Tamoxifen is widely used as an adjuvant
therapy for patients with
estrogen receptor (ERα)-positive
tumors. However, the clinical benefit is often limited because of the emergence of drug resistance. In this study, overexpression of
ribonucleotide reductase M2 (RRM2) in MCF-7
breast cancer cells resulted in a reduction in the effectiveness of
tamoxifen, through downregulation of ERα66 and upregulation of the 36-kDa variant of ER (ERα36). We identified that NF-κB, HIF1α, and MAPK/JNK are the major pathways that are affected by RRM2 overexpression and result in increased NF-κB activity and increased
protein levels of EGFR, HER2, IKKs, Bcl-2, RelB, and p50. RRM2-overexpressing cells also exhibited higher migratory and invasive properties. Through time-lapse microscopy and
protein profiling studies of
tamoxifen-treated MCF-7 and T-47D cells, we have identified that RRM2, along with other key
proteins, is altered during the emergence of acquired
tamoxifen resistance. Inhibition of RRM2 using siRRM2 or the
ribonucleotide reductase (RR) inhibitor
didox not only eradicated and effectively prevented the emergence of
tamoxifen-resistant populations but also led to the reversal of many of the
proteins altered during the process of acquired
tamoxifen resistance. Because
didox also appears to be a potent inhibitor of NF-κB activation, combining
didox with
tamoxifen treatment cooperatively reverses ER-α alterations and inhibits NF-κB activation. Finally, inhibition of RRM2 by
didox reversed
tamoxifen-resistant in vivo
tumor growth and decreased in vitro migratory and invasive properties, revealing a beneficial effect of combination
therapy that includes RRM2 inhibition to delay or abrogate
tamoxifen resistance.