Complete Protection against Aflatoxin B₁-Induced Liver Cancer with a Triterpenoid: DNA Adduct Dosimetry, Molecular Signature, and Genotoxicity Threshold

Abstract

In experimental animals and humans, aflatoxin B₁ (AFB₁) is a potent hepatic toxin and carcinogen. The synthetic oleanane triterpenoid 1-[2-cyano-3-,12-dioxooleana-1,9(11)-dien-28-oyl]imidazole (CDDO-Im), a powerful activator of Keap1-Nrf2 signaling, protects against AFB₁-induced toxicity and preneoplastic lesion formation (GST-P–positive foci). This study assessed and mechanistically characterized the chemoprotective efficacy of CDDO-Im against AFB₁-induced hepatocellular carcinoma (HCC). A lifetime cancer bioassay was undertaken in F344 rats dosed with AFB₁ (200 μg/kg rat/day) for four weeks and receiving either vehicle or CDDO-Im (three times weekly), one week before and throughout the exposure period. Weekly, 24-hour urine samples were collected for analysis of AFB₁ metabolites. In a subset of rats, livers were analyzed for GST-P foci. The comparative response of a toxicogenomic RNA expression signature for AFB₁ was examined. CDDO-Im completely protected (0/20) against AFB₁-induced liver cancer compared with a 96% incidence (22/23) observed in the AFB₁ group. With CDDO-Im treatment, integrated level of urinary AFB₁-N⁷-guanine was significantly reduced (66%) and aflatoxin-N-acetylcysteine, a detoxication product, was consistently elevated (300%) after the first AFB₁ dose. In AFB₁-treated rats, the hepatic burden of GST-P–positive foci increased substantially (0%–13.8%) over the four weeks, but was largely absent with CDDO-Im intervention. The toxicogenomic RNA expression signature characteristic of AFB₁ was absent in the AFB₁ + CDDO-Im–treated rats. The remarkable efficacy of CDDO-Im as an anticarcinogen is established even in the face of a significant aflatoxin adduct burden. Consequently, the absence of cancer requires a concept of a threshold for DNA damage for cancer development. Cancer Prev Res; 7(7); 1–8. ©2014 AACR.