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. Author manuscript; available in PMC: 2012 Jun 5.
Published in final edited form as: Discov Med. 2012 Feb;13(69):135–142.

Targeting prostate cancer stem cells for cancer therapy

Guocan Wang 1, Zhiwei Wang 2, Fazlul H Sarkar 3, Wenyi Wei 2,4
PMCID: PMC3367460  NIHMSID: NIHMS380534  PMID: 22369972

Abstract

Prostate cancer (PCa) is the most common malignant neoplasm in men and the second most frequent cause of cancer death for males in the United States. Recently, emerging evidence suggests that prostate cancer stem cells (CSCs) may play a critical role in the development and progression of PCa. Therefore, targeting prostate CSCs for the prevention of tumor progression and treatment of PCa could become a novel strategy for better treatment of patients diagnosed with PCa. In this review article, we will summarize the most recent advances in the prostate CSCs field, with particular emphasis on targeting prostate CSCs to treat prostate cancer.

Introduction

Prostate cancer (PCa) is one of the leading causes of death worldwide and is the most common malignant neoplasm in men in the United States (Jemal et al., 2009; Shen & Abate-Shen, 2010). In recent years, a number of tumor suppressor genes and proto-oncogenes have been found to play a critical role in the development and progression of PCa, such as tumor suppressor gene NKX3.1, PTEN, and proto-oncogene MYC, NOTCH-1 (Shen & Abate-Shen, 2010). In addition, it has been reported that recurrent chromosomal translocation is also involved in prostate tumorigenesis. Specifically, a recurrent translocation of an ETS transcription factor ERG to the TMPRSS2 promoter region resulting in a constitutively active form of the TMPRSS2-ERG fusion protein occurs in approximately 40% of the primary prostate tumors (Mani et al., 2009; Tomlins et al., 2005). Fusions involving other members of the ETS transcription factors, ETV1, ETV4, and ETV5, are also found in human PCa but with a lower frequency (Clark & Cooper, 2009).

Although there has been obvious progress in the diagnosis and treatment of PCa, several major clinical challenges still remain, such as the inability to accurately distinguish indolent from aggressive PCa, the lack of effective therapeutic options for castration-resistant PCa and most importantly, the bone tropism for PCa metastasis (Shen & Abate-Shen, 2010). Therefore, a better understanding of the genetic elements and the underlying molecular mechanisms governing the progression of PCa will lead to the development of new diagnostic tools as well as novel therapeutic strategies for more effective treatment of patients diagnosed with PCa. Recently, emerging evidence has suggested that prostate cancer stem cells (CSCs) play a pivotal role in the development and progression of PCa. Therefore, in this review, we will summarize important recent findings in the field of putative prostate CSCs and the possibility to target them for achieving better survival for patients with PCa.

The theory of cancer stem cells

It has been well accepted that embryonic and adult stem cells are a rare population of cells that have self-renewal capacity and are able to re-constitute each type of tissue by differentiation into mature (or differentiated) cell types that comprise each organ (Bjerkvig et al., 2005; Huntly & Gilliland, 2005; Pardal et al., 2003; Reya et al., 2001). It has been observed that cellular signaling pathways that regulate normal stem cell self-renewal are often deregulated in human cancer, such as the Wnt/β-catein, Notch, Hedgehog, and TGF-β signaling pathways (Bjerkvig et al., 2005; Huntly & Gilliland, 2005; Pardal et al., 2003). Accumulating experimental and clinical evidence supports the notion that a rare subset of cancer cell population with stem cell properties that can self-renew could give rise to a hierarchy of proliferative and differentiated bulk of tumor cells, leading to tumor initiation, progression, recurrence, and metastasis to distant organs (Bjerkvig et al., 2005; Clevers, 2011; Hamburger & Salmon, 1977; Huntly & Gilliland, 2005; Pardal et al., 2003).

