Essential Role of Polo-like Kinase 1 (Plk1) Oncogene in Tumor Growth and Metastasis of Tamoxifen-Resistant Breast Cancer

The most common therapy for estrogen receptor–positive breast cancer is antihormone therapy, such as tamoxifen. However, acquisition of resistance to tamoxifen in one third of patients presents a serious clinical problem. Polo-like kinase 1 (Plk1) is a key oncogenic regulator of completion of G2–M phase of the cell cycle. We assessed Plk1 expression in five chemoresistant cancer cell types and found that Plk1 and its downstream phosphatase Cdc25c were selectively overexpressed in tamoxifen-resistant MCF-7 (TAMR-MCF-7) breast cancer cells. Real-time monitoring of cell proliferation also showed that TAMR-MCF-7 cells were more sensitive to inhibition of cell proliferation by the ATP-competitive Plk1 inhibitor BI2536 than were the parent MCF-7 cells. Moreover, BI2536 suppressed expression of epithelial–mesenchymal transition marker proteins and 3D spheroid formation in TAMR-MCF-7 cells. Using TAMR-MCF-7 cell–implanted xenograft and spleen–liver metastasis models, we showed that BI2536 inhibited tumor growth and metastasis in vivo. Our results suggest that Plk1 could be a novel target for the treatment of tamoxifen-resistant breast cancer. Mol Cancer Ther; 17(4); 825–37.