Heparin is required for cell-free binding of basic fibroblast growth factor to a soluble receptor and for mitogenesis in whole cells

Integrin regulation by endogenous expression of 18-kDa fibroblast growth factor-2

The three high molecular weight (HMW) forms of fibroblast growth factor-2 (FGF-2) have a distinct intracellular localization and differentially affect cell mobility and growth compared with the fourth 18-kDa form. To characterize further the effects of the 18-kDa and HMW forms of FGF-2, we have examined their ability to modulate integrin expression. Transfected NIH 3T3 cells expressing only 18-kDa FGF-2 exhibited increased cell surface levels of alpha5beta1, whereas cells expressing only HMW FGF-2 exhibited cell surface alpha5beta1 levels similar to parental cells. When cells synthesizing 18-kDa FGF-2 were transfected with a cDNA encoding a dominant negative FGF receptor, alpha5beta1 cell surface levels decreased. Immunoprecipitation of biosynthetically labeled cells indicated that expression of 18-kDa FGF-2 increased the biosynthesis and rate of maturation of alpha5. Northern blot analysis showed that 18-kDa FGF-2 increases the level of the alpha5 subunit mRNA but does not affect beta1 subunit transcript levels. Experiments utilizing luciferase reporter gene activity revealed increased alpha5 promoter activity in cells expressing 18-kDa FGF-2 indicating that the enhanced alpha5 transcript level is due to modulation of the transcription rate. Therefore, interaction of 18-kDa FGF-2 with FGF receptors results in changes in alpha5beta1 biosynthesis and processing. In contrast, endogenous expression of HMW FGF-2 does not mediate this effect.

Sulfated malto-oligosaccharides bind to basic FGF, inhibit endothelial cell proliferation, and disrupt endothelial cell tube formation

The interaction of basic FGF (bFGF) with heparin, heparan sulfate and related sugars can potentiate or antagonize bFGF activity, depending on the size of the saccharide used. Oligosaccharides based on heparin structures, as small as six sugar residues, have been demonstrated to bind to bFGF and block its activity, while larger structures (> 10 sugar residues) tend to potentiate bFGF. In this study we have synthesized a series of compounds designed to test the requirements of size and sulfation for binding of oligosaccharides to bFGF. These oligosaccharides are not derived from heparin, but rather, are linear chains of glucose linked alpha 1-4 (malto-oligosaccharides) that have been chemically sulfated. In addition to bFGF binding, these compounds were tested for their ability to block basic functions of endothelial cells that are known to be mediated, at least in part, by bFGF. We report that the ability of sulfated malto-oligosaccharides to block binding of bFGF to heparan sulfate was dependent on the size (at least a tetrasaccharide is required), and the degree of sulfation. The activity profile in the bFGF ELISA closely correlated with the ability of these compounds to block REEC or HMVEC tube formation on Matrigel. There was a similar relationship of size and sulfation to the ability of the sulfated malto-oligosaccharides to inhibit endothelial cell growth for most human and rat EC types tested. The single exception was REEC cell growth. One isolate of these cells was stimulated by sulfated malto-oligosaccharides rather than inhibited by them, while a second isolate was neither stimulated nor inhibited. This stimulation showed no correlation with inhibition of bFGF binding in the ELISA assay, suggesting that growth of this cell type was probably not dependent on bFGF. Compounds derived from this series of sulfated, malto-oligosaccharides have the potential to function as bFGF antagonists, are relatively easy to produce, and possess relatively low anticoagulant properties.

Does the presence of heparin and dexamethasone in the vitrectomy infusate reduce reproliferation in proliferative vitreoretinopathy?

BACKGROUND

Reproliferation following vitrectomy for proliferative vitreoretinopathy (PVR) causes redetachment in up to 55% of cases. Heparin and corticosteroids have each been shown to inhibit the development of proliferative vitreoretinopathy (PVR) in experimental models. However, little information is available on the use of these agents in humans.

METHODS

In this pilot study, 62 eyes of 62 patients with severe PVR requiring vitrectomy were prospectively randomized to receive either BSS Plus (control) or BSS Plus with heparin and dexamethasone (HD) in the infusate.

RESULTS

After one operation, the retina was reattached in 42 (71%) of 59 eyes: 22 (65%) of 34 eyes in the control group and 20 (80%) of 25 eyes in the HD group (P = 0.16). Reproliferation developed in 26.5% of the control group (9 of 34 eyes) and 16% of the HD group (4 of 25 eyes; P = 0.26). Postoperative hemorrhage was significantly more frequent in the HD group (P = 0.02) but did not influence final visual or anatomic outcome. Hypotony was less frequent in the HD group but the difference was not significant (P = 0.063).

CONCLUSION

The trend from this randomized pilot study suggests that HD supplementation of the vitrectomy infusate may reduce the reproliferation rate in PVR and possibly reduce the rate of hypotony. Postoperative hemorrhage was more frequent with HD but did not cause redetachment or alter visual outcome. A multicenter trial involving more patients would be required to better evaluate the efficacy of HD as a pharmacologic adjunct to PVR surgery.

Basic fibroblast growth factor binds its receptors, is internalized, and stimulates DNA synthesis in Balb/c3T3 cells in the absence of heparan sulfate

We have investigated the interaction of basic fibroblast growth factor (bFGF) with its receptors and heparan sulfate proteoglycans (HSPG). It has been suggested that in the absence of HSPG, cells are not able to bind bFGF or respond to treatment with bFGF. In our studies, Balb/c3T3 fibroblasts were treated with 50 mM sodium chlorate to completely inhibit (99%) sulfation of proteoglycans. We found that bFGF was able to bind, be internalized, and stimulate DNA synthesis in the absence of HSPG in a dose-dependent manner. bFGF bound to its receptors on chlorate-treated cells with a lower apparent affinity and no change in receptor number. To determine if this decreased affinity bFGF-receptor interaction is functional, we quantitatively analyzed bFGF internalization and stimulation of DNA synthesis in control and chlorate-treated cells. Endocytotic rate constants (ke) for chlorate-treated and control cells were ke = 0. 078 +/- 0.022 min-1 and ke = 0.043 +/- 0.012 min-1, respectively, suggesting that the process of bFGF internalization is not dramatically altered by HSPG. bFGF stimulated DNA synthesis to the same maximal level under both conditions, but chlorate-treated cells were significantly less responsive at low bFGF doses (approximately 10-fold increase in ED50). The differences observed for control and chlorate-treated cells in the dose-response curves for stimulation of DNA synthesis and receptor binding correlated directly, suggesting that receptors are equally capable of eliciting a mitogenic signal under both conditions. It is unlikely that these results are due to residual HSPG since heparinase (I and III) digestion of chlorate-treated cells had little effect. Although the presence of HSPG on the cell surface increases the affinity of bFGF for its receptors, our observations suggest that HSPG are not "absolutely" required for binding, internalization, or stimulation of mitogenic activity.

