Ligand-induced conformational alterations of the androgen receptor analyzed by limited trypsinization. Studies on the mechanism of antiandrogen action

Regulating Androgen Receptor Function in Prostate Cancer: Exploring the Diversity of Post-Translational Modifications

Androgen receptor (AR) transcriptional activity significantly influences prostate cancer (PCa) progression. In addition to ligand stimulation, AR transcriptional activity is also influenced by a variety of post-translational modifications (PTMs). A number of oncogenes and tumor suppressors have been observed leveraging PTMs to influence AR activity. Subjectively targeting these post-translational modifiers based on their impact on PCa cell proliferation is a rapidly developing area of research. This review elucidates the modifiers, contextualizes the effects of these PTMs on AR activity, and connects these cellular interactions to the progression of PCa.

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.

Predicting the Activation of the Androgen Receptor by Mixtures of Ligands Using Generalized Concentration Addition

Concentration/dose addition is widely used for compounds that act by similar mechanisms. But it cannot make predictions for mixtures of full and partial agonists for effect levels above that of the least efficacious component. As partial agonists are common, we developed generalized concentration addition, which has been successfully applied to systems in which ligands compete for a single binding site. Here, we applied a pharmacodynamic model for a homodimer receptor system with 2 binding sites, the androgen receptor, that acts according to the classic homodimer activation model: Each cytoplasmic monomer protein binds ligand, undergoes a conformational change that relieves inhibition of dimerization, and binds to DNA response elements as a dimer. We generated individual dose-response data for full (dihydroxytestosterone, BMS564929) and partial (TFM-4AS-1) agonists and a competitive antagonist (MDV3100) using reporter data generated in the MDA-kb2 cell line. We used the Schild method to estimate the binding affinity of MDV3100. Data for individual compounds fit the homodimer pharmacodynamic model well. In the presence of a full agonist, the partial agonist had agonistic effects at low effect levels and antagonistic effects at high levels, as predicted by pharmacological theory. The generalized concentration addition model fits the empirical mixtures data-full/full agonist, full/partial agonist, and full agonist/antagonist-as well or better than relative potency factors or effect summation. The ability of generalized concentration addition to predict the activity of mixtures of different types of androgen receptor ligands is important as a number of environmental compounds act as partial androgen receptor agonists or antagonists.

Identification of Competing Endogenous RNA and Micro-RNA Profiles and Regulatory Networks in 4-Nonylphenol-induced Impairment of Sertoli Cells

The xenoestrogens nonylphenols (NPs), which are materials used in the plastic polymer industry, are considered endocrine disruptors in a wide range of organisms. Studies have shown that human health problems, such as infertility and reproductive toxicology, are linked with NPs. However, the mechanism by which NPs interfere with male reproduction is not fully elucidated. Here, we found that 4-NP can result in male reproductive impairment and reduce androgen receptor (AR) protein levels in rat sertoli cells in vitro and in vivo. Moreover, we performed RNA sequencing to assess the differential expression of ceRNAs in rat primary sertoli cells treated with 4-NP. Bioinformatics methods, such as Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) database and ceRNA functional network analyses, were used to investigate the sequencing data and gain further understanding of the biological processes. Our analysis revealed a core set of mRNAs (Ar, Atf6 and Cbp), and circRNAs (circ673, circ1377, circ1789, and circPTEN) that were selected and validated by RT-qPCR. In addition, the head-to-tail splicing of circ673, circ1377, circ1789, and circPTEN was identified by Sanger sequencing. These findings provide the first insight into the ceRNA expression profiles of rat sertoli cells and reveal that ceRNAs participate in 4-NP-induced impairment of sertoli cell function, thereby indicating potential therapies for both reproductive toxicology and male infertility.

Posttranslational regulation of androgen dependent and independent androgen receptor activities in prostate cancer

Prostate cancer (PCa) is the most commonly diagnosed cancer among men in western countries. Androgen receptor (AR) signaling plays key roles in the development of PCa. Androgen deprivation therapy (ADT) remains the standard therapy for advanced PCa. In addition to its ligand androgen, accumulating evidence indicates that posttranscriptional modification is another important mechanism to regulate AR activities during the progression of PCa, especially in castration resistant prostate cancer (CRPC). To date, a number of posttranscriptional modifications of AR have been identified, including phosphorylation (e.g. by CDK1), acetylation (e.g. by p300 and recognized by BRD4), methylation (e.g. by EZH2), ubiquitination (e.g. by SPOP), and SUMOylation (e.g. by PIAS1). These modifications are essential for the maintenance of protein stability, nuclear localization and transcriptional activity of AR. This review summarizes posttranslational modifications that influence androgen-dependent and -independent activities of AR, PCa progression and therapy resistance. We further emphasize that in addition to androgen, posttranslational modification is another important way to regulate AR activity, suggesting that targeting AR posttranslational modifications, such as proteolysis targeting chimeras (PROTACs) of AR, represents a potential and promising alternate for effective treatment of CRPC. Potential areas to be investigated in the future in the field of AR posttranslational modifications are also discussed.

Modulation of nuclear receptor activity by the F domain

The F domain located at the C-terminus of proteins is one of the least conserved regions of the estrogen receptors alpha and beta, members of the nuclear hormone receptor superfamily. Indeed, many members of the superfamily lack the F domain. However, when present, removing the F domain entirely or mutating it alters transactivation, dimerization, and the responses to agonist and antagonist ligands. This review focuses on the functions of the F domain of the estrogen receptors, particularly in relation to other members of the superfamily.

Co-targeting AR and HSP90 suppresses prostate cancer cell growth and prevents resistance mechanisms

Persistent androgen receptor (AR) signaling in castration resistant prostate cancer (CRPC) underpins the urgent need for therapeutic strategies that better target this pathway. Combining classes of agents that target different components of AR signaling has the potential to delay resistance and improve patient outcomes. Many oncoproteins, including the AR, rely on the molecular chaperone heat shock protein 90 (Hsp90) for functional maturation and stability. In this study, enhanced anti-proliferative activity of the Hsp90 inhibitors 17-allylamino-demethoxygeldanamycin (17-AAG) and AUY922 in androgen-sensitive and CRPC cells was achieved when the agents were used in combination with AR antagonists bicalutamide or enzalutamide. Moreover, significant caspase-dependent cell death was achieved using sub-optimal agent doses that individually have no effect. Expression profiling demonstrated regulation of a broadened set of AR target genes with combined 17-AAG and bicalutamide compared with the respective single agent treatments. This enhanced inhibition of AR signaling was accompanied by impaired chromatin binding and nuclear localization of the AR. Importantly, expression of the AR variant AR-V7 that is implicated in resistance to AR antagonists was not induced by combination treatment. Likewise, the heat shock response that is typically elicited with therapeutic doses of Hsp90 inhibitors, and is a potential mediator of resistance to these agents, was significantly reduced by combination treatment. In summary, the co-targeting strategy in this study more effectively inhibits AR signaling than targeting AR or HSP90 alone and prevents induction of key resistance mechanisms in prostate cancer cells. These findings merit further evaluation of this therapeutic strategy to prevent CRPC growth.

