G Protein-Coupled Receptor 87 (GPR87) Promotes the Growth and Metastasis of CD133+ Cancer Stem-Like Cells in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is a prevalent disease worldwide, and the majority of HCC-related deaths occur due to local invasion and distant metastasis. Cancer stem cells (CSCs) are a small subpopulation of cancer cells that have been hypothesized to be responsible for metastatic disease. Recently, we and others have identified a CSC population from human HCC cell lines and xenograft tumors characterized by their expression of CD133. However, the precise molecular mechanisms by which CD133⁺ cancer stem-like cells mediate HCC metastasis have not been sufficiently analyzed. Here, we have sorted HCC cells using CD133 as a cancer stem cell (CSC) marker by magnetic-activated cell sorting (MACS) and demonstrated that the CD133⁺ HCC cells not only possess greater migratory and invasive capacity in vitro but are also endowed with enhanced metastatic capacity in vivo and in human HCC specimens when compared to CD133⁻ HCC cells. Gene expression analysis of the CD133⁺ and CD133⁻ cells of the HCC line SMMC-7721 revealed that G protein-coupled receptor 87 (GPR87) is highly expressed in CD133⁺ HCC cells. In this study, we explored the role of GPR87 in the regulation of CD133 expression. We demonstrated that the overexpression of GPR87 up-regulated CD133 expression, promoted CSC-associated migratory and invasive properties in vitro, and increased tumor initiation in vivo. Conversely, silencing of GPR87 expression reduced the levels of CD133 expression. Conclusion: GPR87 promotes the growth and metastasis of CD133⁺ cancer stem-like cells, and our findings may reveal new targets for HCC prevention or therapy.     

Combination of dasatinib and curcumin eliminates chemo-resistant colon cancer cells

Metastatic colorectal cancer remains a serious health concern with poor patient survival. Although 5-Fluorouracil (5-FU) or 5-FU plus oxaliplatin (FOLFOX) is the standard therapy for colorectal cancer, it has met with limited success. Recurrence of the tumor after chemotherapy could partly be explained by the enrichment of the chemo-resistant sub-population of cancer stem cells (CSCs) that possess the ability for self-renewal and differentiation into different lineages in the tumor. Therefore development of therapeutic strategies that target CSCs for successful treatment of this malignancy is warranted. The current investigation was undertaken to examine the effectiveness of the combination therapy of dasatinib (a Src inhibitor) and curcumin (a dietary agent with pleiotropic effect) in inhibiting the growth and other properties of carcinogenesis of chemo-resistant colon cancer cells that are enriched in CSCs sub-population. Remnants of spontaneous adenomas from APC ^{Min +/-} mice treated with dasatinib and/or curcumin were analyzed for several cancer stem cell markers (ALDH, CD44, CD133 and CD166). Human colon cancer cells HCT-116 (p53 wild type; K-ras mutant) and HT-29 (p53 mutant; K-ras wild type) were used to generate FOLFOX resistant (referred to as CR) cells. The effectiveness of the combination therapy in inhibiting growth, invasive potential and stemness was examined in colon cancer CR cells. The residual tumors from APC ^{Min +/-} mice treated with dasatinib and/or curcumin showed 80-90% decrease in the expression of the CSC markers ALDH, CD44, CD133, CD166. The colon cancer CR cells showed a higher expression of CSCs markers, cell invasion potential and ability to form colonospheres, compared to the corresponding parental cells. The combination therapy of dasatinib and curcumin demonstrated synergistic interactions in CR HCT-116 and CR HT-29 cells, as determined by Calcusyn analysis. The combinatorial therapy inhibited cellular growth, invasion and colonosphere formation and also reduced CSC population as evidenced by the decreased expression of CSC specific markers: CD133, CD44, CD166 and ALDH. Our data suggest that the combination therapy of dasatinib and curcumin may be a therapeutic strategy for re-emergence of chemo-resistant colon cancer by targeting CSC sub-population.     