Receptor specificity of the fibroblast growth factor family

Fibroblast growth factors (FGFs) are essential molecules for mammalian development. The nine known FGF ligands and the four signaling FGF receptors (and their alternatively spliced variants) are expressed in specific spatial and temporal patterns. The activity of this signaling pathway is regulated by ligand binding specificity, heparan sulfate proteoglycans, and the differential signaling capacity of individual FGF receptors. To determine potentially relevant ligand-receptor pairs we have engineered mitogenically responsive cell lines expressing the major splice variants of all the known FGF receptors. We have assayed the mitogenic activity of the nine known FGF ligands on these cell lines. These studies demonstrate that FGF 1 is the only FGF that can activate all FGF receptor splice variants. Using FGF 1 as an internal standard we have determined the relative activity of all the other members of the FGF family. These data should serve as a biochemical foundation for determining developmental, physiological, and pathophysiological processes that involve FGF signaling pathways.

Involvement of heparan sulfate and related molecules in sequestration and growth promoting activity of fibroblast growth factor

Heparan sulfate proteoglycans (HSPGs) are ubiquitous macromolecules associated with the cell surface and extracellular matrix (ECM) of a wide range of cells of vertebrate and invertebrate tissues [1, 2]. The basic HSPG structure consists of a protein core to which several linear heparan sulfate (HS) chains are covalently attached. The polysaccharide chains are typically composed of repeating hexuronic and D-glucosamine disaccharide units that are substituted to a varying extent with N- and O-linked sulfate moieties and N-linked acetyl groups [1, 2]. Beside serving as a scaffold for the attachment of various ECM components (e.g., collagen, laminin, fibronectin), the binding of HS to certain proteins has been suggested to induce a conformational change which may lead to the exposure of novel reactive determinants or conversely stabilize an inert protein configuration [1-4]. Of particular significance is the interaction of HS with fibroblast growth factors (FGFs), mediating their sequestration, stabilization and high affinity receptor binding and signaling [3-7]. Cellular responses to FGFs may hence be modulated by metabolic inhibitors of HS synthesis and sulfation, HS-degrading enzymes, and synthetic mimetics of heparin/HS. In the present review we focus on the involvement of HS in basic FGF (bFGF) receptor binding and mitogenic activity and its modulation by species of heparin, HS, and synthetic polyanionic 'heparin-mimicking' compounds. The results are discussed in relation to the current thoughts on the dual involvement of low and high affinity receptor sites in the growth promoting and angiogenic activities of bFGF and other heparin-binding growth factors.

Graded activation of fibroblast growth factor receptor 3 by mutations causing achondroplasia and thanatophoric dysplasia

The longitudinal growth of the skeleton arises from the continuous process of endochondral ossification occurring at the ends of growing long bones. Dwarfism results when this process is disrupted, as in the autosomal dominant human skeletal diseases hypochondroplasia (HCH), achondroplasia (ACH) and thanatophoric dysplasia (TD). Interestingly, these disorders display a graded spectrum of phenotypic severity and are the result of distinct missense mutations in the fibroblast growth factor receptor 3 gene (FGFR3). TD, characterized by neonatal lethality and profound dwarfism, is the result of FGFR3 mutations, including an R248C substitution in the extracellular domain or a K650E substitution in the tyrosine kinase (TK) domain. ACH, which is non-lethal and presents less severe dwarfism, results almost exclusively from a G380R substitution in the transmembrane domain. Homozygous achondroplasia resembles the phenotype of TD. In this report the effect of the ACH and TD mutations on the activity and regulation of FGFR3 are analysed. We showed that each of the mutations constitutively activate the receptor, as evidenced by ligand-independent receptor tyrosine phosphorylation and cell proliferation. Moreover, the mutations that are responsible for TD were more strongly activating than the mutation causing ACH, providing a biochemical explanation for the observation that the phenotype of TD is more severe than that of ACH.

Schwann cell heparan sulfate proteoglycans play a critical role in glial growth factor/neuregulin signaling

Glial growth factors are proteins encoded by the neuregulin gene and are thought to signal via receptor tyrosine kinases. Many neuregulin gene products bind heparin, and we hypothesize that affinity for heparin may implicate cell surface heparan sulfate proteoglycans (HeSPGs) as co-receptors for the soluble neuregulin gene product, recombinant human glial growth factor 2 (rhGGF2). Using primary rat Schwann cell cultures, we show that exogenous heparin and heparan sulfate block rhGGF2-induced phosphorylation of putative neuregulin receptors, and block subsequent DNA synthesis; other glycosaminoglycans show no such effect. Inhibition of Schwann cell HeSPG biosynthesis by administration of beta-xyloside also blocks responsiveness to rhGGF2. In cell-free binding assays, rhGGF2 binds heparin and heparan sulfate with high affinity, while suramin and suramin-like molecules block this binding. These suramin-like molecules reversibly block Schwann cell responsiveness to rhGGF2 with a rank order of potency identical to that in the cell-free binding assay. Thus we demonstrate high affinity and specificity in the interaction of rhGGF2 with heparin-like molecules, and show that three distinct perturbations of this interaction on Schwann cells (exogenous heparin/ heparan sulfate treatment, inhibition of HeSPG biosynthesis, and treatment with suramin-like molecules) result in a loss of responsiveness to rhGGF2. These results support a model in which HeSPGs are critical components that modulate extracellular rhGGF2 signaling interactions with appropriate receptor tyrosine kinases.

Sulfate moieties in the subendothelial extracellular matrix are involved in basic fibroblast growth factor sequestration, dimerization, and stimulation of cell proliferation

The growth promoting activity of the subendothelial extracellular matrix (ECM) is attributed to sequestration of basic fibroblast growth factor (bFGF) by heparan sulfate proteoglycans and its regulated release by heparin-like molecules and heparan sulfate (HS) degrading enzymes. HS is also involved in bFGF receptor binding and activation. The present study focuses on the growth promoting activity and bFGF binding capacity of sulfate-depleted ECM. Corneal endothelial cells (EC) maintained in the presence of chlorate, an inhibitor of phosphoadenosine phosphosulfate synthesis, produced ECM containing 10-15% of the sulfate normally present in ECM. Incorporation of sulfate into HS was reduced by more than 90%. Binding of 125I-bFGF to sulfate-depleted ECM was reduced by 50-60% and only about 10% of the ECM-bound bFGF was accessible to release by heparin. Incubation of 125I-bFGF on top of native ECM resulted in dimerization of the ECM-bound bFGF, but there was a markedly reduced binding and dimerization of bFGF on sulfate-depleted ECM. ECM produced in the presence of chlorate contained a nearly 10-fold less endogenous bFGF as compared to native ECM and exerted little or no mitogenic activity toward vascular EC and 3T3 fibroblasts. In other studies, we investigated the interaction between chlorate-treated vascular EC and either native or sulfate-depleted ECM. Exogenous heparin stimulated the proliferation of chlorate-treated EC seeded on native ECM, suggesting its interaction with ECM-bound bFGF and subsequent presentation to high affinity cell surface receptors. On the other hand, heparin had no effect on chlorate-treated cells seeded in contact with sulfate-depleted ECM or regular tissue culture plastic. Altogether, the present experiments indicate that heparan sulfate proteoglycans associated with the cell surface and ECM act in concert to regulate the bioavailability and growth promoting activity of bFGF. While HS in the subendothelial ECM functions primarily in sequestration of bFGF in the vicinity of responsive cells, HS on cell surfaces is playing a more active role in displacing the ECM-bound bFGF and its subsequent presentation to high affinity signal transducing receptors.

