Androgen receptor: a key molecule in the progression of prostate cancer to hormone independence

Molecular basis of agonicity and antagonicity in the androgen receptor studied by molecular dynamics simulations

Treatment of prostate cancer patients with antiandrogens is initially successful, though the therapy often becomes refractory over the time. This mechanism is not fully understood, but the presence of androgen receptor (AR) mutant forms which are activated by antiandrogens and other endogenous ligands, and overexpression of the receptor have been suggested. In an attempt to explain the molecular basis for agonicity and antagonicity in the androgen receptor, and the changes on biological activity of subtle modifications at the ligand and receptor (mutations) level, molecular dynamics simulations were performed on the androgen receptor wild type (WT), and T877A and W741 mutant forms, complexed with several non-steroidal androgens. The stabilizing role of residues from helices 3, 5, 11 and 12 was observed in non-steroidal androgens R-3, S-1, and R-bicalutamide and hydroxyflutamide in resistant mutations. In the AR WT antiandrogen R-bicalutamide complex, destabilization of M895 by both W741 and the sulfonyl linkage of the ligand may be responsible for reported antagonism. Changes in the ligand or mutations alleviating this effect were observed to stabilize the receptor in the active conformation, thus developing resistance to R-bicalutamide. The results presented provide a plausible explanation for the molecular basis of agonicity and antagonicity in the androgen receptor, and complement previous studies using static crystal structures, incorporating for the first time protein dynamics into the analysis. Thus, our results provide a valuable framework for the structure-based design of improved antiandrogens.

Alterations in beta-catenin expression and localization in prostate cancer

BACKGROUND

Wnt signaling is thought to be important in prostate cancer, in part because proteins such as beta-catenin can also affect androgen receptor signaling. beta-Catenin forms a cell adhesion complex with E-cadherin raising the possibility that loss of expression or a change in beta-catenin distribution in the cell could also alter downstream signaling, decreased inter-cellular adhesion and the promotion of metastasis. A number of studies have reported the altered expression and/or localization of beta-catenin as a biomarker in prostate cancer.

METHODS

Tissue microarrays comprised of BPH and low, moderate and high-grade prostate cancer (n=77) were assessed for beta-catenin expression and distribution using immunohistochemistry. Staining was also performed on a tissue microarray containing tissue from patients before and after hormone manipulation. The effects of fixation and different antibodies was assessed on fixed LNCaP cell pellets and small prostate tissue microarrays.

RESULTS

We have observed increased beta-catenin expression in only high Gleason score (>7) prostate cancer. A nuclear re-distribution of beta-catenin has previously been reported. We noted nuclear beta-catenin in benign prostatic hyperplasia and a gradual loss in nuclear distribution with increasing Gleason grade. We found no evidence for an alteration in beta-catenin expression or re-distribution with hormone ablation. Altered fixation, antibodies and antibody concentration did affect the intensity and specificity of staining.

CONCLUSIONS

A loss of nuclear beta-catenin is the most consistent feature in prostate cancer rather than absolute levels of expression. We also suggest that variation in immunohistochemical protocols may explain variations in the reported literature.

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.

An S296R mutation in the human androgen receptor causes activation of the receptor by non-androgenic steroids and stronger inhibition by the nuclear receptor corepressor N-coR

Mutation of the androgen receptor (AR) is believed to contribute to androgen-independent growth of prostate cancer. In the present study, we examined the functional changes associated with the novel somatic mutation S296R in the N-terminal domain of the AR identified from one recurrent prostate cancer sample. The results indicate that the S296R mutation does not differ obviously from the wild-type AR in its ability to bind the synthetic androgen methyltrienolone, or in its transcriptional activity induced by dihydrotestosterone (DHT) in the absence or presence of the overexpression of coactivators (steroid receptor coactivator-1, transcription intermediary factor-2, cAMP response element-binding protein-binding protein and p300). However, S296R was found to differ from wild-type AR in that its transcriptional activity could be activated by high concentrations (1 micromol/L) of 17beta-oestradiol and progesterone and its transactivity induced by DHT was more obviously inhibited by overexpression of the nuclear receptor corepressor N-coR in CV-1 cells. These findings indicate that a point mutation (S296R) in the N-terminal domain of the AR can decrease the ligand specificity of the AR and alter the interaction between S296R and the corepressor N-coR.

Diverse roles of androgen receptor (AR) domains in AR-mediated signaling

Androgens control male sexual development and maintenance of the adult male phenotype. They have very divergent effects on their target organs like the reproductive organs, muscle, bone, brain and skin. This is explained in part by the fact that different cell types respond differently to androgen stimulus, even when all these responses are mediated by the same intracellular androgen receptor. To understand these tissue- and cell-specific readouts of androgens, we have to learn the many different steps in the transcription activation mechanisms of the androgen receptor (NR3C4). Like all nuclear receptors, the steroid receptors have a central DNA-binding domain connected to a ligand-binding domain by a hinge region. In addition, all steroid receptors have a relatively large amino-terminal domain. Despite the overall structural homology with other nuclear receptors, the androgen receptor has several specific characteristics which will be discussed here. This receptor can bind two types of androgen response elements (AREs): one type being similar to the classical GRE/PRE-type elements, the other type being the more divergent and more selective AREs. The hormone-binding domain has low intrinsic transactivation properties, a feature that correlates with the low affinity of this domain for the canonical LxxLL-bearing coactivators. For the androgen receptor, transcriptional activation involves the alternative recruitment of coactivators to different regions in the amino-terminal domain, as well as the hinge region. Finally, a very strong ligand-induced interaction between the amino-terminal domain and the ligand-binding domain of the androgen receptor seems to be involved in many aspects of its function as a transcription factor. This review describes the current knowledge on the structure-function relationships within the domains of the androgen receptor and tries to integrate the involvement of different domains, subdomains and motifs in the functioning of this receptor as a transcription factor with tissue- and cell-specific readouts.

Differential effects of resveratrol on androgen-responsive LNCaP human prostate cancer cells in vitro and in vivo

Resveratrol is a phytochemical that has been under consideration for use as a prostate cancer chemopreventive agent. However, the efficacy, as well as the mechanisms of action of resveratrol on prostate cancer prevention, remains largely unknown. This study seeks to address these questions and examine the cancer preventive effects of resveratrol using complementary human LNCaP prostate cancer cell culture and xenograft models. In cultured LNCaP cells, we found that resveratrol inhibited cell growth. The growth inhibitory effects of resveratrol appeared to be through modulation of both androgen- and estrogen-mediated events. Global gene expression analysis using microarrays identified androgen-responsive genes as a group of genes universally affected by resveratrol in LNCaP cells in vitro. The effect of resveratrol on expression of these genes appeared to be through inhibition of both androgen- and estrogen-mediated transcription. In a xenograft model, resveratrol delayed LNCaP tumor growth and inhibited expression of a marker for steroid hormone responses. However, exposure to resveratrol also led to increased angiogenesis and inhibition of apoptosis in the xenograft. In summary, resveratrol may act through modulation of steroid hormone-dependent pathways to inhibit prostate cancer cell growth in both culture and xenografts, but exposure in vivo may be of concern.

