The rat albumin promoter is composed of six distinct positive elements within 130 nucleotides

HGF and IL-10 expressing ALB::GFP reporter cells generated from iPSCs show robust anti-fibrotic property in acute fibrotic liver model

BACKGROUND

Cell therapy using hepatocytes derived from stem cells has been regarded as a promising alternate to liver transplantation. However, the heterogeneity of these hepatocytes makes them unsuitable for therapeutic use. To overcome this limitation, we generated homogenous hepatocyte like induced hepatocyte-like (iHep) cells.

METHODS

iHep cells were generated from induced pluripotent stem cells (iPSCs) integrated with the albumin (ALB) reporter gene. The therapeutic properties of these iHep cells were investigated after transplantation in fibrotic liver tissues of a mouse model.

RESULTS

The iHep cells expressed hepatocyte specific genes and proteins, and exhibited high levels of hepatocyte growth factor (HGF) and interleukin (IL)-10 expressions. Transplantation of iHep cells significantly decreased thioacetamide (TAA)-induced liver fibrosis, apoptotic cells in the liver, and ameliorated abnormal liver function. Liver tissues engrafted with iHep cells exhibited decreased expression of pro-inflammatory factors such as transforming growth factor (TGF)-β, IL-6, and monocyte chemo attractant protein (MCP)-1. Furthermore, an increased number of proliferating hepatocytes and human albumin-expressing iHep cells were detected in mice liver.

CONCLUSIONS

This study has investigated and proven the liver regeneration potential of genome-edited iHep cells and promises to be a strong foundation for further studies exploring cell therapy as an alternative therapeutic option for the treatment of liver fibrosis.

Construction of a general albumin promoter reporter system for real-time monitoring of the differentiation status of functional hepatocytes from stem cells in mouse, rat and human

Genetic constructs with promoters fused to reporter genes for simultaneous monitoring of cellular events have been the focus of attention in recent years. Adenoviral vectors, which have distinctive characteristics, have been used to monitor the differentiation of stem cells in vitro. In the present study, a modified adenoviral vector was constructed, containing a mouse, rat, and human general albumin promoter sequence fused to a ZsGreen reporter gene, and evaluated its efficiency in different cell types. Two hepatocyte cell lines (Hepa1-6 and HepG2), rat primary hepatocytes, rat bone marrow mesenchymal stem cells (BM-MSCs) and rat BM-MSCs-derived hepatocyte-like cells were transduced with this vector, and the transfection efficiency and functional capabilities of the promoter were evaluated by fluorescent microscopy. The results demonstrated efficient expression of ZsGreen in Hepa1-6 cells, HepG2 cells, rat primary hepatocytes, and rat BM-MSCs-derived hepatocyte-like cells, but not in rat BM-MSCs. In conclusion, the current study demonstrates a simple, high-efficiency, general tool for real-time monitoring of the differentiation status of hepatocytes from stem cells in mice, rats, and humans. This tool may be useful for evaluating different protocols to generate functional hepatocytes from stem cells in multiple species.

Establishment of a transgenic mouse model with liver-specific expression of secretory immunoglobulin D

Mutation of mevalonate kinase (MVK) is thought to account for most cases of hyperimmunoglobulinemia D syndrome (HIDS) with recurrent fever. However, its mechanism and the relationship between elevated serum immunoglobulin D (IgD) and the clinical features of HIDS are unclear. In this study, we generated by fusion PCR a vector to express high levels of chimeric secretory IgD (csIgD) specifically in the liver. We then generated seven founder lines of transgenic mice by co-microinjection, and verified them using genomic PCR and Southern blotting. We detected the expression of csIgD by reverse transcription PCR, quantitative PCR, western blotting, and enzyme-linked immunosorbent assays. We demonstrated that csIgD could be specifically and stably expressed in the liver. We used flow cytometry to show that overexpression of csIgD in the bone marrow and spleen cells had no effect on B cell development. Morphologic and anatomical observation of the transgenic mice revealed skin damage, hepatosplenomegaly, and nephromegaly in some transgenic mice; in these mice, pathological sections showed high levels of cell necrosis and protein-like sediments in the liver, spleen, and kidney. We demonstrated that the genomic insertion sites of the transgenes did not disrupt the MVK gene on mouse chromosome 5. This transgenic mouse will be useful to explore the pathogenesis of HIDS.

