Genetic analysis of proteoglycan structure, function and metabolism

Extracellular cleavage of the vascular endothelial growth factor 189-amino acid form by urokinase is required for its mitogenic effect

Alternative splicing of vascular endothelial growth factor (VEGF) mRNA results in three distinct molecular forms of 121 or 165 (V165) amino acids that are released in the conditioned medium of cultured cells and one longer isoform of 189 amino acids (V189) that remains cell-associated. V189 has been expressed in wild type CHO-K1 cells and in glycosaminoglycan-deficient pgsA-745 Chinese hamster ovary (CHO) mutant cells. It could be released from CHO-K1 cell membranes by heparin or a synthetic peptide designed on the sequence encoded by exon 6 but was freely released from CHO mutant cells. In both cases, the immunoreactive V189 was mainly released as a 40-kDa cleaved form, provided that the serine protease urokinase, but not plasmin, was active. Recombinant V189 was purified from insect cells infected with a recombinant baculovirus as a nonmitogenic 50-kDa precursor that binds to the receptor Flt-1 but not to Flk-1. It could be matured by urokinase as a 38-kDa fragment able to bind to Flk-1 and to trigger cell proliferation. V165 and V189, however, could be cleaved by plasmin as 34-kDa fragments that exhibit a decreased mitogenic activity. These findings indicate that the carboxyl-terminal domain of V189 masks its binding domain to Flk-1.

Heparin binding and oligomerization of hepatocyte growth factor/scatter factor isoforms. Heparan sulfate glycosaminoglycan requirement for Met binding and signaling

Hepatocyte growth factor/scatter factor (HGF/SF) is a heparin-binding polypeptide that stimulates cell proliferation, motility, and morphogenesis by activation of its receptor, the c-Met tyrosine kinase. HGF/SF consists of a series of structural units, including an amino-terminal segment with a hairpin loop, four kringle domains, and a serine protease-like region. In this study, we demonstrate that the amino-terminal (N) domain retains the heparin-binding properties of full-length HGF/SF. In contrast to a previous hypothesis, selected basic amino acid residues in the hairpin loop are not critical for heparin binding, although alanine substitution at a subset of these sites markedly reduced the biological activity of the HGF/SF isoform, HGF/NK1. Covalent cross-linking experiments performed with wild-type and heparan sulfate glycosaminoglycan (HSGAG)-deficient Chinese hamster ovary (CHO) cells revealed that Met-HGF/NK1 binding was strongly dependent on HSGAG. Addition of heparin to HSGAG-deficient CHO cells not only restored ligand binding, but also increased ligand-dependent Met tyrosine phosphorylation and c-fos expression. Moreover, our results showed that heparin stimulated ligand oligomerization through an interaction with the N domain. These findings establish the importance of the N domain for heparin-ligand and ligand-ligand interactions, and demonstrate a crucial role for HSGAG in receptor binding and signal transduction.

Modulation of fibroblast growth factor-2 receptor binding, dimerization, signaling, and angiogenic activity by a synthetic heparin-mimicking polyanionic compound

Heparan sulfate (HS) proteoglycans play a key role in cell proliferation induced by basic fibroblast growth factor (FGF-2) and other heparin-binding growth factors. To modulate the involvement of HS, we have used a synthetic, nonsulfated polyanionic aromatic compound (RG-13577) that mimics functional features of heparin/HS. FGF-2-stimulated proliferation of vascular endothelial cells was markedly inhibited in the presence of 5-10 microg/ml compound RG-13577 (poly-4-hydroxyphenoxy acetic acid; Mr approximately 5 kD). Direct interaction between RG-13577 and FGF-2 was demonstrated by the ability of the former to compete with heparin on binding to FGF-2. RG-13577 inhibited FGF-2 binding to soluble- and cell surface-FGF receptor 1 (FGFR1). Unlike heparin, RG-13577 alone failed to mediate dimerization of FGF-2. Moreover, it abrogated heparin-mediated dimerization of FGF-2 and FGFR1, as well as FGF-2 mitogenic activity in HS-deficient F32 lymphoid cells. The antiproliferative effect of compound RG-13577 was associated with abrogation of FGF-2-induced tyrosine phosphorylation of FGFR1 and of cytoplasmic proteins involved in FGF-2 signal transduction, such as p90 and mitogen-activated protein kinase. A more effective inhibition of tyrosine phosphorylation was obtained after removal of the cell surface HS by heparinase. In contrast, tyrosine phosphorylation of an approximately 200-kD protein was stimulated by RG-13577, but not by heparin or FGF-2. RG-13577 prevented microvessel outgrowth from rat aortic rings embedded in a collagen gel. Development of nontoxic polyanionic compounds may provide an effective strategy to inhibit FGF-2-induced cell proliferation associated with angiogenesis, arteriosclerosis, and restenosis.

Identification and characterization of a fibroblast growth factor (FGF) binding domain in the cysteine-rich FGF receptor

Three distinct transmembrane glycoproteins bind fibroblast growth factor (FGF) family members. These include heparan sulfate proteoglycans, the tyrosine kinase-containing FGF receptors (FGFRs), and a cysteine-rich FGF receptor (CFR). The four FGFRs are thought to mediate FGF-signaling events but require the participation of the heparan sulfate proteoglycans to bind FGFs and transduce intracellular signals. However, a number of groups have proposed that FGF action requires events independent of FGFR activation. CFR, a high affinity FGF-binding protein, was first isolated from chicken embryos. To better understand the interactions between CFR and FGFs, we have constructed a series of CFR deletion mutants and CFR fragments. Analysis of these has identified a approximately 200-amino acid domain that constitutes a CFR FGF binding site. A CFR fragment of 450 residues, CFR290-740, binds FGF-2 with an affinity indistinguishable from the full-length molecule, whereas smaller fragments display greatly reduced FGF binding. Although CFR binds heparin with high affinity, an analysis of the heparin-CFR interaction failed to identify a linear sequence containing a heparin binding site. Two types of FGF binding sites were identified: an ionic strength and heparin-independent site that represents FGF binding to CFR290-740 and an additional FGF binding site that is heparan sulfate-dependent and sensitive to high ionic strength. This latter site is likely to bind FGF indirectly via heparan sulfate binding to CFR. FGF-2 peptides that encompass a sequence implicated in FGF-2 binding to FGFRs also block FGF-2 binding to CFR. Our data suggest that binding of FGFs to CFR and FGFRs is mutually exclusive, since the CFR FGF binding site does not require heparan sulfate, and similar regions on FGF-2 interact with both FGFRs and CFR.

