Prostate cancer stem cells: a target for new therapies

Impact of STAT Proteins in Tumor Progress and Therapy Resistance in Advanced and Metastasized Prostate Cancer

Signal transducers and activators of transcription (STATs) are a family of transcription factors involved in several biological processes such as immune response, cell survival, and cell growth. However, they have also been implicated in the development and progression of several cancers, including prostate cancer (PCa). Although the members of the STAT protein family are structurally similar, they convey different functions in PCa. STAT1, STAT3, and STAT5 are associated with therapy resistance. STAT1 and STAT3 are involved in docetaxel resistance, while STAT3 and STAT5 are involved in antiandrogen resistance. Expression of STAT3 and STAT5 is increased in PCa metastases, and together with STAT6, they play a crucial role in PCa metastasis. Further, expression of STAT3, STAT5, and STAT6 was elevated in advanced and high-grade PCa. STAT2 and STAT4 are currently less researched in PCa. Since STATs are widely involved in PCa, they serve as potential therapeutic targets. Several inhibitors interfering with STATs signaling have been tested unsuccessfully in PCa clinical trials. This review focuses on the respective roles of the STAT family members in PCa, especially in metastatic disease and provides an overview of STAT-inhibitors evaluated in clinical trials.

Phage display screening identifies a prostate specific antigen (PSA)-/lo prostate cancer cell specific peptide to retard castration resistance of prostate cancer

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To our knowledge, this is the first study to identify a peptide (named as “TAP1”) that specifically binds with PSA^−/lo prostate cancer cells.

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TAP1 inhibited PCa growth both in vitro and in vivo.

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TAP1 also improved the anti-tumor effect of the anti-androgens and chemotherapeutic agents in vitro.

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The effects of TAP1 might at least in part by shortening the lengths of telomeres and decreasing the expression of HOXB9 and TGF-β2.

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Our results indicated that therapeutic peptides that specifically target prostate cancer stem cell might be a very valuable and promising approach to overcome chemoresistance and prevent recurrence in patients with PCa.

Patients with prostate cancer (PCa) will eventually progress to castrate-resistant prostate cancer (CRPC) after androgen deprivation therapy (ADT) treatment. Prostate-specific antigen (PSA)^−/lo cells which harbor self-renewing long-term tumor-propagating cells that can be enriched using ALDH+CD44+α2β1+ and can initiate tumor development may represent a critical source of CRPC cells. Our purpose was to find a peptide that specifically targets PSA^−/lo PCa cells to retard the development of CRPC. PSA⁺ and PSA^−/lo cells were successfully separated from LNCaP xenograft tumors after prostate- PSAP-GFP vector infection and FACS. A variety of PSA^−/lo cells specifically targeting peptide (named as “TAP1” targeted affinity peptide 1) was identified by using phage display library screening. The highest binding rate in TAP1 binding cell subpopulations are identified to be among ALDH⁺CD44⁺CXCR4⁺CD24⁺ cells. TAP1 significantly inhibited PCa growth both in vitro and in vivo. TAP1 significantly improved the anti-proliferation effect of the anti-androgens (Charcoal dextran-stripped serum (CDSS)+Bicalutamide, Enzalutamide) and chemotherapeutic agents (Abiraterone, Docetaxel, Etoposide) in vitro. TAP1 treatment shortens the length of telomeres in ALDH⁺CD44⁺CXCR4⁺CD24⁺ cells and significantly reduces the expression of Homeobox B9 (HOXB9) and TGF-β2. In conclusion, PSA^−/lo PCa cell-specific targeting peptide (TAP1) that suppressed PCa cell growth both in vitro and in vivo and improved the drug sensitivities of anti-androgens and chemotherapeutic agents at least through shortening the length of telomere and reducing the expression of HOXB9 and TGF-β2. Therapeutic peptides that specifically target prostate cancer stem cell might be a very valuable and promising approach to overcome chemoresistance and prevent recurrence in patients with PCa.

Targeting Apoptotic Activity Against Prostate Cancer Stem Cells

Numerous data suggest that an increase of cancer stem cells (CSCs) in tumor mass can be the reason for failure of conventional therapies because of their resistance. CD44+/CD24- cells are a putative cancer stem cells subpopulation in prostate cancer. TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) is an activator of apoptosis in tumor cells. However, some tumors are TRAIL-resistant. Cancer cells can be re-sensitized to TRAIL induced apoptosis by a combination of TRAIL and taxanes. The aim of this work was to analyze the enhancement of the anticancer effect of TRAIL by paclitaxel, cabazitaxel and docetaxel in the whole population of PC3 and DU145 prostate cancer cells, but also in CD44+/CD24- prostate cancer stem cells. We examined the apoptotic effect of TRAIL and taxanes using flow cytometry and Annexin-V-PE staining. The co-treatment with taxanes and TRAIL enhanced significantly the apoptosis in CD44+/CD24- cells only in PC3 cell line but not in DU145 cells. We discovered also that taxanes can increase the expression of death receptor TRAIL-R2 in PC3 prostate cancer cells. The results of our study show that treatment with paclitaxel, cabazitaxel and docetaxel is able to enhance the apoptosis induced by TRAIL even in prostate cancer stem cells.

