The rat albumin promoter: cooperation with upstream elements is required when binding of APF/HNF1 to the proximal element is partially impaired by mutation or bacterial methylation

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.

Investigation of the effects of urea cycle amino acids on the expression of ALB and CEBPB in the human hepatocellular carcinoma cell line FLC-4

Cell lines are powerful tools for research into liver function at the molecular level. However, they are generally unsuitable for rigorously assessing the effects of amino acid composition, because many lines require serum-containing medium for their maintenance. Here, we aimed to investigate the effects of ornithine and arginine, which are included in the characteristic metabolic process in hepatocyte, on a human hepatoma-derived cell line (FLC-4) that can be cultured in serum-free medium. FLC-4 cells were cultured under the following three conditions: + ornithine/ – arginine, – ornithine/ – arginine, and –ornithine/ + arginine. Albumin expression evaluated by quantitative polymerase chain reaction and enzyme-linked immunosorbent assay and showed no obvious differences based on the presence of ornithine or arginine. However, the mRNA levels of two liver-enriched transcription factors (CEBPB and HNF1A), which are involved in regulating albumin expression, were significantly higher in cells grown in medium-containing arginine than that in cells grown in ornithine-containing medium. Western blotting showed that the levels both activating and inhibitory C/EBPβ isoforms were significantly increased in cells grown in arginine medium. Furthermore, we have found that depletion of both ornithine and arginine, the polyamine sources, in the medium did not cause polyamine deficiency. When ornithine and arginine were depleted, albumin production was significantly reduced at the mRNA level, CEBPB mRNA levels were increased, and the level of activating form of C/EBPβ was increased. The results of this study suggest that in hepatocyte, these two amino acids might have different functions, and because of which they elicit disparate cellular responses.

N6-Methyladenosine detected in RNA of testicular germ cell tumors is controlled by METTL3, ALKBH5, YTHDC1/F1/F2, and HNRNPC as writers, erasers, and readers

BACKGROUND

Type II testicular germ cell tumors (GCTs) arise from a common precursor lesion (germ cell neoplasia in situ) and are stratified into seminomas and non-seminomas, which differ considerably in morphology, gene expression, and epigenetic landscape. The N6-methyladenosine (6mA) epigenetic modification is the most abundant modification in mRNA and is also detectable in eukaryotic DNA. The functional role of 6mA is not fully understood, but 6mA residues may influence transcription by affecting splicing, miRNA processing, and mRNA stability. Additionally, the methyl group of 6mA destabilizes Watson-Crick base-pairing affecting RNA structure and protein binding.

OBJECTIVES

Here, we analyzed the presence of the 6mA epigenetic modification in germ cells and GCT tissues and cell lines.

MATERIALS AND METHODS

We screened for the presence of 6mA in DNA and RNA by immunohistochemistry, mass spectrometry or ELISA-based quantification assays. Additionally, expression of 6mA writer-, eraser- and reader-factors was analyzed by microarrays, qRT-PCR, western blotting and screening of public databases.

RESULTS

We demonstrate that 6mA is detectable in RNA, but not DNA, of GCT cell lines and tissues, fibroblasts, and Sertoli cells as well as germ cells of different developmental stages. Based on expression analyses, our results suggest METTL3, ALKBH5, YTHDC1, YTHDF1, YTHDF2 and HNRNPC as main writers, erasers, and readers of the 6mA modification in GCTs.

DISCUSSION

Owing to the lack of 6mA in DNA of GCTs, a functional role in regulating DNA transcription can be excluded. Interestingly, expression levels of 6mA regulators are comparable between tumor and normal tissues/cells, suggesting a similar mechanism of 6mA regulation in RNA. Finally, we demonstrate that 6mA levels in RNA increase upon differentiation of GCT cell lines, suggesting a role of 6mA in cell fate decisions.

CONCLUSION

In summary, our data provide the starting point for further experiments deciphering the role of 6mA in the RNA of GCTs.

N6-Methyladenosine in RNA and DNA: An Epitranscriptomic and Epigenetic Player Implicated in Determination of Stem Cell Fate

Vast emerging evidences are linking the base modifications and determination of stem cell fate such as proliferation and differentiation. Among the base modification markers extensively studied, 5-methylcytosine (5-mC) and its oxidative derivatives (5-hydroxymethylcytosine (5-hmC), 5-formylcytosine (5-fC), and 5-carboxylcytosine (5-caC)) dynamically occur in DNA and RNA and have been acknowledged as important epigenetic markers involved in regulation of cellular biological processes. N6-Methyladenosine modification in DNA (m6dA), mRNA (m6A), tRNA, and other noncoding RNAs has been defined as another important epigenetic and epitranscriptomic marker in eukaryotes in recent years. The mRNA m6A modification has been characterized biochemically, molecularly, and phenotypically, including elucidation of its methyltransferase complexes (m6A writer), demethylases (m6A eraser), and direct interaction proteins (readers), while limited information on the DNA m6dA is available. The levels and the landscapes of m6A in the epitranscriptomes and epigenomes are precisely and dynamically regulated by the fine-tuned coordination of the writers and erasers in accordance with stages of the growth, development, and reproduction as naturally programmed during the lifespan. Additionally, progress has been made in appreciation of the link between aberrant m6A modification in stem cells and diseases, like cancers and neurodegenerative disorders. These achievements are inspiring scientists to further uncover the epigenetic mechanisms for stem cell development and to dissect pathogenesis of the multiple diseases conferred by development aberration of the stem cells. This review article will highlight the research advances in the role of m6A methylation modifications of DNA and RNA in the regulation of stem cell and genesis of the closely related disorders. Additionally, this article will also address the research directions in the future.

