Induction of cell-free, in vitro transcription by recombinant androgen receptor peptides

A Multivalent Peptoid Conjugate Modulates Androgen Receptor Transcriptional Activity to Inhibit Therapy-resistant Prostate Cancer

Prostate cancers adapt to androgen receptor (AR) pathway inhibitors and progress to castration resistance due to ongoing AR expression and function. To counter this, we developed a new approach to modulate the AR and inhibit castration-resistant prostate cancer (CRPC) using multivalent peptoid conjugates (MPC) that contain multiple copies of the AR-targeting ligand ethisterone attached to a peptidomimetic scaffold. Here, we investigated the antitumor effects of compound MPC309, a trivalent display of ethisterone conjugated to a peptoid oligomer backbone that binds to the AR with nanomolar affinity. MPC309 exhibited potent antiproliferative effects on various enzalutamide-resistant prostate cancer models, including those with AR splice variants, ligand-binding mutations, and noncanonical AR gene expression programs, as well as mouse prostate organoids harboring defined genetic alterations that mimic lethal human prostate cancer subtypes. MPC309 is taken up by cells through macropinocytosis, an endocytic process more prevalent in cancer cells than in normal ones, thus providing an opportunity to target tumors selectively. MPC309 triggers a distinct AR transcriptome compared with DHT and enzalutamide, a clinically used antiandrogen. Specifically, MPC309 enhances the expression of differentiation genes while reducing the expression of genes needed for cell division and metabolism. Mechanistically, MPC309 increases AR chromatin occupancy and alters AR interactions with coregulatory proteins in a pattern distinct from DHT. In xenograft studies, MPC309 produced significantly greater tumor suppression than enzalutamide. Altogether, MPC309 represents a promising new AR modulator that can combat resistant disease by promoting an AR antiproliferative gene expression program.

ING3 promotes prostate cancer growth by activating the androgen receptor

BACKGROUND

The androgen receptor (AR) is a major driver of prostate cancer, and increased AR levels and co-activators of the receptor promote the development of prostate cancer. INhibitor of Growth (ING) proteins target lysine acetyltransferase or lysine deacetylase complexes to the histone H3K4Me3 mark of active transcription, to affect chromatin structure and gene expression. ING3 is a stoichiometric member of the TIP60 lysine acetyltransferase complex implicated in prostate cancer development.

METHODS

Biopsies of 265 patients with prostate cancer were stained for ING3, pan-cytokeratin, and DNA. LNCaP and C4-2 androgen-responsive cells were used for in vitro assays including immunoprecipitation, western blotting, Luciferase reporter assay and quantitative polymerase chain reaction. Cell viability and migration assays were performed in prostate cancer cell lines using scrambled siRNA or siRNA targeting ING3.

RESULTS

We find that ING3 levels and AR activity positively correlate in prostate cancer. ING3 potentiates androgen effects, increasing expression of androgen-regulated genes and androgen response element-driven reporters to promote growth and anchorage-independent growth. Conversely, ING3 knockdown inhibits prostate cancer cell growth and invasion. ING3 activates the AR by serving as a scaffold to increase interaction between TIP60 and the AR in the cytoplasm, enhancing receptor acetylation and translocation to the nucleus. Activation is independent of ING3's ability to target the TIP60 complex to H3K4Me3, identifying a previously unknown chromatin-independent cytoplasmic activity for ING3. In agreement with in vitro observations, analysis of The Cancer Genome Atlas (TCGA) data (n = 498) and a prostate cancer tissue microarray (n = 256) show that ING3 levels are higher in aggressive prostate cancers, with high levels of ING3 predicting shorter patient survival in a low AR subgroup. Including ING3 levels with currently used indicators such as the Gleason score provides more accurate prognosis in primary prostate cancer.

CONCLUSIONS

In contrast to the majority of previous reports suggesting tumor suppressive functions in other cancers, our observations identify a clear oncogenic role for ING3, which acts as a co-activator of AR in prostate cancer. Data from TCGA and our previous and current tissue microarrays suggest that ING3 levels correlate with AR levels and that in patients with low levels of the receptor, ING3 level could serve as a useful prognostic biomarker.

Extranuclear Actions of the Androgen Receptor Enhance Glucose-Stimulated Insulin Secretion in the Male

Although men with testosterone deficiency are at increased risk for type 2 diabetes (T2D), previous studies have ignored the role of testosterone and the androgen receptor (AR) in pancreatic β cells. We show that male mice lacking AR in β cells (βARKO) exhibit decreased glucose-stimulated insulin secretion (GSIS), leading to glucose intolerance. The AR agonist dihydrotestosterone (DHT) enhances GSIS in cultured male islets, an effect that is abolished in βARKO(-/y) islets and human islets treated with an AR antagonist. In β cells, DHT-activated AR is predominantly extranuclear and enhances GSIS by increasing islet cAMP and activating the protein kinase A. In mouse and human islets, the insulinotropic effect of DHT depends on activation of the glucagon-like peptide-1 (GLP-1) receptor, and accordingly, DHT amplifies the incretin effect of GLP-1. This study identifies AR as a novel receptor that enhances β cell function, a finding with implications for the prevention of T2D in aging men.

NFI transcription factors interact with FOXA1 to regulate prostate-specific gene expression

Androgen receptor (AR) action throughout prostate development and in maintenance of the prostatic epithelium is partly controlled by interactions between AR and forkhead box (FOX) transcription factors, particularly FOXA1. We sought to identity additional FOXA1 binding partners that may mediate prostate-specific gene expression. Here we identify the nuclear factor I (NFI) family of transcription factors as novel FOXA1 binding proteins. All four family members (NFIA, NFIB, NFIC, and NFIX) can interact with FOXA1, and knockdown studies in androgen-dependent LNCaP cells determined that modulating expression of NFI family members results in changes in AR target gene expression. This effect is probably mediated by binding of NFI family members to AR target gene promoters, because chromatin immunoprecipitation (ChIP) studies found that NFIB bound to the prostate-specific antigen enhancer. Förster resonance energy transfer studies revealed that FOXA1 is capable of bringing AR and NFIX into proximity, indicating that FOXA1 facilitates the AR and NFI interaction by bridging the complex. To determine the extent to which NFI family members regulate AR/FOXA1 target genes, motif analysis of publicly available data for ChIP followed by sequencing was undertaken. This analysis revealed that 34.4% of peaks bound by AR and FOXA1 contain NFI binding sites. Validation of 8 of these peaks by ChIP revealed that NFI family members can bind 6 of these predicted genomic elements, and 4 of the 8 associated genes undergo gene expression changes as a result of individual NFI knockdown. These observations suggest that NFI regulation of FOXA1/AR action is a frequent event, with individual family members playing distinct roles in AR target gene expression.

