Antioxidant regulation of genes encoding enzymes that detoxify xenobiotics and carcinogens

Antioxidants are substances that delay or prevent the oxidation of cellular oxidizable substrates. The various antioxidants exert their effect by scavenging superoxide or by activating a battery of detoxifying/defensive proteins. In this chapter, we have focused on the mechanisms by which antioxidants induce gene expression. Many xenobiotics (e.g., beta-naphthoflavone) activate genes similar to those activated by antioxidants. The promoters of these genes contain a common cis-element, termed the antioxidant response element (ARE), which contains two TRE (TPA response element) or TRE-like elements followed by GC box. Mutational studies have identified GTGAC***GC as the core of the ARE sequence. Many transcription factors, including Nrf, Jun, Fos, Fra, Maf, YABP, ARE-BP1, Ah (aromatic hydrocarbon) receptor, and estrogen receptor bind to the ARE from the various genes. Among these factors, Nrf-Jun heterodimers positively regulate ARE-mediated expression and induction of genes in response to antioxidants and xenobiotics. This Nrf-Jun heterodimerization and binding to the ARE requires unknown cytosolic factors. The mechanism of signal transduction from antioxidants and xenobiotics includes several steps: (1) Antioxidants and xenobiotics undergo metabolism to generate superoxide and related reactive species, leading to the generation of a signal to activate expression of detoxifying/defensive genes. (2) The generation of superoxide and related reactive species is followed by activation of yet to be identified cytosolic factors by unknown mechanism(s). (3). Activated cytosolic factors catalyze modification of Nrf and/or Jun proteins, which bind to the ARE in promoters of the various detoxifying/defensive genes. (4) The transcription of genes encoding detoxifying/defensive proteins is increased. The unknown cytosolic factors are significant molecules because they represent the oxidative sensors within the cells. Identification of the cytosolic factors will be of considerable importance in the field of antioxidants and gene regulation research. Future studies will also be required to completely understand the molecular mechanism of signal transduction from antioxidants and xenobiotics to Nrf-Jun. In addition to the Nrf-Jun pathway, mammalian cells also contain other pathways that activate gene expression in response to oxidative stress. These include NF-KB-, HIF-1-, Mac-1-, and SRF-mediated pathways. It is expected that collectively these pathways increase transcription of more than four dozen genes to protect cells against oxidative stress.