Targeting Visceral Fat by Intraperitoneal Delivery of Novel AAV Serotype Vector Restricting Off-Target Transduction in Liver

It is challenging to genetically manipulate fat in adults. We demonstrate that intraperitoneal (i.p.) injection of an engineered adeno-associated virus (AAV) serotype Rec2 leads to high transduction of multiple visceral fat depots at a dose of 1 to 2 orders lower than commonly used doses for systemic gene delivery. To target adipose tissue, we develop a single AAV vector harboring two expression cassettes: one using the CBA promoter to drive transgene expression and one using the liver-specific albumin promoter to drive a microRNA-targeting WPRE sequence that only exists in this AAV vector. This dual-cassette vector achieves highly selective transduction of visceral fat while severely restricting off-target transduction of liver. As proof of efficacy, i.p. administration of an adipose-targeting Rec2 vector harboring the leptin gene corrects leptin deficiency, obesity, and metabolic syndromes of ob/ob mice. This study provides a powerful tool to genetically manipulate fat for basic research and gene therapies of genetic and acquired diseases.

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.

N6-Methyladenine: A Conserved and Dynamic DNA Mark

Chromatin, consisting of deoxyribonucleic acid (DNA) wrapped around histone proteins, facilitates DNA compaction and allows identical DNA codes to confer many different cellular phenotypes. This biological versatility is accomplished in large part by posttranslational modifications to histones and chemical modifications to DNA. These modifications direct the cellular machinery to expand or compact specific chromatin regions and mark regions of the DNA as important for cellular functions. While each of the four bases that make up DNA can be modified (Iyer et al. 2011), this chapter will focus on methylation of the sixth position on adenines (6mA), as this modification has been poorly characterized in recently evolved eukaryotes, but shows promise as a new conserved layer of epigenetic regulation. 6mA was previously thought to be restricted to unicellular organisms, but recent work has revealed its presence in metazoa. Here, we will briefly describe the history of 6mA, examine its evolutionary conservation, and evaluate the current methods for detecting 6mA. We will discuss the enzymes that bind and regulate this mark and finally examine known and potential functions of 6mA in eukaryotes.

DNA N(6)-methyladenine: a new epigenetic mark in eukaryotes?

DNA N(6)-adenine methylation (N(6)-methyladenine; 6mA) in prokaryotes functions primarily in the host defence system. The prevalence and significance of this modification in eukaryotes had been unclear until recently. Here, we discuss recent publications documenting the presence of 6mA in Chlamydomonas reinhardtii, Drosophila melanogaster and Caenorhabditis elegans; consider possible roles for this DNA modification in regulating transcription, the activity of transposable elements and transgenerational epigenetic inheritance; and propose 6mA as a new epigenetic mark in eukaryotes.

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.

Novel non-specific DNA adenine methyltransferases

The mom gene of bacteriophage Mu encodes an enzyme that converts adenine to N⁶-(1-acetamido)-adenine in the phage DNA and thereby protects the viral genome from cleavage by a wide variety of restriction endonucleases. Mu-like prophage sequences present in Haemophilus influenzae Rd (FluMu), Neisseria meningitidis type A strain Z2491 (Pnme1) and H. influenzae biotype aegyptius ATCC 11116 do not possess a Mom-encoding gene. Instead, at the position occupied by mom in Mu they carry an unrelated gene that encodes a protein with homology to DNA adenine N⁶-methyltransferases (hin1523, nma1821, hia5, respectively). Products of the hin1523, hia5 and nma1821 genes modify adenine residues to N⁶-methyladenine, both in vitro and in vivo. All of these enzymes catalyzed extensive DNA methylation; most notably the Hia5 protein caused the methylation of 61% of the adenines in λ DNA. Kinetic analysis of oligonucleotide methylation suggests that all adenine residues in DNA, with the possible exception of poly(A)-tracts, constitute substrates for the Hia5 and Hin1523 enzymes. Their potential ‘sequence specificity’ could be summarized as AB or BA (where B = C, G or T). Plasmid DNA isolated from Escherichia coli cells overexpressing these novel DNA methyltransferases was resistant to cleavage by many restriction enzymes sensitive to adenine methylation.

