Stimulation of fibroblast growth factor receptor-1 occupancy and signaling by cell surface-associated syndecans and glypican

Thrombin- and Factor Xa–Induced DNA Synthesis Is Mediated by Transactivation of Fibroblast Growth Factor Receptor-1 in Human Vascular Smooth Muscle Cells

Thrombin and factor Xa (FXa) are agonists for G protein–coupled receptors (GPRCs) and may contribute to vascular lesion formation by stimulating proliferation of vascular smooth muscle cells (SMCs). Mitogenic signaling of GPCRs requires transactivation of receptor tyrosine kinases (RTKs). In rat SMCs, thrombin transactivates the epidermal growth factor receptor (EGFR) via a pathway that involves heparin-binding EGF-like growth factor (HB-EGF) as ligand for EGFR. The purpose of this study was to investigate in human SMCs the role of receptor transactivation in the mitogenic response to thrombin and FXa. Thrombin (10 nmol/L) and FXa (100 nmol/L) cause a 3.3- and 2.6-fold increase in DNA synthesis, respectively. In human SMCs, neither thrombin nor FXa causes EGFR phosphorylation, and blockade of EGFR kinase does not inhibit DNA synthesis. However, DNA synthesis and phosphorylation of fibroblast growth factor receptor-1 (FGFR-1) induced by thrombin or FXa are inhibited by antibodies neutralizing basic fibroblast growth factor (bFGF) or by heparin. Hirudin inhibits thrombin-, but not FXa-induced mitogenesis, indicating that FXa acts independently of thrombin. We further demonstrate by ELISA that upon thrombin and FXa stimulation, bFGF is released and binds to the extracellular matrix. Our data suggest that in human vascular SMCs, both thrombin and FXa rapidly release bFGF into the pericellular matrix. This is followed by transactivation of the FGFR-1 and increased proliferation. Heparin may inhibit the mitogenic effects of thrombin and FXa in human SMCs by preventing bFGF binding to FGFR-1.

Different heparan sulfate proteoglycans serve as cellular receptors for human papillomaviruses

Papillomaviruses replicate in stratified epithelia of skin and mucosa. Infection with certain human papillomavirus (HPV) types is the main cause of anogenital neoplasia, in particular cervical cancer. Early events of papillomavirus infectivity are poorly understood. While heparan sulfate proteoglycans (HSPGs) mediate initial binding to the cell surface, the class of proteins carrying heparan sulfates has not been defined. Here we examined two processes of papillomavirus infection, attachment of virus-like particles (VLP) to cells and infection with authentic HPV type 11 (HPV11) virions. Of the HSPGs, syndecan-1 is the major epithelial form and is strongly upregulated in wound edge keratinocytes. We employed K562 cells, which lack HSPGs except minor amounts of endogenous betaglycan, and stable clones that express cDNAs of syndecan-1, syndecan-4, or glypican-1. Binding of VLP correlated with levels of heparan sulfate on the cell surface. Parental K562 bound HPV16 VLP weakly, whereas all three K562 transfectants demonstrated enhanced binding, with the highest binding capacity observed for syndecan-1-transfected cells, which also expressed the most HSPG. For HPV11 infectivity assays, a high virion inoculum was required to infect K562 cells, whereas ectopic expression of syndecan-1 increased permissiveness eightfold and expression of syndecan-4 or glypican-1 fourfold. Infection of keratinocytes was eliminated by treatment with heparitinase, but not phospholipase C, further implicating the syndecan family of integral membrane proteins as receptor proteins. Human keratinocytes with a homozygous deletion of alpha6 integrin are permissive for HPV11 infection. These results indicate that several HSPGs can serve as HPV receptors and support a putative role for syndecan-1, rather than alpha6 integrin, as a primary receptor protein in natural HPV infection of keratinocytes.

Heparan sulfate proteoglycans are increased during skeletal muscle regeneration: requirement of syndecan-3 for successful fiber formation

Skeletal muscle regeneration is a highly complex and regulated process that involves muscle precursor proliferation and differentiation and probably requires the participation of heparin binding growth factors such as FGFs, HGF and TGFβ. Heparan sulfate proteoglycans, key components of cell-surfaces and ECM, modulate growth factor activities and influence cell growth and differentiation. Their expression in forming muscle masses during development and in cell culture, suggest their participation in the regulation of myogenesis. In the present study, heparan sulfate proteoglycan expression in skeletal muscle regeneration induced by barium chloride injection was evaluated. Expression of muscle differentiation markers and neuromuscular junction (NMJ) components was characterized. Immunoblots with anti-Δ-heparan sulfate antibody showed that four major species - perlecan, glypican, syndecan-3 and syndecan-4 - were transiently up-regulated. The first three were detected at the surface or basement membranes of newly formed myotubes by specific indirect immunofluorescence. Syndecan-3, a satellite cell marker, showed the earliest and most significant increase. Experiments involving myoblast grafting into regenerating muscle showed that C2C12 cell clones, with inhibited syndecan-3 expression resulting from antisense transfection, presented a normal proliferation rate but an impaired capacity to fuse and form skeletal muscle fibers. These data constitute the first in vivo evidence suggesting the requirement of a specific heparan sulfate proteoglycan for successful skeletal muscle regeneration.

Surface nucleolin participates in both the binding and endocytosis of lactoferrin in target cells

Lactoferrin (Lf), a multifunctional molecule present in mammalian secretions and blood, plays important roles in host defense and cancer. Indeed, Lf has been reported to inhibit the proliferation of cancerous mammary gland epithelial cells and manifest a potent antiviral activity against human immunodeficiency virus and human cytomegalovirus. The Lf-binding sites on the cell surface appear to be proteoglycans and other as yet undefined protein(s). Here, we isolated a Lf-binding 105 kDa molecular mass protein from cell extracts and identified it as human nucleolin. Medium-affinity interactions ( approximately 240 nm) between Lf and purified nucleolin were further illustrated by surface plasmon resonance assays. The interaction of Lf with the cell surface-expressed nucleolin was then demonstrated through competitive binding studies between Lf and the anti-human immunodeficiency virus pseudopeptide, HB-19, which binds specifically surface-expressed nucleolin independently of proteoglycans. Interestingly, binding competition studies between HB-19 and various Lf derivatives in proteoglycan-deficient hamster cells suggested that the nucleolin-binding site is located in both the N- and C-terminal lobes of Lf, whereas the basic N-terminal region is dispensable. On intact cells, Lf co-localizes with surface nucleolin and together they become internalized through vesicles of the recycling/degradation pathway by an active process. Morever, a small proportion of Lf appears to translocate in the nucleus of cells. Finally, the observations that endocytosis of Lf is inhibited by the HB-19 pseudopeptide, and the lack of Lf endocytosis in proteoglycan-deficient cells despite Lf binding, point out that both nucleolin and proteoglycans are implicated in the mechanism of Lf endocytosis.

