Search PubMed⌕ Search

Biomedical subjects

D Sun

Publications and source records attributed to D Sun.

At least 37 records · Page 2Linked to original sources
PubMed2002

Electrochemical impedance spectroscopy study of high-palladium dental alloys. Part I: behavior at open-circuit potential.

Electrochemical impedance spectroscopy (EIS) was used to study the in vitro corrosion of three representative high-palladium alloys and a gold-palladium alloy for comparison. The corrosion resistances (measured as the charge transfer resistance R(CT) from an equivalent circuit) of the high-palladium alloys and the gold-palladium alloy were comparable in simulated body fluid and oral environments, and under simulated dental plaque. The great similarity in corrosion behavior for the three high-palladium alloys is largely attributed to their substantial palladium content and passivity in the laboratory test media, and possibly to their similar structure at the submicron level. Differences in composition and microstructure at the micron level and greater, including the effects of heat treatment simulating the firing cycles for dental porcelain, do not have noteworthy effects on the in vitro corrosion of the three high-palladium alloys. Good accuracy and convenience of extracting corrosion characteristics from equivalent circuit modeling, along with the capability of providing intrinsic information about the corrosion mechanism, enable EIS to be an excellent alternative method to conventional potentiodynamic polarization for evaluating the corrosion behavior of noble dental alloys.

Journal Article↗
PubMed2002

Electrochemical impedance spectroscopy study of high-palladium dental alloys. Part II: behavior at active and passive potentials.

Electrochemical impedance spectroscopic (EIS) analyses were performed on three high-palladium alloys and a gold-palladium alloy at active and passive potentials in five electrolytes that simulated body fluid and oral environmental conditions. All four alloys were previously found to have excellent corrosion resistance in these in vitro environments. Before performing the EIS analyses, alloy specimens were subjected to a clinically relevant heat treatment that simulated the firing cycles for a dental porcelain. It was found that the EIS spectra varied with test potential and electrolyte. Diffusional effects, related to the dealloying and subsequent surface enrichment in palladium of the high-palladium alloys, along with species adsorption and passivation, were revealed at both active and passive potentials, although these effects were more evident at the passive potentials.

Journal Article↗
PubMed2002

Characterization of genomic DNA encoding cecropins from an Aedes albopictus mosquito cell line.

We used cDNA probes from Aedes albopictus mosquito cecropins AalCecA, B, and C to obtain genomic DNA copies and flanking DNA. Two gene copies (AalCecA1 and A2, AalCecB1 and B2, AalCecC1 and C2) encoding each of the three mature cecropin peptides were recovered. All these genes had a similar organization, into two exons interrupted by a single short intron. AalCecA1 and AalCecA2 encode mature protein products that differ by one amino acid residue, while AalCecB1 and AalCecB2, AalCecC1 and AalCecC2 encode identical mature cecropin peptides, respectively. The AalCecB and C gene pairs each share a common intergenic region of approximately 1 kb, with the two coding regions transcribed in opposite directions. With the exception of small insertions/deletions, the intergenic spacer region was highly conserved between the B1/C1 and B2/C2 clones. In transfected cells, 0.8 kb of upstream sequence was sufficient for inducible expression of AalCecA1. Within this region, a 28 bp sequence at positions -192 to -165 upstream of the transcription initiation site was found to contain a potential regulatory element. In electrophoretic mobility shift assays, synthetic double-stranded DNA containing this 28 bp sequence retarded protein in cytoplasmic and nuclear extracts from C7-10 cells.

Aedes↗
PubMed2002

Traumatic brain injury induced cell proliferation in the adult mammalian central nervous system.

Recent studies indicate the existence of progenitor cells and their potential for neurogenesis in the subventricular zone (SVZ) and the hippocampus of the normal adult mammalian brain. However, the proliferative response and the specific cell types generated following traumatic brain injury have not been examined. This cellular response to CNS injury was investigated using the fluid percussion injury (FPI) model, a widely accepted rat model that simulates moderate head injury sustained in humans. Forty-eight hours following moderate FPI, adult rats received intraperitoneal injections of the thymidine analogs, 5-bromodeoxyuridine (BrdU) or tritiated thymidine (3H-thymidine), which are markers for mitotic activity. Injured and control animals receiving BrdU were used to determine the total number of cells induced to proliferate. To determine the cellular identity of these proliferating cells, animals receiving 3H-thymidine were sacrificed and sections through the injured area were immunostained with markers for immature and mature astrocytes, activated microglia, neural precursors and mature neurons. These studies showed that the total number of proliferating cells was significantly increased in the injury group for both the SVZ and the hippocampus. However, the proliferating cells in the SVZ did not express any of the cellular markers used, suggesting that they have not yet begun to differentiate. In contrast, there was a significant increase in the number of immature astrocytes and activated microglia, but not neurons, at this early time point in the hippocampus. Taken together, these experiments demonstrate the compensatory capacity of the adult brain to injury and should lead to a new generation of studies aimed at enhancing the neuronal proliferative response.