Isolation of Stem-Like Cancer Cells in Primary Endometrial Cancer Using Cell Surface Markers CD133 and CXCR4

Endometrial cancer (EC) is the most common familiar gynecologic malignant tumor identified in the female reproductive system and has been increasing yearly. In this study, we will identify the surface markers and stem cell markers related with cancer stem cells (CSCs) of EC. Tissue samples were obtained from endometrial cancer patients during surgical procedures. Single cells were isolated from the tissues for culturing, transfection into nude mice, and histopathology analysis. RT-PCR demonstrated that the cultured cells strongly expressed stemness-related genes, such as c-Myc, Sox-2, Nanog, Oct 4A, ABCG2, BMI-1, CK-18, Nestin and β-actin. The expression of surface markers CD24, CD133, CD47, CD29, CD44, CXCR4, SSEA3 and SSEA4, CD24, and CD133 and chemokine markers such as CXCR4 were measured by flow cytometry. Then the double percentage of CD133+CXCR4+ cells constituted 7.2% and 9.3% in EC cells originated from two different patients, respectively. The CD133+CXCR4+ primary endometrial cancer cells grew faster, exhibited high expression of mRNA of stemness-related genes, produced more spheres, and had higher clonogenic ability than other subpopulations. They are also more resistant to anti-cancer drugs than other subpopulations. These findings indicate that CD133+CXCR4+ cells may possess some characteristics of CSCs in primary endometrial cancer. These cell surface markers may be useful for the development of drugs against CSC molecular targets or as a predictive marker for poor prognosis in primary endometrial cancer.     

CD44 and CD24 cannot act as cancer stem cell markers in human lung adenocarcinoma cell line A549

Cancer stem cells (CSCs) are subpopulations of tumor cells that are responsible for tumor initiation, maintenance and metastasis. Recent studies suggested that lung cancer arises from CSCs. In this study, the expression of potential CSC markers in cell line A549 was evaluated. We applied flow cytometry to assess the expression of putative stem cell markers, including aldehyde dehydrogenase 1 (ALDH1), CD24, CD44, CD133 and ABCG2. Cells were then sorted according to the expression of CD44 and CD24 markers by fluorescence-activated cell sorting (FACS) Aria II and characterized using their clonogenic and sphere-forming capacity. A549 cells expressed the CSC markers CD44 and CD24 at 68.16% and 54.46%, respectively. The expression of the putative CSC marker ALDH1 was 4.20%, whereas the expression of ABCG2 and CD133 was 0.93%. Double-positive CD44/133 populations were rare. CD44⁺/24⁺ and CD44⁺/CD24^?/low subpopulations respectively exhibited 64% and 27.92% expression. The colony-forming potentials in the CD44⁺/CD24⁺ and CD44⁺/CD24^?/low subpopulations were 84.37 ± 2.86% and 90 ± 3.06%, respectively, while the parental A549 cells yielded 56.65 ± 2.33% using the colony-formation assay. Both isolated subpopulations formed spheres in serumfree medium supplemented with basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF). CD44 and CD24 cannot be considered potential markers for isolating lung CSCs in cell line A549, but further investigation using in vivo assays is required.     

Identification of cancer stem cells from human glioblastomas: growth and differentiation capabilities and CD133/prominin-1 expression

CD133 can be a marker of tumorigenic CSCs (cancer stem cells) in human GBM (glioblastoma multiforme), although tumorigenic CD133-negative CSCs have been also isolated. Additional evidence indicates that CSCs from GBM exhibit different phenotypes, with increasing interest in the potential significance of the different CSCs with respect to diagnosis, prognosis and the development of novel targets for treatment. We have analysed the expression of CD133 in freshly isolated cells from 15 human GBM specimens. Only 4 of them contained cells positive for AC133 by FACS analysis, and all of them yielded distinct CSC lines, whereas only 6 CSC lines were obtained from the other 11 GBMs. Of these 10 CSCs lines, we further characterized 6 CSC lines. Three CSCs grew as fast-growing neurospheres with higher clonogenic ability, whereas the remaining 3 grew as slow-growing semi-adherent spheres of lower clonogenicity. In addition, the former CSC lines displayed better differentiation capabilities than the latter ones. PCR and Western blot analysis showed that all 6 GBM CSC lines expressed CD133/prominin-1, suggesting that cells negative by FACS analysis may actually represent cells expressing low levels of CD133 undetected by FACS. Nevertheless, all the 6 CSC lines were tumorigenic in nude mice. In conclusion, CSCs from human primary GBMs show different phenotypes and variable levels of CD133 expression, but these parameters did not directly correlate with the tumorigenic potential.     

