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Biomedical subjects

Yosef Shaul

Publications and source records attributed to Yosef Shaul.

At least 19 recordsLinked to original sources
PubMed2006

PGC-1alpha controls hepatitis B virus through nutritional signals.

Hepatitis B virus (HBV) is a 3.2-kb DNA virus that replicates preferentially in the liver. Liver-enriched nuclear receptors (NRs) play a major role in the HBV life cycle, operating as essential transcription factors for viral gene expression. Notably, these NRs are also key players in metabolic processes that occur in the liver, serving as central transcription factors for key enzymes of gluconeogenesis, fatty acid beta-oxidation, and ketogenesis. However, the association between these metabolic events and HBV gene expression is poorly understood. Here we show that peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PGC-1alpha), a major metabolic regulator and a coactivator of key gluconeogenic genes, robustly coactivates HBV transcription. We further demonstrate that the liver-enriched NR hepatocyte nuclear factor 4alpha that binds HBV plays an important role in this process. Physiologically, we show that a short-term fast that turns on the gluconeogenic program robustly induces HBV gene expression in vivo. This induction is completely reversible by refeeding and depends on PGC-1alpha. We conclude that HBV is tightly regulated by changes in the body's nutritional state through the metabolic regulator PGC-1alpha. Our data provide evidence for nutrition signaling to control viral gene expression and life cycle and thus ascribe to metabolism an important role in virus-host interaction.

Animal Feed↗
PubMed2006

The crystal structure of NAD(P)H quinone oxidoreductase 1 in complex with its potent inhibitor dicoumarol.

NAD(P)H quinone oxidoreductase 1 (NQO1) is a ubiquitous flavoenzyme that catalyzes two-electron reduction of quinones to hydroquinones utilizing NAD(P)H as an electron donor. NQO1 binds and stabilizes several short-lived proteins including the tumor suppressors p53 and p73 and the enzyme ornithine decarboxylase (ODC). Dicoumarol is a widely used potent competitive inhibitor of NQO1 enzymatic activity, which competes with NAD(P)H for binding to NQO1. Dicoumarol also disrupts the binding of NQO1 to p53, p73, and ODC and induces their ubiquitin-independent proteasomal degradation. We report here the crystal structure of human NQO1 in complex with dicoumarol at 2.75 A resolution. We have identified the interactions of dicoumarol with the different residues of NQO1 and the conformational changes imposed upon dicoumarol binding. The most prominent conformational changes that occur in the presence of dicoumarol involve Tyr 128 and Phe 232 that are present on the surface of the NQO1 catalytic pocket. On the basis of the comparison of the NQO1 structure in complex with different NQO1 inhibitors and our previous analysis of NQO1 mutants, we propose that the specific conformation of Tyr 128 and Phe 232 is important for NQO1 interaction with p53 and other client proteins.

Animals↗
PubMed2006

Clathrin-mediated endocytosis and lysosomal cleavage of hepatitis B virus capsid-like core particles.

The hepatitis B virus (HBV) core particle serves as a protective capsid shell for the viral genome and is highly immunogenic. Recombinant capsid-like core particles are used as effective carriers of foreign T and B cell epitopes and as delivery vehicles for oligonucleotides. The core monomer contains an arginine-rich C terminus that directs core particle attachment to cells via membrane heparan sulfate proteoglycans. Here we investigated the mechanism of recombinant core particle uptake and its intracellular fate following heparan sulfate binding. We found that the core particles are internalized in an energy-dependent manner. Core particle uptake is inhibited by chlorpromazine and by cytosol acidification known to block clathrin-mediated endocytosis but not by nystatin, which blocks lipid raft endocytosis. Particle uptake is abolished by expression of dominant negative forms of eps15 and Rab5, adaptors involved in clathrin-mediated endocytosis and early endosome transport, respectively. Endocytosed particles are transported to lysosomes where the core monomer is endoproteolytically cleaved into its distinct domains. Using protease inhibitors, cathepsin B was identified as the enzyme responsible for core monomer cleavage. Finally we found that monomer cleavage promotes particle dissociation within cells. Together, our results show that HBV capsid-like core particles are internalized through clathrin-mediated endocytosis, leading to lysosomal cleavage of the core monomer and particle dissociation.

Animals↗
PubMed2006

20S proteasomes and protein degradation "by default".

