Chemoenzymatic Synthesis of Heparin Oligosaccharides with both Anti-factor Xa and Anti-factor IIa Activities

Heparan sulfate (HS) and heparin are highly sulfated polysaccharides. Heparin is a commonly used anticoagulant drug that inhibits the activities of factors Xa and IIa (also known as thrombin) to prevent blood clot formation. Here, we report the synthesis of a series of size-defined oligosaccharides to probe the minimum size requirement for an oligosaccharide with anti-IIa activity. The synthesis was completed by a chemoenzymatic approach involving glycosyltransferases, HS sulfotransferases, and C(5)-epimerase. We demonstrate the ability to synthesize highly purified N-sulfo-oligosaccharides having up to 21 saccharide residues. The results from anti-Xa and anti-IIa activity measurements revealed that an oligosaccharide longer than 19 saccharide residues is necessary to display anti-IIa activity. The oligosaccharides also exhibit low binding toward platelet factor 4, raising the possibility of preparing a synthetic heparin with a reduced effect of heparin-induced thrombocytopenia. The results from this study demonstrate the ability to synthesize large HS oligosaccharides and provide a unique tool to probe the structure and function relationships of HS that require the use of large HS fragments.     

Directing the biological activities of heparan sulfate oligosaccharides using a chemoenzymatic approach

Heparan sulfate (HS) and heparin are highly sulfated polysaccharides exhibiting essential physiological functions. The sulfation patterns determine the functional selectivity for HS and heparin. Chemical synthesis of HS, especially those larger than a hexasaccharide, remains challenging. Enzymatic synthesis of HS has recently gained momentum. Here we describe the divergent assembly of HS heptasaccharides and nonasaccharides from a common hexasaccharide precursor. The hexasaccharide precursor was synthesized via a chemical method. The subsequent elongation, sulfation and epimerization were completed by glycosyltransferases, HS sulfotransferases and epimerase. Using the synthesized heptasaccharides, we discovered that the iduronic acid is critical for binding to fibroblast growth factor-2. We also designed a synthetic path to prepare a nonasaccharide with an antithrombin-binding affinity of 3?nM. Our method demonstrated the feasibility of combining chemical and enzymatic synthesis to prepare structurally defined HS oligosaccharides with desired biological activities.     

Preparation of heparin/heparan sulfate oligosaccharides with internal N-unsubstituted glucosamine residues for functional studies

The rare N-unsubstituted glucosamine (GlcNH (3)(+)) residues in heparan sulfate (HS) have important biological and pathophysiological roles. However, it is difficult to prepare naturally-occuring, GlcNH(3)(+)-containing oligosaccharides from HS because of their low abundance, as well as the inherent problems in both excising and identifying them. Therefore, the ability to chemically generate a series of structurally-defined oligosaccharides containing GlcNH(3)(+) residues would greatly contribute to investigating their natural role in HS. In this study, a series of heparin/HS oligosaccharides, from dp6 up to dp16 in length that possess internal GlcNH(3)(+) residues were prepared by a combination of chemical modification and heparinase I digestion. Purification and structural analysis of the major species derived from the octa- to dodeca-saccharide size fractions indicated the introduction of between 1 and 3 internal GlcNH(3)(+) residues per oligosaccharide. In addition, a GlcNH(3)(+) residue was selectively introduced into an internal position in a tetrasaccharide species by direct chemical modification. This selectivity has potential as an alternative procedure for preparing internally-modified oligosaccharides of various lengths. The utility of such oligosaccharides was demonstrated by a comparison of the binding of three different tetrasaccharide species containing 0, 1 and 2 free amino groups to the NK1 truncated variant of hepatocyte growth factor/scatter factor.     

