Structure of the altered oligosaccharide present in glycoproteins from a clone of Chinese hamster ovary cells deficient in N-acetylglucosaminyltransferase activity

Synthesis, Processing, and Function of N-Glycans in N-Glycoproteins

Many membrane-resident and secreted proteins, including growth factors and their receptors are N-glycosylated. The initial N-glycan structure is synthesized in the endoplasmic reticulum (ER) as a branched structure on a lipid anchor (dolicholpyrophosphate) and then co-translationally, "en bloc" transferred and linked via N-acetylglucosamine to asparagine within a specific N-glycosylation acceptor sequence of the nascent recipient protein. In the ER and then the Golgi apparatus, the N-linked glycan structure is modified by hydrolytic removal of sugar residues ("trimming") followed by re-glycosylation with additional sugar residues ("processing") such as galactose, fucose or sialic acid to form complex N-glycoproteins. While the sequence of the reactions leading to biosynthesis, "en bloc" transfer and processing of N-glycans is well investigated, it is still not completely understood how N-glycans affect the biological fate and function of N-glycoproteins. This review will discuss the biology of N-glycoprotein synthesis, processing and function with specific reference to the physiology and pathophysiology of the immune and nervous system, as well as infectious diseases such as Covid-19.

Biological roles of glycans

Simple and complex carbohydrates (glycans) have long been known to play major metabolic, structural and physical roles in biological systems. Targeted microbial binding to host glycans has also been studied for decades. But such biological roles can only explain some of the remarkable complexity and organismal diversity of glycans in nature. Reviewing the subject about two decades ago, one could find very few clear-cut instances of glycan-recognition-specific biological roles of glycans that were of intrinsic value to the organism expressing them. In striking contrast there is now a profusion of examples, such that this updated review cannot be comprehensive. Instead, a historical overview is presented, broad principles outlined and a few examples cited, representing diverse types of roles, mediated by various glycan classes, in different evolutionary lineages. What remains unchanged is the fact that while all theories regarding biological roles of glycans are supported by compelling evidence, exceptions to each can be found. In retrospect, this is not surprising. Complex and diverse glycans appear to be ubiquitous to all cells in nature, and essential to all life forms. Thus, >3 billion years of evolution consistently generated organisms that use these molecules for many key biological roles, even while sometimes coopting them for minor functions. In this respect, glycans are no different from other major macromolecular building blocks of life (nucleic acids, proteins and lipids), simply more rapidly evolving and complex. It is time for the diverse functional roles of glycans to be fully incorporated into the mainstream of biological sciences.

Synthesis, Processing, and Function of N-glycans in N-glycoproteins

Many membrane-resident and secrected proteins, including growth factors and their receptors, are N-glycosylated. The initial N-glycan structure is synthesized in the endoplasmic reticulum (ER) as a branched structure on a lipid anchor (dolichol pyrophosphate) and then co-translationally, "en bloc" transferred and linked via N-acetylglucosamine to asparagine within a specific N-glycosylation acceptor sequence of the nascent recipient protein. In the ER and then the Golgi apparatus, the N-linked glycan structure is modified by hydrolytic removal of sugar residues ("trimming") followed by re-glycosylation with additional sugar residues ("processing") such as galactose, fucose, or sialic acid to form complex N-glycoproteins. While the sequence of the reactions leading to biosynthesis, "en bloc" transfer and processing of N-glycans is well investigated, it is still not completely understood how N-glycans affect the biological fate and function of N-glycoproteins. This review discusses the biology of N-glycoprotein synthesis, processing, and function with specific reference to the physiology and pathophysiology of the nervous system.

Down-regulation of gene transcripts associated with ricin tolerance in human RPMI 2650 cells

