Biomedical subjects

A Cheng

Publications and source records attributed to A Cheng.

At least 19 recordsLinked to original sources

PubMed2001

The landing of the NEAR-Shoemaker spacecraft on asteroid 433 Eros.

The NEAR-Shoemaker spacecraft was designed to provide a comprehensive characterization of the S-type asteroid 433 Eros (refs 1,2,3), an irregularly shaped body with approximate dimensions of 34 x 13 x 13 km. Following the completion of its year-long investigation, the mission was terminated with a controlled descent to its surface, in order to provide extremely high resolution images. Here we report the results of the descent on 12 February 2001, during which 70 images were obtained. The landing area is marked by a paucity of small craters and an abundance of 'ejecta blocks'. The properties and distribution of ejecta blocks are discussed in a companion paper. The last sequence of images reveals a transition from the blocky surface to a smooth area, which we interpret as a 'pond'. Properties of the 'ponds' are discussed in a second companion paper. The closest image, from an altitude of 129 m, shows the interior of a 100-m-diameter crater at 1-cm resolution.

Journal Article↗

PubMed2001

p38 MAP kinase mediates nitric oxide-induced apoptosis of neural progenitor cells.

Neural progenitor cells (NPC) can proliferate, differentiate into neurons or glial cells, or undergo a form of programmed cell death called apoptosis. Although death of NPC occurs during development of the nervous system and in the adult, the underlying mechanisms are unknown. Here we show that nitric oxide (NO) can induce death of C17.2 NPC by a mechanism requiring activation of p38 MAP kinase, poly(ADP-ribose) polymerase, and caspase-3. Nitric oxide causes release of cytochrome c from mitochondria, and Bcl-2 protects the neural progenitor cells against nitric oxide-induced death, consistent with a pivotal role for mitochondrial changes in controlling the cell death process. Inhibition of p38 MAP kinase by SB203580 abolished NO-induced cell death, cytochrome c release, and activation of caspase-3, indicating that p38 activation serves as an upstream mediator in the cell death process. The anti-apoptotic protein Bcl-2 protected NPC against nitric oxide-induced apoptosis and suppressed activation of p38 MAP kinase. The ability of nitric oxide to trigger death of NPC by a mechanism involving p38 MAP kinase suggests that this diffusible gas may regulate NPC fate in physiological and pathological settings in which NO is produced.

Animals↗

PubMed2001

Current indoor allergen levels of fungi and cats, but not house dust mites, influence allergy and asthma in adults with high dust mite exposure.

We assessed the influence of current indoor levels of fungi, house dust mite allergen (Der p 1), and cat allergen (Fel d 1) on sensitization and asthma in adults. A total of 485 adults answered a questionnaire and had skin prick tests and lung function tests. Dust and air samples were collected from their bedrooms. The dust was analyzed for Der p 1, Fel d 1, and fungal biomass (ergosterol). Fungal propagules were measured in air samples. Current asthma was defined as having wheezed during the past 12 mo plus bronchial hyperreactivity (BHR) to methacholine. High exposure to total airborne fungi was associated with increased BHR, but perhaps paradoxically with a lower risk of being sensitized to fungi. Ergosterol levels in floor dust were a risk factor both for being sensitized to fungi and having wheezed within the last year. High Fel d 1 levels in floor dust were found to increase the risk of being sensitized to cats and in beds to increase the risk of current asthma. Although Der p 1 levels in homes were high, people exposed to high Der p 1 levels in floor dust were less likely to be sensitized to house dust mites or to have wheezed within the past year. Current indoor levels of fungi and Fel d 1, but not Der p 1, influenced sensitization and asthma in adults with high dust mite exposure.

Adult↗

PubMed2001

Attenuation of adhesion-dependent signaling and cell spreading in transformed fibroblasts lacking protein tyrosine phosphatase-1B.