In vitro transcription and immobilized template analysis of preinitiation complexes

In the study of gene regulation, it is often necessary to employ functional assays that investigate the action or mechanism of specific promoters or enhancer-binding factors and their role in transcription by RNA polymerase II. Although many assays measure the transcription of a gene under the control of an endogenous or model activator in vivo, it is often useful to recreate transcription in vitro in order to study specific regulatory mechanisms. In this unit, protocols are presented that will allow the investigator to perform in vitro transcription using preinitiation complexes assembled from cellular extracts on either naked DNA or chromatin templates.

FUS/TLS is a co-activator of androgen receptor in prostate cancer cells

Androgen receptor (AR) is a member of the nuclear receptor family of transcription factors. Upon binding to androgens, AR becomes transcriptionally active to regulate the expression of target genes that harbor androgen response elements (AREs) in their promoters and/or enhancers. AR is essential for the growth and survival of prostate cancer cells and is therefore a target for current and next-generation therapeutic modalities against prostate cancer. Pathophysiologically relevant protein-protein interaction networks involving AR are, however, poorly understood. In this study, we identified the protein FUsed/Translocated in LipoSarcoma (FUS/TLS) as an AR-interacting protein by co-immunoprecipitation of endogenous proteins in LNCaP human prostate cancer cells. The hormonal response of FUS expression in LNCaP cells was shown to resemble that of other AR co-activators. FUS displayed a strong intrinsic transactivation capacity in prostate cancer cells when tethered to basal promoters using the GAL4 system. Chromatin immunoprecipitation experiments showed that FUS was recruited to ARE III of the enhancer region of the PSA gene. Data from ectopic overexpression and “knock-down” approaches demonstrated that AR transcriptional activity was enhanced by FUS. Depletion of FUS reduced androgen-dependent proliferation of LNCaP cells. Thus, FUS is a novel co-activator of AR in prostate cancer cells.

Relaxin drives Wnt signaling through upregulation of PCDHY in prostate cancer

BACKGROUND

Relaxin, a potent peptide hormone of the insulin-like family normally produced and secreted by the human prostate, is upregulated in castrate resistant prostate cancer progression. In various tissues, relaxin increases angiogenesis and cell motility through upregulation of vascular endothelial growth factor, matrix metalloproteases, and nitric oxide, and therefore maybe an attractive target for cancer therapeutics.

METHODS

To examine the role of relaxin in prostate cancer progression, LNCaP cells stably transfected with relaxin (LNCaP(RLN)) were used to form xenograft tumors, and microarray expression analysis was subsequently performed to determine novel pathways regulated by relaxin. Prostate cancer tissue microarrays from patient samples were stained by immunohistochemistry for further validation and correlation of the findings.

RESULTS

Expression analysis identified novel relaxin regulated pathways, including the ProtocadherinY (PCDHY)/Wnt pathway. PCDHY, which upregulates Wnt11, has previously been shown to stabilize beta-catenin, causing beta-catenin to translocate from the cytoplasmic membrane to the nucleus and initiate TCF-mediated signaling. LNCaP(RLN) xenografts exhibit increased PCDHY expression and increased cytoplasmic localization of beta-catenin, suggesting relaxin directs Wnt11 overexpression through PCDHY upregulation. Similarly, prostate cancer samples from patients who have undergone androgen ablation have increased Wnt11 expression, which is further upregulated in castrate resistant tissues. Like relaxin, Wnt11, and PCDHY are negatively regulated by androgens, and further analysis indicated that the overexpression of relaxin results in dysregulation of androgen-regulated genes.

CONCLUSIONS

These data suggest that prostate cancer cell motility and altered androgen receptor activity attributed to relaxin may be mediated in part by Wnt11.

Bone morphogenetic protein 7 is expressed in prostate cancer metastases and its effects on prostate tumor cells depend on cell phenotype and the tumor microenvironment

Bone morphogenetic protein (BMP) signaling is important in prostate development and prostate cancer (PCa) progression. However, because of the multiple effects of different BMPs, no final conclusions have been made as to the role of BMPs in PCa. In our studies, we have focused on BMP-7 because it is involved in prostate morphogenesis, and its expression is regulated by androgens. The objective of our study was to determine BMP-7 expression in PCa metastases and investigate the effects of BMP-7 on PCa cells. Our results show that BMP-7 is expressed in metastatic PCa and its levels are increased in castration-resistant PCa versus androgen-dependent PCa, whereas the expression of BMP-7 is decreased in primary PCa versus normal prostate. Our in vitro results show that BMP-7 inhibits proliferation of androgen-sensitive LNCaP cells, stimulates androgen receptor signaling, increases the expression of differentiation-associated genes, and decreases the levels of some wingless-regulated transcripts. Interestingly, these effects were not detected in C4-2 castration-resistant PCa cells. In vivo expression of BMP-7 in castration-resistant C4-2 cells did not alter proliferation when these cells were grown subcutaneously, but their growth was inhibited in the bone environment. In summary, our results show that BMP-7 is expressed at the highest level in advanced castration-resistant PCa cells and that the inhibitory effects of BMP-7 are dependent on the differentiation status of PCa cells and the tumor microenvironment. Further studies are needed to identify changes in BMP-7 signaling that lead to the loss of its control of proliferation during PCa progression.

Human prostate sphere-forming cells represent a subset of basal epithelial cells capable of glandular regeneration in vivo

BACKGROUND

Prostate stem/progenitor cells function in glandular development and maintenance. They may be targets for tumor initiation, so characterization of these cells may have therapeutic implications. Cells from dissociated tissues that form spheres in vitro often represent stem/progenitor cells. A subset of human prostate cells that form prostaspheres were evaluated for self-renewal and tissue regeneration capability in the present study.

