Androgen receptor expression in prostate cancer stem cells: is there a conundrum?

Prostate Cancer Stem Cell Markers Drive Progression, Therapeutic Resistance, and Bone Metastasis

Metastatic or recurrent tumors are the primary cause of cancer-related death. For prostate cancer, patients diagnosed with local disease have a 99% 5-year survival rate; however, this 5-year survival rate drops to 28% in patients with metastatic disease. This dramatic decline in survival has driven interest in discovering new markers able to identify tumors likely to recur and in developing new methods to prevent metastases from occurring. Biomarker discovery for aggressive tumor cells includes attempts to identify cancer stem cells (CSCs). CSCs are defined as tumor cells capable of self-renewal and regenerating the entire tumor heterogeneity. Thus, it is hypothesized that CSCs may drive primary tumor aggressiveness, metastatic colonization, and therapeutic relapse. The ability to identify these cells in the primary tumor or circulation would provide prognostic information capable of driving prostate cancer treatment decisions. Further, the ability to target these CSCs could prevent tumor metastasis and relapse after therapy allowing for prostate cancer to finally be cured. Here, we will review potential CSC markers and highlight evidence that describes how cells expressing each marker may drive prostate cancer progression, metastatic colonization and growth, tumor recurrence, and resistance to treatment.

Prostate Cancer Stem Cells and Nanotechnology: A Focus on Wnt Signaling

Prostate cancer is the most common cancer among men worldwide. However, current treatments for prostate cancer patients in advanced stage often fail because of relapse. Prostate cancer stem cells (PCSCs) are resistant to most standard therapies, and are considered to be a major mechanism of cancer metastasis and recurrence. In this review, we summarized current understanding of PCSCs and their self-renewal signaling pathways with a specific focus on Wnt signaling. Although multiple Wnt inhibitors have been developed to target PCSCs, their application is still limited by inefficient delivery and toxicity in vivo. Recently, nanotechnology has opened a new avenue for cancer drug delivery, which significantly increases specificity and reduces toxicity. These nanotechnology-based drug delivery methods showed great potential in targeting PCSCs. Here, we summarized current advancement of nanotechnology-based therapeutic strategies for targeting PCSCs and highlighted the challenges and perspectives in designing future therapies to eliminate PCSCs.

Cryptotanshinone targets tumor-initiating cells through down-regulation of stemness genes expression

Recent evidence indicates that tumor-initiating cells (TICs), also called cancer stem cells (CSCs), are responsible for tumor initiation and progression, therefore representing an important cell population that may be used as a target for the development of future anticancer therapies. In the present study, Cryptotanshinone (CT), a traditional Chinese herbal medicine, was demonstrated to regulate the behaviors of LNCaP prostate cells and prostate LNCaP TICs. The results demonstrate that treatment with CT alters cellular proliferation, cell cycle status, migration, viability, colony formation and notably, sphere formation and down-regulation of stemness genes (Nanog, OCT4, SOX2, β-catenin, CXCR4) in TICs. The present study demonstrates that CT targets the LNCaP CD44+CD24- population that is representative of prostate TICs and also affects total LNCaP cells as well via down-regulation of stemness genes. The strong effect with which CT has on prostate TICs suggests that CT may potentially function as a novel natural anticancer agent that specifically targets TICs.

The relationship of cancer stem cells in urological cancers

Numerous studies are ongoing to identify and isolate cancer stem cells from cancers of genito-urinary tracts. Better understanding of their role in prostate, urothelial and kidney cancer origin, growth and progression opens new pathways in development of more effective treatment methods. However there are still many issues before advances in this field can be introduced for clinical application. This review addresses current achievements in cancer stem cells research in uro-oncology.

Genomic Analysis of Invasive Human Bone Marrow Derived Mesenchymal Stem Cells

BACKGROUND

Human bone marrow derived mesenchymal stem cells (hMSCs) are capable of differentiation into multiple cell lineages and demonstrate a wide variety of use in various therapeutic applications. Only recently has research begun to understand the gene expression profiles of hMSCs and their differentiated counterparts in vivo and ex vivo.

