Search PubMed⌕ Search

Biomedical subjects

C A Mitchell

Publications and source records attributed to C A Mitchell.

At least 55 records · Page 3Linked to original sources
PubMed1999

Underexpression of the 43 kDa inositol polyphosphate 5-phosphatase is associated with spontaneous calcium oscillations and enhanced calcium responses following endothelin-1 stimulation.

The 43 kDa inositol polyphosphate 5-phosphatase (5-phosphatase) hydrolyses the signalling molecules inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) and inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4, 5)P4) and thereby regulates cellular transformation. To investigate the role Ins(1,4,5)P3-mediated Ca2+ oscillations play in cellular transformation, we studied Ins(1,4,5)P3-mediated Ca2+ responses in cells underexpressing the 43 kDa 5-phosphatase. Chronic reduction in 43 kDa 5-phosphatase enzyme activity resulted in a 2.6-fold increase in the resting Ins(1,4,5)P3 concentration and a 4.1-fold increase in basal intracellular Ca2+. The increased Ins(1,4,5)P3 levels resulted in partial emptying (40%) of the Ins(1,4,5)P3-sensitive Ca2+ store, however, store-operated Ca2+ influx remained unchanged. In addition, Ins(1,4,5)P3 receptors were chronically down-regulated in unstimulated cells, as shown by a 53% reduction in [3H]Ins(1,4,5)P3 binding to microsomal receptor sites. Agonist stimulation with endothelin-1 resulted in the rapid rise and fall of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 levels, with no significant differences in the rates of hydrolysis of these second messengers in antisense- or vector-transfected cells. These studies indicate, in contrast to its predicted action, the 43 kDa 5-phosphatase does not metabolise Ins(1, 4,5)P3 and Ins(1,3,4,5)P4 post agonist stimulation. Cells with decreased 43 kDa 5-phosphatase activity exhibited spontaneous Ca2+ oscillations in the absence of any agonist stimulation, and increased sensitivity and amplitude of intracellular Ca2+ responses to both high and low dose endothelin-1 stimulation. We conclude the 43 kDa 5-phosphatase exerts a profound influence on Ins(1,4, 5)P3-induced Ca2+ spiking, both in the unstimulated cell and following agonist stimulation. We propose the enhanced Ca2+ oscillations may mediate cellular transformation in cells underexpressing the 43 kDa 5-phosphatase.

Animals↗
PubMed1999

Transfer from long to short photoperiods affects production efficiency of day-neutral rice.

The day-neutral, semidwarf rice (Oryza sativa L.) cultivar Ai-Nan-Tsao was grown in a greenhouse under summer conditions using high-pressure sodium lamps to extend the natural photoperiod. After allowing 2 weeks for germination, stand establishment, and thinning to a consistent planting density of 212 plants/m2, stands were maintained under continuous lighting for 35 or 49 days before shifting to 8- or 12-h photoperiods until harvest 76 days after planting. Non-shifted control treatments consisting of 8-, 12-, or 24-h photoperiods also were maintained throughout production. Tiller number increased as duration of exposure to continuous light increased before shifting to shorter photoperiods. However, shoot harvest index and yield efficiency rate were lower for all plants receiving continuous light than for those under the 8- or 12-h photoperiods. Stands receiving 12-h photoperiods throughout production had the highest grain yield per plant and equaled the 8-h-photoperiod control plants for the lowest tiller number per plant. As long as stands were exposed to continuous light, tiller formation continued. Shifting to shorter photoperiods late in the cropping cycle resulted in newly formed tillers that were either sterile or unable to mature grain before harvest. Late-forming tillers also suppressed yield of grain in early-forming tillers, presumably by competing for photosynthate or for remobilized assimilate during senescence. Stands receiving 12-h photoperiods throughout production not only produced the highest grain yield at harvest but had the highest shoot harvest index, which is important for resource-recovery strategies in advanced life-support systems proposed for space.

Biomass↗
PubMed1998

Regulation of endothelial monocyte-activating polypeptide II release by apoptosis.

Endothelial monocyte-activating polypeptide II (EMAP II) is a proinflammatory cytokine and a chemoattractant for monocytes. We show here that, in the mouse embryo, EMAP II mRNA was most abundant at sites of tissue remodeling where many apoptotic cells could be detected by terminal deoxynucleotidyltransferase-mediated dUTP end labeling. Removal of dead cells is known to require macrophages, and these were found to colocalize with areas of EMAP II mRNA expression and programmed cell death. In cultured cells, post-translational processing of pro-EMAP II protein to the mature released EMAP II form (23 kDa) occurred coincidentally with apoptosis. Cleavage of pro-EMAP II could be abrogated in cultured cells by using a peptide-based inhibitor, which competes with the ASTD cleavage site of pro-EMAP II. Our results suggest that the coordinate program of cell death includes activation of a caspase-like activity that initiates the processing of a cytokine responsible for macrophage attraction to the sites of apoptosis.