Role of epigenetics in liver-specific gene transcription, hepatocyte differentiation and stem cell reprogrammation

Controlling both growth and differentiation of stem cells and their differentiated somatic progeny is a challenge in numerous fields, from preclinical drug development to clinical therapy. Recently, new insights into the underlying molecular mechanisms have unveiled key regulatory roles of epigenetic marks driving cellular pluripotency, differentiation and self-renewal/proliferation. Indeed, the transcription of genes, governing cell-fate decisions during development and maintenance of a cell's differentiated status in adult life, critically depends on the chromatin accessibility of transcription factors to genomic regulatory and coding regions. In this review, we discuss the epigenetic control of (liver-specific) gene-transcription and the intricate interplay between chromatin modulation, including histone (de)acetylation and DNA (de)methylation, and liver-enriched transcription factors. Special attention is paid to their role in directing hepatic differentiation of primary hepatocytes and stem cells in vitro.

Sustained liver-specific transgene expression from the albumin promoter in mice following hydrodynamic plasmid DNA delivery

BACKGROUND

To properly study gene expression in vivo, often long-term expression is desired. Previous studies using plasmid DNA (pDNA) vectors have typically resulted in short-term expression. Here, we evaluated combinations of the albumin promoter with different enhancers and untranslated regions for liver-specific expression in mice.

METHODS

A series of pDNA secreted alkaline phosphatase (SEAP) reporter gene expression vectors was constructed using the albumin promoter and various other expression cassette elements. Each was evaluated for level and duration of SEAP expression in mice following hydrodynamic tail vein delivery.

RESULTS

Sustained liver expression was obtained from vectors combining the albumin promoter with an albumin 3' untranslated region (3'UTR). The level of expression was increased by inclusion of enhancers and a 5' intron. The optimal expression vector consisted of the albumin promoter combined with an alpha-fetoprotein MERII enhancer, 5' intron from the factor IX gene, and the 3'UTR from the albumin gene including intron 14. With this vector, SEAP reporter gene expression levels remained high for 1 year, at levels comparable to those obtained from the cytomegalovirus (CMV) promoter on day 1. Expression of human apolipoprotein E3 (hApoE) in ApoE knockout mice provided a dose-dependent correction of their hypercholesterolemia.

CONCLUSIONS

Liver-specific sustained transgene expression can be obtained at very high levels from optimized pDNA vectors, without the use of integration systems. Such vectors will further facilitate biological studies of genes in vivo and may find application in gene therapy.

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.

Undetectable levels of N6-methyl adenine in mouse DNA: Cloning and analysis of PRED28, a gene coding for a putative mammalian DNA adenine methyltransferase

Three methylated bases, 5-methylcytosine, N4-methylcytosine and N6-methyladenine (m6A), can be found in DNA. However, to date, only 5-methylcytosine has been detected in mammalian genomes. To reinvestigate the presence of m6A in mammalian DNA, we used a highly sensitive method capable of detecting one N6-methyldeoxyadenosine per million nucleosides. Our results suggest that the total mouse genome contains, if any, less than 10(3) m6A. Experiments were next performed on PRED28, a putative mammalian N6-DNA methyltransferase. The murine PRED28 encodes two alternatively spliced RNA. However, although recombinant PRED28 proteins are found in the nucleus, no evidence for an adenine-methyltransferase activity was detected.

N6-methyladenine: the other methylated base of DNA

Contrary to mammalian DNA, which is thought to contain only 5-methylcytosine (m5C), bacterial DNA contains two additional methylated bases, namely N6-methyladenine (m6A), and N4-methylcytosine (m4C). However, if the main function of m5C and m4C in bacteria is protection against restriction enzymes, the roles of m6A are multiple and include, for example, the regulation of virulence and the control of many bacterial DNA functions such as the replication, repair, expression and transposition of DNA. Interestingly, even if adenine methylation is usually considered a bacterial DNA feature, the presence of m6A has been found in protist and plant DNAs. Furthermore, indirect evidence suggests the presence of m6A in mammal DNA, raising the possibility that this base has remained undetected due to the low sensitivity of the analytical methods used. This highlights the importance of considering m6A as the sixth element of DNA.

Hepatocyte nuclear factors 1alpha and 4alpha control expression of proline oxidase in adult liver

Adult liver functions are regulated by several hepatocyte nuclear factors (HNFs). HNF4alpha and HNF1alpha are involved in metabolic functions in the liver. The expression of proline oxidase (PO) and proline dehydrogenase was downregulated in the HNF4alpha liver-specific null mice. In addition, the expression of PO was also diminished in the liver derived from HNF1alpha-null mice. The -160 bp proximal promoter region of the PO gene has two HNF4alpha- and HNF1alpha-binding consensus sites. Transactivation, electrophoretic mobility shift and chromatin immunoprecipitation studies revealed that these regions are important for PO promoter activity. These results suggested that HNF4alpha and HNF1alpha regulate proline metabolism in adult liver.

