Thyroid hormone and glucocorticoid regulation of receptor and target gene mRNAs in pituitary GH3 cells

Chronic stress from adolescence to adulthood increases adiposity and anxiety in rats with decreased expression of Krtcap3

We previously identified Keratinocyte-associated protein 3, Krtcap3, as a novel adiposity gene, but subsequently found that its impact on adiposity may depend on environmental stress. To more thoroughly understand the connection between Krtcap3, adiposity, and stress, we exposed wild-type (WT) and Krtcap3 knock-out (KO) rats to chronic stress then measured adiposity and behavioral outcomes. We found that KO rats displayed lower basal stress than WT rats under control conditions and exhibited metabolic and behavioral responses to chronic stress exposure. Specifically, stress-exposed KO rats gained more weight, consumed more food when socially isolated, and displayed more anxiety-like behaviors relative to control KO rats. Meanwhile, there were minimal differences between control and stressed WT rats. At study conclusion stress-exposed KO rats had increased corticosterone (CORT) relative to control KO rats with no differences between WT rats. In addition, KO rats, independent of prior stress exposure, had an increased CORT response to removal of their cage-mate (psychosocial stress), which was only seen in WT rats when exposed to chronic stress. Finally, we found differences in expression of the glucocorticoid receptor, Nr3c1, in the pituitary and colon between control and stress-exposed KO rats that were not present in WT rats. These data support that Krtcap3 expression affects stress response, potentially via interactions with Nr3c1, with downstream effects on adiposity and behavior. Future work is necessary to more thoroughly understand the role of Krtcap3 in the stress response.

Somatotropin response in vitro to corticosterone and triiodothyronine during chick embryonic development: Involvement of type I and type II glucocorticoid receptors

Corticosterone (CORT) can stimulate growth hormone (GH) secretion on embryonic day (e) 12 in the chicken. However, CORT failed to induce GH secretion on e20 in a single report, suggesting that regulation of GH production changes during embryonic development. Secretion in response to CORT during embryonic development is modulated by the thyroid hormones triiodothyronine (T(3)) and thyroxine (T(4)). Growth hormone responses on e12 involve both glucocorticoid (GR) and mineralocorticoid receptors (MR); however, involvement of MR has not been evaluated past e12. To further define changes in somatotroph responsiveness to CORT, pituitary cells obtained on e12-e20 were cultured with CORT alone and in combination with T(3) and GH-releasing hormone (GHRH). Growth hormone mRNA levels and protein secretion were quantified by quantitative real-time polymerase chain reaction (qRT-PCR) and radioimmunoassay (RIA), respectively. Corticosterone significantly increased GH mRNA and protein secretion on e12; however, mRNA concentration and protein secretion were unaffected on e20. Contributions of GR and MR in CORT responses were evaluated using GR and MR antagonists. Treatment with a GR-specific antagonist effectively blocked the CORT-induced increase in GH secretion on e12. The same treatment on e20 had no effect on GH secretion. These findings demonstrate that GR is directly involved in glucocorticoid stimulation of GH secretion at the time of somatotroph differentiation but is not regulatory at the end of embryonic development. We conclude that positive somatotroph responses to CORT are lost during chicken embryonic development and that GR is the primary regulator of CORT-induced GH secretion.

Differential regulation of thyroid hormone receptor-mediated function by endocrine disruptors

It is well known that endocrine disruptors (EDs) act as anti-estrogenic agents and affect the function of reproductive organ. EDs are also thought to affect thyroid hormone (TH) system which is important for biological functions such as growth, development and metabolism. However, it is still not clear how EDs are able to regulate TH receptor (TR)-mediated functions. In this study, therefore, the modulatory effects of representative EDs such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polychlorinated biphenyl (Aroclor 1254) and bisphenol A (BPA) were examined using TR-expressing GH3 cells (a rat pituitary gland epithelial tumor cell line) activated by triiodothyronine (T3). EDs tested significantly blocked T3 binding to TR in a dose-dependent manner. Biochemical characterization by Scatchard and Lineweaver-Burk plot analyses indicated that TCDD and aroclor 1254 bound to TH receptors in a competitive inhibitory manner, whereas BPA bound to TH receptors in a non-competitive pattern. The different inhibitory mode of action by EDs was also found in regulating TR-mediated production of prolactin (PRL). Aroclor 1254 exposure for 48 h enhanced T3-mediated PRL production, but BPA down-regulated. These results suggest that the EDs (TCDD, Aroclor 1254 and BPA) could differentially bind to TR and distinctly regulate the action of TR function, even though EDs are structurally similar.

