Topotecan-induced ABCG2 expression in MCF-7 cells is associated with decreased CD24 and EpCAM expression and a loss of tumorigenicity.

Human breast cancer shows a considerable heterogeneity regarding the expression of CD24, CD44, EpCAM, and HER2. These markers are involved in cell adhesion, migration, and proliferation, and thus affect metastasis and, in turn, patient's outcome. The ATP-driven efflux pump (ABC transporter) breast cancer resistance protein (BCRP, ABCG2) is known to confer resistance to a wide variety of structurally unrelated cytostatics and defines subpopulations with enhanced tumor-initiating capacity. The expression of ABCG2 can be induced by treatment with different cytostatic drugs. Concurrent effects of such treatments on the expression of the aforementioned marker proteins and cellular properties related to cancer-initiating cells have not been examined thoroughly. Here, we investigated the effect of the ABCG2 substrate topotecan on the MCF-7 breast cancer cell line and analyzed CD24, CD44, EpCAM, and HER2 expression by flow cytometry. Moreover, we examined the impact of topotecan treatment on the sphere-forming ability in vitro and the tumorigenicity in immunodeficient NMRI-nu/nu and NSG mice. We found an elevated ABCG2 expression in MCF-7 cells in the presence of 500 nM topotecan. Compared with untreated MCF-7 cells, the application of topotecan induced a subpopulation with decreased CD24/EpCAM expression, whereas CD44 expression remained largely unchanged. Topotecan-treated cells showed an impaired mammosphere formation capacity in vitro and reduced tumorigenicity in immunodeficient mice. The data indicate that ABCG2 induction is not necessarily linked to increased tumorigenicity and suggest a major role of CD24 and EpCAM for the preservation of self-renewal capacity in MCF-7 cells and tumor outgrowth in vivo.