The role of mesenchymal stem/progenitor cells in sarcoma: update and dispute

Introduction

Sarcomas are biologically and clinically heterogeneous malignant connective tissue tumors arising from mesenchymal or ectodermal tissues. They often harbor relatively specific genetic aberrations, the recognition of which can be used as a diagnostic tool as well as a potential prognostic/predictive marker (1,2). For instance, the fusion of PAX3-FOXO1 in ARMS, EWS-FLI1 in Ewing sarcoma, COL1A1-PDGFB fusion in dermatofibrosarcoma protuberans (DFSP) and SYT-SSX in synovial sarcoma can be used as diagnostic makers (3). Mutations in key genes and signaling pathways such as C-KIT, PDGFRA and BRAF have been targeted by specific drugs such as imatinib and vemurafenib (1,2). Co-amplified oncogenes cyclin-dependent kinase 4 (CDK4) and MDM2 can serve as confirmatory diagnostic markers and as potential pharmacological targets in well-differentiated and dedifferentiated liposarcomas (4).

Based on the multiple histological morphology and genetic characteristics, sarcomas have been divided into a broad spectrum of subtypes recognized in the 2013 WHO classification of tumors (3). Despite conventional multi-modality treatment approaches (surgery, chemotherapy, and radiation therapy), sarcoma patients have disproportionally higher rates of morbidity and mortality than those with other cancers. Investigations into the biology of sarcoma resistance to therapy and sarcoma relapses have resulted in the development of the mesenchymal stem/progenitor cell (MSC) hypothesis (5). Investigating the possible relationship between the MSCs and sarcoma will gain a better understanding of sarcoma biology. Moreover, these studies might provide better opportunities to discover novel treatment strategies.

Correlation of MSCs, cancer stem cell (CSCs) and tumor-initiating cells (TICs)

Increasing evidence suggests that MSCs might be the TICs capable of initiating sarcomagenesis (6-9), although there is still great controversy about the nature and relation of MSCs, CSCs and TICs. There are also studies supporting that sarcomas could represent good examples of the CSC model and these sarcoma CSCs display MSCs properties (10,11). CSCs that display tumor re-initiating properties have been recently identified in chondrosarcoma (9), osteosarcoma (8), Ewing’s sarcoma (12) and synovial sarcoma (13). These CSCs are characterized by expression of markers OCT3/4, NANOG and SOX2 (5,8,9,14). These CSCs are also able to self-renew and able to sustain the tumor in the serial transplantation experiments (5,8,9,14). More importantly, many of these CSCs express MSC markers and retain MSC differentiation properties in vitro (13). In addition, these MSC-like CSCs are associated with drug resistance and metastasis, which might be responsible for the relapse of sarcomas (8,15-17). Therefore, MSCs may not only be the TICs in sarcomas, but also a population of altered CSCs which are responsible for maintaining tumor growth and initiating tumorigenesis upon serial transplantation.

Important factors involved in the MSC transformation

Transformation of MSCs has been achieved by several methods including knockout of tumor suppressor genes, overexpression of oncogenes and drug administration to affect signaling pathways (6).

During the transformation process there are remarkable deregulations of the tumor suppressor genes and signaling pathways. For instance, in mouse adipose derived MSCs (ADSC), the loss of tumor suppressor P21 and TP53 could induce in vitro transformation and so-called fibrosarcoma formation in vivo after transplantation (18). In another study, both Tp53^–/–Rb^–/– and Tp53^–/– mouse ADSCs were generated and leiomyosarcoma-like tumors were developed in the in vivo tumorigenicity assays of these two types of mouse ADSCs (19). Furthermore, the combination of Cdkn2a loss and C-myc overexpression in mouse MSCs induced osteosarcomas accompanied by the loss of adipogenic differentiation capacity (20).

In addition to directly targeting in vitro cultured MSCs, the genetically engineered mouse models have been used to explore the roles of such genes in the MSC transformation. In P53-deficient mice, many types of sarcomas occurred in the mesenchymal cells of limb buds, which osteosarcoma was the most common type (21). These induced transformation studies established the importance of the P53 and P53 pathway in preventing mouse MSC transformation (22,23).

Additionally, upregulated oncogenic pathways also can induce or potentiate mouse MSC transformation. For instance, the Fos overexpression transgenic mice resulted in the development of bone tumors, with chondrosarcomas as the main type (24). In mice, overexpression of K-ras in addition to P53 loss induced sarcoma formation more efficiently than with P53 loss alone (25). These studies evaluate the roles of different oncogenic pathways in mouse MSC transformation.

