Sequence and expression of a cDNA encoding the red seabream androgen receptor

Environmental chemicals active as human antiandrogens do not activate a stickleback androgen receptor but enhance a feminising effect of oestrogen in roach

Sexual disruption is reported in wild fish populations living in freshwaters receiving discharges of wastewater treatment works (WwTW) effluents and is associated primarily with the feminisation of males by exposure to oestrogenic chemicals. Antiandrogens could also contribute to the feminisation of male fish, but there are far less data supporting this hypothesis and almost nothing is known for the effects of oestrogens in combination with antiandrogens in fish. We conducted a series of in vivo exposures in two fish species to investigate the potency on reproductive-relevant endpoints of the antiandrogenic antimicrobials triclosan (TCS), chlorophene (CP) and dichlorophene (DCP) and the resin, abietic acid (AbA), all found widely in WwTW effluents. We also undertook exposures with a mixture of antiandrogens and a mixture of antiandrogens in combination with the oestrogen 17α-ethinyloestradiol (EE2). In stickleback (Gasterosteus aculeatus), DCP showed a tendency to reduce spiggin induction in females androgenised by dihydrotestosterone (DHT), but these findings were not conclusive. In roach (Rutilus rutilus), exposures to DCP (178 days), or a mixture of TCS, CP and AbA (185 days), or to the model antiandrogen flutamide (FL, 178 days) had no effect on gonadal sex ratio or on the development of the reproductive ducts. Exposure to EE2 (1.5ng/L, 185 days) induced feminisation of the ducts in 17% of the males and in the mixture of antiandrogens (TCS, CP, AbA) in combination with EE2, almost all (96%) of the males had a feminised reproductive ducts. In stickleback androgen receptor (ARα and ARβ) transactivation assays, the model antiandrogens, FL and procymidone inhibited 11-ketotestosterone (11-KT) induced receptor activation, but none of the human antiandrogens, TCS, CP, DCP and AbA had an effect. These data indicate that antimicrobial antiandrogens in combination can contribute to the feminisation process in exposed males, but they do not appear to act through the androgen receptor in fish.

Disruption of androgen receptor signaling in males by environmental chemicals

Androgen-disruptors are environmental chemicals in that interfere with the biosynthesis, metabolism or action of endogenous androgens resulting in a deflection from normal male developmental programming and reproductive tract growth and function. Since male sexual differentiation is entirely androgen-dependent, it is highly susceptible to androgen-disruptors. Animal models and epidemiological evidence link exposure to androgen disrupting chemicals with reduced sperm counts, increased infertility, testicular dysgenesis syndrome, and testicular and prostate cancers. Further, there appears to be increased sensitivity to these agents during critical developmental windows when male differentiation is at its peak. A variety of in vitro and in silico approaches have been used to identify broad classes of androgen disrupting molecules that include organochlorinated pesticides, industrial chemicals, and plasticizers with capacity to ligand the androgen receptor. The vast majority of these synthetic molecules act as anti-androgens. This review will highlight the evidence for androgen disrupting chemicals that act through interference with the androgen receptor, discussing specific compounds for which there is documented in vivo evidence for male reproductive tract perturbations. This article is part of a Special Issue entitled 'Endocrine disruptors'.

5α-Dihydrotestosterone is a potent androgen in the fathead minnow (Pimephales promelas)

Dihydrotestosterone (DHT) is one of the most physiologically important androgens in many male vertebrates, with the exception of teleost fish, in which 11-ketotestosterone (KT) is generally considered the major circulating male androgen. In the present study, we investigated the effects of KT and DHT on fathead minnow juveniles (Pimephales promelas), with the aim to compare the effects of the two androgens on critical physiological processes, such as somatic growth, male secondary sexual characteristics expression, and gonad maturation. Juvenile fish (60 days post-hatch) were exposed to 20 and 200 ng/L of KT and DHT for 45 days. Exposure to both androgens significantly stimulated somatic growth in both males (20 and 200 ng/L) and females (200 ng/L). Nuptial tubercle formation was induced by both KT and DHT, but only the latter, at 200 ng/L, caused the appearance of dorsal fin spot in 92% of males and 75% of females. Circulating plasma T concentrations showed a sex-specific response; a significant increase was recorded in exposed males and a decrease in females. Both androgens induced a significant advancement of the spermatogenic processes in males at 200 ng/L. In contrast, only DHT caused a severe disruption of ovarian physiology and morphology in females, inducing the development of spermatogenic tissue (intersex). These results show that in fathead minnow juveniles, DHT had in vivo androgenic potency comparable to KT in males, and higher than KT in females, suggesting a potential involvement of DHT in the mediation of fathead minnow androgenic responses.

