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Biomedical subjects

X Zhao

Publications and source records attributed to X Zhao.

At least 487 records · Page 27Linked to original sources
PubMed1996

An in vivo cytokine and endotoxin-independent pathway for induction of nitric oxide synthase II mRNA, enzyme, and nitrate/nitrite in alveolar macrophages.

Dibutyryl cyclic AMP (DB-cAMP, 0.1 and 1.0 mg/kg) and the purine-2-receptor agonist methyl-thio-ATP (MT-ATP mg/kg) given by intratracheal (i.t.) administration to rats two hr before bronchoalveolar lavage (BAL) increased iNOS mRNA to be equal to or greater than that produced by i.t. LPS, without eliciting neutrophil infiltration into the alveolar space or the upregulation of tumor necrosis factor alpha (TNF alpha). Translation of DB-cAMP and MT-ATP stimulated iNOS mRNA into protein and activation of iNOS to produce RNI was slower than that resulting from LPS-stimulated iNOS mRNA. Diethyldithiocarbamate (5 mg/kg, i.t.) a sequestrant of reactive oxygen intermediates and an inhibitor of NFkappaB attenuated LPS-induced upregulation of iNOS mRNA without affecting that produced by DB-cAMP or MT-ATP. We conclude that an LPS and cytokine-independent pathway of transcription of iNOS mRNA exists in vivo, which can be directly activated by DB-cAMP and purine-2 receptor stimulation. It is possible that the increase in iNOS found in asthmatic patients and those with other diseases that are treated with drugs which affect the cAMP and purine systems may be iatrogenic rather than pathogenetic in origin.

Adenosine Triphosphate↗
PubMed1996

Mutations in the C-terminal fragment of DnaK affecting peptide binding.

Escherichia coli DnaK acts as a molecular chaperone through its ATP-regulated binding and release of polypeptide substrates. Overexpressing a C-terminal fragment (CTF) of DnaK (Gly-384 to Lys-638) containing the polypeptide substrate binding domain is lethal in wild-type E. coli. This dominant-negative phenotype may result from the nonproductive binding of CTF to cellular polypeptide targets of DnaK. Mutations affecting DnaK substrate binding were identified by selecting noncytotoxic CTF mutants followed by in vitro screening. The clustering of such mutations in the three-dimensional structure of CTF suggests the model that loops L1,2 and L4,5 form a rigid core structure critical for interactions with substrate.

Bacterial Proteins↗
PubMed1996

Analysis of a salt stable mutant of cowpea chlorotic mottle virus.

An understanding of virion assembly and disassembly requires a detailed understanding of the protein-protein and protein-nucleic acid interactions which stabilize the virion. We have characterized a mutant of cowpea chlorotic mottle virus (CCMV) that is altered in virion stability. The mutant virions resist disassembly in 1.0 M NaCl, pH 7.5, whereas the wild-type virions completely disassociate into RNA and capsid protein components. Sequence analysis of the mutant coat protein gene identified a single A to G nucleotide change at position 1484 of RNA 3 (position 134 of RNA 4), which results in a lysine to arginine change at position 42 of the coat protein. Introduction of the K42R mutation into wild-type CCMV coat protein results in a salt stable virion phenotype. Likewise, expression of the K42R mutant coat protein in Escherichia coli followed by in vitro assembly produces virions that exhibit the salt stable phenotype. Analysis of this mutation demonstrates how a single amino acid change in the primary structure of the coat protein leads to tertiary interactions which stabilize the virion.

Bromovirus↗
PubMed1996

Excitation transfer spectroscopy with two metastable 138Ba+ ions in same trap.

In the absence of lasers approaching trapped ion clock transitions in sharpness we propose to replace the 12.49 m laser field exciting the D3/2-D5/2 transition of the single Ba+ ion A in D3/2 with the near-field of a close by identical ion B in the excited D5/2 state. We tune the frequency of the near-field by the differential Stark shift generated when the center of mass of the tuned ions is slightly moved out of the trap center by a small bias voltage. We demonstrate that the resultant resonant energy exchange can be made considerably faster than the natural lifetime of either metastable level and show how it might be detected.

Journal Article↗
PubMed1996

Structural analysis of substrate binding by the molecular chaperone DnaK.

