Analysis of endogenous and exogenous nuclear translocation of fibroblast growth factor-1 in NIH 3T3 cells

Secreted key regulators (Fgf1, Bmp4, Gdf3) are expressed by PAC1-immunopositive retinal ganglion cells in the postnatal rat retina

Identified as a member of the secretin/glucagon/VIP superfamily, pituitary adenylate cyclase-activating polypeptide (PACAP1-38) has been recognized as a hormone, neurohormone, transmitter, trophic factor, and known to be involved in diverse and multiple developmental processes. PACAP1-38 was reported to regulate the production of important morphogens (Fgf1, Bmp4, Gdf3) through PAC1-receptor in the newborn rat retina. To follow up, we aimed to reveal the identity of retinal cells responsible for the production and secretion of Fgf1, Bmp4, and Gdf3 in response to PACAP1-38 treatment. Newborn (P1) rats were treated with 100 pmol PACAP1-38 intravitreally. After 24 h, retinas were dissected and processed for immunohistochemistry performed either on flat-mounted retinas or cryosections. Brn3a and PAC1-R double labeling revealed that 90% of retinal ganglion cells (RGCs) expressed PAC1-receptor. We showed that RGCs were Fgf1, Bmp4, and Gdf3- immunopositive and PAC1-R was co-expressed with each protein. To elucidate if RGCs release these secreted regulators, the key components for vesicle release were examined. No labeling was detected for synaptophysin, Exo70, or NESP55 in RGCs but an intense Rab3a-immunoreactivity was detected in their cell bodies. We found that the vast majority of RGCs are responsive to PACAP, which in turn could have a significant impact on their development or/and physiology. Although Fgf1, Bmp4, and Gdf3 were abundantly expressed in PAC1-positive RGCs, the cells lack synaptophysin and Exo70 in the newborn retina thus unable to release these proteins. These proteins could regulate postnatal RGC development acting through intracrine pathways.

High-efficiency production of bioactive recombinant human fibroblast growth factor 18 in Escherichia coli and its effects on hair follicle growth

Using fusion tags, expression of recombinant human fibroblast growth factor 18 (rhFGF18) in mammalian cells and Escherichia coli has been extensively used for fundamental research and clinical applications, including chondrogenesis and osteogenesis, hair growth, and neuroprotection. However, high-level rhFGF18 expression is difficult and the products are often not homogeneous. Furthermore, fusion-tagged protein has higher immunogenicity and lower bioactivity, and the removal of the fused tag is expensive. To overcome the limitations of fusion-tagged expression of protein and to prepare soluble highly bioactive rhFGF18, we have developed a rapid and efficient expression strategy. Optimized hFGF18 gene was amplified by polymerase chain reaction and cloned into pET22b and pET3c vectors, then transformed into E. coli strains Origima (DE3) and BL21 (DE3)PlysS. The best combination of plasmid and host strain was selected, and only Origima (DE3)/pET3c-rhFGF18 was screened for high-level expressed rhFGF18. Under optimal conditions in a 30-L fermentor, the average bacterial yield and expression level of rhFGF18 of three batches were more than 652 g and 30 % respectively, after treatment with 1 mM isopropyl-thio-β-galactopyranoside for 10 h at 25 °C. The target protein was purified by CM Sepharose FF and heparin affinity chromatography. The purity of rhFGF18 was shown by HPLC to be higher than 95 %, and the yield was 155 mg/L. In vitro MTT assays demonstrated that the purified rhFGF18 could stimulate significant proliferation of NIH3T3 cells, and animal experiments showed that rhFGF18 could effectively regulate hair growth. In conclusion, this may be a better method of producing rhFGF18 to meet the increasing demand in its pharmacological application.

Nuclear translocation of FGF8 and its implication to induce Sprouty2

Fibroblast growth factor 8 (FGF8) functions as a local organizing signal for the tectum and cerebellum. FGF8 activates Ras-ERK signaling pathway to induce cerebellar development. We paid attention to the difference in the expression pattern of the molecules that are induced by FGF8 in the mid-hind brain region during normal development and after FGF8 misexpression; some are expressed in the area corresponding to the ERK activation domain but the others are expressed corresponding to the Fgf8 expression domain. Since some of the FGF family members are localized in the nucleus, we wondered if FGF8 could localize in the nuclei and function in the nucleus. We first show that in cultured NIH3T3 cells transfected FGF8b could localize in the nucleus. Transfected FGF8b could also localize in the nucleus of the cells in the chick neural tube. In mouse embryonic neural tube, we detected endogenous FGF8 in the nuclei. Implantation of an FGF8b-soaked bead showed that exogenous FGF8b could be translocated to the nuclei in the isthmus. Furthermore, signal-peptide-deletion mutant of FGF8b mainly localized in the nuclei, and induced Sprouty2 without activating ERK in the mesencephalon. Signal-peptide-deletion mutant of FGF8b could not induce Pax2 expression. Taken together, we concluded that FGF8b could be translocated to the nuclei, and that the nuclear FGF8 could function as transcriptional regulator to induce Sprouty2 in the isthmus.

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

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

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

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

Gene transfer and protein dynamics in stem cells using single cell electroporation in a microfluidic device

There is great interest in genetic modification of bone marrow-derived mesenchymal stem cells (MSC), not only for research purposes but also for use in (autologous) patient-derived-patient-used transplantations. A major drawback of bulk methods for genetic modifications of (stem) cells, like bulk-electroporation, is its limited yield of DNA transfection (typically then 10%). This is even more limited when cells are present at very low numbers, as is the case for stem cells. Here we present an alternative technology to transfect cells with high efficiency (>75%), based on single cell electroporation in a microfluidic device. In a first experiment we show that we can successfully transport propidium iodide (PI) into single mouse myoblastic C2C12 cells. Subsequently, we show the use of this microfluidic device to perform successful electroporation of single mouse myoblastic C2C12 cells and single human MSC with vector DNA encoding a green fluorescent-erk1 fusion protein (EGFP-ERK1 (MAPK3)). Finally, we performed electroporation in combination with live imaging of protein expression and dynamics in response to extracellular stimuli, by fibroblast growth factor (FGF-2). We observed nuclear translocation of EGFP-ERK1 in both cell types within 15 min after FGF-2 stimulation. Due to the successful and promising results, we predict that microfluidic devices can be used for highly efficient small-scale 'genetic modification' of cells, and biological experimentation, offering possibilities to study cellular processes at the single cell level. Future applications might be small-scale production of cells for therapeutic application under controlled conditions.

IgG deposits can be detected in cell nuclei of patients with both lupus erythematosus and malignancy

The purpose was to find immunological disturbances in lupus erythematosus (LE) patients with concomitant malignancy. 159 LE patients have been analyzed. Routine laboratory analyses including screening of serum autoantibodies and analyzing peripheral blood mononuclear cells by using flow cytometry have been performed. Malignant diseases have been revealed in 12 (7.5%) cases. All patients suffered from internal malignancies. LE patients with vs without malignancy had significantly decreased anti-double stranded DNA (16.6 vs 31.6%; p < 0.05) and increased anti-SSA/SSB (83.3 vs 32.2%/26.4%; p < 3 x 10(-12)) antibodies. Patients with neoplastic disease had increased IgG within the cell nuclei (76.6% +/- 9.6 vs 51.8 +/- 4.6%; p < 2 x 10(-7)). IgG penetrating living cells has been shown previously in SLE but has so far not been found in association to LE patients with malignant disease.

