Heparan sulfate is required for interaction and activation of the epithelial cell fibroblast growth factor receptor-2IIIb with stromal-derived fibroblast growth factor-7

Enhanced prokaryotic expression, purification, and biological activities of human keratinocyte growth factor

Keratinocyte growth factor (KGF), also known as fibroblast growth factor 7 (FGF7), plays a critical role in embryonic development, cell proliferation, and differentiation. However, efficient production of recombinant KGF remains a challenge due to its low expression levels and high tendency for aggregation in Escherichia coli. This study aimed to enhance the expression and solubility of KGF by employing different protein tags-PDIb'a', MBP, and His-fused to the N-terminus of KGF. Among these, H-PDIb'a'-KGF demonstrated superior stability and was selected for large-scale production and purification. The purified KGF was confirmed through liquid chromatography with tandem mass spectrometry analysis, which showed an 81% fragment mass identification coverage. Biological activity assessments using human breast cancer MCF-7 cells indicated that purified KGF significantly increased cell proliferation, with an EC50 of 6.4 ± 0.5 pM. Interestingly, PDIb'a' alone also exhibited a stimulatory effect on MCF-7 cells. Furthermore, the purified KGF enhanced the wound healing of HaCaT keratinocytes in a dose-dependent manner. These findings provide valuable insights into the efficient production and functional characterization of recombinant KGF for potential applications in therapeutic interventions.

In silico design of fusion keratinocyte growth factor containing collagen-binding domain for tissue engineering application

Keratinocyte growth factor (KGF) is a potential therapeutic factor in wound healing. However, its applications have been restricted due to its low stability, short half-life, and limited target specificity. We aimed to immobilize KGF on collagen-based biomaterials for long-lasting and targeted therapy by designing fusion forms of KGF with collagen-binding domains (CBD) from natural origins. Twelve fusion proteins were designed consisting of KGF and CBDs with different lengths and amino acid compositions. Three-dimensional (3D) structures of the fusions were predicted by homology modeling. Physiochemical properties and secondary structure of the fusions were evaluated by bioinformatics tools. Moreover, the effect of the CBDs on the 3D structure and dynamic behavior of the fusions was investigated by molecular dynamics (MD) simulation. The binding affinity of the fusions to collagen, KGF receptor, and heparin was assessed using docking tools. Our results demonstrated that fusions with small CBDs like CBD of mammalian collagenase and decapeptide CBD of von Willebrand factor (VWF) were more stable and properly folded than those with larger CBDs. On the other hand, the insertion of bulky CBDs, including Fibronectin CBD and CBD of Clostridium histolyticum collagenase, into KGF resulted in stronger binding to collagen. Therefore, very small or large CBDs are inappropriate for constructing KGF fusions because they suffer from low collagen affinity or poor stability. By comparing the results of MD simulation and docking, this study proposed that CBDs belonging to Vibrio mimicus metalloprotease and A3 domain of VWF would be good candidates to produce stable fusions with proper affinities toward collagen and KGF receptors. Moreover, the secondary structure analysis showed that the overall structure of KGF and CBDs was better preserved when CBDs were inserted at the C-terminal of KGF. This computational information about novel KGF fusions may help find the best constructs for experimental studies.

Improvement of FGF7 Thermal Stability by Introduction of Mutations in Close Vicinity to Disulfide Bond and Surface Salt Bridge

Fibroblast Growth Factor 7 (FGF7), a growth factor specific to epithelial cells, has attracted attention as a therapeutic protein. However, FGF7 has a limitation in its use due to low protein stability. Here, the mutations were designed to increase the stability of FGF7 by analyzing its 3D structure and sequence of other FGFs. Palifermin, N-terminal truncated FGF7 is known to have improved stability and was used as control protein in our study. The K126 and K178 were substituted into glutamate to form salt bridge with the neighboring residue R175 respectively and A120C mutation was introduced in close vicinity to disulfide bond between C133 and C137. The data of Circular Dichroism (CD) showed that all mutant proteins tested had higher Tm value than Palifermin and Tm of A120C/K126E/K178E FGF7 mutant protein was 15.24 °C higher than that of Palifermin. The results of cell proliferation activity and soluble protein analyzed by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) after 37 °C or 45 °C incubation exhibited that the stability of A120C mutant protein and A120C-including mutant proteins was improved. These results suggest that the mutation of amino acid in close vicinity to disulfide bond and the salt bridge at the surface of FGF7 enhanced thermal stability and make FGF7 more useful for pharmaceutical and cosmetical application.

