ABSTRACT
The Sialyl-Tn antigen (Neu5Acalpha2-6GalNAc-O-Ser/Thr) is highly expressed in several human carcinomas and is associated with carcinoma aggressiveness and poor prognosis. We characterized two human sialyltransferases, CMP-Neu5Ac:GalNAc-R alpha2,6-sialyltransferase (ST6GalNAc)-I and ST6GalNAc-II, that are candidate enzymes for Sialyl-Tn synthases. We expressed soluble recombinant hST6GalNAc-I and hST6GalNAc-II and characterized the substrate specificity of both enzymes toward a panel of glycopeptides, glycoproteins, and other synthetic glycoconjugates. The recombinant ST6GalNAc-I and ST6GalNAc-II showed similar substrate specificity toward glycoproteins and GalNAcalpha-O-Ser/Thr glycopeptides, such as glycopeptides derived from the MUC2 mucin and the HIVgp120. We also observed that the amino acid sequence of the acceptor glycopeptide contributes to the in vitro substrate specificity of both enzymes. We additionally established a gastric cell line, MKN45, stably transfected with the full length of either ST6GalNAc-I or ST6GalNAc-II and evaluated the carbohydrate antigens expression profile induced by each enzyme. MKN45 transfected with ST6GalNAc-I showed high expression of Sialyl-Tn, whereas MKN45 transfected with ST6GalNAc-II showed the biosynthesis of the Sialyl-6T structure [Galbeta1-3 (Neu5Acalpha2-6)GalNAc-O-Ser/Thr]. In conclusion, although both enzymes show similar in vitro activities when Tn antigen alone is available, whenever both Tn and T antigens are present, ST6GalNAc-I acts preferentially on Tn antigen, whereas the ST6GalNAc-II acts preferentially on T antigen. Our results show that ST6GalNAc-I is the major Sialyl-Tn synthase and strongly support the hypothesis that the expression of the Sialyl-Tn antigen in cancer cells is due to ST6GalNAc-I activity.
Subject(s)
Antigens, Tumor-Associated, Carbohydrate/biosynthesis , Sialyltransferases/metabolism , Stomach Neoplasms/metabolism , Carbohydrate Sequence , Cell Line, Tumor , Enzyme-Linked Immunosorbent Assay , Glycopeptides/metabolism , Humans , Molecular Sequence Data , Recombinant Proteins/biosynthesis , Recombinant Proteins/genetics , Recombinant Proteins/isolation & purification , Recombinant Proteins/metabolism , Sialyltransferases/biosynthesis , Sialyltransferases/genetics , Sialyltransferases/isolation & purification , Stomach Neoplasms/enzymology , Stomach Neoplasms/genetics , Substrate Specificity , TransfectionABSTRACT
We have compared the site-by-site N-glycosylation status of human lactoferrin (Lf) produced in maize, a monocotyledon, and in tobacco, used as a model dicotyledon. Maize and tobacco plants were stably transformed and recombinant Lf was purified from both seeds and leaves. N-glycopeptides were generated by trypsin digestion of recombinant Lf and purified by reverse-phase HPLC. The N-glycosylation pattern of each site was determined by mass spectrometry. Our results indicated that the N-glycosylation patterns of recombinant Lf produced in maize and tobacco share common structural features. In particular, both N-glycosylation sites of each recombinant Lf are mainly substituted by typical plant paucimannose-type N-glycans, with beta1,2-xylose and alpha1,3-linked fucose at the proximal N-acetylglucosamine. However, tobacco Lf shows a significant amount of processed N-glycans with one or two beta1,2GlcNAc linked to the trimannose core, which are weakly expressed in maize Lf. Finally, no Lewisa epitope was observed on tobacco Lf.
Subject(s)
Lactoferrin/biosynthesis , Lactoferrin/chemistry , Lactoferrin/genetics , Nicotiana/genetics , Zea mays/genetics , Asparagine/analysis , Glycopeptides/chemistry , Glycopeptides/isolation & purification , Glycosylation , Humans , Recombinant Proteins/chemistry , Recombinant Proteins/metabolism , Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization , Transformation, Genetic , Trypsin/metabolismABSTRACT
BLAST analysis of the human and mouse genome sequence databases using the sequence of the human CMP-sialic acid:beta-galactoside alpha-2,6-sialyltransferase cDNA (hST6Gal I, EC2.4.99.1) as a probe allowed us to identify a putative sialyltransferase gene on chromosome 2. The sequence of the corresponding cDNA was also found as an expressed sequence tag of human brain. This gene contained a 1590 bp open reading frame divided in five exons and the deduced amino-acid sequence didn't correspond to any sialyltransferase already known in other species. Multiple sequence alignment and subsequent phylogenic analysis showed that this new enzyme belonged to the ST6Gal subfamily and shared 48% identity with hST6Gal-I. Consequently, we named this new sialyltransferase ST6Gal II. A construction in pFlag vector transfected in COS-7 cells gave raise to a soluble active form of ST6Gal II. Enzymatic assays indicate that the best acceptor substrate of ST6Gal II was the free disaccharide Galbeta1-4GlcNAc structure whereas ST6Gal I preferred Galbeta1-4GlcNAc-R disaccharide sequence linked to a protein. The alpha2,6-linkage was confirmed by the increase of Sambucus nigra agglutinin-lectin binding to the cell surface of CHO transfected with the cDNA encoding ST6Gal II and by specific sialidases treatment. In addition, the ST6Gal II gene showed a very tissue specific pattern of expression because it was found essentially in brain whereas ST6Gal I gene is ubiquitously expressed.