METHODS

Prostaspheres were generated from 59 prostatectomy specimens. Lineage marker expression and TMPRSS-ERG status was determined via immunohistochemistry and fluorescence in situ hybridization (FISH). Subpopulations of prostate epithelial cells were isolated by cell sorting and interrogated for sphere-forming activity. Tissue regeneration potential was assessed by combining sphere-forming cells with rat urogenital sinus mesenchyme (rUGSM) subcutaneously in immunocompromised mice.

RESULTS

Prostate tissue specimens were heterogeneous, containing both benign and malignant (Gleason 3-5) glands. TMPRSS-ERG fusion was found in approximately 70% of cancers examined. Prostaspheres developed from single cells at a variable rate (0.5-4%) and could be serially passaged. A basal phenotype (CD44+CD49f+CK5+p63+CK8-AR-PSA-) was observed among sphere-forming cells. Subpopulations of prostate cells expressing tumor-associated calcium signal transducer 2 (Trop2), CD44, and CD49f preferentially formed spheres. In vivo implantation of sphere-forming cells and rUGSM regenerated tubular structures containing discreet basal and luminal layers. The TMPRSS-ERG fusion was absent in prostaspheres derived from fusion-positive tumor tissue, suggesting a survival/growth advantage of benign prostate epithelial cells.

CONCLUSION

Human prostate sphere-forming cells self-renew, have tissue regeneration capability, and represent a subpopulation of basal cells.

Monitoring B cell response to immunoselected phage-displayed peptides by microarrays

Successful adaptation of microarray technology for high-throughput screening of proteins requires a large number of purified recombinant proteins, e.g., antibodies for use as capture molecules. Phage surface display technology has been used for the surface expression of proteins, peptides or cDNA repertoires expressed by tumor cells. It does not require protein purification, as recombinant phages can be spotted on glass slides and used in a high-throughput screening format. Biopanning of phage libraries on patient serum antibodies is expected to enrich for antibody-binding phages for the fabrication of diagnostic and/or prognostic B-cell epitope microarrays. In contrast to other immunological techniques, microarrays can measure the antibody levels against different epitopes in a single test. This chapter highlights the recent advances in phage-based microarray technology to profile humoral immune responses in cancer patients.

14-3-3 sigma increases the transcriptional activity of the androgen receptor in the absence of androgens

The androgen receptor (AR) is a ligand-activated transcription factor that regulates numerous target genes, including prostate-specific antigen (PSA). We examined the ability of each member of the 14-3-3 family to modulate transcription of PSA through the AR. Despite significant homology within the 14-3-3 family we observed differences in the ability of each isoform to alter the transcriptional activity of the AR. Significantly, 14-3-3 sigma activated PSA-luciferase reporters not only at castrate levels of androgens, but also in the complete absence of androgens. 14-3-3 sigma also increased expression of the endogenous PSA gene in the absence of androgens. Knockdown of the AR by siRNA oligonucleotides abolished activation of these reporters by 14-3-3 sigma. These findings may have greatest significance in hormone refractory prostate cancer where the AR may be activated in a ligand-independent manner.

Relaxin becomes upregulated during prostate cancer progression to androgen independence and is negatively regulated by androgens

BACKGROUND

Relaxin is a potent peptide hormone normally secreted by the prostate. This study characterized relaxin expression during prostate cancer progression to androgen independence (AI), and in response to androgens.

METHODS

The prostate cancer cell line, LNCaP, was assayed by microarrays and confirmatory Northern analysis to assess changes in relaxin levels due to androgen treatment and in LNCaP xenografts following castration. Relaxin protein levels were examined by immunohistochemistry (IHC) in tissue microarrays of human prostate cancer samples following androgen ablation.

RESULTS

Relaxin levels decreased in a time and concentration-dependent manner due to androgens in vitro, and increased in xenografts post-castration. Relaxin increased in radical prostatectomy specimens after 6 months of androgen ablation and in AI tumors, was highest in bone metastases.

CONCLUSIONS

Relaxin is negatively regulated by androgens in vitro and in vivo, which correlates to clinical prostate cancer specimens following androgen ablation. The role of relaxin in angiogenesis and tissue remodeling suggests it may contribute to prostate cancer progression.

Foxa1 and Foxa2 interact with the androgen receptor to regulate prostate and epididymal genes differentially

Previous studies from our group have shown that Foxa1 is expressed in the prostate and interacts with the androgen receptor (AR) to regulate prostate-specific genes such as prostate-specific antigen (PSA) and probasin (PB). We report here that Foxa2 but not Foxa1 is expressed in the epididymis. Further, Foxa2 interacts with the AR to regulate the mouse epididymal retinoic acid binding protein (mE-RABP) gene, an epididymis-specific gene. Binding of Foxa2 to the mE-RABP promoter was confirmed by gel-shift and chromatin immunoprecipitation (ChIP) assays. Overexpression of Foxa2 suppresses androgen activation of the mE-RABP promoter while overexpression of Foxa2 with prostate-specific promoters activates gene expression in an androgen-independent manner. GST pull-down assays determined that both Foxa1 and Foxa2 physically interact with the DNA binding domain of the AR. The interaction between Foxa proteins and AR was further confirmed by gel-shift assays where Foxa protein was recruited to AR binding oligomers even when Foxa binding sites were not present, and AR was recruited to Foxa binding oligomers even in the absence of an AR binding site. Given that Foxa1 and Foxa2 proteins are expressed differentially in the prostate and epididymis, these data suggest that the Foxa proteins have distinct effects on AR-regulated genes in different male reproductive accessory organs.

