Role of androgen receptor in progression of LNCaP prostate cancer cells from G1 to S phase.

BACKGROUND: The androgen receptor (AR) plays a critical role in the proliferation of prostate cancer cells. However, its mechanism of action in proliferation remains unknown. An understanding of the mechanism of AR action in proliferation may lead to the development of effective strategies for the treatment of prostate cancer. METHODOLOGY/PRINCIPAL FINDINGS: In this study we report that pulse treatment of synchronized LNCaP cells with Casodex, an AR-antagonist, for 4 hours in mid-G(1) phase was sufficient to prevent cells from entering S phase. Since the assembly of pre-replication complex (pre-RC) in G(1) is required for the progression of cells from G(1) to S phase, the effect of Casodex during mid-G(1) suggested that the role of AR in proliferation might be to regulate the assembly of pre-RC. To test this possibility, we investigated the interaction between AR and Cdc6, an essential component of pre-RC in LNCaP cells. AR co-localized and co-immunoprecipitated with Cdc6, and Casodex treatment disrupted this interaction. AR-immunoprecipitate (AR-IP) also contained cyclin E and cyclin A, which play a critical role in pre-RC assembly and cell cycle entry into S phase, and DNA polymerase-α, PCNA, and ribonucleotide reductase, which are essential for the initiation of DNA synthesis. In addition, in cells in S phase, AR co-sedimented with components of the DNA replication machinery of cells that entered S phase. CONCLUSIONS/SIGNIFICANCE: Together, these observations suggest a novel role of AR as a component of the pre-RC to exert control over progression of LNCaP cells from G(1) to S phase through a mechanism that is independent of its role as a transcription factor.

Calmodulin protects androgen receptor from calpain-mediated breakdown in prostate cancer cells.

Although inactivation of the androgen receptor (AR) by androgen-ablation or anti-androgen treatment has been frontline therapy for disseminated prostate cancer for over 60 years, it is not curative because castration-resistant prostate cancer cells retain AR activity. Therefore, curative strategy should include targeted elimination of AR protein. Since AR binds to calmodulin (CaM), and since CaM-binding proteins are targets of calpain (Cpn)-mediated proteolysis, we studied the role of CaM and Cpn in AR breakdown in prostate cancer cells. Whereas the treatment of prostate cancer cells individually with anti-CaM drug or calcimycin, which increases intracellular Ca(++) and activates Cpn, led to minimal AR breakdown, combined treatment led to a precipitous decrease in AR protein levels. This decrease in AR protein occurred without noticeable changes in AR mRNA levels, suggesting an increase in AR protein turnover rather than inhibition of AR mRNA expression. Thus, CaM inactivation seems to sensitize AR to Cpn-mediated breakdown in prostate cancer cells. Consistent with this possibility, purified recombinant human AR (rhAR) underwent proteolysis in the presence of purified Cpn, and the addition of purified CaM to the incubation blocked rhAR proteolysis. Together, these observations demonstrate that AR is a Cpn target and AR-bound CaM plays an important role in protecting AR from Cpn-mediated breakdown in prostate cancer cells. These observations raise an intriguing possibility that anti-CaM drugs in combination with Cpn-activating agents may offer a curative strategy for the treatment of prostate cancer, which relies on AR for growth and survival.

Role of androgen receptor in prostate cancer cell cycle regulation: interaction with cell cycle regulatory proteins and enzymes of DNA synthesis.

The androgen receptor (AR) plays a critical role in proliferation and viability of prostate cancer cells. Therefore, suppressing AR activity by androgen deprivation or anti-androgen treatment has been the frontline therapy for over six decades. However, these treatment strategies are not curative and patients succumb to castration-resistant disease. Although AR is evidently critical for proliferation of prostate cancer cells, very little is known about its mechanism of action in this process. Over the years, the role of AR in prostate cancer cell proliferation and viability has been studied by focusing primarily on its role as a transcription factor. However, recent observations indicate that besides its role as a transcription factor, AR interacts physically with components of the pre-replication complex (pre-RC) and DNA replication machinery (replitase). These interactions may enable AR to exert control over the process of DNA synthesis. In addition, alterations in the proteins that interact with AR in complexes required for DNA synthesis could lead to the development of hormone-refractory prostate cancer. These observations suggest a paradigm shift for the role of AR in proliferation of prostate cancer cells from its role as a transcription factor to a non-transcriptional role as a component of the replication machinery, interacting with cell cycle regulatory proteins and enzymes of DNA synthesis. We propose that a detailed understanding of the structural interactions between AR and the components of pre-RC and replitase may lead to the development of new strategies for the treatment of prostate cancer.

