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Biomedical subjects

X Liu

Publications and source records attributed to X Liu.

At least 199 records · Page 11Linked to original sources
PubMed2001

Reactions of tetrakis(dimethylamide)-titanium, -zirconium and -hafnium with silanes: synthesis of unusual amide hydride complexes and mechanistic studies of titanium-silicon-nitride (Ti-Si-N) formation.

M(NMe(2))(4) (M = Ti, Zr, Hf) were found to react with H(2)SiR'Ph (R' = H, Me, Ph) to yield H(2), aminosilanes, and black solids. Unusual amide hydride complexes [(Me(2)N)(3)M(mu-H)(mu-NMe(2))(2)](2)M (M = Zr, 1; Hf, 2) were observed to be intermediates and characterized by single-crystal X-ray diffraction. [(Me(2)N)(3)M(mu-D)(mu-NMe(2))(2)](2)M (1-d(2), 2-d(2)) were prepared through reactions of M(NMe(2))(4) with D(2)SiPh(2). Reactions of (Me(2)N)(3)ZrSi(SiMe(3))(3) (5) with H(2)SiR'Ph were found to give aminosilanes and (Me(2)N)(2)Zr(H)Si(SiMe(3))(3) (6). These reactions are reversible through unusual equilibria such as (Me(2)N)(3)ZrSi(SiMe(3))(3) (5) + H(2)SiPh(2) right arrow over left arrow (Me(2)N)(2)Zr(H)Si(SiMe(3))(3) (6) + HSi(NMe(2))Ph(2). The deuteride ligand in (Me(2)N)(2)Zr(D)Si(SiMe(3))(3) (6-d(1)) undergoes H-D exchange with H(2)SiR'Ph (R' = Me, H) to give 6 and HDSiR'Ph. The reaction of Ti(NMe(2))(4) with SiH(4) in chemical vapor deposition at 450 degrees C yielded thin Ti-Si-N ternary films containing TiN and Si(3)N(4). Ti(NMe(2))(4) reacts with SiH(4) at 23 degrees C to give H(2), HSi(NMe(2))(3), and a black solid. HNMe(2) was not detected in this reaction. The reaction mixture, upon heating, gave TiN and Si(3)N(4) powders. Analyses and reactivities of the black solid revealed that it contained -H and unreacted -NMe(2) ligands but no silicon-containing ligand. Ab initio quantum chemical calculations of the reactions of Ti(NR(2))(4) (R = Me, H) with SiH(4) indicated that the formation of aminosilanes and HTi(NR(2))(3) was favored. These calculations also showed that HTi(NH(2))(3) (3b) reacted with SiH(4) or H(3)Si-NH(2) in the following step to give H(2)Ti(NH(2))(2) (4b) and aminosilanes. The results in the current studies indicated that the role of SiH(4) in its reaction with Ti(NMe(2))(4) was mainly to remove amide ligands as HSi(NMe(2))(3). The removal of amide ligands is incomplete, and the reaction thus yielded "=Ti(H)(NMe(2))" as the black solid. Subsequent heating of the black solid and HSi(NMe(2))(3) may then yield TiN and Si(3)N(4), respectively, as the Ti-Si-N materials.

Journal Article↗
PubMed2001

Identification of a secreted superoxide dismutase in Mycobacterium avium ssp. paratuberculosis.

Mycobacterium avium ssp. paratuberculosis (M. paratuberculosis), the causative agent of Johne's disease, is an important animal pathogen that has also been implicated in human disease. The major proteins expressed by M. paratuberculosis were analyzed by two-dimensional gel electrophoresis, and a superoxide dismutase (Sod) was identified from this protein profile. The M. paratuberculosis Sod has a molecular mass of 23 kDa and an isoelectric point of 6.1. Sequence analysis of the corresponding sodA gene from M. paratuberculosis indicates that this protein is a manganese-dependent enzyme. We show that the M. paratuberculosis Sod is actively secreted, suggesting that it may elicit a protective cellular immune response in the host during infection.

Amino Acid Sequence↗
PubMed2001

Identification and characterization of a novel fibronectin in zebrafish.

