Multiple hepatocyte-enriched nuclear factors function in the regulation of transthyretin and alpha 1-antitrypsin genes.

Abstract

Transthyretin (TTR) and al-antitrypsin (al-AT) are expressed at high levels in the liver and also in at least one other cell type. We report here a detailed analysis of the proximal regulatory region of the TTR gene, which has uncovered two new DNA-binding factors that are present mainly (or only) in hepatocytes. One of these new factors, hepatocyte nuclear factor 3 (HNF-3), binds to two sites that are crucial in TTR expression as well as to two additional sites in the al-AT proximal enhancer region. The second new factor, HNF-4, binds to two sites in TTR that are required for gene activity. We had previously identified binding sites for another hepatocyte-enriched DNA-binding protein (C/EBP or a relative thereof), and additional promoter-proximal sites for that protein in both TTR and al-AT are also reported here. From these results it seems clear that cell-specific expression is not simply the result of a single cell-specific factor for each gene but the result of a combination of such factors. The variation and distribution of such factors among different cell types could be an important basis for the coordinate expression of the TTR and al-AT genes in the liver or the discordant transcriptional activation of these genes in a few other cell types. The identification of such cell-enriched factors is a necessary prelude to understanding the basis for cell specificity.Hepatocytes express a large number of proteins that are not expressed at all in most other cell types (8,10,19,21,41). However, occasionally other cell types in adults express one or a few of the same proteins as the liver. For example, transthyretin (TTR [14]) and transferrin (4) are expressed in the choroid plexus, apolipoprotein CIII (43) is expressed in the intestine, al-antitrypsin (al-AT) is expressed in macrophages (42), and all are expressed in hepatocytes. In the cells of the yolk sac, an extraembryonic tissue, a large subset of proteins made in the adult liver are produced (39, 47) and secreted. Transcriptional control most likely underlies the regulation of the various genes in all of these cell types (13). It has long been suggested that cell-specific transcriptional factors might exist as at least part of the explanation of cell-specific regulation (for a review, see Maniatis et al. [36]). However, problems immediately arise when one tries to develop a simple scheme to explain this regulation. First, the invocation of negative-acting factors to suppress inappropriate expression, while not impossible, requires such factors to exist for every gene whose transcriptional activity must be restricted in a large number of noneypressing cell types. Second, with respect to positive-acting factors, for example, since TTR, al-AT, and apolipoproteins all are expressed in the liver but only TTR is expressed in the choroid plexus, only al-AT is expressed in macrophages, and only apolipoprotein is expressed in the gut, then the activation mechanism in the hepatocyte must not be the same as the activation mechanism in the other cell types. Obviously, ...

2-acetylaminofluorene dose-dependent differentiation of rat oval cells into hepatocytes: Confocal and electron microscopic studies

“…HNF-4 also is expressed in the intestinal glands, 37 and intestinal metaplasia is another option for the oval cells. 38 One of the major differences between the intestinal glands and hepatocytes is that the enterocytes reside on a laminin-containing basement membrane.…”

Evolution of the Thyroid Hormone-Binding Protein, Transthyretin

“…The 5Ј flanking region of the gene contains a short DNA segment, the -50 to 190 region, which is extremely well conserved (93%) between human and rodents (Costa et al, 1986;Fung et al, 1988). It has been proposed that this region is important for the cellspecific regulation of the gene and binding sites for HNF-1, C/EBP, HNF-3, HNF-4, and HNF-6 (Costa et al, 1989;Samadani and Costa, 1996). In addition to these, further regulatory elements, at least 2000 bp upstream of the gene, may be involved in the regulation of gene expression in human and mouse liver via both cis-acting and trans-acting factors.…”

“…In addition to these, further regulatory elements, at least 2000 bp upstream of the gene, may be involved in the regulation of gene expression in human and mouse liver via both cis-acting and trans-acting factors. Studies with transgenic mice have identified two regions, a 100-nucleotide enhancer located -2 kb upstream and a proximal Ϫ150 to 90-bp promoter region, sufficient for directing TTR expression in the liver (Costa et al, 1989;Yan et al, 1990). Abundant TTR gene expression also occurs in the choroid plexus of mammals, birds, and reptiles, but little is known about the regulatory mechanisms.…”

Chimeric PSA enhancers exhibit augmented activity in prostate cancer gene therapy vectors

“…The strong cooperative regulation of initiation complex assembly is believed to be responsible for tissue-specific gene expression. 36 The precise mechanism of how the PSA regulatory elements orchestrate an accurate and prostate-specific transcription program is incompletely understood, but one essential component of enhancer activation function is mediated through cooperative AR-AR interactions. 16,23 Studies have also pointed to the involvement of both prostatespecific and ubiquitous transcription factors in cellspecific regulation.…”