Suppression of prostate cancer progression by cancer cell stemness inhibitor napabucasin

A small population of cells with stem cell-like properties in prostate cancer (PCa), called prostate cancer stem cells (PrCSCs) or prostate stemness-high cancer cells, displays highly tumorigenic and metastatic features and may be responsible for the therapy resistance. A small molecule, napabucasin (BBI608), recently have been identified with suppression of stemness-high cancer cells in a variety of cancers. However, the effects of napabucasin on PCa cells as well as PrCSCs isolated from PCa cells have not yet been defined. The effect of napabucasin on PCa cells in cell proliferation, colony formation, and cell migration in vitro were measured by MTS, colony formation assay, and Transwell, respectively. Flow cytometry was employed to evaluate cell cycle and cell apoptosis, and the effect on tumorigenesis in vivo was examined by tumor growth assays. Furthermore, the role of napabucasin on self-renewal and survival of PrCSCs was evaluated by their ability to grow spheres and cell viability assay, respectively. Western Blot and qRT-PCR were used to determine the effect of napabucasin on the expressions of stemness markers. Decrease in cell viability, colony formation, migration, and survival with cell cycle arrest, higher sensitivity to docetaxel in vitro, and repressed tumorigenesis in vivo was observed upon napabucasin treatment. More importantly, napabucasin can obviously inhibit spherogenesis and even kill PrCSCs in vitro. Downregulation of stemness markers was observed after PrCSCs were treated with napabucasin. This study demonstrates that napabucasin may be a novel approach in the treatment of advanced PCa, specifically for castration-resistant prostate cancer (CRPC).

Prostate Cancer Stem Cells: Research Advances

Cancer stem cells have been defined as cells within a tumor that possesses the capacity to self-renew and to cause the heterogeneous lineages of cancer cells that comprise the tumor. Experimental evidence showed that these highly tumorigenic cells might be responsible for initiation and progression of cancer into invasive and metastatic disease. Eradicating prostate cancer stem cells, the root of the problem, has been considered as a promising target in prostate cancer treatment to improve the prognosis for patients with advanced stages of the disease.

Altered gene products involved in the malignant reprogramming of cancer stem/progenitor cells and multitargeted therapies

Recent studies in the field of cancer stem cells have revealed that the alterations in key gene products involved in the epithelial-mesenchymal transition (EMT) program, altered metabolic pathways such as enhanced glycolysis, lipogenesis and/or autophagy and treatment resistance may occur in cancer stem/progenitor cells and their progenies during cancer progression. Particularly, the sustained activation of diverse developmental cascades such as hedgehog, epidermal growth factor receptor (EGFR), Wnt/β-catenin, Notch, transforming growth factor-β (TGF-β)/TGF-βR receptors and/or stromal cell-derived factor-1 (SDF-1)/CXC chemokine receptor 4 (CXCR4) can play critical functions for high self-renewal potential, survival, invasion and metastases of cancer stem/progenitor cells and their progenies. It has also been observed that cancer cells may be reprogrammed to re-express different pluripotency-associated stem cell-like markers such as Myc, Oct-3/4, Nanog and Sox-2 along the EMT process and under stressful and hypoxic conditions. Moreover, the enhanced expression and/or activities of some drug resistance-associated molecules such as Bcl-2, Akt/molecular target of rapamycin (mTOR), nuclear factor-kappaB (NF-κB), hypoxia-inducible factors (HIFs), macrophage inhibitory cytokine-1 (MIC-1) and ATP-binding cassette (ABC) multidrug transporters frequently occur in cancer cells during cancer progression and metastases. These molecular events may cooperate for the survival and acquisition of a more aggressive and migratory behavior by cancer stem/progenitor cells and their progenies during cancer transition to metastatic and recurrent disease states. Of therapeutic interest, these altered gene products may also be exploited as molecular biomarkers and therapeutic targets to develop novel multitargeted strategies for improving current cancer therapies and preventing disease relapse.

