Steroid hormones generally mediate their effects by interacting with specific receptors which then bind to defined DNA sequences in the regulatory regions of target genes to activate expression (see Gronmeyer (1992) and references therein). However, the post-transcriptional regulation of gene expression by steroid hormones is also well documented (see Nielsen & Shapiro 1990). Indeed steroid hormones were amongst the first agents to be demonstrated to play a role in mRNA stabilization (Palmiter & Carey 1974). For example, glucocorticoid hormones have been shown to enhance the stability of growth hormone mRNA (Paek & Axel 1987), testosterone has been reported to induce changes in the poly(A) tail length of the mRNA encoding cystatin-related protein (Vercaeren et al. 1992) and testosterone and/or oestrogen induce changes in the poly(A) tail length of the vasopressin mRNA (Carter & Murphy 1993) associated with changes in mRNA accumulation. However, it is still unclear how steroids mediate these
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- Abstract: Estrogen x
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- Abstract: glucocorticoids x
Luc J Martin and Jacques J Tremblay
It is well established that stress, either physical or psychosocial, causes a decrease in testosterone production by Leydig cells. Glucocorticoids (Gc) are the main mediators of stress response and they convey their repressive effect on Leydig cells through the glucocorticoid receptor (GR). So far, various mechanisms have been proposed to explain the mechanism of action of Gc on Leydig cell steroidogenesis including repression of genes involved in testosterone biosynthesis. Several steroidogenic genes, including steroidogenic acute regulatory (STAR) protein, have been shown to be repressed by Gc in a GR-dependent manner but the underlying mechanisms remain to be fully elucidated. Here, we found that dexamethasone (Dex), a potent synthetic Gc, partly antagonizes the cAMP-dependent stimulation of the mouse Star promoter in MA-10 Leydig cells as revealed by transient transfection assays. This repression requires an element located at −95 bp previously implicated in the activation of the Star promoter by the nuclear receptors, NR4A1 and NR5A1. Dex was found to inhibit NR4A1-dependent transactivation of the Star promoter in Leydig cells by decreasing NR4A1, but not NR5A1, recruitment to the proximal Star promoter as determined by chromatin immunoprecipitation assay. Western blots revealed that Dex did not affect NR4A1 or NR5A1 expression in response to cAMP. These data suggest that NR4A1 would be associated with the GR in a transcriptionally inactive complex as previously demonstrated in pituitary corticotrope cells. Thus, our data provide new molecular insights into the stress-mediated suppression of testosterone production in testicular Leydig cells.
R. M. Haigh, C. T. Jones and G. Milligan
Glucocorticoids are known to influence cardiovascular sensitivity to catecholamines but the molecular mechanisms are undefined. We recently showed that glucocorticoids control the coupling of adrenergic receptors to G protein. Alterations in the amount of G protein is one mechanism by which receptor-G protein coupling may be controlled. Therefore, we set out to measure the levels of G proteins in aorta from normal, adrenalectomized and dexamethasonetreated adrenalectomized rats. G proteins were measured in plasma membrane preparations by immunoblotting and horseradish peroxidase staining. After adrenalectomy there was a 53% (n = 5) decrease in the density of staining for Gi (ANOVA; P<0.05 compared to controls). Conversely, there was a 210% (n = 5) increase in the density of staining for Gs. The levels of Go and the β-subunit of G proteins were not changed by adrenalectomy. Dexamethasone-replacement treatment after adrenalectomy returned Gi and Gs close to control values. Go remained unaltered compared to controls but was 24% (n=3) less than the adrenalectomized values (ANOVA; P<0.05). The levels of β-subunit after dexamethasone replacement were significantly greater (ANOVA; P<0.05) than both the controls and adrenalectomized values. These results show that glucocorticoids can differentially regulate the amounts of G proteins in rat aorta as in other tissues. This may be an important mechanism by which steroids control receptor-G protein coupling and hence transmembrane signalling pathways in vascular smooth muscle.
Junhui Zhang, Francis T F Tsai and David S Geller
Steroid hormone receptor antagonists are widely used in clinical medicine, but their use is often complicated by the lack of receptor specificity to presently available drugs. We previously demonstrated an important role of a widely conserved helix 3 (H3)–helix 5 (H5) interaction in determining the sensitivity and specificity of steroid hormone receptors to receptor agonists. Intriguingly, the same H3 residues also play a crucial role in receptor antagonism; mutation of these residues alters the response of these receptors to antagonists. Given the close interaction of H3 and H5 residues at this site, we asked whether H5 residues might also play a role in the sensitivity of these receptors to antagonists. We demonstrate here that modification of H5 residues produces marked changes in the sensitivities of the glucocorticoid and progesterone receptor (PR) to RU486 antagonism. Moreover, while we confirm previous reports that alteration of the H3 residue, Gly 722 prevents RU486-mediated inhibition of the PR, we show that the corresponding substitution in the glucocorticoid receptor does not inhibit RU486-mediated receptor antagonism. Taken together, our data support the notion that RU486 binds differently to these two receptors, providing a potential target for the design of more specific antiglucocorticoid and antiprogestin drugs.
