Point mutation in the fibroblast growth factor receptor eliminates phosphatidylinositol hydrolysis without affecting neuronal differentiation of PC12 cells

Overexpressed fibroblast growth factor receptors increase 1,25-dihydroxyvitamin D-dependent differentiation of acute myeloid leukemia cells

Many malignancies are driven by mutations within the gene for fibroblast growth factor receptor 1 (FGFR1). Previously, we have shown that signal transduction from the FOP2-FGFR1 fusion protein in acute myeloid leukemia KG1 cells is responsible for a low level of expression of the vitamin D receptor gene. In this paper, we address whether other fibroblast growth factor receptors regulate the vitamin D receptor (VDR) gene. We used the human myeloid leukemia U937 and HL60 cells, the bone cancer cell line U2OS, and cell transfection methods to answer the question. For myeloid leukemia cells, overexpression of FGFRs 1-3 genes caused a shift towards monocytic differentiation; this was extracellular regulated kinase (Erk) 1,2-dependent. Overexpression of FGFRs 1-3 genes also upregulated expression of the VDR gene, further sensitizing these cells to 1,25-dihydroxyvitamin D-induced monocyte differentiation. When we increased expression in bone cells, fibroblast growth factor receptors did not upregulate VDR gene expression, nor influence the activity of VDR. Fibroblast growth factor receptors are overexpressed in many neoplasms. Therefore, it may be reasonable to use vitamin D analogs to treat these cancers, to activate VDR and drive cell differentiation.

Fibroblast growth factor and kidney disease: Updates for emerging novel therapeutics

The discovery of fibroblast growth factors (FGFs) and fibroblast growth factor receptors (FGFRs) provided a profound new insight into physiological and metabolic functions. FGF has a large family by having divergent structural elements and enable functional divergence and specification. FGF and FGFRs are highly expressed during kidney development. Signals from the ureteric bud regulate morphogenesis, nephrogenesis, and nephron progenitor survival. Thus, FGF signaling plays an important role in kidney progenitor cell aggregation at the sites of new nephron formation. This review will summarize the current knowledge about functions of FGF signaling in kidney development and their ability to promote regeneration in injured kidneys and its use as a biomarker and therapeutic target in kidney diseases. Further studies are essential to determine the predictive significance of the various FGF/FGFR deviations and to integrate them into clinical algorithms.

The Potential of FGF-2 in Craniofacial Bone Tissue Engineering: A Review

Bone is a hard-vascularized tissue, which renews itself continuously to adapt to the mechanical and metabolic demands of the body. The craniofacial area is prone to trauma and pathologies that often result in large bone damage, these leading to both aesthetic and functional complications for patients. The "gold standard" for treating these large defects is autologous bone grafting, which has some drawbacks including the requirement for a second surgical site with quantity of bone limitations, pain and other surgical complications. Indeed, tissue engineering combining a biomaterial with the appropriate cells and molecules of interest would allow a new therapeutic approach to treat large bone defects while avoiding complications associated with a second surgical site. This review first outlines the current knowledge of bone remodeling and the different signaling pathways involved seeking to improve our understanding of the roles of each to be able to stimulate or inhibit them. Secondly, it highlights the interesting characteristics of one growth factor in particular, FGF-2, and its role in bone homeostasis, before then analyzing its potential usefulness in craniofacial bone tissue engineering because of its proliferative, pro-angiogenic and pro-osteogenic effects depending on its spatial-temporal use, dose and mode of administration.

A threshold model for receptor tyrosine kinase signaling specificity and cell fate determination

Upon ligand engagement, the single-pass transmembrane receptor tyrosine kinases (RTKs) dimerize to transmit qualitatively and quantitatively different intracellular signals that alter the transcriptional landscape and thereby determine the cellular response. The molecular mechanisms underlying these fundamental events are not well understood. Considering recent insights into the structural biology of fibroblast growth factor signaling, we propose a threshold model for RTK signaling specificity in which quantitative differences in the strength/longevity of ligand-induced receptor dimers on the cell surface lead to quantitative differences in the phosphorylation of activation loop (A-loop) tyrosines as well as qualitative differences in the phosphorylation of tyrosines mediating substrate recruitment. In this model, quantitative differences on A-loop tyrosine phosphorylation result in gradations in kinase activation, leading to the generation of intracellular signals of varying amplitude/duration. In contrast, qualitative differences in the pattern of tyrosine phosphorylation on the receptor result in the recruitment/activation of distinct substrates/intracellular pathways. Commensurate with both the dynamics of the intracellular signal and the types of intracellular pathways activated, unique transcriptional signatures are established. Our model provides a framework for engineering clinically useful ligands that can tune receptor dimerization stability so as to bias the cellular transcriptome to achieve a desired cellular output.

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.

Phosphorylation of serine 779 in fibroblast growth factor receptor 1 and 2 by protein kinase C(epsilon) regulates Ras/mitogen-activated protein kinase signaling and neuronal differentiation

The FGF receptors (FGFRs) control a multitude of cellular processes both during development and in the adult through the initiation of signaling cascades that regulate proliferation, survival, and differentiation. Although FGFR tyrosine phosphorylation and the recruitment of Src homology 2 domain proteins have been widely described, we have previously shown that FGFR is also phosphorylated on Ser(779) in response to ligand and binds the 14-3-3 family of phosphoserine/threonine-binding adaptor/scaffold proteins. However, whether this receptor phosphoserine mode of signaling is able to regulate specific signaling pathways and biological responses is unclear. Using PC12 pheochromocytoma cells and primary mouse bone marrow stromal cells as models for growth factor-regulated neuronal differentiation, we show that Ser(779) in the cytoplasmic domains of FGFR1 and FGFR2 is required for the sustained activation of Ras and ERK but not for other FGFR phosphotyrosine pathways. The regulation of Ras and ERK signaling by Ser(779) was critical not only for neuronal differentiation but also for cell survival under limiting growth factor concentrations. PKCε can phosphorylate Ser(779) in vitro, whereas overexpression of PKCε results in constitutive Ser(779) phosphorylation and enhanced PC12 cell differentiation. Furthermore, siRNA knockdown of PKCε reduces both growth factor-induced Ser(779) phosphorylation and neuronal differentiation. Our findings show that in addition to FGFR tyrosine phosphorylation, the phosphorylation of a conserved serine residue, Ser(779), can quantitatively control Ras/MAPK signaling to promote specific cellular responses.