Keratinocyte growth factor in focus: A comprehensive review from structural and functional aspects to therapeutic applications of palifermin

Palifermin (Kepivance™) is the first therapeutic approved by the Food and Drug Administration for preventing and managing the oral mucositis provoked by myelotoxic and mucotoxic therapies. Palifermin is a recombinant protein generated from human keratinocyte growth factor (KGF) and imitates the function of endogenous KGF. KGF is an epithelial mitogen involved in various biological processes which belongs to the FGF family. KGF possesses a high level of receptor specificity and plays an important role in tissue repair and maintaining of the mucosal barrier integrity. Based on these unique features, palifermin was developed to enhance the growth of damaged epithelial tissues. Administration of palifermin has shown success in the reduction of toxicities of chemotherapy and radiotherapy, and improvement of the patient's quality of life. Notwithstanding all merits, the clinical application of palifermin is limited owing to its instability and production challenges. Hence, a growing number of ongoing researches are designed to deal with these problems and enhance the physicochemical and pharmaceutical properties of palifermin. In the current review, we discuss KGF structure and function, potential therapeutic applications of palifermin, as well as the latest progress in the production of recombinant human KGF and its challenges ahead.

In vitro and in vivo antiangiogenic properties of the serpin protease nexin-1

The serpin protease nexin-1 (PN-1) is expressed by vascular cells and secreted by platelets upon activation, and it is known to interact with several modulators of angiogenesis, such as proteases, matrix proteins, and glycosaminoglycans. We therefore investigated the impact of PN-1 on endothelial cell angiogenic responses in vitro and ex vivo and in vivo in PN-1-deficient mice. We found that PN-1 is antiangiogenic in vitro: it inhibited vascular endothelial growth factor (VEGF)-induced endothelial cell responses, including proliferation, migration, and capillary tube formation, and decreased cell spreading on vitronectin. These effects do not require the antiprotease activity of PN-1 but involve PN-1 binding to glycosaminoglycans. In addition, our results indicated that PN-1 does not act by blocking VEGF binding to its heparan sulfate proteoglycan coreceptors. The results obtained in vitro were supported ex vivo in PN-1-deficient mice, where the microvascular network sprouting from aortic rings was significantly enhanced. Moreover, in vivo, neovessel formation was promoted in the Matrigel plug assay in PN-1-deficient mice compared to wild-type mice, and these effects were reversed by the addition of recombinant PN-1. Taken together, our results demonstrate that PN-1 has direct antiangiogenic properties and is a yet-unrecognized player in the angiogenic balance.

Collagen XIX is expressed by interneurons and contributes to the formation of hippocampal synapses

Extracellular matrix (ECM) molecules contribute to the formation and maintenance of synapses in the mammalian nervous system. We previously discovered a family of nonfibrillar collagens that organize synaptic differentiation at the neuromuscular junction (NMJ). Although many NMJ-organizing cues contribute to central nervous system (CNS) synaptogenesis, whether similar roles for collagens exist at central synapses remained unclear. In the present study we discovered that col19a1, the gene encoding nonfibrillar collagen XIX, is expressed by subsets of hippocampal neurons. Colocalization with the interneuron-specific enzyme glutamate decarboxylase 67 (Gad67), but not other cell-type-specific markers, suggests that hippocampal expression of col19a1 is restricted to interneurons. However, not all hippocampal interneurons express col19a1 mRNA; subsets of neuropeptide Y (NPY)-, somatostatin (Som)-, and calbindin (Calb)-immunoreactive interneurons express col19a1, but those containing parvalbumin (Parv) or calretinin (Calr) do not. To assess whether collagen XIX is required for the normal formation of hippocampal synapses, we examined synaptic morphology and composition in targeted mouse mutants lacking collagen XIX. We show here that subsets of synaptotagmin 2 (Syt2)-containing hippocampal nerve terminals appear malformed in the absence of collagen XIX. The presence of Syt2 in inhibitory hippocampal synapses, the altered distribution of Gad67 in collagen XIX-deficient subiculum, and abnormal levels of gephyrin in collagen XIX-deficient hippocampal extracts all suggest inhibitory synapses are affected by the loss of collagen XIX. Together, these data not only reveal that collagen XIX is expressed by central neurons, but show for the first time that a nonfibrillar collagen is necessary for the formation of hippocampal synapses.

