Protection of quercetin against ethanol-induced human hepatotoxicity is regulated is regulated via HO–1/CO system through MAPK/Nrf2 pathway

Aims: Flavonoids, including quercetin, have been reported to have potent hepatoprotective effects against oxidative stress. The protecting effects have been associated with HO–1 induction. Although a number of studies have demonstrated protecting effects of various flavonoids, the exact signalling pathway of quercetin involved in HO–1 induction during oxidative stress in liver damage is still not fully understood. Thus, we have initiated the following study to investigate the role of HO–1 induction as well as its transcriptional regulation and the effects of carbon monoxide (CO), free iron and bilirubin,–all of them catalytic by-products of HO–1.

Material and Methods: Human hepatocytes were isolated using standard procedures established in our laboratories. Cells were incubated with ethanol and various concentrations of quercetin (10–200 mM). Then, cellular damage (AST & LDH release), generation of malondialdehyde, the cellular glutathione status as well as HO–1 activity were measured, using spectrophotometric assays. Nrf2 expression in cytosolic and nuclear fractions was determinated in the presence or absence of ‘specificrsquor; MAPK inhibitors using Western blot analysis. In addition, we have exposed cells to CO) free iron and bilirubin.

Results: Induction of oxidative stress resulted in a sustained cellular glutathione depletion, malondialdehyde elevation, and an evident release of cellular LDH and AST. Quercetin exerted a dose-dependent protective effect against oxidative stress, which is paralleled to HO–1 induction. Zinc protoporphyrin–9 abrogated the protective effect and dramatically enhanced the cytotoxicity. SB203580 (p38 inhibitor) and especially PD98059 (ERK inhibitor) blocked quercetin-derived HO–1 induction and Nrf2 translocation into the nucleus, and subsequently inhibited the quercetin-related protection. Carbon monoxide has a similar protecting effect as quercetin.

Conclusion: HO–1 up-regulation by quercetin protected human hepatocytes from oxidative stress. Among MAPK signalling pathways, p38 and ERK mediated the quercetin-derived Nrf2 translocation into the nuclei and subsequent induction of HO–1 activity. Our data might be the bases of new therapeutic options in drug development against tissue damage caused by inflammatory reactions such as ischemia/reperfusion injury or traumatic injuries.

Supported by the BMBF (Project No.0313079B)