The epidermal growth factor-like domains of the human EMR2 receptor mediate cell attachment through chondroitin sulfate glycosaminoglycans

Using multivalent protein probes, an evolutionarily conserved endogenous ligand for EMR2, a human myeloid cell-restricted EGF-TM7 receptor, was identified on the surface of a number of adherent cell lines. In addition, in situ staining of the ligand has revealed specific in vivo patterns consistent with a connective tissue distribution. The interaction is conserved across species and mediated exclusively by the largest EMR2 isoform containing 5 epidermal growth factor (EGF)-like modules. Antibody-blocking studies subsequently revealed that the fourth EGF-like module constitutes the major ligand-binding site. The largest isoform of CD97, a related EGF-TM7 molecule containing an identical EGF-like module, also binds to the putative EMR2 ligand. Through the use of mutant Chinese hamster ovary (CHO) cell lines defective in glycosaminoglycans (GAGs) biosynthesis as well as the enzymatic removal of specific cell surface GAGs, the molecular identity of the EMR2 ligand was identified as chondroitin sulfate (CS). Thus, exogenous CS GAGs blocked the EMR2-ligand interaction in a dose-dependent manner. EMR2-CS interaction is Ca2+- and sulphation-dependent and results in cell attachment. This is the first report of a GAG ligand for the TM7 receptors extending the already vast repertoire of stimuli of the GPCR superfamily.

Glypican 4 modulates FGF signalling and regulates dorsoventral forebrain patterning in Xenopus embryos

Heparan sulphate proteoglycans such as glypicans are essential modulators of intercellular communication during embryogenesis. In Xenopus laevis embryos, the temporal and spatial distribution of Glypican 4 (Gpc4) transcripts during gastrulation and neurulation suggests functions in early development of the central nervous system. We have functionally analysed the role of Xenopus Gpc4 by using antisense morpholino oligonucleotides and show that Gpc4 is part of the signalling network that patterns the forebrain. Depletion of GPC4 protein results in a pleiotropic phenotype affecting both primary axis formation and early patterning of the anterior central nervous system. Molecular analysis shows that posterior axis elongation during gastrulation is affected in GPC4-depleted embryos, whereas head and neural induction are apparently normal. During neurulation, loss of GPC4 disrupts expression of dorsal forebrain genes, such as Emx2, whereas genes marking the ventral forebrain and posterior central nervous system continue to be expressed. This loss of GPC4 activity also causes apoptosis of forebrain progenitors during neural tube closure. Biochemical studies establish that GPC4 binds FGF2 and modulates FGF signal transduction. Inhibition of FGF signal transduction, by adding the chemical SU5402 to embryos from neural plate stages onwards, phenocopies the loss of gene expression and apoptosis in the forebrain. We propose that GPC4 regulates dorsoventral forebrain patterning by positive modulation of FGF signalling.

Expression pattern of glypican-1 mRNA after brain injury in mice

Glypican-1, a heparan sulfate proteoglycan, is expressed in various tissues including developing and postnatal central nervous system. It serves as a receptor for heparin-binding molecules such as fibroblast growth factors (FGFs). We investigated whether glypican-1 was expressed after brain injury in adult mice. In situ hybridization study showed that glypican-1 mRNA was expressed in the region surrounding necrotic tissue, and that the signal intensity peaked 7 days after the cryo-injury. In addition, both FGF-2 and amyloid precursor protein (APP) were concurrently upregulated and colocalized with glypican-1 mRNA. Since FGF-2 and APP can bind to glypican-1 in vitro, the present study suggested that their autocrine/paracrine interactions with glypican-1 may be involved in neuronal regeneration and/or neurite-outgrowth inhibition after brain injury.

High syndecan-1 expression in breast carcinoma is related to an aggressive phenotype and to poorer prognosis

BACKGROUND

Syndecan-1 is a transmembrane heparan sulphate proteoglycan that is involved in cell-cell adhesion, organization of cell-matrix adhesion, and regulation of growth factor signaling.

METHODS

Specimens from 254 consecutive breast carcinoma (BC) cases (110 N0, 144 N1/2) with long-term follow-up (median, 95 months) were immunostained for syndecan-1, estrogen receptor (ER), progesterone receptor (PgR), and p53; in 154 cases, c-erbB-2 status was known. Syndecan-1 mRNA and protein expression also were evaluated in 20 breast tissue samples (10 normal and tumor pairs).

RESULTS

Syndecan-1 was expressed at high levels in 106 (42%) BCs; syndecan-1 up-regulation was confirmed by reverse transcriptase-polymerase chain reaction (RT-PCR) studies. High syndecan-1 expression was associated with high histologic grade, large tumor size, high mitotic count, c-erbB-2 overexpression, and ER and PgR negative status. At univariate survival analysis syndecan overexpression was related to poor prognosis (P < 0.01 for both overall survival (OS) and disease-free survival). Bivariate survival analysis showed an additive adverse effect for syndecan-1 and c-erbB-2 overexpression. At multivariate analysis, syndecan-1 overexpression was independently associated with poor OS (hazard ratio [HR], 1.71; 95% confidence interval [CI], 1.08-2.69). High syndecan-1 expression also was of independent prognostic value for OS in the group of 102 ER-negative patients (HR, 2.42; 95% CI, 1.21-4.82). Stratifying patients on the basis of the type of adjuvant therapy given, high syndecan-1 expression was associated with a higher risk of death only in patients treated with the cyclophosphamide-methotrexate-fluorouracil regimen (HR, 1.9; P = 0.09); at multivariate analysis for OS, this association proved to be of independent statistical significance (P = 0.03; HR, 2.15).

CONCLUSIONS

Syndecan-1 is expressed at high levels in a significant percentage of breast carcinomas and is related to an aggressive phenotype and poor clinical behavior.

Localisation of specific heparan sulfate proteoglycans during the proliferative phase of brain development

Early brain development is characterised by the proliferation of neural precursor cells. Several families of signalling molecules such as the fibroblast growth factors (FGFs) and Wnts are known to play important roles in this early phase of brain development. Accumulating evidence demonstrates that signalling of these molecules requires the presence of heparan sulfate chains attached to a proteoglycan core protein (HSPG). However, the specific identity of the HSPG components in the developing brain is unknown. To determine which HSPGs might be involved at this early phase, we analysed the expression of the major cell surface HSPG families in the developing brain at a time of most active proliferation. Syndecan-1 and glypican-4 were the most highly expressed in the developing brain during the time of peak proliferation and localise to ventricular regions of the brain, where the precursor cells are proliferating. Syndecan-4, although less abundant, also localises to cells in the ventricular zone. We have also examined HSPG involvement in brain development using cultures of embryonic neural precursor cells. We find that FGF2 stimulation of proliferation is inhibited in the presence of sodium chlorate, an inhibitor of heparan sulfate synthesis, and is rescued by addition of exogenous heparan sulfate. These data support a requirement for heparan sulfate in FGF signalling for proliferation of brain precursor cells. The expression of these specific HSPGs within the proliferative zone of the brain suggests that they may be involved in regulation of early brain development, such as FGF-stimulated proliferation.