Distribution of fibroblast growth factor (FGF)-2 and FGF receptor-1 messenger RNA expression and protein presence in the mid-trimester human fetus

Fibroblast growth factors (FGF) are known to have key roles in embryonic growth and morphogenesis, but their presence and contributions to fetal development are unclear. In particular, little information exists as to the relevance of FGF and their specific receptors to human fetal development. We studied the anatomical distribution of messenger RNA encoding FGF-2 and one of its high affinity receptors, FGFR1, using in situ hybridization in a variety of human fetal tissues in early second trimester. Corresponding protein distributions were determined by immunohistochemistry. Both FGF-2 and FGFR1 mRNA and proteins were found to be present in every organ and tissue examined, but with defined cellular localizations. In skeletal muscle, both FGF-2 and FGFR1 mRNA and peptides were present in differentiated fibers, and both co-localized to proliferating chondrocytes of the epiphyseal growth plate. FGF-2 and FGFR1 mRNA and peptides were also present within cardiac or gastrointestinal smooth muscle. Within the gastrointestinal tract FGF-2 mRNA and peptide were located in the submucosal tissue, whereas FGFR1 was expressed within the overlying mucosa. Similarly, in skin, FGF-2 was expressed within the dermis whereas FGFR1 mRNA and peptide were most apparent in the stratum germinativum of the epidermis. In kidney and lung, FGFR1 mRNA was located in the tubular and alveolar epithelia respectively, whereas FGF-2 was expressed in both epithelial and mesenchymal cell populations. Both growth factor and receptor were widespread in both neuroblasts and glioblasts in the cerebral cortex of the brain. Immunoreactivity for FGF-2 and FGFR1 was seen in all vascular endothelial cells of major vessels and capillaries. Within the skin, kidney, lung, and intestine FGF-2 immunoreactivity was found in basement membranes underlying epithelia, and was associated with the extracellular matrix and plasma membranes of many cell types. The results show that FGF-2 and one of its receptors are widely expressed anatomically in the mid-trimester human fetus.

Interaction of angiogenic basic fibroblast growth factor with endothelial cell heparan sulfate proteoglycans. Biological implications in neovascularization

Basic fibroblast growth factor is an angiogenic molecule involved in several physiological and pathological processes, including wound repair, embryonic development, and tumor growth. In vitro, basic fibroblast growth factor induces an "angiogenic phenotype" in endothelial cells, which includes chemotaxis, mitogenesis, protease production, beta-integrin expression, and tube formation in three-dimensional gels. It acts by binding to specific tyrosine kinase receptors and to cell-associated heparan sulfate proteoglycans. The physiological significance of the interaction with cell-associated and soluble heparan sulfate proteoglycans is manyfold. Heparan sulfate proteoglycans protect basic fibroblast growth factor from inactivation in the extracellular environment and modulate its bioavailability. At the cell surface, soluble and cell-associated heparan sulfate proteoglycans may play different roles in modulating the dimerization of the growth factor and its interaction with tyrosine kinase receptors. Finally, they affect the internalization and the intracellular fate of basic fibroblast growth factor, suggesting that growth factor slash proteoglycan complexes are involved in intracellular delivery. The bioavailability and the biological activity of basic fibroblast growth factor on endothelial cells strictly depend on the glycosaminoglycan milieu of the extracellular environment. Hence the angiogenic activity of the growth factor in vivo might be modulated by using exogenous glycosaminoglycans. The capacity of glycosaminoglycans to bind to and to influence the biological activity of basic fibroblast growth factor depends on size, degree of sulfation, and disaccharide composition. In the present paper we discuss the physiological significance and the biochemical bases of the interaction of the growth factor with heparan sulfate proteoglycans and exogenous glycosaminoglycans with a view to the possible therapeutic use of heparin-related oligosaccharides as basic fibroblast growth factor agonists or antagonists in angiogenesis-dependent diseases.

Copper and calcium binding motifs in the extracellular domains of fibroblast growth factor receptors

High affinity fibroblast growth factor (FGF) receptors contain a cluster of acidic amino acids in their extracellular domains that is reminiscent of the calcium binding domains of some cell adhesion molecules. Based on this observation, we used a calcium blotting technique to show that FGFR-1 binds calcium and that calcium binding is not observed in a mutagenized form of the receptor that lacks the acidic box region. The acidic box also binds other divalent cations, including copper. This latter interaction appears unique since the binding of copper to FGFR-1 mediates the binding of the receptor to immobilized heparin. While this observation may help explain the angiogenic properties of copper, divalent cation binding to FGF receptors may also mediate the interaction between FGF receptors, cell adhesion molecules and other proteoglycan components of the extracellular matrix.

Expression and biological activity of mouse fibroblast growth factor-9

Receptor specificity is an essential mechanism governing the activity of fibroblast growth factors (FGF). To begin to understand the developmental role of FGF-9/glial activating factor, we have cloned and sequenced the murine FGF-9 cDNA and expressed the protein in mammalian cells and in Escherichia coli. We demonstrate that the FGF-9 protein is highly conserved between mouse and human. Receptor specificity was determined by direct binding to soluble and cell surface forms of FGF receptor (FGFR) splice variants and by the mitogenic activity on cells, which express unique FGF receptor splice variants. Our data demonstrate that FGF-9 efficiently activates the "c" splice forms of FGFR2 and FGFR3, receptors expressed in potential target cells for FGF-9. Significantly, FGF-9 also binds to and activates the "b" splice form of FGFR3, thus becoming the first FGF ligand besides FGF-1 to activate this highly specific member of the FGF receptor family.