Androgen receptor overexpression in prostate cancer linked to Pur alpha loss from a novel repressor complex

Increased androgen receptor (AR) expression and activity are pivotal for androgen-independent (AI) prostate cancer (PC) progression and resistance to androgen-deprivation therapy. We show that a novel transcriptional repressor complex that binds a specific sequence (repressor element) in the AR gene 5'-untranslated region contains Pur alpha and hnRNP-K. Pur alpha expression, its nuclear localization, and its AR promoter association, as determined by chromatin immunoprecipitation analysis, were found to be significantly diminished in AI-LNCaP cells and in hormone-refractory human PCs. Transfection of AI cells with a plasmid that restored Pur alpha expression reduced AR at the transcription and protein levels. Pur alpha knockdown in androgen-dependent cells yielded higher AR and reduced p21, a gene previously shown to be under negative control of AR. These changes were linked to increased proliferation in androgen-depleted conditions. Treatment of AI cells with histone deacetylase and DNA methylation inhibitors restored Pur alpha protein and binding to the AR repressor element. This correlated with decreased AR mRNA and protein levels and inhibition of cell growth. Pur alpha is therefore a key repressor of AR transcription and its loss from the transcriptional repressor complex is a determinant of AR overexpression and AI progression of PC. The success in restoring Pur alpha and the repressor complex function by pharmacologic intervention opens a promising new therapeutic approach for advanced PC.

An AKT activity threshold regulates androgen-dependent and androgen-independent PSA expression in prostate cancer cell lines

The androgen receptor (AR) plays an important role in early prostate cancer by activating transcription of a number of genes participating in cell proliferation and growth and cancer progression. However, as the cancer progresses, prostate cancer cells transform from an androgen-dependent to an androgen-independent state. Androgen-independent prostate cancer can manifest itself in several forms, including a percentage of cancers that show reduced levels of prostate-specific antigen (PSA) and can progress without the need for the ligand or active receptor. Therefore, our goal was to examine the role of intracellular signaling pathways in an androgen-independent prostate cancerin vitromodel. Using the cell line PC3(AR)2, we stimulated cells with 5-α-dihydrotestosterone (DHT) and epidermal growth factor (EGF) and then analyzed PSA expression. We observed lower PSA expression when cells were jointly stimulated with DHT and EGF, and this was associated with an increase in AKT activity. We examined the role of AKT in AR activity and PSA expression by creating stable PC3(AR)2cell lines transfected with a PI3K-Ras-effector loop mutant. These cell lines showed lower DHT-stimulated PSA expression that correlated to changes in the phosphorylated state of AR. Therefore, we propose anin vitroandrogen-independent model in which a PI3K/AKT activity threshold and subsequent AR transactivation regulate PSA expression.

Interleukin-8 signaling promotes androgen-independent proliferation of prostate cancer cells via induction of androgen receptor expression and activation

The aim of our study was to assess the importance of the CXC chemokine and interleukin (IL)-8 in promoting the transition of prostate cancer (CaP) to the androgen-independent state. Stimulation of the androgen-dependent cell lines, LNCaP and 22Rv1, with exogenous recombinant human interleukin-8 (rh-IL-8) increased androgen receptor (AR) gene expression at the messenger RNA (mRNA) and protein level, assessed by quantitative polymerase chain reaction and immunoblotting, respectively. Using an androgen response element-luciferase construct, we demonstrated that rh-IL-8 treatment also resulted in increased AR transcriptional activity in both these cell lines, and a subsequent upregulation of prostate-specific antigen and cyclin-dependent kinase 2 mRNA transcript levels in LNCaP cells. Blockade of CXC chemokine receptor-2 signaling using a small molecule antagonist (AZ10397767) attenuated the IL-8-induced increases in AR expression and transcriptional activity. Furthermore, in 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays, coadministration of AZ10397767 reduced the viability of LNCaP and 22Rv1 cells exposed to bicalutamide. Our data show that IL-8 signaling increases AR expression and promotes ligand-independent activation of this receptor in two androgen-dependent cell lines, describing two mechanisms by which this chemokine may assist in promoting the transition of CaP to the androgen-independent state. In addition, our data show that IL-8-promoted regulation of the AR attenuates the effectiveness of the AR antagonist bicalutamide in reducing CaP cell viability.

Pharmacological exploitation of the peroxisome proliferator-activated receptor gamma agonist ciglitazone to develop a novel class of androgen receptor-ablative agents

On the basis of our finding that the peroxisome proliferator-activated receptor gamma (PPARgamma) agonist ciglitazone at high doses was able to mediate PPARgamma-independent transcriptional repression of androgen receptor (AR) in a tumor cell-specific manner, we used Delta2CG, a PPARgamma-inactive analogue of ciglitazone, to conduct lead optimization to develop a novel class of AR-ablative agents. Structure-activity analysis indicates a high degree of flexibility in realigning Delta2CG's structural moieties without compromising potency in AR repression, as evidenced by the higher AR-ablative activity of the permuted isomer 9 [( Z)-5-(4-hydroxybenzylidene)-3-(1-methylcyclohexylmethyl)thiazolidine-2,4-dione]. Further modifications of 9 gave rise to 12 [( Z)-5-(4-hydroxy-3-trifluoromethylbenzylidene)-3-(1-methylcyclohexylmethyl)thiazolidine-2,4-dione], which completely inhibited AR expression in LNCaP cells at low micromolar concentrations. This AR down-regulation led to growth inhibition in LNCaP cells through apoptosis induction. Moreover, the role of AR repression in the antiproliferative effect of compound 12 was validated by the differential inhibition of cell viability between androgen-responsive and androgen-nonresponsive cells.

Androgen receptor and invasion in prostate cancer

Activation of androgen receptor (AR) stimulates the growth of not only androgen-dependent but also of androgen-refractory prostate cancer. However, neither the role of AR in invasion/metastasis nor the relationship between invasiveness and androgen-refractory status has been established. In this study, we used the androgen-dependent prostate cancer cell line MDA PCa 2b, derived from a human bone metastasis, to generate an invasive subline (MDA-I) using a Matrigel chamber. MDA-I cells expressed higher levels of AR and prostate-specific antigen than their less invasive parental cells. Blocking AR function or removal of androgen suppressed the invasion of MDA-I cells, whereas stimulating AR increased invasion. In addition, forced AR overexpression increased the invasiveness of MDA PCa 2b cells. Next, we showed that an androgen-refractory subline (MDA-hr) of MDA PCa 2b cells also expressed higher levels of AR and were more invasive than their parental androgen-dependent cells. Blocking AR function suppressed the invasiveness of MDA-hr cells. Gelatin zymography indicated that matrix metalloproteinase 2 (MMP-2) and MMP-9 activities were regulated by AR signaling and closely correlated with the invasiveness of the androgen-dependent and androgen-refractory prostate cancer cells. These data suggest that AR promotes the invasiveness of both androgen-dependent and androgen-refractory prostate cancer and that a more invasive phenotype might develop through AR activation during cancer progression. These findings potentially support the use of adjuvant hormonal therapy and the future development of more potent androgen blockade therapy.

Androgen receptor coactivator ARA70alpha and ARA70beta isoform-specific antibodies: new tools for studies of expression and immunohistochemical localization

ARA70 is a coactivator of androgen receptor (AR), a ligand-dependent transcription factor that plays an important role in prostate cancer. There are 2 variants of ARA70, the full length 70 kd ARA70alpha isoform and the internally spliced 35 kd ARA70beta isoform. Recent studies have suggested different expression and roles of the 2 isoforms in several endocrine malignancies, including prostate, breast, and ovarian cancers. To study the roles of these isoforms in cancers, we produced isoform-specific polyclonal antibodies. The anti-ARA70alpha antibody was raised in rabbits against 326 amino acid peptide corresponding to the internal deletion missing from ARA70beta (ARA70id), whereas the anti-ARA70beta antibody was raised against 18 amino acid polypeptide spanning the splice junction, with Gln-Gln motif unique to ARA70beta. The antisera were affinity purified on CNBr-activated sepharose 4B, and their specificity tested against bacterially expressed, Ni-column-purified ARA70alpha, ARA70beta, and ARA70id. The anti-ARA70alpha antibody recognized ARA70alpha and ARA70id, but not ARA70beta. The anti-ARA70beta antibody was specific to ARA70beta and did not cross-react with ARA70alpha or ARA70id. We then used these antibodies to detect ARA70 isoforms in crude extracts made of prostate cancer cell lines and performed immunohistochemical localization of these proteins in prostate tissues. ARA70beta localized to the cytosol, whereas ARA70alpha was found in the nucleus, supporting the notion of their dissimilar functions.