Pattern of FGF-2 isoform expression correlated with its biological action in experimental prolactinomas

Fibroblast growth factor-2 (FGF-2) synthesized in the pituitary is involved in the formation and progression of pituitary tumors. The aim of this study was to analyze the pattern expression of two FGF-2 isoforms at different subcellular levels and to determine its correlation with prolactinoma development. Estrogen administration to male rats for 7, 20, and 60 days generated pituitary tumors, with lactotrophs being the prevalent cell type. Ultrastructural immunolabeling showed FGF-2 in the cytosolic and nuclear compartments of somatotrophs, lactotrophs and gonadotrophs, as well as in folliculo-stellate cells of normal rats. Estrogen stimulation increased FGF-2 immunoreactivity in various tumors and enhanced the expression of two FGF-2 isoforms, 18 and 22 kDa, as quantified by western blot. The 18 kDa isoform observed in cytosol extracts reached the highest levels after 60 days of hormonal stimulation and this was related to lactotroph proliferation. However, the 22 kDa FGF-2 isoform was only detected in the nuclear compartment and achieved the maximum expression at 7 days of estrogen treatment, without any correlation with lactotroph proliferation. These results suggest that the 18 kDa FGF-2 may play a role in the modulation of lactotroph proliferation in prolactinomas induced by estrogen. The overproduction of both FGF-2 isoforms appears to be implicated in autocrine-paracrine-intracrine mitogenic loops; this FGF-2 activity could lead to uncontrolled cell growth, angiogenesis, and tumor formation.

Highly stable mutants of human fibroblast growth factor-1 exhibit prolonged biological action

Fibroblast growth factor 1 (FGF-1) shows strong angiogenic, osteogenic and tissue-injury repair properties that might be relevant to medical applications. Since FGF-1 is partially unfolded at physiological temperature we decided to increase significantly its conformational stability and test how such an improvement will affect its biological function. Using an homology approach and rational strategy we designed two new single FGF-1 mutations: Q40P and S47I that appeared to be the most strongly stabilizing substitutions among those reported so far, increasing the denaturation temperature by 7.8 deg. C and 9.0 deg. C, respectively. As our goal was to produce highly stable variants of the growth factor, we combined these two mutations with five previously described stabilizing substitutions. The multiple mutants showed denaturation temperatures up to 27 deg. C higher than the wild-type and exhibited full additivity of the mutational effects. All those mutants were biologically competent in several cell culture assays, maintaining typical FGF-1 activities, such as binding to specific cell surface receptors and activation of downstream signaling pathways. Thus, we demonstrate that the low denaturation temperature of wild-type FGF-1 is not related to its fundamental cellular functions, and that FGF-1 action is not affected by its stability. A more detailed analysis of the biological behavior of stable FGF-1 mutants revealed that, compared with the wild-type, their mitogenic properties, as probed by the DNA synthesis assay, were significantly increased in the absence of heparin, and that their half-lives were extensively prolonged. We found that the biological action of the mutants was dictated by their susceptibility to proteases, which strongly correlated with the stability. Mutants which were much more resistant to proteolytic degradation always displayed a significant improvement in the half-life and mitogenesis. Our results show that engineered stable growth factor variants exhibit enhanced and prolonged activity, which can be advantageous in terms of the potential therapeutic applications of FGF-1.

Nuclear-targeting autoantibodies induced nuclear PARP cleavage accompanied by more pronounced decrease of peripheral white blood cells than Ro/SSA and La/SSB antigen-targeting autoantibodies

Autoantibody production and leukocytopenia may be linked in patients with lupus erythematosus (LE). Unclear is the ability of different autoantibody species to induce apoptosis and cell loss. Laboratory routine analyses (white blood cell counts, autoantibody detection), and flow cytometry (annexin V, CD3, CD4, CD8) have been performed in 126 consecutive LE-patients. Nuclei of PBMC were investigated flow cytometrically for the presence of the 85 kDa poly-(ADP-ribose)-polymerase (PARP) fragment. Peripheral total white blood cells (WBC), lymphocytes, T-cells, CD3+ CD4+, and CD3+ CD8+ cells were significantly decreased in patients with LE (P from 1.2 x 10(-14) to P < .0008). In the presence of either antinuclear (P from 1.2 x 10(-14) to P < .0008) or anti-dsDNA antibodies (P from 2.9 x 10(-12) to P < .007) were significantly diminished. Differences in cell numbers in LE patients with versus without anti-Ro/SSA were less pronounced: significant differences could be only obtained in lymphocytes and T-cells (P < .02). Anti-La/SSB antibodies were accompanied by significant increased leukocytes (P < .02). PARP cleavage (85 kDa) in nuclei was preferentially observed in cases with nuclear targeting autoantibodies. These results indicate that nuclear targeting autoantibodies are associated to lower peripheral blood cells counts than Ro/SSA, and La/SSB cytoplasmic targeting autoantibodies. This provides an explanation for the pathogenesis of cytopenias associated with SLE.

Environment-dependent down-modulation of CD45 cell surface expression on polymorphonuclear cells

CD45, a transmembrane protein tyrosine phosphatase, regulates important cell functions. Its involvement in clearing tissues from dead or dying cells by polymorphonuclear cells (PMNs) is unclear. Therefore, in this study the influence of exogenous factors (e.g., temperature and phagocytosis) on the CD45 expression on PMNs (in patients with lupus erythematosus (LE) vs. controls) was analysed by flow cytometry. The data showed a significant down-regulation of CD45 at 4 degrees C in controls, but not on LE-PMNs. In the presence of targets, CD45 was significantly decreased (P<0.00002). Substrate-dependent down-regulation was greater in the presence of nuclei than in the presence of dead cells(68.98%+/-2.28% vs. 47.89%+/-2.75%; P<0.002), and on LE-PMNs than on control-PMNs (45.93%+/-4.92% vs. 53.65%+/-2.2%). Target pretreatment with anti-dsDNA antibody-containing serum resulted in a lower CD45 expression compared to treatment with normal serum. This effect was more pronounced on PMNs of LE patients than on those of controls. Phagocytosis was inversely correlated with CD45 expression (P<0.0007). These data suggest that phagocytic clearance of cellular debris by PMNs may modulate the function of the effector cells, and that anti-dsDNA antibodies apparently enhance the phagocytic clearance.