Compensatory paracrine mechanisms that define the urothelial response to injury in partial bladder outlet obstruction

Diseases and conditions affecting the lower urinary tract are a leading cause of dysfunctional sexual health, incontinence, infection, and kidney failure. The growth, differentiation, and repair of the bladder's epithelial lining are regulated, in part, by fibroblast growth factor (FGF)-7 and -10 via a paracrine cascade originating in the mesenchyme (lamina propria) and targeting the receptor for FGF-7 and -10 within the transitional epithelium (urothelium). The FGF-7 gene is located at the 15q15-q21.1 locus on chromosome 15 and four exons generate a 3.852-kb mRNA. Five duplicated FGF-7 gene sequences that localized to chromosome 9 were predicted not to generate functional protein products, thus validating the use of FGF-7-null mice as an experimental model. Recombinant FGF-7 and -10 induced proliferation of human urothelial cells in vitro and transitional epithelium of wild-type and FGF-7-null mice in vivo. To determine the extent that induction of urothelial cell proliferation during the bladder response to injury is dependent on FGF-7, an animal model of partial bladder outlet obstruction was developed. Unbiased stereology was used to measure the percentage of proliferating urothelial cells between obstructed groups of wild-type and FGF-7-null mice. The stereological analysis indicated that a statistical significant difference did not exist between the two groups, suggesting that FGF-7 is not essential for urothelial cell proliferation in response to partial outlet obstruction. In contrast, a significant increase in FGF-10 expression was observed in the obstructed FGF-7-null group, indicating that the compensatory pathway that functions in this model results in urothelial repair.

Control of fibroblast growth factor (FGF) 7- and FGF1-induced mitogenesis and downstream signaling by distinct heparin octasaccharide motifs

Variation in length, disaccharide composition, and sulfation of heparan sulfate (HS) affects fibroblast growth factor (FGF) signaling. However, it is unclear whether the specific distribution of groups within oligosaccharides or random variations in charge density underlies the effects. Recently we showed that a mixture of undersulfated octasaccharides exhibiting 7 and 8 sulfates (7,8-S-OctaF7) generated from heparin had the highest affinity for FGF7 monitored by salt resistance (>0.60 M salt) of octasaccharide-FGF7 complexes. 7,8-S-OctaF7 also had the highest specific activity for formation of a complex with dimeric FGFR2IIIb competent to bind FGF7. Here we show that when endogenous HS was inhibited by chlorate treatment, 7,8-S-OctaF7 specifically supported FGF7-stimulated DNA synthesis and downstream signaling in FGFR2IIIb-expressing mouse keratinocytes. It failed to support FGF1 signaling in both HS-deficient mouse keratinocytes and 3T3 fibroblasts. In contrast, abundant, more highly sulfated and heterogenous mixtures of octasaccharides with lower affinity (0.30-0.60 M salt) for FGF7 supported FGF1-induced signaling in both cell types. In contrast to the two-component 7,8-S-OctaF7 mixture from FGF7, the high affinity octasaccharide fraction from FGF1 was a heterogeneous mixture with components ranging from 8 to 12 sulfates with 11-S-octasaccharides the most abundant. The high affinity fraction exhibited similar properties to the lower affinity fractions from both FGF1 and FGF7. Octasaccharide mixtures eluting from FGF1 between 0.30 and 0.60 M and above 0.60 M salt were nearly equal in support of FGF1 signaling in fibroblasts and keratinocytes. Both were deficient in support of FGF7-induced signaling in keratinocytes. The results show that both variations in overall charge density and specific distribution of charged groups within HS motifs exhibit FGF-specific control over formation of FGF-HS-FGFR complexes and downstream signaling.