Initial evidence supporting the CSCs theory came from a series of transplant experiments, which suggest that cancers are composed of a heterogeneous population of cells with marked differences in their potential to self-renew and reconstitute the tumor upon transplantation (Bruce & Van Der Gaag, 1963; Daniel & Brunschwig, 1961; Hamburger & Salmon, 1977; Sabbath & Griffin, 1985). For example, a series of studies in acute myeloid leukemia (AML) provided strong evidence to support the hierarchical CSCs model (Huntly & Gilliland, 2005; Reya et al., 2001) that explained the initiation of AML. Specifically, a rare population of CD34+ cells contained the leukemia-initiating cells that possess the ability to initiate tumor growth and recapitulate the characteristics of the original tumor heterogeneity (Bonnet & Dick, 1997; Lapidot et al., 1994). Subsequently, putative CSCs or tumor-initiating cells were identified using specific markers in many other types of human cancers including breast (Al-Hajj et al., 2003), brain (Singh et al., 2003; Singh et al., 2004), colon (Ricci-Vitiani et al., 2007), and prostate (Collins et al., 2005; Visvader & Lindeman, 2008). Most importantly, CSCs isolated from a specific tumor type have the ability to give rise to new tumors when xenografted in immunodeficient mice (Bjerkvig et al., 2005; Clevers, 2011; Hamburger & Salmon, 1977; Huntly & Gilliland, 2005; Pardal et al., 2003).

Stem/progenitor cells in the normal prostate epithelium

Human adult prostate lacks an apparent lobular structure and displays a zonal architecture, which includes central, transition, and peripheral zones, together with an anterior fibromuscular stroma (Shen & Abate-Shen, 2010). On the other hand, the mouse prostate consists of anterior, ventral, dorsal and lateral lobes. Although human and mouse prostates have different anatomical structures, they both contain a pseudostratified epithelium with three types of terminally differentiated epithelial cells: luminal, basal, and neuroendocrine (Shen & Abate-Shen, 2010). The luminal epithelial cells, the major component of normal and malignant prostate, are polarized columnar cells that generate secreted proteins such as prostate-specific antigen (PSA) and prostatic acid phosphatase (PAP) (Shen & Abate-Shen, 2010). In addition, they express characteristic markers such as cytokeratin 8 (CK8), CK18, NKX3.1, and high levels of androgen receptor (AR). On the other hand, basal cells, which are located beneath the luminal epithelium and adhere strongly to basement membrane (BM), primarily express biomarkers including p63, CK5 and CK14 (Shen & Abate-Shen, 2010). Finally, neuroendocrine cells, expressing endocrine markers such as chromogranin A and synaptophysin, are a rare population of cells with yet unknown functions (Shen & Abate-Shen, 2010).

The existence of prostate stem cells was first proposed by Isaacs and Coffey (Isaacs & Coffey, 1989) based on the ability of the adult prostate glands to undergo multiple cycles of regression-regeneration in response to androgen-deprivation and androgen-restoration (Tsujimura, 2002). It has been suggested that prostate stem cells may reside in the proximal region of the mouse prostate (Kinbara et al., 1996; Tsujimura, 2002) that are slow cycling cells with a high proliferative potential in vitro and have the ability to reconstitute prostatic structures. However, it is still debatable whether the stem cell population has a luminal or basal phenotype in the mouse prostate, as there is evidence in support of either hypothesis.

