Binding characteristics of the hepatic estrogen receptor of the spotted seatrout, Cynoscion nebulosus

Unraveling the estrogen receptor (er) genes in Atlantic salmon (Salmo salar) reveals expression differences between the two adult life stages but little impact from polychlorinated biphenyl (PCB) load

Estrogen receptors (ers) not only are activated by hormones but also interact with many human-derived environmental contaminants. Here, we present evidence for four expressed er genes in Atlantic salmon cDNA - two more ers (erα2 and erβ2) than previously published. To determine if er gene expression differs between two adult life-stages we sampled 20 adult salmon from the feeding phase in the Baltic Sea and during migration in the River Mörrum, Sweden. Results show that all four er genes are present in the investigated tissues, except for erα2 not appearing in the spleen. Overall, a profile analysis reveals the erα1 gene to be the most highly expressed er gene in both female and male Baltic Sea salmon tissues, and also in female River Mörrum salmon. In contrast, this gene has the lowest gene expression level of the four er genes in male salmon from the River Mörrum. The erα2 gene is expressed at the lowest levels in both female/male Baltic Sea salmon and in female River Mörrum salmon. Statistical analyses indicate a significant and complex interaction where both sex and adult life stage can impact er gene expression. Regression analyses did not demonstrate any significant relationship between polychlorinated biphenyl (PCB) body burden and er gene expression level, suggesting that accumulated pollutants from the Baltic Sea may be deactivated inside the salmon's lipid tissues and have limited impact on er activity. This study is the first comprehensive analysis of four er gene expression levels in two wild salmon populations from two different adult life stages where information about PCB load is also available.

Alternatives to in vivo tests to detect endocrine disrupting chemicals (EDCs) in fish and amphibians--screening for estrogen, androgen and thyroid hormone disruption

Endocrine disruption is considered a highly relevant hazard for environmental risk assessment of chemicals, plant protection products, biocides and pharmaceuticals. Therefore, screening tests with a focus on interference with estrogen, androgen, and thyroid hormone pathways in fish and amphibians have been developed. However, they use a large number of animals and short-term alternatives to animal tests would be advantageous. Therefore, the status of alternative assays for endocrine disruption in fish and frogs was assessed by a detailed literature analysis. The aim was to (i) determine the strengths and limitations of alternative assays and (ii) present conclusions regarding chemical specificity, sensitivity, and correlation with in vivo data. Data from 1995 to present were collected related to the detection/testing of estrogen-, androgen-, and thyroid-active chemicals in the following test systems: cell lines, primary cells, fish/frog embryos, yeast and cell-free systems. The review shows that the majority of alternative assays measure effects directly mediated by receptor binding or resulting from interference with hormone synthesis. Other mechanisms were rarely analysed. A database was established and used for a quantitative and comparative analysis. For example, a high correlation was observed between cell-free ligand binding and cell-based reporter cell assays, between fish and frog estrogenic data and between fish embryo tests and in vivo reproductive effects. It was concluded that there is a need for a more systematic study of the predictive capacity of alternative tests and ways to reduce inter- and intra-assay variability.

Sustained high temperature increases the vitellogenin response to 17α-ethynylestradiol in mummichog (Fundulus heteroclitus)

Mummichog (Fundulus heteroclitus), an estuarine fish of the western Atlantic, were acclimated to three salinities (0, 16 or 32 ppt) or three temperatures (10, 20 or 26 °C) and exposed to nominal 50 or 250 ng/L 17α-ethynylestradiol (EE2) for 14 days. In a separate experiment, fish were exposed to the same levels of EE2 and were subjected to a 1h heat shock (20-30 °C) on the 14th day and allowed to recover for 20 h. We were interested in whether or not susceptibility to EE2 exposure, as indicated by increases in vitellogenin (vtg) gene expression would change with high and low salinity, warm or cold temperature acclimation or acute heat shock. We also investigated the potential role of heat shock proteins (HSPs) under these conditions. Liver vtg1 mRNA was significantly induced in male mummichog exposed to 50 and 250 ng/L EE2, but salinity acclimation or acute heat shock did not further affect this induction. Males acclimated to 26 °C and exposed to 250 ng/L EE2 induced 3.5-fold more vtg1 mRNA than EE2 exposed males acclimated to 10 °C. HSP90 and HSP70 protein were largely unaffected by EE2 exposure. Our findings suggest that mummichog are more susceptible to EE2 under sustained temperature increases that may occur seasonally or with warming of coastal waters.

