Dual mode of signal transduction by externally added acidic fibroblast growth factor

Cloning of multiple chicken FGF1 mRNAs and their differential expression during development of whole embryo and of the lens

Five different 5' untranslated regions (5' UTRs) of FGF1 mRNAs were cloned in chicken. The structure of these transcripts suggests that, as in mammals, distinct 5' untranslated exons are spliced to the first coding exon via alternative splicing and alternative promoter usage. In an attempt to correlate the expression of specific transcripts to distinct biological activities, the distribution of these transcripts in different tissues and during the development of both the whole embryo and the lens was studied. In tissues, we have shown a differential, but not exclusive, expression of these transcripts. In the whole embryo, the expression of one transcript correlates with later developmental processes. In the lens, only two transcripts were detected that are both differently expressed and distributed. These results suggest that the biological properties of FGF1 depend on the expression of specific FGF1 mRNAs. Because these transcripts only differ in their 5' UTRs, they could be involved in distinct translational controls.

FGF-1-dependent proliferative and migratory responses are impaired in senescent human umbilical vein endothelial cells and correlate with the inability to signal tyrosine phosphorylation of fibroblast growth factor receptor-1 substrates

Senescent cells do not proliferate in response to exogenous growth factors, yet the number and affinity of growth factor receptors on the cell surface appear to be similar to presenescent cell populations. To determine whether a defect in receptor signaling exists, we analyzed human umbilical vein endothelial cells (HUVEC) since HUVEC growth is absolutely dependent upon the presence of FGF. We report that in both presenescent and senescent HUVEC populations, FGF-1 induces the expression of cell cycle-specific genes, suggesting that functional FGF receptor (FGFR) may exist on the surface of these cells. However, the tyrosine phosphorylation of FGFR-1 substrates, Src and cortactin, is impaired in senescent HUVEC, and only the presenescent cell populations exhibit a FGF-1-dependent Src tyrosine kinase activity. Moreover, we demonstrate that senescent HUVEC are unable to migrate in response to FGF-1, and these data correlate with an altered organization of focal adhesion sites. These data suggest that the induction of gene expression is insufficient to promote a proliferative or migratory phenotype in senescent HUVEC and that the attenuation of the FGFR-1 signal transduction pathway may be involved in the inability of senescent HUVEC to proliferate and/or migrate.

Nuclear accumulation of fibroblast growth factor receptors is regulated by multiple signals in adrenal medullary cells

In an effort to determine the localization of fibroblast growth factor (FGF) receptors (FGFR) that could mediate the intracellular action of FGF-2, we discovered the presence of high-affinity. FGF-2 binding sites in the nuclei of bovine adrenal medullary cells (BAMC). Western blot analysis demonstrated the presence of 103-, 118-, and 145-kDa forms of FGFR1 in nuclei isolated from BAMC. 125I-FGF-2 cross-linking to nuclear extracts followed by FGFR1 immunoprecipitation showed that FGFR1 can account for the nuclear FGF-2 binding sites. Nuclear FGFR1 has kinase activity and undergoes autophosphorylation. Immunocytochemistry with the use of confocal and electron microscopes demonstrated the presence of FGFR1 within the nuclear interior. Nuclear subfractionation followed by Western blot or immunoelectron microscopic analysis showed that the nuclear FGFR1 is contained in the nuclear matrix and the nucleoplasm. Agents that induce translocation of endogenous FGF-2 to the nucleus (forskolin, carbachol, or angiotensin II) increased the intranuclear accumulation of FGFR1. This accumulation was accompanied by an overall increase in FGF-2-inducible tyrosine kinase activity. Our findings suggest a novel mode for growth factor action whereby growth factor receptors translocate to the nucleus in parallel with their ligand and act as direct mediators of nuclear responses to cell stimulation.

Nuclear Translocation of fibroblast growth factor (FGF) receptors in response to FGF-2

Members of the FGF family of growth factors localize to the nuclei in a variety of different cell types. To determine whether FGF receptors are also present within nuclei and if this localization is regulated by FGFs, nuclei were prepared from quiescent and FGF-2-treated Swiss 3T3 fibroblasts and examined for the presence of FGF receptors by immunoblotting with an antibody produced against the extracellular domain of FGF receptor-1 (FGFR-1). Little or no FGFR-1 is detected in nuclei prepared from quiescent cells. When cells are treated with FGF-2, however, there is a time- and dose-dependent increase in the association of FGFR-1 immunoreactivity with the nucleus. In contrast, treatment with either EGF or 10% serum does not increase the association of FGFR-1 with the nucleus. When cell surface proteins are labeled with biotin, a biotinylated FGFR-1 is detected in the nuclear fraction prepared from FGF-2-treated, but not untreated, cells indicating that the nuclear-associated FGFR-1 immunoreactivity derives from the cell surface. The presence of FGFR-1 in the nuclei of FGF-2-treated cells was confirmed by immunostaining with a panel of different FGFR-1 antibodies, including one directed against the COOH-terminal domain of the protein. Fractionation of nuclei from FGF-2-treated cells indicates that nuclear FGFR-1 is localized to the nuclear matrix, suggesting that the receptor may play a role in regulating gene activity.

