Cell-cell interactions influence oligosaccharide modifications on mucins and other large glycoproteins

Aggregation of a Tetrasaccharide Acceptor Observed by NMR: Synthesis of Pentasaccharide Fragments of the Le(a)Le(x) Tumor-Associated Hexasaccharide Antigen

We report the synthesis of a tetrasaccharide and two pentasaccharide fragments of the Le(a)Le(x) tumor-associated carbohydrate antigen α-L-Fuc-(1→4)-[β-D-Gal-(1→3)]-β-D-GlcNAc-(1→3)-β-D-Gal-(1→4)-[α-L-Fuc-(1→3)]-β-D-GlcNAc-(1→OR). The choice of protecting groups permitted a one-step global deprotection (Na/NH3(l)). The protected chlorohexyl glycoside pentasaccharide was the precursor to the hexyl glycoside, to be used as a soluble inhibitor, and the aminohexyl glycoside analogue, to be conjugated to proteins for surface immobilization and immunization experiments. We observed that a linear tetrasaccharide that contained two N-acetylglucosamine residues and a free OH group gave two distinct sets of (1)H NMR signals when the data were acquired in deuterated chloroform. Data acquisition at variable concentrations and variable temperatures suggests that the second set of NMR signals results from aggregation of the tetrasaccharide driven by the formation of intermolecular H-bonds involving the NHAc. While the formation of intra- and intermolecular H-bonds involving N-acetylgucosamine residues has been reported in non-H-bonding solvents, this is, to our knowledge, the first time that these have lead to the appearance of two distinct sets of signals in the NMR spectra. This aggregation may explain the lack of reactivity observed when an attempt is made to glycosylate such an acceptor using non-H-bonding solvents such as dichloromethane.

Challenging deprotection steps during the synthesis of tetra- and pentasaccharide fragments of the Le(a)Le(x) tumor-associated hexasaccharide antigen

We report the convergent synthesis of two novel tetrasaccharide and two novel pentasaccharide fragments of the Le(a)Le(x) TACA: the tetrasaccharides contain neither the galactose at the Le(a) nonreducing end nor the fucose at the Le(x) reducing end; the pentasaccharides only lack the galactose residue at the Le(a) nonreducing end. Two of the analogues were prepared as hexyl glycosides to be used in NMR experiments and as soluble inhibitors in binding studies and two as 6-aminohexyl glycosides to be conjugated to carrier proteins. Our strategy relied on stepwise extensions using excess monosaccharide glycosyl donors (trichloroacetimidates and thioglycosides) in sequential glycosylation reactions. The protecting groups were chosen to limit the number of deprotection steps required to obtain the final derivatives. While this strategy ensured that all glycosylation reactions proceeded in very good yields (70-84%), deprotection of the oligosaccharide intermediates was challenging. Global deprotection using Birch metal dissolving conditions did not remove the tert-butyldiphenylsilyl group, which indeed was incompatible with such reaction conditions. Attempts to remove the TBDPS with tetrabutylammonium fluoride was unsuccessful and led to a complex mixture of compounds that could not be separated. The desired hexyl and aminohexyl tetrasaccharides were finally obtained after four- and five-step deprotection sequences, respectively. Deprotection of the pentasaccharide intermediate to give the hexyl and aminohexyl analogues also led to unexpected results. Indeed, during Zemplén deacylation, a chloroacetamide chlorine atom was displaced by methoxide ions leading to the corresponding methoxyacetamide. Once the chloroacetamide was fully reduced to an acetamide the pentasaccharides were obtained in four and five steps, respectively.

