Wnt/beta-catenin mediates radiation resistance of Sca1+ progenitors in an immortalized mammary gland cell line

WNT activation by lithium abrogates TP53 mutation associated radiation resistance in medulloblastoma

TP53 mutations confer subgroup specific poor survival for children with medulloblastoma. We hypothesized that WNT activation which is associated with improved survival for such children abrogates TP53 related radioresistance and can be used to sensitize TP53 mutant tumors for radiation. We examined the subgroup-specific role of TP53 mutations in a cohort of 314 patients treated with radiation. TP53 wild-type or mutant human medulloblastoma cell-lines and normal neural stem cells were used to test radioresistance of TP53 mutations and the radiosensitizing effect of WNT activation on tumors and the developing brain. Children with WNT/TP53 mutant medulloblastoma had higher 5-year survival than those with SHH/TP53 mutant tumours (100% and 36.6% ± 8.7%, respectively (p < 0.001)). Introduction of TP53 mutation into medulloblastoma cells induced radioresistance (survival fractions at 2Gy (SF2) of 89% ± 2% vs. 57.4% ± 1.8% (p < 0.01)). In contrast, β-catenin mutation sensitized TP53 mutant cells to radiation (p < 0.05). Lithium, an activator of the WNT pathway, sensitized TP53 mutant medulloblastoma to radiation (SF2 of 43.5% ± 1.5% in lithium treated cells vs. 56.6 ± 3% (p < 0.01)) accompanied by increased number of γH2AX foci. Normal neural stem cells were protected from lithium induced radiation damage (SF2 of 33% ± 8% for lithium treated cells vs. 27% ± 3% for untreated controls (p = 0.05). Poor survival of patients with TP53 mutant medulloblastoma may be related to radiation resistance. Since constitutive activation of the WNT pathway by lithium sensitizes TP53 mutant medulloblastoma cells and protect normal neural stem cells from radiation, this oral drug may represent an attractive novel therapy for high-risk medulloblastomas.

Signaling pathways in breast cancer: therapeutic targeting of the microenvironment

Breast cancer is the most common cancer in women worldwide. Understanding the biology of this malignant disease is a prerequisite for selecting an appropriate treatment. Cell cycle alterations are seen in many cancers, including breast cancer. Newly popular targeted agents in breast cancer include cyclin dependent kinase inhibitors (CDKIs) which are agents inhibiting the function of cyclin dependent kinases (CDKs) and agents targeting proto-oncogenic signaling pathways like Notch, Wnt, and SHH (Sonic hedgehog). CDKIs are categorized as selective and non-selective inhibitors of CDK. CDKIs have been tried as monotherapy and combination therapy. The CDKI Palbocyclib is now a promising therapeutic in breast cancer. This drug recently entered phase III trial for estrogen receptor (ER) positive breast cancer after showing encouraging results in progression free survival in a phase II trials. The tumor microenvironment is now recognized as a significant factor in cancer treatment response. The tumor microenvironment is increasingly considered as a target for combination therapy of breast cancer. Recent findings in the signaling pathways in breast cancer are herein summarized and discussed. Furthermore, the therapeutic targeting of the microenvironment in breast cancer is also considered.

Taxane-induced hedgehog signaling is linked to expansion of breast cancer stem-like populations after chemotherapy

Recurrence of breast cancer after chemotherapy is thought to arise from resistant breast cancer stem cells which are eventually able to repopulate the tumor. The Hedgehog (HH) signaling pathway has been shown to regulate the proliferation and survival of breast cancer stem cells, and has been shown to promote resistance to chemotherapy through the activation of multi-drug resistance and pro survival pathways. Here we report that exposure of heterogenous breast cancer cell lines to docetaxel (DOC) resulted in release of Sonic Hedgehog ligand (SHH) and activation of the HH pathway as evidenced by increased expression and nuclear translocation of the downstream effector Gli-1 at 4-24 h after DOC treatment. This activation had little effect on the bulk of the tumor cell population as inhibition of HH signaling failed to increase apoptosis in response to DOC. However, HH pathway activation was required for clonogenic growth of cell lines after DOC. Increases in stemness markers as well as mammosphere formation were observed after treatment with DOC suggesting an increase in the breast cancer stem cell populations. These increases were similar to that of cell lines cultured in the presence of recombinant SHH and could be eliminated by co-treatment with HH inhibitors. These results suggest that HH pathway activation induced by DOC treatment does not have a chemosensitizing effect on the heterogeneous tumor population, but may be required for survival and expansion of breast cancer stem cells after chemotherapy.

Targeting the Wnt pathway in human cancers: therapeutic targeting with a focus on OMP-54F28

The Wnt signaling pathways are a group of signal transduction pathways that play an important role in cell fate specification, cell proliferation and cell migration. Aberrant signaling in these pathways has been implicated in the development and progression of multiple cancers by allowing increased proliferation, angiogenesis, survival and metastasis. Activation of the Wnt pathway also contributes to the tumorigenicity of cancer stem cells (CSCs). Therefore, inhibiting this pathway has been a recent focus of cancer research with multiple targetable candidates in development. OMP-54F28 is a fusion protein that combines the cysteine-rich domain of frizzled family receptor 8 (Fzd8) with the immunoglobulin Fc domain that competes with the native Fzd8 receptor for its ligands and antagonizes Wnt signaling. Preclinical models with OMP-54F28 have shown reduced tumor growth and decreased CSC frequency as a single agent and in combination with other chemotherapeutic agents. Due to these findings, a phase 1a study is nearing completion with OMP-54F28 in advanced solid tumors and 3 phase 1b studies have been opened with OMP-54F28 in combination with standard-of-care chemotherapy backbones in ovarian, pancreatic and hepatocellular cancers. This article will review the Wnt signaling pathway, preclinical data on OMP-54F28 and other Wnt pathway inhibitors and ongoing clinical trials.

Sox2 promotes tamoxifen resistance in breast cancer cells

Development of resistance to therapy continues to be a serious clinical problem in breast cancer management. Cancer stem/progenitor cells have been shown to play roles in resistance to chemo- and radiotherapy. Here, we examined their role in the development of resistance to the oestrogen receptor antagonist tamoxifen. Tamoxifen-resistant cells were enriched for stem/progenitors and expressed high levels of the stem cell marker Sox2. Silencing of the SOX2 gene reduced the size of the stem/progenitor cell population and restored sensitivity to tamoxifen. Conversely, ectopic expression of Sox2 reduced tamoxifen sensitivity in vitro and in vivo. Gene expression profiling revealed activation of the Wnt signalling pathway in Sox2-expressing cells, and inhibition of Wnt signalling sensitized resistant cells to tamoxifen. Examination of patient tumours indicated that Sox2 levels are higher in patients after endocrine therapy failure, and also in the primary tumours of these patients, compared to those of responders. Together, these results suggest that development of tamoxifen resistance is driven by Sox2-dependent activation of Wnt signalling in cancer stem/progenitor cells.

