Key Event Title
|Level of Biological Organization|
Key Event Components
Key Event Overview
AOPs Including This Key Event
Key Event Description
The reactive exo-epoxide is formed in hepatocytes (or extra-hepatically) by metabolism of the parent AFB1 by CYP450 (Larsson et al., 1990; Larsson and Tjalve, 1993). The reactive metabolite then escapes the endoplasmic reticulum where the CYP450 is located. The reactive metabolite must evade conjugation with GSH in the cytoplasm or binding with other cytoplasmic nucleophiles. It then traverses the nuclear membrane in order to reach the cell nucleus and the nuclear DNA. Once the reactive metabolite is in the cell nucleus, it can bind with nuclear DNA to form DNA adducts.
How It Is Measured or Detected
Formation of the exo-epoxide can be produced with in vitro systems and detected using techniques for structural quantitation of AFB1 metabolites (Himmelstein et al., 2009), including liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). By using subcellular fractions (e.g., microsomes), cellular homogenates, or cells in culture, it is possible to measure formation of AFB1 exo-epoxide. Such data can also be collected from in vivo systems; samples of plasma or blood from AFB1-treated animals can be analyzed for the AFB1 exo-epoxide with similar mass spectrometric based detection systems (e.g., LC-MS/MS). Samples of blood from humans in AFB1-endemic regions have demonstrated presence of AFB1-albumin adducts, which are formed from the AFB1 exo-epoxide. AFB1-treated animals may also provide tissue samples for analysis of AFB1 exo-epoxide. Special trapping techniques may be required as the reactive AFB1 exo-epoxide metabolite has a short half-life in biological matrices.
Domain of Applicability
There are data across phyla demonstrating metabolic activation of AFB1 to the exo-epoxide via CYP450. This includes several mammalian species (humans, non-human primates, rats, mice) in addition to birds (turkeys) and fish (Eaton and Gallagher, 1994; IARC, 1993).
• F.D. Asplin, R.B.A. Carnaghan, (1961). The toxicity of certain groundnut meals for poultry with special reference to their effect on ducklings and chickens. Vet. Rec. 73:1215– 1219.
• Brown KL, Deng JZ, Iyer RS, Iyer LG, Voehler MW, Stone MP, Harris CM, Harris TM (2006). Unraveling the aflatoxin-FAPY conundrum: Structural basis of the formamidopyrimidine-type DNA adduct of aflatoxin B1. J Am Chem Soc 128:15188-15199.
• Croy RG, Essigman JM, Reinhold VN, Wogan GN (1978). Identification of the principal aflatoxin N1-DNA adduct formed in vivo in rat liver. Proc Natl Acad Sci USA 75:1745-1749.
• Degen GH, Neumann HG (1981). Differences in aflatoxin B1-susceptibility of rat and mouse are correlated with the capability in vitro to inactivate aflatoxin B1-epoxide. Carcinogenesis 2:299–306.
• Eaton DL, and Gallagher EP (1994). Mechanisms of aflatoxin carcinogenesis. Annu Rev Pharmacol Toxicol 34:135-172.
• Elegbede JA, and Gould MN. (2002). Monoterpenes reduced adducts formation in rats exposed to aflatoxin B1. African J Biotech, 1, 46–49.
• Gregory 3rd JF, Goldstein SL, Edds GT. (1983). Metabolite distribution and rate of residue clearance in turkeys fed a diet containing aflatoxin B1. Food Chem Toxicol, 21, 463–7.
• Groopman JD, Kensler TW (2005). Role of metabolism and viruses in aflatoxin-induced liver cancer. Toxicol Appl Pharmacol 206:131-137.
• Guengerich FP, Johnson WW, Ueng Y-F, Yamazaki H, Shimada T (1996). Involvement of Cytochrome P450, glutathione S-transferase, and epoxide hydrolase in the metabolism of aflatoxin B1 and relevance to risk of human liver cancer. Environ Health Perspect 104(Suppl 3):557-562.
