Ontogeny of androgen receptor and disruption of its mRNA expression by exogenous estrogens during morphogenesis of the genital tubercle

Spatiotemporal map of key signaling factors during early penis development

The formation of the external genitalia is a highly complex developmental process, considering it involves a wide range of cell types and results in sexually dimorphic outcomes. Development is controlled by several secreted signalling factors produced in complex spatiotemporal patterns, including the hedgehog (HH), bone morphogenic protein (BMP), fibroblast growth factor (FGF) and WNT signalling families. Many of these factors act on or are influenced by the actions of the androgen receptor (AR) that is critical to masculinisation. This complexity of expression makes it difficult to conceptualise patterns of potential importance. Mapping expression during key stages of development is needed to develop a comprehensive model of how different cell types interact in formation of external genitalia, and the global regulatory networks at play. This is particularly true in light of the sensitivity of this process to environmental disruption during key stages of development. The goal of this review is to integrate all recent studies on gene expression in early penis development to create a comprehensive spatiotemporal map. This serves as a resource to aid in visualising potentially significant interactions involved in external genital development.

Diagrams of published RNA and protein localisation data for key secreted signalling factors during early penis development.

Unconventional expression patterns are identified that suggest novel signalling axes during development.

Key research gaps and limitations are identified and discussed.

Prenatal exposure to atrazine induces cryptorchidism and hypospadias in F1 male mouse offspring

The main objective of the present study was to determine whether prenatal exposure to atrazine could affect testicle descent and penile masculinization. Atrazine has been demonstrated with a variety of endocrine disrupting activities and reproductive toxicities. However, the effects of prenatal atrazine exposure on male offspring's genital malformation, such as hypospadias and cryptorchidism, remain poorly understood. In this study, pregnant ICR mice were gavaged from gestational day 12.5-16.5 with different doses of atrazine. Although no sign of systemic toxicity was observed in F1 male pups, prenatal exposure to 100 mg/kg/day atrazine affected penile morphology, urethral meatus position and descent of testis, and reduced anogenital distance and penile size in postnatal day 21 F1 male pups. The comparative study with an androgen receptor (AR) antagonist vinclozolin suggested that these effects of atrazine on male genital development may not be through antagonism of AR. The results also revealed that atrazine exposure significantly reduced maternal serum testosterone levels, decreased AR nuclear translocation, and altered the expression levels of developmental gene networks in developing penis of mice. Atrazine exposure also affected the expression of insulin-like 3 (Insl3) and steroidogenic gene expression in developing reproductive tract. Therefore, our data indicate that prenatal atrazine exposure can induce hypospadias in F1 mice, likely through disruption of testosterone production, decreasing genomic androgen action, and then altering expression of developmental genes during sexual differentiation. Our data also suggest that prenatal atrazine exposure can induce cryptorchidism in F1 mice, possibly through down regulation of Insl3.

Estrogens and development of the mouse and human external genitalia

The Jost hypothesis states that androgens are necessary for normal development of the male external genitalia. In this review, we explore the complementary hypothesis that estrogens can elicit abnormal development of male external genitalia. Herein, we review available data in both humans and mice on the deleterious effects of estrogen on external genitalia development, especially during the "window of susceptibility" to exogenous estrogens. The male and female developing external genitalia in both the human and mouse express ESR1 and ESR2, along with the androgen receptor (AR). Human clinical data suggests that exogenous estrogens can adversely affect normal penile and urethral development, resulting in hypospadias. Experimental mouse data also strongly supports the idea that exogenous estrogens cause penile and urethral defects. Despite key differences, estrogen-induced hypospadias in the mouse displays certain morphogenetic homologies to human hypospadias, including disruption of urethral fusion and preputial abnormalities. Timing of estrogenic exposure, or the "window of susceptibility," is an important consideration when examining malformations of the external genitalia in both humans and mice. In addition to a review of normal human and mouse external genital development, this article aims to review the present data on the role of estrogens in normal and abnormal development of the mouse and human internal and external genitalia. Based on the current literature for both species, we conclude that estrogen-dependent processes may play a role in abnormal genital development.

Androgen Receptor Expression in Hypospadias

Introduction:

The exact mechanism behind the development of hypospadias is unclear. Research studies on androgen receptor (AR) expression are controversial with results stating all possible outcomes – AR elevated, similar, or reduced when compared to normal.

Aims:

The aim is to study the AR expression and hormone levels in hypospadias patients and compare them with children having normal genitalia.

