Identification and assay of fibroblast growth factor receptors

Fibroblast Growth Factor 2 Attenuates Renal Ischemia-Reperfusion Injury via Inhibition of Endoplasmic Reticulum Stress

Acute kidney injury (AKI) is a serious clinical disease that is mainly caused by renal ischemia-reperfusion (I/R) injury, sepsis, and nephrotoxic drugs. The pathologic mechanism of AKI is very complex and may involve oxidative stress, inflammatory response, autophagy, apoptosis, and endoplasmic reticulum (ER) stress. The basic fibroblast growth factor (FGF2) is a canonic member of the FGF family that plays a crucial role in various cellular processes, including organ development, wound healing, and tissue regeneration. However, few studies have reported the potential therapeutic effect of FGF2 in the repair of renal ischemic injury in the past two decades. In the present study, we investigated the protective effect of FGF2 on renal I/R injury using Sprague-Dawley and NRK-52E cells. Our results showed that FGF2 significantly attenuates the apoptosis of kidney tissues after I/R injury through the inhibition of excessive ER stress. Moreover, FGF2 also alleviated the excessive ER stress and apoptosis in cultured NRK-52E cells injured by tert-Butyl hydroperoxide (TBHP). Significantly, phosphatidylinositol 3-kinase (PI3K)-selective inhibitor LY294002 and mitogen-activated protein kinase kinase (MEK)-selective inhibitor U0126 were utilized in the present study to examine the protective mechanism of FGF2. Our in vitro experimental results confirmed that both LY294002 and U0126 largely abolished the protective effect of FGF2. Taken together, the findings of the present study indicated that FGF2 attenuates I/R-induced renal epithelial apoptosis by suppressing excessive ER stress via the activation of the PI3K/AKT and MEK-ERK1/2 signaling pathways.

Vitamin D receptor ApaI polymorphism associated with progression of liver disease in Vietnamese patients chronically infected with hepatitis B virus

Background

Vitamin D derivatives and their receptor (VDR) are potent modulators of immune responses in various diseases including malignancies as well as in metabolic and infectious disorders. The impact of vitamin D receptor polymorphisms on clinical outcomes of hepatitis B virus (HBV) infection is not well understood. This study aims to investigate the potential role of VDR polymorphisms (TaqI, FokI, ApaI, and BsmI) in Vietnamese HBV infected patients and to correlate these polymorphisms with the progression of HBV-related liver disease.

Methods

Four hundred forty-three HBV infected patients of the three clinically well-defined subgroups chronic hepatitis B (CHB, n = 183), liver cirrhosis (LC, n = 89) and hepatocellular carcinoma (HCC, n = 171) and 238 healthy individuals (HC) were enrolled. VDR polymorphisms were genotyped by DNA sequencing and in-house validated ARMS assays. Logistic regression models were applied in order to determine the association of VDR polymorphisms with manifest HBV infection as well as with progression of related liver diseases mulin different genetic models.

Results

The VDR ApaI CA genotype was less frequent in HCC than in CHB patients in different genetic models (codominant model, OR = 0.5, 95%CI = 0.3–0.84, P = 0.004; dominant model, OR = 0.46, 95%CI = 0.27–0.76, P = 0.0023). In the recessive model, the genotype ApaI AA was found more frequently among HCC compared to CHB patients (OR = 2.56, 95%CI = 1.01–6.48, P = 0.04). Similarly, the ApaI CA genotype was less frequent in HCC than in non-HCC group codominant model, OR = 0.6, 95%CI = 0.4–0.98, dominant model, P = 0.04 and OR = 0.6, 95%CI = 0.38–0.90, P = 0.017). The ApaI genotypes CA and AA was significantly associated with higher levels of liver enzymes, bilirubin, and HBV DNA (P < 0.05). No association between TaqI, FokI and BsmI polymorphisms and any clinical outcome as well as liver disease progression was found.

Conclusions

Among the four investigated VDR polymorphisms, ApaI is associated with clinical outcome and liver disease progression in Vietnamese HBV infected patients.