Similarly, most studies of human MSC transformation are also based on genetic methods to knock out important tumor suppressor genes and overexpress certain oncogenes, such as the exogenous expression of hTERT (11,26-28). Consistent with MSC studies in mice, the disruption of P53 and RB pathways are also important for human MSC transformation. For instance, the introduction of SV40-LT, which perturbs both P53 and RB proteins, potently promoted human MSC transformation (26). Furthermore, the overexpression of some oncogenes such as H-RAS has also been shown to contribute to the transformation (26). In all, the deregulation of some important signal pathways including P53 pathway, RB pathway, PI3K-AKT pathway, WNT/β-catenin signaling pathway and MAPK pathway might be involved in the MSC transformation and sarcoma formation (29).

It is worth noting that overexpression of sarcoma-specific fusion proteins in mMSCs and mouse models could reproduce several sarcomas (13,14,19,30-36). Ewing’s sarcoma, MLS, ARMS and synovial sarcoma have been reproduced upon expression of EWS-FLI-1, FUS-CHOP, PAX-FKHR and SYT-SSX, respectively (13,14,19,30-36). For instance in mouse MSCs the exogenous expression of the fusion gene EWS-FLI1 alone could transform these MSCs, which are characterized by in vitro immortalization and in vivo sarcomatous tumor formation, even a secondary genetic alteration was needed (33,34). Similarly, human MSCs with exogenous EWS-FLI1 expression were transformed and expressed neuroectodermal markers (35). On the contrary, the knockdown of EWS-FLI1 expression in Ewing sarcoma cell lines restored the in vitro trilineage differentiation ability of the cells (12). In a transgenic mouse model, by expressing EWS-FLI1 gene specifically in the mesoderm-originated tissues in limbs and simultaneous Tp53 knockout, sarcomas with similar characteristics as Ewing sarcoma occurred (36).

Functional and descriptive assays of MSCs

The stem cell assays including functional and descriptive assays are used to characterize MSCs. The functional assays include the colony-forming assay, tumorigenesis, dye efflux, chemoresistance and differentiation. The descriptive assays include the expression of stem cell genes, aldehyde dehydrogenase activity (ALDH), and side population (SP) (5,7,10,11,17,37,38). Approaches used to identify both normal stem cells and MSC populations include cell surface markers, ALDH, SPs etc. (7,37,38).

MSCs are believed to have an increased ability to form colonies from a single cell and the colony-forming assays are the most commonly used methods although there are several issues about this method (39). The general “gold standard” characteristic of MSCs is the ability to grow serially transplantable tumors in immunodeficient mice (40). ATP-binding cassette (ABC) transporter efflux of DNA-binding dyes is also generally used as an indicator of stem cell properties (41). Somewhat related to drug efflux, chemotherapy resistance is also considered as a hallmark of MSCs (17). Differentiation into mesenchymal cell types such as osteoblasts, adipocytes, and chondrocytes is typically evaluated, based on the theory that a MSC has undergone malignant transformation (42).

Expression of so-called “stem cell genes” such as CD20, CD24, CD34, CD44, CD90, CD117, CD133, OCT4, SOX2, and NANOG is used as markers of MSCs (43,44). High activity of ADLH may play a role in early differentiation of stem cells through oxidizing retinol to retinoic acid and can confer resistance to chemotherapeutic agents such as cyclophosphamide (45,46). Cells termed the “SP” due to its position in the flow cytometry plot were found to have many of the characteristics of stem cells (47). Several groups have used SP as a sign of “stemness” to support the idea that the marker they were studying identified stem cells (38,41).

The MSC origin of sarcomas

Osteosarcoma

Different studies have supported the MSC origin of osteosarcoma (20,22). Tsuchida and colleagues firstly reported that SP cells isolated from the osteosarcoma cell line by exposure of the HOS osteosarcoma cell line to cisplatin showed stem cell-like properties (48). Tirino et al. evaluated the utility of CD133 expression to identify and isolate a subpopulation of cells with stem cell properties in osteosarcoma cell lines. Examination of osteosarcoma cell lines identified a subpopulation of CD133⁺ cells which exhibited self-renewal properties, higher proliferative rates, spherical colony formation, and expression of the stem cell-associated gene OCT3/4 (49,50). Wang et al. demonstrated a subpopulation with high ALDH activity in the osteosarcoma cell line OS99-1 (51). These cells were able to grow xenografts in NOD/SCID mice, and showed characteristic CSC features including self-renewal, ability to produce differentiated progeny, and increased expression of the stem cell genes OCT3/4A, NANOG, and SOX-2 (51). Most importantly, Adhikari and colleagues discovered that mouse and human osteosarcoma cell lines positive for both CD117 and Stro-1 (two MSC markers) demonstrated typical stem cell characteristics (8). Validation of the stem cell-like properties would further strengthen the utility of CD117 and Stro-1 marker expression as specific markers for the identification and isolation of MSC population in osteosarcoma.