Simvastatin radiosensitizes differentiated and stem-like breast cancer cell lines and is associated with improved local control in inflammatory breast cancer patients treated with postmastectomy radiation

Reported rates of local failure after adjuvant radiation for women with inflammatory breast cancer (IBC) and triple-negative non-IBC are higher than those of women with receptor-expressing non-IBC. These high rates of locoregional recurrence are potentially influenced by the contribution of radioresistant cancer stem cells to these cancers. Statins have been shown to target stem cells and improve disease-free survival among IBC patients. We examined simvastatin radiosensitization of multiple subtypes of breast cancer cell lines in vitro in monolayer and mammosphere-based clonogenic assays and examined the therapeutic benefit of statin use on local control after postmastectomy radiation (PMRT) among IBC patients. We found that simvastatin radiosensitizes mammosphere-initiating cells (MICs) of IBC cell lines (MDA-IBC3, SUM149, SUM190) and of the metaplastic, non-IBC triple-negative receptor cell line (SUM159). However, simvastatin radioprotects MICs of non-IBC cell lines MCF-7 and SKBR3. In a retrospective clinical study of 519 IBC patients treated with PMRT, 53 patients used a statin. On univariate analysis, actuarial 3-year local recurrence-free survival (LRFS) was higher among statin users, and on multivariate analysis, triple negative breast cancer, absence of lymphatic invasion, neoadjuvant pathological tumor response to preoperative chemotherapy, and statin use were independently associated with higher LRFS. In conclusion, patients with IBC and triple-negative non-IBC breast cancer have the highest rates of local failure, and there are no available known radiosensitizers. We report significant improvement in local control after PMRT among statin users with IBC and significant radiosensitization across triple-negative and IBC cell lines of multiple subtypes using simvastatin. These data suggest that simvastatin should be justified as a radiosensitizing agent by a prospective clinical trial.

WISP-1 contributes to fractionated irradiation-induced radioresistance in esophageal carcinoma cell lines and mice

Cancer cells that survive fractionated irradiation can be radioresistant and cause tumor recurrence. However, the molecular mechanisms underlying the development of radioresistance in cancer cells remain elusive. The aim of this study was to investigate the role of WISP-1 in the development of radioresistance in esophageal carcinoma during fractionated irradiation. Radioresistant esophageal cancer cells were generated from normal esophageal cancer cells via fractionated irradiation, and expression levels of related proteins were determined by Western blot. Radiosensitivity of cells was established by clonogenic cell survival assays, and cell cycle distribution was evaluated by flow cytometry. Protein distributions were determined by immunofluorescence, and cell toxicity was evaluated by cell counting kit-8 assays. In vivo validations were performed in a xenograft transplantation mouse model. Our data indicate that WISP-1 plays an important role in the development of radioresistance in esophageal cancer cells during fractionated irradiation. The overexression of WISP-1 in esophageal cancer cells was associated with radioresistance. Depletion of extracellular WISP-1 by antibody neutralizing reversed radioresistance and directly induced mitotic catastrophe resulting in cell death. WISP-1 may be a candidate therapeutic target in the treatment of recurrent esophageal carcinoma after radiotherapy.

Preoperative chemoradiotherapy in rectal cancer induces changes in the expression of nuclear β-catenin: prognostic significance

BACKGROUND

Preoperative chemoradiotherapy (CRT) is the cornerstone of treatment for locally advanced rectal cancer (LARC). Although high local control is achieved, overall rates of distant control remain suboptimal. Colorectal carcinogenesis is associated with critical alterations of the Wnt/β-catenin pathway involved in proliferation and survival. The aim of this study was to assess whether CRT induces changes in the expression of β-catenin/E-cadherin, and to determine whether these changes are associated with survival.

METHODS

The Immunohistochemical expression of nuclear β-catenin and membranous E-cadherin was prospectively analysed in tumour blocks from 98 stage II/III rectal cancer patients treated with preoperative CRT. Tumour samples were collected before and after CRT treatment. All patients were treated with pelvic RT (46-50 Gy in 2 Gy fractions) and 5-fluorouracil (5FU) intravenous infusion (225 mg/m2) or capecitabine (825 mg/m2) during RT treatment, followed by total mesorectal excision (TME). Disease-free survival (DFS) was analysed using the Kaplan-Meier method and a multivariate Cox regression model was employed for the Multivariate analysis.

RESULTS

CRT induced significant changes in the expression of nuclear β-catenin (49% of patients presented an increased expression after CRT, 17% a decreased expression and 34% no changes; p = 0.001). After a median follow-up of 25 months, patients that overexpressed nuclear β-catenin after CRT showed poor survival compared with patients that experienced a decrease in nuclear β-catenin expression (3-year DFS 92% vs. 43%, HR 0.17; 95% CI 0.03 to 0.8; p = 0.02). In the multivariate analysis for DFS, increased nuclear β-catenin expression after CRT almost reached the cut-off for significance (p = 0.06).

CONCLUSIONS

In our study, preoperative CRT for LARC induced significant changes in nuclear β-catenin expression, which had a major impact on survival. Finding a way to decrease CRT resistance would significantly improve LARC patient survival.

Current concepts in clinical radiation oncology

Based on its potent capacity to induce tumor cell death and to abrogate clonogenic survival, radiotherapy is a key part of multimodal cancer treatment approaches. Numerous clinical trials have documented the clear correlation between improved local control and increased overall survival. However, despite all progress, the efficacy of radiation-based treatment approaches is still limited by different technological, biological, and clinical constraints. In principle, the following major issues can be distinguished: (1) The intrinsic radiation resistance of several tumors is higher than that of the surrounding normal tissue, (2) the true patho-anatomical borders of tumors or areas at risk are not perfectly identifiable, (3) the treatment volume cannot be adjusted properly during a given treatment series, and (4) the individual heterogeneity in terms of tumor and normal tissue responses toward irradiation is immense. At present, research efforts in radiation oncology follow three major tracks, in order to address these limitations: (1) implementation of molecularly targeted agents and 'omics'-based screening and stratification procedures, (2) improvement of treatment planning, imaging, and accuracy of dose application, and (3) clinical implementation of other types of radiation, including protons and heavy ions. Several of these strategies have already revealed promising improvements with regard to clinical outcome. Nevertheless, many open questions remain with individualization of treatment approaches being a key problem. In the present review, the current status of radiation-based cancer treatment with particular focus on novel aspects and developments that will influence the field of radiation oncology in the near future is summarized and discussed.

Aligned chitosan-polycaprolactone polyblend nanofibers promote the migration of glioblastoma cells

In vitro models that accurately mimic the microenvironment of invading glioblastoma multiform (GBM) cells will provide a high-throughput system for testing potential anti-invasion therapies. Here, the ability of chitosan-polycaprolactone polyblend nanofibers to promote a migratory phenotype in human GBM cells by altering the nanotopography of the nanofiber membranes is investigated. Fibers are prepared with diameters of 200 nm, 400 nm, and 1.1 μm, and are either randomly oriented or aligned to produce six distinct nanotopographies. Human U-87 MG GBM cells, a model cell line commonly used for invasion assays, are cultured on the various nanofibrous substrates. Cells show elongation and alignment along the orientation of aligned fibers as early as 24 h and up to 120 h of culture. After 24 h of culture, human GBM cells cultured on aligned 200 nm and 400 nm fibers show marked upregulation of invasion-related genes including β-catenin, Snail, STAT3, TGF-β, and Twist, suggesting a mesenchymal change in these migrating cells. Additionally, cells cultured on 400 nm aligned fibers show similar migration profiles as those reported in vivo, and thus these nanofibers should provide a unique high-throughput in vitro culture substrate for developing anti-migration therapies for the treatment of GBM.