Sequence analysis of zebrafish fibronectin (FN) cDNAs indicates that at least two forms of the protein exist in fish. One form (FN1) is very similar to FNs identified in other vertebrates possessing 12 type I, 2 type II, and 17 type III repeats including two alternative splice sites (EIIIA and EIIIB) and a variable region (V). Zebrafish FN1 contains the RGD cell adhesion site in type III(10) and a second cell-binding site (LDV) in the V region. In addition to this conserved form of FN, a novel truncated form of zebrafish FN (FN2) was identified. The predicted structure of FN2 is identical to FN1 at the N-terminal region possessing 9 type I, 2 type II, and the first 3 type III repeats. Following III(3), FN2 contains a unique 20-amino-acid C-terminal tail that is different from the C-terminus of FN1, lacking the two cysteines that are usually involved in the formation of interchain disulfide bonds. Genomic sequence analysis has revealed that FN2 is generated by an alternative RNA splicing pattern that has not been described for FN in other organisms. Reverse transcription-polymerase chain reaction analysis and RNase protection assays reveal that FN2 mRNA is present in the zebrafish embryo throughout development as well as in cultures of an established liver cell line. Experiments conducted with recombinant FN2 synthesized in insect cells demonstrate that the protein promotes the attachment and spreading of fish embryo cells in culture.

Alternative Splicing↗
PubMed2001

CD98-mediated links between amino acid transport and beta 1 integrin distribution in polarized columnar epithelia.

In non-polarized cells, CD98 has been shown to both influence beta(1) integrins and heterodimerize with LAT-2, which confers amino acid transport capability on the LAT-2/CD98 heterodimer. Since LAT-2 is most heavily expressed in intestine and CD98 associates with the beta(1) integrin splice form selectively found in such epithelia, we investigated the relationship and polarity of these proteins using the intestinal epithelial model Caco2-BBE. CD98 was found to selectively coimmunoprecipitate with both LAT-2 and beta(1) integrin, and, logically, all three proteins were polarized to the same (basolateral) domain. Furthermore, expression of CD98 in polarized epithelia lacking human CD98 (MDCK cells) disrupted beta(1) integrin surface distribution and cytoskeletal architecture, suggesting that CD98 can influence integrin function. Expression of a CD98 mutant lacking the specific residues conferring LAT-2 binding similarly affected cells, confirming that the latter effect was not due to LAT-2 sequestration. Use of CD98 truncation mutants suggest that a 10-amino acid domain located at the putative cytoplasmic tail/transmembrane domain interface was necessary and sufficient to induce the phenotype change. We conclude that the CD98/LAT-2 amino acid transporter is polarized to the same domain on which beta(1) integrin resides. CD98 appears to associate with beta(1) integrin and, in doing so, may influence its function as revealed by disruption of the outside-in signaling that confers cytoskeletal organization. Furthermore, such findings suggest a link between classic transport events and a critical element of barrier function: integrin-mediated influences on cytoskeletal organization.

Amino Acid Transport System y+↗
PubMed2001

[N-actyltransferase2 polymorphism in recurrence of colorectal polyps and adenomas].

OBJECTIVE: To observe the polymorphism of metabolizing enzyme N-actyltransferase 2 gene in populations with history of colorectal adenoma and polyp and to explore the relationship between NAT2 gene phenotype and colorectal cancer genetic susceptibility. METHODS: A cohort of 4,076 patients with history of colorectal adenoma or polyp was established Fifty-two cases were randomly selected from those with recurrence more than 2 times during the twenty years' follow-up. Fifty-two without recurrence were randomly selected as controls. DNA was isolated from the lymphocytes of peripheral blood of these 104 subjects. NAT2 polymorphism was detected by PCR-RFLP. RESULTS: The frequency of wild-type NAT2 (Wt/Wt) was significantly higher in control group (17/52, 32.7%) than in recurrence group (8/52, 15.4%) (P < 0.05). The frequency of heterogeneity type of NAT2 was significantly higher in recurrence group (40/52, 76.9%) than in control group (30/52, 57.7%) (P < 0.05). If OR of wild-type NAT2 (Wt/Wt) was 1, OR of Wt/M* genotype was 2.96 (95% CI: 1.091-8.009), and OR of M*/M* genotype was 2.125 (95% CI: 0.666-6.781). There was no difference of distribution between rapid enzyme type and slow enzyme type in the two groups. CONCLUSION: The frequency of wild-type NAT2 gene (Wt/Wt) is significantly higher in patients without recurrence of adenoma or polyp than in patients with recurrence. The frequency of heterogeneity genotype of NAT2 is significantly higher in patients with recurrence of adenoma or polyp than in those without recurrence. Wild-genotype (Wt/Wt) may be a protection factor in recurrence of colorectal adenomas. No difference can be seen between the distribution of rapid and slow enzyme types in these two groups.