Ewing sarcoma

Suva and colleagues were the first to report on the isolation of CD133⁺ cells derived from primary Ewing sarcoma tumors. They showed that these CD133⁺ cells demonstrated abilities of initiating and forming tumors in NOD/SCID mice and recapitulating the parental tumor phenotype (5,52). The CD133⁺ Ewing’s sarcoma also expressed significantly higher levels of the stem cell genes OCT4, SOX2, and NANOG. Jiang and colleagues investigated the expression of CD133 in 48 primary Ewing’s tumors and cell lines and found that most of them had very low or absent expression of the CD133^– encoding gene PROM1 while 4 cases had overexpression of PROM1. Of these four cases with overexpression of PROM1, two were found to have quickly developed a chemoresistant tumor while the other two were long-term survivors after receiving chemotherapy (53). These results suggest that heterogeneity of CD133 expression in Ewing’s sarcomas and a variable prognostic impact of the level of CD133 expression.

The utility of SP cells to identify MSCs in Ewing sarcoma has also been explored. Yang and colleagues isolated the SP fraction from the Ewing sarcoma cell line SK-ES-1 and showed that the SP cells exhibited increased stem cell features such as clonogenicity, invasive behavior, and cytotoxic drug resistance (54). Isolation of cells with high ALDH activity has also shown promise in identifying cell populations in Ewing sarcoma enriched for MSCs. Awad and colleagues examined Ewing sarcoma cell lines and patient-derived Ewing sarcoma xenograft tumors and found that the cells from the ALDH^High sub-population demonstrated stem cell properties including highly tumorigenic and more resistant to the chemotherapy (55).

Rhabdomyosarcoma

Similar to Ewing sarcoma, the PAX3/FOXO1and PAX7/FOXO1 have classically characterized a more clinically aggressive subset of RMS referred to as the alveolar variant. Komuro and colleagues showed that RMS possessed SP cells based on Hoechst 33342 dye exclusion (56). More recently, Walter and colleagues demonstrated that some RMS cell lines possessed stem cell properties including elevated expression of stem cell markers (OCT-4, NANOG, c-MYC, SOX2, and PAX3) and formed tumors at lower cell densities compared to adherent cells (57). The investigators further sorted the cells and isolated a CD133⁺ cell fraction which showed typical stem cell features. Furthermore, this study also suggested that CD133 expression in RMS may not only be useful in enriching for candidate sarcoma stem cells, but may also have clinical utility in predicting survival outcomes (53,57).

Synovial sarcoma

In synovial sarcoma, exogenous expression of SYT-SSX2 fusion gene in the skeletal-muscle-specific Myf5 expressing lineage induced the formation of synovial sarcomas in vivo (58). Additionally, in primary synovial sarcoma cells the fusion gene silencing restored both the trilineage differentiation capacity and the MSC marker expression, which strongly suggests the cells of MSC lineage was the origin of synovial sarcoma (13). Terry and Nielsen have shown subpopulations of CD133 expressing cells in primary synovial sarcomas and synovial sarcoma cell lines (59).

Other sarcomas

In chondrosarcoma, a study found that less differentiated tumors were shown to have more similarity with MSCs of pre-chondrogenic stages, while more differentiated tumors share more similarity with fully differentiated chondrocytes (60). This suggests that chondrosarcoma progression probably parallels deregulated chondrocytes differentiation process of MSCs (60,61). Additional, chondrosarcoma cells derived from primary tumors and enriched for CD133, which showed potent stem cell potential (50).

Liposarcoma is also found in MSC origin. In a mouse model of liposarcoma, where FUS-CHOP was able to induce liposarcoma genesis in MSCs, whereas no liposarcoma was formed when FUS-CHOP gene was manipulated to be only expressed in differentiated adipocytes. This study also evaluated the exact cell status as a crucial factor in sarcomagenesis (62,63). Stratford and colleagues have identified candidate sarcoma stem cells in a liposarcoma cell line (SW872) after double enrichment for CD133 and ALDH^Highactivity (ALDH^High) and these cells were more clonogenic and tumorigenic (64). Clear cell sarcoma and other subtype sarcomas were also characterized with MSC origin (65,66).

Therapeutic role of MSCs in sarcomas

Although still in infancy and very controversial, recent work indicates that MSCs could have therapeutical implications in sarcoma. The potential of MSCs for cell-based therapies relies on several key properties such as capacity to differentiate into several cell lineages, lack of immunogenicity, immunomodulatory properties, robust ex vivo expansion potential, ability to secrete factors and homing ability to damaged tissues and tumor sites (67,68). Furthermore, MSCs are resistant to chemotherapy-induced apoptosis, and contribute to generating drug resistance and resistant to ionizing radiation in tumor cells (69-72). Due to these properties, MSCs have considerable therapeutic potential in a variety of clinical applications in several disease processes, including cardiovascular disease, as well as in human malignancies (67-69,73).

