The rat alpha 1-fetoprotein gene: characterization of the 5'-flanking region and tandem organization with the albumin gene

The mouse alpha-albumin (afamin) promoter is differentially regulated by hepatocyte nuclear factor 1α and hepatocyte nuclear factor 1β

Alpha-albumin (AFM), a member of the albumin gene family that also includes albumin, alpha-fetoprotein, and vitamin D-binding protein, is expressed predominantly in the liver and activated at birth. Here, we identify two hepatocyte nuclear factor 1 (HNF1)-binding sites in the AFM promoter that are highly conserved in different mammals. These two sites bind HNF1α and HNF1β. The distal site (centered at -132, relative to AFM exon 1) is more important than proximal site (centered at -58), based on HNF1 binding and mutational analysis in transfected cells. Our data indicate that HNF1α is a more potent activator of AFM promoter than is HNF1β. However, HNF1β can act in a dominant manner to inhibit HNF1α-dependent transactivation of the AFM promoter when both proteins are expressed together. This suggests that the differential timing with which the albumin family genes are activated in the liver may be influenced by their responsiveness to HNF1α and HNF1β. Our comparison of HNF1-binding sites in the promoters in the albumin family of genes indicates that the primordial albumin-like gene contained two HNF1 sites; one of these sites was lost from the albumin promoter, but both sites still are present in other members of this gene family.

The alpha-fetoprotein enhancer region activates the albumin and alpha-fetoprotein promoters during liver development

The four members of the albumin gene family encode the serum transport proteins albumin, alpha-fetoprotein, alpha-albumin, and vitamin D-binding protein. These genes are transcribed primarily in the liver with each having a different pattern of developmental expression. The tight linkage of these genes, particularly that of albumin, alpha-fetoprotein and alpha-albumin, and their liver-specific expression, has led to the suggestion that these genes share common regulatory elements. To directly examine whether the alpha-fetoprotein enhancer region could regulate the albumin gene family, expression of these genes was monitored in mice in which this region was deleted by homologous recombination. Our data indicate that this enhancer region is required for alpha-fetoprotein and albumin activation early in liver development and alpha-fetoprotein reactivation during liver regeneration, but that albumin, alpha-albumin, and vitamin D-binding protein expression later in hepatic development is not affected by the absence of these enhancers. We also demonstrate that RNA polymerase II loading on the alpha-fetoprotein and albumin promoters is reduced in the absence of this enhancer region, indicating a direct role for these enhancers in the assembly of the RNA Polymerase II complex during liver development.

Regulation of alpha-fetoprotein expression by Nkx2.8

The alpha-fetoprotein (AFP) gene is an important model of developmental gene silencing and neoplastic gene reactivation. Nkx2.8 is a divergent homeodomain factor originally cloned through its binding to the promoter-coupling element (PCE), a regulatory region upstream of the AFP promoter that mediates stimulation by distant enhancers. Nkx2.8 is the only developmentally regulated factor that has been associated with AFP gene expression. Fetoprotein transcription factor, an orphan nuclear receptor, has also been shown to bind the PCE but is not developmentally regulated. The binding specificities of both families of transcription factor were determined, and overlapping sites for each were defined in the PCE. After modification of nuclear extract and gel shift analysis procedures, Nkx2.8 was identified in six AFP-positive cell lines. Transient-transfection analysis did not show transcriptional stimulation by Nkx2.8 or other active NK2 factors, which only interfered with gene expression. However, two sets of analysis demonstrated the relationship of Nkx2.8 to AFP expression: chromatin immunoprecipitation demonstrated that Nkx2.8 bound to the active AFP promoter, and antisense inhibition of Nkx2.8 mRNA translation selectively reduced expression of both the endogenous human AFP gene and transfected reporters containing the rat AFP promoter.

