CCAAT enhancer-binding protein alpha suppresses the rat placental glutathione S-transferase gene in normal liver.
The rat placental glutathione S-transferase (GST-P), an isozyme of glutathione S-transferase, is not expressed in normal liver but is highly induced at an early stage of chemical hepatocarcinogenesis and in hepatomas. Recently, we reported that the NF-E2 p45-related factor 2 (Nrf2)/MafK heterodimer binds to GST-P enhancer 1 (GPE1), a strong enhancer of the GST-P gene, and activates this gene in preneoplastic lesions and hepatomas. In addition to the positive regulation during hepatocarcinogenesis, negative regulatory mechanisms might work to repress GST-P in normal liver, but this remains to be clarified. In this work, we identify the CCAAT enhancer-binding protein alpha (C/EBPalpha) as a negative regulator that binds to GPE1 and suppresses GST-P expression in normal liver. C/EBPalpha binds to part of the GPE1 sequence, and the binding of Nrf2/MafK and C/EBPalpha to GPE1 is mutually exclusive. In a transient-transfection analysis, C/EBPalpha activated GPE1 in F9 embryonal carcinoma cells but strongly inhibited GPE1 activity in hepatoma cells. The expression of C/EBPalpha was specifically suppressed in GST-P-positive preneoplastic foci in the livers of carcinogentreated rats. A chromatin immunoprecipitation analysis showed that C/EBPalpha bound to GPE1 in the normal liver in vivo but did not bind in preneoplastic hepatocytes. Introduction of the C/EBPalpha gene fused with the estrogen receptor ligand-binding domain into hepatoma cells, and subsequent activation by beta-estradiol led to the suppression of endogenous GST-P expression. These results indicate that C/EBPalpha is a negative regulator of GST-P gene expression in normal liver.
Ikeda
Hiromi
H
Departments of Biochemistry and Ophthalmology, Hokkaido University Graduate School of Medicine, N15, W7, Kita-Ku, Sapporo 060-8638, Japan.
Omoteyama
Kazuki
K
Yoshida
Kazuhiko
K
Nishi
Shinzo
S
Sakai
Masaharu
M
D000818:Animals; D022763:CCAAT-Enhancer-Binding Protein-alpha / Q000502:physiology*; D006528:Carcinoma, Hepatocellular / Q000201:enzymology / Q000235:genetics; D045744:Cell Line, Tumor; D004742:Enhancer Elements, Genetic; D005982:Glutathione Transferase / Q000037:antagonists & inhibitors* / Q000096:biosynthesis / Q000235:genetics*; D008099:Liver / Q000201:enzymology*; D008114:Liver Neoplasms, Experimental / Q000201:enzymology / Q000235:genetics; D051379:Mice; D011257:Pregnancy Proteins / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000235:genetics*; D051381:Rats
D022763:CCAAT-Enhancer-Binding Protein-alpha; D011257:Pregnancy Proteins; D005982:Glutathione Transferase