Alternatively spliced androgen receptor variants
TL;DR: This review will summarize the various alternatively spliced AR variants that have been discovered, with a focus on their role and origin in the pathologic conditions of AIS and PCa.
Abstract: Alternative splicing is an important mechanism for increasing functional diversity from a limited set of genes Deregulation of this process is common in diverse pathologic conditions The androgen receptor (AR) is a steroid receptor transcription factor with functions critical for normal male development as well as the growth and survival of normal and cancerous prostate tissue Studies of AR function in androgen insensitivity syndrome (AIS) and prostate cancer (PCa) have demonstrated loss-of-function AR alterations in AIS and gain-of-function AR alterations in PCa Over the past two decades, AR gene alterations have been identified in various individuals with AIS, which disrupt normal AR splicing patterns and yield dysfunctional AR protein variants Recently, altered AR splicing patterns have been identified as a mechanism of PCa progression and resistance to androgen depletion therapy Several studies have described the synthesis of alternatively spliced transcripts encoding truncated AR isoforms that lack the ligand-binding domain, which is the ultimate target of androgen depletion Many of these truncated AR isoforms function as constitutively active, ligand-independent transcription factors that can support androgenindependent expression of AR target genes, as well as the androgen-independent growth of PCa cells In this review, we will summarize the various alternatively spliced AR variants that have been discovered, with a focus on their role and origin in the pathologic conditions of AIS and PCa
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TL;DR: The aim of this review is to summarize the current literature on the biology and treatment of castrate-resistant prostate cancer.
Abstract: Prostate cancer is the second-leading cause of cancer-related mortality in men in Western societies. Androgen receptor (AR) signaling is a critical survival pathway for prostate cancer cells, and androgen-deprivation therapy (ADT) remains the principal treatment for patients with locally advanced and metastatic disease. Although a majority of patients initially respond to ADT, most will eventually develop castrate resistance, defined as disease progression despite serum testosterone levels of <20 ng/dl. The recent discovery that AR signaling persists during systemic castration via intratumoral production of androgens led to the development of novel anti-androgen therapies including abiraterone acetate and enzalutamide. Although these agents effectively palliate symptoms and prolong life, metastatic castration-resistant prostate cancer remains incurable. An increased understanding of the mechanisms that underlie the pathogenesis of castrate resistance is therefore needed to develop novel therapeutic approaches for this disease. The aim of this review is to summarize the current literature on the biology and treatment of castrate-resistant prostate cancer.
650 citations
TL;DR: This study shows that cells with AR gene rearrangements expressing both full-length and AR-Vs are androgen independent and enzalutamide resistant, but selective knock-down of AR-V expression inhibited androgen-independent growth and restored responsiveness to androgens and antiandrogens.
Abstract: Persistent androgen receptor (AR) transcriptional activity underlies resistance to AR-targeted therapy and progression to lethal castration resistant prostate cancer (CRPC). Recent success in re-targeting persistent AR activity with next-generation androgen/AR axis inhibitors such as enzalutamide (MDV3100) has validated AR as a master regulator during all stages of disease progression. However, resistance to next-generation AR inhibitors limits therapeutic efficacy for many patients. One emerging mechanism of CRPC progression is AR gene rearrangement, promoting synthesis of constitutively-active truncated AR splice variants (AR-Vs) that lack the AR ligand binding domain. In this study, we demonstrate that cells with AR gene rearrangements expressing both full-length and AR-Vs are androgen-independent and enzalutamide-resistant. However, selective knock-down of AR-V expression inhibited androgen-independent growth and restored responsiveness to androgens and antiandrogens. In heterogeneous cell populations, AR gene rearrangements marked individual AR-V-dependent cells that were resistant to enzalutamide. Gene expression profiling following knock-down of full-length AR or AR-Vs demonstrated that AR-Vs drive resistance to AR-targeted therapy by functioning as constitutive and independent effectors of the androgen/AR transcriptional program. Further, mitotic genes deemed previously to be unique AR-V targets were found to be biphasic targets associated with a proliferative level of signaling output from either AR-Vs or androgen-stimulated AR. Overall, these studies highlight AR-Vs as key mediators of persistent AR signaling and resistance to the current arsenal of conventional and next-generation AR-directed therapies, advancing the concept of AR-Vs as therapeutic targets in advanced disease.
