Thyroid hormone potentiates estrogen activation of vitellogenin genes and autoinduction of estrogen receptor in adult Xenopus hepatocytes

Thyroid and reproduction in amphibians and reptiles

The relationship between the thyroid gland and reproduction in amphibians and reptiles has been studied for more than 100 years. Most studies suggest a positive involvement of thyroid hormones with some aspects of reproduction, but some studies support a negative role for thyroid hormones at certain life stages. Comprehensive studies of gene activation/suppression by thyroid hormones and their absence at various levels of the hypothalamo-pituitary-gonadal axis coupled with observations of adrenocorticoid activity, reproductive performance, and metabolic involvement are needed to understand this complex relationship.

Interaction of nuclear ERs and GPER in vitellogenesis in zebrafish

Estrogens exert their biological functions through the estrogen receptors (ERs). In zebrafish, three nuclear estrogen receptors (nERs) named ERα, ERβ1 and ERβ2 and one membrane-bound G protein-coupled estrogen receptor (GPER) are identified. Vitellogenin (Vtg) is predominantly expressed in liver and strongly response to the stimulation of estrogen. It has been proposed that all three nERs are functionally involved in vitellogenesis and ERα may act as the major mediator in teleost. However, the role of GPER and its interaction with nERs in this process are not yet defined in teleost species. In the present study, we provide genetic evidence for the functional significance of ERα that the expression of Vtg genes (vtg1, vtg2, vtg3) and their response to estradiol stimulation were significantly decreased in esr1 mutant zebrafish. Activation of ERβ1 and ERβ2 induced Vtg expression through ERα. Moreover, the involvement of GPER in vitellogenesis and its interaction with nERs in zebrafish were firstly proposed in this work. Activation of GPER induced Vtg genes expression while inhibition of GPER significantly attenuated the estrogenic effect on Vtg. Both treatments altered the expression levels of nERs, suggesting GPER acts interactively with nERs. Collectively, the involvement of both nERs and GPER in regulation of vitellogenesis is demonstrated. ERα is the central factor, acting interactively with ERβ1, ERβ2 and GPER, and GPER regulates vitellogenesis directly and interactively with nERs.

Thyroid hormone regulates vitellogenin by inducing estrogen receptor alpha in the goldfish liver

Vitellogenin (Vtg) is an egg-yolk precursor protein that is synthesized in the liver of oviparous species and taken up from the circulation by the ovary. It is well known that Vtg is induced by circulating estrogens. However, other endocrine factors that regulate the expression of Vtg are less well characterized; factors that might play significant roles, especially in seasonal spawners such as the goldfish which require increased quantities of Vtg for the development of hundreds of follicles. In this regard, thyroid hormones have been shown to cycle with the reproductive season. Therefore, we hypothesized that the thyroid hormones might influence the synthesis of Vtg. Treatment of female goldfish with triiodothyronine (T3) resulted in increased Vtg, an observation that was absent in males. Furthermore, T3 failed to induce Vtg in cultured hepatocytes of either sex. Interestingly however, T3 consistently up-regulated the expression of the estrogen receptor alpha (ERα). The T3 mediated upregulation of ERα requires the presence of both thyroid receptor (TR) α-1 and TRβ. When goldfish or cultured hepatocytes were treated with T3 followed by estradiol, there was a synergistic increase in Vtg, a response which is dependent on the presence of ERα. Therefore, by upregulating ERα, T3 serves to prime the liver to subsequent stimuli from estradiol. This cross-talk likely reveals an important physiologic mechanism by which thyroid hormones, whose circulating levels are high during early gonadal recrudescence, facilitate the production of large amounts of Vtg required for egg development.

Mechanisms of crosstalk between endocrine systems: regulation of sex steroid hormone synthesis and action by thyroid hormones

Thyroid hormones (THs) are well-known regulators of development and metabolism in vertebrates. There is increasing evidence that THs are also involved in gonadal differentiation and reproductive function. Changes in TH status affect sex ratios in developing fish and frogs and reproduction (e.g., fertility), hormone levels, and gonad morphology in adults of species of different vertebrates. In this review, we have summarized and compared the evidence for cross-talk between the steroid hormone and thyroid axes and present a comparative model. We gave special attention to TH regulation of sex steroid synthesis and action in both the brain and gonad, since these are important for gonad development and brain sexual differentiation and have been studied in many species. We also reviewed research showing that there is a TH system, including receptors and enzymes, in the brains and gonads in developing and adult vertebrates. Our analysis shows that THs influences sex steroid hormone synthesis in vertebrates, ranging from fish to pigs. This concept of crosstalk and conserved hormone interaction has implications for our understanding of the role of THs in reproduction, and how these processes may be dysregulated by environmental endocrine disruptors.