In response to androgen deprivation, the majority of the luminal cells (~90%) undergo apoptosis in the regression phase (English et al., 1987; Evans & Chandler, 1987), suggesting that the normal prostate stem cells may be basal-cell origin. Consistent with this notion, it was found that in both human and rodents, basal cells have a much higher proliferative index than luminal cells (Bonkhoff & Remberger, 1996). In addition, ‘intermediate’ cells identified in the basal layer co-expressed both basal-specific and luminal-specific CKs, as well as the luminal marker PSA (Bonkhoff et al., 1994; van Leenders et al., 2000; Verhagen et al., 1988), supporting that basal stem cells may give rise to the intermediate ‘transit-amplifying’ cells that are progenitors for the luminal cells (Bonkhoff & Remberger, 1996). Furthermore, murine basal prostate epithelial cells purified by FACS based on various combinations of stem cell surface markers, such as Lin−/Sca-1+/CD49fhigh and Lin-CD49f+Trop2+, display bipotentiality and self-renewal features (Burger, 2005; Frame et al., 2010; Goldstein, 2008; Kasper, 2008; Lawson et al., 2007; Lawson et al., 2010; Richardson, 2004; Schmelz et al., 2005; Wang et al., 2006; Xin et al., 2005). Moreover, a different panel of markers (Lin-Sca-1+ CD133+CD44+CD117+) was used to purify murine prostate stem cells, which was able to reconstitute functional prostatic structures when these putative stem cells were transplanted in vivo (Leong et al., 2008). Alternatively, aldehyde dehydrogenase (ALDH) activity has also been used to isolate prostate stem cells (Burger et al., 2009).

On the contrary, there are some lines of evidence supporting that mouse prostate contains luminal stem cells. First, bromodeoxyuridine (BrdU) labeled cells are present both in the luminal and basal epithelium in BrdU pulse-chase experiment, indicating that there are slow-proliferating stem cells existing in the luminal cell population as well (Tsujimura et al., 2002). Second, although p63−/− mice fail to form prostate (Mills et al., 1999; Signoretti et al., 2000; Yang et al., 1999), grafted tissues from p63-deficient mice have the capability to form prostate tissues in the absence of basal cells (Kurita et al., 2004). Third, Wang et al. (Wang et al., 2009) demonstrated that castration-resistant Nkx3.1-expressing cells (CARNs) are rare luminal cells that can display stem cell properties during prostate regeneration. Specifically, these CARNs display bipotentiality and long-term self-renewal capacity during the prostate regeneration, and are also capable of reconstituting prostatic ducts. Interestingly, a recent report demonstrated that regenerated luminal cells are derived from pre-existing luminal stem cells in the murine prostate (Liu et al., 2011), arguing against the role of basal stem cells to give rise to both luminal and basal cells in the regression-regeneration experiments.

Recently, a number of cell surface markers, such as α2β1 integrins (Collins et al., 2001; Richardson et al., 2004) and the CD133 antigen (Meregalli et al., 2010), have been characterized and they help researchers to identify and isolate normal prostate stem cells. For example, it has been shown that the α2β1/integrinhigh/CD133+ sub-population identifies a cell population with a high proliferative potential and high colony-forming efficiency in vitro that can reconstruct functional prostate glands in immunodeficient mice, accompanied by differentiation into AR+, PSA-expressing luminal cells (Richardson, 2004). More recently, Trop2+CD44+CD49f+ were used as the markers to identify basal stem cells with enhanced prostasphere-forming and tissue regenerating abilities (Garraway et al., 2010). Moreover, a modified Hoechst 33342 dye efflux assay was used to isolate basal stem cells from normal human prostate tissue, which can form spheroids with acinus like morphology and expressing basal cell markers (Brown et al., 2007).

Prostate cancer stem cells in human and mouse

Although PCa is the most frequently diagnosed cancer in men, its etiology remains unclear, which could be partly attributed to multiple genetic and epigenetic mechanisms and the heterogeneous nature of this disease (Maitland & Collins, 2005; Maitland et al., 2011; Oldridge et al., 2011; Shen & Abate-Shen, 2010). High-grade prostatic intraepithelial neoplasia (PIN), often considered as a precursor to PCa, has histological characteristics of luminal epithelial hyperplasia and a progressive loss of basal cells (Maitland et al., 2011; Oldridge et al., 2011; Shen & Abate-Shen, 2010). The normal histological structure of the prostate is disrupted when PIN progresses into adenocarcinoma, such as the loss of the glandular structure and destruction of the BM (basement membrane), which were described in detail by Gleason (Gleason, 1966). In addition, the number of basal cells progressively decreases to less than 1% during this transition, whereas AR+ luminal cells eventually constitute the bulk of prostate tumor mass (>99%) (Grisanzio & Signoretti, 2008).

