Nuclear export signal of androgen receptor (NESAR) regulation of androgen receptor level in human prostate cell lines via ubiquitination and proteasome-dependent degradation

Differentially disrupted spinal cord and muscle energy metabolism in spinal and bulbar muscular atrophy

Prior studies showed that polyglutamine-expanded androgen receptor (AR) is aberrantly acetylated and that deacetylation of the mutant AR by overexpression of nicotinamide adenine dinucleotide-dependent (NAD+-dependent) sirtuin 1 is protective in cell models of spinal and bulbar muscular atrophy (SBMA). Based on these observations and reduced NAD+ in muscles of SBMA mouse models, we tested the therapeutic potential of NAD+ restoration in vivo by treating postsymptomatic transgenic SBMA mice with the NAD+ precursor nicotinamide riboside (NR). NR supplementation failed to alter disease progression and had no effect on increasing NAD+ or ATP content in muscle, despite producing a modest increase of NAD+ in the spinal cords of SBMA mice. Metabolomic and proteomic profiles of SBMA quadriceps muscles indicated alterations in several important energy-related pathways that use NAD+, in addition to the NAD+ salvage pathway, which is critical for NAD+ regeneration for use in cellular energy production. We also observed decreased mRNA levels of nicotinamide riboside kinase 2 (Nmrk2), which encodes a key kinase responsible for NR phosphorylation, allowing its use by the NAD+ salvage pathway. Together, these data suggest a model in which NAD+ levels are significantly decreased in muscles of an SBMA mouse model and intransigent to NR supplementation because of decreased levels of Nmrk2.

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.

Androgen receptor nucleocytoplasmic trafficking - A one-way journey

The androgen receptor (AR) is a key regulator of the growth and proliferation of prostate cancer. The majority of lethal castration-resistant prostate cancer (CRPC) growth is still dependent on AR activity. The AR need to be in the nucleus to exert its biological action as a transcription factor. As such, defining the mechanisms that regulate the subcellular localization of AR are important. Previously it was believed that AR was imported into the nucleus in a ligand-dependent manner and subsequently exported out of the nucleus upon ligand withdrawal. Recent evidence has challenged this decades-old paradigm and showed that the AR is degraded, not exported, in the nucleus. This review discusses the current understanding of how AR nucleocytoplasmic localization is regulated by import and through nuclear degradation.

Mechanisms and targeting of proteosome-dependent androgen receptor degradation in prostate cancer

The androgen receptor (AR) remains to be a key target for the treatment of prostate cancer, including the majority of castration-resistant prostate cancer (CRPC). AR is stabilized in CRPC and the ubiquitin-proteasome system (UPS) plays a major role in AR degradation. Targeting AR for degradation provides a potential approach to overcome the resistance of CRPC to current AR antagonists, including the next generation AR signaling inhibitors. Different types of AR degraders have been developed, including the proteolysis-targeting chimeras (PROTACs), selective AR degraders (SARDs), and novel AR degraders, with several AR PROTACs currently in clinical trials. The present mini-review discusses the regulation of AR degradation by the UPS, the potential role of a novel nuclear degradation signal in AR, and different types of AR degraders.

The role of ubiquitination in spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a neurodegenerative and neuromuscular genetic disease caused by the expansion of a polyglutamine-encoding CAG tract in the androgen receptor (AR) gene. The AR is an important transcriptional regulator of the nuclear hormone receptor superfamily; its levels are regulated in many ways including by ubiquitin-dependent degradation. Ubiquitination is a post-translational modification (PTM) which plays a key role in both AR transcriptional activity and its degradation. Moreover, the ubiquitin-proteasome system (UPS) is a fundamental component of cellular functioning and has been implicated in diseases of protein misfolding and aggregation, including polyglutamine (polyQ) repeat expansion diseases such as Huntington’s disease and SBMA. In this review, we discuss the details of the UPS system, its functions and regulation, and the role of AR ubiquitination and UPS components in SBMA. We also discuss aspects of the UPS that may be manipulated for therapeutic effect in SBMA.

