Oligosaccharide sequences of endothelial cell surface heparan sulfate proteoglycan with affinity for lipoprotein lipase

AAV-mediated hepatic LPL expression ameliorates severe hypertriglyceridemia and acute pancreatitis in Gpihbp1 deficient mice and rats

GPIHBP1 plays an important role in the hydrolysis of triglyceride (TG) lipoproteins by lipoprotein lipases (LPLs). However, Gpihbp1 knockout mice did not develop hypertriglyceridemia (HTG) during the suckling period but developed severe HTG after weaning on a chow diet. It has been postulated that LPL expression in the liver of suckling mice may be involved. To determine whether hepatic LPL expression could correct severe HTG in Gpihbp1 deficiency, liver-targeted LPL expression was achieved via intravenous administration of the adeno-associated virus (AAV)-human LPL gene, and the effects of AAV-LPL on HTG and HTG-related acute pancreatitis (HTG-AP) were observed. Suckling Gpihbp1-/- mice with high hepatic LPL expression did not develop HTG, whereas Gpihbp1-/- rat pups without hepatic LPL expression developed severe HTG. AAV-mediated liver-targeted LPL expression dose-dependently decreased plasma TG levels in Gpihbp1-/- mice and rats, increased post-heparin plasma LPL mass and activity, decreased mortality in Gpihbp1-/- rat pups, and reduced the susceptibility and severity of both Gpihbp1-/- animals to HTG-AP. However, the muscle expression of AAV-LPL had no significant effect on HTG. Targeted expression of LPL in the liver showed no obvious adverse reactions. Thus, liver-targeted LPL expression may be a new therapeutic approach for HTG-AP caused by GPIHBP1 deficiency.

The glycocalyx: a central regulator of vascular function

The endothelial glycocalyx is a specialized extracellular matrix that covers the apical side of vascular endothelial cells, projecting into the lumen of blood vessels. The composition of the glycocalyx has been studied in great detail, and it is known to be composed of a mixture of proteoglycans, glycosaminoglycans, and glycoproteins. Although this structure was once believed to be a passive physical barrier, it is now recognized as a multifunctional and dynamic structure that participates in many vascular processes, including but not limited to vascular permeability, inflammation, thrombosis, mechanotransduction, and cytokine signaling. Because of its participation in many physiological and pathophysiological states, comprehensive knowledge of the glycocalyx will aid future vascular biologists in their research. With that in mind, this review discusses the biochemical structure of the glycocalyx and its function in many vascular physiological processes. We also briefly review a more recent discovery in glycocalyx biology, the placental glycocalyx.

Endocytosis of very low-density lipoproteins: an unexpected mechanism for lipid acquisition by breast cancer cells

We previously described the expression of CD36 and LPL by breast cancer (BC) cells and tissues and the growth-promoting effect of VLDL observed only in the presence of LPL. We now report a model in which LPL is bound to a heparan sulfate proteoglycan motif on the BC cell surface and acts in concert with the VLDL receptor to internalize VLDLs via receptor-mediated endocytosis. We also demonstrate that gene-expression programs for lipid synthesis versus uptake respond robustly to triglyceride-rich lipoprotein availability. The literature emphasizes de novo FA synthesis and exogenous free FA uptake using CD36 as paramount mechanisms for lipid acquisition by cancer cells. We find that the uptake of intact lipoproteins is also an important mechanism for lipid acquisition and that the relative reliance on lipid synthesis versus uptake varies among BC cell lines and in response to VLDL availability. This metabolic plasticity has important implications for the development of therapies aimed at the lipid dependence of many types of cancer, in that the inhibition of FA synthesis may elicit compensatory upregulation of lipid uptake. Moreover, the mechanism that we have elucidated provides a direct connection between dietary fat and tumor biology.-.

Disorders of lipid metabolism in nephrotic syndrome: mechanisms and consequences

Nephrotic syndrome results in hyperlipidemia and profound alterations in lipid and lipoprotein metabolism. Serum cholesterol, triglycerides, apolipoprotein B (apoB)-containing lipoproteins (very low-density lipoprotein [VLDL], immediate-density lipoprotein [IDL], and low-density lipoprotein [LDL]), lipoprotein(a) (Lp[a]), and the total cholesterol/high-density lipoprotein (HDL) cholesterol ratio are increased in nephrotic syndrome. This is accompanied by significant changes in the composition of various lipoproteins including their cholesterol-to-triglyceride, free cholesterol-to-cholesterol ester, and phospholipid-to-protein ratios. These abnormalities are mediated by changes in the expression and activities of the key proteins involved in the biosynthesis, transport, remodeling, and catabolism of lipids and lipoproteins including apoproteins A, B, C, and E; 3-hydroxy-3-methylglutaryl-coenzyme A reductase; fatty acid synthase; LDL receptor; lecithin cholesteryl ester acyltransferase; acyl coenzyme A cholesterol acyltransferase; HDL docking receptor (scavenger receptor class B, type 1 [SR-B1]); HDL endocytic receptor; lipoprotein lipase; and hepatic lipase, among others. The disorders of lipid and lipoprotein metabolism in nephrotic syndrome contribute to the development and progression of cardiovascular and kidney disease. In addition, by limiting delivery of lipid fuel to the muscles for generation of energy and to the adipose tissues for storage of energy, changes in lipid metabolism contribute to the reduction of body mass and impaired exercise capacity. This article provides an overview of the mechanisms, consequences, and treatment of lipid disorders in nephrotic syndrome.

Identification and characterization of two novel mutations in the LPL gene causing type I hyperlipoproteinemia

BACKGROUND

Type 1 hyperlipoproteinemia is a rare autosomal recessive disorder most often caused by mutations in the lipoprotein lipase (LPL) gene resulting in severe hypertriglyceridemia and pancreatitis.

OBJECTIVES

The aim of this study was to identify novel mutations in the LPL gene causing type 1 hyperlipoproteinemia and to understand the molecular mechanisms underlying the severe hypertriglyceridemia.

METHODS

Three patients presenting classical features of type 1 hyperlipoproteinemia were recruited for DNA sequencing of the LPL gene. Pre-heparin and post-heparin plasma of patients were used for protein detection analysis and functional test. Furthermore, in vitro experiments were performed in HEK293 cells. Protein synthesis and secretion were analyzed in lysate and medium fraction, respectively, whereas medium fraction was used for functional assay.

RESULTS

We identified two novel mutations in the LPL gene causing type 1 hyperlipoproteinemia: a two base pair deletion (c.765_766delAG) resulting in a frameshift at position 256 of the protein (p.G256TfsX26) and a nucleotide substitution (c.1211 T > G) resulting in a methionine to arginine substitution (p.M404 R). LPL protein and activity were not detected in pre-heparin or post-heparin plasma of the patient with p.G256TfsX26 mutation or in the medium of HEK293 cells over-expressing recombinant p.G256TfsX26 LPL. A relatively small amount of LPL p.M404 R was detected in both pre-heparin and post-heparin plasma and in the medium of the cells, whereas no LPL activity was detected.

CONCLUSIONS

We conclude that these two novel mutations cause type 1 hyperlipoproteinemia by inducing a loss or reduction in LPL secretion accompanied by a loss of LPL enzymatic activity.

