Hydroxytyrosol reduces intracellular reactive oxygen species levels in vascular endothelial cells by upregulating catalase expression through the AMPK–FOXO3a pathway

Reactive oxygen species are critically involved in the endothelial dysfunction that contributes to atherosclerosis development. Hydroxytyrosol (HT), a main phenolic compound in olive oil and leaves from Olea europaea L., has antiatherogenic properties with powerful antioxidant activity. The present study verifies the antioxidant activity of HT on H2O2-induced intracellular reactive oxygen species in porcine pulmonary artery endothelial cells (VECs) and the involved molecular mechanisms. Incubation of VECs with HT prevented the increase in intracellular reactive oxygen species levels in the presence of H2O2. HT increased catalase mRNA, protein and activity. Catalase siRNA suppressed HT-dependent reduction of intracellular reactive oxygen species. HT increased both cytosolic and nuclear protein levels of forkhead transcription factor 3a (FOXO3a), as well as the phosphorylation of AMP-activated protein kinase (AMPK) that translocates FOXO3a to the nucleus. AMPK siRNA and a specific inhibitor suppressed HT-induced FOXO3a upregulation and catalase expression. Moreover, FOXO3a siRNA blocked HT-dependent increase in catalase expression. Taken together, our findings strongly demonstrate that HT positively regulates the antioxidant defense system in VECs by inducing the phosphorylation of AMPK with subsequent activation of FOXO3a and catalase expression, and provides a molecular basis for the prevention of cardiovascular diseases by HT.