Androgen receptor coregulators and their involvement in the development and progression of prostate cancer

A novel androgen receptor amino terminal region reveals two classes of amino/carboxyl interaction-deficient variants with divergent capacity to activate responsive sites in chromatin

The androgen receptor (AR) is an important signaling molecule in multiple tissues, yet its mode of action and cell-specific activities remain enigmatic. AR function has been best studied in the prostate, in which it is essential for growth and homeostasis of the normal organ as well as each stage of cancer development. Investigation of mechanisms responsible for continued AR action that evolve during prostate cancer progression or after hormonal management of the disease have been instructive in defining AR signaling pathways. In the current paper, we use sequence similarity and the collocation of somatic mutations in prostate cancer to define residues 501-535 of the AR amino-terminal domain as an important mediator of receptor function. Specifically, the 501-535 region is required for optimal interaction of the amino-terminal domain with both the p160 coactivator, nuclear receptor coactivator-2, and the AR-ligand binding domain in the amino/carboxyl (N/C) interaction. The N/C interaction is decreased by deletion of the 501-535 region but is distinct from deletion of the (23)FQNLF(27) peptide in that it does not affect the capacity of the AR to activate transcription from a chromatin integrated reporter or recruitment of the receptor to androgen-responsive loci in vivo. Collectively, we have been able to outline two classes of N/C-deficient AR variant that are divergent in their capacity to act in a chromatin context, thereby further defining the interplay between N/C interaction and coregulator recruitment via multiple receptor domains. These mechanisms are likely to be key determinants of the cell and promoter specific activities of the AR.

Pleiotropic functional properties of androgen receptor mutants in prostate cancer

The androgen receptor (AR) signaling pathway plays an important role during the development of the normal prostate gland, but also during the progression of prostate cancer on androgen ablation therapy. Mutations in the AR gene emerge to keep active the AR signaling pathway and to support prostate cancer cells growth and survival despite the low levels of circulating androgens. Indeed, mutations affecting the ligand binding domain (LBD) of the AR have been shown to generate so-called "promiscuous" receptors that present widened ligand specificity and allow the stimulation of these receptors by a larger spectrum of endogenous hormones. Another class of mutations, arising in the amino-terminal domain (NTD) of the receptor, modulate AR interactions with coregulators involved in cell proliferation regulation. Besides characteristics of these well-known types of mutations, the properties of other classes of AR mutants recently described in prostate cancer are currently under investigation. Most interestingly, in addition to their potential role in the mechanisms which allow prostate cancer cells to escape androgen ablation therapy, data suggest that certain AR mutations are present early in the natural history of the disease and may play a role in many aspects of prostate cancer progression. Surprisingly, singular truncated AR devoid of their carboxy-terminal end (CTE) region seem to exert specific paracrine effects and to induce a clonal cooperation with neighboring prostate cancer cells, which may facilitate both the invasion and metastasis processes. In this article, we review the functional properties of different classes of AR mutants and their potential impact on the natural history of prostate cancer. Hum Mutat 0, 1-14, 2008. (c) 2008 Wiley-Liss, Inc.

Androgen-induced coactivator ANCCA mediates specific androgen receptor signaling in prostate cancer

Androgen receptor (AR) plays a pivotal role in prostate cancer, primarily by regulating different gene expression programs elicited by androgen, which is important for cancer cell proliferation, survival, and differentiation. It is believed that the transcriptional function of AR is mediated largely by distinct nuclear coregulators. We report here the identification of ANCCA (also known as ATAD2), a new member of the AAA+ ATPase family proteins, as a novel AR coactivator. ANCCA interacts directly with AR and enhances its transcriptional activity, and is required for androgen-stimulated expression of a specific subgroup of genes including IGF1R, IRS-2, SGK1, and survivin. Upon androgen stimulation, ANCCA together with AR is recruited to the specific AR target genes. Suppression of ANCCA expression strongly inhibited the proliferation of androgen-responsive or androgen-independent, AR-positive prostate cancer cells and caused a significant increase of cellular apoptosis. Strikingly, the ANCCA gene itself, located at chromosome 8q24, is highly induced by androgen in androgen-dependent prostate cancer cells and xenograft tumors. Although ANCCA is hardly detected in normal human prostate tissue, high levels of ANCCA are found in hormone-independent prostate cancer cell lines, xenograft tumor, and a subset of prostate cancers with high Gleason scores. Together, these findings suggest that ANCCA plays an important role in prostate cancer by mediating specific AR functions in cancer cell survival and proliferation. The possession of ATPase and bromodomain by ANCCA makes it an attractive target for the development of therapeutics for the disease.

