Study of Androgen Receptor Functions by Genetic Models

Exploring the Role of Posttranslational Modifications in Spinal and Bulbar Muscular Atrophy

Spinal and Bulbar Muscular Atrophy (SBMA) is an X-linked adult-onset progressive neuromuscular disease that affects the spinal and bulbar motor neurons and skeletal muscles. SBMA is caused by expansion of polymorphic CAG trinucleotide repeats in the Androgen Receptor (AR) gene, resulting in expanded glutamine tract in the AR protein. Polyglutamine (polyQ) expansion renders the mutant AR protein toxic, resulting in the formation of mutant protein aggregates and cell death. This classifies SBMA as one of the nine known polyQ diseases. Like other polyQ disorders, the expansion of the polyQ tract in the AR protein is the main genetic cause of the disease; however, multiple other mechanisms besides the polyQ tract expansion also contribute to the SBMA disease pathophysiology. Posttranslational modifications (PTMs), including phosphorylation, acetylation, methylation, ubiquitination, and SUMOylation are a category of mechanisms by which the functionality of AR has been found to be significantly modulated and can alter the neurotoxicity of SBMA. This review summarizes the different PTMs and their effects in regulating the AR function and discusses their pathogenic or protective roles in context of SBMA. This review also includes the therapeutic approaches that target the PTMs of AR in an effort to reduce the mutant AR-mediated toxicity in SBMA.

Androgen-dependent and DNA-binding-independent association of androgen receptor with chromatic regions coding androgen-induced noncoding RNAs

Androgen induces the binding of its receptor (AR) to androgen-responsive elements (AREs), while genome-wide studies showed that most androgen-induced AR binding sites on chromatin were unrelated to AREs. Enhancer RNAs (eRNAs), a class of noncoding RNAs (ncRNAs), are transcribed from superenhancers (SEs) and trigger the formation of large ribonucleoprotein condensates of transcription factors. By in silico search, an SE is found to be located on the locus of KLK3 that encodes prostate specific antigen. On the KLK3 SE, androgen-induced expression of ncRNAs was detected and designated as KLK3eRNAs in LNCaP cells, and androgen-induced association of AR and FOXA1 on the KLK3eRNA coding regions was detected. Such androgen-induced association of an AR mutant lacking DNA binding activity on the KLK3eRNA coding regions was undetectable on an exogenous ARE. Thus, the present findings suggest a molecular basis of androgen-induced association of AR with chromatin on ARE-unrelated sequences.

Defining the Construct of Synthetic Androgen Intoxication: An Application of General Brain Arousal

Synthetic androgens (i. e., anabolic-androgenic steroids) are the primary component to the majority of problematic appearance and performance enhancing drug (APED) use. Despite evidence that these substances are associated with increased risk for aggression, violence, body image disturbances, and polypharmacy and can develop a pattern of chronic use consistent with drug dependence, there are no formal definitions of androgen intoxication. Consequently, the purpose of this paper is to establish a testable theory of androgen intoxication. We present evidence and theorize that synthetic androgen intoxication can be defined by a pattern of poor self-regulation characterized by increased propensity for a range of behaviors (e.g., aggression, sex, drug seeking, exercise, etc.) via androgen mediated effects on general brain arousal. This theory posits that androgens reduce threshold for emotional reactivity, motor response, and alertness to sensory stimuli and disrupt inhibitory control over the behaviors associated with synthetic androgen use. These changes result from alteration to basic neurocircuitry that amplifies limbic activation and reduces top-down cortical control. The implications for this definition are to inform APED specific hypotheses about the behavioral and psychological effects of APED use and provide a basis for establishing clinical, legal, and public health guidelines to address the use and misuse of these substances.

