Harmful effect of oxysterols in the gut and at systemic level: modulatory activity of olive oil phenolics

A healthy and proper diet is essential to prevent the onset of various diseases, as with the diet we can also introduce harmful substances. Among others, the oxysterols are particularly relevant; they can be absorbed from the diet and are the oxidized products of cholesterol present in cholesterol-containing foodstuffs after cooking, processing and storage, such as meat, fish, dairy products and eggs. They can be also generated endogenously from either the spontaneous or enzymatic oxidation of cholesterol. Oxysterols have been shown to exert several in vitro and in vivo biochemical activities, physiological but mostly pathological. In particular, they may induce cytotoxicity, cell death (necrosis and apoptosis), pro-oxidant and pro-inflammatory effects in different cells and tissues and have been linked with the onset and development of major chronic diseases. Only in the last few years, oxysterols have been shown to interfere with the homeostasis of the human digestive tract, demonstrating their involvement in the pathogenesis of human inflammatory bowel disease and colon rectal cancer. Experimental studies suggest that the ingestion of compounds with antioxidant action, such as phenolics, is able to counteract the oxidative stress and inflammatory response in the intestine and inhibit the onset of the main related diseases. Extra virgin olive oil polyphenols in particular, recognized for their strong antioxidant property and related anti-inflammatory action, may display this effect in the intestinal lumen where they concentrate before absorption, preventing intestinal diseases. The first aim of this study was to evaluate the ability of different extra virgin olive oil phenolic extracts to counteract the pro-oxidant, pro-apoptotic and pro- inflammatory action of oxysterols in the human colon adenocarcinoma cell line (Caco-2) and the molecular mechanism involved. Oxysterols treatment, depending on time of exposure and concentrations used, significantly altered the cellular redox status (ROS production increase, GSH decrease, GPX activity enhancement), induced an oxidative damage to the membrane lipid fraction (MDA production increase) and cell death (Caspase 3 increase); cytokines production (IL-6, IL-8) was indicative of a pro-inflammatory effect. Phenolic extracts were able to significantly counteract the oxysterols harmful effects, at least in part by modulating intracellular signaling pathways (Akt/PKB, MAPK) involved in the cellular response to oxidative stress, apoptosis and inflammation. Several studies in animals and humans have shown that dietary oxysterols, after digestion can be absorbed from the gut and transported into the circulation within chylomicrons and other lipoproteins. Furthermore, the presence of oxysterols in plasma can derive from the oxidation of endogenous cholesterol through enzymatic or spontaneous reactions. Oxysterols have been found at increased levels in the plasma of hypercholesterolemic subjects and have been linked with the atherosclerotic process. In this context the second part of this study was specifically focused on the protective effect of pure olive oil phenolics, which can be found in the blood stream after absorption (hydroxytyrosol, tyrosol, and homovanillc alcohol), against the pro-oxidant and pro-inflammatory activity of oxysterols in blood cells (PBMCs). It was demonstrated that all phenolic compounds tested, HT, TYR and HVA were able to inhibit oxysterol-induced pro-inflammatory cytokines production. According to the hypotesis that pro-inflammatory cytokine release may be induced by changes in intracellular redox status, it was observed that, in human PBMCs treated with the oxysterols mixture, simple phenols were also able to inhibit ROS production as well as to suppress both redox-based MAPK phosphorylation (JNK, p38). These data suggest that olive oil phenolic extracts may significantly contribute to preserve the integrity of intestinal mucosa against oxidative damage and inflammation related disorders and simple phenols found in the blood after metabolism and absorption counteract the oxysterols-induced inflammatory effect at systemic level.