622 citations
Cites background from "Alternatively spliced androgen rece..."
...Androgen depletion therapy (ADT) achieves clinical regression or disease stabilization for men with advanced prostate cancer (PCa)....
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...Altered AR mRNA splicing and synthesis of COOH-terminally truncated AR variant (AR-V) proteins lacking the AR ligand-binding domain (LBD) is one mechanism that has been postulated to drive an overall resistance to conventional and next generation ADT (5)....
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TL;DR: The recent genomic characterization of cancers has revealed recurrent somatic point mutations and copy number changes affecting genes encoding RNA splicing factors, which may create novel vulnerabilities in cancer cells that can be therapeutic exploited using compounds that can influence the splicing process.
Abstract: This Review discusses the current genetic and functional links between dysregulated and/or mutated RNA splicing factors and cancer, as well as the therapeutic opportunities presented by alterations in alternative splicing in cancer. The recent genomic characterization of cancers has revealed recurrent somatic point mutations and copy number changes affecting genes encoding RNA splicing factors. Initial studies of these 'spliceosomal mutations' suggest that the proteins bearing these mutations exhibit altered splice site and/or exon recognition preferences relative to their wild-type counterparts, resulting in cancer-specific mis-splicing. Such changes in the splicing machinery may create novel vulnerabilities in cancer cells that can be therapeutically exploited using compounds that can influence the splicing process. Further studies to dissect the biochemical, genomic and biological effects of spliceosomal mutations are crucial for the development of cancer therapies targeted at these mutations.
508 citations
TL;DR: Multiple mechanisms of resistance to ADT and currently approved CRPC treatments will help guide future research into targeted therapies, and some of the mechanisms by which these agents fail are unique, many share similarities to the mechanisms contributing to CRPC progression.
Abstract: Despite advances in prostate cancer diagnosis and management, morbidity from prostate cancer remains high. Approximately 20% of men present with advanced or metastatic disease, while 29,000 men continue to die of prostate cancer each year. Androgen deprivation therapy (ADT) has been the standard of care for initial management of advanced or metastatic prostate cancer since Huggins and Hodges first introduced the concept of androgen-dependence in 1972, but progression to castration-resistant prostate cancer (CRPC) occurs within 2-3 years of initiation of ADT. CRPC, previously defined as hormone-refractory prostate cancer, is now understood to still be androgen dependent. Multiple mechanisms of resistance help contribute to the progression to castration resistant disease, and the androgen receptor (AR) remains an important driver in this progression. These mechanisms include AR amplification and hypersensitivity, AR mutations leading to promiscuity, mutations in coactivators/corepressors, androgen-independent AR activation, and intratumoral and alternative androgen production. More recently, identification of AR variants (ARVs) has been established as another mechanism of progression to CRPC. Docetaxel chemotherapy has historically been the first-line treatment for CRPC, but in recent years, newer agents have been introduced that target some of these mechanisms of resistance, thereby providing additional survival benefit. These include AR signaling inhibitors such as enzalutamide (Xtandi, ENZA, MDV-3100) and CYP17A1 inhibitors such as abiraterone acetate (Zytiga). Ultimately, these agents will also fail to suppress CRPC. While some of the mechanisms by which these agents fail are unique, many share similarities to the mechanisms contributing to CRPC progression. Understanding these mechanisms of resistance to ADT and currently approved CRPC treatments will help guide future research into targeted therapies.
331 citations
Cites background or result from "Alternatively spliced androgen rece..."
...The AR gene on Xq11-12 encodes for a 110 kDa nuclear receptor with four distinct functional motifs—the aminoterminal domain (NTD), DNA-binding domain, hinge region, and ligand-binding domain (LBD) (5-7)....
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...A more recent development has been the identification of splice variants of the AR (AR-Vs), which are constitutively active, typically due to the loss of the C-terminal LBD (94-97)....