Animals↗
PubMed1998

Increased levels of phosphatidylinositol 3-kinase activity in colorectal tumors.

BACKGROUND: Phosphatidylinositol 3-kinase (PI 3-kinase), an enzyme that phosphorylates inositol phospholipids at the D-3 position of the inositol ring, has been implicated in the signaling pathways regulating cell growth by virtue of its activation in response to various mitogenic stimuli. In spite of the considerable attention PI 3-kinase has received with regard to its possible role in the mitogenic pathways in hematopoietic malignancies, there are few reports of investigations into PI 3-kinase activity in solid tumors. METHODS: Colorectal tumor tissue and normal-appearing colonic mucosa from the same patients were homogenized and solubilized and adjusted to equal protein levels. PI 3-kinase then was immunoprecipitated from 200 microg of the solubilized tissue using a polyclonal antibody to the p85 subunit of PI 3-kinase. PI 3-kinase activity was assessed using phosphatidylinositol as the substrate and the assay product analyzed by thin-layer chromatography. Phosphorylation of phosphatidylinositol in the D-3 position was confirmed by high performance liquid chromatography analysis of deacylated and deglycerated products. RESULTS: Thirty-two of the 37 tumors tested (86%) demonstrated increased PI 3-kinase activity compared with normal-appearing mucosa from the same patients (overall mean increase+/-standard error of the mean=3.8+/-0.6-fold; P < 0.05, Student's t test for paired data). The frequency and extent of increased PI 3-kinase enzyme activity in tumors did not correlate with clinical parameters or the presence of oncogenic ras mutations. CONCLUSIONS: In this study colorectal tumors exhibited enhanced PI 3-kinase activity compared with normal colonic mucosa, raising the possibility that PI 3-kinase may be a potential target for new strategies for the treatment of colorectal carcinoma.

Colorectal Neoplasms↗
PubMed1998

Distinct membrane and cytosolic forms of inositol polyphosphate 5-phosphatase II. Efficient membrane localization requires two discrete domains.

The 75-kDa inositol polyphosphate 5-phosphatase (5-phosphatase II) hydrolyzes various signaling molecules including the following: inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate, phosphatidylinositol 4,5-bisphosphate, and phosphatidylinositol 3,4, 5-trisphosphate. Although studied extensively, a demonstrably full-length cDNA encoding 5-phosphatase II has yet to be isolated. In this study we used a human partial 2.3-kilobase pair (kb) cDNA to screen mouse brain and kidney cDNA libraries, resulting in the isolation of a 3.7-kb cDNA (M5), which by multiple criteria represents a full-length cDNA encoding a 115-kDa 5-phosphatase II. We also isolated a smaller cDNA (M22) with a unique N terminus that encodes a 104-kDa polypeptide. Analysis of these cDNAs suggests a further 87-kDa isoform may arise from differential splicing resulting in translation at methionine 234 in M5. RNA analysis of tissues demonstrates expression of two mRNA species of approximately 4.0 or 3.0 kb, respectively. Probes unique to the 5' end of M5 or M22 hybridized to the 4.0- or 3.0-kb transcripts, respectively. RNA analysis using probes derived from sequence 3' to the potential splice site in M5 and M22 hybridized to both transcripts. Expression of the recombinant 115-kDa protein, or a smaller recombinant protein lacking the N terminus transiently in COS-7 cells, showed localization of enzyme activity to the membrane. Removal of the C-terminal CAAX motif resulted in a significant translocation of the protein lacking the N terminus but not the 115-kDa 5-phosphatase to the cytosol. Western blot analysis of membrane and cytosolic fractions of multiple mouse tissues confirmed the 115-kDa 5-phosphatase II was located in the membrane, whereas the 104- and 87-kDa isoforms were prominent in the cytosol. Collectively these studies demonstrate the widespread expression of at least three isoforms of 5-phosphatase II derived from RNA splicing events. This allows differential distribution of the 5-phosphatase II activity between the membrane and cytosol of the cell and thereby may regulate enzyme access to phosphoinositide-derived signaling molecules.

Amino Acid Sequence↗
PubMed1998

Regression of vessels in the tunica vasculosa lentis is initiated by coordinated endothelial apoptosis: a role for vascular endothelial growth factor as a survival factor for endothelium.

