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Biomedical subjects

S C Ng

Publications and source records attributed to S C Ng.

At least 73 records · Page 4Linked to original sources
PubMed2000

Evidence for de novo cholesterol synthesis by term human fetal amnion and chorion: a comparative study using the reverse-isotope dilution technique.

The cholesterol biosynthetic activity was assessed using [2-(14)C]-acetate as substrate in the homogenates of amnion and chorion obtained from women (n = 6, age 26-39 years) after spontaneous labour at term (37-40 weeks of gestation) having uncomplicated pregnancies. Reverse-isotope dilution analysis gave positive identification of [(14)C]-cholesterol acetate in all incubations of viable tissues. This metabolite was not evident in heat-denatured homogenates which served as controls. The extent of enzymic conversion for amnion at 2.6 x 10(-3) to 0.19% was persistently higher than that of the chorion at 1.7 x 10(-3) to 9.0 x 10(-3)%. The results indicate that human term fetal membranes possess the full complement of enzymes to catalyze the transformation of acetate to cholesterol. This study provides evidence that fetal membranes possess the capacity for de novo cholesterol biosynthesis, the sterol being essential for steroidogenesis as well as in embryo viability during pregnancy.

Acetates↗
PubMed2000

Rapid colchicine competition-binding scintillation proximity assay using biotin-labeled tubulin.

We have developed a rapid [3H]colchicine competition-binding scintillation proximity assay (SPA) to evaluate antimitotic compounds that bind to the colchicine-binding site on tubulin. The premise of our assay is that compounds will compete with radiolabeled colchicine for the tubulin-binding domain. Biotin-labeled tubulin is incubated first with unlabeled compound and radiolabeled ligand. Streptavidin-labeled SPA beads are added, and the radiolabel associated with tubulin is directly counted with no separation steps. Under our experimental conditions, the dissociation constant of binding (Kd) for colchicine to tubulin was determined to be 1.4 microM, which was consistent with previously reported values. Assay validation was performed by competitively inhibiting [3H]colchicine binding to tubulin with known microtubule inhibitors and comparing their inhibition constants (Ki). Our SPA bead method is a powerful tool since it overcomes the disadvantage of traditional filtration techniques, as there are no separation steps. It is extremely easy to set up, multiple samples can be assayed and supply and labor costs are reduced because of the minimal volume and test reagents used.

Aminophenols↗
PubMed2000

Effect of T-helper 1 cytokines on secretion of T-helper 2 cytokines by term trophoblast cells in culture.

A successful pregnancy has been postulated to be the result of a discrete balance between T-helper 1 (Th1) and T-helper 2 (Th2) type cytokines involved in growth and development of the conceptus. The aim of the present study was to examine the effect of Th1 cytokines (interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF alpha)) on the release of Th2 cytokines including IL-6 and IL-10 by trophoblast cells obtained from term placenta. Trophoblast cells isolated by enzymatic disaggregation and Percoll gradient fractionation were cultured in supplemented medium alone or with varying concentrations of the selected recombinant cytokines. After 48 h of incubation, samples of the culture supernatant were analyzed for the Th2 cytokines IL-6 and IL-10 using specific ELISA assays. Both IL-1 beta and TNF alpha had no effect on the cell number and viability as determined by MTT assay. IL-1 beta significantly stimulated trophoblast release of IL-6 in a dose-dependent manner (3.3-, 5.5-, 10.3- and 22.4-fold higher compared to the control at 10, 50, 100, 500 U IL-1 beta/ml respectively, p < 0.05). TNF alpha also stimulated release of IL-6 by these cells. However, the stimulation at lower concentrations was not very high and a significant (p < 0.05) stimulation was observed only at higher concentrations (1.1-, 1.3-, 2.6- and 5.9-fold higher at 500, 1000, 1500, 2000 U TNF alpha/ml respectively). In contrast, neither IL-1 beta or TNF alpha exerted any significant effect on IL-10 release by term trophoblast cells (p > 0.05). The results of this study provide evidence that production of Th2 cytokines might be under the control of different regulatory pathways.

Cells, Cultured↗
PubMed2000

Proerectile pharmacological effects of Tribulus terrestris extract on the rabbit corpus cavernosum.