Posttranslational modification of the androgen receptor in prostate cancer

The androgen receptor (AR) is important in the development of the prostate by regulating transcription, cellular proliferation, and apoptosis. AR undergoes posttranslational modifications that alter its transcription activity, translocation to the nucleus and stability. The posttranslational modifications that regulate these events are of utmost importance to understand the functional role of AR and its activity. The majority of these modifications occur in the activation function-1 (AF1) region of the AR, which contains the transcriptional activation unit 1 (TAU1) and 5 (TAU5). Identification of the modifications that occur to these regions may increase our understanding of AR activation in prostate cancer and the role of AR in the progression from androgen-dependent to castration-resistant prostate cancer (CRPC). Most of the posttranslational modifications identified to date have been determined using the full-length AR in androgen dependent cells. Further investigations into the role of posttranslational modifications in androgen-independent activation of full-length AR and constitutively active splicing variants are warranted, findings from which may provide new therapeutic options for CRPC.

Increased PrLZ-mediated androgen receptor transactivation promotes prostate cancer growth at castration-resistant stage

Most advanced prostate cancers (PCa) will develop into the castration-resistant stage following androgen deprivation therapy, yet the molecular mechanisms remain unclear. In this study, we found PrLZ, a newly identified Prostate Leucine Zipper gene that is highly expressed in PCa could interact with the androgen receptor (AR) directly leading to enhance AR transactivation in the castration-resistant condition. PrLZ might enhance AR transactivation via a change of AR conformation that leads to promotion of AR nuclear translocation and suppression of AR degradation via modulating the proteasome pathway, which resulted in increased prostate-specific antigen expression and promoted PCa growth at the castration-resistant stage. Clinical PCa sample survey from same-patient paired specimens found increased PrLZ expression in castration-resistant PCa following the classical androgen deprivation therapy. Targeting the AR-PrLZ complex via ASC-J9® or PrLZ-siRNA resulted in suppression of PCa growth in various human PCa cells and in vivo mouse PCa models. Together, these data not only strengthen PrLZ roles in the transition from androgen dependence to androgen independence during the castration-resistant stage, but they may also provide a new potential therapy to battle PCa at the castration-resistant stage.

FoxA1 specifies unique androgen and glucocorticoid receptor binding events in prostate cancer cells

The forkhead protein FoxA1 has functions other than a pioneer factor, in that its depletion brings about a significant redistribution in the androgen receptor (AR) and glucocorticoid receptor (GR) cistromes. In this study, we found a novel function for FoxA1 in defining the cell-type specificity of AR- and GR-binding events in a distinct fashion, namely, for AR in LNCaP-1F5 cells and for GR in VCaP cells. We also found different, cell-type and receptor-specific compilations of cis-elements enriched adjacent to the AR- and GR-binding sites. The AR pathway is central in prostate cancer biology, but the role of GR is poorly known. We find that AR and GR cistromes and transcription programs exhibit significant overlap, and GR regulates a large number of genes considered to be AR pathway-specific. This raises questions about the role of GR in maintaining the AR pathway under androgen-deprived conditions in castration-resistant prostate cancer patients. However, in the presence of androgen, ligand-occupied GR acts as a partial antiandrogen and attenuates the AR-dependent transcription program. .

Enhancing the effectiveness of androgen deprivation in prostate cancer by inducing Filamin A nuclear localization

As prostate cancer (CaP) is regulated by androgen receptor (AR) activity, metastatic CaP is treated with androgen deprivation therapy (ADT). Despite initial response, patients on ADT eventually progress to castration-resistant CaP (CRPC), which is currently incurable. We previously showed that cleavage of the 280 kDa structural protein Filamin A (FlnA) to a 90 kDa fragment, and nuclear localization of the cleaved product, sensitized CRPC cells to ADT. Hence, treatment promoting FlnA nuclear localization would enhance androgen responsiveness. Here, we show that FlnA nuclear localization induced apoptosis in CRPC cells during ADT, identifying it as a treatment tool in advanced CaP. Significantly, the natural product genistein combined polysaccharide (GCP) had a similar effect. Investigation of the mechanism of GCP-induced apoptosis showed that GCP induced FlnA cleavage and nuclear localization and that apoptosis resulting from GCP treatment was mediated by FlnA nuclear localization. Two main components of GCP are genistein and daidzein: the ability of GCP to induce G2 arrest was due to genistein whereas sensitivity to ADT stemmed from daidzein; hence, both were needed to mediate GCP's effects. FlnA cleavage is regulated by its phosphorylation; we show that ADT enhanced FlnA phosphorylation, which prevented its cleavage, whereas GCP inhibited FlnA phosphorylation, thereby sensitizing CaP cells to ADT. In a mouse model of CaP recurrence, GCP, but not vehicle, impeded relapse following castration, indicating that GCP, when administered with ADT, interrupted the development of CRPC. These results demonstrate the efficacy of GCP in promoting FlnA nuclear localization and enhancing androgen responsiveness in CaP.

Numerical framework to model temporally resolved multi-stage dynamic systems

Numerical modeling of steroid hormone signaling presents an exciting challenge involving spatiotemporal coordination of multiple events. Ligand binding in cytoplasm triggers dissociation and/or association of coregulators which subsequently regulate DNA binding and transcriptional activity in nucleus. In order to develop a comprehensive multi-stage model, it is imperative to follow not only the transcriptional outcomes but also the intermediate protein complexes. Accordingly, we developed a software toolkit for simulating complex biochemical pathways as a set of non-linear differential equations in LabVIEW (Laboratory Virtual Instrumentation and Engineering Workbench, National Instruments, Austin, TX) environment. The toolkit is visual, highly modular, loosely coupled with the rest of LabVIEW, scalable and extensible. The toolkit can be used to develop and validate biochemical models and estimate model parameters from existing experimental data. We illustrate the application of the toolkit for simulation of steroid hormone response in cells, and demonstrate how the toolkit can be employed for other biological and chemical systems as well. The software module presented here can be used stand-alone as well as built into data collection and analysis applications.