Decrease in stromal androgen receptor associates with androgen-independent disease and promotes prostate cancer cell proliferation and invasion

Androgen receptor (AR) is expressed in both stromal and epithelial cells of the prostate. The majority of studies on AR expression and function in prostate cancer is focused on malignant epithelial cells rather than stromal cells. In this study, we examined the levels of stromal AR in androgen-dependent and -independent prostate cancer and the function of stromal AR in prostate cancer growth and invasion. We showed that stromal AR levels were decreased in the areas surrounding cancerous tissue, especially in androgen-independent cancer. Using two telomerase-immortalized human stromal cell lines, one AR-positive and the other AR-negative, we demonstrated that stromal cells lacking AR stimulated cell proliferation of co-cultured prostate cancer cells in vitro and enhanced tumour growth in vivo when co-injected with PC3 epithelial cells in nude mice. In contrast, stromal cells expressing AR suppressed prostate cancer growth in vitro and in vivo. In parallel with cancer growth, in vitro invasion assays revealed that stromal cells lacking AR increased the invasion ability of PC3 cell by one order of magnitude, while stromal cells expressing AR reduced this effect. These results indicate a negative regulation of prostate cancer growth and invasion by stromal AR. This provides potentially new mechanistic insights into the failure of androgen ablation therapy, and the reactivation of stromal AR could be a novel therapeutic approach for treating hormone refractory prostate cancer.

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.

Castration-recurrent prostate cancer is not androgen-independent

An American man is diagnosed with prostate cancer (PC) every 3 min and dies from the disease every 17 min. Although androgen receptor (AR) expression is diminished following androgen deprivation therapy (ADT) that induces clinical remission in most patients, castration-recurrent PC expresses levels of AR protein similar to those found in androgen-stimulated PC and benign prostate. This observation suggests that the AR may be as important for growth regulation in castration-recurrent PC, as it is in androgen-stimulated PC and benign hyperplasia. Neither ligand-independence, point mutations, glutamine and/or glycine repeat expansion nor amplification have explained AR activation in most cases of castration-recurrent PC. Castration-recurrent PC tissue has levels of testosterone (T) similar to androgen-stimulated benign prostate and levels of dihydrotestosterone (DHT), the most active androgen for AR activation that are approximately 10% of androgen-stimulated benign prostate. These levels of tissue androgens appear capable of activating the AR since prostate-specific antigen (PSA), the classic androgen-regulated gene, is expressed at similar tissue levels in castration-recurrent and androgen-stimulated PC. These startling findings suggest a paradigm shift; PC that recurs during ADT is not androgen-independent.

Stimulation of prostate cancer cellular proliferation and invasion by the androgen receptor co-activator ARA70

ARA70 was first identified as a gene fused to the ret oncogene in thyroid carcinoma and subsequently as a co-activator for androgen receptor (AR). Two isoforms of ARA70 have been identified: a 70-kDa version called ARA70 alpha and an internally spliced 35-kDa variant termed ARA70 beta. We have previously reported that ARA70 alpha expression is reduced in prostate cancer, and its overexpression inhibits proliferation of LNCaP prostate cancer cells. However, the function of the ARA70 beta isoform in prostate cancer is not understood. In this report we examined the effects of ARA70 beta on AR transcriptional regulation as well as prostate cancer cellular proliferation and invasion. Although both ARA70 alpha and ARA70 beta functioned as transcriptional co-activators of AR in cell-based reporter assays, ARA70 beta overexpression, in contrast to ARA70 alpha, promoted prostate cancer cellular proliferation and invasion through Matrigel. Interestingly, genome-wide expression profiling of cells expressing ARA70 beta revealed an increase in the expression of genes involved in the control of cell division and adhesion, compatible with a role for ARA70 beta in proliferation and invasion. Consistent with its function in promoting cell growth and invasion, ARA70 beta expression was increased in prostate cancer. Our findings implicate ARA70 beta as a regulator of tumor cell growth and metastasis by affecting gene expression.

ZIP kinase plays a crucial role in androgen receptor-mediated transcription

The androgen receptor (AR) is a ligand-dependent transcription factor that plays a crucial role in the development and homeostasis of the prostate and in prostate cancer. The transcriptional activity of AR is mediated by interaction with multiple co-activators, which serve in chromatin modification or remodeling, or provide a link between specific and general transcription factors. We have identified zipper interacting protein (ZIP) kinase as a novel transcriptional co-activator of the AR. ZIP kinase enhanced expression of AR-responsive promotor/luciferase reporter constructs in a hormone- and kinase-dependent manner. Similar results were obtained for glucocorticoid receptor but not for progesterone receptor and estrogen receptor. Following hormone treatment, AR and ZIP kinase formed physical complexes and associated with the promoter and enhancer of the prostate-specific antigen gene, as revealed by chromatin immunoprecipitation. Strikingly, depletion of ZIP kinase by siRNA led to significant reduction of AR-mediated transactivation. The interaction of ZIP kinase with AR seems to be mediated in part by apoptosis antagonizing transcription factor and in part by direct binding. Interestingly, AR was not phosphorylated by ZIP kinase in vitro, suggesting that it phosphorylates other co-activators or chromatin proteins.

Ligand-independent activation of androgen receptors by Rho GTPase signaling in prostate cancer

Prostate cancer invariably recurs after androgen deprivation therapy. Growth of this recurrent/androgen-independent form of prostate cancer may be due to increased androgen receptor (AR) transcriptional activity in the absence of androgen. This ligand-independent AR activation is promoted by some growth factors but the mechanism is not well understood. Vav3, a Rho guanosine triphosphatase guanine nucleotide exchange factor, which is activated by growth factors, is up-regulated in human prostate cancer. We show here that Vav3 levels increase during in vivo progression of prostate cancer to androgen independence. Vav3 strikingly enhanced growth factor activation of AR in the absence of androgen. Because Vav3 may be chronically activated in prostate cancer by growth factor receptors, we examined the effects of a constitutively active (Ca) form of Vav3 on AR transcriptional activity. Ca Vav3 caused nuclear localization and ligand-independent activation of AR via the Rho guanosine triphosphatase, Rac1. Ca Rac1 activation of AR occurred, in part, through MAPK/ERK signaling. Expression of active Rac1 conferred androgen-independent growth of prostate cancer cells in culture, soft agar, and mice. These findings suggest that Vav3/Rac 1 signaling is an important modulator of ligand-independent AR transcriptional activity in prostate cancer progression.

