Patients with triple-negative breast cancer (TNBC) have a poor prognosis because TNBC often metastasizes, leading to death. Among patients with TNBC, those with ERK2 (extracellular signal-regulated kinase 2)-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 2-dimensional culture. However, in 3-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 to prevent metastasis in certain cases in which tumors are MAPK dependent.

There is an unmet need in triple-negative breast cancer (TNBC) patients for targeted therapies. Maternal embryonic leucine zipper kinase (MELK) is a promising target for inhibition based on the abundance of correlative and functional data supporting its role in various cancer types. Areas covered: This review endeavors to outline the role of MELK in cancer. Studies covering a range of biological functions including proliferation, apoptosis, cancer stem cell phenotypes, epithelial-to-mesenchymal transition, metastasis, and therapy resistance are discussed here in order to understand the potential of MELK as a clinically significant target for TNBC patients. Expert opinion: Targeting MELK may offer a novel therapeutic opportunity in TNBC and other cancers. Despite the abundance of correlative data, there is still much we do not know. There are a lack of potent, specific inhibitors against MELK, as well as an insufficient understanding of MELK's downstream substrates. Addressing these issues is the first step toward identifying a patient population that could benefit from MELK inhibition in combination with other therapies.

We have identified that the ganglioside GD2 is a marker for breast cancer stem cells (BCSCs), and that targeting the enzyme GD3 synthase (GD3S, which regulates GD2 biosynthesis) reduces breast tumorigenesis. The pathways regulating GD2 expression, and their anomalous functions in BCSC, are unclear. Proteomic analysis of GD2+ and GD2- cells from breast cancer cell lines revealed the activation of NFκB signaling in GD2+ cells. Dose- and time-dependent suppression of NFκB signaling by the small molecule inhibitor BMS-345541 reduced GD2+ cells by > 90%. Likewise, BMS-345541 inhibited BCSC GD3S expression, mammosphere formation, and cell migration/invasion in vitro. Breast tumor-bearing mice treated with BMS-345541 showed a statistically significant decrease in tumor volume and exhibited prolonged survival compared to control mice, with a median survival of 78 d for the BMS-345541-treated group vs. 58 d for the controls. Moreover, in an experimental metastases model, treatment with BMS-345541 reduced the lung metastases by > 5-fold. These data suggest that GD2 expression and function, and NFκB signaling, are related, and they control BCSCs tumorigenic characteristics. Thus, the suppression of NFκB signaling by BMS-345541 is a potentially important advance in controlling breast cancer growth and metastases.

Centrosome amplification (CA) is a contributor to carcinogenesis, generating aneuploidy, and chromosome instability. Previous work shows that breast adenocarcinomas have a higher frequency of centrosome defects compared to normal breast tissues. Abnormal centrosome phenotypes are found in pre-malignant lesions, suggesting an early role in breast carcinogenesis. However, the role of CA in breast cancers remains elusive. Identification of pathways and regulatory molecules involved in the generation of CA is essential to understanding its role in breast tumorigenesis. We established a breast cancer model of CA using Her2-positive cells. Our goal was to identify centrosome cycle molecules that are deregulated by aberrant Her2 signaling and the mechanisms driving CA. Our results show some Her2+ breast cancer cell lines harbor both CA and binucleation. Abolishing the expression of Cdk4 abrogated both CA and binucleation in these cells. We also found the source of binucleation in these cells to be defective cytokinesis that is normalized by downregulation of Cdk4. Protein levels of Nek2 diminish upon Cdk4 knockdown and vice versa, suggesting a molecular connection between Cdk4 and Nek2. Knockdown of Nek2 reduces CA and binucleation in this model while its overexpression further enhances centrosome amplification. We conclude that CA is modulated through Cdk4 and Nek2 signaling and that binucleation is a likely source of CA in Her2+ breast cancer cells.