Expression of heparan sulfate sulfotransferases in Kluyveromyces lactis and preparation of 3'-phosphoadenosine-5'-phosphosulfate

Heparan sulfate (HS) belongs to a major class of glycans that perform central physiological functions. Heparin is a specialized form of HS and is a clinically used anticoagulant drug. Heparin is a natural product isolated from pig intestine. There is a strong demand to replace natural heparin with a synthetic counterpart. Although a chemoenzymatic approach has been employed to prepare synthetic heparin, the scale of the synthesis is limited by the availability of sulfotransferases and the cofactor, 3'-phosphoadenosine-5'-phosphosulfate (PAPS). Here, we present a novel method to produce secreted forms of sulfotransferases in the yeast cells, Kluyveromyces lactis. Five sulfotransferases including N-sulfotransferase, 2-O-sulfotransferase, 3-O-sulfotransferase 1 and 6-O-sulfotransferases 1 and 3 were expressed using this method. Unlike bacterial-expressed sulfotransferases, the yeast proteins can be directly used to modify polysaccharides without laborious purification. The yeast-expressed sulfotransferases also tend to have higher specific activity and thermostability. Furthermore, we demonstrated the possibility for the gram-scale synthesis of PAPS from adenosine 5'-triphosphate at only 1/5000th of the price purchased from a commercial source. Our results pave the way to conduct the enzymatic synthesis of heparin in large quantities.     

Chemoenzymatic synthesis of neoglycoproteins using transglycosylation with endo-beta-N-acetylglucosaminidase A

A novel chemoenzymatic approach to synthesize neoglycoproteins containing high-mannose-type oligosaccharides is described. p-Isothiocyanatophenyl-beta-d-glucopyranoside (Glc-ITC) was transferred to the reducing end of the high-mannose-type oligosaccharides using a transglycosylation activity of endo-beta-N-acetylglucosaminidase A (Endo-A). A novel oligosaccharide, Man(6)GlcNAc-Glc-ITC, was synthesized as a coupling reagent for lysyl and N-terminal residues of the protein moiety. The neoglycoconjugate was coupled with several nonglycosylated proteins such as ribonuclease A, lysozyme, and alpha-lactalbumin. Between one and four high-mannose-type oligosaccharides were incorporated per molecule of these proteins. This method should be very useful for the synthesis of neoglycoproteins with homogeneous high-mannose-type oligosaccharides.     

Oligosaccharide Substrate Preferences of Human Extracellular Sulfatase Sulf2 Using Liquid Chromatography-Mass Spectrometry Based Glycomics Approaches

Sulfs are extracellular endosulfatases that selectively remove the 6-O-sulfate groups from cell surface heparan sulfate (HS) chain. By altering the sulfation at these particular sites, Sulfs function to remodel HS chains. As a result of the remodeling activity, HSulf2 regulates a multitude of cell-signaling events that depend on interactions between proteins and HS. Previous efforts to characterize the substrate specificity of human Sulfs (HSulfs) focused on the analysis of HS disaccharides and synthetic repeating units. In this study, we characterized the substrate preferences of human HSulf2 using HS oligosaccharides with various lengths and sulfation degrees from several naturally occurring HS sources by applying liquid chromatography mass spectrometry based glycomics methods. The results showed that HSulf2 preferentially digests highly sulfated HS oligosaccharides with zero acetyl groups and this preference is length dependent. In terms of length of oligosaccharides, HSulf2 digestion induced more sulfation decrease on DP6 (DP: degree of polymerization) compared to DP2, DP4 and DP8. In addition, the HSulf2 preferentially digests the oligosaccharide domain located at the non-reducing end (NRE) of the HS and heparin chain. In addition, the HSulf2 digestion products were altered only for specific isomers. HSulf2 treated NRE oligosaccharides also showed greater decrease in cell proliferation than those from internal domains of the HS chain. After further chromatographic separation, we identified the three most preferred unsaturated hexasaccharide for HSulf2.     

