Basic fibroblast growth factor (FGF-2) ()belongs to a family of structurally related heparin-binding growth factors sharing 30-80% sequence homology. It is widely distributed, and regulates mesenchymal-derived cells and epithelial cells. It promotes angiogenesis, cell proliferation, and differentiation and also increases production of proteases and plasminogen activators(1, 2, 3, 4) . The biology of FGF-2 is complex. Multiple isoforms of the growth factor (from 18 to 24 kDa) can be produced by the same cell from a single mRNA species, as a result of initiation of translation either at the AUG or at different CUG codons. FGF-2, like FGF-1, lacks the hydrophobic signal peptide and is therefore concentrated within its cell of origin. The shorter 18-kDa FGF-2 isoform initiated at the AUG codon is predominantly localized in the cytosol and is also found at the cell surface after secretion via a cellular pathway distinct from classical secretion(5, 6, 7, 8, 9) . In contrast, polypeptides initiated at CUG codons are preferentially localized in the nucleus. They are larger than the 18-kDa form, possessing an additional amino-terminal sequence particularly rich in arginine residues which allows for their nuclear targeting(10, 11) . Furthermore, when the 18-kDa form is provided to cells exogenously it is internalized and specifically translocated to nucleoli probably through a putative nuclear localization signal located between residues 27 and 32(10) .

Production of multiple FGF-2 isoforms with different cellular localizations raises the possibility that each form may exert a specialized function. The internalized 18-kDa form has been shown to increase RNA polymerase I transcriptional activity(12) . Furthermore, the intracellularly stored forms may participate in intracrine regulations leading to some of the broad biological effects exerted by FGF-2(2, 5, 13, 14) . For instance, FGF-2-transfected cells expressing only the nuclear-translocated high molecular weight forms grow in the absence of serum and display the properties of immortalized cells through still unknown mechanisms(5, 13) . The cytosolic 18-kDa form appears also to exert some control at the intracellular level(1, 13) . However, the specific biological functions of the multiple FGF-2 isoforms and their mechanism of action still remain to be defined.

The biological effects of exogenous FGF-2 are mediated by high and low affinity receptors(3, 4) . Four FGFR cDNAs, encoding high affinity receptors possessing tyrosine kinase activity, have been identified and cloned. Different variants of FGFR-1 to 3 arising from alternative splicing of the primary transcript have also been described(3, 4) . There are also low affinity receptors (K in the nM range) corresponding mainly to heparan sulfate proteoglycans(15, 16, 17) . Cooperation between high and low affinity receptors plays an essential role in the induction of the biological effects of the extracellular FGF-2(3, 4, 16) .

High affinity FGFRs are down-regulated by exogenously added FGF-2 (18, 19, 20) , and cells transfected with FGF-2 cDNA show decreased levels of high affinity FGFR. This may be due to down-regulation induced by secretion of the 18-kDa form. Transfected cells treated with suramin, an inhibitor of FGF binding to the cell surface receptors, exhibit higher binding capacities than untreated cells, further suggesting that secreted FGF-2 may down-regulate FGFR(19, 20) . However, it is not known whether high molecular weight intracellular forms of FGF-2 are involved in FGFR modulation. Accordingly, in the present study we examined the respective roles of the 18- and 22.5-kDa FGF-2 forms on the expression of the type I high affinity FGF receptors (FGFR-1). We used a retroviral infection system previously shown to induce the production of low amounts of FGF-2 in the endothelial cell lines ABAE (13) and in the pancreatic acinar cell line AR4-2J(21) . AR4-2J cells do not produce FGF-2 (21) and possess low levels of FGFR (22) . These cells were induced to stably express the two molecular forms of FGF-2 under the control of the -actin promoter. We now report that both FGF-2 molecular forms up-regulate the high affinity receptors and the levels of the FGFR-1 mRNA.

EXPERIMENTAL PROCEDURES

Cell Culture

Transfections and Infections

Binding of I-FGF-2

Cross-linking of I-FGF-2

Effect of Cell Density, Exogenous FGF-2, and Suramin on I-FGF-2 Binding

To assess the effect of neutralizing anti-FGF-2 antibodies or suramin on I-FGF-2 binding, 24 h after plating cells were incubated in fresh serum-free medium containing 0.3 µg/ml of a monoclonal anti-FGF-2 antibody (UBI, Lake Placid, NY) or in 10% fetal calf serum containing 40 µM suramin (gift of Bayer Pharma, Sens, France). Based on cell detachment, this suramin concentration was found to be the maximal nontoxic concentration. Binding studies were then carried out 24 h later.