Androgen receptor functions as a negative transcriptional regulator of DEPTOR, mTOR inhibitor

It has been noticed that crosstalk between androgen receptor (AR) and mammalian target of rapamycin (mTOR) signaling pathways plays a crucial role in the proliferation of prostate cancer cells. To clarify this mechanism, we focused on DEPTOR, a naturally occurring inhibitor of mTOR. The treatment of a human AR-positive prostate cancer cell line, LNCaP, with the AR-agonist dihydrotestosterone (DHT) repressed DEPTOR mRNA expression in a time-dependent manner. This repression was abrogated by treatment with the AR-antagonist bicalutamide. Knockdown of DEPTOR mRNA by siRNA resulted in the increased phosphorylation of 70 kDa ribosomal protein S6 kinase 1 (S6K), a substrate of mTORC1, accompanied by the elevated expression of cyclin D1, a positive regulator of cell proliferation. Furthermore, the ChIP assay demonstrated that AR could bind to AR-responsible element-like region within the 4th intron of the DEPTOR gene. The amount of acetylated histone H3 (Lys9, Lys14) was reduced by the DHT treatment in this region. Taken together, these results propose that AR-dependent prostate cancer cell proliferation requires decreased DEPTOR transcription directly controlled by AR.

Differential ligand selectivity of androgen receptors α and β from Murray-Darling rainbowfish (Melanotaenia fluviatilis)

Androgen receptors (ARs) mediate the physiological effects of androgens in vertebrates. In fishes, AR-mediated pathways can be modulated by aquatic contaminants, resulting in the masculinisation of female fish or diminished secondary sex characteristics in males. The Murray-Darling rainbowfish (Melanotaenia fluviatilis) is a small-bodied freshwater teleost used in Australia as a test species for environmental toxicology research. We determined concentration-response profiles for selected agonists and antagonists of rainbowfish ARα and ARβ using transient transactivation assays. For both ARα and ARβ, the order of potency of natural agonists was 11-ketotestosterone (11-KT)>5α-dihydrotestosterone>testosterone>androstenedione. Methyltestosterone was a highly potent agonist of both receptors relative to 11-KT. The relative potency of the veterinary growth-promoting androgen, 17β-trenbolone, varied by more than a factor of 5 between ARα and ARβ. The non-steroidal anti-androgen bicalutamide exhibited high inhibitory potency relative to the structurally related model anti-androgen, flutamide. The inhibitory potency of the agricultural fungicide, vinclozolin, was approximately 1.7-fold relative to flutamide for ARα, but over 20-fold in the case of ARβ. Fluorescent protein tagging of ARs showed that the rainbowfish ARα subtype is constitutively localised to the nucleus, while ARβ is cytoplasmic in the absence of ligand, an observation which agrees with the reported subcellular localisation of AR subtypes from other teleost species. Collectively, these data suggest that M. fluviatilis ARα and ARβ respond differently to environmental AR modulators and that in vivo sensitivity to contaminants may depend on the tissue distribution of the AR subtypes at the time of exposure.

Prenatal testosterone and dihydrotestosterone exposure disrupts ovine testicular development

Androgens play important roles during the first trimester of intrauterine life, coinciding with genital tract differentiation, during virilization and maintenance of secondary male characteristics, and during initiation of spermatogenesis. Little is known about the impact of inappropriate exposure to excess androgens during fetal development on male sexual maturation and reproduction. The objectives of this study were to determine the effects of prenatal 5α-dihydrotestosterone (DHT) and testosterone treatment during ovine sexual differentiation on post-pubertal testicular formation and subsequent potential for fertility as assessed by epididymal sperm characteristics. Rams prenatally treated with testosterone exhibited increased testicular weight relative to age-matched controls and prenatal DHT-treated rams (P<0.05), as well as elevated total and free testosterone concentrations compared with DHT-treated rams (P=0.07 and P<0.05 respectively). The percentage of progressively motile sperm from the epididymis was significantly reduced in prenatal DHT-treated but not testosterone-treated rams compared with control rams (P<0.05). The testosterone-treated rams had a greater number of germ cell layers than DHT-treated rams, but comparable to the controls. Prenatal testosterone-treated rams had significantly larger seminiferous tubule diameter and lumen diameter compared with prenatal DHT-treated (P<0.05). Significantly, more prenatal DHT- and testosterone-treated rams (P<0.05) had occluded tubule lumen than control rams. Findings from this study demonstrate that exposure to excess testosterone/DHT during male fetal sexual differentiation have differential effects on post-pubertal testicular size, seminiferous tubule size and function, sperm motility, and testosterone concentrations.

