Cell transformation by fibroblast growth factors can be suppressed by truncated fibroblast growth factor receptors

All Good Things Must End: Termination of Receptor Tyrosine Kinase Signal

Receptor tyrosine kinases (RTKs) are membrane receptors that regulate many fundamental cellular processes. A tight regulation of RTK signaling is fundamental for development and survival, and an altered signaling by RTKs can cause cancer. RTKs are localized at the plasma membrane (PM) and the major regulatory mechanism of signaling of RTKs is their endocytosis and degradation. In fact, RTKs at the cell surface bind ligands with their extracellular domain, become active, and are rapidly internalized where the temporal extent of signaling, attenuation, and downregulation are modulated. However, other mechanisms of signal attenuation and termination are known. Indeed, inhibition of RTKs’ activity may occur through the modulation of the phosphorylation state of RTKs and the interaction with specific proteins, whereas antagonist ligands can inhibit the biological responses mediated by the receptor. Another mechanism concerns the expression of endogenous inactive receptor variants that are deficient in RTK activity and take part to inactive heterodimers or hetero-oligomers. The downregulation of RTK signals is fundamental for several cellular functions and the homeostasis of the cell. Here, we will review the mechanisms of signal attenuation and termination of RTKs, focusing on FGFRs.

Adamantinomatous craniopharyngioma as a model to understand paracrine and senescence-induced tumourigenesis

Cellular senescence is a process that can prevent tumour development in a cell autonomous manner by imposing a stable cell cycle arrest after oncogene activation. Paradoxically, senescence can also promote tumour growth cell non-autonomously by creating a permissive tumour microenvironment that fuels tumour initiation, progression to malignancy and metastasis. In a pituitary tumour known as adamantinomatous craniopharyngioma (ACP), cells that carry oncogenic β-catenin mutations and overactivate the WNT signalling pathway form cell clusters that become senescent and activate a senescence-associated secretory phenotype (SASP). Research in mouse models of ACP has provided insights into the function of the senescent cell clusters and revealed a critical role for SASP-mediated activities in paracrine tumour initiation. In this review, we first discuss this research on ACP and subsequently explore the theme of paracrine tumourigenesis in other tumour models available in the literature. Evidence is accumulating supporting the notion that paracrine signalling brought about by senescent cells may underlie tumourigenesis across different tumours and cancer models.

Purification of recombinant human fibroblast growth factor 13 in E. coli and its molecular mechanism of mitogenesis

Fibroblast growth factor (FGF) 13, a member of the FGF11 subfamily, is a kind of intracrine protein similar to other family members including FGF11, FGF12, and FGF14. Unlike classical FGF, FGF13 exerts its bioactivities independent of fibroblast growth factor receptors (FGFRs). However, the effect of exogenous administration of FGF13 still remains further investigated. In the present study, we established an Escherichia coli expression system for the large-scale production of FGF13 and then obtained two isoform proteins including recombinant human FGF13A (rhFGF13A) and rhFGF13B with a purity greater than 90% by column chromatography, respectively. Otherwise, soluble analysis indicated that both rhFGF13A and rhFGF13B expressed in E. coli BL21 (DE3) pLysS were soluble. Furthermore, cellular-based experiments demonstrated that rhFGF13A, rather than rhFGF13B, could promote the proliferation of NIH3T3 cells in the presence of heparin. Mechanistically, the mitogenic effect of FGF13 was mediated by activation of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK), but not p38. Moreover, blockage of FGFRs also significantly attenuated the mitogenic effects of rhFGF13A, implying that FGFRs are still related to FGF13. Thus, our research shows that exogenous FGF13 can act as secreted FGF to participate in cell signal transmission and heparin is still required as an ancillary cofactor for the mitogenic effects of FGF13, which may help people to discover more potential functions of FGF13 in cell life activities.

Expression and purification of an FGF9 fusion protein in E. coli, and the effects of the FGF9 subfamily on human hepatocellular carcinoma cell proliferation and migration

Fibroblast growth factor (FGF) 9 has oncogenic activity and plays an important role in the development of ovarian, lung, prostate, and gastric cancers. In the present study, with the aim of reducing the cost of utilizing growth factors in cancer research, a simple and efficient method for the preparation of recombinant human (rh)FGF9 in Escherichia coli was established. The rhFGF9 fusion protein (6 × His-TEV-rhFGF9) and the native protein released by tobacco etch virus (TEV) protease were obtained using a Ni-NTA system, with > 95% purity. Both purified forms of rhFGF9, with and without fusion tags, significantly stimulated the proliferation of NIH3T3 cells. The FGF9 subfamily, including FGF9, FGF16, and FGF20, in addition to rhFGF16, rhFGF9, and rhFGF20, were shown to stimulate the proliferation and migration of HuH7 human hepatocellular carcinoma (HCC) cells. Mechanistic studies revealed that the stimulation of HuH7 cell proliferation and migration with rhFGF9 and rhFGF20 were associated with the activation of the extracellular signal-regulated kinase (ERK) and nuclear factor κB (NF-κB) pathways and matrix metalloproteinase-26 (MMP26). Inhibition of the ERK and NF-κB pathways blocked cell migration, and NF-κB was demonstrated to be regulated by ERK. Therefore, the present study demonstrates a simple method for the preparation of biologically active rhFGF9 protein. Furthermore, the results indicate that exogenous rhFGF9- and rhFGF20-activated ERK/NF-κB signal transduction pathways play important roles in the regulation of HCC cell proliferation and migration, and this discovery helps to find the potential for new solutions of the treatment of liver cancer.

