Biomedical subjects

Y Shaul

Publications and source records attributed to Y Shaul.

At least 19 recordsLinked to original sources

PubMed1992

c-abl has a sequence-specific enhancer binding activity.

The enhancers of several distinct viruses contain a common functional element, termed EP. This element binds ubiquitous cellular proteins and generates specific complexes in gel retardation analysis. Ultraviolet cross-linking and Southwestern analysis showed that a 140 kd polypeptide is the major EP DNA-binding protein. Using a combination of DNA binding and immunological techniques, we have identified the c-abl protein in a nuclear complex that binds to the EP element. abl was found to have both a specific and high affinity DNA binding activity. The ability to bind DNA is abolished in the mutant abl protein, p210bcr-abl, consistent with its cytoplasmic localization in chronic myelogenous leukemia.

Base Sequence↗

PubMed1991

Structure and function of human jun-D.

A jun related cDNA and its corresponding genomic fragment were cloned from human cells and sequenced. Polymerase chain reaction analysis showed that this gene is the human homologue of the mouse jun-D gene despite the fact that the degree of amino acid sequence conservation between the two is much poorer (77.3%) than that found between the homologues of c-jun and jun-B (95-98%). The product of this gene binds an AP-1 site and upon cotransfection stimulates the activity of a promoter that bears an AP-1 site. The level of activation is comparable to that of v-jun and the activity of both is further stimulated by v-fos. Deletion mutants of the gene that lack the best conserved region in the activating domain are poorly active. However, our data suggest that the activating domain is not confined exclusively to the conserved regions. Interestingly, at high concentrations human jun-D displays decreased activity which cannot be explained by a simple self squelching model.

Amino Acid Sequence↗

PubMed1990

The X protein of the hepatitis B virus acts as a transcription factor when targeted to its responsive element.

The X protein of hepatitis B virus (HBV) stimulates transcription of a large number of viral enhancers. This protein augments the activity of the HBV enhancer through a specific cis element, termed X responsive element (XRE). Multimers of XRE exhibit enhancer activity which is further stimulated by X. XRE binds multiple cellular transcription factors one of which is the C/EBP. We have constructed the DB gene containing the DNA-binding domain of the C/EBP. This gene efficiently represses the enhancer activity of the XRE by competitive displacement of the XRE-binding factors. Under these conditions, X was found to have only a partially stimulatory effect on transcription, suggesting that the XRE-binding proteins are required for the activity of X. In contrast, an X-DB hybrid protein that binds to the XRE is a strong transcription factor and acts without additional XRE-binding proteins. Furthermore, studies of X mutants revealed that the carboxy-terminus of the protein is required for this activation. These data show that X directly stimulates the cellular transcription machinery, possibly by protein-protein interaction with the XRE-binding factors.

Amino Acid Sequence↗

PubMed1990

A single element within the hepatitis B virus enhancer binds multiple proteins and responds to multiple stimuli.

The hepatitis B virus enhancer can be dissected into multiple functional elements, one of which is the E element. We show here that the E element binds multiple nuclear proteins that are essential for its enhancer activity. These findings, together with the ability of this element to respond to at least two different viral transactivators, suggest that the E element is an enhancer modulator capable of binding different factors and responding to multiple stimuli.

Animals↗

PubMed1990

Functional organization of the hepatitis B virus enhancer.

We have studied the functional constituents of the hepatitis B virus enhancer in a number of cell lines. The sequence of this enhancer, being embedded within an open reading frame of the virus, is in part evolutionarily frozen and therefore serves as a good model to investigate the fundamental enhancer elements. The hepatitis B virus enhancer contains three functionally important DNA sequence elements, EP, E, and NF-1a, each of which is bound by a distinct protein(s). The synergistic action of these elements accounts for all of the enhancer activity in a nonliver cell line and for most, but not all, of the activity in liver-derived cell lines. Multimers of the E but not of the EP element act as an autonomous enhancer. Conversely, a single element of either the E or the NF-1a element can act only when linked to the EP element. These results suggest that EP is a crucial enhancer element that acts only in interaction with a second enhancer element with intrinsic enhancer activity. Interestingly, a highly similar enhancer structure is found in a number of distinct viruses.