Association of testicular p63 expression and spermatogenesis in androgen receptor knockout (ARKO) mice

OBJECTIVE

To study changes of testicular p63 expression and its effect on spermatogenic function in seminiferous tubules in androgen receptor knockout (ARKO) mice.

METHODS

A total of 28 ARKO mice (ARKO group) screened by Cre-lox and 28 male Wistar mice without ARKO (controlled group) were enrolled in our study. Route pathology was performed and p63 examination was detected by immunohistochemistry in testes. Linear correlations were used to explore potential associations between p63 protein expression and spermatogenic function (TMS score).

RESULTS

In ARKO group, inner diameter of seminiferous tubules was decreased (62 +/- 1.3 microm vs. 91 +/- 1.2 microm), thickness of the basal membrane of the tubules (4 +/- 0.3 microm vs. 2.7 +/- 0.5 microm), cellular population within tubules was reduced (2 +/- 0.4 vs. 4 +/- 0.1 layers), degree of spermatogenesis within the tubules turned to disturbance (3 +/- 1.0 vs. 5 +/- 0.1), Testicular Makler score was lower than controlled group (7 +/- 0.2 vs.15 +/- 0.3), they had significant differences (p <0.01). P63 expressed significantly lower in ARKO group than that in Wistar group, and was limited at stages from spermatocyte to round spermatid. (Percentage of positive cells ? 68.1 +/- 3.7 vs. 81.7 +/- 5.1, p<0.001). The HSCORE yielded similar results (HSCORE 3.7 +/- 0.3 vs. 2.0 +/- 0.2, p<0.001). p63 protein expression was significantly positively correlated with spermatogenic function (r=0.87, p<0.01).

CONCLUSIONS

p63 developed important effect on spermatogenesis and the regulatory effect of p63 on spermatogenesis mainly occurred in the early stage of spermiogenesis in testis.

Involvement of three glutamine tracts in human androgen receptor transactivation

The androgen receptor (AR) possesses a polymorphic polyglutamine tract (polyQ), whose length is inversely correlated with its transcriptional activity. Here, we investigated whether 6 and 5 repetitive glutamine tracts (Q6 and Q5, respectively) in the N-terminal domain of AR also have effects on AR transactivation. In a reporter gene assay using two-tandem repeats of an androgen response element, deletion of glutamine tracts significantly increased AR transactivation in the following order: wild-type<a single deletion of polyQ or Q5<double deletion of polyQ and Q6<double deletion of polyQ and Q5<triple deletion. Deletion of polyQ alone or combined deletion of polyQ and Q5 from an AR mutant lacking the ligand-binding domain, which is constitutively active due to activation function-1, increased AR transactivation. However, the glutamine tracts had no influence on activation function-1 activity, suggesting that the glutamine tracts modulate the binding of AR to DNA. Q5, like polyQ, was found to be involved in the interaction between the NH(2)- and COOH-terminal regions of AR (N-C interaction). These results indicate that the inhibitory effects of polyQ and Q5 on AR transactivation are the due, at least in part, to their negative regulation of N-C interaction.

Steroidal androgens and nonsteroidal, tissue-selective androgen receptor modulator, S-22, regulate androgen receptor function through distinct genomic and nongenomic signaling pathways

Androgen receptor (AR) ligands are important for the development and function of several tissues and organs. However, the poor oral bioavailability, pharmacokinetic properties, and receptor cross-reactivity of testosterone, coupled with side effects, place limits on its clinical use. Selective AR modulators (SARMs) elicit anabolic effects in muscle and bone, sparing reproductive organs like the prostate. However, molecular mechanisms underlying the tissue selectivity remain ambiguous. We performed a variety of in vitro studies to compare and define the molecular mechanisms of an aryl propionamide SARM, S-22, as compared with dihydrotestosterone (DHT). Studies indicated that S-22 increased levator ani muscle weight but decreased the size of prostate in rats. Analysis of the upstream intracellular signaling events indicated that S-22 and DHT mediated their actions through distinct pathways. Modulation of these pathways altered the recruitment of AR and its cofactors to the PSA enhancer in a ligand-dependent fashion. In addition, S-22 induced Xenopus laevis oocyte maturation and rapid phosphorylation of several kinases, through pathways distinct from steroids. These studies reveal novel differences in the molecular mechanisms by which S-22, a nonsteroidal SARM, and DHT mediate their pharmacological effects.