Control of organization and function of muscle and tendon by thrombospondin-4

Thrombospondins (TSPs) are multifunctional proteins that are deposited in the extracellular matrix where they directly affect the function of vascular and other cell types. TSP-4, one of the 5 TSP family members, is expressed abundantly in tendon and muscle. We have examined the effect of TSP-4 deficiency on tendon collagen and skeletal muscle morphology and function. In Thbs4(-/-) mice, tendon collagen fibrils are significantly larger than in wild-type mice, and there is no compensatory over-expression of TSP-3 and TSP-5, the two TSPs most highly homologous to TSP-4, in the deficient mice. TSP-4 is expressed in skeletal muscle, and higher levels of TSP-4 protein are associated with the microvasculature of red skeletal muscle with high oxidative metabolism. Lack of TSP-4 in medial soleus, red skeletal muscle with predominant oxidative metabolism, is associated with decreased levels of several specific glycosaminoglycan modifications, decreased expression of a TGFβ receptor beta-glycan, decreased activity of lipoprotein lipase, which associates with vascular cell surfaces by binding to glycosaminoglycans, and decreased uptake of VLDL. The soleus muscle is smaller and hind- and fore-limb grip strength is reduced in Thbs4(-/-) mice compared to wild-type mice. These observations suggest that TSP-4 regulates the composition of the ECM at major sites of its deposition, tendon and muscle, and the absence of TSP-4 alters the organization, composition and physiological functions of these tissues.

Mass spectrometric method for determining the uronic acid epimerization in heparan sulfate disaccharides generated using nitrous acid

Heparan sulfate (HS) glycosaminoglycans (GAGs) regulate a host of biological functions. To better understand their biological roles, it is necessary to gain understanding about the structure of HS, which requires identification of the sulfation pattern as well as the uronic acid epimerization. In order to model HS structure, it is necessary to quantitatively profile depolymerization products. To date, liquid chromatography-mass spectrometry (LC-MS) methods for profiling heparin lyase decomposition products have been shown. These enzymes, however, destroy information about uronic acid epimerization. Deaminative cleavage using nitrous acid (HONO) is a classic method for GAG depolymerization that retains uronic acid epimerization. Several chromatographic methods have been used for analysis of deaminative cleavage products. The chromatographic methods have the disadvantage that there is no direct readout on the structures producing the observed peaks. This report demonstrates a porous graphitized carbon (PGC)-MS method for the quantification of HONO generated disaccharides to obtain information about the sulfation pattern and uronic acid epimerization. Here, we demonstrate the separation and identification of uronic acid epimers as well as geometric sulfation isomers. The results are comparable to those expected for benchmark HS and heparin samples. The data demonstrate the utility of PGC-MS for quantification of HS nitrous acid depolymerization products for structural analysis of HS and heparin.

Enhanced osteogenic activity of bone morphogenetic protein-2 by 2-O-desulfated heparin

The objective of this study was to investigate the effect of heparin sulfate groups on the osteogenic activity of bone morphogenetic protein-2 (BMP-2) in vitro and in vivo. Three types of desulfated (DS) derivatives of heparin (2-O-DS, 6-O-DS, and N-DS) were prepared and their bioactivity in rat bone marrow derived mesenchymal stem cells (MSC) in the absence or presence of BMP-2 was evaluated. When cultured with the 2-O-DS derivative and BMP-2 MSC showed enhanced proliferation, alkaline phosphatase activity, and Runx2 mRNA expression, compared with heparin and other derivatives. A similar tendency was observed for MSC cultured on two-dimensional substrates coated with heparin or the derivatives and in three-dimensional hydrogels containing heparin or the derivatives. A binding experiment demonstrated a greater binding affinity of 2-O-DS for BMP-2 than that of heparin and the other derivatives. Following implantation into the back subcutis of mice significantly greater ectopic bone formation in terms of bone weight, amount of calcium, and histology were observed for the gelatin hydrogels incorporating 2-O-DS and containing BMP-2. In addition, the gelatin hydrogels incorporating 2-O-DS showed controlled release of BMP-2 in vitro and in vivo. These findings demonstrated that the 2-O-DS derivative of heparin has a synergistic effect on the in vitro and in vivo osteogenic activity of BMP-2.

Lipoprotein lipase deficiency in chronic kidney disease is accompanied by down-regulation of endothelial GPIHBP1 expression

BACKGROUND

Chronic renal failure (CRF) is associated with hypertriglyceridemia and impaired clearance of very low density lipoprotein (VLDL) and chylomicrons which are largely due to lipoprotein lipase (LPL) deficiency/dysfunction. After its release from myocytes and adipocytes, LPL binds to the endothelium in the adjacent capillaries where it catalyzes hydrolysis of triglycerides in VLDL and chylomicrons. The novel endothelium-derived molecule, glycosylphosphatidylinositol-anchored binding protein 1 (GPIHBP1), plays a critical role in LPL metabolism and function by anchoring LPL to the endothelium and binding chylomicrons. GPIHBP1-deficient mice and humans exhibit severe hypertriglyceridemia and diminished heparin-releasable LPL, pointing to the critical role of GPIHBP1 in regulation of LPL activity. Given its central role in regulation of LPL activity and triglyceride metabolism, we explored the effect of chronic kidney disease (CKD) on GPIHBP1 expression.

METHODS

Expression of GPIHBP1 and LPL were determined by reverse transcriptase-polymerase chain reaction, Western blot and immunohistochemical analyses in the adipose tissue, skeletal muscle and myocardium of rats 12 weeks after 5/6 nephrectomy (CRF) or sham-operation (control).

RESULTS

Compared to the controls, the CRF group exhibited severe hypertriglyceridemia, significant reduction of the skeletal muscle, myocardium and adipose tissue LPL mRNA and protein abundance. This was accompanied by parallel reductions of GPIHBP1 mRNA abundance and immunostaining in the tested tissues.

CONCLUSIONS

LPL deficiency in CKD is associated with and compounded by GPIHBP1 deficiency. Together these abnormalities contribute to impaired clearance of triglyceride-rich lipoproteins, diminished availability of lipid fuel for energy storage in adipocytes and energy production in myocytes and consequent hypertriglyceridemia, cachexia, muscle weakness and atherosclerosis.

Lipoprotein lipase mediated fatty acid delivery and its impact in diabetic cardiomyopathy

Although cardiovascular disease is the leading cause of diabetes-related death, its etiology is still not understood. The immediate change that occurs in the diabetic heart is altered energy metabolism where in the presence of impaired glucose uptake, glycolysis, and pyruvate oxidation, the heart switches to exclusively using fatty acids (FA) for energy supply. It does this by rapidly amplifying its lipoprotein lipase (LPL-a key enzyme, which hydrolyzes circulating lipoprotein-triglyceride to release FA) activity at the coronary lumen. An abnormally high capillary LPL could provide excess fats to the heart, leading to a number of metabolic, morphological, and mechanical changes, and eventually to cardiac disease. Unlike the initial response, chronic severe diabetes "turns off" LPL, this is also detrimental to cardiac function. In this review, we describe a number of post-translational mechanisms that influence LPL vesicle formation, actin cytoskeleton rearrangement, and transfer of LPL from cardiomyocytes to the vascular lumen to hydrolyze lipoprotein-triglyceride following diabetes. Appreciating the mechanism of how the heart regulates its LPL following diabetes should allow the identification of novel targets for therapeutic intervention, to prevent heart failure. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Hepatic heparan sulfate proteoglycans and endocytic clearance of triglyceride-rich lipoproteins

Hypertriglyceridemia, characterized by the accumulation of triglyceride-rich lipoproteins in the blood, affects 10-20% of the population in western countries and increases the risk of atherosclerosis, coronary artery disease, and pancreatitis. The etiology of hypertriglyceridemia is complex, and much interest exists in identifying and characterizing the biological and environmental factors that affect the synthesis and turnover of plasma triglycerides. Genetic studies in mice have recently identified that heparan sulfate proteoglycans are a class of receptors that mediate the clearance of triglyceride-rich lipoproteins in the liver. Heparan sulfate proteoglycans are expressed by endothelial cells that line the hepatic sinusoids and the underlying hepatocytes, and are present in the perisinusoidal space (space of Disse). This chapter discusses the dependence of lipoprotein binding on heparan sulfate structure and the identification of hepatocyte syndecan-1 as the primary proteoglycan that mediates triglyceride-rich lipoprotein clearance.