INTRODUCTION: The objective of the present study was to investigate the effect of oral treatment of Tribulus terrestris (TT) extract on the isolated corpus cavernosal tissue of New Zealand white (NZW) rabbits and to determine the mechanism by which protodioscin (PTN), a constituent of the TT, exerts its pharmacological effects. MATERIALS AND METHODS: Twenty-four NZW rabbits were randomly assigned to 4 experimental groups of 6 each. Group I served as control. Groups II to IV were treated with the extract at different dose levels, i.e. 2.5 mg/kg, 5 mg/kg and 10 mg/kg body weight, respectively. The TT extract was administered orally, once daily, for a period of 8 weeks. The rabbits were then sacrificed and their penile tissue isolated to evaluate the responses to both contracting and relaxing pharmacological agents and electrical field stimulation (EFS). RESULTS: PTN on its own had no effect on the isolated corpus cavernosal strips. The relaxant responses to EFS, acetylcholine and nitroglycerin in noradrenaline precontracted tissues from treated groups showed an increase in relaxation of a concentration dependent nature compared to that of the tissues from control group. However, the contractile, anti-erectile response of corpus cavernosal tissue to noradrenaline and histamine showed no significant change between the treatment and the control groups. CONCLUSIONS: The relaxant responses to acetylcholine, nitroglycerin and EFS by more than 10%, 24% and 10% respectively compared to their control values and the lack of such effect on the contractile response to noradrenaline and histamine indicate that PTN has a proerectile activity. The enhanced relaxant effect observed is probably due to increase in the release of nitric oxide from the endothelium and nitrergic nerve endings, which may account for its claims as an aphrodisiac. However, further study is needed to clarify the precise mechanism of its action.

Animals↗
PubMed2000

Crystal structure and mutagenic analysis of the inhibitor-of-apoptosis protein survivin.

The coupling of apoptosis (programmed cell death) to the cell division cycle is essential for homeostasis and genomic integrity. Here, we report the crystal structure of survivin, an inhibitor of apoptosis, which has been implicated in both control of cell death and regulation of cell division. In addition to a conserved N-terminal Zn finger baculovirus IAP repeat, survivin forms a dimer through a symmetric interaction with an intermolecularly bound Zn atom located along the molecular dyad axis. The interaction of the dimer-related C-terminal alpha helices forms an extended surface of approximately 70 A in length. Mutagenesis analysis revealed that survivin dimerization and an extended negatively charged surface surrounding Asp-71 are required to counteract apoptosis and preserve ploidy. These findings may provide a structural basis for a dual role of survivin in inhibition of apoptosis and regulation of cell division.

Amino Acid Motifs↗
PubMed2000

Deep vein thrombosis after total knee replacement.

INTRODUCTION: The prevalence of deep vein thrombosis after total knee replacement has been quoted to be between 46% and 84% in the Western literature. The aims of this study were to determine its prevalence in the Singapore population and to assess the need for prophylaxis against deep vein thrombosis. MATERIALS AND METHODS: We examined data on 100 consecutive patients undergoing total knee replacement at the Adult Reconstructive Service, Department of Orthopaedic Surgery, Singapore General Hospital and assessed the possible risk factors: age, sex, weight, previous surgery, unilateral or bilateral surgery, postoperative rehabilitation, tourniquet and operating time. Functional and knee scores based on the Knee Society Clinical Rating System were also assessed. No prophylaxis was given to these patients. These patients underwent a duplex scan of both lower limbs on the seventh postoperative day. Treatment was instituted only if proximal deep vein thrombosis was detected. RESULTS: The overall incidence of deep vein thrombosis was 14% with 64.3% of it occurring distally. Deep vein thrombosis was more common in bilateral total knee replacement (22.2%) compared to unilateral total knee replacement (13.2%). Partial thrombosis was present in 71.4% and occurred predominantly in the ipsilateral leg. There was no evidence of propagation. Only 1 patient developed pulmonary embolism and was treated successfully but there was no evidence of deep vein thrombosis on duplex scan in this patient. CONCLUSION: There was no significant difference in the risk factors between patients who did and those who did not develop deep vein thrombosis.

Adult↗
PubMed2000

Use of low molecular weight heparin for prevention of deep vein thrombosis in total knee arthroplasty--a study of its efficacy in an Asian population.

