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M Chen

Publications and source records attributed to M Chen.

At least 523 records · Page 29Linked to original sources
PubMed1996

Protective antibodies in murine Lyme disease arise independently of CD40 ligand.

Borrelia burgdorferi-infected mice develop acute arthritis that undergoes Ab-mediated resolution. To further investigate the role of B. burgdorferi-specific Abs in Lyme borreliosis, CD40 ligand-deficient (CD40L-deficient) mice were infected with B. burgdorferi. The development and regression of arthritis were similar in CD40L-deficient and control mice. Although CD40L-deficient mice have defects in Ig class switching, infected CD40L-deficient mice developed B. burgdorferi-specific IgG2b Abs. Moreover, the transfer of serum from B. burgdorferi-infected CD40L-deficient animals prevented infection in severe combined immunodeficient mice. These data show that B. burgdorferi-infected CD40L-deficient mice are capable of producing Abs that are protective, despite the inability of these mice to mediate T-dependent immune responses.

Animals↗
PubMed1996

Sublethal levels of oxidative stress stimulate transcriptional activation of c-jun and suppress IL-2 promoter activation in Jurkat T cells.

Sublethal levels of oxidative stress are well known to alter T cell functional responses, but the underlying mechanisms are unknown. The current study examined the effects of oxidative stress on transcriptional activities mediated by c-Fos/c-Jun AP-1 and the nuclear factor of activated T cells (NF-AT). The present results show that Jurkat T cells acutely exposed to micromolar concentrations of H2O2 exhibit substantial increases in AP-1 binding activity and the expression of c-jun but not c-fos mRNA. The preferential induction of c-jun by H2O2 did not represent redox stabilization of mRNA transcripts, and oxidative signals closely resembled PHA/PMA stimulation by effectively transactivating the full length c-jun promoter via the proximal jun1 tumor promoter-responsive element (TRE)-like promoter element. Similarly, the complexes binding the consensus AP-1 TRE and jun TRE-like motifs in cells exposed to oxidative signals or PHA/PMA were indistinguishable, being composed of c-Fos, c-Jun, and JunD. However, PHA/PMA but not oxidative signals induced the coordinate activation of reporter constructs containing the AP-1-TRE, NF-AT, and IL-2 promoter regions along with IL-2 mRNA expression. Furthermore, sublethal levels of H2O2 actively suppressed the transcriptional activation of NF-AT and IL-2 reporters as well as the expression of IL-2 mRNA in cells stimulated with PHA/PMA. Gel shift analysis revealed that oxidative suppression of NF-AT represented inhibition in the early generation of NFAT complexes rather than the binding of preformed NF-AT complexes. These results suggest that oxidative signals can positively and negatively regulate T cell transcriptional events and that changes in cellular redox can uncouple AP-1 regulation of c-jun from transcriptional up-regulation of IL-2 via NF-AT.

Base Sequence↗
PubMed1996

Agonist recognition by proteinase-activated receptor 2 and thrombin receptor. Importance of extracellular loop interactions for receptor function.

Thrombin receptor and proteinase-activated receptor 2 (PAR2) define a family of G protein-coupled receptors that are activated by a novel proteolytic mechanism. Specific cleavage of their amino-terminal exodomains unmasks a new amino terminus which then serves as a tethered ligand, docking intramolecularly to the body of the receptor to effect signaling. Identification of the docking interactions between tethered ligand domain and receptor is critical for understanding transmembrane signaling by these receptors. Synthetic "agonist peptides" that mimic the tethered ligand domains of thrombin receptor and PAR2 act as agonists at their respective receptors. Toward defining the docking interactions which mediate receptor activation, we determined the specificity of the thrombin receptor and PAR2 for their respective agonist peptides and used receptor chimeras to identify the receptor domains responsible for such specificity. PAR2 responded to both thrombin receptor and PAR2 agonist peptides. In contrast, thrombin receptor was selective for its own agonist peptide. Substitution of the extracellular face of PAR2, its amino-terminal exodomain and three extracellular loops, for the cognate thrombin receptor structures yielded a chimeric receptor with PAR2-like agonist specificity. Substitution of individual extracellular domains revealed that the primary determinant of agonist specificity was extracellular loop 2. Strikingly, substitution of either the amino-terminal exodomain or third extracellular loop alone caused marked loss of receptor function, but the double substitution yielded a functional receptor. Thus, the extracellular domains of these G protein-coupled receptors are more than simply passive links between transmembrane domains. They participate in agonist recognition and must interact, directly or indirectly, for proper receptor function.

