Molecular cloning of human and rabbit UDP-N-acetylglucosamine: alpha-3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I

Comparing N-glycan processing in mammalian cell lines to native and engineered lepidopteran insect cell lines

In the past decades, a large number of studies in mammalian cells have revealed that processing of glycoproteins is compartmentalized into several subcellular organelles that process N-glycans to generate complex-type oligosaccharides with terminal N -acetlyneuraminic acid. Recent studies also suggested that processing of N-glycans in insect cells appear to follow a similar initial pathway but diverge at subsequent processing steps. N-glycans from insect cell lines are not usually processed to terminally sialylated complex-type structures but are instead modified to paucimannosidic or oligomannose structures. These differences in processing between insect cells and mammalian cells are due to insufficient expression of multiple processing enzymes including glycosyltransferases responsible for generating complex-type structures and metabolic enzymes involved in generating appropriate sugar nucleotides. Recent genomics studies suggest that insects themselves may include many of these complex transferases and metabolic enzymes at certain developmental stages but expression is lost or limited in most lines derived for cell culture. In addition, insect cells include an N -acetylglucosaminidase that removes a terminal N -acetylglucosamine from the N-glycan. The innermost N -acetylglucosamine residue attached to asparagine residue is also modified with alpha(1,3)-linked fucose, a potential allergenic epitope, in some insect cells. In spite of these limitations in N-glycosylation, insect cells have been widely used to express various recombinant proteins with the baculovirus expression vector system, taking advantage of their safety, ease of use, and high productivity. Recently, genetic engineering techniques have been applied successfully to insect cells in order to enable them to produce glycoproteins which include complex-type N-glycans. Modifications to insect N-glycan processing include the expression of missing glycosyltransferases and inclusion of the metabolic enzymes responsible for generating the essential donor sugar nucleotide, CMP- N -acetylneuraminic acid, required for sialylation. Inhibition of N -acetylglucosaminidase has also been applied to alter N-glycan processing in insect cells. This review summarizes current knowledge on N-glycan processing in lepidopteran insect cell lines, and recent progress in glycoengineering lepidopteran insect cells to produce glycoproteins containing complex N-glycans.

Suppression of a sialyltransferase by antisense DNA reduces invasiveness of human colon cancer cells in vitro

Transfer of terminal alpha 2,6-linked sialic acids to N-glycans is catalyzed by beta-galactoside alpha 2,6-sialyltransferase (ST6Gal I). Expression of ST6Gal I and its products is reportedly increased in colon cancers. To investigate directly the functional effects of ST6Gal I expression, human colon cancer (HT29) cells were transfected with specific antisense DNA. ST6Gal I mRNA and protein were virtually undetectable in six strains of transfected HT29 cells. ST6Gal activity was reduced to 14% of control (P<0.005) in transfected cells. Expression of terminal alpha 2,6- and alpha 2,3-linked sialic acids, and unmasked N-acetyllactosamine oligosaccharides, respectively, was assessed using flow cytometry and fluoresceinated Sambucus nigra, Maackia amurensis and Erythrina cristagalli lectins. Results indicated a major reduction in expression of alpha 2,6-linked sialic acids and counterbalancing increase in unmasked N-acetyllactosamines in antisense DNA-transfected cells, without altered expression of alpha 2,3-linked sialic acids or ganglioside profiles. The ability of transfected cells to form colonies in soft agar and to invade extracellular matrix material (Matrigel), respectively, in vitro was reduced by approx. 98% (P<0.0001) and more than 3-fold (P<0.005) compared to parental HT29 cells. These results indicate that N-glycans bearing terminal alpha 2,6-linked sialic acids may enhance the invasive potential of colon cancer cells.

An Arabidopsis thaliana cDNA complements the N-acetylglucosaminyltransferase I deficiency of CHO Lec1 cells

N-Acetylglucosaminyltransferase I (GlcNAcT-I, EC 2.4.1.101) is the enzyme which initiates the formation of complex N-linked glycans in eukaryotes by transforming GlcNAc to the oligo-mannosyl acceptor Man(5)GlcNAc(2)-Asn. The enzymatic activity and the structure that is synthesised by this enzyme are found in animals and plants but not in yeast. cDNAs encoding the enzyme have already been cloned from several mammals and the nematode Caenorhabditis elegans. In this article the cloning of an Arabidopsis thaliana GlcNAcT-I cDNA with homology to animal cDNAs is described. By expression of the plant cDNA in CHO Lec1 cells, a mammalian cell line deficient in GlcNAcT-I, it was shown that it encodes an active enzyme with the same enzymatic activity as the animal homologue. It has already been shown that a human GlcNAcT-I can complement an A. thaliana mutant (cgl-1). Here it is shown that the reverse is also true, the plant glycosyltransferase is able to complement a mammalian mutant (Lec1) deficient in GlcNAcT-I.

Downregulation of a human colonic sialyltransferase by a secondary bile acid and a phorbol ester

Fecal constituents such as bile acids and increased sialylation of membrane glycoproteins by alpha-2,6-sialyltransferase (HST6N-1) may contribute to colorectal tumorigenesis. We hypothesized that bile acids and phorbol ester [12-O-tetradecanoylphorbol-13-acetate (TPA)] would upregulate HST6N-1 in colonic cells. However, deoxycholate (DOC) (300 mumol/l), a secondary bile acid, and TPA (20 ng/ml) decreased expression of an approximately 100-kDa glycoprotein bearing alpha-2,6-linked sialic acid in a colon cancer cell line (T84) in vitro. HST6N-1 mRNA levels were reduced approximately 80% by treatment (< or = 24 h) with DOC or TPA but not by cholate, a primary bile acid. Treatment (24 h) with DOC or TPA decreased activity of this enzyme to 30% and 13% of control, respectively. These effects of DOC and TPA were transcriptional and were mediated by Ca2+ and protein kinase C, respectively. Thus DOC and TPA both downregulated, and did not upregulate, alpha-2,6-sialyltransferase expression in vitro, but by different transduction pathways. As colorectal tumors grow, their progressive removal from the fecal milieu that normally downregulates this enzyme may favor invasion and metastasis.

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.

Complementation of an Arabidopsis thaliana mutant that lacks complex asparagine-linked glycans with the human cDNA encoding N-acetylglucosaminyltransferase I

N-Acetylglucosaminyltransferase I (EC 2.4.1.101) initiates the conversion of high-mannose asparagine-linked glycans to complex asparagine-linked glycans in plant as well as in animal cells. This Golgi enzyme is missing in the cgl mutant of Arabidopsis thaliana, and the mutant cells are unable to synthesize complex glycans. Transformation of cells from the mutant plants with the cDNA encoding human N-acetylglucosaminyltransferase I restores the wild-type phenotype of the plant cells. Fractionation of the subcellular organelles on isopycnic sucrose gradients shows that the human enzyme in the complemented cells bands at the same density, 1.14 g/cm3, typical of Golgi cisternae, as the enzyme in the wild-type plant cells. These results demonstrate that complementation results from the presence of the human enzyme in the plant Golgi apparatus, where it is functionally integrated into the biosynthetic machinery of the plant cell. In addition, given the evolutionary distance between plants and mammals and the great diversity of glycoproteins that are modified in each, there is probably no specific recognition between this Golgi enzyme and the polypeptide domains of the proteins it modifies.