AIM To investigate the effects of different concentrations of calycosin on estrogen receptor-positive human breast cancer cells in vitro and in vivo.METHODS In the present study, ER-positive MCF-7, T47D cells and ER-negative MDA-MB-435S cells were treated with different concentrations of calycosin or/and MAPK inhibitor.Proliferation of the cells treated with calycosin was assayed by MTT.Apoptosis in the treated cells was measured by flow cytometry and Hoechst 33258 staining.The mRNA and protein levels of Ras, Raf, Bcl-2, and Bax in MCF-7 cells were also determined by reverse transcription-polymerase chain reaction (RT-PCR), realtime-PCR, immunohistochemical staining and Western blot, respectively.In addition, the protein expression of ERKI/2, JNK, p38 in MCF-7 cells (neatment with different concentrations of calycosin) was determined by Western blot.In vivo,ovariectomized (OVX) mice were treated with different concentrations of calycosin, the uterine weight and the expression of ERα in the uterine tissues was assessed, on the other hand, the anti-tumor activity and induced-apoptosis of calycosin was evaluated in nude mice bearing orthotopic tumor implants.RESULTS Calycosin stimulated proliferation of ER-positive MCF-7 cells compared with solvent control at low concentrations (< 16 μmol·L-1).Furthermore, low concentrations of calycosin decreased the percentage of early apoptosis in MCF-7 cells, down-regulated mRNA and protein levels of Box and up-regulated those of Bcl-2 at low concentrations, the level of p-ERK1/2 was also upregulated at these low concentrations, and we found that an ERK1/2 inhibitor significantly blocked the effect of low concentrations of calycosin in MCF-7 ceils.In the in vivo studies, calycosin stimulated a dramatic increase in uterine weight and upregulated the level of ERα protein in OVX mice.On the other hand, calycosin at high concentrations (> 25 μmol· L-1) inhibited ER-positive MCF-7 and T47D cells proliferation.However, calycosin at high concentrations could not inhibit the cell of growth of ER-negative breast cancer cells such as MDA-MB-435S cells.We further found that high concentrations of calycosin activated Ras-Rafp38 MAPK signaling pathway, which resulted in increased the ratio of Box / Bcl-2, and induced apoptosis on MCF-7 cells.However,when MCF-7 cells were pretreated with p38MAPK inhibitor SB203580 before calycosin, apoptosis induced by calycosin was significantly attenuated.Moreover, in xenografts experiment, calycosin induced apoptosis and prevented the tumor growth of human breast cancer cells.CONCLUSION Calycosin had stimulatory effects on ER-positive cells growth due to its estrogenic action at low concentrations, in contrast, our data suggested that apoptosis induced by high concentrations of calycosin on human breast cancer cells was related to Ras-Raf-p38MAPK pathway.Considering that Radix Astragali is widely used chnically, our results provided the foundation for future development of different concentrations calycosin for treatment of ER-positive breast cancer.