The classical and updated models of androgen receptor nucleocytoplasmic trafficking

This mini-review covers the classical model of androgen receptor (AR) nucleocytoplasmic trafficking and provides an overview of new data that updates the existing paradigm. The classical model of androgen receptor trafficking involves AR translocating to the nucleus in the presence of androgens and subsequently being exported back to the cytoplasm following the withdrawal of androgens. New data challenges and updates the fate of nuclear AR. In the updated model, the AR can be imported into the nucleus in the absence of androgens and nuclear AR is degraded, not exported. Further, androgens can enhance AR nuclear import and inhibit AR degradation in the nucleus; androgen withdrawal causes nuclear AR degradation, but not export. Enhanced androgen-independent AR nuclear localization and AR nuclear stability may be a hallmark of castration-resistant prostate cancer (CRPC). Further characterization of AR trafficking may aid in the development of new therapies for patients with CRPC.

Regulation and targeting of androgen receptor nuclear localization in castration-resistant prostate cancer

Nuclear localization of the androgen receptor (AR) is necessary for its activation as a transcription factor. Defining the mechanisms regulating AR nuclear localization in androgen-sensitive cells and how these mechanisms are dysregulated in castration-resistant prostate cancer (CRPC) cells is fundamentally important and clinically relevant. According to the classical model of AR intracellular trafficking, androgens induce AR nuclear import and androgen withdrawal causes AR nuclear export. The present study has led to an updated model that AR could be imported in the absence of androgens, ubiquitinated, and degraded in the nucleus. Androgen withdrawal caused nuclear AR degradation, but not export. In comparison with their parental androgen-sensitive LNCaP prostate cancer cells, castration-resistant C4-2 cells exhibited reduced nuclear AR polyubiquitination and increased nuclear AR level. We previously identified 3-(4-chlorophenyl)-6,7-dihydro-5H-pyrrolo[1,2-a]imidazole (CPPI) in a high-throughput screen for its inhibition of androgen-independent AR nuclear localization in CRPC cells. The current study shows that CPPI is a competitive AR antagonist capable of enhancing AR interaction with its E3 ligase MDM2 and degradation of AR in the nuclei of CRPC cells. Also, CPPI blocked androgen-independent AR nuclear import. Overall, these findings suggest the feasibility of targeting androgen-independent AR nuclear import and stabilization, two necessary steps leading to AR nuclear localization and activation in CRPC cells, with small molecule inhibitors.

Quantification of neural and hormonal receptors at the prostate of long-term sexual behaving male rats after lesion of pelvic and hypogastric nerves

Prostate function is regulated by androgens and a neural control via the pelvic and hypogastric nerves. As such, this sexual gland contains receptors for acetylcholine and noradrenaline, although it is unknown whether the expression of these receptors is affected by sexual behavior and even less by denervation of the gland. Thus, the purpose of this work was to evaluate the effect of repeated sexual behavior on the expression of noradrenaline, acetylcholine, and androgen receptors at the prostate, and how they are affected by denervation. To achieve this, we used sexually experienced males denervated at the pelvic or hypogastric nerves, or both. The messenger (mRNA) and protein for androgen, noradrenergic, and cholinergic receptors were evaluated. The weight of the gland and the levels of serum testosterone were also measured. We found that: (1) sexual behavior was not affected by denervation; (2) blood testosterone levels increased due to sexual behavior but such increase is prevented by denervation; (3) the weight of the ventral prostate increased with sexual behavior but was not affected by denervation; (4) AR messenger levels increased with sexual behavior but were not altered by denervation; (5) the messenger for noradrenergic and cholinergic receptors decreased after denervation, and those for muscarinic receptors increased, and (6) only AR protein decreased after denervation of both nerves, while those for other receptors remained unchanged. In summary, we show that the three receptors have different regulatory mechanisms, and that only androgen receptors are regulated by both autonomic systems.

SUMO3 modification by PIAS1 modulates androgen receptor cellular distribution and stability

Background

Abnormal reactivation of androgen receptor (AR) signaling in castration-resistant prostate cancer (CRPC) mainly results from overexpression and down-regulation of AR. Sumoylation of AR can influence its function. However, regulation of AR sumoylation by SUMO E3 ligases PIASs to modify AR distribution and stability are not well understood.

