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G Chen

Publications and source records attributed to G Chen.

At least 451 records · Page 25Linked to original sources
PubMed1998

Elasticity of single-crystal MgO to 8 gigapascals and 1600 kelvin

The cross pressure (P) and temperature (T) dependence of the elastic moduli (Cij) of single-crystal samples of periclase (MgO) from acoustic wave travel times was measured with ultrasonic interferometry: partial differential2C11/ partial differentialP partial differentialT = (-1.3 +/- 0.4) x 10(-3) per kelvin; partial differential2C110/ partial differentialP partial differentialT = (1. 7 +/- 0.7) x 10(-3) per kelvin; and partial differential2C44/ partial differentialP partial differentialT = (-0.2 +/- 0.3) x 10(-3) per kelvin. The elastic anisotropy of MgO decreases with increasing pressure at ambient temperature, but then increases as temperature is increased at high pressure. An assumption of zero cross pressure and temperature derivatives for the elastic moduli underestimates the elastic anisotropy and overestimates the acoustic velocities of MgO at the extrapolated high-pressure and high-temperature conditions of Earth's mantle.

Journal Article↗
PubMed1998

Regulation of cyclo-oxygenase gene expression in rat smooth muscle cells by catalase.

We have studied, in detail, the effect of catalase, one of the naturally occurring antioxidant enzymes, on the expression of cyclo-oxygenase (COX) mRNA and protein in rat aortic smooth muscle cells (RASMC). The activity of COX enzyme within the cells was also determined. Catalase either alone or in combination with interleukin-1beta (IL-1beta) enhanced mRNA and protein expression for cyclo-oxygenase 2 (COX-2) in a concentration-dependent manner. However, it did not affect the expression of mRNA or protein for cyclo-oxygenase 1 (COX-1). The expression of mRNA for COX-2 induced by catalase was blocked completely by actinomycin D (ACT) or cycloheximide (CHX). In comparison, expression of mRNA for COX-2 stimulated by IL-1beta was inhibited by actinomycin D, but not by cycloheximide. This suggests that induction of the synthesis of mRNA for COX-2 by catalase and IL-1beta involves different mechanisms. In particular, the induction of mRNA for COX-2 by catalase requires on-going protein and RNA synthesis, but the induction following exposure to IL-1beta does not. The increase in expression of mRNA for COX-2 induced by catalase may be related to the ability of catalase to stimulate cyclic AMP response element (CRE) and NF-IL6 transcription factors, but not nuclear factor kappa B (NF-kappaB), for electrophoretic mobility shift assays (EMSA) showed that catalase enhanced nuclear factor binding to cyclic AMP response element and NF-IL6 but not to NF-kappaB. Catalase exerted a biphasic effect on prostaglandin synthesis. At low concentrations it enhanced prostaglandin production, but at high concentrations it tended to inhibit it. These findings suggest that catalase has differential and multiple effects on COX expression and activity in rat aortic smooth muscle cells.

Animals↗
PubMed1998

Conformationally defined retinoic acid analogues. 4. Potential new agents for acute promyelocytic and juvenile myelomonocytic leukemias.