New emerging roles of CD133 in cancer stem cell: Signaling pathway and miRNA regulation

Cancer stem cells (CSC) are rare immortal cells within a tumor that are able to initiate tumor progression, development, and resistance. Advances studies show that, like normal stem cells, CSCs can be both self-renewed and given rise to many cell types, therefore form tumors. A number of cell surface markers, such as CD44, CD24, and CD133 are frequently used to identify CSCs. CD133, a transmembrane glycoprotein, either alone or in collaboration with other markers, has been mainly considered to identify CSCs from different solid tumors. However, the exactness of CD133 as a cancer stem cell biomarker has not been approved yet. The clinical importance of CD133 is as a CSC marker in many cancers. Also, it contributes to shorter survival, tumor progression, and tumor recurrence. The expression of CD133 is controlled by many extracellular or intracellular factors, such as tumor microenvironment, epigenetic factors, signaling pathways, and miRNAs. In this study, it was attempted to determine: 1) CD133 function; 2) the role of CD133 in cancer; 3) CD133 regulation; 4) the therapeutic role of CD133 in cancers.     

Heterogeneous phenotype of human glioblastoma: In vitro study

Glioblastomas (GBMs) are the most lethal primary brain tumours. Increasing evidence shows that brain tumours contain the population of stem cells, so‐called cancer stem cells (CSCs). Stem cell marker CD133 was reported to identify CSC population in GBM. Further studies have indicated that CD133 negative cells exhibiting similar properties and are able to initiate the tumour, self‐renew and undergo multilineage differentiation. GBM is a highly heterogeneous tumour and may contain different stem cell populations with different functional properties. We characterized five GBM cell lines, established from surgical samples, according to the marker expression, proliferation and differentiation potential. CD133 positive cell lines showed increased proliferation rate in neurosphere condition and marked differentiation potential towards neuronal lineages. Whereas two cell lines low‐expressing CD133 marker showed mesenchymal properties in vitro, that is high proliferation rate in serum condition and differentiation in mesenchymal cell types. Further, we compared therapy resistance capacity of GBM cell lines treated with hydroxyurea. Our results suggest that CSC concept is more complex than it was believed before, and CD133 could not define entire stem cell population within GBM. At least two different subtypes of GBM CSCs exist, which may have different biological characteristics and imply different therapeutic strategies. Copyright © 2013 John Wiley & Sons, Ltd.     

CD166 promotes the cancer stem-like properties of primary epithelial ovarian cancer cells

Cancer stem cells (CSCs) or tumor-initiating cells are thought to play critical roles in tumorigenesis, metastasis, drug resistance, and tumor recurrence. For the diagnosis and targeted therapy of CSCs, the molecular identity of biomarkers or therapeutic targets for CSCs needs to be clarified. In this study, we identified CD166 as a novel marker expressed in the sphere-forming CSC population of A2780 epithelial ovarian cancer cells and primary ovarian cancer cells. The CD166⁺ cells isolated from A2780 cells and primary ovarian cancer cells highly expressed CSC markers, including ALDH1a1, OCT4, and SOX2, and ABC transporters, which are implicated in the drug resistance of CSCs. The CD166⁺ cells exhibited enhanced CSC-like properties, such as increased sphere-forming ability, cell migration and adhesion abilities, resistance to conventional anti-cancer drugs, and high tumorigenic potential in a xenograft mouse model. Knockdown of CD166 expression in the sphere-forming ovarian CSCs abrogated their CSC-like properties. Moreover, silencing of CD166 expression in the sphere-forming CSCs suppressed the phosphorylation of focal adhesion kinase, paxillin, and SRC. These results suggest that CD166 plays a key role in the regulation of CSC-like properties and focal adhesion kinase signaling in ovarian cancer.     