Animals↗

PubMed1990

Hierarchic and cooperative binding of the rat liver nuclear protein C/EBP at the hepatitis B virus enhancer.

We used the enhancer-binding protein C/EBP as a model to study the nature and the complexity of interaction of an enhancer-binding protein with its target DNA. We found that bacterially expressed C/EBP binds the hepatitis B virus enhancer at multiple sites in a hierarchic and cooperative manner. At low concentrations, only the E element is occupied, but at higher concentrations, additional sites are filled including a site that binds EP, a crucial enhancer-activating protein. This pattern of C/EBP binding may explain the concentration-dependent effect of C/EBP on enhancer activity.

Animals↗

PubMed1990

The identification of hepatitis B virus X gene responsive elements reveals functional similarity of X and HTLV-I tax.

The human hepatitis B virus (HBV) X gene encodes a general transactivator which was suggested to be a potential factor in viral hepatocarcinogenesis. We show here that this protein transactivates the HBV enhancer linked either to the X gene promoter or heterologous promoters. Analysis of individual elements of the HBV enhancer revealed that the E element is sufficient to respond to X and is termed hence the X responsive element (XRE). Interestingly, XRE shares sequence similarity with the HTLV-I taxI responsive element (21 bp repeat or taxRE), and both elements bind similar nuclear proteins. The functional significance of this sequence similarity was demonstrated by the ability of XRE to respond to taxI. We also show that both X and taxI have the capacity to activate transcription through a second cis element, the NF-kappa B binding site. The response pattern of these viral regulators is also similar and both act in a concentration dependent manner. They are very active in low amounts, but almost inactive at high concentrations. Based on these observations, we suggest a common mechanism of action by regulator genes of distinct viruses.

Base Sequence↗

PubMed1989

Liver-specific expression of hepatitis B virus is determined by the combined action of the core gene promoter and the enhancer.

The hepatitis B virus (HBV) enhancer and the core gene promoter regulate the expression of the core and polymerase genes, as well as of the 3.5-kilobase pregenomic RNA. RNA analysis and chloramphenicol acetyltransferase gene expression by plasmids carrying the HBV enhancer linked to the heterologous beta-globin or simian virus 40 early promoter demonstrated that the HBV enhancer is 3- to 20-fold preferentially expressed in human liver cells. Core gene promoter activity was mapped to a 100-base-pair fragment which was shown to be sufficient for accurate initiation of transcription. The partial tissue specificity of this promoter was demonstrated by transient transfection into various cell lines with a plasmid containing the core gene promoter linked to the heterologous simian virus 40 enhancer. When the HBV core gene promoter was examined under the control of the HBV enhancer, there was high tissue specificity in that activity could be observed only in differentiated human liver cells. These results suggest that the strict tissue specificity of HBV gene expression is determined by the combinatorial action of these two elements.

Animals↗

PubMed1989

Cellular factors that interact with the hepatitis B virus enhancer.

An 83-base-pair-long hepatitis B virus DNA fragment efficiently activates the transcription of the heterologous globin gene promoter. This fragment contains binding sites for at least four distinct cellular factors termed E, TGT3, EP, and NF-I. E is a positively acting factor, responsive to phorbol ester. EP is apparently identical to the factor EF-C that binds to the polyomavirus enhancer. The conservation of the binding site sequences for most of these factors in the genomes of other members of the hepadnavirus family suggests that these viruses share common enhancer elements.

Base Sequence↗

PubMed1988

Regulation of hepatitis B virus S gene promoter in transfected cell lines.

Hepatitis B virus (HBV) contains an enhancer element that activates the viral core and X gene promoters. To investigate the transcriptional regulation of the viral S gene promoter, we transfected SK-Hep1 cells with circularized forms of HBV DNAs and their enhancerless mutants. We have found that expression of the S gene, determined by measurement of the appearance of HBsAg in the media and by RNA analysis, is to a large extent enhancer-dependent. This observation was further confirmed by analysis of a series of plasmids containing the chloramphenicol acetyl-transferase (CAT) gene under the control of the S gene promoter and the HBV enhancer element. Interestingly, in contrast to its behavior in SK-Hep1 cells, the S gene promoter is highly active in Alexander cells, in the absence of the enhancer element. This implies that activity of the S gene promoter is cell-type specific.

