H. Yasuda et al., ATBF1, A MULTIPLE-HOMEODOMAIN ZINC-FINGER PROTEIN, SELECTIVELY DOWN-REGULATES AT-RICH ELEMENTS OF THE HUMAN ALPHA-FETOPROTEIN GENE, Molecular and cellular biology, 14(2), 1994, pp. 1395-1401
ATBF1 is a 306-kDa protein containing four homeodomains, 17 zinc finge
r motifs, and several segments potentially involved in transcriptional
regulation (T. Morinaga, H. Yasuda, T. Hashimoto, K. Higashio, and T.
Tamaoki, Mel. Cell. Biol. 11:6041-6049, 1991). At least one of the ho
meodomains of ATBF1 binds to an AT-rich element in the human alpha-fet
oprotein (AFP) enhancer (enhancer AT motif). In the present work, we a
nalyzed the transcriptional regulatory activity of ATBF1 with respect
to the enhancer AT motif and similar AT-rich elements in the human AEP
promoter and the human albumin promoter and enhancer. Gel retardation
assays showed that ATBF1 binds to the AFP enhancer AT motif efficient
ly; however, it binds weakly or not at all to other AT-rich elements i
n the AFP and albumin regulatory regions studied. Alterations of the e
nhancer AT motif by site-specific mutagenesis resulted in the loss of
binding of ATBF1. Cotransfection experiments with an ATBF1 expression
plasmid and the chloramphenicol acetyltransferase (CAT) gene fused to
AFP promoter or enhancer fragments Showed that ATBF1 suppressed the ac
tivity of AFP enhancer and promoter regions containing AT-rich element
s. This suppression was reduced when the mutated AT motifs with low af
finity to ATBF1 were linked to the CAT gene. The ATBF1 suppression of
AFP promoter and enhancer activities appeared to be due, at least in p
art, to competition between ATBF1 and HNF1 for the same binding site.
In contrast to the AFP promoter and enhancer, the albumin promoter and
enhancer were not affected by ATBF1, although they contain homologous
AT-rich elements. These results show that ATBF1 is able to distinguis
h AFP and albumin AT-rich elements, leading to selective suppression o
f the AEP promoter and enhancer activities.