Heparan sulfate proteoglycans as regulators of fibroblast growth factor-2 signaling in brain endothelial cells. Specific role for glypican-1 in glioma angiogenesis

Fibroblast growth factor-2 (FGF2) is a potent angiogenic factor in gliomas. Heparan sulfate promotes ligand binding to receptor tyrosine kinase and regulates signaling. The goal of this study was to examine the contribution of heparan sulfate proteoglycans (HSPGs) to glioma angiogenesis. Here we show that all brain endothelial cell HSPGs carry heparan sulfate chains similarly capable of forming a ternary complex with FGF2 and fibroblast growth factor receptor-1c and of promoting a mitogenic signal. Immunohistochemical analysis revealed that glypican-1 was overexpressed in glioma vessel endothelial cells, whereas this cell-surface HSPG was consistently undetectable in normal brain vessels. To determine the effect of increased glypican-1 expression on FGF2 signaling, we transfected normal brain endothelial cells, which express low base-line levels of glypican-1, with this proteoglycan. Glypican-1 expression enhanced growth of brain endothelial cells and sensitized them to FGF2-induced mitogenesis despite the fact that glypican-1 remained a minor proteoglycan. In contrast, overexpression of syndecan-1 had no effect on growth or FGF2 sensitivity. We conclude that the glypican-1 core protein has a specific role in FGF2 signaling. Glypican-1 overexpression may contribute to angiogenesis and the radiation resistance characteristic of this malignancy.

Structural and functional changes in heparan sulfate proteoglycan expression associated with the myofibroblastic phenotype

The principal cells implicated as the source of the extracellular matrix in areas of progressive fibrosis are fibroblasts with the phenotypic appearance of myofibroblasts. This report describes differences in heparan sulfate proteoglycan expression between myofibroblasts and normal fibroblasts, associated with impaired responses to fibroblast growth factor-2 (FGF-2). Although both cell types responded to platelet-derived growth factor, myofibroblasts, unlike fibroblasts, did not proliferate to FGF-2. A response was acquired, however, when myofibroblasts were incubated with FGF-2 in the presence of heparan sulfate (HS) and heparin. Selective digestion with pronase, NaOH/NaBH(4), heparinase I, or low pH nitrous acid showed that each HS-glycosaminoglycan region comprised a pronase-resistant peptide separating two HS chains. The HS-glycosaminoglycan chains from myofibroblasts were larger (K(av), 0.32; molecular weight, 50 kd) than those from fibroblasts (K(av), 0.4; molecular weight, 33 kd), although their disaccharide composition was identical. The chains from myofibroblasts, however, contained three, compared to two, heparinase 1-resistant sequences separated by larger contiguous areas of low sulfation. Furthermore, although there was no difference in FGF-2-binding affinity between the two cell types, the chains secreted by myofibroblasts had twice the binding capacity of those from fibroblasts. Thus, it is likely that the difference in response to FGF-2 is because of a difference in FGF-2 sequestration and receptor interaction with FGF-2-HS complexes. A comparative investigation into HS fine structure is being undertaken to examine these findings in more detail.

Glypican-3, overexpressed in hepatocellular carcinoma, modulates FGF2 and BMP-7 signaling

The Glypican (GPC) family is a prototypical member of the cell-surface heparan sulfate proteoglycans (HSPGs). The HSPGs have been demonstrated to interact with growth factors, act as coreceptors and modulate growth factor activity. Here we show that based on oligonucleotide array analysis, GPC3 was upregulated in hepatocellular carcinoma (HCC). By northern blot analysis, GPC3 mRNA was found to be upregulated in 29 of 52 cases of HCC (55.7%). By Western blot analysis carried out with a monoclonal anti-GPC3 antibody we generated, the GPC3 protein was found to be overexpressed in 6 hepatoma cell lines, HepG2, Hep3B, HT17, HuH6, HuH7 and PLC/PRF/5, as well as 22 tumors (42.3%). To investigate the role of overexpressed GPC3 in liver cancer, we analyzed its effects on cell growth of hepatoblastoma-derived cells. Overexpression of GPC3 modulated cell proliferation by inhibiting fibroblast growth factor 2 (FGF2) and bone morphogenetic protein 7 (BMP-7) activity. An interaction of GPC3 and FGF2 was revealed by co-immunoprecipitation, while GPC3 was found to inhibit BMP-7 signaling through the Smad pathway by reporter gene assay. The modulation of growth factors by GPC3 may help explain its role in liver carcinogenesis. In addition, the ability of HCC cells to express GPC3 at high levels may serve as a new tumor marker for HCC.

Cytoplasmic interactions of syndecan-4 orchestrate adhesion receptor and growth factor receptor signalling

Syndecan-4 is a ubiquitous transmembrane proteoglycan that localizes to the focal adhesions of adherent cells and binds to a range of extracellular ligands, including growth factors and extracellular-matrix proteins. Engagement of syndecan-4 is essential for adhesion formation in cells adhering via certain integrins, and for cell proliferation and migration in response to growth factors. The cytoplasmic domain of syndecan-4 interacts with a number of signalling and structural proteins, and both extracellular and cytoplasmic domains are necessary for regulated activation of associated transmembrane receptors. PDZ domain-containing scaffold proteins (syntenin and CASK) bind to the C-terminus of the syndecan-4 cytoplasmic domain and co-ordinate clustering of receptors and connection to the actin cytoskeleton. Syndecan-4 also binds and activates protein kinase Calpha in the presence of phosphatidylinositol 4,5-bisphosphate, and regulates signalling by Rho-family GTPases and focal adhesion kinase. This review discusses the cytoplasmic interactions of syndecan-4 and how they affect cell behaviour as a consequence of the interaction with extracellular ligands. These conclusions also offer an insight into the role of syndecan-4 in vivo, and are consistent with phenotypes generated as a consequence of abnormal syndecan-4 expression in pathologies and gene disruption studies.

Augmented synthesis and differential localization of heparan sulfate proteoglycans in Duchenne muscular dystrophy

Muscular dystrophies are characterized by continuous cycles of degeneration and regeneration that result in extensive fibrosis and a progressive diminution of muscle mass. Cell surface heparan sulfate proteoglycans are found almost ubiquitously on the surface and in the extracellular matrix (ECM) of mammalian cells. These macromolecules interact with a great variety of ligands, including ECM constituents, adhesion molecules, and growth factors. In this study, we evaluated the expression and localization of three heparan sulfate proteoglycans in the biopsies of Duchenne muscular dystrophy (DMD) patients. Through SDS-PAGE analyses followed by specific identification of heparitinase-digested proteins with an anti-Delta-heparan sulfate specific monoclonal antibodies, we observed an increase of three forms of heparan sulfate proteoglycans, corresponding to perlecan, syndecan-3, and glypican-1. Immunohistochemistry analyses indicated a differential localization for these proteoglycans: glypican-1 and perlecan were found mainly associated to ECM structures, while syndecan-3 was associated to muscle fibers. These results suggest that the amount of specific heparan sulfate proteoglycans is augmented in skeletal muscle in DMD patients presenting a differential localization.

Increased syndecan expression by pleiotrophin and FGF receptor-expressing astrocytes in injured brain tissue

Syndecan-1, -2, -3, and -4 are heparan sulfate proteoglycans that are differentially expressed during development and wound repair. To determine whether syndecans are also involved in brain injury, we examined the expression of syndecan core proteins genes in cryo-injured mouse brain, using in situ hybridization. All syndecan mRNA transcripts were similarly expressed in the region surrounding the necrotic tissue, exhibiting peak levels at day 7 after injury. Comparison with cellular markers showed that reactive astrocytes were the primary source of syndecans. Syndecans serve as co-receptors for fibroblast growth factor (FGF) and as a reservoir for another heparin-binding growth factor, pleiotrophin (PTN, or heparin-binding growth-associated molecule. In our model, FGF receptor1 (FGFR1) and PTN mRNA levels were upregulated in reactive astrocytes. The distribution patterns of FGFR1 and PTN overlapped considerably with those of syndecan-1 and -3 mRNAs, respectively. These results suggest that syndecans are expressed primarily in reactive astrocytes, and may provide a supportive environment for regenerating axons in concert with heparin-binding growth factors (e.g., FGF and PTN) in the injured brain.