Dynamics of coregulator-induced conformational perturbations in androgen receptor ligand binding domain

Androgen receptor (AR) coregulators modulate ligand-induced gene expression in a tissue specific manner. The molecular events that follow coactivator binding to AR and the mechanisms that govern the sequence-specific effects of AR coregulators are poorly understood. Using consensus coactivator sequence D11-FxxLF and biophysical techniques, we show that coactivator association is followed by conformational rearrangement in AR ligand binding domain (AR-LBD) that is enthalpically and entropically favorable with activation energy of 29.8±4.2 kJ/mol. Further characterization of ARA70 and SRC3-1 based consensus sequences reveal that each coactivator induces a distinct conformational state in the dihydrotestosterone:AR-LBD:coactivator complex. Complementary computational modeling revealed that coactivator induced specific alterations in the backbone flexibility of AR-LBD distant from the site of coactivator binding and that the intramolecular rearrangements in AR-LBD backbone induced by the two coactivator peptides were different. These data suggest that coactivators may impart specificity in the transcriptional machinery by changing the steady-state conformation of AR-LBD. These data provide direct evidence that even in the presence of same ligand, AR-LBD can occupy distinct conformational states depending on its interactions with specific coactivators in the tissues. We posit that this coactivator-specific conformational gating may then dictate subsequent binding partners and interaction/affinity for the DNA-response elements.

Molecular mechanisms of androgen action--a historical perspective

Androgens and the androgen receptor (AR) are indispensable for expression of the male phenotype. The two most important androgens are testosterone and 5α-dihydrotestosterone. The elucidation of the mechanism of androgen action has a long history starting in the 19th century with the classical experiments by Brown-Séquard. In the 1960s the steroid hormone receptor concept was established and the AR was identified as a protein entity with a high affinity and specificity for testosterone and 5α-dihydrotestosterone. In addition, the enzyme 5α-reductase type 2 was discovered and found to catalyze the conversion of testosterone to the more active metabolite 5α-dihydrotestosterone. In the second half of the 1980s, the cDNA cloning of all steroid hormone receptors, including that of the AR, has been another milestone in the whole field of steroid hormone action. Despite two different ligands (testosterone and 5α-dihydrotestosterone), only one AR cDNA has been identified and cloned. The AR (NR3C4) is a ligand-dependent transcription factor and belongs to the family of nuclear hormone receptors which has 48 members in human. The current model for androgen action involves a multistep mechanism. Studies have provided insight into AR association with co-regulators involved in transcription initiation and on intramolecular interactions of the AR protein during activation. Knowledge about androgen action in the normal physiology and in disease states has increased tremendously after cloning of the AR cDNA. Several diseases, such as androgen insensitivity syndrome (AIS), prostate cancer and spinal bulbar muscular atrophy (SBMA), have been shown to be associated with alterations in AR function due to mutations in the AR gene or dysregulation of androgen signalling. A historical overview of androgen action and salient features of AR function in normal and disease states are provided herein.

Efficacy of maximal androgen blockade versus castration alone in the treatment of advanced prostate cancer: a retrospective clinical experience from a Chinese medical centre

In this retrospective study, we evaluated and compared the efficacy and toxicities of maximal androgen blockade (MAB) versus castration alone in Chinese patients with advanced prostate cancer. From 1996 to 2004, 608 patients with advanced prostate cancer were included in the study. Patients were retrospectively divided into two groups according to different therapeutic regimens. Of the 608 patients, 300 patients were treated with MAB (castration plus nonsteroidal antiandrogens) and the remaining 308 were treated with castration alone. The 2- and 5-year overall survival rates of these patients were 73.7% and 56%, respectively. Multivariate analysis showed that, in patients with metastatic prostate cancer, MAB was associated with not only the improvement of progression-free survival (PFS) (increased by 10 months) but also a 20.6% reduction in mortality risk compared with castration alone. In contrast, the efficacy of MAB was not superior to castration alone for patients with nonmetastatic prostate cancer. Interestingly, among patients with MAB, those using bicalutamide had a longer PFS than those using flutamide; this was especially so in patients with metastatic prostate cancer. Almost all of the toxicities due to the hormone therapy were mild to moderate and manageable. To conclude, in China, hormone therapies, including MAB and castration alone, have been standard treatments for advanced prostate cancer. For patients with nonmetastatic prostate cancer, castration alone might be adequately practical and efficient. In patients with metastatic prostate cancer, however, MAB has superior efficacy over castration alone. It is clear that MAB should be considered the first-line standard treatment for patients with metastatic prostate cancer.

Structural characteristics of anabolic androgenic steroids contributing to binding to the androgen receptor and to their anabolic and androgenic activities. Applied modifications in the steroidal structure

Anabolic androgenic steroids (AAS) are synthetic derivatives of testosterone introduced for therapeutic purposes providing enhanced anabolic potency with reduced androgenic effects. Androgens mediate their action through their binding to the androgen receptor (AR) which is mainly expressed in androgen target tissues, such as the prostate, skeletal muscle, liver and central nervous system. This paper reviews some of the wide spectrum of testosterone and synthetic AAS structure modifications related to the intended enhancement in anabolic activity. The structural features of steroids necessary for effective binding to the AR and those which contribute to the stipulation of the androgenic and anabolic activities are also presented.

Maximal androgen blockade for advanced prostate cancer

Maximal androgen blockade (MAB) refers to the combination of medical (gonadotrophin-releasing hormone agonist) or surgical castration with an anti-androgen for the treatment of advanced prostate cancer. A substantial body of basic research has improved our understanding of the interactions between the anti-androgens, the androgen receptor, and androgen response elements in the genome. Anti-androgens act by two primary mechanisms: inhibition of ligand (androgen) binding to the androgen receptor, and inhibition of androgen-independent activation of the receptor. The latter mechanism occurs via several pathways, including inhibiting nuclear co-activators, activating co-suppressors, and inhibiting transcription of a variety of androgen-regulated genes. It is more accurate to refer to these compounds as androgen-receptor antagonists, since they inhibit activation whether this is androgen-mediated or not. Within the class of non-steroidal anti-androgens, there is variation in the degree to which ligand-independent activation is inhibited. Over the last 25 years, approximately 30 clinical trials have addressed the benefit of MAB versus monotherapy. Most of these trials have evaluated flutamide or nilutamide. Several meta-analyses suggest a modest survival benefit of these drugs, amounting to an 8% mortality reduction at 5 years. Preclinical data and two randomized trials -- one historic and one current -- suggest that bicalutamide may be a more effective drug in this respect. This requires confirmation pending further maturity of the current trial, which is the only one directly comparing bicalutamide plus castration to castration alone. In prostate cancer patients at high risk for mortality (based on extent of disease or prostate-specific antigen kinetics), combination therapy with bicalutamide should be considered in preference to monotherapy.

