A novel form of fibroblast growth factor receptor 2. Alternative splicing of the third immunoglobulin-like domain confers ligand binding specificity

Changes in fibroblast growth factor receptors-1c, -2c, -3c, and -4 mRNA in granulosa and theca cells during ovarian follicular growth in dairy cattle

The various fibroblast growth factors (FGF) regulate their function via binding to 4 main FGF receptor (FGFR) subtypes and their splice variants, FGFR1b, FGF1c, FGFR2b, FGFR2c and FGFR3c and FGFR4, but which of these FGFR are expressed in the granulosa (GC) and theca cells (TC), the 2 main cell layers of ovarian follicles, or change during follicular development is unknown. We hypothesized that FGFR1c, FGFR2c and FGFR3c (but not FGFR4) gene expression in GC (but not TC) would change with follicular development. Hence, the objective of this study was to determine if abundance of FGFR1c, FGFR2c, FGFR3c, and FGFR4 mRNA change according to follicular size, steroidogenic status, and days post-ovulation during growth of first-wave dominant follicles in Holstein cattle exhibiting regular estrous cycles. Estrous cycles of non-lactating dairy cattle were synchronized, and ovaries were collected on either d 3 to 4 (n = 8) or d 5 to 6 (n = 8) post-ovulation for GC and TC RNA extraction from small (1-5 mm), medium (5.1 to 8 mm) or large (8.1-18 mm) follicles for real-time PCR analysis. In GC, FGFR1c and FGFR2c mRNA relative abundance was greater in estrogen (E2)-inactive (ie, concentrations of E2 < progesterone, P4) follicles of all sizes than in GC from large E2-active follicles (ie, E2 > P4), whereas FGFR3c and FGFR4 mRNA abundance did not significantly differ among follicle types or days post-estrus. In TC, medium E2-inactive follicles had greater FGFR1c and FGFR4 mRNA abundance than large E2-active and E2-inactive follicles on d 5 to 6 post-ovulation whereas FGFR2c and FGFR3c mRNA abundance did not significantly differ among follicle types or day post-estrus. In vitro experiments revealed that androstenedione increased abundance of FGFR1c, FGFR2c and FGFR4 mRNA in GC whereas estradiol decreased FGFR2c mRNA abundance. Neither androstenedione nor estradiol affected abundance of the various FGFR mRNAs in cultured TC. Taken together, the findings that FGFR1c and FGFR2c mRNA abundance was less in GC of E2-active follicles and FGFR1c and FGFR4 mRNA was greater in TC of medium inactive follicles at late than at early growing phase of the first dominant follicle support an anti-differentiation role for FGF and their FGFR as well as support the idea that steroid-induced changes in FGF and their receptors may regulate selection of dominant follicles in cattle.

Antitumor effect of memantine is related to the formation of the splicing isoform of GLG1, a decoy FGF‑binding protein

Drug repositioning is a strategy for repurposing the approved or investigational drugs that are outside the scope of the original medical indication. Memantine is used as a non‑competitive N‑methyl‑D‑aspartate receptor antagonist to prevent glutamate‑mediated excitotoxicity in Alzheimer's disease, and is one of the promising agents which is utilized for the purpose of cancer therapy. However, the association between memantine and Golgi glycoprotein 1 (GLG1), an intracellular fibroblast growth factor receptor, in cancers has not yet been clarified. The present study analyzed the expression and location of GLG1 in tumor cells treated with memantine. Memantine was found to suppress the growth of malignant glioma and breast cancer cells in a concentration‑dependent manner. The mRNA expression of GLG1 was upregulated in a concentration‑dependent manner, and the splicing variant profiles were altered in all cell lines examined. The results of western blot analysis revealed an increase in the full‑length and truncated forms of GLG1. Moreover, GLG1 spread in the cytosol of memantine‑treated cells, whereas it localized in the Golgi apparatus in control cells. Since GLG1 functions as a decoy FGF receptor, the modulation of GLG1 may prove to be one of the mechanisms underlying the cancer‑suppressive effects of memantine.

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.

Biological Significance and Targeting of the FGFR Axis in Cancer

The pleiotropic effects of fibroblast growth factors (FGFs), the widespread expression of all seven signalling FGF receptors (FGFRs) throughout the body, and the dramatic phenotypes shown by many FGF/R knockout mice, highlight the diversity, complexity and functional importance of FGFR signalling. The FGF/R axis is critical during normal tissue development, homeostasis and repair. Therefore, it is not surprising that substantial evidence also pinpoints the involvement of aberrant FGFR signalling in disease, including tumourigenesis. FGFR aberrations in cancer include mutations, gene fusions, and amplifications as well as corrupted autocrine/paracrine loops. Indeed, many clinical trials on cancer are focusing on targeting the FGF/FGFR axis, using selective FGFR inhibitors, nonselective FGFR tyrosine kinase inhibitors, ligand traps, and monoclonal antibodies and some have already been approved for the treatment of cancer patients. The heterogeneous tumour microenvironment and complexity of FGFR signalling may be some of the factors responsible for the resistance or poor response to therapy with FGFR axis-directed therapeutic agents. In the present review we will focus on the structure and function of FGF(R)s, their common irregularities in cancer and the therapeutic value of targeting their function in cancer.

Bioactivity Determination of a Therapeutic Recombinant Human Keratinocyte Growth Factor by a Validated Cell-based Bioassay

The therapeutic recombinant human keratinocyte growth factor 1 (rhKGF-1) was approved by the FDA for oral mucositis resulting from hematopoietic stem cell transplantation for hematological malignancies in 2004. However, no recommended bioassay for rhKGF-1 bioactivity has been recorded in the U.S. Pharmacopoeia. In this study, we developed an rhKGF-1-dependent bioassay for determining rhKGF-1 bioactivity based on HEK293 and HaCat cell lines that stably expressed the luciferase reporter driven by the serum response element (SRE) and human fibroblast growth factor receptor (FGFR2) IIIb. A good responsiveness to rhKGF-1 and rhKGF-2 shared by target HEK293/HaCat cell lines was demonstrated. Our stringent validation was completely focused on specificity, linearity, accuracy, precision, and robustness according to the International Council for Harmonization (ICH) Q2 (R1) guidelines, AAPS/FDA Bioanalytical Workshop and the Chinese Pharmacopoeia. We confirmed the reliability of the method in determining rhKGF bioactivity. The validated method is highly timesaving, sensitive, and simple, and is especially valuable for providing information for quality control during the manufacture, research, and development of therapeutic rhKGF.

High molecular weight FGF2 isoforms demonstrate canonical receptor-mediated activity and support human embryonic stem cell self-renewal

Basic fibroblast growth factor (FGF2) is a highly pleiotropic member of a large family of growth factors with a broad range of activities, including mitogenesis and angiogenesis (Ornitz et al., 1996; Zhang et al., 2006), and it is known to be essential for maintenance of balance between survival, proliferation, and self-renewal in human pluripotent stem cells (Eiselleova et al., 2009; Zoumaro-Djayoon et al., 2011). A single FGF2 transcript can be translated into five FGF2 protein isoforms, an 18kDa low molecular weight (LMW) isoform and four larger high molecular weight (HMW) isoforms (Arese et al., 1999; Arnaud et al., 1999). As they are not generally secreted, high molecular weight (HMW) FGF2 isoforms have predominantly been investigated intracellularly; only a very limited number of studies have investigated their activity as extracellular factors. Here we report over-expression, isolation, and biological activity of all recombinant human FGF2 isoforms. We show that HMW FGF2 isoforms can support self-renewal of human embryonic stem cells (hESCs) in vitro. Exogenous supplementation with HMW FGF2 isoforms also activates the canonical FGFR/MAPK pathway and induces mitogenic activity in a manner similar to that of the 18kDa FGF2 isoform. Though all HMW isoforms, when supplemented exogenously, are able to recapitulate LMW FGF2 activity to some degree, it appears that certain isoforms tend to do so more poorly, demonstrating a lesser functional response by several measures. A better understanding of isoform-specific FGF2 effects will lead to a better understanding of developmental and pathological FGF2 signaling.

