Control of chondroitin sulphate biosynthesis. beta-D-Xylopyranosides as substrates for UDP-galactose: D-xylose transferase from embryonic-chicken cartilage

Synthetic Xylosides: Probing the Glycosaminoglycan Biosynthetic Machinery for Biomedical Applications

Glycosaminoglycans (GAGs) are polysaccharides ubiquitously found on cell surfaces and in the extracellular matrix (ECM). They regulate numerous cellular signaling events involved in many developmental and pathophysiological processes. GAGs are composed of complex sequences of repeating disaccharide units, each of which can carry many different modifications. The tremendous structural variations account for their ability to bind many proteins and thus, for their numerous functions. Although the sequence of GAG biosynthetic events and the enzymes involved mostly were deduced a decade ago, the emergence of tissue or cell specific GAGs from a nontemplate driven process remains an enigma. Current knowledge favors the hypothesis that macromolecular assemblies of GAG biosynthetic enzymes termed "GAGOSOMEs" coordinate polymerization and fine structural modifications in the Golgi apparatus. Distinct GAG structures arise from the differential channeling of substrates through the Golgi apparatus to various GAGOSOMEs. As GAGs perform multiple regulatory roles, it is of great interest to develop molecular strategies to selectively interfere with GAG biosynthesis for therapeutic applications. In this Account, we assess our present knowledge on GAG biosynthesis, the manipulation of GAG biosynthesis using synthetic xylosides, and the unrealized potential of these xylosides in various biomedical applications. Synthetic xylosides are small molecules consisting of a xylose attached to an aglycone group, and they compete with endogenous proteins for precursors and biosynthetic enzymes to assemble GAGs. This competition reduces endogenous proteoglycan-bound GAGs while increasing xyloside-bound free GAGs, mostly chondroitin sulfate (CS) and less heparan sulfate (HS), resulting in a variety of biological consequences. To date, hundreds of xylosides have been published and the importance of the aglycone group in determining the structure of the primed GAG chains is well established. However, the structure-activity relationship has long been cryptic. Nonetheless, xylosides have been designed to increase HS priming, modified to inhibit endogenous GAG production without priming, and engineered to be more biologically relevant. Synthetic xylosides hold great promise in many biomedical applications and as therapeutics. They are small, orally bioavailable, easily excreted, and utilize the host cell biosynthetic machinery to assemble GAGs that are likely nonimmunogenic. Various xylosides have been shown, in different biological systems, to have anticoagulant effects, selectively kill tumor cells, abrogate angiogenic and metastatic pathways, promote angiogenesis and neuronal growth, and affect embryonic development. However, most of these studies utilized the commercially available one or two β-D-xylosides and focused on the impact of endogenous proteoglycan-bound GAG inhibition on biological activity. Nevertheless, the manipulation of cell behavior as a result of stabilizing growth factor signaling with xyloside-primed GAGs is also reckonable but underexplored. Recent advances in the use of molecular modeling and docking simulations to understand the structure-activity relationships of xylosides have opened up the possibility of a more rational aglycone design to achieve a desirable biological outcome through selective priming and inhibitory activities. We envision these advances will encourage more researchers to explore these fascinating xylosides, harness the GAG biosynthetic machinery for a wider range of biomedical applications, and accelerate the successful transition of xyloside-based therapeutics from bench to bedside.

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 β4GalT7: 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 β4GalT7. Some β-d-xylosides, such as 2-naphthyl β-d-xylopyranoside, can induce GAG synthesis by serving as acceptor substrates for β4GalT7 and by that also compete with the GAG synthesis on core proteins. Here we present structure-activity relationships for β4GalT7 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 β-glycosidic linkages function as good substrates for β4GalT7. 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 β4GalT7 is dependent on subtle differences in orientation of the xylose moiety.

