A role of androgen receptor protein in cell growth of an androgen-independent prostate cancer cell line

Androgen receptor molecular biology and potential targets in prostate cancer

The androgen receptor (AR) is a key transcriptional regulator and therapeutic target in prostate cancer. During androgen deprivation therapy to treat metastatic prostate cancer, surviving cells acquire increased AR signaling through a variety of mechanisms, one of which is enhanced interactions with AR coactivators. One recently identified AR-specific coregulator expressed only in human and nonhuman primates is the melanoma antigen gene protein-A11 (MAGE-11). MAGE-11 increases AR transcriptional activity through direct interactions with AR and other coactivators, and its levels increase during prostate cancer progression to castration-recurrent growth. The MAGE-11 gene is located at Xq28 on the human X chromosome as part of an X-linked MAGE gene family of cancer-testis antigens. MAGE-11 stabilizes AR when androgen levels are low, and functions in a transcriptional hub to promote AR-mediated gene activation. The evolutionary development and organization of the MAGE-11 gene within the cancer-testis antigen family suggests that MAGE-11 provides a gain-of-function to AR among primates in both normal physiology and cancer, and may serve as a therapeutic target in the treatment of advanced prostate cancer.

Modulation of androgen receptor signaling in hormonal therapy-resistant prostate cancer cell lines

BACKGROUND

Prostate epithelial cells depend on androgens for survival and function. In (early) prostate cancer (PCa) androgens also regulate tumor growth, which is exploited by hormonal therapies in metastatic disease. The aim of the present study was to characterize the androgen receptor (AR) response in hormonal therapy-resistant PC346 cells and identify potential disease markers.

METHODOLOGY/PRINCIPAL FINDINGS

Human 19K oligoarrays were used to establish the androgen-regulated expression profile of androgen-responsive PC346C cells and its derivative therapy-resistant sublines: PC346DCC (vestigial AR levels), PC346Flu1 (AR overexpression) and PC346Flu2 (T877A AR mutation). In total, 107 transcripts were differentially-expressed in PC346C and derivatives after R1881 or hydroxyflutamide stimulations. The AR-regulated expression profiles reflected the AR modifications of respective therapy-resistant sublines: AR overexpression resulted in stronger and broader transcriptional response to R1881 stimulation, AR down-regulation correlated with deficient response of AR-target genes and the T877A mutation resulted in transcriptional response to both R1881 and hydroxyflutamide. This AR-target signature was linked to multiple publicly available cell line and tumor derived PCa databases, revealing that distinct functional clusters were differentially modulated during PCa progression. Differentiation and secretory functions were up-regulated in primary PCa but repressed in metastasis, whereas proliferation, cytoskeletal remodeling and adhesion were overexpressed in metastasis. Finally, the androgen-regulated genes ENDOD1, MCCC2 and ACSL3 were selected as potential disease markers for RT-PCR quantification in a distinct set of human prostate specimens. ENDOD1 and ACSL3 showed down-regulation in high-grade and metastatic PCa, while MCCC2 was overexpressed in low-grade PCa.

CONCLUSIONS/SIGNIFICANCE

AR modifications altered the transcriptional response to (anti)androgens in therapy-resistant cells. Furthermore, selective down-regulation of genes involved in differentiation and up-regulation of genes promoting proliferation and invasion suggest a disturbed balance between the growth and differentiation functions of the AR pathway during PCa progression. These findings may have implications in the current treatment and development of novel therapeutical approaches for metastatic PCa.

Increased expression of androgen receptor coregulator MAGE-11 in prostate cancer by DNA hypomethylation and cyclic AMP

Melanoma antigen gene protein-A11 (MAGE-11) of the MAGE family of cancer germ-line antigens increases androgen receptor (AR) transcriptional activity through its interaction with the AR NH(2)-terminal FXXLF motif. The present study investigated the regulatory mechanisms that control MAGE-11 expression during androgen deprivation therapy and prostate cancer progression. Studies include the CWR22 xenograft model of human prostate cancer, clinical specimens of benign and malignant prostate, and prostate cancer cell lines. MAGE-11 mRNA levels increased 100- to 1,500-fold during androgen deprivation therapy and prostate cancer progression, with highest levels in the castration-recurrent CWR22 xenograft and clinical specimens of castration-recurrent prostate cancer. Pyrosequencing of genomic DNA from prostate cancer specimens and cell lines indicated the increase in MAGE-11 resulted from DNA hypomethylation of a CpG island in the 5' promoter of the MAGE-11 gene. Sodium bisulfite sequencing of genomic DNA from benign and malignant prostate tumors and prostate cancer cell lines revealed DNA hypomethylation at individual CpG sites at the transcription start site were most critical for MAGE-11 expression. Cyclic AMP (cAMP) also increased MAGE-11 expression and AR transcriptional activity in prostate cancer cell lines. However, cAMP did not alter DNA methylation of the promoter and its effects were inhibited by extensive DNA methylation in the MAGE-11 promoter region. Increased expression of the AR coregulator MAGE-11 through promoter DNA hypomethylation and cAMP provides a novel mechanism for increased AR signaling in castration-recurrent prostate cancer.