RNA Extraction and mRNA Determination

To determine FGFR mRNA half-life, cells were grown for the indicated times in the presence of 5 µg/ml actinomycin D(26) . Poly(A) RNA fractions were then extracted, electrophoresed under denaturing conditions, and hybridized as described above. The amount of radioactivity of each band was quantified by using a PhosphorImager (Molecular Dynamics, Sunnyvale, Ca). The glyceraldehyde-3-phosphate dehydrogenase mRNA was analyzed in parallel.

Confocal Analysis of FGFR-1

For control experiments, cells were incubated: (i) with anti-FGFR-1 antibodies preadsorbed with the synthetic peptide used for immunization, (ii) with fluorescein isothiocyanate-conjugated anti-rabbit -globulins (1/80) in the absence of the primary antibody. The specificity of the antibody was checked by immunoprecipitating cell extracts in the presence of increasing concentrations of the immunizing peptide and protein A-Sepharose. The cross-linking of the immunoprecipitates with I-FGF-2 was followed by SDS-PAGE separation and autoradiography. As expected, one single band of about 165 kDa was observed corresponding to the receptor and the intensity of the band decreased with increasing peptide concentrations.

To evaluate the FGFR-1 distribution in the different cell compartments, confocal laser scanning microscopic studies were performed using a LSM10 Carl Zeiss microscope equipped with a 63 objective. Fluorescein-stained cells were illuminated with the 488 nm line of the Argon laser. Optical sections of 0.1 µm were collected and analyzed. Each section was scanned eight times. Color pictures from screen images were taken on Ektachrome Kodak iso/100 films. 10-15 cells from three independent experiments were analyzed.

RT-PCR of the FGFR-1 Receptor

Sizes of amplified products corresponded to those expected according to the position of the primers in the FGFR-1 cDNA sequence. The specificity of the PCR products was also checked by using restriction enzymes, selected according to the sequence of the rat FGFR-1 cDNA using computer analysis (PC/GENE, IntelliGenetics, Mountain View, CA). After gel electrophoresis, bands were extracted with the GeneClean II kit (BIO 101 Inc., Vista, CA), digested with restriction enzymes. BstEII was used for the SAS1 and XhoI for the SAS2 amplified products. Both enzymes were selected for their ability to digest each sequence at only one selected restriction point. The two enzymes gave rise to two fragments at the expected sizes. As negative controls we used HO to replace RNA during RT-PCR and the restriction enzyme EcoRV which was unable to digest the PCR products according to the cDNA sequence.

Calcium- and Phospholipid-dependent Protein Kinases C Activity

RESULTS

Up-regulation of FGFR in FGF-2-expressing Cells

Figure 1: Association-dissociation kinetics for I-FGF-2 binding to CAT and A3 cells. Cells were plated at 60,000 cells/cm in 35-mm dishes. Ligand bound to the high affinity binding sites was first removed at pH 4.0. The cells were then incubated at 4 °C, in Krebs-Hepes buffer, pH 7.4, containing 0.2% gelatin, protease inhibitors, and 50 pMI-FGF-2, with or without 500 nM unlabeled FGF-2. At the indicated times cells were washed and then lysed. T, total binding; NS, nonspecific binding. The dissociation kinetics were determined by adding 500 nM FGF-2 to cells previously incubated in the absence of unlabeled FGF-2. The dissociation curves are given as specific binding. Each point represents the average of results obtained with triplicate cultures.

Equilibrium saturation binding experiments were conducted in order to analyze the high affinity receptors. Typical ligand saturation isotherms and Scatchard plots are shown in Fig. 2. Nonspecific binding increased linearly with increasing concentrations of unlabeled FGF-2. A 3-fold increase in maximal I-FGF-2 binding capacity was observed in A3 cells and a 2-fold increase in A5 cells, compared to those of parental AR4-2J and CAT cells (Table 1). Maximal number of high affinity binding sites per cell were around 1200 in mock cells, comparable to values reported for other cell types expressing low levels of FGFR(15) . They increased to about 3700 in A3 cells. No significant modifications were detected in the K values for the high affinity receptors (about 50 pM) among the different cell lines (Table 1).

