The ion transporter Na-K-ATPase plays a decisive role in the selective inhibition of breast cancer stem-like cells by the ionophore salinomycin

Background:

The cancer stem cell hypothesis postulates that recurrences arise exclusively from cancer cells with stem cell properties (CSCs). It is therefore important to specifically target these stem cells. The antibiotic salinomycin, an ionophore for monovalent cations, proved to be particularly effective here. Since it probably destroys intracellular potassium homeostasis, we have investigated whether the transport protein Na-K-ATPase is involved.

Methods:

CSCs with the marker profile CD44^high CD24^low were isolated via spheroids from MDA-MB 231 breast cancer cells. A subclone with similar characteristics but epithelial morphology, expressing E-cadherin, was obtained by transfection of keratin 18 into MDA-231 wild type cells. Both cell types were incubated with increasing concentrations of different ionophores. Proliferation was determined by a MTT-test, intracellular ATP by a chemiluminescence assay. Hellebrin was used as an inhibitor of the ATPase. ATPase integrity and density was tested by binding of tritium-labelled ouabain.

Results:

CSCs were inhibited effectively and by a factor of ten more than epithelial cancer cells. If the Na-K-ATPase of epithelial cells was partially inhibited with a cardiac glycoside, their dose-response curve approached the stem cells. The intracellular concentration of ATP, an important cofactor, was significantly lower in stem cells than in epithelial cells. Furthermore the ATPase density at the cell surface was also lower.

Conclusion:

Salinomycin is a potent inhibitor of breast cancer CSCs in our model system and thus a potential candidate for adjuvant therapy. The ion pump Na-K-ATPase appears to be involved in this inhibition and could therefore also be a therapeutic target.