DnaK and other members of the 70-kilodalton heat-shock protein (hsp70) family promote protein folding, interaction, and translocation, both constitutively and in response to stress, by binding to unfolded polypeptide segments. These proteins have two functional units: a substrate-binding portion binds the polypeptide, and an adenosine triphosphatase portion facilitates substrate exchange. The crystal structure of a peptide complex with the substrate-binding unit of DnaK has now been determined at 2.0 angstroms resolution. The structure consists of a beta-sandwich subdomain followed by alpha-helical segments. The peptide is bound to DnaK in an extended conformation through a channel defined by loops from the beta sandwich. An alpha-helical domain stabilizes the complex, but does not contact the peptide directly. This domain is rotated in the molecules of a second crystal lattice, which suggests a model of conformation-dependent substrate binding that features a latch mechanism for maintaining long lifetime complexes.

Amino Acid Sequence↗
PubMed1996

Corticosterone promotes increased heme oxygenase-2 protein and transcript expression in the newborn rat brain.

Heme oxygenase-2 (HO-2) is the predominant heme oxygenase isozyme in neurons in the brain, the enzyme cleaves the heme molecule at the alpha-meso carbon bridge to form CO, Fe and biliverdin. Recently, in the promotor region of the HO-2 gene a consensus sequence of the glucocorticoid response element (GRE) has been identified. Presently, we have investigated the potential relevance of the GRE to the expression of the isozyme, at the transcript and protein levels, in the 14 day old rat brain, by examining the effect of postparturition corticosterone treatment (4 days, starting 24-36 h after birth) on the developmental pattern of HO-2 expression. Northern blot analysis showed that HO-2 transcripts (approximately 1.3 and approximately 1.9 kb) in brain increase with age. In many brain nuclei, HO-2 protein, as visualized by immunohistochemistry, was detected at low levels in neurons in the 14 day old rat brain. Postparturition exposure to corticosterone resulted in a marked enhancement of HO-2 immunoreactivity in several neuronal populations, including, among others, the cerebellum, the hippocampal formation, and the oculomotor and red nuclei. The response to elevated levels of corticosterone was particularly striking in the Purkinje neurons of the cerebellum and the CA3 region of the hippocampus. This was linked to an increase in gene transcription, as indicated by in situ hybridization analysis, which revealed an increase in the signal for HO-2 transcripts in these regions. Elevated levels of heme oxygenase activity and HO-2 protein were consistent with an increase in catalytically active protein expression. These data point to the intimate involvement of the adrenal steroids in developmentally-linked HO-2 expression in the neurons involved in motor function and cognition, and hence, identify a potentially important aspect of the adrenal steroids' effect on brain growth and differentiation.

Aging↗
PubMed1996

Functional complementation of xeroderma pigmentosum complementation group E by replication protein A in an in vitro system.

Xeroderma pigmentosum (XP) is caused by a defect in nucleotide excision repair. Patients in the complementation group E (XP-E) have the mildest form of the disease and the highest level of residual repair activity. About 20% of the cell strains derived from XP-E patients lack a damaged DNA-binding protein (DDB) activity that binds to ultraviolet-induced (6-4) photoproducts with high affinity. We report here that cell-free extracts prepared from XP-E cell strains that either lacked or contained DDB activity were severely defective in excising DNA damage including (6-4) photoproducts. However, this excision activity defect was not restored by addition of purified DDB that, in fact, inhibited removal of (6-4) photoproducts by the human excision nuclease reconstituted from purified proteins. Extensive purification of correcting activity from HeLa cells revealed that the correcting activity is inseparable from the human replication/repair protein A [RPA (also known as human single stranded DNA binding protein, HSSB)]. Indeed, supplementing XP-E extracts with recombinant human RPA purified from Escherichia coli restored excision activity. However, no mutation was found in the genes encoding the three subunits of RPA in an XP-E (DDB-) cell line. It is concluded that RPA functionally complements XP-E extracts in vitro, but it is not genetically altered in XP-E patients.

Base Sequence↗
PubMed1996

The structure of d(TpA), the major photoproduct of thymidylyl-(3'5')-deoxyadenosine.

Irradiation of the dinucleotide TpdA and TA-containing oligonucleotides and DNA produces the TA* photoproduct which was proposed to be the [2+2] cyclo-addition adduct between the C5-C6 double bonds of the T and the A [Bose,S.N., Kumar,S., Davies,R.J.H., Sethi,S.K. and McCloskey,J.A. (1984) Nucleic Acids Res. 12, 7929-7947]. The proposed structure was based on a variety of spectroscopic and chemical degradation studies, and the assignment of a trans-syn-I stereochemistry was based on an extensive 1H-NMR and molecular modeling study of the dinucleotide adduct [Koning,T.M.G., Davies,R.J.H. and Kaptein,R. (1990) Nucleic Acids Res. 18, 277-284]. However, a number of properties of TA* are not in accord with the originally proposed structure, and prompted a re-evaluation of the structure. To assign the 13C spectrum and establish the bond connectivities of the TA* photoproduct of TpdA [d(TpA)*], 1H-13C heteronuclear multiple-quantum coherence (HMQC) and heteronuclear multiple bond correlation (HMBC) spectra were obtained. The 13C shifts and connectivities were found to be inconsistent with the originally proposed cyclobutane ring fusion between the thymine and adenine, but could be explained by a subsequent ring-expansion reaction to give an eight-membered ring valence isomer. The new structure for the d(TpA)* resolves the inconsistencies with the originally proposed structure, and could have a stereochemistry that arises from the anti, anti glycosyl conformation found in B form DNA.