We recently synthesized several conformationally constrained retinoic acid (RA) analogues [8-(2'-cyclohexen-1'-ylidene)-3, 7-dimethyl-2,4,6-octatrienoic acids with different alkyl substituents at 2' (R1) and 3' (R2) positions on the cyclohexene ring] (Muccio et al. J. Med. Chem. 1996, 39, 3625) as cancer chemopreventive agents. UAB8 (R1 = Et; R2 = iPr), which contains sufficient steric bulk at the terminal end of the polyene chain to mimic the trimethylcyclohexenyl ring of RA, displayed biological properties similar to those of RA. To explore the efficacy of this retinoid in acute promyelocytic leukemia (APL) and juvenile myelomonocytic leukemia (JMML), we evaluated UAB8 isomers in in vitro assays which measure the capacity of retinoids to inhibit aberrant myeloid colony growth from blood or bone marrow cells obtained from human JMML patients and in assays measuring the potential of retinoids to differentiate NB4 cells (an APL cell line). Both (all-E)- and (13Z)-UAB8 were 2-fold more active than RA in the NB4 cell differentiation assay; however, only (all-E)-UAB8 had comparable activity to the natural retinoids in the JMML cell assays. These results were compared to the biological effectiveness of a new retinoid, UAB30 [8-(3', 4'-dihydro-1'(2'H)-naphthalen-1'-ylidene)-3,7-dimethyl-2,4, 6-octatrienoic acid], which had different nuclear receptor binding and transactivational properties than UAB8. Relative to (all-E)-RA and (all-E)-UAB8, (all-E)-UAB30 bound well to RARalpha but did not activate transcription-mediated RARalpha homodimers, even though it was effective in RARbeta- and RARgamma-mediated transactivational assays. In APL assays, this retinoid had much reduced activity and was only moderately effective in JMML assays and in cancer chemoprevention assays.

Animals↗
PubMed1998

Dopamine levels of two classes of vesicles are differentially depleted by amphetamine.

Differential depletion of neurotransmitter by amphetamine in two classes of vesicles, termed large vesicles and small vesicles, has been studied with amperometry. Carbon fiber microelectrodes have been used to monitor and quantify exocytotic events. Current transients, corresponding to individual exocytotic events, have been obtained from the cell body of the dopamine-containing neuron of Planorbis corneus. The dopamine released from individual vesicles of these cells has been compared for cells treated with D-amphetamine vs. control cells. Our results show that amphetamine has differential effects on the release of dopamine from the two classes of vesicles. Thus, it is concluded that at low concentrations, amphetamine preferentially depletes the large vesicles with a minimal effect on the small vesicles. At high concentrations, amphetamine depletes small vesicles more strongly than large vesicles although amphetamine continues to deplete the large vesicles in a dose-dependent manner. Our data appear to indicate that the two classes of vesicles observed in the Planorbis dopamine neuron might have different mechanisms associated with transmitter depletion.

Amphetamine↗
PubMed1998

Regulation of N-arginine dibasic convertase activity by amines: putative role of a novel acidic domain as an amine binding site.

Peptide sequence analysis and cDNA cloning indicate that a previously described mouse arginine-specific dibasic cleaving enzyme (dynorphin converting enzyme) [Csuhai et al. (1995) Biochemistry 34, 12411] is the homologue of N-arginine dibasic convertase (NRDc) isolated from rat testis [Chesneau et al. (1994) J. Biol. Chem. 269, 2056]. A mouse NRDc cDNA exhibited 98% amino acid identity with the rat cDNA. However, within a 74 residue acidic stretch, this identity drops to 82%. Likewise, the corresponding acidic stretch of human NRDc is only 73% identical with that of rat NRDc. To reconcile previously observed kinetic differences between rat and mouse NRDc, the hydrolysis of peptide substrates by the rat, human, and mouse enzymes was compared using phosphate and Tris as buffers. Although the three NRDc's behaved similarly, Tris had a pronounced effect on the kinetics of peptide hydrolysis. With BAM-8, alpha-neoendorphin, and dynorphin B as substrates, Tris increased KM up to 40-fold with little change in Vmax, while with dynorphin A or somatostatin 28 as substrate, Tris caused a decrease in KM of up to 100 fold, again with only a modest change in Vmax. Other amines, including the polyamines putrescine, spermidine, and spermine, all affected NRD convertase activity. It is proposed that amines bind to the acidic stretch found in NRDc, and that quantitative differences in the sensitivity to amines between the rat, mouse, and human enzymes can be at least partially accounted for by differences in their acidic stretch. The role of polyamines as physiological modulators of N-arginine dibasic convertase is considered.