Fibroblast growth factors, old kids on the new block

The fibroblast growth factors (FGFs) are a family of cell intrinsic regulatory peptides that control a broad spectrum of cellular activities. The family includes canonic FGFs that elicit their activities by activating the FGF receptor (FGFR) tyrosine kinase and non-canonic members that elicit their activities intracellularly and via FGFR-independent mechanisms. The FGF signaling axis is highly complex due to the existence of multiple isoforms of both ligands and receptors, as well as cofactors that include the chemically heterogeneous heparan sulfate (HS) cofactors, and in the case of endocrine FGFs, the Klotho coreceptors. Resident FGF signaling controls embryonic development, maintains tissue homeostasis, promotes wound healing and tissue regeneration, and regulates functions of multiple organs. However, ectopic or aberrant FGF signaling is a culprit for various diseases, including congenital birth defects, metabolic disorder, and cancer. The molecular mechanisms by which the specificity of FGF signaling is achieved remain incompletely understood. Since its application as a druggable target has been gradually recognized by pharmaceutical companies and translational researchers, understanding the determinants of FGF signaling specificity has become even more important in order to get into the position to selectively suppress a particular pathway without affecting others to minimize side effects.

Differential specificity of endocrine FGF19 and FGF21 to FGFR1 and FGFR4 in complex with KLB

BACKGROUND

Recent studies suggest that betaKlotho (KLB) and endocrine FGF19 and FGF21 redirect FGFR signaling to regulation of metabolic homeostasis and suppression of obesity and diabetes. However, the identity of the predominant metabolic tissue in which a major FGFR-KLB resides that critically mediates the differential actions and metabolism effects of FGF19 and FGF21 remain unclear.

METHODOLOGY/PRINCIPAL FINDINGS

We determined the receptor and tissue specificity of FGF21 in comparison to FGF19 by using direct, sensitive and quantitative binding kinetics, and downstream signal transduction and expression of early response gene upon administration of FGF19 and FGF21 in mice. We found that FGF21 binds FGFR1 with much higher affinity than FGFR4 in presence of KLB; while FGF19 binds both FGFR1 and FGFR4 in presence of KLB with comparable affinity. The interaction of FGF21 with FGFR4-KLB is very weak even at high concentration and could be negligible at physiological concentration. Both FGF19 and FGF21 but not FGF1 exhibit binding affinity to KLB. The binding of FGF1 is dependent on where FGFRs are present. Both FGF19 and FGF21 are unable to displace the FGF1 binding, and conversely FGF1 cannot displace FGF19 and FGF21 binding. These results indicate that KLB is an indispensable mediator for the binding of FGF19 and FGF21 to FGFRs that is not required for FGF1. Although FGF19 can predominantly activate the responses of the liver and to a less extent the adipose tissue, FGF21 can do so significantly only in the adipose tissue and adipocytes. Among several metabolic and endocrine tissues, the response of adipose tissue to FGF21 is predominant, and can be blunted by the ablation of KLB or FGFR1.

CONCLUSIONS

Our results indicate that unlike FGF19, FGF21 is unable to bind FGFR4-KLB complex with affinity comparable to FGFR1-KLB, and therefore, at physiological concentration less likely to directly and significantly target the liver where FGFR4-KLB predominantly resides. However, both FGF21 and FGF19 have the potential to activate responses of primarily the adipose tissue where FGFR1-KLB resides.

FGFR1 abrogates inhibitory effect of androgen receptor concurrent with induction of androgen-receptor variants in androgen receptor-negative prostate tumor epithelial cells

BACKGROUND

Despite dramatic positive effects, there is evidence that the androgen receptor (AR) may negatively influence prostate tumor progression. Understanding the AR repressor function and how it is subverted is of particular importance in anti-androgen and AR intervention strategies.

METHODS

AR, resident FGFR2IIIb, and ectopic FGFR1 were expressed by transfection in the AR-negative epithelial cell line DTE that predominates in cell culture of AR-positive androgen-responsive model Dunning R3327 rat prostate tumors. Androgen-responsiveness at transcription was measured by a luciferase reporter. Cell population growth rates were assessed by cell counts, DNA synthesis, and expression of cell cycle genes. AR variants (ARVs) were assessed by immunochemistry and nuclease protection of mRNA.

RESULTS

Expression of AR inhibited cell population growth of AR-negative DTE cells at the G1-S phase of the cell cycle. Ectopic FGFR1, but not resident FGFR2IIIb abrogated the growth inhibitory effects of AR. Appearance of ARVs was coincident with co-expression of FGFR1 and AR and abrogation of the AR-dependent inhibition of cell growth.

CONCLUSIONS

DTE cells may represent non-malignant AR-negative progenitors whose population is restricted by activation of AR in vivo. Ectopic expression of epithelial FGFR1, a common observation in tumors, overrides the inhibition of AR and thus may contribute to evolution of androgen and AR independent tumors. These results are consistent with the notion that some tumor cells are negatively restricted by AR and are unleased by androgen-deprivation or ectopic expression of FGFR1. ARV's may play a role in the bypass of the negative restrictions of AR.

