AR-Q640X, a model to study the effects of constitutively active C-terminally truncated AR variants in prostate cancer cells

A compendium of Androgen Receptor Variant 7 target genes and their role in Castration Resistant Prostate Cancer

Persistent androgen receptor (AR) signalling is the main driver of prostate cancer (PCa). Truncated isoforms of the AR called androgen receptor variants (AR-Vs) lacking the ligand binding domain often emerge during treatment resistance against AR pathway inhibitors such as Enzalutamide. This review discusses how AR-Vs drive a more aggressive form of PCa through the regulation of some of their target genes involved in oncogenic pathways, enabling disease progression. There is a pressing need for the development of a new generation of AR inhibitors which can repress the activity of both the full-length AR and AR-Vs, for which the knowledge of differentially expressed target genes will allow evaluation of inhibition efficacy. This review provides a detailed account of the most common variant, AR-V7, the AR-V7 regulated genes which have been experimentally validated, endeavours to understand their relevance in aggressive AR-V driven PCa and discusses the utility of the downstream protein products as potential drug targets for PCa treatment.

[The impact of the androgen receptor splice variant AR-V7 on the prognosis and treatment of advanced prostate cancer]

A recently discovered mechanism enabling prostate cancer cells to escape the effects of endocrine therapies consists in the synthesis of C-terminally truncated, constitutively active androgen receptor (AR) splice variants (AR-V). Devoid of a functional C-terminal hormone/ligand binding domain, various AR-Vs are insensitive to therapies targeting the androgen/AR signalling axis. Preliminary studies suggest that AR-V7, the most common AR-V, is a promising predictive tumour marker and a relevant selection marker for the treatment of advanced prostate cancer. This review critically outlines recent advances in AR-V7 diagnostics and presents an overview of current AR-V7 targeted therapies.

Antibody selection influences the detection of AR-V7 in primary prostate cancer

BACKGROUND

The androgen receptor (AR) splice variant V7 (AR-V7) is an emerging marker to aid clinical decision-making in patients with castration-resistant prostate cancer (CRPC). A number of studies have shown that a subset of patients also express AR-V7 in the primary tumor. These findings have recently been challenged by a study showing that AR-V7 becomes only detectable in CRPC but is virtually absent in castration-naïve prostate cancer.

METHODS

Herein, we directly compare the two relevant antibodies used for the immunodetection of AR-V7 in the conflicting studies (clones AG10008 and RM7) in a predominantly high-risk prostate cancer patient cohort with primary tumor specimens assembled in a tissue microarray (TMA).

RESULTS

The overall rate of AR-V7 positive TMA cores was comparable (AG10008, 24.9%; RM7, 21%). However, the percentage agreement of identical staining intensities of positive cores was only 7%. In contrast, the percentage agreement of negative cores was 62.8%. In approximately 30% of the cores, the antibodies produced discordant staining intensities. Only one of the two antibody stainings (AG10008) conveyed prognostic information and was associated with a shorter progression-free patient survival.

CONCLUSIONS

Our study underscores that nuclear AR-V7 expression can be detected in primary prostate cancer prior to long-term androgen deprivation and castration resistance. There are staining differences between the two antibodies in tumor tissue, for which we currently have no explanation. Clearly, improvements in the detection of functional AR-V7 in prostate cancer are urgently needed.

Quantitative Evaluation of the Transcriptional Activity of Steroid Hormone Receptor Mutants and Variants Using a Single Vector With Two Reporters and a Receptor Expression Cassette