Syndecan-4: dispensable or indispensable?

Studies examining the role of the cell-surface heparan sulfate proteoglycan syndecan-4 have yielded a plethora of information regarding its role in both cell-matrix and growth-factor mediated signaling events. Many of the initial conclusions drawn from such research placed syndecan-4 in a keystone position within various signaling pathways though the generation of syndecan-4 null mice have surprised many in the field by indicating otherwise. These contradictory results place researchers in the frustrating and yet exhilarating position of still asking the question, "What role does syndecan-4 play in life?"

Enhanced bacterial virulence through exploitation of host glycosaminoglycans

Present in the extracellular matrix and membranes of virtually all animal cells, proteoglycans (PGs) are among the first host macromolecules encountered by infectious agents. Because of their wide distribution and direct accessibility, it is not surprising that pathogenic bacteria have evolved mechanisms to exploit PGs for their own purposes, including mediating attachment to target cells. This is achieved through the expression of adhesins that recognize glycosaminoglycans (GAGs) linked to the core protein of PGs. Some pathogens, such as Bordetella pertussis and Chlamydia trachomatis, may express more than one GAG-binding adhesin. Bacterial interactions with PGs may also facilitate cell invasion or systemic dissemination, as observed for Neisseria gonorrhoeae and Mycobacterium tuberculosis respectively. More-over, pathogenic bacteria can use PGs to enhance their virulence via a shedding of PGs that leads to there lease of effectors that weaken the host defences. The exploitation of PGs by pathogenic bacteria is thus a multifaceted mechanistic process directly related to the potential virulence of a number of microorganisms.

Cell surface heparan sulfate proteoglycans: target and partners of the basic fibroblast growth factor in rat Sertoli cells

Basic fibroblast growth factor (bFGF) regulates diversified biological functions in rat Sertoli cells. This report demonstrates that bFGF inhibits steroidogenesis in developing rat Sertoli cells. Follicle stimulating hormone (FSH)-stimulated estradiol production was reduced by bFGF. Moreover, the amount of cytochrome P450 aromatase, responsible for the irreversible transformation of androgens into estrogens, is decreased by bFGF at the transcriptional level. The bFGF inhibitory effect was also observed in the presence of dibutyryl-cAMP, cholera toxin or RO-20-1724, all inducing high levels of cAMP, the second messenger of FSH. Heparan sulfate proteoglycans (HSPGs) were shown to be required as cofactors for bFGF signaling. Indeed, sodium chlorate, described to drastically decrease proteoglycan sulfation, abolishes the bFGF downregulation of FSH-stimulated estradiol synthesis previously observed. Glypican-1, syndecan-1 and -4, potential bFGF coreceptors, are mainly regulated at the transcriptional level. This report shows that the bFGF regulation of their expression specifically depends on the nature of HSPG and of the Sertoli cell developmental stage. In conclusion, HSPG are partners and the target of bFGF in rat Sertoli cells.

Heparan sulfate proteoglycans as regulators of fibroblast growth factor-2 receptor binding in breast carcinomas

Binding of fibroblast growth factors (FGFs) to their tyrosine kinase-signaling receptors (FGFRs) requires heparan sulfate (HS). HS proteoglycans (HSPGs) determine mitogenic responses of breast carcinoma cells to FGF-2 in vitro. For this study, we examined the role of HSPGs as modulators of FGF-2 binding to FGFR-1 in situ and in vitro. During stepwise reconstitution of the FGF-2/HSPG/FGFR-1 complex in situ, we identified an elevated ability of breast carcinoma cell HSPGs to promote receptor complex formation compared to normal breast epithelium. HSPGs isolated from the MCF-7 breast-carcinoma cell line were then fractionated according to their ability to assemble the FGF-2 receptor complex. All MCF-7 HSPGs are decorated with HS chains similarly capable of promoting FGF-2 receptor complex formation. In this in vitro model, syndecan-1 and syndecan-4 are the cell surface HSPGs contributing most to the complex formation. Relative expression levels of these syndecans in human breast carcinoma tissues correlate well with receptor complex formation in situ, indicating that in breast carcinomas, core protein levels determine FGF-2 receptor complex formation. However, variances in syndecan expression levels do not explain the difference in FGF-2 receptor complex formation between normal and malignant epithelial cells, suggesting that alterations in HS structure occur during malignant transformation.

Membrane heparan sulfate proteoglycan-supported FGF2-FGFR1 signaling: evidence in support of the "cooperative end structures" model

Fibroblast growth factor 2 (FGF2)-initiated FGF receptor (FGFR)-signaling requires the assistance of heparin/heparan sulfate. Here, we evaluated the effects of different heparan sulfate proteoglycan (HSPG)-expressing cell lines and HSPGs derived from these cells on FGF2-induced FGFR1-phosphorylation in heparan sulfate-negative BaF3 cells. HSPGs supplied in membrane-associated form, by presenting cells, were all effective promotors of FGF2-initiated FGFR1 phosphorylation, independently of their nature (syndecan/glypican) or cellular origin (human lung fibroblasts, transfected Namalwa cells, or transfected K562 cells). A treatment with heparitinase initially stimulated, but finally completely inhibited, the activity of these presenting cells. In comparison, equivalent amounts of soluble HSPGs, obtained by trypsinization of these cells or by immunopurification from cell extracts, did not promote FGF2-induced FGFR1-phosphorylation, yet removal of the less anionic species or a further treatment with heparitinase converted these soluble fractions into potent activators of FGF2/FGFR1 signaling. Extrapolating from current structural models, we suggest that FGFR dimerization and autophosphorylation is supported by cooperative "heparin-like end structures," and that cell surface association and concentration compensate for the relative scarcity of such end structures in native HSPGs. In this model, "proteolytic" shedding of heparan sulfate would act as a diluting, down-regulatory mechanism, while "heparanolytic" shedding might act as an up-regulatory mechanism, by increasing the concentration of these end structures.

Syndecans serve as attachment receptors for human immunodeficiency virus type 1 on macrophages

Macrophages are thought to represent one of the first cell types in the body to be infected during the early stage of human immunodeficiency virus type 1 (HIV-1) transmission and represent a potential viral reservoir in vivo. Thus, an understanding of HIV-1 attachment to these cells is fundamental to the development of novel anti-HIV-1 therapies. Although one of the major targets of HIV-1 in vivo--CD4(+) T lymphocytes--express high CD4 levels, other major targets such as macrophages do not. We asked in this study whether this low CD4 level on macrophages is sufficient to support HIV-1 attachment to these cells or whether cell surface proteins other than CD4 are required for this process. We show that CD4 alone is not sufficient to support the initial adsorption of HIV-1 to macrophages. Importantly, we find that heparan sulfate proteoglycans (HSPGs) serve as the main class of attachment receptors for HIV-1 on macrophages. Most importantly, we demonstrate that a single family of HSPGs, the syndecans, efficiently mediates HIV-1 attachment and represents an abundant class of attachment receptors on macrophages.