8-bromo-7-methoxychrysin inhibits properties of liver cancer stem cells via downregulation of β-catenin

AIM: To evaluate whether 8-bromo-7-methoxychrysin (BrMC), a synthetic analogue of chrysin, inhibits the properties of cancer stem cells derived from the human liver cancer MHCC97 cell line and to determine the potential mechanisms.

METHODS: CD133⁺ cells were sorted from the MHCC97 cell line by magnetic activated cell sorting, and amplified in stem cell-conditioned medium to obtain the enriched CD133⁺ sphere forming cells (SFCs). The stem cell properties of CD133⁺ SFCs were validated by the tumorsphere formation assay in vitro and the xenograft nude mouse model in vivo, and termed liver cancer stem cells (LCSCs). The effects of BrMC on LCSCs in vitro were evaluated by MTT assay, tumorsphere formation assay and transwell chamber assay. The effects of BrMC on LCSCs in vivo were determined using a primary and secondary xenograft model in Balb/c-nu mice. Expressions of the stem cell markers, epithelial-mesenchymal transition (EMT) markers and β-catenin protein were analyzed by western blotting or immunohistochemical analysis.

RESULTS: CD133⁺ SFCs exhibited stem-like cell properties of tumorsphere formation and tumorigenesis capacity in contrast to the parental MHCC97 cells. We found that BrMC preferentially inhibited proliferation and self-renewal of LCSCs (P < 0.05). Furthermore, BrMC significantly suppressed EMT and invasion of LCSCs. Moreover, BrMC could efficaciously eliminate LCSCs in vivo. Interestingly, we showed that BrMC decreased the expression of β-catenin in LCSCs. Silencing of β-catenin by small interfering RNA could synergize the inhibition of self-renewal of LCSCs induced by BrMC, while Wnt3a treatment antagonized the inhibitory effects of BrMC.

CONCLUSION: BrMC can inhibit the functions and characteristics of LCSCs derived from the liver cancer MHCC97 cell line through downregulation of β-catenin expression.

Transcriptional regulation of the survivin gene

Survivin, a small member of the inhibitors of the apoptosis protein family, is highly deregulated in cancer. It is weakly expressed in normal tissues but very strongly expressed in malignant lesions. Survivin is involved in cell-cycle progression, especially in the G2/M transition, and has anti-apoptotic activity, which correlates with its strong expression in cases with a poor cancer treatment response and poor outcomes. Several therapies that target the survivin transcript or protein are currently being tested in clinical trials. However, focusing new therapies on the origins of survivin overexpression and targeting these upstream deregulations could be more effective. For this reason, it seems important to make an inventory of the transcriptional (de)regulation of survivin. This review will gather the important points concerning the regulation of survivin mRNA expression: structure of the survivin promoter, epigenetic modifications and genetic abnormalities, transcription factors, and signalling pathways that affect survivin mRNA expression.

Overexpression of nuclear β-catenin in rectal adenocarcinoma is associated with radioresistance

AIM: To investigate the association between nuclear β-catenin overexpression in rectal adenocarcinoma and radioresistance.

METHODS: A retrospective analysis was conducted. The analysis involved 136 patients with locally advanced rectal adenocarcinoma who underwent short-course preoperative radiotherapy and radical resection. The expression of β-catenin in both pretreatment biopsy specimens and resected primary tumor tissues was examined by immunohistochemistry. The correlation of β-catenin expression with radioresistance was evaluated using the tumor regression grading (TRG) system. The relationship between β-catenin expression and clinicopathological characteristics was also analyzed. Univariate and logistic multivariate regression analyses were adopted to determine the independent factors of radioresistance.

RESULTS: Nuclear β-catenin overexpression was more evident in radioresistant rectal adenocarcinoma than in radiosensitive rectal adenocarcinoma (57.6% vs 16.7%, P < 0.001). Nuclear β-catenin was overexpressed in favor of poor TRG (≤ 2), whereas membrane β-catenin was expressed in favor of good TRG (≥ 3). Nuclear β-catenin expression in tumor cell differentiation (P = 0.018), lymph node metastasis (P = 0.022), and TRG (P < 0.001) showed significant differences. Univariate analyses demonstrated that radioresistance is associated with nuclear β-catenin overexpression (P < 0.001). In addition, logistic multivariate regression analysis indicated that only three factors, namely, tumor size (P < 0.001), tumor cell differentiation (P < 0.001), and nuclear β-catenin overexpression (P < 0.001), are associated with radioresistance. By using radioresistance as a prediction target, nuclear β-catenin-based prediction alone achieved 83% accuracy, 65% sensitivity, and 88% specificity.

CONCLUSION: Nuclear β-catenin overexpression may be a valuable candidate to predict the response of rectal adenocarcinoma to preoperative radiotherapy.

Wnt pathway activity in breast cancer sub-types and stem-like cells

INTRODUCTION

Wnt signalling has been implicated in stem cell regulation however its role in breast cancer stem cell regulation remains unclear.

METHODS

We used a panel of normal and breast cancer cell lines to assess Wnt pathway gene and protein expression, and for the investigation of Wnt signalling within stem cell-enriched populations, mRNA and protein expression was analysed after the selection of anoikis-resistant cells. Finally, cell lines and patient-derived samples were used to investigate Wnt pathway effects on stem cell activity in vitro.

RESULTS

Wnt pathway signalling increased in cancer compared to normal breast and in both cell lines and patient samples, expression of Wnt pathway genes correlated with estrogen receptor (ER) expression. Furthermore, specific Wnt pathway genes were predictive for recurrence within subtypes of breast cancer. Canonical Wnt pathway genes were increased in breast cancer stem cell-enriched populations in comparison to normal breast stem cell-enriched populations. Furthermore in cell lines, the ligand Wnt3a increased whilst the inhibitor DKK1 reduced mammosphere formation with the greatest inhibitory effects observed in ER+ve breast cancer cell lines. In patient-derived metastatic breast cancer samples, only ER-ve mammospheres were responsive to the ligand Wnt3a. However, the inhibitor DKK1 efficiently inhibited both ER+ve and ER-ve breast cancer but not normal mammosphere formation, suggesting that the Wnt pathway is aberrantly activated in breast cancer mammospheres.

CONCLUSIONS

Collectively, these data highlight differential Wnt signalling in breast cancer subtypes and activity in patient-derived metastatic cancer stem-like cells indicating a potential for Wnt-targeted treatment in breast cancers.