Nuclear and Nucleolar Localization of 18-kDa Fibroblast Growth Factor-2 Is Controlled by C-terminal Signals

Members of high (22-, 22.5-, 24-, and 34-kDa) and low (18-kDa) molecular mass forms of fibroblast growth factor-2 (FGF-2) regulate cell proliferation, differentiation, and migration. FGF-2s have been previously shown to accumulate in the nucleus and nucleolus. Although high molecular weight forms of FGF-2 contain at least one nuclear localization signal (NLS) in their N-terminal extension, the 18-kDa FGF-2 does not contain a standard NLS. To determine signals controlling the nuclear and subnuclear localization of the 18-kDa FGF-2, its full-length cDNA was fused to that of green fluorescent protein (GFP). The fusion protein was primarily localized to the nucleus of COS-7 and HeLa cells and accumulated in the nucleolus. The subcellular distribution was confirmed using wild type FGF-2 and FGF-2 tagged with a FLAG epitope. A 17-amino acid sequence containing two groups of basic amino acid residues separated by eight amino acid residues directed GFP and a GFP dimer into the nucleus. We systematically mutated the basic amino acid residues in this nonclassical NLS and determined the effect on nuclear and nucleolar accumulation of 18-kDa FGF-2. Lys(119) and Arg(129) are the key amino acid residues in both nuclear and nucleolar localization, whereas Lys(128) regulates only nucleolar localization of 18-kDa FGF-2. Together, these results demonstrate that the 18-kDa FGF-2 harbors a C-terminal nonclassical bipartite NLS, a portion of which also regulates its nucleolar localization.

Thromboxane A 2 Receptor Agonists Antagonize the Proangiogenic Effects of Fibroblast Growth Factor-2

Thromboxane (TX) A 2 is released from multiple cell types and is a prime mediator of the pathogenesis of many vascular events, including angiogenesis. Endothelial cells express TXA 2 receptors (TP) but the effects of TP stimulation on angiogenesis remain controversial. In this study, we show that stimulation of endothelial cell TP impairs ligand-induced FGF receptor internalization and consequently abrogates FGF-2-induced endothelial cell migration in vitro and angiogenesis in vivo. Prevention of FGF-2-induced angiogenesis was associated with expression of the TPβ isoform. The deficit in FGFR1 internalization was mediated through activation of TPβ preventing the FGF-2-mediated decrease in p53 expression, thus enhancing thrombospondin-1 (TSP-1) release from EC and reducing FGFR1 internalization. Once released TSP-1 interacted with the α v β 3 integrin on the EC surface. On stimulation, FGFR1 and α v β 3 were found to associate in a complex. We determined that complex formation was important for receptor internalization as conditions that inhibit FGFR1 internalization, such as inappropriate ligation of α v β 3 by either TSP-1 or a neutralizing antibody, disrupted the complex. These results establish a novel role for isoform specific regulation of angiogenesis by TP, provide the first functional significance for the existence of two TP isoforms in humans, and clarify the mechanism by which TP signaling regulates FGFR1 kinetics and signaling.

LE cell phenomenon: nuclear IgG deposits inhibit enzymatic cleavage of the nucleus of damaged cells and support its phagocytic clearance by PMN

Apoptotic cell death, phagocytic uptake, and their interplay may induce/trigger autoantibody production and autoimmunity. Herein the role of immunoglobulin G (IgG) anti-dsDNA autoantibodies in nucleo-phagocytosis by polymorpho-nuclear cells (PMN) has been analyzed. Necrotaxis, phagocytosis and PMN with engulfed nuclei can be imaged by vital microscopy. Vital microscopy revealed dead neutrophils with persistence of nuclei or ingested nuclei in 8/10 patients with lupus erythematosus (LE) and in 4/20 healthy controls (P < 0.0006). The phagocytic clearance of dead cells/nuclei did not correspond to any of the analyzed apoptosis markers (annexin V, CD95, and active caspase 3). IgG staining of isolated PBMC nuclei as detected by flow cytometry was significantly higher in parallel to NUC+ (P < 0.02). In vitro phagocytosis of substrates pre-treated with anti-dsDNA containing serum induced an increased phagocytic capacity as compared to normal serum. These data suggest that anti-dsDNA antibodies seem to inhibit enzymatic cleavage of nuclei from damaged cells, and improve their phagocytic uptake by PMN, which finally results in LE cell formation.

Deletion mutant of FGFR4 induces onion-like membrane structures in the nucleus

The expression of several deletion mutants of fibroblast growth factor receptor 4 (FGFR4) was studied in COS-1 cells. FGFR4-mutants lacking most of the extracellular region did not efficiently reach the plasma membrane but accumulated in the endoplasmic reticulum (ER) and Golgi body. A mutant FGFR4 lacking the kinase domain as well as most of the extracellular region (DeltaExt/R4Tth) had a distinct intracellular distribution. It localized in part to the nucleus, where it exhibited a striking spotted pattern. Ultrastructural studies showed that the nuclear spots consisted of several layers of membrane that were folded into onion-like structures at the nucleoplasmic side of the nuclear envelope. These intranuclear structures did not contain nuclear pores but were positive for the ER proteins calreticulin and protein disulfide isomerase, in addition to abundant DeltaExt/R4Tth. Formation of the intranuclear structures was sensitive to inhibition of protein kinase C. Live microscopy of a green-fluorescent-protein/DeltaExt/R4Tth fusion protein showed that the intranuclear structures were stable and immobile, suggesting that they function as deposits of the overexpressed mutant and associated membrane. The DeltaExt/R4Tth protein also induced formation of densely packed membrane stacks in the cytosol and we suggest a model were the intranuclear structures are formed by invagination of ER-derived membrane stacks into the nucleus.

Proteasomal degradation of the nuclear targeting growth factor midkine

It is widely held that growth factor signaling is terminated by lysosomal degradation of its activated receptor and the endocytosed growth factor is transported to lysosomes. Nuclear targeting is another important pathway through which signals of growth factors are mediated. However, mechanisms underlying desensitization of nuclear targeting growth factors are poorly understood. Here we report that the nuclear targeting pathway is down-regulated by the proteasome system. Degradation of endocytosed midkine, a heparin-binding growth factor, was suppressed by both proteasome and lysosome inhibitors to similar extents. By contrast, a proteasome inhibitor, but not lysosome ones, accelerated the nuclear accumulation of midkine. An expression vector of signal sequence-less midkine, which is produced in the cytosol, was constructed because endocytosed midkine may be translocated to the cytosol from cellular compartments before entering the nucleus. The cytosol-produced midkine underwent proteasomal degradation and accumulated in the nucleus as did the endocytosed midkine. It was polyubiquitinated, and its nuclear accumulation was enhanced by a proteasome inhibitor. We further dissected the midkine molecule to investigate roles in degradation and trafficking. The N-terminal half-domain of midkine was significantly more susceptible to proteasomal degradation, whereas the C-terminal half-domain was sufficient for nuclear localization. Together, these data highlight the desensitization of nuclear targeting by growth factors and indicate a critical role of the proteasome system in it.

Nonclassical transport proteins and peptides: an alternative to classical macromolecule delivery systems

The number of peptides and proteins known to exhibit nonclassical transport activity has increased significantly in recent years. In most cases, these entities have been studied in relation to their ability to deliver high molecular weight compounds, including proteins and DNA, for the ultimate purpose of developing new drug delivery strategies. In this review, an overview of the various types of vectors is presented. The in vitro and in vivo delivery successes of this technology, as well as preliminary therapeutic efforts, are described. Although a comprehensive mechanism of nonclassical transport has not yet been clearly established, we propose a straightforward model based on the cationic nature of the vectors and the need for lack of highly organized structure. In this hypothesis we suggest that the movement of polycations is mediated by a network of extra- and intracellular polyanions while transport across the bilayer is facilitated by cation-pi interactions between the vectors' basic groups and aromatic amino acid side chains in the bilayer spanning helices of membrane proteins.