Deleted in breast cancer 1, a novel androgen receptor (AR) coactivator that promotes AR DNA-binding activity

Androgen receptor (AR) plays a critical role in development and maintenance of male reproductive functions and the etiology of prostate cancer. As a ligand-regulated transcription factor, identification and characterization of AR coregulators are essential for understanding the molecular mechanisms underlying its diverse biological functions. Here we reported the identification of a novel AR coactivator, deleted in breast cancer 1 (DBC1), through a biochemical approach. DBC1 interacts with AR in a ligand-stimulated manner and facilitates AR transcriptional activation in transfected cells as well as in Xenopus oocytes. In in vitro gel shift experiments, recombinant DBC1 drastically enhanced AR DNA-binding activity. Expression of DBC1 also enhanced the binding of AR to chromatinized template in vivo, whereas knockdown of DBC1 impaired the binding of AR to endogenous prostate-specific antigen (PSA) gene in the prostate cancer cell line LNCaP. Thus, our data identify DBC1 as a novel AR coactivator.

Androgen modulation of coregulator expression in prostate cancer cells

Aberrant coregulator expression that occurs during prostate cancer (PCa) progression correlates with poor prognosis and aggressive disease. This has been attributed to the ability to regulate androgen receptor-mediated transcription. We have shown previously that the androgenic milieu regulates the expression of the coactivators p300 and FHL2, with severe consequences for PCa cell proliferation and androgen receptor transcriptional activity. To determine the extent of androgen dependency of coregulator genes, we designed a cDNA-mediated annealing, selection, extension, and ligation RNA profiling array that probes the expression of 186 coregulators. Using this assay, we demonstrated androgen control over approximately 30% of coregulator genes in PCa cells. For a subset of 15 functionally diverse coregulators, androgen regulation was confirmed using real-time RT-PCR and immunoblotting. The extent, dose dependency, and kinetics by which androgens affect coregulator expression differed widely, indicating diverse molecular mechanisms underlying these effects. Moreover, differences in coregulator expression were observed between isogenic androgen-dependent and castration-recurrent PCa cells. Small interfering RNA-mediated changes in coregulator expression had profound effects on cell proliferation, which were most pronounced in castration-recurrent cells. Taken together, our integrated approach combining expression profiling, characterization of androgen-dependent coregulator expression, and validation of the importance of altered coregulator expression for cell proliferation identified several potential novel therapeutic targets for PCa treatment.

Inhibitory effect of matrine on the expression of PSA and AR in prostate cancer cell line LNCaP

In order to investigate the inhibitory effect of matrine on the expression of prostate specific antigen (PSA) and androgen receptor (AR) in prostate cancer cell line LNCaP in vitro, LNCaP cells were treated with matrine at different concentrations (0.5, 1.0, 1.5, 2.0 g/L) for 12-36 h. The growth activities of cancer cells were determined by MTT colorimetric assay. The AR level was measured by Western blotting. The expression of PSA was detected by using AXSYM system-chemical luciferase methods. The results showed that matrine could effectively inhibit the growth of androgen-dependent prostate cancer cell line LNCaP in vitro in a time- and dose-dependent manner (P<0.05). It could obviously decrease the level of AR (P<0.01) and inhibit the expression of PSA in a dose-dependent manner (P<0.05) in LNCaP cells. It was concluded that matrine could significantly suppress the growth of LNCaP cells and inhibit the expression of PSA and AR of prostate cancer cells.