Emergence of androgen independence at early stages of prostate cancer progression in Nkx3.1; Pten mice

Although androgen deprivation therapy is a widely used treatment for patients with advanced prostate cancer, it ultimately results in the emergence of a hormone-refractory disease that is invariably fatal. To provide insights into the genesis of this disease, we have employed an in vivo model to investigate how and when prostate epithelial cells can acquire the ability to survive and proliferate in the absence of androgens. In particular, we have been studying the evolution of androgen independence in Nkx3.1; Pten mutant mice, which develop prostatic intraepithelial neoplasia and adenocarcinoma as a consequence of aging, as well as androgen-independent phenotypes following castration. We now find that the prostate epithelial cells from these Nkx3.1; Pten mutant mice are capable of surviving and proliferating in the absence of androgens and that they develop androgen-independent phenotypes well before they display overt prostatic intraepithelial neoplasia or cancer phenotypes. Our findings in this mouse model show that acquisition of androgen independence can be uncoupled from overt cancer progression and raise the possibility that hormone-refractory disease can arise at early stages of prostate carcinogenesis.

Evidence for calpain-mediated androgen receptor cleavage as a mechanism for androgen independence

Prostate carcinoma is the most commonly diagnosed cancer in men and the second leading cause of death due to cancer in Western civilization. Androgen ablation therapy is effective in treating androgen-dependent tumors, but eventually, androgen-independent tumors recur and are refractory to conventional chemotherapeutics. Hence, the emergence of androgen independence is the most challenging problem in managing prostate tumors. We report a novel mechanism of androgen independence: calpain cleaves the androgen receptor (AR) into an androgen-independent isoform. In vitro and in vivo analyses show that calpain removes the COOH-terminal ligand binding domain generating a constitutively active molecule. Analysis of human prostate tumors indicates that several tumors express higher levels of this truncated AR than noncancerous prostate tissue. In transient transfection studies, the truncated AR is three to five times more potent than the full-length receptor in transactivating transcription. The androgen-independent Rv1 cells express high levels of the truncated AR, and treatment of these cells with a calpain inhibitor reduces truncated AR expression. In the absence of androgen, inhibition of calpain activity induces apoptosis. The HIV protease inhibitor amprenavir inhibits calpain activity and is also effective in inducing apoptosis in the Rv1 cell line. The cell culture studies were reproduced in a mouse xenograft model, where, in the absence of androgens, amprenavir significantly reduces tumor growth. Together, these studies indicate that calpain-dependent proteolysis of the AR may be a mechanism of androgen independence. The calpain inhibition studies suggest that inhibiting this activity may be a potential treatment for some androgen-independent prostate tumors.

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.

A splicing variant of the androgen receptor detected in a metastatic prostate cancer exhibits exclusively cytoplasmic actions

The androgen receptor (AR) is a ligand-activated transcription factor that displays genomic actions characterized by binding to androgen-response elements in the promoter of target genes as well as nongenomic actions that do not require nuclear translocation and DNA binding. In this study, we report exclusive cytoplasmic actions of a splicing variant of the AR detected in a metastatic prostate cancer. This AR variant, named AR23, results from an aberrant splicing of intron 2, wherein the last 69 nucleotides of the intronic sequence are retained, leading to the insertion of 23 amino acids between the two zinc fingers in the DNA-binding domain. We show that the nuclear entry of AR23 upon dihydrotestosterone (DHT) stimulation is impaired. Alternatively, DHT-activated AR23 forms cytoplasmic and perinuclear aggregates that partially colocalize with the endoplasmic reticulum and are devoid of genomic actions. However, in LNCaP cells, this cytoplasmic DHT-activated AR23 remains partially active as evidenced by the activation of transcription from androgen-responsive promoters, the stimulation of NF-kappaB transcriptional activity and by the decrease of AP-1 transcriptional activity. Our data reveal novel cytoplasmic actions for this splicing AR variant, suggesting a contribution in prostate cancer progression.

Oriental herbs as a source of novel anti-androgen and prostate cancer chemopreventive agents

Androgen and androgen receptor (AR) signaling are crucial for the genesis of prostate cancer (PCa), which can often develop into androgen-ligand-independent diseases that are lethal to the patients. Recent studies show that even these hormone-refractory PCa require ligand-independent AR signaling for survival. As current chemotherapy is largely ineffective for PCa and has serious toxic sideeffects, we have initiated a collaborative effort to identify and develop novel, safe and naturally occurring agents that target AR signaling from Oriental medicinal herbs for the chemoprevention and treatment of PCa. We highlight our discovery of decursin from an Oriental formula containing Korean Angelica gigas Nakai (Dang Gui) root as a novel anti-androgen/AR agent. We have identified the following mechanisms to account for the specific anti-AR actions: rapid block of AR nuclear translocation, inhibition of binding of 5alpha-dihydrotestesterone to AR and increased proteasomal degradation of AR protein. Furthermore, decursin lacks the agonist activity of the "pure" anti-androgen bicalutamide and is more potent than bicalutamide in inducing PCa apoptosis. Structure-activity analyses reveal a critical requirement of the side-chain on decursin or its structural isomer decursinol angelate for anti-AR, cell cycle arrest and proapoptotic activities. This work demonstrates the feasibility of using activity-guided fractionation in cell culture assays combined with mechanistic studies to identify novel anti-androgen/ AR agents from complex herbal mixtures.

Ectopic expression of neurotrophic peptide derived from saposin C increases proliferation and upregulates androgen receptor expression and transcriptional activity in human prostate cancer cells

AIM

To determine the effects of the functional domain of saposin C (neurotrophic peptide [NP]) on androgen receptor (AR) expression and transcriptional activity.

METHODS

We constructed DNA vectors expressing NP or a chimeric peptide of the viral TAT transduction domain and NP (TAT-NP) using gene cloning technology. The effects of ectopic expression of NP or TAT-NP on cell growth were examined by 3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. Reverse transcription-polymerase chain reaction (RT-PCR), Western blot, transient transfection and reporter gene assays were used to determine the effects of NP on AR expression and activation.

RESULTS

NP stimulated proliferation of androgen responsive LNCaP cells in the absence of androgens. RT-PCR and Western blot analyses showed that ectopic expression of NP resulted in induction of AR gene expression, and that the NP-stimulated expression of AR could be synergistically enhanced in the presence of androgens. Furthermore, reporter gene assay results showed that NP could enhance AR transactivation by increasing androgen-inducible gene reporter activity.

CONCLUSION

We provided evidence that ectopic expression of saposin C-originated NP could upregulate AR gene expression and activate the AR transcriptional function in an androgen-independent manner in prostate cancer cells.

Increased expression of cyclin B1 sensitizes prostate cancer cells to apoptosis induced by chemotherapy

Chemotherapeutic drugs ideally should take advantage of the differences between transformed and normal cells and induce apoptosis only in cancer cells. One such difference may be the overexpression of cyclin B1 protein in cancer cells, which is required for the proper progression through mitosis. Previously, we showed that treatment of human prostate cancer cells with 2-methoxyestradiol (2-ME) or docetaxel results in an accumulation of cyclin B1 protein and an increase in cyclin B1 kinase activity, followed by induction of apoptotic cell death. Inhibition of cyclin B1 kinase lowers apoptosis induced by 2-ME and docetaxel. In this study, we established a positive correlation between cyclin B1 protein and apoptosis induced by chemotherapy in prostate cancer cells. There is minimal cyclin B1 and induction of apoptosis by chemotherapy in nontransformed cells. LNCaP and PC-3 prostate cancer cells stably overexpressing cyclin B1 are more sensitive to apoptosis induced by chemotherapy. LNCaP cells expressing cyclin B1 small interfering RNA to lower cyclin B1 protein or dominant negative cyclin-dependent kinase 1 to inhibit cyclin B1 kinase show a decrease in apoptosis. Increased sensitivity to apoptosis by overexpression of cyclin B1 may be due to lower Bcl-2, higher p53, and decreased neuroendocrine differentiation. We suggest that a cancer-specific mechanism whereby 2-ME and docetaxel may exert anti-prostate cancer activity is the deregulated activation of cyclin B1 kinase, leading to the induction of apoptotic cell death. Our results also suggest that higher levels of cyclin B1 in prostate cancer cells may be a good prognostic marker for chemotherapy.