Distant enhancers stimulate the albumin promoter through complex proximal binding sites

The albumin-alpha-fetoprotein locus epitomizes the main features of transcriptional regulation of fetal and adult hepatocyte-specific genes: developmentally regulated promoters and strong distant enhancers. Full enhancer activity required only a proximal albumin-promoter region containing the TATA box, hepatic nuclear factor 1 (HNF1), and nuclear factor Y (NF-Y) sites. Deletion of the HNF1 site abrogated enhancer and promoter activity, whereas methylation of the site reduced all activity by about 3-fold. Deletion of the NF-Y site attenuated activity by about half, but much of the activity could be replaced by juxtaposition of an upstream region (designated distal element IV). Gel shift and competition analysis demonstrated that binding of architectural factors overlapped NF-Y binding. Moreover, a mutation that eliminated NF-Y binding but only minimally perturbed the surrounding region did not affect enhancer function. In plasmids with a second promoter, the enhancers simultaneously stimulated both albumin and alpha-fetoprotein promoters with minimal competition, but surprisingly some mutations in the albumin promoter attenuated expression from both promoters, whereas another uncoupled their expression. With single promoters, the function of the proximal promoter region was controlled by three parameters in the following hierarchy: HNF1 binding > local architecture > NF-Y binding, but integrated two-promoter function had a much greater dependence on NF-Y.

Physical and meiotic mapping of the region of human chromosome 4q11-q13 encompassing the vitamin D binding protein DBP/Gc-globulin and albumin multigene cluster

The vitamin D binding protein/Gc-globulin (DBP) gene is a member of a multigene cluster that includes albumin (ALB), alpha-fetoprotein (AFP), and alpha-albumin/afamin (AFM). All four genes have structural and functional similarities and map to the same chromosomal regions in humans (4q11-q13), mice, and rats. An accurate physical map of the region encompassing these genes is a prerequisite for study of their respective transcriptional regulation and identification of potential shared regulatory elements. By refining the physical and meiotic maps of the 4q11-q13 region and creating a local PAC contig, the order and transcriptional orientations of these four genes were determined to be centromere-3'-DBP-5'-5'-ALB-3'-5'-AFP-3'-5'-AFM3'-telomere. The ancestral DBP gene was separated from the ALB gene by >1.5 Mb. This organization and spacing establishes a foundation for ongoing functional studies in this region.

Physical and Meiotic Mapping of the Region of Human Chromosome 4q11–q13 Encompassing the Vitamin D Binding Protein DBP/Gc-Globulin and Albumin Multigene Cluster

The vitamin D binding protein/Gc-globulin (DBP) gene is a member of a multigene cluster that includes albumin (ALB), α-fetoprotein (AFP), and α-albumin/afamin (AFM). All four genes have structural and functional similarities and map to the same chromosomal regions in humans (4q11–q13), mice, and rats. An accurate physical map of the region encompassing these genes is a prerequisite for study of their respective transcriptional regulation and identification of potential shared regulatory elements. By refining the physical and meiotic maps of the 4q11–q13 region and creating a local PAC contig, the order and transcriptional orientations of these four genes were determined to be centromere–3′-DBP-5′–5′-ALB-3′–5′-AFP-3′–5′-AFM3′–telomere. The ancestral DBP gene was separated from the ALBgene by >1.5 Mb. This organization and spacing establishes a foundation for ongoing functional studies in this region.

Chicken ovalbumin upstream promoter-transcription factor, hepatocyte nuclear factor 3, and CCAAT/enhancer binding protein control the far-upstream enhancer of the rat alpha-fetoprotein gene

We have further characterized the most distal of the three alpha-fetoprotein (AFP) enhancers required for expression of the AFP gene in fetal hepatocytes and yolk sac endodermal cells. Almost total rat AFP enhancer 3 (E3) activity is driven by a 160-bp fragment at -6 kb containing three target regions for nuclear proteins that cooperate to stimulate transcription from the AFP and the thymidine kinase promoters in HepG2 hepatoma cells. Region 1, recently shown to be crucial for correct function of the enhancer in liver of transgenic mice, is recognized by two sets of transcription factors that bind to partly overlapping sites, 1a and 1b, in a noncooperative and nonexclusive manner. Site 1a contains a motif, AGGTCA, which is recognized by chicken ovalbumin upstream promoter transcription factors (COUP-TFs), but not by hepatocyte nuclear factor 4. Hepatocyte nuclear factor 3 (HNF3) and CCAAT/enhancer binding protein (C/EBP), which bind to regions 2 and 3, respectively, are likely responsible for the liver-specific E3 action. They play a key role by acting in synergy. The participation of nuclear receptors such as COUP-TFs, with C/EBP and HNF3, in the tight control of the distal AFP enhancer is a new, and perhaps key, step toward understanding the regulation and function of this enhancer, which may remain active throughout development.