N6-Methyldeoxyadenosine, a nucleoside commonly found in prokaryotes, induces C2C12 myogenic differentiation

N(6)-methyl-2(')-deoxyadenosine (MedAdo) is a nucleoside naturally found in prokaryotic DNA. Interestingly, the N(6)-methylation of adenine in DNA seems to have been counter-selected during the course of evolution since MedAdo has not been detected in mammalian DNA until now. We show here that treatment with MedAdo induces myogenesis in C2C12 myoblasts. The presence of MedAdo in C2C12 DNA was investigated using a method based on HPLC coupled to electrospray ionization tandem mass spectrometry which is several thousand fold more sensitive than assays used previously. By this procedure, MedAdo is detected in the DNA from MedAdo-treated cells but remains undetectable in the DNA from control cells. Furthermore, MedAdo regulates the expression of p21, myogenin, mTOR, and MHC. Interestingly, in the pluripotent C2C12 cell line, MedAdo drives the differentiation towards myogenesis only. Thus, the biological effect of MedAdo is suppressed in the presence of BMP-2 which transdifferentiates C2C12 from myogenic into osteogenic lineage cells. Taken together these results point to MedAdo as a novel inducer of myogenesis and further extends the differentiation potentialities of this methylated nucleoside. Furthermore, these data raise the intriguing possibility that the biological effects of MedAdo on cell differentiation may have led to its counter-selection in eukaryotes.

Induction of neurite outgrowth in PC12 cells by the bacterial nucleoside N6-methyldeoxyadenosine is mediated through adenosine A2a receptors and via cAMP and MAPK signaling pathways

We have previously shown that N(6)-methyldeoxyadenosine (MDA) is an inducer of differentiation in several tumor cells. Here we show that in addition to its ability to induce neurite-outgrowth in PC12 cells, MDA also significantly enhances the nerve-growth factor-mediated neurite outgrowth of these cells. Thus, MDA acts synergistically with NGF to repress cdc2 and cdk2 synthesis and to enhance tyrosine hydroxylase synthesis. To further elucidate the mechanisms of action of MDA, we investigated the effect of this drug on various signaling pathways. The neuritogenesis observed in PC12 following MDA treatment is mediated through activation of adenylyl cyclase in a PKA independent process and through the recruitment of the p44/p42 MAPK pathway. Furthermore, the adenosine A(2a) receptor antagonist ZM 241385 prevents the MDA-induced neuritogenesis, suggesting that MDA mediates its effect via this adenylyl cyclase-coupled A(2a) receptor. Collectively, these findings suggest that, in PC12 cells, the MDA-induced neuritogenesis requires the recruitment of adenosine A(2a) receptor, the stimulation of adenylate cyclase, and the activation of the p44/42MAP kinase cascade.

What is, mutatis mutandis, the sequence of plasmid DNAs used in gene therapy?

Mutation is a fundamental biological process occurring in each living organism. Plasmid DNA which is used in gene therapy protocols or DNA vaccination passes through two different living cells which are, respectively, the producing cell (bacterial) and the target cell (eukaryotic). Hence, modifications in the nucleotide sequence of plasmids are likely to occur both in bacteria during the amplification step of plasmid DNA and in eukaryotic cells following gene transfer. In addition to these biological modifications resulting from the physical passage of the plasmid into two different living organisms, an additional source of sequence alteration resides in our mode of representation of the nucleotide sequence of plasmid DNA which uses a four letters code, whereas, bacterial DNA is made of six different nucleosides. Indeed, the therapeutic DNA paradigm seems to have neglected the qualitative importance of these DNA sequence alterations. In this review we discuss the importance and the role of these DNA sequence modifications in the context of non-viral gene therapy approaches.

Mechanisms of down-regulation of CYP2E1 expression by inflammatory cytokines in rat hepatoma cells

CYP2E1 is one of the major cytochrome P450 forms whose expression is strongly inhibited by inflammatory cytokines in humans and rodents. In the present study, we have used the Fao rat hepatoma cell line that constitutively expresses CYP2E1 enzyme to investigate mechanisms of cytokine action. The cells were treated with interleukin (IL)-1beta, tumor necrosis factor-alpha (TNFalpha), or IL-6 for 24 or 72 h, and the expression of CYP2E1 was monitored at the transcriptional, mRNA, and protein levels. All three cytokines decreased the CYP2E1 mRNA levels after 24 h, and the effect was even stronger after 72 h. In contrast, significant inhibition of CYP2E1 protein was seen only after 72 h. In transfection assays using a CYP2E1 5' -3685 to +29-luciferase construct, it was found that IL-6 inhibited gene transcription after 24 h, but a similar effect by IL-1beta and TNFalpha was registered only after 72 h. Using 5' deletions of the CYP2E1 5'-reporter construct a responsive region for the IL-6 effect was located to -669 to -507 base pairs in the CYP2E1 5'-flanking region. Interestingly, IL-1beta, but not TNFalpha, was found to reduce hepatocyte nuclear factor (HNF)-1alpha binding to the CYP2E1 promotor. However, the transactivation function of HNF-1alpha was found to be impaired in Fao cells. In mouse primary hepatocytes, IL-1beta decreased HNF-1alpha-mediated transactivation. In conclusion, our data indicate that inflammatory cytokines inhibit CYP2E1 expression by multiple mechanisms, including control of HNF-1alpha function and regulation of other transcriptional factors acting on the CYP2E1 5'-upstream regulatory region. In addition, regulation of factors of importance for the CYP2E1 mRNA stability may be involved.

