Kinetics of proteoheparan sulfate synthesis, secretion, endocytosis, and catabolism by a hepatocyte cell line

Catalytic and substrate promiscuity: distinct multiple chemistries catalysed by the phosphatase domain of receptor protein tyrosine phosphatase

The presence of latent activities in enzymes is posited to underlie the natural evolution of new catalytic functions. However, the prevalence and extent of such substrate and catalytic ambiguity in evolved enzymes is difficult to address experimentally given the order-of-magnitude difference in the activities for native and, sometimes, promiscuous substrate/s. Further, such latent functions are of special interest when the activities concerned do not fall into the domain of substrate promiscuity. In the present study, we show a special case of such latent enzyme activity by demonstrating the presence of two mechanistically distinct reactions catalysed by the catalytic domain of receptor protein tyrosine phosphatase isoform δ (PTPRδ). The primary catalytic activity involves the hydrolysis of a phosphomonoester bond (C─O─P) with high catalytic efficiency, whereas the secondary activity is the hydrolysis of a glycosidic bond (C─O─C) with poorer catalytic efficiency. This enzyme also displays substrate promiscuity by hydrolysing diester bonds while being highly discriminative for its monoester substrates. To confirm these activities, we also demonstrated their presence on the catalytic domain of protein tyrosine phosphatase Ω (PTPRΩ), a homologue of PTPRδ. Studies on the rate, metal-ion dependence, pH dependence and inhibition of the respective activities showed that they are markedly different. This is the first study that demonstrates a novel sugar hydrolase and diesterase activity for the phosphatase domain (PD) of PTPRδ and PTPRΩ. This work has significant implications for both understanding the evolution of enzymatic activity and the possible physiological role of this new chemistry. Our findings suggest that the genome might harbour a wealth of such alternative latent enzyme activities in the same protein domain that renders our knowledge of metabolic networks incomplete.

Beta-elimination for release of O-linked glycosaminoglycans from proteoglycans

O-linked glycosaminoglycan (GAG) chains in proteoglycans are readily released from their core proteins by treatment with alkali at room temperature. This beta-elimination is the same type of reaction as that for releasing O-linked oligosaccharides from their core proteins. Under the reaction conditions described here, N-linked oligosaccharides remain attached to the core protein, but any O-linked oligosaccharides will be released along with the GAG chains. The procedure can be used to isolate the free GAG chains, the free O-linked oligosaccharides, and the core protein (which will still have any N-linked oligosaccharides that were originally present).

Tripeptidyl-peptidase I in health and disease

The lysosomal lumen contains numerous acidic hydrolases involved in the degradation of carbohydrates, lipids, proteins, and nucleic acids, which are basic cell components that turn over continuously within the cell and/or are ingested from outside of the cell. Deficiency in almost any of these hydrolases causes accumulation of the undigested material in secondary lysosomes, which manifests itself as a form of lysosomal storage disorder (LSD). Mutations in tripeptidyl-peptidase I (TPP I) underlie the classic late-infantile form of neuronal ceroid lipofuscinoses (CLN2), the most common neurodegenerative disorders of childhood. TPP I is an aminopeptidase with minor endopeptidase activity and Ser475 serving as an active-site nucleophile. The enzyme is synthesized as a highly glycosylated precursor transported by mannose-6-phosphate receptors to lysosomes, where it undergoes proteolytic maturation. This review summarizes recent progress in understanding of TPP I biology and molecular pathology of the CLN2 disease process, including distribution of the enzyme, its biosynthesis, glycosylation, transport and activation, as well as catalytic mechanisms and their potential implications for pathogenesis and treatment of the underlying disease. Promising data from gene and stem cell therapy in laboratory animals raise hope that CLN2 will be the first neurodegenerative LSD for which causative treatment will become available for humans.

Heparan sulfate regulates the anabolic activity of MC3T3-E1 preosteoblast cells by induction of Runx2

The transcription factor Runx2 can be controlled by a number of upstream regulators involved in intracellular signalling, including the activation ERK1/2 signaling by fibroblast growth factor-2 (FGF-2). FGFs interact with their cell surface receptors (FGFRs) through an obligate cross-binding interaction with heparan sulfate proteoglycan (HSPG) co-receptors; exogenous HS sugar chains have been shown to potently modulate changes in cell phenotype depending on the stage of tissue differentiation when the HS is harvested, suggesting that HS chain structure and function varies depending on the stage of cell maturity. This study examined the potential of bone-derived heparan sulfate (HS), harvested from differentiating osteoblasts, for the enhancement of preosteoblast growth and differentiation. HS was harvested from conditioned media, cell surface and matrix compartments of postconfluent (differentiating) MC3T3-E1 osteoblasts and dosed back onto preconfluent MC3T3-E1 cells. We show that HS can increase the expression Runx2, ALP, and OPN in preosteoblast cells, suggesting the potential for exogenous HS to shift cells from proliferative to differentiative phenotypes. In line with their structural differences, only HS released into the media was found to co-stimulate the mitogenic effect of FGF-2, whilst exogenous application of all the HSs together with FGF-2 served to increase the expression of OPN. Only the application of cell surface-derived HS triggered a synergistic increase in FGFR1 expression together with FGF-2, although all three HS preparations could trigger transient increases in PI3K, ERK1/2, and stat3 phosphorylation levels. These findings demonstrate that the compartmentally distinct HS species expressed by differentiating MC3T3-E1 cells act in complex ways to coordinate the extracellular conditions that lead to osteoblast differentiation, with the cell surface species coordinating the FGF response.

Glycosaminoglycans Modulate Activation, Activity, and Stability of Tripeptidyl-peptidase I in Vitro and in Vivo

Tripeptidyl-peptidase I (TPP I, CLN2 protein) is a lysosomal exopeptidase that sequentially removes tripeptides from the N termini of polypeptides and shows a minor endoprotease activity. Mutations in TPP I lead to classic late-infantile neuronal ceroid lipofuscinosis, a neurodegenerative lysosomal storage disease. TPP I proenzyme is converted in lysosomes into a mature enzyme with the assistance of another protease and is able to autoactivate in acidic pH in vitro via a unimolecular mechanism. Because autoactivation in vitro at the pH values reported for lysosomes generated inactive enzyme, we intended to determine whether physiologically relevant factors can modify this process to also make it plausible in vivo. Here, we report that high ionic strength and glycosaminoglycans (GAGs) increase yields (ionic strength) or yields and rates (GAGs) of activation, enhance degradation of liberated TPP I prosegment fragments, and switch effective autoactivation of TPP I proenzyme toward less acidic pH values (up to pH 6.0). Although ionic strength and GAGs also inhibited TPP I activity in vitro and in living cells, the degree of inhibition (from 20 to 60%) appears to be of rather limited functional significance. Importantly, binding to GAGs improved thermal stability of TPP I and protected the enzyme against alkaline pH-induced denaturation in vitro (t((1/2)) of mature enzyme at pH 7.4 increased by approximately 8-fold in the presence of heparin) and in vivo ( approximately 2-fold higher loss of TPP I in cells deficient in GAGs than in control cells after bafilomycin A1 treatment). These findings elucidate a potent physiologically relevant mechanism of TPP I regulation by GAGs and suggest that generation of the active enzyme via autoactivation can be accomplished not only in vitro but in vivo as well.