The metabolism of triglyceride-rich lipoproteins revisited: new players, new insight

Peripheral lipoprotein lipase (LPL)-mediated lipolysis of triglycerides is the first step in chylomicron/VLDL clearance involving heparan sulfate proteoglycans (HSPGs) displayed at the cell surface of the capillaries in adipose tissue, heart and skeletal muscle. The newly generated chylomicron remnant particles are then cleared by the liver, whereas VLDL remnant particles are either further modified, through the action of hepatic lipase (HL) and cholesteryl ester transfer protein (CETP), into LDL particles or alternatively directly cleared by the liver. Two proteins, lipase maturation factor 1 (LMF1) and glycosylphosphatidylinositol-anchored high density lipoprotein binding protein 1 (GPIHBP1), have been recently identified and have revised our current understanding of LPL maturation and LPL-mediated lipolysis. Moreover, new insights have been gained with respect to hepatic remnant clearance using genetically modified mice targeting the sulfation of HSPGs and even deletion of the most abundant heparan sulfate proteoglycan: syndecan1. In this review, we will provide an overview of novel data on both peripheral TG hydrolysis and hepatic remnant clearance that will improve our knowledge of plasma triglyceride metabolism.

Heparan sulfate 2-O-sulfotransferase is required for triglyceride-rich lipoprotein clearance

Hepatic clearance of triglyceride-rich lipoproteins depends on heparan sulfate and low density lipoprotein receptors expressed on the basal membrane of hepatocytes. Binding and uptake of the lipoproteins by way of heparan sulfate depends on the degree of sulfation of the chains based on accumulation of plasma triglycerides and delayed clearance of triglyceride-rich lipoproteins in mice bearing a hepatocyte-specific alteration of N-acetylglucosamine (GlcNAc) N-deacetylase-N-sulfotransferase 1 (Ndst1) (MacArthur, J. M., Bishop, J. R., Stanford, K. I., Wang, L., Bensadoun, A., Witztum, J. L., and Esko, J. D. (2007) J. Clin. Invest. 117, 153-164). Inactivation of Ndst1 led to decreased overall sulfation of heparan sulfate due to coupling of uronyl 2-O-sulfation and glucosaminyl 6-O-sulfation to initial N-deacetylation and N-sulfation of GlcNAc residues. To determine whether lipoprotein clearance depends on 2-O-and 6-O-sulfation, we evaluated plasma triglyceride levels in mice containing loxP-flanked conditional alleles of uronyl 2-O-sulfotransferase (Hs2st(f/f)) and glucosaminyl 6-O-sulfotransferase-1 (Hs6st1(f/f)) and the bacterial Cre recombinase expressed in hepatocytes from the rat albumin (Alb) promoter. We show that Hs2st(f/f)AlbCre(+) mice accumulated plasma triglycerides and exhibited delayed clearance of intestinally derived chylomicrons and injected human very low density lipoproteins to the same extent as observed in Ndst1(f/f)AlbCre(+) mice. In contrast, Hs6st1(f/f)AlbCre(+) mice did not exhibit any changes in plasma triglycerides. Chemically modified heparins lacking N-sulfate and 2-O-sulfate groups did not block very low density lipoprotein binding and uptake in isolated hepatocytes, whereas heparin lacking 6-O-sulfate groups was as active as unaltered heparin. Our findings show that plasma lipoprotein clearance depends on specific subclasses of sulfate groups and not on overall charge of the chains.

Syndecan-1 is the primary heparan sulfate proteoglycan mediating hepatic clearance of triglyceride-rich lipoproteins in mice

Elevated plasma triglyceride levels represent a risk factor for premature atherosclerosis. In mice, accumulation of triglyceride-rich lipoproteins can occur if sulfation of heparan sulfate in hepatocytes is diminished, as this alters hepatic lipoprotein clearance via heparan sulfate proteoglycans (HSPGs). However, the relevant HSPG has not been determined. In this study, we found by RT-PCR analysis that mouse hepatocytes expressed the membrane proteoglycans syndecan-1, -2, and -4 and glypican-1 and -4. Analysis of available proteoglycan-deficient mice showed that only syndecan-1 mutants (Sdc1-/- mice) accumulated plasma triglycerides. Sdc1-/- mice also exhibited prolonged circulation of injected human VLDL and intestinally derived chylomicrons. We found that mice lacking both syndecan-1 and hepatocyte heparan sulfate did not display accentuated triglyceride accumulation compared with single mutants, suggesting that syndecan-1 is the primary HSPG mediating hepatic triglyceride clearance. Immunoelectron microscopy showed that syndecan-1 was expressed specifically on the microvilli of hepatocyte basal membranes, facing the space of Disse, where lipoprotein uptake occurs. Abundant syndecan-1 on wild-type murine hepatocytes exhibited saturable binding of VLDL and inhibition by heparin and facilitated degradation of VLDL. Furthermore, adenovirus-encoded syndecan-1 restored binding, uptake, and degradation of VLDL in isolated Sdc1-/- hepatocytes and the lipoprotein clearance defect in Sdc1-/- mice. These findings provide the first in vivo genetic evidence that syndecan-1 is the primary hepatocyte HSPG receptor mediating the clearance of both hepatic and intestinally derived triglyceride-rich lipoproteins.

The repertoire of glycan determinants in the human glycome

The number of glycan determinants that comprise the human glycome is not known. This uncertainty arises from limited knowledge of the total number of distinct glycans and glycan structures in the human glycome, as well as limited information about the glycan determinants recognized by glycan-binding proteins (GBPs), which include lectins, receptors, toxins, microbial adhesins, antibodies, and enzymes. Available evidence indicates that GBP binding sites may accommodate glycan determinants made up of 2 to 6 linear monosaccharides, together with their potential side chains containing other sugars and modifications, such as sulfation, phosphorylation, and acetylation. Glycosaminoglycans, including heparin and heparan sulfate, comprise repeating disaccharide motifs, where a linear sequence of 5 to 6 monosaccharides may be required for recognition. Based on our current knowledge of the composition of the glycome and the size of GBP binding sites, glycoproteins and glycolipids may contain approximately 3000 glycan determinants with an additional approximately 4000 theoretical pentasaccharide sequences in glycosaminoglycans. These numbers provide an achievable target for new chemical and/or enzymatic syntheses, and raise new challenges for defining the total glycome and the determinants recognized by GBPs.

Heparan sulfate proteoglycans and triglyceride-rich lipoprotein metabolism

PURPOSE OF REVIEW

Clearance of triglyceride-rich lipoprotein remnants by the liver is a key step in preventing hypertriglyceridemia, an independent risk factor for cardiovascular disease. We review recent genetic evidence that heparan sulfate proteoglycans work in concert with the LDL receptor in the liver to facilitate binding and clearance of both triglyceride and cholesterol-rich lipoproteins from the circulation.

RECENT FINDINGS

Partial reduction of sulfation of liver heparan sulfate using the Cre-loxP system caused accumulation of hepatic and dietary triglyceride-rich lipoprotein particles due to delayed clearance. Compounding the mutation with LDL receptor deficiency caused enhanced accumulation of both cholesterol and triglyceride-rich particles compared with mice lacking only LDL receptors. These findings provide the first genetic evidence that hepatic heparan sulfate proteoglycans play a central role in the clearance of lipoproteins by the liver and work independently of LDL receptors.

