The PPARγ ligand ciglitazone regulates androgen receptor activation differently in androgen-dependent versus androgen-independent human prostate cancer cells

The androgen receptor (AR) regulates growth and progression of androgen-dependent as well as androgen-independent prostate cancer cells. Peroxisome proliferator-activated receptor gamma (PPARγ) agonists have been reported to reduce AR activation in androgen-dependent LNCaP prostate cancer cells. To determine whether PPARγ ligands are equally effective at inhibiting AR activity in androgen-independent prostate cancer, we examined the effect of the PPARγ ligands ciglitazone and rosiglitazone on C4-2 cells, an androgen- independent derivative of the LNCaP cell line. Luciferase-based reporter assays and Western blot analysis demonstrated that PPARγ ligand reduced dihydrotestosterone (DHT)-induced increases in AR activity in LNCaP cells. However, in C4-2 cells, these compounds increased DHT-induced AR driven luciferase activity. In addition, ciglitazone did not significantly alter DHT-mediated increases in prostate specific antigen (PSA) protein or mRNA levels within C4-2 cells. siRNA-based experiments demonstrated that the ciglitazone-induced regulation of AR activity observed in C4-2 cells was dependent on the presence of PPARγ. Furthermore, overexpression of the AR corepressor cyclin D1 inhibited the ability of ciglitazone to induce AR luciferase activity in C4-2 cells. Thus, our data suggest that both PPARγ and cyclin D1 levels influence the ability of ciglitazone to differentially regulate AR signaling in androgen-independent C4-2 prostate cancer cells.     

Pomegranate polyphenols down-regulate expression of androgen-synthesizing genes in human prostate cancer cells overexpressing the androgen receptor

Prostate cancer is dependent on circulating testosterone in its early stages and is treatable with radiation and surgery. However, recurrent prostate tumors advance to an androgen-independent state in which they progress in the absence of circulating testosterone, leading to metastasis and death. During the development of androgen independence, prostate cancer cells are known to increase intracellular testosterone synthesis, which maintains cancer cell growth in the absence of significant amounts of circulating testosterone. Overexpression of the androgen receptor (AR) occurs in androgen-independent prostate cancer and has been proposed as another mechanism promoting the development of androgen independence. The LNCaP-AR cell line is engineered to overexpress AR but is otherwise similar to the widely studied LNCaP cell line. We have previously shown that pomegranate extracts inhibit both androgen-dependent and androgen-independent prostate cancer cell growth. In this study, we examined the effects of pomegranate polyphenols, ellagitannin-rich extract and whole juice extract on the expression of genes for key androgen-synthesizing enzymes and the AR. We measured expression of the HSD3B2 (3beta-hydroxysteroid dehydrogenase type 2), AKR1C3 (aldo-keto reductase family 1 member C3) and SRD5A1 (steroid 5alpha reductase type 1) genes for the respective androgen-synthesizing enzymes in LNCaP, LNCaP-AR and DU-145 human prostate cancer cells. A twofold suppression of gene expression was considered statistically significant. Pomegranate polyphenols inhibited gene expression and AR most consistently in the LNCaP-AR cell line (P=.05). Therefore, inhibition by pomegranate polyphenols of gene expression involved in androgen-synthesizing enzymes and the AR may be of particular importance in androgen-independent prostate cancer cells and the subset of human prostate cancers where AR is up-regulated.     

The significance of Her2 on androgen receptor protein stability in the transition of androgen requirement in prostate cancer cells

Androgen ablation therapy is the most common strategy for suppressing prostate cancer progression; however, tumor cells eventually escape androgen dependence and progress to an androgen-independent phase. The androgen receptor (AR) plays a pivotal role in this transition. To address this transition mystery in prostate cancer, we established an androgen-independent prostate cancer cell line (LNCaPdcc), by long-term screening of LNCaP cells in androgen-deprived conditions, to investigate changes of molecular mechanisms before and after androgen withdrawal. We found that LNCaPdcc cells displayed a neuroendocrine morphology, less aggressive growth, and lower expression levels of cell cycle-related factors, although the cell cycle distribution was similar to parental LNCaP cells. Notably, higher protein expression of AR, phospho-Ser(81)-AR, and PSA in LNCaPdcc cells were observed. The nuclear distribution and protein stability of AR increased in LNCaPdcc cells. In addition, cell proliferation results exhibited the biphasic nature of the androgen (R1881) effect in two cell lines. On the other hand, LNCaPdcc cells expressed higher levels of Her2, phospho-Tyr(1221/1222)-Her2, ErbB3, and ErbB4 proteins than parental LNCaP cells. These two cell lines exhibited distinct responses to Her2 activation (by heregulin treatment) on Her2 phosphorylation and Her2 inhibition (by AG825 or Herceptin treatments) on proliferation. In addition, the Her2 inhibitor more effectively caused AR degradation and diminished AR Ser(81) phosphorylation in LNCaPdcc cells. Taken together, our data demonstrate that Her2 plays an important role in the support of AR protein stability in the transition of androgen requirement in prostate cancer cells. We hope these findings will provide novel insight into the treatment of hormone-refractory prostate cancer.     

