Reversing paclitaxel resistance in ovarian cancer cells via inhibition of the ABCB1 expressing side population

The majority of deaths in ovarian cancer are caused by recurrent metastatic disease which is usually multidrug resistant. This progression has been hypothesised to be due in part to the presence of cancer stem cells, a subset of cells which are capable of self-renewal and are able to survive chemotherapy and migrate to distant sites. Side population (SP) cells, identified by the efflux of the DNA-binding dye Hoechst 33342 through ATP-binding cassette (ABC) transporters, are a known adult stem cell group and have been suggested as a cancer stem cell in various cancers. Despite the identification of SP cells in cancer cell lines and patient samples, little attention has been paid to the identification of specific ABC transporters within this cell fraction which efflux Hoechst dye and thus may facilitate drug resistance. In this study, we demonstrate that SP cells can be detected in both ovarian cancer cell lines and ascitic fluid samples, and these SP cells possess stem cell and drug resistance properties. We show that ABCB1 is the functioning ABC transporter in ovarian cancer cell lines, and expression of ABCB1 is associated with a paclitaxel-resistant phenotype. Moreover, silencing of ABCB1 using a specific morpholino oligonucleotide results in an inhibition of the SP phenotype and a sensitising of ovarian cancer cell lines to paclitaxel. ABCB1 should therefore be considered as a therapeutic target in ovarian cancer.