Combinatorial treatment of mammospheres with trastuzumab and salinomycin efficiently targets HER2-positive cancer cells and cancer stem cells

A major obstacle in the successful treatment of cancer is the occurrence of chemoresistance. Cancer cells surviving chemotherapy and giving rise to a recurrence of the tumor are termed cancer stem cells and can be identified by elevated levels of certain stem cell markers. Eradication of this cell population is a priority objective in cancer therapy. Here, we report elevated levels of stem cell markers in MCF-7 mammospheres. Likewise, an upregulation of HER2 and its differential expression within individual cells of mammospheres was observed. Sorting for HER2^high and HER2^low cells revealed an upregulation of stem cell markers NANOG, OCT4 and SOX2 in the HER2^low cell fraction. Accordingly, HER2^low cells also showed reduced proliferation, ductal-like outgrowths and an increased number of colonies in matrigel. Xenografts from subcutaneously injected HER2^low sorted cells exihibited earlier onset but slower growth of tumors and an increase in stem cell markers compared to tumors developed from the HER2^high fraction. Treatment of mammospheres with salinomycin reduced the expression of SOX2 indicating a selective targeting of cancer stem cells. Trastuzumab however, did not reduce the expression of SOX2 in mammospheres. Furthermore, a combinatorial treatment of mammospheres with trastuzumab and salinomycin was superior to single treatment with each drug. Thus, targeting HER2 expressing tumors with anti-HER2 therapies will not necessarily eliminate cancer stem cells and may lead to a more aggressive cancer cell phenotype. Our study demonstrates efficient killing of both HER2 positive cells and cancer stem cells, hence opening a possibility for a new combinatorial treatment strategy.