The nuclear trafficking of extracellular fibroblast growth factor (FGF)-1 correlates with the perinuclear association of the FGF receptor-1alpha isoforms but not the FGF receptor-1beta isoforms

The alternatively spliced fibroblast growth factor receptor (FGFR)-1 isoforms, FGFR-1alpha and FGFR-1beta, are characterized by the presence of either three or two Ig-like loops in the extracellular domain and are differentially expressed during embryonic development and tumor progression. We have previously shown that in cells irreversibly committed to DNA synthesis by FGF-1, approximately 15% of cell surface FGFR-1 traffics to a perinuclear locale as a structurally intact and functional tyrosine kinase (Prudovsky, I., Savion, N., Zhan, X., Friesel, R., Xu, J., Hou, J., McKeehan, W. L., and Maciag, T. (1994) J. Biol. Chem. 269, 31720-31724). In order to define the structural requirement for association of FGFR-1 with the nucleus, the expression and trafficking of FGFR-1 in FGFR-1alpha and FGFR-1beta L6 myoblast transfectants was studied. Although FGFR-1alpha was expressed as p145 and p125 forms, FGFR-1beta was expressed as p120 and p100 forms in the L6 myoblast transfectants. Tunicamycin and N-glyconase experiments suggest that these forms of FGFR-1alpha and FGFR-1beta are the result of differential glycosylation. However, only the p145 form of FGFR-1alpha and the p120 form of FGFR-1beta were able to bind FGF-1 and activate tyrosine phosphorylation. Pulse-chase analysis of FGFR-1 biosynthesis suggests that the p125 and p100 proteins are the precursor forms of p145 FGFR-1alpha and p120 FGFR-1beta, respectively. Because ligand-chase analysis demonstrated that FGFR-1beta L6 myoblast transfectants exhibited a reduced efficiency of nuclear translocation of exogenous FGF-1 when compared with FGFR-1alpha transfectants, the intracellular trafficking of the FGFR-1alpha and FGFR-1beta isoforms was studied using an in vitro kinase assay to amplify immunoprecipitated FGFR-1. Indeed, the appearance of the FGFR-1alpha but not FGFR-1beta isoform in the nuclear fraction of L6 myoblast transfectants suggests that the distal Ig-like loop in FGFR-1alpha mediates the differential nuclear association of FGFR-1alpha as a structurally intact and functional tyrosine kinase. Further, the FGFR-1beta L6 myoblast transfectants but not the FGFR-1alpha myoblast transfectants exhibited a pronounced morphologic change in response to exogenous FGF-1. Because this phenotype change involves the induction of a rounded cellular shape, it is possible that the FGFR-1alpha and FGFR-1beta may ultimately exhibit differential trafficking to adhesion sites.

Serum-starvation induces the extracellular appearance of FGF-1

Autocrine/paracrine stimulation of cell growth by members of the fibroblast growth factor (FGF) family of polypeptides is dependent upon extracellular interactions with specific high affinity receptors at the cell surface. Acidic FGF (FGF-1) lacks a classical signal sequence for secretion, suggesting that intrinsic levels of this mitogen may not stimulate cell growth and utilizes a non-classical pathway to gain access to the extracellular compartment. To evaluate the biological potential of intracellular FGF-1 more rigorously, human cDNA sequences for the growth factor were introduced into primary murine embryonic fibroblasts using retrovirally mediated gene transfer. Heparin affinity, Western analysis, mitogenic assays, in situ immunohistochemical techniques, induction of tyrosine phosphorylation and antibody inhibition studies were used to demonstrate functionality of the FGF-1 transgene in this experimental model. Under normal culture conditions, cells constitutively expressing intracellular FGF-1 exhibited a slight growth advantage. In contrast, when maintained in reduced serum, these cells adopted a transformed phenotype and demonstrated an enhanced growth potential, induction of FGF-specific phosphotyrosyl proteins and the nuclear association of the growth factor. Analysis of the conditioned media from these stressed cells indicated that serum starvation induces the secretion of FGF-1 as latent high molecular mass complexes requiring reducing agents to activate its full biological potential.

Nuclear localization of functional FGF receptor 1 in human astrocytes suggests a novel mechanism for growth factor action

Fractionation of human astrocytes revealed the presence of 103, 118, and 145 kDa forms of FGF receptor 1 (FGFR1) in isolated nuclei. Only trace amounts of FGFR1 proteins were detected in the cell membrane or cytoplasmic fractions. Nuclear FGFR1 is found in the nucleoplasm and nuclear matrix but not in chromatin. Immuno-confocal microscopy further demonstrates the intranuclear presence of FGFR1 and its colocalization with FGF-2. Nuclear FGFR1 binds to FGF-2 and has tyrosine kinase activity. Translocation of functional growth factor receptors into the cell nucleus offers a novel mechanism for growth factor action.

Potential role of bFGF and retinoic acid in the regeneration of chicken cochlear hair cells