Based on the similarities between normal stem cells and CSCs, it is hypothesized that prostate CSCs may originate from oncogenic transformation of normal prostate stem cells. The identification of these candidate stem cells in the prostate has led to examination of whether these populations in the mouse or analogous populations in humans can serve as cells of origin for PCa, which is highly relevant for understanding the applicability of the CSC model (Maitland et al., 2011; Oldridge et al., 2011; Shen & Abate-Shen, 2010; Wang & Shen, 2011). Many studies have focused on the differential transformability of various cell populations in the human and mouse prostate, as this could serve as a starting point for subsequent studies to determine whether genetic alterations of the normal stem cells may confer tumor-initiating properties (Shen & Abate-Shen, 2010; Wang & Shen, 2011). However, different laboratories have identified potentially non-overlapping candidate stem cell populations as the cell of origin for PCa. These differences can be attributed, at least in part, to the distinct experimental methodologies used. Another possibility is that there may be distinct cells of origin for PCa, and different cells of origin may give rise to clinically relevant subtypes that differ in their prognosis and treatment outcome (Maitland et al., 2011; Oldridge et al., 2011; Shen & Abate-Shen, 2010; Wang & Shen, 2011).

It has been argued that PCa arises from the AR+ luminal cells, as these cells constitute the majority of cells in PCa and the population of basal cells dramatically decreases to less than 1%. To support this notion, several studies in murine models have shown that PCa can arise from luminal cells (Shen & Abate-Shen, 2010; Wang & Shen, 2011). First, it has been shown that candidate luminal progenitor cells in the prostate tumor derived from a PTEN knockout mouse can act as tumor-initiating cells (Ma et al., 2005). In addition, such genetic alterations were first observed in a subset of luminal cells expressing the progenitor markers Trop2 and Sca-1, suggesting that the luminal cells are the cell-of-origin in this model (Korsten et al., 2009). Second, a more recent paper identified murine CARNs, which can self-renew in vivo and reconstitute prostate ducts in the renal grafts. Moreover, deletion of the Pten tumor suppressor gene in CARNs results in a rapid formation of carcinoma following androgen-mediated regeneration (Wang et al., 2009). However, it remains to be determined whether CARNs exist in the hormonally intact prostate epithelium and whether these cells can serve as cells of origin. Without a doubt, the identification of other molecular markers for CARNs will help to visualize these cells in the hormonally intact prostate and address this critical question in future studies.