Androgen regulated TRPM8 expression: a potential mRNA marker for metastatic prostate cancer detection in body fluids.

Identification of sensitive and specific biomarkers for early detection and prognosis of prostate cancer is essential for timely and appropriate treatment of the disease in individual patients. We identified an RNA transcript with sequence homology to TRPM8 (melastatin-related transient receptor potential member 8) that was overexpressed in tumor vs. patient-matched non-tumor prostate tissues by RT-PCR differential display (DD). Semi-quantitative RT-PCR analysis revealed that TRPM8 levels were higher in tumor than in non-tumor tissue from 31 of 40 (>75%) patients examined. Overexpression of TRPM8 was independent of changes in androgen receptor (AR) mRNA levels in tumor tissue. However, in studies with established cell lines, TRPM8 expression was detectable only in AR-positive, but not in AR-negative cells, and it was suppressed by steroid deprivation or anti-androgen bicalutamide (Casodex) treatment, suggesting the requirement of AR activity for TRPM8 expression in prostate cancer cells. TRPM8 mRNA was also detected in body fluids of men. Most importantly, its levels were significantly higher (p<0.001, n=18) in urine and blood of patients with metastatic disease than in those of healthy men. However, there was no significant difference (p>0.05, n=10) in its levels between prostate cancer patients with localized disease and healthy men. Together, these studies demonstrate that TRPM8 expression is androgen regulated in prostate cancer cells and that, while tissue TRPM8 mRNA levels can be used for detection of prostate cancer, urine and blood TRPM8 mRNA levels may prove to be useful for distinguishing metastatic disease from clinically localized prostate cancer at the time of diagnosis.

Calpain-mediated androgen receptor breakdown in apoptotic prostate cancer cells.

Since androgen receptor (AR) plays an important role in prostate cancer development and progression, androgen-ablation has been the frontline therapy for treatment of advanced prostate cancer even though it is rarely curative. A curative strategy should involve functional and structural elimination of AR from prostate cancer cells. We have previously reported that apoptosis induced by medicinal proteasome-inhibitory compound celastrol is associated with a decrease in AR protein levels. However celastrol-stimulated events contributing to this AR decrease have not been elucidated. Here, we report that a variety of chemotherapeutic agents, including proteasome inhibitors, a topoisomerase inhibitor, DNA-damaging agents and docetaxel that cause cell death, decrease AR levels in LNCaP prostate cancer cells. This decrease in AR protein levels was not due to the suppression of AR mRNA expression in these cells. We observed that a proteolytic activity residing in cytosol of prostate cancer cells is responsible for AR breakdown and that this proteolytic activity was stimulated upon induction of apoptosis. Interestingly, proteasome inhibitor celastrol- and chemotherapeutic drug VP-16-stimulated AR breakdown was attenuated by calpain inhibitors calpastatin and N-acetyl-L-leucyl-L-leucyl-L-methioninal. Furthermore, AR proteolytic activity pulled down by calmodulin-agarose beads from celastrol-treated PC-3 cells showed immunoreactivity to a calpain antibody. Taken together, these results demonstrate calpain involvement in proteasome inhibitor-induced AR breakdown, and suggest that AR degradation is intrinsic to the induction of apoptosis in prostate cancer cells.

Regulation of FOXO3a/beta-catenin/GSK-3beta signaling by 3,3'-diindolylmethane contributes to inhibition of cell proliferation and induction of apoptosis in prostate cancer cells.

Previous studies from our laboratory have shown anti-proliferative and pro-apoptotic effects of 3,3'-diindolylmethane (DIM) through regulation of Akt and androgen receptor (AR) in prostate cancer cells. However, the mechanism by which DIM regulates Akt and AR signaling pathways has not been fully investigated. It has been known that FOXO3a and glycogen synthase kinase-3beta (GSK-3beta), two targets of activated Akt, interact with beta-catenin, regulating cell proliferation and apoptotic cell death. More importantly, FOXO3a, GSK-3beta, and beta-catenin are all AR coregulators and regulate the activity of AR, mediating the development and progression of prostate cancers. Here, we investigated the molecular effects of B-DIM, a formulated DIM with higher bioavailability, on Akt/FOXO3a/GSK-3beta/beta-catenin/AR signaling in hormone-sensitive LNCaP and hormone-insensitive C4-2B prostate cancer cells. We found that B-DIM significantly inhibited the phosphorylation of Akt and FOXO3a and increased the phosphorylation of beta-catenin, leading to the inhibition of cell growth and induction of apoptosis. We also found that B-DIM significantly inhibited beta-catenin nuclear translocation. By electrophoretic mobility shift and chromatin immunoprecipitation assays, we found that B-DIM inhibited FOXO3a binding to the promoter of AR and promoted FOXO3a binding to the p27(KIP1) promoter, resulting in the alteration of AR and p27(KIP1) expression, the inhibition of cell proliferation, and the induction of apoptosis in both androgen-sensitive and -insensitive prostate cancer cells. These results suggest that B-DIM-induced cell growth inhibition and apoptosis induction are partly mediated through the regulation of Akt/FOXO3a/GSK-3beta/beta-catenin/AR signaling. Therefore, B-DIM could be a promising non-toxic agent for possible treatment of hormone-sensitive but most importantly hormone-refractory prostate cancers.

