Stem Cell-like Molecular and Morphological Properties in a Subpopulation of Human Oral Cancer Cells

Background: Side population (SP) cells are present in many human cancers and cancer cell lines, including human oral cancer SCC-4 cells. SP cells were identified as a side branch of Hoechstlow cells existing in a fraction darker than cells in G0/G1 phase. The ratio of isolated SP cells accounted for approximately 1.5% of the SCC-4 cell population. Here, we describe the stem cell-like properties of this subpopulation of SCC-4 cancer cells. Methods: The expression levels of the stem cell markers OCT4, SOX2 and NANOG in SP cells were measured and compared with those of non-SP cells as indicators of pluripotency. Additionally, sphere formation ability and alkaline phosphatase activity were investigated as indicators of self-renewal and undifferentiated state, respectively. Results: OCT4 and NANOG levels were significantly higher in SP cells than in non-SP cells, while SOX2 levels did not differ between the two cell types. After induction for 2 weeks, sphere formation was notably observed and alkaline phosphatase activity was detected in spheres of SP cells. Conclusions: Our findings reveal that SP cells have stem cell-like properties including pluripotency, self-renewal, and retention of undifferentiated state. These results suggest that a subpopulation of oral cancer cells has the potential to act as stem cells. The stem cell-like properties revealed in this study could be useful in future applications targeting intractable cancer stem cells.