Androgen receptor represses the neuroendocrine transdifferentiation process in prostate cancer cells

Androgen-ablation therapy is an effective method for treating prostate cancer. However, prostate tumors that survive long-term androgen-ablation therapy are classified as androgen-independent as they proliferate in the absence of androgens, and they tend to be enriched for neuroendocrine (NE) cells. Androgen withdrawal causes androgen-dependent prostate cancer cells to adopt a pronounced NE phenotype, suggesting that androgen receptor (AR) represses an intrinsic NE transdifferentiation process in prostate cancer cells. In this report we show that short interfering RNA-induced AR silencing induced a NE phenotype that manifested itself in the growth of dendritic-like processes in both the androgen-dependent LNCaP and androgen-independent LNCaP-AI human prostate cancer cells. Western blot analysis revealed that neuronal-specific enolase, a marker of the neuronal lineage, was increased by AR knockdown in LNCaP cells. The expression levels of the neuronal-specific cytoskeletal proteins beta-tubulin III, nestin, and glial acidic fibrillary protein were also characterized in AR knockdown cells. Most interestingly, AR silencing induced beta-tubulin III expression in LNCaP cells, while AR knockdown increased glial acidic fibrillary protein levels in both LNCaP and LNCaP-AI cells. Lastly, AR silencing reduced the proliferative capacity of LNCaP and LNCaP-AI cells. Our data demonstrate that AR actively represses an intrinsic NE transdifferentiation process in androgen-responsive prostate cancer cells and suggest a potential link between AR inactivation and the increased frequency of NE cells in androgen-independent tumors.     

Benzyldihydroxyoctenone, a novel nonsteroidal antiandrogen, shows differential apoptotic induction in prostate cancer cells in response to their androgen responsiveness

The molecular mechanisms of apoptotic induction by benzyldihydroxyoctenone (BDH), a nonsteroidal antiandrogen, isolated from the culture broth of Streptomyces sp., have been previously published in prostate cancer LNCaP cells. Apoptotic induction of BDH-treated LNCaP cells was associated with downregulation of Bcl-xL that caused, in turn, cytochrome c release from mitochondria, and activation of procaspases and specific proteolytic cleavage of poly(ADP-ribose) polymerase (PARP). The purpose of the present study was to investigate the patterns of apoptotic induction by BDH in non-prostate, ovarian cancer PA-1 (androgen-independent and -insensitive) cells and prostate cancer cells with different androgen responsiveness, such as C4-2 (androgen-independent and -sensitive), 22Rv1 (androgen-dependent and -low sensitive), and LNCaP (androgen-dependent and -high sensitive) cells. We found that BDH-treated LNCaP cell proliferation was significantly inhibited in a time-dependent manner and induced apoptosis via downregulation of the androgen receptor (AR) and prostate-specific antigen (PSA), as well as antiapoptotic Bcl-xL protein. However, the levels of BDH-mediated apoptotic induction and growth inhibition in 22Rv1 cells were apparently lower than those of LNCaP cells. In contrast, the induction of apoptosis and antiproliferative effect in BDH-treated non-prostate cancer PA-1 and hormone refractory C4-2 cells were not detectable and marginal, respectively. Therefore, BDH-mediated differential apoptotic induction and growth inhibition in a cell type seem to be obviously dependent on its androgen responsiveness; primarily on androgen-dependency, and then on androgen sensitivity.     

Activation of AMP-Activated Protein Kinase by 3,3′-Diindolylmethane (DIM) Is Associated with Human Prostate Cancer Cell Death In Vitro and In Vivo