Syndecan-1 and -4 synthesized simultaneously by mouse mammary gland epithelial cells bear heparan sulfate chains that are apparently structurally indistinguishable

Many of the biological functions attributed to cell surface heparan sulfate (HS) proteoglycans, including the Syndecan family, are elicited through the interaction of their HS chains with soluble extracellular molecules. Tightly controlled, cell-specific sulfation and epimerization of HS precursors endows these chains with highly sulfated, iduronate-rich regions, which are major determinants of cytokine and matrix-protein binding and which are interspersed by N-acetylated, poorly sulfated regions. Until this study, there have been no comprehensive structural comparisons made on HS chains decorating simultaneously expressed, but different, syndecan core proteins. In this paper we demonstrate that the HS chains on affinity-purified syndecan-1 and -4 from murine mammary gland cells are essentially identical by a number of parameters. Size determination, disaccharide analyses, enzymatic and chemical scission methods, and affinity co-electrophoresis all failed to reveal any significant differences in fine structure, domain organization, or ligand-binding properties of these HS species. These findings lead us to suggest that the imposition of the fine structure onto HS occurs independently of the core protein to which it is attached and that these core proteins, in addition to the HS chains, may play a pivotal role in the various biological functions ascribed to these macromolecules.

Cathepsin B activity regulation. Heparin-like glycosaminogylcans protect human cathepsin B from alkaline pH-induced inactivation

It has been shown that lysosomal cysteine proteinases, specially cathepsin B, has been implicated in a variety of diseases involving tissue remodeling states, such as inflammation, parasite infection, and tumor metastasis, by degradation of extracellular matrix components. Recently, we have shown that heparin and heparan sulfate bind to papain specifically; this interaction induces an increase of its alpha-helix content and stabilizes the enzyme structure even at alkaline pH (Almeida, P. C., Nantes, I. L., Rizzi, C. C. A., Júdice, W. A. S., Chagas, J. R., Juliano, L., Nader, H. B., and Tersariol, I. L. S. (1999) J. Biol. Chem. 274, 30433-30438). In the present work, a combination of circular dichroism analysis, affinity chromatography, cathepsin B mutants, and fluorogenic substrate assays were used to characterize the interaction of human cathepsin B with glycosaminoglycans. The nature of the cathepsin B-glycosaminoglycans interaction was sensitive to the charge and type of polysaccharide. Like papain, heparin and heparan sulfate bind cathepsin B specifically, and this interaction reduces the loss of cathepsin B alpha-helix content at alkaline pH. Our data show that the coupling of cathepsin B with heparin or heparan sulfate can potentiate the endopeptidase activity of the cathepsin B, increasing 5-fold the half-life (t(12)) of the enzyme at alkaline pH. Most of these effects are related to the interaction of heparin and heparan sulfate with His(111) residue of the cathepsin B occluding loop. These results strongly suggest that heparan sulfate may be an important binding site for cathepsin B at cell surface, reporting a novel physiological role for heparan sulfate proteoglycans.

Heparan sulfate chains with antimitogenic properties arise from mesangial cell-surface proteoglycans

Heparan sulfate (HS) chains accumulate in both the medium and the cell layer of mesangial cell cultures. When given in fresh medium to quiescent cultures at naturally occurring concentrations, they suppress entry into the cell cycle and progression to DNA synthesis. We have attempted to identify the proteoglycan (PG) source of the antimitogenic HS chains from mesangial cell layers (HS(c)) and medium (HS(c)). When cells were labeled for 16 hours with [35S]sulfate, 25% of the label was found in intracellular HS chains and 5% in extracellular HSPGs. Cell-surface HSPGs accounted for the remaining 70% of the label associated with cell-layer HS and were released by either trypsin or 2% Triton X-100. About 20% of this cell-surface fraction was released by treatment with phosphatidylinositol-specific phospholipase C (PI-PLC), and probably represents glypican-like PG; glypican mRNA was present in the cells. The remainder of this fraction could be incorporated into liposomes, indicating the presence of hydrophobic transmembrane regions suggestive of syndecans. Upon purification and deglycosylation, an antiserum to rat liver HSPGs that reacts primarily with syndecan-2 showed a strong signal corresponding to this protein and three weaker bands that may represent additional syndecans. mRNAs for syndecan-1, -2, and -4 were present in the cultures. Syndecan-1 and -2 mRNAs were increased 30 minutes after stimulation of quiescent rat mesangial cells (RMCs) with serum. Heparin, HS(c), and HS(m) all prevented this increase. Syndecan-4 mRNA was not affected by serum, heparin, or HS. In pulse-chase experiments, the amount of 35S appearing in the cellular protein-free HS fraction was accounted for almost entirely by cell-surface PGs, as matrix-associated label was a minor contribution at the end of the pulse-labeling. The appearance of [35S]HS in cell extracts was unaffected by phospholipase C treatment, indicating that turnover of the newly labeled syndecan fraction is the source of the antimitogenic HS chains.

Natural hepatocyte growth factor (HGF) from human serum and a bound form of recombinant HGF with heparan sulfate are indistinguishable in their physicochemical properties

Natural hepatocyte growth factor (nHGF) purified from human serum showed a difference in molecular mass (Mr) between SDS PAGE (76-90 kDa) and gel filtration chromatography on a Sephadex G-200 column (> 200 kDa), whereas nHGF or recombinant HGF (rHGF) from cell culture medium did almost the same Mr (74-100 kDa). A bound form of rHGF with heparan sulfate (or heparin), and an aggregate form of rHGF itself showed a homogeneous band with a Mr of 76-90 kDa on SDS PAGE, but showed a Mr larger than 200 kDa on a Sephadex G-200 column. Both nHGFs, rHGF and the bound form were basic, but the aggregate form was acidic in ionic nature. No significant difference was found in affinity for heparin among these HGF preparations. The bound form treated by the procedures for purification of nHGF from human serum still showed a larger molecular form. The bound form mimicked physicochemical properties of nHGF purified from human serum. These results suggest that a possible form of nHGF in human serum may be a bound form with heparin-like molecules such as heparan sulfate, which are found in the circulation and on cell surface, and purified as the bound form.

