Abstract: ObjectiveTo investigate the influence of Smac to the chemosensitivity of cyclophosphamide (CTX) and doxorubicin (DOX) in MCF-7 cells. MethodsMCF-7 cells were exposed to CTX， DOX and the combination of both. 3(4,5dimethyl2thiazoly)2,5diphenyl2Htetrazolium bromide (MTT) assay was used to estimate the cell viability. Apoptosis was measured by acridine orange staining and Ho.33342/PI double staining. The mRNA and protein expressions of Smac were determined by RTPCR and Western blotting. The study also analyzed the changes of proapoptotic proteins active caspase3 and active caspase9. ResultsCTX, DOX and the combination of both drugs reduced the cell survival rates in a concentrationdependent manner. The cell viability after being treated with 4.0 μg/ml CTX or 0.2 μg/ml DOX or 2.0 μg/ml CTX and 0.1 μg/ml DOX for 48 hours was (52.90±8.78)%, (53.35±6.29)% and (34.19±5.43)%, respectively. The drug combination developed a stronger inhibitory effect compared to the single drugs (t=9.051, P=0.014; t=9.074, P=0.014). The Smac mRNA and protein levels in 2.0 μg/ml CTX and 0.1 μg/ml DOX group were 7.47±0.82 and 4.13±0.36, which were higher than those in 4.0 μg/ml CTX group (3.27±0.40 and 2.28±0.27; t=-50.120, P=0.000; t=-42.588，P=0.000) and 0.2 μg/ml DOX group (3.34±0.62 and 2.45±0.40; t=-46.233, P=0.000; t=-39.541, P=0.000). Furthermore, proapoptotic proteins active caspase3 and active caspase9 increased activity was confirmed by Western blotting. ConclusionSmac plays a vital role in enhancing the sensitivity of chemotherapeutic drugs CTX and DOX in MCF-7 cell line.

Key words: Breast neoplasms, Cyclophosphamide, Doxorubicin, Apoptosis, Smac