Regulation of glutathione transferase and DT-diaphorase mRNAs in persistent hepatocyte nodules during chemical hepatocarcinogenesis.

Abstract

We have utilized cDNA probes and in vitro translation analysis to quantitate the levels of rat liver glutathione transferase (glutathione S-aralkyltransferase; RX:glutathione R-transferase, EC 2.5.1.18) and DT-diaphorase [NAD-(P)H:quinone-acceptor oxidoreductase, EC 1.6.99.2] mRNAs in persistent hepatocyte nodules induced by chemical carcinogens. Our results indicate that within the nodules, glutathione transferase mRNAs specific for the Ya/Yc and Yb subunits are increased 3-fold and 5-fold, respectivqly, over the levels observed in normal liver or in the liver tissue surrounding the nodules. Similarly, the level of DT-diaphorase mRNA is increased 5-to 7-fold within the nodules as compared to surrounding liver tissue or normal liver. When animals were administered 3-methylcholanthrene, a typical inducer of these nRNAs in normal animals, a further increase in the glutathione transferase Yb mRNA(s) and DT-diaphorase mRNA was observed in the nodules; however, the Ya/Yc mRNA levels remained unaffected. Our data indicate that during chemically induced neoplastic transformation, the mRNA levels for the Yb subunit of glutathione transferase and DT-diaphorase arp increased in the nodules but still retain the capacity to be regulated by 3-methylcholanthrene. Although the glutathione transferase Ya/Yc mRNAs are also increased in the nodules, they lost their ability to be regulated by 3-methylcholanthrene.These latter data suggest that within the nodules there is a specific defect in the regulatory mechanism(s) that leads to an induction of the Ya/Yc mRNAs in normal tissue by xenobiotics.The administration of a single initiating dose of diethylnitrosamine to rats followed by a low level of 2-acetylaminofluorene plus a partial hepatectomy induces foci of altered hepatocytes, which have been referred to as persistent hepatocyte nodules, hyperplastic nodules, or preneoplastic nodules (1). Persistent hepatocyte nodules are thought to represent precursors for the subsequent development of hepatocellular carcinomas and, thus, have been utilized as a model system to study chemically induced neoplastic transformation (1, 2). Biochemically the nodules have elevated levels of various drug-metabolizing enzymes {e.g., glutathione transferases (glutathione S-aralkyltransferase; RX:glutathione R-transferase, EC 2.5.1.18), DT-diaphorase [NAD(P)H:quinone-acceptor oxidoreductase, EC 1.6.99.2], and epoxide hydrolase}; however, cytochrome P-450 levels appear to be depressed (3-11). The elevated levels of some phase II drugmetabolizing enzyme activities and a decreased level of cytochrome P-450 are consistent with the observation that persistent hepatocyte nodules are more resistant to the cytotoxic effects of carcinogens as compared to normal liver (12). In fact, Farber (2) has hypothesized that an early or even first biological cellular event in liver carcinogenesis is the induction in a rare hepatocyte of resistance to the cytotoxic effect of carcinogens.In order to understand the mechanisms of chemical carcinogenesis, a basic unde...

Metabolism of Xenobiotics in Hepatocyte Nodules

“…The isoenzymes composed of subunits 1, 2, and 3, expressed in normal liver tissue, all occurred at increased concentrations in nodular tissue, whereas the level of transferase 4-4 was decreased. The 2.3-to 4.6-fold increase in the levels of proteins reacting with antibodies to transferases 1-1 , 2-2, and 3-3 was in agreement with the reported 3-to 5-fold increase of mRNAs for the corresponding protein subunits (66). Furthermore, results of chromatography on S-hexylglutathione-Sepharose-6B and immunodiffusion tests indicated that isoenzyme 5-5 was decreased in the nodules.…”

“…The cytosolic forms of glutathione transferase using 1 -chloro-2,4-dinitrobenzene as substrate occur in remarkably high concentrations in hepatocyte nodules (5, 25,70). Also, Pickett et a1 (66) recently showed that the mRNA levels of the Yay Yb, and Yc subunits of glutathione transferase were elevated 3 to 5 times in hepatocyte nodules from the resistant hepatocyte model compared to normal liver values. Their work also describes deviations in regulation of the nodular levels of glutathione transferase isoenzymes upon induction with xenobiotic substances.…”

“…Histochemical investigations confirm the differences in activity of this enzyme between nodular and surrounding tissue (77). Pickett et a1 (65) consolidated the data on DT-diaphorase by showing that the mRNA for the cytosolicenzyme in nodules was elevated to a level 5 to 7 times that in normal liver.…”

Selective expression of glutathione transferase isoenzymes in chemically induced preneoplastic rat heptocyte nodules

“…The 2.3-4.6fold increase in the levels of proteins reacting with antibodies to transferases l-l, 2-2, and 3-3 (table 2) is in good agreement with the 3-.5-fold increase of mRNAs for the corresponding protein subunits [21].…”

Influences of Dietary Deoxycholic Acid on Progression of Hepatocellular Neoplasms and Expression of Glutathione S-Transferases in Rats

“…Previous studies have examined the possible roles of DCA as a genotoxin that can generate preneoplastic &dquo;initiated&dquo; populations (26,44) and as a cytotoxic tumor promotor that enhances the selective proliferation of resistant preneoplastic populations (44,45). However, long-term administration of DCA might also have some direct effect on the progression of established preneoplastic populations (1 (1,26) and y-glutamyl transpeptidase (7,26), each of which typify the xenobiotic-resistant phenotype of preneoplastic hepatocytes generated by various initiating carcinogens and promoted by various regimens (8,11,12,27,34,36). These observations suggest that the resistant phenotype of preneoplastic hepatocytes might favor their selective expansion during exposure to the toxic effects of bile acids.…”