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 G-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 Our results suggest that Plk1 could be a novel target for the treatment of tamoxifen-resistant breast cancer. .