Mechanisms and significance of nuclear receptor auto- and cross-regulation

The number of functional hormone receptors expressed by a cell in large part determines its responsiveness to the hormonal signal. The regulation of hormone receptor gene expression is therefore a central component of hormone action. Vertebrate steroid and thyroid hormones act by binding to nuclear receptors (NR) that function as ligand-activated transcription factors. Nuclear receptor genes are regulated by diverse and interacting intracellular signaling pathways. Nuclear receptor ligands can regulate the expression of the gene for the NR that mediates the hormone's action (autoregulation), thus influencing how a cell responds to the hormone. Autoregulation can be either positive or negative, the hormone increasing or decreasing, respectively, the expression of its own NR. Positive autoregulation (autoinduction) is often observed during postembryonic development, and during the ovarian cycle, where it enhances cellular sensitivity to the hormonal signal to drive the developmental process. By contrast, negative autoregulation (autorepression) may become important in the juvenile and adult for homeostatic negative feedback responses. In addition to autoregulation, a NR can influence the expression other types of NRs (cross-regulation), thus modifying how a cell responds to a different hormone. Cross-regulation by NRs is an important means to temporally coordinate cell responses to a subsequent (different) hormonal signal, or to allow for crosstalk between hormone signaling pathways.

The xenoestrogen bisphenol A inhibits postembryonic vertebrate development by antagonizing gene regulation by thyroid hormone

Bisphenol A (BPA), a chemical widely used to manufacture plastics, is estrogenic and capable of disrupting sex differentiation. However, recent in vitro studies have shown that BPA can also antagonize T(3) activation of the T(3) receptor. The difficulty in studying uterus-enclosed mammalian embryos has hampered the analysis on the direct effects of BPA during vertebrate development. This study proposed to identify critical T(3) pathways that may be disrupted by BPA based on molecular analysis in vivo. Because amphibian metamorphosis requires T(3) and encompasses the postembryonic period in mammals when T(3) action is most critical, we used this unique model for studying the effect of BPA on T(3)-dependent vertebrate development at both the morphological and molecular levels. After 4 d of exposure, BPA inhibited T(3)-induced intestinal remodeling in premetamorphic Xenopus laevis tadpoles. Importantly, microarray analysis revealed that BPA antagonized the regulation of most T(3)-response genes, thereby explaining the inhibitory effect of BPA on metamorphosis. Surprisingly, most of the genes affected by BPA in the presence of T(3) were T(3)-response genes, suggesting that BPA predominantly affected T(3)-signaling pathways during metamorphosis. Our finding that this endocrine disruptor, well known for its estrogenic activity in vitro, functions to inhibit T(3) pathways to affect vertebrate development in vivo and thus not only provides a mechanism for the likely deleterious effects of BPA on human development but also demonstrates the importance of studying endocrine disruption in a developmental context in vivo.

The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Frogs and Its Role in Frog Development and Reproduction

Metamorphosis of the amphibian tadpole is a thyroid hormone (TH)-dependent developmental process. For this reason, the tadpole is considered to be an ideal bioassay system to identify disruption of thyroid function by environmental contaminants. Here we provide an in-depth review of the amphibian thyroid system with particular focus on the role that TH plays in metamorphosis. The amphibian thyroid system is similar to that of mammals and other tetrapods. We review the amphibian hypothalamic-pituitary-thyroid (HPT) axis, focusing on thyroid hormone synthesis, transport, and metabolism. We also discuss the molecular mechanisms of TH action, including the role of TH receptors, the actions of TH on organogenesis, and the mechanisms that underlie the pleiotropic actions of THs. Finally, we discuss methods for evaluating thyroid disruption in frogs, including potential sites of action, relevant endpoints, candidate protocols for measuring thyroid axis disruption, and current gaps in our knowledge. The utility of amphibian metamorphosis as a model for evaluating thyroid axis disruption has recently led to the development of a bioassay using Xenopus laevis.

Hormone cross-regulation in the tadpole brain: developmental expression profiles and effect of T3 exposure on thyroid hormone- and estrogen-responsive genes in Rana pipiens

