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 O-2(center dot) 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 5 10 rim, fluorescence at 580 rim 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 S-1-S-2 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 all argon atmosphere, followed by acidic destruction of remaining borohydride and neutralization and introduction of O-2 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 O-2(center dot) that dismutates to H2O2, that Subsequently, by direct or indirect means, effects the nuclear translocation of transcription factor Nrf2, that upregulates phase 2 enzyme expression.