Distinguishing androgen receptor agonists and antagonists: distinct mechanisms of activation by medroxyprogesterone acetate and dihydrotestosterone

Feed-forward inhibition of androgen receptor activity by glucocorticoid action in human adipocytes

We compared transcriptomes of terminally differentiated mouse 3T3-L1 and human adipocytes to identify cell-specific differences. Gene expression and high content analysis (HCA) data identified the androgen receptor (AR) as both expressed and functional, exclusively during early human adipocyte differentiation. The AR agonist dihydrotestosterone (DHT) inhibited human adipocyte maturation by downregulation of adipocyte marker genes, but not in 3T3-L1. It is interesting that AR induction corresponded with dexamethasone activation of the glucocorticoid receptor (GR); however, when exposed to the differentiation cocktail required for adipocyte maturation, AR adopted an antagonist conformation and was transcriptionally repressed. To further explore effectors within the cocktail, we applied an image-based support vector machine (SVM) classification scheme to show that adipocyte differentiation components inhibit AR action. The results demonstrate human adipocyte differentiation, via GR activation, upregulates AR but also inhibits AR transcriptional activity.

Structure-based virtual screening and identification of a novel androgen receptor antagonist

Hormonal therapies, mainly combinations of anti-androgens and androgen deprivation, have been the mainstay treatment for advanced prostate cancer because the androgen-androgen receptor (AR) system plays a pivotal role in the development and progression of prostate cancers. However, the emergence of androgen resistance, largely due to inefficient anti-hormone action, limits the therapeutic usefulness of these therapies. Here, we report that 6-(3,4-dihydro-1H-isoquinolin-2-yl)-N-(6-methylpyridin-2-yl)nicotinamide (DIMN) acts as a novel anti-androgenic compound that may be effective in the treatment of both androgen-dependent and androgen-independent prostate cancers. Through AR structure-based virtual screening using the FlexX docking model, fifty-four compounds were selected and further screened for AR antagonism via cell-based tests. One compound, DIMN, showed an antagonistic effect specific to AR with comparable potency to that of the classical AR antagonists, hydroxyflutamide and bicalutamide. Consistent with their anti-androgenic activity, DIMN inhibited the growth of androgen-dependent LNCaP prostate cancer cells. Interestingly, the compound also suppressed the growth of androgen-independent C4-2 and CWR22rv prostate cancer cells, which express a functional AR, but did not suppress the growth of the AR-negative prostate cancer cells PPC-1, DU145, and R3327-AT3.1. Taken together, the results suggest that the synthetic compound DIMN is a novel anti-androgen and strong candidate for useful therapeutic agent against early stage to advanced prostate cancer.

Androgen receptor exon 1 mutation causes androgen insensitivity by creating phosphorylation site and inhibiting melanoma antigen-A11 activation of NH2- and carboxyl-terminal interaction-dependent transactivation

Naturally occurring germ line mutations in the X-linked human androgen receptor (AR) gene cause incomplete masculinization of the external genitalia by disrupting AR function in males with androgen insensitivity syndrome. Almost all AR missense mutations that cause androgen insensitivity syndrome are located in the highly structured DNA and ligand binding domains. In this report we investigate the functional defect associated with an AR exon 1 missense mutation, R405S, that caused partial androgen insensitivity. The 46,XX heterozygous maternal carrier had a wild-type Arg-405 CGC allele but transmitted an AGC mutant allele coding for Ser-405. At birth, the 46,XY proband had a bifid scrotum, hypospadias, and micropenis consistent with clinical stage 3 partial androgen insensitivity. Androgen-dependent transcriptional activity of AR-R405S expressed in CV1 cells was less than wild-type AR and refractory in androgen-dependent AR NH(2)- and carboxyl interaction transcription assays that depend on the coregulator effects of melanoma antigen-A11. This mutation created a Ser-405 phosphorylation site evident by the gel migration of an AR-R405S NH(2)-terminal fragment as a double band that converted to the wild-type single band after treatment with λ-phosphatase. Detrimental effects of the R405S mutation were related to the proximity of the AR WXXLF motif (433)WHTLF(437) required for melanoma antigen-A11 and p300 to stimulate transcriptional activity associated with the AR NH(2)- and carboxyl-terminal interaction. We conclude that the coregulator effects of melanoma antigen-A11 on the AR NH(2)- and carboxyl-terminal interaction amplify the androgen-dependent transcriptional response to p300 required for normal human male sex development in utero.

Neuroprotective effects of estrogens and androgens in CNS inflammation and neurodegeneration

Multiple sclerosis (MS) is a disease characterized by inflammation and demyelination. Currently, the cause of MS is unknown. Experimental autoimmune encephalomyelitis (EAE) is the most common mouse model of MS. Treatments with the sex hormones, estrogens and androgens, are capable of offering disease protection during EAE and are currently being used in clinical trials of MS. Beyond endogenous estrogens and androgens, treatments with selective estrogen receptor modulators (SERMs) for estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) are also capable of providing disease protection. This protection includes, but is not limited to, prevention of clinical disease, reduction of CNS inflammation, protection against demyelination, and protection against axonal loss. In EAE, current efforts are focused on using conditional cell specific knockouts of sex hormone receptors to identify the in vivo targets of these estrogens and androgens as well as downstream molecules responsible for disease protection.

Androgen receptor molecular biology and potential targets in prostate cancer

The androgen receptor (AR) is a key transcriptional regulator and therapeutic target in prostate cancer. During androgen deprivation therapy to treat metastatic prostate cancer, surviving cells acquire increased AR signaling through a variety of mechanisms, one of which is enhanced interactions with AR coactivators. One recently identified AR-specific coregulator expressed only in human and nonhuman primates is the melanoma antigen gene protein-A11 (MAGE-11). MAGE-11 increases AR transcriptional activity through direct interactions with AR and other coactivators, and its levels increase during prostate cancer progression to castration-recurrent growth. The MAGE-11 gene is located at Xq28 on the human X chromosome as part of an X-linked MAGE gene family of cancer-testis antigens. MAGE-11 stabilizes AR when androgen levels are low, and functions in a transcriptional hub to promote AR-mediated gene activation. The evolutionary development and organization of the MAGE-11 gene within the cancer-testis antigen family suggests that MAGE-11 provides a gain-of-function to AR among primates in both normal physiology and cancer, and may serve as a therapeutic target in the treatment of advanced prostate cancer.

Potential prostate cancer drug target: bioactivation of androstanediol by conversion to dihydrotestosterone

High-affinity binding of dihydrotestosterone (DHT) to the androgen receptor (AR) initiates androgen-dependent gene activation, required for normal male sex development in utero, and contributes to prostate cancer development and progression in men. Under normal physiologic conditions, DHT is synthesized predominantly by 5α-reduction of testosterone, the major circulating androgen produced by the testis. During androgen deprivation therapy, intratumoral androgen production is sufficient for AR activation and prostate cancer growth, even though circulating testicular androgen levels are low. Recent studies indicate that the metabolism of 5α-androstane-3α, 17β-diol by 17β-hydroxysteroid dehydrogenase 6 in benign prostate and prostate cancer cells is a major biosynthetic pathway for intratumoral synthesis of DHT, which binds AR and initiates transactivation to promote prostate cancer growth during androgen deprivation therapy. Drugs that target the so-called backdoor pathway of DHT synthesis provide an opportunity to enhance clinical response to luteinizing-hormone-releasing hormone (LHRH) agonists or antagonists, AR antagonists, and inhibitors of 5α-reductase enzymes (finasteride or dutasteride), and other steroid metabolism enzyme inhibitors (ketoconazole or the recently available abiraterone acetate).

