Differential responses to ligands of overexpressed thyroid hormone and retinoid X receptors in a Xenopus cell line and in vivo

Transcriptome and Methylome Analysis Reveal Complex Cross-Talks between Thyroid Hormone and Glucocorticoid Signaling at Xenopus Metamorphosis

Background: Most work in endocrinology focus on the action of a single hormone, and very little on the cross-talks between two hormones. Here we characterize the nature of interactions between thyroid hormone and glucocorticoid signaling during Xenopus tropicalis metamorphosis. Methods: We used functional genomics to derive genome wide profiles of methylated DNA and measured changes of gene expression after hormonal treatments of a highly responsive tissue, tailfin. Clustering classified the data into four types of biological responses, and biological networks were modeled by system biology. Results: We found that gene expression is mostly regulated by either T₃ or CORT, or their additive effect when they both regulate the same genes. A small but non-negligible fraction of genes (12%) displayed non-trivial regulations indicative of complex interactions between the signaling pathways. Strikingly, DNA methylation changes display the opposite and are dominated by cross-talks. Conclusion: Cross-talks between thyroid hormones and glucocorticoids are more complex than initially envisioned and are not limited to the simple addition of their individual effects, a statement that can be summarized with the pseudo-equation: TH ∙ GC > TH + GC. DNA methylation changes are highly dynamic and buffered from genome expression.

Hormonal regulation and characterisation of the aldehyde oxidase-like gene of hamster Harderian gland

The HG is a compound tubulo-alveolar gland located in the orbital cavity of the majority of vertebrates. In the golden hamster it shows a clear cut sexual dimorphism in both morphological and biochemical parameters such as cell types, protein pattern, lipid metabolism, porphyrin content, steroid hormone receptor expression. In a previous study we found that in primary culture of male hamster Harderian gland (HG), androgens (A) increase the MHG07 (male Harderian gland) expression and this effect is abrogated by both flutamide and cycloheximide. The present study represents a deeper analysis on MHG07 regulation by other members of steroid/thyroid hormone superfamily. Estrogens (E) impair the stimulatory effect of A and after the addition of a pure anti-estrogen, ICI 164,384, the negative effect of E is abrogated. Dexamethasone (Dex), used alone or in combination with A negatively affect the MHG07 expression. Also T(3) increases the expression of MHG07 mRNA. Progesterone (P) does not affect the expression of MHG07 mRNA. The use of cycloheximide abrogates the effect of steroids, suggesting that the latter act through their own receptors. Dose-response experiments show that low steroid concentrations (10(-12)M) are sufficient to affect the MHG07 expression. It is argued that the expression of MHG07 is under a highly coordinate relationship between androgen, estrogen, glucocorticoid, retinoic acid and thyroid hormones.

Bisphenol A in the Aquatic Environment and Its Endocrine-Disruptive Effects on Aquatic Organisms

Bisphenol A [BPA; 2,2-bis(4-hydroxyphenyl)propane], which is mainly used in the production of epoxy resins and polycarbonate plastics, is a known endocrine disruptor and is acutely toxic to aquatic organisms. Due to intensified usage of these products, exposure of organisms to BPA via several routes, such as the environment and food, has increased. The aquatic environment is an important area for the study of BPA. This report reviews the literature concerning contamination routes and degradation of BPA in the aquatic environment and its endocrine-disruptive effects on aquatic organisms.

A synchronized amphibian metamorphosis assay as an improved tool to detect thyroid hormone disturbance by endocrine disruptors and apolar sediment extracts

