Tanshinone IIA Inhibits Triple-Negative Breast Cancer Cells MDA-MB-231 via G Protein-Coupled Estrogen Receptor- (GPER-) Dependent Signaling Pathway

Due to the lack of classic estrogen receptors, there has been a shortage of targeted therapy for triple-negative breast cancer (TNBC), resulting in a poor prognosis. However, the newly discovered G protein-coupled estrogen receptor (GPER) has been found to be expressed in TNBC cells. Salvia miltiorrhiza (Danshen) is an essential Chinese medicine for gynecological disorders, and its component tanshinone IIA (Tan IIA) exerts an anticancer effect. Therefore, this study attempted to investigate whether GPER is involved in the inhibitory effect of Tan IIA on TNBC. We applied various databases and GO pathway analysis to predict the possible mechanism of Tan IIA. We identified 39 overlapping targets, including c-Jun, c-Fos, and caspase-3, and enriched cell cycle-related pathways. Next, we demonstrated the strong binding ability of Tan IIA to GPER by molecular docking assay. In the subsequent validation tests, Cell Counting Kit-8 (CCK8) assay showed that Tan IIA inhibited proliferation of MDA-MB-231 cells time and dose dependently without affecting normal cells. Using Transwell plate, flow cytometry, and Western blot assays, we showed that Tan IIA inhibited migration and induced apoptosis of MDA-MB-231 dose dependently. Importantly, protein expressions of GPER, epidermal growth factor receptor (EGFR), extracellular regulated protein kinases (ERK), c-Fos, and c-Jun were all decreased by Tan IIA dose dependently. Administration of GPER inhibitor partly abolished these effects. Furthermore, nuclear translocation of c-Fos and c-Jun as well as cell cycle-related proteins was downregulated by Tan IIA dose dependently. In summary, Tan IIA could inhibit the proliferation and migration of MDA-MB-231 cells and induce apoptosis, and the possible mechanism may be the regulation of GPER-mediated pathways, suggesting that GPER could be a therapeutic target for TNBC.