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R Liu

Publications and source records attributed to R Liu.

At least 19 recordsLinked to original sources
PubMed1999

Purification and cloning of aggrecanase-1: a member of the ADAMTS family of proteins.

We purified, cloned, and expressed aggrecanase, a protease that is thought to be responsible for the degradation of cartilage aggrecan in arthritic diseases. Aggrecanase-1 [a disintegrin and metalloproteinase with thrombospondin motifs-4 (ADAMTS-4)] is a member of the ADAMTS protein family that cleaves aggrecan at the glutamic acid-373-alanine-374 bond. The identification of this protease provides a specific target for the development of therapeutics to prevent cartilage degradation in arthritis.

ADAM Proteins↗
PubMed1999

Expression of human alpha(1,3)fucosyltransferase antisense sequences inhibits selectin-mediated adhesion and liver metastasis of colon carcinoma cells.

The initial steps of leukocyte and tumor cell adhesion involve selectin receptor/ligand interactions. The selectin ligand components sialyl Lewis x and sialyl Lewis a are oncodevelopmental antigens involved in progression of adenocarcinoma. Interrupting biosynthesis of these surface glycans by inhibition of alpha(1,3)fucosyltransferase (FUT) gene expression is an attractive goal for functional and therapeutic studies. We report here the inhibition of E-selectin-mediated adenocarcinoma cell adhesion by stable transfection of antisense sequences directed at the human Lewis alpha(1,3/1,4)fucosyltransferase gene, FUT3. The metastatic parental cell line, HT-29LMM, expressed high levels of sialyl Lewis x, sialyl Lewis a, alpha(1,3/1,4)fucosyltransferase activity, and FUT3 transcript, but antisense transfectant cell lines did not. When injected into the spleens of nude mice, the stable antisense clones were unable to colonize the liver. These results provide target validation for inhibition of carcinoma metastasis with antisense FUT sequences and confirm the primacy of alpha(1,3)fucosyltransferases in the synthesis of selectin ligands.

Adenocarcinoma↗
PubMed1999

Diet and brain cancer in adults: a case-control study in northeast China.

A hospital-based case-control study was conducted in the Heilongjiang Province of northeast China between May 1993 and May 1995. A total of 129 histologically confirmed brain cancer cases (73 gliomas and 56 meningiomas) and 258 matched controls were interviewed in 6 major hospitals to examine the influence of dietary factors in developing brain cancer. Information was obtained about frequency of consumption of 57 food items. Odds ratios (ORs) were obtained from conditional logistic regression, including allowance for socio-demographic factors, alcohol, tobacco and total energy intake. Consumption of fresh vegetables (OR = 0.29 for the highest quartile compared with the lowest one), and specifically of Chinese cabbage and onion, fruit (OR = 0.15), fresh fish (OR = 0.38) and poultry (OR = 0.16) was inversely related to the risk of developing brain cancer. A protective effect was also seen for vitamin E intake, calcium and, although non-significantly, beta-carotene and vitamin C. Risk of brain cancer increased with consumption of salted vegetables (OR = 2.54) and salted fish.

Adult↗
PubMed1999

Novel reverse-turn mimics inhibit farnesyl transferase.

Reverse-turn inducing bicyclic lactams were incorporated into the substrate sequence recognized by farnesyl transferase to create inhibitors of RAS farnesylation. While the free peptides did not show any effect on the farnesylation, their Fmoc-protected counterparts impede the transformation of RAS with IC50's in the low micromolar range.

Alkyl and Aryl Transferases↗
PubMed1999

Differential influence of tyrosine residues of the common receptor beta subunit on multiple signals induced by human GM-CSF.

