Selectively O-acylated glycosaminoglycan derivatives

Chemical Modification of Glycosaminoglycan Polysaccharides

The linear anionic class of polysaccharides, glycosaminoglycans (GAGs), are critical throughout the animal kingdom for developmental processes and the maintenance of healthy tissues. They are also of interest as a means of influencing biochemical processes. One member of the GAG family, heparin, is exploited globally as a major anticoagulant pharmaceutical and there is a growing interest in the potential of other GAGs for diverse applications ranging from skin care to the treatment of neurodegenerative conditions, and from the treatment and prevention of microbial infection to biotechnology. To realize the potential of GAGs, however, it is necessary to develop effective tools that are able to exploit the chemical manipulations to which GAGs are susceptible. Here, the current knowledge concerning the chemical modification of GAGs, one of the principal approaches for the study of the structure-function relationships in these molecules, is reviewed. Some additional methods that were applied successfully to the analysis and/or processing of other carbohydrates, but which could be suitable in GAG chemistry, are also discussed.

Formation of Amphiphilic Molecules from the Most Common Marine Polysaccharides, toward a Sustainable Alternative?

Marine polysaccharides are part of the huge seaweeds resources and present many applications for several industries. In order to widen their potential as additives or bioactive compounds, some structural modifications have been studied. Among them, simple hydrophobization reactions have been developed in order to yield to grafted polysaccharides bearing acyl-, aryl-, alkyl-, and alkenyl-groups or fatty acid chains. The resulting polymers are able to present modified physicochemical and/or biological properties of interest in the current pharmaceutical, cosmetics, or food fields. This review covers the chemical structures of the main marine polysaccharides, and then focuses on their structural modifications, and especially on hydrophobization reactions mainly esterification, acylation, alkylation, amidation, or even cross-linking reaction on native hydroxyl-, amine, or carboxylic acid functions. Finally, the question of the necessary requirement for more sustainable processes around these structural modulations of marine polysaccharides is addressed, considering the development of greener technologies applied to traditional polysaccharides.

Preparation and characterization of O-acylated fucosylated chondroitin sulfate from sea cucumber

Fucosylated chondroitin sulfate (FuCS), a kind of complex glycosaminoglycan from sea cucumber, has potent anticoagulant activity. In order to understand the relationship between structures and activity, the depolymerized FuCS (dFuCS) was chosen to prepare its derivates by selective substitution at OH groups. Its O-acylation was carried out in a homogeneous way using carboxylic acid anhydrides. The structures of O-acylated derivatives were characterized by NMR. The results indicated that the 4-O-sulfated fucose residues may be easier to be acylated than the other ones in the sulfated fucose branches. But the O-acylation was always accompanied by the β-elimination, and the degree of elimination was higher as that of acylation was higher. The results of clotting assay indicated that the effect of partial O-acylation of the dFuCS on their anticoagulant potency was not significant and the O-acylation of 2-OH groups of 4-O-sulfated fucose units did not affect the anticoagulant activity.

Case study: contamination of heparin with oversulfated chondroitin sulfate

In late 2007 and early 2008, a cluster of adverse events in patients receiving Heparin Sodium Injection occurred in the United States and in some countries in Europe. The adverse events were reported as being "allergic type" reactions, chiefly characterized by acute hypotension, nausea, and shortness of breath. The root cause of the cluster of adverse events was determined to be a contamination of the heparin by oversulfated chondroitin sulfate. The isolation and structure determination of this contaminant was accomplished by an FDA-led consortium of academic and government laboratories and independently by Baxter Healthcare, whose vial products were first identified in the USA as being associated with the adverse events. Oversulfated chondroitin sulfate was shown to produce acute hypotension in animal models, demonstrating that it was most likely the causative agent responsible for certain of the reported adverse events in patients receiving the contaminated heparin products.

Semi-synthetic heparin derivatives: chemical modifications of heparin beyond chain length, sulfate substitution pattern and N-sulfo/N-acetyl groups

The glycosaminoglycan heparin is a polyanionic polysaccharide most recognized for its anticoagulant activity. Heparin binds to cationic regions in hundreds of prokaryotic and eukaryotic proteins, termed heparin-binding proteins. The endogenous ligand for many of these heparin-binding proteins is a structurally similar glycosaminoglycan, heparan sulfate (HS). Chemical and biosynthetic modifications of heparin and HS have been employed to discern specific sequences and charge-substitution patterns required for these polysaccharides to bind specific proteins, with the goal of understanding structural requirements for protein binding well enough to elucidate the function of the saccharide-protein interactions and/or to develop new or improved heparin-based pharmaceuticals. The most common modifications to heparin structure have been alteration of sulfate substitution patterns, carboxyl reduction, replacement N-sulfo groups with N-acetyl groups, and chain fragmentation. However, an accumulation of reports over the past 50 years describe semi-synthetic heparin derivatives obtained by incorporating aliphatic, aryl, and heteroaryl moieties into the heparin structure. A primary goal in many of these reports has been to identify heparin-derived structures as new or improved heparin-based therapeutics. Presented here is a perspective on the introduction of non-anionic structural motifs into heparin structure, with a focus on such modifications as a strategy to generate novel reduced-charge heparin-based bind-and-block antagonists of HS-protein interactions. The chemical methods employed to synthesize such derivatives, as well as other unique heparin conjugates, are reviewed.

