cDNA cloning and expression of a human FGF receptor which binds acidic and basic FGF

Different abilities of the four FGFRs to mediate FGF-1 translocation are linked to differences in the receptor C-terminal tail

Members of the fibroblast growth factor family bind to one or more of the four closely related membrane-spanning FGF receptors. In addition to signaling through the receptors, exogenous FGF-1 and FGF-2 are endocytosed and translocated to the cytosol and nucleus where they stimulate RNA and DNA synthesis. Here we have studied the ability of the four FGF receptors to facilitate translocation of exogenous FGF-1 to the cytosol and nucleus. FGFR1 and FGFR4 were able to mediate translocation, whereas FGFR2 and FGFR3 completely lacked this ability. By analyzing mutant FGFRs we found that the tyrosine kinase domain could be deleted from FGFR1 without abolishing translocation, whereas the C-terminal tail of the FGFRs, constituted by approximately 50 amino acids downstream of the kinase domain, plays a crucial role in FGF-1 translocation. Three amino acids residues within the C-terminal tail were found to be of particular importance for translocation. For FGFR2, the two amino acid substitutions Q774M and P800H were sufficient to enable the receptor to support FGF-1 translocation. The results demonstrate a striking diversity in function of the four FGFRs determined by their C-terminal domain.

Different intracellular trafficking of FGF1 endocytosed by the four homologous FGF receptors

Many growth factors and cytokines bind to more than one receptor, but in many cases the different roles of the separate receptors in signal transduction are unclear. Intracellular sorting of ligand-receptor complexes may modulate the signalling, and we have here studied the intracellular trafficking of ligand bound to receptors for fibroblast growth factors (FGFs). For this purpose, we transfected HeLa cells with any one of the four tyrosine kinase FGF receptors (FGFR1-4). In cells expressing any one of these receptors, externally added FGF1 was localized to sorting/early endosomes after 15 minutes at 37 degrees C. After longer incubation times, FGF1 internalized in cells expressing FGFR1 was localized mainly to late endosomes/lysosomes, similarly to EGF. By contrast, FGF1 internalized in cells expressing FGFR4 followed largely the same intracellular pathway as the recycling ligand, transferrin. In cells expressing FGFR2 or FGFR3, sorting of FGF1 to lysosomes was somewhat less efficient than that observed for FGFR1. Furthermore, FGF1 was more slowly degraded in cells expressing FGFR4 than in cells expressing FGFR1-3 and in addition, internalized FGFR4 as such was more slowly degraded than the other receptors. The data indicate that after endocytosis, FGFR4 and its bound ligand are sorted mainly to the recycling compartment, whereas FGFR1-3 with ligand are sorted mainly to degradation in the lysosomes. Alignment of the amino acid sequence of the intracellular part of the four FGFRs revealed several lysines conserved in FGFR1-3 but absent in FGFR4. Lysines are potential ubiquitylation sites and could thus target a receptor to lysosomes for degradation. Indeed, we found that FGFR4 is less ubiquitylated than FGFR1, which could be the reason for the different sorting of the receptors.

Differential tyrosine phosphorylation of fibroblast growth factor (FGF) receptor-1 and receptor proximal signal transduction in response to FGF-2 and heparin

The sulfated regions in heparan sulfate and heparin are known to affect fibroblast growth factor (FGF) function. We have studied the mechanism whereby heparin directs FGF-2-induced FGF receptor-1 (FGFR-1) signal transduction. FGF-2 alone stimulated maximal phosphorylation of Src homology domain 2 tyrosine phosphatase (SHP-2) and the adaptor molecule Crk, in heparan sulfate-deficient Chinese hamster ovary (CHO) 677 cells expressing FGFR-1. In contrast, for phospholipase Cgamma(1) (PLCgamma(1)) and the adaptor molecule Shb to be maximally tyrosine-phosphorylated, cells had to be stimulated with both FGF-2 and heparin (100 ng/ml). Tyrosine residues 463 in the juxtamembrane domain and 766 in the C-terminal tail in FGFR-1 are known to bind Crk and PLCgamma(1), respectively. Analysis of tryptic phosphopeptide maps of FGFR-1 from cells stimulated with FGF-2 alone and FGF-2 together with heparin showed that FGF-2 alone stimulated a several-fold increase in tyrosine 463 in the juxtamembrane domain. In contrast, heparin had to be included in order for tyrosine 766 to be phosphorylated to the same fold level. Our data imply that tyrosine 463 is phosphorylated and able to transduce signals in response to FGF-2 treatment alone; furthermore, we suggest that FGFR-1 dimerization/kinase activation is stabilized by heparin.

