[Effects of mammalian-target-of-rapamycin pathway on lapatinib resistance in breast cancer MDA-MB-231 cells].

OBJECTIVE: To establish a lapatinib resistance cell line for elucidating the mechanisms of drug resistance of lapatinib in human breast cancer cells. METHODS: The human breast cancer MDA-MB-231 cells were exposed in an incremental dose of lapatinib to establish a lapatinib resistance rMDA-MB-231 cell line. The assay of methyl thiazolyl tetrazolium (MTT) was used to detect the cytotoxic activity of lapatinib against MDA-MB-231 and rMDA-MB-231 cells. The protein expression was detected by Western blot. Small interfering RNA was used to specifically knock down mammalian-target-of-rapamycin (mTOR) in rMDA-MB-231 cells. Apoptosis was determined by fluorescein isothiocyanate (FITC)-annexin V/PI staining and flow cytometry. RESULTS: The human breast cancer lapatinib resistance cell line rMDA-MB-231 was induced by lapatinib. The half maximal inhibitory concentration (IC50) values of lapatinib against MDA-MB-231 and rMDA-MB-231 cells were (6.1 ± 0.6) and (34.9 ± 2.7) µmol/L respectively (P < 0.01). Compared with MDA-MB-231 cells, the protein expression of mTOR in rMDA-MB-231 cells was significantly up-regulated. The protein expression of mTOR was significantly down-regulated by specific siRNA duplexes in rMDA-MB-231 cells. After siRNA interference, 20 µmol/L lapatinib was added into control, negative siRNA control and mTOR-targeted siRNA groups respectively. The percents of cell apoptosis in control, negative control and targeted siRNA groups were 13.4% ± 2.5%, 14.2% ± 2.8% and 34.6% ± 5.8% respectively, there was no significance between the first two groups (P > 0.05) , and there was significant difference between the control and targeted siRNA group (P < 0.01) . CONCLUSIONS: The up-regulation of mTOR plays an important role in the lapatinib-resistant phenotype of human breast cancer rMDA-MB-231 cells. And the down-regulation of mTOR increases the apoptotic death of lapatinib against rMDA-MB-231 cells.