QSAR models for anti-androgenic effect--a preliminary study

Three modelling systems (MultiCase®, LeadScope® and MDL® QSAR) were used for construction of androgenic receptor antagonist models. There were 923-942 chemicals in the training sets. The models were cross-validated (leave-groups-out) with concordances of 77-81%, specificity of 78-91% and sensitivity of 51-76%. The specificity was highest in the MultiCase® model and the sensitivity was highest in the MDL® QSAR model. A complementary use of the models may be a valuable tool when optimizing the prediction of chemicals for androgenic receptor antagonism. When evaluating the fitness of the model for a particular application, balance of training sets, domain definition, and cut-offs for prediction interpretation should also be taken into account. Different descriptors in the modelling systems are illustrated with hydroxyflutamide and dexamethasone as examples (a non-steroid and a steroid anti-androgen, respectively). More research concerning the mechanism of anti-androgens would increase the possibility for further optimization of the QSAR models. Further expansion of the basis for the models is in progress, including the addition of more drugs.

Regression of castrate-recurrent prostate cancer by a small-molecule inhibitor of the amino-terminus domain of the androgen receptor

Castration-recurrent prostate cancer (CRPC) is suspected to depend on androgen receptor (AR). The AF-1 region in the amino-terminal domain (NTD) of AR contains most, if not all, of the transcriptional activity. Here we identify EPI-001, a small molecule that blocked transactivation of the NTD and was specific for inhibition of AR without attenuating transcriptional activities of related steroid receptors. EPI-001 interacted with the AF-1 region, inhibited protein-protein interactions with AR, and reduced AR interaction with androgen-response elements on target genes. Importantly, EPI-001 blocked androgen-induced proliferation and caused cytoreduction of CRPC in xenografts dependent on AR for growth and survival without causing toxicity.

Effect of anabolic-androgenic steroids and glucocorticoids on the kinetics of hAR and hGR nucleocytoplasmic translocation

Although the qualitative nucleocytoplasmic transport of nuclear hormone receptors (NHRs) has been studied, there is little documentation of the cellular kinetics of this transport. Here, translocation studies using the human androgen receptor (hAR) and the human glucocorticoid receptor (hGR) were performed to aid in identifying the mechanism by which anabolic-androgenic steroids (AAS) were activating hAR and potentially interacting with hGR and how glucocorticoid ligands were interacting with the hGR and hAR. The real-time analysis of EGFP-labeled hAR and hGR ligand-induced cytoplasm-to-nucleus translocation was performed using fluorescence microscopy to better understand the action of these NHRs in a physiologically relevant cell-based model. After transient transfection, the hAR and hGR individually translocate as expected (i.e., transport is ligand-induced and dose-dependent) in this model biological system. Testosterone (TEST) had the fastest translocation rate for the hAR of 0.0525 min(-1). The other endogenous steroids, androstenedione (ANE) and dihydrotestosterone (DHT), had considerably lower hAR transport rates. The rates of hAR transport for the exogenous steroids methyltrienelone (MET), nandrolone (NAN), and oxandrolone (OXA) are lower than that of testosterone and similar to those of the endogenous steroids ANE and DHT. The hGR transport rates for cortisol (COR) and dexamethasone (DEX) are also presented. The synthetic GC, DEX, had a more rapid translocation rate (0.1599 min(-1)) at the highest dose of 100 nM compared to the endogenous GC COR (0.0431 min(-1)). The data obtained agrees with the existing qualitative data and adds an important ligand-dependent kinetic component to hAR and hGR transport. These kinetic data can aid our understanding of NHR action and interaction with other regulatory proteins, and can be useful in the development of new therapies.

Effects of conjugated equine estrogens on cognition and affect in postmenopausal women with prior hysterectomy

CONTEXT

Different menopausal hormone therapies may have varied effects on specific cognitive functions. We previously reported that conjugated equine estrogens (CEE) with medroxyprogesterone acetate had a negative impact on verbal memory but tended to impact figural memory positively over time in older postmenopausal women.

OBJECTIVE

The objective of the study was to determine the effects of unopposed CEE on changes in domain-specific cognitive function and affect in older postmenopausal women with prior hysterectomy.

DESIGN

This was a randomized, double blind, placebo-controlled clinical trial.

SETTING

The study was conducted at 14 of 40 Women's Health Initiative (WHI) clinical centers.

PARTICIPANTS

Participants were 886 postmenopausal women with prior hysterectomy, aged 65 yr and older (mean 74 yr), free of probable dementia, and enrolled in the WHI and WHI Memory Study (WHIMS) CEE-Alone trial for a mean of 3 yr and followed up for a mean of 2.70 yr.

INTERVENTION

Intervention was 0.625 mg of CEE daily or placebo.

MAIN OUTCOME MEASURES

Annual rates of change in specific cognitive functions and affect, adjusted for time since randomization, were measured.

RESULTS

Compared with placebo, unopposed CEE was associated with lower spatial rotational ability (P < 0.01) at initial assessment (after 3 yr of treatment), a difference that diminished over 2.7 yr of continued treatment. CEE did not significantly influence change in other cognitive functions and affect.

CONCLUSIONS

CEE did not improve cognitive functioning in postmenopausal women with prior hysterectomy. CEE was associated with lower spatial rotational performance after an average of 3 yr of treatment. Overall, CEE does not appear to have enduring effects on rates of domain-specific cognitive change in older postmenopausal women.

Androgen receptor inhibits estrogen receptor-alpha activity and is prognostic in breast cancer

There is emerging evidence that the balance between estrogen receptor-alpha (ER(alpha)) and androgen receptor (AR) signaling is a critical determinant of growth in the normal and malignant breast. In this study, we assessed AR status in a cohort of 215 invasive ductal breast carcinomas. AR and (ER(alpha)) were coexpressed in the majority (80-90%) of breast tumor cells. Kaplan-Meier product limit analysis and multivariate Cox regression showed that AR is an independent prognostic factor in (ER(alpha))-positive disease, with a low level of AR (less than median of 75% positive cells) conferring a 4.6-fold increased risk of cancer-related death (P = 0.002). Consistent with a role for AR in breast cancer outcome, AR potently inhibited (ER(alpha))transactivation activity and 17beta-estradiol-stimulated growth of breast cancer cells. Transfection of MDA-MB-231 breast cancer cells with either functionally impaired AR variants or the DNA-binding domain of the AR indicated that the latter is both necessary and sufficient for inhibition of (ER(alpha)) signaling. Consistent with molecular modeling, electrophoretic mobility shift assays showed binding of the AR to an estrogen-responsive element (ERE). Evidence for a functional interaction of the AR with an ERE in vivo was provided by chromatin immunoprecipitation data, revealing recruitment of the AR to the progesterone receptor promoter in T-47D breast cancer cells. We conclude that, by binding to a subset of EREs, the AR can prevent activation of target genes that mediate the stimulatory effects of 17beta-estradiol on breast cancer cells.

Differential presentation of protein interaction surfaces on the androgen receptor defines the pharmacological actions of bound ligands

The pharmacological activity of different nuclear receptor ligands is reflected by their impact on receptor structure. Thus, we asked whether differential presentation of protein-protein interaction surfaces on the androgen receptor (AR), a surrogate assay of receptor conformation, could be used in a prospective manner to define the pharmacological activity of bound ligands. To this end, we identified over 150 proteins/polypeptides whose ability to interact with AR is influenced in a differential manner by ligand binding. The most discriminatory of these protein-AR interactions were used to develop a robust compound-profiling tool that enabled the separation of ligands into functionally distinguishable classes. Importantly, the ligands within each class exhibited similar pharmacological activities, a result that highlights the relationship between receptor structure and activity and provides direction for the discovery of novel AR modulators.

The activity of medroxyprogesterone acetate, an androgenic ligand, in ovarian cancer cell invasion

OBJECTIVES

An epithelial ovarian cancer cell line constitutively expressing the androgen receptor was created to evaluate the mechanism and effects of androgen receptor activation on epithelial ovarian cancer cell invasion.

METHODS

Immunocytochemistry and Western blot analyses confirmed androgen receptor expression. Boyden chamber invasion assays were performed using cells treated with the androgen receptor ligands medroxyprogesterone acetate or dihydrotestosterone. The matrix metalloproteinases associated with invasion were investigated using zymographic assays.