Messenger RNAs (mRNA) of several growth factor receptors and relate genes were examined with reverse transcriptase polymerase chain reaction (RT-PCR) in normal and noise-damaged chicken basilar papillae (BP). Analysis of the amplification products indicated the presence of mRNAs for epidermal growth factor receptor (EGFR), fibroblast factor receptor (FGFR), insulin-like growth factor receptor (IGFR), insulin receptor (IR), retinoic acid receptor beta (RAR beta), retinoic acid receptor gamma (RXR gamma), and basic fibroblast growth factor (BFGF) in both normal and noise-damaged BP. The RT-PCR products generated were characterized by size and sequencing analysis to confirm the identities of the target molecules. The subcellular localization of the mature protein analogs for EGFR, FGFR, IGFR, RAR beta, and BFGF were identified using fluorescence immunocytochemistry and confocal laser scanning microscopy. These experiments indicated that EGFR is present in the stereociliary bundles in the hair cells, IGFR is not present in the cells of the BP, BFGF localizes in the nuclei of supporting cells in the BP, but not hair cells or hyaline cells, and that RAR beta localizes in the perinuclear regions of hair cells. The subcellular distributions of these proteins were consistent in both noise-damaged and control BP. FGFR, in contrast, changed its distribution in the tissue after noise damage. In normal BP, FGFR is concentrated in the stereocilia of hair cells. However, in damaged regions of noise-exposed chick cochleae, FGFR is heavily expressed in the expanded apical regions of the supporting cells. These findings suggest that BFGF and retinoic acid may potentially play a role in the mechanisms which regulate the regeneration of chicken cochlear hair cells.

Transcription factor hoxa-5 is taken up by cells in culture and conveyed to their nuclei

Homeoproteins are transcription factors known to be involved in the early patterning of the nervous system and in lineage decisions. While studying a possible role for homeoproteins at later stages of neuronal differentiation, we observed that the Antennapedia homeodomain is internalized by neurons, translocated to their nuclei, and enhances neurite outgrowth. Studies with mutant homeodomains showed that neurite elongation by post-mitotic vertebrate neurons is regulated by homeoproteins. An intriguing possibility suggested by these results, is that full length homeoproteins might be able to translocate through neuronal membranes. We now report that the entire Hoxa-5 homeoprotein is taken up by fibroblasts and neurons in culture and conveyed to their nuclei. Internalization occurs at 4 and 37 degrees C, and at concentrations as low as 10 pM compatible with a physiological mechanism.

A distinct basic fibroblast growth factor (FGF-2)/FGF receptor interaction distinguishes urokinase-type plasminogen activator induction from mitogenicity in endothelial cells

Basic fibroblast growth factor (FGF-2) induces cell proliferation and urokinase-type plasminogen activator (uPA) production in fetal bovine aortic endothelial GM 7373 cells. In the present paper we investigated the role of the interaction of FGF-2 with tyrosine-kinase (TK) FGF receptors (FGFRs) in mediating uPA up-regulation in these cells. The results show that FGF-2 antagonists suramin, protamine, heparin, the synthetic peptide FGF-2(112-155), and a soluble form of FGFR-1 do not inhibit FGF-2-mediated uPA up-regulation at concentrations that affect growth factor binding to cell surface receptors and mitogenic activity. In contrast, tyrosine phosphorylation inhibitors and overexpression of a dominant negative TK- mutant of FGFR-1 abolish the uPA-inducing activity of FGF-2, indicating that FGFR and its TK activity are essential in mediating uPA induction. Accordingly, FGF-2 induces uPA up-regulation in Chinese hamster ovary cells transfected with wild-type FGFR-1, -2, -3, or -4 but not with TK- FGFR-1 mutant. Small unilamellar phosphatidyl choline:cholesterol vesicles loaded with FGF-2 increased uPA production in GM 7373 cells in the absence of a mitogenic response. Liposome-encapsulated FGF-2 showed a limited but significant capacity, relative to free FGF-2, to interact with FGFR both at 4 degrees C and 37 degrees C and to be internalized within the cell. uPA up-regulation by liposome-encapsulated FGF-2 was quenched by neutralizing anti-FGF-2 antibodies, indicating that the activity of liposome-delivered FGF-2 is mediated by an extracellular action of the growth factor. Taken together, the data indicate that a distinct interaction of FGF-2 with FGFR, quantitatively and/or qualitatively different from the one that leads to mitogenicity, is responsible for the uPA-inducing activity of the growth factor.

The neurotrophic activity of fibroblast growth factor 1 (FGF1) depends on endogenous FGF1 expression and is independent of the mitogen-activated protein kinase cascade pathway

The expression of fibroblast growth factor (FGF) 1, a potent neurotrophic factor, increases during differentiation and remains high in adult neuronal tissues. To examine the importance of this expression on the neuronal phenotype, we have used PC12 cells, a model to study FGF-induced neuronal differentiation. After demonstrating that FGF1 and FGF2 are synthesized by PC12 cells, we investigated if FGF1 expression could be a key element in differentiation. Using the cell signaling pathway to determine the effects of FGF1 alone, FGF1 plus heparin, or a mutated FGF1, we showed an activation to the same extent of mitogen-activated protein (MAP) kinase kinase and MAP kinase (extracellular regulated kinase 1). However, only FGF1 plus heparin could promote PC12 cell differentiation. Thus, the MAP kinase pathway is insufficient to promote differentiation. Analysis of the PC12 cells after the addition of FGF1 plus heparin or FGF2 demonstrated a significant increase in the level of FGF1 expression with the same time course as the appearance of the neuritic extensions. Transfection experiments were performed to enhance constitutivly or after dexamethasone induction the level of FGF1 expression. The degree of differentiation achieved by the cells correlated directly with the amount of FGF1 expressed. The MAP kinase pathway did not appear to be involved. Interestingly, a 5-fold increase in FGF1 in constitutive transfected cells extended dramatically their survival in serum-free medium, suggesting that the rise of FGF1 synthesis during neuronal differentiation is probably linked to their ability to survive in the adult. All of these data demonstrate that, in contrast to the MAP kinase cascade. FGF1 expression is sufficient to induce in PC12 cells both differentiation and survival. It also shows that auto- and trans-activation of FGF1 expression is involved in the differentiation process stimulated by exogenous FGFs through a new pathway which remains to be characterized.

