An Nrf2/Small Maf Heterodimer Mediates the Induction of Phase II Detoxifying Enzyme Genes through Antioxidant Response Elements

The induction of phase II detoxifying enzymes is an important defense mechanism against intake of xenobiotics. While this group of enzymes is believed to be under the transcriptional control of antioxidant response elements (AREs), this contention is experimentally unconfirmed. Since the ARE resembles the binding sequence of erythroid transcription factor NF-E2, we investigated the possibility that the phase II enzyme genes might be regulated by transcription factors that also bind to the NF-E2 sequence. The expression profiles of a number of transcription factors suggest that an Nrf2/small Maf heterodimer is the most likely candidate to fulfill this rolein vivo.To directly test these questions, we disrupted the murinenrf2 genein vivo.While the expression of phase II enzymes (e.g., glutathione S-transferase and NAD(P)H: quinone oxidoreductase) was markedly induced by a phenolic antioxidantin vivoin both wild type and heterozygous mutant mice, the induction was largely eliminated in the liver and intestine of homozygousnrf2-mutant mice. Nrf2 was found to bind to the ARE with high affinity only as a heterodimer with a small Maf protein, suggesting that Nrf2/small Maf activates gene expression directly through the ARE. These results demonstrate that Nrf2 is essential for the transcriptional induction of phase II enzymes and the presence of a coordinate transcriptional regulatory mechanism for phase II enzyme genes. Thenrf2-deficient mice may prove to be a very useful model for thein vivoanalysis of chemical carcinogenesis and resistance to anti-cancer drugs.