N-linked glycoprotein synthesis and transport during G1 are necessary for astrocytic proliferation

Effect of decorin and selected glycosylation inhibitors on the adhesion of caruncular epithelial cells of pregnant cows-part I

Adhesion process ensures the formation of the appropriate connection between mother and foetus during placentation and further placental development, which determines physiological pregnancy course. Extracellular matrix of foetal membranes are a rich source of biologically active proteins, the synthesis of which is regulated by hormones. Depending on the stage of pregnancy, the protein profile of the placenta changes, thanks to which its remodelling is possible. The aim of the study was to evaluate the effect of decorin, as well as selected glycosylation inhibitors on the adhesion of caruncular epithelial cells derived from cows during pregnancy. Placental cells were isolated from healthy, pregnant (2nd and 4th month) cows after slaughter, which allowed for the establishment of 4 primary cell cultures without visible cells of fibroblast morphology. The presence of decorin in cell monolayer and cell lysates was determined by the use of immunocytochemistry and Western blotting, respectively. The viability of cells was evaluated by MTT assay. The adhesion of cells to fibronectin was measured spectrophotometrically. Protein N-glycosylation and O-glycosylation have a modulating effect on the adhesion and viability of placental cells during early-mid pregnancy. Decorin and tunicamycin were shown to have anti-adhesive properties with respect to caruncular cells of the pregnant bovine uterus.

Noninvasive imaging of sialyltransferase activity in living cells by chemoselective recognition

To elucidate the biological and pathological functions of sialyltransferases (STs), intracellular ST activity evaluation is necessary. Focusing on the lack of noninvasive methods for obtaining the dynamic activity information, this work designs a sensing platform for in situ FRET imaging of intracellular ST activity and tracing of sialylation process. The system uses tetramethylrhodamine isothiocyanate labeled asialofetuin (TRITC-AF) as a ST substrate and fluorescein isothiocyanate labeled 3-aminophenylboronic acid (FITC-APBA) as the chemoselective recognition probe of sialylation product, both of which are encapsulated in a liposome vesicle for cellular delivery. The recognition of FITC-APBA to sialylated TRITC-AF leads to the FRET signal that is analyzed by FRET efficiency images. This strategy has been used to evaluate the correlation of ST activity with malignancy and cell surface sialylation, and the sialylation inhibition activity of inhibitors. This work provides a powerful noninvasive tool for glycan biosynthesis mechanism research, cancer diagnostics and drug development.

Cellobiase from Termitomyces clypeatus: activity and secretion in presence of glycosylation inhibitors

In presence of the glycosylation inhibitors, 2-deoxy-D-glucose (1 mg/ml), tunicamycin (30 microg/ml), 1-deoxynojirimycin (30 microg/ml) and D-glucono-delta-lactone (1 mg/ml), total cellobiase activity, in the extracellular, intracellular and cell bound fractions, of the fungus Termitomyces clypeatus grown in 20 ml cellobiose medium (1%, w/v) increased by 50-, 1.8-, 2.4-, 1.3-fold, respectively, with respect to control medium (16.3 U). The inhibitors also stimulated secretion of 95% of the total protein in culture medium, except D-glucono-delta-lactone which released 60% of the total protein. 2-Deoxy-D-glucose (1 mg/ml) led to production of extracellular cellobiase up to 40 U/ml, whereas in absence of the inhibitors only 0.59 U/ml enzyme was detected.

Unlike 2-deoxy-D-glucose, 3-O-methyl-D-glucose does not induce long-term potentiation in rat hippocampal slices

Equimolar replacement of 10 mM glucose by 2-deoxy-D-glucose (2-DG) causes substantial depression followed by a sharp and sustained potentiation of CA1 field EPSPs. In the present experiments, similar applications of 3-O-methyl-D-glucose, which is also taken up by cells but is not phosphorylated, had only a weak blocking action and elicited no potentiation. Possible explanations for the marked effects of 2-DG include a more rapid block of glycolysis and the production of phosphorylated derivatives of 2-DG.