Immunotherapy of radioresistant mammary tumors with early metastasis using molecular chaperone vaccines combined with ionizing radiation

In the current study, exposure of mammary tumor cells derived from mice transgenic for the polyomavirus middle T oncogene to ionizing radiation resulted in the generation of a tumor cell population that preferentially expressed cancer stem cell markers. In addition, these cells were more resistant to subsequent radiation treatments and appeared to acquire an enhanced capacity for dissemination to the lungs of mice. Therefore, we tested an immunotherapy approach to the treatment of local and disseminated mammary tumor cells in a murine model using a recently developed molecular chaperone-based vaccine that specifically targets the radioresistant subpopulation of tumor cells. Heat shock protein 70-peptide complexes (Hsp70.PC-F) were extracted from fusions of dendritic cells and radiation-enriched tumor cells, and the resulting chaperone vaccines were used to treat mice with pre-existing lung metastases. Immunization of mice with the Hsp70.PC-F vaccine resulted in a T cell-mediated immune response, including a significant increase in CD4 and CD8 T cell proliferation and the induction of effector T cells capable of targeting radioresistant tumor cells. Importantly, the growth of primary tumors was inhibited, and the number of tumor cells metastasizing to lung was reduced significantly by combining chaperone vaccine with radiotherapy. These results indicate that Hsp70.PC-F vaccine can induce specific immunity to radioresistant populations of mammary tumor cells and, thus, can complement radiotherapy, leading to synergistic killing.

CD44+ gastric cancer cells with stemness properties are chemoradioresistant and highly invasive

CD44 has been confirmed as a cancer stem cell marker in a variety of human cancer cell lines and primary tumours, but whether this marker is applicable to gastric cancer (GC) remains unknown. The responses of CD44⁺ GC stem-like cells to chemoradiation and the roles they play in cancer invasion are not well understood. In the present study, cell sorting was applied to the poorly differentiated human GC cells to isolate a pure concentration of the CD44⁺ cell populations (<1% CD44⁻ cells). The stemness properties of the CD44⁺ cell population were confirmed by two ‘gold standard’ methods; an in vivo tumourigenicity assay and an in vitro spheroid colony formation assay. In addition, the treatment response was evaluated in CD44⁺ and CD44⁻ cell fractions that underwent chemoradiation. In general, CD44⁺ stem-like cells tended to respond more poorly to chemoradiation than their non-stem-like counterparts. Further experimentation revealed that the CD44⁺ stem-like cells that recorded positive scores in the migration and invasion assay in vitro formed invasive tumours in vivo. Therefore, we hypothesized that CD44⁺ stem-like cells may significantly express invasion-associated genes. Consistent with this prediction, increased expression of the cancer invasion-related genes matrix metalloproteinase (MMP)-1, MMP-2, epidermal growth factor receptor (EGFR) and cyclooxygenase 2 (COX-2) were detected in the CD44⁺ stem-like cells. To the best of our knowledge, this is the first study that reveals the correlation between CD44⁺ GC cells and cancer invasion. By selectively eliminating CD44⁺ stem-like cells, it may be possible to treat patients with aggressive, non-resectable GCs, as well as preventing the tumours from metastasizing.

New biological insights on the link between radiation exposure and breast cancer risk

Radiation exposure is a well-documented risk factor for breast cancer in women. Compelling epidemiological evidence in different exposed populations around the world demonstrate that excess breast cancer increases with radiation doses above 10 cGy. Both frequency and type of breast cancer are affected by prior radiation exposure. Many epidemiological studies suggest that radiation risk is inversely related to age at exposure; exposure during puberty poses the greatest risk while exposures past the menopause appear to carry very low risk. These observations are supported by experimental studies in mice and rats, which together provide the basis for the pubertal 'window of susceptibility' hypothesis for carcinogenic exposure. One line of experimental investigation suggests that the pubertal epithelium is more sensitive because DNA damage responses are less efficient, an other suggests that radiation affects stem cells self-renewal. A recent line of investigation suggests that the irradiated microenvironment mediates cancer risk. Studying the biological basis for radiation effects provides potential routes for protection in vulnerable populations, which include survivors of childhood cancers, as well as insights into the biology for certain types of sporadic cancer.

The implications of cancer stem cells for cancer therapy

Surgery, radiotherapy and chemotherapy are universally recognized as the most effective anti-cancer therapies. Despite significant advances directed towards elucidating molecular mechanisms and developing clinical trials, cancer still remains a major public health issue. Recent studies have showed that cancer stem cells (CSCs), a small subpopulation of tumor cells, can generate bulk populations of nontumorigenic cancer cell progeny through the self-renewal and differentiation processes. As CSCs are proposed to persist in tumors as a distinct population and cause relapse and metastasis by giving rise to new tumors, development of CSC-targeted therapeutic strategies holds new hope for improving survival and quality of life in patients with cancer. Therapeutic innovations will emerge from a better understanding of the biology and environment of CSCs, which, however, are largely unexplored. This review summarizes the characteristics, evidences and development of CSCs, as well as implications and challenges for cancer treatment.

Breast cancer stem cell-like cells are more sensitive to ionizing radiation than non-stem cells: role of ATM

There are contradictory observations about the different radiosensitivities of cancer stem cells and cancer non-stem cells. To resolve these contradictory observations, we studied radiosensitivities by employing breast cancer stem cell (CSC)-like MDA-MB231 and MDA-MB453 cells as well as their corresponding non-stem cells. CSC-like cells proliferate without differentiating and have characteristics of tumor-initiating cells [1]. These cells were exposed to γ-rays (1.25–8.75 Gy) and survival curves were determined by colony formation. A final slope, D₀, of the survival curve for each cell line was determined to measure radiosensitivity. The D₀ of CSC-like and non-stem MDA-MB-453 cells were 1.16 Gy and 1.55 Gy, respectively. Similar results were observed in MDA-MB-231 cells (0.94 Gy vs. 1.56 Gy). After determination of radiosensitivity, we investigated intrinsic cellular determinants which influence radiosensitivity including cell cycle distribution, free-radical scavengers and DNA repair. We observed that even though cell cycle status and antioxidant content may contribute to differential radiosensitivity, differential DNA repair capacity may be a greater determinant of radiosensitivity. Unlike non-stem cells, CSC-like cells have little/no sublethal damage repair, a low intracellular level of ataxia telangiectasia mutated (ATM) and delay of γ-H2AX foci removal (DNA strand break repair). These results suggest that low DNA repair capacity is responsible for the high radiosensitivity of these CSC-like cells.

Salinomycin as a drug for targeting human cancer stem cells

Cancer stem cells (CSCs) represent a subpopulation of tumor cells that possess self-renewal and tumor initiation capacity and the ability to give rise to the heterogenous lineages of malignant cells that comprise a tumor. CSCs possess multiple intrinsic mechanisms of resistance to chemotherapeutic drugs, novel tumor-targeted drugs, and radiation therapy, allowing them to survive standard cancer therapies and to initiate tumor recurrence and metastasis. Various molecular complexes and pathways that confer resistance and survival of CSCs, including expression of ATP-binding cassette (ABC) drug transporters, activation of the Wnt/β-catenin, Hedgehog, Notch and PI3K/Akt/mTOR signaling pathways, and acquisition of epithelial-mesenchymal transition (EMT), have been identified recently. Salinomycin, a polyether ionophore antibiotic isolated from Streptomyces albus, has been shown to kill CSCs in different types of human cancers, most likely by interfering with ABC drug transporters, the Wnt/β-catenin signaling pathway, and other CSC pathways. Promising results from preclinical trials in human xenograft mice and a few clinical pilote studies reveal that salinomycin is able to effectively eliminate CSCs and to induce partial clinical regression of heavily pretreated and therapy-resistant cancers. The ability of salinomycin to kill both CSCs and therapy-resistant cancer cells may define the compound as a novel and an effective anticancer drug.