Cyclin D1 repressor domain mediates proliferation and survival in prostate cancer

Regulation of the androgen receptor (AR) is critical to prostate cancer (PCa) development; therefore, AR is the first line therapeutic target for disseminated tumors. Cell cycle dependent accumulation of cyclin D1 negatively modulates the transcriptional regulation of the AR through discrete, CDK4-independent mechanisms. The transcriptional co-repressor function of cyclin D1 resides within a defined motif termed ther repressor domain (RD), and it was hypothesized that this motif could be utilized as a platform to develop new strategies for blocking AR function. Here, we demonstrate that expression of the RD peptide is sufficient to disrupt AR transcriptional activation of multiple, prostate-specific AR target genes. Importantly, these actions are sufficient to specifically inhibit S-phase progression in AR-positive PCa cells, but not in AR-negative cells or tested AR-positive cells of other lineages. As expected, impaired cell cycle progression resulted in a suppression of cell doubling. Additionally, cell death was observed in AR-positive cells that maintain androgen dependence and in a subset of castrate-resistant PCa cells, dependent on Akt activation status. Lastly, the ability of RD to cooperate with existing hormone therapies was examined, which revealed that RD enhanced the cellular response to an AR antagonist. Together, these data demonstrate that RD is sufficient to disrupt AR-dependent transcriptional and proliferative responses in PCa, and can enhance efficacy of AR antagonists, thus establishing the impetus for development of RD-based mimetics.

The RNA helicase p68 is a novel androgen receptor coactivator involved in splicing and is overexpressed in prostate cancer

The androgen receptor (AR) is a member of the nuclear steroid hormone receptor family and is thought to play an important role in the development of both androgen-dependent and androgen-independent prostatic malignancy. Elucidating roles by which cofactors regulate AR transcriptional activity may provide therapeutic advancement for prostate cancer (PCa). The DEAD box RNA helicase p68 (Ddx5) was identified as a novel AR-interacting protein by yeast two-hybrid screening, and we sought to examine the involvement of p68 in AR signaling and PCa. The p68-AR interaction was verified by colocalization of overexpressed protein by immunofluorescence and confirmed in vivo by coimmunoprecipitation in the PCa LNCaP cell line. Chromatin immunoprecipitation in the same cell line showed AR and p68 recruitment to the promoter region of the androgen-responsive prostate-specific antigen (PSA) gene. Luciferase reporter, minigene splicing assays, and RNA interference (RNAi) were used to examine a functional role of p68 in AR-regulated gene expression, whereby p68 targeted RNAi reduced AR-regulated PSA expression, and p68 enhanced AR-regulated repression of CD44 splicing (P = 0.008). Tyrosine phosphorylation of p68 was found to enhance coactivation of ligand-dependent transcription of AR-regulated luciferase reporters independent of ATP-binding. Finally, we observe increased frequency and expression of p68 in PCa compared with benign tissue using a comprehensive prostate tissue microarray (P = 0.003; P = 0.008). These findings implicate p68 as a novel AR transcriptional coactivator that is significantly overexpressed in PCa with a possible role in progression to hormone-refractory disease.

Androgen levels increase by intratumoral de novo steroidogenesis during progression of castration-resistant prostate cancer

Although systemic androgen deprivation prolongs life in advanced prostate cancer, remissions are temporary because patients almost uniformly progress to a state of a castration-resistant prostate cancer (CRPC) as indicated by recurring PSA. This complex process of progression does not seem to be stochastic as the timing and phenotype are highly predictable, including the observation that most androgen-regulated genes are reactivated despite castrate levels of serum androgens. Recent evidence indicates that intraprostatic levels of androgens remain moderately high following systemic androgen deprivation therapy, whereas the androgen receptor (AR) remains functional, and silencing the AR expression following castration suppresses tumor growth and blocks the expression of genes known to be regulated by androgens. From these observations, we hypothesized that CRPC progression is not independent of androgen-driven activity and that androgens may be synthesized de novo in CRPC tumors leading to AR activation. Using the LNCaP xenograft model, we showed that tumor androgens increase during CRPC progression in correlation to PSA up-regulation. We show here that all enzymes necessary for androgen synthesis are expressed in prostate cancer tumors and some seem to be up-regulated during CRPC progression. Using an ex vivo radiotracing assays coupled to high-performance liquid chromatography-radiometric/mass spectrometry detection, we show that tumor explants isolated from CRPC progression are capable of de novo conversion of [(14)C]acetic acid to dihydrotestosterone and uptake of [(3)H]progesterone allows detection of the production of six other steroids upstream of dihydrotestosterone. This evidence suggests that de novo androgen synthesis may be a driving mechanism leading to CRPC progression following castration.

