Effective targeting of epithelial mesenchymal transition-derived breast cancer stem cells using ophiobolin A, a natural product.

Abstract

Triple negative breast cancer (TNBC) is characterized by the lack of progesterone and estrogen hormone receptors and lack of HER2 overexpression, resulting in limited treatment options. Not only do TNBC patients face limited treatment options, but a TNBC diagnosis also associates with poor diagnostic outcomes due to high rates of metastasis and recurrence, processes ascribed to a subpopulation of cells called cancer stem cells (CSCs). CSCs act as highly-adaptable tumor-initiating cells, which colonize tumor growth in secondary metastatic sites. Cancer stem cells may arise naturally from dedifferentiated adult cells, yet, in tumors, the epithelial-to-mesenchymal transition (EMT) has been shown to yield CSCs. EMT is a normal biological process that becomes dysregulated in tumor formation, whereby stationary epithelial tumor cells gain a migratory mesenchymal phenotype. These mesenchymal cells are adapted to survive the process of metastasis and chemotherapy treatment, and they also display increased stem cell markers and properties. It may be through this de-differentiation process that cancer cells gain their stemness, making EMT an attractive process to target in the attempts to eliminate CSCs. Many conventional therapies fail to successfully target CSCs due to the cells’ reliance on inducing apoptotic cell death, upregulation of drug transporters, and slowed cell cycle. Compounds with cytotoxic activity directed towards CSCs are gaining interest in cancer biology, in particular natural products. Several natural products demonstrate promising effects against cancer hallmarks, contrasted by targeted therapies which fall victim to genetic redundancy, mutation, or epigenetic modification. Ophiobolin A (OpA) is a molecule that shows promise in targeting the elusive CSC population in TNBC. We demonstrate that CSC-derived EMT cells are differentially sensitive to OpA, and that treatment reduces EMT and CSC phenotypes such as migration, mammosphere formation, and chemoresistance. Moreover, OpA induces cellular changes consistent with nonapoptotic cell death, alters the morphology and function of mitochondria, and acts in a mitochondria-dependent manner. Finally, OpA is effective in reducing primary tumor volume in mice. In summary, we show that OpA, a natural product, demonstrates directed cytotoxicity against CSCs and may be useful for clinical application for TNBC patients.