Chimeric molecules facilitate the degradation of androgen receptors and repress the growth of LNCaP cells

Post-translational degradation of protein plays an important role in cell life. We employed chimeric molecules (dihydrotestosterone-based proteolysis-targeting chimeric molecule [DHT-PROTAC]) to facilitate androgen receptor (AR) degradation via the ubiquitin-proteasome pathway (UPP) and to investigate the role of AR in cell proliferation and viability in androgen-sensitive prostate cancer cells. Western blot analysis and immunohistochemistry were applied to analyse AR levels in LNCaP cells after DHT-PROTAC treatment. Cell counting and the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) cell viability assay were used to evaluate cell proliferation and viability after AR elimination in both LNCaP and PC-3 cells. AR was tagged for elimination via the UPP by DHT-PROTAC, and this could be blocked by proteasome inhibitors. Degradation of AR depended on DHT-PROTAC concentration, and either DHT or an ALAPYIP-(arg)(8) peptide could compete with DHT-PROTAC. Inhibition of cell proliferation and decreased viability were observed in LNCaP cells, but not in PC-3 or 786-O cells after DHT-PROTAC treatment. These data indicate that AR elimination is facilitated via the UPP by DHT-PROTAC, and that the growth of LNCaP cells is repressed after AR degradation.

A test of rats' tolerance for 3beta-acetoxyandrosta-1,5-dien-17-one ethylene ketal (ADEK), a new anti-androgen

Following the demonstration that the androgen activity of androsta-5-ene-3beta,17beta-diol (Adiol) is not inhibited by the anti-androgens currently used to treat prostate cancer, we sought agents that would inhibit the androgenic function of Adiol as well as of dihydrotestosterone. The steroid 3beta-acetoxyandrosta-1,5-dien-17-one ethylene ketal (ADEK) met this criterion. Its tolerance was assessed in rats by oral and by subcutaneous administration for four weeks. Neither route of ADEK administration resulted in any behavioral changes. There was no effect on weight gain during the 28 days of steroid intake and no effect on the weight of the kidneys, heart, liver, testes, adrenals or the ventral lobe of the prostate glands. The seminal vesicles of the treated rats were 23-29% and the weights of the anterior prostates of the respective groups were 17-26% smaller than the controls. In contrast, the dorsolateral prostates were increased 26-55% as compared with the controls. There were no detectable changes in the histology of the kidneys, hearts, livers, testes and adrenals of any of the rats, but both groups of ADEK-treated rats had mild atrophic changes in their seminal vesicles and in the ventral lobe of their prostate glands. Both ADEK-treated groups showed focal glandular epithelial hyperplasia in the dorsolateral lobes in comparison with the control group. Orally administered ADEK was rapidly converted to several metabolites, which were nearly completely cleared from the blood within 4h.

Site-specific androgen receptor serine phosphorylation linked to epidermal growth factor-dependent growth of castration-recurrent prostate cancer

The androgen receptor (AR) is required for prostate cancer development and contributes to tumor progression after remission in response to androgen deprivation therapy. Epidermal growth factor (EGF) increases AR transcriptional activity at low levels of androgen in the CWR-R1 prostate cancer cell line derived from the castration-recurrent CWR22 prostate cancer xenograft. Here we report that knockdown of AR decreases EGF stimulation of prostate cancer cell growth and demonstrate a mechanistic link between EGF and AR signaling. The EGF-induced increase in AR transcriptional activity is dependent on phosphorylation at mitogen-activated protein kinase consensus site Ser-515 in the AR NH(2)-terminal region and at protein kinase C consensus site Ser-578 in the AR DNA binding domain. Phosphorylation at these sites alters the nuclear-cytoplasmic shuttling of AR and AR interaction with the Ku-70/80 regulatory subunits of DNA-dependent protein kinase. Abolishing AR Ser-578 phosphorylation by introducing an S578A mutation eliminates the AR transcriptional response to EGF and increases both AR binding of Ku-70/80 and nuclear retention of AR in association with hyperphosphorylation of AR Ser-515. The results support a model in which AR transcriptional activity increases castration-recurrent prostate cancer cell growth in response to EGF by site-specific serine phosphorylation that regulates nuclear-cytoplasmic shuttling through interactions with the Ku-70/80 regulatory complex.

