Biomedical subjects

L Liu

Publications and source records attributed to L Liu.

At least 325 records · Page 18Linked to original sources

PubMed2000

Evidence that DNA polymerase delta isolated by immunoaffinity chromatography exhibits high-molecular weight characteristics and is associated with the KIAA0039 protein and RPA.

DNA polymerase delta, the key enzyme for eukaryotic chromosomal replication, has been well characterized as consisting of a core enzyme of a 125 kDa catalytic subunit and a smaller 50 kDa subunit. However, less is known about the other proteins that may comprise additional subunits or participate in the macromolecular protein complex that is involved in chromosomal DNA replication. In this study, the properties of calf thymus pol delta preparations isolated by immunoaffinity chromatography were investigated. It is demonstrated for the first time using highly purified preparations that the pol delta heterodimer is associated with other polypeptides in high-molecular weight species that range from 260000 to >500000 in size, as determined by FPLC gel filtration. These preparations are associated with polypeptides of ca. 68-70, 34, 32, and 25 kDa. Similar findings were revealed with glycerol gradient ultracentrifugation. The p68 polypeptide was shown to be a PCNA binding protein by overlay methods with biotinylated PCNA. Protein sequencing of the p68, p34, and p25 polypeptide bands revealed sequences that correspond to the hypothetical protein KIAA0039. KIAA0039 displays a small but significant degree of homology to Schizosaccharomyces pombe Cdc27, which, like Saccharomyces cerevisiae Pol32p, has been described as the third subunit of yeast pol delta. These studies provide evidence that p68 is a subunit of pol delta. In addition, the p68-70 and p32 polypeptides were found to be derived from the 70 and 32 kDa subunits of RPA, respectively.

Amino Acid Sequence↗

PubMed2000

Reversible electromechanical characteristics of carbon nanotubes under local-probe manipulation

The effects of mechanical deformation on the electrical properties of carbon nanotubes are of interest given the practical potential of nanotubes in electromechanical devices, and they have been studied using both theoretical and experimental approaches. One recent experiment used the tip of an atomic force microscope (AFM) to manipulate multi-walled nanotubes, revealing that changes in the sample resistance were small unless the nanotubes fractured or the metal-tube contacts were perturbed. But it remains unclear how mechanical deformation affects the intrinsic electrical properties of nanotubes. Here we report an experimental and theoretical elucidation of the electromechanical characteristics of individual single-walled carbon nanotubes (SWNTs) under local-probe manipulation. We use AFM tips to deflect suspended SWNTs reversibly, without changing the contact resistance; in situ electrical measurements reveal that the conductance of an SWNT sample can be reduced by two orders of magnitude when deformed by an AFM tip. Our tight-binding simulations indicate that this effect is owing to the formation of local sp3 bonds caused by the mechanical pushing action of the tip.

Journal Article↗

PubMed2000

The Drosophila takeout gene is a novel molecular link between circadian rhythms and feeding behavior.

We report the characterization of a novel Drosophila circadian clock-regulated output gene, takeout (to). The to amino acid sequence shows similarity to two ligand binding proteins, including juvenile hormone binding protein. to mRNA is expressed in the head and the cardia, crop, and antennae-structures related to feeding. to expression is induced by starvation, which is blocked in all arrhythmic central clock mutants, suggesting a direct molecular link between the circadian clock and the feeding/starvation response. A to mutant has aberrant locomotor activity and dies rapidly in response to starvation, indicating a link between locomotor activity, survival, and food status. We propose that to participates in a novel circadian output pathway that conveys temporal and food status information to feeding-relevant metabolisms and activities.

Amino Acid Sequence↗

PubMed2000

The reduced folate carrier in L1210 murine leukemia cells is a 58-kDa protein.

The reduced folate carrier (RFC1) is a major transporter for both natural reduced folates and antifolate chemotherapeutics. Using polyclonal antibodies targeted to epitopes at the loop between the sixth and seventh predicted transmembrane domains or the distal C-terminus, we were able to demonstrate by Western blot analysis that the molecular size of RFC1 expressed in murine leukemia L1210 cells is 58 kDa as predicted by the open reading frame of its cDNA. 46- and 38-kDa proteins detected only in plasma membrane preparations were proteolytic degradation products that appeared during membrane preparation or treatment with the conventional SDS-PAGE loading buffer. These data resolve discrepancies reported previously for the molecular size of RFC1.

Animals↗

PubMed2000

Leaf morphology and cuticular features of Sphenophyllum in the Gigantopteris flora from South China.

