Biomedical subjects

H Xiong

Publications and source records attributed to H Xiong.

At least 37 records · Page 2Linked to original sources

PubMed2000

The first regioselective alpha-deprotonation and functionalization of allenamides. An application in intramolecular Pauson-Khand-type cycloadditions.

[rection: see text] The first regioselective alpha-deprotonation and functionalization of electron-deficient allenamines are described here. The acidities of alpha- and gamma-allenic protons of these allenamides are readily differentiated using strong bases, thereby allowing regioselective substitutions at either the alpha- or gamma-allenic position. A specific synthetic application of the novel alpha-substituted allenamides in intramolecular Pauson-Khand-type cycloadditions is also described here.

Journal Article↗

PubMed2000

Analysis of acyclovir by high performance capillary electrophoresis with on-column amperometric detection.

The separation of acyclovir (ACV) by high performance capillary electrophoresis (HPCE) with on-column amperometric detection using alpha-amino-5-mercapto-3,4-dithiazole (AMD) as internal standard is described. The calibration line was linear in the range of 0.5-20 mg/L of ACV. The detection limit was 0.15 mg/L of ACV. Its recovery ranged from 98 to 101% with relative standard deviations (RSDs) from 1.9 to 3.2% (n = 5). This method was successfully used for determining ACV in some pharmaceuticals and human urine. Comparable results with HPCE with ultraviolet (UV) detection and amperometric detection were obtained.

Acyclovir↗

PubMed2000

Membrane-type 1 matrix metalloproteinase mRNA expression in colorectal cancer.

PURPOSE: Membrane-type matrix metalloproteinases are recently described proteolytic enzymes belonging to the matrix metalloproteinase family. Initial studies have indicated that membrane-type matrix metalloproteinases are involved in tumor invasion and metastasis. Membrane-type 1 matrix metalloproteinase is the first membrane-type matrix metalloproteinase to be described. The aim of this study was to investigate the expression of membrane-type 1 matrix metalloproteinase mRNA in colorectal cancer. METHODS: Samples were collected from surgical specimens of patients with colorectal adenocarcinoma and were immediately frozen in liquid nitrogen and stored at -80 degrees C until processed. Both normal and cancer tissue was taken from each patient. TNM stage, tumor differentiation, mucin production, and vascular invasion were assessed. Northern blotting was used to quantify membrane-type 1 matrix metalloproteinase mRNA levels in the samples using a membrane-type 1 matrix metalloproteinase cDNA clone. X-ray film images were digitized and densitometry was used to quantify bands. All samples were normalized against 18S rRNA levels. Results are expressed as the ratio of cancer to normal tissue levels. Statistical analysis was performed using analysis of variance, with P < 0.05 accepted as the level of significance. RESULTS: A total of 32 samples were prospectively analyzed. The correlation between TNM stage and increased expression of membrane-type 1 matrix metalloproteinase mRNA in cancer tissue over normal tissue is expressed in the mean ratio of cancer to normal tissue expression for Stages I through IV, respectively: 1.4 +/- 0.2 (12 patients); 4.1 +/- 2.6 (8 patients); 3.4 +/- 3 (7 patients); and 4.5 +/- 5 (5 patients). Stage I is significantly different from Stages II and IV (P < 0.05). These preliminary results show an overall increasing trend in membrane-type 1 matrix metalloproteinase expression with increasing tumor stage. However, there was no correlation between membrane-type 1 matrix metalloproteinase expression and mucin production, degree of tumor differentiation, or vascular invasion. CONCLUSION: Preliminary results indicate that membrane-type 1 matrix metalloproteinase levels correlate with increasing tumor stage.

Adenocarcinoma↗

PubMed2000

Injectable chemotherapeutic microspheres and glioma II: enhanced survival following implantation into deep inoperable tumors.