Metabolic regulator betaKlotho interacts with fibroblast growth factor receptor 4 (FGFR4) to induce apoptosis and inhibit tumor cell proliferation

In organs involved in metabolic homeostasis, transmembrane α and βklothos direct FGFR signaling to control of metabolic pathways. Coordinate expression of βklotho and FGFR4 is a property of mature hepatocytes. Genetic deletion of FGFR4 or βklotho in mice disrupts hepatic cholesterol/bile acid and lipid metabolism. The deletion of FGFR4 has no effect on the proliferative response of hepatocytes after liver injury. However, its absence results in accelerated progression of dimethynitrosamine-initiated hepatocellular carcinomas, indicating that FGFR4 suppresses hepatoma proliferation. The mechanism underlying the FGFR4-mediated hepatoma suppression has not been addressed. Here we show that βklotho expression is more consistently down-regulated in human and mouse hepatomas than FGFR4. Co-expression and activation by either endocrine FGF19 or cellular FGF1 of the FGFR4 kinase in a complex with βklotho restricts cell population growth through induction of apoptotic cell death in both hepatic and nonhepatic cells. The βklotho-FGFR4 partnership caused a depression of activated AKT and mammalian target of rapamycin while activating ERK1/2 that may underlie the pro-apoptotic effect. Our results show that βklotho not only interacts with heparan sulfate-FGFR4 to form a complex with high affinity for endocrine FGF19 but also impacts the quality of downstream signaling and biological end points activated by either FGF19 or canonical FGF1. Thus the same βklotho-heparan sulfate-FGFR4 partnership that mediates endocrine control of hepatic metabolism plays a role in cellular homeostasis and hepatoma suppression through negative control of cell population growth mediated by pro-apoptotic signaling.

Structural specificity in a FGF7-affinity purified heparin octasaccharide required for formation of a complex with FGF7 and FGFR2IIIb

Variations in sulfation of heparan sulfate (HS) affect interaction with FGF, FGFR, and FGF-HS-FGFR signaling complexes. Whether structurally distinct HS motifs are at play is unclear. Here we used stabilized recombinant FGF7 as a bioaffinity matrix to purify size-defined heparin oligosaccharides. We show that only 0.2%-4% of 6 to 14 unit oligosaccharides, respectively, have high affinity for FGF7 based on resistance to salt above 0.6M NaCl. The high affinity fractions exhibit highest specific activity for interaction with FGFR2IIIb and formation of complexes of FGF7-HS-FGFR2IIIb. The majority fractions with moderate (0.30-0.6M NaCl), low (0.14-0.30M NaCl) or no affinity at 0.14M NaCl for FGF7 supported no complex formation. The high affinity octasaccharide mixture exhibited predominantly 7- and 8-sulfated components (7,8-S-OctaF7) and formed FGF7-HS-FGFR2IIIb complexes with highest specific activity. Deduced disaccharide analysis indicated that 7,8-S-OctaF7 comprised of DeltaHexA2SGlcN6S in a 2:1 ratio to a trisulfated and a variable unsulfated or monosulfated disaccharide. The inactive octasaccharides with moderate affinity for FGF7 were much more heterogenous and highly sulfated with major components containing 11 or 12 sulfates comprised of predominantly trisulfated disaccharides. This suggests that a rare undersulfated motif in which sulfate groups are specifically distributed has highest affinity for FGF7. The same motif also exhibits structural requirements for high affinity binding to dimers of FGFR2IIIb prior to binding FGF7 to form FGF7-HS-FGFR2IIIb complexes. In contrast, the majority of more highly sulfated HS motifs likely play FGFR-independent roles in stability and control of access of FGF7 to FGFR2IIIb in the tissue matrix.

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.

Hormone Refractory Prostate Cancer and Fibroblast Growth Factor Receptor

In prostate cancer, a distinct series of alterations in the fibroblast growthfactor (FGF) family occurs during the progression from a hormone-dependent to independent state that disrupts communication between stroma and epithelium and results in autonomy of cancer cells. Changes include (i) loss of FGFR2IIIb, whichbinds stromal-derived FGF-7, which promotes growth, growth limitation and differentiation and (ii) activation of FGFR1, the expression of which is normally limited to stroma, along with activation of FGFs that act on FGFR1 in an autocrine manner. Transfection of the FGFR2IIIb isoform into hormone-independent prostate cancer cells not only causes growth inhibition, but also induces differentiation. However, introduction of FGFR1 by transfection in hormone-dependent prostate cancer cells accelerates their progression to malignancy. These results suggest distinct targets for therapy aimed at both inhibition of the malignant phenotype and restoration of homeostasis.

