Deregulation of let-7e in epithelial ovarian cancer promotes the development of resistance to cisplatin.

Drug resistance remains a major clinical obstacle to successful treatment in ovarian cancer patients, and the evidence of microRNAs involvement in drug resistance has been emerging recently. In this report, we investigated the role of let-7e in the development of cisplatin-resistant ovarian cancer. On the cellular level, let-7e expression was significantly reduced in cisplatin-resistant human epithelial ovarian cancer (EOC) cell line A2780/CP compared with parental A2780 cell and decreased in a concentration-dependent manner in A2780, SKOV3 and ES2 cells treated with cisplatin. Overexpression of let-7e by transfection of agomir could resensitize A2780/CP and reduce the expression of cisplatin-resistant-related proteins enhancer of zeste 2 (EZH2) and cyclin D1 (CCND1), whereas let-7e inhibitors increased resistance to cisplatin in parental A2780 cells. Quantitative methylation-specific PCR analysis showed hypermethylation of the CpG island adjacent to let-7e in A2780/CP cells, and demethylation treatment with 5-aza-CdR or transfection of pYr-let-7e-shRNA plasmid containing unmethylated let-7e DNA sequence could restore let-7e expression and partly reduce the chemoresistance. In addition, cisplatin combined with let-7e agomirs inhibited the growth of A2780/CP xenograft more effectively than cisplatin alone. Diminished expression of EZH2 and CCND1 and higher cisplatin concentrations in tumor tissue of mice subjected to administration of let-7e agomirs in addition to cisplatin were revealed by immunohistochemistry and atomic absorption spectroscopy, respectively. Taken together, our findings suggest that let-7e may act as a promising therapeutic target for improvement of the sensibility to cisplatin in EOC.