BACKGROUND: GM-CSF induces the proliferation and differentiation of a broad range of hematopoietic cells. Stimulation with this cytokine results in the induction of rapid tyrosine phosphorylation of cellular proteins, including the common beta subunit of its receptor. However, the significance of beta subunit phosphorylation with regard to regulation of signaling molecules and the functions of GM-CSF is less well understood. OBJECTIVE: To investigate the role of cytoplasmic tyrosine residues, we performed mutational analyses of the beta subunit. METHODS: A series of beta subunit mutants with 1, some, or all of 8 cytoplasmic tyrosines converted to phenylalanines were constructed. These beta subunit mutants were expressed in murine BA/F3 cells together with the wild-type human alpha subunit, and their potentials to transmit signals were examined. RESULTS: A mutant beta subunit lacking all cytoplasmic tyrosines (Fall) activated JAK2, but not Shc, SHP-2, the Raf/ERK cascade, signal transducer and activator of transcription 5 (STAT5), or the c-fos promoter, nor did it effectively induce proliferation. Adding back each tyrosine to Fall revealed that (1) Tyr577 was necessary and sufficient for Shc phosphorylation; (2) Tyr577, Tyr612, and Tyr695 were involved in activation of SHP-2, the Raf/ERK cascade, and c-fos transcription; and (3) all tyrosines, but particularly Tyr612, Tyr695, Tyr750, and Tyr806, facilitated STAT5 activation. Impaired cell growth was also partly restored by any of the tyrosines. Interestingly, the Fall mutant was still capable of suppressing apoptosis and maintaining cell viability. CONCLUSION: These results provide evidence that beta subunit tyrosines possess distinct, yet overlapping, roles in the activation of multiple signaling pathways induced by GM-CSF. In addition, GM-CSF may activate both beta subunit tyrosine-dependent and -independent pathways, with the latter being sufficient to transmit survival signals.

Animals↗
PubMed1999

HIV-1 infectability of CD4+ lymphocytes with relation to beta-chemokines and the CCR5 coreceptor.

CD4+ lymphocytes exhibit variable permissiveness to the replication of HIV-1. A cohort of sexually-exposed-yet-uninfected individuals were previously shown to have CD4+ lymphocytes refractory for M-tropic viral replication. In particular, two individuals from this population whose CD4+ lymphocytes exhibited complete resistance to M-tropic viral replication were later shown to be homozygous for a 32bp (delta32) deletion in the gene encoding for CCR5. In screening diverse populations, HIV-1 infected individuals heterozygous for the delta32 allele were statistically favored in their disease course to harbor lower viral loads and exhibit slower rates of CD4+ cell loss when compared to control CCR5 wild-type individuals. Further comparative analysis between individuals in the exposed but uninfected cohort who demonstrated intermediate levels of in vitro viral replication and CD4+ lymphocytes isolated from uninfected delta32 heterozygous individuals indicate that reduced levels of in vitro M-tropic replication are a CCR5-related phenomenon: CD4+ lymphocytes from these individuals were more sensitive to the HIV-1 blocking effects of recombinant chemokines, displayed lower CCR5 cell surface expression levels and a proportionate increase in the production of RANTES when compared to CD4+ lymphocytes from control individuals. These results suggest that the CCR5 phenotype is important in determining the replicative capacity of M-tropic HIV-1 in vitro. The implications of these results with relation to HIV-1 transmission and disease progression are discussed.

CD4-Positive T-Lymphocytes↗
PubMed1999

Molecular forceps from combinatorial libraries prevent the farnesylation of Ras by binding to its carboxyl terminus.

INTRODUCTION: Ras is one of the major oncogenes. In order to function properly it has to undergo post-translational processing at its carboxyl terminus. It has been shown that inhibitors of farnesyl transferase, the first enzyme in the processing chain, can suppress the transforming activity of oncogenic Ras. RESULTS: We have identified molecular forceps, branched peptidic molecules, from combinatorial libraries that bind to the carboxyl terminus of Ras and interfere with its farnesylation without inhibiting the farnesyl transferase. The active molecules were selected by a screening against the carboxy-terminal octapeptide of Ras. CONCLUSIONS: The implications of our findings are twofold. First, we demonstrate that it is possible to prevent enzymatic transformations by blocking the enzyme's access to its substrate using a synthetic small molecule to mask the substrate. Second, we show that it is feasible to derive molecules from combinatorial libraries that bind a specific epitope on a protein by selecting these molecules with the isolated peptide epitope.