Amino Acid Sequence↗
PubMed1998

Unusual properties of thermally sensitive oligomer-enzyme conjugates of poly(N-isopropylacrylamide)-trypsin.

Reversible soluble-insoluble oligomer-enzyme conjugates have been prepared by conjugating a thermally sensitive oligomer, poly(N-isopropylacrylamide) [poly(NIPAAm)] to trypsin. The conjugates can catalyze enzymatic reactions in solution and then may be separated from the solution by thermal precipitation. One special feature of the conjugates is that every poly(NIPAAm) chain has only one end attachment to the enzyme, so that the loss of enzymatic activity due to steric hindrance should be minimized. Conjugates with various numbers of oligomer chains per trypsin molecule were prepared. Surprisingly, the conjugates increased in enzymatic activity with increasing oligomer conjugation to the native trypsin. The trypsin active sites in the conjugates were accessible to large molecules, such as soybean trypsin inhibitor (MW = 21,500). The enzyme conjugates were more stable than native trypsin, both in solution and in the precipitated phase. On the other hand, the conjugates lost enzymatic activity faster than native trypsin when the temperature was repeatedly cycled through the lower critical solution temperature (LCST) of the poly(NIPAAm). The recovery of the conjugates by thermal precipitation in each cycle was over 95% even after 14 cycles through the LCST.

Acrylamides↗
PubMed1998

Presynaptic GABAB autoreceptor modulation of P/Q-type calcium channels and GABA release in rat suprachiasmatic nucleus neurons.

GABA is the primary transmitter released by neurons of the suprachiasmatic nucleus (SCN), the circadian clock in the brain. Whereas GABAB receptor agonists exert a significant effect on circadian rhythms, the underlying mechanism by which GABAB receptors act in the SCN has remained a mystery. We found no GABAB receptor-mediated effect on slow potassium conductance, membrane potential, or input resistance in SCN neurons in vitro using whole-cell patch-clamp recording. In contrast, the GABAB receptor agonist baclofen (1-100 microM) exerted a large and dose-dependent inhibition (up to 100%) of evoked IPSCs. Baclofen reduced the frequency of spontaneous IPSCs but showed little effect on the frequency or amplitude of miniature IPSCs in the presence of tetrodotoxin. The activation of GABAB receptors did not modulate postsynaptic GABAA receptor responses. The depression of GABA release by GABAB autoreceptors appeared to be mediated primarily through a modulation of presynaptic calcium channels. The baclofen inhibition of both calcium currents and evoked IPSCs was greatly reduced (up to 100%) by the P/Q-type calcium channel blocker agatoxin IVB, suggesting that P/Q-type calcium channels are the major targets involved in the modulation of GABA release. To a lesser degree, N-type calcium channels were also involved. The inhibition of GABA release by baclofen was abolished by a pretreatment with pertussis toxin (PTX), whereas the inhibition of whole-cell calcium currents by baclofen was only partially depressed by PTX, suggesting that G-protein mechanisms involved in GABAB receptor modulation at the soma and axon terminal may not be identical. We conclude that GABAB receptor activation exerts a strong presynaptic inhibition of GABA release in SCN neurons, primarily by modulating P/Q-type calcium channels at axon terminals.

Animals↗
PubMed1998

A mathematical model for the generation and control of a pH gradient in an immobilized enzyme system involving acid generation.

An optimal pH control technique has been developed for multistep enzymatic synthesis reactions where the optimal pH differs by several units for each step. This technique separates an acidic environment from a basic environment by the hydrolysis of urea within a thin layer of immobilized urease. With this technique, a two-step enzymatic reaction can take place simultaneously, in proximity to each other, and at their respective optimal pH. Because a reaction system involving an acid generation represents a more challenging test of this pH control technique, a number of factors that affect the generation of such a pH gradient are considered in this study. The mathematical model proposed is based on several simplifying assumptions and represents a first attempt to provide an analysis of this complex problem. The results show that, by choosing appropriate parameters, the pH control technique still can generate the desired pH gradient even if there is an acid-generating reaction in the system.

