ATBF1, a multiple-homeodomain zinc finger protein, selectively down-regulates AT-rich elements of the human alpha-fetoprotein gene.

Abstract

ATBF1 is a 306-kDa protein containing four homeodomains, 17 zinc finger motifs, and several segments potentially involved in transcriptional regulation (T. Morinaga, H. Yasuda, T. Hashimoto, K. Higashio, and T. Tamaoki, Mol. Cell. Biol. 11:6041-6049, 1991). At least one of the homeodomains of ATBF1 binds to an AT-rich element in the human a-fetoprotein (AFP) enhancer (enhancer AT motif). In the present work, we analyzed the transcriptional regulatory activity of ATBF1 with respect to the enhancer AT motif and similar AT-rich elements in the human AFP promoter and the human albumin promoter and enhancer. Gel retardation assays showed that ATBF1 binds to the AFP enhancer AT motif efficiently; however, it binds weakly or not at all to other AT-rich elements in the AFP and albumin regulatory regions studied. Alterations of the enhancer 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 activity of AFP enhancer and promoter regions containing AT-rich elements. This suppression was reduced when the mutated AT motifs with low affinity to ATBF1 were linked to the CAT gene. The ATBF1 suppression of AFP promoter and enhancer activities appeared to be due, at least in part, 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 distinguish AFP and albumin AT-rich elements, leading to selective suppression of the AFP promoter and enhancer activities.The regulatory regions of all genes so far studied contain multiple cis-acting elements which interact with trans-acting factors (22,32). Liver-specific gene expression is also achieved through a combinatorial action of regulatory DNA elements and interacting transcription factors (20,21,30). One characteristic feature of a number of liver-specific promoters is the presence of DNA sequences having a well-organized array of 10 to 11 adenine (A) and thymine (T) residues sandwiched by a guanine (G) and a cytosine (C) (AT-rich element) (4-7, 11, 15, 23, 26, 40). These elements have been shown to be the binding sites of a liver-enriched transcription factor, HNF1 (also called LF-B1 and APF) (3-6, 12, 31). Mutations of the AT-rich element cause drastic reduction of liver-specific transcriptional activity both in vitro and in vivo (21, 28, 33, 42), indicating that the AT-rich element and HNF1 play a crucial role in liver-specific promoter activation.In the promoters of a-fetoprotein (AFP) genes of mice, rats, and humans, two HNF1-binding sites are conserved. In the enhancers, on the other hand, the AT-rich element is present in the human AFP gene but not in the mouse and rat genes (13). The human AFP enhancer AT motif is also unique in that HNF1 binds to it 10-fold...

A Sleeping Beauty mutagenesis screen reveals a tumor suppressor role for Ncoa2/Src-2 in liver cancer

“…Atbf1, which encodes AT-motif binding factor-1 protein, also was identified in our screen. Also known as "zinc-finger homeobox 3" (ZFHX3), this protein has been shown previously to negatively regulate the expression of the liver tumor marker α-fetoprotein (AFP) in hepatocytes (31). Moreover, it was demonstrated previously that ATBF1 expression was significantly reduced in 55 of 76 HCC samples (32).…”

Homeotic factor ATBF1 induces the cell cycle arrest associated with neuronal differentiation

“…Absence or dysfunction of the ATM protein causes ataxia-telangiectasia (AT), a human disease that shows remarkable elevation of AFP expression (Chun and Gatti, 2004). The elevation of AFP may be explained by the dysfunction of ATBF1, as ATBF1 is the major suppressive factor for Afp gene transcription (Morinaga et al, 1991;Yasuda et al, 1994). These observations suggest that the expression of ATM, a member of the PI(3)K superfamily, is highly correlated with the function of ATBF1 as a gene regulatory factor in the nucleus.…”

“…ATBF1 has four homeotic domains and 23 zincfinger motifs in a single molecule that should control various target genes. Previously, the Afp (Morinaga et al, 1991;Yasuda et al, 1994) and aminopeptidase N (APN) (Kataoka et al, 2000) genes were identified as targets for ATBF1. The present study was designed based on the predicted roles for ATBF1 in neuronal differentiation both in vitro and in vivo.…”

Subcellular localization of ATBF1 regulates MUC5AC transcription in gastric cancer

“…ATBF1 down regulates AFP gene expression in human hepatic cells through binding to the AT motif competitively with hepatocyte nuclear factor 1 (HNF1), which itself activates the transcription of AFP. 13,14 ATBF1 protein is known to interact with various proteins including c-Myb 15 and PIAS3. 16 ATBF1 represses c-Myb transcription activity through protein-protein interactions, which may in turn result in the suppression of cell growth and differentiation.…”