Structural characterization and functional effects of a circulating heparan sulfate in a patient with hepatocellular carcinoma

A circulating anticoagulant was isolated from the plasma of a 42-year-old man with cirrhosis and hepatocellular carcinoma who had an unusual coagulation test profile. The patient developed a fatal coagulopathy, unresponsive to protamine therapy or plasma exchange following liver biopsy. However, at presentation, routine hemostasis assays were normal. The patient had mucocutaneous bleeding but the sole laboratory abnormality was a prolonged thrombin time (TT = 99 s, normal 25-35 s). Protamine titration indicated activity equivalent to a heparin concentration of 6-7 U/ml. Antithrombin III (AT III) antigen and activity were markedly elevated. The anticoagulant activity, purified from plasma by DEAE chromatography, was identified as a glycosaminoglycan (GAG). GAG anti-thrombin activity was completely abolished by heparin lyase III. Based on the degree of sulfation and HPLC pattern, the GAG was classified as heparan sulfate. Low levels (4 microM) of purified GAG markedly prolonged the TT (>120 s) but not the activated partial thromboplastin time (PTT) (31.4 s). In a Factor Xa assay, the GAG exhibited a potency equivalent to 0.06 U of low molecular weight heparin per nmol of uronic acid. Patients with endogenous circulating glycosaminoglycans can present with unusual laboratory coagulation test profiles. These reflect complex dysfunction of hemostasis, leading to difficulty in providing diagnosis and effective care.

Lysosomal glycosaminoglycan storage as induced by dicationic amphiphilic drugs: investigation into the mechanisms underlying the slow reversibility

Several dicationic amphiphilic compounds, such as the immunomodulator tilorone and analogues, impair the lysosomal catabolism of sulphated glycosaminoglycans (GAGs). Thereby they cause lysosomal GAG storage in rats and in cultured fibroblasts of several species including man. The GAG storage is rather slowly reversible in vivo; it persists for months after discontinuance of drug treatment. In the present study, we investigated the mechanisms underlying the slow reversibility. Cultured bovine corneal fibroblasts were pretreated for 4 days with tilorone (5 and 20 microM) or with compound CL-90.100 (3 and 10 microM) and further cultured in drug-free medium for periods up to 11 days. The intracellular GAG storage was analysed biochemically and demonstrated histochemically. The subcellular drug distribution (CL-90.100) was demonstrated by fluorescence microscopy. Dermatan sulphate (DS) provided the predominant contribution towards the GAG storage. After pretreatments with the low, as well as the high concentrations of either drug, the storage of DS was irreversible during the period of observation, whereas the minor storage of heparan sulphate was resolved. The enhanced secretion of the lysosomal enzyme beta-hexosaminidase (E.C. 3.2.1.52) caused by pretreatment with the high concentration of tilorone was also readily reversible. Thus, enzyme deprivation could not be the explanation for the sustained DS storage. The localization of the drug-related fluorescence within perinuclear cell organelles, presumably lysosomes, resembled that of the stored GAGs as visualized by histochemical staining. Both, the fluorescence and the positive GAG staining persisted with unchanged intracellular distribution throughout the recovery period. The present results suggest that the persistence of the DS storage is due to the formation of long-lived, non-degradable DS-drug complexes within the lysosomes.

Structural modification of fibroblast growth factor-binding heparan sulfate at a determinative stage of neural development

Heparan sulfate (HS) glycosaminoglycans are essential modulators of fibroblast growth factor (FGF) activity and appear to act by coupling particular forms of FGF to appropriate FGF receptors. During neural development, one particular HS proteoglycan is able to rapidly switch its potentiating activity from FGF-2, as neural precursor cell proliferation occurs, to FGF-1, as neuronal differentiation occurs. Using various analytical techniques, including chemical and enzymatic cleavage, low pressure chromatography, and strong anion-exchange high performance liquid chromatography, we have analyzed the different HSs expressed during these crucial developmental stages. There are distinct alterations in patterns of 6-O-sulfation, total chain length, and the number of sulfated domains of the HS from the more mature embryonic brain. These changes correlate with a switch in the ability of the HS to potentiate the actions of FGF-1 in triggering cell differentiation. It thus appears that each HS pool is designed to function in the modulation of an intricate interaction with a specific growth factor and its cognate receptor, and suggests tightly regulated expression of specific, bioactive disaccharide sequences. The data can be used to construct a simple model of controlled variations in HS chain structure which have functional consequences at a crucial stage of neuronal maturation.

Visualization of several binding sites for basic fibroblast growth factor (FGF-2) on fibroblasts by photoaffinity labeling: evidence for intracellular complexes

The internalization of basic fibroblast growth factor (FGF-2) was studied in Chinese hamster lung fibroblasts (CCL39). Recombinant FGF-2 was derivatized with a photoactivable agent, N-hydroxysuccinimidyl-4-azidobenzoate (HSAB), iodinated, and used to visualize intracellular FGF-2-affinity-labeled molecules after internalization at 37 degrees C. Iodinated HSAB-FGF-2 maintained the properties of natural FGF-2 such as affinity for heparin, binding to Bek and Fig receptors, interaction with high- and low-affinity binding sites, and reinitiating of DNA synthesis in CCL39 cells. Affinity-labeling experiments at 4 degrees C with 125I-HSAB-FGF-2 led to the detection of several FGF-cell surface complexes with apparent molecular mass of 80, 100, 125, 150, 170-180, 220, 260, and about 320 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), whereas two specific bands at 80 and 130-160 kDa were obtained using the homobifunctional cross-linking reagent, disuccinimidyl suberate. When the cells, preincubated with 125I-HSAB-FGF-2 at 4 degrees C and then washed, were shifted to 37 degrees C, irradiation of the internalized labeled FGF-2 led to detection of a similar but fainted profile with one major specific band at 80 kDa. Heparitinase II treatment of the cells reduced binding of 125I-HSAB-FGF-2 to its cell surface sites by 80% and internalization by 55%, indicating the involvement of heparan sulfate proteoglycans in these processes. Among the heparitinase-sensitive bands was the 80-kDa complex.

Heparin binding, internalization, and metabolism in vascular smooth muscle cells: I. Upregulation of heparin binding correlates with antiproliferative activity

Vascular smooth muscle cell (SMC) hyperplasia is an important component in the pathogenesis of arteriosclerotic lesions and is responsible for the failure of many vascular surgical procedures. SMC proliferation is inhibited by the glycosaminoglycan heparin; however, the precise mechanisms of action are still not understood. One important question in this regard is whether binding, internalization, and metabolism of heparin are necessary for the antiproliferative activity. In this study, we have analyzed SMC rendered resistant to the antiproliferative effect of heparin by drug selection and retroviral infection of SMC. We first examined the ability of heparin to bind to SMC. Experiments using [3H]heparin indicate the presence of saturable, heparin-displaceable, protease-sensitive binding sites on both sensitive and resistant SMC. The affinity of heparin binding does not correlate with the antiproliferative response. Using fluorescent and radiolabeled heparin probes, we observed that early heparin internalization kinetics in both sensitive and resistant SMC is similar, indicating that resistance to heparin is not due to changes in the ability of cells to take up heparin. In contrast, we observed during the continuous incubation with heparin that binding to resistant SMC is rapidly downregulated, whereas sensitive cells continue to bind and internalize heparin. These results suggest that upregulation of heparin binding to the SMC surface is required for an antiproliferative response. In an accompanying paper (Letourneur et al. [1995] J. Cell Physiol., 165:687-695, this issue), we describe the degradation and secretion of internalized heparin in both sensitive and resistant SMC.

