Identification and isolation of human prostate epithelial stem cells based on α2β1-integrin expression

Cyclin A1 and P450 Aromatase Promote Metastatic Homing and Growth of Stem-like Prostate Cancer Cells in the Bone Marrow

Bone metastasis is a leading cause of morbidity and mortality in prostate cancer. While cancer stem-like cells have been implicated as a cell of origin for prostate cancer metastasis, the pathways that enable metastatic development at distal sites remain largely unknown. In this study, we illuminate pathways relevant to bone metastasis in this disease. We observed that cyclin A1 (CCNA1) protein expression was relatively higher in prostate cancer metastatic lesions in lymph node, lung, and bone/bone marrow. In both primary and metastatic tissues, cyclin A1 expression was also correlated with aromatase (CYP19A1), a key enzyme that directly regulates the local balance of androgens to estrogens. Cyclin A1 overexpression in the stem-like ALDH(high) subpopulation of PC3M cells, one model of prostate cancer, enabled bone marrow integration and metastatic growth. Further, cells obtained from bone marrow metastatic lesions displayed self-renewal capability in colony-forming assays. In the bone marrow, cyclin A1 and aromatase enhanced local bone marrow-releasing factors, including androgen receptor, estrogen and matrix metalloproteinase MMP9 and promoted the metastatic growth of prostate cancer cells. Moreover, ALDH(high) tumor cells expressing elevated levels of aromatase stimulated tumor/host estrogen production and acquired a growth advantage in the presence of host bone marrow cells. Overall, these findings suggest that local production of steroids and MMPs in the bone marrow may provide a suitable microenvironment for ALDH(high) prostate cancer cells to establish metastatic growths, offering new approaches to therapeutically target bone metastases. Cancer Res; 76(8); 2453-64. ©2016 AACR.

The Androgen Receptor Bridges Stem Cell-Associated Signaling Nodes in Prostate Stem Cells

The therapeutic potential of stem cells relies on dissecting the complex signaling networks that are thought to regulate their pluripotency and self-renewal. Until recently, attention has focused almost exclusively on a small set of "core" transcription factors for maintaining the stem cell state. It is now clear that stem cell regulatory networks are far more complex. In this review, we examine the role of the androgen receptor (AR) in coordinating interactions between signaling nodes that govern the balance of cell fate decisions in prostate stem cells.

Harvesting Human Prostate Tissue Material and Culturing Primary Prostate Epithelial Cells

In order to fully explore the biology of a complex solid tumor such as prostate cancer, it is desirable to work with patient tissue. Only by working with cells from a tissue can we take into account patient variability and tumor heterogeneity. Cell lines have long been regarded as the workhorse of cancer research and it could be argued that they are of most use when considered within a panel of cell lines, thus taking into account specified mutations and variations in phenotype between different cell lines. However, often very different results are obtained when comparing cell lines to primary cells cultured from tissue. It stands to reason that cells cultured from patient tissue represents a close-to-patient model that should and does produce clinically relevant data. This chapter aims to illustrate the methods of processing, storing and culturing cells from prostate tissue, with a description of potential uses.

Prostate cancer stem cells: deciphering the origins and pathways involved in prostate tumorigenesis and aggression

The cells of the prostate gland are dependent on cell signaling pathways to regulate their growth, maintenance and function. However, perturbations in key signaling pathways, resulting in neoplastic transformation of cells in the prostate epithelium, are likely to generate subtypes of prostate cancer which may subsequently require different treatment regimes. Accumulating evidence supports multiple sources of stem cells in the prostate epithelium with distinct cellular origins for prostate tumorigenesis documented in animal models, while human prostate cancer stem-like cells (PCSCs) are typically enriched by cell culture, surface marker expression and functional activity assays. As future therapies will require a deeper understanding of its cellular origins as well as the pathways that drive PCSC maintenance and tumorigenesis, we review the molecular and functional evidence supporting dysregulation of PI3K/AKT, RAS/MAPK and STAT3 signaling in PCSCs, the development of castration resistance, and as a novel treatment approach for individual men with prostate cancer.

The many ways to make a luminal cell and a prostate cancer cell

Research in the area of stem/progenitor cells has led to the identification of multiple stem-like cell populations implicated in prostate homeostasis and cancer initiation. Given that there are multiple cells that can regenerate prostatic tissue and give rise to prostate cancer, our focus should shift to defining the signaling mechanisms that drive differentiation and progenitor self-renewal. In this article, we will review the literature, present the evidence and raise important unanswered questions that will help guide the field forward in dissecting critical mechanisms regulating stem-cell differentiation and tumor initiation.

Cellular recognition and macropinocytosis-like internalization of nanoparticles targeted to integrin α2β1

Targeting nanoparticles to desired intracellular compartments is a major challenge. Integrin-type adhesion receptors are connected to different endocytosis routes in a receptor-specific manner. According to our previous observations, the internalization of an α2β1-integrin-echovirus-1 complex takes place via a macropinocytosis-like mechanism, suggesting that the receptor could be used to target nanoparticles to this specific entry route. Here, silica-based nanoparticles, carrying monoclonal antibodies against the α2β1 integrin as address labels, were synthesized. Studies with flow cytometry, atomic force microscopy and confocal microscopy showed the particles to attach to the cell surface via the α2β1 integrin. Furthermore, quantitative analysis of nanoparticle trafficking inside the cell performed with the BioImageXD software indicated that the particles enter cells via a macropinocytosis-like process and end up in caveolin-1 positive structures. Thus, we suggest that different integrins can guide particles to distinct endocytosis routes and, subsequently, also to specific intracellular compartments. In addition, we show that with the BioImageXD software it is possible to conduct sensitive and complex analyses of the behavior of small fluorescent particles inside cells, using basic confocal microscopy images.