Age Factors↗
PubMed2001

Quadruplex-interactive agents as telomerase inhibitors: synthesis of porphyrins and structure-activity relationship for the inhibition of telomerase.

The cationic porphyrin 5,10,15,20-tetra-(N-methyl-4-pyridyl)porphyrin (TMPyP4) binds to quadruplex DNA and is thereby an inhibitor of human telomerase (Wheelhouse et al. J. Am. Chem. Soc. 1998, 120, 3261-3262). Herein the synthesis and telomerase-inhibiting activity of a wide range of analogues of TMPyP4 are reported, from which rules for a structure-activity relationship (SAR) have been discerned: (1) stacking interactions are critical for telomerase inhibition, (2) positively charged substituents are important but may be interchanged and combined with hydrogen-bonding groups, and (3) substitution is tolerated only on the meso positions of the porphyrin ring, and the bulk of the substituents should be matched to the width of the grooves in which they putatively lie. This SAR is consistent with a model presented for the complexation of TMPyP4 with human telomeric quadruplex DNA.

Antineoplastic Agents↗
PubMed2001

The proteinase-activated receptor 2 is involved in nociception.

The proteinase-activated receptor 2 is expressed on a subset of primary afferent neurons and may participate in the neurogenic component of inflammation. We hypothesized that this receptor may also play a role in neuronal sensitization and contribute to the pathogenesis of pain in inflammatory conditions such as pancreatitis. Using a specific proteinase-activated receptor 2 activating peptide, we found evidence of such sensitization in vitro in the form of enhanced capsaicin- and KCl-evoked release of calcitonin gene-related peptide, a marker for nociceptive signaling. We then demonstrated that injection of the proteinase-activated receptor 2 activating peptide into the pancreatic duct can activate and sensitize pancreas-specific afferent neurons in vivo, as measured by Fos expression in the dorsal horn of the spinal cord. These observations suggest that proteinase-activated receptor 2 contributes to nociceptive signaling and may provide a novel link between inflammation and pain.

Animals↗
PubMed2001

Re-engineering monovalent cation binding sites of methylamine dehydrogenase: effects on spectral properties and gated electron transfer.

Methylamine dehydrogenase (MADH) is a tryptophan tryptophylquinone (TTQ)-dependent enzyme that catalyzes the oxidative deamination of primary amines. Monovalent cations are known to affect the spectral properties of MADH and to influence the rate of the gated electron transfer (ET) reaction from substrate-reduced MADH to amicyanin. Two putative monovalent cation binding sites in MADH have been identified by X-ray crystallography [Labesse, G., Ferrari, D., Chen, Z.-W., Rossi, G.-L., Kuusk, V., McIntire, W. S., and Mathews, F. S. (1998) J. Biol. Chem. 273, 25703-25712]. One requires cation-pi interactions involving residue alpha Phe55. An alpha F55A mutation differentially affects these two monovalent cation-dependent phenomena. The apparent K(d) associated with spectral perturbations increases 10-fold. The apparent K(d) associated with enhancement of the gated ET reaction becomes too small to measure, indicating that either it has decreased more than 1000-fold or the mutation has caused a conformational change that eliminates the requirement for the cation for the gated ET. These results show that of the two binding sites revealed in the structure, cation binding to the distal site, which is stabilized by the cation-pi interactions, is responsible for the spectral perturbations. Cation binding to the proximal site, which is stabilized by several oxygen ligands, is responsible for the enhancement of the rate of gated ET. Another site-directed mutant, alpha F55E MADH, exhibited cation binding properties that were the same as those of the native enzyme, indicating that interactions with the carboxylate of Glu can effectively replace the cation-pi interactions with Phe in stabilizing monovalent cation binding to the distal site.

Bacterial Proteins↗
PubMed2001

G-quadruplex DNA binding by a series of carbocyanine dyes.

