MicroRNAs have been implicated in tumorigenesis, drug resistance, and prognosis in cancer. We investigated the role of microRNA-21 (miR-21) in regulating ovarian cancer drug resistance.

We used parental and cisplatin resistant ovarian cell lines to demonstrate the role of miR-21 in drug resistance and investigated the gene targets of miR-21. Fresh tumor specimens were used to validate our in vitro findings.

Cisplatin resistant ovarian cells were four-fold more resistant compared to the parental cell line. MiR-21 was overexpressed in the resistant cell line on microRNA microarray, which was subsequently validated with qRT-PCR. Using anti-microRNA inhibitors, we demonstrated that miR-21 attenuation reversed the drug resistant phenotype in both the resistant and parental cell lines. The inhibition of miR-21 induced apoptosis based on annexin V-FITC immunostaining. Using western blot analysis, miR-21 knockdown enhanced the expression of tumor suppressor PDCD4, and attenuated apoptosis inhibitor c-IAP2. Using 101 specimens from advanced ovarian cancer patients enrolled in The Cancer Genome Atlas, we found that women with tumors that overexpressed miR-21 were associated with a shorter progression-free survival.

Our data suggest that miR-21 regulates drug resistance via apoptosis and cellular survival pathways. Targeting miR-21 may have clinical utility in the treatment of resistant ovarian cancer.