Metformin and prostate cancer stem cells: a novel therapeutic target

Prostate cancer is the second most frequently diagnosed cancer in the world. Localized disease can be effectively treated with radiation therapy or radical prostatectomy. However, advanced prostate cancer is more difficult to treat and if metastatic, is incurable. There is a need for more effective therapy for advanced prostate cancer. One potential target is the cancer stem cell (CSC). CSCs have been described in several solid tumors, including prostate cancer, and contribute to therapeutic resistance and tumor recurrence. Metformin, a common oral biguanide used to treat type 2 diabetes, has been demonstrated to have anti-neoplastic effects. Specifically, metformin targets CSCs in breast cancer, pancreatic cancer, glioblastoma and colon cancer. Metformin acts directly on the mitochondria to inhibit oxidative phosphorylation and reduce mitochondrial ATP production. This forces tumor cells to compensate by increasing the rate of glycolysis. CSCs rely heavily on mitochondrial oxidative phosphorylation for energy production. The glycolytic switch results in an energy crisis in these cells. Metformin could be used to exploit this metabolic weakness in CSCs. This would increase CSC sensitivity to conventional cancer therapies, circumventing treatment resistance and enhancing treatment efficacy. This review will explore the characteristics of prostate CSCs, their role in tumor propagation and therapeutic resistance and the role of metformin as a potential prostate CSC sensitizer to current anticancer therapies.

miR-25 Modulates Invasiveness and Dissemination of Human Prostate Cancer Cells via Regulation of αv- and α6-Integrin Expression

Altered microRNA (miRNA; miR) expression is associated with tumor formation and progression of various solid cancers. A major challenge in miRNA expression profiling of bulk tumors is represented by the heterogeneity of the subpopulations of cells that constitute the organ, as well as the tumor tissue. Here, we analyzed the expression of miRNAs in a subpopulation of epithelial stem/progenitor-like cells in human prostate cancer [prostate cancer stem cell (PCSC)] and compared their expression profile to more differentiated cancer cells. In both cell lines and clinical prostate cancer specimens, we identified that miR-25 expression in PCSCs was low/absent and steadily increased during their differentiation into cells with a luminal epithelial phenotype. Functional studies revealed that overexpression of miR-25 in prostate cancer cell lines and selected subpopulation of highly metastatic and tumorigenic cells (ALDH(high)) strongly affected the invasive cytoskeleton, causing reduced migration in vitro and metastasis via attenuation of extravasation in vivo. Here, we show, for the first time, that miR-25 can act as a tumor suppressor in highly metastatic PCSCs by direct functional interaction with the 3'-untranslated regions of proinvasive αv- and α6-integrins. Taken together, our observations suggest that miR-25 is a key regulator of invasiveness in human prostate cancer through its direct interactions with αv- and α6-integrin expression.

Defined conditions for the isolation and expansion of basal prostate progenitor cells of mouse and human origin

Methods to isolate and culture primary prostate epithelial stem/progenitor cells (PESCs) have proven difficult and ineffective. Here, we present a method to grow and expand both murine and human basal PESCs long term in serum- and feeder-free conditions. The method enriches for adherent mouse basal PESCs with a Lin⁻SCA-1⁺CD49f⁺TROP2^high phenotype. Progesterone and sodium selenite are additionally required for the growth of human Lin⁻CD49f⁺TROP2^high PESCs. The gene-expression profiles of expanded basal PESCs show similarities to ESCs, and NF-kB function is critical for epithelial differentiation of sphere-cultured PESCs. When transplanted in combination with urogenital sinus mesenchyme, expanded mouse and human PESCs generate ectopic prostatic tubules, demonstrating their stem cell activity in vivo. This novel method will facilitate the molecular, genomic, and functional characterization of normal and pathologic prostate glands of mouse and human origin.

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Basal prostate epithelial stem/progenitor cells (PESCs) are expanded in vitro

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Expanded PESCs can differentiate into glandular structures in vitro and in vivo

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A surface-marker screen identifies marker candidates for prostate basal stem cells

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Gene-expression analysis shows a role of NF-kB signaling in PESC differentiation

Role of MicroRNAs in Prostate Cancer Pathogenesis

Prostate cancer (PCa) remains the most commonly diagnosed malignant tumor in men, and is the second highest cause of cancer mortality after lung tumors in the United States. Accumulating research indicates that microRNAs (miRNAs) are increasingly being implicated in PCa. miRNAs are conserved small noncoding RNAs that control gene expression posttranscriptionally. Recent profiling research suggests that miRNAs are aberrantly expressed in PCa, and these have been implicated in the regulation of apoptosis, cell cycle, epithelial to mesenchymal transition, PCa stem cells, and androgen receptor pathway. All of these might provide the basis for new approaches for PCa. Here, we review current findings regarding miRNA research in PCa to provide a strong basis for future study aimed at promising contributions of miRNA in PCa.

Cancer stem cell immunology: key to understanding tumorigenesis and tumor immune escape?

Cancer stem cell (CSC) biology and tumor immunology have shaped our understanding of tumorigenesis. However, we still do not fully understand why tumors can be contained but not eliminated by the immune system and whether rare CSCs are required for tumor propagation.

Long latency or recurrence periods have been described for most tumors. Conceptually, this requires a subset of malignant cells which is capable of initiating tumors, but is neither eliminated by immune cells nor able to grow straight into overt tumors. These criteria would be fulfilled by CSCs. Stem cells are pluripotent, immune-privileged, and long-living, but depend on specialized niches. Thus, latent tumors may be maintained by a niche-constrained reservoir of long-living CSCs that are exempt from immunosurveillance while niche-independent and more immunogenic daughter cells are constantly eliminated.

The small subpopulation of CSCs is often held responsible for tumor initiation, metastasis, and recurrence. Experimentally, this hypothesis was supported by the observation that only this subset can propagate tumors in non-obese diabetic/scid mice, which lack T and B cells. Yet, the concept was challenged when an unexpectedly large proportion of melanoma cells were found to be capable of seeding complex tumors in mice which further lack NK cells. Moreover, the link between stem cell-like properties and tumorigenicity was not sustained in these highly immunodeficient animals. In humans, however, tumor-propagating cells must also escape from immune-mediated destruction. The ability to persist and to initiate neoplastic growth in the presence of immunosurveillance – which would be lost in a maximally immunodeficient animal model – could hence be a decisive criterion for CSCs.

Consequently, integrating scientific insight from stem cell biology and tumor immunology to build a new concept of “CSC immunology” may help to reconcile the outlined contradictions and to improve our understanding of tumorigenesis.

MicroRNAs in cancer stem cells: current status and future directions

The presence of stem-like cells in cancer, popularly known as cancer stem cells, have been known for a long time but it was the research of Bonnet and Dick in leukemia which got cancer researchers interested in them. Over the past few years, a lot of research has gone into the characterization of cancer stem cells (CSCs) from different tumors. CSCs have been elucidated in almost all solid tumors. The growth of this field has not been without controversies as many researchers considered CSCs to be a transient population of little consequence. The field has nevertheless progressed providing us not only a better understanding of cancer and its related facets like proliferation, EMT, and metastasis but also generating a hope for new generation therapeutics with CSCs as their targets. This search for drugs which target CSCs has also focused on miRNAs. miRNAs are small non-coding regulatory RNA molecules capable of fine-tuning the gene expression. The miRNA profile of CSCs is remarkably different from non-stem cancer cells and many miRNAs have also been shown to regulate self-renewal and differentiation properties of CSCs. The differential miRNA profile in CSCs make them probable biomarkers for the prognosis of cancer and their specificity in targeting the properties of CSCs make them potential targets for therapeutic intervention. This review critically analyzes the advancement of the miRNA research in CSC context and also explores the prospect of miRNA therapies against CSCs.

