High Sensitivity of Nrf2 Knockout Mice to Acetaminophen Hepatotoxicity Associated with Decreased Expression of ARE-Regulated Drug Metabolizing Enzymes and Antioxidant Genes

Nrf2, which belongs to the basic leucine zipper (bZip) transcription factor family, has been implicated as a key molecule involved in antioxidant-responsive element (ARE)-mediated gene expression. In order to examine the role of Nrf2 in protection against xenobiotic toxicity, the sensitivity of nrf2 knockout mice to acetaminophen (N-acetyl-4-aminophenol (APAP)) was analyzed. The saturation of detoxification pathways after high levels of exposure to APAP is known to induce hepatotoxicity. Two factors important in its detoxification are UDP-glucuronosyltransferase (UDP-GT), an ARE-regulated phase-II drug-metabolizing enzyme, and glutathione (GSH), an antioxidant molecule whose synthesis depends on ARE-regulated γ-glutamylcysteine synthetase (yGCS). Two- to 4-month-old male mice were orally administered a single dose of APAP at 0, 150, 300, or 600 mglkg. Doses of 300 mglkg APAP or greater caused death in the homozygous knockout mice only, and those that survived showed a greater severity in hepatic damage than the wild-type mice, as demonstrated by increased plasma alanine aminotransferase activity, decreased hepatic non-protein sulfhydryl (NPSH) content, and centrilobular hepatocellular necrosis. The high sensitivity of Nrf2-deficient mice was confirmed from observations made at 0, 2, 8, and 24 h after dosing with 300 mg/kg APAP; increased anti-APAP immunoreactivity was also noted in their livers at 2 h. Untreated homozygous knockout mice showed both a lower UDP-GT activity and NPSH content, which corresponded to decreased mRNA levels of UDP-GT (Ugtla6) and the heavy chain of yGCS, respectively. These results show that Nrf2 plays a protective role against APAP hepatotoxicity by regulating both drug metabolizing enzymes and antioxidant genes through the ARE.