Fibroblast growth factors: biology, function, and application for tissue regeneration

Fibroblast growth factors (FGFs) that signal through FGF receptors (FGFRs) regulate a broad spectrum of biological functions, including cellular proliferation, survival, migration, and differentiation. The FGF signal pathways are the RAS/MAP kinase pathway, PI3 kinase/AKT pathway, and PLCγ pathway, among which the RAS/MAP kinase pathway is known to be predominant. Several studies have recently implicated the in vitro biological functions of FGFs for tissue regeneration. However, to obtain optimal outcomes in vivo, it is important to enhance the half-life of FGFs and their biological stability. Future applications of FGFs are expected when the biological functions of FGFs are potentiated through the appropriate use of delivery systems and scaffolds. This review will introduce the biology and cellular functions of FGFs and deal with the biomaterials based delivery systems and their current applications for the regeneration of tissues, including skin, blood vessel, muscle, adipose, tendon/ligament, cartilage, bone, tooth, and nerve tissues.

Indirect recruitment of the signalling adaptor Shc to the fibroblast growth factor receptor 2 (FGFR2)

The adaptor protein Shc (Src homology and collagen-containing protein) plays an important role in the activation of signalling pathways downstream of RTKs (receptor tyrosine kinases) regulating diverse cellular functions, such as differentiation, adhesion, migration and mitogenesis. Despite being phosphorylated downstream of members of the FGFR (fibroblast growth factor receptor) family, a direct interaction of Shc with this receptor family has not been described to date. Various studies have suggested potential binding sites for the Shc PTB domain (phosphotyrosine-binding domain) and/or the SH2 (Src homology 2) domain on FGFR1, but no interaction of full-length Shc with these sites has been reported in vivo. In the present study, we investigated the importance of the SH2 domain and the PTB domain in recruitment of Shc to FGFR2(IIIc) to characterize the interaction of these two proteins. Confocal microscopy revealed extensive co-localization of Shc with FGFR2. The PTB domain was identified as the critical component of Shc which mediates membrane localization. Results from FLIM (fluorescence lifetime imaging microscopy) revealed that the interaction between Shc and FGFR2 is indirect, suggesting that the adaptor protein forms part of a signalling complex containing the receptor. We identified the non-RTK Src as a protein which potentially mediates the formation of such a ternary complex. Although an interaction between Src and Shc has been described previously, in the present study we implicate the Shc SH2 domain as a novel mediator of this association. The recruitment of Shc to FGFR2 via an indirect mechanism provides new insight into the regulation of protein assembly and activation of various signalling pathways downstream of this RTK.

Fibroblast growth factor receptor 2 phosphorylation on serine 779 couples to 14-3-3 and regulates cell survival and proliferation

The fibroblast growth factors (FGFs) exert their diverse (or pleiotropic) biological responses through the binding and activation of specific cell surface receptors (FGFRs). While FGFRs are known to initiate intracellular signaling through receptor tyrosine phosphorylation, the precise mechanisms by which the FGFRs regulate pleiotropic biological responses remain unclear. We now identify a new mechanism by which FGFR2 is able to regulate intracellular signaling and cellular responses. We show that FGFR2 is phosphorylated on serine 779 (S779) in response to FGF2. S779, which lies adjacent to the phospholipase Cgamma binding site at Y766, provides a docking site for the 14-3-3 phosphoserine-binding proteins and is essential for the full activation of the phosphatidylinositol 3-kinase and Ras/mitogen-activated protein kinase pathways. Furthermore, S779 signaling is essential for promoting cell survival and proliferation in both Ba/F3 cells and BALB/c 3T3 fibroblasts. This new mode of FGFR2 phosphoserine signaling via the 14-3-3 proteins may provide an increased repertoire of signaling outputs to allow the regulation of pleiotropic biological responses. In this regard, we have identified conserved putative phosphotyrosine/phosphoserine motifs in the cytoplasmic domains of diverse cell surface receptors, suggesting that they may perform important functional roles beyond the FGFRs.

Role of phospholipase C-gamma in NGF-stimulated differentiation and gene induction

The PC12 phaeochromocytoma cell line provides a useful model to study nerve growth factor-induced neuronal differentiation. The central signaling route of this process is mediated by the Ras-dependent extracellular signal-regulated kinase cascade. However, Ras-independent pathways are also stimulated by nerve growth factor and may contribute to differentiation signaling. One mediator for Ras-independent signal transduction in PC12 cells is phospholipase C-gamma that generates the second messengers diacylglycerol and inositol-trisphosphate. To probe the possible involvement of this enzyme in nerve growth factor-promoted differentiation, we used the phospholipase C inhibitor U73122 and the inositol-trisphosphate-receptor inhibitor Xestospongin C. Our results show that both chemicals block nerve growth factor-promoted neurite outgrowth, but the blockage of phospholipase C does not inhibit nerve growth factor-induced expression of c-fos, zif268 and transin genes. In addition, induction of these genes by nerve growth factor plus dibutyryl-cAMP is comparable in wild-type PC12 cells as well as in cells in which both Ras- and phospholipase C-gamma-mediated pathways are inhibited. The phospholipase C-gamma pathway thus belongs to those nerve growth factor receptor-originated signaling routes that contribute to the biological response of PC12 cells to nerve growth factor, but its gene activating potential does not have a major role in its neuritogenic effect.

Differential signal transduction of alternatively spliced FGFR2 variants expressed in human mammary epithelial cells

Gene amplification and protein overexpression of fibroblast growth factor receptor 2 (FGFR2) characterize the SUM-52 breast cancer cell line developed in our laboratory. SUM-52 cells express nine distinct alternatively spliced isoforms of FGFR2. Among these isoforms are two otherwise identical FGFR2 variants that express either the C1 or C3 carboxyl terminus. FGFR2-C3 variants are not normally expressed by human mammary epithelial (HME) cells, and we have shown that overexpression of FGFR2-C3 in HME cells results in potent transformation. In particular, FGFR2-C3 expression leads to robust levels of constitutively tyrosine phosphorylated FRS2 in the absence of ligand stimulation. In contrast, overexpressed FGFR2-C1 requires constant stimulation with exogenous keratinocyte growth factor (KGF) to mimic the signaling capability of FGFR2-C3. However, activation of FRS2 that results from KGF-stimulated FGFR2-C1 signaling is transient and is associated with a mobility shift of FRS2 not observed when this signaling molecule is activated by the C3 isoform of FGFR2. Mutation of the only tyrosine phosphorylated site present in the C1 terminus and absent from C3, Tyr769, did not yield a receptor that rivaled the potent signaling of FGFR2-C3. We therefore conclude that aberrant expression of alternatively spliced isoforms of FGFR2 with the C3 carboxyl terminus in the SUM-52 breast cancer cells results in sustained activation of signal transduction leading to transformation.