Evidence that both receptor- and heparan sulfate proteoglycan-bound basic fibroblast growth factor are internalized by cultured immature Leydig cells

The present studies examined how 125I-labeled basic fibroblast growth factor (bFGF) bound to high affinity receptors and with lower affinity to heparan sulfate proteoglycans (HSPG) of cultured immature rat Leydig cells was processed. Following incubation for 2 h at 4 degrees C with 125I-bFGF, cells were washed to remove unbound radioactivity. Fresh medium was added, and cells were incubated at 4 degrees and/or 37 degrees C. At time zero and at specific intervals over the next 6 h, the incubation medium was saved and cells washed to quantitate 125I-bFGF released into the medium, associated with HSPG of the cell surface or extracellular matrix (radioactivity released by washing cells with 2 M NaCl, pH 7.4), associated with cell surface receptors (radioactivity released by washing cells with 2 M NaCl, pH 4.0) or internalized (radioactivity resistant to high salt and acid washes, and solubilized with 0.5 M NaOH). Radioactivity released into the initial medium and the pooled washes was further divided into a trichloroacetic acid (TCA)-precipitated form (radioactivity precipitated by 10% TCA) and a TCA-soluble form (radioactivity remaining in the TCA supernatant). 125I-bFGF associated with both HSPG and surface receptors declined progressively during the first 4 h of incubation before stabilizing when cells were transferred to 37 degrees C. These declines were associated with a corresponding increase in intracellular 125I-bFGF. These changes were blocked by maintaining cells at 4 degrees C. The majority of internalized 125I-bFGF appeared to originate from the HSPG-bound fraction as there was a greater decline in HSPG-associated radioactivity and most of the increase in internalized radioactivity could be blocked by the inclusion of 10 micrograms/ml heparin (which mainly blocks 125I-bFGF binding to HSPG but not to high affinity receptors) during the initial incubation with 125I-bFGF for 2 h at 4 degrees C. Furthermore, HSPG-mediated internalization appeared to have two components: the major fraction was blocked by the inclusion of 10 micrograms/ml heparin, while a heparin-resistant fraction, appeared to be closely linked both quantitatively and temporarily to receptor-mediated internalization. A minor fraction of internalized 125I-bFGF was metabolized in lysosomes, as the inclusion of 50 microM chloroquine during the 6 h incubation at 37 degrees C inhibited most of the increase in TCA-soluble radioactivity appearing in the incubation medium.(ABSTRACT TRUNCATED AT 400 WORDS)

Basic fibroblast growth factor-induced increase in 125I-human chorionic gonadotropin binding to luteinizing hormone receptors in cultured immature Leydig cells is mediated by binding to heparan sulfate proteoglycans

Previous studies have shown that basic fibroblast growth (bFGF) has a biphasic effect on 125I-hCG binding to LH receptors in cultured Leydig cells from immature rats. Low concentrations of bFGF (0.1-1.0 ng/ml) progressively decreased binding, while higher concentrations (10-100 ng/ml) progressively increased binding above nadir levels. In the present studies, treatment of cultured immature Leydig cells with heparinase I and/or heparinase III, which enzymatically remove heparan sulfate proteoglycans, had no effect on basal binding of 125I-hCG to LH receptors or the decrease in binding due to treatment with low bFGF concentrations; however, this treatment dramatically reduced the secondary increase in binding following the addition of higher bFGF concentrations. These results strongly support the idea that the secondary increase in 125I-hCG binding to LH receptors elicited by treatment with higher bFGF concentrations is mediated by bFGF binding to heparan sulfate proteoglycans associated with the plasma membrane and/or extracellular matrix.

Accumulation of heparan sulfate in the culture of human melanoma cells with different metastatic ability

Glycosaminoglycans were metabolically labeled in subconfluent cultures of highly metastatic 7Gp122 and poorly metastatic IC8 variants and of the low metastatic parental M4Be human melanoma cell line. Proteoglycans were separated by DEAE Trisacryl chromatography from the culture medium, from the heparin extract of the cell layer and from the heparin-extracted cell residue lyzed with detergents. Glycosaminoglycans were released from the proteoglycans by reductive alkaline hydrolysis and heparan sulfate (HS) was detected by deaminative cleavage with nitrous acid. Expressed on cell protein basis, the labeled HS content in the medium and in the cell layer decreased with increasing metastatic ability. The extraction of HS with heparin from the 7Gp122 cells indicated that this variant was enriched in (polypeptide bound) HS non inserted into the plasma membrane, compared with the low metastatic IC8 and M4Be cells. The HS fraction in heparin extract and in the heparin-extracted cell residue exhibited molecular mass heterogeneity on gel permeation chromatography and it contained HS fragments. Scission with nitrous acid followed by molecular sieve chromatography of the degradation products indicated that the tetra- and disaccharide repeats separated by the N-sulfated glucosamine residue were present in about equal amounts and constituted 60% of the HS chains in the IC8 and M4Be cells. HS from 7Gp122, IC8 and M4Be cells did not bind antithrombin III with high affinity but it was capable of binding bFGF in in vitro assay.

Differentially expressed patterns of glycosaminoglycan structure in heparan sulfate proteoglycans and free chains

The metabolic relationships between heparan sulfate proteoglycans, free chains, and oligosaccharides in different cell locations were evaluated by comparing their glycosaminoglycan structure. Metabolically labeled heparan sulfate proteoglycans of BALB/c 3T3 cell layers and in conditioned medium were compared with the heparan sulfate free chains (modal mass = 10 kDa) and oligosaccharides (modal mass = 3 kDa) of the cells. Nonlytic, in situ digestion with heparitinase I indicated that 90% of proteoglycans, 70% of the free chains, and 20% of the oligosaccharides were enzyme accessible, but there was no evidence using competitive ligands for binding of the products to the cell surface via the glycosaminoglycan moieties. Structurally, the membrane proteoglycans were the most O-/N-sulfated and yielded more tri- and tetra-sulfated di- and tetra-saccharides by nitrous acid degradation. In contrast, the side chains of medium proteoglycans were less sulfated and more polydisperse in mass, suggesting that most medium proteoglycans are not processed from membrane precursors. The heparan sulfate free chains were of lower mass, less sulfated, and more heterogeneous in distribution of the anionic groups than were proteoglycan side chains. Corroborating analytical heparitinase I digestion indicated that generation of di- and tetra-saccharides proportionately increased from membrane proteoglycan, to cell free chain, to medium proteoglycan categories. Because the structural patterns of the heparan sulfate free chains did not reveal a clear relationship with the side chains of the major proteoglycans, their origin was further probed by [3H]BH4-labeling of the reducing terminus under varying stringencies. The end-labeled residues obtained by nitrous or strong acid hydrolysis of the free chains showed insignificant amounts of galactose and xylose, but rather glucosamine N-sulfate and a residue likely generated from glucuronate. The effective labeling that was achieved with weak alkali indicated that covalent oligopeptide is not present. In summary, the heparan sulfate free chains, which in part are components of the cell surface, are of relatively low mass, are unassociated with covalent peptide, and most probably have a disaccharide motif of glucosamine N-sulfate and a uronate residue at the reducing end. Taken together, these observations suggest that the free chains originate by processing of precursor heparan sulfate proteoglycans on the cell surface via an endoglycosidase acting on an N-sulfated portion of the original polymer.

