Synthesis of 4-deoxy analogues of 2-acetamido-2-deoxy-D-glucose and 2-acetamido-2-deoxy-D-xylose and their effects on glycoconjugate biosynthesis

Inhibition of microbial β-N-acetylhexosaminidases by 4-deoxy- and galacto-analogues of NAG-thiazoline

NAG-thiazoline is a well-established competitive inhibitor of two physiologically relevant glycosidase families-β-N-acetylhexosaminidases (GH20) and β-N-acetylglucosaminidases (GH84). Based on the different substrate flexibilities of these enzyme groups, we designed and synthesized the 4-deoxy derivative of NAG-thiazoline aiming at the selective inhibition of GH20 β-N-acetylhexosaminidases. One GH84 and two GH20 microbial glycosidases were employed as model enzymes for the inhibition assays. Surprisingly, the new compound 4-deoxy-thiazoline exhibited no activity inhibition with either of the enzyme families of interest. Unlike with the substrates, the 4-hydroxyl group of the inhibitor's sugar ring seems to be crucial for binding the inhibitor to the active sites of these enzymes.

Interfering with UDP-GlcNAc metabolism and heparan sulfate expression using a sugar analogue reduces angiogenesis

Heparan sulfate (HS), a long linear polysaccharide, is implicated in various steps of tumorigenesis, including angiogenesis. We successfully interfered with HS biosynthesis using a peracetylated 4-deoxy analogue of the HS constituent GlcNAc and studied the compound's metabolic fate and its effect on angiogenesis. The 4-deoxy analogue was activated intracellularly into UDP-4-deoxy-GlcNAc, and HS expression was inhibited up to ∼96% (IC50 = 16 μM). HS chain size was reduced, without detectable incorporation of the 4-deoxy analogue, likely due to reduced levels of UDP-GlcNAc and/or inhibition of glycosyltransferase activity. Comprehensive gene expression analysis revealed reduced expression of genes regulated by HS binding growth factors such as FGF-2 and VEGF. Cellular binding and signaling of these angiogenic factors was inhibited. Microinjection in zebrafish embryos strongly reduced HS biosynthesis, and angiogenesis was inhibited in both zebrafish and chicken model systems. All of these data identify 4-deoxy-GlcNAc as a potent inhibitor of HS synthesis, which hampers pro-angiogenic signaling and neo-vessel formation.

A 4-deoxy analogue of N-acetyl-D-glucosamine inhibits heparan sulphate expression and growth factor binding in vitro

Heparan sulphate (HS) is a long, linear polysaccharide, which has a basic backbone of -beta1-4GlcA-alpha1-4GlcNAc- units. The involvement of HS in many steps of tumourigenesis, including growth and angiogenesis, makes it an appealing target for cancer therapy. To target the biosynthesis of HS by interfering with its chain elongation, a 4-deoxy analogue of N-acetyl-D-glucosamine (4-deoxy-GlcNAc) was synthesized. Using immunocytochemistry and agarose gel electrophoresis it was shown that incubation with the 4-deoxysugar resulted in a dose dependent reduction of HS expression of MV3 melanoma cells, 1 mM resulting in an almost nullified HS expression. The parent sugar GlcNAc had no effect. 4-deoxysugar treated cells were viable and proliferated at the same rate as control cells. Other glycan structures appeared to be only mildly affected, as staining by various lectins was generally not or only modestly inhibited. At 1 mM of the 4-deoxysugar, the capacity of cells to bind the HS-dependent pro-angiogenic growth factors FGF-2 and VEGF was greatly compromised. Using an in vitro angiogenesis assay, 4-deoxysugar treated endothelial cells showed a sharp reduction of FGF-2-induced sprout formation. Combined, these data indicate that an inexpensive, easily synthesized, water-soluble monosaccharide analogue can interfere with HS expression and pro-angiogenic growth factor binding.