Platelet factor 4 disrupts the intracellular signalling cascade induced by vascular endothelial growth factor by both KDR dependent and independent mechanisms

The mechanism by which the CXC chemokine platelet factor 4 (PF-4) inhibits endothelial cell proliferation is unclear. The heparin-binding domains of PF-4 have been reported to prevent vascular endothelial growth factor 165 (VEGF(165)) and fibroblast growth factor 2 (FGF2) from interacting with their receptors. However, other studies have suggested that PF-4 acts via heparin-binding independent interactions. Here, we compared the effects of PF-4 on the signalling events involved in the proliferation induced by VEGF(165), which binds heparin, and by VEGF(121), which does not. Activation of the VEGF receptor, KDR, and phospholipase Cgamma (PLCgamma) was unaffected in conditions in which PF-4 inhibited VEGF(121)-induced DNA synthesis. In contrast, VEGF(165)-induced phosphorylation of KDR and PLCgamma was partially inhibited by PF-4. These observations are consistent with PF-4 affecting the binding of VEGF(165), but not that of VEGF(121), to KDR. PF-4 also strongly inhibited the VEGF(165)- and VEGF(121)-induced mitogen-activated protein (MAP) kinase signalling pathways comprising Raf1, MEK1/2 and ERK1/2: for VEGF(165) it interacts directly or upstream from Raf1; for VEGF(121), it acts downstream from PLCgamma. Finally, the mechanism by which PF-4 may inhibit the endothelial cell proliferation induced by both VEGF(121) and VEGF(165), involving disruption of the MAP kinase signalling pathway downstream from KDR did not seem to involve CXCR3B activation.

The Shb adaptor protein binds to tyrosine 766 in the FGFR-1 and regulates the Ras/MEK/MAPK pathway via FRS2 phosphorylation in endothelial cells

Stimulation of fibroblast growth factor receptor-1 (FGFR-1) is known to result in phosphorylation of tyrosine 766 and the recruitment and subsequent activation of phospholipase C-gamma (PLC-gamma). To assess the role of tyrosine 766 in endothelial cell function, we generated endothelial cells expressing a chimeric receptor, composed of the extracellular domain of the PDGF receptor-alpha and the intracellular domain of FGFR-1. Mutation of tyrosine 766 to phenylalanine prevented PLC-gamma activation and resulted in a reduced phosphorylation of FRS2 and reduced activation of the Ras/MEK/MAPK pathway relative to the wild-type chimeric receptor. However, FGFR-1-mediated MAPK activation was not dependent on PKC activation or intracellular calcium, both downstream mediators of PLC-gamma activation. We report that the adaptor protein Shb is also able to bind tyrosine 766 in the FGFR-1, via its SH2 domain, resulting in its subsequent phosphorylation. Overexpression of an SH2 domain mutant Shb caused a dramatic reduction in FGFR-1-mediated FRS2 phosphorylation with concomitant perturbment of the Ras/MEK/MAPK pathway. Expression of the chimeric receptor mutant and the Shb SH2 domain mutant resulted in a similar reduction in FGFR-1-mediated mitogenicity. We conclude, that Shb binds to tyrosine 766 in the FGFR-1 and regulates FGF-mediated mitogenicity via FRS2 phosphorylation and the subsequent activation of the Ras/MEK/MAPK pathway.

Fibroblast growth factors and their receptors in hematopoiesis and hematological tumors

Fibroblast growth factors (FGFs) belong to a family of pleiotropic heparin-binding growth factors. They exert multiple functions on various cell types of mesodermal and neuroectodermal origin, affecting cell proliferation, motility, survival, and differentiation. FGF's exert their activity by interacting with tyrosine kinase receptors (FGFRs) and cell-surface heparan sulfate proteoglycans. This article reviews recent studies on the role of the FGF/FGFR system in embryonic hematopoietic development, hematopoiesis, and hematological tumors. FGFs exert both autocrine and paracrine functions in these biological processes by acting on blood cells and their precursors and accessory cells in the bone marrow, including stromal and endothelial cells.

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

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

The role of tyrosine residues in fibroblast growth factor receptor 1 signaling in PC12 cells. Systematic site-directed mutagenesis in the endodomain

To assess the contribution of the intracellular domain tyrosine residues to the signaling capacity of fibroblast growth factor receptor 1 (FGFR1), stably transfected chimeras bearing the ectodomain of the platelet-derived growth factor receptor (PDGFR) and the endodomain of FGFR1 were systematically altered by a tyrosine to phenylalanine bloc and individual conversions. The 15 tyrosine residues of the endodomain of this construct (PFR1) were divided into four linear segments (labeled A, B, C, and D) that contained 4, 4, 2, and 5 tyrosine residues, respectively. When stimulated by platelet-derived growth factor, derivatives in which the A, B, or A + B blocs of tyrosines were mutated were about two-thirds as active as the unmodified chimera at 48 h but achieved full activity by 96 h in a neurite outgrowth assay in transfected PC12 cells. Elimination of only the two activation loop tyrosines (C bloc) also inactivated the receptor. All derivatives in which 4 (or 5) of the D bloc tyrosines were mutated were inactive in producing differentiation but showed low levels of kinase activity in in vitro assays. Derivatives in which 1, 2, or 3 tyrosines of the D bloc in different combinations were systematically changed demonstrated that 2 residues (Tyr(677) and Tyr(701), using hFGFR1 numbering) were essential for bioactivity, but the remaining 3 residues, including Tyr(766), the previously identified site for phospholipase C gamma (PLC gamma) activation, were not. Differentiation activity was paralleled by the activation (phosphorylation) of FRS2, SOS, and ERK1/2. PLC gamma activity was dependent on the presence of Tyr(766) but also required Tyr(677) and/or Tyr(701). Although fully active chimeras did not require PLC gamma, the responses of chimeras showing reduced activation of FRS2 were significantly enhanced by this activity. These results establish that PFR1 does not utilize any tyrosine residues, phosphorylated or not, to activate FRS2. However, it does require Tyr(677) and/or Tyr(701), which may function to stabilize the active conformation directly or indirectly.