There is a large body of scientific evidence suggesting that 3,3′-Diindolylmethane (DIM), a compound derived from the digestion of indole-3-carbinol, which is abundant in cruciferous vegetables, harbors anti-tumor activity in vitro and in vivo. Accumulating evidence suggests that AMP-activated protein kinase (AMPK) plays an essential role in cellular energy homeostasis and tumor development and that targeting AMPK may be a promising therapeutic option for cancer treatment in the clinic. We previously reported that a formulated DIM (BR-DIM; hereafter referred as B-DIM) with higher bioavailability was able to induce apoptosis and inhibit cell growth, angiogenesis, and invasion of prostate cancer cells. However, the precise molecular mechanism(s) for the anti-cancer effects of B-DIM have not been fully elucidated. In the present study, we investigated whether AMP-activated protein kinase (AMPK) is a molecular target of B-DIM in human prostate cancer cells. Our results showed, for the first time, that B-DIM could activate the AMPK signaling pathway, associated with suppression of the mammalian target of rapamycin (mTOR), down-regulation of androgen receptor (AR) expression, and induction of apoptosis in both androgen-sensitive LNCaP and androgen-insensitive C4-2B prostate cancer cells. B-DIM also activates AMPK and down-regulates AR in androgen-independent C4-2B prostate tumor xenografts in SCID mice. These results suggest that B-DIM could be used as a potential anti-cancer agent in the clinic for prevention and/or treatment of prostate cancer regardless of androgen responsiveness, although functional AR may be required.     

Vav3 oncogene is overexpressed and regulates cell growth and androgen receptor activity in human prostate cancer

The purpose of this research was to investigate the role of Vav3 oncogene in human prostate cancer. We found that expression of Vav3 was significantly elevated in androgen-independent LNCaP-AI cells in comparison with that in their androgen-dependent counterparts, LNCaP cells. Vav3 expression was also detected in other human prostate cancer cell lines (PC-3, DU145, and 22Rv1) and, by immunohistochemistry analysis, was detected in 32% (26 of 82) of surgical specimens of human prostate cancer. Knockdown expression of Vav3 by small interfering RNA inhibited growth of both androgen-dependent LNCaP and androgen-independent LNCaP-AI cells. In contrast, overexpression of Vav3 promoted androgen-independent growth of LNCaP cells induced by epidermal growth factor. Overexpression of Vav3 enhanced androgen receptor (AR) activity regardless of the presence or absence of androgen and stimulated the promoters of AR target genes. These effects of Vav3 could be attenuated by either phosphatidylinositol 3-kinase (PI3K) inhibitors or dominant-negative Akt and were enhanced by cotransfection of PI3K. Moreover, phosphorylation of Akt was elevated in LNCaP cells overexpressing Vav3, which could be blocked by PI3K inhibitors. Finally, we ascertained that the DH domain of Vav3 was responsible for activation of AR. Taken together, our data show that overexpression of Vav3, through the PI3K-Akt pathway, inappropriately activates AR signaling axis and stimulates cell growth in prostate cancer cells. These findings suggest that Vav3 overexpression may be involved in prostate cancer development and progression.     

Antioxidants Abrogate Alpha-Tocopherylquinone-Mediated Down-Regulation of the Androgen Receptor in Androgen-Responsive Prostate Cancer Cells

Tocopherylquinone (TQ), the oxidation product of alpha-tocopherol (AT), is a bioactive molecule with distinct properties from AT. In this study, AT and TQ are investigated for their comparative effects on growth and androgenic activity in prostate cancer cells. TQ potently inhibited the growth of androgen-responsive prostate cancer cell lines (e.g., LAPC4 and LNCaP cells), whereas the growth of androgen-independent prostate cancer cells (e.g., DU145 cells) was not affected by TQ. Due to the growth inhibitory effects induced by TQ on androgen-responsive cells, the anti-androgenic properties of TQ were examined. TQ inhibited the androgen-induced activation of an androgen-responsive reporter and inhibited the release of prostate specific antigen from LNCaP cells. TQ pretreatment was also found to inhibit AR activation as measured using the Multifunctional Androgen Receptor Screening assay. Furthermore, TQ decreased androgen-responsive gene expression, including TM4SF1, KLK2, and PSA over 5-fold, whereas AT did not affect the expression of androgen-responsive genes. Of importance, the antiandrogenic effects of TQ on prostate cancer cells were found to result from androgen receptor protein down-regulation produced by TQ that was not observed with AT treatment. Moreover, none of the androgenic endpoints assessed were affected by AT. The down-regulation of androgen receptor protein by TQ was abrogated by co-treatment with antioxidants. Overall, the biological actions of TQ were found to be distinct from AT, where TQ was found to be a potent inhibitor of cell growth and androgenic activity in androgen-responsive prostate cancer cells.     