Figure 2: Saturation isotherms and Scatchard plots for I-FGF-2 binding to CAT, A5, and A3 cells. Upper panel, the cells were incubated with increasing concentrations of radioligand, for 4 h at 4 °C, with and without 500 nM unlabeled FGF-2. The data reported correspond to the I-FGF-2 specifically bound to the high affinity receptors. Each value is the mean ± S.D. of three separate experiments performed in triplicate. Lower panel, a representative Scatchard analysis of the high affinity binding sites.

Competition binding experiments were carried out to analyze the low affinity sites, by using I-FGF-2 and unlabeled FGF-2 or FGF-1. Typical competition curves are reported in Fig. 3. In all cell lines, FGF-2 inhibited I-FGF-2 binding with a slight higher potency than that displayed by FGF-1. Scatchard analysis of the inhibition data ( Fig. 3and Table 1) revealed a greater binding capacity for FGF-2 only in A3 cells. K values (about 2 nM) were comparable among the different cell lines and similar to the data of the literature(15) . Experiments performed with three different clones gave identical results (not shown).

Figure 3: Competitive inhibition of the I-FGF-2 binding by unlabeled FGF-2 and FGF-1. Upper panel, increasing concentrations of unlabeled FGF-2 and FGF-1 were used to inhibit I-FGF-2 (50 pM) binding to CAT, A5, and A3 cells. The cells were incubated 4 h at 4 °C. Values represent the percentage of specific binding determined at different concentrations of competitors. Lower panel, a representative Scatchard analysis of the low affinity binding sites for FGF-2.

Hill plot analysis of the experimental data obtained for both high and low affinity sites indicated that Hill coefficients were not far from 1, suggesting that there was no cooperativity between these two classes of receptors in any cell line.

FGF-2 Expression Did Not Modify the Apparent Molecular Weight of the High Affinity Receptors

Figure 4: Cross-linking of I-FGF-2 to high affinity receptors on the different cell lines. Cells were incubated 4 h at 4 °C in the presence of the radioligand (50 pM), then washed and cross-linked to I-FGF-2 in PBS containing 0.5 mM bis(sulfosuccinimidyl)suberate for 30 min at room temperature. After cross-linking, cells were scraped off, sedimented, lysed with the electrophoresis sample buffer, and analyzed on a 6.5% SDS-PAGE, followed by autoradiography. Lane NS corresponds to the cross-linking of A3 cells in the presence of 500 nM unlabeled FGF-2. The migration of some molecular weight markers is shown on the left.

The High Affinity Receptors Are Still Down-regulated

The effect of exogenous FGF-2 was analyzed by incubating CAT and A3 cells at 37 °C for 3 h with increasing concentrations of FGF-2(19) . Ligand binding was carried out after extensive washings with 2 M NaCl at pH 4.0 to remove the FGF-2 bound to the cells. Cell morphology after these washings was found unmodified. A dose-dependent decrease in specific binding was observed with IC of 0.1 nM for A3 and 0.7 nM for CAT cells (not shown). Thus, endogenous FGF-2 expression did not modify the process of receptor down-regulation.

To analyze the effects on FGF receptor levels of the FGF-2 eventually secreted by transfected AR4-2J cells, cells were grown in the presence of 30 µg/ml of a neutralizing anti-FGF-2 antibody or 40 µM suramin, which is known to reduce the FGFR internalization by interacting with the extracellular FGF-2(19, 20) . Both suramin (Fig. 5) and anti-FGF-2 (not shown) did not alter binding in CAT and A3 cells and caused a further 80 and 60% increase, respectively, in A5 cells. Thus, the 22.5-kDa peptide remains in the intracellular compartments whereas as expected, the 18-kDa form is secreted and decreases the receptor level.

Figure 5: Effect of suramin on FGF receptor levels. Cells were grown 24 h with and without suramin at the concentration of 40 µM before the experiment. The data reported correspond to the modification of I-FGF-2 binding to the high affinity receptors for the same cell number. Data correspond to the mean of three independent experiments.

FGF-2 Expression Increases the FGFR-1 mRNA Levels

Figure 6: FGFR mRNA expression. Northern blot analysis of the FGFR were performed with poly(A) mRNA fractions. After transfer, the nylon membranes were hybridized as described under ``Experimental Procedures'' with P-labeled human FGFR probes. Three µg of mRNA from various cell lines was subjected to Northern blot and hybridized with 20 10 cpm/ml of FGFR-2, -3, and -4 probes and 10 cpm/ml glyceraldehyde-3-phosphate dehydrogenase (GAPDH) probe. FGFR-4 was undetectable.

