Sensitivity of docetaxel-resistant MCF-7 breast cancer cells to microtubule-destabilizing agents including vinca alkaloids and colchicine-site binding agents

INTRODUCTION: One of the main reasons for disease recurrence in the curative breast cancer treatment setting is the development of drug resistance. Microtubule targeted agents (MTAs) are among the most commonly used drugs for the treatment of breaset cancer and therefore overcoming taxane resistance is of primary clinical importance. Our group has previously demonstrated that the microtubule dynamics of docetaxel-resistant MCF-7(TXT) cells are insensitivity to docetaxel due to the distinct expression profiles of β-tubulin isotypes in addition to the high expression of p-glycoprotein (ABCB1). In the present investigation we examined whether taxane-resistant breast cancer cells are more sensitive to microtubule destabilizing agents including vinca alkaloids and colchicine-site binding agents (CSBAs) than the non-resistant cells. METHODS: Two isogenic MCF-7 breast cancer cell lines were selected for resistance to docetaxel (MCF-7(TXT)) and the wild type parental cell line (MCF-7(CC)) to examine if taxane-resistant breast cancer cells are sensitive to microtubule-destabilizing agents including vinca alkaloids and CSBAs. Cytotoxicity assays, immunoblotting, indirect immunofluorescence and live imaging were used to study drug resistance, apoptosis, mitotic arrest, microtubule formation, and microtubule dynamics. RESULTS: MCF-7(TXT) cells were demonstrated to be cross resistant to vinca alkaloids, but were more sensitive to treatment with colchicine compared to parental non-resistant MCF-7(CC) cells. Cytotoxicity assays indicated that the IC(50) of MCF-7(TXT) cell to vinorelbine and vinblastine was more than 6 and 3 times higher, respectively, than that of MCF-7(CC) cells. By contrast, the IC(50) of MCF-7(TXT) cell for colchincine was 4 times lower than that of MCF-7(CC) cells. Indirect immunofluorescence showed that all MTAs induced the disorganization of microtubules and the chromatin morphology and interestingly each with a unique pattern. In terms of microtubule and chromain morphology, MCF-7(TXT) cells were more resistant to vinorelbine and vinblastine, but more sensitive to colchicine compared to MCF-7(CC) cells. PARP cleavage assay further demonstrated that all of the MTAs induced apoptosis of the MCF-7 cells. However, again, MCF-7(TXT) cells were more resistant to vinorelbine and vinblastine, and more sensitive to colchicine compared to MCF-7(CC) cells. Live imaging demonstrated that the microtubule dynamics of MCF-7(TXT) cells were less sensitive to vinca alkaloids, and more sensitive to colchicine. MCF-7(TXT) cells were also noted to be more sensitive to other CSBAs including 2MeOE(2), ABT-751 and phosphorylated combretastatin A-4 (CA-4P). CONCLUSION: Docetaxel-resistant MCF-7(TXT) cells have demonstrated cross-resistance to vinca alkaloids, but appear to be more sensitive to CSBAs (colchicine, 2MeOE(2), ABT-751 and CA-4P) compared to non-resistant MCF-7(CC) cells. Taken together these results suggest that CSBAs should be evaluated further in the treatment of taxane resistant breast cancer.