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

Structural basis for FGF hormone signalling

α/βKlotho coreceptors simultaneously engage fibroblast growth factor (FGF) hormones (FGF19, FGF21 and FGF23)^1,2 and their cognate cell-surface FGF receptors (FGFR1-4) thereby stabilizing the endocrine FGF-FGFR complex^3-6. However, these hormones still require heparan sulfate (HS) proteoglycan as an additional coreceptor to induce FGFR dimerization/activation and hence elicit their essential metabolic activities⁶. To reveal the molecular mechanism underpinning the coreceptor role of HS, we solved cryo-electron microscopy structures of three distinct 1:2:1:1 FGF23-FGFR-αKlotho-HS quaternary complexes featuring the 'c' splice isoforms of FGFR1 (FGFR1c), FGFR3 (FGFR3c) or FGFR4 as the receptor component. These structures, supported by cell-based receptor complementation and heterodimerization experiments, reveal that a single HS chain enables FGF23 and its primary FGFR within a 1:1:1 FGF23-FGFR-αKlotho ternary complex to jointly recruit a lone secondary FGFR molecule leading to asymmetric receptor dimerization and activation. However, αKlotho does not directly participate in recruiting the secondary receptor/dimerization. We also show that the asymmetric mode of receptor dimerization is applicable to paracrine FGFs that signal solely in an HS-dependent fashion. Our structural and biochemical data overturn the current symmetric FGFR dimerization paradigm and provide blueprints for rational discovery of modulators of FGF signalling² as therapeutics for human metabolic diseases and cancer.

Extracellular calcium increases fibroblast growth factor 2 gene expression via extracellular signal-regulated kinase 1/2 and protein kinase A signaling in mouse dental papilla cells

We previously reported that elevated extracellular calcium (Ca²⁺) levels increase bone morphogenetic protein 2 expression in human dental pulp (hDP) cells. However, it is unknown whether extracellular Ca²⁺ affects the expression of other growth factors such as fibroblast growth factor 2 (FGF2).

Fibroblast Growth Factor (FGF) 23 Regulates the Plasma Levels of Parathyroid Hormone In Vivo Through the FGF Receptor in Normocalcemia, But Not in Hypocalcemia

The calcium and phosphate homeostasis is regulated by a complex interplay between parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and calcitriol. Experimental studies have demonstrated an inhibitory effect of FG23 on PTH production and secretion; the physiological role of this regulation is however not well understood. Surprisingly, in uremia, concomitantly elevated FGF23 and PTH levels are observed. The parathyroid gland rapidly loses its responsiveness to extracellular calcium in vitro and a functional parathyroid cell line has currently not been established. Therefore, the aim of the present investigation was to study the impact of FGF23 on the Ca²⁺/PTH relationship in vivo under conditions of normocalcemia and hypocalcemia. Wistar rats were allocated to treatment with intravenous recombinant FGF23 and inhibition of the FGF receptor in the setting of normocalcemia and acute hypocalcemia. We demonstrated that FGF23 rapidly inhibited PTH secretion and that this effect was completely blocked by inhibition of the FGF receptor. Furthermore, inhibition of the FGF receptor by itself significantly increased PTH levels, indicating that FGF23 has a suppressive tonus on the parathyroid gland's PTH secretion. In acute hypocalcemia, there was no effect of either recombinant FGF23 or FGF receptor inhibition on the physiological response to the low ionized calcium levels. In conclusion, FGF23 has an inhibitory tonus on PTH secretion in normocalcemia and signals through the FGF receptor. In acute hypocalcemia, when increased PTH secretion is needed to restore the calcium homeostasis, this inhibitory effect of FGF23 is abolished.

Recombinant expression in E. coli of human FGFR2 with its transmembrane and extracellular domains

Fibroblast growth factor receptors (FGFRs) are a family of receptor tyrosine kinases containing three domains: an extracellular receptor domain, a single transmembrane helix, and an intracellular tyrosine kinase domain. FGFRs are activated by fibroblast growth factors (FGFs) as part of complex signal transduction cascades regulating angiogenesis, skeletal formation, cell differentiation, proliferation, cell survival, and cancer. We have developed the first recombinant expression system in E. coli to produce a construct of human FGFR2 containing its transmembrane and extracellular receptor domains. We demonstrate that the expressed construct is functional in binding heparin and dimerizing. Size exclusion chromatography demonstrates that the purified FGFR2 does not form a complex with FGF1 or adopts an inactive dimer conformation. Progress towards the successful recombinant production of intact FGFRs will facilitate further biochemical experiments and structure determination that will provide insight into how extracellular FGF binding activates intracellular kinase activity.

