The passenger strand, miR-21-3p, plays a role in mediating cisplatin resistance in ovarian cancer cells.

Ovarian cancer is the deadliest gynaecological cancer. A major contributor to the poor survival rate is the development of chemoresistance to platinum-based therapies such as cisplatin and carboplatin. Here we aimed to test the role of miRNAs in the acquisition of drug resistance in ovarian cancer. We used microarrays to measure miRNA levels in the ovarian cancer cell line A2780 and its cisplatin-resistant derivative CP70. The role of miRNAs and the mRNA targets were tested using transfected miRNA mimics and siRNAs, respectively. Potential in vivo significance was investigated by analysing RNA levels in cohorts of ovarian cancer patients. We identified several miRNAs that are increased in cisplatin-resistant cells. We show that most of these do not directly contribute to cisplatin resistance. Interestingly, miR-21-3p, the passenger strand of the known oncomiR, directed increased resistance to cisplatin in a range of ovarian cell lines. This effect was specific to the star strand, as miR-21-5p had the opposite effect and actually increased sensitivity of A2780 cells to cisplatin. We identify NAV3 as a potential target of miR-21-3p and show that knockdown of NAV3 increases resistance. Exosomes released by CP70 cells were also capable of increasing resistance in A2780 cells, although this was independent of miR-21-3p. Finally, we use publically available transcriptomic data to demonstrate that miR-21-3p is raised, while NAV3 is reduced, in ovarian tumours that are resistant to platinum treatment. Our data suggest that miR-21-3p can induce cisplatin resistance in ovarian tumours, potentially by targeting the NAV3 gene.