Structural basis for the nuclear import of the human androgen receptor

Ligand-dependent nuclear import is crucial for the function of the androgen receptor (AR) in both health and disease. The unliganded AR is retained in the cytoplasm but, on binding 5alpha-dihydrotestosterone, it translocates into the nucleus and alters transcription of its target genes. Nuclear import of AR is mediated by the nuclear import factor importin-alpha, which functions as a receptor that recognises and binds to specific nuclear localisation signal (NLS) motifs on cargo proteins. We show here that the AR binds to importin-alpha directly, albeit more weakly than the NLS of SV40 or nucleoplasmin. We describe the 2.6-angstroms-resolution crystal structure of the importin-alpha-AR-NLS complex, and show that the AR binds to the major NLS-binding site on importin-alpha in a manner different from most other NLSs. Finally, we have shown that pathological mutations within the NLS of AR that are associated with prostate cancer and androgen-insensitivity syndrome reduce the binding affinity to importin-alpha and, subsequently, retard nuclear import; surprisingly, however, the transcriptional activity of these mutants varies widely. Thus, in addition to its function in the nuclear import of AR, the NLS in the hinge region of AR has a separate, quite distinct role on transactivation, which becomes apparent once nuclear import has been achieved.

Coprinus comatus and Ganoderma lucidum interfere with androgen receptor function in LNCaP prostate cancer cells

In this study, we screened a total of 201 diethyl ether, ethanol, and ethyl acetate fungal Basidiomycetes extracts for anti-androgenic activity. Based on our screened results in combination with the selective inhibition of prostate cancer LNCaP cells, we selected Coprinus comatus and Ganoderma lucidum for further evaluation. We demonstrated that ethanol and ethyl acetate extracts from C. comatus and G. lucidum, respectively, selectively inhibit dihydrotestosterone-induced LNCaP cell viability, suppress levels of secreted prostate-specific antigen in a dose-dependent manner, and cause a G1 phase arrest in LNCaP, but not in DU 145 and PC-3 cells. For the first time, to the best of our knowledge, we demonstrated that C. comatus and G. lucidum decreased androgen and glucocorticoide receptors transcriptional activity in breast cancer MDA-kb2 cells in a dose-dependent manner, and suppressed androgen receptor (AR) protein level in LNCaP and MDA-kb2 cells. Our findings suggest that AR and non-AR mediated mechanisms underlie the effects of C. comatus and G. lucidum.

Calmodulin-androgen receptor (AR) interaction: calcium-dependent, calpain-mediated breakdown of AR in LNCaP prostate cancer cells

Chemotherapy of prostate cancer targets androgen receptor (AR) by androgen ablation or antiandrogens, but unfortunately, it is not curative. Our attack on prostate cancer envisions the proteolytic elimination of AR, which requires a fuller understanding of AR turnover. We showed previously that calmodulin (CaM) binds to AR with important consequences for AR stability and function. To examine the involvement of Ca(2+)/CaM in the proteolytic breakdown of AR, we analyzed LNCaP cell extracts that bind to a CaM affinity column for the presence of low molecular weight forms of AR (intact AR size, approximately 114 kDa). Using an antibody directed against the NH(2)-terminal domain (ATD) of AR on Western blots, we identified approximately 76-kDa, approximately 50-kDa, and 34/31-kDa polypeptides in eluates of CaM affinity columns, suggesting the presence of CaM-binding sites within the 31/34-kDa ATD of AR. Under cell-free conditions in the presence of phenylmethylsulfonyl fluoride, AR underwent Ca(2+)-dependent degradation. AR degradation was inhibited by N-acetyl-leu-leu-norleu, an inhibitor of thiol proteases, suggesting the involvement of calpain. In intact cells, AR breakdown was accelerated by raising intracellular Ca(2+) using calcimycin, and increased AR breakdown was reversed with the cell-permeable Ca(2+) chelator bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetra-(acetoxymethyl)-ester. In CaM affinity chromatography studies, the Ca(2+)-dependent protease calpain was bound to and eluted from the CaM-agarose column along with AR. Caspase-3, which plays a role in AR turnover under stress conditions, did not bind to the CaM column and was present in the proenzyme form. Similarly, AR immunoprecipitates prepared from whole-cell extracts of exponentially growing LNCaP cells contained both calpain and calpastatin. Nuclear levels of calpain and calpastatin (its endogenous inhibitor) changed in a reciprocal fashion as synchronized LNCaP cells progressed from G(1) to S phase. These reciprocal changes correlated with changes in AR level, which increased in late G(1) phase and decreased as S phase progressed. Taken together, these observations suggest potential involvement of AR-bound CaM in calcium-controlled, calpain-mediated breakdown of AR in prostate cancer cells.

Combined androgen blockade: an update

The use of combined androgen blockade therapy in prostate cancer management remains controversial. This article reviews the effect of the different non-steroid androgens in blocking androgen-independent activation of the androgen receptor in the androgen-depleted environment, and the potential benefit of bicalutamide in comparison to the first generation of anti-androgens (flutamide and nilutamide). An estimate of the benefit of combined therapy with bicalutamide suggests there is a high probability that bicalutamide 50 mg as combined therapy provides a survival advantage over castration alone. This treatment must be balanced against the potential for an increase in side-effects and a consequent adverse effect on the patient's quality of life.

Aggregation and proteasome: the case of elongated polyglutamine aggregation in spinal and bulbar muscular atrophy

Aggregates, a hallmark of most neurodegenerative diseases, may have different properties, and possibly different roles in neurodegeneration. We analysed ubiquitin-proteasome pathway functions during cytoplasmic aggregation in polyglutamine (polyQ) diseases, using a unique model of motor neuron disease, the SpinoBulbar Muscular Atrophy. The disease, which is linked to a polyQ tract elongation in the androgen receptor (ARpolyQ), has the interesting feature that ARpolyQ aggregation is triggered by the AR ligand, testosterone. Using immortalized motor neurons expressing ARpolyQ, we found that a proteasome reporter, YFPu, accumulated in absence of aggregates; testosterone treatment, which induced ARpolyQ aggregation, allowed the normal clearance of YFPu, suggesting that aggregation contributed to proteasome de-saturation, an effect not related to AR nuclear translocation. Using AR antagonists to modulate the kinetic of ARpolyQ aggregation, we demonstrated that aggregation, by removing the neurotoxic protein from the soluble compartment, protected the proteasome from an excess of misfolded protein to be processed.

