The identification of highly tumorigenic subpopulations of cells, the cancer stem cells, in solid tumors, has significant implications regarding cancer biology, response to therapy and the development of new cancer treatments. Therapies based upon tumor regression may produce treatments effective against the majority of more differentiated cancer cells while sparing the cancer stem cell subpopulation8
. The development of more effective cancer therapeutics will require the cancer stem cells to be selectively targeted and eliminated. For this strategy to be successful the cancer stem cell cells must be reliably identified and isolated so their characteristics can be studied.
CD44 is a cell surface marker that identifies a subpopulation of cancer cells from HNSCC that are highly tumorigenic. However, the CD44 subpopulation of HNSCC likely contains both non cancer stem cells and cancer stem cells as shown by the need to inject in the order of 5×103
cells to produce a tumor in the animal model17
. In other cancers combinations of cell surface markers have been used successfully to isolate cancer stem cells, and smaller numbers of the isolated cells have been shown to produce tumors in an animal model suggesting this methodology is capable of isolating the cancer stem cells more selectively. These findings indicated a need to identify a more selective single marker or combination of markers for cancer stem cells in HNSCC.
Wicha recently demonstrated that ALDH expression can be used to identify breast cancer stem cells, and that injections of small numbers of these cells produce tumors in an animal model26
. Interestingly the normal breast stem/progenitor cells also have high expression of ALDH strongly supporting the concept that stem and progenitor cells are the targets of malignant transformation in breast cancer26
We have shown here that ALDH expression isolates a subpopulation of cancer cells from HNSCC that are highly tumorigenic. The ALDH positive cells are able to produce tumors at a 10 fold reduction over that which was possible with CD44+ cells alone17
. The tumors grown from ALDHhigh
cells recreate the original tumor heterogeneity and histology and the ALDHhigh
cells can be passaged in the animal model fulfilling the requirements for CSC phenotype. This data along with the recent report by Chen et. al. strongly supports the use of ALDH expression as a method to select cancer stem cells in HNSCC27
As expected the ALDH subpopulation of HNSCC cells mainly comprise a small subpopulation of the CD44+ cells, suggesting ALDH expression isolates a subset of CD44+ cells that contain the actual tumorigenic cancer stem cells (). Thus we conclude that the tumorigenic cells have both phenotypic markers. The explanation for why CD44 and ALDH are expressed by the stem cell population is not known, however, one could speculate that the ability to oxidize retinoic acid, a proposed function of ALDH, is a requirement for stem cell activity. It is also not certain that both markers are always expressed on the tumorigenic stem cells since there appear to be a small fraction of ALDH positive cells that are CD44-. However, the limited number of cells precluded a separate selection of these cells for testing, and our prior experiments had shown that even large numbers of CD44-negative cells are not tumorigenic. No doubt there are other phenotypic markers that are expressed by cancer stem cells. For example in breast cancer CD24 is also a stem cell marker. In HNSCC this does not appear to be the case, thus there seem to be tissue specific stem cell markers as well as general stem cell markers. ALDH expression may represent a CSC that is generally applicable to all cancer stem cells although this is yet to be proven.
Proposed model for the cancer stem cell compartment in HNSCC.
The small numbers of cancer stem cells obtained from HNSCC limit the critical experiments that must be carried out to understand the role of these cells in cancer persistence, recurrence and resistance to therapy. Repeated re-cultivation in the NOD/SCID mouse or in vitro culture methods would allow for repeated experiments and our future work will target these mechanisms.