RESULTS

Androgen receptor-mediated invasion is ligand dependent. While both medroxyprogesterone acetate and dihydrotestosterone signal through androgen receptor, medroxyprogesterone acetate is more effective at stimulating invasion of epithelial ovarian cancer cells. Unlike the wild-type epithelial ovarian cancer cells, this increase in invasion in androgen receptor + epithelial ovarian cancer cells does not seem to be dependent on matrix metalloproteinase 2 or 9 activation.

CONCLUSION

Although classified as a progestin, medroxyprogesterone acetate has significant androgenic activity unique from the pure androgen dihydrotestosterone. Our studies suggest that pharmacologic doses of medroxyprogesterone acetate may actually increase the invasive potential of epithelial ovarian cancer cells.

FoxO1 mediates PTEN suppression of androgen receptor N- and C-terminal interactions and coactivator recruitment

FoxO (mammalian forkhead subclass O) proteins are transcription factors acting downstream of the PTEN (phosphatase and tensin homolog deleted on chromosome 10) tumor suppressor. Their activity is negatively regulated by AKT-mediated phosphorylation. Our previous studies showed that the transcriptional activity of the androgen receptor (AR) was inhibited by PTEN in an AKT-sensitive manner. Here, we report the repression of the activity of the full-length AR and its N-terminal domain by FoxO1 and the participation of FoxO1 in AR inhibition by PTEN. Ectopic expression of active FoxO1 decreased the transcriptional activity of AR as well as androgen-induced cell proliferation and production of prostate-specific antigen. FoxO1 knock down by RNA interference increased the transcriptional activity of the AR in PTEN-intact cells and relieved its inhibition by ectopic PTEN in PTEN-null cells. Mutational analysis revealed that FoxO1 fragment 150-655, which contains the forkhead box and C-terminal activation domain, was required for AR inhibition. Mammalian two-hybrid and glutathione-S-transferase pull-down assays demonstrated that the inhibition of AR activity by PTEN through FoxO1 involved the interference of androgen-induced interaction of the N- and C-termini of the AR and the recruitment of the p160 coactivators to its N terminus and to the androgen response elements of natural AR target genes. These studies reveal new mechanisms for the inhibition of AR activity by PTEN-FoxO axis and establish FoxO proteins as important nuclear factors that mediate the mutual antagonism between AR and PTEN tumor suppressor in prostate cancer cells.

Functional AR signaling is evident in an in vitro mouse follicle culture bioassay that encompasses most stages of folliculogenesis

Androgens have distinct physiological functions within the ovary. The biological action of androgens is primarily exerted through transcriptional regulation by the nuclear androgen receptor (AR), but the molecular cascades governed by AR remain largely unknown. At present, there is imminent concern that environmental man-made chemicals with antiandrogenic properties, among others, are capable of modulating hormonal responses, thereby interfering with normal physiological processes that are critical to fertility. In the present study, we aimed to further characterize a standardized and reproducible follicle culture system in terms of AR expression during in vitro folliculogenesis to be able to use it as a bioassay to study effects of antiandrogens on follicular and oocyte growth, steroid secretion profile, and oocyte meiotic maturation capacity. Immunohistochemical analysis revealed that cytoplasmic AR protein was translocated to the nucleus of granulosa and theca cells in response to endogenous androgen production in theca cells during preantral follicular development. During the antral phase in vitro, AR was differentially expressed in mural and cumulus cells, implying an oocyte-mediated regulation. Treatment of follicles with hydroxyflutamide or bicalutamide, two model antiandrogenic compounds, resulted in reduced follicular growth during the preantral phase, altered steroidogenic environment, and arrest in oocyte meiotic maturation in response to human chorionic gonadotropin. Androgen receptor expression in the culture model corresponded well to what is described in vivo, and this system revealed several ovarian functions targeted by AR antagonists that can be further investigated using more in-depth molecular techniques.

Integrated Decision-tree Testing Strategies for Developmental and Reproductive Toxicity with Respect to the Requirements of the EU REACH Legislation

Liverpool John Moores University and FRAME conducted a research project, sponsored by Defra, on the status of alternatives to animal testing with regard to the European Union REACH (Registration, Evaluation and Authorisation of Chemicals) system for the safety testing and risk assessment of chemicals. The project covered all the main toxicity endpoints associated with the REACH system. This paper focuses on the prospects for the use of alternative methods (both in vitro and in silico) in developmental and reproductive toxicity testing. It considers many tests based on primary cells and cell lines, and the available expert systems and QSARs for developmental and reproductive toxicity, and also covers tests for endocrine disruption. Ways in which reduction and refinement measures can be used are also discussed, particularly the use of an enhanced one-generation reproductive study, which could potentially replace the two-generation study, and therefore considerably reduce the number of animals required in reproductive toxicity. Decision-tree style integrated testing strategies are also proposed for developmental and reproductive toxicity and for endocrine disruption, followed by a number of recommendations for the future facilitation of developmental and reproductive toxicity testing, with respect to human risk assessment.

The contribution of different androgen receptor domains to receptor dimerization and signaling

The androgen receptor (AR) is a ligand-activated transcription factor of the nuclear receptor superfamily that plays a critical role in male physiology and pathology. Activated by binding of the native androgens testosterone and 5alpha-dihydrotestosterone, the AR regulates transcription of genes involved in the development and maintenance of male phenotype and male reproductive function as well as other tissues such as bone and muscle. Deregulation of AR signaling can cause a diverse range of clinical conditions, including the X-linked androgen insensitivity syndrome, a form of motor neuron disease known as Kennedy's disease, and male infertility. In addition, there is now compelling evidence that the AR is involved in all stages of prostate tumorigenesis including initiation, progression, and treatment resistance. To better understand the role of AR signaling in the pathogenesis of these conditions, it is important to have a comprehensive understanding of the key determinants of AR structure and function. Binding of androgens to the AR induces receptor dimerization, facilitating DNA binding and the recruitment of cofactors and transcriptional machinery to regulate expression of target genes. Various models of dimerization have been described for the AR, the most well characterized interaction being DNA-binding domain- mediated dimerization, which is essential for the AR to bind DNA and regulate transcription. Additional AR interactions with potential to contribute to receptor dimerization include the intermolecular interaction between the AR amino terminal domain and ligand-binding domain known as the N-terminal/C-terminal interaction, and ligand-binding domain dimerization. In this review, we discuss each form of dimerization utilized by the AR to achieve transcriptional competence and highlight that dimerization through multiple domains is necessary for optimal AR signaling.

Minireview: nuclear receptors and breast cancer

Until recently, the study of nuclear receptor (NR) function in breast cancer biology has been largely limited to estrogen and progesterone receptors. The development of reliable gene expression arrays, real-time quantitative RT-PCR, and immunohistochemical techniques for studying NR superfamily members in primary human breast cancers has now revealed the presence and potential importance of several additional NRs in the biology of breast cancer. These include receptors for steroid hormones (including androgens and corticosteroids), fat-soluble vitamins A and D, fatty acids, and xenobiotic lipids derived from diet. It is now clear that after NR activation, both genomic and nongenomic NR pathways can coordinately activate growth factor signaling pathways. Advances in our understanding of both NR functional networks and epithelial cell growth factor signaling pathways have revealed a frequent interplay between NR and epithelial cell growth factor family signaling that is clinically relevant to breast cancer. Understanding how growth factor receptors and their downstream kinases are activated by NRs (and vice-versa) is a central goal for maximizing treatment opportunities in breast cancer. In addition to the estrogen receptor, it is predicted that modulating the activity of other NRs will soon provide novel prevention and treatment approaches for breast cancer patients.

