Regulation of rat-liver glycoprotein: N-acetylneuraminic acid transferase activity by pyrimidine nucleotides

High-throughput profiling of nucleotides and nucleotide sugars to evaluate their impact on antibody N-glycosylation

Recent advances in miniaturized cell culture systems have facilitated the screening of media additives on productivity and protein quality attributes of mammalian cell cultures. However, intracellular components are not routinely measured due to the limited throughput of available analytical techniques. In this work, time profiling of intracellular nucleotides and nucleotide sugars of CHO-S cell fed-batch processes in a micro-scale bioreactor system was carried out using a recently developed high-throughput method based on matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry (TOF-MS). Supplementation of various media additives significantly altered the intracellular nucleotides and nucleotide sugars that are inextricably linked to the process of glycosylation. The results revealed that UDP-Gal synthesis appeared to be particularly limiting whereas the impact of elevated UDP-GlcNAc and GDP-Fuc levels on the final glycosylation patterns was only marginally important. In contrast, manganese and asparagine supplementation altered the glycan profiles without affecting intracellular components. The combination of miniaturized cell cultures and high-throughput analytical techniques serves therefore as a useful tool for future quality driven media optimization studies.

Biosynthetic Machinery Involved in Aberrant Glycosylation: Promising Targets for Developing of Drugs Against Cancer

Cancer cells depend on altered metabolism and nutrient uptake to generate and keep the malignant phenotype. The hexosamine biosynthetic pathway is a branch of glucose metabolism that produces UDP-GlcNAc and its derivatives, UDP-GalNAc and CMP-Neu5Ac and donor substrates used in the production of glycoproteins and glycolipids. Growing evidence demonstrates that alteration of the pool of activated substrates might lead to different glycosylation and cell signaling. It is already well established that aberrant glycosylation can modulate tumor growth and malignant transformation in different cancer types. Therefore, biosynthetic machinery involved in the assembly of aberrant glycans are becoming prominent targets for anti-tumor drugs. This review describes three classes of glycosylation, O-GlcNAcylation, N-linked, and mucin type O-linked glycosylation, involved in tumor progression, their biosynthesis and highlights the available inhibitors as potential anti-tumor drugs.

The CMP-sialic acid transporter is localized in the medial-trans Golgi and possesses two specific endoplasmic reticulum export motifs in its carboxyl-terminal cytoplasmic tail

The addition of sialic acid to glycoproteins and glycolipids requires Golgi sialyltransferases to have access to their glycoconjugate substrates and nucleotide sugar donor, CMP-sialic acid. CMP-sialic acid is transported into the lumen of the Golgi complex through the CMP-sialic acid transporter, an antiporter that also functions to transport CMP into the cytosol. We localized the transporter using immunofluorescence and deconvolution microscopy to test the prediction that it is broadly distributed across the Golgi stack to serve the many sialyltransferases involved in glycoconjugate sialylation. The transporter co-localized with ST6GalI in the medial and trans Golgi, showed partial overlap with a medial Golgi marker and little overlap with early Golgi or trans Golgi network markers. Endoplasmic reticulum-retained forms of sialyltransferases did not redistribute the transporter from the Golgi to the endoplasmic reticulum, suggesting that transporter-sialyltransferase complexes are not involved in transporter localization. Next we evaluated the role of the transporter's N- and C-terminal cytoplasmic tails in its trafficking and localization. The N-tail was not required for either endoplasmic reticulum export or Golgi localization. The C-tail was required for endoplasmic reticulum export and contained di-Ile and terminal Val motifs at its very C terminus that function as independent endoplasmic reticulum export signals. Deletion of the last four amino acids of the C-tail (IIGV) eliminated these export signals and prevented endoplasmic reticulum export of the transporter. This form of the transporter supplied limited amounts of CMP-sialic acid to Golgi sialyltransferases but was unable to completely rescue the transporter defect of Lec2 Chinese hamster ovary cells.