A permeable FGF-1 nuclear localization sequence peptide induces DNA synthesis independently of Ras activation

A 26-amino-acid peptide (designated PFNP) composed of the nuclear localization signal of fibroblast growth factor (FGF)-1 and a membrane-permeable peptide is known to mimic FGF-1's ability to stimulate DNA synthesis in various cell types at low cell densities. The underlying molecular mechanism is unknown, however. Here we show that PFNP activity is inhibited in murine fibroblasts by a tyrosine kinase inhibitor, that PFNP does not bind to the FGF receptor, and that PFNP does not induce phosphorylation of the FGF receptor substrate. In addition, expression of a dominant-negative form of Ras, which abolished the activities of epidermal growth factor (EGF) and heparin-binding EGF, had no affect on PFNP-induced DNA synthesis. Despite this apparent Ras independence, PFNP activity correlated with phosphorylation of ERK1/2 MAP kinases and was concentration dependently inhibited by inhibitors of ERK1/2 MAP kinase phosphorylation. These results indicate that whereas Ras activation is dispensable for PFNP-induced DNA synthesis, activation of tyrosine kinases and ERK1/2 kinases, albeit independently of the FGF receptor system, is crucial. Interestingly, FGF-1 signaling was predominantly Ras-independent when the cell density was optimum for PFNP, suggesting that PFNP and FGF-1 share the same signaling mechanism.

Differential intranuclear localization of fibroblast growth factor-2 isoforms and specific interaction with the survival of motoneuron protein

Fibroblast growth factor 2 (FGF-2) is an important modulator of cell growth and differentiation and a neurotrophic factor. FGF-2 occurs in isoforms, at a low molecular weight of 18,000 and at least two high molecular weight forms (21,000 and 23,000), representing alternative translation products from a single mRNA. In addition to its role as an extracellular ligand, FGF-2 localizes to the nuclei of cells. Here we show differential localization of the 18- and 23-kDa isoforms in the nuclei of rat Schwann cells. Whereas the 18-kDa isoform was found in the nucleoli, nucleoplasm, and Cajal bodies, the 23-kDa isoform localized in a punctuate pattern and associates with mitotic chromosomes suggesting different functional roles of the isoforms. Moreover, we show here that the 23-kDa FGF-2 isoform co-immunoprecipitates specifically with the survival of motor neuron protein (SMN). SMN is an assembly and recycling factor of the splicing machinery and locates to the cytoplasm, the nucleoplasm, and nuclear gems, where it co-localizes with 23-kDa FGF-2. Patients with spinal muscular atrophy suffer from fatal degeneration of motoneurons because of mutations and deletions of the gene for the SMN protein.

Nuclear targeting by the growth factor midkine

Ligand-receptor internalization has been traditionally regarded as part of the cellular desensitization system. Low-density lipoprotein receptor-related protein (LRP) is a large endocytosis receptor with a diverse array of ligands. We recently showed that LRP binds heparin-binding growth factor midkine. Here we demonstrate that LRP mediates nuclear targeting by midkine and that the nuclear targeting is biologically important. Exogenous midkine reached the nucleus, where intact midkine was detected, within 20 min. Midkine was not internalized in LRP-deficient cells, whereas transfection of an LRP expression vector restored midkine internalization and subsequent nuclear translocation. Internalized midkine in the cytoplasm bound to nucleolin, a nucleocytoplasmic shuttle protein. The midkine-binding sites were mapped to acidic stretches in the N-terminal domain of nucleolin. When the nuclear localization signal located next to the acidic stretches was deleted, we found that the mutant nucleolin not only accumulated in the cytoplasm but also suppressed the nuclear translocation of midkine. By using cells that overexpressed the mutant nucleolin, we further demonstrated that the nuclear targeting was necessary for the full activity of midkine in the promotion of cell survival. This study therefore reveals a novel role of LRP in intracellular signaling by its ligand and the importance of nucleolin in this process.

Vesicle transmembrane potential is required for translocation to the cytosol of externally added FGF-1

Externally added fibroblast growth factor-1 (FGF-1) is capable of crossing cellular membranes to reach the cytosol and the nucleus in a number of cell types. We have monitored the translocation of the growth factor by two methods: phosphorylation of FGF-1, and prenylation of an FGF-1 mutant that contains a C-terminal prenylation signal. Inhibition of endosomal acidification by ammonium chloride or monensin did not block the translocation of FGF-1, whereas bafilomycin A1, a specific inhibitor of vacuolar proton pumps, blocked translocation completely. A combination of ionophores expected to dissipate the vesicular membrane potential (valinomycin plus monensin) also fully inhibited the translocation. The inhibition of translocation by bafilomycin A1 was overcome in the presence of monensin or nigericin, while ouabain blocked translocation under these conditions. The data indicate that translocation of FGF-1 to cytosol occurs from the lumen of intracellular vesicles possessing vacuolar proton pumps, and that a vesicular membrane potential is required. Apparently, activation of vesicular Na+/K+-ATPase by monensin or nigericin generates a membrane potential that can support translocation when the proton pump is blocked.

Antiproliferative effect of neomycin in glioma cells

The angiogenic growth factor (AGF) family of signaling molecules has been implicated in normal development and in physiological process as well as in human malignancy. Since blockage of nuclear translocation of AGF in endothelial cells with neomycin resulted in inhibition of the growth factor capacity to induce angiogenesis, we treated glioma cells with neomycin and assessed its effects on cell proliferation. Administration of 10mM neomycin during two days resulted in a 56% inhibition of glioma cells proliferation. This result may provide the basis for the development of a novel adjuvant therapeutic strategy forgliomas.

Fibroblast growth factor-2 interacts with free ribosomal protein S19

Exogenous FGF-2 added to cells is internalized and part of it translocates to the nucleus of the cells. To get a better understanding of the FGF-2-induced signaling pathway, we looked for proteins associated with FGF-2 in the cytoplasm of the target cells. We first used the GST-FGF-2 to isolate cytoplasmic proteins complexes containing FGF-2 from S100 extract (supernatant 100,000g). Among the retrieved proteins, we focused our studies on RPS19, a protein of the 40S small ribosomal subunit. We showed that FGF-2 interacts directly with RPS19 in vitro. Second, we coimmunoprecipitated RPS19 and FGF-2 from a S240 extract (240,000g supernatant) prepared from FGF-2-stimulated cells and devoid of 40S ribosomal subunit. The result of these experiments suggest that a pool of free RPS19 exists in cells and that FGF-2 interacts in vivo with free RPS19.