Figure 7: FGFR-1 mRNA up-regulation. Six µg of the mRNA fraction from CAT and 3 µg from A5 and A3 cells were hybridized with 10 10 cpm/ml FGFR-1 probe. FGFR-1 mRNA was found overexpressed in the FGF-2-expressing cells. kb, kilobase(s).

The High Molecular Weight FGF-2 Increases the FGFR-1 mRNA Stability

Figure 8: Increase in half-life of FGFR-1 mRNA. Cells were incubated in the presence of actinomycin D at 5 µg/ml. At the indicated times, the poly(A) fraction was extracted and probed by Northern blot hybridization. The radioactivity was quantified by a PhosphorImager. Results correspond to the mean of three independent experiments. , glyceraldehyde-3-phosphate dehydrogenase; , A3 cells; , CAT cells.

To determine whether the FGFR-1 mRNA regulation occurred also at the transcriptional level, some nuclear run-on transcription assays were performed in CAT and A3 cells. While lipase transcription (performed as control) was clearly detectable in both cell lines, that of the FGFR-1 was too low to be quantified.

Confocal Analysis of the FGF Receptor Type 1

Figure 9: Analysis of the FGFR-1 by confocal microscopy. Cells were fixed with 3% paraformaldehyde and permeabilized with 0.25% Triton X-100. After washing with PBS containing 0.1% bovine serum albumin, cells were incubated overnight at 4 °C with a 1:250 dilution in PBS, 0.1% bovine serum albumin, of rabbit polyclonal antibody to a sequence of the extracellular domain of the FGFR-1 and then with fluorescein isothiocyanate-conjugated goat anti-rabbit secondary antibody. After 30 min at room temperature, the cells were washed and analyzed for fluorescence. From a to d: four different focal planes, at 1.5-µm intervals from the bottom of the culture coverslips. Arrows indicate the cell surface immunofluorescence on the different planes of the same cells. No reactivity was observed at the nuclear level and in the extracellular space. The micrographs correspond to the cell line AR4-2J.

RT-PCR Analysis of the FGFR-1 Variants Expressed by the FGF-2-producing Cells

Figure 10: RT-PCR amplification of the FGFR-1 receptor mRNA. The RT-PCR was performed as described under ``Experimental Procedures'' with two sets of primers. The amplified products were separated by electrophoresis in a 5% polyacrylamide gel and visualized by staining with ethidium bromide. Left, the amplification products of the extracellular region obtained with the primer pair SAS1. Two bands of about 1100 (the major band) and 800 bp were obtained. Right, the cytosolic sequence containing the membrane-spanning domain was amplified with the primer pair SAS2. Two PCR products evaluated at about 2100 and 950 bp were obtained. 1, CAT; 2, A5; 3, A3. 1.5 µg of the cDNA obtained by the reverse transcriptase were used for PCR amplifications.

To study the intracellular domain of FGFR-1, the nucleotide sequence 1075-2028 containing the transmembrane region was amplified (Fig. 10, right). An amplification product of about 950 bp was obtained in all cell lines, corresponding to the expected size of the membrane-spanning and the intracellular domain. Some partially spliced forms of the primary transcript were observed in the extracellular domain of the receptor in A3 cells and also in the intracellular domain, in all cell lines (at 2100 bp). In AR4-2J cells, immature forms of some other mRNA have already been observed. ()

Thus, PCR amplifications revealed a FGFR-1 variant containing 2 Ig-like domains. According to the whole data the transfected cells did not appear to display major modifications in the expression of the FGFR-1 spliced variants.

Translocation of the Ca-Phospholipid-dependent PKC by Exogenous FGF-2 in FGF-2-expressing Cells

Figure 11: Effect of exogenous FGF-2 on the translocation of the Ca-phospholipid-dependent PKC. Cells were plated at 60,000 cells/cm in 10% fetal calf serum-containing medium. 24 h later cells were washed and medium was replaced by serum-deprived medium buffered with 20 mM Hepes at pH 7.2. FGF-2 at the concentration of 1 nM was added and PKC activity was measured 20 min later when PKC activity in the particulate fraction reached the plateau value. The particulate fractions were prepared as described under ``Experimental Procedures.'' Figures correspond to the particulate fractions and are representative of one experiment, repeated two times.