Amphibian metamorphosis assays are used to evaluate potential effects of endocrine disrupting compounds on the thyroid hormone axis. In this study, Xenopus laevis tadpoles are kept in a solution of 0.2% thiourea (TU) to arrest and synchronise them in their development. The advantage of this synchronized amphibian metamorphosis assays is that synchronised tadpoles are available at any time to start metamorphosis experiments, and experimental groups are much more homogenous at the start of experimental exposure compared with groups selected from an untreated pool of animals. The water volume per animal was kept constant throughout the experimental period to overcome the influence of declining numbers of animals per aquarium due to metamorphosis and mortality on the density dependent development of the remaining tadpoles. Clophen A50 (a technical PCB mixture), the single congener 3,3',4,4'-tetrachlorobiphenyl (PCB 77) and apolar sediment extracts that were previously tested positive in the T-Screen, an in vitro proliferation assay for thyroid hormone disruption, were tested in the Synchronized Amphibian Metamorphosis Assay. Endpoints studied were mortality, malformations, body weight, and percentage of metamorphosed froglets at the end of the 60-day experimental period, percentage of tadpoles in different developmental stages, and developmental stage-dependent awarded penalty points. Dietary exposure to Clophen A50 (0.2-50mg/kg food) resulted in a significant increased percentage of tadpoles that did not pass metamorphosis at concentrations higher than 2mg/kg food. Time until metamorphosis in those animals that were able to metamorphose after the 60-days experimental period was significantly decreased. Dietary exposure to PCB 77, a congener that can be readily metabolised, did not result in significant effects in any exposure group (2-500 microg/kg food). Apolar sediment extracts from two of the three sites that are contaminated with a wide variety of chemicals significantly decreased the percentage of metamorphosed animals and significantly increased the number of tadpoles that remained in early and late metamorphic stages. These effects already occurred when the extracts where diluted more than 1000 times (on an organic carbon base) compared to environmental concentrations. The rank of potency was comparable to results obtained with the T-screen. This suggests the presence of thyroid hormone disrupting compounds in the aquatic environment and possible effects of such compounds on animal development in the wild.

Delayed effects of environmentally relevant concentrations of 3,3',4,4'-tetrachlorobiphenyl (PCB-77) and non-polar sediment extracts detected in the prolonged-FETAX

In the prolonged-FETAX (prolonged-Frog Embryo Teratogenic Assay-Xenopus) tadpoles are allowed to develop until metamorphosis after an initial 4 day early life-stage exposure (FETAX). PCB 77 (3,4,3',4'-tetrachlorobiphenyl) and sediment extracts were used in the presented experiments. Concentrations of PCB 77 (0.03 nM=8.55 ng/L; 3 nM=855 ng/L; 300 nM=85.5 microg/L, 30,000 nM=8.55 mg/L) were derived on the basis of dioxin-like toxic equivalency concentrations known to be present in pore-water. The results were compared to those obtained with the standard FETAX. In the prolonged-FETAX the percentage of animals passing metamorphosis within 115 days was significantly decreased in the groups exposed to 300 or 30,000 nM PCB 77. Significantly increased percentages of tadpoles were halted in thyroid hormone dependent early metamorphic NF stages <or=55 (3 nM PCB 77) and metamorphic stages 56-60 tadpoles (300 nM PCB 77 and 30,000 nM PCB 77). Non-polar sediment extracts from two contaminated Dutch sites significantly decreased, and one significantly increased the percentage of animals that passed metamorphosis. These results were in accordance with earlier in vitro results in a thyroid hormone dependent assay and strongly suggest the presence of thyroid hormone disrupting compounds in the aquatic environment. In the classical FETAX neither PCB 77 nor non-polar sediment extracts induced any effects. The results of the present study indicate that the prolonged-FETAX is an important in vivo tool in addition to the in vitro T-screen to assess long-term effects of early life-stage exposure to environmentally relevant concentrations of (mixtures of) compounds and that affect thyroid hormone dependent physiology.

The hamster androgen receptor promoter: a molecular analysis

The steroid/thyroid hormone receptors are members of a very large family of nuclear-activated transcription factors. These receptors play a crucial role in most biological function, including regulation of development, metabolism, behaviour and reproduction. Among androgen receptor (AR), we have recently demonstrated that its expression in the Harderian gland (HG) of the male hamster is under a well-co-ordinated cross-talk between various steroid hormone receptors. Here, are presented data on the sequence of hamster AR promoter region (5'UTR) and the molecular tools of its regulation. The 5'UTR is 1585 bp. The promoter region shows various responsive elements. Two putative CREM elements are present at -71 and -1576 bp. A putative retinoic acid responsive element is present at -1476 bp. An androgen/glucocorticoid responsive element is present at -473 bp. A putative thyroid hormone-responsive element at -381 bp and an estrogen responsive element at -230 bp. Also, a homopurinic stretch is evident between -1199 and -1118. Furthermore, Sp1 sites are also spread along the sequence. As well as for human, mouse, rat and pig, the hamster lacks the canonical promoter TATA and CCAAT boxes. Gel retardation experiments confirm the presence of active responsive elements for AR, estrogen receptor, glucocorticoid receptor and thyroid hormone receptor. Previous data on the regulation of expression of AR by other members of steroid/thyroid hormone receptors well correlate with sequence analysis and gel retardation experiments. Thus, androgens, thyroid hormone, stimulate the AR transcription, while synthetic glucocorticoid (Dex) and estrogen are potent inhibitors of AR expression. The comparison of hamster AR promoter sequence with other AR promoter shows an 89, 82, 84 and 84% identity with human, rat, mouse and pig AR promoter, respectively. These results, in the light of the extreme plasticity of hamster HG, suggest that the comparative study of expression and regulation of AR gene in the HG of the hamster offers a useful tool to approach the normal and pathological phenotype in human.