Major influence of cell differentiation status on characteristics of proteoglycans synthesized by cultured rabbit renal proximal tubule cells: role of insulin and dexamethasone

To analyze the influence of epithelial cell differentiation and the effects of hormones on the characteristics of cell-associated and secreted proteoglycans (PGs), we studied their distribution, synthesis, and biochemical features in a model of renal proximal tubule cells in primary culture in which cell differentiation could be controlled by medium composition. In cells cultured in serum-free, hormonally defined medium supplemented with insulin and dexamethasone that exhibited a high degree of morphological and functional proximal differentiation (Ronco et al., 1990), cell-associated PGs were similar to those extracted in vivo by their size estimated by Sepharose CL-6B chromatography (Kav = 0.27, vs. 0.26), composition (heparan-sulfate), and localization in a continuous basal layer of extra-cellular matrix (ECM). In contrast, major quantitative and qualitative anomalies of cell-associated PGs were observed in poorly differentiated cells grown in 1% fetal calf serum-supplemented medium (FCS). PGs alterations included: (1) reduced and irregular expression of PGs at the cell basal pole, (2) a 2.8-fold decrease in [35S]-sulfate incorporation into cell-associated PGs, (3) a 3.1-fold increase in trypsin-releasable PGs, and (4) the emergence of a high MW PG composed exclusively of chondroitin-sulfate (CS) (Kav = 0.09 on Sepharose CL-6B) as well as of putative free CS-glycosaminoglycan (GAG) chains (Kav = 0.49 on Sepharose CL-6B). The same alterations were identified in the basal defined medium devoid of hormones but were partially or totally abolished by addition of insulin and dexamethasone, respectively. At variance with cell-associated PGs, production and biochemical features of secreted PGs were not influenced by cell differentiation status and medium composition.

Internalization of basic fibroblast growth factor (bFGF) in cultured endothelial cells: role of the low affinity heparin-like bFGF receptors

We have shown (Presta et al., Cell Regul., 2:719-726, 1991) that a long-lasting interaction of basic fibroblast growth factor (bFGF) with endothelial GM 7373 cells is required to induce cell proliferation. In the present work, we have investigated the interaction of 125I-bFGF with GM 7373 cells, its pathway of internalization, and its intracellular fate under the same experimental conditions previously utilized to assess the mitogenic activity of the growth factor. Cell cultures were incubated with 10 ng/ml 125I-bFGF for 2 h at 4 degrees C. Then, cells were shifted to 37 degrees C without changing the medium. A rapid down-regulation of high affinity sites, paralleled by a rapid internalization of 125I-bFGF, was observed during the first 1-2 h at 37 degrees C. After 6-8 h, also low affinity sites down-regulate. This was paralleled by a continuous internalization of 125I-bFGF and by a slow disappearance of the growth factor from the culture medium. This suggests that GM 7373 cells activate, when exposed to bFGF for a long period of time, a late internalization pathway mediated by low affinity sites. This hypothesis was supported by the following experimental evidence: 1) soluble heparin inhibited the prolonged internalization of 125I-bFGF and its binding to low affinity sites with the same potency; 2) treatment of GM 7373 cells with heparinase, which removes most of the low affinity sites, also inhibited the prolonged internalization of 125I-bFGF. 125I-bFGF internalized via low affinity sites was partially protected from lysosomal degradation. This was the case also when 125I-bFGF was internalized in the presence of soluble heparin, suggesting that the complexes bFGF-cell surface glycosaminoglycan and bFGF-soluble heparin are maintained during the internalization of the growth factor. Moreover, the capacity of soluble heparin to inhibit the mitogenic activity of bFGF also when added to cell cultures several hours after the growth factor indicates that the requirement for a prolonged interaction of bFGF with GM 7373 cells in order to induce cell proliferation might be related to the late internalization of the growth factor via low affinity sites.

Effects of cycloheximide, brefeldin A, suramin, heparin and primaquine on proteoglycan and glycosaminoglycan biosynthesis in human embryonic skin fibroblasts

(1) We have isolated radiolabelled proteoglycans and glycosaminoglycans produced by human embryonic skin fibroblasts in the presence of (a) cycloheximide to inhibit protein synthesis or (b) brefeldin A to impede transport between the endoplasmic reticulum and the Golgi complex or (c) suramin, heparin or primaquine to interfere with internalization, recycling and degradation. Effects on glycosaminoglycan synthesis were assayed separately by using exogenous p-nitrophenyl beta-D-xylopyranoside (and [3H]galactose) or 125I-labelled p-hydroxyphenyl beta-D-xylopyranoside as initiators. (2) Inhibition of protein synthesis or blocking of transport to the Golgi complex prevented production of most of the proteoglycans with one exception: Cell-associated heparan sulphate-proteoglycan was still produced at 20% of the control level. (3) Treatment with suramin or heparin resulted in decreased deposition of proteoglycan in the pericellular matrix but increased accumulation of cell-associated proteoglycan. Primaquine blocked all proteoglycan synthesis. (4) In the presence of cycloheximide, exogenous beta-D-xyloside initiated galactosaminoglycan production. In contrast, in brefeldin A-treated cells, synthesis was completely abolished. Not even formation of the linkage-region trisaccharide could be detected. (5) These results suggest that exogenous xyloside enters the endoplasmic reticulum and is subsequently transported to the trans-Golgi complex where all further steps involved in glycosaminoglycan assembly takes place. (6) Heparan sulphate proteoglycan produced by brefeldin A-treated cells could be derived from (a) an intracellular pool of preformed core protein located to the trans-Golgi complex, or (b) resident proteoglycan that was either deglycanated/reglycanated or chain-extended. As combined treatment with suramin and brefeldin A markedly reduced cell-associated proteoglycan production, the latter possibility is favoured.

Heparan sulfate-degrading enzymes induce modulation of smooth muscle phenotype

Macrophages cocultured with rabbit aortic smooth muscle cells at a ratio of 1:3 degraded all the 35S-labeled heparan sulfate proteoglycan from the smooth muscle surface into free sulfate (Kav of 0.84 on Sepharose 6B). Concomitantly, the same macrophages induced a decrease in the volume fraction of myofilaments (Vvmyo) of the smooth muscle cells and a decrease in alpha-actin mRNA as a percentage of total actin mRNA. Both macrophage lysosomal lysate at neutral pH and heparinase degraded cell-free 35S-labeled matrix deposited by smooth muscle cells into fragments which eluted at a Kav of 0.63 and which were identified as heparan sulfate chains by their complete degradation in the presence of low pH nitrous acid. At acid pH the macrophage lysosomal lysate completely degraded the heparan sulfate to free sulfate (Kav 0.84). Both macrophage lysosomal lysate and commercial heparinase at neutral pH induced smooth muscle phenotypic change while other enzymes such as trypsin and chondroitin ABC lyase had no effect. It was therefore suggested that the active factor present in the macrophages is a lysosomal heparan sulfate-degrading endoglycosidase (heparinase). Only a small amount of heparan sulfate-degrading activity was released into the incubation medium by living macrophages, and there was no heparinase activity on their isolated plasma membranes, although proteolytic enzymes were evident in both instances. In pulse-chase studies, high Vvmyo smooth muscle cells were seen to constantly internalize and degrade 35S-labeled heparan sulfate proteoglycan from their own pericellular compartment, suggesting that this may be the mechanism by which smooth muscle phenotype is maintained under normal circumstances and that removal of heparan sulfate from the surface of smooth muscle cells and its degradation by macrophages temporarily interrupts this process, inducing smooth muscle phenotypic change.

