Abstract
Summary
Starting from human musculoskeletal sarcomas, we isolated a subset of cells that display cancer stem cell properties. The control of culture conditions is crucial to enhance the isolation of this cell population.
Introduction
Cancer stem cells (CSCs) have emerged as the real responsible for the development, chemoresistance, and metastatic spread of different human cancers, including musculoskeletal sarcomas. However, unlike most leukemias and solid tumors, so far, data on musculoskeletal sarcomas refer to CSCs obtained from established cell lines, and only a few authors have reported on the isolation of CSCs from tissue samples [1-7]. Reasonably due to some peculiar features of mesenchymal tumors, including the lack of unique surface markers that identify tumor progenitors, there are still partial clues on the existence of a CSC population in these cancers. Here, we report the identification of putative CSCs in musculoskeletal sarcomas using the most general accepted isolation method, the sphere culture system. Accordingly to recent reports, we also analyzed the effects of reduced oxygen availability on the behavior of sarcoma CSCs.
Patients & Methods
Between 2009 and 2012, we collected fresh tissue samples from 49 patients (25 males and 24 females, age 6–85 yr) with musculoskeletal sarcomas. Cells obtained from samples were cultured in anchorage-independent serum-starved conditions, in the presence of adequate growth factors, until the formation of floating spheres, here called ‘sarcospheres’. To obtain parental tumor cell cultures, single cells obtained from biopsies were in parallel seeded in anchorage-dependent conditions, in the presence of fetal bovine serum until the formation of cell monolayers. The obtained sarcospheres were characterised in terms of gene expression and in vivo tumorigenic potential. We then exposed sarcospheres obtained from a rhabdomyosarcoma model (RD cells) to a hypoxic environment (1% O2), and analyzed their growth and gene expression to that of sarcospheres grown at standard 21% O2.
Results
Using a sphere-forming assay, we established sphere cultures in 5 out of 49 cases (10.2 %). All sarcosphere cultures expressed consistent mRNA levels for OCT3/4, Nanog, and SOX2. CSCs from a chondrosarcoma and from a rhabdomyosarcoma also showed the ability to recapitulate the original tumor morphology in a mouse model. Finally, we observed that hypoxia induced a significant increase of the number and size of CSCs from RD.
Discussion/Conclusion
Starting from human sarcoma biopsies and established cell lines, we were able to characterise the CSC subset of musculoskeletal sarcomas, that were isolated through the sphere system assay. These cells had stem-like properties, and showed in vivo tumorigenic ability. We also observed that exposure of CSCs to low oxygen conditions increased the number and size of spheres and the expression of stem cell-related markers, suggesting that the culture in hypoxic conditions could improve the yield of the isolation method here used, and that the oxygen availability is a crucial element in the physiological maintenance of CSCs of musculoskeletal sarcomas.