Immobilisation on polystyrene of diazirine derivatives of mono- and disaccharides: biological activities of modified surfaces

The potential of surface glycoengineering for biomaterials and biosensors originates from the importance of carbohydrate-protein interactions in biological systems. The strategy employed here utilises carbene generated by illumination of diazirine to achieve covalent bonding of carbohydrates. Here, we describe the synthesis of an aryl diazirine containing a disaccharide (lactose). Surface analysis techniques [X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (ToF-SIMS)] demonstrate its successful surface immobilisation on polystyrene (PS). Results are compared to those previously obtained with an aryl diazirine containing a monosaccharide (galactose). The biological activity of galactose- or lactose-modified PS samples is studied using rat hepatocytes, Allo A lectin and solid-phase semi-synthesis with alpha-2,6-sialyltransferase. Allo A shows some binding to galactose-modified PS but none to lactose-modified surfaces. Similar results are obtained with rat hepatocytes. In contrast, sialylation of lactose-modified PS is achieved but not with galactose-modified surfaces. The different responses indicate that the biological activity depends not only on the carbohydrate per se but also on the structure and length of the spacer.

Studies on the inhibition of sialyl- and galactosyltransferases

The inhibition of the alpha-2,6-sialyltransferase from rat liver, the alpha-2,3-sialyltransferase from porcine submandibular gland and of the galactosyltransferase from human milk were studied using monosaccharide-, nucleoside- and nucleotide-derivatives of their naturally occurring donor substrates cytidine 5'-monophosphate-N-acetylneuraminic acid and uridine 5'-diphosphate-galactose, respectively. Only the corresponding nucleosides/nucleotides showed inhibitory activity. Periodate oxidation of CMP or CMP-Neu5Ac and of UMP or UDP-Gal led to reduced inhibitory efficiency with the respective transferase. The type and reversibility of the inhibition of some of these compounds, as well as the corresponding Ki values were determined.

Sialylation of lipopolysaccharide and loss of absorption of bactericidal antibody during conversion of gonococci to serum resistance by cytidine 5'-monophospho-N-acetyl neuraminic acid

Changes in lipopolysaccharide (LPS) which occur when serum susceptible gonococci are converted to resistance by incubation with cytidine 5'-monophospho-N-acetyl neuraminic acid (CMP-NANA) have been investigated. Transfer of radioactivity to bacterial LPS from CMP-NANA labelled with 14C in the NANA moiety was detected by fluorography following lysis, proteinase K digestion and SDS-PAGE. Incorporation of radioactivity was inhibited by cytidine 5'-monophosphate (CMP). Both the radioactivity of the LPS and the resistance of gonococci to fresh human serum were largely lost after incubation with neuraminidase. No evidence was obtained to suggest that CMP-NANA is an inducer of new protein synthesis as well as a substrate for the sialylation of LPS. Little radioactivity was incorporated into components other than LPS. Sialylated, serum resistant gonococci were less able than serum susceptible gonococci to absorb the bactericidal activity of fresh human serum. Hence, we conclude that serum resistance conferred on gonococci by CMP-NANA is due to transfer of sialyl groups to surface LPS sites and this inhibits their reaction with bactericidal antibody in human serum.

Cytidine 5'-monophospho-N-acetyl neuraminic acid and a low molecular weight factor from human blood cells induce lipopolysaccharide alteration in gonococci when conferring resistance to killing by human serum

Recently evidence has been obtained that a minute amount of cytidine 5'-monophospho-N-acetyl neuraminic acid (CMP-NANA) or a closely related compound is the low Mr factor in human red blood cells which induces Neisseria gonorrhoeae (BS4(agar] to resistance to killing by fresh human serum. Induction of gonococci to resistance by both CMP-NANA and semi-purified low Mr factor from red blood cells was accompanied by a 35-55% reduction of silver staining of lipopolysaccharide separated in SDS-PAGE gels of proteinase K digests. These alterations in lipopolysaccharide are probably responsible for conferring serum resistance. However, lipopolysaccharide-containing digests from resistant as well as from susceptible gonococci neutralised serum bactericidal activity. These observations may have wider implications since CMP-NANA is a sialylating agent wide-spread in mammalian tissues and LPS is ubiquitous amongst Gram-negative pathogens.