Age-related changes in hyaluronan, proteoglycan, collagen, and osteonectin synthesis by human bone cells

Human bone cells grown in culture, representative of a preosteoblastic stage of maturation, produce an extracellular matrix composed of collagen, several noncollagenous glycoproteins, hyaluronan, and four distinct proteoglycans (PGs). The influence of donor age on the levels of expression of these molecules in vitro has not been well characterized. In this study, human bone cells derived from sources ranging from fetal to 60-year-old donors were grown in culture, radiolabeled for 24 h, and the amount of incorporation of [35S]sulfate into PGs, [3H]glucosamine into hyaluronan, [3H]leucine/proline into osteonectin, and [3H]proline into collagen was determined. Cell proliferation was most rapid in fetal-derived bone cells and decreased with increasing age. Total protein and PG synthesis also decreased with increasing age, falling to 1/3 and 1/4, respectively, of fetal levels after age 30. A large chondroitin sulfate PG (Mr approximately 600,000 Da) was the major fetal PG and its levels were highly correlated with cellular proliferation. [3H]Collagen and [35S]decorin levels increased with the increasing age of the donor, reached a maximum in puberty-derived cells, and decreased to 1/3 maximal levels after age 20. The heparan sulfate PG (Mr approximately 400,000 Da) exhibited steady-state levels regardless of donor age. [3H]Osteonectin and [35S]biglycan levels were high in fetal-derived cells and in cells derived from pubescent donors. The percentage of collagen and four proteoglycans associated with the cell layer pool changed with donor age. All fetal-derived PG core proteins possessed more N- and O-linked oligosaccharides than newborn or adult derived PGs.

Internalization of basic fibroblast growth factor by Chinese hamster lung fibroblast cells: involvement of several pathways

Subconfluent Chinese hamster lung fibroblast cells (CCL39) which express high- and low-affinity binding sites for basic fibroblast growth factor (bFGF) were used to study bFGF internalization. Kinetics at 37 degrees C indicated that this process was complex and involved various pathways with regard to the ligand concentration used. Internalization with 6 to 45 pM of 125I-r-bFGF led to a steady state that lasted up to 3 h without any appearance of 125I-labeled degradation products in the cell-culture medium, suggesting that the endocytosis reached equilibrium. Furthermore, binding data at steady state, at 37 degrees C, revealed a two-phase Scatchard curve suggesting the involvement of two families of interaction sites in the process of internalization. Apparent dissociation constants were estimated to be 20 pM and 58 nM, respectively, and the number of bFGF molecules involved per cell, 4300 and 1.3 x 10(6), respectively. These data were in good agreement with those obtained from binding experiments at equilibrium at 4 degrees C. Besides, higher concentrations of 125I-r-bFGF (greater than 47 pM) induced an internalization process which did not reach steady state and was not saturable. These results suggest that CCL39 cells could internalize bFGF by various pathways involving high- and low-affinity binding sites.

Metabolism and turnover of cell surface-associated heparan sulfate proteoglycan and chondroitin sulfate proteoglycan in normal and cholesterol-enriched macrophages

Analysis of sulfur-35-labeled proteoglycans indicated that cholesterol-enriched pigeon peritoneal macrophages synthesized 42% more 35S-labeled proteoglycan when compared with control macrophages during a 24-hour incubation. Proteoglycan turnover was subsequently studied in radiolabeled macrophage cultures after a 1-, 3-, 6-, 12-, or 24-hour chase with fresh media. During the chase, intracellular proteoglycan disappeared rapidly, whereas there was a small accumulation of 35S-labeled proteoglycan in the media that plateaued at about 6 hours and remained relatively constant thereafter. Pericellular heparan sulfate proteoglycan and chondroitin sulfate proteoglycan disappeared throughout the chase and did not appear to accumulate in the media or in the intracellular compartment. The rapid disappearance of intracellular proteoglycans along with the relative lack in metabolism of media proteoglycans indicated that the majority of pericellular proteoglycans were metabolized via an intracellular degradative pathway. Kinetic analysis of pericellular proteoglycans revealed the presence of a single pool of heparan sulfate proteoglycan (half-life [t1/2] = 6.9 hours) and a single pool of chondroitin sulfate proteoglycan (t1/2 = 11.5 hours) in control macrophage cultures. Cholesterol-enriched macrophage cultures also contained a single pool of pericellular heparan sulfate proteoglycan (t1/2 = 7.3 hours) but contained two pools of chondroitin sulfate proteoglycan (t1/2 = 0.8 hour and 25.9 hours).

Proliferation and transformation of cultured liver fat-storing cells (perisinusoidal lipocytes) under conditions of beta-D-xyloside-induced abrogation of proteoglycan synthesis

Fat-storing cells (perisinusoidal lipocytes, Ito cells) are the major connective tissue-producing cell type in liver. In areas of necroinflammation the cells proliferate and transform into desmin and smooth muscle alpha-actin-positive myofibroblast-like cells which synthesize a broad spectrum of significant amounts of collagens, proteoglycans, and matrix glycoproteins. Available data suggest a central role for these cells in the pathogenesis of fibrosis. Beta-D-Xyloside, an artificial initiation site for galactose-linked glycosaminoglycans, thereby uncoupling the synthesis of core protein and GAG, was used as a probe to study main cellular functions under conditions of abrogated proteoglycan synthesis. The exposure for 48 hr of fat-storing cells to p-nitrophenyl beta-D-xyloside (PNP-Xyl) increased dose-dependently the synthesis of [35S]sulfate-labeled medium GAG. Maximum stimulation of fivefold above normal was reached at 1.0 mM PNP-Xyl. Higher concentrations of PNP-Xyl progressively decreased the stimulatory effect on GAG synthesis. The relative composition of GAG in medium (60% chondroitin sulfate, 34% dermatan sulfate), at the cell surface, and intracellularly (mainly heparan sulfate) was not changed significantly by PNP-Xyl. The amounts of intracellular and cell surface-bound GAG were reduced by 40 and 30%, respectively, by PNP-Xyl leading to a depletion of heparan sulfate at the cell surface. Pulse-chase experiments revealed that xyloside-initiated GAG were secreted immediately after synthesis into the medium. GAG synthesized in the presence of 1 and 5 mM PNP-Xyl were free of core protein, and the molecular size of the GAG chains was smaller than that of GAG obtained from beta-eliminated proteoglycans synthesized in control cultures. At concentrations above 3 mM PNP-Xyl generated a dose-dependent inhibition of cell proliferation, which was at any stage of culture fully reversible upon removal of the drug. Viability and general protein synthesis were not reduced, but fat-storing cell transformation and deposition of matrix glycoproteins were retarded. Only a very small fraction of drug-treated cells (5 mM PNP-Xyl) did express on the 11th culture day smooth muscle iso-alpha-actin- and desmin-containing cytoskeletal filaments, which are important indicators of transformation into myofibroblast-like cells. Furthermore, the synthesis of hyaluronan and the expression of immunostained fibronectin, laminin, and tenascin were reduced in cultures exposed to 5 mM PNP-Xyl. The described cellular functions were not affected by exposure of fat-storing cells to p-nitrophenyl beta-D-galactoside.(ABSTRACT TRUNCATED AT 400 WORDS)

