Evolution of vertebrate steroid receptors from an ancestral estrogen receptor by ligand exploitation and serial genome expansions

Structure and function of steroid receptor AF1 transactivation domains: induction of active conformations

Steroid hormones are important endocrine signalling molecules controlling reproduction, development, metabolism, salt balance and specialized cellular responses, such as inflammation and immunity. They are lipophilic in character and act by binding to intracellular receptor proteins. These receptors function as ligand-activated transcription factors, switching on or off networks of genes in response to a specific hormone signal. The receptor proteins have a conserved domain organization, comprising a C-terminal LBD (ligand-binding domain), a hinge region, a central DBD (DNA-binding domain) and a highly variable NTD (N-terminal domain). The NTD is structurally flexible and contains surfaces for both activation and repression of gene transcription, and the strength of the transactivation response has been correlated with protein length. Recent evidence supports a structural and functional model for the NTD that involves induced folding, possibly involving alpha-helix structure, in response to protein-protein interactions and structure-stabilizing solutes.

Mineralocorticoid receptors: distribution and activation

Mineralocorticoid receptors (MR) bind both mineralocorticoids and glucocorticoids with high affinity (deoxycorticosterone = corticosterone >/= aldosterone = cortisol), and are found in both Na(+) transporting epithelia (e.g. kidney, colon) and nonepithelial tissues (e.g. heart, brain). MR evolved before aldosterone synthase, consistent with their acting in nonepithelial tissues as high affinity glucocorticoid receptors, essentially always occupied by normal levels of endogenous glucocorticoids. In epithelial tissues the enzyme 11beta hydroxysteroid dehydrogenase Type 2 (11betaHSD2) allows aldosterone to selectively activate MR, by converting cortisol to cortisone and NAD to NADH. 11betaHSD2 debulks intracellular cortisol by 90%, to levels approximately 10-fold those of aldosterone, so that when the enzyme is operating most epithelial MR are occupied but not activated by cortisol. When intracellular redox state is changed-by inhibition of 11beta HSD2, generation of reactive oxygen species, or intracellular introduction of oxidised glutathione (GSSG)-cortisol changes from an MR antagonist to an MR agonist. This bivalent activity of cortisol appears to underlie the therapeutic efficacy of MR blockade in heart failure (RALES, EPHESUS) and in essential hypertension, providing a rationale for MR blockade in cardiovascular disease not characterized by elevated aldosterone levels. Its wider (patho)physiologic implications, particularly for neurobiology, remain to be explored.

Gene regulation profiles by progesterone and dexamethasone in human endometrial cancer Ishikawa H cells

OBJECTIVE

Progesterone and glucocorticoids such as dexamethasone mediate distinct biological functions, yet they bind to receptors that recognize the same consensus DNA response element. In breast cancer, progestins are associated with the incidence and progression of tumors, whereas glucocorticoids are growth-suppressive in mammary cancer cells; the differential effects of these two steroids are less well understood in the hormone-dependent disease cancer of the uterine endometrium. We set out to identify genes that are regulated by progesterone through progesterone receptors and dexamethasone through glucocorticoid receptors in a well-differentiated human endometrial cancer cell line.

METHODS

PR- and GR-positive Ishikawa H endometrial cancer cells were treated with vehicle, dexamethasone (100 nM) or progesterone (100 nM) for 2 h, 6 h, 12 h and 24 h, and RNA was isolated. Affymetrix microarrays were performed using the human HG-U133A chip, querying the expression of 22,000 genes. Expression of genes of particular interest was confirmed by real-time RT-PCR.

RESULTS

Expression analysis demonstrated that dexamethasone and progesterone regulate overlapping but distinct sets of genes and presumably exert many similar but also unique biological effects. Using real-time RT-PCR, we confirmed three particular genes of interest: the transcript for cysteine 1 (legumain), a gene associated with metastasis, that is strongly downregulated by progesterone, upstream c-fos relating transcription factor-2 (USF-2), an anti-proliferative factor that is induced by both progesterone and dexamethasone and N-cadherin, a cellular adhesion molecule downregulated by dexamethasone.

CONCLUSION

These studies provide new insight into the effects of progesterone and dexamethasone in endometrial cancer cells and provide an extensive list of regulated pathways which can be assessed in the future as biomarkers and molecular targets for new therapies. Taken together, our findings indicate that progesterone and dexamethasone are primarily growth inhibitors in Ishikawa H endometrial cancer cells.

The Smad6-histone deacetylase 3 complex silences the transcriptional activity of the glucocorticoid receptor: potential clinical implications

Glucocorticoids play pivotal roles in the maintenance of homeostasis but, when dysregulated, may also have deleterious effects. Smad6, one of the transforming growth factor beta (TGFbeta) family downstream transcription factors, interacts with the N-terminal domain of the glucocorticoid receptor (GR) through its Mad homology 2 domain and suppresses GR-mediated transcriptional activity in vitro. Adenovirus-mediated Smad6 overexpression inhibits glucocorticoid action in rat liver in vivo, preventing dexamethasone-induced elevation of blood glucose levels and hepatic mRNA expression of phosphoenolpyruvate carboxykinase, a well known rate-limiting enzyme of liver gluconeogenesis. Smad6 suppresses GR-induced transactivation by attracting histone deacetylase 3 to DNA-bound GR and by antagonizing acetylation of histone H3 and H4 induced by p160 histone acetyltransferase. These results indicate that Smad6 regulates glucocorticoid actions as a corepressor of the GR. From our results and known cross-talks between glucocorticoids and TGFbeta family molecules, it appears that the anti-glucocorticoid actions of Smad6 may contribute to the neuroprotective, anticatabolic and pro-wound healing properties of the TGFbeta family of proteins.

CAR and PXR: xenosensors of endocrine disrupters?

The pregnane X-receptor (PXR) and the constitutive androstane receptor (CAR) are orphan nuclear receptors activated by a variety of ligands. Currently it remains uncertain whether these receptors have a high-affinity ligand or instead function as more generalized steroid/xenobiotic sensors. Both receptors are important regulators of several steroid and xenobiotic detoxification enzymes and transporters (phases I-III) in the liver and intestine and thus are important regulators of adaptation to chemical stress. The detoxification proteins induced are responsible for the metabolism, deactivation and transport of bile acids, thyroid and steroid hormones, numerous environmental chemicals, and several drugs. PXR and CAR received their names because of steroid ligands that activate and inhibit their transcriptional activity, respectively. Interestingly, some steroids and steroid mimics activate one or both receptors, including several endocrine disrupting chemicals. Environmental estrogens, such as the pesticides methoxychlor, endosulfan, dieldrin, DDT, and the plasticizer nonylphenol activate either PXR or both PXR and CAR. Because PXR and CAR are activated by numerous steroids and endocrine disrupters, it appears that these receptors protect the integrity of the endocrine system. They recognize an increase in steroid-like chemicals and, in turn, induce detoxification. Furthermore, PXR and CAR induce enzymes, such as the CYP2B and CYP3A family members, responsible for the metabolism of steroid and thyroid hormones and this may alter their normal physiological function. This review summarizes the available data on the activity of endocrine disrupters and endocrine active chemicals on PXR and CAR, examines the role of PXR and CAR in protection from these chemicals, and evaluates potential adverse physiological consequences of PXR and CAR activation.

