Patients with triple-negative breast cancer (TNBC) have a poor prognosis because TNBC often metastasizes, leading to death. Among patients with TNBC, those with extracellular signal-regulated kinase 2 (ERK2)-overexpressing tumors were at higher risk of death than those with low-ERK2-expressing tumors (hazard ratio, 2.76; 95% confidence interval, 1.19-6.41). The MAPK pathway has been shown to be a marker of breast cancer metastasis, but has not been explored as a potential therapeutic target for preventing TNBC metastasis. Interestingly, when we treated TNBC cells with the allosteric MEK inhibitor selumetinib, cell viability was not reduced in two-dimensional culture. However, in three-dimensional culture, selumetinib changed the mesenchymal phenotype of TNBC cells to an epithelial phenotype. Cells that undergo epithelial-mesenchymal transition (EMT) are thought to contribute to the metastatic process. EMT leads to generation of mesenchymal-like breast cancer cells with stem cell-like characteristics and a CD44(+)CD24(-/low) expression pattern. We tested the hypothesis that targeted inhibition of the MAPK pathway by selumetinib inhibits acquisition of the breast cancer stem cell phenotype and prevents lung metastasis of TNBC. TNBC cells treated with selumetinib showed inhibition of anchorage-independent growth, an indicator of in vivo tumorigenicity (P < 0.005), and decreases in the CD44(+)CD24(-/low) fraction, ALDH1 activity, and mammosphere-forming efficiency. Mice treated with selumetinib formed significantly fewer lung metastases than control mice injected with vehicle (P < 0.05). Our data demonstrate that MEK inhibitors can inhibit breast cancer stem cells and may have clinical potential for the prevention of metastasis in certain cases in which tumors are MAPK dependent.
©2015 American Association for Cancer Research.