The fate of the duplicated androgen receptor in fishes: a late neofunctionalization event?

BACKGROUND

Based on the observation of an increased number of paralogous genes in teleost fishes compared with other vertebrates and on the conserved synteny between duplicated copies, it has been shown that a whole genome duplication (WGD) occurred during the evolution of Actinopterygian fish. Comparative phylogenetic dating of this duplication event suggests that it occurred early on, specifically in teleosts. It has been proposed that this event might have facilitated the evolutionary radiation and the phenotypic diversification of the teleost fish, notably by allowing the sub- or neo-functionalization of many duplicated genes.

RESULTS

In this paper, we studied in a wide range of Actinopterygians the duplication and fate of the androgen receptor (AR, NR3C4), a nuclear receptor known to play a key role in sex-determination in vertebrates. The pattern of AR gene duplication is consistent with an early WGD event: it has been duplicated into two genes AR-A and AR-B after the split of the Acipenseriformes from the lineage leading to teleost fish but before the divergence of Osteoglossiformes. Genomic and syntenic analyses in addition to lack of PCR amplification show that one of the duplicated copies, AR-B, was lost in several basal Clupeocephala such as Cypriniformes (including the model species zebrafish), Siluriformes, Characiformes and Salmoniformes. Interestingly, we also found that, in basal teleost fish (Osteoglossiformes and Anguilliformes), the two copies remain very similar, whereas, specifically in Percomorphs, one of the copies, AR-B, has accumulated substitutions in both the ligand binding domain (LBD) and the DNA binding domain (DBD).

CONCLUSION

The comparison of the mutations present in these divergent AR-B with those known in human to be implicated in complete, partial or mild androgen insensitivity syndrome suggests that the existence of two distinct AR duplicates may be correlated to specific functional differences that may be connected to the well-known plasticity of sex determination in fish. This suggests that three specific events have shaped the present diversity of ARs in Actinopterygians: (i) early WGD, (ii) parallel loss of one duplicate in several lineages and (iii) putative neofunctionalization of the same duplicate in percomorphs, which occurred a long time after the WGD.

11-Ketotestosterone and IGF-I increase the size of previtellogenic oocytes from shortfinned eel, Anguilla australis, in vitro

Previtellogenic ovarian fragments from eel,Anguilla australis, were culturedin vitroin a chemically defined medium containing steroids and/or peptide hormones for 18 days in order to investigate their involvement in control of early oocyte growth. 11-Ketotestosterone (11-KT), but not estradiol-17β, induced a significant 10–20% increase in diameters of previtellogenic oocytes and oocyte nuclei in a dose-dependent manner. Effects were greatest for 100 nM 11-KT, a dose that is within the physiological range seen in very early vitellogenic eels in the wild. The effect was not accompanied by obvious ultrastructural changes in the oocytes other than an apparent increase in nuclear size. Similarly, treatment with recombinant human IGF-I resulted in increased oocyte diameters, whereas no such effect was seen after treatment with heterologous insulin, GH, leptin, or human chorionic gonadotropin. Interestingly, lipid supplementation also resulted in an increase in oocyte diameter, and greater radioactivity in ovarian explants following incubation with14C-triglycerides and 11-KT, but not FSH, suggesting that the androgen may play a role in lipid accumulation into the oocyte. Our results implicate hormones from both the reproductive and the metabolic axes in control of previtellogenic oocyte growth in a teleost fish.

Molecular cloning and characterization of a nuclear androgen receptor activated by 11-ketotestosterone

