Potent inhibition of aflatoxin-induced hepatic tumorigenesis by the monofunctional enzyme inducer 1,2-dithiole-3-thione.
1,2-Dithiole-3-thiones are five-membered cyclic sulfur-containing compounds with antioxidant, chemotherapeutic, radioprotective and chemoprotective properties. Several substituted 1,2-dithiole-3-thiones are used medicinally and one of these, oltipraz [5-(2-pyrazinyl)-4-methyl-1,2-dithiole-3-thione], has been recently shown to be an inhibitor of aflatoxin B1 (AFB1) hepatocarcinogenesis in the rat. Structure-activity studies have been undertaken to probe the mechanisms by which dithiolethiones inhibit carcinogenesis. Such studies revealed that unsubstituted 1,2-dithiole-3-thione was more effective than oltipraz at inhibiting aflatoxin-DNA adduct formation in vivo and at inducing electrophile detoxication enzymes in cell culture. In the present studies the effects of dietary administration of 1,2-dithiole-3-thione on the induction of xenobiotic metabolizing enzymes and inhibition of aflatoxin-induced hepatic tumorigenesis were examined. Male F344 rats were fed graded doses of 1,2-dithiole-3-thione (0.001-0.03%) for 4 weeks. During the second and third weeks of 1,2-dithiole-3-thione feeding, rats were dosed by gavage with 250 micrograms of AFB1/kg five times a week. Rats were then restored to control AIN-76A diet 1 week after cessation of AFB1 dosing. At 4 months, focal areas of hepatocellular alteration were identified and quantified by staining sections of liver for gamma-glutamyltranspeptidase (GGT) activity and glutathione S-transferase P (GST-P) expression. Treatment with 1,2-dithiole-3-thione at the lowest dose (0.001%) reduced by greater than 80% the volume of liver occupied by GGT or GST-P foci; higher dietary concentrations provided greater than 98% reductions in the volume per cent of these markers for presumptive preneoplastic lesions. All dietary concentrations of 1,2-dithiole-3-thione resulted in significant elevations in hepatic GST activities. In accord with the protective effects against tumorigenesis, 4- to 6-fold increases in the specific activities of aflatoxin-glutathione conjugation were observed in cytosols prepared from livers of animals fed 1,2-dithiole-3-thione. By contrast, 1,2-dithiole-3-thione did not have any detectable inductive effects on hepatic microsomal cytochrome P450 levels or activities. Dietary administration of 1,2-dithiole-3-thione also elevated activities of GSTs and other phase II enzymes in several extrahepatic organs. This broad pattern of induction of detoxication enzymes by 1,2-dithiole-3-thione supports the potential widespread use of this compound as a protective agent against chemical carcinogenesis and other forms of electrophile toxicity.
Kensler
T W
TW
Department of Environmental Health Sciences, Johns Hopkins School of Hygiene and Public Health, Baltimore, MD 21205.
Groopman
J D
JD
Eaton
D L
DL
Curphey
T J
TJ
Roebuck
B D
BD
D016604:Aflatoxin B1 / Q000378:metabolism / Q000633:toxicity*; D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology*; D004790:Enzyme Induction / Q000187:drug effects; D005982:Glutathione Transferase / Q000032:analysis; D008099:Liver / Q000201:enzymology; D008114:Liver Neoplasms, Experimental / Q000139:chemically induced / Q000517:prevention & control*; D008297:Male; D011230:Precancerous Conditions / Q000517:prevention & control; D011719:Pyrazines / Q000494:pharmacology*; D051381:Rats; D011916:Rats, Inbred F344; D013871:Thiones / Q000494:pharmacology*; D013876:Thiophenes / Q000494:pharmacology*; D005723:gamma-Glutamyltransferase / Q000032:analysis
D000970:Antineoplastic Agents; D011719:Pyrazines; D013871:Thiones; D013876:Thiophenes; C026209:oltipraz; D016604:Aflatoxin B1; D005723:gamma-Glutamyltransferase; D005982:Glutathione Transferase; C049325:1,2-dithiol-3-thione