Locally reduced levels of acidic FGF lead to decreased expression of 28-kda calbindin and contribute to the selective vulnerability of the neurons in the entorhinal cortex in Alzheimer's disease

Recent studies demonstrate that a disturbed calcium-homeostasis leading to increased susceptibility to excitotoxic triggers plays a major role in the neurodegenerative process initiating in layer 2 of the entorhinal cortex (EC2) during Alzheimer's disease (AD). Thus, proteins binding free Ca++ (i.e. calbindin) and factors regulating these proteins are of great importance for the neuroprotective-neurotoxic balance in the affected brain regions. In the present combined human and in vitro study evidence is provided that altered levels of the acidic fibroblast growth factor (aFGF) and calbindin expression are concomitantly present in EC2 neurons and have interactive effects. A dramatic loss of aFGF- and calbindin-labeled EC2 neurons was found. Further analysis of the surviving EC2 neurons revealed a strong immunoreactivity to calbindin and aFGF. In vitro experiments show that aFGF regulates calbindin expression, because treatment of differentiating neurons with recombinant aFGF increases calbindin expression in a time-dependent fashion. The data imply that a reduced expression of aFGF in EC2 neurons of AD brains leads to lower levels of calbindin resulting in decreased neuroprotective capacity.

Nuclear accumulation of exogenous DNA fragments in viable cells mediated by FGF-2 and DNA release upon cellular injury

We and others have previously shown that basic fibroblast growth factor (FGF-2 or bFGF) can be used as a targeting molecule to help carry plasmid DNA into cells when the growth factor molecule is physically coupled to the DNA molecule being delivered. Herein we report our observations on the FGF-mediated uptake of exogenous labeled DNA into cultured cells in a manner that is representative of that which may occur under physiological conditions at sites of wounded tissue. Cellular debris at such sites contains nucleic acid fragments released from dead cells, as well as growth factors such as FGF-2 that function early in the wound repair process. Using a cell culture model designed to mimic the local environment of a wound with respect to the presence of soluble FGF-2 and DNA fragments, we have shown that FGF-2 is able to direct the cellular uptake and nuclear localization of fragments of exogenous DNA via the FGF receptor into intact and healthy cells. Furthermore, we can monitor and quantitate this type of FGF-mediated DNA delivery by using indirect immunofluorescence of bromodeoxyuridine-labeled exogenous DNA. Our results suggest that this type of FGF-mediated DNA fragment uptake could allow for the transduction of viable nearest neighbor cells at sites of injury in vivo. Such a phenomenon may lead to mutational aberrations in the recipient cells and enhance the probability of wound carcinogenesis.

Cell-cycle dependent tyrosine phosphorylation on mortalin regulates its interaction with fibroblast growth factor-1

We previously reported that endogenously expressed, intracellularly localized fibroblast growth factor (FGF)-1 interacts with mortalin. Here we report that FGF-1 added to the culture medium of quiescent BALB/c3T3 cells is taken up by the cells and interacts with mortalin in the cells in a regulated manner. Although both the internalized FGF-1 and mortalin were present at high levels throughout the FGF-1-initiated cell cycle, their interaction became apparent only in late G1 phase. Interestingly, mortalin was preferentially tyrosine phosphorylated at the same time, and when its normally weak phosphorylation in early G1 phase was augmented by treating the cells with vanadate, a strong interaction between mortalin and FGF-1 was established. Conversely, when phosphorylated mortalin was treated with tyrosine phosphatase, its interaction with FGF-1 was abrogated. These results indicate that FGF-1 taken up by cells preferentially interacts with mortalin in late G1 phase of the cell cycle, and that tyrosine phosphorylation of mortalin regulates this interaction.

Regulation of proliferation-survival decisions is controlled by FGF1 secretion in retinal pigmented epithelial cells

Fibroblast growth factor 1 (FGF1) induces proliferation and differentiation in a wide variety of cells of mesodermal and neuroectodermal origin. FGF1 has no 'classical' signal sequence to direct its secretion, and there has been considerable debate concerning FGF1 secretion and its role in the biological activities of FGF1. We investigated the effects of FGF1 secretion and the signalling induced by signal peptide (SP)-containing FGFI and SP-less FGF1, on the proliferation and the apoptosis in retinal pigmented epithelial (RPE) cells. Primary RPE cell cultures were transfected with FGF1 (FGF1 cells) and SP-FGF1 (SP-FGF1 cells) cDNAs. SP-FGF1 cells secreted large amount of FGF1 and actively proliferated, whereas FGF1 and control cells did not. Secreted FGF1 induced short-term activation of both FGFR1 and ERK2, which were required for cell proliferation. In contrast, SP-FGF1 cells stopped secreting FGF1 and died rapidly, if cultured in the absence of serum. Surprisingly, FGF1 cells, but not control cells, secreted FGF1 and were resistant to apoptosis induced by serum depletion. Secreted FGF1 induced long-term activation of FGFR1 and ERK2, which was necessary to induce a constant and high level of Bcl-x production, and to induce cell survival in FGFI cells. Downregulation of ERK2 and Bcl-x increased apoptosis. Thus, the proliferation and survival activities of FGF1 depend on the secretion of FGF1 which is determined by the cell culture conditions. Cell proliferation was SP-dependent, whereas cell survival was not. The signal peptide controls the level and duration, 'whispering or shouting', of ERK2 activation cells which determines FGF1 biological function and may have important implications for anti-degenerative and anti-proliferative treatments.

Acidic fibroblast growth factor in synovial cells

OBJECTIVE

To characterize the production and regulation of acidic fibroblast growth factor (aFGF) in type B (fibroblast-like) synoviocytes cultured from both inflammatory and noninflammatory synovial lesions.

METHODS

Immunohistochemistry, Western blotting, and reverse transcriptase-polymerase chain reaction were used to examine the expression of aFGF by synovial cells in vitro. Incorporation of 3H-thymidine by NIH3T3 cells in the presence or absence of neutralizing antibody to aFGF was used to measure bioactive aFGF levels in culture media.

RESULTS

Acidic FGF was detected in all synovial cell lines during growth in vitro; however, synoviocytes from rheumatoid arthritis (RA) patients sustained more abundant production of cytoplasmic and nuclear aFGF. Acidic FGF production persisted after multiple passages and did not depend on the presence of serum. Both RA and noninflammatory synovial cells were competent to release aFGF into the media, even though aFGF lacks a signal peptide. Tumor necrosis factor alpha, interleukin-6, and epidermal growth factor did not increase aFGF expression in vitro; in contrast, transforming growth factor beta1 (TGFbeta1) was found to markedly increase aFGF production by cultured synovial cells.

CONCLUSION

Acidic FGF synthesis and release is a component of synovial cell growth that is markedly increased in RA. TGFbeta1, and not proinflammatory cytokines, is a potent inducer of aFGF production by synoviocytes in vitro. These findings suggest that in RA, interactions between TGFbeta1 and aFGF may contribute to angiogenesis and fibroblast proliferation, potentially independently of inflammatory mediators.

The mitogenic activity of fibroblast growth factor-1 correlates with its internalization and limited proteolytic processing

The fibroblast growth factor-1 (FGF-1) mitogenic signal transduction pathway is not well characterized, and evidence indicates that FGF-1 binding to and activation of cell-surface receptors is not solely sufficient for a full mitogenic response. Although initiation of the phosphorylation signaling cascades are likely important in FGF-1-induced mitogenic signaling, there appear to be additional signaling requirements. In this study, we demonstrate that FGF-1 internalization and subsequent processing correlates with the mitogenic potential of the growth factor on NIH 3T3 cells. Using site-directed mutants of FGF-1 and inhibitors of the endocytic and degradative pathways, we provide evidence for growth factor internalization and exposure to an acidic environment as necessary components of FGF-1-induced mitogenesis. In addition, a protease-sensitive event(s) appears critical for a complete mitogenic response to FGF-1, whereas, this protease sensitivity was not detected under the same conditions for serum-stimulated mitogenesis. Therefore, proteolytic modification of internalized FGF-1 may result in the activation of additional, intracellular signaling events.