Role of proteoglycans in cell adhesion of prostate cancer cells: from review to experiment

Development and progression of prostate cancer is a multistep process of cumulative genetic damage, acquired during a life-time. However, the altered genotype acts against an appropriate background of epigenetic control mechanisms. Several mechanisms of mitotically heritable, epigenetic control of differential gene transcription have been noted, such as stromal-epithelial and cell-cell interactions. In prostate cancer, an important, supporting and/or inhibiting role of stromal-epithelial interaction has been implicated in tumor growth, angiogenesis and metastasis, which includes cell proliferation, adhesion and motility. Within these processes, data mainly obtained in systems other than the prostate have shown a crucial (regulatory) role of proteoglycans (PGs) acting at the level of cell-cell and cell-pericellular matrix interactions. Although little information has been recorded from normal, benign hyperplastic and malignant prostate tissue, PGs are components of both the cell surface and the extracellular matrix (ECM) that form associations with other molecules, such as fibronectin and laminin. On the basis of cell-ECM adhesion/interaction as a prerequisite for both cell proliferation and motility, and the involvement of PGs, the purpose of this study was to investigate the possible biological relevance of (free) glycosaminoglycans (GAGs), as major functional substructures of PGs, on cell adhesion of a series of human prostatic cell lines cultured in vitro. The effects of a series of exogenously applied GAGs on cell adhesion and proliferation were studied in the human cell lines LNCaP, DU 145 and PC-3, cultured on tissue culture plastic as substratum. The applied GAGs were the natural GAGs heparin, heparan, dermatan, chondroitin-4 and chondroitin-6 sulfate, and the semisynthetic, GAG-like pentosan polysulfate (PPS). Addition of GAGs (1-300 micrograms/ml) to cultures that were allowed to adhere for 24 h prior to GAG addition did not affect cell proliferation. In contrast, whereas the natural GAG added during cell adhesion had no effect. PPS strongly inhibited proliferation of LNCaP and DU145, but not the less anchorage-dependent PC-3 cells. Under the latter conditions, after 6 days of culturing the IC50 of proliferation were determined to be < 1 and 50 micrograms PPS/ml for LNCaP and DU145, respectively, corresponding with a profound effect on cell morphology. Direct measurements of cell adhesion confirmed that, in contrast to the natural GAGs, PPS inhibited cell adhesion. In conclusion, the interference of a nonnatural, GAG-like structure with cell adhesion may be interpreted as the involvement of PGs of the cell surface in cell adhesion, possibly affecting the various processes (proliferation, angiogenesis and metastasis) of prostate tumor progression. Although similar interferences of nonnatural GAGs with cell-adhesion-associated proliferation of anchorage-dependent cells remain to be established under in vivo conditions, this type of compounds deserves further attention as a tool with which to study the role of cell adhesion in the progression of prostate cancer and as a potential candidate for the development of a stromal-epithelial targeted therapy.

Release of malaria circumsporozoite protein into the host cell cytoplasm and interaction with ribosomes

To date, the circumsporozoite (CS) protein has been implicated in guiding malaria sporozoites to the liver [Cerami et al., Cell 70, 1992, 1021-1033]. Here we show that shortly after invasion, P. berghei and P. yoelii sporozoites lie free in the invaded cell and release considerable amounts of CS protein into the cytoplasm. The intracytoplasmic deposition of CS protein begins during the attachment of the sporozoite to the host cell surface and reaches its peak during the first 4-6 h after invasion. Initially, the CS protein spreads over the entire cytoplasm of the infected cell where it interacts with cytosolic as well as endoplasmic reticulum-associated ribosomes. During the subsequent development of the parasites to exoerythrocytic forms, the CS protein binding becomes gradually restricted to ribosomes lining the outer membrane of the nuclear envelope of the host cell. The distribution pattern of the parasite-released CS protein in the host cell cytoplasm is independent of the permissiveness of the host cell for the development of the parasites to exoerythrocytic forms. It requires neither the host cell metabolism nor does it involve the endocytotic machinery. Recombinant P. falciparum CS protein interacts with RNAse-sensitive sites on endoplasmic reticulum-associated ribosomes as shown by microinjection and immunoelectron microscopy. The generalized interaction of the CS protein with host cell ribosomes suggests that the CS protein has an intracellular function during the hepatic phase in the life cycle of Plasmodium and may also explain the generation of a CD8+ T cell response in the course of rodent malaria infections.

A recombinant Chlamydia trachomatis major outer membrane protein binds to heparan sulfate receptors on epithelial cells

Chlamydial attachment to columnar conjunctival or urogenital epithelial cells is an initial and critical step in the pathogenesis of chlamydial mucosal infections. The chlamydial major outer membrane protein (MOMP) has been implicated as a putative chlamydial cytoadhesin; however, direct evidence supporting this hypothesis has not been reported. The function of MOMP as a cytoadhesin was directly investigated by expressing the protein as a fusion with the Escherichia coli maltose binding protein (MBP-MOMP) and studying its interaction with human epithelial cells. The recombinant MBP-MOMP bound specifically to HeLa cells at 4 degrees C but was not internalized after shifting the temperature to 37 degrees C. The MBP-MOMP competitively inhibited the infectivity of viable chlamydiae for epithelial cells, indicating that the MOMP and intact chlamydiae bind the same host receptor. Heparan sulfate markedly reduced binding of the MBP-MOMP to cells, whereas chondroitin sulfate had no effect on binding. Enzymatic treatment of cells with heparitinase but not chondroitinase inhibited the binding of MBP-MOMP. These same treatments were also shown to reduce the infectivity of chlamydiae for epithelial cells. Mutant cell lines defective in heparan sulfate synthesis but not chondroitin sulfate synthesis showed a marked reduction in the binding of MBP-MOMP and were also less susceptible to infection by chlamydiae. Collectively, these findings provide strong evidence that the MOMP functions as a chlamydial cytoadhesin and that heparan sulfate proteoglycans are the host-cell receptors to which the MOMP binds.

Binding and degradation of thrombospondin-1 mediated through heparan sulphate proteoglycans and low-density-lipoprotein receptor-related protein: localization of the functional activity to the trimeric N-terminal heparin-binding region of thrombospondin-1

Thrombospondin-1 (TSP-1) is a multimodular trimeric protein involved in cell adhesion, motility and growth. TSP-1 binds to cells and is internalized and degraded in a process that requires the presence of heparan sulphate proteoglycan; the process is inhibited by heparin or receptor-associated protein (RAP), an antagonist of the low-density-lipoprotein receptor (LDLR) family. We characterized the attributes of TSP-1 that mediate the process. TSP277, which is truncated at Gln-277 of TSP-1 and contains the heparin-binding domain and the heptad repeat region that mediates trimerization, bound to and was degraded by a variety of cells with kinetics similar to those of the binding and degradation of intact TSP-1. Degradation of TSP277 was inhibited by heparin or RAP with dose responses similar to those for inhibition of degradation of TSP-1. Binding and degradation of TSP277 were decreased in Chinese hamster ovary cells lacking heparan sulphate. These results indicate that the N-terminal heparin-binding domain in a trivalent configuration is sufficient to mediate binding and degradation of TSP-1 via the proteoglycan-LDLR family pathway.

Expression of avian glypican is developmentally regulated

An avian cDNA homologue of human and rat glypicans has been cloned from a stage 17 chicken heart cDNA library and used to analyze the distribution of this proteoglycan during development by Northern analysis and whole mount in situ hybridization. At stages 7-12, strong signals were detected in the cephalic region of the neural folds, rostral portion of paraxial mesoderm, and newly formed epithelial somites. At stages 20-25, strong expression was observed in the mantle zone of the telencephalon, the apical epidermal ridge and proximal region of developing limb. Transcripts also were found in the truncus arteriosus and arteriovenous-canal region of the heart, but not in the myocardium. This distribution pattern suggests that the avian glypican may be involved in the morphogenesis of limb, somite, heart, and brain. The expression of glypican also overlaps FGFs in limb bud, FGF receptors in heart and somite, and NGF receptors in forebrain. The affinity of heparan sulfate proteoglycans for growth factors and the distribution of the avian glypican are consistent with a role for this molecule in growth factor-mediated signals.