Role of cancer stem cells in spine tumors: review of current literature

The management of spinal column tumors continues to be a challenge for clinicians. The mechanisms of tumor recurrence after surgical intervention as well as resistance to radiation and chemotherapy continue to be elucidated. Furthermore, the pathophysiology of metastatic spread remains an area of active investigation. There is a growing body of evidence pointing to the existence of a subset of tumor cells with high tumorigenic potential in many spine cancers that exhibit characteristics similar to those of stem cells. The ability to self-renew and differentiate into multiple lineages is the hallmark of stem cells, and tumor cells that exhibit these characteristics have been described as cancer stem cells (CSCs). The mechanisms that allow nonmalignant stem cells to promote normal developmental programming by way of enhanced proliferation, promotion of angiogenesis, and increased motility may be used by CSCs to fuel carcinogenesis. The purpose of this review is to discuss what is known about the role of CSCs in tumors of the osseous spine. First, this article reviews the fundamental concepts critical to understanding the role of CSCs with respect to chemoresistance, radioresistance, and metastatic disease. This discussion is followed by a review of what is known about the role of CSCs in the most common primary tumors of the osseous spine.

Targeting a cornerstone of radiation resistance: cancer stem cell

In radiation oncology, cancer stem cells (CSCs) have become an important research field. In fact, it appears that most cancer types contain populations of cells that exhibit stem-cell properties. CSCs have the ability to renew indefinitely, which can drive tumor development and metastatic invasion. As those cells are classically resistant to conventional chemotherapy and to radiation therapy, they may contribute to treatment failure and relapse. Over past decades, preclinical research has highlighted that variations in the CSCs content within tumor could affect their radiocurability by interfering with mechanisms of DNA repair, redistribution in the cell cycle, tumor cells repopulation, and hypoxia. It is now possible to isolate particular cells expressing specific surface markers and thus better investigating CSCs pathways. Numerous inhibitory agents targeting these specific signaling pathways, such as Notch and Wnt/B-catenin, are currently evaluated in early clinical trials. By targeting CSCs, tumor radioresistance could be potentially overcome to improve outcome for patients with solid malignancies. Radiation therapy using ion particles (proton and carbon) may be also more effective than classic photon on CSCs. This review presents the major pathophysiological mechanisms involved in CSCs radioresistance and recent developments for targeted strategies.

Concurrent transient activation of Wnt/β-catenin pathway prevents radiation damage to salivary glands

PURPOSE

Many head and neck cancer survivors treated with radiotherapy suffer from permanent impairment of their salivary gland function, for which few effective prevention or treatment options are available. This study explored the potential of transient activation of Wnt/β-catenin signaling in preventing radiation damage to salivary glands in a preclinical model.

METHODS AND MATERIALS

Wnt reporter transgenic mice were exposed to 15 Gy single-dose radiation in the head and neck area to evaluate the effects of radiation on Wnt activity in salivary glands. Transient Wnt1 overexpression in basal epithelia was induced in inducible Wnt1 transgenic mice before together with, after, or without local radiation, and then saliva flow rate, histology, apoptosis, proliferation, stem cell activity, and mRNA expression were evaluated.

RESULTS

Radiation damage did not significantly affect activity of Wnt/β-catenin pathway as physical damage did. Transient expression of Wnt1 in basal epithelia significantly activated the Wnt/β-catenin pathway in submandibular glands of male mice but not in those of females. Concurrent transient activation of the Wnt pathway prevented chronic salivary gland dysfunction following radiation by suppressing apoptosis and preserving functional salivary stem/progenitor cells. In contrast, Wnt activation 3 days before or after irradiation did not show significant beneficial effects, mainly due to failure to inhibit acute apoptosis after radiation. Excessive Wnt activation before radiation failed to inhibit apoptosis, likely due to extensive induction of mitosis and up-regulation of proapoptosis gene PUMA while that after radiation might miss the critical treatment window.

CONCLUSION

These results suggest that concurrent transient activation of the Wnt/β-catenin pathway could prevent radiation-induced salivary gland dysfunction.

Stem cell antigen-1 deficiency enhances the chemopreventive effect of peroxisome proliferator-activated receptorγ activation

Stem cell antigen-1 (Sca-1, Ly6A) is a glycerophosphatidylinositol (GPI)-anchored protein that was identified as a murine marker of bone marrow stem cells. Although Sca-1 is widely used to enrich for stem and progenitor cells in various tissues, little is known about its function and associated signaling pathways in normal and malignant cells. Here, we report that the absence of Sca-1 in the mammary gland resulted in higher levels of PPARγ and PTEN, and a reduction of pSer84PPARγ, pERK1/2, and PPARδ. This phenotype correlated with markedly increased sensitivity of Sca-1 null mice to PPARγ agonist GW7845 and insensitivity to PPARδ agonist GW501516. Reduction of Sca-1 expression in mammary tumor cells by RNA interference resulted in a phenotype similar to the Sca-1 deficient mammary gland, as evidenced by increased PPARγ expression and transcriptional activity, resulting in part from a lesser susceptibility to proteasomal degradation. These data implicate Sca-1 as a negative regulator of the tumor suppressor effects of PPARγ.

Resistance to chemotherapy: short-term drug tolerance and stem cell-like subpopulations

Personalized medicine in cancer treatment has been a major goal for decades. Recently, the development of several therapies that specifically target key genetic alterations in different malignancies has dramatically improved patient outcome and brought the goal of personalized medicine closer to practicality. Despite the improved specificity of these treatment options, resistance to targeted therapy is common and remains a major obstacle to long-term management of a patient's disease. Often patient relapse is a result of the positive selection of cells with certain genetic alterations that result in a bypass of the therapeutic intervention. Once this occurs, patient relapse is inevitable and further treatment options are limited. The time to relapse is often quite rapid indicating that cancer cells may be primed for adapting to cytotoxic stimuli. Recently, it has been suggested that small subpopulations of cells allow resistance to occur more rapidly. It is thought that these cells are capable of surviving strong apoptotic stimuli until more permanent mechanisms of long-term resistance are developed. In order to decrease the rate of patient relapse, more studies are required in order to identify these subpopulations of cells, understand the mechanisms underlying their drug tolerance, and develop strategies to prevent them from evading treatment.

Wnt/β-catenin Pathway Modulates the Sensitivity of the Mutant FLT3 Receptor Kinase Inhibitors in a GSK-3β Dependent Manner

The FLT3 tyrosine kinase receptor is involved in both hematopoiesis and hematological malignancies. The Wnt/β-catenin pathway has been shown to participate in the self-renewal of hematopoietic stem cells and to cooperate with the mutant FLT3 receptors in leukemic transformation. However, the detailed biological impact of such a constitutively activated Wnt pathway remains to be further explored. Here, the authors report that activating mutations of FLT3 constitutively activate β-catenin by inhibition of GSK-3β in a PI3 kinase pathway-dependent manner. Ectopic expression of a dominant negative form of GSK-3β in FLT3-ITD-expressing cells activated β-catenin and blocked the downregulation of the TCF/β-catenin transcriptional activity induced by inhibition of FLT3 kinase. Furthermore, inhibition of cell proliferation and colony formation induced by such suppression of FLT3 kinase activity could be partially reversed by knockdown of GSK-3β and restored by knockdown of either TCF4 or β-catenin. Moreover, exogenous activation of the Wnt pathway also attenuated such inhibitory effect. These findings indicate that the potencies of the inhibitors of FLT3 kinase activity could be modulated by the activity of the Wnt/β-catenin pathway in the cells harboring FLT3-ITD mutations, and FLT3-ITDs signal through GSK-3β to activate β-catenin that this is likely to directly contribute to the leukemic phenotype.