Suppression of glioblastoma cell growth following antisense oligonucleotide-mediated inhibition of fibroblast growth factor receptor expression

Astrocytes exhibit significant changes in fibroblast growth factor receptor (FGFR) gene expression during malignant progression. These changes include induction of FGFR1 and concomitant loss of FGFR2 expression. The induction of FGFR1 is believed to endow malignant astrocytes with a selective growth advantage. Glioblastoma (the most malignant form of astrocytoma) cell lines, which exhibit the same pattern of FGFR gene expression as glioblastoma biopsies, were used to evaluate the contribution of FGFR1 expression to glioblastoma cell growth. Addition of phosphorothioate-modified antisense oligonucleotides complementary to the initiation site or the alpha exon of the FGFR1 gene suppressed growth of human glioblastoma-derived cell lines. Reverse antisense controls or antisense oligonucleotide complementary to FGFR2 had no effect on proliferation. Consistent with its growth-suppressive effect, FGFR1 antisense oligonucleotides markedly reduced expression of both FGFR1 mRNA and high-affinity bFGF binding sites, whereas FGFR1 reverse antisense control oligonucleotide had no effect. Antisense oligonucleotide targeted to the alpha exon of the FGFR1 gene suppressed alpha and beta alternatively spliced FGFR1 mRNA isoforms but did not alter the expression of related FGFR family members. Fluorescein-labeled antisense and reverse control oligonucleotides demonstrated cellular uptake and nuclear accumulation. These results indicate that alterations in FGFR expression may contribute to malignant proliferation in human astrocytomas. These findings also illustrate the high degree of selectivity that can be obtained with antisense oligonucleotides, a property that is essential for employing these reagents therapeutically.

Requirement for anticoagulant heparan sulfate in the fibroblast growth factor receptor complex

A divalent cation-dependent association between heparin or heparan sulfate and the ectodomain of the fibroblast growth factor (FGF) receptor kinase (FGFR) restricts FGF-independent trans-phosphorylation between self-associated FGFR and determines specificity for and mediates binding of activating FGF. Here we show that only the fraction of commercial heparin or rat liver heparan sulfate which binds to immobilized antithrombin formed an FGF-binding binary complex with the ectodomain of the FGFR kinase. Conversely, only the fraction of heparin that binds to immobilized FGFR inhibited Factor Xa in the presence of antithrombin. Only the antithrombin-bound fraction of heparin competed with (3)H-heparin bound to FGFR in absence of FGF, whereas both antithrombin-bound and unretained fractions competed with radiolabeled heparin bound independently to FGF-1 and FGF-2. The antithrombin-bound fraction of heparin was required to support the heparin-dependent stimulation of DNA synthesis of endothelial cells by FGF-1. The requirement for divalent cations and the antithrombin-binding motif distinguish the role of heparan sulfate as an integral subunit of the FGFR complex from the wider range of effects of heparan sulfates and homologues on FGF signaling through FGFR-independent interactions with FGF.

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.

Vascular remodeling in response to altered blood flow is mediated by fibroblast growth factor-2

Vascular structures adapt to changes in blood flow by adjusting their diameter accordingly. The factors mediating this process are only beginning to be identified. We have recently established a mouse model of arterial remodeling in which flow in the common carotid artery is interrupted by ligation of the vessel near the carotid bifurcation, resulting in a dramatic reduction in vessel diameter as a consequence of inward remodeling and intimal lesion formation. In the present study, we used this model to determine the role of fibroblast growth factor-2 (FGF-2) in the remodeling response by maintaining neutralizing serum levels of a mouse monoclonal antibody against FGF-2 for 4 weeks. Morphometric analysis revealed that intimal lesion formation was not affected by the antibody. However, lumen narrowing was significantly inhibited, resulting in a greater than 3-fold increase in lumen area in anti-FGF-2-treated animals compared with controls. Treatment with anti-FGF-2 antibody significantly inhibited the reduction in vessel diameter (inward remodeling) and shortening of the internal elastic lamina in the ligated vessel. In addition, anti-FGF-2 treatment also caused outward remodeling of the contralateral carotid artery. These findings identify FGF-2 as an important factor in vascular remodeling, and its effects are likely to be mediated by increasing vascular tone. The results are consistent with the recent observation of reduced vascular tone in the FGF-2-deficient mouse.