Effects of triclosan and triclocarban, two ubiquitous environmental contaminants, on anatomy, physiology, and behavior of the fathead minnow (Pimephales promelas)

Triclosan (TCS) and triclocarban (TCC) are two common antimicrobial agents found in many personal care products and subsequently are detected ubiquitously in wastewater effluent and receiving waters. Both compounds are of recent regulatory interest due to their omnipresence in the environment, including in humans. Although TCS and TCC have been suggested to be endocrine active, little information exists about their effects on organismal end points in development (growth, escape performance), anatomy (morphological indices, histology), physiology (vitellogenin), and behavior of exposed aquatic organisms. In this study, newly hatched fathead minnows were exposed for 12 days, and mature male and female fathead minnows (Pimephales promelas) were exposed for 21 days to environmentally realistic concentrations (nanograms per liter) of these two compounds singularly and in mixtures. At the end of the exposure, larvae were assessed for growth and predator-avoidance performance, and a subset of mature fish was assessed for plasma vitellogenin induction, expression of secondary sexual characteristics, relative size of liver and gonads, and histopathological changes to both organs. The remaining exposed mature fish were placed in breeding pairs of one male and one female minnow from the same treatment to assess their ability to defend a nest site and reproduce. Exposure to either antimicrobial compound, alone or as a mixture, caused no changes to larval fish, gonad size, or vitellogenin concentrations in mature fathead minnows. In contrast, decreased aggression was seen in adult male fathead minnows exposed to TCC (1.6 μg/l) or a mixture (560 ng/l TCS + 179 ng/l TCC and 1.6 μg/l TCS + 450 ng/l TCC). Decreased aggression would likely decrease their ability to defend and hold a nest site needed for spawning and reproduction. Substantial variability was found in the severity of observed effects within treatments, suggesting that environmentally realistic concentrations of these compounds may only affect particularly sensitive individuals.

Variations in hepatic estradiol-17beta receptor concentrations during the annual reproductive cycle of diploid and triploid female catfish, Heteropneustes fossilis (Bloch)

Concentrations of hepatic estradiol-17beta (E2) receptors (ER) in cytosolic and nuclear fractions were evaluated in diploid and triploid female catfish Heteropneustes fossilis (Bloch) during four different reproductive periods of a complete reproductive cycle. Basal level of ER concentration was noted in the resting period of both diploids and triploids. Receptor level gradually elevated through the preparatory period and reached a peak in the pre-spawning period in both diploids and triploids. However, ER concentrations were overall reduced in triploid to that of diploid females. In a single point assay, in diploids, ER concentration showed about a 3-fold rise (p<0.001) in the cytosolic and a 4-fold rise (p<0.001) in the nuclear extracts from resting to the pre-spawning period. In triploids, only a 2-fold rise was observed both in cytosolic (p<0.01) and nuclear (p<0.05) ER concentration during the same span. Finally, a sudden fall of receptor level was observed in the spawning period in both the ploidy groups with a lower concentration in the triploids. The K(d) value did not differ between the females of diploids (cytosolic 1.12+/-0.21 nM and nuclear 6.9+/-0.9 nM) and triploids (cytosolic--1.13+/-0.17 nM, nuclear--6.8+/-2 nM). However, B(max) of the diploid showed about double the value than triploid females both in the cytosolic (diploid--367.4+/-33.24 pmol/mg protein, triploid--187.3+/-13.20 pmol/mg protein, p<0.001) and nuclear extracts (diploid--946+/-66 pmol/mg DNA, triploid-558+/-98 pmol/mg DNA, p<0.01) of liver. Lower E2 binding capacity and lower amount of E2 receptors of triploid catfish liver with a stunted vitellogenic status could be one of the major causes for reduced gonadal development and sterility in female triploids.

