Spheroid body-forming cells in the human gastric cancer cell line MKN-45 possess cancer stem cell properties

The cancer stem cell theory hypothesizes that cancer stem cells (CSCs), which possess self-renewal and other stem cell properties, are regarded as the cause of tumor formation, recurrence and metastasis. The isolation and identification of CSCs could help to develop novel therapeutic strategies specifically targeting CSCs. In this study, we enriched gastric cancer stem cells through spheroid body formation by cultivating the human gastric cancer cell line MKN-45 in defined serum-free medium. The stemness characteristics of spheroid body-forming cells, including self-renewal, proliferation, chemoresistance, tumorigenicity of the MKN-45 spheroid body-forming cells were evaluated, and the expression levels of stemness genes and related proteins in the MKN-45 spheroid body-forming cells were assessed. Furthermore, immunofluorescence staining for the stem cell markers on spheroid body-forming cells was examined to evaluate the association between stemness factors (Oct4, Sox2, Nanog) and the proposed CSC marker CD44. Our data demonstrated that non-adherent spheroid body-forming cells from the gastric cancer cell line MKN-45 cultured in stem cell-conditioned medium possessed gastric CSC properties, such as persistent self-renewal, extensive proliferation, drug resistance, high tumorigenic capacity and overexpression of CSC-related genes and proteins (Oct4, Sox2, Nanog and CD44), compared with the parental cells. More importantly, CD44-positive cells co-expressing the pluripotency genes Oct4, Sox2 and Nanog may represent gastric CSCs. Further experiments using more refined selection criteria such as a combination of two or multiple markers would be useful to specifically identify and purify CSCs.