Multiple mechanisms underlying acquired resistance to taxanes in selected docetaxel-resistant MCF-7 breast cancer cells

Background: Chemoresistance is a major factor involved in a poor response and reduced overall survival in patients with advanced breast cancer. Although extensive studies have been carried out to understand the mechanisms of chemoresistance, many questions remain unanswered. Methods: In this research, we used two isogenic MCF-7 breast cancer cell lines selected for resistance to doxorubicin (MCF-7(DOX)) or docetaxel (MCF-7(TXT)) and the wild type parental cell line (MCF-7(CC)) to study mechanisms underlying acquired resistance to taxanes in MCF-7(TXT) cells. Cytotoxicity assay, immunoblotting, indirect immunofluorescence and live imaging were used to study the drug resistance, the expression levels of drug transporters and various tubulin isoforms, apoptosis, microtubule formation, and microtubule dynamics. Results: MCF-7(TXT) cells were cross resistant to paclitaxel, but not to doxorubicin. MCF-7(DOX) cells were not cross-resistant to taxanes. We also showed that multiple mechanisms are involved in the resistance to taxanes in MCF-7(TXT) cells. Firstly, MCF-7(TXT) cells express higher level of ABCB1. Secondly, the microtubule dynamics of MCF-7(TXT) cells are weak and insensitive to the docetaxel treatment, which may partially explain why docetaxel is less effective in inducing M-phase arrest and apoptosis in MCF-7(TXT) cells in comparison with MCF-7(CC) cells. Moreover, MCF-7(TXT) cells express relatively higher levels of beta 2- and beta 4-tubulin and relatively lower levels of beta 3-tubulin than both MCF-7(CC) and MCF-7(DOX) cells. The subcellular localization of various beta-tubulin isoforms in MCF-7(TXT) cells is also different from that in MCF-7(CC) and MCF-7(DOX) cells. Conclusion: Multiple mechanisms are involved in the resistance to taxanes in MCF-7(TXT) cells. The high expression level of ABCB1, the specific composition and localization of beta-tubulin isoforms, the weak microtubule dynamics and its insensitivity to docetaxel may all contribute to the acquired resistance of MCF-7(TXT) cells to taxanes.