Low intensity focused ultrasound (LOFU) modulates unfolded protein response and sensitizes prostate cancer to 17AAG

The hypoxic tumor microenvironment generates oxidative Endoplasmic Reticulum (ER) stress, resulting in protein misfolding and unfolded protein response (UPR). UPR induces several molecular chaperones including heat-shock protein 90 (HSP90), which corrects protein misfolding and improves survival of cancer cells and resistance to tumoricidal therapy although prolonged activation of UPR induces cell death. The HSP90 inhibitor, 17AAG, has shown promise against various solid tumors, including prostate cancer (PC). However, therapeutic doses of 17AAG elicit systemic toxicity. In this manuscript, we describe a new paradigm where the combination therapy of a non-ablative and non-invasive low energy focused ultrasound (LOFU) and a non-toxic, low dose 17AAG causes synthetic lethality and significant tumoricidal effects in mouse and human PC xenografts. LOFU induces ER stress and UPR in tumor cells without inducing cell death. Treatment with a non-toxic dose of 17AAG further increased ER stress in LOFU treated PC and switch UPR from a cytoprotective to an apoptotic response in tumors resulting significant induction of apoptosis and tumor growth retardation. These observations suggest that LOFU-induced ER stress makes the ultrasound-treated tumors more susceptible to chemotherapeutic agents, such as 17AAG. Thus, a novel therapy of LOFU-induced chemosensitization may be designed for locally advanced and recurrent tumors.

Validation of stem cell markers in clinical prostate cancer: α6-integrin is predictive for non-aggressive disease

BACKGROUND

Stem cells are postulated to mediate prostate cancer progression, and represent a small fraction of the entire tumor. Various proteins (α2-integrin, α6-integrin, CD117, CD133, EZH2, OCT3/4) are associated with a prostate cancer stem cell phenotype in cell lines and xenografts. Our objective was to investigate expression of stem cell markers in clinical prostate cancer in relation to outcome.

METHODS

We validated immunohistochemical expression of stem cell markers in 481 prostate cancer patients and correlated expression with clinicopathologic parameters.

RESULTS

Sporadic expression of α2-integrin was present in a fraction of tumor cells (<5%) in 94.7% of tumors and associated with PSA > 10 ng/ml (P = 0.04). α6-Integrin expression (<5%) occurred in 28.4% patients, while ≥5% α6-integrin expression was associated with PSA≤10 ng/ml (P = 0.01), Gleason score <7 (P < 0.01) and pT2-disease (P = 0.02). α6-integrin was predictive for biochemical recurrence (P < 0.01), local recurrence (P = 0.03) and disease specific death (P = 0.03). EZH2 expression was generally low with 2.6% of tumors showing ≥1% positive cells. EZH2 was associated with Gleason score ≥7 (P = 0.01) and biochemical recurrence (P = 0.01). We did not identify expression of CD117, CD133, and OCT3/4 in prostate cancer samples.

CONCLUSIONS

Expression of α2-integrin and EZH2 in a small fraction of prostate cancer cells is supportive for their role as stem cell marker. Although α6-integrin was not a unique stem cell marker, it was predictive for prostate cancer biochemical and local recurrence, and disease specific death. The validity of CD117, CD133, and OCT3/4 as prostate cancer stem cell marker is questionable since these proteins were not expressed in clinical prostate cancer.

Prostate progenitor cells proliferate in response to castration

Androgen-deprivation is a mainstay of therapy for advanced prostate cancer but tumor regression is usually incomplete and temporary because of androgen-independent cells in the tumor. It has been speculated that these tumor cells resemble the stem/progenitor cells of the normal prostate. The purpose of this study was to examine the response of slow-cycling progenitor cells in the adult mouse prostate to castration. Proliferating cells in the E16 urogenital sinus were pulse labeled by BrdU administration or by doxycycline-controlled labeling of the histone-H2B GFP mouse. A small population of labeled epithelial cells in the adult prostate localized at the junction of the prostatic ducts and urethra. Fluorescence-activated cell sorting (FACS) showed that GFP label-retaining cells were enriched for cells co-expressing stem cell markers Sca-1, CD133, CD44 and CD117 (4- marker cells; 60-fold enrichment). FACS showed, additionally, that 4-marker cells were androgen receptor positive. Castration induced proliferation and dispersal of E16 labeled cells into more distal ductal segments. When naïve adult mice were administered BrdU daily for 2 weeks after castration, 16% of 4-marker cells exhibited BrdU label in contrast to only 6% of all epithelial cells (P<0.01). In sham-castrated controls less than 4% of 4-marker cells were BrdU labeled (P<0.01). The unexpected and admittedly counter-intuitive finding that castration induced progenitor cell proliferation suggests that androgen deprivation therapy in men with advanced prostate cancer could not only exert pleiotrophic effects on tumor sub-populations but may induce inadvertent expansion of tumor stem cells.

Heterogeneity of limbal basal epithelial progenitor cells

Although corneal epithelial stem cells (SCs) are located at the limbus between the cornea and the conjunctiva, not all limbal basal epithelial cells are SCs. Using 2 dispase digestions to remove different amounts of limbal basal epithelial cells for cross-sections, flat mounts, and cytospin preparations, double immunostaining to pancytokeratins (PCK) and vimentin (Vim) identified 3 p63+ epithelial progenitors such as PCK-/Vim+, PCK/Vim, and PCK-/Vim+ and 1 p63+ mesenchymal cell, PCK-/Vim+. PCK-/Vim- progenitors had the smallest cell size were 10-20 times more enriched on collagen I-coated dishes in the 5-minute rapid adherent fraction that contained the highest percentage of p63+ cells but the lowest percentage of cytokeratin12+ cells, and gave rise to high Ki67 labeling and vivid clonal growth. In contrast, PCK+/Vim+ and PCK+/Vim- progenitors were found more in the slow-adherent fraction and yielded poor clonal growth. PCK/Vim progenitors and clusters of PCK-/Vim+ mesenchymal cells, which were neither melanocytes nor Langerhans cells, were located in the limbal basal region. Therefore, differential expression of PCK and Vim helps identify small PCK-/Vim- cells as the most likely candidate for SCs among a hierarchy of heterogeneous limbal basal progenitors, and their close association with PCK-/Vim+ presumed "niche" cells.