Leptin deficiency suppresses MMTV-Wnt-1 mammary tumor growth in obese mice and abrogates tumor initiating cell survival

Obesity increases both the risk and mortality associated with many types of cancer including that of the breast. In mice, obesity increases both incidence of spontaneous tumors and burden of transplanted tumors. Our findings identify leptin, an adipose secreted cytokine, in promoting increased mammary tumor burden in obese mice and provide a link between this adipokine and cancer. Using a transplantable tumor that develops spontaneously in the murine mammary tumor virus-Wnt-1 transgenic mice, we show that tumors transplanted into obese leptin receptor (LepRb)-deficient (db/db) mice grow to eight times the volume of tumors transplanted into lean wild-type (WT) mice. However, tumor outgrowth and overall tumor burden is reduced in obese, leptin-deficient (ob/ob) mice. The residual tumors in ob/ob mice contain fewer undifferentiated tumor cells (keratin 6 immunopositive) compared with WT or db/db mice. Furthermore, tumors in ob/ob mice contain fewer cells expressing phosphorylated Akt, a growth promoting kinase activated by the LepRb, compared with WT and db/db mice. In vivo limiting dilution analysis of residual tumors from ob/ob mice indicated reduced tumor initiating activity suggesting fewer cancer stem cells (CSCs). The tumor cell populations reduced by leptin deficiency were identified by fluorescence-activated cell sorting and found to express LepRb. Finally, LepRb expressing tumor cells exhibit stem cell characteristics based on the ability to form tumorspheres in vitro and leptin promotes their survival. These studies provide critical new insight on the role of leptin in tumor growth and implicate LepRb as a CSC target.

Human pancreatic cancer stem cells: implications for how we treat pancreatic cancer

Pancreatic cancer has the worst prognosis of any major malignancy, with an annual death rate that approximates the annual incidence rate. Delayed diagnosis, relative chemotherapy and radiation resistance and an intrinsic biologic aggressiveness all contribute to the abysmal prognosis associated with pancreatic cancer. Answers to the frustrating effort to find effective therapies for pancreatic cancer may be gained through a renewed perspective on tumorigenesis as a process governed by a select population of cells, termed cancer stem cells (CSCs). Cancer stem cells, like their normal counterparts, have the properties of self-renewal and multilineage differentiation and possess inherently heightened DNA damage response and repair mechanisms that make them difficult to eradicate. Initially discovered in leukemias, researchers have identified CSCs in several solid-organ malignancies including breast, brain, prostate, and colon cancers. We have recently identified a CSC population in human pancreatic cancers. These pancreatic CSC represent 0.5% to 1.0% of all pancreatic cancer cells and express the cell surface markers CD44, CD24, and epithelial-specific antigen. Pancreatic CSCs have been shown to be resistant to standard chemotherapy and radiation, and devising specific therapies to target this distinct cell population is likely needed to identify effective therapies to treat this dismal disease.

Wnt and mammary stem cells: hormones cannot fly wingless

The mammary stem cell and its local microenvironment are central for the maintenance of proper tissue homeostasis during normal development. Defining the hierarchical organization of the epithelial subtypes in the mammary gland and the molecular pathways guiding their development has begun to provide a framework for understanding how cancer stem cells sustain the progression and heterogeneity of breast cancers. The Wnt pathway plays a fundamental role in multiple adult stem cells, as well as in orchestrating proper mammary gland development and maintenance. These processes are intricately guided by the influence of systemic hormones and local factors. Alterations in Wnt signaling can skew the homeostatic balance of the mammary epithelium to drive malignant progression; however, complexities of Wnt pathway components present a challenge in understanding their physiological function.

Repression of mammary stem/progenitor cells by p53 is mediated by Notch and separable from apoptotic activity

Breast cancer is the most common tumor among women with inherited mutations in the p53 gene (Li-Fraumeni syndrome). The tumors represent the basal-like subtype, which has been suggested to originate from mammary stem/progenitor cells. In mouse mammary epithelium, mammosphere-forming potential was increased with decreased dosage of the gene encoding the p53 tumor suppressor protein (Trp53). Limiting dilution transplantation also showed a 3.3-fold increase in the frequency of long-term regenerative mammary stem cells in Trp53-/- mice. The repression of mammospheres by p53 was apparent despite the absence of apoptotic responses to radiation indicating a dissociation of these two activities of p53. The effects of p53 on progenitor cells were also observed in TM40A cells using both mammosphere-forming assays and the DsRed-let7c-sensor. The frequency of long-term label-retaining epithelial cells was decreased in Trp53-/- mammary glands indicating that asymmetric segregation of DNA is diminished and contributes to the expansion of the mammary stem cells. Treatment with an inhibitor of γ-secretase (N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester) reduced the number of Trp53-/- mammospheres to the level found in Trp53+/+ cells. These results demonstrate that basal levels of p53 restrict mammary stem/progenitor cells through Notch and that the Notch pathway is a therapeutic target to prevent expansion of this vulnerable pool of cells.

The radiosensitization effect of NS398 on esophageal cancer stem cell-like radioresistant cells

This study aimed to investigate the cancer stem cell (CSC) properties of radioresistant esophageal cancer cells and the radiosensitization effect of NS398, a cyclooxygenase (COX)-2 inhibitor, on them. Fractionated irradiation was applied to acquire radioresistant esophageal cancer cells. Clone formation assay was employed to detect cell radiosensitivity and cloning formation ability. Cell viability was determined by methyl tetrazolium colorimetry assay. Cell cycle distribution and apoptosis were detected by flow cytometry. Tumorigenicity was investigated by xenograft tumorigenicity assay. Expression levels of β-catenin were detected by reverse transcription polymerase chain reaction or Western blot. As results, radioresistant Eca109R50Gy cells were obtained through fractional irradiation from Eca109 cells; Eca109R50Gy cells displayed higher ability of proliferation, colony-formation, and 40 times tumorigenic ability as high as that of the Eca109 cells in vivo. Meantime stem cell marker β-catenin was elevated in Eca109R50Gy cells. All of the above implied that Eca109R50Gy cells have some properties of CSCs. NS398 enhanced the radiosensitivity of Eca109R50Gy cells accompanied by down-regulating the expression of β-catenin. In conclusion, radioresistant Eca109R50Gy cells carried some CSC-like properties; NS398 enhanced the radiosensitivity of CSC-like Eca109R50Gy cells and this function may partly through down-regulating the expression of β-catenin. These findings both stress the important role of CSCs in esophageal cancer radioresistance and provide new insight on possible application of COX-2 inhibitors on CSCs.