Uncoupling the Mitogenic and Metabolic Functions of FGF1 by Tuning FGF1-FGF Receptor Dimer Stability

The recent discovery of metabolic roles for fibroblast growth factor 1 (FGF1) in glucose homeostasis has expanded the functions of this classically known mitogen. To dissect the molecular basis for this functional pleiotropy, we engineered an FGF1 partial agonist carrying triple mutations (FGF1ΔHBS) that diminished its ability to induce heparan sulfate (HS)-assisted FGF receptor (FGFR) dimerization and activation. FGF1ΔHBS exhibited a severely reduced proliferative potential, while preserving the full metabolic activity of wild-type FGF1 in vitro and in vivo. Hence, suboptimal FGFR activation by a weak FGF1-FGFR dimer is sufficient to evoke a metabolic response, whereas full FGFR activation by stable and sustained dimerization is required to elicit a mitogenic response. In addition to providing a physical basis for the diverse activities of FGF1, our findings will impact ongoing drug discoveries targeting FGF1 and related FGFs for the treatment of a variety of human diseases.

High-yield of biologically active recombinant human fibroblast growth factor-16 in E. coli and its mechanism of proliferation in NCL-H460 cells

Fibroblast growth factor-16 (FGF16) is a member of FGF9 subfamily, which plays key role in promoting mitosis and cell survival, and also involved in embryonic development, cell growth, tissue repair, morphogenesis, tumor growth, and invasion. However, the successful high-yield purification of recombinant human fibroblast growth factor-16 (rhFGF16) protein has not been reported. In addition, lung cancer is a major cause of cancer-related deaths, which threats people's lives and its incidence has continued to rise. Learning pathways or proteins, which involved in lung tumor progression will contribute to the development of early diagnosis and targeted therapy. FGF16 promoted proliferation and invasion behavior of SKOV-3 ovarian cancer cells, whose function may be similar in lung cancer. The hFGF16 was cloned into pET-3d and expressed in Escherichia coli BL21 (DE3) pLysS. Finally, obtained two forms of FGF16 that exhibited remarkable biological activity and the purity is over 95%, meanwhile, the yield of soluble 130 mg/100 g and insoluble 240 mg/100 g. Experiments demonstrated FGF16 could promote proliferation of NCL-H460 cells by activating Akt, Erk1/2, and p38 MAPK signaling, whereas JNK had no significant effect. In total, this optimized expression strategy enables significant quantity and activity of rhFGF16, thereby meeting its further pharmacological and clinical usages.

High production in E. coli of biologically active recombinant human fibroblast growth factor 20 and its neuroprotective effects

Fibroblast growth factor 20 (FGF20) has a wide range of biological activities; its expression is most pronounced in neural tissues where it has functions in development and neuroprotection. Given these activities, interest in the clinical applications of FGF20 is rising, which will lead to increasing demand for active recombinant human FGF20 (rhFGF20). To improve the production of rhFGF20, an artificial gene encoding fgf20 was cloned into pET3a and expressed in E. coli BL21(DE3)pLysS. By optimizing induction conditions, we successfully induced large amounts of insoluble rhFGF20. Following solubilization and refolding of the rhFGF20 from inclusion bodies, it was purified by HiTrap heparin affinity chromatography to a purity of over 96% with a yield of 218 mg rhFGF20/100 g wet cells. The purified rhFGF20 could stimulate proliferation of both NIH 3T3 cells and PC-12 cells, measured by the MTT assay. In a model of Aβ25-35-induced apoptosis on PC-12 cells, rhFGF20 had a clear protective effect. RT-PCR and Western blot analysis of apoptosis-related genes and proteins revealed that the FGF20-derived protective mechanism was likely due to the relief of endoplasmic reticulum stress (ER stress). In conclusion, the approach described here may be a better means to produce active rhFGF20 in good quantity, thereby allowing for its future pharmacological and clinical use.

Cardiomyocyte FGF signaling is required for Cx43 phosphorylation and cardiac gap junction maintenance

Cardiac remodeling resulting from impairment of myocardial integrity leads to heart failure, through still incompletely understood mechanisms. The fibroblast growth factor (FGF) system has been implicated in tissue maintenance, but its role in the adult heart is not well defined. We hypothesized that the FGF system plays a role in the maintenance of cardiac homeostasis, and the impairment of cardiomyocyte FGF signaling leads to pathological cardiac remodeling. We showed that FGF signaling is required for connexin 43 (Cx43) localization at cell-cell contacts in isolated cardiomyocytes and COS7 cells. Lack of FGF signaling led to decreased Cx43 phosphorylation at serines 325/328/330 (S325/328/330), sites known to be important for assembly of gap junctions. Cx43 instability induced by FGF inhibition was restored by the Cx43 S325/328/330 phospho-mimetic mutant, suggesting FGF-dependent phosphorylation of these sites. Consistent with these in vitro findings, cardiomyocyte-specific inhibition of FGF signaling in adult mice demonstrated mislocalization of Cx43 at intercalated discs, whereas localization of N-cadherin and desmoplakin was not affected. This led to premature death resulting from impaired cardiac remodeling. We conclude that cardiomyocyte FGF signaling is essential for cardiomyocyte homeostasis through phosphorylation of Cx43 at S325/328/330 residues which are important for the maintenance of gap junction.