Contributions of epithelial-mesenchymal transition and cancer stem cells to the development of castration resistance of prostate cancer

An important clinical challenge in prostate cancer therapy is the inevitable transition from androgen-sensitive to castration-resistant and metastatic prostate cancer. Albeit the androgen receptor (AR) signaling axis has been targeted, the biological mechanism underlying the lethal event of androgen independence remains unclear. New emerging evidences indicate that epithelial-to-mesenchymal transition (EMT) and cancer stem cells (CSCs) play crucial roles during the development of castration-resistance and metastasis of prostate cancer. Notably, EMT may be a dynamic process. Castration can induce EMT that may enhance the stemness of CSCs, which in turn results in castration-resistance and metastasis. Reverse of EMT may attenuate the stemness of CSCs and inhibit castration-resistance and metastasis. These prospective approaches suggest that therapies target EMT and CSCs may cast a new light on the treatment of castration-resistant prostate cancer (CRPC) in the future. Here we review recent progress of EMT and CSCs in CRPC.

Plasticity of in vitro-generated urothelial cells for functional tissue formation

Tissue-engineering and regenerative medicine strategies for the bladder and urinary tract are dependent on the ability to generate adequate numbers of differentiation-competent uro-epithelial cells. In situ, urothelium is a mitotically quiescent, but highly regenerative epithelium. Although evidence supports a resident, basally located urothelial progenitor population, no specific stem cell has been identified. Our aim was to isolate basal and suprabasal urothelial subpopulations and characterize their regenerative and differentiation potentials in vitro. We showed that the low-affinity nerve growth factor receptor (NGFR) is a cell surface-expressed marker that is restricted to basal cells in normal human and porcine urothelia in situ. We used NGFR immunoseparation and differential adherence to collagen to isolate subpopulations of urothelial cells for culture. Isolated basal-derived porcine NGFR⁺ urothelial cells initially showed a higher proliferative and clonogenic phenotype than their suprabasal NGFR⁻ counterparts in vitro. However, after a short period of adaptation to culture, both NGFR⁺ and NGFR⁻ subpopulations became indistinguishable and displayed similar long-term growth and differentiation potentials. Both populations generated hierarchically organized, differentiated tissue equivalents similar to native urothelium, including a fully reconstituted NGFR⁺ basal cell layer by the NGFR⁻ suprabasal-derived population. Similarly, slow collagen-adherent human urothelial cells initially displayed a longer lag phase than rapid-adherent cultures, but after adaptation, both populations showed similar long-term proliferation, exponential growth rates, and capacity to form a functional barrier urothelium. Our results support a model where urothelial cell phenotype is plastic and determined by the niche or local environment. This has direct implications for tissue-engineering strategies requiring urothelial cell expansion and provides a new perspective toward understanding urothelial regeneration and differentiated tissue hierarchy.

Enrichment of human prostate cancer cells with tumor initiating properties in mouse and zebrafish xenografts by differential adhesion

BACKGROUND

Prostate tumor-initiating cells (TICs) have intrinsic resistance to current therapies. TICs are commonly isolated by cell sorting or dye exclusion, however, isolating TICs from limited primary prostate cancer (PCa) tissues is inherently inefficient. We adapted the collagen adherence feature to develop a combined immunophenotypic and time-of-adherence assay to identify human prostate TICs.

METHODS

PCa cells from multiple cell lines and primary tissues were allowed to adhere to several matrix molecules, and fractions of adherent cells were examined for their TIC properties.

RESULTS

Collagen I rapidly-adherent PCa cells have significantly higher clonogenic, migration, and invasion abilities, and initiated more tumor xenografts in mice when compared to slowly-adherent and no-adherent cells. To determine the relative frequency of TICs among PCa cell lines and primary PCa cells, we utilized zebrafish xenografts to define the tumor initiation potential of serial dilutions of rapidly-adherent α2β1(hi) /CD44(hi) cells compared to non-adherent cells with α2β1(low) /CD44(low) phenotype. Tumor initiation from rapidly-adherent α2β1(hi) /CD44(hi) TICs harboring the TMPRSS2:ERG fusion generated xenografts comprising of PCa cells expressing Erg, AMACR, and PSA. Moreover, PCa-cell dissemination was consistently observed in the immune-permissive zebrafish microenvironment from as-few-as 3 rapidly-adherent α2β1(hi) /CD44(hi) cells. In zebrafish xenografts, self-renewing prostate TICs comprise 0.02-0.9% of PC3 cells, 0.3-1.3% of DU145 cells, and 0.22-14.3% of primary prostate adenocarcinomas.

CONCLUSION

Zebrafish PCa xenografts were used to determine that the frequency of prostate TICs varies among PCa cell lines and primary PCa tissues. These data support a paradigm of utilizing zebrafish xenografts to evaluate novel therapies targeting TICs in prostate cancer.

Cellular signaling by collagen-binding integrins

The four collagen-binding αI domain integrins form their own subgroup among cell adhesion receptors. The signaling functions of α1β1 and α2β1 integrins have been analyzed in many experimental models, whereas less studies are available about the more recently found α10β1 and α11β1 heterodimers. Interestingly, collagen binding by α1β1 and α2β1 often generates opposite cellular responses. For example α1β1 has often been reported to promote cell proliferation and to suppress collagen synthesis, whereas α2β1 can in many model systems inhibit growth and promote collagen synthesis. There are obviously cell type dependent factors modifying the signaling. Additionally the structure and the organization of collagenous matrix play a critic role. Many recent studies have also stressed the importance of the crosstalk between the integrins and other cell surface receptors.

HDAC inhibitor confers radiosensitivity to prostate stem-like cells

Background:

Radiotherapy can be an effective treatment for prostate cancer, but radiorecurrent tumours do develop. Considering prostate cancer heterogeneity, we hypothesised that primitive stem-like cells may constitute the radiation-resistant fraction.

Methods:

Primary cultures were derived from patients undergoing resection for prostate cancer or benign prostatic hyperplasia. After short-term culture, three populations of cells were sorted, reflecting the prostate epithelial hierarchy, namely stem-like cells (SCs, α₂β₁integrin^hi/CD133⁺), transit-amplifying (TA, α₂β₁integrin^hi/CD133⁻) and committed basal (CB, α₂β₁integrin^lo) cells. Radiosensitivity was measured by colony-forming efficiency (CFE) and DNA damage by comet assay and DNA damage foci quantification. Immunofluorescence and flow cytometry were used to measure heterochromatin. The HDAC (histone deacetylase) inhibitor Trichostatin A was used as a radiosensitiser.

Results:

Stem-like cells had increased CFE post irradiation compared with the more differentiated cells (TA and CB). The SC population sustained fewer lethal double-strand breaks than either TA or CB cells, which correlated with SCs being less proliferative and having increased levels of heterochromatin. Finally, treatment with an HDAC inhibitor sensitised the SCs to radiation.