In vitro changes in plasma membrane heparan sulfate proteoglycans and in perlecan expression participate in the regulation of fibroblast growth factor 2 mitogenic activity

Fibroblast growth factor 1 (FGF1) and 2 (FGF2) bind to two classes of receptors: the high affinity receptors, a family of four known transmembrane tyrosine kinases (FGF R1-R4), and the low affinity receptors, cell surface and basement membrane heparan sulfate proteoglycan (HSPG). During early (first and second) passages of retinal pigmented epithelial (RPE) cells, both FGF1 and FGF2 exhibited low mitogenic activity, while in later (fifth to ninth) passages the activity of FGF1 remained constant but FGF2 activity increased two- to threefold. We have investigated aspects of FGF receptor interactions and the role of heparin/heparan sulfate which modulates FGF activity on RPE cells during in vitro senescence. Northern blot analysis demonstrated that FGF receptor type 1 (FGF R1) is the major high affinity receptor expressed in RPE cells and that its level of expression did not change during serially passage. Both the FGF R1 and the FGF low affinity receptors' binding characteristics (i.e., Kd and number of sites per cell) for FGF1 were unaffected by passage number, whereas the capacity of FGF2 binding to FGF R1 and to the low affinity receptors increased by two- and fivefold, respectively, in late passages, although the affinities were unchanged. This change in the capacity of FGF2 to bind to FGF R1 and to HSPG was not due to a switch of the IIIc splice form of FGF R1 to the IIIb splice form since the exon IIIc was the most predominant splice form of FGF R1 during RPE cell cultures. Furthermore the ratio of the IIIb to the IIIc splice form was not modified during cell subcultures. In parallel in the older RPE cell passages, expression of perlecan, the major FGF low affinity binding site localized on the extracellular matrix of RPE cells, was much elevated compared to early RPE cell passages. Moreover, the cell surface of late passage RPE cells had 79% more HSPG than early passage cells. Therefore, it is suggested that the increase in the number of FGF low affinity receptors present on the cell surface or basement membrane could account for a part of the greater proliferative response of aged RPE cells to FGF2.

Characterization of fibroblast growth factor-2 binding to ribosomes

We have examined the binding of FGF-2 to ribosomes and have found that in NIH 3T3 cells that synthesize high amounts of all FGF-2 forms, both 18 kDa and HMW forms of FGF-2 bind to ribosomes. Ribosomes purified from these cells were treated with RNase or puromycin to identify the binding site of FGF-2 on the ribosome. Neither RNase nor puromycin treatment affected the in vivo binding of FGF-2 to ribosomes suggesting that FGF-2 binds ribosomal protein or rRNA, but not mRNA. The stoichiometry of binding in these cells was approximately 1 FGF-2 molecule bound per 1 ribosome. Binding was unaffected by high salt treatment indicating that FGF-2 tightly associates with polysomes. An in vitro binding experiment performed with purified ribosomes and recombinant FGF-2 suggested that the binding site is saturable. HBNF, a protein with similar charge and size to FGF-2, bound 15-fold less than FGF-2 to purified ribosomes. These results indicate that the binding of FGF-2 to ribosomes is specific.

Chimeric molecules between keratinocyte growth factor and basic fibroblast growth factor define domains that confer receptor binding specificities

Basic fibroblast growth factor (FGF) and keratinocyte growth factor (KGF) are structurally related fibroblast growth factors, yet they exhibit distinct receptor binding specificity. Basic FGF binds with high affinity to FGFR1, FGFR2, and FGFR4, whereas KGF does not interact with these receptors and can only bind an isoform of FGFR2 known as the KGFR. Basic GFG binds KGFR but with lower affinity than KGF. In order to identify domains that confer this specificity, four reciprocal chimeras were generated between the two growth factors and were analyzed for receptor recognition and biological activity. The chimeras are designated BK1 (bFGF1-54:KGF91-194), BK2 (bFGF1-74:KGF111-194), KB1 (KGF31-90:bFGF55-155), and KB2 (KGF31-110:bFGF75-155). The two BK chimera similarly interacted with FGFR1 and FGFR4 but differed from each other with respect to KGFR recognition. BK1 displayed a slightly better affinity for KGFR than BK2 and induced a higher level of DNA synthesis in keratinocytes compared with bFGF and BK2. A neutralizing monoclonal antibody directed against bFGF specifically neutralized the biological activity of the BK chimeras. The reciprocal chimeras, KB1 and KB2, exhibited KGF-like receptor binding and activation properties. However, KB2 displayed higher affinity for KGFR and was significantly more potent mitogen that KB1. Altogether, our results suggest that the amino-terminal part of KGF and bFGF plays an important role in determining their receptor binding specificity. In addition, the results point to the contribution of a segment from the middle part of KGF (residues 91-110) for recognition and activation of the KGFR, as the two chimeras containing these residues (BK1 and KB2) displayed an enhanced interaction with the KGFR.

Degradation of cell surface heparan sulfates decreases the high affinity binding of basic FGF to endothelial cells, but not to FRTL-5 rat thyroid cells

The role of cell surface heparan sulfate proteoglycans in the effect of basic fibroblast growth factor (bFGF) on FRTL-5 rat thyroid cells was investigated and compared with that of endothelial cells. FRTL-5 cells were incubated for 2 h with heparitinase (0.5-5.0 mU/mL), which specifically degrades heparan sulfate proteolgycans, and then stimulated by bFGF. The mitogenic effect of bFGF was estimated by measuring [3H]thymidine incorporation. Although cell surface heparan sulfates have been believed to be necessary for bFGF binding to its high affinity receptors, the heparitinase treatment had no significant effect on the DNA synthesis of FRTL-5 cells stimulated by bFGF. The binding study revealed that heparitinase treatment decreased low affinity bindings of [125I]bFGF to FRTL-5 cells by only 50% and did not attenuate the high affinity binding, while the same treatment abolished the high and low affinity binding to bovine pulmonary artery endothelial (CPAE) cells. Analysis of trypsin accessible cell surface 35SO4-labeled materials by Q-sepharose anion-exchange column chromatography showed that heparan sulfate proteoglycans, peaked at 0.55 M NaCl elution, disappeared from the surface of FRTL-5 cells after treatment with 2.0 mU/mL of heparitinase, indicating that the heparitinase resistant low-affinity binding sites are not heparan sulfates. These results demonstrate that cell surface heparan sulfates are not required for the high affinity binding of bFGF to FRTL-5 rat thyroid cells, while proteoglycans are necessary for binding to endothelial cells, and suggest that the mechanism of the action of bFGF is different in rat thyroid cells compared with endothelial cells.

