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 carcino genesis in rodents, A principal mechanism by which these chemopreventive co mpounds exert their protective effects is likely to be via induction of car cinogen detoxification, This can be mediated by conjugation with glutathion e, which is synthesized by the sequential actions of glutamate-cysteine lig ase (GLCL) and glutathione synthetase. We have demonstrated that dietary ad ministration of the naturally occurring chemopreventive agents, ellagic aci d, coumarin or alpha -angelicalactone caused an increase in GLCL activity o f between similar to3- and 5-fold in rat liver. Treatment with the syntheti c antioxidant ethoxyquin or the classic inducer phenobarbital caused < 2-fo ld induction of GLCL activity in rat liver, which was not found to be signi ficant. The increases in GLCL activity were accompanied by increases (betwe en 2- and 4-fold) in levels of both the catalytic heavy subunit (GLCLC) and regulatory light subunit (GLCLR), No substantial induction of GLCL was obs erved in rat kidney. The glutathione S-transferase (GST) subunits A1, A3, A 4, 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 Fl, followi ng treatment with coumarin (20-fold), <alpha>-angelicaliactone (10-fold) or ellagic acid (6-fold), and GST A5, following treatment with coumarin (7-fo ld), alpha -angelicalactone (6-fold) and ethoxyquin (6-fold), Glutathione s ynthetase was induced similar to 14-fold by coumarin, alpha -angelicalacton e, ellagic acid and ethoxyquin, The expression of glutathione-related enzym es was also examined in preneoplastic lesions induced in rat liver by aflat oxin B-1. The majority of gamma -glutamyltranspeptidase (GGT)-positive pren eoplastic foci contained increased levels of GLCLC relative to the surround ing tissue. This was usually found to be accompanied by an increase in GLCL R, Cells in the inner cortex of rat kidney were found to contain the highes t levels of both GLCLC and GLCLR, The same cells showed the strongest stain ing for GGT activity.