Carbon Radioisotopes↗
PubMed1996

Mutation spectra of TA*, the major photoproduct of thymidylyl-(3'5')-deoxyadenosine, in Escherichia coli under SOS conditions.

The biological activity of TA*, the major photoproduct of thymidylyl-(3',5')-deoxyadenosine, has remained speculative since it was identified a decade ago. To determine the mutagenicity of TA* in Escherichia coli, we constructed the replicative form of an M13mp18-derived phage containing TA* in the (-)-strand by polymerase-catalyzed elongation of a TA*-containing 49mer opposite a uracil-containing (+)-strand of the phage. The in vitro synthesis mixture was transfected into an ung+, phr- E.coli host and the progeny were screened with a hybridization probe unique for the (-)-strand. TA* was found to block DNA replication substantially in the absence of SOS, but under SOS, TA* was bypassed more efficiently and was highly mutagenic. Among 56 analyzed (-)-strand progeny from two transfections, 46 (82%) were mutants, including six (11%) tandem mutants. The most abundant mutation was a 3'A-->T substitution (31/46, 56%). The possible biological consequences of TA* formation in the highly conserved TATA box consensus sequence on gene expression are discussed in light of the mutagenicity of TA*.

Bacteriophage M13↗
PubMed1996

Mutation spectra of M13 vectors containing site-specific Cis-Syn, Trans-Syn-I, (6-4), and Dewar pyrimidone photoproducts of thymidylyl-(3'-->5')-thymidine in Escherichia coli under SOS conditions.

The mutations spectra of cis-syn, trans-syn-I, (6-4), and Dewar pyrimidone photoproducts of the TT site of AATTAA and TATTAT in the (-) strand of a heteroduplex M13 vector were obtained in an excision and photoreversal repair deficient Escherichia coli host under SOS conditions. Oligonucleotides containing site-specific photoproducts were annealed to a complementary uracil-containing (+) strand that contained one or more unique pairs of nucleotide mismatches and used to prime (-) strand synthesis with a DNA polymerase and dNTPs. Following DNA synthesis, the reaction mixtures were incubated with T4 DNA ligase and ATP and then used to transfect SOS-induced competent CSRO6F' cells (uvrA6 and phr-1). The transfectants were plated, gridded, and probed by oligonucleotides specific for progeny of the (-) and (+) strands. Individual progeny of the photoproduct-containing (-) strands were plaque purified and sequenced by the dideoxy method. The cis-syn and trans-syn-I dimers were found not to be very mutagenic (<9%), the Dewar product more so (<33%), and the (6-4) product the most mutagenic (<73%). The mutation spectra were similar to those previously reported for the same photoproducts of the TT site of AGTTGG in the (+) strand of an M13 vector [Lawrence, C. W., et al. (1990) Mol. Gen Genet. 222, 166-168; LeClerc, J. E., et al. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 9685-9689] except that -1 deletion mutations were not observed for the trans-syn-I photoproducts, and a lower frequency of 3'-T-->C mutations was observed for the (6-4) photoproduct. Evidence that a small percentage of (+) strand repair of a double mismatch to the 3'-side of the photoproduct. Evidence that a small percentage of (+) strand repair of a double mismatch to the 3'-side was obtained from transfection experiments in which a second double mismatch was introduced opposite or flanking the photoproduct. Analysis of the minor tandem mutations induced by the (6-4) and Dewar products suggests that the SOS polymerase complex is able to elongate what amounts to double mismatches opposite these photoproducts and is consistent with the action of a highly processive polymerase that lacks proofreading ability.

Bacteriophage M13↗
PubMed1996

Differential effects of two hydrocephalus/MASA syndrome-related mutations on the homophilic binding and neuritogenic activities of the cell adhesion molecule L1.

