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

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. 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. 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 β2- and β4-tubulin and relatively lower levels of β3-tubulin than both MCF-7_CC and MCF-7_DOX cells. The subcellular localization of various β-tubulin isoforms in MCF-7_TXT cells is also different from that in MCF-7_CC and MCF-7_DOX cells. 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 β-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