Alkyl and Aryl Transferases↗
PubMed1999

Control of inducible chemoresistance: enhanced anti-tumor therapy through increased apoptosis by inhibition of NF-kappaB.

Programmed cell death (apoptosis) seems to be the principal mechanism whereby anti-oncogenic therapies such as chemotherapy and radiation effect their responses. Resistance to apoptosis, therefore, is probably a principal mechanism whereby tumors are able to overcome these cancer therapies. The transcription factor NF-kappaB is activated by chemotherapy and by irradiation in some cancer cell lines. Furthermore, inhibition of NF-kappaB in vitro leads to enhanced apoptosis in response to a variety of different stimuli. We show here that inhibition of NF-kappaB through the adenoviral delivery of a modified form of IkappaBalpha, the inhibitor of NF-kappaB, sensitizes chemoresistant tumors to the apoptotic potential of TNFalpha and of the chemotherapeutic compound CPT-11, resulting in tumor regression. These results demonstrate that the activation of NF-kappaB in response to chemotherapy is a principal mechanism of inducible tumor chemoresistance, and establish the inhibition of NF-kappaB as a new approach to adjuvant therapy in cancer treatment.

Adenoviridae↗
PubMed1999

In vivo kinetics of human apolipoprotein A-I variants in rabbits.

BACKGROUND: Genetic variants of human apolipoprotein (apo) A-I, the major protein component of high-density lipoprotein (HDL), with a single amino acid substitution have been reported, and some of these result in very low plasma HDL-cholesterol (C) levels. Examining the kinetics of radiolabelled apolipoprotein is a straightforward technique for determining its metabolism in vivo. In this study, we investigated the in vivo kinetics of several human apo A-I variants, which we had identified previously, in rabbits. MATERIALS AND METHODS: Apo A-I variants from heterozygous carriers of Lys-107-->0, Lys-107-->Met, Pro-3-->Arg, Pro-4-->Arg, Pro-165-->Arg and Glu-198-->Lys and the corresponding normal apo A-I were purified and then radioiodinated with 131I and 125I. A kinetic study of apo A-I variants was performed in normolipidaemic rabbits after simultaneous injection of the two isotopes that had been incorporated into HDL. The fractional catabolic rate (FCR) was calculated from the radioactive decay curve. RESULTS: Acidic mature (negatively charged) apo A-I variants caused by a single amino acid substitution (Lys-107-->0, and Lys-107-->Met) were catabolized faster (FCR, 1.931 +/- 0.539 per day vs. 1.636 +/- 0.460 per day, P </= 0.01 using the Wilcoxon signed-rank test) and basic mature (positively charged) apo A-I variants (Pro-3-->Arg, Pro-4-->Arg, Pro-165-->Arg and Glu-198-->Lys) were catabolized more slowly (FCR 1.470 +/- 0.380 per day vs. 1.654 +/- 0.430 per day, P </= 0.01) than the corresponding normal mature apo A-I in vivo in rabbits. In addition, an inverse linear relationship was observed between the deviation in the FCR of variant human apo A-I from that of normal human apo A-I and the number of electric charges that the apo A-I variant carried (r = -0. 90, k = -0.188, P = 0.0003), as assessed by a linear regression analysis, suggesting that the electric charge of apo A-I variants may determine, at least in part, its in vivo kinetics in rabbits. CONCLUSIONS: Genetic variants of apo A-I with a single amino acid substitution show abnormal kinetics, and the electric charge of a apo A-I variant could contribute to determining its kinetics in vivo in this xenologous model.

Animals↗
PubMed1999

Development of muscarinic analgesics derived from epibatidine: role of the M4 receptor subtype.

