Relationship of the rate of transcription to the level of nuclear thyroid hormone-receptor complexes.

Using cultured GH1 cells, we reported that stimulation (3- to 5-fold) of growth hormone synthesis and mRNA levels by thyroid hormone is mediated by a chromatin-associated receptor. Thyroid hormone also elicits a rapid reduction of homologous receptor in GH1 cells primarily by decreasing the synthetic rate of receptor ( Raaka , B. M., and Samuels , H. H. (1981) J. Biol. Chem. 256, 6883-6889). Without 3,5,3'-triiodo-L-thyronine (L-T3), glucocorticoid agonists induced a limited and delayed effect while L-T3 + glucocorticoid synergistically stimulated the response an additional 2- to 4-fold compared to L-T3. In this study, we utilized GC cells, a related cell line, to compare the abundance of L-T3-receptor complexes to the rate of growth hormone mRNA synthesis and gene transcription. Gene transcription was assessed by in vitro labeling of nuclei with [alpha-32P]UTP which were derived from cells incubated with hormone(s), while mRNA synthesis was determined in intact cells by [3H]uridine labeling. Labeled growth hormone mRNA and gene transcripts were quantitated by filter hybridization to plasmid containing growth hormone cDNA. L-T3 rapidly decreased receptor levels in GC cells with kinetics similar to that in GH1 cells. Both the L-T3 and the synergistic L-T3 + glucocorticoid stimulation of growth hormone mRNA synthesis changed in parallel with the level of L-T3-receptor complexes. Glucocorticoid hormones alone elicited a variable response which resulted in minimal stimulation or inhibition of growth hormone mRNA synthesis or gene transcription rates. No apparent lag was identified between the kinetics of L-T3 binding to receptor and stimulation of growth hormone gene transcription. L-T3 stimulated growth hormone gene transcription rates maximally in 1 h which then progressively decreased in parallel with L-T3-receptor levels. Using [3H]uridine pulse-chase, growth hormone mRNA was found to have a half-life of approximately 50 h in agreement with the decay curve of growth hormone production of deinduced cells. Our studies suggest that regulation of the growth hormone response is predominantly determined by positive control of growth hormone gene transcription which is proportional to the concentration of thyroid hormone-receptor complexes.