The present study sought to determine if novel therapeutic approaches against ricin intoxication could be identified from human respiratory tract cells selected for increased resistance to this toxin. Initial studies indicated that the RPMI 2650 line was an appropriate model, owing to its sensitivity to ricin. Tolerant cultures were developed by exposing cells to a graded series of ricin concentrations from 6 to 192 pM. This resulted in the generation of cultures whose LC(50) values were increased by up to 4-fold following exposure to up to 96 pM ricin and by up to 6-fold following exposure to up to 192 pM ricin, compared to control cultures. DNA microarrays were employed to determine the gene transcript expression profile of cultures with increased resistance to ricin to investigate which gene products mediate ricin resistance. Transcripts (10) were identified that were greater than 2-fold down-regulated in the cells tolerant to 96 pM ricin, whereas 48 transcripts were seen to be down-regulated in cultures tolerant to 192 pM ricin. Gene transcripts (5) were up-regulated 2-fold or more in the 192 pM tolerant cultures in comparison to unexposed cells. The results indicate that ricin tolerance is the product of complex changes in gene expression profiles, most of which were found to involve down-regulation of transcript expression. It may be possible to modulate the gene expression profiles associated with ricin tolerance for potential therapeutic purposes using drugs and antisense technologies.

The role of the GlcNAc(beta)1,2Man(alpha)- moiety in mammalian development. Null mutations of the genes encoding UDP-N-acetylglucosamine:alpha-3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I and UDP-N-acetylglucosamine:alpha-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I.2 cause embryonic lethality and congenital muscular dystrophy in mice and men, respectively

The GlcNAc(beta)1,2Man(alpha)- moiety can be synthesized by at least two mammalian glycosyltransferases, UDP-GlcNAc:alpha-3-D-mannoside beta1,2-N-acetylglucosaminyltransferase I (GnT I, EC 2.4.1.101) and UDP-GlcNAc:alpha-D-mannoside beta1,2-N-acetylglucosaminyltransferase I.2 (GnT I.2). GnT I adds a GlcNAc residue in beta1,2 glycosidic linkage to the Man(alpha)1,3 arm of the N-glycan core to initiate the biosynthesis of hybrid and complex N-glycans. GnT I.2 can add GlcNAc in beta1,2 linkage to any alpha-linked terminal Man residue but has a strong preference for the Man(alpha)1-O-Thr- moiety which occurs in alpha-dystroglycan and other O-mannosylated glycoproteins. Mouse embryos lacking a functional GnT I gene (MgatI) were unable to synthesize complex N-glycans and none survived past 10.5 days after fertilization. The embryos showed multisystemic defects in various morphogenic processes such as neural tube formation, vascularization and the determination of left-right body plan asymmetry. Six human patients with muscle-eye-brain disease (MEB) were recently shown to have point mutations in the gene encoding GnT I.2 (MGATI.2). MEB is an autosomal recessive disease characterized by congenital muscular dystrophy, ocular abnormalities, brain malformations and other multisystemic defects. Both the MGATI.2 gene and MEB disease have been mapped to chromosome 1p32-p34. At least one of the biochemical sites affected by the MGATI.2 mutations is probably the interaction between laminin in the extracellular matrix and the peripheral membrane glycoprotein alpha-dystroglycan since this interaction is believed to require the presence of the sialyl(alpha)2,3Gal(beta)1,4GlcNAc(beta)1,2Man(alpha)1-O-Ser/Thr moiety on alpha-dystroglycan. It can be concluded that the GlcNAc(beta)1,2Man(alpha)- moiety is important for mammalian development due to an essential role in two distinct biochemical pathways.

Golgi alpha-mannosidase II deficiency in vertebrate systems: implications for asparagine-linked oligosaccharide processing in mammals

The maturation of N-glycans to complex type structures on cellular and secreted proteins is essential for the roles that these structures play in cell adhesion and recognition events in metazoan organisms. Critical steps in the biosynthetic pathway leading from high mannose to complex structures include the trimming of mannose residues by processing mannosidases in the endoplasmic reticulum (ER) and Golgi complex. These exo-mannosidases comprise two separate families of enzymes that are distinguished by enzymatic characteristics and sequence similarity. Members of the Class 2 mannosidase family (glycosylhydrolase family 38) include enzymes involved in trimming reactions in N-glycan maturation in the Golgi complex (Golgi mannosidase II) as well as catabolic enzymes in lysosomes and cytosol. Studies on the biological roles of complex type N-glycans have employed a variety of strategies including the treatment of cells with glycosidase inhibitors, characterization of human patients with enzymatic defects in processing enzymes, and generation of mouse models for the enzyme deficiency by selective gene disruption approaches. Corresponding studies on Golgi mannosidase II have employed swainsonine, an alkaloid natural plant product that causes "locoism", a phenocopy of the lysosomal storage disease, alpha-mannosidosis, as a result of the additional targeting of the broad-specificity lysosomal mannosidase by this compound. The human deficiency in Golgi mannosidase II is characterized by congenital dyserythropoietic anemia with splenomegaly and various additional abnormalities and complications. Mouse models for Golgi mannosidase II deficiency recapitulate many of the pathological features of the human disease and confirm that the unexpectedly mild effects of the enzyme deficiency result from a tissue-specific and glycoprotein substrate-specific alternate pathway for synthesis of complex N-glycans. In addition, the mutant mice develop symptoms of a systemic autoimmune disorder as a consequence of the altered glycosylation. This review will discuss the biochemical features of Golgi mannosidase II and the consequences of its deficiency in mammalian systems as a model for the effects of alterations in vertebrate N-glycan maturation during development.