Acids↗
PubMed1998

Studies on an affinity label for the sulfuryl acceptor binding site in an aryl sulfotransferase.

Active site-directed affinity labeling was utilized to elucidate peptide sequences at the binding site for sulfuryl acceptors in rat hepatic aryl sulfotransferase (AST) IV (also known as tyrosine-ester sulfotransferase, EC 2.8.2.9). The affinity labeling reagent, N-bromoacetyl-4-hydroxyphenylamine, was designed on the basis of substrate specificity studies with para-substituted phenols, utilization of a bromoacetamido group for reactivity with active site amino acid residues and its similarity to acetaminophen, a known substrate for aryl (phenol) sulfotransferases. AST IV utilized N-bromoacetyl-4-hydroxyphenylamine as a substrate with kinetic constants that compared favorably to those obtained with acetaminophen. Incubation of AST IV with N-bromoacetyl-4-hydroxyphenylamine at pH 7.0 in the absence of PAPS and other substrates resulted in an irreversible inactivation of the enzyme that was both time- and concentration-dependent. [14C]-N-bromoacetyl-4-hydroxyphenylamine was synthesized and used to analyze the regions of protein sequence that were involved in the binding of the affinity label. AST IV was incubated with [14C]-N-bromoacetyl-4-hydroxyphenylamine, hydrolyzed with endoproteinase Lys-C and the labeled peptides were purified by HPLC. Control incubations of AST IV with the affinity label in the presence of 4-propylphenol and PAP were utilized to ascertain the specificity of the interaction. Sequence analysis of the labeled peptides, carried out by automated Edman degradation, revealed labeling sites on cysteine (Cys-232, Cys-283 and Cys-289) and lysine (Lys-286) residues near the C-terminus of the protein. The locations of these labeling sites were further evaluated both by sequence-alignment with other sulfotransferases and by theoretical calculations on predicted secondary structure.

Affinity Labels↗
PubMed1998

An AB block copolymer of oligo(methyl methacrylate) and poly(acrylic acid) for micellar delivery of hydrophobic drugs.

An AB block copolymer of oligo(methyl methacrylate) (oMMA) and poly(acrylic acid) (PAAc) has been synthesized. The block copolymer forms micelles in an aqueous medium, as confirmed by a fluorescence probe technique using pyrene. Doxorubicin hydrochloride was incorporated into the micelle and the release profile of doxorubicin hydrochloride was investigated. Slow and prolonged release of doxorubicin hydrochloride from the micelle was observed. The AB block copolymer micelle can be useful for prolonged mucosal drug delivery of hydrophobic drugs.

Acrylic Resins↗
PubMed1998

Protection against cyclophosphamide-induced alopecia and inhibition of mammary tumor growth by topical 1,25-dihydroxyvitamin D3 in mice.

Twenty-one-day-old BALB/c mice were shaved on the back to synchronize hair growth. On day 30 or 31, when at least 90% of mice exhibited hair regrowth in the shaved area, 1,25(OH)2D3 was applied topically to the shaved area daily for 5 days. On the 6th day, cyclophosphamide (Cytoxan, CTX) was injected i.p. to induce hair loss in the shaved area. Alopecia was induced in a dose-dependent manner by CTX treatment within 1 to 2 weeks. This effect was reduced significantly if mice were pre-treated with 1,25(OH)2D3, though only slight protection was observed in female mice. Interestingly, this 1,25(OH)2D3-mediated protection against hair loss was attenuated in male mice but became more significant in female mice when they were inoculated with the EMT-6 murine mammary tumor prior to treatment. More importantly, topical treatment with 1,25(OH)2D3 alone was able to inhibit EMT-6 tumor growth in both male and female BALB/c mice. Furthermore, 1,25(OH)2D3 pre-treatment also augmented the anti-tumor effect of CTX. Our results demonstrate that topical application of 1,25(OH)2D3 can protect against CTX-induced alopecia both in tumor-free and in tumor-bearing mice in a sex-dependent manner. Moreover, 1,25(OH)2D3 was shown, either alone or in combination with CTX, to inhibit tumor growth.