• Himmelstein MW, Boogaard PJ, Cadet J, et al. (2009). Creating context for the use of DNA adduct data in cancer risk assessment: II.Overview of methods of identification and quantitation of DNA damage. Crit Rev Toxicol, 39, 679–694.
• IARC (1993). Some Naturally Occurring Substances: Food Items and Constituents, Heterocyclic Aromatic Amines and Mycotoxins. IARC Monographs on the Evaluation of Carcinogenic Risk to Humans. Vol. 56, 245-395.
• Johnson NM, Egner PA, Baxter VK, Sporn MB, Wible RS, Sutter TR, Groopman JD, Kensler TW, Roebuck BD. (2014). Complete protection against aflatoxin B(1)-induced liver cancer with a triterpenoid: DNA adduct dosimetry, molecular signature, and genotoxicity threshold. Cancer Prev Res. 7(7):658-665.
• Kensler TW, He X, Otieno M, et al. (1998). Oltipraz chemoprevention trial in Qidong, People’s Republic of China: Modulation of serum aflatoxin albumin adduct biomarkers. Cancer Epidemiol Biomarkers Prev, 7, 127–34.
• Larsson P, Hoedaya, WI, Tjalve H. (1990). Disposition of 3H-aflatoxin H in mice: formation and retention of tissue bound metabolites in nasal glands. Pharmacol Toxicol, 67, 162–71.
• Larsson P, and Tjalve H. (1993). Distribution and metabolism of aflatoxin B1 in the marmoset monkey (Callithrix jacchus). Carcinogenesis, 14, 1–6.
• Monroe DH, Eaton DL. (1987). Comparative effects of butylated hydroxyanisole on hepatic in vivo DNA binding and in vitro biotransformation of aflatoxin B1 in the rat and the mouse. Toxicol Appl Pharmacol, 90, 401–409.
• Pottenger, L.H., Andrews LS, Bachman AN, Boogaard PJ, Cadet J, Embry MR, Farmer PB, Himmelstein MW, Jarabek AM, Martin EA, Mauthe RJ, Persaud R, Preston RJ, Schoeny R, Skare J, Swenberg JA, Williams GM, Zeiger E, Zhang F, Kim JH. (2014). An organizational approach for the assessment of DNA adduct data in risk assessment: case studies for aflatoxin B1, tamoxifen and vinyl chloride. Crit. Rev. Toxicol. 44(4):348-391.
• Primiano T, Egner PA, Sutter TR, et al. (1995). Intermittent dosing with oltipraz: relationship between chemoprevention of aflatoxin-induced tumorigenesis and induction of glutathione-S-transferases. Cancer Res, 55, 4319–4324.
• Roebuck BD, Liu Y-L, Rogers AE, et al. (1991). Protection against aflatoxin B1-induced hepatocarcinogenesis in F344 rats by 5-(2-pyrazinyl)-4-methyl-1,2-dithiole-3-thione (oltipraz): predictive role for short term molecular dosimetry. Cancer Res, 51, 5501–5506.
• Ueng Y-F, Shimada T, Yamazaki H, Guengerich FP (1995). Oxidation of aflatoxin B1 by bacterial recombinant human cytochrome P450 enzymes. Chem Res Toxiol 8:218-225.
• Wang J-S, Shen X, He X, et al. (1999). Protective alerations in phase 1 and 2 metabolism of aflatoxin B1 by oltipraz in residents of Qidong, People’s Republic of China. J Natl Cancer Inst, 91, 347–354.
• Yates MS, Kwak M-K, Egner PA, et al. (2006). Potent protection against aflatoxin-induced tumorigenesis through induction of Nrf2-regulated pathways by the triterpenoid 1-[2-cyano-3-,12-dioxooleana-1,9 (11)-dien-28-oyl] imidazole. Cancer Res, 66, 2488–2494.