Expression of cancer stem cell marker during 4-nitroquinoline 1-oxide-induced rat tongue carcinogenesis

One of the theories regarding oral carcinogenesis is that the tumor growth is initiated from cancer stem cells (CSCs) that self-renew and give rise to differentiated tumor cells, like stem cells do in normal tissues. The most common methods of CSC identification are based on CSC marker expression in carcinogenesis. This study examined the expression of CD133 and CD44, the most commonly used CSC biomarkers in oral squamous cell sarcoma (SCC), with the goal of identifying molecular biomarkers whose expression is associated with the multistep oral carcinogenesis. The expression of CD133, CD44, proliferating cell nuclear antigen (PCNA), and Cytokeratin (CK) was examined by Western blot analysis and confirmed by immunohistochemistry in a 4-nitroquinoline 1-oxide-induced rat tongue carcinogenesis model. Also, the expression of aldehyde dehydrogenase 1 (ALDH1), OCT-4 and Nanog were investigated for alteration of cancer cell stemness by Western blot. Along with the progress of multistep carcinogenesis, there were slight increases of CD133 and CD44 expression in the dysplasia group compared with normal rats. However, CD133 protein level was significantly overexpressed in SCC. The expression of PCNA and CK were low in normal group, but sequentially increased in SCC. ALDH1, Nanog and OCT-4 expression were significantly increased according to SCC grade during carcinogenesis. The findings indicate that CD133 is useful in identifying oral CSCs, which suggests that CD133 may serve as a predictor to identify CSCs with a high risk of oral cancer development.     

Autophagy promotes resistance to photodynamic therapy-induced apoptosis selectively in colorectal cancer stem-like cells

Recent studies have indicated that cancer stem-like cells (CSCs) exhibit a high resistance to current therapeutic strategies, including photodynamic therapy (PDT), leading to the recurrence and progression of colorectal cancer (CRC). In cancer, autophagy acts as both a tumor suppressor and a tumor promoter. However, the role of autophagy in the resistance of CSCs to PDT has not been reported. In this study, CSCs were isolated from colorectal cancer cells using PROM1/CD133 (prominin 1) expression, which is a surface marker commonly found on stem cells of various tissues. We demonstrated that PpIX-mediated PDT induced the formation of autophagosomes in PROM1/CD133⁺ cells, accompanied by the upregulation of autophagy-related proteins ATG3, ATG5, ATG7, and ATG12. The inhibition of PDT-induced autophagy by pharmacological inhibitors and silencing of the ATG5 gene substantially triggered apoptosis of PROM1/CD133⁺ cells and decreased the ability of colonosphere formation in vitro and tumorigenicity in vivo. In conclusion, our results revealed a protective role played by autophagy against PDT in CSCs and indicated that targeting autophagy could be used to elevate the PDT sensitivity of CSCs. These findings would aid in the development of novel therapeutic approaches for CSC treatment.     

Glutathione metabolism is essential for self-renewal and chemoresistance of pancreatic cancer stem cells

