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Albert W. Tam

Bio: Albert W. Tam is an academic researcher from Government of the United States of America. The author has contributed to research in topics: Hepatitis E virus & Virus. The author has an hindex of 16, co-authored 35 publications receiving 2049 citations.

Papers
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Journal ArticleDOI
01 Nov 1991-Virology
TL;DR: Findings on the genetic organization and expression strategy of HEV suggest that it is the prototype human pathogen for a new class of RNA virus or perhaps a separate genus within the Caliciviridae family.

1,016 citations

Journal ArticleDOI
01 Dec 1992-Virology
TL;DR: Knowing the extent of the sequence variation encountered with HEV will not only aid in the future development of diagnostic and vaccine reagents but also further the understanding of how HEV strain variation might impact the pathological outcome of infection.

326 citations

Journal ArticleDOI
TL;DR: Data support the hypothesis that activation of apoptosis is at least one essential step in the TNF lytic pathway in the U937 model system.
Abstract: The hypothesis that activation of apoptosis and DNA fragmentation is involved in TNF-mediated cytolysis of U937 tumor cells was investigated. Morphological, biochemical, and kinetic criteria established that TNF activates apoptosis as opposed to necrosis. Within 2-3 h of exposure to TNF, U937 underwent the morphological alterations characteristic of apoptosis. This was accompanied by cleavage of DNA into multiples of nucleosome size fragments. Both of these events occurred 1-2 h prior to cell death as defined by trypan blue exclusion or 51Cr release. DNA fragmentation was not a non-specific result of cell death since U937 cells lysed under hypotonic conditions did not release DNA fragments. The percentage of cells undergoing apoptosis depended on the concentration of TNF and was augmented by the addition of cycloheximide. A TNF-resistant variant derived from U937 did not undergo apoptosis in response to TNF, even in the presence of cycloheximide. Furthermore, TNF could still activate NFkB in this variant, suggesting that this pathway is not involved in TNF-mediated cytotoxicity. Two agents known to inhibit TNF-mediated cytotoxicity, ZnSO4 and 3-aminobenzamide, were shown to inhibit TNF-induced apoptosis. Taken altogether, these data support the hypothesis that activation of apoptosis is at least one essential step in the TNF lytic pathway in the U937 model system.

151 citations

Journal ArticleDOI
TL;DR: A Western blot assay which uses a baculovirus-expressed ORF2 protein is both sensitive and specific for diagnosing acute hepatitis E.
Abstract: : The full-length putative structural proteins encoded by open reading frame 2 (ORF2) and ORF3 of hepatitis E virus have been cloned and expressed in recombinant baculovirus. Sera obtained from 28 Sudanese pediatric patients with acute hepatitis and 19 pediatric control patients were analyzed for reactivity to hepatitis E virus by using the baculovirus-expressed ORF2 and ORF3 proteins in a Western blot (immunoblot) format. Seventeen of the 18 patients classified as having non-A, non-B hepatitis, without acute antibody markers for hepatitis A, B, or C viruses, Epstein-Barr virus, or cytomegalovirus, were shown to have immunoglobulin M (IgM) antibodies to the recombinant ORF2 protein, as did two patients with chronic hepatitis B, three of seven patients with acute hepatitis A, and one patient with acute hepatitis B. None of the 19 control patients had IgM antibodies against the ORF2 or ORF3 proteins. The Western blot assay using the baculovirus-expressed ORF3 protein did not appear to be as sensitive as the assay based on the ORF2 protein. Only 10 of the patients classified as having non-A, non-B hepatitis had IgM antibodies to the baculovirus-expressed ORF3 protein. We conclude that a Western blot assay which uses a baculovirus- expressed ORF2 protein is both sensitive and specific for diagnosing acute hepatitis E.

76 citations

Journal ArticleDOI
01 Jan 1996-Virology
TL;DR: The capacity of this hepatocyte culture system to replicate HEV in vitro is demonstrated, thus providing an experimental means to study the replicative process of the virus.