Development of novel monoclonal antibodies for the analysis of functional sites in FGF-2

Fibroblast growth factor 2 (FGF-2) can function as a potent mitogen, as well as a survival factor for a variety of mammalian cell types. The biological effects of FGF-2 are mediated by its interaction with two types of cellular binding sites: (1) high affinity tyrosine kinase receptors; and (2) low affinity heparan sulfate proteoglycans (HSPGs) on the cell surface. Although numerous FGF-2 antibodies have been used previously to analyze its biological actions, few studies have utilized antibodies to analyze domains within FGF-2 involved in its interactions with the two binding sites. In this report, we describe the generation and use of two monoclonal antibodies against human recombinant FGF-2 (254F1 and 256A12) that inhibit FGF-2 function. However, these antibodies appear to target preferentially different domains within the FGF-2 molecule, and therefore differentially influence the interactions of FGF-2 with its low and high affinity receptors. 254F1 is a more effective inhibitor of the high affinity, receptor tyrosine kinase binding site, whereas 256A12 appears to be a better inhibitor of the low affinity, HSPG interactions. We also demonstrate that the two antibodies are potent inhibitors of FGF-2 stimulated vascular cell proliferation, and as such have potential use in the treatment of vascular hyperproliferative diseases.

The heparan sulfate-fibroblast growth factor family: diversity of structure and function

The fibroblast growth factor (FGF) receptor complex is a ubiquitous regulator of development and adult tissue homeostasis that bridges the peri-cellular matrix and the intracellular environment. Diverse members of the FGF polypeptide family, the FGF receptor tyrosine kinase (FGFRTK) family and the FGF receptor heparan sulfate proteoglycan (FGFRHS) family combine to result in active and specific FGFR signal transduction complexes. Regulated alternate splicing and combination of variant subdomains give rise to diversity of FGFRTK monomers. Divalent cations cooperate with the FGFRHS to conformationally restrict FGFRTK trans-phosphorylation, which causes depression of kinase activity and facilitates appropriate activation of the FGFR complex by FGF. Diffusional and conformational molecular models of the oligomeric FGFR complex are presented to explain how different point mutations in the FGFRTK commonly cause craniofacial and skeletal abnormalities of graded severity by graded increases in FGF-independent activity of total FGFR complexes. The role of the FGF family in liver growth and function and in prostate tumor progression is discussed.

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

Fibroblast growth factor-7 (FGF-7) and a specific splice variant of the FGF tyrosine kinase receptor family (FGFR2IIIb) constitute a paracrine signaling system from stroma to epithelium. Different effects of the manipulation of cellular heparan sulfates and heparin on activities of FGF-7 relative to FGF-1 in epithelial cells suggest that pericellular heparan sulfates may regulate the activity of FGF-7 by a different mechanism than other FGFs. In this report, we employ the heparan sulfate-binding protein, protamine sulfate, to reversibly block cellular heparan sulfates. Protamine sulfate, which does not bind significantly to FGF-7 or FGFR2IIIb, inhibited FGF-7 activities, but not those of epidermal growth factor. The inhibition was overcome by increasing the concentrations of FGF-7 or heparin. Heparin was essential for binding of FGF-7 to recombinant FGFR2IIIb expressed in insect cells or FGFR2IIIb purified away from cell products. These results suggest that, similar to other FGF polypeptides, heparan sulfate within the pericellular matrix is required for activity of FGF-7. Differences in response to heparin and alterations in the BULK heparan sulfate content of cells likely reflect FGF-specific differences in the cellular repertoire of multivalent heparan sulfate chains required for assembly and activation of the FGF signal transduction complex.

A homeo-interaction sequence in the ectodomain of the fibroblast growth factor receptor

Interaction of fibroblast growth factor receptors (FGFR) sufficient for a trans-phosphorylation event in which one intracellular domain is substrate for the other is essential for signal transduction. By analysis of the direct interaction of recombinant constructions co-expressed in baculoviral-infected insect cells, we identified a 17-amino acid sequence that is required for the stable interaction between ectodomains of FGFR. The sequence 160ERSPHRPILQAGLPANK176 (Glu160-Lys176) connects immunoglobulin modules II and III. In insect cells, the interaction between Glu160-Lys176 domains occurs independently of intact heparin or FGF binding domains. The sequence is not required for the binding of heparin or FGF-1, but is essential for mitogenic activity of the FGFR kinase in mammalian cells. The results support a model in which the homeo-interaction between Glu160-Lys176 in the ectodomain contributes to the interaction between intracellular domains in mammalian cell membranes (Kan, M., Wang, F., Kan, M., To, B., Gabriel, J. L., and McKeehan, W. L. (1996) J. Biol. Chem. 271, 26143-26148). We propose that the Glu160-Lys176 domain plays a pivotal role in restriction of the interaction between kinases by pericellular matrix heparan sulfate proteoglycan and divalent cations. Restrictions are overcome by FGF or constitutively by diverse gain of function mutations which cause skeletal and craniofacial abnormalities.

