Regulation of the expression of some genes for enzymes of glutathione metabolism in hepatotoxicity and hepatocarcinogenesis.
The reversible stage of tumor promotion, which follows the stage of initiation and precedes that of progression in multistage carcinogenesis, is a unique example of reversible toxicity in biological systems. In order to study the molecular mechanisms involved in the action of promoting agents during this stage, the regulation of the expression of genes for two enzymes of glutathione metabolism, gamma-glutamyl transpeptidase (GGT) and the placental isozyme of glutathione S-transferase (GST-P), was studied under several different conditions of promotion during multistage hepatocarcinogenesis in the rat. Promotion by phenobarbital caused an increased expression of both of these genes in altered hepatic focal lesions, although this was somewhat more variable in the case of the GGT gene. C.I. Solvent Yellow 14, an industrial dye, served as an effective promoting agent. Feeding this dye resulted in a dramatic increase in the expression of GST-P, but not that of GGT in altered hepatic foci. Factors in crude, cereal-based diets inhibited the stage of promotion by diethylnitrosamine, but enhanced promotion by phenobarbital in a synergistic manner. In contrast, at least one purified diet had the converse effect during this stage. The mRNA levels of GST-P were uniformly elevated dramatically in reversible nodules and neoplasms of rat liver that had been induced by diethylnitrosamine and phenobarbital promotion. In contrast, the level of GGT mRNA was somewhat variable, with an occasional neoplasm exhibiting almost a background level of expression of this gene. Therefore, the altered regulation of multiple genes in hepatocytes during the stage of promotion can vary with the promoting agent itself; this process may be related to the heterogeneous gene expression seen in hepatic neoplasms. A possible role for specific DNA sequences in the 5' flanking regions of such genes is considered. In addition, a cDNA clone to the mRNA of human liver GGT was isolated and sequenced. The homology of the coding sequence of the human liver GGT mRNA to that of rat kidney GGT mRNA was striking.
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McArdle Laboratory for Cancer Research, Department of Oncology, Medical School, University of Wisconsin, Madison 53706.
Goodspeed
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Dunn
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Hendrich
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Maronpot
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Moran
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D000818:Animals; D001483:Base Sequence; D003043:Cocarcinogenesis; D004247:DNA / Q000032:analysis; D004032:Diet; D004052:Diethylnitrosamine; D005260:Female; D005786:Gene Expression Regulation / Q000187:drug effects*; D005982:Glutathione Transferase / Q000235:genetics* / Q000378:metabolism; D008114:Liver Neoplasms, Experimental / Q000139:chemically induced / Q000201:enzymology*; D008297:Male; D008969:Molecular Sequence Data; D009284:Naphthols; D010634:Phenobarbital / Q000494:pharmacology; D011230:Precancerous Conditions / Q000139:chemically induced / Q000201:enzymology*; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D011916:Rats, Inbred F344; D005723:gamma-Glutamyltransferase / Q000235:genetics* / Q000378:metabolism
D009284:Naphthols; D012333:RNA, Messenger; D004052:Diethylnitrosamine; C024336:1-phenylazo-2-naphthol; D004247:DNA; D005723:gamma-Glutamyltransferase; D005982:Glutathione Transferase; D010634:Phenobarbital