Evolutionary selection across the nuclear hormone receptor superfamily with a focus on the NR1I subfamily (vitamin D, pregnane X, and constitutive androstane receptors)

BACKGROUND

The nuclear hormone receptor (NR) superfamily complement in humans is composed of 48 genes with diverse roles in metabolic homeostasis, development, and detoxification. In general, NRs are strongly conserved between vertebrate species, and few examples of molecular adaptation (positive selection) within this superfamily have been demonstrated. Previous studies utilizing two-species comparisons reveal strong purifying (negative) selection of most NR genes, with two possible exceptions being the ligand-binding domains (LBDs) of the pregnane X receptor (PXR, NR1I2) and the constitutive androstane receptor (CAR, NR1I3), two proteins involved in the regulation of toxic compound metabolism and elimination. The aim of this study was to apply detailed phylogenetic analysis using maximum likelihood methods to the entire complement of genes in the vertebrate NR superfamily. Analyses were carried out both across all vertebrates and limited to mammals and also separately for the two major domains of NRs, the DNA-binding domain (DBD) and LBD, in addition to the full-length sequences. Additional functional data is also reported for activation of PXR and the vitamin D receptor (VDR; NR1I1) to gain further insight into the evolution of the NR1I subfamily.

RESULTS

The NR genes appear to be subject to strong purifying selection, particularly in the DBDs. Estimates of the ratio of the non-synonymous to synonymous nucleotide substitution rates (the omega ratio) revealed that only the PXR LBD had a sub-population of codons with an estimated omega ratio greater than 1. CAR was also unusual in showing high relative omega ratios in both the DBD and LBD, a finding that may relate to the recent appearance of the CAR gene (presumably by duplication of a pre-mammalian PXR gene) just prior to the evolution of mammals. Functional analyses of the NR1I subfamily show that human and zebrafish PXRs show similar activation by steroid hormones and early bile salts, properties not shared by sea lamprey, mouse, or human VDRs, or by Xenopus laevis PXRs.

CONCLUSION

NR genes generally show strong sequence conservation and little evidence for positive selection. The main exceptions are PXR and CAR, genes that may have adapted to cross-species differences in toxic compound exposure.

The distributions of the duplicate oestrogen receptors ER-beta a and ER-beta b in the forebrain of the Atlantic croaker (Micropogonias undulatus): evidence for subfunctionalization after gene duplication

Teleost fishes have three distinct oestrogen receptor (ER) subtypes: ER-alpha, ER-beta a (or ER-gamma) and ER-beta b. ER-beta a and ER-beta b arose from a duplication of an ancestral ER-beta gene early in the teleost lineage. Here, we describe the distribution of the three ER mRNAs in the hypothalamus and cerebellum of the Atlantic croaker to address two issues: the specific functions of multiple ERs in the neuroendocrine system and the evolution and fate of duplicated genes. ER-alpha was detected in nuclei of the preoptic area (POA) and hypothalamus previously shown to possess ER-alphas in teleosts. AcER-beta b, but not ER-beta a, labelling was detected in the magnocellular neurons of the POA, nucleus posterior tuberis, the nucleus recessus posterior and cerebellum. By contrast, acER-beta a, but not ER-beta b, was detected in the dorsal anterior parvocellular POA and suprachiasmatic nucleus. Both ER-betas were found in posterior parvocellular and ventral anterior POA nuclei, the ventral hypothalamus, and periventricular dorsal hypothalamus. The differences we observed in ER subtype mRNA distribution within well-characterized brain nuclei suggest that ER-beta a and ER-beta b have distinct functions in the neuroendocrine control of reproduction and behaviour, and provide evidence that the teleost ER-beta paralogues have partitioned functions of the ancestral ER-beta gene they shared with tetrapods.

Translational regulatory mechanisms generate N-terminal glucocorticoid receptor isoforms with unique transcriptional target genes

Glucocorticoids regulate diverse physiological functions ranging from mitosis to apoptosis, although only one glucocorticoid receptor (GR) gene has been discovered. We report here that one single GR mRNA species unexpectedly produces at least eight functional GR N-terminal isoforms via translational mechanisms. These GR isoforms display diverse cytoplasm-to-nucleus trafficking patterns and distinct transcriptional activities. In human osteosarcoma cells, the transcriptional responses to glucocorticoids closely reflect the identity and abundance of the GR isoforms. In addition, each GR isoform regulates both a common and a unique set of genes in the same cell. Interestingly, the levels of these GR isoforms differ significantly among tissues. Based on these observations, we propose that cell-type specific GR isoforms generate specificity in glucocorticoid control of transcription in different tissues.

Sea bass (Dicentrarchus labrax) androgen receptor: cDNA cloning, tissue-specific expression, and mRNA levels during early development and sex differentiation

Androgens play key roles in vertebrate sex differentiation, gonadal maturation and reproductive behaviour and their actions are generally mediated through specific nuclear receptors. The present study describes the isolation, sequencing and characterization of the cDNA encoding the androgen receptor (AR) in the European sea bass. AR was cloned from a sea bass testis cDNA library and encoded a predicted protein of 767 residues, with a calculated molecular weight of 86.4 kDa and a theoretical pI of 6.34. Several domains present in all cloned ARs were identified. The domains corresponded to an amino-terminal hypervariable transcriptional activation domain (TAD), a central highly conserved DNA-binding domain (DBD), and a carboxy-terminal ligand-binding domain (LBD). Percentages of homology-similarity among these functional domains in teleost fish ranged between 9 and 75% for the TAD, 73 and 98% for the DBD, and 78 and 96% for the LBD when compared to those of the sea bass. Tissue-specific expression showed that AR was preferentially expressed in testis, ovaries, and brain. Some other tissues such as the head kidney, liver and spleen also showed AR expression although at very low levels. A semiquantitative PCR was developed to study the expression of AR mRNA during the period of development encompassed between 50 and 300 DPH in sea bass gonads. An experimental design, involving repeated size gradings, based on the fact that sea bass females are larger than males already at sex differentiation, was set to obtain a group consisting of the largest fish (female-dominant) and a group consisting of the smallest fish (male-dominant). The results showed very low mRNA expression levels of AR in the gonads during early development. Differences in AR expression between groups were first encountered at 150 DPH and became especially marked at 250 DPH with much higher levels in the male-dominant group. These sex-related differences in expression profiles between males and females by the time of sex differentiation, suggest an important role for AR controlling this process in the sea bass.

Oestrogen functions in skin and skin appendages

Oestrogens have significant effects on different cell types important in skin physiology, including the epidermal keratinocytes, dermal fibroblasts and melanocytes. In addition, they can also modulate skin appendages such as the hair follicle, the sebaceous gland and the apocrine glands. Oestrogens may also have important modulatory roles in events such as skin ageing, pigmentation, hair growth, sebum production and skin cancer. It is now recognised that oestrogens can modulate their actions via two distinct intracellular receptors (ERalpha and ERbeta) or via cell surface receptors, which activate specific second messenger signalling pathways. This paper highlights the effects of oestrogens on different components of the skin and reviews some of the more recent developments in terms of receptor expression and cell signalling pathways.

