Elements and factors involved in tissue-specific and embryonic expression of the liver transcription factor LFB1 in Xenopus laevis.

Abstract

LFB1 (HNF1) is a tissue-specific transcription factor found in the livers, stomachs, intestines, and kidneys of vertebrates. By analyzing the promoter of the Xenopus LFB1 gene, we identified potential autoregulation by LFB1 and regulation by HNF4, a transcription factor with a tissue distribution similar to that of LFB1.Injection of LFB1 promoter-chloramphenicol acetyltransferase constructs into Xenopus eggs revealed embryonic activation that is restricted to the region of the developing larvae expressing endogeneous LFB1. Proper embryonic activation was also observed with a rat LFB1 promoter. Deletion analysis of the Xenopus and rat promoters revealed that in both promoters embryonic activation is absolutely dependent on the presence of an element that contains CCNCTCTC as the core consensus sequence. Since this element is recognized by the maternal factor OZ-1 previously described by N. Ovsenek, A. M. Zorn, and P. A. Krieg (Development 115:649-655, 1992), we might have identified the main constituents of a hierarchy that leads via LFB1 to the activation of tissue-specific genes during embryogenesis.Tissue-specific expression of genes is achieved to a large extent by the interaction of transcription factors with cisacting elements present in the promoters and enhancers of genes that are expressed differently in different tissues. These transcription factors themselves are tissue specific. This restricted expression pattern is established sometime during embryogenesis, and we assume that it plays a key role in the differentiation processes. Molecular analysis of early vertebrate development has established a complex hierarchy of regulatory factors involved in establishing the body plan (for recent reviews, see references 12 and 15). Some of the genes encoding these factors are the homeobox genes known to be conserved between Drosophila melanogaster and mammals (for a recent review, see reference 26). There is increasing evidence that these genes form a very complex regulatory network in which specific genes influence the activities of other genes. Typically these regulatory genes are expressed in distinct areas of the embryo and may be active only transiently during embryogenesis. There is, at least in the case of D. melanogaster, clear evidence that some transcription factors are already present in fertilized eggs, and it is assumed that these maternal factors are responsible for initiating gene activation in early embryogenesis (for a recent review, see reference 16). So far no direct link between the early active transcription factors and the expression of tissue-specific transcription factors could be made. In one approach to this question, we analyzed the regulatory elements and factors involved in embryonic activation of the promoter of the liver transcription factor LFB1 (HNF1) in Xenopus laevis.

The Mouse Fetoprotein Transcription Factor (FTF) Gene Promoter Is Regulated by Three GATA Elements with Tandem E Box and Nkx Motifs, and FTF in Turn Activates the Hnf3β, Hnf4α, and Hnf1α Gene Promoters

“…Furthermore, the FTF site in the xHnf1␣ promoter lies in a 30-bp domain essential to promoter activity (deletant 238/207 in Ref. 51), and it is contained in a short 69-bp promoter segment inducible by activin A (53), also a cell surface receptor ligand of the TGF-␤ superfamily. The 69-bp domain carries the conserved HNF4-binding site (5, 51) which, however, is dispensable for activation of the 69-bp domain in endodermal cells (54).…”

Inhibitor of the Tissue-Specific Transcription Factor HNF4, a Potential Regulator in Early Xenopus Development

“…Gel retardation assays were performed as described (16) using the following oligonucleotide probes: the HNF4 binding site of the Xenopus HNF1␣ promoter (44), the HNF1 binding site of the Xenopus albumin promoter (31), an AP1 binding site (37), an ATF/CREB binding site (37), an Sp1 site (22), and the HNF3 binding site of the transthyretin promoter (Ϫ111/Ϫ85 [18]). …”

“…(A) Elution profile of a heat-treated Xenopus egg extract on a MonoQ column using a salt gradient from 0.2 to 1.0 M NaCl. An aliquot of 0.1 l of each column fraction was added to a gel retardation assay using rat liver extract and the HNF4 binding site of the Xenopus HNF1␣ promoter (44). The results are given for fractions 6 to 12 only, as the other fractions did not contain any inhibitory activity.…”

“…The importance of HNF4␣ in gene control in tissues distinct from the liver has been documented by the fact that an inherited human disease is based on the expression of a mutated HNF4␣ gene in the ␤ cells of the endocrine pancreas, leading to maturity-onset diabetes of the young (MODY1 [42]). Most interestingly, another MODY gene identified in humans represents the tissuespecific transcription factor HNF1␣ (43), known to be tightly regulated by HNF4 (17,39,44).In addition to its role as a tissue-specific transcription factor, HNF4␣ is also a maternal component in the egg of Drosophila melanogaster (46) as well as of the amphibian species Xenopus laevis (12). In the mouse, an early embryonic function is implied by the fact that HNF4␣ transcripts are present in the primary endoderm at day 4.5 (6) and can be detected in totipotent embryonic stem cells (25).…”

“…The importance of HNF4␣ in gene control in tissues distinct from the liver has been documented by the fact that an inherited human disease is based on the expression of a mutated HNF4␣ gene in the ␤ cells of the endocrine pancreas, leading to maturity-onset diabetes of the young (MODY1 [42]). Most interestingly, another MODY gene identified in humans represents the tissuespecific transcription factor HNF1␣ (43), known to be tightly regulated by HNF4 (17,39,44).…”