Polymer chain flexibility-induced differences in fetuin A adsorption and its implications on cell attachment and proliferation

Tissue cells are known to respond to the stiffness of the polymer substrate on which they are grown. It has been suggested that material stiffness influences the composition of the protein layer that adsorbs to the material surface, which affects subsequent cell behavior. Previously, the stiffness of a biomaterial elastomer formed from an acrylated star-poly(d,l lactide-co-ε-caprolactone) was found to influence both fibroblast proliferation as well as the adsorption of certain proteins. However, it remained unresolved as to whether material stiffness influenced protein adsorption from serum supplemented environments and which protein(s) may have been responsible for the difference in fibroblast proliferation. Using quantitative proteomics, we show that polymer stiffness influenced the composition of the protein layers that adsorb from serum supplemented media. Fetuin A was identified as a protein that influenced fibroblast proliferation and, when combined with basic fibroblast growth factor as a medium supplement, improved fibroblast proliferation over 14days. This study is the first to correlate cell proliferation to surface adsorbed fetuin A and presents the potential new application for fetuin A as biomaterial coating or surface modifier. This work also demonstrates a novel application of quantitative proteomics for the investigation of competitive protein adsorption to biomaterial surfaces.

STATEMENT OF SIGNIFICANCE

Cells are able to respond to the stiffness of their material substrate, but the method by which they sense material stiffness is still under investigation. Previously, material stiffness was found to impact the individual adsorption of fibronectin, a protein associated with cell attachment; however, it was unclear if stiffness was able to affect protein adsorption in environments with multiple proteins. This study shows that material stiffness affects the compositions of protein layers adsorbed from supplemented media, and suggests that cells may sense material stiffness via the adsorbed protein layer. Interestingly, fetuin A was found to be affecting cell proliferation and not fibronectin. Finally, this research demonstrates the use of relative quantitation proteomics as a potentially powerful method to improve biomaterial compatibility.

Chitosan-based copper nanocomposite accelerates healing in excision wound model in rats

Copper possesses efficacy in wound healing which is a complex phenomenon involving various cells, cytokines and growth factors. Copper nanoparticles modulate cells, cytokines and growth factors involved in wound healing in a better way than copper ions. Chitosan has been shown to be beneficial in healing because of its antibacterial, antifungal, biocompatible and biodegradable polymeric nature. In the present study, chitosan-based copper nanocomposite (CCNC) was prepared by mixing chitosan and copper nanoparticles. CCNC was applied topically to evaluate its wound healing potential and to study its effects on some important components of healing process in open excision wound model in adult Wistar rats. Significant increase in wound contraction was observed in the CCNC-treated rats. The up-regulation of vascular endothelial growth factor (VEGF) and transforming growth factor-beta1(TGF-β1) by CCNC-treatment revealed its role in facilitating angiogenesis, fibroblast proliferation and collagen deposition. The tumor necrosis factor-α (TNF-α) and interleukin-10 (IL-10) were significantly decreased and increased, respectively, in CCNC-treated rats. Histological evaluation showed more fibroblast proliferation, collagen deposition and intact re-epithelialization in CCNC-treated rats. Immunohistochemistry of CD31 revealed marked increase in angiogenesis. Thus, we concluded that chitosan-based copper nanocomposite efficiently enhanced cutaneous wound healing by modulation of various cells, cytokines and growth factors during different phases of healing process.

Activation of FGF receptors is a new mechanism by which strontium ranelate induces osteoblastic cell growth

BACKGROUND/AIMS

Strontium ranelate (SrRan) is an anti-osteoporotic treatment that reduces the risk of vertebral and hip fractures. Recent in vitro studies suggest that the effect of strontium ranelate on osteoblastic cell growth likely involves two processes including activation of the calcium sensing receptor (CaSR) and a yet undefined mechanism. In the present study, we investigated the CaSR-independent molecular mechanism by which SrRan stimulates osteoblast growth.

METHODS

MC3T3-E1 and primary osteoblastic cells, specific inhibitors of receptor tyrosine kinases (RTK) and western blot analysis were used to characterize the CaSR-independent mechanism in osteoblastic cells.

