Table 1.

Summary of hydroxytyrosol (HT) properties in different study models. GCL: γ-glutamyl cysteine ligase; GR: GSH reductase; HO-1: heme oxygenase-1; NQO1: NAD(P)H quinone oxidoreductase; CAT: catalase; ROS: reactive oxygen species; FOXO3a: forkhead transcription factor 3a; NO: nitric oxide; PGE2: prostaglandin E2; iNOS: inducible NO synthase; PGES: prostaglandin E2 synthase; NAFLD: non-alcoholic fatty liver disease; HFD: high-fat diet; PPAR-α: peroxisome proliferator-activated receptor-α; GSH: glutathione-ethyl ester; CHOP: CCAAT-enhancer-binding protein homologous protein; BiP: binding immunoglobulin protein; hECs: human umbilical vein endothelial cells; IL: interleukin; EGFR: epidermal growth factor receptor; LDL: low density lipoproteins; UPR: unfolded protein response.

Properties	Reference	Model	Intervention	Results/Findings
Antioxidant	Giordano et al., 2014 [37]	Animal/cellular	C57BL/6 mice were randomly assigned to a standard diet without HT or the same diet supplemented with HT (0.03 gm%) for 8 weeks. Murine 3T3-L1 pre-adipocytes, after differentiation, were cultured with H2O2 and 1 or 5 μM HT.	Modulated gene expression of pathways related to oxidative stress (OS) in adipose tissue. Decreased GSSG/GSH ratio in cultured adipocytes.
	Granados-Principal et al., 2014 [38]	Animal	Thirty-six female Sprague–Dawley rats with induced mammary tumors were divided into four groups: control, HT (0.5 mg/kg, 5 days/week), doxorubicin (1 mg/kg/week), and doxorubicin + HT.	Improved drug-induced cardiac alterations induced by doxorubicin by reducing mitochondrial damage and OS.
	Zhu et al., 2010 [40]	Cellular	Cultured ARPE-19 cells were pretreated with HT dissolved in dimethylsulfoxide (DMSO; final DMSO concentration ≤ 0.025%) and treated with acrolein to induce OS.	Increased translocation of Nrf2 to the nucleus in cells not challenged with acrolein. Increased expression of phase II detoxifying enzymes (GCL, GR, GSH GPx, HO-1, NQO1) and of genes involved in oxidative defense (indirect effect). Increased expression and activity of CAT.
	Zrelli et al., 2011 [41]	Cellular	VECs with OS induced by H2O2 were incubated with HT (10, 30 and 50 μM).	Prevented intracellular increase of ROS levels. Increased CAT mRNA expression levels. Increased expression and activity of FOXO3a.
Anti-inflammatory	Richard et al., 2011 [9]	Cellular	Murine macrophages (RAW264.7 cells) were stimulated with lipopolysaccharide (LPS) and treated with olive vegetation water containing 2.5% HT.	Inhibited of NO and PGE2 production. Decreased secretion of cytokines (interleukin (IL)-1α, IL-1β, IL-6, IL-12, TNF-α, and chemokines (CXCL10/IP-10, CCL2/MPC-1)). Decreased gene expression of iNOS, IL-1α, CXCL10/IP-10, MIP-1β, matrix metalloproteinase-9 and PGES.
	Pirozzi et al., 2016 [42]	Animal	Rats with NAFLD were divided into three groups: Control diet, HFD and HFD + HT (10 mg/kg/d) for 5 weeks.	Increased levels of PPAR-α. Reduced the expression of pro-inflammatory cytokines TNF-α and IL-6.
	Lopez et al., 2017 [46]	Cellular	hECs were cultured in the presence of HT and HT metabolites and treated with TNF-α.	HT and its metabolites suppressed production of ROS induced by TNF-α. Down-regulated TNF-α-induced phosphorylation of NF-κB signaling proteins (IKKαβ, IκBα, and p65) in hECs.
