Although many
cancers are maintained by tumor-initiating cells, this has not been shown for mesenchymal
tumors, in part due to the lack of unique surface markers that identify mesenchymal progenitors. An alternative technique to isolate stem-like cells is to isolate side population (SP) cells based on efflux of
Hoechst 33342 dye. We examined 29 mesenchymal
tumors ranging from benign to high-grade
sarcomas and identified SP cells in all but six samples. There was a positive correlation between the percentage of SP cells and the grade of the
tumor. SP cells preferentially formed
tumors when grafted into immunodeficient mice, and only cells from
tumors that developed from the SP cells had the ability to initiate
tumor formation upon serial
transplantation. Although SP cells are able to efflux
rhodamine dye in addition to
Hoechst 33342, we found that the ability to efflux
rhodamine dye did not identify a population of cells enriched for
tumor-initiating capacity. Here, we identify a subpopulation of cells within a broad range of benign and malignant mesenchymal
tumors with
tumor-initiating capacity. In addition, our data suggest that the proportion of SP cells could be used as a prognostic factor and that therapeutically targeting this subpopulation of cells could be used to improve patient outcome.