UDP-Gal: GlcNAc-R beta1,4-galactosyltransferase--a target enzyme for drug design. Acceptor specificity and inhibition of the enzyme

Galactosyltransferases are important enzymes for the extension of the glycan chains of glycoproteins and glycolipids, and play critical roles in cell surface functions and in the immune system. In this work, the acceptor specificity and several inhibitors of bovine beta1,4-Gal-transferase T1 (beta4GalT, EC 2.4.1.90) were studied. Series of analogs of N-acetylglucosamine (GlcNAc) and GlcNAc-carrying glycopeptides were synthesized as acceptor substrates. Modifications were made at the 3-, 4- and 6-positions of the sugar ring of the acceptor, in the nature of the glycosidic linkage, in the aglycone moiety and in the 2-acetamido group. The acceptor specificity studies showed that the 4-hydroxyl group of the sugar ring was essential for beta4GalT activity, but that the 3-hydroxyl could be replaced by an electronegative group. Compounds having the anomeric beta-configuration were more active than those having the alpha-configuration, and O-, S- and C-glycosyl compounds were all active as substrates. The aglycone was a major determinant for the rate of Gal-transfer. Derivatives containing a 2-naphthyl aglycone were inactive as substrates although quinolinyl groups supported activity. Several compounds having a bicyclic structure as the aglycone were found to bind to the enzyme and inhibited the transfer of Gal to control substrates. The best small hydrophobic GlcNAc-analog inhibitor was found to be 1-thio-N-butyrylGlcNbeta-(2-naphthyl) with a K(i) of 0.01 mM. These studies help to delineate beta4GalT-substrate interactions and will aid in the development of biologically applicable inhibitors of the enzyme.

A comparative study of the influence of some protecting groups on the reactivity of d-glucosamine acceptors with a galactofuranosyl donor

Competitive glycosylation experiments with a galactofuranosyl trichloroacetimidate donor were performed with glucosamine acceptors having a free 4-OH group and carrying different protecting groups at N-2, O-3, and O-6. The most reactive acceptor is the N-dimethylmaleimido 3,6-di-O-benzylated derivative (6c), which reacts even faster than the oxazolidinone 1a. Molecular orbital calculations have helped to rationalize these experimental facts in terms of a hard-hard reaction occurring between the donor and the acceptor.

Biological evaluation of a series of 2-acetamido-2-deoxy-D-glucose analogs towards cellular glycosaminoglycan and protein synthesis in vitro

Using primary hepatocytes in culture, various 2-acetamido-2-deoxy-D-glucose (GlcNAc) analogs were examined for their effects on the incorporation of D-[3H]glucosamine, [35S]sulfate, and L-[14C]leucine into cellular glycoconjugates. A series of acetylated GlcNAc analogs, namely methyl 2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-alpha-(3) and beta-D-glucopyranoside (4) and 2-acetamido-1,3,4,6-tetra-O-acetyl-2-deoxy-D-glucopyranose (5), exhibited a concentration-dependent reduction of D-[3H]glucosamine, but not of [35S]sulfate incorporation into isolated glycosaminoglycans (GAGs), without affecting L-[14C]leucine incorporation into total protein synthesis. These results suggest that analogs 3-5 exhibit an inhibitory effect on D-[3H]glucosamine incorporation into isolated GAGs by diluting the specific activity of cellular D-[3H]glucosamine and by competing for the same metabolic pathways. In the case of the corresponding series of 4-deoxy-GlcNAc analogs, namely methyl 2-acetamido-3,6-di-O-acetyl-2,4-dideoxy-alpha-(6) and beta-D-xylo-hexopyranoside (7) and 2-acetamido-1,3,6-tri-O-acetyl-2,4-dideoxy-D-xylo-hexopyranose (8), compound 8 at 1.0 mM exhibited the greatest reduction of D-[3H]glucosamine and [35S]sulfate incorporation into isolated GAGs, namely to approximately 7% of controls, and a moderate inhibition of total protein synthesis, namely to 60% of controls. Exogenous uridine was able to restore the inhibition of total protein synthesis by compound 8 at 1.0 mM. Isolated GAGs from cultures treated with compound 8 were shown to be smaller in size (approximately 40 kDa) than for control cultures (approximately 77 kDa). These results suggest that the inhibitory effects of compound 8 on cellular GAG synthesis may be mediated by the incorporation of a 4-deoxy moiety into GAGs resulting in premature chain termination and/or by its serving as an enzymatic inhibitor of the normal sugar metabolites. The inhibition of total protein synthesis from cultures treated with compound 8 suggests a uridine trapping mechanism which would result in the depletion of UTP pools and cause the inhibition of total protein synthesis. A 1-deoxy-GlcNAc analog, namely 2-acetamido-3,4,6-tri-O-acetyl-1,5-anhydro-2-deoxy-D-glucitol (9), also exhibited a reduction in both D-[3H]glucosamine and [35S]sulfate incorporation into isolated GAGs by 19 and 57%, of the control cells, respectively, at 1.0 mM without affecting total protein synthesis. The inability of compound 9 to form a UDP-sugar and, hence, be incorporated into GAGs presents another metabolic route for the inhibition of cellular GAG synthesis. Potential metabolic routes for each analog's effects are presented.

