[Let-7i reduces chemotherapy resistance in breast cancer cells through down-regulation of K-Ras and Bcl2 expression].

Objective To investigate the effect of microRNA let-7i on the drug-resistant cells of human breast cancer and its potential molecular mechanisms. Methods Reverse transcription PCR was performed to detect the relative levels of let-7i expressed in breast cancer cell line MCF-7 as well as MCF-7 resistant to both cisplatin (CDDP) and adriamycin (ADR) (MCF-7/CDDP, MCF-7/ADR) cells. CCK-8 assay was used to measure the varied sensitivity of cell MCF-7, MCF-7/CDDP and MCF-7/ADR to ADR following chemotherapy. CCK-8 assay and colony formation assay were carried out to observe the change of sensitivity to ADR after let-7i was over-expressed or inhibited, and flow cytometry was used to determine the induced apoptosis of breast cancer cells resistant to ADR following over-expression of let-7i. ELISA was conducted to measure the activity of caspase-3/9 against breast cancer drug. Reverse transcription-PCR was performed to detect the mRNA levels of Bcl2 and K-Ras, and Western blot analysis to examine the protein levels of Bcl2, BAX and K-Ras. Results Let-7i was down-regulated in drug-resistant breast cancer cells as compared with simple MCF-7 cell line, and negatively correlated with chemotherapy resistance. Compared with negative control group, over-expression of let-7i resulted in improved cell viability and colony formation of drug-resistant breast cancer cells, and facilitated the cancer cell apoptosis induced by ADR, yet raised caspase-3/9 activity. Bcl2 level decreased, whereas BAX increased with added ADR concentration. Over-expressed let-7i significantly inhibited Bcl2 and K-Ras expression in the drug-resistant cells. Conclusion Let-7i is down-regulated in drug-resistant breast cancer cells, and negatively correlated with chemotherapy resistance. The findings suggest that let-7i may inhibit the resistance of drug-resistant breast cancer cells via inhibiting the expression of K-Ras and Bcl2.