Determining heparan sulfate structure in the vicinity of specific sulfotransferase recognition sites by mass spectrometry

Sulfated motifs on heparan sulfate (HS) are involved in various extracellular processes from cell signaling to enzymatic regulation, but the structures of these motifs are obscure. We have developed a strategy to determine the structure of sulfotransferase recognition sites which constitute these motifs. Stable isotope is first introduced into specific sites on HS with HS sulfotransferases and the modified HS is then digested into oligosaccharides of differing sizes. The overlapping oligosaccharides containing the introduced stable isotope are identified by changes in the m/z profiles by mass spectrometry, and their relationships are elucidated. In this way, the HS structures in the vicinity of the sulfotransferase recognition site are quickly determined and groups on precursor structures of HS that direct the action of HS sulfotransferases are pinpointed.     

Use of biosynthetic enzymes in heparin and heparan sulfate synthesis

Heparan sulfate and heparin are highly sulfated polysaccharides consisting of repeating disaccharide units of glucuronic acid or iduronic acid that is linked to glucosamine. Heparan sulfate displays a range of biological functions, and heparin is a widely used anticoagulant drug in hospitals. It has been known to organic chemists that the chemical synthesis of heparan sulfate and heparin oligosaccharides is extremely difficult. Recent advances in the study of the biosynthesis of heparan sulfate/heparin offer a chemoenzymatic approach to synthesize heparan sulfate and heparin. Compared to chemical synthesis, the chemoenzymatic method shortens the synthesis and improves the product yields significantly, providing an excellent opportunity to advance the understanding of the structure and function relationships of heparan sulfate. In this review, we attempt to summarize the progress of the chemoenzymatic synthetic method and its application in heparan sulfate and heparin research.     

Fluorescent-tagged heparan sulfate precursor oligosaccharides to probe the enzymatic action of heparitinase I

Heparitinase I, a key lyase enzyme essential for structural analysis of heparan sulfate (HS), degrades HS domains that are undersulfated at glucuronyl residues through an elimination mechanism. Earlier studies employed viscosimetric measurements and electrophoresis to deduce the mechanism of action of heparitinase I and two other related lyases, heparitinase II and heparitinase III. However, these findings lack molecular evidence for the intermediates formed and could not distinguish whether the cleavage occurred from the reducing end or the nonreducing end. In the current study, 2-aminoacridone (2-AMAC)-labeled HS precursor oligosaccharides of various sizes were prepared to investigate the mechanism of heparitinase I-mediated depolymerization using sensitive and quantitative methodologies. Furthermore, fluorescent (2-AMAC) tagging of HS precursor oligosaccharides allowed us to distinguish fragments that result from cleavage of the substrates at various time intervals and sites farther away from the reducing and nonreducing ends of oligosaccharide substrates. This study provides the first direct molecular evidence for a predominantly random endolytic mechanism of cleavage of HS precursor oligosaccharides by heparitinase I. This robust strategy can be adapted to deduce the mechanism of action of other heparitinases and also to deduce structural information of complex HS oligosaccharides of biological importance.     

Metabolic pathways of heparan sulfate proteoglycans in a rat parathyroid cell line.

The distribution of heparan sulfate (HS) proteoglycans in clonal rat parathyroid cells is regulated by the extracellular Ca2+ concentration, which is a principal factor for parathyroid cell function (Takeuchi, Y., Sakaguchi, K., Yanagishita, M., Aurbach, G. D., and Hascall, V. C. (1990) J. Biol. Chem. 265, 13661-13668). Increasing the concentration of extracellular Ca2+ in the physiological range redistributes HS proteoglycans from the cell surface to an intracellular compartment. We have now examined effects of the extracellular Ca2+ concentration on the metabolism of the HS proteoglycans in detail using [35S]sulfate metabolic labeling-chase experiments. Two distinct metabolic pathways were demonstrated: (i) the intracellular generation of HS chains from HS proteoglycans in prelysosomal compartments followed by their release into the medium (pathway 1), and (ii) intracellular generation of HS oligosaccharides from HS chains in prelysosomal compartments, which are eventually degraded into free sulfate in lysosomes (pathway 2). The HS oligosaccharides were exclusively present within the cells, whereas HS chains were found primarily in the medium. The cells do not internalize either HS proteoglycans or HS chains from the medium. These observations indicate that these two degradation pathways are independent. In addition to these pathways, approximately 15% of the HS proteoglycans were released into the medium as a proteoglycan form. Treatment of cells with chloroquine, a lysosomotropic agent, did not affect generation of HS chains but inhibited conversion of HS chains to HS oligosaccharides or to free sulfate and resulted in the release of HS chains from the cells. The drug did not affect metabolic pathway 1. The extracellular Ca2+ concentration did not alter these intracellular degradation pathways for HS proteoglycans in the parathyroid cells. Thus, extracellular Ca2+ appears to regulate only the distribution of HS proteoglycans between the cell surface and intracellular compartments, and the process of cycling between these compartments when extracellular Ca2+ is low.     

