Met909 plays a key role in the activation of the progesterone receptor and also in the high potency of 13-ethyl progestins.

Many progestins have been developed for use in contraception, menopausal hormone therapy, and treatment of gynecological diseases. They are derived from either progesterone or testosterone, and they act by binding to the progesterone receptor (PR), a hormone-inducible transcription factor belonging to the nuclear receptor superfamily. Unlike mineralocorticoid, glucocorticoid, and androgen receptors, the steroid-receptor contacts that trigger the switch of the ligand-binding domain from an inactive to an active conformation have not yet been identified for the PR. With this aim, we solved the crystal structure of the ligand-binding domain of the human PR complexed with levonorgestrel, a potent testosterone-derived progestin characterized by a 13-ethyl substituent. Via mutagenesis analysis and functional studies, we identified Met909 of the helix 12 as the key residue for PR activation by both testosterone- and progesterone-derived progestins with a 13-methyl or a 13-ethyl substituent. We also showed that Asn719 contributes to PR activation by testosterone-derived progestins only, and that Met759 and Met909 are responsible for the high potency of 19-norprogestins and of 13-ethyl progestins, respectively. Our findings provide a structural guideline for the rational synthesis of potent PR agonist and antagonist ligands that could have therapeutic uses in women's health.

A rapid, physiologic protocol for testing transcriptional effects of thyroid-disrupting agents in premetamorphic Xenopus tadpoles.

Increasing numbers of substances present in the environment are postulated to have endocrine-disrupting effects on vertebrate populations. However, data on disruption of thyroid signaling are fragmentary, particularly at the molecular level. Thyroid hormone (TH; triiodothyronine, T3) acts principally by modulating transcription from target genes; thus, thyroid signaling is particularly amenable to analysis with a transcriptional assay. Also, T3 orchestrates amphibian metamorphosis, thereby providing an exceptional model for identifying thyroid-disrupting chemicals. We combined these two advantages to develop a method for following and quantifying the transcriptional action of T3 in Xenopus laevis tadpoles. This technology provides a means of assessing thyroid activity at the molecular level in a physiologically relevant situation. Moreover, translucent tadpoles are amenable to "on-line" imaging with fluorescent reporter constructs that facilitate in vivo measurement of transcriptional activity. We adapted transgenesis with TH-responsive elements coupled to either luciferase or green fluorescent protein to follow T3-dependent transcription in vivo. To reduce time of exposure and to synchronize responses, we optimized a physiologic pretreatment protocol that induced competence to respond to T3 and thus to assess T3 effects and T3 disruption within 48 hr. This pretreatment protocol was based on a short (24 hr), weak (10(-12) M) pulse of T3 that induced TH receptors, facilitating and synchronizing the transcriptional responses. This protocol was successfully applied to somatic and germinal transgenesis with both reporter systems. Finally, we show that the transcriptional assay allows detection of the thyroid-disrupting activity of environmentally relevant concentrations (10(-8) M) of acetochlor, a persistent herbicide.

Assessment of estrogenic endocrine-disrupting chemical actions in the brain using in vivo somatic gene transfer.

Estrogenic endocrine-disrupting chemicals abnormally stimulate vitellogenin gene expression and production in the liver of many male aquatic vertebrates. However, very few studies demonstrate the effects of estrogenic pollutants on brain function. We have used polyethylenimine-mediated in vivo somatic gene transfer to introduce an estrogen response element-thymidine kinase-luciferase (ERE-TK-LUC) construct into the brain. To determine if waterborne estrogenic chemicals modulate gene transcription in the brain, we injected the estrogen-sensitive construct into the brains of Nieuwkoop-Faber stage 54 Xenopus laevis tadpoles. Both ethinylestradiol (EE2; p < 0.002) and bisphenol A (BPA; p < 0.03) increased luciferase activity by 1.9- and 1.5-fold, respectively. In contrast, low physiologic levels of 17ss-estradiol had no effect (p > 0.05). The mixed antagonist/agonist tamoxifen was estrogenic in vivo and increased (p < 0.003) luciferase activity in the tadpole brain by 2.3-fold. There have been no previous reports of somatic gene transfer to the fish brain; therefore, it was necessary to optimize injection and transfection conditions for the adult goldfish (Carassius auratus). Following third brain ventricle injection of cytomegalovirus (CMV)-green fluorescent protein or CMV-LUC gene constructs, we established that cells in the telencephalon and optic tectum are transfected. Optimal transfections were achieved with 1 microg DNA complexed with 18 nmol 22 kDa polyethylenimine 4 days after brain injections. Exposure to EE2 increased brain luciferase activity by 2-fold in males (p < 0.05) but not in females. Activation of an ERE-dependent luciferase reporter gene in both tadpole and fish indicates that waterborne estrogens can directly modulate transcription of estrogen-responsive genes in the brain. We provide a method adaptable to aquatic organisms to study the direct regulation of estrogen-responsive genes in vivo.

An innovative continuous flow system for monitoring heavy metal pollution in water using transgenic Xenopus laevis tadpoles.

While numerous detection methods exist for environmental heavy metal monitoring, easy-to-use technologies combining rapidity with in vivo measurements are lacking. Multiwell systems exploiting transgenic tadpoles are ideal but require time-consuming placement of individuals in wells. We developed a real-time flow-through system, based on Fountain Flow cytometry, which measures in situ contaminant-induced fluorescence in transgenic amphibian larvae immersed in water samples. The system maintains the advantages of transgenic amphibians, but requires minimal human intervention. Portable and self-contained, it allows on-site measurements. Optimization exploited a transgenic Xenopus laevis bearing a chimeric gene with metal responsive elements fused to eGFP. The transgene was selectively induced by 1 microM Zn(2+). Using this tadpole we show the continuous flow method to be as rapid and sensitive as image analysis. Flow-through readings thus accelerate the overall process of data acquisition and render fluorescent monitoring of tadpoles suitable for on-site tracking of heavy metal pollution.

An in vivo multiwell-based fluorescent screen for monitoring vertebrate thyroid hormone disruption.

There is a pressing need for high throughput methods to assess potential effects of endocrine disrupting chemicals (EDCs). released into the environment. Currently our ability to identify effects in vitro exceeds that for in vivo monitoring. However, only in vivo analysis provides the full spectrum of physiological impacts exerted by a given chemical. With the aim of finding a physiological system compatible with automatic plate reading we tested the capacity of early embryonic stage Xenopus laevis tadpoles to monitor thyroid hormone (TH) disruption. Fluorescent transgenic X. laevis embryos bearing a TH/bZIP-eGFP construct, placed in 96 well plates, were used for a physiological-based screen for potential TH signaling disruptors. Using stage NF-45 embryos (time of thyroid gland formation) allowed rapid detection of chemical interference with both peripheral TR signaling and production of endogenous TH. Nanomolar concentrations of TH receptor agonists could be detected within 72 h. Moreover, when testing against a 5nM T3 challenge, the effects of inhibitors of TH production were revealed, including inhibitors of TH synthesis, (methimazole: 1 mM or sodium perchlorate: 3.56 microM), as well as antagonists acting at the receptor level (NH3: 2 microM) and a deiodinase inhibitor (iopanoic acid: 10 microM). Finally, we show that the thyroid disrupting activities of BPA (10 microM) and TBBPA (1 microM) can also be detected in this rapid screening protocol. Finally, this noninvasive technology using an automatic reading system shows low variability (around 5%) and permits detection of subtle changes in signaling by EDCs that either inhibit or activate TH signaling in vivo.