DISCUSSION

FGF-2 is synthesized as different molecular weight isoforms lacking the signal peptide sequence for secretion. The high molecular weight forms possess a nuclear-targeting sequence. All these peptides are concentrated in the cells. The multiple biological functions of FGF-2 have been suggested to result from the cooperation of these isoforms acting at specific cell compartments. The 18-kDa form can be secreted and the subsequent activation of the FGF receptors-tyrosine kinase elicits the cascade of intracellular events(3, 4) . The functions of the intracellular isoforms, including the non-secreted 18-kDa peptide, are still unknown. Previous studies reported that cells transfected by plasmidic vectors containing the FGF-2 cDNA expressed high amounts of FGF-2 and exhibited low levels of high affinity FGF receptors(19, 20) . Secretion of the 18-kDa form in the extracellular space has been suggested to be responsible for receptor internalization(18, 19, 20) . By contrast, in human malignant tissues, such as pancreatic cancers, glioma, and other brain tumors, up-regulation of high affinity FGFR concomitantly with an increased production of FGF-2 has been reported(31, 32, 33) .

Transfected cells often express very high levels of FGF-2 (for instance in the range of 75-600 ng of FGF-2/million cells)(19, 20) , compared to normal and tumor cells(34) . In these transfected cells, the FGFR down-regulation induced by secreted FGF-2 might mask the increased receptor biosynthesis by the FGF-2 forms localized in the intracellular compartments. Therefore, we used a retroviral infection system to obtain the production of lower levels of FGF-2 (about 0.5-2 ng/million cells) (13, 34) and we chose a pancreatic acinar cell line (AR4-2J) in which cell differentiation can be easily determined by morphological analysis and by measuring the biosynthesis rate of the secretory enzymes(24) . Inasmuch as AR4-2J cells do not produce FGF-2(21) , the biological effects resulting from the induction of low level expression of each isoform can be more easily analyzed. In contrast to cells overexpressing FGF-2, retroviral-infected A3 and A5 cells did not show any modification in cell morphology or differentiation state, biosynthesis and secretion of digestive enzymes, and cell regulation by glucocorticoids(21) . On the other hand, by using this infection system identical properties are often observed in the different cell clones (35) as we indeed observed among the different clones isolated for each FGF-2 infection(21) .

The present study provides evidence that the low level expression of either the cytosolic 18-kDa or the nuclear-targeted 22.5-kDa FGF-2 isoform increased the cell surface high affinity receptors, by 2- and 3-fold, respectively. By contrast, exogenously added FGF-2 exerted opposite effects resulting in FGFR down-regulation. Only the 22.5-kDa peptide was found to up-regulate the low affinity binding sites. Binding experiments performed on other clones expressing the FGF-2 peptides confirmed the above results. In control CAT cells, maximal binding capacities of high and low affinity receptors were identical to those of parental cells, indicating that the vector did not contribute to the modifications observed. Thus, the increase in the high affinity receptors reported in the present study on cultured cells resemble that which was published on tumor cells in vivo(31, 32, 33) . In all cell lines the affinities of both classes of receptors remained unchanged as well as the greater affinity for FGF-2 compared to FGF-1.

Altering the cell density and adding exogenous 18-kDa FGF-2 down-regulated the high affinity receptors as in control cells. Cell treatment by neutralizing anti-FGF-2 antibodies and by suramin did not increase the high affinity binding sites on control and A3 cells expressing the nuclear-translocated isoform. A5 cells which synthesize the secretory 18-kDa FGF-2 molecular form(36) , displayed as expected, a further increase (about 60-80%) in the cell surface binding sites. On the other hand, confocal analysis of A3 cells with anti-FGF-2 antibodies confirmed that the 22.5-kDa form was localized in the nucleus and was absent at the cell surface and in the extracellular space (data not shown). Taken together these data suggest that the transfected FGF-2 up-regulated the high affinity receptors by acting at a level other than the down-regulation process.

FGF-2-producing cells and mock cells shared comparable cell diameters and doubling times(21) , hence these parameters should do not play a role in receptor up-regulation. On the other hand, modifications of high affinity binding sites during cell differentiation have been reported both in vivo(37) and in vitro(38) in the absence of significant modulations of FGFR-1 mRNA levels(20, 38) . By contrast, morphological and biochemical data showed that the differentiation state of A3 and A5 cells was strictly comparable to that of control cells (21) and as reported below, FGFR-1 mRNA levels were found increased in the present study. These data suggest that cell differentiation is not responsible for the receptor modulation.