Cooperative signaling between cytokine receptors and the glucocorticoid receptor in the expansion of erythroid progenitors: molecular analysis by expression profiling

Erythroid progenitors undergo renewal (proliferation without apparent differentiation) in response to erythropoietin (Epo), stem cell factor (SCF), and glucocorticoids (dexamethasone) (Dex). SCF and Dex cooperate with Epo to promote proliferation and inhibit differentiation of erythroid progenitors, while Epo alone is required to protect erythroid cells from apoptosis during terminal red cell maturation. To examine the mechanism of the synergistic interactions of Epo, SCF, and Dex, we analyzed gene expression patterns using DNA chip-based large-scale comparative gene profiling using microarrays enriched in hematopoietic transcripts or containing randomly selected genes. Differentially regulated genes were validated by real-time reverse transcription-polymerase chain reaction (RT-PCR). The results reveal cooperative regulation of gene expression by glucocorticoids and Epo/SCF on a number of genes, such as CIS, BTG1, VDUP1, CXCR4, GILZ, and RIKEN29300106B05. While Epo and SCF never showed opposite effects on gene expression, Dex either enhanced or attenuated the effect of Epo and/or SCF. Several glucocorticoid receptor (GR)-target genes were regulated by Dex only in the presence of Epo and/or SCF, suggesting that the GR functions in the context of a larger transactivation complex to regulate these genes. The data also suggest that modulation of cytokine-induced signals by the GR is an important mechanism in erythroid progenitor renewal.

Transcriptional regulation of the GnRH receptor gene by glucocorticoids

Expression of the gonadotropin-releasing hormone (GnRH) receptor gene is stimulated by dexamethasone in GnRH-deficient rodents. In this study we identify a 1226 bp sequence at the 5'-flanking region of the mouse GnRH-R gene that confers dexamethasone responsiveness when expressed in host cells. Further, a glucocorticoid antagonist blocks transcriptional activity of the mGnRHR promoter. Progressive 5'-deletion of the mGnRHR promoter localized the response sequence between the -331/-255 region. Analysis of this region revealed binding sites for the AP-1 transcription factor. Mutation in AP-1 modified the functional activity of the mGnRHR promoter following GnRH agonist or dexamethasone-stimulation. Using an electrophoretic mobility shift assay, a protein complex is shown to bind to the AP-1 site. These results suggest that AP-1 proteins and glucocorticoid receptor regulate transcription of the GnRH-R promoter in a heterologous system.

Regulation of chicken embryonic growth hormone secretion by corticosterone and triiodothyronine: evidence for a negative synergistic response

We reported that growth hormone (GH)-secreting cells differentiated by d 16 of chick embryonic development and that these somatotrophs were responsive to GH-releasing hormone and thyrotropin-releasing hormone. The present experiments evaluated effects of corticosterone and triiodothyronine (T3) on embryonic GH secretion. Anterior pituitary cells from embryonic day (e) 16, e18, and e20 were subjected to reverse hemolytic plaque assays (RHPAs) for GH in the absence or presence of corticosterone or T3. Corticosterone increased GH secretion from embryonic somatotrophs, an effect particularly evident on e16 and e18. T3 decreased GH secretion on e16, while no effect of T3 was significant on e18 or e20. Next, pituitary cells were subjected to RHPAs with T3 and corticosterone alone or in combination. Combined treatment with these hormones suppressed GH secretion from e16, e18, and e20 somatotrophs to levels below those found under basal conditions. We conclude that corticosterone can stimulate GH secretion in vitro at all embryonic ages tested. Furthermore, T3 can suppress basal GH secretion on e16, and the combination of T3 and corticosterone can suppress GH secretion at all ages. These findings indicate that GH secretion during the end of chicken embryonic development may be regulated by the interactions of endogenous glucocorticoids and thyroid hormones that increase prior to hatching.

Differential expression of thyroid hormone receptor isoforms by thyroid hormone and lithium in rat GH3 and B103 cells

BACKGROUND

The interaction between lithium, a mood stabilizer, and the thyroid axis has been extensively studied; however, the regulation of thyroid hormone receptors by lithium is yet to be investigated.

METHODS

To test whether lithium affects thyroid hormones at the receptor level, we examined the effects of lithium in combination with triiodothyronine (T3) on gene expression of thyroid hormone receptor isoforms in GH3 and B103 cells.

RESULTS

The pattern of expression as well as the magnitude of regulation of the different thyroid hormone receptor isoforms appeared to be cell line specific. Whereas T3 regulated all four isoforms in GH3 cells at both time points, T3 did not alter thyroid hormone receptor TR alpha 1 and TR alpha 2 mRNA in B103 cells. Addition of lithium to thyroid hormone-deficient GH3 cells decreased TR alpha 1, alpha 2, and beta 2 expression without affecting TR beta 1 expression at 2 but not 5 days. Addition of lithium to T3-treated GH3 cells did not further modulate gene expression of TR alpha 1, alpha 2, beta 1, or beta 2 when compared to cells treated with T3 alone. The effects of lithium in B103 cells appeared to be isoform specific as well as time dependent, since TR alpha 1 expression was selectively decreased in B103 cells, when treated with T3 in the presence of lithium.