Previous biochemical evidence has yielded conflicting models for the role of protein tyrosine phosphatase-1B (PTP-1B) in the regulation of integrin signaling. Thus, to establish the physiological relevance for such a role, we employed a genetic approach by generating embryonic fibroblasts from PTP-1B knockout mice. Both primary fibroblasts and their derived cell lines were used in this study. Immortalization of wild-type primary cells with the SV40 Large T antigen resulted in a dramatic increase in the endogenous expression of PTP-1B, suggesting a role during transformation. Moreover, the absence of PTP-1B in the transformed cell lines led to a more pronounced effect on different pathways of fibronectin-mediated signaling compared with the untransformed state. Specifically, p130(Cas) phosphorylation, Erk activation as well as cell spreading were delayed in PTP-1B-deficient cells, compared with their wild-type counterparts. Interestingly, this attenuation in integrin-mediated events closely resembles that of Src-deficient fibroblasts. Indeed, PTP-1B deficient, transformed fibroblasts held in suspension do exhibit a hyperphosphorylation of the inhibitory site (Tyr-527) of Src, compared with their wild-type counterparts. These results establish PTP-1B as a positive physiological regulator of integrin signaling in transformed cells, acting upstream of Src Tyr-527 dephosphorylation that leads to several adhesion-dependent events.

Animals↗

PubMed2001

Imaging of small-scale features on 433 Eros from NEAR: evidence for a complex regolith.

On 25 October 2000, the Near Earth Asteroid Rendevous (NEAR)-Shoemaker spacecraft executed a low-altitude flyover of asteroid 433 Eros, making it possible to image the surface at a resolution of about 1 meter per pixel. The images reveal an evolved surface distinguished by an abundance of ejecta blocks, a dearth of small craters, and smooth material infilling some topographic lows. The subdued appearance of craters of different diameters and the variety of blocks and different degrees of their burial suggest that ejecta from several impact events blanketed the region imaged at closest approach and led to the building up of a substantial and complex regolith consisting of fine materials and abundant meter-sized blocks.

Journal Article↗

PubMed2001

Visualization of a water-selective pore by electron crystallography in vitreous ice.

The water-selective pathway through the aquaporin-1 membrane channel has been visualized by fitting an atomic model to a 3.7-A resolution three-dimensional density map. This map was determined by analyzing images and electron diffraction patterns of lipid-reconstituted two-dimensional crystals of aquaporin-1 preserved in vitrified buffer in the absence of any additive. The aqueous pathway is characterized by a size-selective pore that is approximately 4.0 +/- 0.5A in diameter, spans a length of approximately 18A, and bends by approximately 25 degrees as it traverses the bilayer. This narrow pore is connected by wide, funnel-shaped openings at the extracellular and cytoplasmic faces. The size-selective pore is outlined mostly by hydrophobic residues, resulting in a relatively inert pathway conducive to diffusion-limited water flow. The apex of the curved pore is close to the locations of the in-plane pseudo-2-fold symmetry axis that relates the N- and C-terminal halves and the conserved, functionally important N76 and N192 residues.

Amino Acid Sequence↗

PubMed2001

Ebp1, an ErbB-3 binding protein, interacts with Rb and affects Rb transcriptional regulation.

Ebp1, an ErbB-3 binding protein, inhibits the proliferation and induces the differentiation of human breast cancer cells. The mechanisms of these effects are unknown. Rb, the product of the retinoblastoma gene, is an important modulator of cell cycle progression and cellular differentiation. We report that Rb is a binding target for Ebp1. Ebp1 was localized to both the nucleus and the cytoplasm of logarithmically growing AU565 breast cancer cells and HeLa cells as determined by confocal immunofluorescent microscopy. Ebp1 was present in Rb immunoprecipitates derived from AU565 breast cancer cells. GST-Rb also bound endogenous Ebp1. Using GST-Ebp1 constructs, we determined that the 72 C-terminal amino acids of Ebp1 were sufficient to bind Rb. Dephosphorylation of Ebp1 enhanced the interaction of Ebp1 with Rb. The overexpression of Ebp1 in MCF-7 and AU565 (Rb(+)) cells inhibited the activity of the E2F1 regulated cyclin-E promoter. Ebp1 bound E2F1 indirectly via Rb in lysates of MCF-7 cells. The interaction of Ebp1 with Rb may prove to be an important mechanism of Ebp1 induced changes in cell proliferation and differentiation.