Molecular biomarkers of cancer stem/progenitor cells associated with progression, metastases, and treatment resistance of aggressive cancers

The validation of novel diagnostic, prognostic, and predictive biomarkers and therapeutic targets in tumor cells is of critical importance for optimizing the choice and efficacy of personalized therapies. Importantly, recent advances have led to the identification of gene-expression signatures in cancer cells, including cancer stem/progenitor cells, in the primary tumors, exosomes, circulating tumor cells (CTC), and disseminated cancer cells at distant metastatic sites. The gene-expression signatures may help to improve the accuracy of diagnosis and predict the therapeutic responses and overall survival of patients with cancer. Potential biomarkers in cancer cells include stem cell-like markers [CD133, aldehyde dehydrogenase (ALDH), CD44, and CD24], growth factors, and their cognate receptors [epidermal growth factor receptor (EGFR), EGFRvIII, and HER2], molecules associated with epithelial-mesenchymal transition (EMT; vimentin, N-cadherin, snail, twist, and Zeb1), regulators of altered metabolism (phosphatidylinositol-3' kinase/Akt/mTOR), and drug resistance (multidrug transporters and macrophage inhibitory cytokine-1). Moreover, different pluripotency-associated transcription factors (Oct3/4, Nanog, Sox2, and Myc) and microRNAs that are involved in the epigenetic reprogramming and acquisition of stem cell-like properties by cancer cells during cancer progression may also be exploited as molecular biomarkers to predict the risk of metastases, systemic treatment resistance, and disease relapse of patients with cancer.

TLR2 enhances ovarian cancer stem cell self-renewal and promotes tumor repair and recurrence

Primary ovarian cancer is responsive to treatment, but chemoresistant recurrent disease ensues in majority of patients. Recent compelling evidence demonstrates that a specific population of cancer cells, the cancer stem cells, initiates and sustains tumors. It is therefore possible that this cell population is also responsible for recurrence. We have shown previously that CD44+/MyD88+ epithelial ovarian cancer stem cells (CD44+/MyD88+ EOC stem cells) are responsible for tumor initiation. In this study, we demonstrate that this population drives tumor repair following surgery- and chemotherapy-induced tumor injury. Using in vivo and in vitro models, we also demonstrate that during the process of tumor repair, CD44+/MyD88+ EOC stem cells undergo self-renewal as evidenced by upregulation of stemness-associated genes. More importantly, we show that a pro-inflammatory microenvironment created by the TLR2-MyD88-NFκB pathway supports EOC stem cell-driven repair and self-renewal. Overall, our findings point to a specific cancer cell population, the CD44+/MyD88+ EOC stem cells and a specific pro-inflammatory pathway, the TLR2-MyD88-NFκB pathway, as two of the required players promoting tumor repair, which is associated with enhanced cancer stem cell load. Identification of these key players is the first step in elucidating the steps necessary to prevent recurrence in EOC patients.

Pathobiological implications of the expression of EGFR, pAkt, NF-κB and MIC-1 in prostate cancer stem cells and their progenies

The progression of prostate cancers (PCs) to locally invasive, androgen-independent and metastatic disease states is generally associated with treatment resistance and disease relapse. The present study was undertaken to establish the possibility of using a combination of specific oncogenic products, including epidermal growth factor receptor (EGFR), pAkt, nuclear factor-kappaB (NF-κB) and macrophage inhibitory cytokine-1 (MIC-1) as biomarkers and therapeutic targets for optimizing the management of patients with localized PC at earlier disease stages. The immunohistochemical and immunofluorescence data have revealed that the expression levels of EGFR, Ser⁴⁷³-pAkt, NF-κB p65 and MIC-1 proteins were significantly enhanced in the same subset of 76 cases of prostatic adenocarcinoma specimens during the disease progression and these biomarkers were expressed in a small subpopulation of CD133⁺ PC cells and the bulk tumor mass of CD133⁻ PC cells. Importantly, all of these biomarkers were also overexpressed in 80–100% of 30 PC metastasis bone tissue specimens. Moreover, the results have indicated that the EGF-EGFR signaling pathway can provide critical functions for the self-renewal of side population (SP) cells endowed with stem cell-like features from highly invasive WPE1-NB26 cells. Of therapeutic interest, the targeting of EGFR, pAkt, NF-κB or MIC-1 was also effective at suppressing the basal and EGF-promoted prostasphere formation by SP WPE1-NB26 cells, inducing disintegration of SP cell-derived prostaspheres and decreasing the viability of SP and non-SP WPE1-NB26 cell fractions. Also, the targeting of these oncogenic products induced the caspase-dependent apoptosis in chemoresistant SP WPE1-NB26 cells and enhanced their sensibility to the cytotoxic effects induced by docetaxel. These findings suggest that the combined use of EGFR, pAkt, NF-κB and/or MIC-1 may represent promising strategies for improving the accuracy of current diagnostic and prognostic methods and efficacy of treatments of PC patients in considering the disease heterogeneity, thereby preventing PC progression to metastatic and lethal disease states.