Enhanced channelling of sulphate through a rapidly exchangeable sulphate pool in response to stimulated glycosaminoglycan synthesis in pancreatic epithelial cells

The ability of cells to decorate glycosaminoglycans (GAGs) with sulphate in highly specific patterns is important to extracellular matrix biogenesis and placing appropriate glycosulphated ligands on the cell surface. We have examined sulphate metabolism in two pancreatic duct epithelial cell lines - PANC-1 and CFPAC-1 (derived from a cystic fibrosis patient) with a view to understanding how pancreatic cells utilise intracellular sulphate. [35S]Sulphate uptake was rapid and reached near steady state levels within 10 min. However, the intracellular specific activity of [35S]sulphate for PANC-1 and CFPAC-1 reached only 35 and 10%, respectively, of the medium specific activity at 10 min. Therefore, sulphate appears to reside within two compartments; a rapidly exchangeable sulphate pool (RESP) and a slowly exchangeable sulphate pool (SESP). Reducing chloride in the medium, increased the specific activity of [35S]sulphate within cells and increased the size of the inorganic sulphate pool, suggesting that the RESP was enlarged. Sulphate pools were not different in size between the two cell lines in physiological NaCl. Increasing the size of the sulphate pool had no effect on [35S]sulphate:[3H]glucosamine ratios incorporated into glycosaminoglycans (GAGs); however, stimulating the synthesis of GAGs with 4-methylumbelliferyl-beta-d-xyloside, stably elevated [35S]:[3H] ratios. This was due to higher [35S]sulphate incorporation. [35S]Cysteine contributed less than 0.1% of the cells' sulphate requirements. We conclude that in the face of elevated demand for sulphate, pancreatic cells appear to channel a greater proportion through the RESP.

Identification of a novel glycosaminoglycan core-like molecule. II. Alpha-GalNAc-capped xylosides can be made by many cell types

The accompanying article (Manzi, A., Salimath, P. V., Spiro, R. C., Keifer, P. A., and Freeze, H. H. (1995) J. Biol. Chem. 270, 9154-9163) reported the complete structure of a novel molecule made by human melanoma cells incubated with 1 mM 4-methylumbelliferyl-beta Xyl (Xyl beta MU). The product resembles a common pentasaccharide core region found in chondroitin/dermatan sulfate glycosaminoglycans, except that a terminal alpha-Gal-NAc residue is found in a location normally occupied by beta-GalNAc in these chains or alpha-GlcNAc in heparan sulfate chains. In this paper we show that several other human cancer cell lines and Chinese hamster ovary cells also make alpha-GalNAc-capped xylosides. The [6-3H]galactose-labeled Xyl beta MU product binds to immobilized alpha-GalNAc-specific lectin from Helix pomatia and the binding is competed by GalNAc, but not by Glc. Binding to the lectin is destroyed by digestion with alpha-N-acetylgalactosaminidase, but not beta-hexosaminidase. The nature of the aglycone influences the amount and relative proportion of this material made, with p-nitrophenyl-beta-xyloside being a better promoter of alpha-GalNAc-terminated product than Xyl beta MU. This novel oligosaccharide accounts for 45-65% of xyloside-based products made by both human melanoma and Chinese hamster ovary cells when they are incubated with 30 microM Xyl beta MU, but at 1 mM both the total amount and the proportion decreases to only 5-10%. In both cell lines this product is replaced by a corresponding amount of Sia alpha 2,3Gal beta 4Xyl beta MU. Preferential synthesis of the alpha-GalNAc-capped material at very low xyloside concentration argues that it is a normal biosynthetic product and not an experimental artifact. This pentasaccharide may be a previously unrecognized intermediate in glycosaminoglycan chain biosynthesis. Since this alpha-GalNAc residue occurs at a position that determines whether chondroitin or heparan chains are added to the acceptor, it may influence the timing, type, and extent of further chain elongation.

Identification of a novel glycosaminoglycan core-like molecule. I. 500 MHz 1H NMR analysis using a nano-NMR probe indicates the presence of a terminal alpha-GalNAc residue capping 4-methylumbelliferyl-beta-D-xylosides