Loss of endogenous androgen receptor protein accelerates motor neuron degeneration and accentuates androgen insensitivity in a mouse model of X-linked spinal and bulbar muscular atrophy

X-linked spinal and bulbar muscular atrophy (SBMA; Kennedy's disease) is a polyglutamine (polyQ) disease in which the affected males suffer progressive motor neuron degeneration accompanied by signs of androgen insensitivity, such as gynecomastia and reduced fertility. SBMA is caused by CAG repeat expansions in the androgen receptor (AR) gene resulting in the production of AR protein with an extended glutamine tract. SBMA is one of nine polyQ diseases in which polyQ expansion is believed to impart a toxic gain-of-function effect upon the mutant protein, and initiate a cascade of events that culminate in neurodegeneration. However, whether loss of a disease protein's normal function concomitantly contributes to the neurodegeneration remains unanswered. To address this, we examined the role of normal AR function in SBMA by crossing a highly representative AR YAC transgenic mouse model with 100 glutamines (AR100) and a corresponding control (AR20) onto an AR null (testicular feminization; Tfm) background. Absence of endogenous AR protein in AR100Tfm mice had profound effects upon neuromuscular and endocrine-reproductive features of this SBMA mouse model, as AR100Tfm mice displayed accelerated neurodegeneration and severe androgen insensitivity in comparison to AR100 littermates. Reduction in size and number of androgen-sensitive motor neurons in the spinal cord of AR100Tfm mice underscored the importance of AR action for neuronal health and survival. Promoter-reporter assays confirmed that AR transactivation competence diminishes in a polyQ length-dependent fashion. Our studies indicate that SBMA disease pathogenesis, both in the nervous system and the periphery, involves two simultaneous pathways: gain-of-function misfolded protein toxicity and loss of normal protein function.

The role of forkhead box A2 to restrict androgen-regulated gene expression of lipocalin 5 in the mouse epididymis

Murine epididymal retinoic acid-binding protein [or lipocalin 5 (Lcn5)] is synthesized and secreted by the principal cells of the mouse middle/distal caput epididymidis. A 5-kb promoter fragment of the Lcn5 gene can dictate androgen-dependent and epididymis region-specific gene expression in transgenic mice. Here, we reported that the 1.8-kb Lcn5 promoter confers epididymis region-specific gene expression in transgenic mice. To decipher the mechanism that directs transcription, 14 chimeric constructs that sequentially removed 100 bp of 1.8-kb Lcn5 promoter were generated and transfected into epididymal cells and nonepididymal cells. Transient transfection analysis revealed that 1.3 kb promoter fragment gave the strongest response to androgens. Between the 1.2-kb to 1.3-kb region, two androgen receptor (AR) binding sites were identified. Adjacent to AR binding sites, a Foxa2 [Fox (Forkhead box) subclass A] binding site was confirmed by gel shift assay. Similar Foxa binding sites were also found on the promoters of human and rat Lcn5, indicating the Foxa binding site is conserved among species. We previously reported that among the three members of Foxa family, Foxa1 and Foxa3 were absent in the epididymis whereas Foxa2 was detected in epididymal principal cells. Here, we report that Foxa2 displays a region-specific expression pattern along the epididymis: no staining observed in initial segment, light staining in proximal caput, gradiently heavier staining in middle and distal caput, and strongest staining in corpus and cauda, regions with little or no expression of Lcn5. In transient transfection experiments, Foxa2 expression inhibits AR induction of the Lcn5 promoter, which is consistent with the lack of expression of Lcn5 in the corpus and cauda. We conclude that Foxa2 functions as a repressor that restricts AR regulation of Lcn5 to a segment-specific pattern in the epididymis.

Cyclin G-associated kinase: A novel androgen receptor-interacting transcriptional coactivator that is overexpressed in hormone refractory prostate cancer

The androgen receptor (AR), a steroid receptor family member, is a ligand-dependent transcription factor that has an integral role in normal prostate development. Alterations in AR-mediated activity can result in abnormal gene expression, dysregulated cell growth and prostate cancer. Coregulator proteins that interact with AR to influence activity and specificity of the AR-response may also have an important role in prostate cancer progression. Since the NH(2)-terminal domain (NTD) of AR encodes the ligand-independent activation function (AF)-1, this domain is incompatible with conventional yeast two-hybrid systems. Therefore, we have used the Tup1 repressed transactivator (RTA) system, which exploits the intrinsic transactivation properties of AR.NTD, for identification of novel AR-interacting proteins. Using this system, cyclin G-associated kinase (GAK) was identified as an AR interacting protein, and GST pull-down assays were used to confirm the interaction. GAK was shown to enhance the AF-1 function of AR activity in a ligand-dependent manner. Additionally, GAK enhanced the AR transcriptional response even at low concentrations of androgens, which is relevant to AR activity in androgen-independent prostate cancer. Finally, neo-adjuvant hormone therapy (NHT) tissue microarray analysis demonstrated that GAK expression increased significantly with prostate cancer progression to androgen independence, which suggests a prognostic role for GAK in advanced disease.

Ku is a novel transcriptional recycling coactivator of the androgen receptor in prostate cancer cells

The androgen receptor (AR) dynamically assembles and disassembles multicomponent receptor complexes in order to respond rapidly and reversibly to fluctuations in androgen levels. We are interested in identifying the basal factors that compose the AR aporeceptor and holoreceptor complexes and impact the transcriptional process. Using tandem mass spectroscopy analysis, we identified the trimeric DNA-dependent protein kinase (DNA-PK) complex as the major AR-interacting proteins. AR directly interacts with both Ku70 and Ku80 in vivo and in vitro, as shown by co-immunoprecipitation, glutathione S-transferase pull-down, and Sf9 cell/baculovirus expression. The interaction was localized to the androgen receptor ligand binding domain and is independent of DNA interactions. Ku interacts with AR in the cytoplasm and nucleus regardless of the presence or absence of androgen. Ku acts as a coactivator of AR activity in a luciferase reporter assay employing both Ku-defective cells and Ku small interfering RNA knock-down in a prostate cancer cell line. DNA-PK catalytic subunit (DNA-PKcs) also acts as a coactivator of androgen receptor activity in a luciferase reporter assay employing DNA-PKcs defective cells. AR nuclear translocation is not affected in Ku defective cells, implying Ku functionality may be mainly nuclear. Chromatin immunoprecipitation experiments demonstrated that both Ku70 and Ku80 interact with the prostate-specific antigen promoter in an androgen-dependant manner. Finally, in vitro transcription assays demonstrated Ku involvement in transcriptional recycling with androgen dependent promoters.