Epibatidine, a neurotoxin isolated from the skin of Epipedobates tricolor, is an efficacious antinociceptive agent with a potency 200 times that of morphine. The toxicity of epibatidine, because of its nonspecificity for both peripheral and central nicotinic receptors, precludes its development as an analgesic. During the synthesis of epibatidine analogs we developed potent antinociceptive agents, typified by CMI-936 and CMI-1145, whose antinociception, unlike that of epibatidine, is mediated via muscarinic receptors. Subsequently, we used specific muscarinic toxins and antagonists to delineate the muscarinic receptor subtype involved in the antinociception evoked by these agents. Thus, the antinociception produced by CMI-936 and CMI-1145 is inhibited substantially by 1) intrathecal injection of the specific muscarinic M4 toxin, muscarinic toxin-3; 2) intrathecally administered pertussis toxin, which inhibits the G proteins coupled to M2 and M4 receptors; and 3) s.c. injection of the M2/M4 muscarinic antagonist himbacine. These results demonstrate that the antinociception elicited by these epibatidine analogs is mediated via muscarinic M4 receptors located in the spinal cord. Compounds that specifically target the M4 receptor therefore may be of substantial value as alternative analgesics to the opiates.

Alkaloids↗
PubMed1999

Isoproterenol, forskolin, and cAMP-induced nitric oxide production in pig ciliary processes.

PURPOSE: To investigate whether isoproterenol and forskolin, two adenylylcyclase activators, or 8-bromo-cAMP, an adenosine 3',5'-cyclic monophosphate (cAMP) analog, increase nitric oxide (NO) production in isolated porcine ciliary processes. METHODS: Nitrite (an NO metabolite) was measured (Griess reaction) before and 2 hours after exposure to 0.1 to 100 microM isoproterenol (a beta-adrenoreceptor agonist), 0.01 to 100 microM forskolin, or 0.1 to 1000 microM 8-bromo-cAMP. Some experiments were conducted in the presence of 0.5 mM N(G)-nitro-L-arginine methyl ester (L-NAME; a nitric oxide synthase [NOS] inhibitor), 10 microM propranolol (a beta-adrenoreceptor antagonist), or 1 microM KT 5720 (a cAMP-dependent protein kinase inhibitor). cAMP production was also measured (by immunoassay). RESULTS: Nitrite production was increased by isoproterenol (maximum, 10 microM: 164%; P < 0.001), forskolin (maximum, 10 microM: 254%; P < 0.001), and 8-bromo-cAMP (maximum, 100 microM: 184%; P < 0.001), an effect prevented by L-NAME (P < 0.05-0.001). Propranolol inhibited only isoproterenol-induced (10 microM) nitrite production (P < 0.05), whereas KT 5720 (P < 0.05) inhibited isoproterenol- (10 microM) and 8-bromo-cAMP-induced (10 microM) nitrite production. Furthermore, cAMP production evoked by isoproterenol (10 microM, P < 0.05) but not by forskolin (10 microM, P < 0.001) was inhibited by propranolol (P < 0.05). CONCLUSIONS: In isolated porcine ciliary processes, drugs activating adenylylcyclase or mimicking cAMP increase the production of NO by a mechanism that appears to involve both a cAMP-dependent protein kinase and NOS.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1999

Increased hydroxyl radical production and apoptosis in PC12 neuron cells expressing the gain-of-function mutant G93A SOD1 gene.

Mutations of the SOD1 gene (formerly known as Cu,Zn-SOD) are frequently associated with the familial form of amyotrophic lateral sclerosis (ALS). The G93A mutation of SOD1 with substitution of Gly to Ala at residue 93 results in gain of a peroxidative function. Here we report that transfection of PC12 neuron precursor cells with the G93A mutation of SOD1 results in increased production of hydroxyl radicals (*OH) and an enhanced rate of cell death by apoptosis. Notably, PC12 cells transfected with the H63C/G93A mutant of SOD1 with a mutation in the catalytic site that converts histidine at position 63 to cysteine showed a dramatically reduced production of *OH and rate of death by apoptosis. Thus the gain of function of the mutant G93A SOD1 can be reduced by an active site mutation. These results provide additional genetic evidence for the hypothesis that the increased *OH production and induced cytotoxicity in neuron cells expressing the mutant G93A SOD1 results from the gain of peroxidative function by the enzyme's catalytic site.