Inhibition of cell migration by 24-kDa fibroblast growth factor-2 is dependent upon the estrogen receptor

The single-copy gene for fibroblast growth factor-2 (FGF-2) encodes for multiple forms of the protein with molecular masses of 24, 22.5, 22, and 18 kDa. We reported previously that the 24-22-kDa FGF-2 forms inhibit the migration of endothelial and MCF-7 cells by 50% and 70%, respectively. Here we show that this inhibition of migration is mediated by the estrogen receptor (ER). We have found that depletion of the receptor in either cell line abrogates the inhibitory activity of 24-kDa FGF-2 while re-introduction of the ER into deficient cells once again promotes the inhibitory response. To determine whether exposure to 24-kDa FGF-2 resulted in the activation of the estrogen receptor, 3T3 cells were cotransfected with estrogen receptor cDNA and an estrogen regulatory element-luciferase gene reporter construct and treated with 24- and 18-kDa FGF-2. The high molecular weight form stimulated luciferase activity 5-fold while 18-kDa FGF-2 at the same concentration had no effect. Treatment of ER-positive MCF-7 cells transfected with the reporter construct only showed the same results. Inclusion of the pure estrogen antagonist ICI 182,780 blocked the increase in luciferase activity by 24-kDa FGF-2, further indicating that the response was estrogen receptor dependent. Expression of dominant negative FGF receptor 1 inhibited ER activation, indicating that this was the cell surface receptor mediating the effect. Although growth factor-dependent activation of the ER was reported to require mitogen-activated protein kinase-induced phosphorylation at Ser(118) in COS and HeLa cells, this mechanism is not involved with the activation by 24-kDa FGF-2. These results suggest that the addition of 55 amino acids to the amino-terminal end of 18-kDa FGF-2 by alternative translation alters FGF-2 function and allows for the activation of a second signaling pathway involving the estrogen receptor.

Transformation and Stat activation by derivatives of FGFR1, FGFR3, and FGFR4

The fibroblast growth factor receptor (FGFR) family members mediate a number of important cellular processes, and are mutated or overexpressed in several forms of human cancer. Mutation of Lys650-->Glu in the activation loop of the FGFR3 kinase domain causes the lethal human skeletal disorder thanatophoric dysplasia type II (TDII) and is also found in patients with multiple myeloma, bladder and cervical carcinomas. This mutation leads to constitutive activation of FGFR3. To compare the signaling activity of FGFR family members, this activating mutation was generated in FGFR1, FGFR3, and FGFR4. We show that the kinase domains of FGFR1, FGFR3, and FGFR4 containing the activation loop mutation, when targeted to the plasma membrane by a myristylation signal, can transform NIH3T3 cells and induce neurite outgrowth in PC12 cells. Phosphorylation of Shp2, PLC-gamma, and MAPK was also stimulated by all three 'TDII-like' FGFR derivatives. Additionally, activation of Stat1 and Stat3 was observed in cells expressing the activated FGFR derivatives. Finally, we demonstrate that FGFR1, FGFR3, and FGFR4 derivatives can stimulate PI-3 kinase activity. Our comparison of these activated receptor derivatives reveals a significant overlap in the panel of effector proteins used to mediate downstream signals. This also represents the first demonstration that activation of FGFR4, in addition to FGFR1 and FGFR3, can induce cellular transformation. Moreover, our results suggest that Stat activation by FGFRs is important in their ability to act as oncogenes.

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.

Association of fibroblast growth factor receptor 1 with the adaptor protein Grb14. Characterization of a new receptor binding partner

Using the cytoplasmic domain of fibroblast growth factor receptor 1 (FGFR1) as bait in a yeast two-hybrid screen, Grb14 was identified as a FGFR1 binding partner. A kinase-inactive mutant of FGFR1 failed to interact with Grb14, indicating that activation of FGFR1 is necessary for binding. Deletion of the C-tail or mutation of both C-tail tyrosine residues of FGFR1 to phenylalanine abolished binding, and deletion of the juxtamembrane domain of the receptor reduced binding, suggesting that Grb14 binds to FGFR1 at multiple sites. Co-immunoprecipitation and in vitro binding assays demonstrated that binding of Grb14 to FGFR1 in mammalian cells was dependent on receptor activation by fibroblast growth factor-2 (FGF-2). Deletion of the Src homology 2 (SH2) domain of Grb14 reduced but did not block binding to FGFR1 and eliminated dependence on receptor activation. The SH2 domain alone bound both FGFR1 and platelet-derived growth factor receptor, whereas full-length Grb14 bound only FGFR1, suggesting that regions upstream of the SH2 domain confer specificity for FGFR1. Grb14 was phosphorylated on serine and threonine residues in unstimulated cells, and treatment with FGF-2 enhanced this phosphorylation. Expression of exogenous Grb14 inhibited FGF-2-induced cell proliferation, whereas a point-mutated form of Grb14 incapable of binding to FGFR1 enhanced FGF-2-induced mitogenesis. These data demonstrate an interaction between activated FGFR1 and Grb14 and suggest a role for Grb14 in FGF signaling.

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.

Replacing two conserved tyrosines of the EphB2 receptor with glutamic acid prevents binding of SH2 domains without abrogating kinase activity and biological responses

Eph receptor tyrosine kinases play key roles in pattern formation during embryonic development, but little is known about the mechanisms by which they elicit specific biological responses in cells. Here, we investigate the role of tyrosines 605 and 611 in the juxtamembrane region of EphB2, because they are conserved Eph receptor autophosphorylation sites and demonstrated binding sites for the SH2 domains of multiple signaling proteins. Mutation of tyrosines 605 and 611 to phenylalanine impaired EphB2 kinase activity, complicating analysis of their function as SH2 domain binding sites and their contribution to EphB2-mediated signaling. In contrast, mutation to the negatively charged glutamic acid disrupted SH2 domain binding without reducing EphB2 kinase activity. By using a panel of EphB2 mutants, we found that kinase activity is required for the changes in cell-matrix and cell - cell adhesion, cytoskeletal organization, and activation of mitogen-activated protein (MAP) kinases elicited by EphB2 in transiently transfected cells. Instead, the two juxtamembrane SH2 domain binding sites were dispensable for these effects. These results suggest that phosphorylation of tyrosines 605 and 611 is critical for EphB2-mediated cellular responses because it regulates EphB2 kinase activity.

