RESUMEN
BACKGROUND: MicroRNAs (miRNAs) are small non-coding RNAs which have been considered as major players in the process of carcinogenesis and drug responsiveness of breast cancer. OBJECTIVES: In this study, we aimed to investigate the expression pattern of let-7a and miR-205 tumorsuppressor miRNAs in breast cancer cell lines under treatment with paclitaxel. MATERIAL AND METHODS: The half maximal inhibitory concentration (IC50) of paclitaxel was determined for 4 breast cancer cell lines, including MCF-7, MDA-MB-231, SKBR-3, and BT-474 by an MTT assay. The expression level of let-7a and miR-205, and their targets, K-RAS and HER3, was determined before and after treatment with paclitaxel, using quantitative reverse transcriptase real-time polymerase chain reaction (qRT-PCR). RESULTS: After treatment, the expression level of both let-7a and miR-205 was significantly increased in HER2- overexpressing cell line BT-474 (26.4- and 7.2-fold, respectively). In contrast, the HER2-negative cell lines MCF-7 and MDA-MB-231 showed a significantly decreased expression of both let-7a (30.3- and 13.5-fold, respectively) and miR-205 (20- and 18.1-fold, respectively). Controversially, SKBR-3 revealed a significantly decreased expression of both let-7a (1.3-fold) and miR-205 (1.3-fold). The expression level of K-RAS as a target of let-7a decreased in all cell lines significantly, but the pattern of alteration in the expression level of HER3 as a target of miR-205 in all cell lines was the reverse of the pattern of alteration in the expression level of miR-205. CONCLUSIONS: Our results confirmed a better response of HER2-overexpressing breast cancer to paclitaxel at the miRNA level. One putative reason could be the upregulation of tumor-suppressor miRNAs after treatment with paclitaxel. On the other hand, HER2-negative breast cancer cell lines showed a significantly decreased expression of tumor-suppressor miRNAs, a putative mechanism of resisting the therapy.