Androgen receptor (AR) expression in AR-negative prostate cancer cells results in differential effects of DHT and IGF-I on proliferation and AR activity between localized and metastatic tumors

BACKGROUND

Two features of the progression from organ-confined to metastatic prostate cancer are dysregulation of the androgen receptor (AR) and a decrease in insulin-like growth factor-type-I receptor (IGF-IR) expression. The purpose of this study was to determine the effect of changes in IGF-IR expression on AR activity.

METHODS

M12 human prostate cells were stably transfected with an AR expression construct to produce the M12-AR parental (PAR) cell line. PAR cells were implanted orthotopically into nude mice and M12-AR primary (PRI) cell lines were derived from intraprostatic tumors and metastatic cell lines (MET) were derived from PRI tumors that had metastasized to diaphragm or lung.

RESULTS

Tumor formation in the prostate by PAR cells was decreased significantly compared to M12 controls. PAR, PRI, and MET cells expressed equivalent amounts of AR protein; however, IGF-IR expression was increased significantly in PAR and PRI cells. IGF-IR expression decreased in MET lines to the levels seen in M12 control cells. IGF-I significantly enhanced dihydrotestosterone (DHT)-stimulated, but not basal, AR transcriptional activity in PRI cells. In MET cells, IGF-I significantly suppressed DHT-stimulated transcriptional activity. In MET cells in which the IGF-IR was re-expressed from a retroviral vector, the effects of DHT and IGF-I on AR activity were similar to those seen in PRI cells.

CONCLUSIONS

This study demonstrates that the changes in IGF-IR expression exhibited by this model of metastatic progression cause significant alterations in AR signaling and suggest that this interaction may be an important aspect of the changes seen in AR function in disease progression in vivo.

Identification of a negative regulatory cis-element in the enhancer core region of the prostate-specific antigen promoter: implications for intersection of androgen receptor and nuclear factor-kappaB signalling in prostate cancer cells

The NF-kappaB (nuclear factor-kappaB) transcription factors mediate activation of a large number of gene promoters containing diverse kappaB-site sequences. Here, PSA (prostate-specific antigen) was used as an AR (androgen receptor)-responsive gene to examine the underlying mechanism by which the NF-kappaB p65 transcription factor down-regulates the transcriptional activity of AR in cells. We observed that activation of NF-kappaB by TNFalpha (tumour necrosis factor alpha) inhibited both basal and androgen-stimulated PSA expression, and that this down-regulation occurred at the promoter level, as confirmed by the super-repressor IkappaBalpha (S32A/S36A), a dominant negative inhibitor of NF-kappaB. Using a linker-scanning mutagenesis approach, we identified a cis -element, designated XBE (X-factor-binding element), in the AREc (androgen response element enhancer core) of the PSA promoter, which negatively regulated several AR-responsive promoters, including that of PSA. When three copies of XBE in tandem were juxtaposed to GRE4 (glucocorticoid response element 4), a 4-6-fold reduction of inducible GRE4 activity was detected in three different cell lines, LNCaP, ARCaP-AR and PC3-AR. Bioinformatics and molecular biochemical studies indicated that XBE is a kappaB-like element that binds specifically to the NF-kappaB p65 subunit; consistent with these observations, only NF-kappaB p65, but not the NF-kappaB p50 subunit, was capable of inhibiting AR-mediated PSA promoter transactivation in LNCaP cells. In addition, our data also showed that AR binds to XBE, as well as to the kappaB consensus site, and that the transfection of AR inhibits the kappaB-responsive promoter in transient co-transfection assays. Collectively, these data indicate that cross-modulation between AR and NF-kappaB p65 transcription factors may occur by a novel mechanism involving binding to a common cis -DNA element.

Osteoblast-Derived Factors Induce Androgen-Independent Proliferation and Expression of Prostate-Specific Antigen in Human Prostate Cancer Cells

PURPOSE

Prostate cancer metastasizes to the skeleton to form osteoblastic lesions. Androgen ablation is the current treatment for metastatic prostate cancer. This therapy is palliative, and the disease will return in an androgen-independent form that is preceded by a rising titer of prostate-specific antigen (PSA). Here, we investigated the possibility that human osteoblasts might secrete factors that contribute to the emergence of androgen-independent prostate cancer.

EXPERIMENTAL DESIGN

Primary cultures of human osteoblasts were used as a source of conditioned medium (OCM). Proliferation, expression of androgen-regulated genes, and transactivation of the androgen receptor (AR) were monitored in LNCaP human prostate cancer cells in response to OCM using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, Northern blot analysis, and reporter gene constructs. Levels of interleukin-6 (IL-6) present in OCM were measured, and its contribution to proliferation and expression of PSA were investigated by neutralization studies with anti IL-6 antibodies.

RESULTS

OCM increased the proliferation and expression of PSA at both the protein and RNA levels in LNCaP cells. Synergistic increases in the activities of PSA (6.1 kb)- and pARR(3)-tk-luciferase reporters were measured in cells cotreated with both OCM and androgen. OCM targeted the NH(2)-terminal domain of the AR. The effect of OCM on transcriptional activity of the AR was inhibited by an antiandrogen. Neutralizing antibodies to IL-6 blocked proliferation and expression of PSA by OCM.

CONCLUSION

Osteoblasts secrete factors, such as IL-6, that cause androgen-independent induction of PSA gene expression and proliferation of prostate cancer cells by a mechanism that partially relies on the AR. Identifying such molecular mechanisms may lead to improved clinical management of metastatic prostate cancer.

Isolation and identification of L-dopa decarboxylase as a protein that binds to and enhances transcriptional activity of the androgen receptor using the repressed transactivator yeast two-hybrid system

The AR (androgen receptor) is a ligand-regulated transcription factor, which belongs to the steroid receptor family and plays an essential role in growth and development of the prostate. Transcriptional activity of steroid receptors is modulated by interaction with co-regulator proteins and yeast two-hybrid analysis is commonly used to identify these steroid receptor-interacting proteins. However, a limitation of conventional two-hybrid systems for detecting AR protein partners has been that they only allow for analysis of the ligand- and DNA-binding domains of the receptor, as its NTD (N-terminal domain) possesses intrinsic transactivation activity. To identify AR N-terminus-interacting proteins, its NTD was used in the RTA (repressed transactivator) system, which is specifically designed for transactivator bait proteins and was shown to be suitable for two-hybrid analysis with the AR NTD. DDC (L-dopa decarboxylase) was detected multiple times as a novel AR-interacting protein, which was subsequently confirmed in vitro and in vivo. Furthermore, transient transfection of DDC in prostate cancer cells strongly enhanced ligand-dependent AR transcriptional activity, an effect that was antagonized using high concentrations of the anti-androgen bicalutamide. Glucocorticoid receptor activity was also strongly enhanced with DDC co-transfection, while oestrogen receptor activity was only mildly affected. Together, our data demonstrate that DDC interacts with AR to enhance steroid receptor transactivation, which may have important implications in prostate cancer progression.