Blocking Fibroblast Growth Factor receptor signaling inhibits tumor growth, lymphangiogenesis, and metastasis

Fibroblast Growth Factor receptor (FGFR) activity plays crucial roles in tumor growth and patient survival. However, FGF (Fibroblast Growth Factor) signaling as a target for cancer therapy has been under-investigated compared to other receptor tyrosine kinases. Here, we studied the effect of FGFR signaling inhibition on tumor growth, metastasis and lymphangiogenesis by expressing a dominant negative FGFR (FGFR-2DN) in an orthotopic mouse mammary 66c14 carcinoma model. We show that FGFR-2DN-expressing 66c14 cells proliferate in vitro slower than controls. 66c14 tumor outgrowth and lung metastatic foci are reduced in mice implanted with FGFR-2DN-expressing cells, which also exhibited better overall survival. We found 66c14 cells in the lumen of tumor lymphatic vessels and in lymph nodes. FGFR-2DN-expressing tumors exhibited a decrease in VEGFR-3 (Vascular Endothelial Growth Factor Receptor-3) or podoplanin-positive lymphatic vessels, an increase in isolated intratumoral lymphatic endothelial cells and a reduction in VEGF-C (Vascular Endothelial Growth Factor-C) mRNA expression. FGFs may act in an autocrine manner as the inhibition of FGFR signaling in tumor cells suppresses VEGF-C expression in a COX-2 (cyclooxygenase-2) or HIF1-α (hypoxia-inducible factor-1 α) independent manner. FGFs may also act in a paracrine manner on tumor lymphatics by inducing expression of pro-lymphangiogenic molecules such as VEGFR-3, integrin α9, prox1 and netrin-1. Finally, in vitro lymphangiogenesis is impeded in the presence of FGFR-2DN 66c14 cells. These data confirm that both FGF and VEGF signaling are necessary for the maintenance of vascular morphogenesis and provide evidence that targeting FGFR signaling may be an interesting approach to inhibit tumor lymphangiogenesis and metastatic spread.

Conserved intron positions in FGFR genes reflect the modular structure of FGFR and reveal stepwise addition of domains to an already complex ancestral FGFR

We have analyzed the evolution of fibroblast growth factor receptor (FGFR) tyrosine kinase genes throughout a wide range of animal phyla. No evidence for an FGFR gene was found in Porifera, but we tentatively identified an FGFR gene in the placozoan Trichoplax adhaerens. The gene encodes a protein with three immunoglobulin-like domains, a single-pass transmembrane, and a split tyrosine kinase domain. By superimposing intron positions of 20 FGFR genes from Placozoa, Cnidaria, Protostomia, and Deuterostomia over the respective protein domain structure, we identified ten ancestral introns and three conserved intron groups. Our analysis shows (1) that the position of ancestral introns correlates to the modular structure of FGFRs, (2) that the acidic domain very likely evolved in the last common ancestor of triploblasts, (3) that splicing of IgIII was enabled by a triploblast-specific insertion, and (4) that IgI is subject to substantial loss or duplication particularly in quickly evolving genomes. Moreover, intron positions in the catalytic domain of FGFRs map to the borders of protein subdomains highly conserved in other serine/threonine kinases. Nevertheless, these introns were introduced in metazoan receptor tyrosine kinases exclusively. Our data support the view that protein evolution dating back to the Cambrian explosion took place in such a short time window that only subtle changes in the domain structure are detectable in extant representatives of animal phyla. We propose that the first multidomain FGFR originated in the last common ancestor of Placozoa, Cnidaria, and Bilateria. Additional domains were introduced mainly in the ancestor of triploblasts and in the Ecdysozoa.

Fibroblast growth factor 16 and 18 are expressed in human cardiovascular tissues and induce on endothelial cells migration but not proliferation

Endothelial cells line the blood vessel and precursor endothelial cells appear to have a pivotal effect on the organ formation of the heart, the embryonic development of the kidney, and the liver. Several growth factors including the fibroblast growth factors (FGF) seem to be involved in these processes. Ligands such as basic FGF produced and secreted by endothelial cells may also coordinate cellular migration, differentiation, and proliferation under pathological conditions including wound healing, tumorgenesis, and fibrogenesis in the adult. Recently we demonstrated the expression of two secreted FGFs, FGF16, and FGF18, in HUVEC and in rat aortic tissue. In the present report, we confirmed by RT-PCR analysis that FGF18 is wildly expressed in the cardiovascular tissue, while FGF16 showed a more restricted expression pattern. HUVEC clearly demonstrated chemotaxis towards FGF16 and FGF18. Both FGFs also enhanced cell migration in response to mechanical damage. However, recombinant FGF16 and FGF18 failed to induce endothelial cell proliferation or sprouting in a three-dimensional in vitro angiogenesis assay. Fgf18 expression was earlier reported in the liver, and we detected FGF18 expression in liver vascular and liver sinusoidal endothelial cells (LSECs), but not in hepatic parenchymal cells. Recombinant FGF18 stimulated DNA synthesis in primary hepatocytes, suggesting, that endothelial FGF18 might have a paracrine function in promoting growth of the parenchymal tissue. Interestingly, FGF2, which is mitogenic on endothelial cells and hepatocytes stimulates a sustained MAPK activation in both cell types, while FGF18 causes a short transient activation of the MAPK pathway in endothelial cells but a sustained activation in hepatocytes. Therefore, the difference in the time course of MAPK activation by the different FGFs appears to be the cause for the different cellular responses.

Autocrine signaling through Ras regulates cell survival activity in human glioma cells: potential cross-talk between Ras and the phosphatidylinositol 3-kinase-Akt pathway

Autocrine fibroblast growth factor (FGF) signaling mediates an uncontrollable growth of human gliomas. We investigated the intracellular signaling of FGF on cell survival activity. U251MG human glioma cells were infected with adenovirus vectors expressing dominant negative type I FGF receptor (DNFR), constitutive active Ras (RasL61), or dominant negative Ras (RasN17). DNFR reduced glioma cell accumulation with apoptosis and this reduction was alleviated with exogenous epidermal growth factor (EGF), which can activate Ras independent of FGFR but not with bFGF. RasL61 prevented but RasN17-enhanced DNFR-induced apoptosis. Reportedly, cell survival signaling through Akt was constitutively active in U251MG cells and this effect may be dependent on autocrine signaling and dysfunction of PTEN, a tumor suppressor gene limiting phosphatidylinositol 3-kinase (PI3K) activity. DNFR dose-dependently inhibited Akt activity and this inhibition was recovered by RasL61, whereas RasN17 inhibited Akt activity. Wortmannin (a PI3K inhibitor) inhibited Akt activity and mildly promoted apoptosis. RasL61 prevented the down-regulation of Akt activity and apoptosis induced by wortmannin, but RasN17 plus wortmannin strongly inhibited Akt activity and promoted marked apoptosis. Our data suggested that the cell survival activity of human gliomas is largely dependent on cross-talk between Ras and the PI3K-Akt pathway, and this cross-talk could be a potential target for molecular-based therapeutics.