A. Stephanou, N. J. Sarlis, R. A. Knight, S. L. Lightman and H. S. Chowdrey
Adjuvant arthritis (AA) in the rat leads to chronic stimulation of the hypothalamic-pituitary-adrenal (HPA) axis and the loss of its diurnal rhythmicity. We have investigated the effects of adrenalectomy (ADX) and different levels of corticosterone replacement upon plasma ACTH levels and anterior pituitary pro-opiomelanocortin (POMC), GH and prolactin mRNAs during the development of AA. In control ADX animals, we observed the negative feedback effects of exogenous corticosterone on plasma ACTH and anterior pituitary POMC mRNA. In the ADX animal with AA, however, the increased POMC mRNA which was observed was not reduced by exogenous corticosterone on day 7 of AA, although the negative feedback effect of corticosterone on plasma ACTH was intact. On day 14, however, even high dose corticosterone replacement failed to have a significant feedback effect on the raised levels of plasma ACTH.
In control ADX animals, corticosterone replacement resulted in increased anterior pituitary GH mRNA and reduced prolactin mRNA. In contrast, in ADX animals with AA, GH mRNA was reduced and there was a further decrease in prolactin mRNA. In these animals, corticosterone replacement did not affect GH or prolactin mRNA expression.
These data demonstrate a disruption of the normal mechanisms underlying feedback inhibition of the HPA axis by glucocorticoids during AA. Similarly, the glucocorticoid-dependent regulation of GH and prolactin mRNA expression is altered in AA.
Russell A Prough, Barbara J Clark and Carolyn M Klinge
Dehydroepiandrosterone (3β-hydroxy-5-androsten-17-one, DHEA), secreted by the adrenal cortex, gastrointestinal tract, gonads, and brain, and its sulfated metabolite DHEA-S are the most abundant endogeneous circulating steroid hormones. DHEA actions are classically associated with age-related changes in cardiovascular tissues, female fertility, metabolism, and neuronal/CNS functions. Early work on DHEA action focused on the metabolism to more potent sex hormones, testosterone and estradiol, and the subsequent effect on the activation of the androgen and estrogen steroid receptors. However, it is now clear that DHEA and DHEA-S act directly as ligands for many hepatic nuclear receptors and G-protein-coupled receptors. In addition, it can function to mediate acute cell signaling pathways. This review summarizes the molecular mechanisms by which DHEA acts in cells and animal models with a focus on the ‘novel’ and physiological modes of DHEA action.
K Alheim, J Corness, MK Samuelsson, LG Bladh, T Murata, T Nilsson and S Okret
Glucocorticoids are known regulators of the cell cycle, normally exerting an anti-proliferative effect. We have previously shown that glucocorticoids stimulate expression of p57(Kip2), a member of the Cip/Kip family of cyclin-dependent kinase inhibitors which, in some cell types, may account for the anti-proliferative responses seen after glucocorticoid treatment. The induction of p57(Kip2) involves primary transcriptional effects where no de novo protein synthesis is necessary, suggesting a direct interaction of the glucocorticoid receptor with the p57(Kip2) gene. In this study we have identified a functional glucocorticoid response element (GRE), located 5 kilo bases (kb) upstream of the transcription start site in the human p57(Kip2) promoter. This GRE was functional also when isolated, suggesting a direct transcriptional effect of the glucocorticoid receptor. Furthermore, mutation of this GRE abolished glucocorticoid induction of the reporter gene, whereas mutation of a nearby Sp1 site did not. Using electrophoretic mobility shift assays, we have shown that the -5 kb p57(Kip2) promoter GRE was able to compete with a well-known GRE for glucocorticoid receptor binding. Sequence comparisons with the mouse genome showed that this GRE is highly conserved, further strengthening the biological importance of this site. All these data emphasize the involvement of this GRE in the glucocorticoid-mediated induction of p57(Kip2) expression.