We have examined a number of carbocyanine dyes for their ability to bind intramolecular G-quadruplex DNA structures (G4'-DNA) using a Taq polymerase stop assay. Of the five dyes examined, only one, N,N'-diethylthiacarbocyanine iodide (DTC), was found to bind to G4'-DNA. DTC was also the only dye found to inhibit human telomerase at 50 microM concentration.

Binding Sites↗
PubMed2001

Expression of Na(+)-K(+)-Cl(-) cotransporter in rat brain during development and its localization in mature astrocytes.

Na(+)-K(+)-Cl(-) cotransporter has been proposed to play an important role in the regulation of intracellular Cl(-) concentration in neurons during development. In this study, the expression pattern of the cotransporter in different regions of rat brain was examined at birth (P0), postnatal days 7 (P7), P14, P21, and adult by Western blotting analysis. In cortex, thalamus, cerebellum and striatum, the cotransporter expression level was low at P0 and significantly increased at P14 (P<0.05). The expression peaked at P21 and was maintained at the same level in adulthood. However, in hippocampus, a peak level of the cotransporter expression was detected in adult brain. The immunocytochemistry study of adult rat brain revealed that an intense staining of the Na(+)-K(+)-Cl(-) cotransporter protein was observed in dendritic processes of CA1-CA3 hippocampal pyramidal neurons. In contrast, abundant immuno-reactive signals of the cotransporter were found in somata of thalamic nucleus. Immunofluorescence double staining demonstrates that the Na(+)-K(+)-Cl(-) cotransporter was expressed in astrocytes within cortex, corpus callosum, hippocampus and cerebellum. In addition, co-localization of the cotransporter and glial fibrillary acidic protein (GFAP), or with aquaporin 4, was found in perivascular astrocytes of cortical cortex and white matter. The results indicate that a time-dependent expression of the Na(+)-K(+)-Cl(-) cotransporter protein occurs not only in cortex but also in hippocampus, striatum, thalamus and cerebellum. In addition, the cotransporter is expressed in astrocytes and perivascular astrocytes of adult rat brain.

Aging↗
PubMed2001

Mediation of tubuloglomerular feedback by adenosine: evidence from mice lacking adenosine 1 receptors.

Adenosine is a determinant of metabolic control of organ function increasing oxygen supply through the A2 class of adenosine receptors and reducing oxygen demand through A1 adenosine receptors (A1AR). In the kidney, activation of A1AR in afferent glomerular arterioles has been suggested to contribute to tubuloglomerular feedback (TGF), the vasoconstriction elicited by elevations in [NaCl] in the macula densa region of the nephron. To further elucidate the role of A1AR in TGF, we have generated mice in which the entire A1AR coding sequence was deleted by homologous recombination. Homozygous A1AR mutants that do not express A1AR mRNA transcripts and do not respond to A1AR agonists are viable and without gross anatomical abnormalities. Plasma and urinary electrolytes were not different between genotypes. Likewise, arterial blood pressure, heart rates, and glomerular filtration rates were indistinguishable between A1AR(+/+), A1AR(+/-), and A1AR(-/-) mice. TGF responses to an increase in loop of Henle flow rate from 0 to 30 nl/min, whether determined as change of stop flow pressure or early proximal flow rate, were completely abolished in A1AR(-/-) mice (stop flow pressure response, -6.8 +/- 0.55 mmHg and -0.4 +/- 0.2 in A1AR(+/+) and A1AR(-/-) mice; early proximal flow rate response, -3.4 +/- 0.4 nl/min and +0.02 +/- 0.3 nl/min in A1AR(+/+) and A1AR(-/-) mice). Absence of TGF responses in A1AR-deficient mice suggests that adenosine is a required constituent of the juxtaglomerular signaling pathway. A1AR null mutant mice are a promising tool to study the functional role of A1AR in different target tissues.

Adenosine↗
PubMed2001

Mutant p53 cooperates with ETS and selectively up-regulates human MDR1 not MRP1.

The most frequently expressed drug resistance genes, MDR1 and MRP1, occur in human tumors with mutant p53. However, it was unknown if mutant p53 transcriptionally regulated both MDR1 and MRP1. We demonstrated that mutant p53 did not activate either the MRP1 promoter or the endogenous gene. In contrast, mutant p53 strongly up-regulated the MDR1 promoter and expression of the endogenous MDR1 gene. Notably, cells that expressed either a transcriptionally inactive mutant p53 or the empty vector showed no endogenous MDR1 up-regulation. Transcriptional activation of the MDR1 promoter by mutant p53 required an Ets binding site, and mutant p53 and Ets-1 synergistically activated MDR1 transcription. Biochemical analysis revealed that Ets-1 interacted exclusively with mutant p53s in vivo but not with wild-type p53. These findings are the first to demonstrate the induction of endogenous MDR1 by mutant p53 and provide insight into the mechanism.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed2001

Myelin antigen-specific CD8+ T cells are encephalitogenic and produce severe disease in C57BL/6 mice.