beta-Xylosides compete with endogenous proteoglycan core proteins and act as alternate acceptors for synthesizing protein-free glycosaminoglycan chains. Their assembly on these alternate acceptors utilizes the same glycosyltransferases that make the protein-bound chains. Most studies using alternate acceptors focus on the production of sulfated glycosaminoglycan chains that are thought to be the major products. However, we previously showed that labeling melanoma cells with [6-3H]galactose in the presence of 4-methylumbelliferyl (MU) or p-nitrophenyl (pNP) beta-xylosides led to the synthesis of mostly di- to tetrasaccharide products including incomplete core structures. We have solved the structure of one of the previously unidentified products as, GalNAc alpha(1,4)GlcA beta(1,3)Gal beta(1,3)Gal beta(1,4)Xyl beta MU, based on compositional analysis by high performance liquid chromatography, fast atom bombardment, electrospray mass spectrometry, and one-dimensional and two-dimensional 1H NMR spectroscopy. The novel aspect of this molecule is the presence of a terminal alpha-Gal-NAc residue at a position that is normally occupied by beta-GalNAc in chondroitin/dermatan sulfate or by alpha-Glc-NAc in heparin or heparan sulfate chains. An alpha-GalNAc residue at this critical location may prevent further chain extension or influence the type of chain subsequently added to the common tetrasaccharide core.

The effect of the beta-D-xyloside naroparcil on circulating plasma glycosaminoglycans. An explanation for its known antithrombotic activity in the rabbit

Beta-D-Xylosides are known to initiate or prime free glycosaminoglycan (GAG) chain synthesis in cell and tissue culture. As such, the effect of the venous antithrombotic beta-D-xyloside, naroparcil, was investigated on the plasma GAG profile in the rabbit after oral administration. Using dose-response experiments, we showed that antithrombin activity via antithrombin III and heparin cofactor II was increased in parallel with GAG plasma levels compared to control. A more detailed qualitative examination of plasma GAGs by cellulose acetate electrophoresis and ion-exchange chromatography, following oral administration of naroparcil at 400 mg/kg, revealed the presence of higher density charged molecules compared to control. The extracted GAGs were found to activate inhibition of thrombin by heparin cofactor II and contained approximately 25% of a dermatan sulfate-like compound (undetectable in control), which could be responsible for the antithrombotic effect. Using radiolabeled naroparcil, we found radiolabeled GAG fractions and the fact that naroparcil was a substrate for galactosyltransferase I, the second enzyme responsible for GAG chain polymerization, suggested that the compound could initiate in vivo the biosynthesis of antithrombotic free GAG chains. This is, to our knowledge, the first description of the in vivo effect of a beta-D-xyloside on GAG biosynthesis; furthermore, this is correlated with an antithrombotic action.

A method for determination of galactosyltransferase I activity synthesizing the proteoglycan linkage region

An assay method was devised for measuring the activity of galactosyltransferase I (UDP-D-galactose:D-xylose galactosyltransferase), which is one of the enzymes synthesizing the linkage region between the core protein and glycosaminoglycan chains of proteoglycan. For this method, the reaction mixture contained a fluorescent substrate, 4-methylumbelliferyl-beta-D-xyloside as an acceptor, UDP-galactose as a donor and D-galactal as a competitive inhibitor of endogenous beta-galactosidase in the enzyme solution. The reaction mixture was incubated at 37 degrees C with enzyme solution prepared from an extract of cultured cells, and galactosyl-xylosyl-4-methylumbelliferone was produced as a reaction product. Measurement of galactosyltransferase I activity was performed by separation and quantitative analysis of this reaction product using high-performance liquid chromatography. Utilizing this method, easier and more sensitive detection of galactosyltransferase I activity in a cell-free system became possible. Application of the method revealed that cultured human skin fibroblasts contained galactosyltransferase I activity.

The effect of beta-D-xylosides on the proliferation and proteoglycan biosynthesis of monoblastic U-937 cells