Lycopene and Lutein Inhibit Proliferation in Rat Prostate Carcinoma Cells

Consumption of lycopene, a carotenoid without provitamin A activity, has been associated with a lower risk of prostate and breast cancer. Lutein is another carotenoid that may be associated with a reduced risk of age-related macular degeneration, the leading cause of blindness in adults 65 years of age and older. Bioactive compounds such as lycopene and lutein, derived from natural plant sources, have been shown to act at low substrate levels through the action of intrinsic cytokines and growth factors and their receptors within tissues, particularly those of the fibroblast growth factor and transforming growth factor beta families. The effects of grapefruit-derived and commercial lycopene and lutein preparations on androgen independent cultured malignant type II tumor cells [Dunning R3327AT3 or AT3 cells (androgen-responsive, slow-growing tumor cells with well developed epithelium and stroma)] were compared to their benign parent type I tumor epithelial cells (DTE). Results demonstrated that both lycopene, in an alpha -cyclodextrin water soluble carrier, and lutein inhibited malignant AT3 cells in a concentration and time-dependent manner. No such effect was observed when benign DTE cells were examined, demonstrating selective inhibition of extremely malignant AT3 prostate cancer cells relative to their benign parent. Lutein demonstrated a similar but slightly diminished response as lycopene. When cells were treated with cocktails of lycopene and lutein, no synergistic or additive effect occurred. These studies are consistent with epidemiological studies that show inverse relationships of these carotenoids with prostate cancer.

A novel class of pyranocoumarin anti-androgen receptor signaling compounds

Androgen and the androgen receptor (AR)-mediated signaling are crucial for prostate cancer development. Novel agents that can inhibit AR signaling in ligand-dependent and ligand-independent manners are desirable for the chemoprevention of prostate carcinogenesis and for the treatment of advanced prostate cancer. We have shown recently that the pyranocoumarin compound decursin from the herb Angelica gigas possesses potent anti-AR activities distinct from the anti-androgen bicalutamide. Here, we compared the anti-AR activities and the cell cycle arrest and apoptotic effects of decursin and two natural analogues in the androgen-dependent LNCaP human prostate cancer cell culture model to identify structure-activity relationships and mechanisms. Decursin and its isomer decursinol angelate decreased prostate-specific antigen expression with IC(50) of approximately 1 mumol/L. Both inhibited the androgen-stimulated AR nuclear translocation and transactivation, decreased AR protein abundance through proteasomal degradation, and induced G(0/1) arrest and morphologic differentiation. They also induced caspase-mediated apoptosis and reactive oxygen species at higher concentrations. Furthermore, they lacked the agonist activity of bicalutamide in the absence of androgen and were more potent than bicalutamide for suppressing androgen-stimulated cell growth. Decursinol, which does not contain a side chain, lacked the reactive oxygen species induction and apoptotic activities and exerted paradoxically an inhibitory and a stimulatory effect on AR signaling and cell growth. In conclusion, decursin and decursinol angelate are members of a novel class of nonsteroidal compounds that exert a long-lasting inhibition of both ligand-dependent and ligand-independent AR signaling. The side chain is critical for sustaining the anti-AR activities and the growth arrest and apoptotic effects.

The hinge region regulates DNA binding, nuclear translocation, and transactivation of the androgen receptor

The androgen receptor (AR) encoding gene can undergo mutations during the development and treatment of prostate cancer. Even in hormone-independent stages, mutations in the receptor paradoxically seem to result in an increased AR function. Two such point mutations have been described in the part of the AR involved in DNA binding and nuclear translocation, namely the hinge region. Despite a decreased nuclear translocation, these mutant ARs display increased transactivating potencies. Through detailed analysis of the hinge region, we found that deletion of residues 629 to 636 resulted in a stronger androgen response on different reporters, although this mutant displays an extremely low in vitro affinity for androgen response elements. This superactivity is independent of nuclear localization and can be inhibited by antiandrogens. Surprisingly, the AR activation functions, AF1 and AF2, are not dramatically affected when the inhibitory region (629-RKLKKLGN-636) is deleted, although cotransfected p160 coactivator TIF2 had a stronger potentiating effect in the absence of this motif. The ligand-dependent interaction between the amino-terminal domain and the ligand-binding domain (N/C interaction) plays an important role in transactivation by the AR. We found that this interaction is strongly enhanced by deletion of the inhibitory region. In conclusion, the description of prostate cancer mutations has led to the discovery of a complex role of the hinge region in nuclear localization, DNA binding, coactivator recruitment, and N/C interaction of the AR.

The zinc finger protein ras-responsive element binding protein-1 is a coregulator of the androgen receptor: implications for the role of the Ras pathway in enhancing androgenic signaling in prostate cancer

Androgen receptor (AR) plays an important role in normal prostate function as well as in the etiology of prostate cancer. Activation of AR is dictated by hormone binding and by interactions with coregulators. Several of these coregulators are known targets of Ras-related signals. Recent evidence suggests that Ras activation may play a causal role in the progression of prostate cancer toward a more malignant and hormone-insensitive phenotype. In the present study, we used a transcription factor-transcription factor interaction array method to identify the zinc finger protein Ras-responsive element binding protein (RREB-1) as a partner and coregulator of AR. In LNCaP prostate cancer cells, RREB-1 was found to be present in a complex with endogenous AR as determined by coimmunoprecipitation, glutathione S-transferase pull down, and immunofluorescence analyses. RREB-1 bound to the prostate-specific antigen (PSA) promoter as assessed by chromatin immunoprecipitation. Transient expression of RREB-1 down-regulated AR-mediated promoter activity and suppressed expression of PSA protein. The repressor activity of RREB-1 was significantly attenuated by cotransfection of activated Ras. Moreover, expression of the dominant-negative N-17-Ras or, alternatively, use of the MAPK kinase inhibitor PD98059 [2-(2-amino-3-methyoxyphenyl)-4H-1-benzopyran-4-one] abolished the effect of Ras in attenuating RREB-1-mediated repression. Furthermore, inhibition of RREB-1 expression by RNA interference enhanced the effect of Ras on PSA promoter activity and PSA expression. In addition, activation of the Ras pathway depleted AR from the RREB-1/AR complex. Collectively, our data for the first time identify RREB-1 as a repressor of AR and further implicate the Ras/MAPK kinase pathway as a likely antagonist of the inhibitory effects of RREB-1 on androgenic signaling.

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

The androgen receptor signaling axis plays an essential role in the development, function and homeostasis of male urogenital structures including the prostate gland although the mechanism by which the AR axis contributes to the initiation, progression and metastatic spread of prostate cancer remains somewhat enigmatic. A number of molecular events have been proposed to act at the level of the AR and associated coregulators to influence the natural history of prostate cancer including deregulated expression, somatic mutation, and post-translational modification. The purpose of this article is to review the evidence for deregulated expression and function of the AR and associated coactivators and corepressors and how such events might contribute to the progression of prostate cancer by controlling the selection and expression of AR targets.