Although the rapid development of high-throughput sequencing has led to the identification of a large number of truncated or mutated steroid hormone receptor (SHR) variants, their clinical relevance remains to be defined. A platform for functional analysis of these SHR variants in cells would be instrumental for better assessing their impact on normal physiology and SHR-associated diseases. Here we have developed a new reporter system that allows rapid and accurate assessment of the transcriptional activity of SHR variants in cells. The reporter is a single construct containing a firefly luciferase reporter gene, whose expression is under the control of a promoter with multiple steroid hormone responsive elements, and a Renilla luciferase reporter gene, that is constitutively expressed under the control of an internal ribosome entry site (IRES) and is not regulated by steroid hormones. The corresponding SHR (wildtype or mutant/variant) is also expressed from the same construct. Using this improved reporter system, we revealed a large spectrum of transactivation activities within a set of previously identified mutations and variations of the androgen receptor (AR), the estrogen receptor α (ERα) and the glucocorticoid receptor (GR). This novel reporter system enables functional analysis of SHR mutants and variants in physiological and pathological settings, offering valuable preclinical, or diagnostic information for the understanding and treatment of associated diseases.

Role of androgen receptor splice variants, their clinical relevance and treatment options

PURPOSE

In this review, we summarize the importance of AR variants with a particular focus on clinically relevant members of this family.

METHODS

A non-systematic literature review was performed based on Medline and PubMed.

RESULTS

Endocrine therapy represents the central paradigm for the management of prostate cancer. Eventually, in response to androgen ablation therapy, several resistance mechanisms against the endocrine therapy might develop that can circumvent the therapy approaches. One specific resistance mechanism that has gained increasing attention is the generation of alternatively spliced variants of the androgen receptor, with AR-V7 being the most prominent. More broadly, AR-V7 is one member of a group of alternatively spliced AR variants that share a common feature, the missing ligand-binding domain. These ΔLBD androgen receptor variants have shown the capability to induce androgen receptor-mediated gene transcription even under conditions of androgen deprivation and to drive cancer progression.

CONCLUSION

The methods used for detecting AR-Vs, at least on the mRNA level, are well-advanced and harbor the potential to be introduced into clinical diagnostics. It is important to note, that the testing, especially of AR-V7 has its limitations in predicting treatment response. More promising is the great number of active clinical trials aimed at reducing the AR-Vs, and using this to re-sensitize CRPC towards endocrine treatment might provide additional treatment options for CRPC patients in the future.

The BET-inhibitor PFI-1 diminishes AR/AR-V7 signaling in prostate cancer cells

OBJECTIVE

The bromodomain and extra-terminal (BET) family of proteins provides a scaffolding platform for the recruitment and tethering of transcription factors to acetylated chromatin, thereby modulating gene expression. In this study, we evaluated the efficacy of the BET-inhibitor PFI-1 to diminish AR/AR-V7 signaling and proliferation in castration-resistant prostate cancer cells.

METHODS

Prostate-specific antigen and androgen receptor (AR) protein were quantified by means of two commercial ELISAs. Transactivation of the AR, AR-V7 and Q641X was determined by reporter gene assays. Cell proliferation was measured using a colorimetric MTT-assay.

RESULTS

PFI-1 dose-dependently inhibited transactivation of full-length AR (non- mutated, i.e., wild-type or point-mutated/promiscuous forms) without affecting their cellular protein levels. Moreover, PFI-1 was active against C-terminally truncated constitutively active ARs like AR-V7 and Q641X. Prostate cancer cells exhibiting a transcriptionally active AR-signaling complex (LNCaP, 22Rv1) were more susceptible to the growth-inhibitory effects than the AR-negative PC-3 cells.

CONCLUSION

The quinazolinone PFI-1 is a highly efficient inhibitor of AR-signaling-competent prostate cancer cells in vitro. PFI-1 could serve as a lead compound for the development of new therapeutics able to block AR/AR-V7 signaling in advanced prostate cancer.

Dissecting the roles of the androgen receptor in prostate cancer from molecular perspectives

Androgen receptor plays a pivotal role in prostate cancer progression, and androgen deprivation therapy to intercept androgen receptor signal pathway is an indispensable treatment for most advanced prostate cancer patients to delay cancer progression. However, the emerging of castration-resistant prostate cancer reminds us the alteration of androgen receptor, which includes androgen receptor mutation, the formation of androgen receptor variants, and androgen receptor distribution in cancer cells. In this review, we introduce the process of androgen receptor and also its variants' formation, translocation, and function alteration by protein modification or interaction with other pathways. We dissect the roles of androgen receptor in prostate cancer from molecular perspective to provide clues for battling prostate cancer, especially castration-resistant prostate cancer.

