Cancer stem cell‐like SP cells have a high adhesion ability to the peritoneum in gastric carcinoma

Cancer stem cells (CSCs) are considered to be responsible for cancer metastasis, but the evidence to conclusively prove this hypothesis remains uncertain. The side population (SP), as evaluated by a flow cytometric analysis using Hoechst 33342, has been known as CSC‐rich population. The aim of this study was to clarify the characterization of the SP cells in peritoneal metastasis of gastric carcinoma. Gastric cancer cell lines OCUM‐2M, OCUM‐2D, and OCUM‐2MD3 (a daughter cell line with high potential for peritoneal metastasis) were used. We isolated SP cells from OCUM‐2M and OCUM‐2D using flow cytometry. Serial sorting was performed three times to enrich SP cells, and they were designated as OCUM‐2M/SP and OCUM‐2D/SP cells. Flow cytometric analysis showed 0.46%, 0.29%, 5.24%, 6.49%, and 11.3% of the SP cells to be found in OCUM‐2M, OCUM‐2D, OCUM‐2MD3, OCUM‐2M/SP, and OCUM‐2D/SP cells, respectively. The intraperitoneal inoculation of SP cells and OCUM‐2MD3 cells produced peritoneal metastasis, but parent cells did not. The adhesion ability of SP and OCUM‐2MD3 cells was significantly high in comparison to that of parent cells. The expression level of adhesion molecules α2‐, α5‐, β3‐, and β5‐integrin, and CD44, was high in SP cells compared to parent cells. The expression of stemness markers, Oct3/4 and Sox2, increased in the SP‐cell‐injected tumors. These findings suggested that CSC‐like SP cells expressing α2‐, α5‐, β3‐, and β5‐integrin, and CD44, may play an important role for peritoneal metastasis in gastric carcinoma. Oct3/4 and Sox2 may be associated with CSC in gastric cancer. (Cancer Sci 2009)