In a mouse model using Ewing’s sarcoma, the beneficial effect of MSCs was related to their ability to locate and migrate to the tumors and deliver interleukin-12 (74). Survival advantage was shown in Ewing’s sarcoma patients treated with autologous stem cell transplantation (75). Furthermore human MSCs were reported to exert anti- tumorigenic effects in a model of Kaposi’s sarcoma (73). This effect is mediated by direct cell contact leading to the inhibition of Akt activation within KS cells (73). The result suggests that in contrast to other stem cells or normal stromal cells, MSCs possess intrinsic antineoplastic properties and that this stem cell population might be of particular utility for treating those human malignancies characterized by dysregulated Akt (73).

At the same time, the safety issues using MSCs in clinical settings should be paid with more attention (76). This is mainly because MSCs were reported to promote growth and pulmonary metastasis of osteosarcoma and also provide a niche for cancer metastasis in breast cancer (77,78). Also sarcoma formation in bone marrow recipients treated for unrelated diseases has been also reported (79) and highly unexpected osteosarcoma recurrence was related to an autologous fat graft (80). Moreover, cross-contamination of human MSCs with established cancer cell lines was very recently reported (81). All these issues indicate that there is a long way to go.

Debates about the origin of sarcoma tumor cell

The exact cell of origin for sarcomas has not been conclusively identified yet. By now two main models have been conceptualized to support the MSCs as the cell of origin for sarcomas. The first theory presumes that sarcoma is a differentiation disease, caused by mutations hampering terminal differentiation of MSCs (82). The second theory argues that sarcoma is more likely to originate from a primitive MSC than a differentiated one, and the primitive MSC acquires relevant mutations, which directs sarcoma genesis (26).

As for the first theory, different sarcomas could be initiated depending on the mutations according to different lineage and different stage of differentiation (7). This hypothesis is suggested to explain the variable subtypes of osteosarcoma (83). In addition, most powerful evidence supporting this theory is based on studies which show overlap of the gene expression signatures of differentiation stages of MSCs from sarcomas and normal tissues. For instance, differentiated chondrosarcoma was shown to share similarities with fully differentiated chondrocytes while less differentiated chondrosarcoma showed overlap with prechondrogenic stages of MSCs (7,60,84). Similar studies were found in leiomyosarcoma, pleomorphic/dedifferentiated liposarcoma (7,85).

However, this hypothesis is debated because the overlap of the signature might be due to the similar stroma of the different tissues (26,86). Therefore, another hypothesis has been suggested, which considers that sarcoma is more likely to originate from a primitive MSC rather than a differentiated MSC (26). Evidences supporting this hypothesis come from the experimental spontaneous malignant transformation of human and murine MSCs (87,88). Others also have shown that targeting deletion or expression of certain genes in MSCs indeed induce sarcoma formation (76,89). This hypothesis has been confirmed in depth in Ewing’s sarcoma where the expression of EWS-FLI-1 chimeric gene in human MSCs induces Ewing’s sarcoma formation (31). Similarly, transformation of MSCs and leiomyosarcoma formation were initiated by deleting p53 in certain human MSCs (22). Other studies have also indicated MSCs transformation resulting into other subtypes of sarcoma (9,22).

All these studies do not provide a direct evidence for the transformation of different sarcoma subtypes from MSCs. It is more acceptable that some sarcomas originate from mutated MSCs which are vulnerable for subsequent mutations. Depending on the impact of the initial and/or subsequent additional mutations and/or different environmental factors, the MSCs may differentiate into different sarcoma subtypes (90).

Summary

Osteosarcoma, ewing sarcoma, rhabdomyosarcoma, synovial sarcoma, chondrosarcoma, liposarcoma and other sarcomas are found shown MSC origin. It is more acceptable that sarcomas originate from mutated MSCs which are vulnerable for subsequent mutations. Transformation of MSCs can be achieved by several methods including knockout of tumor suppressor genes, overexpression of oncogenes and drug administration to affect signaling pathways. The potential of MSCs for cell-based therapies relies on several key properties such as capacity to differentiate into several cell lineages, lack of immunogenicity, immunomodulatory properties, robust ex vivo expansion potential, ability to secrete factors and homing ability to damaged tissues and tumor sites. Due to these properties, MSCs have considerable therapeutic potential in a variety of clinical applications in human malignancies. These recent accomplishments have broadened our knowledge of the role of MSCs in sarcomas.

Acknowledgements

Funding: This work was partly supported by the National Nature Science Foundation of China (No. 81372872 to J Yang, No. 81402215 to X Du and No. 81320108022 to K Chen) and funds from the University Cancer Foundation via the Sister Institution Network Fund (SINF) (to J Yang).