Differentially expressed genes in the prostate cancer cell line LNCaP after exposure to androgen and anti-androgen

Androgens play an important role in growth and maintenance of prostate cells. The actions of androgens are mediated by the androgen receptor (AR), a transcription factor member of the super-family of nuclear hormone receptors. Androgen regulated genes (ARGs) are potential markers for early diagnosis and treatment of prostate cancer patients. In the present study, we used DDRT-PCR (differential display reverse transcriptase polymerase chain reaction) technique in order to investigate differentially expressed genes in the prostate cancer cell line LNCaP after treatment with dihydrotestosterone and bicalutamide for 6, 24, and 48 hours. Fifty-five differentially expressed fragments were isolated, cloned, and sequenced. Sequencing analysis of these fragments revealed 56 different transcripts that showed homology to transcription factors, cell cycle regulators, metabolic enzymes, and hypothetical proteins. Among the differentially expressed genes, SPA17 and DDEF2 were further validated using quantitative real time RT-PCR (qPCR) in a series of 25 prostate tumor samples. The DDEF2 gene is involved in adhesion and cell migration of monocytes, and the SPA17 gene might be involved in cellular signal transduction. The transcripts of both, SPA17 and DDEF2 genes, showed altered pattern of expression in the group of prostate tumors analyzed by qPCR. The differentially expressed genes identified in this study might provide new insights into the androgen signaling pathways in prostate cells.

Mutation of histidine 874 in the androgen receptor ligand-binding domain leads to promiscuous ligand activation and altered p160 coactivator interactions

The androgen receptor (AR) signaling pathway is a major therapeutic target in the treatment of prostate cancer. The AR functions as a ligand-activated transcription factor in the presence of the cognate hormone ligands testosterone and dihydrotestosterone (DHT). We have characterized a highly conserved sequence at the C-terminal end of helix 10/11 in the ligand-binding domain (LBD), which is prone to receptor point mutations in prostate cancer. This sequence includes threonine 877 that is involved in hydrogen bonding to the D ring of the steroid molecule and leads to promiscuous ligand activation of the AR when mutated to alanine or serine. A second mutation in this region, H874Y, also results in a receptor protein that has broadened ligand-binding specificity, but retains an affinity for DHT (K(d) = 0.77 nm) similar to that of the wild-type receptor. The structure of the mutant LBD, expressed in Escherichia coli, is not dramatically altered compared with the wild-type AR-LBD in the presence of DHT, but shows a modestly increased sensitivity to protease digestion in the absence of hormone. This mutant AR showed wild-type AR-LBD/N-terminal domain interactions, but significantly enhanced binding and transactivation activity with all three members of the p160 family of coactivator proteins. Together, these phenotypic changes are likely to confer a selective advantage for tumor cells in a low androgen environment resulting from hormone therapy.

Novel biosensors for the detection of estrogen receptor ligands

There exists a significant need for the detection of novel estrogen receptor (ER) ligands for pharmaceutical uses, especially for treating complications associated with menopause. We have developed fluorescence resonance energy transfer (FRET)-based biosensors that permit the direct in vitro detection of ER ligands. These biosensors contain an ER ligand-binding domain (LBD) flanked by the FRET donor fluorophore, cyan fluorescent protein (CFP), and the acceptor fluorophore, yellow fluorescent protein (YFP). The ER-LBD has been modified so that Ala 430 has been changed to Asp, which increases the magnitude of the FRET signal in response to ligand-binding by more than four-fold compared to the wild-type LBD. The binding of agonists can be distinguished from that of antagonists on the basis of the distinct ligand-induced conformations in the ER-LBD. The approach to binding equilibrium occurs within 30min, and the FRET signal is stable over 24h. The biosensor demonstrates a high signal-to-noise, with a Z' value (a statistical determinant of assay quality) of 0.72. The affinity of the ER for different ligands can be determined using a modified version of the biosensor in which a truncated YFP and an enhanced CFP are used. Thus, we have developed platforms for high-throughput screens for the identification of novel estrogen receptor ligands. Moreover, we have demonstrated that this FRET technology can be applied to other nuclear receptors, such as the androgen receptor.

Homologous up-regulation of androgen receptor expression by androgen in vascular smooth muscle cells

OBJECTIVE

Androgens play an important role in the arterial vascular system, and androgen receptors (AR) have been identified in vascular smooth muscle cells (VSMCs). This study examined the effects of testosterone exposure on AR gene expression in cultured rat aortic smooth muscle cells.

METHODS

Changes in AR protein and messenger RNA (mRNA) levels after androgen exposure were determined using immunoblotting and Northern blotting analysis respectively.

RESULTS

Treatment of synchronized VSMCs with testosterone increased both cytoplasmic and nuclear AR protein expression in a dose- and time-dependent fashion, whereas exposure of VSMCs to androgen for 10 min induced a transient down-regulation of AR protein. Meanwhile, AR mRNA level was also up-regulated, but to a much smaller extent. Pretreatment with transcription inhibitor and translation inhibitor repressed cytoplasmic AR protein levels to 46 and 12% (means) of the androgen treatment control level respectively. Furthermore, androgen up-regulation of intracellular AR protein was partially inhibited by androgen antagonist.

CONCLUSIONS

Androgen increases AR expression in VSMCs at the level of both transcription and non-transcription.

Differential modulation of androgen receptor transcriptional activity by the nuclear receptor co-repressor (N-CoR)

Antiandrogens are widely used agents in the treatment of prostate cancer, as inhibitors of AR (androgen receptor) action. Although the precise mechanism of antiandrogen action is not yet elucidated, recent studies indicate the involvement of nuclear receptor co-repressors. In the present study, the regulation of AR transcriptional activity by N-CoR (nuclear receptor co-repressor), in the presence of different ligands, has been investigated. Increasing levels of N-CoR differentially affected the transcriptional activity of AR occupied with either agonistic or antagonistic ligands. Small amounts of co-transfected N-CoR repressed CPA (cyproterone acetate)- and mifepristone (RU486)-mediated AR activity, but did not affect agonist (R1881)-induced AR activity. Larger amounts of co-transfected N-CoR repressed AR activity for all ligands, and converted the partial agonists CPA and RU486 into strong AR antagonists. In the presence of the agonist R1881, co-expression of the p160 co-activator TIF2 (transcriptional intermediary factor 2) relieved N-CoR repression up to control levels. However, in the presence of RU486 and CPA, TIF2 did not functionally compete with N-CoR, suggesting that antagonist-bound AR has a preference for N-CoR. The AR mutation T877A (Thr877-->Ala), which is frequently found in prostate cancer and affects the ligand-induced conformational change of the AR, considerably reduced the repressive action of N-CoR. The agonistic activities of CPA- and hydroxyflutamide-occupied T877A-AR were hardly affected by N-CoR, whereas TIF2 strongly enhanced their activities. These results indicate that lack of N-CoR action allows these antiandrogens to act as strong agonists on the mutant AR.