Expression of Fibroblast Growth Factor 10 Is Correlated with Poor Prognosis in Gastric Adenocarcinoma

Fibroblast growth factor receptor 2 (FGFR2) has been proved to be a significant prognostic factor and a potential therapeutic target in several types of cancer, including gastric cancer. FGFR2 consists two isoforms: FGFR2-IIIb and FGFR2-IIIc, which can be stimulated by different ligands and trigger different downstream signaling pathways. As a specific ligand to FGFR2-IIIb, fibroblast growth factor 10 (FGF10) is expressed in the gastric mesenchyme cell and is involved in stomach development and morphogenesis, but its expression and clinical significance is not well elucidated in gastric cancer. We analyzed FGF10 expression by immunohistochemistry in 178 samples of gastric adenocarcinoma (134 male and 44 female patients, with the average age of 63.2 years old and the average follow-up of 21.6 months). Using the arbitrarily scoring method based on positive cell percentage and staining intensity, we sub-divided the patients into FGF10 high-expression group (58 patients) and low-expression group (120 patients). We thus found that FGF10 expression is significantly associated with lymph node invasion (P = 0.004) and distant metastasis (P = 0.032). Importantly, FGF10 expression is an independent unfavorable prognostic factor (P = 0.042). Moreover, FGF10 knockdown significantly decreased the migration of cultured gastric adenocarcinoma cells, suggesting that FGF10 could promote the invasion of gastric adenocarcinoma. In conclusion, FGF10 expression was identified as a poor prognostic biomarker in gastric adenocarcinoma, and FGF10 could promote the invasion of gastric cancer cells. We suggest that FGF10 could be a potential and promising drug target in gastric adenocarcinoma.

A meta-analysis of genome-wide association studies for adiponectin levels in East Asians identifies a novel locus near WDR11-FGFR2

Blood levels of adiponectin, an adipocyte-secreted protein correlated with metabolic and cardiovascular risks, are highly heritable. Genome-wide association (GWA) studies for adiponectin levels have identified 14 loci harboring variants associated with blood levels of adiponectin. To identify novel adiponectin-associated loci, particularly those of importance in East Asians, we conducted a meta-analysis of GWA studies for adiponectin in 7827 individuals, followed by two stages of replications in 4298 and 5954 additional individuals. We identified a novel adiponectin-associated locus on chromosome 10 near WDR11-FGFR2 (P = 3.0 × 10(-14)) and provided suggestive evidence for a locus on chromosome 12 near OR8S1-LALBA (P = 1.2 × 10(-7)). Of the adiponectin-associated loci previously described, we confirmed the association at CDH13 (P = 6.8 × 10(-165)), ADIPOQ (P = 1.8 × 10(-22)), PEPD (P = 3.6 × 10(-12)), CMIP (P = 2.1 × 10(-10)), ZNF664 (P = 2.3 × 10(-7)) and GPR109A (P = 7.4 × 10(-6)). Conditional analysis at ADIPOQ revealed a second signal with suggestive evidence of association only after conditioning on the lead SNP (Pinitial = 0.020; Pconditional = 7.0 × 10(-7)). We further confirmed the independence of two pairs of closely located loci (<2 Mb) on chromosome 16 at CMIP and CDH13, and on chromosome 12 at GPR109A and ZNF664. In addition, the newly identified signal near WDR11-FGFR2 exhibited evidence of association with triglycerides (P = 3.3 × 10(-4)), high density lipoprotein cholesterol (HDL-C, P = 4.9 × 10(-4)) and body mass index (BMI)-adjusted waist-hip ratio (P = 9.8 × 10(-3)). These findings improve our knowledge of the genetic basis of adiponectin variation, demonstrate the shared allelic architecture for adiponectin with lipids and central obesity and motivate further studies of underlying mechanisms.

Functional roles of fibroblast growth factor receptors (FGFRs) signaling in human cancers

The fibroblast growth factor receptors (FGFRs) regulate important biological processes including cell proliferation and differentiation during development and tissue repair. Over the past decades, numerous pathological conditions and developmental syndromes have emerged as a consequence of deregulation in the FGFRs signaling network. This review aims to provide an overview of FGFR family, their complex signaling pathways in tumorigenesis, and the current development and application of therapeutics targeting the FGFRs signaling for treatment of refractory human cancers.

O-GlcNAc cycling: a link between metabolism and chronic disease

To maintain homeostasis under variable nutrient conditions, cells rapidly and robustly respond to fluctuations through adaptable signaling networks. Evidence suggests that the O-linked N-acetylglucosamine (O-GlcNAc) posttranslational modification of serine and threonine residues functions as a critical regulator of intracellular signaling cascades in response to nutrient changes. O-GlcNAc is a highly regulated, reversible modification poised to integrate metabolic signals and acts to influence many cellular processes, including cellular signaling, protein stability, and transcription. This review describes the role O-GlcNAc plays in governing both integrated cellular processes and the activity of individual proteins in response to nutrient levels. Moreover, we discuss the ways in which cellular changes in O-GlcNAc status may be linked to chronic diseases such as type 2 diabetes, neurodegeneration, and cancers, providing a unique window through which to identify and treat disease conditions.

Fluorescence-based alternative splicing reporters for the study of epithelial plasticity in vivo

Alternative splicing generates a vast diversity of protein isoforms from a limited number of protein-coding genes, with many of the isoforms possessing unique, and even contrasting, functions. Fluorescence-based splicing reporters have the potential to facilitate studies of alternative splicing at the single-cell level and can provide valuable information on phenotypic transitions in almost real time. Fibroblast growth factor receptor 2 (FGFR2) pre-mRNA is alternatively spliced to form the epithelial-specific and mesenchymal-specific IIIb and IIIc isoforms, respectively, which are useful markers of epithelial-mesenchymal transitions (EMT). We have used our knowledge of FGFR2 splicing regulation to develop a fluorescence-based reporter system to visualize exon IIIc regulation in vitro and in vivo. Here we show the application of this reporter system to the study of EMT in vitro in cell culture and in vivo in transgenic mice harboring these splicing constructs. In explant studies, the reporters revealed that FGFR2 isoform switching is not required for keratinocyte migration during cutaneous wound closure. Our results demonstrate the value of the splicing reporters as tools to study phenotypic transitions and cell fates at single cell resolution. Moreover, our data suggest that keratinocytes migrate efficiently in the absence of a complete EMT.

Characterization of FGF receptor expression in human neutrophils and their contribution to chemotaxis

Several members of the fibroblast growth factor (FGF) family are potent endothelial cell (EC) mitogens and angiogenic factors, and their activities can be mediated by four tyrosine kinase receptors (FGFR1–4). In addition, FGFs can induce the release of inflammatory mediators by ECs and the expression of adhesion molecules at their surface, thereby favoring the recruitment and transvascular migration of inflammatory cells such as neutrophils. Neither the expression nor the biological activities that could be mediated by FGFRs have been investigated in human neutrophils. By biochemical and cytological analyses, we observed that purified circulating human neutrophils from healthy individuals expressed varying levels of FGFRs in their cytosol and at their cytoplasmic membrane. FGFR-2 was identified as the sole cell surface receptor, with FGFR-1 and -4 localizing in the cytosol and FGFR-3 being undetectable. We assessed the capacity of FGF-1 and FGF-2 to induce neutrophil chemotaxis in a modified Boyden microchamber and observed that they increase neutrophil transmigration at 10−10 and 10−9 M and by 1.77- and 2.34-fold, respectively, as compared with PBS-treated cells. Treatment with a selective anti-FGFR-2 antibody reduced FGF-1-mediated chemotaxis by 75% and abrogated the effect of FGF-2, while the blockade of FGFR-1 and -4 partially inhibited (15–40%) FGF-chemotactic activities. In summary, our data are the first to report the expression of FGF receptors in human neutrophils, with FGF-1 and FGF-2 promoting neutrophil chemotaxis mainly through FGFR-2 activation.

