Thyroid hormone receptors bind to defined regions of the growth hormone and placental lactogen genes.

Abstract

The intracellular receptor for thyroid hormone is a protein found in chromatin. Since thyroid hormone stimulates transcription of the growth hormone gene through an unknown mechanism, the hypothesis that the thyroid hormone-receptor complex interacts with defined regions of this gene has been investigated in a cell-free system. Nuclear extracts from human lymphoblastoid IM-9 cells containing thyroid hormone receptors were incubated with L-3,5,3'-tri[('I]iodothyronine and calf thymus DNA-cellulose. Restriction fragments of the human growth hormone gene were added to determine their ability to inhibit labeled receptor binding to DNA-cellulose. These fragments encompassed nucleotide sequences from about three kilobase pairs upstream to about four kilobase pairs downstream from the transcription initiation site. The thyroid hormone-receptor complex bound preferentially to the 5'-flanking sequences of the growth hormone gene in a region between nucleotide coordinates -290 and -129. The receptor also bound to an analogous promoter region in the human placental lactogen gene, which has 92% nucleotide sequence homology with the growth hormone gene. These binding regions appear to be distinct from those that are recognized by the receptor for glucocorticoids, which stimulate growth hormone gene expression synergistically with thyroid hormone. The presence of thyroid hormone was required for binding of its receptor to the growth hormone gene promoter, suggesting that thyroid hormone renders the receptor capable of recognizing specific gene regions. L-3,5,3'-Triiodothyronine (T3), the active form of thyroid hormone, modulates the expression of a number of genes (1, 2), and this is ascribed to the association of T3 with receptors localized in chromatin (3). In rat pituitary tumor cells the concentration of growth hormone (GH) mRNA increases in response to physiological concentrations ofT3 (refs. 4 and 5).Although additional mechanisms have not been ruled out, this response reflects an increased rate of transcription of the GH gene (6, 7), presumably through a direct effect of the receptor (8). Moreover, T3 acts synergistically with glucocorticoid hormones in stimulating GH gene transcription (6, 9). Binding regions for the rat (10, 11) and human (12) glucocorticoid receptors have been identified in the human GH gene upstream and downstream from the transcription initiation site (cap site). Homologous binding regions have also been detected (12) in the human placental lactogen (chorionic somatomammotropin, CS) gene, which has a 92% overall nucleotide sequence homology with the human GH gene (13). In contrast, the question of specific recognition sites for thyroid hormone receptors on any DNA has remained open. We have now searched for such sites in the human GH and CS genes (12) using the human thyroid hormone receptor in a cell-free system. This system contained [1251]T3-labeled nuclear extracts from cultured human lymphoblastoid cells of the IM-9 line and cloned fragments from the genes of interest. MATERIALS AND M...

Growth Hormone Gene Regulation by <i>trans</i><i>-</i>Acting Factors

“…Although T 3 reduced basal GH secretion by human fetal pituitary' cells [6]. Tj had no effect on GH mRNA in cultured GH-secreting human pituitary adenoma cells [35], despite the fact that the T3 receptor binds to the human GH gene promoter [41], However, by testing different portions of the human GH gene in transfected cells, a promoter-mediated inhibition was seen, while the structural part of the gene mediated a probable mRNA stabilization, these opposite effects of T 3 cancelling each other when the entire gene with its pro moter was used [42], As shown in figure 1, the T3 receptor might inhibit promoter activity by interference with Spl or distal GHF-1 binding [42],…”

Characterization of thyroid hormone receptors in human IM-9 lymphocytes

“…Identification of a function for the IM-9 T3 receptor is beyond the scope of this study. However, the recently des¬ cribed interaction of these binding sites with the promotor region of T3 responsive genes (Barlow et al 1986) suggests that they have a role in the regulation of these genes.…”

Evolutionary implications of two rainbow trout growth hormone genes

“…The 5' region of the GH gene contains a consensus promoter sequence (5'-TATAAAAA-3'), 22 nucleotides from the putative transcription start site. Analysis of the 5' sequence, up to 332 nucleotides upstream from the cap site, did not reveal the existence of any sequeence similar to the DNA binding sites of the glucocorticoid and thyroid hormone-receptor complexes found in mammalian GH gene (Moore et al 1985;Barlow et al 1986). However, it is not clear whether thyroid and glucocorticoid hormones control the expression of rainbow trout GH gene in a manner similar to that in mammals.…”