Glucocorticoids antagonize induction of prolactin-gene expression by calcitriol in rat pituitary tumour cells

Culture requirements for optimal expression of 1 alpha,25-dihydroxyvitamin D3-enhanced thyrotropin secretion

An effect of the hormone, 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3] on hormone secretion by normal rat pituitary cells was investigated in vitro. Based on previous findings using GH4C1 cells, dispersed anterior pituitary cell cultures were prepared and maintained in serum-free conditions for up to 6 d. Under these circumstances, there was no effect of 1,25(OH)2D3 to alter medium or cell-associated levels of thyrotropin (TSH), prolactin (PRL), or growth hormone (GH). Cultures maintained under these conditions had lower medium and cell-associated hormone levels and lesser responses to agonists than cultures maintained in serum-supplemented medium. In the presence of 10% charcoal-treated fetal bovine serum, treatment with 10(-8) M 1,25(OH)2D3 for 24 h selectively increased TRH (10(-10) to 10(-7) M)-induced TSH secretion (P less than 0.001), with maximal enhancement observed at 10(-9) M TSH-releasing hormone (TRH). Enhancement of TSH secretion by 1,25(OH)2D3 was detected after 15 min exposure to TRH. There was no effect on agonist-induced PRL or GH secretion or on cell-associated hormone levels. The effect was evident after 24 h treatment with 1,25(OH)2D3, and decreased thereafter. Several other steroid hormones had no effect on 10(-9) M TRH-induced TSH secretion. These data contrast with the effect of 1,25(OH)2D3 in GH cells. They suggest that 1,25(OH)2D3 may act selectively in the normal pituitary to modulate TSH secretion.

Steroid inhibition of calcitriol-induced prolactin production in GH4C1 cells. Specificity and sensitivity

The induction of prolactin (PRL)-gene expression by calcitriol (1,25-dihydroxyvitamin D3, 1,25-dihydroxycholecalciferol) in clonal rat pituitary tumour (GH4C1) cells was selectively inhibited by cortisol [IC50 (concentration causing 50% inhibition) = 3.2-4.1 nM]. The steroid specificity of this effect was investigated and various steroids were found to inhibit calcitriol-stimulated PRL production with the following relative potencies: cortisol, 1; dexamethasone, 8; 11-deoxycortisol, 0.5; corticosterone, 0.4; aldosterone, 0.07; testosterone and oestradiol, less than 0.003. The steroid antagonist RU 38486 did not affect basal or calcitriol-stimulated PRL production, but antagonized the effect of 10 nM-cortisol in a concentration-dependent manner. Neither progesterone nor 11-deoxycortisol antagonized the effect of 10 nM-cortisol. Calcitriol-induced PRL production was 14 times more sensitive to dexamethasone inhibition than was non-stimulated PRL production. Growth-hormone production was stimulated by dexamethasone, in the presence or absence of calcitriol, with a concentration-dependence similar to that of dexamethasone inhibition of basal PRL production. These data indicate that steroid inhibition of calcitriol-stimulated PRL production is a specific glucocorticoid effect. The sensitivity of calcitriol-stimulated PRL production to dexamethasone was 14-26-fold greater than that of other measured responses in the same cells. Two of the possible explanations for this selectively increased sensitivity to glucocorticoids are: amplification of the glucocorticoid effect via an induced mediator; and the presence of very-high-affinity glucocorticoid-receptor-binding sites on DNA.

Approach to the molecular mechanisms of the modulation of growth hormone gene expression by glucocorticoid and thyroid hormones

Glucocorticoid and thyroid hormones modulate the expression of the growth hormone gene. To investigate this control mechanism, we have determined whether this gene contains sites that bind the human glucocorticoid and thyroid hormone receptors in vitro. To do so, we have designed a novel assay for studying binding of the purified glucocorticoid receptor to cloned fragments of the human growth hormone (hGH) gene, and have adapted a DNA-competition assay for the thyroid receptor in nuclear cell extracts. Two glucocorticoid receptor binding regions were found in the hGH gene, one of high affinity in a fragment of the gene containing the first intron, and one of low affinity located within a 290 bp-fragment of 5'-flanking DNA. In contrast, the thyroid receptor bound with high affinity to the 5'-flanking fragment. Homologous binding regions for the two types of receptor were found in the human placental lactogen (chronic somatomammotropin) gene. DNA binding of the two receptor types appeared to depend on the presence of the hormone, yet antagonist-bound glucocorticoid receptor was still capable of interacting specifically with DNA. There was no evidence for synergism or antagonism of the two receptor types in binding to their respective sites on the hGH gene. The data also make it unlikely that the thyroid receptor negatively controls gene transcription and that the stimulatory effect of thyroid hormone results from a derepression mechanism.