Acetyltransferases↗

PubMed1988

The S promoter of hepatitis B virus is regulated by positive and negative elements.

The S promoter, one of the major hepatitis B virus (HBV) promoters, directs the synthesis of mRNA for surface antigen. Transient expression studies revealed that this promoter is highly active in the Alexander hepatoma cell line but not in SK-Hep1 and HeLa cells. We found that a distal element of the promoter (-103 to -48) confers this cell-type-specific behavior through a mechanism in which the promoter activity is repressed in HeLa and SK-Hep1 cells but increased in Alexander cells. By using an inhibitor of protein synthesis, we obtained evidence that a labile repressor(s) confers the negative effect in SK-Hep1 cells. We also found an enhancerlike activity associated with a small DNA segment of the S promoter (-27 to + 30). This proximal element was active in HeLa and SK-Hep1 cells only in the absence of the distal negative element. Finally, analysis of S promoter deletion mutants demonstrated that the -27 to -17 region of the S promoter is crucial for its activity.

Acetyltransferases↗

PubMed1988

The glucocorticoid receptor recognizes a specific nucleotide sequence in hepatitis B virus DNA causing increased activity of the HBV enhancer.

The hepatitis B virus (HBV) genome contains a specific DNA binding site for the glucocorticoid receptor. Using DNase I footprinting, this binding site was localized at HBV map positions 341-370 clockwise from the EcoRI site. The DNA sequence protected in the footprint contains two tandem copies of the GRE core hexanucleotide 5'-TGTTCCT-3'. Deletion analysis and reconstruction experiments in plasmid expression vectors demonstrated that this glucocorticoid receptor binding sequence serves as a signal for augmenting glucocorticoid-dependent activity of the HBV enhancer, which is located approximately 730 nucleotides downstream in the HBV genome. Even though it does not serve as an independent enhancer element, the HBV glucocorticoid receptor domain can therefore be categorized as a functional GRE.

Base Sequence↗

PubMed1987

Integration of hepatitis B virus: analysis of unoccupied sites.

Hepatitis B virus (HBV) sequences integrated in the PLC/PRF/5 cell line (Alexander cells), which was derived from a human primary liver carcinoma, were previously extensively studied. Here we describe the analysis of the unoccupied sites of two linearly integrated forms of HBV DNA, AL-14 and AL-26, that were characterized previously. No major cellular DNA rearrangements were seen at the integration sites except for small deletions of host sequences: 2 kilobases of DNA in AL-14 and 17 base pairs (bp) in AL-26. The unoccupied site of AL-26 was found to be missing 182 bp, which previously mapped next to the right end of the integration sites of several independent clones. These were believed to be of cellular origin, but we show here that these 182 bp are in fact from unusual HBV sequences. Surprisingly, a region of this newly detected HBV DNA sequence is more homologous to that of woodchuck HBV DNA. Our analysis shows that the normal counterparts of both AL-14 and AL-26 contain minisatellite-like repetitive sequences. Based on the data presented here and our previous finding of HBV DNA integration at satellite sequences, we propose that genomic simple repetitive sequences are hot spots for HBV DNA integration.

Base Sequence↗

PubMed1986

High affinity binding site for nuclear factor I next to the hepatitis B virus S gene promoter.

The hepatitis B virus (HBV) surface antigen (HBsAG) is encoded by the S gene under the regulation of a promoter in the pre-S1 region. The S gene promoter does not contain a 'TATA' box-like sequence, but there is a sequence resembling, in part, the late promoter of Simian virus 40 (SV40). In an attempt to study the regulation of the S gene promoter we looked for cellular proteins which bind to this region. We report here that a nuclear protein is tightly bound to the HBV genome at a position approximately 190 bases upstream from the S gene promoter. Evidence is provided to show that (a) this nuclear protein is the nuclear factor I (NF-I) that was previously found to be bound to the inverted terminal repeat of the adenovirus (Ad) DNA and to enhance Ad DNA replication in vitro and (b) this NF-I binding site is required for optimal activity of the S gene promoter.