During metamorphosis, the tadpole neuroendocrine brain is a major target for the organisational effects of hormones acting via both endocrine feedback mechanisms and local hormone production. While the receptor-mediated actions of thyroid hormones in brain development have been well described, there is evidence that thyroid hormones could also be an important modulator of estrogen action during metamorphosis. To better understand hormone action and potential cross-regulation between thyroid hormone and estrogen, we examined changes in thyroid hormone receptors (TRalpha and TRbeta) and the estrogen receptor (ERalpha) in the brain of Rana pipiens throughout metamorphosis and in response to 48 h waterborne triiodothyronine (T3) exposure (0.5, 5 and 50 nM). We also measured mRNA levels of iodothyronine deiodinase (D2 and D3) and aromatase, key enzymes responsible for local synthesis and availability of thyroid hormones and estrogen, respectively. A real-time PCR strategy targeting these genes was developed using either a fluorescent dual-labelled probe- or SYBR Green I-based method. TRbeta mRNA levels were increased during development and in response to T3 exposure. Deiodinase (D2 and D3) enzymes were differentially regulated during development, but mRNA levels of both were increased with 50 nM T3 exposure. ERalpha and aromatase mRNA levels significantly increased at metamorphic climax, but whereas estrogen receptor alpha mRNA levels were increased by 50 nM T3, aromatase mRNA levels were decreased. These results (1) demonstrate that the developing amphibian brain is an important site for stage-specific thyroid hormone regulation of nuclear receptors and hormone synthesis enzymes and (2) provide the basis for further studies exploring the physiological and functional significance of the cross-regulation between thyroid status and estrogen-sensitive genes in the brain during amphibian metamorphosis.

Long-term exposure of European flounder (Platichthys flesus) to the flame-retardants tetrabromobisphenol A (TBBPA) and hexabromocyclododecane (HBCD)

Tetrabromobisphenol A (TBBPA) and hexabromocyclododecane (HBCD) are widely used flame retardants that have increasingly been found as contaminants in the aquatic environment. In the present study, European flounder (Platichthys flesus) were chronically exposed to TBBPA; (105 days) and HBCD (78 days), in a wide range including environmentally relevant concentrations. TBBPA was administered via the water, whereas HBCD was administered in food and sediment, or in sediment alone. Chemical analysis of muscle showed an average increase in internal concentrations of approximately two orders of magnitude for both compounds tested. Animals exposed to HBCD via sediment alone (8000 microg/g total organic carbon, TOC) showed a proportional increase of alpha-HBCD in muscle compared to animals exposed via food and sediment. In both studies, exposure to the test compounds did not affect general health and toxicity parameters (behavior, survival, growth rate, relative liver and gonad weight). Hepatic microsomal enzyme activities (TBBPA: EROD; HBCD: EROD, PROD, and BROD) were not induced by any of the tested chemicals. Aromatase activity in male gonads showed a mild increase with rising TBBPA levels. There were no morphological and immunohistochemical indications for increased production of the yolk precursor protein vitellogenin (VTG) in animals exposed to TBBPA and HBCD; immunochemical analysis of plasma VTG levels showed no dose response in animals exposed to TBBPA. In animals exposed to TBBPA, levels of the thyroid hormone thyroxin (T(4)) increased with internal concentrations of the test compound, possibly indicating competition of TBBPA for plasma protein binding. Triiodothyronin (T(3)) levels were not affected and histology showed no signs of altered thyroid gland activity. Other organs investigated (liver, gills, kidney, skin, and gonads) revealed no histological changes related to TBBPA or HBCD exposure. Overall, the present results indicate limited endocrine effects of these widely used flame retardants in a test species representative of European estuaries at environmentally relevant exposure levels and at internal levels up to 4300 ng TBBPA/g wet weight, and 446 microg HBCD/g lipid weight in flounder muscle.

Regulation of estrogen receptors in primary cultured hepatocytes of the amphibian Xenopus laevis as estrogenic biomarker and its application in environmental monitoring

The present study aims to introduce the regulation of estrogen receptors (ER) in primary cultured hepatocytes of the amphibian Xenopus laevis as a further potential estrogenic biomarker. Time courses of free ER in cell cultures treated with 17beta-estradiol (E2), nonylphenol (NP), and bisphenol A (BPA) were determined by means of radioreceptorassay (RARA). All compounds led to an immediate drop of free ER followed by a significant increase. The estrogen specific induction of ER-mRNA in vitro during time course was verified by using semiquantitative RT-PCR demonstrating greatest differences after 36 h. Dose-response curves of ER-mRNA for E2, NP, and BPA revealed that E2 possessed highest estrogenicity starting at 10(-9) M, while NP and BPA induced significant increases at 10(-8) and 10(-7) M, respectively. Extracts of the river Alb were subjected to RARA for ER binding to cytosolic liver fraction as well as to primary cultured hepatocytes for assessment of ER-mRNA induction. The results by RARA demonstrated clearly that binding to ER was highest in sewage treatment plant effluents and increased during the course of the river. These findings could be correlated with induction of ER-mRNA levels in vitro indicating that both techniques are suitable for application in monitoring of estrogenic EDC.