Permian specimens of Sphenophyllum with preserved cuticular anatomy have been discovered in Cathaysia and have prompted a detailed re-evaluation of the genus in the Gigantopteris flora in South China. New specimens are described and previously published material is re-examined to clarify taxonomic diversity and to establish geographic and stratigraphic ranges for each species. Recognized taxa include Sphenophyllum apiciserratum sp. nov., S. koboense Kobatake, S. meridionale sp. nov., S. minor (Sterzel) Gu and Zhi, S. sinocoreanum Yabe, S. cf. sinocoreanum Yabe, and S. aff. speciosum (Royle) McClelland. Cuticles of Sphenophyllum apiciserratum and S. koboense are hypostomatic with randomly disposed ordinary cells in intercostal regions. Specific concepts are clarified and unresolved taxonomic problems are discussed. The South China taxa are compared with those from Euramerican, Gondwanan and Angaran floras, and the South China Cathaysian species are found to be taxonomically distinct.

Journal Article↗

PubMed2000

Effects of arachidonic acid on unitary calcium currents in rat sympathetic neurons.

We have characterized the actions of arachidonic acid (AA) on whole cell and unitary calcium (Ca2+) currents in rat neonatal superior cervical ganglion (SCG) neurons using barium (Ba2+) as the charge carrier. Whole cell currents were elicited by stepping the membrane potential from -90 mV to +10 mV. Arachidonic acid (5 microM) was introduced into the bath in the continued presence of 1 microM (+)-202-791, an L-type Ca2+ channel agonist. Under these conditions, the peak current, comprised mainly of N-type current, and a slow, (+)-202-791-induced component of the tail current were inhibited by 67 +/- 6 and 60 +/- 10 %, respectively, indicating that AA inhibits both N- and L-type currents. At a test potential of +30 mV, AA (5 microM) decreased unitary L- and N-type Ca2+ channel open probability (Po) in cell-attached patches that contained a single channel. For both channels, the underlying causes of the decrease in Po were similar. Arachidonic acid caused an increase in the percentage of null sweeps and in the number of null sweeps that clustered together. In sweeps with activity, the average number of openings per sweep decreased, while first latency and mean closed time increased. Arachidonic acid had no significant effect on unitary current amplitude or mean open time. Our findings are the first description of the inhibition of unitary L- and N-type Ca2+ channel activity by AA and are consistent with both channels spending more time in their null mode and with increased dwell time in one or more closed states.

Animals↗

PubMed2000

Protective effect of high density lipoprotein on endothelium-dependent vasodilatation.

Low concentrations of high-density lipoprotein cholesterol (HDL-C) have been associated with increased risk of coronary heart disease (CHD) even when the total cholesterol (TC) and triglyceride (TG) levels are not elevated. The mechanism by which HDL confers protection against atherosclerosis remains speculative. Using high-resolution ultrasound, we measured the dilatation changes of brachial arteries during reactive hyperemia and after sublingual glyceryl trinitrate (GTN) in 63 patients with established (CHD) and 45 controls, in which the serum TC level was normal. The results showed that both flow-mediated dilatation (FMD) and GTN-induced dilatation of brachial arteries in patients with CHD were much reduced compared with control group (2.31+/-2.46% vs. 7.43+/-4.10% and 16.41+/-6.15% vs. 22.44+/-8.63%, respectively, P<0.001 for all). Univariate analysis indicated that FMD of brachial arteries was inversely related to age (r=-0.226, P<0.05), hypertension (r=-0.229, P<0.05), baseline diameter (r=-0.299, P<0.01) and LDL-C (r=-0.237, P<0.05) and positively related to HDL-C (r=0.491, P<0.01). GTN induced vasodilatation was inversely related to age (r=-0.216, P<0. 05) and baseline diameter (-0.476, P<0.01). Multiple stepwise regression analyses in two groups taken together showed that HDL-C and age were the independent predictors of the FMD of brachial arteries (beta=0.466, P=0.000 and beta=-0.184, P=0.020, respectively). Baseline diameter was significant predictor of GTN-induced vasodilatation (beta=-0.390, P=0.000). The analysis in the group of CHD patients showed that only HDL-C was significantly relate to the FMD of brachial arteries (beta=0.295, P=0.018 ) and in controls that hypertension and HDL-C were significantly relate to the FMD of brachial arteries (beta=-0.395, P=0.004 and beta=0.344, P=0.011, respectively). These finding suggest that endothelium-dependent and endothelium-independent vasodilatation are impaired in the patients with CHD. HDL exerts a protective effect on endothelium-dependent vasodilatation in TC being relatively normal population.

Adult↗

PubMed2000

Emergence of H3N2 reassortant influenza A viruses in North American pigs.

