Relationship between oleuropein metabolism, apoptosis, and HER2 expression in SKBR3 breast cancer cells. In order to evaluate how oleuropein aglycone's metabolism affects breast cancer cell viability, breast cancer apoptotic cell death, and HER2 oncogene expression, we performed MTT-based metabolic assays (as described in Figure 1), Cell Death ELISAs (as described in Figure 2), and HER2-specific ELISAs (as described in Figure 5) in serum-starved SKBR3 cells cultured in the absence or presence of equimolar concentrations of oleuropein glycoside (i.e., the precursor form of oleuropein aglycone), oleuropein aglycone, hydroxytyrosol (i.e., the dihydroxy moiety of oleuropein aglycone), and elenolic acid (i.e., the iridoid moiety of oleuropein aglycone). Results for HER2 protein expression are means (columns) and 95% confidence intervals (bars) of three independent experiments made in triplicate. Statistically significant differences (one-factor ANOVA analysis) between experimental conditions and unsupplemented control cells are shown by asterisks (* P < .01, ** P < .001). All statistical tests were two-sided. Figure shows both the chemical structures and the metabolic pathway of the EVOO phenolic compounds studied.