Footnote

Conflicts of Interest: The authors have no conflicts of interest to declare.

References

1. Hameed M. Molecular diagnosis of soft tissue neoplasia: clinical applications and recent advances. Expert Rev Mol Diagn 2014;14:961-77. [PubMed]
2. Taylor BS, Barretina J, Maki RG, et al. Advances in sarcoma genomics and new therapeutic targets. Nat Rev Cancer 2011;11:541-57. [PubMed]
3. Fletcher CD, Bridge JA, Hogendoorn P, et al. eds. WHO Classification of Tumours of Soft Tissue and Bone. 1 ed. Lyon: IARC; 2013.
4. Barretina J, Taylor BS, Banerji S, et al. Subtype-specific genomic alterations define new targets for soft-tissue sarcoma therapy. Nat Genet 2010;42:715-21. [PubMed]
5. Dela Cruz FS. Cancer stem cells in pediatric sarcomas. Front Oncol 2013;3:168. [PubMed]
6. Xiao W, Mohseny AB, Hogendoorn PC, et al. Mesenchymal stem cell transformation and sarcoma genesis. Clin Sarcoma Res 2013;3:10. [PubMed]
7. Mohseny AB, Hogendoorn PC. Concise review: mesenchymal tumors: when stem cells go mad. Stem Cells 2011;29:397-403. [PubMed]
8. Adhikari AS, Agarwal N, Wood BM, et al. CD117 and Stro-1 identify osteosarcoma tumor-initiating cells associated with metastasis and drug resistance. Cancer Res 2010;70:4602-12. [PubMed]
9. Levings PP, McGarry SV, Currie TP, et al. Expression of an exogenous human Oct-4 promoter identifies tumor-initiating cells in osteosarcoma. Cancer Res 2009;69:5648-55. [PubMed]
10. Visvader JE. Cells of origin in cancer. Nature 2011;469:314-22. [PubMed]
11. Rodriguez R, Rubio R, Menendez P. Modeling sarcomagenesis using multipotent mesenchymal stem cells. Cell Res 2012;22:62-77. [PubMed]
12. Tirode F, Laud-Duval K, Prieur A, et al. Mesenchymal stem cell features of Ewing tumors. Cancer Cell 2007;11:421-9. [PubMed]
13. Naka N, Takenaka S, Araki N, et al. Synovial sarcoma is a stem cell malignancy. Stem Cells 2010;28:1119-31. [PubMed]
14. Riggi N, Suva ML, De Vito C, et al. EWS-FLI-1 modulates miRNA145 and SOX2 expression to initiate mesenchymal stem cell reprogramming toward Ewing sarcoma cancer stem cells. Genes Dev 2010;24:916-32. [PubMed]
15. Fujii H, Honoki K, Tsujiuchi T, et al. Sphere-forming stem-like cell populations with drug resistance in human sarcoma cell lines. Int J Oncol 2009;34:1381-6. [PubMed]
16. Liu B, Ma W, Jha RK, et al. Cancer stem cells in osteosarcoma: recent progress and perspective. Acta Oncol 2011;50:1142-50. [PubMed]
17. Honoki K. Do stem-like cells play a role in drug resistance of sarcomas? Expert Rev Anticancer Ther 2010;10:261-70. [PubMed]
18. Rodriguez R, Rubio R, Masip M, et al. Loss of p53 induces tumorigenesis in p21-deficient mesenchymal stem cells. Neoplasia 2009;11:397-407. [PubMed]
19. Rubio R, Garcia-Castro J, Gutierrez-Aranda I, et al. Deficiency in p53 but not retinoblastoma induces the transformation of mesenchymal stem cells in vitro and initiates leiomyosarcoma in vivo. Cancer Res 2010;70:4185-94. [PubMed]
20. Shimizu T, Ishikawa T, Sugihara E, et al. c-MYC overexpression with loss of Ink4a/Arf transforms bone marrow stromal cells into osteosarcoma accompanied by loss of adipogenesis. Oncogene 2010;29:5687-99. [PubMed]
21. Lin PP, Pandey MK, Jin F, et al. Targeted mutation of p53 and Rb in mesenchymal cells of the limb bud produces sarcomas in mice. Carcinogenesis 2009;30:1789-95. [PubMed]
22. Mohseny AB, Szuhai K, Romeo S, et al. Osteosarcoma originates from mesenchymal stem cells in consequence of aneuploidization and genomic loss of Cdkn2. J Pathol 2009;219:294-305. [PubMed]
23. Li H, Fan X, Kovi RC, et al. Spontaneous expression of embryonic factors and p53 point mutations in aged mesenchymal stem cells: a model of age-related tumorigenesis in mice. Cancer Res 2007;67:10889-98. [PubMed]
24. Rüther U, Komitowski D, Schubert FR, et al. c-fos expression induces bone tumors in transgenic mice. Oncogene 1989;4:861-5. [PubMed]