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...As described above, it lacks the LBD, is located in the nucleus, and is constitutively active....
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...The Phe876Leu mutation in the LBD of AR has been reported to make enzalutamide into an agonist of AR, © Translational Andrology and Urology....
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...It has been show to regulate both AR-regulated genes and a unique set of AR-independent genes (96), suggesting it has an overlapping but distinct role compared to full-length AR in prostate cancer cells (94)....
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TL;DR: AR functions that contribute to PCa development and progression, the roles of intratumoral androgen synthesis and AR structural alterations in driving AR activity in CRPC, mechanisms of action for abiraterone and enzalutamide, and possible mechanisms of resistance to these agents are outlined.
Abstract: The metabolic functions of androgen receptor (AR) in normal prostate are circumvented in prostate cancer (PCa) to drive tumor growth, and the AR also can acquire new growth-promoting functions during PCa development and progression through genetic and epigenetic mechanisms. Androgen deprivation therapy (ADT, surgical or medical castration) is the standard treatment for metastatic PCa, but patients invariably relapse despite castrate androgen levels (castration-resistant PCa, CRPC). Early studies from many groups had shown that AR was highly expressed and transcriptionally active in CRPC, and indicated that steroids from the adrenal glands were contributing to this AR activity. More recent studies showed that CRPC cells had increased expression of enzymes mediating androgen synthesis from adrenal steroids, and could synthesize androgens de novo from cholesterol. Phase III clinical trials showing a survival advantage in CRPC for treatment with abiraterone (inhibitor of the enzyme CYP17A1 required for androgen synthesis that markedly reduces androgens and precursor steroids) and for enzalutamide (new AR antagonist) have now confirmed that AR activity driven by residual androgens makes a major contribution to CRPC, and led to the recent Food and Drug Administration approval of both agents. Unfortunately, patients treated with these agents for advanced CRPC generally relapse within a year and AR appears to be active in the relapsed tumors, but the molecular mechanisms mediating intrinsic or acquired resistance to these AR-targeted therapies remain to be defined. This review outlines AR functions that contribute to PCa development and progression, the roles of intratumoral androgen synthesis and AR structural alterations in driving AR activity in CRPC, mechanisms of action for abiraterone and enzalutamide, and possible mechanisms of resistance to these agents.
314 citations
References
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Université Paris-Saclay1, Cross Cancer Institute2, University of British Columbia3, Nevada Cancer Institute4, Université de Montréal5, Cardiff University6, University of Colorado Denver7, Baylor University Medical Center8, University of Calgary9, Sarah Cannon Research Institute10, University of California, San Francisco11, Memorial Sloan Kettering Cancer Center12
TL;DR: The inhibition of androgen biosynthesis by abiraterone acetate prolonged overall survival among patients with metastatic castration-resistant prostate cancer who previously received chemotherapy.
Abstract: BACKGROUND Biosynthesis of extragonadal androgen may contribute to the progression of castration-resistant prostate cancer. We evaluated whether abiraterone acetate, an inhibitor of androgen biosynthesis, prolongs overall survival among patients with metastatic castration-resistant prostate cancer who have received chemotherapy. METHODS We randomly assigned, in a 2:1 ratio, 1195 patients who had previously received docetaxel to receive 5 mg of prednisone twice daily with either 1000 mg of abiraterone acetate (797 patients) or placebo (398 patients). The primary end point was overall survival. The secondary end points included time to prostate-specific antigen (PSA) progression (elevation in the PSA level according to prespecified criteria), progression-free survival according to radiologic findings based on prespecified criteria, and the PSA response rate. RESULTS After a median follow-up of 12.8 months, overall survival was longer in the abiraterone acetate–prednisone group than in the placebo–prednisone group (14.8 months vs. 10.9 months; hazard ratio, 0.65; 95% confidence interval, 0.54 to 0.77; P<0.001). Data were unblinded at the interim analysis, since these results exceeded the preplanned criteria for study termination. All secondary end points, including time to PSA progression (10.2 vs. 6.6 months; P<0.001), progression-free survival (5.6 months vs. 3.6 months; P<0.001), and PSA response rate (29% vs. 6%, P<0.001), favored the treatment group. Mineralocorticoid-related adverse events, including fluid retention, hypertension, and hypokalemia, were more frequently reported in the abiraterone acetate–prednisone group than in the placebo–prednisone group. CONCLUSIONS The inhibition of androgen biosynthesis by abiraterone acetate prolonged overall survival among patients with metastatic castration-resistant prostate cancer who previously received chemotherapy. (Funded by Cougar Biotechnology; COU-AA-301 ClinicalTrials.gov number, NCT00638690.)