BACKGROUNDCellular metabolism regulates stemness in health and disease.  A reduced redox state is essential for self-renewal of normal and cancer stem cells (CSCs). However, while stem cells rely on glycolysis, different CSCs, including pancreatic CSCs, favor mitochondrial metabolism as their dominant energy-producing pathway. This suggests that powerful antioxidant networks must be in place to detoxify mitochondrial reactive oxygen species (ROS) and maintain stemness in oxidative CSCs. Since glutathione metabolism is critical for normal stem cell function and CSCs from breast, liver and gastric cancer show increased glutathione content, we hypothesized that pancreatic CSCs also rely on this pathway for ROS detoxification.AIMTo investigate the role of glutathione metabolism in pancreatic CSCs.METHODSPrimary pancreatic cancer cells of patient-derived xenografts (PDXs) were cultured in adherent or CSC-enriching sphere conditions to determine the role of glutathione metabolism in stemness. Real-time polymerase chain reaction (PCR) was used to validate RNAseq results involving glutathione metabolism genes in adherent vs spheres, as well as the expression of pluripotency-related genes following treatment. Public TCGA and GTEx RNAseq data from pancreatic cancer vs normal tissue samples were analyzed using the webserver GEPIA2. The glutathione-sensitive fluorescent probe monochlorobimane was used to determine glutathione content by fluorimetry or flow cytometry. Pharmacological inhibitors of glutathione synthesis and recycling [buthionine-sulfoximine (BSO) and 6-Aminonicotinamide (6-AN), respectively] were used to investigate the impact of glutathione depletion on CSC-enriched cultures. Staining with propidium iodide (cell cycle), Annexin-V (apoptosis) and CD133 (CSC content) were determined by flow cytometry. Self-renewal was assessed by sphere formation assay and response to gemcitabine treatment was used as a readout for chemoresistance.RESULTSAnalysis of our previously published RNAseq dataset E-MTAB-3808 revealed up-regulation of genes involved in the KEGG (Kyoto Encyclopedia of Genes and Genomes) Pathway Glutathione Metabolism in CSC-enriched cultures compared to their differentiated counterparts. Consistently, in pancreatic cancer patient samples the expression of most of these up-regulated genes positively correlated with a stemness signature defined by NANOG, KLF4, SOX2 and OCT4 expression (P < 10^-5). Moreover, 3 of the upregulated genes (MGST1, GPX8, GCCT) were associated with reduced disease-free survival in patients [Hazard ratio (HR) 2.2-2.5; P = 0.03-0.0054], suggesting a critical role for this pathway in pancreatic cancer progression. CSC-enriched sphere cultures also showed increased expression of different glutathione metabolism-related genes, as well as enhanced glutathione content in its reduced form (GSH). Glutathione depletion with BSO induced cell cycle arrest and apoptosis in spheres, and diminished the expression of stemness genes. Moreover, treatment with either BSO or the glutathione recycling inhibitor 6-AN inhibited self-renewal and the expression of the CSC marker CD133. GSH content in spheres positively correlated with intrinsic resistance to gemcitabine treatment in different PDXs r = 0.96, P = 5.8 × 10¹¹). Additionally, CD133⁺ cells accumulated GSH in response to gemcitabine, which was abrogated by BSO treatment (P < 0.05). Combined treatment with BSO and gemcitabine-induced apoptosis in CD133⁺ cells to levels comparable to CD133^- cells and significantly diminished self-renewal (P < 0.05), suggesting that chemoresistance of CSCs is partially dependent on GSH metabolism.CONCLUSIONOur data suggest that pancreatic CSCs depend on glutathione metabolism. Pharmacological targeting of this pathway showed that high GSH content is essential to maintain CSC functionality in terms of self-renewal and chemoresistance.     

Photochemical internalization of CD133-targeting immunotoxins efficiently depletes sarcoma cells with stem-like properties and reduces tumorigenicity

Background

The normal stem cell marker CD133 is also a putative marker of cancer stem cells (CSCs) in different types of cancers. Hence, a major challenge when targeting CD133-expressing CSCs is to prevent depletion of the normal stem cell pool. We hypothesized that the site-specific and light-controlled drug delivery method photochemical internalization (PCI) may have the potential to enhance selectivity and endosomal escape of CD133-targeting immunotoxins in stem-like sarcoma cells.

Methods

We have used a sarcoma model, SW872 cells isolated from xenografts harboring CSCs within a ~ 2% CD133^high subpopulation to investigate the potential of PCI of CD133-targeting toxin as a novel strategy to kill CSCs. Model immunotoxins were generated by binding the ribosome-inactivating protein toxin saporin to each of the monoclonal antibodies CD133/1 (AC133) or CD133/2 (293C), specific for individual CD133-epitopes. Cellular targeting, intracellular co-localization with the PCI photosensitizer, disulfonated meso-tetraphenylchlorin (TPCS_2a), and cytotoxic efficacy of PCI of the CD133-targeting toxins were evaluated.

Results

PCI of CD133–saporin efficiently targets CD133-expressing SW872 and HT1080 sarcoma cells and results in loss of cell viability. Following sub-toxic treatment, surviving SW872 cells, depleted of the CD133-expressing population, display reduced proliferative capacity and attenuated CSC properties, such as reduced colony-forming ability and tumorigenicity.

Conclusion

Here we present a proof-of-concept study, where PCI enables light-triggered delivery of CD133-targeting antibody-drug conjugates, resulting in decreased sarcoma tumor-initiating capacity.

General significance

PCI of CD133-targeting toxins may be used as a minimal invasive strategy in the treatment of sarcomas, and potentially as a therapeutic for other solid tumors expressing CD133.     