Stem cell antigen-1 enhances tumorigenicity by disruption of growth differentiation factor-10 (GDF10)-dependent TGF-beta signaling

Stem cell antigen (Sca)-1/Ly6A, a glycerophosphatidylinositol-linked surface protein, was found to be associated with murine stem cell- and progenitor cell-enriched populations, and also has been linked to the capacity of tumor-initiating cells. Despite these interesting associations, this protein's functional role in these processes remains largely unknown. To identify the mechanism underlying the protein's possible role in mammary tumorigenesis, Sca-1 expression was examined in Sca-1(+/EGFP) mice during carcinogenesis. Mammary tumor cells derived from these mice readily engrafted in syngeneic mice, and tumor growth was markedly inhibited on down-regulation of Sca-1 expression. The latter effect was associated with significantly elevated expression of the TGF-β ligand growth differentiation factor-10 (GDF10), which was found to selectively activate TGF-β receptor (TβRI/II)-dependent Smad3 phosphorylation. Overexpression of GDF10 attenuated tumor formation; conversely, silencing of GDF10 expression reversed these effects. Sca-1 attenuated GDF10-dependent TGF-β signaling by disrupting the heterodimerization of TβRI and TβRII receptors. These findings suggest a new functional role for Sca-1 in maintaining tumorigenicity, in part by acting as a potent suppressor of TGF-β signaling.

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.

Pharmacological inhibition of AKT sensitizes MCF-7 human breast cancer-initiating cells to radiation

BACKGROUND

Caner-initiating cells (CICs or cancer stem cells) have been shown both experimentally and clinically to be resistant to radiation. The mechanism underlying radioresistance remains unclear.

METHODS

In the present study, we screened 51 genes which are potentially important in mediating radioresistance of breast CICs.

RESULTS

The expression of AKT1 and AKT2 at protein and mRNA levels was dramatically increased among the screened genes by 8 Gy radiation treatment in MCF-7 mammosphere cells (predominantly CD24(-/low)/CD44(+) CICs), but not in the bulk population of MCF-7 cells (predominantly CD24(+)/CD44(+)). Using apoptosis and clonogenic survival assays, we found pharmacological inhibition of AKT with selective inhibitors of AKT sensitized MCF-7 mammosphere cells, but not MCF-7 monolayer cells to radiation.

CONCLUSION

The present findings suggest that treatment with AKT inhibitors prior to ionizing radiation treatment may be a potential benefit to patients with breast cancer, in particular to eradiate breast CICs.

EZH2 promotes expansion of breast tumor initiating cells through activation of RAF1-β-catenin signaling

It has been proposed that an aggressive secondary cancer stem cell population arises from a primary cancer stem cell population through acquisition of additional genetic mutations and drives cancer progression. Overexpression of Polycomb protein EZH2, essential in stem cell self-renewal, has been linked to breast cancer progression. However, critical mechanism linking increased EZH2 expression to BTIC (breast tumor initiating cell) regulation and cancer progression remains unclear. Here, we identify a mechanism in which EZH2 expression-mediated downregulation of DNA damage repair leads to accumulation of recurrent RAF1 gene amplification in BTICs, which activates p-ERK-β-catenin signaling to promote BTIC expansion. We further reveal that AZD6244, a clinical trial drug that inhibits RAF1-ERK signaling, could prevent breast cancer progression by eliminating BTICs.

Phenotypic heterogeneity of breast cancer stem cells

Many types of tumors are organized in a hierarchy of heterogeneous cell populations, with only a small proportion of cancer stem cells (CSCs) capable of sustaining tumor formation and growth, giving rise to differentiated cells, which form the bulk of the tumor. Proof of the existence of CSC comes from clinical experience with germ-cell cancers, where the elimination of a subset of undifferentiated cells can cure patients (Horwich et al., 2006), and from the study of leukemic cells (Bonnet and Dick, 1997; Lapidot et al., 1994; and Yilmaz et al., 2006). The discovery of CSC in leukemias as well as in many solid malignancies, including breast carcinoma (Al-Hajj et al. 2003; Fang et al., 2005; Hemmati et al., 2003; Kim et al., 2005; Lawson et al., 2007; Li et al., 2007; Ricci-Vitiani et al., 2007; Singh et al., 2003; and Xin et al., 2005), has suggested a unifying CSC theory of cancer development. The reported general insensitivity of CSC to chemotherapy and radiation treatment (Bao et al., 2006) has suggested that current anticancer drugs, which inhibit bulk replicating cancer cells, may not effectively inhibit CSC. The clinical relevance of targeting CSC-associated genes is supported by several recent studies, including CD44 targeting for treatment of acute myeloid leukemia (Jin et al., 2006), CD24 targeting for treatment of colon and pancreatic cancer (Sagiv et al., 2008), and CD133 targeting for hepatocellular and gastric cancer (Smith et al., 2008). One promising approach is to target CSC survival signaling pathways, where leukemia stem cell research has already made some progress (Mikkola et al., 2010).

ID3 mediates X-ray-induced apoptosis of keratinocytes through the regulation of β-catenin

BACKGROUND

Ionizing radiation is used to treat many of cancers, however, it also produces unwanted side effect on normal tissues, such as radiodermatitis. We previously established an animal model for radiodermatitis, and found that X-ray irradiation induced the expression of ID3 in hairless mouse skin by cDNA microarray.

OBJECTIVE

The aim of this study is to investigate the functional role of ID3 in X-ray irradiated keratinocytes.

METHODS

Immunohistochemistry, RT-PCR and Western blot were performed to demonstrate the ID3 induction by X-ray irradiation. HaCaT keratinocytes were transduced with the recombinant adenovirus expressing HA-ID3, and then effects on apoptosis were analyzed.

RESULTS

X-ray irradiation increased markedly the ID3 protein level in epidermis of mouse skin. X-ray irradiation also induced the expression of ID3 in HaCaT keratinocytes cultured in vitro, at both mRNA and protein levels. When ID3 was overexpressed by recombinant adenovirus, apoptosis of keratinocytes were induced even in the absence of X-ray irradiation. Furthermore, overexpression of ID3 sensitized X-ray-induced apoptosis. Interestingly, X-ray irradiation significantly reduced the endogenous β-catenin level, which was related with induction of apoptosis. Similarly, overexpression of ID3 led to remarkable reduction in β-catenin level.

CONCLUSION

These results suggest that ID3 plays a role as an apoptosis inducer in response to X-ray irradiation via the regulation of endogenous β-catenin level.

Characterizing cancer cells with cancer stem cell-like features in 293T human embryonic kidney cells

BACKGROUND

Since the first suggestion of prospectively identifiable cancer stem cells in solid tumors, efforts have been made to characterize reported cancer stem cell surrogates in existing cancer cell lines, and cell lines rich with these surrogates have been used to screen for cancer stem cell targeted agents. Although 293T cells were derived from human embryonic kidney, transplantation of these cells into the mammary fat pad yields aggressive tumors that self-renew as evidenced by serial xenograft passages through transplantation. Herein we fully characterize cancer stem cell-like features in 293T human embryonic kidney cells.

