Side population is enriched in tumorigenic, stem-like cancer cells, whereas ABCG2+ and ABCG2- cancer cells are similarly tumorigenic

Recently, several human cancers including leukemia and breast and brain tumors were found to contain stem-like cancer cells called cancer stem cells (CSC). Most of these CSCs were identified using markers that identify putative normal stem cells. In some cases, stem-like cancer cells were identified using the flow cytometry-based side population technique. In this study, we first show that similar to 30% of cultured human cancer cells and xenograft tumors examined (similar to 30 in total) possess a detectable side population. Purified side population cells from two cell lines (U373 glioma and MCF7 breast cancer) and a xenograft prostate tumor (LAPC-9) are more tumorigenic than the corresponding non-side population cells. These side population cells also possess some intrinsic stem cell properties as they generate non-side population cells in vivo, can be further transplanted, and preferentially express some stemness genes, including Notch-1 and beta-catenin. Because the side population phenotype is mainly mediated by ABCG2, an ATP-binding cassette half-transporter associated with multidrug resistance, we subsequently studied ABCG2(+) and ABCG2(-) cancer cells with respect to their tumorigenicity in vivo. Although side population cells show increased ABCG2 mRNA expression relative to the non-side population cells and all cancer cells and xenograft tumors examined express ABCG2 in a small fraction (0.5-3%) of the cells, highly purified ABCG2(+) cancer cells, surprisingly, have very similar tumorigenicity to the ABCG2(-) cancer cells. Mechanistic studies indicate that ABCG2 expression is associated with proliferation and ABCG2(+) cancer cells can generate ABCG2(-) cells. However, ABCG2(-) cancer cells can also generate ABCG2(+) cells. Furthermore, the ABCG2(-) cancer cells form more and larger clones in the long-term clonal analyses and the ABCG2(-) population preferentially expresses several stemness genes. Taken together, our results suggest that (a) the side population is enriched with tumorigenic stem-like cancer cells, (b) ABCG2 expression identifies mainly fast-cycling tumor progenitors, and (c) the ABCG2(-) population contains primitive stem-like cancer cells.