Amino Acid Sequence↗
PubMed1996

Reconstitution of the hippocampal mossy fiber and associational-commissural pathways in a novel dissociated cell culture system.

Synapses of the hippocampal mossy fiber pathway exhibit several characteristic features, including a unique form of long-term potentiation that does not require activation of the N-methyl-D-aspartate receptor by glutamate, a complex postsynaptic architecture, and sprouting in response to seizures. However, these connections have proven difficult to study in hippocampal slices because of their relative paucity (<0.4%) compared to commissural-collateral synapses. To overcome this problem, we have developed a novel dissociated cell culture system in which we have enriched mossy fiber synapses by increasing the ratio of granule-to-pyramidal cells. As in vivo, mossy fiber connections are composed of large dynorphin A-positive varicosities contacting complex spines (but without a restricted localization). The elementary synaptic connections are glutamatergic, inhibited by dynorphin A, and exhibit N-methyl-D-aspartate-independent long-term potentiation. Thus, the simplicity and experimental accessibility of this enriched in vitro mossy fiber pathway provides a new perspective for studying nonassociative plasticity in the mammalian central nervous system.

Animals↗
PubMed1996

Age-related decreases in IL-2 production by human T cells are associated with impaired activation of nuclear transcriptional factors AP-1 and NF-AT.

Although transcriptional factors AP-1 and nuclear factor of activated T cells (NF-AT) are important for the normal induction of IL-2, it is unknown if the age-related decline in IL-2 production by activated human T cells may be associated with aberrancies in transcriptional regulatory proteins. In the current studies, IL-2 production by T cells from elderly (mean 78 years) and young (mean 37 years) humans was measured in cultures stimulated with PHA, PHA plus PMA, crosslinked anti-CD3 mAB OKT3 plus PMA, or PMA plus ionomycin. Substantial decreases of IL-2 production were observed for cell cultures from 7 of 12 elderly individuals in response to the different stimuli, whereas the levels of IL-2 produced by stimulated T cells from other elderly individuals were equivalent to those observed for stimulated T cells of young subjects. Analyses of nuclear extracts by electrophoretic DNA mobility shift assays showed that decreased IL-2 production by stimulated T cells of elderly individuals was closely associated with impairments in the activation of both AP-1 and NF-AT. By contrast, T cells from elderly subjects with normal levels of IL-2 production exhibited normal activation of AP-1 and NF-AT. In addition, the results of competition experiments analyzing the normal components of NF-AT showed that the age-related reductions in stimulus-dependent NF-AT complexes corresponded to the slow migrating complexes that were composed of c-Fos/c-Jun AP-1. The resting and stimulated levels of NF kappa B were reduced in T cells from certain elderly individuals; however, alterations of NF kappa B did not correlate with changes in IL-2 expression. Thus, these results show that age-related impairments in the activation of AP-1 and NF-AT are closely associated with decreased expression of IL-2 and further suggest that aberrancies in the signaling pathways important for the induction of transcriptionally active c-Fos/c-Jun AP-1 may contribute to the impaired activation of NF-AT.

Adult↗
PubMed1996

Thrombin receptor activating mutations. Alteration of an extracellular agonist recognition domain causes constitutive signaling.

Constitutively active thrombin receptors were generated while constructing chimeric receptors to identify the structural basis for thrombin receptor agonist specificity. Substitution of eight amino acids from the Xenopus receptor's second extracellular loop (XECL2B) for the cognate sequence in the human thrombin receptor was sufficient to confer robust constitutive activity. Smaller substitutions within the XECL2B site yielded less constitutive activation, and substitution of several unrelated sequences at this site caused no activation. Expression of the XECL2B receptor caused high basal 45Ca efflux in Xenopus oocytes and high basal phosphoinositide hydrolysis and reporter gene induction in COS cells. Of note, a mutant receptor in which all four of the Xenopus thrombin receptor's extracellular segments replaced the cognate human sequences showed much less constitutive activity than XECL2B and preserved responsiveness to agonist. This partial complementation of the XECL2B phenotype by addition of other Xenopus extracellular structures suggests that the XECL2B mutation causes constitutive activation by altering interactions among the human receptor's extracellular domains. Thus, a change in an extracellular loop of a G protein-coupled receptor can transmit information across the cell membrane to cause signaling, perhaps via a conformational change similar to that caused by agonist binding. Indeed, the site of the activating mutation in XECL2B coincides with a putative agonist-docking site, supporting the hypothesis that agonist interactions with the thrombin receptor's extracellular loops contribute to receptor activation.