Aroclor 1254 modulates gene expression of nuclear transcription factors: implications for albumin gene transcription and protein synthesis in rat hepatocyte cultures

Human exposure to polychlorinated biphenyls (PCBs) may lead to increased albumin serum levels, but little is known about the underlying events. Certain PCBs are also ligands for the aryl hydrocarbon receptor (Ahr) and this receptor regulates transcriptional activation of many different genes, including CYP1A1. We tested our hypothesis that expression of certain nuclear transcription factors is altered upon treatment of rat hepatocyte cultures with Aroclor 1254 and we studied the gene expression of albumin and liver-enriched transcription factors simultaneously. We correlate albumin gene expression with protein synthesis and we used CYP1A1 gene expression and enzyme activity as a surrogate endpoint for aryl hydrocarbon receptor activation. We found mRNA transcripts of CCAAT/enhancer binding protein alpha and gamma, hepatic nuclear factor 1, and hepatic nuclear factor 4 to be increased up to 62-fold, whereas albumin gene expression and secretion was increased 3-fold. Noticeably, expression of c-fos, c-jun (AP-1), HNF-6, CCAAT/enhancer binding protein beta and delta, tissue-specific enhancer-1, Ah-receptor, and albumin D-site-binding protein was unchanged. We show coordinate albumin gene expression and protein secretion in primary rat hepatocyte cultures and propose a relationship between induction of certain liver-enriched transcription factors and of the albumin gene via an Ahr-mediated mechanism.

Induced synthesis of albumin-like protein in damaged rat reticulocytes

In this study, we present evidence that red blood cell (RBC) membrane p68 in the Belgrade (b/b) rat is similar if not identical to rat serum albumin (RSA). Structural homology between RSA and the RBC p68 has been determined by a variety of biochemical and immunological criteria. This albumin-like protein is a normal constituent of rat RBC and it is partially exported by exosomes during erythroid differentiation. The endogenous origin of rat RBC albumin-like protein was demonstrated by monitoring protein synthesis in reticulocytes and by identification of reticulocyte mRNA for albumin. Haemolytic anaemia, either hereditary (b/b rat) or drug-induced (phenylhydrazine-treated normal rat), results in increased accumulation of the albumin-like protein in rat RBCs as a result of its induced synthesis.

In vivo and in vitro regulation of sterol 27-hydroxylase in the liver during the acute phase response. potential role of hepatocyte nuclear factor-1

The host response to infection is associated with several alterations in lipid metabolism that promote lipoprotein production. These changes can be reproduced by lipopolysaccharide (LPS) administration. LPS stimulates hepatic cholesterol synthesis and suppresses the conversion of cholesterol to bile acids. LPS down-regulates hepatic cholesterol 7alpha-hydroxylase, the rate-limiting enzyme in the classic pathway of bile acid synthesis. We now demonstrate that LPS markedly decreases the activity of sterol 27-hydroxylase, the rate-limiting enzyme in the alternate pathway of bile acid synthesis, in the liver of Syrian hamsters. Moreover, LPS progressively decreases hepatic sterol 27-hydroxylase mRNA levels by 75% compared with controls over a 24-h treatment period. LPS also decreases oxysterol 7alpha-hydroxylase mRNA levels in mouse liver. In vitro studies in HepG2 cells demonstrate that tumor necrosis factor and interleukin (IL)-1 decrease sterol 27-hydroxylase mRNA levels by 48 and 80%, respectively, whereas IL-6 has no such effect. The IL-1-induced decrease in sterol 27-hydroxylase mRNA expression occurs early, is sustained for 48 h, and requires very low doses. In vivo IL-1 treatment also lowers hepatic sterol 27-hydroxylase mRNA levels in Syrian hamsters. Studies investigating the molecular mechanisms of LPS-induced decrease in sterol 27-hydroxylase show that LPS markedly decreases mRNA and protein levels of hepatocyte nuclear factor-1 (HNF-1), a transcription factor that regulates sterol 27-hydroxylase, in the liver. Moreover, LPS decreases the binding activity of HNF-1 by 70% in nuclear extracts in hamster liver, suggesting that LPS may down-regulate sterol 27-hydroxylase by decreasing the binding of HNF-1 to its promoter. Coupled with our earlier studies on cholesterol 7alpha-hydroxylase, these data indicate that LPS suppresses both the classic and alternate pathways of bile acid synthesis. A decrease in bile acid synthesis in liver would reduce cholesterol catabolism and thereby contribute to the increase in hepatic lipoprotein production that is induced by LPS and cytokines.

Characterization of the genomic structure and tissue-specific promoter of the human nuclear receptor NR5A2 (hB1F) gene

The human homologue of the Drosophila melanogaster orphan nuclear receptor fushi tarazu factor 1 (Ftz-F1), NR5A2 (hB1F), was initially identified as a regulatory factor that binds and activates enhancer II of hepatitis B virus. NR5A2 (hB1F) is expressed specifically in pancreas and liver, playing important roles in the regulation of several liver-specific genes. A detailed analysis on the genomic structure and promoter activity will greatly promote future studies on the function of the NR5A2 (hB1F) gene. In this report, a bacterial artificial chromosome clone and several phage clones covering the NR5A2 (hB1F) gene were isolated and the complete genomic sequence was obtained. Alignment of different cDNAs of the NR5A2 (hB1F) gene with the genomic sequence facilitated the delineation of its structural organization, which spans over 150 kb and consists of eight exons interrupted by seven introns. RT-PCR and 3'-RACE revealed that utilization of two polyadenylation signals results in the 3.8 and 5.2 kb transcripts that were observed previously. The transcription start site of the NR5A2 (hB1F) gene was mapped downstream of a canonical TATA box. An upstream fragment containing binding sites for several liver-specific and ubiquitous transcription factors exhibits hepatocyte-specific promoter activity. Transient transfections indicated that hepatocyte nuclear factors HNF1 and HNF3beta could activate NR5A2 (hB1F) 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.