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

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

Fibroblast growth factors in cancer: therapeutic possibilities

The fibroblast growth factor (FGF) family of signalling molecules and its receptors (FGFRs) contribute to normal developmental and physiological processes. However, the subversion of this powerful growth stimulatory pathway has been implicated in the generation of a variety of pathological conditions. This review focuses on the role of FGF/FGFRs in cancer. The case will be made that this signalling pathway is associated with and functionally important for the growth of some human tumours. As such, FGF/FGFRs can be viewed as rational therapeutic oncology targets and strategies used to inhibit these molecules are discussed. The therapeutic exploitation of tumour-associated FGFR expression to deliver toxins or antiproliferative signals to tumour cells is also reviewed, as is the use of FGFs as protein therapeutics to alleviate the side effects of cancer therapy.

Biological activity of substrate-bound basic fibroblast growth factor (FGF2): recruitment of FGF receptor-1 in endothelial cell adhesion contacts

Substrate-bound FGF2 promotes endothelial cell adhesion by interacting with alpha(v)beta(3) integrin. Here, endothelial GM7373 cells spread and organize focal adhesion plaques on immobilized FGF2, fibronectin (FN), and vitronectin (VN). alpha(v)beta(3) integrin, paxillin, focal adhesion kinase, vinculin and pp60(src) localize in cell-substratum contact sites on FGF2, FN or VN. However, only immobilized FGF2 induces a long-lasting activation of extracellular signal-regulated kinases(1/2) (ERK(1/2)) and cell proliferation that was inhibited by the ERK(1/2) inhibitor PD 098059 and the tyrosine kinase (TK) inhibitor tyrphostin 23, pointing to the engagement of FGF receptor (FGFR) at the basal side of the cell. To assess this hypothesis, GM7373 cells were transfected with a dominant negative TK(-)-DeltaFGFR1 mutant (GM7373-DeltaFGFR1 cells) or with the full-length receptor (GM7373-FGFR1 cells). Both transfectants adhere and spread on FGF2 but GM7373-DeltaFGFR1 cells do not proliferate. Also, parental and GM7373-FGFR1 cells, but not GM7373-DeltaFGFR1 cells, undergo morphological changes and increased motility on FGF2-coated plastic. Finally, FGFR1, but not TK(-)-DeltaFGFR1, localizes in cell adhesion contacts on immobilized FGF2. In conclusion, substrate-bound FGF2 induces endothelial cell proliferation, motility, and the recruitment of FGFR1 in cell-substratum contacts. This may contribute to the cross talk among intracellular signaling pathways activated by FGFR1 and alpha(v)beta(3) integrin in endothelial cells.

Integrative nuclear FGFR1 signaling (INFS) pathway mediates activation of the tyrosine hydroxylase gene by angiotensin II, depolarization and protein kinase C

The integrative nuclear FGFR1 signaling (INFS) pathway functions in association with cellular growth, differentiation, and regulation of gene expression, and is activated by diverse extracellular signals. Here we show that stimulation of angiotensin II (AII) receptors, depolarization, or activation protein kinase C (PKC) or adenylate cyclase all lead to nuclear accumulation of fibroblast growth factor 2 (FGF-2) and FGFR1, association of FGFR1 with splicing factor-rich domains, and activation of the tyrosine hydroxylase (TH) gene promoter in bovine adrenal medullary cells (BAMC). The up-regulation of endogenous TH protein or a transfected TH promoter-luciferase construct by AII, veratridine, or PMA (but not by forskolin) is abolished by transfection with a dominant negative FGFR1TK-mutant which localizes to the nucleus and plasma membrane, but not by extracellularly acting FGFR1 antagonists suramin and inositolhexakisphosphate (IP6). Mechanism of TH gene activation by FGF-2 and FGFR1 was further investigated in BAMC and human TE671 cultures. TH promoter was activated by co-transfected HMW FGF-2 (which is exclusively nuclear) but not by cytoplasmic FGF-1 or extracellular FGFs. Promoter transactivation by HMWFGF-2 was accompanied by an up-regulation of FGFR1 specifically in the cell nucleus and was prevented FGFR1(TK-) but not by IP6 or suramin. The TH promoter was also transactivated by co-transfected wild-type FGFR1, which localizes to both to the nucleus and the plasma membrane, and by an exclusively nuclear, soluble FGFR1(SP-/NLS) mutant with an inserted nuclear localization signal. Activation of the TH promoter by nuclear FGFR1 and FGF-2 was mediated through the cAMP-responsive element (CRE) and was associated with induction of CREB- and CBP/P-300-containing CRE complexes. We propose a new model for gene regulation in which nuclear FGFR1 acts as a mediator of CRE transactivation by AII, cell depolarization, and PKC.

Role of fibroblast growth factor receptor signaling in prostate cancer cell survival

BACKGROUND

Expression of fibroblast growth factors (FGFs) is increased in a substantial fraction of human prostate cancers in vivo and in prostate cancer cell lines. Altered FGF signaling can potentially have a variety of effects, including stimulating cell proliferation and inhibiting cell death. To determine the biologic significance of altered FGF signaling in human prostate cancer, we disrupted signaling by expression of a dominant-negative (DN) FGF receptor in prostate cancer cell lines.