Androgen regulation of the androgen receptor coregulators

Background

The critical role of the androgen receptor (AR) in the development of prostate cancer is well recognized. The transcriptional activity of AR is partly regulated by coregulatory proteins. It has been suggested that these coregulators could also be important in the progression of prostate cancer. The aim of this study was to identify coregulators whose expression is regulated by either the androgens and/or by the expression level of AR.

Methods

We used empty vector and AR cDNA-transfected LNCaP cells (LNCaP-pcDNA3.1, and LNCaP-ARhi, respectively), and grew them for 4 and 24 hours in the presence of dihydrotestosterone (DHT) at various concentrations. The expression of 25 AR coregulators (SRC1, TIF2, PIAS1, PIASx, ARIP4, BRCA1, β-catenin, AIB3, AIB1, CBP, STAT1, NCoR1, AES, cyclin D1, p300, ARA24, LSD1, BAG1L, gelsolin, prohibitin, JMJD2C, JMJD1A, MAK, PAK6 and MAGE11) was then measured by using real-time quantitative RT-PCR (Q-RT-PCR).

Results

Five of the coregulators (AIB1, CBP, MAK, BRCA1 and β-catenin) showed more than 2-fold induction and 5 others (cyclin D1, gelsolin, prohibitin, JMJD1A, and JMJD2C) less than 2-fold induction. Overexpression of AR did not affect the expression of the coregulators alone. However, overexpression of AR enhanced the DHT-stimulated expression of MAK, BRCA1, AIB1 and CBP and reduced the level of expression of β-catenin, cyclinD1 and gelsolin.

Conclusion

In conclusion, we identified 5 coactivators whose expression was induced by androgens suggesting that they could potentiate AR signaling. Overexpression of AR seems to sensitize cells for low levels of androgens.

Progression of prostate cancer: multiple pathways to androgen independence

Prostate cancer remains one of the most commonly diagnosed cancers and a leading cause of cancer death in men. Initially, prostate tumors respond to hormonal therapies, but androgen-independent tumors refractory to these therapies emerge. Identifying the mechanisms responsible for the emergence of androgen independence has been the subject of multiple studies. This article reviews the multiple pathways that have been shown to promote androgen independence, including a recently described mechanism that involves androgen receptor proteolysis to a constitutively active ligand-independent isoform. Identifying the underlying mechanisms of androgen independence is crucial in the design of appropriate therapies for hormonally refractive neoplasms.

Protein kinase D1 (PKD1) influences androgen receptor (AR) function in prostate cancer cells

Protein kinase D1 (PKD1), founding member of PKD protein family, is down-regulated in advanced prostate cancer (PCa). We demonstrate that PKD1 and androgen receptor (AR) are present as a protein complex in PCa cells. PKD1 is associated with a transcriptional complex which contains AR and promoter sequence of the Prostate Specific Antigen (PSA) gene. Ectopic expression of wild type PKD1 and the kinase dead mutant PKD1 (K628W) attenuated the ligand-dependent transcriptional activation of AR in prostate cancer cells and yeast cells indicating that PKD1 can affect AR transcription activity, whereas knocking down PKD1 enhanced the ligand-dependent transcriptional activation of AR. Co-expression of kinase dead mutant with AR significantly inhibited androgen-mediated cell proliferation in both LNCaP and DU145 PC cells. Our data demonstrate for the first time that PKD1 can influence AR function in PCa cells.

Androgens and prevention of prostate cancer

PURPOSE OF REVIEW

This review provides a description of recent insights into the role of androgens in prostate cancer prevention.

RECENT FINDINGS

Many studies have elucidated a variety of molecular mechanisms involved in the initiation and progression of prostate cancer with many directly or indirectly related to the androgen signaling pathway. Both well known and novel agents for targeting the androgen pathway are under investigation, though very few are in clinical trials. After a review of recent papers describing these mechanisms, their results and implications were summarized.

SUMMARY

Finasteride remains the only agent proven to reduce the risk of prostate cancer, though there are currently two other ongoing phase III trials with vitamin E, selenium, and dutasteride. An enhanced understanding of complex interactions with the androgen pathways is leading to the exploration of additional promising approaches to mitigating the risk of prostate cancer.

Longitudinal analysis of androgen deprivation of prostate cancer cells identifies pathways to androgen independence

BACKGROUND

Following androgen ablation therapy, the majority of prostate cancer patients develop treatment resistance with a median time of 18-24 months to disease progression.