The degradation of the majority of cellular proteins is mediated by the proteasomes. Ubiquitin-dependent proteasomal protein degradation is executed by a number of enzymes that interact to modify the substrates prior to their engagement with the 26S proteasomes. Alternatively, certain proteins are inherently unstable and undergo "default" degradation by the 20S proteasomes. Puzzlingly, proteins are by large subjected to both degradation pathways. Proteins with unstructured regions have been found to be substrates of the 20S proteasomes in vitro and, therefore, unstructured regions may serve as signals for protein degradation "by default" in the cell. The literature is loaded with examples where engagement of a protein into larger complexes increases protein stability, possibly by escaping degradation "by default". Our model suggests that formation of protein complexes masks the unstructured regions, making them inaccessible to the 20S proteasomes. This model not only provides molecular explanations for a recent theoretical "cooperative stability" principle, but also provokes new predictions and explanations in the field of protein regulation and functionality.

Animals↗
PubMed2006

RNA interference for antiviral therapy.

Silencing gene expression through a process known as RNA interference (RNAi) has been known in the plant world for many years. In recent years, knowledge of the prevalence of RNAi and the mechanism of gene silencing through RNAi has started to unfold. It is now believed that RNAi serves in part as an innate response against invading viral pathogens and, indeed, counter silencing mechanisms aimed at neutralizing RNAi have been found in various viral pathogens. During the past few years, it has been demonstrated that RNAi, induced by specifically designed double-stranded RNA (dsRNA) molecules, can silence gene expression of human viral pathogens both in acute and chronic viral infections. Furthermore, it is now apparent that in in vitro and in some in vivo models, the prospects for this technology in developing therapeutic applications are robust. However, many key questions and obstacles in the translation of RNAi into a potential therapeutic platform still remain, including the specificity and longevity of the silencing effect, and, most importantly, the delivery of the dsRNA that induces the system. It is expected that for the specific examples in which the delivery issue could be circumvented or resolved, RNAi may hold promise for the development of gene-specific therapeutics.

Animals↗
PubMed2006

Ubiquitin-independent degradation: lessons from the p53 model.

Ubiquitin-proteasome degradation is a key cellular process involved in almost every aspect of cell life. According to the current concept, proteins are stable unless they are marked by poly-ubiquitination for degradation by the 26S proteasomes. A new twist in the concept became evident while studying the degradation of the tumor suppressor p53, a protein that appeared to satisfy this principle. We have discovered that native p53 is also prone to ubiquitin-independent 20S proteasomal degradation, suggesting that certain proteins are inherently unstable. We further found that this process of degradation is mediated by 20S proteasomes and inhibited by NADH quinone oxidoreductase 1. Our recent findings together with previous observations of ubiquitin-independent degradation suggest the existence of ubiquitin-independent mechanisms for proteasomal protein degradation in the cells.

Humans↗
PubMed2005

Mechanisms of protein degradation: an odyssey with ODC.

Intracellular proteolysis plays an important role in regulating fundamental cellular processes such as cell cycle, immune and inflammation responses, development, differentiation, and transformation. The ubiquitin-proteasome system accounts for the degradation of the majority of cellular short-lived proteins. This system involves the conjugation of multiple ubiquitin residues to the target protein and its recognition by the 26S proteasome through the poly-ubiquitin chain. Studies on the degradation of ornithine decarboxylase (ODC) demonstrated that poly-ubiquitin is not the only signal recognized by the 26S proteasome. The recognition of ODC by the 26S proteasome is mediated by interaction with a polyamine-induced protein termed, antizyme (Az). While the degradation of ODC is ubiquitin-independent, the degradation of its regulator Az, and of antizyme-inhibitor (AzI), an ODC homologous protein that regulates Az availability, are ubiquitin dependent. Interestingly, ODC undergoes another type of ubiquitin-independent degradation by the 20S proteasome that is regulated by NAD(P)H quinone oxidoreductase 1 (NQO1). Considering the prevalence of the ubiquitin system in the process of cellular protein degradation it is rather remarkable that a key cellular enzyme is subjected to two different proteolytic pathways that are different from the ubiquitin dependent one. This exceptional behavior of ODC provides us with valuable insights regarding protein degradation in general.

Cell Proliferation↗
PubMed2005

Cytokine induction by the hepatitis B virus capsid in macrophages is facilitated by membrane heparan sulfate and involves TLR2.

The hepatitis B virus (HBV) core Ag (HBcAg) serves as the structural subunit of the highly immunogenic capsid shell. HBcAg harbors a unique arginine-rich C terminus that was implicated in immune responses induced by the capsid. In this study, we examined the capacity of the HBV capsid to induce proinflammatory and regulatory cytokines in human THP-1 macrophages and the possible underlying mechanism. Full-length HBc capsids, but not HBc-144 capsids lacking the arginine-rich domain of HBcAg, efficiently bound differentiated THP-1 macrophages and strongly induced TNF-alpha, IL-6, and IL-12p40. Capsid binding to macrophages and cytokine induction were independent of the RNA associated with the arginine-rich domain. Soluble heparin and heparan sulfate but not chondroitin sulfates greatly diminished cytokine induction through inhibition of capsid binding to THP-1 macrophages. Furthermore, serine phosphorylation in the arginine-rich domain modulates capsid binding to macrophages and the cytokine response. Induction of cytokines by the capsid involved activation of NF-kappaB, ERK-1/2, and p38 MAPK and did not require endosomal acidification. Finally, NF-kappaB activation by the capsid in HEK 293 cells specifically required expression of TLR2 and was compromised by soluble heparin. Thus, cytokine induction by the HBV capsid in macrophages is facilitated by interaction of its arginine-rich domain with membrane heparan sulfate and involves signaling through TLR2.