Functional localization and competition between the androgen receptor and T-cell factor for nuclear beta-catenin: a means for inhibition of the Tcf signaling axis

Recent reports suggest that the beta-catenin-T-cell factor (Tcf) (BCT) signaling pathway is important in the progression of prostate cancer. Evidence suggests that the androgen receptor (AR) can repress BCT-mediated transcription both in prostate cancer and colon cancer cells (Chesire and Isaacs, 2002). In this study, we validate such findings and show that repression of BCT signaling is facilitated by competition between the AR and Tcf. Measurements of the Tcf transcriptional reporter (TOPFLASH) indicated that AR+DHT-mediated repression can inhibit BCT transcription in the presence of WT and exogenous activating beta-catenin (Delta1-130 bp). Transient transfections in SW480 cells (APC(mut/mut)) showed that this mode of repression is functionally independent of APC-mediated beta-catenin ubiquitination. Using a recently developed red flourescent protein (HcRed), we demonstrate novel observations about the nuclear distribution of Tcf. Furthermore, with the use of red (HcRed-AR and HcRed-Tcf) and green fusion proteins (beta-catenin-EGFP), we provide morphological evidence of a reciprocal balance of nuclear beta-catenin-EGFP (BC-EGFP). By cotransfecting in LNCaP prostate tumor cells and using quantitative imaging software, we demonstrated a 62.0% colocalization of HcRed-AR and BC-EGFP in the presence of DHT and 63.3% colocalization of HcRed-Tcf/BC-EGFP in the absence of DHT. Costaining for activated RNA Pol II (phosphoserine 2) and HcRed-Tcf suggested that Tcf foci contain transcriptional 'hotspots' validating that these sites have the capacity for transcriptional activity. Given this apparent androgen-dependent competition for nuclear BC-EGFP, we chose to assess our hypothesis by in vivo and in vitro binding assays. SW480 cells transiently transfected with an AR expression construct, treated with DHT and immunoprecipitated for Tcf showed less associated beta-catenin when compared to Tcf precipitates from untreated cells. Furthermore, by treating cells with DHT+Casodex, we were able to abrogate the androgen-sensitive AR/beta-catenin interaction, in addition to relieving transcriptional repression of the TOPFLASH reporter. In vitro binding assays, with increasing amounts of AR(S35), resulted in decreased Tcf(S35) association with immunoprecipitated recombinant beta-catenin-HIS. These data suggest that in steady-state conditions, AR has the ability to compete out Tcf binding for beta-catenin. Finally, using SW480 cells, we show that AR-mediated repression of the BCT pathway has implications for cell cycle progression and in vitro growth. Using FACs analysis, we observed a 26.1% increase in accumulation of cells in the G1 phase of the cell cycle, while in vitro growth assays showed a 35% reduction in viable cells transfected with AR+DHT treatment. Together, our data strongly suggest that a reciprocal balance of nuclear beta-catenin facilitates AR-mediated repression of BCT-driven transcription and cell growth.

The role of hepatocyte nuclear factor-3 alpha (Forkhead Box A1) and androgen receptor in transcriptional regulation of prostatic genes

Androgens and mesenchymal factors are essential extracellular signals for the development as well as the functional activity of the prostate epithelium. Little is known of the intraepithelial determinants that are involved in prostatic differentiation. Here we found that hepatocyte nuclear factor-3 alpha (HNF-3 alpha), an endoderm developmental factor, is essential for androgen receptor (AR)-mediated prostatic gene activation. Two HNF-3 cis-regulatory elements were identified in the rat probasin (PB) gene promoter, each immediately adjacent to an androgen response element. Remarkably, similar organization of HNF-3 and AR binding sites was observed in the prostate-specific antigen (PSA) gene core enhancer, suggesting a common functional mechanism. Mutations that disrupt these HNF-3 motifs significantly abolished the maximal androgen induction of PB and PSA activities. Overexpressing a mutant HNF-3 alpha deleted in the C-terminal region inhibited the androgen-induced promoter activity in LNCaP cells where endogenous HNF-3 alpha is expressed. Chromatin immunoprecipitation revealed in vivo that the occupancy of HNF-3 alpha on PSA enhancer can occur in an androgen-depleted condition, and before the recruitment of ligand-bound AR. A physical interaction of HNF-3 alpha and AR was detected through immunoprecipitation and confirmed by glutathione-S-transferase pull-down. This interaction is directly mediated through the DNA-binding domain/hinge region of AR and the forkhead domain of HNF-3 alpha. In addition, strong HNF-3 alpha expression, but not HNF-3 beta or HNF-3 gamma, is detected in both human and mouse prostatic epithelial cells where markers (PSA and PB) of differentiation are expressed. Taken together, these data support a model in which regulatory cues from the cell lineage and the extracellular environment coordinately establish the prostatic differentiated response.

RanBPM, a nuclear protein that interacts with and regulates transcriptional activity of androgen receptor and glucocorticoid receptor

The androgen receptor (AR) is a ligand-dependent transcription factor that has an essential role in the normal growth, development, and maintenance of the prostate gland. The AR is part of a large family of steroid receptors that also includes the glucocorticoid, progesterone, and mineralocorticoid receptors. Steroid receptor family members share significant homology at their DNA and ligand-binding domains. However, these receptors exhibit a high degree of sequence variability at their NH(2)-terminal domain, which suggests the possibility of receptor-specific interactions with co-regulator proteins. Transcriptional co-regulators that interact with the AR may have a role in defining AR activity and may be involved in directing AR-specific responses. Here we have identified Ran-binding protein in the microtubule-organizing center (RanBPM) to be a novel AR-interacting protein by yeast two-hybrid assay and have confirmed this interaction by glutathione S-transferase- and His-tagged pull-down assays. In addition, transient overexpression of RanBPM in prostate cancer cell lines resulted in enhanced AR activity in a ligand-dependent fashion. Glucocorticoid receptor activity was also enhanced when RanBPM was overexpressed, whereas estrogen receptor activity remained unchanged. These data demonstrate that RanBPM interacts with steroid receptors to selectively modify their activity.