Estradiol-17β: Tracing its metabolic significance in female fatbody of fifth instar larvae of silkworm, Bombyx mori L (race: Nistari)

In recent years, various vertebrate peptide and steroid hormones have been identified in invertebrates, estradiol-17beta (E2) being a major one. We have specifically shown NADP-malate dehydrogenase (NADP-MDH) activity in fifth instar larval fatbody of female silkworm, Bombyx mori, as an E2 responsive parameter. Interestingly enough, estradiol-induced increase in the enzyme activity could be counteracted by simultaneous application of specific E2-inhibitor, ICI-182780. Further, a nice correlation was obtained among the E2 titre, specific *E2 binding and expression of NADP-MDH activity in fatbody during different days of normal fifth instar larval development. Though the nature of the binding sites is quite similar to known steroid receptors of vertebrate, the reported absence of estrogen receptor gene in some insects poses a question. A recent finding regarding the presence of an estrogen-related receptor ortholog in fruit fly may provide some answers. The specific effects elicited by estradiol in the female fatbody of this insect support its possibility of having an important metabolic function. This role played by E2, whether hormonal or not, is yet to be identified.

Intracellular Localization of hsp90 Is Influenced by Developmental Stage and Environmental Estrogens in Rainbow TroutOncorhynchus mykiss

In this study, we investigated the intracellular localization of heat shock proteins hsp90 and hsp70 in adult and juvenile rainbow trout (Oncorhynchus mykiss) and in juvenile trout exposed to estrogen or one of its mimics, 4-nonylphenol (4-NP). Livers were harvested from each group and analyzed directly or separated into nuclear and nonnuclear fractions. We found that hsp70 was predominantly nonnuclear in mature and juvenile fish regardless of treatment. Mature fish had significantly greater levels of hsp90 outside the nucleus, while juvenile fish had similar levels of hsp90 inside and outside the nucleus. Treatment with estradiol or 4-NP resulted in a translocation of hsp90 out of the nucleus in juvenile fish. To our knowledge, this is the first study to demonstrate a development- and/or estrogen-dependent shift in intracellular localization of hsp90 in fish. This change in subcellular distribution points to important roles for this hsp in fish estrogen signaling and development.

Differential effects of temperature and maturity stage on hepatic estrogen receptor characteristics of Atlantic salmon

In order to determine if elevated temperature during vitellogenesis had a detrimental effect on hepatic estrogen receptors of Atlantic salmon (Salmo salar), 3H-estradiol saturation binding analysis, using one- and two-site binding models, was carried out on extracts of hepatic cytosols from fish held at 14, 18 or 22 degrees C over the austral period of peak vitellogenesis (February to April). With one-site binding analysis, no temperature related difference in either receptor affinity (Kd) or number (Bmax) was found at each sampling point, but there was an apparent decrease in both affinity and number at each temperature over the period of the study. However, some analyses, notably at 22 degrees C during February, were best described using a two-site binding model. At this temperature and time, there was a clear separation of binding affinity into high and low components (Kd = 0.67+/-S.E. 0.05 and 20+/-S.E. 5.6 nM, respectively) (n = 4), which suggests that February was a critical time of temperature related hepatic sensitivity to estrogen. These results support those of other studies where we found that February was also a sensitive time with respect to temperature impairment of in vitro follicular estrogen synthesis, and the greatest period of in vivo temperature sensitivity.

Modeling vitellogenesis in female fish exposed to environmental stressors: predicting the effects of endocrine disturbance due to exposure to a PCB mixture and cadmium

A wide variety of chemical and physical environmental stressors have been shown to alter the reproductive processes in fish by interfering with endocrine function. Most endocrine indicators or biomarkers are static measures from dynamic hormonally-mediated processes, and often do not directly relate to reproductive endpoints of ecological significance. Adequate production of the yolk precursor protein, vitellogenin, is critical for the survival and normal development of the sensitive egg and yolk-sac larval fish life stages. We developed a model that simulates vitellogenesis in a mature female sciaenid fish. The model simulates the major biochemical reactions over a 6-month period from the secretion of gonadotropin (GtH) into the blood to the production of vitellogenin. We simulated the effects of two endocrine disrupting chemicals (EDCs) that have different actions on vitellogenin production: a PCB mixture and cadmium. Predicted changes in steroid concentrations and cumulative vitellogenin production compared favorably with changes reported in laboratory experiments. Simulations illustrate the potential utility of our model for interpreting reproductive endocrine biomarkers measured in fish collected from degraded environments.