Interpretation:

Prostate SCs are more radioresistant than more differentiated cell populations. We suggest that the primitive cells survive radiation therapy and that pre-treatment with HDAC inhibitors may sensitise this resistant fraction.

Monoallelic expression of TMPRSS2/ERG in prostate cancer stem cells

While chromosomal translocations have a fundamental role in the development of several human leukaemias, their role in solid tumour development has been somewhat more controversial. Recently, it was shown that up to 80% of prostate tumours harbour at least one such gene fusion, and that the most common fusion event, between the prostate-specific TMPRSS2 gene and the ERG oncogene, is a critical, and probably early factor in prostate cancer development. Here we demonstrate the presence and expression of this significant chromosomal rearrangement in prostate cancer stem cells. Moreover, we show that in the prostate epithelial hierarchy from both normal and tumour tissues, TMPRSS2 transcription is subjected to tight monoallelic regulation, which is retained upon asymmetric division and relaxed during epithelial cell differentiation. The presence and expression of TMPRSS2/ERG in prostate stem cells would provide ERG-driven survival advantages, allowing maintenance of this mutated genotype.

Surface CD24 distinguishes between low differentiated and transit-amplifying cells in the basal layer of human prostate

BACKGROUND

Benign prostatic hyperplasia (BPH) and prostate cancer (PCa) are common abnormalities in elderly men. It is considered that epithelial stem cells are involved in the etiology and development of both diseases. To distinguish aberrant from normal cells, the knowledge about primary epithelial stem/progenitor cells (ES/P) is essential. The aim of this study was to examine the role of surface markers to distinguish between different subsets of prostate basal epithelium.

METHODS

The expression pattern of prostate tissue single cell suspensions was analyzed by flow cytometry using different markers. Sorted cell populations were examined for their clonogenic capacity and the resulted colonies were analyzed with flow cytometry, Western blot, and qPCR for stem cell, basal, and luminal epithelium markers. Additionally, the histological localization of the examined markers was determined using immunofluorescence.

RESULTS

Using the combination of CD49f, Trop-2, and surface CD24, basal cell subsets with distinct differentiation capacities were dissected (CD49f(+) Trop-2(+) CD24(-) and CD49f(+) Trop-2(+) CD24(+) ). Although cells from the two subsets gave rise to similar basal colonies, qPCR of primary tissue revealed that higher levels of basal marker expression were detected in the CD49f(+) Trop-2(+) CD24(-) subset. Immunofluorescence analysis showed a prominent expression of CD24 by luminal and basal cells.

CONCLUSIONS

Subsets with distinct differentiation capacities within the basal epithelium (CD49f(+) Trop-2(+) CD24(-) and CD49f(+) Trop-2(+) CD24(+) ) can be distinguished in human prostate. CD24 is a marker expressed on the basal transit-amplifying cells (transition cells) and may play a role in the differentiation and migration of ES/P cells to the luminal layer. The knowledge of this mechanism is of relevance for treatment of both diseases.

Early human prostate adenocarcinomas harbor androgen-independent cancer cells

Although blockade of androgen receptor (AR) signaling represents the main treatment for advanced prostate cancer (PrCa), many patients progress to a lethal phenotype of "Castration-Resistant" prostate cancer (CR-PrCa). With the hypothesis that early PrCa may harbor a population of androgen-unresponsive cancer cells as precursors to CR-recurrent disease, we undertook the propagation of androgen-independent cells from PrCa-prostatectomy samples of early, localized (Stage-I) cases. A collection of 120 surgical specimens from prostatectomy cases was established, among which 54 were adenocarcinomas. Hormone-free cell culture conditions were developed allowing routine propagation of cells expressing prostate basal cell markers and stem/progenitor cell markers, and which proliferated as spheres/spheroids in suspension cultures. Colonies of androgen-independent epithelial cells grew out from 30/43 (70%) of the adenocarcinoma cases studied in detail. Fluorescence microscopy and flow cytometry showed that CR-PrCa cells were positive for CD44, CD133, CK5/14, c-kit, integrin α2β1, SSEA4, E-Cadherin and Aldehyde Dehydrogenase (ALDH). All 30 CR-PrCa cell cultures were also TERT-positive, but negative for TMPRSS2-ERG. Additionally, a subset of 22 of these CR-PrCa cell cultures was examined by orthotopic xenografting in intact and castrated SCID mice, generating histologically typical locally-invasive human PrCa or undifferentiated cancers, respectively, in 6-8 weeks. Cultured PrCa cells and orthotopically-induced in vivo cancers lacked PSA expression. We report here the propagation of Cancer Initiating Cells (CIC) directly from Stage I human PrCa tissue without selection or genetic manipulation. The propagation of stem/progenitor-like CR-PrCa cells derived from early human prostate carcinomas suggests the existence of a subpopulation of cells resistant to androgen-deprivation therapy and which may drive the subsequent emergence of disseminated CR-PrCa.

Characterization and clinical relevance of ALDHbright populations in prostate cancer

PURPOSE

High aldehyde dehydrogenase (ALDH) has been suggested to selectively mark cells with high tumorigenic potential in established prostate cancer cell lines. However, the existence of cells with high ALDH activity (ALDH(bright)) in primary prostate cancer specimens has not been shown so far. We investigated the presence, phenotype, and clinical significance of ALDH(bright) populations in clinical prostate cancer specimens.

EXPERIMENTAL DESIGN

We used ALDEFLUOR technology and fluorescence-activated cell-sorting (FACS) staining to identify and characterize ALDH(bright) populations in cells freshly isolated from clinical prostate cancer specimens. Expression of genes encoding ALDH-specific isoforms was evaluated by quantitative real-time PCR in normal prostate, benign prostatic hyperplasia (BPH), and prostate cancer tissues. ALDH1A1-specific expression and prognostic significance were assessed by staining two tissue microarrays that included more than 500 samples of BPH, prostatic intraepithelial neoplasia (PIN), and multistage prostate cancer.

RESULTS

ALDH(bright) cells were detectable in freshly excised prostate cancer specimens (n = 39) and were mainly included within the EpCAM((+)) and Trop2((+)) cell populations. Although several ALDH isoforms were expressed to high extents in prostate cancer, only ALDH1A1 gene expression significantly correlated with ALDH activity (P < 0.01) and was increased in cancers with high Gleason scores (P = 0.03). Most importantly, ALDH1A1 protein was expressed significantly more frequently and at higher levels in advanced-stage than in low-stage prostate cancer and BPH. Notably, ALDH1A1 positivity was associated with poor survival (P = 0.02) in hormone-naïve patients.