Amino Acid Sequence↗
PubMed1996

Enhancement of TAT-induced transactivation of the HIV-1 LTR by two genomic fragments of HHV-6.

Clinical and experimental observations suggest that human herpesvirus-6 (HHV-6), a T-lymphotropic herpesvirus, may act as a cofactor in the acquired immunodeficiency syndrome (AIDS). Moreover, a possible role of HHV-6 in the increased incidence and severity of cervical carcinoma in human immunodeficiency virus (HIV)-infected women was suggested by the recent observation that HHV-6 can infect cervical carcinoma cells, accelerating their tumorigenicity in vivo. Therefore, the ability of four HHV-6 genomic clones derived from HHV-6 to transactivate the long terminal repeat (LTR) of HIV-1 in two cervical carcinoma cell lines and in a T-lymphoid cell line was tested. Two HHV-6 clones, pZVH-14 and pZVB-70, which were previously shown to increase the expression of human papillomavirus (HPV)-transforming genes, were, per se, weak transactivators of the HIV-1 LTR. However, an increased effect occurred when these clones were combined with the HIV-1 transactivator TAT-1. No such effect was seen with two other HHV-6 clones used as controls. Analysis with HIV-1 LTR deletion mutants indicated that this enhancing effect requires the presence of elements contained in both the enhancer region and the TAT activation region (TAR) of HIV-1. This data may have implications for the potential role of HHV-6 in AIDS and AIDS-related cervical carcinoma.

Genes, tat↗
PubMed1996

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Journal Article↗
PubMed1996

Enhanced lipid peroxidation by extracellular ATP in PC12 cells.

Recently we have demonstrated that extracellular ATP acts as an excitatory neurotransmitter and enhances cell death in the presence of ferrous ions. By using a newly developed cis-parinaric acid fluorescence technique, we demonstrated that ATP, in a dose dependent manner, enhanced the increased membrane lipid peroxidation in PC12 cells when cells were incubated with micromolar FeCl2/DTP. P2 purinoceptor agonists, alpha,beta-methylene ATP and 2-methylthio-ATP, induced PC12 cell lipid peroxidation, but to a lesser extent than ATP. ATP-induced Ca(2+) influx via P2 purinoceptor activation significantly increased the intracellular Ca(2+)concentration, which may have triggered a free radical generating cascade(s), and led to membrane lipid peroxidation and cell death. Since oxidative stress has been implicated in certain neurodegenerative diseases such as aging, extracellular ATP may contribute to neuronal cell death by an oxidative mechanism involving lipid peroxidation.

Adenosine Triphosphate↗
PubMed1996

Cerebrospinal fluid sodium concentration and osmosensitive sites related to arterial pressure in anaesthetized rats.

Intracerebroventricular injection of hypertonic saline induces experimental hypertension. To measure [Na] in the vicinity of osmosensitive sites, we continuously measured [Na] in cerebrospinal fluid ([Na]csf) in the lateral ventricle (LV, n = 6), in the third ventricle (V3, n = 6) and in the medial preoptic nucleus (MPO, n = 6) ([Na]MPO) with a Na-sensitive electrode together with mean arterial pressure (MAP) during infusion of hypertonic artificial cerebrospinal fluid (ACSF, [Na] = 1,000 meq/kg H2O) at 5 "mu"l/min for 3 min into the LV in urethane- anaesthetized rats. [Na]csf in the LV began to increase at the beginning of infusion, reaching a peak of 48 +/- 9 meq/kg H2O (mean +/- SE) around the end of infusion, then recovering to the pre-infusion level by 17 min. [Na]csf in V3 changed similarly to that in the LV without any delay, although the peak value was reduced (61% , P < 0.05). In the MPO, in contrast the increase in [Na]MPO was delayed (3 min, P < 0.002) and the peak reduced even further (to 37%, P < 0.01) compared with that in V3. Thereafter, it remained higher than the pre-infusion level until the end of recovery (P < 0.05). MAP began to increase at the onset of infusion (P < 0.05); the maximum increase of 16 +/- 2 mm Hg (n = 18) was reached at the end of infusion, whereafter this level was almost sustained until the end of the 22-min recovery period. To analyse quantitatively the relationship between MAP and [Na]csf, hypertonic ACSF was infused at 2.5 "mu"l/min into the LV. [Na]csf in the LV and MAP increased at half the rates seen with 5 "mu"l/min. These results suggest that the first increase in MAP after hypertonic infusion into the LV is due to the increase in [Na] in the LV and V3, and that the subsequent sustained increase in MAP is related to the delayed increase in [Na] in the periventricular tissues of the V3.