RESULTS

A selective inhibitor of FGF receptor but not other RTK inhibitors markedly blunted cell growth induced by SrRan in osteoblastic cells. Associated with this observation, SrRan induced rapid activation of FGFR signaling pathways such as PLCγ, FRS2, Akt, ERK1,2 and p38. FGFR-dependent stimulation of osteogenic cell growth was also observed with other cations but not with neomycin, a selective CaSR agonist. Also, in cultured conditions used in this study, MC3T3-E1 cells and primary osteoblasts did not express the CaSR.

CONCLUSION

data presented in this study suggest that activation of FGFRs is a new potential mechanism by which strontium can stimulate osteoblastic cell growth. Activation of FGFR-dependent cell growth is also observed in response to other cations suggesting that activation of FGF receptors is a new cation sensing mechanism in osteoblasts.

Phase I study of copper-binding agent ATN-224 in patients with advanced solid tumors

PURPOSE

Copper chelation reduces the secretion of many angiogenic factors and reduces tumor growth and microvascular density in animal models. ATN-224 is a second-generation analogue of ammonium tetrathiomolybdate. The aim of our phase I study was to reduce serum copper levels, as measured by ceruloplasmin, to 5 to 15 mg/dL (normal 16-60) in 14 to 21 days, to determine the pharmacokinetic profile of ATN-224 and to evaluate dose-limiting toxicities.

PATIENTS AND METHODS

Cohorts of patients were treated with escalating oral doses of ATN-224 until copper depletion followed by a titrated maintenance dose.

RESULTS

Eighteen patients received 78 cycles of ATN-224. Mean baseline ceruloplasmin was 39.6 mg/dL. The maximum administered dose was 330 mg/d where grade 3 fatigue was dose-limiting. At the maximum tolerated dose of 300 mg/d, the median time to achieve target ceruloplasmin was 21 days, and toxicities included grade 3 anemia, grade 3 neutropenia, fatigue, and sulfur eructation. ATN-224 treatment caused a significant reduction (> 90%) in RBC superoxide dismutase 1 activity and circulating endothelial cells. Pharmacokinetic data indicate greater absorption of ATN-224 and more rapid ceruloplasmin reduction when administered with a proton pump inhibitor. Stable disease of > 6 months was observed in 2 patients.

CONCLUSIONS

Oral ATN-224 is a well-tolerated therapy and at a loading dose of 300 mg/d leads to a reduction of serum ceruloplasmin levels in 80% patients within 21 days. A loading dose of 300 mg/d for 2 weeks followed by a titrated maintenance dose will be the recommended starting dose for phase II study.

Trace elemental analysis of normal, benign hypertrophic and cancerous tissues of the prostate gland using the particle-induced X-ray emission technique

Trace elemental analysis was carried out in the tissue samples of normal, benign hypertrophic and carcinoma prostate using the particle-induced X-ray emission technique. A proton beam of 3 MeV energy was used to excite the samples. The elements Cl, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Se, and Br were identified and their concentrations were estimated. It is observed that in benign tissues the concentrations of Cl, K, Zn, and Se are lower (P<0.05) and those of Cr, Fe, Ni, and Cu are higher (P<0.05 ) than in normal tissues. The concentrations of K, Ca, Zn, Se, and Br are lower (P<0.01) and those of Ti, Cr, Mn, Fe, Ni, and Cu are significantly higher (P<0.0005) in cancerous tissues than in normal tissues. Free radicals generated by elevated levels of Cr, Fe, Ni, and Cu possibly initiate and promote prostate cancer by oxidative DNA damage. The excess Cu levels in cancerous tissues support the fact that Cu promotes cancer through angiogenesis. The higher levels of Fe observed in cancerous tissues might be a consequence of tumor growth through angiogenesis. Significantly higher levels of Ni and Cr observed in carcinoma tissues support the well-established role of Ni and Cr as carcinogens. It is likely that the observed low levels of Zn and Se in cancerous tissues lead to the development of prostate cancer owing to a decrease in antioxidative defense capacity and impaired immune function of cells and also suggest that the inability to retain high levels of Zn and Se may possibly be an important factor in the development and progression of malignant prostate cells. In order to substantiate the observed elevated or deficient levels of trace elements in initiating, promoting, and inhibiting prostate cancer, several cellular and molecular studies are required.

