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Dose Response Group, Society of Risk Assessment: Webinar (noon EST, Tuesday, 5 April 16)
Complete Protection against Aflatoxin B1-Induced Liver Cancer with a Triterpenoid: DNA Adduct
Dosimetry and Genotoxicity Threshold
Bill D. Roebuck1, John D. Groopman2, and Thomas W. Kensler2,3
1
Department of Pharmacology and Toxicology, The Geisel School of Medicine at Dartmouth, Hanover, New Hampshire
Department of Environmental Health Sciences, Bloomberg School of Public Health, Baltimore, Maryland
3
Department of Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, Pennsylvania
2
In the early 1960s, aflatoxins were discovered in food grains and shown to be both acutely toxic and to cause liver cancer.
Aflatoxins are produced by molds (such as, Aspergillus flavus) widely occurring on crops such as maize (corn) and
groundnuts (peanuts) and they are found in foods produced from these crops. In high exposures (mg/kg body weight),
aflatoxins cause acute toxicity particularly to the liver with growth retardation and death depending upon the dose and
duration of aflatoxin exposure. With lower exposures (µg/kg) of aflatoxins (particularly aflatoxin B1; AFB1) in a wide
variety of domestic and livestock animals as well as a wide array of experimental species, aflatoxins cause liver diseases
including hepatocellular cancer (HCC). Mold growth and aflatoxin production can be controlled by a variety of pre- and
post-harvest agricultural practices, but not completely eliminated. Worldwide HCC is the second leading cause of cancer
deaths. We have been interested in using chemicals some of which are approved drugs, to block, retard, or reverse
carcinogenic processes, a strategy termed cancer chemoprevention. Our work was facilitated by the development of
highly reproducible and quantitative rat models of liver carcinogenesis. In this setting, the antischistosomal drug oltipraz
produced marked reductions in incidence of HCC from 20 to 0% with low doses of AFB1 and 83 to 48% with AFB1 at
higher doses and longer duration of exposure. In both experiments, significant, but incomplete reduction of hepatic
aflatoxin-derived DNA adducts were observed in parallel cohorts of rats. Reduction in levels of hepatic DNA adducts
underestimated the efficacy of the chemopreventive intervention, suggesting a background of adducts not aligned with
risk of HCC.
This presentation focuses on the same AFB1-rat model, but with a new generation of cancer chemoprevention agents. 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 mg/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. 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, the integrated level of urinary AFB1-N7-guanine was significantly reduced (66%) and aflatoxin-Nacetylcysteine, 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-week dosing period, but was
largely absent with CDDO-Im intervention. The remarkable efficacy of CDDO-Im as an anticarcinogen is established
even in the face of a significant aflatoxin DNA adduct burden. Consequently, the absence of cancer requires a concept of
a threshold for DNA damage for cancer development.
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* Johnson NM, Egner PA, Baxter VK, Sporn MB, Wible RS, Sutter TR, Groopman JD, Kensler TW, Roebuck BD. Complete
protection against aflatoxin B(1)-induced liver cancer with a triterpenoid: DNA adduct dosimetry, molecular signature, and
genotoxicity threshold. Cancer Prev Res (Phila) 2014; 7(7):658-65.