Brain microglia express steroid-converting enzymes in the mouse

In the CNS, steroid hormones play a major role in the maintenance of brain homeostasis and it's response to injury. Since activated microglia are the pivotal immune cell involved in neurodegeneration, we investigated the possibility that microglia provide a discrete source for the metabolism of active steroid hormones. Using RT-PCR, our results showed that mouse microglia expressed mRNA for 17beta-hydroxysteroid dehydrogenase type 1 and steroid 5alpha-reductase type 1, which are involved in the metabolism of androgens and estrogens. Microglia also expressed the peripheral benzodiazepine receptor and steroid acute regulatory protein; however, the enzymes required for de novo formation of progesterone and DHEA from cholesterol were not expressed. To test the function of these enzymes, primary microglia cultures were incubated with steroid precursors, DHEA and AD. Microglia preferentially produced delta-5 androgens (Adiol) from DHEA and 5alpha-reduced androgens from AD. Adiol behaved as an effective estrogen receptor agonist in neuronal cells. Activation of microglia with pro-inflammatory factors, LPS and INFgamma did not affect the enzymatic properties of these proteins. However, PBR ligands reduced TNFalpha production signifying an immunomodulatory role for PBR. Collectively, our results suggest that microglia utilize steroid-converting enzymes and related proteins to influence inflammation and neurodegeneration within microenvironments of the brain.

High-throughput mammalian two-hybrid screening for protein-protein interactions using transfected cell arrays

Background

Most of the biological processes rely on the formation of protein complexes. Investigation of protein-protein interactions (PPI) is therefore essential for understanding of cellular functions. It is advantageous to perform mammalian PPI analysis in mammalian cells because the expressed proteins can then be subjected to essential post-translational modifications. Until now mammalian two-hybrid assays have been performed on individual gene scale. We here describe a new and cost-effective method for the high-throughput detection of protein-protein interactions in mammalian cells that combines the advantages of mammalian two-hybrid systems with those of DNA microarrays.

Results

In this cell array protein-protein interaction assay (CAPPIA), mixtures of bait and prey expression plasmids together with an auto-fluorescent reporter are immobilized on glass slides in defined array formats. Adherent cells that grow on top of the micro-array will become fluorescent only if the expressed proteins interact and subsequently trans-activate the reporter. Using known interaction partners and by screening 160 different combinations of prey and bait proteins associated with the human androgen receptor we demonstrate that this assay allows the quantitative detection of specific protein interactions in different types of mammalian cells and under the influence of different compounds. Moreover, different strategies in respect to bait-prey combinations are presented.

Conclusion

We demonstrate that the CAPPIA assay allows the quantitative detection of specific protein interactions in different types of mammalian cells and under the influence of different compounds. The high number of preys that can be tested per slide together with the flexibility to interrogate any bait of interest and the small amounts of reagents that are required makes this assay currently one of the most economical high-throughput detection assays for protein-protein interactions in mammalian cells.

Integrated Decision-tree Testing Strategies for Developmental and Reproductive Toxicity with Respect to the Requirements of the EU REACH Legislation

Liverpool John Moores University and FRAME conducted a research project, sponsored by Defra, on the status of alternatives to animal testing with regard to the European Union REACH (Registration, Evaluation and Authorisation of Chemicals) system for the safety testing and risk assessment of chemicals. The project covered all the main toxicity endpoints associated with the REACH system. This paper focuses on the prospects for the use of alternative methods (both in vitro and in silico) in developmental and reproductive toxicity testing. It considers many tests based on primary cells and cell lines, and the available expert systems and QSARs for developmental and reproductive toxicity, and also covers tests for endocrine disruption. Ways in which reduction and refinement measures can be used are also discussed, particularly the use of an enhanced one-generation reproductive study, which could potentially replace the two-generation study, and therefore considerably reduce the number of animals required in reproductive toxicity. Decision-tree style integrated testing strategies are also proposed for developmental and reproductive toxicity and for endocrine disruption, followed by a number of recommendations for the future facilitation of developmental and reproductive toxicity testing, with respect to human risk assessment.

Epidermal-growth-factor-dependent phosphorylation and ubiquitinylation of MAGE-11 regulates its interaction with the androgen receptor

The androgen receptor (AR) is a ligand-activated transcription factor that interacts with coregulatory proteins during androgen-dependent gene regulation. Melanoma antigen gene protein 11 (MAGE-11) is an AR coregulator that specifically binds the AR NH(2)-terminal FXXLF motif and modulates the AR NH(2)- and carboxyl-terminal N/C interaction to increase AR transcriptional activity. Here we demonstrate that epidermal growth factor (EGF) signaling increases androgen-dependent AR transcriptional activity through the posttranslational modification of MAGE-11. EGF in the presence of dihydrotestosterone stabilizes the AR-MAGE complex through the site-specific phosphorylation of MAGE-11 at Thr-360 and ubiquitinylation at Lys-240 and Lys-245. The time-dependent EGF-induced increase in AR transcriptional activity by MAGE-11 is mediated through AR activation functions 1 and 2 in association with the increased turnover of AR and MAGE-11. The results reveal a dynamic mechanism whereby growth factor signaling increases AR transcriptional activity through the covalent modification of an AR-specific coregulatory protein. Sequence conservation of the MAGE-11 phosphorylation and ubiquitinylation sites throughout the MAGE gene family suggests common regulatory mechanisms for this group of cancer-testis antigens.

Mechanisms of androgen receptor activation in advanced prostate cancer: differential co-activator recruitment and gene expression

Prostate tumour growth depends on androgens; hence treatment includes androgen ablation and anti-androgens. Eventually tumours progress and in approximately 30% of patients this is associated with mutation of the androgen receptor. Several receptor variants associated with advanced disease show promiscuous activation by other hormones and anti-androgens. Such loss of specificity could promote receptor activation, hence tumour growth, in the absence of conventional ligands, explaining therapy failure. We aimed to elucidate mechanisms by which alternative ligands promote receptor activation. The three most commonly identified variants in tumours (with amino-acid substitutions H874Y, T877A and T877S) and wild-type receptor showed differences in co-activator recruitment dependent upon ligand and the interaction motif utilized. Co-expression and knockdown of co-activators that bind via leucine or phenylalanine motifs, combined with chromatin immunoprecipitation and quantitative PCR, revealed these preferences extend to co-activator recruitment in vivo and affect receptor activity at the transcriptional level, with subsequent effects on target gene regulation. The findings suggest that mutant receptors, activated by alternative ligands, drive growth via different mechanisms to androgen-activated wild-type receptor. Tumours may hence behave differently dependent upon any androgen receptor mutation present and what ligand is driving growth, as distinct subsets of genes may be regulated.

Activity of androgen receptor antagonist bicalutamide in prostate cancer cells is independent of NCoR and SMRT corepressors

The mechanisms by which androgen receptor (AR) antagonists inhibit AR activity, and how their antagonist activity may be abrogated in prostate cancer that progresses after androgen deprivation therapy, are not clear. Recent studies show that AR antagonists (including the clinically used drug bicalutamide) can enhance AR recruitment of corepressor proteins [nuclear receptor corepressor (NCoR) and silencing mediator of retinoid and thyroid receptors (SMRT)] and that loss of corepressors may enhance agonist activity and be a mechanism of antagonist failure. We first show that the agonist activities of weak androgens and an AR antagonist (cyproterone acetate) are still dependent on the AR NH(2)/COOH-terminal interaction and are enhanced by steroid receptor coactivator (SRC)-1, whereas the bicalutamide-liganded AR did not undergo a detectable NH(2)/COOH-terminal interaction and was not coactivated by SRC-1. However, both the isolated AR NH(2) terminus and the bicalutamide-liganded AR could interact with the SRC-1 glutamine-rich domain that mediates AR NH(2)-terminal binding. To determine whether bicalutamide agonist activity was being suppressed by NCoR recruitment, we used small interfering RNA to deplete NCoR in CV1 cells and both NCoR and SMRT in LNCaP prostate cancer cells. Depletion of these corepressors enhanced dihydrotestosterone-stimulated AR activity on a reporter gene and on the endogenous AR-regulated PSA gene in LNCaP cells but did not reveal any detectable bicalutamide agonist activity. Taken together, these results indicate that bicalutamide lacks agonist activity and functions as an AR antagonist due to ineffective recruitment of coactivator proteins and that enhanced coactivator recruitment, rather than loss of corepressors, may be a mechanism contributing to bicalutamide resistance.