In late summer through early winter of 1998, there were several outbreaks of respiratory disease in the swine herds of North Carolina, Texas, Minnesota and Iowa. Four viral isolates from outbreaks in different states were analyzed, both antigenically and genetically. All of the isolates were identified as H3N2 influenza viruses with antigenic profiles similar to those of recent human H3 strains. Genotyping and phylogenetic analysis demonstrated that the four swine viruses had emerged through two different pathways. The North Carolina isolate is the product of genetic reassortment between human and swine influenza viruses, while the others arose from reassortment of human, swine and avian viral genes. The hemagglutinin genes of the four isolates were all derived from the human H3N2 virus circulating in 1995. It remains to be determined if either of these recently emerged viruses will become established in the pigs in North America and whether they will become an economic burden.

Animals↗

PubMed2000

Controllable reversibility of an sp(2) to sp(3) transition of a single wall nanotube under the manipulation of an AFM tip: A nanoscale electromechanical switch?

A simulation of the deflection of a single wall nanotube under the manipulation of an atomic force microscope tip revealed the key feature characterizing the deformation at relatively small bending angles to be a reversible transition from sp(2) to sp(3) bonding configurations in the bending region, leading to a 2 orders of magnitude reduction in conductance consistent with our most recent experimental observation. A local analysis elucidating the underlying physics of the findings is also discussed.

Journal Article↗

PubMed2000

A study on the enthalpy-entropy compensation in protein unfolding.

A large number of thermodynamic data including the free energy, enthalpy, entropy, and heat capacity changes were collected for the denaturation of various proteins. Regression indicated that remarkable enthalpy-entropy compensation occurred in protein unfolding, which meant that the change in enthalpy was almost compensated by a corresponding change in entropy resulting in a smaller net free energy change. This behavior was proposed to result from the water molecule reorganization, which contributed significantly to the enthalpy and entropy changes but little to the free energy change in protein unfolding. It turned out that the enthalpy-entropy compensation could provide novel insights into the problem of enthalpy and entropy convergence in protein unfolding.

Calorimetry, Differential Scanning↗

PubMed2000

Antibody-catalyzed removal of the p-nitrobenzyl ester protecting group: the molecular basis of broad substrate specificity.

Antibody catalysts for the removal of the p-nitrobenzyl ester protecting group have been generated to accommodate a broad range of substrates. Antibody 7B9, which was elicited against p-nitrobenzyl phosphonate 1, catalyzed the hydrolyses of p-nitrobenzyl monoesters of nonsubstituted, and beta- and gamma-substituted glutaric acids with almost identical Km and kcat values. In addition, 7B9 displayed substrate tolerance towards the a-substituents and accepted the p-nitrobenzyl esters of Leu, Norleu, and Phe. To define the molecular basis of the broad substrate tolerance, we have cloned and sequenced the antibody and constructed a model of the active-site-hapten complex. The model showed a relatively shallow pocket of the antigen-combining site that accommodates the p-nitrobenzyl moiety, and this is consistent with the observed substrate specificity. Thus, in the antibody-catalyzed reaction, the alpha-, beta-, and gamma-substituents of the substrates should be outside the combining site and ignored by the antibody recognition. A structural comparison of 7B9 with antibody D2.3, elicited against the structurally similar haptenic phosphonate, suggests the significance of the linker moiety in hapten design, which endows antibody catalysts with broad substrate specificity. These investigations provide new strategies for the generation of catalytic antibodies that accept a broad range of substrates for practical applications in organic synthetic chemistry.

Amino Acid Sequence↗

PubMed2000

Degradation of transcription factor RFX5 during the inhibition of both constitutive and interferon gamma-inducible major histocompatibility complex class I expression in chlamydia-infected cells.

We have previously shown that the obligate intracellular pathogen chlamydia can suppress interferon (IFN)-gamma-inducible major histocompatibility complex (MHC) class II expression in infected cells by degrading upstream stimulation factor (USF)-1. We now report that chlamydia can also inhibit both constitutive and IFN-gamma-inducible MHC class I expression in the infected cells. The inhibition of MHC class I molecule expression correlates well with degradation of RFX5, an essential downstream transcription factor required for both the constitutive and IFN-gamma-inducible MHC class I expression. We further demonstrate that a lactacystin-sensitive proteasome-like activity identified in chlamydia-infected cell cytosolic fraction can degrade both USF-1 and RFX5. This proteasome-like activity is dependent on chlamydial but not host protein synthesis. Host preexisting proteasomes may not be required for the unique proteasome-like activity. These observations suggest that chlamydia-secreted factors may directly participate in the proteasome-like activity. Efforts to identify the chlamydial factors are underway. These findings provide novel information on the molecular mechanisms of chlamydial evasion of host immune recognition.