SUMMARY

A role for hepatocyte heparan sulfate in lipoprotein metabolism has now been genetically established in mice. Given this finding, mild, but clinically relevant, hyperlipidemias in human patients may be a result of alterations in heparan sulfate structure or possible genetic polymorphisms in the relevant biosynthetic genes.

Glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 plays a critical role in the lipolytic processing of chylomicrons

The triglycerides in chylomicrons are hydrolyzed by lipoprotein lipase (LpL) along the luminal surface of the capillaries. However, the endothelial cell molecule that facilitates chylomicron processing by LpL has not yet been defined. Here, we show that glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) plays a critical role in the lipolytic processing of chylomicrons. Gpihbp1-deficient mice exhibit a striking accumulation of chylomicrons in the plasma, even on a low-fat diet, resulting in milky plasma and plasma triglyceride levels as high as 5000 mg/dl. Normally, Gpihbp1 is expressed highly in heart and adipose tissue, the same tissues that express high levels of LpL. In these tissues, GPIHBP1 is located on the luminal face of the capillary endothelium. Expression of GPIHBP1 in cultured cells confers the ability to bind both LpL and chylomicrons. These studies strongly suggest that GPIHBP1 is an important platform for the LpL-mediated processing of chylomicrons in capillaries.

Liver heparan sulfate proteoglycans mediate clearance of triglyceride-rich lipoproteins independently of LDL receptor family members

We examined the role of hepatic heparan sulfate in triglyceride-rich lipoprotein metabolism by inactivating the biosynthetic gene GlcNAc N-deacetylase/N-sulfotransferase 1 (Ndst1) in hepatocytes using the Cre-loxP system, which resulted in an approximately 50% reduction in sulfation of liver heparan sulfate. Mice were viable and healthy, but they accumulated triglyceride-rich lipoprotein particles containing apoB-100, apoB-48, apoE, and apoCI-IV. Compounding the mutation with LDL receptor deficiency caused enhanced accumulation of both cholesterol- and triglyceride-rich particles compared with mice lacking only LDL receptors, suggesting that heparan sulfate participates in the clearance of cholesterol-rich lipoproteins as well. Mutant mice synthesized VLDL normally but showed reduced plasma clearance of human VLDL and a corresponding reduction in hepatic VLDL uptake. Retinyl ester excursion studies revealed that clearance of intestinally derived lipoproteins also depended on hepatocyte heparan sulfate. These findings show that under normal physiological conditions, hepatic heparan sulfate proteoglycans play a crucial role in the clearance of both intestinally derived and hepatic lipoprotein particles.

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.

HSulf-2, an extracellular endoglucosamine-6-sulfatase, selectively mobilizes heparin-bound growth factors and chemokines: effects on VEGF, FGF-1, and SDF-1

BACKGROUND

Heparin/heparan sulfate (HS) proteoglycans are found in the extracellular matrix (ECM) and on the cell surface. A considerable body of evidence has established that heparin and heparan sulfate proteoglycans (HSPGs) interact with numerous protein ligands including fibroblast growth factors, vascular endothelial growth factor (VEGF), cytokines, and chemokines. These interactions are highly dependent upon the pattern of sulfation modifications within the glycosaminoglycan chains. We previously cloned a cDNA encoding a novel human endosulfatase, HSulf-2, which removes 6-O-sulfate groups on glucosamine from subregions of intact heparin. Here, we have employed both recombinant HSulf-2 and the native enzyme from conditioned medium of the MCF-7-breast carcinoma cell line. To determine whether HSulf-2 modulates the interactions between heparin-binding factors and heparin, we developed an ELISA, in which soluble factors were allowed to bind to immobilized heparin.

RESULTS

Our results show that the binding of VEGF, FGF-1, and certain chemokines (SDF-1 and SLC) to immobilized heparin was abolished or greatly diminished by pre-treating the heparin with HSulf-2. Furthermore, HSulf-2 released these soluble proteins from their association with heparin. Native Sulf-2 from MCF-7 cells reproduced all of these activities.

CONCLUSION

Our results validate Sulf-2 as a new tool for deciphering the sulfation requirements in the interaction of protein ligands with heparin/HSPGs and expand the range of potential biological activities of this enzyme.

A unique role for 6-O sulfation modification in zebrafish vascular development

Heparan sulfate proteoglycans are important modulators of growth factor signaling in a variety of patterning processes. Secreted growth factors that play critical roles in angiogenesis bind to heparan sulfate, and this association is affected by 6-O-sulfation of the heparan sulfate chains. Addition of 6-O-sulfate is catalyzed by a family of sulfotransferases (HS6STs), and genetic manipulation of their function permits an assessment of their contribution to vascular assembly. We report on the biochemical activity and expression patterns of two zebrafish HS6ST genes. In situ hybridization reveals dynamic and distinct expression patterns of these two genes during development. Structural analysis of heparan sulfate from wild-type and morpholino antisense 'knockdown' embryos suggests that HS6ST-1 and HS6ST-2 have similar biochemical activity. HS6ST-2, but not HS6ST-1, morphants exhibit abnormalities in the branching morphogenesis of the caudal vein during embryonic development of the zebrafish. Our finding that HS6ST-2 is required for the branching morphogenesis of the caudal vein is the first in vivo evidence for an essential role of a gene encoding a heparan sulfate modifying enzyme in vertebrate angiogenesis. Our analysis of two zebrafish HS6ST genes suggests that a wide range of biological processes may be regulated by an array of sulfation-modifying enzymes in the vertebrate genome.

Specific structural features of heparan sulfate proteoglycans potentiate neuregulin-1 signaling

Neuregulins are a family of growth and differentiation factors that act through activation of cell-surface erbB receptor tyrosine kinases and have essential functions both during development and on the growth of cancer cells. One alternatively spliced neuregulin-1 form has a distinct heparin-binding immunoglobulin-like domain that enables it to adhere to heparan sulfate proteoglycans at key locations during development and substantially potentiates its activity. We examined the structural specificity needed for neuregulin-1-heparin interactions using a gel mobility shift assay together with an assay that measures the ability of specific oligosaccharides to block erbB receptor phosphorylation in L6 muscle cells. Whereas the N-sulfate group of heparin was most important, the 2-O-sulfate and 6-O-sulfate groups also contributed to neuregulin-1 binding in these two assays. Optimal binding to neuregulin-1 required eight or more heparin disaccharides; however, as few as two disaccharides were still able to bind neuregulin-1 to a lesser extent. The physiological importance of this specificity was shown both by chemical and siRNA treatment of cultured muscle cells. Pretreatment of muscle cells with chlorate that blocks all sulfation or with an siRNA that selectively blocks N-sulfation significantly reduced erbB receptor activation by neuregulin-1 but had no effect on the activity of neuregulin-1 that lacks the heparin-binding domain. These results suggest that the regulation of glycosaminoglycan sulfation is an important biological mechanism that can modulate both the localization and potentiation of neuregulin-1 signaling.

Association of hepatic lipase with proteoglycans stimulates the production of proteoglycans in vivo and in vitro

HL is synthesized in hepatocytes and functions while bound to heparan sulfate proteoglycans (HSPGs) in sinusoidal endothelial cells. The HL-mediated uptake of lipoprotein requires cell-surface HSPG. The present study tested whether HL plays a role in the production of HSPG. The production of HSPG in Chinese hamster ovary (CHO) cells was determined by measuring the incorporation of (35)SO(4) into PGs. HL-producing HL-CHO cells showed approximately 30% more cellular PG than did wild-type (WT) cells. In contrast, PG production in cells producing a membrane-anchored HL-glycophosphatidylinositol (GPI) that was not bound to HSPG was virtually identical to that in WT cells. When purified HL was added to the WT- or HL-GPI cells, PG production increased significantly to a level similar to that of the HL-secreting cells, suggesting that the binding of HL to HSPG triggered the increased HSPG production. Heparin reduced PG production in HL-producing cells, confirming that PG production is stimulated only when HL is present as a ligand for HSPG. Real-time PCR and Northern blots demonstrated that PG production was significantly reduced in animals lacking HL. Together, these data suggest that the binding of HL to PG on the cell surface exerts a positive feedback on cellular PG production.