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.

Regulation of interleukin-6-mediated PI3K activation and neuroendocrine differentiation by androgen signaling in prostate cancer LNCaP cells

BACKGROUND

Neuroendocrine (NE) differentiation in prostate cancer has been suggested to be one of the early events in the development of androgen independence. In the human prostate cancer LNCaP cell line, treatment with interleukin-6 (IL-6) induces NE-like differentiation, which is similar to the phenomena observed in advanced stages of prostate cancer progression. In this study, we investigate how androgen plays a role in IL-6-mediated NE differentiation in LNCaP cell line.

METHODS

Western blot, co-immunoprecipitation (co-IP), and GST pull-down assays were performed to detect the protein expression and protein-protein interaction. PI3K kinase assay was used to measure PI3K activity.

RESULTS

Addition of androgen blocks IL-6-mediated PI3K activation and NE differentiation in LNCaP cells. In vivo and in vitro protein interaction assays suggested that androgen receptor (AR) can directly interact with IL-6 transducer gp130. In addition, androgen treatment enhances the interaction between AR and gp130, interrupts the IL-6-induced gp130-mediated PI3K activation, which may lead to inhibition of IL-6-mediated NE differentiation in LNCaP cells.

CONCLUSIONS

Our results suggest androgen and AR can regulate IL-6-mediated LNCaP cell NE differentiation via directly modulating the IL-6-PI3K pathway.

Comparative evaluation of androgen and progesterone receptor transcription selectivity indices of 19-nortestosterone-derived progestins

Synthetic 19-nortestosterone-derived progestins show affinity for the androgen receptor (AR) and retain varying degrees of androgenic activity. In this study, AR- and progesterone receptor (PR)-dependent transcriptional activation induced by norethisterone (NET), levonorgestrel (LNG) and gestodene (GSD), and their 5alpha-reduced derivatives, including limited trypsin digestion of AR in the presence of natural and synthetic progestins were investigated. The results confirmed the progestogenic activity of the three 19-nortestosterone derivatives, which decreases after reduction of the 4-ene-double bound. These compounds were able to activate AR-dependent reporter gene expression, LNG and GSD being the stronger activators. 5alpha-Reduction of LNG and GSD did not change their androgenic transcriptional activity; however, the activation of AR by 5alpha-NET was four-fold higher than NET. The highest selectivity transcriptional index, as a measure of progestogenicity versus androgenicity, was obtained for NET. The 5alpha-reduced derivatives had values significantly lower than those of their parent compounds. Non-reduced and 5alpha-reduced 19-nortestosterone progestins induced virtually identical proteolysis fragmentation patterns of the AR to those observed with DHT.

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.

Coregulator function: a key to understanding tissue specificity of selective receptor modulators

Ligands for the nuclear receptor superfamily control many aspects of biology, including development, reproduction, and homeostasis, through regulation of the transcriptional activity of their cognate receptors. Selective receptor modulators (SRMs) are receptor ligands that exhibit agonistic or antagonistic biocharacter in a cell- and tissue context-dependent manner. The prototypical SRM is tamoxifen, which as a selective estrogen receptor modulator, can activate or inhibit estrogen receptor action. SRM-induced alterations in the conformation of the ligand-binding domains of nuclear receptors influence their abilities to interact with other proteins, such as coactivators and corepressors. It has been postulated, therefore, that the relative balance of coactivator and corepressor expression within a given target cell determines the relative agonist vs. antagonist activity of SRMs. However, recent evidence reveals that the cellular environment also plays a critical role in determining SRM biocharacter. Cellular signaling influences the activity and subcellular localization of coactivators and corepressors as well as nuclear receptors, and this contributes to gene-, cell-, and tissue-specific responses to SRM ligands. Increased understanding of the effect of cellular environment on nuclear receptors and their coregulators has the potential to open the field of SRM discovery and research to many members of the nuclear receptor superfamily.

Agonist-antagonist induced coactivator and corepressor interplay on the human androgen receptor

The human androgen receptor (AR) is a member of the nuclear hormone receptor superfamily. However, in contrast to other members of this family the amino-(N)-terminus of AR harbors the major transactivation function. Previously we have shown that hormone antagonists that bind to the carboxy-terminal ligand-binding domain repress AR through recruitment of corepressors that are recruited to the receptor N-terminus. Here we show by a modified mammalian two-hybrid system that both the AR interacting domains of the coactivator SRC1 and of the corepressor SMRT compete for interaction with the AR N-terminus. In contrast to other members of the nuclear receptor superfamily the LXXLL motifs of SRC1e are not required for this interaction, instead a stretch of 135 amino acids of the glutamine rich region (Qr) of SRC1e is essential to bind to the AR N-terminus. We show that the Qr-region of SRC1 is able to inhibit the interaction of SMRT with AR. Also, we demonstrate that the corepressor mediated repression decreases the antagonist-induced transactivation while, surprisingly, it increases the agonist-induced transactivation. This may indicate that coactivators and corepressors act in concert to dictate the overall receptor-mediated action dependent on the type of ligand.

Functional androgen receptor confers sensitization of androgen-independent prostate cancer cells to anticancer therapy via caspase activation

Therapeutic resistance remains an unresolved problem in the clinical management of human prostate cancer (PC). Despite initial positive response to androgen ablation therapy (AAT), virtually all PC patients will relapse due to acquisition of hormone refractory disease and selective outgrowth of tumor cells with multidrug resistance phenotype. We here provide the first experimental evidence that restoring a functional androgen receptor (AR) in the androgen-independent prostate cancer PC3 cells enhances their sensitivity to growth arrest and suppresses their colony-forming ability in response to paclitaxel and gamma-irradiation. Furthermore, functional AR increases the susceptibility of these cells to the apoptotic potentials of therapeutic agents, as evidenced by an increase in caspase activity, annexin V binding, and internucleosomal DNA fragmentation, by inducing caspase activation. The abrogation of the cytotoxic effects by 4-hydroxyflutamide suggests a crucial role for AR activation in enhancing the therapeutic sensitivity of these cells in a ligand-independent fashion. Our data thus demonstrate that a functional AR is a prerequisite for effective therapeutic response and that aberrant expression or blockade by AAT may trigger pathways leading to emergence of PC cells with therapeutic resistance phenotype. Since the mainstay of primary therapy for PC has been AAT by pharmaco-therapeutic or surgical means, this study thus provides a new frontier for revising the AAT therapeutic strategy in conjunction with radiation and/or chemotherapeutic agents.