An animal cell mutant defective in heparan sulfate hexuronic acid 2-O-sulfation

The interaction of heparan sulfate with protein ligands depends on unique oligosaccharide sequences containing iduronic acid (IdUA), N-sulfated glucosamine residues, and O-sulfated sugars. To study the role of O-sulfation in greater detail, we isolated a Chinese hamster ovary cell mutant defective in 2-O-sulfation of iduronic acid. The mutant, pgsF-17, was identified by a colony blotting assay in which colonies of mutagen-treated cells were replica plated to two disks of polyester cloth. One disk was blotted with 125I-labeled basic fibroblast growth factor (bFGF) to measure binding to cell surface proteoglycans. The other disk was incubated with 35SO4 to measure proteoglycan biosynthesis. Autoradiography revealed a colony that did not bind 125I-bFGF, but incorporated 35SO4 normally (mutant pgsF-17). Complete deaminative cleavage of heparan sulfate revealed that material from pgsF-17 lacked IdUA(2OSO3)-GlcNSO3 and IdUA(2OSO3)-GlcNSO3(6OSO3), but contained a higher proportion of glucuronic acid GlcUA-GlcNSO3(6OSO3) and IdUA-GlcNSO3(6OSO3). Assay of the 2-O-sulfotransferase that acts on IdUA residues showed that mutant 17 lacked enzyme activity. Interestingly, the alteration resulted in accumulation of GlcNSO3 groups, suggesting that under normal conditions 2-O-sulfation decreases GlcNAc N-deacetylation/N-sulfation, and that the reactions occur simultaneously. The formation of IdUA and 6-O-sulfated glucosaminyl residues appears to be independent of 2-O-sulfation. pgsF-17 also lacks 2-O-sulfated GlcUA residues, suggesting that the same enzyme is responsible for 2-O-sulfation of IdUA and GlcUA residues. Mutant 17 provides a useful tool for studying the regulation of heparan sulfate biosynthesis and the relationship of heparan sulfate fine structure to its biological function.

Divergent secretory behavior of the opposite ends of aggrecan

The proteoglycan, aggrecan has a globular domain, G1, at the N terminus and a different globular domain, G3, at the C terminus. Aggrecan produced by mutant nanomelic chickens is truncated due to a premature stop codon and consequently lacks G3 and a minor portion of its chondroitin sulfate domain (Li, H., Schwartz, N. B., and Vertel, B. M.(1993) J. Biol. Chem. 268, 23504-23511). The mutant protein is retained in the endoplasmic reticulum and fails to enter the Golgi stacks (Vertel, B. M., Walters, L. M., Grier, B., Maine, N. , and Goetinck, P. F.(1993) J. Cell Sci. 104, 939-948). The homozygous mutant is lethal because of failure of chondrogenesis and osteogenesis, while the heterozygous mutant is dwarfed. To further elucidate the pathogenetic mechanisms underlying nanomelia and to determine if G1 and G3 are themselves secreted, we expressed them in transfected host cells. Expression was performed in wild type Chinese hamster ovary (CHO) cells and in mutant CHO cells which are unable to link glycosaminoglycan (GAG) chains to core proteins. We compared: (a) secretion of expressed G1 and G3 constructs containing contiguous GAG chain consensus sites and (b) GAG chain modification of the secreted proteins. We find that: 1) G3 is 24-100 times more rapidly secreted than G1; 2) secreted G3 contains contiguous chondroitin sulfate GAG chains, while secreted G1 lacks contiguous GAG chains; 3) G3 secretion is not coupled to xylosylation of contiguous GAG chain consensus sites. These results imply that G1 and G3 intrinsically differ in passage through the cell secretory route.

Matrix metalloproteinase 2 releases active soluble ectodomain of fibroblast growth factor receptor 1

Recent studies have demonstrated the existence of a soluble fibroblast growth factor (FGF) receptor type 1 (FGFR1) extracellular domain in the circulation and in vascular basement membranes. However, the process of FGFR1 ectodomain release from the plasma membrane is not known. Here we report that the 72-kDa gelatinase A (matrix metalloproteinase type 2, MMP2) can hydrolyze the Val368-Met369 peptide bond of the FGFR1 ectodomain, eight amino acids upstream of the transmembrane domain, thus releasing the entire extracellular domain. Similar results were obtained regardless of whether FGF was first bound to the receptor or not. The action of MMP2 abolished binding of FGF to an immobilized recombinant FGFR1 ectodomain fusion protein and to Chinese hamster ovary cells overexpressing FGFR1 The released recombinant FGFR1 ectodomain was able to bind FGF after MMP2 cleavage, suggesting that the cleaved soluble receptor maintained its FGF binding capacity. The activity of MMP2 could not be reproduced by the 92-kDa gelatinase B (MMP9) and was inhibited by tissue inhibitor of metalloproteinase type 2. These studies demonstrate that FGFR1 may be a specific target for MMP2 on the cell surface, yielding a soluble FGF receptor that may modulate the mitogenic and angiogenic activities of FGF.

Characterization of glycosaminoglycan-binding domains present in insulin-like growth factor-binding protein-3

Matrix metalloproteinase 3 cleaves insulin-like growth factor-binding protein-3 (IGFBP-3) into six fragments, four of which bind heparin-Sepharose (Fowlkes, J. L., Enghild, J. J., Suzuki, K., and Nagase, H. (1994) J. Biol. Chem. 269, 25742-25746). Sequence analysis of IGFBP-3 heparin-binding fragments shows that all fragments contain at least one of two highly basic, putative heparin-binding consensus sequences present in IGFBP-3. Epitope-specific antibodies generated against synthetic peptides containing these domains recognized IGFBP-3, yet were significantly inhibited from binding in the presence of heparin, demonstrating that these regions of IGFBP-3 contain functional heparin-binding domains. IGFBP-3 peptides containing one of the two heparin-binding consensus sequences bound heparin in a solid phase binding assay in a dose-dependent and saturable manner. However, the IGFBP-3 peptide containing the heparin-binding consensus sequence 149KKGHA153 bound heparin with approximately 4-fold less affinity than the IGFBP-3 peptide containing the longer heparin-binding consensus sequence 219YKKKQCRP226. Examination of several well characterized glycosaminoglycans to inhibit the binding of heparin to both heparin-binding IGFBP-3 peptides revealed that the most potent inhibitors were heparin, heparan sulfate, and dermatan sulfate; chondroitin sulfate A and hyaluronic acid were intermediate in their inhibitory activities; and chondroitin sulfate C caused no inhibition. These studies identify and characterize the glycosaminoglycan-binding domains in IGFBP-3, providing a basis for the better understanding of IGFBP-3-glycosaminoglycan interactions at the cellular and extracellular interface.

Specific involvement of glypican in thrombin adhesive properties

We have previously demonstrated that thrombin possesses an active yet cryptic Arg-Gly-Asp (RGD) site which upon exposure induces endothelial cell (EC) adhesion via alpha nu beta 3 integrin [Bar-Shavit et al. (1991): J Cell Biol 112:335]. This was achieved in the presence of cell surface-associated heparan sulfate proteoglycans (HSPG) and exceedingly low concentrations of plasmin [Bar-Shavit et al. (1993): J Cell Biol 123:1279]. A portion of the cell surface-associated HSPG (glypican) is anchored via a covalently linked glycosyl-phosphatidylinositol (PI) residue, which can be released by treatment with glycosyl-PI-specific phospholipase C (PI-PLC). We report here that exposure of either bovine aortic EC, smooth muscle cells (SMC), or wild-type CHO cells to PI-PLC released HSPG involved in the conversion of thrombin to an adhesive molecule. The adhesion-promoting activity of the released HSPG was abolished following treatment with heparinase but not chondroitinase ABC. Incubation of thrombin with heparan sulfate-deficient CHO cells or cells that were pretreated with PI-PLC failed to induce its conversion to an adhesive molecule, indicating that glypican was playing a major role in this conversion. Moreover, affinity-purified glypican, but not syndecan or fibroglycan, elicited efficient conversion of plasmin-treated thrombin into an adhesive molecule. Antibodies raised against the RGD site in thrombin failed to interact with native thrombin, prothrombin, or the RGD site in other adhesive proteins such as vitronectin, fibrinogen, or fibronectin. Anti-thrombin-RGD antibodies which blocked the adhesion-promoting activity of thrombin were also capable of recognizing thrombin that was first incubated with a suboptimal concentration of plasm in in the presence of PI-PLC-released HSPG. Heparin, heparan sulfate, and PI-PLC-released HSPG had no effect on other cellular properties of thrombin such as receptor binding and growth-promoting activity. Altogether we have demonstrated that the heparin binding domain in thrombin plays a specific role in promoting thrombin adhesive properties and that membrane-associated glypican is likely to be the major physiological inducer of this property.