FGF10/FGFR2b signaling is essential for cardiac fibroblast development and growth of the myocardium

The epicardium serves as a source of growth factors that regulate myocardial proliferation and as a source of epicardial-derived cells (EPDC), which give rise to interstitial cardiac fibroblasts and perivascular cells. These progenitors populate the compact myocardium to become part of the mature coronary vasculature and fibrous skeleton of the heart. Little is known about the mechanisms that regulate EPDC migration into the myocardium or the functions carried out by these cells once they enter the myocardium. However, it has been proposed that cardiac fibroblasts are important for growth of the heart during late gestation and are a source of homeostatic factors in the adult. Here, we identify a myocardial to epicardial fibroblast growth factor (FGF) signal, mediated by FGF10 and FGFR2b, that is essential for movement of cardiac fibroblasts into the compact myocardium. Inactivation of this signaling pathway results in fewer epicardial derived cells within the compact myocardium, decreased myocardial proliferation and a resulting smaller thin-walled heart.

Analysis of the alternative splicing of an FGFR2 transcript due to a novel 5' splice site mutation (1084+1G>A): case report

Craniosynostosis is characterized by premature fusion of one or more cranial sutures and is associated with mutations in fibroblast growth factor receptor (FGFR) genes. Here we describe a novel mutation (1084+1G>A) in the FGFR2 gene of a patient with isolated bicoronal synostosis. We detected two isoforms that result from the mutation and are characterized, respectively, by exon skipping and the use of a cryptic splice site. Interestingly, the alternatively spliced forms of FGFR2 appear to induce fusion of the cranial sutures suggesting that the mutation acts via a gain-of-function mechanism rather than a loss of protein functionality.

Reduced expression of fibroblast growth factor receptor 2IIIb in hepatocellular carcinoma induces a more aggressive growth

Fibroblast growth factor receptor 2 isoform b (FGFR2-IIIb) is highly expressed in hepatocytes and plays an important role in liver homeostasis and regeneration. Here, we analyzed the expression and function of FGFR2-IIIb in hepatocellular carcinoma (HCC). FGFR2-IIIb expression in HCC tissues and cell lines was lower than in primary human hepatocytes and nontumorous tissue. FGFR2-IIIb-negative HCCs showed a significantly higher Ki-67 labeling index, and loss of FGFR2-IIIb expression correlated significantly with vascular invasion and more advanced tumor stages. A decrease in FGFR-2IIIb expression in HCC cell lines was not related to promoter hypermethylation. However, PCR analysis indicated that chromosomal deletion at 10q accounted for the loss of FGFR2 expression in a subset of HCC cells. FGFR2-IIIb re-expression in stable transfected HCC cell lines induced a higher basal apoptosis rate and a significantly reduced proliferation and migratory potential in vitro. In nude mice, FGFR2-IIIb re-expressing HCC cells grew significantly slower, and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assay revealed higher apoptosis rates. The antitumorigenic effects of FGFR2-IIIb expression in HCC cells were not affected by keratinocyte growth factor or an inhibitor of FGFR-phosphorylation, indicating that they are independent of tyrosine kinase activation. In conclusion, our data indicate that FGFR2-IIIb inhibits tumorigenicity of HCC cells. Identification of the molecular mechanisms promoting regeneration in normal tissue while suppressing malignancy may lead to novel therapeutic targets of this highly aggressive tumor.

Action, localization and structure-function relationship of growth factors and their receptors in the prostate

Whereas the direct action of sex steroids, namely of androgens, on prostate cell division was questioned as early as in the 1970s, and remains so, the interest in prostatic growth factors (GFs) is rather recent but has expanded tremendously in the last five years. This lag period can be partly explained by the fact that, at the time, androgen receptors had just been discovered, and newly developed hormonal regimens or strategies to treat patients with prostate carcinoma (PCa) or epithelioma had generated great enthusiasm and hopes in the medical and scientific community. Another point to consider was the difficulty in maintaining prostate tissues in organ cultures and the relative novelty of culturing prostate epithelial cells in monolayers. Failures of sex steroids to elicit a direct positive response on prostate cell divisionin vitro, as seenin vivo, were interpreted as resulting from inappropriate models or culture conditions. However, the increasing number of reports confirming the lack of mitogenic activity of sex steroidsin vitro, coupled with the powerful mitogenic activity of GFs displayed in other systems, the discovery of GF receptors (GF-Rs), and the elucidation of their signalling pathways showing sex steroid receptors as potential substrates of GF-activated protein kinases gradually led to an increased interest in the putative role of GFs in prostate physiopathology. Of utmost importance was the recognition that hormone refractiveness was responsible for PCa progression, and for the poor outcome of patients with advanced disease under endocrine therapies. This problem remains a major issue and it raises several key questions that need to be solved at the fundamental and clinical levels.

Keratinocyte growth factor accelerates compensatory growth in the remaining lung after trilobectomy in rats

OBJECTIVE

In rats pulmonary resection is followed by lung compensatory growth. However, the molecular mechanism underlying lung compensatory growth remains unclear. Keratinocyte growth factor is expressed in lung tissue and is considered a possible mitogen for lung epithelial cells. The objectives of this study were to define the role of keratinocyte growth factor and its receptor in rat lung compensatory growth after trilobectomy and the effect of exogenous keratinocyte growth factor gene transfection.

METHODS

Adult Lewis rats were used. Right trilobectomy was performed in the operation group and sham thoracotomy in the sham group. In the operation group, keratinocyte growth factor-FLAG or FLAG expression vector was transfected directly into the lung by means of electroporation. Expression of keratinocyte growth factor and its receptor and alveolar cell proliferation index based on proliferating cell nuclear antigen levels were measured in the right lung at day 14 after the operation.

RESULTS

Proliferating cell nuclear antigen, keratinocyte growth factor, and keratinocyte growth factor receptor expression in lung epithelial cells was significantly increased at day 4 after trilobectomy. Transfection of keratinocyte growth factor-FLAG expression vector resulted in further significant enhancement of proliferating cell nuclear antigen at day 4 after trilobectomy; however, the transfection of FLAG expression vector did not alter the enhancement of proliferating cell nuclear antigen. Exogenous expression of keratinocyte growth factor in the remaining lung by means of electroporation significantly augmented epithelial proliferation and decreased the average airspace distance (mean linear intercept).

CONCLUSION

Our results implicate keratinocyte growth factor in the induction of alveolar epithelial cell proliferation for compensatory lung growth and indicate that overexpression of keratinocyte growth factor in the remaining lung by means of electroporation significantly augmented lung epithelial proliferation.

Aberrant receptor internalization and enhanced FRS2-dependent signaling contribute to the transforming activity of the fibroblast growth factor receptor 2 IIIb C3 isoform

Alternative splice variants of fibroblast growth factor receptor 2 (FGFR2) IIIb, designated C1, C2, and C3, possess progressive reduction in their cytoplasmic carboxyl termini (822, 788, and 769 residues, respectively), with preferential expression of the C2 and C3 isoforms in human cancers. We determined that the progressive deletion of carboxyl-terminal sequences correlated with increasing transforming potency. The highly transforming C3 variant lacks five tyrosine residues present in C1, and we determined that the loss of Tyr-770 alone enhanced FGFR2 IIIb C1 transforming activity. Because Tyr-770 may compose a putative YXXL sorting motif, we hypothesized that loss of Tyr-770 in the 770YXXL motif may cause disruption of FGFR2 IIIb C1 internalization and enhance transforming activity. Surprisingly, we found that mutation of Leu-773 but not Tyr-770 impaired receptor internalization and increased receptor stability and activation. Interestingly, concurrent mutations of Tyr-770 and Leu-773 caused 2-fold higher transforming activity than caused by the Y770F or L773A single mutations, suggesting loss of Tyr and Leu residues of the 770YXXL773 motif enhances FGFR2 IIIb transforming activity by distinct mechanisms. We also determined that loss of Tyr-770 caused persistent activation of FRS2 by enhancing FRS2 binding to FGFR2 IIIb. Furthermore, we found that FRS2 binding to FGFR2 IIIb is required for increased FRS2 tyrosine phosphorylation and enhanced transforming activity by Y770F mutation. Our data support a dual mechanism where deletion of the 770YXXL773 motif promotes FGFR2 IIIb C3 transforming activity by causing aberrant receptor recycling and stability and persistent FRS2-dependent signaling.