SIRNA-directed in vivo silencing of androgen receptor inhibits the growth of castration-resistant prostate carcinomas

Background

Prostate carcinomas are initially dependent on androgens, and castration or androgen antagonists inhibit their growth. After some time though, tumors become resistant and recur with a poor prognosis. The majority of resistant tumors still expresses a functional androgen receptor (AR), frequently amplified or mutated.

Methodology/Principal Findings

To test the hypothesis that AR is not only expressed, but is still a key therapeutic target in advanced carcinomas, we injected siRNA targeting AR into mice bearing exponentially growing castration-resistant tumors. Quantification of siRNA into tumors and mouse tissues demonstrated their efficient uptake. This uptake silenced AR in the prostate, testes and tumors. AR silencing in tumors strongly inhibited their growth, and importantly, also markedly repressed the VEGF production and angiogenesis.

Conclusions/Significance

Our results demonstrate that carcinomas resistant to hormonal manipulations still depend on the expression of the androgen receptor for their development in vivo. The siRNA-directed silencing of AR, which allows targeting overexpressed as well as mutated isoforms, triggers a strong antitumoral and antiangiogenic effect. siRNA-directed silencing of this key gene in advanced and resistant prostate tumors opens promising new therapeutic perspectives and tools.

Locus-wide chromatin remodeling and enhanced androgen receptor-mediated transcription in recurrent prostate tumor cells

Prostate cancers (PCas) become resistant to hormone withdrawal through increased androgen receptor (AR) signaling. Here we show increased AR-mediated transcription efficiency in PCa cells that have acquired the ability to grow in low concentrations of androgen. Compared to androgen-dependent PCa cells, these cells showed increased activity of transiently transfected reporters and increased mRNA synthesis relative to levels of AR occupancy of the prostate-specific antigen (PSA) gene. The locus also displayed up to 10-fold-higher levels of histone H3-K9/K14 acetylation and H3-K4 methylation across the entire body of the gene. Although similar increased mRNA expression and locus-wide histone acetylation were also observed at another kallikrein locus (KLK2), at a third AR target locus (TMPRSS2) increased gene expression and locus-wide histone acetylation were not seen in the absence of ligand. Androgen-independent PCa cells have thus evolved three distinctive alterations in AR-mediated transcription. First, increased RNA polymerase initiation and processivity contributed to increased gene expression. Second, AR signaling was more sensitive to ligand. Third, locus-wide chromatin remodeling conducive to the increased gene expression in the absence of ligand was apparent and depended on sustained AR activity. Therefore, increased AR ligand sensitivity as well as locus-specific chromatin alterations contribute to basal gene expression of a subpopulation of specific AR target genes in androgen-independent PCa cells. These features contribute to the androgen-independent phenotype of these cells.

C19-Steroids as androgen receptor modulators: Design, discovery, and structure-activity relationship of new steroidal androgen receptor antagonists

Dehydroepiandrosterone (DHEA), the most abundant steroid in human circulating blood, is metabolized to sex hormones and other C19-steroids. Our previous collaborative study demonstrated that androst-5-ene-3beta,17beta-diol (Adiol) and androst-4-ene-3,17-dione (Adione), metabolites of DHEA, can activate androgen receptor (AR) target genes. Adiol is maintained at a high concentration in prostate cancer tissue; even after androgen deprivation therapy and its androgen activity is not inhibited by the antiandrogens currently used to treat prostate cancer patients. We have synthesized possible metabolites of DHEA and several synthetic analogues and evaluated their role in androgen receptor transactivation to identify AR modulators. Steroids with low androgenic potential in PC-3 cell lines were evaluated for anti-dihydrotestosterone (DHT) and anti-Adiol activity. We discovered three potent antiandrogens: 3beta-acetoxyandrosta-1,5-diene-17-one 17-ethylene ketal (ADEK), androsta-1,4-diene-3,17-dione 17-ethylene ketal (OAK), and 3beta-hydroxyandrosta-5,16-diene (HAD) that antagonized the effects of DHT as well as of Adiol on the growth of LNCaP cells and on the expression of prostate-specific antigen (PSA). In vivo tests of these compounds will reveal their potential as potent antiandrogens for the treatment of prostate cancer.