Adenomatous Polyposis Coli↗
PubMed2001

Medullary A1 noradrenergic neurones may mediate oxytocin release after noxious stimuli.

Noxious stimuli facilitate oxytocin release from the pituitary. Oxytocin cells receive excitatory synaptic inputs from the noradrenergic neurones located in the medulla oblongata. Oxytocin release after noxious stimuli is blocked by noradrenaline depletion in the brain. Here, we examined effects of noxious stimuli upon noradrenaline release within the supraoptic nucleus. Electric footshocks or mustard oil application to the foot pad facilitated noradrenaline release in the nucleus. Noradrenaline release after noxious stimuli was impaired by microinjections with a GABA(A) receptor agonist, muscimol, or an alpha 2 adrenoceptor agonist, clonidine, into the A1 noradrenergic cell regions. From these and reported data, we conclude that the medullary A1 noradrenergic neurones contribute, at least in part, to oxytocin release from the pituitary after noxious stimuli.

Adrenergic alpha-Agonists↗
PubMed2001

Conditional epidermal expression of TGFbeta 1 blocks neonatal lethality but causes a reversible hyperplasia and alopecia.

To study the role of transforming growth factor type beta1 (TGFbeta1) in epidermal growth control and disease, we have generated a conditional expression system by using the bovine keratin 5 promoter to drive expression of the tetracycline-regulated transactivators tTA and rTA, and a constitutively active mutant of TGFbeta1 linked to the tetO target sequence for the transactivator. This model allows for induction or suppression of exogenous TGFbeta1 with oral doxycycline. Maximal expression of TGFbeta1 during gestation caused embryonic lethality, whereas partial suppression allowed full-term development with neonatal lethality characterized by runting, epidermal hypoproliferation, and blocked hair follicle growth. With complete suppression, phenotypically normal double transgenic (DT) mice were born. Acute induction of TGFbeta1 in the epidermis of adult mice inhibited basal and follicular keratinocyte proliferation and reentry of telogen hair follicles into anagen. However, chronic expression of TGFbeta1 in adult DTs caused severe alopecia characterized by epidermal and follicular hyperproliferation, apoptosis, as well as dermal fibrosis and inflammation. Readministration of doxycycline to tTA DT mice caused hair regrowth within 14 days. The mRNA and protein for Smad7, an inhibitor of TGFbeta signaling, were up-regulated in the epidermis and hair follicles of alopecic skin and rapidly induced in rTA mice in parallel with the TGFbeta1 transgene, suggesting that the hyperproliferative phenotype may result in part from development of a sustained negative feedback loop. Thus, this conditional expression system provides an important model for understanding the role of TGFbeta1 during development, in normal skin biology, and in disease.

Alopecia↗
PubMed2001

Capillary electrochromatography-laser-induced fluorescence method for separation and detection of dansylated dialkylamine tags in encoded combinatorial libraries.

LC-fluorescence and LC-MS methods have been previously reported for use in decoding bead-based combinatorial libraries. We present the use of capillary electrochromatography (CEC) for highly selective decoding in combination with laser-induced fluorescence (LIF) detection for high sensitivity. The results are compared to prior data obtained using HPLC with fluorescence detection. The use of CEC shows promise for miniaturization and multiplexing for future applications, and the use of LIF detection can allow for detection at sub-pmol amounts.