Phytoestrogens modulate prostaglandin production in bovine endometrium: cell type specificity and intracellular mechanisms

Prostaglandins (PGs) are known to modulate the proper cyclicity of bovine reproductive organs. The main luteolytic agent in ruminants is PGF2alpha, whereas PGE2 has luteotropic actions. Estradiol 17beta (E2) regulates uterus function by influencing PG synthesis. Phytoestrogens structurally resemble E2 and possess estrogenic activity; therefore, they may mimic the effects of E2 on PG synthesis and influence the reproductive system. Using a cell-culture system of bovine epithelial and stromal cells, we determined cell-specific effects of phytoestrogens (i.e., daidzein, genistein), their metabolites (i.e., equol and para-ethyl-phenol, respectively), and E2 on PGF2alpha and PGE2 synthesis and examined the intracellular mechanisms of their actions. Both PGs produced by stromal and epithelial cells were significantly stimulated by phytoestrogens and their metabolites. However, PGF2alpha synthesis by both kinds of cells was greater stimulated than PGE2 synthesis. Moreover, epithelial cells treated with phytoestrogens synthesized more PGF2alpha than stromal cells, increasing the PGF2alpha to PGE2 ratio. The epithelial and stromal cells were preincubated with an estrogen-receptor (ER) antagonist (i.e., ICI), a translation inhibitor (i.e., actinomycin D), a protein kinase A inhibitor (i.e., staurosporin), and a phospholipase C inhibitor (i.e., U73122) for 0.5 hrs and then stimulated with equol, para-ethyl-phenol, or E2. Although the action of E2 on PGF2alpha synthesis was blocked by all reagents, the stimulatory effect of phytoestrogens was blocked only by ICI and actinomycin D in both cell types. Moreover, in contrast to E2 action, phytoestrogens did not cause intracellular calcium mobilization in either epithelial or stromal cells. Phytoestrogens stimulate both PGF2alpha and PGE2 in both cell types of bovine endometrium via an ER-dependent genomic pathway. However, because phytoestrogens preferentially stimulated PGF2alpha synthesis in epithelial cells of bovine endometrium, they may disrupt uterus function by altering the PGF2alpha to PGE2 ratio.

Rapid Creation of a Novel Protein Function by in Vitro Coevolution

We have developed a simple and efficient method for creation of novel protein functions in an existing protein scaffold. The in vitro coevolution method involves design of a hypothetical pathway for the target function followed by stepwise directed evolution of the corresponding protein along the pathway. As a test case, this strategy was used to engineer variants of human estrogen receptor alpha ligand-binding domain (hERalphaLBD) with novel corticosterone activity. Two steroids, testosterone and progesterone, that provide a progressive structural bridge between 17beta-estradiol and corticosterone, were chosen to assist the directed evolution of hERalphaLBD. A total of approximately 10(6) variants were screened in four rounds of random mutagenesis, resulting in two hERalphaLBD variants that respond to corticosterone. Creation of this new ligand activity required the presence of four simultaneous mutations. In addition, several required mutations were located outside the ligand binding pocket and yet exerted important action on ligand binding. Our results demonstrate the ability of in vitro coevolution to create novel protein function that is difficult or impossible to achieve by existing protein engineering approaches and also shed light on the natural evolution of nuclear hormone receptors. This in vitro coevolution approach should provide a powerful, broadly applicable tool for engineering biological molecules and systems with novel functions.

A Novel Antiestrogenic Mechanism in Progesterone Receptor-transfected Breast Cancer Cells

The expression of progesterone receptor (PR) is normally estrogen-dependent, and progesterone is only active in target cells following estrogen exposure. This study revealed that the effect of estrogen was markedly disrupted by estrogen-independent expression of PR. Transfection of PR in estrogen receptor (ER)-positive MCF-7 cells abolished the estradiol-17beta growth stimulatory effect that was observed in the parental cells and the vector-transfected controls in a ligand-independent manner. The antiestrogenic effect was also observed at the level of gene transcription. Estradiol-17beta (E2)-induced gene expression of pS2 and GREB1 was impaired by 50-75% after 24-72 h of E2 treatment in PR-transfected cells. Promoter interference assay revealed that PR transfection drastically inhibited E2-mediated ER binding to estrogen response elements (ERE). The antiestrogenic effects of transfected PR are associated with enhanced metabolism of E2. HPLC analysis of [3H]E2 in the samples indicated that the percentage of [3H]E2 metabolized by PR-transfected cells in 6 h is similar to that by vector-transfected control cells in 24 h (77 and 80%, respectively). The increased metabolism of E2 may, in turn, be caused by increased cellular uptake of E2, as demonstrated by whole cell binding of [3H]E2. The findings open up a new window for a hitherto unknown functional relationship between the PR and ER. The antiestrogenic effect of transfected PR also provides a potential therapeutic strategy for estrogen-dependent breast cancer.

Evolution of the pregnane x receptor: adaptation to cross-species differences in biliary bile salts

The pregnane X receptor (PXR) regulates the metabolism and elimination of bile salts, steroids, and xenobiotics. The sequence of the PXR ligand-binding domain diverges extensively between different animals, suggesting interspecies differences in ligands. Of the endogenous ligands known to activate PXR, biliary bile salts vary the most across vertebrate species, ranging from 27-carbon (C27) bile alcohol sulfates (early fish, amphibians) to C24 bile acids (birds, mammals). Using a luciferase-based reporter assay, human PXR was activated by a wide variety of bile salts. In contrast, zebrafish PXR was activated efficiently only by cyprinol sulfate, the major zebrafish bile salt, but not by recent bile acids. Chicken, mouse, rat, and rabbit PXRs were all activated by species-specific bile acids and by early fish bile alcohol sulfates. In addition, phylogenetic analysis using maximum likelihood demonstrated evidence for nonneutral evolution of the PXR ligand-binding domain. PXR activation by bile salts has expanded from narrow specificity for C27 bile alcohol sulfates (early fish) to a broader specificity for recent bile acids (birds, mammals). PXR specificity for bile salts has thus paralleled the increasing complexity of the bile salt synthetic pathway during vertebrate evolution, an unusual example of ligand-receptor coevolution in the nuclear hormone receptor superfamily.

Identity of an estrogen membrane receptor coupled to a G protein in human breast cancer cells

Although nonclassical estrogen actions initiated at the cell surface have been described in many tissues, the identities of the membrane estrogen receptors (mERs) mediating these actions remain unclear. Here we show that GPR30, an orphan receptor unrelated to nuclear estrogen receptors, has all the binding and signaling characteristics of a mER. A high-affinity (dissociation constant 2.7 nm), limited capacity, displaceable, single binding site specific for estrogens was detected in plasma membranes of SKBR3 breast cancer cells that express GPR30 but lack nuclear estrogen receptors. Progesterone-induced increases and small interfering RNA-induced decreases in GPR30 expression in SKBR3 cells were accompanied by parallel changes in specific estradiol-17beta (E2) binding. Plasma membranes of human embryonic kidney 293 cells transfected with GPR30, but not those of untransfected cells, and human placental tissues that express GPR30 also displayed high-affinity, specific estrogen binding typical of mERs. E2 treatment of transfected cell membranes caused activation of a stimulatory G protein that is directly coupled to the receptor, indicating GPR30 is a G protein-coupled receptor (GPCR), and also increased adenylyl cyclase activity. The finding that the antiestrogens tamoxifen and ICI 182,780, and an environmental estrogen, ortho,para-dichlorodiphenyldichloroethylene (o,p'-DDE), have high binding affinities to the receptor and mimic the actions of E2 has important implications for both the development and treatment of estrogen-dependent breast cancer. GPR30 is structurally unrelated to the recently discovered family of GPCR-like membrane progestin receptors. The identification of a second distinct class of GPCR-like steroid membrane receptors suggests a widespread role for GPCRs in nonclassical steroid hormone actions.