Current status and perspective of antiangiogenic therapy for cancer: urinary cancer

Angiogenesis is considered a prerequisite for solid tumor growth. Antiangiogenic therapy reduces tumor size and extends host survival in a number of preclinical animal models. However, in humans antiangiogenic therapy is a poor promoter of tumor regression and has shown minimal effect on patient survival. In urinary cancers, such as renal cell cancer, prostate cancer, and bladder cancer, advanced refractory disease is a good candidate for antiangiogenic therapy because of its resistance to ordinary chemotherapy, radiotherapy, and hormonal therapy. Unique characteristics of molecular mechanisms underlie the induction of angiogenesis in urinary cancers. In this review, we summarize these unique mechanisms and review the results of clinical trials of antiangiogenic therapy for these cancers, discussing prospects and problems relating to antiangiogenic therapy.

The role of copper in tumour angiogenesis

Copper stimulates the proliferation and migration of endothelial cells and is required for the secretion of several angiogenic factors by tumour cells. Copper chelation decreases the secretion of many of these factors. Serum copper levels are upregulated in many human tumours and correlate with tumour burden and prognosis. Copper chelators reduce tumour growth and microvascular density in animal models. New orally active copper chelators have enabled clinical trials to be undertaken, and there are several studies ongoing. A unifying mechanism of action by which copper chelation inhibits endothelial cell proliferation and tumour secretion of angiogenic factors remains to be elucidated, but possible targets include copper-dependent enzymes, chaperones, and transporters.

A requirement for copper in angiogenesis

Although two decades have passed since copper was shown to stimulate blood vessel formation in the avascular cornea of rabbits, only recently have clinical trials established that Cu privation by diet or by Cu chelators diminishes a tumor's ability to mount an angiogenic response. These data have shed new light on the functional role of Cu in microvessel development and, of equal importance, stimulated new nutritional models of cancer therapeutic intervention.

The copper chelator trientine has an antiangiogenic effect against hepatocellular carcinoma, possibly through inhibition of interleukin-8 production

Recent studies have revealed that copper is an important cofactor for several angiogenic agents. We examined the antiangiogenic effect against hepatocellular carcinoma (HCC) of the copper chelator trientine, especially focusing on the relationship between copper and interleukin-8 (IL-8), a potent angiogenic factor produced by hepatoma cells. HuH-7 hepatoma cells were transplanted into nude mice and the growth of xenograft tumors was compared to and without administration of trientine. Using the resected tumor, microvessel density, apoptotic potential and proliferative activity were analyzed histologically and IL-8 mRNA was semiquantified by RT-PCR. In addition, HuH-7 cells were cultured in control medium, medium supplemented with copper, medium supplemented with trientine and medium supplemented with both copper and trientine. Human IL-8 levels were measured in the supernatants by ELISA. Using the extracts from cultured cells, IL-8 mRNA was semiquantified by RT-PCR. Trientine suppressed the growth of xenograft tumors significantly. Histologically, apoptotic potential was increased significantly and microvessel density, decreased. The production of IL-8 from the tumor was suppressed by trientine. In vitro, IL-8 production by HuH-7 cells in copper-containing medium was significantly greater than that in copper-free medium, and this effect was weakened when trientine was added. However, no significant change was apparent when trientine was added to the medium alone. In conclusion, the chelating effect of trientine prevented copper from functioning as a cofactor in angiogenesis, which resulted in reduced IL-8 production from HuH-7 cells.

Ca(2+) binding to both the heavy and light chains of factor VIII is required for cofactor activity

Previously, we demonstrated that Ca(2+) was necessary for the generation of cofactor activity following reconstitution of factor VIII from its isolated light chain (LC) and heavy chain (HC) but that Ca(2+) did not affect HC-LC binding affinity (Wakabayashi et al. (2001) Biochemistry 40, 10293-10300). Titration of EDTA-treated factor VIII with Ca(2+) followed by factor Xa generation assay showed a two-site binding pattern, with indicated high-affinity (K(d) = 8.9 +/- 1.8 microM) and low-affinity (K(d) = 4.0 +/- 0.6 mM) sites. Analysis by equilibrium dialysis using (45)Ca and <400 microM free Ca(2+) verified a high-affinity binding (K(d) = 18.9 +/- 3.7 microM). Preincubation of either HC or LC with 6 mM Ca(2+) followed by reassociation with the untreated complementary chain in the presence of 0.12 mM Ca(2+) failed to generate significant cofactor activity (<0.5 nM min(-1) (nM LC)(-1)). However, pretreatment of both HC and LC with 6 mM Ca(2+) followed by reassociation (at 0.12 mM Ca(2+)) generated high activity (7.5 +/- 0.4 nM min(-1) (nM LC)(-1)). Progress curves for activity regain following factor VIII-Ca(2+) association kinetics fitted well to a series reaction scheme rather than one of simple association (p < 0.0001), suggesting a multistep process which may include a Ca(2+)-dependent conformational change. These results suggest that factor VIII contains two Ca(2+) binding sites with different affinities and that active factor VIII can be reconstituted from HC and LC only when both chains are preactivated by Ca(2+).