Although 11-ketotestosterone is a potent androgen and induces male secondary sex characteristics in many teleosts, androgen receptors with high binding affinity for 11-ketotestosterone or preferential activation by 11-ketotestosterone have not been identified. So, the mechanism by which 11-ketotestosterone exhibits such high potency remains unclear. Recently we cloned the cDNA of an 11-ketotestosterone regulated protein, spiggin, from three-spined stickleback renal tissue. As spiggin is the only identified gene product regulated by 11-ketotestosterone, the stickleback kidney is ideal for determination of the mechanism of 11-ketotestosterone gene regulation. A single androgen receptor gene with two splicing variants, belonging to the androgen receptor-beta subfamily was cloned from stickleback kidney. A high affinity, saturable, single class of androgen specific binding sites, with the characteristics of an androgen receptor, was identified in renal cytosolic and nuclear fractions. Measurement of ligand binding moieties in the cytosolic and nuclear fractions as well as to the recombinant receptor revealed lower affinity for 11-ketotestosterone than for dihydrotestosterone. Treatment with different androgens did not up-regulate androgen receptor mRNA level or increase receptor abundance, suggesting that auto-regulation is not involved in differential ligand activation. However, comparison of the trans-activation potential of the stickleback androgen receptor with the human androgen receptor, in both human HepG2 cells and zebrafish ZFL cells, revealed preferential activation by 11-ketotestosterone of the stickleback receptor, but not of the human receptor. These findings demonstrate the presence of a receptor preferentially activated by 11-ketotestosterone in the three-spined stickleback, so far the only one known in any animal.

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.

Interactions Between Androgen and Growth Factors in Granulosa Cell Subtypes of Porcine Antral Follicles1

Androgens acting via the androgen receptor (AR) have been implicated in regulation of folliculogenesis in many animal species. These effects are possibly mediated via enhancement of FSH and/or insulin-like growth factor (IGF)-I activity in granulosa cells, which contain high levels of AR protein. We examined the in vitro effect of dihydrotestosterone (DHT) on DNA synthesis and progesterone secretion by follicular cells in response to FSH and IGF-I, alone or in combination. Cells from separate pools of 1- to 3-mm and 3- to 5-mm antral follicles were aspirated from gilt ovaries and fractioned into mural granulosa cells (MGCs) and cumulus-oocyte complexes (COCs) for subsequent cell culture. Androgen alone or with any combination of mitogen had minimal effect on proliferative and no effect on steroidogenic responses of MGCs from 3- to 5-mm antral follicles. Conversely, in MGCs from 1- to 3-mm follicles, DHT significantly enhanced IFG-I-stimulated proliferation and had variable influence on progesterone secretion. The effects of DHT on proliferative responses of COCs were also dependent on follicle size: DHT significantly augmented either IGF-I-stimulated proliferation (1- to 3-mm follicles) or FSH-stimulated proliferation (3- to 5-mm follicles). However, the steroidogenic responses of all COCs were identical, whereby DHT significantly suppressed progesterone secretion, predominantly in the presence of FSH. Addition of an AR antagonist, hydroxyflutamide, generally reversed the proliferative responses invoked by DHT but not the steroidogenic responses. We conclude that androgen-receptor-mediated activity in granulosa cells of antral follicles is dependent on follicle size, is influenced by proximity of cells to the oocyte, and possibly involves both classic and nonclassic steroid mechanisms.

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.

Differential messenger RNA transcription of androgen receptor and estrogen receptor in gonad in relation to the sex change in protandrous black porgy, Acanthopagrus schlegeli

Black porgy, Acanthopagrus schlegeli Bleeker, is a marine protandrous hermaphrodite fish. All are functional males at 1-2 yr of age and then become either males or females at 3 yr of age. To study the process of sex change in this species, mRNA transcripts of two estrogen receptors (ERalpha and ERbeta) and an androgen receptor (AR) were monitored. An AR cDNA was cloned and characterized. ERalpha, ERbeta, and AR were differentially transcribed in bisexual testicular and ovarian tissue according to reverse transcription polymerase chain reaction (RT-PCR) and Southern analysis. A real-time quantification PCR analysis was further developed for the measurement of AR, ERalpha, and ERbeta transcripts. ERalpha and AR transcripts were significantly more plentiful in bisexual testis than in bisexual ovary in 1(+)- and 2(+)-yr-old fish. ERalpha, ERbeta, and AR transcripts decreased in the functional testis of 3-yr-old fish. Similar levels of ERbeta and AR were detected in the ovary of sex-changed females and in functional testis of 3-yr-old males. Significantly decreased AR transcripts were found in testicular tissue of bisexual and functional male and female gonads in 3-yr-old fish as compared with 1- and 2-yr-old fish. In contrast, increased ERalpha transcripts were detected in the bisexual ovary and sex-changed ovary of 3-yr-old fish as compared with the bisexual ovary of 1- and 2-yr-old fish. The data suggest a differential sensitivity in the bisexual testicular and ovarian tissue of black porgy.

