Hydrogen peroxide is a second messenger in phase 2 enzyme induction by cancer chemopreventive dithiolethiones

The ability of three dithiolethione cancer chemopreventives, oltipraz 1, anetholedithione (ADT) 2, 1,2-dithiole-3-thione (D3T) 3, and the major metabolite, 4, of 1, to induce the cytoprotective enzyme NQO1 in Hepa 1c1c7 cells and the inhibition of this induction by catalase are demonstrated. The ability of 1, 3, and 4 to form O₂^• has been reported, and it is here demonstrated that 2 decomposes in the presence of GSH to form, upon addition of the nitrone spin trap DMPO, the DMPO-OH adduct that is detectable by EPR. Decomposition of 2 in the presence of GSH elicits, upon the addition of hydroethidine and excitation at 510 nm, fluorescence at 580 nm that is diminished by the addition of superoxide dismutase. The compound 4, is a product of the reduction of 1, and it is demonstrated that 2 and 3 decompose in the presence of reductants such as thiolates and NaBH₄, followed by addition of CH₃I, to form the dimethylated products of reductive cleavage of the S₁-S₂ bond. The same products are isolated subsequent to lysis in buffer containing CH₃I of Hepa 1c1c7 cells treated with 2 or 3. Reductive cleavage of 2 and 3 in aqueous ethanol by NaBH₄ in an argon atmosphere, followed by acidic destruction of remaining borohydride and neutralization and introduction of O₂ results in the reformation of 2 and 3 to the extent of 80 and 33%, respectively. The data in toto are consistent with a model in which dithiolethiones, generally, undergo reductive cleavage in Hepa 1c1c7 cells, thereby resulting in the generation of O₂^• that dismutates to H ₂O₂, that subsequently, by direct or indirect means, effects the nuclear translocation of transcription factor Nrf2, that upregulates phase 2 enzyme expression.