Anticancer	Warleta et al., 2011 [47]	Cellular	Three human breast cell lines were treated with HT and tyrosol: mammary epithelial cells (MCF10A) and breast cancer cells (MDA-MB-231, MCF7).	Decreased ROS production in a dose-dependent manner in MCF10A cells. Decreased H2O2 induced ROS level in breast cancer cells. Reduced DNA damage significantly in MCF7, MDA-MB-231 and MCF10A (in unexposed cells).
	Terzuoli et al., 2016 [48]	Cellular	Human colorectal adenocarcinoma cells (HT-29, CaCo2, WiDr) or human colon fibroblast cells (CCD18Co) treated with HT.	Down-regulated EGFR expression in colon tumor cells by promoting its degradation via lysosomal and proteasomal mechanisms.
	Notarnicola et al., 2011 [49]	Cellular	Human colon adenocarcinoma cell lines (HT-29 and SW-620) were treated with HT and oleuropein at different concentrations (10, 25, 50 and 100 μM) for 24 and 72 h.	Reduced both gene expression and activity of FAS (starting from 10 μM). Showed an anti-proliferative effect in SW-620 both cell lines Had a pro-apoptotic effect in HT-29 cells.
	Han et al., 2009 [50]	Cellular	MCF-7 human breast cancer cells were treated with HT (6.25, 12.5, 25, 50 μg/mL) and oleuropein for 12 h.	Inhibited cell proliferation, reducing the cell viability in a time and concentration-dependent manner. Induced cell apoptosis by caspases activation and also blocked the cell cycle in G1 phase.
Endothelial and vascular function	Valls et al., 2015 [51]	Human	Thirteen pre- and stage-1 hypertensive patients received a single dose of 30 ml of functional virgin olive oil (FVOO) (phenolic content = 961 mg/kg) or regular virgin olive oil (VOO) (phenolic content = 289 mg/kg) in a postprandial randomized, double blind, crossover trial.	FVOO improved human endothelial function (ischemic reactive hyperemia values were higher with FVOO than with VOO). Postprandial values of PAI-I, and hsCRP were lower after FVOO versus VOO. FVOO reduced oxidized LDL.
	Lockyer et al., 2015 [52]	Human	Eighteen healthy volunteers who consumed either OLE (51 mg oleuropein; 10 mg HT), or a matched control (separated by a 4-week wash out) on a single occasion were studied in a randomized, double-blind, placebo-controlled, cross-over, acute intervention trial.	OLE reduced arterial stiffness. OLE reduced IL-8 production.
	Catalán et al., 2015 [11]	Cellular	Human aortic endothelial cells (HAEC) were treated with HT and the mixture of its metabolites (1, 2, 5, and 10 μM) and co-incubated with TNF-α for 18 and 24 h.	HT and its metabolites reduced the secretion of E-selectin, ICAM-1, and VCAM-1. Free HT and HT metabolites were effective in the reduction of the endothelial dysfunction biomarkers.
	Catalán et al., 2016 [53]	Animal	Twelve female Wistar rats were separated in three groups: standard diet, diet supplemented with HT or diet supplemented with secoroids (SEC) in doses of 5 mg/kg/d for 21 days.	HT was detected in heart tissue mainly in its free form after supplementation with HT or SEC. Supplementation changed heart and aorta proteome. These proteins are related to cardiovascular function, improving endothelial and vascular function.
Anti-steatotic	Park et al., 2011 [54]	Animal	Male C57BL and 6N mice were separated into three groups: normal diet, HFD, HFD supplemented with oleuropein (the precursor of HT), for 10 weeks.	Had a protective effect in reversing the negative effects induced by a HFD, regularizing: hepatic steatosis, increased plasma lipids, and increased body weight and liver. Down-regulated transcription factors and their target genes involved in adipogenesis.
	Priore et al., 2014 [55]	Cellular	Rat-liver cells were treated with HT, tyrosol and oleuropein (EVOO phenols).	Cholesterol synthesis and fatty acids (FA) synthesis were inhibited by the treatment. Reduced the activity of ACC.
