Let-7e Suppresses DNA Damage Repair And Sensitizes Ovarian Cancer To Cisplatin Through Targeting PARP1

Backgroud:Resistance to platinum-based chemotherapy remains a great challenge for ovarian cancer treatment.Platinum drugs exert cytotoxic effects via inducing DNA damage including single and double strand breaks.The enhanced DNA damage repair is one of the mechanisms of platinum resistance.Poly(ADP-ribose)polymerases 1(PARP1)is thought to play a key role in DNA damage repair and is a candidate gene targeted by let-7e.Our previous study showed that up-regulation of let-7e sensitized ovarian cancer to cisplatin.In the present study,we aimed to investigate the role of let-7e in the repair of cisplatin-induced DNA damage and evaluate if the effect of let-7e on DNA damage repair and cisplatin sensitiviey is mediated by PARP1.Methods:The expression of let-7e,PARP1,BRCA1,and Rad51 were assayed by in situ hybridization and immunohistochemistry in ovarian cancer tissues.MTT and colony-formation assays were performed to examine the cytotoxicity of cisplatin to ovarian cancer cells.Alkaline and neutral comet assay and immunofluorescence of y-H2AX foci formation were used to evaluate the repair of cisplatin-induced DNA damage in ovarian cancer cells.Dual luciferase reporter assay was performed to detect whether let-7e could directly target the 3’UTR of PARP1.Fisher exact tests,Kaplan-Meier analyses,Receiver operating characteristic(ROC)curve analyses,cox and logistic regression analyses were performed to explore the clinical values of let-7e,PARP1,BRCA1 and Rad51 in predicting chemo-response and prognosis in ovarian cancer.Results:let-7e expression was decreased in chemo-resistant ovarian cancer tissues compared with chemo-sensitive ones.Increased let-7e expression sensitized ovarian cancer cells to cisplatin and led to a dramatically delayed DNA damage repair,while the inhibition of let-7e accelerated DNA damage repair and promote cisplatin resistance.And let-7e could negatively regulate the expression of BRCA1 and Rad51,the key factors implicated in homologous recombination repair.The above-mentioned effect of let-7e was mediated by directly targeting PARP1.In vivo experiments showed that both let-7e agomir and olaparib,the inhibitor of PARP1,could sensitize ovarian cancer to cisplatin.The low expression of let-7e and high expression of PARP1,BRCA1 and Rad51 were significantly associated with chemotherapy resistance and poor survival of ovarian cancer patients indicated by Fisher exact tests,Kaplan-Meier,and univariate regression analyses.ROC analyses indicated that all the four factors could predict the chemotherapy response in ovarian cancer patients.However,only let-7e was proved to be an independent factor to predict prognosis and chemotherapy response indicated by multivariable cox and logistic analyses.Conclusions:In ovarian cancer,low expression of let-7e releases the suppression to PARP1 and subsequently enhances DNA damage repair,which in turn results in cisplatin-resistance.Similar to PARP inhibitor,re-expression of let-7e could sensitize ovarian cancer to cisplatin,indicating that let-7e might be an effective strategy for overcoming chemo-resistance.