Administration, Topical↗
PubMed1998

The roles of glycine residues in the ATP binding site of human brain hexokinase.

Mutants of hexokinase I (Arg539 --> Lys, Thr661 --> Ala, Thr661 --> Val, Gly534 --> Ala, Gly679 --> Ala, and Gly862 --> Ala), located putatively in the vicinity of the ATP binding pocket, were constructed, purified to homogeneity, and studied by circular dichroism (CD) spectroscopy, fluorescence spectroscopy, and initial velocity kinetics. The wild-type and mutant enzymes have similar secondary structures on the basis of CD spectroscopy. The mutation Gly679 --> Ala had little effect on the kinetic properties of the enzyme. Compared with the wild-type enzyme, however, the Gly534 --> Ala mutant exhibited a 4000-fold decrease in kcat and the Gly862 --> Ala mutant showed an 11-fold increase in Km for ATP. Glucose 6-phosphate inhibition of the three glycine mutants is comparable to that of the wild-type enzyme. Inorganic phosphate is, however, less effective in relieving glucose 6-phosphate inhibition of the Gly862 --> Ala mutant, relative to the wild-type enzyme and entirely ineffective in relieving inhibition of the Gly534 --> Ala mutant. Although the fluorescence emission spectra showed some difference for the Gly862 --> Ala mutant relative to that of the wild-type enzyme, indicating an environmental alteration around tryptophan residues, no change was observed for the Gly534 --> Ala and Gly679 --> Ala mutants. Gly862 --> Ala and Gly534 --> Ala are the first instances of single residue mutations in hexokinase I that affect the binding affinity of ATP and abolish phosphate-induced relief of glucose 6-phosphate inhibition, respectively.

Adenosine Triphosphate↗
PubMed1998

Treatment of severe aplastic anemia with an immunosuppressive agent plus recombinant human granulocyte-macrophage colony-stimulating factor and erythropoietin.

To evaluate the therapeutic potential of hematopoietic growth factors (HGFs) during immunosuppressive treatment (IST) of severe aplastic anemia (SAA), 38 patients with newly diagnosed SAA received IST alone (group I), or IST plus recombinant human erythropoietin and granulocyte-macrophage colony-stimulating factor (rhEPO + rhGM-CSF) (group II). Eleven patients in each group received antilymphocyte globulin (ALG) for IST, and eight patients in each group received cyclosporine (CSA). Complete remission rates at one year were 26% and 74% for group I and group II patients, respectively. The ALG-treated subgroup showed the greatest differences between treatments. Compared with patients receiving ALG alone, patients treated with ALG plus HGFs had significantly better one-year survival (100% vs. 54.5%, P < 0.05), complete remission rates (91% vs. 36%, P < 0.05), more rapid and complete hematologic recovery, greater reductions in transfusion requirements, and lower infection rates. The data suggest a potential role for rhEPO + rhGM-CSF therapy in SAA patients receiving IST.

Adolescent↗
PubMed1998

Effect of protein and cell behavior on pattern-grafted thermoresponsive polymer.