Radiotherapy and antiangiogenic TM in lung cancer

Tetrathiomolybdate (TM) is a potent nontoxic orally delivered copper complexing agent under development for the last several years for the treatment of Wilson's disease. It has been shown to block angiogenesis in primary and metastatic tumors. Therefore, the combination of cytotoxic radiotherapy (RT) and antiangiogenic TM could target both the existing tumor and the tumor microvasculature in a comprehensive strategy. Using a Lewis lung high metastatic (LLHM) carcinoma mouse tumor model, we demonstrate that the combination of TM and RT is more effective than either used as monotherapy. We also show that their therapeutic effects are additive, with no additional toxicity. We show that TM has no significant cytotoxicity in vitro against LLHM tumor cells, further supporting the antiangiogenic mechanism for its action.

FGFR3 isoforms have distinct functions in the regulation of growth and cell morphology

We have previously cloned the alternatively spliced isoform of fibroblast growth factor receptor 3 (FGFR3DeltaAB) that lacks the acid box in the extracellular region. To understand the biological functions and signal transduction of these FGFR3 isoforms, we analyzed the effect of FGF1 in ATDC5 cells, chondroprogenitor cell lines overexpressing these isoforms. In response to FGF1, FGFR3 induced a marked cell-morphology change to a round shape, while FGFR3DeltaAB did not. Furthermore, FGFR3 induced complete growth arrest, whereas FGFR3DeltaAB induced only moderate growth inhibition. Both receptors induced the expression of the CDK inhibitor p21(CIP1). However, only FGFR3 induced STAT1 phosphorylation that mediates the transcriptional induction of p21(CIP1), although both FGFR3 isoforms could induce a strong activation of mitogen-activated protein (MAP) kinases. Taken together, the different biological responses mediated by FGFR3 and FGFR3DeltaAB appear to be due to a difference in their ability to utilize STAT1 pathway and signals involved in cell rounding.

Binding of heparin/heparan sulfate to fibroblast growth factor receptor 4

Fibroblast growth factors (FGFs) are heparin-binding polypeptides that affect the growth, differentiation, and migration of many cell types. FGFs signal by binding and activating cell surface FGF receptors (FGFRs) with intracellular tyrosine kinase domains. The signaling involves ligand-induced receptor dimerization and autophosphorylation, followed by downstream transfer of the signal. The sulfated glycosaminoglycans heparin and heparan sulfate bind both FGFs and FGFRs and enhance FGF signaling by mediating complex formation between the growth factor and receptor components. Whereas the heparin/heparan sulfate structures involved in FGF binding have been studied in some detail, little information has been available on saccharide structures mediating binding to FGFRs. We have performed structural characterization of heparin/heparan sulfate oligosaccharides with affinity toward FGFR4. The binding of heparin oligosaccharides to FGFR4 increased with increasing fragment length, the minimal binding domains being contained within eight monosaccharide units. The FGFR4-binding saccharide domains contained both 2-O-sulfated iduronic acid and 6-O-sulfated N-sulfoglucosamine residues, as shown by experiments with selectively desulfated heparin, compositional disaccharide analysis, and a novel exoenzyme-based sequence analysis of heparan sulfate oligosaccharides. Structurally distinct heparan sulfate octasaccharides differed in binding to FGFR4. Sequence analysis suggested that the affinity of the interaction depended on the number of 6-O-sulfate groups but not on their precise location.

The Role of Copper Suppression as an Antiangiogenic Strategy in Head and Neck Squamous Cell Carcinoma

OBJECTIVE

To determine whether tetrathiomolybdate (TM), a powerful chelator of copper, is capable of lowering the body stores of copper and suppressing the growth of head and neck squamous cell carcinoma in an orthotopic murine model.