Chondroitin sulfate anticoagulant activity is linked to water transfer: relevance to proteoglycan structure in atherosclerosis

OBJECTIVE

Changes in chondroitin sulfate (CS) proteoglycan (PG) during atherosclerosis are associated with chronic inflammatory changes and increased incidence of thrombosis. To explore how glycosaminoglycan changes could influence the thrombogenicity of atherosclerotic lesions, water-transfer reactions were examined during activation of antithrombin by CS.

METHODS AND RESULTS

Advanced type IV atherosclerotic lesions prone to thrombosis contained CSPG (versican) with undersulfated CS relative to CS of the adjacent healthy aorta. Approximately 11% of the CS disaccharide in versican from healthy arteries was oversulfated, but this proportion decreased markedly to 3% in atherosclerotic lesions. Oversulfated CS functionally bound antithrombin with a dissociation constant of 3.3+/-1.9 micromol/L. Measured by osmotic stress (OS) techniques with an approximately 26-A probe, the reaction was linked to transfer of approximately 2500 mol water per mole of coagulation factor Xa inhibited. Under OS, the anticoagulant efficiency of CS was 1.3 (micromol/L)(-1) x s(-1), approximately 5- and 15-fold higher than heparan sulfate efficiency measured under OS and standard conditions, respectively.

CONCLUSIONS

Decreased sulfation of high molecular weight CSPG in the advancing atherosclerotic lesions may predispose the lesions to thrombosis by disrupting osmotic regulation, limiting avidity for antithrombin and decreasing activation efficiency.

Order out of chaos: assembly of ligand binding sites in heparan sulfate

Virtually every cell type in metazoan organisms produces heparan sulfate. These complex polysaccharides provide docking sites for numerous protein ligands and receptors involved in diverse biological processes, including growth control, signal transduction, cell adhesion, hemostasis, and lipid metabolism. The binding sites consist of relatively small tracts of variably sulfated glucosamine and uronic acid residues in specific arrangements. Their formation occurs in a tissue-specific fashion, generated by the action of a large family of enzymes involved in nucleotide sugar metabolism, polymer formation (glycosyltransferases), and chain processing (sulfotransferases and an epimerase). New insights into the specificity and organization of the biosynthetic apparatus have emerged from genetic studies of cultured cells, nematodes, fruit flies, zebrafish, rodents, and humans. This review covers recent developments in the field and provides a resource for investigators interested in the incredible diversity and specificity of this process.

Inhibition of leukocyte and platelet aggregation in vitro by antithrombin

Glycosaminoglycan-mediated aggregation of cells occurs through adhesion mechanisms in which heparan sulfate chains bind to counter receptors on these cells. As antithrombin interacts with heparan sulfate proteoglycans through its heparin-binding domain and inhibits leukocyte adhesion in ischaemia/reperfusion, it may affect leukocyte aggregation. Leukocyte aggregation was therefore monitored in vitro as the increase in transmission of light through stirred suspensions in a platelet aggregometer. Aggregation curves were quantified as the area under the curve in the first 6 min following stimulation. Leukocytes in platelet-rich plasma were obtained from heparinized whole blood of healthy donors by centrifugation; the ratio of leukocytes to platelets was about 1:45, and the final concentration of autologous plasma was 80%. Neutrophils were purified by dextran sedimentation, density centrifugation, and hypotonic lysis of erythrocytes. Aggregation was induced by phytohaemaglutinin (0.24 mg/mL) or formyl-Met-Leu-Phe (0.2 x 10(-6) M), with or without various concentrations of antithrombin. During the observation period (6 min) no aggregation of leukocytes in platelet-rich plasma or isolated neutrophils could be induced either with medium or with antithrombin (0.2 x 10(0) to 0.2 x 10(-6) IU/mL). Addition of phytohaemagglutinin or formyl-Met-Leu-Phe stimulated aggregation of leukocytes in platelet-rich plasma and neutrophils to different extents. Additional presence of anti-thrombin significantly inhibited phytohaemagglutinin-induced aggregation of leukocytes in platelet-rich plasma, whereas formyl-Met-Leu-Phe-induced aggregation was not affected by antithrombin. Data show that in the presence of plasma and platelets, aggregation of normal white blood cells after stimulation with lectin but not with chemotaxin is inhibited by antithrombin, suggesting specific interactions of antithrombin with lectin-activated processes of neutrophil aggregation that occur in the presence of platelets and/or plasma.

Hydration effects of heparin on antithrombin probed by osmotic stress

Antithrombin is a key inhibitor of blood coagulation proteases and a prototype metastable protein. Heparin binding to antithrombin induces conformational transitions distal to the binding site. We applied osmotic stress techniques and rate measurements in the stopped flow fluorometer to investigate the possibility that hydration changes are associated with these transitions. Water transfer was identified from changes in the free energy of activation, Delta G(++), with osmotic pressure pi. The Delta G(++) was determined from the rate of fluorescence enhancement/decrease associated with heparin binding/release. The volume of water transferred, Delta V, was determined from the relationship, Delta G/pi = Delta V. With an osmotic probe of 4 A radius, the volumes transferred correspond to 158 +/- 11 water molecules from reactants to bulk during association and 162 +/- 22 from bulk to reactants during dissociation. Analytical characterization of water-permeable volumes in x-ray-derived bound and free antithrombin structures were correlated with the volumes measured in solution. Volume changes in water permeable pockets were identified at the loop-insertion and heparin-binding regions. Analyses of the pockets' atomic composition indicate that residues Ser-79, Ala-86, Val-214, Leu-215, Asn-217, Ile-219, and Thr-218 contribute atoms to both the heparin-binding pockets and to the loop-insertion region. These results demonstrate that the increases and decreases in the intrinsic fluorescence of antithrombin during heparin binding and release are linked to dehydration and hydration reactions, respectively. Together with the structural analyses, results also suggest a direct mechanism linking heparin binding/release to loop expulsion/insertion.

Regulation of glycosaminoglycan function by osmotic potentials. Measurement of water transfer during antithrombin activation by heparin

The sulfated glycosaminoglycan heparin is an important anticoagulant, widely used to treat and to prevent arterial thrombosis. Heparin triggers conformational changes in, and the functional activation of, the serine proteinase inhibitor antithrombin. We investigated water-transfer reactions during the activation process to explore the possibility that functional interaction between antithrombin and sulfated glycosaminoglycans can be regulated by osmotic potentials. Volume of water transferred upon heparin binding was measured from differences in free energy change, Delta(Delta G), with osmotic stress, pi. Osmotic stress was induced with chemically inert probes that are geometrically excluded from the water-permeable spaces of antithrombin and from intermolecular spaces formed during the association reaction. The free energy change, Delta G, for the antithrombin/heparin interaction was calculated from the dissociation constant, determined by functional titrations of heparin with antithrombin at fixed concentrations of the coagulation protease factor Xa. The effect of osmotic stress was independent of the chemical nature of osmotic probes but correlated with their radius up to radius >17 A. In mixtures including a large and a small probe, the effect of the large probe was not modified by the small probe added at a large molar excess. With an osmotic probe of 4-A radius, the Delta(Delta G)/pi slope corresponds to a transfer of 119 +/- 25 water molecules to bulk solution on formation of the complex. Analytical characterization of water-permeable volumes in x-ray-derived bound and free antithrombin structures revealed complex surfaces with smaller hydration volumes in the bound relative to the free conformation. The residue distribution in, and atomic composition of, the pockets containing atoms from residues implicated in heparin binding were distinct in the bound versus free conformer. The results demonstrate that the heparin/antithrombin interaction is linked to net water transfer and, therefore, can be regulated in biological gels by osmotic potentials.