PURPOSE: Delivery of chemotherapeutics using implantable, biodegradable polymers provides a potentially powerful method of treating brain tumors. The present studies examined the ability of injectable microspheres, formulated to release carboplatin or BCNU for 2-3 weeks, to enhance survival in a rodent model of deep, inoperable glioma. METHODS: Rat glioma (RG2) cells were implanted into the striatum of rats. In a first experiment, the tumors were allowed to grow for 3 days, followed by either no treatment, bolus chemotherapy (100 microg), or implantation of microspheres containing 10, 50, or 100 microg of carboplatin. The microspheres were implanted, via hypodermic injection, directly into the center of the small, 3-day-old tumors. In a second experiment, tumors grew for 8 days prior to treatment with either carboplatin- or BCNU-loaded microspheres. The microspheres were then injected either directly into the center of these larger tumors or into three sites along the perimeter of the tumor. Separate sets of animals received bolus chemotherapy (100 microg) into either the tumor center or around the tumor perimeter. RESULTS: Injection of carboplatin-loaded microspheres into the center of the small 3 day old, tumors produced dose-related increases in survival. When injections of carboplatin- or BCNU-loaded microspheres were made into the center of the larger, 8-day-old tumors, survival was not enhanced. However, when the microspheres were injected along the perimeter of the larger tumors, sustained-release chemotherapy did significantly prolong survival. Bolus chemotherapy was less effective than sustained release chemotherapy. CONCLUSIONS: Together, these data: (1) demonstrate that sustained delivery of chemotherapy in or near the tumor site is superior to equipotent bolus doses in inoperable tumors, (2) demonstrate that injection of sustained release microspheres into the tissue surrounding a growing tumor may provide superior effects over injections directly into the tumor mass, and (3) suggest that this approach may provide a useful means of selectively delivering chemotherapeutics to tumors or portions of tumors that cannot otherwise be treated with conventional surgical approaches.

Animals↗

PubMed2000

Involvement of nitric oxide in pentylenetetrazole-induced kindling in rats.

We investigated the role of nitric oxide (NO) and brain-derived neurotrophic factor (BDNF) in the pentylenetetrazole (PTZ)-induced kindling in rats. Seizures were induced by single administration of PTZ, which was associated with an increase in levels of NO metabolites (NOx) in the hippocampus. Pretreatment with a neuronal NO synthase inhibitor, 7-nitroindazole (7-NI), diminished the PTZ-induced increase in NOx levels without affecting the seizure intensity. Repeated administration of PTZ produced a gradual increase in the seizure intensity, leading to the development of kindling. In the kindled rats, PTZ at a dose of 40 mg/kg increased NOx levels in the hippocampus, whereas it had no effect in control animals. Cotreatment of 7-NI with PTZ blocked the development of kindling and attenuated the PTZ-induced increase in NOx levels. A significant increase in BDNF levels was observed in the hippocampus of the kindled rats, which returned to the control levels following seizures induced by PTZ. 7-NI reduced the hippocampal BDNF levels in control rats and suppressed the increase of BDNF levels in the kindled rats. Our findings suggest that NO plays a role in the development of PTZ-induced kindling and that BDNF may contribute to the NO-dependent plastic changes in neuronal excitability.

Animals↗

PubMed2000

HIV-1 infected mononuclear phagocyte secretory products affect neuronal physiology leading to cellular demise: relevance for HIV-1-associated dementia.

Viral and cellular products from HIV-1-infected and/or immune competent mononuclear phagocytes (MP) (brain macrophages and microglia) affect neuronal function during HIV-1-associated dementia (HAD). Neurotoxic MP factors include, but are not limited to, pro-inflammatory cytokines, chemokines, platelet activating factor, arachidonic acid and its metabolites, nitric oxide, progeny virions and viral structural and regulatory proteins. The mechanisms for immune-mediated neural injury in HAD, only now, are being unraveled. In this regard, we reviewed the current knowledge of how postmitotic neurons, which can neither divide nor be replaced, are damaged by MP secretory activities. Linking neuronal function with brain MP activation was made possible by placing viral and/or immune products onto neurons and measuring cell signaling events or through ex vivo electrophysiological tests on MP-treated brain slices. Such linkages are shown, in this report, by select demonstrations of MP factors which cause neuronal dysfunction in HAD.

AIDS Dementia Complex↗

PubMed2000

Altered hepatobiliary disposition of acetaminophen glucuronide in isolated perfused livers from multidrug resistance-associated protein 2-deficient TR(-) rats.