2-deoxyglucose induces LTP in layer I of rat somatosensory cortex in vitro

Temporary replacement of glucose by 2-deoxyglucose (2-DG) induces a long-term potentiation (2-DG-LTP) of excitatory synaptic transmission in hippocampal slices. We therefore examined the effects of 2-DG on monosynaptic field excitatory postsynaptic potentials (fEPSPs) in slices of somatosensory cortex from rats. Monosynaptic fEPSPs were elicited in layer I by stimulating horizontal projections in the same layer. Replacement of glucose (10 mM) in the artificial cerebrospinal fluid with 10 mM 2-DG for 15-17 min produced a minor reduction (by 10-30%), followed by a sustained increase (by approximately 150%) in synaptic responses that could last for over 2 hours. Equimolar replacement of glucose with sucrose did not induce potentiation. The addition of 5 or even 2.5 mM glucose to 10 mM 2-DG largely suppressed the effects of 2-DG; but topically-applied GABA antagonists bicuculline and CGP 35348 did not prevent 2-DG-LTP. Unlike hippocampal 2-DG-LTP, neocortical 2-DG-LTP was: (1) not sensitive to 2-amino-5-phosphonopentanoic acid (AP5); and (2) usually not depotentiated by stimulation at 1 Hz. We conclude that 2-DG produces a robust and sustained increase in synaptic transmission in the neocortex through mechanisms that are independent of NMDA receptor activation.

Cell cycle-dependent regulation of N-acetylglucosaminyltransferase-III in a human colon cancer cell line, Colo201

The mechanism for cell-cycle-dependent regulation of N-acetylglucosaminyltransferase III (GnT-III) activity was investigated using synchronized culture of Colo201, a human colon cancer cell line. In the synchronized culture, it was found that GnT-III activity rapidly increased in the M phase and the maximal activity was five times higher than the basal level found in the G1 phase. Northern blot and Western blot analyses revealed that the increase in the activity is due not to an increase in expression level of its mRNA but, rather, to the level of protein. Furthermore, it was shown by a pulse-chase experiment that the increased protein level of GnT-III is the result of its prolonged turnover rate. Lectin blotting with erythroagglutinating phytohemagglutinin showed that the content of bisecting N-acetylglucosamine structure in glycoproteins was transiently increased during the M phase in conjunction with the increased activity of GnT-III. These results suggest that GnT-III activity undergoes a cell-cycle-dependent regulation and thereby oligosaccharide structures of N-glycans vary specifically during the M phase of the cell cycle. Thus, it is possible that the cell-cycle-dependent alteration of N-glycans by GnT-III might play a role in biological events, such as the progression of cell cycle and cell division.

Protein N-glycosylation: molecular genetics and functional significance

Protein N-glycosylation is a metabolic process that has been highly conserved in evolution. In all eukaryotes, N-glycosylation is obligatory for viability. It functions by modifying appropriate asparagine residues of proteins with oligosaccharide structures, thus influencing their properties and bioactivities. N-glycoprotein biosynthesis involves a multitude of enzymes, glycosyltransferases, and glycosidases, encoded by distinct genes. The majority of these enzymes are transmembrane proteins that function in the endoplasmic reticulum and Golgi apparatus in an ordered and well-orchestrated manner. The complexity of N-glycosylation is augmented by the fact that different asparagine residues within the same polypeptide may be modified with different oligosaccharide structures, and various proteins are distinguished from one another by the characteristics of their carbohydrate moieties. Furthermore, biological consequences of derivatization of proteins with N-glycans range from subtle to significant. In the past, all these features of N-glycosylation have posed a formidable challenge to an elucidation of the physiological role for this modification. Recent advances in molecular genetics, combined with the availability of diverse in vivo experimental systems ranging from yeast to transgenic mice, have expedited the identification, isolation, and characterization of N-glycosylation genes. As a result, rather unexpected information regarding relationships between N-glycosylation and other cellular functions--including secretion, cytoskeletal organization, proliferation, and apoptosis--has emerged. Concurrently, increased understanding of molecular details of N-glycosylation has facilitated the alignment between N-glycosylation deficiencies and human diseases, and has highlighted the possibility of using N-glycan expression on cells as potential determinants of disease and its progression. Recent studies suggest correlations between N-glycosylation capacities of cells and drug sensitivities, as well as susceptibility to infection. Therefore, knowledge of the regulatory features of N-glycosylation may prove useful in the design of novel therapeutics. While facing the demanding task of defining properties, functions, and regulation of the numerous, as yet uncharacterized, N-glycosylation genes, glycobiologists of the 21st century offer exciting possibilities for new approaches to disease diagnosis, prevention, and cure.