Methods

We assessed the potential effect of SUMO3 modification on AR intracellular localization by immunostaining in AR-negative prostate cancer DU145 cells, and detected the effect of PIAS1/SUMO3 overexpression on AR sumoylation related degradation. Then we characterized AR sumoylation sites involved modified by SUMO3, and the key residue of PIAS1 involved in itself sumoylation and further mediated AR sumoylation (sumo3-conjugated), translocation and degradation. Finally we detected the recognition of PIAS1 (sumoylation ligase) to MDM2, a ubiquin ligase mediated AR degradation.

Results

We demonstrate that SUMO E3 ligase PIAS1, along with SUMO3, mediates AR cytosolic translocation and subsequent degradation via a ubiquitin-proteasome pathway. Although AR sumoylation occurs prior to ubiquitination, the SUMO-acceptor lysine 386 on AR, together with ubiquitin-acceptor lysine 845, contribute to PIAS1/SUMO3-induced AR nuclear export, ubiquitination and subsequent degradation. Moreover, PIAS1 itself is modified by SUMO3 overexpression, and mutation of SUMO-acceptor lysine 117 on PIAS1 can impair AR cytoplasmic distribution, demonstrating the essential role of sumoylated PIAS1 in AR translocation. We further determine that sumoylated PIAS1 interacts with AR lysine 386 and 845 to form a binary complex. Consistent with the effect on AR distribution, SUMO3 modification of PIAS1 is also required for AR ubiquitination and degradation by recruiting ubiquitin E3 ligase MDM2.

Conclusion

Taken together, SUMO3 modification of PIAS1 modulates AR cellular distribution and stability. Our study provided the evidence the crosstalk between AR sumoylation and ubquitination mediated by PIAS1 and SUMO3.

Molecular Mechanisms and Therapeutics for SBMA/Kennedy's Disease

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disease caused by a polyglutamine (polyQ) expansion in the androgen receptor (AR). Despite the fact that the monogenic cause of SBMA has been known for nearly 3 decades, there is no effective treatment for this disease, underscoring the complexity of the pathogenic mechanisms that lead to a loss of motor neurons and muscle in SBMA patients. In the current review, we provide an overview of the system-wide clinical features of SBMA, summarize the structure and function of the AR, discuss both gain-of-function and loss-of-function mechanisms of toxicity caused by polyQ-expanded AR, and describe the cell and animal models utilized in the study of SBMA. Additionally, we summarize previously conducted clinical trials which, despite being based on positive results from preclinical studies, proved to be largely ineffective in the treatment of SBMA; nonetheless, these studies provide important insights as researchers develop the next generation of therapies.

The tumor suppressor OVCA1 is a short half-life protein degraded by the ubiquitin-proteasome pathway

Ovarian cancer gene 1 (OVCA1) is a tumor suppressor associated with ovarian cancer, which is involved in cell proliferation regulation, embryonic development and tumorigenesis. Loss of heterozygosity in the OVCA1 gene occurs in 50-86% of cases of ovarian cancer; however, the physiological and biochemical functions of OVCA1 are not yet clear. In the present study, the stability and degradation of OVCA1 were investigated in A2780, Hela and 293 cells. The results revealed that the OVCA1 protein was unstable by MG132 inhibiting proteasome mediated degradation, co-immunoprecipitation and half-life measurement experiments. The cellular protein levels of endogenous OVCA1 were too low to be detected by western blotting. In addition, carbobenzoxy-L-leucyl-L-leucyl-L-leucinal inhibited the degradation of OVCA1 in cells. The co-immunoprecipitation assay revealed that the OVCA1 protein interacted with ubiquitin to form a poly-ubiquitinated complex in cells. The half-life of OVCA1, measured by inhibiting protein synthesis with cycloheximide, was <2 h. The present study demonstrated that OVCA1 may be degraded by the ubiquitin-mediated proteasome pathway and may be considered a short half-life protein. In conclusion, the regulation of OVCA1 protein degradation via the ubiquitin-proteasome pathway may represent a novel direction in the development of ovarian cancer therapy.