A simple mathematical model based on the cancer stem cell hypothesis suggests kinetic commonalities in solid tumor growth

Background

The Cancer Stem Cell (CSC) hypothesis has gained credibility within the cancer research community. According to this hypothesis, a small subpopulation of cells within cancerous tissues exhibits stem-cell-like characteristics and is responsible for the maintenance and proliferation of cancer.

Methodologies/Principal Findings

We present a simple compartmental pseudo-chemical mathematical model for tumor growth, based on the CSC hypothesis, and derived using a “chemical reaction” approach. We defined three cell subpopulations: CSCs, transit progenitor cells, and differentiated cells. Each event related to cell division, differentiation, or death is then modeled as a chemical reaction. The resulting set of ordinary differential equations was numerically integrated to describe the time evolution of each cell subpopulation and the overall tumor growth. The parameter space was explored to identify combinations of parameter values that produce biologically feasible and consistent scenarios.

Conclusions/Significance

Certain kinetic relationships apparently must be satisfied to sustain solid tumor growth and to maintain an approximate constant fraction of CSCs in the tumor lower than 0.01 (as experimentally observed): (a) the rate of symmetrical and asymmetrical CSC renewal must be in the same order of magnitude; (b) the intrinsic rate of renewal and differentiation of progenitor cells must be half an order of magnitude higher than the corresponding intrinsic rates for cancer stem cells; (c) the rates of apoptosis of the CSC, transit amplifying progenitor (P) cells, and terminally differentiated (D) cells must be progressively higher by approximately one order of magnitude. Simulation results were consistent with reports that have suggested that encouraging CSC differentiation could be an effective therapeutic strategy for fighting cancer in addition to selective killing or inhibition of symmetric division of CSCs.

Flow cytometry in cancer stem cell analysis and separation

In recent years, a special type of cancer cell--the cancer stem cell (CSC)--has been identified and characterized for different tumors. CSCs may be responsible for the recurrence of a tumor following a primarily successful therapy and are thought to bear a high metastatic potential. For the development of efficient treatment strategies, the establishment of reliable methods for the identification and effective isolation of CSCs is imperative. Similar to their stem cell counterparts in bone marrow or small intestine, different cluster of differentiation surface antigens have been characterized, thus enabling researchers to identify them within the tumor bulk and to determine their degree of differentiation. In addition, functional properties characteristic of stem cells can be measured. Side population analysis is based on the stem cell-specific activity of certain ATP-binding cassette transporter proteins, which are able to transport fluorescent dyes out of the cells. Furthermore, the stem cell-specific presence of aldehyde dehydrogenase isoform 1 can be used for CSC labeling. However, the flow cytometric analysis of these CSC functional features requires specific technical adjustments. This review focuses on the principles and strategies of the flow cytometric analysis of CSCs and provides an overview of current protocols as well as technical requirements and pitfalls. A special focus is set on side population analysis and analysis of ALDH activity. Flow cytometry-based sorting principles and future flow cytometric applications for CSC analysis are also discussed.

Prevalence of epithelial ovarian cancer stem cells correlates with recurrence in early-stage ovarian cancer

Epithelial ovarian cancer stem cells (EOC stem cells) have been associated with recurrence and chemoresistance. CD44 and CK18 are highly expressed in cancer stem cells and function as tools for their identification and characterization. We investigated the association between the number of CD44+ EOC stem cells in ovarian cancer tumors and progression-free survival. EOC stem cells exist as clusters located close to the stroma forming the cancer stem cell “niche”. 17.1% of the samples reveled high number of CD44+ EOC stem cells (>20% positive cells). In addition, the number of CD44+ EOC stem cells was significantly higher in patients with early-stage ovarian cancer (FIGO I/II), and it was associated with shorter progression-free survival (P = 0.026). This study suggests that quantification of the number of EOC stem cells in the tumor can be used as a predictor of disease and could be applied for treatment selection in early-stage ovarian cancer.

Immunotherapy of prostate cancer: should we be targeting stem cells and EMT?

