Lapatinib sensitizes quiescent MDA-MB-231 breast cancer cells to doxorubicin by inhibiting the expression of multidrug resistance-associated protein-1

The quiescent state plays an important role in tumor recurrence because it protects cancer cells from chemotherapy. Previously, we optimized tumorsphere cultures for in vitro screening methods for targeting quiescent cell population since the majority of cells in tumorspheres are quiescent. In this study, we analyzed efficacies of current chemotherapeutics in tumorsphere assays to seek better strategies for eradicating quiescent cell population. Tumorspheres generated from MDA-MB-231 cells exhibited accumulations of cells in the G0/G1 phase as compared with cells in monolayer culture, suggesting that sphere formation contributes to an increase of quiescent cells. As a result of a decreased doxorubicin uptake, MDA-MB-231 tumorspheres exhibited chemoresistance to both doxorubicin and paclitaxel. Since we found that the enhanced EGFR signaling is characteristics of MDA-MB-231 tumorspheres, the combination effects of chemotherapy with lapatinib, a dual ErbBl/ErbB2 inhibitor, were accessed in tumorsphere assays. Western blot analysis revealed that lapatinib inhibited the phosphorylation of EGFR, AKT and p38 in doxorubicin-treated tumorspheres. The inhibition of EGFR signaling by the treatment with lapatinib suppressed the expression of multidrug resistance-associated protein-1 (MRP-1), leading to increased cytotoxicity of doxorubicin to tumorspheres. Furthermore, blockade of the PI3K/AKT and p38 MAPK signaling pathways resulted in a remarkable decrease in the expression of MRP-1 in doxorubicin-treated tumorspheres. These results demonstrate that lapatinib sensitizes quiescent MDA-MB-231 breast cancer cells to doxorubicin by inhibiting doxorubicin-induced MRP-1 expression via PI3K/AKT and p38 MAPK signaling pathways. Thus, this study suggests that treatment with lapatinib in combination with anti-mitotic drugs maybe a useful approach to improve clinical response by eradicating the quiescent cancer cell population.