Teratogenic and anti-metamorphic effects of bisphenol A on embryonic and larval Xenopus laevis

Effects of bisphenol A (BPA) on embryonic and larval development were investigated. In Xenopus laevis blastulae treated with 2.5-3.0 x 10(-5) M BPA or with 10(-5) M 17beta estradiol (E2), malformation of the head region, scoliosis (curved vertebrate), and suppression of organogenesis were observed. In addition, 10(-5)-10(-4) M BPA blocked tri-iodothyronine (T3)-inducible resorption of the tail segments from premetamorphic (stage 52-54) larvae in vitro. When stage 52 tadpoles were immersed in 1.0-2.5 x 10(-5) M BPA, deceleration of both spontaneous and thyroxin (T4)-induced metamorphic changes occurred. Furthermore, BPA suppressed thyroid hormone receptor (TR) beta gene expression both in vivo and in vitro. Thus, we concluded that BPA at the concentrations examined affects both embryonic development and larval metamorphosis.

Regulation of androgen receptor mRNA expression in primary culture of Harderian gland cells: cross-talk between steroid hormones

The androgen receptor (AR) must be considered a transcription factor belonging to the steroid-thyroid hormones receptor superfamily. Previous results gained from the Harderian gland, a tubulo-alveolar gland located in the orbital cavity of the golden hamster, indicate that Harderian gland cells express mRNAs encoding for androgen, glucocorticoid, thyroid hormone (T(3)), and estrogen receptors, respectively. Since in other systems, these receptors have been related to the expression of the androgen receptor, we have studied the regulation of AR expression in primary cultures of the male hamster Harderian gland. Our in vitro experiments show that androgen, and thyroid hormones increase the expression of AR. Retinoic acids also show a positive effect on AR expression, while exposure to glucocorticoid or estrogen blocks AR expression. Since these steroids differently modulate AR expression, our results must be considered in the context of multi-hormonal control of gene expression that could act through cross-talk between members of the steroid-thyroid hormones.

Regional pattern of retinoid X receptor-alpha gene expression in the central nervous system of the chicken embryo and its up-regulation by exposure to 9-cis retinoic acid

We have investigated the expression of the retinoid X receptor-alpha (RXRalpha) gene in the developing chicken embryo by using nonradioactive wholemount in situ hybridization. At the earliest stage of development examined (stage 9; Hamburger and Hamilton [1951] J. Morphol. 88:49-92), we detect RXRalpha transcripts in a stretch of neuroepithelium corresponding roughly to the presumptive caudal hindbrain. Upon formation of the rhombomeres at stage 12, a strongly RXRalpha-positive region extends from a sharp rostral limit at the boundary between rhombomeres 6 and 7 caudad to at least the level of somite 9. This pattern of highest expression continues at least until stage 22 but with some variability in the caudal extent. A lower level of expression extends throughout the spinal cord. Transverse sections show that RXRalpha transcripts are expressed in a gradient, with the highest levels near the roof plate and decreasing toward the floor plate. At later stages, the level of expression is highest in the proliferative ventricular zone. However, at reduced levels, RXRalpha transcripts are also detectable in the mantle zone as well as outside the developing central nervous system, for example, in the neural crest and the limb buds. Nine-cis-retinoic acid up-regulates RXRalpha transcripts at stages 19.5-22.0 within a few hours, augmenting but not expanding the expression pattern. Northern blots demonstrate the potential expression of multiple RXRalpha isoforms in the central nervous system at posthatch stages. These results implicate the RXRalpha receptor in both rostrocaudal and transverse patterning of the neural tube.