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 i s believed to be under the transcriptional control of antioxidant resp onse elements (AREs), this contention is experimentally unconfirmed. S ince the ARE resembles the binding sequence of erythroid transcription factor NF-E2, we investigated the possibility that the phase II enzym e genes might be regulated by transcription factors that also kind to the NF-E2 sequence. The expression profiles of a number of transcripti on factors suggest that an Nrf2/small Maf heterodimer is the most like ly candidate to fulfill this role in vivo. To directly test these ques tions, we disrupted the murine nrf2 gene in vivo. While the expression of phase II enzymes (e.g., glutathione S-transferase and NAD(P)H: qui none oxidoreductase) was markedly induced by a phenolic antioxidant in vivo in both wild type and heterozygous mutant mice, the induction wa s largely eliminated in the liver and intestine of homozygous nrf2-mut ant 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 Ma f activates gene expression directly through the ARE. These results de monstrate that Nrf2 is essential for the transcriptional induction of phase II enzymes and the presence of a coordinate transcriptional regu latory mechanism for phase II enzyme genes. The nrf2-deficient mice ma y prove to be a very useful model for the in vivo analysis of chemical carcinogenesis and resistance to anti-cancer drugs. (C) 1997 Academic Press.