The development of the embryonic lens is dependent on the formation and regression of the tunica vasculosa lentis (TVL), which is a transiently occurring capillary plexus that surrounds the posterior part of the lens. In this study, by using the terminal deoxy-nucleotidyl transferase mediated nick end-labelling technique (TUNEL), electron microscopy, radioactive end-labelling of DNA extracted from TVL, and the Comet assay, we show that widespread apoptosis of the endothelial cells that constitute the TVL is occurring already at embryonic day 17.5 (E17.5) of mouse development, much earlier than was reported previously (Jack [1972a] Am. J. Ophthalmol. 74:261-272; Lang [1997] Cell Death Diff. 4:12-20). In addition to apoptotic cell death, regression of this structure is associated with loss of capillary integrity, leakage of erythrocytes into the vitreal compartment, and phagocytosis of the apoptotic endothelium by tissue macrophages (hyalocytes). In situ hybridization experiments with probes for the flk-1 receptor and its high-affinity ligand, vascular endothelial growth factor (VEGF; Terman et al. [1992] Biochem. Biophys. Res. Commun. 187:1579-1586; Millauer et al. [1993] Cell 72:835-846), revealed strong endothelial cell expression for flk-1 in the eyes of E13.5-E17.5 embryos. VEGF mRNA was detected in lens epithelial cells located at the posterior pole of the developing lens in E13.5 embryos, in close proximity to the TVL capillaries. At later times (E14.5-E17.5), when the lens epithelial cells have differentiated into primary lens fiber cells, and a thick lenticular capsule is formed, the expression of VEGF mRNA becomes restricted to the anterior and equatorial portions of the lens. The physical separation of the VEGF-producing cells from the flk-1-expressing endothelium (due to the differentiation of the lens epithelial cells into lens fiber cells and the formation of the lenticular capsule) may deprive the endothelium of an essential survival factor and, thus, may constitute the primary mechanism that is responsible for the induction of endothelial cell apoptosis in this model.

Animals↗
PubMed1998

Capsulated versus hand-mixed glass-ionomer luting cements for post retention.

OBJECTIVES: Glass-ionomer luting cements are supplied in two forms, as loose powder and liquid to be hand-mixed (HM) or pre-proportioned in a capsule to be mechanically mixed (MM). This study was to determine if post retention in pull-out tests was affected by the method of mixing the cement. METHODS: Two hundred stainless steel posts of diameter 1.75 mm were cemented within post-channels prepared in stainless steel cylinders using two hand-mixed cements FJL and KCL (Fuji I Luting Cement and Ketac-Cem Luting Cement) and two capsulated cements FJC and KCM (Fuji Cap I and Ketac-Cem Maxicap). Three groups of test specimens were prepared. In Group I each cement was mixed as recommended by the manufacturer, Group II cements were placed within a capsule and mechanically mixed and in Group III cements were removed from the capsule and mixed by hand. Specimens were stored for 1 h at 37 degrees C and 100% humidity prior to post pull-out tests at a crosshead speed of 10 mm min-1. The maximum loads at failure were subjected to Weibull analysis and Mann-Whitney tests to determine probabilities of survival and significant differences between the groups. RESULTS: Significant differences (P < 0.05) were found between all pairs of cements tested except KCM(MM) vs FJL(HM), FJL(MM) vs KCL(HM), KCL(MM), vs KCL(HM), FJC(HM) vs FJC(MM), KCL(MM) vs FJL(MM). CONCLUSIONS: The capsulated cements as supplied by the manufacturers are preferable to the equivalent hand-mixed formulations, as they give higher probabilities of survival when subjected to a given load. Both capsulated and hand-mixed formulations of Fuji had higher probabilities of survival compared to the corresponding Ketac cements. The probability of post survival can be altered by the method of mixing the cement.

Capsules↗
PubMed1998

Comparison of conventional and resin-modified glass-ionomer luting cements in the retention of post-crowns by fatigue loading.

Fatigue testing may be used for in vitro evaluation of luting cements, allowing comparison of materials under controlled conditions. It is recognized that glass-ionomer cements are materials which are susceptible to microcracking, even during curing prior to load application, and their failure can be related to crack propagation. The aim of this study was to compare the retention of post-crowns cemented with conventional and resin-modified glass-ionomer cements, the latter having significantly greater fracture toughness, under cyclic loads which are representative of physiological service. A custom-designed fatigue machine was used for three tests each comprising 18 specimens in a modified randomized complete block programme. Correlation of load amplitude to endurance was low, as expected from static test experience, but cement comparison was made through Kaplan-Meier survival and cumulative hazard functions. Differences in the performance of the cements were indicated, but were not statistically significant in this study, although analysis by Cox's proportional hazards model indicated that significance may be gained by a larger study.