RESULTS

293T cells can be readily cultured and passaged as spheres in serum-free stem cell promoting culture conditions. Cells cultured in vitro as three-dimensional spheres (3D) were shown to contain higher ALDH1 and CD44+/CD24- population compared to monolayer cells. These cells were also resistant to radiation and upregulate stem cell survival signaling including beta-catenin, Notch1 and Survivin in response to radiation. Moreover, 3D spheres generated from the 293T cells have increased expression of mesenchymal genes including vimentin, n-cadherin, zeb1, snail and slug as well as pro-metastatic genes RhoC, Tenascin C and MTA1. In addition, microRNAs implicated in self-renewal and metastases were markedly reduced in 3D spheres.

CONCLUSIONS

293T cells exhibit a cancer stem cell-like phenotype when cultured as 3D spheres and represent an important research tool for studying the molecular and biological mechanisms of cancer stem cells and for testing and developing novel targets for cancer therapy.

Do stem-like cells play a role in drug resistance of sarcomas?

Stem cells are defined by their unique characteristics, which include their abilities to self-renew and differentiate. Normal somatic stem cells have been isolated from various tissues such as bone marrow, adipose tissue, mammary glands and the nervous system. They are considered naturally resistant to chemotherapeutic agents because they express high levels of membrane transporter molecules, detoxifying enzymes and DNA repair proteins. Several recent studies have identified the presence of side populations in various cancer tissues, the so-called 'cancer stem cells', which are defined as the counterparts of stem cells in tumor tissues. These cancer stem cells possess stem-like properties, such as self-renewal and differentiation abilities, as well as playing a role in tumor initiation. Most sarcomas, which are thought to originate from mesenchymal stem cells, are highly malignant and approximately 30-40% of them show local and/or distant relapse (metastasis), even in the case of relatively chemosensitive tumors such as osteosarcomas and Ewing sarcomas. Several studies have suggested the presence of stem-like cell populations in sarcomas, based on their tumorigenicity and drug resistance. This review explores the issues of drug resistance of cancer stem cells in sarcomas and the possibilities of targeting cancer stem cells for the future treatment of sarcomas.

Wnt/beta-catenin pathway activation is enriched in basal-like breast cancers and predicts poor outcome

Although Wnt/beta-catenin pathway activation has been implicated in mouse models of breast cancer, there is contradictory evidence regarding its importance in human breast cancer. In this study, invasive and in situ breast cancer tissue microarrays containing luminal A, luminal B, human epidermal growth factor receptor 2 (HER2)(+)/ER(-) and basal-like breast cancers were analyzed for beta-catenin subcellular localization. We demonstrate that nuclear and cytosolic accumulation of beta-catenin, a read-out of Wnt pathway activation, was enriched in basal-like breast cancers. In contrast, membrane-associated beta-catenin was observed in all breast cancer subtypes, and its expression decreased with tumor progression. Moreover, nuclear and cytosolic localization of beta-catenin was associated with other markers of the basal-like phenotype, including nuclear hormone receptor and HER2 negativity, cytokeratin 5/6 and vimentin expression, and stem cell enrichment. Importantly, this subcellular localization of beta-catenin was associated with a poor outcome and is more frequently observed in tumors from black patients. In addition, beta-catenin accumulation was more often observed in basal-like in situ carcinomas than other in situ subtypes, suggesting that activation of this pathway might be an early event in basal-like tumor development. Collectively, these data indicate that Wnt/beta-catenin activation is an important feature of basal-like breast cancers and is predictive of worse overall survival, suggesting that it may be an attractive pharmacological target for this aggressive breast cancer subtype.

Molecular biology of breast cancer stem cells: potential clinical applications

Breast cancer stem cells (CSC) have been postulated recently as responsible for failure of breast cancer treatment. The purpose of this study is to review breast CSCs molecular biology with respect to their mechanism of resistance to conventional therapy, and to develop treatment strategies that may improve survival of breast cancer patients. A literature search has identified in vitro and in vivo studies of breast CSCs. Breast CSCs overexpress breast cancer resistance protein (BCRP) which allows cancer cells to transport actively chemotherapy agents out of the cells. Radioresistance is modulated through activation of Wnt signaling pathway and overexpression of genes coding for glutathione. Lapatinib can selectively target HER-2 positive breast CSCs and improves disease-free survival in these patients. Metformin may target basal type breast CSCs. Parthenolide and oncolytic viruses are promising targeting agents for breast CSCs. Future clinical trials for breast cancer should include anti-cancer stem cells targeting agents in addition to conventional chemotherapy. Hypofractionation radiotherapy may be indicated for residual disease post chemotherapy.

Cancer stem cells at the crossroads of current cancer therapy failures--radiation oncology perspective

Despite continuous improvements in cancer management, locoregional recurrence or metastatic spread still occurs in a high proportion of patients after radiotherapy or combined treatments. One underlying reason might be a low efficacy of current treatments on eradication of cancer stem cells (CSCs). It has been recognised for a long time, that only the small subpopulation of CSCs can cause recurrences and that all CSCs need to be killed for permanent tumour cure. However, only recently novel technologies have allowed to enrich CSCs and to investigate their biology. An emerging experimental and clinical database provides first hints that cell populations accumulated by putative stem cell markers or tumours that highly express such markers may be more radioresistant than their marker-negative counterparts. Other data support a higher tolerance of CSCs to hypoxia and preferential location in specific microenvironmental niches. However, conflicting data, methodological problems of the assays and a generally small database on only few tumour types necessitate further large and well-designed prospective experimental and clinical investigations that specifically address this question to corroborate this hypothesis. If such investigations confirm biological differences between CSCs and non-CSCs, this would imply that novel treatment strategies need to be tested specifically for their effect on CSCs. Another implication is that also biomarkers for prediction of local tumour control after radiotherapy or combined treatments need to reflect the behaviour of CSCs and not of the bulk of all cancer cells. This review discusses the importance of CSCs for treatment failure and challenges occurring from the CSC concept for cancer diagnosis, treatment and prediction of outcome. It is concluded that CSC-based endpoints and biomarkers are eventually expected to considerably improve tumour cure rates in the clinics through individualised tailoring of treatment.

A putative role for microRNA-205 in mammary epithelial cell progenitors

In an effort to understand the potential role of microRNAs (miRNAs) in mammary-gland stem or progenitor cells, miRNA microarrays were performed on subpopulations of the mouse mammary epithelial cell (MEC) line COMMA-DbetaGeo. This cell line contains a heterogeneous subpopulation of progenitors characterized by the expression of stem cell antigen 1 (Sca-1; encoded by Ly6a). Microarray analysis indicated that the Sca-1 subpopulations have distinct miRNA expression profiles. Functional studies were performed on miR-205, which was highly expressed in the Sca-1-positive (Sca-1(+)) cells. When miR-205 was overexpressed in vitro, the COMMA-DbetaGeo cells underwent several significant morphological and molecular changes. miR-205 overexpression led to an expansion of the progenitor-cell population, decreased cell size and increased cellular proliferation. In addition, the colony-forming potential of the two Sca-1 subpopulations was increased. Target prediction for miR-205 indicated that it might regulate the expression of the tumor-suppressor protein PTEN. Overexpression studies using reporter constructs confirmed that PTEN expression is regulated by miR-205. In addition to PTEN, several other putative and previously validated miR-205 targets were identified by microarray analysis, including the previously reported miR-205 targets ZEB1 and ZEB2. Additionally, in normal mouse MECs, high expression of miR-205 was observed in stem-cell-enriched cell populations isolated by FACS using established cell-surface markers.