Biospecific extraction and neutralization of anticoagulant heparin with fibroblast growth factors (FGF)

The polyanionic sulfated carbohydrate heparin is a mixture of anticoagulant and nonanticoagulant activity that is best known for its pharmacological benefit as an anticoagulant. The objective of this study was to design and evaluate a simple purification method for an anticoagulant fraction of heparin from a crude heparin mixture as an alternative to antithrombin. Similar to blood clotting, the fibroblast growth factor signaling system is heparan sulfate-regulated and comprised of components with structurally distinct heparin-binding domains. A rare and highly specific motif within a single heparan sulfate chain has been proposed to tether both FGF and the FGFR ectodomain together. The diversity of heparin-binding motifs within the large FGF family of polypeptides and receptors provides a repertoire of diverse templates for capture of diverse heparin/heparan sulfate motifs in biology. We show here that, similar to antithrombin, a member of the FGF family, FGF7, selectively captures anti-Factor Xa and anti-Factor IIa activity from commercially and clinically applied heparin mixtures. In the presence of purified anticoagulant heparin and derivative, FGF7 has the similar activity as protamine sulfate for reversal of anticoagulant effect, while FGF1 is much less potent than FGF7. This may provide a novel cost-effective, bioaffinity-based alternative to antithrombin for concurrent enrichment and recovery of anticoagulant and nonanticoagulant heparin from the same heparin mixture. In addition, FGF7 and homologues may be useful in pharmaceutical neutralization of anticoagulant heparin and heparan sulfate.

Improved production of recombinant fibroblast growth factor 7 (FGF7/KGF) from bacteria in high magnesium chloride

Because of specificity for both heparin/heparan sulfate and the receptor complex on epithelial cells relative to other fibroblast growth factor (FGF) homologues, there is considerable interest in clinical and commercial applications of FGF7 (also called keratinocyte growth factor or KGF) that require large quantities at reasonable cost. Production of recombinant FGF7 from bacteria suffers from lower yields and recovery relative to FGF1 and FGF2. Fusion of FGF7 at the N-terminus with glutathione-S-transferase (GST) followed by removal of GST by proteolysis while bound to natural ligand heparin improved the intrinsically low yields from Escherichia coli hosts to 3.2 mg per liter per OD(600), which was still only 10% of that for FGF1. Yield of the GST-FGF7 fusion product was improved to about 17 mg per liter per OD(600) in strain BL21(DE3)pLysS by inclusion of 10-100mM magnesium chloride (MgCl(2)) in the culture medium. This improved by about five times the yields of fully active 54ser-FGF7 after proteolytic excision of the GST portion from GST-FGF7 immobilized on heparin-Sepharose. This simple enhancement improves the cost-effectiveness of production of recombinant FGF7 in bacteria for clinical and commercial applications.

Loss of ligand-binding specificity of fibroblast growth factor receptor 2 by RNA splicing in human chondrosarcoma cells

We have previously reported the alternatively spliced transcripts of fibroblast growth factor receptor (FGFR) 3 derived by aberrant splicing in human cancers. Here, we describe a novel splice variant of FGFR2 (FGFR2DeltaIII) arising from skipping exons 7-10, resulting in the deletion of Ig-like-III domain in human chondrosarcoma cell. Sf9 cells expressing FGFR2DeltaIII were able to bind FGF1, FGF2, and FGF7, leading to loss of ligand-binding specificity. Together with our previous findings, the present studies suggest that mRNA splicing plays an important role in the regulation of FGFRs' function.

The epithelial mitogen keratinocyte growth factor binds to collagens via the consensus sequence glycine-proline-hydroxyproline

The binding of certain growth factors and cytokines to components of the extracellular matrix can regulate their local availability and modulate their biological activities. We show that mesenchymal cell-derived keratinocyte growth factor (KGF), a key stimulator of epithelial cell proliferation during wound healing, preferentially binds to collagens I, III, and VI. Binding is inhibited in a dose-dependent manner by denatured single collagen chains and collagen cyanogen bromide peptides. This interaction is saturable with dissociation constants of approximately 10(-8) to 10(-9) m and estimated molar ratios of up to three molecules of KGF bound to one molecule of triple helical collagen. Furthermore, collagen-bound KGF stimulated the proliferation of transformed keratinocyte or HaCaT cells. Ligand blotting of collagen-derived peptides points to a limited set of collagenous consensus sequences that bind KGF. By using synthetic collagen peptides, we defined the consensus sequence (Gly-Pro-Hyp)(n) as the collagen binding motif. We conclude that the preferential binding of KGF to the abundant collagens leads to a spatial pattern of bioavailable KGF that is dictated by the local organization of the collagenous extracellular matrix. The defined collagenous consensus peptide or its analogue may be useful in wound healing by increasing KGF bioactivity and thus modulating local epithelial remodeling and regeneration.