Role of Fgf receptor 2c in adipocyte hypertrophy in mesenteric white adipose tissue

Fgf receptor 2c (Fgfr2c) was expressed in mature adipocytes of mouse white adipose tissue (WAT). To examine the role of Fgfr2c in mature adipocytes, we generated adipocyte-specific Fgfr2 knockout (Fgfr2 CKO) mice. The hypertrophy impairment of adipocytes in the mesenteric WAT but not in the subcutaneous WAT and decreased plasma free fatty acid (FFA) levels were observed in Fgfr2 CKO mice. Although the expression of genes involved in adipocyte differentiation and lipid metabolism in the mesenteric WAT was essentially unchanged, the expression of uncoupling protein 2 potentially involved in energy dissipation was significantly increased. Among potential Fgf ligands for Fgfr2c, Fgf9 was preferentially expressed in the mesenteric WAT. The present findings indicate that Fgfr2c potentially activated by Fgf9 plays a role in the adipocyte hypertrophy in the mesenteric WAT and FFA metabolism and/or energy dissipation in the mesenteric WAT might be involved in the hypertrophy impairment.

Evidence that heparin saccharides promote FGF2 mitogenesis through two distinct mechanisms

Heparin-like saccharides play an essential role in binding to both fibroblast growth factors (FGF) and their receptors at the cell surface. In this study we prepared a series of heparin oligosaccharides according to their size and sulfation level. We then investigated their affinity for FGF2 and their ability to support FGF2 mitogenesis of heparan sulfate-deficient cells expressing FGFR1c. Tetra- and hexasaccharides bound FGF2, but failed to dimerize the growth factor. Nevertheless, these saccharides promoted FGF2-mediated cell growth. Furthermore, whereas enzymatic removal of the non-reducing end 2-O-sulfate group had little effect on the 1:1 interaction with FGF2, it eliminated the mitogenic activity of these saccharides. This evidence supports the symmetric two-end model of ternary complex formation. In contrast, even at very low concentrations, octasaccharide and larger heparin fragments conferred a potent mitogenic activity that was independent of terminal 2-O-sulfation. This correlated with the ability to dimerize FGF2 in an apparently cooperative manner. This data suggests that potent mitogenic signaling results from heparin-mediated trans-dimerization of FGF2, consistent with the asymmetric model of ternary complex formation. We propose that, depending on saccharide structure, there are different architectures and modes of ternary complex assembly that differ in stability and/or efficiency of transmembrane signaling.

Identification of an intronic splicing enhancer essential for the inclusion of FGFR2 exon IIIc

The ligand specificity of fibroblast growth factor receptor 2 (FGFR2) is determined by the alternative splicing of exons 8 (IIIb) or 9 (IIIc). Exon IIIb is included in epithelial cells, whereas exon IIIc is included in mesenchymal cells. Although a number of cis elements and trans factors have been identified that play a role in exon IIIb inclusion in epithelium, little is known about the activation of exon IIIc in mesenchyme. We report here the identification of a splicing enhancer required for IIIc inclusion. This 24-nucleotide (nt) downstream intronic splicing enhancer (DISE) is located within intron 9 immediately downstream of exon IIIc. DISE was able to activate the inclusion of heterologous exons rat FGFR2 IIIb and human beta-globin exon 2 in cell lines from different tissues and species and also in HeLa cell nuclear extracts in vitro. DISE was capable of replacing the intronic activator sequence 1 (IAS1), a known IIIb splicing enhancer and vice versa. This fact, together with the requirement for DISE to be close to the 5'-splice site and the ability of DISE to promote binding of U1 snRNP, suggested that IAS1 and DISE belong to the same class of cis-acting elements.

Translocation of fibroblast growth factor-10 and its receptor into nuclei of human urothelial cells

Fibroblast growth factor-10 (FGF-10), a mitogen for the epithelial cells lining the lower urinary tract, has been identified inside urothelial cells, despite its acknowledged role as an extracellular signaling ligand. Recombinant (r)FGF-10 was determined by fluorescence microscopy optical sectioning to localize strongly to nuclei inside cultured urothelial cells. To clarify the possible role of a nuclear localization signal (NLS) in this translocation, a variant of rFGF-10 was constructed which lacked this sequence. rFGF-10(no NLS) was found in cytoplasm to a far greater degree than rFGF-10, identifying this motif as a possible NLS. Furthermore, this variant displayed poor or non-existent bioactivity compared to the wild-type protein in triggering mitogenesis in quiescent urothelial cells. The presence of rFGF-10(no NLS) in the nucleus suggested that additional interactions were also responsible for the nuclear accumulation of rFGF-10. The FGF-10 receptor was observed in cell nuclei regardless of the presence or concentration of exogenous rFGF-10 ligand. Co-localization studies between rFGF-10 and the FGF-10 receptor revealed a strong intracellular relationship between the two. This co-localization was seen in nuclei for both rFGF-10 and for rFGF-10(no NLS), although the correlation was weaker for rFGF-10(no NLS). These data show that an NLS-like motif of rFGF-10 is a partial determinant of its intracellular distribution and is necessary for its mitogenic activity. These advancements in the understanding of the activity of FGF-10 present an opportunity to engineer the growth factor as a therapeutic agent for the healing of damaged urothelial tissue.

Potentiation of fibroblast growth factor activity by synthetic heparin oligosaccharide glycodendrimers

Heparin is a highly sulfated polysaccharide that regulates a variety of cellular processes by interaction with a host of proteins. We report the preparation of synthetic heparin oligosaccharide glycodendrimers and their use as heparin mimetics to regulate heparin-protein interactions. The multivalent display of sugar epitopes mimics the naturally occurring glycans found on cell surfaces and enhances their binding capacity. Binding of the heparin dendrimers to basic fibroblast growth factor (FGF-2) was analyzed using heparin microarray experiments and surface plasmon resonance measurements on gold chips. Heparin-coated dendrimers bind FGF-2 significantly more effectively than monovalent heparin oligosaccharides. Dendrimer 1, which displays multiple copies of the sulfated hexasaccharide (GlcNSO(3)[6-OSO(3)]-IdoA[2-OSO(3)])3, was employed to promote FGF-2-mediated mitogen-activated kinase activation, demonstrating the utility of glycodendrimers to modulate heparin-protein interactions.

Osteogenic differentiation of murine embryonic stem cells is mediated by fibroblast growth factor receptors

The mechanisms involved in the control of embryonic stem (ES) cell differentiation are yet to be fully elucidated. However, it has become clear that the family of fibroblast growth factors (FGFs) are centrally involved. In this study we examined the role of the FGF receptors (FGFRs 1-4) during osteogenesis in murine ES cells. Single cells were obtained after the formation of embryoid bodies, cultured on gelatin-coated plates, and coaxed to differentiate along the osteogenic lineage. Upregulation of genes was analyzed at both the transcript and protein levels using gene array, relative-quantitative PCR (RQ-PCR), and Western blotting. Deposition of a mineralized matrix was evaluated with Alizarin Red staining. An FGFR1-specific antibody was generated and used to block FGFR1 activity in mES cells during osteogenic differentiation. Upon induction of osteogenic differentiation in mES cells, all four FGFRs were clearly upregulated at both the transcript and protein levels with a number of genes known to be involved in osteogenic differentiation including bone morphogenetic proteins (BMPs), collagen I, and Runx2. Cells were also capable of depositing a mineralized matrix, confirming the commitment of these cells to the osteogenic lineage. When FGFR1 activity was blocked, a reduction in cell proliferation and a coincident upregulation of Runx2 with enhanced mineralization of cultures was observed. These results indicate that FGFRs play critical roles in cell recruitment and differentiation during the process of osteogenesis in mES cells. In particular, the data indicate that FGFR1 plays a pivotal role in osteoblast lineage determination.

