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Project description:Prostate cancer growth depends on androgen receptor signaling. Androgen ablation therapy induces expression of constitutively active androgen receptor splice variants that drive disease progression. Taxanes are a standard of care therapy in castration-resistant prostate cancer (CRPC); however, mechanisms underlying the clinical activity of taxanes are poorly understood. Recent work suggests that the microtubule network of prostate cells is critical for androgen receptor nuclear translocation and activity. In this study, we used a set of androgen receptor deletion mutants to identify the microtubule-binding domain of the androgen receptor, which encompasses the DNA binding domain plus hinge region. We report that two clinically relevant androgen receptor splice variants, ARv567 and ARv7, differentially associate with microtubules and dynein motor protein, thereby resulting in differential taxane sensitivity in vitro and in vivo. ARv7, which lacks the hinge region, did not co-sediment with microtubules or coprecipitate with dynein motor protein, unlike ARv567. Mechanistic investigations revealed that the nuclear accumulation and transcriptional activity of ARv7 was unaffected by taxane treatment. In contrast, the microtubule-interacting splice variant ARv567 was sensitive to taxane-induced microtubule stabilization. In ARv567-expressing LuCap86.2 tumor xenografts, docetaxel treatment was highly efficacious, whereas ARv7-expressing LuCap23.1 tumor xenografts displayed docetaxel resistance. Our results suggest that androgen receptor variants that accumulate in CRPC cells utilize distinct pathways of nuclear import that affect the antitumor efficacy of taxanes, suggesting a mechanistic rationale to customize treatments for patients with CRPC, which might improve outcomes.
| S-EPMC4012562 | biostudies-literature
Project description:Mechanisms mediating androgen receptor (AR) reactivation in prostate cancer that progresses after castration (castration-resistant prostate cancer; CRPC) and subsequent treatment with abiraterone (CYP17A1 inhibitor that further suppresses androgen synthesis) remain unclear.Prostate cancer xenografts were examined to identify mechanism of progression after castration and abiraterone.AR reactivation in abiraterone-resistant VCaP xenografts was not associated with restoration of intratumoral androgens or alterations in AR coregulators. In contrast, mRNA encoding full-length AR (AR-FL) and a constitutively active splice variant (AR-V7) were increased compared with xenografts before castration, with an increase in AR-V7 relative to AR-FL. This shift toward AR-V7 was due to a feedback mechanism whereby the androgen-liganded AR stimulates expression of proteins that suppress generation of AR-V7 relative to AR-FL transcripts. However, despite the increases in AR-V7 mRNA, it remained a minor transcript (<1%) relative to AR-FL in resistant VCaP xenografts and CRPC clinical samples. AR-V7 protein expression was similarly low relative to AR-FL in castration-resistant VCaP xenografts and androgen-deprived VCaP cells, but the weak basal AR activity in these latter cells was further repressed by AR-V7 siRNA.AR-V7 at these low levels is not adequate to restore AR activity, but its rapid induction after androgen deprivation allows tumors to retain basal AR activity that may be needed for survival until more potent mechanisms emerge to activate AR. Agents targeting AR splice variants may be most effective when used very early in conjunction with therapies targeting the AR ligand-binding domain.
| S-EPMC4022291 | biostudies-literature
Project description:Therapeutic interventions for advanced prostate cancer (PCa) center on inhibiting androgen receptor (AR) and downstream signaling pathways. Resistance to androgen deprivation therapy and/or AR antagonists is inevitable and molecular mechanisms driving castration-resistant PCa (CR-PCa) primarily involve alterations in AR expression and activity. Detailed molecular biology work over the past decade, discussed at length in this review article, has revealed several AR transcripts that result from alternative splicing. These AR splice variants are increased in cell and mouse models of CR-PCa and in CR-PCa tumors. Several AR variants lack the ligand binding domain, but retain their ability to bind DNA and activate transcription-linking constitutive AR function and therapeutic failure. ARV7 is the only variant endogenously detected at the protein level and thus has undergone more thorough molecular characterization. Clinical trials in PCa are currently investigating ARV7 utility as a biomarker and new therapeutics that inhibit ARV7 . Overall, this review will illustrate the historical perspectives of AR splice variant discovery using fundamental molecular biology techniques and how it changed the clinical approach to both therapeutic decisions and strategy. The body of work investigating AR splice variants in PCa represents a true example of translational research from bench to bedside.
| S-EPMC5392349 | biostudies-literature
Project description:Androgen receptor (AR) splice variants lacking the ligand binding domain (ARVs), originally isolated from prostate cancer cell lines derived from a single patient, are detected in normal and malignant human prostate tissue, with the highest levels observed in late stage, castration-resistant prostate cancer. The most studied variant (called AR-V7 or AR3) activates AR reporter genes in the absence of ligand and therefore, could play a role in castration resistance. To explore the range of potential ARVs, we screened additional human and murine prostate cancer models using conventional and next generation sequencing technologies and detected several structurally diverse AR isoforms. Some, like AR-V7/AR3, display gain of function, whereas others have dominant interfering activity. We also find that ARV expression increases acutely in response to androgen withdrawal, is suppressed by testosterone, and in some models, is coupled to full-length AR (AR-FL) mRNA production. As expected, constitutively active, ligand-independent ARVs such as AR-V7/AR3 are sufficient to confer anchorage-independent (in vitro) and castration-resistant (in vivo) growth. Surprisingly, this growth is blocked by ligand binding domain-targeted antiandrogens, such as MDV3100, or by selective siRNA silencing of AR-FL, indicating that the growth-promoting effects of ARVs are mediated through AR-FL. These data indicate that the increase in ARV expression in castrate-resistant prostate cancer is an acute response to castration rather than clonal expansion of castration or antiandrogen-resistant cells expressing gain of function ARVs, and furthermore, they provide a strategy to overcome ARV function in the clinic.
