Abstract: 【Abstract】Objective To investigate the expression of miR-490-3p in adriamycin（ADM）resistant human breast cancer cell line MCF-7／ADM and its effect on the cell proliferation，apoptosis and resistance to ADM of MCF-7／ADM cell line. Methods Cultured wild-type MCF-7 human breast cancer cells and its resistant cells MCF-7／ADM were selected as research objectives. The realtime fluorescent quantitative PCR（qPCR）was used to compare the level of miR-490-3p in both cell lines. The recombinant plasmid pcDNA3.1（+）miR-490-3p to over-express miR-490-3p was transiently transfected into MCF-7／ADM cells（overexpression group）and then the transfection effect was verified by qPCR. Meanwhile，the pcDNA3.1（-）empty control group and blank control group were set up. MTT method was used to determine the effect of pcDNA3.1（+）miR-490-3p on the proliferation of each group. The inhibitory rate of ADM on the proliferation of MCF-7／ADM cells was determined, and the inhibition concentration （IC50） and the reversal factor were calculated to evaluate the sensitivity to ADM. The cells in each group were collected at 48 h post-transfection for the detection of apoptotic rate by flow cytometry with Annexin FITC／PI double staining. Meanwhile, the expression of resistance proteins including P-glycoprotein（P-gp），breast cancer resistance protein （BCRP）and multidrug resistance associated protein 1（MRP1）were tested by Western blotting，intracellular ADM concentration was detected by fluorescence photometer and Pgp activity by flow cytometry. Results The level of miR-490-3p in MCF-7／ADM cells was 0.24±0.07，significantly lower than 1.02±0.03 of wild type MCF-7 cells（P＜005）. The levels of miR-490-3p in the overexpression group were higher than those in the other two groups during 24-96 h post-transfection（P＜005）. The proliferation inhibition rates were（17.52 ±1.87）％，（31.67±2.79）％，（45.09±1.88）％ and（61.82±2.52）％ at 48，24，72 and 96 h post-transfection， all higher than those of empty control group（P＜0.05）. IC50 value of ADM for MCF-7／ADM cells was（11.27±2.34）μg／ml，lower than that of blank control group and empty control group（P＜0.05）. Moreover，reversal index was 3.35and 3.39fold of blank control group and empty control group for the overexpression group. Compared with the other two groups，there were increased apoptotic rates and intracellular drug concentration but decreased expression of P-gp，BCRP and MRP1 as well as P-gp activity in over-expression group （P＜0.05）.Conclusion Upregulation of miR-490-3p exhibits the reversal effect of resistance to ADM in MCF-7／ADM cell，possibly by reducing the resistance gene protein expression and inhibition of P-gp activity. Elevated level of miR-490-3p can inhibit proliferation and induce apoptosis.