Fringe modulation of Jagged1-induced Notch signaling requires the action of beta 4galactosyltransferase-1

Fringe modulates Notch signaling resulting in the establishment of compartmental boundaries in developing organisms. Fringe is a beta 3N-acetylglucosaminyltransferase (beta 3GlcNAcT) that transfers GlcNAc to O-fucose in epidermal growth factor-like repeats of Notch. Here we use five different Chinese hamster ovary cell glycosylation mutants to identify a key aspect of the mechanism of fringe action. Although the beta 3GlcNAcT activity of manic or lunatic fringe is shown to be necessary for inhibition of Jagged1-induced Notch signaling in a coculture assay, it is not sufficient. Fringe fails to inhibit Notch signaling if the disaccharide generated by fringe action, GlcNAc beta 3Fuc, is not elongated. The trisaccharide, Gal beta 4GlcNAc beta 3Fuc, is the minimal O-fucose glycan to support fringe modulation of Notch signaling. Of six beta 4galactosyltransferases (beta 4GalT) in Chinese hamster ovary cells, only beta 4GalT-1 is required to add Gal to GlcNAc beta 3Fuc, identifying beta 4GalT-1 as a new modulator of Notch signaling.

Sterols regulate cycling of SREBP cleavage-activating protein (SCAP) between endoplasmic reticulum and Golgi

The proteolytic cleavage of sterol regulatory element-binding proteins (SREBPs) is regulated by SREBP cleavage-activating protein (SCAP), which forms complexes with SREBPs in membranes of the endoplasmic reticulum (ER). In sterol-depleted cells, SCAP facilitates cleavage of SREBPs by Site-1 protease, thereby initiating release of active NH(2)-terminal fragments from the ER membrane so that they can enter the nucleus and activate gene expression. In sterol-overloaded cells, the activity of SCAP is blocked, SREBPs remain bound to membranes, and transcription of sterol-regulated genes declines. Here, we provide evidence that sterols act by inhibiting the cycling of SCAP between the ER and Golgi. We use glycosidases, glycosidase inhibitors, and a glycosylation-defective mutant cell line to demonstrate that the N-linked carbohydrates of SCAP are modified by Golgi enzymes in sterol-depleted cells. After modification, SCAP returns to the ER, as indicated by experiments that show that the Golgi-modified forms of SCAP cofractionate with ER membranes on density gradients. In sterol-overloaded cells, the Golgi modifications of SCAP do not occur, apparently because SCAP fails to leave the ER. Golgi modifications of SCAP are restored when sterol-overloaded cells are treated with brefeldin A, which causes Golgi enzymes to translocate to the ER. These studies suggest that sterols regulate the cleavage of SREBPs by modulating the ability of SCAP to transport SREBPs to a post-ER compartment that houses active Site-1 protease.

Molecular dynamics simulations of hybrid and complex type oligosaccharides

Conformational preferences of hybrid (GlcNAc1Man5GlcNAc2) and complex (GlcNAc1Man3GlcNAc2; GlcNAc2Man3GlcNAc2) type asparagine-linked oligosaccharides and the corresponding bisected oligosaccharides have been studied by molecular dynamics simulations for 2.5 ns. The fluctuations of the core Man-alpha 1,3-Man fragment are restricted to a region around (-30 degrees, -30 degrees) due to a 'face-to-face' arrangement of bisecting GlcNAc and the beta 1,2-GlcNAc on the 1,3-arm. However, conformations where such a 'face-to-face' arrangement is disrupted are also accessed occasionally. The orientation of the 1,6-arm is affected not only by changes in chi, but also by changes in phi and psi around the core Man-alpha 1,6-Man linkage. The conformation around the core Man-alpha 1,6-Man linkage is different in the hybrid and the two complex types suggesting that the preferred values of phi, psi, and chi are affected by the addition or deletion of saccharides to the alpha 1,6-linked mannose. The conformational data are in agreement with the available experimental studies and also explain the branch specificity of galactosyltransferases.