Fibroblast growth factor-2 stimulates phospholipase Cβ in adult cardiomyocytes

Although fibroblast growth factor-2 (FGF-2) plays an important role in cardioprotection and growth, little is known about the signals triggered by it in the adult heart. We therefore examined FGF-2-induced effects on phosphoinositide-specific phospholipase C (PI-PLC) isozymes, which produce second messengers linked to the inotropic and hypertrophic response of the myocardium. FGF-2, administered by retrograde perfusion to the isolated heart, induced an increase in inositol-1,4,5-trisphosphate levels in the cytosol, as well as an increase in total PI-PLC activity associated with sarcolemmal and cytosolic fractions. Furthermore FGF-2 induced a time-dependent elevation in cardiomyocyte membrane-associated PLC gamma1 and PLC β1 activities, assayed in immunoprecipitated fractions, and moreover, increased the membrane levels of PLC β1 and PLC β3. Activation of PLC β is suggestive of FGF-2-induced cross-talk between FGF-receptor tyrosine kinase and G-protein-coupled signaling in adult cardiomyocytes and underscores the importance of FGF-2 in cardiac physiology.Key words: FGF-2, signal transduction, PLC gamma, PLC β, cardiomyocytes.

The role of individual SH2 domains in mediating association of phospholipase C-gamma1 with the activated EGF receptor

The two SH2 (Src homology domain 2) domains present in phospholipase C-gamma1 (PLC-gamma1) were assayed for their capacities to recognize the five autophosphorylation sites in the epidermal growth factor receptor. Plasmon resonance and immunological techniques were employed to measure interactions between SH2 fusion proteins and phosphotyrosine-containing peptides. The N-SH2 domain recognized peptides in the order of pY1173 > pY992 > pY1068 > pY1148 >> pY1086, while the C-SH2 domain recognized peptides in the order of pY992 > pY1068 > pY1148 >> pY1086 and pY1173. The major autophosphorylation site, pY1173, was recognized only by the N-SH2 domain. Contributions of the N-SH2 and C-SH2 domains to the association of the intact PLC-gamma1 molecule with the activated epidermal growth factor (EGF) receptor were assessed in vivo. Loss of function mutants of each SH2 domain were produced in a full-length epitope-tagged PLC-gamma1. After expression of the mutants, cells were treated with EGF and association of exogenous PLC-gamma1 with EGF receptors was measured. In this context the N-SH2 is the primary contributor to PLC-gamma1 association with the EGF receptor. The combined results suggest an association mechanism involving the N-SH2 domain and the pY1173 autophosphorylation site as a primary event and the C-SH2 domain and the pY992 autophosphorylation site as a secondary event.

Fibroblast growth factor receptors participate in the control of mitogen-activated protein kinase activity during nerve growth factor-induced neuronal differentiation of PC12 cells

The current paradigm for the role of nerve growth factor (NGF) or FGF-2 in the differentiation of neuronal cells implies their binding to specific receptors and activation of kinase cascades leading to the expression of differentiation specific genes. We examined herein the hypothesis that FGF receptors (FGFRs) are involved in NGF-induced neuritogenesis of pheochromocytoma-derived PC12 cells. We demonstrate that in PC12 cells, FGFR expression and activity are modulated upon NGF treatment and that a dominant negative FGFR-2 reduces NGF-induced neuritogenesis. Moreover, FGF-2 expression is modulated by NGF, and FGF-2 is detected at the cell surface. Oligonucleotides that specifically inhibit FGF-2 binding to its receptors are able to significantly reduce NGF-induced neurite outgrowth. Finally, the duration of mitogen-activated protein kinase (MAPK) activity upon FGF or NGF stimulation is shortened in FGFR-2 dominant negative cells through inactivation of signaling from the receptor to the Ras/MAPK pathway. In conclusion, these results demonstrate that FGFR activation is involved in neuritogenesis induced by NGF where it contributes to a sustained MAPK activity in response to NGF.

Physiological requirement for both SH2 domains for phospholipase C-gamma1 function and interaction with platelet-derived growth factor receptors

Two approaches have been utilized to investigate the role of individual SH2 domains in growth factor activation of phospholipase C-gamma1 (PLC-gamma1). Surface plasmon resonance analysis indicates that the individual N-SH2 and C-SH2 domains are able to specifically recognize a phosphotyrosine-containing peptide corresponding to Tyr 1021 of the platelet-derived growth factor (PDGF) beta receptor. To assess SH2 function in the context of the full-length PLC-gamma1 molecule as well as within the intact cell, PLC-gamma1 SH2 domain mutants, disabled by site-directed mutagenesis of the N-SH2 and/or C-SH2 domain(s), were expressed in Plcg1(-/-) fibroblasts. Under equilibrium incubation conditions (4 degrees C, 40 min), the N-SH2 domain, but not the C-SH2 domain, was sufficient to mediate significant PLC-gamma1 association with the activated PDGF receptor and PLC-gamma1 tyrosine phosphorylation. When both SH2 domains in PLC-gamma1 were disabled, the double mutant did not associate with activated PDGF receptors and was not tyrosine phosphorylated. However, no single SH2 mutant was able to mediate growth factor activation of Ca2+ mobilization or inositol 1,4,5-trisphosphate (IP3) formation. Subsequent kinetic experiments demonstrated that each single SH2 domain mutant was significantly impaired in its capacity to mediate rapid association with activated PDGF receptors and become tyrosine phosphorylated. Hence, when assayed under physiological conditions necessary to achieve a rapid biological response (Ca2+ mobilization and IP3 formation), both SH2 domains of PLC-gamma1 are essential to growth factor responsiveness.

p38 mitogen-activated protein kinase activation is required for fibroblast growth factor-2-stimulated cell proliferation but not differentiation

Basic fibroblast growth factor (FGF-2) is a member of a family of polypeptides that have roles in a wide range of biological processes. To determine why different cell types show distinct responses to treatment with FGF-2, the array of FGF receptors present on the surface of a cell which differentiates in response to FGF-2 (PC12 cells) was compared with that present on the surface of a cell that proliferates in response to FGF-2 (Swiss 3T3 fibroblasts). Both cell types express exclusively FGFR1, suggesting that there are cell type-specific FGFR1 signaling pathways. Since mitogen-activated protein kinases function as mediators of cellular responses to a variety of stimuli, the roles of these proteins in FGF-mediated responses were examined. FGF-2 activates extracellular signal-regulated kinases with similar kinetics in both fibroblasts and PC12 cells, and a specific inhibitor of extracellular signal-regulated kinase activation blocks differentiation but has little effect on proliferation. In contrast, while p38 mitogen-activated protein kinase is activated weakly and transiently in PC12 cells treated with FGF-2, a much stronger and sustained activation of this kinase is seen in FGF-2-treated fibroblasts. Furthermore, specific inhibitors of this kinase block proliferation but have no effect on differentiation. This effect on proliferation is specific for FGF-2 since the same concentrations of inhibitors have little or no effect on proliferation induced by serum.

