In vitro and in vivo interplay between NAD(P)H: quinone oxidoreductase 1 and flavonoids

Flavonoids are naturally occurring, health-promoting, bioactive compounds, omnipresent in the human diet. The protective effect of these phytochemicals is accomplished for an important part by modulating the activity of enzyme systems responsible for deactivation of chemical carcinogens, such as NAD(P)H: quinone oxidoreductase 1 (NQO1). Several flavonoids act as NQO1 inducers by increasing the NQO1 gene expression level through the electrophile-responsive element (EpRE). On the other hand certain flavonoids are efficient inhibitors of the NQO1 enzyme activity in vitro. The objective of this thesis is to elucidate the complex interplay between flavonoids and NQO1. First, inhibition of NQO1 by flavonoids, pointing at a mechanism contradicting the proven beneficial properties of these natural compounds was studied. Kinetic and molecular dynamics studies were conducted and a method to monitor NQO1 activity in living cells was developed. These studies revealed that although flavonoids possess the potential to inhibit NQO1 activity, inhibition of NQO1 is not likely to happen in cellular systems due to intracellular physiological conditions. Furthermore, the mechanism by which flavonoids are able to induce the EpRE- mediated expression of NQO1 was studied. Reporter gene assays elucidated that upstream XRE-mediated gene expression is not nessessary to induce EpRE-mediated gene expression and quantum-mechanical calculations revealed that flavonoids with a higher intrinsic potential to generate oxidative stress and redox cycling, are the most potent inducers of NQO1. Radioactive binding studies showed Keap1 modification by the flavonoid quercetin, resulting in switching on of EpRE-mediated gene transcription activation. In addition, in vivo metabolites of quercetin were studied on their ability to induce EpRE-mediated gene expression. The results show, that, although quercetin-derived glucuronides are the major metabolites present in the systemic circulation, the deglucuronidated parent compound and its methylated derivatives are the active compounds responsible for the beneficial EpRE-mediated gene expression effects. Overall, the studies presented in this thesis provide insight in the complex interplay between NQO1 and flavonoids on the protein as well as on the gene expression level.