METHODS

PC-3, LNCaP, and DU145 prostate cancer cells were stably transfected with DN FGFR constructs, and LNCaP and DU145 cells were infected with a recombinant adenovirus expressing DN FGFR-1. The effect of DN FGFR-1 expression was assessed by colony-formation assays, cell proliferation assays, flow cytometry, and cytogenetic analysis. Key regulators involved in the G(2)-to-M cell cycle transition were assessed by western blotting to examine cyclin B1 expression and by in vitro kinase assay to assess cdc2 kinase activity.

RESULTS

Stable transfection of the DN FGFR-1 construct inhibited colony formation by more than 99% in all three cell lines. Infection of LNCaP and DU145 prostate cancer cells with adenovirus expressing DN FGFR-1 led to extensive cell death within 48 hours. Flow cytometry and cytogenetic analysis revealed that the DN FGFR-1 receptor led to arrest in the G(2) phase of the cell cycle before cell death. Cyclin B1 accumulated in DN FGFR-1-infected LNCaP cells, but cdc2 kinase activity was decreased.

CONCLUSIONS

These findings reveal an unexpected dependence of prostate cancer cells on FGF receptor signal transduction to traverse the G(2)/M checkpoint. The mechanism for the G(2) arrest is not clear. Our results raise the possibility that FGF-signaling antagonists might enhance the cell death induced by other prostate cancer therapies.

Fgf receptor signaling plays a role in lens induction

We describe experiments showing that fibroblast growth factor receptor (Fgfr) signaling plays a role in lens induction. Three distinct experimental strategies were used: (1) using small-molecule inhibitors of Fgfr kinase activity, we showed that both the transcription level and protein expression of Pax6, a transcription factor critical for lens development, was diminished in the presumptive lens ectoderm; (2) transgenic mice (designated Tfr7) that expressed a dominant-negative Fgf receptor exclusively in the presumptive lens ectoderm showed defects in formation of the lens placode at E9.5 but in addition, showed reduced levels of expression for Pax6, Sox2 and Foxe3, all markers of lens induction; (3) by performing crosses between Tfr7 transgenic and Bmp7-null mice, we showed that there is a genetic interaction between Fgfr and Bmp7 signaling at the induction phases of lens development. This manifested as exacerbated lens development defects and lower levels of Pax6 and Foxe3 expression in Tfr7/Tfr7, Bmp7+/– mice when compared with Tfr7/Tfr7 mice alone. As Bmp7 is an established lens induction signal, this provides further evidence that Fgfr activity is important for lens induction. This analysis establishes a role for Fgfr signaling in lens induction and defines a genetic pathway in which Fgfr and Bmp7 signaling converge on Pax6 expression in the lens placode with the Foxe3 and Sox2 genes lying downstream.

Novel nuclear signaling pathway mediates activation of fibroblast growth factor-2 gene by type 1 and type 2 angiotensin II receptors

In bovine adrenal medullary cells synergistically acting type 1 and type 2 angiotensin II (AII) receptors activate the fibroblast growth factor-2 (FGF-2) gene through a unique AII-responsive promoter element. Both the type 1 and type 2 AII receptors and the downstream cyclic adenosine 1',3'-monophosphate- and protein kinase C-dependent signaling pathways activate the FGF-2 promoter through a novel signal-transducing mechanism. This mechanism, which we have named integrative nuclear FGF receptor-1 signaling, involves the nuclear translocation of FGF receptor-1 and its subsequent transactivation of the AII-responsive element in the FGF-2 promoter.

In vivo modulation of FGF biological activity alters cranial suture fate

Gain-of-function mutations in fibroblast growth factor receptors have been identified in numerous syndromes associated with premature cranial suture fusion. Murine models in which the posterior frontal suture undergoes programmed fusion after birth while all other sutures remain patent provide an ideal model to study the biomolecular mechanisms that govern cranial suture fusion. Using adenoviral vectors and targeted in utero injections in rats, we demonstrate that physiological posterior frontal suture fusion is inhibited using a dominant-negative fibroblast growth factor receptor-1 construct, whereas the normally patent coronal suture fuses when infected with a construct that increases basic fibroblast growth factor biological activity. Our data may facilitate the development of novel, less invasive treatment options for children with craniosynostosis.

Signaling by fibroblast growth factors (FGF) and fibroblast growth factor receptor 2 (FGFR2)-activating mutations blocks mineralization and induces apoptosis in osteoblasts

Fibroblast growth factors (FGF) play a critical role in bone growth and development affecting both chondrogenesis and osteogenesis. During the process of intramembranous ossification, which leads to the formation of the flat bones of the skull, unregulated FGF signaling can produce premature suture closure or craniosynostosis and other craniofacial deformities. Indeed, many human craniosynostosis disorders have been linked to activating mutations in FGF receptors (FGFR) 1 and 2, but the precise effects of FGF on the proliferation, maturation and differentiation of the target osteoblastic cells are still unclear. In this report, we studied the effects of FGF treatment on primary murine calvarial osteoblast, and on OB1, a newly established osteoblastic cell line. We show that FGF signaling has a dual effect on osteoblast proliferation and differentiation. FGFs activate the endogenous FGFRs leading to the formation of a Grb2/FRS2/Shp2 complex and activation of MAP kinase. However, immature osteoblasts respond to FGF treatment with increased proliferation, whereas in differentiating cells FGF does not induce DNA synthesis but causes apoptosis. When either primary or OB1 osteoblasts are induced to differentiate, FGF signaling inhibits expression of alkaline phosphatase, and blocks mineralization. To study the effect of craniosynostosis-linked mutations in osteoblasts, we introduced FGFR2 carrying either the C342Y (Crouzon syndrome) or the S252W (Apert syndrome) mutation in OB1 cells. Both mutations inhibited differentiation, while dramatically inducing apoptosis. Furthermore, we could also show that overexpression of FGF2 in transgenic mice leads to increased apoptosis in their calvaria. These data provide the first biochemical analysis of FGF signaling in osteoblasts, and show that FGF can act as a cell death inducer with distinct effects in proliferating and differentiating osteoblasts.