Previous studies have demonstrated that phenobarbital treatment impairs the biliary excretion of acetaminophen glucuronide (AG), although the transport system(s) responsible for AG excretion into bile has not been identified. Initial studies in rat canalicular liver plasma membrane vesicles indicated that AG uptake was stimulated modestly by ATP, but not by membrane potential, HCO(3)(-), or pH gradients. To examine the role of the ATP-dependent canalicular transporter multidrug resistance-associated protein 2 (Mrp2)/canalicular multispecific organic anion transporter (cMOAT) in the biliary excretion of AG, the hepatobiliary disposition of acetaminophen, AG, and acetaminophen sulfate (AS) was examined in isolated perfused livers from control and TR(-) (Mrp2-deficient) Wistar rats. Mean bile flow in TR(-) livers was approximately 0.3 microl/min/g of liver ( approximately 4-fold lower than control). AG biliary excretion was decreased (>300-fold) to negligible levels in TR(-) rat livers, indicating that AG is an Mrp2 substrate. Similarly, AS biliary excretion in TR(-) livers was decreased ( approximately 5-fold); however, concentrations were still measurable, suggesting that multiple mechanisms, including Mrp2-mediated active transport, may be involved in AS biliary excretion. AG and AS perfusate concentrations were significantly higher in livers from TR(-) compared with control rats. Pharmacokinetic modeling of the data revealed that the rate constant for basolateral egress of AG increased significantly from 0.028 to 0.206 min(-1), consistent with up-regulation of a basolateral organic anion transporter in Mrp2-deficient rat livers. In conclusion, these data indicate that AG biliary excretion is mediated by Mrp2, and clearly demonstrate that substrate disposition may be influenced by alterations in complementary transport systems in transport-deficient animals.

ATP Binding Cassette Transporter, Subfamily B↗

PubMed2000

[Pathophysiological alterations in cultured astrocytes exposed to hypoxia/reoxygenation].

While the research of hypoxic-ischemic injury to the central nervous system has been focused on neurons, astrocytes are also critically involved in and contribute to the hypoxic-ischemic process. The role these cells play appears to be more and more important. There are considerable progress in characterizing the pathophysiological alterations of these cells during hypoxic-ischemic brain damage.

Animals↗

PubMed1999

Mechanistic studies of the processing of human S-adenosylmethionine decarboxylase proenzyme. Isolation of an ester intermediate.

Human S-adenosylmethionine decarboxylase is synthesized as a proenzyme that undergoes an autocatalytic cleavage reaction generating the alpha and beta subunits and forming the pyruvate prosthetic group, which is derived from an internal Ser residue (Ser-68). The mechanism of this processing reaction was studied using site-directed mutagenesis of conserved residues (His-243 and Ser-229) located close to the cleavage site. Mutant S229A failed to process, and mutant S229C cleaved very slowly, whereas mutant S229T processed normally, suggesting that the hydroxyl group of residue 229 is required for the processing reaction where Ser-229 may act as a proton acceptor. Mutant His-243A cleaved very slowly, forming a small amount of the correctly processed pyruvoyl enzyme but a much larger proportion of the alpha subunit with an amino-terminal Ser. The cleavage to form the latter was greatly enhanced by hydroxylamine. This result suggests that the N-O acyl shift needed for ester formation occurs normally in this mutant but that the next step, which is a beta-elimination reaction leading to the two subunits, does not occur. His-243 may therefore act as the basic residue that extracts the hydrogen of the alpha-carbon of Ser-68 in the ester in order to facilitate this reaction. The availability of the recombinant H243A S-adenosylmethionine decarboxylase proenzyme provides a useful model system to examine the processing reaction in vitro and test the design of specific inactivators aimed at blocking the production of the pyruvoyl prosthetic group.

Adenosylmethionine Decarboxylase↗

PubMed1999

Genomic sequence, structural organization, molecular evolution, and aberrant rearrangement of promyelocytic leukemia zinc finger gene.