Adaptor Proteins, Signal Transducing↗

PubMed2001

Adipose tissue reduction in mice lacking the translational inhibitor 4E-BP1.

All nuclear-encoded mRNAs contain a 5' cap structure (m7GpppN, where N is any nucleotide), which is recognized by the eukaryotic translation initiation factor 4E (eIF4E) subunit of the eIF4F complex. The eIF4E-binding proteins constitute a family of three polypeptides that reversibly repress cap-dependent translation by binding to eIF4E, thus preventing the formation of the eIF4F complex. We investigated the biological function of 4E-BP1 by disrupting its gene (Eif4ebp1) in the mouse. Eif4ebp1-/- mice manifest markedly smaller white fat pads than wild-type animals, and knockout males display an increase in metabolic rate. The males' white adipose tissue contains cells that exhibit the distinctive multilocular appearance of brown adipocytes, and expresses the uncoupling protein 1 (UCP1), a specific marker of brown fat. Consistent with these observations, translation of the peroxisome proliferator-activated receptor-gamma co-activator 1 (PGC1), a transcriptional co-activator implicated in mitochondrial biogenesis and adaptive thermogenesis, is increased in white adipose tissue of Eif4ebp1-/- mice. These findings demonstrate that 4E-BP1 is a novel regulator of adipogenesis and metabolism in mammals.

Adaptor Proteins, Signal Transducing↗

PubMed2001

Lipid disorders: justification of methods and goals of treatment.

Dyslipidemia is a major risk factor for coronary heart disease (CHD). While some uncertainty exists about the clinical significance of improving high-density lipoprotein cholesterol and triglyceride levels, large primary- and secondary-prevention studies aimed at lowering low-density lipoprotein cholesterol levels with statins have convincingly reduced CHD events and total mortality. Despite the strong clinical evidence and widely publicized treatment guidelines, many hyperlipidemic patients receive inadequate lipid-lowering treatment. This failure to achieve clinical treatment goals may be due to poor physician adherence to treatment guidelines, patient noncompliance, and the presence of concomitant medical conditions that modify typical hyperlipidemia management. This review considers the challenges and available strategies to optimize lipid management in patients at risk for CHD.

Adult↗

PubMed2001

Impairment of endothelial function--a possible mechanism for atherosclerosis of a high-fat meal intake.

INTRODUCTION: Endothelial dysfunction is known to occur in patients with coronary artery disease. Flow-mediated dilation of the brachial artery using Doppler ultrasound is a non-invasive technique for the assessment of endothelial function. The objective of the study was to use the above method to evaluate the pathophysiology of high-fat (HF) intake on endothelial function in a local population. A popular local dish "nasi-lemak", a source of high saturated fat content from coconut milk, was chosen to represent a local high-fat meal (LHF). In addition, the effects of a Western high-fat (WHF) ("McDonald's") meal and a low-fat (LF) meal control on endothelial function were studied. MATERIALS AND METHODS: The study population consisted of 10 healthy male non-smoker (mean age 22 +/- 2 years) with normal body mass index, normal fasting sugar and lipid profiles. Nitric oxide dependent flow-mediated dilation and nitric oxide independent (GTN) dilation was assessed by Doppler flow in the brachial artery before and 4 hours after each meal on separate occasions by 2 experienced sonographers blinded to the type of meals. RESULTS: The baseline brachial artery size, baseline vessel flow and increase in flow after cuff deflation were similar for each of the six arterial studies. In response to reactive hyperaemia after cuff deflation, the endothelium-dependent dilation was significantly different between the meals. There was a marked decrease in endothelium-dependent dilation after the WHF meal compared to the LF meal (8.6 +/- 2.2% vs. -0.8 +/- 1.1%, P < 0.006). There was also a marked decrease in endothelium-dependent dilation after the LHF meal compared to the LF meal (7.7 +/- 2.1% vs. -0.8 +/- 1.1%, P < 0.001). When comparing between the two HF meals, the change in endothelium-dependent dilation was not significant (7.7 vs. 8.6%, P = 0.678). GTN-induced dilation was not significantly different before and after the LF, WHF or LHF (0.1 +/- 0.5% vs. 0.2 +/- 0.9% vs. 1.3 +/- 0.5%, P = 0.094). CONCLUSION: The results suggest that in a local population, impairment of endothelial function is a possible mechanism in the pathophysiology of atherosclerosis from HF intake, beyond just affecting lipid levels. This effect is observed after both a LHF and a WHF meal intake. This technique to study endothelial function may be a useful non-invasive screening tool in the study of other HF diet choices and provides further information for the education of the influence of dietary choices on atherosclerosis.