Toxicity of glyphosate-based pesticides to four North American frog species

Glyphosate-based herbicides are among the most widely used pesticides in the world. We compared the acute toxicity of the glyphosate end-use formulation Roundup Original to four North American amphibian species (Rana clamitans, R. pipiens, R. sylvatica, and Bufo americanus) and the toxicity of glyphosate technical, the polyethoxylated tallowamine surfactant (POEA) commonly used in glyphosate-based herbicides, and five newer glyphosate formulations to R. clamitans. For R. clamitans, acute toxicity values in order of decreasing toxicity were POEA > Roundup Original > Roundup Transorb > Glyfos AU; no significant acute toxicity was observed with glyphosate technical material or the glyphosate formulations Roundup Biactive, Touchdown, or Glyfos BIO. Comparisons between the four amphibian species showed that the toxicity of Roundup Original varied with species and developmental stage. Rana pipiens tadpoles chronically exposed to environmentally relevant concentrations of POEA or glyphosate formulations containing POEA showed decreased snout-vent length at metamorphosis and increased time to metamorphosis, tail damage, and gonadal abnormalities. These effects may be caused, in some part, by disruption of hormone signaling, because thyroid hormone receptor beta mRNA transcript levels were elevated by exposure to formulations containing glyphosate and POEA. Taken together, the data suggest that surfactant composition must be considered in the evaluation of toxicity of glyphosate-based herbicides.

Functional genomics and sexual differentiation in amphibians

In Xenopus laevis the basic mechanisms underlying sexual differentiation were investigated by determining time courses of sexual steroids and their corresponding receptors during complete larval development from egg to juveniles. Androgens as well as estradiol (E2) are derived from maternal origin and accumulate in hatching tadpoles. Sexual steroid contents decreased rapidly after hatching and rose again at the end of metamorphosis indicating endogenous production. In parallel the mRNA expression for corresponding androgen (AR) and estrogen receptors (ER) was measured by means of semiquantitative RT-PCR. Both receptor mRNAs increased dramatically just after hatching and decreased only moderately until end of metamorphosis. In female juveniles E2 and ER-mRNA levels were higher compared with males. Treatment by exogenous E2 elevated both, ER- and AR-mRNA, indicating stimulatory functions of E2 for gene expression of both receptors. Effects on sexual differentiation during larval development were achieved by treatment with E2 and the antiandrogen cyproterone acetate both causing feminization, the antiestrogen tamoxifen resulting in neutralization, and the androgens, methyltestosterone and dihydrotestosterone, but not testosterone, leading to masculinization. The data presented are in accordance with further recent findings and suggest a new hypothesis for functional genomics in sexual differentiation of amphibians.

Estrogen and thyroid hormone receptor interactions: physiological flexibility by molecular specificity

The influence of thyroid hormone on estrogen actions has been demonstrated both in vivo and in vitro. In transient transfection assays, the effects of liganded thyroid hormone receptors (TR) on transcriptional facilitation by estrogens bound to estrogen receptors (ER) display specificity according to the following: 1) ER isoform, 2) TR isoform, 3) the promoter through which transcriptional facilitation occurs, and 4) cell type. Some of these molecular phenomena may be related to thyroid hormone signaling of seasonal limitations upon reproduction. The various combinations of these molecular interactions provide multiple and flexible opportunities for relations between two major hormonal systems important for neuroendocrine feedbacks and reproductive behaviors.

Development of an ER action indicator mouse for the study of estrogens, selective ER modulators (SERMs), and Xenobiotics

We have developed a transgenic mouse that functions as a reporter of ER activity, termed ER action indicator (ERIN), by incorporating a transgene with an estrogen-responsive promoter (three copies of the vitellogenin estrogen response element with a minimal thymidine kinase promoter) linked to the reporter gene beta-galactosidase. Evaluation of ER activity in female ERIN mice demonstrated estrogen-inducible expression of the reporter gene in the uterus, pituitary, and hypothalamus; established targets of estrogen action. Importantly, we also identified ER activity in a number of nonclassical estrogen target tissues, including kidney, liver, adrenal, and thyroid gland. ERIN provides a system to measure the same end point (transgene regulation) in different target tissues, permitting separation of the contributions of cell- and promoter-specific factors in determining ER pharmacology. In this regard we observed that on this specific promoter the pituitary gland was 25-fold more sensitive than the uterus to the estrogen diethylstilbestrol, implying the existence of cell-specific factors that influence ligand sensitivity. Our studies also identified considerable difference in the efficacy and potency of ER ligands in the uterus when ER transcriptional activity was assayed vs. uterine weight gain. Specifically, we observed that the environmental estrogen bisphenol A was a potent agonist in stimulating ER transcriptional activity, whereas it exhibited little uterotropic activity. In contrast to bisphenol A, tamoxifen significantly increased uterine weight, but minimally induced ER reporter activity in this tissue. Given the results of these studies, we believe that ERIN will be a useful model to evaluate ER ligand pharmacology and will assist in defining the cellular and molecular mechanisms that determine agonist and antagonist activity.