Cancer stem cells have been implicated in a number of solid malignancies including prostate cancer. In the case of localised prostate cancer, patients are often treated with surgery (radical prostatectomy) and/or radiotherapy. However, disease recurrence is an issue in about 30% of patients, who will then go on to receive hormone ablation therapy. Hormone ablation therapy is often palliative in a vast proportion of individuals, and for hormone-refractory patients, there are several immunotherapies targeting a number of prostate tumour antigens which are currently in development. However, clinical responses in this setting are inconsistent, and it is believed that the failure to achieve full and permanent tumour eradication is due to a small, resistant population of cells known as 'cancer stem cells' (CSCs). The stochastic and clonal evolution models are among several models used to describe cancer development. The general consensus is that cancer may arise in any cell as a result of genetic mutations in oncogenes and tumour suppressor genes, which consequently result in uncontrolled cell growth. The cancer stem cell theory, however, challenges previous opinion and proposes that like normal tissues, tumours are hierarchical and only the rare subpopulation of cells at the top of the hierarchy possess the biological properties required to initiate tumourigenesis. Furthermore, where most cancer models infer that every cell within a tumour is equally malignant, i.e. equally capable of reconstituting new tumours, the cancer stem cell theory suggests that only the rare cancer stem cell component possess tumour-initiating capabilities. Hence, according to this model, cancer stem cells are implicated in both tumour initiation and progression. In recent years, the role of epithelial--mesenchymal transition (EMT) in the advancement of prostate cancer has become apparent. Therefore, CSCs and EMT are both likely to play critical roles in prostate cancer tumourigenesis. This review summarises the current immunotherapeutic strategies targeting prostate tumour antigens taking into account the need to consider treatments that target cancer stem cells and cells involved in epithelial--mesenchymal transition.

Frequent gene products and molecular pathways altered in prostate cancer- and metastasis-initiating cells and their progenies and novel promising multitargeted therapies

Recent gene expression profiling analyses and gain- and loss-of-function studies performed with distinct prostate cancer (PC) cell models indicated that the alterations in specific gene products and molecular pathways often occur in PC stem/progenitor cells and their progenies during prostate carcinogenesis and metastases at distant sites, including bones. Particularly, the sustained activation of epidermal growth factor receptor (EGFR), hedgehog, Wnt/β-catenin, Notch, hyaluronan (HA)/CD44 and stromal cell-derived factor-1 (SDF-1)/CXC chemokine receptor 4 (CXCR4) during the epithelial-mesenchymal transition (EMT) process may provide critical functions for PC progression to locally invasive, metastatic and androgen-independent disease states and treatment resistance. Moreover, an enhanced glycolytic metabolism in PC stem/progenitor cells and their progenies concomitant with the changes in their local microenvironment, including the induction of tumor hypoxia and release of diverse soluble factors by tumor myofibroblasts, also may promote the tumor growth, angiogenesis and metastases. More particularly, these molecular transforming events may cooperate to upregulate Akt, nuclear factor (NF)-κB, hypoxia-inducible factors (HIFs) and stemness gene products such as Oct3/4, Sox2, Nanog and Bmi-1 in PC cells that contribute to their acquisition of high self-renewal, tumorigenic and invasive capacities and survival advantages during PC progression. Consequently, the molecular targeting of these deregulated gene products in the PC- and metastasis-initiating cells and their progenies represent new promising therapeutic strategies of great clinical interest for eradicating the total PC cell mass and improving current antihormonal treatments and docetaxel-based chemotherapies, thereby preventing disease relapse and the death of PC patients.

Wnt/β-catenin Signaling in Normal and Cancer Stem Cells

The ability of Wnt ligands to initiate a signaling cascade that results in cytoplasmic stabilization of, and nuclear localization of, β-catenin underlies their ability to regulate progenitor cell differentiation. In this review, we will summarize the current knowledge of the mechanisms underlying Wnt/β-catenin signaling and how the pathway regulates normal differentiation of stem cells in the intestine, mammary gland, and prostate. We will also discuss how dysregulation of the pathway is associated with putative cancer stem cells and the potential therapeutic implications of regulating Wnt signaling.

Animal models relevant to human prostate carcinogenesis underlining the critical implication of prostatic stem/progenitor cells