Deer antlers as a model of Mammalian regeneration

Deer antlers are cranial appendages that develop after birth as extensions of a permanent protuberance (pedicle) on the frontal bone. Pedicles and antlers originate from a specialized region of the frontal bone; the 'antlerogeneic periosteum' and the systemic cue which triggers their development in the fawn is an increase in circulating androgen. These primary antlers are then shed and regenerated the following year in a larger, more complex form. Antler growth is extremely rapid-an adult red deer can produce a pair of antlers weighing approximately 30kg in three months, and involves both endochondral and intramembranous ossification. Since antlers are sexual secondary characteristics, their annual cycles of growth have evolved to be closely coordinated to the reproductive cycle which, in temperate species, is linked to the photoperiod. Cessation of antler growth and death of the overlying skin (velvet) coincides with a rise in circulating testosterone as the autumn breeding season approaches. The 'dead' antlers remain attached to the pedicle until they are shed (cast) the following spring when circulating testosterone levels fall. In red deer, the species that we study, casting of the old set of antlers is followed immediately by growth of the new set. Although the anatomy of antler growth and the endocrine changes associated with it have been well documented, the molecular mechanisms involved remain poorly understood. The case for continuing to decipher them remains compelling, despite the obvious limitations of using deer as an experimental model, because this research will help provide insight into why humans and other mammals have lost the ability to regenerate organs. From the information so far available, it would appear that the signaling pathways that control the development of skeletal elements are recapitulated in regenerating antlers. This apparent lack of any specific 'antlerogenic molecular machinery' suggests that the secret of deers' ability to regenerate antlers lies in the particular cues to which multipotential progenitor/stem cells in an antler's 'regeneration territory' are exposed. This in turn suggests that with appropriate manipulation of the environment, pluripotential cells in other adult mammalian tissues could be stimulated to increase the healing capacity of organs, even if not to regenerate them completely. The need for replacement organs in humans is substantial. The benefits of increasing individuals' own capacity for regeneration and repair are self evident.

In situ Characterization of Gonadotropin- Releasing Hormone-I, -III, and Glutamic Acid Decarboxylase Expression in the Brain of the Sea Lamprey, Petromyzon marinus

The distribution of lamprey gonadotropin-releasing hormone (GnRH)-I and -III has been extensively characterized by immunocytochemistry in the forebrain of the sea lamprey, Petromyzon marinus. However, the cellular location of lamprey GnRH-III mRNA expression by in situ hybridization in the lamprey brain has not been determined. We show for the first time the location of expression of lamprey GnRH-III, as well as provide a more comprehensive in situ study of lamprey GnRH-I and glutamic acid decarboxylase (GAD; GABA-synthesizing enzyme) mRNA expression in the brain of the lamprey in different reproductive life stages. Colorimetric and dual-label fluorescent amplification methods of in situ hybridization were used on brain tissue sections of adult, juvenile, and larval sea lamprey. In each life stage of the lamprey, expression of lamprey GnRH-I was shown in the preoptic area (POA) and the hypothalamus forming the characteristic arc-like cell population extending from the preoptic nucleus (NPO) to the neurohypophysis. Lamprey GnRH-III expression was also seen in the POA of each life stage in close proximity to lamprey GnRH-I mRNA containing neurons. GAD expression was shown in distinct cell clusters in and around the POA, in the olfactory bulb, in the dorsal thalamus beneath the habenular region, and also in the ventral-medial hypothalamus stretching from the periventricular region to the anterior portion of the rhombencephalon. Using dual-label in situ hybridization, we have shown that lamprey GnRH-I and -III mRNA are colocalized in the same cells in the POA in adult lampreys. Dual-label in situ hybridization also showed close proximity of GAD mRNA containing neurons and GnRH containing neurons in the POA. These data suggest that gamma-aminobutyric acid (GABA) may directly affect GnRH release in the brain of the sea lamprey.

Structural basis for androgen receptor interdomain and coactivator interactions suggests a transition in nuclear receptor activation function dominance

The androgen receptor (AR) is required for male sex development and contributes to prostate cancer cell survival. In contrast to other nuclear receptors that bind the LXXLL motifs of coactivators, the AR ligand binding domain is preferentially engaged in an interdomain interaction with the AR FXXLF motif. Reported here are crystal structures of the ligand-activated AR ligand binding domain with and without bound FXXLF and LXXLL peptides. Key residues that establish motif binding specificity are identified through comparative structure-function and mutagenesis studies. A mechanism in prostate cancer is suggested by a functional AR mutation at a specificity-determining residue that recovers coactivator LXXLL motif binding. An activation function transition hypothesis is proposed in which an evolutionary decline in LXXLL motif binding parallels expansion and functional dominance of the NH(2)-terminal transactivation domain in the steroid receptor subfamily.

Androgen dependent development of a modified anal fin, gonopodium, as a model to understand the mechanism of secondary sexual character expression in vertebrates

Male external genitalia show structural variations among species. Androgenic hormones are essential for the morphological specification of male type copulatory organs, while little is known about the developmental mechanisms of such secondary sexual characters. Western mosquitofish Gambusia affinis may offer a clue to the sexual differentiation researches, because they show a prominent masculine sexual character for appendage development, anal fin to gonopodium (GP) transition, and its formation could be induced in early juvenile fry by exogenously supplied androgens. We show that GP development is promoted by androgen dependent augmentation of sonic hedgehog (Shh) expression. Two AR cDNAs were cloned and identified as ARalpha and ARbeta from western mosquitofish. Both ARs were predominantly expressed in the distal region of outgrowing anal fin rays. Exposure of fry to androgen caused anal fin outgrowth concomitant with the Shh induction in the distal anal fin ray epithelium. When AR signaling was inhibited by its antagonist flutamide in fry, the initial induction of the Shh was suppressed accompanying retarded anal fin outgrowth. Similar suppression of anal fin outgrowth was induced by treatment with cyclopamine, an inhibitor of Shh signaling. These observations indicate that androgen dependent Shh expression is required for anal fin outgrowth leading to the formation of a genital appendage, the GP in teleost fishes. Androgen-induced GP formation may provide insights into the expression mechanism regulating the specification of sexual features in vertebrates.

Genome duplication in the teleost fish Tetraodon nigroviridis reveals the early vertebrate proto-karyotype

Tetraodon nigroviridis is a freshwater puffer fish with the smallest known vertebrate genome. Here, we report a draft genome sequence with long-range linkage and substantial anchoring to the 21 Tetraodon chromosomes. Genome analysis provides a greatly improved fish gene catalogue, including identifying key genes previously thought to be absent in fish. Comparison with other vertebrates and a urochordate indicates that fish proteins have diverged markedly faster than their mammalian homologues. Comparison with the human genome suggests approximately 900 previously unannotated human genes. Analysis of the Tetraodon and human genomes shows that whole-genome duplication occurred in the teleost fish lineage, subsequent to its divergence from mammals. The analysis also makes it possible to infer the basic structure of the ancestral bony vertebrate genome, which was composed of 12 chromosomes, and to reconstruct much of the evolutionary history of ancient and recent chromosome rearrangements leading to the modern human karyotype.

Convergence of lipid homeostasis through liver X and thyroid hormone receptors

Members of the nuclear receptor gene family act as biological rheostats to maintain metabolic homeostasis in response to endocrine and nutritional changes. The liver X (LXR) and thyroid hormone (TR) receptors have been shown to regulate overlapping but distinct metabolic pathways important for overall lipid homeostasis. Dyslipidemia is one out of four key determinants for cardiovascular risk and both LXRs and TRs may provide attractive targets for intervention of cardiovascular disease. In this review we will compare the two receptor systems to highlight similarities and differences in structure and function with implications for development of novel treatments for dyslipidemia and atherosclerosis.

Progesterone: the forgotten hormone in men?