Modulation of androgen receptor activation function 2 by testosterone and dihydrotestosterone

The androgen receptor (AR) is transcriptionally activated by high affinity binding of testosterone (T) or its 5alpha-reduced metabolite, dihydrotestosterone (DHT), a more potent androgen required for male reproductive tract development. The molecular basis for the weaker activity of T was investigated by determining T-bound ligand binding domain crystal structures of wild-type AR and a prostate cancer somatic mutant complexed with the AR FXXLF or coactivator LXXLL peptide. Nearly identical interactions of T and DHT in the AR ligand binding pocket correlate with similar rates of dissociation from an AR fragment containing the ligand binding domain. However, T induces weaker AR FXXLF and coactivator LXXLL motif interactions at activation function 2 (AF2). Less effective FXXLF motif binding to AF2 accounts for faster T dissociation from full-length AR. T can nevertheless acquire DHT-like activity through an AR helix-10 H874Y prostate cancer mutation. The Tyr-874 mutant side chain mediates a new hydrogen bonding scheme from exterior helix-10 to backbone protein core helix-4 residue Tyr-739 to rescue T-induced AR activity by improving AF2 binding of FXXLF and LXXLL motifs. Greater AR AF2 activity by improved core helix interactions is supported by the effects of melanoma antigen gene protein-11, an AR coregulator that binds the AR FXXLF motif and targets AF2 for activation. We conclude that T is a weaker androgen than DHT because of less favorable T-dependent AR FXXLF and coactivator LXXLL motif interactions at AF2.

A novel synthetic compound that interrupts androgen receptor signaling in human prostate cancer cells

The purpose of this study was to determine the effects of 6-amino-2-[2-(4-tert-butyl-phenoxy)-ethylsulfonyl]-1H-pyrimidine-4-one (DL3), a novel synthetic compound with small-molecule drug properties, on androgen-regulated gene expression and cell growth in human prostate cancer cells. LNCaP, 22Rv1, and LAPC-4 cells were used in the studies. Expression of prostate-specific antigen (PSA) and androgen receptor (AR) was determined by ELISA, Western blotting, real-time reverse transcription-PCR, nuclear run-on, and/or promoter luciferase reporter assays. Effects of DL3 on cell growth were determined by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide staining. DL3 inhibited dihydrotestosterone (DHT)-induced PSA expression in a dose-dependent fashion. The inhibitory effects of DL3 were more potent than those of flutamide, nilutamide, and bicalutamide. Moreover, DL3 blocked the stimulatory effects of nilutamide on PSA expression in LNCaP cells. Unlike the three classic antiandrogens, DL3 did not show intrinsic AR agonist activity. Nuclear run-on and PSA promoter reporter assays revealed that DL3 blocked DHT-induced PSA gene transcription. Consistent with its effects on PSA expression, DL3 inhibited DHT-stimulated cell growth with a potency significantly superior to flutamide, nilutamide, or bicalutamide. Furthermore, cells resistant to flutamide or nilutamide were as susceptible as their parental counterparts to the inhibitory effects of DL3 on both PSA expression and cell growth. DL3 did not inhibit AR nuclear localization and the NH(2)- and COOH-terminal interaction of AR induced by DHT. These data show that DL3 is a novel inhibitor of the AR signaling axis and a potentially potent therapeutic agent for the management of advanced human prostate cancer.

Disruption of androgen receptor signaling by synthetic progestins may increase risk of developing breast cancer

There is now considerable evidence that using a combination of synthetic progestins and estrogens in hormone replacement therapy (HRT) increases the risk of breast cancer compared with estrogen alone. Furthermore, the World Health Organization has recently cited combination contraceptives, which contain synthetic progestins, as potentially carcinogenic to humans, particularly for increased breast cancer risk. Given the above observations and the current trend toward progestin-only contraception, it is important that we have a comprehensive understanding of how progestins act in the millions of women worldwide who regularly take these medications. While synthetic progestins, such as medroxyprogesterone acetate (MPA), which are currently used in both HRT and oral contraceptives were designed to act exclusively through the progesterone receptor, it is clear from both clinical and experimental settings that their effects may be mediated, in part, by binding to the androgen receptor (AR). Disruption of androgen action by synthetic progestins may have serious deleterious side effects in the breast, where the balance between estrogen signaling and androgen signaling plays a critical role in breast homeostasis. Here, we review the role of androgen signaling in the normal breast and in breast cancer and present new data demonstrating that androgen receptor function can be perturbed by low doses of MPA, similar to doses achieved in serum of women taking HRT. We propose that the observed excess of breast malignancies associated with combined HRT may be explained, in part, by synthetic progestins such as MPA acting as endocrine disruptors to negate the protective effects of androgen signaling in the breast. Understanding the role of androgen signaling in the breast and how this is modulated by synthetic progestins is necessary to determine how combined HRT alters breast cancer risk, and to inform the development of optimal preventive and treatment strategies for this disease.

Complex actions of sex steroids in adipose tissue, the cardiovascular system, and brain: Insights from basic science and clinical studies

Recent publications describing the results of the Women's Health Initiative (WHI) and other studies reporting the impact of hormone therapy on aging women have spurred reexamination of the broad use of estrogens and progestins during the postmenopausal years. Here, we review the complex pharmacology of these hormones, the diverse and sometimes opposite effects that result from the use of different estrogenic and progestinic compounds, given via different delivery routes in different concentrations and treatment sequence, and to women of different ages and health status. We examine our new and growing appreciation of the role of estrogens in the immune system and the inflammatory response, and we pose the concept that estrogen's interface with this system may be at the core of some of the effects on multiple physiological systems, such as the adipose/metabolic system, the cardiovascular system, and the central nervous system. We compare and contrast clinical and basic science studies as we focus on the actions of estrogens in these systems because the untoward effects of hormone therapy reported in the WHI were not expected. The broad interpretation and publicity of the results of the WHI have resulted in a general condemnation of all hormone replacement in postmenopausal women. In fact, careful review of the extensive literature suggests that data resulting from the WHI and other recent studies should be interpreted within the narrow context of the study design. We argue that these results should encourage us to perform new studies that take advantage of a dialogue between basic scientists and clinician scientists to ensure appropriate design, incorporation of current knowledge, and proper interpretation of results. Only then will we have a better understanding of what hormonal compounds should be used in which populations of women and at what stages of menopausal/postmenopausal life.

Therapeutic potential of the SARMs: revisiting the androgen receptor for drug discovery

Selective androgen receptor modulators (SARMS) bind to the androgen receptor and demonstrate anabolic activity in a variety of tissues; however, unlike testosterone and other anabolic steroids, these nonsteroidal agents are able to induce bone and muscle growth, as well as shrinking the prostate. The potential of SARMS is to maximise the positive attributes of steroidal androgens as well as minimising negative effects, thus providing therapeutic opportunities in a variety of diseases, including muscle wasting associated with burns, cancer, end-stage renal disease, osteoporosis, frailty and hypogonadism. This review summarises androgen physiology, the current status of the R&D of SARMS and potential therapeutic indications for this emerging class of drugs.

