Differential Effect of Anti-apoptotic Genes Bcl-xL and c-FLIP on Sensitivity of MCF-7 Breast Cancer Cells to Paclitaxel and Docetaxel

Abstract

Background: Intrinsic or acquired resistance to chemotherapy is a major clinical problem leading to the fatality of patients with advanced and metastatic breast cancer. The overexpression of anti-apoptotic genes is believed to play a role in the resistance to chemotherapy. In the present study, we tested the sensitivity of MCF-7 breast cancer cells overexpressing anti-apoptotic genes TRAF-1, c-FLIP, Bcl-xL, cIAP-2 or Mn-SOD to paclitaxel and docetaxel. Materials and Methods: MTT and trypan blue dye exclusion assays were performed to examine the sensitivity of different cell lines to docetaxel and paclitaxel. Cell cycle analysis and carboxyfluorescein FLICA assay were employed to determine whether defects in the cell cycle arrest or apoptotic pathway are responsible for the resistance of cells overexpressing Bcl-xL or c-FLIP. Caspase 8 and 9 activities were measured in cells overexpressing Bcl-xL or c-FLIP exposed to docetaxel and paclitaxel using fluorescent substrate cleavage assay. Results: MCF-7 cells overexpressing Bcl-xL but not TRAF-1, cIAP-2 or Mn-SOD were less sensitive to both paclitaxel and docetaxel compared to vector-transfected control cells. Resistance of Bcl-xL-overexpressing cells to taxanes correlated with the failure to activate caspase 9. 2-Methoxyantimycin A3, a chemical inhibitor of Bcl-xL, sensitized Bcl-xL-overexpressing cells to paclitaxel and docetaxel, which suggests the drugs that inhibit Bcl-xL activity can be used as sensitizers to taxanes. MCF-7 cells overexpressing c-FLIP were less sensitive to paclitaxel but not to docetaxel. Paclitaxel failed to induce caspase 8 in c-FLIP-overexpressing cells. Conclusion: Because c-FLIP inhibits the extrinsic pathway of cell death whereas Bcl-xL inhibits the intrinsic pathway of cell death, these results suggest that overexpression of anti-apoptotic genes that inhibit either the extrinsic or intrinsic cell death pathways can reduce sensitivity of cancer cells to paclitaxel, whereas anti-apoptotic genes that inhibit only the intrinsic pathway reduce sensitivity to docetaxel.