Peroxisome proliferator-activated receptor gamma-independent suppression of androgen receptor expression by troglitazone mechanism and pharmacologic exploitation

Previously, we showed that the peroxisome proliferator-activated receptor gamma (PPARgamma) agonist troglitazone at high doses was able to suppress androgen receptor (AR) expression in LNCaP prostate cancer cells independently of PPARgamma. Pharmacologic exploitation of this finding led to STG28, a PPARgamma-inactive analogue of troglitazone with substantially higher potency in AR repression. Considering the pivotal role of AR in prostate tumorigenesis, this study investigates the mechanism by which troglitazone and derivatives suppress AR expression in LNCaP cells. Reverse transcription-PCR and reporter gene assays indicate that this drug-induced AR repression occurs at both mRNA and protein levels. Evidence suggests that troglitazone and derivatives mediate the transcriptional repression of AR by facilitating the ubiquitin-dependent proteasomal degradation of the transcriptional factor Sp1. These agents also cause the proteolysis of two proteins that regulate Sp1-mediated transcription (i.e., the TATA-binding protein-associated factor TAF(II)250 and cyclin D1). However, their involvement in the transcriptional repression of AR is refuted by the finding that small interfering RNA knockdown of these two regulatory proteins does not cause AR down-regulation. STG28 does not cause significant reduction in Sp1 or AR expression in normal prostate epithelial cells. This discriminatory effect underscores the differential susceptibility of malignant versus normal cells to the inhibitory effect of STG28 on cell viability. From a translational perspective, STG28 provides a proof of principle that potent AR-ablative agents could be developed through structural modifications of troglitazone. Moreover, as the control of Sp1 degradation remains unclear, STG28 represents a unique pharmacologic probe to investigate the ubiquitin-proteasome system that regulates Sp1 proteolysis.

Impact of differential cyclin D1 expression and localisation in prostate cancer

Cyclin D1 is a critical regulator of androgen-dependent transcription and cell cycle progression in prostate cancer cells. Despite the influence of D-type cyclins on prostate cancer proliferation, few studies have examined the expression of cyclin D1 in localised tumours or challenged its relevance to disease progression. Cyclin D1 status was characterised using immunohistochemistry in 38 non-neoplastic prostate samples, 138 primary human prostate carcinomas, and three lymph node metastatic specimens. Relevance of cyclin D1 to preoperative prostate-specific antigen (PSA) levels, Ki-67 index, and p21^Cip1 status was also examined. Cyclin D1-positive phenotype was increased in primary carcinoma compared to non-neoplastic tissue, and was evident in all lymph node metastases cases. Interestingly, at least three distinct localisation patterns were observed in the cyclin D1-positive cohort, wherein cytoplasmic localisation was identified in a large fraction, and this pattern was predominant in lower grade tumours. Relevance of altered cyclin D1 status was observed, wherein cyclin D1-positive tumours were associated with low preoperative PSA levels, consistent with in vitro reports that cyclin D1 may alter the expression of this tumour marker. Moreover, tumours with predominantly cytoplasmic cyclin D1 showed the lowest Ki-67 index, whereas nuclear cyclin D1 was associated with higher grade, elevated Ki-67, and increased nuclear p21^Cip1. These data demonstrate that differential cyclin D1 status may influence clinicopathological parameters, and reveal new insight as to the regulation and potential consequence of cyclin D1 expression in prostate cancer.

Maspin augments proteasome inhibitor-induced apoptosis in prostate cancer cells

Proteasome inhibitors are known to induce apoptosis in a variety of cancer cells. On the other hand, maspin, a non-inhibitory serine protease inhibitor, is shown to sensitize cancer cells to therapeutic agents that induce apoptosis. We examined the consequence of maspin expression in prostate cancer cells targeted for treatment with various proteasome inhibitors. We observed that proteasome inhibitors induced apoptosis more effectively in maspin transfected human prostate cancer DU145 cells than in control cells. Interestingly, increased apoptosis in these cells was associated with a significant induction of maspin expression. MG-132, a proteasome inhibitor, induced endogenous and ectopic [cytomegalovirus promoter (CMV)-driven] maspin expression, and maspin siRNA attenuated MG-132-induced apoptosis. Proteasome inhibitor-induced maspin expression was inhibited by actinomycin D (Act D) and cyclohexamide (CHX), and by the inhibitors of p38MAPK, but not ERK1/2 or NF-kappaB. Electrophoretic mobility-shift assay (EMSA) and promoter-reporter activity analyses suggested that p38MAPK activated transcription factor AP-1 is responsible for proteasome inhibitor-induced maspin expression. Taken together, these observations demonstrate that proteasome inhibitors induce maspin expression by activating p38MAPK pathway, and that maspin thus expressed, in turn, augments proteasome inhibitor-induced apoptosis in prostate cancer cells. Our results suggest that gene therapy involving ectopic maspin expression may dramatically improve the efficacy of proteasome inhibitors for the treatment of prostate cancer.

Drug Insight: role of the androgen receptor in the development and progression of prostate cancer

Functional androgen receptor (AR) signaling is necessary for the development of prostate cancer. The therapeutic effect of androgen deprivation therapy for prostate cancer was described over 60 years ago and this treatment remains the mainstay of systemic therapy despite its transient response duration. It has become clear that AR expression and signaling remains intact as the disease evolves from androgen-sensitive cancer to classically (but perhaps inaccurately) termed hormone refractory prostate cancer. Through several genetic and epigenetic adaptations, prostate tumors continue to rely on AR growth signaling and they thus remain targets of 'hormonal' therapy. The development of new strategies and drugs that can abrogate AR signaling will probably result in important clinical benefits. The biology of androgen independence and the development of new approaches targeting AR signaling are reviewed herein.

The up-regulation of angiotensin II receptor type 1 and connective tissue growth factor are involved in high-glucose-induced fibronectin production by cultured human dermal fibroblasts

BACKGROUND

The expansion of extracellular matrix in diabetes occur in many tissues, including skin, but the underlying mechanisms are poorly understood. We were interested to study whether the activation of angiotensin II/receptor type 1 pathway with the consequent involvement of CTGF may be the possible cause of high-glucose-induced matrix abnormalities in cultured dermal fibroblasts.

OBJECTIVE

We aimed to investigate the effect of high glucose on the generation of angiotensin II and the expression of angiotensin II receptors, and on the expression of CTGF mediating the fibronectin production in cultured human dermal fibroblasts.

METHODS

Cell culturing, ELISA, semi-quantitative RT-PCR, Western blotting.

RESULTS

High glucose treatment of cultured dermal fibroblasts led to: (1) the angiotensin II receptor type 1 was up-regulated at the level of mRNA and protein, unlike the receptor type 2; (2) the generation of angiotensin II and the mRNA expression of all components of the local renin-angiotensin system were not altered; (3) the mRNA and protein expression of CTGF was up-regulated, and this effect was cancelled by losartan; (4) the fibronectin production was increased, also was cancelled by losartan, while an anti-CTGF-neutralizing antibody only partly reduced it; (5) TGFbeta1 expression, the secretion of total and active TGFbeta1 were not changed; (6) the hyperosmotic action of high glucose had no effect.

CONCLUSION

The up-regulation of angiotensin II receptor type 1 and the consequent increase of CTGF expression, independently of TGFbeta1, participate in high-glucose-induced fibronectin production in cultured human dermal fibroblasts.