Amines↗
PubMed2001

Inhibition of Wilms' tumor growth by intramuscular administration of tissue inhibitor of metalloproteinases-4 plasmid DNA.

Extracellular matrix (ECM) degrading matrix metalloproteinases (MMPs) lead to ECM turnover, a key event in cancer growth and progression. The tissue inhibitors of matrix metalloproteinases (TIMPs) limit the activity of MMPs, which suggests their use for cancer gene therapy. Here we report that systemic administration of naked TIMP-4 DNA significantly inhibited Wilms' tumor growth in nude mice. TIMP-4, whose expression was lost in Wilms' tumor, inhibited the growth of G401 Wilms' tumor cells at a concentration lower than those required for MMP inhibition. This inhibition was associated with internalization of exogenous recombinant TIMP-4. Electroporation-mediated intramuscular injection of TIMP-4 expression plasmid resulted in sustained plasma TIMP-4 levels and significant tumor suppression. Our data demonstrate a tumor suppressive effect of TIMP-4 against Wilms' tumor and the potential utility of intramuscular delivery of TIMP gene for treatment of kidney derived cancers.

Adult↗
PubMed2001

Myosin I mutants with only 1% of wild-type actin-activated MgATPase activity retain essential in vivo function(s).

The single class I myosin (MYOA) of Aspergillus nidulans is essential for hyphal growth. It is generally assumed that the functions of all myosins depend on their actin-activated MgATPase activity. Here we show that MYOA mutants with no more than 1% of the actin-activated MgATPase activity of wild-type MYOA in vitro and no detectable in vitro motility activity can support fungal cell growth, albeit with a delay in germination time and a reduction in hyphal elongation. From these and other data, we conclude that the essential role(s) of myosin I in A. nidulans is probably structural, requiring little, if any, actin-activated MgATPase or motor activity, which have long been considered the defining characteristics of the myosin family.

Actins↗
PubMed2001

A transmission/disequilibrium test that allows for genotyping errors in the analysis of single-nucleotide polymorphism data.

The present study assesses the effects of genotyping errors on the type I error rate of a particular transmission/disequilibrium test (TDT(std)), which assumes that data are errorless, and introduces a new transmission/disequilibrium test (TDT(ae)) that allows for random genotyping errors. We evaluate the type I error rate and power of the TDT(ae) under a variety of simulations and perform a power comparison between the TDT(std) and the TDT(ae), for errorless data. Both the TDT(std) and the TDT(ae) statistics are computed as two times a log-likelihood difference, and both are asymptotically distributed as chi(2) with 1 df. Genotype data for trios are simulated under a null hypothesis and under an alternative (power) hypothesis. For each simulation, errors are introduced randomly via a computer algorithm with different probabilities (called "allelic error rates"). The TDT(std) statistic is computed on all trios that show Mendelian consistency, whereas the TDT(ae) statistic is computed on all trios. The results indicate that TDT(std) shows a significant increase in type I error when applied to data in which inconsistent trios are removed. This type I error increases both with an increase in sample size and with an increase in the allelic error rates. TDT(ae) always maintains correct type I error rates for the simulations considered. Factors affecting the power of the TDT(ae) are discussed. Finally, the power of TDT(std) is at least that of TDT(ae) for simulations with errorless data. Because data are rarely error free, we recommend that researchers use methods, such as the TDT(ae), that allow for errors in genotype data.

Alleles↗
PubMed2001

HIV Nef-mediated cellular phenotypes are differentially expressed as a function of intracellular Nef concentrations.

Nef is a regulatory protein encoded by the genome of both human and simian immunodeficiency virus. Its expression in T cells leads to CD4 and major histocompatibility complex class I modulation and either enhancement or suppression of T cell activation. How this viral protein achieves multiple and at times opposing activities has been unclear. Through direct measurements of Nef and the Nef-GFP fusion protein, we find that these events are mediated by different Nef concentrations. Relative to the intracellular concentration that down-modulates surface CD4, an order of magnitude increase in Nef-GFP expression is required for a comparable modulation of major histocompatibility complex class I, and a further 3-fold increase is necessary to suppress T cell activation.