Molecular modeling and deletion mutagenesis implicate the nuclear translocation sequence in structural integrity of fibroblast growth factor-1

The sequence NYKKPKL in the NH2 terminus of fibroblast growth factor (FGF)-1 has been proposed to affect the long term activities of FGF-1 through its function as a nuclear translocation signal or its role in stabilization of the structure required to sustain binding and activation of the transmembrane receptor kinase. A dynamic molecular model of FGF-1 docked into a duplex of the FGF receptor ectodomain and a hexadecameric heparin chain suggests that the NYKKPKL sequence does not directly interact with heparin or the receptor, but rather the lysine-leucine residues within the sequence indirectly stabilize a major receptor-binding domain. Concurrent with a marked increase in dependence on exogenous heparin for optimal activity, sequential deletion of residues in the NYKKPKL sequence in FGF-1 resulted in a progressive loss of thermal stability, resistance to protease, mitogenic activity, and affinity for the transmembrane receptor. The largest change resulted from deletion of the entire sequence through the lysine-leucine residues. In the presence of sufficiently high concentrations of heparin, the deletion mutants exhibited mitogenic activity equal to wild-type FGF-1. The results confirm that a primary role of the NYKKPKL sequence domain is to maintain the structural integrity of FGF-1 required for optimal binding to and activation of the heparan sulfate-transmembrane receptor complex.

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.

Basic fibroblast growth factor activates calcium channels in neonatal rat cardiomyocytes

Basic fibroblast growth factor (bFGF) is a potent mitogen for many cell lineages including fetal cardiomyocytes. Furthermore, bFGF has been shown to modify gene expression, in vitro, in adult nonproliferative ventricular myocytes. This effect is suspected to be partly responsible for the genetic modifications that occur in vivo under pathophysiological conditions such as ischemia or pressure overload and that lead to myocardial hypertrophy. However, little is known about the first steps of the molecular mechanisms that take place soon after cell activation by bFGF. In this study, using biochemical and electrophysiological approaches, we have established, on cardiomyocytes cultured from neonatal rat ventricles, that (i) differentiated beating cells express at least two classes of bFGF-receptors having high and low affinity (Kd = 10 +/- 2 pM and 1 +/- 0.5 nM); (ii) the stimulation of these bFGF receptors promotes an increase in the beating frequencies of cultured cardiomyocytes (40 +/- 10%); (iii) bFGF provokes the activation of poorly specific and voltage-independent calcium channels (12pS); (iv) inositol 1,4,5-trisphosphate enhances similar bFGF-induced Ca2+ currents and is therefore suspected to be a second messenger triggering this activation. These results support the presence, in cultured cardiomyocytes, of new calcium channels whose activation after bFGF binding may be partly responsible for the cell response to this growth factor.

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.

Inositolhexakisphosphate (InsP6): an antagonist of fibroblast growth factor receptor binding and activity

Fibroblast growth factors (FGF), which have been implicated in tumor cell growth and angiogenesis, have biological activities that appear to be mediated by both heparinlike extracellular matrix sites and transmembrane tyrosine kinase receptor sites. In the present study, we demonstrated that inositolhexakisphosphate (InsP6) inhibits basic FGF (bFGF) binding to heparin. Our spectrofluorometric analyses demonstrated that InsP6 not only bound to bFGF, presumably within the bFGF heparin-binding domain, but also protected bFGF from degradation by trypsin. Also, InsP6 inhibited the cellular binding of bFGF and other fibroblast growth factor family members such as acidic FGF (aFGF) and K-FGF in a saturable and dose-dependent manner. Furthermore, concentrations as low as 100 microM InsP6 inhibited bFGF-induced DNA synthesis in AKR-2B fibroblasts, as well as the growth of bFGF- and K-FGF-transfected NIH/3T3 cells. Together, these results indicate that InsP6 may serve as a useful antagonist of FGF activity.