Transgenic alternative-splicing reporters reveal tissue-specific expression profiles and regulation mechanisms in vivo

Alternative splicing of pre-mRNAs allows multicellular organisms to create a huge diversity of proteomes from a finite number of genes. But extensive studies in vitro or in cultured cells have not fully explained the regulation mechanisms of tissue-specific or developmentally regulated alternative splicing in living organisms. Here we report a transgenic reporter system that allows visualization of expression profiles of mutually exclusive exons in Caenorhabditis elegans. Reporters for egl-15 exons 5A and 5B showed tissue-specific profiles, and we isolated mutants defective in the tissue specificity. We identified alternative-splicing defective-1 (asd-1), encoding a new RNA-binding protein of the evolutionarily conserved Fox-1 family, as a regulator of the egl-15 reporter. Furthermore, an asd-1;fox-1 double mutant was defective in the expression of endogenous egl-15 (5A) and phenocopied egl-15 (5A) mutant. This transgenic reporter system can be a powerful experimental tool for the comprehensive study of expression profiles and regulation mechanisms of alternative splicing in metazoans.

Fox-2 mediates epithelial cell-specific fibroblast growth factor receptor 2 exon choice

Alternative splicing of fibroblast growth factor receptor 2 (FGFR2) transcripts occurs in a cell-type-specific manner leading to the mutually exclusive use of exon IIIb in epithelia or exon IIIc in mesenchyme. Epithelial cell-specific exon choice is dependent on (U)GCAUG elements, which have been shown to bind Fox protein family members. In this paper we show that FGFR2 exon choice is regulated by (U)GCAUG elements and Fox protein family members. Fox-2 isoforms are differentially expressed in IIIb+ cells in comparison to IIIc+ cells, and expression of Fox-1 or Fox-2 in the latter led to a striking alteration in FGFR2 splice choice from IIIc to IIIb. This switch was absolutely dependent on the (U)GCAUG elements present in the FGFR2 pre-mRNA and required critical residues in the C-terminal region of Fox-2. Interestingly, Fox-2 expression led to skipping of exon 6 among endogenous Fox-2 transcripts and formation of an inactive Fox-2 isoform, which suggests that Fox-2 can regulate its own activity. Moreover, the repression of exon IIIc in IIIb+ cells was abrogated by interfering RNA-mediated knockdown of Fox-2. We also show that Fox-2 is critical for the FGFR2(IIIb)-to-FGFR2(IIIc) switch observed in T Rex-293 cells grown to overconfluency. Overconfluent T Rex-293 cells show molecular and morphological changes consistent with a mesenchymal-to-epithelial transition. If overconfluent cells are depleted of Fox-2, the switch from IIIc to IIIb is abrogated. The data in this paper place Fox-2 among critical regulators of gene expression during mesenchymal-epithelial transitions and demonstrate that this action of Fox-2 is mediated by mechanisms distinct from those described for other cases of Fox activity.

The fibroblast growth factor binding protein is a novel interaction partner of FGF-7, FGF-10 and FGF-22 and regulates FGF activity: implications for epithelial repair

The fibroblast growth factor-binding protein (FGF-BP) binds and activates FGF-1 and FGF-2, thereby contributing to tumor angiogenesis. In this study, we identified novel binding partners of FGF-BP, and we provide evidence for a role of this protein in epithelial repair processes. We show that expression of FGF-BP increases after injury to murine and human skin, in particular in keratinocytes. This upregulation is most likely achieved by major keratinocyte mitogens present at the wound site. Most importantly, we demonstrate that FGF-BP interacts with FGF-7, FGF-10, and with the recently identified FGF-22, and enhances the activity of low concentrations of ligand. Due to the important functions of FGF-7 and FGF-10 for repair of injured epithelia, our findings suggest that upregulation of FGF-BP expression after injury stimulates FGF activity at the wound site, thus enhancing the process of epithelial repair.

Characterization of the intronic splicing silencers flanking FGFR2 exon IIIb

The cell type-specific alternative splicing of FGFR2 pre-mRNA results in the mutually exclusive use of exons IIIb and IIIc, which leads to critically important differences in receptor function. The choice of exon IIIc in mesenchymal cells involves activation of this exon and repression of exon IIIb. This repression is mediated by the function of upstream and downstream intronic splicing silencers (UISS and DISS). Here we present a detailed characterization of the determinants of silencing function within UISS and DISS. We used a systematic mutational analysis, introducing deletions and substitutions to define discrete elements within these two silencers of exon IIIb. We show that UISS requires polypyrimidine tract-binding protein (PTB)-binding sites, which define the UISS1 sub-element, and an eight nucleotide sequence 5'-GCAGCACC-3' (UISS2) that is also required. Even though UISS2 does not bind PTB, the full UISS can be replaced with a synthetic silencer designed to provide optimal PTB binding. DISS is composed of a 5'-conserved sub-element (5'-CE) and two regions that contain multiple PTB sites and are functionally redundant (DISS1 and DISS2). DISS1 and DISS2 are separated by the activator sequence IAS2, and together these opposing elements form the intronic control element. Deletion of DISS in the FGFR2 exon IIIb context resulted in the near full inclusion of exon IIIb, and insertion of this silencer downstream of a heterologous exon with a weak 5' splice site was capable of repressing exon inclusion. Extensive deletion analysis demonstrated that the majority of silencing activity could be mapped to the conserved octamer CUCGGUGC within the 5'CE. Replacement of 5'CE and DISS1 with PTB-binding elements failed to restore repression of exon IIIb. We tested the importance of the relative position of the silencers and of the subelements within each silencer. Whereas UISS1, UISS2, DISS1, and DISS2 appear somewhat malleable, the 5'CE is rigid in terms of relative position and redundancy. Our data defined elements of function within the ISSs flanking exon IIIb and suggested that silencing of this exon is mediated by multiple trans-acting factors.

Insights into the molecular basis for fibroblast growth factor receptor autoinhibition and ligand-binding promiscuity

The prototypical fibroblast growth factor receptor (FGFR) extracellular domain consists of three Ig domains (D1-D3) of which the two membrane-proximal D2 and D3 domains and the interconnecting D2-D3 linker bear the determinants of ligand binding and specificity. In contrast, D1 and the D1-D2 linker are thought to play autoinhibitory roles in FGFR regulation. Here, we report the crystal structure of the three-Ig form of FGFR3c in complex with FGF1, an FGF that binds promiscuously to each of the seven principal FGFRs. In this structure, D1 and the D1-D2 linker are completely disordered, demonstrating that these regions are dispensable for FGF binding. Real-time binding experiments using surface plasmon resonance show that relative to two-Ig form, the three-Ig form of FGFR3c exhibits lower affinity for both FGF1 and heparin. Importantly, we demonstrate that this autoinhibition is mediated by intramolecular interactions of D1 and the D1-D2 linker with the minimal FGF and heparin-binding D2-D3 region. As in the FGF1-FGFR2c structure, but not the FGF1-FGFR1c structure, the alternatively spliced betaC'-betaE loop is ordered and interacts with FGF1 in the FGF1-FGFR3c structure. However, in contrast to the FGF1-FGFR2c structure in which the betaC'-betaE loop interacts with the beta-trefoil core region of FGF1, in the FGF1-FGFR3c structure, this loop interacts extensively with the N-terminal region of FGF1, underscoring the importance of the FGF1 N terminus in conferring receptor-binding affinity and promiscuity. Importantly, comparison of the three FGF1-FGFR structures shows that the flexibility of the betaC'-betaE loop is a major determinant of ligand-binding specificity and promiscuity.

Translocation of FGF-1 and FGF-2 across vesicular membranes occurs during G1-phase by a common mechanism

The entry of exogenous fibroblast growth factor 2 (FGF-2) to the cytosolic/nuclear compartment was studied and compared with the translocation mechanism used by FGF-1. To differentiate between external and endogenous growth factor, we used FGF-2 modified to contain a farnesylation signal, a CaaX-box. Because farnesylation occurs only in the cytosol and nucleoplasm, farnesylation of exogenous FGF-2-CaaX was taken as evidence that the growth factor had translocated across cellular membranes. We found that FGF-2 translocation occurred in endothelial cells and fibroblasts, which express FGF receptors, and that the efficiency of translocation was increased in the presence of heparin. Concomitantly with translocation, the 18-kDa FGF-2 was N-terminally cleaved to yield a 16-kDa form. Translocation of FGF-2 required PI3-kinase activity but not transport through the Golgi apparatus. Inhibition of endosomal acidification did not prevent translocation, whereas dissipation of the vesicular membrane potential completely blocked it. The data indicate that translocation occurs from intracellular vesicles containing proton pumps and that an electrical potential across the vesicle membrane is required. Translocation of both FGF-1 and FGF-2 occurred during most of G(1) but decreased shortly before the G(1)-->S transition. A common mechanism for FGF-1 and FGF-2 translocation into cells is postulated.