Acetyltransferases↗

PubMed1986

Hepatitis B virus DNA contains a glucocorticoid-responsive element.

It has recently been shown that hepatitis B virus (HBV) contains a transcriptional enhancer element. In order to determine whether this enhancer responds to glucocorticoids, a series of derivatives of plasmid pA10CAT2 was constructed containing the HBV enhancer and variable lengths of further upstream sequences. Transient expression of chloramphenicol acetyltransferase (CAT) was determined after introduction of these plasmids into PLC/PRF/5, Hep 3B, Hep G2, HeLa, and mouse L cells. Highest CAT activity was noted in the human hepatocellular carcinoma line PLC/PRF/5, which contains integrated HBV DNA sequences. Dexamethasone augmented CAT expression in all cell lines tested with 40% of maximal induction at 10 nM and maximum stimulation (3- to 8-fold) at 1 microM dexamethasone. Dexamethasone augmentation of CAT expression was observed only when constructs contained HBV DNA sequences residing upstream to map position 735 from the EcoRI site. This indicates that the glucocorticoid-responsive region is distinct from the previously defined HBV enhancer sequence located at map position 1080-1234. These studies suggest that HBV DNA contains a glucocorticoid-responsive element, which may mediate expression of HBV genes in infected mammalian cells.

Acetyltransferases↗

PubMed1986

Integration of hepatitis B virus DNA in chromosome-specific satellite sequences.

We previously reported the cloning and detailed analysis of the integrated hepatitis B virus sequences in a human hepatoma cell line. We report here the integration of at least one of hepatitis B virus at human satellite DNA sequences. The majority of the cellular sequences identified by this satellite DNA were organized as a multimeric composition of a 0.6-kilobase EcoRI fragment. This clone hybridized in situ almost exclusively to the centromeric heterochromatin of chromosomes 1 and 16 and to a lower extent to chromosome 2 and to the heterochromatic region of the Y chromosome. The immediate flanking host sequence appeared as a hierarchy of repeating units which were almost identical to a previously reported human satellite III DNA sequence.

Carcinoma, Hepatocellular↗

PubMed1985

Homologous recombination between a defective virus and a chromosomal sequence in mammalian cells.

Replacement of the early region of simian virus 40 results in virus that cannot replicate in a normal host, CV-1 cells, but can replicate in COS cells, a derivative of CV-1 cells that constitutively express simian virus 40 tumor antigen (T antigen). However, passage of such an early replacement simian virus 40 mutant in COS cells results in the emergence of virus that can propagate in CV-1 cells. Analysis of this virus revealed that the mutant rescued the integrated T-antigen gene from the COS cell genome. Comparison of the sequence of the recovered virus with that of the viral DNA resident in COS cells (strain 776) and the mutant used in our studies (derived from strain 777) proves that the mutant virus acquired the T-antigen gene from the COS cell chromosome via homologous recombination. Most probably this process was mediated by a direct genetic exchange.

Amino Acid Sequence↗

PubMed1985

Sequence of hepatitis B virus DNA incorporated into the genome of a human hepatoma cell line.

Seven copies of integrated hepatitis B virus (HBV) DNA and contiguous genomic DNA from a human hepatoma cell line (PLC/PRF/5) have been isolated by molecular cloning and have been partially sequenced. The HBV sequences are fragmented and rearranged. Thus, the surface antigen gene is the only intact HBV transcription unit present in these integrated sequences. The sites of integration-recombination are dispersed over the entire viral genome; there is some preference for integration within the double-stranded portion of the genome. There are no repeats at the ends of the integrated HBV DNA fragments. Thus, recombination does not take place in a manner resembling the integration of retroviruses. The sequence data suggest that each HBV fragment is of the adw subtype. However, the integrated DNAs show an unexpected degree of sequence divergence. Direct evidence for the duplication, transposition, and subsequent divergence of two sequences is presented. The data surprisingly suggest that infection-integration of four distinct adw strains occurred.

Base Sequence↗