Fibroblast growth factors are translocated to the nucleus of human endothelial cells in a microtubule- and lysosome-independent pathway

Exogenous acidic and basic fibroblast growth factors undergo rapid nuclear translocation in human umbilical vein endothelial cells. When nuclear translocation reaches saturation, more than 70% of the internalized growth factors are in the nuclear fraction. Lysosomal inhibitors, such as leupeptin and chloroquine, and microtubule inhibitors including colchicine and 2-methoxyl-beta-estradiol neither increase nor decrease nuclear translocation. The results suggest that nuclear translocation of fibroblast growth factors does not require cytosolic accumulation or lysosomal processing and that the transportation of exogenous growth factors across the cytoplasm is independent of microtubules.

Requirement for C-terminal end of fibroblast growth factor receptor 4 in translocation of acidic fibroblast growth factor to cytosol and nucleus

The ability of COS cells to bind and internalise acidic fibroblast growth factor (aFGF) was studied after transient transfection of the cells with wild-type and mutated fibroblast growth factor receptor 4. In one case the tyrosine kinase of the receptor was inactivated by a point mutation in the active site, whereas in other cases parts of the receptor were deleted to remove various parts of the cytoplasmic domain. In all cases the receptors were expressed at the cell surface at a high level and the cells bound labelled growth factor efficiently and internalised it by endocytosis. Translocation of externally added aFGF across cellular membranes to reach the cytosol and nucleus was measured as transport of labelled growth factor to the nuclear fraction obtained by centrifugation, by farnesylation of growth factor modified to carry a CAAX motif, and by phosphorylation of the growth factor at a site specific for protein kinase C. Whereas both full-length receptors (with and without an active kinase domain) facilitated translocation of the growth factor to the cytosol and nucleus, as assessed by these methods, the mutants of the receptor where the C terminus was deleted, were unable to do so. In contrast, a receptor containing only the 57 most C-terminal amino acids of the cytoplasmic domain in addition to the juxtamembrane, transmembrane and extracellular domains, was in fact able to mediate translocation of aFGF to the cytosol. These data indicate that information contained in the C terminus of the receptor is required for translocation.

Requirement of phosphatidylinositol 3-kinase activity for translocation of exogenous aFGF to the cytosol and nucleus

Acidic fibroblast growth factor (aFGF) is a potent mitogen for many cells. Exogenous aFGF is able to enter the cytosol and nucleus of sensitive cells. There are indications that both activation of the receptor tyrosine kinase and translocation of aFGF to the nucleus are of importance for mitogenesis. However, the mechanism of transport of aFGF from the cell surface to the nucleus is poorly understood. In this work we demonstrate that inhibition of phosphatidylinositol (PI) 3-kinase by chemical inhibitors and by expression of a dominant negative mutant of PI 3-kinase blocks translocation of aFGF to the cytosol and nucleus. Translocation to the cytosol and nucleus was monitored by cell fractionation, by farnesylation of aFGF modified to contain a farnesylation signal, and by phosphorylation by protein kinase C of aFGF added externally to cells. If aFGF is fused to diphtheria toxin A-fragment, it can be artificially translocated from the cell surface to the cytoplasm by the diphtheria toxin pathway. Upon further incubation, the fusion protein enters the nucleus due to a nuclear localization sequence in aFGF. We demonstrate here that upon inhibition of PI 3-kinase the fusion protein remains in the cytosol. We also provide evidence that the phosphorylation status of the fusion protein does not regulate its nucleocytoplasmic distribution.

Intracellular and extracellular leukemia inhibitory factor proteins have different cellular activities that are mediated by distinct protein motifs

Although many growth factors and cytokines have been shown to be localized within the cell and nucleus, the mechanism by which these molecules elicit a biological response is not well understood. The cytokine leukemia inhibitory factor (LIF) provides a tractable experimental system to investigate this problem, because translation of alternatively spliced transcripts results in the production of differentially localized LIF proteins, one secreted from the cell and acting via cell surface receptors and the other localized within the cell. We have used overexpression analysis to demonstrate that extracellular and intracellular LIF proteins can have distinct cellular activities. Intracellular LIF protein is localized to both nucleus and cytoplasm and when overexpressed induces apoptosis that is inhibited by CrmA but not Bcl-2 expression. Mutational analysis revealed that the intracellular activity was independent of receptor interaction and activation and reliant on a conserved leucine-rich motif that was not required for activation of cell surface receptors by extracellular protein. This provides the first report of alternate intracellular and extracellular cytokine activities that result from differential cellular localization of the protein and are mediated by spatially distinct motifs.

Fibroblast growth factor 9 secretion is mediated by a non-cleaved amino-terminal signal sequence

Fibroblast growth factors are a family of intercellular signaling molecules with multiple and varied roles in animal development. Most are exported from cells by means of a classical amino-terminal signal sequence that is cleaved from the mature protein during its passage through the secretory pathway. Fibroblast growth factor-9 (Fgf-9) does not contain a recognizable signal sequence, although it is efficiently secreted. In this study, we show that Fgf-9 enters the endoplasmic reticulum and traverses the Golgi complex in a similar manner to other constitutively secreted proteins. Deletion and point mutation analysis has revealed an atypical non-cleaved signal sequence within the amino-terminal region of Fgf-9. Moreover, the first 28 amino acids of Fgf-9 can function as an efficient non-cleaved signal peptide when appended to the amino terminus of green fluorescent protein.

Nuclear localization of the type 1 parathyroid hormone/parathyroid hormone-related peptide receptor in MC3T3-E1 cells: association with serum-induced cell proliferation

We have recently demonstrated that the receptor for parathyroid hormone (PTH) and PTH-related peptide (PTHrP), PTHR, can be localized to the nucleus of cells within the liver, kidney, uterus, gut, and ovary of the rat. We set out to determine the localization of the PTHR in cultured osteoblast-like cells. MC3T3-E1, ROS 17/2.8, UMR106, and SaOS-2 cells were cultured in alpha-modified eagle medium containing 15% fetal calf serum under standard conditions. Untreated cells were grown on glass coverslips to 75-95% confluence and fixed in 1% paraformaldehyde. For experiments designed to examine cells synchronized by serum starvation, cells were grown on glass coverslips, starved of serum for 46 h, and then fixed at 2-h intervals for a total of 26 h after the addition of serum to the medium. Parallel sets of cells were pulsed with [3H]thymidine to track the DNA duplication interval. The PTHR was localized by immunocytochemistry using a primary antibody raised against a portion of the N-terminal extracellular domain of the PTHR. The results presented herein indicate that the PTHR attains a nuclear localization in each cell line examined. In UMR106 cells, PTHR immunoreactivity was restricted to the nucleolus. After cell synchronization, MC3T3-E1 cells double approximately 24 h after the addition of serum. Immunocytochemistry for the PTHR in these cells showed that the receptor staining is initially diffuse for the first 6 h, then becomes more perinuclear in distribution by 12-16 h. Nuclear localization of the receptor is achieved approximately 16-20 h after the addition of serum and remains there throughout the mitotic phase. Intense staining of mitotic and postmitotic cells was observed. No change in cell proliferation kinetics was observed in MC3T3-E1 cells cultured in the presence of 25 nM PTH(1-34). These data suggest an important role for the PTHR in the nucleus of MC3T3-E1 cells at the time of DNA synthesis and mitosis.