Recent development of animal models relevant to human prostate cancer (PC) etiopathogenesis has provided important information on the specific functions provided by key gene products altered during disease initiation and progression to locally invasive, metastatic and hormone-refractory stages. Especially, the characterization of transgenic mouse models has indicated that the inactivation of distinct tumor suppressor proteins such as phosphatase tensin homolog deleted on chromosome 10 (PTEN), Nkx3.1, p27(KIP1), p53 and retinoblastoma (pRb) may cooperate for the malignant transformation of prostatic stem/progenitor cells into PC stem/progenitor cells and tumor development and metastases. Moreover, the sustained activation of diverse oncogenic signaling elements, including epidermal growth factor receptor (EGFR), sonic hedgehog, Wnt/β-catenin, c-Myc, Akt and nuclear factor-kappaB (NF-κB) also may contribute to the acquisition of more aggressive and hormone-refractory phenotypes by PC stem/progenitor cells and their progenies during disease progression. Importantly, it has also been shown that an enrichment of PC stem/progenitor cells expressing stem cell-like markers may occur after androgen deprivation therapy and docetaxel treatment in the transgenic mouse models of PC suggesting the critical implication of these immature PC cells in treatment resistance, tumor re-growth and disease recurrence. Of clinical interest, the molecular targeting of distinct gene products altered in PC cells by using different dietary compounds has also been shown to counteract PC initiation and progression in animal models supporting their potential use as chemopreventive or chemotherapeutic agents for eradicating the total tumor cell mass, improving current anti-hormonal and chemotherapies and preventing disease relapse.

Sulforaphane synergizes with quercetin to inhibit self-renewal capacity of pancreatic cancer stem cells

According to the cancer stem cell hypothesis, the aggressive growth and early metastasis of cancer may arise through dysregulation of self-renewal of stem cells. The objectives of this study were to examine the molecular mechanisms by which sulforaphane (SFN, an active compound in cruciferous vegetables) inhibits self-renewal capacity of pancreatic cancer stem cells (CSCs), and synergizes with quercetin, a major polyphenol and flavonoid commonly detected in many fruits and vegetables. Our data demonstrated that SFN inhibited self-renewal capacity of pancreatic CSCs. Inhibition of Nanog by lentiviral-mediated shRNA expression enhanced the inhibitory effects of sulforaphane on self-renewal capacity of CSCs. SFN induced apoptosis by inhibiting the expression of Bcl-2 and XIAP, phosphorylation of FKHR, and activating caspase-3. Moreover, SFN inhibited expression of proteins involved in the epithelial-mesenchymal transition (beta-catenin, vimentin, twist-1, and ZEB1), suggesting the blockade of signaling involved in early metastasis. Furthermore, the combination of quercetin with SFN had synergistic effects on self-renewal capacity of pancreatic CSCs. These data suggest that SFN either alone or in combination with quercetin can eliminate cancer stem cell-characteristics.

New advances on critical implications of tumor- and metastasis-initiating cells in cancer progression, treatment resistance and disease recurrence

Accumulating lines of experimental evidence have revealed that the malignant transformation of multipotent tissue-resident adult stem/progenitor cells into cancer stem/progenitor cells endowed with a high self-renewal capacity and aberrant multilineage differentiation potential may be at origin of the most types of human aggressive and recurrent cancers. Based on new cancer stem/progenitor cell concepts of carcinogenesis, it is suggested that a small subpopulation of highly tumorigenic and migrating cancer stem/progenitor cells, also designated as cancer- and metastasis-initiating cells, can provide critical roles for primary tumor growth, metastases at distant tissues and organs, treatment resistance and disease relapse. Particularly, cancer initiation and progression to locally invasive and metastatic stages is often associated with a persistent activation of distinct developmental signaling pathways in these immature cells during epithelial-mesenchymal transition program. The signaling cascades that are often deregulated in cancer stem/progenitor cells include hedgehog, epidermal growth factor receptor (EGFR), Wnt/beta-catenin, NOTCH, polycomb gene product BMI-1 and/or stromal cell-derived factor-1 (SDF-1)/CXC chemokine receptor 4 (CXCR4). Importantly, the results from recent investigations have also indicated that different cancer subtypes may harbor distinct subsets and/or number of cancer-initiating cells during cancer progression as well as before or after therapy initiation and disease recurrence. Therefore, the identification of the molecular transforming events that frequently occur in cancer- and metastasis-initiating cells versus their differentiated progenies is of immense interest to develop new targeting approach for improving current therapies against aggressive, metastatic, recurrent and lethal cancers.

Novel therapies against aggressive and recurrent epithelial cancers by molecular targeting tumor- and metastasis-initiating cells and their progenies

A growing body of experimental evidence has revealed that the highly tumorigenic cancer stem/progenitor cells endowed with stem cell-like properties might be responsible for initiation and progression of numerous aggressive epithelial cancers into locally invasive, metastatic and incurable disease states. The malignant transformation of tissue-resident adult stem/progenitor cells or their progenies into tumorigenic and migrating cancer stem/progenitor cells and their resistance to current cancer therapies have been associated with their high expression levels of specific oncogenic products and drug resistance-associated molecules. In this regard, we describe the tumorigenic cascades that are frequently activated in cancer stem/progenitor cells versus their differentiated progenies during the early and late stages of the epithelial cancer progression. The emphasis is on the growth factor signaling pathways involved in the malignant behavior of prostate and pancreatic cancer stem/progenitor cells and their progenies. Of clinical interest, the potential molecular therapeutic targets to eradicate the tumor- and metastasis-initiating cells and their progenies and develop new effective combination therapies against locally advanced and metastatic epithelial cancers are also described.

