Nrf2-Keap1 regulation of cellular defense mechanisms against electrophiles and reactive oxygen species

A simple model for understanding xenobiotic metabolism is to divide the biotransformation process into two consecutive reactions. The phase 1 reaction is mediated by cytochrome P450 systems. Phase 2 enzymes promote conjugation of phase 1 products with various hydrophilic moieties. Phase 2 enzyme genes are often induced by the metabolites of phase 1 reactions, which are highly electrophilic. Characterization of the regulatory elements of phase 2 enzyme genes, such as NQO-1 and GSTs, revealed that electrophiles transcriptionally activate the expression of these genes through the ARE. Studies on the regulation of erythroid-specific gene expression originally identified p45 NF-E2 as a molecule interacting with the NF-E2 binding motif. Subsequently, Nrf1, Nrf2, and Nrf3 were identified. These factors are referred to as the CNC family of transcription factors. The NF-E2 binding motif and the ARE share a high level of sequence similarity, suggesting that one of the CNC family members may activate transcription through the ARE. Nrf2 is expressed in metabolic and detoxification organs, such as liver, kidney, and intestine, and in organs continuously exposed to the environment, such as skin, lung, and the digestive tract. It was therefore assumed that Nrf2 acts as a transcriptional activator interacting with the ARE. The coordinated induction of a battery of genes encoding phase 2 detoxifying enzymes and antioxidant enzymes is a pivotal cellular defense mechanism against the toxicity of electrophiles and reactive oxygen species. Targeted disruption of the mouse Nrf2 gene revealed that Nrf2 is essential for the coordinated induction of these defense enzymes. Detailed analysis of the regulatory mechanisms governing Nrf2 activity led to the identification of a new protein, Keap1, which represses Nrf2 activity by binding to the N-terminal Neh2 domain. Electrophiles liberate Nrf2 from Keap1 repression and provoke the nuclear accumulation of Nrf2, suggesting that the Nrf2-Keap1 system acts as a sensor for xenobiotics and oxidative stress. Targeted disruption of the mouse Keap1 gene induced the constitutive expression of Nrf2-target genes in the Keap1-deficient mouse. Nrf2 is turned over rapidly through the proteasome pathway, whereas electrophiles stabilize Nrf2. These results support the contention that Keap1 regulates the rapid proteolysis of Nrf2 and this process provides an important basis for the tight regulation of cellular defense enzymes by Nrf2.