CD4 Antigens↗
PubMed2001

Efficient asymmetric synthesis of biologically important tryptophan analogues via a palladium-mediated heteroannulation reaction.

A novel and concise synthesis of optically active tryptophan derivatives was developed via a palladium-catalyzed heteroannulation reaction of substituted o-iodoanilines with an internal alkyne. The required internal alkyne 14a or 25 was prepared in greater than 96% de via alkylation of the Schöllkopf chiral auxiliary 19 employing diphenyl phosphate as the leaving group. The Schöllkopf chiral auxiliary was chosen here for the preparation of L-tryptophans would be available from D-valine while the D-isomers required for natural product total synthesis would originate from the inexpensive L-valine (300-g scale). Applications of the palladium-catalyzed heteroannulation reaction were extended to the first asymmetric synthesis of L-isotryptophan 38 and L-benz[f]tryptophan 39. More importantly, the optically pure 6-methoxy-D-tryptophan 62 was prepared by this protocol on a large scale (>300 g). This should permit entry into many ring-A oxygenated indole alkaloids when coupled with the asymmetric Pictet-Spengler reaction. In addition, an improved total synthesis of tryprostatin A (9a) was accomplished in 43% overall yield employing this palladium-mediated process.

Alkylation↗
PubMed2001

Arg(1098) is critical for the chloride dependence of human angiotensin I-converting enzyme C-domain catalytic activity.

Angiotensin (Ang) I-converting enzyme (ACE) is a Zn(2+) metalloprotease with two homologous catalytic domains. Both the N- and C-terminal domains are peptidyl dipeptidases. Hydrolysis by ACE of its decapeptide substrate Ang I is increased by Cl(-), but the molecular mechanism of this regulation is unclear. A search for single substitutions to Gln among all conserved basic residues (Lys/Arg) in human ACE C-domain identified R1098Q as the sole mutant that lacked Cl(-) dependence. Cl(-) dependence is also lost when the equivalent Arg in the N-domain, Arg(500), is substituted with Gln. The Arg(1098) to Lys substitution reduced Cl(-) binding affinity by approximately 100-fold. In the absence of Cl(-), substrate binding affinity (1/K(m)) of and catalytic efficiency (k(cat)/K(m)) for Ang I hydrolysis are increased 6.9- and 32-fold, respectively, by the Arg(1098) to Gln substitution, and are similar (<2-fold difference) to the respective wild-type C-domain catalytic constants in the presence of optimal [Cl(-)]. The Arg(1098) to Gln substitution also eliminates Cl(-) dependence for hydrolysis of tetrapeptide substrates, but activity toward these substrates is similar to that of the wild-type C-domain in the absence of Cl(-). These findings indicate that: 1) Arg(1098) is a critical residue of the C-domain Cl(-)-binding site and 2) a basic side chain is necessary for Cl(-) dependence. For tetrapeptide substrates, the inability of R1098Q to recreate the high affinity state generated by the Cl(-)-C-domain interaction suggests that substrate interactions with the enzyme-bound Cl(-) are much more important for the hydrolysis of short substrates than for Ang I. Since Cl(-) concentrations are saturating under physiological conditions and Arg(1098) is not critical for Ang I hydrolysis, we speculate that the evolutionary pressure for the maintenance of the Cl(-)-binding site is its ability to allow cleavage of short cognate peptide substrates at high catalytic efficiencies.

Allosteric Site↗
PubMed2001

Effect of H-7 on cultured human trabecular meshwork cells.