Tyrosine kinase signalling in breast cancer: fibroblast growth factors and their receptors

The fibroblast growth factors [Fgfs (murine), FGFs (human)] constitute a large family of ligands that signal through a class of cell-surface tyrosine kinase receptors. Fgf signalling has been associated in vitro with cellular differentiation as well as mitogenic and motogenic responses. In vivo, Fgfs are critical for animal development, and some have potent angiogenic properties. Several Fgfs have been identified as oncogenes in murine mammary cancer, where their deregulation is associated with proviral insertions of the mouse mammary tumour virus (MMTV). Thus, in some mammary tumours of MMTV-infected mouse strains, integration of viral genomic DNA into the somatic DNA of mammary epithelial cells was found to have caused the inappropriate expression of members of this family of growth factors. Although examination of human breast cancers has shown an altered expression of FGFs or of their receptors in some tumours, their role in the causation of breast disease is unclear and remains controversial.

Human angiogenin is rapidly translocated to the nucleus of human umbilical vein endothelial cells and binds to DNA

Human angiogenin is translocated to the nucleus of human umbilical vein endothelial cells in a time-dependent manner. Exogenous angiogenin appears in the nucleus in 2 min, reaches saturation in 15 min when 85% of the internalized angiogenin is in the nuclei, and remains associated with the nucleus for at least 4 h. Endothelial cells cultured at low density have a much higher capacity to translocate angiogenin to the nucleus than do those cultured at high density. This observation is consistent with previous findings that both the ability of endothelial cells to proliferate in response to angiogenin and the expression of an angiogenin receptor on the cell surface depend on cell density. Nuclear (125)I-angiogenin is not degraded and is neither spontaneously dissociated nor replaced by unlabeled angiogenin. It is, however, released by deoxyribonuclease I, but not by ribonuclease A, suggesting that angiogenin binds to DNA in the nucleus. These results suggest that in addition to acting as a ribonuclease, angiogenin may play a role in regulating gene expression by direct binding to DNA.

Uptake and intracellular transport of acidic fibroblast growth factor: evidence for free and cytoskeleton-anchored fibroblast growth factor receptors

Endocytic uptake and intracellular transport of acidic FGF was studied in cells transfected with FGF receptor 4 (FGFR4). Acidification of the cytosol to block endocytic uptake from coated pits did not inhibit endocytosis of the growth factor in COS cells transfected with FGFR4, indicating that it is to a large extent taken up by an alternative endocytic pathway. Fractionation of the cells demonstrated that part of the growth factor receptor was present in a low-density, caveolin-containing fraction, but we were unable to demonstrate binding to caveolin in immunoprecipitation studies. Upon treatment of the cells with acidic FGF, the activated receptor, together with the growth factor, moved to a juxtanuclear compartment, which was identified as the recycling endosome compartment. When the cells were lysed with Triton X-100, 3-([3-chloramidopropyl]dimethylammonio)-2-hydroxy-1-propanesulfona te, or 2-octyl glucoside, almost all surface-exposed and endocytosed FGFR4 was solubilized, but only a minor fraction of the total FGFR4 in the cells was found in the soluble fraction. The data indicate that the major part of FGFR4 is anchored to detergent-insoluble structures, presumably cytoskeletal elements associated with the recycling endosome compartment.

Effects of mutations of a phosphorylation site in an exposed loop in acidic fibroblast growth factor

Acidic fibroblast growth factor (aFGF) contains a phosphorylation site recognized by protein kinase C. A non-mitogenic mutant growth factor is devoid of this phosphorylation site. We have changed amino acids in and close to the phosphorylation site and studied the consequences of this for binding of the growth factor to high affinity receptors as well as to heparin. We have also studied the ability of the mutants to stimulate DNA synthesis and cell proliferation as well as phosphorylation of mitogen-activated protein kinase and the ability of the growth factor mutants to be transported to the nucleus. The results indicate that while the mutations strongly affect the ability of the growth factor to bind to heparin, they do not affect much the binding to the specific FGF receptors, activation of mitogen-activated protein kinase or transport of the growth factor to the nucleus. The mutations affect to various extents the ability of the growth factor to stimulate DNA synthesis and to induce cell multiplication. We find that phosphorylation of aFGF is not required for mitogenic activity. The data suggest that altered interaction of the growth factor with a cellular component different from the receptor, possibly a component in the nucleus, is the reason for the different mitogenicity of the different growth factor mutants.

Evidence for control of nitric oxide synthesis by intracellular transforming growth factor-beta1 in tumor cells. Implications for tumor development

Transforming growth factor-beta1 (TGF-beta1) has been shown to down-regulate NO synthesis in a variety of normal cells. In the present study, we investigated the influence of TGF-beta1 upon NO production in tumor cells and its consequences for tumor development. During the growth of PROb colon carcinoma cells intraperitoneally injected in syngeneic BDIX rats, intratumoral concentration of TGF-beta1 increases while NO concentration stays very low. Tumor regression induced by intraperitoneal injections of a lipid A is associated with a decrease in TGF-beta1 and an increase in NO intratumoral concentration. In these tumors, PROb tumor cells are the NO- and TGF-beta1-secreting cells. Using PROb cells transfected with an expression vector coding for TGF-beta1 antisense mRNA, we demonstrate in vitro that there is an inverse correlation between the amount of TGF-beta1 secreted and the ability of PROb cells to secrete NO. As the same results were obtained in the presence of an anti-TGF-beta type II receptor neutralizing antibody, and as exogenous TGF-beta1 is without any effect on NO secretion by PROb cells, TGF-beta1 apparently down-regulates NO synthesis in PROb cells by an intracellular mechanism. These results suggest that endogenous TGF-beta1 constitutes a potential target in a search for new antitumoral agents.

Nuclear activities of basic fibroblast growth factor: potentiation of low-serum growth mediated by natural or chimeric nuclear localization signals

Human basic fibroblast growth factor (FGF-2) occurs in four isoforms: a low molecular weight (LMW FGF-2, 18 kDa) and three high molecular weight (HMW FGF-2, 22, 22.5, and 24 kDa) forms. LMW FGF-2 is primarily cytoplasmic and functions in an autocrine manner, whereas HMW FGF-2s are nuclear and exert activities through an intracrine, perhaps nuclear, pathway. Selective overexpression of HMW FGF-2 forms in fibroblasts promotes growth in low serum, whereas overexpression of LMW FGF-2 does not. The HMW FGF-2 forms have two functional domains: an amino-terminal extension and a common 18-kDa amino acid sequence. To investigate the role of these regions in the intracrine signaling of HMW FGF-2, we produced stable transfectants of NIH 3T3 fibroblasts overexpressing either individual HMW FGF-2 forms or artificially nuclear-targeted LMW FGF-2. All of these forms of FGF-2 localize to the nucleus/nucleolus and induce growth in low serum. The nuclear forms of FGF-2 trigger a mitogenic stimulus under serum starvation conditions and do not specifically protect the cells from apoptosis. These data indicate the existence of a specific role for nuclear FGF-2 and suggest that LMW FGF-2 represents the biological messenger in both the autocrine/paracrine and intracrine FGF-2 pathways.