Purification and characterization of an endogenous inhibitor of the sialyltransferase CMP-N-acetylneuraminate: lactosylceramide alpha 2,6-N-acetylneuraminyltransferase (EC 2.4.99.-)

The presence in the 100,000 g supernatant of rat brain homogenate of an inhibitor of the sialyltransferase has been confirmed. It is also present in chicken and bovine brain and in other rat and bovine organs. The inhibitor has been purified, a preparation with a specific activity 130-fold higher than that of the original 100,000 g supernatant of brain being obtained. It runs as a single peak in polyacrylamide-gel electrophoresis; when run in the presence of SDS, two components appeared. The apparent Mr of the components were 14,800 and 22,400. The inhibitor has been characterized as a heat-stable protein of acidic nature. It has effect on the glycolipid and the glycoprotein sialyltransferase activities but has no effect on the galactosaminyltransferase activity.

Role of sialic acid in insulin action and the insulin resistance of diabetes mellitus

Adipocytes treated with neuraminidase show markedly reduced responsiveness to insulin without any alteration in insulin binding. In addition, several studies have separately demonstrated both insulin resistance and decreases in membrane sialic acid content and associated biosynthetic enzymes in diabetes mellitus. In the present study, we investigated the role that sialic acid residues may play in insulin action and in the hepatic insulin resistance associated with nonketotic diabetes. Primary cultures of hepatocytes from normal rats treated with neuraminidase demonstrated a dose-dependent decrease in insulin-stimulated lipogenesis. At a concentration of neuraminidase that decreases insulin action by 50%, 23% of total cellular sialic acid content was released. Neuraminidase-releasable sialic acid was significantly decreased in hepatocytes from diabetic rats and this was associated with significant insulin resistance. Treatment of hepatocytes from diabetic rats with cytidine 5'-monophospho-N-acetylneuraminic acid (CMP-NANA) enhanced insulin responsiveness 39%. The enhanced insulin responsiveness induced by CMP-NANA was blocked by cytidine 5'-monophosphate (CMP) suggesting that the CMP-NANA effect was catalyzed by a cell surface sialyltransferase. CMP reduced neuraminidase-releasable [14C]sialic acid incorporation into hepatocytes by 43%. The data demonstrate a role for cell surface sialic acid residues in hepatic insulin action and support a role for decreased cell surface sialic acid residues in the insulin resistance of diabetes mellitus.

Ganglioside biosynthesis in rat liver: effect of UDP-amino sugars on individual transfer reactions

Several glycosyltransferases participating in ganglioside biosynthesis were measured in Golgi-rich fractions from rat liver. Addition of those UDP-amino sugars to the enzyme assays which accumulate in liver after treatment of rats with D-galactosamine inhibited the transferases to different degrees. The simultaneous presence of UDP-GalN, UDP-GalNAc, UDP-GlcN, and UDP-GlcNAc in concentrations resembling their overall content in livers 6 h after D-galactosamine administration led to an inhibition of the glycolipid galactosyltransferases, GL2 and GM1 synthases of 44 and 64%, respectively. GM2 synthase was moderately inhibited whereas the sialyltransferases (GM3, GD3, and GD1a synthases) were almost unimpaired. Induction of liver cell damage by D-galactosamine did not cause any change of glycosyltransferase activities as determined in rat liver homogenates and Golgi-rich fractions. These results indicate a possible role for UDP-amino sugars in the depression of ganglioside biosynthesis observed in vivo after GalN administration.