Reconstitution of fibroblast growth factor receptor interactions in the yeast two hybrid system

Fibroblast growth factors (FGF) activate their receptors through the formation of trimolecular complexes, composed of a ligand, a receptor, and a heparan sulfate oligosaccharide, all of which are members of particularly large families capable of multiple interactions in a combinatorial fashion. Understanding this large network of interactions not only presents a great challenge, but is practically beyond the capacity of most classical techniques routinely used to study ligand receptor interactions. We have used the yeast two hybrid system to study protein-protein interactions in the FGF family. Both ligand and receptor ectodomains are properly folded and functional in the yeast. Basic FGF (bFGF) expressed in the yeast dimerizes spontaneously. This self-assembly occurs at low affinity, which can be greatly enhanced by the introduction of heparin, supporting a defined role for heparin in bFGF dimerization. Screening a rat embryo cDNA library with bFGF in the yeast two hybrid system identified a short variant of FGF receptor 1, found most frequently in embryonal and tumor cells and which possesses affinity toward bFGF that is significantly greater than that of the more abundant, full-length receptor. We find the yeast two hybrid system, a most suitable alternative method for the analysis of growth factor-receptor interactions as well as for screening for novel interacting proteins and modulators of FGF and its receptors.

Different tyrosine autophosphorylation requirements in fibroblast growth factor receptor-1 mediate urokinase-type plasminogen activator induction and mitogenesis

Among the seven tyrosine autophosphorylation sites identified in the intracellular domain of tyrosine kinase fibroblast growth factor receptor-1 (FGFR1), five of them are dispensable for FGFR1-mediated mitogenic signaling. The possibility of dissociating the mitogenic activity of basic FGF (FGF2) from its urokinase-type plasminogen activator (uPA)-inducing capacity both at pharmacological and structural levels prompted us to evaluate the role of these autophosphorylation sites in transducing FGF2-mediated uPA upregulation. To this purpose, L6 myoblasts transfected with either wild-type (wt) or various FGFR1 mutants were evaluated for the capacity to upregulate uPA production by FGF2. uPA was induced in cells transfected with wt-FGFR1, FGFR1-Y463F, -Y585F, -Y730F, -Y766F, or -Y583/585F mutants. In contrast, uPA upregulation was prevented in L6 cells transfected with FGFR1-Y463/583/585/730F mutant (FGFR1-4F) or with FGFR1-Y463/583/585/730/766F mutant (FGFR1-5F) that retained instead a full mitogenic response to FGF2; however, preservation of residue Y730 in FGFR1-Y463/583/585F mutant (FGFR1-3F) and FGFR1-Y463/583/585/766F mutant (FGFR1-4Fbis) allows the receptor to transduce uPA upregulation. Wild-type FGFR1, FGFR1-3F, and FGFR1-4F similarly bind to a 90-kDa tyrosine-phosphorylated protein and activate Shc, extracellular signal-regulated kinase (ERK)2, and JunD after stimulation with FGF2. These data, together with the capacity of the ERK kinase inhibitor PD 098059 to prevent ERK2 activation and uPA upregulation in wt-FGFR1 cells, suggest that signaling through the Ras/Raf-1/ERK kinase/ERK/JunD pathway is necessary but not sufficient for uPA induction in L6 transfectants. Accordingly, FGF2 was able to stimulate ERK1/2 phosphorylation and cell proliferation, but not uPA upregulation, in L6 cells transfected with the FGFR1-Y463/730F mutant, whereas the FGFR1-Y583/585/730F mutant was fully active. We conclude that different tyrosine autophosphorylation requirements in FGFR1 mediate cell proliferation and uPA upregulation induced by FGF2 in L6 cells. In particular, phosphorylation of either Y463 or Y730, dispensable for mitogenic signaling, represents an absolute requirement for FGF2-mediated uPA induction.

FGF signal transduction in PC12 cells: comparison of the responses induced by endogenous and chimeric receptors

Rat phaeochromocytoma (PC12) cells respond to many growth factors and produce different phenotypes, including neurite outgrowth. Receptor tyrosine kinases (RTK), which activate multiple signalling pathways in response to ligand binding, initiate many of these. One such family of receptors, the fibroblast growth factor receptor (FGFR), has four different members and expresses at least three of these in PC12 cells. A chimeric tyrosine kinase receptor, consisting of the extracellular domain of human plasma-derived growth factor receptor-beta (hPDGFR-beta) and the transmembrane and intracellular region of FGFR1 (designated PFR1), was constructed and was stably transfected into cloned PC12 cell lines. This chimera, which can be activated without stimulating endogenous RTK including other FGFR, induces neurite outgrowth in a PDGF-dependent manner. By altering the protocol for preparing the retroviral vectors, cells with a wide range of expression levels can be obtained. The amount of these chimeric receptors seems to correlate with the time and the intensity of response as observed in neurite outgrowth assays. Analysis of proteins implicated in FGFR1 signalling indicates that upon stimulation, a tyrosine phosphorylated protein designated FRS2 associates with SOS, Grb2 and the receptor. The chimeric receptor appears entirely similar to that observed for the stimulation of native PC12 cells with FGF2. These results support the view that FRS2 is the dominant FGFR1 signalling entity in PC12 cells.

Estrogen stimulates delayed mitogen-activated protein kinase activity in human endothelial cells via an autocrine loop that involves basic fibroblast growth factor

BACKGROUND

Estrogen plays a significant role in protecting premenopausal women from cardiovascular disease. We have found that estradiol augments endothelial cell activities related to vascular healing and that human coronary artery and umbilical vein endothelial cells express estrogen receptors (ERs). Classically, the ER functions as a transcription factor, but the cytoplasmic targets of this genomic effect have not been defined for endothelial cells. In the present study, we examined the potential role of the mitogen-activated protein (MAP) kinases ERK1 and ERK2 as mediators of estrogen action.