73 citations


Cited by
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Journal ArticleDOI
15 Apr 1994-Cancer
TL;DR: C‐myc expression has been shown to be involved in the initiation of apoptosis in some situations, and bcl‐2 has emerged as a new type of proto‐oncogene that inhibits apoptosis, rather than stimulating mitosis.
Abstract: Apoptosis is a distinct mode of cell death that is responsible for deletion of cells in normal tissues; it also occurs in specific pathologic contexts. Morphologically, it involves rapid condensation and budding of the cell, with the formation of membrane-enclosed apoptotic bodies containing well-preserved organelles, which are phagocytosed and digested by nearby resident cells. There is no associated inflammation. A characteristic biochemical feature of the process is double-strand cleavage of nuclear DNA at the linker regions between nucleosomes leading to the production of oligonucleosomal fragments. In many, although not all of the circumstances in which apoptosis occurs, it is suppressed by inhibitors of messenger RNA and protein synthesis. Apoptosis occurs spontaneously in malignant tumors, often markedly retarding their growth, and it is increased in tumors responding to irradiation, cytotoxic chemotherapy, heating and hormone ablation. However, much of the current interest in the process stems from the discovery that it can be regulated by certain proto-oncogenes and the p53 tumor suppressor gene. Thus, c-myc expression has been shown to be involved in the initiation of apoptosis in some situations, and bcl-2 has emerged as a new type of proto-oncogene that inhibits apoptosis, rather than stimulating mitosis. In p53-negative tumor-derived cell lines transfected with wild-type p53, induction of the gene has, in rare cases, been found to cause extensive apoptosis, instead of growth arrest. Finally, the demonstration that antibodies against a cell-surface protein designated APO-1 or Fas can enhance apoptosis in some human lymphoid cell lines may have therapeutic implications.

2,157 citations

Journal ArticleDOI
19 Mar 1993-Science
TL;DR: The effects of C2-ceramide on DNA fragmentation were prevented by the protein kinase C activator phorbol 12-myristate 13-acetate, which suggests the existence of two opposing intracellular pathways in the regulation of apoptosis.
Abstract: Sphingomyelin hydrolysis and ceramide generation have been implicated in a signal transduction pathway that mediates the effects of tumor necrosis factor-alpha (TNF-alpha) and other agents on cell growth and differentiation In many leukemic cells, TNF-alpha causes DNA fragmentation, which leads to programmed cell death (apoptosis) C2-ceramide (06 to 5 microM), a synthetic cell-permeable ceramide analog, induced internucleosomal DNA fragmentation, which was inhibited by zinc ion Other amphiphilic lipids failed to induce apoptosis The closely related C2-dihydroceramide was also ineffective, which suggests a critical role for the sphingolipid double bond The effects of C2-ceramide on DNA fragmentation were prevented by the protein kinase C activator phorbol 12-myristate 13-acetate, which suggests the existence of two opposing intracellular pathways in the regulation of apoptosis