Regulation of angiogenesis and tumorigenesis by signal transduction cascades: lessons from benign and malignant endothelial tumors

Oncogenes and tumor suppressor genes are implicated in the regulation of the angiogenic switch. Much of the data accumulated to date uses NIH 3T3 cells, which are deficient in the tumor suppressor gene p16, as models for these studies. We have used a novel system, derived by sequential introduction of a temperature-sensitive SV40 large T antigen and oncogenic H-ras, to study the angiogenic switch. The results from our studies differ from those using NIH3T3 cells, but have been confirmed by multiple other groups. The data from all of these studies suggest that there is synergy between inactivation of the p53 tumor suppressor gene and activation of the phosphoinositol-3-kinase pathway (PI-3-K), as well as synergy between inactivation of the p16 tumor suppressor gene and activation of the MAP kinase pathway. These findings suggest that there are predictable behaviors of tumors that may be assessed by the status of p53 or p16 in a biopsy, and that these predictable changes in signal transduction may be useful both prognostically and in the design of rationally based drug therapy of benign and malignant tumors.

Selectively desulfated heparin inhibits fibroblast growth factor-induced mitogenicity and angiogenesis

Fibroblast growth factors (FGFs) are known to induce formation of new blood vessels, angiogenesis. We show that FGF-induced angiogenesis can be modulated using selectively desulfated heparin. Chinese hamster ovary cells (CHO677) deficient in heparan sulfate biosynthesis were employed to assess the function of heparin/heparan sulfate in FGF receptor-1 (FGFR-1) signal transduction and biological responses. In the presence of FGF-2, FGFR-1 kinase and subsequent mitogen-activated protein kinase Erk2 activities were augmented in a dose-dependent manner, whereas high concentrations of heparin resulted in decreased activity. The length of the heparin oligomer, minimally an 8/10-mer, was critical for the ability to enhance FGFR-1 kinase activity. The N- and 2-O-sulfate groups of heparin were essential for binding to FGF-2, whereas stimulation of FGFR-1 and Erk2 kinases by FGF-2 also required the presence of 6-O-sulfate groups. Sulfation at 2-O- and 6-O-positions was moreover a prerequisite for binding of heparin to a lysine-rich peptide corresponding to amino acids 160-177 in the extracellular domain of FGFR-1. Selectively 6-O-desulfated heparin, which binds to FGF-2 but fails to bind the receptor, decreased FGF-2-induced proliferation of CHO677 cells, presumably by displacing intact heparin. Furthermore, FGF-2-induced angiogenesis in chick embryos was inhibited by 6-O-desulfated heparin. Thus, formation of a ternary complex of FGF-2, heparin, and FGFR-1 appears critical for the activation of FGFR-1 kinase and downstream signal transduction. Preventing complex formation by modified heparin preparations may allow regulation of FGF-2 functions, such as induction of angiogenesis.

Tyrosine 766 in the fibroblast growth factor receptor-1 is required for FGF-stimulation of phospholipase C, phospholipase D, phospholipase A(2), phosphoinositide 3-kinase and cytoskeletal reorganisation in porcine aortic endothelial cells

Fibroblast growth factor-mediated signalling was studied in porcine aortic endothelial cells expressing either wild-type fibroblast growth factor receptor-1 or a mutant receptor (Y766F) unable to bind phospholipase C-(γ). Stimulation of cells expressing the wild-type receptor resulted in activation of phospholipases C, D and A(2) and increased phosphoinositide 3-kinase activity. Stimulation of the wild-type receptor also resulted in stress fibre formation and a cellular shape change. Cells expressing the Y766F mutant receptor failed to stimulate phospholipase C, D and A(2) as well as phosphoinositide 3-kinase. Furthermore, no stress fibre formation or shape change was observed. Both the wild-type and Y766F receptor mutant activated MAP kinase and elicited proliferative responses in the porcine aortic endothelial cells. Thus, fibroblast growth factor receptor-1 mediated activation of phospholipases C, D and A(2) and phosphoinositide 3-kinase was dependent on tyrosine 766. Furthermore, whilst tyrosine 766 was not required for a proliferative response, it was required for fibroblast growth factor receptor-1 mediated cytoskeletal reorganisation.