Localization of a phenobarbital-responsive element (PBRE) in the 5'-flanking region of the rat CYP2B2 gene

Cytochrome P450 2B1, encoded by CYP2B1, and cytochrome P450 2B2, encoded by CYP2B2, are inducible in rat liver by phenobarbital (PB). We have used cultured adult rat hepatocytes to study molecular mechanisms regulating CYP2B1/CYP2B2 transcription. Northern blot analysis demonstrated that the endogenous CYP2B1/CYP2B2 genes were inducible by PB in such cultures. A PB-responsive element (PBRE) conferring PB inducibility on a reporter gene was identified in the CYP2B2 5'-flanking region. The PBRE was localized to a 163-bp Sau3AI fragment situated between 2155 and 2318 bp upstream from the CYP2B2 transcription start point (tsp). The PBRE also conferred PB responsiveness on an enhancerless heterologous promoter and was active in both orientations both upstream and downstream from the heterologous promoter; hence, it has the properties of a transcriptional enhancer. Gel-retardation assays showed that nuclear extracts of liver cells of untreated and PB-treated rats contained sequence-specific DNA-binding factors that interact with a PBRE-containing DNA fragment. These results may open the way to identifying one or more transcription factors mediating induction of CYP2B2 and CYP2B1 in rat liver.

Enhancer sharing in a plasmid model containing the alpha-fetoprotein and albumin promoters

It has been proposed that the alpha-fetoprotein (AFP) enhancers also regulate the adjacent albumin gene, since the -10 kbp albumin enhancer is inactive in a number of cell lines that express albumin. In transfection experiments, the AFP enhancers strongly stimulate the albumin promoter in cells that silence the AFP promoter. These observations led us to develop a plasmid model of AFP-albumin gene switching, in which the albumin and AFP promoters would compete for the three AFP enhancers. However, when AFPCAT + ALBgal genes were combined with the AFP enhancers in one plasmid, both genes were driven at full activity. There was no change in the relative promoter expression over a wide range of transfected DNA concentrations, demonstrating that relative promoter activity was independent of DNA concentration and of promoter concentration, and that neither promoter was limiting the expression of the other. In contrast, a control plasmid containing two albumin promoters showed mutual inhibition, indicating the expected promoter competition. The albumin and AFP promoters noncompetitively shared the three enhancers on this plasmid, resulting in high levels of transcription from both promoters.

Rat liver cytochrome P450 2B3: structure of the CYP2B3 gene and immunological identification of a constitutive P450 2B3-like protein in rat liver

The cytochrome P450 2B subfamily in the rat contains an estimated eight to eleven members at the genomic level. Synthesis in the liver of the prototypic forms P450 2B1 and P450 2B2 is dramatically induced by phenobarbital. The 1.9-kb mRNA for P450 2B3, a third member of the P450 2B subfamily, is constitutively present in rat liver but is not inducible by phenobarbital. We have now cloned and sequenced exonic sequences corresponding to the entire 2B3 mRNA and determined their exon-intron structure, which is identical to that of CYP2B1/CYP2B2 and other CYP2B genes. A putative CYP2B3 transcription start site was identified and CYP2B3 5'- and 3'-flanking sequences were compared to those of CYP2B1 and CYP2B2. CYP2B3, like CYP2B1 and CYP2B2, has a modified TATA box preceding the transcription start site and lacks the canonical polyadenylation signal preceding the poly(A) site. A 2B3 expression vector, pMT2-2B3, directed the synthesis in COS-1 cells of an approximately 50-kD protein detectable on Western blots with a polyclonal antibody and with one of four monoclonal antibodies raised against 2B1 but not with a polyclonal antibody raised against P450 PB6. The 2B3 protein migrated with a slightly higher electrophoretic mobility than 2B1 and comigrated with a protein detected by anti-2B1 antibodies in liver microsomes from untreated rats. The results indicate that a 2B3-like protein is present in rat liver and that it is distinct from P450 PB6 and other known constitutive rat hepatic P450s.