Ligand-independent activation of the androgen receptor by interleukin-6 and the role of steroid receptor coactivator-1 in prostate cancer cells

The androgen receptor (AR) can be activated in the absence of androgens by interleukin-6 (IL-6) in human prostate cancer cells. The events involved in ligand-independent activation of the AR are unknown, but have been suggested to involve phosphorylation of the AR itself or a receptor-associated protein. Steroid receptor coactivator-1 (SRC-1) has been shown to interact with the human AR and to modulate ligand-dependent AR transactivation and is regulated by phosphorylation by MAPK. To date, no one has examined the role of SRC-1 in ligand-independent activation of the AR by IL-6 or other signaling pathways known to activate the full-length receptor. This study addressed this and has revealed the following. 1) SRC-1 similarly enhanced ligand-independent activation of the AR by IL-6 to the same magnitude as that obtained via ligand-dependent activation. 2) Androgen and IL-6 stimulated the MAPK pathway. 3) MAPK was required for both ligand-dependent and ligand-independent activation of the AR. 4) Phosphorylation of SRC-1 by MAPK was required for optimal ligand-independent activation of the AR by IL-6. 5) Protein-protein interaction between endogenous AR and SRC-1 was dependent upon treatment of LNCaP cells with IL-6 or R1881. 6) Protein-protein interaction between the AR N-terminal domain and SRC-1 was independent of MAPK. 7) Ligand-independent activation of the AR did not occur by a mechanism of overexpression of either solely wild-type SRC-1 or mutant SRC-1 that mimics its phosphorylated form.

Activation of the androgen receptor N-terminal domain by interleukin-6 via MAPK and STAT3 signal transduction pathways

The androgen receptor (AR) is a ligand-activated transcription factor that mediates the biological responses of androgens. However, non-androgenic pathways have also been shown to activate the AR. The mechanism of cross-talk between the interleukin-6 (IL-6) and AR signal transduction pathways was investigated in LNCaP human prostate cancer cells. IL-6 induced several androgen-response element-driven reporters that are dependent upon the AR, increased the phosphorylation of mitogen-activated protein kinase (MAPK), and activated the AR N-terminal domain (NTD). Inhibitors to MAPK and JAK decreased the IL-6-induced phosphorylation of MAPK and activation of the AR NTD. Immunoprecipitation and transactivation studies showed a direct interaction between amino acids 234-558 of the AR NTD and STAT3 following IL-6 treatment of LNCaP cells. These results demonstrate that activation of the human AR NTD by IL-6 was mediated through MAPK and STAT3 signal transduction pathways in LNCaP prostate cancer cells.

Prostate cancer and the androgen receptor: strategies for the development of novel therapeutics

The early demonstrations that prostate cancer was hormone-sensitive initiated a therapeutic strategy of hormone ablation that is still in use today. Although chemical or surgical castration reduces androgen stimulation of the androgen receptor (AR) and produces tumor regression, little survival benefit is achieved. Patients with metastatic cancer eventually relapse as their tumors progress to hormone independence. The AR is a member of the steroid receptor family; however, it manifests many unique features including: N-terminal, C-terminal interactions and antiparallel dimerization, unique N-terminal domains for co-factor recruitment, AR-specific co-activators and upstream promoter/enhancer response elements that amplify AR-mediated responses. The AR is regulated by phosphorylation and cross-talk with several signaling pathways, including MAP kinases, PKA and PKC. Non-genomic effects of AR to regulate transcription factors elk-1 and -2 have also been demonstrated. These unique features suggest mechanisms by which novel therapeutics might target and influence AR-mediated actions. Progress in this direction has been realized with the recent synthesis of non-steroidal androgen agonists that may have tissue-selective effects.

Cooperative assembly of androgen receptor into a nucleoprotein complex that regulates the prostate-specific antigen enhancer

Prostate cancer is characterized by elevated serum levels of prostate-specific antigen (PSA). PSA gene expression is controlled by an androgen-responsive transcriptional enhancer. Our study suggests that formation of a nucleoprotein complex, encompassing 170 base pairs of enhancer DNA, mediates androgen-responsive PSA enhancer activity. The complex is assembled by cooperative binding of androgen receptor to at least four tandem, nonconsensus androgen response elements (AREs). Systematic mutagenesis of the AREs demonstrated that they act synergistically to stimulate androgen receptor-responsive gene expression. We discuss a mechanism whereby a combination of high androgen receptor levels in the prostate and low affinity AREs contribute to the cell type specificity and activity of the enhancer.

Androgen-independent induction of prostate-specific antigen gene expression via cross-talk between the androgen receptor and protein kinase A signal transduction pathways

Transcription of the prostate-specific antigen (PSA) gene escapes regulation by androgens in advanced prostate cancer. To determine the molecular mechanism(s) of androgen-independent regulation of the PSA gene, the possibility that the androgen receptor (AR) is activated in the absence of androgen by stimulation of protein kinase A (PKA) was investigated. Activation of PKA by forskolin resulted in elevated expression of the PSA gene in androgen-depleted LNCaP cells, an effect that was blocked by the antiandrogen, bicalutamide. Further evidence that induction of PSA gene expression was dependent on AR was obtained from experiments using PC3 cells devoid of AR. Neither PSA, PB, nor ARR3 androgen-responsive reporters could be induced by activation of PKA in the absence of transfected AR. In addition, when nuclear AR from forskolin-treated LNCaP cells was incubated with oligonucleotides encoding an androgen response element of the PSA promoter and examined by electromobility shift assay, an increase in AR-androgen response element complex formation was observed. Lastly, cotransfection of an expression vector for a chimeric protein encoding the amino-terminal domain of the human AR linked to Gal4 and a 5xGal4UAS reporter gene construct resulted in activation of the amino-terminal domain of the AR by stimulation of PKA activity. These results demonstrate androgen-independent induction of PSA gene expression in prostate cancer cells by an AR-dependent pathway.