The promyelocytic leukemia zinc finger gene (PLZF) is involved in chromosomal translocation t(11;17) associated with acute promyelocytic leukemia. In this work, a 201-kilobase genomic DNA region containing the entire PLZF gene was sequenced. Repeated elements account for 19.83%, and no obvious coding information other than PLZF is present over this region. PLZF contains six exons and five introns, and the exon organization corresponds well with protein domains. There are at least four alternative splicings (AS-I, -II, -III, and -IV) within exon 1. AS-I could be detected in most tissues tested whereas AS-II, -III, and -IV were present in the stomach, testis, and heart, respectively. Although splicing donor and acceptor signals at exon-intron boundaries for AS-I and exons 1-6 were classical (gt-ag), AS-II, -III, and -IV had atypical splicing sites. These alternative splicings, nevertheless, maintained the ORF and may encode isoforms with absence of important functional domains. In mRNA species without AS-I, there is a relatively long 5' UTR of 6.0 kilobases. A TATA box and several transcription factor binding sites were found in the putative promoter region upstream of the transcription start site. PLZF is a well conserved gene from Caenorhabditis elegans to human. PLZF paralogous sequences are found in human genome. The presence of two MLL/PLZF-like alignments on human chromosome 11q23 and 19 suggests a syntenic replication during evolution. The chromosomal breakpoints and joining sites in the index acute promyelocytic leukemia case with t(11;17) also were characterized, which suggests the involvement of DNA damage-repair mechanism.

Alternative Splicing↗

PubMed1999

Intracellular CXCR4 signaling, neuronal apoptosis and neuropathogenic mechanisms of HIV-1-associated dementia.

The mechanism(s) by which HIV-1 affects neural injury in HIV-1-associated dementia (HAD) remains unknown. To ascertain the role that cellular and viral macrophage products play in HAD neurotoxicity, we explored one potential route for neuronal demise, CXCR4. CXCR4, expressed on lymphocytes and neurons, is both a part of neural development and a co-receptor for HIV-1. Its ligand, stromal cell-derived factor-1alpha (SDF-1alpha), affects neuronal viability. GTP binding protein (G-protein) linked signaling after neuronal exposure to SDF-1alpha, virus-infected monocyte-derived macrophage (MDM) secretory products, and virus was determined. In both human and rat neurons, CXCR4 was expressed at high levels. SDF-1alpha/beta was detected predominantly in astrocytes and at low levels in MDM. SDF-1beta/beta was expressed in HAD brain tissue and upregulated in astrocytes exposed to virus infected and/or immune activated MDM conditioned media (fluids). HIV-1-infected MDM secretions, virus and SDF-1beta induced a G inhibitory (Gi) protein-linked decrease in cyclic AMP (cAMP) and increase inositol 1,4, 5-trisphosphate (IP3) and intracellular calcium. Such effects were partially blocked by antibodies to CXCR4 or removal of virus from MDM fluids. Changes in G-protein-coupled signaling correlated, but were not directly linked, to increased neuronal synaptic transmission, Caspase 3 activation and apoptosis. These data, taken together, suggest that CXCR4-mediated signal transduction may be a potential mechanism for neuronal dysfunction during HAD.

AIDS Dementia Complex↗

PubMed1999

The Monolayer Behavior of Amphiphilic Polymer and Heterostructure of Polymer LB Film/CdS Clusters.

In this paper we report the behavior of an amphiphilic polymer monolayer on pure water and Cd2+ subphase. This polymer was composed of hydrophilic ethylene diamine epichlorohydrin slightly crosslinked microgel and hydrophobic stearic chains, noted as ES-1. The introduction of Cd2+ ions in subphase had a marked effect on the process of the organization of the amphiphilic polymer at the air/water interface due to the association of Cd2+ ions with the hydrophilic network, which could be indicated by the pressure-area isotherms and Brewster Angle Microscopy. Ordered ES-1/Cd2+ LB multilayers were fabricated. After the multilayers reacted with H2S gas, CdS clusters were synthesized within the film, which was characterized by X-ray diffraction and UV-visible spectroscopy. Copyright 1999 Academic Press.

Journal Article↗

PubMed1999

Role of cysteine-82 in the catalytic mechanism of human S-adenosylmethionine decarboxylase.

