Relationship between thyroid and glucocorticoid hormone receptor occupancy, growth hormone gene transcription, and mRNA accumulation.

Abstract

The temporal relationship between the occupancy of the thyroid hormone (3,5,3'-triiodo-L-thyronine (T3)) and glucocorticoid (dexamethasone) receptors and the rate of growth hormone (GH) gene transcription and mRNA accumulation were investigated. The GC line of cultured rat pituitary tumor cells was used in these studies. The rate of GH gene transcription was measured by elongation of in vivo initiated RNA chains in isolated nuclei using radioactive precursors and hybridization of labeled transcripts to immobilized GH gene sequences. T3 receptor occupancy was directly proportional to the rate of GH gene transcription during the initial phase of hormone induction. These results suggest that a single occupied receptor is sufficient to fully activate the gene and that the unoccupied receptor resides close to the gene regulatory site. A decrease in GH gene transcription rapidly followed the initial increase in activity. This decrease occurred in the absence of new protein synthesis and was not accompanied by a proportional decrease in the level of occupied T3 receptor. An analysis of dexamethasone activation of GH gene transcription found maximum stable binding of occupied receptor to nuclei within 5 min of hormone addition, while transcription of the gene did not become fully activated until 30 to 60 min later. Transcription of the gene then declined to a rate 2 to 3 times that observed before addition of dexamethasone. These results suggest that at least one relatively slow molecular step precedes glucocorticoid hormone-receptor enhancement of gene transcription and that a second molecular event later attenuates the response. The simultaneous addition of both hormones produced two peaks of enhanced transcription, each apparently due to the separate effects of the hormones. GH mRNA levels were closely linked to the rate of transcription of the gene under all induction conditions. Comparison of the rates of decay of GH mRNA in cells cultured in the presence or the absence of hormones suggests that GH mRNA is much more stable when T3 is present. The glucocorticoid responses of the gene were consistently obtained only in the presence of T3. These results and the transcriptional synergism of the two hormones suggest a direct interaction of the occupied receptors at their regulatory sites. The transcriptional effects of glucocorticoids were observed only in about half the experiments performed in the absence of T3. When transcriptional activation did not occur, GH mRNA usually still increased 24 h after hormone treatment was begun, but not early in the induction.(ABSTRACT TRUNCATED AT 400 WORDS)