METHODS AND RESULTS

Human umbilical vein endothelial cells were estrogen depleted by culturing in hormone-free medium for 48 hours before experiments. 17Beta-estradiol (E2) stimulated a delayed (3 hours) 5- to 7-fold induction of ERK1/2 activity requiring activation of ER and new transcription/translation. Conditioned media from cells stimulated for 3 hours with E2 induced immediate ERK1/2 activation and phosphorylation of the basic fibroblast growth factor (bFGF) receptor. Moreover, ERK1/2 activation by E2 or by conditioned media was abrogated by treatment with neutralizing anti-bFGF antibody.

CONCLUSIONS

These data describe an autocrine mechanism for E2 induction of ERK1/2 in HUVEC. Because our previous studies suggested that certain cardioprotective effects of estrogen are genomic in nature, the results are consistent with the hypothesis that autocrine stimulation of endothelial ERK1/2 activity by bFGF may play a role in the beneficial effects of estrogen on cardiovascular biology.

Opposite phenotypes of hypomorphic and Y766 phosphorylation site mutations reveal a function for Fgfr1 in anteroposterior patterning of mouse embryos

Intercellular communication is needed for both the generation of the mesodermal germ layer and its division into distinct subpopulations. To dissect the functions of fibroblast growth factor receptor-1 (FGFR1) during mouse gastrulation as well as to gain insights into its possible roles during later embryonic development, we have introduced specific mutations into the Fgfr1 locus by gene targeting. Our results show functional dominance of one of the receptor isoforms and suggest a function for the autophosphorylation of site Y766 in the negative regulation of FGFR1 activity. Y766F and hypomorphic mutations in Fgfr1 generate opposite phenotypes in terms of homeotic vertebral transformations, suggesting a role for FGFR1 in patterning the embryonic anteriorposterior axis by way of regulation of Hox gene activity.

Identification of the cytoplasmic regions of fibroblast growth factor (FGF) receptor 1 which play important roles in induction of neurite outgrowth in PC12 cells by FGF-1

Fibroblast growth factor 1 (FGF-1) induces neurite outgrowth in PC12 cells. Recently, we have shown that the FGF receptor 1 (FGFR-1) is much more potent than FGFR-3 in induction of neurite outgrowth. To identify the cytoplasmic regions of FGFR-1 that are responsible for the induction of neurite outgrowth in PC12 cells, we took advantage of this difference and prepared receptor chimeras containing different regions of the FGFR-1 introduced into the FGFR-3 protein. The chimeric receptors were introduced into FGF-nonresponsive variant PC12 cells (fnr-PC12 cells), and their ability to mediate FGF-stimulated neurite outgrowth of the cells was assessed. The juxtamembrane (JM) and carboxy-terminal (COOH) regions of FGFR-1 were identified as conferring robust and moderate abilities, respectively, for induction of neurite outgrowth to FGFR-3. Analysis of FGF-stimulated activation of signal transduction revealed that the JM region of FGFR-1 conferred strong and sustained tyrosine phosphorylation of several cellular proteins and activation of MAP kinase. The SNT/FRS2 protein was demonstrated to be one of the cellular substrates preferentially phosphorylated by chimeras containing the JM domain of FGFR-1. SNT/FRS2 links FGF signaling to the MAP kinase pathway. Thus, the ability of FGFR-1 JM domain chimeras to induce strong sustained phosphorylation of this protein would explain the ability of these chimeras to activate MAP kinase and hence neurite outgrowth. The role of the COOH region of FGFR-1 in induction of neurite outgrowth involved the tyrosine residue at amino acid position 764, a site required for phospholipase C gamma binding and activation, whereas the JM region functioned primarily through a non-phosphotyrosine-dependent mechanism. In contrast, assessment of the chimeras in the pre-B lymphoid cell line BaF3 for FGF-1-induced mitogenesis revealed that the JM region did not play a role in this cell type. These data indicate that FGFR signaling can be regulated at the level of intracellular interactions and that signaling pathways for neurite outgrowth and mitogenesis use different regions of the FGFR.

Binding of Shp2 tyrosine phosphatase to FRS2 is essential for fibroblast growth factor-induced PC12 cell differentiation

FRS2 is a lipid-anchored docking protein that plays an important role in linking fibroblast growth factor (FGF) and nerve growth factor receptors with the Ras/mitogen-activated protein (MAP) kinase signaling pathway. In this report, we demonstrate that FRS2 forms a complex with the N-terminal SH2 domain of the protein tyrosine phosphatase Shp2 in response to FGF stimulation. FGF stimulation induces tyrosine phosphorylation of Shp2, leading to the formation of a complex containing Grb2 and Sos1 molecules. In addition, a mutant FRS2 deficient in both Grb2 and Shp2 binding induces a weak and transient MAP kinase response and fails to induce PC12 cell differentiation in response to FGF stimulation. Furthermore, FGF is unable to induce differentiation of PC12 cells expressing an FRS2 point mutant deficient in Shp2 binding. Finally, we demonstrate that the catalytic activity of Shp2 is essential for sustained activation of MAP kinase and for potentiation of FGF-induced PC12 cell differentiation. These experiments demonstrate that FRS2 recruits Grb2 molecules both directly and indirectly via complex formation with Shp2 and that Shp2 plays an important role in FGF-induced PC12 cell differentiation.

soc-2 encodes a leucine-rich repeat protein implicated in fibroblast growth factor receptor signaling

Activation of fibroblast growth factor (FGF) receptors elicits diverse cellular responses including growth, mitogenesis, migration, and differentiation. The intracellular signaling pathways that mediate these important processes are not well understood. In Caenorhabditis elegans, suppressors of clr-1 identify genes, termed soc genes, that potentially mediate or activate signaling through the EGL-15 FGF receptor. We demonstrate that three soc genes, soc-1, soc-2, and sem-5, suppress the activity of an activated form of the EGL-15 FGF receptor, consistent with the soc genes functioning downstream of EGL-15. We show that soc-2 encodes a protein composed almost entirely of leucine-rich repeats, a domain implicated in protein-protein interactions. We identified a putative human homolog, SHOC-2, which is 54% identical to SOC-2. We find that shoc-2 maps to 10q25, shoc-2 mRNA is expressed in all tissues assayed, and SHOC-2 protein is cytoplasmically localized. Within the leucine-rich repeats of both SOC-2 and SHOC-2 are two YXNX motifs that are potential tyrosine-phosphorylated docking sites for the SEM-5/GRB2 Src homology 2 domain. However, phosphorylation of these residues is not required for SOC-2 function in vivo, and SHOC-2 is not observed to be tyrosine phosphorylated in response to FGF stimulation. We conclude that this genetic system has allowed for the identification of a conserved gene implicated in mediating FGF receptor signaling in C. elegans.