C-terminally truncated constitutively active androgen receptor variants and their biologic and clinical significance in castration-resistant prostate cancer

A mechanism allowing castration resistant prostate cancer cells to escape the effects of conventional anti-hormonal treatments is the synthesis of constitutively active, C-terminally truncated androgen receptor (AR)-variants. Lacking the entire or vast parts of the ligand binding domain, the intended target of traditional endocrine therapies, these AR-variants (termed ARΔLBD) are insensitive to all traditional treatments including second generation compounds like abiraterone, enzalutamide or ARN-509. Although ARΔLBD are predominantly products of alternative splicing, they can also be products of nonsense mutations or proteolytic cleavage. In this review, we will discuss the etiology and function of c-terminally truncated AR-variants and their clinical significance as markers/targets for the treatment of castration resistant prostate cancer.

Inhibition of IGF-1R diminishes transcriptional activity of the androgen receptor and its constitutively active, C-terminally truncated counterparts Q640X and AR-V7

PURPOSE

Failure of endocrine treatment in castration-resistant prostate cancer (CRPC) is often associated with the emergence of C-terminally truncated androgen receptor variants that function as constitutively active transcription factors (i.e., AR∆LBD). The mechanisms involved in the regulation of AR∆LBD signaling are largely unknown. Since the IGF-1 pathway was repeatedly shown to affect AR function, we studied whether an inhibition of IGF-1R could also affect AR∆LBD signaling.

METHODS

Regulation of androgen receptor (AR) and AR∆LBD signaling was analyzed by reporter gene assays, immunoblotting, ELISA and quantitative RT-PCR.

RESULTS

Inhibition of IGF-1R with the small-molecule inhibitor NVP-AEW541 reduced the transcriptional activity of the AR and its truncated counterparts Q640X and AR-V7. As shown in Q640X, the inhibition of transcriptional activity was paralleled by a decreased receptor phosphorylation.

CONCLUSIONS

Inhibition of IGF-1R leads to a down-regulation of AR∆LBD signaling and provides a rationale for CRPC therapies targeting growth factor receptors.

Therapy escape mechanisms in the malignant prostate

Androgen receptor (AR) is the main target for prostate cancer therapy. Clinical approaches for AR inactivation include chemical castration, inhibition of androgen synthesis and AR antagonists (anti-androgens). However, treatment resistance occurs for which an important number of therapy escape mechanisms have been identified. Herein, we summarise the current knowledge of molecular mechanisms underlying therapy resistance in prostate cancer. Moreover, the tumour escape mechanisms are arranged into the concepts of target modification, bypass signalling, histologic transformation, cancer stem cells and miscellaneous mechanisms. This may help researchers to compare and understand same or similar concepts of therapy resistance in prostate cancer and other cancer types.

Androgen receptor aberrations in the era of abiraterone and enzalutamide

Prostate cancer is the most prevalent non-skin cancer and the second leading cause of cancer death in men of the western world. As growth and differentiation of prostate cancer largely depend on androgens, inhibition of the androgen/androgen receptor signaling axis is the main treatment for locally advanced and/or metastatic tumors. Although first-line androgen deprivation therapies like chemical/surgical castration and/or administration of anti-androgens are able to control the disease for several years, prostate cancer almost invariably recurs as castration-resistant prostate cancer. This stage of the disease is characterized by a sustained AR-signaling despite castrate levels of circulating androgens. Various molecular mechanisms were shown to induce castration resistance. This review will discuss the most recent and relevant experimental findings on AR-signaling in castration-resistant prostate cancer in order to provide a comprehensive interpretation of the clinical behavior of this tumor entity following treatments with abiraterone, enzalutamide, ARN-509 or taxanes.