Functional analysis of developmentally regulated chromatin-hypersensitive domains carrying the alpha 1-fetoprotein gene promoter and the albumin/alpha 1-fetoprotein intergenic enhancer

During liver development, the tandem alpha 1-fetoprotein (AFP)/albumin locus is triggered at the AFP end and then asymmetrically enhanced; this is followed by autonomous repression of the AFP-encoding gene. To understand this regulation better, we characterized the two early developmental stage-specific DNase I-hypersensitive (DH) sites so far identified in rat liver AFP/albumin chromatin: an intergenic DH-enhancer site and the AFP DH-promoter site. Mutation-transfection analyses circumscribed the DH-enhancer domain to a 200-bp DNA segment stringently conserved among species. Targeted mutations, DNA-protein-binding assays, and coexpression experiments pinpointed C/EBP as the major activatory component of the intergenic enhancer. Structure-function relationships at the AFP DH-promoter site defined a discrete glucocorticoid-regulated domain activated cooperatively by HNF1 and a highly specific AFP transcription factor, FTF, which binds to a steroid receptor recognition motif. The HNF1/FTF/DNA complex is deactivated by glucocorticoid receptors or by the ubiquitous factor NF1, which eliminates HNF1 by competition at an overlapping, high-affinity binding site. We propose that the HNF1-NF1 site might serve as a developmental switch to direct autonomous AFP gene repression in late liver development. We also conclude that the intergenic enhancer is driven by C/EBP alpha primarily to fulfill albumin gene activation functions at early developmental stages. Factor FTF seems to be the key regulator of AFP gene-specific functions in carcinoembryonic states.

Functional analysis of developmentally regulated chromatin-hypersensitive domains carrying the alpha 1-fetoprotein gene promoter and the albumin/alpha 1-fetoprotein intergenic enhancer

During liver development, the tandem alpha 1-fetoprotein (AFP)/albumin locus is triggered at the AFP end and then asymmetrically enhanced; this is followed by autonomous repression of the AFP-encoding gene. To understand this regulation better, we characterized the two early developmental stage-specific DNase I-hypersensitive (DH) sites so far identified in rat liver AFP/albumin chromatin: an intergenic DH-enhancer site and the AFP DH-promoter site. Mutation-transfection analyses circumscribed the DH-enhancer domain to a 200-bp DNA segment stringently conserved among species. Targeted mutations, DNA-protein-binding assays, and coexpression experiments pinpointed C/EBP as the major activatory component of the intergenic enhancer. Structure-function relationships at the AFP DH-promoter site defined a discrete glucocorticoid-regulated domain activated cooperatively by HNF1 and a highly specific AFP transcription factor, FTF, which binds to a steroid receptor recognition motif. The HNF1/FTF/DNA complex is deactivated by glucocorticoid receptors or by the ubiquitous factor NF1, which eliminates HNF1 by competition at an overlapping, high-affinity binding site. We propose that the HNF1-NF1 site might serve as a developmental switch to direct autonomous AFP gene repression in late liver development. We also conclude that the intergenic enhancer is driven by C/EBP alpha primarily to fulfill albumin gene activation functions at early developmental stages. Factor FTF seems to be the key regulator of AFP gene-specific functions in carcinoembryonic states.

Characterization of a rat variant alpha-fetoprotein

The rat adult liver and hepatocyte lines express an 1.7-kb variant alpha-fetoprotein (AFP) mRNA which differs from the 2.2-kb fetal AFP transcript in sequence in the 5' region. Here we report the characterization of a variant AFP cDNA, ARFP9, which is 1349 bp in length and encodes a 325 amino acid polypeptide. Nucleotides 225 to the 3' end (1125 bp) in ARFP9 corresponds exactly to nucleotide 873 to the 3' end of the fetal AFP mRNA. However, the first 224-bp of ARFP9, which is located in the 7th intron (designated the V exon) of the rat AFP gene, is not present in the 2.2-kb fetal AFP transcript. The size of the V exon is about 266-bp. In vitro expression experiments showed that the variant AFP is an unglycosylated intracellular protein of 37 kDa. Methyl-isobutyl-xanthine (MIX) stimulated expression of the fetal AFP mRNA but inhibited expression of the variant AFP mRNA, suggesting that in the rat, the two AFP transcripts are developmentally and differentially regulated.

Structural analysis of the putative regulatory region of the rat gene encoding poly(ADP-ribose) polymerase

A lambda EMBL3 clone containing the first three exons along with part of the 4th exon of the rat poly(ADP-ribose) polymerase gene was isolated from a genomic DNA library. This clone also contains 6.6 kbp of upstream sequences. Nucleotide sequence analysis of the proximal 5' 670 nucleotides flanking the major RNA start site of the rat gene does not reveal significant global homology with the same region of the human gene, but a series of short sequences are identical. Among these sequences are found two putative Sp1 binding sites along with a decanucleotide sequence responsible for the attachment of the transcription factor AP-2.