Occurrence of vertebrate steroids, estradiol 17beta and progesterone in the reproducing females of the mud crab Scylla serrata

In crustaceans, vitellogenesis is known to be controlled by eyestalk neuropeptides, biogenic amines, ecdysteroids and a juvenile hormone-like compound, methyl farnesoate. In recent years, the occurrence of vertebrate steroid hormones, estradiol 17beta (E2) and progesterone (PG) has also been reported in a few decapods, although their precise role in female reproduction is yet to be determined. The levels of E2 and PG in the ovary, hepatopancreas and the hemolymph of the red mud crab, Scylla serrata were analyzed in different vitellogenic stages in order to establish a correlation between hormone profile and stages of vitellogenesis. It was observed that the levels of both the steroids increased steeply in the tissues at the onset of vitellogenesis (vitellogenic stage I). Maximum levels of estradiol were present in the hepatopancreas whereas the highest concentration of progesterone was seen in the ovary, suggesting dichotomous roles for these hormones in vitellogenesis. Furthermore, levels of these hormones were estimated in different embryonic stages of the eggs of the sand crab Emerita asiatica and mud crab S. serrata. Their levels fluctuated, following a definite pattern in the different stages, suggesting a possible functional role as morphogenetic hormones. This study, in addition, also reports the presence of E2 and PG on lipovitellin purified from ovary and eggs as well as vitellogenin purified from the hemolymph implicating a role for these lipoproteins as steroid carriers.

Molecular analysis of estrogen induction of preproenkephalin gene expression and its modulation by thyroid hormones

Estrogen receptors (ER) and thyroid hormone receptors (TR) are ligand-dependent nuclear transcription factors. Estrogen-induced preproenkephalin (PPE) gene expression in the hypothalamus is directly related to estrogen-induced lordosis behavior in the rat. In the present study, we showed that the PPE mRNA level in the ventromedial hypothalamus of female rats was significantly decreased by ovariectomy. This decrease was reversed by estrogen replacement in a dose- and time-dependent manner. Using transient transfection and electrophoretic mobility shift assays (EMSA), functional estrogen response elements (ERE) were identified between -437 and -145 base pairs (bp) of the rat PPE gene promoter region. Two ERE-like elements are present between -405 and -364 of the rat PPE gene promoter, which bind ERalpha as demonstrated by EMSA. Estrogen produced a dose-dependent increase in CAT activity in cotransfection assays with ERalpha expression vector and a 437PPE-CAT reporter construct containing 437 bp of the rat PPE gene promoter and the CAT reporter gene. This estrogen-induced PPE promoter activity was inhibited by liganded-TR in transient cotransfection assays. Analysis of DNA-protein interactions by EMSA revealed that both ERalpha and TR (alpha1 and beta1) could bind to the EREs in the rat PPE gene promoter. Furthermore, estrogen induction of PPE mRNA in the ventromedial hypothalamus of the ovariectomized female rat was significantly attenuated by concomitant administration of triiodothyronine. These results suggest that estrogen regulation of the hypothalamic PPE gene expression is mediated through an estrogen-receptor complex directly interacting with the functional EREs in its promoter region; and that this estrogen effect can be modified by thyroid hormones.

Oestradiol improves arterial endothelial function in healthy men receiving testosterone

OBJECTIVE

To assess prospectively the effects of low dose oestradiol on arterial endothelial and smooth muscle function in healthy men. Oestrogen use is associated with reduced cardiovascular disease in oestrogen-deficient women, however, the vascular effects of low-dose oestradiol in healthy men have not been investigated previously.

PATIENTS AND DESIGN

Twenty-three men (aged 32 +/- 8 years) were randomized to receive depot implants of testosterone (T) alone (group 1, n = 10), or T with either 10 mg (group 2, n = 7) or 20 mg (group 3, n = 6) of oestradiol (E).

MEASUREMENTS

Hormone levels, lipids and vascular reactivity were measured before, 1 month and 6 months after hormone implantation. Using high-resolution ultrasound, brachial artery diameter was measured at rest, during reactive hyperaemia (leading to flow-mediated dilatation, FMD, which is endothelium-dependent) and after sublingual nitroglycerin (GTN, an endothelium-independent dilator).

RESULTS

Oestradiol produced a dose-dependent increase in plasma oestradiol (at 1 month 96 +/- 7, 149 +/- 6, 192 +/- 23 pmol/l in the 3 groups, respectively, P < 0.001 by ANOVA for trend). Minor side-effects (gynaecomastia, nipple tenderness) indicated that 20 mg oestradiol was the maximum tolerated dose. There was also a dose-dependent increase in FMD with oestradiol dose: at 1 month, - 0.2, + 0.2 and + 1.8% for groups 1-3, respectively (P = 0.31 by ANOVA for trend); and at 6 months, - 0.8, + 0.4 and + 2.2% (P = 0.02). The rise in oestradiol levels following treatment correlated with the improvement in FMD (P = 0.01). GTN responses were similar in the 3 groups throughout the study.