Methods:

Group 1 (controls) involved patients who underwent circumcision for phimosis while Group 2 involved hypospadias patients who did not receive any preoperative testosterone. Preoperative hormonal assay included luteinizing hormone, follicle-stimulating hormone, and free testosterone levels in all the patients. The foreskin specimen was analyzed for AR expression using immunohistochemistry (anti-AR antibody PathnSitu, clone R441, 1/100 dilution). AR staining was expressed as H score. The H score was calculated by multiplying the intensity of staining and the percentage of stained cells showing cytoplasmic positivity at high power (×40).

Results:

There were 27 patients in Group 1 while 16 in Group 2 (distal 10; proximal 6).There was no significant difference in the age distribution. The mean H score was significantly higher (189.5) in hypospadias patients compared to controls (97.5) and was significantly higher in proximal (220) compared to distal (159) hypospadias. There was no significant difference in hormone levels between groups.

Conclusion:

AR expression was significantly elevated in hypospadias patients. It was higher in proximal compared to distal hypospadias, probably due to end-organ overexpression. Further larger trials are likely to through light into this controversial subject.

Development of the human penis and clitoris

The human penis and clitoris develop from the ambisexual genital tubercle. To compare and contrast the development of human penis and clitoris, we used macroscopic photography, optical projection tomography, light sheet microscopy, scanning electron microscopy, histology and immunohistochemistry. The human genital tubercle differentiates into a penis under the influence of androgens forming a tubular urethra that develops by canalization of the urethral plate to form a wide diamond-shaped urethral groove (opening zipper) whose edges (urethral folds) fuse in the midline (closing zipper). In contrast, in females, without the influence of androgens, the vestibular plate (homologue of the urethral plate) undergoes canalization to form a wide vestibular groove whose edges (vestibular folds) remain unfused, ultimately forming the labia minora defining the vaginal vestibule. The neurovascular anatomy is similar in both the developing human penis and clitoris and is the key to successful surgical reconstructions.

Contrasting mechanisms of penile urethral formation in mouse and human

This paper addresses the developmental mechanisms of formation of the mouse and human penile urethra and the possibility that two disparate mechanisms are at play. It has been suggested that the entire penile urethra of the mouse forms via direct canalization of the endodermal urethral plate. While this mechanism surely accounts for development of the proximal portion of the mouse penile urethra, we suggest that the distal portion of the mouse penile urethra forms via a series of epithelial fusion events. Through review of the recent literature in combination with new data, it is unlikely that the entire mouse urethra is formed from the endodermal urethral plate due in part to the fact that from E14 onward the urethral plate is not present in the distal aspect of the genital tubercle. Formation of the distal portion of the mouse urethra receives substantial contribution from the preputial swellings that form the preputial-urethral groove and subsequently the preputial-urethral canal, the later of which is subdivided by a fusion event to form the distal portion of the mouse penile urethra. Examination of human penile development also reveals comparable dual morphogenetic mechanisms. However, in the case of human, direct canalization of the urethral plate occurs in the glans, while fusion events are involved in formation of the urethra within the penile shaft, a pattern exactly opposite to that of the mouse. The highest incidence of hypospadias in humans occurs at the junction of these two different developmental mechanisms. The relevance of the mouse as a model of human hypospadias is discussed.

Computational modeling and simulation of genital tubercle development

Hypospadias is a developmental defect of urethral tube closure that has a complex etiology involving genetic and environmental factors, including anti-androgenic and estrogenic disrupting chemicals; however, little is known about the morphoregulatory consequences of androgen/estrogen balance during genital tubercle (GT) development. Computer models that predictively model sexual dimorphism of the GT may provide a useful resource to translate chemical-target bipartite networks and their developmental consequences across the human-relevant chemical universe. Here, we describe a multicellular agent-based model of genital tubercle (GT) development that simulates urethrogenesis from the sexually-indifferent urethral plate stage to urethral tube closure. The prototype model, constructed in CompuCell3D, recapitulates key aspects of GT morphogenesis controlled by SHH, FGF10, and androgen pathways through modulation of stochastic cell behaviors, including differential adhesion, motility, proliferation, and apoptosis. Proper urethral tube closure in the model was shown to depend quantitatively on SHH- and FGF10-induced effects on mesenchymal proliferation and epithelial apoptosis-both ultimately linked to androgen signaling. In the absence of androgen, GT development was feminized and with partial androgen deficiency, the model resolved with incomplete urethral tube closure, thereby providing an in silico platform for probabilistic prediction of hypospadias risk across combinations of minor perturbations to the GT system at various stages of embryonic development.

Do endocrine disruptors cause hypospadias?