INTRODUCTION: The aims of this paper were to study the incidence of deep vein thrombosis following total knee replacement in an Asian population and to evaluate the role of low molecular weight heparin for deep vein thrombosis in this setting. MATERIALS AND METHODS: We prospectively studied two groups of 100 consecutive patients undergoing total knee replacement separately. Group 1 did not receive any low molecular weight heparin and group 2 received low molecular weight heparin, nodraparin calcium (Fraxiparine) according to body weight. The sex distribution, age group, weight, preoperative knee and function scores, and postoperative rehabilitation were similar for both groups. A single ultrasound technician performed ultrasound duplex scan of both lower limbs on the seventh postoperative day. RESULTS: The incidence of deep vein thrombosis in group 1 was 14% (14 patients, 5 proximal vein thromboses and 9 distal vein thromboses) while in group 2, no patients developed deep vein thrombosis. There was no increased incidence, either local or systemic, of major bleeding complications with the use of low molecular weight heparin. CONCLUSION: While the incidence of deep vein thrombosis following total knee replacement in an Asian population appears lower compared to Western populations, the use of low molecular weight heparin for thromboprophylaxis appears to further reduce the incidence without major bleeding complications.

Activities of Daily Living↗
PubMed2000

Physiological significance of nitrergic transmission in human penile erection.

The corpora cavernosa (CC) muscles of the human penis and their structural arrangements are essential for the physiology of erection. Contraction of this muscle causes detumescence, and relaxation, tumescence. The motor excitatory neurotransmission is adrenergic, acting through the alpha adrenoceptors. Continuous adrenergic transmitter (noradrenaline) release is necessary for the maintenance of non-erectile (contractile) state of the penis. The inhibitory neurotransmitter that relaxes CC muscle to produce erection is nitrergic i.e., the chemical messenger being nitric oxide (NO). The latter can also be released from cavernous endothelium. Presence of NO increases intracellular cGMP through activation of the enzyme guanylate cyclase. This causes relaxation of CC muscle. Phosphodiesterase type 5 (PDE5) is responsible for the degradation of cGMP and regulation of CC muscle tone. Specific PDE inhibitors such as sildenafil enhance the intracellular cGMP to improve erection. Increase in intracellular cAMP can also bring about pharmacological erection in man (e.g. PGE1, papaverine and histamine). Inhibition of excessive adrenergic tone with appropriate alpha-adrenergic blocking agents (e.g. phentolamine) can also contribute to the onset of pharmacological erection.

Humans↗
PubMed1999

Discovery of a series of cyclohexylethylamine-containing protein farnesyltransferase inhibitors exhibiting potent cellular activity.

Synthesis of a library of secondary benzylic amines based on the Sebti-Hamilton type peptidomimetic farnesyltransferase (FTase) inhibitor FTI-276 (1) led to the identification of 6 as a potent enzyme inhibitor (IC(50) of 8 nM) which lacked the problematic thiol residue which had been a common theme in many of the more important FTase inhibitors reported to date. It has previously been disclosed that addition of o-tolyl substitution to FTase inhibitors of the general description 2 had a salutary effect on both FTase inhibition and inhibition of Ras prenylation in whole cells. Combination of these two observations led us to synthesize 7, a potent FTase inhibitor which displayed an IC(50) of 0.16 nM for in vitro inhibition of FTase and an EC(50) of 190 nM for inhibition of whole cell Ras prenylation. Modification of 7 by classical medicinal chemistry led to the discovery of a series of potent FTase inhibitors, culminating in the identification of 25 which exhibited an IC(50) of 0.20 nM and an EC(50) of 4.4 nM. In vivo tests in a nude mouse xenograft model of human pancreatic cancer (MiaPaCa cells) showed that oral dosing of 25 gave rise to impressive attenuation of the growth of this aggressive tumor cell line.

Alkyl and Aryl Transferases↗
PubMed1999

Fertilizing ability of DNA-damaged spermatozoa.

In order to investigate the fertilizing ability of DNA-damaged sperm, they were exposed to gamma radiation prior to insemination. The presence of DNA-strand breaks were detected by the TUNEL test. Fertilization rates of 64.3, 59.9, 58.5, and 61.1% were achieved when sperm were subjected to 5, 10, 50, and 100 GY, respectively. This rate was 53.2% in the control group with no significant difference (P > 0.01). The blastocyst development was decreased from 49.8% in the control group to 20.3, 7.8, 3.4, and 2.3% with sperm exposed to doses of 5, 10, 50, and 100 GY, respectively. Of the transferred blastocyst in the control group, 69.8% were implanted and 33.9% developed into live fetuses. These rates were 57.1 and 21. 4%, 20 and 0% when sperm were exposed to doses of 5 and 10 GY with a significant difference (P < 0.01). The present study clearly shows that DNA-damaged sperm (regardless of degree of damage) have the ability to fertilize the oocyte, but that embryonic development is very much related to the degree of DNA damage. However, the oocyte has the capacity to repair DNA damage of sperm when it is damaged less than 8%. Damage beyond this level will result in low rate of embryonic development and high early pregnancy loss. J. Exp. Zool. 284:696-704, 1999.