CONCLUSIONS

The present study provides direct evidence that T3 and/or lithium regulate TR gene expression in vitro in a both time-dependent and cell line-specific manner.

Responsiveness of gill Na+/K+-ATPase to cortisol is related to gill corticosteroid receptor concentration in juvenile rainbow trout

A positive relationship between receptor concentration and tissue responsiveness is an often-assumed and rarely tested principle in endocrinology. In salmonids, seasonal changes in levels of plasma cortisol and gill corticosteroid receptors (CRs) during the spring indicate a potential role for this hormone in the parr-smolt transformation. It is not known whether these seasonal changes result in alterations in gill responsiveness to cortisol. The relationship between CR concentration and tissue responsiveness was, therefore, examined in the gills of juvenile rainbow trout (Oncorhynchus mykiss). Gill CR concentration (Bmax) and affinity (Kd) were assessed using a radioligand binding assay with the synthetic glucocorticoid triamcinolone acetonide. Gill responsiveness to cortisol was quantified by measuring in vitro Na+/K+-ATPase activity. Gill CR concentration was manipulated by stress or hormonal treatments. Repeated handling stresses resulted in a significant reduction in CR numbers. The decrease in CR Bmax corresponded to a reduction in gill responsiveness to cortisol. Triiodothyronine, but not growth hormone, treatment was found to increase CR Bmax significantly. The increase in CR numbers was correlated with a marked increase in gill responsiveness to cortisol. A significant positive linear relationship exists between the in vitro gill Na+/K+-ATPase activity response to cortisol and CR Bmax (r2=0.614, P<0.001). We have demonstrated that binding sites for cortisol in the gills of rainbow trout have high affinity, high specificity and saturable binding and that the number of binding sites is correlated with the tissue response to cortisol.

Changes in prolactin secretion in postnatal rats and effect of neonatal thyroidectomy

The prolactin (PRL) gene expression and PRL accumulation in rat pituitaries during the neonatal period were measured by means of radioimmunoassay and Northern blot analyses. The effects of neonatal thyroidectomy (Tx) on PRL synthesis and release were also investigated. The pituitary PRL mRNA level at 20 fetal days had already reached levels sufficient to detect and was increased markedly in the early neonatal period. The PRL concentrations in sera were at almost the same level during the early neonatal period, but the increases of PRL contents and PRL mRNA levels were marked in this period. Neonatal Tx effectively decreased the PRL mRNA level and PRL contents in the pituitary after 10 postnatal days. A significant decrease in the serum PRL of the Tx group was first observed at 20 postnatal days. These data indicate that thyroid hormone dependent PRL secretion is established in the neonatal period.

Regulation of growth hormone expression in fetal rat pituitary gland by thyroid or glucocorticoid hormone

The regulation of growth hormone (GH) cell development by thyroid and glucocorticoid hormones in the fetal rat pituitary gland was examined. Dexamethasone (Dex) treatment of dams induced GH and GH mRNA accumulation in the fetal pituitary gland on day 17 or 18 of gestation when substantial GH expression has not yet occurred in the control fetus. The additional thyroxine injections apparently enhanced the effect of Dex, whereas it exhibited no effect when given alone. The reduction of fetal thyroid hormone level by methimazole suppressed either the Dex induction of GH expression on day 18 or the spontaneous onset of GH expression on day 19 of gestation. The results suggest that 1) thyroid hormone exerts its stimulatory action on fetal GH gene expression only in the presence of glucocorticoid, 2) this synergistic action of these two hormones is evident as early as day 17 of gestation, and 3) rapid maturation of GH cells seen on day 19 in the normal fetus is considered to be induced by concomitant increase in both serum thyroid and glucocorticoid hormone levels.