25. Tsumura H, Yoshida T, Saito H, et al. Cooperation of oncogenic K-ras and p53 deficiency in pleomorphic rhabdomyosarcoma development in adult mice. Oncogene 2006;25:7673-9. [PubMed]
26. Li N, Yang R, Zhang W, et al. Genetically transforming human mesenchymal stem cells to sarcomas: changes in cellular phenotype and multilineage differentiation potential. Cancer 2009;115:4795-806. [PubMed]
27. Serakinci N, Guldberg P, Burns JS, et al. Adult human mesenchymal stem cell as a target for neoplastic transformation. Oncogene 2004;23:5095-8. [PubMed]
28. Funes JM, Quintero M, Henderson S, et al. Transformation of human mesenchymal stem cells increases their dependency on oxidative phosphorylation for energy production. Proc Natl Acad Sci U S A 2007;104:6223-8. [PubMed]
29. Hernando E, Charytonowicz E, Dudas ME, et al. The AKT-mTOR pathway plays a critical role in the development of leiomyosarcomas. Nat Med 2007;13:748-53. [PubMed]
30. Charytonowicz E, Cordon-Cardo C, Matushansky I, et al. Alveolar rhabdomyosarcoma: is the cell of origin a mesenchymal stem cell? Cancer Lett 2009;279:126-36. [PubMed]
31. Riggi N, Suva ML, Suva D, et al. EWS-FLI-1 expression triggers a Ewing’s sarcoma initiation program in primary human mesenchymal stem cells. Cancer Res 2008;68:2176-85. [PubMed]
32. Rodriguez R, Rubio R, Gutierrez-Aranda I, et al. FUS-CHOP fusion protein expression coupled to p53 deficiency induces liposarcoma in mouse but not in human adipose-derived mesenchymal stem/stromal cells. Stem Cells 2011;29:179-92. [PubMed]
33. Castillero-Trejo Y, Eliazer S, Xiang L, et al. Expression of the EWS/FLI-1 oncogene in murine primary bone-derived cells Results in EWS/FLI-1-dependent, ewing sarcoma-like tumors. Cancer Res 2005;65:8698-705. [PubMed]
34. Riggi N, Cironi L, Provero P, et al. Development of Ewing’s sarcoma from primary bone marrow-derived mesenchymal progenitor cells. Cancer Res 2005;65:11459-68. [PubMed]
35. Miyagawa Y, Okita H, Nakaijima H, et al. Inducible expression of chimeric EWS/ETS proteins confers Ewing’s family tumor-like phenotypes to human mesenchymal progenitor cells. Mol Cell Biol 2008;28:2125-37. [PubMed]
36. Lin PP, Pandey MK, Jin F, et al. EWS-FLI1 induces developmental abnormalities and accelerates sarcoma formation in a transgenic mouse model. Cancer Res 2008;68:8968-75. [PubMed]
37. Veselska R, Skoda J, Neradil J. Detection of cancer stem cell markers in sarcomas. Klin Onkol 2012;25 Suppl 2:2S16-20.
38. Trucco M, Loeb D. Sarcoma stem cells: do we know what we are looking for? Sarcoma 2012;2012:291705.
39. Franken NA, Rodermond HM, Stap J, et al. Clonogenic assay of cells in vitro. Nat Protoc 2006;1:2315-9. [PubMed]
40. Baiocchi M, Biffoni M, Ricci-Vitiani L, et al. New models for cancer research: human cancer stem cell xenografts. Curr Opin Pharmacol 2010;10:380-4. [PubMed]
41. Challen GA, Little MH. A side order of stem cells: the SP phenotype. Stem Cells 2006;24:3-12. [PubMed]
42. Sell S. Stem cell origin of cancer and differentiation therapy. Crit Rev Oncol Hematol 2004;51:1-28. [PubMed]
43. Schatton T, Frank NY, Frank MH. Identification and targeting of cancer stem cells. Bioessays 2009;31:1038-49. [PubMed]
44. Chambers I. The molecular basis of pluripotency in mouse embryonic stem cells. Cloning Stem Cells 2004;6:386-91. [PubMed]
45. Armstrong L, Stojkovic M, Dimmick I, et al. Phenotypic characterization of murine primitive hematopoietic progenitor cells isolated on basis of aldehyde dehydrogenase activity. Stem Cells 2004;22:1142-51. [PubMed]
46. Chute JP, Muramoto GG, Whitesides J, et al. Inhibition of aldehyde dehydrogenase and retinoid signaling induces the expansion of human hematopoietic stem cells. Proc Natl Acad Sci U S A 2006;103:11707-12. [PubMed]
47. Goodell MA, Brose K, Paradis G, et al. Isolation and functional properties of murine hematopoietic stem cells that are replicating in vivo. J Exp Med 1996;183:1797-806. [PubMed]