Effects of early diabetes on uridine diphosphosugar synthesis in the rat renal cortex

To assess the effects of streptozotocin-induced diabetes on the substrates utilized in the formation of glycoproteins, the pools of uridine 5'-diphosphoglucose (UDPG), uridine 5'-diphosphogalactose (UDP-GAL), uridine 5'-diphosphoglucuronic acid (UDPGA), and uridine 5'-diphospho N-acetyl galactosamine (UDPA-GAL) were measured in the renal cortex of control and over a 48-hr period in diabetic rats. In control rats these pools measured: UDPG, 256 +/- 23; UDP-GAL, 75 +/- 14; UDPGA, 147 +/- 16; UDPAG, 367 +/- 23; UDPG-GAL, 131 +/- 13 nmoles/mg DNA. In diabetic rats, except for UDP-GAL, all pools were increased 41 to 68%. The incorporation of radiolabeled orotate was increased in all pools, except UDP-GAL, in diabetic rats by 41 to 77% compared to control rats. The incorporation into UDPG and UDPAG was increased even after correction for the specific radioactivity of their immediate precursor, uridine 5'-triphosphate (UTP). Expansion of the UTP pool after orotate infusion was associated with an increase in the size of the UDPG and UDPAG pools in both control and diabetic rats. Depletion of the UTP pool after adenine infusion in controls was associated with a decrease in all pools. This study demonstrates that after the induction of diabetes there is a rapid increase in the bioavailability of substrates utilized in the synthesis of glycoproteins and glycosaminoglycans. It is theorized that this increase is necessary for the augmented synthesis of basement membrane-like material in the diabetic kidney.

Glycosyltransferases of the human cervical epithelium. II. Characterization of a CMP-N-acetylneuraminate: galactosyl-glycoprotein sialyltransferase

GMP-N-Acetylneuraminate: galactosyl-glycoprotein sialytransferase (CMP-N-acetylneuraminate: D-galactosyl-glycoprotein N-acetylneuraminyltransferase, EC 2.4.99.1) activity was identified in the human cervical epithelium. The enzyme has a pH optimum of 6.0, a temperature optimum of 28 degrees C, and demonstrates a partial requirement for Triton X-100. Michaelis constants for asialofetuin and CMP-N-acetyl[14C]neuraminic acid are 0.64 . 10(-5) M (expressed as the concentration of terminal galactose residues) and 2.05 . 10(-5) M, respectively. Sialytransferase demonstrated minimal affinity for the low molecular weight acceptors tested, and may have a requirement for a glycoprotein acceptor having a terminal N-acetyllactosamine (Gal beta (1 leads to 4)GlcNAc) type structure. Cytidine nucleotides are potent inhibitors of the sialyltransferase reaction; CMP acts as a competitive inhibitor.

Serum levels of glycosyltransferases and related glycoproteins as indicators of cancer: biological and clinical implications

Many studies have suggested that malignant transformation is associated with fundamental changes in the cell surface; similar changes have been described for normal stem cells and cells of embryonic or fetal origin. There is now evidence that the tumor cell secretes or sheds glycoproteins and glycosyltransferases into the surrounding medium and into serum. There are claims that some of these serum glycoproteins and glycosyltransferases are associated with, or specifically related to, the extent of tumor growth and may serve as a cancer marker. A cancer-associated galactosyltransferase isoenzyme (GT-II) has been described and purified. Different isoelectric forms of fucosyltransferase have also been described as indicative of malignancy. The articles to be published in CRC Critical Reviews in Clinical Laboratory Sciences will analyze the evidence for the association of these membrane factors with tumor growth. In order to better understand the possible significance of altered glycoproteins and of increased or different forms of glycosyltransferases during tumor growth, recent data on glycoprotein synthesis will be discussed including the new concepts on the control of glycoprotein synthesis through lipid intermediates. The possible mechanisms whereby malignant transformation could alter glycoprotein synthesis will be discussed with particular emphasis on the significance of these alterations to the biology of the malignant cell. Changes in surface membrane glycoproteins have long been implicated in the ability of a cell to metastasize. Secretion and/or shedding of the cell surface may also be important in the process of metastasis and in altering the host immune response. Detection and the study of these "shed" materials in patients appear to be indicating a new approach to cancer biology detection and therapy.