FGF-8 isoforms activate receptor splice forms that are expressed in mesenchymal regions of mouse development

The Fgf8 gene is expressed in developing limb and craniofacial structures, regions known to be important for growth and patterning of the mouse embryo. Although Fgf8 is alternatively spliced to generate at least 7 secreted isoforms that differ only at their mature amino terminus, the biological significance of these multiple isoforms is not known. In this report, we demonstrate that multiple FGF-8 isoforms are present at sites of Fgf8 expression during mouse development. To address the possibility that the FGF-8 isoforms might interact with different fibroblast growth factor receptors, we prepared recombinant FGF-8 protein isoforms. We examined the ability of these proteins to activate alternatively spliced forms of fibroblast growth factor receptors 1–3, and fibroblast growth factor receptor 4. Recombinant FGF-8b and FGF-8c activate the ‘c’ splice form of FGFR3, and FGFR4, while FGF-8b also efficiently activates ‘c’ splice form of FGFR2. No activity could be detected for recombinant or cell expressed FGF-8a. Furthermore, none of the isoforms tested interact efficiently with ‘b’ splice forms of FGFR1-3, or the ‘c’ splice form of FGFR1. These results indicate that the FGF-8b and FGF-8c isoforms, produced by ectodermally derived epithelial cells, interact with mesenchymally expressed fibroblast growth factor receptors. FGF-8b and FGF-8c may therefore provide a mitogenic signal to the underlying mesenchyme during limb and craniofacial development.

The division abnormally delayed (dally) gene: a putative integral membrane proteoglycan required for cell division patterning during postembryonic development of the nervous system in Drosophila

We have devised a genetic screen to obtain mutants affecting cell division patterning in the developing central nervous system of Drosophila. The division abnormally delayed (dally) locus was identified using a combination of “enhancer trap” and behavioral screening methods. The ordered cell cycle progression of lamina precursor cells, which generate synaptic target neurons for photoreceptors, is disrupted in dally mutants. The first of two lamina precursor cell divisions shows a delayed entry into mitosis. The second division, one that is triggered by an intercellular signal from photoreceptor axons, fails to take place. Similar to lamina precursors, cells that generate the ommatidia of the adult eye show two synchronized divisions found along the morphogenetic furrow in the eye disc and the first division cycle in dally mutants displays a delayed progression into M phase like that found in the first lamina precursor cell division. dally mutations also affect viability and produce morphological defects in several adult tissues, including the eye, antenna, wing and genitalia. Sequencing of a dally cDNA reveals a potential open reading frame of 626 amino acids with homology to a family of Glypican-related integral membrane proteoglycans. These heparan sulfate-containing proteins are attached to the external leaflet of the plasma membrane via a glycosylphosphatidylinositol linkage. Heparan sulfate proteoglycans may serve as co-receptors for a variety of secreted proteins including fibroblast growth factor, vascular endothelial growth factor, hepatocyte growth factor and members of the Wnt, TGF-beta and Hedgehog families. The cell division defects found in dally mutants implicate the Glypican group of integral membrane proteoglycans in the control of cell division during development.

Analysis of putative heparin-binding domains of fibroblast growth factor-1. Using site-directed mutagenesis and peptide analogues

The contribution of individual basic amino acids within three putative "consensus sequences" for heparin binding of fibroblast growth factor-1 have been examined by site-directed mutagenesis. The results indicate that a significant reduction in the apparent affinity of fibroblast growth factor-1 for heparin is only observed when basic residues in one of the three regions are mutated. Mutation in the other regions are without affect on heparin binding. The heparin binding properties of synthetic peptides based on the three "consensus sequences" paralleled the mutagenesis results. That is, synthetic peptides corresponding to regions of the protein that were affected by mutagenesis with respect to heparin binding exhibited a relatively high affinity for immobilized heparin, whereas those corresponding to regions of similar charge density that were unaffected by mutagenesis did not. In addition, amino acid substitution of a nonbasic residue in the heparin-binding peptide could abolish its heparin binding capacity. The heparin-binding peptide could antagonize the mitogenic activity of FGF-1, probably because of the heparin dependence of this activity. Together these data demonstrate that the heparin binding properties of fibroblast growth factor-1 are dictated by structural features more complex than clusters of basic amino acids. The results of these and other studies indicate that consensus motifs for heparin-binding require further definition. More importantly, the results provide a basis for the design of peptide-based inhibitors of FGF-1.

Receptor binding and mitogenic properties of mouse fibroblast growth factor 3. Modulation of response by heparin

fgf3 has been implicated in the embryonic and fetal development of the mouse and as an oncogene in murine breast cancer. We describe a procedure to purify the product of the mouse fgf3 gene and show it to be a potent mitogen for some epithelial cell lines. Using a receptor binding competition assay, Fgf3 was shown to bind with high affinity to the IIIb isoforms of Fgf receptor (FgfR) 1 and FgfR2 (ID50 = approximately 0.8 nM) and with a lower affinity to the IIIc variant of FgfR2 (ID50 = approximately 9 nM). No competition for the binding of 125I-Fgf1 was observed for FgfR1 (IIIc), FgfR3 (IIIb and IIIc), or FgfR4. Mitogenicity assays using BaF3 cells containing individual Fgf receptors showed a pattern of response in agreement with the receptor binding results. A comparison of two mammary epithelial cell lines showed a marked difference of potency and dependence upon heparin in their response to mouse Fgf3, suggesting a complex interaction between the ligand and its low and high affinity receptors.

Regulation of prostate growth by fibroblast growth factors

Growth of the prostate is controlled by androgen. However, there is information indicating that androgen may not act directly, but may act indirectly through polypeptide growth factors, to control prostate growth. This review will focus on the involvement of members of the fibroblast growth factor (FGF) family in this process. The properties of FGFs and FGF-receptors are described that implicate these molecules in growth control. Information is provided that prostate stromal cells synthesize FGF2 and FGF7. FGF2 is a potent mitogen for stromal cells; whereas, FGF7 is exclusively a mitogen for epithelial cells. Transforming growth factor beta (TGF beta), also produced by prostate cells, inhibit cell growth. This suggests that prostate growth is controlled by autocrine and paracrine mechanisms. Evidence is presented that altered FGF expression accompanies benign prostatic hyperplasia and prostate cancer. A model is proposed whereby androgen regulates TGF beta, influencing FGF2 and FGF7 expression, and in turn regulating growth of the prostatic stroma and epithelium. An imbalance in the influence of these growth factors may contribute to prostate disease.

Increased mitogenic response to heparin-binding epidermal growth factor-like growth factor in vascular smooth muscle cells of diabetic rats

We investigated the mitogenic effects of heparin-binding epidermal growth factor-like growth factor (HB-EGF) in vascular smooth muscle cells (SMCs) obtained from rats with streptozotocin (STZ)-induced diabetes and evaluated the role of heparan sulfate proteoglycan (HSPG) in inducing these effects. HB-EGF significantly increased DNA synthesis in the SMCs of diabetic rats (STZ-SMCs) compared with control rats (control SMCs). However, the mitogenic effects of EGF, which shares EGF receptors with HB-EGF, and basic fibroblast growth factor, another heparin-binding growth factor, were similar in STZ-SMCs and control SMCs. The mitogenic response to HB-EGF in SMCs of insulin-treated diabetic rats was similar to the response in control SMCs. HB-EGF-induced autophosphorylation of EGF receptors was increased in STZ-SMCs compared with control SMCs, although the number of EGF receptors in STZ-SMCs was 40% of that in controls. This increased mitogenic response to HB-EGF in STZ-SMCs was completely inhibited by treatment with heparitinase, chlorate, and a synthetic peptide corresponding to the heparin-binding domain of HB-EGF. Compared with heparan sulfate isolated from control SMCs, heparan sulfate isolated from STZ-SMCs was of smaller molecular size and caused a greater mitogenic effect of HB-EGF. These findings suggest that the mitogenic response to HB-EGF is increased in SMCs of diabetic rats. Changes in cell-associated heparan sulfate in STZ-SMCs may be related to the increased mitogenic response to HB-EGF.