Multiple receptor domains interact to permit, or restrict, androgen-specific gene activation

A critical problem within transcription factor families is how diverse regulatory programs are directed by highly related members. Androgen and glucocorticoid receptors (AR, GR) recognize a consensus DNA hormone response element (HRE), but they activate target genes with precise specificity, largely dependent on the promoter and cell context. We have assessed the role of different receptor domains in hormone-specific response by testing chimeras of AR and GR for their ability to activate the androgen-specific enhancer of the mouse sex-limited protein (Slp) gene. Although all of the mutant receptors activated simple HREs, only a few activated the androgen-specific element. One component shared by receptors functional on the AR-specific target was the AR DNA binding domain. Activation was not due to differential DNA affinity but rather to the AR DNA binding domain escaping suppression directed at the GR DNA binding domain in this enhancer context. A further mechanism increasing specific activation was cooperation of receptors at multiple and weak HREs, which was accentuated in the presence of both the AR N terminus and ligand binding domain. These domains together increased recognition of weak HREs, as demonstrated by in vitro DNase I footprinting and transactivation of mutant enhancers. Further, AR N-terminal subdomains reported to interact directly with the ligand binding domain relieved an inhibitory effect imposed by that domain. Therefore, functions intrinsic to AR augment steroid-specific gene activation, by evading negative regulation operating on the domains of other receptors and by enhancing cooperativity through intra- and inter-receptor domain interactions. These subtle distinctions in AR and GR behavior enforce transcriptional specificity established by the context of nonreceptor factors.

Differential transactivation by the androgen receptor in prostate cancer cells

BACKGROUND

The purpose of this study was to determine the contribution of different transactivating regions of the androgen receptor (AR) to the induction of androgen-regulated promoters in poorly (PC3 cells) and well-differentiated (LNCaP cells) prostate cancer cell lines.

METHODS

PC3 and LNCaP cells were co-transfected with plasmids expressing full-length AR or deletion mutants together with luciferase reporters linked to the probasin (PB) and PSA promoters; as well as to ARR3tk, a PB-derived recombinant promoter.

RESULTS

Androgen induction of the ARR3tk promoter in the presence of AR was 8- to 10-fold higher than that seen with the PB promoter. Activation of ARR3tk was greatest with an androgen-independent construct in which the first 231 amino acids and the ligand binding domain had been removed, indicating that this promoter is more responsive to activating functions in the N-terminal domain than in the ligand binding domain. By comparison, induction of the PB promoter was greatest with the full-length AR, which suggests that the ligand binding domain also makes a major contribution to the activation of this promoter. In similar analyses with the PSA promoter, AR regions required for promoter induction was dependent on the host cell type. In PC3 cells, the predominant AR transactivation function was androgen-independent and resided in the N-terminal domain, whereas in LNCaP cells, the highest level of induction was androgen dependent and also required participation of the ligand binding domain.

CONCLUSIONS

Our results indicate that the relative utilization of transactivating functions in N-terminal and ligand binding domains of the AR is promoter and cell specific.

Interaction of the human androgen receptor transactivation function with the general transcription factor TFIIF

The human androgen receptor (AR) is a ligand-activated transcription factor that regulates genes important for male sexual differentiation and development. To better understand the role of the receptor as a transcription factor we have studied the mechanism of action of the N-terminal transactivation function. In a protein-protein interaction assay the AR N terminus (amino acids 142-485) selectively bound to the basal transcription factors TFIIF and the TATA-box-binding protein (TBP). Reconstitution of the transactivation activity in vitro revealed that AR142-485 fused to the LexA protein DNA-binding domain was competent to activate a reporter gene in the presence of a competing DNA template lacking LexA binding sites. Furthermore, consistent with direct interaction with basal transcription factors, addition of recombinant TFIIF relieved squelching of basal transcription by AR142-485. Taken together these results suggest that one mechanism of transcriptional activation by the AR involves binding to TFIIF and recruitment of the transcriptional machinery.

Androgenic induction of prostate-specific antigen gene is repressed by protein-protein interaction between the androgen receptor and AP-1/c-Jun in the human prostate cancer cell line LNCaP

In exploring the possible mechanisms of androgen independence of prostate-specific antigen (PSA) gene expression, we investigated the effect of elevating AP-1 by both 12-O-tetradecanoylphorbol 13-acetate (TPA) treatment and transfection of the c-Jun expression vector in LNCaP cells. Transcription of PSA is initiated when ligand-activated androgen receptor (AR) binds to a region in the PSA promoter that contains an androgen-responsive element (ARE). It was found that TPA inhibited androgen-induced PSA gene expression by a mechanism that did not alter nuclear levels of AR protein. Overexpression of AP-1 (jun and fos proteins) also inhibited androgen-induced PSA promoter activity. These observations were apparently related to the disruption of AR.ARE complexes as demonstrated by the results of electrophoretic mobility shift assays. Specifically, c-Jun inhibited the formation of AR.ARE complexes and conversely that AR-glutathione S-transferase proteins inhibited the formation of c-Jun.TPA-responsive element (TRE) complexes. Consistent with the inhibitory effect of both proteins, anti-c-Jun antibody blocked the inhibition of AR.ARE complex formation by c-Jun. A similar, but less marked, effect was obtained when anti-AR antibody was used to prevent AR inhibition of c-Jun.TRE complex formation. These findings together with results obtained from co-immunoprecipitation experiments strongly suggest that mutual repression of DNA binding activity is due to direct interaction between the two proteins and that the degree of repression may be determined by the ratio of AR to c-Jun. The mechanism of repression studied in mutant analysis experiments yielded evidence of an interaction between the DNA- and ligand-binding domains of AR and the leucine zipper region of c-Jun. Thus, the AR is similar to other nuclear receptors in its ability to interact with AP-1. This association provides a link between AP-1 and AR signal transduction pathways and may play a role in the regulation of the androgen-responsive PSA gene.