Decreased expression of hepatocyte nuclear factor 3 alpha during the acute-phase response influences transthyretin gene transcription.
Three distinct hepatocyte nuclear factor 3 (HNF-3) proteins (alpha, beta, and gamma) are known to regulate the transcription of numerous liver-specific genes. The HNF-3 proteins bind to DNA as monomers through a winged-helix motif, which is also utilized by a number of developmental regulators, including the Drosophila homeotic fork head (fkh) protein. We have previously characterized a strong-affinity HNF-3S site in the transthyretin (TTR) promoter region which is essential for expression in human hepatoma (HepG2) cells. In the current study, we identify an activating protein 1 (AP-1) site which partially overlaps the HNF-3S sequence in the TTR promoter. We show that in HepG2 cells the AP-1 sequence confers 12-O-tetradecanoylphorbol-13-acetate inducibility to the TTR promoter and contributes to normal TTR transcriptional activity. We also demonstrate that the HNF-3 proteins and AP-1 bind independently to the TTR AP-1-HNF-3 site, and cotransfection experiments suggest that they do not cooperate to activate an AP-1-HNF-3 reporter construct. In addition, 12-O-tetradecanoylphorbol-13-acetate exposure of HepG2 cells results in a reciprocal decrease in HNF-3 alpha and -3 gamma expression which may facilitate interaction of AP-1 with the TTR AP-1-HNF-3 site. In order to explore the role of HNF-3 in the liver, we have examined expression patterns of TTR and HNF-3 during the acute-phase response and liver regeneration. Partial hepatectomy produced minimal fluctuation in HNF-3 and TTR expression, suggesting that HNF-3 expression is not influenced by proliferative signals induced during liver regeneration. In acute-phase livers, we observed a dramatic reduction in HNF-3 alpha expression which correlates with a decrease in the expression of its target gene, the TTR gene. Furthermore, consistent with previous studies, the acute-phase livers are induced for c-jun but not c-fos expression. We propose that the reduction in TTR gene expression during the acute phase is likely due to lower HNF-3 alpha expression levels and that the induction of primarily c-jun homodimers, which are poor transcriptional activators, is insufficient to maintain normal TTR expression levels. We also discuss the role of reduced HNF-3 alpha expression in mediating decreased transcription of HNF-3 target genes which respond negatively to cytokine signalling.
Qian
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Department of Biochemistry, College of Medicine, University of Illinois at Chicago 60612-7334.
Samadani
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U
Porcella
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Costa
R H
RH
D000818:Animals; D016375:Antisense Elements (Genetics); D001483:Base Sequence; D006528:Carcinoma, Hepatocellular; D015500:Chloramphenicol O-Acetyltransferase / Q000096:biosynthesis; D017931:DNA Primers; D004268:DNA-Binding Proteins / Q000096:biosynthesis*; D004330:Drosophila; D051858:Forkhead Transcription Factors; D015870:Gene Expression; D015972:Gene Expression Regulation, Neoplastic; D006498:Hepatectomy; D051541:Hepatocyte Nuclear Factor 3-alpha; D006801:Humans; D007700:Kinetics; D008070:Lipopolysaccharides / Q000633:toxicity; D008099:Liver / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D008113:Liver Neoplasms; D051379:Mice; D008815:Mice, Inbred Strains; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009687:Nuclear Proteins / Q000096:biosynthesis* / Q000378:metabolism; D016133:Polymerase Chain Reaction; D011228:Prealbumin / Q000096:biosynthesis*; D011401:Promoter Regions, Genetic; D016755:Proto-Oncogene Proteins c-jun / Q000096:biosynthesis / Q000378:metabolism; D015347:RNA Probes; D051381:Rats; D011994:Recombinant Proteins / Q000096:biosynthesis; D016385:TATA Box; D013755:Tetradecanoylphorbol Acetate; D014157:Transcription Factors / Q000096:biosynthesis / Q000378:metabolism; D014158:Transcription, Genetic; D014407:Tumor Cells, Cultured
D016375:Antisense Elements (Genetics); D017931:DNA Primers; D004268:DNA-Binding Proteins; C496499:FOXA1 protein, human; D051858:Forkhead Transcription Factors; C496500:Foxa1 protein, mouse; C496501:Foxa1 protein, rat; D051541:Hepatocyte Nuclear Factor 3-alpha; D008070:Lipopolysaccharides; D009687:Nuclear Proteins; D011228:Prealbumin; D016755:Proto-Oncogene Proteins c-jun; D015347:RNA Probes; D011994:Recombinant Proteins; D014157:Transcription Factors; D015500:Chloramphenicol O-Acetyltransferase; D013755:Tetradecanoylphorbol Acetate