Calmodulin-androgen receptor (AR) interaction: calcium-dependent, calpain-mediated breakdown of AR in LNCaP prostate cancer cells.

Chemotherapy of prostate cancer targets androgen receptor (AR) by androgen ablation or antiandrogens, but unfortunately, it is not curative. Our attack on prostate cancer envisions the proteolytic elimination of AR, which requires a fuller understanding of AR turnover. We showed previously that calmodulin (CaM) binds to AR with important consequences for AR stability and function. To examine the involvement of Ca(2+)/CaM in the proteolytic breakdown of AR, we analyzed LNCaP cell extracts that bind to a CaM affinity column for the presence of low molecular weight forms of AR (intact AR size, approximately 114 kDa). Using an antibody directed against the NH(2)-terminal domain (ATD) of AR on Western blots, we identified approximately 76-kDa, approximately 50-kDa, and 34/31-kDa polypeptides in eluates of CaM affinity columns, suggesting the presence of CaM-binding sites within the 31/34-kDa ATD of AR. Under cell-free conditions in the presence of phenylmethylsulfonyl fluoride, AR underwent Ca(2+)-dependent degradation. AR degradation was inhibited by N-acetyl-leu-leu-norleu, an inhibitor of thiol proteases, suggesting the involvement of calpain. In intact cells, AR breakdown was accelerated by raising intracellular Ca(2+) using calcimycin, and increased AR breakdown was reversed with the cell-permeable Ca(2+) chelator bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetra-(acetoxymethyl)-ester. In CaM affinity chromatography studies, the Ca(2+)-dependent protease calpain was bound to and eluted from the CaM-agarose column along with AR. Caspase-3, which plays a role in AR turnover under stress conditions, did not bind to the CaM column and was present in the proenzyme form. Similarly, AR immunoprecipitates prepared from whole-cell extracts of exponentially growing LNCaP cells contained both calpain and calpastatin. Nuclear levels of calpain and calpastatin (its endogenous inhibitor) changed in a reciprocal fashion as synchronized LNCaP cells progressed from G(1) to S phase. These reciprocal changes correlated with changes in AR level, which increased in late G(1) phase and decreased as S phase progressed. Taken together, these observations suggest potential involvement of AR-bound CaM in calcium-controlled, calpain-mediated breakdown of AR in prostate cancer cells.

Replitase: complete machinery for DNA synthesis.

Replication of nuclear DNA in eukaryotes presents a tremendous challenge, not only due to the size and complexity of the genome, but also because of the time constraint imposed by a limited duration of S phase during which the entire genome has to be duplicated accurately and only once per cell division cycle. A challenge of this magnitude can only be met by the close coupling of DNA precursor synthesis to replication. Prokaryotic systems provide evidence for multienzyme and multiprotein complexes involved in DNA precursor synthesis and DNA replication. In addition, fractionation of nuclear proteins from proliferating mammalian cells shows co-sedimentation of enzymes involved in DNA replication with those required for synthesis of deoxynucleoside triphosphates (dNTPs). Such complexes can be isolated only from cells that are in S phase, but not from cells in G(0)/G(1) phases of cell cycle. The kinetics of deoxynucleotide metabolism supporting DNA replication in intact and permeabilized cells reveals close coupling and allosteric interaction between the enzymes of dNTP synthesis and DNA replication. These interactions contribute to channeling and compartmentation of deoxynucleotides in the microvicinity of DNA replication. A multienzyme and multiprotein megacomplex with these unique properties is called "replitase." In this article, we summarize some of the relevant evidence to date that supports the concept of replitase in mammalian cells, which originated from the observations in Dr. Pardee's laboratory. In addition, we show that androgen receptor (AR), which plays a critical role in proliferation and viability of prostate cancer cells, is associated with replitase, and that identification of constituents of replitase in androgen-dependent versus androgen-independent prostate cancer cells may provide insights into androgen-regulated events that control proliferation of prostate cancer cells and potentially offer an effective strategy for the treatment of prostate cancer.