Impaired Nuclear Export of Polyglutamine-Expanded Androgen Receptor in Spinal and Bulbar Muscular Atrophy

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disease caused by polyglutamine (polyQ) expansion in the androgen receptor (AR). Prior studies have highlighted the importance of AR nuclear localization in SBMA pathogenesis; therefore, in this study, we sought to determine the role of AR nuclear export in the pathological manifestations of SBMA. We demonstrate here that the nuclear export of polyQ-expanded AR is impaired, even prior to the formation of intranuclear inclusions of aggregated AR. Additionally, we find that promoting AR export with an exogenous nuclear export signal substantially reduces its aggregation and blocks hormone-induced toxicity. Moreover, we show that these protective effects are conferred by destabilization of the mutant protein due to an increase in proteasomal degradation of the cytoplasmic AR. Despite a growing body of evidence that global disruption of nucleo/cytoplasmic transport occurs in ALS and HD, our data suggest that no such global disruption occurs in models of SBMA; rather, AR-specific mechanisms, including reduced phosphorylation at Serine 650, are likely responsible for the impaired nuclear export of polyQ-expanded AR.

Sertad1 promotes prostate cancer progression through binding androgen receptor ligand binding domain

Androgen receptor (AR) signaling is involved in the initiation and progression of prostate cancer (PCa), which is the most frequently diagnosed nonskin cancer and remains a leading cause of cancer-related death in men. Further investigation of the involvement of AR signaling in PCa progression is urgently needed. In the present study, we performed a yeast two-hybrid screen and demonstrated that SERTA domain-containing protein 1 (Sertad1) is a novel AR-binding protein that binds to the AR ligand binding domain (LBD). The binding between AR-LBD and Sertad1 was confirmed by glutathione S-transferase (GST) pull-down assays and immunoprecipitation (IP) and confocal immunofluorescence co-localization experiments. Furthermore, we demonstrated that DHT inhibited Sertad1 protein degradation in prostate cancer cell lines and that Sertad1 knockdown inhibited the proliferation of prostate cancer cells in vitro. In human PCa tumor tissues, Sertad1 expression is positively correlated with AR expression and the Gleason score. Taken together, this report is the first to show that Sertad1 is a novel AR-LBD-binding protein, and DHT-liganded AR-LBD inhibits Sertad1 degradation. Thus, Sertad1 may represent a novel therapeutic target for the treatment of AR-positive PCa.

Molecular pathways involved in the transport of nuclear receptors from the nucleus to cytoplasm

Nuclear receptors (NRs) are transcription regulators that direct the expression of many genes linked to cellular processes, such as proliferation, differentiation, and apoptosis. Additionally, some cellular events are also modulated by signaling pathways induced by NRs outside of the nucleus. Hence, the subcellular transport of NRs is dynamic and is modulated by several signals, protein-protein interactions, and posttranslational modifications. Particularly, the exit of NRs from the nucleus to cytoplasm and/or other compartments is transcendental, as it is this export event, which determines their abundance in the cells' compartments, the activation or attenuation of nuclear or extranuclear pathways, and the magnitude and duration of their effects inside or outside of the nucleus. Consequently, an adequate control of the distribution of NRs is critical for homeostasis, because a deregulation in the nucleo-cytoplasmic transport of NRs could be involved in diseases including cancer as well as metabolic and vascular alterations. In this review, we investigated the pathways and molecular and biological aspects that have been described for the nuclear export of NRs so far and their functional relevance in some diseases. This information suggests that the transport of NRs out of the nucleus is a key mechanism for the identification of new therapeutic targets for alterations associated with the deregulation of the function of NRs.

DHX15 promotes prostate cancer progression by stimulating Siah2-mediated ubiquitination of androgen receptor

Androgen receptor (AR) activation is critical for prostate cancer development and progression, including castration-resistance. The nuclear export signal of AR (NES^AR) plays an important role in AR intracellular trafficking and proteasome-dependent degradation. Here, we identified the RNA helicase DHX15 as a novel AR co-activator using a yeast mutagenesis screen and revealed that DHX15 regulates AR activity by modulating E3 ligase Siah2-mediated AR ubiquitination independent of its ATPase activity. DHX15 and Siah2 form a complex with AR, through NES^AR. DHX15 stabilized Siah2 and enhanced its E3 ubiquitin ligase activity, resulting in AR activation. Importantly, DHX15 was upregulated in prostate cancer specimens and its expression was correlated with Gleason scores and PSA recurrence. Furthermore, DHX15 immunostaining correlated with Siah2. Finally, DHX15 knockdown inhibited the growth of C4-2 prostate tumor xenografts in mice. Collectively, our data argue that DHX15 enhances AR transcriptional activity and contributes to prostate cancer progression through Siah2.