Acidic substitution of the activation loop tyrosines in TrkA supports nerve growth factor-independent cell survival and neuronal differentiation

TrkA is the receptor tyrosine kinase (RTK) for nerve growth factor (NGF) and stimulates NGF-dependent cell survival and differentiation in primary neurons. TrkA expression in neuronal tumors also supports NGF-dependent differentiation of neuroblastomas and apoptosis of medulloblastomas. Phosphorylation of the activation loop tyrosines in RTK's are essential to activation as well as allosteric changes that facilitate substrate interaction and phosphorylation. Acidic amino acid substitution of the activation loop tyrosines in TrkA, Tyr683Tyr684, was performed to mimic the negative charges normally induced by ligand activation and receptor phosphorylation. A total of eight independent mutants containing single or double substitutions were generated for comparison. Herein, we demonstrate that acidic substitution of the activation loop tyrosines is sufficient to induce allosteric changes required for constitutive TrkA kinase activity as well as phosphorylation of TrkA signaling proteins such as Shc, PLCgamma-1, FRS-2 and erk1/2. The strongest constitutively active TrkA mutants, GluAsp and AspGlu, support NGF-independent neuritogenesis and cell survival to levels approximately 65 and 80-100%, respectively, of NGF-activated wild type TrkA. Thus, constitutively active TrkA may provide a useful strategy in future therapeutic approaches to limit the development and progression of neuronal tumors.

FGF signaling and the anterior neural induction in Xenopus

We previously showed that FGF was capable of inducing Xenopus gastrula ectoderm cells in culture to express position-specific neural markers along the anteroposterior axis in a dose-dependent manner. However, conflicting results have been obtained concerning involvement of FGF signaling in the anterior neural induction in vivo using the same dominant-negative construct of Xenopus FGF receptor type-1 (delta XFGFR-1 or XFD). We explored this issue by employing a similar construct of receptor type-4a (XFGFR-4a) in addition, since expression of XFGFR-4a was seen to peak between gastrula and neurula stages, when the neural induction and patterning take place, whereas expression of XFGFR-1 had not a distinct peak during that period. Further, these two FGFRs are most distantly related in amino acid sequence in the Xenopus FGFR family. When we injected mRNA of a dominant-negative version of XFGFR-4a (delta XFGFR-4a) into eight animal pole blastomeres at 32-cell stage, anterior defects including loss of normal structure in telencephalon and eye regions became prominent as examined morphologically or by in situ hybridization. Overexpression of delta XFGFR-1 appeared far less effective than that of delta XFGFR-4a. Requirement of FGF signaling in ectoderm for anterior neural development was further confirmed in culture: when ectoderm cells that were overexpressing delta XFGFR-4a were cocultured with intact organizer cells from either early or late gastrula embryos, expression of anterior and posterior neural markers was inhibited, respectively. We also showed that autonomous neuralization of the anterior-type observed in ectoderm cells that were subjected to prolonged dissociation was strongly suppressed by delta XFGFR-4a, but not as much by delta XFGFR-1. It is thus indicated that FGF signaling in ectoderm, mainly through XFGFR-4, is required for the anterior neural induction by organizer. We may reconcile our data to the current "neural default model," which features the central roles of BMP4 signaling in ectoderm and BMP4 antagonists from organizer, simply postulating that the neural default pathway in ectoderm includes constitutive FGF signaling step.

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.

Fas gene mutation in the progression of adult T cell leukemia

Fas antigen (Apo-1/CD95) is an apoptosis-signaling cell surface receptor belonging to the tumor necrosis factor receptor superfamily. Adult T cell leukemia (ATL) cells express Fas antigen and show apoptosis after treatment with an anti-Fas monoclonal antibody. We established the ATL cell line KOB, which showed resistance to Fas-mediated apoptosis, and found that KOB expressed two forms of Fas mRNA, the normal form and a truncated form. The truncated transcript lacked 20 base pairs at exon 9, resulting in a frame shift and the generation of a premature stop codon at amino acid 239. The same mutation was detected in primary ascitic cells and peripheral blood cells. The mutation was not detected in lymph node cells, however, although all of the primary ATL cells were of the same clonal origin. A retroviral-mediated gene transfer of the truncated Fas to Jurkat cells rendered the cells resistant to Fas-mediated apoptosis, suggesting a dominant negative interference mechanism. These results indicate that an ATL subclone acquires a Fas mutation in the lymph nodes, enabling the subclone to escape from apoptosis mediated by the Fas/Fas ligand system and proliferate in the body. Mutation of the Fas gene may be one of the mechanisms underlying the progression of ATL.

The community effect in FGF-1 mediated tumor progression of a rat bladder carcinoma does not involve a direct paracrine signaling

A community effect was found to occur between heterogeneous tumor cell populations leading to an overall increased tumorigenicity without a clonal dominance of the more tumorigenic clone. In the rat bladder carcinoma cell line NBT-II, this effect appears mediated by the Fibroblast Growth Factor-1 (FGF-1) through either a direct or an indirect signaling pathway. Neovascularization induced by FGF-1 was found not to be responsible for the community effect. The present study shows that the community effect does not involve a direct FGF-1 signaling since tumor cells expressing a dominant-negative FGF receptor mutant were still responding to the highly tumorigenic FGF-1 expressing cells. Tumors arising from inoculates of the FGF-1 producing NBT-II cells mixed with non tumorigenic epithelial MDCK cells contain only the tumorigenic cells indicating that MDCK cells may exerce a helper effect for the growth of the tumor not dependant on their own growth. Therefore the helper function of MDCK cells must be distinguished from a community effect where the contribution of low tumorigenic cells not only provides an in vivo growth advantage to few highly tumorigenic cells but become themselves highly tumorigenic indicating that the community effect may require cell-cell specific cooperativity independent from an helper effect.