Bacterial Proteins↗

PubMed2000

Molecular engineering of matrix-targeted retroviral vectors incorporating a surveillance function inherent in von Willebrand factor.

A major obstacle that limits the potential of human gene therapy is the inefficiency of gene delivery to appropriate sites in vivo. Previous studies demonstrated that the physiological surveillance function performed by von Willebrand factor (vWF) could be incorporated into retroviral vectors by molecular engineering of the MuLV ecotropic envelope (Env) protein. To advance the application of vWF targeting technology beyond laboratory animals, we prepared an extensive series of Env proteins bearing modified vWF-derived matrix-binding sequences and assembled these chimeric proteins into targeted vectors that are capable of transducing human cells. Initially, a dual envelope configuration was utilized, which required coexpression of a wild-type amphotropic Env. Subsequently, streamlined "escort" Env proteins were constructed wherein the inoperative receptor-binding domain of the targeting partner was replaced by the vWF-derived collagen-binding motif. Ultimately, an optimal construct was developed that exhibited properties of both extracellular matrix (ECM)-targeting and near wild-type amphotropic infectivity, and could be arrayed as a single envelope on a retroviral particle. On intraarterial instillation, enhanced focal transduction of neointimal cells (approximately 20%) was demonstrated in a rat model of balloon angioplasty. Moreover, transduction of tumor foci (approximately 1-3%) was detected after portal vein infusion of a matrix-targeted vector in a nude mouse model of liver metastasis. We conclude that the unique properties of these targeted injectable retroviral vectors would be suitable for improving therapeutic gene delivery in numerous clinical applications, including vascular restenosis, laser and other surgical procedures, orthopedic injuries, wound healing, ischemia, arthritis, inflammatory disease, and metastatic cancer.

Amino Acid Sequence↗

PubMed2000

Protection from nitrosative stress by yeast flavohemoglobin.

Yeast hemoglobin was discovered close to half a century ago, but its function has remained unknown. Herein, we report that this flavohemoglobin protects Saccharomyces cerevisiae from nitrosative stress. Deletion of the flavohemoglobin gene (YHB1) abolished the nitric oxide (NO)-consuming activity of yeast cells. Levels of protein nitrosylation were more than 10-fold higher in yhb1 mutant yeast than in isogenic wild-type cells after incubation with NO donors. Growth of mutant cells was inhibited by a nitrosative challenge that had little effect on wild-type cells, whereas the resistance of mutant cells to oxidative stress was unimpaired. Protection conferred by yeast flavohemoglobin against NO and S-nitrosothiols was seen under both anaerobic and aerobic conditions, consistent with a primary function in NO detoxification. A phylogenetic analysis indicated that protection from nitrosative stress is likely to be a conserved function among microorganismal flavohemoglobins. Flavohemoglobin is therefore a potential target for antimicrobial therapy.

Aerobiosis↗

PubMed2000

Pulse pressure not mean pressure determines cardiovascular risk in older hypertensive patients.

BACKGROUND: Current guidelines for the management of hypertension rest almost completely on the measurement of systolic and diastolic blood pressure. However, the arterial blood pressure wave is more correctly described as consisting of a pulsatile (pulse pressure) and a steady (mean pressure) component. OBJECTIVE: To explore the independent roles of pulse pressure and mean pressure as determinants of cardiovascular prognosis in older hypertensive patients. METHODS: This meta-analysis, based on individual patient data, pooled the results of the European Working Party on High Blood Pressure in the Elderly trial (n = 840), the Systolic Hypertension in Europe Trial (n = 4695), and the Systolic Hypertension in China Trial (n = 2394). The relative hazard rates associated with pulse pressure and mean pressure were calculated using Cox regression analysis, with stratification for the 3 trials and with adjustments for sex, age, previous cardiovascular complications, smoking, and treatment group. RESULTS: A 10-mm Hg wider pulse pressure increased the risk of major cardiovascular complications; after controlling for mean pressure and the other covariates, the increase in risk ranged from approximately 13% for all coronary end points (P = .02) to nearly 20% for cardiovascular mortality (P = .001). In a similar analysis, mean pressure predicted the incidence of cardiovascular complications but only after removal of pulse pressure as an explanatory variable from the model. Furthermore, the probability of a major cardiovascular end point increased with higher systolic blood pressure; at any given level of systolic blood pressure, it also increased with lower diastolic blood pressure, suggesting that the wider pulse pressure was driving the risk of major complications. CONCLUSIONS: In older hypertensive patients, pulse pressure not mean pressure is the major determinant of cardiovascular risk. The implications of these findings for the management of hypertensive patients should be further investigated in randomized controlled outcome trials in which the pulsatile component of blood pressure is differently affected by antihypertensive drug treatment.