Inhibition of high affinity basic fibroblast growth factor binding by oligonucleotides

Oligonucleotides can be used to inhibit the binding of basic fibroblast growth factor to cells. Though standard phosphodiester oligonucleotides show a slight inhibition of binding, the oligonucleotides with phosphorothioate internucleoside linkages have inhibition levels equivalent to that of the polyanion heparin. Variations in sequence of the oligonucleotides does lead to differences in the inhibitory action of the oligonucleotides. This inhibition of basic fibroblast growth factor by phosphorothioate oligonucleotides may account for much of the published data on inhibition of various genes by proposed antisense oligonucleotides and needs to be taken into account when considering the mechanism of action of oligonucleotides in biological systems.

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.

Binding of platelet-derived growth factor and low density lipoproteins to glycosaminoglycan species produced by human arterial smooth muscle cells

The platelet-derived growth factor (PDGF) binds via a defined amino acid sequence to heparin (Fager et al., 1992, In Vitro Cell. Dev. Biol., 28A:176-180) and the protein moiety of low density lipoproteins (LDL; apo B-100) via a similar sequence to chondroitin sulfate (Camejo et al., 1988, Arteriosclerosis Thromb., 8:368-377). In this study, synthetic oligopeptides were used to explore the capacity of smooth muscle cell-derived glycosaminoglycans to bind to the critical sequences of PDGF and apo B-100. In vitro, proliferating human arterial smooth muscle cells synthesized twice as much proteoglycans as did quiescent cells. The dominating glycosaminoglycan side chains were chondroitin and heparan sulfates in secreted and cell-associated proteoglycans, respectively. The chondroitin sulfate-rich proteoglycans had a higher molecular size and were to a larger extent secreted into the culture medium than the heparan and dermatan sulfate-rich proteoglycans. Heparan, dermatan, and chondroitin sulfates bound to the PDGF-derived oligopeptide with affinities similar to those of heparin. However, while heparan and dermatan sulfates both inhibited DNA synthesis in human arterial smooth muscle cells, chondroitin sulfate had no such inhibitory effect. Like the PDGF-derived oligopeptide, the apo B-100-derived oligopeptide bound to these glycosaminoglycans. At the same time, both oligopeptides displaced bound LDL from chondroitin sulfate in vitro and released the block on DNA synthesis in smooth muscle cells that heparin induced in culture. Thus, chondroitin, dermatan, and heparan sulfates produced by arterial smooth muscle cells may bind LDL and PDGF competitively in atherogenesis but only heparan and dermatan sulfates inhibit cellular DNA synthesis. LDL and PDGF deposition may occur by binding to similar binding sites on glycosaminoglycans derived from smooth muscle cells within atherosclerotic lesions.

Carboxyl-terminal truncation of leucine76 converts heparin-binding EGF-like growth factor from a heparin-enhancible to a heparin-suppressible growth factor

Previous studies have indicated that heparin differentially regulates heparin-binding EGF-like growth factor (HB-EGF) and amphiregulin (AR) mitogenic activity. To further explore this phenomenon, these mitogens were compared under identical cell culture conditions in two different assays. The results of our present investigation demonstrated that AR-mediated mitogenic activity in the murine AKR-2B fibroblast-like cell line was inhibited by heparin, while HB-EGF activity was enhanced. However, the absolute effect of heparin appeared to be cell type specific since HB-EGF mitogenic activity was not dramatically affected by coincubation with heparin when tested on human dermal fibroblasts. Several studies have indicated that mutation of a conserved leucine in the carboxyl-terminal region of both EGF and transforming growth factor-alpha results in decreased affinity for EGF receptors. Since this leucine is present in the analogous position of HB-EGF, but absent in AR, we examined the effect of deleting this residue by carboxyl-terminal truncation of HB-EGF. Analysis of recombinant forms of HB-EGF demonstrated that HB-EGF can be converted to a heparin-inhibited growth factor if the putative mature form of the protein is truncated by two residues (leucine76 and proline77) at the carboxyl terminus. Further analysis demonstrated that only leucine76 appears to be required for heparin-dependent enhancement of HB-EGF-mediated mitogenic activity, indicating that this amino acid may play a pivotal role in controlling the response of HB-EGF to heparin or related glycosaminoglycan sulfates. Our results also suggest that expression of different HB-EGF forms in vivo could result in the production of HB-EGFs with divergent responses to sulfated glycosaminoglycans and proteoglycans.

Differential effects of a heparin antagonist (hexadimethrine) or chlorate on amphiregulin, basic fibroblast growth factor, and heparin-binding EGF-like growth factor activity

Amphiregulin (AR) and heparin-binding EGF-like growth factor (HB-EGF) are two recently identified members of the EGF family. Both AR and HB-EGF share with EGF the ability to interact with the type-1 EGF receptor; however, AR and HB-EGF differ from EGF in that both of these mitogens bind to heparin while EGF does not. To determine whether interactions with heparin-like molecules on the cell surface influence binding of AR and HB-EGF with EGF receptors and the subsequent mitogenic activity exerted by these growth factors, murine AKR-2B and Balb/MK-2 cells were treated with either an inhibitor of proteoglycan sulfation (chlorate) or a heparin antagonist (hexadimethrine). As expected, neither treatment significantly altered the specific binding of 125I-EGF on AKR-2B cells. Interestingly, treatment with either chlorate or hexadimethrine inhibited the ability of AR to compete with 125I-EGF for cell surface binding and also attenuated AR-mediated DNA synthesis. Thus, as has been suggested for other heparin-binding growth factors such as basic fibroblast growth factor (bFGF), the interaction of AR with an EGF-binding receptor appears to be facilitated by interaction with cell-associated sulfated glycosaminoglycans or proteoglycans. Unexpectedly, however, neither chlorate nor hexadimethrine treatment caused an inhibition of HB-EGF-induced mitogenic activity. Chlorate treatment did not significantly alter the ability of HB-EGF to compete with 125I-EGF for cell surface binding sites, however, heparin and hexadimethrine reduced the ability of HB-EGF to compete for 125I-EGF binding. These results suggest that, in AKR-2B cells, HB-EGF may mediate its mitogenic response at least in part through a receptor which appears to be selective for HB-EGF and permits HB-EGF-mediated mitogenic responses in the presence of hexadimethrine or heparin. Finally, hexadimethrine inhibited the specific binding and mitogenic activity of bFGF, suggesting that this cationic polymer can function as an antagonist of heparin-binding mitogens other than AR.