Role of transiently altered sarcolemmal membrane permeability and basic fibroblast growth factor release in the hypertrophic response of adult rat ventricular myocytes to increased mechanical activity in vitro

One of the trophic factors that has been implicated in initiating or facilitating growth in response to increased mechanical stress in several tissues and cell types is basic fibroblast growth factor (bFGF; FGF-2). Although mammalian cardiac muscle cells express bFGF, it is not known whether it plays a role in mediating cardiac adaptation to increased load, nor how release of the cytosolic 18-kD isoform of bFGF would be regulated in response to increased mechanical stress. To test the hypothesis that increased mechanical activity induces transient alterations in sarcolemmal permeability that allow cytosolic bFGF to be released and subsequently to act as an autocrine and paracrine growth stimulus, we examined primary isolates of adult rat ventricular myocytes maintained in serum-free, defined medium that were continually paced at 3 Hz for up to 5 d. Paced myocytes, but not nonpaced control cells, exhibited a "hypertrophic" response, which was characterized by increases in the rate of phenylalanine incorporation, total cellular protein content, and cell size. These changes could be mimicked in control cells by exogenous recombinant bFGF and could be blocked in continually paced cells by a specific neutralizing anti-bFGF antibody. In addition, medium conditioned by continually paced myocytes contained significantly more bFGF measured by ELISA and more mitogenic activity for 3T3 cells, activity that could be reduced by a neutralizing anti-bFGF antibody. The hypothesis that transient membrane disruptions sufficient to allow release of cytosolic bFGF occur in paced myocytes was examined by monitoring the rate of uptake into myocytes from the medium of 10-kD dextran linked to fluorescein. Paced myocytes exhibited a significantly higher rate of fluoresceinlabeled dextran uptake. These data are consistent with the hypothesis that nonlethal, transient alterations in sarcolemmal membrane permeability with release of cytosolic bFGF is one mechanism by which increased mechanical activity could lead to a hypertrophic response in cardiac myocytes.

Ability of methotrexate to inhibit translocation to the cytosol of dihydrofolate reductase fused to diphtheria toxin

A fusion protein consisting of dihydrofolate reductase and diphtheria toxin A-fragment was made by genetically linking cDNA for the two proteins followed by in vitro transcription and translation in a rabbit reticulocyte lysate system. The dihydrofolate reductase in the fusion protein exhibited enzyme activity and, in the presence of methotrexate which imposes a tight structure on dihydrofolate reductase, it was trypsin resistant, indicating that it was correctly folded. When reconstituted with diphtheria toxin B-fragment, it bound specifically to diphtheria toxin receptors and was translocated into cells upon exposure to low pH. Methotrexate prevented the translocation. Protein synthesis was inhibited in cells incubated with the reconstituted fusion protein, but the inhibition was reduced in the presence of methotrexate. We also made a fusion protein containing a mutated dihydrofolate reductase with much lower affinity to methotrexate. Methotrexate did not prevent translocation of this protein. The data indicate that methotrexate prevents translocation of the fusion protein containing wild-type dihydrofolate reductase by imposing a tight structure on to the enzyme.

Stimulation of proliferation of a human osteosarcoma cell line by exogenous acidic fibroblast growth factor requires both activation of receptor tyrosine kinase and growth factor internalization

U2OS Dr1 cells, originating from a human osteosarcoma, are resistant to the intracellular action of diphtheria toxin but contain toxin receptors on their surfaces. These cells do not have detectable amounts of fibroblast growth factor receptors. When these cells were transfected with fibroblast growth factor receptor 4, the addition of acidic fibroblast growth factor to the medium induced tyrosine phosphorylation, DNA synthesis, and cell proliferation. A considerable fraction of the cell-associated growth factor was found in the nuclear fraction. When the growth factor was fused to the diphtheria toxin A fragment, it was still bound to the growth factor receptor and induced tyrosine phosphorylation but did not induce DNA synthesis or cell proliferation, nor was any fusion protein recovered in the nuclear fraction. On the other hand, when the fusion protein was associated with the diphtheria toxin B fragment to allow translocation to the cytosol by the toxin pathway, the fusion protein was targeted to the nucleus and stimulated both DNA synthesis and cell proliferation. In untransfected cells containing toxin receptors but not fibroblast growth factor receptors, the fusion protein was translocated to the cytosol and targeted to the nucleus, but in this case, it stimulated only DNA synthesis. These data indicate that the following two signals are required to stimulate cell proliferation in transfected U2OS Dr1 cells: the tyrosine kinase signal from the activated fibroblast growth factor receptor and translocation of the growth factor into the cell.

Translocation of cytosol of exogenous, CAAX-tagged acidic fibroblast growth factor

Acidic fibroblast growth factor (aFGF) added externally to cells has been proposed to enter the nucleus and stimulate DNA synthesis, but it has remained controversial whether or not exogenous aFGF has the capability of crossing cellular membranes. To test this, a novel principle to study translocation of proteins to the cytosol was developed by fusing a C-terminal farnesylation signal, a CAAX tag (C = Cys, A = an aliphatic amino acid, and X = any amino acid), onto aFGF. Farnesylation is only known to occur in the cytosol and possibly in the nucleus. When incubated with NIH3T3 cells overnight, about one-third of the cell-associated, CAAX-tagged growth factor was farnesylated, indicating that efficient translocation had taken place. Binding to specific FGF receptors was required for translocation to occur. Part of the farnesylated growth factor was found in the nuclear fraction. The data indicate that CAAX-tagged aFGF added externally to cells is able to cross cellular membranes and enter the cytosol and the nucleus.