CONCLUSION

In healthy young men, oestradiol supplementation is associated with enhanced arterial endothelial function, a key marker of vascular health.

Interaction of thyroxine and estrogen on the expression of estrogen receptor alpha, cholecystokinin, and preproenkephalin messenger ribonucleic acid in the limbic-hypothalamic circuit

To study thyroid hormone and estrogen interactions in the central nervous system (CNS), the expression of estrogen sensitive genes was examined within the limbic-hypothalamic circuit. Estrogen up-regulates the expression of reproductively relevant neuropeptide messenger RNAs (mRNAs) encoding cholecystokinin (CCK) and enkephalin, peptides that stimulate lordosis. Estrogen down-regulates the expression of the estrogen receptor alpha (ER alpha) mRNA in the nuclei of the circuit. We examined the possibility that thyroid hormone treatment would block the estrogen modulation of these messages. Estradiol benzoate (EB), EB + thyroxine (T4), T4, or oil were administered to ovariectomized, adult female rats for 10 days. Isotopic in situ hybridization histochemistry revealed that within the limbic-hypothalamic nuclei, levels of CCK and preproenkephalin (PPE) mRNA levels were significantly higher in EB and EB + T4-treated animals compared with T4 or oil-treated animals. ER alpha mRNA levels were low in EB treated animals, elevated in T4 or oil-treated animals and further elevated in EB + T4-treated animals. In summary, T4 treatment had neither an independent nor an antagonistic effect on estrogen induced expression of CCK or PPE mRNA in the circuit. However, T4 did prevent the normal estrogenic decrease of ER alpha mRNA levels in the nuclei of the limbic-hypothalamic circuit.

Prolactin inhibits auto- and cross-induction of thyroid hormone and estrogen receptor and vitellogenin genes in adult Xenopus (Amphibia) hepatocytes

It is well known that virtually every tissue of the amphibian larvae is highly sensitive to the mutually antagonistic actions of thyroid hormone (TH) and prolactin (PRL), but it is not known if adult amphibian tissues respond similarly to these two hormones. We have previously shown that very low doses of triiodothyronine (T3) rapidly and strongly potentiate the activation of silent vitellogenin (Vit) genes by estrogen (E2) and the autoinduction of estrogen receptor (ER) transcripts in primary cultures of adult Xenopus hepatocytes. This response to T3 is accompanied by the upregulation of thyroid hormone receptor <FONT FACE="Symbol">b</FONT> (TR<FONT FACE="Symbol">b</FONT>) mRNA. Using Northern blot and RNase protection assays, we now show that ovine PRL added for 12 h along with 2 x 10-9 M T3 will completely prevent potentiation of E2 induction of Vit mRNA in primary cultures of adult Xenopus hepatocytes. PRL also abolished the auto-upregulation of TR<FONT FACE="Symbol">b</FONT> mRNA and the cross-activation of autoinduction of ER mRNA. Thus, we show for the first time that the anti-TH action of PRL that is manifested in Xenopus tadpole tissues during metamorphosis is retained in adult liver, and suggest that the mutually antagonistic actions of the two hormones may be brought about by similar molecular mechanisms in larval and adult amphibian tissues

Hormonal regulation of the estrogen receptor in primary cultures of hepatocytes from female rats

Estrogen treatment affects the hepatic synthesis and/or secretion of several proteins involved in clinically important pathological processes such as atherosclerosis, hypertension, and thrombosis. The endocrine regulation of the estrogen receptor (ER) concentration in primary cultures of rat hepatocytes was studied. Human growth hormone (hGH) and dexamethasone (DEX) in combination increased ER concentration 6-fold and ER mRNA levels 2.5-fold. These effects were not significantly different from those observed after treatment with the purely somatogenic bovine growth hormone (GH) in combination with DEX. Treatment with the lactogen ovine prolactin in the presence or absence of DEX did not significantly affect ER or ER mRNA concentrations. Triiodothyronine treatment at the most effective concentration (50 nM) increased ER and ER mRNA levels twofold. Medium supplementation with estradiol (0.1 nM) throughout the experiment did not affect the response to treatment with hGH and DEX. Treatment with high concentrations of ethinylestradiol in combination with hGH and DEX, however, increased the ER level twice as much as hGH and DEX without addition of estradiol or ethinylestradiol, whereas the ER mRNA concentration was the same in both the GH+DEX group and GH+ DEX+ (estradiol or ethinylestradiol) groups. These data indicate the importance of GH in combination with glucocorticoids for the maintenance of ER concentrations in the rat liver. Thyroid hormones may be of some, although minor importance, whereas the data suggest that prolactin is not directly involved in hepatic ER regulation.