Isolation and identification of L-dopa decarboxylase as a protein that binds to and enhances transcriptional activity of the androgen receptor using the repressed transactivator yeast two-hybrid system

The AR (androgen receptor) is a ligand-regulated transcription factor, which belongs to the steroid receptor family and plays an essential role in growth and development of the prostate. Transcriptional activity of steroid receptors is modulated by interaction with co-regulator proteins and yeast two-hybrid analysis is commonly used to identify these steroid receptor-interacting proteins. However, a limitation of conventional two-hybrid systems for detecting AR protein partners has been that they only allow for analysis of the ligand- and DNA-binding domains of the receptor, as its NTD (N-terminal domain) possesses intrinsic transactivation activity. To identify AR N-terminus-interacting proteins, its NTD was used in the RTA (repressed transactivator) system, which is specifically designed for transactivator bait proteins and was shown to be suitable for two-hybrid analysis with the AR NTD. DDC (L-dopa decarboxylase) was detected multiple times as a novel AR-interacting protein, which was subsequently confirmed in vitro and in vivo. Furthermore, transient transfection of DDC in prostate cancer cells strongly enhanced ligand-dependent AR transcriptional activity, an effect that was antagonized using high concentrations of the anti-androgen bicalutamide. Glucocorticoid receptor activity was also strongly enhanced with DDC co-transfection, while oestrogen receptor activity was only mildly affected. Together, our data demonstrate that DDC interacts with AR to enhance steroid receptor transactivation, which may have important implications in prostate cancer progression.

Gene expression changes following androgen receptor elimination in LNCaP prostate cancer cells

We have shown recently that inhibition of androgen receptor (AR) expression with an antisense AR oligonucleotide (ODN) inhibits LNCaP prostate tumor cells in vitro as well as in vivo. In this study, we investigated gene expression changes that occur after AR signaling blockade, either through AR elimination by antisense treatment or through complete androgen receptor inhibition by androgen deprivation combined with the antiandrogen bicalutamide, in order to search for genes that are directly or indirectly regulated through the AR. Gene expression changes were investigated with cDNA NIH 10K gene microarrays in response to treatment over 48 h. Expression of selected genes was further analyzed by real-time reverse transcriptase (RT)-polymerase chain reaction (PCR), Western blotting, and radioimmunoassay. A comparison of antisense-treated and androgen-deprived cells revealed several concordances such as significant downregulation of prostate-specific genes, cell-cycle regulatory genes, genes of the cholesterol biosynthesis pathway, and several cytoskeletal genes. However, there were also several genes that were differentially regulated. Among the genes that were exclusively changed by treatment with the antisense AR ODN were the insulin-like growth factor binding protein 2 (IGFBP2) and the phosphatidylinositol-4-phosphate 5-kinase type I alpha (PIP5KIA). On the other hand, complete androgen receptor blockade induced changes in the expression of the prostate overexpressed gene 1 and the S100 calcium binding protein P. In summary, we identified a cohort of interesting genes whose expression was highly affected by elimination of the AR in LNCaP prostate cancer cells. Further investigations are warranted to clarify their role in the AR signaling pathway and their susceptibility as a target for the treatment of prostate cancer.

The androgen receptor: structure, mutations, and antiandrogens

Androgens play a critical role not only in the physiological development of the prostate but also in the genesis of prostate cancer. The effects of androgen on the prostate gland and on the other tissues of the body are mediated by activation of the androgen receptor. The androgen receptor is a member of the superfamily of hormone receptors with a DNA-binding site, two zinc finger domains, and a hormone-binding site. Mutations in this receptor can be associated with loss of function or chronic endogeneous activation, depending upon the site of change. Androgens effect a conformal change in the structure of the androgen receptor associated with a change in protein phosphorylation. The androgen receptor can be activated by additional ligands affecting the hormone-binding site besides androgens. Activators and repressors of the androgen receptor modify this protein's function and are very delicately balanced such that disruptions of either function are associated with a disease state. Antiandrogens, which bind to the receptor and thus down-regulate the effects of endogeneous circulating androgens, remain the first line treatment for palliation of advanced prostate cancer. Mutations in the receptor are associated with a change in function of such compounds from antagonist to agonist in vitro. Newer evidence suggests there may be a role of intermittent androgen suppression rather than continuous suppression, perhaps by preventing overgrowth of hormone independent tumor cells. Future research focuses on the development of drugs directed at suppressing the androgen drive of the androgen sensitive clone of the tumor and making the nonsensitive subset more susceptible to cytotoxics.

Agonist but not antagonist ligands induce conformational change in the mouse aryl hydrocarbon receptor as detected by partial proteolysis

The cytosolic transcription factor known as the aryl hydrocarbon receptor (AhR) undergoes transformation to a DNA-binding form by a series of processes initiated by binding of ligand. Subsequent steps include dissociation of several proteins that are complexed with the inactive receptor, nuclear translocation, and dimerization with Arnt. We have used limited proteolysis of the in vitro-translated mouse AhR to determine whether this technique can detect conformational change(s) associated with AhR transformation and whether the effect of agonist and antagonist ligands can be distinguished by this assay. Limited digestion of [(35)S]AhR/AhR nuclear translocator (Arnt) by trypsin produced a peptide of approximately 40 kDa that was more resistant to proteolysis in the presence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) than vehicle and was also Arnt-dependent. This trypsin-resistant peptide was also elicited in the presence of other agonist ligands, but not with antagonist ligands that do not form the DNA-binding AhR/Arnt complex. Immunoblot of trypsin-treated AhR/Arnt +/- TCDD indicated that the trypsin-resistant peptide did not include the N-terminal portion of the AhR against which the antibody was made. Truncated AhRs were also subjected to limited trypsinization. From AhR(1-399), a TCDD-dependent peptide of approximately 35 kDa was observed; from the constitutively active AhR(1-348), a band of approximately 30 kDa was produced from vehicle- and TCDD-treated protein. From these observations, we hypothesize that the trypsin-resistant peptide from full-length AhR spans approximately from amino acid 80 to 440. We conclude that agonist ligands initiate structural alteration in AhR that is Arnt-dependent and at least partially involves the ligand-binding/Per-Arnt-Sim domain.

