CD133 expression is not restricted to stem cells, and both CD133 ⁺ and CD133^- metastatic colon cancer cells initiate tumors

Colon cancer stem cells are believed to originate from a rare population of putative CD133⁺ intestinal stem cells. Recent publications suggest that a small subset of colon cancer cells expresses CD133, and that only these CD133⁺ cancer cells are capable of tumor initiation. However, the precise contribution of CD133⁺ tumor-initiating cells in mediating colon cancer metastasis remains unknown. Therefore, to temporally and spatially track the expression of CD133 in adult mice and during tumorigenesis, we generated a knockin lacZ reporter mouse (CD133^lacZ/+), in which the expression of lacZ is driven by the endogenous CD133 promoters. Using this model and immunostaining, we discovered that CD133 expression in colon is not restricted to stem cells; on the contrary, CD133 is ubiquitously expressed on differentiated colonic epithelium in both adult mice and humans. Using Il10 ^-/-CD133^lacZ mice, in which chronic inflammation in colon leads to adenocarcinomas, we demonstrated that CD133 is expressed on a full gamut of colonic tumor cells, which express epithelial cell adhesion molecule (EpCAM). Similarly, CD133 is widely expressed by human primary colon cancer epithelial cells, whereas the CD133^- population is composed mostly of stromal and inflammatory cells. Conversely, CD133 expression does not identify the entire population of epithelial and tumor-initiating cells in human metastatic colon cancer. Indeed, both CD133⁺ and CD133^- metastatic tumor subpopulations formed colonospheres in in vitro cultures and were capable of long-term tumorigenesis in a NOD/SCID serial xenotransplantation model. Moreover, metastatic CD133^- cells form more aggressive tumors and express typical phenotypic markers of cancer-initiating cells, including CD44 (CD44⁺CD24^-), whereas the CD133⁺ fraction is composed of CD44^lowCD24⁺ cells. Collectively, our data suggest that CD133 expression is not restricted to intestinal stem or cancer-initiating cells, and during the metastatic transition, CD133⁺ tumor cells might give rise to the more aggressive CD133^- subset, which is also capable of tumor initiation in NOD/SCID mice.