Introduction

Endocrine disruptors or environmental agents, disrupt the endocrine system, leading to various adverse effects in humans and animals. Although the phenomenon has been noted historically in the cases of diethylstilbestrol (DES) and dichlorodiphenyltrichloroethane (DDT), the term “endocrine disruptor” is relatively new. Endocrine disruptors can have a variety of hormonal activities such as estrogenicity or anti-androgenicity. The focus of this review concerns on the induction of hypospadias by exogenous estrogenic endocrine disruptors. This has been a particular clinical concern secondary to reported increased incidence of hypospadias. Herein, the recent literature is reviewed as to whether endocrine disruptors cause hypospadias.

Methods

A literature search was performed for studies involving both humans and animals. Studies within the past 5 years were reviewed and categorized into basic science, clinical science, epidemiologic, or review studies.

Results

Forty-three scientific articles were identified. Relevant sentinel articles were also reviewed. Additional pertinent studies were extracted from the reference of the articles that obtained from initial search results. Each article was reviewed and results presented. Overall, there were no studies which definitely stated that endocrine disruptors caused hypospadias. However, there were multiple studies which implicated endocrine disruptors as one component of a multifactorial model for hypospadias.

Conclusions

Endocrine disruption may be one of the many critical steps in aberrant development that manifests as hypospadias.

The Genetic and Environmental Factors Underlying Hypospadias

Hypospadias results from a failure of urethral closure in the male phallus and affects 1 in 200-300 boys. It is thought to be due to a combination of genetic and environmental factors. The development of the penis progresses in 2 stages: an initial hormone-independent phase and a secondary hormone-dependent phase. Here, we review the molecular pathways that contribute to each of these stages, drawing on studies from both human and mouse models. Hypospadias can occur when normal development of the phallus is disrupted, and we provide evidence that mutations in genes underlying this developmental process are causative. Finally, we discuss the environmental factors that may contribute to hypospadias and their potential immediate and transgenerational epigenetic impacts.

Specific morphogenetic events in mouse external genitalia sex differentiation are responsive/dependent upon androgens and/or estrogens

The objective of this study was to perform a comprehensive morphologic analysis of developing mouse external genitalia (ExG) and to determine specific sexual differentiation features that are responsive to androgens or estrogens. To eliminate sex steroid signaling postnatally, male and female mice were gonadectomized on the day of birth, and then injected intraperitoneally every other day with DES (200 ng/g), DHT (1 μg/g), or oil. On day-10 postnatal male and female ExG were dissected, fixed, embedded, serially sectioned and analyzed. We identified 10 sexually dimorphic anatomical features indicative of normal penile and clitoral differentiation in intact mice. Several (but not all) penile features were impaired or abolished as a result of neonatal castration. Those penile features remaining after neonatal castration were completely abolished with attendant clitoral development in androgen receptor (AR) mutant male mice (X(Tfm)/Y and X/Y AR-null) in which AR signaling is absent both pre- and postnatally. Administration of DHT to neonatally castrated males restored development of all 10 masculine features to almost normal levels. Neonatal ovariectomy of female mice had little effect on clitoral development, whereas treatment of ovariectomized female mice with DHT induced partial masculinization of the clitoris. Administration of DES to neonatally gonadectomized male and female mice elicited a spectrum of development abnormalities. These studies demonstrate that the presence or absence of androgen prenatally specifies penile versus clitoral identity. Differentiated penile features emerge postnatally and are sensitive to and dependent upon prenatal or pre- and postnatal androgen. Emergence of differentiated clitoral features occurs postnatally in either intact or ovariectomized females. It is likely that each penile and clitoral feature has a unique time-course of hormonal dependency/sensitivity.

Androgen receptor is overexpressed in boys with severe hypospadias, and ZEB1 regulates androgen receptor expression in human foreskin cells

INTRODUCTION

ZEB1 is overexpressed in patients with severe hypospadias. We examined the interaction between ZEB1 and the androgen receptor (AR) in vitro and the expression of AR in boys with hypospadias.

RESULTS

ZEB1 and AR colocalize to the nucleus. Estrogen upregulated ZEB1 and AR expression. Chromatin immunoprecipitation (ChIP) demonstrated that ZEB1 binds to an E-box sequence in the AR gene promoter. AR expression is higher in subjects with severe hypospadias than those with mild hypospadias and control subjects (P < 0.05). ZEB1 physically interacts with AR in human foreskin cells.

DISCUSSION

AR is overexpressed in patients with severe hypospadias. Environmental estrogenic compounds may increase the risk of hypospadias by facilitating the interaction between ZEB1 and AR.

