A mechanism of inhibition of aflatoxin B1-DNA binding in the liver by phenobarbital pretreatment of rats.
The effect of phenobarbital (PB) pretreatment of rats on both hepatic aflatoxin B1 (AFB1)-DNA binding and AFB1-glutathione (AFB1-SG) conjugation have been examined in studies in vivo and in vitro. Male Sprague-Dawley rats fed a commercial diet with 0.1% PB in their drinking water for 1 week had total wet liver weight and microsomal protein content about 27% and 38% higher, respectively, than controls. Hepatic cytochrome P-450 content, microsomal cytochrome P-450 mediated AFB1 binding to exogenous DNA and formation of hydroxy metabolites of AFB1 were also about threefold higher in PB-treated rats and cytosolic reduced glutathione S-transferase activities were about doubled. Microsome-mediated AFB1-DNA binding, when examined at 2 microM and 10 microM levels of AFB1, was inhibited two-to threefold more by cytosols of treated rats whereas AFB1-SG conjugation was two- to threefold higher by cytosols of treated rats. In reconstitution experiments with 2 microM AFB1, with intact nuclei serving as a source of endogenous DNA, addition of microsomes from either group generated a large amount of AFB1-DNA binding (68-105 pmol) and a smaller amount of AFB1-SG conjugate (12-21 pmol). The presence of cytosol from the controls reduced AFB1-DNA binding to a much lesser extent than the cytosol from the treated group whereas AFB1-SG conjugation was much higher with the cytosol from the treated group. These results are in agreement with the studies in vivo. In isolated hepatocytes at 33 nM, 2 microM and 10 microM AFB1 levels, AFB1-DNA binding was decreased 50 to 70% by prior PB-treatment whereas AFB1-SG conjugation was two- to threefold higher in treated compared to control hepatocytes. In hepatocytes, addition of 1 mM diethylmaleate increased DNA binding two- to threefold with a corresponding decrease in AFB1-SG conjugation. Addition of 1 mM styrene oxide caused 5- to 10-fold increases in AFB1-DNA binding at levels of AFB1 of 33 nM and 2 microM; but at 10 microM AFB1, increases in AFB1-DNA binding were two- to threefold. In intact rats, PB treatment reduced hepatic AFB1-DNA binding to 30% of controls with concomitant increase in biliary excretion of AFB1-SG conjugate. It appears that the induced cytosolic GSH S-transferases after PB treatment of rats plays a significant role in inhibiting hepatic AFB1-DNA binding and hepatocarcinogenesis presumably by inactivation of the reactive AFB1-epoxide.(ABSTRACT TRUNCATED AT 400 WORDS)
Lotlikar
P D
PD
Fels Research Institute, Temple University School of Medicine, Philadelphia, Pennsylvania 19140.
Raj
H G
HG
Bohm
L S
LS
Ho
L L
LL
Jhee
E C
EC
Tsuji
K
K
Gopalan
P
P
D016604:Aflatoxin B1; D000348:Aflatoxins / Q000378:metabolism*; D000818:Animals; D002273:Carcinogens / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D004247:DNA / Q000378:metabolism*; D005978:Glutathione / Q000378:metabolism; D005982:Glutathione Transferase / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D010634:Phenobarbital / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D013347:Subcellular Fractions / Q000378:metabolism
D000348:Aflatoxins; D002273:Carcinogens; D004247:DNA; D016604:Aflatoxin B1; D005982:Glutathione Transferase; D005978:Glutathione; D010634:Phenobarbital