Animals↗
PubMed2005

p53 proteasomal degradation: poly-ubiquitination is not the whole story.

Protein degradation is a key cellular process involved in almost every aspect of the living cell. The current prevailing concept is that proteins are stable unless marked by poly-ubiquitination for degradation by the proteasomes. Studies on the tumor suppressor p53 have indeed demonstrated that poly-ubiquitination of p53 by different E3 ubiquin ligases targets p53 for degradation by the 26S proteasomes. Recent findings suggest that p53 also undergoes ubiquitin-independent degradation by the 20S proteasomes and that this process is regulated by NAD(P)H quinone oxidoreductase 1 (NQO1) together with NADH. This "degradation by default" mechanism sheds new light on our understanding of p53 degradation and possibly on protein degradation in general and may establish a new principle in protein stability with wide physiological implications.

Cell Line, Tumor↗
PubMed2005

Inhibition of NAD(P)H:quinone oxidoreductase 1 activity and induction of p53 degradation by the natural phenolic compound curcumin.

NAD(P)H:quinone oxidoreductase 1 (NQO1) regulates the stability of the tumor suppressor WT p53. NQO1 binds and stabilizes WT p53, whereas NQO1 inhibitors including dicoumarol and various other coumarins and flavones induce ubiquitin-independent proteasomal p53 degradation and thus inhibit p53-induced apoptosis. Here, we show that curcumin, a natural phenolic compound found in the spice turmeric, induced ubiquitin-independent degradation of WT p53 and inhibited p53-induced apoptosis in normal thymocytes and myeloid leukemic cells. Like dicoumarol, curcumin inhibited the activity of recombinant NQO1 in vitro, inhibited the activity of endogenous cellular NQO1 in vivo, and dissociated NQO1-WT p53 complexes. Neither dicoumarol nor curcumin dissociated the complexes of NQO1 and the human cancer hot-spot p53 R273H mutant and therefore did not induce degradation of this mutant. NQO1 knockdown by small-interfering RNA induced degradation of both WT p53 and the p53 R273H mutant. The results indicate that curcumin induces p53 degradation and inhibits p53-induced apoptosis by an NQO1-dependent pathway.

Animals↗
PubMed2005

20S proteasomal degradation of ornithine decarboxylase is regulated by NQO1.

Ornithine decarboxylase (ODC), a key enzyme in the biosynthesis of polyamines, is a very labile protein. ODC is a homodimeric enzyme that undergoes ubiquitin-independent proteasomal degradation via direct interaction with antizyme, a polyamine-induced protein. Binding of antizyme promotes the dissociation of ODC homodimers and marks ODC for degradation by the 26S proteasomes. We describe here an alternative pathway for ODC degradation that is regulated by NAD(P)H quinone oxidoreductase 1 (NQO1). We show that NQO1 binds and stabilizes ODC. Dicoumarol, an inhibitor of NQO1, dissociates ODC-NQO1 interaction and enhances ubiquitin-independent ODC proteasomal degradation. We further show that dicoumarol sensitizes ODC monomers to proteasomal degradation in an antizyme-independent manner. This process of NQO1-regulated ODC degradation was recapitulated in vitro by using purified 20S proteasomes. Finally, we show that the regulation of ODC stability by NQO1 is especially prominent under oxidative stress. Our findings assign to NQO1 a role in regulating ubiquitin-independent degradation of ODC by the 20S proteasomes.

Animals↗
PubMed2005

Recombinant viral capsids as an efficient vehicle of oligonucleotide delivery into cells.

Delivery of oligonucleotides (ON) into cells is a technical challenge. In this study, we utilized the capsid of the hepatitis B virus (HBV) to meet this goal. A single and short open reading frame of the virus programs efficient capsid production in bacteria. We show that these capsids can encapsulate ON in vitro and then mediate their delivery into cells with extreme efficiency. This process is cell type non-specific, rendering the recombinant HBV capsid a potentially valuable vehicle for ON delivery into a wide range of cultured cells.

Amino Acid Sequence↗
PubMed2005

A mechanism of ubiquitin-independent proteasomal degradation of the tumor suppressors p53 and p73.