Aged↗

PubMed2000

The 5' flank of mouse H19 in an unusual chromatin conformation unidirectionally blocks enhancer-promoter communication.

BACKGROUND: During mouse prenatal development, the neighbouring insulin-like growth factor II (Igf2) and H19 loci are expressed monoallelically from the paternal and maternal alleles, respectively. Identical spatiotemporal expression patterns and enhancer deletion experiments show that the Igf2 and H19 genes share a common set of enhancers. Deletion of a differentially methylated region in the 5' flank of the H19 gene partially relieves the repression of the maternal Igf2 and paternal H19 alleles in the soma. The mechanisms underlying the function of the 5' flank of the H19 gene are, however, unknown. RESULTS: Chromatin analysis showed that the 5' flank of the mouse H19 gene contains maternal-specific, multiple nuclease hypersensitive sites that map to linker regions between positioned nucleosomes. These features could be recapitulated in an episomal-based H19 minigene, which was propagated in human somatic cells. Although the 5' flank of the H19 promoter has no intrinsic silencer activity under these conditions, it unidirectionally extinguished promoter-enhancer communications in a position-dependent manner, without directly affecting the enhancer function. CONCLUSIONS: The unmethylated 5' flank of the H19 gene adopts an unusual and maternal-specific chromatin conformation in somatic cells and regulates enhancer-promoter communications, thereby providing an explanation for its role in manifesting the repressed state of the maternally inherited Igf2 allele.

Alleles↗

PubMed2000

Inhibition of transplant vasculopathy in a rat aortic allograft model after infusion of anti-inflammatory viral serpin.

BACKGROUND: Transplant vasculopathy remains a difficult therapeutic problem, resulting in the majority of late cardiac graft losses. This chronic vascular disease is thought to be triggered by alloantigen-dependent and alloantigen-independent inflammatory factors. Despite improved 1-year survival, the incidence of transplant vasculopathy has not improved with current immunosuppressive protocols. Highly effective strategies have evolved in the large DNA viruses that shield infecting viruses from host inflammatory responses. Serp-1 is a secreted myxoma virus anti-inflammatory serine proteinase inhibitor. Serp-1 inhibits plasminogen activators in a manner similar to plasminogen activator inhibitor (PAI-1), a vascular protein that plays a pivotal regulatory role in vascular wound healing. In this study, we tested the ability of purified Serp-1 protein to ameliorate posttransplant vasculopathy after rat aortic allograft surgery. METHODS AND RESULTS: Serp-1 protein or controls were infused into 98 rats immediately after segmental aortic allograft transplantation. After either late (28 days, 64 rats) or early (12 to 48 hours, 24 rats) follow-up, transplanted aortic segments were harvested for morphological and immunohistochemical analysis. Significant reductions in intimal plaque growth (P<0.002) and mononuclear cell invasion (P<0.033) were detected after Serp-1 infusion at nanogram doses. Serp-1 reduced early macrophage (P<0.0016) and nonspecific lymphocyte (P<0.0179) invasion into medial and adventitial layers and inhibited associated depletion of medial smooth muscle cells (P<0.0006). CONCLUSIONS: Infusion of a viral anti-inflammatory serpin, Serp-1, significantly reduces early inflammatory responses and later luminal occlusion in a rat aortic allograft model.

Animals↗

PubMed2000

Differential regulation of circadian pacemaker output by separate clock genes in Drosophila.

Regulation of the Drosophila pigment-dispersing factor (pdf) gene products was analyzed in wild-type and clock mutants. Mutations in the transcription factors CLOCK and CYCLE severely diminish pdf RNA and neuropeptide (PDF) levels in a single cluster of clock-gene-expressing brain cells, called small ventrolateral neurons (s-LN(v)s). This clock-gene regulation of specific cells does not operate through an E-box found within pdf regulatory sequences. PDF immunoreactivity exhibits daily cycling, but only within terminals of axons projecting from the s-LN(v)s. This posttranslational rhythm is eliminated by period or timeless null mutations, which do not affect PDF staining in cell bodies or pdf mRNA levels. Therefore, within these chronobiologically important neurons, separate elements of the central pacemaking machinery regulate pdf or its product in novel and different ways. Coupled with contemporary results showing a pdf-null mutant to be severely defective in its behavioral rhythmicity, the present results reveal PDF as an important circadian mediator whose expression and function are downstream of the clockworks.

Animals↗