Design and synthesis of structurally defined heparan sulfate (HS)-FK506 conjugates as an exogenous approach to investigate biological functions of nucleus HS

Although heparan sulfate (HS) is widely implicated in numerous physiological and pathological processes, the biological function of nucleus HS remains underexplored, largely due to its complex structure and high hydrophilic property. To supplement these efforts, ideal vehicles are drawing attention as they combine attractive features including lipid solubility for penetrating cell membrane, high affinity binding to its target receptor, metabolic stability, and no cellular actions resulting in toxicity. Herein, we develop a convenient and promising strategy to prepare HS-FK506 conjugates for membrane transport and entry into nucleus, where click chemistry takes easily place between the exocyclic allyl group of a clinic drug FK506 and thiol as a handle incorporated into HS analogues. HS derivatives for constructing the conjugates were synthesized using a cutting-edge chemoenzymatic method. Meantime, [³⁵S] labeled 3′-phosphoadenosine 5′-phosphosulfate (PAP³⁵S) and [¹⁴C] glucuronic acid (Glc A) were adopted to label HS-FK506 conjugates, respectively, to evaluate their efficiency of nucleus entry, as a result, ¹⁴C Glc A was sensitive, effective and reliable whereas PAP³⁵S gave rise to a mixture of labeled compounds, hampering the understanding of structure-function relationship of nucleus HS. Compared with the corresponding HS, the amount of HS-FK506 conjugates to translocate into nucleus from radioactive assay experiments sharply increased, e.g. tridecasaccharide-FK506 1d increased by approximate 10 folds, offering a simple and robust platform for enabling hydrophilic compounds including carbohydrates to translocate into nucleus and shedding light on their biological functions.     

Thiosugar nucleotide analogues: synthesis of uridine 5'-(2,3,6-tri-O-acetyl-4-S-acetyl-4-thio-alpha-D-galactopyranosyl diphosphate)

The synthesis of a novel uridine diphosphate galactose (UDP-Gal) analog, (UDP-2,3,6-tri-O-acetyl-4-S-acetyl-4-thio-alpha-D-galactopyranose) (10) is described. Compound 10 contains a sulfur in the place of oxygen at the 4-position of the galactose moiety. Compound 10 represents a protected form of a novel sugar nucleotide analog that can potentially be used during chemoenzymatic synthesis to modify complex oligosaccharides.     

A defect in exodegradative pathways provides insight into endodegradation of heparan and dermatan sulfates

