Ovarian cancer stem-like side-population cells are tumourigenic and chemoresistant

BACKGROUND: Ovarian cancer is the most lethal gynaecological malignancy. Although ovarian cancer patients often respond initially to chemotherapy, they usually develop chemoresistance. We hypothesised that a small portion of ovarian cancer cells have stem-like cell properties that contribute to tumourigenesis and drug resistance. METHODS: Flow cytometry and Hoechst 33342 efflux isolated side-population (SP) cells from ascites derived from ovarian cancer patients and from mice inoculated with human ovarian cancer cell lines. The SP cells were examined for stem cell markers OCT4, NANOG, STELLAR, and ABCG2/BCRP1 by immunocytochemistry and RT–PCR. The SP cells and non-SP cells were studied for tumourigenesis and chemoresistance in vitro and in vivo. RESULTS: The SP cells expressed ABCG2/BCRP1, OCT4, STELLAR, and NANOG, detected by immunocytochemistry and RT–PCR. ABCG2/BCRP1 expression was higher in SP than in non-SP cells. Xenogeneic mice inoculated with SP cells yielded more tumours than did mice inoculated with non-SP cells. In parallel, SP cell culture resulted in extensive cell proliferation, which was markedly more than in non-SP cells. SP cells resisted chemotherapy compared with non-SP cells, both in vivo and in vitro. CONCLUSION: Ovarian cancer SP cells are tumourigenic and chemoresistant. ABCG2/BCRP1 has an important role in chemoresistance, which has implications for new therapeutic approaches.