Adult↗

PubMed2000

Dephosphorylation of human cyclin-dependent kinases by protein phosphatase type 2C alpha and beta 2 isoforms.

We previously reported that the activating phosphorylation on cyclin-dependent kinases in yeast (Cdc28p) and in humans (Cdk2) is removed by type 2C protein phosphatases. In this study, we characterize this PP2C-like activity in HeLa cell extract and determine that it is due to PP2C beta 2, a novel PP2C beta isoform, and to PP2C alpha. PP2C alpha and PP2C beta 2 co-purified with Mg(2+)-dependent Cdk2/Cdk6 phosphatase activity in DEAE-Sepharose, Superdex-200, and Mono Q chromatographies. Moreover, purified recombinant PP2C alpha and PP2C beta 2 proteins efficiently dephosphorylated monomeric Cdk2/Cdk6 in vitro. The dephosphorylation of Cdk2 and Cdk6 by PP2C isoforms was inhibited by the binding of cyclins. We found that the PP2C-like activity in HeLa cell extract, partially purified HeLa PP2C alpha and PP2C beta 2 isoforms, and the recombinant PP2Cs exhibited a comparable substrate preference for a phosphothreonine containing substrate, consistent with the conservation of threonine residues at the site of activating phosphorylation in CDKs.

Amino Acid Sequence↗

PubMed2000

The effects of changing the site of activating phosphorylation in CDK2 from threonine to serine.

Cyclin-dependent kinases (CDKs) that control cell cycle progression are regulated in many ways, including activating phosphorylation of a conserved threonine residue. This essential phosphorylation is carried out by the CDK-activating kinase (CAK). Here we examine the effects of replacing this threonine residue in human CDK2 by serine. We found that cyclin A bound equally well to wild-type CDK2 (CDK2(Thr-160)) or to the mutant CDK2 (CDK2(Ser-160)). In the absence of activating phosphorylation, CDK2(Ser-160)-cyclin A complexes were more active than wild-type CDK2(Thr-160)-cyclin A complexes. In contrast, following activating phosphorylation, CDK2(Ser-160)-cyclin A complexes were less active than phosphorylated CDK2(Thr-160)-cyclin A complexes, reflecting a much smaller effect of activating phosphorylation on CDK2(Ser-160). The kinetic parameters for phosphorylating histone H1 were similar for mutant and wild-type CDK2, ruling out a general defect in catalytic activity. Interestingly, the CDK2(Ser-160) mutant was selectively defective in phosphorylating a peptide derived from the C-terminal domain of RNA polymerase II. CDK2(Ser-160) was efficiently phosphorylated by CAKs, both human p40(MO15)(CDK7)-cyclin H and budding yeast Cak1p. In fact, the k(cat) values for phosphorylation of CDK2(Ser-160) were significantly higher than for phosphorylation of CDK2(Thr-160), indicating that CDK2(Ser-160) is actually phosphorylated more efficiently than wild-type CDK2. In contrast, dephosphorylation proceeded more slowly with CDK2(Ser-160) than with wild-type CDK2, either in HeLa cell extract or by purified PP2Cbeta. Combined with the more efficient phosphorylation of CDK2(Ser-160) by CAK, we suggest that one reason for the conservation of threonine as the site of activating phosphorylation may be to favor unphosphorylated CDKs following the degradation of cyclins.