A novel splice variant of fibroblast growth factor receptor 2 in human leukemia HL-60 cells

Fibroblast growth factor receptors (FGFRs) genes have been shown to be translocated in multiple myeloma (MM) and myeloproliferative disorder (MPD), indicating an important role for the FGFRs in hematologic malignancies. Here, we describe a novel splice variant of FGFR2 (FGFR2AT-I) arising from skipping exons 7-10 in human myeloid leukemia HL-60 cells, encoding a FGFR2 in which the Ig-like-III domain is deleted while the remainder of the mature molecule is fused in-frame to the transmembrane and COOH-terminal cytoplasmic kinases. Binding assays demonstrated that the FGFR2AT-I was able to bind FGF1, FGF2, and FGF7, leading to loss of ligand binding specificity. Furthermore, overexpression of FGFR2AT-I resulted in increased AKT and MAPK activation, conferring a survival advantage. Taken together, these findings indicate that the dysregulation of FGFRs' function by aberrant mRNA splicing contributes to tumor progression.

Identification and characterization of soluble isoform of fibroblast growth factor receptor 3 in human SaOS-2 osteosarcoma cells

We have previously reported the alternatively spliced transcripts of fibroblast growth factor 3 (FGFR3 ATs and MTs) derived by aberrant splicing and usage of cryptic splicing sites. Here, we describe a soluble variant of FGFR3 (FGFR3 AT-III) arising from skipping exons 8, 9, and 10 in human SaOS-2 osteosarcoma cell. This splicing event leads to the generation of an mRNA encoding a FGFR3 in which the COOH-terminal portion of the Ig-like-III domain and transmembrane domain are deleted while the remainder of the mature molecule is fused in-frame to the COOH-terminal cytoplasmic kinases domains. Sf9 cells transfected with the corresponding cDNA express the soluble form of FGFR3 AT-III into the condition medium and its secreted form was able to bind both FGF-1 and FGF-2 leading to loss of ligand binding specificity. These results indicate that the FGFR3 AT-III mRNAs are transcribed due to exon skipping with altered ligand binding specificity. These results suggest that the presence of soluble transcripts of FGFRs gene is a common feature due to mRNA splicing and this splicing plays an important role in the regulation of FGFRs function.

Growth inhibition by keratinocyte growth factor receptor of human salivary adenocarcinoma cells through induction of differentiation and apoptosis

We have reported that normal human salivary gland-derived epithelial cells exclusively express keratinocyte growth factor receptor (KGFR). In the process of malignant transformation of human salivary gland tumors, KGFR gene expression disappeared concomitantly with the de novo expression of the fibroblast growth factor receptor 1 (FGFR1) and FGFR4 genes. In the present study, we introduced wild-type KGFR cDNA or chimeric KGFR/FGFR1 cDNA, which encoded the extracellular domain of KGFR and the intracellular domain of FGFR1, into the HSY human salivary adenocarcinoma cell line. The KGFR tyrosine kinase suppressed the activity of FGF receptor substrate 2 (FRS2) and inhibited the growth of HSY by inducing differentiation and apoptosis in vitro and in vivo. Our results provided significant insight into the mechanism of KGFR tumor suppression and suggest that KGFR gene therapy might be a viable method of inhibiting human salivary adenocarcinoma growth.

Evidence that the intracellular domain of FGF receptor 2IIIb affects contact of the ectodomain with two FGF7 ligands

Models of the oligomeric FGF signaling complex, including those derived from crystal structures, vary in stoichiometry and arrangement of the three subunits comprised of heparin/heparan sulfate chains, FGFR tyrosine kinase and activating FGF. Here, using covalent affinity crosslinking of radiolabeled FGF7 to binary complexes of FGFR2IIIb and heparin, we show that two molecules of FGF7 contact each FGFR2IIIb. This supports models that propose a dimeric complex of two units with stoichiometry 1 FGF:1 FGFR in which each FGF contacts both FGFR. The bivalent FGF7 contact was dependent on the full-length amino terminus of FGF7alpha and the intracellular domain of FGFR2IIIb extending through the juxtamembrane domain and the beta1 and beta2 strands of the kinase which is required for ATP binding. We propose that the differences in crosslinking report differences in relationships among subunits in the ectodomain of the complex that are affected by the amino terminus of FGF and the FGFR intracellular domain. From this, we suggest the corollary that conformational relationships among subunits in the ectodomain are transmitted to the intracellular and ATP binding domains during activation of the complex.