| S-EPMC2947883 | biostudies-other
Project description: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.
| S-EPMC4308035 | biostudies-literature
Project description:In 2015, as part of the Prostate Cancer Foundation-Movember Foundation Reproducibility Initiative, we published a Registered Report (Shan et al., 2015) that described how we intended to replicate selected experiments from the paper "Androgen Receptor Splice Variants Determine Taxane Sensitivity in Prostate Cancer" (Thadani-Mulero et al., 2014). Here we report the results of those experiments. Growth of tumor xenografts from two prostate cancer xenograft lines, LuCaP 86.2, which expresses wild-type androgen receptor (AR) and AR variant 567, and LuCaP 23.1, which expresses wild-type AR and AR variant 7, were not affected by docetaxel treatment. The LuCaP 23.1 tumor xenografts grew slower than in the original study. This result is different from the original study, which reported significant reduction of tumor growth in the LuCaP 86.2. Furthermore, we were unable to detect ARv7 in the LuCaP 23.1, although we used the antibody as stated in the original study and ensured that it was detecting ARv7 via a known positive control (22rv1, Hörnberg et al., 2011). Finally, we report a meta-analysis of the result.
| S-EPMC5907780 | biostudies-literature
Project description:The Prostate Cancer Foundation-Movember Foundation Reproducibility Initiative seeks to address growing concerns about reproducibility in scientific research by conducting replications of recent papers in the field of prostate cancer. This Registered Report describes the proposed replication plan of key experiments from "Androgen Receptor Splice Variants Determine Taxane Sensitivity in Prostate Cancer" by Thadani-Mulero and colleagues (2014) published in Cancer Research in 2014. The experiment that will be replicated is reported in Fig. 6A. Thadani-Mulero and colleagues generated xenografts from two prostate cancer cell lines; LuCaP 86.2, which expresses predominantly the ARv567 splice variant of the androgen receptor (AR), and LuCaP 23.1, which expresses the full length AR as well as the ARv7 variant. Treatment of the tumors with the taxane docetaxel showed that the drug inhibited tumor growth of the LuCaP 86.2 cells but not of the LuCaP 23.1 cells, indicating that expression of splice variants of the AR can affect sensitivity to docetaxel. The Prostate Cancer Foundation-Movember Foundation Reproducibility Initiative is a collaboration between the Prostate Cancer Foundation, the Movember Foundation and Science Exchange, and the results of the replications will be published by PeerJ.
| S-EPMC4579034 | biostudies-literature
Project description:Persistent androgen receptor (AR) transcriptional activity underlies resistance to AR-targeted therapy and progression to lethal castration-resistant prostate cancer (CRPC). Recent success in retargeting 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-V) that lack the AR ligand-binding domain. In this study, we show 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 showed 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.
| S-EPMC3549016 | biostudies-literature
Project description:Androgen receptor (AR) variants are associated with resistance to anti androgen therapy both in human prostate cancer cell lines and clinical samples. These observations support the hypothesis that AR isoform accumulation is a consequence of selective therapeutic pressure on the full length AR. The Pten deficient prostate cancer model proceeds with well-defined kinetics including progression to castration resistant prostate cancer (CRPC). While surgical castration and enzalutamide treatments yield an initial therapeutic response, Pten-/-epithelia continue to proliferate yielding locally invasive primary tumor pathology. That most epithelium remains AR positive, but ligand independent, suggests the presence of oncogenic AR variants. To address this hypothesis, we have used a panel of recently described Pten-/- tumor cell lines derived from both from hormone intact (E4, E8) and castrated Pten mutants (cE1, cE2) followed by RACE PCR to identify and characterize three novel truncated, amino terminus containing AR variants (mAR-Va, b, c). Variants appear not only conserved throughout progression but are correlated with nearly complete loss of full length AR (AR-FL) at castrate androgen levels. The overexpression of variants leads to enhanced transcriptional activity of AR while knock down studies show reduced transcriptional output. Collectively, the identification of truncated AR variants in the conditional PTEN deletion model supports a role for maintaining the CRPC phenotype and provides further therapeutic applications of this preclinical model.
| S-EPMC4510390 | biostudies-literature
Project description:Synthesis of truncated androgen receptor (AR) splice variants has emerged as an important mechanism of prostate cancer (PCa) resistance to AR-targeted therapy and progression to a lethal castration-resistant phenotype. However, the precise role of these factors at this stage of the disease is not clear due to loss of multiple COOH-terminal AR protein domains, including the canonical nuclear localization signal (NLS) in the AR hinge region. Despite loss of this NLS, we show that diverse truncated AR variant species have a basal level of nuclear localization sufficient for ligand-independent transcriptional activity. Whereas full-length AR requires Hsp90 and importin-? for active nuclear translocation, basal nuclear localization of truncated AR variants is independent of these classical signals. For a subset of truncated AR variants, this basal level of nuclear import can be augmented by unique COOH-terminal sequences that reconstitute classical AR NLS activity. However, this property is separable from ligand-independent transcriptional activity. Therefore, the AR splice variant core consisting of the AR NH(2)-terminal domain and DNA binding domain is sufficient for nuclear localization and androgen-independent transcriptional activation of endogenous AR target genes. Indeed, we show that truncated AR variants with nuclear as well as nuclear/cytoplasmic localization patterns can drive androgen-independent growth of PCa cells. Together, our data demonstrate that diverse truncated AR species with varying efficiencies of nuclear localization can contribute to castration-resistant PCa pathology by driving persistent ligand-independent AR transcriptional activity.
| S-EPMC3366007 | biostudies-other