Fibroblast growth factor (FGF) soluble receptor 1 acts as a natural inhibitor of FGF2 neurotrophic activity during retinal degeneration

Fibroblast growth factors (FGF) 1 and 2 and their tyrosine kinase receptor (FGFR) are present throughout the adult retina. FGFs are potential mitogens, but adult retinal cells are maintained in a nonproliferative state unless the retina is damaged. Our work aims to find a modulator of FGF signaling in normal and pathological retina. We identified and sequenced a truncated FGFR1 form from rat retina generated by the use of selective polyadenylation sites. This 70-kDa form of soluble extracellular FGFR1 (SR1) was distributed mainly localized in the inner nuclear layer of the retina, whereas the full-length FGFR1 form was detected in the retinal Muller glial cells. FGF2 and FGFR1 mRNA levels greatly increased in light-induced retinal degeneration. FGFR1 was detected in the radial fibers of activated retinal Muller glial cells. In contrast, SR1 mRNA synthesis followed a biphasic pattern of down- and up-regulation, and anti-SR1 staining was intense in retinal pigmented epithelial cells. The synthesis of SR1 and FGFR1 specifically and independently regulated in normal and degenerating retina suggests that changes in the proportion of various FGFR forms may control the bioavailability of FGFs and thus their potential as neurotrophic factors. This was demonstrated in vivo during retinal degeneration when recombinant SR1 inhibited the neurotrophic activity of exogenous FGF2 and increased damaging effects of light by inhibiting endogenous FGF. This study highlights the significance of the generation of SR1 in normal and pathological conditions.

Suppression of fibroblast growth factor receptor signaling inhibits pancreatic cancer growth in vitro and in vivo

BACKGROUND & AIMS

Fibroblast growth factors (FGFs) are mitogenic polypeptides that activate specific cell surface FGF receptors (FGFRs). Pancreatic cancers overexpress basic FGF (bFGF) and the type I FGF receptor (FGFR-1), and overexpression of bFGF has been correlated with decreased patient survival. The aim of this study was to examine the effects of abrogation of FGFR-1-dependent signaling on pancreatic cancer cell growth.

METHODS

PANC-1 human pancreatic cancer cells were transfected with a truncated FGFR-1 complementary DNA (FGFR405), resulting in the expression of a kinase-deficient receptor. Activation of endogenous FGFR-1 was assessed in immunoblot studies with antiphosphotyrosine and anti-active mitogen-activated protein (MAP) kinase antibodies. Effects on cell growth were determined in vitro and in nude mice.

RESULTS

PANC-1 clones expressing the truncated receptor showed attenuated receptor tyrosine phosphorylation and MAP kinase activation in response to bFGF, decreased basal cell growth, and a marked decrease in tumor-forming potential in vivo. Confirmatory experiments with MIA PaCa-2 pancreatic cancer cells indicated that FGFR405 also attenuated FGF-dependent MAP kinase activation in this cell line.

CONCLUSIONS

The findings suggest that FGFR-dependent signaling is crucial for pancreatic cancer growth and raise the possibility that inhibition of FGFR signaling may ultimately prove useful as a therapeutic option in patients with pancreatic cancer.

Critical periods of basic fibroblast growth factor and brain-derived neurotrophic factor in the development of the chicken cochleovestibular ganglion in vitro

The temporal roles of brain-derived neurotrophic factor (BDNF) and fibroblast growth factor-2 (FGF-2) in the development of sensory neurons have been studied in a cell culture preparation which models normal embryonic inner ear development (normocytic). Previous studies showed that FGF-2 stimulated migration and differentiation of ganglion cells for the first 2 days in vitro, but after 5 days led to degeneration, implicating other factors in their later development. To see if BDNF could be such a factor, otocysts were explanted from white leghorn embryos at the time when ganglion cell precursors normally start migrating from the otic epithelium. Cultures were grown in a defined medium, either with or without human recombinant FGF-2 for 2 days or with BDNF. On Day 3, FGF-2 was replaced either with BDNF in defined medium or with defined medium only. Measurements of neuroblast migration and neurite outgrowth were made by time-lapse imaging in living cultures. In cultures receiving BDNF on Day 3, cell migration and neurite outgrowth from the explant increased for more than 3 weeks but not in cultures receiving only defined medium from Day 3. Cultures did not survive more than 3-4 days when receiving either BDNF in defined medium or defined medium alone from the first day. A neutralizing antibody to BDNF inhibited neuronal migration and neurite outgrowth, and it also blocked the effects of exogenous BDNF. BDNF did not enhance the effects of FGF-2 by interacting with it. These experiments defined a temporal sequence in which FGF-2 acts early in development, while BDNF affects a later stage.

Basic fibroblast growth factor (FGF-2) affects development of acoustico-vestibular neurons in the chick embryo brain in vitro

The effects of basic fibroblast growth factor (FGF-2) on presumptive auditory and vestibular neurons from the medulla were studied in primary cell cultures. The part of the rhombic lip that forms nucleus magnocellularis (homologue of the mammalian anteroventral cochlear nucleus) was explanted from white leghorn chicken embryos at Hamburger-Hamilton stage 28 (E5.5), the time when precursors of the magnocellularis bushy cells migrate and begin to differentiate in situ. In vitro the neuroblasts migrated onto 2-D substrates of purified collagen, differentiated, and expressed neuronal markers. One-half of the cultures were supplemented with human recombinant FGF-2 (10 ng/ml daily) for 5-7 days; the others, with fetal bovine serum. FGF-2 more than doubled the length of neurite outgrowth during the first 3 day treatment compared to serum, but the number of migrating neuroblasts was unaffected. Although neurites attained greater lengths in FGF-2, they usually degenerated after 4-5 days; in serum their growth continued for several weeks. Differentiation of neuronal structure, including axons and dendrites, began within 1-2 days in bFGF but required at least 5-7 days in serum. Histochemical observations in vitro and in situ with antibodies to FGF receptor demonstrated immunopositive patches on acoustico-vestibular neuroblasts at stage 28, when they are migrating and first forming their axons. The findings suggest that FGF-2 stimulates neurite outgrowth in the cochlear and vestibular nuclei. FGF-2 may accelerate cell death by overstimulating neuroblasts, but other factors are needed to sustain their further development.