Inhibition of Androgen Receptor Function and Level in Castration-Resistant Prostate Cancer Cells by 2-[(isoxazol-4-ylmethyl)thio]-1-(4-phenylpiperazin-1-yl)ethanone

The androgen receptor (AR) plays a critical role in the development of castration-resistant prostate cancer (CRPC) as well as in the resistance to the second-generation AR antagonist enzalutamide and the selective inhibitor of cytochrome P450 17A1 (CYP17A1) abiraterone. Novel agents targeting AR may inhibit the growth of prostate cancer cells resistant to enzalutamide and/or abiraterone. Through a high-throughput/high-content screening of a 220,000-member small molecule library, we have previously identified 2-[(isoxazol-4-ylmethyl)thio]-1-(4-phenylpiperazin-1-yl)ethanone (IMTPPE) (SID 3712502) as a novel small molecule capable of inhibiting AR transcriptional activity and protein level in C4-2 prostate cancer cells. In this study, we show that IMTPPE inhibits AR-target gene expression using real-time polymerase chain reaction, Western blot, and luciferase assays. IMTPPE inhibited proliferation of AR-positive, but not AR-negative, prostate cancer cells in culture. IMTPPE inhibited the transcriptional activity of a mutant AR lacking the ligand-binding domain (LBD), indicating that IMTPPE inhibition of AR is independent of the LBD. Furthermore, animal studies showed that IMTPPE inhibited the growth of 22Rv1 xenograft tumor, a model for enzalutamide-resistant prostate cancer. These findings suggest that IMTPPE is a potential lead compound for developing clinical candidates for the treatment of CRPC, including those resistant to enzalutamide.

Allosteric alterations in the androgen receptor and activity in prostate cancer

Organisms have evolved to generate biological complexity in their proteome and transcriptome from a limited number of genes. This concept holds true for the androgen receptor, which displays a diversity of inclusion/exclusion events in its structural motifs as a mechanism of resistance to the most forefront anti-androgen therapies. More than 20 androgen receptor variants that lack various portions of ligand-binding domain have been identified in human prostate cancer (PCa) samples. Most of the variants are inactive on their own, with a few exceptions displaying constitutive activity. The full-length receptor and one or more variants can be co-expressed in the same cell under many circumstances, which raises the question of how these variants physically and functionally interact with the full-length receptor or one another in the course of PCa progression. To address this issue, in this review, we will characterize and discuss androgen receptor variants, including the novel variants discovered in the last couple of years (i) individually, (ii) with respect to their physical and functional interaction with one another and (iii) in clinical relevance. Here, we also introduce the very recent understanding of AR-Vs obtained through successful development of some AR-V-specific antibodies as well as identification of novel AR-Vs by data mining approaches.

Inhibition of Androgen Receptor Nuclear Localization and Castration-Resistant Prostate Tumor Growth by Pyrroloimidazole-based Small Molecules

The androgen receptor (AR) is a ligand-dependent transcription factor that controls the expression of androgen-responsive genes. A key step in androgen action, which is amplified in castration-resistant prostate cancer (CRPC), is AR nuclear translocation. Small molecules capable of inhibiting AR nuclear localization could be developed as novel therapeutics for CRPC. We developed a high-throughput screen and identified two structurally-related pyrroloimidazoles that could block AR nuclear localization in CRPC cells. We show that these two small molecules, 3-(4-ethoxyphenyl)-6,7-dihydro-5H-pyrrolo[1,2-a]imidazole (EPPI) and 3-(4-chlorophenyl)-6,7-dihydro-5H-pyrrolo[1,2-a]imidazole (CPPI) can inhibit the nuclear localization and transcriptional activity of AR and reduce the proliferation of AR-positive but not AR-negative prostate cancer cell lines. EPPI and CPPI did not inhibit nuclear localization of the glucocorticoid receptor or the estrogen receptor, suggesting they selectively target AR. In LNCaP tumor xenografts, CPPI inhibited the proliferation of relapsed LNCaP tumors. These findings suggest that EPPI and CPPI could serve as lead structures for the development of therapeutic agents for CRPC. Mol Cancer Ther; 16(10); 2120-9. ©2017 AACR.