Differential gene expression between human neurons and neuronal progenitor cells in culture: an analysis of arrayed cDNA clones in NTera2 human embryonal carcinoma cell line as a model system

To elucidate the highly complex expression pattern of the genes involved in human neuronal differentiation, differential gene expression between human neurons and neuronal progenitor cells was investigated by analysis of a cDNA expression array in a pluripotent human embryonal carcinoma cell line NTera2 (NT2), a model system of human neuronal differentiation. Among 588 arrayed cDNA clones, 87 genes showed a differential expression pattern between undifferentiated neuronal progenitor cells (NT2-U) and NT2-derived differentiated neurons induced by treatment with retinoic acid (RA) (NT2-N), while 26 genes could not be analyzed due to high background signals. The levels of expression of 76 genes, including those encoding a group of transcription factors, intracellular signal-transducing proteins, cell death-regulatory proteins, and growth factors/cytokines/neurotransmitters and their receptors, were elevated after neuronal differentiation, while the levels of 11 genes, including those coding for cellular proliferation-related proteins, were decreased. Among the differentially expressed genes following induction of neuronal differentiation, significant up-regulation of the growth-associated protein (GAP-43), low-affinity nerve growth factor receptor p75 (LNGFR), and defender against apoptotic cell death (DAD1) mRNAs and substantial down-regulation of the proliferation-associated gene (PAG), fibroblast growth factor receptor-1 (FGFR-1), and cellular RA-binding protein-II (CRABP-II) mRNAs were verified by Northern blot analysis. These results indicate that the analysis of cDNA expression arrays provides a useful approach for screening and identification of a set of distinct genes that undergo highly complex regulation during human neuronal differentiation.

Fibroblast growth factor receptor-1-mediated endothelial cell proliferation is dependent on the Src homology (SH) 2/SH3 domain-containing adaptor protein Crk

Stimulation of fibroblast growth factor receptor-1 (FGFR-1) expressed on endothelial cells leads to cellular migration and proliferation. We have examined the role of the Src homology (SH) 2/SH3 domain-containing adaptor protein Crk in these processes. Transient tyrosine phosphorylation of Crk in fibroblast growth factor-2-stimulated endothelial cells was dependent on the juxtamembrane tyrosine residue 463 in FGFR-1, and a Crk SH2 domain precipitated FGFR-1 via phosphorylated Tyr-463, indicating direct complex formation between Crk and FGFR-1. Furthermore, Crk SH2 and SH3 domains formed ligand-independent complexes with Shc, C3G, and the Crk-associated substrate (Cas). Tyrosine phosphorylation of C3G and Cas increased as a consequence of growth factor treatment. We examined the role of Crk in FGFR-1-mediated cellular responses by use of cells expressing chimeric platelet-derived growth factor receptor-alpha/FGFR-1 (alphaR/FR) wild type and mutant Y463F receptors. The kinase activity of alphaR/FR Y463F was intact, but both Crk and the adaptor FRS-2 were no longer tyrosine-phosphorylated in the mutant cells. Both wild type and mutant receptor cells migrated efficiently, whereas cells expressing the mutant alphaR/FR Y463F failed to proliferate and Erk2 and Jun kinase activities were suppressed in these cells. In wild type alphaR/FR cells transiently expressing an SH2 domain mutant of Crk, Erk and Jun kinase activities as well as DNA synthesis were attenuated. Our data indicate that Crk participates in signaling complexes downstream of FGFR-1, which propagate mitogenic signals.

Inhibition of angiogenesis and breast cancer xenograft tumor growth by VEGI, a novel cytokine of the TNF superfamily

Recently, we reported a novel protein of the tumor necrosis factor (TNF) superfamily, named vascular endothelial cell growth inhibitor (VEGI), which is expressed predominantly in endothelial cells. When a secreted form of this new protein was overexpressed in mouse colon cancer cells, the growth of tumors formed by these cells in black mice was inhibited. We now report that recombinant VEGI inhibits the proliferation of endothelial cells but not that of other types of cells examined. The protein also inhibits formation of capillary-like structures by endothelial cells in collagen gels, and the growth of capillaries into collagen gels placed on the chick chorioallantoic membrane. The anticancer potential of VEGI was examined in a breast cancer xenograft tumor model in which the cancer cells were co-injected with Chinese hamster ovary cells overexpressing a secreted form of the protein. The co-injection resulted in potent inhibition of xenograft tumor growth. Our findings are consistent with the view that VEGI is an endothelial cell-specific negative regulator of angiogenesis.