CONCLUSIONS

Our data indicate that ALDH contributes to the identification of subsets of prostate cancer cells of potentially high clinical relevance.

Prostate stem cells in the development of benign prostate hyperplasia and prostate cancer: emerging role and concepts

Benign Prostate hyperplasia (BPH) and prostate cancer (PCa) are the most common prostatic disorders affecting elderly men. Multiple factors including hormonal imbalance, disruption of cell proliferation, apoptosis, chronic inflammation, and aging are thought to be responsible for the pathophysiology of these diseases. Both BPH and PCa are considered to be arisen from aberrant proliferation of prostate stem cells. Recent studies on BPH and PCa have provided significant evidence for the origin of these diseases from stem cells that share characteristics with normal prostate stem cells. Aberrant changes in prostate stem cell regulatory factors may contribute to the development of BPH or PCa. Understanding these regulatory factors may provide insight into the mechanisms that convert quiescent adult prostate cells into proliferating compartments and lead to BPH or carcinoma. Ultimately, the knowledge of the unique prostate stem or stem-like cells in the pathogenesis and development of hyperplasia will facilitate the development of new therapeutic targets for BPH and PCa. In this review, we address recent progress towards understanding the putative role and complexities of stem cells in the development of BPH and PCa.

JAK-STAT blockade inhibits tumor initiation and clonogenic recovery of prostate cancer stem-like cells

Interleukin (IL)-6 overexpression and constitutive STAT3 activation occur in many cancers, including prostate cancer. However, their contribution to prostate stem and progenitor cells has not been explored. In this study, we show that stem-like cells from patients with prostate cancer secrete higher levels of IL-6 than their counterparts in non-neoplastic prostate. Tumor grade did not influence the levels of expression or secretion. Stem-like and progenitor cells expressed the IL-6 receptor gp80 with concomitant expression of pSTAT3. Blockade of activated STAT3, by either anti-IL-6 antibody siltuximab (CNTO 328) or LLL12, a specific pSTAT3 inhibitor, suppressed the clonogenicity of the stem-like cells in patients with high-grade disease. In a murine xenograft model used to determine the in vivo effects of pSTAT3 suppression, LLL12 treatment effectively abolished outgrowth of a patient-derived castrate-resistant tumor. Our results indicate that the most primitive cells in prostate cancer require pSTAT3 for survival, rationalizing STAT3 as a therapeutic target to treat advanced prostate cancer.

Evaluating baculovirus as a vector for human prostate cancer gene therapy

Gene therapy represents an attractive strategy for the non-invasive treatment of prostate cancer, where current clinical interventions show limited efficacy. Here, we evaluate the use of the insect virus, baculovirus (BV), as a novel vector for human prostate cancer gene therapy. Since prostate tumours represent a heterogeneous environment, a therapeutic approach that achieves long-term regression must be capable of targeting multiple transformed cell populations. Furthermore, discrimination in the targeting of malignant compared to non-malignant cells would have value in minimising side effects. We employed a number of prostate cancer models to analyse the potential for BV to achieve these goals. In vitro, both traditional prostate cell lines as well as primary epithelial or stromal cells derived from patient prostate biopsies, in two- or three-dimensional cultures, were used. We also evaluated BV in vivo in murine prostate cancer xenograft models. BV was capable of preferentially transducing invasive malignant prostate cancer cell lines compared to early stage cancers and non-malignant samples, a restriction that was not a function of nuclear import. Of more clinical relevance, primary patient-derived prostate cancer cells were also efficiently transduced by BV, with robust rates observed in epithelial cells of basal phenotype, which expressed BV-encoded transgenes faster than epithelial cells of a more differentiated, luminal phenotype. Maximum transduction capacity was observed in stromal cells. BV was able to penetrate through three-dimensional structures, including in vitro spheroids and in vivo orthotopic xenografts. BV vectors containing a nitroreductase transgene in a gene-directed enzyme pro-drug therapy approach were capable of efficiently killing malignant prostate targets following administration of the pro-drug, CB1954. Thus, BV is capable of transducing a large proportion of prostate cell types within a heterogeneous 3-D prostate tumour, can facilitate cell death using a pro-drug approach, and shows promise as a vector for the treatment of prostate cancer.

Genetic instability in human embryonic stem cells: prospects and caveats

Human embryonic stem cells (hESCs) display a leaky G1/S checkpoint and inefficient nucleotide excision repair activity. Maintenance of genomic stability in these cells mostly relies on the elimination of damaged cells by high rates of apoptosis. However, a subpopulation survives and proliferates actively, bypassing DNA damage by translesion synthesis, a known mutagenic process. Indeed, high levels of damage-induced mutations were observed in hESCs, similar to those in repair-deficient cells. The surviving cells also become more resistant to further damage, leading to a progressive enrichment of cultures in mutant cells. In long-term cultures, hESCs display features characteristic of neoplastic progression, including chromosomal anomalies often similar to those observed in embryo carcinoma. The implication of these facts for stem cell-based therapy and cancer research are discussed.

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.

Retinoic acid represses invasion and stem cell phenotype by induction of the metastasis suppressors RARRES1 and LXN

The mouse haematopoietic stem cell (SC) regulator Latexin (LXN) is the only known homologue of the retinoic acid receptor responder 1 (RARRES1) gene. Both genes lie adjacent on chromosome 3 and differ mostly by the presence of a transmembrane domain in RARRES1. Despite their homology, it is not known whether they possess similar regulatory mechanisms, cellular localization and function. Here, we identified RARRES1 and LXN as highly significantly downregulated genes in human prostate SCs, whose expression was induced by the pro-differentiation agent all-trans retinoic acid (atRA). AtRA induced expression in the most differentiated cells compared with the SC fraction, suggesting that this subpopulation was less responsive to atRA. Small interfering RNA suppression of RARRES1 and LXN enhanced the SC properties of primary prostate cultures, as shown by a significant increase in their colony-forming ability. Expression of both RARRES1 and LXN was co-ordinately repressed by DNA methylation in prostate cancer cell lines and inhibition of RARRES1 and LXN increased the invasive capacity of primary prostate cultures, which also fully rescued an inhibitory effect induced by atRA. Moreover, we showed that RARRES1 and LXN reside within different sub-cellular compartments, providing evidence that RARRES1 is not a plasma membrane protein as previously supposed but is located primarily in the endoplasmic reticulum; whereas LXN was detected in the nucleus of prostate epithelial cells. Thus, LXN and RARRES1 are potential tumour suppressor genes, which are co-ordinately regulated, SC-silenced genes functioning to suppress invasion and colony-forming ability of prostate cancer cells; yet the proteins reside within different sub-cellular compartments.