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.

Opposite transcriptional activity between the wild type c-myc gene coding for c-Myc1 and c-Myc2 proteins and c-Myc1 and c-Myc2 separately

E-cadherin expression was previously shown to be activated by RB and c-myc specifically in epithelial cells, through interaction with the AP-2 transcription factor. Here we show that only a wild type c-myc gene, coding for the two c-Myc proteins c-Myc2 and c-Myc1, was able to transactivate the E-cadherin promoter, in contrast to c-Myc2 or c-Myc1 alone which strongly repressed E-cadherin in both epithelial cells and fibroblasts. In addition, overexpression of c-myc2 or c-myc1 inhibited c-myc and RB-mediated activation in a dose-dependent manner, suggesting that the ratio of the two c-Myc proteins is essential for transactivation. We also showed by using several mutants of the E-cadherin promoter, that the AP-2 binding sites were the main target of c-myc2- and c-myc1-mediated repression. AP-2-mediated inhibition was cell-type specific, as was the activation. Nevertheless, when high amounts of c-myc2 and c-myc1 were used, a second c-myc-mediated repression was observed, possibly mediated by the Inr sequence of the E-cadherin promoter. However, this repression was independent of cell type. Our results suggest a new way to regulate c-myc transcriptional activity by interfering with the ratio of the two c-myc proteins, which has already been found to be disrupted in vivo in several tumor types.

The glucocorticoid response element II is functionally homologous in rat and human insulin-like growth factor-binding protein-1 promoters

In vivo, insulin-like growth factor-binding protein-1 (IGFBP-1) modulates the IGFs' bioavailability and may contribute to their delivery to peripheral tissues. In rat and human hepatocytes, glucocorticoids stimulate IGFBP-1 gene transcription through homologous glucocorticoid response units (GRU). Transfection experiments have shown that one of these, GRU2 (nucleotide (nt) -121 to -85 and nt -111 to -74 in human and rat promoters, respectively), was on its own able to mediate the glucocorticoid response in rat but not in human species (Suwanichkul, A., Allander, S., Morris, S. L. & Powell, D. R. (1994) J. Biol. Chem. 269, 30835-30841, Goswami, R., Lacson, R., Yang, E., Sam, R. & Unterman, T. (1994) Endocrinology 134, 736-743, and Suh, D. S., Ooi, G. T. & Rechler, M. M. (1994) Mol. Endocrinol. 8, 794-805). A close comparison of GRU2 sequences has pointed out a C to A transition in the underlying GREII, which creates a GATC tetranucleotide in rat species. This tetranucleotide is submitted to adenosyl methylation (dam methylation) in most Escherichia coli bacterial strains, but not in eucaryotic cells. We showed (i) that on its own, the unmethylated rat GRU2 (propagated in dam E. coli strains) was inactive, as is the case for its human counterpart (nonsignificant glucocorticoid inductions, 1.48 +/- 0.23 and 1.7 +/- 0.35-fold in Chinese hamster ovary cells, respectively) and (ii) that its adenosyl methylation in standard dam+ bacterial strains yielded a functional GRU (6.5 +/- 1. 1 and 13.1 +/- 3.9-fold glucocorticoid inductions in Chinese hamster ovary and HepG2 cells, respectively). Transient transfection in HepG2 hepatoma cells clearly showed that the interaction of liver-enriched trans-acting factor(s) with the 5'-overlapping insulin response element does not enable the unmethylated rat GRU2 or the human GRU2 to become responsive to glucocorticoids (nonsignificant 2.21 +/- 0.48 and 1.20 +/- 0.06-fold induction, respectively). Furthermore, we have correlated these functional data with in vitro DNA-protein interaction studies: the dam methylated rat GREII displayed a 2.8-fold higher affinity for the glucocorticoid receptor than its unmethylated counterpart.

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.

Effects of diabetes mellitus on hepatocyte nuclear factor 1 decrease albumin gene transcription

We have previously reported that albumin gene transcription is reduced in diabetes mellitus (DM). The present study explored the mechanism by which albumin gene transcription is down-regulated in DM. Deletional studies and displacement of factors binding to site B of the albumin promoter indicated that the repressive effects of DM are mediated by nuclear factors binding to this site. Since hepatocyte nuclear factor 1 (HNF1) activates albumin promoter activity and is the predominant factor binding to site B, we examined HNF1. The abundance and binding activity of HNF1 were reduced in hepatonuclear extracts from diabetic compared to control rats. However, HNF1 mRNA levels were unchanged, suggesting that the effect of DM on HNF1 is at the post-transcriptional level. Extracts from diabetic animals also contained another protein, distinct from HNF1 and vHNF1, which bound to site B in gel retardation studies. In summary, our studies demonstrate that the reduced abundance and binding activity of HNF1 correlates with decreased albumin gene transcription in DM.