Stochastic searches and NMR experiments on four Lewis A analogues: NMR experiments support some flexibility around the fucosidic bond

We have compared the conformational behavior of three Le(a) analogues with that of Le(a) using stochastic searches (MOE2005) and selective ROESY experiments. In the analogues either or both the β-d-Gal and α-l-Fuc residues were replaced by β-d-Glc and α-l-Rha units, respectively. All compounds showed similar behavior and even though four conformational families were identified, the calculations and NMR experiments support that the 'stacked conformation' known for Le(a) is predominant for all analogues. Interestingly, ROESY showed a correlation between H-1 Fuc/Rha and H-3 GlcNAc which, although small, could be seen in all analogues. For two compounds, the corresponding distance was measured and found to be shorter (∼3.7Å) than that found in the global minimum (4.5Å). While one published study suggests some motion around the fucosidic bond, this constitutes the first experimental evidence supporting such flexibility. Our MD simulation (Amber10/Glycam06) on Le(a) was in full agreement with previous studies which described a rigid conformation for this branched trisaccharide. Thus, NMR seems to indicate that these dynamic studies are underestimating flexibility around the fucosidic bond.

Synthesis of Lewis A trisaccharide analogues in which D-glucose and L-rhamnose replace D-galactose and L-fucose, respectively

In our effort to design a safe anti-cancer vaccine based on the tumor associated carbohydrate antigen Le(a)Le(x), we are studying the cross-reactivity between the Le(a) natural trisaccharide antigen and analogues in which the L-fucose, D-galactose, and/or D-glucosamine residues are replaced by L-rhamnose or D-glucose, respectively. We describe here the chemical synthesis of two such Le(a) trisaccharide analogues. In one trisaccharide, D-glucose replaces D-galactose and in the second analogue L-rhamnose and D-glucose replace L-fucose and D-galactose, respectively. Introduction of the rhamnose and fucose moiety onto the poorly reactive 4-OH group of the N-acetylglucosamine residue in a disaccharide acceptor was successful after bis-N-acetylation of the amine group. These analogues will be used in competitive binding experiments with anti-Le(a) antibodies and their solution conformations will be studied.

Identifying novel homozygous deletions by microsatellite analysis and characterization of tumor suppressor candidate 1 gene, TUSC1, on chromosome 9p in human lung cancer

Loss of heterozygosity (LOH) studies indicate that genetic alterations of chromosome 9p occur in numerous tumor types, suggesting the presence of tumor suppressor genes (TSGs) on chromosome 9p critical in carcinogenesis. Our previous LOH analyses in primary lung tumors led us to propose that chromosome 9p harbors other TSGs important in lung tumorigenesis. In this study, 30 non-small-cell lung cancer and 12 small-cell lung cancer cell lines were screened with 55 markers to identify new regions of homozygous deletion (HD) on chromosome 9p. Three novel noncontiguous homozygously deleted regions were detected and ranged in size from 840 kb to 7.4 Mb. One gene identified in the deletion at D9S126, TUSC1 (tumor suppressor candidate 1), is an intronless gene. Multiplex polymerase chain reaction and Southern blot confirmed the HD of TUSC1. Northern blot analysis of TUSC1 demonstrated two transcripts of approximately 2 and 1.5 kb that are likely generated by alternative polyadenylation signals. Both transcripts are expressed in several human tissues and share an open-reading frame encoding a peptide of 209 amino acids. Analysing cell line cDNAs by reverse transcriptase (RT)-PCR demonstrated downregulation of TUSC1 in cell lines with or without HDs, suggesting that TUSC1 may play a role in lung tumorigenesis.

Sulfated lewis X determinants as a major structural motif in glycans from LS174T-HM7 human colon carcinoma mucin

This article describes oligosaccharide structures of mucin isolated from nude mouse xenograft tumors produced by LS174T-HM7 cells, a subline of the human colon carcinoma LS174T with higher metastatic tendency and higher mucin production. A striking feature of the oligosaccharides of the LS174T-HM7 xenograft tumor mucin was a predominance of sulfated Lewis X determinants: HSO3-Galbeta1-4(Fucalpha1-3)GlcNAc. In addition to one previously known saccharide with one sulfated Lewis X determinant, the HM7 xenograft tumor mucin contained multiple novel structures containing one, two, or three sulfated Lewis X determinants. This determinant, known to act as a selectin ligand, has been found previously in minor saccharide components of human milk as well as mucins, but never before as a predominant structure in one mucin source.