Ganglioside biosynthesis in rat liver: alteration of sialyltransferase activities by nucleotides

CMP-NAcNeu:GM3 ganglioside sialyltransferase (GD3 synthase) was characterized with respect to regulation of activity by nucleotides and compared in this regard with other sialyltransferases of ganglioside biosynthesis. Nucleotides preferentially inhibited the activity of GD3 synthase. Di- and trinucleotides inhibited most strongly and cytidine nucleotides were the most inhibitory class. The mode of inhibition by CMP (competitive or noncompetitive) varied with storage conditions of Golgi apparatus membranes; CMP inhibition was decreased during a series of consecutive freeze-thawings of membranes. Also, GD3 synthase inhibition by CDP was only partially relieved by excess Mg2+. With lactosylceramide as the in vitro precursor, synthesis of GM3 was always less inhibited by cytidine nucleotides than was that of GD3 and GT3. An 8-fold reduction in the ratio GD3/GM3 in the reaction products was obtained at 1.5 mM CTP. Separate incubations for the sialylation of GM3 or GM1 showed cytidine nucleotides increased synthesis of GD1a relative to GD3 by 3.5-fold.

Orotic acid prevents changes in cardiac sarcolemmal glycoproteins and contractility associated with muscular dystrophy in hamsters

Orotic acid included in the diet of cardiomyopathic hamsters during the myolytic phase of the disease (30-60 days of age) prevented the reduction in cardiac sarcolemmal sialic acid, calcium binding, sialyltransferase activity and contractile activity associated with the cardiomyopathy.

Effect of partial hepatectomy and hydrocortisone administration on liver and serum sialyltransferase activities

Partial hepatectomy of rats was followed by a rise in liver sialyltransferase activity. The maximum (2.5-fold increase) was reached on the third day after the operation, after which the level started to decline, returning to normal by day 6. Determination of serum sialyltransferase in these animals showed a parallel pattern. Daily injection of 5 mg hydrocortisone to adrenalectomized rats led to a maximal 3-fold elevation in liver sialyltransferase within 3 days, but failed to elicit any change in the corresponding enzyme in the serum. Results from these two experiments suggest that the elevations of sialyltransferase in the tissue and in the circulation are independently regulated.

A second gene affecting the sialylation of lysosomal alpha-mannosidase in mouse liver

We have previously reported on a mouse liver-specific genetic polymorphism associated with altered sialylation of lysosomal alpha-mannosidase. A second electrophoretic polymorphism for liver lysosomal alpha-mannosidase has now been found and characterized. This variation, between SWR/J and SM/JCv inbred mice, is determined by a single genetic locus (Map-2) on chromosome 17 and appears to be the result of further differences in sialylation of the lysosomal enzyme. The Map-2 gene appears to affect the processing of liver, spleen, and lung lysosomal alpha-mannosidase, whereas the Map-1 gene appears to be specific to the processing of liver lysosomal alpha-mannosidase (Dizik and Elliott, 1977). The more negatively charged electrophoretic liver phenotype (MA-A) characteristic of the SM/JCv strain is recessive to the phenotype (MA-B) characteristic of the SWR/J strain. In contrast, at the Map-1 locus, the more negatively charged phenotype is dominant. The electrophoretic pattern of development of the liver enzyme from SM/JCv mice is described.

Factors affecting glucosyl and mannosyl transfer to dolichyl monophosphate by liver cell-free preparations

GDP-mannose and UDP-mannose (each at less than 1 micrometer) markedly inhibit glucosyl transfer from UDP-glucose (1.6 micrometer( to dolichyl phosphate in liver microsomal preparations. The biphasic response suggests the presence of two glucosyl transferases only one of which is inhibited. The inhibition appears to be a property of the intact nucleotide phosphate sugars and not due to competition for a limited pool of dolichyl phosphate. UDP-galactose and UDP-xylose cause a less marked inhibition of the same enzyme. The failure of UDP-glucose to inhibit mannosyl transfer suggests that the pool of dolichol monophosphate used by mannosyl transferase is not available to the glucosyl transferase. The relationship between the degree to which an exogenous prenol phosphate acts as an acceptor of mannose and the degree to which it inhibits mannosylation of endogenous dolichyl monophosphate varies among different prenyl phosphates. Mannosyl transferase exhibits two pH optima.