PURPOSE

The research presented here aimed at gaining a better understanding of gene expression patterns present during hMSC invasion through a basement membrane.

METHODS

Changes in gene expression were evaluated between invasive and non-invasive cells using Agilent's gene expression arrays and Matrigel invasion chambers. The cells were specifically attracted to a defined stem cell media called SCM.

RESULTS

A total 435 genes were up-regulated by 2- fold or more in the invasive population of cells and classified into developmental programs and immunological/inflammatory signaling pathways determined by Ingenuity Pathway Analysis (IPA). This list included a variety of regulators of growth and differentiation including NANOG, STAT3 and STAT5A and members of the polycomb repressive complex-2 (PCRC2) EZH2 and SUZ12. The known regulator of inflammation and hypoxia HIF-1α was also increased suggesting that regulation of the microenvironment is important during this process. Finally, the invasion process could be reversed using the STAT3 inhibitor Static.

CONCLUSIONS

Overall these data will increase the understanding of the genetic pathways functioning during hMSC invasion and aid in the development of their therapeutic applications.

Growth kinetics of CD133-positive prostate cancer cells

BACKGROUND

In the adult human prostate CD133 expression is thought to mark rare prostate epithelial stem cells and malignant tumor stem/initiating cells. Such putative stem cell populations are thought to proliferate slowly, but possess unlimited proliferative potential. Based on this, we hypothesized that CD133(pos) prostate cancer cells proliferate slower than CD133(neg) cells.

METHODS

Human prostate cancer cell lines were analyzed for CD133 expression and DNA content using flow cytometry. Rates of cell division and DNA synthesis were determined using CFSE cell tracing and BrdU uptake, respectively. Changes in cell cycle distribution and the percentage of CD133(pos) cells were assayed under conditions of different cell density and AR-pathway modulation. Lastly, we over-expressed lentiviral CD133 to measure whether CD133 regulates the cell cycle.

RESULTS

The cell cycle distribution differs between CD133(pos) and CD133(neg) cells in all three human prostate cancer cell lines studied. CD133(pos) cells have a greater proportion of cells in G2 and proliferate faster than CD133(neg) cells. High cell density increases the percentage of CD133(pos) cells without changing CD133(pos) cell cycle progression. Treatment with the AR agonist R1881, or the anti-androgen MDV3100, significantly changed the percentage and proliferation of CD133(pos) cells. Finally, ectopic over-expression of CD133 had no effect on cell cycle progression.

CONCLUSIONS

Contrary to our hypothesis, we demonstrate that CD133(pos) cells proliferate faster than CD133(neg) cells. This association of CD133 expression with increased cell proliferation is not directly mediated by CD133, suggesting that surface CD133 is a downstream target gene of an undefined pathway controlling cell proliferation.

Side population rather than CD133(+) cells distinguishes enriched tumorigenicity in hTERT-immortalized primary prostate cancer cells

BACKGROUND

Subpopulations of cancer cells with the capacity of generating solid tumors have been characterized. In various cancer types, including prostate cancer cells, a side population (SP) and CD133-expressing cells have been proposed as containing a population cancer cells with stem-like ability. Therefore the aim of this work was to determine, in prostate cancer cell lines, the frequency and tumorigenic potential of SP and CD133+ cells.

RESULTS

In vitro 2D colony-forming assay and sphere-forming assay, Flow cytometry analysis and magnetic cell sorting were utilized to sort CD133+, CD133- and Side population (SP) cells. Our findings indicate that CD44 and integrin α-6 are uniformly expressed in the hTERT cell lines; however, CD133 is expressed only in a small population (< 0.1%). FACS-sorted CD133+ and CD133- cells exhibited similar tumorigenicity in vitro and in vivo. Additionally, for the hTERT cells, SP rather than CD133 expression showed an 8-fold enhanced tumorigenic potential. The data suggest that SP cells, rather than those with CD133 marker, contain the rare population of CSC capable of producing prostate tumors.