Linking ligand-induced alterations in androgen receptor structure to differential gene expression: a first step in the rational design of selective androgen receptor modulators

We have previously identified a family of novel androgen receptor (AR) ligands that, upon binding, enable AR to adopt structures distinct from that observed in the presence of canonical agonists. In this report, we describe the use of these compounds to establish a relationship between AR structure and biological activity with a view to defining a rational approach with which to identify useful selective AR modulators. To this end, we used combinatorial peptide phage display coupled with molecular dynamic structure analysis to identify the surfaces on AR that are exposed specifically in the presence of selected AR ligands. Subsequently, we used a DNA microarray analysis to demonstrate that differently conformed receptors facilitate distinct patterns of gene expression in LNCaP cells. Interestingly, we observed a complete overlap in the identity of genes expressed after treatment with mechanistically distinct AR ligands. However, it was differences in the kinetics of gene regulation that distinguished these compounds. Follow-up studies, in cell-based assays of AR action, confirmed the importance of these alterations in gene expression. Together, these studies demonstrate an important link between AR structure, gene expression, and biological outcome. This relationship provides a firm underpinning for mechanism-based screens aimed at identifying SARMs with useful clinical profiles.

Drug insight: Testosterone and selective androgen receptor modulators as anabolic therapies for chronic illness and aging

Several regulatory concerns have hindered development of androgens as anabolic therapies, despite unequivocal evidence that testosterone supplementation increases muscle mass and strength in men; it induces hypertrophy of type I and II muscle fibers, and increases myonuclear and satellite cell number. Androgens promote differentiation of mesenchymal multipotent cells into the myogenic lineage and inhibit their adipogenic differentiation, by facilitating association of androgen receptors with beta-catenin and activating T-cell factor 4. Meta-analyses indicate that testosterone supplementation increases fat-free mass and muscle strength in HIV-positive men with weight loss, glucocorticoid-treated men, and older men with low or low-normal testosterone levels. The effects of testosterone on physical function and outcomes important to patients have not, however, been studied. In older men, increased hematocrit and increased risk of prostate biopsy and detection of prostate events are the most frequent, testosterone-related adverse events. Concerns about long-term risks have restrained enthusiasm for testosterone use as anabolic therapy. Selective androgen-receptor modulators that are preferentially anabolic and that spare the prostate hold promise as anabolic therapies. We need more studies to determine whether testosterone or selective androgen-receptor modulators can induce meaningful improvements in physical function and patient-important outcomes in patients with physical dysfunction associated with chronic illness or aging.

Probing the Functional Link between Androgen Receptor Coactivator and Ligand-binding Sites in Prostate Cancer and Androgen Insensitivity

The androgen receptor (AR) is a ligand-activated transcription factor required for male sex development and virilization and contributes to prostate cancer initiation and progression. High affinity androgen binding triggers conformational changes required for AR transactivation. Here we characterized naturally occurring AR gene mutations in the region of activation function 2 (AF2) that decrease or increase AR transcriptional activity by altering the region bounded by AF2 and the ligand binding pocket without affecting equilibrium androgen binding affinity. In the androgen insensitivity syndrome, germ line AR mutations increase the androgen dissociation rate and reduce AR FXXLF motif binding and the recruitment of steroid receptor coactivator (SRC)/p160 coactivator LXXLL motifs. In prostate cancer, somatic AR mutations in AF2 or near the bound ligand slow androgen dissociation and increase AR stabilization and coactivator recruitment. Crystal structures of the AR ligand binding domain bound to R1881 and FXXLF or LXXLL motif peptide indicate the mutations are proximal to the AF2 bound peptide, adjacent to the ligand pocket, or in a putative ligand gateway. The results suggest a bidirectional structural relay between bound ligand and coactivator that establishes AR functional potency in vivo.

Decreased Androgen Receptor Levels and Receptor Function in Breast Cancer Contribute to the Failure of Response to Medroxyprogesterone Acetate

Previously, we reported that androgen receptor (AR), but not estrogen receptor (ER) or progesterone receptor (PR), is predictive of response to the synthetic progestin, medroxyprogesterone acetate (MPA), in a cohort of 83 patients with metastatic breast cancer. To further investigate the role of AR in determining response to MPA in this cohort, we analyzed AR levels by immunohistochemistry with two discrete antisera directed at either the NH2 or the COOH termini of the receptor. Compared with tumors that responded to MPA (n = 31), there was a significant decrease in the intensity and extent of AR immunoreactivity with both AR antisera in tumors from nonresponders (n = 52). Whereas only a single AR immunostaining pattern was detected in responders to MPA, reflecting concordance of immunoreactivity with the two AR antisera, tumors from nonresponders exhibited four distinct AR immunostaining patterns: (a) concordance with the two antibodies (31%), (b) staining only with the COOH-terminal antibody (33%), (c) staining only with the NH2-terminal antibody (22%), or (d) no immunoreactivity with either NH2- or COOH-terminal antibody (14%). DNA sequencing and functional analysis identified inactivating missense gene mutations in the ligand-binding domain of the AR in tumors from two of nine nonresponders positive with the NH2-terminal AR antisera but negative for COOH-terminal immunoreactivity and lacking specific, high-affinity dihydrotestosterone binding in tumor cytosol fractions. Tumors with more AR than the median level (37 fmol/mg protein) had significantly lower levels of PR (30 fmol/mg protein) than tumors with low AR (PR; 127 fmol/mg protein) despite comparable levels of ER. Ligand-dependent activation of the AR in human T47D and MCF-7 breast cancer cells resulted in inhibition of estradiol-stimulated cell proliferation and a reduction in the capacity of the ER to induce expression of the PR. These effects could be reversed using a specific AR antisense oligonucleotide. Increasing the ratio of AR to ER resulted in a greater androgen-dependent inhibition of ER function. Collectively, these data suggest that reduced levels of AR or impaired AR function contribute to the failure of MPA therapy potentially due to abrogation of the inhibitory effect of AR on ER signaling.

The structural basis of androgen receptor activation: intramolecular and intermolecular amino-carboxy interactions

Nuclear receptors (NRs) are ligand-regulated transcription factors important in human physiology and disease. In certain NRs, including the androgen receptor (AR), ligand binding to the carboxy-terminal domain (LBD) regulates transcriptional activation functions in the LBD and amino-terminal domain (NTD). The basis for NTD-LBD communication is unknown but may involve NTD-LBD interactions either within a single receptor or between different members of an AR dimer. Here, measurement of FRET between fluorophores attached to the NTD and LBD of the AR established that agonist binding initiated an intramolecular NTD-LBD interaction in the nucleus and cytoplasm. This intramolecular folding was followed by AR self-association, which occurred preferentially in the nucleus. Rapid, ligand-induced intramolecular folding and delayed association also were observed for estrogen receptor-alpha but not for peroxisome proliferator activated receptor-gamma2. An antagonist ligand, hydroxyflutamide, blocked the NTD-LBD association within AR. NTD-LBD association also closely correlated with the transcriptional activation by heterologous ligands of AR mutants isolated from hormone-refractory prostate tumors. Intramolecular folding, but not AR-AR affinity, was disrupted by mutation of an alpha-helical ((23)FQNLF(27)) motif in the AR NTD previously described to interact with the AR LBD in vitro. This work establishes an intramolecular NTD-LBD conformational change as an initial component of ligand-regulated NR function.

Melanoma antigen gene protein MAGE-11 regulates androgen receptor function by modulating the interdomain interaction

Gene activation by steroid hormone receptors involves the recruitment of the steroid receptor coactivator (SRC)/p160 coactivator LXXLL motifs to activation function 2 (AF2) in the ligand binding domain. For the androgen receptor (AR), AF2 also serves as the interaction site for the AR NH(2)-terminal FXXLF motif in the androgen-dependent NH(2)-terminal and carboxyl-terminal (N/C) interaction. The relative importance of the AR AF2 site has been unclear, since the AR FXXLF motif interferes with coactivator recruitment by competitive inhibition of LXXLL motif binding. In this report, we identified the X chromosome-linked melanoma antigen gene product MAGE-11 as an AR coregulator that specifically binds the AR NH(2)-terminal FXXLF motif. Binding of MAGE-11 to the AR FXXLF alpha-helical region stabilizes the ligand-free AR and, in the presence of an agonist, increases exposure of AF2 to the recruitment and activation by the SRC/p160 coactivators. Intracellular association between AR and MAGE-11 is supported by their coimmunoprecipitation and colocalization in the absence and presence of hormone and by competitive inhibition of the N/C interaction. AR transactivation increases in response to MAGE-11 and the SRC/p160 coactivators through mechanisms that include but are not limited to the AF2 site. MAGE-11 is expressed in androgen-dependent tissues and in prostate cancer cell lines. The results suggest MAGE-11 is a unique AR coregulator that increases AR activity by modulating the AR interdomain interaction.

