On the reduction of dithiolethiones and dithiolylium ions by NADPH and glutathione reductase

Chemoprevention through the Keap1-Nrf2 signaling pathway by phase 2 enzyme inducers

One successful strategy for cancer chemoprevention is modulation of drug metabolizing enzymes, leading to a facilitated elimination of endogenous and environmental carcinogens. Inducers of phase 2 enzymes such as dithiolethiones inhibit tumorigenesis of environmental carcinogens in various animal models and modulate the metabolism of the carcinogen aflatoxin B1 in human clinical trials. Increasing lines of evidence show that the Keap1-Nrf2 complex is a key molecular target of chemopreventive phase 2 enzyme inducers. The transcription factor Nrf2 is a member of the basic leucine-zipper NF-E2 family and interacts with the antioxidant response element (ARE) in the promoter region of phase 2 detoxifying enzymes. A cytoplasmic actin-binding protein, Keap1, is an inhibitor of Nrf2 that sequesters it in the cytoplasm. Inducers dissociate this complex, allowing Nrf2 to translocate to the nucleus. Disruption of the nrf2 gene in mice leads to the loss of chemopreventive efficacy by inducers. This review focuses on (1) the role of Nrf2 in the regulation of phase 2 and antioxidative genes, (2) the molecular actions of dithiolethiones on the Keap1-Nrf2 pathway, and (3) the contribution of Nrf2-regulated gene families to the cytoprotective actions of dithiolethiones and other inducers. Rapidly accumulating data on this pathway is providing insight into the coordinated mammalian defense systems against electrophiles and oxidative stresses and the means by which it may be targeted by small molecules.

Modulation of gene expression by cancer chemopreventive dithiolethiones through the Keap1-Nrf2 pathway. Identification of novel gene clusters for cell survival

Enzyme inducers such as 3H-1,2-dithiole-3-thione (D3T) enhance the detoxication of environmental carcinogens and protect against neoplasia. The putative molecular sensor for inducers is Keap1, a sulfhydryl-rich protein that sequesters the transcription factor Nrf2 in the cytoplasm. Expression of these detoxication enzymes is blunted in nrf2-deficient mice; moreover, these mice are more sensitive to carcinogenesis, and the protective actions of dithiolethiones are lost with nrf2 disruption. Hepatic gene expression profiles were examined by oligonucleotide microarray analysis in vehicle- or D3T-treated wild-type mice as well as in nrf2 single and keap1-nrf2 double knockout mice to identify those genes regulated by the Keap1-Nrf2 pathway. Transcript levels of 292 genes were elevated in wild-type mice 24 h after treatment with D3T; 79% of these genes were induced in wild-type, but not nrf2-deficient mice. These nrf2-dependent, D3T-inducible genes included known detoxication and antioxidative enzymes. Unexpected clusters included genes for chaperones, protein trafficking, ubiquitin/26 S proteasome subunits, and signaling molecules. Gene expression patterns in keap1-nrf2 double knockout mice were similar to those in nrf2-single knockout mice. D3T also led to nrf2-dependent repression of 31 genes at 24 h; principally genes related to cholesterol/lipid biosynthesis. Collectively, D3T increases the expression of genes through the Keap1-Nrf2 signaling pathway that directly detoxify toxins and generate essential cofactors such as glutathione and reducing equivalents. Induction of nrf2-dependent genes involved in the recognition and repair/removal of damaged proteins expands the role of this pathway beyond primary control of electrophilic and oxidative stresses into secondary protective actions that enhance cell survival.