The proliferative and migratory activities of breast cancer cells can be differentially regulated by heparan sulfates

To explore how heparan sulfate (HS) controls the responsiveness of the breast cancer cell lines MCF-7 and MDA-MB-231 to fibroblast growth factors (FGFs), we have exposed them to HS preparations known to have specificity for FGF-1 (HS glycosaminoglycan (HSGAG A)) or FGF-2 (HSGAGB). Proliferation assays confirmed that MCF-7 cells were highly responsive to FGF-2 complexed with GAGB, whereas migration assays indicated that FGF-1/HSGAGA combinations were stimulatory for the highly invasive MDA-MB-231 cells. Quantitative polymerase chain reaction for the levels of FGF receptor (FGFR) isoforms revealed that MCF-7 cells have greater levels of FGFR1 and that MDA-MB-231 cells have greater relative levels of FGFR2. Cross-linking demonstrated that FGF-2/HSGAGB primarily activated FGFR1, which in turn up-regulated the activity of mitogen-activated protein kinase; in contrast, FGF-1/HSGAGA led to the phosphorylation of equal proportions of both FGFR1 and FGFR2, which in turn led to the up-regulation of Src and p125(FAK). MDA-MB-231 cells were particularly responsive to vitronectin substrates in the presence of FGF-1/HSGAGA, and blocking antibodies established that they used the alpha(v)beta(3) integrin to bind to it. These results suggest that the clustering of particular FGFR configurations on breast cancer cells induced by different HS chains leads to distinct phenotypic behaviors.

Up-regulation of the chondrogenic Sox9 gene by fibroblast growth factors is mediated by the mitogen-activated protein kinase pathway

Recent experiments have established that Sox9 is required for chondrocyte differentiation. Here, we show that fibroblast growth factors (FGFs) markedly enhance Sox9 expression in mouse primary chondrocytes as well as in C3H10T1/2 cells that express low levels of Sox9. FGFs also strongly increase the activity of a Sox9-dependent chondrocyte-specific enhancer in the gene for collagen type II. Transient transfection experiments using constructs encoding FGF receptors strongly suggested that all FGF receptors, FGFR1-R4, can transduce signals that lead to the increase in Sox9 expression. The increase in Sox9 levels induced by FGF2 was inhibited by a specific mitogen-activated protein kinase kinase (MAPKK)/mitogen-activated protein kinase/ERK kinase (MEK) inhibitor U0126 in primary chondrocytes. In addition, coexpression of a dual-specificity phosphatase, CL100/MKP-1, that is able to dephosphorylate and inactivate mitogen-activated protein kinases (MAPKs) inhibited the FGF2-induced increase in activity of the Sox9-dependent enhancer. Furthermore, coexpression of a constitutively active mutant of MEK1 increased the activity of the Sox9-dependent enhancer in primary chondrocytes and C3H10T1/2 cells, mimicking the effects of FGFs. These results indicate that expression of the gene for the master chondrogenic factor Sox9 is stimulated by FGFs in chondrocytes as well as in undifferentiated mesenchymal cells and strongly suggest that this regulation is mediated by the MAPK pathway. Because Sox9 is essential for chondrocyte differentiation, we propose that FGFs and the MAPK pathway play an important role in chondrogenesis.

Similarities and differences between the effects of heparin and glypican-1 on the bioactivity of acidic fibroblast growth factor and the keratinocyte growth factor

The keratinocyte growth factor (KGF or FGF-7) is unique among its family members both in its target cell specificity and its inhibition by the addition of heparin and the native heparan-sulfate proteoglycan (HSPG), glypican-1 in cells expressing endogenous HSPGs. FGF-1, which binds the FGF-7 receptor with a similar affinity as FGF-7, is stimulated by both molecules. In the present study, we investigated the modulation of FGF-7 activities by heparin and glypican-1 in HS-free background utilizing either HS-deficient cells expressing the FGF-7 receptor (designated BaF/KGFR cells) or soluble extracellular domain of the receptor. At physiological concentrations of FGF-7, heparin was required for high affinity receptor binding and for signaling in BaF/KGFR cells. In contrast, binding of FGF-7 to the soluble form of the receptor did not require heparin. However, high concentrations of heparin inhibited the binding of FGF-7 to both the cell surface and the soluble receptor, similar to the reported effect of heparin in cells expressing endogenous HSPGs. The difference in heparin dependence for high affinity interaction between the cell surface and soluble receptor may be due to other molecule(s) present on cell surfaces. Glypican-1 differed from heparin in that it stimulated FGF-1 but not FGF-7 activities in BaF/KGFR cells. Glypican-1 abrogated the stimulatory effect of heparin, and heparin reversed the inhibitory effect of glypican-1, indicating that this HSPG inhibits FGF-7 activities by acting, most likely, as a competitive inhibitor of stimulatory HSPG species for FGF-7. The regulatory effect of glypican-1 is mediated at the level of interaction with the growth factor as glypican-1 did not bind the KGFR. The effect of heparin and glypican-1 on FGF-1 and FGF-7 oligomerization was studied employing high and physiological concentrations of growth factors. We did not find a correlation between the effects of these glycosaminoglycans on FGFs biological activity and oligomerization. Altogether, our findings argue against the heparin-linked dimer presentation model as key in FGFR activation, and support the notion that HSPGs primarily affect high affinity interaction of FGFs with their receptors.