Chinese hamster ovary cells deficient in N-acetylglucosaminyltransferase I activity are resistant to Entamoeba histolytica-mediated cytotoxicity

To study the relationship between carbohydrate-specific amebic cytoadherence and ameba-mediated cytotoxicity, we measured Entamoeba histolytica trophozoite-mediated cytolysis directed against a panel of four Chinese hamster ovary (CHO) cell lines that have defined alterations in their glycosylation patterns. We recently measured amebic trophozoite adherence to this panel of CHO cells and showed that trophozoites bind variant cells (RICR 15B), which are deficient in Asn-linked N-acetyllactosamine units, at 12% of the level observed for wild-type cells (E. Li, A. Becker, and S. L. Stanley, J. Exp. Med 167:1725-1730, 1988). Using a 51Cr release assay to measure trophozoite-mediated cytolysis, we demonstrate in this study that RICR 15B cells are less susceptible to trophozoite-mediated cytolysis than are wild-type cells. In addition, we found that N-acetyllactosamine, which inhibits trophozoite adherence to CHO cells, also inhibited trophozoite-mediated cytolysis of wild-type cells. These studies indicate that surface carbohydrates on target cells can influence susceptibility to ameba-mediated cytotoxicity. This panel of CHO cells provides a useful model system for investigating the role of glycoconjugates in mediating amebic interactions with mammalian cells.

Natural killer cells discriminate between high mannose- and complex-type asparagine-linked oligosaccharides

Chinese hamster ovary cell lines with different types of N-linked oligosaccharides were tested as targets for control and lymphokine treated natural killer (NK) cells. The targets tested were parent cells, Lec1 mutants and Lec4 mutants. Due to an apparent defect in GlcNAc transferase V, Lec4 cells produce complex-type N-linked oligosaccharides devoid of GlcNAc beta(1-6) linked branches. Lec1 cells form only high mannose-type N-linked oligosaccharides because they lack GlcNAc transferase I activity. Lec1 cells are very sensitive to lysis by beta-interferon treated human NK cells, but both parent and Lec4 cells are resistant to NK lysis. The ability to discriminate between parent and Lec1 targets was demonstrated with untreated control effectors as well as those which were pretreated with either beta-interferon, gamma-interferon or interleukin-2. Both control and lymphokine-boosted NK cells exhibit much greater lytic activity against targets having only high mannose-type N-linked oligosaccharides. Five oligosaccharide structures resembling those found on N-linked glycoproteins were tested for their ability to block NK lysis of Lec1 targets. Only the high mannose-type glycopeptide from 7S soybean glycoprotein was inhibitory in the mu molar range. At the same concentration, none of the complex-type oligosaccharides had any effect on lytic activity. The results suggest that a high mannose-type N-linked oligosaccharides is recognized at some step in NK cell-mediated lysis.

The biosynthesis of highly branched N-glycans: studies on the sequential pathway and functional role of N-acetylglucosaminyltransferases I, II, III, IV, V and VI