Animals↗
PubMed1999

NMR structure and mutagenesis of the inhibitor-of-apoptosis protein XIAP.

The inhibitor-of-apoptosis (IAP) family of proteins, originally identified in baculoviruses, regulate programmed cell death in a variety of organisms. IAPs inhibit specific enzymes (caspases) in the death cascade and contain one to three modules of a common 70-amino-acid motif called the BIR domain. Here we describe the nuclear magnetic resonance structure of a region encompassing the second BIR domain (BIR2) of a human IAP family member, XIAP (also called hILP or MIHA). The structure of the BIR domain consists of a three-stranded antiparallel beta-sheet and four alpha-helices and resembles a classical zinc finger. Unexpectedly, conserved amino acids within the linker region between the BIR1 and BIR2 domains were found to be critical for inhibiting caspase-3. The absence or presence of these residues may explain the differences in caspase inhibition observed for different truncated and full-length IAPs. Our data further indicate that these residues may bind to the active site and that the BIR domain may interact with an adjacent site on the enzyme.

Amino Acid Sequence↗
PubMed1999

Second-generation peptidomimetic inhibitors of protein farnesyltransferase demonstrating improved cellular potency and significant in vivo efficacy.

The synthesis and evaluation of analogues of previously reported farnesyltransferase inhibitors, pyridyl benzyl ether 3 and pyridylbenzylamine 4, are described. Substitution of 3 at the 5-position of the core aryl ring resulted in inhibitors of equal or less potency against the enzyme and decreased efficacy in a cellular assay against Ras processing by the enzyme. Substitution of 4 at the benzyl nitrogen yielded 26, which showed improved efficacy and potency and yet presented a poor pharmacokinetic profile. Further modification afforded 30, which demonstrated a dramatically improved pharmacokinetic profile. Compounds 26 and 29 demonstrated significant in vivo efficacy in nude mice inoculated with MiaPaCa-2, a human pancreatic tumor-derived cell line.

Alkyl and Aryl Transferases↗
PubMed1999

Reactive oxygen intermediates enhance Fc gamma receptor signaling and amplify phagocytic capacity.

Receptors for the Fc region of IgG (Fc gamma R) mediate internalization of opsonized particles by human neutrophils (PMN) and mononuclear phagocytes. Cross-linking of Fc gamma R leads to activation of protein tyrosine kinases and phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) within Fc gamma R subunits, both obligatory early signals for phagocytosis. Human PMN constitutively express two structurally distinct Fc gamma R, Fc gamma RIIa and Fc gamma RIIIb, and can be induced to express Fc gamma RI by IFN-gamma. We have previously shown that stimulation of PMN through Fc gamma RIIIb results in enhanced Fc gamma RIIa-mediated phagocytic activity that is inhibited by catalase. In the present study, we have tested the hypothesis that reactive oxygen intermediates (ROI) have the capacity to regulate Fc gamma R responses and defined a mechanism for this effect. We show that H2O2 augmented phagocytosis mediated by Fc gamma RIIa and Fc gamma RI in PMN and amplified receptor-triggered tyrosine phosphorylation of Fc gamma R-associated ITAMs and signaling elements. Generation of endogenous oxidants in PMN by cross-linking Fc gamma RIIIb similarly enhanced phosphorylation of Fc gamma RIIa and Syk, a tyrosine kinase required for phagocytic function, in a catalase-sensitive manner. Our results provide a mechanism for priming phagocytes for enhanced responses to receptor-driven effects. ROI generated in an inflammatory milieu may stimulate quiescent cells to rapidly increase the magnitude of their effector function. Indeed, human monocytes incubated in the presence of stimulated PMN showed oxidant-induced increases in Fc gamma RIIa-mediated phagocytosis. Definition of the role of oxidants as amplifiers of Fc gamma R signaling identifies a target for therapeutic intervention in immune complex-mediated tissue injury.

Adjuvants, Immunologic↗
PubMed1999

Potent and orally bioavailable noncysteine-containing inhibitors of protein farnesyltransferase.