Distinct regulation of myoblast differentiation by intracellular and extracellular fibroblast growth factor-1

We studied the role of fibroblast growth factor (FGF)-1 in the physiology of myoblast differentiation. We found that, while endogenous FGF-1 in L6-10 rat myoblasts did not suppress the progress of differentiation, the addition of FGF-1 to the culture medium suppressed it. Moreover, L6-10 cells stably transfected with full length FGF-1 undergo enhanced differentiation. The latter was well correlated with myogenin expression and myotube formation. Constitutive expression of a mutant FGF-1 (FGF-1U) that lacked a nuclear localization signal, promoted the differentiation of the myoblasts even more strongly. Furthermore, the expression of FGF-1U in an inducible expression system enhanced myogenin expression promptly. In L6-10 transfectants expressing a dominant-negative mutant of FGF receptor, stable transfection of FGF-1 promoted differentiation as it did in parent cells. Studies with FGF receptors and MAP kinase suggest that both are involved in the effect of FGF-1 when it is supplemented to culture medium but not during the effect of endogenous FGF-1 synthesized in cells. We conclude that intracellular (endogenous) and extracellular (exogenous) FGF-1 have differential effects on the regulation of myogenic differentiation of L6-10 cells.

Permeable FGF-1 nuclear localization signal peptide stimulates DNA synthesis in various cell types but is cell-density sensitive and unable to support cell proliferation

An earlier report indicated that a 26-amino-acid peptide (SA), comprised of the nuclear localization signal (NLS) of fibroblast growth factor-1 (FGF-1) and a membrane-permeable peptide, was able to stimulate DNA synthesis after it was taken up by NIH3T3 fibroblasts. Here, we report that SA, but not a mutant with the NLS motif destroyed, induced DNA synthesis in BALB/c3T3 murine fibroblasts, human vascular endothelial (HUVE) cells, and primary cultured hepatocytes, although the activity was weaker than that of FGF-1. The kinetics of SA-induced DNA synthesis and G1 cyclin expression were similar to those elicited by FGF-1, indicating that SA induces cell cycle progression. Kinetic analysis also suggested that SA stimulates only a fraction of the DNA replication in BALB/c3T3 cells. At high cell densities, SA-induced G1 cyclin expression and DNA synthesis were more strongly inhibited than those induced by FGF-1. SA did not induce cell division in HUVE and BALB/c3T3 cells and did not interfere with FGF-1-stimulated proliferation of HUVE cells. These results indicate that SA is able to partially induce cell cycle progression through a contact-inhibition sensitive signaling pathway, but it is insufficient to support cell mitosis. We also suggest that signaling by SA does not interfere with that of FGF-1.

Signals mediating nuclear targeting and their regulation: application in drug delivery

The recent progress with respect to understanding the signals mediating the transport of proteins in both directions through the NPC, and cellular proteins interacting with these signals to effect the transport process has made possible a number of advances in terms of the use of this information in a clinical setting. In particular, our knowledge of the mechanism of regulation of the process, and of how we may exploit the cellular transport machinery itself in a therapeutic situation, especially where there may be transport pathways specific to particular viruses, has advanced considerably. In this context, this review expounds current understanding of the signals conferring targeting to the nucleus, and their practical and potential use in delivering molecules of interest to the nucleus in a clinical context. It also deals with targeting signals conferring nuclear protein export/ shuttling between nuclear and cytoplasmic compartments as well as with those conferring nuclear or cytoplasmic retention, and with the specific mechanisms regulating the activity of these signals, and in particular those regulating signal-dependent nuclear protein import. Detailed understanding of the processes of signal-mediated nuclear protein import/export and its regulation enables the considered application and optimization of approaches to target molecules of interest, such as plasmid DNA or toxic molecules, efficiently to the nucleus according to need in a clinical or research context, and enhance the expression or efficiency of their action, respectively. The use of nuclear targeting signals in this context is reviewed, and future possibilities in terms of the application of our growing understanding of nuclear transport and its regulation are discussed.

Activation of the MAP kinase pathway by FGF-1 correlates with cell proliferation induction while activation of the Src pathway correlates with migration

FGF regulates both cell migration and proliferation by receptor-dependent induction of immediate-early gene expression and tyrosine phosphorylation of intracellular polypeptides. Because little is known about the disparate nature of intracellular signaling pathways, which are able to discriminate between cell migration and proliferation, we used a washout strategy to examine the relationship between immediate-early gene expression and tyrosine phosphorylation with respect to the potential of cells either to migrate or to initiate DNA synthesis in response to FGF-1. We demonstrate that transient exposure to FGF-1 results in a significant decrease in Fos transcript expression and a decrease in tyrosine phosphorylation of the FGFR-1, p42(mapk), and p44(mapk). Consistent with these biochemical effects, we demonstrate that attenuation in the level of DNA synthesis such that a 1.5-h withdrawal is sufficient to return the population to a state similar to quiescence. In contrast, the level of Myc mRNA, the activity of Src, the tyrosine phosphorylation of cortactin, and the FGF-1-induced redistribution of cortactin and F-actin were unaffected by transient FGF-1 stimulation. These biochemical responses are consistent with an implied uncompromised migratory potential of the cells in response to growth factor withdrawal. These results suggest a correlation between Fos expression and the mitogen-activated protein kinase pathway with initiation of DNA synthesis and a correlation between high levels of Myc mRNA and Src kinase activity with the regulation of cell migration.

Inability of the acidic fibroblast growth factor mutant K132E to stimulate DNA synthesis after translocation into cells

Acidic fibroblast growth factor (aFGF) is a potent mitogen. It acts through activation of specific cell surface receptors leading to intracellular tyrosine phosphorylation cascades, but several reports also indicate that aFGF enters cells and that it has an intracellular function as well. The aFGF(K132E) mutant binds to and activates fibroblast growth factor receptors equally strongly as the wild-type, but it is a poor mitogen. We demonstrate that aFGF(K132E) enters NIH 3T3 cells and is transported to the nuclear fraction like wild-type aFGF. A fusion protein of aFGF(K132E) and diphtheria toxin A-fragment (aFGF(K132E)-DT-A) and a similar fusion protein containing wild-type aFGF (aFGF-DT-A) were reconstituted with diphtheria toxin B-fragment. Both fusion proteins were translocated to the cytosol by the diphtheria toxin pathway and subsequently recovered from the nuclear fraction. Whereas translocation of aFGF-DT-A stimulated DNA synthesis in U2OSDR1 cells lacking functional fibroblast growth factor receptors, aFGF(K132E)-DT-A did not. The mutation disrupts a protein kinase C phosphorylation site in the growth factor making it unable to be phosphorylated. The data indicate that a defect in the intracellular action of aFGF(K132E) is the reason for its strongly reduced mitogenicity, possibly due to inability to be phosphorylated.