A preclinical xenograft model of prostate cancer using human tumors

Most cases of prostate cancer are now diagnosed as moderate-grade localized disease. These tumor specimens are important tools in the discovery and translation of prostate cancer research; however, unlike more advanced tumors, they are notoriously difficult to grow in the laboratory. We developed a system for efficiently xenografting localized human prostate cancer tissue, and we adapted this protocol to study the interactions between the specific subsets of epithelial and stromal cells. Fresh prostate tissues or isolated epithelial cells are recombined with mouse seminal vesicle mesenchyme (SVM) and grafted under the renal capsule of immunodeficient mice for optimum growth and survival. Alternatively, mouse mesenchyme can be replaced with human prostate fibroblasts in order to determine their contribution to tumor progression. Grafts can be grown for several months to determine the effectiveness of novel therapeutic compounds when administered to host mice, thereby paving the way for personalizing the treatment of individual prostate cancers.

New insights into prostate cancer stem cells

Prostate cancer (PCa) remains one of the most prevalent malignancies affecting men in the western world. The etiology for PCa development and molecular mechanisms underlying castration-resistant progression are incompletely understood. Emerging evidence from many tumor systems has shown the existence of distinct subpopulations of stem like-cancer cells termed cancer stem cells (CSCs), which may be involved in tumor initiation, progression, metastasis and therapy resistance. Prostate cancer stem cells (PCSCs) have also been identified using different experimental strategies in distinct model systems. In this brief review, we summarize our current knowledge of normal prostate stem/progenitor cells, highlight recent progress on PCSCs, expound on the potential cell-of-origin for PCa and discuss the involvement of PCSCs in PCa progression and castration resistance. Elucidation of the phenotypic and functional properties and molecular regulation of PCSCs will help us better understand PCa biology and may lead to development of novel therapeutics targeting castration-resistant PCa cells.

Human prostate side population cells demonstrate stem cell properties in recombination with urogenital sinus mesenchyme

Stem cell enrichment provides a tool to examine prostate stem cells obtained from benign and malignant tissue. Functional assays can enrich stem cells based on common stem cell phenotypes, such as high ATP binding cassette (ABC) transporter mediated efflux of Hoechst substrates (side population assay). This functional assay is based upon mechanisms that protect cells from environmental insult thus contributing to the survival and protection of the stem cell population. We have isolated and analyzed cells digested from twelve clinical prostate specimens based on the side population assay. Prostate stem cell properties of the isolated cells were tested by serial recombination with rat urogenital mesenchyme. Recombinants with side population cells demonstrate an increase in the frequency of human ductal growth and the number of glands per recombinant when compared to recombinants with non-side population cells. Isolated cells were capable of prostatic growth for up to three generations in the recombination assay with as little as 125 sorted prostate cells. The ability to reproducibly use cells isolated by fluorescence activated cell sorting from human prostate tissue is an essential step to a better understanding of human prostate stem cell biology. ABC transporter G2 (ABCG2) was expressed in recombinants from side population cells indicating the side population cells have self-renewal properties. Epithelial cell differentiation of recombinants was determined by immunohistochemical analysis for expression of the basal, luminal, and neuroendocrine markers, p63, androgen receptor, prostate specific antigen, and chromogranin A, respectively. Thus, the ABCG2 expressing side population demonstrates multipotency and self-renewal properties indicating stem cells are within this population.

Prominin-1 (CD133) Expression in the Prostate and Prostate Cancer: A Marker for Quiescent Stem Cells

The origin and phenotype of stem cells in human prostate cancer remains a subject of much conjecture. In this scenario, CD133 has been successfully used as a stem cell marker in both normal prostate and prostate cancer. However, cancer stem cells have been identified without the use of this marker, opening up the possibility of a CD133 negative cancer stem cell. In this chapter, we review the current literature regarding prostate cancer stem cells, with specific reference to the expression of CD133 as a stem cell marker to identify and purify stem cells in normal prostate epithelium and prostate cancer.

Role of the microenvironment in ovarian cancer stem cell maintenance

Despite recent progresses in cancer therapy and increased knowledge in cancer biology, ovarian cancer remains a challenging condition. Among the latest concepts developed in cancer biology, cancer stem cells and the role of microenvironment in tumor progression seem to be related. Indeed, cancer stem cells have been described in several solid tumors including ovarian cancers. These particular cells have the ability to self-renew and reconstitute a heterogeneous tumor. They are characterized by specific surface markers and display resistance to therapeutic regimens. During development, specific molecular cues from the tumor microenvironment can play a role in maintaining and expanding stemness of cancer cells. The tumor stroma contains several compartments: cellular component, cytokine network, and extracellular matrix. These different compartments interact to form a permissive niche for the cancer stem cells. Understanding the molecular cues underlying this crosstalk will allow the design of new therapeutic regimens targeting the niche. In this paper, we will discuss the mechanisms implicated in the interaction between ovarian cancer stem cells and their microenvironment.

Integrin alpha2beta 1 (α2β1) promotes prostate cancer skeletal metastasis

Men who die of prostate cancer (PCa) do so because of systemic metastases, the most frequent of which are within the skeleton. Recent data suggest that the colonization of the skeleton is mediated in part by collagen type I, the most abundant protein within the bone. We have shown that enhanced collagen I binding through increased expression of integrin α2β1 stimulated in vitro invasion and promoted the growth of PCa cells within the bone. Accordingly, we sought to determine whether α2β1 integrin is a potential mediator of skeletal metastasis. To examine whether α2β1 integrin mediates PCa metastasis, α2 integrin was over-expressed in low-tumorigenic LNCaP PCa cells or selectively knocked-down in highly metastatic LNCaPcol PCa cells. We document that the over-expression of α2 cDNA stimulated whereas α2 shRNA inhibited the ability of transduced cells to bind to or migrate towards collagen in vitro. Correspondingly, α2 integrin knock-down reduced the tumor burden of intra-osseous tumors compared to control-transduced cells. To investigate the clinical significance of α2β1 expression in PCa, α2β1 protein was measured in prostatic tissues and in soft tissue and bone metastases. The data demonstrate that α2β1 protein was elevated in PCa skeletal metastases compared to either PCa primary lesions or soft tissue metastases suggesting that α2β1 contributes to the selective metastasis to the bone. Taken together, these data support that α2β1 integrin is needed for the efficient metastasis of PCa cells to the skeleton.