STUDY DESIGN

In vivo, murine model.

METHODS

Twelve 8-week-old male C3H/HeJ mice were assigned to either a TM treatment group (n = 7) or a control group (n = 5). Serum samples were obtained from a single mouse in each group to measure the level of ceruloplasmin as a surrogate marker of total body copper on days 0, 4, and 7. Mice in both groups received a floor-of-mouth injection of 1.5 x 105 SCC VII/SF cells. After 7 to 10 days of tumor growth the treatment group received fresh water daily, to which TM was added to achieve an oral intake of 50 mg per mouse. The control group received only fresh drinking water daily. Tumor volume measurements were obtained every other day. Microvessel density counts were assessed in the tumors by Factor VIII analysis.

RESULTS

Measurable tumor growth was achieved in 100% of the mice by the tenth day. Total body copper was reduced by 28% from baseline levels in mice in the treatment group. The difference in mean tumor volume in the control group was 4.7 times greater than the TM-treated group at the completion of treatment (3004 mm3 and 633mm3, respectively). This accounted for an overall suppression rate of 79% (P =.008; two-tailed Student t test). In addition, microvessel density was reduced by 50% in the TM-treated group.

CONCLUSION

In this initial study, the first of its kind in head and neck squamous cell carcinoma, we have demonstrated the ability of TM to significantly suppress both the growth of squamous cell carcinoma and tumor vascularity in this orthotopic murine model, suggesting its potential for efficacy in the treatment of this disease in humans.

Cu(II) and Zn(II) complexes with hyaluronic acid and its sulphated derivative. Effect on the motility of vascular endothelial cells

With the aim of improving the compatibility of biomaterials to be used for the construction of cardiovascular prosthesis, we have designed bioactive macromolecules resulting from chemical modifications of hyaluronic acid (Hyal). The stability constants of Cu(II) and Zn(II) complexes with the sulphated derivative of hyaluronic acid (HyalS3.5) were evaluated. Two different complexes have been found for each metal ion, CuL, Cu(OH)2L and ZnL, Zn(OH)2L (L means the disaccharide unit of the ligands) in aqueous solution at 37 degrees C. The dihydroxo Cu(II) complex was present in high percentage at pH=7.4. On the contrary, the Zn(II) ion was present with a relatively low percentage of both complexes. The ability to stimulate endothelial cell adhesion and migration was evaluated for Hyal, HyalS3.5 and their complexes with Cu(II) and Zn(II) ions. The results revealed that Hyal and [Cu(OH)2HyalS3.5](4.5)- induced cell adhesion, while [ZnHyalS3.5](2.5)- and [Zn(OH)2HyalS3.5](4.5)- inhibited the process. The chemotactic activity of increasing concentrations of the above complexes was also evaluated, demonstrating that [Cu(OH)2HyalS3.5](4.5)- complex at 1 microM concentration was the most active in inducing cell migration. These results have been also strengthened by analysing adherent cell migration in agarose. In conclusion, sulphated hyaluronic acid coordinated to Cu(II) seems to be a promising matrix molecule for the construction of cardiovascular prosthesis.

A dimeric ternary complex of FGFR [correction of FGFR1], heparin and FGF-1 leads to an 'electrostatic sandwich' model for heparin binding

BACKGROUND

Fibroblastic growth factors (FGFs) are a family of cytokines involved in regulation of cell growth, differentiation and chemotaxis in a variety of tissue types. High-affinity FGF receptors (FGFRs) are transmembrane proteins that consist of three extracellular immunoglobulin-like domains, a transmembrane helix and an intracellular protein tyrosine kinase signalling domain. FGFRs are activated through ligand-dependent dimerization that allows trans-autophosphorylation of the tyrosine kinase domains. Heparin or heparin-like molecules, such as heparan sulphate proteoglycans, bind to both FGFs and FGFRs and are required for FGF signal transduction. At present no structure of the ternary complex for FGFR, FGF and heparin exists.

RESULTS

We have used the type-1 interleukin-1 receptor-interleukin-1 beta complex crystal structure, in which both the ligand and the receptor are homologous to those of the FGF-FGFR pair, to identify potential interactions in the FGFR-heparin-FGF ternary complex. A key feature of the modelled complex is the 'electrostatic sandwich' that is formed between the positively charged surfaces of FGF and the receptor, with the negatively charged heparin captured in between. The ternary complex places limits on the range of likely modes of receptor dimerization: one of five different dimeric receptor complexes built from the ternary complex correlates best with the experimental data.