11-Oxygenated androgens in female teleosts: prevalence, abundance, and life history implications

Although 11-ketotestosterone (11-KT) has been found in blood of females of several diadromous fish species, the importance, abundance, and prevalence of this and related 11-oxygenated androgens in females have not been investigated. To address this issue and to determine whether the differences among androgen profiles relate to specific life history strategies, particularly diadromous migrations, fish (males and females) of around 30 species were sampled and 5 androgens were measured by radioimmunoassay. Levels of 17beta-estradiol and cortisol were also determined to evaluate ovarian and interrenal activity at the time of sampling. Testosterone (T) was the predominant androgen in most sexually recrudescent females. Only in female eel and sturgeon were 11-oxygenated androgens present in levels as high as, or higher than, those of T, although substantial amounts were also found in blood of mullet and salmonids. 11-KT was generally the most abundant 11-oxyandrogen, levels being higher than those of 11beta-hydroxytestosterone or 11beta-hydroxyandrostenedione. It is concluded that 11-oxygenated androgens are quantitatively minor steroids in most female fish. There was no convincing evidence to support the notion that the presence of 11-oxygenated androgens in blood is an adaptation specific to migratory fishes.

Dependence of selective gene activation on the androgen receptor NH2- and COOH-terminal interaction

The agonist-induced androgen receptor NH(2)- and COOH-terminal (N/C) interaction is mediated by the FXXLF and WXXLF NH(2)-terminal motifs. Here we demonstrate that agonist-dependent transactivation of prostate-specific antigen (PSA) and probasin enhancer/promoter regions requires the N/C interaction, whereas the sex-limited protein gene and mouse mammary tumor virus long terminal repeat do not. Transactivation of PSA and probasin response regions also depends on activation function 1 (AF1) in the NH(2)-terminal region but can be increased by binding an overexpressed p160 coactivator to activation function 2 (AF2) in the ligand binding domain. The dependence of the PSA and probasin enhancer/promoters on the N/C interaction for transactivation allowed us to demonstrate that in the presence of androgen, the WXXLF motif with the sequence (433)WHTLF(437) contributes as an inhibitor to AR transactivation. We further show that like the FXXLF and LXXLL motifs, the WXXLF motif interacts in the presence of androgen with AF2 in the ligand binding domain. Sequence comparisons among species indicate greater conservation of the FXXLF motif compared with the WXXLF motif, paralleling the functional significance of these binding motifs. The data provide evidence for promoter-specific differences in the requirement for the androgen receptor N/C interaction and in the contributions of AF1 and AF2 in androgen-induced gene regulation.

Sequence and expression of androgen receptor and estrogen receptor gene in the sex types of protogynous wrasse, Halichoeres trimaculatus

Sex steroid hormones play important roles in sex change and behavior of wrasses. Their actions are considered to be mediated through the nuclear hormone receptors. In this study, to elucidate the roles of estrogen receptor (ER) and androgen receptor (AR) in the reproduction of the protogynous wrasse, Halichoeres trimaculatus, AR and ER genes were partially cloned using 5'- and 3'-RACE and their transcript levels in the gonads and the brain were measured by competitive RT-PCR. The amino acid sequence (563 a.a) deduced from 5' truncated cDNA encoding wrasse AR shows about 81%, 69%, 66%, 64%, and 58% identity with those of red seabream (Chrysophrys major) androgen receptor subtype, AR1, and rainbow trout (Oncorhynchus mykiss) ARalpha, ARbeta, Japanese eel (Anguilla japonica) ARalpha, and ARbeta, respectively. The amino acid sequence (458 a.a) deduced from 5(') truncated cDNA encoding wrasse ER shows about 81%, 79%, 73%, 66%, and 63% identity with those of red seabream estrogen receptor subtype, ERalpha, Atlantic croaker (Micropogonias undulatus) ERalpha, tilapia (Oreochromis niloticus) ERalpha, rainbow trout ERalpha, and catfish (Ictalurus punctatus) ERalpha, respectively. Among the various tissues tested, AR and ER mRNAs were highly expressed in the gonads and brains. When the transcript levels of ER were measured in the gonads and the brains of females (F), initial phase male (IP), and terminal phase male (TP), no significant changes in the gene expression were observed. The transcript levels of AR in the gonads did not change among different sex types, while those in the brains of TP were higher than F and IP. These results suggest that higher expression of AR in the brains of TP is strongly correlated with behavioral change.