Cell surface glycosaminoglycans are not obligatory for Plasmodium berghei sporozoite invasion in vitro

The malaria circumsporozoite (CS) protein binds to glycosaminoglycan chains from heparan sulfate proteoglycans present on the basolateral surface of hepatocytes and hepatoma cells in vitro. When injected into mice, CS protein is rapidly cleared from the blood circulation by hepatocytes. The binding region for the HSPGs is the evolutionarily conserved region II-plus of the CS protein. Here we have asked whether the presence of glycosaminoglycans on the plasma membrane of target cells is required for sporozoite invasion in vitro. Two types of target cells were used: HepG2 cells, which are permissive for Plasmodium berghei sporozoite development into mature exoerythrocytic forms, and CHO cells, in which the intracellular development of the parasites is arrested early after penetration. The invasion of mutant CHO cells expressing undersulfated glycosaminoglycans or no glycosaminoglycans was only inhibited 41-49% or 24-32%, respectively, in comparison to invasion of CHO-K1 cells. Previous cleavage of HepG2 surface membrane glycosaminoglycans with heparinase or heparitinase had no significant inhibitory effect on subsequent P. berghei sporozoite invasion and EEF development in these cells, although the glycosaminoglycan lyase treatments removed over 80% of CS binding sites from the cell surface. These results suggest that although the presence of glycosaminoglycans on the target cell surface enhances sporozoite invasion, glycosaminoglycans are not required for sporozoite penetration or the development of exoerythrocytic forms in vitro.

Biosynthesis of dermatan sulphate. Defructosylated Escherichia coli K4 capsular polysaccharide as a substrate for the D-glucuronyl C-5 epimerase, and an indication of a two-base reaction mechanism

The capsular polysaccharide from Escherichia coli K4 consists of a chondroitin ([GlcA(beta 1-->3)GalNAc(beta 1-->4)]n) backbone, to which beta-fructofuranose units are linked to C-3 of D-glucuronic acid (GlcA) residues. Removal of the fructose units by mild acid hydrolysis provided a substrate for the GlcA C-5 epimerase, which is involved in the generation of L-iduronic acid (IdoA) units during dermatan sulphate biosynthesis. Incubation of this substrate with solubilized fibroblast microsomal enzyme in the presence of 3H2O resulted in the incorporation of tritium at C-5 of hexuronyl units. A Km of 67 x 10(-6) M hexuronic acid (equivalent to disaccharide units) was determined, which is similar to that (80 x 10(-6) M) obtained for dermatan (desulphated dermatan sulphate). Vmax was about 4 times higher with dermatan than with the K4 substrate. A defructosylated K4 polysaccharide isolated after incubation of bacteria with D-[5-3H]glucose released 3H2O on reaction with the epimerase, and thus could be used to assay the enzyme. Incubation of a K4 substrate with solubilized microsomal epimerase for 6 h in the presence of 3H2O resulted in the formation of about 5% IdoA and approximately equal amounts of 3H in GlcA and IdoA. A corresponding incubation of dermatan yielded approx. 22% GlcA, which contained virtually all the 3H label. These results are tentatively explained in terms of a two-base reaction mechanism, involving a monoprotic L-ido-specific base and a polyprotic D-gluco-specific base. Most of the IdoA residues generated by the enzyme occurred singly, although some formation of two or three consecutive IdoA-containing disaccharide units was observed.

Sulfated glycans stimulate endocytosis of the cellular isoform of the prion protein, PrPC, in cultured cells

There is currently no effective therapy for human prion diseases. However, several polyanionic glycans, including pentosan sulfate and dextran sulfate, prolong the incubation time of scrapie in rodents, and inhibit the production of the scrapie isoform of the prion protein (PrPSc), the major component of infectious prions, in cultured neuroblastoma cells. We report here that pentosan sulfate and related compounds rapidly and dramatically reduce the amount of PrPC, the non-infectious precursor of PrPSc, present on the cell surface. This effect results primarily from the ability of these agents to stimulate endocytosis of PrPC, thereby causing a redistribution of the protein from the plasma membrane to the cell interior. Pentosan sulfate also causes a change in the ultrastructural localization of PrPC, such that a portion of the protein molecules are shifted into late endosomes and/or lysosomes. In addition, we demonstrate, using PrP-containing bacterial fusion proteins, that cultured cells express saturable and specific surface binding sites for PrP, many of which are glycosaminoglycan molecules. Our results raise the possibility that sulfated glycans inhibit prion production by altering the cellular localization of PrPC precursor, and they indicate that endogenous proteoglycans are likely to play an important role in the cellular metabolism of both PrPC and PrPSc.

A stereocontrolled synthetic approach to glycopeptides corresponding to the carbohydrate-protein linkage region of cell-surface proteoglycans

The glycopeptides 1 (Gly[O-beta-D-Xylp)-L-Ser-Gly-L-Glu) and 2 (Gly[O-[beta-D-GlcpA-(1-->3)-beta-D-Galp-(1-->3)-beta-D-Galp]-(1-- >4) -beta-D-Xylp)-L-Ser-Gly-L-Glu), carrying the core structure of serine-linked cell-surface proteoglycans were synthesized in a stereocontrolled manner. The carbohydrate key imidate xylosyl donors 3 and glycotetraosyl donors 4 and 5, as well as a tetrapeptide glycosyl acceptor 6, were coupled in the crucial glycosylation step. In these reactions, the application of either trimethylsilyl trifluoromethanesulfonate (TMSOTf) or borontrifluoride etherate (BF3-Et2O) as catalysts proved to be highly efficient. The serine linked glycopeptides 34, 36 and 37 thus obtained yielded target compounds 1 and 2 on complete deprotection.

Identification of a new membrane-bound heparan sulphate proteoglycan

The morphological changes that occur during intestinal development have been extensively described, but the molecular basis of these changes is largely unknown. As a result of our efforts to identify molecules that play a role in intestinal morphogenesis during development, we have previously isolated a cDNA that is developmentally regulated in the intestine. This cDNA, named OCI-5, was recently shown to have 20-25% identity at the protein-sequence level with glypican and cerebroglycan, two heparan sulphate proteoglycans (HSPG) that are attached to the cell membrane by a glycosyl-phosphatidylinositol (GPI) anchor. Here we provide experimental evidence indicating that OCI-5 is also a GPI-linked HSPG. We demonstrate this by showing that OCI-5 can be labelled with radioactive sulphate and can be digested by heparitinase, but not by chondroitinase. We also show that treatment with phosphatidylinositol-specific phospholipase C releases OCI-5 from the cell surface of COS cells transfected with an OCI-5 expression vector. The identification of OCI-5 as a GPI-linked HSPG confirms that this proteoglycan belongs to the same family of HSPGs that include glypican and cerebroglycan.