Acrylic Resins↗
PubMed1998

Bonding to molars--the effect of etch time (an in vitro study).

This study evaluated the effect of etch time on the shear bond strength obtained when bonding to the buccal enamel of first molar teeth. Recently extracted first molar teeth were etched with 37 per cent phosphoric acid gel for 15, 30 and 60 seconds. Preformed cylinders of Concise composite resin were then bonded to the buccal surfaces of the molar teeth. After storage in water for 24 hours at 37 degrees C, the specimens were debonded in a direction parallel to the buccal surface. Examination of the shear bond strengths showed significant differences in shear bond strength between 15 and 30 seconds (P < 0.01) and between 15 and 60 seconds (P < 0.001). Weibull analysis revealed that the probability of bond survival increased as etch time increased, however, the difference between 30 and 60 seconds was not statistically significant. The results indicate that, despite current recommendations of a 15-second etch for premolars, canines and anterior teeth, an etching time of at least 30 seconds should be used when bonding to the buccal surfaces of first molars. A further increase in etching time to 60 seconds produces no significant increase in bond strength.

Acid Etching, Dental↗
PubMed1998

Genetic and environmental risk factors for asthma: a cotwin-control study.

In complex diseases of genetic etiology such as asthma and atopy, it is difficult to differentiate causes of disease from consequences, and quantitate the importance of such causative factors. We examined possible risk factors for the development of wheezing and bronchial hyperresponsiveness in a cotwin-control study nested within a larger community-based twin-family study. In 62 monozygotic (MZ) twin pairs discordant for a history of wheezing, skin prick test to house dust extract was the most important discriminator, followed by sensitization to cat and cockroach allergens. In contrast, 62 dizygotic (DZ) discordant twin pairs differed additionally in sensitization to grass pollens and fungi. Markers such as serum haptoglobin, serum magnesium, and alpha-1-antitrypsin levels did not differ significantly between discordant twins. This MZ/DZ difference suggests that pollen allergy in asthmatics is more an epiphenomenon due to a genetic correlation between asthma and the allergic diathesis, whereas indoor allergens are likely to be direct environmental causes of asthma.

Adult↗
PubMed1998

Intracanopy lighting of cowpea canopies in controlled environments.

Traditional designs for plant-growth lighting in space life support systems irradiate tops of closed foliar canopies while canopy understories are light limited. "Intracanopy lighting," a technique whereby plants are allowed to grow up and around multiple layers of low-intensity lamps that irradiate interior portions of canopies, can potentially enhance productivity while reducing overall energy consumption. Intracanopy lighting of cowpea (Vigna unguiculata L. Walp) was optimized by varying stand densities and lining growth compartments with light-scattering or reflective films. Yield rates using intracanopy lighting were less than those obtained with traditional lighting strategies. However, yield efficiencies and energetic conversion efficiencies, parameters that put edible yield in terms of inedible biomass, energetic, spatial, and temporal penalties, indicate intracanopy lighting is more efficient in crop production. Single-leaf photosynthetic rates indicate all leaves participate in net carbon gain regardless of age and position within a canopy.

Biomass↗
PubMed1998

Predicting lettuce canopy photosynthesis with statistical and neural network models.

An artificial neural network (NN) and a statistical regression model were developed to predict canopy photosynthetic rates (Pn) for 'Waldman's Green' leaf lettuce (Latuca sativa L.). All data used to develop and test the models were collected for crop stands grown hydroponically and under controlled-environment conditions. In the NN and regression models, canopy Pn was predicted as a function of three independent variables: shootzone CO2 concentration (600 to 1500 micromoles mol-1), photosynthetic photon flux (PPF) (600 to 1100 micromoles m-2 s-1), and canopy age (10 to 20 days after planting). The models were used to determine the combinations of CO2 and PPF setpoints required each day to maintain maximum canopy Pn. The statistical model (a third-order polynomial) predicted Pn more accurately than the simple NN (a three-layer, fully connected net). Over an 11-day validation period, average percent difference between predicted and actual Pn was 12.3% and 24.6% for the statistical and NN models, respectively. Both models lost considerable accuracy when used to determine relatively long-range Pn predictions (> or = 6 days into the future).

Carbon Dioxide↗
PubMed1998

Optimizing the calcium content of a copolymer acrylamide gel matrix for dark-grown seedlings.