Phosphatidylinositol 3'-kinase-independent p70 S6 kinase activation by fibroblast growth factor receptor-1 is important for proliferation but not differentiation of endothelial cells

p70(s6k) has a role in cell cycle progression in response to specific extracellular stimuli. The signal transduction pathway leading to activation of p70(s6k) by fibroblast growth factor receptor-1 (FGFR-1) was examined in FGF-2-treated rat L6 myoblasts. p70(s6k) was activated in a biphasic and rapamycin-sensitive manner. Although phosphatidylinositol 3'-kinase was not activated in the FGF-2 treated cells, as judged from in vitro and in vivo analyses, wortmannin and LY294002 treatment inhibited p70(s6k) activation. Inhibition of protein kinase C (PKC), by bisindolylmaleimide or by chronic phorbol ester treatment of the FGFR-1 cells, suppressed but did not block p70(s6k) activation. In cells expressing a point-mutated FGFR-1, Y766F, unable to mediate PKC activation, p70(s6k) was still activated, in a bisindolylmaleimide- and phorbol ester-resistant manner. The involvement of S6 kinase in FGFR-1-dependent biological responses was examined in murine brain endothelial cells. In response to FGF-2, these cells differentiate to form tube-like structures in collagen gel cultures and proliferate when cultured on fibronectin. p70(s6k) was not activated in endothelial cells on collagen, whereas activation was observed during proliferation on fibronectin. In agreement with this finding, rapamycin inhibited the proliferative but not the differentiation response. Our results indicate that FGFR-1 mediates p70(s6k) activation by a phosphatidylinositol 3'-kinase-independent mechanism that does not require PKC activation and, furthermore, proliferation, but not differentiation of endothelial cells in response to FGF-2, is associated with p70(s6k) activation.

Basic fibroblast growth factor stimulates phosphatidylcholine-hydrolyzing phospholipase D in osteoblast-like cells

We examined the effect of basic fibroblast growth factor (bFGF) on the activation of phosphatidylcholine-hydrolyzing phospholipase D in osteoblast-like MC3T3-E1 cells. bFGF stimulated both the formations of choline (EC50 was 30 ng/ml) and inositol phosphates (EC50 was 10 ng/ml). Calphostin C, an inhibitor of protein kinase C (PKC), had little effect on the bFGF-induced formation of choline. bFGF stimulated the formation of choline also in PKC down regulated cells. Genistein and methyl 2,5-dihydroxycinnamate, inhibitors of protein tyrosine kinases, significantly suppressed the bFGF-induced formation of choline. Sodium orthovanadate, an inhibitor of protein tyrosine phosphatases, enhanced the bFGF-induced formation of choline. In vitro kinase assay for FGF receptors revealed that FGF receptor 1 and 2 were autophosphorylated after FGF stimulation. bFGF dose-dependently stimulated DNA synthesis of these cells. These results strongly suggest that bFGF activates phosphatidylcholine-hydrolyzing phospholipase D through the activation of tyrosine kinase, but independently of PKC activated by phosphoinositide hydrolysis in osteoblast-like cells.

Stimulation of fibroblast growth factor receptor-1 occupancy and signaling by cell surface-associated syndecans and glypican