S-Adenosylmethionine decarboxylase is a pyruvate-dependent enzyme. The enzyme forms a Schiff base with substrate, S-adenosylmethionine, through the pyruvoyl moiety. This facilitates the release of CO2 from the substrate, which must then be protonated on the alpha carbon in order to permit hydrolysis of the Schiff base to release the product. The catalytic mechanism of human S-adenosylmethionine decarboxylase was investigated via mutagenic and kinetic approaches. The results of enzyme kinetic studies indicated that Cys-82 is a crucial residue for activity and this residue has a basic pKa. Iodoacetic acid inhibited wild-type enzyme activity in a time- and pH-dependent manner but did not affect the already reduced activity of mutant C82A. Reaction of this mutant with iodoacetic acid led to approximately one less mole of reagent being incorporated per mole of enzyme alphabeta dimer than with wild-type S-adenosylmethionine decarboxylase. Both wild-type and C82A mutant S-adenosylmethionine decarboxylases were inactivated by substrate-mediated transamination, but this reaction occurred much more frequently with C82A than with wild-type enzyme. A major proportion of the recombinant C82A mutant protein was in the transaminated form in which the pyruvoyl cofactor is converted into alanine. This suggests that incorrect protonation of the pyruvate, rather than the substrate, occurs much more readily when Cys-82 is altered. On the basis of these results, it was postulated that residue Cys-82 may be the proton donor of the decarboxylation reaction catalyzed by S-adenosylmethionine decarboxylase.

Adenosylmethionine Decarboxylase↗

PubMed1999

ClC-2 activation modulates regulatory volume decrease.

ClC-2 belongs to a large family of chloride channels and its expression in certain cell types is associated with the appearance of swelling-activated chloride (Cl-) currents. In the present report, we examined the hypothesis that ClC-2 plays a role in regulatory volume decrease by expressing ClC-2 in Sf9 cells using the baculovirus system. First, we showed that ClC-2 protein expression is associated with appearance of a Cl- conductance which is activated by hypo-osmotic shock and can be distinguished from swelling-activated chloride currents endogenous to Sf9 cells on the basis of its pharmacology and specific inhibition by an anti-ClC-2 antibody. Second, we show that the rate of regulatory volume decrease is significantly enhanced in Sf9 cells expressing ClC-2 protein. Hence, our data support the hypothesis that ClC-2 is capable of mediating regulatory volume decrease.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗

PubMed1999

Unraveling the mechanisms of neurotoxicity in HIV type 1-associated dementia: inhibition of neuronal synaptic transmission by macrophage secretory products.

The cognitive and motor impairments of HIV-1-associated dementia (HAD) often result from neuronal damage of drop-out. In the infected human host, virus-infected immune-competent mononuclear phagocytes (MPs) (brain macrophages and microglia) are the target cells for HIV-1 and the producers of bioactive molecules that mediate neural damage. Indeed, in laboratory experiments, activated HIV-1-infected macrophages placed into human or rodent brain tissues induce neuronal apoptosis. Nonetheless, the mechanisms for neuronal dysfunction in HAD have yet to be discerned. To these ends, we studied the effects of HIV-1-infected monocyte-derived macrophage (MDM) secretions, electrophysiologically, on neuronal synaptic transmission. Bath application of HIV-1-infected MDM culture fluids onto rat hippocampal brain slices resulted in inhibition of evoked field excitatory postsynaptic potentials (EPSPs). In contrast, fluids from uninfected MDMs showed mild effects on the EPSPs. HIV-1-associated inhibition of EPSPs was enhanced by LPS activation, both for HIV-1-infected and uninfected MDMs. Importantly, paired-pulse facilitation ratio tests showed that factors secreted by HIV-1-infected MDMs acted transiently on presynaptic terminals, providing insights into the site of action and mechanism of the MDM-induced neuronal dysfunction. These results, taken together, demonstrate that factors produced as a consequence of MDM infection and activation affect neuronal synaptic transmission.

AIDS Dementia Complex↗

PubMed1999

Enhanced delivery of carboplatin into brain tumours with intravenous Cereport (RMP-7): dramatic differences and insight gained from dosing parameters.

