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Thyrotropic tumors (TtT97) contain mRNA transcripts for three T3-receptor (TR) isoforms, alpha 1, beta 1 and beta 2, and a non-receptor alpha 2-variant. We administered T4 (5 mg/l of drinking water) for one month to TtT97-bearing mice, to examine its effect on tumor growth and tumor TR isoform steady-state mRNA levels. Baseline mice were killed at the start of the experiment, and placebo mice were maintained hypothyroid. The treated tumors were 30-35% smaller than the baseline tumors (p = NS), while the placebo tumors were 2- to 7-fold larger than the baseline tumors (p < 0.05). TR beta 1 mRNA increased 5- to 6-fold, while TR beta 2 mRNA decreased by 76%. TR alpha 1 and the alpha 2-variant decreased by 52% and 70%, respectively. Therefore, the tumors decreased their growth rate in response to T4 administration, and increased the ratio of TR beta 1 to TR beta 2 mRNA. This raises the intriguing possibility of a correlation between the relative abundance of the TR beta isoforms and tumor growth.

Hormonal regulation of adenohypophyseal messenger ribonucleic acids (mRNAs) encoding preprotachykinin (PPT), prolactin (PRL) and thyrotropin beta subunit (TSH beta) was examined in juvenile and pubertal female rats. Hypothyroidism, initiated on day 2 (d2) or 22 (d22) of life, increased PPT and TSH beta mRNAs but decreased PRL mRNA 17 days later. Exogenous estradiol given for 3 days reduced PPT mRNA in pubertal (d38) but not juvenile (d18) euthyroid females; conversely, estradiol increased PRL mRNA on d18 but not d38. In hypothyroid females however, estradiol decreased PPT and TSH beta mRNAs at both ages and increased PRL mRNA in pubertal but not juvenile females. Thus, regulation of adenohypophyseal mRNAs by estradiol varies with age and thyroid status. In previous studies, adenohypophyseal tachykinins increased in male, but not female rats at puberty. This sex difference was not reproduced here by neonatal androgenization of females, suggesting that it is not mediated by hypothalamic sexual differentiation. However, PRL mRNA increased in androgenized females; this increase was prevented by ovariectomy, suggesting its medication by estradiol.

Thyroid hormones suppress transcription of the gene for the beta-subunit of thyrotropin (TSH beta). Since the TSH beta gene in both the mouse and the rat contains two start sites of transcription in exon 1, we have investigated whether expression of the gene from each start site is differentially regulated by thyroid hormones in each species. RNase protection analysis was used to assay the levels of mRNA specifically transcribed from the upstream (TSS 1) and downstream (TSS 2) transcription start sites in the mouse and rat pituitary. In euthyroid and hypothyroid pituitaries there was an approximately 5-fold and 2-fold greater abundance of mRNA derived from TSS 2 than TSS 1, respectively. Hypothyroidism induced an 18- and a 9-fold increase in TSH beta gene expression from TSS 1 and TSS 2, respectively. Treatment of hypothyroid animals for 1 day with triiodothyronine (T3) reduced expression from both start sites by about 50%; after 4 days of T3 treatment, TSH beta mRNAs derived from both start sites were below detectable levels. These results were confirmed in the rat by primer extension analysis. Expression from TSS 1 in the mouse was also shown to be dependent on thyroid status using the polymerase chain reaction (PCR) technique. In contrast to previous results from primer extension studies, PCR analysis demonstrated that alternative splicing of the TSH beta RNA primary transcript can occur when transcription is initiated at the upstream start site. We conclude that, in both the mouse and the rat pituitary, expression of the TSH beta gene from both transcription start sites is regulated by thyroid hormones.

Glucocorticoid hormones suppress circulating concentrations of thyrotrophin (TSH), but their effect on synthesis of TSH in the pituitary gland is unclear. We have examined the influence of the glucocorticoid dexamethasone on serum TSH, pituitary TSH content and TSH beta- and alpha-subunit mRNA concentrations in pituitary cytoplasm in both the euthyroid and hypothyroid rat, and following triiodothyronine (T3) treatment in the hypothyroid rat. The rise in serum TSH in hypothyroidism was attenuated in animals treated with dexamethasone; in addition the suppression of serum TSH 6 h after T3 administration to hypothyroid rats was enhanced by dexamethasone. In contrast to the changes in serum TSH, pituitary TSH content was unaffected by dexamethasone. Furthermore dexamethasone had no significant effect on changes in pituitary cytoplasmic TSH beta- and alpha-subunit mRNA levels with thyroid status. These findings demonstrate that dexamethasone exerts differential effects on serum TSH levels and TSH biosynthesis which contrast with those of thyroid hormones.

The immune system's production of and response to thyrotropin (TSH) suggests the existence of a hypothalamic-lymphoid-thyroid regulatory axis (HLT). To evaluate the possible roles of lymphocyte-derived TSH we characterized its structure and regulation of production. Using an in vitro system, we screened T cell lines for the production of TSH in response to thyrotropin-releasing hormone (TRH). We identified MOLT 4 T cells as producers of intact TSH based on TSH radioimmunoassay (RIA) displacement and de novo synthesized protein structure similar to pituitary TSH. Northern blot analysis using a cDNA probe to TSH beta showed the existence of an mRNA species similar in molecular size to pituitary mRNA of TSH beta. The MOLT 4-derived TSH is induced in a dose-dependent fashion by TRH and that induction is significantly inhibited by the thyroid hormone, triiodothyronine (T3). HUT 78 T cells express an mRNA species similar to TSH beta but do not express detectable protein suggesting a lack of translation of the mRNA species. The evidence suggests that the MOLT 4 T cells serve as an in vitro model for TSH production and may be used to study differential expression of the genes for TSH in the immune system. Elucidation of this in vitro system will provide information on mechanisms of regulation of an HLT axis that are unique to the immune system as well as pathways common to the immune and neuroendocrine systems.

The effects of in vivo TRH administered either as bolus or continuous doses on TSH secretion, synthesis, and glycosylation were studied in normal and hypothyroid rats. Nine-week-old normal or 3-week postthyroidectomy rats were administered bolus doses of saline or TRH (0.5 mg/kg) twice daily or continuous saline or TRH (1 mg/kg/day) via an osmotic pump. After 5 days, pituitaries were removed and incubated with [35S]methionine (MET) and [3H]glucosamine (GLCN), with or without 10(-8) M TRH, for 6 and 24 h. Samples were precipitated with anti-TSH beta sera and then analyzed by gel electrophoresis. In normal rats, plasma TSH, T4 and T3 increased with continuous in vivo TRH but not with bolus TRH; in hypothyroid rats, plasma TSH, T4 and T3 were not altered by continuous or bolus doses of TRH. Additionally, in normal rats, continuous in vivo TRH increased incorporation of MET in secreted TSH (477 vs. 212 X 10(3) dpm/mg DNA; P less than 0.05) and intrapituitary TSH (5035 vs. 2124 X 10(3) dpm/mg DNA; P less than 0.05), and GLCN in secreted TSH (148 vs. 50 dpm/mg DNA; P less than 0.05) and intrapituitary TSH (2344 vs. 744 X 10(3) dpm/mg DNA; P less than 0.05). In contrast, in hypothyroid animals, continuous in vivo TRH did not alter MET or GLCN incorporation in TSH. Bolus TRH did not alter secreted or intrapituitary MET or GLCN incorporation into TSH in the normal rat. However, bolus TRH in the intrapituitary MET or GLCN incorporation into TSH in the normal rat.(ABSTRACT TRUNCATED AT 250 WORDS)