Hepatocyte nuclear factor 1 inactivation results in hepatic dysfunction, phenylketonuria, and renal Fanconi syndrome

HNF1 is a transcriptional activator of many hepatic genes including albumin, alpha1-antitrypsin, and alpha- and beta-fibrinogen. It is related to the homeobox gene family and is predominantly expressed in liver and kidney. Mice lacking HNF1 fail to thrive and die around weaning after a progressive wasting syndrome with a marked liver enlargement. The transcription rate of genes like albumin and alpha1-antitrypsin is reduced, while the gene coding for phenylalanine hydroxylase is totally silent, giving rise to phenylketonuria. Mutant mice also suffer from severe Fanconi syndrome caused by renal proximal tubular dysfunction. The resulting massive urinary glucose loss leads to energy and water wasting. HNF1-deficient mice may provide a model for human renal Fanconi syndrome.

The evolution of small DNA viruses of eukaryotes: past and present considerations

Historically, viral evolution has often been considered from the perspective of the ability of the virus to maintain viral pathogenic fitness by causing disease. A predator-prey model has been successfully applied to explain genetically variable quasi-species of viruses, such as influenza virus and human immunodeficiency virus (HIV), which evolve much faster rates than the host. In contrast, small DNA viruses (polyomaviruses, papillomaviruses, and parvoviruses) are species specific but are stable genetically, and appear to have co-evolved with their host species. Genetic stability is attributable primarily to the ability to establish and maintain a benign persistent state in vivo and not to the host DNA proofreading mechanisms. The persistent state often involves a cell cycle-regulated episomal state and a tight linkage of DNA amplification mechanisms to cellular differentiation. This linkage requires conserved features among viral regulatory proteins, with characteristic host-interactive domains needed to recruit and utilize host machinery, thus imposing mechanistic constrains on possible evolutionary options. Sequence similarities within these domains are seen amongst all small mammalian DNA viruses and most of the parvo-like viruses, including those that span the entire spectrum of evolution of organisms from E. coli to humans that replicate via a rolling circle-like mechanism among the entire spectrum of organisms throughout evolution from E. coli to humans. To achieve benign inapparent viral persistence, small DNA viruses are proposed to circumvent the host acute phase reaction (characterized by minimal inflammation) by mechanisms that are evolutionarily adapted to the immune system and the related cytokine communication networks. A striking example of this is the relationship of hymenoptera to polydnaviruses, in which the crucial to the recognition of self, development, and maintenance of genetic identity of both the host and virus. These observations in aggregate suggest that viral replicons are not recent "escapies" of host replication, but rather provide relentless pressure in driving the evolution of the host through cospeciation.

Cloning and developmental expression of LFB3/HNF1 beta transcription factor in Xenopus laevis

We have cloned the Xenopus laevis homologue of the LFB3/HNF1 beta transcription factor. RNase protection and in situ hybridisation experiments show that XLFB3 transcription starts in the gastrulating endoderm at stage 10.5 (mid-gastrula). At later stages, XLFB3 transcripts within the endoderm are restricted to mid- and hindgut and to their derivative organs and tissues. XLFB3 is also expressed in the neuroectoderm and in the pronephros anlage. XLFB3 is not expressed in the rostral part of all three germ layers, with coincident anterior borders that are shifted anteriorly by treatment of developing embryos with retinoic acid. XLFB3 is a useful marker of early endoderm differentiation and its expression pattern along the antero-posterior axis, as well as the response to retinoic acid treatment, suggests a role in early morphogenesis.

Regulation of expression of transgenes in developing fish

The transcriptional regulatory elements of the beta-actin gene of carp (Cyprinus carpio) have been examined in zebrafish and goldfish harbouring transgenes. The high sequence conservation of the putative regulatory elements in the beta-actin genes of animals suggested that their function would be conserved, so that transgenic constructs with the same transcriptional control elements would promote similar levels of transgene expression in different species of transgenic animals. To test this assumption, we analysed the temporal expression of a reporter gene under the control of transcriptional control sequences from the carp beta-actin gene in zebrafish (Brachydanio rerio) and goldfish (Carrasius auratus). Our results indicated that, contrary to expectations, combinations of different transcriptional control elements affected the level, duration, and onset of gene expression differently in developing zebrafish and goldfish. The major differences in expression of beta-actin/CAT (chloramphenicol acetyltransferase) constructs in zebrafish and goldfish were: (1) overall expression was almost 100-fold higher in goldfish than in zebrafish embryos, (2) the first intron had an enhancing effect on gene expression in zebrafish but not in goldfish, and (3) the serum-responsive/CArG-containing regulatory element in the proximal promoter was not always required for maximal CAT activity in goldfish, but was required in zebrafish. These results suggest that in the zebrafish, but not in the goldfish, there may be interactions between motifs in the proximal promoter and the first intron which appear to be required for maximal enhancement of transcription.