Animals↗
PubMed1998

The three-dimensional structure of 4-hydroxybenzoyl-CoA thioesterase from Pseudomonas sp. Strain CBS-3.

The soil-dwelling microbe, Pseudomonas sp. strain CBS-3, has attracted recent attention due to its ability to survive on 4-chlorobenzoate as its sole carbon source. The biochemical pathway by which this organism converts 4-chlorobenzoate to 4-hydroxybenzoate consists of three enzymes: 4-chlorobenzoyl-CoA ligase, 4-chlorobenzoyl-CoA dehalogenase, and 4-hydroxybenzoyl-CoA thioesterase. Here we describe the three-dimensional structure of the thioesterase determined to 2.0-A resolution. Each subunit of the homotetramer is characterized by a five-stranded anti-parallel beta-sheet and three major alpha-helices. While previous amino acid sequence analyses failed to reveal any similarity between this thioesterase and other known proteins, the results from this study clearly demonstrate that the molecular architecture of 4-hydroxybenzoyl-CoA thioesterase is topologically equivalent to that observed for beta-hydroxydecanoyl thiol ester dehydrase from Escherichia coli. On the basis of the structural similarity between these two enzymes, the active site of the thioesterase has been identified and a catalytic mechanism proposed.

Amino Acid Sequence↗
PubMed1998

Functional analysis of the proximal CCR5 promoter.

Two promoters for the CCR5 gene, termed Pu and Pd, corresponding to the upstream and downstream initiation sites, respectively, have been described. We show here that the proximal promoter, Pd, is used two- to fivefold more frequently than Pu in primary activated T cells and in the transformed T cell line PM1. Because of its importance in CCR5 transcription we characterized the transcriptional activity of this promoter. Pd contains a pair of consensus TATA elements (nt -19 and -31) and several potential regulatory elements and transcription factor-binding sites, including those for STAT, NF-kappaB, AP-1, NF-AT, and CD28RE. Using a transfected reporter vector, we found the promoter to be highly active and cell type specific. By 5' deletion analysis, the minimal CCR5 promoter was localized to a 225-nucleotide region (nt -189 to +36). This region contained the two TATA elements, a CD28RE consensus sequence, an AP-1-binding site, and two STAT-binding sites. The 1.9-kb intron appeared to have a negative influence on reporter gene activity, suggesting the presence of a negative element in this region. In addition, an upstream negative element was detected in the region nt -988 to -588. Mutagenesis of the TATA elements, of the NF-kappaB-, and AP-1-, and STAT-binding sites, and of the CD28RE indicated the importance of each of these in transcription. Finally, the NF-kappaB/Rel family member, p65(RelA), was a potent activator of the CCR5 promoter.

Base Sequence↗
PubMed1998

Predictive models for GABAA/benzodiazepine receptor subtypes: studies of quantitative structure-activity relationships for imidazobenzodiazepines at five recombinant GABAA/benzodiazepine receptor subtypes [alphaxbeta3gamma2 (x = 1-3, 5, and 6)] via comparative molecular field analysis.