Although prostate tumors display a strongly luminal phenotype, other studies have demonstrated that basal cells could also serve as the cell of origin for PCa. The population of p63+ basal cells expands, accompanied by increased number of progenitor cells in the prostate-specific conditional deletion of Pten mouse model, suggesting the existence of a basal cell of origin (Wang et al., 2006). In addition, mouse Lin−Sca-1+CD49fhigh cells, a predominantly basal population, can differentiate into luminal cells in xenografts (Lawson et al., 2007). Lin−Sca-1+CD49fhigh cells from a Pten−/− mouse model displayed CSCs properties, which gave rise to adenocarcinoma in the resulting grafts (Mulholland et al., 2009). Furthermore, basal Lin−Sca-1highCD49fhigh cells have the capacity to form tumor-like spheroids in vitro and grafts in vivo (Lawson et al., 2010). In addition, cancer-associated fibroblasts play an important role in supporting and potentiating the stemness and growth properties of these CSCs, suggesting a critical role of the microenvironment in the modulation of the phenotype of the initiating cell (Liao et al., 2010). Moreover, lentiviral overexpression of ERG1 in Lin−Sca-1+ CD49fhigh cells resulted in a PIN phenotype, while co-activation of Akt and AR signaling pathways resulted in adenocarcinoma. Interestingly, these mouse tumors do not always recapitulate the luminal features of human PCa, as expression of the basal marker p63 is frequently observed in these tumors (Lawson et al., 2010). Importantly, a recent study has shown that basal cells, but not the luminal cells, are the possible cells of origin for human PCa (Goldstein et al., 2010). Specifically, luminal (CD49floTrop2hi) and basal (CD49fhiTrop2hi) cells were combined with mouse UGM and subcutaneously injected into immunodeficient NOD-SCID-IL-2Rγ−/− mice. Interestingly, only basal cells, but not the luminal cells, formed prostatic ducts after 16 weeks. In addition, high-grade PIN was formed when activated Akt and ERG were expressed in CD49fhiTrop2hi cells whereas adenocarcinoma was formed when they were co-expressed together with AR. On the contrary, no prostate ducts, PIN, or adenocarcinomas were formed using either infected or uninfected luminal cells in this study (Goldstein et al., 2010), which could be due to experimental approach used in this system. Luminal cells failed to form prostate ducts when combined with UGM and matrigel because this heterologous environment may be not the optimal condition for luminal cell outgrowth (Wang & Shen, 2011). In addition, it is possible that luminal cells derived from basal stem cells are the actual target of neoplastic transformation within these grafts, and overexpression of AR may promote basal cell differentiation toward the luminal cells and their subsequent transformation (Wang & Shen, 2011).

Studies using flow cytometry-based approach with various markers also demonstrated a basal origin for PCa. Putative basal CSCs have been isolated from human PCa biopsies with a CD44+α2β1integrinhighCD133+ cell surface markers (Collins et al., 2005), which were able to self-renew in vitro. In addition, AR+PAP+CK18+ luminal cells could be identified in these cultures under differentiating conditions, indicating that they were derived from the more primitive population. ALDHhigh is also used as a marker for cancer stem/progenitor cells in PCa cell lines. A subpopulation of human PCa cells with high ALDH activity (ALDHhighα2+/α6+/αv+-integrinCD44+) showed both enhanced clonogenicity and invasiveness in vitro as well as enhanced tumorigenicity and increased metastatic ability in vivo (van den Hoogen et al., 2010). CD44+CD24− basal prostate stem-like cells isolated from LNCaP PCa cells formed colonies in soft agar and formed tumors in NOD/SCID mice when only 100 cells were injected (Hurt et al., 2008). In addition, CD44+ population from xenograft tumors and cell lines displayed enhanced proliferative potential and tumor-initiating ability in vivo (Patrawala et al., 2006). Prostate CSCs cells were also isolated from DU145 cell line with CD44+/α2β1hi/CD133+ markers (Wei et al., 2007) and human telomerase reverse transcriptase-immortalized primary tumor-derived prostate epithelial cell lines with CD133hi phenotype (Miki et al., 2007), respectively. TMPRSS2–ERG, a recurrent genomic alteration in PCa (Tomlins et al., 2005), is expressed in CD44+α2β1integrinhighCD133+ cells from prostate tumors (Birnie et al., 2008), indicating that cell-of-origin of PCa can be a basal stem cells. Recently, a small population of TRA-1-60+ CD151+ CD166+ tumor initiating cells isolated from human prostate xenograft tumors expressed basal cell markers and displayed stem-like cell features with increased NF-κB signaling, and recapitulated the cellular hierarchy of the original tumor (Rajasekhar et al., 2011).