Diversity-oriented chemical modification of heparin: Identification of charge-reduced N-acyl heparin derivatives having increased selectivity for heparin-binding proteins

The diversity-oriented chemical modification of heparin is shown to afford charge-reduced heparin derivatives that possess increased selectivity for binding heparin-binding proteins. Variable N-desulfonation of heparin was employed to afford heparin fractions possessing varied levels of free amine. These N-desulfonated heparin fractions were selectively N-acylated with structurally diverse carboxylic acids using a parallel synthesis protocol to generate a library of 133 heparin-derived structures. Screening library members to compare affinity for heparin-binding proteins revealed unique heparin-derived structures possessing increased affinity and selectivity for individual heparin-binding proteins. Moreover, N-sulfo groups in heparin previously shown to be required for heparin to bind specific proteins have been replaced with structurally diverse non-anionic moieties to afford identification of charge-reduced heparin derivatives that bind these proteins with equivalent or increased affinity compared to unmodified heparin. The methods described here outline a process that we feel will be applicable to the systematic chemical modification of natural polyanionic polysaccharides and the preparation of synthetic oligosaccharides to identify charge-reduced high affinity ligands for heparin-binding proteins.

A novel strategy to generate biologically active neo-glycosaminoglycan conjugates

Heparin and heparan sulfate are structurally related polysaccharides with a variety of biological effects/functions. Most of these effects are due to interactions, of varying specificity, between the negatively charged polysaccharide chains and proteins. While such interactions generally involve a single saccharide domain of decasaccharide size or less, ternary complexes of two protein molecules binding to separate domains on a single polysaccharide chain are known to occur. To facilitate studies on domain organization and its importance for biological function a strategy was developed to chemically conjugate defined heparin oligomers in linear and chemoselective fashion. The procedure requires that the oligosaccharide to provide the reducing-terminal domain of the conjugate is generated by lyase degradation of a parent polysaccharide, whereas the nonreducing-terminal domain is obtained through deaminative cleavage with nitrous acid. The applicability of the method was demonstrated by constructing a conjugate composed of two heparin 12-mers, of which the reducing-terminal component contained the antithrombin-binding region, whereas the nonreducing-terminal domain did not. Contrary to any of the unconjugated oligomers, the product was found to efficiently promote the inactivation of thrombin by antithrombin.

Influence of heparin(s) on the interleukin-1-beta-induced expression of collagenase, stromelysin-1, and tissue inhibitor of metalloproteinase-1 in human gingival fibroblasts

Here, we describe the influence of heparin(s) on the interleukin-1-beta (IL-1beta)-induced expression of collagenase (matrix metalloproteinase-1, MMP-1), stromelysin-1 (matrix metalloproteinase-3, MMP-3) and tissue inhibitor of matrix metalloproteinase-1 (TIMP-1) in human gingival fibroblasts (HGF). Amounts of secreted enzymes and inhibitors as well as their mRNA steady-state levels increased significantly following supplementation of HGF culture medium with 2 ng/mL of IL-1 beta1. Addition of heparin to cell culture medium 1 hour following IL-1beta decreased MMP and TIMP-1 expression in a dose-dependent manner. The inhibitory effect of heparin was significant at a concentration as low as 1 microg/mL. These findings could be reproduced with a low Mr heparin fragment devoid of anticoagulant activity. Heparin and fragments might therefore reduce the excessive proteolytic capacity of the gingival fibroblast during inflammation and could be useful as pharmacological agent(s) in gingivitis and periodontitis.

Introducing a C-interglycosidic bond in a biologically active pentasaccharide hardly affects its biological properties

We describe here the synthesis and the biological activity of a 'C-pentasaccharide', a new analogue of the antithrombin III (AT III) binding region of heparin containing a methylene bridge in place of an interglycosidic oxygen atom. The affinity for AT III and the anti-factor Xa activity of this compound have been compared with that of the corresponding selected 'O-pentasaccharide'. Such a structural modification slightly decreased the affinity of this compound for AT III as well as its anti-factor Xa activity (880 +/- 40 anti-Xa units versus 1180 +/- 30 anti-Xa units for the C-pentasaccharide and the O-pentasaccharide, respectively). This compound therefore represents the first example of a new class of anti-factor Xa pentasaccharides containing a C-interglycosidic bond.