The formation of distinctive basic FGF-heparan sulfate complexes is essential for the binding of bFGF to its cognate receptor. In previous experiments, cell-surface heparan sulfate proteoglycans extracted from human lung fibroblasts could not be shown to promote high affinity binding of bFGF when added to heparan sulfate-deficient cells that express FGF receptor-1 (FGFR1) (Aviezer, D., D. Hecht, M. Safran, M. Eisinger, G. David, and A. Yayon. 1994. Cell 79:1005-1013). In alternative tests to establish whether cell-surface proteoglycans can support the formation of the required complexes, K562 cells were first transfected with the IIIc splice variant of FGFR1 and then transfected with constructs coding for either syndecan-1, syndecan-2, syndecan-4 or glypican, or with an antisense syndecan-4 construct. Cells cotransfected with receptor and proteoglycan showed a two- to three- fold increase in neutral salt-resistant specific 125I-bFGF binding in comparison to cells transfected with only receptor or cells cotransfected with receptor and anti-syndecan-4. Exogenous heparin enhanced the specific binding and affinity cross-linking of 125I-bFGF to FGFR1 in receptor transfectants that were not cotransfected with proteoglycan, but had no effect on this binding and decreased the yield of bFGFR cross-links in cells that were cotransfected with proteoglycan. Receptor-transfectant cells showed a decrease in glycophorin A expression when exposed to bFGF. This suppression was dose-dependent and obtained at significantly lower concentrations of bFGF in proteoglycan-cotransfected cells. Finally, complementary cell-free binding assays indicated that the affinity of 125I-bFGF for an immobilized FGFR1 ectodomain was increased threefold when the syndecan-4 ectodomain was coimmobilized with receptor. Equimolar amounts of soluble syndecan-4 ectodomain, in contrast, had no effect on this binding. We conclude that, at least in K562 cells, syndecans and glypican can support bFGF-FGFR1 interactions and signaling, and that cell-surface association may augment their effectiveness.

Suppression of platelet-derived growth factor alpha- and beta-receptor mRNA levels in human fibroblasts by SV40 T/t antigen

It is known that down-regulation of cell surface platelet-derived growth factor (PDGF) receptors accompanies transformation by SV40. In this work human embryonic lung fibroblasts were used as a model system to study the effects of SV 40 on PDGF receptor expression. It is shown that transformation by SV 40 early region leads to a total loss of PDGF alpha-receptor and partial loss of beta-receptor mRNA. Microinjection experiments revealed that receptor down-regulation was a primary effect, and not only secondary to transformation and clonal selection. Total loss of PDGF alpha-receptor expression requires both large T and small t, and down-regulation of the PDGF alpha-receptor occurs independently of p53 and Rb binding to large T.

Expression of mRNAs of multiple growth factors and receptors by neuronal cell lines: detection with RT-PCR

Neurons and glia are capable of both secreting and responding to a large variety of growth factors. However, information on multiple expression of growth factors and their receptors was usually obtained from uncorrelated observations, using cells from various animals of origin, developmental stages, growth phases, culture ages and culture conditions. Because of its specificity and extreme sensitivity, reverse transcription-polymerase chain reaction (RT-PCR) is uniquely suitable to study a large panel of growth factors and their receptors from a limited cell sample, free of these intervening variables. In this paper we evaluate the expression of mRNA of a total of 35 growth factor-related proteins by conducting RT-PCR on three neuronal cell lines: the PC12 rat pheochromocytoma line, the MAH rat sympathoadrenal progenitor line, and the N18 mouse neuroblastoma line. Three types of results are presented. The first confirms the existing knowledge such as the presence of Trk-A (NFG receptor) in PC12. The second consists of new information that expands and extends earlier observations, such as the presence of CNTF receptor complex in PC12, which explains our previous report that CNTF enhances the biological effects of NGF on these cells. The third consists of novel information that leads the way to further experimentation by the more conventional methods. These include the strong expression of Trk-B by MAH, predicting the biological responsiveness of MAH to BDNF and NT-4, and the expression of CNTF receptor in N18. Our results also suggest that CNTF is an autocrine factor for PC12 and MAH, since both lines express the growth factor as well as the receptor. Thus, RT-PCR is a valuable tool in growth factor research that can be used in complement to, and interactively with, other approaches such as bioassay, receptor binding, and immunochemical determination. It will be particularly useful for screening a large number of growth factors in minute areas of the brain in patients suffering from neurodegenerative diseases such as Parkinson's and Alzheimer's.