Within cells, dermatan sulfate (DS) and heparan sulfate (HS) are degraded in two steps. The initial endohydrolysis of these polysaccharides is followed by the sequential action of lysosomal exoenzymes to reduce the resulting oligosaccharides to monosaccharides and inorganic sulfate. Mucopolysaccharidosis (MPS) type II is a lysosomal storage disorder caused by a deficiency of the exoenzyme iduronate-2-sulfatase (I2S). Consequently, partially degraded fragments of DS and HS have been shown to accumulate in the lysosomes of affected cells and are excreted in the urine. Di- to hexadecasaccharides, isolated from the urine of a MPS II patient using anion exchange and gel filtration chromatography, were identified using electrospray ionization-tandem mass spectrometry (ESI-MS/MS). These oligosaccharides were shown to have non-reducing terminal iduronate-2-sulfate residues by digestion with recombinant I2S. A pattern of growing oligosaccharide chains composed of alternating uronic acid and N-acetylhexosamine residues was identified and suggested to originate from DS. A series of oligosaccharides consisting of hexosamine/N-acetylhexosamine alternating with uronic acid residues was also identified and on the basis of the presence of unacetylated hexosamine; these oligosaccharides are proposed to derive from HS. The presence of both odd and even-length oligosaccharides suggests both endo-beta-glucuronidase and endo-N-acetylhexosaminidase activities toward both glycosaminoglycans. Furthermore, the putative HS oligosaccharide structures identified indicate that heparanase activities are directed toward regions of both low and high sulfation, while the N-acetylhexosaminidase activity acted only in regions of low sulfation in this polysaccharide.     

Isolation and characterization of low sulfated heparan sulfate sequences with affinity for lipoprotein lipase

Lipoprotein lipase (LPL), which is an important enzyme in lipid metabolism, binds to heparan sulfate (HS) proteoglycans. This interaction is crucial for several aspects of LPL function, such as intracellular/extracellular transport and high capacity attachment to cell surfaces. Retention of LPL on the capillary walls, and elsewhere, via HS chains is most likely affected by the quality and quantity of HS present. Earlier studies have demonstrated that LPL interacts with highly sulfated HS and heparin oligosaccharides. Since such structures are relatively rare in endothelial HS, we have re-addressed the question of physiological ligand structures for LPL by affinity purification of end-labeled oligosaccharides originating from heparin and HS on immobilized LPL. By a combination of chemical modification and fragmentation of the bound material we identified that the bound fraction contained modestly sulfated oligosaccharides with an average sulfation of one O-sulfate per disaccharide unit and tolerates N-acetylated glucosamine residues. Therefore LPL, containing several clusters of positive charges on each subunit, may constitute an ideal structure for a protein that needs to bind with reasonable affinity to a variety of modestly sulfated sequences of the type that is abundant in HS chains.     

Regulation of FGF-1 mitogenic activity by heparan sulfate oligosaccharides is dependent on specific structural features: differential requirements for the modulation of FGF-1 and FGF-2

The interaction of heparan sulfate (HS) (and the closely related molecule heparin) with FGF-1 is a requirement for enabling the growth factor to activate its cell surface tyrosine kinase receptor. However, little is known about the regulatory role of naturally occurring cell surface HS in FGF-1 activation. We have addressed this issue by utilizing a library of HS oligosaccharides, which are defined in both length and sulfate content. Mitogenic activation assays using these oligosaccharides showed that HS contained both FGF-1 activatory and inhibitory sugar sequences. Further analysis of these oligosaccharides showed a clear correlation between FGF-1 promoting activity and their 6-O-sulfate content. The results, in particular with the dodecasaccharide sequences, suggested that specific positioning of 6-O-sulfate groups may be required for the promotion of FGF-1 mitogenic activity. This may also be true for 2-O-sulfate groups though the evidence was not as conclusive. Differential activation of FGF-1 and FGF-2 was also observed and found to be mediated by both oligosaccharide length and sulfation pattern, with different specific O-sulfate positioning being implicated for the promotion of different growth factors. These results suggest that variation and tight control of the fine structure of HS may allow cells to not only control their positive/negative responses to individual FGFs but also to change specificity towards promotion of different members of the FGF family.     