METHODS

To identify molecular targets that promote prostate cancer cell survival and contribute to androgen independence, we evaluated changes in LNCaP cell gene expression during 12 months of androgen deprivation. At time points reflecting critical growth and phenotypic changes, we performed Affymetrix expression array analysis to examine the effects of androgen deprivation during the acute response, during the period of apparent quiescence, and following the emergence of a highly proliferative, androgen-independent prostate cancer cell phenotype (LNCaP-AI).

RESULTS

We discovered alterations in gene expression for molecules associated with promoting prostate cancer cell growth and survival, and regulating cell cycle progression and apoptosis. Additionally, expression of AR co-regulators, adrenal androgen metabolizing enzymes, and markers of neuroendocrine disease were significantly altered.

CONCLUSIONS

These findings contribute greatly to our understanding of androgen-independent prostate cancer. The value of this longitudinal approach lies in the ability to examine gene expression changes throughout the adaptive response to androgen deprivation; it provides a more dynamic illustration of genes which contribute to disease progression in addition to specific genes which constitute an androgen-independent phenotype.

Effects of castration on the development of prostate adenocarcinoma from its precursor HGPIN and on the occurrence of androgen-independent, poorly differentiated carcinoma in TRAMP mice

Androgen ablation is thought to exert selective pressure for the development of androgen-independent forms of prostate cancer. This study was set up to investigate the effects of surgical castration on the development of prostate adenocarcinoma (ADC) from its precursor (high-grade prostate intraepithelial neoplasia (HGPIN)) and on the occurrence of androgen-independent, poorly differentiated carcinoma (PDC) in (C57Bl/6 transgenic adenocarcinoma of mouse prostate) TRAMP mice. It was found that castration cures HGPIN and ADC and prevents their further occurrence and growth, but has no effect on PDC. This indicates that in this model, PDC is not the progression of ADC favoured by androgen ablation and that its initiating cells are different from those of HGPIN and ADC.

AR, the cell cycle, and prostate cancer

The androgen receptor (AR) is a critical effector of prostate cancer development and progression. The dependence of this tumor type on AR activity is exploited in treatment of disseminated prostate cancers, wherein ablation of AR function (achieved either through ligand depletion and/or the use of AR antagonists) is the first line of therapeutic intervention. These strategies are initially effective, and induce a mixed response of cell cycle arrest or apoptosis in prostate cancer cells. However, recurrent, incurable tumors ultimately arise as a result of inappropriately restored AR function. Based on these observations, it is imperative to define the mechanisms by which AR controls cancer cell proliferation. Mechanistic investigation has revealed that AR acts as a master regulator of G1-S phase progression, able to induce signals that promote G1 cyclin-dependent kinase (CDK) activity, induce phosphorylation/inactivation of the retinoblastoma tumor suppressor (RB), and thereby govern androgen-dependent proliferation. These functions appear to be independent of the recently identified TMPRSS2-ETS fusions. Once engaged, several components of the cell cycle machinery actively modulate AR activity throughout the cell cycle, thus indicating that crosstalk between the AR and cell cycle pathways likely modulate the mitogenic response to androgen. As will be discussed, discrete aberrations in this process can alter the proliferative response to androgen, and potentially subvert hormonal control of tumor progression.

Mechanisms of androgen receptor activation in advanced prostate cancer: differential co-activator recruitment and gene expression

Prostate tumour growth depends on androgens; hence treatment includes androgen ablation and anti-androgens. Eventually tumours progress and in approximately 30% of patients this is associated with mutation of the androgen receptor. Several receptor variants associated with advanced disease show promiscuous activation by other hormones and anti-androgens. Such loss of specificity could promote receptor activation, hence tumour growth, in the absence of conventional ligands, explaining therapy failure. We aimed to elucidate mechanisms by which alternative ligands promote receptor activation. The three most commonly identified variants in tumours (with amino-acid substitutions H874Y, T877A and T877S) and wild-type receptor showed differences in co-activator recruitment dependent upon ligand and the interaction motif utilized. Co-expression and knockdown of co-activators that bind via leucine or phenylalanine motifs, combined with chromatin immunoprecipitation and quantitative PCR, revealed these preferences extend to co-activator recruitment in vivo and affect receptor activity at the transcriptional level, with subsequent effects on target gene regulation. The findings suggest that mutant receptors, activated by alternative ligands, drive growth via different mechanisms to androgen-activated wild-type receptor. Tumours may hence behave differently dependent upon any androgen receptor mutation present and what ligand is driving growth, as distinct subsets of genes may be regulated.