Synthesis of proteoglycans and hyaluronic acid by long-term cultures of testicular cells from immature and pubertal rats

Long-term cultures of somatic testicular cells derived from immature and pubertal rats were used to study the synthesis of proteoglycans (PG) and hyaluronic acid (HA). Labelled PG and HA in the culture medium, membrane-associated and intracellular pools were characterized by gel filtration, ion exchange chromatography and selected enzymatic and chemical treatments. Somatic cells synthesize a PG containing both heparan and chondroitin/dermatan sulfate (CS/DS) chains and a PG containing only CS/DS chains. No major qualitative changes in the type of PG were observed in cells derived from immature and pubertal animals. However, significant age-dependent differences in the cell distribution pattern of PG and HA were determined. This may have implications in the regulation of spermatogenesis.

A quantitative solid-phase assay for identifying radiolabeled glycosaminoglycans in crude cell extracts

Extraction of radiosulfate-labeled cell layers in denaturing urea and nonionic detergent allows the quantitative binding of GAG-containing materials from up to 96 discrete samples to a single cationic nylon blot. Free sulfate and/or sulfated lipids fail to bind. Washing the blot with differential salt concentrations discriminates between native proteoglycans and free glycosaminoglycan chains or fragments. In addition, chondroitin sulfates and heparan sulfate are identified either by prior digestion with chondroitin ABC or AC lyase, as generated disaccharides fail to bind to the blot, or by treatment of the entire blot with nitrous acid following binding. Similarly, heparan sulfate can be identified on chromatograms or Western transfers from polyacrylamide gel electrophoresis by autoradiography before and after treatment of the blot with nitrous acid.

Chondroitin sulfate proteoglycan synthesis and reutilization of beta-D-xyloside-initiated chondroitin/dermatan sulfate glycosaminoglycans in fetal kidney branching morphogenesis

Branching morphogenesis and chondroitin sulfate proteoglycan synthesis by explanted fetal mouse kidneys were previously shown to be inhibited by p-nitrophenyl beta-D-xylopyranoside (beta-D-xyloside) while glomerular development and heparan sulfate proteoglycan synthesis were unaffected. The metabolic fate of fetal kidney explant proteoglycans was investigated to determine whether or not recovery of proteoglycan synthesis and morphogenesis occur after exposure to beta-D-xyloside. Chondroitin sulfate proteoglycan synthesis resumed within 4 hr of removal of beta-D-xyloside and was enhanced once beta-D-xyloside-initiated chondroitin/dermatan-35SO4 glycosaminoglycans (GAGs) were released from the tissue. Radioactivity incorporated into beta-D-xyloside-initiated chondroitin/dermatan-35SO4 GAGs during labeling in the presence of beta-D-xyloside was reutilized in the synthesis of chondroitin-35SO4 proteoglycan during a 24-hr chase in nonradioactive medium without beta-D-xyloside. Further, highly purified beta-D-xyloside-initiated chondroitin/dermatan-35SO4 GAGs were taken up by kidneys more avidly than was free [35S]sulfate. These 35S-GAGs were degraded and reutilized in the synthesis of chondroitin-35SO4 proteoglycan. Ureteric bud branching resumed 48 hr after beta-D-xyloside was removed from the incubation medium. These findings support the idea that both chondroitin sulfate proteoglycan synthesis and proteoglycan processing may be involved in branching morphogenesis.

Control of cell division in hepatoma cells by exogenous heparan sulfate proteoglycan

The effects of cell surface heparan sulfate proteoglycan (HSPG) prepared from log and confluent monolayers of a rat hepatoma cell line on hepatoma cell growth were studied. When HSPG isolated from confluent cells was added exogenously to log phase cells, it was internalized and free heparan sulfate (HS) chains appeared transiently in the nucleus. Concurrently, the growth of the treated cells was inhibited, but the cells resumed logarithmic growth as the level of nuclear HS fell, and the cells grew to confluence and became contact inhibited. When HSPG prepared from log-phase hepatoma cells was added exogenously to log phase cells, it was internalized but very little of the internalized HS appeared in the nucleus, and there was no change in the rate of cell growth. However, when the rate of cell growth was reduced by culture of the cells in serum- and insulin-deficient medium, HSPG prepared from log-phase cells stimulated the growth rate of these slow-growing cells. The cell cycle dependency of HSPG uptake and growth inhibition was studied in cultures synchronized by a thymidine/aphidicolin double block. When [35SO4]HSPG from confluent cells was added to synchronized cells just as they were released from the second block, a portion of the [35SO4]HSPG was internalized and [35SO4]HS appeared in the nucleus. However, at mitosis the [35SO4]HS disappeared almost completely from all of the cellular pools, and after mitosis, more of the [35SO4]HSPG was taken up and [35SO4]HS reappeared in the nucleus and remained in the nucleus until the cells divided again. When cultures were released from the aphidicolin block, both control and HSPG-treated cells progressed through the S, the G2, and the M phases of the cell cycle. However, the length of the G1 phase of the cycle was increased in the HSPG-treated cells. The treated cultures then progressed through the second S, G2, and M phases. Thus, the inhibition of cell division occurred in the G1 phase of the cell cycle, prior to the G1/S boundary. Addition of the HSPG to the synchronized cultures just after the first mitosis resulted in an immediate arrest of the cell cycle in G1.(ABSTRACT TRUNCATED AT 400 WORDS)

