OVER THE PAST DECADE, evidence that stem cells could become malignant and that only certain cancer cells shared a variety of traits with stem cells strengthened the idea that the driving force underlying tumor growth might be a subpopulation of stemlike cancer cells. The theory has a longer history, but in the past the technology to prove it was lacking.
By the 1960s a few scientists were already beginning to note that groups of cells within the same tumor differed in their ability to produce new tumor tissue. In 1971 C. H. Park and his colleagues at the University of Toronto showed that within a culture of cells taken from an original, or “primary,” myeloma (a cancer affecting plasma cells in bone marrow), the cells displayed significant differences in their ability to proliferate. At the time, Park's group could not interpret this phenomenon decisively, because at least two explanations were possible: all the cells might have had the ability to multiply in culture but by chance only some of them did, or else a hierarchy of cells was present in the tumor and cancer stem cells were giving rise to cells that were nontumorigenic, or incapable of proliferation.
Philip J. Fialkow of the University of Washington had already demonstrated in 1967 that the stem cell model was probably the correct one for leukemia. Using a cell-surface protein marker called G-6-PD, which can identify a cell's lineage, Fialkow showed that in some women with leukemia, both the tumorigenic cells as well as their more differentiated nontumorigenic progeny had all arisen from the same parent cell.
These early studies were critical in the development of the stem cell model for cancer, but they were still limited by researchers' inability to isolate and examine different cell populations within a tumor. A key event in stem cell biology, therefore, was the commercial availability, beginning in the 1970s, of an instrument called a flow cytometer, which can automatically sort different living cell populations based on the unique surface markers they bear.
A second crucial event in the evolution of cancer stem cell studies was the advent during the 1990s of conclusive tests for self-renewal. Assays to establish self-renewal in human cells did not exist until Weissman of Stanford and John E. Dick of the University of Toronto developed methods that allowed normal human stem cells to grow in mice. Using flow cytometry and this new mouse model, Dick began in 1994 to publish a series of seminal reports identifying cancer stem cells in leukemia. In 2003 Richard Jones of Johns Hopkins University identified a cancer stem cell population in multiple myeloma.
Earlier the same year our own laboratory group at the University of Michigan at Ann Arbor had published the first evidence of cancer stem cells in solid tumors. By transplanting sorted populations of cells from human breast tumors into mice, we were able to confirm that not all human breast cancer cells have the same capacity to generate new tumor tissue. Only one subpopulation of the cells was able to re-create the original tumor in the new environment. We then compared the phenotype, or physical traits, of those new tumors with that of the patient samples and found that the profile of the new tumors recapitulated the original. This finding indicated that the transplanted tumorigenic cells could both self-renew and give rise to all the different cell populations present in the original tumor, including the nontumorigenic cells.
Our study attested to the presence of a hierarchy of cells within a breast cancer similar to those identified in blood malignancies. Since then, the investigation of cancer stem cell biology has exploded, as labs across the world continue to find similar subpopulations of tumorigenic cells in other forms of cancer. In 2004, for example, the laboratory of Peter Dirks of the University of Toronto identified cells from primary human central nervous system tumors with the capacity to regenerate the entire tumor in mice. Since then, the phenotypes of cancer stem cell populations for colorectal cancer, lung cancer, melanoma, prostate cancer and pancreatic cancer have been reported.
A related area of recent intensive investigation is also providing support for the cancer stem cell model. The signaling environment, or niche, in which tumors reside appears to strongly influence the initiation and maintenance of malignancy. Studies of normal body cells as well as of stem cells have already established the essential role of signals emanating from surrounding tissue and the supportive extracellular matrix in sustaining a given cell's identity and in directing its behavior. Normal cells removed from their usual context in the body and placed in a dish have a tendency to lose some of their differentiated functional characteristics, for example. Stem cells, in contrast, must be cultured on a medium that provides signals telling them to remain undifferentiated, or they will quickly begin proliferating and differentiating—seemingly as though that is their default programmed behavior, and only the niche signals hold it in check.
In the body, stem cell niches are literal enclaves surrounded by specific cell types, such as stromal cells that form connective tissue in the bone marrow. With a few exceptions, stem cells always remain in their niche and are sometimes physically attached to it by adhesion molecules. Progenitor cells, on the other hand, move away from the niche, often under escort by guardian cells, as they become increasingly differentiated.
The importance of niche signaling in maintaining stem cells' undifferentiated state and in keeping them quiescent until they are called on to produce new cells suggests that these local environmental signals could exert similar regulatory control over cancer stem cells. Intriguing experiments have shown, for example, that when transplanted into a new niche, stem cells predisposed to malignancy because of oncogenic mutations will nonetheless fail to produce a tumor. Conversely, normal stem cells transplanted into a tissue environment that has been previously damaged by radiation do give rise to tumors.
Many of the same genetic pathways identified with signaling between stem cells and their niche have been associated with cancer, which also suggests a role for the niche in the final transition to malignancy. For example, if malignant stem cells were being held in check by the niche but the niche was somehow altered and expanded, the malignant stem cell pool would have room to grow as well. Another possibility is that certain oncogenic mutations within cancer stem cells could permit them to adapt to a different niche, again letting them increase their numbers and expand their territory. Still a third alternative is that mutations might allow the cancer stem cells to become independent of niche signals altogether, lifting environmental controls on both self-renewal and proliferation.