Shc and a novel 89-kDa component couple to the Grb2-Sos complex in fibroblast growth factor-2-stimulated cells

A major pathway for mitogenicity is gated via the small GTP-binding protein Ras. Receptor tyrosine kinases couple to Ras through the Src homology 2 (SH2) domain protein Grb2. The activated fibroblast growth factor receptor-1 (FGFR-1) expressed in L6 myoblasts did not bind Grb2 directly, but indirectly, through the small adaptor protein Shc, which was tyrosine-phosphorylated in response to fibroblast growth factor-2 (FGF-2) stimulation. A FGFR-1 mutant in which Tyr766, a known autophosphorylation site, was changed to Phe, mediated less efficient tyrosine phosphorylation of Shc. FGF-2 stimulation of mutant FGFR-1-expressing cells still allowed formation of complexes containing Shc, Grb2, and the nucleotide exchange factor Sos and mediation of a mitogenic signal. Another pool of Grb2 was found in complex with a tyrosine-phosphorylated 89-kDa component after FGF-2 stimulation. Stimulation with other growth factors did not lead to tyrosine phosphorylation of p89. As shown by "far-Western" analysis, p89 bound directly to the Grb2 SH2 domain, and this interaction was inhibited by a peptide containing the Y(P)-X-N motif. Tyrosine-phosphorylated p89 was found exclusively in the membrane fraction, indicating its role in bringing Grb2, as well as Sos, to the plasma membrane. These data support the concept of growth factor-specific coupling of Grb2 to the Ras pathway.

Expression of mRNAs of multiple growth factors and receptors by astrocytes and glioma cells: detection with reverse transcription-polymerase chain reaction

1. Although glial cells in culture are known to secrete growth factors and are also known to be responsive to some of them, detailed comparisons are difficult because the bulk of information was based on various animals of origin, developmental stages, growth properties, culture age, and culture conditions. 2. To present a unified picture of the growth factors and their receptors found in glial cells, we surveyed the expression of messenger RNAs of a panel of growth factors and receptors, using reverse transcription-polymerase chain reaction (RT-PCR), in three common glial cell types: rat astrocytes in primary culture, rat glioma line C6, and human glioma line A172. 3. We observed that normal and neoplastic glial cells in culture express multiple growth factors and also possess most of the receptors to the factors, suggesting multiple autocrine functions. In addition, glia produce growth factors known to be capable of acting on neurons, implicating paracrine function involving glia-neuron interaction. Glial cells also produce growth factors and receptors that are capable of communicating with hematopoietic cells, suggesting neuroimmunologic interaction. What is most interesting is that glial cells express receptors for growth factors previously thought to be acting on neurons only. 4. The current study demonstrates the feasibility of screening from a small sample a large number of growth factors and receptors. The method portends future clinical application to biopsy or necropsy samples from brain tumors or pathologic brains suffering from degenerative diseases such as Alzheimer's or Parkinson's disease.

Effects of oxygen on wound responses to growth factors: Kaposi's FGF, but not basic FGF stimulates repair in ischemic wounds

Kaposi's fibroblast growth factor (K-FGF, FGF-4) is a newer member of FGF family with uncharacterized wound healing properties. Basic fibroblast growth factor (bFGF, FGF-2) has been well studied and accelerates repair in normal and impaired wound healing models. K-FGF and bFGF are known to have similar biological effects in tissue culture, and both stimulate fibroblast and endothelial cell proliferation. The rabbit dermal ulcer model was used to examine the effects of bFGF and K-FGF under ischemic and nonischemic conditions. We found bFGF was ineffective in stimulating healing under ischemic conditions even at high doses (30 micrograms/wound). However, when the ischemic wounds were treated with bFGF (5 micrograms/wound) plus hyperbaric oxygen therapy, it was highly effective again as previously found under nonischemic conditions (P < 0.05). In contrast K-FGF stimulated repair in both nonischemic and ischemic wounds (P < 0.05). These results suggest that wound oxygen content differentially regulates responsiveness to bFGF and that K-FGF is biologically active in hypoxic wounds.

Characteristics of FGF-receptors expressed by stromal and epithelial cells cultured from normal and hyperplastic prostates