A copolymer acrylamide acrylate gel was investigated as the sole root matrix for dark-grown seedlings of soybean (Glycine max Merr. 'Century 84'). Increasing Ca2+ in the hydrating solution of the hydrogel from 1 to 10 mM decreased its water-holding capacity from 97 to 46 mL g-1, yet water potential of the medium remained high, sufficient for normal plant growth at all Ca2+ concentrations tested. Elongation rate of dark-grown soybean seedlings over a 54-hour period was 0.9, 1.5, and 1.8 mm h-1 with 1.0, 2.5, or 5.0 mM Ca2+, respectively, but did not increase with further increases in Ca2+ concentration. Further study revealed that Na+ was released from the hydrogel medium and was taken up by the seedlings as Ca2+ increased in the medium. In dry hypocotyl tissue, sodium content correlated negatively with calcium content. Despite the presence of Na+ in the hydrogel, seedling growth was normal when adequate Ca2+ was added in the hydrating solution. Acrylamide hydrogels hold good potential as a sole growth matrix for short-term experiments with dark-grown seedlings without irrigation.

Acrylamides↗
PubMed1997

Activation of the 43 kDa inositol polyphosphate 5-phosphatase by 14-3-3zeta.

The 43 kDa inositol polyphosphate 5-phosphatase (5-phosphatase) hydrolyzes and thereby inactivates the second messenger molecules inositol 1,4,5-trisphosphate -Ins(1,4,5)P3- and inositol 1,3,4,5-tetrakisphosphate in a signal terminating reaction. Recent studies have shown that the platelet protein pleckstrin forms a complex with the 43 kDa 5-phosphatase and activates Ins(1,4,5)P3 hydrolysis 2-fold [Auethavekiat, V., Abrams, C. S., & Majerus, P. W. (1997) J. Biol. Chem. 272, 1786-1790]. We now show that another platelet protein, 14-3-3zeta, forms a complex with the 43 kDa 5-phosphatase and thereby activates the hydrolysis of Ins(1,4,5)P3. Both pleckstrin and 14-3-3zeta contain one or more pleckstrin-homology domains, both are present in platelet cytosol, and both dimerize and form complexes with other signalling proteins. Purified platelet pleckstrin and 14-3-3zeta enhanced the rate of the hydrolysis of Ins(1,4,5)P3 by the 43 kDa 5-phosphatase 1.9- and 3.8-fold, respectively, but did not activate the 75 kDa 5-phosphatase. We have demonstrated that the mechanism of 5-phosphatase activation by 14-3-3zeta results from specific complex formation between the 43 kDa 5-phosphatase and 14-3-3zeta. Recombinant 43 kDa 5-phosphatase bound to recombinant glutathione S-transferase (GST)/14-3-3zeta fusion protein, but not GST alone, immobilized on glutathione-Sepharose. A potential 14-3-3 binding motif was located in the 43 kDa, but not the 75 kDa, 5-phosphatase. The motif "363RSESEE" is present in close proximity to the proposed catalytic domain of the 43 kDa 5-phosphatase. A synthetic peptide corresponding to the putative 14-3-3 binding motif demonstrated specific, saturable binding to purified 125I-14-3-3, with a Kd of 92 nM. In addition, platelet cytosolic 5-phosphatase bound to recombinant 14-3-3zeta immobilized on glutathione-Sepharose. Thus, 14-3-3zeta serves in human platelets to activate the 43 kDa 5-phosphatase and may thereby function to prevent generation of Ins(1,4,5)P3 -mediated calcium release in unstimulated platelets.

14-3-3 Proteins↗
PubMed1997

Comparison of the porosity of hand-mixed and capsulated glass-ionomer luting cements.

The strength of dental glass-ionomer cements will be influenced by defects present within its structure. This study measured the surface area porosity, percentage surface area porosity, and mean surface area of small bubbles (<0.01 mm2) and the surface area porosity, percentage surface area porosity and diameter of large bubbles within 40-microm-thick layers of four cements, using image analysis software. Two hand-mixed cements (Fuji I and KetacCem) and two capsulated cements (Fuji Cap I and KetacCem Maxicap) were viewed under transmitted light at x117.6 magnification. For each selected area (64.75 mm2) of each cement sample, five independent measurements were made of each of these parameters. Analysis of variance (ANOVA) indicated that there were no significant differences between the four cements in the small bubble parameters measured, whilst there were significant differences in the surface area porosity, percentage surface area porosity and diameter of the large bubbles. It was concluded that the hand-mixed cements tested had a greater number of larger diameter bubbles compared with the capsulated cements.

Analysis of Variance↗