Enforced expression of FGF-7 promotes epithelial hyperplasia whereas a dominant negative FGFR2iiib promotes the emergence of neuroendocrine phenotype in prostate glands of transgenic mice

The minimal rat probasin (PB) promoter was used to target expression of human fibroblast growth factor-7 (FGF-7)/keratinocyte growth factor (KGF) directly to prostatic epithelium of transgenic mice, converting FGF-7 from a paracrine to an autocrine factor. Four independent lines were established that expressed the transgene (PKS) in the prostate. Upon histologic analysis, the prostatic epithelium of PKS mice was found to be hyperplastic. Many of the prostatic ducts were filled with secretory epithelial cells tightly associated with a highly enfolded basement membrane. Distortions of the ductal smooth muscle layer were also observed. Prostates from year-old PKS mice had significantly more abnormal ducts than their wild-type nontransgenic littermates. The minimal rat PB promoter was also used to target a truncated FGFR2iiib receptor to prostatic epithelium to functionally abrogate endogenous FGF-7 signaling. Three lines were established that expressed the transgene (KDNR) in the prostate. Upon dissection it was noted that all four lobes of the prostates of KDNR mice were present but smaller in size. Histologic analysis indicated that the epithelium in many of the prostatic ducts was disorganized and contained numerous rounded cytokeratin-positive cells that were not tightly associated with the basement membrane. The stroma was disorganized and did not form a tight layer of smooth muscle around the epithelial ducts. Surprisingly, abrogation of FGF signaling in KDNR mice correlated with the emergence of a neuroendocrine-like phenotype that was not observed as a consequence of enforced FGF-7 expression in the PKS mice.

FGF3 Attached to a Phosholipid Membrane Anchor Gains a High Transforming Capacity

NIH3T3 cells transformed by mouse FGF3-cDNA (DMI cells) selected for their ability to grow as anchorage-independent colonies in soft agar and in defined medium lacking growth factors exhibit a highly transformed phenotype. We have used dominant negative (DN) fibroblast growth factor (FGF) receptor 2 (FGFR2) isoforms to block the FGF response in DMI cells. When the DN-FGFR was expressed in DMI cells, their transformed phenotype can be reverted. The truncated FGFR2(IIIb), the high affinity FGFR for FGF3, is significantly more efficient at reverting the transformed phenotype as the IIIc isoform, reaffirming the notion that the affinity of the ligand to the DN-FGFR2 isoform determines the effect. Heparin or heparan sulfate displaces FGF3 from binding sites on the cell surface inhibiting the growth of DMI cells and reverts the transformed phenotype (). However, the presence of heparin is necessary to induce a mitogenic response in NIH3T3 cells when stimulated with soluble purified mouse FGF3. We have investigated the importance of cell surface binding of FGF3 for its ability to transform NIH3T3 cells by creating an FGF3 mutant anchored to the membrane via glycosylphosphatidylinositol (GPI). The GPI anchor renders the cell surface association of FGF3 independent from binding to heparan sulfate-proteoglycan of the cell surface membrane. Attachment of a GPI anchor to FGF3 also confers a much higher transforming potential to the growth factor. Even more, the purified GPI-attached FGF3 is as much transforming as the secreted protein acting in an autocrine mode. Because NIH3T3 cells do not express the high affinity tyrosine kinase FGF receptors for FGF3, these findings suggest that FGF3 attached to GPI-linked heparan sulfate-proteoglycan may have a broader biological activity as when bound to transmembrane or soluble heparan sulfate-proteoglycan.

Vascular smooth muscle cells spontaneously adopt a skeletal muscle phenotype: a unique Myf5(-)/MyoD(+) myogenic program

Smooth and skeletal muscle tissues are composed of distinct cell types that express related but distinct isoforms of the structural genes used for contraction. These two muscle cell types are also believed to have distinct embryological origins. Nevertheless, the phenomenon of a phenotypic switch from smooth to skeletal muscle has been demonstrated in several in vivo studies. This switch has been minimally analyzed at the cellular level, and the mechanism driving it is unknown. We used immunofluorescence and RT-PCR to demonstrate the expression of the skeletal muscle-specific regulatory genes MyoD and myogenin, and of several skeletal muscle-specific structural genes in cultures of the established rat smooth muscle cell lines PAC1, A10, and A7r5. The skeletal muscle regulatory gene Myf5 was not detected in these three cell lines. We further isolated clonal sublines from PAC1 cultures that homogeneously express smooth muscle characteristics at low density and undergo a coordinated increase in skeletal muscle-specific gene expression at high density. In some of these PAC1 sublines, this process culminates in the high-frequency formation of myotubes. As in the PAC1 parental line, Myf5 was not expressed in the PAC1 sublines. We show that the PAC1 sublines that undergo a more robust transition into the skeletal muscle phenotype also express significantly higher levels of the insulin-like growth factor (IGF1 and IGF2) genes and of FGF receptor 4 (FGFR4) gene. Our results suggest that MyoD expression in itself is not a sufficient condition to promote a coordinated program of skeletal myogenesis in the smooth muscle cells. Insulin administered at a high concentration to PAC1 cell populations with a poor capacity to undergo skeletal muscle differentiation enhances the number of cells displaying the skeletal muscle differentiated phenotype. The findings raise the possibility that the IGF signaling system is involved in the phenotypic switch from smooth to skeletal muscle. The gene expression program described here can now be used to investigate the mechanisms that may underlie the propensity of certain smooth muscle cells to adopt a skeletal muscle identity.(J Histochem Cytochem 48:1173-1193, 2000)

Fibroblast growth factor (FGF) receptor 1-IIIb is a naturally occurring functional receptor for FGFs that is preferentially expressed in the skin and the brain

Fibroblast growth factors (FGFs) transmit their signals through four transmembrane receptors that are designated FGFR1-4. Alternative splicing in the extracellular region of FGFR1-3 generates receptor variants with different ligand binding affinities. Thus two types of transmembrane receptors (IIIb and IIIc isoforms) have been identified for FGFR2 and FGFR3, and the existence of analogous variants has been postulated for FGFR1 based on its genomic structure. However, only a single full-length transmembrane FGFR1 variant (FGFR1-IIIc) has been identified so far. Here we describe the cloning of a full-length cDNA encoding FGFR1-IIIb from a mouse skin wound cDNA library. This receptor isoform was expressed at the highest levels in a subset of sebaceous glands of the skin and in neurons of the hippocampus and the cerebellum. FGFR1-IIIb was expressed in L6 rat skeletal muscle myoblasts and used in cross-linking and receptor binding studies. FGF-1 was found to bind the receptor with high affinity, whereas FGF-2, -10, and -7 bound with significantly lower affinities. Despite their apparently similar but low affinities, FGF-10 but not FGF-7 induced the activation of p44/42 mitogen-activated protein kinase in FGFR1-IIIb-expressing L6 myoblasts and stimulated mitogenesis in these cells, demonstrating that this new receptor variant is a functional transmembrane receptor for FGF-10.