Basic fibroblast growth factor (FGF-2) affects development of acoustico-vestibular neurons in the chick embryo brain in vitro

The effects of basic fibroblast growth factor (FGF-2) on presumptive auditory and vestibular neurons from the medulla were studied in primary cell cultures. The part of the rhombic lip that forms nucleus magnocellularis (homologue of the mammalian anteroventral cochlear nucleus) was explanted from white leghorn chicken embryos at Hamburger-Hamilton stage 28 (E5.5), the time when precursors of the magnocellularis bushy cells migrate and begin to differentiate in situ. In vitro the neuroblasts migrated onto 2-D substrates of purified collagen, differentiated, and expressed neuronal markers. One-half of the cultures were supplemented with human recombinant FGF-2 (10 ng/ml daily) for 5-7 days; the others, with fetal bovine serum. FGF-2 more than doubled the length of neurite outgrowth during the first 3 day treatment compared to serum, but the number of migrating neuroblasts was unaffected. Although neurites attained greater lengths in FGF-2, they usually degenerated after 4-5 days; in serum their growth continued for several weeks. Differentiation of neuronal structure, including axons and dendrites, began within 1-2 days in bFGF but required at least 5-7 days in serum. Histochemical observations in vitro and in situ with antibodies to FGF receptor demonstrated immunopositive patches on acoustico-vestibular neuroblasts at stage 28, when they are migrating and first forming their axons. The findings suggest that FGF-2 stimulates neurite outgrowth in the cochlear and vestibular nuclei. FGF-2 may accelerate cell death by overstimulating neuroblasts, but other factors are needed to sustain their further development.

FGF suppresses apoptosis and induces differentiation of fibre cells in the mouse lens

To determine whether fibroblast growth factor (FGF) has a role in lens development, we have generated transgenic mice expressing a dominant-negative form of the murine FGF receptor-1 (FGFRDN) in the lens. Using the fibre cell-specific alpha A-crystallin promoter to express the FGFRDN, we have asked whether FGF is required for fibre cell differentiation. The transgenic mice display diminished differentiation of fibre cells as indicated by their reduced elongation. In addition, transgenic lenses have an unusual refractile anomaly that morphological and biochemical data show results from the apoptosis of fibre cells in the central region of the lens. These results show that lens fibre cells are dependent on FGF for their survival and differentiation, and demonstrate that growth factor deprivation in vivo can lead to apoptosis.

Changes in the expression of fibroblast growth factor receptors mark distinct stages of chondrogenesis in vitro and during chick limb skeletal patterning

Members of the fibroblast growth factor (FGF) family of growth factors are key regulators of limb skeletal patterning and growth. Abnormal expression of FGFs or mutations in their receptors (fgfrs) result in skeletal disorders. Here we show that changes in the expression of fgfrs are intrinsic properties of differentiating cartilage. In mesenchymal micromass cultures differentiating into cartilage, as in ovo, fgfr 1 mRNA was found predominantly in undifferentiated, proliferating mesenchyme, fgfr 2 in precartilage cell aggregates, and fgfr 3 in differentiating cartilage nodules. Thus, our data suggest that switches in the expression of fgfr 1, 2, and 3 mRNAs are associated with phases of cartilage patterning both in vitro and in ovo, and mark distinct stages in the development of the limb skeleton.

Fluorescence resonance energy transfer reveals interleukin (IL)-1-dependent aggregation of IL-1 type I receptors that correlates with receptor activation

Fluorescence resonance energy transfer (FRET) was used to investigate whether interleukin-1 (IL-1) causes the aggregation of IL-1 type I receptors (IL-1 RI) at the cell surface. For these experiments, a noncompetitive anti-IL1 RI monoclonal antibody, M5, was labeled separately with a donor probe, fluorescein isothiocyanate, or with an acceptor carbocyanine probe, Cy3. Donor-labeled M5 and acceptor-labeled M5 were simultaneously bound to transfected mouse IL-1 RI on either C-127 mouse mammary carcinoma cells or on Chinese hamster ovary (CHO)-K1 cells, and the ratio of acceptor emission at 590 nm to donor emission at 525 nm (excitation at 488 and 514 nm) was monitored with flow cytometry as an indicator of FRET. Addition of a saturating concentration of human IL-1 alpha at 22 degrees C causes a time-dependent increase in FRET for both cell lines that indicates IL-1-dependent self-association of IL-1 RI. Binding of the IL-1 receptor antagonist at 22 degrees C causes little or no FRET for both cell lines, indicating a correlation between receptor aggregation and the ability of the ligand to stimulate a functional response. When donor-labeled and acceptor-labeled Fab fragments of M5 are used to monitor FRET, IL-1 alpha causes efficient energy transfer in the CHO-K1 cells at 22 degrees C, but not at 4 degrees C. In contrast, IL-1 alpha causes much less FRET at 22 degrees C in C-127 cells when the M5 Fab fragments are used instead of the intact bivalent M5. In a striking parallel, IL-1 alpha-dependent activation of prostaglandin E2 production depends on the bivalent M5 antibody in the C-127 cells, but is independent of this monoclonal antibody in the CHO-K1 cells. These results provide a strong correlation between the ability of IL-1 to cause the aggregation of IL-1 RI and the stimulation of a functional response.