A role for the perlecan protein core in the activation of the keratinocyte growth factor receptor

Perlecan, a widespread heparan sulphate (HS) proteoglycan, is directly involved in the storing of angiogenic growth factors, mostly members of the fibroblast growth factor (FGF) gene family. We have previously shown that antisense targeting of the perlecan gene causes a reduced growth and responsiveness to FGF7 [also known as keratinocyte growth factor (KGF)] in human cancer cells, and that the perlecan protein core interacts specifically with FGF7. In the present paper, we have investigated human colon carcinoma cells in which the perlecan gene was disrupted by targeted homologous recombination. After screening over 1000 clones, we obtained two clones heterozygous for the null mutation with no detectable perlecan, indicating that the other allele was non-functioning. The perlecan-deficient cells grew more slowly, did not respond to FGF7 with or without the addition of heparin, and were less tumorigenic than control cells. Paradoxically, the perlecan-deficient cells displayed increased FGF7 surface binding. However, the perlecan protein core was required for functional activation of the KGF receptor and downstream signalling. Because heparin could not substitute for perlecan, the HS chains are not critical for FGF7-mediated signalling in this cell system. These results provide the first genetic evidence that the perlecan protein core is a molecular entity implicated in FGF7 binding and activation of its receptor.

Identifying intercellular signaling genes expressed in malignant plasma cells by using complementary DNA arrays

In multiple myeloma (MM), the growth of primary plasma cells depends not only on interleukin-6 (IL-6), but also on additional unidentified signals delivered by the bone marrow environment. Using Atlas complementary DNA (cDNA) arrays comprising 268 genes coding for intercellular signaling molecules, this study identified genes that are overexpressed in myeloma cells compared to autologous B-lymphoblastoid cell lines. These genes encode the oncogenic Tyro3 tyrosine kinase receptor, the heparin-binding epidermal growth factor-like growth factor (HB-EGF) that is an epithelial autocrine tumor growth factor, the thrombin receptor (TR) that is linked to HB-EGF and syndecan-1 processing and to cell invasion, chemokine receptors CCR1 and CCR2, the Wnt pathway actor Frizzled-related protein (FRZB), and the Notch receptor ligand Jagged 2. These data, obtained with the Atlas cDNA array, were confirmed by reverse transcriptase-polymerase chain reaction or protein analysis or both. Furthermore, Tyro3, HB-EGF, TR, and FRZB gene expression was documented in purified primary malignant plasma cells from patients with plasma cell leukemia or MM. HB-EGF and FRZB were poorly expressed in purified polyclonal plasma cells. Finally, HB-EGF was proved to be an essential autocrine growth factor for the XG-1 myeloma cells. This study shows the potency and the biologic relevance of cDNA arrays used to analyze simultaneously a large panel of intercellular signaling genes and, by identifying several genes overexpressed in malignant plasma cells, opens new fields of investigation in MM biology. (Blood. 2001;98:771-780)

Transgenic expression of syndecan-1 uncovers a physiological control of feeding behavior by syndecan-3

Transgenic expression in the hypothalamus of syndecan-1, a cell surface heparan sulfate proteoglycan (HSPG) and modulator of ligand-receptor encounters, produces mice with hyperphagia and maturity-onset obesity resembling mice with reduced action of alpha melanocyte stimulating hormone (alphaMSH). Via their HS chains, syndecans potentiate the action of agouti-related protein and agouti signaling protein, endogenous inhibitors of alphaMSH. In wild-type mice, syndecan-3, the predominantly neural syndecan, is expressed in hypothalamic regions that control energy balance. Food deprivation increases hypothalamic syndecan-3 levels several-fold. Syndecan-3 null mice, otherwise apparently normal, respond to food deprivation with markedly reduced reflex hyperphagia. We propose that oscillation of hypothalamic syndecan-3 levels physiologically modulates feeding behavior.

Heparin therapy for ulcerative colitis? Effects and mechanisms

Despite intensive medical treatment with steroids and immunosuppressants, acute colitis is still associated with a colectomy rate of up to 15%. Following the observation that a patient with severe steroid-resistant colitis went into remission when treated with heparin for a deep vein thrombosis, there have been a number of reports on the use of heparin in acute ulcerative colitis. Although small and uncontrolled, these studies consistently demonstrate the beneficial effects of heparin, with surprisingly few side-effects in a disease characterized by mucosal haemorrhage. The mechanisms by which heparin may ameliorate ulcerative colitis remain unclear. A simple anticoagulant effect may be responsible, but similar effects are not seen with warfarin. As a result of their intense negative charge, the glycosaminoglycans that constitute heparin have diverse biological effects. These include potent anti-inflammatory actions, in vitro and in vivo, and the potentiation of the activity of the peptide growth factors necessary for mucosal regeneration and repair. This review summarizes the clinical reports on heparin treatment for ulcerative colitis and explores the mechanisms by which this novel form of treatment may exert its effects.

Heparan sulfate proteoglycans in invasion and metastasis

Because heparan sulfate proteoglycans mediate cell adhesion and control the activities of numerous growth and motility factors, they play a critical role in regulating the metastatic behavior of tumor cells. Due to their utilitarian nature, heparan sulfate proteoglycans may at times act as inhibitors of cell invasion and at other times as promoters of cell invasion, with their function being determined by their location (cell surface or extracellular matrix), the heparin-binding molecules they associate with, the presence of modifying enzymes (proteases, heparanases) and the precise structural characteristics of the proteoglycan. Also, the tissue type and pathophysiological state of the tumor influence proteogylcan function. This review summarizes our current knowledge of the role heparan sulfate proteoglycans play in regulating tumor cell metastasis, proposes mechanisms of how these molecules function and examines the potential for discovery of new therapeutic approaches designed to block metastatic cancer.

Developmental roles of the glypicans

Glypicans are proteins with very characteristic structures that are substituted with heparan sulfate and that are linked to the cell surface via glycosylphosphatidylinositol. The modular structure of the glypicans has been highly conserved throughout evolution. Six glypicans have been identified so far in vertebrates. Mutations in Drosophila, humans and mice reveal a role for these cell surface molecules in the control of cell growth and differentiation. Their mechanism of action is not yet clear. Most likely, glypicans activate or determine the activity ranges of morphogens and growth factors such as FGFs, BMPs, Wnts, Hhs and IGFs.

Expression and localization of proteoglycans during limb myogenic activation

After arriving at the limb bud, migrating myogenic precursor cells express transcription factors responsible for the induction of terminal skeletal muscle differentiation. One such factor is myogenin, a member of the basic helix-loop-helix family, known to activate the expression of muscle-specific genes. The extracellular signals involved in activating the myogenic program in the muscle precursor cells that reach the limb in vivo are not known. However, in vitro, it has been shown that proteoglycans, macromolecules composed of a core protein and glycosaminoglycan chains, modulate the triggering of myogenin activity. To understand the role of proteoglycans during limb muscle development, we assessed the synthesis of proteoglycans in limb bud explants at 10.5 days post coitum, when migrating cells arrive, evaluated the expression and nature of these macromolecules during in vivo early limb bud formation, and determined the colocalization of myoblasts expressing myogenin with specific proteoglycans. We found that the expression of myogenin was temporally and spatially coincident with the expression of syndecan-3 and decorin, two essential proteoglycans in the modulation of skeletal muscle differentiation. This article is the first report of myogenic activation and proteoglycan expression during limb muscle formation.

