Cancer Therapy: Preclinical Dicer-Mediated Upregulation of BCRP Confers Tamoxifen Resistance in Human Breast Cancer Cells

Purpose: Tamoxifen (Tam) is the most prescribed hormonal agent for treatment of estrogen receptor a (ERa)-positive breast cancer patients. Using microarray analysis, we observed that metastatic breast tumors resistant to Tam therapy had elevated levels of Dicer. Experimental Design: We overexpressed Dicer in ERa-positive MCF-7 human breast cancer cells and observed a concomitant increase in expression of the breast cancer resistance protein (BCRP). We thus hypothesized that Tam resistance associated with Dicer overexpression in ERa-positive breast cancer cells may involve BCRP. We analyzed BCRP function in Dicer-overexpressing cells using growth in soft agar and mammosphere formation and evaluated intracellular Tam efflux. Results: In the presence of Tam, Dicer-overexpressing cells formed resistant colonies in soft agar, and treatment with BCRP inhibitors restored Tam sensitivity. Tumor xenograft studies confirmed that Diceroverexpressing cells were resistant to Tam in vivo. Tumors and distant metastases could be initiated with as few as five mammosphere cells from both vector and Dicer-overexpressing cells, indicating that the mammosphere assay selected for cells with enhanced tumor-initiating and metastatic capacity. Diceroverexpressing cells with elevated levels of BCRP effluxed Tammore efficiently than control cells, and BCRP inhibitors were able to inhibit efflux. Conclusion:Dicer-overexpressing breast cancer cells enriched for cellswith enhancedBCRP function.We hypothesize that it is thispopulationwhichmaybe involved in the emergenceof Tam-resistant growth.BCRP may be a novel clinical target to restore Tam sensitivity. Clin Cancer Res; 17(20); 6510–21. 2011 AACR.