The examination of hydrogen peroxide as a general signaling agent of dithiolethione cancer chemopreventives and its effects on human Keap1

Dithiolethiones are a class of cancer chemopreventives which manifest some of their biological activity through the activation of transcription factor Nrf2, resulting in the stimulation of ARE dependant transcription of a large family of cytoprotective enzymes. In determining the mechanism by which these compounds activate Nrf2 it was observed that all dithiolethiones undergo reductive metabolism. These reductive metabolites can reduce molecular oxygen forming superoxide anion. Subsequent dismutation of superoxide anion results in the formation of hydrogen peroxide which has been shown to be a second messenger for enzyme induction of dithiolethiones. Oxidants such as hydrogen peroxide have been hypothesized to encumber Keap1, a negative regulator of Nrf2, thus enhancing Nrf2 activity. Low levels of hydrogen peroxide can react directly with Keap1 forming three Type 2 disulfides, Cys23-Cys38, Cys257-Cys297 and Cys319-Cys319, all of which lie in functional domains of Keap1. Higher levels of hydrogen peroxide which lead to a perturbation of the GSH/GSSG redox couple also cause multiple modifications to Keap1. Some of the modifications, Type 1 disulfides at Cys77, Cys297, Cys319, Cys368 and Cys434, and Type 2 disulfides involving Cys23-Cys38, Cys257-Cys297 and Cys319-Cys319, form in a physiologically relevant range of redox potentials, and again lie in functional domains of Keap1. Through mathematical modeling Type 1 disulfides Cys368 and Cys434 have been shown to alter the structure of the kelch domain, the Nrf2 binding domain, of Keap1. These modifications may cause the observed activation of Nrf2 and subsequent induction of cytoprotective enzymes, which give dithiolethiones their cancer chemopreventive characteristics.