Amino Acid Substitution↗

PubMed2000

NEAR at eros: imaging and spectral results

Eros is a very elongated (34 kilometers by 11 kilometers by 11 kilometers) asteroid, most of the surface of which is saturated with craters smaller than 1 kilometer in diameter. The largest crater is 5.5 kilometers across, but there is a 10-kilometer saddle-like depression with attributes of a large degraded crater. Surface lineations, both grooves and ridges, are prominent on Eros; some probably exploit planes of weakness produced by collisions on Eros and/or its parent body. Ejecta blocks (30 to 100 meters across) are abundant but not uniformly distributed over the surface. Albedo variations are restricted to the inner walls of certain craters and may be related to downslope movement of regolith. On scales of 200 meters to 1 kilometer, Eros is more bland in terms of color variations than Gaspra or Ida. Spectra (800 to 2500 nanometers) are consistent with an ordinary chondrite composition for which the measured mean density of 2.67 +/- 0.1 grams per cubic centimeter implies internal porosities ranging from about 10 to 30 percent.

Journal Article↗

PubMed2000

Three-dimensional fold of the human AQP1 water channel determined at 4 A resolution by electron crystallography of two-dimensional crystals embedded in ice.

Here, we present a three-dimensional (3D) density map of deglycosylated, human erythrocyte aquaporin 1 (AQP1) determined at 4 A resolution in plane and approximately 7 A resolution perpendicular to the bilayer. The map was calculated by analyzing images and electron diffraction patterns recorded from tilted (up to 60 degrees ), ice-embedded, frozen-hydrated 2D crystals of AQP1 in lipid bilayer membranes. This map significantly extends the findings related to the folding of the AQP1 polypeptide chain determined by us at a lower, 7 A by approximately 20 A, resolution. The solvent-accessible volume within a monomer has a vestibular architecture, with a narrow, approximately 6.5 A diameter constriction near the center of the bilayer, where the location of the water-selective channel is postulated to exist. The clearly resolved densities for the transmembrane helices display the protrusions expected for bulky side-chains. The density in the interior of the helix barrel (putative NPA box region) is better resolved compared to our previous map, suggesting clearer linkage to some of the helices, and it may harbor short stretches of alpha-helix. At the bilayer extremities, densities for some of the inter-helix hydrophilic loops are visible. Consistent with these observed inter-helix connections, possible models for the threading of the AQP1 polypeptide chain are presented. A preferred model is deduced that agrees with the putative locations of a group of aromatic residues in the amino acid sequence and in the 3D density map.

Aquaporin 1↗

PubMed2000

Serine/threonine protein phosphatase type 1gamma1 is required for the completion of cytokinesis in human A549 lung carcinoma cells.

In lower eukaryotic organisms, the loss of serine/threonine protein phosphatase type 1 (PP1) results in growth arrest after the onset of mitosis. In humans, four highly homologous isoforms of PP1 (PP1alpha, PP1delta, PP1gamma1, and PP1gamma2) have been identified. Determining the roles of these phosphatases, however, has proven difficult due to the lack of subtype-specific inhibitors. In this study, we developed chimeric antisense 2'-O-(2-methoxy)ethylphosphothioate oligonucleotides targeting human PP1gamma1 that specifically inhibit PP1gamma1 gene expression. Two potent antisense oligonucleotides (ISIS 14435 and 14439; IC(50) approximately 50 nM) were then employed to elucidate the cellular functions of PP1gamma1 during cell cycle progression. In A549 cells, the inhibition of PP1gamma1 expression resulted in a dose-dependent inhibition of cellular proliferation, with growth arrest occurring after approximately 36-48 h, when PP1gamma1 mRNA expression was inhibited by >85%. Fluorescence-activated cell sorter analysis revealed that ISIS 14435/14439-induced growth arrest was associated with an increase in the number of cells containing 4N DNA. Immunostaining of treated cells revealed that the inhibition of PP1gamma1 expression had no apparent effect on the formation of mitotic spindles. However, decreased expression was associated with the failure of cell division in a late stage of cytokinesis and the formation of dikaryons.

