Generation of MCF-7 Tamoxifen Resistant Cell Line (MCF-7/TR)

The MCF-7/TR cell line was selected after the long-term cultivation of parental ERα+ MCF- 7 cells. To induce resistance to tamoxifen, cells were exposed stepwise against increasing concentrations of 4-hydroxytamoxifen (4-OHT, Sigma-Aldrich, Italy), starting from 10^–9M up to a final concentration of 10^–6M. Cells were re-fed with a fresh growth medium containing the drug every 2–3 days. The acquired resistance to the antiestrogen was checked on a scheduled basis by the lack of any inhibitory effect on the proliferation of resistant cell lines compared to the parental ones (Supplementary Figures S1A,B, Supporting information).