Tanshinone IIA is a widely used Chinese herbal medicine isolated from Danshen (Salvia miltiorrhiza). Recent studies indicate that tanshinone IIA may have anti-inflammatory and anti-oxidant properties, as well as cytotoxic activities against multiple human cancer cell lines. This study was performed to determine the anti-cancer activity of tanshinone IIA on human breast cancer cells in vitro and in vivo and to elucidate the underlying mechanism of this activity. Human breast cancer cell lines (estrogen receptor-positive and -negative) were treated with tanshinone IIA and tamoxifen. The inhibitory effects of tanshinone IIA and tamoxifen on breast cancer cell proliferation were examined using MTT assays, BrdU incorporation, immunohistochemistry and flow cytometry. Upon treatment with tanshinone IIA, breast cancer cell proliferation was significantly inhibited in a dose- and time-dependent manner (IC50 = 0.25 µg/ml) and apoptotic cell populations increased, while tamoxifen inhibited only ER-positive breast cancer cells prominently and had no effect on ER-negative cells. In addition, tamoxifen had significantly weaker inhibitory effect than tanshinone IIA on ER-positive breast cancer cells in vitro and in vivo. Furthermore, tanshinone IIA decreased the expression of P53 and bcl-2, but not of cerbB-2, in estrogen receptor-positive and negative xenografted nude mice. Our findings suggest that tanshinone IIA might have potential anti-cancer activity that is stronger than tamoxifen in both ER-positive and ER-negative breast cancers. This function could be attributed in part to its inhibition of proliferation and apoptosis induction in cancer cells via the downregulation of multiple genes involved in cell cycle regulation, cell proliferation, apoptosis and DNA synthesis.

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