The monoblastic cell line U-937 was cultured in the presence of C-ethyl beta-D-xyloside (E-xyl), hexyl beta-D-thioxyloside (HX-xyl), p-nitrophenyl beta-D-xyloside, phenyl beta-D-xyloside or phenyl alpha-D-xyloside. All of the beta-D-xylosides inhibited proliferation, but HX-xyl was by far the most efficient, and had a maximum effect at 1 mM concentration. The inhibitory effect of HX-xyl could be reversed; after washing, the HX-xyl-treated cells proliferated with a pattern similar to that of control cells. For more detailed analysis of the effects of beta-D-xylosides on cell proliferation and chondroitin sulphate (CS)/chondroitin sulphate proteoglycan (CSPG) structure, a comparison between the effects of E-xyl and HX-xyl was made. Treating the cells with 1 mM-HX-xyl resulted in a large increase in CS synthesis, whereas 1 mM-E-xyl had only minor effects on the rate of PG/glycosaminoglycan synthesis. Sepharose CL-6B gel chromatography of medium and cell fractions from 35S-labelled cells revealed that HX-xyl treatment resulted in the expression of only free CS chains, whereas E-xyl exposure leads to the synthesis of both large and small CSPGs, as well as some free CS chains. The expression of elevated levels of free CS chains was clearly correlated to the inhibition of proliferation. The proliferation of U-937-4, a clone of U-937 synthesizing ten times more CSPG/CS than the parent line, was equally inhibited by HX-xyl treatment. With this clone, however, there was no stimulation of CS synthesis after xyloside exposure, indicating that the elevated level of CS evident after xyloside treatment of the parent cell line is not causing the inhibition of proliferation. Furthermore, the biosynthesis of hyaluronate was shown not to be implicated in the xyloside-induced decrease in proliferation. The inhibition of proliferation observed in the presence of 1 mM-HX-xyl did not lead to differentiation of the cells into macrophage-like cells, as is observed when the cells are cultured in the presence of phorbol esters, agents also known to inhibit proliferation of U-937 cells.

p-nitrophenyl-beta-D-xyloside modulates proteoglycan synthesis and secretory differentiation in mouse mammary epithelial cell cultures

Primary cultures of mouse mammary epithelial cells synthesize significant quantities of chondroitin and heparan sulfate proteoglycans (16). Long term treatment of such cultures with p-nitrophenyl-beta-D-xylopyranoside leads to a 10-20 fold increase in the synthesis and secretion of free chondroitin sulfate glycosaminoglycan (GAG) chains and assembly of a cell-associated matrix that is relatively enriched in heparan sulfate proteoglycan. This modulation of cell-synthesized proteoglycans leads to significant changes in cell morphology and cellular differentiation. Notably cells cultured on plastic culture dishes change from being flattened to cuboidal. The synthesis of the milk proteins alpha 1, and beta-casein is also increased as is the formation of fat droplets and fat droplet membrane components. Promotion of differentiation increases with increasing xyloside concentration in the range 0-1.5 mM, but there may be a block in secretion at higher xyloside concentrations. While the detailed mechanisms remain to be elucidated, we conclude that the composition of proteoglycans incorporated into the matrix (and possibly the glycosaminoglycans secreted into the medium), may play a significant role in maintaining the phenotypic characteristics of terminally differentiated mammary epithelial cells.

The effect of retinoic acid on proteoglycan biosynthesis in bovine articular cartilage cultures

The addition of retinoic acid to adult bovine articular cartilage cultures produces a concentration-dependent decrease in both proteoglycan synthesis and the proteoglycan content of the tissue. Total protein synthesis was not affected by the presence of retinoic acid, indicating that the inhibition of proteoglycan synthesis was not due to cytotoxicity. The proteoglycans synthesized in the presence of retinoic acid were similar in hydrodynamic size, ability to form aggregates with hyaluronate, and glycosaminoglycan composition to those of control cultures. However, the presence of larger glycosaminoglycan chains suggests that the core protein was substituted with fewer but longer glycosaminoglycan chains. In cultures maintained with retinoic acid, a decreased ratio of the large proteoglycan was synthesized relative to the small proteoglycan compared to that measured in control cultures. In cultures maintained with retinoic acid for 1 day and then switched to medium with 20% (v/v) fetal calf serum, the rate of proteoglycan synthesis and hexuronate contents increased within 5 days to levels near those of control cultures. Within 2 days of switching to medium with 20% (v/v) fetal calf serum, the relative proportions of the proteoglycan species were similar to those produced in cultures maintained in medium with 20% (v/v) fetal calf serum throughout. The rate of proteoglycan synthesis by bovine articular cartilage cultures exhibited an exponential decay following exposure to retinoic acid, with estimated half-lives of 11.5 and 5.3 h for tissue previously maintained in medium alone or containing 20% (v/v) fetal calf serum, respectively. The addition of 1 mM benzyl beta-D-xyloside only partially reversed the retinoic acid-mediated inhibition of proteoglycan synthesis. This indicates that the inhibition of proteoglycan synthesis by retinoic acid was due to both a decreased availability of xylosylated core protein and a decreased capacity of the chondrocytes to synthesize chondroitin sulfate chains.

