Ovarian cancer stem-like cells and drug resistance

Ovarian cancer is characterised by late diagnosis, relatively poor survival and high recurrence due to the development of multidrug resistance (MDR – resistance to various types of drug). Cancer stem-like cells (CSCs) are thought to contribute to drug resistance due to the over-expression of ABC transporters which efflux chemotherapeutic drugs. Frequently overexpressed ABC transporters include Multidrug resistance protein 1 (MRP1), P-glycoprotein (Pgp) and Breast cancer resistance protein (BCRP). No definitive stem cell marker panel has been determined for ovarian CSC detection; investigation of multiple putative stem cell markers (CD133, CD24, CD44 and Aldehyde dehydrogenase (ALDH)) may prove beneficial to the identification of these cells. All three ABC transporters are heterogeneously expressed in the ovarian cancer cell lines studied. Functional MRP1 and Pgp activity was identified utilising the Calcein-AM assay. Stem cell markers CD133 and ALDH were undetectable by western blot (WB). However small populations of ALDH expressing cells were detected using the ALDEFLUOR™ assay. Low-level CD44 was observed by WB, and confirmed by IF. RT-qPCR and microarray analysis confirmed the gene expression of all three ABC transporters and stem cell markers. A potential CSC population of high Hoechst effluxing cells, called a side population (SP) was identified. However, the viability of the SP was greatly reduced. Chemotherapeutic resistance to carboplatin, paclitaxel and doxorubicin was determined in ovarian cancer cell lines. The ovarian cancer cell line 1847 supports CSC and MDR theories, showing greater colony and spheroid formation and is most resistant to chemotherapeutics. Heterogeneous ABC transporter and stem cell marker expression was confirmed in primary samples. IHC analysis confirmed marker expression was not correlated with patient survival. Correlations were observed between MRP1 and CD44 and ALDH, CD44 and Pgp, BCRP and ALDH expression. Cells derived from patient ascites were also capable of forming colonies and spheroids, indicative of CSC properties.