The midline glia of Drosophila: a molecular genetic model for the developmental functions of glia

The Midline Glia of Drosophila are required for nervous system morphogenesis and midline axon guidance during embryogenesis. In origin, gene expression and function, this lineage is analogous to the floorplate of the vertebrate neural tube. The expression or function of over 50 genes, summarised here, has been linked to the Midline Glia. Like the floorplate, the cells which generate the Midline Glia lineage, the mesectoderm, are determined by the interaction of ectoderm and mesoderm during gastrulation. Determination and differentiation of the Midline Glia involves the Drosophila EGF, Notch and segment polarity signaling pathways, as well as twelve identified transcription factors. The Midline Glia lineage has two phases of cell proliferation and of programmed cell death. During embryogenesis, the EGF receptor pathway signaling and Wrapper protein both function to suppress apoptosis only in those MG which are appropriately positioned to separate and ensheath midline axonal commissures. Apoptosis during metamorphosis is regulated by the insect steroid, Ecdysone. The Midline Glia participate in both the attraction of axonal growth cones towards the midline, as well as repulsion of growth cones from the midline. Midline axon guidance requires the Drosophila orthologs of vertebrate genes expressed in the floorplate, which perform the same function. Genetic and molecular evidence of the interaction of attractive (Netrin) and repellent (Slit) signaling is reviewed and summarised in a model. The Midline Glia participate also in the generation of extracellular matrix and in trophic interactions with axons. Genetic evidence for these functions is reviewed.

The cell biology of thrombospondin-1

Thrombospondin-1 (TSP-1) is a matricellular protein that regulates cellular phenotype during tissue genesis and repair. It acts as a molecular facilitator by bringing together cytokines, growth factors, matrix components, membrane receptors and extracellular proteases. TSP-1 binds to a wide variety of integrin and non-integrin cell surface receptors. The binding sites for these receptors on TSP-1 are dispersed throughout the molecule, with most domains binding multiple receptors. In some cases, TSP-1 binds to multiple receptors concurrently, and recent data indicate that there is cross-talk between the receptor systems. Thus, TSP-1 may function to direct the clustering of receptors to specialized domains for adhesion and signal transduction.

High levels of soluble syndecan-1 in myeloma-derived bone marrow: modulation of hepatocyte growth factor activity

Syndecan-1 is a heparan sulfate proteoglycan expressed on the surface of, and actively shed by, myeloma cells. Hepatocyte growth factor (HGF) is a cytokine produced by myeloma cells. Previous studies have demonstrated elevated levels of syndecan-1 and HGF in the serum of patients with myeloma, both of negative prognostic value for the disease. Here we show that the median concentrations of syndecan-1 (900 ng/mL) and HGF (6 ng/mL) in the marrow compartment of patients with myeloma are highly elevated compared with healthy controls and controls with other diseases. We show that syndecan-1 isolated from the marrow of patients with myeloma seems to exist in an intact form, with glucosaminoglycan chains. Because HGF is a heparan-sulfate binding cytokine, we examined whether it interacted with soluble syndecan-1. In supernatants from myeloma cells in culture as well as in pleural effusions from patients with myeloma, HGF existed in a complex with soluble syndecan-1. Washing myeloma cells with purified soluble syndecan-1 could effectively displace HGF from the cell surface, suggesting that soluble syndecan-1 can act as a carrier for HGF in vivo. Finally, using a sensitive HGF bioassay (interleukin-11 production from the osteosarcoma cell line Saos-2) and intact syndecan-1 isolated from the U-266 myeloma cell line, we found that the presence of high concentrations of syndecan-1 (more than 3 μg/mL) inhibited the HGF effect, whereas lower concentrations potentiated it. HGF is only one of several heparin-binding cytokines associated with myeloma. These data indicate that soluble syndecan-1 may participate in the pathology of myeloma by modulating cytokine activity within the bone marrow.

High levels of soluble syndecan-1 in myeloma-derived bone marrow: modulation of hepatocyte growth factor activity

Syndecan-1 is a heparan sulfate proteoglycan expressed on the surface of, and actively shed by, myeloma cells. Hepatocyte growth factor (HGF) is a cytokine produced by myeloma cells. Previous studies have demonstrated elevated levels of syndecan-1 and HGF in the serum of patients with myeloma, both of negative prognostic value for the disease. Here we show that the median concentrations of syndecan-1 (900 ng/mL) and HGF (6 ng/mL) in the marrow compartment of patients with myeloma are highly elevated compared with healthy controls and controls with other diseases. We show that syndecan-1 isolated from the marrow of patients with myeloma seems to exist in an intact form, with glucosaminoglycan chains. Because HGF is a heparan-sulfate binding cytokine, we examined whether it interacted with soluble syndecan-1. In supernatants from myeloma cells in culture as well as in pleural effusions from patients with myeloma, HGF existed in a complex with soluble syndecan-1. Washing myeloma cells with purified soluble syndecan-1 could effectively displace HGF from the cell surface, suggesting that soluble syndecan-1 can act as a carrier for HGF in vivo. Finally, using a sensitive HGF bioassay (interleukin-11 production from the osteosarcoma cell line Saos-2) and intact syndecan-1 isolated from the U-266 myeloma cell line, we found that the presence of high concentrations of syndecan-1 (more than 3 μg/mL) inhibited the HGF effect, whereas lower concentrations potentiated it. HGF is only one of several heparin-binding cytokines associated with myeloma. These data indicate that soluble syndecan-1 may participate in the pathology of myeloma by modulating cytokine activity within the bone marrow.

Expression of c-Met and heparan-sulfate proteoglycan forms of CD44 in colorectal cancer

In colorectal cancer patients, prognosis is not determined by the primary tumor but by the formation of distant metastases. Molecules that have been implicated in the metastatic process are the proto-oncogene product c-Met and CD44 glycoproteins. Recently, we obtained evidence for functional collaboration between these two molecules: CD44 isoforms decorated with heparan sulfate chains (CD44-HS) can bind the c-Met ligand, the growth and motility factor hepatocyte growth factor/scatter factor (HGF/SF). This interaction strongly promotes signaling through the receptor tyrosine kinase c-Met. In the present study, we explored the expression of CD44-HS, c-Met, and HGF/SF in the normal human colon mucosa, and in colorectal adenomas and carcinomas, as well as their interaction in colorectal cancer cell lines. Compared to the normal colon, CD44v3 isoforms, which contain a site for HS attachment, and c-Met, were both overexpressed on the neoplastic epithelium of colorectal adenomas and on most carcinomas. Likewise, HGF/SF was expressed at increased levels in tumor tissue. On all tested colorectal cancer cell lines CD44v3 and c-Met were co-expressed. As was shown by immunoprecipitation and Western blotting, CD44 on these cells lines was decorated with HS. Interaction with HS moieties on colorectal carcinoma (HT29) cells promoted HGF/SF-induced activation of c-Met and of the Ras-MAP kinase pathway. Interestingly, survival analysis showed that CD44-HS expression predicts unfavorable prognosis in patients with invasive colorectal carcinomas. Taken together, our findings indicate that CD44-HS, c-Met, and HGF/SF are simultaneously overexpressed in colorectal cancer and that HS moieties promote c-Met signaling in colon carcinoma cells. These observations suggest that collaboration between CD44-HS and the c-Met signaling pathway may play an important role in colorectal tumorigenesis.

