Aflatoxin B1 Carcinogenesis and its Relationship to DNA Adduct Formation and Adduct Persistence in Sensitive and Resistant Salmonid Fish

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Oxford University Press

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Rainbow trout (Salmo gairdneri)and coho salmon (Oncorhyn-chus kisutch) were exposed to aflatoxin B1(AFB1) either by passive embryo uptake or by dietary treatment after hatching and feeding onset. Trout exposed as embryos to an aqueous solution of 0.5 p.p.m. AFB1 for 15 min showed a 62% tumor incidence 12 months later, whereas coho salmon exposed to a similar solution for 30 min showed only a 9% incidence. The difference between salmon and trout response was even greater by dietary AFB1 treatment. Trout exposed for 4 weeks to 20 p.p.b. dietary AFB1 had a 62% tumor response 12 months later, whereas salmon exposed to 40 p.p.b. dietary AFB1 for 4 weeks failed to develop tumors. A 5% tumor incidence was observed in salmon 12 months after 3 weeks exposure to 5000 p.p.b. dietary AFB1, a lethal dose for trout. In addition to a lower tumor incidence when compared to trout, the neoplastic response of salmon to AFB1 is to produce benign hepatic adenomas in contrast to the malignant hepatocellular carcinomas seen in trout. AFB1 metabolism, DNA adduct formation, adduct persistence in vivo and in vitro and cytochrome P-450 isozyme composition were compared in livers of trout and salmon to understand the role of metabolism and initiation in this species difference. AFB1-DNA binding was 7–56 times greater in trout than salmon liver at various times after AFB1 injection, 20 times greater in embryos or in freshly isolated trout hepatocyte preparations after a 1 h incubation with aflatoxin Bl, and 18 times greater in trout liver after a three week dietary (80 p.p.b.) exposure. The major AFB1-DNA adduct was 8, 9-dihydro-8-(N7-guanyl)-9-hydroxyaflatoxin B1 in both species. Persistence of AFB1-DNA adducts in vivo in liver was high compared to mamalian systems, implying that active enzymatic removal of bulky DNA adducts is low in both species and probably not a factor in their differential response to aflatoxin. Species differences in other phase I and phase II metabolism pathways and in AFB1 elimination were, overall, much less striking than those previously observed for trout fed inhibitors of aflatoxin carcinogenesis. Rates of bileelimination of AFB1 detoxication products, and total excretion of aflatoxins into water after AFB1 exposure, were not significantly different between trout and salmon. Since detoxication differences were not observed, the species difference in AFB1-DNA binding appears to reflect less efficient cytochrome P-450 metabolism of aflatoxin to the reactive 8, 9-epoxide in salmon, compared to trout. In support of this hypothesis, trout liver microsomes displayed a Km (7.5 µM)for AFB1-DNA adduction in vitro that was 7-fold lower than salmon (52 µM). Furthermore, immunoquantitation of various P-450 isozymes suggest that salmon liver microsomes have much lower amounts of an isozyme immunochemically related to trout P-450 LM2 which has previously been shown to be the major isozyme catalyzing AFB1 8, 9-epoxidation. Other, post-initiation differences were not ruled out by these studies and may contribute to the differential response of rainbow trout and coho salmon to AFB1 hepatocarcino-genesis.


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