Background and purpose: Resistance of cancer cells to chemotherapy is a major clinical obstacle to successful treatment and leads to poor prognosis for the patients. MicroRNA (miRNA) plays a vital role in tumor cells response to chemotherapeutic agents. This study plans to investigate the expressions of miRNA27a and miRNA451 in ovarian cancer and breast cancer cells and its correlation with drug resistance. Methods: A2780/Taxol cells were established using stepwise selection; Stem-loop quantitative real-time PCR (stem-loop RT-PCR) was used to detect expression of miRNA27a and miRNA451 in ovarian cancer and breast cancer cells. The A2780 and A2780/Taxol cells were transfected with the mimics or inhibitors of miRNA27a or negative control (NC) RNA and the mimics of miRNA451 or NC were transfected into MCF-7/ADM cells by Lipofectamine

TM

2000. The expression levels of MDR1 mRNA and P-glycoprotein (P-gp) were examined using RT-PCR and Western blot respectively. MTT method was used to analyze drug sensitivity. Results: The expression of miRNA27a was an average of (2.2±0.30) times higher in A2780/ Taxol cells than in A2780 cells

with a significant difference between the two groups (P0.05). The expression of miRNA451 was lower by 84% in MCF-7/ADM cells than in MCF-7 cells

with a significant difference between the two groups (P0.05). A2780/Taxol cells transfection with inhibitors of miRNA27a showed that the levels of MDR1 mRNA was decreased by (39±0.14)%

P-gp level [(26±5.3)%

]

decreased compared with the NC group [(43±6.7)%

]

the IC

50

(0.53 μmol/L) was less than the NC group (6.8 μmol/L)

and there was a significant difference between two groups (P0.05). Transfection of

A2780 cells with mimics of miRNA27a led to increase MDR1 mRNA expression by (121±0.11)% and decrease the sensitivity to paclitaxel (IC

50

: 0.2 μmol/L vs 0.06 μmol/L). There was a significant difference between two groups (P0.05). Transfection of MCF-7/ADM cells with mimics of miRNA451 showed that expression of MDR1 mRNA was decreased by (65±12)%

P-gp [(31±19)%

]

was less than the NC group [(83±12)%

]

the sensitivity of cells to adriamycin enhanced and the IC

50

of adriamycin (4.61 μmol/L) was less than the NC group (26 μmol/L)

and there was a significant difference between two groups (P0.05). Conclusion: The expressions of miRNA27a and miRNA451 are deregulated in A2780/Taxol and MCF-7/ADM cells respectively

which may play vital roles in drug resistance by regulating MDR1/P-gp expression directly or indirectly.