PURPOSE: To determine the effect of the serine-threonine kinase inhibitor H-7, which blocks actomyosin contractility and increases outflow facility in live monkeys, on morphology, cytoskeleton, and cellular adhesions of human trabecular meshwork (HTM) cells in culture. METHODS: Cultured HTM cells were videographically recorded and evaluated before and after exposure to H-7 at different concentrations. The subcellular distribution of the actin-based cytoskeleton and associated anchor proteins including vinculin, paxillin, and beta-catenin, as well as phosphotyrosine-containing proteins were evaluated by fluorescence immunocytochemistry and digital fluorescence microscopy. RESULTS: H-7 induced pronounced but reversible HTM cell thickening toward the cell center and deterioration of the actin cytoskeletal network. Cell-extracellular matrix (ECM) and cell-cell adhesions were also affected, but the beta-catenin-rich, vinculin-containing adherens junctions were clearly more resistant than focal contacts. Phosphotyrosine labeling in focal contacts was highly sensitive to H-7. CONCLUSIONS: H-7 induces alterations in cell shape, actin cytoskeleton, and associated focal adhesions in cultured HTM cells, which may be responsible for the effects of H-7 on outflow facility in live monkey eyes.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed2001

Trishomocubanes: novel sigma-receptor ligands modulate amphetamine-stimulated [3H]dopamine release.

Several trishomocubane analogues of the type 4-azahexacyclo [5.4.1.0(2,6).0(3,10).0(5,9).0(8,11)]dodecane exhibited moderate to high affinity at sigma-receptor subtypes and low or negligible affinity at dopamine and serotonin transporters (SERT). Selected compounds were examined for their effects on amphetamine-stimulated [3H]dopamine release from striatal slices in vitro. Compounds 1, 2, 3 and 4 significantly enhanced amphetamine-stimulated release in a concentration-dependent manner. Compound 4, with the highest affinity and selectivity for the sigma(2)-receptor subtype, displayed the greatest potency. The enhancement produced by 1 and 2 was fully reversed by the selective sigma(2) antagonists 1'-[4-[1-(4-fluorophenyl)-1-H-indol-3-yl]-1-butyl]spiro[iso-benzofuran-1(3H), 4'piperidine] (Lu28-179), endo-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2,3-dihydro-(1-methyl)ethyl-2-oxo-1-H-benzimidazole-1-carboxyamidehydrochloride (BIMU-8) and the non-subtype selective antagonist N-[2-(3,4-dichlorophenyl)-ethyl]-N-methyl-2-pyrrolidinyl)ethylamine (BD1008). These data suggested a potential role for compounds 1 through 4 as sigma(2)-receptor agonists in functional studies. In addition, a D(3)-trishomocubane compound 5 displayed low affinity at sigma receptors (K(i)=3 microM) and moderate affinity at dopamine transporters (K(i)=623 nM). Compound 5 significantly inhibited the potentiation mediated by compound 2, presumably through sigma(2)-receptor antagonism, or a direct action on dopamine transporters.

Amphetamine↗
PubMed2001

Cobalt chloride induces PC12 cells apoptosis through reactive oxygen species and accompanied by AP-1 activation.

Reactive oxygen species (ROS) are supposed to play an important role in hypoxia- and ischemia/reperfusion-mediated neuronal injury with the characteristics of apoptosis. There are many reports showing that cobalt chloride (CoCl(2)) could mimic the hypoxic responses in some aspects including production of ROS in cultured cells. The cytotoxicity of CoCl(2) and its molecular mechanisms have yet to be elucidated. We report that CoCl(2) triggered neuronal PC12 cells apoptosis in a dose- and time-dependent manner. Apoptosis was demonstrated by morphological changes and DNA fragmentation, and was dependent on macromolecular synthesis. Apoptosis was also confirmed by the decrease of the expression of Bcl-X(L). To our knowledge, this is the first documentation of the apoptotic induction of CoCl(2) on PC12 cells. Furthermore, ROS production in PC12 cells was increased during CoCl(2) treatment. Antioxidants, which could inhibit ROS production, significantly blocked CoCl(2)-induced apoptosis, suggesting that apoptosis is mediated by ROS production. We also observed a significant increase of the DNA-binding activity of AP-1 in response to CoCl(2) and this increase was blocked by antioxidants, showing that CoCl(2)-induced apoptosis is accompanied by ROS-activated AP-1. CoCl(2)-treated PC12 cells may serve as an in vitro model for studies of molecular mechanisms in ROS-linked neuronal disorders.

Animals↗