CONCLUSION

Collectively, our data suggest that although CD133 is expressed only in a small population of hTERT-immortalized prostate cancer cells, it is not likely to be associated with stem cells, as CD133- and CD133+ cells exhibited similar tumorigenicity. However, SP isolated cells, appear to be enriched with tumorigenic stem-like cells capable of generating palpable tumors.

Neoplastic stem cells: current concepts and clinical perspectives

Neoplastic stem cells have initially been characterized in myeloid leukemias where NOD/SCID mouse-repopulating progenitors supposedly reside within a CD34+/Lin- subset of the malignant clone. These progenitors are considered to be self-renewing cells responsible for the in vivo long-term growth of neoplastic cells in leukemic patients. Therefore, these cells represent an attractive target of therapy. In some lymphoid leukemias, NOD/SCID mouse-repopulating cells were also reported to reside within the CD34+/Lin- subfraction of the clone. More recently, several attempts have been made to transfer the cancer stem cell concept to solid tumors and other non-hematopoietic neoplasms. In several of these tumors, the cell surface antigens AC133 (CD133) and CD44 are considered to indicate the potential of a cell to initiate permanent tumor formation in vivo. However, several questions concerning the phenotype, self-renewal capacity, stroma-dependence, and other properties of cancer- or leukemia-initiating cells remain to be solved. The current article provides a summary of our current knowledge on neoplastic (cancer) stem cells, with special emphasis on clinical implications and therapeutic options as well as a discussion about conceptual and technical limitations.

Cell-autonomous intracellular androgen receptor signaling drives the growth of human prostate cancer initiating cells

BACKGROUND

The lethality of prostate cancer is due to the continuous growth of cancer initiating cells (CICs) which are often stimulated by androgen receptor (AR) signaling. However, the underlying molecular mechanism(s) for such AR-mediated growth stimulation are not fully understood. Such mechanisms may involve cancer cell-dependent induction of tumor stromal cells to produce paracrine growth factors or could involve cancer cell autonomous autocrine and/or intracellular AR signaling pathways.

METHODS

We utilized clinical samples, animal models and a series of AR-positive human prostate cancer cell lines to evaluate AR-mediated growth stimulation of prostate CICs.

RESULTS

The present studies document that stromal AR expression is not required for prostate cancer growth, since tumor stroma surrounding AR-positive human prostate cancer metastases (N = 127) are characteristically AR-negative. This lack of a requirement for AR expression in tumor stromal cells is also documented by the fact that human AR-positive prostate cancer cells grow equally well when xenografted in wild-type versus AR-null nude mice. AR-dependent growth stimulation was documented to involve secretion, extracellular binding, and signaling by autocrine growth factors. Orthotopic xenograft animal studies documented that the cellautonomous autocrine growth factors which stimulate prostate CIC growth are not the andromedins secreted by normal prostate stromal cells. Such cell autonomous and extracellular autocrine signaling is necessary but not sufficient for the optimal growth of prostate CICs based upon the response to anti-androgen plus/or minus preconditioned media.

CONCLUSIONS

AR-induced growth stimulation of human prostate CICs requires AR-dependent intracellular pathways. The identification of such AR-dependent intracellular pathways offers new leads for the development of effective therapies for prostate cancer.

Androgen receptor expression is associated with prostate cancer-specific survival in castrate patients with metastatic disease

OBJECTIVE

To investigate whether baseline (before treatment) clinical variables and tumour specimen characteristics (including the androgen receptor, AR) from patients with castrate-resistant metastatic prostate cancer can be used to predict the time to prostate cancer-specific mortality and overall survival, as AR levels in prostate cancer have been associated with disease progression, including prostate-specific antigen (PSA) recurrence and systemic metastasis.

