Regulation of the alpha-fetoprotein gene by the isoforms of ATBF1 transcription factor in human hepatoma.
We investigated mechanisms regulating expression of alpha-fetoprotein (AFP) in 3 human hepatoma cell lines, HuH-7, HepG2, and huH-1, producing high, medium, and low levels of AFP, respectively. The silencer, a negative cis-acting element of the AFP gene, was highly activated in huH-1 and HepG2 to repress AFP enhancer activity by 91%, whereas only 26% repression was observed in HuH-7. To account for the difference in AFP production between HepG2 and huH-1, we investigated the roles of two isoforms of the AT motif-binding factor 1 (ATBF1) transcription factor, ATBF1-A and -B. Cotransfection assays showed that the ATBF1 isoforms regulated the AFP gene differently in HepG2 and huH-1. In huH-1 and HuH-7, both ATBF1 isoforms suppressed strongly enhancer activity and slightly promoter activity. In HepG2, on the other hand, ATBF1-A suppressed the enhancer and promoter activities, but surprisingly, ATBF1-B was found to stimulate enhancer activity while showing no effect on the promoter. Levels of ATBF1-A mRNA were similar in all 3 cell lines, whereas the expression ATBF1-B mRNA varied greatly, with the highest level seen in HepG2 followed by huH-1 and HuH-7. These results suggest that, in HepG2, ATBF1-B may have a dominant negative effect to relieve the transcriptional repression caused by its isoform. In support of this view, we found that the N-terminal region specific to the ATBF1-A molecule possessed transcriptional repressor activity. Thus, the use of the ATBF1 variants as well as the silencer may provide a unique mechanism that contributes to the determination of AFP levels in human hepatoma cell lines.
Ninomiya
Toshiaki
T
Department of Biochemistry and Molecular Biology, University of Calgary, Calgary, Alberta, Canada. ninomiya@med.kobe-u.ac.jp
Mihara
Koichiro
K
Fushimi
Kazuo
K
Hayashi
Yoshitake
Y
Hashimoto-Tamaoki
Tomoko
T
Tamaoki
Taiki
T
D006528:Carcinoma, Hepatocellular / Q000378:metabolism*; D004742:Enhancer Elements, Genetic / Q000235:genetics; D005786:Gene Expression Regulation; D020868:Gene Silencing; D018398:Homeodomain Proteins / Q000235:genetics / Q000502:physiology*; D006801:Humans; D008113:Liver Neoplasms / Q000378:metabolism*; D011401:Promoter Regions, Genetic / Q000235:genetics; D020033:Protein Isoforms / Q000235:genetics / Q000502:physiology*; D014158:Transcription, Genetic; D014162:Transfection; D014407:Tumor Cells, Cultured; D000509:alpha-Fetoproteins / Q000235:genetics*
D018398:Homeodomain Proteins; D020033:Protein Isoforms; D000509:alpha-Fetoproteins; C071660:ZFHX3 protein, human