Characterization of Distant Enhancers and Promoters in the Albumin-α-Fetoprotein Locus during Active and Silenced Expression

The albumin and alpha-fetoprotein genes are adjacent and express closely related serum proteins. Both genes are strongly expressed in fetal liver, primarily through activation by distant enhancers, but the AFP gene selectively undergoes developmental silencing. We used chromatin immunoprecipitation to study enhancers and promoters during active and silenced gene expression. In adult phenotype cells, the silenced AFP gene was actively repressed at the promoter and two proximal enhancers, characterized by the absence of coactivators and acetylated histone 4, and the presence of corepressors and K9-methylated histone 3. Specific transcription factors, TBP, and RNA polymerase II were all detected on both active and silenced genes, indicating that both states were actively regulated. Surprisingly, promoter-specific factors were also detected on enhancers, especially with reduced chromatin shearing. Under these conditions, an enhancer-specific factor was also detected on the albumin promoter. Association of promoter- and enhancer-specific factors was confirmed by sequential immunoprecipitation. Because no binding was detected on intervening segments, these promoter-enhancer associations suggest looping.

Identification of neutrophil gelatinase-associated lipocalin (NGAL) as a discriminatory marker of the hepatocyte-secreted protein response to IL-1beta: a proteomic analysis

The liver is the major source of proteins used throughout the body for various functions. Upon injury or infection, an acute phase response (APR) is initiated in the liver that is primarily mediated by inflammatory cytokines such as interleukin-1beta (IL-1beta) and interleukin-6. Among others, the APR is characterized by an altered protein synthetic profile. We used two-dimensional gel electrophoresis to study the dynamics of changes in protein synthesis in hepatocytes exposed to these inflammatory cytokines. Protein profiles were quantified using image analysis and further analyzed using multivariate statistical methods. Our results indicate that IL-1beta and IL-6 each induces secreted protein responses with distinct dynamics and dose-dependence. Parallel stimulation by IL-1beta and IL-6 results in a protein pattern indistinguishable from the IL-1beta pattern, indicating a dominant effect of IL-1beta over IL-6 at the doses tested. Multidimensional scaling (MDS) of correlation distances between protein secretion levels revealed two protein pairs that are robustly co-secreted across the various cytokine stimulation conditions, suggesting shared regulatory pathways. Finally, we also used multivariate alternating conditional expectation (MACE) to identify transformation functions that discriminated the cytokine-stimulated and untreated hepatocyte-secreted protein profiles. Our analysis indicates that the expression of neutrophil gelatinase-associated lipocalin (NGAL) was sufficient to discriminate between IL-1beta and IL-6 stimulation. The combination of proteomics and multivariate analysis is expected to provide new information on the cellular regulatory networks involved in generating specific cellular responses.

Effect of the molar ratio of branched-chain to aromatic amino acids on growth and albumin mRNA expression of human liver cancer cell lines in a serum-free medium

Supplementation of branched-chain amino acids (BCAAs) is often used for the treatment of hepatic encephalopathy and low albuminemia in Japan. In this scenario, although many cases are complicated with hepatocellular carcinoma in chronic viral infection, the effect of BCAA levels on hepatocellular carcinoma cells remains unclear. We investigated the effect of the molar ratios of BCAAs to aromatic amino acids (AAAs) on the growth and albumin mRNA expression of cultured human liver cancer cell lines, HCC-M, HCC-T, PLC/PRF/5, and Hep G2. To exclude the effect of fetal serum in culture media on modification of the growth and albumin transcription of cell lines, we used a synthetic serum-free medium. We found that an increase in the molar ratio of BCAAs to AAAs reduced the growth of Hep G2 cells, and it increased albumin mRNA expression in this cell line at a molar ratio of 0.1-10. These results suggest that the molar ratio of BCAAs to AAAs affect the growth and mRNA expression of some liver cancer cells, and supplementation of BCAAs may at least be beneficial to patients with cirrhosis, even complicated with liver cancer.

Identification of retinoic acid-responsive elements on the HNF1alpha and HNF4alpha genes

Hepatocyte nuclear factor 1alpha (HNF1alpha) and HNF4alpha are liver-selective transcription factors and are essential for hepatocyte differentiation. This study demonstrates that HNF1alpha as well as HNF4alpha genes contain a direct repeat with a space of one nucleotide (DR1)-retinoic acid (RA) response element that can be bound and regulated by RA and retinoid x receptor alpha (RXRalpha) complex. Transient transfection experiments showed that RA increased the promoter activity of the HNF1alpha and HNF4alpha genes in Hep3B cells. Overexpression of RXRalpha further enhanced the activities of both genes. Two putative RXRalpha binding sites on the HNF1alpha (-295 to -276) and HNF4alpha (-418 to -399) genes have been characterized. By transient transfection, both sites positively responded to RA, and overexpression of RXRalpha in Hep3B cells increased the regulatory effect. Gel mobility shift assay demonstrated that these two DR-1 sites could be bound by RXRalpha specifically. These data suggest that the differentiation effect of RA on hepatocyte may be due to direct interaction of RXRalpha with the RA-responsive elements on the HNF1alpha and HNF4alpha genes.