48. Tsuchida R, Das B, Yeger H, et al. Cisplatin treatment increases survival and expansion of a highly tumorigenic side-population fraction by upregulating VEGF/Flt1 autocrine signaling. Oncogene 2008;27:3923-34. [PubMed]
49. Tirino V, Desiderio V, d’Aquino R, et al. Detection and characterization of CD133+ cancer stem cells in human solid tumours. PLoS One 2008;3:e3469. [PubMed]
50. Tirino V, Desiderio V, Paino F, et al. Human primary bone sarcomas contain CD133+ cancer stem cells displaying high tumorigenicity in vivo. FASEB J 2011;25:2022-30. [PubMed]
51. Wang L, Park P, Zhang H, et al. Prospective identification of tumorigenic osteosarcoma cancer stem cells in OS99-1 cells based on high aldehyde dehydrogenase activity. Int J Cancer 2011;128:294-303. [PubMed]
52. Suvà ML, Riggi N, Stehle JC, et al. Identification of cancer stem cells in Ewing’s sarcoma. Cancer Res 2009;69:1776-1781. [PubMed]
53. Jiang X, Gwye Y, Russell D, et al. CD133 expression in chemo-resistant Ewing sarcoma cells. BMC Cancer 2010;10:116. [PubMed]
54. Yang M, Zhang R, Yan M, et al. Detection and characterization of side population in Ewing’s sarcoma SK-ES-1 cells in vitro. Biochem Biophys Res Commun 2010;391:1062-6. [PubMed]
55. Awad O, Yustein JT, Shah P, et al. High ALDH activity identifies chemotherapy-resistant Ewing’s sarcoma stem cells that retain sensitivity to EWS-FLI1 inhibition. PLoS One 2010;5:e13943. [PubMed]
56. Komuro H, Saihara R, Shinya M, et al. Identification of side population cells (stem-like cell population) in pediatric solid tumor cell lines. J Pediatr Surg 2007;42:2040-5. [PubMed]
57. Walter D, Satheesha S, Albrecht P, et al. CD133 positive embryonal rhabdomyosarcoma stem-like cell population is enriched in rhabdospheres. PLoS One 2011;6:e19506. [PubMed]
58. Haldar M, Hancock JD, Coffin CM, et al. A conditional mouse model of synovial sarcoma: insights into a myogenic origin. Cancer Cell 2007;11:375-88. [PubMed]
59. Terry J, Nielsen T. Expression of CD133 in synovial sarcoma. Appl Immunohistochem Mol Morphol 2010;18:159-65. [PubMed]
60. Boeuf S, Kunz P, Hennig T, et al. A chondrogenic gene expression signature in mesenchymal stem cells is a classifier of conventional central chondrosarcoma. J Pathol 2008;216:158-66. [PubMed]
61. de Andrea CE, Hogendoorn PC. Epiphyseal growth plate and secondary peripheral chondrosarcoma: the neighbours matter. J Pathol 2012;226:219-28. [PubMed]
62. Pérez-Losada J, Sanchez-Martin M, Rodriguez-Garcia MA, et al. Liposarcoma initiated by FUS/TLS-CHOP: the FUS/TLS domain plays a critical role in the pathogenesis of liposarcoma. Oncogene 2000;19:6015-22. [PubMed]
63. Pérez-Mancera PA, Bermejo-Rodriguez C, Sanchez-Martin M, et al. FUS-DDIT3 prevents the development of adipocytic precursors in liposarcoma by repressing PPARgamma and C/EBPalpha and activating eIF4E. PLoS One 2008;3:e2569. [PubMed]
64. Stratford EW, Castro R, Wennerstrom A, et al. Liposarcoma Cells with Aldefluor and CD133 Activity have a Cancer Stem Cell Potential. Clin Sarcoma Res 2011;1:8. [PubMed]
65. Straessler KM, Jones KB, Hu H, et al. Modeling clear cell sarcomagenesis in the mouse: cell of origin differentiation state impacts tumor characteristics. Cancer Cell 2013;23:215-27. [PubMed]
66. Wu C, Wei Q, Utomo V, et al. Side population cells isolated from mesenchymal neoplasms have tumor initiating potential. Cancer Res 2007;67:8216-22. [PubMed]
67. García-Castro J, Trigueros C, Madrenas J, et al. Mesenchymal stem cells and their use as cell replacement therapy and disease modelling tool. J Cell Mol Med 2008;12:2552-65. [PubMed]
68. Rubio D, Garcia S, De la Cueva T, et al. Human mesenchymal stem cell transformation is associated with a mesenchymal-epithelial transition. Exp Cell Res 2008;314:691-8. [PubMed]