Although mRNAs encoding for FGFR-1, -2, and -3 were observed in all the cell lines, only the level of the FGFR-1 transcript was found increased, suggesting that FGF-2 exerts a positive control on the expression of this receptor. Cells producing the secretory 18-kDa form expressed lower levels of FGFR mRNA compared to A3 cells (a 3-fold increase in A5 cells against a 6-fold increase in A3 cells). This finding was the opposite of what was expected if the mRNA induction occurred via the activation of cell surface receptors. Therefore, these data further suggest that the receptor up-regulation did not occur through the activation of the cell surface receptors but rather through an intracrine mechanism. Similar conclusions were recently reached concerning proliferation of cells expressing the high molecular weight isoforms. Indeed, by using dominant negative FGF receptors it has been shown that proliferation was independent from the activation of the cell surface receptors(39) .

We chose A3 cells to analyze the FGFR-1 mRNA stability since the 22.5-kDa form is not secreted leading therefore to an easier interpretation of the results obtained because of the absence of any receptor occupancy. A 3-fold increase in the FGFR-1 mRNA half-life was found compared to control cells. These data show first that the 22.5-kDa form is able to exert a control on the stability of the mRNA transcript(s), second that the increase in FGFR-1 mRNA level occurred at least in part, through a modulation of its half-life. The 22.5-kDa FGF-2 although chiefly concentrated in the nucleus, is also localized in the cytoplasm close to the endoplasmic reticulum (14) , suggesting some possible function at the cytoplasmic level. However, whether the control exerted on the mRNA occurs at the cytoplasmic level remains to be confirmed. The existence of a control on mRNA stability by the 22.5-kDa isoform which has not previously been reported, opens new avenues for investigating the molecular mechanisms whereby this FGF peptide exerts its effects.

Immunofluorescent staining and confocal analysis revealed the presence of the FGFR-1 subtype at the cell surface of AR4-2J and transfected cells and did not detect a secretory FGFR-1 isoform in the extracellular space. PCR analysis revealed in all cell lines the FGFR-1 isoform containing the 3 Ig-like domains. However, it also revealed in all cell lines a mRNA species encoding the variant with 2 Ig-like regions, undetected by Northern blots. The corresponding protein was not observed by cross-linking studies, raising the possibility that this mRNA species was probably untranslated. Thus, the present data did not show any important modification of the FGFR-1 variant expressed in FGF-2-transfected cells. The higher affinity of the FGFR for FGF-2 than for FGF-1 in all cell lines, agrees with the presence of the IIIa sequence in the mRNA region encoding the third Ig-like domain(4) , irrespective of the FGF-2 isoform expressed.

The in vivo relevance of the up-regulation of the FGFR-1 could be suggested taking into account that the different FGF-2 isoforms are indeed produced by the same cell. The biosynthesis of each of them is under specific controls inducing preferentially the initiation of translation at the AUG or CUG codons(10) . According to the predominant expression of the 18-kDa or the 22.5-kDa isoform, either the high affinity receptor or both high and low affinity receptors will be up-regulated. Thus, the endogenous isoforms appear to cooperate with the regulations evoked by exogenous FGF-2 and to amplify cell responses. The increase of the PKC translocation in A3 cells after exogenous FGF-2 stimulation, agrees with that hypothesis. In addition to the control exerted by FGF-2 isoforms at FGFR-1 level, it will be of biological relevance to analyze also their regulations on the different second messenger levels involved in the transduction pathway evoked by exogenous FGF-2.

FOOTNOTES

*

This research was supported by Association pour la Recherche sur le Cancer Grant 6165 (to F. C.) and by United States Public Health Service Grant DK-44948 awarded by the National Institutes of Health (to M. K.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked ``advertisement'' in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

§

To whom correspondence should be addressed: INSERM U 151, CHU Rangueil, Bat L3, 31054 Toulouse Cedex, France. Tel.: 33-61-32-24-01; Fax: 33-62-26-40-12.

()

The abbreviations used are: FGF-2, basic fibroblast growth factor; FGF-1, acidic fibroblast growth factor; FGFR, FGF receptors; PBS, phosphate-buffered saline; PKC, protein kinase C; CAT, chloramphenicol acetyltransferase; RT-PCR, reverse transcriptase-polymerase chain reaction; bp, base pair(s).

()

M. Svoboda, personal communication.

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