3,875 citations
"Alternatively spliced androgen rece..." refers background in this paper
...This drug blocks intra-tumor conversion of progesterone and adrenal androgens to DHT (Attard et al. 2009) and a recent phase III trial demonstrated an overall survival benefit for abiraterone in men with metastatic CRPCa (de Bono et al. 2011)....
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TL;DR: The diarylthiohydantoins RD162 and MDV3100 are characterized, two compounds optimized from a screen for nonsteroidal antiandrogens that retain activity in the setting of increased androgen receptor expression that appear to be promising candidates for treatment of advanced prostate cancer.
Abstract: Metastatic prostate cancer is treated with drugs that antagonize androgen action, but most patients progress to a more aggressive form of the disease called castration-resistant prostate cancer, driven by elevated expression of the androgen receptor. Here we characterize the diarylthiohydantoins RD162 and MDV3100, two compounds optimized from a screen for nonsteroidal antiandrogens that retain activity in the setting of increased androgen receptor expression. Both compounds bind to the androgen receptor with greater relative affinity than the clinically used antiandrogen bicalutamide, reduce the efficiency of its nuclear translocation, and impair both DNA binding to androgen response elements and recruitment of coactivators. RD162 and MDV3100 are orally available and induce tumor regression in mouse models of castration-resistant human prostate cancer. Of the first 30 patients treated with MDV3100 in a Phase I/II clinical trial, 13 of 30 (43%) showed sustained declines (by >50%) in serum concentrations of prostate-specific antigen, a biomarker of prostate cancer. These compounds thus appear to be promising candidates for treatment of advanced prostate cancer.
2,046 citations
"Alternatively spliced androgen rece..." refers background in this paper
...MDV3100 is a non-steroidal antiandrogen that can inhibit AR even under conditions of protein overexpression R187 Downloaded from Bioscientifica.com at 12/21/2018 02:37:08PM via free access (Tran et al. 2009)....
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TL;DR: A historical perspective on a body of steroid receptor research dealing with the structure and physiological significance of the untransformed 9S receptor is provided, and it is shown that hsp90 itself exists in a variety of native multiprotein heterocomplexes independent of steroid receptors and other 'substrate' proteins.
Abstract: We have provided a historical perspective on a body of steroid receptor research dealing with the structure and physiological significance of the untransformed 9S receptor that has often confused both novice and expert investigators. The frequent controversies and equivocations of earlier studies were due to the fact that the native, hormone-free state of these receptors is a large multiprotein complex that resisted description for many years because of its unstable and dynamic nature. The untransformed 9S state of the steroid and dioxin receptors has provided a unique system for studying the function of the ubiquitous, abundant, and conserved heat shock protein, hsp90. The hormonal control of receptor association with hsp90 provided a method of manipulating the receptor heterocomplex in a manner that was physiologically meaningful. For several steroid receptors, binding to hsp90 was required for the receptor to be in a native hormone-binding state, and for all of the receptors, hormone binding promoted dissociation of the receptor from hsp90 and conversion of the receptor to the DNA-binding state. Although the complexes between tyrosine kinases and hsp90 were discovered earlier, the hormonal regulation or steroid receptor association with hsp90 permitted much more rapid and facile study of hsp90 function. The observations that hsp90 binds to the receptors through their HBDs and that these domains can be fused to structurally different proteins bringing their function under hormonal control provided a powerful linkage between the hormonal regulation of receptor binding to hsp90 and the initial step in steroid hormone action. Because the 9S receptor hsp90 heterocomplexes could be physically stabilized by molybdate, their protein composition could be readily studied, and it became clear that these complexes are multiprotein structures containing a number of unique proteins, such as FKBP51, FKBP52, CyP-40, and p23, that