Cereport (RMP-7) is a selective bradykinin B2 receptor agonist which increases the permeability of the 'blood-brain tumour barrier' (BBTB) to increase delivery of chemotherapeutic agents to brain tumours. A series of experiments was performed in an RG2 rodent model of glioma to evaluate and refine intravenous (i.v.) parameters to optimize Cereport's clinical utility. The first experiment demonstrated that while carboplatin levels were increased by twofold when given as a bolus during the Cereport infusion, no increase in carboplatin levels were seen when Cereport and carboplatin were simultaneously co-infused for 15 min. A subsequent experiment established that a major factor responsible for the lack of an effect with the co-infusion paradigm was tachyphylaxis to Cereport during the 15 min infusion, for a progressively diminished response to Cereport occurred over that time frame, as plasma levels of carboplatin were rising. A final experiment adjusted the timing of the Cereport and carboplatin infusions so that higher plasma carboplatin levels were achieved prior to initiating the Cereport infusion. Significant uptake effects were achieved when the carboplatin infusion preceded the Cereport infusion by 10 min (i.e. 5 min overlap in the delivery of the two agents). Collectively, these data provide the first systematic evaluation of dosing parameters involving receptor-mediated changes in BBTB permeability and provide new information regarding the pharmacodynamics and potential clinical use of Cereport.

Animals↗

PubMed1999

Mutual regulation between the intercellular messengers nitric oxide and brain-derived neurotrophic factor in rodent neocortical neurons.

The diffusible factors, nitric oxide (NO) and brain-derived neurotrophic factor (BDNF) are both suggested to be intercellular messengers that have similar synaptic activities and developmental influences in the brain. In the present study, we have analysed their mutual regulation with respect to their production in rodent neocortical neurons. Some of the cultured rat neocortical neurons exhibited immunoreactivity for both neuronal NO synthase (NOS) and the BDNF receptor trkB. Neuronal NOS appeared to be activated autonomously and produced NO in culture as monitored by nitrite accumulation. Inhibition of the endogenous NO production in culture by a NOS inhibitor, NG-monomethyl-L-arginine (NMMA), enhanced basal expression of BDNF mRNA and protein. Similarly, cerebroventricular administration of another NOS inhibitor, N-omega-nitro-L-arginine methylester (L-NAME), but not D-NAME or saline, increased BDNF content in the neocortex. In the opposite direction, however, BDNF appeared to function as a positive regulator for NO synthesis. Addition of BDNF upregulated the neuronal NOS expression as well as NO production in neocortical culture. In agreement, BDNF knock-out mice exhibited significant impairment of neuronal NOS expression in the neocortex. Taken together, these observations suggest that the trans-synaptic signalling molecules, NO and BDNF, influence the production of each other and mutually regulate the strength of their intercellular communications.

Animals↗

PubMed1999

Lysophosphatidic acid acyltransferase from coconut endosperm mediates the insertion of laurate at the sn-2 position of triacylglycerols in lauric rapeseed oil and can increase total laurate levels

Expression of a California bay laurel (Umbellularia californica) 12:0-acyl-carrier protein thioesterase, bay thioesterase (BTE), in developing seeds of oilseed rape (Brassica napus) led to the production of oils containing up to 50% laurate. In these BTE oils, laurate is found almost exclusively at the sn-1 and sn-3 positions of the triacylglycerols (T.A. Voelker, T.R. Hayes, A.C. Cranmer, H.M. Davies [1996] Plant J 9: 229-241). Coexpression of a coconut (Cocos nucifera) 12:0-coenzyme A-preferring lysophosphatitic acid acyltransferase (D.S. Knutzon, K.D. Lardizabal, J.S. Nelsen, J.L. Bleibaum, H.M. Davies, J.G. Metz [1995] Plant Physiol 109: 999-1006) in BTE oilseed rape seeds facilitates efficient laurate deposition at the sn-2 position, resulting in the acccumulation of trilaurin. The introduction of the coconut protein into BTE oilseed rape lines with laurate above 50 mol % further increases total laurate levels.

Journal Article↗