Correlations between heparan sulfate metabolism and hepatoma growth

A rat hepatoma cell line (Gershenson et al., Science, 170:859-861, 1970) contains a dynamic steady-state pool of free heparan sulfate (HS) chains in the nucleus that increases in amount when growing cells reach confluence (Fedarko and Conrad, J. Cell Biol., 102:587-599, 1986). In logarithmically growing cells labeled with 35SO4(2-) steady-state levels of [35SO4]HS in the nucleus are altered by a variety of culture conditions. Rapidly dividing cells (doubling time = 18-22 h) growing under optimized conditions had steady-state levels of nuclear HS within the range of 40-50 pmol 35SO4 in nuclear HS/10(6) cells. The steady-state levels of nuclear HS were lowered by several changes in culture conditions, including 1) additions of 1 mM p-nitrophenyl-beta-D-xyloside, 0.25-0.5 mM (+)-catechin, 0.5 ng/ml transforming growth factor beta, 20 ng/ml phorbol-12-myristate-13-acetate, 1 mM dibutyryl cAMP, or 1 mM inositol-2-PO4; 2) decreased levels of D-glucose; or 3) deletions of serum, insulin, or inositol. In all cases lowering of the nuclear HS level was accompanied by an increase in the cell doubling times, suggesting a correlation in which nuclear HS levels must be optimized for maximal growth rates. When cells cultured under optimal growth conditions reached confluence, the level of nuclear HS increased threefold and the cells stopped dividing. The same culture conditions that lowered the steady-state levels of HS in the logarithmically growing cells prevented this rise in the nuclear HS as the cells reached confluence and resulted in loss of contact inhibition and overgrowth of the confluent cultures. These observations suggest a second correlation in which elevated nuclear HS levels are found when cell growth is inhibited at confluence; prevention of this rise results in continued growth. Consistent with this correlation between elevated nuclear HS and reduced growth rates, it was observed that addition of either 0.5 microgram/ml hydrocortisone or 0.05 microgram/ml retinoic acid to the culture medium of logarithmically growing cultures resulted in increases in steady-state levels of nuclear HS that were accompanied by increased cell doubling times. The two agents that increased the levels of nuclear HS in logarithmically growing cultures had little effect on levels of nuclear HS in confluent cells or on contact inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell biology of arterial proteoglycans

Although proteoglycans constitute a minor component of vascular tissue, these molecules have been shown to influence a number of arterial properties such as viscoelasticity, permeability, lipid metabolism, hemostasis, and thrombosis. A hallmark of early and late atherosclerosis is the accumulation of proteoglycans in the intimal lesions. Yet, it is not clear why this accumulation occurs. This article reviews the classes of proteoglycans synthesized by the two major cell types of the arterial wall--the endothelial and smooth muscle cell. Detailed consideration is then given to the modulation of proteoglycan metabolism and the role that proteoglycans play in a number of cellular events such as adhesion, migration, and proliferation--important processes in both the development and the pathogenesis of blood vessels. Last, the involvement of proteoglycans in two critical vascular wall processes--hemostasis and lipid metabolism--is reviewed, because these events pertain to atherogenesis. This review emphasizes the importance of proteoglycans in regulating several key events in normal and pathophysiological processes in the vascular tissue.

The disaccharide composition of heparins and heparan sulfates

Heparin and heparan sulfate can be cleaved selectively at their N-sulfated glucosamine residues by direct treatment with nitrous acid at pH 1.5. These polymers can also be cleaved selectively at their N-acetylated glucosamine residues by first N-deacetylating with hydrazine and then treating the products with nitrous acid at pH 4. These procedures have been combined and optimized for the conversion of these glycosaminoglycan chains into their disaccharide units. A modified hydrazinolysis procedure in which the glycosaminoglycans were heated with hydrazine:water (70:30) containing 1% hydrazine sulfate gave rapid rates of N-deacetylation and minimal conversion of the uronic acid residues to their hydrazide derivatives. Under these conditions, N-deacetylation was complete in 4 h and the beta-eliminative cleavage of the polymer chains that occurs during hydrazinolysis (P. N. Shaklee and H. E. Conrad (1984) Biochem. J. 217, 187-197) was eliminated. Treatment of the N-deacetylated polymer with nitrous acid at pH 3 for 15 h at 25 degrees C then gave simultaneous cleavage at the N-unsubstituted glucosamine residues and the N-sulfated glucosamine residues. These deamination conditions minimized, but did not eliminate, the side reaction in which nitrous acid-reactive glucosamine residues undergo ring contraction without glucosaminide bond cleavage. Thus, the disaccharides were obtained in a yield of 90% of those originally present in the glycosaminoglycan chains. Since the ring contraction side reaction occurs randomly at the diazotized glucosamine residues, the disaccharides formed in the pH 3 nitrous acid reaction were recovered in proportions equal to those in the original glycosaminoglycan chain.(ABSTRACT TRUNCATED AT 250 WORDS)

Heparin and heparan sulfate binding sites on B-16 melanoma cells

We have reported previously that the production of a tumor cell factor that stimulates synthesis of fibroblast collagenase is influenced by a fibroblast-deposited matrix component, possibly heparan sulfate-proteoglycan. In this study, binding sites for heparin and heparan sulfate on mouse B-16 melanoma cells have been demonstrated. Binding of 3H-heparin and 35S-heparan sulfate has been shown to occur to whole cells, isolated membranes, and to a component(s) of detergent extracts of the membranes. Scatchard analysis of binding of 3H-heparin yielded a Kd of 2-5 x 10(-8) M and a Bmax of 0.5 x 10(7) heparin molecules bound per cell. Binding of 35S-heparan sulfate was of at least an order of magnitude lower affinity than heparin, but the Bmax was similar to that for heparin. Competition studies showed that 35S-heparan sulfate binding was inhibited totally by heparin and heparan sulfate and partially by dermatan sulfate, but no inhibition was obtained with hyaluronate or chondroitin sulfate. Binding of 3H-heparin was inhibited totally by heparin but to different extents by preparations of heparan sulfate from different tissue sources. The heparin/heparan sulfate binding activity is a protein(s) because it is destroyed by treatment with trypsin. Binding of 3H-heparin to transblots of the detergent extract of the B-16 cell membranes indicated that at least part of the binding activity is a 14,000-dalton protein.

Evidence for independent metabolism and cell surface localization of cell surface localization of cellular proteoglycans and glycosaminoglycan free chains

The synthesis and turnover of metabolically labeled proteoglycans from medium, cell layer, and substratum-associated compartments were characterized in four cell lines of fibroblastic origin, including a fibrosarcoma line, and in the murine melanoma cell type, B16.F10. Substantial differences were apparent between the various cell types with regard to quantities, hydrodynamic sizes, and compartmentalization of labeled product. Such variations were greater between the different cell lines than between separately labeled cultures of the same cell type. Greater than 85% of cell-associated proteoglycans were accessible to glycosaminoglycan-degrading enzymes added to the medium of monolayer cultures, demonstrating their principal location to be external to the cell membrane. Apparent glycosaminoglycan free chains, determined by a lack of change in hydrodynamic size following alkaline elimination, were among the products from each cell line and were similarly found to be in a principally pericellular location. Results from label-chase studies demonstrated apparent independent kinetics for proteoglycans and glycosaminoglycan free chains, with little conclusive evidence for precursor-product relationships. Also, their processing by the cells was different, since the proteoglycans were shed largely unchanged into the medium for the three cell lines evaluated, whereas the free chains were not recoverable from the medium in significant amounts. The latter observation suggests the internalization of cell surface-associated free chains and their depolymerization at an intracellular site. The results, which indicate that the content, cellular disposition, and turnover of proteoglycans are quite variable between the cell lines studied, may reflect fundamental cell type-specific specialization in the metabolism of these complex substances. Furthermore, the data raise the interesting possibility that glycosaminoglycan free chains may have biological functions at the cellular level, independent of proteoglycans.