New insights into the heparan sulfate proteoglycan-binding activity of apolipoprotein E

Defective binding of apolipoprotein E (apoE) to heparan sulfate proteoglycans (HSPGs) is associated with increased risk of atherosclerosis due to inefficient clearance of lipoprotein remnants by the liver. The interaction of apoE with HSPGs has also been implicated in the pathogenesis of Alzheimer's disease and may play a role in neuronal repair. To identify which residues in the heparin-binding site of apoE and which structural elements of heparan sulfate interact, we used a variety of approaches, including glycosaminoglycan specificity assays, (13)C nuclear magnetic resonance, and heparin affinity chromatography. The formation of the high affinity complex required Arg-142, Lys-143, Arg-145, Lys-146, and Arg-147 from apoE and N- and 6-O-sulfo groups of the glucosamine units from the heparin fragment. As shown by molecular modeling, using a high affinity binding octasaccharide fragment of heparin, these findings are consistent with a binding mode in which five saccharide residues of fully sulfated heparan sulfate lie in a shallow groove of the alpha-helix that contains the HSPG-binding site (helix 4 of the four-helix bundle of the 22-kDa fragment). This groove is lined with residues Arg-136, Ser-139, His-140, Arg-142, Lys-143, Arg-145, Lys-146, and Arg-147. In the model, all of these residues make direct contact with either the 2-O-sulfo groups of the iduronic acid monosaccharides or the N- and 6-O-sulfo groups of the glucosamine sulfate monosaccharides. This model indicates that apoE has an HSPG-binding site highly complementary to heparan sulfate rich in N- and O-sulfo groups such as that found in the liver and the brain.

Substrate specificity of the heparan sulfate hexuronic acid 2-O-sulfotransferase

The interaction of heparan sulfate with different ligand proteins depends on the precise location of O-sulfate groups in the polysaccharide chain. We have previously shown that overexpression in human kidney 293 cells of a mouse mastocytoma 2-O-sulfotransferase (2-OST), previously thought to catalyze the transfer of sulfate from 3'-phosphoadenosine 5'-phosphosulfate to C2 of L-iduronyl residues, preferentially increases the level of 2-O-sulfation of D-glucuronyl units [Rong, J., Habuchi, H., Kimata, K., Lindahl, U., and Kusche-Gullberg, M. (2000) Biochem. J. 346, 463-468]. In the study presented here, we further investigated the substrate specificity of the mouse mastocytoma 2-OST. Different polysaccharide acceptor substrates were incubated with cell extracts from 2-OST-transfected 293 cells together with the sulfate donor 3'-phosphoadenosine 5'-phospho[(35)S]sulfate. Incubations with O-desulfated heparin, predominantly composed of [(4)alphaIdoA(1)-(4)alphaGlcNSO(3)(1)-](n)(), resulted in 2-O-sulfation of iduronic acid. When, on the other hand, an N-sulfated capsular polysaccharide from Escherichia coli K5, with the structure [(4)betaGlcA(1)-(4)alphaGlcNSO(3)(1)-](n)(), was used as an acceptor, sulfate was transferred almost exclusively to C2 of glucuronic acid. Substrates containing both iduronic and glucuronic acid residues in about equal proportions strongly favored sulfation of iduronic acid. In agreement with these results, the 2-OST was found to have a approximately 5-fold higher affinity for iduronic acid-containing substrate disaccharide units (K(m) approximately 3.7 microM) than for glucuronic acid-containing substrate disaccharide units (K(m) approximately 19.3 microM).

Heparin-binding defective lipoprotein lipase is unstable and causes abnormalities in lipid delivery to tissues

Lipoprotein lipase (LpL) binding to heparan sulfate proteoglycans (HSPGs) is hypothesized to stabilize the enzyme, localize LpL in specific capillary beds, and route lipoprotein lipids to the underlying tissues. To test these hypotheses in vivo, we created mice expressing a human LpL minigene (hLpL(HBM)) carrying a mutated heparin-binding site. Three basic amino acids in the carboxyl terminal region of LpL were mutated, yielding an active enzyme with reduced heparin binding. Mice expressing hLpL(HBM) accumulated inactive human LpL (hLpL) protein in preheparin blood. hLpL(HBM) rapidly lost activity during a 37 degrees C incubation, confirming a requirement for heparin binding to stabilize LPL: Nevertheless, expression of hLpL(HBM) prevented the neonatal demise of LpL knockout mice. On the LpL-deficient background hLpL(HBM) expression led to defective targeting of lipids to tissues. Compared with mice expressing native hLpL in the muscle, hLpL(HBM) transgenic mice had increased postprandial FFAs, decreased lipid uptake in muscle tissue, and increased lipid uptake in kidneys. Thus, heparin association is required for LpL stability and normal physiologic functions. These experiments confirm in vivo that association with HSPGs can provide a means to maintain proteins in their stable conformations and to anchor them at sites where their activity is required.

Transcytosis of lipoprotein lipase across cultured endothelial cells requires both heparan sulfate proteoglycans and the very low density lipoprotein receptor

Lipoprotein lipase (LPL), the major enzyme responsible for the hydrolysis of circulating lipoprotein triglyceride molecules, is synthesized in myocytes and adipocytes but functions while bound to heparan sulfate proteoglycans (HSPGs) on the luminal surface of vascular endothelial cells. This requires transfer of LPL from the abluminal side to the luminal side of endothelial cells. Studies were performed to investigate the mechanisms of LPL transcytosis using cultured monolayers of bovine aortic endothelial cells. We tested whether HSPGs and members of the low density lipoprotein (LDL) receptor superfamily were involved in transfer of LPL from the basolateral to the apical side of cultured endothelial cells. Heparinase/heparinitase treatment of the basolateral cell surface or addition of heparin to the basolateral medium decreased the movement of LPL. This suggested a requirement for HSPGs. To assess the role of receptors, we used either receptor-associated protein, the 39-kDa inhibitor of ligand binding to the LDL receptor-related protein and the very low density lipoprotein (VLDL) receptor, or specific receptor antibodies. Receptor-associated protein reduced (125)I-LPL and LPL activity transfer across the monolayers. When the basolateral surface of the cells was treated with antibodies, only anti-VLDL receptor antibodies inhibited transcytosis. Moreover, overexpression of the VLDL receptor using adenoviral-mediated gene transfer increased LPL transcytosis. Thus, movement of active LPL across endothelial cells involves both HSPGs and VLDL receptor.

Lipoprotein lipase-mediated interactions of small proteoglycans and low-density lipoproteins

According to numerous studies low-density lipoproteins (LDL) are supposed to interact with the glycosaminoglycan chain(s) of proteoglycans, e.g. with decorin and biglycan, which themselves are subject to receptor-mediated endocytosis. We tested, therefore, whether complexes of LDL and small proteoglycans can be endocytosed by either the LDL- or the small proteoglycan uptake mechanism. However, neither was the endocytosis of LDL significantly influenced by proteoglycans nor that of proteoglycans by LDL. This negative result could be explained by the observation that in vitro complex formation takes place only in buffers of low ionic strength. Under physiological conditions additional molecules may be necessary for complex stabilization. Lipoprotein lipase (LpL) which binds LDL was also able to interact with high affinity with decorin and its glycosaminoglycan-free core protein, both interactions being heparin-sensitive. Regardless of the presence or absence of LDL, LpL stimulated the endocytosis of decorin 1.5-fold, whereas LpL mediated a 4-fold stimulation of LDL uptake in the absence of decorin. No significant additional effect was seen in the presence of small concentrations of proteoglycans whereas in the presence of 1 microM decorin the endocytosis of [125I]LDL was reduced in normal as well as in LDL receptor-deficient fibroblasts. These observations could best be explained by assuming that LpL/LDL complexes are internalized upon binding to membrane-associated heparan sulphate and that small proteoglycans interfere with this process. It could not be ruled out, however, that a small proportion of the complexes is also taken up by the small proteoglycan receptor.