Targeting prostate CSCs for cancer therapy

Although both the origin and the precise impact of CSCs on tumorigenesis are still in debate, it is widely accepted that cancers can arise from normal stem cells which may accumulate genetic/epigenetic changes that disrupt the tightly control of self-renewal capacity. In addition, progenitor cells that are blocked from terminal differentiation may also be responsible for the initiation and progression of cancer. Up to now, it is clear that the research community has not reached a consensus on the exact cell of origin for PCa. Results from the murine models of PCa suggest the co-existence of multiple cells-of-origin in the mouse; however, the current evidence is rather overwhelming in the human system implicating basal cells as the cell-of-origin for human PCa. As a result, there remains no consensus to conclude that there is only one cell-of-origin, since different genetic alterations may have the capacity to transform different target cells, and different clinical sub-types of cancer may arise from different cell types. There is considerable information concerning prostate and PCa development resulting from the modeling of human disease in the murine prostate, but these opposing results suggest that the mouse is perhaps not the ideal model for studying human prostate CSCs (Visvader, 2011). Given the complexity of human PCa as well as the anatomic differences between human and mouse prostates, it is likely that distinct mouse models may only recapitulate properties of specific subtypes of human PCa. As in the case of the normal prostate epithelial stem cell, it is important to note that these studies on the cell of origin for PCa are not mutually inconsistent, in part because they employed distinct functional assays experimentally. However, there may also be multiple cells of origin for PCa. By analogy with breast cancer (Visvader, 2009; Visvader & Smith, 2011), it may be the case that distinct cells of origin give rise to PCa that display different subtypes. Such subtypes might correspond to rare pathological variants that in total account for less than 5–10% of disease cases (Mazzucchelli et al., 2008), or correspond to molecular subtypes that are now being well defined through the comprehensive oncogenomic analyses (Taylor et al., 2010). Importantly, as such subtypes might differ in their prognosis and/or response to clinical treatments, the investigation of cells of origin for PCa should have important clinical implications.

It should also be noted that there are many distinct properties between normal and cancer prostate stem cells. In contrast to normal stem cells that remain largely constant in cell number, the number of CSCs can increase over time as the tumors grow. In addition, CSCs are slow-cycling cells that divide much more slowly. Such properties would allow them to escape from traditional radio- and chemotherapies, which leads to cancer recurrence and metastasis. More importantly, the ability of CSCs to efficiently pump out drugs makes them highly resistant to most conventional therapies. Current treatments might therefore only hit the bulk of the tumor cells but spare CSCs, thus leading to a later on recurrence and metastasis. Since CSCs may represent the bad seeds of tumors, eliminating CSCs offers the potential to completely eradicate the disease. Hence, the targeting of cancer stem cells will probably require the identification of novel drugs. This is an enormous challenge because of the paucity of CSCs, the technical difficulties of keeping them in culture, and their unusual drug resistance (Dodge & Lum, 2011).

However, the identification of proliferation and differentiation pathways that are active in CSCs but not in normal, differentiated cells may offer unique opportunities for selective therapies (Dodge & Lum, 2011). Therefore, the discovery of prostate CSCs may offer a great opportunity to develop novel therapeutics targeting prostate CSCs to treat PCa. To achieve this goal, it is critical to further define which specific markers, probably on the surface, or pathways associated with prostate CSC could be potentially served as therapy targets. Studies towards addressing this important question are currently undergoing with recent report identifying CD133, a prostate CSC marker, as a novel therapeutic target for the treatment of PCa (Shepherd et al. 2008). Without a doubt, further in-depth investigation and discovery of additional molecular targets for prostate CSCs will lead to a better clinical application for men with PCa in the future. However, it should be recognized that tissue-specific stem cell therapy for PCa is still in this infant stage and much more studies and optimizations are required to execute its full power in clinical treatment of prostate cancer patients. Therefore, more thorough understanding of the molecular signatures and biological features of CSCs is required, which will ultimately help us to build further knowledge base for targeting the critical signaling pathways in the treatment of PCa.

Acknowledgments

The authors were supported by funding from the National Institute of General Medicines, NIH (GM089763) (to W.W.), Massachusetts Life Science Center New Investigator award (to W.W.), and Department of Defense Prostate New Investigator award to W.W. W.W is an American Cancer Society Scholar. Z.W. is supported by NIH NRSA fellowship.

Footnotes

Conflict of Interest

The authors declare that they have no conflict of interest.

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