Concomitant elevations in serum sialytransferase activity and sialic acid content in rats with metastasizing mammary tumors

Rats with transplantable spontaneously metastasizing mammary tumors have elevated levels of both serum sialoglycoconjugate and serum sialytransferase activity compared with normal female rats or rats with various nonmetastasizing mammary tumors. A direct relationship was observed between the amount of serum protein-bound sialic acid and serum sialyltransferase activity in all rats studied. Serum sialyltransferase activity in rats with a representative metastasizing mammary tumor, SMT-2A, was also correlated with tumor age. Microsomes prepared from the SMT-2A tumor have a sixfold higher sialyltransferase activity than do microsomes prepared from the nonmetastasizing mammary tumor MT-W9B. Normal rat liver microsomes have the same level of activity as microsomes prepared from livers of animals with either SMT-2A or MT-W9B tumors. The data indicate that spontaneously metastasizing mammary tumor cells have an increased production and release, perhaps through cell surface shedding, of a sialyltransferase. It is suggested that this sialyltransferase may increase the serum half-life of certain tumor-specific circulating glycoconjugates by increasing the content of protein-bound sialic acid and may thereby play a role in the immune escape mechanism of metastasizing tumor cells.

Biochemical characteristics, metabolism, and antitumor activity of several acetylated hexosamines

We have synthesized several potential inhibitors and/or modifiers of the carbohydrate portion of plasma membrane glycoconjugates. These include fluorinated and actylated analogs of D-glucosamine, D-galactosamine, and D-mannosamine. These compounds have been tested to determine their effects on both[14C] glucosamine and [3H] leucine incorporation into glycoconjugate and on cell growth and viability using P-288 murine lymphoma cells maintained in tissue culture. The most cytotoxic agent tested was 2-acetamido-2-deoxy-1,3,4,6-tetra-O-acetyl-beta-D-glucopyranose or simply beta-pentaacetylglucosamine which prevented cell growth at 10(-4)-10(-3) M. beta-Pentaacetylglucosamine cytotoxicity was correlated with its high lipid solubility, having an octanol/water partition coefficient of 0.424 as compared with 0.278 for the alpha-anomer and 0.017 for N-acetylglucosamine. In vitro metabolism studies with [4C]- and/or [3H]-labeled pentaacetylglucosamine have indicated intracellular de-O-acetylation leading to the biosynthesis of UDP-N-acetylglucosamine, followed by the incorporation of this sugar into cellular glycoprotein. Concomitant with the formation of increased amounts of this nucleotide sugar, intracellular UTP and CTP pools fell to one third normal within 3 h after the administration of 1 mM pentaacetylglucosamine. At present it is unclear whether the cytotoxicity of beta-pentaacetylglucosamine or other similar agents is due to alterations in nucleotide and nucleotide-sugar pools causing a decrease in energy charge and polynucleotide biosynthesis or is due to a direct effect on membrane glycoconjugate biosynthesis.

[Sialyltransferase in human malignant melanoma]

A glycosyltransferase, CMP-N-acetylneuraminic acid : glycoprotein sialyltransferase was found in human malignant melanoma. Activities were measured with desialized glycoprotein as an exogenous acceptor. The enzyme was characterized by means of its pH optimum, 5.5, temperature optimum, 30 degrees C, KM values, 10 muM for the sugar nucleotide and 0.3 mM for desialized glycoprotein. It did not require exogenously added metal ions but was slightly stimulated by Mg2+. It required detergent for optimal activity. The effect of nucleotides and sugar nucleotides on enzyme activity has been investigated.