458. Sensitizing Ovarian Cancer Cells To Chemotherapy By Interfering With Pathways That Are Involved in the Formation and Maintenance of Cancer Stem Cells

Abstract

It has been suggested that tumor re-growth, as well as chemotherapy resistance and metastasis, are dependent on a small sub-population of cancer cells within the tumor that are thought to represent cancer stem cells (CSCs), i.e. self-renewing cells with pluripotent potential and tumor forming ability. We and others have shown that putative ovarian CSCs are in a transitional phase between epithelial and mesenchymal cell stages and that considerable plasticity exists between non-CSCs and CSCs. These transitions are based on epigenetic changes involving either epithelial-to-mesenchymal transition (EMT) or the reverse process mesenchymal-to-epithelial transition (MET). Here we report that primary ovarian cancer cells treated with the chemotherapeutic drug cisplatin undergo partial EMT/MET-like changes in culture and in xenograft tumors. Concurrently with the induction of partial EMT/MET, cisplatin treatment of primary ovarian cancer cells enriched for CSCs that were positive for the stem cell markers Nanog, Oct4, and Sox2, indicating that the formation of CSC involves partial EMT/MET-like events. In vivo cisplatin treatment of mice with xenograft tumors showed an increase in Nanog and E-cadherin expression as well as in MAPK signaling. Notably, MET initiates and is required for the nuclear reprogramming of fibroblasts into induced pluripotent stem (iPS) cells indicating similarities in the formation of iPS cells and CSCs. We therefore screened 30 compounds from a library of small molecules, which has previously been generated to study EMT/MET signaling in iPS cells. We identified compounds (e.g. compound GHDM1516) that blocked EMT and the formation of CSCs in primary ovarian cancer cultures. Co-treatment of cancer cells with GHDM1516 significantly increased cisplatin induced apoptosis/necrosis in CSCs as assessed by flow cytometry for Annexin V/7-AAD. In contrast, compounds that promoted MET (e.g. compound SD83) acted synergistically with cisplatin and increased the number of CSCs and cisplatin-resistant cancer cells. We are currently performing in vivo studies with a combination GHDM1516 and cisplatin in xenograft models of ovarian cancer. Furthermore, a “wound healing”, functional EMT assays showed the EMT inhibitory potential of GHDM1516 after treatment of ovarian cancer cells with other chemotherapy drugs such as Paclitaxel and Etoposide. Our findings suggest that similarities in iPS and CSC formation can be exploited to find new cancer treatments that are targeted towards CSCs. Our data indicate that it might be possible to sensitize late stage tumors to chemotherapy by interfering with pathways that are involved in the formation and maintenance of CSCs.