Mechanistic basis for estrogenic effects in fathead minnow (Pimephales promelas) following exposure to the androgen 17alpha-methyltestosterone: conversion of 17alpha-methyltestosterone to 17alpha-methylestradiol

Exposure of adult fathead minnows (Pimephales promelas) to the androgen 17alpha-methyltestosterone (MT) produces both androgenic and estrogenic effects, manifested as nuptial tubercle formation in females, and vitellogenin production in males and females, respectively. The present study was conducted to determine if the unanticipated estrogenic effects are produced by conversion of MT via aromatase activity to 17alpha-methylestradiol (ME2). Aromatase activity at the end of a 7-day waterborne MT exposure (20, 200microg/l) was significantly decreased in ovarian microsomes and brain homogenates from exposed fish, to about 30-50% of control activity. Although aromatase activity was decreased by 7 days, it is possible that the conversion of MT to ME2 occurred soon after initial exposure. In support of this, ME2 was detected in plasma samples of the fish following the 7-day exposure, confirming their ability convert the androgen MT to the estrogen ME2. The concentration of ME2 in plasma was within the range of plasma 17ss-estradiol (E2) found in control female fathead minnows (4-5ng/ml). These results, in conjunction with competitive binding assays that indicate ME2 binds to the fathead minnow estrogen receptor with a relative binding affinity of 68.3% of E2, support the hypothesis that aromatization of MT to ME2 contributes to the estrogenic effects in fathead minnows following exposure to this androgen.

Molecular cloning, characterisation, and tissue distribution of oestrogen receptor alpha in eelpout (Zoarces viviparus)

A cDNA encoding the eelpout (Zoarces viviparus) oestrogen receptor alpha (eERalpha) has been isolated from eelpout liver, cloned and sequenced. The cDNA contains a complete open reading frame encoding 570 amino acid residues (mw: 63.0 kDa). The amino acid sequence of eERalpha showed a high degree of identity to ERalpha of other teleost species. The tissue distribution of eERalpha mRNA was examined using Northern blotting, RT-PCR and in situ hybridisation (ISH). All three methods identified a pronounced expression of eERalpha in liver, pituitary, testis and ovary. In the brain ISH experiments showed that ERalpha mRNA was highly expressed in distinct regions of the preoptic area and the mediobasal hypothalamus. We have provided evidence that the receptor is auto-regulated by 17beta-oestradiol (E(2)) not only in liver but also in the testis, indicating an important role for E(2) during spermatogenesis in male eelpout. RT-PCR analysis showed a broader expression pattern including significant expression in the brain, kidney, heart, and gut of adult eelpout. In eelpout embryos eERalpha expression has also been identified, indicating a possible role for the receptor in early development. This study contributes to the accumulating evidence that in fish E(2) is not only involved in the regulation of liver specific proteins, but has a much broader range of targets.

Binding of xenobiotics to hepatic estrogen receptor and plasma sex steroid binding protein in the teleost fish, the common carp (Cyprinus carpio)

Competitive receptor binding assays have been suggested as an in vitro screening tool for assessing the activity of alleged estrogenic substances. In this study, we determined the ability of steroidal and nonsteroidal substances to inhibit the binding of [(3)H]17 beta-estradiol (E2) to the hepatic estrogen receptor (ER) and the plasma sex steroid binding protein (SBP) of the teleost fish, the common carp (Cyprinus carpio). The objectives of the study were (1) to characterize ER binding in the liver cytosol of male and female carp, (2) to establish complete [(3)H]E2 displacement curves from carp ER for a range of natural and xenobiotic substances and to compare the ligand data of carp ER with published data from other vertebrate species to reveal possible species differences, and (3) to determine the interaction of natural and xenobiotic substances with the steroid binding site of SBP in carp plasma. The results indicate the presence of a single class of estrogen binding sites with high affinity and limited capacity in liver cytosol of carp. The various test agents showed partly quantitative differences in their binding affinities, with the xenobiotics generally showing limited ability to displace [(3)H]E2 from the hepatic ER or from plasma SBP of carp. However, we found no evidence that a compound is an ER ligand exclusively in one species. The findings of this study indicate that interspecies extrapolation of steroid receptor binding data is possible on a yes/no basis but not on a quantitative basis.