Modification of alternative messenger RNA splicing of fibroblast growth factor receptors in human cardiac allografts during rejection

Accelerated coronary atherosclerosis in cardiac transplants (cardiac allograft vasculopathy, CAV) is characterized by coronary intimal hyperplasia. Acidic fibroblast growth factor (aFGF) is a potent mitogen for vascular smooth muscle cells and endothelial cells, and its expression is increased in cardiac allografts, suggesting it may play a role in the pathogenesis of CAV. The activity of aFGF is dependent on binding to transmembrane receptors. To investigate whether receptors for aFGF are also induced after transplantation, polymerase chain reaction, in situ hybridization, and immunohistochemistry were used to analyze expression of four receptors for aFGF (FGFR1-FGFR4). Expression of mRNA encoding extracellular immunoglobulin-like domains of FGFR1 was increased 35-fold in cardiac allografts compared with normal hearts and was predominantly present in cardiac myocytes and vascular structures. Alternatively spliced mRNA that encodes transmembrane forms of FGFR1, which contain the signal-transducing tyrosine kinase domains, was induced in allografts during rejection, in infiltrating cells, vascular structures, and myocytes. In vitro experiments showed that differential expression of FGF receptor isoforms was induced by aFGF, and also by IL-6 and TGF-beta, which are expressed in cardiac allografts during rejection. The results show that expression of both aFGF and its receptors is altered in cardiac allografts and suggest that these events are important in the pathogenesis of CAV.

Control of fibroblast growth factor receptor kinase signal transduction by heterodimerization of combinatorial splice variants

A differentiated liver cell (HepG2), which exhibits a dose-dependent growth-stimulatory and growth-inhibitory response to heparin-binding fibroblast growth factor type 1 (FGF-1), displays high- and low-affinity receptor phenotypes and expresses specific combinatorial splice variants alpha 1, beta 1, and alpha 2 of the FGF receptor (FGF-R) gene (flg). The extracellular domains of the alpha and beta variants consist of three and two immunoglobulin loops, respectively, while the intracellular variants consist of a tyrosine kinase (type 1) isoform and a kinase-defective (type 2) isoform. The type 2 isoform is also devoid of the two major intracellular tyrosine autophosphorylation sites (Tyr-653 and Tyr-766) in the type 1 kinase. An analysis of ligand affinity, dimerization, autophosphorylation, and interaction with src homology region 2 (SH2) substrates of the recombinant alpha 1, beta 1, and alpha 2 isoforms was carried out to determine whether dimerization of the combinatorial splice variants might explain the dose-dependent opposite mitogenic effects of FGF. Scatchard analysis indicated that the alpha and beta isoforms exhibit low and high affinity for ligand, respectively. The three combinatorial splice variants dimerized in all combinations. FGF enhanced dimerization and kinase activity, as assessed by receptor autophosphorylation. Phosphopeptide analysis revealed that phosphorylation of Tyr-653 was reduced relative to phosphorylation of Tyr-766 in the type 1 kinase component of heterodimers of the type 1 and type 2 isoforms. The SH2 domain substrate, phospholipase C gamma 1 (PLC gamma 1), associated with the phosphorylated type 1-type 2 heterodimers but was phosphorylated only in preparations containing the type 1 kinase homodimer. The results suggest that phosphorylation of Tyr-653 within the kinase catalytic domain, but not Tyr-766 in the COOH-terminal domain, may be stringently dependent on a trans intermolecular mechanism within FGF-R kinase homodimers. Although phosphotyrosine 766 is sufficient for interaction of PLC gamma 1 and other SH2 substrates with the FGF-R kinase, phosphorylation and presumably activation of substrates require the kinase homodimer and phosphorylation of Tyr-653. We propose that complexes of phosphotyrosine 766 kinase monomers and SH2 domain signal transducers may constitute unactivated presignal complexes whose active or inactive fate depends on homodimerization with a kinase or heterodimerization with a kinase-defective monomer, respectively. The results suggest a mechanism for control of signal transduction by different concentrations of ligand through heterodimerization of combinatorial splice variants from the same receptor gene.

