Regulation of rat glutamate-cysteine ligase (γ-glutamylcysteine synthetase) subunits by chemopreventive agents and in aflatoxin B1-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 α-angelicalactone caused an increase in GLCL activity of between ~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), α-angelicalactone (10-fold) or ellagic acid (6-fold), and GST A5, following treatment with coumarin (7-fold), α-angelicalactone (6-fold) and ethoxyquin (6-fold). Glutathione synthetase was induced ~1.5-fold by coumarin, α-angelicalactone, ellagic acid and ethoxyquin. The expression of glutathione-related enzymes was also examined in preneoplastic lesions induced in rat liver by aflatoxin B1. The majority of γ-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.