New promising drug targets in cancer- and metastasis-initiating cells

The unique properties of cancer- and metastasis-initiating cells endowed with a high self-renewal and aberrant differentiation potential (including their elevated expression levels of anti-apoptotic factors, multidrug transporters, and DNA repair and detoxifying enzymes) might be associated with their resistance to current clinical cancer therapies and disease recurrence. The eradication of cancer- and metastasis-initiating cells by molecular targeting of distinct deregulated signaling elements that might contribute to their sustained growth, survival, and treatment resistance, therefore, is of immense therapeutic interest. These novel targeted approaches should improve the efficacy of current therapeutic treatments against highly aggressive, metastatic, recurrent, and lethal cancers.

Cytotoxic effects induced by docetaxel, gefitinib, and cyclopamine on side population and nonside population cell fractions from human invasive prostate cancer cells

The present study has been undertaken to establish the therapeutic benefit of cotargeting epidermal growth factor receptor (EGFR) and sonic hedgehog pathways by using gefitinib and cyclopamine, respectively, for improving the efficacy of the current chemotherapeutic drug docetaxel to counteract the prostate cancer progression from locally invasive to metastatic and recurrent disease stages. The data from immuofluorescence analyses revealed that EGFR/Tyr(1173)-pEGFR, sonic hedgehog ligand, smoothened coreceptor, and GLI-1 were colocalized with the CD133(+) stem cell-like marker in a small subpopulation of prostate cancer cells. These signaling molecules were also present in the bulk tumor mass of CD133(-) prostate cancer cells with a luminal phenotype detected in patient's adenocarcinoma tissues. Importantly, the results revealed that the CD133(+)/CD44(high)/AR(-/low) side population (SP) cell fraction endowed with a high self-renewal potential isolated from tumorigenic and invasive WPE1-NB26 cells by the Hoechst dye technique was insensitive to the current chemotherapeutic drug, docetaxel. In contrast, the docetaxel treatment induced significant antiproliferative and apoptotic effects on the CD133(-)/CD44(low)/AR(+) non-SP cell fraction isolated from the WPE1-NB26 cell line. Of therapeutic interest, the results have also indicated that combined docetaxel, gefitinib, and cyclopamine induced greater antiproliferative and apoptotic effects on SP and non-SP cell fractions isolated from WPE1-NB26 cells than individual drugs or two-drug combinations. Altogether, these observations suggest that EGFR and sonic hedgehog cascades may represent the potential therapeutic targets of great clinical interest to eradicate the total prostate cancer cell mass and improve the current docetaxel-based therapies against locally advanced and invasive prostate cancers, and thereby prevent metastases and disease relapse.

Activation of rapid signaling pathways does not contribute to 1 alpha,25-dihydroxyvitamin D3-induced growth inhibition of mouse prostate epithelial progenitor cells

The active form of vitamin D, 1 alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D) inhibits the growth of prostate epithelial cells, however the underlying mechanisms have not been clearly delineated. In the current study, the impact of 1,25(OH)(2)D on the rapid activation of extracellular-regulated kinase (ERK) 1/2 and protein kinase C alpha (PKC alpha), and the role of these pathways in growth inhibition was examined in immortalized mouse prostate epithelial cells, MPEC3, that exhibit stem/progenitor cell characteristics. 1,25(OH)(2)D treatment suppressed the growth of MPEC3 in a dose and time dependent manner (e.g., 21% reduction at three days with 100 nM 1,25(OH)(2)D treatment). However, ERK1/2 activity was not altered by 100 nM 1,25(OH)(2)D treatment for time points from 1 min to 1 h in either serum-containing or serum-free medium. Similarly, PKC alpha activation (translocation onto the plasma membrane) was not regulated by short-term treatment of 100 nM 1,25(OH)(2)D. In conclusion, 1,25(OH)(2)D did not mediate rapid activation of ERK1/2 or PKC alpha in MPEC3 and therefore the growth inhibitory effect of 1,25(OH)(2)D is independent of rapid activation of these signaling pathways in this cell type.