Potent and orally bioavailable nonthiol-containing inhibitors of protein farnesyltransferase are described. Oral bioavailability was achieved by replacement of the pyridyl ether moiety of 1 with a 2-substituted furan ether to give 4. Potency was regained with 2,5-disubstituted furan ethers while maintaining the bioavailability inherent in 4. p-Chlorophenylfuran ether 24 is 0.7 nM in vitro (FTase) and is 32% bioavailable in the mouse, 30% bioavailable in rats, and 21% bioavailable in dogs.

Alkyl and Aryl Transferases↗
PubMed1999

Potent inhibitors of protein farnesyltransferase: heteroarenes as cysteine replacements.

Synthesis and biological evaluation of heteroarenes as reduced cysteine replacements are described. Of the heteroaryl groups examined with respect to FT inhibitor FTI-276 (1), pyridyl was the replacement found to be most effective. Substitutions at C4 of the pyridyl moiety did not affect the in vitro activity. Compound 9a was found to have moderate in vivo bioavailability.

3T3 Cells↗
PubMed1999

Ovarian response after laparoscopic ovarian cystectomy for endometriotic cysts in 132 monitored cycles.

OBJECTIVE: To investigate the follicular response of ovaries after laparoscopic ovarian cystectomy for endometriotic cysts. DESIGN: A retrospective, controlled study. SETTING: Obstetrics and Gynaecology Department of a university hospital. PATIENT(S): Patients with history of infertility who had laparoscopic ovarian cystectomy for endometriotic cysts. INTERVENTION(S): Laparoscopic ovarian cystectomy, transvaginal ultrasound monitoring of follicles in natural cycles, stimulated cycles using clomiphene citrate (CC), chronic low-dose FSH, and "flare down" regimen. MAIN OUTCOME MEASURE(S): Follicular response of postcystectomy and normal ovaries. RESULT(S): For women < 35 years of age, the mean follicular response of postcystectomy ovaries was reduced significantly when compared with normal ovaries in natural cycles (0.30 versus 1.00 follicle per cycle) and in CC-stimulated cycles (0.87 versus 1.27 follicles per cycle). The mean follicular response was not statistically significantly different when the ovaries were stimulated with chronic low-dose FSH or with the flare down regimen. For women > or = 35 years, postcystectomy ovaries responded with a comparable number of follicles as the normal ovaries in natural cycles and the three different ovarian stimulation regimens. CONCLUSION(S): Postcystectomy ovaries showed reduced follicular response in natural and CC-stimulated cycles for women < 35 years of age. Postcystectomy ovaries produced a comparable number of follicles as normal ovaries when stimulated with gonadotropins.

Adult↗
PubMed1999

Effect of novel CAAX peptidomimetic farnesyltransferase inhibitor on angiogenesis in vitro and in vivo.

Ras oncogenes can contribute to tumour development by stimulating vascular endothelial growth factor (VEGF)-dependent angiogenesis. The effect of Ras on angiogenesis may be affected by farnesyltransferase inhibitors (FTI) since farnesylation of Ras is required for its biological activity. In this paper we evaluated the effect of A-170634, a novel and potent CAAX FTI on angiogenesis. Human umbilical vein endothelial cell (HUVEC) tube formation and VEGF secretion were used to assess the effect of A-170634 on angiogenesis in vitro. In vivo, nude mice were injected with the K-ras mutant colon carcinoma cell line HCT116 and treated subcutaneously with A-170634 using osmotic minipump infusion for 10 days. The effect of A-170634 on corneal angiogenesis in vivo was assessed using pellets containing hydron, VEGF, A-170634 or vehicle. In vitro, A-170634 selectively inhibited farnesyltransferase activity over the closely related geranylgeranyltransferase I, inhibited Ras processing, blocked anchorage-dependent and -independent growth of HCT116 K-ras mutated cells, decreased HUVEC capillary structure formation, decreased VEGF secretion from tumour cells and HUVEC growth stimulating activity in a dose-dependent manner. In vivo, tumour growth was decreased by 30% and vascularisation in and around the tumours was reduced by 41% following drug-treatment with no apparent toxicity to the animals. VEGF-induced corneal neovascularisation was reduced by 80% following A-170634 treatment for 7 days. The data presented here demonstrated that A-170634 was a potent and selective peptidomimetic CAAX FTI with anti-angiogenic properties. These results implied that A-170634 may affect tumour growth in vivo by one or more antitumour pathways.

Alkyl and Aryl Transferases↗