Specificity for fibroblast growth factors determined by heparan sulfate in a binary complex with the receptor kinase

A divalent cation-dependent association between heparin or heparan sulfate and the ectodomain of the FGF receptor kinase (FGFR) restricts FGF-independent trans-phosphorylation and supports the binding of activating FGF to self-associated FGFR. Here we show that in contrast to heparin, cellular heparan sulfate forms a binary complex with FGFR that discriminates between FGF-1 and FGF-2. FGFR type 4 (FGFR4) in liver parenchymal cells binds only FGF-1, whereas FGFR1 binds FGF-1 and FGF-2 equally. Cell-free complexes of heparin and recombinant FGFR4 bound FGF-1 and FGF-2 equally. However, in contrast to FGFR1, when recombinant FGFR4 was expressed back in epithelial cells by transfection, it failed to bind FGF-2 unless heparan sulfate was depressed by chlorate or heparinase treatment. Isolated heparan sulfate proteoglycan (HSPG) from liver cells in cell-free complexes with FGFR4 restored the specificity for FGF-1 and supported the binding of both FGF-1 and FGF-2 when complexed with FGFR1. In contrast, FGF-2 bound equally well to complexes of both FGFR1 and FGFR4 formed with endothelial cell-derived HSPG, but the endothelial HSPG was deficient for the binding of FGF-1 to both FGFR complexes. These data suggest that a heparan sulfate subunit is a cell type- and FGFR-specific determinant of the selectivity of the FGFR signaling complex for FGF. In a physiological context, the heparan sulfate subunit may limit the redundancy among the current 18 FGF polypeptides for the 4 known FGFR.

Fibroblast growth factor-10. A second candidate stromal to epithelial cell andromedin in prostate

Fibroblast growth factor (FGF)-10, a homologue of FGF-7, is expressed significantly in normal rat prostate tissue, well differentiated rat prostate tumors with an epithelial and stromal compartment and only in derived prostate stromal cells in culture. Similar to FGF-7, recombinant rat FGF-10 was a specific mitogen for prostate epithelial cells. In contrast to FGF-7 which is widely expressed among stromal cells in tissues, the expression of FGF-10 correlated with the presence of stromal cells of muscle origin. Radioreceptor binding assays and covalent cross-linking analysis revealed that FGF-10 binds with an affinity equal to FGF-7 to resident epithelial cell receptor, FGFR2IIIb, but unlike FGF-7 also binds the IIIb splice variant of FGFR1. Analysis of mRNA expression by RNase protection revealed that, similar to FGF-7, the expression of FGF-10 was responsive to androgen in stromal cells from normal prostate and non-malignant differentiated tumors. Although FGF-10 cDNA exhibits a signal sequence for secretion, cultured stromal cells exhibit strictly a cell-associated FGF-10 antigen that correlates with an alternately translated intracellular isoform. FGF-10 requires 1.4 times higher NaCl for elution from immobilized heparin than does FGF-7 and binds to four times the number of sites on the pericellular matrix of epithelial cells. The results show that prostate stromal cell-derived FGF-10, like FGF-7, exhibits the properties of an andromedin which may indirectly mediate control of epithelial cell growth and function by androgen. Although FGF-10 and FGF-7 bind and activate the same resident epithelial cell receptor (FGFR2IIIb), differences in cell type of origin, compartmentation by alternate translation, the affinity for FGFR1IIIb, and access to FGFR by differential interaction with pericellular matrix heparan sulfate suggest they may play both independent and compensatory roles in prostate homeostasis.