Structure-based virtual screening and identification of a novel androgen receptor antagonist

Hormonal therapies, mainly combinations of anti-androgens and androgen deprivation, have been the mainstay treatment for advanced prostate cancer because the androgen-androgen receptor (AR) system plays a pivotal role in the development and progression of prostate cancers. However, the emergence of androgen resistance, largely due to inefficient anti-hormone action, limits the therapeutic usefulness of these therapies. Here, we report that 6-(3,4-dihydro-1H-isoquinolin-2-yl)-N-(6-methylpyridin-2-yl)nicotinamide (DIMN) acts as a novel anti-androgenic compound that may be effective in the treatment of both androgen-dependent and androgen-independent prostate cancers. Through AR structure-based virtual screening using the FlexX docking model, fifty-four compounds were selected and further screened for AR antagonism via cell-based tests. One compound, DIMN, showed an antagonistic effect specific to AR with comparable potency to that of the classical AR antagonists, hydroxyflutamide and bicalutamide. Consistent with their anti-androgenic activity, DIMN inhibited the growth of androgen-dependent LNCaP prostate cancer cells. Interestingly, the compound also suppressed the growth of androgen-independent C4-2 and CWR22rv prostate cancer cells, which express a functional AR, but did not suppress the growth of the AR-negative prostate cancer cells PPC-1, DU145, and R3327-AT3.1. Taken together, the results suggest that the synthetic compound DIMN is a novel anti-androgen and strong candidate for useful therapeutic agent against early stage to advanced prostate cancer.     

Difference in Protein Expression Profile and Chemotherapy Drugs Response of Different Progression Stages of LNCaP Sublines and Other Human Prostate Cancer Cells

Androgen ablation therapy is the primary treatment for metastatic prostate cancer. However, 80-90% of the patients who receive androgen ablation therapy ultimately develop recurrent tumors in 12-33 months after treatment with a median overall survival time of 1-2 years after relapse. LNCaP is a commonly used cell line established from a human lymph node metastatic lesion of prostatic adenocarcinoma. We previously established two relapsed androgen receptor (AR)-rich androgen-independent LNCaP sublines 104-R1 (androgen depleted for 12 months) and 104-R2 cells (androgen depleted for 24 months) from AR-positive androgen-dependent LNCaP 104-S cells. LNCaP 104-R1 and 104-R2 mimics the AR-positive hormone-refractory relapsed tumors in patients receiving androgen ablation therapy. Androgen treatment stimulates proliferation of 104-S cells, but causes growth inhibition and G1 cell cycle arrest in 104-R1 and 104-R2 cells. We investigated the protein expression profile difference between LNCaP 104-S vs. LNCaP 104-R1, 104-R2, PC-3, and DU-145 cells as well as examined the sensitivity of these prostate cancer cells to different chemotherapy drugs and small molecule inhibitors. Compared to 104-S cells, 104-R1 and 104-R2 cells express higher protein levels of AR, PSA, c-Myc, Skp2, BCL-2, P53, p-MDM2 S166, Rb, and p-Rb S807/811. The 104-R1 and 104-R2 cells express higher ratio of p-Akt S473/Akt, p-EGFR/EGFR, and p-Src/Src, but lower ratio of p-ERK/ERK than 104-S cells. PC-3 and DU-145 cells express higher c-Myc, Skp2, Akt, Akt1, and phospho-EGFR but less phospho-Akt and phospho-ERK. Overexpression of Skp2 increased resistance of LNCaP cells to chemotherapy drugs. Paclitaxel, androgen, and inhibitors for PI3K/Akt, EGFR, Src, or Bcl-2 seem to be potential choices for treatment of advanced prostate cancers. Our study provides rationale for targeting Akt, EGFR, Src, Bcl-2, and AR signaling as a treatment for AR-positive relapsed prostate tumors after hormone therapy.     

5alpha-androstane-3alpha,17beta-diol supports human prostate cancer cell survival and proliferation through androgen receptor-independent signaling pathways: implication of androgen-independent prostate cancer progression

Androgen and androgen receptor (AR) are involved in growth of normal prostate and development of prostatic diseases including prostate cancer. Androgen deprivation therapy is used for treating advanced prostate cancer. This therapeutic approach focuses on suppressing the accumulation of potent androgens, testosterone and 5alpha-dihydrotestosterone (5alpha-DHT), or inactivating the AR. Unfortunately, the majority of patients with prostate cancer eventually advance to androgen-independent states and no longer respond to the therapy. In addition to the potent androgens, 5alpha-androstane-3alpha,17beta-diol (3alpha-diol), reduced from 5alpha-DHT through 3alpha-hydroxysteroid dehydrogenases (3alpha-HSDs), activated signaling may represent a novel pathway responsible for the progression to androgen-independent prostate cancer. Androgen sensitive human prostate cancer LNCaP cells were used to compare 5alpha-DHT and 3alpha-diol activated androgenic effects. In contrast to 5alpha-DHT, 3alpha-diol regulated unique patterns of beta-catenin and Akt expression as well as Akt phosphorylation in parental and in AR-silenced LNCaP cells. More significantly, 3alpha-diol, but not 5alpha-DHT, supported AR-silenced LNCaP cells and AR negative prostate cancer PC-3 cell proliferation. 3alpha-diol-activated androgenic effects in prostate cells cannot be attributed to the accumulation of 5alpha-DHT, since 5alpha-DHT formation was not detected following 3alpha-diol administration. Potential accumulation of 3alpha-diol, as a result of elevated 3alpha-HSD expression in cancerous prostate, may continue to support prostate cancer growth in the presence of androgen deprivation. Future therapeutic strategies for treating advanced prostate cancer might need to target reductive 3alpha-HSD to block intraprostatic 3alpha-diol accumulation.     