beta-D-xylosides and their analogues as artificial initiators of glycosaminoglycan chain synthesis. Aglycone-related variation in their effectiveness in vitro and in ovo

A series of aryl and alkyl O-beta-D-xylosides and their analogues with S, NH or CH2 in the glycosidic linkage were prepared and examined for their ability to act as artificial chain initiators of chondroitin (dermatan) sulphate synthesis in embryonic chick cartilage, foetal rat skin and 6-week-old-rat aorta under conditions where normal protein-core synthesis was inhibited by cycloheximide. For all these tissues in culture, phenyl O-beta-D-xyloside and phenyl beta-D-thioxyloside were clearly more effective than the corresponding N-xyloside and homo-C-xyloside. Introduction of a carboxy group to the para position of their aglycone yielded derivatives with far lower initiator activity. In a concentration range lower than 0.1 mM, the effectiveness of alkyl beta-D-thioxyloside was greatly influenced by the carbon number (n) of the alkyl group and was at a maximum at n = 7 or 8 for the cartilage, at n = 5 for the skin and at n = 4 for the aorta. In the beta-xyloside-treated cartilages, the average length of newly formed chondroitin sulphate chains reflected the chain-initiator activity of added xyloside, i.e. the higher the initiator activity, the shorter the average chain length. In the skin and aorta, none of the drugs could relieve the inhibition of heparan sulphate synthesis caused by cycloheximide. Fertilized hens' eggs were each injected on day 9 with 9.2 mumol of beta-xyloside and the skeletal systems of embryos were examined a week later. The embryos treated with beta-xylosides of relatively high initiator activity showed a 30-40% decrease in the overall growth rate of skeletons, whereas those treated with beta-xylosides of low initiator activity showed little or no decrease in the growth rate. The results are consistent with the notion that the observed change in skeletal morphology results mainly, if not completely, from beta-xyloside-induced synthesis of core-protein-free chondroitin sulphate, and further suggest that a procedure employing a series of beta-xyloside homologues with various initiator activities will furnish an easily applied criterion on which to test the specificity of xyloside action on biological processes.

Proteoglycan biosynthesis in chondrocytes: protein A-gold localization of proteoglycan protein core and chondroitin sulfate within Golgi subcompartments

The intracellular pathway of cartilage proteoglycan biosynthesis was investigated in isolated chondrocytes using a protein A-gold electron microscopy immunolocalization procedure. Proteoglycans contain a protein core to which chondroitin sulfate and keratan sulfate chains and oligosaccharides are added in posttranslational processing. Specific antibodies have been used in this study to determine separately the distribution of the protein core and chondroitin sulfate components. In normal chondrocytes, proteoglycan protein core was readily localized only in smooth-membraned vesicles which co-labeled with ricin, indicating them to be galactose-rich medial/trans-Golgi cisternae, whereas there was only a low level of labeling in the rough endoplasmic reticulum. Chondroitin sulfate was also localized in medial/trans-Golgi cisternae of control chondrocytes but was not detected in other cellular compartments. In cells treated with monensin (up to 1.0 microM), which strongly inhibits proteoglycan secretion (Burditt, L.J., A. Ratcliffe, P. R. Fryer, and T. Hardingham, 1985, Biochim. Biophys. Acta., 844:247-255), there was greatly increased intracellular localization of proteoglycan protein core in both ricin-positive vesicles, and in ricin-negative vesicles (derived from cis-Golgi stacks) and in the distended rough endoplasmic reticulum. Chondroitin sulfate also increased in abundance after monensin treatment, but continued to be localized only in ricin-positive vesicles. The results suggested that the synthesis of chondroitin sulfate on proteoglycan only occurs in medial/trans-Golgi cisternae as a late event in proteoglycan biosynthesis. This also suggests that glycosaminoglycan synthesis on proteoglycans takes place in a compartment in common with events in the biosynthesis of both O-linked and N-linked oligosaccharides on other secretory glycoproteins.