Are androgen receptor variants a substitute for the full-length receptor?

Androgen receptor splice variants (AR-Vs)--which are expressed in castration-resistant prostate cancer (CRPC) cell lines and clinical samples--lack the C-terminal ligand-binding domain and are constitutively active. AR-Vs are, therefore, resistant to traditional androgen deprivation therapy (ADT). AR-Vs are induced by several mechanisms, including ADT, and might contribute to the progression of CRPC and resistance to ADT. AR-Vs could represent a novel therapeutic target for prostate cancer, especially in CRPC.

Stilbene induced inhibition of androgen receptor dimerization: implications for AR and ARΔLBD-signalling in human prostate cancer cells

Background

Advanced castration resistant prostate cancer (CRPC) is often characterized by an increase of C-terminally truncated, constitutively active androgen receptor (AR) variants. Due to the absence of a ligand binding domain located in the AR-C-terminus, these receptor variants (also termed ARΔLBD) are unable to respond to all classical forms of endocrine treatments like surgical/chemical castration and/or application of anti-androgens.

Methodology

In this study we tested the effects of the naturally occurring stilbene resveratrol (RSV) and (E)-4-(2, 6-Difluorostyryl)-N, N-dimethylaniline, a fluorinated dialkylaminostilbene (FIDAS) on AR- and ARΔLBD in prostate cancer cells. The ability of the compounds to modulate transcriptional activity of AR and the ARΔLBD-variant Q640X was shown by reporter gene assays. Expression of endogenous AR and ARΔLBD mRNA and protein levels were determined by qRT-PCR and Western Blot. Nuclear translocation of AR-molecules was analyzed by fluorescence microscopy. AR and ARΔLBD/Q640X homo-/heterodimer formation was assessed by mammalian two hybrid assays. Biological activity of both compounds in vivo was demonstrated using a chick chorioallantoic membrane xenograft assay.

Results

The stilbenes RSV and FIDAS were able to significantly diminish AR and Q640X-signalling. Successful inhibition of the Q640X suggests that RSV and FIDAS are not interfering with the AR-ligand binding domain like all currently available anti-hormonal drugs. Repression of AR and Q640X-signalling by RSV and FIDAS in prostate cancer cells was caused by an inhibition of the AR and/or Q640X-dimerization. Although systemic bioavailability of both stilbenes is very low, both compounds were also able to downregulate tumor growth and AR-signalling in vivo.

Conclusion

RSV and FIDAS are able to inhibit the dimerization of AR and ARΔLBD molecules suggesting that stilbenes might serve as lead compounds for a novel generation of AR-inhibitors.

On the origins of the androgen receptor low molecular weight species

Prostate cancer (CaP), a commonly diagnosed malignancy, is readily treated by androgen ablation. This treatment temporarily halts the disease, but castration-resistant neoplasms that are refractory to current therapies emerge. While these neoplasms are no longer dependent on physiological levels of androgens, they remain reliant on the expression of the androgen receptor (AR). There are multiple mechanisms by which CaP cells circumvent androgen ablation therapies. These include AR mutations that broaden ligand specificity, AR overexpression, AR activation by growth factors and cytokines, overexpression of AR co-activators, altered steroid metabolism, and a locus-wide histone transcriptional activation of some AR targets. This review focuses on a more recently described mechanism: the expression of low molecular weight AR species that are missing the ligand-binding domain and function independently of ligand to drive proliferation. The etiology, biological activity, unique features, predictive value, and therapeutic implication of these androgen receptor isoforms are discussed in depth.

Enzalutamide in castration-resistant prostate cancer patients progressing after docetaxel and abiraterone

BACKGROUND

Abiraterone, an androgen synthesis inhibitor, has been successfully used in the treatment of castration-resistant prostate cancer (CRPC) for 2 yr. Enzalutamide is a second-generation nonsteroidal antiandrogen that has recently been approved for the same indication.

