Regulation of the α-fetoprotein gene by the isoforms of ATBF1 transcription factor in human hepatoma

Abstract

We investigated mechanisms regulating expression of α‐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.