Affinities of a series of substituted imidazobenzodiazepines at recombinant alpha1beta3gamma2, alpha2beta3gamma2, alpha3beta3gamma2, alpha5beta3gamma2, and alpha6beta3gamma2 GABAA/benzodiazepine receptor subtypes are reported. Many of these ligands displayed high affinities (low-nanomolar to subnanomolar scale) at all five receptor subtypes. Furthermore, a number of imidazobenzodiazepines exhibited relatively good selectivity at the alpha5-containing receptor isoform. For example, ligand 27 (RY-023) demonstrated a 55-fold higher selectivity at alpha5beta3gamma2 isoforms in comparison to other receptor subtypes. The affinity ratio of alpha1 (the most prevalent subtype in the brain) to alpha5 of this series of ligands ranged from 60- to 75-fold for the most selective ligands. Studies of quantitative structure-activity relationships (QSAR) by means of comparative molecular field analysis (CoMFA) were carried out. As a result, examination of CoMFA models for all five receptor subtypes demonstrated their predictability for affinities of imidazobenzodiazepines at the five receptor subtypes. Regions of molecular fields which would favor or disfavor the binding affinity of a ligand at a specific receptor subtype were examined via CoMFA for alpha1, alpha2, alpha3, alpha5, and alpha6 subtypes. A CoMFA regression analysis was applied to predict the ratio of Ki alpha1/Ki alpha5, an index for the selectivity of a ligand at the alpha5 subtype. All of the CoMFA models offered good cross-validated correlations for the ligands in the test set as well as the ratios of Ki alpha1/Ki alpha5, which demonstrated their potential for prediction.

Animals↗
PubMed1998

Linear and cyclic peptides as substrates and modulators of P-glycoprotein: peptide binding and effects on drug transport and accumulation.

One cause of multidrug resistance (MDR) in human cancers is the overexpression of the P-glycoprotein multidrug transporter, a member of the ABC superfamily of membrane proteins. Natural products and chemotherapeutic drugs are pumped out of the cell by P-glycoprotein in an ATP-dependent fashion. There is growing evidence that many hydrophobic peptides are also P-glycoprotein substrates. With the use of a fluorescence-quenching assay, we have shown that some linear and cyclic hydrophobic peptides interact with P-glycoprotein, whereas others do not. The measured values of the quenching constant, Kq, for interaction of peptides with P-glycoprotein ranged from 200 nM for cyclosporine A to 138 microM for the tripeptide N-acetyl-leucyl-leucyl-norleucinal. Peptides that interacted with P-glycoprotein in the fluorescence assay also blocked colchicine transport into plasma membrane vesicles from MDR cells. The values of Dm, the peptide concentration causing 50% inhibition of drug uptake, were highly correlated with the values of Kq, over three orders of magnitude. The P-glycoprotein ATPase stimulation/inhibition profile of the peptides was not helpful in making a quantitative assessment of the ability of a peptide to interact with P-glycoprotein or to block drug transport. Some hydrophobic peptides were able to restore accumulation in MDR cells of the chemotherapeutic drug daunorubicin and the fluorescent dye rhodamine 123 to the levels observed in the drug-sensitive parent. Peptides that interacted with P-glycoprotein also displayed a relatively low overall toxicity to intact MDR cells, and inhibited drug transport at concentrations below the toxic range. Hydrophobic peptides should be given serious consideration for development as clinical chemosensitizing agents.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1998

Adenosine mediates spinal norepinephrine-produced antinociception as revealed by nociceptive discharges in parafascicular neurons in rats.

The effects of intrathecal pretreatment with aminophylline on intrathecal norepinephrine-produced or serotonin-produced suppression of noxiously evoked discharges in thalamic parafascicular neurons were investigated in 35 urethane-anesthetized rats. The results showed that: (1) both intrathecal norepinephrine (15 nmol) or serotonin (20 nmol) produced significant suppression of noxiously evoked discharges in parafascicular neurons; (2) intrathecal aminophylline (120 nmol) blocked the norepinephrine-produced suppression of noxiously evoked discharges, while the same dose of aminophylline exhibited no significant effect on the serotonin-produced suppression of these discharges in parafascicular neurons. The results suggest that spinal norepinephrine-produced, but not serotonin-produced, antinociceptive effects may be mediated by adenosine as one of successive chemical links in the spinal dorsal horn circuitry.

Adenosine↗