Encephalitogenic T cells that mediate experimental autoimmune encephalomyelitis (EAE) are commonly assumed to be exclusively CD4+, but formal proof is still lacking. In this study, we report that synthetic peptides 35-55 from myelin oligodendrocyte glycoprotein (pMOG(35-55)) consistently activate a high proportion of CD8+ alphabetaTCR+ T cells that are encephalitogenic in C57BL/6 (B6) mice. The encephalitogenic potential of CD8+ MOG-specific T cells was established by adoptive transfer of CD8-enriched MOG-specific T cells. These cells induced a much more severe and permanent disease than disease actively induced by immunization with pMOG(35-55). CNS lesions in pMOG(35-55) CD8+ T cell-induced EAE were progressive and more destructive. The CD8+ T cells were strongly pathogenic in syngeneic B6 and RAG-1(-/-) mice, but not in isogeneic beta2-microglobulin-deficient mice. MOG-specific CD8+ T cells could be repeatedly reisolated for up to 287 days from recipient B6 or RAG-1(-/-) mice in which disease was induced adoptively with <1 x 10(6) T cells sensitized to pMOG(35-55). It is postulated that MOG induces a relapsing and/or progressive pattern of EAE by eliciting a T cell response dominated by CD8+ autoreactive T cells. Such cells appear to have an enhanced tissue-damaging effect and persist in the animal for long periods.

Adoptive Transfer↗
PubMed2001

Novel inhibitors of poly(ADP-ribose) polymerase/PARP1 and PARP2 identified using a cell-based screen in yeast.

Multicellular organisms must have means of preserving their genomic integrity or face catastrophic consequences such as uncontrolled cell proliferation or massive cell death. One response is a modification of nuclear proteins by the addition and removal of polymers of ADP-ribose that modulate the properties of DNA-binding proteins involved in DNA repair and metabolism. These ADP-ribose units are added by poly(ADP-ribose) polymerase (PARP) and removed by poly(ADP-ribose) glycohydrolase. Although budding yeast Saccharomyces cerevisiae does not possess proteins with significant sequence similarity to the human PARP family of proteins, we identified novel small molecule inhibitors against two family members, PARP1 and PARP2, using a cell-based assay in yeast. The assay was based on the reversal of growth inhibition caused by the heterologous expression of either PARP1 or PARP2. Validation of the assay was achieved by showing that the growth inhibition was relieved by a mutation in a single residue in the catalytic site of PARP1 or PARP2 or exposure of yeast to a known PARP1 inhibitor, 6(5H)-phenanthridinone. In separate experiments, when a putative protein regulator of PARP activity, human poly(ADP-ribose) glycohydrolase, was coexpressed with PARP1 or PARP2, yeast growth was restored. Finally, the inhibitors identified by screening the yeast assay are active in a mammalian PARP biochemical assay and inhibit PARP1 and PARP2 activity in yeast cell extracts. Thus, our data reflect the strength of using yeast to identify small molecule inhibitors of therapeutically relevant gene families, including those that are not found in yeast, such as PARP. The resultant inhibitors have two critical uses (a) as leads for drug development and (b) as tools to dissect cellular function.

ATP-Binding Cassette Transporters↗
PubMed2001

Protein and DNA oxidation in spinal injury: neurofilaments--an oxidation target.

This study measured the time courses of protein and DNA oxidation following spinal cord injury (SCI) in rats and characterized oxidative degradation of proteins. Protein carbonyl content-a marker of protein oxidation-significantly increased at 3-9 h postinjury and the ratio 8-hydroxy-2-deoxyguanosine/deoxyguanosine-an indicator of DNA oxidation-was significantly higher at 3-6 h postinjury in the injured cords than in the sham controls. This suggests that oxidative modification of proteins and DNA contributes to secondary damage in SCI. Densities of selected bands on coomassie-stained gels indicated that most proteins were degraded. Neurofilament protein (NFP) was particularly evaluated immunohistochemically; its light chain (NFP-68) was gradually degraded in nerve fibers, neuron bodies, and large dendrites following SCI. A mixture of Mn (III) tetrakis (4-benzoic acid) porphyrin (10 mg/kg)-a novel SOD mimetic-and nitro-L-arginine (1 mg/kg)-an inhibitor of nitric oxide synthase-injected intraperitoneally, increased NFP-68 immunoreactivity and the numbers of NFP-positive nerve fibers post-SCI, correlating NFP degradation in SCI to free radical-triggered oxidative damage for the first time. Therefore, blockage of protein and DNA oxidation in the secondary injury stage may improve long-term recovery-important information for development of the SCI therapies.