Ligand-specific structural domains in the fibroblast growth factor receptor

Two tandem immunoglobulin-like disulfide loops (Loops II and III) linked by a short connecting sequence in the ectodomain of the fibroblast growth factor receptor kinase compose the binding sites for glycosaminoglycan and fibroblast growth factor (FGF) ligands. Alternate splicing of exons IIIb and IIIc coding for the COOH-terminal half of Loop III confers high affinity for FGF-7 or FGF-2, respectively, on the fibroblast growth factor receptor ectodomain without effect on the binding of FGF-1. Here we show that a 139-amino acid fragment composed of Loop II, the inter-Loop II/III sequence, and a short segment of the NH2 terminus of Loop III is sufficient and near the minimal requirement for binding of FGF-1, FGF-2, and FGF-7. Extension of the fragment by five additional highly conserved residues (SD(P/A)QP) within a distinct constitutive structural domain (fl1) in Loop III restricts the binding of FGF-7 without effect on FGF-1 and FGF-2. Since the presence of exon IIIc in the full-length ectodomain does not change this ligand binding profile, we suggest that alternately spliced exon IIIc plays no active role in binding of the three ligands. In contrast, exon IIIb actively abrogates the restriction on the binding of FGF-7 and concurrently lowers the affinity for FGF-2.

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.

A model of the complex between interleukin-4 and its receptors

A three-dimensional model of interleukin-4 (IL-4) bound to one molecule each of the high- and low-affinity receptors (IL-4R and IL-2R gamma) was built, using the crystal structure of the complex of human growth hormone (HGH) with its receptor (HGHR) as a starting model. The modeling of IL-4 with its receptors was based on the conservation of the sequences and on the predicted structural organization for cytokine receptors, and assuming that the binding mode of the ligands would be similar. Analysis of the interface between IL-4 and both receptor molecules was carried out to reveal which residues are important for complex formation. The modeling procedures showed that there were no major problems in maintaining a reasonable fit of IL-4 with the two receptor molecules, in a manner analogous to the complex of HGH-HGHR. Many of the residues that appear by modeling to be important for binding between IL-4 and the receptors have been previously implicated in that role by different methods. A striking motif of aromatic and positively charged residues on the surface of the C-terminal domains of the receptors is highly conserved in the structure of HGH-HGHR and in the models of IL-4 complexed with its receptors.

Keratinocyte growth factor as a cytokine that mediates mesenchymal-epithelial interaction

Keratinocyte growth factor (KGF) is a member of the heparin-binding fibroblast growth factor family (FGF-7) with a distinctive pattern of target-cell specificity. Studies performed in cell culture suggested that KGF was mitogenically active only on epithelial cells, though from a variety of tissues. In contrast, KGF was produced solely by cells of mesenchymal origin, leading to the hypothesis that it might function as a paracrine mediator of mesenchymal-epithelial communication. Biochemical analysis and molecular cloning established that the KGF receptor (KGFR) was a tyrosine kinase isoform encoded by the fgfr-2 gene. Many detailed investigations of KGF and KGFR expression in whole tissue and cell lines largely substantiated the pattern initially perceived in vitro of mesenchymal and epithelial distribution, respectively. Moreover, functional assays in organ culture and in vivo and analysis of agents regulating KGF expression reinforced the idea that KGF acts predominantly on epithelial cells. While the data do not implicate a KGF autocrine loop in neoplasia, paracrine sources of factor or ligand-independent signaling by the KGFR might contribute to malignancy. Alternatively, because of its differentiation-promoting effects, KGF may retard processes that culminate in uncontrolled cell growth.

Identification of fibroblast growth factor 9 (FGF9) as a high affinity, heparin dependent ligand for FGF receptors 3 and 2 but not for FGF receptors 1 and 4

Fibroblast growth factors (FGF) are multifunctional, heparin binding polypeptides that share structural similarity, but differ in their target cell specificity and expression pattern. Here we describe the cloning and expression of the mouse homologue of FGF9, and the use of a panel of soluble FGF receptors and genetically engineered cells to study its receptor binding specificity. FGF9 is found to bind with high affinity (kd: 0.25 nM) to FGFR3, for which a specific ligand has not yet been identified. FGF9 can also bind, albeit with a lower affinity, to FGFR2 but does not bind FGFR1 or FGFR4. There is no significant binding to either FGFR3 or FGFR2, expressed either as soluble receptors or in heparin sulfate deficient cells, in the absence of heparin. Moreover, receptor binding of FGF9 requires heparin in a manner specific to the receptor type. In conclusion FGF9 presents a unique case of ligand-receptor specificity and fulfills the criteria as a high affinity, heparin-dependent ligand for FGFR3.

Interactions between the Flk-1 receptor, vascular endothelial growth factor, and cell surface proteoglycan identified with a soluble receptor reagent

Fetal liver kinase-1 (Flk-1) is a transmembrane tyrosine kinase that was identified in endothelial cells and populations of cells enriched in hematopoietic progenitors. To characterize the interaction of Flk-1 with potential ligands the receptor extracellular domain was genetically fused to an alkaline phosphatase (AP) tag. A soluble ligand for Flk-1 was identified in the supernatants of numerous mesenchymal cell lines by co-immunoprecipitation with the Flk1-AP fusion protein. This polypeptide was shown by N-terminal sequencing to be vascular endothelial growth factor (VEGF). Receptor-AP fusion proteins can thus be used to identify soluble ligands as well as transmembrane ligands, and this approach is therefore likely to be widely applicable to many types of orphan receptor. The Flk1-AP soluble receptor was also found to bind to cell surfaces, showing two apparent classes of binding site with different affinities. This interaction could be reconstructed by introducing a VEGF expression plasmid into cells. These results indicate that VEGF presented at the cell surface can bind to the Flk-1 receptor, and could mediate a direct cell-cell interaction. The Flk1-AP fusion protein was also found to bind heparin, implying that ligand binding by the Flk-1 receptor may involve a three way interaction between the Flk-1 receptor, VEGF, and heparin-like cell surface proteoglycans.