Characterization of METH-1/ADAMTS1 processing reveals two distinct active forms

METH-1/ADAMTS1 is a member of a newly described family of genes that contain metalloprotease, disintegrin, and thrombospondin-like motifs. We have recently shown that METH-1 protein is a potent inhibitor of angiogenesis. Here, we demonstrate that secreted human pro-METH-1 is processed in two consecutive steps to release both p87 and p65 active forms. The p87 form lacks the N-terminal prodomain and p65 results from an additional processing event in the C-terminal end. Generation of p87 was blocked with specific inhibitors of furin, and incubation of pro-METH-1 with purified furin released the p87 fragment but not p65. Generation of p65 required preformation of p87 and was suppressed by inhibitors of matrix metalloproteases. We demonstrate that matrix metalloproteases 2, 8, and 15 were able to release p65 when p87 was used as substrate. This second processing step removes two thrombospondin repeats from the carboxyl-terminal end of p87-METH-1 and alters the affinity of the protein to heparin and endothelial cultures. Furthermore, this deletion was associated with a reduced activity upon suppression of endothelial cell proliferation. We hypothesize that METH-1 processing is relevant for the modulation of the anti-angiogenic properties displayed by the protein.

Proteoglycans in the developing brain: new conceptual insights for old proteins

Proteoglycans are a heterogeneous class of proteins bearing sulfated glycosaminoglycans. Some of the proteoglycans have distinct core protein structures, and others display similarities and thus may be grouped into families such as the syndecans, the glypicans, or the hyalectans (or lecticans). Proteoglycans can be found in almost all tissues being present in the extracellular matrix, on cellular surfaces, or in intracellular granules. In recent years, brain proteoglycans have attracted growing interest due to their highly regulated spatiotemporal expression during nervous system development and maturation. There is increasing evidence that different proteoglycans act as regulators of cell migration, axonal pathfinding, synaptogenesis, and structural plasticity. This review summarizes the most recent data on structures and functions of brain proteoglycans and focuses on new physiological concepts for their potential roles in the developing central nervous system.

Fibroblast growth factor (FGF)-2 mediates cell attachment through interactions with two FGF receptor-1 isoforms and extracellular matrix or cell-associated heparan sulfate proteoglycans

In the presence of FGF-2, cells in suspension expressing FGF receptor-1 will attach to monolayers of cells expressing heparan sulfates. This attachment provides physical evidence for the formation of a trimolecular complex between FGF-2, heparan sulfate, and FGF receptors. We have used this system to determine if receptor isoforms containing or lacking the first of three immunoglobulin-like domains are equally able to form complexes with FGF-2 and heparan sulfates. In the presence of FGF-2, cells expressing either isoform of the receptor were able to attach to monolayers of CHO cells expressing heparan sulfates. No attachment was observed in the absence of FGF-2 or if heparin was included in the incubation medium. Attachment of cells expressing the two receptor isoforms occurred at similar concentrations of FGF-2, and similar concentrations of heparin were required to disrupt the interactions. Thus, there appeared to be little difference between these receptor isoforms in their ability to form trimolecular complexes with FGF-2 and cell-associated heparan sulfates. We also found that, in the presence of FGF-2, cells expressing FGF receptor-1 are able to form complexes with both extracellular matrix and cell-surface heparan sulfates.

Functions of cell surface heparan sulfate proteoglycans

The heparan sulfate on the surface of all adherent cells modulates the actions of a large number of extracellular ligands. Members of both cell surface heparan sulfate proteoglycan families, the transmembrane syndecans and the glycosylphosphoinositide-linked glypicans, bind these ligands and enhance formation of their receptor-signaling complexes. These heparan sulfate proteoglycans also immobilize and regulate the turnover of ligands that act at the cell surface. The extracellular domains of these proteoglycans can be shed from the cell surface, generating soluble heparan sulfate proteoglycans that can inhibit interactions at the cell surface. Recent analyses of genetic defects in Drosophila melanogaster, mice, and humans confirm most of these activities in vivo and identify additional processes that involve cell surface heparan sulfate proteoglycans. This chapter focuses on the mechanisms underlying these activities and on the cellular functions that they regulate.

Extracellular matrix cell adhesion peptides: functional applications in orthopedic materials

This review describes research on selected peptide sequences that affect cell adhesion as it applies in orthopedic applications. Of particular interest are the integrin-binding RGD peptides and heparin-binding peptides. The influence of these peptides on cell adhesion is described. Cell adhesion is defined as a sequence of four steps: cell attachment, cell spreading, organization of an actin cytoskeleton, and formation of focal adhesions. RGD sequences clearly influence cell attachment and spreading, whereas heparin-binding sequences appear to be less efficient. Collectively, these sequences appear to promote all steps of cell adhesion in certain cell types. This review also addresses issues related to peptide immobilization, as well as potential complexities that may develop as a result of using these versatile cell-binding sequences. Also described are future directions in the field concerning use of existing and more sophisticated peptide substrata.

Heterogeneity in growth and differentiation characteristics in male and female satellite cells isolated from turkey lines with different growth rates

The effects of growth- and gender-related differences on satellite cell proliferation and differentiation were investigated using satellite cells isolated from the pectoralis major muscle of a turkey line selected for increased 16-week body weight (F-line) and its unselected randombred control (RBC2-line). Proliferation rates within the F- and RBC2-lines did not differ between sexes. The F-line male and female satellite cells when compared to the RBC2-line male and female satellite cells proliferated at a faster rate. Differentiation rates were increased for the F-line male cells compared to both the F-line female and RBC2-line male satellite cells. No difference in differentiation rate was noted within the RBC2-line satellite cells. For satellite cells from females, the RBC2-line differentiated faster than the F-line. Morphological data on myotube length and the number of nuclei per myotube supported the differentiation data in that F-line male satellite cells had the longest myotubes with the most nuclei, there was no significant difference between myotubes within the RBC2-line, and female-derived myotubes from the RBC2-line were longer than those of the F-line by 96 h of fusion. These data are suggestive of both growth- and gender- related differences in satellite cell proliferation and differentiation.

Expression of glypican-1, syndecan-1 and syndecan-4 mRNAs protein kinase C-regulated in rat immature Sertoli cells by semi-quantitative RT-PCR analysis

In seminiferous tubules, Sertoli cells provide structural and nutritional support for the developing germinal cells. Cell to cell signalization and cell adhesion require proteoglycans expressed at the cell membrane. A preliminary biochemical and structural approach indicated that cell surface proteoglycans are mostly heparan sulfate (HSPG) in immature rat Sertoli cells. The present study focused on the qualitative and quantitative expression of three membrane HSPG, syndecan-1, syndecan-4 and glypican-1 in Sertoli cells of 20-day-old rat. A semi-quantitative multiplex RT-PCR strategy was developed to appreciate the effect of PKC activation on the mRNA expression of the three HSPG. Our data show that the syndecan-1 and glypican-1 mRNA expression is increased by the phorbol myristate acetate (PMA) suggesting a regulation of their expression by the phosphatidyl inositol pathway, as previously hypothesized (Fagen et al., Biochim. Biophys. Acta, 1472 (1999) 250-261). In addition, a physiological effector of the PKC as ATP gave similar effects. Thus, this over-expression could be related with paracrine factors secreted by germ cells.