Key structural features of nonsteroidal ligands for binding and activation of the androgen receptor

The purposes of the present studies were to examine the androgen receptor (AR) binding ability and in vitro functional activity of multiple series of nonsteroidal compounds derived from known antiandrogen pharmacophores and to investigate the structure-activity relationships (SARs) of these nonsteroidal compounds. The AR binding properties of sixty-five nonsteroidal compounds were assessed by a radioligand competitive binding assay with the use of cytosolic AR prepared from rat prostates. The AR agonist and antagonist activities of high-affinity ligands were determined by the ability of the ligand to regulate AR-mediated transcriptional activation in cultured CV-1 cells, using a cotransfection assay. Nonsteroidal compounds with diverse structural features demonstrated a wide range of binding affinity for the AR. Ten compounds, mainly from the bicalutamide-related series, showed a binding affinity superior to the structural pharmacophore from which they were derived. Several SARs regarding nonsteroidal AR binding were revealed from the binding data, including stereoisomeric conformation, steric effect, and electronic effect. The functional activity of high-affinity ligands ranged from antagonist to full agonist for the AR. Several structural features were found to be determinative of agonist and antagonist activities. The nonsteroidal AR agonists identified from the present studies provided a pool of candidates for further development of selective androgen receptor modulators (SARMs) for androgen therapy. Also, these studies uncovered or confirmed numerous important SARs governing AR binding and functional properties by nonsteroidal molecules, which would be valuable in the future structural optimization of SARMs.

Antiandrogens: selective androgen receptor modulators

Antiandrogens can efficiently block androgen receptor (AR) mediated gene expression, and are therefore useful tools in the treatment of androgen dependent prostate cancer. Antiandrogens are either complete or partial inhibitors of AR activity, depending on the nature of the compound. As compared to androgens, antiandrogens induce a different AR conformation, thereby influencing the recruitment of co-regulators (coactivators and corepressors). This ligand-selective modulation of AR activity is affected by an AR mutation (Thr877Ala substitution) found in prostate cancer. In contrast to the wild-type AR, the mutant AR conformation induced by cyproterone acetate (CPA) and hydroxyflutamide (OHF) is comparable to that induced by androgens. As a consequence, this might affect recruitment of co-regulators, thereby allowing CPA and OHF to act as strong agonists on the mutant AR.

RanBPM, a nuclear protein that interacts with and regulates transcriptional activity of androgen receptor and glucocorticoid receptor

The androgen receptor (AR) is a ligand-dependent transcription factor that has an essential role in the normal growth, development, and maintenance of the prostate gland. The AR is part of a large family of steroid receptors that also includes the glucocorticoid, progesterone, and mineralocorticoid receptors. Steroid receptor family members share significant homology at their DNA and ligand-binding domains. However, these receptors exhibit a high degree of sequence variability at their NH(2)-terminal domain, which suggests the possibility of receptor-specific interactions with co-regulator proteins. Transcriptional co-regulators that interact with the AR may have a role in defining AR activity and may be involved in directing AR-specific responses. Here we have identified Ran-binding protein in the microtubule-organizing center (RanBPM) to be a novel AR-interacting protein by yeast two-hybrid assay and have confirmed this interaction by glutathione S-transferase- and His-tagged pull-down assays. In addition, transient overexpression of RanBPM in prostate cancer cell lines resulted in enhanced AR activity in a ligand-dependent fashion. Glucocorticoid receptor activity was also enhanced when RanBPM was overexpressed, whereas estrogen receptor activity remained unchanged. These data demonstrate that RanBPM interacts with steroid receptors to selectively modify their activity.

Amino acids 3-13 and amino acids in and flanking the 23FxxLF27 motif modulate the interaction between the N-terminal and ligand-binding domain of the androgen receptor

The N-terminal domain (NTD) and the ligand-binding domain (LBD) of the androgen receptor (AR) exhibit a ligand-dependent interaction (N/C interaction). Amino acids 3-36 in the NTD (AR3-36) play a dominant role in this interaction. Previously, it has been shown that a PhixxPhiPhi motif in AR3-36, 23FxxLF27, is essential for LBD interaction. We demonstrate in the current study that AR3-36 can be subdivided into two functionally distinct fragments: AR3-13 and AR16-36. AR3-13 does not directly interact with the AR LBD, but rather contributes to the transactivation function of the AR.NTD-AR.LBD complex. AR16-36, encompassing the 23FxxLF27 motif, is predicted to fold into a long amphipathic alpha-helix. A second PhixxPhiPhi candidate protein interaction motif within the helical structure, 30VREVI34, shows no affinity to the LBD. Within AR16-36, amino acid residues in and flanking the 23FxxLF27 motif are demonstrated to modulate N/C interaction. Substitution of Q24 and N25 by alanine residues enhances N/C interaction. Substitution of amino acids flanking the 23FxxLF27 motif by alanines are inhibitory to LBD interaction.

A novel androgen receptor mutant, A748T, exhibits hormone concentration-dependent defects in nuclear accumulation and activity despite normal hormone-binding affinity

Functional analysis of androgen receptor (AR) gene mutations isolated from prostate cancer has led to the identification of residues that play important roles in the structure and function of the receptor. Here we report the characteristics of a novel AR mutation A748T located in helix 5 of the ligand-binding domain, which was identified in metastatic prostate cancer. Despite a normal hormone-binding affinity, A748T causes hormone concentration-dependent defects in nuclear accumulation and transcriptional activation. Moreover, when equivalent amounts of DNA are transfected, the mutant is expressed at much lower levels than the wild-type AR (ARWT). Treatment with geldanamycin to disrupt receptor-heat shock protein complexes rapidly decreases the levels of ARWT but not A748T, suggesting that the lower expression and rapid degradation rate of A748T is due to weaker interactions with heat shock proteins. Further analysis revealed that hormone dissociates from A748T five times faster than from ARWT. Loss of the ability to form stable amino/carboxyl-terminal interactions causes accelerated dissociation rates in some AR mutants. However, A748T exhibits normal amino/carboxyl-terminal interactions at high hormone concentrations, suggesting that the mutation alters interactions with ligand. Consistent with this conclusion, our structural model predicts that A748T disrupts crucial contact points with ligand, thereby altering the conformation of the ligand-binding domain.