Enzymatic degradation of glycosaminoglycans

Glycosaminoglycans (GAGs) play an intricate role in the extracellular matrix (ECM), not only as soluble components and polyelectrolytes, but also by specific interactions with growth factors and other transient components of the ECM. Modifications of GAG chains, such as isomerization, sulfation, and acetylation, generate the chemical specificity of GAGs. GAGs can be depolymerized enzymatically either by eliminative cleavage with lyases (EC 4.2.2.-) or by hydrolytic cleavage with hydrolases (EC 3.2.1.-). Often, these enzymes are specific for residues in the polysaccharide chain with certain modifications. As such, the enzymes can serve as tools for studying the physiological effect of residue modifications and as models at the molecular level of protein-GAG recognition. This review examines the structure of the substrates, the properties of enzymatic degradation, and the enzyme substrate-interactions at a molecular level. The primary structure of several GAGs is organized macroscopically by segregation into alternating blocks of specific sulfation patterns and microscopically by formation of oligosaccharide sequences with specific binding functions. Among GAGs, considerable dermatan sulfate, heparin and heparan sulfate show conformational flexibility in solution. They elicit sequence-specific interactions with enzymes that degrade them, as well as with other proteins, however, the effect of conformational flexibility on protein-GAG interactions is not clear. Recent findings have established empirical rules of substrate specificity and elucidated molecular mechanisms of enzyme-substrate interactions for enzymes that degrade GAGs. Here we propose that local formation of polysaccharide secondary structure is determined by the immediate sequence environment within the GAG polymer, and that this secondary structure, in turn, governs the binding and catalytic interactions between proteins and GAGs.

BK1: an FGF-responsive central nervous system-derived cell line

Fibroblast growth factors (FGF) are expressed at high levels in the central nervous system (CNS), however their function in the CNS is not well understood. The immortalized neuronal cell line (BK1), derived from a transgenic mouse central nervous system tumor, expresses high levels of FGF receptor 1 (FGFR1) and demonstrates both morphologic and biochemical changes when treated with basic FGF (FGF-2). We have derived subclones of BK1 cells with varying degrees of FGF responsiveness by transfecting either a wild type (FRW) or a truncated (FRX) form of FGFR1. Cells expressing high levels of FGFR1 rapidly and uniformly respond to FGF, while cells expressing FRX fail to respond to FGF, either morphologically or by the expression of molecular markers. These BK1 subclones will prove useful to study FGFR mediated signal transduction and FGFR responsive genes in a CNS derived cell. These studies also demonstrate that a dominant negative FGF receptor can be used as a tool to elucidate the function of FGF in the central nervous system.

Perlecan, basal lamina proteoglycan, promotes basic fibroblast growth factor-receptor binding, mitogenesis, and angiogenesis

A survey of defined species of cell surface and extracellular matrix heparan sulfate proteoglycans (HSPG) was performed in a search for cellular proteoglycans that can promote bFGF receptor binding and biological activity. Of the various affinity-purified HSPGs tested, perlecan, the large basement membrane HSPG, is found to induce high affinity binding of bFGF both to cells deficient in HS and to soluble FGF receptors. Heparin-dependent mitogenic activity of bFGF is strongly augmented by perlecan. Monoclonal antibodies to perlecan extract the receptor binding promoting activity from active HSPG preparations. In a rabbit ear model for in vivo angiogenesis, perlecan is a potent inducer of bFGF-mediated neovascularization. These results identify perlecan as a major candidate for a bFGF low affinity, accessory receptor and an angiogenic modulator.

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.

Heparin and its non-anticoagulant analogues inhibit human keratinocyte growth without inducing differentiation

In addition to its anti-coagulant effect, heparin inhibits the growth of several types of cells. Recent studies suggest that heparin inhibition of proliferation of cultured human keratinocytes, results primarily from interaction with keratinocyte-generated, heparin-binding autocrine growth factors. In this study, we evaluated whether non-anticoagulant heparin analogs, and oligosaccharide fragments of heparin, retain the growth-inhibitory properties of whole heparin on human keratinocytes. Second-passage neonatal keratinocytes were grown in serum-free keratinocyte growth medium, and the effect of heparin analogs was studied in the absence of exogenous growth factors using keratinocyte-conditioned medium. Cell proliferation was assessed by measurement of both DNA content and [3H]-thymidine incorporation. The addition of heparin inhibited the conditioned medium-stimulated keratinocyte proliferation in a dose-dependent manner, with 80% inhibition at or above 10 micrograms/ml. Moreover, heparin was not toxic to keratinocytes (as detected by propidium-iodide fluorescence and by retention of normal protein synthetic rate) and it did not induce terminal differentiation (as measured by cornified envelope formation). Furthermore, heparin stimulated protein secretion by keratinocytes without altering rates of protein synthesis. The growth-inhibitory effects of heparin oligosaccharides were directly proportional to their chain length. The hexasaccharide unit represented the minimum requirement for inhibition, whereas decasaccharide units demonstrated nearly equivalent growth inhibition to native heparin. Finally, two non-anticoagulant heparin analogs were equipotent with heparin in inhibiting autocrine-induced keratinocyte growth. These studies show that the growth-inhibitory activities of heparin are independent of the anticoagulant effects and that decasaccharides contain the optimal oligosaccharide chain length for the antiproliferative effect in human keratinocytes.

Regulation of signal transduction and signal diversity by receptor oligomerization

Receptor oligomerization was initially proposed as a mechanism by which epidermal growth factor activates the protein tyrosine kinase activity of its receptor. It is now well established that ligand-induced receptor oligomerization plays an important role in transmembrane signaling by a large number of receptors for hormones, cytokines and growth factors. Heterodimerization of the extracellular domains of two members of the same receptor family, or interaction with an accessory molecule, can increase the diversity of ligands recognized by individual receptors. Heterodimerization of cytoplasmic domains permits the recruitment of different complements of SH2-domain-containing signaling molecules, increasing the repertoire of signaling pathways that can be activated by a given receptor.

Heparan sulfate proteoglycans as transducers of FGF-2 signalling

The fibroblast growth factor-2 (FGF-2) low-affinity binding sites, heparan sulfate proteoglycans (HSPGs), function as modulators of FGF-2 activity. It is noteworthy that HSPG binding protects FGF-2 from denaturation and proteolytic degradation, provides a matrix-bound or cell-surface reservoir of this factor for the cells and is required for the activation of FGF high-affinity receptors. In our study we investigated the biological meaning of FGF-2 internalization mediated through its low-affinity binding sites, HSPGs. Using as model system L6 myoblasts lacking endogenous FGF receptors (FGFRs), we demonstrated that these cells internalize FGF-2 efficiently through an HSPG-mediated pathway. FGF-2 internalization occurring through HSPGs was paralleled by an increase in the activity of urokinase plasminogen activator (u-PA). The u-PA-inducing activity of FGF-2 was strictly correlated to its internalization, as chlorate treatment, which causes a strong inhibition of FGF-2 internalization, abrogated the u-PA-inducing activity of FGF-2. In addition, expression of functional FGF high-affinity receptors (FGFR-1) did not enhance u-PA in L6 myoblasts upon FGF-2 stimulation. According to our results we propose that FGF-2 internalization mediated through HSPGs may transduce FGF-2 signalling such as u-PA-activity stimulation. Thus, HSPGs may act as direct transducers of FGF signalling and indeed, different FGF-signalling pathways must exist.