Binding characteristics of [3H]17beta-estradiol in the hypothalamus of juvenile rainbow trout, Oncorhynchus mykiss

Gonadal steroids in the salmonid brain, acting through cellular receptors, may be responsible for the modulation of neuronal activity and organization of reproductive behaviors. We report our findings on the use of [3H]17beta-estradiol (E2) to identify intracellular estrogen receptors (ERs) in the hypothalamus of juvenile rainbow trout, Oncorhynchus mykiss. Specific binding (B(SP)) of [3H]E2 was tissue dependent between 0.5 and 2.25 hypothalamus equivalents for cytosol and nuclear extract preparations, respectively. B(SP) in cytosol fractions increased with time and reached maximum levels (4.18 nM) at 2.5 h incubation; by contrast, B(SP) in nuclear extract increased with time to achieve maximum levels (3.9 nM) by 2 h incubation. The association rate constants (k(+1)) for cytosol and nuclear extract preparations were 1.10 +/- 0.02 x 10(8) M(-1) min(-1) and 1.27 +/- 0.04 x 10(8) M(-1) min(-1), respectively. Equilibrium bound B(SP) dissociated from cytosol preparations with a half life (t1-2) of 42 min and a dissociation rate constant (k(-1)) of 1.01 +/- 0.03 min(-1). B(SP) dissociated from nuclear extract preparations with a t1-2 = 45 min and k(-1)= 0.92 +/- 0.01 min(-1) x B(SP) was saturable in both extract preparations with a calculated equilibrium dissociation constant (Kd) of 1.46 +/- 0.1 nM (cytosol) and 2.37 +/- 0.2 nM (nuclear), and a maximum number of binding sites (B(MAX)) of 50.85 +/- 3.2 fmol mg(-1) protein and 61.74 +/- 2.65 fmol mg(-1) protein, respectively. In both preparations, B(SP) was differentially displaced by structurally similar compounds with a rank order of potency of E2 > estrone > estriol > 17alpha-ethynyl estradiol > testosterone >> progesterone = tamoxifen >> cortisol > dexamethasone >> > beta-sitosterol. These properties of specifically bound [3H]E2 suggest the presence of an ER in the hypothalamus of juvenile rainbow trout comparable with ERs identified in salmonid liver.

Sex steroids relative to alternative mating behaviors in the simultaneous hermaphrodite Serranus subligarius (Perciformes: Serranidae)

This study is the first investigation of reproductive endocrinology in a simultaneously hermaphroditic teleost, the belted sandfish (Serranus subligarius). We address two questions: (1) Do steroid hormone levels vary during the spawning season or during the daily spawning cycle of sandfish? (2) Do hormone levels vary relative to an individual's phenotype-size, frequency of spawning and aggressive behaviors, and proportion of testis in the gonad? We analyzed circulating estradiol-17beta (E2), testosterone (T), 11-ketotestosterone (11KT), 17alpha,20beta,21-trihydroxy-4-pregnen-3-one (20betaS), and 17alpha,20beta-dihydroxy-4-pregnen-3-one (DHP) concentrations in a field population. Only E2 levels were significantly higher at the new and full moon, suggesting peak periods of vitellogenesis at these times. Naturally spawning sandfish were sampled every 2 h during the photophase of a 25-h period (12 pm to 1 pm the following day) and gonadosomatic index, degree of oocyte hydration and ovulation, and plasma levels of E2, T, DHP, and 20betaS were analyzed. E2 and T levels did not vary during photophase, suggesting continuous recruitment of oocytes into vitellogenesis. The 20betaS levels peaked around the time of final oocyte maturation. Since frequency of spawning behaviors changes with body size, we captured individuals of various sizes throughout the spawning season and analyzed circulating levels of hormones. 11KT and 20betaS levels increased significantly with body size. In 1992, we quantified frequency of spawning and aggressive behaviors, circulating T and 11KT levels and testicular mass relative to ovotestis mass in focal animals. 11KT levels tended to be positively correlated with frequency of courting male behavior, but were unrelated to the frequency of aggressive behavior or testis mass. Because hormone levels increased with size and frequency of each spawning behavior changes with size, we propose that sex steroids influence growth-related changes in spawning tactics of individuals.

