Regulation of rat glutamate-cysteine ligase (gamma-glutamylcysteine synthetase) subunits by chemopreventive agents and in aflatoxin B(1)-induced preneoplasia.

Certain dietary constituents can protect against chemically induced carcinogenesis in rodents. A principal mechanism by which these chemopreventive compounds exert their protective effects is likely to be via induction of carcinogen detoxification. This can be mediated by conjugation with glutathione, which is synthesized by the sequential actions of glutamate-cysteine ligase (GLCL) and glutathione synthetase. We have demonstrated that dietary administration of the naturally occurring chemopreventive agents, ellagic acid, coumarin or alpha-angelicalactone caused an increase in GLCL activity of between approximately 3- and 5-fold in rat liver. Treatment with the synthetic antioxidant ethoxyquin or the classic inducer phenobarbital caused < 2-fold induction of GLCL activity in rat liver, which was not found to be significant. The increases in GLCL activity were accompanied by increases (between 2- and 4-fold) in levels of both the catalytic heavy subunit (GLCLC) and regulatory light subunit (GLCLR). No substantial induction of GLCL was observed in rat kidney. The glutathione S-transferase (GST) subunits A1, A3, A4, A5, P1 and M1 were all found to be inducible in rat liver by most of the agents. The greatest levels of induction were observed for GST P1, following treatment with coumarin (20-fold), alpha-angelicalactone (10-fold) or ellagic acid (6-fold), and GST A5, following treatment with coumarin (7-fold), alpha-angelicalactone (6-fold) and ethoxyquin (6-fold). Glutathione synthetase was induced approximately 1.5-fold by coumarin, alpha-angelicalactone, ellagic acid and ethoxyquin. The expression of glutathione-related enzymes was also examined in preneoplastic lesions induced in rat liver by aflatoxin B(1). The majority of gamma-glutamyltranspeptidase (GGT)-positive preneoplastic foci contained increased levels of GLCLC relative to the surrounding tissue. This was usually found to be accompanied by an increase in GLCLR. Cells in the inner cortex of rat kidney were found to contain the highest levels of both GLCLC and GLCLR. The same cells showed the strongest staining for GGT activity.

Chemoprevention of aflatoxin B1-induced carcinogenesis by indole-3-carbinol in rat liver--predicting the outcome using early biomarkers.

Indole-3-carbinol (I3C) was examined for its ability to inhibit aflatoxin B1 (AFB1)-induced hepatocarcinogenesis in male Fischer rats when administered either before or after the carcinogen. After 13 weeks, animals pretreated with I3C (0.5% in the diet) for 2 weeks prior to administration of AFB1 and with continuing treatment during exposure to the carcinogen were protected from development of preneoplastic lesions, as determined by the classical markers gamma-glutamyltranspeptidase (GGT) and glutathione S-transferase (GST) P. In animals receiving AFB1 for 6 weeks before treatment with I3C, there was no obvious protective effect at 13 weeks compared with animals receiving only AFB1. Using cytokeratin 18 expression as a marker, animals fed AFB1 alone had a small number of positive foci at 13 weeks. However, no cytokeratin-positive foci were visible in the majority of livers from either group receiving I3C in combination with AFB1 and after 43 weeks all animals in these groups were protected from liver tumour formation. These results suggest that expression of cytokeratin 18, a later phenotypic change in foci than induction of GST-P and GGT, correlates more closely with tumour outcome in this model. I3C appeared to retard progression of AFB1-induced carcinogenesis at both the initiation and promotion stages. Continuous treatment with I3C for 13 weeks caused significant induction of CYP1A1, 1A2, 3A and 2B1/2, GST Yc2, aflatoxin B1 aldehyde reductase and quinone reductase. Such alteration of the drug metabolizing capacity of the liver by I3C contributes to blocking of initiation, while the observed inhibition of ornithine decarboxylase, a rate limiting enzyme in polyamine biosynthesis, and of tyrosine kinase activity may contribute to the suppressive effect of I3C.

Mechanism of action of dietary chemoprotective agents in rat liver: induction of phase I and II drug metabolizing enzymes and aflatoxin B1 metabolism.

A range of potential chemoprotective agents, most of them natural dietary constituents, has been examined for ability to modulate both phase I (cytochrome P450 1A1, 1A2, 2B1/2, 2C11, 2E1, 3A, 4A) and phase II drug metabolizing enzymes (glutathione S-transferases, in particular subunits Yc2 and P, aflatoxin B1-aldehyde reductase and quinone reductase) in rat liver. In addition to assays of total enzyme activity and Western blots for individual isozymes, the ability of microsomes to metabolize aflatoxin B1, and of cytosols to conjugate aflatoxin B1 (AFB1)-epoxide to GSH and to produce AFB1-dialcohol, were measured. Induction of gamma-glutamyl transpeptidase activity was examined by histochemistry. Differing patterns of induction were observed, reflecting differences in the control of expression of the individual enzymes studied. Of the compounds examined, butylated hydroxytoluene, ethoxyquin, indole-3-carbinol and phenethyl isothiocyanate were the most potent bifunctional agents (inducing both phase I and II activities). Oltipraz, while only weakly inducing CYP1A2 and 2B1/2, was a potent inducer of phase II enzymes. Caffeic acid, garlic oil, sinigrin and propyl gallate all showed some ability to induce phase II enzymes. 4-Methyl catechol, alpha-tocopherol and red wine decreased certain phase I enzyme activities, while inducing total GST activity. Butylated hydroxytoluene, ethoxyquin, garlic oil and indole-3-carbinol induced gamma glutamyltranspeptidase in periportal hepatocytes. Particularly because of their ability to induce the detoxifying activities of glutathione S-transferase Yc2 and aldehyde reductase, butylated hydroxytoluene, ethoxyquin, indole-3-carbinol, oltipraz, phenethyl isothiocyanate and sinigrin will be effective blocking agents in rodents, if administered prior to AFB1. While these studies indicate the relative contributions of phase I and II metabolism in the overall protective effect in rat, care should be taken that a similar balance is achieved in man, and that relevant enzymes or iso forms are induced.