A Comparative Study of Oleuropein Extraction from Wild Olive Leaves (Olea europea subsp. oleaster, Hoffmanns. & Link), Its Gastrointestinal Stability, and Biological Potential

Olive leaves are the richest source of phenolic compounds, particularly oleuropein, which has many beneficial effects on human health. This study compares the effect of three extraction techniques: ultrasonic-solvent extraction, microwave-assisted extraction, and supercritical CO2 extraction of freeze-dried wild olive leaves (Olea europea subps. oleaster, Hoffmanns. & Link) on oleuropein concentration in obtained extracts. The extract with the highest concentration of oleuropein was obtained after low-frequency ultrasound solvent extraction (WOLE-S) (71.97 mu g/mL). Oleuropein from that extract showed low stability after simulated gastric digestion with human gastric juices (35.66%) and high stability after 60 min of intestinal phase (65.97%). The highest antiproliferative activity of WOLE-S and pure oleuropein was detected against T24 cancer cells. The antiproliferative activity of WOLE-S against MD-MBA-123, A549, and A1235 cells was moderate, while the activity of oleuropein was high. WOLE-S significantly down-regulated the expression of IL-6 in A549 cells, indicating a possible anti-inflammatory effect in lung cancer cells. The expression of IL-8 was slightly reduced by treating the cells with WOLE-S, while the expression of IL-10 was not altered by the treatment with WOLE-S or oleuropein.