METHODS

Hs68 cells, a fibroblast cell line derived from neonatal human foreskin, were exposed to 0, 10, and 100 nmol/l of estrogen, after which the cellular localization of ZEB1 and AR was assessed using immunocytochemistry. To determine if ZEB1 interacted with the AR gene, ChIP was performed using ZEB1 antibody and polymerase chain reaction (PCR) for AR. Second, AR expression was quantified using real-time PCR and western blot in normal subjects (n = 32), and subjects with mild (n = 16) and severe hypospadia (n = 16).

Bisphenol A: developmental toxicity from early prenatal exposure

Bisphenol A (BPA) exposure has been documented in pregnant women, but consequences for development are not yet widely studied in human populations. This review presents research on the consequences for offspring of BPA exposure during pregnancy. Extensive work in laboratory rodents has evaluated survival and growth of the conceptus, interference with embryonic programs of development, morphological sex differentiation, sex differentiation of the brain and behavior, immune responsiveness, and mechanism of action. Sensitive measures include RAR, aryl hydrocarbon receptor, and Hox A10 gene expression, anogenital distance, sex differentiation of affective and exploratory behavior, and immune hyperresponsiveness. Many BPA effects are reported at low doses (10-50 µg/kg d range) by the oral route of administration. At high doses (>500,000 µg/kg d) fetal viability is compromised. Much of the work has centered around the implications of the estrogenic actions of this agent. Some work related to thyroid mechanism of action has also been explored. BPA research has actively integrated current knowledge of developmental biology, concepts of endocrine disruption, and toxicological research to provide a basis for human health risk assessment.

ZEB1 is estrogen responsive in vitro in human foreskin cells and is over expressed in penile skin in patients with severe hypospadias

PURPOSE

We determined the effect of estrogen on ZEB1 in vitro and tested the hypothesis that ZEB1 is over expressed in the penile skin of subjects with hypospadias.

MATERIALS AND METHODS

Hs68 cells, a fibroblast cell line derived from human foreskin, were exposed to 0, 1, 10 and 100 nM estrogen, and the expression level of ZEB1 was assessed using reverse transcription real-time polymerase chain reaction, Western blot and immunocytochemical analysis. Next, preputial skin was prospectively collected from case and control subjects at hypospadias repair (37 cases) and circumcision (11). Hypospadias was classified as severe (13 cases) or mild (24) based on the position of the urethral meatus. ZEB1 expression was quantified using reverse transcription real-time polymerase chain reaction, Western blot and immunohistochemical analysis.

RESULTS

Estrogen increased ZEB1 expression at the mRNA and protein levels in Hs68 cells in a concentration dependent fashion (p <0.01). Subjects with severe hypospadias had significantly higher ZEB1 mRNA levels and protein expression compared to controls or subjects with mild hypospadias (both p <0.01). Subjects with severe hypospadias had increased expression of ZEB1 in the basal layers of the preputial epidermis.

CONCLUSIONS

Estrogen increases ZEB1 expression in a human foreskin fibroblast cell line in vitro. Furthermore, ZEB1 is significantly over expressed in the penile skin of subjects with severe hypospadias. We propose that ZEB1 overexpression may contribute to development of hypospadias and may mediate the effect of estrogen on developing external male genitalia.

Cell lineage analysis demonstrates an endodermal origin of the distal urethra and perineum

Congenital malformations of anorectal and genitourinary (collectively, anogenital) organs occur at a high frequency in humans, however the lineage of cells that gives rise to anogenital organs remains poorly understood. The penile urethra has been reported to develop from two cell populations, with the proximal urethra developing from endoderm and the distal urethra forming from an apical ectodermal invagination, however this has never been tested by direct analysis of cell lineage. During gut development, endodermal cells express Sonic hedgehog (Shh), which is required for normal patterning of digestive and genitourinary organs. We have taken advantage of the properties of Shh expression to genetically label and follow the fate of posterior gut endoderm during anogenital development. We report that the entire urethra, including the distal (glandar) region, is derived from endoderm. Cloacal endoderm also gives rise to the epithelial linings of the bladder, rectum and anterior region of the anus. Surprisingly, the lineage map also revealed an endodermal origin of the perineum, which is the first demonstration that endoderm differentiates into skin. In addition, we fate mapped genital tubercle ectoderm and show that it makes no detectable contribution to the urethra. In males, formation of the urethral tube involves septation of the urethral plate by continued growth of the urorectal septum. Analysis of cell lineage following disruption of androgen signaling revealed that the urethral plate of flutamide-treated males does not undergo this septation event. Instead, urethral plate cells persist to the ventral margin of the tubercle, mimicking the pattern seen in females. Based on these spatial and temporal fate maps, we present a new model for anogenital development and suggest that disruptions at specific developmental time points can account for the association between anorectal and genitourinary defects.