Binding affinities of vascular endothelial growth factor (VEGF) for heparin-derived oligosaccharides

Heparin and HS (heparan sulfate) exert their wide range of biological activities by interacting with extracellular protein ligands. Among these important protein ligands are various angiogenic growth factors and cytokines. HS binding to VEGF (vascular endothelial growth factor) regulates multiple aspects of vascular development and function through its specific interaction with HS. Many studies have focused on HS-derived or HS-mimicking structures for the characterization of VEGF165 interaction with HS. Using a heparinase 1-prepared small library of heparin-derived oligosaccharides ranging from hexasaccharide to octadecasaccharide, we systematically investigated the heparin-specific structural features required for VEGF binding. We report the apparent affinities for the association between the heparin-derived oligosaccharides with both VEGF165 and VEGF55, a peptide construct encompassing exclusively the heparin-binding domain of VEGF165. An octasaccharide was the minimum size of oligosaccharide within the library to efficiently bind to both forms of VEGF and a tetradecasaccharide displayed an effective binding affinity to VEGF165 comparable to unfractionated heparin. The range of relative apparent binding affinities among VEGF and the panel of heparin-derived oligosaccharides demonstrate that the VEGF binding affinity likely depends on the specific structural features of these oligosaccharides, including their degree of sulfation, sugar-ring stereochemistry and conformation. Notably, the unique 3-O-sulfo group found within the specific antithrombin binding site of heparin is not required for VEGF165 binding. These findings afford new insight into the inherent kinetics and affinities for VEGF association with heparin and heparin-derived oligosaccharides with key residue-specific modifications and may potentially benefit the future design of oligosaccharide-based anti-angiogenesis drugs.     

Characterization of the structure of antithrombin-binding heparan sulfate generated by heparan sulfate 3-O-sulfotransferase 5

The 3-O-sulfation of glucosamine is a key modification step during the biosynthesis of anticoagulant heparan sulfate (HS). Both heparan sulfate 3-O-sulfotransferase -1 (3-OST-1) and 3-O-sulfotransferase-5 (3-OST-5) transfer sulfate to the 3-OH group of glucosamine to generate antithrombin-binding heparan sulfate (HS(act)). Here, we reported the isolation and characterization of the antithrombin-binding HS oligosaccharides generated by 3-OST-5 (3-OST-5 oligo(act)). (3)H-labeled HS of Chinese hamster ovary cells was exhaustively modified by 3-OST-1 to remove the 3-OST-1 modification sites followed by antithrombin-affinity fractionation. The non-antithrombin-binding fraction of 3-OST-1 pretreated HS was further modified by 3-OST-5 to generate additional antithrombin-binding HS, which was designated as 3-OST-5 HS(act). Structural analysis of 3-OST-5 HS(act) revealed that the antithrombin-binding site of 3-OST-5 HS(act) is located within a domain clustered with N-sulfated glucosamine units. We also isolated 3-OST-5 antithrombin-binding oligosaccharides (3-OST-5 oligo(act)) from high pH nitrous acid degraded 3-OST-5 HS(act). A disaccharide analysis revealed that 3-OST-5 oligo(act) were composed of multiple 3-O-sulfated glucosamine units. Our results provide additional insights on the relationship between the anticoagulant activity and structure of HS.     

Rapid chemoenzymatic synthesis of monodisperse hyaluronan oligosaccharides with immobilized enzyme reactors

We describe the chemoenzymatic synthesis of a variety of monodisperse hyaluronan (beta 4-glucuronic acid-beta 3-N-acetylglucosamine (HA)) oligosaccharides. Potential medical applications for HA oligosaccharides (approximately 10-20 sugars in length) include killing cancerous tumors and enhancing wound vascularization. Previously, the lack of defined oligosaccharides has limited the exploration of these sugars as components of new therapeutics. The Pasteurella multocida HA synthase, pmHAS, a polymerizing enzyme that normally elongates HA chains rapidly (approximately 1-100 sugars/s), was converted by mutagenesis into two single-action glycosyltransferases (glucuronic acid transferase and N-acetylglucosamine transferase). The two resulting enzymes were purified and immobilized individually onto solid supports. The two types of enzyme reactors were used in an alternating fashion to produce extremely pure sugar polymers of a single length (up to HA20) in a controlled, stepwise fashion without purification of the intermediates. These molecules are the longest, non-block, monodisperse synthetic oligosaccharides hitherto reported. This technology platform is also amenable to the synthesis of medicant-tagged or radioactive oligosaccharides for biomedical testing. Furthermore, these experiments with immobilized mutant enzymes prove both that pmHAS-catalyzed polymerization is non-processive and that a monomer of enzyme is the functional catalytic unit.     

