A position-dependent silencer plays a major role in repressing alpha-fetoprotein expression in human hepatoma.
A large percentage of human hepatomas produce alpha-fetoprotein (AFP), but the levels of AFP expression vary greatly among hepatomas. To understand the molecular basis for this variation, we analyzed transcriptional regulatory activities associated with the 5'-flanking region of the AFP gene in two human hepatoma cell lines, HuH-7 and huH-1/cl-2, which produce a high and a low level of AFP, respectively. We found that the low level of AFP production in huH-1/cl-2 is due to the action of at least two silencer regions located between the enhancer and the promoter of the AFP gene. In contrast, no silencer activity is expressed in HuH-7. We identified 5'-CTTCATAACTAATACTT-3' to be a core sequence responsible for the negative regulatory activity. This sequence is repeated four times in a strong, distal silencer region, Sd, whereas one copy is present in a weak, proximal silencer region, Sp. The silencer reduces transcriptional initiation by blocking enhancer activation of the AFP promoter in a position-dependent manner. The silencer functions in the presence of positive transcription factors and may play a key role in developmental repression as well as variable expression of the AFP gene in hepatomas.
Nakabayashi
H
H
Department of Medical Biochemistry, University of Calgary, Alberta, Canada.
Hashimoto
T
T
Miyao
Y
Y
Tjong
K K
KK
Chan
J
J
Tamaoki
T
T
D000418:Albumins / Q000378:metabolism; D001483:Base Sequence; D006528:Carcinoma, Hepatocellular / Q000235:genetics*; D003001:Cloning, Molecular; D004273:DNA, Neoplasm; D004742:Enhancer Elements, Genetic; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D008113:Liver Neoplasms / Q000235:genetics*; D008969:Molecular Sequence Data; D011401:Promoter Regions, Genetic; D012045:Regulatory Sequences, Nucleic Acid; D012689:Sequence Homology, Nucleic Acid; D014158:Transcription, Genetic; D014162:Transfection; D014407:Tumor Cells, Cultured; D000509:alpha-Fetoproteins / Q000235:genetics* / Q000378:metabolism
D000418:Albumins; D004273:DNA, Neoplasm; D000509:alpha-Fetoproteins