Entry and distribution of fluorescent antiproliferative heparin derivatives into rat vascular smooth muscle cells: comparison between heparin-sensitive and heparin-resistant cultures

We studied the binding and entry of fluorescein (FITC)-labeled heparin derivatives into rat aortic smooth muscle cells (SMC) by confocal microscopy. FITC-labeled heparin fractions or FITC-labeled SR 80037A, a potent antiproliferative heparin derivative (Bârzu et al., Eur. J. Pharmacol., 219:225-233 1992), were prepared and their antiproliferative activity was confirmed. By incubating SMC with FITC-labeled heparins, a specific cell-associated fluorescence was found. Cellular fluorescence was mostly located around the nucleus and at the level of cell contacts or cell adhesion. The fluorescence was displaced neither by chasing with excess of unlabeled heparins nor by washing with 1 M NaCl, which proved that labeled heparins had been internalized by SMC. Kinetics of internalization of FITC-heparins suggested receptor-mediated endocytosis of heparins by SMC. Double labeling of SMC with biotinylated Concanavalin A and FITC-SR 80037A also indicated that heparin derivative enters the endocytic pathway. The process was accelerated when serum was present in the incubation medium. Treatment of cells with chloroquine (50 microM) induced accumulation of FITC-SR 80037A in the late endosomes, around the nucleus. No fluorescence labeling could be evidenced inside the nucleus. Neither electron microscopy nor cell fractionation experiments performed with SMC previously incubated with [3H]-heparin were able to ascertain nuclear uptake of heparin, as proposed by other workers (Busch et al., Cell Biol., 116:31-42; 1992; Sing et al., Drug Dev. Res., 29:129-136 1993). The cell-associated fluorescence was very weak in SMC resistant to the antiproliferative activity of heparin, selected by long-term heparin treatment (HT-SMC) as previously shown [Bârzu et al., J. Cell. Physiol., 160:239-248, 1994]. The HT-SMC differed from control SMC with regard to expression of extracellular matrix proteins. These cells exhibited very low expression of fibronectin and prevalent expression of laminin and synthesized less cell-associated glycosaminoglycans. From our results, the following conclusions can be drawn: (1) the antiproliferative heparins are bound and internalized by SMC without being taken up into the nucleus; (2) there is a correlation between the binding and/or the internalization process and the sensitivity of SMC to the antiproliferative activity of heparins; and (3) selection of heparin-resistant SMC by long treatment with heparin results in particular growth pattern of SMC (absence of focal overgrowth), associated with changes in the expression of the extracellular matrix components (fibronectin, laminin, and cell-bound glycosaminoglycans).

Characterization of rat aortic smooth muscle cells resistant to the antiproliferative activity of heparin following long-term heparin treatment

Vascular smooth muscle cells (SMC) do not represent a homogeneous population (Schwartz et al., 1990, Am. J. Pathol. 136: 1417-1428). Cellular clones resistant to the antiproliferative activity of heparin were isolated from rat aortic SMC cultures (Pukac et al., 1990, Cell Regul., 1:435-443; San Antonio et al., 1993, Arterioscler. Thromb., 13:748-757) and from explant of human arterial restenotic lesions (Chan et al., 1993, Lancet, 341:341-342). We have shown in the present study that long-term treatment (growth medium supplemented with 200 micrograms/ml heparin, from the second to the tenth passage) of rat aortic SMC, without cell cloning, resulted in a significant loss of sensitivity to the growth inhibition by heparin and its derivatives. The heparin resistance was stable after growing cells for two passages in heparin-free medium, suggesting the selection of a particular phenotype. We tried to characterize these cells and to determine the causes of the resistance to the growth inhibition by heparin. Heparin-treated SMC (HT-SMC) were smaller than their control culture at the same passage, expressed less alpha-SM actin, and did not overgrow after reaching confluence. As in the heparin-resistant clones (San Antonio et al., 1993, Cell Regul., 1:435-443) expression of alpha-SM actin could be increased in HT-SMC by heparin addition before Western blotting. Heparin resistance was associated with a tenfold decrease in [3H]-heparin binding capacity (Bmax = 1.9 x 10(6) sites per cell) compared to control cultures (Bmax = 1.7 x 10(7) sites per cell), which was irreversible after growing the cells for two additional passages in heparin-free medium. We also investigated protein kinase C (PKC) in HT-SMC in terms of both enzymatic activity and protein expression (evaluated by [3H]-staurosporine and [3H]-phorbol-12,13-dibutyrate binding). We found that HT-SMC had only half the PKC activity and expression as control SMC. Therefore, long-term treatment of rat aortic SMC with heparin allowed the selection of a less differentiated subpopulation of cells, exhibiting low sensitivity to the growth inhibition by heparin, which could be related to the low capacity of binding heparin and to a lower PKC activity and/or expression.

O-acylated heparin derivatives with low anticoagulant activity decrease proliferation and increase alpha-smooth muscle actin expression in cultured arterial smooth muscle cells

Selectively O-acylated derivatives of various glycosaminoglycans were prepared and tested in vitro for their anticoagulant activity and their antiproliferative effect on rat and rabbit smooth muscle cells. When O-acylation (butyrylation or hexanoylation) had been performed on periodate-depolymerized heparin fragments having very low anticoagulant activity, the antiproliferative potency was markedly increased (IC50 = 2 and 1 micrograms/ml respectively, versus 31 micrograms/ml for starting compound) without an increase in anticoagulant activity. The antiproliferative activity was related to the degree of acylation. The O-acylated derivatives of heparin fragments were also very active in reversing the de-differentiation of smooth muscle cell in culture, as estimated by the increase in the expression of alpha-smooth muscle actin and alpha-smooth muscle actin mRNA.