Fibroblast growth factor-2 can mediate cell attachment by linking receptors and heparan sulfate proteoglycans on neighboring cells

The myeloid 32D cell line, which grows in suspension and does not express FGF receptors or heparan sulfate proteoglycans, was transfected with the cDNA encoding FGF receptor-1 (32D-flg cells). When co-cultured with glutaraldehyde-fixed Chinese hamster ovary (CHO) cells, the 32D-flg cells remained in suspension in the absence of FGF-2 but attached to the CHO monolayer in the presence of 10 ng/ml FGF-2. In contrast, 32D cells transfected with the vector alone did not attach to the CHO monolayer in the presence of FGF-2. FGF-2-dependent attachment of 32D-flg cells was prevented by inclusion of 10 micrograms/ml heparin in the incubation medium and was diminished when CHO mutants in glycosaminoglycan synthesis or wild-type CHO cells treated with heparinase were used, indicating that the attachment occurred through FGF-2 interactions with heparan sulfates on the CHO cells. Attachment of 32D-flg cells to wild-type CHO cells was half-maximal at 0.4 ng/ml FGF-2 and was also observed with FGF-1 but not FGF-4. 32D-flg cells also attached to living CHO cells in a FGF-2-dependent manner, but attachment was transient at 37 degrees C. Induction of new proteins was not required for FGF-2-dependent attachment, since attachment occurred when the co-cultures were incubated at 4 degrees C and when the 32D-flg cells were preincubated with cycloheximide. FGF-2-dependent attachment of 32D-flg cells was also observed with Balb/C 3T3, NIH 3T3, and bovine capillary endothelial cells. We conclude that attachment is due to FGF-2 binding simultaneously to receptors on the 32D-flg cells and heparan sulfates on the CHO monolayers; thus, the FGF-2 acts as a bridge between receptor-expressing cells and heparan sulfate-bearing cells. In addition, induction of DNA synthesis in 32D-flg cells in response to FGF-2 was potentiated by the CHO-associated heparan sulfates to the same extent as by soluble heparin, indicating that this interaction has functional significance.

Laminarin sulfate mimics the effects of heparin on smooth muscle cell proliferation and basic fibroblast growth factor-receptor binding and mitogenic activity

Heparin and heparin-like molecules may function, apart from their effect on hemostasis, as regulators of cell growth and neovascularization. We investigated whether similar effects are exerted by laminarin sulfate, an unrelated polysulfated saccharide isolated from the cell wall of seaweed and composed of chemically O-sulfated beta-(1,3)-linked glucose residues. Laminarin sulfate exhibits about 30% of the anticoagulant activity of heparin and is effective therapeutically in the prevention and treatment of cerebrovascular diseases. We characterized the effect of laminarin sulfate on interaction of the heparin-binding angiogenic factor, basic fibroblast growth factor (bFGF), with a naturally produced subendothelial extra-cellular matrix (ECM) and with cell surface receptor sites. Laminarin sulfate (1-2 micrograms/ml) inhibited the binding of bFGF to ECM and to the surface of vascular smooth muscle cells (SMC) in a manner similar to that observed with heparin. Likewise, laminarin sulfate efficiently displaced both ECM- and cell-bound bFGF at concentrations as low as 1 microgram/ml. Both laminarin sulfate and heparin efficiently induced restoration of bFGF receptor binding in xylosyltransferase-deficient CHO cell mutants defective in initiation of glycosaminoglycan synthesis. Moreover, laminarin sulfate elicited bFGF receptor activation and mitogenic response in heparan sulfate (HS)-deficient, cytokine-dependent lymphoid cells. These results indicate that laminarin sulfate effectively replaced the need for heparin and HS in the induction of bFGF receptor binding and signaling. In other experiments, laminarin sulfate was found to inhibit the proliferation of vascular SMC in a manner similar to that observed with heparin. These effects of laminarin sulfate may have potential clinical applications in diverse situations such as wound healing, angiogenesis, and atherosclerosis.

Effect of transforming growth factor-beta 1 and basic fibroblast growth factor on the expression of cell surface proteoglycans in human lung fibroblasts. Enhanced glycanation and fibronectin-binding of CD44 proteoglycan, and down-regulation of glypican

We have tested the effects of transforming growth factor-beta 1 (TGF-beta 1), basic fibroblast growth factor (bFGF) and TGF-beta 1 + bFGF on the expression of the cell surface proteoglycans (CD44, syndecans and glypican) in cultures of human lung fibroblasts (HLF). Cell surface proteoglycan expression was monitored by quantitative immunoprecipitation from metabolically labelled cells. Western and Northern blotting and evaluation of the glycanation of the proteoglycans. Stimulation of the cells with TGF-beta 1 increased the length of the chondroitin sulphate (CS) chains on CD44 (approximately 1.6-fold). bFGF, administered solely, also increased the length of the CS chains on CD44 (approximately 1.4-fold), whereas the combination of TGF-beta 1 + bFGF nearly doubled both the length and the number of the CS chains on CD44. None of these treatments lead to changes in CD44 message or core-protein expression. This enhanced glycanation of CD44 after the TGF-beta 1, bFGF and combined treatments correlated with a 2-fold increase in the affinity of the proteoglycan for fibronectin but had no influence on the binding to type I collagen. TGF-beta 1, alone or in combination with bFGF, also stimulated the CS content of syndecan-1, but none of the other syndecans was significantly affected by any of the factors or combinations tested. The expression of glypican however was significantly decreased (nearly halved) by the combination of TGF-beta 1 + bFGF, less so by TGF-beta 1 and not at all by bFGF. This decrease occurred both at the level of the message and of the core protein. These data demonstrate specific and differential effects of TGF-beta 1 and bFGF on the structure, expression and interactions of the cell surface proteoglycans of HLF.

Chondroitin sulfate A is a cell surface receptor for Plasmodium falciparum-infected erythrocytes

Adherence of Plasmodium falciparum-infected erythrocytes to cerebral postcapillary venular endothelium is believed to be a critical step in the development of cerebral malaria. Some of the possible receptors mediating adherence have been identified, but the process of adherence in vivo is poorly understood. We investigated the role of carbohydrate ligands in adherence, and we identified chondroitin sulfate (CS) as a specific receptor for P. falciparum-infected erythrocytes. Parasitized cells bound to Chinese hamster ovary (CHO) cells and C32 melanoma cells in a chondroitin sulfate-dependent manner, whereas glycosylation mutants lacking chondroitin sulfate A (CSA) supported little or no binding. Chondroitinase treatment of wild-type CHO cells reduced binding by up to 90%. Soluble CSA inhibited binding to CHO cells by 99.2 +/- 0.2% at 10 mg/ml and by 72.5 +/- 3.8% at 1 mg/ml, whereas a range of other glycosaminoglycans such as heparan sulfate had no effect. Parasite lines selected for increased binding to CHO cells and most patient isolates bound specifically to immobilized CSA. We conclude that P. falciparum can express or expose proteins at the surface of the infected erythrocyte that mediate specific binding to CSA. This mechanism of adherence may contribute to the pathogenesis of P. falciparum malaria, but has wider implications as an example of an infectious agent with the capacity to bind specifically to cell-associated or immobilized CS.

