The medicinal properties of the leaves and fruit of Olea Europaea (olive tree) have been known since antiquity. Numerous contemporary studies have linked the
Mediterranean diet with increased health. In particular, consumption of
olive oil has been associated with a decreased risk of
cardiovascular disease and certain
cancers. Increasingly, there has been an interest in the
biological properties of
polyphenols, which are minor constituents of
olive oil. For example,
hydroxytyrosol has been shown to be a potent
antioxidant and has anti-atherogenic and anti-
cancer properties. The overall aim of this study was to provide insights into the molecular mechanisms of action of
hydroxytyrosol using genome-wide
mRNA-Seq. Initial experiments were aimed at assessing cytotoxicity, apoptosis and cell cycle effects of
hydroxytyrosol in various cell lines. The findings indicated a dose-dependent reduction in cell viability in human erythroleukemic K562 and human keratinocytes. When comparing the viability in parental CEM-CCRF and R100 cells (which overexpress the
P-glycoprotein pump), it was determined that the R100 cells were more resistant to effects of
hydroxytyrosol suggesting efflux by the multi-drug resistance pump. By comparing the uptake of
Hoechst 33342 in the two cell lines that had been pretreated with
hydroxytyrosol, it was determined that the
polyphenol may have
P-glycoprotein-modulating activity. Further, initial studies indicated modest radioprotective effects of relatively low doses of
hydroxytyrosol in human keratinocytes. Analysis of
mRNA sequencing data identified that treatment of keratinocytes with 20 μM
hydroxytyrosol results in the upregulation of numerous
antioxidant proteins and
enzymes, including
heme oxygenase-1 (15.46-fold upregulation),
glutaredoxin (1.65) and
glutathione peroxidase (1.53). This may account for the radioprotective activity of the compound, and reduction in oxidative stress suggests a mechanism for
chemoprevention of
cancer by
hydroxytyrosol. Alteration in the expression of
transcription factors may also contribute to the anti-
cancer effects described in numerous studies. These include changes in the expression of STAT3, STAT6, SMAD7 and ETS-1. The
telomerase subunit TERT was also found to be downregulated in K562 cells. Overall, our findings provide insights into the mechanisms of action of
hydroxytyrosol, and more generally, we identify potential gene candidates for further exploration.