The transcription control region of the rat alpha-fetoprotein gene. DNA sequence and homology studies

The alpha-fetoprotein (AFP) gene, an important system for studying developmental and tissue-specific gene expression, is regulated mostly through the control of transcription. The promoter and cis-acting DNA elements which regulate the rat gene lie within a 7 kbp region upstream of the cap site. We have determined the sequence of this entire region. It contains several repetitive elements and a species-specific distribution of DNA methylation sites. We aligned our rat AFP sequence with fragmentary mouse and human AFP sequences to define blocks of highly conserved sequence, which we then analyzed for homology to known transcription regulatory sequences. Our analysis demonstrates that the regulatory region of the rat AFP gene is unusually complex.

The role of cis-acting promoter elements in tissue-specific albumin gene expression

The mouse albumin gene promoter has six closely spaced binding sites for nuclear proteins that are located between the TATA motif and nucleotide position -170. In vitro transcription with liver or spleen nuclear extracts of templates containing either mutated or polymerized albumin promoter elements establishes a hierarchy of the different protein binding sites for tissue-specific albumin gene transcription. The HNF-1 and C/EBP binding sites strongly activate transcription in a tissue-specific manner. The NF-Y binding site has a lower activation potential and is less specific, being equally efficient in liver and spleen nuclear extracts. The remaining elements are relatively weak activator sites.

Postnatal repression of the alpha-fetoprotein gene is enhancer independent

The mammalian liver undergoes a number of dramatic changes in gene expression during development. One of these is typified by the alpha-fetoprotein (AFP) gene, which is activated in the fetal liver but undergoes a transcriptional decline at birth. In contrast, although activated at the same time during fetal development, albumin gene transcription is maintained at high levels in adult animals. To determine whether the postnatal decline in AFP gene transcription is mediated through its distal enhancers or through more proximal elements surrounding the promoter or structural gene, chimeric genes bearing substitutions of albumin gene cis-acting elements for the equivalent AFP gene elements were introduced into the germ line of mice. The expression of the transgenes was then analyzed at various stages of development. Our results indicate that the AFP gene enhancers are not involved in the postnatal decline in AFP transcription. Rather, a region within the first kilobase of DNA upstream of the AFP gene, including its promoter, and/or portions of the structural gene is sufficient to direct postnatal repression of the gene.

The organization of repetitive sequences in the albumin and alpha-fetoprotein gene loci in the rat

The distribution of middle repetitive sequences in the genic and extragenic regions of the rat albumin and alpha-fetoprotein genes was analyzed. Their presence was determined by probing Southern blots of restriction fragments of albumin and alpha-fetoprotein genomic subclones with 32P-labeled total rat DNA. Repetitive sequences were detected in both genes. They were classified as weak, moderate and intense hybridizing elements according to the intensity of hybridization. Weak repetitive sequences were characterized as dG.dT repeats by using 32P-labeled poly-(dG.dT)(dC.dA) oligomer probe. They occurred in 5' and 3' extragenic regions of the two genes and in introns 4 and 5 of the albumin gene. The moderate repetitive sequence present in intron 6 of the albumin gene was identified as the rat SINES element. 4D12. The intense repetitive sequence, localized in the 3' non-coding region of the albumin gene, corresponded to the terminal segment of a rat high repeat long interspersed DNA family, L1Rn. 4D12 and L1Rn sequences were also scattered throughout the alpha-fetoprotein locus as moderate and intense repetitive elements, respectively, but their distribution was different from that of the albumin genomic region. These results indicate that repetitive sequences invaded the two loci in a non-conservative manner.