Activation of mitogen-activated protein kinase in estrogen receptor alpha-positive breast cancer cells in vitro induces an in vivo molecular phenotype of estrogen receptor alpha-negative human breast tumors.

Breast cancer presents as either estrogen receptor alpha (ERalpha) positive or negative, with ERalpha+ tumors responding to antiestrogen therapy and having a better prognosis. By themselves, mRNA expression signatures of estrogen regulation in ERalpha+ breast cancer cells do not account for the vast molecular differences observed between ERalpha+ and ERalpha- cancers. In ERalpha- tumors, overexpression of epidermal growth factor receptor (EGFR) or c-erbB-2, leading to increased growth factor signaling, is observed such that mitogen-activated protein (MAP) kinase (MAPK) is significantly hyperactivated compared with ERalpha+ breast cancer. In ERalpha+/progesterone receptor-positive, estrogen-dependent MCF-7 breast cancer cells, we stably overexpressed EGFR or constitutively active erbB-2, Raf, or MAP/extracellular signal-regulated kinase kinase, resulting in cell lines exhibiting hyperactivation of MAPK, estrogen-independent growth, and the reversible down-regulation of ERalpha expression. By global mRNA profiling, we found a "MAPK signature" of approximately 400 genes consistently up-regulated or down-regulated in each of the MAPK+ cell lines. In several independent profile data sets of human breast tumors, the in vitro MAPK signature was able to accurately distinguish ER+ from ER- tumors. In addition, our in vitro mRNA profile data revealed distinct mRNA signatures specific to either erbB-2 or EGFR activation. A subset of breast tumor profiles was found to share extensive similarities with either the erbB-2-specific or the EGFR-specific signatures. Our results confirm that increased MAPK activation causes loss of ERalpha expression and suggest that hyperactivation of MAPK plays a role in the generation of the ERalpha- phenotype in breast cancer. These MAPK+ cell lines are excellent models for investigating the underlying mechanisms behind the ERalpha- phenotype.

Androgen receptor signaling and vitamin D receptor action in prostate cancer cells.

BACKGROUND: 1,25(OH)2D3 inhibits the growth of prostate cancer cells; previous reports suggest that 1,25(OH)2D3 actions in LNCaP prostate cancer cells are androgen dependent. This is due in part to the observation that the androgen receptor (AR) antagonist, Casodex, modestly inhibits LNCaP cell growth, but reduces the greater growth inhibition induced by 1,25(OH)2D3 to the level of Casodex alone. Because androgen ablation therapy is used for metastatic prostate cancer, we sought to better characterize this androgen dependence. METHODS: We have assessed the requirement for endogenous androgens in 1,25(OH)2D3 mediated growth inhibition of AR+ prostate cancer cell lines. We have also sought the mechanism for anti-androgen mediated reversal of 1,25(OH)2D3 dependent growth inhibition in LNCaP cells. RESULTS: Although 1,25(OH)2D3 does not inhibit the growth of LNCaP cells grown in medium lacking androgens, we find that growth of androgen independent derivatives of LNCaP cells is inhibited by 1,25(OH)2D3. Despite this independence, Casodex treatment reduced the response of these cells to 1,25(OH)2D3 suggesting a unique function for Casodex-bound AR. Because Casodex does not directly inhibit the transcriptional activity of the vitamin D receptor (VDR) we sought a common primary target of VDR and AR action whose VDR dependent transcription could be repressed by Casodex. We report that AS3 (APRIN), a novel gene required for androgen dependent growth arrest, is a primary target for 1,25(OH)2D3 and androgens. Moreover, Casodex reduces induction of AS3 by 1,25(OH)2D3 suggesting that it is a candidate for the Casodex effect. Analysis of functional interactions between AR and VDR in other AR containing prostate cancer cells lines (PC-3 AR, LAPC-4, and 22Rv1) revealed that Casodex reversal was unique to LNCaP derived cells. CONCLUSION: Anti-androgen mediated reversal of 1,25(OH)2D3 dependent growth inhibition is limited to LNCaP derived prostate cancer cell lines. Moreover, the growth of androgen independent derivatives of LNCaP cells in medium depleted of androgens is strongly inhibited by 1,25D. Therefore, most forms of androgen ablation should not eliminate the utility of VDR agonist treatment in most prostate cancers.