clr-1 encodes a receptor tyrosine phosphatase that negatively regulates an FGF receptor signaling pathway in Caenorhabditis elegans

Receptor tyrosine phosphatases have been implicated in playing important roles in cell signaling events by their ability to regulate the level of protein tyrosine phosphorylation. Although the catalytic activity of their phosphatase domains has been well established, the biological roles of these molecules are, for the most part, not well understood. Here we show that the Caenorhabditis elegans protein CLR-1 (CLeaR) is a receptor tyrosine phosphatase (RTP) with a complex extracellular region and two intracellular phosphatase domains. Mutations in clr-1 result in a dramatic Clr phenotype that we have used to study the physiological requirements for the CLR-1 RTP. We show that the phosphatase activity of the membrane-proximal domain is essential for the in vivo function of CLR-1. By contrast, we present evidence that the membrane-distal domain is not required to prevent the Clr phenotype in vivo. The Clr phenotype of clr-1 mutants is mimicked by activation of the EGL-15 fibroblast growth factor receptor (FGFR) and is suppressed by mutations that reduce or eliminate the activity of egl-15. Our data strongly indicate that CLR-1 attenuates the action of an FGFR-mediated signaling pathway by dephosphorylation.

Lipid metabolism in fibroblast growth factor-stimulated L6 myoblasts: a receptor mutation (Y766F) abrogates phospholipase D and diacylglycerol kinase activities

Phosphatidylcholine (PC) hydrolysis induced by basic fibroblast growth factor (bFGF) was studied in rat L6 myoblasts expressing the wild-type FGF receptor-1 (FGFR-1) or a mutant (Y766F) that is incapable of activating phospholipase C-gamma (PLCgamma). Stimulation of FGFR-1 activated phospholipase D (PLD) rapidly and transiently, but did not induce PC-specific PLC activity. Downregulation of protein kinase C blocked bFGF-induced PLD activation but not phosphatidic acid formation by diacylglycerol (DG) kinase. Only phosphoinositide (PI)-derived DG, not PC-derived DG, appeared to be a substrate for DG kinase. Stimulation of FGFR-1(Y766F) did not activate PLD or DG kinase, both of which apparently require initial PLCgamma activation. The Y766F mutation reduced mitogen-activated protein kinase activation but not cell proliferation. We conclude that both PI turnover and PC hydrolysis are dispensable for bFGF-induced mitogenesis.

Signal transduction pathways activated by RET oncoproteins in PC12 pheochromocytoma cells

Gene alterations in the ret proto-oncogene, which encodes a receptor tyrosine kinase, have been found to associate with several human diseases. In this study, we showed that induction of the vgf promoter activity is a good molecular indicator for RET activation in PC12 cells, a rat pheochromocytoma cell line. We demonstrated that all forms of RET oncoprotein, including RET chimeric oncoproteins found in human papillary thyroid carcinomas (RET/PTC) as well as RET oncoproteins found in patients with multiple endocrine neoplasia type 2A and 2B (2A/RET and 2B/RET) can induce vgf promoter activity in PC12 cells. In contrast, a RET mutant found in a patient with Hirschsprung's disease, as well as a RET/PTC1 mutant with deletion of the dimerization domain, failed to induce vgf promoter activity in PC12 cells. We further determined that the signaling events mediated by phosphorylated Tyr294 and phosphorylated Tyr451 binding sites are essential for RET/PTC1 to induce vgf promoter activity in PC12 cells. We also showed that RET/PTC1, 2A/RET, and 2B/RET induce ELK-, cAMP-responsive element binding protein (CREB), or JUN-mediated gene expression in PC12 cells, and these three signaling events are mediated by phosphorylated Tyr294 and phosphorylated Tyr451 binding sites in RET/PTC1.

Fibroblast variants nonresponsive to fibroblast growth factor 1 are defective in its nuclear translocation

Fibroblast growth factors (FGF) elicit biological effects by binding to high affinity cell-surface receptors and activation of receptor tyrosine kinase. We previously reported that two NIH/3T3 derivatives, NR31 and NR33 (NR cells), express high levels of full-length FGF-1 and exhibit a complete spectrum of transformed phenotype. In the present study, we report that NR cells respond to the mitogenic stimulation of truncated FGF-1 but not to the full-length FGF-1. Incubation of the NR cells with either form of FGF-1 resulted in its binding to cell-surface FGF receptors, activation of mitogen-activated protein (MAP) kinase, and induction of c-fos and c-myc. These data demonstrate that the FGF receptor-mediated, MAP kinase-dependent signaling pathway is not defective in the NR cells. Our data further suggest that the activation of MAP kinase in response to full-length FGF-1 is not sufficient for mitogenesis. Subcellular distribution of exogenously added FGF-1 demonstrated that full-length FGF-1 fails to translocate to the nuclei of NR31 cells. Although the full-length FGF-1 was detected in the nuclear fractions of both NIH/3T3 and NR33 cells, its half-life is much shortened in NR33 than in NIH/3T3 cells. These observations suggest that non-responsiveness of the two NR cell lines may be due to defectiveness at different steps of nuclear translocation mechanism of FGF-1.

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.

Induction of urokinase-type plasminogen activator by fibroblast growth factor (FGF)-2 is dependent on expression of FGF receptors and does not require activation of phospholipase Cgamma1

The roles of heparan sulfate proteoglycans and tyrosine kinase fibroblast growth factor (FGF) receptors in mediating the induction of plasminogen activator (PA) by FGF-2 were investigated using L6 myoblast cells that normally do not express detectable FGF receptors. PA was induced by FGF-2 in a dose-dependent manner in L6 cells expressing transfected FGF receptor-1 but not in nontransfected cells or cells transfected with the vector alone. The PA produced in these cells was characterized as urokinase-type PA (uPA). Thus, expression of a tyrosine kinase FGF receptor was required for induction of uPA. Internalization of FGF through heparan sulfates does not seem to be involved in this response as soluble heparin and suramin at concentrations which inhibited FGF-2 binding to heparan sulfates but not receptors did not affect the induction of uPA by FGF-2. Mutant receptors in which the tyrosine kinase was inactivated were not able to respond to FGF-2. In contrast, mutation of the site of phospholipase Cgamma1 (PLCgamma) binding in the receptor, which causes loss of PLCgamma activation, had no effect on uPA induction by FGF-2. These results suggest that PLCgamma activation is not required for induction of uPA by FGF-2.