Human epithelial basal cells are cells of origin of prostate cancer, independent of CD133 status

Normal prostatic epithelium is composed of basal and luminal cells. Prostate cancer can be initiated in both benign basal and luminal stem cells, but because basal cell markers are not expressed in patient tumors, the former result was unexpected. Since the cells of origin of prostate cancer are important therapeutic targets, we sought to provide further proof that basal stem cells have tumorigenic potential. Prostatic basal cells were enriched based on α2β1integrin(hi) expression and further enriched for stem cells using CD133 in nontumorigenic BPH-1 cells. Human embryonic stem cells (hESCs) were also used as a source of normal stem cells. To test their tumorigenicity, we used two alternate stromal-based approaches; (a) recombination with human cancer-associated fibroblasts (CAFs) or (b) recombination with embryonic stroma (urogenital mesenchyme) and treated host mice with testosterone and 17β-estradiol. Enriched α2β1integrin(hi) basal cells from BPH-1 cells resulted in malignant tumor formation using both assays of tumorigenicity. Surprisingly, the tumorigenic potential did not reside in the CD133(+) stem cells but was consistently observed in the CD133(-) population. CAFs also failed to induce prostatic tumors from hESCs. These data confirmed that benign human basal cells include cells of origin of prostate cancer and reinforced their importance as therapeutic targets. In addition, our data suggested that the more proliferative CD133(-) basal cells are more susceptible to tumorigenesis compared to the CD133(+)-enriched stem cells. These findings challenge the current dogma that normal stem cells and cells of origin of cancer are the same cell type(s).

Human α(2)β(1)(HI) CD133(+VE) epithelial prostate stem cells express low levels of active androgen receptor

Stem cells are thought to be the cell of origin in malignant transformation in many tissues, but their role in human prostate carcinogenesis continues to be debated. One of the conflicts with this model is that cancer stem cells have been described to lack androgen receptor (AR) expression, which is of established importance in prostate cancer initiation and progression. We re-examined the expression patterns of AR within adult prostate epithelial differentiation using an optimised sensitive and specific approach examining transcript, protein and AR regulated gene expression. Highly enriched populations were isolated consisting of stem (α₂β₁^HI CD133^+VE), transiently amplifying (α₂β₁^HI CD133^–VE) and terminally differentiated (α₂β₁^LOW CD133^–VE) cells. AR transcript and protein expression was confirmed in α₂β₁^HI CD133^+VE and CD133^–VE progenitor cells. Flow cytometry confirmed that median (±SD) fraction of cells expressing AR were 77% (±6%) in α₂β₁^HI CD133^+VE stem cells and 68% (±12%) in α₂β₁^HI CD133^–VE transiently amplifying cells. However, 3-fold lower levels of total AR protein expression (peak and median immunofluorescence) were present in α₂β₁^HI CD133^+VE stem cells compared with differentiated cells. This finding was confirmed with dual immunostaining of prostate sections for AR and CD133, which again demonstrated low levels of AR within basal CD133^+VE cells. Activity of the AR was confirmed in prostate progenitor cells by the expression of low levels of the AR regulated genes PSA, KLK2 and TMPRSS2. The confirmation of AR expression in prostate progenitor cells allows integration of the cancer stem cell theory with the established models of prostate cancer initiation based on a functional AR. Further study of specific AR functions in prostate stem and differentiated cells may highlight novel mechanisms of prostate homeostasis and insights into tumourigenesis.

The roots of cancer: stem cells and the basis for tumor heterogeneity

Recent studies of prostate cancer and other tumor types have revealed significant support, as well as unexpected complexities, for the application of concepts from normal stem cell biology to cancer. In particular, the cell of origin and cancer stem cell models have been proposed to explain the heterogeneity of tumors during the initiation, propagation, and evolution of cancer. Thus, a basis of intertumor heterogeneity has emerged from studies investigating whether stem cells and/or non-stem cells can serve as cells of origin for cancer and give rise to tumor subtypes that vary in disease outcome. Furthermore, analyses of putative cancer stem cells have revealed the genetically diverse nature of cancers and expanded our understanding of intratumor heterogeneity and clonal evolution. Overall, the principles that have emerged from these stem cell studies highlight the challenges to be surmounted to develop effective treatment strategies for cancer.

Targeting the unique methylation pattern of androgen receptor (AR) promoter in prostate stem/progenitor cells with 5-aza-2'-deoxycytidine (5-AZA) leads to suppressed prostate tumorigenesis

Androgen receptor (AR) expression surveys found that normal prostate/prostate cancer (PCa) stem/progenitor cells, but not embryonic or mesenchymal stem cells, expressed little AR with high methylation in the AR promoter. Mechanism dissection revealed that the differential methylation pattern in the AR promoter could be due to differential expression of methyltransferases and binding of methylation binding protein to the AR promoter region. The low expression of AR in normal prostate/PCa stem/progenitor cells was reversed after adding 5-aza-2'-deoxycytidine, a demethylating agent, which could then lead to decreased stemness and drive cells into a more differentiated status, suggesting that the methylation in the AR promoter of prostate stem/progenitor cells is critical not only in maintaining the stemness but also critical in protection of cells from differentiation. Furthermore, induced AR expression, via alteration of its methylation pattern, led to suppression of the self-renewal/proliferation of prostate stem/progenitor cells and PCa tumorigenesis in both in vitro assays and in vivo orthotopic xenografted mouse studies. Taken together, these data prove the unique methylation pattern of AR promoter in normal prostate/PCa stem/progenitor cells and the influence of AR on their renewal/proliferation and differentiation. Targeting PCa stem/progenitor cells with alteration of methylated AR promoter status might provide a new potential therapeutic approach to battle PCa because the PCa stem/progenitor cells have high tumorigenicity.

Prostate cancer stem cells: are they androgen-responsive?

The prostate gland is highly dependent on androgens for its development, growth and function. Consequently, the prostatic epithelium predominantly consists of androgen-dependent luminal cells, which express the androgen receptor at high levels. In contrast, androgens are not required for the survival of the androgen-responsive, but androgen-independent, basal compartment in which stem cells reside. Basal and luminal cells are linked in a hierarchical pathway, which most probably exists as a continuum with different stages of phenotypic change. Prostate cancer is also characterised by heterogeneity, which is reflected in its response to treatment. The putative androgen receptor negative cancer stem cell (CSC) is likely to form a resistant core after most androgen-based therapies, contributing to the evolution of castration-resistant disease. The development of CSC-targeted therapies is now of crucial importance and identifying the phenotypic differences between CSCs and both their progeny will be key in this process.

Biology of castration-recurrent prostate cancer

Although androgen-deprivation therapy is the standard therapy for advanced and metastatic prostate cancer, this treatment is only palliative. Prostate cancer recurs then grows despite low circulating testicular androgens, using several mechanisms that remain dependent on androgen-receptor signaling in most cases. This article reviews the diversity of mechanisms used for growth by castration-recurrent prostate cancer.