'Classical' genomic progesterone receptors appear relatively late in phylogenesis, i.e. it is only in birds and mammals that they are detectable. In the different species, they mediate manifold effects regarding the differentiation of target organ functions, mainly in the reproductive system. Surprisingly, we know little about the physiology, endocrinology, and pharmacology of progesterone and progestins in male gender or men respectively, despite the fact that, as to progesterone secretion and serum progesterone levels, there are no great quantitative differences between men and women (at least outside the luteal phase). In a prospective cohort study of 1026 men with and without cardiovascular disease, we were not able to demonstrate any age-dependent change in serum progesterone concentrations. Progesterone influences spermiogenesis, sperm capacitation/acrosome reaction and testosterone biosynthesis in the Leydig cells. Other progesterone effects in men include those on the central nervous system (CNS) (mainly mediated by 5alpha-reduced progesterone metabolites as so-called neurosteroids), including blocking of gonadotropin secretion, sleep improvement, and effects on tumors in the CNS (meningioma, fibroma), as well as effects on the immune system, cardiovascular system, kidney function, adipose tissue, behavior, and respiratory system. A progestin may stimulate weight gain and appetite in men as well as in women. The detection of progesterone receptor isoforms would have a highly diagnostic value in prostate pathology (benign prostatic hypertrophy and prostate cancer). The modulation of progesterone effects on typical male targets is connected with a great pharmacodynamic variability. The reason for this is that, in men, some important effects of progesterone are mediated non-genomically through different molecular biological modes of action. Therefore, the precise therapeutic manipulation of progesterone actions in the male requires completely new endocrine-pharmacological approaches.

The origin and functions of multiple human glucocorticoid receptor isoforms

Glucocorticoid hormones are necessary for life and are essential in all aspects of human health and disease. The actions of glucocorticoids are mediated by the glucocorticoid receptor (GR), which binds glucocorticoid hormones and regulates gene expression, cell signaling, and homeostasis. Decades of research have focused on the mechanisms of action of one isoform of GR, GRa. However, in recent years, increasing numbers of human GR (hGR) isoforms have been reported. Evidence obtained from this and other laboratories indicates that multiple hGR isoforms are generated from one single hGR gene via mutations and/or polymorphisms, transcript alternative splicing, and alternative translation initiation. Each hGR protein, in turn, is subject to a variety of posttranslational modifications, and the nature and degree of posttranslational modification affect receptor function. We summarize here the processes that generate and modify various hGR isoforms with a focus on those that impact the ability of hGR to regulate target genes. We speculate that unique receptor compositions and relative receptor proportions within a cell determine the specific response to glucocorticoids. Unchecked expression of some isoforms, for example hGRbeta, has been implicated in various diseases.

Directed Evolution of Human Estrogen Receptor Variants with Significantly Enhanced Androgen Specificity and Affinity

Human estrogen receptor alpha (hERalpha) and human androgen receptor exhibit exquisite ligand specificity, which underlies their remarkable ability to effect ligand-regulated gene transcription in a highly distinctive and specific manner. Here we used a directed evolution approach to create hERalpha variants with enhanced androgen specificity and affinity with the goal to better understand the molecular basis of ER ligand specificity and the evolutionary mechanism of nuclear receptors. We developed a sensitive yeast two-hybrid system to screen for hERalpha variants with increased transactivation potency toward testosterone. After two rounds of directed evolution, we identified five hERalpha variants with dramatically improved transactivation potency toward testosterone in both yeast and mammalian cells. These variants showed up to 7,600-fold improvement in the binding affinity for testosterone and only slightly reduced affinity toward 17beta-estradiol. Detailed analysis of these evolved variants and a few site-directed mutants generated de novo led to several unexpected findings including the following. 1) Only two beneficial mutations were needed to create hERalpha variants with near nanomolar affinity for testosterone. 2) Some beneficial mutations were synergistic, context-dependent, or non-additive. 3) Of the five identified beneficial mutations, four of them were not in the ER ligand binding pocket and yet exerted important action on ligand specificity. 4) The single ligand-contacting mutation E353Q plays a dominant role in discriminating androgens and estrogens. These results, viewed in conjunction with the ligand exploitation model of nuclear receptor evolution, suggest that the mutation E353Q may represent a key event in the evolution of androgen receptors from an ancestral estrogen receptor and that ligand promiscuity may play an important role in the creation of new nuclear receptors via divergent evolution.

15Alpha-hydroxyprogesterone in male sea lampreys, Petromyzon marinus L

There is growing evidence that sea lampreys, Petromyzon marinus L., produce gonadal steroids differing from those of other vertebrates by possessing an additional hydroxyl group at the C15 position. Here we demonstrate that sea lamprey testes produce 15alpha-hydroxyprogesterone (15alpha-P) in vitro when incubated with tritiated progesterone, that 15alpha-P is present in the plasma of sea lampreys, and that plasma concentrations of immunoreactive (ir) 15alpha-P rise dramatically in response to injections of gonadotropin-releasing hormone (GnRH). The identity of the tritiated 15alpha-P produced in vitro was confirmed by co-elution with standard 15alpha-P on high performance liquid chromatography, co-elution with standard and acetylated 15alpha-P on thin layer chromatography, and specific binding to antibodies raised against standard 15alpha-P. The in vitro conversion was used to produce tritiated 15alpha-P label for a radioimmunoassay (RIA), which is able to detect 15alpha-P in amounts as low as 2 pg per tube. The RIA has been used to measure the plasma concentrations of 15alpha-P in males given two serial injections, 24 h apart, of either lamprey GnRH I or GnRH III (50, 100, or 200 microg/kg) or saline control, with plasma being sampled 8 and 24 h after the second injection. Plasma concentrations of ir-15alpha-P rose from < 1 to 36 ng/ml (mean of all treatments) 8 h after injection and declined within 24 h. This is the first time that an RIA has detected such high steroid concentrations in lampreys. This finding is suggestive of a role for 15alpha-P in control of reproduction in the sea lamprey.

An Androgen Receptor NH2-terminal Conserved Motif Interacts with the COOH Terminus of the Hsp70-interacting Protein (CHIP)

The NH2-terminal sequence of steroid receptors is highly variable between different receptors and in the same receptor from different species. In this study, a primary sequence homology comparison identified a 14-amino acid NH2-terminal motif of the human androgen receptor (AR) that is common to AR from all species reported, including the lower vertebrates. The evolutionarily conserved motif is unique to AR, with the exception of a partial sequence in the glucocorticoid receptor of higher species. The presence of the conserved motif in AR and the glucocorticoid receptor and its absence in other steroid receptors suggests convergent evolution. The function of the AR NH2-terminal conserved motif was suggested from a yeast two-hybrid screen that identified the COOH terminus of the Hsp70-interacting protein (CHIP) as a binding partner. We found that CHIP functions as a negative regulator of AR transcriptional activity by promoting AR degradation. In support of this, two mutations in the AR NH2-terminal conserved motif previously identified in the transgenic adenocarcinoma of mouse prostate model reduced the interaction between CHIP and AR. Our results suggest that the AR NH2-terminal domain contains an evolutionarily conserved motif that functions to limit AR transcriptional activity. Moreover, we demonstrate that the combination of comparative sequence alignment and yeast two-hybrid screening using short conserved peptides as bait provides an effective strategy to probe the structure-function relationships of steroid receptor NH2-terminal domains and other intrinsically unstructured transcriptional regulatory proteins.