	Pirozzi et al., 2016 [42]	Animal	Male rats were divided into three groups: standard diet, HFD, HFD + HT (10 mg/kg/d) for 5 weeks.	Reduced AST, ALT and cholesterol levels in serum, and reduced liver steatosis. Increased the activity of PPAR-α. Increased phosphorylation of ACC, increasing hepatic metabolism and oxidation of FA.
ER stress and autophagy	Giordano et al., 2014 [13]	Cellular	Human hepatocarcinoma cells (HepG2) were treated with HT (1 μM and 5 μM) and 100 μM lipoic acid (LA) and glutathione-ethyl ester (GSH), for 24 h. UPR was induced tunicamycin for 4 h.	Reduced mRNA levels of CHOP and BiP compared with LA and GSH and with tunicamycin alone. Reduced protein levels of BiP and levels of eIF2a. Modulated the antiapoptotic Bcl2 protein levels. Prevented ER stress in hepatic cells.
	Giordano et al., 2015 [57]	Cellular	Human hepatocarcinoma cells (HepG2) were treated with two HT hepatic metabolites: i.e., 3-O-HT glucuronide and 4-O-HT glucuronide. UPR was induced tunicamycin.	Both metabolites reduced mRNA expression levels of CHOP and BiP. The treatment also decreased BiP and CHOP protein levels.
	Cetrullo et al., 2016 [14]	Cellular	Primary cultures of chondrocytes obtained from patients with knee arthroplasty were incubated in the absence or presence of 100 μM H2O2 and treated with HT.	Enhanced SIRT-1 expression, positively regulating autophagy.
	Feng et al., 2011 [58]	Animal	Eight-week-old male Sprague–Dawley rats were selected for the experiment by their ability to perform 1 week of running exercise at low speed. Rats were divided into four groups: sedentary with or without HT, and endurance exercise with or without HT (25 mg/kg/d). After eight weeks of exercise the analyses were done.	Reduced OS and thereby mitochondrial impairment. Reduced muscular atrophy induced by autophagy and mitochondrial fission and decreased expression of PGC-1α. Up-regulated autophagy.
Mitochondrial function	Hao et al., 2010 [60]	Cellular	Murine 3T3-L1 pre-adipocytes were treated with HT 0.1–50 μmol/L.	Stimulated activation and expression of PGC1α. Increased the mRNA expression levels of Nrf1, Nrf2 and Tfam (1 μmol/L HT). Promoted protein expression of complex I, II, III and V. Promoted the activity of complexes I, II, III, IV and V, increasing oxygen consumption in adipocytes. Increased mitochondrial mass.
	Granados-Principal et al., 2014 [38]	Animal	Thirty-six female Sprague–Dawley rats with induced mammary tumors were divided into four groups: control, HT (0.5 mg/kg, 5 d/week), doxorubicin (1 mg/kg/week), and doxorubicin + HT.	Improved the mitochondrial electron transport chain in rats with cardiotoxicity induced by doxorubicin. Increased complexes II and III protein concentrations.
	Zheng et al., 2015 [15]	Animal	Male db/db C57BL/6J mice were separated into three groups: control, HT (10 mg/mg/d) and HT (50 mg/kg(d)	Improved expression of complexes I, II, and IV. Increased activity of complex I. Induced phase II antioxidant systems and inhibited protein oxidation in mice brain. Increased the expression of p-AMPK/AMPK, PGC-1α and SIRT1.
	Zhu et al., 2010 [40]	Cellular	Human retinal pigment epithelial cells (ARPE-19) were incubated with acrolein. The protective effects of HT were studied by pre-treating cells with HT for 48 h, followed by 24-h acrolein treatment in the absence of HT.	Increased the expression of PGC1α. Increased protein expression of mitochondrial transcription factor A and uncoupling protein 2 (UCP2). Increased the expression of complexes. Increased Nrf2 nuclear protein levels and its nuclear translocation. Enhanced phase II detoxifying enzymes.