were discovered because of their presence in these structures. Further analysis showed that hsp90 itself exists in a variety of native multiprotein heterocomplexes independent of steroid receptors and other 'substrate' proteins. Cell-free systems can now be used to study the formation of receptor heterocomplexes. As we outlined in the scheme of Fig. 1, the multicomponent receptor-hsp90 heterocomplex assembly system is being reconstituted, and the importance of individual proteins, such as hsp70, p60, and p23, in the assembly process is becoming recognized. It should be noted that our understanding of the mechanism and purpose of steroid receptor heterocomplex assembly is still at an early stage. We can now speculate on the roles of receptor-associated proteins in receptor action, both as individuals and as a group, but their actual functions are still vague or unknown. We can make realistic models about the chaperoning and trafficking of steroid receptors, but we don't yet know how these processes occur, we don't know where chaperoning occurs in the cell (e.g. Is it limited to the cytoplasm? Is it a diffuse process or does chaperoning occur in association with structural elements?), and, with the exception of the requirement for hormone binding, we don't know the extent to which the hsp90-based chaperone system impacts on steroid hormone action. It is not yet clear how far the discovery of this hsp90 heterocomplex assembly system will be extended to the development of a general understanding of protein processing in the cell. Because this assembly system is apparently present in all eukaryotic cells, it probably performs an essential function for many proteins. The bacterial homolog of hsp90 is not an essential protein, but hsp90 is essential in eukaryotes, and recent studies indicate that the development of the cell nucleus from prokaryotic progenitors was accompanied by the duplication of genes for hsp90 and hsp70 (698). (ABSTRACT TRUNCATED)
1,829 citations
"Alternatively spliced androgen rece..." refers background in this paper
...However, AR45 was also demonstrated to harbor transcriptional activity under conditions of overexpression of the AR co-activators b-catenin or TIF-2 (Ahrens-Fath et al. 2005), which is a phenomenon that has been demonstrated for DBD/CTD fragments derived from the AR in other studies (Alen et al. 1999, He et al. 1999)....
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...It is also unclear whether AR45 mRNA is translated into a protein in these tissues, although western blot analysis of LNCaP lysates demonstrated the presence of a w45 kDa species that was immunoreactive with an antibody specific for the AR CTD. Functionally, ectopically expressed AR45 protein was shown to bind androgen, localize to the nucleus, interact with the full-length AR NTD, and inhibit full-length AR activity in a ligand- and DBDdependent manner....
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...Exons 4–8 encode a short flexible hinge region and 11 a-helices folded in an a-helical sandwich to form the well-characterized AR COOH-terminal domain (CTD), which harbors the AR ligand-binding domain (LBD) and transcriptional activation function 2 (AF2) co-regulator binding interface (Matias et al. 2000, Sack et al. 2001, He et al. 2004, Hur et al. 2004, Estebanez-Perpina et al. 2005, Bain et al. 2006)....
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...…version via http://www.endocrinology-journals.org Downloaded from Bioscientifica.com at 12/21/2018 02:37:08PM via free access is engaged with Hsp90, other molecular chaperones, and high-molecular-weight immunophilins by virtue of interaction with the AR CTD (Pratt & Toft 1997, Smith & Toft 2008)....
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...AR protein in the ligand-free state is localized in the cytoplasm, where it t Britain DOI: 10.1530/ERC-11-0141 Online version via http://www.endocrinology-journals.org Downloaded from Bioscientifica.com at 12/21/2018 02:37:08PM via free access is engaged with Hsp90, other molecular chaperones, and high-molecular-weight immunophilins by virtue of interaction with the AR CTD (Pratt & Toft 1997, Smith & Toft 2008)....
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TL;DR: AR remains important in the development and progression of prostate cancer and the inhibition of AR activity through mechanisms in addition to androgen ablation, such as modulation of signal transduction pathways, may delay prostate cancer progression.