Aldehyde dehydrogenase discriminates the CD133 liver cancer stem cell populations

Recent efforts in our study of cancer stem cells (CSC) in hepatocellular carcinoma (HCC) have led to the identification of CD133 as a prominent HCC CSC marker. Findings were based on experiments done on cell lines and xenograft tumors where expression of CD133 was detected at levels as high as 65%. Based on the CSC theory, CSCs are believed to represent only a minority number of the tumor mass. This is indicative that our previously characterized CD133(+) HCC CSC population is still heterogeneous, consisting of perhaps subsets of cells with differing tumorigenic potential. We hypothesized that it is possible to further enrich the CSC population by means of additional differentially expressed markers. Using a two-dimensional PAGE approach, we compared protein profiles between CD133(+) and CD133(-) subpopulations isolated from Huh7 and PLC8024 and identified aldehyde dehydrogenase 1A1 as one of the proteins that are preferentially expressed in the CD133(+) subfraction. Analysis of the expression of several different ALDH isoforms and ALDH enzymatic activity in liver cell lines found ALDH to be positively correlated with CD133 expression. Dual-color flow cytometry analysis found the majority of ALDH(+) to be CD133(+), yet not all CD133(+) HCC cells were ALDH(+). Subsequent studies on purified subpopulations found CD133(+)ALDH(+) cells to be significantly more tumorigenic than their CD133(-)ALDH(+) or CD133(-)ALDH(-) counterparts, both in vitro and in vivo. These data, combined with those from our previous work, reveal the existence of a hierarchical organization in HCC bearing tumorigenic potential in the order of CD133(+)ALDH(+) > CD133(+)ALDH(-) > CD133(-)ALDH(-). ALDH, expressed along CD133, can more specifically characterize the tumorigenic liver CSC population.     

Plasma Membrane Proteomics of Tumor Spheres Identify CD166 as a Novel Marker for Cancer Stem-like Cells in Head and Neck Squamous Cell Carcinoma

