let-7d-3p is associated with apoptosis and response to neoadjuvant chemotherapy in ovarian cancer

: Altered expression of microRNAs contributes to the heterogeneous biological behavior of human malignancies and it may correlate with the clinical pathological features of patients. The let-7 microRNA family is frequently downregulated in human cancers and its aberrant expression may be a useful marker for prediction of the clinical response to therapy in patients. In the present study, we analyzed the expression of three members of the let-7 family (let-7a-3p, let-7d-3p and let-7f), which remains largely uncharacterized in ovarian cancer tissues. We also investigated the function of let-7d-3p in the apoptosis and sensitization to chemotherapy in ovarian cancer cells. Our data from stem-loop quantitative RT-PCR showed that expression of let-7a-3p and let-7d-3p, but not let-7f, was significantly (P<0.04) upregulated in ovarian tumors relative to that noted in normal ovarian tissues. Markedly, an increased expression of let‑7d-3p (also known as let-7d-3*) was associated with positive response to carboplatin/paclitaxel treatment in ovarian cancer patients. To investigate the biological relevance of let‑7d-3p, we knocked down its expression in SKOV-3 ovarian cancer cell line using antagomiRs. Loss of function analysis showed that inhibition of let-7d-3p significantly (P<0.05) impaired cell proliferation and activated apoptosis. In contrast, scratch/wound healing and Transwell chamber assays showed that migration and invasion abilities were not affected in the let-7d-3p-deficient SKOV-3 cancer cells. Notably, Annexin V assays showed a significant (P<0.05) increase in cell death of cancer cells treated with the let-7d-3p inhibitor plus carboplatin indicating a synergistic effect of the drug with antagomiR therapy. Gene ontology classification of predicted targets of let-7d-3p identified a number of genes involved in cellular pathways associated with therapy resistance such as ABC transporters, HIF-1, RAS and ErbB signaling. In summary, our findings showed that inhibition of let-7d-3 activates apoptosis and that its upregulation is associated with a positive response of ovarian cancer patients to carboplatin/paclitaxel chemotherapy.