Characterization of a cytosolic estrogen receptor and its up-regulation by 17 beta-estradiol and the xenoestrogen 4-tert-octylphenol in the liver of eelpout (Zoarces viviparus)

Estrogen binding activity was revealed in the cytosolic fraction of hepatic extracts from adult male and female eelpout (Zoarces viviparus). The binding moiety was characterized by a single class of high affinity binding sites (K(d)=0.59+/-0.05 nM in males and 1.06+/-0.10 nM in females). The affinity was significantly higher in males. Binding sites were satiable and binding capacity was significantly elevated in vitellogenic females (2.92+/-0.28 pmol/g) compared to males (1.67+/-0.11 pmol/g). The binding was specific to known estrogens but not to other tested steroids. The binding moiety was able to bind to DNA-cellulose and was extractable by high salt concentrations. A time-course study of estrogen binding activity in liver cytosol and of vitellogenin (Vtg) in plasma, after intraperitoneal (i.p.) injections of 17 beta-estradiol (E(2)) in male eelpout, was carried out. It was shown that both are inducible by E(2). Estrogen binding activity was significantly elevated 48 h and Vtg 72 h after E(2) treatment. The binding moiety was hereafter designated as a cytosolic estrogen receptor (ER). The estrogenicity of 4-tert-octylphenol (OP) was evaluated by measuring ER and Vtg after i.p. treatment. OP-treatment increased both receptor levels and Vtg concentrations in male fish, indicating that OP acts as an estrogen in male eelpout.

Interactions of nitromusk parent compounds and their amino-metabolites with the estrogen receptors of rainbow trout (Oncorhynchus mykiss) and the South African clawed frog (Xenopus laevis)

Nitromusks, musk xylene (MX), musk ketone (MK) and musk moskene (MM) are synthetic fragrances. 4-Amino-MX (4X), 2-amino-MX (2X) and 2-amino-MK (2K) are nitromusk metabolites formed during the sewage treatment process and have been detected in effluent and surface water at concentrations four to 40 times higher than their parent compounds. To date, data to the aquatic toxicity of nitromusk compounds are limited to the parent compounds and the determination of acute and subacute effects in aquatic organisms. No data are available regarding the potential endocrine modulating effects of these compounds and/or their metabolites in aquatic organisms. Therefore, the competitive binding capability of nitromusks and their metabolites to the estrogen receptors (ER) in rainbow trout and xenopus was investigated. No binding of MX, MK and MM to the ER of either species was observed. In contrast, binding to the ER was observed for 4X, 2X and 2K in both species. The IC50 (competitive binding at the ER) of 2X in rainbow trout was 1.3 +/- 1.1 mM. In contrast, 4X, 2X and 2K bound to the xenopus ER with an IC50 of 30.8 +/- 28.5, 12.9 +/- 10.3 and 70.1 +/- 88.3 microM, respectively.

Binding characteristics of estrogen receptor (ER) in Atlantic croaker (Micropogonias undulatus) testis: different affinity for estrogens and xenobiotics from that of hepatic ER

An estrogen receptor (ER) was identified in cytosolic and nuclear fractions of the testis in a marine teleost, Atlantic croaker (Micropogonias undulatus). A single class of high affinity, low capacity, and displaceable binding sites was identified by saturation analysis, with a Kd of 0.40 nM in cytosolic extracts and a Kd of 0.33 nM in nuclear extracts. Competition studies demonstrated that the receptor was highly specific for estrogens (diethylstilbestrol > estradiol >> estriol = estrone) and also bound several antiestrogens. Testosterone and 5alpha-dihydrotestosterone had much lower affinities for the receptor, whereas no displacement of specific binding occurred with 11-ketotestosterone or any of the C21 maturation-inducing steroids. A variety of xenoestrogens, including o,p'-dichlorodiphenyltrichloroethane (DDT), chlordecone (Kepone), nonylphenol, hydroxylated polychlorinated biphenyls (PCBs), and the mycotoxin zearalenone, bound to the receptor with relatively low binding affinities, 10(-3) to 10(-5) that of estradiol. A comparison of the binding affinities of various ligands for the testicular ER and the hepatic ER in this species revealed that the testicular ER was saturated at a lower [3H]estradiol concentration (1 nM vs. 4 nM). The binding affinities of several compounds, including testosterone and nafoxidine, exhibited marked differences for the two ERs; and most of the estrogens and xenoestrogens tested had higher binding affinities for the testicular receptor. Minor amounts of estradiol (0.12 ng/g tissue/h) were produced by testicular tissue fragments incubated in vitro, and estradiol was detected in male Atlantic croaker plasma. The identification of a testicular ER and evidence that estradiol is produced by the testes in croaker suggest that estrogens participate in the hormonal control of testicular function in teleosts.