Positive regulation of steroidogenic acute regulatory protein gene expression through the interaction between Dlx and GATA-4 for testicular steroidogenesis

Split hand/foot malformation (SHFM) is syndromic ectrodactyly often associated with mental retardation and/or craniofacial defects. Several clinical reports previously described urogenital dysplasia such as micropenis, hypospadias, and small testis in SHFM patients. Genetic lesions in the Dlx5 and Dlx6 (Dlx5/6) locus are associated with the human genetic disorder SHFM type 1. Although Dlx5/6 are expressed in the testis, their possible function of Dlx5/6 during testis differentiation has not been described. In this study, we show that Dlx5/6 are expressed in the fetal Leydig cells during testis development. We examined the effect of Dlx5 expression on the promoter activation of the steroidogenic acute regulatory protein (StAR) gene, which is essential for gonadal and adrenal steroidogenesis, in a Leydig cell line. Dlx5 efficiently activates the StAR promoter when GATA-4, another transcription factor essential for testicular steroidogenesis, was coexpressed. The transcriptional activation required the GATA-4-recognition element in the StAR promoter region and Dlx5 can physically interact with GATA-4. Furthermore, we herein show that the double inactivation of Dlx5 and Dlx6 in the mouse leads to decreased testosterone level and abnormal masculinization phenotype. These results suggest that Dlx5 and Dlx6 participate in the control of steroidogenesis during testis development. The findings of this study may open the way to analyze human congenital birth defects.

Subcellular Localization Modulates Activation Function 1 Domain Phosphorylation in the Androgen Receptor

Although the steady-state distribution of the androgen receptor (AR) is predominantly nuclear in androgen-treated cells, androgen-bound AR shuttles between the nucleus and the cytoplasm. In the present study we have addressed how nucleocytoplasmic shuttling contributes to the regulation of AR. Nuclear transport signal fusions were used to force AR localization to the nucleus or cytoplasm of prostate cancer cells, and the effect of localization on shuttling, transcription, androgen binding, and phosphorylation was determined. Fusing the simian virus 40 nuclear localization signal or c-Abl nuclear export signal to AR resulted in androgen-independent localization to the nucleus or cytoplasm, respectively. AR forced to the nucleus was transcriptionally active on prostate-specific antigen and mouse mammary tumor virus promoters driving reporter genes. AR forced to the cytoplasm was largely inactive on the prostate-specific antigen promoter, but, surprisingly, AR was active on the mouse mammary tumor virus promoter and on two endogenous genes examined. Thus, highly transient nuclear localization of AR is sufficient to activate transcription. Androgen dissociation rates and the dissociation constant (KD) of AR for androgen were similar whether AR was localized to the cytoplasm or the nucleus, suggesting the ligand-binding cycle of AR is not strictly linked to its compartmentalization. Using phosphosite antibodies, we found that compartmentalization influences the phosphorylation state of AR. We show there is a bias for androgen-dependent phosphorylation of Ser81, Ser256, and Ser308 in the nucleus and androgen-independent phosphorylation of Ser94 in the cytoplasm. We propose that one function of nucleocytoplasmic shuttling is to integrate the signaling environment in the cytoplasm with AR activity in the nucleus.

Sex steroid effects at target tissues: mechanisms of action

Our understanding of the mechanisms of sex hormone action has changed dramatically over the last 10 years. Estrogens, progestins, and androgens are the steroid hormones that modulate reproductive function. Recent data have shown that many other tissues are targets of sex hormones in addition to classical reproductive organs. This review outlines new advances in our understanding of the spectrum of steroid hormone ligands, newly recognized target tissues, structure-function relationships of steroid receptors, and, finally, their genomic and nongenomic actions. Sex-based specific effects are often related to the different steroid hormone mileu in men compared with women. Understanding the mechanisms of sex steroid action gives insight into the differences in normal physiology and disease states.