Inhibition of amyloid A amyloidogenesis in vivo and in tissue culture by 4-deoxy analogues of peracetylated 2-acetamido-2-deoxy-alpha- and beta-d-glucose: implications for the treatment of various amyloidoses

Two novel sugars, 2-acetamido-1,3,6-tri-O-acetyl-2,4-dideoxy-alpha- and beta-D-xylo-hexopyranoses, have been synthesized and their effects on heparan sulfate biosynthesis using primary mouse hepatocytes in tissue culture have been assessed. At concentrations of 0.1 and 1.0 mmol/L a mixture of both anomers significantly inhibited the biosynthesis of heparan sulfate by 60% and 99%, respectively. At 1.0 mmol/L the average molecular weight of the heparan sulfate synthesized is reduced from 77 kd to 40 kd. The biosynthetic inhibition is apparent within 1 hour (the earliest time point examined) of exposure of the hepatocytes to the analogues and appears virtually complete throughout a 24-hour incubation period. Using a radiolabeled version of the beta-anomer we demonstrate that the analogue is incorporated into growing heparan sulfate chains. The nature of the analogue, the quantity of analogue isotope incorporated, and the reduction in the size of the heparan sulfate polysaccharide are consistent with UDP activation and incorporation of the analogue and truncation of the growing heparan sulfate chain. At 0.1 mmol/L, and in the presence of a constant concentration of serum amyloid A (the precursor to AA amyloid), each analogue inhibited amyloid deposition (by 95 to 99%) in a tissue culture model of AA amyloidogenesis. At 6 mg/dose twice daily each analogue inhibited in vivo splenic AA amyloid deposition by 65 to 70% when using a rapid induction model of mouse AA amyloidogenesis. These data indicate that polysaccharides, such as heparan sulfate, play an integral part in the pathogenesis of AA amyloid deposition, and potentially other forms of amyloid. These data support our previous work that demonstrated that agents that mimic aspects of heparan sulfate structure and that interfere with heparan sulfate:amyloid protein binding inhibit AA amyloid deposition. They emphasize that heparan sulfate likely plays a critical role in amyloidogenesis, and compounds that interfere with heparan sulfate biosynthesis may provide leads for the development of anti-amyloid therapeutic agents.

Synthesis of two oligosaccharides, the GPI anchor glycans from S. cerevesiae and A. fumigatus

Two oligosaccharides, alpha-D-Manp-(1-->2)-alpha-D-Manp-(1-->2)-alpha-D-Manp-(1-->6)-alpha-D-Manp-(1-->4)-alpha-D-GlcpNAc (I) and alpha-D-Manp-(1-->3)-alpha-D-Manp-(1-->2)-alpha-D-Manp-(1-->2)-alpha-D-Manp-(1-->6)-alpha-D-Manp-(1-->4)-alpha-D-GlcpNAc (II), the glycosylphosphatidylinositol (GPI) anchor glycans from S. cerevesiae and A. fumigatus were synthesized as their methyl glycosides in a regio- and stereoselective manner. The pentasaccharide I was obtained from 6-O-selective glycosylation of methyl 2,3-di-O-benzoyl-alpha-D-mannopyranosyl-(1-->4)-2-acetamido-3,6-di-O-benzoyl-2-deoxy-alpha-D-glucopyranoside (8) with 2-O-acetyl-3,4,6-tri-O-benzoyl-alpha-D-mannopyranosyl-(1-->2)-3,4,6-tri-O-benzoyl-alpha-D-mannopyranosyl trichloroacetimidate (9), followed by benzoylation, deacetylation, and mannosylation, and then by deprotection. The hexasaccharide (II) was obtained via condensation of allyl 3,4,6-tri-O-benzoyl-alpha-D-mannopyranosyl-(1-->2)-3,4,6-tri-O-benzoyl-alpha-D-mannopyranoside (17) with 2,3,4,6-tetra-O-benzoyl-alpha-D-mannopyranosyl-(1-->3)-2,4,6-tri-O-acetyl-alpha-D-mannopyranosyl trichloroacetimidate (16), followed by deallylation, trichloroacetimidation, and coupling with acceptor (8), and finally by deprotection.