A novel type I fibroblast growth factor receptor activates mitogenic signaling in the absence of detectable tyrosine phosphorylation of FRS2

A novel variant of the fibroblast growth factor receptor type 1 (FGFR-1) was identified in human placental RNA. In this receptor (FGFR-1L) portions of the second and third immunoglobulin-like (Ig-like) domains are deleted. To determine whether FGFR-1L was functional, full-length variant (pSV/FGFR-1L) and wild-type (pSV/FGFR-1) receptors were stably transfected into rat L6 myoblasts cells. Transfected L6 clones expressed respective proteins and bound (125)I-labeled FGF-2 with K(d) values of 99 pm (FGFR-1) and 26 pm (FGFR-1L). FGF-1 and FGF-2 competed efficiently with (125)I-FGF-2 for binding to FGFR-1 and FGFR-1L, whereas FGF-4 was less efficient. FGF-1, FGF-2, and FGF-4 enhanced mitogen-activated protein kinase (MAPK) activity, increased steady-state c-fos mRNA levels, and stimulated proliferation through either receptor, whereas KGF was without effect. FGFR-1 expressing clones exhibited ligand-induced tyrosine phosphorylation of fibroblast growth factor receptor substrate 2 (FRS2), a 90-kDa adaptor protein that links FGFR-1 activation to the MAPK cascade. In contrast, tyrosine phosphorylation of FRS2 was not evident with FGFR-1L. In addition, phospholipase C-gamma was not tyrosine phosphorylated via activated FGFR-1L. These findings indicate that FGFR-1L binds FGF-1 and FGF-2 with high affinity and is capable of mitogenic signaling, but may activate MAPK to occur via non-classical signaling intermediates.

Crystal structures of two FGF-FGFR complexes reveal the determinants of ligand-receptor specificity

To elucidate the structural determinants governing specificity in fibroblast growth factor (FGF) signaling, we have determined the crystal structures of FGF1 and FGF2 complexed with the ligand binding domains (immunoglobulin-like domains 2 [D2] and 3 [D3]) of FGF receptor 1 (FGFR1) and FGFR2, respectively. Highly conserved FGF-D2 and FGF-linker (between D2-D3) interfaces define a general binding site for all FGF-FGFR complexes. Specificity is achieved through interactions between the N-terminal and central regions of FGFs and two loop regions in D3 that are subject to alternative splicing. These structures provide a molecular basis for FGF1 as a universal FGFR ligand and for modulation of FGF-FGFR specificity through primary sequence variations and alternative splicing.

Characterization of fibroblast growth factor receptor 2 overexpression in the human breast cancer cell line SUM-52PE

The fibroblast growth factor receptor (FGFR)2 gene has been shown to be amplified in 5-10% of breast cancer patients. A breast cancer cell line developed in our laboratory, SUM-52PE, was shown to have a 12-fold amplification of the FGFR2 gene, and FGFR2 message was found to be overexpressed 40-fold in SUM-52PE cells as compared with normal human mammary epithelial (HME) cells. Both human breast cancer (HBC) cell lines and HME cells expressed two FGFR2 isoforms, whereas SUM-52PE cells overexpressed those two isoforms, as well as several unique FGFR2 polypeptides. SUM-52PE cells expressed exclusively FGFR2-IIIb isoforms, which are high-affinity receptors for fibroblast growth factor (FGF)-1 and FGF-7. Differences were identified in the expression of the extracellular Ig-like domains, acid box and carboxyl termini, and several variants not previously reported were isolated from these cells.

Mapping ligand binding domains in chimeric fibroblast growth factor receptor molecules. Multiple regions determine ligand binding specificity

Fibroblast growth factors (FGFs) mediate essential cellular functions by activating one of four alternatively spliced FGF receptors (FGFRs). To determine the mechanism regulating ligand binding affinity and specificity, soluble FGFR1 and FGFR3 binding domains were compared for activity. FGFR1 bound well to FGF2 but poorly to FGF8 and FGF9. In contrast, FGFR3 bound well to FGF8 and FGF9 but poorly to FGF2. The differential ligand binding specificity of these two receptors was exploited to map specific ligand binding regions in mutant and chimeric receptor molecules. Deletion of immunoglobulin-like (Ig) domain I did not effect ligand binding, thus localizing the binding region(s) to the distal two Ig domains. Mapping studies identified two regions that contribute to FGF binding. Additionally, FGF2 binding showed positive cooperativity, suggesting the presence of two binding sites on a single FGFR or two interacting sites on an FGFR dimer. Analysis of FGF8 and FGF9 binding to chimeric receptors showed that a broad region spanning Ig domain II and sequences further N-terminal determines binding specificity for these ligands. These data demonstrate that multiple regions of the FGFR regulate ligand binding specificity and that these regions are distinct with respect to different members of the FGF family.

Tumour suppressive properties of fibroblast growth factor receptor 2-IIIb in human bladder cancer

FGFRs (fibroblast growth factor receptors) are encoded by four genes (FGFR1-4). Alternative splicing results in various receptor isoforms. The FGFR2-IIIb variant is present in a wide variety of epithelia, including the bladder epithelium. Recently, we have shown that FGFR2-IIIb is downregulated in a subset of transitional cell carcinomas of the bladder, and that this downregulation is associated with a poor prognosis. We investigated possible tumour suppressive properties of FGFR2-IIIb by transfecting two human bladder tumour cell lines, J82 and T24, which have no endogenous FGFR2-IIIb expression, with FGFR2-IIIb cDNA. No stable clones expressing FGFR2-IIIb were isolated with the J82 cell line. For the T24 cell line, stable transfectants expressing FGFR2-IIIb had reduced growth in vitro and formed fewer tumours in nude mice which, in addition, grew more slowly. The potential mechanisms leading to decreased FGFR2-IIIb mRNA levels were also investigated. The 5' region of the human FGFR2 gene was isolated and found to contain a CpG island which was partially methylated in more than half the cell lines and tumours which do not express FGFR2-IIIb. No homozygous deletion was identified in any of the tumours or cell lines with reduced levels of FGFR2-IIIb. Mutational analysis of the entire coding region of FGFR2-IIIb at the transcript level was performed in 33 bladder tumours. In addition to normal FGFR2-IIIb mRNA, abnormal transcripts were detected in two tumour samples. These abnormal mRNAs resulted from exon skipping which affected the region encoding the kinase domain. Altogether, these results show that FGFR2-IIIb has tumour growth suppressive properties in bladder carcinomas and suggest possible mechanisms of FGFR2 gene inactivation.

FGF-19, a novel fibroblast growth factor with unique specificity for FGFR4

We have identified a novel fibroblast growth factor, FGF-19, the most distant member of the FGF family described to date. FGF-19 is a high affinity, heparin dependent ligand for FGFR4 and is the first member of the FGF family to show exclusive binding to FGFR4. Human FGF-19 maps to chromosome 11 q13.1, a region associated with an osteoporosis-pseudoglioma syndrome of skeletal and retinal defects. FGF-19 message is expressed in several tissues including fetal cartilage, skin, and retina, as well as adult gall bladder and is overexpressed in a colon adenocarcinoma cell line.

Structural basis for FGF receptor dimerization and activation

The crystal structure of FGF2 bound to a naturally occurring variant of FGF receptor 1 (FGFR1) consisting of immunoglobulin-like domains 2 (D2) and 3 (D3) has been determined at 2.8 A resolution. Two FGF2:FGFR1 complexes form a 2-fold symmetric dimer. Within each complex, FGF2 interacts extensively with D2 and D3 as well as with the linker between the two domains. The dimer is stabilized by interactions between FGF2 and D2 of the adjoining complex and by a direct interaction between D2 of each receptor. A positively charged canyon formed by a cluster of exposed basic residues likely represents the heparin-binding site. A general model for FGF- and heparin-induced FGFR dimerization is inferred from the crystal structure, unifying a wealth of biochemical data.