At least 6 N-acetylglucosaminyltransferases (GlcNAc-T I, II, III, IV, V and VI) are involved in initiating the synthesis of the various branches found in complex asparagine-linked oligosaccharides (N-glycans), as indicated below: GlcNAc beta 1-6 GlcNAc-T V GlcNAc beta 1-4 GlcNAc-T VI GlcNAc beta 1-2Man alpha 1-6 GlcNAc-T II GlcNAc beta 1-4Man beta 1-4-R GlcNAc T III GlcNAc beta 1-4Man alpha 1-3 GlcNAc-T IV GlcNAc beta 1-2 GlcNAc-T I where R is GlcNAc beta 1-4(+/- Fuc alpha 1-6)GlcNAcAsn-X. HPLC was used to study the substrate specificities of these GlcNAc-T and the sequential pathways involved in the biosynthesis of highly branched N-glycans in hen oviduct (I. Brockhausen, J.P. Carver and H. Schachter (1988) Biochem. Cell Biol. 66, 1134-1151). The following sequential rules have been established: GlcNAc-T I must act before GlcNAc-T II, III and IV; GlcNAc-T II, IV and V cannot act after GlcNAc-T III, i.e., on bisected substrates; GlcNAc-T VI can act on both bisected and non-bisected substrates; both Glc-NAc-T I and II must act before GlcNAc-T V and VI; GlcNAc-T V cannot act after GlcNAc-T VI. GlcNAc-T V is the only enzyme among the 6 transferases cited above which can be essayed in the absence of Mn2+. In studies on the possible functional role of N-glycan branching, we have measured GlcNAc-T III in pre-neoplastic rat liver nodules (S. Narasimhan, H. Schachter and S. Rajalakshmi (1988) J. Biol. Chem. 263, 1273-1281). The nodules were initiated by administration of a single dose of carcinogen 1,2-dimethyl-hydrazine.2 HCl 18 h after partial hepatectomy and promoted by feeding a diet supplemented with 1% orotic acid for 32-40 weeks. The nodules had significant GlcNAc-T III activity (1.2-2.2 nmol/h/mg), whereas the surrounding liver, regenerating liver 24 h after partial hepatectomy and control liver from normal rats had negligible activity (0.02-0.03 nmol/h/mg). These results suggest that GlcNAc-T III is induced at the pre-neoplastic stage in liver carcinogenesis and are consistent with the reported presence of bisecting GlcNAc residues in N-glycans from rat and human hepatoma gamma-glutamyl transpeptidase and their absence in enzyme from normal liver of rats and humans (A. Kobata and K. Yamashita (1984) Pure Appl. Chem. 56, 821-832).

Phytohemagglutinin (PHA)-resistant Chinese hamster ovary cell mutants acquire sensitivity to the lectin after fusion with liposomes containing PHA receptor glycoproteins

Phytohemagglutinin receptor glycoproteins have been inserted into phospholipid vesicles and these have been fused with phytohemagglutinin-resistant chinese hamster ovary cells. Our results show that the fused cells acquire "neoreceptors" for the lectin phytohemagglutinin. Fluorescence activated cell sorting analyses show that approximately 40% of the cells fused with the receptor-containing vesicles. Studies with 125I-labelled lectin showed that fused cells bound three times more ligand than untreated mutant cells. Furthermore, lectin receptors were functionally inserted in the mutant cell plasma membrane. Fused cells cultured in the presence of lectin (200 micrograms ml-1) lost rapidly (8 hours or less) their ability to incorporate [3H] thymidine. Whereas mutant cells cultured for 16 hours in the presence of 50-400 micrograms ml-1 of lectin remained viable, fused cells showed a 45% decrease in 3H-labelled nucleotide incorporation. The method described here should be of general applicability for the study of lectin-dependent cytotoxicity in chinese hamster ovary cell lines.

Chinese hamster ovary cell variants resistant to monensin, an ionophoric antibiotic. I. Isolation and altered endocytosis of ricin

Chinese hamster ovary (CHO) cell variants resistant to a carboxylic ionophore, monensin, have been isolated. Two monensin-resistant variants (MonR-31 and MonR-32) showed a three- to fourfold higher resistance to monensin than did CHO. These MonR clones also showed fourfold higher resistance to another carboxylic ionophore, nigericin, and twofold higher resistance to valinomycin. They were also slightly more resistant to other unrelated drugs such as adriamycin, colchicine, bleomycin, and chloroquine, and in particular, they showed about threefold higher resistance to ricin, a toxin of Ricinus communis. MonR clones were found to retain a normal level of [125I]ricin binding, but internalization of [125I]ricin into the MonR clones was one-half or less than with CHO. Present data suggest that drug-resistant clones selected in culture may provide a way to isolate cells with altered response to various bioactive molecules.

