Antisense glutaminase inhibition modifies the O-GlcNAc pattern and flux through the hexosamine pathway in breast cancer cells.

Glutamine behaves as a key nutrient for tumors and rapidly dividing cells. Glutaminase is the main glutamine-utilizing enzyme in these cells, and its activity correlates with glutamine consumption and growth rate. We have carried out the antisense L-type glutaminase inhibition in human MCF7 breast cancer cells, in order to study its effect on the hexosamine pathway and the pattern of protein O-glycosylation. The antisense mRNA glutaminase expressing cells, named ORF19, presented a 50% lower proliferation rate than parental cells, showing a more differentiated phenotype. ORF19 cells had an 80% reduction in glutamine:fructose-6-P amidotransferase activity, which is the rate-limiting step of the hexosamine pathway. Although the overall cellular protein O-glycosylation did not change, the O-glycosylation status of several key proteins was altered. O-glycosylation of O-GlcNAc transferase (OGT), the enzyme that links N-acetylglucosamine to proteins, was fivefold lower in ORF19 than in wild type cells. Inhibition of glutaminase also provoked a 10-fold increase in Sp1 expression, and a significant decrease in the ratio of O-glycosylated to total protein for both Sp1 and the Rpt2 proteasome component. These changes were accompanied by a higher Sp1 transcriptional activity. Proteome analysis of O-glycosylated proteins permitted the detection of two new OGT target proteins: the chaperonin TCP-1 theta and the oncogene Ets-related protein isoform 7. Taken together, our results support the hexosamine pathway and the O-glycosylation of proteins being a sensor mechanism of the nutritional and energetic states of the cell.

Inositolphosphoceramide is not a substrate for the first steps in the biosynthesis of glycoinositolphospholipids in Trypanosoma cruzi.

The major free glycoinositolphospholipids and protein-linked glycoinositolphospholipids in Trypanosoma cruzi contain ceramide as the lipid moiety. Ceramide was not found in mammalian glycosylphosphatidylinositol (GPI)-anchors. An alkylglycerol, either as a lyso species or acylated has been also found in T. cruzi anchors. However, unlike African trypanosomes, no diacylglycerol was detected in the GPI-anchors. Using a membrane preparation from epimastigotes upon labelling with UDP[3H]GlcNAc we identified [3H]GlcNAcPI as the first step of GPI biosynthesis. Both, alkylacylglycerol (major) and diacylglycerol are constituents of the lipid. Although inositolphosphoceramide is the main inositolphospholipid in epimastigotes, it does not incorporate GlcNAc. The de-N-acetylation step afforded [3H]GlcN(alkylacylglycerol)PI and we also detected the [3H]GlcN(lysoacyl)PI. A new metabolite, phosphoGlcN(lysoacyl)PI, which was formed on long incorporation times, was characterized by chemical and enzymatic degradations. Several [3H]-Man labelled GPI precursors were obtained by in vitro GDP[3H]-Man labelling in the presence of UDPGlcNAc. All of them were sensitive to PI-PLC and to saponification conditions, thus, supporting a glycerolipid structure.

Uridine diphosphate-glucose transport into the endoplasmic reticulum of Saccharomyces cerevisiae: in vivo and in vitro evidence.

It has been proposed that synthesis of beta-1,6-glucan, one of Saccharomyces cerevisiae cell wall components, is initiated by a uridine diphosphate (UDP)-glucose-dependent reaction in the lumen of the endoplasmic reticulum (ER). Because this sugar nucleotide is not synthesized in the lumen of the ER, we have examined whether or not UDP-glucose can be transported across the ER membrane. We have detected transport of this sugar nucleotide into the ER in vivo and into ER-containing microsomes in vitro. Experiments with ER-containing microsomes showed that transport of UDP-glucose was temperature dependent and saturable with an apparent Km of 46 microM and a Vmax of 200 pmol/mg protein/3 min. Transport was substrate specific because UDP-N-acetylglucosamine did not enter these vesicles. Demonstration of UDP-glucose transport into the ER lumen in vivo was accomplished by functional expression of Schizosaccharomyces pombe UDP-glucose:glycoprotein glucosyltransferase (GT) in S. cerevisiae, which is devoid of this activity. Monoglucosylated protein-linked oligosaccharides were detected in alg6 or alg5 mutant cells, which transfer Man9GlcNAc2 to protein; glucosylation was dependent on the inhibition of glucosidase II or the disruption of the gene encoding this enzyme. Although S. cerevisiae lacks GT, it contains Kre5p, a protein with significant homology and the same size and subcellular location as GT. Deletion mutants, kre5Delta, lack cell wall beta-1,6 glucan and grow very slowly. Expression of S. pombe GT in kre5Delta mutants did not complement the slow-growth phenotype, indicating that both proteins have different functions in spite of their similarities.

Characterization and partial purification of a novel enzymatic activity. UDP-GlcNAc:Ser-protein N-acetylglucosamine-1-phosphotransferase from the cellular slime mold Dictyostelium discoideum.