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.

Functional activity of hepatocyte nuclear factor-1 is specifically decreased in amino acid-limited hepatoma cells

Limitation of cultured rat hepatoma cells for an essential amino acid results in a specific decrease in expression of several genes that are preferentially expressed in the liver, including the serum albumin and transthyretin genes. In the work presented here, we examined whether the coordinate repression of these genes is caused by decreased activity of one or more of the liver-enriched transcription factors, hepatocyte nuclear factor-1 (HNF-1), HNF-3, HNF-4 or C/EBP. To address this question, HepG2 human hepatoma cells were transiently transfected with luciferase reporter constructs containing multiple copies of individual transcription factor binding sites. Limitation for an essential amino acid resulted in specific repression of a construct in which luciferase expression was directed by HNF-1. A single HNF-1 binding site located adjacent to the TATA box plays a major role in transcription directed by the serum albumin promoter in transient transfection assays. Amino acid limitation of cells transfected with an albumin promoter/luciferase reporter construct resulted in specific repression of promoter activity. In addition, bacterial methylation or site-directed mutagenesis of the HNF-1 binding site in the albumin proximal promoter region eliminated the regulation of an albumin promoter-luciferase reporter construct under conditions of amino acid limitation. These results demonstrated that the HNF-1 binding site played a major role in regulation of the albumin promoter by amino acid availability. Deletion analysis of the albumin promoter confirmed regulation through the HNF-1 binding site and also identified a second amino acid regulatory element in the upstream region of the albumin promoter, which has been shown previously to contain a functional binding site for HNF-3. The repression of albumin promoter and HNF-1 reporter constructs in amino acid-limited cells occurred without a change in the DNA binding activity of HNF-1. Moreover, HNF-3 DNA binding activity was also not decreased in amino acid-limited cells. These results suggest that the regulation of transcription by amino acids occurs at the level of transcriptional activation by HNF-1 and HNF-3, rather than by alteration of the DNA binding activity of either factor.

Upstream region of rat serum albumin gene promoter contributes to promoter activity: presence of functional binding site for hepatocyte nuclear factor-3

Transcription of the serum albumin gene occurs almost exclusively in the liver and is controlled in part by a strong liver-specific promoter. The upstream region of the serum albumin gene promoter is highly conserved among species and is footprinted in vitro by a number of nuclear proteins. However, the role of the upstream promoter region in regulating transcription and the identity of the transcription factors that bind to this region have not been established. In the present study, deletion analysis of the rat serum albumin promoter in transiently transfected HepG2 cells demonstrated that elimination of the region between -207 and -153 bp caused a two-fold decrease in promoter activity (P<0.05). Additional analysis of the -207 to -124 bp promoter interval led to the identification of two potential binding sites for hepatocyte nuclear factor-3 (HNF-3) located at -168 to -157 bp (site X) and -145 to -134 bp (site Y). Electrophoretic mobility-shift assays performed with the HNF-3 X and Y sites demonstrated that both sites are capable of binding HNF-3alpha and HNF-3beta. Placement of a single copy of the HNF-3 X site upstream from a minimal promoter increased promoter activity by about four-fold in HepG2 cells, and the reporter construct containing this site could be transactivated if co-transfected with an HNF-3 expression construct. Furthermore, inactivation of the HNF-3 X site by site-directed mutagenesis within the context of the -261 bp albumin promoter construct resulted in a 40% decrease in transcription (P<0.05). These results indicate that the positive effect of the -207 to -153 bp promoter interval is attributable to the presence of the HNF-3 X site within this interval. Additional results obtained with transfected HepG2 cells suggest that the HNF-3 Y site plays a lesser role in activation of transcription than the X site.