Fibroblast growth factor receptors (FGFRs) localize in different cellular compartments. A splice variant of FGFR-3 localizes to the nucleus

We have raised specific antibodies to the second immunoglobulin-like domain of fibroblast growth factor receptors (FGFRs) and used these to investigate the expression and subcellular localization of FGFR-1, -2, -3, and -4 in breast epithelial cells. All four receptors classes could be detected in breast cell lines; however, FGFR-4 and FGFR-2 appeared to be expressed at a higher level in breast cancer cell lines than in normal epithelial cells. Surprisingly, FGFR-3 localized in the cell nucleus by immunofluorescence. A second antibody to a separate epitope confirmed this finding and showed that the form of FGFR-3 present must contain an intact kinase domain as well as the growth factor binding domain. Western analysis of fractionated cells revealed the presence of two forms of FGFR-3 of 135 and 110 kDa. The 110-kDa form was predominantly found in the nucleus, whereas the 135 kDa form was sometimes found in the nucleus. RT-PCR analysis of FGFR-3 mRNA showed the presence of a splice variant in which exons 7 and 8 are deleted. This results in the translation of FGFR-3 missing the transmembrane domain but with an intact kinase domain, which could be a soluble, intracellular receptor. Transfection experiments showed that FGFR-3 containing this deletion and no signal peptide gave an identical nuclear staining pattern to that seen in breast epithelial cells. We conclude that two forms of FGFR-3 are present in breast epithelial cells; a full-length 135-kDa receptor, which has a conventional membrane localization, and a novel soluble form of 110 kDa.

Insulin-induced egr-1 expression in Chinese hamster ovary cells is insulin receptor and insulin receptor substrate-1 phosphorylation-independent. Evidence of an alternative signal transduction pathway

Insulin's effects primarily are initiated by insulin binding to its plasma membrane receptor and the sequential tyrosine phosphorylation of the insulin receptor and intracellular substrates, such as insulin receptor substrate-1 (IRS-1). However, studies suggest some insulin effects, including those at the nucleus, may not be regulated by this pathway. The present study compared the levels of insulin binding, insulin receptor and IRS-1 tyrosine phosphorylation, and phosphatidylinositol 3'-kinase activity to immediate early gene c-fos and egr-1 mRNA expression in Chinese hamster ovary (CHO) cells expressing only neomycin-resistant plasmid (CHONEO), overexpressing wild type human insulin receptor (CHOHIRc) or ATP binding site-mutated insulin receptors (CHOA1018K). Insulin binding in CHONEO cells was markedly lower than that in other cell types. 10 nM insulin significantly increased tyrosine phosphorylation of insulin receptor and IRS-1 in CHOHIRc cells. Phosphorylation of insulin receptor and IRS-1 in CHONEO and CHOA1018K cells was not detected in the presence or absence of insulin. Similarly, insulin increased phosphatidylinositol 3-kinase activity only in CHOHIRc cells. As determined by Northern blot, nuclear run-on analysis, and in situ hybridization, insulin induced c-fos mRNA expression, through transcription, in CHOHIRc cells but not in CHONEO and CHOA1018K cells, consistent with previous reports. In contrast, all three cell types showed a similar insulin dose-dependent increase of egr-1 mRNA expression through transcription. These data indicated that insulin-induced egr-1 mRNA expression did not correlate with the levels of insulin binding to insulin receptor or phosphorylation of insulin receptor and IRS-1. These results suggest that different mechanisms are involved in induction of c-fos and egr-1 mRNA expression by insulin, the former by the more classic insulin receptor tyrosine kinase pathway and the latter by a yet to be determined alternative signal transduction pathway.

Entry of exogenous polypeptides into the nucleus of living cells: facts and speculations

Although the plasma membrane acts as an impermeable barrier to most macromolecules, some exogenous proteins (for example fibroblast growth factor, HIV-1 Tat and lactoferrin) can gain access into the cytosol and reach the nucleus of living cells. How are these exogenous polypeptides selected over and above other extracellular proteins? How and where do they cross the cell membrane? Why do cells need to take up exogenous transcription factors when sophisticated signal-transduction pathways are available? Here, we review the current knowledge on these issues and discuss some mechanistic and physiological implications of this unconventional and direct way of taking messages to the nucleus.

Farnesylation of CaaX-tagged diphtheria toxin A-fragment as a measure of transfer to the cytosol

Diphtheria toxin binds to receptor-positive cells through its B-fragment, the toxin is then endocytosed, and the low pH in endosomes triggers the translocation of the enzymatically active A-fragment to the cytosol. A synchronous release of A-fragments into the cytosol can be induced by exposing cells with surface-bound toxin to low pH. We have used this protein translocation system to develop a novel method to study whether or not a protein is exposed to the cytosol. Protein farnesylation is a cytosolic modification signaled by a C-terminal CaaX motif, and to visualize the translocation process, we added a farnesylation signal to the toxin A-fragment. The A-fragment with an added CaaX motif was farnesylated within 1 h after exposure of cells with surface-bound toxin to low pH, and also A-fragment translocated from endosomes was quantitatively farnesylated. The results indicate that all cell-mediated reduction of the toxin implicates translocation of the A-fragment to the cytosol. The farnesylation was inhibited by lovastatin, the alkylating agent NEM, and the peptidomimetic farnesylation inhibitor B581. Farnesylated A-fragment partitioned preferentially into the detergent phase upon extraction with Triton X-114. Our data suggest that farnesylation of a CaaX tag is generally applicable as a cytosolic marker, and this strategy for monitoring protein transfer to the cytosol may have considerable potential for studying the transport to the cytosol of proteins added externally to cells.