Chemical communication threatened by endocrine-disrupting chemicals

Communication on a cellular level--defined as chemical signaling, sensing, and response--is an essential and universal component of all living organisms and the framework that unites all ecosystems. Evolutionarily conserved signaling "webs," existing both within an organism and between organisms, rely on efficient and accurate interpretation of chemical signals by receptors. Therefore, endocrine-disrupting chemicals (EDCs), which have been shown to disrupt hormone signaling in laboratory animals and exposed wildlife, may have broader implications for disrupting signaling webs that have yet to be identified as possible targets. In this article, I explore common evolutionary themes of chemical signaling (e.g., estrogen signaling in vertebrates and phytoestrogen signaling from plants to symbiotic soil bacteria) and show that such signaling systems are targets of disruption by EDCs. Recent evolutionary phylogenetic data have shown that the estrogen receptor (ER) is the ancestral receptor from which all other steroid receptors have evolved. In addition to binding endogenous estrogens, ERs also bind phytoestrogens, an ability shared in common with nodulation D protein (NodD) receptors found in Rhizobium soil bacteria. Recent data have shown that many of the same synthetic and natural environmental chemicals that disrupt endocrine signaling in vertebrates also disrupt phytoestrogen-NodD receptor signaling in soil bacteria, which is necessary for nitrogen-fixing symbiosis. Bacteria-plant symbiosis is an unexpected target of EDCs, and other unexpected nontarget species may also be vulnerable to EDCs found in the environment.

15alpha-hydroxytestosterone induction by GnRH I and GnRH III in Atlantic and Great Lakes sea lamprey (Petromyzon marinus L.)

The sea lamprey (Petromyzon marinus L.) represents one of the two most ancient classes of vertebrates and possesses a functional hypothalamus-pituitary-gonadal axis. However, the presence and functionality of androgens in the sea lamprey remain elusive. Recently, 15alpha-hydroxytestosterone (15alpha-T) has been found in sea lamprey gonads and blood plasma. In this study we examined changes of circulatory concentrations of 15alpha-T in response to gonadotropin releasing hormone (GnRH) treatments. Plasma concentrations of 15alpha-T in sea lamprey increased 2-5 times for all GnRH-injected sea lamprey compared to controls (P < 0.001). However, there were no differences among responses: (1) to the two forms of GnRH (lamprey GnRH I or lamprey GnRH III), (2) to the doses delivered (50, 100, or 200 microg/kg), or (3) between post-injection sample intervals (8 or 24 h). Between lampreys from the Atlantic Ocean and Great Lakes sites, two of seven GnRH form and dosage comparisons showed between-site differences, but were not believed to represent an overall between-site difference. These are the first data to show a response of a C19 steroid to GnRH stimulation in sea lamprey.

Estrogen-inducible uterine flavonoid binding sites: is it time to reconsider?

Epidemiological data support the beneficial effect of plant flavonoids on human health including anti-inflammatory and cancer preventing actions. The phytoestrogen flavonoids might interfere with estrogen action. The possible relations between the steroid- and the flavonoid-signalling in animal and plant cells have been addressed in numerous studies in the past decade. In search for possible sites of conjunction between these phenomena the post-receptor targets must not be disregarded. The estrogen-inducible type II estrogen binding sites of rat uteri have first been reported 25 years ago by Clark and coworkers [Biochem. Biophys. Res. Commun. 81 (1978) 1]. These sites are known to bind catecholic flavonoids with considerable affinity. Behaviour of the tyrosinase-like enzymatic activity associated with these sites appeared reminiscent to the recently described dopachrome oxidase or tautomerase activity exhibited by the cytokine macrophage migration inhibitory factor (MIF) inasmuch as it also accepts a broad range of catecholic melanogenic precursors. Therefore we assessed, whether the known type II ligand flavonoids interfere with the MIF tautomerase. We report here, that luteolin and quercetin have a biphasic effect on the enol-keto conversion of phenylpyruvate mediated by MIF tautomerase. We also demonstrate the presence of MIF immunoreactivity by Western blotting in rat uterine nuclear extracts prepared according to the method that yields high type II binding activity. These data support the possible participation of MIF in type II estrogen binding phenomena.

Minireview: Neuronal steroid hormone receptors: they're not just for hormones anymore

The ovarian steroid hormones have numerous effects on the brain, many of which are mediated, at least in part, by interaction with intracellular steroid hormone receptors acting as regulators of transcription. These intracellular steroid hormone receptors have often been considered to be activated solely by cognate hormone. However, during the past decade, numerous studies have shown that the receptors can be activated by neurotransmitters and intracellular signaling systems, through a process that does not require hormone. Although most of these have been in vitro experiments, others have been in vivo. Evidence from a wide variety of tissues and cells suggests that steroid hormone receptors are transcription factors that can be activated by a wide variety of factors, only one of which is cognate hormone. Furthermore, ligand-independent activation of neural steroid hormone receptors, rather than being a pharmacological or in vitro curiosity, seems to be a process that occurs in the normal physiology of animals. Thinking of steroid hormone receptors only as ligand-activated proteins may constrain our thinking about the many factors that may activate the receptors and cause receptor-dependent changes in neural gene expression and neuroendocrine function.

Anatomy of the estrogen response element

Estrogens exert their regulatory potential on gene expression through different nuclear and non-nuclear mechanisms. A direct nuclear approach is the interaction of estrogen with specific target sequences of DNA, estrogen response elements (ERE) or units. EREs can be grouped into perfect and imperfect palindromic sequences with the imperfect sequences differing from the consensus sequence in one or more nucleotides and being less responsive to the activated estrogen-estrogen receptor (ER) complex. Differences in the ERE sequence and the ER subtype involved can substantially alter ER-ERE interaction. In addition, cross-talk between ERs and other nuclear transcription factors profoundly influences gene expression. Here, we focus on the recent advances in the understanding of the structure of EREs and how ERs are recruited to these. Identifying known target genes for estrogen action could help us to understand the potential risks and benefits of the administration of this steroid to humans.

Glucocorticoid and mineralocorticoid cross-talk with progesterone receptor to induce focal adhesion and growth inhibition in breast cancer cells

Progesterone receptor (PR), glucocorticoid receptor, and mineralocorticoid receptor belong to a subfamily of nuclear receptor superfamily with similar sequence and structural characteristics. Many reports have documented glucocorticoid-like effects of progesterone in various tissues. This study addresses the issue of cross-talk between corticosteroids and PR using PR-transfected MDA-MB-231 cells ABC28 and vector-transfected control cells CTC15. At physiological concentrations, dexamethasone, cortisol, and aldosterone mimic the effects of progesterone by inducing significant growth inhibition, cell spreading, and focal adhesions in PR-positive ABC28 cells. These hormones also induce progesterone-like effects in increasing the expression of p21(CIP1/WAF1) protein and decreasing the level of phospho-p42/p44 mAPK. Two lines of evidence suggest that these effects are mediated by cross-talk with PR. First, these compounds do not exhibit the same progesterone-like effects in PR-negative CTC15 cells. Second, PR blocker ZK98299 abolishes their effect on cell spreading and focal adhesion in ABC28 cells. The cross-talk is corticosteroid specific because estradiol and thyroid hormone triiodothyronine have no effect on PR-transfected cells ABC28. It is also interesting to note that dexamethasone induces a small but detectable increase of focal adhesions and limited growth stimulation in vector-transfected cells CTC15. In contrast, progesterone exhibits no detectable effect on CTC15 cells. This study provides evidence that glucocorticoid and mineralocorticoid cross-talk with PR to produce progesterone-like effects in breast cancer cells. Glucocorticoid receptor and PR share some overlapping activity in mediating focal adhesion but not in regulating cell proliferation.