Hepatocyte nuclear factor-4 controls transcription from a TATA-less human sex hormone-binding globulin gene promoter

Hepatocytes are the major source of sex hormone-binding globulin (SHBG), a glycoprotein that transports sex steroids in the blood and regulates their access to target tissues. The human SHBG proximal promoter was analyzed by DNase I footprinting, and the functional significance of 6 footprinted regions (FP1-FP6) within the proximal promoter was studied in human HepG2 hepatoblastoma cells. Two footprinted regions (FP1 and FP3) contain binding sites for the chicken ovalbumin upstream promoter-transcription factor (COUP-TF) and hepatocyte nuclear factor-4 (HNF-4). In experiments where SHBG promoter-luciferase reporter gene constructs were co-transfected into HepG2 cells with COUP-TF and/or HNF-4 expression vectors, HNF-4 markedly increased transcription, whereas COUP-TF suppressed this probably by displacing HNF-4 from their common FP1-binding site. This COUP-TF/HNF-4-binding site within FP1 includes a TTTAA sequence, located at nucleotides -30/-26 upstream of the transcription start site, which fails to interact with human TFIID, TATA-binding protein in vitro. When this sequence was replaced with an idealized HNF-4-binding site, the transcriptional activity of the promoter increased in HepG2 cells. Taken together, these data imply that an interplay between COUP-TF and HNF-4 at a site within FP1 regulates human SHBG expression and that HNF-4 controls transcription from this TATA-less promoter by somehow substituting for TATA-binding protein in the recruitment of a transcription preinitiation complex.

Vitamin D-binding protein gene transcription is regulated by the relative abundance of hepatocyte nuclear factors 1alpha and 1beta

Vitamin D-binding protein (DBP)/Gc-globulin, the major carrier of vitamin D and its metabolites in blood, is synthesized predominantly in the liver in a developmentally regulated fashion. By transient transfection analysis, we identified three regions in the 5'-flanking region of the rat DBP gene, segments F-2, B, and A, that contain tissue-specific transcriptional determinants. Gel mobility shift and DNase I footprinting analyses showed that all three regions contained binding sites for the hepatocyte nuclear factor 1 (HNF1), a transcriptional regulator composed of HNF1alpha and HNF1beta hetero- and homodimers. The activity of the most proximal segment A (coordinates -141 to -43) was DBP promoter-specific, position-dependent, and positively controlled by HNF1alpha. In contrast, the two more distal determinants (segments F-2 and B; coordinates -1844 to -1621 and -254 to -140, respectively) acted as classical enhancers in transfected hepatocyte-derived HepG2 cells; their activities were promoter- and orientation-independent, and disruption of their respective HNF1-binding sites resulted in marked loss of DBP gene expression. Remarkably, the activities of these two distal elements depended upon the relative levels of HNF1alpha and HNF1beta; HNF1alpha had a major stimulatory effect, whereas HNF1beta acted as a trans-dominant inhibitor of HNF1alpha-mediated enhancer activity. These results suggested that the net expression of the DBP gene reflected a balance between the two major HNF1 species; the relative abundance of HNF1alpha and HNF1beta proteins in a cell may thus play a critical role in determining the pattern of gene expression.

Two rainbow trout (Oncorhynchus mykiss) albumin genes are differentially regulated

Two distinct albumin cDNAs (rtALB1 and rtALB2) were isolated from the rainbow trout (Oncorhynchus mykiss) liver cDNA library. The rtALB1 cDNA (2761 bp) contains a 69 bp 5' untranslated region (UTR), a 1821 bp reading region, and a long 3' UTR of 872 bp. The rtALB2 cDNA (2250 bp) contains a 78 bp 5' UTR, a 1824 bp coding region, and a 348 bp 3' UTR. The two albumins are 81.5% and 77.5% identical in their nucleotides and protein sequences, respectively. Both rtALB1 and rtALB2 genes are expressed only in the liver. The albumin mRNA was first detected in 5-week-old embryos and was tissue-specific. The two albumin genes were differentially expressed, with the rtALB1 transcripts being 3 to 10 times more abundant than the rtALB2 transcripts. This differential expression was partially regulated at the transcriptional level. Promoter analysis showed that the rtALB1 gene had a typical albumin promoter structure. However, the rtALB2 promoter was abnormal in the TATA box region and was less effective in activating the reporter gene in the mammalian cell lines. These variations in rainbow trout albumin promoter sequences might account for their differences in transcriptional efficiency.