Nucleo-cytoplasmic transport of estrogen receptor alpha in breast cancer cells

Approximately 70% cases of breast cancers exhibit high expression and activity levels of estrogen receptor alpha (ERα), a transcription regulator that induces the expression of genes associated with cellular proliferation and survival. These nuclear functions of the receptor are associated with the development of breast cancer. However, ERα localization is not static, but rather, dynamic with continuous shuttling between the nucleus and the cytoplasm. Interestingly, both the nuclear import and export of ERα are modulated by several stimuli that include estradiol, antiestrogens, and growth factors. As ERα nuclear accumulation is critical to the regulation of gene expression, nuclear export of this receptor modulates the intensity and duration of its transcriptional activity. Thus, the subcellular spatial distribution of ERα ensures tight modulation of its concentration in cellular compartments, as well as of its nuclear and extranuclear functions. In this review, we will discuss current findings regarding the biological importance of molecular mechanisms of, and proteins responsible for, the nuclear import and export of ERα in breast cancer cells.

Reciprocal stabilization of ABL and TAZ regulates osteoblastogenesis through transcription factor RUNX2

Cellular identity in metazoan organisms is frequently established through lineage-specifying transcription factors, which control their own expression through transcriptional positive feedback, while antagonizing the developmental networks of competing lineages. Here, we have uncovered a distinct positive feedback loop that arises from the reciprocal stabilization of the tyrosine kinase ABL and the transcriptional coactivator TAZ. Moreover, we determined that this loop is required for osteoblast differentiation and embryonic skeletal formation. ABL potentiated the assembly and activation of the RUNX2-TAZ master transcription factor complex that is required for osteoblastogenesis, while antagonizing PPARγ-mediated adipogenesis. ABL also enhanced TAZ nuclear localization and the formation of the TAZ-TEAD complex that is required for osteoblast expansion. Last, we have provided genetic data showing that regulation of the ABL-TAZ amplification loop lies downstream of the adaptor protein 3BP2, which is mutated in the craniofacial dysmorphia syndrome cherubism. Our study demonstrates an interplay between ABL and TAZ that controls the mesenchymal maturation program toward the osteoblast lineage and is mechanistically distinct from the established model of lineage-specific maturation.

Targeting CDK9: a promising therapeutic opportunity in prostate cancer

Cyclin-dependent kinase 9 (CDK9) is a key transcriptional regulator and a lucrative target for cancer treatment. Targeting CDK9 can effectively confine the hyperactivity of androgen receptor and the constitutive expression of anti-apoptotic proteins; both being main causes of prostate cancer (PCa) development and progression. In castrate-resistant PCa, traditional therapies that only target androgen receptor (AR) have become obsolete due to reprograming in AR activity to make the cells independent of androgen. CDK9 inhibitors may provide a new and better therapeutic opportunity over traditional treatment options by targeting both androgen receptor activity and anti-apoptotic proteins, improving the chances of positive outcomes, especially in patients with the advanced disease. This review focuses on biological functions of CDK9, its involvement with AR and the potential for therapeutic opportunities in PCa treatment.

A calreticulin-dependent nuclear export signal is involved in the regulation of liver receptor homologue-1 protein folding

LRH-1 (liver receptor homologue-1) contained a calreticulin-dependent NES (nuclear export signal) that regulate shutting, protein folding and transactivity.