Characterization of two nuclear androgen receptors in Atlantic croaker: comparison of their biochemical properties and binding specificities

Two distinct androgen receptors (ARs) with different characteristics were identified in the brain and ovary of Atlantic croaker, Micropogonias undulatus. A nuclear AR, AR1, was identified in the brain that had high affinity binding sites for testosterone (T; Kd, 1.1 +/- 0.15 nM; binding capacity, 1.4 +/- 0.14 pmol/g tissue; n = 16). A second nuclear AR, AR2, was found in the ovary that had high affinity binding sites for 5alpha-dihydrotestosterone (DHT; Kd, 0.62 +/- 0.1 nM; binding capacity, 0.38 +/- 0.06 pmol/g tissue; n = 14). AR2 has physiochemical properties similar to those of other vertebrate ARs. AR2 has high affinity binding for a broad spectrum of natural and synthetic androgens, including 17alpha-methyl-5alpha-dihydrotestosterone, which has a relative binding affinity of DHT = 100% > T > mibolerone > 11-ketotestosterone = 16%, a rapid association (t1/2, 44 min) and a slow dissociation (t1/2, 45 h) rate, as well as specific binding to purified DNA. The cytosolic AR2 interacts with heat shock proteins in a manner similar to other steroid receptors, as sodium molybdate stabilizes the receptor, and it has a 7.4-7.8S sedimentation coefficient in a 5-20% sucrose gradient. In contrast, AR1 is highly specific for only a few androgens, with T = 100% relative binding affinity >> DHT >> 11-ketotestosterone > mibolerone > 17alpha-methyl-5alpha-dihydrotestosterone = 0, has rapid association (t1/2, 15 min) and dissociation (t1/2, 2.6 +/- 0.7 h) rates, and has specific binding to purified DNA upon heat activation. The cytosolic binding component sediments at 5.6-5.7S in a 5-20% sucrose gradient and is not affected by sodium molybdate, which suggests that AR1 does not interact with heat shock proteins in the usual manner. This is the first report of the presence of two different nuclear ARs displaying markedly different steroid binding specificities within a single vertebrate species.

The ovarian progestogen receptor in the spotted seatrout, Cynoscion nebulosus, demonstrates steroid specificity different from progesterone receptors in other vertebrates

A nuclear progestogen receptor has previously been characterized in the ovary of the spotted seatrout. The steroid specificity of this receptor was further defined in the present study by determining the binding affinity of a wide variety of progestin and corticosteroid agonists and antagonists. The addition of a hydroxyl or keto group to the 11 position resulted in a 10-100-fold decrease in relative binding affinity (RBA). The significance of the 17, 20, and 21 positions in determining the RBA of closely related steroids was investigated in detail. Modification of the 17alpha-hydroxyl to an acetyl or carbyne group resulted in a 10-fold decrease in RBA. The substitution of a ketone group with a hydroxyl group at the 20 position increased binding, whereas the addition of a 21-hydroxyl group consistently decreased RBA by 40-60%. The effect of the 17alpha-hydroxyl group on RBA was dependent on what functional group was present at the 20 position. The addition of a 17alpha-hydroxyl decreased affinity by one- to 10-fold if a ketone group was present at position 20. However, the RBA increased five- to 10-fold upon addition of the 17alpha-hydroxyl group if a hydroxyl was present at the 20 position. The effects of the different substitutions at the 17, 20 and 21 positions explain why the two teleost maturation-inducing steroids 17alpha,20beta-dihydroxy-4-pregnen-3-one (17alpha,20beta-P) and 17alpha,20beta,21-trihydroxy-4-pregnen-3-one (20beta-S) have higher affinities than progesterone for this receptor. It is concluded that the seatrout progestogen receptor demonstrates steroid specificity different from progesterone receptors in other vertebrates.

Up-regulation of estrogen receptor mRNA and estrogen receptor activity by estradiol in liver of rainbow trout and other teleostean fish

Injection of estradiol (E2) into immature rainbow trout resulted in the induction of the hepatic vitellogenin gene mediated by the nuclear estrogen receptor (ER). Liver ER mRNA rose markedly on E2 treatment in three groups of trout kept at different temperatures. Only in the group kept at 4 degrees C did the total cellular ER, as measured by [3H]estradiol-binding activity in nuclear and cytosol fractions, parallel the ER mRNA level. In fish kept at 9 degrees C and 15 degrees C, the ratio of total ER activity to ER mRNA fell during chronic E2 treatment, probably reflecting translational of post-translational control mechanisms. Upregulation of ER mRNA also occurred in sea raven, sculpin, winter flounder, and Atlantic salmon after E2 treatment. Intrahepatic ER activity rose proportionately in Atlantic salmon kept at 6-9 degrees C but not in sea raven, sculpin, or flounder. We conclude that the regulation of ER expression in teleosts is complex and includes transcriptional, translational, and post-translational elements and is influenced by environmental temperature.