Androgen receptor (AR) NH2- and COOH-terminal interactions result in the differential influences on the AR-mediated transactivation and cell growth

Early reports showed that androgen receptor (AR) NH2- and COOH-terminal (N-C) interaction was important for full AR function. However, the influence of these interactions on the AR in vivo effects remains unclear. Here we tested some AR-associated peptides and coregulators to determine their influences on AR N-C interaction, AR transactivation, and AR coregulator function. The results showed that AR coactivators such as ARA70N, gelsolin, ARA54, and SRC-1 can enhance AR transactivation but showed differential influences on the N-C interaction. In contrast, AR corepressors ARA67 and Rad9 can suppress AR transactivation, with ARA67 enhancing and Rad9 suppressing AR N-C interaction. Furthermore, liganded AR C terminus-associated peptides can block AR N-C interaction, but only selective peptides can block AR transactivation and coregulator function. We found all the tested peptides can suppress prostate cancer LNCaP cell growth at different levels in the presence of 5alpha-dihydrotestosterone, but only the tested FXXLF-containing peptides, not FXXMF-containing peptides, can suppress prostate cancer CWR22R cell growth. Together, these results suggest that the effects of AR N-C interactions may not always correlate with similar effects on AR-mediated transactivation and/or AR-mediated cell growth. Therefore, drugs designed by targeting AR N-C interaction as a therapeutic intervention for prostate cancer treatment may face unpredictable in vivo effects.

Effects of medroxyprogesterone and estradiol on the recovery of spermatogenesis in irradiated rats

Suppression of intratesticular testosterone (ITT) levels is required for spermatogenic recovery in rats after irradiation, but maintenance of peripheral testosterone (T) levels is important for many male functions. Considering the preservation of peripheral T while suppressing ITT, we tested the effects of a combination of a progestin, medroxyprogesterone acetate (MPA), plus T on spermatogenic recovery after irradiation, and compared its effects to those of T alone or T combined with estradiol (E2). Rats were given testicular irradiation (6 Gy) and treated during wk 3-7 after irradiation with MPA + T, or the individual steroids with or without GnRH antagonist (GnRH-ant), or GnRH-ant alone, or T + E2. Whereas GnRH-ant alone stimulated differentiation in 55% of tubules 13 wk after irradiation compared with 0% in irradiated-only rats, the addition of MPA reduced the percentage of tubules showing differentiation to 18%. However, T or MPA alone or the combination of the two induced germ cell differentiation in only 2-4% of tubules. In contrast, E2 stimulated differentiation in 88% of tubules, and T combined with E2 still resulted in differentiation in 30% of tubules. Although both MPA and E2 suppressed ITT levels to approximately 2% of control (2 ng/g testis), MPA was a less effective stimulator of spermatogenic recovery than E2 or GnRH-ant alone. MPA's function as a weak androgen was likely responsible for inhibiting spermatogenic recovery, as was the case for all other tested androgens. Thus, for clinical protection or restoration of spermatogenesis after radiation or chemotherapy by suppressing T production, MPA, at least in the doses used in the present study, is suboptimal. The combination of an estrogen with T appears to be most effective for stimulating such recovery.

Biochemical mechanisms of drug action: what does it take for success?

Drug discovery is extremely difficult. There are many unanticipated scientific, medical and business challenges to every drug discovery programme. It is important to increase our understanding of the fundamental properties of effective drugs so that we can anticipate potential problems in developing new agents. This article addresses potential drug discovery and development risks associated with the biochemical mechanism of drug action, and proposes simple rules to minimize these risks.

Recruitment of beta-catenin by wild-type or mutant androgen receptors correlates with ligand-stimulated growth of prostate cancer cells

Prostate cancers respond to treatments that suppress androgen receptor (AR) function, with bicalutamide, flutamide, and cyproterone acetate (CPA) being AR antagonists in clinical use. As CPA has substantial agonist activity, it was examined to identify AR coactivator/corepressor interactions that may mediate androgen-stimulated prostate cancer growth. The CPA-liganded AR was coactivated by steroid receptor coactivator-1 (SRC-1) but did not mediate N-C terminal interactions or recruit beta-catenin, indicating a nonagonist conformation. Nonetheless, CPA did not enhance AR interaction with nuclear receptor corepressor, whereas the AR antagonist RU486 (mifepristone) strongly stimulated AR-nuclear receptor corepressor binding. The role of coactivators was further assessed with a T877A AR mutation, found in LNCaP prostate cancer cells, which converts hydroxyflutamide (HF, the active flutamide metabolite) into an agonist that stimulates LNCaP cell growth. The HF and CPA-liganded T877A ARs were coactivated by SRC-1, but only the HF-liganded T877A AR was coactivated by beta-catenin. L-39, a novel AR antagonist that transcriptionally activates the T877A AR, but still inhibits LNCaP growth, similarly mediated recruitment of SRC-1 and not beta-catenin. In contrast, beta-catenin coactivated a bicalutamide-responsive mutant AR (W741C) isolated from a bicalutamide-stimulated LNCaP subline, further implicating beta-catenin recruitment in AR-stimulated growth. Androgen-stimulated prostate-specific antigen gene expression in LNCaP cells could be modulated by beta-catenin, and endogenous c-myc expression was repressed by dihydrotestosterone, but not CPA. These results indicate that interactions between AR and beta-catenin contribute to prostate cell growth in vivo, although specific growth promoting genes positively regulated by AR recruitment of beta-catenin remain to be identified.

The Ability of a Gonadotropin-Releasing Hormone Antagonist, Acyline, to Prevent Irreversible Infertility Induced by the Indenopyridine, CDB-4022, in Adult Male Rats: The Role of Testosterone1

Intratesticular testosterone (ITT) is known to play a critical role in the maintenance of spermatogenesis. We have used acyline, a GnRH antagonist, to suppress testosterone (T) production, and acyline and T implants to study the prevention of irreversible infertility induced by CDB-4022. Vehicle or acyline was administered to proven fertile male rats (n = 5/group) at a dose (210 microg/day) that completely suppressed (P < 0.05) T production, as measured by serum T, and testicular function, either before, concurrent with, or after vehicle or a single oral dose of 2.5 mg CDB-4022/kg (Week 0). Vehicle-treated males remained fertile, whereas acyline-treated males exhibited transitory infertility. CDB-4022 alone caused irreversible infertility in all males. Importantly, CDB-4022-treated males recovered fertility when acyline was started before CDB-4022 (Weeks -4 to 0; Weeks -4-9), but not when acyline was administered concurrently with or after CDB-4022 (Weeks 0-9; Weeks 10-19). At the end of this study (Week 34), testes weights, spermatid head counts (SHC), and tubule differentiation indices (TDI) were suppressed (P < 0.05) in infertile CDB-4022-treated males, but in rats that recovered fertility, these parameters were similar (P > 0.05) to those in vehicle-treated males. In addition, serum inhibin B and epididymal androgen-binding protein levels were nondetectable in infertile CDB-4022-treated rats. To test whether suppression of ITT was critical for prevention of CDB-4022-induced infertility, proven fertile rats (n = 7-8/group) received vehicle, acyline alone, or acyline and a T implant for 4 wk before CDB-4022 (Week 0). The T implant increased ITT in acyline-treated rats. Although ITT was lower (P < 0.05) in the T-implanted males than in untreated rats, it was sufficient to sustain spermiogenesis. Serum rFSH levels were also elevated in rats treated with acyline + T as compared with acyline alone during the treatment interval, but rFSH was still lower than in vehicle-treated rats. Rats in all treatment groups were rendered infertile initially, but the acyline + CDB-4022-treated rats recovered fertility by Week 10. In contrast, rats treated with CDB-4022 alone or acyline + T + CDB-4022 remained infertile until at least Week 16. Testes weights, SHC, and TDI were within normal ranges for acyline + CDB-4022-treated rats, but were decreased (P < 0.05) in CDB-4022- or acyline + T + CDB-4022-treated rats. Serum inhibin B levels were nondetectable by Week 1 in males rendered irreversibly infertile by CDB-4022; levels increased transiently and returned to baseline in rats protected by acyline pretreatment. These data indicate that pretreatment with acyline was able to prevent irreversible infertility in CDB-4022-treated rats, whereas posttreatment with acyline did not promote spermatogonial differentiation, as has been observed by others in rats that received GnRH analogs and various other testicular toxicants. Suppression of ITT and possibly rFSH by acyline appeared to be crucial in preventing irreversible infertility induced by CDB-4022. In this regard, our results are similar to those of investigators who have studied other testicular toxicants. Continued development of CDB-4022 as a potential male contraceptive will depend largely on its safety profile and whether its antispermatogenic activity is reversible in primates.

