Cancer stem cells (CSCs) are a collection of small numbers of cells that have the potential to induce all cell types within the tumor mass and have self-renewal capacity. Today, the reasons for the failure of conventional cancer therapies lie in the fact that they are unable to target cancer stem cells. Targeting the cancer stem cell is thought to provide a very important and revolutionary advance in cancer cell targeting and therapy

 Tumor hypoxia is a characteristic of solid tumors and has been associated with poor prognosis and resistance to radiotherapy and chemotherapy. HIF-1 (Hypoxia Inducible Factor-1) is the major transcription factor activated in hypoxic conditions and allows transcriptional activation of various genes that are effective for the adaptation of the cell to the hypoxic condition. Experimental studies have provided evidence that also hypoxia and HIF-1α promote the cancer stem cell phenotype and targeting of HIF-1α may reduce or eliminate cancer stem cells.

Breast cancer is the most common form of cancer in women worldwide and affects 10% of the world's female population. 25% to 30% of patients with invasive breast cancer still die from this disease. The recurrence frequency of the disease varies between 60% and 80% within the first 3 years after treatment. In order to target breast cancer stem cells more effectively, in this study we aimed to reveal whether the hypoxic conditions in the tumor, which act as the stem cell production area, at the same time creates resistance to therapy. Thus, we evaluated effect of CSCs targeting agent Salinomycin alone or in combination with Everolimus which is an m-TOR and HIF-1α inhibitor on parental MCF-7 and MDA-231 breast cancer cells and their isolated CSCs in hypoxic conditions.

Here it is presented that starting with 2 mM, increased concentrations of salinomycin significantly inhibits proliferation and induce apoptosis in hypoxia, in both parental MCF-7 and MDA-231 breast cancer cells and in their isolated CSCs. The most effective concentration of salinomycin was 10 mM and induced around 35% and 45% of apoptosis in both parental MCF-7 and MDA-231 and their isolated CSCs, respectively. Whereas everolimus alone was not as effective as salinomycin, as 25 mM everolimus induced 30% and 15% of growth inhibition or apoptosis in both parental and CSCs of MCF-7 and MDA-231 cellsin hypoxia, respectively. When lower concentrations of salinomycin (2mM) and everolimus (5mM) was used in combination they show synergistic effect and able to inhibit proliferation at least 35% and 45% in both parental and CSCs of MCF-7 and MDA-231 cells in hypoxia, respectively. Similar results were also obtained for induction of apoptosis in response to salinomycin + everolimus treatment in hypoxia in both parental and CSCs of MCF-7 and MDA-231 cells. Hence using lower concentrations of salinomycin and everolimus together may provide an effective targeting strategy for hypoxic CSCs and may contribute to the development of novel strategies for therapeutic intervention in breast cancer.