Cytoskeletal abnormalities in chondrocytes with EXT1 and EXT2 mutations

The EXT genes are a group of putative tumor suppressor genes that previously have been shown to participate in the development of hereditary multiple exostoses (HME), HME-associated and isolated chondrosarcomas. Two HME disease genes, EXT1 and EXT2, have been identified and are expressed ubiquitously. However, the only known effect of mutations in the EXT genes is on chondrocyte function as evidenced by aberrant proliferation of chondrocytes leading to formation of bony, cartilage-capped projections (exostoses). In this study, we have characterized exostosis chondrocytes from three patients with HME (one with EXT1 and two with EXT2 germline mutations) and from one individual with a non-HME, isolated exostosis. At the light microscopic level, exostosis chondrocytes have a stellate appearance with elongated inclusions in the cytoplasm. Confocal and immunofluorescence of in vitro and in vivo chondrocytes showed that these massive accumulations are composed of actin bundled by 1.5-microm repeat cross-bridges of alpha-actinin. Western blot analysis shows that exostosis chondrocytes from two out of three patients aberrantly produce high levels of muscle-specific alpha-actin, whereas beta-actin levels are similar to normal chondrocytes. These findings suggest that mutations in the EXT genes cause abnormal processing of cytoskeleton proteins in chondrocytes.

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

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

Syndecan-1 and syndecan-4 can mediate the invasion of OpaHSPG-expressing Neisseria gonorrhoeae into epithelial cells

Neisseria gonorrhoeae (Ngo) expressing the outer membrane protein OpaHSPG can adhere to and invade epithelial cells via binding to heparan sulphate proteoglycan (HSPG) receptors. In this study, we have investigated the role of syndecan-1 and syndecan-4, two members of the HSPG family, in the uptake of Ngo by epithelial cells. When overexpressed in HeLa cells, both syndecans co-localize with adherent Ngo on the host cell surface. This overexpression of syndecan-1 and syndecan-4 leads to a three- and sevenfold increase in Ngo invasion respectively. In contrast, transfection with the syndecan-1 and syndecan-4 mutant constructs lacking the intracellular domain results in an abrogation of the invasion process, characteristic of a dominant-negative mode of action. A concomitant loss of the capacity to mediate Ngo uptake was also observed with syndecan-4 mutant constructs carrying lesions in the dimerization motif necessary for the binding of protein kinase C (PKC) and phosphatidylinositol 4,5-bisphosphate (PIP2), and mutants that are deficient in a C-terminal EFYA amino acid motif responsible for binding to syntenin or CASK. We conclude that syndecan-1 and syndecan-4 can both mediate Ngo uptake into epithelial cells, and that their intracellular domains play a crucial role in this process, perhaps by mediating signal transduction or anchorage to the cytoskeleton.

Expression of GPC3, an X-linked recessive overgrowth gene, is silenced in malignant mesothelioma

Gene expression changes in rat asbestos-induced malignant mesothelioma (MM) cells were investigated by differential mRNA display. A mRNA transcript identified by this approach was abundant in normal rat mesothelial cells but not expressed in rat MM cell lines. Northern blot analysis confirmed that this transcript is uniformly silenced in rat MM cell lines and primary tumors. Nucleotide sequence analysis revealed that this transcript is encoded by the rat glypican 3 gene (GPC3), whose human homolog is mutated in the Simpson-Golabi-Behmel overgrowth syndrome. Allelic loss at the GPC3 locus was infrequent (6.9%) in MM cell lines, and no mutations were found. GPC3 transcript levels were markedly decreased in 16 of 18 primary tumors and 17 of 22 human MM cell lines. Most of the cell lines were shown to have aberrant methylation of the GPC3 promoter region. In two of four human MM cell lines tested, GPC3 expression was restored after 2-deoxy 5-azacytidine (DAC)-mediated demethylation of its promoter region. Ectopic expression of GPC3 inhibited in vitro colony formation of human MM cells. Collectively, these data suggest that down-regulation of GPC3 is a common occurrence in MM and that GPC3, an X-linked recessive overgrowth gene, may encode a negative regulator of mesothelial cell growth.

Differential ability of heparan sulfate proteoglycans to assemble the fibroblast growth factor receptor complex in situ

Fibroblast growth factors (FGFs) require heparan sulfate proteoglycans (HSPGs) as cofactors for signaling. The heparan sulfate chains (HS) mediate stable high affinity binding of FGFs to their receptor tyrosine kinases (FR) and may specifically regulate FGF activity. A novel in situ binding assay was developed to examine the ability of HSPGs to promote FGF/FR binding using a soluble FR fusion construct (FR1-AP). This fusion protein probe forms a dimer in solution, simulating the dimerization or oligomerization that is thought to occur at the cell surface physiologically. In frozen sections of human skin, FGF-2 binds to keratinocytes and basement membranes of epidermis and dermal blood vessels. In contrast, in skin preincubated with FGF-2, FR1-AP binds avidly to FGF-2 immobilized on keratinocyte cell surfaces, but fails to bind to basement membranes at the dermo-epidermal junction or dermal microvessels despite the fact that these structures bind large amounts of FGF-2. Apparently, basement membrane and cell surface HSPGs differ in their ability to mediate the assembly of a FGF/FR signaling complex presumably due to structural differences of the heparan sulfate chains.

Phenotypic alterations in Kaposi's sarcoma cells by antisense reduction of perlecan

Metastasis is a sequence of events including proliferation, migration, adhesion, invasion and subsequent metastatic growth of tumour cells in distant organs. We previously showed that highly metastatic variants of murine melanoma cells express higher levels of the basement membrane proteoglycan perlecan than low or non metastatic variants and expression of an antisense perlecan can reduce metastatic potential. In contrast, antisense expression of perlecan in fibrosarcoma cells was reported to enhance tumorigenesis. To better understand the role of perlecan in angiogenesis we have transfected KS-IMM, an immortalized cell line derived from a human Kaposi s sarcoma, with an antisense perlecan construct and investigated the positive/negative role of perlecan in KS. KS-IMM cells were transfected with either empty vector (neo) or the antisense perlecan construct and clones were isolated. Immuno-blot analysis showed a reduction of perlecan levels in two (AP3 and AP4) isolated clones, in Northern blot analysis endogenous perlecan was undetectable in the AP3 and AP4 clones, while it was present in the neo control clones. AP clones had a reduced migration to HGF in Boyden chambers as compared to neo clones. Proliferation in low serum or serum-free conditions was strongly reduced in the AP clones as compared to the neo control cells. The neotransfected cells showed rapid proliferation in low serum supplemented with HGF and VEGF, while antisense transfected clones showed little response. Finally, AP-trasfected KS-IMM cells had significantly reduced migration to VEGF and HGF with respect to controls. In contrast, when the AP transfected cells were injected in nude mice they paradoxically showed enhanced tumor growth as compared to controls. Our preliminary data indicate that perlecan reduction plays a crucial role on Kaposi s sarcoma cell migration and proliferation in vitro. However, in vivo KS-IMM depleted of perlecan had a growth advantage. A possible hypothesis is that perlecan is necessary for growth of KS-IMM cells in vitro, however its down-regulation might promote angiogenesis through increased angiogenic growth factor diffusion, resulting in enhanced tumor growth in vivo.