The spacing of S-domains on HS glycosaminoglycans determines whether the chain is a substrate for intracellular heparanases

Heparanases are mammalian endoglucuronidases that degrade heparan sulfate (HS) glycosaminoglycans to short 5-6 kDa pieces. In the Golgi, HS glycosaminoglycans are modified by a series of interdependent reactions which result in chains that have regions rich in N- and O-sulfate groups and iduronate residues (S-domains), separated by regions that are nearly devoid of sulfate. Structural analysis of the short HS chains produced by Chinese hamster ovary (CHO) cell heparanases indicate that the enzymes recognize differences in sulfate content between S-domains and unmodified sequences, and cleave the chain at junctions between these regions. To look more closely at whether the spacing of S-domains on the gly- cosaminoglycan influences its ability to be cleaved by heparanases, we examined the susceptibility of the HS chains synthesized by the proteoglycan synthesis mutant, pgsE-606. PGS:E-606 cells are deficient in the modification enzyme N-deacetylase/N-sulfotransferase I, and synthesize HS chains that have fewer N- and O-sulfate groups and iduronate residues compared to wild-type (Bame et al., (1991), J. Biol. Chem., 266, 10287). HS glycosaminoglycans were isolated from wild-type and pgsE-606 cells and separated into populations based on sulfate content. Compared to wild-type HS, which has 14 S-domains, pgsE-606 cells synthesize three HS species, 606-1, 606-2, and 606-3, with 1, 4, and 8 S-domains, respectively. The spacing of the S-domains on the pgsE-606 HS chains is similar to the spacing the modified sequences on wild-type HS, indicating that each mutant glycosaminoglycan is composed of wild-type-like sequences and sequences devoid of S-domains. When incubated with partially purified CHO heparanases, only the portion of the mutant HS chains that had S-domains were degraded. Structural analysis of the heparanase-products confirmed that both the number and the arrangement of S-domains on the HS glycosaminoglycan are important for heparanase susceptibility. The structure of the different pgsE-606 HS chains also suggests mechanisms for the placement of S-domains when the gly- cosaminoglycan is synthesized.

Altered dermatan sulfate structure and reduced heparin cofactor II-stimulating activity of biglycan and decorin from human atherosclerotic plaque

Biglycan and decorin are small dermatan sulfate-containing proteoglycans in the extracellular matrix of the artery wall. The dermatan sulfate chains are known to stimulate thrombin inhibition by heparin cofactor II (HCII), a plasma proteinase inhibitor that has been detected within the artery wall. The purpose of this study was to analyze the HCII-stimulatory activity of biglycan and decorin isolated from normal human aorta and atherosclerotic lesions type II through VI and to correlate activity with dermatan sulfate chain composition and structure. Biglycan and decorin from plaque exhibited a 24-75% and 38-79% loss of activity, respectively, in thrombin-HCII inhibition assays relative to proteoglycan from normal aorta. A significant negative linear relationship was observed between lesion severity and HCII stimulatory activity (r = 0.79, biglycan; r = 0.63, decorin; p < 0.05). Biglycan, but not decorin, from atherosclerotic plaque contained significantly reduced amounts of iduronic acid and disulfated disaccharides DeltaDi-2,4S and DeltaDi-4,6S relative to proteoglycan from normal artery. Affinity coelectrophoresis analysis of a subset of samples demonstrated that increased interaction of proteoglycan with HCII in agarose gels paralleled increased activity in thrombin-HCII inhibition assays. In conclusion, both biglycan and decorin from atherosclerotic plaque possessed reduced activity with HCII, but only biglycan demonstrated a correlation between activity and specific glycosaminoglycan structural features. Loss of the ability of biglycan and decorin in atherosclerotic lesions to regulate thrombin activity through HCII may be critical in the progression of the disease.

Interaction of thrombospondin-1 and heparan sulfate from endothelial cells. Structural requirements of heparan sulfate

Cell surface-associated heparan sulfate proteoglycans, predominantly perlecan, are involved in the process of binding and endocytosis of thrombospondin-1 (TSP-1) by vascular endothelial cells. To investigate the structural properties of heparan sulfate (HS) side chains that mediate this interaction, the proteoglycans were isolated from porcine endothelial cells and HS chains obtained thereof by beta-elimination. To characterize the structural composition of the HS chains and to identify the TSP-1-binding sequences, HS was disintegrated by specific chemical and enzymatic treatments. Cell layer-derived HS chains revealed the typical structural heterogeneity with domains of non-contiguously arranged highly sulfated disaccharides separated by extended sequences containing predominantly N-acetylated sequences of low sulfation. Affinity chromatography on immobilized TSP-1 demonstrated that nearly all intact HS chains possessed binding affinity, whereas after heparinase III treatment only a small proportion of oligosaccharides were bound with similar affinity to the column. Size fractioning of the bound and unbound oligosaccharides revealed that only a specific portion of deca- to tetradecasaccharides possessed TSP-1-binding affinity. The binding fraction contained over 40% di- and trisulfated disaccharide units and was enriched in the content of the trisulfated 2-O-sulfated L-iduronic acid-N-sulfated-6-O-sulfated glucosamine disaccharide unit. Comparison with the disaccharide composition of the intact HS chains and competition experiments with modified heparin species indicated the specific importance of N- and 6-O-sulfated glucosamine residues for binding. Further depolymerization of the binding oligosaccharides revealed that the glucosamine residues within the TSP-1-binding sequences are not continuously N-sulfated. The present findings implicate specific structural properties for the HS domain involved in TSP-1 binding and indicate that they are distinct from the binding sequence described for basic fibroblast growth factor, another HS ligand and a potential antagonist of TSP-1.

Heparan sulfate proteoglycans on the cell surface: versatile coordinators of cellular functions

Heparan sulfate proteoglycans are complex molecules composed of a core protein with covalently attached glycosaminoglycan chains. While the protein part determines localization of the proteoglycan on the cell surfaces or in the extracellular matrix, the glycosaminoglycan component, heparan sulfate, mediates interactions with a variety of extracellular ligands such as growth factors and adhesion molecules. Through these interactions, heparan sulfate proteoglycans participate in many events during cell adhesion, migration, proliferation and differentiation. We are determining the multitude of proteoglycan functions, as their intricate roles in many pathways are revealed. They act as coreceptors for growth factors, participate in signalling during cell adhesion, modulate the activity of a broad range of molecules, and partake in many developmental and pathological processes, including tumorigenesis and wound repair. This review concentrates on biological roles of cell surface heparan sulfate proteoglycans, namely syndecans and glypicans, and outlines the progress achieved during the last decade in unraveling the molecular interactions behind proteoglycan functions.