Distribution of acidic fibroblast growth factor mRNA-expressing neurons in the adult mouse central nervous system

The distribution of acidic fibroblast growth factor (aFGF) mRNA-expressing neurons was studied throughout the adult mouse central nervous system (CNS) with in situ hybridization histochemistry using a radiolabelled synthetic oligodeoxynucleotide probe complementary to the mRNA of human aFGF. We report here a widespread distribution of aFGF mRNA in several defined functional systems of the adult mouse brain, whereby the highest levels of aFGF mRNA were found in large somatomotor neurons in the nuclei of the oculomotor, trochlear, abducens, and hypoglossal nerves; in the motoneurons of the ventral spinal cord and the special visceromotor neurons in the motor nucleus of the trigeminal nerve; and in the facial and ambiguus nuclei. Labelled perikarya were also detected in all central structures of the auditory pathway including the level of the inferior colliculus, i.e., the lateral and medial superior nuclei; the trapezoid, cochlear, and lateral lemniscal nuclei; and parts of the anterior colliculus. Furthermore, many aFGF-positive cell bodies were found in the vestibular system and other structures projecting to the cerebellum, in the deep cerebellar nuclei, in somatosensory structures of the medulla (i.e., in the gracile, cuneate, and external cuneate nuclei), as well as in the spinal nucleus of the trigeminal nerve. The findings that aFGF mRNA is expressed in all components of several well-defined systems (i.e., in sensory structures) as well as in central neurons that process sensory information and, finally, in some efferent projections point towards a concept of aFGF expression primarily within certain neuronal circuitries.

A novel human c-sis mRNA species is transcribed from a promoter in c-sis intron 1 and contains the code for an alternative PDGF B-like protein

The human platelet-derived growth factor (PDGF) B chain precursor is usually translated from a 3.5 kb c-sis/PDGF B gene transcript. The first exon of the c-sis/gene contains the code for the signal peptide of the PDGF B chain precursor, preceded by a 1 kb long untranslated sequence with potent translation inhibitory activity. In this paper we show that a novel 2.6 kb c-sis mRNA present in the human choriocarcinoma cell line JEG-3 initiates at an alternative exon 1, which we refer to as exon 1a. The 90 bp long exon 1a is located in the center of the first intron of the gene. It coincides with a very pronounced DNase-I-hypersensitive site and is preceded by a functional promoter. Of the three ATG codons present in exon 1a, the third one perfectly matches the criteria of a consensus start codon. It initiates an open reading frame that is continuous with the code for the PDGF B chain precursor but lacks the code for a signal peptide. We conclude that this novel 2.6 kb c-sis mRNA species lacks the strong translation inhibitory potential of the regular exon 1 and contains the code for a PDGF B-like protein that may be targeted to the cell nucleus.

Interaction of partially structured states of acidic fibroblast growth factor with phospholipid membranes

Although acidic fibroblast growth factor (aFGF) lacks a conventional signal sequence, it is often found complexed to sulfated proteoglycans on the external surface of cells. The protein also forms a "molten globule"-like state at neutral pH and physiological temperatures as well as at acidic pH in the presence of physiological ionic strength or moderate quantities of polyanions. These states display a marked tendency to aggregate. Such observations suggest that related partially structured states might be involved in the membrane translocation of aFGF. To explore this hypothesis, we examined the interaction of this growth factor with lipid vesicles as well as the effect of such surfaces on the structure of the protein. We find that these states interact with negatively charged but not neutral phospholipid unilammelar vesicles at acidic pH, inducing bilayer disruption. The rate of leakage of a liposome-entrapped fluorescent probe is proportional to the logarithm of the aFGF concentration, suggesting competition between protein self-association and membrane binding. Liposome leakage can be also induced at neutral pH by partial unfolding of aFGF at or above physiological temperature in contrast to most control proteins. The importance of partially folded hydrophobic surfaces in aFGF self-association and membrane binding is further suggested by the fact that thermally unfolded aFGF does not aggregate, in contrast to states observed at intermediate temperatures or transiently during unfolding at high temperatures. In contrast to heparin, a polyanion which stabilizes the native structure of aFGF, negatively charged phospholipid membranes appear to enhance the disruption of aFGF tertiary structure at submicellar concentrations of sodium dodecyl sulfate but stabilize the remaining secondary structure.(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin receptor alpha-subunit: a putative gene regulatory molecule

Previous studies have demonstrated the presence of the insulin receptor in the cell nucleus. Recently, it was shown that the insulin receptor also exhibits nuclear tyrosine kinase activity. In the present investigation, I have searched for structural correlates to a nuclear localization of the insulin receptor as well as to other potential nuclear actions of this molecule. Interestingly, this analysis yielded that the insulin receptor (alpha-subunit) contains a bipartite nuclear localization signal (consistent with the preceding experimental data), several zinc finger-like motifs and an RGG box. These findings have intriguing implications with regard to a presumable role of the insulin receptor (alpha-subunit) as a gene regulatory molecule.