Alterations in N-linked oligosaccharides of glycoproteins during rat liver regeneration

[3H]Mannose-labeled glycopeptides in the slices after partial hepatectomy were characterized by column chromatography using Sephadex G-50, DE-52 and Con A-Sepharose, and further by digestion with alpha-mannosidase and endo-beta-N-acetylglucosaminidase H. They contained both 'complex type' and 'high-mannose type' oligosaccharides. A higher proportion of 'complex type' oligosaccharides was contained in regenerating liver 24 h after partial hepatectomy than in control. This tendency was increased gradually with time and was most pronounced at 144 h. In our previous studies, the activities of microsomal N-acetylglucosaminyltransferase towards endogenous and exogenous acceptors at 144 h after partial hepatectomy were shown to exceed most prominently that in control. No differences in the oligosaccharides were observed at 240 h when the deficit of liver had been restored. The oligosaccharides of glycopeptides in the incubation media were mostly 'complex type' and the differences between regenerating liver and control were observed only at 144 h. These results suggest that oligosaccharide processing of glycoproteins is regulated at the transfer step of peripheral N-acetylglucosamine to core oligosaccharides 144 h after partial hepatectomy, and that these alterations in oligosaccharides of glycoproteins may be related to hypertrophy and hyperplasia of hepatic cells in liver regeneration.

Isolation and characterization of peanut agglutinin-resistant embryonal carcinoma cell-surface variants

The isolation and characterization of variant embryonal carcinoma (EC) cells possessing altered cell-surface structures is described. The lectin peanut agglutinin (PNA), which binds to EC cells but not their differentiated derivatives, was used to select the variants. Clones resistant to the cytotoxic effect of PNA were isolated at a frequency of 4 X 10(-5) following mutagenesis. The resistant phenotype was stable in the absence of selection in all eight clones tested. The increased frequency of resistant clones following mutagenesis and the stability of the phenotype suggests a mutational origin. Somatic cell hybrids constructed between wild-type cells and two different PNA-resistant cell lines were sensitive to PNA; this suggests that the resistant phenotype is recessive. Binding assays demonstrated that resistant cells exhibited a twofold to fourfold reduction in the total amount of PNA bound. Together with the recessive behavior of the phenotype, this suggests that resistant cells are deficient for PNA receptors. The PNA-resistant cells also showed reduced binding of monoclonal antibody against stage-specific embryonic antigen 1 (SSEA-1) in indirect cytotoxicity tests. All eight PNA-resistant lines isolated were tumorigenic in syngeneic mice and gave rise to well-differentiated teratocarcinomas. The PNA-resistant cells behaved like their wild-type parents in a cell recognition assay; when incubated in suspension with endodermal cells, they sorted out to form simple embryoid bodies (a core of EC cells surrounded by an endodermal rind). Thus, EC cells can form tumors, differentiate, and recognize differentiated cells in a sorting assay despite a reduction in expression of the embryo-specific cell surface structures (s) that bind PNA and anti-SSEA-1 antibody.

Product-identification and substrate-specificity studies of the GDP-L-fucose:2-acetamido-2-deoxy-beta-D-glucoside (FUC goes to Asn-linked GlcNAc) 6-alpha-L-fucosyltransferase in a Golgi-rich fraction from porcine liver

Golgi-rich membranes from porcine liver have been shown to contain an enzyme that transfers L-fucose in alpha-(1 goes to 6) linkage from GDP-L-fucose to the asparagine linked 2-acetamido-2-deoxy-D-glucose residue of a glycopeptide derived from human alpha 1-acid glycoprotein. Product identification was performed by high resolution, 1H-n.m.r. spectroscopy at 360 MHz and by permethylation analysis. The enzyme has been named GDP-L-fucose: 2-acetamido-2-deoxy-beta-D-glucoside (Fuc goes to Asn-linked GlcNAc) 6-alpha-L-fucosyltransferase, because the substrate requires a terminal beta-(1 goes to 2)-linked GlcNAc residue on the alpha-Man (1 goes to 3) arm of the core. Glycopeptides with this residue were shown to be acceptors whether they contain 3 or 5 Man residues. Substrate-specificity studies have shown that diantennary glycopeptides with two terminal beta-(1 goes to 2)-linked GlcNAc residues and glycopeptides with more than two terminal GlcNAc residues are also excellent acceptors for the fucosyltransferase. An examination of four pairs of glycopeptides differing only by the absence or presence of a bisecting GlcNAc residue in beta-(1 goes to 4) linkage to the beta-linked Man residue of the core showed that the bisecting GlcNAc prevented 6-alpha-L-fucosyltransferase action. These findings probably explain why the oligosaccharides with a high content of mannose and the hybrid oligosaccharides with a bisecting GlCNAc residue that have been isolated to date do not contain a core L-fucosyl residue.