Valproic acid suppresses G1 phase-dependent sialylation of a 65kDa glycoprotein in the C6 glioma cell cycle

The influence of valproate on in vitro glycosylation events in C6 glioma has been investigated, as this major human teratogen restricts proliferation in the mid-G1 phase of the cycle and alters the prevalence and/or glycosylation state of cell surface glycoproteins with the potential to mediate cell-cell and cell matrix interactions critical to development. C6 glioma cultured continuously in the presence of 1 mM valproate exhibited a significant depression of exponential growth but attained confluency one day later, when the majority of cells entered the G1 phase of the cycle. Glycoprotein sialyltransferase, which exhibited a four-fold increase during exponential growth and a small decrease at confluency, was markedly attenuated in valproate-exposed cells in a manner which was indirect. This was associated with an inhibition of transient alpha2,3 sialylation of a 65 kDa glycoprotein expressed maximally at 4 hr into the G1 phase of the cell cycle. This effect was cell-cycle phase-specific, as exposure of synchronized cells to valproate inhibited transient sialylation at 4 and 5 hr into the G1 phase. Inhibition of the 65 kDa glycoprotein sialylation by valproate is suggested to arise from impaired signal transduction preceding the eventual arrest by the drug at a 5-6 hr G1 phase restriction point.

Tunicamycin inhibits the initiation of DNA synthesis stimulated by prostaglandin F2 alpha in Swiss mouse 3T3 cells

Tunicamycin, an inhibitor of the asparagine-linked protein N-glycosylation, blocks the initiation of DNA synthesis in Swiss 3T3 cells stimulated by prostaglandin F2 alpha alone or with insulin. This effect is exerted only when tunicamycin is added from 0 to 8 h after stimulation and it decreases the rate of entry into S phase. Blocking of labeled sugar incorporation to proteins occurs regardless of the time of PGF2 alpha stimulation. In contrast tunicamycin does not inhibit protein synthesis. These results suggest that N-glycoprotein synthesis early during the prereplicative phase is an important event controlling the mitogenic action of PGF2 alpha.

The anticonvulsant valproate teratogen restricts the glial cell cycle at a defined point in the mid-G1 phase

Direct cell counting and extent of [3H]thymidine incorporation demonstrated valproate to inhibit C6 glioma proliferation rate in a dose-dependent manner with a 1 mM concentration achieving 50% inhibition. The antiproliferative effect was reversible and could not be attributed to cytotoxicity at the valproate concentrations employed. The site of valproate action within the cell cycle was determined to be in the G1 phase, at a point 6-6.5 h prior to S phase, by estimating the time to increased [3H]thymidine incorporation following release from a 70% proliferative arrest. Synchronised cells obtained by a mitotic selection procedure required 11-12 h to enter S phase and demonstrated the valproate restriction point to be 5 h into the G1 phase of the C6 cell cycle. Exposure of valproate to the part of the G1 period which follows the restriction point was without effect on cell entry into S phase.