Synthetic Heparan Sulfate Oligosaccharides Inhibit Endothelial Cell Functions Essential for Angiogenesis

Background

Heparan sulfate (HS) is an important regulator of the assembly and activity of various angiogenic signalling complexes. However, the significance of precisely defined HS structures in regulating cytokine-dependent angiogenic cellular functions and signalling through receptors regulating angiogenic responses remains unclear. Understanding such structure-activity relationships is important for the rational design of HS fragments that inhibit HS-dependent angiogenic signalling complexes.

Methodology/Principal Findings

We synthesized a series of HS oligosaccharides ranging from 7 to 12 saccharide residues that contained a repeating disaccharide unit consisting of iduronate 2-O-sulfate linked to glucosamine with or without N-sulfate. The ability of oligosaccharides to compete with HS for FGF2 and VEGF₁₆₅ binding significantly increased with oligosaccharide length and sulfation. Correspondingly, the inhibitory potential of oligosaccharides against FGF2- and VEGF₁₆₅-induced endothelial cell responses was greater in longer oligosaccharide species that were comprised of disaccharides bearing both 2-O- and N-sulfation (2SNS). FGF2- and VEGF₁₆₅-induced endothelial cell migration were inhibited by longer 2SNS oligosaccharide species with 2SNS dodecasaccharide activity being comparable to that of receptor tyrosine kinase inhibitors targeting FGFR or VEGFR-2. Moreover, the 2SNS dodecasaccharide ablated FGF2- or VEGF₁₆₅-induced phosphorylation of FAK and assembly of F-actin in peripheral lamellipodia-like structures. In contrast, FGF2-induced endothelial cell proliferation was only moderately inhibited by longer 2SNS oligosaccharides. Inhibition of FGF2- and VEGF₁₆₅-dependent endothelial tube formation strongly correlated with oligosaccharide length and sulfation with 10-mer and 12-mer 2SNS oligosaccharides being the most potent species. FGF2- and VEGF₁₆₅-induced activation of MAPK pathway was inhibited by biologically active oligosaccharides correlating with the specific phosphorylation events in FRS2 and VEGFR-2, respectively.

Conclusion/Significance

These results demonstrate structure-function relationships for synthetic HS saccharides that suppress endothelial cell migration, tube formation and signalling induced by key angiogenic cytokines.     

Role of heparan sulfate domain organization in endostatin inhibition of endothelial cell function

The anti-angiogenic activity of endostatin (ES) depends on interactions with heparan sulfate (HS). In the present study, intact HS chains of >/=15 kDa bound quantitatively to ES whereas N-sulfated HS decasaccharides, with affinity for several fibroblast growth factor (FGF) species, failed to bind. Instead, ES-binding oligosaccharides composed of mixed N-sulfated and N-acetylated disaccharide units were isolated from pig intestinal HS. A 10/12mer ES-binding epitope was identified, with two N-sulfated regions separated by at least one N-acetylated glucosamine unit (SAS-domain). Cleavage at the N-acetylation site disrupted ES binding. These findings point to interaction between discontinuous sulfated domains in HS and arginine clusters at the ES surface. The inhibitory effect of ES on vascular endothelial growth factor-induced endothelial cell migration was blocked by the ES-binding SAS-domains and by heparin oligosaccharides (12mers) similar in length to the ES-binding SAS-domains, but not by 6mers capable of FGF binding. We propose that SAS-domains modulate the biological activities of ES and other protein ligands with extended HS-binding sites. The results provide a rational explanation for the preferential interaction of ES with certain HS proteoglycan species.