Molecular cloning of the estrogen and progesterone receptors of the American alligator

Steroid hormones perform many essential roles in vertebrates during embryonic development, reproduction, growth, water balance, and responses to stress. The estrogens are essential for normal reproductive activity in female and male vertebrates and appear to have direct actions during sex determination in some vertebrates. To begin to understand the molecular mechanisms of estrogen action in alligators, we have isolated cDNAs encoding the estrogen receptors (ER) from the ovary. Degenerate PCR primers specific to ER were designed and used to amplify alligator ovary RNA. Two different DNA fragments (ERalpha and ERbeta) were obtained and the full-length alligator ERalpha cDNA was obtained using 5' and 3' RACE. The inferred amino acid sequence of alligator ERalpha (aERalpha) was very similar to the chicken ERalpha (91% identity), although phylogenetic analyses suggested profound differences in the rate of sequence evolution for vertebrate ER sequences. We also isolated partial DNA fragments encoding ERbeta and the progesterone receptor (PR) of the alligator, both of which show strong sequence similarities to avian ERbeta and PR. We examined the expression levels of these three steroid receptors (ERalpha, ERbeta, and PR) in the ovary of juvenile alligators and observed detectable levels of all three receptors. Quantitative RT-PCR showed that gonadal ERalpha transcript levels in juvenile alligators decreased after E2 treatment whereas ERbeta and PR transcripts were not changed. These results provide tools that will allow future studies examining the regulation and ontogenic expression of steroid receptors in alligators and expand our knowledge of vertebrate steroid receptor evolution.

Estrogens in the Nervous System: Mechanisms and Nonreproductive Functions

The past decade has witnessed a growing interest in estrogens and their activity in the central nervous system, which was originally believed to be restricted to the control of reproduction. It is now well accepted that estrogens modulate the activity of all types of neural cells through a multiplicity of mechanisms. Estrogens, by binding to two cognate receptors ERalpha and ERbeta, may interact with selected promoters to initiate the synthesis of target proteins. Alternatively, the hormone receptor complex may interfere with intracellular signaling at both cytoplasmic and nuclear levels. The generation of cellular and animal models, combined with clinical and epidemiological studies, has allowed us to appreciate the neurotrophic and neuroprotective effects of estrogens. These findings are of major interest because estradiol might become an important therapeutic agent to maintain neural functions during aging and in selected neural diseases.

Co-evolution of steroidogenic and steroid-inactivating enzymes and adrenal and sex steroid receptors

Receptors for the adrenal and sex steroids arose by a series of gene duplications from an ancestral nuclear receptor in a primitive vertebrate, at least 540 million years ago. Sequence analysis indicates many steroidogenic and steroid-inactivating enzymes, including cytochrome P450s and hydroxysteroid dehydrogenases (HSDs), arose at the same time. The estrogen receptor (ER) appears to be the ancestral steroid receptor. Initially, the redundant duplicated ER had a low specificity for its new ligand. This raises the question: "How was specificity for responses to different steroids regulated early in the evolution of steroid receptors?" Selective expression of these steroid-metabolizing enzymes provided specificity for different steroid responses in primitive vertebrates. 17 beta-Hydroxysteroid dehydrogenase-type 1 (17 beta-HSD-type 1) and 17 beta-HSD-type 2, which preferentially catalyze the reduction and oxidation at C17 of androgens and estrogens, respectively, provide an example of this mechanism. Selective expression of either 17 beta-HSD-type 1 or 17 beta-HSD-type 2 can regulate synthesis or inactivation of androgens or estrogens in specific cells. Steroids also were important in the evolution of land animals, which began about 400 million years ago. Steroidogenic and steroid-inactivating enzymes were recruited to regulate steroid-mediated responses as organ function became more complex. For example, in the kidney 11 beta-HSD-type 2 prevents binding of glucocorticoids to the mineralocorticoid receptor (MR), which is crucial for aldosterone-mediated regulation of electrolyte transport in the distal tubule. We propose that Delta 5 steroids, such as dehydroepiandrosterone and its metabolites, were the ligands for the ancestral ER. Understanding the actions of Delta 5 steroids in amphioxus and lamprey may shed light on adrenarche and neurosteroid actions in humans.

Regulatory nodes that integrate and coordinate signaling as potential targets for breast cancer therapy

Blockade of the estrogen receptor (ER) with antiestrogens and aromatase inhibitors is effective in the treatment of breast cancer. Why ER plays such a dominant role in breast cancer and represents such an excellent target remains to be defined. The ability of ER to respond to multiple inputs and to control expression of multiple downstream genes may be one of the reasons why ER is such a powerful target for breast cancer treatment. The recent modest performance of a number of targeted therapies in breast cancer has raised the question whether we will ever develop therapies that have such success as antiestrogens. Targeted therapies tend to inhibit a single pathway that is probably altered in only a subset of patients. Even within this subset, only a limited number of patients respond. The evidence that virtually all pathways can cross-talk and that they exhibit several layers of redundancy reveals a complexity of signaling networks that may defy the generation of targeted therapies with efficacy similar to antiestrogens. However, there are clearly regulatory nodes that can integrate multiple upstream inputs and elicit diverse downstream outputs. We provide evidence and rationales for integrins, insulin receptor substrates (IRSs), and cyclin D1 as potential therapeutic targets. These proteins, similar to ER, can integrate and coordinate multiple signals in breast cancer cells and thus mediate diverse aspects of breast cancer progression. New treatment targets will emerge in light of more global models of signal transduction that fully integrate all aspects of cell biology such as the role of the extracellular matrix and will hopefully result in the development of targeted therapies that show efficacy similar to antiestrogens.

Resurrecting the ancestral steroid receptor: ancient origin of estrogen signaling

Receptors for sex and adrenal steroid hormones are absent from fully sequenced invertebrate genomes and have not been recovered from other invertebrates. Here we report the isolation of an estrogen receptor ortholog from the mollusk Aplysia californica and the reconstruction, synthesis, and experimental characterization of functional domains of the ancestral protein from which all extant steroid receptors (SRs) evolved. Our findings indicate that SRs are extremely ancient and widespread, having diversified from a primordial gene before the origin of bilaterally symmetric animals, and that this ancient receptor had estrogen receptor-like functionality. This gene was lost in the lineage leading to arthropods and nematodes and became independent of hormone regulation in the Aplysia lineage.

Molecular markers of endocrine disruption in aquatic organisms

A wide range of organic contaminant compounds prevalent in the aquatic environment has been shown to exhibit hormone-disrupting activity. The actual potency of such compounds are low compared with endogenous hormones, such as 17beta-estradiol, but may still produce detrimental biological effects. Induced hormone levels are routinely measured using commercial testing kits, though these fail to relate to actual effects. Field and laboratory studies on the biological effects of environmental estrogens have, in the past, largely relied on assays of vitellogenin (vtg) induction in male fish, reduced growth in testes formation, and intersex incidence. Here, we critically review the current and potential application of molecular techniques in assessing the adverse biological reproductive effects of endocrine-disrupting chemicals in aquatic organisms. The role of fish (estrogen, androgen, and progestogen) hormone receptors and invertebrate (ecdysone) hormone receptor, egg production (vtg and chorion) proteins, steroid biosynthesis enzymes (aromatase, sulfotransferase, and hydroxysteroid dehydrogenase), DNA damage, apoptosis, and their potential development as biomarkers are discussed in turn. In each case, the sequences characterized are presented and homologies across species are highlighted. Molecular methods of gauging vtg and zona radiata (ZR) expression and protein concentrations have included immunoassay and reverse transcription polymerase chain reaction (RT-PCR). Suggestions for the isolation for key gene expression products (aromatase, ZR, and vtg, for instance), from a wider range of fish species using degenerate primers, are given. Endocrine disruption in invertebrates has received less attention compared with fish, partly because the knowledge regarding invertebrate endocrinology is limited. Here we review and suggest alternate isolation strategies for key players in the imposex induction process: vitellin (Vn), aromatase, and Ala-Pro-Gly-Trp (APGW) amide neurohormone. Current molecular-level techniques rely on ligand-binding assays, enzyme-linked immunosorbent assay (ELISA), and, more recently, gene expression. In the future, more reliance will be placed on the development of gene expression assays using reporter systems combined with cross-species PCR-based or polyclonal antibody-based assays. We discuss the use of recombinant receptors as a means of primary screening of environmental samples for estrogenicity and antiestrogenicity, which avoids species and seasonal variation in receptor response to ligand binding, a recognized problem of earlier bioassays. Most exciting is the potential that microarray and proteomics approaches have to offer. Such techniques are now used routinely in medical research to identify specific genes and proteins affected by treatment with endocrine disruptors, including estradiol. The technique has yet to be used to screen aquatic organisms, but it has the potential to implicate previously unsuspected estradiol-sensitive genes that may later become molecular markers of endocrine disruption.