Structure and acute-phase regulation of the rat alpha 2-macroglobulin gene

Seven genomic DNA clones representing the rat alpha 2-macroglobulin gene were isolated and characterized. The cloned sequence covered the entire gene (48 kilobases) plus 2 kilobases of 3'- and 13.7 kilobases of 5'-flanking sequences. A restriction cleavage map of the gene was produced, and the restriction cleavage pattern of genomic DNA suggested that the alpha 2-macroglobulin gene is a single-copy gene. A 7.7-kilobase fragment from the 5'-terminal region and a 250 base pair fragment from the 3'-terminal region of the gene were sequenced, and the 3' end of the gene was mapped. The sequenced 5'-terminal fragment contained 4.5 kilobases of 5'-flanking sequences plus the first three exons and two introns of the gene. Two transcription start sites, a minor and a major site, located 65 nucleotides apart, were defined by primer extension, S1 mapping, and RNaseH experiments. During an acute-phase response, transcription from both sites was induced in the liver, and over 90% of the transcripts originated from the major site. Very high concentrations of alpha 2-macroglobulin mRNA originating from both start sites were also found in the uterus but not in the liver of pregnant females. A glucocorticoid response element (GRE), a conserved consensus sequence for a potential glucocorticoid receptor DNA binding site, was found by computer search in the promoter-proximal 5'-flanking region of the alpha 2-macroglobulin gene.

Mapping of the hooded, Gc protein, and albumin gene loci in linkage group VI of the laboratory rat

Crosses to determine the position of the three gene loci, h, Gc, and Alb, in the sixth linkage group of the rat used three strains, the TM strain, the ACI-alb analbuminemic congenic strain, and the abh-alb tester strain established by crossing the abh coat color tester strain and analbuminemic rats. Their genotypes were [C/C, h/h, GcB/GcB, Alb/Alb], [C/C, hi/hi, GcA/GcA, alb/alb] and [C/C, h/h, GcA/GcA, alb/alb], respectively. Determination of genotypes was performed by coat color and polyacrylamide gel electrophoresis (PAGE of serum protein for the Gc and albumin genes. The positions of the three gene loci in the VI linkage group were calculated from the recombination values from the phenotypes of progenies. According to this data, the three gene loci were in h-Gc-Alb tandem and the distances were 15.5 +/- 1.0% in h-Gc, 15.8 +/- 1.0% in h-Alb, and 0.32 +/- 0.16% in Gc-Alb. These data confirmed the relationship among the Gc, Alb, and Afp genes in the rat as well as in humans.

A liver-specific factor essential for albumin transcription differs between differentiated and dedifferentiated rat hepatoma cells

We have identified and characterized two mutually exclusive nuclear proteins that interact with a single crucial element of the albumin promoter. One, albumin proximal factor (APF), is found only in liver or differentiated hepatoma cells and is probably identical to the liver-specific factor named HNF1, alpha 1TFB, or HP1-binding protein. The other, variant albumin proximal factor (vAPF), is present in dedifferentiated hepatoma cells as well as in somatic cell hybrids that show extinction of the expression of liver-specific proteins, including albumin. Reversion to the hepatic phenotype of either a dedifferentiated variant or an extinguished somatic hybrid clone is accompanied by loss of vAPF and reappearance of APF. These two proteins differ in their thermostability and in their molecular weight, while displaying identical sequence specificities. Both proteins interact with a homologous motif present in promoter regions of several other liver-specific genes. In vitro transcription assays, using a rat liver nuclear extract, indicate that the binding of APF to its target sequence is required for albumin transcription. These results suggest that a modification in the primary structure of a transcription factor is correlated with the differentiated state of the hepatic cell.

Enhancer and promoter elements directing activation and glucocorticoid repression of the alpha 1-fetoprotein gene in hepatocytes

Mutations were introduced in 7 kilobases of 5'-flanking rat alpha 1-fetoprotein (AFP) genomic DNA, linked to the chloramphenicol acetyltransferase gene. AFP promoter activity and its repression by a glucocorticoid hormone were assessed by stable and transient expression assays. Stable transfection assays were more sensitive and accurate than transient expression assays in a Morris 7777 rat hepatoma recipient (Hepa7.6), selected for its strong AFP repression by dexamethasone. The segment of DNA encompassing a hepatocyte-constitutive chromatin DNase I-hypersensitive site at -3.7 kilobases and a liver developmental stage-specific site at -2.5 kilobases contains interacting enhancer elements sufficient for high AFP promoter activity in Hepa7.6 or HepG2 cells. Deletions and point mutations define an upstream promoter domain of AFP gene activation, operating with at least three distinct promoter-activating elements, PEI at -65 base pairs, PEII at -120 base pairs, and DE at -160 base pairs. PEI and PEII share homologies with albumin promoter sequences, PEII is a near-consensus nuclear factor I recognition sequence, and DE overlaps a glucocorticoid receptor recognition sequence. An element conferring glucocorticoid repression of AFP gene activity is located in the upstream AFP promoter domain. Receptor-binding assays indicate that this element is the glucocorticoid receptor recognition sequence which overlaps with promoter-activating element DE.

