Modulation of LPS-induced nitric oxide production in intestinal cells by hydroxytyrosol and tyrosol metabolites: Insight into the mechanism of action

At intestinal level, after acute or chronic exposure to iNOS-derived NO, a toxic mechanism of action leads to inflammation and degenerative diseases. The aim of this study was to investigate the effect of glucuronide and sulfate metabolites of the extra virgin olive oil phenols tyrosol (Tyr) and hydroxytyrosol (HT), in comparison with their parent compounds, on the release of NO following exposure to a pro-inflammatory stimulus, the bacterial lipopolysaccharide (LPS). Human colon adenocarcinoma cells (Caco-2), differentiated as normal enterocytes, were treated with pathological concentrations of LPS, in order to stimulate iNOS pathway, which involves NF-ĸB activation through IĸBα phosphorylation and subsequent degradation induced by Akt or MAPKs. All the tested metabolites inhibited NO release induced by LPS, acting as inhibitors of iNOS expression, with an efficacy comparable to that of the parent compounds. HT and Tyr metabolites were effective in the inhibition of IĸBα degradation. No one of the compounds was able to inhibit Akt activation, whereas they modulated p38 and ERK1/2 MAPK. Obtained data show that HT and Tyr metabolites are able to prevent a pathological NO overproduction at intestinal level, where they concentrate, thus significantly contributing to the protective activity exerted by their parent compounds against inflammation.