Glycosyl transferases of baby-hamster-kidney (BHK) cells and ricin-resistant mutants. N-glycan biosynthesis

Five cell lines of ricin-resistant BHK cells have been assayed for gross carbohydrate analysis of cellular glycoproteins, for the activities of several glycosidases and of specific glycosyl transferases active in assembly of N-glycans of glycoproteins. The latter enzymes include sialyl transferase using asialofetuin as glycosyl acceptor, fucosyl transferases using asialofetuin and asialoagalactofetuin acceptors, galactosyl transferases using ovalbumin, ovomucoid and N-acetylglucosamine as acceptors and N-acetylglucosaminyl transferases using ovalbumin and glycopeptides as acceptors. Cell line RicR14, binding less ricin than normal BHK cells, contains reduced amounts of sialic acid, galactose and N-acetylglucosamine in cellular glycoproteins and lacks almost completely N-acetylglucosamine transferase I, an essential enzyme in assembly of ricin-binding carbohydrate sequences of N-glycans. These cells also contain reduced levels of N-acetylglucosamine transferase II active on a product of N-acetylglucosamine transferase I action. Sialyl transferase activity is severely depressed while fucose-(alpha 1 leads to 6)-N-acetylglucosamine fucosyl transferase activity is increased. Cell lines RicR15, 17, 19 and 21 showed partial deficiencies in galactosyl and N-acetylglucosaminyl transferases. A hypothesis is put forward to account for the different carbohydrate compositions and ricin binding properties of glycoproteins synthesised by these cells in terms of the determined enzyme defects, the normal level of sialyl transferases detected in RicR15 and RicR21 cells and the elevated levels of sialyl and fucosyl transferases detected in RicR17 and 19 cells. None of the above changes in glycosyl transfer reactions in the RicR cell lines are due to enhanced glycosidase or sugar nucleotidase activities in the mutant cells.

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.

The membrane glycoproteins of the malignant cell

Experiments are described and reviewed demonstrating that the bound carbohydrates of glycoproteins of many forms of malignant cells differ from their normal counterpart. The difference involves many oligosaccharide groups and is essentially quantitative. The characteristics of the difference are discussed. Despite the consistency of the finding its significance is unknown because the function of bound carbohydrates is largely unknown. Some properties of protein-bound carbohydrates that may be of special relevance to malignancy and other pathological processes are considered. The array of structures found in the cell is highly complex but seems to be similar in man, hamster, mouse, chick and fish. On the other hand, the biosynthesis of these structures can be influenced and altered by the environment and by drugs; the cell is tolerant of variation in its bound carbohydrate; microheterogeneity of the carbohydrates is probably the rule rather than the exception; experiments to test the function of bound carbohydrate show only small effects. A role for the bound carbohydrates in evolution is proposed that is consistent with these characteristics. It is also postulated that altered, bound carbohydrates of most glycoprotein does not endanger the life of the cell but may be responsible for involvement and change of many processes some of which permit the malignant cell to divide persistently and to prosper.

CHO cells selected for phytohemagglutinin and con A resistance are defective in both early and late stages of protein glycosylation

The lipid-linked and asparagine-linked oligosaccharides of two lectin-resistant and one parental Chinese hamster ovary (CHO) cell line have been compared by glycosidase digestion and gel filtration analysis of radiolabeled glycopeptides and oligosaccharides. The additional glycosylation defect in a double mutant cell line (CHO-PhaRConAR) selected from a phytohemagglutinin-resistant single mutant cell line (CHO-PhaR) for resistance to concanavalin A has been identified as a block in the synthesis of the lipid-linked oligosaccharide precursor, resulting in a structure with seven instead of the normal nine mannose units. Both the CHO-PhaRConR and CHO-PhaR cells were completely blocked in the synthesis of complex, acidic type oligosaccharides because of a previously demonstrated deficiency in a particular N--acetylglucosamine transferase activity. In addition, an altered collection of neutral type oligosaccharides (Man4-7GlcNAc2) accumulated in the glycoproteins of the double mutant.