Sex steroids and prostate carcinogenesis: integrated, multifactorial working hypothesis

Androgens are thought to cause prostate cancer, but there is little epidemiological support for this notion. Animal studies, however, demonstrate that androgens are very strong tumor promotors for prostate carcinogenesis after tumor-initiating events. Even treatment with low doses of testosterone alone can induce prostate cancer in rodents. Because testosterone can be converted to estradiol-17beta by the enzyme aromatase, expressed in human and rodent prostate, estrogen may be involved in prostate cancer induction by testosterone. When estradiol is added to testosterone treatment of rats, prostate cancer incidence is markedly increased and even a short course of estrogen treatment results in a high incidence of prostate cancer. The active testosterone metabolite 5alpha-dihydrotestosterone cannot be aromatized to estrogen and hardly induces prostate cancer, supporting a critical role of estrogen in prostate carcinogenesis. Estrogen receptors are expressed in the prostate and may mediate some or all of the effects of estrogen. However, there is also evidence that in the rodent and human prostate conversion occurs of estrogens to catecholestrogens. These can be converted to reactive intermediates that can adduct to DNA and cause generation of reactive oxygen species, and thus estradiol can be a weak DNA damaging (genotoxic) carcinogen. In the rat prostate DNA damage can result from estrogen treatment; this occurs prior to cancer development and at exactly the same location. Inflammation may be associated with prostate cancer risk, but no environmental carcinogenic risk factors have been definitively identified. We postulate that endogenous factors present in every man, sex steroids, are responsible for the high prevalence of prostate cancer in aging men, androgens acting as strong tumor promoters in the presence of a weak, but continuously present genotoxic carcinogen, estradiol-17beta.

First pharmacophore-based identification of androgen receptor down-regulating agents: discovery of potent anti-prostate cancer agents

A qualitative 3D pharmacophore model (a common feature based model or Catalyst HipHop algorithm) was developed for well-known natural product androgen receptor down-regulating agents (ARDAs). The four common chemical features identified included: one hydrophobic group, one ring aromatic group, and two hydrogen bond acceptors. This model served as a template in virtual screening of the Maybridge and NCI databases that resulted in identification of six new ARDAs (EC(50) values 17.5-212 microM). Five of these molecules strongly inhibited the growth of human prostate LNCaP cells. These novel compounds may be used as leads to develop other novel anti-prostate cancer agents.

[Development of novel nuclear receptor ligands based on receptor-folding inhibition hypothesis]

Nuclear receptors are ligand-inducible transcriptional factors, and regulate various significant biological phenomena such as cell differentiation, proliferation, metabolism, and homeostasis. By the elucidation of the physiological functions of nuclear receptors, they have become one of the most significant molecular targets for drug discovery in the fields of cancer, autoimmune diseases, and metabolic syndrome. In this study, several novel nuclear receptor ligands have been developed, based on the receptor-folding inhibition hypothesis is discussed. In this hypothesis, the antagonists for nuclear receptors are classified into two types, the misfolding inducers and the folding inhibitors, related to the helix 12 (AF-2 region) conformation of the receptor that is significant for the receptor activation. Then, in order to overcome the resistance in the treatment of prostate tumors with androgen antagonists, the novel folding-inhibitor type antagonists such as isoxazole and pyrrolecarboxamide derivatives were designed and synthesized. Some of them exhibited the androgen antagonistic activities in LNCaP cells with mutated androgen receptor in which conventional antagonists such as flutamide and RU56187 were inactive. The folding-inhibitor type vitamin D3 antagonists (DLAM series) are similarly developed. Further, novel non-seco-steroidal vitamin D3 analogs were designed and synthesized by using a 3,3-diphenylpropane derivative, LG190178 as lead compound. The aza analogs exhibited both potent vitamin D agonistic and androgen antagonistic activities. The results indicate the drug design based on the receptor-folding inhibition hypothesis is efficient in medicinal chemistry of nuclear receptors.

Estrogen signaling and disruption of androgen metabolism in acquired androgen-independence during cadmium carcinogenesis in human prostate epithelial cells

BACKGROUND

Lethal prostate cancers often become androgen-independent due to androgen receptor (AR) overexpression. The role of cadmium in prostate tumor progression was determined.

METHODS

Control and cadmium-transformed prostate epithelial cells (CTPE) were compared for steroid-induced proliferation, steroid receptor expression, and androgen metabolism.

RESULTS

CTPE cells showed rapid proliferation in complete medium and sustained proliferation in steroid-reduced medium. Androgens stimulated significantly less cell proliferation and AR-related genes expression in CTPE cells. 5alpha-Dihydrotestosterone increased PSA expression more effectively in control cells. Flutamide reduced 5alpha-dihydrotestosterone-stimulated growth less effectively in CTPE cells compared to control. CTPE cells showed decreased p27 expression. Estrogen receptors were overexpressed and estradiol markedly stimulated proliferation in CTPE cells. In CTPE cells 5alpha-aromatase was markedly increased, while 5alpha-reductase was decreased.

CONCLUSIONS

Cadmium-induced malignant transformation stimulates androgen independence, unrelated to AR expression or activity. Increased estrogen receptor and 5alpha-aromatase expression suggest estrogen signaling may be critical to this process.

Mechanisms of acquired androgen independence during arsenic-induced malignant transformation of human prostate epithelial cells

BACKGROUND

Prostate cancer progression often occurs with overexpression of growth factors and receptors, many of which engage the Ras/mitogen-activated protein MAP kinase (MAPK) pathway.

OBJECTIVES

In this study we used arsenic-transformed human prostate epithelial cells, which also show androgen-independent growth, to study the possibility that chronic activation of Ras/MAPK signaling may contribute to arsenic-induced prostate cancer progression.

METHODS

Control and chronic arsenic-transformed prostate epithelial cells (CAsE-PE) were compared for Ras/MAPK signaling capacities using reverse transcription-polymerase chain reaction and Western blot analyses.

RESULTS

We found activation of HER-2/neu oncogene in transformed CAsE-PE cells, providing molecular evidence of androgen independence in the transformed cells. CAsE-PE cells displayed constitutively increased expression of unmutated K-Ras (6-fold), and the downstream MAP kinases A-Raf and B-Raf (2.2-fold and 3.2-fold, respectively). There was also increased expression of phosphorylated MEK1/2 and Elk1 in the transformant cells. The MEK1/2 inhibitor, U0126, blocked PSA overexpression in CAsE-PE cells.

CONCLUSION

Thus, arsenic-induced malignant transformation and acquired androgen independence are linked to Ras signaling activation in human prostate epithelial cells. Chronic activation of this pathway can sensitize the androgen receptor to subphysiologic levels of androgen. This may be important in arsenic carcinogenesis and provide a mechanism that may be common for prostate cancer progression driven by diverse agents.

Identification of Putative Androgen Receptor Interaction Protein Modules

We have developed a novel androgen receptor (AR) expression system in the 293 human embryonic kidney cell line that recapitulates AR biochemical activity as a steroid hormone receptor in prostate cancer cells. We used this system to identify putative AR-binding proteins in the cytosolic and nuclear compartments of mammalian cells using a large scale co-immunoprecipitation strategy coupled to quantitative mass spectrometry. For example, the heat shock 70 and 90 chaperones, which are known regulators of steroid hormone receptor, were identified as AR-binding proteins. AR purification enriched for proteins involved in RNA processing, protein transport, and cytoskeletal organization, suggesting a functional link between AR and these protein modules in mammalian cells. For example, AR purification in the nuclear compartment led to the specific enrichment of alpha-actinin-4, clathrin heavy chain, and serine-threonine protein kinase C delta. Short interfering RNA knockdown studies and co-transcriptional reporter assays revealed that clathrin heavy chain possessed co-activator activity during AR-mediated transcription, whereas alpha-actinin-4 and protein kinase C delta displayed both co-activator and co-repressor activity during AR-mediated transcription that was dependent upon their relative expression levels. Lastly immunohistochemical staining of prostate tissue showed that alpha-actinin-4 levels decreased in the nucleus of high grade cancerous prostate samples, suggesting its possible deregulation in advanced prostate cancers as previously observed in late stage metastatic breast cancers. Taken together, these findings suggest AR binds to specific protein modules in mammalian cells and that these protein modules may provide a molecular framework for interrogating AR function in normal and cancerous prostate epithelial cells.