CONCLUSIONS

The ternary complex of FGFR, FGF and heparin, derived on the basis of the homologous interleukin-1 receptor complex, is in agreement with much of the published experimental data, as is the dimeric receptor complex (FGFR-heparin-FGF)2. This work suggests that the FGF interactions seen in crystal structures, which have previously been used to predict the mode of FGF dimerization, might not be relevant to the biologically active dimeric FGFR-heparin-FGF complex.

Involvement of intermediary metabolites in the pathway of extracellular Ca2+-induced mitogen-activated protein kinase activation in human fibroblasts

Human fibroblasts in culture will grow in serum-free medium containing serum replacement factors, but without protein growth factors, as long as the Ca2+ level is 1.0-2.0 mM. When the Ca2+ is reduced to 0.1 mM, the cells stop cycling, but they can be reinduced to cycle by raising the Ca2+ level to 1.0 mM Ca2+ or to higher concentrations that result in activation of mitogen-activated protein kinase (MAPK). We now report that exposure of human fibroblasts to extracellular Ca2+ increased the level of inositol (1,4,5)-trisphosphate in the cytoplasm and caused a transient rise in the concentration of intracellular free Ca2+. Ca2+-induced MAPK activation was partly abolished by treatment of the cells with pertussis toxin. It was also decreased by treatment of cells with thapsigargin, which depletes intracellular Ca2+ stores; with phorbol 12-myristyl 13-acetate (PMA), which down-regulates protein kinase C (PKC); with the calmodulin antagonists N-(6-aminohexyl)-5-chloro-1-naphthalenesulphonamide HCl (W-7), and calmidazolium (24571); as well as with lanthanum, a Ca2+ channel inhibitor. Ca2+ stimulation did not result in phosphorylation of the c-raf-1 protein. Our results suggest that extracellular Ca2+ stimulates MAPK activation through a pathway(s) involving a pertussis toxin-sensitive G protein, phospholipase C, intracellular free Ca2+, calmodulin, and PKC.

Fibroblast growth factors as multifunctional signaling factors

The fibroblast growth factor (FGF) family consists of at least 15 structurally related polypeptide growth factors. Their expression is controlled at the levels of transcription, mRNA stability, and translation. The bioavailability of FGFs is further modulated by posttranslational processing and regulated protein trafficking. FGFs bind to receptor tyrosine kinases (FGFRs), heparan sulfate proteoglycans (HSPG), and a cysteine-rich FGF receptor (CFR). FGFRs are required for most biological activities of FGFs. HSPGs alter FGF-FGFR interactions and CFR participates in FGF intracellular transport. FGF signaling pathways are intricate and are intertwined with insulin-like growth factor, transforming growth factor-beta, bone morphogenetic protein, and vertebrate homologs of Drosophila wingless activated pathways. FGFs are major regulators of embryonic development: They influence the formation of the primary body axis, neural axis, limbs, and other structures. The activities of FGFs depend on their coordination of fundamental cellular functions, such as survival, replication, differentiation, adhesion, and motility, through effects on gene expression and the cytoskeleton.

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

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

Divalent cations and heparin/heparan sulfate cooperate to control assembly and activity of the fibroblast growth factor receptor complex

Polypeptides of the fibroblast growth factor (FGF) family are ubiquitous bioregulators within tissues whose activity is controlled by heparan sulfates within the pericellular matrix. FGF and the ectodomain of their transmembrane tyrosine kinase receptors (FGFR) exhibit heparin-binding domains that when juxtaposed in a FGF middle dotFGFR complex can accommodate a single, potentially bivalent, decameric polysaccharide chain in a ternary complex. Here we show that the interaction of heparin with FGF ligands is not affected by divalent cations. In contrast, the high affinity interaction (apparent Kd = 10 nM) of heparin with FGFR requires Ca2+ or Mg2+ at physiological concentrations. Divalent cations maintain FGFR in a heparan sulfate-dependent state in respect to FGF binding and an FGF- and heparan sulfate-dependent state in respect to autophosphorylation. A model is proposed where divalent cations and heparan sulfate cooperate to maintain FGFR in a conformation that restricts trans-phosphorylation between intracellular kinase domains. The restriction is overcome by FGF or constitutively as a common consequence of diverse mutations in FGFR associated with skeletal and craniofacial abnormalities.