Oestrogen control of the sexual dimorphism in the Harderian gland of Xenopus laevis

Xenopus laevis shows a sexual dimorphism of the electrophoretic pattern of Harderian gland (HG) proteins. The male pattern displays three protein fractions whose molecular sizes are approx. 205, 180 and 78 kDa, respectively, and which are absent in the female pattern. Conversely, the female pattern displays two protein fractions of approx. 190 and 76 kDa, respectively. This sexual dimorphism led us to hypothesize a sex steroid control of the HG. Administration of 17beta-oestradiol to male Xenopus converts the male protein pattern into the female one, while the administration of testosterone to the female has no effect. In this respect neither Northern analysis nor the RNase-protection assay performed using a 213 bp encoding for the androgen-binding domain reveals the presence of an androgen receptor mRNA in Xenopus HG. Conversely, Northern analysis has shown an oestrogen receptor mRNA whose size is approx. 6.5 kb and the RNase-protection assay performed by using a 197 bp encoding for the oestrogen-binding domain has also displayed the presence of an oestrogen receptor mRNA in the female HG but not in the male one. In addition, the oestrogen administration to male Xenopus induces the appearance of an oestrogen receptor mRNA. Androgen administration to female toad is ineffective. Taken together, all these findings suggest that in Xenopus laevis oestrogens are involved into the HG physiology. The appearance of an oestrogen receptor mRNA in the oestradiol treated males supports the hypothesis of the occurrence of autoinduction of oestrogen receptor mRNA expression in the HG.

Vitellogenesis as a biomarker for estrogenic contamination of the aquatic environment

A rapidly increasing number of chemicals, or their degradation products, are being recognized as possessing estrogenic activity, albeit usually weak. We have found that effluent from sewage treatment works contains a chemical, or mixture of chemicals, that induces vitellogenin synthesis in male fish maintained in the effluent, thus indicating that the effluent is estrogenic. The effect was extremely pronounced and occurred at all sewage treatment works tested. The nature of the chemical or chemicals causing the effect is presently not known. However, we have tested a number of chemicals known to be estrogenic to mammals and have shown that they are also estrogenic to fish; that is, no species specificity was apparent. Many of these weakly estrogenic chemicals are known to be present in effluents. Further, a mixture of different estrogenic chemicals was considerably more potent than each of the chemicals when tested individually, suggesting that enhanced effects could occur when fish are exposed simultaneously to various estrogenic chemicals (as is likely to occur in rivers receiving effluent). Subsequent work should determine whether exposure to these chemicals at the concentrations present in the environment leads to any deleterious physiological effects.

Potential interactions between estrogen receptor and thyroid receptors relevant for neuroendocrine systems

Environmental signals can profoundly affect reproductive behavior, physiology and responses to steroids. One consequence of nutritional or temperature stress is altered plasma concentrations of thyroid hormone. Recent in vivo and in vitro data indicate that manipulations of estrogen and thyroid hormone levels can alter each other's functions. One possible mechanism for interaction may be that thyroid and estrogen receptors bind to parts of the same hormone response elements of target genes and compete with each other, thus serving to integrate environmental signals with neuroendocrine responses.

Molecular cloning and sequencing of the hormone-binding domain of Oreochromis aureus estrogen receptor gene

The nucleotide sequence of the estrogen receptor gene of Oreochromis aureus (OaER) indicates that the hormone-binding E domain is composed of 4 exons interspersed by short introns of only 0.18-1.3 kb each. All 4 E exons exhibit consensus sequences flanking the donor and acceptor splice sites. Analysis of introns revealed (i) numerous palindromic and half-palindromic steroid responsive elements including ERE, TRE and GRE, (ii) six alternative polyadenylation signals and (iii) putative control regions identified by the clustering of transcription factor binding sites. Of particular interest is the presence of a TATA and CAAT box in intron IV. The hydropathicity profile shows that the E exons are relatively hydrophobic. Two receptor dimerization regions have been observed: a conserved heptad repeat of hydrophobic residues (R168-M193) and a perfect leucine zipper (L36-L57). The presence of multiple sites for kinase activity in these regions suggests the importance of phosphorylation in the regulation of receptor functions and ligand-affinity.

Thyroid hormone and glucocorticoid independently regulate the expression of estrogen receptor in male Xenopus liver cells

Earlier studies from our laboratory had shown that triiodothyronine (T3) strongly potentiates the activation by estradiol (E2) of silent vitellogenin (Vit) genes and the autoinduction of estrogen receptor (ER) mRNA in primary cultures of male Xenopus hepatocytes (Rabelo and Tata, 1993). It was, however, not known if T3, or other hormones, could up-regulate ER mRNA in the absence of exogenous E2. We now show that T3 and dexamethasone (Dex), but not progesterone and testosterone, directly induce ER mRNA within 4 h by separate pathways, at doses compatible with the Kd values of their receptors. This induction of ER mRNA is accompanied by a marked enhancement of the activation of the silent Vit B1 gene if E2 is added by 12 h after T3 and Dex, thus suggesting an elevated level of functional ER induced by the two hormones. This conclusion was supported by a higher rate of transcription from an estrogen response element (ERE)-tk-CAT construct transfected into cultured hepatocytes pre-treated with T3 and Dex before incubation with estrogen. Our findings emphasize the importance of hormonal interplay via auto- and cross-regulation of nuclear hormone receptors.