1,735 citations

Journal ArticleDOI
TL;DR: Assessment of mitochondrial ROS generation provides an accurate picture of PCD-mediated lymphocyte depletion and indicates alterations of mitochondrial function constitute an important feature of early PCD.
Abstract: Programmed cell death (PCD) is a physiological process commonly defined by alterations in nuclear morphology (apoptosis) and/or characteristic stepwise degradation of chromosomal DNA occurring before cytolysis. However, determined characteristics of PCD such as loss in mitochondrial reductase activity or cytolysis can be induced in enucleated cells, indicating cytoplasmic PCD control. Here we report a sequential disregulation of mitochondrial function that precedes cell shrinkage and nuclear fragmentation. A first cyclosporin A-inhibitable step of ongoing PCD is characterized by a reduction of mitochondrial transmembrane potential, as determined by specific fluorochromes (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolcarbocyanine++ + iodide; 3,3'dihexyloxacarbocyanine iodide). Cytofluorometrically purified cells with reduced mitochondrial transmembrane potential are initially incapable of oxidizing hydroethidine (HE) into ethidium. Upon short-term in vitro culture, such cells acquire the capacity of HE oxidation, thus revealing a second step of PCD marked by mitochondrial generation of reactive oxygen species (ROS). This step can be selectively inhibited by rotenone and ruthenium red yet is not affected by cyclosporin A. Finally, cells reduce their volume, a step that is delayed by radical scavengers, indicating the implication of ROS in the apoptotic process. This sequence of alterations accompanying early PCD is found in very different models of apoptosis induction: glucocorticoid-induced death of lymphocytes, activation-induced PCD of T cell hybridomas, and tumor necrosis factor-induced death of U937 cells. Transfection with the antiapoptotic protooncogene Bcl-2 simultaneously inhibits mitochondrial alterations and apoptotic cell death triggered by steroids or ceramide. In vivo injection of fluorochromes such as 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolcarbocyanine iodide; 3,3'dihexyloxacarbocyanine iodide; or HE allows for the detection of cells that are programmed for death but still lack nuclear DNA fragmentation. In particular, assessment of mitochondrial ROS generation provides an accurate picture of PCD-mediated lymphocyte depletion. In conclusion, alterations of mitochondrial function constitute an important feature of early PCD.

1,561 citations

Journal ArticleDOI
TL;DR: It is hypothesized that all positive-strand RNA viruses and some related double-stranded RNA viruses could have evolved from a common ancestor virus that contained genes for RNA-dependent RNA polymerase, a chymotrypsin-related protease that also functioned as the capsid protein, and possibly an RNA helicase.
Abstract: Despite the rapid mutational change that is typical of positive-strand RNA viruses, enzymes mediating the replication and expression of virus genomes contain arrays of conserved sequence motifs. Proteins with such motifs include RNA-dependent RNA polymerase, putative RNA helicase, chymotrypsin-like and papain-like proteases, and methyltransferases. The genes for these proteins form partially conserved modules in large subsets of viruses. A concept of the virus genome as a relatively evolutionarily stable "core" of housekeeping genes accompanied by a much more flexible "shell" consisting mostly of genes coding for virion components and various accessory proteins is discussed. Shuffling of the "shell" genes including genome reorganization and recombination between remote groups of viruses is considered to be one of the major factors of virus evolution. Multiple alignments for the conserved viral proteins were constructed and used to generate the respective phylogenetic trees. Based primarily on the tentative phylogeny for the RNA-dependent RNA polymerase, which is the only universally conserved protein of positive-strand RNA viruses, three large classes of viruses, each consisting of distinct smaller divisions, were delineated. A strong correlation was observed between this grouping and the tentative phylogenies for the other conserved proteins as well as the arrangement of genes encoding these proteins in the virus genome. A comparable correlation with the polymerase phylogeny was not found for genes encoding virion components or for genome expression strategies. It is surmised that several types of arrangement of the "shell" genes as well as basic mechanisms of expression could have evolved independently in different evolutionary lineages. The grouping revealed by phylogenetic analysis may provide the basis for revision of virus classification, and phylogenetic taxonomy of positive-strand RNA viruses is outlined. Some of the phylogenetically derived divisions of positive-strand RNA viruses also include double-stranded RNA viruses, indicating that in certain cases the type of genome nucleic acid may not be a reliable taxonomic criterion for viruses. Hypothetical evolutionary scenarios for positive-strand RNA viruses are proposed. It is hypothesized that all positive-strand RNA viruses and some related double-stranded RNA viruses could have evolved from a common ancestor virus that contained genes for RNA-dependent RNA polymerase, a chymotrypsin-related protease that also functioned as the capsid protein, and possibly an RNA helicase.

1,107 citations