The HIV-1 TAT protein induces the expression and extracellular appearance of acidic fibroblast growth factor

Mounting experimental evidence suggests that the TAT protein, released from human immunodeficiency virus-1 (HIV-1)-infected inflammatory cells, may genetically reprogram targeted cells within a localized environment to develop highly vascularized tumors of mesenchymal origin. The fibroblast growth factor (FGF) family of polypeptides has gained general acceptance as initiators of angiogenesis and functions as potent mitogens for mesoderm-derived cells. To evaluate a potential biological relationship between TAT and acidic FGF (FGF-1), primary murine embryonic fibroblasts either were transfected with the viral transactivator or were transduced (retrovirally mediated) with a secreted, chimeric form of the human polypeptide growth factor, human stomach tumor/Kaposi's sarcoma (hst/KS)FGF-1. Reverse transcriptase-polymerase chain reaction, Western blotting, in situ immunohistochemical, heparin affinity, DNA synthesis, and transient transfection techniques were used to confirm expression, localization, and functionality of the transgenes. Both transfected and transduced cells constitutively expressing either TAT or (hst/KS)FGF-1 adopted a transformed phenotype, maintained aggressive growth behavior, and demonstrated both induction of FGF-specific phosphotyrosyl proteins and nuclear association of FGF-1 and FGF-1 receptor. Increased levels of endogenous, murine FGF-1 mRNA (reverse transcriptase-polymerase chain reaction) and protein (immunoblot analysis) were apparent in both (hst/KS)FGF-1- and TAT-transformed cells. Medium conditioned by (hst/KS)FGF-1-transduced cells contained steady-state levels of biologically active FGF-1 which exhibited a representative molecular weight. Limited sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis of the conditioned medium from TAT-transformed cells demonstrated the appearance of FGF-1 as latent, high molecular weight complexes requiring reducing agents to activate full biological activity. Collectively, these results suggest that TAT induces the expression and secretion of FGF-1, which may be potentially relevant to the pathophysiological development of AIDS-Kaposi's sarcoma.

A requirement for fibroblast growth factor in regulation of skeletal muscle growth and differentiation cannot be replaced by activation of platelet-derived growth factor signaling pathways

The distinct effects of cytokines on cellular growth and differentiation suggest that specific signaling pathways mediate these diverse biological activities. Fibroblast growth factors (FGFs) are well-established inhibitors of skeletal muscle differentiation and may operate via activation of specific signaling pathways distinct from recently identified mitogen signaling pathways. We examined whether platelet-derived growth factor (PDGF)-activated signaling pathways are sufficient to mediate FGF-dependent repression of myogenesis by introducing the PDGF beta receptor into a mouse skeletal muscle cell line. Addition of PDGF-BB to cells expressing the PDGF beta receptor activated the PDGF beta receptor tyrosine kinase, stimulated mitogen-activated protein (MAP) kinase, and increased the steady-state levels of junB and c-fos mRNAs. Despite the activation of these intracellular signaling molecules, PDGF beta receptor activation elicited no detectable effect on cell proliferation or differentiation. In contrast to PDGF-BB, addition of FGF-2 to myoblasts activated signaling pathways that resulted in DNA synthesis and repression of differentiation. Because of the low number of endogenous FGF receptors expressed, FGF-stimulated signaling events, including tyrosine phosphorylation and activation of MAP kinase, could be detected only in cells expressing higher levels of a transfected FGF receptor cDNA. As the PDGF beta receptor- and FGF receptor-stimulated signaling pathways yield different biological responses in these skeletal muscle cells, we hypothesize that FGF-mediated repression of skeletal muscle differentiation activates signaling pathways distinct from those activated by the PDGF beta receptor. Activation of PDGF beta receptor tyrosine kinase activity, stimulation of MAP kinase, and upregulation of immediate-early gene expression are not sufficient to repress skeletal muscle differentiation.

Acidic FGF and FGF receptors are specifically expressed in neurons of developing and adult rat dorsal root ganglia

Employing complementary technical approaches, we have studied the expression of acidic fibroblast growth factor (aFGF) and FGF receptors in rat dorsal root ganglia. The results clearly showed that within spinal nerves aFGF and two high-affinity FGF receptors, FGFR-1 and FGFR-2, were prominently expressed in neurons, while expression in Schwann cells was undetectable. FGFR-3 and FGFR-4 were not expressed in dorsal root ganglia. Acidic FGF mRNA was detected in the majority of dorsal root ganglion neurons, including all size classes: FGFR-1 and FGFR-2 transcripts were only detected in subpopulations of mainly large and medium size neurons. In subcellular fractionation studies on dorsal root ganglion and spinal root tissue, aFGF was recovered in the soluble fraction and was thus not tightly associated with neuronal membranes. During development FGFR-1 and FGFR-2 mRNAs were found to be present at all stages examined (embryonic days 15-21 and postnatal days 1-120). Acidic FGF mRNA and protein were first detected at embryonic day 18, and their expression then increased progressively up to postnatal levels. In cultures of dorsal root ganglion neurons derived from day 15 embryos, aFGF expression was first detected 3 days after plating. The resulting neuron cultures continued to express aFGF in a Schwann cell-independent manner. In combination, these results indicate that aFGF expression in dorsal root ganglia is initiated and maintained in postmitotic neurons. Furthermore, the data suggest that the physiological function of aFGF in the peripheral nervous system is connected to processes specific to the mature sensory (and motor) system, such as the maintenance and survival of peripheral nerve neurons.