Patients with advanced head and neck squamous cell carcinoma (HNSCC) have a poor prognosis with the currently available therapy, and tumor recurrence is frequently observed. The discovery of specific membrane-associated cancer stem cell (CSC) markers is crucial for the development of novel therapeutic strategies to target these CSCs. To address this issue, we established sphere cultures to enrich CSCs and used them for plasma membrane proteomics to identify specific membrane signatures of the HNSCC spheres. Of a dataset that included a total of 376 identified proteins, 200 were bona fide membrane proteins. Among them, 123 proteins were at least 1.5-fold up- or down-regulated in the spheres relative to the adherent cultures. These proteins included cell adhesion molecules, receptors, and transporter proteins. Some of them play key roles in wnt, integrin, and TGFβ signaling pathways. When we compared our dataset with two published hESC membrane protein signatures, we found 18 proteins common to all three of the databases. CD166 and CD44 were two such proteins. Interestingly, the expression of CD166, rather than that of the well-established HNSCC CSC marker CD44, was significantly related to the malignant behavior of HNSCC. Relative to CD166^low HNSCC cells, CD166^high HNSCC cells had a greater sphere-formation ability in vitro and tumor formation ability in vivo. Patients whose tumors expressed high levels of CD166 had a significantly poorer clinical outcome than those whose tumors expressed low levels of CD166 (cohort 1: 96 cases, p = 0.040), whereas the level of CD44 expression had only a marginal influence on the clinical outcome of patients with HNSCC (p = 0.078). The level of CD166 expression in HNSCC tumors was also associated with the tumor recurrence rate (cohort 2: 104 cases, p = 0.016). This study demonstrates that CD166 is a valuable cell surface marker for the enrichment of HNSCC stem cells and that plasma membrane proteomics is a promising biological tool for investigating the membrane proteins of CSCs.Head and neck squamous cell carcinoma (HNSCC)¹ is the sixth most common cancer worldwide. Despite ongoing improvement in traditional treatments, the long-term survival rate of patients with HNSCC has not significantly improved over the past several decades. More than 60% of patients with advanced tumors or localized lymph node metastases die within five years of their diagnosis (1). Tumor recurrence and resistance to therapy are the major causes of death. Recently, newly recognized cancer stem cells (CSCs) or tumor-initiating cells have been associated in a cause-and-effect manner with tumor recurrence and resistance to therapy. The concept of CSCs was established because of the heterogeneous nature of cancer and suggests that CSCs are a subpopulation of cancer cells with stem-cell-like traits and the source of all cells in the cancer. Conventional cancer therapies such as chemotherapy and radiotherapy may destroy only those cells that form the bulk of the tumor, leaving the CSCs intact and able to give rise to tumor recurrence. Based on this theory, researchers are searching for therapies that would destroy CSCs in the hope of finally curing cancer (2). In order to develop strategies that target CSCs, experimental assays are required to determine how to distinguish CSCs from their progeny. Different methods have been used to isolate CSCs from a range of hematopoietic and solid tumors, and some CSC-specific cell surface markers have been found. These markers are primarily selected from the corresponding normal stem-cell markers based on their heterogeneous expression in the pertinent cancers. Despite some controversy, the CD34+CD38- marker signature was chosen to define the CSCs of leukemia (3), the CD44+CD24- signature was chosen to define breast cancer CSCs (4), and the CD44 marker was chosen to define the CSCs of HNSCC (5). Though membrane proteins represent only one-third of the proteins encoded by the human genome, they represent more than two-thirds of the known protein targets of drugs. These cell surface markers are not only useful for enriching CSCs from different tumors, but also of significant interest for drug discovery.However, as more cell surface markers for different cancers have been identified, conflicting results have been reported regarding the usefulness of some of the markers and the reproducibility of some of the marker profiles (6). Quintana et al. examined the expression of 22 common CSC markers in melanoma and found that none of them were exclusively enriched in tumorigenic cells relative to non-tumorigenic cells derived from melanoma (7). CD133 is a widely accepted cell surface marker for glioblastoma CSCs, but Beier et al. found that some glioblastoma CSCs were CD133- (8). CD44 is a CSC marker that is commonly expressed by different malignancies of hematopoietic and epithelial origin, including HNSCC (5). However, increasing data have demonstrated a high level of expression of CD44 in the great majority of cells in head and neck tissues, including normal mucosa and carcinomas, and its subsequent expression could not be used to distinguish normal from benign or malignant epithelia of the head and neck. These observations suggest the need for a comprehensive investigation and greater understanding of the cell surface molecules of CSCs.Many different “omic” technologies have shown promise as means to identify markers for cancer stem cells and tumors (9). Among them, membrane proteomics can directly detect changes in the cell surface content and provide insights into the post-translational regulation of cell surface functions. Therefore, in this study, we chose to use membrane proteomics both to investigate the cell surface molecules of CSCs that were enriched from the HNSCC cell populations based on their ability to form spheres and to relate their expression to that of stem cell traits. Our results may contribute to further clinical applications of CSCs by providing tools for purifying and identifying CSCs.     

CD133 positive embryonal rhabdomyosarcoma stem-like cell population is enriched in rhabdospheres

Cancer stem cells (CSCs) have been identified in a number of solid tumors, but not yet in rhabdomyosarcoma (RMS), the most frequently occurring soft tissue tumor in childhood. Hence, the aim of this study was to identify and characterize a CSC population in RMS using a functional approach. We found that embryonal rhabdomyosarcoma (eRMS) cell lines can form rhabdomyosarcoma spheres (short rhabdospheres) in stem cell medium containing defined growth factors over several passages. Using an orthotopic xenograft model, we demonstrate that a 100 fold less sphere cells result in faster tumor growth compared to the adherent population suggesting that CSCs were enriched in the sphere population. Furthermore, stem cell genes such as oct4, nanog, c-myc, pax3 and sox2 are significantly upregulated in rhabdospheres which can be differentiated into multiple lineages such as adipocytes, myocytes and neuronal cells. Surprisingly, gene expression profiles indicate that rhabdospheres show more similarities with neuronal than with hematopoietic or mesenchymal stem cells. Analysis of these profiles identified the known CSC marker CD133 as one of the genes upregulated in rhabdospheres, both on RNA and protein levels. CD133(+) sorted cells were subsequently shown to be more tumorigenic and more resistant to commonly used chemotherapeutics. Using a tissue microarray (TMA) of eRMS patients, we found that high expression of CD133 correlates with poor overall survival. Hence, CD133 could be a prognostic marker for eRMS. These experiments indicate that a CD133(+) CSC population can be enriched from eRMS which might help to develop novel targeted therapies against this pediatric tumor.     