Androgen and estrogen receptors: potential of crystallography in the fight against cancer

Androgen (AR) and estrogen (ERalpha and ERbeta) receptors are primary targets in the treatment of hormone-sensitive tumors such as prostate or breast cancers. Because of their diverse and important roles in normal and pathologic physiology, these nuclear receptors have prompted intense research. Here, we review how structural studies conducted over the past several years on AR and ERs have provided significant advances in our comprehension of androgen and estrogen signaling and how this information can be used in the fight against cancer.

Role of Zn2+ in epigallocatechin gallate affecting the growth of PC-3 cells

Green tea has chemo-preventive effects to human carcinoma including prostate cancer. Epigallocatechin gallate (EGCG) is the major active component in green tea. Zn(2+) is indispensable to our health, and plays an important role in the normal function and pathology of the prostate gland, and might be a good marker for diagnosing prostate cancer. Effects of Zn(2+), EGCG and their interactions on the growth of androgen-insensitive prostate cancer cell (PC-3) were investigated in the present paper. The results show that Zn(2+) and EGCG inhibited the growth of PC-3 cells in a time- and dose-dependent manner, but effects of interactions of EGCG with Zn(2+) were extremely dependent on their concentrations and added orders. Inhibitory effects of Zn(2+) were significantly decreased in the presence of EGCG on PC-3 cell growth. Therefore, we hypothesize that complexation of EGCG with Zn(2+) might be responsible for the observed decrease of the bioactivities of Zn(2+) against PC-3 cells.

Bisphenol A facilitates bypass of androgen ablation therapy in prostate cancer

Prostatic adenocarcinomas depend on androgen for growth and survival. First line treatment of disseminated disease exploits this dependence by specifically targeting androgen receptor function. Clinical evidence has shown that androgen receptor is reactivated in recurrent tumors despite the continuance of androgen deprivation therapy. Several factors have been shown to restore androgen receptor activity under these conditions, including somatic mutation of the androgen receptor ligand-binding domain. We have shown previously that select tumor-derived mutants of the androgen receptor are receptive to activation by bisphenol A (BPA), an endocrine-disrupting compound that is leached from polycarbonate plastics and epoxy resins into the human food supply. Moreover, we have shown that BPA can promote cell cycle progression in cultured prostate cancer cells under conditions of androgen deprivation. Here, we challenged the effect of BPA on the therapeutic response in a xenograft model system of prostate cancer containing the endogenous BPA-responsive AR-T877A mutant protein. We show that after androgen deprivation, BPA enhanced both cellular proliferation rates and tumor growth. These effects were mediated, at least in part, through androgen receptor activity, as prostate-specific antigen levels rose with accelerated kinetics in BPA-exposed animals. Thus, at levels relevant to human exposure, BPA can modulate tumor cell growth and advance biochemical recurrence in tumors expressing the AR-T877A mutation.

Short hairpin RNA knockdown of the androgen receptor attenuates ligand-independent activation and delays tumor progression

Progression to androgen independence is the lethal end stage of prostate cancer. We used expression of androgen receptor (AR)-targeted short hairpin RNAs (shRNA) to directly test the requirement for AR in ligand-independent activation of androgen-regulated genes and hormone-independent tumor progression. Transient transfection of LNCaP human prostate cancer cells showed that AR shRNA decreased R1881 induction of the prostate-specific antigen (PSA)-luciferase reporter by 96%, whereas activation by forskolin, interleukin-6, or epidermal growth factor was inhibited 48% to 75%. Whereas the antiandrogen bicalutamide provided no further suppression, treatment with the mitogen-activated protein kinase (MAPK) inhibitor U0126 completely abrogated the residual activity, indicating a MAPK-dependent, AR-independent pathway for regulating the PSA promoter. Expression of doxycycline-inducible AR shRNA expression in LNCaP cells resulted in decreased levels of AR and PSA as well as reduced proliferation in vitro. When these cells were grown as xenografts in immunocompromised mice, induction of AR shRNA decreased serum PSA to below castration nadir levels and significantly retarded tumor growth over the entire 55-day experimental period. This is the first demonstration that, by inducibly suppressing AR expression in vivo, there is an extensive delay in progression to androgen independence as well as a dramatic inhibition of tumor growth and decrease in serum PSA, which exceeds that seen with castration alone. Based on these findings, we propose that suppressing AR expression may provide superior therapeutic benefit in reducing tumor growth rate than castration and may additionally be very effective in delaying progression to androgen independence.

Androgen receptor coactivators lysine-specific histone demethylase 1 and four and a half LIM domain protein 2 predict risk of prostate cancer recurrence

Prostate cancer biology varies from locally confined tumors with low risk for relapse to tumors with high risk for progression even after radical prostatectomy. Currently, there are no reliable biomarkers to predict tumor relapse and poor clinical outcome. In this study, we correlated expression patterns of the androgen receptor (AR) coactivators lysine-specific histone demethylase 1 (LSD1) and four and a half LIM-domain protein 2 (FHL2), AR, Gleason score, Gleason grade, and p53 expression in clinically organ confined prostate cancers with relapse after radical prostatectomy. Our data reveal that high levels of LSD1, nuclear expression of the FHL2 coactivator, high Gleason score and grade, and very strong staining of nuclear p53 correlate significantly with relapse during follow-up. No correlation exists with relapse and the expression of AR and cytoplasmic expression of FHL2. To confirm these data, we did quantitative reverse transcription-PCR and Western blot analyses in a subset of tumor specimens. Consistently, both LSD1 mRNA and protein levels were significantly up-regulated in high-risk tumors. We previously identified LSD1 and FHL2 as nuclear cofactors interacting specifically with the AR in prostate cells and showed that both stimulate androgen-dependent gene transcription. Our present study suggests that LSD1 and nuclear FHL2 may serve as novel biomarkers predictive for prostate cancer with aggressive biology and point to a role of LSD1 and FHL2 in constitutive activation of AR-mediated growth signals.

Interaction of IGF signaling and the androgen receptor in prostate cancer progression

The insulin-like growth factor type I receptor (IGF-IR) has been suggested to play an important role in prostate cancer progression and possibly in the progression to androgen-independent (AI) disease. The term AI may not be entirely correct, in that recent data suggest that expression of androgen receptor (AR) and androgen-regulated genes is the primary association with prostate cancer progression after hormone ablation. Therefore, signaling through other growth factors has been thought to play a role in AR-mediated prostate cancer progression to AI disease in the absence of androgen ligand. However, existing data on how IGF-IR signaling interacts with AR activation in prostate cancer are conflicting. In this Prospect article, we review some of the published data on the mechanisms of IGF-IR/AR interaction and present new evidence that IGF-IR signaling may modulate AR compartmentation and thus alter AR activity in prostate cancer cells. Inhibition of IGF-IR signaling can result in cytoplasmic AR retention and a significant change in androgen-regulated gene expression. Translocation of AR from the cytoplasm to the nucleus may be associated with IGF-induced dephosphorylation. Since fully humanized antibodies targeting the IGF-IR are now in clinical trials, the current review is intended to reveal the mechanisms of potential therapeutic effects of these antibodies on AI prostate cancers.