An enzymatic activity that transfers N-acetylglucosamine-1-phosphate residues from UDP-GlcNAc to serine units in proteins (UDP-GlcNAc:Ser-protein N-acetylglucosamine-1-phosphotransferase) was detected in membranes of the cellular slime mold Dictyostelium discoideum. The enzyme was partially purified by affinity chromatography in concanavalin A-Sepharose and ion exchange chromatography in a Mono Q column. The enzyme showed an absolute requirement for bivalent cations, Mn2+ being more effective than Mg2+. It had a broad optimum pH value (6.5-9.0). The Km for UDP-GlcNAc was 18 microM. In cell free assays it used apomucin and native or 8 M urea-denatured thyroglobulin but neither bovine serum albumin nor native or denatured uteroferrin as exogenous acceptors. Analysis of proteins isolated from cells grown in the presence of [32P]phosphate and from the culture medium showed that the majority of proteins bearing the structure Glc-NAc-1-P-Ser were secreted. In equilibrium density centrifugations of microsomes, the enzyme appeared in membranes having lighter densities than the enzyme that phosphorylates high mannose-type oligosaccharides. This showed that the activity that phosphorylates serine residues in proteins (UDP-GlcNAc:Ser-protein N-acetylglucosamine-1-phosphotransferase) is different from that phosphorylating protein-linked high mannose-type oligosaccharides (UDP-GlcNAc:glycoprotein N-acetylglucosamine-1-phosphotransferase).

Hexosamine and cell wall biogenesis in the aquatic fungus Blastocladiella emersonii.

Chitin, a beta-(1-->4) polymer of N-acetyl-glucosamine, is an important constituent of fungal cell walls. This polymer is synthesized by the incorporation of N-acetyl-D-glucosamine units from the precursor UDP-N-acetyl-D-glucosamine (UDP-GlcNAc) in a reaction catalyzed by chitin synthase. In the aquatic fungus Blastocladiella emersonii, chitin, the major component of the cell wall, is synthesized and incorporated in the cell surface of the free-swimming zoospore during the abrupt transition from this wall-less cell to the sessile, wall-containing cyst. Studies with cycloheximide indicate that chitin synthesis occurs in the apparent absence of protein synthesis, and thus posttranslational controls presumably regulate the cell wall biogenesis during encystment. Glutamine: fructose 6-phosphate amidotransferase, first enzyme of the hexosamine biosynthetic pathway, was found to play a central role in the regulation of chitin synthesis in this fungus. This enzyme exists in two forms, which are interconvertible by phosphorylation or dephosphorylation of serine residues. It is allosterically inhibited in the phosphorylated form, as it is in the zoospore, by UDP-GlcNAc. In addition, UDP-GlcNAc inhibits the dephosphorylation of amidotransferase catalyzed by protein phosphatases 2A and 2C. Thus, UDP-GlcNAc plays a dual role in hexosamine and chitin synthesis in zoospore: it not only inhibits the phosphorylated form of the enzyme but also prevents its dephosphorylation. The available data suggest that substrate availability plays a role in the control of chitin synthesis during zoospore differentiation.

Phosphorylation-dependent regulation of amidotransferase during the development of Blastocladiella emersonii.

The enzyme amidotransferase [2-amino-2-deoxy-D-glucose-6-phosphate ketol isomerase (amino-transferring); EC 2.6.1.16] catalyzes the first step in the hexosamine biosynthetic pathway. In Blastocladiella emersonii the sensitivity of the enzyme to the inhibitor uridine-5'-diphospho-N-acetylglucosamine (UDP-GlcNAc) is developmentally regulated. The inhibitable form of amidotransferase activity present in the zoospore is converted to a noninhibitable form during germination. The latter form is present throughout the growth phase and sensitivity to UDP-GlcNAc gradually returns to the zoospore level during sporulation [C.P. Selitrennikoff, N.E. Dalley, and D.R. Sonneborn (1980) Proc. Natl. Acad. Sci. USA 77, 5998-6002]. The following evidence suggests that a phosphorylation/dephosphorylation mechanism underlies this interconversion: (i) Both the vegetative and zoospore enzymes have the same molecular weight of 140,000, but the vegetative enzyme elutes significantly earlier on a DEAE-cellulose column than does the zoospore enzyme. (ii) The increased sensitivity to UDP-GlcNAc occurring in vivo and in vitro correlates with increased phosphorylation of a polypeptide of apparent Mr 76,000. This component copurifies with amidotransferase activity through ion-exchange chromatography and sucrose density gradient centrifugation. (iii) Desensitization and concurrent dephosphorylation of sensitive amidotransferase can be observed in vitro after treatment with a partially purified magnesium-dependent phosphoprotein phosphatase from zoospores.

Chitin synthesis in Triatoma infestans and other insects.

In vivo and in vitro synthesis of chitin in Triatoma infestans was studied. For in vivo experiments, [14C] sugars were injected through the abdominal wall. Maximal incorporation of radioactivity into the cuticle was attained immediately after the ecdysis. The identification of in vivo synthesized chitin was performed by the enzymatic hydrolysis of the alkali-insoluble material from the cuticle with Helix chitinase. The main water-soluble compound obtained was N-acetylglucosamine as demonstrated by chromatographic procedures. In vitro synthesis of chitin was carried out with enzymatic crude extracts from Triatoma infestans, and UDP-N-acetylglucosamine was used as "source" of glycosyl moieties. Higher amounts of [14C] N-acetylglucosamine incorporation to chitin than those previously reported by others, were obtained. The identity of the product was confirmed in a similar way as that from in vivo synthesis. Radioactivity was also found in a liposoluble fraction concomitantly with chitin synthesis. This compound had an anionic behavior, was acid labile and had similar chromatographic properties as dolichol pyrophosphate N-acetylglucosamine obtained with pig liver extracts. Knowledge about dolichol phosphate sugars mediated glycoprotein synthesis in eukaryotes, suggests the involvement of this type of N-acetylglucosaminyl-phospholipid in macromolecule "building" even in insects.