Differential modulation of cell phenotype by different molecular weight forms of basic fibroblast growth factor: possible intracellular signaling by the high molecular weight forms

To study possible functional differences of the 18-kD and high molecular weight forms of basic fibroblast growth factor (bFGF), we have examined the effect of endogenous production of different bFGF forms on the phenotype of NIH 3T3 cells. Cells transfected with cDNAs coding for either 18-kD bFGF (18-kD bFGF) or all four molecular forms (18, 22, 22.5, 24 kD; wild type [WT] bFGF) exhibit increased migration and decreased FGF receptor number compared to parental cells. However, migration and FGF receptor number of cells transfected with a cDNA coding only for high molecular weight bFGF (22, 22.5, and 24 kD; HMW bFGF) were similar to that of parental cells transfected with vector alone. Cells expressing HMW, 18 kD, or WT bFGF grew to high saturation densities in 10% serum. However, only cells expressing HMW or WT bFGF grew in low serum. Cell surface or metabolic labeling of the different cell types followed by immunoprecipitation with anti-bFGF antibody showed primarily cell surface-associated 18-kD bFGF. In addition, when cells expressing exclusively HMW bFGF were transfected with a cDNA coding for 18-kD bFGF, migration was increased, bFGF receptors were down-regulated, and 18-kD bFGF was found on the cell surface. Cells expressing 18-kD bFGF transfected with a cDNA encoding FGF receptor-2 lacking the COOH-terminal domain (dominant negative bFGF receptor) exhibited a flat morphology and decreases in migration and saturation density. Cells expressing HMW bFGF transfected with the dominant negative bFGF receptor continued to grow to a high saturation density, proliferated in low serum, and exhibited no morphological changes. These results indicate that increased cell migration and FGF receptor down-regulation are mediated by the extracellular interaction of 18-kD bFGF with its cell surface receptor. Growth in low serum may be stimulated by the intracellular action of HMW bFGF through mechanisms independent of the presence of a cell surface receptor. Thus, the different molecular forms of bFGF may act through distinct but convergent pathways.

Nucleo-cytoplasmic translocation and secretion of fibroblast growth factor-2 during avian gastrulation

The expression and distribution of the fibroblast growth factor-2 (FGF-2 or bFGF) proteins during early avian embryogenesis has been analysed in detail. Three FGF-2 protein isoforms of 18.5, 20.0 and 21.5 kDa are expressed during gastrulation of chicken embryos. Using whole mount immunohistochemistry, these proteins were found to be predominantly nuclear in prestreak blastodiscs during mesoderm induction. Distribution of positive cells in the epiblast was mosaic, whereas all cells of the forming hypoblast expressed the FGF-2 proteins. During primitive streak formation, the proteins started to translocate to the cytoplasm in epiblast cells but remained nuclear in the hypoblast. The FGF-2 proteins became predominantly cytoplasmic in all cells during the subsequent developmental stages. Their highest levels were detected in endodermal cells underlying Hensen's node and the newly formed notochord, the dorsal apex of all epiblast cells and, most interestingly, in the extra-cellular basal lamina separating the epiblast from newly formed mesoderm. Heparin and suramin treatment of these advanced embryos (stage 4) revealed a dose-dependent inhibition on the regression of Hensen's node and formation of mesodermal derivatives such as somites. The results are discussed with respect to current models on FGF-mediated functions during vertebrate mesoderm induction and regionalization.

Nuclear/growth factors

The now classical model for cell-cell communication espouses that information travels between cells in the form of molecules that bind specific cell-surface receptors and trigger signal-transducing mechanisms that eventually lead to transcriptional modifications. Here we gather the available information suggesting that some growth factors may also act by interfering directly with gene transcription, following their internalization and nuclear translocation. Among these factors are bona fide growth factors such as Fibroblast Growth Factor-1 and -2 and Schwannoma Derived Growth Factor, for which internalization and nuclear translocation have been demonstrated. Conversely, we propose that some isoforms of nuclear factors of the homeoprotein family could pass from cell to cell. The implications of the model are presented in the context of the specificity of cellular interactions.

Fibroblast growth factors and their receptors: an information network controlling tissue growth, morphogenesis and repair

The stimulation of cellular metabolism by the nine fibroblast growth factors (FGFs) is mediated by a dual-receptor system. This comprises a family of four receptor tyrosine kinases (FGFR) and heparan sulphate proteoglycans (HSPG). The stimulation of cell division by FGFs has an obligate requirement for both partners of the dual-receptor system. The binding of the nine FGFs to the FGFRs is marked by a pattern of overlapping specificity despite alternative splicing events generating a large number of FGFR proteins. Thus many of the FGFR isoforms bind several FGFs. It is likely that each FGF requires a different pattern of sulphation within the heparan sulphate chains for binding. Therefore, the HSPG receptors may provide additional specificity, allowing a cell to fine tune its response to the FGFs present in the extracellular milieu. The HSPG receptors also control the availability of FGFs and hence regulate the transport of FGFs within a tissue. FGF-stimulated cell division would appear to have a mandatory requirement for the FGFs to be translocated to the nucleus via the cytosol after interacting with the dual-receptor system. The consequences of the potential direct action of FGFs in stimulating cell division are examined in the light of current models of signal transduction.