A cytoplasmic substrate of mitogen-activated protein kinase is responsible for estrogen receptor-alpha down-regulation in breast cancer cells: the role of nuclear factor-kappaB.

Estrogen receptor alpha (ERalpha) negative breast tumors often present with enhanced expression and/or activation of growth factor receptors, resulting in increased growth factor signaling and hyperactivation of MAPK (ERK1 and ERK2). We have pre-viously shown that ERalpha(+) MCF-7 cells with elevated growth factor signaling lose expression of ERalpha without any ligand-independent transcriptional activation, and this is a reversible effect attributable to ERK1/2 hyperactivation. Here, we show that down-regulation of ERalpha is not mediated by a specific ERK-1 vs. ERK-2 substrate. Despite up-regulated activator protein-1 activity in response to ERK1/2 activation, and in ERalpha(-) and hormone-independent breast cancers, we find that increased activator protein-1 activity is not responsible for ERalpha down-regulation. Interestingly, our findings implicate a cytoplasmic substrate of ERK1/2. However, RSK1, the best-characterized cytoplasmic ERK1/2 substrate, does not down-regulate ERalpha in our models. On the other hand, inhibition of nuclear factor-kappaB (which is linked to chemoresistance in cancer in general and has elevated activity in hormone-independent and ERalpha- breast cancer) significantly enhances ERalpha activity, suggesting that indirect elevation in nuclear factor-kappaB activity (due to hyperactive ERK1/2) is at least partially responsible for ERalpha down-regulation in these cell line models.

1alpha,25-dihydroxyvitamin D3 induced growth inhibition of PC-3 prostate cancer cells requires an active transforming growth factor beta signaling pathway.

BACKGROUND: Prostate cancer growth inhibition by 1alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) is best characterized in the androgen dependent LNCaP cell line, where treatment with this hormone causes cell cycle arrest and apoptosis. 1,25(OH)2D3 also inhibits the growth of PC-3 prostate cancer cells, but not through the induction of G1 arrest or apoptosis. In this study, we have sought to elucidate the mechanism/s involved in PC-3 cell growth inhibition by 1,25(OH)2D3. EXPERIMENTAL METHODS: We determined the effect of transforming growth factor beta (TGFbeta) blocking antibodies on 1,25(OH)2D3 mediated growth inhibition of PC-3 cells. In addition, we also studied the effects of 1,25(OH)2D3 on TGFbeta signaling and receptor expression. Finally, we assessed the role of TGFbeta signaling in the induction of the growth inhibitory protein, insulin like growth factor binding protein 3 (IGFBP-3), by 1,25(OH)2D3. RESULTS: We find that 1,25(OH)2D3 action in PC-3 cells is mediated through at least two distinct pathways, the TGFbeta pathway and the IGFBP-3 pathway. We show that 1,25(OH)2D3 treatment elevates TGFbeta production and signaling, as well as receptor levels, in PC-3 cells. Further, using a blocking antibody against TGFbeta substantially reduces 1,25(OH)2D3 mediated growth inhibition without affecting IGFBP-3 induction, suggesting that IGFBP-3, alone, is insufficient to inhibit the growth of PC-3 cells.

Stable expression of full length human androgen receptor in PC-3 prostate cancer cells enhances sensitivity to retinoic acid but not to 1alpha,25-dihydroxyvitamin D3.

BACKGROUND: PC-3 prostate cancer cell growth is inhibited by 1alpha,25-dihydroxyvitamin D(3) (1,25 D) and retinoids, but not to the same extent as the androgen receptor (AR) positive LNCaP prostate cancer cells. Previous reports suggest a role for AR in growth inhibition of LNCaP cells by 1,25 D and retinoids. PC-3 cells do not express AR. We therefore asked whether re-expression of AR would enhance the response of PC-3 cells to 1,25 D and retinoids. METHODS: PC-3 cells were stably transfected with wild type human AR cDNA. Pooled cells expressing AR protein at levels comparable to LNCaP cells were used to analyze response to 1,25 D, retinoids, androgens, and anti-androgens. RESULTS: AR re-expression in PC-3 cells restored response to androgens and anti-androgens, but growth inhibition by 1,25 D was not significantly altered. However, cells were sensitized to low levels of retinoids, and, in contrast to the parental PC-3 cells, sub-optimal levels of 1,25 D and retinoids caused additive growth inhibition. CONCLUSIONS: Restoring AR expression and activity in PC-3 cells results in enhanced sensitivity to low levels of retinoids while the response to 1,25 D remains unaltered. Thus, the involvement of AR in prostate cancer growth inhibition by 1,25 D appears to be cell line specific.