Recent insights into the molecular mechanisms involved in aging and the malignant transformation of adult stem/progenitor cells and their therapeutic implications

Recent advancements in tissue-resident adult stem/progenitor cell research have revealed that enhanced telomere attrition, oxidative stress, ultraviolet radiation exposure and oncogenic events leading to severe DNA damages and genomic instability may occur in these immature and regenerative cells during chronological aging. Particularly, the alterations in key signaling components controlling their self-renewal capacity and an up-regulation of tumor suppressor gene products such as p16(INK4A), p19(ARF), ataxia-telangiectasia mutated (ATM) kinase, p53 and/or the forkhead box O (FOXOs) family of transcription factors may result in their dysfunctions, growth arrest and senescence or apoptotic death during the aging process. These molecular events may culminate in a progressive decline in the regenerative functions and the number of tissue-resident adult stem/progenitor cells, and age-related disease development. Conversely, the telomerase re-activation and accumulation of numerous genetic and/or epigenetic alterations in adult stem/progenitor cells with advancing age may result in their immortalization and malignant transformation into highly leukemic or tumorigenic cancer-initiating cells and cancer initiation. Therefore, the cell-replacement and gene therapies and molecular targeting of aged and dysfunctional adult stem/progenitor cells including their malignant counterpart, cancer-initiating cells, hold great promise for treating and even curing diverse devastating human diseases. These diseases include premature aging diseases, hematopoietic, cardiovascular, musculoskeletal, pulmonary, ocular, urogenital, neurodegenerative and skin disorders and aggressive and recurrent cancers.

Glioma formation, cancer stem cells, and akt signaling

Several recent reports have provided evidence that cancer is initiated by a rare fraction of cells called "cancer stem cells" which are multipotent, self-renewing subset of the tumor. However, several issues regarding the biology and techniques of isolating these cells from solid tumors remain to be clarified. In addition, experimental data supports two possibilities for glioma cell of origin. First, that stem cells or early progenitors are transformed and show variable differentiation of their progeny during tumor development. Second, that more differentiated glia are transformed by genetic events that lead to a loss of differentiation maintenance. In human gliomas, these two theories are not mutually exclusive. In this review we will summarize both theories, and highlight outstanding issues that remain to be resolved.

Prostate cancer stem cells: a new target for therapy

The existence of prostate cancer stem cells offers a theoretical explanation for many of the enduring uncertainties surrounding the etiology and treatment of the most commonly diagnosed tumor in US males. The study of cancer stem cells in prostate, as in other complex tissues, is critically dependent on the availability of pure cell populations, a situation complicated by the heterogeneity of prostate tumors. However, selection of cells with a CD133(+)/alpha 2 beta 1 integrin/ CD44(+) phenotype enriches for a tumor-initiating population from human prostate cancers. Among the most pressing needs is for enduring therapy in patients who have experienced failure of hormonal treatments. Because the putative cancer stem cell does not express androgen receptor, it is likely to be immune from most androgen-based therapies, and an inherent genetic instability would enable the tumor to develop the new variants present in hormone-refractory disease. Prostate cancer stem cells have a unique gene expression signature that can also be related to Gleason grade and patient outcome. The scarcity of cancer stem cells in a prostate tumor will probably limit their usefulness in cancer diagnosis and prognosis. However, the emergence of new stem-cell therapeutic targets not only will require new assays for efficacy (because of their relatively quiescent nature), but also holds real promise of more lasting treatments to augment those currently directed against the remaining tumor cells, which comprise 99.9% of tumor mass, but paradoxically have a poor tumor-initiating capacity.

Functions of normal and malignant prostatic stem/progenitor cells in tissue regeneration and cancer progression and novel targeting therapies

This review summarizes the recent advancements that have improved our understanding of the functions of prostatic stem/progenitor cells in maintaining homeostasis of the prostate gland. We also describe the oncogenic events that may contribute to their malignant transformation into prostatic cancer stem/progenitor cells during cancer initiation and progression to metastatic disease stages. The molecular mechanisms that may contribute to the intrinsic or the acquisition of a resistant phenotype by the prostatic cancer stem/progenitor cells and their differentiated progenies with a luminal phenotype to the current therapies and disease relapse are also reviewed. The emphasis is on the critical functions of distinct tumorigenic signaling cascades induced through the epidermal growth factor system, hedgehog, Wnt/beta-catenin, and/or stromal cell-derived factor-1/CXC chemokine receptor-4 pathways as well as the deregulated apoptotic signaling elements and ATP-binding cassette multidrug transporter. Of particular therapeutic interest, we also discuss the potential beneficial effects associated with the targeting of these signaling elements to overcome the resistance to current treatments and prostate cancer recurrence. The combined targeted strategies toward distinct oncogenic signaling cascades in prostatic cancer stem/progenitor cells and their progenies as well as their local microenvironment, which could improve the efficacy of current clinical chemotherapeutic treatments against incurable, androgen-independent, and metastatic prostate cancers, are also described.