A bipartite activation domain is responsible for the activity of transcription factor HNF1/LFB1 in cells of hepatic and nonhepatic origin

HNF1/LFB1 is a transcription factor that controls the expression of several liver-specific genes. Previous in vitro experiments allowed us to identify two different regions in the carboxy-terminal portion of the protein responsible for most of the transcription activation potential: the first, ADI, between amino acids 546 and 628 and the second, ADII, between amino acids 281 and 318. To characterize the molecular anatomy of HNF1/LFB1 better, we have analyzed its trans-activating properties in vivo. Several HNF1/LFB1 deletion mutants were tested for their ability to induce transcription from HNF1/LFB1-dependent synthetic promoters in cells of hepatic and nonhepatic origin. These last recipient cells provide an HNF1/LFB1-deficient environment that is useful for a precise quantification of the recombinant protein. Our results confirm the importance of ADI and indicate that no activating property can be assigned to ADII in vivo. Moreover, a novel glutamine/proline-rich activation domain (ADIII) has been identified between amino acids 440 and 506. These findings are confirmed by domain-swapping experiments, carried out with the heterologous GAL4 DNA-binding domain, which also show that the activity of each individual activation domain is influenced by combining adjacent HNF1/LFB1 sequences. The data presented indicate that HNF1/LFB1 transcription activating potential relies on a complex structure and also provide important clues to understanding the different functions exerted by transcription factors of this family.

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.

HNF1, a homeoprotein member of the hepatic transcription regulatory network

Numerous liver specific genes are transcriptionally activated by the binding to their promoter or enhancer of Hepatic Nuclear Factor 1 (HNF1). HNF1 contains a variant homeo-domain and binds to DNA as either a homodimer or a heterodimer with the vHNF1 protein. Surprisingly, HNF1 is not restricted to hepatocytes but is expressed in epithelial cells of several endoderm derived organs and in mesoderm derived kidney tubules. Hence, HNF1 alone can not account for the differentiated state of the hepatic cells. In fact, several other liver-enriched transcription factors have been cloned. The hepatic phenotype could result from the combinatorial expression of these regulators. Possible involvement of these trans-acting factors in liver organogenesis and hepatic differentiation is discussed.

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.

Response to heat shock of gene 1, a Drosophila melanogaster small heat shock gene, is developmentally regulated

The expression of gene 1, a member of the small heat shock gene family from the Drosophila melanogaster chromosomal locus 67B was studied. In contrast to the other heat shock genes, the response of gene 1 to stress was modulated during development. In the absence of stress, gene 1 was expressed at the beginning of pupation, and at a very low level in adult males. Expression of gene 1 was substantially increased by heat shock in pupae, but was one to two orders of magnitude lower in adults or in embryos. Under the same conditions, hsp70 or hsp26 were induced to similar levels in all stages. This developmental effect could be mimicked in cultured Drosophila cells: expression of gene 1 was stimulated by heat shock in the presence, but not in the absence, of the moulting hormone ecdysterone, while the level of expression of hsp26 and hsp70 in response to heat shock was independent of the presence of the hormone. Thus, the presence and activity of the heat shock transcription factor are not sufficient for the maximal response of gene 1 to stress. These results suggest that the heat shock activator protein requires additional factors, which are developmentally regulated, to activate transcription of gene 1. Furthermore, S1 nuclease mapping analysis revealed several gene 1 mRNA species, which are generated by the use of alternative polyadenylation sites and by the use of differentially regulated transcriptional initiation sites.

Cloning of human hepatic nuclear factor 1 (HNF1) and chromosomal localization of its gene in man and mouse

HNF1 is a transcription factor that is required for hepatocyte-specific expression of several genes, including albumin and fibrinogen. Rat HNF1-encoding cDNAs have recently been cloned, revealing that this factor is a distant member of the homeoprotein family. We have now isolated HNF1 clones from a human liver cDNA library by using a rat HNF1 cDNA-derived probe. The longest clone, HCL20, contains a sequence corresponding to the intact rat HNF1-coding region followed by a 3' nontranslated region and a poly(A) tail, hence representing an almost full-length HNF1 cDNA. Alignment of the human and rat sequences shows that HNF1 is highly conserved between the two species. The HNF1 gene was mapped by in situ hybridization and by RFLP analysis of interspecific mouse backcrosses to chromosomes 12q24.3 and 5F in human and mouse, respectively, establishing a new segmental homology between these two chromosomes.

A myosin-like dimerization helix and an extra-large homeodomain are essential elements of the tripartite DNA binding structure of LFB1

The transcription activator LFB1 is a major determinant of hepatocyte-specific expression of many genes. To study the mechanisms underlying LFB1 transcriptional selectivity, we have initiated its biochemical characterization. By in vitro complementation assays we have defined two distinct regions required for high levels of transcription, which resemble previously described activation domains. In contrast, the region of LFB1 necessary for DNA binding displays several novel features. The DNA binding domain is tripartite, including a homeodomain of unusual length (81 amino acids) and an N-terminal helix similar to part of myosin. This helical region mediates dimerization, which is shown to be essential for DNA binding.