Characterization of the promoter of SF-1, an orphan nuclear receptor required for adrenal and gonadal development

Steroidogenic factor 1 (SF-1) is a transcription factor shown to be critical for regulation of adrenal and gonadal development and function. To dissect the mechanisms that direct expression of this regulator, we have studied the promoter of the SF-1 gene and have identified cis-acting elements that recognize a basic-helix-loop-helix transcription factor; the CAAT binding factor; and Sp1. We demonstrate in Y1 adrenocortical cells that a 90-bp proximal promoter fragment is sufficient to direct steroidogenic-specific expression and that all three elements are required for activity of the SF-1 promoter. Functional analysis of the binding sites on a heterologous TATA box-containing promoter demonstrates that the CAAT box and Sp1 site are not essential for promoter activity when a TATA box is present, whereas the E box is absolutely required for gene expression and is most likely the steroidogenic cell-specific element. We also demonstrate that SF-1 itself does not significantly affect the transcription of its own gene, and thus conclude that the E box, CAAT box, and Sp1 site of the proximal promoter direct expression of the SF-1 gene.

Sex- and tissue-specific expression of a cytochrome P450 2C2-luciferase transgene

To study the transcriptional regulation of the rabbit cytochrome P450 2C2 gene (CYP2C2), transgenic mice were generated using a DNA fragment containing the CYP2C2 5'-flanking region fused to a luciferase reporter gene. In one of three transgenic lines, this transgene was expressed in the liver and at a lower level in the kidney, similar to the tissue-specific pattern of this gene in rabbits. Unlike expression in rabbits, the transgene was expressed in a male-specific pattern in liver and kidney, and in the brain of both sexes. Expression of the transgene in male mice was repressed after castration, while expression in females was stimulated after testosterone treatment and periodic injection of rat growth hormone (GH). The expression of this transgene was also regulated developmentally in the liver. These results suggest that this 3500 bp of CYP2C2 5'-flanking region contains the basal transcriptional regulation elements for tissue-specific and developmental expression. Although a position effect can not be ruled out, this transgene may also include the cis-regulatory element for sexual dimorphic expression.

cis-regulatory elements within the proximal promoter of the rat gene encoding corticosteroid-binding globulin

Corticosteroid-binding globulin (CBG) transports and modulates the bioavailability of glucocorticoids in blood plasma. It is produced predominantly by the liver, but is also produced in a complex spatial and temporal pattern during development and is regulated hormonally. The rat Cbg promoter (pCbg) has therefore been cloned to allow identification of cis-acting sequence elements that could contribute to its regulation. Five protein-binding sites (P1 to P5) were identified within 236 bp immediately 5' of the transcription start point by DNase I footprinting with rat liver nuclear extracts. These P1-P5 sites are highly conserved in the human pCbg, and resemble recognition sequences for HNF-1, CP-2, DBP, HNF-3 and C/EBP or NF-1L6, respectively. Electrophoretic mobility-shift assays indicted that the P1 element most likely binds HNF-1, and transient transfection assays with luciferase reporter plasmids demonstrated that P1-P5 represent a positive component of rat pCbg activity, whereas additional 5' sequences repressed promoter activity 2-4-fold in H4IIEC3 rat hepatoblastoma cells.

The transcriptional regulation of human aldehyde dehydrogenase I gene. The structural and functional analysis of the promoter

Human cytosolic aldehyde dehydrogenase 1 (ALDH1) plays a role in the biosynthesis of retinoic acid that is a modulator for gene expression and cell differentiation. Northern blot analysis showed that liver tissue, pancreas tissue, hepatoma cells, and genital skin fibroblast cells expressed high levels of ALDH1. Sequence analysis showed that the 5'-flanking region contains a number of putative regulatory elements, such as NF-IL6, HNF-5, GATA binding sites, and putative response elements for interleukin-6, phenobarbital and androgen, in addition to a noncanonical TATA box (ATAAA) and a CCAAT box. Functional characterization of the 5'-regulatory region of the human ALDH1 gene was carried out by a fusion to the chloramphenicol acetyltransferase gene. A construct containing 2.6 kilobase pairs of the 5'-flanking region was efficiently expressed in hepatoma Hep3B cells, but not in erythroleukemic K562 cells or in fibroblast LTK- cells, which do not express ALDH1. Within this region, we define a minimal promoter (-91 to +53) that contains positive regulatory elements. The study using site-directed mutagenesis demonstrated that the CCAAT box region is the major cis-acting element involved in basal ALDH1 promoter activity in Hep3B cells. Gel mobility shift assays showed that NF-Y and other octamer factors bound CCAAT box and an octamer motif sequence, but not GATA site existing in the minimal promoter region. Two additional DNA binding activities associated with the minimal promoter were found in the nuclear extract from Hep3B cells, but not from K562 cells. These results offer the possible molecular mechanism of the cell type-specific expression of ALDH1 gene.

