**2.1 Antioxidant activity**

The antioxidant activity is the most studied property of olive phenolic compounds. The interest of hydroxytyrosol is based on its remarkable pharmacological and antioxidant activities. Reactive oxygen species, which are continuously being formed as a result of metabolic processes in the organism, may cause oxidation and damage of cellular macromolecules, and therefore, may contribute to the development of degenerative diseases, such as atherosclerosis, cancer, diabetes, rheumatoid arthritis and other inflammatory diseases (Balsano & Alisi, 2009).

The high antioxidant efficiency of HT, attributed to the presence of the *o*-dihydroxyphenyl moiety, is due to its high capacity for free radical scavenging during the oxidation process and to its reducing power on Fe3+ (Torres de Pinedo et al., 2007). The antioxidant properties of the *o*-diphenols are associated with their ability to form intramolecular hydrogen bonds between the hydroxyl group and the phenoxy radical (Visioli et al., 1998); therefore, the catechol avoids the chain propagation by donating a hydrogen radical to alkylperoxyl radicals (ROO· ) formed in the initiation step of lipid oxidation (Scheme 1).

Scheme 1. Mechanism of free radical scavenging by hydroxytyrosol

Oxidation of low-density lipoproteins (LDLs) is a lipid peroxidation chain reaction, which is initiated by free radicals. It has been shown that hydroxytyrosol can inhibit LDL oxidation efficiently due to its capacity to scavenge peroxyl radicals (Arouma et al., 1998; Turner et al., 2005). Hydroxytyrosol reduces oxidation of the low-density lipoproteins carrying cholesterol (LDL-C), which is a critical step in the development of atherosclerosis and other cardiovascular diseases (Gonzalez-Santiago et al., 2010; Vázquez-Velasco et al., 2011); hydroxytyrosol has also a potential protective effect against oxidative stress induced by *tert*butyl hydroperoxide (Goya et al., 2007).

It has been reported that hydroxytyrosol enhances the lipid profile and antioxidant status preventing the development of atherosclerosis. This compound may also reduce the expression of vascular cell adhesion molecules (Carluccio et al., 2007) and inhibit platelet aggregation in rats (González-Correa et al., 2008a) and hypercholesterolaemia in humans (Ruano et al., 2007).

#### **2.2 Anticancer activity**

Numerous studies about the relationship between olive oil consumption and cancer prevention have been carried out (Pérez-Jiménez et al., 2005). Antioxidant compounds supplied in the diet can reduce the risk of cancer due to the fact that they can minimize DNA damage, lipid peroxidation and the amount of ROS generated (Omar, 2010a; Hillestrom, 2006; Manna, 2005).

It has been reported that HT may exert a pro-apoptotic effect by modulating the expression of genes involved in tumor cell proliferation of promyelocytes (HL60 cells) (Fabiani et al., 2006, 2008, 2009, 2011). Moreover, it has been shown that HT inhibits proliferation of human MCF-7 breast cancer cells (Siriani et al., 2010; Bulotta et al. 2011; Bouallagui et al., 2011a), human HT29 colon carcinoma cells (Guichard et al., 2006), human M14 melanoma cells (D'Angelo et al., 2005) and human PC3 prostate cells (Quiles et al., 2002).

Pre-treatment of HepG2 cells with hydroxytyrosol prevented cell damage, what could be due to the fact that hydroxytyrosol may prepare the antioxidant defense system of the cell to face oxidative stress conditions (Goya et al., 2007, 2010).