Changes in hepatic estrogen-receptor concentrations during the annual reproductive and ovarian cycles of a marine teleost, the spotted seatrout, Cynoscion nebulosus

The concentrations of estrogen and vitellogenin in plasma, and hepatic estrogen receptor in cytosolic (ERc) and nuclear (ERn) extracts were elevated throughout the reproductive season in females from a wild population of spotted seatrout, Cynoscion nebulosus. There were significant correlations between plasma estradiol and ERc and between ERn and plasma vitellogenin during the period of ovarian recrudescence (January-May), but not during the remainder of the reproductive season (June-September). During ovarian recrudescence ERc concentrations increased fourfold from 0.78 +/- 0.15 pmol/g liver (N = 26) in nonvitellogenic females to 3.23 +/- 0.26 pmol/g liver (N = 77) in late vitellogenic females and ERn concentrations increased eightfold from 0.16 +/- 0.07 pmol/g liver (N = 26) in nonvitellogenic females to 1.12 +/- 0.45 pmol/g (N = 10) in mid vitellogenic females. However, following this period of ovarian recrudescence, estrogen-receptor concentrations and plasma vitellogenin titers did not fluctuate during the remaining stages of the ovulatory cycle (hydration, ovulation, and spawning). In addition, both hepatic ERc and ERn were elevated for the rest of the seasonal reproductive cycle, during which several ovulatory cycles may have occurred in this multiple-spawning species. The affinity of ERc for estradiol did not vary (Kd = 1.26 +/- 0.06 nM, N = 68) regardless of the stage of ovarian development. Plasma estradiol titers declined in August, near the end of the reproductive season, prior to the decline in estrogen-receptor concentrations. The persistence of the estrogen receptor during the ovulatory cycle suggests that even if plasma estradiol titers declined between successive ovulatory periods, the hepatic responsiveness to estrogenic stimuli would not be diminished and thus vitellogenin synthesis, if interrupted at all, could be resumed soon after spawning.

Characterization of membrane receptor activity for 17 alpha, 20 beta, 21-trihydroxy-4-pregnen-3-one in ovaries of spotted seatrout (Cynoscion nebulosus)

The proposed maturation-inducing substance (MIS) of spotted seatrout (Cynoscion nebulosus) is 17 alpha, 20 beta, 21-trihydroxy-4-pregnen-3-one (20 beta-S). In this study, we characterized the binding of radioactive 20 beta-S to plasma membranes from the ovaries of spotted seatrout. Bound 20 beta-S was isolated by filtration of membrane suspensions and quantified by measurement of the radioactivity content of the filters. The saturable component of 20 beta-S binding reached equilibrium within 5 min at 0 degree, showed linearity with membrane concentration, and was pH dependent (optimum, 7.5-7.8). Scatchard analyses suggested a single class of high-affinity (KD, 10(-9) M), low-capacity (10(-13)-10(-12) mol/g ovary) binding sites for 20 beta-S. High levels of saturable binding were found in membrane preparations from the ovary, testis, and liver, but not from the gills. 17 alpha, 20 beta-Dihydroxy-4-pregnen-3-one (17 alpha, 20 beta-P) showed relatively little affinity for the 20 beta-S binding site. However, this steroid was converted to a compound immunologically and chromatographically similar to 20 beta-S by intact ovarian follicles, a finding which may explain its previously reported high potency in an in vitro oocyte maturation bioassay. Conversely, although reportedly a weak inducer of oocyte maturation, progesterone readily displaced 20 beta-S from its binding site. Thus, progesterone appears to be a relatively inactive ligand with high affinity for the 20 beta-S receptor. The concentration of 20 beta-S binding sites in ovaries was significantly higher during final oocyte maturation (germinal vesicle migration) than at earlier stages of development. These results strongly suggest that the 20 beta-S binding activity characterized in our study represents authentic MIS receptors. A distinct, soluble binding site for 17 alpha, 20 beta-P was also identified in seatrout ovaries, but its biological function remains unclear. A hypothesis is presented for the significance of this 17 alpha,20 beta-P binding site.