Medroxyprogesterone acetate antagonizes the effects of estrogen treatment on social and sexual behavior in female macaques

Medroxyprogesterone acetate (MPA) commonly is used in contraception and hormone replacement therapy. However, little is known about its effects within the central nervous system. Using ovariectomized pigtail macaques (Macaca nemestrina), we evaluated the potential for MPA to antagonize estradiol (E2) effects on female sociosexual behavior. Subjects (n = 6) were treated sequentially with placebo, E2 alone, E2 + progesterone (P4), and E2 + MPA. The order of treatments was balanced among subjects, and equimolar quantities of P4 and MPA were administered. During each treatment period, female sexual initiation rates, anxiety-related behavior, and aggression were recorded. Treatment with E2 alone induced a substantial rise in female sexual initiation rates. Although concurrent P4 treatment failed to significantly inhibit sexual behavior, MPA treatment markedly antagonized E2's effects. Neither the E2-only nor the E2 + P4 treatment had an impact on aggression rates, but the E2 + MPA treatment induced a significant rise in this behavior. Both MPA and P4 counteracted the effect of E2 on measures of anxiety. These findings suggest that MPA antagonizes certain behavioral effects of E2 that may be beneficial to women, and that it does so more profoundly or in ways that endogenous P4 does not. The marked increase in aggression seen during MPA treatment suggests that production of negative affect may be a particularly serious side effect of MPA.

Mechanistic relationship between androgen receptor polyglutamine tract truncation and androgen-dependent transcriptional hyperactivity in prostate cancer cells

Androgen receptor (AR) signaling pathways mediate critical events in normal and neoplastic prostate growth. Shortening of the polymorphic N-terminal polyglutamine (poly(Q)) tract of the AR gene leads to transcriptional hyperactivity and has been correlated with an increased risk of prostate cancer. The underlying mechanisms for these effects are poorly understood. We show here that androgen-dependent cellular proliferation and transcription in prostate cancer cells is inversely correlated to the length of the AR poly(Q) region. We further show that AR proteins containing a shortened poly(Q) region functionally respond to lower concentrations of androgens than wild type AR. Whereas DNA binding activity is relatively unaffected by AR poly(Q) variation, we found that ligand binding affinity and the ligand-induced NH(2)- to COOH-terminal intramolecular interaction is enhanced when the poly(Q) region is shortened. Importantly, we show that AR proteins containing a shortened poly(Q) region associate in vivo with higher levels of specific p160 coactivators and components of the SWI/SNF chromatin remodeling complex as compared with the wild type AR. Collectively, our findings suggest that the AR transcriptional hyperactivity associated with shortened poly(Q) length stems from altered ligand-induced conformational changes that enhance coactivator recruitment.

Repressors of androgen and progesterone receptor action

Androgen and progesterone receptors (AR and PR) are two determining factors in gonadal differentiation that are highly expressed in developing and mature gonads. Loss of AR results in XY sex reversal and mutations causing reduced AR activity lead to varying degrees of defects in masculinization. Female PR knockout mice are infertile due to ovarian defects. While much has been discovered about positive regulation of these receptors by coactivators little is known about repression of the transcriptional activity of AR and PR in the presence of agonists. In this study we assessed the effect of SMRT and DAX-1 on AR and PR activity in the presence of both agonists and partial antagonists. We show that SMRT and DAX-1 repress agonist-dependent activity of both receptors, and the mechanism of repression includes disruption of the receptor dimer interactions rather than recruitment of histone deacetylases. We demonstrate that endogenous agonist-bound PR and DAX-1 in T47D breast cancer cells and endogenous AR and DAX-1 in LNCaP prostate cancer cells can be coimmunoprecipitated suggesting that the interaction is physiological. Surprisingly, although DAX-1 represses partial antagonist activity of AR, it was ineffective in repressing partial antagonist induced activity of PR. In contrast to most reported repressors, the expression of DAX-1 is restricted. We found that although DAX-1 is expressed in normal human prostate, its expression is strongly reduced in benign prostatic hyperplasia suggesting that DAX-1 plays a role in limiting AR activity in prostate.

A direct beta-catenin-independent interaction between androgen receptor and T cell factor 4

T cell factor (Tcf) proteins bind beta-catenin and are downstream effectors of Wnt/beta-catenin signals. A recently demonstrated interaction between beta-catenin and the androgen receptor (AR) ligand binding domain has suggested that AR may be a Tcf-independent Wnt/beta-catenin effector. This study demonstrates that there is a direct interaction between the AR DNA binding domain (DBD) and Tcf4. Tcf4 bound specifically to a glutathione S-transferase-ARDBD fusion protein and could be coimmunoprecipitated with beta-catenin and transfected AR or endogenous AR in prostate cancer cells. Transfected Tcf4 repressed the transcriptional activity of full-length AR and a VP16-ARDBD fusion protein, and this repression was only partially reversed by transfected beta-catenin. AR activation by cyproterone acetate, a partial agonist that did not support beta-catenin binding to the AR, was also repressed by Tcf4, further indicating that repression was not due to beta-catenin sequestration. Tcf4 could recruit beta-catenin to the AR DBD in vitro and to the cyproterone acetate-liganded AR in vivo. Chromatin immunoprecipitation experiments in LNCaP prostate cancer cells showed that endogenous AR was bound to a Tcf4-responsive element in the c-myc promoter. These findings indicate that AR and Tcf4 can interact directly and that this interaction may occur on the promoters or enhancers of particular genes. The direct AR-Tcf4 interaction, in conjunction AR- and Tcf4-beta-catenin binding, provides a mechanism for cooperative and selective gene regulation by AR and the Wnt/beta-catenin-Tcf pathway that may contribute to normal and neoplastic prostate growth.

Electrostatic modulation in steroid receptor recruitment of LXXLL and FXXLF motifs

Coactivator recruitment by activation function 2 (AF2) in the steroid receptor ligand binding domain takes place through binding of an LXXLL amphipathic alpha-helical motif at the AF2 hydrophobic surface. The androgen receptor (AR) and certain AR coregulators are distinguished by an FXXLF motif that interacts selectively with the AR AF2 site. Here we show that LXXLL and FXXLF motif interactions with steroid receptors are modulated by oppositely charged residues flanking the motifs and charge clusters bordering AF2 in the ligand binding domain. An increased number of charged residues flanking AF2 in the ligand binding domain complement the two previously characterized charge clamp residues in coactivator recruitment. The data suggest a model whereby coactivator recruitment to the receptor AF2 surface is initiated by complementary charge interactions that reflect a reversal of the acidic activation domain-coactivator interaction model.