Importance of phase 2 gene regulation in protection against electrophile and reactive oxygen toxicity and carcinogenesis

A growing body of evidence supports the view that induction of phase 2 enzymes is an effective strategy for lowering the susceptibility of animals and their cells to the carcinogenic effects of electrophiles and reactive oxygen intermediates, and thereby decreasing the risk of developing cancer. Many inducers of phase 2 genes are present in edible plants. They belong to a variety of chemical classes that have few apparent chemical similarities, and are therefore unlikely to react with a structurally complementary cellular receptor. However, all inducers are chemically reactive and can react with thiol groups by alkylation or by oxidoreduction. Measurement of potency of induction of NAD(P)H: quinone reductase in murine hepatoma cells provides a simple and reliable bioassay for isolating inducers from plant extracts and for directing a synthetic program seeking to develop more effective analogues of known inducers. The mechanism of induction involves the ability of inducers to enter the cytoplasm and disrupt the complex between the transcription factor Nrf2 and Keap1, a cysteine-rich protein that is anchored to the actin cytoskeleton. Upon disruption of the complex, Nrf2 migrates to the nucleus where it binds to the ARE (Antioxidant Response Element) present in the upstream regions of many phase 2 genes and activates transcription of these genes. With the use of overexpresssed and purified Keap1, we have established that this cysteine-rich protein contains a small number of highly reactive cysteine residues that appear to be the sensors for inducers. Reaction of these thiol groups with inducers could cause a conformational change leading to dissociation of the Nrf2-Keap1 complex.