Accumulation of a pentasaccharide terminating in alpha-N-acetylglucosamine in an animal cell mutant defective in heparan sulfate biosynthesis

Heparan sulfate biosynthesis initiates by the transfer of alpha-D-GlcNAc from UDP-GlcNAc to the D-GlcA moiety of the linkage tetrasaccharide, GlcA beta 1-3Gal beta 1-3Gal beta 1-4Xyl beta 1-core protein. The enzyme catalyzing this reaction differs from the alpha-GlcNAc transferase involved in chain polymerization based on genetic and enzymatic studies of an animal cell mutant defective in chain polymerization (Fritz, T. A., Gabb, M. M., Wei, G., and Esko, J. D. (1994) J. Biol. Chem. 269, 28809-28814). In this report we show that this mutant also accumulates a pentasaccharide intermediate containing alpha-GlcNAc. A fusion protein was made from the IgG-binding domain of protein A and a segment of the proteoglycan, betaglycan. This segment contained one glycosaminoglycan attachment site that primes only chondroitin sulfate and another that primes both heparan sulfate and chondroitin sulfate (Zhang, L., and Esko, J. D. (1994) J. Biol. Chem. 264, 19295-19299). Expression of the chimera in the mutant resulted in the accumulation of an oligosaccharide that labeled with [6-3H]GlcN. The oligosaccharide comigrated with a pentasaccharide standard derived from chondroitin sulfate, but acid hydrolysis gave 98% [3H]GlcN. Heparin lyase III digestion yielded [3H]GlcNAc, suggesting that the GlcNAc residue was alpha-linked to the nonreducing terminus. Enzymatic treatment of [6-3H]Gal-labeled material yielded the tetrasaccharide, delta GlcA-[3H]Gal-[3H]Gal-xylitol. These findings suggest that pentasaccharide had the structure, GlcNAc alpha 1-4GlcA beta 1-3Gal beta 1-3Gal beta 1-4Xyl. Its accumulation in a Chinese hamster ovary cell mutant defective in the polymerizing alpha-GlcNAc transferase provides in vivo evidence that two alpha-GlcNAc transferases catalyze the formation of heparan sulfate.

Endothelial heparan sulfate proteoglycans that bind to L-selectin have glucosamine residues with unsubstituted amino groups

We earlier reported calcium-dependent, heparin-like L-selectin ligands in cultured bovine endothelial cells (Norgard-Sumnicht, K. E., Varki, N. M., and Varki, A. (1993) Science 261,480-483). Here we show that these are heparan sulfate proteoglycans (HSPGs) associated either with the cultured cells or secreted into the medium and extracellular matrix. Activation of the endothelial cells with bacterial lipopolysaccharide (LPS) does not markedly alter the amount or distribution of this material. A major portion of the glycosaminoglycan (GAG) chains released from these HSPGs by alkaline beta-elimination rebinds to L-selectin in the presence of calcium, indicating that these saccharides alone can mediate the high affinity recognition. Heparin lyase digestions indicate that these GAG chains are enriched in heparan sulfate, not heparin sequences. Current understanding of the biosynthesis of heparan sulfate chains indicates that all glucosamine amino groups must be either N-acetylated or N-sulfated. However, nitrous acid deamination at pH 4.0 suggests the presence of some unsubstituted amino groups in these L-selectin-binding GAG chains from endothelial cell HSPGs. This is confirmed by chemical N-reacetylation and by reactivity with sulfo-N-hydroxysuccinimide-biotin. These unsubstituted amino groups are also found on HSPGs from human umbilical vein endothelial cells, but are not detected in those from Chinese hamster ovary cells. In both bovine and human endothelial cells, these novel groups are enriched for in the HS-GAG chains which bind to L-selectin. Despite this, studies with N-reacetylation and nitrous acid deamination do not show conclusive evidence for the direct involvement of the unsubstituted amino groups in L-selectin binding. This may be because the chemical reactions used to modify the amino groups do not go to completion. Alternatively, the unsubstituted amino groups may only be indirectly involved in generating binding, by dictating the biosynthesis of another critical group. Regardless, these studies shown that HSPGs from cultured endothelial cells which can bind to L-selectin are enriched with unsubstituted amino groups on their GAG chains. The possible biochemical mechanisms for generation of these novel groups are discussed.

Chondroitin sulfate proteoglycans in the developing cerebral cortex: the distribution of neurocan distinguishes forming afferent and efferent axonal pathways

The first thalamocortical axons to arrive in the developing cerebral cortex traverse a pathway that is separate from the adjacent intracortical pathway for early efferents, suggesting that different molecular signals guide their growth. We previously demonstrated that the intracortical pathway for thalamic axons is centered on the subplate (Bicknese et al. [1994] J. Neurosci. 14:3500-3510), which is rich in chondroitin sulfate proteoglycans (CSPGs; Sheppard et al. [1991] J. Neurosci. 11:3928-3942), whereas efferent axons cross the subplate to exit in a zone containing much less CSPG. To define the molecular composition of the subplate further, we used antibodies against CSPG core proteins and chondroitin sulfate disaccharides in an immunohistochemical analysis of their distribution in the developing neocortex of the rat. Immunolabeling for neurocan, a central nervous system-specific CSPG (Rauch et al. [1992] J. Biol. Chem. 267:19537-19547), and for chondroitin 6-sulfate and unsulfated chondroitin becomes prominent in the subplate before the arrival of thalamic afferents. Immunolabeling is initially sparse in the cortical plate but appears later in maturing cortical layers. A postnatal decline in immunolabeling occurs uniformly for most proteoglycans, but, in the somatosensory cortex, labeling for neurocan, phosphacan, and chondroitin 4- and 6-sulfate declines in the centers of the whisker barrels before the walls. In contrast to neurocan, immunolabeling for other proteoglycans is either uniformly distributed (syndecan-1, N-syndecan, 5F3, phosphacan, chondroitin 4-sulfate), restricted to axons (PGM1), distributed exclusively on nonneuronal elements (2D6, NG2, and CD44), or undetectable (9.2.27, aggrecan, decorin). Thus, neurocan is a candidate molecule for delineating the intracortical pathway of thalamocortical axons and distinguishing it from that of cortical efferents.

Infection of cells by varicella zoster virus: inhibition of viral entry by mannose 6-phosphate and heparin

Envelope glycoproteins of varicella zoster virus (VZV) contain mannose 6-phosphate (Man6P) residues. We now report that Man6P competitively and selectively inhibits infection of cells in vitro by cell-free VZV; furthermore, dephosphorylation of VZV by exposure to alkaline phosphatase rapidly destroys infectivity. Cells are also protected from VZV in a concentration-dependent manner by heparin (ED50 = 0.23 micrograms/ml; 95% confidence limits = 0.16-0.26 microgram/ml) but not by chondroitin sulfate. Both heparin and Man6P are protective only when present about the time of inoculation. Heparin but not Man6P interferes with the attachment of VZV to cell surfaces; moreover, VZV binds to heparin-affinity columns. These data are compatible with a working hypothesis, whereby VZV attaches to cell surfaces by binding to a heparin sulfate proteoglycan. This binding stabilizes VZV, making possible a low-affinity interaction with another Man6P-dependent receptor, which is necessary for viral entry.

Immunostaining of a heterodimeric dermatan sulphate proteoglycan is correlated with smooth muscles and some basement membranes

A heterodimeric 760-kDa dermatan sulphate proteoglycan tentatively named PG-760 was characterized as a product of keratinocytes, endothelial cells, and fibroblasts. The two core proteins of 460 kDa and 300 kDa are linked by disulphide bridges, and both carry one or only very few dermatan sulphate chains. Different antisera against PG-760 were used in the present study to investigate the distribution in selected murine tissues by light and electron microscopy. PG-760 immunostaining was observed in cornea (epithelium including basement membrane, stroma, and Descemet's membrane), skin, mucosa of the small intestine, Engelbreth-Holm-Swarm (EHS)-tumour (matrix and cells), and the smooth muscle layers of uterus, small intestine, and blood vessels. No staining was observed in capillaries, striated muscles, and liver parenchyma including the central vein. The expression of PG-760 in EHS-tumour was also demonstrated after extraction with 4 M guanidine and partial purification by diethylaminoethyl (DEAE)-chromatography. We conclude that this novel proteoglycan exhibits a unique tissue distribution being a constituent of some but not all basement membranes, of some other extracellular matrices, and additionally, of all investigated smooth muscle layers.