Role of the liver-enriched transcription factor HNF-1 alpha in expression of the CYP2E1 gene

The role of the trans-acting factor HNF-1 alpha in activating CYP2E1 gene expression was confirmed by transient co-transfection of an HNF-1 alpha expression plasmid and the CYP2E1 promoter fused to the chloramphenicol acetyl transferase (CAT) reporter gene. Only HNF-1 alpha, and not HNF-1 beta, HNF-4, C/EBP alpha, C/EBP beta, or DBP, was able to activate the CYP2E1 promoter. The extent of activation was proportional to the number of copies of the HNF-1 binding sequence upstream of the promoter. Removal or mutation of the HNF-1 binding sequence led to inactivation of the promoter in response to HNF-1 alpha. Gel-shift Western blot analysis using a synthetic HNF-1 binding sequence derived from CYP2E1 and rat liver nuclear extract revealed that the protein-DNA complex obtained with adult rat liver nuclear extract consisted of both HNF-1 alpha and HNF-1 beta proteins. The shifted bands produced by nuclear extracts from adult, where the endogenous CYP2E1 gene is active, and fetal rat liver, where the gene is inactive, were found to migrate differently, suggesting that the population of factors, possibly including different ratios of HNF-1 alpha and HNF-1 beta proteins, may change during development. However, the co-transfection study did not show cooperativity between the two factors. Elements upstream of the HNF-1 binding site were found to affect the activity of the promoter negatively in the transfection assay. DNase I hypersensitive site mapping revealed a hypersensitive site in this inhibiting element in the adult rat liver sample but not in liver from newborn animals.

Characterization of the functional role of E-box elements for the transcriptional activity of rat acetylcholine receptor epsilon-subunit and gamma-subunit gene promoters in primary muscle cell cultures

The expression of gamma and epsilon subunits of the acetylcholine receptor from mammalian skeletal muscle is regulated independently during myogenic differentiation and innervation. Genomic DNA fragments containing 5'-flanking sequences of the epsilon-subunit and gamma-subunit genes were characterised by a series of 5' deletions fused to the chloramphenicol-acetyltransferase gene and transiently expressed by transfection of primary cultures of rat muscle cells and non-muscle cells. A 6.3-kb epsilon-subunit fragment can be reduced to yield a 270-bp fragment that confers 5-10-times higher expression levels in muscle cells compared to in non-muscle cells. The region composed of nucleotides -185 to -128 increases the transcriptional activity moderately while the 14-bp palindrome containing a single E box at nucleotides -88 to -83 may interact with the promoter but has no enhancer properties in muscle cells. From a 1.1-kb genomic fragment of the gamma-subunit gene, 167 bp were sufficient for muscle-specific expression. Two promoter-proximal E-box elements enhance promoter activity in muscle and mediate transactivation by myogenic factors. Myogenin and myf5 were much more efficient than MRF4 or MyoD1 which exerted only little transactivation. Cotransfection experiments show that increased expression of Id in primary muscle cells inhibits chloramphenicol-acetyltransferase expression mediated by the gamma-subunit gene promoter and support the view that myogenic factors play an important role in the transcriptional regulation of the gamma-subunit gene.

Alterations of the p53 tumor-suppressor gene in transformed mouse liver cells

Mutational inactivation of p53, a potential tumor-suppressor gene, has been found in many tumors of humans as well as rodents. The p53 status in normal and transformed mouse liver cell lines has, however, not been investigated. We examined possible point mutations and compared mRNA and protein expression of the p53 gene in normal vs. transformed mouse liver cells. The transformed cells studied included lines spontaneously transformed by sub-culture, virally transformed by simian virus 40 (SV40), and chemically transformed by N-methyl-N-nitro-N-nitrosoguanidine (MNNG) or methylcholanthrene epoxide (MC). A heterozygous G-->A point mutation at codon 241, position 1, of p53 was detected in MNNG-transformed cells after screening of 5 evolutionarily conserved regions where mutation hot-spots are clustered. The mutation causes a gly-->arg substitution. No mutations were found in normal or other transformed cells. The steady-state levels of p53 mRNA were decreased in chemically transformed (both MNNG- and MC-transformed) cells. Elevated levels of p53 protein were found in spontaneously transformed and SV40-transformed cells, an observation that may reflect a longer half-life of the protein, as has been shown in other transformed lines. The low level of the p53 protein in MC-transformed cells may result from transcriptional depression of the p53 gene. We conclude from these data that abnormal p53 status, such as point mutation or altered expression, may play a role during the malignant transformation of mouse liver cells.