Three fibroblast growth factors (FGFs), acidic FGF (FGF1), basic FGF (FGF2), and keratinocyte growth factor (FGF7) have been identified in prostate. To understand how FGFs regulate growth of the prostate, and to determine if regulation is altered in benign prostatic hyperplasia (BPH), the mitogenic potential of FGFs, receptor binding, and FGF-receptor (FGFR) gene expression of stromal (PS) and epithelial cells (PE) cultured from normal human prostate and BPH where determined. FGF1 and FGF2, but not FGF7, were mitogens for PS. FGF1 and FGF7 were potent mitogens for PE, but FGF2 was a weak mitogen for these cells. Both PS and PE exhibited high affinity binding (pM K) of iodinated-FGF2. The K was 4-fold and 12-fold higher for PS than for PE cultured from normal prostate and BPH, respectively. Northern analysis indicated that PS, but not PE, expressed FGFR type 1 (FGFR1) mRNA. The reverse transcriptase polymerase chain reaction (RT-PCR) was used to evaluate FGFR type 2 (FGFR2) expression. The size of amplified DNA fragments, and nucleotide sequences, indicated that PS also expressed transcripts for the exon IIIc RNA splice variant of FGFR2. A RT-PCR product with the FGFR2 exon IIIb nucleotide sequence joined with the exon IIIc sequence was amplified with poly A+ RNA from PE and primers spanning both exons. Thus, PE did not alternatively splice mRNA for FGFR2 exon IIIb and exon IIIc. No differences in the mitogenic potential of FGFs, receptor binding (K or number of sites), or FGFR gene expression were found in cells cultured from normal prostate and BPH.

Structural and functional specificity of FGF receptors

Fibroblast growth factors (FGFs) represent a group of polypeptide mitogens eliciting a wide variety of responses depending on the target cell type. The knowledge of the cell surface receptors mediating the effects of FGFs has recently expanded remarkably. Perhaps not surprisingly, the complexity of the FGF family and FGF induced responses is reflected in the diversity and redundancy of the FGF receptors. The molecular cloning of the signal transducing receptors for fibroblast growth factors has revealed a tyrosine kinase gene family with at least four members. Differential splicing and polyadenylation creates further diversity in the FGF receptor system. These numerous receptor forms have both distinct and redundant properties. We are only now beginning to understand how the different receptors are activated by the various FGFs and how they are expressed by various cells and tissues. FGF binding to the tyrosine kinase receptors needs the assistance of heparan sulphate side chains of proteoglycans present at the cell surface and in the extracellular matrix. As several other growth factors share the heparin binding property of FGFs, the dual receptor system for FGFs might be an example of a more widely used principle.

A chimera between platelet-derived growth factor beta-receptor and fibroblast growth factor receptor-1 stimulates pancreatic beta-cell DNA synthesis in the presence of PDGF-BB

This study was undertaken to characterize the expression of a chimeric growth factor receptor composed of the extracellular and transmembrane domains of the platelet-derived growth factor (PDGF) beta-receptor (PDGFR-beta) fused to the intracellular domain of the fibroblast growth factor receptor-1 (FGFR-1) and to assess its effect on the growth potential of pancreatic islet cells. For this purpose rat pancreatic islets or monolayers of pancreatic islet cells were transfected with recombinant DNA constructs coding for the PDGF B-chain, the PDGFR-beta, the FGFR-1 and the chimera between PDGFR-beta and FGFR-1. DNA synthesis, monitored as the percentage of labelled nuclei and [3H]thymidine incorporation, was stimulated in pancreatic islet cells cotransfected with the constructs coding for the PDGF B-chain and the PDGFR-beta or the chimeric PDGFR-beta/FGFR-1 as compared with that determined after transfection with control plasmid. PDGF-BB stimulated DNA synthesis when islet cells had been transfected with PDGFR-beta or PDGFR-beta/FGFR-1. Cotransfection of the PDGFR-beta and the chimeric PDGFR-beta/FGFR-1 constructs attenuated the stimulation of DNA synthesis in response to PDGF-BB. Receptor binding studies showed binding with a Kd of 0.7 nM to the chimeric receptor. The present findings show that when the chimeric PDGFR-beta/FGFR-1 construct is expressed in beta-cells it is efficient in increasing DNA synthesis when stimulated with ligand.

Diverse receptors for fibroblast growth factors

The development and maintenance of multicellular organisms requires a complex interplay between cells in different tissues. Many of the factors mediating cell-cell communication are polypeptides, which were originally identified because of their ability to stimulate cell growth. In addition to growth signalling several of these factors have been observed to modulate cell survival, chemotaxis and differentiation both in vitro and in vivo. Fibroblast growth factors are a good example of polypeptide mitogens eliciting a wide variety of responses depending on the target cell type. Our knowledge of the cell surface receptors mediating the effects of FGFs has recently expanded remarkably. Perhaps not surprisingly, the complexity of the FGF family and FGF induced responses is reflected as diversity and redundancy of the FGF receptors.