Progesterone Receptors in the Developing Genital Tubercle: Implications for the Endocrine Disruptor Hypothesis as the Etiology of Hypospadias

PURPOSE

In fetal mice genital tubercles the ontogenetic expression of progesterone receptors and the effect of in utero estrogen and testosterone exposure were investigated.

MATERIALS AND METHODS

To evaluate ontogenetic progesterone receptor expression genital tubercles from untreated fetuses at gestational days 12, 14, 16 and 18, and newborn pups were prepared for real-time reverse transcriptase-polymerase chain reaction or immunohistochemistry. To evaluate estrogen and testosterone effects pregnant dams were gavaged once daily with corn oil (vehicle), ethinyl estradiol or testosterone propionate from gestational days 12 through 17. At gestational day 19 the genital tubercles of delivered fetuses were harvested for morphological examination and then pooled for real-time reverse transcriptase-polymerase chain reaction.

RESULTS

Progesterone receptor protein was first detected at gestational day 12 in the urethral plate and mesenchyma. At later stages staining intensity increased with a greater progesterone receptor signal, especially in the urethra. Progesterone receptor mRNA expression showed different increasing patterns in each sex until birth. However, no difference was noted between male and female genital tubercles in terms of the distribution and quantity of progesterone receptor expression. In utero ethinyl estradiol led to 8.2, 9.7 and 5.2-fold increases in progesterone receptor mRNA in females and in males with and without hypospadias, respectively. Testosterone propionate significantly decreased progesterone receptor mRNA levels in females and males.

CONCLUSIONS

Progesterone receptors are expressed in developing genital tubercles, suggesting a direct role of progesterone in normal genital tubercle patterning. Their increasing expression until birth also implies increasing sensitivity of the genital tubercles to the effects of estrogenic and progestogenic endocrine disruptors during fetal life. Ethinyl estradiol and testosterone propionate lead to opposing effects on progesterone receptor expression, in addition to their opposing morphological effects on the genital tubercles. These findings expand our knowledge of genital tubercle morphogenesis and provide important information for understanding the effects of endocrine disruptors.

Estrogen receptor-alpha and beta are differentially distributed, expressed and activated in the fetal genital tubercle

PURPOSE

We examined the ontogenic and sex specific expression of estrogen receptor-alpha and beta in mouse genital tubercles and assessed the effects of in utero estrogen exposure on these parameters.

MATERIALS AND METHODS

Expression of the 2 genes was detected in mouse genital tubercles from fetuses collected on gestational days 12, 14, 16 and 18, and from newborns using immunohistochemistry and quantitative polymerase chain reaction. Pregnant dams were exposed to ethinyl estradiol or corn oil as the control.

RESULTS

Estrogen receptor-alpha and beta proteins first appeared on gestational days 12 and 14, respectively. The 2 proteins were expressed in the urethral plate and mesenchyma. Staining intensity was more prominent in the mesenchyma for estrogen receptor-alpha and in the urethral plate for estrogen receptor-beta. Female genital tubercles expressed more estrogen receptor-alpha than male genital tubercles (p <0.01), while estrogen receptor-alpha expression increased gradually in the 2 sexes until birth. Estrogen receptor-beta expression did not differ between males and females, and it showed no notable variation during fetal life. Ethinyl estradiol led to a 2.1 and 3.8-fold increase in estrogen receptor-alpha expression in females and in males with hypospadias (p = 0.002 and 0.04, respectively). Estrogen receptor-beta expression did not change in response to ethinyl estradiol.

CONCLUSIONS

This study provides in vivo evidence that estrogen receptor-alpha expression in the genital tubercles of each sex increases until parturition but estrogen receptor-beta expression does not, implying genital tubercle sensitivity to estrogen increases during fetal life. Exogenous administration of estrogens results in a response of increased expression of estrogen receptor-alpha but not of estrogen receptor-beta. These differential findings for estrogen receptor-alpha and beta imply that the 2 receptors may have different roles in normal or anomalous genital tubercle development.

Candidate genes and their response to environmental agents in the etiology of hypospadias

The molecular events that lead to isolated hypospadias remain largely unknown, and the etiology of this common congenital anomaly seems to be multifactorial. We have explored the response of several candidate genes to environmental agents that cause hypospadias in a mouse model. Here, we provide an overview of current findings in relation to candidate genes and their response to environmental agents, including the results of genomic analyses of both mouse and human tissues. In addition to steroid-hormone receptors, one gene of specific interest is activating transcription factor 3 (ATF3). We hypothesize a potential mechanism of action for ATF3 and other identified genes, including TGF-B.