Sources of estrogen and their importance

In premenopausal women, the ovaries are the principle source of estradiol, which functions as a circulating hormone to act on distal target tissues. However, in postmenopausal women when the ovaries cease to produce estrogen, and in men, this is no longer the case, because estradiol is no longer solely an endocrine factor. Instead, it is produced in a number of extragonadal sites and acts locally at these sites as a paracrine or even intracrine factor. These sites include the mesenchymal cells of adipose tissue including that of the breast, osteoblasts and chondrocytes of bone, the vascular endothelium and aortic smooth muscle cells, and numerous sites in the brain. Thus, circulating levels of estrogens in postmenopausal women and in men are not the drivers of estrogen action, they are reactive rather than proactive. This is because in these cases circulating estrogen originates in the extragonadal sites where it acts locally, and if it escapes local metabolism then it enters the circulation. Therefore, circulating levels reflect rather than direct estrogen action in postmenopausal women and in men. Tissue-specific regulation of CYP19 expression is achieved through the use of distinct promoters, each of which is regulated by different hormonal factors and second messenger signaling pathways. Thus, in the ovary, CYP19 expression is regulated by FSH which acts through cyclic AMP via the proximal promoter II, whereas in placenta the distal promoter I.1 regulates CYP19 expression in response to retinoids. In adipose tissue and bone by contrast, another distal promoter--promoter I.4--drives CYP19 expression under the control of glucocorticoids, class 1 cytokines and TNFalpha. The importance of this unique aspect of the tissue-specific regulation of aromatase expression lies in the fact that the low circulating levels of estrogens which are observed in postmenopausal women have little bearing on the concentrations of estrogen in, for example, a breast tumor, which can reach levels at least one order of magnitude greater than those present in the circulation, due to local synthesis within the breast. Thus, the estrogen which is responsible for breast cancer development, for the maintenance of bone mineralization and for the maintenance of cognitive function is not circulating estrogen but rather that which is produced locally at these specific sites within the breast, bone and brain. In breast adipose of breast cancer patients, aromatase activity and CYP19 expression are elevated. This occurs in response to tumor-derived factors such as prostaglandin E2 produced by breast tumor fibroblasts and epithelium as well as infiltrating macrophages. This increased CYP19 expression is associated with a switch in promoter usage from the normal adipose-specific promoter I.4 to the cyclic AMP responsive promoter, promoter II. Since these two promoters are regulated by different cohorts of transcription factors and coactivators, it follows that the differential regulation of CYP19 expression via alternative promoters in disease-free and cancerous breast adipose tissue may permit the development of selective aromatase modulators (SAMs) that target the aberrant overexpression of aromatase in cancerous breast, whilst sparing estrogen synthesis in other sites such as normal adipose tissue, bone and brain.

Estrogen receptors in Xenopus: duplicate genes, splice variants, and tissue-specific expression

The estrogenic steroid hormones, acting primarily through the nuclear estrogen receptors ERalpha and ERbeta, regulate sexual differentiation in a wide variety of vertebrates. In the frog Xenopus laevis, estrogen regulates the strength of vocal neuromuscular synapses and contributes to the physiological basis of sexually differentiated songs. To understand the mechanisms by which estrogen produces these effects, we have characterized the ERs of X. laevis and their expression in laryngeal muscle and other tissues. We found a remarkable molecular diversity in the estrogen receptor population within individuals. First, we have identified two distinct ERalpha genes, xlERalpha1 and xlERalpha2, which represent, to our knowledge, the first discovery of retained duplicates of the ERalpha gene in any species. These two genes are highly conserved at the amino acid level but have distinct nucleotide sequences; moreover, ERalpha2 has no N-terminal domain. Cloning of ERalpha and ERbeta in the related species Xenopus tropicalis and phylogenetic analysis indicate that the two xlERalpha loci were generated by a duplication specific to the X. laevis lineage-most likely the genome duplication that led to a doubling of the X. laevis chromosome number about 30 million years ago. The primary ER expressed in X. laevis laryngeal muscle is the novel gene xlERalpha2; ERalpha1 is primarily expressed in liver, forebrain, and oviduct. Alternatively spliced transcripts of both xlERalpha1 and xlERalpha2 are also expressed in a tissue-specific manner. We propose that complementary spatial expression of these two genes and their alternatively spliced transcripts contributes to their conservation over such a long period of time, consistent with the subfunctionalization model for evolution after gene duplication.

Nuclear receptors are markers of animal genome evolution

Nuclear hormone receptors form one evolutionary related super-family of proteins, which mediate the interaction between hormones (or other ligands) and gene expression in animals. Early phylogenetic analyses showed two main periods of gene duplication which gave rise to present-day diversity in most animals: one at the origin of the family, and another specifically in vertebrates. Moreover this second period is composed itself by, probably, two rounds of duplication, as proposed by Susumu Ohno at the origin of vertebrates. There are indeed often two, three or four vertebrate orthologs of each invertebrate nuclear receptor, in accordance with this theory. The complete genome of Drosophila melanogaster contains 21 nuclear receptors, compared to 49 in the human genome. In addition, many nuclear receptors have more paralogs in the zebrafish than in mammals, and a genome duplication has been proposed at the origin of ray-finned fishes. Nuclear receptors are a very good model to investigate the dating and functional role of these duplications, since they are dispersed in the genome, allow robust phylogenetic reconstruction, and are functionnaly well characterized, with different adaptations for different paralogs. We illustrate this with examples from differents nuclear receptors and different groups of species.

15 alpha-Hydroxytestosterone produced in vitro and in vivo in the sea lamprey, Petromyzon marinus

Prior research has shown that the testes of lampreys are able to synthesize 15-hydroxylated steroid hormones in vitro. Here we show that testes of the sea lamprey Petromyzon marinus L. are able to convert tritiated testosterone into tritiated 15alpha-hydroxytestosterone (15alpha-T) in high yield. The identity of the tritiated 15alpha-T has been confirmed by: co-elution with standard 15alpha-T on high performance liquid chromatography (HPLC); co-elution on thin layer chromatography (TLC); co-elution of acetylated tritiated and standard 15alpha-T on TLC; and strong binding to an antiserum developed against 15alpha-T. The strong reaction between the tritiated 15alpha-T and the antiserum has been used to develop a radioimmunoassay (RIA). The RIA operates over the range of 500-2pg per tube; and can be applied directly to plasma samples. This assay has been used to demonstrate that 15alpha-T is present in blood plasma of the sea lamprey. The concentrations of 15alpha-T in captive lamprey were found to be as follows (pg/ml; mean+/-SEM, n): parasitic stage (reproductively immature), <20, n=7; pre-ovulatory females, 156+/-30, n=8; ovulated females, 62+/-9, n=5; pre-spermiating males, 275+/-19, n=8; spermiating males, 216+/-48, n=8. When spermiating male plasma was fractionated on HPLC, immunoreactivity was found exclusively in the expected elution position of 15alpha-T. The biological significance of this steroid has yet to be established.