A thermoresponsive copolymer, poly(Nisopropylacrylamide-co-acrylic acid), was coupled with azidoaniline. The azidophenyl-derivatized copolymer was grafted in a specific pattern on a polystyrene matrix by photolithography. The surface micropattern appeared and disappeared interchangeably, as observed under a phase-contrast microscope, by varying the temperature between 10 degrees C and 37 degrees C. The copolymer-grafted polystyrene surface was hydrophobic at 37 degrees C and hydrophilic at 10 degrees C. Albumin and fibronectin adsorption on the matrix was investigated using the fluorescent-labeling method. Fibronectin adsorbed onto both the grafted and nongrafted regions, while albumin adsorbed more onto the nongrafted regions than the grafted regions. Protein adsorption did not affect surface wettability. Mouse fibroblast STO cells were cultured on tissue culture plates pattern-grafted with the thermoresponsive copolymer. Fibronectin adsorption enhanced cell spreading, while albumin reduced it. When the temperature was lowered, the cells selectively detached from the surface areas grafted with the thermoresponsive copolymer when cultured in serum-free medium; the cells partially detached from these areas when cultured in serum-containing medium. The effect of serum proteins on cell detachment was similar to that caused by a mixture of albumin and fibronectin. Albumin adsorption did not affect the detachment of cells, while fibronectin adsorption inhibited it. The results of the present study indicate that a pattern-grafted, thermoresponsive, azidophenyl-derivatized copolymer can effectively facilitate selective cell detachment under some conditions such as serum-free culture or preadsorption of albumin. The pattern-grafting technique will be useful for qualitative microscopic comparison of surfaces prepared differently on one chip under the same conditions.

Acrylamides↗
PubMed1998

Influence of dioxin and metal-contaminated sediment on phase I and II biotransformation enzymes in silver crucian carp.

Several biochemical responses were measured in silver crucian carp (Carassius auratus gibelio) after exposure to sediments obtained from contaminated Ya-Er Lake, No. 1 pond, and an unpolluted reference site, Honglian Lake. After 1 week of exposure, a significant induction of the phase I biotransformation enzyme (ethoxylresorufin-o-deethylase, EROD) was found (83-fold of control), whereas the phase II biotransformation enzyme (glutathione S-transferase, GST) exhibited a slight, but significant induction (1.4-fold of control) after 4 weeks of exposure. The level of cellular glutathione in the liver was also slightly elevated after 4 weeks of exposure. The delayed response of GST to the contaminants indicates that the phase I and phase II biotransformation enzymes are regulated differently in fish. The results suggest that EROD is a sensitive bioindicator to assess the toxicity of dioxin-contamined sediment in the laboratory.

Animals↗
PubMed1998

Glutathione S-transferase activity in aquatic macrophytes with emphasis on habitat dependence.

Glutathione S-transferase activity of both the microsomal and soluble fractions was determined in a variety of aquatic macrophytes. The examined enzyme extract was prepared from a combination of leaves and shoots. Four different model substrates were used. The highest conjugation rate was obtained for 1-iodo-2,4-dinitrobenzene, followed by 1-chloro-2,4-dinitrobenzene, p-nitrobenzoyl chloride, and 1,2-dichloro-4-nitrobenzene. Comparison of several samples of Nuphar lutea L. from two different lake areas revealed increased glutathione S-transferase activity in plants from the site contaminated with polyaromatic hydrocarbons.

Environment↗
PubMed1998

Chemoattractive effect on the effector cells of the supernatants from melanoma cells transfected with the interleukin-2 (IL-2), IL-4 or IL-6 gene.

By using the Boyden chamber system, the chemoattractive activity of the supernatants from the B16 melanoma cells transfected with the interleukin-2 (IL-2), IL-4 or IL-6 gene was investigated. We found that supernatants from IL-2- or IL-6-gene-transfected B16 melanoma cells showed chemoattractive activity on natural killer (NK) cells, CD4+ T cells and CD8+ T cells, while the supernatants from IL-4-gene-transfected B16 melanoma cells showed chemotactic activity on macrophages but not on NK cells and T cells. Further, the chemoattractive activity of the supernatants from cytokine-gene-transfected B16 cells might be the direct effects of the cytokine that they have been engineered to secrete. These results indicate that reduced tumorigenicity and increased immunogenicity of cytokine-gene-transfected B16 cells may be attributed to the direct recruitment of immune effector cells by the secreted cytokine, which plays an important role in the induction of the local antitumor immunity.

Animals↗