8-Hydroxy-2'-Deoxyguanosine↗
PubMed2001

Spermidine/spermine N1-acetyl transferase activity in rat brain following transient focal cerebral ischemia and reperfusion.

The polyamine system is very sensitive to different pathological states of brain and is perturbed after central nervous system (CNS) injury. Spermidine/Spermine N(1)-acetyl transferase (SSAT) is the key enzyme responsible for interconversion of spermine and spermidine to spermidine and putrescine respectively. In the present study, SSAT activity was evaluated in the rat CNS, following transient focal cerebral ischemia and reperfusion. The middle cerebral artery (MCA) was occluded for 2 h in male spontaneously hypertensive rats by an intraluminal suture technique. Animals were sacrificed at 3-24 h reperfusion following the MCA occlusion and SSAT activity was assayed in cortex and striatum. Results showed that SSAT activity was significantly increased at 12 h reperfusion in cortex and at 9, 12 and 18 h reperfusion in striatum following ischemia compared to sham or contralateral controls. These results demonstrate that polyamine catabolism in the rat CNS is altered following MCA occlusion. In the in vitro ischemia study, SSAT activity was evaluated in primary cortical neuronal cultures at 6-24 h re-oxygenation intervals following oxygen-glucose deprivation for 1 h, and the results from this group show that the enzyme activity increased by about 62% (P<0.05) at 24 h re-oxygenation. This study suggests that the increased SSAT activity may contribute to the increase in putrescine during the post-ischemic period.

Acetyltransferases↗
PubMed2001

Exploration of mosquito immunity using cells in culture.

The propagation of immune-responsive cells in vitro has provided the basis for substantial contributions to our understanding of many aspects of the mammalian immune response. In contrast, the potential for exploring the innate immune response of insects using cultured cells is only beginning to be developed, particularly with various mosquito cell lines from the genera Aedes and Anopheles. Immune-reactive mosquito cell lines express various defensive factors, including transferrin, lysozyme, cecropin, defensin, and prophenoloxidase activities. In this review, we discuss insect immunity in the context of key concepts that have emerged in the study of the mammalian immune system, with emphasis on the properties of the cells that participate in the immune response. The nature of established cell lines and their contributions to our understanding of immune functions in humans and insects is described, with emphasis on our own work with the C7-10 and Aag-2 mosquito cell lines from Aedes albopictus and Aedes aegypti, respectively. Finally, we offer some speculation on further advances in insect immunology that may be facilitated by work with cells in culture.

Amino Acid Sequence↗
PubMed2001

Mdr1b facilitates p53-mediated cell death and p53 is required for Mdr1b upregulation in vivo.

The mdr1b gene is thought to be a "stress-responsive" gene, however it is unknown if this gene is regulated by p53 in the whole animal. Moreover, it is unknown if overexpression of mdr1b affects cell survival. The dependence of mdr1b upon p53 for upregulation was evaluated in p53 knockout mice. Wild-type (wt) or p53-/- mice were treated singly or in combination with gamma irradiation (IR) and/or the potent DNA damaging agent, diethylnitrosoamine (DEN). Both IR and DEN induced mdr1b in wild-type animals, but not in the p53-/- mice. IR also upregulated endogenous mdr1b in the H35 liver cell line, and the mdr1b promoter was activated by IR and activation correlated with p53 levels; moreover activation required an intact p53 binding site. Colony survival studies revealed that co-transfection of both mdr1b and p53 dramatically reduced colony numbers compared to cells transfected with either p53 or mdr1b alone and cells microinjected with both mdr1b and p53 had a more dramatic loss in viability compared to cells injected with either expression vector alone. Further studies using acridine orange and ethidium bromide to measure apoptosis revealed that mdr1b caused apoptosis and this was enhanced by p53, however the increased apoptosis required a functional p53 transactivation domain. These studies indicate that mdr1b is a downstream target of p53 in the whole animal and expression of mdr1b facilitates p53-mediated cell death.

ATP Binding Cassette Transporter, Subfamily B↗