Novel glycosaminoglycan precursors as anti-amyloid agents, part III

In vivo amyloids consist of two classes of constituents. The first is the disease-defining protein, e.g., amyloid beta (Abeta) in Alzheimer's disease (AD). The second is a set of common structural components that usually are the building blocks of basement membrane (BM), a tissue structure that serves as a scaffold onto which cells normally adhere. In vitro binding interactions between one of these BM components and amyloidogenic proteins rapidly change the conformation of the amyloidogenic protein into amyloid fibrils. The offending BM component is a heparan sulfate (HS) proteoglycan (HSPG), part of which is protein, and the remainder is a specific linear polysaccharide that is the portion responsible for binding and imparting the typical amyloid structure to the amyloid precursor protein/peptide. Our past work has demonstrated that agents that inhibit the binding between HS and the amyloid precursor are effective antiamyloid compounds both in vitro and in vivo. Similarly, 4-deoxy analogs of glucosamine (a precursor of HS biosynthesis) are effective antiamyloid compounds both in culture and in vivo. Our continuing work concerns (1) the testing of our 4-deoxy compounds in a mouse transgenic model of AD, and (2) the continuing design and synthesis of modified sugar precursors of HS, which when incorporated into the polysaccharide will alter its structure so that it loses its amyloid-inducing properties. Since our previous report, 14 additional compounds have been designed and synthesized based on the known steps involved in HS biosynthesis. Of these, eight have been assessed for their effect on HS biosynthesis in hepatocyte tissue cultures, and the two anomers of a 4-deoxy-D-glucosamine analog have been assessed for their inflammation-associated amyloid (AA amyloid) inhibitory properties in vivo. The promising in vivo results with these two compounds have prompted studies using a murine transgenic model of brain Abeta amyloidogenesis. A macrophage tissue-culture model of AA amyloidogenesis has been devised based on the work of Kluve-Beckerman et al. and modified so as to assess compounds in the absence of potential in vivo confounding variables. Preliminary results indicate that the anomers of interest also inhibit AA amyloid deposition in macrophage tissue culture. Finally, an in vitro technique, using liver Golgi (the site of HS synthesis) rather than whole cells, has been devised to directly assess the effect of analogs on HS biosynthesis. The majority of the novel sugars prepared to date are analogs of N-acetylglucosamine. They have been modified either at the 2-N, C-3, C-4, or C-3 and C-4 positions. Results with the majority of the 2-N analogs suggest that hepacyte N-demethylases remove the N-substituent removal. Several of these have the desired effect on HS biosynthesis using hepatocyte cultures and will be assessed in the culture and in vivo AA amyloid models. To date 3-deoxy and 3,4-dideoxy analogs have failed to affect HS synthesis significantly. Compounds incorporating the 6-deoxy structural feature are currently being designed and synthesized.