Abstract: The normal development and maintenance of the prostate is dependent on androgen acting through the androgen receptor (AR). AR remains important in the development and progression of prostate cancer. AR expression is maintained throughout prostate cancer progression, and the majority of androgen-independent or hormone refractory prostate cancers express AR. Mutation of AR, especially mutations that result in a relaxation of AR ligand specificity, may contribute to the progression of prostate cancer and the failure of endocrine therapy by allowing AR transcriptional activation in response to antiandrogens or other endogenous hormones. Similarly, alterations in the relative expression of AR coregulators have been found to occur with prostate cancer progression and may contribute to differences in AR ligand specificity or transcriptional activity. Prostate cancer progression is also associated with increased growth factor production and an altered response to growth factors by prostate cancer cells. The kinase signal transduction cascades initiated by mitogenic growth factors modulate the transcriptional activity of AR and the interaction between AR and AR coactivators. The inhibition of AR activity through mechanisms in addition to androgen ablation, such as modulation of signal transduction pathways, may delay prostate cancer progression.
1,569 citations
TL;DR: Novel forms of AR alteration that are prevalent in HRPC are reported, suggesting a novel mechanism for the development of HRPC that warrants further investigation and may be explored as potential biomarkers and therapeutic targets for advanced PCa.
Abstract: Suppression of androgen production and function provides palliation but not cure in men with prostate cancer (PCa). Therapeutic failure and progression to hormone-refractory PCa (HRPC) are often accompanied by molecular alterations involving the androgen receptor (AR). In this study, we report novel forms of AR alteration that are prevalent in HRPC. Through in silico sequence analysis and subsequent experimental validation studies, we uncovered seven AR variant transcripts lacking the reading frames for the ligand-binding domain due to splicing of "intronic" cryptic exons to the upstream exons encoding the AR DNA-binding domain. We focused on the two most abundantly expressed variants, AR-V1 and AR-V7, for more detailed analysis. AR-V1 and AR-V7 mRNA showed an average 20-fold higher expression in HRPC (n = 25) when compared with hormone-naive PCa (n = 82; P < 0.0001). Among the hormone-naive PCa, higher expression of AR-V7 predicted biochemical recurrence following surgical treatment (P = 0.012). Polyclonal antibodies specific to AR-V7 detected the AR-V7 protein frequently in HRPC specimens but rarely in hormone-naive PCa specimens. AR-V7 was localized in the nuclei of cultured PCa cells under androgen-depleted conditions, and constitutively active in driving the expression of canonical androgen-responsive genes, as revealed by both AR reporter assays and expression microarray analysis. These results suggest a novel mechanism for the development of HRPC that warrants further investigation. In addition, as expression markers for lethal PCa, these novel AR variants may be explored as potential biomarkers and therapeutic targets for advanced PCa.
1,028 citations
"Alternatively spliced androgen rece..." refers background in this paper
...2009) AR-V4 (Hu et al. 2009) AR 1/2/2b (Dehm et al....
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...For example, one transcript was reported with exon 2b (referred to as exon CE4 in the study) spliced between exons 2 and 3, with exon CE1 at the 3 0 terminus (Hu et al. 2009)....
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...Two AR mRNA species composed of contiguously spliced AR exons 1/2/3/CE1 (termed AR-V1) and AR exons 1/2/3/3/CE1 (termed AR-V2) were demonstrated to be expressed in 22Rv1 cells following the bioinformatic identification of AR exon CE1 as an expressed sequence tag mapping to AR intron 3 (Hu et al. 2009)....
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...Two separate AR mRNA species composed of contiguously spliced AR exons 1/2/3/CE2 have been identified, with different locations for the splice acceptor site used for incorporation of exon CE2 into these mRNAs (Hu et al. 2009)....
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...…of LNCaP cells transfected with an expression vector encoding AR 1/2/3/CE3 demonstrated that the ligand-independent transcriptional activity of this isoform could largely recapitulate the gene expression program activated by ligand-induced full-length AR, including the PSA gene (Hu et al. 2009)....
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