The cell adhesion molecule L1 plays an important role in neural development. We have previously demonstrated that the second immunoglobulin-like domain (Ig2) of L1 contains both homophilic binding and neuritogenic activities (Zhao, X., and Siu, C.-H. (1995) J. Biol. Chem. 270, 29413-29421). Recently, two mutations (R184Q and H210Q) within the Ig2 region of the human L1 gene have been shown to be responsible for X-linked hydrocephalus and the related MASA (mental retardation, aphasia, shuffling gait, and adducted thumbs) syndrome. Glutathione S-transferase-Ig2 fusion proteins containing these mutations were used to evaluate their effects on L1. The homophilic binding activity of fusion proteins and their ability to promote neurite outgrowth from retinal cells were examined. The R184Q mutation led to a complete loss of both homophilic binding and neuritogenic activities, while the H210Q mutation resulted only in a partial loss. These results provide, for the first time, direct demonstration of the deleterious effects of hydrocephalus/MASA mutations on two intrinsic properties of L1.

Animals↗
PubMed1996

Structural and enzymatic aspects of rhodopsin phosphorylation.

Photoactivated rhodopsin (Rho*) is phosphorylated near the C terminus at multiple sites, predominantly at Ser334, Ser338, and Ser343. We systematically examined the sites of phosphorylation upon flash activation of Rho in rod outer segment (ROS) homogenates. Addition of an inhibitory antibody against rhodopsin kinase (RK) lowered phosphorylation at Ser334, Ser338, and Ser343, without changing the ratio between phosphorylation sites. In contrast, no effect of protein kinase C was detected after stimulation (by a phorbol ester), inhibition (with H7), or reconstitution of protein kinase C with purified ROS membranes. The stoichiometry and the ratio between different phosphorylation sites in purified Rho were also reproduced using RK, purified to apparent homogeneity from ROS or from an insect cell expression system. Thus, we conclude that light-dependent phosphorylation of Rho is mediated primarily by RK. Depalmitoylation of Rho at Cys322 and Cys323 altered the conformation of the C terminus of Rho, as observed by phosphorylation by casein kinase I, but did not affect phosphorylation by RK. The sites of phosphorylation were influenced, however, by the presence of four conserved amino acids at the C terminus of Rho. The accumulation of phosphorylated Ser334 observed in vivo could result from slower dephosphorylation of this site as compared with dephosphorylation of Ser338 and Ser343. These data provide a molecular mechanism for the site-specific phosphorylation of Rho observed in vivo.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1996

Induction of autologous tumor killing by heat treatment of fresh human tumor cells: involvement of gamma delta T cells and heat shock protein 70.

Autologous tumor killing (ATK) has been implicated as an important prognostic factor in cancer patients since the ability of blood lymphocytes to kill freshly isolated autologous tumor cells was strongly associated with good prognosis of the patients. The present study was designed to induce or enhance ATK sensitivity of fresh human tumor cells by heat stress. Brief exposure of fresh human tumor cells to elevated temperature increased their susceptibility to lysis by autologous blood lymphocytes in a short-term (51)Cr release assay. In addition, the heat-elevated ATK sensitivity was confirmed by clonogenic assays. An increase in ATK was observed with unstimulated lymphocytes in 42% of the cases and OK432 (streptococcal preparation)-activated lymphocytes in 80% of the cases. Stimulation of blood lymphocytes with autologous, heat-stressed tumor cells and OK432 resulted in an increase in number of gamma delta T cells, which was associated with elevated ATK activity against the stressed tumor cells. At the clonal level, three gamma delta T-cell clones (V gamma 9/V delta 2+) proliferated in response to autologous, heat stressed tumor cells and/or OK432 and exhibited elevated cytotoxicity against the tumor cells. Western blot analysis revealed an increased expression of heat shock protein (HSP) 70 in heat- treated tumor cells. Some of them expressed HSP70 on their surfaces. The elevated cytoxicity against heat-stressed tumor cells was inhibited by treatment of targets with anti-HSP70 monoclonal antibody (mAb) or of effector cells with anti-V delta2 mAb. Reactivity of gamma delta T cells to autologous, heat- stressed tumor cells was also inhibited by anti-HSP70 mAb. These results indicate that exposure to heat of tumor cells induces ATK susceptibility, especially to OK432-activated effector cells, and suggest that gamma delta T cells may be involved in ATK against stressed tumor cells through recognition of HSP70 on the target cells.

Adult↗
PubMed1996

Inhibition of cellular proliferation and modulation of insulin-like growth factor binding proteins by retinoids in a bovine mammary epithelial cell line.

