Cancer protection associated with the consumption of olive products is well established, but not for leukemia. The protective effects of olive (Olea europaea L.) leaves were investigated by incubating human promyelocytic leukemia HL-60 cells with olive leaf extracts (OLEs) from seven principal Tunisian olive varieties, namely, Chemchali, Chemlali, Chétoui, Gerboui, Sayali, Zalmati and Zarrazi. The results showed significant growth inhibition of HL-60 cells incubated for 48 h with a 100-fold dilution of each OLE which had been obtained by incubating 10 g of dried leaves in 100 ml of 70% ethanol for one week with subsequent ultrafiltration. DNA fragmentation was observed in the cells incubated for 19 h with a 100-fold dilution of the Chemchali, Chemlali and Zalmati extracts. The results of a nitroblue tetrazolium (NBT) assay revealed NBT reduction, a differentiation marker, by the OLE-treated cells after an overnight incubation. The Gerboui extract showed the highest NBT reduction ability at more than 90%. An HPLC analysis revealed the presence of apigenin 7-glucoside in the extract, which was found in subsequent experiments to be responsible for the Gerboui extract-mediated cell differentiation.

The aim of this study was to investigate the influence of the solvent type on the extraction of phenolics and the antioxidant properties of the extracts obtained from Chétoui olive leaves. Extraction was conducted at room temperature using four solvents: deionized water (ddH2O), 80% methanol (80% MeOH), 70% ethanol (70% EtOH), and 80% acetone. Total phenols and total flavonoids were measured using the Folin-Ciocalteau and aluminum chloride colorimetric methods, respectively. The antioxidant properties have been determined by two scavenging activity methods, DPPH and ABTS. The total phenol and flavonoid contents of the olive leaf extracts (OLEs) ranged from 16.52 to 24.93 mg gallic acid g–1 DM and from 6.23 to 21.47 mg catechin g–1 DM, respectively. The IC50 values of DPPH varied from 0.17 to 0.97 mg mL–1 and the antioxidant activity values determined by ABTS (trolox equivalent antioxidant capacity) were between 629.87 and 1064.25 μmol TE g–1 DM. Our results revealed that extracting solvents have a significant influence on the antioxidant properties of olive leaves and that a methanol mixture is recommended for extracts with high levels of flavonoids and important antioxidant activities. Moreover, it was noticed that the trolox equivalent antioxidant capacity (TEAC) depends more on the flavonoids (r = 0.821) than on the total phenols (r = 0.399).