Quercetin-3-methyl ether suppresses human breast cancer stem cell formation by inhibiting the Notch1 and PI3K/Akt signaling pathways

Breast cancer is a leading cause of mortality among women with cancer worldwide. Quercetin-3-methyl ether, a natural compound occurring in various plants, has been indicated to have potent anticancer activity. Breast cancer cell growth and survival were examined by CCK-8 and colony formation assay, whilst cell cycle and apoptosis were determined by flow cytometry. Cell invasion and migration were assessed by wound-healing assay and Transwell assay. Cancer stem cell formation was analyzed by mammosphere formation assay and related signaling pathways were detected by western blotting. In the present study, it was observed that treatment with quercetin-3-methyl ether significantly inhibited cell growth, induced apoptosis and cell cycle arrest at the G(2)-M phase, and suppressed invasion and migration in human breast cancer cells, including the triple negative MDAMB-231 cell line, and the estrogen receptor-positive/progesterone receptor-positive/human epidermal growth factor receptor 2-negative MCF-7 and T47D cell lines. This compound also markedly suppressed the epithelial-mesenchymal transition process as evidenced by the upregulated expression of E-cadherin, and the concomitant downregulated expression of vimentin and MMP-2. Furthermore, it was demonstrated that quercetin-3-methyl ether treatment inhibited mammo-sphere formation and the expression of the stemness-related genes, SRY-box 2 and Nanog. Mechanistically, this compound decreased the expression of Notch1, and induced the phosphorylation of phosphoinositide 3-kinase (PI3K) and Akt. It also attenuated the human insulin growth factor 1-induced phosphorylation of PI3K, Akt and glycogen synthase kinase . Additionally, the combination of quercetin-3-methyl ether and a secretase inhibitor (DAPT) exhibited additive suppression of the expression of Notch1, PI3K, Akt and mammalian target of rapamycin and a more marked inhibitory effect on cell proliferation and colony formation compared with either drug alone. Treatment with quercetin-3-methyl ether alone markedly suppressed the levels of tri-methyl histone H3 (Lys27), but had no effect on the expression of enhancer of zeste homolog 2. Overall, these findings indicated that quercein-3-methyl ether may be a potential therapeutic compound for the treatment of triple negative and hormone-sensitive breast cancer.