Cancer Stem Cells in Sarcoma

An ESUN Editorial by C. Parker Gibbs, MD

Historically, all cancer cells within a tumor have been assumed to be created equal, derived from a single cancerous cell of origin that divides over and over again producing identical daughter cells that together make up the mass of the tumor. As all of the cells within the tumor come from a single parent cell (clonal), it was felt that each cell had the same characteristics including the capacity to proliferate and metastasize. However, it is also known that in many cancers, cells look and behave differently both within and between individual tumors. One theoretical explanation for this apparent contradiction is that the original cancer cell has the capacity to generate not only identical offspring as in most cells, but offspring of differing appearance and function, much like a stem cell.

The cancer stem cell theory holds that there is a small sub-population of cells within a tumor, which like normal stem cells, has the ability to self-renew. These few cells can divide asymmetrically, producing an identical daughter stem-like cell and a more differentiated cell which upon subsequent divisions generates the vast majority of the tumor bulk, which is essentially benign. This stem-like cell is responsible for initiating and maintaining the growth of the tumor, and if not completely eradicated by surgical extirpation or chemotherapy, is responsible for local and distant recurrence.¹ Along these lines, Weismann, drawing parallels between cancer stem cells and normal stem cells, has suggested that tumorigenesis can be viewed as aberrant organogenesis.² Both types of cells self-renew; proliferate extensively, and give rise to heterogeneous tissue. However, normal stem cells do so in a tightly regulated fashion, and cancer stem cells in a dysregulated manner. As cancer is felt to be of clonal origin, the cancer stem cell also must be capable of generating diverse offspring of both indefinite and limited proliferative potential.³ Implicating the involvement of stem-like cells in tumorigenesis, many tumors reflect functional and physical aspects of the tissue from which they purportedly arose. Osteosarcoma is a typical example in that it generates a matrix that is clearly recognizable as bone under the microscope as well as radiographic examination. However, it is also abundantly clear that this bone is aberrant in both form and function.

The first definitive work describing a cancer stem cell was performed by John Dick and colleagues in studies of acute myeloid leukemia (AML).⁴ They identified a rare population of human SCID leukemia initiating cells that were able to propagate AML in an immunocompromised mouse model. The leukemic grafts generated were representative of the patients' original disease phenotype. They demonstrated that the human AML stem cells purified from patient samples were CD34+,CD38-, resembling the normal hematopoietic stem cell phenotype. Cells from the CD34+,CD38+ fraction could not transfer the disease despite having a leukemic blast phenotype. Others have subsequently implicated stem-like cells in the pathogenesis of brain and breast malignancies suggesting a broader involvement of stem cells in carcinogenesis.^5-9 More recently, others have identified tumor initiating cells in both brain and breast malignancies using antibodies to the purported stem cell markers CD44 and CD133.

Whether these cells represent "stem cells gone bad" or cancer cells that have acquired some of the molecular machinery of stem cells to facilitate their tumor initiating capacity is a matter of some debate. Originally, it was felt that a cancer stem cell was derived from an adult stem cell that had acquired cancer causing mutations. Support for this came from two observations. One, most tumor initiating cells have been identified by cell surface markers seen in adult stem cells. Two, stem cells, unlike more differentiated cells, are long lived and therefore more likely to experience mutations over time. Although this seems a reasonable argument for adult tumors such as breast cancer, it does not seem to hold up as well in sarcoma. Most of the evidence for a tumor initiating cell in sarcoma, has come from studies in osteosarcoma and more recently Ewing’s sarcoma.^10-12 Both of these are tumors of childhood and adolescence thus mitigating the long lived argument. In the case of Ewing’s sarcoma (and several other malignancies), highly tumorigenic cells have been identified by the expression of the stem cell surface marker CD133. However, the molecular function of this blood stem cell marker in Ewing’s sarcoma is unknown. Unlike almost all other cancer stem cells, no cell surface markers have been found to identify the Osteosarcoma stem cell. However two groups have identified highly tumorigenic populations in this disease using biologic functional assays. Wu et.al. found a population of cells in Osteosarcoma that formed tumors more readily in mice identified by their ability, when treated with a certain toxic dye, to actively pump this out of the cell.¹² More recently, Levings et. al. identified an Osteosarcoma stem cell by its ability to activate the Oct-4 gene promoter, a function normally restricted to the most primitive embryonic stem cells. Interestingly, Oct-4 is one of a small group of genes recently shown that when inserted into a normal skin cell can convert that cell back to a primitive stem-like cell.¹³ They hypothesized that the tumorigenic cell of Osteosarcoma is not derived from an embryonic stem cell as might be suggested by the activation of the Oct-4 promoter, but is able to reactivate parts of the stem cell molecular machinery thus providing them with stem-like traits.¹¹ This logic is supported by the observation that most solid tumors, to a greater or lesser degree, resemble the tissues from which they arose, and not a tissue from a disparate cell lineage as might be the case if derived from a true stem cell. For example, colon cancer tissue can be recognized as glandular tissue reminiscent of normal colonic epithelium and osteosarcoma makes disorganized bone and its unmineralized precursor, osteoid.

Regardless of whether cancer stem cells are derived from a true stem cell or have acquired the ability to reactivate primitive stem cell programs, it is becoming clearer that they likely play a critical role in the origin, propagation, and persistence of sarcomas. The existence of this subpopulation of cells may very well be responsible for the failure of current anti-sarcoma therapies. Further studies to better identify them and understand the molecular basis of their activity are needed to develop novel therapies targeted against this critical subset of cancer cells. Two such studies in Ewing’s sarcoma and Neurosarcoma are currently being funded by the Liddy Shriver Sarcoma Initiative.^14,15