OBJECTIVE

This is the first study to evaluate the effectiveness of enzalutamide after failure of abiraterone.

DESIGN, SETTING, AND PARTICIPANTS

Thirty-five patients were identified as having received sequential therapy with abiraterone followed by enzalutamide. All patients had undergone prior docetaxel chemotherapy, and no patient had received ketoconazole.

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS

Posttreatment changes in prostate-specific antigen (PSA) were used to determine the activity of enzalutamide in patients who had received prior abiraterone.

RESULTS AND LIMITATIONS

The median duration of abiraterone treatment was 9.0 mo (range: 2.0-19.0 mo). Of the 35 patients, 16 (45.7%) achieved a >50% decline in PSA, and 14 (40%) had a rising PSA as the best response. The median duration of subsequent enzalutamide treatment was 4.9 mo (Kaplan-Meier estimate; 95% confidence interval [CI], 2.4-7.4). Seven of 16 CRPC patients who were initially abiraterone-sensitive (43.8%) and 3 of 19 CRPC patients who were initially abiraterone-insensitive (15.8%) showed a >50% PSA decline while taking enzalutamide. Of the 35 patients, 17 (48.6%) were primarily enzalutamide-resistant and showed a rising PSA as the best response. Median time to progression was 4.0 mo (95% CI, 2.0-6.0) for 18 of 35 patients with at least one declining PSA value while taking enzalutamide (51.4%). Of the 17 patients who were assessable radiologically, only 1 (2.9%) attained a confirmed partial response. Small sample size was the major limitation.

CONCLUSIONS

Enzalutamide treatment achieved only a modest response rate in patients progressing after abiraterone. Although cross-resistance between abiraterone and enzalutamide was a common phenomenon, it was not inevitable, and a small but significant number of patients showed significant benefit from sequential treatment.

Effects of sorafenib on C-terminally truncated androgen receptor variants in human prostate cancer cells

Recent evidence suggests that the development of castration resistant prostate cancer (CRPCa) is commonly associated with an aberrant, ligand-independent activation of the androgen receptor (AR). A putative mechanism allowing prostate cancer (PCa) cells to grow under low levels of androgens, is the expression of constitutively active, C-terminally truncated AR lacking the AR-ligand binding domain (LBD). Due to the absence of a LBD, these receptors, termed ARΔLBD, are unable to respond to any form of anti-hormonal therapies. In this study we demonstrate that the multikinase inhibitor sorafenib inhibits AR as well as ARΔLBD-signalling in CRPCa cells. This inhibition was paralleled by proteasomal degradation of the AR- and ARΔLBD-molecules. In line with these observations, maximal antiproliferative effects of sorafenib were achieved in AR and ARΔLBD-positive PCa cells. The present findings warrant further investigations on sorafenib as an option for the treatment of advanced AR-positive PCa.

Cistrome plasticity and mechanisms of cistrome reprogramming

Mammalian genomes contain thousands of cis-regulatory elements for each transcription factor (TF), but TFs only occupy a relatively small subset referred to as cistrome. Recent studies demonstrate that a TF cistrome might differ among different organisms, tissue types and individuals. In a cell, a TF cistrome might differ among different physiological states, pathological stages and between physiological and pathological conditions. It is, therefore, remarkable how highly plastic these binding profiles are, and how massively they can be reprogrammed in rapid response to intra/extracellular variations and during cell identity transitions and evolution. Biologically, cistrome reprogramming events tend to be followed by changes in transcriptional outputs, thus serving as transformative mechanisms to synchronically alter the biology of the cell. In this review, we discuss the molecular basis of cistrome plasticity and attempt to integrate the different mechanisms and biological conditions associated with cistrome reprogramming. Emerging data suggest that, when altered, these reprogramming events might be linked to tumor development and/or progression, which is a radical conceptual change in our mechanistic understanding of cancer and, potentially, other diseases.