Control of fibroblast growth factor receptor kinase signal transduction by heterodimerization of combinatorial splice variants

A differentiated liver cell (HepG2), which exhibits a dose-dependent growth-stimulatory and growth-inhibitory response to heparin-binding fibroblast growth factor type 1 (FGF-1), displays high- and low-affinity receptor phenotypes and expresses specific combinatorial splice variants alpha 1, beta 1, and alpha 2 of the FGF receptor (FGF-R) gene (flg). The extracellular domains of the alpha and beta variants consist of three and two immunoglobulin loops, respectively, while the intracellular variants consist of a tyrosine kinase (type 1) isoform and a kinase-defective (type 2) isoform. The type 2 isoform is also devoid of the two major intracellular tyrosine autophosphorylation sites (Tyr-653 and Tyr-766) in the type 1 kinase. An analysis of ligand affinity, dimerization, autophosphorylation, and interaction with src homology region 2 (SH2) substrates of the recombinant alpha 1, beta 1, and alpha 2 isoforms was carried out to determine whether dimerization of the combinatorial splice variants might explain the dose-dependent opposite mitogenic effects of FGF. Scatchard analysis indicated that the alpha and beta isoforms exhibit low and high affinity for ligand, respectively. The three combinatorial splice variants dimerized in all combinations. FGF enhanced dimerization and kinase activity, as assessed by receptor autophosphorylation. Phosphopeptide analysis revealed that phosphorylation of Tyr-653 was reduced relative to phosphorylation of Tyr-766 in the type 1 kinase component of heterodimers of the type 1 and type 2 isoforms. The SH2 domain substrate, phospholipase C gamma 1 (PLC gamma 1), associated with the phosphorylated type 1-type 2 heterodimers but was phosphorylated only in preparations containing the type 1 kinase homodimer. The results suggest that phosphorylation of Tyr-653 within the kinase catalytic domain, but not Tyr-766 in the COOH-terminal domain, may be stringently dependent on a trans intermolecular mechanism within FGF-R kinase homodimers. Although phosphotyrosine 766 is sufficient for interaction of PLC gamma 1 and other SH2 substrates with the FGF-R kinase, phosphorylation and presumably activation of substrates require the kinase homodimer and phosphorylation of Tyr-653. We propose that complexes of phosphotyrosine 766 kinase monomers and SH2 domain signal transducers may constitute unactivated presignal complexes whose active or inactive fate depends on homodimerization with a kinase or heterodimerization with a kinase-defective monomer, respectively. The results suggest a mechanism for control of signal transduction by different concentrations of ligand through heterodimerization of combinatorial splice variants from the same receptor gene.

An essential heparin-binding domain in the fibroblast growth factor receptor kinase

Heparin or heparin-like heparan sulfate proteoglycans are obligatory for activity of the heparin-binding fibroblast growth factor (FGF) family. Heparin interacts independently of FGF ligand with a specific sequence (K18K) in one of the immunoglobulin-like loops in the extracellular domain of the FGF receptor tyrosine kinase transmembrane glycoprotein. A synthetic peptide corresponding to K18K inhibited heparin and heparin-dependent FGF binding to the receptor. K18K and an antibody to K18K were antagonists of FGF-stimulated cell growth. Point mutations of lysine residues in the K18K sequence abrogated both heparin- and ligand-binding activities of the receptor kinase. The results indicate that the FGF receptor is a ternary complex of heparan sulfate proteoglycan, tyrosine kinase transmembrane glycoprotein, and ligand.

Receptor phenotype underlies differential response of hepatocytes and nonparenchymal cells to heparin-binding fibroblast growth factor type 1 (aFGF) and type 2 (bFGF)

Heparin-binding fibroblast growth factors (HBGF) have been implicated in the regeneration of both parenchymal and nonparenchymal cells of the liver. The response to and phenotype of hepatocyte receptors for HBGF-1 (acidic fibroblast growth factor) and HBGF-2 (basic fibroblast growth factor) were compared to keratinocytes, fibroblasts, and endothelial cells. HBGF-1 stimulated DNA synthesis in hepatocytes, keratinocytes, fibroblasts, and endothelial cells whereas activity of HBGF-2 was limited to fibroblasts and endothelial cells. HBGF-2 antagonized the mitogenic activity of HBGF-1 for hepatocytes and keratinocytes. Hepatocytes and keratinocytes exhibited both high- and low-affinity, nonmatrix receptor sites for HBGF-1, but only low-affinity sites for HBGF-2. The mesenchymal cells displayed only high-affinity sites for both HBGF-1 and HBGF-2. Northern blot and immunochemical analysis revealed that the expression of HBGF receptor genes bek and flg are partitioned between normal hepatocytes and nonparenchymal cells, respectively. Expression of epithelial cell-specific, mesenchymal cell-derived HBGF-7 (keratinocyte growth factor) mRNA in regenerating liver tissue was undetectable relative to HBGF-1. The results support a multifunctional role of HBGF-1 acting through different receptor phenotypes in hepatocyte and nonparenchymal cells during liver regeneration.