Heparan sulfate-mediated cell aggregation. Syndecans-1 and -4 mediate intercellular adhesion following their transfection into human B lymphoid cells

Because syndecans are present at sites of cell-cell contact in vivo it has been hypothesized that they play a role in mediating cell-cell adhesion. However, there has been no direct evidence to support this notion. To address this question, B lymphoid (ARH-77) cells were transfected with the cDNA for murine syndecan-1. Unlike the parental cells, the transfectants form large multicellular aggregates in suspension cultures and stain intensely for syndecan-1 at sites of cell-cell contact. Using rotation-mediated aggregation assays, we find that aggregation of syndecan-1-transfected cells is dependent on divalent cations and is inhibited by the following: (i) addition of heparin and heparin-like glycosaminoglycans, (ii) removal of heparan sulfate from the cell surface, or (iii) addition of exogenous purified syndecan-1. Mixing of syndecan-1-transfected and control-transfected cells results in aggregates containing both cell types indicating that aggregation occurs through a heterophilic adhesion mechanism in which heparan sulfate chains bind to a counter-receptor present on these cells. Importantly, syndecan-4-transfected cells also aggregate in a heparan sulfate-dependent manner, while in contrast, betaglycan-transfected cells aggregate poorly. Thus, syndecans may be important mediators of cell-cell adhesion, but this function may not be common to all transmembrane heparan sulfate-bearing proteoglycans.

Hemagglutination and proteoglycan binding by the Lyme disease spirochete, Borrelia burgdorferi

The ability of the Lyme disease spirochete to attach to host components may contribute to its ability to infect diverse tissues. We present evidence that the Lyme disease spirochete expresses a lectin activity that promotes agglutination of erythrocytes and bacterial attachment to glycosaminoglycans. Among a diverse collection of 21 strains of Lyme disease spirochete, hemagglutinating activity was easily detected in all but 3 strains, and these three strains were noninfectious. The ability to agglutinate erythrocytes was associated with the ability of the spirochete to bind to the sulfated polysaccharide dextran sulfate and to mammalian cells. Soluble dextran sulfate was a potent inhibitor of both hemagglutination and attachment to mammalian cells, while dextran had no effect on either activity, suggesting that dextran sulfate may inhibit attachment by mimicking host cell glycosaminoglycans. Consistent with this, the spirochete bound to immobilized heparin, and soluble heparin inhibited bacterial adhesion to mammalian cells. The bacterium did not bind efficiently to Vero cells treated with heparinase or heparitinase or to mutant CHO cell lines that are deficient in proteoglycan synthesis. Sulfation of glycosaminoglycans was critical for efficient bacterial recognition, as Vero cells treated with an inhibitor of sulfation, or a mutant CHO cell line that produces undersulfated heparan sulfate, did not mediate maximal spirochetal binding. Binding of the spirochete to extracellular matrix also appeared to be dependent upon this attachment pathway. These findings suggest that a glycosaminoglycan-binding activity which can be detected by hemagglutination contributes to the attachment of the Lyme disease spirochete to host cells and matrix.

Heparan sulfate primed on beta-D-xylosides restores binding of basic fibroblast growth factor

Heparan sulfate proteoglycans (HSPG) are obligatory for receptor binding and mitogenic activity of basic fibroblast growth factor (bFGF). Mutant Chinese hamster ovary cells (pgsA-745) deficient in xylosyltransferase are unable to initiate glycosaminoglycan synthesis and hence can not bind bFGF to low- and high-affinity cell surface receptors. Exposure of pgsA-745 cells to beta-D-xylopyranosides containing hydrophobic aglycones resulted in restoration of bFGF binding in a manner similar to that induced by soluble heparin or by heparan sulfate (HS) normally associated with cell surfaces. Restoration of bF-GF binding correlated with the ability of the beta-D-xylosides to prime the synthesis of heparan sulfate. Thus, both heparan sulfate synthesis and bFGF receptor binding were induced by low concentrations (10-30 microM) of estradiol-beta-D-xyloside and naphthyl-beta-D-xyloside, but not by cis/trans-decahydro-2-naphthyl-beta-D-xyloside, which at low concentration primes mainly chondroitin sulfate. The obligatory involvement of xyloside-primed heparan sulfate in restoration of bFGF-receptor binding was also demonstrated by its sensitivity to heparinase treatment and by the lack of restoration activity in CHO cell mutants that lack enzymatic activities required to form the repeating disaccharide unit characteristic of heparan sulfate. Xyloside-primed heparan sulfate binds to the cell surface. Restoration of bFGF receptor binding was induced by both soluble and cell bound xyloside-primed heparan sulfate and was abolished in cells that were exposed to 0.5-1.0 M NaCl prior to the bFGF binding reaction. These results indicate that heparan sulfate chains produced on xyloside primers behave like heparan sulfate chains attached to cellular core proteins in terms of affinity for bFGF and ability to function as low-affinity sites in a dual receptor mechanism characteristic of bFGF and other heparin-binding growth promoting factors.

Heparin-induced oligomerization of FGF molecules is responsible for FGF receptor dimerization, activation, and cell proliferation

Heparin is required for fibroblast growth factor (FGF) stimulation of biological responses. Using isothermal titration calorimetry, we show that acidic FGF (aFGF) forms a 1:1 complex with the soluble extracellular domain of FGF receptor (FGFR). Heparin exerts its effect by binding to many molecules of aFGF. The resulting aFGF-heparin complex can bind to several receptor molecules, leading to FGFR dimerization. In two cell lines lacking endogenous heparan sulfate, exogenous heparin is required for FGFR dimerization, tyrosine kinase activation, c-fos mRNA transcription, and cell proliferation. Moreover, a synthetic heparin analog that binds monovalently to aFGF blocks FGFR dimerization, activation, and signaling via FGFR. We propose that heparin causes oligomerization of aFGF such that its binding to FGFR results in dimerization and activation. This represents a novel mechanism for transmembrane signaling and may account for the action of many heparin-bound growth factors.