Human abdominal aortic aneurysm is closely associated with compositional and specific structural modifications at the glycosaminoglycan level

Human abdominal aortic aneurysm (AAA) is a commonly occuring disease of blood vessels and is related to alterations in extracellular matrix molecules. In this study we report on the type and fine structural characterization of glycosaminoglycans (GAGs) present in AAA as compared with those present in normal abdominal aorta. Hyaluronan (HA), the galactosaminoglycans-chondroitin sulfate (CS) and dermatan sulfate (DS) with average molecular size (Mr) of 35-kDa-as well as heparan sulfate (HS) with Mr of 40-kDa were identified in both tissues. No significant intrabatch differences in total GAG content were identified in normal and aneurysmal aortas. Comparing, however, tissue composition and structure of GAGs between AAAs and normal aortas, significant differences (P < or = 0.001) were found. The overall GAG content in AAAs was approx. 60% lower than the normal ones. A 90% decrease in HS content, and 65 and 73% in CS and HA, respectively, were also recorded. In contrast, only a slight decrease in the amount of DS was noted (8%). Structural alterations in disaccharide composition of GAGs correspond mainly to significant decreases (P < or = 0.001) of HS-derived N-sulfated disaccharides, CS-derived 6-sulfated disaccharide and DS-derived disulfated disaccharides. These results demonstrate that the development of AAA is related to dramatic quantitative and structural modifications at the GAG level and this may well be attributed to the destruction of arterial wall architecture and further significant functional inadequacies of the tissue.

Anticoagulant heparan sulfate precursor structures in F9 embryonal carcinoma cells

To understand the mechanisms that control anticoagulant heparan sulfate (HSact) biosynthesis, we previously showed that HSact production in the F9 system is determined by the abundance of 3-O-sulfotransferase-1 as well as the size of the HSact precursor pool. In this study, HSact precursor structures have been studied by characterizing [6-3H]GlcN metabolically labeled F9 HS tagged with 3-O-sulfates in vitro by 3'-phosphoadenosine 5'-phospho-35S and purified 3-O-sulfotransferase-1. This later in vitro labeling allows the regions of HS destined to become the antithrombin (AT)-binding sites to be tagged for subsequent structural studies. It was shown that six 3-O-sulfation sites exist per HSact precursor chain. At least five out of six 3-O-sulfate-tagged oligosaccharides in HSact precursors bind AT, whereas none of 3-O-sulfate-tagged oligosaccharides from HSinact precursors bind AT. When treated with low pH nitrous or heparitinase, 3-O-sulfate-tagged HSact and HSinact precursors exhibit clearly different structural features. 3-O-Sulfate-tagged HSact hexasaccharides were AT affinity purified and sequenced by chemical and enzymatic degradations. The 3-O-sulfate-tagged HSact hexasaccharides exhibited the following structures, DeltaUA-[6-3H]GlcNAc6S-GlcUA-[6-3H]GlcNS3(35)S+/-6S-++ +IdceA2S-[6-3H]Glc NS6S. The underlined 6- and 3-O-sulfates constitute the most critical groups for AT binding in view of the fact that the precursor hexasaccharides possess all the elements for AT binding except for the 3-O-sulfate moiety. The presence of five potential AT-binding precursor hexasaccharides in all HSact precursor chains demonstrates for the first time the processive assembly of specific sequence in HS. The difference in structures around potential 3-O-sulfate acceptor sites in HSact and HSinact precursors suggests that these precursors might be generated by different concerted assembly mechanisms in the same cell. This study permits us to understand better the nature of the HS biosynthetic pathway that leads to the generation of specific saccharide sequences.

Apolipoprotein E containing high density lipoprotein stimulates endothelial production of heparan sulfate rich in biologically active heparin-like domains. A potential mechanism for the anti-atherogenic actions of vascular apolipoprotein e

Reduced heparin and heparan sulfate (HS) proteoglycans (PG) have been observed in both inflammation and atherosclerosis. Methods to increase endogenous heparin and heparan sulfate are not known. We found that incubation of endothelial cells with 500-1,000 micrograms/ml high density lipoprotein (HDL) increased 35SO4 incorporation into PG by 1.5-2.5-fold. A major portion of this increase was in HS and was the result of increased synthesis. Total PG core proteins were not altered by HDL; however, the ratio of 35SO4 to [3H]glucosamine was increased by HDL, suggesting increased sulfation of glycosaminoglycans. In addition, HDL increased the amount of highly sulfated heparin-like HS in the subendothelial matrix. HS from HDL-treated cells bound 40 +/- 5% more 125I-antithrombin III (requires 3-O sulfated HS) and 49 +/- 3% fewer monocytes. Moreover, the HS isolated from HDL-treated cells inhibited smooth muscle cell proliferation (by 83 +/- 5%) better than control HS (56 +/- 6%) and heparin (42 +/- 6%). HDL isolated from apolipoprotein E (apoE)-null mice did not stimulate HS production unless apoE was added. ApoE also stimulated HS production in the absence of HDL. ApoE did not increase 35SO4 incorporation in macrophages and fibroblasts, suggesting that this is an endothelial cell-specific process. Receptor-associated protein inhibited apoE-mediated stimulation of HS only at higher (20 micrograms/ml) doses, suggesting the involvement of a receptor-associated protein-sensitive pathway in mediating apoE actions. In summary, our data identify a novel mechanism by which apoE and apoE-containing HDL can be anti-atherogenic. Identification of specific apoE peptides that stimulate endothelial heparin/HS production may have important therapeutic applications.

Thymic epithelial cells synthesize a heparan sulfate with a highly sulfated region

Epithelial cells are important components of the thymus microenvironment and are involved in thymocyte differentiation. The production and secretion of sulfated glycosaminoglycans by these cells grown in culture were investigated using labeling with radioactive 35S-Na2SO4 and 3H-glucosamine. The major glycosaminoglycans synthesized by these cells are heparan sulfate and hyaluronic acid. The structure of the heparan sulfate was investigated by the pattern of degradation products formed by deaminative cleavage with nitrous acid. The ratio 35S-sulfate/ H-glucosamine is high in the segments of the heparan sulfate released during the deaminative cleavage with nitrous acid but low in the resistant portion of the molecule. Thus, the heparan sulfate synthesized by the thymic epithelial cells contains a highly sulfated region. Digestion with heparitinase reveals that this highly sulfated region is a heparin-like segment of the molecule. The heparan sulfate is rapidly incorporated into the cell surface but its secretion to the extracellular medium requires a longer incubation period. Finally, heparin was used to mimic the possible effect of this heparan sulfate with a highly sulfated region, as ascertained by its ability to modulate thymocyte adhesion to thymic epithelial cells. Since heparin actually enhanced thymocyte adhesion, it is suggested that the heparan sulfate described herein, secreted by the thymic epithelium, may play a role upon intrathymic heterotypic cellular interactions.

Differentiation-associated modulation of heparan sulfate structure and function in CaCo-2 colon carcinoma cells

Heparan sulfate species expressed by different cell and tissue types differ in their structural and functional properties. Limited information is available on differences in regulation of heparan sulfate biosynthesis within a single tissue or cell population under different conditions. We have approached this question by studying the effect of cell differentiation on the biosynthesis and function of heparan sulfate in human colon carcinoma cells (CaCo-2). These cells undergo spontaneous differentiation in culture when grown on semipermeable supports; the differentiated cells show phenotypic similarity to small intestine enterocytes. Metabolically labeled heparan sulfate was isolated from the apical and basolateral media from cultures of differentiated and undifferentiated cells. Compositional analysis of disaccharides, derived from the contiguous N-sulfated regions of heparan sulfate, indicated a greater proportion of 2-O-sulfated iduronic acid units and a smaller amount of 6-O-sulfated glucosamine units in differentiated than in undifferentiated cells. By contrast, the overall degree of sulfation, the chain length and the size distribution of the N-acetylated regions were similar regardless the differentiation status of the cells. The structural changes were found to affect the binding of heparan sulfate to the long isoform of platelet-derived growth factor A chain but not to fibroblast growth factor 2. These findings show that heparan sulfate structures change during cell differentiation and that heparan sulfate-growth factor interactions may be affected by such changes.