Effect of CD44 Binding Peptide Conjugated to an Engineered Inert Matrix on Maintenance of Breast Cancer Stem Cells and Tumorsphere Formation

Introduction

As cancer cells are affected by many factors in their microenvironment, a major challenge is to isolate the effect of a specific factor on cancer stem cells (CSCs) while keeping other factors unchanged. We have developed a synthetic inert 3D polyethylene glycol diacrylate (PEGDA) gel culture system as a unique tool to study the effect of microenvironmental factors on CSCs response. We have reported that CSCs formed in the inert PEGDA gel by encapsulation of breast cancer cells maintain their stemness within a certain range of gel stiffness. The objective was to investigate the effect of CD44 binding peptide (CD44BP) conjugated to the gel on the maintenance of breast CSCs.

Methods

4T1 or MCF7 breast cancer cells were encapsulated in PEGDA gel with CD44BP conjugation. Control groups included dissolved CD44BP and the gel with mutant CD44BP conjugation. Tumorsphere size and density, and expression of CSC markers were determined after 9 days. For in vivo, cell encapsulated gels were inoculated in syngeneic Balb/C mice and tumor formation was determined after 4 weeks. Effect of CD44BP conjugation on breast CSC maintenance was compared with integrin binding RGD peptide (IBP) and fibronectin-derived heparin binding peptide (FHBP).

Results

Conjugation of CD44BP to the gel inhibited breast tumorsphere formation in vitro and in vivo. The ability of the encapsulated cells to form tumorspheres in the peptide-conjugated gels correlated with the expression of CSC markers. Tumorsphere formation in vitro was enhanced by FHBP while it was abolished by IBP.

Conclusion

CD44BP and IBP conjugated to the gel abolished tumorsphere formation by encapsulated 4T1 cells while FHBP enhanced tumorsphere formation compared to cells in the gel without peptide. The PEGDA hydrogel culture system provides a novel tool to investigate the individual effect of factors in the microenvironment on CSC maintenance without interference of other factors.     

Aggressive Phenotype of Cells Disseminated via Hematogenous and Lymphatic Route in Breast Cancer Patients

Intratumoral heterogeneity of breast cancer remains a major challenge in successful treatment. Failure of cancer therapies can also be accredited to inability to systemically eradicate cancer stem cells (CSCs). Recent evidence points to the role of epithelial-mesenchymal transition (EMT) in expanding the pool of tumor cells with CSCs features. Thus, we assessed expression level as well as heterogeneity of CSCs markers in primary tumors (PT), lymph node metastasis (LNM), and circulating tumor cells (CTCs)–enriched blood fractions in order to correlate them with signs of EMT activation as well as clinicopathological data of breast cancer patients. Level of CSCs markers (ALDH1, CD44, CD133, OCT-4, NANOG) and EMT markers was quantified in PT (N=107), LNM (N=56), and CTCs-enriched blood fractions (N=85). Heterogeneity of CSCs markers expression within each PT and LNM was assessed by calculating Gini Index. Percentage of ALDH1-positive cells was elevated in PT in comparison to LNM (P = .005). However, heterogeneity of the four CSCs markers: ALDH1 (P = .019), CD133 (P = .009), OCT-4 (P = .027), and CD44 (P < .001) was decreased in LNM. Samples classified as mesenchymal (post-EMT) showed elevated expression of CSCs markers (OCT-4 and CD44 in PT; OCT-4 in LNM; ALDH1, OCT-4, NANOG, CD44 in CTCs). Patients with mesenchymal-like CTCs had worse prognosis than patients with epithelial-like or no CTCs (P = .0025). CSCs markers are enriched in PT, LNM, and CTCs with mesenchymal features, but their heterogeneity is decreased in metastatic lymph nodes. Mesenchymal CTCs phenotype correlates with poor prognosis of the patients.