Synchronized prostate cancer cells for studying androgen regulated events in cell cycle progression from G1 into S phase

Androgen-ablation is a most commonly prescribed treatment for metastatic prostate cancer but it is not curative. Development of new strategies for treatment of prostate cancer is limited partly by a lack of full understanding of the mechanism by which androgen regulates prostate cancer cell proliferation. This is due, mainly, to the limitations in currently available experimental models to distinguish androgen/androgen receptor (AR)-induced events specific to proliferation from those that are required for cell viability. We have, therefore, developed an experimental model system in which both androgen-sensitive (LNCaP) and androgen-independent (DU145) prostate cancer cells can be reversibly blocked in G(0)/G(1) phase of cell cycle by isoleucine deprivation without affecting their viability. Pulse-labeling studies with (3)H-thymidine indicated that isoleucine-deprivation caused LNCaP and DU145 cells to arrest at a point in G(1) phase which is 12-15 and 6-8 h, respectively, before the start of S phase and that their progression into S phase was dependent on serum factors. Furthermore, LNCaP, but not DU145, cells required AR activity for progression from G(1) into S phase. Western blot analysis of the cell extracts prepared at regular intervals following release from isoleucine-block revealed remarkable differences in the expression of cyclin E, p21(Cip1), p27(Kip1), and Rb at the protein level between LNCaP and DU145 cells during progression from G(1) into S phase. However, in both cell types Cdk-2 activity associated with cyclin E and cyclin A showed an increase only when the cells transited from G(1) into S phase. These observations were further corroborated by studies using exponentially growing cells that were enriched in specific phases of the cell cycle by centrifugal elutriation. These studies demonstrate usefulness of the isoleucine-deprivation method for synchronization of androgen-sensitive and androgen-independent prostate cancer cells, and for examining the role of androgen and AR in progression of androgen-sensitive prostate cancer cells from G(1) into S phase.     

Androgen Suppresses the Proliferation of Androgen Receptor-Positive Castration-Resistant Prostate Cancer Cells via Inhibition of Cdk2, CyclinA,and Skp2

The majority of prostate cancer (PCa) patient receiving androgen ablation therapy eventually develop castration-resistant prostate cancer (CRPC). We previously reported that androgen treatment suppresses Skp2 and c-Myc through androgen receptor (AR) and induced G1 cell cycle arrest in androgen-independent LNCaP 104-R2 cells, a late stage CRPC cell line model. However, the mechanism of androgenic regulation of Skp2 in CRPC cells was not fully understood. In this study, we investigated the androgenic regulation of Skp2 in two AR-positive CRPC cell line models, the LNCaP 104-R1 and PC-3^AR Cells. The former one is an early stage androgen-independent LNCaP cells, while the later one is PC-3 cells re-expressing either wild type AR or mutant LNCaP AR. Proliferation of LNCaP 104-R1 and PC-3^AR cells is not dependent on but is suppressed by androgen. We observed in this study that androgen treatment reduced protein expression of Cdk2, Cdk7, Cyclin A, cyclin H, Skp2, c-Myc, and E2F-1; lessened phosphorylation of Thr14, Tyr15, and Thr160 on Cdk2; decreased activity of Cdk2; induced protein level of p27^Kip1; and caused G1 cell cycle arrest in LNCaP 104-R1 cells and PC-3^AR cells. Overexpression of Skp2 protein in LNCaP 104-R1 or PC-3^AR cells partially blocked accumulation of p27^Kip1 and increased Cdk2 activity under androgen treatment, which partially blocked the androgenic suppressive effects on proliferation and cell cycle. Analyzing on-line gene array data of 214 normal and PCa samples indicated that gene expression of Skp2, Cdk2, and cyclin A positively correlates to each other, while Cdk7 negatively correlates to these genes. These observations suggested that androgen suppresses the proliferation of CRPC cells partially through inhibition of Cyclin A, Cdk2, and Skp2.