Multiple binding sites for nuclear factors in the 5'-upstream region of two alpha 2u-globulin genes: implications for hormone-regulated and tissue-specific control

In order to understand the tissue- and hormone-specific control of alpha 2u-globulin synthesis we isolated the 5'-upstream putative regulatory region of two alpha 2u-globulin genes: RAP 01 and RAO 01. Both clones seem to be expressed in rat liver. DNAseI footprinting analysis after incubation with rat liver nuclear extracts was used to identify regions of potential interest. Particular attention was paid to protected regions located in the neighbourhood of domains which, according to our previous studies, interact specifically with androgen- and glucocorticoid-receptor complexes. Fifteen DNAseI footprints could be mapped in clone RAO 01 (bp -758 up to the cap site). Nineteen footprints were observed in the corresponding region of RAP 01. Differences in the footprinting patterns were mainly observed in the more distal regions. Our data confirm the presence in both clones of two binding sites for the liver enriched factor pseudo-NF1 and one site for C/EBP previously observed in other alpha 2u-globulin genes. In addition we have been able to demonstrate, in RAP 01 only, a binding site for transferrin-liver factor 1. No differences in footprinting patterns could be demonstrated using liver nuclear extracts derived from animals with a high hepatic expression of alpha 2u-globulins (normal male rats) and animals with low to absent expression (prepubertal rats, female rats, rats with the testicular feminization syndrome, diabetic rats and hypophysectomized animals). Transfection experiments indicate that a fragment of RAP 01 (bp -643 up to -617) is able to act as a glucocorticoid and as an androgen response element. Larger fragments of RAP 01 and fragments of RAO 01 are ineffective. It is concluded that the expression of individual alpha 2u-globulin genes is probably the result of combinatorial interactions of several trans-acting factors with appropriate cis-acting elements. Moreover, important sites for tissue-specific and hormone-regulated expression may be situated outside the regions investigated.

A1F-B, a novel CCAAT-binding transcription activator that interacts with the aldolase B promoter

We describe here a 70 kDa transcription factor A1F-B, which preferentially binds to an element encompassing a CCAAT motif on the rat aldolase B promoter. Comparison of binding specificities, relative molecular masses, and subunit compositions with those of other known CCAAT-binding factors indicated that A1F-B is a novel member of CCAAT-binding factors.

Regulation of the HNF-1 homeodomain proteins by DCoH

The pattern of expression of homeodomain proteins often exceeds their apparent domain of activity. Tissue-specific proteins that modulate the in vivo activity of homeodomain proteins have been proposed to account for this functional restriction. The first identified example of such an accessory protein is DCoH, which confers transcriptional activity to the hepatocyte nuclear factor 1 and provides a model of how other accessory factors might modulate the function of homeodomain proteins.

Cell phenotype, binding affinity and promoter structure modulate transactivation by HNF1 and LAP

To evaluate the importance of the transcription factors known to bind to the albumin promoter as well as the parameters involved in their activity, we have used cotransfections with an albumin promoter-cat plasmid combined with expression vectors driving the expression of cDNAs coding for liver-enriched factors known to interact with this promoter. We describe the characteristics of a set of clones of hepatic origin: well differentiated, partial variants or pleiotropic dedifferentiated variants. These lines have been characterized for the accumulation of RNAs corresponding to each of the albumin promoter-binding factors. Only HNF1, and to a lesser extent C/EBP, show differences depending upon the differentiation state of the cells. Overexpression of exogenous HNF1 in these cells reveals that this factor is able to transactivate the albumin promoter only in variant cells where the endogenous protein is limiting. By contrast, if the HNF1-binding site is of weak affinity, overexpression of exogenous HNF1 stimulates the albumin promoter even in the HNF1-rich differentiated cells. Overexpression of exogenous LAP strongly transactivates an artificial promoter containing one LAP-binding site, but surprisingly in all the cell lines, it has little effect upon the albumin promoter. These results demonstrate that the transactivation potential of a given transcription factor depends on the degree of differentiation of the recipient cells, on the promoter structure, and on the affinity of the binding site for this factor.

