Mini reviewRegulatory mechanism of glutathione S-transferase P-form during chemical hepatocarcinogenesis: old wine in a new bottle
Introduction
The rat glutathione S-transferase P-form (GST-P) has been found to be dramatically up-regulated in its expression in preneoplastic and neoplastic cells [1], [2] and is widely used as a specific marker in the basic analysis of chemical carcinogenesis [3]. However, the mechanism by which the expression of GST-P is induced in the rat liver during carcinogenic treatment has remained to be solved. The aim of this review is to highlight recent advances in our understanding of how GST-P gene expression is regulated during the early stages of hepatocarcinogenesis by chemical carcinogens and to obtain a hint as to why GST-P gene expression is the end point marker in most cases of chemical carcinogenesis [4].
Although GST-P expression has been routinely employed as a preneoplastic marker during chemical hepatocarcinogenesis in rats, it must be noted that there is no induction of the GST-P gene in preneoplastic foci and hepatoma by nongenotoxic proliferators such as clofibrate [5]. On the contrary, adult rat brain [6] and adult male mouse liver [7] constitutively expressed GST-P at significant levels without any tumor formation. Human hepatocellular carcinomas do not consistently overexpress GST P-form, but it is found in cholangiocarcinoma, presumably reflecting the tissue of origin [8]. Therefore, it is unlikely that GST-P expression is essentially associated with the transformation of hepatocytes among these species.
To distinguish between the two alternative possibilities in the liver, that is, either the GST-P gene is coactivated with a closely located transforming gene by a cis mechanism or it is activated in trans by a common trans-acting factor, Moriyama et al. carried out carcinogenesis experiments using transgenic rats harboring the chloramphenycol acetyltransferase (CAT) reporter gene ligated to the upstream regulatory sequence of the rat GST-P gene [9]. Although different lines of transgenic rats had multiple copies of the CAT gene, immunohistochemical staining demonstrated the co-expression of CAT with endogenous GST-P-positive foci, indicating clearly that the GST-P gene is activated position independently by a trans-mechanism [9].
Section snippets
GPEI as an enhancer element for GST-P gene expression
As with the expression of most eukaryotic genes, GST-P gene expression depends on at least two regulatory elements: a promoter and a far-upstream enhancer. GST-P enhancer I (GPEI) located at ā2.5 kb, consists of imperfect TPA (12-o-tetradecanoyl-phorbol-13-acetate) responsive element (TRE)-like sequences that are palindromically oriented [10]. The AP-1 DNA-binding site, or TRE, contains the sequence 5ā²-TGACTCA-3ā² which shares some homology with the core consensus ARE (antioxidant responsive
Changes in regulatory network of transcription factors during carcinogenesis
In the 5ā²-flanking region of the rat GST-P gene, the negative regulatory elements located around 400 bp upstream from the cap site are also significant, and these elements would aid the repressive mechanism of the GST-P gene in normal rat liver [23]. One of the binding factors, a member of the CCAAT/enhancer binding protein (C/EBP) family, binds to GST-P silencer-1 (GPS-1). The ratio of C/EBP alpha to C/EBP-beta is an important factor for GST-P silencer activity and a decrease in the ratio
Possible participation of CCAAT/enhancer-binding proteins(C/EBPs)
At least six members of C/EBP family have been isolated and characterized to date (C/EBP-alpha-C/EBP-zeta), particularly in hepatocytes, adipocytes and hematopoietic cells [32]. The expression of C/EBPs is regulated at multiple levels under several physiological and pathological conditions through the actions of a variety of factors, including hormones, mitogens, cytokines, nutrients and certain toxins.
C/EBP-alpha is expressed at high levels in terminally differentiated cells and the expression
Specific genetic change is not necessary for GST-P expression
To return to our hypothesis, the induction of GST-P gene expression during the early stages of chemical hepatocarcinogenesis is not necessarily associated with the mutation of any specific gene(s), but could be caused similarly by a variety of hepatocarcinogens with different chemical structures and also by different protocols. Randomly occurring mutations, if any exist, may be incidental to the early stage of carcinogenesis.
Possible explanation is as follows: metabolically activated
Genetic analysis of GST-P gene expression in carcinogen-resistant DRH rats
A final question is whether the formation of GST-P-positive foci is a prerequisite for the development of hepatocellular carcinoma (HCC). Genetic linkage analysis was carried out on the GST-P-positive foci in the livers of (F344 x DRH)F2 rats at early (7 weeks) and later (20 weeks) stage in the development of HCC with 3ā²-Me-DAB administration. Our previous study showed that the size of GST-P-positive foci in the livers of carcinogen-resistant DRH rats at 7 weeks of 3ā²-Me-DAB administration was
Acknowledgements
We are grateful to Dr Masaharu Sakai (Hokkaido University, School of Medicine) for helpful discussion and Ms Junko Iuchi for preparing this manuscript.
References (60)
- et al.
Medium-term liver and multi-organ carcinogenesis and chemopreventive agents
Exp. Toxicol. Pathol.
(1996) - et al.
Developmental and hormonal regulation of the major form of hepatic glutathione S-transferase in male mice
Biochem. Biophys. Res. Comm.
(1986) - et al.
In vivo cross-linking and immunoprecipitation for studying dynamic protein:DNA association in a chromatin environment
Methods
(1999) - et al.
An Nrf2/small Maf heterodimer mediates the induction of phase II detoxifying genes through antioxidant response elements
Biochem. Biophys. Res. Comm.
(1997) - et al.
Transcriptional regulation of the rat NAD(P)H:quinine reductase gene. Identification of regulatory elements controlling basal level expression and inducible expression by plant aromatic compounds and phenolic antioxidants
J. Biol. Chem.
(1991) - et al.
C/EBP-alpha arrests cell proliferation through direct inhibition of cdk2 and cdk4
Mol. Cell
(2001) - et al.
Phosphorylation of rat serine 105 or mouse threonine 217 in C/EBP-beta is required for hepatocyte proliferation induced by TGF-alpha
Mol. Cell
(1999) - et al.
Bile acid-induced activation of activator protein kinase C signaling
J. Biol. Chem.
(2000) - et al.
Cellular response to the redox active lipid peroxidation products: induction of glutathione S-transferase P by 4-hydroxy-2-nonenal
Biochem. Biophys. Res. Comm.
(1997) - et al.
Lipid peroxidation end products-responded induction of a preneoplastic marker enzyme glutathione S-transferase P-form (GST-P) in liver on administration via the portal vein
Mut. Res.
(2001)