Abstract 1843: An unintended consequence of Nrf2-directed cytoprotection against a major human bladder carcinogen and a counteracting solution

Abstract

NF-E2 related factor 2 (Nrf2) plays a pivotal role in cytoprotection against carcinogens and oxidants. Activating Nrf2 has been widely suggested as a key chemopreventive strategy against cancer and other diseases. Here, we show that Nrf2 faces a serious dilemma in dealing with 4-aminobiphenyl (ABP), a major human bladder carcinogen from tobacco smoke and other sources. ABP is metabolized/activated primarily in the liver. While Nrf2 significantly protected mouse liver against ABP in vivo, bladder DNA damage was approximately 2.6 fold higher in Nrf2+/+ mice than in Nrf2-/- mice after ABP exposure, as measured by dG-C8-ABP, the predominant ABP-DNA adduct formed in bladder cells and tissues. Interestingly, Nrf2 significantly protected bladder cells against ABP in vitro. Subsequent experiments revealed that UDP-glucuronosyltransferase (UGT) might be the principal, if not the only mediator of the dichotomous effects of Nrf2. Nrf2 upregulated UGT, promoted glucuronide conjugation of ABP in the liver, and increased urinary excretion of the conjugate. While glucuronidation of ABP and its metabolites is generally a detoxification process, these conjugates, which are excreted in urine, are known to be unstable in acidic urine and deliver the parent compounds to bladder. Hence, higher liver UGT activity may protect liver against ABP but increases bladder exposure to ABP. We further show that 5,6-dihydrocyclopenta[c][1,2]-dithiole-3(4H)-thione (CPDT), a promising cancer chemopreventive dithiolethione, was able to significantly activate Nrf2 cytoprotective signaling and to inhibit dG-C8-ABP formation in the bladder in vivo, but did not modulate liver UGT expression. Thus, CPDT exemplifies a counteracting solution to the dilemma faced by Nrf2 with regard to ABP. In conclusion, our study shows that Nrf2 increases ABP genotoxicity in the bladder in vivo and that UGT may be the key mediator. Using CPDT, we also demonstrate how the adverse effect of Nrf2 may be mitigated. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1843. doi:10.1158/1538-7445.AM2011-1843