Novel glycosaminoglycan precursors as anti-amyloid agents part II

In vivo amyloids consist of two classes of constituents. The first is the disease defining protein, e.g., A beta in Alzheimer's disease. The second is a set of common structural components that usually are the building blocks of basement membrane (BM), a tissue structure that serves as a scaffold onto which cells normally adhere. In vitro binding interactions between one of these BM components and amyloidogenic proteins rapidly change the conformation of the amyloidogenic protein into amyloid fibrils. The offending BM component is a heparan sulfate (HS) proteoglycan (HSPG), part of which is protein and the remainder a specific linear polysaccharide, which is the portion responsible for binding, and imparting the typical amyloid structure, to the amyloid precursor protein/peptide. Our past work has demonstrated that agents that inhibit the binding between HS and the amyloid precursor are effective anti-amyloid compounds both in vitro and in vivo. The present work is concerned with the design and synthesis of modified sugar precursors of HS, which, when incorporated into the polysaccharide, will alter its structure so that it loses its amyloid precursor protein/peptide-binding and fibril-inducing properties. As part of our continuing study, since our previous report, 17 additional compounds have been designed and synthesized based primarily on the known steps involved in HS biosynthesis. In addition to the 4 reported last year, 10 more have been assessed in tissue culture for their inhibitory effect on heparan sulfate synthesis, and one of these has been assessed for its AA-amyloid inhibitory properties. The majority of the novel sugars are analogues of N-acetylglucosamine. They have been modified either at the 4-OH, 3-OH, or 2-N positions. The majority of the 2-N analogues provide data suggesting that hepatocyte N-demethylases remove the N-substituents converting the 2-N analogues into the natural sugar, a process that dilutes the D-[3H] glucosamine tracer used to track heparan sulfate synthesis and thereby gives the impression that biosynthetic inhibition is occurring. To date 3-deoxy analogues have failed to affect heparan sulfate synthesis significantly. Compounds incorporating the 3,4-dideoxy structural feature are currently being assessed. Using primary hepatocyte cultures, we reported previously that a 4-deoxy analogue is incorporated into HS and terminates its elongation. From the 4-deoxy series, one of the compounds has now been assessed in an in vivo model of AA-amyloid induction. This 4-deoxy analogue inhibited splenic AA amyloid deposition by at least 50%, and liver AA amyloid deposition by 85% when measured as amyloid/unit area of tissue. Furthermore, the spleen weights of the treated group were 1/2-1/3 of that in the untreated group indicating that the total splenic amyloid was 1/4-1/6 of that in the untreated group. The results provide further evidence that heparan sulfate is a critical factor in amyloidogenesis and modifications of sugar precursors of heparan sulfate synthesis may provide leads for therapeutic intervention in amyloidogenesis.

Synthesis and biological evaluation of a radiolabeled analog of methyl 2-acetamido-2,4-dideoxy-beta-D-xylo-hexopyranoside directed towards influencing cellular glycosaminoglycan biosynthesis

Two methods are presented for the synthesis of methyl 2-acetamido-2,4-dideoxy-beta-D-xylo-hexopyranoside. The first method employs the Barton-McCombie deoxygenation methodology, and the second method utilizes an oxidation-beta-elimination methodology that allows for the incorporation of hydrogen isotopes into the title compound. Hence, methyl 2-acetamido-2,4-dideoxy-beta-D-xylo-hexopyranoside (4) and methyl 2-acetamido-2,4-dideoxy-beta-D-xylo-hexopyranoside-6-t (14) were synthesized and evaluated for their ability to inhibit hepatocyte, cell-surface glycosaminoglycan biosynthesis and to incorporate a [(3)H] radiolabel into isolated glycosaminoglycans, respectively. Compound 4, at a concentration of 1.0 mM, demonstrated a reduction of D-[(3)H]glucosamine and [(35)S]sulfate incorporation into isolated glycosaminoglycans by 69 and 59%, of the control cultures, respectively. At 10 and 20 mM, 4 demonstrated a maximum inhibition of incorporation of both radiolabels to approximately 10% of the control cultures. Compound 14 demonstrated a maximum incorporation of a [(3)H] radiolabel into isolated cell-surface glycosaminoglycans at 10 and 20 mM. The mechanism of inhibition of glycosaminoglycan biosynthesis is due, in part, to the incorporation of a 4-deoxy moiety into glycosaminoglycan chains resulting in premature chain termination.

Synthesis of 4-deoxy-4-fluoro analogues of 2-acetamido-2-deoxy-D-glucose and 2-acetamido-2-deoxy-D-galactose and their effects on cellular glycosaminoglycan biosynthesis

4-Deoxy-4-fluoro analogues of 2-acetamido-2-deoxy-D-glucose and 2-acetamido-2-deoxy-D-galactose were synthesized and evaluated as inhibitors of hepatic glycosaminoglycan biosynthesis. 2-Acetamido-1,3,6-tri-O-acetyl-2,4-dideoxy-4-fluoro-D-glucopyranose (16) exhibited a reduction of [3H]GlcN and [35S]SO4 incorporation into hepatocyte cellular glycosaminoglycans to 12 and 18%, respectively, of the control cells, at 1.0 mM. Similarly, 2-acetamido-1,3,6-tri-O-acetyl-2,4-dideoxy-4-fluoro-D-galactopyranose (31) exhibited a reduction of [3H]GlcN and [35S]SO4 incorporation to 1 and 9%, respectively, of the control cells, at 1.0 mM. Unlike 16, 31 exhibited a reduction of [14C]Leu incorporation into cellular protein to 57% of control cells, at 1.0 mM.