Wnt/β-catenin Signaling Inhibitors suppress the Tumor-initiating properties of a CD44(+)CD133(+) subpopulation of Caco-2 cells

Tumor-initiating cells or cancer stem cells are a subset of cancer cells that have tumorigenic potential in human cancer. Although several markers have been proposed to distinguish tumor-initiating cells from colorectal cancer cells, little is known about how this subpopulation contributes to tumorigenesis. Here, we characterized a tumor-initiating cell subpopulation from Caco-2 colorectal cancer cells. Based on the findings that Caco-2 cell subpopulations express different cell surface markers, we were able to discriminate three main fractions, CD44(-)CD133(-), CD44(-)CD133(+), and CD44(+)CD133(+) subsets, and characterized their biochemical and tumorigenic properties. Our results show that CD44(+)CD133(+) cells possessed an unusual capacity to proliferate and could form tumors when transplanted into NSG mice. Additionally, primary tumors grown from CD44(+)CD133(+) Caco-2 cells contained mixed populations of CD44(+)CD133(+) and non-CD44(+)CD133(+) Caco-2 cells, indicating that the full phenotypic heterogeneity of the parental Caco-2 cells was re-created. Notably, only the CD44(+)CD133(+) subset of Caco-2-derived primary tumors had tumorigenic potential in NSG mice, and the tumor growth of CD44(+)CD133(+) cells was faster in secondary xenografts than in primary transplants. Gene expression analysis revealed that the Wnt/β-catenin pathway was over-activated in CD44(+)CD133(+) cells, and the growth and tumorigenic potential of this subpopulation were significantly suppressed by small-molecule Wnt/β-catenin signaling inhibitors. Our findings suggest that the CD44(+)CD133(+) subpopulation from Caco-2 cells was highly enriched in tumorigenic cells and will be useful for investigating the mechanisms leading to human colorectal cancer development.