PATIENTS AND METHODS

Haematoxylin and eosin (H&E) slides/blocks and outcome data from a 104 castrate patients with metastatic disease (43 prostatectomy and 61 prostate needle biopsy samples), were independently reviewed; H&E morphometry and quantitative immunofluorescence were used to assess the samples. Sections were analysed with a multiplex quantitative immunofluorescence (IF) assay for cytokeratin-18 (epithelial cells), 4',6-diamidino-2-phenylindole (nuclei), p63/high molecular weight keratin (basal cells), AR and alpha-methyl CoA-racemase. Images were acquired with spectral imaging software and processed for quantification with IF algorithms.

RESULTS

The median follow-up was 12 years from diagnosis; 49 men (47%) baseline PSA levels of > or = 20 ng/mL, 55 (53%) had a Gleason sum of 8, 63 (60%) died from the disease and 40% were alive (censored). In all, 66 patients had evaluable IF features, and the association with outcome was evaluated by univariate Cox modelling and support-vector regression. PSA was the only clinical variable associated with outcome (concordance index, CoI, 0.41; P < 0.05, log-rank test). The amount of AR present within tumour nuclei (regardless of tissue provenance and primary treatment) significantly correlated with a greater risk of a shorter time to prostate cancer-specific mortality (CoI 0.36; P < 0.05 log-rank test). There were no H&E features that correlated with mortality.

CONCLUSION

By univariate analysis, increased nuclear AR expression in either the diagnostic biopsy and/or radical prostatectomy specimen, from patients with advanced disease, was associated with a reduced time to prostate cancer-specific mortality.

The expression of pluripotency marker Oct 3/4 in prostate cancer and benign prostate hyperplasia

BACKGROUND

Oct 3/4 (Octamer 3/4), a member of POU family has been considered as an important stem cell marker and essential transcription factor during human embryogenesis. In recent years, there have also been reports on presence of Oct 3/4 in differentiated benign and malignant human cells. The objective of this study was to investigate the transcription and the protein expression of Oct 3/4 isoforms in prostate cancer and benign prostate tissue.

METHODS

Thirty sex adenocarcinomas and eight cases of benign prostate hyperplasia were studied. The transcription of Oct 3/4 was analyzed using RT-PCR approach associated with restriction digestion analysis. Oct 3/4 protein expression was studied by immunohistochemistry on paraffin sections using two different antibodies.

RESULTS

We identified only the transcript 2 of Oct 3/4 in prostate tumors and benign prostate hyperplasia. Immunohistochemistry verified these results, demonstrating only cytoplasmic localization of Oct 3/4. Transcription of type 1 of Oct 3/4 as well as protein expression with nuclear localization of Oct 3/4 isoform 1 were not detected. Oct 3/4 immunopositive tumors were also displayed neuroendocrine differentiation and showed androgen receptor immunopositivity. The stem cell markers CD44 and CD117 were not detected in Oct 3/4 immunopositive cells.

CONCLUSION

Our results indicate that only the cytoplasmic isoform 2 of Oct 3/4 is present in prostate cancer and benign prostate hyperplasia. The malignant and benign prostate cells, which are immunopositive for variant 2 of Oct 3/4, lack other stem cell markers supporting previously published data that variant 2 of Oct 3/4 is not a pluripotency marker.

Molecular biology of androgen-independent prostate cancer: the role of the androgen receptor pathway

Prostate cancer (PC) cells express the androgen receptor (AR) and need the presence of androgens to survive. Androgen suppression is the gold standard first-line therapy for metastatic disease. Almost all PC patients initially respond to hormonal therapy, but most of them gradually develop resistance to castration. There is evidence that these tumours that are considered castration-resistant continue to depend on AR signalling. Several mechanisms that enhance AR signalling in an androgen-depleted environment have been elucidated: (1) AR mutations that allow activation by low androgen levels or by other endogenous steroids, (2) AR amplification and/or overexpression, (3) increased local intracrine synthesis of androgens, (4) changes in AR cofactors and (5) cross-talk with cytokines and growth factors. Today, there are a number of novel agents targeting the AR signalling pathway under development, including more effective antiandrogens; inhibitors of CYP17, inhibitors of HSP90, inhibitors of histone deacetylases and inhibitors of tyrosine kinase inhibitors.