Triple-negative breast cancer (TNBC) has high relapse and metastasis rates and a high proportion of cancer stem-like cells (CSCs), which possess self-renewal and tumor initiation capacity. MELK (maternal embryonic leucine zipper kinase), a protein kinase of the Snf1/AMPK kinase family, is known to promote CSC maintenance and malignant transformation. However, the role of MELK in TNBC metastasis is unknown; we sought to address this in the current study. We found that MELK mRNA levels were higher in TNBC tumors (8.11 [3.79-10.95]) than in HR+HER2- tumors (6.54 [2.90-9.26]; P < 0.001). In univariate analysis, breast cancer patients with high-MELK-expressing tumors had worse overall survival (P < 0.001) and distant metastasis-free survival (P < 0.01) than patients with low-MELK-expressing tumors. In a multicovariate Cox regression model, high MELK expression was associated with shorter overall survival after adjusting for other baseline risk factors. MELK knockdown using siRNA or MELK inhibition using the MELK inhibitor MELK-In-17 significantly reduced invasiveness, reversed epithelial-to-mesenchymal transition, and reduced CSC self-renewal and maintenance in TNBC cells. Nude mice injected with CRISPR MELK-knockout MDA-MB-231 cells exhibited suppression of lung metastasis and improved overall survival compared with mice injected with control cells (P < 0.05). Furthermore, MELK-In-17 suppressed 4T1 tumor growth in syngeneic BALB/c mice (P < 0.001). Our findings indicate that MELK supports metastasis by promoting epithelial-to-mesenchymal transition and the CSC phenotype in TNBC. Significance: These findings indicate that MELK is a driver of aggressiveness and metastasis in TNBC.

<p>Supplementary Figure S4. Graphical representation of the physical protein-protein interaction network involved in modulating MELK activity in MDA-MB-231 cells.</p>

<p>Supplementary Table S1. Full list of candidate regulators that were significantly enriched for 60 Reactome and WikiPathways, as identified by Affymetrix Genechip microarray analysis.</p>

<p>Supplementary Figure S4. Graphical representation of the physical protein-protein interaction network involved in modulating MELK activity in MDA-MB-231 cells.</p>

<p>Supplementary Table S1. Full list of candidate regulators that were significantly enriched for 60 Reactome and WikiPathways, as identified by Affymetrix Genechip microarray analysis.</p>