Protein degradation is an essential and highly regulated process. The proteasomal degradation of the tumor suppressors p53 and p73 is regulated by both polyubiquitination and by an ubiquitin-independent process. Here, we show that this ubiquitin-independent process is mediated by the 20S proteasomes and is regulated by NQO1. NQO1 physically interacts with p53 and p73 in an NADH-dependent manner and protects them from 20S proteasomal degradation. Remarkably, the vast majority of NQO1 in cells is found in physical association with the 20S proteasomes, suggesting that NQO1 functions as a gatekeeper of the 20S proteasomes. We further show that this pathway plays a role in p53 accumulation in response to ionizing radiation. Our findings provide the first evidence for in vivo degradation of p53 and p73 by the 20S proteasomes and its regulation by NQO1 and NADH level.

Animals↗
PubMed2005

Role of c-Abl in the DNA damage stress response.

c-Abl has been implicated in many cellular processes including differentiation, division, adhesion, death, and stress response. c-Abl is a latent tyrosine kinase that becomes activated in response to numerous extra- and intra-cellular stimuli. Here we briefly review the current knowledge about c-Abl involvement in the DNA-damage stress response and its implication on cell physiology.

Animals↗
PubMed2005

Autorepression of rfx1 gene expression: functional conservation from yeast to humans in response to DNA replication arrest.

The yeast Saccharomyces cerevisiae Crt1 transcription repressor is an effector of the DNA damage and replication checkpoint pathway. Crt1 binds and represses genes encoding ribonucleotide reductase (RNR) and its own promoter, establishing a negative-feedback pathway. The role of Rfx1, the mammalian Crt1 homologue, remained uncertain. In this study we investigated the possibility that Rfx1 plays a similar function in animal cells. We show here that, like Crt1, Rfx1 binds and represses its own promoter. Furthermore, Rfx1 binding to its promoter is reduced upon induction of a DNA replication block by hydroxyurea, which led to a release of repression. Significantly, like Crt1, Rfx1 binds and represses the RNR-R2 gene. Upon blocking replication and UV treatment, expression of both Rfx1 and RNR-R2 is induced; however, unlike the results seen with the RNR-R2 gene, the derepression of the RFX1 gene is only partially blocked by inhibiting Chk1, the DNA checkpoint kinase. This report provides evidence for a common mechanism for Crt1 and Rfx1 expression and for the conservation of their mode of action in response to a DNA replication block. We suggest that Rfx1 plays a role in the DNA damage response by down-regulating a subset of genes whose expression is increased in response to replication blocking and UV-induced DNA damage.

Cell Cycle↗
PubMed2004

Direct interaction of NRSF with TBP: chromatin reorganization and core promoter repression for neuron-specific gene transcription.

Neural restrictive silencer factor, NRSF (also known as REST) binds a neuronal cell type selective silencer element to mediate transcriptional repression of neuron-specific genes in non-neuronal cells and neuronal progenitors. Two repression domains (RD-1 and RD-2) occur in its N-terminal and C-terminal regions, respectively. RD-1 recruits mSin3 and HDAC, thereby inhibiting transcription by inducing reorganization of the chromatin structure. However, little is known about how such global repression becomes promoter-specific repression or whether the NRSF-HDAC complex can interact with transcriptional core factors at each specific promoter. Here we show evidence that NRSF interacts with core promoter factors, including TATA-binding protein (TBP). The NRSF-TBP interaction occurred between the linear segments of the N- and C-terminal-most portions of NRSF and the C-terminal half of TBP. A RD-2 mutant of NRSF lost the TBP-binding activity and was unable to repress transcription at an exogenously introduced TGTA promoter. These results indicate that the direct interaction between the NRSF C-terminal domain and TBP is essential for the C-terminal repression mechanism of NRSF. Thus, the RD-1 and RD-2 repression domains of NRSF utilize both chromatin-dependent and chromatin-independent mechanisms, which may be segregated at various stages of neural development and modulation.

Binding Sites↗
PubMed2004

RNA interference--small RNAs effectively fight viral hepatitis.

RNA interference (RNAi) is the process of sequence-specific gene silencing, initiated by double-stranded RNA that is homologous in sequence to the target gene. This unique phenomenon has been extensively investigated during the last few years not only in the context of its mechanism and its possible role in the regulation of gene expression and cell function, but also as a potential powerful tool for gene therapy. Targeting essential viral genes or oncogenic alleles are only some of the possible applications of RNAi in the field of gene-directed therapy. This review covers the potential use of RNAi against hepatitis B and hepatitis C viruses, the main pathogens causing chronic liver disease. The major milestones along the discovery of RNAi will also be covered.

Animals↗