Calycosin, a Phytoestrogen Isoflavone, Induces Apoptosis of Estrogen Receptor-Positive MG-63 Osteosarcoma Cells via the Phosphatidylinositol 3-Kinase (PI3K)/AKT/Mammalian Target of Rapamycin (mTOR) Pathway

Background: Osteosarcoma is the most common primary bone malignancy and often presents at an early age. Calycosin is a phytoestrogen isoflavone, which has previously been reported to inhibit tumor cell growth. The aim of this study was to investigate the effects of calycosin on apoptosis of estrogen receptor (ER)-positive and ER-negative human osteosarcoma cell lines and tumor xenografts in mice. Material/Methods: Cultured ER-positive MG-63 human osteosarcoma cells and ER-negative U2-05 human osteosarcoma cells were treated with increasing doses of calycosin (0, 25, 50, and 100 mu m). Cell viability and apoptosis were studied by an MTT assay and flow cytometry. Western blot measured the expression levels of the apoptosis-related protein p-PI3K, p-Akt, and p-mTOR in MG-63 cells, with and without pretreatment with the PI3K inhibitor, LY294002, the AKT inhibitor, MK-2206, or the mTOR inhibitor, rapamycin. MG-63 tumor-bearing nude mice were used to evaluate the effects of treatment with calycosin. Results: Calycosin treatment inhibited proliferation and induced apoptosis in MG-63 cells, but had no effect on U2-05 cells. In MG-63 cells, calycosin treatment increased the expression of the PI3K/AKT/mTOR pathway proteins; inhibitor assays showed that expression of the PI3K protein was most strongly associated with the antitumor effects of calycosin. In the nude mouse MG-63 tumor xenografts, calycosin inhibited tumor growth and regulated the expression levels of apoptosis-related PI3K/AKT/mTOR pathway proteins. Conclusions: The phytoestrogen, calycosin, induced apoptosis of cells of the ER-positive osteosarcoma cell line, MG-63, via the PI3K/AKT/mTOR pathway, with these effects being mainly due to PI3K.