69. Cruet-Hennequart S, Prendergast AM, Barry FP, et al. Human mesenchymal stem cells (hMSCs) as targets of DNA damaging agents in cancer therapy. Curr Cancer Drug Targets 2010;10:411-21. [PubMed]
70. Mueller LP, Luetzkendorf J, Mueller T, et al. Presence of mesenchymal stem cells in human bone marrow after exposure to chemotherapy: evidence of resistance to apoptosis induction. Stem Cells 2006;24:2753-65. [PubMed]
71. Meads MB, Hazlehurst LA, Dalton WS. The bone marrow microenvironment as a tumor sanctuary and contributor to drug resistance. Clin Cancer Res 2008;14:2519-26. [PubMed]
72. Christensen R, Alsner J, Brandt Sorensen F, et al. Transformation of human mesenchymal stem cells in radiation carcinogenesis: long-term effect of ionizing radiation. Regen Med 2008;3:849-61. [PubMed]
73. Khakoo AY, Pati S, Anderson SA, et al. Human mesenchymal stem cells exert potent antitumorigenic effects in a model of Kaposi’s sarcoma. J Exp Med 2006;203:1235-47. [PubMed]
74. Duan X, Guan H, Cao Y, et al. Murine bone marrow-derived mesenchymal stem cells as vehicles for interleukin-12 gene delivery into Ewing sarcoma tumors. Cancer 2009;115:13-22. [PubMed]
75. Fraser CJ, Weigel BJ, Perentesis JP, et al. Autologous stem cell transplantation for high-risk Ewing’s sarcoma and other pediatric solid tumors. Bone Marrow Transplant 2006;37:175-81. [PubMed]
76. Helman LJ, Meltzer P. Mechanisms of sarcoma development. Nat Rev Cancer 2003;3:685-94. [PubMed]
77. Xu WT, Bian ZY, Fan QM, et al. Human mesenchymal stem cells (hMSCs) target osteosarcoma and promote its growth and pulmonary metastasis. Cancer Lett 2009;281:32-41. [PubMed]
78. Karnoub AE, Dash AB, Vo AP, et al. Mesenchymal stem cells within tumour stroma promote breast cancer metastasis. Nature 2007;449:557-63. [PubMed]
79. Berger M, Muraro M, Fagioli F, et al. Osteosarcoma derived from donor stem cells carrying the Norrie’s disease gene. N Engl J Med 2008;359:2502-4. [PubMed]
80. Perrot P, Rousseau J, Bouffaut AL, et al. Safety concern between autologous fat graft, mesenchymal stem cell and osteosarcoma recurrence. PLoS One 2010;5:e10999. [PubMed]
81. Torsvik A, Rosland GV, Svendsen A, et al. Spontaneous malignant transformation of human mesenchymal stem cells reflects cross-contamination: putting the research field on track - letter. Cancer Res 2010;70:6393-6. [PubMed]
82. Hauben EI, Weeden S, Pringle J, et al. Does the histological subtype of high-grade central osteosarcoma influence the response to treatment with chemotherapy and does it affect overall survival? A study on 570 patients of two consecutive trials of the European Osteosarcoma Intergroup. Eur J Cancer 2002;38:1218-25. [PubMed]
83. Tang N, Song WX, Luo J, et al. Osteosarcoma development and stem cell differentiation. Clin Orthop Relat Res 2008;466:2114-30. [PubMed]
84. Bovée JV, Hogendoorn PC, Wunder JS, et al. Cartilage tumours and bone development: molecular pathology and possible therapeutic targets. Nat Rev Cancer 2010;10:481-8. [PubMed]
85. Nielsen TO, West RB, Linn SC, et al. Molecular characterisation of soft tissue tumours: a gene expression study. Lancet 2002;359:1301-7. [PubMed]
86. Wunder JS, Nielsen TO, Maki RG, et al. Opportunities for improving the therapeutic ratio for patients with sarcoma. Lancet Oncol 2007;8:513-24. [PubMed]
87. Tolar J, Nauta AJ, Osborn MJ, et al. Sarcoma derived from cultured mesenchymal stem cells. Stem Cells 2007;25:371-9. [PubMed]
88. Rubio D, Garcia-Castro J, Martin MC, et al. Spontaneous human adult stem cell transformation. Cancer Res 2005;65:3035-9. [PubMed]
89. Calo E, Quintero-Estades JA, Danielian PS, et al. Rb regulates fate choice and lineage commitment in vivo. Nature 2010;466:1110-4. [PubMed]
90. Li Q, Hisha H, Takaki T, et al. Transformation potential of bone marrow stromal cells into undifferentiated high-grade pleomorphic sarcoma. J Cancer Res Clin Oncol 2010;136:829-38. [PubMed]