Genomic organization of the human fibroblast growth factor receptor 2 (FGFR2) gene and comparative analysis of the human FGFR gene family

The human fibroblast growth factor receptor (FGFR) genes play important roles in normal vertebrate development. Mutations in the human FGFR2 gene have been associated with many craniosynostotic syndromes and malformations, including Crouzon, Pfeiffer, Apert, Jackson-Weiss, Beare-Stevenson cutis gyrata, and Antley-Bixler syndromes, and Kleeblaatschadel (cloverleaf skull) deformity. The mutations identified to date are concentrated in the previously characterized region of FGFR2 that codes for the extracellular IgIII domain of the receptor protein. The search for mutations in other regions of the gene, however, has been hindered by lack of knowledge of the genomic structure. Using a combination of genomic library screening, long-range PCR, and genomic walking, we have characterized the genomic structure of nearly the entire human FGFR2 gene, including a delineation of the organization and size of all introns and exons and determination of the DNA sequences at the intron/exon boundaries. Comparative analysis of the human FGFR gene family reveals that the genomic organization of the FGFRs is relatively conserved. Moreover, alignment of the amino acid sequences shows that the four corresponding proteins share 46% identity overall, with up to 70% identity between individual pairs of FGFR proteins. However, the FGFR2 gene contains an additional exon not found in other members of the family, and it also has much larger intronic sequences throughout the gene. Remarkable similarities in genomic organization, intron/exon boundaries, and intron sizes are found between the human and mouse FGFR2 genes. Knowledge gained from this study of the human FGFR2 gene structure may prove useful in future screening studies designed to find additional mutations associated with craniosynostotic syndromes, and in understanding the molecular and cell biology of this receptor family.

De novo alu-element insertions in FGFR2 identify a distinct pathological basis for Apert syndrome

Apert syndrome, one of five craniosynostosis syndromes caused by allelic mutations of fibroblast growth-factor receptor 2 (FGFR2), is characterized by symmetrical bony syndactyly of the hands and feet. We have analyzed 260 unrelated patients, all but 2 of whom have missense mutations in exon 7, which affect a dipeptide in the linker region between the second and third immunoglobulin-like domains. Hence, the molecular mechanism of Apert syndrome is exquisitely specific. FGFR2 mutations in the remaining two patients are distinct in position and nature. Surprisingly, each patient harbors an Alu-element insertion of approximately 360 bp, in one case just upstream of exon 9 and in the other case within exon 9 itself. The insertions are likely to be pathological, because they have arisen de novo; in both cases this occurred on the paternal chromosome. FGFR2 is present in alternatively spliced isoforms characterized by either the IIIb (exon 8) or IIIc (exon 9) domains (keratinocyte growth-factor receptor [KGFR] and bacterially expressed kinase, respectively), which are differentially expressed in mouse limbs on embryonic day 13. Splicing of exon 9 was examined in RNA extracted from fibroblasts and keratinocytes from one patient with an Alu insertion and two patients with Pfeiffer syndrome who had nucleotide substitutions of the exon 9 acceptor splice site. Ectopic expression of KGFR in the fibroblast lines correlated with the severity of limb abnormalities. This provides the first genetic evidence that signaling through KGFR causes syndactyly in Apert syndrome.

Characterization of recombinant human fibroblast growth factor (FGF)-10 reveals functional similarities with keratinocyte growth factor (FGF-7)

A newly identified member of the fibroblast growth factor (FGF) family, designated FGF-10, is expressed during development and preferentially in adult lung. The predicted FGF-10 protein is most related to keratinocyte growth factor (KGF, or FGF-7). The latter is unique among FGFs in that it binds and signals only through the FGF receptor (FGFR2b) isoform KGF receptor (KGFR) expressed specifically by epithelial cells. In order to examine the biological and biochemical properties of human FGF-10, we isolated the cDNA and expressed its encoded protein in bacteria. The recombinant protein (rFGF-10) was a potent mitogen for Balb/MK mouse epidermal keratinocytes with activity detectable at 0.1 nM and maximal at around 5 nM. Within this concentration range, FGF-10 did not stimulate DNA synthesis in NIH/3T3 mouse fibroblasts. rFGF-10 bound the KGFR with high affinity comparable to that of KGF, and did not bind detectably to either the FGFR1c (Flg) or FGFR2c (Bek) receptor isoforms. The mitogenic activity of FGF-10 could be distinguished from that of KGF by its different sensitivity to heparin and lack of neutralization by a KGF monoclonal antibody. These results indicate that FGF-10 and KGF have similar receptor binding properties and target cell specificities, but are differentially regulated by components of the extracellular matrix.

FGFR2 mutation associated with clinical manifestations consistent with Antley-Bixler syndrome

The Antley-Bixler syndrome (ABS) is a rare syndrome with synostosis of cranial sutures and elbow joints as minimal diagnostic criteria. The inheritance has been suggested to be autosomal recessive based on two families with sib recurrence with both sexes being affected, and two cases born to consanguineous parents. We report the first case of ABS associated with an apparent dominant de novo mutation in the fibroblast growth factor receptor 2 (FGFR2) gene. The patient was found to be heterozygous for a C-->G transversion at nucleotide 1064, which predicts a Ser351Cys amino acid substitution in the IgIII domain of FGFR2. Apart from the craniosynostosis and elbow ankylosis, our patient also presented with severe spinal dysraphism, the first report of such a finding in association with ABS. This suggests that FGFR2 is expressed as early as the fourth week of embryogenesis when somite formation occurs. We propose that the Antley-Bixler syndrome is an autosomal dominant condition with possible gonadal mosaicism. Alternatively, there may be two types of ABS: an autosomal dominant form and an autosomal recessive form. In light of our findings, FGFR mutations should be looked for in other craniosynostosis patients with elbow synostosis.

Establishment of epitope-defined monoclonal antibodies with specificity for fibroblast growth factor receptor types 1 and 2

The development of specific antibody probes for characterizing the expression of the family of 4 fibroblast growth factor receptor (FGFR) types has been difficult because of their close homology to each other and high degree of evolutionary conservation. Of the existing anti-FGFR monoclonal antibodies (MAbs), there are few that are useful for staining paraffin-embedded tissues. We have raised MAbs against human FGFR1 and FGFR2 in both rats and mice using bacterial recombinant receptor fusion proteins as immunogens. We used peptide epitope mapping to characterize the immune sera and the selected MAbs. Immunized animals were selected that displayed the broadest reactivity against epitopes unique to the immunizing receptor type. We produced FGFR1 specific MAbs that bind epitopes in immunoglobulin domain I (Ig-I) and FGFR2 specific MAbs that bind epitopes in Ig-I, Ig-II, and the acid box. The specificity of the antibodies was demonstrated by ELISA and immunoblot analysis of purified recombinant FGFR1 and FGFR2 extracellular domains produced both in E. coli and in eucaryotic cells. Based on the lack of epitope homology, these MAbs would not be expected to cross-react with FGFR3 or FGFR4. We isolated MAbs that bound to paraffin embedded tissue and immunoblots of recombinant receptor. These epitope-defined MAbs can distinguish between members of the FGF receptor family and should be useful as tools for assessing FGF receptor expression in a variety of normal and diseased tissues.

Developmental response by Leydig cells to acidic and basic fibroblast growth factor

The present study examines the effects of acidic (FGF-1) and basic (FGF-2) fibroblast growth factors on Leydig cell steroidogenesis by cells from 5-, 21- and 90-day-old rats. These ages represent three distinct time points in Leydig cell development: fetal Leydig cells (day 5), immature Leydig cells (day 21) and adult Leydig cells (day 90). The results demonstrate that the actions of the two growth factors on steroidogenesis are developmentally regulated, and require the presence of heparan sulphate proteoglycans (HSPG). FGF-1 and FGF-2 both had stimulatory effects on basal, but not maximally LH-stimulated, testosterone production by fetal Leydig cells, and both growth factors stimulated basal 5 alpha-androstane-3 alpha, 17 beta-diol production by immature Leydig cells. These effects were mediated by heparan sulphate-proteoglycans (HSPG), as they were blocked by the addition of protamine sulphate and sodium chlorate. FGF-1 and FGF-2 had no effect on basal testosterone production by adult Leydig cells, however, FGF-1 alone inhibited LH-stimulated testosterone production by adult Leydig cells in a dose-dependent manner. These data demonstrate that the effects of FGF-1 and FGF-2 are dependent on the specific stage of Leydig cell differentiation and development and may vary accordingly. Furthermore, although FGF-1 and FGF-2 are closely related structurally, a different effect of these two growth factors can be observed on the same type of Leydig cells. The data therefore suggest that these growth factors may have different but specific roles in the regulation of Leydig cell steroidogenesis, at different stages of development.