L K Beitel, N Sabbaghian, A Alarifi, C Alvarado, L Pinsky and M Trifiro
The baculovirus system is able to generate large amounts of a protein, permitting detailed analysis of structure-function relations. We have used this system to overexpress and characterize normal human androgen receptor (hAR) and mutant hARs from humans with complete or partial androgen insensitivity. Maximum specific binding of [3H]mibolerone (MB) in recombinant baculovirus-infected Spodoptera frugiperda (Sf9) cells varied from 15 to 40 pmol/mg protein, about 1000-fold higher than in genital skin fibroblasts, and peaked 48–72 h after infection. In contrast, Coomassie blue staining and Western blotting revealed maximum accumulation of 110–120 kDa hAR proteins 96 h post-infection. Normal and mutant hARs were specifically photoaffinity-labeled with [3H]methyltrienolone (MT), and had normal steroid-binding selectivity: the order of competition was androgen>estrogen>progestin>glucocorticoid. Normal hAR was phosphorylated in Sf9 cells, reacted with antibodies against phosphoserine and phosphothreonine after purification using testosterone-biotin, and transactivated a transfected androgen response element-luciferase reporter in infected Sf9 cells. Two mutant hARs had increased rates of dissociation from MB and MT that were in accord with the associated degree of clinical androgen insensitivity: complete, Pro903Ser>partial, Leu820Val; the third, Ile663Asn, was not abnormal. Our data extend the characterization of normal hAR produced by baculovirus-infected Sf9 cells, and demonstrate, for the first time, that point-mutated hARs so produced can display distinctive biochemical phenotypes.
Kazue Nagasawa, Christopher Presslauer, Lech Kirtiklis, Igor Babiak and Jorge M O Fernandes
The role of sex steroid regulation in gonadal maturation is a very complex process that is far from being fully understood. Hence, we have investigated seasonal changes in gonadal expression of estrogen receptors (ERs) in Atlantic cod (Gadus morhua L.), a batch spawner, throughout the annual reproductive cycle. Three nuclear ER partial cDNA sequences (esr1, esr2a, and esr2b) were cloned and all esr transcripts were detected mainly in liver and gonads of fish of both sexes. In situ hybridization of esrs along with germ cell (vasa) and gonadal somatic cell markers (gonadal soma-derived factor (gsdf), 3β-hydroxysteroid dehydrogenase (3 β hsd), and anti-Müllerian hormone (amh) for testicular, or gsdf for ovarian somatic cells) showed that all three esrs were preferentially localized within interstitial fibroblasts composed of immature and mature Leydig cells in testis, whereas they were differentially expressed in both follicular cells and oocytes in ovary. Quantitative real-time PCR analysis revealed a sexually dimorphic expression pattern of the three esr paralogs in testis and ovary. A significant increase in esr2a expression was identified in testis and of esr2b in ovary, whereas e sr1 transcripts were elevated in both testis and ovary in February and March before the spawning period. The localization and sexually dimorphic expression of esr genes in gonads indicate a direct function of estrogen via ERs in gonadal somatic cell growth and differentiation for Leydig cell in testis and follicular cells in ovary throughout the annual reproductive cycle in Atlantic cod.
He Jiang, Xiao-Ping Ye, Zhong-Yin Yang, Ming Zhan, Hai-Ning Wang, Huang-Min Cao, Hui-Jun Xie, Chun-Ming Pan, Huai-Dong Song and Shuang-Xia Zhao
There is a high incidence of metabolic syndrome among patients with primary aldosteronism (PA), which has recently been associated with an unfavorable cardiometabolic profile. However, the underlying mechanisms have not been clarified in detail. Characterizing aldosterone (Ald) target genes in adipocytes will help us to elucidate the deleterious effects associated with excess Ald. Apelin, a novel adipokine, exerts beneficial effects on obesity-associated disorders and cardiovascular homeostasis. The objective of this study was to investigate the effects of high Ald levels on apelin expression and secretion and the underlying mechanisms involved in adipocytes. In vivo, a single-dose Ald injection acutely decreased apelin serum levels and adipose tissue apelin production, which demonstrates a clear inverse relationship between the levels of plasma Ald and plasma apelin. Experiments using 3T3-L1 adipocytes showed that Ald decreased apelin expression and secretion in a time- and dose-dependent manner. This effect was reversed by glucocorticoid receptor (GR) antagonists or GR (NR3C1) knockdown; furthermore, putative HREs were identified in the apelin promoter. Subsequently, we verified that both glucocorticoids and mineralocorticoids regulated apelin expression through GR activation, although no synergistic effect was observed. Additionally, detailed potential mechanisms involved a p38 MAPK signaling pathway. In conclusion, our findings strengthen the fact that there is a direct interaction between Ald and apelin in adipocytes, which has important implications for hyperaldosteronism or PA-associated cardiometabolic syndrome and hoists apelin on the list of potent therapeutic targets for PA.