Carcinoma↗

PubMed2000

Polymorphism in the packing of aquaporin-1 tetramers in 2-D crystals.

Hitherto, the packing arrangement of the aquaporin-1 (AQP1) tetramer in 2-dimensional (2-D) crystals (two-sided plane group p42(1)2) was observed to be largely similar (canonical crystal form) despite the difference in the source of the protein, the glycosylation state of the protein, the type of lipids, and the ratio of lipid to protein in the crystallization mixture. We report here our observation that the packing of AQP1 tetramers shows polymorphism in 2-D crystals generated in dioleoyl phosphatidylcholine bilayers. Apart from the canonical form, three additional allomorphs were identified. One was observed when small (0.25) lipid to protein ratio was used in the crystallization mixture while the other two were observed when the divalent cation content in the canonical crystals was modified. The various allomorphs were distinguished by different relative orientations of the AQP1 tetramer viewed in projection. The same, two-sided plane group p42(1)2 and similar unit cell dimensions were maintained in the different allomorphs as established by analysis of images of frozen-hydrated, nominally untilted crystals. Our results indicate that the interaction between the AQP1 monomers at the interface of the tetramers is flexible and is also strongly influenced by Mg(2+) ions with the cation effect materializing because of the intrinsic fluidity of the membrane.

Aquaporin 1↗

PubMed2000

Modulation of insulin signaling by protein tyrosine phosphatases.

Non-insulin-dependent diabetes mellitus (NIDDM) is a worldwide endocrine disorder afflicting persons of all races and age groups. At the molecular level NIDDM is often characterized by impaired insulin action on peripheral tissues. One important mechanism in regulating insulin signaling is mediated by protein tyrosine phosphatases, which may act on the insulin receptor itself and/or its substrates. Understanding the molecular events triggered by insulin has undoubtedly provided important clues in the treatment of NIDDM. In particular, the use of mouse models has helped us to focus on specific gene targets that are involved in the onset and progression of diabetes. Here we present an overview of the biochemical and genetic evidence supporting the role of five protein tyrosine phosphatases in insulin-mediated responses.

Animals↗

PubMed2000

Epidemiology of cervical and trochanteric fractures of the proximal femur in 1994 in Tangshan, China.

The purpose of this study was to determine the incidence of cervical and trochanteric fractures of the proximal femur in 1994 in Tangshan City, China. There are many reports on hip fracture incidence in many countries, suggesting that there are many factors affecting hip fractures. We visited 15 hospitals with an orthopaedic department within Tangshan City, and reviewed the medical records and radiographs of all patients with hip fractures occurring between January 1 and December 31, 1994. The population of Tangshan in 1994 was determined to be 1,454,543 (746,015 males and 708,528 females). The population of those over 65 years of age was 88,490 (41,519 males and 46,971 females), representing 6.08% of the total population. This study detected 184 cervical and trochanteric fractures of the proximal femur in 1994 in Tangshan (127 men and 57 women). The overall incidence or rate of the combined number of cervical and trochanteric fractures was 25 fractures per 100,000 population per year for men and 12 for women. There were a total of 147 cervical fractures (80%) and 37 trochanteric fractures (20%). The incidence of the combined number of cervical and trochanteric fractures in patients over 70 years of age increased to 108 for men and 156 for women. The incidence of hip fractures increased with age in both sex groups, especially in women over 65. Severe trauma fractures happened more often in younger groups, and mainly occurred in men, which may be a result of the particular composition of the population in Tangshan, which is young and male dominated. In addition, because Tangshan is an industrial city, many of its citizens are involved in occupations requiring a high level of physical activity.

Age Distribution↗