Initiation of chondroitin sulphate synthesis by beta-D-galactosides. Substrates for galactosyltransferase II

beta-Galactosides were found to initiate chondroitin sulphate chain synthesis in chick-embryo cartilage in vitro and thereby relieve inhibition by cycloheximide of [3H]-acetate incorporation into chondroitin sulphate. beta-Galactosides with an apolar aglycan group such as phenyl O-beta-galactoside were active, whereas those with a charged or polar aglycan group such as pyridine 3-O-beta-galactoside or those with sulphur instead of oxygen in the glycosidic linkage (phenyl beta-thiogalactoside) were not. beta-Galactosides also serve as substrates for microsomal galactosyltransferase activity from chick-embryo cartilage. Phenyl O-beta-galactoside and pyridine 3-O-beta-galactoside were effective substrates for this enzyme, but phenyl S-beta-thiogalactoside and pyridine 2-S-beta-thiogalactoside were only slightly active. This galactosyltransferase was shown to be a separate enzyme from galactosyltransferase I, which catalyses transfer of galactose from UDP-galactose to beta-xylosides. It is proposed that the enzyme catalysing this reaction is galactosyltransferase II, responsible for transfer of the second galactose residue of the chondroitin sulphate linkage oligosaccharide. No transfer of glucuronic acid from UDP-glucuronic acid to beta-galactosides, catalysed by the microsomal preparation could be detected.

The relation of protein synthesis to chondroitin sulphate biosynthesis in cultured bovine cartilage

The effect of cycloheximide on chondroitin sulphate biosynthesis was studied in bovine articular cartilage maintained in culture. Addition of 0.4 mM-cycloheximide to the culture medium was followed, over the next 4h, by a first-order decrease in the rate of incorporation of [35S]sulphate into glycosaminoglycan (half-life, t 1/2 = 32 min), which is consistent with the depletion of a pool of proteoglycan core protein. Addition of 1.0 mM-benzyl beta-D-xyloside increased the rate of incorporation of [35S]sulphate and [3H]acetate into glycosaminoglycan, but this elevated rate was also diminished by cycloheximide. It was concluded that cycloheximide exerted two effects on the tissue; not only did it inhibit the synthesis of the core protein, but it also lowered the tissue's capacity for chondroitin sulphate chain synthesis. Similar results were obtained with chick chondrocytes grown in high-density cultures. Although the exact mechanism of this secondary effect of cycloheximide is not known, it was shown that there was no detectable change in cellular ATP concentration or in the amount of three glycosyltransferases (galactosyltransferase-I, N-acetylgalactosaminyltransferase and glucuronosyltransferase-II) involved in chondroitin sulphate chain synthesis. The sizes of the glycosaminoglycan chains formed in the presence of cycloheximide were larger than those formed in control cultures, whereas those synthesized in the presence of benzyl beta-D-xyloside were consistently smaller, irrespective of the presence of cycloheximide. These results suggest that beta-D-xylosides must be used with caution to study chondroitin sulphate biosynthesis as an event entirely independent of proteoglycan core-protein synthesis, and they also indicate a possible involvement of the core protein in the activation of the enzymes of chondroitin sulphate synthesis.

Regulation of haemopoiesis in long-term bone marrow cultures. IV. Glycosaminoglycan synthesis and the stimulation of haemopoiesis by beta-D-xylosides

Sulfated glycosaminoglycans (GAGs) are distributed in consistent and distinctive patterns between the cell surface and the growth medium of haemopoietically active long-term bone marrow cultures. Heparan sulfate is the main cell surface component and chondroitin sulfate is the major sulfated species in the medium. When the cultures are supplemented with beta-D-xylosides a significant increase in chondroitin sulfate synthesis is observed but no stimulation of heparan sulfate synthesis occurs. The chondroitin sulfate accumulates in the culture medium in beta-D-xyloside-treated cultures but the composition of sulfated GAGs in cell-surface derived material is unaffected. beta-D-xylosides also stimulate the production of haemopoietic cells without any apparent alteration in the adherent stromal cells of the marrow cultures. Equivalent increases are obtained in cells at all stages of development so that a fivefold increase in pluripotent stem cells (CFU-S) is matched by fivefold increase in the granulocyte-macrophage progenitors (GM-CFC) and in mature granulocytes. The stimulation persists for many weeks in beta-D-xyloside-treated cultures. These results indicate that the sulfated GAGs may play an important role in the regulation of haemopoiesis.