Disubstituted naphthyl ß-D-xylopyranosides: Synthesis, GAG priming, and histone acetyltransferase (HAT) inhibition.

Xylosides are a group of compounds that can induce glycosaminoglycan (GAG) chain synthesis independently of a proteoglycan core protein. We have previously shown that the xyloside 2-(6-hydroxynaphthyl)ß-D-xylopyranoside has a tumor-selective growth inhibitory effect both in vitro and in vivo, and that the effect in vitro was correlated to a reduction in histone H3 acetylation. In addition, GAG chains have previously been reported to inhibit histone acetyltransferases (HAT). To investigate if xylosides, or the corresponding xyloside-primed GAG chains, can be used as HAT inhibitors, we have synthesized a series of naphthoxylosides carrying structural motifs similar to the aromatic moieties of the known HAT inhibitors garcinol and curcumin, and studied their biological activities. Here, we show that the disubstituted naphthoxylosides induced GAG chain synthesis, and that the ones with at least one free phenolic group exhibited moderate HAT inhibition in vitro, without affecting histone H3 acetylation in cell culture. The xyloside-primed GAG chains, on the other hand, had no effect on HAT activity, possibly explaining why the effect of the xylosides on histone H3 acetylation was absent in cell culture as the xylosides were recruited for GAG chain synthesis. Further investigations are required to find xylosides that are effective HAT inhibitors or xylosides producing GAG chains with HAT inhibitory effects.

Chemistry of xylopyranosides.

Xylose is one of the few monosaccharidic building blocks that are used by mammalian cells. In comparison with other monosaccharides, xylose is rather unusual and, so far, only found in two different mammalian structures, i.e. in the Notch receptor and as the linker between protein and glycosaminoglycan (GAG) chains in proteoglycans. Interestingly, simple soluble xylopyranosides can not only initiate the biosynthesis of soluble GAG chains but also function as inhibitors of important enzymes in the biosynthesis of proteoglycans. Furthermore, xylose is a major constituent of hemicellulosic xylans and thus one of the most abundant carbohydrates on Earth. Altogether, this has spurred a strong interest in xylose chemistry. The scope of this review is to describe synthesis of xylopyranosyl donors, as well as protective group chemistry, modifications, and conformational analysis of xylose.

Exploration of the active site of ß4GalT7: modifications of the aglycon of aromatic xylosides.

Proteoglycans (PGs) are macromolecules that consist of long linear polysaccharides, glycosaminoglycan (GAG) chains, covalently attached to a core protein by the carbohydrate xylose. The biosynthesis of GAG chains is initiated by xylosylation of the core protein followed by galactosylation by the galactosyltransferase ß4GalT7. Some ß-d-xylosides, such as 2-naphthyl ß-d-xylopyranoside, can induce GAG synthesis by serving as acceptor substrates for ß4GalT7 and by that also compete with the GAG synthesis on core proteins. Here we present structure-activity relationships for ß4GalT7 and xylosides with modifications of the aromatic aglycon, using enzymatic assays, cell studies, and molecular docking simulations. The results show that the aglycons reside on the outside of the active site of the enzyme and that quite bulky aglycons are accepted. By separating the aromatic aglycon from the xylose moiety by linkers, a trend towards increased galactosylation with increased linker length is observed. The galactosylation is influenced by the identity and position of substituents in the aromatic framework, and generally, only xylosides with ß-glycosidic linkages function as good substrates for ß4GalT7. We also show that the galactosylation ability of a xyloside is increased by replacing the anomeric oxygen with sulfur, but decreased by replacing it with carbon. Finally, we propose that reaction kinetics of galactosylation by ß4GalT7 is dependent on subtle differences in orientation of the xylose moiety.

Exploration of conformational flexibility and hydrogen bonding of xylosides in different solvents, as a model system for enzyme active site interactions.

The predominantly populated conformation of carbohydrates in solution does not necessarily represent the biologically active species; rather, any conformer accessible without too large an energy penalty may be present in a biological pathway. Thus, the conformational preferences of a naphthyl xyloside, which initiates in vivo synthesis of antiproliferative glycosaminoglycans, have been studied by using NMR spectroscopy in a variety of solvents. Equilibria comprising the conformations (4)C1, (2)SO and (1)C4 were found, with a strong dependence on the hydrogen bonding ability of the solvent. Studies of fluorinated analogues revealed a direct hydrogen bond from the hydroxyl group at C2 to the fluorine atom at C4 by a (1h)JF4,HO2 coupling. Hydrogen bond directionality was further established via comparisons of fluorinated levoglucosan molecules.

Synthesis, conformation and biology of naphthoxylosides.

Proteoglycans (PG) are polyanionic proteins consisting of a core protein substituted with carbohydrate chains, that is, glycosaminoglycans (GAG). The biosynthesis of GAG can be manipulated by simple xylosides carrying hydrophobic aglycons, which can enter the cell and initiate the biosynthesis. While the importance of the aglycon is well investigated, there is far less information on the effect of modifications in the xylose residue. We have developed a new synthetic protocol, based on acetal protection and selective benzylation, for modification of the three hydroxyl groups in xylose. Thus we have synthesized twelve analogs of 2-naphthyl ß-d-xylopyranoside (XylNap), where each hydroxyl group has been epimerized or replaced by methoxy, fluoro, or hydrogen. To gain more information about the properties of xylose, conformational studies were made on some of the analogs. It was found that the (4)C(1) conformation is highly predominant, accompanied by a nonnegligible population of the (2)S(0) conformation. However, deoxygenation at C3 results in a large portion of the (1)C(4) conformation. The GAG priming ability and proliferation activity of the twelve analogs, were investigated using a matched pair of human breast fibroblasts and human breast carcinoma cells. None of the analogs initiated the biosynthesis of GAG, but an inhibitory effect on endogenous PG production was observed for analogs fluorinated or deoxygenated at C4. From our data it seems reasonable that all three hydroxyl groups in XylNap are essential for the priming of GAG chains and for selective toxicity for tumor cells.