Splicing Factor Prp8 Interacts With NES(AR) and Regulates Androgen Receptor in Prostate Cancer Cells

Androgen receptor (AR) plays a pivotal role in the development of primary as well as advanced castration-resistant prostate cancer. Previous work in our lab identified a novel nuclear export signal (NES) (NES(AR)) in AR ligand-binding domain essential for AR nucleocytoplasmic trafficking. By characterizing the localization of green fluorescence protein (GFP)-tagged NES(AR), we designed and executed a yeast mutagenesis screen and isolated 7 yeast mutants that failed to display the NES(AR) export function. One of those mutants was identified as the splicing factor pre-mRNA processing factor 8 (Prp8). We further showed that Prp8 could regulate NES(AR) function using short hairpin RNA knockdown of Prp8 coupled with a rapamycin export assay in mammalian cells and knockdown of Prp8 could induce nuclear accumulation of GFP-tagged AR in PC3 cells. Prp8 expression was decreased in castration-resistant LuCaP35 xenograft tumors as compared with androgen-sensitive xenografts. Laser capture microdissection and quantitative PCR showed Prp8 mRNA levels were decreased in human prostate cancer specimens with high Gleason scores. In prostate cancer cells, coimmunoprecipitation and deletion mutagenesis revealed a physical interaction between Prp8 and AR mainly mediated by NES(AR). Luciferase assay with prostate specific antigen promoter-driven reporter demonstrated that Prp8 regulated AR transcription activity in prostate cancer cells. Interestingly, Prp8 knockdown also increased polyubiquitination of endogenous AR. This may be 1 possible mechanism by which it modulates AR activity. These results show that Prp8 is a novel AR cofactor that interacts with NES(AR) and regulates AR function in prostate cancer cells.

The link between androgen receptor splice variants and castration-resistant prostate cancer

Resistance to the latest advanced prostate cancer therapies, including abiraterone and enzalutamide, is associated with increased expression of constitutively active androgen receptor splice variants (AR-Vs). The exact mechanism by which these therapies result in AR-Vs is unknown, but may include genomic rearrangement of the androgen receptor gene as well as alternative splicing of the AR pre-messenger RNA (mRNA). An additional complication that hinders further development of effective AR strategies is that the mechanisms by which the directed therapies are bypassed may vary. Finally, the question must be addressed as to whether the androgen receptor remains to be the driver of most castration resistant disease or whether truly AR-independent tumors arise after successful androgen ablation therapy. In this review, we will examine androgen receptor splice variants as an alternative mechanism by which prostate cancer becomes resistant to androgen receptor-directed therapy.

Preclinical evaluation of the supercritical extract of azadirachta indica (neem) leaves in vitro and in vivo on inhibition of prostate cancer tumor growth

Azadirachta indica, commonly known as neem, has gained worldwide prominence because of its medical properties, namely antitumor, antiviral, anti-inflammatory, antihyperglycemic, antifungal, and antibacterial activities. Despite these promising results, gaps remain in our understanding of the molecular mechanism of action of neem compounds and their potential for use in clinical trials. We investigated supercritical extract of neem leaves (SENL) for the following: molecular targets in vitro, in vivo efficacy to inhibit tumor growth, and bioactive compounds that exert antitumor activity. Treatment of LNCaP-luc2 prostate cancer cells with SENL suppressed dihydrotestosterone-induced androgen receptor and prostate-specific antigen levels. SENL inhibited integrin β1, calreticulin, and focal adhesion kinase activation in LNCaP-luc2 and PC3 prostate cancer cells. Oral administration of SENL significantly reduced LNCaP-luc2 xenograft tumor growth in mice with the formation of hyalinized fibrous tumor tissue, reduction in the prostate-specific antigen, and increase in AKR1C2 levels. To identify the active anticancer compounds, we fractionated SENL by high-pressure liquid chromatography and evaluated 16 peaks for cytotoxic activity. Four of the 16 peaks exhibited significant cytotoxic activity against prostate cancer cells. Mass spectrometry of the isolated peaks suggested the compounds with cytotoxic activity were nimbandiol, nimbolide, 2',3'-dihydronimbolide, and 28-deoxonimbolide. Analysis of tumor tissue and plasma samples from mice treated with SENL indicated 28-deoxonimbolide and nimbolide as the bioactive compounds. Overall, our data revealed the bioactive compounds in SENL and suggested that the anticancer activity could be mediated through alteration in androgen receptor and calreticulin levels in prostate cancer.

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.