Anesthesia↗
PubMed1996

Protective immunization against Helicobacter stimulates long-term immunity.

Immunization with an oral vaccine composed of whole cell sonicates of Helicobacter felis plus cholera toxin (CT) can protect mice from H. felis challenge. The aim of this study was to determine whether this protective immunity was long-lived. Mice were given the vaccine then left for up to 15 months before challenge. After 15 months, all mice were still protected from H. felis infection, indicating that oral immunization using bacterial antigens plus CT can stimulate long-term local immunological memory.

Animals↗
PubMed1996

Effects of beta-blockade and atropine on ischemic responses in left ventricular regions subtending coronary stenosis during dobutamine stress echocardiography.

OBJECTIVES: This study was designed to examine the effects of a beta-adrenergic blocking agent on the ischemic response to dobutamine stress and to determine the degree to which these effects can be abolished by the addition of atropine. BACKGROUND: Whether beta-blockade affects the sensitivity of dobutamine stress echocardiography for the diagnosis of coronary artery disease has been controversial. METHODS: In nine pigs, a left anterior descending coronary artery stenosis was created to reduce flow reserve (maximal/rest flow) to 1.1 to 1.9 without baseline regional wall motion abnormalities. This corresponded to a 50% to 90% diameter stenosis. Wall thickening was measured using epicardial echocardiography. Regional lactate production and coronary venous pH were monitored from an adjacent cardiac vein. A standard protocol of dobutamine stress echocardiography was first performed. After normalization of the ischemic abnormalities elicited with this infusion, esmolol was infused at 50 micrograms/kg body weight per min and the dobutamine test was repeated, with 1.0 mg of atropine added at the maximal dobutamine dose. RESULTS: Without esmolol, dobutamine stress induced myocardial ischemia with a reduction in regional wall thickening and lactate production in all nine pigs. Multiple regression analysis revealed that coronary flow per heartbeat (p < 0.01) and lactate production (p < 0.05) independently correlated with regional wall thickening during dobutamine stress. The beta-blocker significantly reduced heart rate and regional oxygen consumption and altered the relation between coronary flow per heartbeat and regional wall thickening (p < 0.05) during dobutamine stress. Esmolol prevented dobutamine-induced ischemia (lactate production and wall motion abnormalities) in seven of nine pigs. The addition of atropine induced lactate production and a reduction in wall thickening in five of seven pigs in which ischemia had been prevented by beta-blockade. However, lactate production was higher and regional venous pH was lower with the baseline dobutamine infusion than with that performed after esmolol with atropine added at the maximal dobutamine dose (p < 0.05). CONCLUSIONS: A correlation between regional wall thickening and coronary flow per heartbeat was demonstrated during baseline dobutamine stress. Beta-blockade shifted this relation so that dobutamine stress-induced myocardial ischemia was attenuated. The mechanisms by which beta-blockade prevents dobutamine-induced ischemia appeared to be mainly through decreases in heart rate and rate of rise in left ventricular pressure, improvement of regional coronary flow per heartbeat and attenuation of regional ischemic lactate production. Adding atropine in conventional doses enhanced the ability of dobutamine stress to induce myocardial ischemia but did not completely abolish the effects of beta-blockade on either the severity of dobutamine-induced wall thickening abnormalities or regional metabolic disturbances.

Adrenergic beta-Antagonists↗
PubMed1996

Leonticins A-C, three octasaccharide saponins from Leontice kiangnanensis.

Three octasaccharide saponins, leonticins A, B, and C (1-3), were isolated from the tubers of Leontice kiangnanensis. Their structures were elucidated by a combination of chemical degradation and spectral methods including negative FABMS and NMR measurements as 3-O-beta-D-glucopyranosyl (1-->2)-alpha-L-arabinopyranosylhederagenin 28-O-alpha-L-rhamnopyranosyl (1-->4)-beta-D-glucopyranosyl (1-->6)-beta-D-glucopyranosyl(1-->4)-alpha-L-rhamnopyranosyl (1-->4)-beta-D-glucopyranosyl-(1-->6)-beta-D-glucopyranoside (1), 3-O-[beta-D-xylopyranosyl(1-->3)-beta-D-galactopyranosyl(1-->4)-beta-D- glucopyranosyl (1-->3)][beta-D-glucopyranosyl(1-2)]-alpha-L-arabinopyranosyloleanoli c acid 28-O-alpha-L-rhamnopyranosyl (1-->4)-beta-D-glucopyranosyl(1-->6)-beta-D-glucopyranoside (2), and 3-O-[beta-D-xylopyranosyl (1-->3)-beta-D-galactopyranosyl(1-->4)-beta-D-glucopyranosyl(1-->3)] [beta-D-glucopyranosyl(1-->2)]-alpha-L-arabinopyranosylechinocystic++ + acid 28-O-alpha-L-rhamnopyranosyl(1-->4)-beta-D-glucopyranosyl- (1-->6)-beta-D-glucopyranoside (3), respectively. The complete assignments of the proton and carbon resonances for 1-3 were achieved based on extensive 2D NMR analysis (DQF-COSY, TOCSY, ROESY, HSQC, and HMBC).