Lactoferrin binding to heparan sulfate proteoglycans and the LDL receptor-related protein. Further evidence supporting the importance of direct binding of remnant lipoproteins to HSPG

Bovine lactoferrin inhibits the clearance of remnant lipoproteins from the plasma and competes with the cell-surface binding of apolipoprotein (apo) E-enriched remnants. We established that lactoferrin inhibits remnant binding and uptake by interacting with both heparan sulfate proteoglycans (HSPG) and the low-density lipoprotein receptor-related protein (LRP). The binding of 125I-lactoferrin was inhibited 45% to 60% in HepG2 hepatocytes and wild-type Chinese hamster ovary (CHO) cells treated with heparinase to remove HSPG. In mutant CHO cells (pgsD-677) lacking HSPG, the level of 125I-lactoferrin binding was approximately 50% that seen with wild-type CHO cells; thus, about one half of lactoferrin binding appears to be mediated through cell-surface HSPG. A significant fraction of the residual binding of the lactoferrin appears to be mediated through the LRP. The 39-kd protein known to bind to the LRP and to block ligand interaction inhibited 125I-lactoferrin degradation in wild-type CHO cells by 60% to 65%. The addition of the 39-kd protein plus heparinase treatment reduced the binding by 85% to 90% (this combination blocks direct interaction with both the LRP and HSPG). However, it was also shown that the 39-kd protein bound to HSPG and the LRP. Heparinase treatment of wild-type CHO cells decreased the binding of the 125I-39-kd protein by approximately 40%, and the mutant CHO cells lacking HSPG bound half as much 125I-39-kd protein as wild-type CHO cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Sulfated glycoconjugate receptors for the Bordetella pertussis adhesin filamentous hemagglutinin (FHA) and mapping of the heparin-binding domain on FHA

Filamentous hemagglutinin (FHA) is a major adhesin present on the surface of the gram-negative respiratory pathogen Bordetella pertussis. A number of binding mechanisms have been described for the interaction of FHA with eukaryotic cells. We have focused on its function as a sulfated polysaccharide-binding protein and on identifying potential receptors for FHA on the epithelial cell surface. Using a thin-layer overlay technique, we found that FHA binds specifically to sulfated glycolipids but not to gangliosides or other neutral glycolipids. These results suggest that epithelial cell surface sulfated glycolipids function as receptors for FHA. Further studies demonstrated that a Chinese hamster ovary (CHO) cell strain deficient in glycosaminoglycan expression exhibits greatly diminished attachment to FHA. By FHA-Affi-Gel chromatography, a putative receptor for FHA that has characteristics consistent with a heparan sulfate proteoglycan was isolated from epithelial cell extracts. In addition, by using recombinant FHA fusion proteins, a specific glycosaminoglycan-binding domain located near the N terminus of the FHA molecule was identified. Our results indicate that the B. pertussis adhesin FHA may utilize sulfated glycolipids and proteoglycans commonly found on the surface of human cells and tissues to initiate infection.

The biology of perlecan: the multifaceted heparan sulphate proteoglycan of basement membranes and pericellular matrices

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Diminished heparin binding of a basic fibroblast growth factor mutant is associated with reduced receptor binding, mitogenesis, plasminogen activator induction, and in vitro angiogenesis

Using modeling of heparin-fibroblast growth factor interactions, we replaced four basic residues of basic fibroblast growth factor (FGF-2) with neutral glutamine residues by site-specific mutagenesis to give the mutants K128Q, K138Q, K128Q-K138Q, R129Q, K134Q, and R129Q-K134Q. The FGF mutants were characterized for their receptor and heparin binding affinities, mitogenic and cell proliferation activities, and their ability to induce plasminogen activator (PA) production and in vitro angiogenesis by cultured endothelial cells. Heparin binding properties and biological activities of the three mutants involving R129 and K134 remained essentially unchanged; however, significant changes for three mutants involving K128 and K138 were found. The KD values for heparin binding for K128Q and K138Q mutants were increased about 10-fold, and that for the K128Q-K138Q double mutant was increased by about 100-fold. The mutant K128Q-K138Q required a 10-fold higher concentration of heparin to promote binding to heparan sulfate proteoglycan (HSPG)-deficient CHO cells transfected with fibroblast growth factor receptor-1 (FGFR1) or to induce DNA synthesis in HSPG-deficient myeloid cells transfected with FGFR1. Binding affinities of the mutants to cell surface receptors on BHK-21 cells, however, were similar to that of wild-type FGF-2. In endothelial cell proliferation assays the activities of K128Q and K128Q-K138Q were about 10-fold lower than that of the wild-type protein, whereas the K138Q mutant exhibited wild-type activity. In addition, the K128Q-K138Q mutant displayed a markedly lowered capacity to induce PA activity in cultured endothelial cells and to form capillary-like structures in an in vitro angiogenesis model.(ABSTRACT TRUNCATED AT 250 WORDS)

Heparan sulfate proteoglycans are required for mesoderm formation in Xenopus embryos

Mesoderm forms in the vertebrate embryo as a result of inductive interactions involving secreted growth factors and cell surface molecules. Proteoglycans have recently been implicated in the control of cell adhesion, migration and growth factor responsiveness. We have found that removal of glycosaminoglycan chains of proteoglycans from Xenopus ectodermal explants by heparinase, but not by chondroitinase, results in inhibition of elongation and mesodermal differentiation in response to signaling factors: activin, FGF and Wnt. Heparinase treatment differentially affected expression of early general and region-specific mesodermal markers, suggesting that mesodermal cell fates become specified in the early embryo via at least two signaling pathways which differ in their requirements for heparan sulfate proteoglycans. Addition of soluble heparan sulfate restored activin-mediated induction of muscle-specific actin gene in heparinase-treated explants. Finally, heparinase inhibited autonomous morphogenetic movements and mesodermal, but not neural, differentiation in dorsal marginal zone explants, which normally give rise to mesoderm in the embryo. These results directly demonstrate that heparan sulfate proteoglycans participate in gastrulation and mesoderm formation in the early embryo.

High-molecular-weight proteins of nontypeable Haemophilus influenzae mediate bacterial adhesion to cellular proteoglycans

A family of high-molecular-weight (HMW) surface-exposed proteins of nontypeable Haemophilus influenzae (NT H. influenzae) mediated adherence of these organisms to human epithelium. To better understand the molecular basis for this adherence, the role of glycosaminoglycans (GAGs), substances commonly expressed on cell surfaces, was examined. Bacterial adherence to cells with specific deficiencies in GAG biosynthesis was measured. HMW protein-dependent bacterial adherence to normal cells was significantly greater than adherence to cells deficient in sulfated GAGs or to cells deficient in heparan sulfate but overexpressing chondroitin sulfate. Cells expressing undersulfated heparan sulfate exhibited intermediate levels of bacterial adherence. The addition of exogenous dextran sulfate or heparin inhibited over 70% of the adherence of NT H. influenzae to normal cells, whereas hyaluronic acid and chondroitin sulfate tested at the same concentration (100 micrograms/ml) inhibited bacterial adherence by less than 11%. Treatment of cells with heparinase significantly reduced bacterial adherence. Following electrophoretic separation, HMW proteins were shown to bind directly to radiolabeled heparin. These results indicate that HMW protein-dependent adherence of NT H. influenzae is mediated by cellular sulfated GAGs and that heparan sulfate may be the predominant GAG involved in this process. However, the decreased adherence of bacteria to cells expressing undersulfated heparan sulfate and the inhibition of bacterial adherence by the addition of exogenous dextran sulfate suggest that bacterial adhesion to mammalian cells is likely to be influenced by a variety of factors, including the degree of sulfation and the specificity of the carbohydrate moieties contained in the cellular proteoglycans.

Proteoglycans and glycosaminoglycans in tumor growth and migration: first experience with tumors of bladder and prostate origin

Proteoglycans (PGs), macromolecules that contain bound glycosaminoglycans (GAGs), are an abundant and ubiquitously distributed group of proteins with a large variety of heterogeneous structures. In recent years a whole range of functions, varying from structural/mechanical support to involvement in the regulation of cell proliferation and cell mobility, cell-cell interactions, and angiogenesis and modulation of the extracellular matrix, have been attributed to these compounds. In this communication an overview is presented dealing with the available knowledge of the (regulatory) properties of PGs and GAGs as participants in tumor growth and metastasis. Acquaintance with the biochemical, biophysical, and biological characteristics of PGs and GAGs may provide a rational basis of the therapeutic use of GAGs in the prevention of tumor growth and invasion. The scarcely available information related to the role of PGs and GAGs in uro-oncology and the potential application of GAGs in the treatment of urological tumors are discussed.