Physiology of the steroid-thyroid hormone nuclear receptor superfamily

Glucocorticoids, other steroid hormones, thyroid hormones and vitamin-derived hormones (including retinoids) all exert their effects by the regulation of hormone-responsive target genes within the cell nucleus. These hormones bind to a series of specific nuclear receptor proteins that function as hormone-inducible transcription factors. The receptors are structurally homologous, are related to the avian erythroblastosis oncogene v-erbA, and exhibit remarkable evolutionary conservation. Together they form the steroid-thyroid hormone nuclear receptor superfamily. This chapter describes the structure and functions of the various family members and highlights the differences and similarities that occur between individual receptor proteins. Type I receptors, which include glucocorticoid receptor and other steroid receptor proteins, interact as homodimers with target sequences of DNA containing two receptor binding sites arranged as a palindrome. Type II receptors, which include receptors for retinoids, thyroid hormone and vitamin D3, bind as heterodimers (or homodimers) to DNA sequences in which two or more receptor-binding sites are arranged as a direct repeat or as other more complex configurations. The complexity of both receptor-DNA and receptor-receptor interactions predicts the potential for considerable cross-talk between various hormone-activated pathways. Thus, the specificity of hormone action and its regulation is discussed in relation to the structural and functional characteristics of the receptors and their molecular mechanisms of action. Finally, potential sites of regulation of hormone action, from circulating hormone levels in the periphery to their delivery to the cell and final site of action in the nucleus, are highlighted to provide a perspective for the following chapters in this volume and to indicate their clinical significance.

11 beta-Hydroxysteroid dehydrogenase activity and corticosteroid hormone action

In normal physiology 11 beta-hydroxysteroid dehydrogenase (11 beta-OHSD) protects the mineralocorticoid receptor (MR) from glucocorticoid excess. In the rat, however, 11 beta-OHSD mRNA and activity is widespread, suggesting that it may also play a role in regulating ligand access to the glucocorticoid receptor (GR). We have studied the role of the 11 beta-OHSD in modulating corticosteroid hormone action in rat pituitary GH3 cells (glucocorticoids inhibit prolactin gene transcription) and renal epithelial NRK-52E cells (mineralocorticoids increase Na-K ATPase subunit gene expression) in culture. Both cell lines express high levels of 11 beta-OHSD activity, and Northern/Western blot analyses using a rat cDNA probe and antisera raised against rat liver 11 beta-OHSD reveal a single 1.4 Kb mRNA encoding an enzyme of molecular size 34 kDa. In GH3 cells, prolactin gene transcription was unaffected by corticosterone (B) in doses of 10(-8) to 10(-6) M. When 11 beta-OHSD activity was inhibited with the licorice derivative, glycyrrhetinic acid (GE); however, 10(-6) M B inhibited prolactin (PRL) mRNA levels to the same degree as an equimolar concentration of the GR agonist RU 28362. This effect was blocked by co-incubation with the GR antagonist RU 38486. In NRK-52E cells, co-incubation with B and GE resulted in a marked increase in alpha 1/beta 1 Na-K ATPase subunit mRNA levels when compared with GE and/or B alone and this effect could be blocked by administration of the MR antagonist RU 26752.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue specific effects of thyroid hormone on 11 beta-hydroxysteroid dehydrogenase gene expression

11 beta-Hydroxysteroid dehydrogenase (11 beta-HSD) by converting active glucocorticoid to an inactive metabolite confers specificity upon the mineralocorticoid receptor (MR) and regulates ligand access to the glucocorticoid receptor (GR). Factors which influence 11 beta-HSD activity seem likely to be of considerable importance in the modulation of both mineralocorticoid and glucocorticoid hormone action. The administration of tri-iodothyronine (T3) to rats has previously been shown to reduce 11 beta-HSD activity in liver but not in kidney. We have studied the effect of T3 on 11 beta-HSD gene expression in vivo in rat liver, kidney, distal colon and pituitary. In addition the effects of T3 on 11 beta-HSD gene expression in vitro in the rat pituitary GH3 cell line have been studied. T3 administration to normal adult rats (40 micrograms/day, s.c. for 1, 3 and 7 days) resulted in a marked decline in liver and pituitary 11 beta-HSD mRNA levels and activity following 3 and 7 days of treatment. These reduced levels were maintained for 3 days following withdrawal of T3 treatment, but returned to control levels after 7 days. In contrast 11 beta-HSD mRNA and activity in kidney and distal colon were unaffected by T3 treatment at each time point studied. In vitro, levels of 11 beta-HSD mRNA and activity in GH3 cells were unchanged following 8, 24 and 72 h treatment with T3 (10(-8) to 10(-6) M). T3 bio-activity was confirmed by a marked dose-dependent decline in the expression of the T3 and glucocorticoid responsive gene, prolactin. T3 inhibits 11 beta-HSD gene expression in both liver and pituitary at a pre-translational level. This effect is absent in the predominantly mineralocorticoid target tissues, kidney and distal colon, i.e. it is tissue specific and as such is consistent with the existence of multiple differentially regulated isoforms of 11 beta-HSD. The time course of the T3 effect in liver and pituitary in vivo and the lack of any effect in vitro suggests that this action is indirect, and not as a result of interaction between the T3 receptor and the putative thyroid hormone response element on the rat 11 beta-HSD gene.