Oncotic pressure regulates gene transcriptions of albumin and apolipoprotein B in cultured rat hepatoma cells

The mechanism of the accelerated syntheses of albumin and apolipoprotein B (apo B) in response to decreased oncotic pressure was investigated in cultured rat hepatoma H4-II-E cells. Addition of dextran (mol wt 6-9 x 10(4)) to the culture medium decreased the levels of albumin and apo B mRNAs in an oncotic pressure-dependent manner. The reductions of both mRNAs were attenuated with increase in the molecular weight of dextran, which resulted in a decrease in oncotic pressure. Addition of macromolecule increased the viscosity in medium; however, alteration of viscosity appeared not to correlate with albumin and apo B mRNA levels. Transcriptional run-on assays with isolated nuclei from dextran-treated vs. untreated hepatoma cells indicated that the changes in steady-state mRNA levels were mainly controlled at the transcriptional step. Treatment with cycloheximide increased albumin mRNA to the basal level, which was effectively suppressed by dextran, and resulted in superinduction of apo B mRNA. These changes occurred primarily at the transcriptional step. These results suggest that regulations of the expressions of the albumin and apo B genes for adaptive increases in the mRNAs may require the continued synthesis of a labile protein(s) or a limiting transcription factor(s). We conclude that oncotic pressure plays an important role in regulation of expression of the albumin and apo B genes at the transcriptional step.

Interaction of a nuclear factor 1-like protein with a cAMP response element-binding protein in rat liver

1. The existence of both cAMP-responsive element binding factor and a nuclear factor 1-like (NF-1-like) protein in nuclear extracts from liver of cAMP-treated rat has been revealed. 2. Binding of these proteins to a DNA fragment containing both elements was cooperative, and 50% binding was achieved with considerably less protein than with a fragment bearing either element alone. 3. Cleavage of the fragment between the two elements abolished the apparent cooperative interaction. 4. Southwestern blot analysis showed that the NF-1-like protein has a molecular weight in the 28-30-kDa range. 5. The NF-1-like binding activity was very stable.

Importance of the CArG box in regulation of beta-actin-encoding genes

The beta-actin-encoding gene (Act) in carp is regulated by several cis-acting regulatory elements including the evolutionarily conserved CC(A/T)6GG (CArG box or serum-response element) sequences positioned in the promoter region between the CAAT and TATA boxes and in the first intron. To address the roles of the two CArG boxes on gene expression, we replaced them with linker sequences. The CArG box in the proximal promoter was not required for promoter activity in tissue-cultured cells, but was required in conjunction with a second CArG box in the first intron to give full expression in transgenic embryos. Likewise, the geometry of cis-acting transcriptional elements in the proximal promoter was more important for expression of transgenic constructs in developing embryos than in tissue-cultured fibroblasts. Mobility-shift and exonuclease mapping experiments indicated that the same or similar protein factors bind around the two CArG boxes, suggesting that interactions between the promoter and the first intron are involved in Act regulation.

NFY or a related CCAAT binding factor can be replaced by other transcriptional activators for co-operation with HNF1 in driving the rat albumin promoter in vivo

Like many eukaryotic genes, the rat albumin promoter contains a CCAAT consensus motif at position -80. In transfected H4II hepatoma cells the strength of this promoter depends to a large extent on the integrity of a hepatic nuclear factor 1 (HNF1) binding site located at position -60 and to a lesser extent on the CCAAT element. However, if the affinity for HNF1 is reduced, the CCAAT-box becomes essential for high, and tissue specific, promoter activity. We wished to determine which, among the different CCAAT binding factors co-existing in eukaryotic cells, was responsible for this co-operativity with HNF1. To this end we prepared a series of mutants of the CCAAT sequence and compared their effects on albumin promoter activity in vivo and on the binding of different CCAAT binding factors in vitro. Our results strongly suggest that a ubiquitous factor NFY (also designated CBF, ACF, CP1) interacts with this CCAAT element in vivo. We propose that during development NFY could facilitate transcription of the albumin gene in hepatocytes when the concentration of HNF1 is limiting. This co-operativity in transcriptional activation is not due to strict co-operativity in DNA binding between the two proteins and is not limited to NFY or a closely related factor, as the CCAAT-box can be replaced by AP1, SP1 or E2 target sites without significantly affecting the final activity.

One subunit of the transcription factor NF-Y maps close to the major histocompatibility complex in murine and human chromosomes

The genes coding for the A and B subunits of the transcription factor NF-Y are assigned by a combination of in situ hybridization and analysis of somatic cell hybrids and recombinant mouse strains. NF-YA is assigned to human chromosome 6p21 and to mouse chromosome 17. NF-YB is assigned to human chromosome 12 and to mouse chromosome 10.