Anti-Inflammatory Agents↗
PubMed1996

Automaticity of social behavior: direct effects of trait construct and stereotype-activation on action.

Previous research has shown that trait concepts and stereotype become active automatically in the presence of relevant behavior or stereotyped-group features. Through the use of the same priming procedures as in previous impression formation research, Experiment 1 showed that participants whose concept of rudeness was printed interrupted the experimenter more quickly and frequently than did participants primed with polite-related stimuli. In Experiment 2, participants for whom an elderly stereotype was primed walked more slowly down the hallway when leaving the experiment than did control participants, consistent with the content of that stereotype. In Experiment 3, participants for whom the African American stereotype was primed subliminally reacted with more hostility to a vexatious request of the experimenter. Implications of this automatic behavior priming effect for self-fulfilling prophecies are discussed, as is whether social behavior is necessarily mediated by conscious choice processes.

Affect↗
PubMed1996

Inactivation of Fac in mice produces inducible chromosomal instability and reduced fertility reminiscent of Fanconi anaemia.

Fanconi anaemia (FA) is an autosomal recessive disease characterized by bone marrow failure, variable congenital malformations and predisposition to malignancies. Cells derived from FA patients show elevated levels of chromosomal breakage and an increased sensitivity to bifunctional alkylating agents such as mitomycin C (MMC) and diepoxybutane (DEB). Five complementation groups have been identified by somatic cell methods, and we have cloned the gene defective in group C (FAC)(7). To understand the in vivo role of this gene, we have disrupted murine Fac and generated mice homozygous for the targeted allele. The -/- mice did not exhibit developmental abnormalities nor haematologic defects up to 9 months of age. However, their spleen cells had dramatically increased numbers of chromosomal aberrations in response to MMC and DEB. Homozygous male and female mice also had compromised gametogenesis, leading to markedly impaired fertility, a characteristic of FA patients. Thus, inactivation of Fac replicates some of the features of the human disease.

Animals↗
PubMed1996

Laparoscopy for the Acute Abdomen.

The application of laparoscopy for therapy is well established in biliary tract disease, inguinal herniorrhaphy, and fundoplication. Frequently, the diagnostic capability of laparoscopy is overlooked. A review of the literature and an institution's experience in laparoscopy for the acute abdomen is presented. Therapy was completed laparoscopically in a substantial number of cases.

Journal Article↗
PubMed1996

Membrane integration of in vitro-translated gap junctional proteins: co- and post-translational mechanisms.

Connexins (Cx) are protein components of gap junction channels that permit the passage of small molecules between neighboring cells. cDNAs of a large family of connexins have been isolated and sequenced. A gap junction channel consists of two connexons, one from each cell in contact, composed of six connexin subunits. It has been suggested by Musil and coworkers that the oligomerization of formation of a connexon occurs at the level of the trans-Golgi network. In the present study, we initiated an analysis of the early stages of protein synthesis and membrane insertion of Cx32 and Cx26, two connexins that we have demonstrated are co-expressed in the same junctions in hepatocytes. Using an in vitro transcription and a coupled cell-free translation and translocation system, we observed that both Cx32 and Cx26 could insert into microsome membranes co-translationally, producing a topological structure indistinguishable from that in isolated gap junctions. To our surprise, Cx26 could also insert into membranes post-translationally with a native orientation. This post-translational membrane insertion process is dependent on nucleotides but not their hydrolysis. Cx32, on the other hand, could not insert into membranes post-translationally. These disparate properties of Cx32 and Cx26 are not due to the significant difference in the lengths of their C-terminal domains, but rather to their internal amino acid sequences. These observations raise the possibility that there may be another pathway for Cx26 to insert into membranes in cells and this feature may be important for the regulation of its functions. These findings may also lead us to a new approach to reconstitution without detergent extraction.

Adenosine Triphosphate↗