Abstract
Putative cancer stem cells are a subpopulation of cancer cells that give rise to chemotherapy resistance and are therefore of prognostic and therapeutic interest, though their identification remains elusive in colon cancer due to lack of reliable and accurate markers. We previously identified a p53-dependent putative cancer stem cell population, the calcein low population (C(lo)P), based on their exclusive efflux of the fluorescent dye Calcein. This functional identification method enables comparative live cell studies of subpopulations without differential toxicity that occurs with traditional Hoechst methods, which has confounded conclusions and limited the utility of this cancer stem cell marker. In this study, we examined the cancer stem cell-like properties of the C(lo)P population in vivo in comparison with the parental and calcein-high population (C(hi)P) in human colon cancer xenografts. Serial dilution xenograft experiments in NOD/SCID mice revealed that the C(lo)P is only marginally more tumorigenic compared to the C(hi)P or parental cells. However, serial passage of these tumors revealed that the C(lo)P is uniquely enriched for self-renewal capacity in vivo compared to the other populations. Immunohistochemical analysis of these tumors revealed that the C(lo)P possesses increased levels of nuclear β-catenin and furthermore, siRNA-mediated knockdown of β-catenin significantly reduced the C(lo)P population. These findings highlight the C(lo)P as an important subpopulation of tumor cells that are exclusively endowed with the ability to self-renew and propagate tumors. The dependency of the C(lo)P on β-catenin provides a molecular explanation for this ability and suggests that this population can and should be therapeutically targeted by inhibition of Wnt signaling.
Highlights
The heterogeneity of tumors has become increasingly apparent along with the emerging importance of the tumor microenvironment
We examined the overlap of the calcein low population (CloP) with CD133 and CD26 surface expression in human colon cancer cell lines (Table 1; Figure S1)
Comparing the overlap of the CloP with these canonical markers did not reveal any striking enrichment for CD133 or CD26 within the CloP except for HT-29 cells, which were highly enriched for CD133+ cells
Summary
The heterogeneity of tumors has become increasingly apparent along with the emerging importance of the tumor microenvironment. Other approaches have focused on functional assays for CSC identification In colon cancer, such methods include enhanced activity of the enzyme aldehyde dehydrogenase 1 (ALDH1) amongst CSCs [2, 3] or the increased efflux of small molecules by ABC transporters [4]. The latter property is often identified by efflux of the DNA-binding dye Hoechst 33342 using flow cytometry analysis to identify the side population (SP) that does not retain this fluorescent molecule [5, 6]. The SP has been identified in numerous tumor types and directly linked to therapy resistance due to its efflux of several www.impactjournals.com/oncotarget
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