Male fetal germ cells are targets for androgens that physiologically inhibit their proliferation

In adulthood, the action of androgens on seminiferous tubules is essential for full quantitatively normal spermatogenesis and fertility. In contrast, their role in the fetal testis, and particularly in fetal germ cell development, remains largely unknown. Using testicular feminized (Tfm) mice, we investigated the effects of a lack of functional androgen receptor (AR) on fetal germ cells, also named gonocytes. We demonstrated that endogenous androgens/AR physiologically control normal gonocyte proliferation. We observed an increase in the number of gonocytes at 17.5 days postconception resulting from an increase in proliferative activity in Tfm mice. In a reciprocal manner, gonocyte proliferation is decreased by the addition of DHT in fetal testis organotypic culture. Furthermore, the AR coregulator Hsp90alpha (mRNA and protein) specifically expressed in gonocytes was down-regulated in Tfm mice at 15.5 days postconception. To investigate whether these effects could result from direct action of androgens on gonocytes, we collected pure gonocyte preparations and detected AR transcripts therein. We used an original model harboring a reporter gene that specifically reflects AR activity by androgens and clearly demonstrated the presence of a functional AR protein in fetal germ cells. These data provide in vivo and in vitro evidence of a new control of endogenous androgens on gonocytes identified as direct target cells for androgens. Finally, our results focus on a new pathway in the fetal testis during the embryonic period, which is the most sensitive to antiandrogenic endocrine disruptors.

ATAXIN-1 Interacts with the Repressor Capicua in Its Native Complex to Cause SCA1 Neuropathology

Spinocerebellar ataxia type 1 (SCA1) is one of several neurodegenerative diseases caused by expansion of a polyglutamine tract in the disease protein, in this case, ATAXIN-1 (ATXN1). A key question in the field is whether neurotoxicity is mediated by aberrant, novel interactions with the expanded protein or whether its wild-type functions are augmented to a deleterious degree. We examined soluble protein complexes from mouse cerebellum and found that the majority of wild-type and expanded ATXN1 assembles into large stable complexes containing the transcriptional repressor Capicua. ATXN1 directly binds Capicua and modulates Capicua repressor activity in Drosophila and mammalian cells, and its loss decreases the steady-state level of Capicua. Interestingly, the S776A mutation, which abrogates the neurotoxicity of expanded ATXN1, substantially reduces the association of mutant ATXN1 with Capicua in vivo. These data provide insight into the function of ATXN1 and suggest that SCA1 neuropathology depends on native, not novel, protein interactions.

Molecular genetic cascades for external genitalia formation: an emerging organogenesis program

External genitalia are anatomical structures located at the posterior embryonic region as part of several urogenital/reproductive organs. The embryonic anlage of the external genitalia, the genital tubercle (GT) develops as a bud-shaped structure with an initial urethral plate and later urethra. Embryonic external genitalia are considered to be one of the appendages. Recent experiments suggest that essential regulatory genes possess similar functions for the outgrowth regulation of the GT and limb appendages. The transient embryonic epithelia located in the distal GT are called the distal urethral epithelium (DUE) regulating, at least in part, the (distal) GT development. This review covers the available data about early patterning of GT and discusses the molecular developmental similarities and points of divergence between the different appendages. Development of the male and female external genitalia is also reviewed.

Ovarian follicle development and transgenic mouse models

Ovarian follicle development is a complex process that begins with the establishment of what is thought to be a finite pool of primordial follicles and culminates in either the atretic degradation of the follicle or the release of a mature oocyte for fertilization. This review highlights the many advances made in understanding these events using transgenic mouse models. Specifically, this review describes the ovarian phenotypes of mice with genetic mutations that affect ovarian differentiation, primordial follicle formation, follicular growth, atresia, ovulation and corpus luteum (CL) formation. In addition, this review describes the phenotypes of mice with mutations in a variety of genes, which affect the hormones that regulate folliculogenesis. Because studies using transgenic animals have revealed a variety of reproductive abnormalities that resemble many reproductive disorders in women, it is likely that studies using transgenic mouse models will impact our understanding of ovarian function and fertility in women.