Retinoids are potent inhibitors of growth and tumor progression in many mammary carcinoma cell lines, though regulation of growth in nontumorigenic mammary epithelial cells by retinoids is less clear. Here, we have characterized the inhibition of MAC-T (a nontransformed bovine mammary epithelial cell line) cellular proliferation by retinoids and their role in regulating insulin-like growth factor binding proteins (IGFBPs). Retinoic acid (RA) (100 nM) was a potent inhibitor of MAC-T cell proliferation. Retinol was 10-100 times less effective. Neither retinoid could completely arrest growth at noncytotoxic concentrations. Retinoic acid inhibited cellular proliferation by 1 h (P < .05), but inhibition was fivefold greater by 24 h (P < .01). This second stage of growth inhibition (after 12 h) was dependent upon protein synthesis. However, RA-induced inhibition of cellular proliferation did not persist, with thymidine incorporation increasing toward control levels by 4 days in culture. Retinoic acid was less effective in inhibiting thymidine incorporation when cells were stimulated with insulin, des(1-3) IGF-I, or Long(R3) IGF-I when compared to cells stimulated with native IGF-I or serum. Inhibition of proliferation by RA was associated with increased levels of IGFBP-2 in conditioned media and in plasma membrane preparations. Treatment with insulin or des(1-3) IGF-I resulted in the appearance of IGFBP-3 in conditioned media and on the cell surface. However, RA significantly reduced IGFBP-3 levels in conditioned media and eliminated IGFBP-3 associated with the plasma membrane. Thus, RA is a potent but transient inhibitor of bovine mammary epithelial cell proliferation, and this growth inhibition is correlated with increased IGFBP-2 accumulation and inhibition of IGF-I stimulated IGFBP-3 protein secretion.

Animals↗
PubMed1996

Polymorphism and concerted evolution in a tandemly repeated gene family: 5S ribosomal DNA in diploid and allopolyploid cottons.

5S RNA genes and their nontranscribed spacers are tandemly repeated in plant genomes at one or more chromosomal loci. To facilitate an understanding of the forces that govern 5S rDNA evolution, copy-number estimation and DNA sequencing were conducted for a phylogenetically well-characterized set of 16 diploid species of cotton (Gossypium) and 4 species representing allopolyploid derivatives of the diploids. Copy number varies over twentyfold in the genus, from approximately 1,000 to 20,000 copies/2C genome. When superimposed on the organismal phylogeny, these data reveal examples of both array expansion and contraction. Across species, a mean of 12% of nucleotide positions are polymorphic within individual arrays, for both gene and spacer sequences. This shows, in conjunction with phylogenetic evidence for ancestral polymorphisms that survive speciation events, that intralocus concerted evolutionary forces are relatively weak and that the rate of interrepeat homogenization is approximately equal to the rate of speciation. Evidence presented also shows that duplicated 5S rDNA arrays in allopolyploids have retained their subgenomic identity since polyploid formation, thereby indicating that interlocus concerted evolution has not been an important factor in the evolution of these arrays. A descriptive model, one which incorporates the opposing forces of mutation and homogenization within a selective framework, is outlined to account for the empirical data presented. Weak homogenizing forces allow equivalent levels of sequence polymorphism to accumulate in the 5S gene and spacer sequences, but fixation of mutations is nearly prohibited in the 5S gene. As a consequence, fixed interspecific differences are statistically underrepresented for 5S genes. This result explains the apparent paradox that despite similar levels of gene and spacer diversity, phylogenetic analysis of spacer sequences yields highly resolved trees, whereas analyses based on 5S gene sequences do not.

Base Sequence↗
PubMed1996

Production, characterization and application of the cellulase-free xylanase from Aspergillus niger.

The effects of carbon source on xylanase and cellulase production were studied. The extract of steam-exploded corn stover was found to be the best raw material for producing cellulase-free xylanase. The xylanolytic enzymes of Aspergillus niger An-76 were purified by chromatography, and the properties of the four purified components were analyzed. When the enzyme was used to treat the birch Kraft pulp, and followed by a subsequent CEH bleaching, a brightness of 6.8% SBD more than that of the untreated one using the same chlorine dosage, or a saving of nearly 50% of chlorine consumption with the same brightness, was realized.

Aspergillus niger↗
PubMed1996

Lineage-specific CaN19 expression in human skin: lack of expression in normal melanocytes.

Although expression of CaN19 is reduced in cell lines derived from tumors of diverse origin, its expression in their normal counterparts has not been studied in detail. We find that CaN19 mRNA is expressed at very low to undetectable levels in normal melanocytes as well as malignant melanoma cell lines, confirming that CaN19 expression in normal cells is lineage-dependent.

Cells, Cultured↗