Hepatocyte-specific promoter element HP1 of the Xenopus albumin gene interacts with transcriptional factors of mammalian hepatocytes

By transfecting various Xenopus albumin-CAT fusion genes into the mouse hepatoma cell line BW1J a 13 base-pair hepatocyte-specific promoter element (HP1) could be identified. A similar sequence element is also present in the promoter of the albumin and alpha-fetoprotein genes of other vertebrates. Introduction of single point mutations into HP1 destroys its function. Binding studies with nuclear proteins identify a factor interacting with HP1 which is specific for hepatic cells. In-vitro transcription in a rat liver nuclear extract demonstrates that HP1 leads to an increased transcriptional activity. This increased transcription is specifically inhibited by the addition of an HP1-containing oligonucleotide, establishing that the interaction of factors with HP1 is essential for increased transcription. Since HP1 derived from a Xenopus gene functions in mammalian hepatocytes, we conclude that a regulatory system involved in liver-specific gene expression has been conserved during evolution.

Enhancer and promoter elements directing activation and glucocorticoid repression of the alpha 1-fetoprotein gene in hepatocytes

Mutations were introduced in 7 kilobases of 5'-flanking rat alpha 1-fetoprotein (AFP) genomic DNA, linked to the chloramphenicol acetyltransferase gene. AFP promoter activity and its repression by a glucocorticoid hormone were assessed by stable and transient expression assays. Stable transfection assays were more sensitive and accurate than transient expression assays in a Morris 7777 rat hepatoma recipient (Hepa7.6), selected for its strong AFP repression by dexamethasone. The segment of DNA encompassing a hepatocyte-constitutive chromatin DNase I-hypersensitive site at -3.7 kilobases and a liver developmental stage-specific site at -2.5 kilobases contains interacting enhancer elements sufficient for high AFP promoter activity in Hepa7.6 or HepG2 cells. Deletions and point mutations define an upstream promoter domain of AFP gene activation, operating with at least three distinct promoter-activating elements, PEI at -65 base pairs, PEII at -120 base pairs, and DE at -160 base pairs. PEI and PEII share homologies with albumin promoter sequences, PEII is a near-consensus nuclear factor I recognition sequence, and DE overlaps a glucocorticoid receptor recognition sequence. An element conferring glucocorticoid repression of AFP gene activity is located in the upstream AFP promoter domain. Receptor-binding assays indicate that this element is the glucocorticoid receptor recognition sequence which overlaps with promoter-activating element DE.

The regulation of albumin and alpha-fetoprotein gene expression in mammals

Albumin and alpha-fetoprotein (AFP) are two plasma proteins synthesized by the liver and the yolk sac. The production of these major proteins is subject to considerable and characteristic variations during both the course of development and hepatic carcinogenesis. It is therefore a system of choice for the analysis of genetic expression during normal differentiation and the cancerous state of eukaryotic cells. The knowledge of regulatory mechanisms at the cellular and molecular levels of the albumin and AFP genes has recently made great progress: 1) the cells which are responsible for the synthesis of albumin and AFP in the liver and other organs have been defined by conjointly using in vitro and in vivo molecular hybridization techniques; 2) the organization of these genes and their adjoining regions has been established in the rat, the mouse and man; 3) the level at which the synthesis of these two proteins is regulated has been determined; it is the transcriptional level. The transcriptional regulation of the albumin and AFP genes could be the result of genome and/or chromatin conformation level modifications. Different groups have shown that: 1) the global structure of the albumin and AFP genes does not change during the course of development and hepatic carcinogenesis; 2) modifications at the level of the methylation of certain specific cytosines could be associated with the variations in the transcription of these genes; 3) global or local (hypersensitive sites with DNase I) changes of chromatin conformation could be correlated to the potential or the overt activity of the transcription of these genes. Very recently certain 'regulatory' regions having cis 'enhancer' or 'silencer' properties have been detected upstream from the albumin and AFP genes. These regions are hypothesized to be DNA 'target' sequences on which trans-acting regulatory factors are fixed and which control the transcription of these genes. Starting from the framework of this recent work, a model of albumin and AFP gene regulation is proposed.