Olive oil oleuropein has anti-breast cancer properties with higher efficiency on ER-negative cells

► Oleuropein is toxic against different breast cancer cell lines. ► Oleuropein triggers apoptosis efficiently in ER-negative MDA-MB-231 cells. ► The intrinsic pathway is involved in the apoptosis process. ► Oleuropein delays the cell cycle of breast cancer cells at S phase. ► Oleuropein down-regulates NF-κB and its main oncogenic target cyclin D1.