Tunicamycin potentiates paclitaxel-induced apoptosis through inhibition of PI3K/AKT and MAPK pathways in breast cancer

Paclitaxel has been reported to upregulate both AKT and MAPK signaling pathways and thereby compromises its antitumor efficacy. However, tunicamycin has the ability to downregulate AKT and MAPK pathways. The aim of the study is to investigate the antitumor activity of the combination treatment of paclitaxel with tunicamycin and the mechanisms involving the changes of antitumor efficacy. Sulforhodamine B (SRB) assay was used to examine the cell viability upon treatment of breast cancer cells with paclitaxel, tunicamycin and the combination of both. Cell cycle distributions and apoptosis were detected by flow cytometry. Western blotting and immunofluorescence staining were used to analyze the effect of drugs on tubulin polymerization. The antitumor growth of combined treatment was measured in nude mice bearing MDA-MB-231 xenograft. Western blotting was performed to explore the alteration of AKT and MAPK pathways in vitro and in vivo. SRB assay and nude mice experiment showed that tunicamycin synergistically enhanced paclitaxel-induced inhibition of cell proliferation and tumor growth. Tunicamycin had no clear effect on paclitaxel-induced cell cycle arrest, demonstrating that cell cycle distribution was not involved in the enhanced antitumor activity. Both annexin V-FITC/propidium iodide assay and TUNEL assay indicated that the combination of tunicamycin with paclitaxel resulted in significant increased cell apoptosis as compared with individual treatment in vitro and in vivo. Tunicamycin decreased paclitaxel-induced microtubulin polymerization, suggesting that enhanced antitumor effect of paclitaxel was not dependent of microtubulin polymerization. Western blotting analysis confirmed that tunicamycin decreased paclitaxel-induced upregulation of survival signal pathways such as AKT and MAPK. These results revealed that tunicamycin synergistically enhanced the antitumor effects of paclitaxel through potentiating apoptosis via inhibiting paclitaxel-induced elevation of AKT and MAPK pathways. This study raised the possibility that the combination of paclitaxel with tunicamycin may be a promising approach for improving the clinical activity of paclitaxel.