Autoregulation and crossregulation of nuclear receptor genes

Whereas crossregulation of nuclear receptors has been known for some time, recently several examples of autoregulation have been described, especially during development and specific gene expression. In this review, I discuss both these phenomena, based on some studies from our laboratory on amphibian metamorphosis and egg protein gene expression. These include autoinduction of estrogen receptor (ER) accompanying egg protein gene expression in adult and larval Xenopus; autoinduction of thyroid hormone receptor (TR) during metamorphosis and in adult Xenopus; crossregulation by triiodothyronine (T(3)) and dexamethasone of autoinduction of ER; and inhibition by PRL of autoinduction and crossinduction of TR and ER genes. A dual receptor threshold model to explain the interplay between T(3), estrogen and PRL is presented and its significance to the general question of nuclear receptor autoregulation and crossregulation during development is also discussed.

Autoinduction of nuclear receptor genes and its significance

Although downregulation of receptors by their respective hormonal ligands is a well-studied phenomenon, relatively less is known about autoupregulation of receptors. However, an increasing number of observations of the latter process are now being reported. Here, we discuss the phenomenon of autoinduction of nuclear receptors of the steroid/thyroid hormone gene family, and its significance in the context of the developmental and gene regulatory function of the ligands. Much of this review is illustrated by recent work from our laboratory on the autoregulation of Xenopus estrogen (ER) and thyroid hormone (TR) receptors and their transcripts, accompanying or anticipating vitellogenesis and metamorphosis, respectively. The activation by estrogen (E2) of the silent vitellogenin genes and the induction of FOSP-1 genes in primary cultures of hepatocytes from male Xenopus and oviduct cells, respectively, are tightly coupled to a substantial upregulation of ER protein and its transcript. The developmental competence to activate vitellogenin in response to E2 was found to be acquired during late metamorphosis. Since the latter process is obligatorily controlled by thyroid hormones (TH), we extended our studies to the developmental and hormonal regulation of Xenopus TR genes. Although very low levels of TR alpha and beta mRNAs are detectable in embryos and early larvae, there is a large increase in the accumulation of both transcripts before the onset of metamorphosis (stage 54 tadpoles), by which time the larval thyroid gland has first begun to secrete TH. Filter and in situ hybridization revealed that the two transcripts were differentially regulated and were not equally distributed in all regions or tissues of the tadpole. Their concentration peaks at metamorphic climax and drops to low levels in froglets and adult Xenopus. Exogenous TH given to pre-metamorphic tadpoles is known to induce metamorphosis precociously. Administration of triiodothyronine (T3) to early tadpoles (stages 50/52) caused a rapid upregulation of TR alpha and beta mRNAs which was particularly marked for the beta transcript (20- to 50-fold increase in steady-state levels). This autoinduction, which is the earliest response to T3, is mimicked to variable degrees in some Xenopus cell lines. In XTC-2 cells, in which the in vivo process is fully reproduced, it was possible to show with cycloheximide that the increase in TR mRNA requires protein synthesis. It was also possible to show by transfection of XTC-2 cells with a reporter-promoter construct of Xenopus albumin gene, which is a target for T3, that the extra TR mRNA increases functional receptor in the cell. Although the role of TH is well-known in metamorphosis, we found that TR is also autoinduced in primary culture of adult male Xenopus hepatocytes. The significance of this finding lies in the fact that T3 potentiates the autoinduction of ER and the de novo activation of vitellogenin genes by E2. Prolactin (PRL) is known to exert a "juvenilizing" action by preventing the induction of amphibian metamorphosis by TH. It is therefore highly significant that PRL prevented both the autoinduction of TR alpha and beta mRNAs in whole tadpoles and organ cultures and the activation of TR target genes, such as those encoding albumin and 63 kDa adult-type keratin. Although how PRL exerts its antimetamorphic effect is not known, these findings lead us to propose a dual threshold model for the autoinduction of TR, whereby the autoinduction of TR genes requires a very low level of TR and TH to rapidly augment the amount of functional TR. This higher amount of receptor would be required to achieve a higher threshold to activate "downstream" or target genes which specify the adult phenotype at the end of metamorphosis. Finally, a survey of recent literature shows that the phenomenon of autoinduction is not restricted to Xenopus ER and TR but is more widespread among members of the nuclear receptor family.