Nuclear receptor interaction protein, a coactivator of androgen receptors (AR), is regulated by AR and Sp1 to feed forward and activate its own gene expression through AR protein stability

Previously, we found a novel gene, nuclear receptor interaction protein (NRIP), a transcription cofactor that can enhance an AR-driven PSA promoter activity in a ligand-dependent manner in prostate cancer cells. Here, we investigated NRIP regulation. We cloned a 413-bp fragment from the transcription initiation site of the NRIP gene that had strong promoter activity, was TATA-less and GC-rich, and, based on DNA sequences, contained one androgen response element (ARE) and three Sp1-binding sites (Sp1-1, Sp1-2, Sp1-3). Transient promoter luciferase assays, chromatin immunoprecipitation and small RNA interference analyses mapped ARE and Sp1-2-binding sites involved in NRIP promoter activation, implying that NRIP is a target gene for AR or Sp1. AR associates with the NRIP promoter through ARE and indirectly through Sp1-binding site via AR–Sp1 complex formation. Thus both ARE and Sp1-binding site within the NRIP promoter can respond to androgen induction. More intriguingly, NRIP plays a feed-forward role enhancing AR-driven NRIP promoter activity via NRIP forming a complex with AR to protect AR protein from proteasome degradation. This is the first demonstration that NRIP is a novel AR-target gene and that NRIP expression feeds forward and activates its own expression through AR protein stability.

Roles of androgen-dependent and -independent activation of signal transduction pathways for cell proliferation of prostate cancer cells

Prostate cancer is one of the most frequently diagnosed cancers in the western world and this malignant neoplasm is the second-leading cause of cancer death among men in the USA. In the early 1940s, Huggins and Hodges demonstrated that growth and survival of prostate cancer depends on androgens. The mainstay of treatment for advanced prostate cancer is currently androgen ablation. Over the past few decades, several compounds, such as luteinizing hormone-releasing hormone analogues and anti-androgens, were developed and widely used in clinics. Then, the new treatment strategy, maximum androgen blockade (MAB) was introduced. In fact, MAB improved the prognosis of patients with advanced prostate cancer to some extent; however, most of those patients finally relapse after a period of initial response to this therapy, developing androgen-independent prostate cancer (AIPC). Once patients develop AIPC, effective therapeutic modalities are extremely limited and, therefore, the prognosis of this disease is very poor. It is strongly desirable to explore novel therapeutic concepts for AIPC, based on detailed molecular mechanisms for progression to androgen independency. As for the molecular mechanisms involved in the emergence of AIPC, mutations in the androgen receptor have been examined most extensively. These days, evidence is accumulating that demonstrates activation of signal transduction pathways, such as Src, PI3K and mTOR/S6K, are involved in the acquisition of the androgen-independent cell proliferation of prostate cancer cells. In addition, animal models using transgenic and gene-knockout techniques have confirmed these results. The development of therapies targeting against the signal transduction pathways is critical for the improvement of the prognosis of patients with AIPC. In this article, we review recent understandings on molecular mechanisms of androgen-dependent proliferation of prostate cancer cells, whose aberrant activation is proposed as a critical event for progression to AIPC.

The androgen receptor in prostate cancer: therapy target in search of an integrated diagnostic test

Despite the near ubiquitous use of therapies targeting the androgen receptor (AR) axis in prostate cancer, there remains, at this date, no integrated diagnostic test designed to select and individualize patients for hormonal-based treatments. From the article by Chmelar and coauthors, we may have a glimpse as to a way forward to the development of a test that can successfully guide the use of androgen ablation therapy. By turning our attention away from the AR gene and protein itself and focusing on coregulators of the AR pathway we may have a new opportunity to develop a single or group of slide-based tests designed to assess AR pathway status on the needle biopsy of a newly diagnosed prostate cancer patient and predict the clinical benefit of total androgen blockade treatment strategies.